The dual role of factor VII in blood coagulation. Initiation and inhibition of a proteolytic system by a zymogen

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.

A new regulatory function of activated factor V: inhibition of the activation by tissue factor/factor VII(a) of factor X

BACKGROUND

We observed that minute amounts of thrombin or the enzyme Russell's viper venom activating factor V (RVV-V) added to plasma strongly diminish the potential of that plasma to generate thrombin after being triggered by tissue factor.

OBJECTIVE

To find the mechanism behind this phenomenon.

METHODS AND RESULTS

Thrombin generation (TG) initiated by tissue factor (TF) is strongly and dose-dependently inhibited by addition of activated factor V (FVa) or by addition of a factor V activator (thrombin or RVV-V). No inhibition is seen when TG is triggered via the intrinsic pathway or by direct activation of factor X. The effect is independent of proteins C and S and tissue factor pathway inhibitor (TFPI). In factor VII-deficient plasma the effect is seen when it is spiked with recombinant factor VII (FVII) and to a much lesser extent when spiked with recombinant FVIIa. In a purified system, FVa also dose-dependently inhibits the activation of FX by TF/FVII(a). The inhibitory effect is neutralized by antibodies against the light chain of FVa but not by antibodies against the heavy chain.

CONCLUSIONS

Our observations can be explained by assuming that FVa, via its light chain, binds to the complex TF/FVII(a) and prevents it from activating FX. We assume that this mechanism reduces the possibility that thrombin and factor Xa escaping from a wound area into the circulation, together with blood-borne tissue factor, would trigger intravascular coagulation.

Unifying the mechanism of recombinant FVIIa action: dose dependence is regulated differently by tissue factor and phospholipids

Recombinant factor VIIa (rFVIIa) is used for treatment of hemophilia patients with inhibitors, as well for off-label treatment of severe bleeding in trauma and surgery. Effective bleeding control requires supraphysiological doses of rFVIIa, posing both high expense and uncertain thrombotic risk. Two major competing theories offer different explanations for the supraphysiological rFVIIa dosing requirement: (1) the need to overcome competition between FVIIa and FVII zymogen for tissue factor (TF) binding, and (2) a high-dose-requiring phospholipid-related pathway of FVIIa action. In the present study, we found experimental conditions in which both mechanisms contribute simultaneously and independently to rFVIIa-driven thrombin generation in FVII-deficient human plasma. From mathematical simulations of our model of FX activation, which were confirmed by thrombin-generation experiments, we conclude that the action of rFVIIa at pharmacologic doses is dominated by the TF-dependent pathway with a minor contribution from a phospholipid-dependent mechanism. We established a dose-response curve for rFVIIa that is useful to explain dosing strategies. In the present study, we present a pathway to reconcile the 2 major mechanisms of rFVIIa action, a necessary step to understanding future dose optimization and evaluation of new rFVIIa analogs currently under development.

Dosing with recombinant factor viia based on current evidence

Recombinant factor VIIa (rFVIIa; NovoSeven(R), Novo Nordisk, Bagsvaerd, Denmark) induces hemostasis in patients with severe hemophilia and inhibitors, and has been found to control hemorrhage associated with severe trauma and surgery in patients with basically normal hemostatic mechanisms from the start. By enhancing the generation of thrombin on activated platelets, rFVIIa facilitates the formation of a tight, stable fibrin plug that is resistant to premature lysis. Clinical efficacy has been achieved with doses of rFVIIa much lower than originally proposed by in vitro models. Based on early clinical experiences, a dosing schedule of 90 to 120 microg/kg every 2 hours for the first 24 hours was recommended for serious bleeds and surgical cover. This schedule has been shown to induce and maintain hemostasis in 83% to 95% of serious bleeding episodes, and in 90% to 100% of major surgical cases. However, "mega" doses of rFVIIa may demonstrate greater efficacy in the treatment of joint bleeds, as they are more likely to evoke a full thrombin burst. Interpatient variation in recovery rates, clearance rates, and the ability to generate thrombin on the activated platelet surface may influence the efficacy of rFVIIa. Optimal doses may thus vary not only between hemophilia patients, but also between patients treated for other bleeding disorders.

The interaction between platelets and factor VII/VIIa

Blood coagulation normally occurs when factor VII interacts with its specific cellular receptor, tissue factor, which is exposed when a blood vessel is severed. The factor VII/tissue factor complex then initiates a cascade of proteolytic reactions involving factors IX, X, prothrombin and fibrinogen, culminating in the formation of a fibrin clot. The role of platelets in the initiation phase of blood coagulation is still unclear. It has been postulated that platelets bind activated factor VIIa independently of tissue factor, and that this interaction forms the basis of the usefulness of high-dose recombinant factor VIIa in treating hemophiliacs with inhibitory antibodies, and other thrombocytopenia-like syndromes. In this review, we will examine the evidence for and against such an hypothesis, as well as discuss an alternative mechanism for the efficacy of high-dose factor VIIa in treating hemophilic patients with inhibitors.

