Regulation of factor VIIa activity in plasma: evidence that antithrombin III is the sole plasma protease inhibitor of human factor VIIa

Post-Translational Oxidative Modifications of Hemostasis Proteins: Structure, Function, and Regulation

Reactive oxygen species (ROS) are constantly generated in a living organism. An imbalance between the amount of generated reactive species in the body and their destruction leads to the development of oxidative stress. Proteins are extremely vulnerable targets for ROS molecules, which can cause oxidative modifications of amino acid residues, thus altering structure and function of intra- and extracellular proteins. The current review considers the effect of oxidation on the structural rearrangements and functional activity of hemostasis proteins: coagulation system proteins such as fibrinogen, prothrombin/thrombin, factor VII/VIIa; anticoagulant proteins - thrombomodulin and protein C; proteins of the fibrinolytic system such as plasminogen, tissue plasminogen activator and plasminogen activator inhibitor-1. Structure and function of the proteins, oxidative modifications, and their detrimental consequences resulting from the induced oxidation or oxidative stress in vivo are described. Possible effects of oxidative modifications of proteins in vitro and in vivo leading to disruption of the coagulation and fibrinolysis processes are summarized and systematized, and the possibility of a compensatory mechanism in maintaining hemostasis under oxidative stress is analyzed.

Five Challenging Cases of Hereditary Antithrombin Deficiency Characterized by Thrombosis or Complicated Pregnancy

Hereditary antithrombin deficiency (ATD) is a rare autosomal dominant condition (estimated prevalence 1:500-1:5000). Most ATD patients have AT activity levels 40-60% of normal. We present treatments for venous thromboembolism (VTE) in five cases of hereditary ATD. Four patients had a family history of ATD, and one had a de novo mutation. The majority of patients had a VTE while on prophylactic anticoagulation. AT concentrate augmentation was added in these cases to treat the VTE and for prophylaxis against further episodes. Two patients had significant bleeding events, one had permanent physical sequelae. Two of the patients were pregnant. VTE is a common cause of morbidity and mortality during pregnancy. Although low molecular weight heparins are the drugs of choice during pregnancy, this treatment was inadequate in one patient (developed VTE on therapy). These cases emphasize the need to screen for ATD in young patients (<55 years) presenting with VTE. AT augmentation therapy may be necessary in patients inadequately treated with conventional anticoagulants. Careful monitoring and individualized care are needed in ATD patients, especially those with demonstrated bleeding tendencies.

Two SERPINC1 variants affecting N-glycosylation of Asn224 cause severe thrombophilia not detected by functional assays

Antithrombin deficiency, the most severe congenital thrombophilia, might be underestimated, as some pathogenic variants are not detected by routine functional methods. We have identified two new SERPINC1 variants, p.Glu227Lys and p.Asn224His, in four unrelated thrombophilic patients with early and recurrent thrombosis that had normal antithrombin activity. In one case, the mutation was identified by whole genome sequencing, while in the 3 remaining cases, the mutation was identified by sequencing SERPINC1 based on a single functional positive finding supporting deficiency. The two variants shared a common functional defect, an impaired or null N-glycosylation of Asn224 according to a eukaryotic expression model. Carriers had normal anti-FXa or anti-FIIa activities, but impaired anti-FVIIa activity and a detectable loss of inhibitory function when incubating the plasma 1 hour at 41ºC. Moreover, the beta glycoform of the variants, lacking two N-glycans, had reduced secretion, increased heparin affinity, no inhibitory activity, and a potential dominant negative effect. These results explain the increased thrombin generation observed in carriers. Mutation experiments reflected the role that Lysine residues close to the N-glycosylation sequon have in impairing the efficacy of N-glycosylation. Our study shows new elements involved in the regulation of N-glycosylation, a key post-translational modification that, according to our results affects folding, secretion and function, providing new evidence of the pathogenic consequence of an incorrect N-glycosylation of antithrombin. This study supports that antithrombin deficiency is underestimated and encourages the development of new functional and genetic tests to diagnose this severe thrombophilia.

