The therapeutic potential of GPR43: a novel role in modulating metabolic health

Abstract

GPR43 is a receptor for short-chain fatty acids. Preliminary data suggest a putative role for GPR43 in regulating systemic health via processes including inflammation, carcinogenesis, gastrointestinal function, and adipogenesis. GPR43 is involved in secretion of gastrointestinal peptides, which regulate appetite and gastrointestinal motility. This suggests GPR43 may have a role in weight control. Moreover, GPR43 regulates plasma lipid profile and inflammatory processes, which further indicates that GPR43 could have the ability to modulate the etiology and pathogenesis of metabolic diseases such as obesity, type 2 diabetes mellitus, and cardiovascular disease. This review summarizes the current evidence regarding the ability of GPR43 to mediate both systemic and tissue specific functions and how GPR43 may be modulated in the treatment of metabolic disease.

Introduction

G-protein-coupled receptors (GPCRs) are at the forefront of research aimed at the treatment of a wide spectrum of disease states. This heterogeneous family of receptors is able to modulate metabolic function by a variety of mechanisms, largely due to the pleiotropic nature of the endogenous ligands for these receptors. As a result of this, GPCRs form the target of approximately 50 % of recently developed pharmaceuticals [1].

G-protein-coupled receptor 43 (GPR43; also designated free fatty acid receptor 2) is a GPCR identified as a cognate receptor for short-chain fatty acids (SCFA) [2, 3]. SCFAs are produced via colonic fermentation of indigestible carbohydrates, with the major products being acetate, propionate, and butyrate [4]. SCFAs elicit a plethora of effects on systemic health including regulation of gastrointestinal motility, appetite, inflammation, carcinogenesis, and both tissue-specific as well as systemic metabolic health [5–11]. As a receptor for SCFAs, GPR43 may be in part responsible for the diverse regulatory effects these nutrients impart. This further indicates that the functionality of GPR43 can be altered in response to differential production of luminal SCFAs. Thus, GPR43 has been suggested to be an appealing pharmaceutical target with broad therapeutic applications across a spectrum of disease states [12]. The scope of this review examines the current knowledge of GPR43 functioning and the subsequent appropriateness of GPR43 as a novel pharmaceutical target. Particular attention is given to a novel role for this receptor as a mediator of metabolic disease states such as the twin epidemics of obesity and type 2 diabetes mellitus (T2DM), which are critical health priority areas.

GPR43: a receptor for short-chain fatty acids

SCFAs containing a chain of two to three carbon atoms have been shown to demonstrate the greatest specificity for GPR43 [2]. Of this class of lipid, acetate, a two-carbon SCFA (C2), and propionate (3) display a similar magnitude of effect, however, the potency of propionate exceeds that of acetate [2]. Butyrate (C4) also activates GPR43, although the reported potency as compared to acetate or propionate is varied [2, 13]. Importantly, while acetate has a comparable effect with propionate at human GPR43, this SCFA demonstrated the lowest affinity of the three ligands to GPR41, which is also activated endogenously by SCFAs and regulates parameters of metabolic health [2, 14–16]. This indicates acetate to be preferable in terms of studying the functionality of human GPR43 specifically. However, subsequent work has shown that the selectivity of acetate for GPR43 over GPR41 is not replicated in the mouse ortholog of GPR43 [17], suggesting that caution needs to be applied when considering the results of studies in rodents using acetate to probe the effects of GPR43. Medium- and long-chain fatty acids (C ≥ 9) are inactive at this receptor [3]. SCFAs form a minor component of dietary lipid intake given their low prevalence in foods. However, SCFAs including acetate and propionate are produced from dietary breakdown of ingested lipids and by colonic fermentation of insoluble carbohydrates by gut microbiota [4, 18]. SCFAs liberated by intestinal digestion act locally on colonic receptors and are readily absorbed into the bloodstream and circulated around the body, functioning as bioactive ligands and substrates for energy metabolism [5, 18, 19]. Dietary composition has been shown to regulate the colonic SCFA production through alterations in the ratio of gut microflora and SCFA production from dietary intake [18, 20]. High-fat diets and obesity are associated with dysbiosis leading to a reduction in the ratio of bacteroidetes to firmicutes in the colon [21–23]. Since bacteroidetes are the main contributors to acetate production [18], this suggests acetate production would be reduced when consuming a high-fat diet. Further, hepatic metabolism of ethanol subsequent to alcohol consumption liberates acetate, which has been shown to correlate to increases in circulating plasma acetate levels [20]. Thus the consumption of a high-fat diet or presence of obesity has the capacity to reduce GPR43 activity via reductions in ligand availability, while hyperactivity of GPR43 may result subsequent to excessive alcohol consumption due to increased acetate production. Together, this indicates dietary intake and metabolic health to be important in maintaining homeostatic regulation of GPR43′s function. Such postulations are supported by Kimura et al. [24] who show that the effects of GPR43 in lean and diet-induced obese mice are regulated by gut microbiota.

