Prothrombinase is protected from inactivation by tissue factor pathway inhibitor: competition between prothrombin and inhibitor

Activated protein C, protein S, and tissue factor pathway inhibitor cooperate to inhibit thrombin activation

INTRODUCTION

Thrombin, the enzyme which converts fibrinogen into a fibrin clot, is produced by the prothrombinase complex, composed of factor Xa (FXa) and factor Va (FVa). Down-regulation of this process is critical, as excess thrombin can lead to life-threatening thrombotic events. FXa and FVa are inhibited by the anticoagulants tissue factor pathway inhibitor alpha (TFPIα) and activated protein C (APC), respectively, and their common cofactor protein S (PS). However, prothrombinase is resistant to either of these inhibitory systems in isolation.

MATERIALS AND METHODS

We hypothesized that these anticoagulants function best together, and tested this hypothesis using purified proteins and plasma-based systems.

RESULTS

In plasma, TFPIα had greater anticoagulant activity in the presence of APC and PS, maximum PS activity required both TFPIα and APC, and antibodies against TFPI and APC had an additive procoagulant effect, which was mimicked by an antibody against PS alone. In purified protein systems, TFPIα dose-dependently inhibited thrombin activation by prothrombinase, but only in the presence of APC, and this activity was enhanced by PS. Conversely, FXa protected FVa from cleavage by APC, even in the presence of PS, and TFPIα reversed this protection. However, prothrombinase assembled on platelets was still protected from inhibition, even in the presence of TFPIα, APC, and PS.

CONCLUSIONS

We propose a model of prothrombinase inhibition through combined targeting of both FXa and FVa, and that this mechanism enables down-regulation of thrombin activation outside of a platelet clot. Platelets protect prothrombinase from inhibition, however, supporting a procoagulant environment within the clot.

Polyphosphate as an antithrombotic target and hemostatic agent

Polyphosphate (PolyP) is a polymer comprised of linear phosphate units connected by phosphate anhydride bonds. PolyP exists in a diverse range of eukaryotes and prokaryotes with varied chain lengths ranging from six to thousands of phosphate units. Upon activation, human platelets and neutrophils release short-chain PolyP, along with other components, to initiate the coagulation pathway. Long-chain PolyP derived from cellular or bacterial organelles exhibits higher proinflammatory and procoagulant effects compared to short-chain PolyP. Notably, PolyP has been identified as a low-hemorrhagic antithrombotic target since neutralizing plasma PolyP suppresses the thrombotic process without impairing the hemostatic functions. As an inorganic polymer without uniform steric configuration, PolyP is typically targeted by cationic polymers or recombinant polyphosphatases rather than conventional antibodies, small-molecule compounds, or peptides. Additionally, because of its procoagulant property, PolyP has been incorporated in wound-dressing materials to facilitate blood hemostasis. This review summarizes current studies on PolyP as a low-hemorrhagic antithrombotic target and the development of hemostatic materials based on PolyP.

Role of exosite binding modulators in the inhibition of Fxa by TFPI

Tissue factor pathway inhibitor (TFPI) down-regulates the extrinsic coagulation pathway by inhibiting FXa and FVIIa. Both TFPI and FXa interact with several plasma proteins (e. g. prothrombin, FV/FVa, protein S) and non-proteinaceous compounds (e. g. phospholipids, heparin). It was our aim to investigate effects of ligands that bind to FXa and TFPI on FXa inhibition by full-length TFPI (designated TFPI) and truncated TFPI (TFPI1-150). Inhibition of FXa by TFPI and TFPI1-150 and effects of phospholipids, heparin, prothrombin, FV, FVa, and protein S thereon was quantified from progress curves of conversion of the FXa-specific chromogenic substrate CS11-(65). Low concentrations negatively charged phospholipids (~10 μM) already maximally stimulated (up to 5- to 6-fold) FXa inhibition by TFPI. Unfractionated heparin at concentrations (0.2–1 U/ml) enhanced FXa inhibition by TFPI ~8-fold, but impaired inhibition at concentrations > 1 U/ml. Physiological protein S and FV concentrations both enhanced FXa inhibition by TFPI 2- to 3-fold. In contrast, thrombin-activated FV (FVa) impaired the ability of TFPI to inhibit FXa. FXa inhibition by TFPI1–150 was not affected by FV, FVa, protein S, phospholipids and heparin. TFPI potently inhibited FXa-catalysed prothrombin activation in the absence of FVa, but hardly inhibited prothrombin activation in the presence of thrombin-activated FVa. In conclusion, physiological concentrations TFPI (0.25–0.5 nM TFPI) inhibit FXa with a t1/2 between 3–15 minutes. Direct FXa inhibition by TFPI is modulated by physiological concentrations prothrombin, FV, FVa, protein S, phospholipids and heparin indicating the importance of these modulators for the in vivo anticoagulant activity of TFPI.

