Deletion of the membrane anchoring region of tissue factor abolishes autoactivation of factor VII but not cofactor function. Analysis of a mutant with a selective deficiency in activity

A bispecific antibody approach for the potential prophylactic treatment of inherited bleeding disorders

Inherited bleeding disorders such as Glanzmann thrombasthenia (GT) lack prophylactic treatment options. As a result, serious bleeding episodes are treated acutely with blood product transfusions or frequent, repeated intravenous administration of recombinant activated coagulation factor VII (rFVIIa). Here we describe HMB-001, a bispecific antibody designed to bind and accumulate endogenous FVIIa and deliver it to sites of vascular injury by targeting it to the TREM (triggering receptor expressed on myeloid cells)-like transcript-1 (TLT-1) receptor that is selectively expressed on activated platelets. In healthy nonhuman primates, HMB-001 prolonged the half-life of endogenous FVIIa, resulting in its accumulation. Mouse bleeding studies confirmed antibody-mediated potentiation of FVIIa hemostatic activity by TLT-1 targeting. In ex vivo models of GT, HMB-001 localized FVIIa on activated platelets and potentiated fibrin-dependent platelet aggregation. Taken together, these results indicate that HMB-001 has the potential to offer subcutaneous prophylactic treatment to prevent bleeds in people with GT and other inherited bleeding disorders, with a low-frequency dosing regimen.

Lyophilized apoptotic vesicle-encapsulated adhesive hydrogel sponge as a rapid hemostat for traumatic hemorrhage in coagulopathy

Rapid hemostasis of uncontrolled bleeding following traumatic injuries, especially accompanied by coagulopathies, remains a significant clinical challenge. Extracellular vesicles (EVs) show therapeutic effects for fast clotting. However, low yield, specific storage conditions, and lack of proper carriers have hindered EVs' clinical application. Herein, we establish an optimized procedure method to generate lyophilized mesenchymal stem cell-derived apoptotic vesicles (apoVs) with adhesive hydrogel sponge to show superior procoagulant activity for traumatic hemorrhage. Mechanistically, apoVs' procoagulant ability stems from their high tissue factor (TF) and phosphatidylserine (PS) expression independent of hemocytes and circulating procoagulant microparticles (cMPs). Their stable hemostatic capability was maintained after 2-month room temperature storage. Subsequently, we mixed apoVs with both phenylboronic acid grafted oxidized hyaluronic acid (PBA-HA) and poly(vinyl alcohol) (PVA) simultaneously, followed by lyophilization to construct a novel apoV-encapsulated hydrogel sponge (apoV-HS). Compared to commercial hemostats, apoV-HS exhibits rapid procoagulant ability in liver-laceration and femoral artery hemorrhage in rat and rabbit models of coagulopathies. The combination of high productivity, physiological stability, injectability, plasticity, excellent adhesivity, biocompatibility, and rapid coagulant property indicates that apoV-HS is a promising therapeutic approach for heavy hemorrhage in civilian and military populations.

Targeting Tissue Factor to Tumor Vasculature to Induce Tumor Infarction

Simple Summary

Among multiple other functional roles of tissue factor (TF) and other coagulation proteins in the development and targeting of malignant disease, some scientific groups are attempting to modify TF and target the molecule or truncated forms of the molecule to tumor vasculature to selectively induce local blood vessel thromboembolic occlusion resulting in tumor infarction. This review briefly describes the characteristics and development of some of these proteins and structures, including tTF-NGR, which as the first drug candidate from this class has entered clinical trials in cancer patients.

Abstract

Besides its central functional role in coagulation, TF has been described as being operational in the development of malignancies and is currently being studied as a possible therapeutic tool against cancer. One of the avenues being explored is retargeting TF or its truncated extracellular part (tTF) to the tumor vasculature to induce tumor vessel occlusion and tumor infarction. To this end, multiple structures on tumor vascular wall cells have been studied at which tTF has been aimed via antibodies, derivatives, or as bifunctional fusion protein through targeting peptides. Among these targets were vascular adhesion molecules, oncofetal variants of fibronectin, prostate-specific membrane antigens, vascular endothelial growth factor receptors and co-receptors, integrins, fibroblast activation proteins, NG2 proteoglycan, microthrombus-associated fibrin-fibronectin, and aminopeptidase N. Targeting was also attempted toward cellular membranes within an acidic milieu or toward necrotic tumor areas. tTF-NGR, targeting tTF primarily at aminopeptidase N on angiogenic endothelial cells, was the first drug candidate from this emerging class of coaguligands translated to clinical studies in cancer patients. Upon completion of a phase I study, tTF-NGR entered randomized studies in oncology to test the therapeutic impact of this novel therapeutic modality.

Structure of human factor VIIa-soluble tissue factor with calcium, magnesium and rubidium

Coagulation factor VIIa (FVIIa) consists of a γ-carboxyglutamic acid (GLA) domain, two epidermal growth factor-like (EGF) domains and a protease domain. FVIIa binds three Mg2+ ions and four Ca2+ ions in the GLA domain, one Ca2+ ion in the EGF1 domain and one Ca2+ ion in the protease domain. Further, FVIIa contains an Na+ site in the protease domain. Since Na+ and water share the same number of electrons, Na+ sites in proteins are difficult to distinguish from waters in X-ray structures. Here, to verify the Na+ site in FVIIa, the structure of the FVIIa-soluble tissue factor (TF) complex was solved at 1.8 Å resolution containing Mg2+, Ca2+ and Rb+ ions. In this structure, Rb+ replaced two Ca2+ sites in the GLA domain and occupied three non-metal sites in the protease domain. However, Rb+ was not detected at the expected Na+ site. In kinetic experiments, Na+ increased the amidolytic activity of FVIIa towards the synthetic substrate S-2288 (H-D-Ile-Pro-Arg-p-nitroanilide) by ∼20-fold; however, in the presence of Ca2+, Na+ had a negligible effect. Ca2+ increased the hydrolytic activity of FVIIa towards S-2288 by ∼60-fold in the absence of Na+ and by ∼82-fold in the presence of Na+. In molecular-dynamics simulations, Na+ stabilized the two Na+-binding loops (the 184-loop and 220-loop) and the TF-binding region spanning residues 163-180. Ca2+ stabilized the Ca2+-binding loop (the 70-loop) and Na+-binding loops but not the TF-binding region. Na+ and Ca2+ together stabilized both the Na+-binding and Ca2+-binding loops and the TF-binding region. Previously, Rb+ has been used to define the Na+ site in thrombin; however, it was unsuccessful in detecting the Na+ site in FVIIa. A conceivable explanation for this observation is provided.

