GPCRs get fatty: the role of G protein-coupled receptor signaling in the development and progression of nonalcoholic fatty liver disease

Abstract

Nonalcoholic fatty liver disease (NAFLD), characterized by the abnormal deposition of lipids within the liver not due to alcohol consumption, is a growing epidemic affecting over 30% of the United States population. Both simple fatty liver and its more severe counterpart, nonalcoholic steatohepatitis, represent one of the most common forms of liver disease. Recently, several G protein-coupled receptors have emerged as targets for therapeutic intervention for these disorders. These include those with known hepatic function as well as those involved in global metabolic regulation. In this review, we highlight these emerging therapeutic targets, focusing on several common themes including their activation by microbial metabolites, stimulatory effect on insulin and incretin secretion, and contribution to glucose tolerance. The overlap in ligands, localization, and downstream effects of activation indicate the interdependent nature of these receptors and highlight the importance of this signaling family in the development and prevention of NAFLD.

INTRODUCTION

Diabetes is a worldwide epidemic, and by 2030, it is estimated that it will become the seventh leading cause of death in the world (1). Out of all cases, 90%–95% of diabetics have type 2 diabetes (T2D), and this can be explained by both lifestyle and genetic factors (1–3). The global prevalence of nonalcoholic fatty liver disease (NAFLD) is also on the rise, with an estimated ∼30% of the United States population presenting with this disorder (4–7). By definition, NAFLD is characterized by the presence of hepatic steatosis not associated with causes such as chronic alcohol consumption, autoimmune disorders, or the use of lipid-altering medications (4–7). Abnormal storage of triglycerides within hepatocytes is a result of the imbalance in lipid metabolism where de novo lipogenesis and fatty acid uptake is favored over fatty acid oxidation and export of VLDL particles. Although its pathogenesis is incompletely understood, NAFLD is associated with insulin resistance, alterations in the gut microbiota, decreased release of glucagon-like peptide 1 (GLP-1), inflammation, cholestasis, and hyperlipidemia; these factors are all at play as it progresses to nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis, all of which can lead to liver failure and hepatocellular carcinoma (5–8). The progression from NAFLD to NASH is driven, in part, by the activation of the resident hepatic macrophages, Kupffer cells, and hepatic stellate cells (HSCs). These cells promote inflammation, fibrogenesis, and activate a variety of signaling pathways including Hedgehog and the Toll-like receptors (TLRs), the latter of which is described in NON-GPCR SIGNALING.

G protein-coupled receptors (GPCRs) are the largest gene family in the genome and are involved in almost every aspect of physiology. These seven transmembrane domain receptors represent the largest class of “druggable” proteins; in fact, 20%–30% of all FDA-approved drugs target GPCRs (9, 10). As their name implies, these receptors are coupled to a membrane-bound heterotrimeric G protein. In canonical GPCR signaling cascades, ligand binding promotes the exchange of GDP for GTP and the dissociation of the G protein complex (α and βγ) leading to the regulation of downstream effectors (11, 12). Cross talk with other signaling pathways [including the mitogen-activated protein kinase (MAPK) kinase cascade] and β-arrestin signaling contribute to the diversity of this class of receptors (11, 12). Although the functions of this protein family are vast, only a subset are being actively studied, leaving behind an extensive list of sensory and orphan receptors with unknown or emerging physiological functions. In fact, many of these receptors play key roles in metabolic pathways and respond to naturally occurring metabolites (13–18).

The liver is the largest metabolic organ in the body and is responsible for maintaining homeostasis by sensing and detoxifying xenobiotics, producing and metabolizing glucose, synthesizing and secreting bile acids, and removing bacteria from the blood. In the past few years, the number of identified understudied GPCRs that influence liver function has drastically increased. Here, we highlight some of the emerging roles that sensory GPCRs (and select nonsensory receptors) have on the development and progression of NAFLD. Given the global changes that accompany obesity and T2D, it is often difficult to parse out whether cellular signaling has a direct effect on liver function or contributes to NAFLD in a more circuitous fashion. For these reasons, this review will focus on the major GPCRs that have been linked, both directly and indirectly, to NAFLD.

SENSORY GPCRs

GPR91 (SUCNR1)

GPR91, commonly known as succinate receptor 1 (SUCNR1), was identified as a member of the GPCR superfamily and cloned by Wittenberger et al. (19) in 2001. Years later, a seminal study by He et al. (13) identified the tricarboxylic acid (TCA) cycle intermediate succinate as the endogenous ligand for GPR91, paving the way for a myriad of studies to deduce physiological actions of succinate and identify additional agonists (Table 1) through this receptor (21, 38, 54, 75). Succinate alone activates G_q/11 and G_i/o pathways, inhibiting cAMP and mediating intracellular calcium mobilization (102). It also upregulates extracellular signal-regulated kinases (ERK) 1/2, c-Jun NH₂-terminal kinase (JNK), and p38 MAPK, playing a role in tumorigenesis, signal transduction, inflammation, and paracrine modulation (13, 23, 103, 104).

Table 1.

