Human platelets express endothelial protein C receptor, which can be utilized to enhance localization of factor VIIa activity

Essentials Factor VIIa binds activated platelets to promote hemostasis in hemophilia patients with inhibitors. The interactions and sites responsible for platelet-FVIIa binding are not fully understood. Endothelial cell protein C receptor (EPCR) is expressed on activated human platelets. EPCR binding enhances the efficacy of a FVIIa variant and could impact design of new therapeutics.

SUMMARY

Background High-dose factor VIIa (FVIIa) is routinely used as an effective bypassing agent to treat hemophilia patients with inhibitory antibodies that compromise factor replacement. However, the mechanism by which FVIIa binds activated platelets to promote hemostasis is not fully understood. FVIIa-DVQ is an analog of FVIIa with enhanced tissue factor (TF)-independent activity and hemostatic efficacy relative to FVIIa. Our previous studies have shown that FVIIa-DVQ exhibits greater platelet binding, thereby suggesting that features in addition to lipid composition contribute to platelet-FVIIa interactions. Objectives Endothelial cell protein C receptor (EPCR) also functions as a receptor for FVIIa on endothelial cells. We therefore hypothesized that an interaction with EPCR might play a role in platelet-FVIIa binding. Methods/results In the present study, we used flow cytometric analyses to show that platelet binding of both FVIIa and FVIIa-DVQ is partially inhibited in the presence of excess protein C or an anti-EPCR antibody. This decreased binding results in a corresponding decrease in the activity of both molecules in FXa and thrombin generation assays. Enhanced binding to EPCR was sufficient to account for the increased platelet binding of FVIIa-DVQ compared with wild-type FVIIa. As EPCR protein expression has not previously been shown in platelets, we confirmed the presence of EPCR in platelets using immunofluorescence, flow cytometry, immunoprecipitation, and mass spectrometry. Conclusions This work represents the first demonstration that human platelets express EPCR and suggests that modulation of EPCR binding could be utilized to enhance the hemostatic efficacy of rationally designed FVIIa analogs.

Effect of oral contraceptives on the anticoagulant activity of protein S in plasma

We determined anticoagulant parameters that depend on protein S function in plasma, i.e.the APC-independent anticoagulant activity of protein S (expressed as pSR) and APC resistance determined with thrombin generation-based tests (expressed as APCsr) as well as plasma levels of total and free protein S and prothrombin in men, women not using oral contraceptives (OC), and in women using second or third generation OC. Thrombin generation in the APC resistance assays was initiated either with factor Xa (Xa-APCsr) or tissue factor (TF-APCsr). The APC-independent anticoagulant activity of protein S was highest in men (pSR=1.69) and gradually decreased from women not using OC (pSR=1.49) via women using second generation (pSR=1.35) to women using third generation OC (pSR=1.27). The pSR correlated inversely with nAPCsr determined with the tissue factor-based APC resistance test (TF-APCsr) but not with nAPCsr determined with the factor Xa-based assay (Xa-APCsr). Multiple linear regression analysis in which sex, OC use, and protein S and prothrombin levels were included as independent variables and the pSR, TF-APCsr or Xa-APCsr as dependent variables indicated that plasma protein S levels poorly predict the pSR and the TF-APCsr, but are the main determinant of the Xa-APCsr. This indicates that OC use alters the expression of protein S activity. This phenomenon can be caused by differences in modulation of the activity of protein S by other plasma proteins that change during OC use or by OC-induced changes in the protein S molecule that impair its anticoagulant activity. Functional impairment of protein S as a result of hormonal influence may, at least in part, contribute to the thrombotic risk of OC users.

Antithrombotic and antiplatelet activities of pelargonidin in vivo and in vitro

Pelargonidin is a well-known red pigment found in plants, and has been reported as having important biological activities that are potentially beneficial for human health. However, the possible roles of pelargonidin as an anticoagulant and the underlying mechanism have not yet been elucidated. We tested the effect of pelargonidin and its glucoside-conjugated form, pelargonidin-3-glucoside, on the clotting times, such as activated partial thromboplastin time (aPTT) and prothrombin time (PT), and the activities and productions of thrombin and activated factor X (FXa). Furthermore, the effects of pelargonidin on the fibrin polymerization, platelet aggregation, and the ratio of plasminogen activator inhibitor-1 (PAI-1) to tissue plasminogen activator were determined. Pelargonidin, but not pelargonidin-3-glucoside, prolonged the aPTT and PT, and inhibited the activity and production of thrombin and FXa in human umbilical vein endothelial cells. Furthermore, pelargonidin inhibited thrombin-catalyzed fibrin polymerization and platelet aggregation and elicited anticoagulant effects in mice. In addition, pelargonidin significantly reduced PAI-1 to t-PA ratio. Collectively, these results indicate that the anthocyanin pelargonidin possesses antithrombotic activity, and can be beneficial in preventing thrombus formation, thus improving blood circulation.

