Role of free fatty acid receptors in the regulation of energy metabolism

Free fatty acids (FFAs) are energy-generating nutrients that act as signaling molecules in various cellular processes. Several orphan G protein-coupled receptors (GPCRs) that act as FFA receptors (FFARs) have been identified and play important physiological roles in various diseases. FFA ligands are obtained from food sources and metabolites produced during digestion and lipase degradation of triglyceride stores. FFARs can be grouped according to ligand profiles, depending on the length of carbon chains of the FFAs. Medium- and long-chain FFAs activate FFA1/GPR40 and FFA4/GPR120. Short-chain FFAs activate FFA2/GPR43 and FFA3/GPR41. However, only medium-chain FFAs, and not long-chain FFAs, activate GPR84 receptor. A number of pharmacological and physiological studies have shown that these receptors are expressed in various tissues and are primarily involved in energy metabolism. Because an impairment of these processes is a part of the pathology of obesity and type 2 diabetes, FFARs are considered as key therapeutic targets. Here, we reviewed recently published studies on the physiological functions of these receptors, primarily focusing on energy homeostasis.     

Free fatty acids inhibit serum deprivation-induced apoptosis through GPR120 in a murine enteroendocrine cell line STC-1

Free fatty acids (FFAs) provide an important energy source and also act as signaling molecules. FFAs are known to exert a variety of physiological responses via their G protein-coupled receptors (GPCRs), such as the GPR40 family. Recently, we identified a novel FFA receptor, GPR120, that promotes secretion of glucagon-like peptide-1 (Hirasawa, A., Tsumaya, K., Awaji, T., Katsuma, S., Adachi, T., Yamada, M., Sugimoto, Y., Miyazaki, S., and Tsujimoto, G. (2005) Nat. Med. 11, 90-94). Here we showed that FFAs inhibit serum deprivation-induced apoptosis of murine enteroendocrine STC-1 cells, which express two types of GPCRs, GPR120 and GPR40, for unsaturated long chain FFA. We first found that linolenic acid potently activated ERK and Akt/protein kinase B (Akt) in STC-1 cells. ERK kinase inhibitors significantly reduced the anti-apoptotic effects of linolenic acid. Inhibitors for phosphatidylinositol 3-kinase (PI3K), a major target of which is Akt, significantly reduced the anti-apoptotic effects. Transfection of STC-1 cells with the dominant-negative form of Akt also inhibited the anti-apoptotic effect. These results suggested that the activation of ERK and PI3K-Akt pathways is required for FFA-induced anti-apoptotic effects on STC-1 cells. Transient transfection of STC-1 cells with GPR120 cDNA, but not GPR40 cDNA, enhanced inhibition of caspase-3 activation. RNA interference experiments showed that reduced expression of GPR120, but not GPR40, resulted in reduced ERK activation and reduced effects of FFAs on caspase-3 inhibition. Collectively, these results demonstrated that FFAs promote the activation of ERK and PI3K-Akt pathways mainly via GPR120, leading to the anti-apoptotic effect of STC-1 cells.     

Inflammatory changes in adipose tissue enhance expression of GPR84, a medium-chain fatty acid receptor: TNFα enhances GPR84 expression in adipocytes

In this study we aimed to identify the physiological roles of G protein-coupled receptor 84 (GPR84) in adipose tissue, together with medium-chain fatty acids (MCFAs), the specific ligands for GPR84. In mice, high-fat diet up-regulated GPR84 expression in fat pads. In 3T3-L1 adipocytes, co-culture with a macrophage cell line, RAW264, or TNFα remarkably enhanced GPR84 expression. In the presence of TNFα, MCFAs down-regulated adiponectin mRNA expression in 3T3-L1 adipocytes. Taken together, our results suggest that GPR84 emerges in adipocytes in response to TNFα from infiltrating macrophages and exacerbates the vicious cycle between adiposity and diabesity.     

