GPR43/FFA2: physiopathological relevance and therapeutic prospects

Research interest in free fatty acid-binding receptors has been growing during the past decade, with an aim to better understand the modulation of host physiology in response to nutrition. G-protein-coupled receptor 43 (GPR43), also called free fatty acid receptor 2 (FFA2/FFAR2), binds short-chain fatty acids (SCFAs) produced by the microbial fermentation of carbohydrates and has shown promising therapeutic potential. This review presents current knowledge regarding the pharmacological properties of GPR43 and addresses its functions in selected organs (adipose tissue, intestine and immune cells). Furthermore, the demonstration of GPR43 involvement in several pathological conditions such as obesity, inflammatory disease, and cancer suggests new fields of interest related to this receptor. Finally, GPR43 could be a key player in gut microbes–host crosstalk, although further research is needed to clearly evaluate its role in the management of host health by nutrients or treatments targeting the gut microbiota. Research interest in free fatty acid-binding receptors has been growing during the past decade, with an aim to better understand the modulation of host physiology in response to nutrition. G-protein-coupled receptor 43 (GPR43), also called free fatty acid receptor 2 (FFA2/FFAR2), binds short-chain fatty acids (SCFAs) produced by the microbial fermentation of carbohydrates and has shown promising therapeutic potential. This review presents current knowledge regarding the pharmacological properties of GPR43 and addresses its functions in selected organs (adipose tissue, intestine and immune cells). Furthermore, the demonstration of GPR43 involvement in several pathological conditions such as obesity, inflammatory disease, and cancer suggests new fields of interest related to this receptor. Finally, GPR43 could be a key player in gut microbes–host crosstalk, although further research is needed to clearly evaluate its role in the management of host health by nutrients or treatments targeting the gut microbiota. G-protein-coupled receptors (GPCRs) are seven-transmembrane (7TM) receptors that mediate cellular responses to the majority of hormones and neurotransmitters, and are therefore attractive targets for drug discovery [1Shoichet B.K. Kobilka B.K. Structure-based drug screening for G-protein-coupled receptors.Trends Pharmacol. Sci. 2012; 33: 268-272Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar]. Free fatty acids (FFAs) have long been considered as key signaling molecules in numerous physiological and pathological processes. The recent identification of a family of GPCRs that bind FFAs has highlighted new potential mechanisms of action for FFAs in health and disease [2Stoddart L.A. et al.International Union of Pharmacology. LXXI. Free fatty acid receptors FFA1, -2, and -3: pharmacology and pathophysiological functions.Pharmacol. Rev. 2008; 60: 405-417Crossref PubMed Scopus (275) Google Scholar]. Among these FFAs receptors, GPR43 is present in a large variety of tissues, including adipose tissue, inflammatory cells, and gastrointestinal (GI) tract and is activated by SCFAs [3Brown A.J. et al.The orphan G protein-coupled receptors GPR41 and GPR43 are activated by propionate and other short chain carboxylic acids.J. Biol. Chem. 2003; 278: 11312-11319Crossref PubMed Scopus (1542) Google Scholar, 4Le Poul E. et al.Functional characterization of human receptors for short chain fatty acids and their role in polymorphonuclear cell activation.J. Biol. Chem. 2003; 278: 25481-25489Crossref PubMed Scopus (1076) Google Scholar, 5Nilsson N.E. et al.Identification of a free fatty acid receptor, FFA2R, expressed on leukocytes and activated by short-chain fatty acids.Biochem. Biophys. Res. Commun. 2003; 303: 1047-1052Crossref PubMed Scopus (403) Google Scholar, 6Maslowski K.M. et al.Regulation of inflammatory responses by gut microbiota and chemoattractant receptor GPR43.Nature. 