Ligands and localization of GPCRs associated with NAFLD

GPCR	Endogenous Ligands	Select Synthetic Ligands	G Protein Coupling	Localization
GPR91 (SUCNR1)	Succinate (13)	cESA (20); cCPDA (20); CMPD131 (21)	Gα (13, 22,23)ERK1/2, c-JUN	Kidney (mRNA, protein) (17, 23,24), Liver (mRNA, protein) (19, 25), Retina (mRNA, protein) (26), Adipose (mRNA, protein) (27), Heart (mRNA, protein) (28), Dendritic cells (mRNA, protein) (29), Breast (mRNA) (19), Blood vessels (mRNA) (30)
GPR55 (LPIR1)	Cannabinoids (31–35), LPI (36), OEA (37)	AM251 (37), AM281 (37), SR141716A (37), HU210 (38), JWH015 (39), CP55940 (37), O1602 (37), Abnormal cannabidiol (35), ML‐193 (40), O‐1918 (40), CID16020046 (40)	Gα12 (41)RhoAGαq (42)PKCβIIG13 (35)RhoA, CDC42, RAC1β-Arrestin (43)ERK 1/2	CNS (mRNA) (35, 39, 44), Neutrophils (mRNA) (45), Gastrointestinal tract (mRNA) (35), Adipose (mRNA, protein) (46,47), Liver (mRNA, protein) (46,47), Skeletal muscle (mRNA) (46), Pancreas (protein) (48)
GPR119 (GPCR2)	OEA (49, 50), LEA (49, 51, 150), 2OG (51,52), LPC (38), LPEA (53), 1-Oleoyl-LPEA (53), LPI (53), LPS (53), LPA (53), SPC (53), Oleic acid (53), Oleamide (53), PEA (53), Anandamide (53), ODA (53), NADA (53), N-Oleoyl-tyrosine (53), 2-Linoleoyl glycerol (53), 2-Palmitoyl glycerol (53), 2-AG (53), 1-Oleoyl glycerol (53), 1-LG(53)	PSN632408 (38), AR231453 (38), APD668 (53), APD597 (53), GSK-1292263 (53), MBX-2982 (53), PSN632408 (54), Sitagliptin (41), Linagliptin (41), Tenegliptin (41)	Gαs (55), Gαi (55), Gαq (55), β-Arrestin (55)	Pancreas (mRNA) (52), Small intestine (mRNA) (52), Stomach (mRNA) (52), Colon (mRNA) (52), Liver (mRNA) (51), Hepatocytes (mRNA) (51), Small intestine (mRNA) (56,57), Macrophages (58,59)
GPR109a (HCA2, MB74b)	Butyrate (60–62), β-Hydroxybutyrate (63)	Niacin (64), Monomethylfumarate (65), Dimethylfumarate (66)	Gαi (64)	Adipose (mRNA) (64), Lung (mRNA) (64), Spleen (mRNA) (64), Kidney (protein) (60), Macrophages (protein) (60), Liver (mRNA, protein) (67), Colon (mRNA) (68), Pancreas (mRNA, protein) (69), Brain (mRNA) (70)
GPR142 (GPRg1b, PGR2)	Tryptophan (71,72), Phenylalanine (71)	CLP-3094 (37), CpdA (73), Benzo-[1,2,4]-triazolo-[1,4]-oxazepine (74), LY3325656 (75)	Gαs (76), Gαi (73), Gαq (71)	Brain (mRNA) (72, 77), Hypothalamus (mRNA) (72), Pancreas (mRNA) (72, 78), Pituitary (mRNA) (72), Lung (mRNA) (72), Spleen (mRNA) (77), Liver (mRNA) (72, 77), Kidneys (mRNA) (72, 77), Stomach (mRNA) (72), Small intestine (mRNA) (72), Colon (mRNA) (72), Adipose (mRNA) (72), Heart (mRNA) (72), Testis (mRNA) (77)
GPR41 (FFAR3, Gm478)	Propionate (79), Butyrate (79), Pentanoate (79), Acetate (79), Formate (79)	AR420626 (80)	Gαi/o (81), p38JNKGβγ (82), PLCb, MAPK, p38, JNK	White adipose (mRNA) (79, 83), Small intestine (mRNA) (16), Pancreas (mRNA) (79, 84), Spleen (mRNA, protein) (79, 85,86), PBMCs (mRNA) (79), Placenta (mRNA) (79), Lung (mRNA, protein) (79, 85), Pituitary (mRNA) (79)v Brain (mRNA) (79), Liver (mRNA, protein) (79, 85), Stomach (mRNA) (79), Kidney (mRNA) (79, 86), Bone marrow (mRNA) (79), Prostate (mRNA) (79, 85), Colon (mRNA, protein) (87)
GPR43 (FFAR2)	Acetate (79), Propionate (79), Butyrate (79), Pentanoate (79), Hexanoate (79), Formate (79)	Compound 1 (80), Compound 2 (80), Compound 3 (80), GLPG0974 (80), CATPB (80), Cmp71 (80), MeCmp71 (80), BTI-A-404 (80), BTI-A-292 (80), 4-CMTB (80), AZ1729 (80)	Gαi/o (80), Gαq/11(80), Gβγ(80), p38 (87), JNK (87), β-Arrestin (80), NF-κB	Immune cells (mRNA, protein) (79, 85), Pancreas (mRNA) (88), Liver (protein) (85), Small intestine (Protein) (89), Mucosal mast cells (protein) (89), Adipose (mRNA) (86, 90), Colon (mRNA, protein) (86, 89), Spleen (mRNA, protein) (85,86), Stomach (mRNA) (86), Lung (mRNA, protein) (85,86), Heart (mRNA) (140) Muscle (mRNA) (140) Bone marrow (mRNA) (90)
GPR40 (FFAR1)	Medium-/long-chain FFAs (91–93)	Fasiglifam (40), RLA-8 (88), SCO-267 (94), Fezagepras (95)	Gαq (96)	Pancreas (mRNA, protein) (91, 97,98), Brain (mRNA) (91), Hepatocytes (mRNA) (99), Immune cells (mRNA) (91), Small intestine (mRNA, protein) (57, 100), Taste buds (protein) (101)

