Combined effects of eptifibatide and anticoagulants: differences between LMWH and UH or rH in thrombin generation inhibition but not in platelet aggregation inhibition

Aim of our study was to investigate effects of eptifibatide and anticoagulants on platelet aggregation and thrombin generation under low and high coagulant challenge in tissue factor-activated platelet rich plasma using a model allowing simultaneous determination of the time course of platelet aggregation and thrombin generation. Eptifibatide exerted a dose-dependent anti-aggregating effect under both high and significantly stronger under low coagulant challenge. Combination of eptifibatide and anticoagulants resulted in significant additive prolongation of the lag phase until the onset of platelet aggregation, more pronounced under low coagulant challenge. Under high, but not under low coagulant challenge combination of eptifibatide and anticoagulants had a significant synergistic inhibitory effect on platelet aggregation. Under low coagulant challenge combination of eptifibatide with LMWH, but not with UH, or rH, resulted in significantly reduced thrombin potential, F 1+2 generation, and FXa formation compared to measurements in the absence of eptifibatide. We demonstrate a synergistic effect of eptifibatide and anticoagulants on platelet aggregation inhibition and an additional inhibitory effect of LMWH and eptifibatide on thrombin generation. Our results support the notion that combination of eptifibatide and anticoagulants might be beneficial in atherosclerotic disease to palliate the thrombogenic potency of ruptured atherosclerotic plaques.

Inhibition of tissue factor gene induction and activity using a hairpin ribozyme

BACKGROUND

Tissue factor (TF) is a membrane-bound glycoprotein that initiates the clotting cascade. Inhibition of the TF pathway has been shown to prevent thrombosis and restenosis after arterial injury in a variety of animal models.

METHODS AND RESULTS

We describe a novel approach to inhibiting the expression of the TF protein that involves the targeted destruction of cellular TF mRNA with the use of a tetraloop hairpin ribozyme. After construction of the ribozyme and determination of its optimal length and kinetic parameters, a ribozyme expression vector that used the retroviral vector pMV12 was constructed. The ability of this expression vector to generate anti-TF ribozyme was further augmented by positioning of the anti-TF ribozyme downstream of a rat tRNA val (RNA polymerase II) promoter. The resultant construct containing the anti-TF ribozyme was then used to transfect vascular smooth muscle cells and generate a variety of clonal cell lines. Northern blot analyses performed on 3 transfected and 3 untransfected clones demonstrated markedly reduced TF mRNA levels in the transfected clones both during quiescence and after serum stimulation. Cell lysates analyzed for total TF activity by monitoring factor Xa generation similarly demonstrated a statistically significant and concordant reduction in TF activity in smooth muscle cells transfected with the ribozyme expression vector compared with both untransfected clones and clones transfected with the empty vector.

CONCLUSIONS

These results demonstrate the feasibility of an antithrombotic strategy based on ribozyme technology.

Blood coagulation

The process of tissue factor initiated blood coagulation is discussed. Reactions of the blood coagulation cascade are propagated by complex enzymes containing a vitamin K-dependent serine protease and an accessory cofactor protein that are assembled on a membrane surface in a calcium-dependent manner. These complexes are 105-109-fold more efficient in proteolyses of their natural substrates than enzymes alone. Based upon data acquired using several in vitro models of blood coagulation, tissue factor initiated thrombin generation can be divided into two phases: an initiation phase and a propagation phase. The initiation phase is characterized by the generation of nanomolar amounts of thrombin, femto- to picomolar amounts of factors VIIa, IXa, Xa, and XIa, partial activation of platelets, and almost quantitative activation of procofactors, factors V and VIII. The duration of this phase is primarily influenced by concentrations of tissue factor and TFPI. The characteristic features of the propagation phase are: almost quantitative prothrombin activation at a high rate, completion of platelet activation, and solid clot formation. This phase is primarily regulated by antithrombin III and the protein C system. Thrombin generation during the propagation phase is remarkably suppressed in the absence of factor VIII and IX (hemophilia A and B, respectively) and at platelet counts <5% of mean plasma concentration. The majority of data accumulated in in vitro models and discussed in this review are in good agreement with the results of in vivo observations.

