The role of LDL in hemostasis

Hemophilia A is a hereditary bleeding disorder resulting from the absence or deficiency of coagulation factor VIII (FVIII). More than 300,000 people worldwide are afflicted with hemophilia A. The severity of hemophilia A is related to the residual FVIII activity (severe less than 1%, moderate 1-5%, and mild between 5-40% FVIII activity); half of the patients have severe hemophilia. However, bleeding severity varies even within the same category of hemophilia. The mechanism is unknown. Compared to an age-matched general population, hemophilia patients have lower overall cardiovascular risk and lower levels of atherogenic low-density lipoprotein (LDL)-cholesterol; the lowest LDL cholesterol levels being observed in those with the most severe bleeding. Also associated with high LDL is impaired fibrinolysis, the primary mechanism to dissolve blood clots. Fibrinolysis is initiated by the serine protease tissue-type plasminogen activator (tPA). A subgroup of patients with severe hemophilia demonstrates a higher tPA concentration and increased fibrinolysis, associated with severe bleeding. Recently we have made several observations that link LDL to hemostasis and thrombosis. We have showed that: 1) LDL promotes thrombosis by enhancing von Willebrand factor (VWF) self-association and subsequent platelet adhesion; 2) tPA binds apolipoprotein B (apoB), the core protein of LDL. The N- terminus of apoB binds tPA’s Kringle 2 domain partially through its lysine-binding site, an interaction that may interfere with tPA’s ability to initiate fibrinolysis. Therefore, we hypothesize that the low LDL in hemophilia may lead to reduced VWF self-association and increased fibrinolysis. Our new preliminary data showed that 1) hemophilia A patients have lower LDL-cholesterol and apoB with the lowest level in the severe group; 2) LDL particles isolated from healthy volunteers or from wild-type mice accelerate clotting time in the whole blood of hemophilia A mice; 3) LDL is associated with circulating FVIII and VWF; 4) lower apoB levels correlate with faster clot lysis by recombinant tPA in hemophilia patients; and 5) a lysine analog partially blocks the interaction between tPA and apoB. Based on these compelling data, the overarching objective of this proposal is to test the central hypothesis that LDL improves hemostasis by increasing platelet adhesion, accelerating clotting time, increasing clot strength, and inhibiting fibrinolysis. We propose three specific aims to test our hypothesis. Aim 1. To test the hypothesis that LDL associates with FVIII-VWF and improves hemostasis. Aim 2. To test the hypothesis that LDL extends FVIII half-life with a slower clearance rate. Aim 3. To test the hypothesis that LDL stabilizes clots by inhibiting fibrinolysis. In summary, by completing the proposed study, we expect to elucidate the important role of LDL in hemostasis and characterize LDL as an antifibrinolytic factor. It has not escaped our notice that the same mechanism by which LDL improves hemostasis in hemophilia may also be pro-thrombotic when there are no hemostatic defects. This study's results have broad implications for understanding the potential pro-thrombotic effects of LDL in cardiovascular diseases.