Oxidized LDL Modify the Human Adipocyte Phenotype to an Insulin Resistant, Proinflamatory and Proapoptotic Profile

Little information exists in humans on the regulation that oxidized low-density lipoprotein (oxLDL) exerts on adipocyte metabolism, which is associated with obesity and type 2 diabetes. The aim was to analyze the oxLDL effects on adipocytokine secretion and scavenger receptors (SRs) and cell death markers in human visceral adipocytes. Human differentiated adipocytes from visceral adipose tissue from non-obese and morbidly obese subjects were incubated with increasing oxLDL concentrations. mRNA expression of SRs, markers of apoptosis and autophagy, secretion of adipocytokines, and glucose uptake were analyzed. In non-obese and in morbidly obese subjects, oxLDL produced a decrease in insulin-induced glucose uptake, a significant dose-dependent increase in tumor necrosis factor-α (TNF-α), IL-6, and adiponectin secretion, and a decrease in leptin secretion. OxLDL produced a significant increase of Lox-1 and a decrease in Cxcl16 and Cl-p1 expression. The expression of Bnip3 (marker of apoptosis, necrosis and autophagy) was significantly increased and Bcl2 (antiapoptotic marker) was decreased. OxLDL could sensitize adipocytes to a lower insulin-induced glucose uptake, a more proinflammatory phenotype, and could modify the gene expression involved in apoptosis, autophagy, necrosis, and mitophagy. OxLDL can upregulate Lox-1, and this could lead to a possible amplification of proinflammatory and proapoptotic effects of oxLDL.

A candidate activation pathway for coagulation factor VII

The mechanism of generation of factor VIIa, considered the initiating protease in the tissue factor-initiated extrinsic limb of blood coagulation, is obscure. Decreased levels of plasma VIIa in individuals with congenital factor IX deficiency suggest that generation of VIIa is dependent on an activation product of factor IX. Factor VIIa activates IX to IXa by a two-step removal of the activation peptide with cleavages occurring after R191 and R226. Factor IXaα, however, is IX cleaved only after R226, and not after R191. We tested the hypothesis that IXaα activates VII with mutant IX that could be cleaved only at R226 and thus generate only IXaα upon activation. Factor IXaα demonstrated 1.6% the coagulant activity of IXa in a contact activation-based assay of the intrinsic activation limb and was less efficient than IXa at activating factor X in the presence of factor VIIIa. However, IXaα and IXa had indistinguishable amidolytic activity, and, strikingly, both catalyzed the cleavage required to convert VII to VIIa with indistinguishable kinetic parameters that were augmented by phospholipids, but not by factor VIIIa or tissue factor. We propose that IXa and IXaα participate in a pathway of reciprocal activation of VII and IX that does not require a protein cofactor. Since both VIIa and activated IX are equally plausible as the initiating protease for the extrinsic limb of blood coagulation, it might be appropriate to illustrate this key step of hemostasis as currently being unknown.

Associations of activated coagulation factor VII and factor VIIa-antithrombin levels with genome-wide polymorphisms and cardiovascular disease risk

ESSENTIALS

Essentials A fraction of coagulation factor VII circulates in blood as an activated protease (FVIIa). We evaluated FVIIa and FVIIa-antithrombin (FVIIa-AT) levels in the Cardiovascular Health Study. Polymorphisms in the F7 and PROCR loci were associated with FVIIa and FVIIa-AT levels. FVIIa may be an ischemic stroke risk factor in older adults and FVIIa-AT may assess mortality risk.

SUMMARY

Background A fraction of coagulation factor (F) VII circulates as an active protease (FVIIa). FVIIa also circulates as an inactivated complex with antithrombin (FVIIa-AT). Objective Evaluate associations of FVIIa and FVIIa-AT with genome-wide single nucleotide polymorphisms (SNPs) and incident coronary heart disease, ischemic stroke and mortality. Patients/Methods We measured FVIIa and FVIIa-AT in 3486 Cardiovascular Health Study (CHS) participants. We performed a genome-wide association scan for FVIIa and FVIIa-AT in European-Americans (n = 2410) and examined associations of FVII phenotypes with incident cardiovascular disease. Results In European-Americans, the most significant SNP for FVIIa and FVIIa-AT was rs1755685 in the F7 promoter region on chromosome 13 (FVIIa, β = -25.9 mU mL-1 per minor allele; FVIIa-AT, β = -26.6 pm per minor allele). Phenotypes were also associated with rs867186 located in PROCR on chromosome 20 (FVIIa, β = 7.8 mU mL-1 per minor allele; FVIIa-AT, β = 9.9 per minor allele). Adjusted for risk factors, a one standard deviation higher FVIIa was associated with increased risk of ischemic stroke (hazard ratio [HR], 1.12; 95% confidence interval [CI], 1.01, 1.23). Higher FVIIa-AT was associated with mortality from all causes (HR, 1.08; 95% CI, 1.03, 1.12). Among European-American CHS participants the rs1755685 minor allele was associated with lower ischemic stroke (HR, 0.69; 95% CI, 0.54, 0.88), but this association was not replicated in a larger multi-cohort analysis. Conclusions The results support the importance of the F7 and PROCR loci in variation in circulating FVIIa and FVIIa-AT. The findings suggest FVIIa is a risk factor for ischemic stroke in older adults, whereas higher FVIIa-AT may reflect mortality risk.

