2.1.1 Innate Immune System

The state of OS exerts a direct influence on the functionality of immune cells through the regulation of various cellular signaling pathways. In the context of the innate immune system, the polarization and activity of macrophages are significantly modulated by the concentrations of ROS. Moreover, the formation of neutrophil extracellular traps (NETs) is similarly impacted by ROS levels. Additionally, in dendritic cells (DCs), ROS plays a pivotal role in the regulation of cross-presentation capacity [27-29]. Despite these insights, a comprehensive understanding of the multifaceted roles of OS in immune responses remains to be fully elucidated, highlighting the need for further exploration into the interconnected mechanisms driving these processes.

2.1.2 Adaptive Immune System

In the context of adaptive immunity, the levels of ROS significantly influence the activation and metabolic processes of T cells and B cells, thereby impacting the progression of immune responses. This modulation underscores the critical role of ROS in shaping the functional dynamics of adaptive immune cells [58, 59]. However, despite these findings, a comprehensive understanding of the intricate mechanisms by which ROS affect T cell and B cell function remains to be fully elucidated, calling for further investigation into the interconnected pathways that govern immune response modulation.

2.1.3 ROS Function in Host Defense

ROS play a crucial role in host defense, acting both as essential antimicrobial agents and, when in excess, as mediators of OS that can impair tissue repair. During the immune response to wounds and infections, professional phagocytes such as neutrophils and macrophages generate ROS through the NOX complex as part of the respiratory burst [78-80]. This enzymatic complex reduces molecular oxygen to O₂•−, which is rapidly converted to H₂O₂ and other reactive species within phagosomes, effectively killing engulfed pathogens [78-80]. Moreover, H₂O₂ can diffuse extracellularly, creating antimicrobial gradients extending hundreds of micrometers from the wound site, further restricting bacterial growth and preventing infection spread [80].

However, ROS are not only microbicidal agents but also signaling molecules that regulate immune cell recruitment and inflammatory responses. For instance, quercetin decreased ROS-induced OS and inflammation by suppressing NOX2 production [81].

ROS also mediate host defense through the activation of key inflammatory signaling pathways [82]. They modulate NLRP3 inflammasome activation by oxidizing mitochondrial thioredoxin and releasing thioredoxin-interacting protein, facilitating inflammatory cytokine maturation and pyroptosis [83, 84]. ROS influence NF-κB signaling by promoting IKKβ activation and facilitating nuclear translocation, thereby amplifying proinflammatory gene expression [85, 86]. MAPK pathways (including ERK, JNK, and p38) are activated via ROS-dependent oxidation of upstream kinases, sustaining inflammatory responses [87, 88]. Additionally, ROS regulate Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling through modulation of STAT phosphorylation, affecting cytokine-driven immune functions [89].

2.2.1 Nuclear Factor Erythroid 2-Related Factor 2

The Keap1–Nrf2–antioxidant response elements (ARE) pathway plays a critical role in maintaining cellular redox balance and metabolism, as well as in inducing adaptive responses to OS, thereby being closely associated with the pathogenesis of various diseases [90, 91]. When the levels of ROS increase in cells, they oxidize cysteine residues in Keap1, leading to the dissociation of Nrf2. This dissociation allows Nrf2 to escape ubiquitination and the subsequent proteasomal degradation, facilitating its translocation to the nucleus [92]. In the nucleus, Nrf2 forms heterodimers with Maf proteins to activate the transcription of antioxidant enzyme genes regulated by ARE, thereby preventing OS [93]. The Keap1–Nrf2–ARE pathway established by this process is pivotal in combating OS and maintaining cellular metabolic balance, and it is associated with adaptive responses in various inflammatory diseases. Furthermore, OS also influences the phosphorylation of Nrf2 and the nuclear export of Keap1 by activating other kinases, thereby further regulating Nrf2 activity and degradation, which finely tunes cellular responses to OS.

Multiple studies have revealed sophisticated mechanisms by which mtROS modulate the Keap1–Nrf2 pathway [94]. Increased mtROS induce modifications of Keap1 cysteine residues, disrupting this interaction and permitting Nrf2 activation [94]. And there is multiple proteins serving as a critical mediator linking mtROS levels to the regulation of the antioxidant TF Nrf2. For instance, mtROS promote the accumulation of phosphoglycerate mutase 5, which interacts with Keap1 and facilitates its translocation to the outer mitochondrial membrane. This interaction impairs Keap1-mediated ubiquitination and proteasomal degradation of Nrf2, resulting in increased Nrf2 stability and enhanced transcriptional activity [95, 96]. Furthermore, mtROS influence selective mitophagy through an Nrf2-dependent positive feedback loop involving p62/SQSTM1. By competing with Nrf2 for Keap1 binding, p62 sustains Nrf2 activation, integrating mtROS signaling with the autophagic removal of damaged mitochondria [97].

Furthermore, Nrf2's regulatory influence extends to ferroptosis. Activation of Nrf2 enhances cellular antioxidant defenses by upregulating genes such as GPX4 and FTH1, which reduce both ROS levels and free iron availability, thereby inhibiting ferroptosis [98, 99]. Nrf2 also modulates ferroptosis indirectly through noncoding RNAs, including lncRNAs and microRNAs (miRNAs), which influence Keap1 expression and Nrf2 nuclear translocation [100-102].

2.2.2 Nuclear Factor Kappa-B

There is a significant interaction between ROS and NF-κB. As a crucial TF, NF-κB is involved in various cellular processes, including immunity, inflammation, cell proliferation, and apoptosis, and is associated with numerous diseases. Both its classical and nonclassical activation pathways are strictly regulated, with the classical pathway primarily activating NF-κB through IκB kinase β (IKKβ)-mediated phosphorylation of inhibitors of NF-κB α (IκBα), which leads to its degradation and allows NF-κB to translocate to the nucleus to activate target gene transcription [103, 104]. Studies have shown that ROS mainly inhibit the phosphorylation of IκBα, thereby impairing its ubiquitination and degradation processes, including the influence of exogenous H₂O₂ on the phosphorylation of IκBα at tyrosine residues [103, 104]. Furthermore, ROS suppress IKK-activating kinases while covalently inhibiting Ubc12's ubiquitin-conjugating activity [104].

2.2.3 Hypoxia-Inducible Factor 1-Alpha

ROS play a critical role in regulating the activity of HIF-1α [105]. Under normal conditions, HIF-1α is hydroxylated by its key inhibitor, prolyl hydroxylase 2 (PHD2), leading to its degradation. However, elevated levels of ROS can inactivate PHD2 through redox-dependent dimerization, thereby stabilizing HIF-1α even in the presence of sufficient oxygen [105]. Additionally, ROS can directly oxidize cysteine residues in the PHD protein, further inhibiting its activity, reducing HIF-1α degradation, and reinforcing its stabilization. Meanwhile, the Cys520 residue in HIF-1α itself can also be oxidized by ROS, thereby inhibiting the ubiquitination mediated by von Hippel–Lindau tumour suppressor protein, which further stabilizes HIF-1α. These ROS-mediated oxidative modifications result in the accumulation of HIF-1α within the cell and promote its translocation to the nucleus [ 106]. In the nucleus, HIF-1α forms a complex with HIF-1β and is recruited to hypoxia response elements, thereby transactivating hypoxia-responsive transcriptional programs, such as genes involved in immune response, energy metabolism, iron metabolism, glucose metabolism and transport, as well as cell proliferation, survival, and angiogenesis [105, 107, 108].

2.2.4 FOXO

ROS have a complex regulatory effect on FOXO TFs, involving both direct redox control, such as the acetylation of FOXO mediated by acetyltransferase p300, which reduces its DNA-binding capacity, and indirect pathways, such as the modulation of FOXO phosphorylation state and other posttranslational modifications via the influence of upstream factors like growth factor receptor activity, protein kinases, and phosphatases [109]. In the PI3K–protein kinase B (PKB) signaling pathway, ROS are generally associated with the activation of growth factor receptors, which suppresses FOXO activity through the activation of this pathway, promoting cellular metabolism and growth [110]. Additionally, while OS may lead to the inactivation of FOXO, it also enhances the activity of certain kinases, such as members of the MAPK family [111-113], which further regulate FOXO phosphorylation and function, thereby playing a crucial role in maintaining balance at the cellular and organismal levels.

2.4.1 Redox Regulation of mtROS Production

Mitochondria are the primary sites for aerobic respiration in cells, synthesizing ATP through the process of oxidative phosphorylation. The ETC consists of four core complexes (i.e., complex I, II, III, and IV) and coenzyme Q (CoQ), which work together to form a coherent electron transport system [126, 127] that catalyzes chemiosmotic ATP biosynthesis. In this system, electrons originate from nicotinamide adenine dinucleotide [reduced form] (NADH) or flavine adenine dinucleotide, reduced (FADH₂) and are gradually transferred through these complexes, ultimately being transferred to oxygen, leading to the generation of water [126, 127]. However, there is a phenomenon of electron leakage within the ETC, which typically occurs at specific sites in complex I and III, resulting in the combination of electrons with oxygen to produce superoxide anion radicals (O₂•−) and other ROS such as H₂O₂ and hydroxyl radicals (•OH) [127].

The regulation of the intracellular antioxidant system involves the synergistic action of various antioxidant enzymes and nonenzymatic antioxidants, which effectively eliminate ROS and prevent their excessive accumulation. SOD, as a crucial enzyme, converts O₂•− into H₂O₂, which is subsequently decomposed into water and nontoxic metabolites by CAT and GSH peroxidase (GSH-Px), thereby maintaining redox balance within the cell [128-130]. In addition, the active states of ETC complexes are finely regulated through posttranslational modification mechanisms such as phosphorylation, acetylation, and nitration, which in turn affect ROS production. For example, the phosphorylation state of ETC complex I is directly correlated with its activity and ROS generation levels [131]. Furthermore, the regulation of mitochondrial dynamics, including the processes of mitochondrial fission and fusion, significantly impacts ROS generation. Mitochondrial fission promotes ROS production by increasing the surface area of the ETC, while mitochondrial fusion aids in ROS clearance and mitochondrial function restoration, thereby enabling a complex regulation of ROS production at the cellular level [132].

2.4.2 Redox Regulation of Energy Metabolism

ROS play a crucial role in regulating mitochondrial energy metabolism. As signaling molecules, ROS can activate or inhibit specific signaling pathways, thereby modulating mitochondrial metabolic pathways. For example, ROS can activate AMPK, which is a key energy sensor that regulates cellular energy metabolism [133]. Additionally, ROS can influence various metabolic pathways within mitochondria, including the TCA cycle, fatty acid β-oxidation, and amino acid metabolism. The regulation of these pathways alters the generation and consumption of metabolic products, significantly impacting the energy status of the cell [7]. Moreover, ROS are involved in the regulation of mitochondrial biogenesis processes, such as the replication, transcription, and translation of mitochondrial DNA (mtDNA), as well as mitochondrial dynamics, including mitochondrial fission and fusion. These processes are vital for maintaining normal mitochondrial function and fine-tuning energy metabolism [7].

2.5.1 Redox Regulation of mRNA Translation

The entire process of mRNA translation is precisely regulated by the redox state. Studies have shown that the activity of initiation factors during translation is modulated by the redox state. For example, the phosphorylation state of eukaryotic translation initiation factor 4E (eIF4E) can influence its ability to bind to the 5′ cap structure of mRNA, and this phosphorylation process may be regulated by ROS [134, 135]. Furthermore, the GTPase activity of eukaryotic translation initiation factor 2 (eIF2) is also regulated by redox-sensitive modifications, thereby affecting the efficiency of translation initiation [135-138]. Translation termination involves the recognition of stop codons and the release of nascent proteins, and this process is similarly regulated by the redox state. For instance, the activities of release factors eukaryotic release factor 1 (eRF1) and eRF3 may be modulated by ROS, affecting the efficiency of translation termination [135]. Research indicates that H₂O₂ may directly impact ribosome biogenesis and the enrichment of proteins in cytoplasmic translation, thereby regulating the activity of ribosomal proteins and translation initiation factors [138].

2.5.2 Redox Regulation of ER Homeostasis

ER is a core site for protein folding and modification within the cell, crucial for maintaining physiological functions. Within the ER, membrane proteins and secretory proteins undergo precise folding, especially the formation of disulfide bonds, a process reliant on the oxidative environment of the ER, termed “oxidative protein folding.” Protein disulfide isomerase (PDI) and the oxidoreductase ERO1-like alpha protein (ERO1A) serve as key enzymes, working together to maintain the oxidative environment of the ER and induce disulfide bond formation [139, 140]. The active site of PDI effectively captures electrons from free thiols of nascent proteins, facilitating the precise construction of disulfide bonds [139, 140]. Accumulation of ROS can disrupt the redox homeostasis of the ER, leading to the accumulation of misfolded proteins and ER stress, which ultimately activates the unfolded protein response (UPR) [141, 142]. Conversely, the UPR can lead to the formation of additional ROS and affect mitochondrial function [141, 142].

2.5.3 Redox Regulation of Protein Degradation

Under conditions of OS, cells enhance the removal of misfolded and/or oxidized proteins through a series of finely tuned regulatory mechanisms such as the ubiquitin–proteasome system and autophagy. The 26S proteasome is responsible for degrading ubiquitin-tagged proteins under nonstress conditions. However, during OS and low ATP levels, it splits into the 20S and 19S subunits, a process supported by heat shock 70 kDa protein (Hsp70) [143]. S-glutathionylation, as a redox regulatory mechanism, can directly act on the 20S proteasome, opening its gate structure and thus enhancing its proteolytic activity [144]. Additionally, the ubiquitin–proteasome system regulates cellular redox balance through the degradation of Nrf2 and the activation of NF-κB, both of which can mediate ROS levels via their downstream antioxidant proteins [145]。

In the regulation of autophagy, the redox state plays a crucial role. Specifically, ROS produced by mitochondria can activate AMPK by oxidizing cysteine residues within the enzyme, leading to the phosphorylation of UNC51-like kinase 1 (ULK1) kinase, which inactivates it; the inactivation of ULK1 is a key step in the formation of autophagosomes [146]. Moreover, the p62/Keap1/Nrf2 system reveals a collaborative mechanism between autophagy and redox regulation, where p62 activates Nrf2 in a redox-independent manner by regulating Keap1 degradation, promoting Nrf2 nuclear translocation and inducing antioxidant gene transcription [146]. Multiple key components in the autophagy pathways are also directly regulated by the redox state. The protease autophagy-related 4 (ATG4), which regulates microtubule-associated protein 1A/1B-light chain 3 (LC3)/ATG8 cleavage and lipidation, contains catalytic and redox-sensitive regulatory cysteine residues [147]. The oxidation state of these cysteine residues can affect the activity of ATG4, thereby regulating the cleavage and lipidation of LC3/ATG8, critical steps in autophagosome formation and maturation [147]. Additionally, ATG3 and ATG7, two enzymes containing catalytic cysteine, undergo inhibition under oxidative conditions, which affects the progression of the autophagy process [148].

4.1.1 Alzheimer's Disease

AD is a prevalent neurodegenerative disorder characterized by progressive cognitive dysfunction and behavioral changes, with OS recognized as key factors in the pathogenesis of AD. In the brain tissue of AD patients, the level of OS is significantly elevated, and this phenomenon is closely associated with degenerative changes in neurons and declines in cognitive abilities. The increase in ROS in AD patients primarily arises from multiple factors, including mitochondrial dysfunction, the aggregation of amyloid-beta (Aβ), and neuroinflammation.