Recombinant human factor VIIa (rFVIIa) in hemophilia: mode of action and evidence to date

Recombinant activated factor VII (rFVIIa) is a bypassing agent widely used both in the treatment and prevention of hemorrhagic complications due to hemophilia with inhibitor. In such cases, antihemophilic factors cannot be used. The normal physiology of factor VII/ factor VIIa (FVII/FVIIa) in the hemostatic process requires the presence of tissue factor (TF) that links to FVII leading to a FVIIa-TF complex which activates both factor X and factor IX. The therapeutic use of rFVIIa requires high amount of FVIIa. Some studies demonstrate that FVIIa at high doses still requires tissue factor for function, whereas others suggest that FVIIa activates FX directly on the platelet surface, in a TF-independent manner. In the present article, we discuss the arguments supporting both TF-dependent and TF-independent modes of action. Finally, the coexistence of both TF-dependent and TF-independent mechanisms cannot be excluded.

Factor VIIa-antithrombin complex: a possible new biomarker for activated coagulation

The activation of the extrinsic coagulation pathway occurs after endothelial injury when the tissue factor (TF), a transmembrane protein located outside the vasculature, binds factor VII (FVII) or activated FVII (FVIIa). Once formed, the TF-VIIa complex activates both factor IX and X and initiates the coagulation process. The TF-VIIa complex is inhibited by both TF pathway inhibitor (TFPI) and antithrombin (AT). The interaction between TF-VIIa and AT induces FVIIa-AT complex formation, which is released into the plasma. Because AT reacts with FVIIa only when it is bound to TF, the circulating levels of FVIIa-AT reflect the degree of exposure of TF to blood. Preliminary clinical studies have shown higher plasma levels of FVIIa-AT complex both in patients with a prior arterial or venous thrombotic event. Increased plasma levels of FVIIa-AT have also been reported in a number of other prothrombotic conditions – antiphospholipid antibodies, solid and hematological malignancies, pre-eclampsia (PE), obesity and cardiac surgery. However, most of the studies published so far are retrospective and with a limited sample size. Larger prospective clinical studies are needed to confirm these findings and to assess the prognostic role of this possible new biomarker for activated coagulation.

Structure-Function Relationship of the Interaction between Tissue Factor and Factor VIIa

Interactions between tissue factor and factor VIIa are the primary initiators of coagulation in hemostasis and certain thrombotic diseases. Tissue factor, an integral membrane protein expressed extensively outside of the vasculature, is the regulatory protein cofactor for coagulation factor VIIa. Factor VIIa, a trypsin-like serine protease homologous with other blood coagulation proteases, is weakly active when free in solution and must bind its membrane-bound cofactor for physiologically relevant activity. Tissue factor allosterically activates factor VIIa by several mechanisms such as active site positioning, spatial stabilization, and direct interactions with the substrate. Protein-membrane interactions between tissue factor, factor VIIa, and substrates all play critical roles in modulating the activity of this enzyme complex. Additionally, divalent cations such as Ca(2+) and Mg(2+) are critical for correct protein folding, as well as protein-membrane and protein-protein interactions. The contributions of these factors toward tissue factor-factor VIIa activity are discussed in this review.

How it all starts: Initiation of the clotting cascade

The plasma coagulation system in mammalian blood consists of a cascade of enzyme activation events in which serine proteases activate the proteins (proenzymes and procofactors) in the next step of the cascade via limited proteolysis. The ultimate outcome is the polymerization of fibrin and the activation of platelets, leading to a blood clot. This process is protective, as it prevents excessive blood loss following injury (normal hemostasis). Unfortunately, the blood clotting system can also lead to unwanted blood clots inside blood vessels (pathologic thrombosis), which is a leading cause of disability and death in the developed world. There are two main mechanisms for triggering the blood clotting, termed the tissue factor pathway and the contact pathway. Only one of these pathways (the tissue factor pathway) functions in normal hemostasis. Both pathways, however, are thought to contribute to thrombosis. An emerging concept is that the contact pathway functions in host pathogen defenses. This review focuses on how the initiation phase of the blood clotting cascade is regulated in both pathways, with a discussion of the contributions of these pathways to hemostasis versus thrombosis.

