Free Fatty acid receptor 4 (FFAR4), also termedG-protein coupled receptor 120 (GPR120), is aprotein that in humans is encoded (i.e., its formation is directed) by theFFAR4gene.[5] This gene is located on the long (i.e. "q") arm of chromosome 10 at position 23.33 (position notated as 10q23.33).G protein-coupled receptors (also termed GPRs or GPCRs) reside on their parent cells' surfacemembranes, bind any one of the specific set ofligands that they recognize, and thereby are activated to trigger certain responses in their parent cells.[6] FFAR4 is arhodopsin-like GPR in the broad family of GPRs[7] which in humans are encoded by more than 800 different genes.[8] It is also a member of a small family of structurally and functionally related GPRs that include at least three otherfree fatty acid receptors (FFARs)viz.,FFAR1 (also termed GPR40),FFAR2 (also termed GPR43), andFFAR3 (also termed GPR41). These four FFARs bind and thereby are activated by certainfatty acids.[9]
FFAR4 protein is expressed in a wide range of cell types. Studies conducted primarily on human and rodentcultured cells and in animals (mostly rodents) suggest that FFAR4 acts in these cells to regulate many normal bodily functions such as food preferences, food consumption, food tastes, body weight,blood sugar (i.e., glucose) levels,inflammation,atherosclerosis, andbone remodeling. Studies also suggest that the stimulation or suppression of FFAR4 alters the development and progression of several types of cancers.[10] In consequence, agents that activate or inhibit FFAR4 may be useful for treating excessive fatty food consumption, obesity,type 2 diabetes, pathological inflammatory reactions, atherosclerosis, atherosclerosis-inducedcardiovascular disease, repair of damaged bones,[11]osteoporosis.[12][13] and some cancers.[10] These findings have made FFAR4 a potentially attractive therapeuticbiological target for treating these disorders[11] and therefore lead to the development of drugs directed at regulating FFAR4's activities.[14][15]
Certainfatty acids, including in particular theomega-3 fatty acids,docosahexaenoic andeicosapentaenoic acids,[16] have been taken indiets andsupplements to prevent or treat thediseases and tissueinjuries that recent studies suggest are associated with abnormalities in FFAR4's functions. It is now known that these fatty acids activate FFAR4. While dietary and supplemental omega-3 fatty acids have had little or only marginal therapeutic effects on these disorders (seehealth effects of omega-3 fatty acid supplementation), many drugs have been found that are more potent and selective in activating FFAR4 than the omega-3 fatty acids[16][14] and one drug is a potent inhibitor of FFAR4.[15] This raised a possibility that the drugs may be more effective in treating these disorders[11] and prompted initial studies testing the effectiveness of them in disorders targeted by the omega-3 fatty acids.[17] These studies, which are mostlypreclinical studies oncultured cells oranimal models of disease with only a few preliminaryclinical studies, are reviewed here.
The genes for FFAR1,[18] FFAR2, and FFAR3[19] are located close to each other on the short (i.e., "p") arm ofchromosome 19 at position 13.12 (location notated as 19p13.12); theFFAR4 gene is located on the "q" (i.e., long) arm ofchromosome 10 at position 23.33 (location notated as 10q23.33).[5] Humans express a long FFAR4protein isoform consisting of 377amino acids and a shortsplice variant protein isoform consisting of 361 amino acids. However, rodents, non-human primates, and other studied animals express only the short protein. The twoisoforms operate through different cell-stimulating pathways to elicit different responses.[15] Furthermore, humans express the long FFAR4 protein only in theircolon and colon cancer tissues. The consequences of these differences for the studies reported here have not been determined.[10]
FFARs are activated by certainstraight-chain fatty acids.[9] FFAR2 and FFAR3 are activated byshort-chain fatty acids, i.e., fatty acid chains consisting of 2 to 5 carbon atoms, mainlyacetic,butyric, andpropionic acids.[20] FFAR1 and FFAR4 are activated by1)medium-chain fatty acids i.e., fatty acids consisting of 6-12 carbon atoms such ascapric andlauric acids;2) long-chainunsaturated fatty acids consisting of 13 to 21 carbon atoms such asmyristic andsteric acids;3)monounsaturated fatty acids such asoleic andpalmitoleic acids; and4)polyunsaturated fatty acids such as theomega-3 fatty acidsalpha-linolenic,eicosatrienoic,eicosapentaenoic, anddocosahexaenoic acids oromega-6 fatty acids such aslinoleic,gamma-linolenic,dihomo-gamma-linolenic,arachidonic, anddocosatetraenoic acids.[21] Docosahexaenoic and eicosapentaenoic acid are commonly regarded as the main dietary fatty acids that activate FFAR4.[16] Since all of the FFAR1- and FFAR4-activating fatty acids have similar potencies in activating FFAR4 and FFAR1 and have FAR-independent means of influencing cells, it can be difficult to determine if their actions involve FFAR4, FFAR1, both FFARs, or FFAR-independent mechanisms.[14]