In AD, the accumulation of Aβ has a pronounced impact on cellular metabolism and survival, particularly regarding the dynamic balance of mitochondrial function and OS [209]. The accumulation of Aβ compromises the integrity of the mitochondrial membrane, leading to a reduction in mitochondrial inner membrane potential. This process not only activates the mitochondrial permeability transition but also enhances the membrane potential, thereby exacerbating the production of ROS [206, 210]. The accumulated ROS can directly damage mitochondria, resulting in functional impairments and triggering a series of apoptotic pathways involving cytochrome c [206]. Additionally, ROS can activate various cellular stress kinase signaling pathways, such as JNK [211] and p38–MAPK [212], through the damaging of intracellular lipids, proteins, and DNA, thus promoting apoptosis and accelerating the progression of AD [206].

Moreover, OS is closely linked to the immune responses of microglia and astrocytes in AD. Microglia, acting as immune sentinels in the brain, are typically activated as a protective response. However, during the pathological process of AD, overactive microglia release large amounts of proinflammatory factors such as tumor necrosis factor-α (TNF-α), IL-1β, and IL-6 [192]. These proinflammatory factors activate signaling pathways like NF-κB and the NLRP3 inflammasome, leading to further inflammatory responses and creating a vicious cycle of interaction [192, 213]. This neuroinflammation may result in neurons being unable to effectively counter oxidative damage, which is a significant contributor to cognitive decline in AD patients [192, 213].

In AD, reactive nitrogen species (RNS) also contributes to neurodegeneration primarily through nitrative and nitrosative modifications of key proteins involved in disease pathology [192, 213]. RNS-mediated S-nitrosylation of tau promotes tau hyperphosphorylation, aggregation, and Aβ oligomerization, accelerating plaque and tangle formation [214-216]. Peroxynitrite (ONOO⁻)-induced tyrosine nitration further destabilizes tau and amyloid precursor protein, exacerbating protein misfolding and synaptic dysfunction [214, 215, 217, 218].

RNS also impair neurotrophic signaling by nitrating proNGF, reducing its binding to TrkA and downregulating TrkA expression, thereby shifting signaling toward p75NTR-mediated apoptosis [214]. Concurrently, nitrative damage to motor proteins and microtubule-associated tau disrupts axonal transport of survival signals, especially affecting basal forebrain cholinergic neurons [214, 215]. Additionally, NO-related pathways dysregulation, including impaired NO/cGMP signaling, contributes to synaptic failure and cognitive deficits [215, 219, 220]. The interplay of RNS-induced protein modifications, neurotrophic receptor imbalance, and transport deficits underlies a pathogenic cascade linking oxidative/nitrative stress with neuronal loss and dysfunction in AD [215].

In the context of AD, the dysregulation of iron homeostasis is also closely related to heightened OS. Existing studies have indicated a significant increase in iron levels in the brains of AD patients. This excess iron can accelerate ROS production via the Fenton reaction, leading to LPO and cell death, and may also activate ferroptosis, a newly recognized mechanism of cell death [221, 222]. Iron promotes the accumulation of lipid peroxides and inhibits GPX4 activity by reacting with intracellular ROS, thereby enhancing cellular oxidative damage [221-224]. Furthermore, excessive iron accumulation damages mitochondria and interferes with mitochondrial energy metabolism, further exacerbating oxidative damage [221, 222, 225]. Therefore, regulating iron homeostasis in the brain has become a crucial research direction for AD therapy, with the development of drugs aimed at reducing iron accumulation and inhibiting OS being a current focus of preclinical studies.

4.1.2 Parkinson's Disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by various motor impairments including muscle rigidity and resting tremor. OS emerges as a central pathogenic mechanism in PD, intricately linked with multiple pathological hallmarks. In the PD brain, heightened OS correlates with dopaminergic neuron loss, α-synuclein (α-syn) aggregation, aberrant iron deposition, and neuroinflammatory activation.

Iron dysregulation plays a pivotal role in PD progression through OS induction and ferroptosis association [226]. Excessive iron catalyzes free radical generation via Fenton reactions, exacerbating cellular damage [226, 227]. Dopaminergic neurons exhibit upregulated transferrin and transferrin receptor (TFR) expression, promoting iron overload that triggers proinflammatory cytokine release and accelerates neuronal injury [226-228]. Critical iron accumulation induces ferroptosis—an iron-dependent cell death pathway. In PD substantia nigra, iron deposition stimulates ROS production and initiates irreversible ferroptotic processes through disrupted iron homeostasis, enhanced LPO, and mitochondrial dysfunction [226, 229].

OS concurrently drives α-syn pathological aggregation. Aggregated α-syn compromises membrane integrity, alters permeability, and induces ROS generation [230]. Membrane-bound α-syn aggregates facilitate abnormal calcium influx, activating ROS-producing signaling cascades [230]. These changes deplete endogenous GSH reserves, increasing neuronal vulnerability to oxidative damage. This self-perpetuating cycle disrupts chaperone-mediated autophagy and accelerates neuronal destruction [230, 231].

Dopamine (DA) serves dual roles as a neurotransmitter and antioxidant modulator. It stabilizes GPX4 to inhibit iron-mediated ROS production, maintaining redox homeostasis [232]. However, DA depletion in PD patients weakens antioxidant defenses, increasing neuronal susceptibility to oxidative membrane damage and apoptosis. The DA transporter (DAT) critically regulates synaptic DA levels and neuronal protection [233]. DAT-mediated DA reuptake prevents oxidative toxicity from synaptic DA excess while supporting intracellular GSH maintenance for ROS neutralization [233-235]. PD-associated DAT downregulation causes synaptic DA accumulation, exacerbating OS and creating a pathogenic loop that promotes ROS accumulation and neuronal death [233, 235].

Mitochondrial dysfunction significantly amplifies OS in PD. Genetic mutations (e.g., Parkin) impair mitophagy, permitting damaged mitochondria to accumulate ROS [231]. Compromised mitochondrial function reduces ATP synthesis and disrupts membrane integrity, worsening oxidative environments [231]. Enhanced calcium influx through mitochondrial l-type channels accelerates basal OS in substantia nigra dopaminergic neurons via increased DA metabolism, promoting age-related mitochondrial oxidative damage and cell death in PD pathogenesis [236, 237].

In PD, RNS contribute to dopaminergic neurodegeneration primarily through nitrative damage to mitochondrial proteins, leading to impaired electron transport and energy failure [238, 239]. ONOO⁻ nitrates mitochondrial and cellular proteins, exacerbating OS and neuronal loss in the substantia nigra [238, 239]. Concurrently, activated glial cells release proinflammatory NO, amplifying oxidative/nitrative stress and sustaining neuroinflammation, which further damages dopaminergic neurons [238, 239].

RNS-mediated nitration disrupts DA metabolism and exacerbates mitochondrial dysfunction, while genetic variations in NO synthase (nNOS) may increase susceptibility by disturbing NO balance [215, 240, 241]. Experimental models show that reducing nitrative stress through NOS inhibition lessens neuronal death and motor deficits, suggesting nitrative stress as a significant mechanism in PD pathogenesis [215, 240-242].

4.1.3 Huntington's Disease

Huntington's disease (HD) is a fatal autosomal dominant neurodegenerative disorder primarily caused by abnormal CAG repeat expansions in the huntingtin gene. The disease is characterized by selective degeneration of the striatum, manifesting as choreiform movements, progressive dementia, and dystonia. HD pathogenesis involves complex mechanisms attributed to mutant huntingtin protein (mHTT), which induces cellular dysfunction and neuronal death. Emerging evidence highlights the critical role of OS in HD pathology, particularly through iron dysregulation and mitochondrial dysfunction [215, 243-245].

Iron accumulation demonstrates a strong association with OS in HD patients and animal models. Postmortem analyses reveal elevated iron levels in the caudate nucleus and striatum of HD patients [246]. This iron overload exacerbates OS and disrupts cellular redox homeostasis. Mechanistically, reduced expression of iron regulatory proteins (IRP1/IRP2) coupled with increased TFR levels indicates impaired iron metabolism regulation. Concurrent upregulation of ferroportin may paradoxically exacerbate intracellular iron accumulation, potentiating ferroptosis [247]. Therapeutic interventions using iron chelators like desferrioxamine (DFO) improve motor and cognitive deficits in HD mice, substantiating the pathogenic role of iron-mediated OS [248].

OS further intersects with mitochondrial dysfunction in HD pathogenesis. As primary ROS production sites, damaged mitochondria in HD exhibit enhanced ROS generation. mHTT induces mtDNA damage and compromises respiratory chain activity, aggravating OS [209, 243]. Through direct suppression of peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α), mHTT disrupts mitochondrial biogenesis and amplifies ROS production [243, 249]. Additionally, calcium homeostasis dysregulation and altered mPTP dynamics promote apoptotic signaling cascades [209, 243, 249].

LPO driven by OS critically impacts neuronal survival. Ferroptosis inhibitors (e.g., Fer-1) significantly attenuate oxidative lipid damage and neuronal death in HD cellular models, identifying LPO as a potential therapeutic target [250]. RNAi screening implicates arachidonate 5-lipoxygenase (ALOX5) as a key mediator in mHTT-induced ferroptosis, with ALOX5 deficiency abolishing polyglutamine of mHTT (HTTQ94)-triggered ferroptosis under oxidative conditions. GPX4 further emerges as a crucial antioxidant defense component, modulating LPO and ferroptosis to support neuroprotective mechanisms [247].

4.1.4 Major Depressive Disorder

Major depressive disorder (MDD), a psychiatric condition characterized by persistent low mood and anhedonia, has emerged as a leading global cause of mental and physical disability. Growing evidence implicates OS as a critical pathogenic contributor to MDD through mechanisms involving cellular damage, inflammatory responses, and eventual cell death. Excessive ROS disrupt neural signaling pathways and interact with cellular lipids, proteins, and nucleic acids, compromising neuronal structural integrity and functional capacity.

Epidemiological investigations and animal studies collectively demonstrate the association between disrupted metal ion homeostasis and emotional dysregulation. Iron accumulation and dysregulated iron metabolism show significant correlations with depressive symptom severity [251]. Experimental evidence reveals that ferroptosis inhibitors (e.g., Fer-1) ameliorate depression-like behaviors and promote neuronal growth, suggesting OS–ferroptosis crosstalk as a potential regulatory axis in depression pathology [251].

Beyond neuronal death, OS interacts with neuroinflammatory processes in MDD pathogenesis. Oxidative damage-derived molecules activate innate immune responses and sterile inflammation, driving proinflammatory cytokine production [252, 253]. Clinical and preclinical studies consistently demonstrate elevated OS markers (8-OHdG and malondialdehyde [MDA]) correlating with depressive symptom severity in MDD patients and animal models, highlighting oxidative-neuroinflammatory interplay as a potential disease mechanism [254, 255].

MDD patients exhibit altered antioxidant enzyme activities, with SOD and GPX levels correlating with clinical severity. Concurrent depletion of nonenzymatic antioxidants (e.g., vitamins C/E) further compromises systemic antioxidant capacity, exacerbating oxidative damage. Dietary supplementation with bioactive antioxidants (e.g., N-acetylcysteine [NAC]) shows potential adjunctive benefits in mitigating depressive symptoms through redox homeostasis restoration [252].

4.1.5 Neuropathic Pain

Neuropathic pain (NeP) represents a prevalent and progressive neurological disorder characterized by spontaneous or evoked pain accompanied by heightened pain hypersensitivity and hyperreactivity. Emerging evidence implicates OS as a critical contributor to NeP pathogenesis through cellular damage promotion and chronic pain development and maintenance. The primary endogenous sources of OS involve NOX enzymes and mitochondrial ETCs, with NOX-derived ROS constituting a major pathogenic component [252]. Experimental studies demonstrate that OS exacerbates neuronal damage and modulates nociceptive signaling pathways, significantly enhancing pain sensitivity in murine models [256].

OS interacts synergistically with LPO in NeP progression. ROS-induced LPO degrades membrane fatty acids, compromising neural cell integrity. Concurrent neuroinflammatory responses amplify oxidative damage, establishing a vicious cycle that perpetuates pain perception. Spinal cord neurons exhibit direct functional impairment correlated with accumulated LPO products [256-258]. Furthermore, OS extends beyond neuronal effects to microglial activation, where ROS-mediated crosstalk between activated microglia and astrocytes drives neuroinflammation through cytokine release and secondary ROS generation [256-258].

Clinical investigations document reduced antioxidant enzyme activity and elevated OS biomarkers in chronic pain patients. This redox imbalance influences neuronal survival while inducing maladaptive plasticity in central nervous system pain processing pathways. ROS accumulation parallels increased proinflammatory cytokine levels, suggesting mechanistic involvement in peripheral and central sensitization processes [256-258]. The progressive nature of oxidative damage underscores its dual role as both consequence and driver of NeP pathophysiology.

4.1.6 Traumatic Brain Injury

Traumatic brain injury (TBI) represents a major global cause of disability and mortality, with OS serving as a critical pathological mediator postinjury. Following TBI, ROS production surges dramatically, overwhelming endogenous antioxidant defenses. This redox imbalance induces oxidative damage to cellular membranes, proteins, and glycolipids, ultimately compromising neuronal structural integrity and functionality while amplifying inflammatory responses and exacerbating secondary ischemic-reperfusion injuries [259].

Emerging evidence links cerebral iron dyshomeostasis to OS pathogenesis in TBI. Mechanical trauma disrupts blood–brain barrier (BBB) integrity and induces intracranial hemorrhage, facilitating iron accumulation that perpetuates ROS generation through self-amplifying cycles. Such iron overload not only correlates with neuronal dysfunction but also potentiates neurodegenerative processes [260, 261]. The postsynaptic density protein 95 (PSD95) critically mediates N-methyl-d-aspartate receptor–PKCα coupling and interacts with neuronal nNOS to enhance NO production, thereby intensifying OS [260-262]. Post-TBI temporal analyses reveal progressive reductions in synaptic markers including PSD95, synapsin I, and synapse-associated protein 97 (SAP-97), reflecting dynamic synaptic remodeling [260-262]. Concurrent glutamate excitotoxicity—driven by BBB disruption, vesicular release mechanisms, and transporter dysregulation—induces profound synaptic alterations that compound neural damage [260-262].

Acute-phase cytokine storms involving IL-1β, TNF-α, and transforming growth factor (TGF-β) induce cerebral inflammation and compromise BBB integrity post-TBI. IL-1β elevation promotes cerebral edema and neuronal loss, while TNF-α dysregulation disrupts synaptic plasticity and central nervous system ion homeostasis, directly correlating with neurological deficits [260]. Therapeutic strategies targeting oxidative pathways show promise: melanin derivatives mitigate neuronal injury through MT2/IL-33 pathway activation and ROS reduction, while Nrf2 overexpression counteracts TBI-induced ferroptosis and synaptic damage via enhanced antioxidant responses [260].

Iron chelation therapy demonstrates clinical potential by reducing cerebral iron deposition and apoptosis, alleviating acute edema and chronic neurotoxicity. Natural Nrf2 activators like curcumin exhibit neuroprotective effects in preclinical models by boosting neuronal antioxidant capacity [263, 264]. These interventions highlight redox modulation as a viable therapeutic axis in TBI management.

4.1.7 Spinal Cord Injury

Spinal cord injury (SCI) constitutes a severe neurological trauma characterized by high mortality and significant disability. Emerging evidence highlights the critical involvement of OS during the acute phase of SCI, with this process involving not only increased ROS production but also compromised antioxidant defense mechanisms [265, 266]. Post-SCI, ROS levels surge dramatically, encompassing elevated superoxide, H₂O₂, and hydroxyl radical concentrations that directly correlate with pathological progression [265, 266].

