Neutral sphingomyelinase increases the binding, internalization, and degradation of low density lipoproteins and synthesis of cholesteryl ester in cultured human fibroblasts

I have investigated the effects of human urinary neutral sphingomyelinase (N-SMase) (Chatterjee, S., and Ghosh, N. (1989) J. Biol. Chem. 264, 12554- 12561) on the cell-surface binding, internalization, and degradation of ¹²⁵I-low density lipoprotein (LDL) and on cholesteryl ester synthesis in cultured human fibroblasts. N-SMase exerted a concentration-dependent continuous stimulation of ¹²⁵I-LDL cell-surface binding, internalization, and degradation in normal human fibroblasts. A 3-fold increase in binding, internalization, and degradation was observed at the maximum amount (600 units of N-SMase/ml) examined. This phenomenon was accompanied by a continuous stimulation of cholesteryl ester synthesis. A 5-fold increase in cholesteryl ester synthesis was observed after incubation for 4 h with N- SMase. Antibody against N-SMase and heat inactivation of N-SMase compromised the stimulatory effects of N-SMase on ¹²⁵I-LDL metabolism and cholesteryl ester synthesis in these cells. Incubation of cells with phospholipase D and phospholipase C did not alter ¹²⁵I-LDL binding, internalization, or degradation. This finding suggests that the stimulatory effects of N-SMase on LDL metabolism and on cholesteryl ester synthesis in fibroblasts is specific. Moreover, unlabeled LDL competitively displaced ¹²⁵I-LDL from binding to N-SMase-treated cells. None of the precursors of sphingomyelin could mimic the stimulatory effects of N-SMase on ¹²⁵I-LDL metabolism in these cells. Taken together, these studies suggest that one of the biological roles of N- SMase involves modulating LDL metabolism and cholesterol metabolism in fibroblasts.