TFPIα interacts with FVa and FXa to inhibit prothrombinase during the initiation of coagulation

Tissue factor pathway inhibitor α (TFPIα) inhibits prothrombinase, the thrombin-generating complex of factor Xa (FXa) and factor Va (FVa), during the initiation of coagulation. This inhibition requires binding of a conserved basic region within TFPIα to a conserved acidic region in FXa-activated and platelet-released FVa. In this study, the contribution of interactions between TFPIα and the FXa active site and FVa heavy chain to prothrombinase inhibition were examined to further define the inhibitory biochemistry. Removal of FXa active site binding by mutation or by deletion of the second Kunitz domain (K2) of TFPIα produced 17- or 34-fold weaker prothrombinase inhibition, respectively, establishing that K2 binding to the FXa active site is required for efficient inhibition. Substitution of the TFPIα basic region uncharged residues (Leu252, Ile253, Thr255) with Ala (TFPI-AAKA) produced 5.8-fold decreased inhibition. This finding was confirmed using a basic region peptide (Leu252-Lys261) and Ala substitution peptides, which established that the uncharged residues are required for prothrombinase inhibitory activity but not for binding the FVa acidic region. This suggests that the uncharged residues mediate a secondary interaction with FVa subsequent to acidic region binding. This secondary interaction seems to be with the FVa heavy chain, because the FV Leiden mutation weakened prothrombinase inhibition by TFPIα but did not alter TFPI-AAKA inhibitory activity. Thus, efficient inhibition of prothrombinase by TFPIα requires at least 3 intermolecular interactions: (1) the TFPIα basic region binds the FVa acidic region, (2) K2 binds the FXa active site, and (3) Leu252-Thr255 binds the FVa heavy chain.

Reduced Prothrombinase Inhibition by Tissue Factor Pathway Inhibitor Contributes to the Factor V Leiden Hypercoagulable State

Activated factor V (FVa) and factor X (FXa) form prothrombinase, which converts prothrombin to thrombin. The α isoform of tissue factor (TF) pathway inhibitor (TFPI) dampens early procoagulant events, partly by interacting with FV. FV Leiden (FVL) is the most common genetic thrombophilia in Caucasians. Thrombosis risk is particularly elevated in women with FVL taking oral contraceptives, which produce acquired TFPIα deficiency. In mice, FVL combined with 50% reduction in TFPI causes severe thrombosis and perinatal lethality. However, a possible interaction between FVL and TFPIα has not been defined in humans. Here, we examined this interaction using samples from patients with FVL in thrombin generation and fibrin formation assays. In dilute TF- or FXa-initiated reactions, these studies exposed a TFPI-dependent activation threshold for coagulation initiation that was greatly reduced by FVL. The reduced threshold was progressively overcome with higher concentrations of TF or FXa. Plasma assays using anti-TFPI antibodies or a TFPI peptide that binds and inhibits FVa demonstrated that the decreased activation threshold resulted from reduced TFPIα inhibition of prothrombinase. In assays using purified proteins, TFPIα was a 1.7-fold weaker inhibitor of prothrombinase assembled with FVL than with FV. Thus, FVL reduces the threshold for initiating coagulation, and this threshold is further reduced in situations of low TFPIα concentration. Individuals with FVL are likely prone to thrombosis in response to weak procoagulant stimuli that would not initiate blood clot formation in individuals with FV.