Decidual cell-expressed tissue factor maintains hemostasis in human endometrium

We showed that decidualized stromal cells of luteal phase and pregnant human endometrium express tissue factor (TF), the primary initiator of hemostasis, thereby suggesting a mechanism by which perivascular decidual cells can mitigate the risk of hemorrhage during endovascular trophoblast invasion. Progestins enhanced TF mRNA and protein levels in monolayers of human endometrial stromal cells (HESCs), with estradiol (E2) + progestin, further enhancing TF levels despite a lack of response to E2 alone. This differential ovarian steroid response has been found for several decidualization markers. Further studies with cultured HESCs established that elevated TF levels are mediated by the progesterone receptor and are maintained for weeks in response to E2 plus progestin, thus simulating the chronic upregulation of TF levels observed in decidualized HESCs in vivo. Recent studies revealed that elevated TF expression during in vitro decidualization of HESCs involved both the EGFR and progesterone receptor. Thus, enhancement of TF mRNA and protein levels in the HESCs required co-incubation with a progestin (MPA) and an EGFR agonist such as EGF or TGF-alpha. In correspondence with co-elevation of EGFR and TF in decidualized HESCs in sections of luteal phase and pregnant endometrium, EGFR levels proved to be progestin-enhanced in the cultured HESCs. We established that progestin-enhanced TF expression in HESCs was trancriptionally regulated, then evaluated the relative roles of SP and EGR-1 sites on the TF promoter in regulating this expression. Transient transfections with a series of promoter constructs containing overlapping SP and EGR-1 sites and with constructs in which the EGR-1 and SP sites were systematically inactivated by site-directed mutagenesis established the dominance of SP sites in both basal and progestin-enhanced TF transcriptional activity. Additional experiments involving transient transfections with SPloverexpressing vectors and with a specific blocker of if Sp1 binding to its corresponding GC box specified the importance of the Sp1 transcription factor. These results were further validated by immunostaining, which revealed that the ratio of Sp1 to Sp3 increased during progestin-regulated decidualization of HESCs in vitro and in vivo. The absence of canonical estrogen and progesterone response elements from either the TF or Sp1 gene promoters suggests that the EGFR may help to mediate progestin-enhanced TF expression during decidualization of HESCs.

Inhibition of thrombin generation by the zymogen factor VII: implications for the treatment of hemophilia A by factor VIIa

Factor VII circulates as a single chain inactive zymogen (10 nmol/L) and a trace (∼10-100 pmol/L) circulates as the 2-chain form, factor VIIa. Factor VII and factor VIIa were studied in a coagulation model using plasma concentrations of purified coagulation factors with reactions initiated with relipidated tissue factor (TF). Factor VII (10 nmol/L) extended the lag phase of thrombin generation initiated by 100 pmol/L factor VIIa and low TF. With the coagulation inhibitors TFPI and AT-III present, factor VII both extended the lag phase of the reaction and depressed the rate of thrombin generation. The inhibition of factor Xa generation by factor VII is consistent with its competition with factor VIIa for TF. Thrombin generation with TF concentrations >100 pmol/L was not inhibited by factor VII. At low tissue factor concentrations (<25 pmol/L) thrombin generation becomes sensitive to the absence of factor VIII. In the absence of factor VIII, factor VII significantly inhibits TF-initiated thrombin generation by 100 pmol/L factor VIIa. In this hemophilia A model, approximately 2 nmol/L factor VIIa is needed to overcome the inhibition of physiologic (10 nmol/L) factor VII. At 10 nmol/L, factor VIIa provided a thrombin generation response in the hemophilia model (0% factor VIII, 10 nmol/L factor VII) equivalent to that observed with normal plasma, (100% factor VIII, 10 nmol/L factor VII, 100 pmol/L factor VIIa). These results suggest that the therapeutic efficacy of factor VIIa in the medical treatment of hemophiliacs with inhibitors is, in part, based on overcoming the factor VII inhibitory effect.

Factor VII Deficiency Caused by a Structural Variant N57D of the First Epidermal Growth Factor Domain

We have previously described a kindred with factor VII (FVII) deficiency whose members exhibited reduced procoagulant activity relative to FVII antigen concentration. In this report, the molecular genetic basis of the FVII defect has been determined to be a heterozygous substitution of Asp for Asn at position 57 in the first epidermal growth factor (EGF) domain. Recombinant FVII (N57D) cDNA was created by site-directed mutagenesis and transiently expressed in human 293 cells. The transfected cells synthesized an immunoprecipitable protein with an apparent molecular weight of 50 kD. Quantitation of expression by FVII enzyme-linked immunosorbent assay indicated that mutant protein yields were consistently low, typically 10% to 30% of wild-type FVII. FVII (N57D) protein did not accumulate intracellularly, and Northern blot analysis indicated equivalent FVII mRNA levels in 293 cells expressing either wild-type FVII or FVII (N57D). Secreted FVII (N57D) protein did not bind tissue factor, exhibited no procoagulant activity, and failed to bind a conformation-dependent monoclonal antibody specific for the first EGF domain of FVII. Molecular modeling of the first EGF domain of FVII predicted that the N57D amino acid substitution would disrupt tertiary bonding structure. We conclude that the N57D mutation affects folding of the first EGF domain of FVII resulting in decreased cellular secretion of a mutant FVII molecule, which is unable to bind tissue factor and is therefore biologically inactive.