G蛋白偶联受体介导游离脂肪酸的信号通路及生理功能

游离脂肪酸（free fatty acid,FFA）是动物一种重要能量来源,同时它还是一种重要的信号分子,其生理功能和作用机制长期以来倍受关注. 最近研究表明,细胞膜存在FFA的特定孤儿型G蛋白偶联膜受体家族.中长链游离脂肪酸是GPR40和GPR120的配基,而短链游离脂肪酸则是GPR41和GPR43的配基. 该受体家族可以介导游离脂肪酸,通过ERK、PI3K-Akt和MAPK信号通路,在维持机体内的葡萄糖稳态、脂肪形成、白细胞功能和细胞增殖等生理过程中发挥重要作用. 本文就游离脂肪酸G蛋白偶联受体的结构、分布、配体选择性、下游信号通路,及其介导FFA生理功能的最新研究进展进行简要综述.     

Free fatty acids regulate gut incretin glucagon-like peptide-1 secretion through GPR120

Diabetes, a disease in which the body does not produce or use insulin properly, is a serious global health problem. Gut polypeptides secreted in response to food intake, such as glucagon-like peptide-1 (GLP-1), are potent incretin hormones that enhance the glucose-dependent secretion of insulin from pancreatic beta cells. Free fatty acids (FFAs) provide an important energy source and also act as signaling molecules in various cellular processes, including the secretion of gut incretin peptides. Here we show that a G-protein-coupled receptor, GPR120, which is abundantly expressed in intestine, functions as a receptor for unsaturated long-chain FFAs. Furthermore, we show that the stimulation of GPR120 by FFAs promotes the secretion of GLP-1 in vitro and in vivo, and increases circulating insulin. Because GLP-1 is the most potent insulinotropic incretin, our results indicate that GPR120-mediated GLP-1 secretion induced by dietary FFAs is important in the treatment of diabetes.     

The FFA receptor GPR40 links hyperinsulinemia, hepatic steatosis, and impaired glucose homeostasis in mouse

Obesity is typically associated with elevated levels of free fatty acids (FFAs) and is linked to glucose intolerance and type 2 diabetes. FFAs exert divergent effects on insulin secretion from beta cells: acute exposure to FFAs stimulates insulin secretion, whereas chronic exposure impairs insulin secretion. The G protein-coupled receptor GPR40 is selectively expressed in beta cells and is activated by FFAs. We show here that GPR40 mediates both acute and chronic effects of FFAs on insulin secretion and that GPR40 signaling is linked to impaired glucose homeostasis. GPR40-deficient beta cells secrete less insulin in response to FFAs, and loss of GPR40 protects mice from obesity-induced hyperinsulinemia, hepatic steatosis, hypertriglyceridemia, increased hepatic glucose output, hyperglycemia, and glucose intolerance. Conversely, overexpression of GPR40 in beta cells of mice leads to impaired beta cell function, hypoinsulinemia, and diabetes. These results suggest that GPR40 plays an important role in the chain of events linking obesity and type 2 diabetes.     

GRK2 selectively attenuates the neutrophil NADPH-oxidase response triggered by β-arrestin recruiting GPR84 agonists

In order to avoid a prolonged pro-inflammatory neutrophil response, signaling downstream of an agonist-activated G protein-coupled receptor (GPCR) has to be rapidly terminated. Among the family of GPCR kinases (GRKs) that regulate receptor phosphorylation and signaling termination, GRK2, which is highly expressed by immune cells, plays an important role. The medium chain fatty acid receptor GPR84 as well as formyl peptide receptor 2 (FPR2), receptors expressed in neutrophils, play a key role in regulating inflammation. In this study, we investigated the effects of GRK2 inhibitors on neutrophil functions induced by GPR84 and FPR2 agonists. GRK2 was shown to be expressed in human neutrophils and analysis of subcellular fractions revealed a cytosolic localization. The GRK2 inhibitors enhanced and prolonged neutrophil production of reactive oxygen species (ROS) induced by GPR84- but not FPR2-agonists, suggesting a receptor selective function of GRK2. This suggestion was supported by β-arrestin recruitment data. The ROS production induced by a non β-arrestin recruiting GPR84 agonist was not affected by the GRK2 inhibitor. Termination of this β-arrestin independent response relied, similar to the response induced by FPR2 agonists, primarily on the actin cytoskeleton. In summary, we show that GPR84 utilizes GRK2 in concert with β-arrestin and actin cytoskeleton dependent processes to fine-tune the activity of the ROS generating NADPH-oxidase in neutrophils.     

