2.1 Periodontitis

Periodontitis is a chronic bacterial infection disease characterized by primary gingival and extended alveolar bone inflammation, accompanied by periodontal tissue damage 31, 32 with connective tissue degradation and even tooth loss.33 Consistent links between fatty acids and periodontitis have been established by evidence derived from animal and human subjects. Investigations in animal models from different groups such as Bendyk et al34 and Azuma et al35, 36 come to conclusions that tissue levels of ω-3 LCPUFAs is inversely associated with periodontic alveolar bone loss,34 and ω-3 LCPUFAs EPA metabolite RvE1 is also established to enhance bone formation and reduce bone resorption in rabbit periodontitis models.37 As for the anti-inflammatory effects, LCMUFAs oleic acid (OA) exhibits anti-inflammatory potentials to decrease alveolar bone loss, while LCSFAs PA shows contrary effects with elevated tumour necrosis factor α (TNF-α) levels in obesity mice models,38 suggesting that the anti-inflammatory potentials of fatty acids in periodontitis might be varied based on specific fatty acids types. In human subjects, increased concentrations of specific SCFAs (lactic acid, propionic acid, butyric acid, isovaleric acid) have been found in the gingival fluid of periodontitis patients, demonstrating a possible association between SCFAs and inflammatory alveolar bone loss.39, 40 Moreover, a preliminary clinical study by El-Sharkawy et al suggests that dietary supplementation of ω-3 LCPUFAs might have therapeutic values against periodontitis.41-45 However, there are also clinical investigations report that benefits of dietary ω-3 LCPUFAs might not be applied to periodontitis prevention and treatment.44, 46

In mechanism, fatty acids might exert effects on periodontitis pathogenesis and intervention via direct and indirect mechanisms. Fatty acids could directly affect periodontitis-associated bone destruction. LCSFAs such as PA could trigger P gingivalis-induced alveolar bone loss directly.31 In benefit, EPA metabolite RvE1 could target BLT1 receptors in osteoclasts to inhibit osteoclast fusion and maturation, and RvE1 can induce the release of osteoprotegerin (OPG) to antagonize the proresorptive role of osteoclast-stimulating receptor activator of nuclear kappa-β ligand (RANKL), and thus facilitates the prevention of alveolar bone loss and enhances periodontal bone regeneration in periodontitis patients.47 The indirect effects of fatty acids in periodontitis are mainly through inflammatory response. Studies have shown that LCSFAs (such as PA) at high levels in plasma may facilitate P gingivalis-induced chemokine production in human gingival fibroblasts and further promote inflammatory response in periodontium.31 PA-induced chemokine secretion in human gingival fibroblasts could be inhibited by LCPUFAs (such as DHA), and such effects presumably involving the suppression of toll-like receptor (TLR) dimerization as well as nuclear factor-kappa B (NF-κB) activation.48 In addition to exert effects on chemokine, fatty acids such as RvE1 could also act on inflammatory cells by enhancing the migration of monocytes and neutrophils and promoting the clearance of apoptotic neutrophils to enhance pro-inflammatory response.31 Last but not the least, ω-3 fatty acids such as DHA and EPA exhibit extensive antibacterial effects against putative periodontal pathogens including F nucleatum and P gingivalis, and SCFA butyrate derived from anaerobic bacterial metabolism could inhibit the differentiation of gingival fibroblasts to promote chronic periodontitis.49 Given that refractory periodontitis significantly decreases the life quality of patients, studies investigating the interaction between fatty acids and periodontitis are required to develop novel intervention strategies.