Development of synthetic ligands at GPR43

In addition to SCFAs, a number of synthetic compounds have been identified as activators or inhibitors of GPR43. Given the lack of specificity and species differences in selectivity of GPR43 to acetate, the development of synthetic agonists and antagonists at this receptor is of utmost importance in furthering research into the therapeutic applications of GPR43. To address this need, any small-molecule agents developed need to be devoid of activity at other receptors including the closely related GPR41 receptor. Initial studies demonstrated a group of small-molecule phenylacetamides as allosteric agonists selective for GPR43 [25, 26]. These compounds were shown to act cooperatively with the orthosteric agonist, acetate, indicating this group of phenylacetamides to be ago-allosteric modulators of GPR43 activity [25, 27, 28]. Smith et al. [28] further showed that the extracellular loop 2 (ECL2) of GPR43 houses key residues in regulating the allosteric effects of the phenylacetamide compound (termed 4-CMTB in this paper) however, the direct agonistic effects were unaffected by mutational analysis at ECL2. Such work provides great insight into future development of synthetic agonists at GPR43 and indicates a potential for development of small-molecule compounds that target GPR43 with greater potency and selectivity than SCFAs including acetate. This is supported by the recent commercial availability of the phenylacetamide agonist for GPR43 (S)-2-(4-chlorophenyl)-3,3-dimethyl-N-(5-phenylthiazol-2-yl) butanamide from Merck Millipore (Darmstadt, Germany).

Schmidt et al. [29] then provided insight into the chemical properties of orthosteric agonists selective for GPR43 with conjugated unsaturated carboxylic acids demonstrating selectivity for GPR43 over GPR41. Through mutational analysis, they showed that binding of small carboxylic acids at the orthosteric binding site of GPR43 requires two arginine residues Arg¹⁸⁰ (V:05/5.39) and Arg²⁵⁵ (VII:08/7.35) located in the transmembrane domains 5 and 7, respectively, which are conserved in both GPR43 and GPR41, while the non-conserved residues Glu¹⁶⁶ (ECL2), Leu¹⁸³ (V:08/5.42), and Cys¹⁸⁴ (V:09/5.43) within the orthosteric binding site of GPR43 confer specificity to this receptor [27, 29]. Hudson et al. [30] further describe two agonistic compounds for GPR43 with 4-oxobutanoic acid backbones which bind to the orthosteric site of the GPR43 receptor, with ECL2 again being important in modulating potency and specificity of these compounds. Interestingly, the agonistic activity of the two oxobutanoic acid compounds was not enhanced by the addition of the previously described ago-allosteric modulator 4-CMTB while the effects of propionate were [30]. This indicates that there is a difference in action between the endogenous and synthetic ligands, which may in turn lead to divergence in the net effect elicited by these compounds. Nonetheless, given the highly conserved nature of the orthosteric binding site between GPR43 and GPR41, exploitation of this knowledge can enable the development of molecules displaying greater specificity and potency than natural SCFA agonists. While the majority of work thus far has described synthetic agonists for GPR43, Brantis et al. [31] describe a novel GPR43 antagonist referred to as CATPB. Hudson et al. [17] further qualify this compound as an inverse agonist for GPR43 and support the application of CATPB in antagonizing the effects of propionate, 4-CMTB, and the oxobutanoic acid compounds [17, 30]. Interestingly, while CATPB antagonized the human ortholog of GPR43, the compound was not active at the mouse ortholog of GPR43 [17]. Given this research is still in its relative infancy, the lack of activity for murine GPR43 limits in vivo testing of the compound. In addition to CATPB, another series of patented azetidine derivatives have recently been proposed as having therapeutic applications as GPR43 antagonists [32], which may overcome the shortfalls of CATPB in this regard. In view of the conflicting data published regarding the functional outcomes of GPR43, the development of selective antagonists is equally important to that of GPR43 agonists to clarify the therapeutic potential of this receptor in pathophysiological states.