The kidneys play an important role in the clearance of rFVIIa in rats

INTRODUCTION

Previous distribution and histological studies have indicated that the kidneys and renal proximal tubular cells play a role in clearance of rFVIIa. However, the relative importance of the kidneys in clearance of rFVIIa has not previously been addressed. The objective of the present study was to evaluate the importance of the kidneys in the clearance process of rFVIIa after iv administration to rats using a nephrectomy model.

MATERIALS AND METHODS

A nephrectomized rat model was established and validated using inulin, a compound primarily cleared by the kidneys, as a test substance and several physiological parameters were monitored to ensure viability and robustness of the model. The model was then used for pharmacokinetic evaluation of renal clearance of rFVIIa. The pharmacokinetic parameters for rFVIIa were evaluated both by use of standard non-compartmental methods and by use of mixed effects methods, where a pharmacokinetic model was used to simultaneously model all data from healthy, sham operated, and nephrectomized rats.

RESULTS

Nephrectomized animals showed stable rectal temperature, SpO2 and pulse and as expected, clearance of inulin was essentially abolished compared to control animals (p<0.001). For rFVIIa, nephrectomy resulted in a clearance and terminal half-life of 34mL/h/kg and 2.8h compared to 68mL/h/kg and1.9h in rats exposed to sham surgery (p<0.0001 for both parameters).

CONCLUSION

The present data show that about 50% of the total clearance of rFVIIa from circulation in rats under isoflurane anaesthesia is due to renal clearance.

Tissue Factor/Factor FVII Complex Inhibitors in Cardiovascular Disease. Are Things Going Well?

Blood coagulation is a complex biological mechanism aimed to avoid bleeding in which a highly regulated and coordinated interplay of specific proteins and cellular components respond quickly to a vascular injury. However, when this mechanisms occurs in the coronary circulation, it has not a “protective” effect, but rather, it plays a pivotal role in determining acute coronary syndromes. Coagulation recognizes Tissue Factor (TF), the main physiological initiator of the extrinsic coagulation pathway, as its starter.

Since TF:VIIa complex is the critical point of the blood coagulation cascade, it is a pharmacological attractive issue for the development of agents with anti thrombotic properties that can exert their activity by inhibiting complex formation and/or its catalytic activity. In fact, it is intuitive that an antithrombotic agent able to inhibit this initial step of the coagulation pathway has several theoretical, extremely important, advantages if compared with drugs active downstream the coagulation pathway, such as FXa or thrombin. The present report gives a brief overview of TF pathophysiology, highlighting the most recent advances in the field of inhibitors of the complex TF/VIIa potentially useful in cardiovascular disease.

Intravascular inhibition of factor VIIa and the analogue NN1731 by antithrombin

NN1731 is a recombinant activated factor VII (rFVIIa) analogue with increased intrinsic activity. This also applies to its reactivity towards antithrombin (AT), the role of which was investigated in a pharmacokinetic (PK) study. NN1731 or rFVIIa was administered to normal and haemophilia A dogs and elimination was measured by FVIIa clot activity, FVIIa- and FVIIa-AT antigen. In vitro AT complex formation was studied in canine plasma spiked with NN1731 or rFVIIa. Based on FVIIa antigen concentrations, PK profiles in normal and haemophilia A dogs were similar for NN1731 and rFVIIa with antigen half lives, t(½) ≈1·8 h. In contrast, PK profiles based on activity measurements were distinctly different. NN1731 induced a strong, short lasting (t(½) ≈0·5 h) pro-coagulant response, whereas rFVIIa induced a lower, longer lasting (t(½) ≈1·1 h) response. Western Blot and FVIIa-AT antigen analysis demonstrated in vivo AT complex formation that accounted for these divergences. AT complex formation with FVIIa or NN1731 in vitro in canine plasma was considerably slower than the in vivo reaction. The results suggest that in vivo inhibition by AT contributes significantly to define drug duration in haemophilia treatment with rFVIIa and in particular with the NN1731 analogue.

