2.1 Obesity-associated DNAm and cancer development

Cancer initiation and development are mainly characterized by hypermethylation on CGIs of tumor suppressor gene promoters and global DNA hypomethylation.²⁸ The expression levels of obesity-related genes due to DNAm changes might affect cancer progression. Some overexpressed obesity-related genes were associated with tumor-promoting factors, while other underexpressed counterparts could inhibit cancer development.²⁹ Specific CpG methylation patterns are associated with obesity.³⁰ Thus, DNAm in diverse specific genes in cancers is conducive to differential diagnosis and targeted therapy. For instance, the combined methylation levels of zinc finger protein genes ZNF397OS and ZNF543 could distinguish obesity-associated colorectal tumors from other colorectal tumors, contributing to precise medical management.³¹ Additionally, ZNF577 hypermethylation might be the epigenetic biomarker of obesity-associated BC and is also relevant to diet patterns.^{32, 33} Secreted frizzled-related protein type 2 (SFRP2) is a potential epigenetic biomarker of colorectal cancer, which could inhibit Wnt signaling. BMI could influence the DNAm level of SFRP2 in patients with colorectal cancer.³⁴ Long interspersed nucleotide element 1 （LINE-1）is often used to represent the global DNAm level, showing hypomethylation in tumors, and was considered to be statistically significant with BMI in colon cancer and BC.^{35, 36} Ten-eleven translocation methylcytosine dioxygenases (TETs) are the main DNA demethylases that catalyze 5-methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC) and play an important role in maintaining pluripotency of stem cells.²⁶ Obesity promoted the self-renewal of triple-negative breast cancer (TNBC) stem cells by inducing oxidative stress, which was triggered by the increased expression of TAR DNA binding protein (TARDBP) mediated by ten-eleven translocation methylcytosine dioxygenase 1 (TET1).³⁷

Obesity can increase oncogenes expression or inhibit expression of tumor suppressor genes through DNAm regulation.^{38, 39} For instance, obesity-induced inflammation led to hypomethylation of the promoter regions of oncogenes such as Nanog homeobox, POU class 5 homeobox 1 (POU5F1), and MYC proto-oncogene (MYC) in colon cells, which may eventually cause colon cancer.⁴⁰ In tumors of obese patients with colon cancer, the promoter of oncogene histone linker 1 with histone H3.1 (HIST1H3I) is hypomethylated, while the promoter of tumor suppressor gene Slit-Robo Rho GTPase-activating protein 2C (SRGAP2C) is hypermethylated.⁴¹ Additionally, the intron of oncogene nuclear factor of activated T-cells cytoplasmic 4 (NFATC4) demonstrates hypermethylation.⁴¹ The alterations in gene expression resulting from differential methylation regions will contribute to the diagnosis of obesity-associated cancers. Nonalcoholic steatohepatitis (NASH) is highly correlated with overnutrition and IR. In some cases, patients with NASH might give rise to hepatocellular carcinoma (HCC). A genome-wide DNAm analysis in liver tissue samples exhibited a decrease in DNAm of tumor-related genes in NASH-associated precancerous tissue, and this phenomenon continued and intensified in NASH-related HCC.⁴² These overexpressed tumor-related genes such as tripartite motif-containing 4 (TRIM4), protein regulator of cytokinesis 1 (PRC1), tubulin alpha 1b (TUBA1B), are closely related to inflammation and malignant differentiation.

Epigenetic modifications may influence both lipid metabolism and immune response. Epigenetic research showed significantly different epigenome methylation profiles of periprostatic AT between obese/overweight and normal-weight patients with prostate cancer. Thirty-eight genes were found to be related to fatty acid accumulation in AT and tumor immune evasion.⁴³ AT, as the primary endocrine and metabolic organ, playing a significant role in obesity-associated cancers by secreting inflammatory factors, adipocytokines and hormones that can stimulate an adverse TME. Colorectal cancer patients with elevated DNAm levels of inflammatory factors, specifically interleukin-6 (IL-6), in their visceral AT (VAT) are at an increased risk of developing colorectal cancer.⁴⁴ It is suggested that 25-hydroxyvitamin D partly mediates this process. Various adipokines have been proven to stimulate tumor cell expansion through intracellular signal pathways.⁴⁵ For instance, leptin exerts its effects on antiapoptotic, inflammation promotion, and angiogenesis through signaling pathways such as JAK/STAT3, MAPK, and PI3K.⁴⁶ On the other hand, resistin promotes tumor cells proliferation, migration, and adhesion by affecting MAPK, PI3K, and NF-κB signaling pathways.^{47, 48} Recent research suggested that adipokines also impact cellular communication in the cancerous process. For instance, adipose stromal cells (ASCs) could secrete adipokines to affect the gap junction loci, which affected the intercellular communication in endometrial tumors of obese patients.⁴⁹ Mechanistically, adipokines caused promoter hypermethylation and long-term silence of gap junction protein alpha 1 (GJA1) that encoded gap junction protein connexin-43. In addition, organic pollutants accumulating in AT might cause an oncogenic impact through the endocrine pathway.⁵⁰ Exposure to endocrine-disrupting compounds might lead to intergenerational epigenetic alterations.⁵¹ Bisphenol A (BPA) is an endocrine disruptor with estrogenic activity and has been confirmed as a common carcinogenic chemical substance in environment that promote the progression of both hormone-sensitive and nonhormone-sensitive cancers.^{52, 53} A high-fat diet (HFD) can increase the incidence of offspring BC induced by BPA, which is associated with DNAm-mediated changes in gene expression.⁵⁴ BPA can accumulate in AT, and the accumulated BPA can cause changes in lipid metabolism, further aggravating the obese state, and this vicious cycle may be one of the causes of cancer.⁵⁵

