OXIDIZED LOW-DENSITY-LIPOPROTEIN INDUCES APOPTOSIS IN CULTURED HUMAN UMBILICAL VEIN ENDOTHELIAL-CELLS BY COMMON AND UNIQUE MECHANISMS

Oxidized low density lipoprotein (oxLDL) induces apoptosis in vascular cells, To elucidate the mechanisms involved in this apoptosis, we stu died the apoptosis-inducing activity in lipid fractions of oxLDL and t he roles of two common mechanisms, ceramide generation and the activat ion of caspases, in apoptosis in human umbilical vein endothelial cell s treated with oxLDL. We also studied the effects of antioxidants and cholesterol. oxLDL induced endothelial apoptosis in a time-and dose-de pendent fashion. Apoptosis-inducing activity was recovered in the neut ral lipid fraction of oxLDL. Various oxysterols in this fraction induc ed endothelial apoptosis. Neither the phospholipid fraction nor its co mponent lysophosphatidylcholine induced apoptosis. oxLDL induced ceram ide accumulation temporarily at 15 min in a dose-dependent fashion. Tw o inhibitors of acid sphinogomyelinase inhibited both the increase in ceramide and the apoptosis induced by oxLDL. Furthermore, a membrane-p ermeable ceramide (C-2-ceramide) induced endothelial apoptosis. These findings demonstrated that ceramide generation by acid sphingomyelinas e is indispensable for the endothelial apoptosis induced by oxLDL, Inh ibitors of both caspase-1 and caspase-3 inhibited the apoptosis, sugge sting that oxLDL induced apoptosis by activating these cysteine protea ses. The antioxidants butylated hydroxytoluene and superoxide dismutas e but not catalase inhibited the apoptosis induced by oxLDL or 25-hydr oxycholesterol. This suggests not only that superoxide plays an import ant role but also that a critical interaction between oxLDL and the ce ll takes place on the outer surface of the membrane, because superoxid e dismutase is not membrane-permeable, Exogenous cholesterol also inhi bited the apoptosis. Our study demonstrated that neutral lipids in oxL DL induce endothelial apoptosis by activating membrane sphingomyelinas e in a superoxide-dependent manner, as well as by activating caspases.