cCPDA, cis-1,2-cyclopropanedicarboxylic acid; cESA, cis-epoxysuccinic acid; Compound 1, 3-benzyl-4-(cyclopropyl-(4-(2,5-dichlorophenyl)thiazol-2-yl)amino)-4-oxobutanoic acid; Compound 2, (R)-3-(cyclopentylmethyl)-4-(cyclopropyl-(4-(2,6-dichlorophenyl)thiazol-2-yl)amino)-4-oxobutanoic acid; Compound 3, (2S,5R)-5-(2-chlorophenyl)-1-1(2′-methoxy-[1,1′-biphenyl]-4-carbonyl)pyrrolidine-2-carboxylic acid; LEA, linoleoyl ethanolamide; LPA, lysophosphatidic acid; LPEA, lysophosphatidylethanolamine; LPC, lysophosphatidylcholines; LPI, lysophosphatidylinositol; LPS, lysophosphatidylserine; OEA, oleoylethanolamide; 2OG, 2-oleoylglycerol; PEA, palmitoylethanolamide; ODA, N-oleoyl-dopamine; NADA, N-arachidonoyl-dopamine; 2-AG, 2-arachidonoyl glycerol; 1-LG, 1-linoleoyl glycerol; AM251, 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide; AM281, 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide; SR141716, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichloro-phenyl)-4-methyl-pyrazole-3-carboxamide; HU210, (−) 11-OH-8-tetrahydrocannabinol-dimethylheptyl; JWH015, (2-methyl-1-propyl-1H-indol-3-yl)-1-naphthalenylmethanone; CP55940, (−)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol; O1602, 5-methyl-4-[(1R,6R)-3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]-1,3-benzenediol; ML‐193, N-[4-[[(3,4-dimethyl-5-isoxazolyl)amino]sulfonyl]phenyl]-6,8-dimethyl-2-(2-pyridinyl)-4-quinolinecarboxamide; O‐1918, 1,3-dimethoxy-5-methyl-2-[(1R,6R)-3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]benzene; CID16020046, 4-[4,6-dihydro-4-(3-hydroxyphenyl)-3-(4-methylphenyl)-6-oxopyrrolo[3,4-c]pyrazol-5(1H)-yl]benzoic acid; PSN632408, tert-butyl 4-((3-(pyridin-4-yl)-1,2,4-oxadiazol-5-yl)methoxy)piperidine-1-carboxylate; AR231453, (2-fluoro-4-methanesulfonylphenyl)-(6-[4-(3-isopropyl-[1,2,4]oxadiazol-5-yl)-piperidin-1-yl]-5-nitropyrimidin-4-yl)amine, AR-231,453, AR231453, N-(2-fluoro-4-(methylsulfonyl)phenyl)-6-(4-(3-isopropyl-1,2,4-oxadiazol-5-yl)piperidin-1-yl)-5-nitropyrimidin-4-amine; APD668, isopropyl 4-(1-(2-fluoro-4-(methylsulfonyl)phenyl)-1H-pyrazolo[3,4-d]pyrimidin-4-yloxy)piperidine-1-carboxylate; APD597, 4-[[5-methoxy-6-[[2-methyl-6-(methylsulfonyl)-3-pyridinyl]amino]-4-pyrimidinyl]oxy]-1-piperidinecarboxylic acid, 1-methylethyl ester; GSK-1292263, 3-isopropyl-5-(4-(((6-(4-(methylsulfonyl)phenyl)pyridin-3-yl)oxy)methyl)piperidin-1-yl)-1,2,4-oxadiazole; MBX-2982, 2-[1-(5-ethylpyrimidin-2-yl)piperidin-4-yl]-4-[[4-(tetrazol-1-yl)phenoxy]methyl]-1,3-thiazole; PSN632408, 4-[[3-(4-pyridinyl)-1,2,4-oxadiazol-5-yl]methoxy]-1-piperidinecarboxylic acid, 1,1-dimethylethyl ester; CpdA; N-[(3-methylimidazol-4-yl)methyl]-1-[5-methyl-4-(2-thienyl)pyrimidin-2-yl]-5-propyl-pyrazole-4-carboxamide); LY3325656, N-((2S,4R)-2-(5-(1,4-dimethyl-1H-imidazol-5-yl)-4H-1,2,4-triazol-3-yl)tetrahydro-2H-pyran-4-yl)-N-methyl-3-(trifluoromethyl)benzamide; AR420626, N-(2,5-dichlorophenyl)-4-(furan-2-yl)-2-methyl-5-oxo-1,4,5,6,7,8-hexahydro-quinoline-3-carboxamide; GLPG0974; CATPB, (s)-3-(2-(3-chlorophenyl)acetamido)-4-(4-(trifluoromethyl)phenyl)butanoic acid; Cmp71, (4-(1-(benzo[b]thiophene-3-carbonyl)-2-methyl-N-(4-trifluoromethylbenzyl)azetidine-2-carboxamido)butanoic acid); MeCmp71, (methyl 4-(1-(benzo[b]thiophene-3-carbonyl)-2-methyl-N-(4-trifluoromethylbenzyl)azetidine-2-carboxamido)butanoate); BTI-A-404, [4-[4-(dimethylamino)phenyl]-N-(3,5-dimethylphenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydro-5-pyrimidinecarboxamide]; BTI-A-292, [4-[4-(dimethylamino) phenyl]-N-(4,5-dimethylphenyl)-6-methyl-2-oxo-1,2,3,4-tetrahydro-5-pyrimidinecarboxamide]; 4-CMTB, 4-Chloro-α-(1-methylethyl)-N-2-thiazolyl-benzeneacetamide; AZ1729, 4-fluoro-N-[3-[2-[(aminoiminomethyl)amino]-4-methyl-5-thiazolyl]phenyl]benzamide; RLA-8, (E)-8-(3-methoxy-5-(4-methoxystyryl)phenoxy)octanoic acid; SCO-267, (3S)-3-cyclopropyl-3-{2-[(1-{2-[(2,2-dimethylpropyl)(6-methylpyridin-2-yl)carbamoyl]-5-methoxyphenyl}piperidin-4-yl)methoxy]pyridin-4-yl}propanoic acid; CLP-3094, 2-((2-(4-chlorophenoxy)ethyl)thio)-1H-benzo[d]imidazole; SPC, sphingosylphosphorylcholine.

Since its discovery, GPR91 has been shown to be widely expressed (Table 1), most notably within the kidneys (17, 23, 24), liver (19, 25), and white adipose tissue (27) of humans and rodents. Given its extensive expression pattern, this receptor is implicated in multiple physiological processes, including the mediation of blood pressure and plasma renin levels (13), lipolysis (27), and angiogenesis (26). Of interest, GPR91 has been localized to quiescent HSCs (25), the activation of which has been linked to succinate-GPR91 signaling and the release of proinflammatory cytokines IL-6 and TNF-α (105), thus contributing to extensive hepatocyte injury and the fibrogenic response seen in patients with NAFLD and NASH (103, 106). TCA cycle activity and its intermediate metabolites are increased under conditions associated with NAFLD (107), suggesting increased activation of GPR91 through succinate as a result. Indeed, in cultured HSCs treated with succinate, GPR91 protein expression was upregulated, accompanying an increase in activity of α-smooth muscle actin (α-SMA), a marker of fibrogenesis. This observation was matched in murine HSCs isolated from C57BL/6 mice fed a methionine/choline-deficient (MCD) diet, a technique used to induce NASH in animal models, and in liver biopsies from patients with NAFLD (106, 108). In both cases, GPR91 expression correlated with areas of increased fibrosis and markedly elevated the levels of succinate, collectively implicating GPR91 signaling in the fibrotic progression associated with NASH (Fig. 1). These results, matched with the noted upregulation of TCA cycle intermediates associated with NAFLD, indicate a direct relationship between this receptor and these disorders.

Figure 1.