2009; 461: 1282-1286Crossref PubMed Scopus (2083) Google Scholar]. SCFAs are the major anions present in the large intestine of non-ruminant mammals. They are produced by the gut microbiota through the fermentation of undigested carbohydrates and dietary fibers (Box 1) [7Roberfroid M. et al.Prebiotic effects: metabolic and health benefits.Br. J. Nutr. 2010; 104: S1-S63Crossref PubMed Scopus (1440) Google Scholar]. The identification of these endogenous ligands of GPR43 has led the scientific community to propose a new appellation for GPR43, namely FFA2 or FFAR2 [2Stoddart L.A. et al.International Union of Pharmacology. LXXI. Free fatty acid receptors FFA1, -2, and -3: pharmacology and pathophysiological functions.Pharmacol. Rev. 2008; 60: 405-417Crossref PubMed Scopus (275) Google Scholar, 5Nilsson N.E. et al.Identification of a free fatty acid receptor, FFA2R, expressed on leukocytes and activated by short-chain fatty acids.Biochem. Biophys. Res. Commun. 2003; 303: 1047-1052Crossref PubMed Scopus (403) Google Scholar]. SCFAs bind GPR43 in the following rank order of potency: propionate ≥ acetate ≈ butyrate > valerate > formate (Figure 1, panel 1) [3Brown A.J. et al.The orphan G protein-coupled receptors GPR41 and GPR43 are activated by propionate and other short chain carboxylic acids.J. Biol. Chem. 2003; 278: 11312-11319Crossref PubMed Scopus (1542) Google Scholar, 4Le Poul E. et al.Functional characterization of human receptors for short chain fatty acids and their role in polymorphonuclear cell activation.J. Biol. Chem. 2003; 278: 25481-25489Crossref PubMed Scopus (1076) Google Scholar, 5Nilsson N.E. et al.Identification of a free fatty acid receptor, FFA2R, expressed on leukocytes and activated by short-chain fatty acids.Biochem. Biophys. Res. Commun. 2003; 303: 1047-1052Crossref PubMed Scopus (403) Google Scholar]. Importantly, SCFAs also activate another receptor of the same family, GPR41, with propionate and butyrate being the most potent agonists [3Brown A.J. et al.The orphan G protein-coupled receptors GPR41 and GPR43 are activated by propionate and other short chain carboxylic acids.J. Biol. Chem. 2003; 278: 11312-11319Crossref PubMed Scopus (1542) Google Scholar, 4Le Poul E. et al.Functional characterization of human receptors for short chain fatty acids and their role in polymorphonuclear cell activation.J. Biol. Chem. 2003; 278: 25481-25489Crossref PubMed Scopus (1076) Google Scholar]. Both receptors can couple to Gαi/o resulting in inhibition of the adenylate cyclase pathway, but only GPR43 is also able to couple to Gαq, thus leading to activation of the phospholipase C (PLC) pathway and increased intracellular calcium levels [3Brown A.J. et al.The orphan G protein-coupled receptors GPR41 and GPR43 are activated by propionate and other short chain carboxylic acids.J. Biol. Chem. 2003; 278: 11312-11319Crossref PubMed Scopus (1542) Google Scholar, 4Le Poul E. et al.Functional characterization of human receptors for short chain fatty acids and their role in polymorphonuclear cell activation.J. Biol. Chem. 2003; 278: 25481-25489Crossref PubMed Scopus (1076) Google Scholar].Box 1Introducing the gut microbiota and prebioticsThe microbiota consists of 100 trillion microorganisms, which outnumber human cells in the body by at least tenfold. The majority of the microbes reside in the gut, where they exert diverse and crucial functions (e.g., energy extraction from undigested food, bile acid metabolism, regulation of mucosal immunity and gut barrier function, production of metabolic regulators) [7Roberfroid M. et al.Prebiotic effects: metabolic and health benefits.Br. J. Nutr. 2010; 104: S1-S63Crossref PubMed Scopus (1440) Google Scholar, 43Delzenne N.M. Cani P.D. Interaction between obesity and the gut microbiota: relevance in nutrition.Annu. Rev. Nutr. 2011; 31: 15-31Crossref PubMed Scopus (306) Google Scholar]. Gut microbiota composition is influenced by various factors coming from the host and the environment, such as diet, age, genetic background, or immunity [43Delzenne N.M. Cani P.D. Interaction between obesity and the gut microbiota: relevance in nutrition.Annu. Rev. Nutr. 2011; 31: 15-31Crossref PubMed Scopus (306) Google Scholar].The whole gut bacterial gene set, defined as the microbiome, is around 150 times larger than the human genome. This extensive gene set confers to the gut microbes a huge metabolic capacity [56Qin J. et al.A human gut microbial gene catalogue established by metagenomic sequencing.Nature. 