Leptin and adiponectin secreted by adipocytes are crucial in development of cancer. In comparison with normal-weight individuals, obese/overweight individuals get higher levels of leptin and lower levels of adiponectin in their serum.⁵⁶ When the normal organism is in a state of hunger or reduction of body fat, the serum leptin content significantly decreases to store energy.⁵⁷ On the contrary, the rising leptin content in response to increased body fat resists the obese state by inhibiting appetite. This negative feedback between adipocytes and the central nervous system helps maintain the lipid metabolism of the organism to keep weight. Previous research has indicated that overexpression of leptin and leptin receptors significantly affect tumor proliferation, invasion, and metastasis, accelerating the process of tumor deterioration and decrease the survival rates.^58-61 Intriguingly, leptin not only promotes tumor progression by regulating angiogenesis, inhibiting apoptosis, and remodeling the TME but also plays a critical role in antitumor immunotherapy through innate and adaptive immune response.^{62, 63} Some obese/overweight cancer patients experience better prognoses and responses to immunotherapy due to the “leptin paradox” than their normal-weight counterparts.⁶² In untreated and newly diagnosed patients with renal cell carcinoma, promoter methylation in the leptin gene (LEP), as well as leptin receptor gene (LEPR), is remarkably higher in tumor tissues compared with normal adjacent tissues.⁶⁴ Additionally, hypermethylation of leptin receptor gene correlated with recurrence and shorter recurrence-free survival. In contrast, increased expression of LEP in ovarian cancer patients was related to longer lifetimes and lower recurrence rates.⁶⁵ In addition to solid tumors, the degree of LEP methylation could also be used to predict the prognosis of acute myeloid leukemia (AML).⁶⁶ Leptin has a dual effect on cancer progression, its antitumor immunomodulatory effects and mechanisms remain unclear. Correspondingly, due to the pleiotropic effects of leptin, its complex regulation has become an obstacle to its widespread clinical application.

Adiponectin is mainly involved in glucose and lipid metabolism, and it is known to bind to three receptors, namely adiponectin receptor 1 (ADIPOR1), adiponectin receptor 2 (ADIPOR2), and T-cadherin. Adiponectin exerts an inhibitory effect on tumors, and promoter methylation of the adiponectin gene (ADIPOQ) is associated with cancer risk.⁶⁷ Adiponectin can counteract the effects of leptin, exerting an antitumor effect by inhibiting angiogenesis, cell proliferation, tumor invasion, and metastasis.⁶⁸ For patients with pancreatic cancer, treatment with the methylation-blocking drug 5-Azacytidine has been shown to improve the transcription level of ADIPOR1 and ADIPOR2 to enhance adiponectin signaling.⁶⁹ This inhibitory effect on tumors by adiponectin is mainly reflected in antiproliferation. Furthermore, studies have demonstrated that chronic energy restriction can impede mammary tumor growth by altering the DNAm of LEP and ADIPOR1.⁷⁰ Obesity was found to promote tumor progression by increasing leptin expression synergized with decreasing adiponectin expression. Meanwhile, the increased leptin levels can also directly reduce adiponectin transcription. Consequently, the alteration in leptin-to-adiponectin ratio plays a crucial role in linking obesity to cancer progression.⁷¹

2.2 DNAm profiles in T2D and complications

By examining the DNAm profiles of individuals with T2D, researchers have identified novel candidate genes associated with T2D and enhanced comprehension of the disease's pathogenesis.⁷² Obesity primarily leads to the development of T2D by inducing IR and disrupting lipid metabolism. Researchers have reported hypermethylation of the hepcidin antimicrobial peptide (HAMP) gene promoter in the livers of mice predisposed to T2D and obese women exhibiting IR.⁷³ HAMP can serve as a candidate gene for assessing susceptibility to T2D and potentially mitigating excessive insulin secretion. CD36 plays a role in the uptake of fatty acids and the regulation of lipid metabolism. Hypermethylation and polymorphism of the CD36 promoter may be linked to the progression of obesity and T2D.⁷⁴ In obese and T2D patients, there is an increase in CD36 promoter methylation compared with thin individuals. Additionally, reduced plasma sCD36 levels lead to an increase in low-density lipoprotein cholesterol (LDL-C) levels.⁷⁴ In obese individuals, those with T2D have hypermethylation of the insulin receptor substrate 2 (IRS2) promoter CpG sites in the liver, and the reduced expression of IRS2 is closely related to IR.⁷⁵ Among patients with T2D, the elevated expression of DNA damage-inducible β (Gadd45β), in conjunction with TET1, stimulates the DNA demethylation of the PPARG coactivator 1 alpha (PPARGC1A) promoter, resulting in heightened gluconeogenesis and reduced glucose tolerance among patients.⁷⁶ DNAm alterations are evident across all three stages of adipogenesis in obese individuals, and these differentially methylated genes may play a role in the development of T2D primarily by impairing the function of mature adipocytes.⁷⁷

DNAm profiles varies among different T2D subgroups, and the initiation of diabetic complications can be predicted according to the methylation risk scores of subgroups.⁷⁸ Advanced diabetes patients commonly experience a range of complications, increasing the complexity of treatment. Compared with normal mice, obese/diabetic mice show different expression patterns of DNMTs and TET proteins in their kidney tissues, which may promote the progression of diabetic nephropathy by regulating DNAm levels.⁷⁹ Ten-eleven translocation methylcytosine dioxygenase 2 (TET2) can demethylate the CGI of transforming growth factor β1 (TGFB1) to promote glomerulosclerosis and accelerating the progression of diabetic nephropathy.⁸⁰ In addition, DNAm markers can also be used to evaluate the risk stratification of T2D nephropathy and predict the occurrence of diabetic retinopathy.^{81, 82} Due to technological limitations, mitochondrial epigenetic regulation has often been ignored. However, recent studies have also demonstrated the utility of mitochondrial DNA (mtDNA) methylation in the pathogenesis of IR in skeletal muscle and diabetic retinopathy.⁸³ Elevated levels of FFAs in the liver can enhance the mitochondrial translocation of DNA methyltransferase 1 (DNMT1), resulting in the hypermethylation of NADH-dehydrogenase 6 (ND6) on mtDNA.⁸⁴ The inhibition of ND6 leads to systemic IR due to mitochondrial dysfunction.