[Assessment of anti-factor Xa activity for heparin and related products by chromogenic assays and thrombin generation tests]

Anticoagulant therapy is widely used for the prevention and treatment of thromboembolism. In addition to established agents such as warfarin, unfractionated heparin and low-molecular-weight heparins, a variety of new anticoagulant drugs has been introduced for clinical use, including direct thrombin inhibitors and factor Xa inhibitors. These new drugs can be given at fixed doses without the need for routine monitoring of the coagulation profile. However, an assays to evaluate anticoagulant strength would be valuable to prevent undesired hemorrhagic or thromboembolic events during treatment. In the present study, we examined the feasibility of several laboratory monitoring tests, including chromogenic-based anti-factor Xa assay and the thrombin generation test to determine the anticoagulant effect of low-molecular-weight heparins and fonda-parinux. Dose-dependent relationship between anti-factor Xa activity and the concentration of fondaparinux was observed by the chromogenic assays. In the thrombin generation test, the peak parameter seemed to be more informative than the endogenous thrombin potential, which corresponds to the total amount of thrombin activity, to assess the pharmacodynamic effects. In summary, our study suggested that both assays may be useful for quantitative determination of factor Xa activity. Further studies are necessary to develop and establish simpler methods that can be used in routine laboratory testing to monitor treatment with the newer anticoagulant drugs.

Effect of ionizing radiation on cellular procoagulability and co-ordinated gene alterations

BACKGROUND AND OBJECTIVES

Ionizing radiation (IR) is associated with thrombotic vascular occlusion predicting a poor clinical outcome. Our study examined whether IR induced tissue factor (TF) expression and procoagulability. We further investigated coordinated gene alterations associated with TF upregulation in the myelomonocytic leukemia THP-1 cells.

DESIGN AND METHODS

TF expression was determined by quantitative Reverse Transcriptase (TaqMan) PCR, TF ELISA and TF activity by a two stage chromogenic assay in the time course of days 1, 3, 7, 10, and 17 post IR. To detect IR-induced alterations in gene expression, Affymetrix HG U133 Plus 2.0 microarrays were used. RESULTS IR induced a significant increase in TF/GAPDH mRNA ratios and cellular TF protein on days 3 and 7 post IR (20 Gy [p>or=0.01] and 40 Gy [p <or=0.01 vs. control]), suggesting a late and persistent induction of TF. An increase in cellular TF activity was already found 1 day post IR (20 Gy and 40 Gy [p>or=0.001] vs. control respectively), suggesting IR immediately alters the cellular thrombogenicity. TF upregulation post IR was confirmed in PBMNCs. Gene expression profiling showed IR increased the expression of inflammatory and apoptosis-related pathways known to be involved in the regulation of TF expression.

INTERPRETATION AND CONCLUSIONS

TF upregulation together with inflammation and apoptosis may increase the thrombogenicity of tissues. The demonstrated upregulation of TF might play a pivotal role in radiation associated thrombosis.

Very severe thrombocytopenia and fragmentation hemolysis mimicking thrombotic thrombocytopenic purpura associated with a giant intracardiac vegetation infected with Staphylococcus epidermidis: role of monocyte procoagulant activity induced by bacterial supernatant

The pathogenesis of very severe thrombocytopenia in bacterial endocarditis is uncertain. We report a 50-year-old male with platelet counts < 10 x 10(9)/l and fragmentation hemolysis complicating Staphylococcus epidermidis pacemaker endocarditis with a giant vegetation. Antibiotics, corticosteroids, high-dose intravenous gammaglobulin, and plasmapheresis (for initially-suspected thrombotic thrombocytopenic purpura) failed to produce significant platelet count increase. However, therapeutic-dose heparin anticoagulation was associated with a platelet count increase from <10 to approximately 40 x 10(9)/l, with parallel reduction in thrombin-antithrombin complexes (from 8.9 to 3.5 microg/l), facilitating surgical intervention. The thrombocytopenia promptly resolved following surgical removal of the vegetation. Culture supernatant from S. epidermidis isolated from the patient's blood induced monocytes to express procoagulant activity (assessed by factor Xa generation) equivalent to lipopolysaccharide (1 microg/ml), with half-maximal activation seen with culture supernatant diluted to 1:12,800. These data are consistent with previous animal models of endocarditis demonstrating staphylococci-induced procoagulant changes in monocytes. This case demonstrates that heparin anticoagulation can be therapeutic in infective endocarditis-associated severe thrombocytopenia in a non-bleeding patient, and that such therapy may ameliorate the platelet count enough to permit surgical intervention.

Activity and regulation of factor VIIa analogs with increased potency at the endothelial cell surface

BACKGROUND

Variants of recombinant factor VIIa (rFVIIa) with increased intrinsic activity have been developed to improve efficacy in the treatment of bleeding disorders in the future. The increased potency of FVIIa variants was demonstrated in limited in vitro and in vivo studies. However, further characterization of FVIIa variants is needed to evaluate their potential clinical use.