Regulation of monocyte procoagulant activity in acute myocardial infarction: role of tissue factor and tissue factor pathway inhibitor-1

In acute myocardial infarction (AMI), monocyte procoagulant activity is increased and may contribute to the risk for recurrence and other thrombotic events. This study sought to investigate the role tissue factor (TF) and tissue factor pathway inhibitor-1 (TFPI-1) in the regulation of monocyte procoagulant activity in AMI. Serial venous blood samples were obtained from 40 patients with AMI undergoing revascularization by stent placement. Twenty patients with elective stenting for stable angina served as control subjects. TF proteolytic activity was measured with spectrozyme factor Xa (FXa), TF and TFPI-1 surface expression on monocytes by flow cytometry, RNA expression in whole blood by reverse transcription-polymerase chain reaction, and concentrations of plasma prothrombin fragments F(1 + 2) by immunoassay. Forty-eight hours after AMI, an increase was found in TF RNA, followed by an increase in TF surface expression by 24% +/- 4% and in plasma concentration of F(1 + 2) by 103% +/- 17% (P <.05). These changes could not be attributed to the intervention because they did not occur in the control group. TFPI-1 RNA and binding to the monocyte surface remained unchanged. FXa generation by monocytes of patients with AMI increased 53.6% +/- 9% in the presence of polyclonal antibodies to TFPI-1, indicating that cell-associated TFPI-1 inhibits monocyte TF activity. The increased monocyte procoagulant activity in AMI was caused by an up-regulation of TF that was partially inhibited by surface-bound TFPI-1. Anticoagulant therapy by direct inhibition of TF activity may, thus, be particularly effective in AMI. (Blood. 2001;97:3721-3726)

Human peripheral blood mononuclear cells require factor X and platelets for expression of prothrombinase activity in response to bacterial lipopolysaccharide

We investigated whether human peripheral blood mononuclear cells (PBMC) express prothrombinase following stimulation with bacterial lipopolysaccharide (LPS). LPS-stimulated PBMC devoid of contaminating platelets failed to activate prothrombin directly. Addition of platelets did not result in expression of prothrombinase in the absence of factor X whereas the combination of platelets and factor X resulted in strong prothrombinase activity on LPS-activated cells. The induced prothrombinase was dependent on tissue factor, as the activity was completely inhibited by an anti-tissue factor antibody. Our data suggest that platelet/monocyte cooperation is important in the generation of prothrombinase activity in response to endotoxin.

Surface-dependent coagulation enzymes. Flow kinetics of factor Xa generation on live cell membranes

The initial surface reactions of the extrinsic coagulation pathway on live cell membranes were examined under flow conditions. Generation of activated coagulation factor X (fXa) was measured on spherical monolayers of epithelial cells with a total surface area of 41-47 cm(2) expressing tissue factor (TF) at >25 fmol/cm(2). Concentrations of reactants and product were monitored as a function of time with radiolabeled proteins and a chromogenic substrate at resolutions of 2-8 s. At physiological concentrations of fVIIa and fX, the reaction rate was 3.05 +/- 0.75 fmol fXa/s/cm(2), independent of flux, and 10 times slower than that expected for collision-limited reactions. Rates were also independent of surface fVIIa concentrations within the range 0.6-25 fmol/cm(2). The transit time of fX activated on the reaction chamber was prolonged relative to transit times of nonreacting tracers or preformed fXa. Membrane reactions were modeled using a set of nonlinear kinetic equations and a lagged normal density curve to track the expected surface concentration of reactants for various hypothetical reaction mechanisms. The experimental results were theoretically predicted only when the models used a slow intermediate reaction step, consistent with surface diffusion. These results provide evidence that the transfer of substrate within the membrane is rate-limiting in the kinetic mechanisms leading to initiation of blood coagulation by the TF pathway.

Structural and functional characteristics of the B-domain-deleted recombinant factor VIII protein, r-VIII SQ