G protein-coupled receptor (GPCR) signaling within the liver, small intestine, adipose tissue, and pancreas all contribute to the development of, or protection from, nonalcoholic fatty liver disease (NAFLD). By promoting insulin release and signaling, regulating lipogenesis and fibrogenesis, impacting hepatic stellate cell activation, and modulating inflammation and the inflammasome, these receptors highlight known and emerging hepatic and metabolic regulatory pathways. The numbers represent the primary sites of expression for the GPCRs (listed by GPR number) included in this review. Green arrows indicate pathways and/or proteins that are activated, whereas red arrow butts represent pathways that are downregulated upon GPCR activation. GLP-1, glucagon-like peptide 1. Figure was created with BioRender.

GPR55 (LPIR1)

GPR55 belongs to the Class A family of GPCRs, sharing low-sequence homology with cannabinoid receptors CB1 and CB2 (35, 39, 109, 110). As such, it has been suggested as a novel cannabinoid receptor (31, 32) and studied extensively in the context of physiological processes involving the endocannabinoid system, notably energy balance and glucose homeostasis (45, 111, 112). Its ability to bind common endocannabinoids (35), atypical cannabinoids (33), and cannabinoid antagonists AM251 and SR141716A (34) have reinforced this belief. However, identification of its endogenous ligand, l-α-lysophosphatidylinositol (LPI), a lipid and noncannabinoid mediator, has led some to question its suggested status as a true cannabinoid receptor (113). To date, the list of GPR55 agonists continues to grow (Table 1) without a clear consensus, although signaling has primarily been attributed to activation of Gα₁₂ and Gα_q family proteins and RhoA (35, 42, 49). Although mRNA expression has been detected throughout the body (Table 1), protein expression has notably been localized to adipose tissue (47), β-cells of the pancreas (48), and the liver of humans and rodents (47), with hepatic expression correlating with levels of circulating LPI (36, 114).

Levels of hepatic LPI have been found to be increased upon consumption of a high-fat diet (HFD) and other “Western” diets (115). In turn, LPI and GPR55 have collectively been implicated in NAFLD, NASH, diabetes, and obesity. Both GPR55 expression and LPI levels are upregulated in the visceral adipose tissue of obese and diabetic patients, with LPI inducing upregulation of genes involved in fatty acid synthesis and adipocyte differentiation (47). LPI has additionally been shown to decrease mRNA expression of several markers of lipopolysaccharide (LPS)-induced macrophage activation through GPR55, notably IL-1β, IL-6, and COX-2, in a murine macrophage cell line, showcasing a possible role of LPI-GPR55 signaling in overall inflammation and macrophage regulation that requires further exploration (116). With regard to NAFLD and NASH, a recent study observed a marked increase in GPR55 expression and circulating levels of LPI in patients with NAFLD and in both HFD- and MCD-fed mice (117). This is further supported by in vitro and in vivo data where LPI-treated hepatic cell lines, THLE2 and HepG2, and wild-type C57BL/6 mice exhibited an upregulation of de novo lipogenesis and β-oxidation. Both of these processes were negated upon silencing of GPR55. In addition, knockdown of hepatic GPR55 ameliorated carbon tetrachloride (CCl₄) injection- or diet-induced liver damage and steatosis in animal models. Finally, GPR55 mRNA was additionally upregulated in primary HSCs isolated from MCD-fed mice, suggesting a possible role for this receptor in their activation and consequent contribution to fibrosis (117). Collectively, these results implicate GPR55 and LPI in the development of hepatic fibrosis and steatosis.

GPR55 has also been implicated in insulin signaling. In rat and human hepatic cell lines H4IIE and HepG2, respectively, treatment with LPI resulted in augmented insulin signaling and increased insulin sensitivity that was subsequently negated upon treatment with GPR55 antagonists (46). Interestingly, whole body GPR55-knockouts (KOs) fed a normal chow diet demonstrated upregulation of lipogenic proteins, impaired insulin signaling, hyperglycemia, and steatosis (46). This detrimental action following the loss of GPR55 contradicts the protective effect exhibited in aforementioned liver-specific knockdowns under HFD or MCD diet conditions. These effects seem to diverge depending on where GPR55 is removed (118), showcasing a possible discrepancy between this receptor’s role in the regulation of insulin signaling in the liver and peripheral metabolic tissues, including adipose.

GPR119 (GPCR2)

GPR119 is a novel cannabinoid receptor that is broadly tuned (see Table 1) but responds best to endogenous ligands oleoylethanolamide (OEA), linoleoyl ethanolamide (LEA), and 2-oleoylglycerol (2-OG) (56, 183). Activation of GPR119 has been shown to lead to increases in cAMP, intracellular calcium, and inositol trisphosphate (IP3) turnover, by coupling with Gα_s, Gα_i, and Gα_q subunits (55). Activation of GPR119 also initiates G protein-independent β-arrestin recruitment (55). GPR119 is predominantly expressed in the α and β cells of the pancreas but is also found in the stomach, small intestine, colon (52), and hepatocytes (Table 1) (51). Ethanolamines, such as OEA and LEA, are crucial lipid nutrient sources that affect lipid metabolism and short-chain fatty acid (SCFA) synthesis (184).

Little is known about ethanolamines and their link to the progression of NAFLD or T2D; however, it is worth noting that phosphatidyl-ethanolamine is the only de novo source of the essential nutrient choline, and choline deficiency can lead to hepatic steatosis (185, 186). LEA was found to suppress the proinflammatory effects of LPS in mouse macrophages, suggesting that GPR119 may affect TLR signaling. Most notably, TNF-α levels decreased in response to LEA in a dose-dependent manner in LPS-treated mouse macrophages (187). OEA is a lipid amide whose synthesis is modulated by bile acids and has been shown to induce satiety and decreased serum cholesterol and triglyceride levels through agonism of PPAR-α (188). Several studies have shown that activation of GPR119 by both OEA and 2-OG, as well as synthetic agonists, has been shown to stimulate GLP-1 release from L cells (56, 189, 190) (Fig. 1). It also bears mentioning that OEA treatment altered the microbiome of mice after only 11 days of dietary treatment, resulting in enriched populations of Bacteroides and reduced populations of Lactobacillus (191).

2-OG is a monoacylglycerol derived from the digestion of triacylglycerol. This is particularly remarkable in consideration of the fact that some microbiota can alter 2-OG content, such as Akkermansia muciniphila (192). In a small human trial, 2-OG bolus increased plasma GLP-1 and GIP compared with controls (189).