2.2 Osteoporosis

Osteoporosis, marked by low bone mineral density (BMD) and deteriorated bone tissue microarchitecture, contributes to a high incidence of bone fracture on average up to 50% of women > 50 years.50 Osteoporosis is mainly caused by excessive bone resorption resulting from imbalance between overactive osteoclasts and inactive osteoblasts.51, 52 Hence, inhibiting bone resorption or promoting bone formation are promising strategies for osteoporosis prevention and treatment.53 It has been well acknowledged that osteoporosis is associated with levels of fatty acid in bone microenvironment.54 As reviewed earlier by Salari et al,55 investigations conducted in humans have shown inconsistent correlations between fatty acids and osteoporosis, while studies in animal models have confirmed that supplementation of ω-3 LCPUFAs alleviates osteoporosis by suppressing bone breakdown, promoting calcium absorption from diet, reducing prostaglandin E2 (PGE2) production and increasing skeletal calcium.56 In mechanism, ω-6 LCPUFAs intake results in a high ratio of ω-6 versus ω-3 LCPUFAs, and thus facilitating osteoporosis by promoting low-grade chronic inflammation and regulating MSC lineage commitment.57 ω-3 LCPUFAs inhibit osteoclastogenesis, decrease PGE2 content, and thus increasing BMD to benefit osteoporosis prevention and alleviation.51, 58 In addition, fatty acids such as palmitate could enhance energy generation for osteoblast differentiation, thus accelerating bone formation.52 Moreover, since LCPUFAs are highly prone to reactive oxygen species (ROS)-induced oxidative damage, adoption of antioxidant CoQ as adjuvant could eliminate the disadvantages of LCPUFAs during osteoporosis therapeutics.59, 60

Estrogen deficiency-induced postmenopausal osteoporosis is the most common type of osteoporosis. Along with decrease in estrogen levels, reduction in OPG delays osteoblast maturation and attenuates bone formation61; also, drop in OPG/RANKL ratio enhances osteoclast differentiation and promotes bone resorption and eventually results in bone loss.47 Moreover, endogenous fatty acids could serve as energy sources of skeletal and bone marrow cells to contribute to postmenopausal women bone health,62 while exogenous supply of fatty acids might favour or harm postmenopausal women bone condition. As illustrated by animal models, supplementation of fatty acids such as and SCFAs63 and ω-3 LCPUFAs56 substantially reduces bone loss and restores bone mass and thus ameliorates postmenopausal bone loss in ovariectomized mice, and the protective roles of SCFAs on bone loss were mainly attributed to the suppression of osteoclast differentiation and function.64 According to data derived from human subjects, although earlier investigations indicate that ω-3 LCPUFAs intake plays positive roles in enhancing bone mass and limiting postmenopausal osteoporosis risks,65 effects of PUFAs on bone are shown to be contradictory in general. For example, there is one study demonstrated that PUFA supplementation significantly enhanced lumbar spine and femoral neck BMD in a population of 65 postmenopausal women; however, another trial reported no significant therapeutic effects in 42 postmenopausal women receiving similar PUFA supplements.56 Taken together, understanding the functions and mechanisms of fatty acids in osteoporosis might help to develop novel preventive or therapeutic strategies to benefit bone health maintenance in osteoporotic patients.60, 66

2.3 Bone fracture

The high risk of bone fractures may result from osteoporosis with low BMD, or more specifically, deterioration of bone structure and loss of bone mass.67-69 Studies in mice models have suggested that endogenously produced ω-3 LCPUFAs could facilitate fracture healing process, and supplementation of ω-3 LCPUFAs exert positive effects on fracture healing.21 Consistently, investigations in human subjects by Sadeghi et al68 and Harris et al 67 have indicated that increased intake of total PUFAs is positively correlated with higher BMD and reduced bone fracture risk in populations including elder men. However, epidemiological investigation by Virtanen et al demonstrates that low total PUFA, ω-6 PUFA or LA intakes might promote the risk of hip fractures in women.70 Apart from heterogeneity in study design, sample inclusion and data process among different studies, diversity in fatty acid types might be an important factor contributing to the conflicting involvement of fatty acids in bone fractures. Correspondingly, specific mechanisms of fatty acid modulation on bone fractures vary a lot. For example, ω-6 LCPUFAs such as arachidonic acid (AA) could stimulate PGE2 production to regulate bone metabolism and fracture healing, while ω-3 PUFAs increase BMD by increasing calcium resorption and bone collagen synthesis, decreasing urinary calcium excretion, and thus inhibiting bone resorption.67, 68 Overall, fatty acids of different types might exert differential effects on bone fractures pathophysiology, and much more work needs to be done on exploiting them for bone fractures prevention and therapeutics.