Regulation of Thrombin Activity—Pharmacologic and Structural Aspects

Thrombin is an essential serine protease for survival. Since the discovery of heparin in the early twentieth century, significant advances have been made in the understanding of thrombin structure and function in coagulation system. Endogenous anticoagulant proteins in blood tightly regulate thrombin generation, but additional anticoagulant agents may be necessary to suppress excessive thrombin formation or defective anticoagulant proteins. Despite the availability of an array of anticoagulant agents based on chemical and biological engineering technologies, anticoagulation therapy remains a challenge for clinicians in terms of balancing bleeding and thrombosis. The aim of this article is to review endogenous serine protease inhibitors and novel antithrombotic agents in relation to pharmacologic regulation of thrombin.

Antithrombin significantly influences platelet adhesion onto immobilized fibrinogen in an in-vitro system simulating low flow

Background

Adhesion of platelets onto immobilized fibrinogen is of importance in initiation and development of thrombosis. According to a recent increase in evidence of a multiple biological property of antithrombin, we evaluated the influence of antithrombin on platelet adhesion onto immobilized fibrinogen using an in-vitro flow system.

Methods

Platelets in anticoagulated whole blood (29 healthy blood donors) were labelled with fluorescence dye and perfused through a rectangular flow chamber (shear rates of 13 s^-1 to 1500 s^-1). Platelet adhesion onto fibrinogen-coated slips was assessed using a fluorescence laser-scan microscope and compared to the plasma antithrombin activity. Additionally the effect of supraphysiological AT supplementation on platelets adhesion rate was evaluated.

Results

Within a first minute of perfusion, an inverse correlation between platelet adhesion and plasma antithrombin were observed at 13 s^-1 and 50 s^-1 (r = -0.48 and r = -0.7, p < 0.05, respectively). Significant differences in platelet adhesion related to low (92 ± 3.3%) and high (117 ± 4.1%) antithrombin activity (1786 ± 516 U vs. 823 ± 331 U, p < 0.05) at low flow rate (13 s^-1, within first minute) have been found. An in-vitro supplementation of whole blood with antithrombin increased the antithrombin activity up to 280% and platelet adhesion rate reached about 65% related to the adhesion rate in a non-supplemented blood (1.25 ± 0.17 vs. 1.95 ± 0.4 p = 0.008, respectively).

Conclusion

It appears that antithrombin in a low flow system suppresses platelet adhesion onto immobilized fibrinogen independently from its antithrombin activity. A supraphysiological substitution of blood with antithrombin significantly reduces platelet adhesion rate. This inhibitory effect might be of clinical relevance.

Tissue Factor: A Key Molecule in Hemostatic and Nonhemostatic Systems

Tissue factor (also known as tissue thromboplastin or CD142) is the protein that activates the blood clotting system by binding to, and activating, the plasma serine protease, factor VIIa, following vascular injury. Because of its essential role in hemostasis, tissue factor plays a role in pathology associated with hemostasis, triggering the coagulation system in many thrombotic diseases and the coagulopathies associated with sepsis and other forms of disseminated intravascular coagulation. Recent research has also implicated tissue factor in a variety of nonhemostatic roles, including cell signaling, inflammation, vasculogenesis, and tumor growth and metastasis. This review focuses on both the well-known roles of tissue factor in hemostasis and thrombosis and the newer concepts of tissue-factor biology including how it functions as a signaling receptor and the possible role of blood-borne tissue factor in thrombosis.

Inhibition of factor VIIa generation and prothrombin activation by treatment with enoxaparin in patients with unstable angina

Factor VIIa (FVIIa) and thrombin generation occur in patients suffering an acute coronary event. We studied the effect of treatment with enoxaparin on FVIIa and prothrombin activation in patients with unstable angina. Anti-Xa activity, FVIIa, FVII coagulant activity (FVII:C) and FVII antigen (FVII:Ag), free tissue factor pathway inhibitor (TFPI), and prothrombin fragments 1 + 2 (F1+2) were measured in patients' plasma, over a 24-h treatment period with enoxaparin. All 14 patients recruited in the study (mean age 68 years) were treated with a subcutaneous injection of enoxaparin, 1 mg/kg twice daily. Blood was drawn just before, and at different time intervals after, the first injection. Before enoxaparin administration, the levels of FVIIa (4.02 +/- 0.8 ng/ml) and F1+2 (2.68 +/- 0.2 nmol/l) were significantly increased as compared with control subjects (2.3 +/- 0.3 ng/ml and 0.9 +/- 0.1 nmol/l respectively, P < 0.05). Free TFPI, FVII:C and FVII:Ag were within normal ranges. One hour after the first injection of enoxaparin, FVIIa and F1+2 levels decreased by 65% and 50%, respectively, and no significant fluctuations were noted throughout the observation period. The concentrations of FVII:C and FVII:Ag were not modified as compared with baseline values. After each injection, the peak concentrations of free TFPI and anti-Xa activity were observed at 2 and 4 h respectively. The kinetics of FVIIa and F1+2 inhibition did not follow those of anti-Xa activity and TFPI release.