GPR43 tissue distribution

Initial studies have indicated that GPR43 is expressed abundantly in immune tissues and cells including peripheral blood leukocytes, neutrophils, monocytes, spleen, and adenoid tissues [3, 13, 33]. Additional work has confirmed the expression of GPR43 in a number of other peripheral tissues including white and brown adipose tissue, hepatic tissue, cardiac muscle, skeletal muscle, colon, myometrium, and breast [3, 34–37]. Further studies have extended on this work to show that the mRNA abundance of GPR43 is selectively altered in different tissues by the induction of diet-related obesity [34, 36, 38]. These studies have shown that the mRNA expression of GPR43 is increased subsequent to diet-induced obesity in rodent models in soleus (oxidative) and extensor digitorum longus (glycolytic) skeletal muscles, the liver, and adipose tissue [34, 36, 38]. Increased consumption of fermentable indigestible carbohydrate in rats also increased the density of GPR43-expressing cells in the proximal colon [39]. Furthermore, GPR43 abundance has been shown to be altered in human colorectal and gastric cancers with one study showing increased GPR43 expression [40] while another reported a decrease in GPR43 expression [41]. Thus this may implicate altered GPR43 expression in the development and/or progression of certain conditions such as obesity, diabetes, and cancer and their co-morbid conditions.

GPR43-mediated intracellular signaling

Characterization of the signaling pathways mediated via GPCRs is confounded by the pleiotropic nature of these proteins. GPR43 is a membrane-bound GPCR [41] that has been shown to exhibit dual coupling to both the G_q and G_i/o pathways leading to induction of intracellular calcium and decreased intracellular cyclic adenosine monophosphate [2, 3, 42]. More recently, studies have shown specific coupling to the G_(i/o)βγ pathway and that GPR43 also couples with beta arrestin2 [24, 30, 43], which may be indicative of receptor internalization after ligand binding. This coupling to multiple signaling pathways may provide an explanation for the variability in the effects observed following manipulation of GPR43 activity. Work to discern this would provide greater insights into how GPR43 functionality can be targeted pharmaceutically and should be the focus of future research.

Downstream targets of the GPR43 signaling cascade have thus far been identified as members of the mitogen-activated protein kinase (MAPK) family with GPR43 agonism leading to increased phosphorylation of p38 in MCF-7 cell- [44] and GPR43-transfected CHO-K1 and HEK293 cells [2, 45]. Further, some [2, 45] but not all [44] studies also show increased phosphorylation of p44/42 (also known as extracellular regulated protein kinase 1/2) following activation of GPR43. It appears that GPR43 activation of p44/42 MAPK is largely dependent on the upstream kinase, mitogen-activated protein kinase kinase 1/2 (MEK1/2). However, partial inhibition of p44/42 activity was also observed following treatment with inhibitors of G_i/o Scr and Rac [45]. The SCFA propionate did not lead to changes in the other main protein of this family c-jun N-terminal kinase (JNK) or Protein kinase B (also known as Akt) in MCF-7 cells [44]. However, in GPR43-transfected HEK293, acetate induced weak phosphorylation of JNK [45]. GPR43 has additionally been demonstrated to regulate phosphorylation of Akt, phosphatase and tensin homologue (PTEN), and liver kinase B1 via the G_(i/o)βγ –phospholipase C-protein kinase B signaling axis [24]. While these studies provide the first insight into the pathways through which GPR43 may elicit its effects, further studies are warranted to clarify the GPR43-mediated signaling cascade in these and other tissues and to determine how these may be selectively targeted to confer health protection.

GPR43 modulation of health

Immunity and inflammation

One of the first functions for GPR43 that was identified was as a modulator of immune function and inflammatory processes [2, 13, 46]. GPR43 expression in immune tissues is consistent with the immunomodulatory effects of SCFAs as the cognate ligands for this receptor. SCFAs have been shown to modulate the production of chemokines and pro-inflammatory markers in a number of cell types (reviewed by [5]). Largely, SCFAs have been shown to exert anti-inflammatory properties, with sodium -acetate, -propionate, and -butyrate all reducing pro-inflammatory markers including nitric oxide, tumor necrosis factor-α, interleukin (IL)—1β, IL-6, and nuclear factor–κβ activation [46, 47]. Similarly, the anti-inflammatory cytokine IL-10 was induced by sodium -acetate, -propionate, and -butyrate in these cells [47]. However, there is a paucity of data regarding the role of acetate in this process, which has implications for interpretation of the involvement of GPR43 in these processes.