Agonism of GPR119 has also been shown to elicit insulin release in vitro, as well as to improve glucose tolerance and increase gastric inhibitory peptide (GIP) levels in a mouse model (193). Synthetic agonists have been shown to increase glucagon release in rat pancreata and human islets when perfused with both low (2 mmol/L) and high (12 mmol/L) concentrations of glucose, and pretreatment with agonist in a rat model of hypoglycemia provoked an increased glucagon secretion in a dose-dependent manner, implying that GPR119 activation can promote glucagon release independent of glucose levels (194) (Fig. 1). GPR119 is highly expressed in human islets; in mouse islets from whole body glucagon and GLP-1 receptor deletions, GPR119 expression is upregulated (195).

GPR119 has also been linked to steatosis (Fig. 1). In cultured hepatocytes, a synthetic agonist inhibited SREBP-1 protein expression. In a high trans-fat diet mouse model of NASH, a synthetic GPR119 agonist reduced plasma AST, ALT, and cholesterol as well as epididymal fat mass (196). Similarly, when this agonist was administered orally to high-fat-diet-fed mice, wild-type mice showed decreased hepatic lipid accumulation, and this was abolished in GPR119 KO mice (51). Taken together, these studies suggest that GPR119 may play a key role in lipogenesis and steatosis in NAFLD.

GPR109a (HCA2, MB74b)

For over 60 years, niacin (nicotinic acid), a water-soluble vitamin, has been used to treat cardiovascular disease due to its anti-inflammatory and anti-lipolytic properties (119). However, the mechanism(s) of action for this therapeutic wasn’t established until much more recently. Initial reports found that an orphan receptor, GPR109a (HCA2 or HM74b), can be activated by niacin via the Gα_i inhibitory signaling pathway (64). Since then, two additional endogenous ligands have been identified: butyrate, a major SCFA produced by gut microbiota, and β-hydroxybutyrate (BHB), the predominant ketone body and by-product of β-oxidation (60, 62, 63). In addition, the action of several pharmacological compounds has been linked to GPR109a (Table 1), and all of these agonists have beneficial effects. In keeping with this, GPR109a has a broad and diverse tissue distribution (Table 1) (60, 64, 67–70, 120). Notably, this receptor is expressed in adipocytes, pancreatic β-cells, and macrophages with activation and subsequent inhibition of adenylate cyclase/PKA linked to a reduction in lipolysis and circulation of free fatty acids (61, 62, 67–69, 121). Use of whole animal KOs and isolated cell lines has linked GPR109a activation to adiposity, lipid accumulation, and inflammation. GPR109a KO mice exhibit significant weight gain, visceral fat accumulation, a reduction in regulatory T cells, and an increase in inflammatory cytokine production, all of which can have indirect effects on the liver and liver function (61, 62, 67–69, 121).

Although GPR109a signaling appears to cast a wide net, it also has direct effects on the liver. Our laboratory and others have reported that GPR109a is expressed in the liver, with protein detected in hepatocytes, HSCs, and Kupffer cells (67, 120). The latter exhibits the highest rate of expression, which aligns well with its localization to macrophages and other cells of the immune system. However, hepatic localization is the most interesting when it comes to assessing its role in NAFLD and NASH. GPR109a expression decreases during the aging process (120), and loss of this receptor within the liver leads to an increase in lipid accumulation, circulation of liver enzymes (AST and ALT), liver weight, and triglyceride accumulation (62, 120) (Fig. 1). It should be noted that studies thus far have utilized a whole animal KO; thus, it is likely that some of these phenotypes are due to complete loss of receptor function. Consumption of an HFD to induce diet-induced obesity and early stages of T2D and NAFLD has been shown to increase hepatic GPR109a expression (67). This likely serves as a protective mechanism given that activation by any of its three ligands decreases de novo hepatic lipogenesis.

Although basal levels of GPR109a in hepatocytes remain low, recent studies have shown that its expression is also increased upon exposure to inflammatory stimuli to combat widespread inflammation (122). Indeed, BHB (but not butyrate) has been shown to prevent NLRP3 inflammasome activation in bone marrow-derived macrophages, although this was found to be independent of GPR109a (123). Nonetheless, many endogenous or exogenous GPR109a ligands have been used for their anti-inflammatory properties, and there is evidence that this is through a receptor-mediated signaling pathway (122–124). Although evidence of attenuated hepatic inflammation is lacking, a recent study did link the butyrate-GPR109a pathway to the mitigation of inflammation in rats fed an HFD (124).

Given the direct role that GPR109a has on the liver, hepatic lipid metabolism, and inflammation, it is tempting to speculate that its signaling pathway serves a protective role against the development of NAFLD and NASH. Although expression is increased upon consumption of an HFD, the availability of its endogenous ligands is often decreased. A reduction in fatty acid oxidation, observed in patients with NAFLD and NASH, leads to a decrease in BHB (6, 7), and NAFLD is associated with an alteration of the gut microbiome (7). Consumption of a “Western diet” has been shown to increase the production of Bacteroidetes and to decrease the abundance of the Firmicutes (125). This change in microbiome diversity has been linked to a decrease in circulating levels of SCFAs, most notably butyrate, which may alter GPR109a activity. Thus, although it is clear that GPR109a signaling promotes anti-inflammatory and antilipolytic pathways, direct experimental studies are required to fully elucidate its important role in hepatic health and disease.

GPR142 (GPRg1b, PGR2)

GPR142 is primarily localized to the α and β cells of the pancreatic islets. It is also found in enteroendocrine cells (EECs, specifically K cells and L cells) (72) and the liver (77) (Table 1). Although GPR142 responds endogenously to aromatic amino acids, most notably l-tryptophan (71), there have also been several synthetic agonists established for use in clinical trials (Table 1). GPR142 has been found to signal through Gα_s (76), Gα_q (73), and Gα_i (73). The Gα_q pathway leads to ERK phosphorylation, whereas the Gα_s and Gα_i coupling modulates intracellular cAMP levels. Activation of GPR142 by a synthetic agonist has been shown to reduce postmeal glucose levels and stimulate glucagon release in humans (Fig. 1). In both control and HFD-induced obese mice, tryptophan improved glucose tolerance and led to glucose-stimulated insulin secretion (GSIS) (72). Although tryptophan’s beneficial effects may be attributed to more than GPR142 activation (197, 198), studies using both tryptophan and a synthetic agonist have shown increased insulin, GIP, GLP-1 secretion, and glucose tolerance in wild-type but not GPR142 KO mice (71, 199).

