Role for malic enzyme, pyruvate carboxylation and mitochondrial malate import in the glucose-stimulated insulin secretion

Pyruvate cycling has been implicated in glucose-stimulated insulin secretion (GSIS) from pancreatic beta-cells. The operation of some pyruvate cycling pathways is proposed to necessitate malate export from the mitochondria and NADP(+)-dependent decarboxylation of malate to pyruvate by cytosolic malic enzyme (ME1). Evidence in favor of and against role of ME1 in GSIS has been presented by others using si-RNA-mediated suppression of ME1. ME1 was also proposed to account for methyl succinate (MS)-stimulated insulin secretion (MSSIS), hypothesized to take place via succinate entry to the mitochondria in exchange for malate, and subsequent malate conversion to pyruvate. In contrast to rat, mouse beta-cells are lacking ME1 activity, which was suggested to explain their lack of MSSIS. However this hypothesis was not tested. In this report, we demonstrate that while adenoviral-mediated over-expression of ME1 greatly augments GSIS in rat insulinoma INS-1 832/13 cells, it does not restore MSSIS or significantly affect GSIS in mouse islets. The increase in GSIS following ME1 over-expression in INS-1 832/13 cells did not alter the ATP/ADP ratio, but was accompanied by increases in malate and citrate levels. Increased malate and citrate levels were also observed after providing INS-1 832/13 cells with the malate permeable analog dimethyl-malate (DMM). These data suggest that while ME1 over-expression augments anaplerosis and GSIS in INS-1 832/13 cells, it is not likely involved in MSSIS and GSIS in pancreatic islets. Key words: insulin secretion, pyruvate carboxylation, malic enzyme, dimethyl-malate.