2010; 464: 59-65Crossref PubMed Scopus (7109) Google Scholar]. Accordingly, the diversity of the produced metabolites, including SCFAs, could explain why these gut microbes are involved in a symbiotic relationship with the host [43Delzenne N.M. Cani P.D. Interaction between obesity and the gut microbiota: relevance in nutrition.Annu. Rev. Nutr. 2011; 31: 15-31Crossref PubMed Scopus (306) Google Scholar]. SCFAs are rapidly absorbed by the colonic mucosa and contribute towards energy requirements of the host. Butyrate is mostly consumed by the intestinal cells, propionate is cleared by the liver, whereas acetate is mainly metabolized in human muscle, kidney, heart, and brain [7Roberfroid M. et al.Prebiotic effects: metabolic and health benefits.Br. J. Nutr. 2010; 104: S1-S63Crossref PubMed Scopus (1440) Google Scholar].The prebiotic concept is defined as the selective stimulation of growth and/or activity(ies) of one or a limited number of microbial genus(era)/species in the gut microbiota that confer(s) health benefits to the host [7Roberfroid M. et al.Prebiotic effects: metabolic and health benefits.Br. J. Nutr. 2010; 104: S1-S63Crossref PubMed Scopus (1440) Google Scholar]. For instance, inulin-type fructans (ITF) are nondigestible carbohydrates fermented in the colon that exhibit prebiotic properties. ITF administration increases bifidobacteria levels – but also modifies other bacteria – and improves health in different pathophysiological conditions [50Cani P.D. et al.Selective increases of bifidobacteria in gut microflora improve high-fat-diet-induced diabetes in mice through a mechanism associated with endotoxaemia.Diabetologia. 2007; 50: 2374-2383Crossref PubMed Scopus (1323) Google Scholar, 51Cani P.D. et al.Changes in gut microbiota control inflammation in obese mice through a mechanism involving GLP-2-driven improvement of gut permeability.Gut. 2009; 58: 1091-1103Crossref PubMed Scopus (1788) Google Scholar, 57Everard A. et al.Responses of gut microbiota and glucose and lipid metabolism to prebiotics in genetic obese and diet-induced leptin-resistant mice.Diabetes. 2011; 60: 2775-2786Crossref PubMed Scopus (749) Google Scholar, 58Dewulf, E.M. et al. Insight into the prebiotic concept: lessons from an exploratory, double blind intervention study with inulin-type fructans in obese women. Gut, (in press) http://dx.doi.org/10.1136/gutjnl-2012-303304Google Scholar]. Of note, SCFA levels in cecal content and serum are increased in rodents and humans after administration of prebiotics or fermentable carbohydrates owing to bacterial fermentation [22Bindels L.B. et al.Gut microbiota-derived propionate reduces cancer cell proliferation in the liver.Br. J. Cancer. 2012; 107: 1337-1344Crossref PubMed Scopus (183) Google Scholar, 59McOrist A.L. et al.Fecal butyrate levels vary widely among individuals but are usually increased by a diet high in resistant starch.J. Nutr. 2011; 141: 883-889Crossref PubMed Scopus (142) Google Scholar, 60Tarini J. Wolever T.M. The fermentable fibre inulin increases postprandial serum short-chain fatty acids and reduces free-fatty acids and ghrelin in healthy subjects.Appl. Physiol. Nutr. Metab. 2010; 35: 9-16Crossref PubMed Scopus (219) Google Scholar]. The microbiota consists of 100 trillion microorganisms, which outnumber human cells in the body by at least tenfold. The majority of the microbes reside in the gut, where they exert diverse and crucial functions (e.g., energy extraction from undigested food, bile acid metabolism, regulation of mucosal immunity and gut barrier function, production of metabolic regulators) [7Roberfroid M. et al.Prebiotic effects: metabolic and health benefits.Br. J. Nutr. 2010; 104: S1-S63Crossref PubMed Scopus (1440) Google Scholar, 43Delzenne N.M. Cani P.D. Interaction between obesity and the gut microbiota: relevance in nutrition.Annu. Rev. Nutr. 2011; 31: 15-31Crossref PubMed Scopus (306) Google Scholar]. Gut microbiota composition is influenced by various factors coming from the host and the environment, such as diet, age, genetic background, or immunity [43Delzenne N.M. Cani P.D. Interaction between obesity and the gut microbiota: relevance in nutrition.Annu. Rev. Nutr. 2011; 31: 15-31Crossref PubMed Scopus (306) Google Scholar]. The whole gut bacterial gene set, defined as the microbiome, is around 150 times larger than the human genome. This extensive gene set confers to the gut microbes a huge metabolic capacity [56Qin J. et al.A human gut microbial gene catalogue established by metagenomic sequencing.Nature. 