Engineering of a membrane-triggered activity switch in coagulation factor VIIa

Coagulation factor VIIa (FVIIa) is an intrinsically poor serine protease that requires assistance from its cofactor tissue factor (TF) to trigger the extrinsic pathway of blood coagulation. TF stimulates FVIIa through allosteric maturation of its active site and by facilitating substrate recognition. The surface dependence of the latter property allowed us to design a potent membrane-triggered activity switch in FVIIa by engineering a disulfide cross-link between an allosterically silent FVIIa variant and soluble TF. These results show that optimization of substrate recognition remote from the active site represents a promising new route to simultaneously enhance and localize the procoagulant activity of FVIIa for therapeutic purposes.

Recombinant factor VIIa (FVIIa) variants with increased activity offer the promise to improve the treatment of bleeding episodes in patients with inhibitor-complicated hemophilia. Here, an approach was adopted to enhance the activity of FVIIa by selectively optimizing substrate turnover at the membrane surface. Under physiological conditions, endogenous FVIIa engages its cell-localized cofactor tissue factor (TF), which stimulates activity through membrane-dependent substrate recognition and allosteric effects. To exploit these properties of TF, a covalent complex between FVIIa and the soluble ectodomain of TF (sTF) was engineered by introduction of a nonperturbing cystine bridge (FVIIa Q64C-sTF G109C) in the interface. Upon coexpression, FVIIa Q64C and sTF G109C spontaneously assembled into a covalent complex with functional properties similar to the noncovalent wild-type complex. Additional introduction of a FVIIa-M306D mutation to uncouple the sTF-mediated allosteric stimulation of FVIIa provided a final complex with FVIIa-like activity in solution, while exhibiting a two to three orders-of-magnitude increase in activity relative to FVIIa upon exposure to a procoagulant membrane. In a mouse model of hemophilia A, the complex normalized hemostasis upon vascular injury at a dose of 0.3 nmol/kg compared with 300 nmol/kg for FVIIa.

Molecular determinants involved in differential behaviour between soluble tissue factor and full-length tissue factor towards factor VIIa

During blood-coagulation, the transmembrane protein tissue factor (TF) binds to its ligand, factor (F)VII, activating and allosterically modifying it to form a mature active binary complex (TF–FVIIa).

Differences in N-glycosylation of recombinant human coagulation factor VII derived from BHK, CHO, and HEK293 cells

BACKGROUND &

METHODS

Recombinant factor VII (rFVII), the precursor molecule for recombinant activated FVII (rFVIIa), is, due to its need for complex post translational modifications, produced in mammalian cells. To evaluate the suitability of a human cell line in order to produce rFVII with post-translational modifications as close as possible to pdFVII, we compared the biochemical properties of rFVII synthesized in human embryonic kidney-derived (HEK)293 cells (HEK293rFVII) with those of rFVII expressed in Chinese hamster ovary (CHO, CHOrFVII) and baby hamster kidney (BHK, BHKrFVII) cells, and also with those of plasma derived FVII (pdFVII), using various analytical methods. rFVII was purified from selected production clones derived from BHK, CHO, and HEK293 cells after stable transfection, and rFVII isolates were analyzed for protein activity, impurities and post-translational modifications. RESULTS &

DISCUSSION

The analytical results showed no apparent gross differences between the various FVII proteins, except in their N-linked glycosylation pattern. Most N-glycans found on rFVII produced in HEK293 cells were not detected on rFVII from CHO and BHK cells, or, somewhat unexpectedly, on pdFVII; all other protein features were similar. HEK293rFVII glycans were mainly characterized by a higher structural variety and a lower degree of terminal sialylation, and a high amount of terminal N-acetyl galactosamines (GalNAc). All HEK293rFVII oligosaccharides contained one or more fucoses (Fuc), as well as hybrid and high mannose (Man) structures.

CONCLUSIONS

From all rFVII isolates investigated, CHOrFVII contained the highest degree of sialylation and no terminal GalNAc, and CHO cells were therefore assumed to be the best option for the production of rFVII.

Absence of Vitamin K-Dependent γ-Carboxylation in Human Periostin Extracted from Fibrotic Lung or Secreted from a Cell Line Engineered to Optimize γ-Carboxylation

Periostin (PN, gene name POSTN) is an extracellular matrix protein that is up-regulated in bronchial epithelial cells and lung fibroblasts by TH-2 cytokines. Its paralog, TGF-β-induced protein (βig-h3, gene name TGFBI), is also expressed in the lung and up-regulated in bronchial myofibroblasts by TGF-β. PN and βig-h3 contain fasciclin 1 modules that harbor putative recognition sequences for γ-glutamyl carboxylase and are annotated in UniProt as undergoing vitamin K-dependent γ-carboxylation of multiple glutamic acid residues. γ-carboxylation profoundly alters activities of other proteins subject to the modification, e.g., blood coagulation factors, and would be expected to alter the structure and function of PN and βig-h3. To analyze for the presence of γ-carboxylation, proteins extracted from fibrotic lung were reacted with monoclonal antibodies specific for PN, βig-h3, or modification with γ-carboxyglutamic acid (Gla). In Western blots of 1-dimensional gels, bands stained with anti-PN or -βig-h3 did not match those stained with anti-Gla. In 2-dimensional gels, anti-PN-positive spots had pIs of 7.0 to >8, as expected for the unmodified protein, and there was no overlap between anti-PN-positive and anti-Gla-positive spots. Recombinant PN and blood coagulation factor VII were produced in HEK293 cells that had been transfected with vitamin K 2, 3-epoxide reductase C1 to optimize γ-carboxylation. Recombinant PN secreted from these cells did not react with anti-Gla antibody and had pIs similar to that found in extracts of fibrotic lung whereas secreted factor VII reacted strongly with anti-Gla antibody. Over 67% coverage of recombinant PN was achieved by mass spectrometry, including peptides with 19 of the 24 glutamates considered targets of γ-carboxylation, but analysis revealed no modification. Over 86% sequence coverage and three modified glutamic acid residues were identified in recombinant fVII. These data indicate that PN and βig-h3 are not subject to vitamin K-dependent γ-carboxylation.

