Platelets in the pathogenesis of atherosclerosis

HDL3-mediated inhibition of thrombin-induced platelet aggregation and fibrinogen binding occurs via decreased production of phosphoinositide-derived second messengers 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate

We demonstrate that physiological concentrations of HDL3 inhibit the thrombin-induced platelet fibrinogen binding and aggregation in a time- and concentration-dependent fashion. The underlying mechanism includes HDL3-mediated inhibition of phosphatidylinositol 4,5-bis-phosphate turnover, 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate formation, and intracellular calcium mobilization. The inhibitory effects of HDL3 on inositol 1,4,5-tris-phosphate formation and intracellular calcium mobilization were abolished after covalent modification of HDL3 with dimethylsuberimidate. Furthermore, they could be blocked by calphostin C and bis-indolylmaleimide, 2 highly selective and structurally unrelated protein kinase C inhibitors. However, the inhibitory effects of HDL3 were not blocked by H89, a protein kinase A inhibitor. In addition, HDL3 failed to induce cAMP formation but stimulated the phosphorylation of the protein kinase C 40- to 47-kD major protein substrate. We observed a close temporal relationship between the HDL3-mediated inhibition of thrombin-induced inositol 1,4,5-tris-phosphate formation, intracellular calcium mobilization, and fibrinogen binding and the phosphorylation of the protein kinase C 40- to 47-kD major protein substrate. Taken together, these findings indicate that the HDL3-mediated inhibition of thrombin-induced fibrinogen binding and aggregation occurs via inhibition of phosphatidylinositol 4,5-bis-phosphate turnover and formation of 1,2-diacylglycerol and inositol 1,4,5-tris-phosphate. Protein kinase C may be involved in this process.

Dissociation of the alphaIIbbeta3-integrin by EGTA stimulates the tyrosine kinase pp72(syk) without inducing platelet activation

Incubation of human platelets with EGTA under conditions that dissociate the alphaIIbbeta3-integrin stimulated tyrosine phosphorylation of pp72(syk) (6.8-fold) and of proteins of 62 (2. 2-fold), 68 (2.5-fold) and 130 kDa (1.4-fold). Stimulation of tyrosine phosphorylation of pp72(syk) was associated with an increase of pp72(syk) kinase activity. In contrast to pp72(syk), tyrosine phosphorylation of the focal adhesion kinase pp125(FAK) was not stimulated by EGTA. Preincubation of platelets with the monoclonal antibody P2, which binds to the alphaIIbbeta3 complex and thus stabilizes it, strongly reduced the increase of tyrosine phosphorylation of pp72(syk), p62, and p68 induced by EGTA. The Y2/51 monoclonal antibody, which recognizes only the beta3 glycoprotein, did not inhibit the stimulation of protein tyrosine phosphorylation evoked by EGTA. Stimulation of tyrosine phosphorylation of pp72(syk), p62, p68, and p130 induced by EGTA was not observed in thrombasthenic platelets, which lack the alphaIIbbeta3-integrin. The results indicate that the dissociation of the alphaIIbbeta3 complex in intact platelets activates pp72(syk). The mechanism of activation was found to be insensitive to inhibition by cAMP and cGMP and only partially dependent on cytosolic Ca2+, suggesting a close functional coupling of alphaIIbbeta3-integrin and pp72(syk). Since platelets retain their discoid shape after EGTA treatment, we further conclude that pp72(syk) stimulation alone is not sufficient for platelet activation.

Interaction of antiplatelet drugs in vitro: aspirin, iloprost, and the nitric oxide donors SIN-1 and sodium nitroprusside

The interaction of three antiplatelet drugs was studied in vitro: aspirin, an inhibitor of the cyclooxygenase pathway of platelet activation; iloprost, a stable analog of prostacyclin that increases platelet cAMP; and the nitrix oxide donors SIN-1 and sodium nitroprusside (SNP), which both raise platelet cGMP. Platelet adhesion and aggregation evoked by collagen/ADP were measured in anticoagulated blood under physiological flow conditions using the new Thrombostat. Aggregation was also measured in platelet-rich plasma (PRP) upon stimulation by a low (2.5 micrograms/ml) and high (20 micrograms/ml) dose of collagen, ADP, or thrombin-receptor activating peptide (TRAP). We found a synergism between iloprost and aspirin in inhibiting platelet adhesion/aggregation in flowing blood and aggregation of PRP stimulated by collagen. The mean inhibitory concentrations (IC50) of iloprost in the presence of aspirin were much lower (0.7 nM and 0.5 nM in flowing blood and low-dose collagen-stimulated PRP, respectively) than in the absence of aspirin (3 and 3.6 nM, respectively). Synergism between SIN-1 and aspirin was observed in inhibiting platelet activation in flowing blood but was much less pronounced in inhibiting collagen-induced aggregation of PRP. SIN-1/SNP and iloprost synergistically inhibited the aggregation of PRP induced by collagen as well as platelet adhesion/aggregation in blood. We found that two protein substrates of cAMP- and cGMP-dependent protein kinases, rap1B and a 50 kD protein, were associated with the functional synergism between SIN-1 and iloprost and were synergistically phosphorylated by platelet treatment with both iloprost and SIN-1. Platelet inhibition by SIN-1, iloprost, and aspirin was synergistic when measured in blood. In contrast, only additive effects of SIN-1 and iloprost were observed when platelet aggregation was measured in aspirin-treated PRP stimulated by ADP, TRAP, or collagen. Our study defines the basis for a more effective antiplatelet therapy using a combination of cGMP- and cAMP-elevating and cyclooxygenase-inhibiting drugs. The results also emphasize the importance of using various methods for the evaluation of antiplatelet drugs.

Mildly oxidized LDL induces platelet aggregation through activation of phospholipase A2

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 CuSO4 (mox-LDL) or 3-(N-morpholino)sydnonimine (SIN-1-LDL). Analysis of mox-LDL and SIN-1-LDL showed a small increase of dienes (E234nm 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 CuSO4 (ox-LDL) did not aggregate platelets. Although mox-LDL was depleted in antioxidants (alpha- and gamma-tocopherol, alpha- and beta-carotene, and other carotenoids), incubation of mox-LDL with exogenous alpha-tocopherol did not reverse its ability to induce platelet aggregation and secretion. Preincubation of platelets with the cyclooxygenase inhibitor aspirin or the phospholipase A2 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 A2/cyclooxygenase-dependent pathway. The complete inhibition of mox-LDL-induced platelet aggregation by aspirin could contribute to its beneficial effect in cardiovascular disease.