In addition, GPR142 agonism leads to increased circulating levels of cholecystokinin, which is known to promote hepatic bile secretion and stimulate insulin and glucagon release from the pancreas (200). This effect was abolished in the KOs. Glucagon levels were also increased by administration of a GPR142 agonist, even in GIP receptor-deficient mice (199). A highly selective, synthetic GPR142 agonist also led to glucose-independent glucagon release from murine islets (78). A GPR142 in vitro knockdown study using islet cells also showed decreased release of cytokines including TNF-α (76). A study utilizing a selective synthetic GPR142 agonist showed a significant, dose-dependent relationship between arthritis scores and agonist treatment in a mouse model, though the effect was minor (37). Pancreatic expression of GPR142 is upregulated in both HFD-fed and ob/ob mice, possibly as a compensatory mechanism, highlighting the importance that this signaling pathway may play in obesity (72). It is also worth noting that GPR142 agonist treatment led to a 60% reduction in liver glycogen stores following a 4-h fast in both lean and obese mice compared with vehicle controls (199). Furthermore, long-term treatment with this agonist led to increased energy expenditure and insulin sensitivity in these same mice (199).

GPR41 (FFAR3, Gm478) & GPR43 (FFAR2)

GPR41 and GPR43, otherwise known as free fatty acid receptors (FFAR3 and FFAR2, respectively), both respond to SCFAs and may prove relevant to NAFLD. In addition, there are many synthetic agonists available for both receptors for use in further research (Table 1). Both receptors are most highly expressed in the appendix (201) and have also been found in adipose tissue, small intestine, colon, and hepatocytes (Table 1) (88–93, 201). It is worth noting, however, that there have been contradictory findings in regard to GPR41 expression in adipose (83, 91, 95, 202). GPR43 has also been found highly expressed in leukocytes and other immune cells, as well as murine pancreas (85); GPR43 may play a role in regulating key β-cell genes (84, 202). Although their affinities do differ, both receptors respond similarly well to propionate and butyrate (propionate EC₅₀: GPR43: 290 μM, GPR41: 127 μM; butyrate EC₅₀: GPR43: 371 μM, GPR41: 158 μM), though GPR43 responds more strongly to acetate (EC₅₀: GPR43: 431 μM, GPR41: 1,020 μM) (79). The most likely source of these ligands would be the microbiota.

GPR43 is known to signal through both Gα_i and Gα_q/11 pathways (135) and β-arrestin-2 recruitment (136), leading to downstream phosphorylation of ERK (extracellular signal-regulated kinase) and reduced phosphorylation of nuclear factor-κB (NF-κB), respectively. GPR41 signals through Gα_i/o, which in turn increases TNF-α expression in HepG2 liver cells (81, 82). GRP41 has also been indirectly implicated in AMPK/mTOR/S6K signaling in epithelial cells (126).

Many groups have found GPR43 to be protective against diet-induced obesity (88, 90, 203, 205). On a normal chow diet, GPR43 KO mice are obese; this phenotype has been attributed to the microbiome as germ-free KOs remain slim. Conversely, overexpression of GPR43 within the adipose tissue prevents the development of obesity even with consumption of an HFD (90). Intestinal SCFAs have been shown to promote GLP-1 release and activation of either GPR41 or GPR43 results in an increase in GLP-1 release from L cells (206) (Fig. 1). An in vitro KO study observed reduced GLP-1 release following SCFA treatment in both GPR41 and GPR43 KOs (203). In keeping with this phenotype, mice deficient in either GPR41 or GPR43 were shown to have impaired glucose tolerance and reduced basal levels of active GLP-1 (203). SCFAs have also been shown to stimulate leptin secretion, through activation of both GPR41 and GPR43 in adipocytes (Fig. 1) (83, 204).

GPR43 was also demonstrated to be essential for mediating a healthy inflammatory response in KO mouse models of colitis, arthritis, and asthma. GPR43 KOs showed exacerbated inflammation and increased immune cell recruitment, which held true for germ-free mice as well (15). Conversely, in mouse intestinal epithelial cells, GPR43 and GPR41 KOs showed a reduced inflammatory response. Both receptors were shown to activate ERK1/2 as well as p38 MAPK pathways, thereby increasing chemokine and cytokine levels in the intestine. Notably, KO mice inoculated with Citrobacter rodentium showed increased translocation into the liver (127). These examples are part of a larger trend of inconsistent findings (129) in relation to the roles of GPR41 and GPR43, as they relate to inflammation, highlighting the importance of further research to determine their detrimental or protective functions in NAFLD/NASH.

It should be noted that the effects of SCFAs are multifarious, and both GPR41 and GPR43 are also expressed in the vasculature where another SCFA receptor, Olfr78, resides (207). GPR41 activation in the vascular endothelium leads to a hypotensive response when propionate levels are low (∼150 μM), whereas Olfr78 activation in smooth muscle cells leads to a hypertensive response when propionate levels are high (∼900 μM) (208). GPR43’s role in this process has yet to be explored, nor is it known if this system is relevant to NAFLD (79, 207).

GPR40 (FFAR1)

GPR40 (FFAR1) is another member of the free fatty acid receptor family responding to saturated and unsaturated medium- and long-chain free fatty acids (91, 128), including 6-octadecynoic acid (6-ODA), 9-octadecynoic acid (9-ODA) (93), and linoleic acid (92), among others (Table 1). Evidence of expression has notably been identified within both human (98) and rodent pancreata, specifically localized to pancreatic β-cells (9, 91), and within type L, K, and I EECs of mouse intestines (57, 100). Given its locality, GPR40 has been associated with fatty acid-induced GSIS from pancreatic β-cells (97) and the release of GLP-1 from EECs (100, 131) (Fig. 1) via Gα_q-coupling and signaling through the inositol 1,4,5-trisphosphate and Ca²⁺ pathway (96).