Le Poul et al. [2] pioneered studies showing GPR43 activation led to neutrophil recruitment to the site of infection indicative of an inflammation/immune modulating role for GPR43. Vinolo et al. [46] supported this finding showing the chemotactic response from bone marrow neutrophils of GPR43^−/− mice was attenuated when compared to GPR43 wild-type mice. Together, these studies establish a role for GPR43 in neutrophil-mediated immune responses. However, the net effect of GPR43 activation on inflammation is currently unclear. Contention exists as to whether GPR43 agonism exerts an anti- or pro-inflammatory effect. A study by Sina et al. [48] supports a role for GPR43 in chemotaxis as an early line of defense against pathogen infiltration, however this study also report a reduction in colonic inflammation and tissue damage subsequent to dextran sodium sulphate (DSS)-induced chronic colitis in GPR43^−/− mice when compared to their wild-type littermates. This is suggestive of a pro-inflammatory role for GPR43. Sina et al. [48] suggest the reduction in tissue damage may be attributed to diminished p38 MAPK activation via SCFAs. This is consistent with p38 being predominately activated by stress and inflammatory stimuli (reviewed by [49]). In contrast, the results of Maslowski et al. [50] show an increase in inflammation in GPR43^−/− mice compared to their wild-type littermates in a similar model of DSS-induced colitis and also in arthritis and allergic airway inflammation. Moreover, acetate administration in colitis and arthritis further reduced inflammatory processes in wild-type mice but was without effect in GPR43^−/− mice [50]. Thus, the results of this study imply an anti-inflammatory effect mediated by GPR43.

Gastrointestinal functionality

Given the high level of expression of GPR43 in enteroendocrine cells of the gastrointestinal tract and the regulatory role of SCFAs on GPR43 function, it follows that GPR43 would have a role in modulating gut function. Indeed, studies have shown that GPR43 co-localizes with peptide YY (PYY) and glucagon-like peptide-1 (GLP-1) in human intestinal L cell and 5-hydroxytryptamine (5-HT) in rodent mast cells but not enteroendocrine cells [35, 39, 51]. SCFAs have previously been reported to mediate intestinal release of the gastrointestinal peptides PYY, 5-HT, and GLP-1 [6, 52, 53]. Cognizant of this, GPR43 has been suggested to mediate the release of PYY, GLP-1, and 5-HT due to the regulatory effects of SCFAs on the release of these peptides and the distinct pattern of co-localization of these peptides with GPR43 [35, 39, 51]. Indeed, basal and glucose-stimulated GLP-1 has been shown to be reduced in vivo in GPR43^−/− mice, which coupled with reduced plasma insulin and increased plasma glucose level [54]. In colonic cultures from GPR43^−/− mice, the SCFAs acetate and propionate have a reduced ability to stimulate GLP-1 release [54]. These peptides are known to modulate a number of metabolic functions both locally in the gut and distally at peripheral tissues to regulate nutrient intake and systemic metabolic health (Table 1). Therefore, regulation of these and potentially other gastrointestinal peptides by intestinal GPR43 provides a plausible mechanism by which GPR43 may be targeted for the treatment of obesity and the associated co-morbid conditions.

Table 1.

Gastrointestinal peptide-mediated effects on local and distal metabolic functions and health

Peptide	Proposed role of GPR43	Gastrointestinal effects	Peripheral metabolic effects	References
GLP-1	↑	↓ Appetite ↓ Gastrointestinal motility ↓ Gastric emptying	↑ Insulin secretion ↑ Glucose tolerance ↑ CM glucose uptake ↑ SM glucose uptake ↑ Cardioprotective effects	[111–116]
PYY	↑	↓ Appetite (PYY3-36) ↑ Appetite (PYY 1-36) ↓ Gastrointestinal motility ↓ Gastric secretions	↓ Pancreatic secretions ↓ Insulin secretion ↑ Insulin sensitivity ↓ Body weight (PYY3-36) ↑ SM glucose uptake ↑ FAO	[113, 115, 117–120]
5-HT	↑	↑ Gastrointestinal motility ↑ GIT inflammation ↓ Appetite ↑ Satiation	↑ SM glucose uptake ↓ Body weight ↓ Hypertension ↑ Cardiac inotropy ↑ Cardiac chronotropy	[53, 121–127]

PYY _3-36 is increased in postprandial state, PYY _1-36 is increasing in fasting state. ↑ is increase, ↓ is decrease, 5-HT 5-hydroxytryptamine, CM cardiac muscle, FAO fatty acid oxidation, GIT gastrointestinal tract, GLP-1 glucagon-like peptide 1, PYY peptide YY, SM skeletal muscle