Similarities and differences between the responses induced in human phagocytes through activation of the medium chain fatty acid receptor GPR84 and the short chain fatty acid receptor FFA2R

GPR84 is a recently de-orphanized member of the G-protein coupled receptor (GPCR) family recognizing medium chain fatty acids, and has been suggested to play important roles in inflammation. Due to the lack of potent and selective GPR84 ligands, the basic knowledge related to GPR84 functions is very limited. In this study, we have characterized the GPR84 activation profile and regulation mechanism in human phagocytes, using two recently developed small molecules that specifically target GPR84 agonistically (ZQ16) and antagonistically (GLPG1205), respectively. Compared to our earlier characterization of the short chain fatty acid receptor FFA2R which is functionally expressed in neutrophils but not in monocytes, GPR84 is expressed in both cell types and in monocyte-derived macrophages. In neutrophils, the GPR84 agonist had an activation profile very similar to that of FFA2R. The GPR84-mediated superoxide release was low in naïve cells, but the response could be significantly primed by TNFα and by the actin cytoskeleton disrupting agent Latrunculin A. Similar to that of FFA2R, a desensitization mechanism bypassing the actin cytoskeleton was utilized by GPR84. All ZQ16-mediated cellular responses were sensitive to GLPG1205, confirming the GPR84-dependency. Finally, our data of in vivo transmigrated tissue neutrophils indicate that both GPR84 and FFA2R are involved in neutrophil recruitment processes in vivo.In summary, we show functional similarities but also some important differences between GPR84 and FFA2R in human phagocytes, thus providing some mechanistic insights into GPR84 regulation in blood neutrophils and cells recruited to an aseptic inflammatory site in vivo.     

Kynurenic acid as a ligand for orphan G protein-coupled receptor GPR35

Local catabolism of the essential amino acid tryptophan is considered an important mechanism in regulating immunological and neurological responses. The kynurenine pathway is the main route for the non-protein metabolism of tryptophan. The intermediates of the kynurenine pathway are present at micromolar concentrations in blood and are regulated by inflammatory stimuli. Here we show that GPR35, a previously orphan G protein-coupled receptor, functions as a receptor for the kynurenine pathway intermediate kynurenic acid. Kynurenic acid elicits calcium mobilization and inositol phosphate production in a GPR35-dependent manner in the presence of G(qi/o) chimeric G proteins. Kynurenic acid stimulates [35S]guanosine 5'-O-(3-thiotriphosphate) binding in GPR35-expressing cells, an effect abolished by pertussis toxin treatment. Kynurenic acid also induces the internalization of GPR35. Expression analysis indicates that GPR35 is predominantly detected in immune cells and the gastrointestinal tract. Furthermore, we show that kynurenic acid inhibits lipopolysaccharide-induced tumor necrosis factor-alpha secretion in peripheral blood mononuclear cells. Our results suggest unexpected signaling functions for kynurenic acid through GPR35 activation.     

Functional Analysis of Free Fatty Acid Receptor GPR120 in Human Eosinophils: Implications in Metabolic Homeostasis