2010; 464: 59-65Crossref PubMed Scopus (7109) Google Scholar]. Accordingly, the diversity of the produced metabolites, including SCFAs, could explain why these gut microbes are involved in a symbiotic relationship with the host [43Delzenne N.M. Cani P.D. Interaction between obesity and the gut microbiota: relevance in nutrition.Annu. Rev. Nutr. 2011; 31: 15-31Crossref PubMed Scopus (306) Google Scholar]. SCFAs are rapidly absorbed by the colonic mucosa and contribute towards energy requirements of the host. Butyrate is mostly consumed by the intestinal cells, propionate is cleared by the liver, whereas acetate is mainly metabolized in human muscle, kidney, heart, and brain [7Roberfroid M. et al.Prebiotic effects: metabolic and health benefits.Br. J. Nutr. 2010; 104: S1-S63Crossref PubMed Scopus (1440) Google Scholar]. The prebiotic concept is defined as the selective stimulation of growth and/or activity(ies) of one or a limited number of microbial genus(era)/species in the gut microbiota that confer(s) health benefits to the host [7Roberfroid M. et al.Prebiotic effects: metabolic and health benefits.Br. J. Nutr. 2010; 104: S1-S63Crossref PubMed Scopus (1440) Google Scholar]. For instance, inulin-type fructans (ITF) are nondigestible carbohydrates fermented in the colon that exhibit prebiotic properties. ITF administration increases bifidobacteria levels – but also modifies other bacteria – and improves health in different pathophysiological conditions [50Cani P.D. et al.Selective increases of bifidobacteria in gut microflora improve high-fat-diet-induced diabetes in mice through a mechanism associated with endotoxaemia.Diabetologia. 2007; 50: 2374-2383Crossref PubMed Scopus (1323) Google Scholar, 51Cani P.D. et al.Changes in gut microbiota control inflammation in obese mice through a mechanism involving GLP-2-driven improvement of gut permeability.Gut. 2009; 58: 1091-1103Crossref PubMed Scopus (1788) Google Scholar, 57Everard A. et al.Responses of gut microbiota and glucose and lipid metabolism to prebiotics in genetic obese and diet-induced leptin-resistant mice.Diabetes. 2011; 60: 2775-2786Crossref PubMed Scopus (749) Google Scholar, 58Dewulf, E.M. et al. Insight into the prebiotic concept: lessons from an exploratory, double blind intervention study with inulin-type fructans in obese women. Gut, (in press) http://dx.doi.org/10.1136/gutjnl-2012-303304Google Scholar]. Of note, SCFA levels in cecal content and serum are increased in rodents and humans after administration of prebiotics or fermentable carbohydrates owing to bacterial fermentation [22Bindels L.B. et al.Gut microbiota-derived propionate reduces cancer cell proliferation in the liver.Br. J. Cancer. 2012; 107: 1337-1344Crossref PubMed Scopus (183) Google Scholar, 59McOrist A.L. et al.Fecal butyrate levels vary widely among individuals but are usually increased by a diet high in resistant starch.J. Nutr. 2011; 141: 883-889Crossref PubMed Scopus (142) Google Scholar, 60Tarini J. Wolever T.M. The fermentable fibre inulin increases postprandial serum short-chain fatty acids and reduces free-fatty acids and ghrelin in healthy subjects.Appl. Physiol. Nutr. Metab. 2010; 35: 9-16Crossref PubMed Scopus (219) Google Scholar]. GPR41 and GPR43 bind the same family of ligands (SCFAs), exhibit some overlapping expression, and partially share signaling pathways (Gαi/o). Furthermore, both receptors represent potentially interesting targets for drug discovery. The pathophysiological roles of GPR41 have been largely described in several recent reviews and will not be discussed here [8Ulven T. Short-chain free fatty acid receptors FFA2/GPR43 and FFA3/GPR41 as new potential therapeutic targets.Front. Endocrinol. (Lausanne). 2012; 3: 111PubMed Google Scholar, 9Blad C.C. et al.G protein-coupled receptors for energy metabolites as new therapeutic targets.Nat. Rev. Drug Discov. 