Given that the local colonic concentrations of acetate exceed that of other SCFAs [4], GPR43 is a leading candidate as a mediator of the effects of SCFAs on gastrointestinal peptide release. However, the potential for GPR43 agonism via SCFAs to positively regulate glucose homeostasis and satiety through induction of the gastrointestinal peptides, GLP-1, PYY, and 5-HT must be considered concomitantly with the effects of GPR43 on intestinal inflammation, which remain ambiguous. Alterations in inflammatory and immune responses in the gut have the potential to regulate the progression of conditions such as Crohn’s disease and ulcerative colitis (reviewed by [55]). Therefore, modulation of GPR43 functions, whether it is through agonism or antagonism, to favorably regulate the inflammatory state of the gastrointestinal tract in these conditions may yield yet another therapeutic function of GPR43. However, if GPR43 agonism is required to increase release of GLP-1, PYY, and 5-HT to yield beneficial effects, yet at the same time agonism of GPR43 increases risk of inflammatory conditions of the gastrointestinal tract leading to deleterious health effects, the net effect on health becomes distorted. Thus, a clearer understanding of the role of GPR43 in regulating gastrointestinal inflammation is first needed before GPR43 agonists or antagonists can be developed with confidence for the treatment of disorders of the gastrointestinal tract.

Carcinogenesis

Alterations of GPR43 expression have been reported in individuals with cancers of the gastrointestinal tract [40, 41]. Interestingly, these two studies present conflicting data as to the effect of cancer on GPR43 expression with Tang et al. [41] reporting significantly diminished GPR43 protein expression in human malignant colorectal adenocarcinomas and to a lesser extent, in the presence of benign colon cancers, polyps, and hyperplasia. This was supported by quantification of in vitro GPR43 mRNA expression in nine colorectal cell lines with the receptor only being expressed in one of these [41]. Subsequent transfection of the colorectal cell line HCT8 with GPR43 lead to increased apoptotic susceptibility, which correlated with an increase in cells in the G₀/G₁ arrest phase of the cell cycle [41]. GPR43 mRNA was expressed in MCF-7, a breast cancer cell line in which treatment with SCFAs selectively increased p38 MAPK and heat shock protein 27 phosphorylation and propionate treatment correlated with reduced cell proliferation [44]. p38 MAPK phosphorylation was not prevented by pertussis toxin treatment and was abrogated by GPR43-specific siRNA, indicating G_q coupling mediated via GPR43 [44]. Taken together, these studies infer a protective role for GPR43 in cancer progression.

Opposing this hypothesis is the work of Hatanaka et al. [40] who report increased GPR43 expression in colorectal and gastric cancer and thus propose an oncogenic potential for GPR43 supported by foci formation in vitro and tumorigenesis in nude mice. Therefore, consistent with the overall conflicts regarding GPR43 agonism versus GPR43 antagonism on health, further research is needed to clarify the effects of GPR43 on carcinogenic processes.

Metabolic health

The prevalence of metabolic diseases is at epidemic proportions in Western societies world-wide [56–58]. It is widely accepted that obesogenic changes in diet and lifestyle have largely driven the substantial increases in the prevalence of obesity, type 2 diabetes mellitus (T2DM), and co-morbid conditions including cardiovascular disease (CVD), which together represent a major cause of morbidity and pre-mature mortality [56–60]. A novel therapeutic potential exists for GPR43 as a pharmaceutical target for the treatment of metabolic diseases. Prospective roles for GPR43 occur at both the tissue-specific and systemic levels through changes in gut microbiota, production of endogenous ligands, and direct influences on GPR43 abundance in specific metabolically active tissues. Of the studies looking at the effect of GPR43 on metabolic homeostasis, Bjursell et al. [61] and Kimura et al. [24] provide the greatest insight of the metabolic phenotype imparted by GPR43. However, the data from these two studies is conflicting. The study by Bjursell et al. [61] quantifies the effect of high-fat feeding in GPR43^−/− mice showing that on a chronic high-fat diet (39.9 % of total energy; >10 weeks), the absence of the GPR43 receptor led to protection from diet-induced obesity compared to wild-type littermates [61]. Contrary to this, Kimura et al. [24] show that GPR43 knockout mice gained significantly more weight when fed a normal or high-fat diet (61.6 % of total energy; 12 weeks) and manifested reduced insulin sensitivity and marked insulin resistance when compared to their wild-type counterparts, indicating that GPR43 protected these mice from diet-induced obesity and associated deficits in glucose metabolism.