2.4 Rheumatoid arthritis

Rheumatoid arthritis, with manifestations of arthralgia, redness and swelling, and limited range of motion,71 is a chronic and autoimmune inflammatory disease affecting 0.5%−1% of the world population.72-74 If left untreated or ineffectively treated, rheumatoid arthritis typically leads to primary joints destruction caused by erosion of cartilage and bone, as well as subsequent systemic complications and even death.72-75 Several studies have investigated the individual and combinational protective effects of LCPUFAs in rheumatoid arthritis. For example, ω-3 LCPUFAs could lower the risk of cardiovascular disease in rheumatoid arthritis patients,23 and combinational utilization of ω-3 LCPUFAs with low-dose vitamin E could substantially reduce the side effects of disease-modifying anti-rheumatic drugs (DMARDs).75 The attenuation effects of ω-3 LCPUFAs on rheumatoid arthritis-induced bone and cartilage destruction are mainly mediated by reduced synthesis of cartilage-degrading enzymes as well as the inflammatory response cytokines. ω-3 LCPUFAs, especially EPA and DHA,23, 76 could alleviate morning stiffness and decrease number of swollen and tender joints in patients with rheumatoid arthritis and thus show anti-inflammatory and restorative effects against rheumatoid arthritis. Importantly, since LCPUFAs AA could drive the synthesis of pro-inflammatory cytokines, restriction of AA enhances ω-3 LCPUFAs-mediated anti-inflammatory responses by decreasing the production of metalloproteinases and pro-inflammatory cytokines as well as the migration of leucocytes in vivo, and thus strengthens the action of ω-3 LCPUFAs in combating rheumatoid arthritis.75, 76 Another kind of LCPUFAs, ω-6 LCPUFAs are eventually metabolized into AA and inflammatory eicosanoids and function as pro-inflammatory agents,75, 77 ω-3 LCPUFAs could reduce the synthesis of ω-6 LCPUFAs by competing with the rate-limiting delta-6 desaturation enzyme and thus exert a therapeutic effect on rheumatoid arthritis.75, 78, 79 Moreover, SCFAs also play crucial roles in bone metabolism and immune responses in pathological bone loss and thus regulate systemic bone mass and protect from rheumatoid arthritis.64 Investigations are needed to further elucidate mechanisms underlying the pharmacological roles and therapeutic potentials of multiple types of fatty acids in arthritis such as temporomandibular joint arthritis.66

2.5 Tumour-associated bone destruction

Multiple myeloma is a destructive cancer that mainly occurs in bone marrow.80 Studies have shown that fatty acids of different types play either pro-death or pro-survival roles in multiple myeloma. For example, PA could activate apoptosis in multiple myeloma cells and thereby serves as a potentially direct anti-myeloma strategy.81 EPA and DHA could also initiate apoptosis and promote drug sensitivity in multiple myeloma cells, with a mechanism involving NF-κB inhibition concomitant with activation of mitochondrial defects leading to caspase-3 activation and apoptosis.82 In addition, EPA and DHA modulate p53/miR-34a/Bcl-2 axis to enhance dexamethasone (Dex)-sensitivity in multiple myeloma cells where they trigger p53 expression and subsequent increase of miR-34a levels in U266 cells, and finally activate Bcl-2 to induce apoptosis of multiple myeloma cells.83-85 By contrast, SFAs and ω-6 LCPUFAs represent energy sources for multiple myeloma cells, and ratio of ω-3/ω-6 fatty acid intake is critical for the maintenance of multiple myeloma cell survival.86, 87