Recent evidence has shown that eosinophils play an important role in metabolic homeostasis through Th2 cytokine production. GPR120 (FFA4) is a G protein-coupled receptor (GPCR) for long-chain fatty acids that functions as a regulator of physiological energy metabolism. In the present study, we aimed to investigate whether human eosinophils express GPR120 and, if present, whether it possesses a functional capacity on eosinophils. Eosinophils isolated from peripheral venous blood expressed GPR120 at both the mRNA and protein levels. Stimulation with a synthetic GPR120 agonist, GW9508, induced rapid down-regulation of cell surface expression of GPR120, suggesting ligand-dependent receptor internalization. Although GPR120 activation did not induce eosinophil chemotactic response and degranulation, we found that GW9508 inhibited eosinophil spontaneous apoptosis and Fas receptor expression. The anti-apoptotic effect was attenuated by phosphoinositide 3-kinase (PI3K) inhibitors and was associated with inhibition of caspase-3 activity. Eosinophil response investigated using ELISpot assay indicated that stimulation with a GPR120 agonist induced IL-4 secretion. These findings demonstrate the novel functional properties of fatty acid sensor GPR120 on human eosinophils and indicate the previously unrecognized link between nutrient metabolism and the immune system.     

Characterization of an orphan G protein-coupled receptor, GPR20, that constitutively activates Gi proteins

GPR20 was isolated as an orphan G protein-coupled receptor from genomic DNA by PCR amplification. Although GPR20 was closely related to nucleotide or lipid receptors, the functional role of this receptor, as well as its endogenous ligand, remains unclear. Here we demonstrate that GPR20 is constitutively active in the absence of ligand, leading to continuous activation of its coupled G proteins. When GPR20 was exogenously expressed in HEK293 cells, both the basal level and the prostaglandin E(2)-induced production of cAMP were significantly decreased. A remarkable increase in [(35)S]guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) binding to membrane preparations was also observed in GPR20-expressing cells. These effects of GPR20 overexpression were diminished in cells treated with pertussis toxin, suggesting that the expression of GPR20 results in the activation of G(i/o) proteins. Involvement of GPR20 in the activation of G(i/o) proteins was also supported by evidence that the disruption of a conserved DRY motif in GPR20 attenuated both [(35)S]GTPgammaS incorporation and inhibition of the prostaglandin E(2)-induced cAMP production. Knockdown of GPR20 in PC12h cells resulted in an elevation of the basal cAMP level, suggesting that the endogenous GPR20 achieves a constitutively or spontaneously active conformation. Furthermore, enhancement of [(3)H]thymidine incorporation was also observed in the GPR20-silencing cells, implying that the GPR20 expression seems to attenuate PC12h cell growth. Taken together, these data indicate that GPR20 constitutively activates G(i) proteins without ligand stimulation. The receptor may be involved in cellular processes, including control of intracellular cAMP levels and mitogenic signaling.     

The relationship between GPR40 and lipotoxicity of the pancreatic β-cells as well as the effect of pioglitazone

Free fatty acids (FFAs) acutely stimulate insulin secretion from pancreatic β-cells, whereas impair β-cell function following long term exposure. GPR40, a FFAs receptor, has been demonstrated to be activated by both medium and long chain FFAs and played an important role in insulin release. This study was performed to determine the contribution of GPR40 to short- and/or long-term effects of FFAs on glucose-stimulated insulin secretion (GSIS) and the expression of PDX-1 and GLUT2 in pancreatic β-cells, as well as the intervenient effects of pioglitazone on lipotoxicity of β-cells. βTC6 cell line stably expressing GPR40shRNA were established and the intervention of FFAs and pioglitazone on GSIS and expression of PDX-1 and GLUT2 in βTC6 cells was investigated. Results showed that 1-h exposure to FFAs significantly enhanced GSIS and increased expression of PDX-1 and GLUT2 in pSilencer-control transfected cells, but not in cells transfected with GPR40shRNA. While 48-h exposure to FFAs significantly impaired GSIS in pSilencer-control transfected cells as well as cells transfected with GPR40shRNA. Furthermore, pioglitazone enhanced insulin secretion in pSilencer-control transfected cells exposed to FFAs for 48 h, but not in cells transfected with GPR40shRNA. These results indicate that GPR40 mediates the short-term effects of FFAs on GSIS, but does not mediate the chronic lipotoxicity on β-cells. The reverse role of pioglitazone on lipotoxicity of β-cells may be related to GPR40.     

