MOLECULAR OR PHARMACOLOGICAL PERTURBATION OF THE LINK BETWEEN GLUCOSEAND LIPID-METABOLISM IS WITHOUT EFFECT ON GLUCOSE-STIMULATED INSULIN-SECRETION - A REEVALUATION OF THE LONG-CHAIN ACYL-COA HYPOTHESIS

The mechanism by which glucose stimulates insulin secretion from the p ancreatic islets of Langerhans is incompletely understood. It has been suggested that malonyl-CoA plays a regulatory role by inhibiting fatt y acid oxidation and promoting accumulation of cytosolic long-chain ac yl-CoA (LC-CoA). In the current study, we have re evaluated this ''lon g-chain acyl-CoA hypothesis'' by using molecular and pharmacologic met hods to perturb lipid metabolism in INS-1 insulinoma cells or rat isle ts during glucose stimulation. First, we constructed a recombinant ade novirus containing the cDNA encoding malonyl-CoA decarboxylase (AdCMV- MCD), an enzyme that decarboxylates malonyl-CoA to acetyl-CoA. INS-1 c ells treated with AdCMV-MCD had dramatically lowered intracellular mal onyl CoA levels compared with AdCMV-beta Gal-treated cells at both 3 a nd 20 mM glucose. Further, at 20 mM glucose, AdCMV-MCD-treated cells w ere less effective at suppressing [1-C-14]palmitate oxidation and inco rporated 43% less labeled palmitate and 50% less labeled glucose into cellular lipids than either AdCMV-beta GAL-treated or untreated INS-1 cells. Despite the large metabolic changes caused by expression of MCD , insulin secretion in response to glucose was unaltered relative to c ontrols. The alternative, pharmacologic approach for perturbing lipid metabolism was to use triacsin C to inhibit long-chain acyl-CoA synthe tase. This agent caused potent attenuation of palmitate oxidation and glucose or palmitate incorporation into cellular lipids and also cause d a 47% decrease in total LC-CoA. Despite this, the drug had no effect on glucose-stimulated insulin secretion in islets or INS-1 cells. We conclude that significant disruption of the link between glucose and l ipid metabolism does not impair glucose-stimulated insulin secretion i n pancreatic islets or INS-1 cells.