OS exhibits a reciprocal relationship with mitochondrial dysfunction in SCI pathogenesis [266-268]. mtROS generation activates phospholipases and related enzymes, triggering membrane fatty acid release and impairing ATP synthesis [267, 268]. This pathological cascade disrupts intracellular ion homeostasis, particularly elevating sodium and calcium concentrations, which amplifies oxidative cytotoxicity and cellular damage while potentiating neutrophil activation and secondary ROS generation [267, 268].

In the context of inflammatory responses, OS drives microglial activation and proliferation, initiating proinflammatory cytokine release that reciprocally enhances ROS production-establishing a self-perpetuating cycle [266]. Upregulated inflammatory mediators including IL-6 and TNF-α demonstrate strong OS correlations, actively promoting neuronal apoptosis and exacerbating injury severity post-SCI [266].

The Nrf2 signaling pathway emerges as a crucial regulatory mechanism in OS management [266]. Nrf2 activation enhances antioxidant enzyme expression through coordinated transcriptional regulation, restoring redox homeostasis and mitigating SCI-associated oxidative damage [266]. This endogenous defense system represents a promising therapeutic target for modulating secondary injury processes.

4.2.1 The Epithelial–Mesenchymal Transition

Epithelial–mesenchymal transition (EMT), a pivotal biological mechanism in cancer pathology, involves the phenotypic and functional transformation of epithelial cells into mesenchymal-like states. EMT activation frequently serves as a critical driver of tumor cell migration, invasion, and metastasis during cancer progression [269, 270], with OS emerging as key regulatory modulators of this process.

Accumulating evidence indicates elevated ROS levels during EMT in multiple cancer types, leading to intracellular labile iron pool accumulation and heightened ferroptosis susceptibility [271, 272]. For instance, in melanoma models, TGF-β1 not only induces EMT phenotypes but also upregulates antioxidant enzymes GPX4 and solute carrier family 7 membrane 11 (SLC7A11) [273]. TGF-β1-driven EMT correlates with ROS generation, where enhanced OS paradoxically reduces ferroptosis resistance in post-EMT cells, underscoring redox regulation in EMT–ferroptosis crosstalk [273].

OS extends beyond direct effects on proliferation/apoptosis to reinforce tumor aggressiveness via EMT-related TFs and signaling pathways [274]. mtROS generation, for example, promotes hypoxia-induced EMT in alveolar epithelial cells [275]. ROS-mediated OS facilitates E-cadherin downregulation—a hallmark EMT event marking epithelial identity loss [274, 276]. A critical regulatory axis involves ROS amplification of TGF-β signaling, where Smad complex activation drives downstream EMT gene networks [274, 277, 278]. Consequently, mesenchymal marker upregulation (vimentin, fibronectin) and E-cadherin suppression exhibit strong OS dependencies [274, 276].

Within tumor microenvironments, ROS modulate cancer-associated fibroblast (CAF) behavior through HIF-1α adaptation mechanisms [274, 279]. CAFs exhibit dual redox regulation by elevating ROS production while simultaneously activating antioxidant gene expression, a strategy influencing epithelial plasticity and potentially enhancing tumor invasiveness and therapy resistance [274, 279, 280]. This bidirectional interplay highlights OS as both a facilitator and consequence of EMT-driven malignancy.

4.2.2 Cancer Growth

The interplay between cancer progression and OS is complex and bidirectional. Excessive ROS generation drives cellular damage, promotes tumor proliferation and metastasis, yet paradoxically may induce apoptosis under specific conditions. Substantial evidence highlights OS as a critical facilitator of tumor cell growth, migration, and chemoresistance across multiple cancer types.

ROS critically influence cellular proliferation, survival, and apoptosis through diverse signaling cascades. The MAPK pathway, a key OS-responsive axis, mediates ROS-induced phosphorylation and activation of TFs AP-1 and NF-κB, thereby regulating survival-, proliferation-, and apoptosis-associated gene networks [212, 281, 282].

The PI3K/Akt survival pathway is similarly modulated by ROS. ROS directly activate PI3K, triggering downstream Akt phosphorylation. Activated Akt suppresses proapoptotic factors (e.g., caspase-9), thereby enhancing tumor cell survival [283-285]. Concurrently, the Keap1–Nrf2 system serves as a master OS regulator. Under redox imbalance, Nrf2 escapes Keap1-mediated degradation, translocates to the nucleus, and activates antioxidant gene expression, bolstering cellular defense against oxidative damage [286-289].

ROS further regulate the JAK/STAT axis by inducing JAK activation and subsequent STAT phosphorylation. Nuclear-translocated STAT proteins modulate genes governing proliferation, inflammation, and apoptosis. Aberrant JAK/STAT activation correlates with tumor progression and poor prognosis in multiple malignancies [290-293]. The Wnt/β-catenin pathway, activated under OS, promotes tumorigenesis through β-catenin nuclear accumulation and transcriptional activation of proproliferative genes [294-297].

The p53 pathway exhibits dual roles in OS responses. ROS-induced DNA damage activates p53-mediated cell cycle control and repair mechanisms. Irreparable damage triggers p53-dependent apoptosis, constraining tumor growth. This tumor-suppressive function underscores critical role of p53 in genomic stability maintenance and cancer cell fate determination [298-301].

Notably, ROS exert concentration-dependent effects: while low levels support tumor proliferation, excessive ROS induce senescence or cell death. This dual regulatory role necessitates precise redox homeostasis in cancer cells to survive high-stress microenvironments, highlighting the delicate balance between protumorigenic and cytotoxic ROS thresholds [302].

Besides, OS can reshape the tumor microenvironment through induction of cellular senescence and the senescence-associated secretory phenotype (SASP). Persistent ROS accumulation triggers irreversible growth arrest in cancer cells, leading to SASP secretion of proinflammatory factors (e.g., IL-6, IL-8, VEGF) that paradoxically fuel tumor progression despite cell cycle exit [301].

4.2.3 Cancer Metastasis

Cancer metastasis, a critical phase in malignant progression, involves the dissemination of primary tumor cells to distant sites. OS plays a multifaceted yet pivotal role in this process, not only promoting cancer cell proliferation and survival but also modulating metastatic efficiency through context-dependent induction of cell death.

The crosstalk between ROS and TGF-β signaling profoundly influences metastatic cascades [303]. As a key driver of advanced cancer progression, TGF-β activity is redox-regulated, with ROS amplifying its signaling to upregulate metastasis-associated genes [303]. Mechanistically, ROS enhance the expression of prometastatic proteins (e.g., SNAIL, SLUG) while suppressing E-cadherin in pancreatic ductal adenocarcinoma, thereby augmenting tumor invasiveness [303].

Hypoxic microenvironments further potentiate metastatic progression by elevating ROS levels. HIFs coordinate with TGF-β signaling to regulate metastasis-related gene networks, enhancing tumor cell aggressiveness. This ROS–hypoxia interplay exacerbates tumor malignancy, enabling cancer cells to thrive in complex microenvironments [303].

Fibrotic remodeling, particularly TGF-β-driven fibrosis, contributes to metastasis by fostering immune suppression [303]. ROS overproduction strongly correlates with fibrogenesis, which remodels the tumor microenvironment and impairs immune cell function, facilitating immune-evasive cancer dissemination [303].

4.2.4 Drug Resistance in Cancer

Cancer drug resistance, a major cause of therapeutic failure, severely impacts patient survival and quality of life. OS plays a dual role in resistance mechanisms during chemotherapy and targeted therapies.

In cancer cells, ROS modulate survival and drug resistance through redox-sensitive signaling pathways. Subtoxic ROS levels promote prosurvival and antiapoptotic pathways, whereas excessive ROS may induce mutagenic adaptations underlying resistance [304]. Drug-resistant tumors often exhibit enhanced antioxidant defenses via upregulated GSH and SOD, mitigating ROS accumulation to evade drug toxicity [305].

ROS-regulated TFs, including Nrf2 and NF-κB, orchestrate resistance networks. Nrf2 activation under OS induces ARE-driven cytoprotective gene expression, correlating with multidrug resistance across cancers [306]. NF-κB concurrently enhances survival by upregulating antiapoptotic genes and antioxidant pathways [304, 306].

Therapeutic strategies targeting redox homeostasis show promise in reversing resistance. ROS-boosting agents or antioxidant inhibitors may sensitize resistant cells to chemotherapy. Preclinical studies demonstrate that selective inhibition of antioxidant enzymes elevates intracellular ROS, restoring chemosensitivity and improving therapeutic efficacy [305]. Combinatorial regimens integrating conventional therapies with redox modulators represent a rational approach to overcome resistance.

4.2.5 Inhibition of ROS-Induced Ferroptosis in Cancers

Ferroptosis is a distinct iron-dependent regulated cell death modality triggered by the accumulation of lipid peroxides produced through ROS. This ROS-induced ferroptosis functions as a powerful tumor-suppressive mechanism by selectively eliminating cancer cells. Excessive ROS and LPO compromise cancer cell viability, thereby inhibiting tumor growth and progression.

Key tumor suppressors, such as p53 and BAP1, promote ferroptosis by repressing the expression of antioxidant defense components like SLC7A11, a subunit of the cystine/glutamate antiporter system Xc− responsible for cystine uptake required for GSH synthesis, and GPX4 [307-309]. Downregulation of SLC7A11 and GPX4 diminishes cellular antioxidant capacity, allowing ROS and lipid peroxide accumulation to trigger ferroptotic cell death. Thus, impairment of the GSH/GPX4 axis facilitates ROS-mediated ferroptosis, effectively suppressing tumor development through iron-dependent oxidative destruction of malignant cells [310-312].

The intrinsic metabolic rewiring of cancer cells often results in elevated ROS levels, which makes them particularly vulnerable to ferroptosis. Moreover, factors within the tumor microenvironment, including immune cells such as cytotoxic CD8+ T cells, enhance ferroptosis by secreting interferon-γ (IFN-γ), which downregulates SLC7A11 expression in cancer cells, further sensitizing them to ROS-induced ferroptosis [313].

4.3.1 Atherosclerosis

AS, a complex chronic disease intricately linked to OS, involves multifaceted redox mechanisms throughout its initiation and progression. This section delineates the distinct roles and molecular pathways of OS in AS pathophysiology.

During early atherogenesis, endothelial cells exposed to hyperglycemia, hyperlipidemia, and hemodynamic shear stress exhibit elevated ROS production [314-316]. ONOO⁻ oxidizes tetrahydrobiopterin (BH4) to BH2, triggering eNOS uncoupling. This shifts eNOS from NO generation toward superoxide production, further diminishing NO bioavailability and exacerbating oxidative and nitrative stress [314-316]. The resulting endothelial dysfunction impairs vasodilation and favors proinflammatory signaling through NF-κB activation, promoting leukocyte recruitment and glycocalyx disruption. Simultaneously, ONOO⁻ modifies LDL components, accelerating LDL oxidation and foam cell formation, key steps in plaque development [314-316]. In vascular smooth muscle cells (VSMCs), RNS-induced mitochondrial damage contributes to cellular calcification and vascular stiffness by disrupting NO–guanylate cyclase signaling [317, 318]. Thus, sustained RNS accumulation perpetuates a proatherogenic environment characterized by endothelial dysfunction, inflammation, and vascular remodeling [317, 318].

LDL oxidation generates atherogenic oxLDL—a pivotal driver of plaque development [316, 319, 320]. oxLDL promotes endothelial dysfunction and proinflammatory responses, facilitating monocyte adhesion and transmigration. Subendothelial accumulation of oxLDL-laden macrophages as foam cells accelerates lipid deposition and fatty streak formation [316, 319, 320].

Macrophages exhibit dual roles in AS progression [316, 321-323]. Initially, oxLDL uptake via lectin-like oxLDL receptor 1 receptors induces foam cell formation and ROS overproduction, which activates NF-κB signaling to release proinflammatory cytokines (TNF-α, IL-6). This chemotactic cascade recruits additional immune cells, amplifying inflammatory feedback loops [316, 321-323].

VSMC dynamics are redox-modulated in AS [324-326]. ROS stimulate VSMC proliferation and migration via MAPK/PI3K/NF-κB pathways, contributing to neointimal hyperplasia [327-329]. Paradoxically, excessive ROS may induce VSMC apoptosis, reducing fibrous cap-forming cells and destabilizing atherosclerotic plaques [324-326].

ER stress, exacerbated by ROS and homocysteine, activates apoptosis signal regulating kinase-1 (Ask1) and upregulates ER stress markers (C/EBP homologous protein, phosphorylated protein kinase R-like ER kinase, glucose regulated protein 78) alongside inflammatory mediators (NLRP3, IL-1β, caspase1) [330-332]. ER stress synergizes with cholesterol dysmetabolism to enhance macrophage apoptosis and inflammatory responses, further accelerating atherogenesis [330-332].

ROS-mediated apoptosis and necrosis critically fuel plaque progression [333]. Oxidative damage to endothelial cells, VSMCs, and macrophages activates p53 and caspase pathways, promoting cellular debris accumulation and lipid core expansion [333]. Necrotic cell-derived prorepair signals paradoxically amplify local inflammation, establishing a self-perpetuating cycle of vascular injury [333].

4.3.2 Heart Failure

Heart failure, a chronic cardiovascular disorder predominantly affecting the elderly, is characterized by pathological alterations including myocardial hypertrophy and cardiac fibrosis. OS plays a pivotal role in its pathogenesis, primarily mediated through ROS generation and accumulation that inflict myocardial damage and pathological remodeling.

Under pathological conditions, mitochondrial ETC dysfunction drives excessive O₂•− production, inducing oxidative cardiomyocyte injury [334]. NOX activity and expression are markedly upregulated by mechanical stretch, angiotensin II, and TNF-α, correlating with heart failure progression [334]. Concurrently, xanthine oxidase hyperactivity contributes to ROS overproduction, while uncoupled NOS generates pathological O₂•− under stress, exacerbating cardiac structural remodeling and contractile dysfunction [334].

A hallmark of heart failure involves the progressive depletion of endogenous antioxidant defenses under chronic OS. Experimental models demonstrate significant reductions in key antioxidant enzymes—SOD, CAT, and GPX—impairing cellular redox homeostasis and amplifying oxidative damage [335]. GSH depletion, closely associated with myocardial dysfunction, correlates with elevated TNF-α levels postinjury and inversely relates to disease severity in advanced dilated or ischemic cardiomyopathy [334].

OS orchestrates myocardial remodeling through redox-sensitive signaling cascades. ROS activate prohypertrophic pathways including Src tyrosine kinase, Ras GTPase, and MAPK, perturbing intracellular calcium homeostasis to induce cardiomyocyte hypertrophy, apoptosis, and fibrosis [334]. Mechanistically, ROS stimulate TFs NF-κB and AP-1, driving proinflammatory cytokine secretion (e.g., IL-1β) that amplifies local inflammation and cardiac tissue injury [336].

ROS-mediated apoptosis critically exacerbates heart failure progression. DNA and mitochondrial damage triggered by ROS activates proapoptotic pathways, accelerating cardiomyocyte loss [337]. This apoptotic cascade synergizes with fibrotic remodeling, as activated fibroblasts deposit collagen via TGF-β signaling, further compromising cardiac architecture and function [337]. The interplay between OS, apoptosis, and fibrosis establishes a self-reinforcing cycle that perpetuates heart failure pathogenesis.

4.3.3 DOX Induced Cardiomyopathy

DOX, a widely used chemotherapeutic agent, is limited by its cardiotoxic effects and propensity to induce cardiomyopathy. OS constitutes a central mechanism in DOX-induced cardiomyopathy, driving biochemical cascades that impair cardiomyocyte function and survival.