2.3 DNAm and metabolic risk factors in CVDs

DNAm markers can be utilized to assess the individual's cardiac metabolic risk and offer assistance in the context of obesity-induced CVDs.^85-87 A meta-analysis of genome-wide DNAm in obese adults shows that BMI-related CpG sites methylation with are associated with metabolic parameters of atherosclerosis and hyperlipidemia.⁸⁸ In the whole-genome association study, one-third of the differentially methylated genes related to CHD are associated with obesity, among which estrogen receptor 1 ATP binding cassette subfamily G member 1 (ABCG1), and IL6 have been identified as candidate genes for CHD in multiple studies.⁸⁹ Krüpple-like factor 14 (KLF14) is a key metabolic-related transcription factor, and its deficiency can accelerate the formation of atherosclerosis.⁹⁰ The high methylation status of KLF14 is closely related to obesity indices, lipid profiles, blood pressure status, and IR status, which can provide reference for the precise treatment of cardiometabolic diseases.⁹¹

Abnormal secretion of adipokines is an important pathogenesis of CVDs caused by obesity. Leptin can stimulate carotid sinus nerve to increase blood pressure. The high expression of leptin and leptin receptor in carotid corpuscles is positively correlated with the promoter demethylation of transient receptor potential melastatin 7 (Trpm7).⁹² Leptin-activated pSTAT3 can regulate the expression of Trmp7 through epigenetic modification and increase the risk of hypertension in obese patients.⁹² Inflammation is the basic feature of atherosclerosis, and the release of inflammatory factors and the recruitment of immune cells are accompanied by the whole course of the disease. The promoters of the IL-6 and nuclear factor kappa B (NF-κ B) genes were found to be hypomethylated in the peripheral leukocytes of obese patients.^{93, 94} Upregulation of IL-6 and NF- κ B in leukocytes is closely associated with a decreased level of adiponectin.⁹³ In obese patients, anti-inflammatory effect of adiponectin is inhibited, leading to an acceleration of the atherosclerosis process. In addition, another study shows that the hypomethylation of IL-6 and tumor necrosis factor-alpha (TNF-α) genes in obese patients can lead to endothelial dysfunction by causing inflammation and oxidative stress, thus promoting the occurrence of atherosclerosis and coronary artery disease.⁹⁴

DNAm changes typically precede changes in gene expression, and detecting DNAm can be beneficial for early disease diagnosis.⁹⁵ Furthermore, in obese/overweight state, DNAm regulates and reprograms gene expression patterns, which means that DNAm will become a pivotal target for treatment and drug development.

3.1 The impact of histone modifications on cancer-related gene expression

Adding a different number of methyl groups to various residue sites of histone has an effect on gene expression, and different methylation degrees further complicate histone modification and regulation of gene expression.⁹⁷ The consumption of a HFD was shown to upregulate the transcription of MYC and led to hypomethylation of histone H4K20 by Jumonji domain-containing Histone Demethylases (JHDM) in the promoter region of the MYC regulatory genes.⁹⁸ This leads to proliferation of prostate cancer cells and increased tumor burden. Histone methyltransferase SET domain-containing 2 (SETD2) could catalyze H3K36me3, which is a well-known conserved epigenetic modification.⁹⁹ Deficiency in SETD2 was known to regulate cholesterol homeostasis and activate c-Jun/activator protein 1 (AP-1) via triggering lipid accumulation to promote HCC progression.¹⁰⁰

Histone acetylation mainly occurs on the lysine residues in histone tails, which is a reversible dynamic equilibrium process, and the disruption of this balance has been linked to tumorigenesis. Chromatin becomes slack due to histone acetylation, thereby increasing DNA accessibility and promoting transcriptional activity. In steatotic hepatocytes, hyperacetylation of genome-wide histones could initiate carcinogenesis by damaging DNA.¹⁰¹ Recent studies have found that the concurrent activation of acetylated and trimethylated H3K9 in mice can lead to NASH-related HCC, and deacetylation of H4K16 promotes cancer progress by silencing genes associated with cell death.^{102, 103} In addition to these findings, histone modifications caused by the interactions between a HFD and intestinal microbiome are significant during gastrointestinal tumor development. Enhancer histone methylation and acetylation induced by intestinal microflora have been associated with the signal pathway of the occurrence and development of colon cancer under obesity exposure.¹⁰⁴

3.2 Histone acetylation and methylation in initiation and progression of T2D

PPAR γ in AT can promote the growth and differentiation of adipocytes, and can reduce insulin secretion by pancreatic β cells and increase insulin sensitivity.¹⁰⁵ The acetylation of PPAR γ is essential for its activation. Targeting Sirtuin 1 (SIRT1) can increase the expression levels of genes related to insulin sensitivity and ameliorate obesity-induced T2D.¹⁰⁶ Furthermore, inhibitors of histone deacetylase 3 (HDAC3) can also activate PPAR γ to reverse IR caused by HFD.¹⁰⁷ Inhibition of histone deacetylases (HDACs) can also be used to prevent cardiomyopathy caused by diabetes. HDAC3 increases the H3 histone deacetylation of dual specificity phosphatase 5 (DUSP5) gene promoter in the heart of diabetic mice, which leads to cardiac dysfunction by inhibiting the expression of DUSP5. Blocking HDACs can serve as a preventive measure against diabetic cardiomyopathy. Specifically, HDAC3 stimulates H3 histone deacetylation of the DUSP5 gene promoter in the hearts of diabetic mice, resulting in cardiac dysfunction through suppressing the expression of DUSP5.¹⁰⁸ HDAC3 inhibitors can also mitigate diabetic liver injury by upregulating nuclear factor erythroid 2-related factor 2 (Nrf2), and alleviate T2D-induced aortic fibrosis and inflammation by enhancing fibroblast growth factor 21 (FGF21) synthesis.¹⁰⁹ The role of HDAC3 inhibitors in the treatment of diabetes has been widely recognized, and selective HDAC3 inhibitors can become potential antidiabetic drugs. The therapeutic potential of HDAC3 inhibitors in managing diabetes is widely acknowledged, and high selective HDAC3 inhibitors have the ability to develop into promising antidiabetic medications.¹¹⁰