Recombinant factor VIII SQ (r-VIII SQ), ReFacto, is a recombinant factor VIII product similar to the smallest active factor VIII protein found in plasma-derived factor VIII (p-VIII) concentrates. The protein comprises two polypeptide chains of 80 and 90 kDa and lacks the major part of the heavily glycosylated B-domain i.e. amino acids Gln744 to Ser1637. r-VIII SQ retains six potential glycosylation sites for N-linked oligosaccharides at asparagine residues 41, 239, 582, 1685, 1810 and 2118. We describe a thorough comparison of the characteristics of r-VIII SQ with those of p-VIII. The primary and secondary structures of r-VIII SQ were in good agreement with that of B-domain-deleted p-VIII (p-VIII-LMW) as shown by SDS-PAGE, Western blotting with antifactor VIII antibodies, tryptic mapping, amino acid sequence analysis and circular dichroism spectroscopy. A few divergences also existed. Thus r-VIII SQ was shown to contain a small amount of the single chain primary translation product of 170 kDa and also the product specific sequence of 14 amino acids, the SQ-link, in the C-terminal end of the 90 kDa chain. It was shown that r-VIII SQ had a high specific activity of about 14,000 IU VIII:C/mg as determined by use of a chromogenic substrate assay. The r-VIII SQ protein was comparable to p-VIII forms with a retained B-domain, in terms of potency measured by a chromogenic substrate or a two-stage clotting assay, in interactions with thrombin, and with activated protein C (APC) in combination with Protein S. The ability of r-VIII SQ to participate as a cofactor in factor Xa generation in a mixture of factors IXa and X, phospholipid and calcium was in conformity with that of p-VIII. Furthermore r-VIII SQ had a good binding capacity for phospholipid vesicles and von Willebrand factor (vWF) as shown in gel filtration studies. The same kinetics in binding to von Willebrand factor was found for r-VIII SQ and p-VIII as determined by real-time biospecific interaction analysis (BIA) with use of the BIAcore instrument. The apparent association rate constant was 4 x 10(6) M(-1)s(-1). Two dissociation rate constants were found, 1 X 10(-2)s(-1) and 4 x 10(-4)s(-1). The results extend the present knowledge that the factor VIII B-domain is dispensable for the factor VIII cofactor function in hemostasis.

Shear stress decreases endothelial cell tissue factor activity by augmenting secretion of tissue factor pathway inhibitor

Monolayers of human umbilical vein endothelial cells were activated with 50 U/mL interleukin-1alpha (IL-1alpha) for 3 hours and simultaneously conditioned with shear stresses of 0, 0.68, or 13.2 dyne/cm(2) in a parallel-plate flow chamber. In the presence of an inflow buffer containing 100 nmol/L factor X and 10 nmol/L factor VII, production of factor Xa, a measure of functional tissue factor (TF), was determined as the product of outflow concentration of factor Xa (chromogenic assay performed under quasi-static flow conditions after the shear period) and flow rate. Similarly, production of TF pathway inhibitor (TFPI) was estimated as the product of antigenic TFPI (by enzyme-linked immunosorbent assay) in the supernatant and flow rate. In parallel experiments, total RNA was isolated for determination of amplification products of TF mRNA by reverse transcription-polymerase chain reaction. We found that shear stress reduced factor Xa production (mean+/-SE; n=number of experiments) from 13.33+/-1.14 (n=16) fmol/minxcm(2) at 0 shear stress to 5.70+/-2.51 (n=5) and 0.54+/-0.54 (n=4) fmol/minxcm(2) at shear stresses of 0.68 and 13.2 dyne/cm(2), respectively. At the same time, immunogold labeling showed that TF antigen on the endothelial surface increased >5-fold with shear stress, whereas TFPI antigen on the surface increased 2-fold. The secretion of TFPI (appearance of new supernatant TFPI) rose from 7.4+/-2.4 (n=12) x10(-)(3) fmol/minxcm(2) at 0 shear stress to 23.7+/-7.3 (n=9) and 50.2+/-14.3 (n=4) x10(-)(3) fmol/minxcm(2) at 0.68 and 13.2 dyne/cm(2), respectively. TF mRNA amplification products were not markedly changed by shear stress. We conclude that acute application of shear stress reduces functional, but not antigenic, expression of TF by intact, activated endothelial cell monolayers in a manner associated with shear stress-augmented endothelial cell secretion of TFPI.