Current research provides significant evidence that implicates GPR43 as an adipogenic factor, with binding of cognate ligands to GPR43 being demonstrated to promote adipocyte development, adipogenesis, and anti-lipolytic activity [36, 62]. GPR43 mRNA is induced in differentiated adipocytes compared to immature cells [36] and importantly, in primary adipocytes derived from GPR43^−/− mice, the SCFAs acetate and propionate failed to inhibit lipolysis, confirming that GPR43 was indeed responsible for mediating these effects [62]. Furthermore, acetate reduced plasma-free fatty acid levels in wild-type but not GPR43^−/− mice [62], thus facilitating the deduction that GPR43 is a potent regulator of adipose tissue lipolysis. In vitro studies show increased lipid droplet accumulation in 3T3-L1 adipocytes following acetate and propionate treatment, which was abrogated by treatment with GPR43-specific siRNA [36]. GPR43 knockdown also reduced the expression of peroxisome proliferator active receptor gamma (PPARγ) in 3T3-L1 cells [36]. Given that PPARγ promotes adipogenesis [63] and is a key regulator of adipocyte metabolism [64], the ability of GPR43 to regulate PPARγ expression provides a mechanism by which GPR43 may elicit its influence on adipogenesis. The effects of GPR43 in regulating adipose tissue metabolism suggest that GPR43 is a valid target for the treatment of dyslipidemia, a hallmark characteristic of metabolic diseases. Importantly, GPR43 activation is reportedly not associated with induction of peripheral vasodilation as a side effect [62] which is associated with flushing of the skin. This is contrary to the commonly used agent for dyslipidemia, nicotinic acid, produces the unwanted side effect of flushing on administration through cutaneous vasodilation [62, 65]. This suggests that GPR43 antagonism may be a preferable target for the treatment of dyslipidemia due to reduced side effects. However, with respect to overall body weight and glucose homeostasis, mice with transgenic adipose-specific expression of GPR43 are lean and manifest an improved profile of insulin sensitivity and resistance [24]. Mechanistically, the latter could be a consequence of the reduced body weight. GPR43 activity was also associated with suppression of white adipose insulin signaling via Akt and PTEN, which may alternately counter the anabolic effects of insulin on fat accumulation [24]. Interestingly, both the increase in body weight in GPR43 knockouts fed a chow or high-fat diet and effects of adipose tissue-specific expression of GPR43 seemed to be reliant on gut microbiota, as these effects were diminished or unobserved under treatment with antibiotics or germ-free conditions [24].

Given that adipose tissue is not inert but a potent regulator of metabolism through the secretion of a large number of adipokines, the role of GPR43 in mediating adipokine secretion is of interest. Zaibi et al. [37] suggest that GPR43 has a selective role in regulating leptin secretion from mesenteric but not epididymal adipocytes through G_i coupling. Leptin has been shown to act as an anorectic agent to reduce energy intake [66], augment systemic energy expenditure [67], enhance insulin sensitivity and glucose tolerance [68], and promote lipid oxidation [69–71]. Together, these effects promote the loss of fat mass while preserving lean body mass [72, 73]. However, further work is needed to delineate the role of GPR43 in controlling leptin secretion as the study by Zaibi et al. [37] examined the effect of SCFAs on leptin secretion in GPR41^−/− and wild-type mice but does not compare the effects in GPR43^−/− and the wild type. Consequently, this study does not provide definite evidence of a role for GPR43 in mediating leptin secretion. Bjursell et al. [61] showed that GPR43^−/− mice fed a high-fat diet for 40 weeks had higher plasma adiponectin and a non-significant trend towards reduced leptin levels. In obesity, leptin levels are positively correlated with adiposity, however the elevated levels of leptin often occurs concomitant to leptin resistance, a condition analogous to insulin resistance in T2DM where the beneficial effects of leptin are markedly attenuated (reviewed by [74, 75]). Compensatory hyperleptinemia subsequently develops in obesity in an attempt to maintain the beneficial functions of leptin attenuated by the resistance [76]. Conversely, the results of Kimura et al. [24] showed no change in plasma leptin levels in GPR43^−/− mice, however mice with adipose-specific transgenic expression of GPR43 exhibited reduced plasma leptin levels. Therefore, further work is needed to delineate the mechanisms behind the potential of GPR43 antagonism to reduce hyperleptinemia in the obese state as such an effect may contribute towards preservation and/or restoration of the beneficial functions of leptin in the treatment of obesity and obesity-related conditions.