DOX undergoes enzymatic reduction via NADH dehydrogenase and eNOS to form semiquinone radicals. These radicals undergo redox cycling in the presence of oxygen, regenerating the parent quinone while generating O₂•− [338]. SOD catalyzes O₂•− conversion to H₂O₂, which subsequently generates •OH via Fenton chemistry, inducing oxidative damage to DNA, lipids, and proteins, ultimately triggering cardiomyocyte apoptosis [338].

DOX further exacerbates OS through iron overload mechanisms. By chelating Fe³⁺ and reducing it to Fe²⁺, DOX facilitates Fenton reactions where Fe²⁺ reacts with H₂O₂ to produce cytotoxic •OH [338-341]. Myocardial iron accumulation correlates with cardiomyopathy progression, as iron–ROS interplay promotes LPO and ferroptosis—a regulated cell death pathway characterized by membrane integrity loss and iron-dependent oxidative damage [338-341].

Endogenous antioxidant defenses are critically compromised in DOX cardiotoxicity [341, 342]. Downregulation of SOD and GPX, particularly the lipid peroxide-detoxifying enzyme GPX4, depletes cellular antioxidant capacity, leading to lethal LPO and cardiomyocyte vulnerability [341, 342].

The Nrf2–Keap1 axis, a master regulator of redox homeostasis, is dysregulated in DOX-induced injury [341, 343, 344]. Under physiological conditions, Keap1 targets Nrf2 for proteasomal degradation. OS triggers Nrf2 nuclear translocation to activate antioxidant genes, yet DOX suppresses Nrf2 signaling, diminishing cytoprotective responses [341, 343, 344].

The redox-sensitive p66Shc protein and its regulator sirtuin 1 (Sirt1) modulate cardiomyocyte ROS susceptibility. DOX upregulates p66Shc while downregulating Sirt1, amplifying oxidative damage [338, 339, 345, 346]. Concurrently, NOX isoforms (NOX2/NOX4) contribute to ROS generation via single-electron transfer mechanisms, exacerbating myocardial injury [338, 339, 345, 346]. These interconnected pathways highlight the multifaceted redox dysregulation underlying DOX cardiotoxicity.

4.3.4 Metabolic Cardiomyopathy

Metabolic cardiomyopathy, characterized by myocardial triglyceride accumulation and hyperglycemia-induced insulin resistance, predominantly manifests in obese individuals. High-glucose and high-fat diets trigger deleterious oxidative modifications of metabolic regulatory proteins, driving maladaptive tissue remodeling that exacerbates left ventricular diastolic dysfunction.

In diabetic patients, elevated blood glucose directly amplifies mtROS production. Hyperglycemic conditions alter mitochondrial metabolism, particularly increasing electron leakage from the ETC, which elevates O₂•− generation [338, 339, 345, 346]. While cardiomyocyte antioxidant systems typically neutralize ROS, diabetic pathophysiology compromises these defenses. Overexpression of poly(ADP-ribose) polymerase (PARP), a biomarker of OS and DNA damage, depletes nicotinamide adenine dinucleotide [oxidized form] (NAD⁺) reserves, disrupting energy metabolism and cellular survival. This establishes a vicious cycle where ROS overaccumulation and DNA damage reciprocally amplify myocardial injury [347, 348].

Mitochondria serve as pivotal mediators of diabetic OS. Cardiac mitochondrial ETC constitutes a major ROS source, with chronic ROS production impairing physiological processes such as NO-mediated vasodilation [347, 348]. Reduced NO bioavailability aggravates myocardial ischemia and cardiac dysfunction. Beyond oxidizing lipids, proteins, and DNA, excessive ROS activate apoptotic pathways, inducing programmed cardiomyocyte death and functional deterioration [347, 348].

Elevated free fatty acid (FFA) levels in diabetes further potentiate oxidative injury. Impaired insulin-mediated glucose metabolism promotes cardiac FFA accumulation, which activates NOX to generate O₂•−, intensifying intracellular OS [347, 348]. NOX-derived O₂•− reacts with NO to form ONOO⁻, a potent oxidant causing cellular damage. FFA metabolism-generated ROS additionally drive membrane LPO, disrupting cellular architecture and exacerbating metabolic dysfunction [349-351].

Despite endogenous antioxidant defenses—including SOD and GSH—diabetic conditions suppress their activity, heightening cellular vulnerability [349-351]. Oxidative byproducts like MDA and oxLDL elicit robust inflammatory responses, establishing a redox-inflammatory feedback loop that accelerates cardiac pathology progression [349]. This interplay underscores the critical role of OS in diabetic metabolic cardiomyopathy pathogenesis.

4.3.5 Heart Transplantation

Cardiac transplantation remains a critical intervention for end-stage heart disease, yet postoperative OS poses significant challenges. OS in this context involves multifaceted mechanisms that may impair graft function and compromise long-term outcomes. This section delineates key pathways underlying OS in cardiac transplantation.

Ischemia/reperfusion (I/R) injury constitutes a primary OS trigger during transplantation. The excision of the donor heart induces ischemic hypoxia, followed by abrupt oxygen reintroduction during reperfusion. This transition triggers uncontrolled mitochondrial ETC activity, generating O₂•− [352]. Concurrent NOX activation further amplifies ROS production, directly damaging cardiomyocytes through apoptosis, necrosis, and functional impairment [352].

Postreperfusion ROS activate innate inflammatory pathways, notably NF-κB signaling, which drives proinflammatory cytokine release (TNF-α, IL-1, IL-6) [352]. These cytokines reciprocally enhance ROS generation and recruit immune cells to the graft, establishing a self-perpetuating cycle of oxidative-inflammatory injury [353-355]. This crosstalk elevates acute graft damage risk and adversely impacts long-term survival [353-355].

Mainstay immunosuppressive therapies paradoxically exacerbate OS. Cyclosporine A, while preventing rejection, suppresses endogenous antioxidant defenses (e.g., GSH reductase), heightening cellular redox vulnerability. Chronic immunosuppressant use thus amplifies oxidative injury risk, potentially accelerating graft dysfunction over time [356, 357].

Posttransplant metabolic derangements (hyperglycemia, dyslipidemia) synergize with OS. Hyperglycemia augments mtROS via enhanced electron leakage and advanced glycation end (AGE)-product formation. Elevated FFA activate NOX-dependent ROS generation, while FFA-β-oxidation generates LPO byproducts [352, 356]. These metabolic–oxidative perturbations impair graft healing, promote maladaptive remodeling, and hasten functional decline.

Collectively, these mechanisms underscore OS as a modifiable therapeutic target to improve cardiac transplantation outcomes. Strategic interventions balancing immunosuppression with redox homeostasis may mitigate graft injury and enhance postoperative recovery.

4.3.6 Aortic Dissection

Aortic dissection (AD), a life-threatening cardiovascular condition, involves structural disintegration of the aortic wall leading to hematoma formation between the intimal and medial layers. Its pathogenesis involves multifactorial contributions including genetic predisposition, environmental triggers, and lifestyle factors, with OS serving as a pivotal mediator through effects on VSMCs, ECM, inflammatory responses, and endothelial function [358].

ROS overproduction induces VSMC dysfunction and programmed cell death in AD progression [358]. While healthy VSMCs maintain contractile phenotypes to withstand hemodynamic stress, chronic OS drives phenotypic switching to synthetic VSMCs characterized by enhanced proliferation and secretory activity [359-361]. This transition, mediated through MAPK pathway activation, correlates with upregulated MMP expression and structural destabilization of the aortic wall [359-361].

OS critically disrupts ECM homeostasis in AD [358, 362, 363]. Histopathological hallmarks including elastic fiber fragmentation and aberrant collagen deposition are redox-regulated. ROS upregulate MMPs to degrade structural proteins (elastin, collagen), while impairing ECM synthesis [362, 363]. Combined VSMC loss and ECM degradation markedly reduce aortic tensile strength, accelerating dissection propagation and rupture risk [362, 363].

AD progression involves macrophage infiltration and activation, establishing a vicious cycle of oxidative-inflammatory damage [358, 363, 364]. Inflammatory cells release ROS, proinflammatory cytokines, and MMPs that synergistically degrade ECM components and induce VSMC apoptosis. OS concurrently exacerbates endothelial dysfunction, further amplifying aortic vulnerability [363, 364].

Endothelial cells critically regulate vascular tone and integrity through NO synthesis [358, 365]. OS diminishes NO bioavailability, promoting VSMC hyperplasia and endothelial dysfunction. NO deficiency disrupts vascular homeostasis, exacerbating aortic wall remodeling and dissection risk [365]. These interconnected mechanisms underscore OS as a therapeutic target for AD management.

4.4.1 Hepatotoxicity

OS plays a central role in hepatotoxicity through mechanisms involving excessive ROS generation and multifaceted cellular damage. Under physiological conditions, the liver maintains redox equilibrium between ROS production and clearance. However, external insults (e.g., drugs, alcohol, viral infections) induce mitochondrial dysfunction, triggering pathological ROS overproduction. Excessive ROS initiate LPO, attacking PUFAs in cellular membranes to generate cytotoxic aldehydes (MDA, 4-HNE) that disrupt membrane integrity, permeability, and function [366, 367].

Oxidative protein damage induces irreversible denaturation and functional loss, activating apoptosis via p53, caspase, and Bcl-2/Bax pathways [366-368]. Concurrently, ROS oxidatively modify DNA to form mutagenic adducts (e.g., 8-OHdG), causing genomic instability, cell cycle arrest, and programmed cell death [366, 367].

OS synergizes with inflammatory responses through NF-κB signaling activation, driving proinflammatory cytokine release (TNF-α, IL-6, IL-1β). This redox-inflammatory crosstalk perpetuates hepatic injury, establishing a self-amplifying cycle that exacerbates liver pathology [366, 367].

Endogenous antioxidant defenses (SOD, GPX, reduced GSH) are overwhelmed under sustained OS. For instance, GPX4 downregulation in alcoholic liver disease (ALD) impairs ROS detoxification, accelerating hepatocyte injury [366, 367]. Mitochondrial integrity critically regulates redox homeostasis, as membrane potential collapse and respiratory chain defects amplify ROS generation, inducing bioenergetic failure and apoptosis [366, 367, 369]. These interconnected mechanisms highlight OS as both a driver and consequence of hepatotoxic injury.

4.4.2 Alcoholic Liver Disease

ALD is a form of liver injury caused by chronic alcohol consumption, characterized by a complex pathogenesis in which OS plays a crucial role. Alcohol consumption not only exacerbates the generation of ROS within hepatocytes but also progressively weakens the antioxidant defense mechanisms of liver, creating a vicious cycle that ultimately leads to escalating levels of OS and progressively worsening liver damage.

First, the metabolic process of alcohol is a key factor in inducing OS. Ethanol is primarily metabolized in the liver by alcohol dehydrogenase (ADH) into acetaldehyde, which is subsequently converted into acetate by aldehyde dehydrogenase. This process significantly increases the production of NADH, enhancing the reductive state within the liver. This reductive state not only increases metabolic pressure on mitochondria but also leads to dysfunction in the ETC, promoting the excessive production of ROS [370, 371]. Particularly, when the NADH/NAD⁺ ratio decreases, the metabolic homeostasis of the liver is disrupted, reducing fatty acid oxidation while increasing fatty acid synthesis due to the enhanced reductive state, ultimately leading to fat accumulation and LPO in the liver [370, 371].

LPO is one of the direct consequences of OS and a significant mechanism of damage in ALD. Enhanced ROS reacts with PUFAs in cell membranes, promoting lipid oxidation and generating toxic LPO products such as LPO and 4-HNE [372, 373]. Additionally, the effects of ethanol and acetaldehyde downregulate adiponectin and STAT3, inhibiting the activity of 5′-AMPK and peroxisome proliferator-activated receptor α (PPARα), ultimately promoting fatty acid accumulation in the liver and leading to LPO [372, 373]. These toxic compounds not only directly damage hepatocyte membranes, disrupting cell structure and function, but also bind to membrane proteins, affecting cell signaling and inducing apoptosis or necrosis, thereby increasing the extent of liver damage.

In the context of OS, DNA and proteins suffer varying degrees of oxidative damage. In ALD, DNA damage in hepatocytes often manifests as the accumulation of oxidative modifications, particularly an increase in 8-OHdG [373]. This DNA damage not only leads to gene mutations but may also affect gene expression, halt DNA synthesis, initiate cell detection mechanisms, and cause cell cycle arrest [373]. Regarding protein oxidative damage, oxidation-induced amino acid modifications alter the structure and function of key proteins. When proteins are oxidized, their structure and function change, potentially being recognized as new antigens by the immune system, further inducing autoimmune responses [373]. Tyrosine oxidation can form dityrosine, while cysteine oxidation produces sulfinic acid, sulfonic acid, and disulfide bonds, leading to protein cross-linking and aggregation. Excessive protein oxidative damage disrupts cellular homeostasis, increasing the risk of hepatotoxicity and cell death [373]. These apoptotic signaling pathways, such as the activation of p53 and the caspase family, are directly related to ROS production, exacerbating hepatocyte damage [373].

OS also plays a role in ALD by modulating inflammatory responses. The excessive production of ROS can activate NF-κB and other proinflammatory signaling pathways, enhancing the secretion of proinflammatory cytokines such as TNF-α, IL-6, and IL-1β [370], not only causing local inflammation in the liver but also potentially triggering systemic immune responses, further aggravating hepatocyte damage. As liver inflammation worsens, the interaction between related cytokines and ROS forms a vicious cycle, leading to progressively worsening liver pathology [370].

Simultaneously, chronic alcohol consumption significantly impacts the antioxidant capacity of liver. This imbalance in antioxidant mechanisms is primarily reflected in the decreased expression levels of antioxidant enzymes. Key antioxidant enzymes such as SOD, GSH-Px, and CAT typically exhibit significantly reduced activity under alcohol stimulation. This renders hepatocytes unable to effectively clear the increased ROS in the body, thereby exacerbating the degree of OS [374]. Additionally, alcohol consumption depletes GSH and other crucial antioxidants in the body, further weakening the resistance of liver to oxidative damage. This damage not only affects hepatocyte survival but also induces intracellular inflammatory responses, aggravating the progression of ALD [374].

4.4.3 Liver Fibrosis and Cirrhosis

Hepatic fibrosis is a pathological condition arising from liver injury, characterized by hepatocyte apoptosis and excessive deposition of collagen and other ECM components in the hepatic stroma. OS plays a pivotal role in the initiation and progression of hepatic fibrosis, primarily through hepatocyte damage and activation of hepatic stellate cells (HSCs).

During fibrogenesis, OS is typically triggered by inflammatory factors, toxins, and viral infections, which induce excessive production of ROS in the liver. When hepatocytes are injured and mitochondrial dysfunction occurs, ROS generation is markedly elevated. These ROS not only directly cause oxidative damage to hepatocyte membranes, proteins, and DNA, thereby inducing apoptosis, but also stimulate the release of proinflammatory cytokines such as TNF-α and IL-1β [374], exacerbating hepatic inflammation.