In addition to histone acetylation, the modification of histones through methylation is critical in pathogenesis and progression of T2D. Premature senescence of adipocyte precursor cells and disruption in adipogenesis can lead to early IR. In adipocytes of individuals with T2D, reduced enrichment of lysine 4 tri-methylated H3-histone (H3K4me3) and subsequent increasing expression of mitochondrial transcription factor A (TFAM) promotes premature senescence of adipocytes, inhibits adipocyte differentiation, and accelerates the progression of T2D.¹¹¹ Pancreatic β cells are crucial cells in the regulation of glucose homeostasis, and the defective adaptive response of β cells is one of the pathogenic mechanisms of T2D. PPAR γ agonists can upregulate the histone lysine methyltransferase SET domain containing 7 (SETD7) in β-cells, which increases pancreatic and duodenal homeobox 1 (PDX1).¹¹² PDX1 drives β-cell adaptation to protect β cell function, thus regulating glucose sensitivity and insulin secretion.¹¹²

3.3 The dual role of histone acetylation in vascular diseases

Lysine acetylation is implicated in numerous cardiac disease processes, including cell metabolism, gene transcription, and enzyme activity, and is associated with metabolic disorders and heart failure.¹¹³ Histone acetylation is involved in the regulation of smooth muscle cells (SMCs) differentiation and remodels vessels, which leads to the initiation of hypertension in individuals with metabolic syndrome.¹¹⁴ Obesity-induced hyperleptinemia may lead to histone acetylation and methylation of Trpm7, thereby promoting hypertension through upregulating Trpm7 expression.⁹² Reduced expression of the histone deacetylase Sirtuin 6 (SIRT6) in SMCs within atherosclerotic plaques may compromise telomere integrity, leading to senescence of SMCs and inflammation.¹¹⁵ Conversely, the lack of histone deacetylase 9 (HDAC9) can mitigate endothelial–mesenchymal transition (EMT) that preserve plaques stability and decelerate the progression of atherosclerosis.¹¹⁶ It is suggested that diverse HDACs may play different roles in vascular diseases.

Abnormal lipid metabolism may prompt modifying enzymes and binding proteins to recognize, produce and eliminate posttranslational modification, thereby regulating the expression of related genes and promoting the advancement of obesity-associated diseases. With the development of mass spectrometry, additional histone acylation modifications have been discovered, which enriches the level of protein posttranslational modification regulations. Currently, various HDAC inhibitors have been approved for clinical application.

4.1 ncRNAs in obesity-associated diseases

LncRNAs are a type of ncRNAs lacking open reading frames and having a length greater than 200 nucleotides. They can be classified into five types: sense, antisense, bidirectional, intronic, and intergenic ncRNAs.¹¹⁷ Although lncRNAs have been regarded as “transcriptional noise” for a long time, studies have shown that lncRNAs affect cell proliferation, differentiation, apoptosis, and carcinogenic pathways.

4.2 MiRNAs in obesity-associated diseases

MiRNAs are endogenous small ncRNAs whose length is about 22 nucleotides. iRNAs repress translation through complete and incomplete base complementary pairing with mRNA, regulating the function of the target genes.

4.2.1 The role of miRNAs in obesity-associated cancers and potential therapeutic applications

The modifications of miRNAs have vital roles in diverse cancerous progress, and differential expression of miRNAs can serve as biomarkers and therapeutic targets.¹⁴⁰ In HCC, miR-148a and miR-22-3p could participate in tumor progression by regulating lipid metabolism.^{141, 142} Peroxisome proliferator-activated receptor γ (PPARγ) is abundant in AT and known for its role in modulating energy metabolism, inflammation, adipogenesis, cell differentiation, and antitumor effect.^{143, 144} Obesity might decrease PPARγ by inducing overexpression of miR-27b, miR-130b, and miRNA-138, which could increase the susceptibility of colorectal cancer.¹⁴⁵ Obesity increases the risk of BC through various molecular mechanisms, which are partly regulated by miRNA. Upregulated miRNA-3184-5p and downregulated miRNA-181c-3p that targeted forkhead box protein P4 (FOXP4) and PPARα, respectively, could promote proliferation and invasive ability of adipocyte-induced BC cells.¹⁴⁶ Many miRNAs are now recognized as valuable diagnostic and prognostic markers for cancer, these miRNAs that play an oncogenic role are defined as oncomiRs. Chronic inflammation induced by obesity can also modulate the expression of cancer-related miRNAs and inflammatory miRNAs (inflamma-miRs). For instance, obese patients with BC have an increased expression of miRNA-21 (oncomiR and inflamma-miR) and a decreased expression of tumor suppressor miRNA-34a compared with normal-weight patients.¹⁴⁷ In addition, miRNA-21 and miRNA-146a (inflamma-miR) were significantly increased in obese patients with lymph node metastasis. MiRNA-10b could mediate the interaction between obesity and BC in postmenopausal women by regulating cancer-related genes.¹⁴⁸ MiRNA is a promising novel noninvasive biomarker, and there are methods for detecting miRNAs in feces, blood, and urine of patients.^{149, 150} Endogenous mature miRNA is highly stable in serum and is not easily degraded by RNA enzymes in the blood. Research showed that miRNA-223 in the urine of patients with endometrial cancer (EC) is evidently increasing, and there were differential expressions of miRNAs between normal-weight and obese patients.¹⁵¹ Detecting miRNAs in body fluid will be a powerful tool for screening, diagnosis, and prognosis of cancer. However, the modification of miRNA is complex and variable. Combined detection is necessary to enhance the diagnostic value, and the differences in expression among diverse individuals may limit detection application. Thus, expanding the sample size for further study can help use miRNAs as diagnostic markers of tumors in clinical applications.