Characterization of the anticoagulant actions of a semisynthetic curdlan sulfate

Sulfation of curdlan, a natural, linear beta-1,3-glucan results in potent anticoagulant and antithrombotic agents. The different activity characteristics in the classical coagulation assays were shown to depend on various structural parameters. To obtain more detailed information about their structure-dependent mechanisms of action, one representative of these beta-1,3-glucan sulfates (CurS), was further investigated using several coagulation assays and amidolytic tests with chromogenic substrates. The mode of action of CurS differs from that of heparin. CurS reduces the thrombin formation by principally inhibiting the intrinsic FXa generation. As shown by amidolytic assays, it eliminates already generated thrombin mainly by accelerating the HCII-mediated thrombin inactivation, whereas its AT-mediated anti-thrombin activity is considerably lower than that of heparin. Further, it prevents the thrombin-mediated fibrin polymerization by directly interfering with the thrombin action on fibrinogen as well as by binding to fibrinogen. Finally, CurS is capable to activate the contact system with the consequence of a potential fibrinolytic effect. In conclusion, beta-1,3-glucan sulfates do not inhibit the blood coagulation nonspecifically due to their anionic character, but in dependence on their individual structure, they interfere specifically with the coagulation process at several sites.

Procoagulant activity of endothelial cells after infection with respiratory viruses

Influenza virus epidemics are associated with excess mortality due to cardiovascular diseases. There are several case reports of excessive coagulation during generalised influenza virus infection. In this study, we demonstrate the ability of respiratory viruses (influenza A, influenza B, parainfluenza-1, respiratory syncytial virus, adenovirus, cytomegalovirus) to infect lung fibroblasts and human umbilical vein endothelial cells in culture. All viral pathogens induced procoagulant activity in infected endothelial cells, as determined in a one-stage clotting assay, by causing an average 55% reduction in the clotting time. When factor VII deficient plasma was used clotting time was not reduced. The induction of procoagulant activity was associated with a 4- to 5-fold increase in the expression of tissue factor, as measured by the generation of factor Xa. Both experiments indicate that the procoagulant activity of endothelial cells in response to infection with respiratory viruses is caused by upregulation of the extrinsic pathway. Although both enveloped viruses and a non-enveloped virus (adenovirus) induced procoagulant activity in endothelial cells by stimulating tissue factor expression, the role of the viral envelope in the assembly of the prothrombinase complex remains uncertain. We conclude that both enveloped and non-enveloped respiratory viruses are capable of infecting cultured human endothelial cells and causing a shift from anticoagulant to procoagulant activity associated with the induction of tissue factor expression.

Sodium binding site of factor Xa: role of sodium in the prothrombinase complex

The nature of residue 225 on a consensus loop in serine proteases determines whether a protease can bind Na(+). Serine proteases with a Pro at this position are unable to bind Na(+), but those with a Tyr or Phe can bind Na(+). Factor Xa (FXa), the serine protease of the prothrombinase complex, contains a Tyr at this position. Na(+) is also known to stimulate the amidolytic activity of FXa toward cleavage of small synthetic substrates, but the role of Na(+) in the prothrombinase complex has not been investigated. In this study, we engineered a Gla-domainless form of FX (GDFX) in which residue Tyr(225) was replaced with a Pro. We found that Na(+) stimulated the cleavage rate of chromogenic substrates by FXa or GDFXa approximately 8-24-fold with apparent dissociation constants [K(d(app))] of 37 and 182 mM in the presence and absence of Ca(2+), respectively. In contrast, Na(+) minimally affected the cleavage rate of these substrates by the mutant, and no K(d(app)) for Na(+) binding to the mutant could be estimated. Unlike the wild-type enzyme, the reactivity of the mutant with antithrombin was independent of Na(+) and impaired approximately 32-fold. Ca(2+) improved the reactivity of the mutant with antithrombin approximately 5-fold. Affinity of the mutant for binding to factor Va was weakened and its ability to activate prothrombin was severely impaired. Further studies with the wild-type prothrombinase complex revealed that FXa binds to factor Va with a similar K(d(app)) of 1. 1-1.8 nM in the presence of Na(+), K(+), Li(+), Ch(+), and Tris(+) and that the catalytic efficiency of prothrombinase is enhanced less than 1.5-fold by the specific effect of Na(+) in the reaction buffer. These results suggest that (1) the loop including residue 225 (225-loop) is a Na(+) binding site in FXa, (2) the Na(+)- and Ca(2+)-binding loops of FXa are allosterically linked, and (3) the Tyr conformer of the 225-loop is critical for factor Xa function; however, both Na(+)-bound and Na(+)-free forms of factor Xa in the prothrombinase complex can efficiently activate prothrombin.