The inhibitors of the tissue factor:factor VII pathway

It is widely accepted that blood coagulation in vivo is initiated during normal hemostasis, as well as during intravascular thrombus formation, when the cell-surface protein, tissue factor (TF), is exposed to the blood as a consequence of vascular injury. In addition to its essential role in hemostasis, tissue factor may be also implicated in several pathophysiological processes, such as intracellular signaling, cell proliferation, and inflammation. For these reasons, the tissue factor:factor VIIa complex has been the subject of intense research focus. Many experimental studies have demonstrated that inhibition of tissue factor:factor VIIa procoagulant activity are powerful inhibitors of in vivo thrombosis and that this approach usually results in less pronounced bleeding tendency, as compared to other "more classical" antithrombotic interventions. Alternative approaches may be represented by transfecting the arterial wall with natural inhibitors of tissue factor:factor VIIa complex, such as tissue factor pathway inhibitor (TFPI), which may result in complete inhibition of local thrombosis without incurring in potentially harmful systemic effects. Additional studies are warranted to determine the efficacy and safety of such approaches in patients.

Tissue factor pathway inhibitor binds to platelet thrombospondin-1

Tissue factor pathway inhibitor (TFPI) is a Kunitz-type serine proteinase inhibitor that down-regulates tissue factor-initiated blood coagulation. The most biologically active pool of TFPI is associated with the vascular endothelium, however, the biochemical mechanisms responsible for its cellular binding are not entirely defined. Proposed cellular binding sites for TFPI include nonspecific association with cell surface glycosaminoglycans and binding to glycosyl phosphatidylinositol-anchored proteins. Here, we report that TFPI binds specifically and saturably to thrombospondin-1 (TSP-1) purified from platelet alpha-granules with an apparent K(D) of approximately 7.5 nm. Binding is inhibited by polyclonal antibodies against TFPI and partially inhibited by the B-7 monoclonal anti-TSP-1 antibody. TFPI bound to immobilized TSP-1 remains an active proteinase inhibitor. Additionally, in solution phase assays measuring TFPI inhibition of factor VIIa/tissue factor catalytic activity, the rate of factor Xa generation was decreased 55% in the presence of TSP-1 compared with TFPI alone. Binding experiments done in the presence of heparin and with altered forms of TFPI suggest that the basic C-terminal region of TFPI is required for TSP-1 binding. The data provide a mechanism for the recruitment and localization of TFPI to extravascular surfaces within a bleeding wound, where it can efficiently down-regulate the procoagulant activity of tissue factor and allow subsequent aspects of platelet-mediated healing to proceed.

Decreased plasma tissue factor pathway inhibitor in women taking combined oral contraceptives

