Fatty acids (FAs) and other lipid molecules are important for many cellular functions, including vesicle exocytosis. For the pancreatic ␤-cell, while the presence of some FAs is essential for glucose-stimulated insulin secretion, FAs have enormous capacity to amplify glucose-stimulated insulin secretion, which is particularly operative in situations of ␤-cell compensation for insulin resistance. In this review, we propose that FAs do this via three interdependent processes, which we have assigned to a "trident model" of ␤-cell lipid signaling. The first two arms of the model implicate intracellular metabolism of FAs, whereas the third is related to membrane free fatty acid receptor (

Glucose metabolism in pancreatic β cells stimulates insulin granule exocytosis, and this process requires generation of a lipid signal. However, the signals involved in lipid amplification of glucose-stimulated insulin secretion (GSIS) are unknown. Here we show that in β cells, glucose stimulates production of lipolysis-derived long-chain saturated monoacylglycerols, which further increase upon inhibition of the membrane-bound monoacylglycerol lipase α/β-Hydrolase Domain-6 (ABHD6). ABHD6 expression in β cells is inversely proportional to GSIS. Exogenous monoacylglycerols stimulate β cell insulin secretion and restore GSIS suppressed by the pan-lipase inhibitor orlistat. Whole-body and β-cell-specific ABHD6-KO mice exhibit enhanced GSIS, and their islets show elevated monoacylglycerol production and insulin secretion in response to glucose. Inhibition of ABHD6 in diabetic mice restores GSIS and improves glucose tolerance. Monoacylglycerol binds and activates the vesicle priming protein Munc13-1, thereby inducing insulin exocytosis. We propose saturated monoacylglycerol as a signal for GSIS and ABHD6 as a negative modulator of insulin secretion.

The malonyl-CoA/long-chain acyl-CoA (LC-CoA) model of glucose-induced insulin secretion (GIIS) predicts that malonyl-CoA derived from glucose metabolism inhibits fatty acid oxidation, thereby increasing the availability of LC-CoA for lipid signaling to cellular processes involved in exocytosis. For directly testing the model, INSr3 cell clones overexpressing malonylCoA decarboxylase in the cytosol (MCDc) in a tetracycline regulatable manner were generated, and INS(832/ 13) and rat islets were infected with MCDc-expressing adenoviruses. MCD activity was increased more than fivefold, and the malonyl-CoA content was markedly diminished. This was associated with enhanced fat oxidation at high glucose, a suppression of the glucoseinduced increase in cellular free fatty acid (FFA) content, and reduced partitioning at elevated glucose of exogenous palmitate into lipid esterification products. MCDc overexpression, in the presence of exogenous FFAs but not in their absence, reduced GIIS in all ␤-cell lines and in rat islets. It also markedly curtailed the stimulation of insulin secretion by other fuel and nonfuel secretagogues. In the absence of MCDc overexpression, the secretory responses to all types of secretagogues were amplified by the provision of exogenous fatty acids. In the presence of exogenous FFAs, the fatty acyl-CoA synthetase inhibitor triacsin C reduced secretion in response to glucose and nonfuel stimuli. The data show the existence of important links between the metabolic coupling factor malonyl-CoA, the partitioning of fatty acids, and the stimulation of insulin secretion to both fuel and nonfuel stimuli. Diabetes 53:

The failure of pancreatic β cells to adapt to an increasing demand for insulin is the major mechanism by which patients progress from insulin resistance to type 2 diabetes (T2D) and is thought to be related to dysfunctional lipid homeostasis within those cells. In multiple animal models of diabetes, females demonstrate relative protection from β cell failure. We previously found that the hormone 17β-estradiol (E2) in part mediates this benefit. Here, we show that treating male Zucker diabetic fatty (ZDF) rats with E2 suppressed synthesis and accumulation of fatty acids and glycerolipids in islets and protected against β cell failure. The antilipogenic actions of E2 were recapitulated by pharmacological activation of estrogen receptor α (ERα) or ERβ in a rat β cell line and in cultured ZDF rat, mouse, and human islets. Pancreas-specific null deletion of ERα in mice (PERα -/-) prevented reduction of lipid synthesis by E2 via a direct action in islets, and PERα -/-mice were predisposed to islet lipid accumulation and β cell dysfunction in response to feeding with a high-fat diet. ER activation inhibited β cell lipid synthesis by suppressing the expression (and activity) of fatty acid synthase via a nonclassical pathway dependent on activated Stat3. Accordingly, pancreas-specific deletion of Stat3 in mice curtailed ER-mediated suppression of lipid synthesis. These data suggest that extranuclear ERs may be promising therapeutic targets to prevent β cell failure in T2D.

Aims/hypothesis The aim of this study was to determine the role of fatty acid signalling in islet beta cell compensation for insulin resistance in the Zucker fatty fa/fa (ZF) rat, a genetic model of severe obesity, hyperlipidaemia and insulin resistance that does not develop diabetes. Materials and methods NEFA augmentation of insulin secretion and fatty acid metabolism were studied in isolated islets from ZF and Zucker lean (ZL) control rats. Results Exogenous palmitate markedly potentiated glucosestimulated insulin secretion (GSIS) in ZF islets, allowing robust secretion at physiological glucose levels (5-8 mmol/l).Exogenous palmitate also synergised with glucagon-like peptide-1 and the cyclic AMP-raising agent forskolin to enhance GSIS in ZF islets only. In assessing islet fatty acid metabolism, we found increased glucose-responsive palmitate esterification and lipolysis processes in ZF islets, suggestive of enhanced triglyceride-fatty acid cycling. Interruption of glucose-stimulated lipolysis by the lipase inhibitor Orlistat (tetrahydrolipstatin) blunted palmitate-augmented GSIS in ZF islets. Fatty acid oxidation was also higher at intermediate glucose levels in ZF islets and steatotic triglyceride accumulation was absent. Conclusions/interpretationThe results highlight the potential importance of NEFA and glucoincretin enhancement of insulin secretion in beta cell compensation for insulin resistance. We propose that coordinated glucose-responsive fatty acid esterification and lipolysis processes, suggestive of triglyceride-fatty acid cycling, play a role in the coupling mechanisms of glucose-induced insulin secretion as well as in beta cell compensation and the hypersecretion of insulin in obesity.