As inflammation intensifies, activated Kupffer cells and neutrophils further contribute to ROS generation, collectively promoting HSCs activation and migration. ROS production primarily depends on NOX. In HSCs, the expression of NOX1, NOX2, and NOX4 drives excessive ROS generation, which in turn enhances HSC activation [376]. Experimental studies demonstrate that mice deficient in NOX regulatory components fail to produce ROS when exposed to angiotensin II, PDGF, leptin stimulation, or apoptotic body treatment, resulting in attenuated hepatic fibrosis in bile duct ligation or carbon tetrachloride-induced models. These findings underscore the critical role of NOX in fibrotic progression [376]. Recent studies further reveal that NOX-mediated effects extend beyond HSCs, as macrophage-derived NOX1 promotes hepatocarcinogenesis through induction of inflammatory cytokines, highlighting the multifaceted roles of NOX in liver pathology.

Under OS, activated HSCs undergo sustained proliferation and transdifferentiation into myofibroblasts via multiple signaling pathways, including TGF-β/Smad, leading to excessive collagen synthesis and ECM deposition that drives stromal hyperplasia and fibrotic scar formation [375]. Notably, OS not only enhances collagen production in HSCs but also inhibits their apoptosis, creating a self-reinforcing loop that accelerates fibrogenesis [375].

A complex interaction network exists between OS and ECM remodeling during hepatic fibrosis. Persistent oxidative damage to hepatocytes amplifies inflammatory responses, further activating HSCs and aggravating fibrotic changes. Concurrently, activated HSCs secrete additional proinflammatory factors and cytokines that sustain elevated OS levels. This vicious cycle drives rapid progression and clinical deterioration of hepatic fibrosis [377].

4.4.4 Liver Transplantation

OS plays a pivotal role in liver transplantation, profoundly influencing functional recovery and long-term prognosis of the grafted liver. Both the surgical procedure itself and postoperative immunosuppressive therapy can induce substantial ROS production, triggering OS that causes structural and functional damage to the hepatic tissue.

During liver transplantation, the organ undergoes ischemia–reperfusion injury (IRI) resulting from temporary cessation of blood flow during donor liver procurement and implantation. Subsequent reperfusion leads to abrupt oxygen influx, driving excessive ROS generation. These radicals oxidize cellular components including lipids, proteins, and DNA, inducing structural and functional alterations that culminate in apoptosis or necrosis. Beyond direct hepatocyte damage, oxidative modifications disrupt cellular signaling pathways. Clinical evidence demonstrates strong correlations between elevated OS and posttransplant complications including hepatic dysfunction, cholestasis, and acute rejection episodes.

The accelerated ROS production during IRI primarily stems from enzymatic activation pathways. Key enzymes such as xanthine oxidase, NOX, and NOS catalyze molecular oxygen reduction to generate superoxide anions, substantially elevating ROS levels [378-380]. These radicals not only inflict direct hepatocellular damage but also initiate localized immune responses through damage-associated molecular pattern (DAMP) release into the extracellular milieu following cellular injury. DAMPs subsequently activate innate immunity, perpetuating inflammatory cascades and secondary tissue damage [378, 379].

Postoperative immunosuppression paradoxically modulates OS dynamics. While mainstay agents like corticosteroids and calcineurin inhibitors effectively prevent graft rejection, certain immunosuppressants exhibit ROS-promoting properties that exacerbate oxidative damage. This pharmacological paradox suppresses hepatic antioxidant defenses, potentially contributing to persistent posttransplant hepatic dysfunction [378, 379].

Chronic OS constitutes a critical mediator of graft deterioration and fibrogenesis. In long-term transplant recipients, sustained oxidative injury promotes chronic rejection and hepatic fibrosis through inflammatory amplification and LPO cascades. The cumulative oxidative damage not only compromises hepatic architecture and function but also impairs regenerative capacity, ultimately diminishing long-term graft survival rates.

4.5.1 Nephrotoxicity

OS plays a pivotal role in the pathogenesis of nephrotoxicity, encompassing a complex interplay of biochemical reactions and cellular processes. Fundamentally, OS arises from an imbalance between the generation of ROS and the cellular antioxidant defense mechanisms, culminating in cellular dysfunction and tissue injury. Within the renal context, OS not only induces direct damage to renal tubular epithelial cells but also triggers inflammatory cascades and programmed cell death, ultimately contributing to the deterioration of renal function.

The molecular mechanisms underlying nephrotoxicity are mediated through multiple signaling pathways, particularly the MAPK family, including p38 MAPK, ERK, and JNK [381]. These pathways are intricately involved in OS responses and renal immune regulation. For example, activation of the p38–MAPK pathway facilitates IκB kinase-mediated degradation of IκBα, leading to nuclear translocation and transcriptional activation of NF-κB. This establishes a positive feedback loop that amplifies the release of proinflammatory cytokines, including TNF-α and IL-6 [381]. Concurrently, Nrf2, a master regulator of antioxidant responses, is activated under OS conditions to orchestrate the expression of cytoprotective genes, thereby maintaining intracellular redox homeostasis [381, 382]. The dynamic interplay between these pathways is critical in modulating OS and inflammatory responses in renal pathophysiology.

Apoptosis of renal tubular epithelial cells represents a hallmark of nephrotoxic injury, particularly in the context of drug-induced or toxin-mediated renal damage [383, 384]. The apoptotic cascade is predominantly initiated by excessive ROS generation, which disrupts intracellular calcium homeostasis and activates the factor-related apoptosis ligand/factor-related apoptosis (Fas ligand/Fas) receptor system. This leads to mitochondrial outer membrane permeabilization, cytochrome c release, and subsequent activation of the caspase-3-dependent apoptotic pathway. Such mechanisms render renal tubular cells particularly vulnerable to toxic insults, contributing to the progression of acute kidney injury (AKI) and renal failure. The loss of tubular epithelial cells compromises their regenerative capacity, thereby exacerbating renal structural and functional impairment and establishing a self-perpetuating cycle of injury [384, 385].

ER stress has emerged as a critical contributor to nephrotoxicity. Under conditions of OS, renal cells experience ER dysfunction, characterized by the accumulation of misfolded or unfolded proteins within the ER lumen [385, 386]. This triggers the UPR, which initially activates autophagy as an adaptive mechanism to mitigate ER stress. However, sustained ER stress leads to the suppression of both ER function and autophagic flux, resulting in the accumulation of damaged cellular components [385, 386]. Mechanistic studies have demonstrated that ER stress-induced cellular injury is closely associated with apoptotic pathways, which simultaneously inhibit renal antioxidant defense systems, ultimately culminating in cellular dysfunction and progressive renal damage [385].

Furthermore, exposure to nephrotoxic agents elicits robust inflammatory responses characterized by the release of proinflammatory mediators, including TNF-α, IL-1β, and IL-6, which further potentiate OS. NF-κB, a central transcriptional regulator of inflammation, plays a critical role in this process by enhancing the expression of proinflammatory cytokines, thereby establishing a vicious cycle between OS and inflammatory signaling. This inflammatory milieu not only exacerbates renal parenchymal injury but also disrupts the renal microenvironment, impairing normal physiological functions and contributing to the progression of chronic kidney disease (CKD) [383, 387-389].

4.5.2 Diabetic Nephropathy

OS plays a pivotal role in the pathogenesis of diabetic kidney disease (DKD), involving multiple biochemical pathways and cellular processes. Diabetes, a metabolic disorder characterized by chronic hyperglycemia, drives excessive ROS generation, leading to redox imbalance. This imbalance not only damages renal tissues but also exacerbates tubular and glomerular dysfunction.

Chronic hyperglycemia, a hallmark of diabetes, significantly promotes ROS overproduction, initiating metabolic disturbances. A critical mechanism involves the formation of AGE products (AGEs). These AGEs bind to cell surface receptors (e.g., RAGE), further stimulating ROS generation and inducing oxidative cellular damage [390-392]. Concurrently, AGEs trigger inflammatory responses by releasing proinflammatory cytokines such as TNF-α and IL-1β, amplifying OS and establishing a self-perpetuating cycle of injury [390-393]. Notably, NOX isoforms, particularly NOX4 and NOX5, serve as primary ROS sources in renal tubular cells and podocytes. Experimental evidence highlights that NOX4 upregulation correlates with DKD progression, as its activation enhances ROS generation and exacerbates oxidative renal injury. This sustained OS disrupts both tubular and glomerular function, accelerating DKD advancement [351].

A tightly interdependent relationship exists between OS and inflammation in DKD [18, 394]. Hyperglycemia-induced ROS overproduction directly damages cells while stimulating the release of inflammatory mediators, including TNF-α and IL-6. Mechanistically, ROS activate NF-κB, a master transcriptional regulator of proinflammatory cytokine expression, thereby amplifying inflammation. Moreover, ROS induce monocyte chemoattractant protein-1 overexpression, promoting macrophage and T-cell infiltration into renal tissues and aggravating injury [390, 391, 394]. TNF-α further perpetuates this cycle by stimulating ROS production and modulating cellular proliferation/apoptosis through feedback mechanisms. This persistent crosstalk between OS and inflammation disrupts the renal microenvironment, impairing physiological function [390, 391, 394].

Renal fibrosis represents a critical pathological feature of DKD. Chronic ROS generation under hyperglycemic conditions activates renal stellate cells, driving excessive collagen and ECM deposition. This fibrotic process is tightly regulated by the activation of TGF-β and connective tissue growth factor [390]. ROS enhance the expression of these profibrotic factors, stimulating fibroblast activity and ECM accumulation, which disrupt renal architecture and function. Progressive fibrosis impairs tubular and glomerular regenerative capacity, culminating in end-stage renal disease [390]. Notably, AGEs synergistically accelerate DKD progression by amplifying inflammatory responses and activating fibrotic signaling pathways.

Mitochondria serve as a major ROS source in DKD pathogenesis. Hyperglycemia-induced mitochondrial dysfunction exacerbates OS by impairing ETC efficiency [392]. Studies demonstrate that mitochondrial damage not only reduces ATP synthesis but also triggers cytochrome c release, activating apoptotic pathways. In renal endothelial cells and podocytes, mitochondrial dysfunction activates signaling cascades such as PI3K/Akt and NF-κB, promoting inflammation, apoptosis, and fibrosis [392]. Furthermore, iron overload and LPO exacerbate mitochondrial injury, depleting antioxidant defenses and worsening diabetic renal damage. Thus, mitochondria act both as targets of OS and as critical mediators linking OS to cellular injury in DKD [392].

4.5.3 Acute Kidney Injury

AKI is a heterogeneous clinical syndrome, which arises from a diverse array of underlying causes, including ischemia, nephrotoxins, and systemic inflammatory responses, leading to varying degrees of renal dysfunction. OS plays a pivotal role in the pathogenesis and progression of AKI, involving multiple signaling pathways and cellular responses.

IRI is a major cause of AKI, typically resulting from renal hypoperfusion followed by reperfusion, which triggers profound OS. During ischemia, ATP synthesis in renal tubular cells is markedly reduced, leading to impaired cellular metabolism and mitochondrial dysfunction [395, 396]. Ischemia disrupts the mitochondrial ETC, increasing ROS production [395, 396]. Upon reperfusion, the sudden influx of oxygen drives the rapid generation of superoxide and H₂O₂ by mitochondria, further exacerbating OS [395, 396]. Elevated ROS levels directly damage cellular membranes, proteins, and DNA, increasing membrane permeability and disrupting cellular integrity, ultimately leading to cell dysfunction and death. Concurrently, IRI activates the NF-κB pathway, promoting the release of proinflammatory cytokines such as TNF-α and IL-6, which amplify local inflammation and oxidative damage, thereby inducing apoptosis [355, 397].

Sepsis is another critical contributor to AKI, often accompanied by systemic inflammatory responses and microcirculatory dysfunction. In that case, the immune system undergoes hyperactivation, with macrophages and neutrophils generating reactive species such as superoxide and ONOO⁻ to eliminate pathogens [398, 399]. However, these ROS also inflict damage on surrounding renal tubular and endothelial cells [398, 399]. Additionally, the activation of iNOS in sepsis produces excessive NO, which reacts with ROS to form ONOO⁻, a highly toxic compound that exacerbates tubular cell injury [398-400]. The release of DAMPs further amplifies inflammation by activating TLRs. The interplay between DAMPs and ROS creates a vicious cycle, intensifying local inflammation and OS [398, 399]. Moreover, sepsis-induced microvascular dysfunction compromises renal blood supply, exacerbating cellular hypoxia and renal injury [398, 399].

Drug-induced AKI involves multiple mechanisms that exacerbate OS [401]. For instance, during drug metabolism, cytochrome P450 enzymes in microsomes generate ROS. The detoxification process may lead to excessive ROS production, increasing the risk of oxidative damage [402]. Furthermore, the depletion of GSH, a critical antioxidant, results in ROS accumulation and impaired cellular antioxidant capacity. Certain drugs directly damage mitochondria, particularly at complexes I and III of the ETC, leading to increased O₂•− generation. Mitochondrial membrane potential and permeability are also altered, impairing ATP synthesis and promoting cell death [403]. Drug-induced ER stress causes the accumulation of misfolded proteins, activating the UPR and increasing ROS production, thereby elevating the risk of apoptosis [404, 405].

Viral infections contribute to AKI through multiple mechanisms, with OS being a central factor. During viral infections, biotransformation enzymes such as cytochrome P450 and xanthine oxidase are activated, leading to increased ROS production. The accumulation of ROS is closely associated with cellular and tissue damage. The respiratory burst of monocytes and macrophages during immune responses generates substantial ROS, which, while aiding viral clearance, also damages surrounding cells [406]. Additionally, certain viral infections may increase intracellular iron availability, enhancing the Fenton reaction to produce hydroxyl radicals, a potent oxidant that damages cellular membranes and biomolecules. Viral infections also induce ER and mitochondrial dysfunction, leading to sustained ROS production, exacerbating OS, and impairing cell survival and function [407].

4.5.4 Chronic Kidney Disease

OS plays a crucial role in the initiation and progression of CKD, with mechanisms involving various biochemical reactions and cellular signaling pathways. In the context of CKD, OS is triggered by multiple factors, including prolonged hyperglycemia, hypertension, obesity, and chronic inflammation. The sustained generation of ROS leads to damage to renal tissue and promotes the progression of pathological changes.

Mitochondria serve as the primary centers for intracellular energy metabolism, and their normal functioning is essential for maintaining cellular health. Mitochondria produce ATP through the process of oxidative phosphorylation, but they also generate ROS. In the context of CKD, the ETC in mitochondria is impaired, resulting in excessive accumulation of ROS [408, 409]. Studies have shown that the overproduction of ROS not only directly causes damage to mitochondrial membranes and disrupts membrane potential, leading to apoptosis, but also induces oxidative damage to mtDNA. This damage is closely associated with the progression of CKD, and the limited repair mechanisms for mtDNA accelerate the loss of mitochondrial function and the decline of renal function [408, 409]. Moreover, the abnormal opening of the mPTP can lead to the release of cytosolic contents under pathological conditions, inducing apoptosis [408, 409].

ER stress is an significant factor in the progression of CKD. The ER is responsible for synthesizing and modifying cellular membrane and secretory proteins. When the ER encounters protein folding disorders or accumulates misfolded proteins, it activates the UPR, which restores ER homeostasis by promoting autophagy or inducing apoptosis [410-412]. The state of ER stress can result in loss of cell function and pathological damage [410-412]. Additionally, peroxisomes serve as the main antioxidant organelles in cells, responsible for removing intracellular ROS. Research has found that in patients with CKD, the decline in peroxisomal function prevents the efficient clearance of ROS, exacerbating OS and triggering a series of cellular injuries and interstitial fibrosis [410-412].