Factor VII Deficiency Caused by a Structural Variant N57D of the First Epidermal Growth Factor Domain

We have previously described a kindred with factor VII (FVII) deficiency whose members exhibited reduced procoagulant activity relative to FVII antigen concentration. In this report, the molecular genetic basis of the FVII defect has been determined to be a heterozygous substitution of Asp for Asn at position 57 in the first epidermal growth factor (EGF) domain. Recombinant FVII (N57D) cDNA was created by site-directed mutagenesis and transiently expressed in human 293 cells. The transfected cells synthesized an immunoprecipitable protein with an apparent molecular weight of 50 kD. Quantitation of expression by FVII enzyme-linked immunosorbent assay indicated that mutant protein yields were consistently low, typically 10% to 30% of wild-type FVII. FVII (N57D) protein did not accumulate intracellularly, and Northern blot analysis indicated equivalent FVII mRNA levels in 293 cells expressing either wild-type FVII or FVII (N57D). Secreted FVII (N57D) protein did not bind tissue factor, exhibited no procoagulant activity, and failed to bind a conformation-dependent monoclonal antibody specific for the first EGF domain of FVII. Molecular modeling of the first EGF domain of FVII predicted that the N57D amino acid substitution would disrupt tertiary bonding structure. We conclude that the N57D mutation affects folding of the first EGF domain of FVII resulting in decreased cellular secretion of a mutant FVII molecule, which is unable to bind tissue factor and is therefore biologically inactive.

A new automated method for continuous registration of factor VII activation in vitro. Activation is accelerated by the concentration of factor VII and the activity state of the protein

When a plasma sample is exposed to tissue factor, single-chain factor VII (FVII) is gradually converted to the active two-chain form (FVIIa). In the present study, we have constructed a measurement system, which allows continuous registration of the activation of FVII to FVIIa in vitro. In this system, FVII activation follows parabolic kinetic after an initial lag-phase. The slope of the linear phase is a measure of the protein concentration of factor VII (FVIItotal), while the length of the non-linear phase represents the velocity of FVII activation. The time required for complete activation of FVII is inversely related to both FVIItotal and the relative amount of FVIIa in the sample. In future studies, this new measurement system will make it possible to study the process of FVII activation in different samples, and to examine how varying concentrations of exogenous added components affect the activation process in vitro.

The role of progestationally regulated stromal cell tissue factor and type-1 plasminogen activator inhibitor (PAI-1) in endometrial hemostasis and menstruation

The physiologic mechanisms whereby the human endometrium maintains hemostasis during endovascular trophoblast invasion, yet permits menstrual hemorrhage, are unknown. This paradoxical relationship was investigated by evaluating endometrial expression of tissue factor (TF), the primary initiator of hemostasis, and plasminogen activator inhibitor-1 (PAI-1), the primary inhibitor of fibrinolysis. We observed increased immunostaining for TF and PAI-1 in sections of decidualized stromal cells from luteal phase and gestational endometrium. To determine whether TF and PAI-1 expression are directly linked to decidualization, both endpoints were monitored in a well described in vitro model of decidualization. Thus, confluent stromal cell cultures were exposed to vehicle control, 10(-8) M estradiol (E2), 10(-8) to 10(-6) M medroxyprogesterone acetate (MPA) or both E2 + MPA for 2-24 days in serum-containing or defined media. The progestin enhanced the content of stromal cell-associated immunoreactive and functionally active TF and PAI-1 released into the medium and elevated levels of stromal cell TF and PAI-1 mRNA. While E2 alone was ineffective, it greatly augmented MPA-enhanced TF and PAI-1 protein and mRNA content. Dose-dependent effects on TF and PAI-1 content were observed between 10(-8) to 10(-6) M MPA +/- E2. Similar results were observed for decidual cells derived from first trimester endometrium and cultured in type 1 collagen gels. Following optimal induction of TF and PAI-1 expression by E2 + MPA in stromal cell cultures, removal of these steroids greatly reduced levels of both TF and PAI-1 protein and mRNA within 4 days. These studies suggest a mechanism whereby endometrial hemostasis is maintained during trophoblast invasion yet reduced at the end of nonfertile cycles to permit menses.

Analysis of the partial nucleotide sequences and deduced primary structures of the protease domains of mammalian blood coagulation factors VII and X

In order to obtain sequence data for the blood coagulation factor VII and factor X in several mammalian species, we amplified and sequenced the DNA segments of exon VIII from each gene by means of the polymerase chain reaction (PCR) method. The DNA segments from the following species were successfully amplified: factor VII from the rhesus monkey and dog, and factor X from the rhesus monkey, Syrian hamster and rat. In each factor, the nucleotide sequences and predicted primary structures of the protease domain showed a high degree of homology among species; amino acid identities of approximately 68%-92% and 80%-98% were demonstrated among species in factor VII and factor X, respectively. The locations of the active site residues and five Cys residues were evolutionarily conserved in both factors. Interestingly, the amino acids involved in the human genetic variants, both factor VII 304-Arg and factor X 326-Arg, were always conserved across species. The data presented here will be helpful for investigating human genetic variants of factor VII or X, and will provide considerable information for constructing in vitro site-specific mutants of these factors.