The drugs which stimulate (i.e., areagonists of) FFAR4 include: GW9508 (the first discovered and most studied FFAR agonist is about 60-fold more potent in activating FFAR1 than FFAR4; it is often used to implicate FFAR4 functions in cells that naturally or after manipulation express no or very low FFAR1 levels);[9] TUG-891 (almost 300-fold more potent in activating FFAR4 than FFAR1 in human cells but only modestly more potent on FFAR4 than FFAR1 in mouse cells);[15][12] TUG-1197 (activates FFAR4 but not FFAR1);[22] metabolex 36 (about 100-fold more potent in activating FFAR4 than FFAR1); GSK137647A (about 50-fold more potent in activating FFAR4 than FFAR1); compound A (Merck & Co.) and compound B (CymaBay Therapeutics) (both potently activate FFAR4 with negligible effects on FFAR1);[14] and GPR120 III (2,000-fold more active on FFAR4 than FFAR1).[23] AH-7614 acts as a negativeallosteric modulator to inhibit FFAR4; it is >100-fold more potent in inhibiting FFAR4 than FFAR1[15][12][24] and the only currently available FFAR4antagonist that inhibits FFAR4.[15] Most of the studies reported to date have examined the effects of two FFAR4 agonists, GW9508 and TUG-891, that have been available far longer than the other listed drugs.
FFAR4 is expressed in a wide variety of tissues and cell types but its highest levels of expression are in certain intestinal cells (i.e.,enteroendocrine K and I cells),[25]taste bud cells,fat cells,respiratory epithelium cells in the lung[15] (i.e.,club cells also termed clara cells[25]), andmacrophages.[5] It is less strongly expressed in other cell types including: various immune cells besides macrophages,[26] cells in brain, heart, and liver tissues,[27]skeletal muscle cells,[15] blood vesselendothelial cells,[11]enteroendocrine L cells of thegastrointestinal tract,delta cells in theislets of the pancreas, cells involved inbone development and remodeling, some cells in thearcuate nucleus andnucleus accumbens of thehypothalamus,[21] and in some types of cancer cells.[10] However, in comparing animal to human studies the cells and tissues that express FFAR4 can differ and many of these studies have measured FFAR4messenger RNA (mRNA) but not the product directed to be made by this mRNA, FFRA4 protein. The significance of these issues requires study.[11]
The two forms offat cells, i.e.,white andbrown fat cells, develop from precursorstem cells. Brown fat cells promotethermogenesis, i.e., the generation of body heat.[28] Studies have reported that:1) FFAR4 levels rose in the fat tissues of mice exposed to cold;[29]2) TUG‐891 and GW9508 stimulated3T3-L1 mouse stem-like cells to mature into fat cells;3) mice lacking a functionalffar4 gene (i.e.,ffar4knockout mice;ffar4 is the term for the mouse equivalent to the humanFFAR4 gene) had fewer brown fat cells in their subcutaneous adipose (i.e. fat) tissues in response to cold exposure,[28] werecold intolerant, and had poor survival rates in cold temperatures;[30]4) GW9508 stimulated increases in the brown fat tissue of normal mice; however, inffar4 gene knockout mice it simulatedhistology, i.e., microscopic, changes in fat tissue suggesting that thermogenesis was impaired;[28][31]5) TUG-891 stimulated cultured mouse fat cells to oxidize fatty acids (this oxidation underlies the development of body heat in thenon-shivering form of thermogenesis);[28][32] and6) FFAR1 has not yet been reported to be expressed in the fat tissue of mice or humans.[28] These studies suggest that FFAR4 contributes to the proliferation of brown fat cells and thermogenesis in mice. Studies are needed to determine if FFAR4 has a similar role in humans.[28][33]
