The Neutrophil Proteases, Elastase and Cathepsin G, May Modulate the Activity of Factor VIII.

Neutrophils and monocytes express cathepsin G and elastase and also can bind to activated platelets, thus they can be localized to the site of active coagulation. Early studies suggested that cathepsin G and elastase inactivated factor VIII (FVIII) and were thus anticoagulant. But other studies have suggested procoagulant functions for cathepsin G and elastase in activation of factor V or activation of platelets among other possible mechanisms. Therefore, we investigated the effects of human neutrophil elastase and human neutrophil cathepsin G on FVIII/VIIIa. Elastase does inactivate both FVIII and FVIIIa but cathepsin G activates FVIII while having very little effect on FVIIIa. Cathepsin G activation of FVIII is enhanced by phospholipid vesicles, apparently due to enhanced rate of cleavage and stabilization of the resulting molecule. The maximum level of activation is less than that of thrombin, but it is still four-fold as measured in an APTT assay. Cleavage sites for both proteases in FVIII were identified by Edman degradation and gel analysis. FVIII cleavages are limited to a few specific sites that are mostly located near known activating and inactivating cleavage sites. A notable exception is a cleavage site for elastase after valine 26 in the A1 domain. Cathepsin G cleavage sites near to thrombin cleavage sites likely contribute to the partial activation of FVIII. The unique elastase cleavage site at valine 26 likely contributes to the inactivation of FVIII and FVIIIa. Therefore, it is possible that neutrophils and monocytes may provide some pro-coagulant effect by activating FVIII and may also provide negative feedback by inactivating FVIIIa as well.