Activation of coagulation factor VII during alimentary lipemia

Dietary studies have established a connection between plasma lipoproteins and coagulation factor VII. The present study was undertaken to specifically examine whether factor VII is activated during alimentary lipemia and to investigate the relations of factor VII mass and activity state with fasting and postprandial lipoproteins and free fatty acids (FFAs). Factor VII levels were therefore determined in plasma samples taken before and after intake of a standardized, oral fat load of a mixed-meal type in 33 men (mean age +/- SD, 48.8 +/- 3.2 years) with a previous myocardial infarction at a young age and 10 healthy, age-matched control subjects. A panel of methods for factor VII determination was used to ensure that changes in all potentially existing forms of the factor during alimentary lipemia would be included. Substantial activation of factor VII was found to occur during alimentary lipemia, whereas the number of factor VII molecules remained constant or even appeared to decrease after the test meal. Activation of factor VII was more pronounced in control subjects than patients, and the proportion of activated factor VII molecules was higher in control subjects. Interestingly, factor VII activation, which correlated quantitatively with the degree of postprandial triglyceridemia, seemed to be related to FFA production during lipolysis of triglyceride-rich lipoproteins that were generated in response to fat intake. Postheparin plasma lipoprotein lipase activity was lower in patients, which could offer one explanation why factor VII activity was lower during alimentary lipemia in these subjects despite their exaggerated postprandial triglyceridemia. Thus, activation of coagulation factor VII during alimentary lipemia may result in a procoagulant state that is likely to promote the formation of a coronary thrombus in individuals with established coronary artery disease.

Specific molecular interaction sites on factor VII involved in factor X activation

Factor VII, a serine-protease zymogen, and tissue factor, the cellular receptor/coenzyme, are the protein components of the macromolecular complex which initiates the extrinsic pathway of the coagulation cascade. Previous studies were directed to the identification of functional sites on factor VII which mediate factor X activation, employing a series of potentially inhibitory synthetic peptides representing the primary structure of factor VII and antibodies to selected peptides. The involvement of at least four high-affinity interactive regions [factor VII (44-50), (196-229), (285-305) and (376-396) peptides] on the surface of factor VII was clearly demonstrated. The minimal sequences for the expression of inhibitory activity of these four molecular recognition domains on factor VII were identified using short and overlapping peptides. The short factor VII-(206-218)-peptide (most inhibitory peptide in the sequence 196-229 on factor VII) inhibited the binding of factor VII to the tissue-factor-expressing J82 cell line. Furthermore, radiolabeled [Tyr201] factor VII-(199-221)-peptide, with a tyrosine substituted for the normal tryptophan residue, was specifically bound to J82 cells, and also the binding of the radiolabeled peptide to this cell line was specifically inhibited by a monoclonal antibody to tissue factor, confirming that the interaction site for tissue factor on factor VII is present within the peptide sequence 196-229. Kinetic analyses suggested that the regions represented by factor VII-(285-305)- and factor VII-(376-396)-peptides are involved in factor X recognition and the chemical cross-linking of the radiolabeled peptides resulted in specific binding to factor X, confirming that these two regions on factor VII represent the substrate-recognition site. Furthermore, these radiolabeled peptides specifically interact with the heavy chain of factor X, suggesting that the complementary binding region for the substrate-recognition site on factor VII are present on the heavy chain of factor X.

An overview of hemostasis

Hemostasis is a remarkable and a remarkably complex mechanism. It can maintain blood in a fluid state intravascularly but very quickly changes blood to a jellylike mass upon disruption of the vasculature. This review will give a synopsis of the 3 phases of hemostasis: platelet, vascular, and coagulation. Fibrinolysis and control mechanisms of hemostasis will also be covered. In addition, brief descriptions of the clinical and laboratory evaluation of patients and the diagnosis of bleeding disorders will be presented.

Amniotic fluid contains tissue factor, a potent initiator of coagulation

A primary clinical manifestation of amniotic fluid embolism is coagulopathy. Prior studies have identified a poorly characterized yet potent procoagulant property in amniotic fluid that increases with gestational age. One possible source of procoagulant activity is tissue factor, a primary biologic initiator of coagulation. We used sensitive immunoassays and functional assays to identify substantial quantities of tissue factor antigen and tissue factor-specific procoagulant activity in amniotic fluid, which increased with gestational age. Moreover, tissue factor accounted for virtually all of the coagulant potential of amniotic fluid. Amniotic tissue factor appeared intact and membrane bound and, when reconstituted into synthetic microvesicles of optimal phospholipid content, displayed nearly full activity. Calcium chelation and sonication experiments suggested that the presence of inhibitors and the physical configuration of membrane-bound tissue factor in amniotic fluid might explain the modest reduction in tissue factor procoagulant activity relative to total antigen levels observed in vivo. We postulate that the substantial quantities of functionally active tissue factor in amniotic fluid account for the coagulation changes accompanying amniotic fluid embolism and could indirectly contribute to the characteristic hemodynamic derangements of amniotic fluid embolism.

