Lipotoxicity of the pancreatic beta-cell is associated with glucose-dependent esterification of fatty acids into neutral lipids

Prolonged exposure of isolated islets to supraphysiologic concentrations of palmitate decreases insulin gene expression in the presence of elevated gl ucose levels. This study was designed to determine whether or not this phen omenon is associated with a glucose-dependent increase in esterification of fatty acids into neutral lipids. Gene expression of sn-glycerol-3-phosphat e acyltransferase (GPAT), diacylglycerol acyltransferase (DGAT), and hormon e-sensitive lipase (HSL), three key enzymes of lipid metabolism, was detect ed in isolated rat islets. Their levels of expression were not affected aft er a 72-h exposure to elevated glucose and palmitate. To determine the effe cts of glucose on palmitate-induced neutral lipid synthesis, isolated rat i slets were cultured for 72 h with trace amounts of [C-14]palmitate with or without 0.5 mmol/l unlabeled palmitate, at 2.8 or 16.7 mmol/l glucose. Gluc ose increased incorporation of [C-14]palmitate into complex lipids. Additio n of exogenous palmitate directed lipid metabolism toward neutral lipid syn thesis. As a result, neutral lipid mass was increased upon prolonged incuba tion with elevated palmitate only in the presence of high glucose. The abil ity of palmitate to increase neutral lipid synthesis in the presence of hig h glucose was concentration-dependent in HIT cells and was inversely correl ated to insulin mRNA levels. 2-Bromopalmitate, an inhibitor of fatty acid m itochondrial beta -oxidation, reproduced the inhibitory effect of palmitate on insulin mRNA levels. In contrast, palmitate methyl ester, which is not metabolized, and the medium-chain fatty acid octanoate, which is readily ox idized, did not affect insulin gene expression, suggesting that fatty-acid inhibition of insulin gene expression requires activation of the esterifica tion pathway. These results demonstrate that inhibition of insulin gene exp ression upon prolonged exposure of islets to palmitate is associated with a glucose-dependent increase in esterification of fatty acids into neutral l ipids.