METHODS

In the present study, we investigated the interactions of two FVIIa variants, FVIIa(Q) and FVIIa(DVQ), with plasma inhibitors, tissue factor pathway inhibitor (TFPI) and antithrombin (AT), and vascular endothelium. TF-FVIIa activity or its inhibition was measured directly in an amidolytic activity assay or for its ability to activate factor X.

RESULTS

Both TFPI and AT/heparin inhibited the FVIIa variants more rapidly than the wild-type (WT) FVIIa in the absence of tissue factor (TF). In the presence of TF, TFPI, TFPI-Xa, and AT/heparin inhibited FVIIa and FVIIa variants at similar rates. Although the WT FVIIa failed to generate significant amounts of FXa on unperturbed endothelial cells, FVIIa variants, particularly FVIIa(DVQ), generated a substantial amount of FXa on unperturbed endothelium. Annexin V fully attenuated the FVIIa-mediated activation of FX on unperturbed endothelial cells. On stimulated human umbilical vein endothelial cells, FVIIa and FVIIa variants activated FX at similar rates, and annexin V blocked the activation only partly. AT/heparin and TFPI-Xa inhibited the activity of FVIIa and FVIIa variants bound to endothelial cell TF in a similar fashion. Interestingly, despite significant differences observed in FXa generation on unperturbed endothelium exposed to FVIIa and FVIIa analogs, no differences were found in thrombin generation when cells were exposed to FVIIa or FVIIa analogs under plasma mimicking conditions.

CONCLUSION

Overall, the present data suggest that although FVIIa variants generate substantial amounts of FXa, they do not generate excessive thrombin on the surface of endothelium.

Calcium ionophore-induced de-encryption of tissue factor in monocytes is associated with extensive cell death

INTRODUCTION

Cell surface tissue factor (TF) is normally encrypted, but can be activated by various cellular perturbations. Exposure of TF bearing cells to calcium ionophore has been reported to increase TF activity, de-encrypt TF, by phosphatidylserine (PS)-dependent and -independent mechanisms. Our aim has been to examine at the single cell level, if increased cell surface PS coincided with increased cell surface TF antigen, and cell death (necrosis, 7-AAD-intercalation), and relate this to monocyte- and microparticle (MP)-associated procoagulant activity.

MATERIALS AND METHODS

We exposed lipopolysaccharide-stimulated, human, elutriation-purified, cryopreserved TF bearing monocytes to increasing concentrations of calcium ionophore (A23187) and measured procoagulant activity in cells and supernatants. These measurements were compared with quantification of cell surface TF and PS (Annexin V) and of cell necrosis (7-AAD) by flow cytometry, and complemented by confocal microscopy.

RESULTS

We observed that calcium ionophore increased cellular and MP-associated TF activity, but not cell surface TF antigen. The discrepancy between TF activity and TF antigen coincided with a dose-dependent increase in the number of cells expressing PS. These cells were to a large extent necrotic and many of them also expressed TF.

CONCLUSIONS

We suggest such TF positive dying cells to contribute to the discordance between TF activity and TF expression. Calcium ionophore also increased MP-associated TF activity and release of MPs may be a way to disseminate procoagulant activity. Our findings emphasize the importance of adequately assessing cell death and taking into consideration its possible role in experiments with calcium ionophore.

Coagulation cascade activation causes CC chemokine receptor-2 gene expression and mononuclear cell activation in hemodialysis patients

Priming of the coagulation cascade during hemodialysis (HD) leads to the release of activated factor X (FXa). The binding of FXa to its specific receptors, effector protease receptor-1 (EPR-1) and protease-activated receptor-2 (PAR-2), may induce the activation of peripheral blood mononuclear cells (PBMC) and promote a chronic inflammatory state that is responsible for several HD-related morbidities. In the attempt to elucidate the mechanisms underlying the coagulation-associated inflammation in HD, 10 HD patients were randomized to be treated subsequently with a cellulose acetate membrane (CA) and Ethylen-vinyl-alcohol (EVAL), a synthetic membrane that has been shown to reduce FXa generation. At the end of each experimental period, surface FXa and thrombin receptors (EPR-1 and PAR-1, -2, and -4) and CCR2 (monocyte chemoattractant protein-1 receptor) gene expression in isolated PBMC were examined. the ability of dialytic membranes to activate protein-tyrosine kinases and the stress-activated kinase JNK and to modulate the generation of terminal complement complex (TCC) was also investigated. EPR-1 and PAR-2 and -4 mRNA expression, barely detectable in normal PBMC, were significantly upregulated in HD patients, particularly in those who were treated with CA. A striking increase of tyrosine-phosphorylated proteins and JNK activation was observed at the end of HD only in CA-treated patients. Simultaneously, an increased gene expression for both splicing isoforms of CCR2, A and B, only in PBMC from CA-treated patients was demonstrated. The increased CCR-2 mRNA abundance was followed by a significant increase in its protein synthesis. The high expression of CCR2 was associated with an increased generation of plasma TCC and a significant drop in leukocyte and monocyte count. By contrast, EVAL treatment slightly lowered TCC generation and normalized leukocyte count. In vitro FXa induced CCR2 A and B expression and JNK activation in freshly isolated PBMC. FXa-induced CCR2 mRNA expression was completely abolished by JNK and tyrosine kinase inhibition. In conclusion, these data suggest that subclinical clotting activation may cause an increased CCR2 gene and protein expression on uremic PBMC, contributing to HD-related chronic microinflammation. The use of the less coagulation-activating membrane, EVAL, may reduce PBMC activation through the modulation of the stress-activated kinase JNK.

