Tissue factor pathway inhibitor-alpha inhibits prothrombinase during the initiation of blood coagulation

Abstract

Tissue factor (TF) pathway inhibitor (TFPI) is a well-characterized activated factor X (FXa)-dependent inhibitor of TF-initiated coagulation produced in two alternatively spliced isoforms, TFPIα and TFPIβ. The TFPIα C terminus has a basic sequence nearly identical to a portion of the factor V (FV) B domain necessary for maintaining FV in an inactive conformation via interaction with an acidic region of the B domain. We demonstrate rapid inhibition of prothrombinase by TFPIα mediated through a high-affinity exosite interaction between the basic region of TFPIα and the FV acidic region, which is retained in FXa-activated FVa and platelet FVa. This inhibitory activity is not mediated by TFPIβ and is lost upon removal of the acidic region of FVa by thrombin. The data identify a previously undescribed, isoform-specific anticoagulant function for TFPIα and are a unique description of physiologically relevant inhibition of prothrombinase. These findings, combined with previous descriptions of differential expression patterns of TFPIα and TFPIβ in platelets and endothelial cells, suggest that the TFPI isoforms may act through distinct mechanisms to inhibit the initial stages of intravascular coagulation, with TFPIβ acting to dampen TF expressed on the surface of vascular cells, whereas TFPIα dampens the initial prothrombinase formed on the activated platelet surface.hemophilia | bleeding | thrombosis

Platelets Contain Tissue Factor Pathway Inhibitor-2 Derived from Megakaryocytes and Inhibits Fibrinolysis

“…These data confirm previous observations (47,54,56) and further suggest that, similar to TFPI-1, the C-terminal basic region of TFPI-2 plays an essential role in its interaction with FV. Our data are also consistent with the earlier observations that TFPI-1 binds to FVa (47) but with 10 -20-fold reduced affinity (56 (70) are involved in this interaction is not known. However, concentrations of full-length TFPI-1 (71) and TFPI-2 in platelets or plasma are quite low to appreciably bind to FVa, suggesting this interaction is not biologically significant.…”

“…Five basic amino acids (KRKRK) in the C-terminal region of TFPI-1 have been implicated in binding to the B-domain AR segment of FV AR , a molecule that lacks the BR region (56). Because TFPI-1 and TFPI-2 compete with each other in binding to FV, and the C-terminal of TFPI-2 also has five conserved basic amino acids (KKKKK; Table 2), we predict that these residues bind to the FV-AR.…”

Factor V‐short and protein S as synergistic tissue factor pathway inhibitor (TFPIα) cofactors

“…It has been proposed that the two highly charged regions in the B domain interact with each other, an interaction that is crucial for keeping FV in a procofactor state 1, 31, 32, 33. In a recombinant B‐domain truncated FV‐variant denoted FV‐810 (deletion of residues 810‐1492), the high affinity‐binding site for TFPIα has been localized to the acidic C‐terminal region of the B domain 12. It is therefore most likely that the same region in FV‐Short is the high affinity‐binding site for TFPIα 23.…”

“…TFPIα, the full‐length isoform of TFPI, comprises three Kunitz domains and a highly charged C‐terminal extension that contains the binding site for the acidic region in the B‐domain of FV 11, 12. The full length TFPIα (≈0.2 nmol L −1 in plasma) constitutes around 20% of total TFPI in circulation 13.…”

“…Family members carrying the mutation have high plasma concentrations of FV‐Short (≈2‐5 nmol L −1 ). The C‐terminus of the truncated B domain of FV‐Short exposes the negatively charged cluster to which TFPIα binds with high affinity 12, 23. As a result, the TFPIα concentration in circulation increases 10‐fold and causes a moderately severe bleeding phenotype.…”