Proteins secreted by activated platelets can adhere to the vessel wall and promote the development of atherosclerosis and thrombosis. Despite this biologic significance, however, the complement of proteins comprising the platelet releasate is largely unknown. Using a proteomics approach, we have identified more than 300 proteins released by human platelets following thrombin activation. Many of the proteins identified were not previously attributed to platelets, including secretogranin III, a potential monocyte chemoattractant precursor; cyclophilin A, a vascular smooth muscle cell growth factor; calumenin, an inhibitor of the vitamin K epoxide reductase-warfarin interaction, as well as proteins of unknown function that map to expressed sequence tags. Secretogranin III, cyclophilin A, and calumenin were confirmed to localize in platelets and to be released upon activation. Furthermore, while absent in normal vasculature, they were identified in human atherosclerotic lesions. Therefore, these and other proteins released from platelets may contribute to atherosclerosis and to the thrombosis that complicates the disease. Moreover, as soluble extracellular proteins, they may prove suitable as novel therapeutic targets.

Pathological and clinical studies have suggested that platelets have a role in the pathogenesis of unstable angina and myocardial infarction. However, the relation of platelet activation to episodic ischemia in patients with unstable angina is unknown. We assessed the biosynthesis of thromboxane and prostacyclin as indexes of platelet activation in patients with stable and unstable coronary disease by physicochemical analysis of metabolites in plasma and urine. Prostacyclin biosynthesis was markedly elevated in patients with acute myocardial infarction and correlated with plasma creatine kinase (r = 0.795; P less than 0.001). The largest rise in thromboxane synthesis was observed in patients with unstable angina, in whom 84 percent of the episodes of chest pain were associated with phasic increases in the excretion of thromboxane and prostacyclin metabolites. However, 50 percent of such increases were not associated with chest pain, possibly reflecting silent myocardial ischemia. These data indicate that platelet activation occurs during spontaneous ischemia in patients with unstable angina. The increment in prostacyclin biosynthesis during such episodes may be a compensatory response of vascular endothelium that limits the degree or effects of platelet activation. If so, biochemically selective inhibition of the synthesis or action of thromboxane A2 would be desirable in the treatment of unstable angina. In contrast, thromboxane inhibitors or antagonists would not be expected to be effective in patients with chronic stable angina, in whom there was no increase in the formation of thromboxane A2.

Summary The ability of Staphylococcus aureus cells to induce platelet aggregation has long been recognized. However, despite several attempts to identify the mechanisms involved in this interaction, the nature of the bacterial receptors required remains poorly understood. Using genetic manipulation, this study for the first time provides clear evidence that several S. aureus surface proteins participate in the inter‐action with platelets. Mutants of S. aureus strain Newman lacking one or more surface proteins were tested for their ability to stimulate platelet aggre‐gation. This approach was complemented by the expression of a number of candidate proteins in the non‐aggregating Gram‐positive bacterium Lacto‐coccus lactis. S. aureus‐induced aggregation was monophasic and was dependent on the platelet receptor GPIIb/IIIa. The fibrinogen‐binding proteins, clumping factors A and B and the serine‐aspartate repeat protein SdrE could each induce aggregation when expressed in L. lactis. Although protein A expressed in L. lactis was not capable of inducing aggregation independently, it enhanced the aggregation response when expressed on the surface of S. aureus. Thus, S. aureus has multiple mechanisms for stimulating platelet aggregation. Such functional redundancy suggests that this phenomenon may be important in the pathogenesis of invasive diseases such as infective endocarditis.

Background-The formation of prostacyclin (PGI 2 ), thromboxane (TX) A 2 , and isoprostanes is markedly enhanced in atherosclerosis. We examined the relative contribution of cyclooxygenase (COX)-1 and -2 to the generation of these eicosanoids in patients with atherosclerosis. Methods and Results-The study population consisted of 42 patients with atherosclerosis who were undergoing surgical revascularization. COX-2 mRNA was detected in areas of atherosclerosis but not in normal blood vessel walls, and there was evidence of COX-1 induction. The use of immunohistochemical studies localized the COX-2 to proliferating vascular smooth muscle cells and macrophages. Twenty-four patients who did not previously receive aspirin were randomized to receive either no treatment or nimesulide at 24 hours before surgery and then for 3 days. Eighteen patients who were receiving aspirin were continued on a protocol of either aspirin alone or a combination of aspirin and nimesulide. Urinary levels of 11-dehydro-TXB 2 and 2,3-dinor-6-keto-PGF 1␣ , metabolites of TXA 2 and PGI 2 , respectively, were elevated in patients with atherosclerosis compared with normal subjects (3211Ϯ533 versus 679Ϯ63 pg/mg creatinine, PϽ0.001; 594Ϯ156 versus 130Ϯ22 pg/mg creatinine, PϽ0.05, respectively), as was the level of the isoprostane 8-iso-PGF 2␣ . Nimesulide reduced 2,3-dinor-6-keto-PGF 1␣ excretion by 46Ϯ5% (378.3Ϯ103 to 167Ϯ37 pg/mg creatinine, PϽ0.01) preoperatively and blunted the increase after surgery. Nimesulide had no significant effect on 11-dehydro-TXB 2 before (2678Ϯ694 to 2110Ϯ282 pg/mg creatinine) or after surgery. The levels of both products were lower in patients who were taking aspirin, and no further reduction was seen with the addition of nimesulide. None of the treatments influenced urinary 8-iso-PGF 2␣ excretion. Conclusions-Both COX-1 and -2 are expressed and contribute to the increase in PGI 2 in patients with atherosclerosis, whereas TXA 2 is generated by COX-1. (Circulation. 2000;102:840-845.)