Dissociation of the Factor-VIII complex during clotting: role of thrombin and phospholipids

Changes in plasma factor VIII complex and serum lipid profile during atherogenesis in cynomolgus monkeys

If endothelial injury plays a prominent role in early atherogenesis, the plasma levels of von Willebrand factor (VWF), which is made within and normally released from endothelial cells, might be expected to rise as a marker of the cellular damage. To evaluate this hypothesis, we measured plasma VWF (as VIIIR:Ag), factor VIII:C, and serum lipids serially up to 37 weeks in 29 adult male cynomolgus monkeys on an atherogenic diet. Factor VIII:C peaked at 113% above baseline by week 10 (p less than 0.0001), then fell and remained 53% below baseline (p less than 0.04) during weeks 20 to 37. However, the overall rise in VWF was not significant. In contrast, serum cholesterol continued to rise after week 21. Serum phospholipids (PL), triglycerides (TG), and free fatty acids (FFA) showed a temporal pattern similar to VIII:C. Significant positive correlations with VIII:C were noted for PL (r = 0.59, p = 0.0001) and TG (r = 0.36, p = 0.0096). At autopsy, small to moderately advanced atherosclerotic lesions were distributed throughout the aortas of the majority of the animals. We conclude that changes in plasma VIIIR:Ag do not correlate with atherogenesis in this model. However, the similar course of VIII:C, TG, and PL suggests that these substances may be involved and perhaps interrelated early in atherogenesis.

Factor VIII influences binding of factor IX and factor X to intact human platelets

To investigate the influence of factor VIII on the binding of factors IX and X to the surface of intact human platelets, washed collagen-stimulated platelets were incubated with factor IX and factor X in the presence or absence of factor VIII. Platelets were then lysed and IX:Ag and X:Ag were determined in the platelet lysate. Platelet bound IX:Ag was higher after incubation of platelets with factor IX in the presence of native or activated factor VIII than after incubation of platelets with factor IX alone. Thrombin degraded factor VIII did not support binding of factor IX to stimulated platelets. Preactivation of factor IX enhanced binding to platelets; influence by factor VIII on binding to platelets was greater with native factor IX than with preactivated factor IX. Presence of native or activated factor VIII also increased binding of factor X to platelets. In contrast to factor IX, preactivation of factor X did not influence binding to platelets, but in the presence of factor VIII preactivation of factor X increased binding of factor X to platelets. Our data suggest that mediation of binding of factors IX and X to the surface of platelets is one of the mechanisms by that factor VIII exerts its cofactor function in the activation of factor X.

Factor VIII coagulant moiety binds to platelets by binding to phospholipids of the platelet membrane

Washed human platelets were incubated with commercial factor VIII concentrate, or with purified factor VIII coagulant moiety. Platelets were then washed again and lysed by sonication. VIII:Ag and vWf:Ag were measured in the platelet lysate prior to and after incubation of the lysate with phospholipase C (PL-C). Platelet bound VIII:Ag was significantly higher after incubation of washed platelets with factor VIII concentrate than after incubation with buffer. Platelet bound VIII:Ag was further increased when platelets had been incubated with concentrate in the presence of thrombin and collagen. In contrast, only a slight increase in platelet bound vWf:Ag was observed after incubation of platelets with concentrate. When washed platelets had been incubated with factor VIII coagulant moiety, also significantly more platelet bound VIII:Ag was observed than after incubation with buffer. Measurable VIII:Ag, but not vWf:Ag, increased significantly after incubation of the platelet lysate with PL-C. When intact washed platelets had been treated with PL-C prior to the incubation with concentrate, binding of VIII:Ag to platelets was nearly completely abolished. Our data suggest that the factor VIII coagulant moiety binds to phospholipids of the platelet membrane and thereby contributes to the assembly of the factor X activating complex on the platelet surface.