Lack of GPR40/FFAR1 does not induce diabetes even under insulin resistance condition

Nutrient regulation of inflammatory signalling in obesity and vascular disease

Despite obesity and diabetes markedly increasing the risk of developing cardiovascular diseases, the molecular and cellular mechanisms that underlie this association remain poorly characterised. In the last 20 years it has become apparent that chronic, low-grade inflammation in obese adipose tissue may contribute to the risk of developing insulin resistance and type 2 diabetes. Furthermore, increased vascular pro-inflammatory signalling is a key event in the development of cardiovascular diseases. Overnutrition exacerbates pro-inflammatory signalling in vascular and adipose tissues, with several mechanisms proposed to mediate this. In this article, we review the molecular and cellular mechanisms by which nutrients are proposed to regulate pro-inflammatory signalling in adipose and vascular tissues. In addition, we examine the potential therapeutic opportunities that these mechanisms provide for suppression of inappropriate inflammation in obesity and vascular disease.

GPR40 deficiency is associated with hepatic FAT/CD36 upregulation, steatosis, inflammation, and cell injury in C57BL/6 mice

G-protein-coupled receptor 40 (GPR40) is highly expressed in pancreatic islets, and its activation increases glucose-stimulated insulin secretion from pancreas. Therefore, GPR40 is considered as a target for type 2 diabetes mellitus (T2DM). Since nonalcoholic fatty liver disease (NAFLD) is associated with T2DM and GPR40 is also expressed by hepatocytes and macrophages, it is important to understand the role of GPR40 in NAFLD. However, the role of GPR40 in NAFLD in animal models has not been well defined. In this study, we fed wild-type or GPR40 knockout C57BL/6 mice a high-fat diet (HFD) for 20 wk and then assessed the effect of GPR40 deficiency on HFD-induced NAFLD. Assays on metabolic parameters showed that an HFD increased body weight, glucose, insulin, insulin resistance, cholesterol, and alanine aminotransferase (ALT), and GPR40 deficiency did not mitigate the HFD-induced metabolic abnormalities. In contrast, we found that GPR40 deficiency was associated with increased body weight, insulin, insulin resistance, cholesterol, and ALT in control mice fed a low-fat diet (LFD). Surprisingly, histology and Oil Red O staining showed that GPR40 deficiency in LFD-fed mice was associated with steatosis. Immunohistochemical analysis showed that GPR40 deficiency also increased F4/80, a macrophage biomarker, in LFD-fed mice. Furthermore, results showed that GPR40 deficiency led to a robust upregulation of hepatic fatty acid translocase (FAT)/CD36 expression. Finally, our in vitro studies showed that GPR40 knockdown by siRNA or a GPR40 antagonist increased palmitic acid-induced FAT/CD36 mRNA in hepatocytes. Taken together, this study indicates that GPR40 plays an important role in homeostasis of hepatic metabolism and inflammation and inhibits nonalcoholic steatohepatitis by possible modulation of FAT/CD36 expression.

Free fatty acid receptors, G protein-coupled receptor 120 and G protein-coupled receptor 40, are essential for oil-induced gastric inhibitory polypeptide secretion

AIMS/INTRODUCTION

Incretin hormone glucose-dependent insulinotropic polypeptide/gastric inhibitory polypeptide (GIP) plays a key role in high-fat diet-induced obesity and insulin resistance. GIP is strongly secreted from enteroendocrine K cells by oil ingestion. G protein-coupled receptor (GPR)120 and GPR40 are two major receptors for long chain fatty acids, and are expressed in enteroendocrine K cells. In the present study, we investigated the effect of the two receptors on oil-induced GIP secretion using GPR120- and GPR40-double knockout (DKO) mice.

MATERIALS AND METHODS

Global knockout mice of GPR120 and GPR40 were crossbred to generate DKO mice. Oral glucose tolerance test and oral corn oil tolerance test were carried out. For analysis of the number of K cells and gene expression in K cells, DKO mice were crossbred with GIP-green fluorescent protein knock-in mice in which visualization and isolation of K cells can be achieved.