Bone metastasis is a pernicious complication88 occurring in virtually 60% of patients with osteolytic breast or osteogenic prostate cancers and at a smaller rate in patients with other cancer types.89, 90 Patients with bone metastasis suffer from severe pain, bone fracture and osteolytic lesions, which symptoms are primarily attributed to aberrant bone resorption mediated by osteoclasts.91, 92 In osteolytic metastasis mice model originating from MDA-MB-231 human breast cancer, researchers found that supplementation with DHA and EPA-enriched fish oil prevented breast cancer metastasis-induced bone osteolysis,93 suggesting potential therapeutic effects of fatty acids for osteolytic bone metastasis. In mechanism, both DHA and EPA reduce the mRNA and protein levels of CD44 in breast cancer cells to inhibit cancer invasion; moreover, compared to EPA, DHA has profound anti-inflammatory effects via inhibiting TNF-α secretion and NF-κB activation in macrophages and thus exhibits stronger suppression of osteoclast activity to attenuate the related osteolysis.94 However, in osteogenic metastasis derived from prostate cancer, fatty acids such as AA could facilitate metastatic cancer cell implantation and propagation via preparation of bone microenvironment “soil” for cancer cells by activating bone marrow adipocyte formation,95 demonstrating promotional roles of fatty acids in favour of osteogenic bone metastasis. This might be explained by the fact that fatty acids synthesized by bone marrow adipocytes could serve as energy source for certain types of tumour cells engaged in metastasis. During bone metastasis of prostate cancer, free fatty acid influx into cells induces the expression of lipid transport mediator fatty acid-binding protein (FABP4), and expression of FABP4 between tumour cells and adipocytes could mediate adipocyte-induced metabolic switch in prostate microenvironment and thus promotes osteogenic prostate cancer metastasis.96 Such roles of fatty acids in facilitating bone metastasis have also been confirmed in melanoma cancer,89 where bone marrow adipocytes play a pivotal role in bone metastasis by releasing free fatty acids to meet the energy demands of metastatic cancer cells for survival and growth. Therefore, fatty acids of different types behave significantly differently in cancer bone metastasis, and osteolytic or osteogenic or mixed bone lesion conditions derived from specific cancer types should be definitely taken into account when employing fatty acids for cancer bone metastasis therapeutics.

2.6 Other bone disorders

Fatty acids are also involved in non-typical skeletal diseases such as osteomyelitis, a bone inflammatory process initiated by infection of pyogenic organisms 97 that predominantly occurs in long bones of children, and in hips, feet, jaws and spine of adults.98-100 This disease is characterized by severe damage to bone tissue and bone marrow, and probably accompanied by high morbidity and mortality.100 Accumulating evidence has shown that ω-3 LCPUFAs could effectively combat microbial pathogenesis in osteomyelitis.101-103 Furthermore, combination of vancomycin and ω-3 LCPUFAs has been suggested to be a reliable therapeutic strategy against S aureus-induced osteomyelitis, with a mechanism involving inflammation alleviation by reducing TNF-α and interleukin 6 (IL-6) levels as well as antioxidant activity by decreasing SOD activity.97

Taken together, according to currently available pre-clinical experiments (Table 2) and clinical studies (Table 3), various factors contribute to implications of different fatty acids types in multiple bone disorders. With most associations between fatty acids and bone disorders remain obscure (Figure 2), much more work needs to be done by collaboration of biological and clinical researchers to maximize the therapeutic potentials and minimize the side effects of fatty acids against bone diseases.

3.1 Receptors involved in fatty acids-modulated bone metabolism

Cellular membrane-bound and nuclear receptors, such as G protein-coupled receptors (GPRs), peroxisome proliferator-activated receptors (PPARs), TLRs and receptors for metabolites such as chemokine-like receptor (ChemR), play essential roles in mediating the effects of fatty acids on bone metabolism.