Herpes simplex virus type 1-encoded glycoprotein C enhances coagulation factor VIIa activity on the virus

Tissue factor (TF) is the blood coagulation initiator, whose cofactor function is required for physiological factor VIIa (FVIIa)-mediated activation of factor X (FX) to FXa. A previous study reported TF on herpes simplex virus type 1 (HSV1), but this explained only part of FVIIa-dependent FXa generation observed on the virus surface (Sutherland et al. (1997) Proc. Natl. Acad. Sci. USA. 94:13510-14). In the current study, we investigated the role of HSV1-encoded glycoprotein C (gC) in this process. Purified gC-deficient HSV1 facilitated several fold less FX activation by FVIIa than either wild type or gC-rescued strains. To confirm the implication of gC in FVIIa-dependent FX activation, purified soluble gC (sgC) enhanced FXa production in the absence of TF. sgC required FVIIa, calcium and anionic phospholipid to participate in FX activation, suggesting similarity to TF. When purified virus was combined with sgC, the sgC-dependent FXa generation was enhanced three orders of magnitude, suggesting synergy with an additional HSV1 component and explaining the relatively low activity of purified sgC compared to the viral counterpart. FX activation on gC-competent HSV1 was inhibited 20% by a gC-specific antibody, inhibited 40% by a TF-specific antibody, inhibited 65% by combining the gC- and TF-specific antibodies, and nearly completely inhibited by the TF antibody alone on gC-deficient HSV-1. Cumulatively, these observations show that two pathways initiating FX activation function in parallel on the virus surface. In addition to the previously described TF-dependent pathway, HSV-1-encoded gC also enhances FXa generation, and like TF, requires FVIIa.

Thrombin-catalyzed activation of factor VIII with His substituted for Arg372 at the P1 site

Thrombin-catalyzed proteolysis at Arg372 of factor VIII is essential for procofactor activation. However, hemophilia A patients with the missense mutation Arg372 to His possess a mild to moderate phenotype yet show no detectable cleavage at this bond. To evaluate this discrepancy, we prepared and stably expressed a recombinant, B-domainless factor VIII mutant (R372H) that possessed approximately 1% the specific activity of wild type. Cleavage at R372H by thrombin occurred with an approximately 80-fold decreased rate compared with wild type. N-terminal sequence analysis of the derived A2 subunit confirmed that cleavage occurred at the His372-Ser373 bond. Factor VIII R372H was activated slowly, attained lower activity levels, and exhibited an apparent reduced inactivation rate compared with factor VIII wild type. These observations were attributed to a reduced cleavage rate at His372. Factor Xa generation assays showed similar Michaelis-Menten constant (K(m), apparent) values for thrombin-catalyzed activation for either factor VIII form, but suggested an approximately 70-fold reduced maximum velocity (V(max)) for factor VIII R372H. However, prolonged reaction with thrombin yielded similar activity and stability values for the mutant and wild-type factor VIIIa forms. These results indicate a markedly reduced rate of cleavage following substitution at the P(1)Arg, and this property likely reflects the severity of the hemophilia A phenotype.

Assembly and regulation of prothrombinase complex on B16F10 melanoma cells

A number of studies indicate that coagulation proteases play significant roles in cancer biology. Melanoma is a highly metastatic cancer, and there is evidence that thrombin contributes to this aggressive pattern. However, few studies correlate this type of cancer with formation of the prothrombinase complex, which is responsible for conversion of prothrombin into thrombin in the coagulation system. The aim of this study was to investigate the assembly and regulation of prothrombinase complex on the murine melanoma cell line, B16F10. B16F10 cells were unable to activate prothrombin except when previously incubated with factor Xa. This effect was dependent on factor Xa binding to cell membranes, since no activation was detected with Gla-domainless factor Xa. The thrombin formation by B16F10-bound factor Xa was enhanced approximately 10 fold in the presence of factor Va, indicating the assembly of prothrombinase complex. Differently from platelets, B16F10-assembled prothrombinase complex was inhibited by prothrombin fragment 1 but not by fragment 2. In addition, bothrojaracin, a specific ligand of proexosite I on prothrombin, caused a significant decrease in the zymogen activation. Our data demonstrate that B16F10 melanoma cells generate thrombin by promoting assembly of the prothrombinase complex. This ability might be correlated with the increased metastatic potential of this cell line. Moreover, B16F10-assembled prothrombinase complex seems to be modulated in a different way from that found for the physiological complex assembled on platelets.