Effects of long-term gestodene-containing oral contraceptive administration on hemostasis

The purpose of this study was to evaluate the behavior of the hemostatic system during treatment with gestodene-containing oral contraceptives in monophasic (SHD 356, n = 15) and triphasic (SHD 415, n = 15) formulations. No changes in platelet (beta-thromboglobulin, platelet aggregate ratio, and megathrombocyte) and routine clotting assays were observed. Factor VIIc/factor VIIag ratio and fibrinopeptide A values showed a significant (p less than 0.005) increase after three cycles of both treatments. A slight, significant increase (p less than 0.01) in antithrombin III activity was observed during triphasic treatment. Protein C was unchanged. Fibrinolytic activity and plasminogen levels were significantly increased (p less than 0.05 and p less than 0.001). After 6 and 9 months, the factor VIIc/factor VIIag ratio was still significantly enhanced, whereas fibrinopeptide A values significantly (p less than 0.05) decreased, even if they were higher (p less than 0.05) than basal values. The persistence of factor VII activation without enhanced thrombin formation after long-term use of oral contraceptives suggests that at that time the activity of antithrombotic mechanisms counteracts the prothrombotic tendency, thus helping to minimize unwanted side effects on hemostasis during long-term drug administration.

Synthesis, purification, and characterization of an Arg152----Glu site-directed mutant of recombinant human blood clotting factor VII

Coagulation factor VII circulates in blood as a single-chain zymogen of a serine protease and is converted to its activated two-chain form, factor VIIa, by cleavage of an internal peptide bond located at Arg152-Ile153. Previous studies using serine protease active-site inhibitors suggest that zymogen factor VII may possess sufficient proteolytic activity to initiate the extrinsic pathway of blood coagulation. In order to assess the putative intrinsic proteolytic activity of single-chain factor VII, we have constructed a site-specific mutant of recombinant human factor VII in which arginine-152 has been replaced with a glutamic acid residue. Mutant factor VII was purified in a single step from culture supernatants of baby hamster kidney cells transfected with a plasmid containing the sequence for Arg152----Glu factor VII using a calcium-dependent, murine anti-factor VII monoclonal antibody column. Purified mutant factor VII was indistinguishable from plasma-derived or recombinant wild-type factor VII by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and migrated as a single band with an apparent molecular weight of 50,000. The average specific activity of several mutant factor VII preparations was 0.00025 unit/micrograms, or 0.01% of that observed for recombinant wild-type factor VII preparations. The clotting activity of mutant factor VII was, however, completely inhibited following incubation with dansyl-Glu-Gly-Arg chloromethyl ketone, suggesting that the apparent clotting activity of mutant factor VII was due to a contaminating serine protease.(ABSTRACT TRUNCATED AT 250 WORDS)

Factor VII and extrinsic pathway inhibitor in acute coronary disease

This report describes studies on the activation of coagulation factor VII (FVII) and the inhibition of the extrinsic coagulation pathway in acute ischaemic heart disease. FVII and the inhibitor of the tissue thromboplastin-FVII complex, called extrinsic pathway inhibitor (EPI), were determined in plasma from 68 patients and compared to findings in 37 normal individuals. The mean FVII amidolytic activity, the mean FVII clotting activity, as well as the FVII clotting/FVII amidolytic ratio were not significantly different in the patient groups as compared to the controls. The fraction of FVII clotting activity that is sensitive to phospholipase C, 'the FVII-phospholipid complex', was 8% in controls, 19% (P less than 0.05) in patients with acute myocardial infarction, 15% (n.s.) in angina pectoris and 13% (n.s.) in heart failure/arrhythmia patients. The 'FVII-phospholipid complex' was highly significantly correlated to triglycerides in plasma in patients with acute myocardial infarction (r = 0.88, P less than 0.001) and angina pectoris (r = 0.89, P less than 0.001). The mean EPI levels were significantly increased in patients with acute myocardial infarction (132%), angina pectoris (134%), and heart failure (150%) as compared to the control population (110%). The FVII clotting/EPI ratio was significantly decreased both in patients with acute myocardial infarction and heart failure, whereas the FVII amidolytic/EPI ratio was significantly decreased only in the heart failure group. Apparently, in patients with acute ischaemic heart disease, a moderate increase in the procoagulant activity is accompanied by a marked increase in the anticoagulant activity of the extrinsic coagulation pathway, suggesting a balanced activation system.

