Effect of sulfonylureas on triglyceride metabolism in the rat liver: possible role of the lysosomes in hepatic lipolysis

Abstract

It has been suggested previously that chlorpropamide and other hypoglycemic sulfonylureas interfere with hepatic triglyceride breakdown. Since ketogenesis from endogenous hepatic lipid stores is a measure of hepatic triglyceride hydrolysis, ketogenesis derived from endogenous lipids as well as ketogenesis derived from exogenously added isotopic oleate was determined in isolated hepatocytes from fasted rats in an attempt to identify the nature of the direct effects of sulfonylureas on hepatic lipid metabolism. Ketogenesis from endogenous lipids was inhibited by 1 mM chlorpropamide, while ketone production from exogenous oleate did not change. The effect of chlorpropamide on hepatic triglyceride metabolism was further studied in the isolated perfused liver of normal rats in the presence of a continuous [3H]oleate infusion and in isolated liver cells incubated in the presence of [3H]oleate. In liver perfusion experiments, 1 mM chlorpropamide enhanced the incorporation of tritium into triglycerides (but not other lipid classes) and increased both liver triglyceride content and triglyceride secretion. Using isolated cells similar effects could be demonstrated at 0.5 mM chlorpropamide. Chlorpropamide, tolbutamide, and carbutamide, all of which inhibited endogenous ketogenesis in isolated liver cells, also inhibited lysosomal triglyceride lipase activity in rat liver homogenates. The drugs were not inhibitory towards alkaline lipase activity. Demethylglycodiazin (2-benzolsulfonamid--5-(beta-hydroxyethoxy)-pyrimidin), which did not inhibit endogenous ketogenesis in isolated liver cells, did not affect lysosomal lipase activity. The lysosomotropic drug chloroquine was markedly antiketogenic when tested in liver cells. The reduction in endogenous ketogenesis, the enhanced accumulation of liver triglycerides, and the stimulation of hepatic triglyceride output by chlorpropamide are ascribed to an interference of the drug with hepatic triglyceride breakdown. The present results also suggest that the lysosomes play a significant role in hepatic lipolysis.