An oligonucleotide decoy for nuclear factor-kappa B inhibits tumor necrosis factor-alpha-induced human umbilical cord vein endothelial cell tissue factor expression in vitro

Abnormal tissue factor (TF) expression on vascular endothelial cells may account for thrombotic events associated with cardiovascular disease. The transcription factor nuclear factor-kappa B (NF-kappa B) activation plays a key role in endothelial cell injury and TF expression. Disruption of NF-kappa B activation in endothelial cells may inhibit TF expression and be protective in thrombosis. The purpose of the study was to determine whether NF-kappa B transcription factor decoy (TFD) could block TF expression. NF-kappa B TFD was transferred into cultured human umbilical cord vein endothelial cells (HUVEC) by liposomes, and the transfection efficiency was detected by flow cytometry. The effect of NF-kappa B TFD on TF mRNA levels was determined by reverse transcription-polymerase chain reaction. The expression of surface TF antigen was analyzed by flow cytometry. TF activity was studied by measuring enzymatic activation of factor X by the TF-activated factor VII complex. The results suggested that NF-kappa B decoy could be successfully transferred into HUVEC by liposome. The NF-kappa B TFD competed with the endogenous kappa B site sequence in the TF promoter for binding to transcription factor NF-kappa B in tumor necrosis factor-alpha-stimulated HUVEC, which could block the tumor necrosis factor-alpha-induced increase in TF mRNA levels, the upregulation of surface TF antigen and TF activity. This study demonstrated that NF-kappa B decoy could block HUVEC TF gene expression. Targeted genetic disruption of endothelial TF expression by NF-kappa B decoy may provide a possible therapeutic method for cardiovascular and thrombosis disease.

Antifactor Xa activity correlates to thrombin generation time, platelet contractile force and clot elastic modulus following ex vivo enoxaparin exposure in patients with and without renal dysfunction

Antifactor Xa activity is the gold standard monitoring parameter for low molecular weight heparin (LMWH) derivatives. It is frequently measured in high-risk populations, such as patients with renal dysfunction. Despite antifactor Xa monitoring, however, bleeding in renal dysfunction patients receiving LMWH remains a problem. This study determined the relationship between antifactor Xa activity and three novel coagulation monitoring parameters: thrombin generation time (TGT), platelet contractile force (PCF) and clot elastic modulus (CEM). This study also assessed the effect of renal dysfunction on these relationships. This was an ex vivo pharmacodynamic study of the relationship between antifactor Xa activity and TGT, PCF and CEM in subjects both with and without renal dysfunction. Thirty subjects completed this study (10 controls, 10 chronic kidney disease subjects, and 10 end-stage renal disease subjects receiving hemodialysis). Blood samples obtained from participants were spiked with increasing enoxaparin concentrations (0.25, 0.5, 1.0 and 3.0 IU mL(-1)). Samples were analyzed for TGT, PCF and CEM. The relationship between antifactor Xa activity and TGT, PCF and CEM was determined by Pearson's correlation. The effect of renal dysfunction on the relationship between antifactor Xa activity and TGT, PCF and CEM was determined by analysis of covariance. There is strong correlation between antifactor Xa activity and TGT, CEM and PCF. The presence of renal dysfunction significantly prolongs the TGT, and decreases the CEM relative to controls. These results suggest that patients with renal dysfunction have a greater pharmacodynamic response to LMWH, independent of the pharmacokinetics of LMWH.

Aggregated low-density lipoprotein uptake induces membrane tissue factor procoagulant activity and microparticle release in human vascular smooth muscle cells

BACKGROUND

Tissue factor (TF) is the main initiator of the arterial blood coagulation system, and aggregated LDL (agLDL) are found in the arterial intima. Our hypothesis is that agLDL internalization by vascular smooth muscle cells (VSMCs) may trigger TF-procoagulant activity.

METHODS AND RESULTS

Cultured human VSMCs were obtained from human coronary arteries of explanted hearts during transplant operations. VSMCs were incubated with native LDL (nLDL) or agLDL. TF mRNA was analyzed by real-time polymerase chain reaction, and cellular and released TF protein antigen were analyzed by Western blot. TF microparticle (MP) content was analyzed by flow cytometry and TF activity by a factor Xa generation test. Both nLDL and agLDL strongly and equally increased TF mRNA and cell membrane protein expression, by approximately 5- and 9-fold, respectively. A sustained TF procoagulant activity was induced by agLDL but not by nLDL (agLDL 2.46+/-0.22 versus nLDL 0.72+/-0.12 mU/mg protein at 12 hours). AgLDL increased TF antigen release (agLDL 5.64+/-0.4 versus nLDL 3.28+/-0.22 AU) and TF MP release (agLDL 89.85+/-8.51 versus nLDL 19.69+/-4.59 TF MP/10(3) cells). TF activation and release induced by agLDL is not related to apoptosis. Blockade of LDL receptor-related protein, a receptor for agLDL, prevented the agLDL-induced release of TF protein and TF MP.