Direct inhibition of factor VIIa by TFPI and TFPI constructs

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is a multi-Kunitz domain protease inhibitor that down-regulates the extrinsic coagulation pathway by inhibiting FXa and FVIIa.

OBJECTIVES

To investigate the role of the three Kunitz domains (KDs) of TFPI in FVIIa inhibition using full-length TFPI (TFPIfl ) and truncated TFPI constructs.

METHODS

Inhibition of FVIIa with/without relipidated tissue factor (TF) or soluble TF (sTF) by TFPIfl /TFPI constructs was quantified with a FVIIa-specific chromogenic substrate.

RESULTS AND CONCLUSIONS

TFPIfl inhibited TF-FVIIa via a monophasic reaction, which is rather slow at low TFPIfl concentrations (t½  ≈ 5 min at 2 nm TFPI) and has a Ki of 4.6 nm. In the presence of sTF and without TF, TFPIfl was a poor FVIIa inhibitor, with Ki values of 122 nm and 1118 nm, respectively. This indicates that phospholipids and TF significantly contribute to FVIIa inhibition by TFPIfl . TFPI constructs without the KD3-c-terminus (TFPI1-150 and KD1-KD2) were 7-10-fold less effective than TFPIfl in inhibiting TF-FVIIa and sTF-FVIIa, indicating that the KD3-C-terminus significantly contributes to direct inhibition of FVIIa by TFPI. Compared with KD1-KD2, KD1 was a poor TF-FVIIa inhibitor (Ki =434 nm), which shows that the KD2 domain of TFPI also contributes to FVIIa inhibition. Protein S stimulated TF-FVIIa inhibition by TFPIfl (Ki =0.7 nm). In the presence of FXa, a tight quaternary TF-FVIIa-TFPI-FXa complex is formed with TFPIfl , TFPI1-150 and KD1-KD2, with Ki values of < 0.15 nm, 0.5 nm and 0.8 nm, respectively, indicating the KD3-C-terminus is not a prerequisite for quaternary complex formation. Phospholipids and the Gla-domain of FXa are required for quaternary complex formation.

Raising the Active Site of Factor VIIa above the Membrane Surface Reduces Its Procoagulant Activity but Not Factor VII Autoactivation

Tissue factor, the physiologic trigger of blood clotting, is the membrane-anchored protein cofactor for the plasma serine protease, factor VIIa. Tissue factor is hypothesized to position and align the active site of factor VIIa relative to the membrane surface for optimum proteolytic attack on the scissile bonds of membrane-bound protein substrates such as factor X. We tested this hypothesis by raising the factor VIIa binding site above the membrane surface by creating chimeras containing the tissue factor ectodomain linked to varying portions of the membrane-anchored protein, P-selectin. The tissue factor/P-selectin chimeras bound factor VIIa with high affinity and supported full allosteric activation of factor VIIa toward tripeptidyl-amide substrates. That the active site of factor VIIa was raised above the membrane surface when bound to tissue factor/P-selectin chimeras was confirmed using resonance energy transfer techniques in which appropriate fluorescent dyes were placed in the active site of factor VIIa and at the membrane surface. The chimeras were deficient in supporting factor X activation by factor VIIa due to decreased k(cat). The chimeras were also markedly deficient in clotting plasma, although incubating factor VII or VIIa with the chimeras prior to the addition of plasma restored much of their procoagulant activity. Interestingly, all chimeras fully supported tissue factor-dependent factor VII autoactivation. These studies indicate that proper positioning of the factor VII/VIIa binding site on tissue factor above the membrane surface is important for efficient rates of activation of factor X by this membrane-bound enzyme/cofactor complex.

High Resolution Structures of p-Aminobenzamidine- and Benzamidine-VIIa/Soluble Tissue Factor

Factor VIIa (FVIIa) consists of a gamma-carboxyglutamic acid (Gla) domain, two epidermal growth factor-like domains, and a protease domain. FVIIa binds seven Ca(2+) ions in the Gla, one in the EGF1, and one in the protease domain. However, blood contains both Ca(2+) and Mg(2+), and the Ca(2+) sites in FVIIa that could be specifically occupied by Mg(2+) are unknown. Furthermore, FVIIa contains a Na(+) and two Zn(2+) sites, but ligands for these cations are undefined. We obtained p-aminobenzamidine-VIIa/soluble tissue factor (sTF) crystals under conditions containing Ca(2+), Mg(2+), Na(+), and Zn(2+). The crystal diffracted to 1.8A resolution, and the final structure has an R-factor of 19.8%. In this structure, the Gla domain has four Ca(2+) and three bound Mg(2+). The EGF1 domain contains one Ca(2+) site, and the protease domain contains one Ca(2+), one Na(+), and two Zn(2+) sites. (45)Ca(2+) binding in the presence/absence of Mg(2+) to FVIIa, Gla-domainless FVIIa, and prothrombin fragment 1 supports the crystal data. Furthermore, unlike in other serine proteases, the amide N of Gly(193) in FVIIa points away from the oxyanion hole in this structure. Importantly, the oxyanion hole is also absent in the benzamidine-FVIIa/sTF structure at 1.87A resolution. However, soaking benzamidine-FVIIa/sTF crystals with d-Phe-Pro-Arg-chloromethyl ketone results in benzamidine displacement, d-Phe-Pro-Arg incorporation, and oxyanion hole formation by a flip of the 192-193 peptide bond in FVIIa. Thus, it is the substrate and not the TF binding that induces oxyanion hole formation and functional active site geometry in FVIIa. Absence of oxyanion hole is unusual and has biologic implications for FVIIa macromolecular substrate specificity and catalysis.