Blocking inhibition of prothrombinase by tissue factor pathway inhibitor alpha: a procoagulant property of heparins

Unfractionated heparin (UFH) has procoagulant activity in antithrombin/heparin cofactor II (HCII)-depleted plasma. UFH prevents tissue factor pathway inhibitor alpha (TFPIα) from inhibiting the procoagulant enzyme complex, prothrombinase, providing a possible mechanism for its procoagulant activity. The procoagulant potential of UFH and various low molecular weight heparins (LMWHs) were characterized for TFPIα dependence, using thrombin generation assays performed with antithrombin/HCII-depleted plasma. UFH, the LMWHs enoxaparin and dalteparin, and the low anticoagulant LMWH 2-O, 3-O desulphated heparin (ODSH) all promoted thrombin generation, but fondaparinux did not, and this activity was blocked by a TFPIα antibody. UFH, enoxaparin, and dalteparin were anticoagulant in reactions containing 1-2% normal plasma. In prothrombinase activity assays, UFH, enoxaparin, dalteparin and ODSH blocked prothrombinase inhibition by TFPIα, while again fondaparinux did not. In both the plasma and purified assays, LMWHs displayed greater procoagulant potential than UFH, even when normalized to saccharide concentration. These biochemical data reveal that UFH and LMWHs, but not fondaparinux, block prothrombinase inhibition by TFPIα, thereby producing their paradoxical procoagulant activity observed in the absence of antithrombin/HCII. The findings may help to understand the complex pathophysiology and treatment of patients that are simultaneously bleeding and clotting, such as those with disseminated intravascular coagulation.

Biology of tissue factor pathway inhibitor

Recent studies of the anticoagulant activities of the tissue factor (TF) pathway inhibitor (TFPI) isoforms, TFPIα and TFPIβ, have provided new insight into the biochemical and physiological mechanisms that underlie bleeding and clotting disorders. TFPIα and TFPIβ have tissue-specific expression patterns and anticoagulant activities. An alternative splicing event in the 5' untranslated region allows for translational regulation of TFPIβ expression. TFPIα has 3 Kunitz-type inhibitor domains (K1, K2, K3) and a basic C terminus, whereas TFPIβ has the K1 and K2 domains attached to a glycosylphosphatidyl inositol-anchored C terminus. TFPIα is the only isoform present in platelets, whereas endothelial cells produce both isoforms, secreting TFPIα and expressing TFPIβ on the cell surface. TFPIα and TFPIβ inhibit both TF-factor VIIa-dependent factor Xa (FXa) generation and free FXa. Protein S enhances FXa inhibition by TFPIα. TFPIα produces isoform-specific inhibition of prothrombinase during the initiation of coagulation, an anticoagulant activity that requires an exosite interaction between its basic C terminus and an acidic region in the factor Va B domain. Platelet TFPIα may be optimally localized to dampen initial thrombin generation. Similarly, endothelial TFPIβ may be optimally localized to inhibit processes that occur when endothelial TF is present, such as during the inflammatory response.

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

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.

Studies on the mechanism of action of the aptamer BAX499, an inhibitor of tissue factor pathway inhibitor

INTRODUCTION

Promoting thrombin generation by inhibiting tissue factor pathway inhibitor (TFPI) is a potentially viable therapeutic approach to the prevention and/or treatment of bleeding in hemophilia. In this report, we studied the interaction between an aptamer (BAX499; formerly ARC19499) and TFPI that resulted in inhibition of TFPI-mediated regulation of the tissue factor pathway.

MATERIALS AND METHODS

Enzyme kinetic analyses were performed to study the interaction between BAX499 and recombinant TFPI against factor Xa, the extrinsic Xase and prothrombinase activities. Diluted prothrombin time assay was used to investigate the effects of BAX499 on factor VIII-deficient plasma collected from hemophilia patients.

RESULTS

Our results indicate that after binding of BAX499 to TFPI, the TFPI/ BAX499 complex retains factor Xa inhibitory activity, albeit with reduced affinity. When tested in an extrinsic Xase activity assay, BAX499 delayed TFPI-mediated inhibition of extrinsic Xase activity. In addition, BAX499 reversed TFPI inhibition of the prothrombinase complex. BAX499 shortened the dilute prothrombin time in factor VIII-deficient plasma, and when added to freshly drawn hemophilia A blood either with or without a factor VIII inhibitor, the whole blood clotting time was also shortened. These results suggest that BAX499 may be a useful addition to the armamentarium of bypassing agents to control bleeding in hemophilic patients with inhibitors.