CONCLUSIONS

VSMC-TF expression is upregulated by both nLDL and agLDL. However, only agLDL engagement to LDL receptor-related protein induced cellular TF procoagulant activity and TF release by human VSMCs.

Clustered basic residues within segment 484-510 of the factor VIIIa A2 subunit contribute to the catalytic efficiency for factor Xa generation

Residues 484-510 of factor (F)VIIIa A2 subunit comprise a prominent epitope for inhibitor antibodies, suggesting that this region is critical for cofactor function. To address the role of this region in catalysis, FVIIIa forms were evaluated following conversion of conserved charged residues to Ala, either in clusters or individually. The two cluster mutants, Lys496Ala/Lys499Ala/Asp500Ala and Glu507Ala/Lys510Ala, were indistinguishable from wild type. The mutation Arg489Ala/Arg490Ala/Lys493Ala (489-3A) possessed near-normal affinity for FIXa and showed no effect on the Km for FX, but exhibited approximately 3-fold and approximately 30-fold reduced kcat values for FXase in the presence and absence of surface, respectively. However, the single-site mutants Arg489Ala, Arg490Ala and Lys493Ala exhibited affinity and kcat values similar to wild type. Furthermore, the 489-3A mutant showed a marked reduction in the positive electrostatic potential within this region of A2, consistent with the hypothesis that the cumulative basic charge in this region of A2 subunit modulates cofactor function.

Molecular and pharmacologic profile of tinzaparin and a comparable low-molecular-weight bacterial sulfaminoheparosan

Low-molecular-weight heparins (LMWH) represent depolymerized porcine mucosal heparin derivatives, which are commonly used for the management of thrombotic disorders. Because of their widespread usage, the supplies of the raw material namely unfractionated heparin are nearly exhausted. Porcine mucosal tissue is almost exclusively used for the preparation of these agents. Thus, there is a timely need for the production of heparin like drugs from other sources. Fermentation techniques have been used to produce carbohydrates such as dextran and innulin for therapeutic purposes. Bacterial cell wall polysaccharide mimics the linear hexose units, which constitute heparin. Utilizing Escherichia coli cell membranes produced by fermentation technology, chemical sulfation and enzymatic epimerization, sulfaminoheparosan type of polymer mimicking the structure of heparin has been produced. These semi-synthetic sulfaminoheparosans exhibit biologic actions comparable to that observed with heparin. The sulfaminoheparosan core can also be degraded to obtain low-molecular-weight (LMW) derivatives mimicking LMWHs. Using this technique, a novel LMW sulfaminoheparosan derivative (Q93C/239) was produced by Inalco, Milan, Italy. To compare this heparin analogue, a LMWH, namely tinzaparin, was used to determine the relative anticoagulant, antiprotease, and molecular profile. Additional studies were carried out to determine the susceptibility of this agent to heparinase-I. These comparative studies exhibited both antiprotease and anticoagulant properties similar to those of tinzaparin. However LMW sulfaminoheparosan resisted heparinase-I digestion at low heparinase-I concentrations. These studies demonstrate that the sulfaminoheparosan derived LMW components exhibit similar molecular and anticoagulant profile as tinzaparin and warrant additional preclinical and clinical development to determine their potential usefulness as antithrombotic agents.

Detection of procoagulant phospholipid interfering in tests for lupus anticoagulant

Excess platelets shorten most clotting tests for lupus anticoagulant (LA). Often it is not clear if a shortened, normal or slightly prolonged result in a test masks a weak LA in combination with activated platelets, which express procoagulant phospholipid (PPL). Our aim was to investigate a new LA-insensitive factor Xa-activated clotting time (XACT) test for detecting PPL in plasma specimens submitted for LA testing. In most clotting tests for PPL, specimens are mixed with human platelet-free plasma (PFP) to correct for factor defects. Such tests are usually very sensitive to prolongation by LA, which act against PPL-human clotting factor complexes. We found that phospholipid-free plasma from pigs could be used instead of human platelet-free plasma as substrate plasma without reducing sensitivity of XACT to PPL. However, the pig plasma-based system was significantly less affected by most LA. Activated platelets were detectable despite the presence of most LA. Since some LA still had significant prolonging effect on the XACT despite the use of pig plasma we investigated this further. ELISAs for IgG and IgM anti-beta2GP1 and anti-prothrombin antibodies were carried out on 23 specimens. We did not find that LA in plasmas displaying anti-prothrombin antibodies had less prolonging effect on the test based on pig plasma than that using human platelet-free plasma. Similarly, there were no subtyping trends apparent among results from anti-beta2GPI-positive samples. Our results do not support the concept that anti-prothrombin-dependent LA might be more species specific than anti-beta2GPI-dependent LA.

