Vitamin E prevents the age-dependent and palmitate-induced disturbances of sphingolipid turnover in liver cells

Sphingolipid turnover has been shown to be activated at old age and in response to various stress stimuli including oxidative stress. Reduction of vitamin E content in the liver under the pro-oxidant action is associated with enhanced sphingolipid turnover and ceramide accumulation in hepatocytes. In the present paper, the correction of sphingolipid metabolism in the liver cells of old rats and in the palmitate-treated young hepatocytes using alpha-tocopherol has been investigated. 3- and 24-month-old rats, [(14) C]palmitic acid, [methyl-(14) C-choline]sphingomyelin (SM), and [(14) C]serine were used. alpha-Tocopherol administration to old rats or addition to the culture medium of old liver slices or hepatocytes prevented age-dependent increase of ceramide synthesis and lipid accumulation, and increased SM content in liver tissue and cells. alpha-Tocopherol treatment of old cells decreased the neutral and acid sphingomyelinase (SMase) activities in hepatocytes and serine palmitoyl transferase activity in the liver cell microsomes. Effect of alpha- or gamma-tocopherol, but not of delta-tocopherol, on the newly synthesized ceramide content in old cells was correlated with the action of inhibitor of serine palmitoyl transferase (SPT) activity (myriocin) and SMase inhibitors (glutathione, imipramine). Addition of alpha-tocopherol as well as myriocin to the culture medium of young hepatocytes, treated by palmitate, abolished ceramide accumulation and synthesis. The data obtained demonstrate that alpha-tocopherol normalized elevated ceramide content in the old liver cells via inhibition of acid and neutral SMase activities and lipid synthesis de novo. alpha-Tocopherol, reducing ceramide synthesis, prevented palmitate-induced aging-like ceramide accumulation in young liver cells.