GPRs are a superfamily of more than 1000 distinct membrane receptors; several GRPs among these have been reported to be modulated by fatty acids. GPR18, GPR41, GPR43 and GPR109A are receptors for SCFAs (C2-C5) found in both osteoclasts and osteoblasts, where GPR41 could regulate leptin production, while GPR43 is the main receptor in mediating effects of SCFAs on osteoclasts.30, 107, 108 GPR40, which is expressed on osteoclasts and could be activated by medium/long-chain fatty acids with a chain length of C8-C22,109-111 positively affects bone metabolism by downregulating osteoclastogenesis, combating bone loss and protecting cartilage.112-114 GPR84, whose expression in macrophages and adipocytes could be enhanced under inflammatory conditions, is a receptor for MCFAs (C9-C14).115-117 GPR120, which is expressed on osteoblasts and osteoclasts and could be stimulated by long-chain saturated (C14-C18) and long-chain unsaturated fatty acids (C16-C22),109-111 has been shown to mediate the anti-inflammatory effects of DHA in macrophages.118 And GRP120 could enhance ω-3 LCPUFAs-induced osteoblastic bone formation by inducing β-catenin activation and reduce osteoclastic bone resorption by suppressing NF-κB signalling,14 and GPR120 could also modulate the bi-potential differentiation of BMMSC in a dose-dependent manner.119 In addition to the acknowledged roles of GPR40 and GPR120 in preventing bone disorders such as osteoporosis and osteoarthritis,120 GPR2 family member parathyroid hormone type 1 receptor (PTH1R) also plays a role in bone metabolism. PTH1R could mediate ω-3 LCPUFAs-induced activation of extracellular signal-regulated kinases (ERK) to enhance osteoblasts proliferation and differentiation121, 122; moreover, EPA and DHA could act as agonists of PTH1R to attenuate osteoblast apoptosis and promote bone formation.123

PPARs, with known ligands including LCPUFAs and metabolites such as PGE2, are nuclear receptors that regulate lipid metabolism by acting as transcription factors in BMMSCs, osteoblasts and osteoclasts.124-128 When BMMSCs are exposed to a mixture of palmitic, oleic and linoleic acids, upregulation of PPARs and reduction of Runx2 facilitate differentiation of towards adipocyte-like cells.129 Influences of PPARs on osteoblasts and osteoclasts depend on specific receptor isoform. Specifically, PPARα/β promotes bone resorption,130 whereas PPARγ is known inhibitors of osteoclastogenesis.131 Roles of PPARγ in osteoblasts are still disputed; it has been shown that conditional deletion of PPARγ in osteoblasts enhances bone mass and increased bone formation by activating mTOR signalling,132 while studies from other groups reported conflicting results.133-135 Moreover, recent findings have indicated that treatment of multiple myeloma cells with PPARs resulted in apoptotic effects,136 suggesting PPARs might serve as promising therapeutic targets for bone diseases.

TLRs mainly mediate the inflammatory action of fatty acids in bone cells. PA particularly activates TLR2 and induces IL-1β expression and secretion to promote inflammatory response.137-139 Binding of SFAs to TLR4 on osteoclasts induces chronic inflammation 140-142 by enhancing the expression of macrophage inflammatory protein-1a, which leads to hyperactivation of NF-κB and subsequent enhancement of osteoclastic activities 143 as well as further decrease in bone size, BMC and BMD.144 Moreover, studies have shown that DHA treatment could block the pro-inflammatory effects of lauric acid-induced TLR2/4 activation in Raw264.7 cells,145 suggesting TLRs might be involved in the crosstalk among multiple downstream signalling pathways of different fatty acids types.

ChemR23 can act as chemerin receptor 146, 147 as well as RvE1 receptor in bone tissue cells such as monocytes.148 Binding of RvE1with ChemR23 could prevent inflammation by inhibiting NF-κB activation,149 enhancing bone formation150 and reducing bone loss via RANKL/OPG ratio modulation,151, 152 while the detailed mechanisms involved in the downstream of Chem23 have yet to be fully elucidated.

