Mildly Oxidized LDL Induces Platelet Aggregation Through Activation of Phospholipase A 2

Abstract

Native LDL and LDL oxidized under various conditions were compared in terms of their ability to activate platelets. Native LDL did not induce platelet shape change or aggregation, even at high concentrations (2 mg protein/mL). LDL was mildly oxidized with either CuSO ₄ (mox-LDL) or 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 0.55±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 electrophoretic 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 aggregation and secretion were induced by a threshold concentration of 0.4 mg protein/mL. In contrast, LDL thoroughly oxidized with CuSO ₄ (ox-LDL) did not aggregate platelets. Although mox-LDL was depleted in antioxidants (α- and γ-tocopherol, α- and β-carotene, and other carotenoids), incubation of mox-LDL with exogenous α-tocopherol did not reverse its ability to induce platelet aggregation and secretion. Preincubation of platelets with the cyclooxygenase inhibitor aspirin or the phospholipase A ₂ inhibitors trifluoperazine, quinacrine, 4-bromophenacyl bromide, and propranolol completely prevented platelet aggregation and secretion caused by mox-LDL or SIN-1–LDL. These results indicate that mildly oxidized LDL activates platelets through a phospholipase A ₂ /cyclooxygenase–dependent pathway. The complete inhibition of mox-LDL–induced platelet aggregation by aspirin could contribute to its beneficial effect in cardiovascular disease.