2012; 11: 603-619Crossref PubMed Scopus (172) Google Scholar, 10Talukdar S. et al.Targeting GPR120 and other fatty acid-sensing GPCRs ameliorates insulin resistance and inflammatory diseases.Trends Pharmacol. Sci. 2011; 32: 543-550Abstract Full Text Full Text PDF PubMed Scopus (200) Google Scholar]. We will focus solely on GPR43 in order to address in detail all aspects related to this receptor. In this review, we briefly present the pharmacological tools currently available to study GPR43. We also evaluate the relevance of GPR43 in the symbiotic relationship between the gut microbiota and its host in adipose tissue, GI tract, and immune cells. Finally, we examine the therapeutic potential of GPR43 for treating diseases such as obesity, diabetes, inflammatory disorders, and cancer. SCFAs exert variable actions through several pathways [11Tolhurst G. et al.Short-chain fatty acids stimulate glucagon-like peptide-1 secretion via the G-protein-coupled receptor FFAR2.Diabetes. 2012; 61: 364-371Crossref PubMed Scopus (1301) Google Scholar, 12Vinolo M.A. et al.SCFAs induce mouse neutrophil chemotaxis through the GPR43 receptor.PLoS ONE. 2011; 6: e21205Crossref PubMed Scopus (194) Google Scholar, 13Siavoshian S. et al.Butyrate and trichostatin A effects on the proliferation/differentiation of human intestinal epithelial cells: induction of cyclin D3 and p21 expression.Gut. 2000; 46: 507-514Crossref PubMed Scopus (237) Google Scholar]. Therefore, the development of selective agonists and antagonists of GPR43 is required to decipher which beneficial effects of the SCFAs may be dependent on their binding to GPR43 and to investigate the therapeutic potential of modulating GPR43 activity. Two other arguments are in favor of the development of pharmacological tools specific for GPR43: firstly, SCFA potency is low, ranging from high micromolar to low millimolar concentrations; secondly, GPR43 knockout (KO) mice could not be an ideal model to investigate GPR43 function because of an altered expression of GPR41 in these mice, which could interfere with the interpretation of the data related to SCFAs [11Tolhurst G. et al.Short-chain fatty acids stimulate glucagon-like peptide-1 secretion via the G-protein-coupled receptor FFAR2.Diabetes. 2012; 61: 364-371Crossref PubMed Scopus (1301) Google Scholar, 14Bjursell M. et al.Improved glucose control and reduced body fat mass in free fatty acid receptor 2-deficient mice fed a high-fat diet.Am. J. Physiol. Endocrinol. Metab. 2011; 300: E211-E220Crossref PubMed Scopus (230) Google Scholar]. Designing selective GPR43 modulators requires characterization of its orthosteric and allosteric binding pockets, including the identification of the requirements for selective activation of GPR43 versus GPR41. Two arginines and one histidine have been established as crucial for recognition of SCFAs by human orthologs of both receptors (R180, R255, H242 for hGPR43 and R185, R258, H245 for hGPR41) [8Ulven T. Short-chain free fatty acid receptors FFA2/GPR43 and FFA3/GPR41 as new potential therapeutic targets.Front. Endocrinol. (Lausanne). 2012; 3: 111PubMed Google Scholar, 15Stoddart L.A. et al.Conserved polar residues in transmembrane domains V, VI, and VII of free fatty acid receptor 2 and free fatty acid receptor 3 are required for the binding and function of short chain fatty acids.J. Biol. Chem. 2008; 283: 32913-32924Crossref PubMed Scopus (87) Google Scholar, 16Swaminath G. et al.Allosteric rescuing of loss-of-function FFAR2 mutations.FEBS Lett. 2010; 584: 4208-4214Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar]. The difference in selectivity at the orthosteric site between hGPR41 and hGPR43 has been explored using structure–activity relationship analysis of small carboxylic acids, and three amino acids (E166, L183, C184) have been identified as critical for selective orthosteric activation of hGPR43 versus hGPR41 [17Schmidt J. et al.Selective orthosteric free fatty acid receptor 2 (FFA2) agonists: identification of the structural and chemical requirements for selective activation of FFA2 versus FFA3.J. Biol. Chem. 2011; 286: 10628-10640Crossref PubMed Scopus (91) Google Scholar]. In addition, residues (e.g., L173) and extracellular loop 2 are also important for positive allosteric effects on hGPR43 [18Smith N.J. et al.Extracellular loop 2 of the free fatty acid receptor 2 mediates allosterism of a phenylacetamide ago-allosteric modulator.Mol. Pharmacol. 