Background-Lysophosphatidic acid (LPA) is a platelet-activating component of mildly oxidized LDL (mox-LDL) and lipids isolated from human atherosclerotic plaques. Specific antagonists of platelet LPA receptors could be useful inhibitors of thrombus formation in patients with cardiovascular disease. Methods and Results-Short-chain analogs of phosphatidic acid (PA) were examined for their effect on two initial platelet responses, platelet shape change and Ca 2ϩ mobilization. Dioctylglycerol pyrophosphate [DGPP(8:0)] and dioctylphosphatidic acid [PA(8:0)], recently described selective antagonists of the LPA 1 and LPA 3 receptors, inhibited platelet activation evoked by LPA but not by other platelet stimuli. DGPP(8:0) was more potent than PA(8:0). DGPP(8:0) also inhibited platelet shape change induced by mox-LDL and lipid extracts from human atherosclerotic plaques. Notably, we demonstrate for the first time that the lipid-rich core isolated from soft plaques was able to directly induce shape change. This effect was completely abrogated by prior incubation of platelets with DGPP(8:0). Moreover, coapplication of the lipid-rich core or LPA together with subthreshold concentrations of ADP or epinephrine synergistically induced platelet aggregation; this effect was inhibited by DGPP(8:0). Analysis by liquid chromatography-mass spectrometry revealed the presence of LPA alkyl-and acyl-molecular species with high platelet-activating potency (

Lipid-rich atherosclerotic plaques are vulnerable, and their rupture can cause the formation of a platelet- and fibrin-rich thrombus leading to myocardial infarction and ischemic stroke. Although the role of plaque-based tissue factor as stimulator of blood coagulation has been recognized, it is not known whether plaques can cause thrombus formation through direct activation of platelets. We isolated lipid-rich atheromatous plaques from 60 patients with carotid stenosis and identified morphologically diverse collagen type I- and type III-positive structures in the plaques that directly stimulated adhesion, dense granule secretion, and aggregation of platelets in buffer, plasma, and blood. This material also elicited platelet-monocyte aggregation and platelet-dependent blood coagulation. Plaques exposed to flowing blood at arterial wall shear rate induced platelets to adhere to and spread on the collagenous structures, triggering subsequent thrombus formation. Plaque-induced platelet thrombus formation was observed in fully anticoagulated blood (i.e., in the absence of tissue factor-mediated coagulation). Mice platelets lacking glycoprotein VI (GPVI) were unable to adhere to atheromatous plaque or form thrombi. Human platelet thrombus formation onto plaques in flowing blood was completely blocked by GPVI inhibition with the antibody 10B12 but not affected by integrin alpha2beta1 inhibition with 6F1 mAb. Moreover, the initial platelet response, shape change, induced by plaque was blocked by GPVI inhibition but not with alpha2beta1 antagonists (6F1 mAb or GFOGER-GPP peptide). Pretreatment of plaques with collagenase or anti-collagen type I and anti-collagen type III antibodies abolished plaque-induced platelet activation. Our results indicate that morphologically diverse collagen type I- and collagen type III-containing structures in lipid-rich atherosclerotic plaques stimulate thrombus formation by activating platelet GPVI. This platelet collagen receptor, essential for plaque-induced thrombus formation, presents a promising new anti-thrombotic target for the prevention of ischemic cardiovascular diseases.

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Lipid-rich atherosclerotic plaques are vulnerable, and upon disruption trigger intraarterial thrombus formation. Tissue factor activating blood coagulation is viewed as the major prothrombotic stimulus within the plaque. We isolated lipid-rich atheromatous plaques from 50 patients with carotid artery stenosis and identified morphologically diverse collagenous structures within in the plaques. They stimulated platelet adhesion, dense granule secretion and aggregation, and triggered thrombus formation in hirudin-anticoagulated blood under arterial flow conditions. Even in fully anticoagulated flowing blood, i.e. in the absence of tissue factor-mediated coagulation, plaques were able to activate platelets. Thrombus formation was more rapid and stable when blood was anticoagulated with a low concentration of heparin, but, although fibrin was detectable within the thrombus, the initial step was always single platelet adhesion and not fibrin formation. In contrast, absence or inhibition of the platelet collagen receptor glycoprotein VI prevented platelet adhesion to atheromatous plaques and thrombus formation. We thus identified platelet glycoprotein VI as being essential and sufficient to mediate plaque-induced thrombus formation. Our study suggests a novel anti-thrombotic strategy to prevent and treat atherothrombosis in patients with vulnerable atherosclerotic plaques. Figure Figure