Identification of residues important for agonist recognition and activation in GPR40

GPR40 was formerly an orphan G protein-coupled receptor whose endogenous ligands have recently been identified as free fatty acids (FFAs). The receptor, now named FFA receptor 1, has been implicated in the pathophysiology of type 2 diabetes and is a drug target because of its role in FFA-mediated enhancement of glucose-stimulated insulin release. Guided by molecular modeling, we investigated the molecular determinants contributing to binding of linoleic acid, a C18 polyunsaturated FFA, and GW9508, a synthetic small molecule agonist. Twelve residues within the putative GPR40-binding pocket including hydrophilic/positively charged, aromatic, and hydrophobic residues were identified and were subjected to site-directed mutagenesis. Our results suggest that linoleic acid and GW9508 are anchored on their carboxylate groups by Arg(183), Asn(244), and Arg(258). Moreover, His(86), Tyr(91), and His(137) may contribute to aromatic and/or hydrophobic interactions with GW9508 that are not present, or relatively weak, with linoleic acid. The anchor residues, as well as the residues Tyr(12), Tyr(91), His(137), and Leu(186), appear to be important for receptor activation also. Interestingly, His(137) and particularly His(86) may interact with GW9508 in a manner dependent on its protonation status. The greater number of putative interactions between GPR40 and GW9508 compared with linoleic acid may explain the higher potency of GW9508.     

Production and characterization of a monoclonal antibody against GPR40 (FFAR1; free fatty acid receptor 1)

GPR40 is G protein-coupled receptor whose endogenous ligands have recently been identified as free fatty acids (FFAs), and it has been implicated to play an important role in FFA-mediated enhancement of glucose-stimulated insulin release. We have developed a monoclonal antibody against the extracellular domain of GPR40. Specificity of the antibody was demonstrated by immunoprecipitation and cell surface staining using GPR40-transfected cells. GPR40 immunoreactivity was highly abundant in mouse pancreatic β-cells and splenocytes, THP-1 cells, and human peripheral blood mononuclear cells. The anti-GPR40 monoclonal antibody should prove valuable for further studying the function of this nutrient sensing receptor.     

TRPM5 is critical for linoleic acid-induced CCK secretion from the enteroendocrine cell line, STC-1

Fatty acid-induced stimulation of enteroendocrine cells leads to release of the hormones such as cholecystokinin (CCK) that contribute to satiety. Recently, the fatty acid activated G protein-coupled receptor GPR120 has been shown to mediate long-chain unsaturated free fatty acid-induced CCK release from the enteroendocrine cell line, STC-1, yet the downstream signaling pathway remains unclear. Here we show that linoleic acid (LA) elicits membrane depolarization and an intracellular calcium rise in STC-1 cells and that these responses are significantly reduced when activity of G proteins or phospholipase C is blocked. LA leads to activation of monovalent cation-specific transient receptor potential channel type M5 (TRPM5) in STC-1 cells. LA-induced TRPM5 currents are significantly reduced when expression of TRPM5 or GPR120 is reduced using RNA interference. Furthermore, the LA-induced rise in intracellular calcium and CCK secretion is greatly diminished when expression of TRPM5 channels is reduced using RNA interference, consistent with a role of TRPM5 in LA-induced CCK secretion in STC-1 cells.     

Distinct Kinetic and Spatial Patterns of Protein Kinase C (PKC)- and Epidermal Growth Factor Receptor (EGFR)-dependent Activation of Extracellular Signal-regulated Kinases 1 and 2 by Human Nicotinic Acid Receptor GPR109A