Serum stimulation of quiescent human fibroblasts induces the synthesis of tissue factor mRNA followed by the appearance of tissue factor antigen and procoagulant activity

Expression of the gene for tissue factor, the cell-surface initiator of blood coagulation, was examined by stimulating growth-arrested human fibroblasts with serum and measuring changes in the cellular content of tissue factor mRNA, antigen, and activity. Maximum tissue factor mRNA levels were reached within 1 h following serum induction and slowly declined to basal levels from 24 to 48 h after stimulation. The appearance of the tissue factor mRNA was followed by an increase in tissue factor antigen and activity. The parallel rise in antigenically positive protein and procoagulant activity was first observed about 2 h after serum stimulation with a peak at 12 h followed by a slow decline during the next 36 h. The serum-induced synthesis of the tissue factor mRNA was independent of de novo protein synthesis as demonstrated by the increased tissue factor mRNA levels generated in the presence of cycloheximide. The results of this study suggest that the synthesis of tissue factor in human fibroblasts is regulated principally at the level of transcription. In one strain of fibroblasts the activity/antigen ratio, during the period of maximum synthesis, was indistinguishable from that of tissue factor which had been immunoaffinity purified from human brain and reconstituted into phospholipid vesicles. However, during serum starvation the activity/antigen ratio in these cells was significantly reduced. Western blot analysis revealed that in serum-starved cells there was an accumulation of truncated forms of the tissue factor antigen while in the serum-stimulated cells only the full-length antigen was observed.

Factor VII hyperactivity in acute myocardial thrombosis. A relation to the coagulation activation

It has been shown that coagulation factor VII (FVII) has an increased coagulant activity (FVIIc) in cardiovascular high risk patients and that it is a important risk factor for the development of ischaemic heart disease (IHD) and cardiovascular death. In this study, we measured FVII coagulant (FVIIc) and immunological (FVIIag) activities during the acute phase of unstable angina (UA) and acute uncomplicated and complicated myocardial infarction (AMI). We have also studied its changes in relation to thrombin formation and coagulation activation, as assessed by determination of thrombin-antithrombin circulating complexes (T-AT) at the same time. Our results show a marked increase in FVIIc in all patients, with highest significant levels in complicated AMI. In fact, this increase was also different between groups, complicated AMI showing a significant degree of increase in FVIIc in relation to UA and uncomplicated AMI. FVIIag did not vary between groups and controls, implicating a progressive activation of FVII. As expected, we found comparable levels of T-AT in UA and in AMI patients, suggesting that a common thrombotic process is involved in both situations. FVIIc was strongly correlated to T-AT in all patients (r = 0. 750; p less than 0.001) and also within groups. This study underlines the important positive contribution of FVIIc to IHD and to the prognosis of its thrombotic acute events, and shows that the increase in FVII activity is associated with an increase of a thrombotic marker (thrombin-antithrombin). Further studies are needed to evaluate if FVII activation is the cause or the consequence of the thrombotic processes.

A method for the determination of activated factor VII using bovine and rabbit brain thromboplastins: demonstration of increased levels in disseminated intravascular coagulation

Factor VII levels as measured in one stage clotting assays are dependent on the degree of activation of factor VII and on the type of thromboplastin used. Bovine thromboplastin (BT) is known to be more sensitive to factor VIIa than human brain thromboplastin and the different sensitivities of these reagents to VIIa have formed the basis of a method for determining VIIa in plasma (16). Since human thromboplastin is no longer widely available, we have re-evaluated this method using bovine and two rabbit thromboplastins (Manchester Reagent, RT MR and Diagen activated, RT Diagen). Activated factor VII was generated in normal plasma by cold activation for 24 hours. During this period, factor VII assays using bovine thromboplastin increased markedly with time whereas the assays using rabbit thromboplastins showed considerably less change. The ratio of factor VII (bovine thromboplastin)/factor VII (rabbit thromboplastin) is a sensitive index of activated factor VII in plasma. The mean ratio in normal plasmas (using BT and RT MR) was found to be 1.02 (+/- s.d., range 0.80-1.24). The ratio was measured in 29 samples from patients with disseminated intravascular coagulation (D.I.C.) and was found to be increased in 24 (mean 1.71, range 0.93-3.38).

The effect of polymerised fibrin on the catalytic activities of one-chain tissue-type plasminogen activator as revealed by an analogue resistant to plasmin cleavage

A one-chain recombinant tissue-type plasminogen activator (EC 2.4.31.-) (tPA) analogue was constructed in which Arg-275 of the activation site was changed to Gly by site-directed mutagenesis. This analogue, tPA-Gly275, was very resistant to plasmin (EC 2.4.21.5) cleavage. It has been used to gain information about the activity of the uncleaved one-chain tPA form, also when plasmin is generated as a result of a plasminogen activation reaction. The amidolytic activity of tPA-Gly275 with less than Glu-Gly-Arg-pNA was investigated and compared to that of one-chain and two-chain wild-type recombinant tPA. A small but significant intrinsic amidolytic activity was observed with the analogue as well as the wild-type one-chain tPA form. However, it was much lower than that of two-chain tPA. Polymerised fibrin enhanced the amidolytic activity of both one-chain tPA forms but not of two-chain tPA. Measurements of the plasminogen activation kinetics in the absence of fibrin revealed that tPA-Gly275 possessed a significant intrinsic activity. However, it was 30-fold lower than that of two-chain tPA. Addition of polymerised fibrin profoundly enhanced the plasminogen activation rate of both tPA-Gly275 and wild-type one- and two-chain tPA to approximately the same maximal level. The results were interpreted to mean that fibrin binding can induce an activated state of the intact tPA one-chain form.