The interactions between miRNAs and adipokines play an important role in biological functions of obesity-associated cancers. Adipokines could upregulate oncogenic miRNAs and downregulate antitumoral miRNAs, ultimately promoting tumor development.¹⁵² Additionally, adipokines can also affect antitumor therapy via miRNAs. Leptin could upregulate coactivator Mediator complex subunit 1 (Med1), an anchor subunit of the human mediator complex, by decreasing miRNA-205 expression to diminish the hormonal therapy effect of tamoxifen in BC.¹⁵³ Overexpressed miRNA-628 could dampen the proliferation of prostate cancer cells and reduce EMT, invasive ability, and the characteristics of stem cells, while leptin can promote the development of prostate cancer by downregulating the expression of miRNA-628.¹⁵⁴ Meanwhile, miRNA-628 could increase the sensitivity of prostate cancer cells to chemotherapeutic drugs by promoting cell apoptosis. In the case of pancreatic ductal adenocarcinoma, leptin can upregulate miRNA-342-3p to inhibit tumor suppressor (KLF6), which was associated with increased gemcitabine resistance.¹⁵⁵ Obesity could increase leptin expression by downregulating tumor suppressor p16^INK4A expression, which promoted the precancerous development of BC.¹⁵⁶ Interestingly, p16^INK4A in breast adipocytes could also mediate miRNA-141 and miRNA-146b-5p to combine with leptin mRNA, negatively regulating leptin expression. Collectively, the complex interaction between miRNAs and adipokines is believed to function in the progression of obesity-associated cancers.

Extracellular vehicles (EVs), including but not limited to microvesicles, exosomes, apoptotic bodies, and oncosomes, are membranous vesicles that could shuttle amongst cells in the extracellular matrix, which participate in cell communication, migration, angiogenesis, and cancer cell proliferation. AT-derived EVs involve the occurrence and metastasis of tumors and can transport miRNAs to mediate cell-to-cell communications¹⁵⁷ (Figure 3). For example, miRNA-23a/b was transported into HCC cells by AT-derived exosomes and played a pivotal role in tumor proliferation, metastasis, and 5-fluorouracil (5-FU) resistance by targeting von Hippel-Lindau (VHL)/hypoxia-inducible factor 1α (HIF-1α) axis.¹⁵⁸ Instead, tumor cells could also secrete exosomes containing miRNAs that act on adipocytes. For instance, BC cells secreted exosomes containing miRNA-155, which could induce beige/brown adipocyte differentiation and alter the surrounding adipocyte metabolism to promote cancer progression.¹⁵⁹ Adipocytes differentiate into white, beige, and brown adipocytes. White adipocytes are primarily used for energy storage, while brown adipocytes can generate energy by burning fat, and beige adipocytes are an intermediate state between them. MiRNA-155 could downregulate PPARγ expression, making adipocytes use energy more efficiently, and this high metabolic state contributed to the growth of breast tumors. In addition, tumor cell-derived EVs could also cause abnormal glucose metabolism leading to disorders of systemic glycemic control. For instance, miRNA-122 carried by BC cells-derived EVs acts on pyruvate kinase M (PKM) in β-cells, thus inhibiting insulin secretion, and contributing to the development of a hyperglycemic state that favors tumor growth.¹⁶⁰ Recent studies have also suggested that the specific miRNome profile of platelets in patients with visceral obesity was involved in the initiation of colon cancer, and the regulation of platelet activation may become another breakthrough in the therapy of obesity-associated cancers.¹⁶¹

MiRNAs are direct or indirect factors in cellular communication between adipocytes and tumor cells^{162, 163}; thus, utilizing the transport function of exosomes may provide a new direction for cancer treatment. Scientists have leveraged AT-derived exosomes to transport miRNAs to tumors which triggers apoptosis of cancer cells and enhances the sensitivity to chemotherapy (Figure 3). Here, miRNAs downregulate the expression of target genes by binding to the mRNA of carcinogenic factors. For example, AT-derived mesenchymal stem cells (AMSCs) secrete exosomes with miR-122 to suppress the expression of a disintegrin and metalloprotease 10 (ADAM10), insulin-like growth factor receptor 1(IGF1R), and cyclin G1 (CCNG1) in HCC cells, resulting in arrest of cell cycle and apoptosis, and improve chemotherapy efficacy.¹⁶⁴ AMSC-derived exosomes can delivery miR-145 to BC cells to prevent cancer progression by suppressing the expression of Rho-associated coiled-coil containing protein kinase 1 (ROCK1), matrix metalloproteinase 9 (MMP9), erb-b2 receptor tyrosine kinase 2 (ERBB2) and upregulation of tumor protein p53 (TP53).¹⁶⁵ Similarly, miR-199a enhances liver cancer sensitivity to doxorubicin by inhibiting the mTOR pathway.¹⁶⁶ MiR-424-5p restrained the expression of programmed death-ligand 1 (PD-L1) on TNBC cells, which create an inflammatory milieu.¹⁶⁷ AMSC-derived exosomes, serving as endogenous carriers, exhibit stability and low immunogenicity, rendering them an innate vehicle for miRNAs transportation. They can effectively counteract the drawbacks of instability and susceptibility to degradation of exogenous miRNAs. Despite the promising prospects, exosomes still face technical impediments in terms of separation, identification, and commercialization, and their clinical applications remain in the nascent stage.

4.3 CircRNAs in obesity-associated diseases

CircRNAs are characterized by their closed loop shape that lacks 3′ polyadenylation (poly(A)) tails and 5′ cap, making them stable and resistant to degradation. circRNA, as a competing endogenous RNA (ceRNA), could competitively combinate with miRNA to attenuate negative regulation of downstream target genes and increase target gene expression.¹⁸⁰ Although the primary function of circRNAs is to act as miRNA “sponges” in regulating target gene expression, some circRNAs could also exert their functions by interacting with proteins.¹⁸¹ CircRNAs have been confirmed to be involved in lipid synthesis and decomposition, and its imbalance can lead to a series of metabolic diseases.^{182, 183}

CircRNAs play a dual role in regulating inflammation in AT. For instance, circRNAs participated in regulating inflammation response through an obesity-associated ceRNA network, which was related to lipid metabolic disorders.¹⁸⁴ On the contrary, circARF3 (ADP-ribosylation factor 3) could also strengthen mitophagy to subdue adipose inflammation by combining miRNA-103.,¹⁸⁵ but whether changes in this inflammatory state affect tumor development have not been confirmed. A recent study identified circRNAs profiles in white and brown AT, which provided circRNA candidates for regulating adipose formation.^{186, 187} Studies suggested that circSAMD4A increased preadipocyte differentiation,¹⁸⁸ and circArhgap5-2 played an essential role in adipogenesis and lipid metabolism.¹⁸⁹