Restoring full biological activity to the isolated ectodomain of an integral membrane protein

Integral membrane proteins, which include many cellular effector proteins and drug targets, can be difficult to produce, purify, and manipulate. Although the isolated ectodomains of many membrane proteins can be expressed as water soluble proteins, biological activity is frequently lost when these proteins are released from the membrane surface. An example is tissue factor, the integral membrane protein that triggers the blood clotting cascade and for which membrane anchoring is essential. Its isolated ectodomain (soluble tissue factor) can be expressed with high yield in bacteria but is orders of magnitude less active than the intact, membrane-anchored protein. We now report full restoration of biological activity to the isolated tissue factor ectodomain via the engineering of a hexahistidine tag onto its C-terminus and its use in combination with membrane bilayers containing nickel-chelating lipids. When soluble tissue factor was tethered to the membrane surface via such metal-chelating lipids, it bound factor VIIa with the same high affinity as wild-type tissue factor, and the resulting factor VIIa-tissue factor complexes supported factor X activation and factor VII autoactivation with essentially wild-type enzyme kinetic constants. Furthermore, when such bilayers were immobilized onto solid supports, they efficiently captured histidine-tagged soluble tissue factor directly from crude culture supernatants, with full biological activity, obviating the need for purification or laborious membrane reconstitution procedures. This strategy is rapid, efficient, scalable, and automatable and should be applicable to other integral membrane proteins, especially those with a single transmembrane domain. Applications include high-throughput screening of mutants or drugs, flow reactors, clinical assays, and point-of-care instrumentation.

Identification and characterization of murine alternatively spliced tissue factor

Tissue factor (TF) is a transmembrane glycoprotein that initiates coagulation and plays a critical role in regulating hemostasis and thrombosis. We have recently reported a naturally occurring, soluble form of human tissue factor (asTF) generated by alternative splicing. This splice variant has a novel C-terminus with no homology to that of the full-length TF (flTF), lacks a transmembrane domain, and is active in the presence of phospholipids. Mouse models offer unique opportunities to examine the relative importance of flTF and asTF in mediating thrombosis, the response to arterial injury, and ischemic damage. To that end, we have identified and characterized murine asTF (masTF). Like the human splice variant, masTF lacks a transmembrane domain and has a unique C-terminus. We have generated antibodies specific to masTF and murine flTF (mflTF) to examine the expression of both forms of TF. masTF antigen is widely and abundantly expressed, with a pattern similar to that of mflTF, in adult tissues, in experimentally induced thrombi, and during development. These studies demonstrate that masTF contributes to the pool of total TF and may thus play an important role in mediating TF-dependent processes.

Positive Feedbacks of Coagulation

Tissue factor (TF), the initiator of coagulation, continuously circulates in the plasma, and the clotting system “idles,” generating very low levels of active clotting enzymes, clotting products, and by-products. Given the enormous amplification potential of the clotting cascade, rigorous control is required to ensure that such low-level stimulation does not cause massive system amplification and response. We propose that among the various mechanisms of regulation, activation thresholds may play a major role. These arise when positive-feedback reactions, of which there are several in the clotting system, are regulated by inhibitors. Such thresholds act like switches, so that small stimuli and/or nonproductive local conditions will generate no response, whereas larger stimuli or the existence of local prothrombotic conditions will produce a full, explosive response. We review here the evidence for system idling, the structures of the various feedback mechanisms of clotting, the mechanisms by which they can produce threshold behavior, and the possible role of thresholds in system regulation.

A soluble tissue factor-annexin V chimeric protein has both procoagulant and anticoagulant properties

Tissue factor (TF) initiates blood coagulation, but its expression in the vascular space requires a finite period of time. We hypothesized that targeting exogenous tissue factor to sites of vascular injury could lead to accelerated hemostasis. Since phosphatidylserine (PS) is exposed on activated cells at sites of vascular injury, we cloned the cDNA for a chimeric protein consisting of the extracellular domain of TF (called soluble TF or sTF) and annexin V, a human PS-binding protein. Both the sTF and annexin V domains had ligand-binding activities consistent with their native counterparts, and the chimera accelerated factor X activation by factor VIIa. The chimera exhibited biphasic effects upon blood coagulation. At low concentrations it accelerated blood coagulation, while at higher concentrations it acted as an anticoagulant. The chimera accelerated coagulation in the presence of either unfractionated or low-molecular-weight heparins more potently than factor VIIa and shortened the bleeding time of mice treated with enoxaparin. The sTF-annexin V chimera is a targeted procoagulant protein that may be useful in accelerating thrombin generation where PS is exposed to the vasculature, such as may occur at sites of vascular injury or within the vasculature of tumors.

The first epidermal growth factor-like domains of factor Xa and factor IXa are important for the activation of the factor VII--tissue factor complex

During tissue factor (TF)-induced coagulation, the factor (F)VIIa-TF complex activates factor (F)X and factor (F)IX. Through positive feedback, the generated FXa and FIXa activate FVII-TF. The first epidermal growth factor-like (EGF1) domains of FX and FIX serve as important TF-recognition motifs when FVIIa-TF activates FX or FIX. Here, we investigated the role of EGF1 domains of FXa and FIXa during the activation of FVII-TF and inhibition by tissue factor pathway inhibitor (TFPI). FXaPCEGF1 (EGF1 domain of FXa replaced with that of protein C), and FXaQ49P (EGF1 domain mutant with impaired calcium-binding), and the corresponding FIXa mutants were generated, and their abilities to activate FVII-TF were compared with the wild-type (WT) enzymes. In the absence of TF, the rates of FVII activation were similar between WT enzymes and mutant FXa and FIXa proteases. In the presence of either soluble TF (sTF) or relipidated TF, each mutant of FXa or FIXa activated FVII-TF at a slower rate than the corresponding WT enzyme. Kinetics of inhibition of the amidolytic activity of WT and the mutant FXa proteases by either two-domain or full-length TFPI were similar. However, compared with the complex of TFPI-FXaWT, the abilities of the complexes of TFPI-FXa mutants to inhibit FVIIa-TF were impaired. We conclude that the EGF1 domains of FXa and FIXa are important for the activation of FVII-TF and for the formation of FVIIa-TF-FXa-TFPI complex.