Initiation of coagulation by tissue factor

Tissue factor (TF) is an integral membrane glycoprotein which functions as an initiator of coagulation. Furthermore, it is probably the principal biological initiator of this essential hemostatic process. This article reviews the studies which form the basis for these assertions. The work on TF is traced from the 19th century discovery of the thromboplastic activity of tissues to the recent purification of the protein from bovine and human tissues and the isolation cDNA clones coding from human TF. The features of TF structure and function which tailor it to the role of initiator of the coagulation cascade are considered. For example, cell-surface TF and factor VII, the plasma serine proteases zymogen, form a proteolytic complex without prior proteolysis of either component. In addition, a kinetic model for the molecular mechanism of TF-initiated clotting is reviewed. The factors which control the expression of TF procoagulant activity by cultured cells are examined in light of the hypothesized role of TF in normal hemostasis. Also, the potential pathological consequences of aberrant TF expression, i.e., thrombosis and hemorrhage, are explored.

Increased factor VII coagulant activity in the rabbit following diet-induced hypercholesterolaemia. Evidence for increased conversion of VII to alpha VIIa and higher flux within the coagulation pathway

Factor VII coagulant activity (VIIc) is considerably higher in rabbits fed a 1% cholesterol-supplemented diet than in rabbits fed a standard diet. This increase was statistically significant 4-6 days from the beginning of treatment and rose to about 300% during the 100 days of treatment. Treatment is also associated with a 20-fold increase in plasma cholesterol concentration with the major fraction of excess cholesterol associated with the very low and intermediate density lipoprotein fractions. In both groups of rabbits, the direction and extent of variation in VIIc generally coincided with variation in cholesterol, so that over time there were significant and positive correlations between plasma cholesterol concentration in both the rabbits fed the standard diet and the rabbits fed the cholesterol-supplemented diet. The increase in VIIc was due to a higher proportion of the more active alpha VIIa in the plasma of hypercholesterolaemic rabbits rather than to an increase in the concentration of the single-chain protein. The plasma concentration of factor X and prothrombin had increased in the hypercholesterolaemic rabbits by 10 days from the beginning of treatment and both proteins were maintained at 150-200% of the concentrations in the plasma of rabbits fed the standard diet. However, these differences were only seen when the factor X and prothrombin were assayed using synthetic substrates. The specific coagulation assays for these two factors revealed no differences between the groups of animals up to 100 days.

The incorporation of a fluorescent probe into the active sites of one- and two-chain tissue-type plasminogen activator

Dansyl-glutamyl-glycyl-arginyl chloromethyl ketone (DEGR-CK) was shown to inactivate both one- and two-chain human, recombinant tissue-type plasminogen activator (t-PA). The interaction of DEGR-CK with both forms of t-PA was accompanied by an identical increase in the fluorescence intensity and a blue shift in the wavelength of maximum emission, which suggests that the environment of the incorporated DEGR is similar in both one- and two-chain t-PA. The kinetics of the interaction of t-PA with DEGR-CK could be followed by both loss of activity and increase in fluorescence. The second order rate constants (k2/Ki) obtained with these two methods agreed quite well. With two-chain t-PA the values were 42 X 10(4) M-1 min-1 and 46 X 10(4) M-1 min-1 by the activity loss and fluorescence methods, respectively. With one-chain t-PA the results were 2.5 X 10(4) M-1 min-1 and 3.1 X 10(4) M-1 min-1. The rate at which one-chain t-PA is inactivated by DEGR-CK is 15 times lower than the rate with two-chain t-PA. The results demonstrated, however, that the cleavage of the one-chain protein to the two-chain form is not required for reactivity with DEGR-CK. This fluorescently labeled t-PA should be useful in probing the interactions of one- and two-chain t-PA with other proteins.

An ordered addition, essential activation model of the tissue factor pathway of coagulation: evidence for a conformational cage

One way in which coagulation may be initiated is by the action of factor VIIa (a plasma serine protease) and tissue factor (a membrane-bound lipid-dependent glycoprotein). We show that in the absence of either factor VIIa or tissue factor, the activation of the natural coagulation substrates, factors IX and X, is not detectable; i.e., tissue factor is an essential activator. We propose that the reaction is fully ordered; that is, the enzyme-activator complex picks up substrate to form a ternary product forming species. Our model precludes the formation of enzyme-substrate and activator-substrate complexes. We have derived equations for the two possible variations of this model: one in which product formation is accompanied by the release of the enzyme-activator complex and the other in which product, free enzyme, and free activator are formed with each catalytic cycle. Our data support only the former which is consistent with both steady-state and rapid equilibrium assumptions. The model is supported by experiments using a monoclonal anti-tissue factor antibody, which affects only the Km app, and a modified form of factor VIIa, which, depending on the sequence in which reagents are added to the reaction, either decreases the Vmax or increases the Km app. We present equations describing the initial velocity of these reactions. Utilizing dilution-jump experiments, we show that the system is hysteretic and suggest that this phenomenon is due to a slow release of enzyme from activator. However, the kinetically determined dissociation constant of enzyme and activator, previously found to be 4.5 nM under equilibrium conditions, was estimated to be 0.04-0.09 nM. Accordingly, we examined other essential activation models in which the product-forming species consists of a complex of enzyme, activator, and substrate at a molar ratio of 1:1:1; none could account for the apparent tight binding of enzyme and activator. We therefore postulate an ordered addition, essential activation model in which the enzyme undergoes two conformational transformations: one as a consequence of binding to tissue factor, resulting in a species which binds to and hydrolyzes its natural substrates. The other conformational change in the enzyme is induced by substrate, resulting in a species which binds more tightly to its activator. Thus, we hypothesize a "conformational cage" which precludes the dissociation of enzyme from activator while significant concentrations of substrate are present.