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 ( approximately 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. (Blood. 2000;95:1330-1335)

Inhibition of the generation of thrombin and factor Xa by a fucoidan from the brown seaweed Ecklonia kurome

The effects of a fucoidan (C-II), which was purified from the brown seaweed Ecklonia kurome, on the generation of thrombin and factor Xa have been investigated by measuring the amidolytic activities by using the respective specific chromogenic substrates in both plasma and purified systems. C-II inhibited significantly the generation of thrombin in both the intrinsic and the extrinsic pathways, although the intrinsic inhibitory effect by C-II was more remarkable than the extrinsic one. On the other hand, C-II was a good inhibitor of the factor Xa generation in the intrinsic pathway, while it was a poor one in the extrinsic pathway. In the purified systems C-II also inhibited the formation of prothrombin-activating complex (i.e., prothrombinase), but not its activity. The concentration of C-II required for 50% inhibition of thrombin generation was about one-tenth to one-seventh of that of the activity of the generated thrombin in plasma. These results indicate that C-II has an inhibitory effect on the generation of thrombin by blocking the formation of prothrombinase and by preventing the generation of intrinsic factor Xa in addition to its antithrombin activity, and also that the generation-inhibitory effect is more remarkable than C-II's enhancement effect on the antithrombin activity by heparin cofactor II in plasma.

Cooperation between VEGF and TNF-alpha is necessary for exposure of active tissue factor on the surface of human endothelial cells

This study was undertaken to characterize tissue factor (TF) induction, localization, and functional activity in cultured human umbilical vein endothelial cells (HUVECs) exposed to recombinant vascular endothelial growth factor (rVEGF) and recombinant tumor necrosis factor-alpha (rTNF-alpha). rVEGF (1 nmol/L) and rTNF-alpha (500 U/mL) synergistically increased TF mRNA, protein, and total activity, as measured in cell lysates. To examine surface TF expression, living cells were treated with antibody to TF and examined microscopically. Almost no staining was seen in control cells or cells treated with a single agent. In contrast, cells treated with both agonists showed intense membrane staining with surface patches, appearing as buds by confocal microscopy. To determine surface TF activity, studies were performed using a parallel-plate flow chamber, which allows detection of factor Xa generation on living cells. rVEGF and rTNF-alpha induced little surface TF activity (0.032+/-0.008 and 0.014+/-0.008 fmol/cm2, respectively). In combination, they significantly increased TF expression on the cell surface (0.429+/-0.094 fmol/cm2, P<0.05). These data indicate that the synergistic effect of rVEGF and rTNF-alpha is necessary to generate functional TF on the surface of endothelial cells. The requirement for multiple agonists to expose active TF may serve to protect endothelial cells from acting as a procoagulant surface, even under conditions of cell perturbation.

Platelet activating factor causes a rapid increase in activity of prior expressed tissue factor on monocyte surface membrane

It is known that tissue factor (TF) activity depends on cell membrane phospholipids. However, the mechanism involved in the regulation of TF activity by the modulation of the phospholipids has not yet been described in detail. To determine whether some mediators regulate TF activity by such a mechanism, we investigated the effect of platelet activating factor (PAF). Addition of PAF to TF-expressed monocytes caused a rapid and marked increase in the activity, but no increase in the antigen. Kinetic analyses were performed on TF-expressed monocytes with or without the addition of PAF, and on purified TF. The former revealed that the activity enhancement by PAF was associated with reduced Km, with Vmax remaining unaltered. The latter showed that the additional phosphatidylserine produced greater TF activity in purified TF, with an alteration pattern of kinetic parameters similar to that observed in the addition of PAF. From these results, we conclude that PAF regulates TF activity at the cell surface by alteration of the phospholipid composition of the membrane, and not by fresh production of TF apoprotein. The role of PAF as described in this paper must be one of the major regulatory systems in TF activity.

