OXYSTEROLS AND TBARS ARE AMONG THE LDL OXIDATION-PRODUCTS WHICH ENHANCE THROMBOXANE A(2) SYNTHESIS BY PLATELETS

In this study, we compared the effects of normal LDL (nLDL) and oxidiz ed LDL (oxLDL) on thromboxane (TXA(2)) release by platelets triggered by low concentration of thrombin, and we determined which component of oxLDL is responsible for that activation. After oxidation of LDL with copper sulfate, the small molecular weight fraction (<10 kDa) which w as high in TEARS was removed; using Amicon Centriprep-10 concentrator membrane. More than 67% of TEARS in the oxLDL preparation was found in solution while the remaining was covalently attached to the oxLDL par ticles. OxLDL contained significantly higher levels of oxysterols and TEARS than the nLDL. Platelets preincubated with low concentrations of oxLDL (33-132 mu g protein/mL) produced significantly higher TXA(2) t han platelets preincubated with equivalent concentrations of nLDL when triggered with thrombin. Platelets treated with oxLDL also contained significantly higher levels of oxysterols than platelets treated with nLDL. Platelets preincubated with pure cholestanetriol (10 mu g/mL) co ntained a high level of cholestanetriol in the membrane, and TXA(2) re lease was significantly increased in these platelets compared to the c ontrol platelets. The TEARS in solution also was very potent in enhanc ing TXA(2) release by thrombin-treated platelets. These results indica te that oxysterols and the free TEARS either in solution or covalently attached to the oxLDL particles are partly responsible for the stimul atory effect of oxLDL on TXA(2) release by platelets. The present stud y also showed that this enhancement of TXA(2) release was due to activ ation of phospholipase A(2) and to the increase of arachidonic acid li beration from the platelet phospholipids.