Although evidence for endogenous hepatic protein expression is lacking (95, 130), GPR40 mRNA has been detected and studied in hepatocytes and in the HepG2 cell line (99). Wu et al. (132) demonstrated that HepG2 cells incubated with oleic acid (OA) exhibited steatosis and an induction of GPR40 expression. The subsequent increase in peroxisome proliferator-activated receptor δ (PPARδ) was found to be controlled through a GPR40-mediated pathway. Elevation of circulating FFAs has additionally been shown to promote lipid accumulation and insulin resistance, both of which contribute to hepatic steatosis through a GPR40-mediated pathway. This was evidenced in a study set to elucidate the role of GPR40 in hepatic dysfunction through an overexpression or deletion of this receptor within pancreatic β-cells. In HFD-fed mice overexpressing GPR40, there was a dramatic increase in liver lipid storage; this was not observed in pancreatic GPR40-deficient mice, which were protected from HFD-induced hypertriglyceridemia, increased hepatic glucose output, and hepatic steatosis (130). Taken together, these results suggest a detrimental effect of GPR40 dysregulation concomitant with certain metabolic disorders, such as diabetes and NAFLD, among others.

Conversely, recent studies suggest that GPR40 signaling serves a protective role within the liver. Using a synthetic PPAR-α/γ/δ agonist RLA8 as a novel ligand for GPR40, NASH symptoms, including a reduction in hepatic FFA and triglyceride levels, oxidative stress, and inflammation, were reversed in treated animals (133). This positive action of GPR40 signaling was supported by Gagnon et al. (95) in a study aimed to identify the role of synthetic GPR40 agonist 3-pentylbenzeneacetic acid sodium salt (PBI-4050) in multiorgan fibrosis, including the liver. In mouse models of kidney fibrosis, GPR40 KOs displayed increased interstitial fibrosis as a long-term response, with PBI-4050 treatment only slightly decreasing fibrosis, indicating a protective and necessary role of GPR40 signaling. In a hepatic mouse model of fibrosis induced by CCl₄ administration, treatment with PBI-4050 significantly attenuated fibrosis and AST levels. This result was matched in renal, pancreatic, and cardiac models of fibrosis, among others. Furthermore, synthetic GPR40 agonist SCO-267 was recently shown to decrease liver weight, triglyceride, and collagen content, and levels of plasma ALT upon oral administration in a nondiabetic mouse model of early-stage NAFLD. Elevated mRNA levels of mitochondrial transcription factor A, which assists with mitochondrial regulation, and PPAR-α and long-chain acyl-CoA dehydrogenase, which play a role in the β-oxidation pathway, were also present. Notably, SCO-267 treatment led to inhibition of molecules with roles in lipogenesis, inflammation, reactive oxygen species generation, and liver fibrosis, giving further evidence to a protective role of GPR40 signaling in the case of NAFLD that remains to be explored (134). Collectively, these results indicate antifibrotic, anti-inflammatory, and antiproliferative actions of synthetic GPR40 agonists (95, 134).

Olfactory and Taste Receptors

It is now appreciated that understudied sensory receptors, including olfactory and taste receptors, are expressed in a variety of seemingly “nonsensory” tissues, including the liver. In a recent study into the role of SREBP-1a phosphorylation on liver disease, an analysis of hepatic gene expression found that six of the top 10 differentially regulated genes with the highest significance between phosphorylation-site-deficient mice (which were protected against fatty liver disease) and liver-specific overexpressed SREBP-1a mice (which suffered from obesity and fatty liver) were for olfactory receptors (138). Indeed, several hepatic olfactory receptors have been linked to hepatic steatosis and lipolysis, including Olfr544 and Olfr43. Olfr544, highly expressed in liver and adipose, triggers lipolysis upon activation in diabetic mice. In addition, Olfr544’s ligand azelaic acid stimulates lipolysis in cultured adipocytes and drives fuel preference toward lipids while reducing adiposity in HFD mice (139). Olfr43 activation similarly reduces hepatic lipid accumulation and PPAR-γ expression and stimulates GLP-1 release from EECs (137, 140). Asprosin, a hormone that triggers hepatic glucose production, also modulates gluconeogenesis and adiposity via Olfr734 (141). In addition, Olfr16 responds to α-cedrene, which reduces triglyceride, cholesterol, free fatty acids, and AST/ALT injury markers in HFD-fed mice (142). Unbiased studies from our laboratory have unveiled that the murine liver expresses an additional seven olfactory receptors, some of which have been linked to cholesterol synthesis and antioxidant activity via their newly identified ligands (143).

Taste receptors also have links to liver function and NAFLD. Apart from our study that identified a total of six hepatic bitter taste receptors (143), Tas2r108 is known to improve glucose tolerance and reduces liver adiposity upon chronic treatment with agonist in a mouse model (144). In humans, this Tas2r108 agonist has recently been shown to increase plasma adiponectin (144). Finally, T1R2, a sweet taste receptor, is implicated in lipogenesis and hepatic triglyceride accumulation (145). As additional functions and ligands emerge for these ectopic sensory receptors, the link to NAFLD will only grow stronger.

NON-GPCR SIGNALING

Although the major focus of this review is on the role of GPCRs in the development and treatment of NAFLD, this signal transduction cascade is not alone in its contribution to liver physiology. Indeed, there are a few notable non-GPCRs that are expressed within the liver, all of which contribute in some way to the pathogenesis of NAFLD and NASH. In the following sections, we briefly highlight several classes of notable receptors.

Pattern Recognition Receptors

As their name implies, pattern recognition receptors (PRRs) are activated by molecules produced or expressed by both pathogens and host cells. These include pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) that are derived from microorganisms and host cells, respectively. The most extensively studied PRRs are the TLRs that are widely expressed including on every cell type of the liver (146, 147). Hepatic TLRs respond to LPS, bacterial DNA, and bacterial cell walls. Under normal conditions, TLR signaling in the liver is quite low; dysbiosis and increased gut permeability (both hallmarks of NAFLD) have been shown to increase TLR signaling in both humans and animal models (148–150). This is especially true for TLRs expressed on Kupffer cells and HSCs, as their activation is linked to the production of proinflammatory cytokines and extracellular matrix deposition. Ultimately, activation of hepatic TLRs increases de novo lipogenesis, triglyceride accumulation, and fibrosis (146). Similarly, NOD-like receptors (NLRs) are intracellular PRRs that respond primarily to DAMPs, including free fatty acids. The most notable NLR is NLRP3, the primary protein involved in inflammasome formation (150–152). Increased circulation of free fatty acids leads to the activation of NLRP3 followed by induction of the inflammasome and caspase 1-dependent pyroptosis (123, 151, 153); this pathway is highly active under conditions of NAFLD. Interestingly, BHB, a ligand for GPR109a (discussed in GPR109a (HCA2, MB74b)), can block NLRP3-mediated inflammatory disease (123), whereas consumption of high-dietary fiber (which is fermented into SCFAs) can activate NLRP3 via GPR43 (Fig. 1). These data imply cross talk between the SCFA-sensing GPCRs and the NLR signaling pathways. Finally, there is also evidence that activation of PRRs may be beneficial to liver function. NLRP2, which is downregulated under conditions of steatosis and HFD-feeding, promotes insulin tolerance and offers protection from inflammation and oxidative stress through regulation of NF-κB (154).