RESULTS

Double knockout mice showed normal glucose-induced GIP secretion, but no GIP secretion by oil. We then investigated the number of K cells and gene characteristics in K cells isolated from GIP-green fluorescent protein knock-in mice. Deficiency of both receptors did not affect the number of K cells in the small intestine or expression of GIP messenger ribonucleic acid in K cells. Furthermore, there was no significant difference in the expression of the genes associated with lipid absorption or GIP secretion in K cells between wild-type and DKO mice.

CONCLUSIONS

Oil-induced GIP secretion is triggered by the two major fatty acid receptors, GPR120 and GPR40, without changing K-cell number or K-cell characteristics.

GPR40 full agonism exerts feeding suppression and weight loss through afferent vagal nerve

GPR40/FFAR1 is a Gq protein-coupled receptor expressed in pancreatic β cells and enteroendocrine cells, and mediates insulin and incretin secretion to regulate feeding behavior. Several GPR40 full agonists have been reported to reduce food intake in rodents by regulating gut hormone secretion in addition to their potent glucose-lowering effects; however, detailed mechanisms of feeding suppression are still unknown. In the present study, we characterized T-3601386, a novel compound with potent full agonistic activity for GPR40, by using in vitro Ca²⁺ mobilization assay in Chinese hamster ovary (CHO) cells expressing FFAR1 and in vivo hormone secretion assay. We also evaluated feeding suppression and weight loss after the administration of T-3601386 and investigated the involvement of the vagal nerve in these effects. T-3601386, but not a partial agonist fasiglifam, increased intracellular Ca²⁺ levels in CHO cells with low FFAR1 expression, and single dosing of T-3601386 in diet-induced obese (DIO) rats elevated plasma incretin levels, suggesting full agonistic properties of T-3601386 against GPR40. Multiple doses of T-3601386, but not fasiglifam, in DIO rats showed dose-dependent weight loss accompanied by feeding suppression and durable glucagon-like peptide-1 elevation, all of which were completely abolished in Ffar1^-/- mice. Immunohistochemical analysis in the nuclei of the solitary tract demonstrated that T-3601386 increased the number of c-Fos positive cells, which also disappeared in Ffar1^-/- mice. Surgical vagotomy and drug-induced deafferentation counteracted the feeding suppression and weight loss induced by the administration of T-3601386. These results suggest that T-3601386 exerts incretin release and weight loss in a GPR40-dependent manner, and that afferent vagal nerves are important for the feeding suppression induced by GPR40 full agonism. Our novel findings raise the possibility that GPR40 full agonist can induce periphery-derived weight reduction, which may provide benefits such as less adverse effects in central nervous system compared to centrally-acting anti-obesity drugs.

Connecting pancreatic islet lipid metabolism with insulin secretion and the development of type 2 diabetes

Obesity is the major contributing factor for the increased prevalence of type 2 diabetes (T2D) in recent years. Sustained positive influx of lipids is considered to be a precipitating factor for beta cell dysfunction and serves as a connection between obesity and T2D. Importantly, fatty acids (FA), a key building block of lipids, are a double-edged sword for beta cells. FA acutely increase glucose-stimulated insulin secretion through cell-surface receptor and intracellular pathways. However, chronic exposure to FA, combined with elevated glucose, impair the viability and function of beta cells in vitro and in animal models of obesity (glucolipotoxicity), providing an experimental basis for the propensity of beta cell demise under obesity in humans. To better understand the two-sided relationship between lipids and beta cells, we present a current view of acute and chronic handling of lipids by beta cells and implications for beta cell function and health. We also discuss an emerging role for lipid droplets (LD) in the dynamic regulation of lipid metabolism in beta cells and insulin secretion, along with a potential role for LD under nutritional stress in beta cells, and incorporate recent advancement in the field of lipid droplet biology.

Metformin Reduces Lipotoxicity-Induced Meta-Inflammation in β-Cells through the Activation of GPR40-PLC-IP3 Pathway

BACKGROUND AND PURPOSE

Metformin, a widely used antidiabetic drug, has been shown to have anti-inflammatory properties; nevertheless, its influence on β-cell meta-inflammation remains unclear. The following study investigated the effects of metformin on meta-inflammatory in β-cells and whether the underlying mechanisms were associated with the G protein-coupled receptor 40-phospholipase C-inositol 1, 4, 5-trisphosphate (GPR40-PLC-IP3) pathway.