Tissue factor activity in whole blood

Tissue factor (TF) is an integral membrane protein essential for hemostasis. During the past several years, a number of studies have suggested that physiologically active TF circulates in blood at concentrations greater than 30 pM either as a component of blood cells and microparticles or as a soluble plasma protein. In our studies using contact pathway-inhibited blood or plasma containing activated platelets, typically no clot is observed for 20 minutes in the absence of exogenous TF. An inhibitory anti-TF antibody also has no effect on the clotting time in the absence of exogenous TF. The addition of TF to whole blood at a concentration as low as 16 to 20 fM results in pronounced acceleration of clot formation. The presence of potential platelet TF activity was evaluated using ionophore-treated platelets and employing functional and immunoassays. No detectable TF activity or antigen was observed on quiescent or ionophore-stimulated platelets. Similarly, no TF antigen was detected on mononuclear cells in nonstimulated whole blood, whereas in lipopolysaccharide (LPS)-stimulated blood a significant fraction of monocytes express TF. Our data indicate that the concentration of physiologically active TF in non-cytokine-stimulated blood from healthy individuals cannot exceed and is probably lower than 20 fM.

The rare coagulation disorders - review with guidelines for management from the United Kingdom Haemophilia Centre Doctors' Organisation

The rare coagulation disorders are heritable abnormalities of haemostasis that may present significant difficulties in diagnosis and management. This review summarizes the current literature for disorders of fibrinogen, and deficiencies of prothrombin, factor V, FV + VIII, FVII, FX, the combined vitamin K-dependent factors, FXI and FXIII. Based on both collective clinical experience and the literature, guidelines for management of bleeding complications are suggested with specific advice for surgery, spontaneous bleeding, management of pregnancy and the neonate. We have chosen to include a section on Ehlers-Danlos Syndrome because haematologists may be consulted about bleeding manifestations in such patients.

Factor VII deficiency

As yet, there have been neither systematic reviews nor reports of randomized, controlled trials involving factor VII (FVII) deficiency. Hence, a picture of this disorder can only be drawn by reviewing and summarizing the data that is available. This article provides an overview of the understanding of this rare, inherited disorder of coagulation. In particular, the status of current knowledge of the disorder's prevalence, clinical presentation, diagnostic characteristics and molecular genetics is reviewed, followed by a summary of currently available options for its treatment and management.

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.

Lactadherin inhibits enzyme complexes of blood coagulation by competing for phospholipid-binding sites

Lactadherin, a glycoprotein of the milk-fat globule membrane, contains tandem C domains with homology to discoidin-type lectins and to membrane-binding domains of blood-clotting factors V and VIII. We asked whether the structural homology confers the capacity to compete for the membrane-binding sites of factor VIII and factor V and to function as an anticoagulant. Our results indicate that lactadherin competes efficiently with factor VIII and factor V for binding sites on synthetic phosphatidylserine-containing membranes with half-maximal displacement at lactadherin concentrations of 1 to 4 nM. Binding competition correlated to functional inhibition of factor VIIIa-factor IXa (factor Xase) enzyme complex. In contrast to annexin V, lactadherin was an efficient inhibitor of the prothrombinase and the factor Xase complexes regardless of the degree of membrane curvature and the phosphatidylserine content. Lactadherin also inhibited the factor VIIa-tissue factor complex efficiently whereas annexin V was less effective. Because the inhibitory concentration of lactadherin was proportional to the phospholipid concentration, and because lactadherin was not an efficient inhibitor in the absence of phospholipid, the major inhibitory effect of lactadherin relates to blocking phospholipid sites rather than forming inhibitory protein-protein complexes. Lactadherin was also an effective inhibitor of a modified whole blood prothrombin time assay in which clotting was initiated by dilute tissue factor; 60 nM lactadherin prolonged the prothrombin time 150% versus 20% for 60 nM annexin V. These results indicate that lactadherin can function as a potent phospholipid-blocking anticoagulant.

Large-scale production and properties of human plasma-derived activated Factor VII concentrate

BACKGROUND AND OBJECTIVES

An activated Factor VII (FVIIa) concentrate, prepared from human plasma on a large scale, has to date not been available for clinical use for haemophiliacs with antibodies against FVIII and FIX. In the present study, we attempted to establish a large-scale manufacturing process to obtain plasma-derived FVIIa concentrate with high recovery and safety, and to characterize its biochemical and biological properties.

MATERIALS AND METHODS

FVII was purified from human cryoprecipitate-poor plasma, by a combination of anion exchange and immunoaffinity chromatography, using Ca2+-dependent anti-FVII monoclonal antibody. To activate FVII, a FVII preparation that was nanofiltered using a Bemberg Microporous Membrane-15 nm was partially converted to FVIIa by autoactivation on an anion-exchange resin. The residual FVII in the FVII and FVIIa mixture was completely activated by further incubating the mixture in the presence of Ca2+ for 18 h at 10 degrees C, without any additional activators. For preparation of the FVIIa concentrate, after dialysis of FVIIa against 20 mm citrate, pH 6.9, containing 13 mm glycine and 240 mm NaCl, the FVIIa preparation was supplemented with 2.5% human albumin (which was first pasteurized at 60 degrees C for 10 h) and lyophilized in vials. To inactivate viruses contaminating the FVIIa concentrate, the lyophilized product was further heated at 65 degrees C for 96 h in a water bath.

RESULTS

Total recovery of FVII from 15 000 l of plasma was approximately 40%, and the FVII preparation was fully converted to FVIIa with trace amounts of degraded products (FVIIabeta and FVIIagamma). The specific activity of the FVIIa was approximately 40 U/ micro g. Furthermore, virus-spiking tests demonstrated that immunoaffinity chromatography, nanofiltration and dry-heating effectively removed and inactivated the spiked viruses in the FVIIa. These results indicated that the FVIIa concentrate had both high specific activity and safety.

CONCLUSIONS

We established a large-scale manufacturing process of human plasma-derived FVIIa concentrate with a high yield, making it possible to provide sufficient FVIIa concentrate for use in haemophiliacs with inhibitory antibodies.