The relationship between OS and renal inflammation is closely intertwined, primarily achieved through the activation of various signaling pathways. ROS can activate TFs such as NF-κB, promoting the release of inflammatory mediators, including TNF-α and IL-6. These inflammatory factors not only directly affect the function of tubular cells and glomerular endothelial cells but also stimulate the proliferation and activation of renal stellate cells, promoting the fibrotic process of the renal interstitium. Furthermore, the sustained release of proapoptotic proteins and other inflammatory mediators exacerbates renal cellular injury and the inflammatory response, forming a vicious cycle of exacerbating negative feedback [413].

eNOS plays a central role in maintaining endothelial function and microcirculatory stability. Dysfunction of eNOS results in a decreased generation of NO while increasing the production of ROS, thereby exacerbating endothelial dysfunction. The activity of endogenous NOS is influenced by various factors, particularly the depletion of BH4 and the accumulation of asymmetric dimethylarginine, which further aggravate eNOS dysfunction [409, 414]. In CKD patients, impaired endothelial function not only affects the microcirculation of the kidneys but also leads to ischemia of renal tissue, thereby exacerbating OS and cellular injury. Therefore, restoring endothelial cell function and eNOS activity may represent potential strategies for reversing the progression of CKD [409, 414].

4.5.5 Kidney Transplantation

In kidney transplantation, OS not only affects the quality of the donor kidney and the restoration of function posttransplant but is also closely associated with postoperative complications such as acute rejection, chronic rejection, and renal failure.

In the context of kidney transplantation, abnormal accumulation of iron generates a large amount of ROS through the Fenton reaction, consequently leading to intracellular OS. During the IRI process of the donor kidney, iron accumulation can trigger ferroptosis by inhibiting the cystine/glutamate antiporter (XC system) [415-417]. The XC system is responsible for transporting cystine into cells for the synthesis of GSH, which is critical for the activity of GPX4 in eliminating lipid peroxides. Therefore, the inhibition of the XC system leads to decreased GPX4 activity, promoting the accumulation of lipid peroxides and initiating ferroptosis [415-417]. Against this backdrop, protective mechanisms against ferroptosis have gradually gained attention, including the modulation of iron metabolism, the application of antioxidants such as vitamin E, and related enzymes like CoQ10, emphasizing the potential to protect cells from OS.

Mitochondria are the main energy metabolism centers within cells, and their proper functioning is key to maintaining cellular physiology and health. However, during the I/R process of kidney transplantation, mitochondrial function is significantly impaired, resulting in excessive ROS generation [418-420]. Under ischemic conditions, the increase in the NADH/NAD⁺ ratio within mitochondria and the elevation of membrane potential promote the generation of O₂•−, further exacerbating oxidative damage [418-420]. After reperfusion, the restoration of oxygen metabolism stimulates mitochondria to produce more ROS, which not only damages the mitochondrial membrane but may also trigger apoptosis and ferroptosis, creating a vicious cycle [418-420].

Throughout the process of kidney transplantation, OS is also closely related to immune rejection responses. OS can activate the host immune system, causing it to attack the graft. A substantial amount of ROS can promote the migration and activation of inflammatory cells, enhancing the immune response and triggering acute rejection. Studies have shown that OS activates signaling pathways such as NF-κB, promoting the release of proinflammatory cytokines, including IL-1, IL-6, and TNF-α, aggravating the rejection response and leading to more severe functional loss following kidney transplantation [421-425]. Additionally, chronic rejection is also associated with OS. Prolonged OS can lead to the activation of renal stellate cells and collagen deposition, ultimately resulting in fibrosis and the progressive decline of renal function [426].

4.6.1 Diabetes

OS plays a significant role in the development and progression of diabetes, with mechanisms involving various cellular pathways and biological reactions. Diabetes is a complex metabolic disorder characterized by hyperglycemia, insulin resistance, and insufficient insulin secretion. OS is one of the core mechanisms of diabetes pathogenesis, affecting several critical metabolic processes, such as glucose oxidation, fat metabolism, and the inflammatory response.

Insulin resistance is one of the primary characteristics of diabetes, and OS suppresses insulin signaling through multiple pathways. In a hyperglycemic state, excessive ROS can oxidize IRS, inhibiting their phosphorylation, which directly interferes with the effective transmission of insulin signals [427]. Studies have shown that ROS can also activate phosphatases, subsequently reducing the activity of IRS and affecting the activation of downstream signaling pathways. This mechanism leads to the impaired translocation of glucose transporter type 4, which affects cellular glucose uptake, thereby creating a vicious cycle that exacerbates insulin resistance [427].

OS also enhances insulin resistance through mitochondrial dysfunction, as normal mitochondrial function is crucial for insulin signaling [428, 429]. As OS increases, the functionality of the mitochondrial respiratory chain becomes impaired, leading to insufficient energy production and increased proton leakage. This process further decreases cellular ATP levels, impacting the ability of insulin-stimulated cells to uptake glucose and increasing the risk of developing insulin resistance [428, 429].

Additionally, OS promotes the formation of insulin resistance through the activation of proinflammatory signaling pathways. ROS can activate NF-κB and JNK/stress-activated protein kinase signaling pathways, leading to the serine phosphorylation of IRS-1 and IRS-2, which further impairs normal insulin signaling [428, 430, 431]. Research has found a close association between low-grade chronic inflammation and insulin resistance, suggesting that OS may contribute to this chronic inflammation, where inflammatory mediators such as TNF-α and IL-6 exacerbate the pathological state of insulin resistance [428, 431].

Another mechanism involves the dysfunction of pancreatic β-cells. OS impairs β-cell function, reducing insulin production and secretion [427, 432, 433]. High concentrations of ROS induce apoptosis in β-cells, resulting in an increase in apoptotic cells. This process is mediated through multiple proapoptotic signaling pathways, such as affecting the expression of TFs like MafA, which in turn suppresses the transcription of the insulin gene [427, 432-434]. Furthermore, free radicals damage ATP-sensitive K channels, and this impairment directly affects insulin secretion capacity, making glycemic control more challenging [427, 432, 433, 435].

Glycolysis is a crucial pathway in carbohydrate metabolism responsible for converting glucose into energy. Under normal physiological conditions, the ROS produced during glycolysis are balanced by antioxidant systems. However, in a hyperglycemic state, the accelerated glycolytic process leads to excessive accumulation of intermediates such as glucose-6-phosphate (G6P) and fructose-6-phosphate (F6P), which trigger OS [436, 437]. G6P can cause DNA damage by activating PARP-1; this damage, in turn, exacerbates ROS generation and affects glycolysis [436-438]. At the same time, the formation of AGEs under high glucose conditions is also closely related to OS, where AGEs induce OS through their receptors, further damaging cellular function [436, 437].

4.6.2 Obesity

Obesity is widely recognized as a global health issue, fundamentally arising from a long-term energy imbalance, primarily characterized by excessive caloric intake and insufficient energy expenditure. In addition to traditional risk factors for obesity, such as excessive energy consumption and a lack of physical activity, OS also plays a key role in the development of obesity.

OS is significantly involved in the metabolic dysregulation associated with obesity. Under the influence of high-calorie diets, levels of FFA and glucose in the body significantly increase, and these metabolites affect the redox status of cells through multiple metabolic pathways. Excessive glucose produces a large amount of reducing coenzymes, such as NADH and FADH₂, via glycolysis and the TCA cycle. These coenzymes lead to electron leakage in the ETC, resulting in the production of superoxide and thereby inducing OS. In addition, excessive glucose can also be converted to sorbitol via the polyol pathway, consuming NADPH and exacerbating ROS generation [441]. In obese individuals, the presence of excess FFA challenges the capacity of mitochondrial energy metabolism, leading to an overflow of electrons in the ETC, causing electron leakage and generating large amounts of ROS. This increase in ROS not only reduces energy generation efficiency but also elevates the level of OS in cells [441, 442]. Furthermore, the excessive accumulation of FFA promotes LPO, a process that causes significant cellular damage. The deposition of FFAs on cell membranes affects membrane integrity and fluidity, leading to cellular dysfunction [441, 442]. Simultaneously, the accumulation of FFA stimulates the release of proinflammatory cytokines, such as TNF-α and IL-6, promoting a systemic low-grade inflammatory response, thereby creating a vicious cycle. Specifically, the presence of FFA activates the NOX pathway in adipose tissue, enhancing ROS production, which exacerbates oxidative damage and affects insulin signaling pathways, leading to increased insulin resistance. Notably, high levels of FFA in tissues such as mitochondria, liver, and muscle are closely correlated with the occurrence of insulin resistance and metabolic syndrome [441, 442].

At the same time, the relationship between OS and inflammatory responses is also extremely close. In the state of obesity, adipose tissue serves not only as an energy reservoir but also as an endocrine organ that releases a large amount of proinflammatory factors, including an increased level of TNF-α, which further promotes the release of IL-6. The increase in these factors leads to the development of systemic low-grade inflammatory responses. The release of proinflammatory factors promotes ROS generation and further activates proinflammatory cellular pathways such as NF-κB and JNK, creating a vicious cycle [441, 443].

4.6.3 Hyperlipidemia

Hyperlipidemia is defined as a state of abnormally elevated concentrations of lipids (such as cholesterol and triglycerides) in the blood, and it is a promoting factor for many metabolic diseases.

In hyperlipidemia, enhanced OS promotes the occurrence of ferroptosis, affecting the function of cardiac cells and VSMCs. Research indicates that excessive iron accumulation can inhibit the activity of lipoprotein lipase, resulting in abnormal triglyceride accumulation. When lipids are esterified in cell membranes, they become substrates for LPO, further exacerbating the cellular OS response [444]. In a hyperlipidemic environment, the generation of oxLDL increases, a process that not only leads to endothelial dysfunction but also promotes iron-dependent cell death by inhibiting the antioxidant enzyme GPX4 [445].

OS enhances both endogenous and exogenous inflammatory responses in hyperlipidemia, creating a vicious cycle. On one hand, excessive FFA in a hyperlipidemic state stimulate adipose tissue to produce various proinflammatory factors, triggering systemic chronic low-grade inflammation. This inflammation activates macrophages and other immune cells, prompting their infiltration into damaged tissues and exacerbating OS. On the other hand, OS itself can also promote the intensification of inflammatory responses. The excessive production of ROS not only stimulates endothelial cells to release inflammatory factors but also exacerbates the atherosclerotic process by upregulating the formation of oxLDL. Furthermore, oxLDL activates the NLRP3 inflammasome, initiating inflammatory signaling pathways that lead to the release of inflammatory factors such as IL-1β, further promoting systemic inflammatory responses [446].

4.6.4 Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is characterized by the abnormal accumulation of fat in the liver, unrelated to alcohol consumption, and is closely associated with obesity, diabetes, and metabolic syndrome. In recent years, OS has garnered widespread attention in the development and progression of NAFLD and has become a key aspect of its pathological mechanism research.

Mitochondria play a crucial role in NAFLD. As the primary energy suppliers of cells, mitochondrial dysfunction is closely linked to the dysregulation of hepatic fat metabolism [447, 448]. In patients with NAFLD, mitochondria often exhibit ultrastructural damage and mtDNA damage. These impairments reduce the activity of respiratory chain complexes, leading to decreased ATP synthesis and an accompanying increase in ROS production. Within mitochondria, the β-oxidation of fatty acids is a vital step for energy generation; however, excess ROS can directly damage mitochondrial membranes, triggering dysregulation of oxidative phosphorylation. This dysregulation not only affects ATP synthesis but also increases the formation of lipid peroxides, leading to further oxidative damage and diminished cellular function. In this process, mitochondria increasingly become a source of OS, creating a vicious cycle that ultimately exacerbates the progression of NAFLD [447, 448].

Fatty acid metabolism plays a central role in the onset of NAFLD, and OS is critical for the regulation of this process. Under normal circumstances, the liver should maintain a dynamic balance between lipogenesis and fatty acid oxidation. However, in NAFLD, excessive accumulation of FFAs stimulates lipogenesis while simultaneously inhibiting fatty acid oxidation. OS interacts with the regulatory pathways of fatty acid metabolism via ROS, promoting enhanced lipogenesis and suppressed fatty acid oxidation. Elevated levels of ROS not only promote the expression of lipogenic genes but can also inhibit the activity of PPARα and carnitine palmitoyltransferase 1, which are associated with fatty acid oxidation, leading to further accumulation of fatty acids in the liver. This metabolic dysregulation not only affects the energy balance of the liver but also exacerbates the severity of fatty liver [449, 450].

4.7.1 Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a multifactorial autoimmune disease characterized by the destruction of bone and cartilage, as well as synovial proliferation, leading to joint pain, swelling, and stiffness. In the pathogenesis of RA, activated fibroblast-like synoviocytes exhibit tumor-like proliferative characteristics, resulting in cartilage erosion and eventual joint destruction. The initial symptoms of RA typically include pain in the fingers and wrists, and as the disease progresses, larger joints such as the shoulders and knees may also be affected, ultimately leading to restricted joint movement, joint deformities, and even disability. Additionally, RA may cause systemic multiorgan damage and extra-articular manifestations.

In the synovial tissue of RA patients, common ROS include O₂•−, H₂O₂, •OH, and NO. These molecules play a significant role in the pathophysiology of RA, particularly by promoting local inflammatory responses that further exacerbate damage to joints and synovial tissues [451]. Studies have shown that the levels of antioxidants such as GSH and SOD are often reduced in RA patients, leading to a significant increase in OS [451].

In the synovial tissue of RA, activated macrophages and T cells induce the generation of ROS by releasing proinflammatory cytokines such as TNF-α and IL-1β. These inflammatory factors not only directly damage synovial cells but also trigger further inflammatory responses by increasing ROS generation. At this stage, a vicious cycle emerges between OS and inflammation, promoting cellular damage and enhancing disease progression [19]. OS also exacerbates local inflammation and contributes to further joint destruction by affecting T cell activation [19, 452].

In the pathogenesis of RA, OS further promotes inflammatory responses and cellular damage by activating specific signaling pathways, such as the MAPK and NF-κB pathways. The MAPK pathways, including P38, ERK, and JNK, are significantly activated during the inflammatory processes of RA, facilitating the release of proinflammatory cytokines [453-455]. NF-κB, as a major regulatory factor in the inflammatory response, is activated under the influence of OS, enhancing the release of IL-1 and TNF-α, thereby forming a self-amplifying feedback loop. With the accumulation of these proinflammatory factors, joint inflammation intensifies, the invasiveness of synovial cells increases, and ultimately leads to joint structural destruction [452].

Patients with RA commonly exhibit endothelial dysfunction, which is manifested in both macrocirculation and microcirculation. Studies have shown a positive correlation between disease activity in early RA patients and microvascular dysfunction. The OS damage to endothelial cells not only affects the integrity of blood vessels but may also lead to the development of comorbidities such as AS and cardiovascular diseases. Research has found that IL-1 can induce OS even in the preclinical stage of RA, further leading to vascular damage [456].

4.7.2 Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a unique autoimmune disease characterized by the excessive production of various autoantibodies targeting the cellular nucleus, cytoplasm, and cell membrane. These autoantibodies form immune complexes that deposit in various tissues and organs, leading to tissue damage, where OS plays a key role.

Treg cells are crucial for maintaining immune balance and preventing autoimmune responses. However, in SLE patients, both the quantity and function of Treg cells are significantly reduced. OS influences the differentiation and function of Treg cells through various mechanisms. Forkhead box protein 3 (Foxp3) is a key TF for Treg cell development and function; OS inhibits the expression of Foxp3 by increasing the production of IL-6, consequently reducing the number of Treg cells. Elevated levels of IL-6 have been widely documented in SLE patients, suggesting that OS may be one of the primary reasons for the reduction of Treg cells [457-460]. Additionally, the mTOR signaling pathway plays a negative regulatory role in the differentiation and function of Treg cells. The mTOR complex 1 (mTORC1) and mTORC2 complexes promote the differentiation of T helper 1 cell (Th1) and Th17 cells by inhibiting the expression of Foxp3 and the expansion of Treg cells, further exacerbating immune imbalance [457, 458, 461-463]. Leptin, a hormone closely related to OS, further weakens Treg cell function by inhibiting Treg cell proliferation and enhancing mTOR activity [464].