Antidote-controlled antithrombotic therapy targeting factor IXa and von Willebrand factor

Thrombotic disorders and their common clinical phenotypes of acute myocardial infarction, ischemic stroke, and venous thromboembolism are the proximate cause of substantial morbidity, mortality, and health care expenditures worldwide. Accordingly, therapies designed to attenuate thrombus initiation and propagation, reflecting integrated platelet-mediated and coagulation protease-mediated events, respectively, represent a standard of care. Unfortunately, there are numerous inherent limitations of existing therapies that include target nonselectivity, variable onset and offset of pharmacodynamic effects, a narrow efficacy-safety profile, and the absence of a safe and reliable platform for either accurate titration, based on existing patient-specific, disease-specific, and clinical conditions, or active reversibility. Herein, we summarize our experience with oligonucleotide antithrombotic agents and their complementary antidotes, targeting the platelet adhesive protein von Willebrand factor and the pivotal coagulation protease factor IXa.

The coagulation cascade in cirrhosis

The coagulation "cascade" model accurately represents the mechanisms of the prothrombin time and activated partial thromboplastin time tests. However, these tests and the "cascade" model do not accurately reflect the risk of hemorrhage or thrombosis in vivo. In hepatic insufficiency, a balanced reduction in the levels of most of pro- and anticoagulant proteins produced in the liver does not impair thrombin generation until levels are quite low. However, the ability of the coagulation system to tolerate or recover from an insult is markedly impaired in liver disease. This allows the coagulation system to be more easily tipped into a state favoring either hemorrhage or thrombosis.

Factor IXa inhibitors as novel anticoagulants

Currently available anticoagulants are limited by modest therapeutic benefits, narrow clinical applications, increased bleeding risk, and drug-induced thrombophilia. Because factor IX plays a pivotal role in tissue factor (TF)-mediated thrombin generation, it may represent a promising target for drug development. Several methods of attenuating factor IX activity, including monoclonal antibodies, synthetic active site-blocked competitive inhibitors, oral inhibitors, and RNA aptamers, have undergone investigation. This review summarizes present knowledge of factor IX inhibitors with emphasis on biology, pharmacology, preclinical data, and early-phase clinical experience in humans.

Fusion proteins comprising annexin V and Kunitz protease inhibitors are highly potent thrombogenic site-directed anticoagulants

The anionic phospholipid, phosphatidyl-L-serine (PS), is sequestered in the inner layer of the plasma membrane in normal cells. Upon injury, activation, and apoptosis, PS becomes exposed on the surfaces of cells and sheds microparticles, which are procoagulant. Coagulation is initiated by formation of a tissue factor/factor VIIa complex on PS-exposed membranes and propagated through the assembly of intrinsic tenase (factor VIIIa/factor IXa), prothrombinase (factor Va/factor Xa), and factor XIa complexes on PS-exposed activated platelets. We constructed a novel series of recombinant anticoagulant fusion proteins by linking annexin V (ANV), a PS-binding protein, to the Kunitz-type protease inhibitor (KPI) domain of tick anticoagulant protein, an aprotinin mutant (6L15), amyloid beta-protein precursor, or tissue factor pathway inhibitor. The resulting ANV-KPI fusion proteins were 6- to 86-fold more active than recombinant tissue factor pathway inhibitor and tick anticoagulant protein in an in vitro tissue factor-initiated clotting assay. The in vivo antithrombotic activities of the most active constructs were 3- to 10-fold higher than that of ANV in a mouse arterial thrombosis model. ANV-KPI fusion proteins represent a new class of anticoagulants that specifically target the anionic membrane-associated coagulation enzyme complexes present at sites of thrombogenesis and are potentially useful as antithrombotic agents.

Platelets and thrombin generation

This review examines the evidence that platelets play a major role in localizing and controlling the burst of thrombin generation leading to fibrin clot formation. From the first functional description of platelets, it has been recognized that platelets supply factors that support the activation of prothrombin. Studies have demonstrated that on activation, the amount of one specific lipid, phosphatidylserine, is significantly increased on the outer leaflet of platelet membranes. When it was found that phosphatidylserine containing lipid extracts could be substituted for platelets in clotting assays, this suggested the possibility that changes in platelet lipid composition were necessary and sufficient to account for platelet surface thrombin generation. Because a growing body of data suggest that platelet-binding proteins provide much of the specificity for platelet thrombin generation, we review in this report data suggesting that changes in lipid composition are necessary but not sufficient to account for platelet surface regulation of thrombin generation. Also, we review data suggesting that platelets from different individuals differ in their capacity to generate thrombin, whereas platelets from a single subject support thrombin generation in a reproducible manner. Individual differences in platelet thrombin generation might be accounted for by differences in platelet-binding proteins.