Evidence suggests that downregulated hsa_circ_000839 in patients with BC exhibits a negative correlation with BMI, but the underlying mechanism is still unclear.¹⁹⁰ A transcriptome study of circRNAs was conducted, focusing on obese and postmenopausal patients with EC and revealed the presence of 174 differentially expressed circRNA in EC tissues compared with adjacent normal tissues.¹⁹¹ Nonetheless, additional functional research is required to determine the circRNAs that can offer diagnostic and prognostic assistance in relation to obesity-related EC. The plasma of TNBC patients with high BFR showed increased expression of exo-cirCRIM1 derived from adipocytes.¹⁹² Exo-cirCRIM1 can increase protein glycosylation of fructose-bisphosphatase 1 (FBP1) by suppressing miR-503-5p and thus weakening its glycolytic inhibition, which promotes TNBC progression. While circRNAs were confirmed to contribute to obesity and cancer development, whether circRNAs play a part in the interaction between obesity and cancer has been rarely explored.

SIRT1 is involved in regulating insulin sensitivity and oxidative stress, and it plays a protective role in T2D.¹⁹³ In T2D patients, the expression of hsa_circ_0115355 was abnormal, and the reduced hsa_circ_0115355 hindered its competitive binding with miR-145, resulting in decreased expression of SIRT1.¹⁹³ The upregulation of circ LRP6 in damaged β cells leads to increased apoptosis, decreased insulin release, and induction of oxidative stress, exacerbating T2D by targeting the miR-9-5p/protein N-arginine methyltransferase-1 (PRMT1) signaling pathway.¹⁹⁴ Forkhead box O1 (FOXO1) is a negative regulator of insulin signal and plays an important role in the regulation of blood glucose.¹⁹⁵ Oleic acid can promote the activation of circ HIPK3, leading to increased fat deposition and IR by sponging miR-192-5p and upregulating FOXO1.¹⁹⁶

Due to their tissue specificity and conservation, circRNAs can serve as potential biomarkers for diagnosing coronary artery disease.¹⁹⁷ For instance, in comparison with healthy individuals, the expression of hsa_circ_0124644 was significantly upregulated in the peripheral blood of patients with coronary artery disease and validated in larger sample sizes.¹⁹⁸ Mechanistically, circRNAs can competitively bind to miRNAs to regulate the expression of genes associated with coronary artery disease.⁸⁷ CircRNAs derived from extracellular vesicles can also elevate the risk of hypertension and atherosclerosis by modulating blood lipids, lipid deposition, and endothelial cell function.¹⁹⁹

Evidence shows that ncRNAs have significant biological functions and participate in forming complex gene regulatory networks. Exosomes assist in transporting ncRNAs to distant organs, amplifying the negative effects of AT. These molecules interact with each other, facilitate intercellular communication and collectively contributing to the progression of obesity-associated diseases.

7.1 Dietary regulation

With the change in dietary habits, high-fat/fast food has become a common dish on the table. Consumption of a HFD can alter the global DNAm patterns, thereby elevating the susceptibility to obesity-associated diseases.^{232, 233} Rats fed a high-sugar and high-fat cafeteria diet exhibited adipocyte hypertrophy and IR in the VAT, which was associated with DNA hypomethylation of the solute carrier family 27 member 3 (Slc27a3).²³⁴ An obese diet could also affect the gut microbiome to stimulate histone modification of enhancers in the colon, which might be relative to the promotion of colon cancer.¹⁰⁴ Moreover, HFD, which is rich in dietary vegetable fat n-6 linoleic acid, was known to regulate DNAm alterations of farnesoid-X-receptor (Fxr) and prostaglandin-endoperoxide synthase-2 (Ptsg-2) genes, which involve in the development of colonic inflammation and cancer.²³⁵ Over-consumption of fructose for an extended period might promote lipid accumulation and downregulated expression of LEP and miR-192, which was also related to prostate cancer pathogenesis.²³⁶ Excessive fructose consumption elevates the risk of developing T2D by impacting the regulatory network of miRNAs, thereby diminishing insulin sensitivity and promoting the accumulation of TG.²³⁷ Consequently, it is crucial to restrict fructose consumption in our regular diet. Proper dietary interventions can mitigate the adverse effects of obesity and decrease the initiation and progress of associated complications.²³² Research demonstrated that following a two-year weight loss dietary intervention, TXNIP DNA hypermethylation, regulating glucose homeostasis, enhanced pancreatic islet function and lowered the risk of T2D.²³⁸ In addition, dietary intervention for individuals in early T2D has the potential to mitigate the impairment on β cells due to lipotoxicity and inflammation by reinstating the chromatin reprogramming.²¹⁰ Improved diet quality can alter the levels of DNAm in cardiometabolism-related genes, which is linked with increased risk of hypertension and CHDs.²³⁹ Energy restriction has always been the main intervention for weight loss, but it poses significant challenges to long-term dietary intervention due to its impact on health and low compliance.