Use of combined oral contraceptives (OC) is associated with a significant risk of thrombosis. The mechanisms of this effect are not clearly defined. Tissue factor pathway inhibitor (TFPI) is a circulating anti-coagulant that inhibits the earliest steps in activation of the extrinsic coagulation pathway. It plays a central role in control of coagulation but its contribution to the thrombotic risk associated with OC has not been assessed. Plasma TFPI antigen and activity, factor VIIa, prothrombin fragments 1&2, von Willebrand antigen, fibrinogen, and low density lipoprotein cholesterol were measured by standard assays in women taking OC (aged 16 to 45 years, n = 40) and age-matched women not taking OC (controls, n = 40). Plasma TFPI antigen did not vary significantly across the menstrual cycle in controls. Women on OC had a 25% reduction in plasma TFPI antigen (median 51.0 ng/ml; 95% confidence intervals [CI] 37.5 to 85.5; control 68.0 ng/ml, CI 61.0 to 95.0; P < 0.001) and a 29% reduction in TFPI activity (78.5 U/ml, CI 57.5 to 107.5; control 111.0 U/ml, CI 79.5 to 171.0; P < 0.001) compared to controls. Plasma factor VIIa activity and prothrombin fragments 1&2 were also significantly increased in women using OC (both P < 0.001), indicating activation of the extrinsic coagulation pathway. These results demonstrate that normal cyclic variations in estrogen and/or progesterone do not significantly alter plasma TFPI levels. However, estrogens and/or progestogens in OC result in activation of the extrinsic coagulation pathway and significantly reduce plasma TFPI, its major circulating inhibitor. Reduced plasma TFPI levels may underlie the thrombotic effects of OC.

Quantification and characterization of human endothelial cell-derived tissue factor pathway inhibitor-2

Tissue factor pathway inhibitor-2 (TFPI-2), also known as placental protein 5, is a serine protease inhibitor consisting of three tandemly-arranged Kunitz-type protease inhibitor domains. While TFPI-2 is a potent inhibitor of trypsin, plasmin, kallikrein, and factor XIa in the test tube, the function of this inhibitor in vivo remains unclear. In the present study, we investigated the synthesis and secretion of TFPI-2 by cultured endothelial cells derived from human umbilical vein, aorta, saphenous vein, and dermal microvessels to gain insight into its biological function. While all endothelial cells examined synthesized and secreted TFPI-2, dermal microvascular endothelial cells synthesized threefold to sevenfold higher levels of TFPI-2. Approximately 60% to 90% of the TFPI-2 secreted by endothelial cells was directed to the subendothelial extracellular matrix (ECM). When cultured human umbilical vein endothelial cells were stimulated with inflammatory mediators such as phorbol 12-myristate,13-acetate; endotoxin; and tumor necrosis factor-alpha, TFPI-2 synthesis by these cells increased twofold to 14-fold. Recombinant TFPI-2 bound to dermal microvascular endothelial cell monolayers and its ECM in a specific, dose-dependent, and saturable manner with Kd values of 21 and 24 nmol/L, respectively. TFPI-2 interacted with 4.5 X 10(10) sites/cm2 (3 X 10[5] sites/cell) and 2.3 X 10(11) sites/cm2 on endothelial cells and ECM, respectively. In the presence of rabbit anti-TFPI-2 IgG, but not preimmune IgG, endothelial cells dissociated from the culture flask in a time- and IgG concentration-dependent manner. Our findings provide evidence that endothelial cell-derived TFPI-2 is primarily secreted into the abluminal space and presumably plays an important role in maintaining the integrity of the ECM essential for cell attachment.

Plasma-activated factor VII level in patients positive for lupus anticoagulant

We examined plasma levels of activated factor VII (F VIla) in 50 patients positive for lupus anticoagulant (LA), in 83 patients negative for LA, and in 10 healthy volunteers as controls. Plasma F VIIa was present in healthy volunteers; its level was significantly increased, compared to the level in the controls, in patients with thrombosis, collagen diseases, and disseminated intravascular coagulation (DIC), suggesting that it reflected a thrombotic state. Plasma F VIIa was correlated with thrombin-antithrombin complex (TAT) in patients negative for LA but showed no such correlation in those positive for LA. Plasma F VIIa was negatively correlated with activated partial thromboplastin time (APTT) in patients positive for LA, but not in those negative for LA, suggesting that LA could inhibit the F VIIa assay system. Plasma F VIIa level was significantly increased in patients with thrombotic diseases; however, in patients positive for LA, it is possible that increased plasma F VIIa level may not be correlated with thrombogenicity.