MATERIALS AND METHODS

Lipotoxicity-induced β-cells and the high-fat diet-induced obese rat model were used in the study.

RESULTS

Metformin-reduced lipotoxicity-induced β-cell meta-inflammatory injury was associated with the expression of GPR40. GPR40 was involved in metformin reversing metabolic inflammation key marker TLR4 activation-mediated β-cell injury. Furthermore, downstream signaling protein PLC-IP3 of GPR40 was involved in the protective effect of metformin on meta-inflammation, and the above process of metformin was partially regulated by AMPK activity. In addition, the anti-inflammatory effects of metformin were observed in obese rats.

CONCLUSION

Metformin can reduce lipotoxicity-induced meta-inflammation in β-cells through the regulation of the GPR40-PLC-IP3 pathway and partially via the regulation of AMPK activity.

Considerations and guidelines for mouse metabolic phenotyping in diabetes research

Mice are the most commonly used species in preclinical research on the pathophysiology of metabolic diseases. Although they are extremely useful for identifying pathways, mechanisms and genes regulating glucose and energy homeostasis, the specificities of the various mouse models and methodologies used to investigate a metabolic phenotype can have a profound impact on experimental results and their interpretation. This review aims to: (1) describe the most commonly used experimental tests to assess glucose and energy homeostasis in mice; (2) provide some guidelines regarding the design, analysis and interpretation of these tests, as well as for studies using genetic models; and (3) identify important caveats and confounding factors that must be taken into account in the interpretation of findings.

20-HETE promotes glucose-stimulated insulin secretion in an autocrine manner through FFAR1

The long-chain fatty acid receptor FFAR1 is highly expressed in pancreatic β-cells. Synthetic FFAR1 agonists can be used as antidiabetic drugs to promote glucose-stimulated insulin secretion (GSIS). However, the physiological role of FFAR1 in β-cells remains poorly understood. Here we show that 20-HETE activates FFAR1 and promotes GSIS via FFAR1 with higher potency and efficacy than dietary fatty acids such as palmitic, linoleic, and α-linolenic acid. Murine and human β-cells produce 20-HETE, and the ω-hydroxylase-mediated formation and release of 20-HETE is strongly stimulated by glucose. Pharmacological inhibition of 20-HETE formation and blockade of FFAR1 in islets inhibits GSIS. In islets from type-2 diabetic humans and mice, glucose-stimulated 20-HETE formation and 20-HETE-dependent stimulation of GSIS are strongly reduced. We show that 20-HETE is an FFAR1 agonist, which functions as an autocrine positive feed-forward regulator of GSIS, and that a reduced glucose-induced 20-HETE formation contributes to inefficient GSIS in type-2 diabetes.

FFAR1 receptor is highly expressed in beta cells and its activation has been suggested as therapy against type-2 diabetes. Here, Tunaru et al. show that 20-hydroxyeicosatetraenoic acid, produced within the islets upon glucose stimulation, acts in an autocrine manner to stimulate insulin secretion via FFAR1 activation.

A novel free fatty acid receptor 1 (GPR40/FFAR1) agonist, MR1704, enhances glucose-dependent insulin secretion and improves glucose homeostasis in rats

Activation of G protein‐coupled receptor 40/Free fatty acid receptor 1 (GPR40/FFAR1), which is highly expressed in pancreatic β cells, is considered an important pharmacologic target for the treatment of type 2 diabetes mellitus. The aim of this study was to determine the effect of MR1704, a novel GPR40/FFAR1 agonist, on glucose homeostasis in rats. MR1704 is a highly potent and selective, orally bioavailable agonist with similar in vitro potencies among humans, mice, and rats. Treatment of rat islets with MR1704 increased glucose‐dependent insulin secretion. Augmentation of glucose‐dependent insulin secretion was abolished by adding a GPR40/FFAR1 antagonist. In mouse, insulinoma MIN6 cells, palmitic acid induced the activity of caspase 3/7 after a 72‐h exposure, while pharmacologically active concentrations of MR1704 did not. In an oral glucose tolerance test in normal Sprague‐Dawley rats, orally administered MR1704 (1–10 mg·kg⁻¹) reduced plasma glucose excursion and enhanced insulin secretion, but MR1704 did not induce hypoglycemia, even at 300 mg·kg⁻¹, in fasted Sprague‐Dawley rats. In addition, orally administered MR1704 reduced plasma glucose excursion and enhanced insulin secretion in diabetic Goto‐Kakizaki rats. Oral administration of MR1704 once daily to Goto‐Kakizaki rats reduced their blood glucose levels during a 5‐week treatment period without reducing pancreatic insulin content; as a result, hemoglobin A1C levels significantly decreased. These results suggest that MR1704 improves glucose homeostasis through glucose‐dependent insulin secretion with a low risk of hypoglycemia and pancreatic toxicity. MR1704 shows promise as a new, glucose‐lowering drug to treat type 2 diabetes mellitus.