Certain nutrients are thought to mitigate the impact of obesity, and their proper intake may help prevent obesity-associated diseases. Animal research suggested that adding grape powder to HFD could reduce oncomiR-34a and oncomiR-21 expression to prevent obesity-induced prostate cancer.²⁴⁰ Genistein (GE) is a bioactive nutritional compound that is abundant in soybean products and is considered a potential antitumor agent. Nowadays, GE consumption could prevent advanced BC through epigenetic modification and even inhibit HFD-induced BC promotion and metabolism disorders in offspring.²⁴¹ Maternal consumption of GE can impact the initial colonization of intestinal microbiota, global DNAm levels and the expression of key cancer-associated genes in offspring.²⁴² Whole grain proso millet (WGPM) has been shown to substantially reduce blood glucose and lipid levels in mice with T2D by altering the expression profile of miRNA and inhibiting gluconeogenesis-related pathways.²⁴³ Flavonoids are prevalent in plants and have demonstrated the ability to enhance endothelial function and decrease the CVDs risk through epigenetic regulation.²⁴⁴ Epigallocatechin-3-gallate (EGCG), which is rich in green tea, can act as an epigenetic regulator to inhibit ROS production and exert an antioxidant effect, contributing to prevent CVDs and diabetes.²⁴⁵ The gut microbiota in obese patients diminishes ethanolamine metabolism, elevates intestinal permeability, and contributes to chronic inflammation. Innovative probiotics have the potential to reinstate microbial ethanolamine metabolism and alleviate intestinal permeability and inflammation in obese and diabetic patients by targeting the AT-rich interaction domain 3a (ARID3a)/miR-101a/zona occludens-1 (Zo1) axis.²⁴⁶ Furthermore, supplementing the diet with new probiotics can decrease the levels of miR-155-5p and miR-125b-5p in the plasma of obese patients, thereby regulating inflammation and lipogenesis.²⁴⁷ While dietary supplements are generally a safe and convenient means of prevention, additional clinical experimental evidence is needed to determine appropriate optimal dosages and supplementation method.

When glucose levels are insufficient under the state of starvation, the lipids are decomposed into glycerol and fatty acids by fat mobilization, producing acetyl-CoA for energy and ketone bodies to supply the brain and heart. The ketogenic diet is characterized by high-fat and low-carbohydrate, similar to the state of hunger, producing a large number of ketone bodies. At present, studies have shown that a ketogenic diet helps to inhibit the progression of metabolic syndrome, diabetes, CVDs, and cancer.^248-250 Nevertheless, some studies indicate that a long-term ketogenic diet may lead to IR and elevated LDL levels.^{251, 252} Consequently, numerous uncertainties persist regarding the effectiveness of ketogenic diet treatment for metabolic diseases. We all know that cancer cells generate energy mainly through glycolysis, and damaged mitochondrial function prevents them from utilizing ketone bodies for energy production like normal cells. Thus, tumor cells are more sensitive to glucose deficiency. A ketogenic diet inhibits the initiation of the primary tumor and systemic metastasis by lower blood glucose levels and increasing ketone bodies. However, the biological mechanisms of the antitumor effect of ketone bodies remain unclear. The latest study showed that β-hydroxybutyric acid, a ketone body, could bind to the cell surface receptor Hcar2, increasing the level of transcription factor Hopx to suppress the proliferation of colon cancer cells.²⁵³ β-hydroxybutyric acid can mediate histone lysine 3-hydroxybutyrylation modification which has been identified as a noval epigenetic modification that links fatty acid metabolism to cancer progession.²⁵⁴ β-Hydroxybutyrate dehydrogenase 1 (BDH1) is the primary rate-limiting enzyme in ketone body metabolism and is closely related to the overall survival rate of patients with HCC. Metastasis-associated protein 2 (MTA2) could inhibit the level of BDH1 and increase histone β-hydroxybutyrylation, thus promoting the proliferation of HCC stem cells.²⁵⁵ β-Hydroxybutyrylation of tumor suppressor factor p53 could inhibit cancer cell apoptosis and dampen the function that maintains cancer cell growth arrest.²⁵⁶ However, β-hydroxybutyrylation might also play an antitumor role in inhibiting the critical enzyme of methionine metabolism, as tumors are highly dependent on methionine metabolism.²⁵⁷ There are diverse results treated with ketone bodies in different tumors under different physiological conditions. While the ketogenic diet may impede the tumor's growth rate, persistent weight loss may engender cachexia. Recent studies indicate that the utilization of glucocorticoids alongside a ketogenic diet may avert the onset of cachexia and correspondingly elongate survival rates.²⁵⁸ To date, most of the experiments conducted have only been based on animal models, and further studies are required to determine how the ketogenic diet can be safely applied in clinical anticancer therapy.

7.2 Physical exercise

In addition to healthy dietary intervention, regular physical exercise is also an effective preventive measure for obesity-associated diseases.^259-262 In a study focusing on postmenopausal overweight/obese women, miR-122 levels in the plasma of the participants decreased significantly following a 12-month weight-loss regimen of diet and exercise that reduced BC risk.²⁶³ Physical exercise can stimulate the release of ncRNAs and alleviate metabolic abnormalities by impacting intertissue communication.²⁶⁴ The myokine irisin, primarily derived from contracted skeletal muscles, plays a key role in improving glucose tolerance and suppressing IR.²⁶⁵ From a mechanistic standpoint, miRNAs released from skeletal muscle may be involved in the regulation of irisin expression.²⁶⁵ Exercise can influence the miRNA expression profile in exosomes derived from myotubes and upregulate the expression of genes associated with anti-inflammatory responses and appetite regulation.²⁶⁶ The effects of exercise are comprehensive, and designing a tailored exercise plan based on individual conditions can enhance insulin sensitivity and cardiopulmonary health. Integrating it with dietary regulation can produce multiplied benefits.

7.3 Bariatric surgery

A sedentary state is ubiquitous during daily study and work, and physical exercise and calorie restriction are two major measures to manage weight. Although caloric restriction can limit the glucose supply to tumors, it can perturb water and electrolyte metabolism, causing physical damage to patients. Therefore, it is not advisable for patients to attempt this method without professional guidance. Furthermore, recent research suggested that long-term weight loss interventions had a better effect on cancer prevention and mitigation than short-term approaches.^{20, 267, 268} Epigenetic modification is a dynamic process that can be reversed by long-term weight loss interventions, thereby reducing the risk of obesity-associated diseases. The potential harmful effects of obesity are well-documented. In young individuals, obesity can lead to changes in DNAm, and over time, tumor-prone gene signature appears in aging individuals.²⁶⁹ Obesity-associated DNAm changes and the upregulation of oncogenes remain unchanged after short-term weight loss, but long-term weight loss reverses the tumor-prone gene signature.