Tissue factor pathway

Blood coagulation is initiated in response to vessel damage in order to preserve the integrity of the mammalian vascular system. The coagulation cascade can also be initiated by mediators of the inflammatory response, and fibrin deposition has been noted in a variety of pathological states. The cascade of coagulation zymogen activations which leads to clot formation is initiated by exposure of flowing blood to tissue factor (TF), the cellular receptor and cofactor for factor VII (FVII). FVII binds to the receptor in a 1:1 stoichiometric complex and is rapidly activated. FVIIa undergoes an active site transition upon binding TF in the presence of calcium which enhances the fundamental properties of the enzyme. This results in rapid autocatalytic activation of FVII to VIIa thereby amplifying the response by generating more TF-VIIa complexes. The TF-VIIa activates both FIX and FX. Further FXa generation by the IXa-VIIIa-Ca(2+)-phospholipid complex is required to sustain the coagulation mechanism, since the TF-VIIa complex is rapidly inactivated. Structure and function studies have identified a number of regions on both TF and FVII involved in this interaction. It is clear, however, that the molecular structures of TF, FVII and the TF-VII complex will have to be solved before we fully understand this complex interaction. The activity of the TF-VIIa complex is controlled by two inhibitors:tissue factor pathway inhibitor (TFPI) and antithrombin III (AT-III). TFPI circulates in plasma, is associated with vascular cell surface and is released from platelets following stimulation by thrombin. TFPI requires the formation of an active TF-VIIa complex and FXa generation before inhibition can occur. Similarly, AT-III which is unable to inhibit circulating FVIIa requires the formation of the TF-VIIa complex. TFPI prevents further participation of TF in the coagulation process by forming a stable quaternary complex, TF-VIIa-Xa-TFPI. In contrast, the AT-III-VIIa complex is thought to dissociate from TF allowing it to interact with additional FVII-VIIa. TFPI has been considered the primary regulator of TF-VIIa activity during haemostasis. Whether AT-III in the presence of glycosaminoglycans on cell surfaces expressing TF can function as an auxiliary second physiological regulator is not known.

Effect of nafamostat mesilate, a synthetic protease inhibitor, on tissue factor-factor VIIa complex activity

Nafamostat mesilate (NM), a synthetic protease inhibitor, is frequently used for the treatment of disseminated intravascular coagulation (DIC) in Japan. NM inhibits several proteases which may be importantly involved in the pathophysiology of DIC. Since tissue factor (TF) plays a critical role in DIC associated with septicemia, inhibition of the extrinsic pathway of coagulation by coagulation inhibitors may be useful for the treatment of DIC. NM inhibited extrinsic pathway activity (TF-F.VIIa mediated-F.Xa generation) in a concentration dependent manner; the IC50 was 1.0 x 10(-7) M. F.Xa was not inhibited by NM at the concentrations used in the experiment, suggesting that NM might inhibit TF-F.VIIa complex activity. When incubated with TF-F.VIIa complex, NM inhibited the complex activity with an IC50 of 1.5 x 10(-7) M, the same value that found for inhibition of extrinsic pathway activity. A Lineweaver-Bulk's plot of the inhibition demonstrated that NM inhibited TF-F.VIIa complex in a competitive fashion, with an inhibition constant (Ki) of 2.0 x 10(-7) M. These findings suggested that NM may be a potent inhibitor of TF-F.VIIa complex and the therapeutic effect of NM in DIC patients could be partly explained by inhibition of the extrinsic pathway of the coagulation system.

Aprotinin is a competitive inhibitor of the factor VIIa-tissue factor complex

A highly purified preparation of human plasma factor VIIa was submitted to chromogenic assays with S-2288 factors IXa, Xa, activated protein C and thrombin being absent. Factor VIIa alone or in the presence of calcium, kept its activity even in the presence of high concentrations of aprotinin, inhibition appeared only in the presence of a factor VIIa-tissue factor complex. A two-stage amidolytic assay using activation of purified factor X and hydrolysis of S-2765 chromogenic substrate by the generated Xa was used to show a competitive inhibition with a Ki value of 30 microM. Aprotinin had no effect on factor Xa amidolytic activity per se. The factor VIIa-tissue factor complex could be adsorbed to immobilized aprotinin and removed by a chaotropic ion like KSCN 3 M. The assays with the DFP inactivated VIIa-tissue factor complex proved that the interaction involved the active site of factor VIIa. The inhibition of the VIIa-tissue factor complex was demonstrated in a clotting assay using aprotinin enriched normal or factor VIII deficient plasma.