Th17 cells are key effector cells in the pathogenesis of SLE, and their amplification is closely related to OS. Ultraviolet (UV) radiation is a significant environmental trigger for SLE, promoting Th17 cell differentiation by inducing DNA damage and the production of proinflammatory cytokines. UV radiation also generates endogenous aryl hydrocarbon receptor (AHR) ligands like 6-formylindolo[3,2-b]carbazole, which activate AHR, further promoting the amplification of Th17 cells. AHR serves as a critical regulator of Th17 cell differentiation, and its activation exacerbates the symptoms of SLE [465-467]. Importantly, the mTOR signaling pathway also plays a significant role; OS activates the mTORC1 signaling pathway, regulating TFs associated with Th17 cells and positively promoting their differentiation, thereby aggravating the progression of SLE [468-470].

In SLE patients, mitochondrial dysfunction is one of the significant manifestations of OS, directly affecting immune cell function. The high-energy state of mitochondria induced by OS results in increased mitochondrial potential and insufficient ATP synthesis, leading to increased ROS production and further intensifying the inflammatory response. Lymphocytes from SLE patients are more prone to apoptosis under OS, resulting in a reduction in lymphocyte numbers, which further exacerbates immune imbalance [471].

OS affects T cell function through various mechanisms, including the alteration of cytokine expression, regulation of gene transcription, and changes in mitochondrial function. OS promotes the production of TH2 cytokines and suppresses the expression of Th1 cytokines, leading to an imbalance in T cell differentiation [472, 473]. GSH deficiency and increased mitochondrial calcium storage exacerbate this imbalance. At the same time, OS alters T cell signaling pathways and lineage development by affecting DNA methylation and histone modification [474].

4.7.3 Psoriasis

Psoriasis is a chronic immune-mediated skin disease characterized by the appearance of red patches covered with silvery scales on the skin. The pathophysiological features of this disease indicate that OS plays a significant role in the development and progression of psoriasis. Studies have shown that the exacerbation of OS is not only associated with the severity of skin lesions but may also impact the effectiveness of treatments.

Research demonstrates that OS markers such as MDA and NO levels are significantly elevated in the blood and skin of psoriatic patients, indicating an increased extent of LPO and protein oxidation. Concurrently, levels of antioxidants such as GSH and vitamin E are generally reduced, reflecting a weakened antioxidant capacity. Additionally, the activity of antioxidant enzymes, including SOD and CAT, is decreased, further indicating dysregulation of the antioxidant system [475, 476].

DCs play a critical role in the pathogenesis of psoriasis. OS enhances the production of proinflammatory cytokines such as IL-8 and TNF-α by DCs, promoting their interaction with T cells. The functional enhancement of these inflammatory DCs drives the immunopathological processes associated with the onset of psoriasis [477, 478]. In psoriasis, Th1-type cytokines such as IFN-γ and IL-2 are significantly increased, while the levels of Th2-type cytokines like IL-10 are relatively low, further promoting the immune response related to psoriasis [478]. Meanwhile, OS also promotes the abnormal proliferation of KCs. The overexpression of cytokines such as TNF-α stimulates the proliferation and differentiation of KCs, leading to impaired skin barrier function and characteristic skin lesions [478-481]. Moreover, OS is closely related to endothelial cells and angiogenesis in psoriatic patients. ROS promote vascular formation through a VEGF-mediated pathway, which further exacerbates the local inflammatory environment in psoriasis [482].

OS plays a role in the pathogenesis of psoriasis by activating multiple signaling pathways. Studies indicate that ROS can activate signaling pathways such as MAPK and NF-κB. The abnormal activation of ERK, JNK, and p38 within the MAPK pathway is closely associated with the excessive proliferation and inflammation of psoriatic lesions. In particular, the activation of p38–MAPK during the pathological process may worsen the inflammatory state [20, 483, 484]. NF-κB, as a key TF, has significantly elevated activation levels in psoriatic lesions, enhancing the expression of proinflammatory genes and perpetuating inflammation [20, 485]. Furthermore, OS triggers the activation of the JAK–STAT signaling pathway, wherein the upregulation of STAT3 plays an significant role in the immunopathology of psoriasis. The abnormal activation of these signaling pathways not only exacerbates inflammatory responses but also promotes the proliferation and differentiation of KCs [486, 487].

4.8.1 Osteoarthritis

Osteoarthritis (OA) is a prevalent joint disease increasingly recognized as a significant health problem, particularly among the elderly population. It is characterized by swelling and pain in the affected joints, often accompanied by restricted mobility and disability. OS is a critical contributor to the progression of OA, with studies indicating that OS levels in damaged joint tissue are significantly higher than those in healthy joints, thereby exacerbating injury and functional impairment in the joints.

In the pathological processes of OA, OS is closely linked to the production of various inflammatory factors, especially IL-1β and TNF-α, which play pivotal roles in the inflammatory response. ROS can induce and amplify the production of these proinflammatory cytokines, creating a vicious cycle that leads to chronic joint inflammation. Specifically, ROS enhance the expression of matrix-degrading proteins, inhibit the synthesis of the ECM, and elevate the risk of apoptosis or necrosis in chondrocytes, thereby worsening cartilage damage and functional impairment [488, 489]. Research has demonstrated increased expression levels of pro-oxidative enzymes, such as NOX2, in the synovium of OA patients, which correlates with collagen metabolism. Notably, the upregulation of NOX2 contributes to collagen degradation by generating ROS, adversely affecting the structural integrity of the joints [488, 489].

Furthermore, OS in the context of OA not only promotes cell apoptosis but also encompasses other regulated forms of cell death, such as ferroptosis. The importance of ferroptosis in OA is being increasingly acknowledged, characterized primarily by excess iron accumulation and LPO. In OA chondrocytes, iron accumulation exacerbates oxidative damage by inducing ROS production. The downregulation of the GPX4 enzyme is closely associated with GSH depletion, leading to diminished cellular resistance to OS and resulting in cell death [489, 490]. The occurrence of ferroptosis further contributes to the degradation of the ECM, facilitating the pathological progression of OA [490]. Additionally, OS promotes apoptosis, which can occur via intrinsic and extrinsic pathways; ROS can activate both routes. In the intrinsic pathway, ROS induce proapoptotic factors, such as Bak and Bax, through the activation of cell cycle regulators like p53 and JNK, leading to increased mitochondrial membrane permeability and subsequent apoptosis. Low ROS levels may induce cell cycle arrest to allow for DNA repair. The extrinsic pathway is activated through transmembrane death receptors like Fas and TNF-related apoptosis-inducing ligand receptors. The ROS produced interact with the intrinsic apoptotic processes, thereby enhancing the extrinsic signaling [489-492].

In OA, ROS presence modulates several signaling pathways, such as the activation of the MAPK and NF-κB pathways. OS initiates the phosphorylation of MAPK, which promotes NF-κB activation, consequently increasing the expression of various proinflammatory cytokines. The activation of these signaling pathways not only fuels the inflammatory response in OA but also influences cell survival and metabolism [493, 494].

4.8.2 Osteoporosis

Osteoporosis is an age-related condition with increasing incidence among the elderly, significantly elevating the risk of fractures. Maintaining the integrity and homeostasis of bone tissue relies on a balance between osteoblasts and osteoclasts. OS is crucial in the development of osteoporosis, as excessive ROS can damage bone cells, affecting both bone resorption and formation, and thus accelerating the progression of the disease.

Mitochondria are critical energy factories within cells and play an essential role in energy metabolism. As individuals age, mitochondrial dysfunction leads to increased ROS production. mtDNA, lacking the protective mechanisms present in nuclear DNA, is particularly vulnerable to oxidative damage and is closely associated with cellular aging. OS damages mitochondrial function and disrupts ATP synthesis, diminishes mitochondrial membrane potential, and alters calcium homeostasis, adversely impacting osteoblast functionality. Additionally, OS influences cellular metabolism by affecting mitochondrial dynamics, including fission and fusion processes. Under conditions of OS, bone mesenchymal stem cells (BMSCs) can transfer intact mitochondria to damaged cells; however, in osteoporosis, the migration of oxidatively damaged mitochondria may negatively affect cellular function [495, 496].

BMSCs are integral to bone metabolism, but their behavior significantly shifts under OS. For instance, AGEs have been shown to inhibit the osteogenic potential of BMSCs while promoting their differentiation into adipocytes. This negative impact can be partially alleviated through the regulation of mitochondrial autophagy [497]. Furthermore, Sirt3, an NAD⁺-dependent enzyme, can protect the osteogenic function of BMSCs by activating mitochondrial autophagy, thereby slowing the aging process. Sirt3 overexpression has demonstrated an inhibitory effect on osteoporosis in mouse models, highlighting its protective role in the OS response of bone cells [498].

OS impacts bone metabolism via multiple mechanisms, particularly by disrupting the balance between osteoblasts and osteoclasts. Research reveals that in osteoporosis, the accumulation of ROS decreases nuclear translocation of the Nrf2 TF, thereby reducing the expression of antioxidant genes, which promotes osteoclastogenesis and aggravates osteoporosis. Specifically, H₂O₂ stimulates the differentiation of osteoclasts while reducing Nrf2 expression through the activation of the NF-κB signaling pathway [499]. In another critical aspect of bone metabolism, OS enhances bone resorption by increasing osteoclast formation. This occurs due to OS-induced upregulation of receptor activator of nuclear factor kappa-B ligand (RANKL) and downregulation of osteoprotegerin (OPG); the rise in RANKL activates osteoclasts, while the decrease in OPG leads to reduced inhibitory control over osteoclast development. Chronic OS and inflammation can significantly accelerate osteoclast formation, resulting in bone loss [499-501].

Additionally, OS affects bone density by promoting apoptosis in bone cells. Excessive ROS have been shown to induce permanent cell cycle arrest and apoptosis in osteoblasts, worsening osteoporosis. In patients with osteoporosis, the apoptosis of bone cells not only increases osteoclast generation by releasing RANKL but also disrupts osteogenic function by inhibiting the Wnt/β-catenin signaling pathway [499, 502, 503]. Finally, excessive iron accumulation and OS can trigger ferroptosis in osteoblasts, a process characterized by LPO and the inactivation of antioxidant enzymes such as GPX4 [499].

4.8.3 Sarcopenia

Sarcopenia is primarily characterized by a decrease in skeletal muscle mass and a decline in muscle function. Primary sarcopenia is mainly age-related, whereas secondary sarcopenia arises due to conditions such as heart failure, kidney failure, malignancies, and chronic obstructive pulmonary disease.

In elderly muscle cells, the accumulation of OS is closely linked to a diminished antioxidant and repair system. As antioxidant defenses weaken, oxidized molecules more readily accumulate within muscle cells, resulting in a loss of the cells' capability to respond to OS. This may be a critical factor in the inadequate response of muscle tissue to oxidative challenges [504, 505]. Additionally, excessive production of reactive oxygen and nitrogen species (RONS), combined with a reduced responsiveness of the antioxidant system, can contribute to the onset of sarcopenia and hinder muscle regeneration. During the pursuit of muscle function recovery, OS not only impedes the regenerative capacity of muscle cells but may also activate the ubiquitin–proteasome system, leading to accelerated muscle atrophy. This system is typically activated in muscle cells, enhancing the degradation of damaged proteins and further diminishing muscle quality and functionality [504, 505].

The high energy demands of muscle contraction lead to substantial production of RONS. For instance, during muscle contraction, mitochondria produce free radicals such as superoxide anions. Moderate levels of RONS stimulate cells to undergo adaptive responses by activating TFs that regulate the expression of antioxidant enzyme genes, thus safeguarding muscle cells against oxidative damage. However, an imbalance between the generation and elimination of RONS can lead to OS, which significantly accelerates the progression of sarcopenia [506, 507]. NOX is one of the principal oxidases in skeletal muscle cells, found in the plasma membrane and transverse tubules, catalyzing the conversion of NADPH to NADP⁺ while releasing superoxide anions. The O₂•− generated by this enzyme during muscle contraction modulates redox signaling, maintaining cellular vitality. However, dysregulation of this signaling may contribute to critical pathways associated with muscle atrophy [506-508].

Mitochondria serve as the main energy producers within cells and are also a key source of oxidants. The susceptibility of mtDNA to oxidative damage, coupled with ineffective repair mechanisms, leads to the accumulation of mtDNA mutations during aging, creating a detrimental cycle that impairs functionality. According to the mitochondrial free radical theory of aging, oxidative damage-induced mitochondrial dysfunction triggers the synthesis of defective ETCs, hampers oxidative phosphorylation, and reduces ATP production, resulting in enhanced ROS generation [509-511]. Evidence indicates a strong correlation between mtDNA damage and muscle atrophy in mouse models, signifying the central role of mitochondrial dysfunction in age-related sarcopenia [509, 512].

In sarcopenia, OS leads to iron overload in muscle cells, further aggravating cellular damage. LPO resulting from iron overload increases intracellular ROS production and initiates ferroptosis through the p53/SLC7A11 pathway [513]. Research has shown that iron overload reduces phosphorylation levels of FOXO3a and Akt while increasing the expression of E3 ubiquitin ligases associated with muscle atrophy, such as muscle RING-finger 1 and atrogin-1 [514-517]. Moreover, the role of macrophages in muscle regeneration is significant. These immune cells facilitate muscle repair by expressing CD163, ferritin, and HO-1 to absorb and store iron. The release of iron by macrophages promotes muscle regeneration, while inhibiting macrophage iron output can diminish regenerative capacity and enhance fat accumulation [514, 515].

4.9.1 Glaucoma

Glaucoma is a chronic ocular disease characterized by optic nerve damage and loss of visual fields, often linked to elevated intraocular pressure (IOP). Recent research has demonstrated that OS plays a critical role in the onset and progression of glaucoma. OS is caused by the excessive generation of ROS and a failure of antioxidant defense mechanisms, leading to damage in various cell types, including ganglion cells and retinal pigment epithelial (RPE) cells.

The trabecular meshwork (TM) is a crucial structure for maintaining stable IOP by regulating the outflow of aqueous humor. In primary open-angle glaucoma, TM cells undergo pathological changes that increase outflow resistance and elevate IOP. This elevation results in pressure on the optic nerve head, promoting the death of retinal ganglion cells (RGCs) and subsequent optic nerve damage. OS exacerbates this pathological process by impairing the normal function of TM cells [518, 519]. ROS can diminish the effectiveness of local antioxidants, enhance aqueous humor outflow resistance, and contribute to raised IOP. Studies have shown that TM cells exhibit high sensitivity to OS, leading to lysosomal dysfunction and disruptions in autophagy, which promote cellular senescence and compromise TM function. In TM cells from glaucoma patients, matrix accumulation and inflammatory responses have been observed, further intensifying oxidative damage. Increased ROS levels can affect the adhesion and integrity of TM cells, leading to blockages in aqueous outflow pathways [518-521].

OS results in mitochondrial dysfunction, which is essential for the survival of RGCs. Both increases in intracellular ROS and the accumulation of mtDNA mutations can trigger apoptosis or necrosis, particularly in RGCs, which are especially vulnerable due to their high metabolic demands. Thus, OS not only directly affects cell survival but also further impairs visual transduction functions [522, 523].