3.2 RANK/RANKL/OPG signalling in fatty acids-modulated bone metabolism

To our knowledge, various signalling pathways including RANKL,153 NF-κB,154 mitogen-activated protein kinase (MAPK),155 Wnt,156 Notch,157 Hedgehog,158 transforming growth factor-β (TGF-β),155 mTOR 159 and bone morphogenetic protein (BMP)155 are involved in bone metabolism. Among these, RANK/RANKL/OPG signalling is most frequently implicated in bone remodelling via modulation by a wide variety of fatty acids160, 161 (Figure 3). Upon activation, RANK/RANKL/OPG signalling substantially inhibits osteoclastogenesis but enhances osteogenesis via downstream signalling cascades such as MAPK, NF-κB and phosphatidylinositol 3-kinase (PI3K)/mTOR.160 Specifically, MAPK signalling162-165 could be activated by ω-7 LCMUFAs,166 PA and MCFAs167 and activation of MAPK signalling normally leads to enhanced proliferation of both osteoblasts and chondrocytes.155, 163, 165 By contrast, ALA,168 ω-7 LCMUFAs166 and MCFAs167 could inhibit NF-κB cascade, and repression of NF-κB cascade attenuates osteoclastogenesis by enhancing both cell death and differentiation.154, 165, 169, 170 Moreover, PI3K/mTOR pathway could be downregulated by EPA or LXA₄ but upregulated by PA159 and thus involved in BMMSC differentiation, osteoblast function and osteocyte formation during bone metabolism.171-177

4.1 Fatty acids and osteoblasts

Osteoblasts are mononuclear cells predominantly involved in bone formation 4, 5. A growing body of evidence supports the promotional or inhibitory action of fatty acids on osteoblasts. In general, fatty acids such as PA suppress osteoblast function, whereas EPA, DHA and RvE1 predominantly promote osteoblastic function. Exploring the modulation effects of fatty acids on osteoblasts might provide new insights into therapeutic intervention targeting skeletal disorders associated with dysregulated bone formation.

4.2 Fatty acids and osteoclasts

Osteoclasts are multinucleated giant cells with bone resorptive activity. Two essential factors secreted by osteoblasts, macrophage colony-stimulating factor (M-CSF) and RANKL, are responsible for osteoclast precursors proliferation and osteoclastogenesis. Importantly, RANKL could prevent apoptosis of osteoclasts3, 4 and induce expression of osteoclast-specific markers and transcription factors such as nuclear factor of activated T cells c1(NFATc1).4, 6 As bone-resorbing cells,192-195 osteoclasts highly express bone resorption-associated proteins including osteoclast-specific markers cathepsin K (CTSK), tartrate resistant acid phosphatase (TRAP) and matrix metalloproteinase 9 (MMP-9).4, 196 Specifically, CTSK breaks down organic components in bone,197, 198 TRAP is implicated in cell adhesion upon activation by CTSK,199, 200 and high levels of MMP-9 commonly occur in resorption lacunae.201 Multiple fatty acids have been found to promote or suppress osteoclast activity, in most cases via regulation of RANKL signalling. Effects of fatty acids on osteoclast functions demonstrate their potential applications as therapeutic reagents against resorption-associated bone disorders such as osteoporosis and rheumatoid arthritis.

4.3 Fatty acids and BMMSCs

BMMSCs are multipotent cells characterized by surface markers of CD105, CD73, CD90, CD44, CD29 and CD1469 with differential potentials into osteoblasts, chondroblasts and bone marrow adipocytes.7 BMMSCs are critical in maintaining the dynamic homeostasis of bone tissue, and deficiencies of BMMSCs proliferation are correlated with reduced bone mass.220, 221 Various signalling pathways including Wnt, Notch, Hedgehog, TGF-β and BMP are involved in BMMSCs osteogenesis. Notably, Runx2 plays the most pivotal role in this process by promoting expression of osteogenesis-related genes, regulating cell cycle progression and improving bone microenvironments.8

4.4 Fatty acids and osteocytes

Osteocytes are osteoblast-derived cells located in lacunae surrounded by mineralized bone matrix, with the ability to support bone structure and receive machine sensation. Importantly, osteocytes can serve as endocrine cells to synthesize and express important regulatory molecules including RANKL, Dickkopf-1 (DKK1) and sclerostin (SOST)233-235 and thus participating in bone resorption and formation regulation by coupling osteoclast and osteoblast activities.6 Studies have shown that fatty acids such as PA and PGE2 have noteworthy influences on osteocyte metabolism, which might provide novel therapeutic strategies for bone diseases like osteoporosis.