Long-Chain Free Fatty Acid Receptor GPR120 Mediates Oil-Induced GIP Secretion Through CCK in Male Mice

Free fatty acid receptors GPR120 and GPR40 are involved in the secretion of gut hormones. GPR120 and GPR40 are expressed in enteroendocrine K cells, and their activation induces the secretion of the incretin glucose-dependent insulinotropic polypeptide (GIP). However, the role of these receptors in fat-induced GIP secretion in vivo and the associated mechanisms are unclear. In this study, we investigated corn oil-induced GIP secretion in GPR120-knockout (GPR120-/-) and GPR40-knockout (GPR40-/-) mice. Oil-induced GIP secretion was reduced by 50% and 80% in GPR120-/- and GPR40-/- mice, respectively, compared with wild-type mice. This was not associated with a significant difference in K-cell number or GIP content in K cells, nor messenger RNA levels of the lipid receptor GPR119, nor bile acid receptors TGR5 and farnesoid X receptor. GPR120-/- and GPR40-/- mice also exhibited substantially decreased levels of cholecystokinin (CCK), a hormone from I cells that promotes bile and pancreatic lipase secretion, and this decrease was associated with impaired gallbladder contraction. Notably, treatment with a CCK analog resulted in recovery of oil-induced GIP secretion in GPR120-/- mice but not in GPR40-/- mice. These results indicate that corn oil-induced GIP secretion from K cells involves both GPR120 and GPR40 signaling pathways, and GPR120-induced GIP secretion is indirectly mediated by CCK.

The Role and Future of FFA1 as a Therapeutic Target

Of the 415 million people suffering from diabetes worldwide, 90% have type 2 diabetes. Type 2 diabetes is characterized by hyperglycemia and occurs in obese individuals as a result of insulin resistance and inadequate insulin levels. Accordingly, diabetes drugs are tailored to enhance glucose disposal or target the pancreatic islet β cell to increase insulin secretion. The majority of the present-day insulin secretagogues, however, increase the risk of iatrogenic hypoglycemia, and hence alternatives are actively sought. The long-chain fatty acid, G protein-coupled receptor FFA1/Gpr40, is expressed in β cells, and its activation potentiates insulin secretion in a glucose-dependent manner. Preclinical data indicate that FFA1 agonism is an effective treatment to restore glucose homeostasis in rodent models of diabetes. This initial success prompted clinical trials in type 2 diabetes patients, the results of which were promising; however, the field suffered a significant setback when the lead compound TAK-875/fasiglifam was withdrawn from clinical development due to liver safety concerns. Nevertheless, recent developments have brought to light a surprising complexity of FFA1 agonist action, signaling diversity, and biological outcomes, raising hopes that with a greater understanding of the mechanisms at play the second round will be more successful.

GPR142 Controls Tryptophan-Induced Insulin and Incretin Hormone Secretion to Improve Glucose Metabolism

GPR142, a putative amino acid receptor, is expressed in pancreatic islets and the gastrointestinal tract, but the ligand affinity and physiological role of this receptor remain obscure. In this study, we show that in addition to L-Tryptophan, GPR142 signaling is also activated by L-Phenylalanine but not by other naturally occurring amino acids. Furthermore, we show that Tryptophan and a synthetic GPR142 agonist increase insulin and incretin hormones and improve glucose disposal in mice in a GPR142-dependent manner. In contrast, Phenylalanine improves in vivo glucose disposal independently of GPR142. Noteworthy, refeeding-induced elevations in insulin and glucose-dependent insulinotropic polypeptide are blunted in Gpr142 null mice. In conclusion, these findings demonstrate GPR142 is a Tryptophan receptor critically required for insulin and incretin hormone regulation and suggest GPR142 agonists may be effective therapies that leverage amino acid sensing pathways for the treatment of type 2 diabetes.