Recombinant factor VIIa does not induce hypercoagulability in vitro

Recombinant factor VIIa (rVIIa) has been reported to be clinically effective and safe in haemophilic patients with inhibitor antibodies. Compared to activated prothrombin complex concentrates the risk of thrombotic complications seems to be very low after rVIIa administration. Determination of free thrombin generation has been shown to identify hypercoagulability. Therefore, free thrombin and prothrombinase activity (Xa generation) were assessed after extrinsic activation of rVIIa supplemented factor VIII and factor IX deficient plasma. Free thrombin generation was also determined after supplementation of (activated) prothrombin complex concentrates. Addition of 150 U rVIIa/ml shortened the clotting times markedly in control, factor VIII, and factor IX deficient plasma. In contrast, free thrombin and Xa generation were not different in the absence or presence of 150 U rVIIa/ml. Addition of (activated) prothrombin complex concentrates resulted in a marked increase of free thrombin generation in all investigated plasmas. Although in vitro studies cannot reflect specific clinical circumstances our results support the notion that rVIIa does not induce a hypercoagulable state as sporadically observed after administration of (activated) prothrombin complex concentrates.

Expression of functional tissue factor on small vesicles of lipopolysaccharide-stimulated human vascular endothelial cells

We examined tissue factor expression on lipopolysaccharide-stimulated endothelial cells and their small vesicles by using specific antibodies and flow cytometry. Tissue factor functional activity was also assessed by activation of factor X. Endothelial cells were stimulated with 10 microg/ml of lipopolysaccharide in M-199/bovine serum albumin. Flow cytometry showed that expression of tissue factor on endothelial cells reached a maximum at 6 hours after stimulation, whereas that on small vesicles reached a maximum after 12 hours. Factor X activation mediated by factor VIIa and tissue factor was observed over a similar time course and was inhibited by the addition of antitissue factor antibody. Immunoelectron microscopy suggested that small vesicles with expression of some tissue factor were produced from the surface of endothelial cells. Our findings thus showed that tissue factor on endothelial cells produced by lipopolysaccharide stimulation was partly released to small vesicles. This may cause disseminated intravascular coagulation and related coagulation disorders.

Factor Xa generation at the surface of cultured rat vascular smooth muscle cells in an in vitro flow system

The purpose of the present investigation was to explore the effects of well-defined flow conditions on the activity of tissue factor (TF) expressed on the surface of cultured rat vascular smooth muscle cells. Cells were cultured to confluence on Permanox brand slides and stimulated to express TF by a 90 min incubation with fresh growth medium containing 10 percent calf serum. The stimulated cells were then placed in a parallel plate flow chamber and perfused with Hank's Balanced Salt Solution containing factor VIIa, factor X (FX), and calcium. The chamber effluent was collected and assayed for factor Xa (FXa) and the steady-state flux of FXa was calculated. The flux values were 68.73, 94.81, 139.75, 138.19, 316.82, and 592.92 fmole/min/cm2 at wall shear rates of 10, 20, 40, 80, 320, and 1280 s-1, respectively. The FXa flux depended on the wall shear rate to a greater degree than predicted by classical mass transport theory. The flux at each shear rate was three to five times less than that calculated according to the Leveque solution. These features of the experimental data imply nonclassical behavior, which may partially result from a direct effect of flow on the cell layer.

Regulation of extrinsic pathway factor Xa formation by tissue factor pathway inhibitor

Tissue factor (TF) pathway inhibitor (TFPI) regulates factor X activation through the sequential inhibition of factor Xa and the VIIa.TF complex. Factor Xa formation was studied in a purified, reconstituted system, at plasma concentrations of factor X and TFPI, saturating concentrations of factor VIIa, and increasing concentrations of TF reconstituted into phosphatidylcholine:phosphatidylserine membranes (TF/PCPS) or PC membranes (TF/PC). The initial rate of factor Xa formation was equivalent in the presence or absence of 2.4 nM TFPI. However, reaction extent was small (<20%) relative to that observed in the absence of TFPI, implying the rapid inhibition of VIIa.TF during factor X activation. Initiation of factor Xa formation using increasing concentrations of TF/PCPS or TF/PC in the presence of TFPI yielded families of progress curves where both initial rate and reaction extent were linearly proportional to the concentration of VIIa.TF. These observations were consistent with a kinetic model in which the rate-limiting step represents the initial inhibition of newly formed factor Xa. Numerical analyses of progress curves yielded a rate constant for inhibition of VIIa.TF by Xa.TFPI (>10(8) M-1.s-1) that was substantially greater than the value (7.34 +/- 0.8 x 10(6) M-1.s-1) directly measured. Thus, VIIa.TF is inhibited at near diffusion-limited rates by Xa.TFPI formed during catalysis which cannot be explained by studies of the isolated reaction. We propose that the predominant inhibitory pathway during factor X activation may involve the initial inhibition of factor Xa either bound to or in the near vicinity of VIIa.TF on the membrane surface. As a result, VIIa.TF inhibition is unexpectedly rapid, and the concentration of active factor Xa that escapes regulation is linearly dependent on the availability of TF.