Nicotinic acid (niacin) has been widely used as a lipid-lowering drug for several decades, and recently, orphan G protein-coupled receptor GPR109A has been identified as a receptor for niacin. Mechanistic investigations have shown that, upon niacin activation, GPR109A couples to a G_i protein and inhibits adenylate cyclase activity, leading to inhibition of liberation of free fatty acid. However, the underlying molecular mechanisms for GPR109A signaling remain largely unknown. Using CHO-K1 cells stably expressing GPR109A and A431 cells, which are a human epidermoid cell line with high levels of endogenous expression of functional GPR109A receptors, we found that activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) by niacin was rapid, peaking at 5 min, and was significantly blocked by pertussis toxin. Furthermore, time course experiments with different kinase inhibitors demonstrated that GPR109A induced ERK1/2 activation via the matrix metalloproteinase/epidermal growth factor receptor transactivation pathway at both early and later time points (2–5 min); this pathway was distinct from the PKC pathway-mediated ERK1/2 phosphorylation that occurs at early time points (≤2 min) in response to niacin. Overexpression of Gβγ subunit scavengers βARK1-CT and the Gα subunit of transducin led to a significant reduction of ERK1/2 phosphorylation, suggesting a critical role for βγ subunits in GPR109A-activated ERK1/2 phosphorylation. Using arrestin-2/3-specific siRNA and an internalization-deficient GPR109A mutant, we found that arrestin-2 and arrestin-3 were not involved in GPR109A-mediated ERK1/2 activation. In conclusion, our findings demonstrate that upon binding to niacin GPR109A receptors initially activate G_i, leading to dissociation of the Gβγ subunit from activated G_i, and subsequently induce ERK1/2 activation via two distinct pathways, one PKC-dependent pathway occurring at a peak time of ≤2 min and the other matrix metalloproteinase-dependent growth factor receptor transactivation occurring at both early and later time points (2–5 min).     

Free fatty acid effects on mitochondrial permeability: an overview

A variety of experimental conditions elicit increases in mitochondrial permeability that can be differentiated from the classic cyclosporin A (CsA)-sensitive mitochondrial permeability transition (MPT). For example, butylated hydroxytoluene, signal peptides, and the hormone thyroxine have been shown to promote increases in mitochondrial permeability that are CsA-insensitive. Our laboratory has recently demonstrated that palmitic acid, a saturated 16-carbon free fatty acid (FFA), can also open a CsA-insensitive pore. This nonclassic permeability transition (NCPT) is further distinguished by a nonselective dependence on divalent cations and by spontaneous closure. To determine if induction of the NCPT is specific to palmitic acid and to resolve conflicting reports as to the mechanisms by which FFAs alter mitochondrial permeability, we examined in detail mitochondrial swelling induced by FFAs that differ in chain length and degree of saturation. The following results were obtained: (1) In the presence of modest Ca2+ concentrations (75 nmol/mg protein), medium-chain FFAs (C12-C18) were more effective in eliciting mitochondrial swelling than were shorter or longer FFAs; medium-chain alkanols and amines had no effect. (2) Under these conditions, saturated FFAs induced CsA-insensitive swelling with all the characteristics of the NCPT, while unsaturated FFAs triggered the MPT. (3) When matrix Ca2+ concentration was further elevated, unsaturated FFAs triggered the NCPT. (4) Mitochondrial swelling induced by saturated FFAs was inhibited by unsaturated FFAs but not by other saturated FFAs or medium-chain alkanols. These results suggest that ambient conditions can greatly influence the nature of the increase in mitochondrial permeability induced by FFAs. They are also consistent with our earlier proposal that Ca2+ (or Sr2+) binding to FFAs in the inner leaflet of the inner mitochondrial membrane underlies the NCPT.     

Therapeutic effect of omega-3 fatty acids on T cell-mediated autoimmune diseases

The present study was to demonstrate that the G protein coupled receptors serve as targets for the treatment of autoimmune disease such as rheumatoid arthritis and multiple sclerosis. Rats received pristane at the base of the tail. Affected joints were counted daily. The T cell mediated autoimmune diseases such as pristine-induced arthritis (PIA) and autoimmune encephalomyelitis (EAE) in a rat model were profoundly ameliorated by treatment with the specific G protein couple receptors 120 (GPR120) stimuli omega-3 fatty acids (ω-3 FAs). Our study further revealed that the activation of GPR120 by ω-3 FAs can result in a decrease of phosphorylated transforming growth factor-β activated kinase 1 (TAK1), and further inhibit the downstream IKKβ/I-κB pathway and the terminal NF-κB activation which serves as a mediator of T cell activation. ω-3 Fatty acids exhibited an inhibitory effect on TAK1 by enhancing the association of β-arrestin2 and TAK1 binding protein 1 (TAB1), thus the disassociation of TAB1 from the TAB1/TAK1 complex renders a limited effect on β-arrestin2 signaling as an innate immunity regulation. GPR120 is a functional receptor of ω-3 fatty acids in T cell-mediated autoimmune disease compared with its effect on innate immunity.     

