The C-terminus of tissue factor pathway inhibitor-α inhibits factor V activation by protecting the Arg¹⁵⁴⁵ cleavage site

Essentials The C-terminus of tissue factor pathway inhibitor (TFPIα) binds to the B-domain of factor V (FV). The functional consequences of this interaction were investigated in plasma and model systems. The TFPIα C-terminus inhibited thrombin generation in plasma, but not in the presence of FVa. The TFPIα C-terminus inhibited FV activation by preventing cleavage at Arg¹⁵⁴⁵ .

Summary: Background Factor V (FV) is a carrier and a cofactor of the anticoagulant protein tissue factor pathway inhibitor-α (TFPIα), whose basic C-terminus binds to an acidic region in the B-domain of FV. Proteolysis of FV at Arg⁷⁰⁹ , Arg¹⁰¹⁸ and Arg¹⁵⁴⁵ by activated FX (FXa) or thrombin removes the B-domain, and converts FV into a procoagulant cofactor (activated FV [FVa]) of FXa in the prothrombinase complex. However, retention of the acidic region in partially activated FV makes prothrombinase activity susceptible to inhibition by TFPIα. Objective/Methods To investigate the effect of the TFPIα C-terminal peptide (TFPIα C-term) on thrombin generation in plasma and on FV activation in model systems. Results TFPIα C-term inhibited tissue factor-initiated and FXa-initiated thrombin generation in a dose-dependent manner. Failure to inhibit thrombin generation in FV-depleted plasma reconstituted with FVa indicated that the peptide effect was mediated by the acidic region of FV, and was localized at the level of FV activation and/or prothrombinase. In model systems, TFPIα C-term inhibited both FV activation and prothrombinase activity. Western blot analysis showed that the peptide impaired cleavage at Arg¹⁵⁴⁵ by both thrombin and FXa. The inhibition was stronger for FV-short, which binds TFPIα with higher affinity. Similar results were obtained with full-length TFPIα. Conclusions Cleavage of FV at Arg¹⁵⁴⁵ , which abolishes the anticoagulant properties of FV and commits FV to the procoagulant pathway, is inhibited by binding of the TFPIα C-terminus to the FV acidic region. Possible targets of this new anticoagulant function of TFPIα are low-abundance FV(a) species retaining the acidic region.