A computational analysis of FXa generation by TF:FVIIa on the surface of rat vascular smooth muscle cells

A computational model was developed to investigate the contribution of classical mass transport and flow parameters to factor X (FX) activation by the tissue factor-factor VIIa complex (TF:VIIa) on one wall of a parallel-plate flow chamber. The computational results were compared to previously obtained experimental data for the generation of factor Xa (FXa) by TF:VIIa on the surface of cultured rat vascular smooth muscle cells. In this study, the complete steady-state convection-diffusion equation was solved using the commercial software package, FLUENT (Fluent Inc., Lebanon, New Hampshire). A user-defined subroutine interfaced with FLUENT implemented the surface reaction which was modeled using classical Michaelis-Menten reaction kinetics. The numerical solutions were obtained for 12 cases which used combinations of three wall shear rates and four reaction rates. The numerically obtained fluxes for a given reaction rate displayed a wall shear rate dependence which ranged from classical kinetic reaction control (no dependence) to pure diffusional control (maximum dependence). The experimental data, however, were not represented by numerical data generated using a single reaction rate. The three numerically obtained fluxes which corresponded most closely to the experimental fluxes were determined using three different Vmax values. This finding supports the hypothesis that there may be a direct effect of flow on the TF:VIIa complex or the cell membrane.

Cancer procoagulant--CP

A review of literature concerning cancer procoagulant (CP) has been carried out. This procoagulant directly activates coagulation factor X to factor Xa. Possibilities of utilising determinations of this activator in diagnostics and prognostics of the cancerous disease are discussed.

Inhibitory mechanism of the protein C pathway on tissue factor-induced thrombin generation. Synergistic effect in combination with tissue factor pathway inhibitor

The effects of the components of the protein C pathway on thrombin generation were studied in a reconstituted model in which thrombin is generated by factor VIIa and relipidated tissue factor (TF) via the activation of the purified coagulation factors X, IX, VIII, V, and prothrombin. The influence of protein C and soluble thrombomodulin on thrombin generation was correlated with factor Xa generation, factor V(a) and factor VIII(a) formation/inactivation, and protein C activation. Thrombin generation initiated by low concentrations of factor VIIa.TF (1.25 pM) occurs in an explosive fashion during a propagation phase which occurs after an initiation phase of approximately 1 min in which only traces of thrombin are formed. In the absence of other inhibitors, protein C (65 nM) in combination with high concentrations of soluble thrombomodulin (10 nM) resulted in a reduced rate of thrombin generation during the propagation phase without affecting the initiation phase; the activated protein C generated failed to neutralize prothrombinase activity and did not prevent prothrombin consumption. In the presence of plasma levels of the tissue factor pathway inhibitor (2. 5 nM recombinant TFPI), the protein C pathway reduced the rate of thrombin generation, initiated by 1.25 pM factor VIIa.TF, and completely eliminated prothrombinase activity at soluble thrombomodulin concentrations of >/=1 nM. The neutralization of prothrombinase activity coincided with cleavages at Arg-506 and subsequent cleavage at Arg-306 of the factor Va heavy chain by activated protein C. Thus, the protein C pathway combined with TFPI creates a minimal inhibitory potential required to shut down TF-initiated thrombin generation. The protein C pathway constituents did not influence factor Xa generation or factor VIIIa degradation over the interval in which prothrombinase activity was neutralized. Our data thus suggest that the protein C pathway regulates thrombin generation solely by the inactivation of factor Va. At low initiating factor VIIa.TF (1.25 pM) and high thrombomodulin concentrations (10 nM), the factor Va heavy chain is cleaved before significant amounts of light chain are generated. The ability of the protein C pathway to inhibit thrombin generation was greatly reduced when the reaction was initiated in the presence of factor Va, supporting the hypothesis that effective down-regulation of thrombin generation by the protein C pathway, in reactions initiated with the procofactor, occurs by prevention of the coexistence of the factor Va heavy and light chains.