Tissue factor alters the pK(a) values of catalytically important factor VIIa residues

Blood coagulation is triggered when the serine protease factor VIIa (fVIIa) binds to cell surface tissue factor (TF) to form the active enzyme-cofactor complex. TF binding to fVIIa allosterically augments the enzymatic activity of fVIIa toward macromolecular substrates and small peptidyl substrates. The mechanism of this enhancement remains unclear. Our previous studies have indicated that soluble TF (sTF; residues 1-219) alters the pH dependence of fVIIa amidolytic activity (Neuenschwander et al. (1993) Thromb. Haemostasis 70, 970), indicating an effect of TF on critical ionizations within the fVIIa active center. The pKa values and identities of these ionizable groups are unknown. To gain additional insight into this effect, we have performed a detailed study of the pH dependence of fVIIa amidolytic activity. Kinetic constants of Chromozym t-PA (MeSO(2)-D-Phe-Gly-Arg-pNA) hydrolysis at various pH values were determined for fVIIa alone and in complex with sTF. The pH dependence of both enzymes was adequately represented using a diprotic model. For fVIIa alone, two ionizations were observed in the free enzyme (pK(E1) = 7.46 and pK(E2) = 8.67), with at least a single ionization apparent in the Michaelis complex (pK(ES1) similar 7.62). For the fVIIa-TF complex, the pK(a) of one of the two important ionizations in the free enzyme was shifted to a more basic value (pK(E1) = 7.57 and pK(E2) = 9.27), and the ionization in the Michaelis complex was possibly shifted to a more acidic pH (pK(ES1) = 6.93). When these results are compared to those obtained for other well-studied serine proteases, K(E1) and K(ES1) are presumed to represent the ionization of the overall catalytic triad in the absence and presence of substrate, respectively, while K(E2) is presumed to represent ionization of the alpha-amino group of Ile(153). Taken together, these results would suggest that sTF binding to fVIIa alters the chemical environment of the fVIIa active site by protecting Ile(153) from deprotonation in the free enzyme while deprotecting the catalytic triad as a whole when in the Michaelis complex.

The role of membrane patch size and flow in regulating a proteolytic feedback threshold on a membrane: possible application in blood coagulation

Positive feedback controls in proteolytic systems are characterized by thresholds which are regulated by the concentration of the initial stimulus and the kinetic parameters for enzyme generation and inhibition. Significant complexity is added when a positive feedback is localized on a membrane in contact with a flowing medium, such as seen in the early steps of blood coagulation. A partial differential equation model of an archetypal feedback loop is examined in which a proteolytic enzyme catalyzes its own formation from a zymogen on a membrane in contact with a flowing medium. As predicted from prior solution-phase and membrane-phase analyses, the threshold conditions for activation of the system are regulated by the kinetics of enzyme generation and inhibition and by the density of reactant-binding sites on the membrane; but the present analysis also establishes how the feedback threshold is controlled by the flow rate of the adjacent medium and the physical size of the membrane patch on which the feedback loop is localized. For given systems of particular kinetic properties, lower flow rates or larger active patches of membrane can result in the activation threshold being exceeded, whereas higher flow rates or smaller membrane patches can prevent initiation. In addition to numerical simulation, a simplified non-flowing model is analyzed to formulate an approximate mathematical statement of the dependence of the minimum activatable patch size on the kinetic and other parameters.

Serum lipids and regulation of tissue factor-induced coagulation in middle-aged men

Formation of an occlusive thrombus by exposure of tissue factor (TF) to circulating blood and subsequent triggering of coagulation by TF-activated factor VII (FVIIa) complexes on ruptured atherosclerotic plaques is thought to be a key event in myocardial infarction. Tissue factor pathway inhibitor (TFPI) is a potent inhibitor of TF-induced coagulation in which the anticoagulant function most probably is restricted to free TFPI in human plasma. The present study was undertaken to assess the interrelations between serum lipids and components of TF-induced coagulation in 234 apparently healthy men aged 36-56 years recruited from the general population. Plasma free TFPI antigen (Ag) was positively correlated (P < .001) with total cholesterol, low-density lipoprotein (LDL) cholesterol, triglycerides, apolipoprotein B (apoB-100), fibrinogen, total amount of FVII (FVIIam), coagulation activity of factor VII (FVIIc), and FVIIa. The significant predictors for free TFPI Ag were total cholesterol, triglycerides, fibrinogen, FVIIc, and age, which explained 33% of the plasma variation in free TFPI Ag assessed by multiple regression analysis. A highly significant (P < .0001) linear trend for increase in atherogenic lipids (i.e., total cholesterol and triglycerides), FVII (i.e., FVIIc and FVIIa), and fibrinogen across quartiles of TFPI Ag was demonstrated after adjustment for confounders. These findings may indicate a compensatory increase in plasma free TFPI with lipid and hemostatic risk factors for atherothrombotic diseases in healthy middle-aged men.

The tissue factor region that interacts with factor Xa in the activation of factor VII

Tissue factor is the cell membrane-anchored cofactor for factor VIIa and triggers the coagulation reactions. The initial step is the conversion of factor VII to factor VIIa which, in vitro, is efficiently catalyzed by low concentrations of factor Xa. To identify the tissue factor region that interacts with the activator factor Xa during this process, we evaluated a panel of soluble tissue factor (1-219) mutants for their ability to support factor Xa-mediated activation of factor VII. The tissue factor residues identified as most important for this interaction (Tyr157, Lys159, Ser163, Gly164, Lys165, Lys166, and Tyr185) were identical to those found to be important for the interaction of substrate factor X with the tissue factor.factor VIIa complex. The residues form a continuous surface-exposed patch with an area of about 500 A(2), which appears to be located outside the tissue factor-factor VII contact zone. In agreement, the two monoclonal antibodies 5G6 and D3H44-F(ab')(2), whose epitopes overlap with this identified region, inhibited the rates of factor VII activation by 86% and 95%, respectively. These antibodies also strongly inhibited the conversion of (125)I-labeled factor VII when cell membrane-expressed, full-length tissue factor (1-263) was employed. Together the results suggest the usage of a common surface region of tissue factor in its dual role-as a cofactor for factor Xa-mediated factor VII activation and as a cofactor for factor VIIa-mediated factor X activation. The finding that factor Xa and factor X may engage in similar, if not identical, molecular interactions with tissue factor further indicates that factor Xa and factor X are similarly oriented toward their respective interaction partners in the ternary catalytic complexes.

