Ceramides Inhibit Phospholipase D-Dependent Insulin Signaling in Liver Cells of Old Rats

Ceramides are a novel class of biologically active molecules involved in the regulation of different signaling pathways. Ceramide is involved in regulation of the phospholipase D (PLD) activity and development of cell resistance to insulin. In this work, we have studied age-related features of insulin regulation of PLD activity and glucose metabolism in intact cells and modeled their resistance to insulin by exogenous ceramide and palmitic acid. Contents of ceramides and of free fatty acids (FFA) are found to increase with age, as well as on incubation of liver cells of young rats in the presence of the ceramide precursor palmitic acid. Under these conditions, the ability of insulin to activate PLD, the cell uptake of glucose, and glycogen synthesis sharply decreased. On incubation of hepatocytes of young animals in the presence of exogenous C2-ceramide, the contents of endogenous ceramides increased but not the contents of FFAs and of neutral lipids. These events were accompanied by suppression of the insulin-induced production of phosphatidylethanol (a result of ethanol transphosphatidylation by PLD), glucose uptake, and glycogen synthesis. Incubation of insulin-resistant liver cells of young rats and also of hepatocytes of old rats in the presence of myriocin (an inhibitor of the de novo synthesis of ceramide) was associated with a decrease in ceramide content in the cells and an increase in the cell sensitivity to insulin. The findings indicate an important role of ceramide in disturbance of insulin signaling due to inhibition of the PLD-dependent link in the liver cells of old animals.