Factor VII binding to tissue factor in reconstituted phospholipid vesicles: induction of cooperativity by phosphatidylserine

The binding of factor VII and tissue factor produces a membrane-associated proteolytic complex which may be the primary biological initiator of coagulation. Homogeneous tissue factor, a glycoprotein purified from bovine brain, was reconstituted into phospholipid vesicles ranging from neutral (100% phosphatidylcholine) to highly charged (40% phosphatidylserine) with octyl glucoside. The vesicles were characterized with respect to size and tissue factor content and orientation. Employing data from protease digestion, we deduced that tissue factor is randomly oriented; thus, its effective concentration in these vesicles was half its total concentration. In all binding experiments, 1 mol of enzyme was bound per mole of available activator at saturation. This stoichiometry was not affected by the form of the enzyme employed or the phospholipid composition of the vesicles. With tissue factor incorporated into phosphatidylcholine vesicles, the Kd was 13.2 +/- 0.72 nM for factor VII and 4.54 +/- 1.37 nM for factor VIIa. Thus, the one-chain zymogen binds to the activator with only slightly less affinity than the more active two-chain enzyme. Active-site modification of factor VII and factor VIIa with diisopropyl fluorophosphate resulted in tighter binding of the derivatized molecules. Inclusion of phosphatidylserine in the vesicles altered the binding both quantitatively and qualitatively. With increasing acidic phospholipid, the concentration of enzyme required to occupy half the activator sites was decreased. In addition, positive cooperativity was observed, the degree of which depended on the vesicle charge and the form of the enzyme. An explicit two-site cooperative binding model is presented which fits these complex data. In this model, tissue factor is at least a dimer with two interacting enzyme binding sites.

Application of factor VII-Sepharose affinity chromatography in the purification of human tissue factor apoprotein

Coagulation factor VII covalently coupled to Sepharose proved to be an effective binding ligand for human tissue factor apoprotein, the specific cofactor of factor VII for the activation of factor X and IX. This interaction is completely calcium-dependent and the calcium ions cannot be replaced by magnesium or barium ions. The binding of the apoprotein to immobilized factor VII seems to be independent of the presence of phospholipid. When factor VII-Sepharose column chromatography is combined with a mild extraction procedure, tissue factor apoprotein could be purified approximately 40,000-fold from an acetone powder of human brain. SDS-PAA gel electrophoresis revealed that with this simple purification scheme human tissue factor apoprotein can be purified to apparent homogeneity and that the apoprotein migrates at a molecular weight of 47,000. The isolated human protein is heterogeneously glycosylated; the two different forms of the apoprotein function as cofactor of factor VII in the activation of both factor X and factor IX.

An association between the factor VII coagulant activity and thrombin activity induced by surface/cold exposure of normal human plasma

To test whether factor VII activation correlated with the generation of thrombin activity when plasma was exposed to a glass surface and reduced temperature, an association was sought between the changes in factor VII clotting activity (VIIc) and fibrinopeptide A concentration (an index of thrombin activity) in platelet-poor citrated plasma from 42 healthy adults. The Spearman rank correlation (rs) between responses was 0.82 (P less than 0.001). The VIIc assay response to surface/cold exposure was unaffected when thrombin was suppressed by hirudin. An assay for factor VII activity based upon its activation of tritiated factor X revealed an association between the increase in fibrinopeptide A concentration and reduction in functional factor VII concentration during activation of a subset of 22 plasma samples (rs = -0.62; P = 0.003). This loss of functional factor VII was probably due to conversion of active factor VII to its non-functional end-product by factor Xa. The results suggest that VIIc is an index of flux within the coagulation system and support the hypothesis that a high VIIc is an indicator of a hypercoagulable state.

Tissue factor accelerates the activation of coagulation factor VII: the role of a bifunctional coagulation cofactor

One way in which coagulation may be initiated is via the tissue factor pathway in which a non-proteolytic enzyme activator, tissue factor, complexes with a plasma protein, factor VII. Factor VII is the zymogen of a serine protease, factor VIIa. While factor VII-tissue factor has coagulant activity, upon conversion to factor VIIa the activity increases about 120-fold. We now show that tissue factor markedly accelerates the conversion of factor VII to factor VIIa. Thus, this essential coagulation cofactor is clearly bifunctional.