4.5 Fatty acids and chondrocytes

Chondrocytes is the main cartilage cell type existing in cartilaginous interstitium and cartilage lacuna, and they can produce cartilage extra cellular matrix that composed mainly of proteoglycans and collagen.243 Fatty acids are integrated into chondrocytes mainly in the form of phosphatidylcholine and triacylglycerols and then mediate downstream signalling pathways via receptors expressed on chondrocytes membrane such as GPR40, GPR120, CD36 and TLR4, as well as a few LRP and PPAR family members.12, 244 As an energy source for chondrocytes, fatty acids participate in chondrocytes energy metabolism245 and further alleviate or enhance chondrocytes damage and cartilage degeneration via multiple mechanisms. Understanding the regulation effects of fatty acids in chondrocytes might help to explore their potential therapeutic values for bone disorders associated with chondrocytes inflammation and cartilage degeneration.

4.5.2 Fatty acids as negative regulators of chondrocytes

SFA and its metabolites

Several studies have shown that animals fed with high-SFAs diet exhibit accelerated cartilage degeneration,260 and long-chain SFAs are considered as important negative regulators of chondrocyte metabolism. Studies have shown that BMMSCs and adipose stem cells-derived chondrocytes which generate long-chain SFAs have decreased cartilaginous matrix production,261 and SFAs with different chain lengths might exert relative effects in chondrocytes. It has been found that diet rich in longer chain SFAs such as PA and SA promotes more expression of collagenase-10 and MMP-13 and increases much more chondrocyte apoptosis than diet rich in shorter chain SFAs.260

PA and SA have been reported to participate in inflammatory reactions by augmenting pro-inflammatory markers such as IL-6 in human chondrocytes.262 In primary mouse chondrocytes, SA could promote lactate dehydrogenase-dependent production of lactate to stabilize HIF1α protein and facilitate pro-inflammatory cytokines expression263 SA-stimulated NF-κβ p65 activation and pro-inflammatory cytokines expression in chondrocytes could be attenuated by miRNA-26a; conversely, NF-κB p65 could also inhibit miRNA-26a production by directly targeting the promoter region of miRNA-26a.264 In addition, PA and SA treatment could enhance autophagy activation in chondrocytes, which is strongly associated with increased activation of NF-κB signalling pathway,265 while opposite effects have been observed upon LA stimulation.

Palmitate has been reported to synergize with IL-1β to induce caspase activation and chondrocyte apoptosis, as well as increase expression of cyclooxygenase 2 and IL-6 in chondrocytes via TLR-4 signalling, which are all involved in the pathological processes of cartilage destruction.266, 267 Lipotoxicity of palmitate could also be mediated by endoplasmic reticulum (ER) stress and further suppresses IGF-1-mediated signalling and succedent proteoglycans and collagen type II synthesis in chondrocytes.268 And utilization of either JNK inhibitor or small molecule chemical chaperone could weaken the effect of palmitate to facilitate cartilage matrix synthesis and chondrocytes survival.269, 270 Importantly, palmitate-induced ER stress could activate unfolded protein response signalling and subsequently promote apoptosis of meniscus cells to affect the development of obesity-related osteoarthritis.271 Moreover, in human chondrocytes, palmitate could induce expression of pro-apoptotic molecules such as cleaved caspase-3 (CC3) and negative cell survival regulators such as tribbles related protein 3 (TRB3) and nuclear protein 1 (Nupr1) and thus induces apoptosis of chondrocytes.272 Taken together, palmitate has potent therapeutic implications for inflammatory bone diseases such as osteoarthritis.