Tissue factor as a tumor procoagulant

Tissue factor is a cell surface glycoprotein responsible for initiating the extrinsic pathway of coagulation. Many tumor cell homogenates and intact tumor cells have been shown to contain tissue factor activity. Immunohistochemical studies show that many tumors associated with Trousseau's syndrome express tissue factor on their cell surfaces. Tumor cells shed membrane fragments which carry tissue factor that can account for the activation of the clotting system. Tumor cells also produce soluble substances that can induce tissue factor expression on host cells, such as endothelium and monocytes, at sites distant from the tumor. Although, all the functional TF molecules are localized on the outer cell membrane in many tumor cells, the procoagulant activity on the intact cell surface is largely dormant and can be greatly enhanced upon cell injury or damage. Tissue factor procoagulant activity on the cell surface can be modulated by alterations in the plasma membrane without loss of cell viability. Tissue factor activity on cell surfaces is largely regulated by a plasma inhibitor, tissue factor pathway inhibitor. This inhibitor binds to both functional and non-functional tissue factor/VIIa complexes on the cell surface and prevents non-functional tissue factor/VIIa complexes from becoming functional after cell injury or lysis. Heparin, but not warfarin, therapy is effective in preventing the occurrence of devastating thrombotic events in patients with Trousseau's syndrome and the reason(s) for this are still unknown.

Activation of factor VII bound to tissue factor: a key early step in the tissue factor pathway of blood coagulation

Whether the factor VII/tissue factor complex that forms in tissue factor-dependent blood coagulation must be activated to factor VIIa/tissue factor before it can activate its substrates, factor X and factor IX, has been a difficult question to answer because the substrates, once activated, back-activate factor VII. Our earlier studies suggested that human factor VII/tissue factor cannot activate factor IX. Studies have now been extended to the activation of factor X. Reaction mixtures were made with purified factor VII, X, and tissue factor; in some experiments antithrombin III and heparin were added to prevent back-activation of factor VII. Factor X was activated at similar rates in reaction mixtures containing either factor VII or factor VIIa after an initial 30-sec lag with factor VII. In reaction mixtures with factor VII a linear activation of factor X was established several minutes before cleavage of 125I-labeled factor VII to the two-chain activated molecule was demonstrable on gel profiles. Adding antithrombin III and heparin blocked activation of factor X by factor VII/tissue factor but not by factor VIIa/tissue factor. When the antithrombin III and heparin were added 1 min after the other reagents, factor VII/tissue factor activation of factor X was not blocked. These data suggest that factor VII/tissue factor cannot activate measurable amounts of factor X over several minutes. Overall, our results support the hypothesis that a rapid preferential activation of factor VII bound to tissue factor by trace amounts of factor Xa is a key early step in tissue factor-dependent blood coagulation.

Inhibition of human factor VIIa-tissue factor activity by placental anticoagulant protein

Previous studies indicated that human placental anticoagulant protein, a member of the lipocortin family, prolonged the clotting time of normal plasma when clotting was induced by brain thromboplastin or by kaolin in the presence of cephalin and calcium. Using a two-stage amidolytic assay to assess factor X activation and a tritiated peptide release assay to assess factor IX activation, we have examined the ability of purified preparations of placental anticoagulant protein (Mr = 36.5 kDa) to inhibit the activation of either factor X or factor IX by a complex of human factor VIIa-tissue factor. Placental anticoagulant protein markedly inhibits factor X and factor IX activation by factor VIIa-tissue factor in a non-competitive manner with Ki values of 40 nM and 70 nM, respectively. Placental anticoagulant protein had no effect on factor Xa amidolytic activity, and its inhibitory activity was not diminished by prior incubation with antibody raised against partially purified plasma extrinsic pathway inhibitor. Binding of placental anticoagulant protein to phospholipid vesicles, crude tissue factor and purified, relipidated human brain tissue factor apoprotein was observed only in the presence of calcium ions. These results indicate that placental anticoagulant protein is a potent factor VIIa-tissue factor inhibitor and suggests that its mechanism of action involves binding to the phospholipid portion of the tissue factor lipoprotein.