Identification of active tissue factor in human coronary atheroma

BACKGROUND

Recent observations suggest that thrombosis in vivo is initiated via the tissue factor (TF) pathway. The TF activity of human coronary atheroma has not been reported.

METHODS AND RESULTS

Directional coronary atherectomy (DCA) specimens from 63 lesions were analyzed with the use of a quantitative TF-specific activity assay. The median content of TF was 10 ng/g plaque (95% CI, 6 to 13 ng/g; range, 0 to 47 ng/g). After homogenization of the specimens, TF activity was detected in 28 of 31 lesions (90%). With a polyclonal anti-human TF antibody, the use of immunohistochemistry detected TF antigen in 43 of 50 lesions (86%); TF antigen was expressed in cellular and acellular areas of the plaque. Histologically defined thrombus was present in 19 of the 43 lesions with detectable TF antigen and in none of the 7 lesions without detectable TF antigen (19 of 43 versus 0 of 7; P < .02). TF antigen was undetectable with immunohistochemistry in 4 of 13 restenotic lesions (31%) and in 3 of 37 de novo lesions (8%) (P < .05).

CONCLUSIONS

TF contributes to the procoagulant activity of most atherosclerotic lesions treated with DCA. The association of immunohistochemically detectable TF with plaque thrombus suggests that TF plays a role in coronary thrombosis. Diminished TF expression in restenotic lesions may in part account for the lower complication rate that has been associated with DCA of restenotic versus de novo lesions. Inhibition of TF may represent a therapeutic goal for the prevention of thrombotic complications associated with percutaneous coronary interventions.

Kinetics of blood coagulation factor Xaalpha autoproteolytic conversion to factor Xabeta. Effect on inhibition by antithrombin, prothrombinase assembly, and enzyme activity

Autoproteolysis of blood coagulation factor Xa (FXa) results in the excision of a 4-kDa fragment (beta-peptide) from the intact subform, factor Xaalpha (FXaalpha), to yield factor Xabeta (FXabeta). In the preceding paper, we showed that generation of FXabeta leads to expression of a plasminogen binding site. FXabeta may consequently participate in fibrinolysis; therefore, the timing of subform conversion compared with thrombin production is important. In the current study we evaluated the kinetics of FXabeta generation, which showed that autoproteolysis of FXaalpha followed a second order mechanism where FXaalpha and FXabeta behaved as identical enzymes. Rate constants of 9 and 172 M-1 s-1 were derived, respectively, in the absence and presence of FXaalpha binding to procoagulant phospholipid. Under identical conditions the latter is estimated to be 6 orders of magnitude slower than thrombin generation by prothrombinase. Since heparin binding and prothrombin recognition have been previously attributed to a region of FXaalpha proximal to the beta-peptide, functional comparisons were conducted using homogeneous and stabilized preparations of FXaalpha and FXabeta. Comparisons included 1) the recognition of small substrates; 2) the rate of interaction with antithrombin/heparin; 3) the assembly of prothrombinase; and 4) the activation of prothrombin by prothrombinase. Although the beta-peptide neighbors a probable functional region in FXaalpha, conversion to FXabeta was not observed to influence these functions. The data support a model where FXaalpha is predominantly responsible for thrombin generation and where slow conversion to FXabeta coordinates coagulation and the initiation of fibrinolysis at sites of prothrombinase assembly.

Efficient synthesis of a 72-kDa mitochondrial polypeptide using the yeast Ty expression system

Using the Ty system from yeast we report the efficient expression of a heterologous eukaryotic gene encoding a 72 kDa mitochondrial polypeptide. The pFM2IIBgIII expression vector was initially modified for this purpose by inserting the factor X(a) protease cleavage site. The TyA gene, which encodes the structural component of the yeast virus-like particles (VLPs), and the eukaryotic yst1 gene, encoding a 72 kDa mitochondrial tyrosyl-tRNA synthetase from the filamentous fungus Podospora anserina, were subsequently fused to the factor X(a) cleavage site. The resulting chimeric gene, in which the two polypeptide coding sequences are separated by the factor X(a) cleavage site, was expressed in yeast. High yield expression of this foreign protein, which was isolated from yeast transformants as hybrid TyVLPs, was verified after factor X(a) treatment by SDS polyacrylamide gel electrophoresis and antibody detection. The strategy presented here should be useful for expressing a wide variety of eukaryotic genes.