Factor VIII and Platelets Synergistically Accelerate Cleavage of von Willebrand Factor by ADAMTS13 under Fluid Shear Stress

Previous studies have demonstrated that factor VIII (FVIII) or platelets alone increase cleavage of von Willebrand factor (VWF) by ADAMTS13 under mechanically induced shear stresses. We show in this study that the combination of FVIII and platelets at the physiological concentrations is more effective than either one alone. In the absence of FVIII, lyophilized platelets increase the formation of cleavage product by 2-3-fold. However, in the presence of physiological concentration of FVIII (1 nM), the formation of VWF cleavage product increases dramatically as a function of increasing platelets with the maximal rate enhancement of similar to 8-fold. Conversely, in the presence of a physiological concentration of lyophilized platelets (150 x 10(3)/mu l), the half-maximal concentration of FVIII required to accelerate VWF proteolysis by ADAMTS13 reduces by similar to 10-fold (to similar to 0.3 nM) compared with that in the absence of platelets (similar to 3.0 nM). Further studies using the FVIII derivative that lacks an acidic region (a3), an antiplatelet glycoprotein 1b alpha IgG, and a purified recombinant VWF-A1 domain or glycoprotein 1b alpha-stripped platelets demonstrate that the synergistic rate-enhancing effect of FVIII and platelets depends on their specific binding interactions with VWF. Our findings suggest that FVIII and platelets are cofactors that regulate proteolysis of multimeric VWF by ADAMTS13 under physiological conditions.