Long-chain fatty acids amplify insulin secretion from the pancreatic beta cell. The G protein-coupled receptor GPR40 is specifically expressed in beta cells and is activated by fatty acids. Loss of function of GPR40 was shown to markedly inhibit fatty-acid stimulation of insulin secretion in vitro. However, the role of GPR40 in acute regulation of insulin secretion in vivo remains unclear. To this aim, we generated GPR40 knock-out (KO) mice and examined glucose homeostasis, insulin secretion in response to glucose and Intralipid in vivo, and insulin secretion in vitro after short-and long-term exposure to fatty acids. Our results show that GPR40 KO mice have essentially normal glucose tolerance and insulin secretion in response to glucose. Insulin secretion in response to Intralipid was reduced by approximately 50%. In isolated islets, insulin secretion in response to glucose and other secretagogues was unaltered, but fatty-acid potentiation of insulin release was markedly reduced. Islets from GPR40 KO mice were as sensitive to fatty-acid inhibition of insulin secretion upon prolonged exposure as islets from wild-type animals. We conclude that GPR40 contributes approximately half of the full insulin secretory response to fatty acids in mice, but does not play a role in the mechanisms of lipotoxicity.Long-chain fatty acids are essential regulators of normal pancreatic beta-cell function, and are likely to play a role in the pathogenesis of beta-cell dysfunction in type 2 diabetes (reviewed in (1)). Under normal circumstances, fatty acids do not initiate insulin release, but amplify glucose-stimulated insulin secretion (GSIS) (2-5). Fatty-acid potentiation of insulin secretion has physiological implications, particularly after a period of fasting (6). Until recently, the prevailing model postulated that the effects of fatty acids on the beta cell were mediated by their intracellular metabolism and the generation of lipid derived signals which, in turn, potentiate GSIS (2;7). According to this hypothesis, fatty acids are transported across the plasma membrane and activated into their long-chain coenzyme A esters, which in turn modulate a number of intracellular targets that influence insulin secretion. Moreover, evidence suggests that intracellular fatty-acid metabolism is a key component of both nutrient-and nonnutrient-induced insulin secretion (7). In contrast to their acute, stimulatory effect on GSIS, prolonged exposure to elevated levels of fatty acids impairs beta-cell function, a phenomenon referred to as lipotoxicity (reviewed in (1)). The mechanisms of lipotoxicity remain poorly understood but have been proposed to also involve intracellular metabolism of fatty acids and the generation of lipid-derived metabolites (8).The models described above have been challenged by the observation that fatty acids activate the G-protein coupled receptor (GPCR) GPR40 (9-11), also referred to as the free fatty-acid 1 receptor (FFA 1 R) (12;13). GPR40 belongs to a class of GPCR with high structural conservation, o...

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OBJECTIVE—The G-protein–coupled receptor GPR40 is expressed in pancreatic β-cells and is activated by long-chain fatty acids. Gene deletion studies have shown that GPR40 mediates, at least in part, fatty acid–amplification of glucose-induced insulin secretion (GSIS) but is not implicated in GSIS itself. However, the role of GPR40 in the long-term effects of fatty acids on insulin secretion remains controversial. This study aimed to test the hypothesis that GPR40 plays a role in insulin secretion after high-fat feeding.RESEARCH DESIGN AND METHODS—GPR40 knockout (KO) mice on a C57BL/6 background and their wild-type (WT) littermates were fed a high-fat diet (HFD) for 11 weeks. Glucose tolerance, insulin tolerance, and insulin secretion in response to glucose and Intralipid were assessed during the course of the diet period.RESULTS—GPR40 KO mice had fasting hyperglycemia. They became as obese, glucose intolerant, and insulin resistant as their WT littermates given HFD and developed a similar degree of liver steatosis. Their fasting blood glucose levels increased earlier than those of control mice during the course of the HFD. The remarkable increase in insulin secretory responses to intravenous glucose and Intralipid seen in WT mice after HFD was of much lower magnitude in GPR40 KO mice.CONCLUSIONS—GPR40 plays a role not only in fatty acid modulation of insulin secretion, but also in GSIS after high-fat feeding. These observations raise doubts on the validity of a therapeutic approach based on GPR40 antagonism for the treatment of type 2 diabetes.

Maladaptive wound healing responses to chronic tissue injury result in organ fibrosis. Fibrosis, which entails excessive extracellular matrix (ECM) deposition and tissue remodelling by activated myofibroblasts, leads to loss of proper tissue architecture and organ function; however the molecular mediators of myofibroblast activation remain to be fully identified. Here we identify soluble ephrin-B2 as a novel pro-fibrotic mediator in lung and skin fibrosis. We provide molecular, functional and translational evidence that the ectodomain of membrane-bound ephrin-B2 is shed from fibroblasts into the alveolar airspace after lung injury. Shedding of soluble ephrin-B2 (sEphrin-B2) promotes fibroblast chemotaxis and activation via EphB3/EphB4 receptor signaling. We found that mice lacking ephrin-B2 in fibroblasts are protected from skin and lung fibrosis and that a distintegrin and metalloproteinase 10 (ADAM10) is the major ephrin-B2 sheddase in fibroblasts. ADAM10 is induced by transforming growth factor-β1 (TGF-β1), and ADAM10-mediated sEphrin-B2 generation is required for TGF-β1–induced myofibroblast activation. Pharmacological inhibition of ADAM10 reduces sEphrin-B2 levels in bronchoalveolar lavage and prevents lung fibrosis in mice. Consistent with the mouse data, ADAM10/sEphrin-B2 signaling is upregulated in fibroblasts from human subjects with idiopathic pulmonary fibrosis. These results uncover a new molecular mechanism of tissue fibrogenesis and identify sEphrin-B2, its receptors Eph3/Eph4, and ADAM10 as potential therapeutic targets in the treatment of fibrotic diseases.

OBJECTIVE—Prolonged exposure of isolated islets of Langerhans to elevated levels of fatty acids, in the presence of high glucose, impairs insulin gene expression via a transcriptional mechanism involving nuclear exclusion of pancreas-duodenum homeobox-1 (Pdx-1) and loss of MafA expression. Whether such a phenomenon also occurs in vivo is unknown. Our objective was therefore to ascertain whether chronic nutrient oversupply inhibits insulin gene expression in vivo. RESEARCH DESIGN AND METHODS—Wistar rats received alternating 4-h infusions of glucose and Intralipid for a total of 72 h. Control groups received alternating infusions of glucose and saline, saline and Intralipid, or saline only. Insulin and C-peptide secretion were measured under hyperglycemic clamps. Insulin secretion and gene expression were assessed in isolated islets, and β-cell mass was quantified by morphometric analysis. RESULTS—Neither C-peptide secretion nor insulin sensitivity was different among infusion regimens. Insulin content and insulin mRNA levels were lower in islets isolated from rats infused with glucose plus Intralipid. This was associated with reduced Pdx-1 binding to the endogenous insulin promoter, and an increased proportion of Pdx-1 localized in the cytoplasm versus the nucleus. In contrast, MafA mRNA and protein levels and β-cell mass and proliferation were unchanged. CONCLUSIONS—Cyclical and alternating infusions of glucose and Intralipid in normal rats inhibit insulin gene expression without affecting insulin secretion or β-cell mass. We conclude that fatty acid inhibition of insulin gene expression, in the presence of high glucose, is an early functional defect that may contribute to β-cell failure in type 2 diabetes.