Fatty acid metabolism and insulin secretion in pancreatic beta cells

Increases in glucose or fatty acids affect metabolism via changes in long-chain acyl-CoA formation and chronically elevated fatty acids increase total cellular CoA. Understanding the response of pancreatic beta cells to increased amounts of fuel and the role that altered insulin secretion plays in the development and maintenance of obesity and Type 2 diabetes is important. Data indicate that the activated form of fatty acids acts as an effector molecule in stimulus-secretion coupling. Glucose increases cytosolic long-chain acyl-CoA because it increases the "switch" compound malonyl-CoA that blocks mitochondrial -oxidation, thus implementing a shift from fatty acid to glucose oxidation. We present arguments in support of the following: (i) A source of fatty acid either exogenous or endogenous (derived by lipolysis of triglyceride) is necessary to support normal insulin secretion; (ii) a rapid increase of fatty acids potentiates glucose-stimulated secretion by increasing fatty acyl-CoA or complex lipid concentrations that act distally by modulating key enzymes such as protein kinase C or the exocytotic machinery; (iii) a chronic increase of fatty acids enhances basal secretion by the same mechanism, but promotes obesity and a diminished response to stimulatory glucose; (iv) agents which raise cAMP act as incretins, at least in part, by stimulating lipolysis via beta-cell hormone-sensitive lipase activation. Furthermore, increased triglyceride stores can give higher rates of lipolysis and thus influence both basal and stimulated insulin secretion. These points highlight the important roles of NEFA, LC-CoA, and their esterified derivatives in affecting insulin secretion in both normal and pathological states.Abbreviations ACS acyl-CoA synthetase - ACC acetyl-CoA carboxylase - BAT brown adipose tissue - CPT carnitine palmitoyl transferase - CL citrate lyase - DAG diacylglycerol - GSIS glucose-stimulated insulin secretion - HSL hormone-sensitive lipase - K_ATP ATP-sensitive K⁺ channel - LC-CoA long chain acyl-CoA - PA phosphatidate - PFK-1 phosphofructokinase-1 - PKC protein kinase C - PMA phorbol myristate acetate - PC pyruvate carboxylase - PS phosphatidylserine - SNAP soluble NSF-associated protein - SNAP-25 synaptosomal-associated protein of 25 kD - t-SNARE target SNAP receptor - v-SNARE vesicle SNAP receptor - VAMP vesicle-associated membrane protein - VDCC voltage-dependent Ca²⁺ channel - WAT white adipose tissue