Two rodent studies suggested that FFAR4 functions to limit excessive weight gains: FFAR4 deficient mice developed obesity[28] and mice treated with the FFAR4 agonist, TUG-891, lost fat tissue.[28][32] FFAR4 might play a similar obesity-suppressing role in humans. One study found that FFAR4 mRNA and protein levels were lower in the visceral fat tissues (i.e., fat around internal organs) of obese than lean individuals.[34] However, another study found that the expression of FFAR4 mRNA was higher in thesubcutaneous andomentalfat tissues of obese than lean individuals.[28] Similarly, one study reported that Europeans who carried asingle-nucleotide variant of theFFAR4 gene which encoded a dysfunctional FFAR4 protein (it has theamino acidarginine rather thanhistidine at position 270 and is notated as p.R270H) increased the risk of becoming obese.[35] However, this relation was not found in later studies on Danish[36] and European populations.[37] It is possible that the loss of FFAR4 expression or activity may contribute to but by itself is insufficient to promote obesity.[17] Other studies have implicated activated FFAR1 in having anti-obese effects in cultured cells, animal models, and possibly humans (seeFFAR1 and obesity). For example,Ffar1 gene knockout mice (i.e., mice made to lackFfar1 genes) became obese when fed a low-fat diet[38] while control mice became obese only when fed a high-fat diet.[39]
The following studies suggest that FFAR4 regulates blood glucose levels and that FFAR4 agonists may be useful for treating individuals withtype 2 diabetes.[40]1) The FFAR4 agonist GSK137647 and docosahexaenoic acid stimulated the release of insulin from cultured mouse and ratpancreatic islets (sites of insulin production and storage)[41] and improved post-feedinghyperglycemia in diabetic mice.[21][41]2) The FFAR1 agonist TUG-891 stimulated cultured mouse fat cells to take up glucose[28] and lowered fasting and post-feeding blood glucose levels in diabetic rats;[21] it also stimulated insulin secretion[14] and lowered blood glucose levels in mice.[42]3) FFAR1 agonist compound A (Merck & Co.)[43] and, with greater efficacy, a dual agonist of FFAR1 and FFAR4, DFL23916,[44] improved blood insulin and glucose levels in mice challenged with aglucose tolerance test.4) Fatty acid activators of FFAR4 promoted the release ofglucagon-like peptide-1 andgastric inhibitory peptide (both stimulate insulin secretion) and reduced secretion ofghrelin (which stimulates the drive to eat) in mice.[25]5)Downregulation (i.e., forced reduction in the cellular levels) of FFAR4 impaired insulin's actions by reducing the levels of the glucose transporterGLUT4 andinsulin receptor substrate in3T3-L1 mouse fat cells.[28]6) FFAR4-deficient mice developed glucose intolerance (a potential form ofprediabetes) when fed a high fat diet.[25]7) A diet rich in omega-3 fatty acids improved insulin sensitivity and glucose uptake in muscle and liver tissues in normal but not FFAR4-deficient mice.[28][45]8) FFAR4 levels in pancreas islets are higher in individuals with higher insulin and lowerHbA1c levels (HbA1c levels rise with higher blood glucose levels averaged over the preceding 3 months).[46] And,9) individuals who carried theFFAR4 gene variant, p.R270H, (codes a hypoactive FFAR4) who regularly consumed low-fat diets had an increased incidence of developingtype 2 diabetes; this association did not occur in p.R270H carriers who regularly consumed high-fat diets.[37]
In aphase II clinical trial (NCT02444910https://www.clinicaltrials.gov), nine adults with previously untreatedinsulin-resistant type 2 diabetes were treated orally with increasing doses of KDT501 (anisohumulone derivative that is a relative weak FFAR4 activator andpartial agonist ofPPARγ) for up to 29 days. After treatment, the participants had significantly lowerblood plasmatriglyceride andTNF-α levels and higher levels ofadiponectin, a regulator of blood glucose levels.[14] However, there were no significant changes in these individuals' oral glucose tolerance test results or measurements of insulin sensitivity.[47] Further studies including the usage of more potent and selectively acting FFAR4 agonists are needed to determine their effectiveness in regulating blood glucose levels and treating type 2 diabetes.[46] Two separate studies have reported that the selective FFAR1 agonists MK‐8666 and TAK-875 greatly improved blood glucose levels in type 2 diabetic patients but also appeared to cause unacceptable liver damage (seeFFAR1 and type 2 diabetes). These studies have been regarded as proof that FFAR1 contributes to regulating glucose levels in patients with type 2 diabetes and therefore is a target for treating these patients with FFAR1 agonists that do not have significantadverse effects such ashepatotoxicity.[48][49] Recentpreclinical studies are examining other FFAR1 agonists for their liver and other toxicities.[15]