2011; 80: 163-173Crossref PubMed Scopus (63) Google Scholar, 19Swaminath G. et al.Mutational analysis of G-protein coupled receptor–FFA2.Biochem. Biophys. Res. Commun. 2011; 405: 122-127Crossref PubMed Scopus (15) Google Scholar]. Schmidt et al. explored the orthosteric activation potential of an expanded set of small carboxylic acids including carboxylic acids with additional branched, unsaturated, and cyclic tails. The authors proposed that compounds with an sp- or sp2-hybridized α-carbon preferentially activate hGPR43, whereas hGPR41-selective ligands contained a substituted sp3-hybridized α-carbon. They showed that the development of small molecules with a higher selectivity for GPR43 versus GPR41 than propionate and acetate was feasible (Figure 1, panel 2), but a gain of potency remains to be achieved [17Schmidt J. et al.Selective orthosteric free fatty acid receptor 2 (FFA2) agonists: identification of the structural and chemical requirements for selective activation of FFA2 versus FFA3.J. Biol. Chem. 2011; 286: 10628-10640Crossref PubMed Scopus (91) Google Scholar]. Two allosteric modulators with agonist activity, derived from phenylacetamide, have been described in 2008. Lee et al. highlighted the existence of nonoverlapping binding sites for the synthetic and endogenous ligands [20Lee T. et al.Identification and functional characterization of allosteric agonists for the G protein-coupled receptor FFA2.Mol. Pharmacol. 2008; 74: 1599-1609Crossref PubMed Scopus (124) Google Scholar]. Two years later, the same team reported a full series of GPR43 agonists based on these two lead molecules, with optimization of pharmacokinetic properties (Figure 1, panel 3) [21Wang Y. et al.The first synthetic agonists of FFA2: discovery and SAR of phenylacetamides as allosteric modulators.Bioorg. Med. Chem. Lett. 2010; 20: 493-498Crossref PubMed Scopus (72) Google Scholar]. Importantly, the synthetic and endogenous ligands seem to activate the intracellular pathways in a different way [17Schmidt J. et al.Selective orthosteric free fatty acid receptor 2 (FFA2) agonists: identification of the structural and chemical requirements for selective activation of FFA2 versus FFA3.J. Biol. Chem. 2011; 286: 10628-10640Crossref PubMed Scopus (91) Google Scholar, 20Lee T. et al.Identification and functional characterization of allosteric agonists for the G protein-coupled receptor FFA2.Mol. Pharmacol. 2008; 74: 1599-1609Crossref PubMed Scopus (124) Google Scholar, 22Bindels L.B. et al.Gut microbiota-derived propionate reduces cancer cell proliferation in the liver.Br. J. Cancer. 2012; 107: 1337-1344Crossref PubMed Scopus (183) Google Scholar]. Phenylacetamide derivatives are equally potent on Gαi–cAMP and Gαq–PLC pathways, whereas propionate and acetate are more potent on Gαi–cAMP than on Gαq–PLC pathways [20Lee T. et al.Identification and functional characterization of allosteric agonists for the G protein-coupled receptor FFA2.Mol. Pharmacol. 2008; 74: 1599-1609Crossref PubMed Scopus (124) Google Scholar]. More recently, Euroscreen has patented several series of GPR43 agonists, with some of them exhibiting interesting potency (Figure 1, panel 4) [8Ulven T. Short-chain free fatty acid receptors FFA2/GPR43 and FFA3/GPR41 as new potential therapeutic targets.Front. Endocrinol. (Lausanne). 2012; 3: 111PubMed Google Scholar, 23Hoveyda, H. et al. Euroscreen. Novel compounds, method for use them and pharmaceutical composition containing them, WO2011151436Google Scholar]. Two series of GPR43 antagonists have also been protected by Euroscreen [24Brantis, C. et al. Euroscreen. Novel amino acid derivatives and their use as GPR43 receptor modulators, WO2011092284Google Scholar] and Galapagos [25Saniere, L.R.M. et al. Galapagos. Azetidine derivatives useful for the treatment of metabolic and inflammatory diseases, WO2012098033Google Scholar], with both patents including pharmacological and functional assays. Their claims are supported by Hudson et al. who described one of these compounds (Figure 1, panel 5) as a full ‘inverse agonist’ at the human GPR43 ortholog with very little, if any, activity at the mouse ortholog [26Hudson B.D. et al.Extracellular ionic locks determine variation in constitutive activity and ligand potency between species orthologs of the free fatty acid