MILDLY OXIDIZED LDL INDUCES PLATELET-AGGREGATION THROUGH ACTIVATION OF PHOSPHOLIPASE A(2)

Native LDL and LDL oxidized under various conditions were compared in terms of their ability to activate platelets. Native LDL did not induc e platelet shape change or aggregation, even at high concentrations (2 mg protein/mL). LDL was mildly oxidized with either CuSO4 (mox-LDL) o r 3-(N-morpholino)sydnonimine (SIN-1-LDL). Analysis of mox-LDL and SIN -1-LDL showed a small increase of dienes (E(234nm) from 0.28+/-0.04 to 0.551+/-0.09, mean+/-SD) and thiobarbituric acid-reactive substance ( from 0 to 10.6+/-1.5 nmol/mg, mean+/-SEM), no change in apo B electrop horetic mobility, and a minor (12% to 30%) decrease in polyunsaturated fatty acid content. Interestingly, this small oxidative modification of LDL dramatically changed its effect on platelets. Irreversible aggr egation and secretion were induced by a threshold concentration of 0.4 mg protein/mL. In contrast, LDL thoroughly oxidized with CuSO4 (ox-LD L) did not aggregate platelets. Although mox-LDL was depleted in antio xidants (alpha- and gamma-tocopherol, alpha- and beta-carotene, and ot her carotenoids), incubation of mox-LDL with exogenous alpha-tocophero l did not reverse its ability to induce platelet aggregation and secre tion. Preincubation of platelets with the cyclooxygenase inhibitor asp irin or the phospholipase A(2) inhibitors trifluoperazine, quinacrine, 4-bromophenacyl bromide, and propranolol completely prevented platele t aggregation and secretion caused by mox-LDL or SIN-1-LDL. These resu lts indicate that mildly oxidized LDL activates platelets through a ph ospholipase A(2)/cyclooxygenase-dependent pathway. The complete inhibi tion of mox-LDL-induced platelet aggregation by aspirin could contribu te to its beneficial effect in cardiovascular disease.