Human and rodent[33]taste buds and other areas of their tongues contain cells that expresstaste receptors which detect the fivetaste perception elements viz.,saltiness,sourness,bitterness,sweetness, andumami. One well-studied site that has these receptor-bearing cells is in the mouse and human taste buds of their tongues'circumvallate papillae.[50][51] TUG-891 stimulated cultured mouse and human taste bud cells to mobilize severalcell activation pathways. Furthermore:1) application of TUG-891 to the tongues of mice caused alterations in their blood levels ofcholecystokinin (one of its functions is to mediatesatiety) andadipokines (i.e., signaling proteins secreted by fat tissues[51]);[14][52]2) dietary fatty acids that activate FFAR4 altered the taste of and preferences for fats in rats;[37]3) FFAR4-deficient mice were less likely to consume fatty meals;[21]4) the injection of the FFAR4 agonist GPR120 III[23] into the arcuate nucleus and nucleus accumbens brain areas of mice reduced their food intake and suppressed the rewarding effects of high-fat and high-sugar foods;[33] and5) TUG-891 enhanced humans' fatty orosensation (i.e., false sensation of taste obtained by tongue stimulation) when added to FFAR4-activating dietary fats but not when added to fat-free mineral oil. The latter finding suggests that in humans FFAR4 agonists enhance the sensation of fats but by themselves do not directly evoke it.[53] However, one study found that mice with non-functionalFfra4 genes retained their preferences for oily solutions and long chain fatty acids.[54] Follow-up studies are essential to confirm the functional roles of FFAR4 in taste perceptions and preferences.[37]
FFAR4 is expressed by a variety of cell types involved ininflammation such asmacrophages,dendritic cells,eosinophils,[26]neutrophils, andT cells.[16] FFAR4 activators inhibited: human eosinophils from secreting a pro-inflammatorycytokine,interleukin 4;[55] mouse RAW 264.7 andperitoneal macrophages from secreting the pro-inflammatory cytokinestumor necrosis factor-α (i.e., TNF-α) andinterleukin-6; bone marrow-derived mouse dendritic cells from secreting the pro-inflammatory cytokinesmonocyte chemoattractant protein 1, TNF-α, interleukin 6,interleukin-12 subunit alpha, andinterleukin-12 subunit beta;[45][56] and mousehelper andcytotoxic T cells from releasing the pro-inflammatory cytokines interferon gamma,interleukin 17,interleukin-2, and TNF-α.[16] These findings suggest that FFAR4 acts to suppress inflammation, a view supported by the following studies. FFAR4-deficient mice have increased levels of inflammation in their fat tissues.[28] Furthermore, FFAR4 agonist drugs and/or omega-3 fatty acids reduced:1) the chronic inflammation that develops in the fat and liver tissues ofdb/db mice;[57]2)cyclophosphamide-inducedinterstitial cystitis (i.e., urinary bladder inflammation) in rats;3) the liver inflammation which follows transient blockage of its blood supply in mice;4) chronicsleep deprivation-induced inflammation of visceral fat tissues in mice;5) diet-induced inflammation in theislets of thepancreas in mice (this reduction did not occur in mice lacking a functionalFFAR4 gene);6)2,4-dinitrochlorobenzene-induced mousecontact dermatitis[14] (this reduction did not occur inFFAR4 gene-deficient mice[26]);7) dextran sodium sulfate-inducedcolitis in mice;[58] and8) brain inflammation due to the reduction of blood flow to the brains of mice caused by experimentally inducedcerebral infarction.[14]
The short-term (i.e., less than 29 days) phase II clinical trial (NCT02444910https://www.clinicaltrials.gov) found that nine diabetic adults treated with the FFAR4 agonist KDT501 developed higher plasma levels of adiponectin. Biopsied specimens of these individual's subcutaneous fat tissues obtained up to 3 days after the end of KDT501 treatment released greater amounts of adiponectin than biopsies obtained before KDT501 treatment.[59] Adiponectin has various anti-inflammatory actions.[60]