Like leptin, adiponectin is an adipokine that improves insulin signaling, promotes systemic euglycemia, and increases fatty acid oxidation [77, 78]. The modulation of glucose and fatty acid metabolism by adiponectin is largely mediated through increasing the activity of 5′ adenosine monophosphate-activated protein kinase and PPARα, which are known to promote both glucose and fatty acid metabolism [77, 79]. Contrary to leptin, adiponectin levels are reduced in obese individuals [80], which is considered pathogenic in the progression of metabolic diseases. Thus, therapies to attenuate hypoadiponectinemia and restore the effects of adiponectin are considered beneficial in the treatment of metabolic disease such as obesity, T2DM and CVD [81, 82]. Despite the fact that both wild-type and knockout rats fed a high-fat diet gained significantly more weight than the animals maintained on a standard chow diet, the GPR43 knockout animals had increased plasma adiponectin [61], which may have contributed to the preservation of normal metabolic health seen in these animals. While the mechanism behind the augmented adiponectin level is unknown, the inverse correlation between adiposity and adiponectin levels supports the reduced fat mass in GPR43^−/− mice as an explanation. Thus, by way of decreasing systemic adiposity, there may be a role for GPR43 antagonists in ameliorating hypoadiponectinemia. Further work would clarify whether the effect on adiponectin level is dependent on adiposity. Furthermore, the effects of GPR43 knockout on leptin and adiponectin were not seen in the mice fed a control diet [61]. Thus GPR43 antagonism may selectively elicit the beneficial effects in conditions which manifest perturbed levels of these adipokines such as obesity. Quantifying the role of GPR43 on these adipokines in models of T2DM or CVD would also be of interest to determine if the effects were dependent on pre-existing metabolic dysfunction.

GPR43 and muscle metabolic function

The above sections provide an overview of the current knowledge concerning the ability of GPR43 to modulate health. The novel potential for GPR43 to mediate metabolism in peripheral metabolically active tissues warrants mention in this review. We propose that selective manipulation of GPR43 functionality will potentially regulate metabolism in skeletal and cardiac muscle. Skeletal muscle is a major site of both fatty acid metabolism and insulin-stimulated glucose disposal, and a major contributor to the maintenance of systemic energy homeostasis [83, 84]. Similarly, the myocardium has very high energetic demands based on its workload, with the adult heart relying predominantly on fatty acid as a metabolic fuel [85]. It is well known that obesity, T2DM, and CVD are involved in the development of perturbed substrate metabolism of skeletal muscle and heart. However to highlight the complexity associated with these conditions, they can also arise secondary to perturbations in substrate metabolism through associated changes in systemic metabolic health. Aberrations of substrate metabolism are associated with myopathies of both muscle types including intramyocellular lipid accretion, insulin resistance, resistance to adiponectin and leptin, impairments in signaling pathways regulating glucose and fatty acid metabolism, chronic low-grade inflammation, and contractile dysfunction [86–96]. Systemically, these factors promote further weight gain, aberrant insulin signaling and blood glucose control, decompensated cardiac hypertrophy and hastens progression to heart failure. Therefore it can be seen that impairments in nutrient metabolism in these tissues perpetuates a self-potentiating cycle that promotes the cause and progression of these disease states.

We [34] and others [13, 36] have shown that GPR43 is expressed in both cardiac and skeletal muscle, which implies that GPR43 may have a direct role in metabolic function in these tissues. Our recent observation of up-regulated GPR43 expression in skeletal muscle of diet-induced obese rats suggests that the disease process in obesity may induce aberrations of GPR43 functionality that alter nutrient metabolism [34]. This postulation may be in line with the protective effect seen when GPR43 knockout mice are maintained on a high-fat diet compared with control-fed animals [61]. A likely candidate in the regulation of these processes is the MAPK family of proteins. GPR43 has been shown to modulate these pathways in cells including CHO-K1, HEK 293, primary polymorphonuclear leukocytes, and the breast cancer cell line MCF-7 [2, 44, 45, 48]. However, this role is yet to be confirmed in skeletal and cardiac muscle. In the heart and skeletal muscle, all three members of the MAPK family (JNK, p44/42 and p38) are involved in regulating aspects of both glucose and fatty acid metabolism [97–103], inflammation [104, 105], and cardiac hypertrophy [106, 107]. However, some contention exists as to the specific consequences of MAPK activation on these processes in skeletal and cardiac muscle. Regardless, this suggests that if GPR43 is able to induce changes in the activity of MAPKs, GPR43 would exert influence over key metabolic processes in cardiac and skeletal muscle, which are critical to the maintenance of systemic metabolic health. Systemic knockout of GPR43-induced suppression of insulin signaling in white adipose tissue but not muscle tissue [24]. In fact, in mice transgenically expressing adipose-specific GPR43, Akt phosphorylation was enhanced in muscle tissue [24]. However, it will be important to assess the effects of directly activating GPR43 in muscle (both skeletal and cardiac), as adaptive/compensatory changes in physiological function may occur when GPR43 is solely expressed in adipose tissue. Further research should therefore confirm whether GPR43 activation occurs to the benefit or detriment of metabolic function in cardiac and skeletal muscle and in turn, systemic health.