Activation of GPR4 by acidosis increases endothelial cell adhesion through the cAMP/Epac pathway

Endothelium-leukocyte interaction is critical for inflammatory responses. Whereas the tissue microenvironments are often acidic at inflammatory sites, the mechanisms by which cells respond to acidosis are not well understood. Using molecular, cellular and biochemical approaches, we demonstrate that activation of GPR4, a proton-sensing G protein-coupled receptor, by isocapnic acidosis increases the adhesiveness of human umbilical vein endothelial cells (HUVECs) that express GPR4 endogenously. Acidosis in combination with GPR4 overexpression further augments HUVEC adhesion with U937 monocytes. In contrast, overexpression of a G protein signaling-defective DRY motif mutant (R115A) of GPR4 does not elicit any increase of HUVEC adhesion, indicating the requirement of G protein signaling. Downregulation of GPR4 expression by RNA interference reduces the acidosis-induced HUVEC adhesion. To delineate downstream pathways, we show that inhibition of adenylate cyclase by inhibitors, 2',5'-dideoxyadenosine (DDA) or SQ 22536, attenuates acidosis/GPR4-induced HUVEC adhesion. Consistently, treatment with a cAMP analog or a G(i) signaling inhibitor increases HUVEC adhesiveness, suggesting a role of the G(s)/cAMP signaling in this process. We further show that the cAMP downstream effector Epac is important for acidosis/GPR4-induced cell adhesion. Moreover, activation of GPR4 by acidosis increases the expression of vascular adhesion molecules E-selectin, VCAM-1 and ICAM-1, which are functionally involved in acidosis/GPR4-mediated HUVEC adhesion. Similarly, hypercapnic acidosis can also activate GPR4 to stimulate HUVEC adhesion molecule expression and adhesiveness. These results suggest that acidosis/GPR4 signaling regulates endothelial cell adhesion mainly through the G(s)/cAMP/Epac pathway and may play a role in the inflammatory response of vascular endothelial cells.     

GPR120 induces regulatory dendritic cells by inhibiting HK2-dependent glycolysis to alleviate fulminant hepatic failure

Fulminant hepatic failure (FHF) is a potentially fatal liver disease that is associated with intrahepatic infiltration of inflammatory cells. As the receptor of polyunsaturated long chain fatty acids, GPR120 can regulate cell differentiation, proliferation, metabolism, and immune response. However, whether GPR120 is involved in FHF remains unknown. Using Propionibacterium acnes (P. acnes)-primed, LPS-induced FHF in mice, we found that interference with GPR120 activity using pharmacological agonist attenuated the severity of the liver injury and mortality of FHF in mice, while a lack of GPR120 exacerbated the disease. GPR120 activation potently alleviated FHF and led to decreased T helper (Th) 1 cell response and expansion of regulatory T cells (Tregs). Interestingly, GPR120 agonist didn’t directly target T cells, but dramatically induced a distinct population of CD11c⁺MHC II^lowCD80^lowCD86^low regulatory DCs in the livers of FHF mice. GPR120 was found to restrict HIF-1α-dependent glycolysis. The augmented HIF-1α stabilization caused by GPR120 antagonism or deletion could be attenuated by the inhibition of ERK or by the activation of AMPK. Through the analysis of the clinical FHF, we further confirmed the activation of GPR120 was negatively associated with the severity in patients. Our findings indicated that GPR120 activation has therapeutic potential in FHF. Strategies to target GPR120 using agonists or free fatty acids (FFAs) may represent a novel approach to FHF treatment.Subject terms: Inflammation, Hepatitis