Arterial atherosclerosis is initiated by damage to these blood vessels'endothelial cells, i.e., their single layers of cells which face the blood. This damage opens a passage for circulatinglow-density lipoproteins to enter the vessel and move to its innermost layer, thetunica intima, where they are metabolized tooxidized low-density lipoproteins (i.e., oxLDL). Circulatingmonocytes attach to the damaged endothelium, move to the tunica intima, ingest the oxLDL, and differentiate intoM1 macrophages, i.e., macrophages that promote inflammation. These M1 cells continue to ingest oxLDL and may eventually becomecholesterol-ladenfoam cells that promote the development ofatheromatous plaques, i.e., hardened accumulations of macrophages, lipids, calcium, and fibrousconnective tissue. Over time, the plaques may grow to sizes that narrow or occlude the arteries in which they reside to causeperipheral artery disease,hypertension,coronary artery diseases, and heart damage.[11] The following studies suggest that the suppression of vascular inflammation by FFAR4 agonists reduces the development of atherosclerosis and its associated disorders.[17]1) The FFAR1/FFAR4 activating drug, GW9508, stimulated culturedhuman THP-1 macrophage foam cells and RAW264.7 mouse macrophages to secrete their cholesterol and reduce their levels ofcholesteryl esters.[61]2) In cell cultures, GW9508 and TUG-891 inhibited THP-1 monocytes from attaching to human aortic endothelial cells.[11]3) Long-term administration of GW9508[62] or TUG-891[63] to APOE−/− mice (these mice develop atherosclerosis due to lack of theapolipoprotein E gene) converted M1 macrophages to inflammation-suppressingM2-macrophages and resulted in less vascular inflammation and smaller atherosclerotic plaques (TUG-891's actions were reversed by the FFAR4 antagonist, AH-7614[63]).4) Following constriction of theiraortas (using an experimental procedure termed transverse aortic constriction which forces the heart to beat against excessively high blood pressures), the hearts of FFAR4-deficient male but not female mice hadpathologically thickened ventricle walls which contracted dysfunctionally compared to mice with normal FFAR4 levels.5) Cardiac tissue FFAR4 levels were lower in humans withcongestive heart failure.[11] And,6) compared to women, men carrying the defective p.R270HFFAR4 gene had several cardiac abnormalities including largerleft ventricle masses, larger left ventricle diameters as measured at the end ofdiastole, increased maximum left cardiacatrium sizes, and a trend toward somewhat lower minimum cardiacejection fractions.[64] Many but not all clinical trials have found that dietary regimens enriched with eicosapentaenoic and docosahexaenoic acids lower the risk of coronary artery heart disease,congestive heart failure, and sudden death due to cardiac disease.[65] Further studies are needed to determine if the therapeutic effects of omega-3 fatty acids in mice and humans involve FFAR4 activation and if potent, selectively acting FFAR4 drugs are more effective than omega-3 fatty acids in preventing and/or treating these and the other cited atherosclerosis-associated disorders.[65]
FFAR4 has been detected in various types of cultured human cancer cells and found to promote or inhibit their proliferation,[9]migration, survival, and/or resistance to anti-cancer drugs.[11] The direction of their effects depended on the type of cancer cell and response examined.[10] Studies have reported that:1) GW9508 (which activates FFAR1 but at higher concentrations also activates FFAR4) stimulated migration of human SW480 andHCT116 colon cancer cells (since neithercell line expressed FFRA1, GW9508 appeared to stimulate this migration by activating FFAR4);[10][66]2) GW9508 inhibited the migration and proliferation of human A375 and G361melanoma cells but was less effective onFFAR4knockdown A375 cells (i.e. cells that have been forced to express low levels of FFAR4; this result suggests that GW5098 acted through both FFAR4 and FFAR1 in these two melanoma cell types);3) GW9508 stimulated the migration and invasiveness of human MG-63 boneosteosarcoma cells but this stimulation did not occur inFFAR4 gene knockdown cells and therefore appeared to involve FFAR4 activation;4) the FFAR4 agonist TUG-891 reduced the ability of docosahexaenoic and eicosapentaenoic acids to stimulate the proliferation of humanDU145 andPC-3 prostate cancer cells (this result suggests that activated FFAR1 inhibited these cells proliferation);[10] and5) the effects ofFFAR4 andFFAR1 gene knockdowns and TUG-891 treatment inPANC-1 humanpancreas cancer cells suggested that FFAR4 stimulated and FFAR1 inhibited these cells motility, invasiveness, and formation of colonies in cell culture assays.[10][67] Activated FFAR1 also stimulates or inhibits the malignant behaviors of various cancer includend some of those discussed here (seeFFAR1 and cancer). Finally, one study reported that individuals carrying asingle-nucleotide polymorphism, i.e. SNP,variant allele of theFFAR4 gene (variant described as 9469C>A in whichadenine replacescytosine at position 9469 of the gene'snucleic acid sequence) had increased family histories and personal risks of developing lung cancer.[10][68] Further animal model, clinical, and gene studies are needed to define the roles of FFAR4 and FFAR1 in these and other cancers.[10]
