As part of our long-term goal to develop new approaches to treat thrombotic diseases, we will investigate a novel mechanism that links platelets, inflammation and thrombosis. Platelets are crucial for the process of thrombosis, but are not widely considered as key elements in inflammation. An emerging view of platelets is that they contain and produce many immunomodulatory and proinflammatory mediators including CD40 ligand (CD40L), prostaglandins and cytokines. Platelets are not thought to possess transcription factors. However, our compelling preliminary data demonstrate that the transcription factor peroxisome proliferators activated receptor gamma (PPARgamma), believed only present in nucleated cells to regulate lipid metabolism, is highly expressed by platelets. Further, our experiments show that small molecule PPARgamma agonists, such as rosiglitazone and prostaglandin J2, inhibit platelet aggregation and ATP release. Remarkably, PPARgamma agonists inhibit the thrombin-induced production and release of key platelet mediators of inflammation including CD40L and thromboxanes. These findings are important because (1) thrombin is an extremely potent platelet activator, and (2) rosiglitazone and similar agents are already widely clinically employed in patients with diabetes. In addition to CD40L's known critical role in inflammation and atherosclerosis, it is now recognized as a primary platelet agonist important in thrombosis. CD40L is also a marker for disease activity and prognosis in cardiac and vascular diseases. Therefore, platelet PPARgamma is a new and previously unsuspected potential target to attenuate thrombosis, vascular injury and inflammation. We have developed the overall hypothesis that the transcription factor PPARgamma is expressed by platelets and modulates their ability to become activated and to produce mediators of inflammation that contribute to vascular injury. To provide a scientific framework to evaluate PPARgamma agonists as a possible therapy for thrombosis and inflammation, we will answer the following questions posed as our specific aims.
Aim 1 : What are the subcellular locations of platelet PPARgamma and what are the effects of PPARgamma agonists on platelet release of key proinflammatory mediators? Aim 2: How do PPARgamma agonists inhibit platelet activation and function? Aim 3: What is the significance of the PPARgamma protein that is expelled from platelets after activation? Completion of these studies will provide a scientific justification for clinical trials that employ existing PPARgamma agonists to attenuate atherothrombotic disease through modulation of platelet thrombotic and inflammatory functions.
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