Bile Acid Receptors

Primary bile acids are synthesized in the liver through metabolism of cholesterol precursors. These primary bile acids are then stored in the gallbladder and released into the intestine to mediate lipid digestion. Apart from their role in digestion, both primary and secondary bile acids are known to regulate the gene expression of lipid and glucose metabolic enzymes. Nuclear farnesoid x receptor (FXR) is the primary bile acid receptor; it is highly expressed in the liver and mediates transcription of genes associated with lipid and glucose metabolism, and inflammation (155–159). Loss of FXR leads to accumulation of hepatic cholesterol and triglycerides, insulin resistance, and lipogenesis (160, 161). Currently, there is an ongoing phase 3 clinical trial to evaluate whether activation of FXR via obeticholic acid (OCA) is a viable therapeutic for patients with fibrosis due to NASH (162, 163). In addition to FXR, GPBAR1 or TGR5, a GPCR, is activated by secondary bile acids. In the liver, this receptor is excluded from hepatocytes but is found on sinusoidal cells and can offer liver protection during xenobiotic processing (94, 164). Thus far, efforts to generate effective therapeutics via TGR5 agonists have been hampered by off-target effects on gallbladder function. Specific activation of TGR5 within the intestine does give rise to GLP-1 and GLP-2 release, leading to an improvement in hepatic steatosis, insulin sensitivity, and a decrease in hepatic glucose production (165, 166).

Adipokine Receptors

Adipokines are hormones secreted directly from the adipose tissue and exert varied metabolic effects on a variety of tissues. Among the list of these most well-studied peptide hormones include leptin, adiponectin, and resistin (167, 168). Although the effects of secretion are widespread, the liver expresses receptors for both leptin (leptin receptor or OB-Rb) and adiponectin (AdipoR1 and AdipoR2), the latter of which is exclusively expressed in hepatocytes (169). The receptor(s) for resistin are still under investigation. Generally speaking, secretion of leptin corresponds with increased energy expenditure and decreased food intake. Conversely, adiponectin is associated with fatty acid oxidation and the regulation of glucose metabolism (170). With respect to liver function, studies have found that overexpression of AdipoR1 and AdipoR2 can decrease obesity and improve insulin sensitivity in leptin-deficient mice (171). Adiponectin itself increases hepatic β-oxidation and can ameliorate the phenotype of NAFLD (172). Finally, under conditions of HFD-feeding, AdipoR2 expression is lowered, presumably through the regulation by a hepatic microRNA, miR-375 (173). The direct role of the leptin receptor on the liver is a bit less clear. Leptin itself has been shown to decrease hepatic glucose output through a variety of possible mechanisms (174). On its own, loss of hepatic leptin receptor does not alter body weight or circulating levels of glucose or insulin (175). Impaired hepatic leptin signaling does increase hepatic cholesterol and triglyceride accumulation in keeping with the whole body KOs (176). Perhaps counterintuitively, loss of leptin receptor within the liver also improves age- and diet-induced glucose intolerance by increasing insulin sensitivity (176). Given that insulin receptor signaling is associated with lipogenesis, this finding suggests that leptin signaling is a double-edged sword simultaneously promoting insulin sensitivity and lipogenesis. Zonal insulin receptor signaling has been implicated in the development of both hyperglycemia and steatosis (177); it is possible that leptin and hepatic leptin receptor differentially regulate insulin receptor leading to both phenotypes. Regardless, it is clear that adipokine receptors have both direct and indirect effects on the development of NAFLD and offer promising therapeutic avenues for treatment.

CONCLUSIONS

This review highlights the importance of a number of GPCRs (and select non-GPCRs) in signaling pathways vital to the development and progression of NAFLD (Fig. 1). Whether directly or indirectly, the balance of gut microbiota, lipogenesis, β-oxidation, insulin signaling, and incretin and glucagon release via these receptors is necessary in modulating liver health. Their ability to respond to ligands naturally produced by the intestinal microbiota and metabolic pathways of the body underscore their importance. Several GPCRs highlighted in this review respond to SCFAs and other microbiome-derived ligands (GPR109a, GPR41, GPR43) or are known to alter the balance of the microbiome (GPR119). Given this, it is quite possible that signaling cross talk may occur between these GPCRs altering their responses and functions in the context of NAFLD and NASH. Interestingly, a unifying theme among NAFLD therapeutics is their ability to alter the microbiota that may directly modulate GPCR signaling (178–181). In addition, olfactory and taste receptors expressed in nonsensory tissues, most notably within the liver itself, have emerged as additional promising targets. However, future studies utilizing liver-specific KOs are required if hepatic signaling is to be separated from the systemic effects resulting from these interconnected pathways. Finally, modulation of incretin signaling, particularly via GLP-1 agonists, has emerged as a promising therapeutic for both patients with diabetes and NAFLD (50, 182). A number of the GPCRs highlighted here induce GLP-1 release (GPR119, GPR142, GPR41, GPR43, GPR40), and given the number of pharmacological and synthetic agonists that have been developed for these and other GPCRs (Table 1), additional therapeutics—whether via GLP-1 or other modulators of insulin receptor signaling—are no doubt ready to emerge. It is here where future studies would be especially helpful to deduce the role of these receptors in the context of NAFLD and other hepatic disorders, specifically.

GRANTS

This work was supported by National Institutes of Health Grants K01-DK106400 and R03-DK123546 (both awarded to B. D. Shepard) and the Dekkers Endowed Chair in Human Science (to B. D. Shepard).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

M.F.A. prepared figures; R.K., M.F.A., and B.D.S. drafted manuscript; R.K., M.F.A., and B.D.S. edited and revised manuscript; R.K., M.F.A., and B.D.S. approved final version of manuscript.