Evidence for an Anaplerotic/Malonyl-CoA Pathway in Pancreatic β-Cell Nutrient Signaling

Abstract

A metabolic model of fuel sensing has been proposed in which malonyl-CoA and long-chain acyl-CoA esters may act as coupling factors in nutrient-induced insulin release (Prentki M, Vischer S, Glennon MC, Regazzi R, Deeney J, Corkey BE: Malonyl-CoA and long chain acyl-CoA esters as metabolic coupling factors in nutrient-induced insulin secretion. J Biol Chem 267:5802–5810, 1992). To gain further insight into the control of malonyl-CoA content in islet tissue, we have studied the short- and long-term regulation of acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) in the β-cell. These enzymes catalyze the formation of malonyl-CoA and its usage for de novo fatty acid biogenesis. ACC mRNA, protein, and enzymatic activity are present at appreciable levels in rat pancreatic islets and clonal β-cells (HIT cells). Glucose addition to HIT cells results in a marked increase in ACC activity that precedes the initiation of insulin release. Fasting does not modify the ACC content of islets, whereas it markedly downregulates that of lipogenic tissues. This indicates differential regulation of the ACC gene in lipogenic tissues and the islets of Langerhans. FAS is very poorly expressed in islet tissue, yet ACC is abundant. This demonstrates that the primary function of malonyl-CoA in the β-cells is to regulate fatty acid oxidation, not to serve as a substrate for fatty acid biosynthesis. The anaplerotic enzyme pyruvate carboxylase, which allows the replenishment of citric acid cycle intermediates needed for malonyl-CoA production via citrate, is abundant in islet tissue. Glucose causes an elevation in β (HIT)-cell citrate that precedes secretion, and only those nutrients that can elevate citrate induce effective insulin release. The results provide new evidence in support of the model and explain why malonyl-CoA rises markedly and rapidly in islets upon glucose stimulation: 1) glucose elevates citrate, the precursor of malonyl-CoA; 2) glucose enhances ACC enzymatic activity; and 3) malonyl-CoA is not diverted to lipids. The data suggest that ACC is a key enzyme in metabolic signal transduction of the β-cell and provide evidence for the concept that an anaplerotic/malonyl-CoA pathway is implicated in insulin secretion.