Except for diet control and physical exercise, long-term weight loss induced by bariatric surgery could change the metabolic status through epigenetic regulation, thus reducing the risk of obesity-related complications.^{270, 271} According to a cohort study of 30318 obese patients (BMI ≥ 35) that showed bariatric surgery significantly reduced the incidence and mortality of obesity-associated cancers.²⁷² The plasma miRNA expression profile in T2D patients undergoes changes following bariatric surgery. This analysis of changes can help identify potential therapeutic targets for T2D. Differentially expressed miRNAs in obese patients tended to be normalized after bariatric surgery, which was thought to reduce the risk of colorectal cancer.²⁷³ Gastric sleeve surgery can prompt the upregulation of tumor suppressor miR-122 in serum, thus reducing the risk of cancer.²⁷⁴ In addition, the marked upregulation of lncRNA Gm19619 triggered by a HFD can enhance hepatic gluconeogenesis and lipid accumulation.²⁷⁵ Following vertical sleeve gastrectomy, the expression of lncRNA Gm19619 is reduced, leading to improved glucose and lipid metabolism.²⁷⁵ Sleeve gastrectomy can reverse the upregulation of lncRNA taurine-upregulated gene 1 (TUG1) induced by high sugar and high fat intake, leading to decreased TUG1 levels which in turn help in regulating blood sugar levels in patients with T2D.²⁷⁶ Hence, bariatric surgery presents a promising approach for preventing complications in obese patients.

7.4 Epigenetic drugs

The diversity and uncertainty of epigenetic modification pose significant challenges to drug therapy related to epigenetics. Resveratrol or 5-aza could reduce triglyceride accumulation in hepatoma cells, and the resveratrol could reverse promoter DNAm of Nrf2-Keap1 (aberrant DNAm in human cancers) to attenuate NAFLD.²⁷⁷ HCC patients induced by NAFLD exhibit overexpression of Squalene Epoxidase (SQLE), which triggers a series of oxidative stress events. These events include the expression of DNMT3A and the silencing of DNMT3A-induced Phosphatase and Tensin Homolog (PTEN), ultimately activating AKT–mTOR.²⁷⁸ Inhibitors of SQLE, such as the antifungal drug terbinafine, could suppress this series of chain reactions and offer a new approach for HCC target therapy. As a target point, the fatty acid-binding (FABP4) could indirectly lead to overexpression of DNMT1 and silence tumor suppressor gene p15^INK4B in AML cells.²⁷⁹ Bioactive compound honokiol and adiponectin could inhibit leptin-induced tamoxifen resistance in BC.¹⁵³ Maternal generation with HFD during pregnancy can induce epigenetic alteration in offspring. DNMT inhibitor hydralazine and HDAC inhibitor valproic acid suppresses BC progress in mice offspring of the high-fat group but increases the incidence and burden of BC in the control group.²⁸⁰ Currently, approved epigenetic drugs are mainly prescribed for treating hematological malignant tumors (Table 1), while a large number of epigenetic drugs for solid tumors are under clinical trials. This will provide a new direction for cancer treatment.

Though there are no epigenetic drugs approved for the treatment of T2D and CVDs in clinical application, some drugs are undergoing clinical trials (Table 2). Investigating the epigenetic genome of obese patients and advancing the development of targeted chromatin-modifying drugs have the potential to minimize side effects and facilitate precision treatment.⁸⁶ Thiazolidinediones are antidiabetic drugs that target PPARγ, but their usage is restricted due to their potential impact on heart and kidney function. HDAC3 inhibitors have the ability to induce posttranslational modifications of PPARγ and could potentially offer a safer therapeutic approach for T2D by enhancing insulin sensitivity.¹⁰⁷ Bromodomain and extraterminal (BET) proteins act as histone acetylation readers in chromatin remodeling. The BET inhibitor apabetalone has the potential to reduce the risk of CVDs and T2D by suppressing monocyte inflammatory activation.²⁸¹

In comparison with traditional drugs, epigenetic drugs show certain advantages, the development of epigenetic drugs faces significant challenges due to high costs and individual variability. Although epigenetic therapy has brought great convenience to diseases treatment, it still has a long way to go before its wide application in clinics.

7.5 Immunotherapy in obesity-associated cancers

Tumor cells evade immune surveillance and malignantly proliferate in the organism due to their low immunogenicity and the release of immune suppressing molecules. Recent years, immunotherapy targeting tumor immune evasion has brought hope to cancer patients. However, the response to immunotherapy varies among patients with obesity-associated cancers, and the specific mechanisms by which obesity affects the tumor immune microenvironment are not clear. In general, with the progression of obesity, the infiltration of myeloid-derived suppressor cells increases in tumors, while CD8+ T cells decrease, leading to inhibition of antitumor immunity.^{282, 283} However, some obese patients show better response to immunotherapy compared with normal-weight patients. For instance, researchers have constructed a DNAm-based biomarker for BMI (DM-BMI) model to investigate the relationship between obesity and BC. They found a positive correlation between DM-BMI and immunotherapy checkpoint inhibitor response markers, indicating that obese cancer patients may be more sensitive to immune checkpoint blockade therapy than normal-weight patients.²⁸⁴ In obese patients, increased interferon-gamma and leptin can upregulate the expression of PD-1/PD-L1, and anti-CTLA-4 therapy has also been demonstrated to be more effective.²⁸⁵ In addition, the expression of Nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome increases in AT of obese patients.²⁸⁶ The NLRP3 inflammasome can produce inflammatory factors, including IL-1β and IL-18, while promoting the upregulation of PD-1/PD-L1 expression, resulting in an immune-suppressive microenvironment.²⁸⁶ Current research indicates that miR-223-3p can downregulate the NLRP3 inflammasome, thereby mitigating tumor progression and offering a novel approach to overcoming immunotherapy resistance.²⁸⁷ Variances in obese cancer patients’ responsiveness to immunotherapy may be associated with obesity-induced epigenetic modifications. Subsequent research may unveil novel therapeutic targets for cancer. Presently, the confluence of immunotherapy and chemotherapy with epigenetic drug therapy has the potential to augment the efficacy of singular treatments while mitigating adverse drug effects, thus emerging as promising anticancer strategies.