receptors FFA2 and FFA3.J. Biol. Chem. 2012; 287: 41195-41209Crossref PubMed Scopus (88) Google Scholar]. Finally, a chemically engineered receptor activated solely by synthetic ligands (RASSL) is a useful alternative for probing receptor function and was provided in an elegant study based on pharmacological variation between species orthologs. Knock-in transgenesis of this RASSL will allow investigating GPR43 function in vivo [27Hudson B.D. et al.Chemically engineering ligand selectivity at the free fatty acid receptor 2 based on pharmacological variation between species orthologs.FASEB J. 2012; 26: 4951-4965Crossref PubMed Scopus (63) Google Scholar]. For therapeutic prospects, the potency, affinity, and pharmacokinetics properties of GPR43 modulators must be improved. The safety of a chronic administration of an agonist or antagonist is not known yet and must be investigated before ruling on the ‘druggability’ of GPR43. The pharmacological challenges related to FFA receptors, including careful description of crucial residues and recent patents, have been discussed in two well-written recent reviews [8Ulven T. Short-chain free fatty acid receptors FFA2/GPR43 and FFA3/GPR41 as new potential therapeutic targets.Front. Endocrinol. (Lausanne). 2012; 3: 111PubMed Google Scholar, 28Hudson B.D. et al.Experimental challenges to targeting poorly characterized GPCRs: uncovering the therapeutic potential for free fatty acid receptors.Adv. Pharmacol. 2011; 62: 175-218Crossref PubMed Scopus (44) Google Scholar]. In 2005, Hong et al. demonstrated that GPR43 was expressed in mouse white adipose tissue (WAT), to a higher extent in adipocytes than in stromal vascular cells, and overexpressed in the adipose tissue of mice fed a high-fat (HF) diet [29Hong Y.H. et al.Acetate and propionate short chain fatty acids stimulate adipogenesis via GPCR43.Endocrinology. 2005; 146: 5092-5099Crossref PubMed Scopus (449) Google Scholar]. GPR43 expression was increased during the differentiation of 3T3-L1 preadipocytes and an agonist of the peroxisome proliferator-activated receptor γ (PPARγ), a master regulator of the differentiation process, reinforced this overexpression. Furthermore, acetate and propionate increased lipid accumulation in differentiating cells and propionate induced GPR43 and PPARγ overexpression during differentiation. 3T3-L1 cells transfected with a small interfering RNA (siRNA) to reduce GPR43 expression exhibited lower mRNA levels of PPARγ and aP2 – a PPARγ target gene, marker of adipocyte differentiation – and accumulated less fat during differentiation. The authors thus suggested that GPR43 and SCFAs play an important role in adipogenesis (Figure 2) [29Hong Y.H. et al.Acetate and propionate short chain fatty acids stimulate adipogenesis via GPCR43.Endocrinology. 2005; 146: 5092-5099Crossref PubMed Scopus (449) Google Scholar]. Moreover, acetate and propionate were shown to inhibit the lipolytic activity in 3T3-L1 differentiated adipocytes. Interestingly, GPR43 siRNA-transfected cells and adipocytes isolated from GPR43 KO mice did not respond to propionate and acetate, suggesting that GPR43 activation can result in lipolysis inhibition [29Hong Y.H. et al.Acetate and propionate short chain fatty acids stimulate adipogenesis via GPCR43.Endocrinology. 2005; 146: 5092-5099Crossref PubMed Scopus (449) Google Scholar, 30Ge H. et al.Activation of G protein-coupled receptor 43 in adipocytes leads to inhibition of lipolysis and suppression of plasma free fatty acids.Endocrinology. 2008; 149: 4519-4526Crossref PubMed Scopus (352) Google Scholar]. Moreover, an intraperitoneal injection of acetate led to reduced plasma FFAs in wild type mice but not in GPR43 KO animals, thus highlighting that the antilipolytic activity of acetate through GPR43 activation also occurred in vivo [30Ge H. et al.Activation of G protein-coupled receptor 43 in adipocytes leads to inhibition of lipolysis and suppression of plasma free fatty acids.Endocrinology. 2008; 149: 4519-4526Crossref PubMed Scopus (352) Google Scholar]. The hypothesis that GPR43 may be implicated in adipogenesis has been recently reinforced by our own results [31Dewulf E.M. et al.Inulin-type fructans with prebiotic properties counteract GPR43 overexpression and PPARγ-related adipogen