Recent studies have identified that miRNAs play significant roles in the changes of ECM synthesis in TM cells induced by OS [518]. miR-29b has been implicated in conferring protective effects under chronic OS and physiological oxygen levels. In normal conditions, miR-29b negatively regulates the expression of key collagens (such as collagen type I alpha 1 chain [COL1A1] and COL1A2) involved in the synthesis and deposition of the ECM in TM cells. However, under chronic OS conditions, the downregulation of miR-29b results in increased expression of these genes, leading to cytotoxic effects [524-526]. Additionally, other miRNAs, including miR-141 and miR-93, have been shown to have close associations with OS in the pathophysiology of glaucoma. miR-141 decreases OS by activating the Nrf2 signaling pathway, while miR-93 inhibits Nrf2, thus promoting apoptosis in RGCs. These findings suggest that the modulation of miRNAs could represent a potential therapeutic approach for glaucoma [527, 528].

Exosomes produced by nonpigmented ciliary epithelial cells (NPCE) are crucial for maintaining TM function and regulating IOP in response to OS. These exosomes can support TM cell metabolism and mitigate oxidative damage, helping to disrupt the vicious cycle between OS and TM dysfunction. Notably, under OS, NPCE-derived exosomes can upregulate the Wnt/β-catenin signaling pathway, which participates in cell proliferation and apoptosis [529, 530]. Studies have revealed that exosomes are rich in proteins that participate in ECM remodeling, influencing the composition of the ECM by regulating the activity of MMPs, thereby reducing resistance to aqueous humor outflow [529].

4.9.2 Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the most common complications among diabetic patients, primarily characterized by retinal vascular lesions and optic nerve damage. Increasing evidence indicates that OS plays a vital role in the onset and progression of DR.

In diabetic patients, mtDNA is subjected to OS, resulting in epigenetic changes. mtDNA is more susceptible to free radical attacks in the diabetic environment due to ineffective protective mechanisms, leading to damage in specific regions, particularly the displacement loop (D-loop). This D-loop contains crucial sequences for regulating mtDNA replication and transcription; damages and increases in mutations in this region directly disrupt normal mitochondrial function [531, 532]. Impaired mtDNA not only diminishes the ability of mitochondria to synthesize ATP but also affects the ETC, consequently leading to excessive ROS production. Moreover, mtDNA damage can result in dysregulation of cellular signaling, triggering inflammatory responses and apoptosis, thus worsening pathological changes in the retina.

Mitochondria are essential for cellular energy metabolism, and their functionality is significantly compromised in diabetic conditions. Continuous hyperglycemia triggers mitochondria to produce excessive ROS, which damages mitochondrial membranes and alters membrane permeability. This alteration enables the release of proapoptotic factors, such as cytochrome c, from the mitochondria into the cytoplasm, activating the caspase cascade and ultimately initiating apoptosis [533, 534]. During this process, the activity of antioxidant enzymes, such as SOD and GPX, is significantly reduced, leading to diminished antioxidant capacity and exacerbating oxidative damage to cells. Therefore, mitochondrial dysfunction not only impacts the survival of retinal cells directly but also affects overall visual function, resulting in persistent damage to retinal neurons.

In the context of hyperglycemia, various metabolic pathways are activated, leading to the overproduction of ROS and the promotion of OS [535]. Under high glucose conditions, the polyol pathway converts glucose to sorbitol via aldose reductase, which is subsequently oxidized to fructose. This process consumes NADPH, leading to reduced NADPH levels. NADPH is a precursor for the synthesis of reduced GSH, and its reduction significantly decreases the antioxidant capabilities of cell, making cells more susceptible to oxidative damage. Additionally, the accumulation of sorbitol and fructose raises intracellular osmotic pressure, resulting in cellular edema and membrane permeability damage, which further drives the development of retinal pathology [536-539]. In the hexosamine pathway, glucose is transformed into F6P. Activation of this pathway in hyperglycemic conditions increases ROS production and inhibits glyceraldehyde-3-phosphate dehydrogenase activity, leading to the overflow of its metabolites into the hexosamine pathway. This process not only increases H₂O₂ production but also promotes changes in endothelial cells, enhancing microvascular permeability and subsequently affecting the normal structure and function of the retina [540-542]. In the PKC pathway, high glucose states activate this pathway through the synthesis of ROS and diacylglycerol, causing the upregulation of PKC isoforms such as PKC-α, PKC-β, PKC-δ, and PKC-ε. These activated PKC isoforms facilitate changes in endothelial cell permeability and induce the expression of VEGF, promoting angiogenesis, endothelial cell damage, and pericyte loss, thereby accelerating the progression of DR [543-546].

In DR, the activation of multiple signaling pathways significantly influences OS, particularly the NF-κB and NOX pathways. NF-κB, a key TF in OS, is activated under hyperglycemic conditions, promoting the expression of inflammatory factors such as TNF-α and IL-6, which further exacerbate inflammatory responses in the retina. The persistent activation of NF-κB not only facilitates apoptosis but also inhibits the expression of antioxidant enzymes, leading to prolonged exposure of retinal cells to an oxidative environment, thereby worsening pathological changes in the retina [547, 548]. The activation of the NOX complex also plays a significant role in DR. One key function of NOX is to generate ROS, and its excessive activation under diabetic conditions leads to increased OS, interacting with the activation of other signaling pathways, such as MMPs. This interaction exacerbates further intracellular damage, affecting retinal microvascular function and strongly correlating with the worsening of retinal pathology [351, 549, 550].

4.9.3 Age-Related Macular Mediated Degeneration

Age-related macular degeneration (AMD) is the most prevalent retinal degenerative disease among the elderly, primarily characterized by the gradual loss of central vision. Recent studies have established that OS is crucial in the onset and progression of AMD. OS results from the excessive production of ROS and a disruption in antioxidant defense mechanisms, leading to detrimental effects on RPE cells and photoreceptor cells.

RPE cells play a vital role in the retina; they maintain the integrity of the blood-retinal barrier and are responsible for phagocytosing shed outer segments of photoreceptors. In this process, the high metabolic activity and oxygen demands of RPE cells generate significant amounts of ROS, increasing their susceptibility to oxidative damage. In high oxygen tension environments, such as the macular region, OS in RPE cells significantly escalates, further contributing to AMD pathogenesis [551-553]. As OS intensifies, oxidative damage to proteins, lipids, and mtDNA occurs within RPE cells. Research has identified that ROS accumulation is closely linked to cell death pathways; low levels of ROS may trigger apoptosis, while high levels can induce necrosis. This dysfunction in RPE cells subsequently leads to degenerative changes in the macula, promoting the development of AMD [553, 554].

Autophagy in RPE cells effectively eliminates oxidatively damaged proteins and organelles, thereby mitigating OS. However, when autophagy is suppressed, oxidatively damaged proteins and organelles cannot be adequately cleared, exacerbating OS and facilitating the progression of AMD [555-558]. Studies suggest that the Nrf2 and PGC-1 pathways are pivotal in regulating autophagy and combating oxidative damage [554]. Nrf2 serves as a key TF that allows cells to respond to OS by activating the expression of various antioxidant and detoxification genes, including HO-1 and NQO1. Under normal conditions, Nrf2 forms a complex with Keap1, which targets Nrf2 for degradation. However, under OS, conformational changes in Keap1 inhibit Nrf2 degradation, allowing it to translocate to the nucleus to activate antioxidant responses. Nrf2 enhances the antioxidant capacity of cells and also contributes to cellular resistance to oxidative damage by influencing autophagy and the ubiquitin–proteasome system [559, 560]. Concurrently, PGC-1α, a critical factor in regulating mitochondrial biogenesis, bolsters RPE cells’ antioxidant defenses by inducing genes involved in oxidative metabolism and antioxidant enzymes. Evidence suggests a cooperative interaction between PGC-1α and Nrf2, as both promote the expression of antioxidant enzymes and enhance cellular responses to OS. Additionally, the activation of Nrf2 and PGC-1α can boost autophagic activity, reduce intracellular protein aggregation, and improve the cellular protective mechanisms [561, 562].

OS not only directly damages RPE cells but also induces inflammatory responses, particularly through the activation of the NLRP3 inflammasome. Elevated levels of ROS stimulate the release of proinflammatory factors such as IL-1β and TNF-α, which contribute to the pathological progression of AMD. There is a bidirectional relationship between inflammation and OS, as inflammatory responses can further exacerbate OS, leading to a vicious cycle that intensifies damage to RPE cells and photoreceptor cells [557].

4.10.1 Male Infertility

Male infertility represents a growing public health concern. Substantial evidence indicates that OS plays a pivotal role in the pathogenesis of male infertility by inducing oxidative damage to germ cells and reproductive tissues, ultimately impairing spermatogenesis and semen quality.

ROS generated during OS encompass O₂•−, H₂O₂, and •OH. Human spermatozoa exhibit particular vulnerability to OS owing to their plasma membrane composition rich in PUFAs and limited endogenous antioxidant defenses [563, 564]. This makes sperm vulnerable to lipid, protein, and DNA damage when ROS is excessive, which in turn affects sperm motility, function, and fertilization ability [564, 565]. This biological predisposition renders sperm susceptible to ROS-mediated damage through three primary mechanisms: (1) LPO of membrane PUFAs compromises membrane integrity, impairing sperm motility and viability [566, 567]; (2) DNA fragmentation and oxidative base modifications jeopardize genetic integrity, reducing fertilization potential and increasing risks of embryonic abnormalities [567-570]; (3) protein carbonylation disrupts structural and functional proteins essential for motility and chromatin packaging [567]. Mitochondrial dysfunction exacerbates these effects, as oxidative damage to ETC components reduces ATP synthesis critical for sperm motility and survival [571, 572].

4.10.2 Female Infertility

OS constitutes as key pathogenic factor in female infertility through multifaceted impacts on reproductive physiology.

Ovarian follicles, comprising oocytes and associated granulosa cells, undergo ROS-mediated regulation during maturation [573-575]. Physiological ROS levels derived from follicular fluid components (macrophages, leukocytes, and paracrine factors) facilitate luteinizing hormone (LH) surge-induced ovulation and oocyte maturation [573-575]. This redox balance demonstrates dual regulatory roles: physiological ROS concentrations support folliculogenesis through steroidogenesis modulation and cytokine signaling, while excessive ROS induces membrane LPO that compromises oocyte developmental competence [573-575].

The peri-ovulatory phase exhibits characteristic ROS fluctuations, with pre-ovulatory follicles demonstrating elevated ROS levels that facilitate follicle rupture and oocyte release [573-575]. While LH surge-associated ROS elevation is physiologically essential, supraphysiological levels impair oocyte quality through oxidative damage mechanisms. Persistent ROS accumulation in unruptured follicles may precipitate apoptotic pathways, adversely affecting subsequent cycle outcomes [573-575].

In polycystic ovary syndrome, a prevalent endocrinopathy characterized by hyperandrogenism, oligo-ovulation, and polycystic ovarian morphology, OS exacerbates ovarian dysfunction through multiple pathways [575]. Insulin resistance-driven ROS overproduction synergizes with antioxidant depletion to establish a pro-oxidant microenvironment. Concurrent chronic inflammation, mediated by TNF-α and IL-6 overexpression, further disrupts follicular development through paracrine mechanisms, ultimately contributing to anovulation and infertility [575].

7.2.1 Toxicity of Antioxidants to Organ Systems

In recent decades, antioxidants have gained recognition for their protective roles against OS caused by ROS, providing defense against a variety of diseases. However, recent studies have highlighted the complexities associated with the use of antioxidants, especially when administered at high doses or inappropriately, as they may result in potential toxic effects across various organ systems in the body.

The toxicity of antioxidants is notably dose dependent. For example, investigations using the antioxidant AO2246 in zebrafish larvae have demonstrated that its toxic effects become more pronounced with increasing doses [711-713]. Moreover, excessive use of antioxidants can lead to “antioxidant stress,” disrupting the physiological balance between peroxides and antioxidants [714]. Such an imbalance may result in the conversion of vitamin C into a pro-oxidant under certain conditions, initiating harmful lipid oxidation. Additionally, superoxide anions and other free radicals can exacerbate toxicity by interfering with iron metabolism, thereby intensifying oxidative damage to cells [714].

Maintaining a dynamic balance between ROS and antioxidants is essential for overall health. Moderate levels of ROS play a role in regulating physiological functions and promoting disease prevention. Nevertheless, current research primarily focuses on the short-term effects of antioxidants and their anticipated therapeutic benefits in disease management. Therefore, a systematic assessment of the risks associated with long-term usage and potential cumulative toxicity is necessary.

7.2.2 Application of OS in Precision Medicine

Recent progress in precision medicine has leveraged OS-related molecular pathways to develop innovative and highly targeted therapeutic strategies across multiple disease contexts.

In oncology, predictive gene signatures derived from OS-associated mitochondrial gene sets and oxidative metabolism-related genes have enabled patient stratification into molecular subtypes with distinct sensitivities to chemotherapeutic and targeted agents [715, 716]. The integration of these signatures with immune checkpoint profiling uncovers OS as a modulator of tumor immune microenvironments, informing combinational approaches that synergize oxidative metabolism modulation with immunotherapy [715, 716].

Mechanistic innovations in nanomedicine exemplify the translational potential of OS-targeted therapies. The development of DNA nanostructure platforms, particularly tetrahedral framework nucleic acids functionalized with aptamers targeting overexpressed receptors (e.g., CD44 on injured renal tubular epithelial cells), enables highly specific cellular targeting and intracellular delivery of antioxidant agents such as baicalein. This approach overcomes limitations imposed by conventional antioxidants’ systemic distribution and rapid clearance by enhancing stability, renal accumulation, and cellular internalization [717]. Functionally, these nanocarriers attenuate mtROS, restore membrane potential homeostasis, and inhibit apoptosis by modulating redox-sensitive signaling cascades including NF-κB and NLRP3 inflammasome pathways. The resultant multimodal effects—combining antioxidation, anti-inflammatory action, and preservation of cellular integrity—demonstrate superior therapeutic outcomes in AKI models when compared with untargeted treatments [717].

In addition, the correlation between OS gene signatures and immune cell infiltration profiles across cancer types provides avenues for integrating oxidative metabolism modulators with immune checkpoint inhibitors [718]. This combinatorial precision strategy aims to reverse immunosuppressive microenvironments fostered by OS-induced metabolic reprogramming, thus enhancing antitumor immunity in patient subsets defined by OS-related molecular phenotypes [718].

Collectively, these advances highlight the evolving landscape of OS-centered precision therapeutics, which synergize molecular profiling, targeted delivery technologies, and mechanistic understanding of redox biology.

7.2.3 The Integration of Multiomics and Systems Biology Approaches

The integration of multiomics and systems biology approaches has also greatly advanced our understanding of OS in complex diseases and biological processes. By combining genomic, epigenomic, transcriptomic, proteomic, and metabolomic data, these integrative methods enable the identification of key OS-related genes, pathways, and molecular mechanisms across diverse conditions such as gastroesophageal reflux disease, atrial fibrillation, pancreatic cancer, parasitic infections, and MDD [719-721]. The use of Mendelian randomization, machine learning, and immune cell profiling further strengthens causal inference and elucidates the interplay between OS, immune responses, metabolic dysregulation, and cellular damage [722]. This holistic strategy uncovers novel biomarkers and potential therapeutic targets by revealing how OS contributes to disease pathogenesis through complex gene-environment and molecular network interactions. Despite challenges such as tissue specificity, data coverage limitations, and the need for functional validation, multiomics integration represents a powerful framework for deciphering OS-related biology and informing precision medicine approaches.