Glucose-regulated anaplerosis and cataplerosis in pancreatic beta-cells - Possible implication of a pyruvate/citrate shuttle in insulin secretion

The hypothesis proposing that anaplerosis and cataplerosis play an importan t role in fuel signaling by providing mitochondrially derived coupling fact ors for stimulation of insulin secretion was tested. A rise in citrate coin cided with the initiation of insulin secretion in response to glucose in IN S-1 beta-cells. The dose dependence of glucose-stimulated insulin release c orrelated closely with those of the cellular contents of citrate, malate, a nd citrate-derived malonyl-CoA.. The glucose-induced elevations in citrate, alpha-ketoglutarate, malonyl-CoA, and the 3-[4,5-dimethylthiazol-2-yl]-2,5 -diphenyltetrazolium reduction state, an index of beta-cell metabolic activ ity, were unaffected by the Ca2+ chelator EGTA. Glucose induced a rise in b oth mitochondrial and cytosolic citrate and promoted efflux of citrate from the cells. The latter amounted to similar to 20% of glucose carbons enteri ng the glycolytic pathway Phenylacetic acid, a pyruvate carboxylase inhibit or, reduced the glucose-induced rise in citrate in INS-1 cells and insulin secretion in both INS-1 cells and rat islets. The results indicate the feas ibility of a pyruvate/citrate shuttle in INS-1 beta-cells, allowing the reg eneration of NAD(+) in the cytosol and the formation of cytosolic acetyl-Co A, malonyl-30A, and NADPH. The data suggest that anaplerosis and catapleros is are early signaling events in beta-cell activation that do not require a rise in Ca2+. It is proposed that citrate is a signal of fuel abundance th at contributes to beta-cell activation in both the mitochondrial and cytoso lic compartments and that a major fate of anaplerotic glucose carbons is ex ternal citrate.