STUDIES OF THE ACTIVATION OF FACTOR-VII BOUND TO TISSUE FACTOR

Experiments were performed to evaluate activation of factor VII bound to relipidated tissue factor (TF) in suspension and to TF constitutive ly expressed on the surface of an ovarian carcinoma cell line (OC-2008 ). Activation was assessed by measuring cleavage of I-125-factor VII a nd by the ability of unlabeled factor VII to catalyze activation of a variant factor IX molecule that, after activation, cannot back-activat e factor VII, Factor Xa was found to effectively activate factor VII b ound to TF relipidated in either acidic or neutral phospholipid vesicl es. Autoactivation of factor VII bound to TF in suspension was depende nt on the preparation of TF apoprotein used and the technique of its r elipidation. This highlights the need for caution in extrapolating dat a from TF in suspension to the activation of factor VII bound to cell surfaces during hemostasis. A relatively slow activation of factor VII bound to OC-2008 monolayers in the absence of added protease was obse rved consistently, Antithrombin in the presence or absence of heparin prevented this basal activation, whereas TF pathway inhibitor (TFPI)/f actor Xa complexes had only a limited inhibitory effect, Adding a subs trate concentration of factor X markedly enhanced basal activation of factor VII, but both TFPI/factor Xa and antithrombin/heparin abolished this enhancement. Overall, our data are compatible with the hypothesi s that not all factor VII/TF complexes formed at a site of tissue inju ry are readily activated to factor VIIa (VIIa)/TF complexes during hem ostasis. The clinical significance of this is discussed. (C) 1996 by T he American Society of Hematology.