A non-immunological phospholipid-dependent coagulation inhibitor associated with IgGlambda-type multiple myeloma

We investigated the rare case of a patient with IgGlambda multiple myeloma for whom both prothrombin time and APTT were significantly prolonged. The IgG inhibited coagulation reactions upstream from prothrombin when coagulation was initiated by mRVVT, but not by FXa, as indicated by a chromogenic substrate for FXa. The mPT and the mAPTT showed inhibition of FXa generation in both the intrinsic and extrinsic pathways. The IgG inhibited both protein C (indicated by APTT) and FX (indicated by RVV) but not amidolysis for either activated protein C or FXa. The addition of excess phospholipid significantly shortened the prolonged RVVT of the patient. It inhibited the coagulation reactions of normal plasma and was dependent on decreasing the PS concentration in the APTT reagent. It was suggested that the IgG showed lupus anticoagulant (LA)-like activity that inhibited phospholipid-dependent coagulation reactions in the intrinsic, extrinsic, and common pathways. However, the IgG did not bind cardiolipin-beta2GPI complex, beta2GPI, or prothrombin in ELISA assays. The IgG did not bind to either PS or phospholipid complexes in the presence or absence of prothrombin, FX, or FXa. Interestingly, the IgG lost its LA like-activity when it was degraded to F(ab')2 and Fc fragments by pepsin. We suspected that the IgG might inhibit the interaction between coagulation factors and acid phospholipid non-immunologically and that this process requires an intact IgG conformation, although the reaction mode is still not clear.

Formation of tissue factor-factor VIIa-factor Xa complex promotes cellular signaling and migration of human breast cancer cells

Tissue factor (TF) is a transmembrane glycoprotein that initiates blood coagulation when complexed with factor (F)VIIa. Recently, TF has been shown to promote cellular signaling, tumor growth, angiogenesis, and metastasis. In the present study, we examined the pathway by which TF-FVIIa complex induces cellular signaling in human breast cancer cells using the Adr-MCF-7 cell line. This cell line has high endogenous TF expression as measured by flow cytometry and expression of protease-activated receptors 1 and 2 (PAR1 and PAR2) as determined by reverse transcriptase-polymerase chain reaction analysis. Both PAR1 and PAR2 are functionally active as determined by induction of p44/42 mitogen-activated protein kinase (MAPK) phosphorylation using specific agonist peptides. We found that MAPK phosphorylation in this cell line was strongly induced by the combination of FVIIa and factor (F)X, but not by FVIIa alone at a concentration of FVIIa that approaches physiological levels. Induction of MAPK phosphorylation involved the formation of TF-FVIIa-FXa complex and occurred by a pathway that did not require thrombin formation, indicating a critical role for FXa generation. In addition, induction of MAPK phosphorylation was found to be independent of PAR1 activation. We then examined whether TF-FVIIa complex formation could promote tumor cell migration using a modified Boyden chamber chemotaxis assay. The combination of FVIIa and FX, but not FVIIa alone, strongly induced migration of tumor cells by a pathway that probably involves PAR2, but not PAR1 activation. MAPK phosphorylation was found to be required for the induction of cell migration by the combination of FVIIa and FX. These data suggest that TF-FVIIa-mediated signaling in human breast cancer cells occurs most efficiently by formation of the TF-FVIIa-FXa complex. One of the physiological consequences of this signaling pathway is enhanced cell migration that is probably mediated by PAR2, but not PAR1 activation.

Coinheritance of Factor V (FV) Leiden enhances thrombin formation and is associated with a mild bleeding phenotype in patients homozygous for the FVII 9726+5G>A (FVII Lazio) mutation

We investigated the role of thrombophilic mutations as possible modifiers of the clinical phenotype in severe factor VII (FVII) deficiency. Among 7 patients homozygous for a cross-reacting material-negative (CRM-) FVII defect (9726+5G>A, FVII Lazio), the only asymptomatic individual carried FV Leiden. Differential modulation of FVII levels by intragenic polymorphisms was excluded by a FVII to factor X (FX) gene haplotype analysis. The coagulation efficiency in the FV Leiden carrier and a noncarrier was evaluated by measuring FXa, FVa, and thrombin generation after extrinsic activation of plasma in the absence and presence of activated protein C (APC). In both patients coagulation factor activation was much slower and resulted in significantly lower amounts of FXa and thrombin than in a normal control. However, more FXa and thrombin were formed in the plasma of the patient carrying FV Leiden than in the noncarrier, especially in the presence of APC. These results were confirmed in FV-FVII doubly deficient plasma reconstituted with purified normal FV or FV Leiden. The difference in thrombin generation between plasmas reconstituted with normal FV or FV Leiden gradually decreased at increasing FVII concentration. We conclude that coinheritance of FV Leiden increases thrombin formation and can improve the clinical phenotype in patients with severe FVII deficiency.