Through eliciting whole-body changes in processes like inflammation, gastrointestinal peptide release, and circulating adipokines, GPR43 also has the ability to indirectly modulate metabolic function of skeletal and cardiac muscle. There is evidence to support increases in PYY, GLP-1, and 5-HT having an insulin-sensitizing effect to enhance muscle glucose uptake and glycemic regulation, increased fatty acid oxidation, and weight control (Table 1). It is also known that the adipokines, leptin, and adiponectin, promote insulin sensitivity and fatty acid oxidation in these tissues [68–71, 77, 78, 108]. Adiponectin is further implicated in attenuating hypertrophic processes of the myocardium and improving contractile dysfunction in myocytes from obese diabetic mice [109, 110]. While the exact nature of GPR43s’ influence over these processes remains to be determined, Fig. 1 summarizes the proposed mechanisms by which GPR43 may directly and indirectly regulate nutrient metabolism in these tissues.

Fig. 1.

Proposed interplay of GPR43-mediated effects on metabolic homeostasis. Proposed pathways that represent the complexity with which GPR43 has the ability to regulate both tissue-specific and systemic metabolic function. These roles indicate GPR43 as a drug target for the treatment of metabolic diseases such as obesity, type 2 diabetes mellitus, and cardiovascular disease. Solid lines represent a direct effect of GPR43; Broken lines represent an indirect or secondary effect of GPR43 activation on metabolic function. The sum of these direct and indirect mechanisms provides multiple points of cross-talk between the distinct effects, suggesting that by targeting one aspect of GPR43 functionality it is likely to modulate other related pathways. 5-HT 5-hydroxytryptamine, Ad adiponectin, GLP-1 glucagon-like peptide 1, Lep leptin, MAPK mitogen activated protein kinase, PYY peptide YY

Conclusions

Data suggest that GPR43 has the potential to modulate a number of processes and disease states which influence overall health. Exciting roles have been identified for GPR43 in inflammatory, gastrointestinal, cancerous, and metabolic conditions, which together plague current society. However, the question remains as to whether GPR43 agonism or antagonism is the appropriate method for targeting these abnormalities. While it is tempting to consider each discrete function of GPR43 in isolation, attention must be given to the systemic effects of selectively activating or deactivating the GPR43 receptor. Crosstalk between these systems, as highlighted in Fig. 1, suggests that targeting GPR43 is likely to influence other pathways affected either directly or indirectly by GPR43, which may produce unwanted side effects.

Further research is needed to characterize both the tissue-specific and systemic effects of altering GPR43 functionality and how this translates to changes in long-term health. This needs to occur before GPR43 agonists or antagonists can be prescribed with confidence in any of the aforementioned disease states. This opens the field to exciting new research aimed at answering this question with significant benefit to be derived from elucidating the roles of GPR43 in more detail.

Abbreviations

5-HT

5-Hydroxytryptamine

C2

2 Carbon fatty acid

C3

3 Carbon fatty acid

C4

4 Carbon fatty acid

CVD

Cardiovascular disease

DSS

Dextran sodium, sulphate

ECL2

Extracellular loop 2

IL

Interleukin

JNK

c-jun N-terminal kinase

GLP-1

Glucagon-like peptide-1

GPCR

G-protein-coupled receptor

GPR43

G-protein-coupled receptor 43

MAPK

Mitogen-activated protein kinase

MEK1/2

Mitogen-activated protein kinase kinase 1/2

PPARα

Peroxisome proliferator active receptor alpha

PPARγ

Peroxisome proliferator active receptor gamma

PTEN

Phosphatase and tensin homologue

PYY

Peptide YY

SCFA

Short-chain fatty acid

T2DM

Type 2 diabetes mellitus