Breast cancer studies on FFAR4 have been more extensive than those on other cancers. Cell culture studies showed that the knockdown of FFAR4 levels in cultured humanMCF-7,MDA-MB-231, andSKBR3 breast cancer cells slowed their proliferation and increased their death byapoptosis and that GW9508 and TUG-891 inhibited the proliferation and migration of MCF-7 and MDA-MB-231 cells.[9][10][69] Animal studies found thatFfar4 gene knockdown MBA-MB-231 cellstransplanted into mice formed more rapidly growing and larger tumors than those formed by normal MBA-MB-231 cell transplants;[9] and that GW9508-treated mice transplanted withFFAR4 gene knockdown MBA-MB-231 cells had more lungmetastases than mice transplanted with normal MBA-MB-231-cells. These findings suggest that FFAR4 and FFAR1 contribute to inhibiting breast tumor growth but FFAR1, not FFAR4, inhibits these cell's metastasis.[9][70] Clinicalobservation studies reported that:1) FFAR4 was expressed in patients' breast cancers but not in the normalepithelium lining their breasts'ducts andlobules;2) the proportions of five fatty acids which activate FFRA4 and FFAR1 (viz., stearic, dihomo-gamma-linolenic, docosatetraenoic, docosapentaenoic, and docosahexaenoic acids) were higher in patients' cancerous than adjacent normal breast tissues;3) patients withER(+) breast cancer, (i.e., breast cancers containing cells that expressestrogen receptors) had higher cancer tissue levels of FFAR4 than patients with estrogen receptor negative, i.e., ER(-), breast cancer;4) among all ER(+) breast cancer patients who were treated withtamoxifen (aselective estrogen receptor modulator commonly used to treat breast cancer), those with high cancer cell FFAR4 levels had a significantly lower 10 year recurrence-free survival rate (percentage of individuals disease-free 10 years after diagnosis) and lower 10 year breast cancer-specific survival rate (percentage of individuals alive 10 years after diagnosis) than those with lower FFAR4 expression levels or ER(-) breast cancer patients;5) individuals with higher cancer tissue FFAR4 levels who had a luminal A, luminal B HER2(–), or luminal B HER(2+) breast cancer subtype (seebreast cancer subtypes) had worse prognoses than individuals with low FFAR4 levels in these respective cancer subtypes (individuals with non-luminal HER2(+) or triple-negative breast cancer subtypes did not show this relationship).[9][71] These clinical findings allow that one or more of the five FFAR4/FFAR1-activating fatty acids in breast cancer tissues contributes to this cancers development and/or progression; that high levels of FFAR4 in ER(+) breast cancers confer resistance to tamoxifen therapy and thereby reduce survival; and that high levels of FFAR4 are associated with poorer survival in certain breast cancer subtypes. Studies are needed to determine if a high FFAR4 level can be a clinically usefulmarker for predicting the severity and prognosis of breast cancers, a contraindication to using tamoxifen to treat breast cancers, and atarget for treating ER(+) breast cancers with, e.g., a FFAR4 inhibitor.[9][71]
Osteoclasts absorb bone tissue in a physiological process needed to maintain, repair, andremodel bones. Osteoclasts develop by a process ofcellular differentiation termedosteoclastogenesis from cells in themononuclear phagocyte system. In a mousebone marrow culture model of bone reabsorption, GW9508 inhibited osteoclast activity by reducing the differentiation of cells to osteoclasts as well as the survival and function of the osteoclasts. Since FFAR4 expression was 100-fold higher than FFAR1 in the osteoclasts and knockdown of FFAR4 in the osteoclasts blocked GW9508's effects,[14] studies suggest that activated FFAR4 functions to block osteoclast-mediatedbone resorption.[14][72] In support of this view,Ffar4 gene knockdown mice developed osteoarthritis more rapidly than control mice in a model of knee osteoarthritis; docosahexaenoic acid inhibited the expression of inflammatory factors in cultured humanchondrocytes; and the levels of FFAR4 protein in the osteoarthritic and/or nearby fat tissue of humans with osteoarthritis were higher than those with non-osteoarthritic bone disease.[16][73] These studies suggest that FFAR4 inhibits bone resorption and may prove to be useful for treating excessive bone resorption, i.e., osteoporosis.[12][13][21]