Anti-heavy-chain monoclonal antibodies directed to the acidic regions of the factor VIII molecule inhibit the binding of factor VIII to phospholipids and von Willebrand factor

Recent studies have shown that inhibitors develop against acidic regions of the FVIII molecule, which contain important functional sites. However, their mechanisms of inhibition are not well understood. In this study, two anti-human FVIII mouse monoclonal antibodies (MAbs), directed towards the exposed acidic regions of the FVIII molecule, were developed, characterised and their mechanisms of inhibition investigated. The two MAbs, F7B4 and F26F6, had inhibitory titres of 32 and 944 BU/mg respectively, had high affinities for the FVIII molecule (K(D) approximately nM range) and recognised sequences V(357)-F(360) on the acidic a1 region and E(724)-L(731) on the acidic a2 region of the FVIII heavy-chain (HC), respectively. F7B4 inhibited the rate of FXa generation by activated FVIII, whilst both antibodies inhibited FVIII activation by thrombin and blocked thrombin cleavage of FVIII. Furthermore, F7B4 and F26F6 inhibited FVIII binding to (a) phospholipids (IC(50): 77 nM and 40 nM respectively), and (b) VWF (IC(50): 93 nM and 267 nM respectively), despite both having HC specificity. Experiments with F(ab')(2) fragments confirmed the above findings. Taken together these data represent novel findings in that anti-acidic HC antibodies can inhibit FVIII function by a variety of mechanisms, in particular by interfering with the binding of FVIII to phospholipids & VWF.

Overexpression of glycosyl phosphatidylinositol-anchored tissue factor pathway inhibitor-1 inhibits tissue factor activity

The cellular initiation of coagulation by the tissue factor (TF)-activated factor VII complex is transiently inhibited by endogenous tissue factor pathway inhibitor-1 (TFPI-1), whereas exogenously added TFPI-1 is targeted to a degradation pathway. This study investigates the relevance of glycosyl phosphatidylinositol (GPI) anchoring for the anticoagulant properties of TFPI-1. Experiments were performed with the human cell line ECV304 using liposomal gene transfer. For GPI anchoring of TFPI-1 we used a fusion protein of TFPI-1 and the GPI attachment sequence of decay-accelerating factor (GPI-TFPI-1), and compared it with wild-type TFPI-1. We measured TF and TFPI-1 surface expression by flow cytometry and TF proteolytic activity by a chromogenic assay for activated factor X generation. After transfection of GPI-TFPI-1, surface expression of TFPI-1 increased to 134 +/- 9% of mock transfected cells (mean +/- SEM, P = 0.004), and transfection with wild-type TFPI-1 did not significantly alter TFPI-1 surface expression. After transfection with GPI-TFPI-1, TF activity was reduced by 18 +/- 9% compared with mock transfections (P = 0.003), whereas after transfection with TFPI-1 wild type no significant inhibition was observed. This effect was not due to altered TF expression. GPI anchoring is an essential prerequisite for surface expression of TFPI-1 and inhibition of TF activity. Gene transfer of GPI-anchored TFPI, therefore, may be an efficient tool to inhibit local TF-induced coagulation.

Soluble tissue factor induces coagulation on tumor endothelial cells in vivo if coadministered with low-dose lipopolysaccharides

OBJECTIVE

This study was performed to evaluate the mechanisms leading to tumor vessel occlusion by tissue factor-based drugs, which are used in vascular targeting approaches for the treatment of malignant tumors.

METHODS AND RESULTS

The effects of nontargeted soluble tissue factor were evaluated in vitro and in vivo. Tumor-bearing mice were treated with (1) the extracellular portion of tissue factor (soluble tissue factor), (2) low nontoxic doses of lipopolysaccharides, or (3) a combination thereof. The combination treatment showed the best effects and resulted in selective thrombosis of tumor vessels. On the basis of our data from subsequent in vitro analyses, including surface plasmon resonance measurements and endothelial cell based coagulation assays, we propose a model on how soluble tissue factor, although lacking its membrane anchor, can promote selective tumor vessel occlusion.

CONCLUSIONS

To our knowledge, this is the first report to describe the molecular mechanisms of coagulation induction by untargeted soluble tissue factor in vivo. Combination treatments including soluble tissue factor might represent an alternative vascular targeting approach for the treatment of malignant tumors.

Low tissue factor pathway inhibitor (TFPI) together with low antithrombin allows sufficient thrombin generation in neonates

Neonates have an excellent hemostasis despite, in comparison to adults, markedly decreased and delayed ability to generate thrombin. Only 30-50% of peak adult thrombin activity can be produced in neonatal plasma by means of conventional in vitro assays. We show that in contrast to conventional activation, activation with small amounts of lipidated tissue factor (<10 pmol L(-1)) results in shorter clotting times and faster activated factor X- and thrombin generation in neonates compared with adults due to the concomitant action of low tissue factor pathway inhibitor and antithrombin. The concentrations of both inhibitors in cord plasma are approximately 50% of the respective adult values. After addition of 2.5 pmol L(-1) lipidated tissue factor, cord plasma clotted approximately 90 s earlier than adult plasma and the amount of free thrombin generated was approximately 90% of adult value (291 +/- 14 vs. 329 +/- 16 nmol L(-1) min(-1), P < 0.01). Our results might help to explain the clinically observed excellent hemostasis of neonates despite low levels of procoagulant factors.

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.