Membrane Phospholipids: The Key Regulators of Tissue Factor Encryption/Decryption

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Upon vascular injury, plasma clotting factor VII (FVII) along with traces of activated FVII (FVIIa) come into contact with the cofactor tissue factor (TF), which is expressed constitutively in cells within the vessel wall. Complex formation of FVIIa with TF results in a marked enhancement of the catalytic activity of FVIIa and triggers TF-mediated blood coagulation. Certain disease conditions induce TF expression in circulating blood cells and vascular endothelial cells and thus allow direct contact between circulating blood and TF that leads to thrombosis. While TF-mediated blood coagulation is essential to maintain hemostasis, the aberrant activation of TF-mediated blood coagulation leads to thrombosis, the precipitating event in acute myocardial infarction, ischemic stroke, and sepsis. Therefore, the proper regulation of TF expression and the activity is critical for not only to the maintenance of the hemostatic balance but also for health in general. Typically, most of the TF expressed in cells stays encrypted with very little procoagulant activity that is sufficient to achieve hemostasis but not to cause intravascular coagulation. Cellular injury enhances TF procoagulant activity greatly without altering TF antigen levels, i.e., transforming cryptic TF to prothrombotic TF. TF procoagulant activity in cells is controlled dynamically by a variety of post-translational mechanisms. Our recent studies revealed that sphingomyelin (SM) in the outer leaflet of the plasma membrane is responsible for maintaining TF in an encrypted state and that hydrolysis of SM activates TF and releases TF+ microvesicles (MVs). SM metabolism is altered in many disease settings, including atherosclerosis, diabetes, sepsis, and cancer, the same disease settings that induce aberrant activation of TF. The current proposal is built on the above novel findings and proposes to investigate the pathophysiologic relevance of SM metabolism in regulation of TF- mediated hemostasis, thrombosis, and inflammation. Aim 1 focuses on elucidating mechanisms by which SM metabolism regulates TF procoagulant activity, whereas Aim 2 investigates whether SM metabolism influences hemostasis and thrombosis. Experiments proposed in Aim 3 will test the hypothesis that acute inflammation-induced alterations in SM metabolism play a key role in TF activation and TF-mediated coagulopathy. Aim 4 focuses on investigating whether altered SM metabolism contributes to inflammation via the regulation of TF activity. In the proposed studies, we will manipulate SM levels in macrophages, endothelial cells, and other cell types by the overexpression or down regulation of various enzymes involved in the SM metabolism or using specific pharmacological inhibitors of these enzymes. We will employ various knock-out mice with altered SM metabolism and murine models of hemostasis and thrombosis to investigate the pathophysiologic relevance of the newly identified mechanism. Our proposed studies will lead to a paradigm shift in our understanding of how TF-mediated coagulation is activated in various disease settings. They may also lead to the development of novel, targeted interventions to prevent thrombosis.