The effect of FFAR1 on pioglitazone-mediated attenuation of palmitic acid-induced oxidative stress and apoptosis in βTC6 cells

OBJECTIVE

We sought to determine whether free fatty acid receptor 1 (FFAR1), a receptor for free fatty acids on the β-cell membrane, can mediate the pioglitazone (PIO)-attenuating effect on lipoapoptosis in β cells and its relationship to oxidative stress.

METHODS

The glucose-sensitive mouse beta pancreatic cell line βTC6 was used to investigate the effect of FFAR1 on PIO-attenuating palmitic acid (PA)-induced oxidative stress and apoptosis.

RESULTS

(1) PIO reduced PA-induced lipoapoptosis in β cells and upregulated the expression of FFAR1 at the mRNA and protein levels in a dose- and time-dependent manner. Silencing of FFAR1 expression was shown to weaken the protective effect of PIO on PA-induced lipoapoptosis in βTC6 cells; while lentiviral-mediated overexpression of FFAR1 was shown to enhance the protective effect of PIO against lipoapoptosis in β cells. (2) Downregulation of FFAR1 expression reduced the attenuating effect of PIO on the expression of NAPDH oxidase subunit p47(phox), Bax, cleaved caspase 3, and the production of reactive oxygen specific (ROS) induced by lipotoxicity, thereby preventing the upregulation of the expression of bcl-2. Inducing the overexpression of FFAR1 enhanced the anti-oxidative stress effect of PIO. Similarly, these effects of FFAR1 on PIO were reproduced under conditions of oxidative stress and apoptosis in βTC6 cells that were induced by H2O2. (3) PIO was found to increase the expression of PLCγ, ERK1/2, and PPARγ in lipotoxic β cells. Silencing FFAR1 expression reduced the PIO-mediated increases in the expression of above proteins; while inducing FFAR1 overexpression showed the opposite effect. Use of an inhibitor of PLCγ, ERK1/2, PPARγ was shown to restrict the protective effect of PIO on oxidative stress and lipoapoptosis of β cells.

CONCLUSIONS

FFAR1 can mediate PIO suppression of β-cell lipoapoptosis through anti-oxidative stress, which may be related to the activation of the PLCγ-ERK1/2-PPARγ pathway.

A novel antidiabetic drug, fasiglifam/TAK-875, acts as an ago-allosteric modulator of FFAR1

Selective free fatty acid receptor 1 (FFAR1)/GPR40 agonist fasiglifam (TAK-875), an antidiabetic drug under phase 3 development, potentiates insulin secretion in a glucose-dependent manner by activating FFAR1 expressed in pancreatic β cells. Although fasiglifam significantly improved glycemic control in type 2 diabetes patients with a minimum risk of hypoglycemia in a phase 2 study, the precise mechanisms of its potent pharmacological effects are not fully understood. Here we demonstrate that fasiglifam acts as an ago-allosteric modulator with a partial agonistic activity for FFAR1. In both Ca²⁺ influx and insulin secretion assays using cell lines and mouse islets, fasiglifam showed positive cooperativity with the FFAR1 ligand γ-linolenic acid (γ-LA). Augmentation of glucose-induced insulin secretion by fasiglifam, γ-LA, or their combination was completely abolished in pancreatic islets of FFAR1-knockout mice. In diabetic rats, the insulinotropic effect of fasiglifam was suppressed by pharmacological reduction of plasma free fatty acid (FFA) levels using a lipolysis inhibitor, suggesting that fasiglifam potentiates insulin release in conjunction with plasma FFAs in vivo. Point mutations of FFAR1 differentially affected Ca²⁺ influx activities of fasiglifam and γ-LA, further indicating that these agonists may bind to distinct binding sites. Our results strongly suggest that fasiglifam is an ago-allosteric modulator of FFAR1 that exerts its effects by acting cooperatively with endogenous plasma FFAs in human patients as well as diabetic animals. These findings contribute to our understanding of fasiglifam as an attractive antidiabetic drug with a novel mechanism of action.