ω-6 PUFAs and their metabolites

A growing body of evidence has shown that a higher ratio of ω-6-to-ω-3 PUFAs might exert negative influences on cartilage.273 As for specific mechanisms, ω-6 PUFAs such as ALA and AA aggravate cartilage damage by serving as precursors for pro-inflammatory prostanoids, while ω-3 PUFAs such as EPA and DHA protect cartilage by being metabolized to anti-inflammatory mediators such as protectins and resolvins.273, 274 Moreover, AA-derived PGE2 could serve as important inflammatory mediator to regulate inflammatory reactions of chondrocytes. Studies have shown that PGE2 could suppress differentiation of chondrocytes by activating downstream receptors protein kinase A (PKA) and protein kinase C (PKC), which might be responsible for activation of transcription factors associated with collagen X production.275

Taken together, fatty acids exert multiple effects on specific bone cell types and thereby associated bone diseases (Table 4), which might be mediated via distinct mechanisms at cellular and molecular levels (Figure 4). Understanding the mechanistic implications of fatty acids in bone cells will greatly benefit their further utilization in related bone disorders.

Table 4. Modulatory roles and therapeutic potentials of fatty acids for bone diseases

Disease	Pathologic mechanism	Fatty acid	Detrimental mechanism	Potential drug formula	Therapeutic mechanism	Reference
Periodontitis	Specific bacterial colonization Increased function of osteoclasts Increased dysfunction of osteoblasts	PA	Induces pro-inflammatory response	ω-3 LCPUFAs	Inhibit putative periodontal pathogens Inhibit PA-induced chemokine secretion	31, 33, 49
		Butyrate	Inhibit differentiation of HGFs	Resolvin D1	Inhibit putative periodontal pathogens Inhibit PA-induced chemokine secretion
		SFAs	Induce oxidative stress Facilitate inflammatory processes	RvE1	Reduce inflammation Inhibit osteoclast activities
Osteoporosis	Increased function of osteoclasts Increased dysfunction of osteoblasts	ω-6 LCPUFAs	Induce chronic inflammation Induce MSC chronic deregulation	ω-3 LCPUFAs	Inhibit osteoclastogenesis Reduce PGE2	51, 52, 58
				PA	Provide energy generation for differentiation of osteoblasts
Osteoporosis	Reduced OPG Facilitated osteoclast differentiation	—	—	ω-3 LCPUFAs	Inhibit osteoclastogenesis Reduce PGE2	47, 61, 64, 65
		—	—	SCFAs	Inhibit osteoclast differentiation Provide energy generation for differentiation of osteoblasts
Bone fracture	Deterioration of bone structure Loss of bone mineral	ω-6 LCPUFAs	Induce PGE2 production	ω-3 LCPUFAs	Increase calcium resorption Increase synthesis of bone collagen Inhibit urinary calcium excretion	67, 68
Rheumatoid arthritis	Autoimmune inflammatory disease of unknown aetiology	ω-6 LCPUFAs	Induce production of pro-inflammatory cytokines	ω-3 LCPUFAs	Reduce inflammation Reduce cartilage-degrading enzymes	75, 76
Osteocarcinoma	Derives from primary bone sarcomas or prostate cancer, breast cancer etc	AA	Supports implantation and propagation of metastatic cells	DHA	Reduce CD44 expression in metastatic cells Inhibit osteoclast formation	88-92, 94-96
				EPA	Reduce CD44 expression in metastatic cells
Osteomyelitis	Bone infection of pyogenic organisms	—	—	ω-3 LCPUFAs	Reduce levels of TNF-α and IL-6 Reduce SOD activity	97, 101-103, 298
Multiple myeloma	Cancer growing in bone marrow	SFAs	—	PA	Activate multiple myeloma cell apoptosis	94-101
		ω-6 LCPUFAs	—	ω-3 LCPUFAs	Promote drug sensitivity of myeloma cell apoptosis Activate multiple myeloma cell apoptosis Inhibit function of ω-6 LCPUFAs