Reduction of factor FVIIa activity during heparin therapy. Evidence for assay interactions with tissue factor pathway inhibitor and antithrombin

Heparin is known to exert its antithrombotic effects by accelerating the effect of antithrombin (AT) and by mobilizing tissue factor pathway inhibitor (TFPI) into the circulation from vascular endothelium. Heparin treatment has been reported to decrease FVIIa activity by 40%; this was suggested as a new antithrombotic action of heparins. The present study was conducted to investigate whether the apparent reduction in FVIIa activity induced by unfractionated heparin (UFH) infusion in vivo is due to interactions between AT and TFPI with the FVIIa assay or due to an actual decrease in FVIIa. Blocking plasma TFPI in affinity purified anti-TFPI IgG caused a 25% increase in plasma FVIIa activity (Staclot VII - rTF, Diagnostica Stago, Aswiéres-sur-Seine, France). In vitro heparinization of plasma caused a dose-dependent decrease in FVIIa (up to 56 +/- 8%) at high heparin concentrations (1.0-5.0 IU/mL UFH), a reduction abolished by Hexadimethine Bromide (HDB) to neutralize heparin-induced activation of AT. Thus, heparin-induced activation of AT is apparently responsible for decreased FVIIa under in vitro conditions. Bolus injection followed by continuous infusion of heparin to healthy volunteers was accompanied by a prompt 50% reduction in FVIIa activity, which was sustained throughout heparin infusion and normalized within 24 hours after discontinuation of treatment. Addition of anti-TFPI IgG to postheparin plasma reversed the heparin-induced reduction in FVIIa by approximately 50%, and combined pretreatment of postheparin plasma with anti-TFPI IgG and HDB brought FVIIa to preheparin levels. The present study shows that the FVIIa assay is sensitive to TFPI and AT, especially during heparin treatment, and thereby indicates that the heparin-induced decrease in FVIIa is affected by interactions between TFPI and AT with the FVIIa assay.

Changes of the coagulation and fibrinolysis system in malignancy: their possible impact on future diagnostic and therapeutic procedures

The interaction between malignant cell growth and the coagulation and fibrinolysis system has been a well known phenomenon for decades. During recent years, this area of research has received new attention. Experimental data suggest a role for the coagulation and fibrinolysis system in tumor development, progression and metastasis. Also, clinical research suggests that targeting the coagulation system or fibrinolysis system might influence the course of malignant disease beneficially. This paper reviews data on various hemostatic and fibrinolytic parameters in malignancy; the possible use of such parameters as risk markers in oncology patients; and possible targets of anti-neoplastic therapies using anticoagulant and/or antifibrinolytic strategies. Current evidence suggests that the tissue factor/factor VIIa pathway mediates the most abundant procoagulant stimulus in malignancy via the increase in thrombin generation. Tissue factor has been suggested to mediate pro-metastatic properties via coagulation-dependent and coagulation-independent pathways; tissue factor has also been implicated in tumor neo-angiogenesis. However, so far no model has been validated that would allow the use of tissue factor in its soluble or insoluble form as a marker for risk stratification in tumor patients. On the other hand, there is now good evidence that parts of the fibrinolytic system, such as urokinase-type plasminogen activator and its receptor ("uPAR"), can be used as strong predictors of outcome in several types of cancer, specifically breast cancer. Observation of various treatment options in patients with thromboembolic disease and cancer as well as attempts to use anticoagulants and/or therapies modulating the fibrinolytic system as anti-neoplastic treatment strategies have yielded exciting results. These data indicate that anticoagulant therapy, and specifically low molecular weight heparin therapy, is likely to have anti-neoplastic effects; and that their use in addition to chemotherapy will probably improve outcome of tumor treatment in certain types of cancer. However, the body of clinical data is still relatively small and the question whether or not we should routinely consider the coagulation and/or fibrinolysis system as therapeutic targets in cancer patients is yet to be answered.

Regulation of cell growth by oxidized LDL

The first reports of the influences of oxidized LDL (oxLDL) on cell function pertained to negative effects on cell growth-growth arrest, injury, and toxicity. Since these studies, it has become apparent that sublethal levels of oxLDL cause some, but not all, cells to proliferate. This review highlights the growth-promoting effects of oxLDL rather than its inhibitory or injurious effects. Smooth muscle cells (SMCs) and monocyte-macrophages proliferate after exposure to oxLDL; endothelial cells do not. Scavenger receptors are involved in the proliferative effects on monocyte-macrophages, whereas the effects of oxLDL on SMCs appear to be receptor independent. Lysophosphatidylcholine (lysoPC), and structurally related lipids are among the growth-promoting constituents of oxLDL. OxLDL exerts at least a part of its effects by inducing expression or causing the release of growth factors. OxLDL (or lysoPC) can cause the release of basic fibroblast growth factor (bFGF) from SMCs; oxLDL (or lysoPC) can induce heparin binding EGF-like growth factor (HB-EGF) synthesis and release from macrophages. An imposing array of changes in cytokine and growth factor expression and/or release can be imposed by oxLDL on a wide variety of cell types. These effects and the studies probing the cell signaling events leading to them are described.

Effect of omega-3 fatty acids and simvastatin on hemostatic risk factors and postprandial hyperlipemia in patients with combined hyperlipemia

Patients with combined hyperlipemia have lipid abnormalities associated with an increased tendency to develop atherosclerosis and thrombosis. This tendency may be accelerated during postprandial hyperlipemia. In the present double-blind parallel study, 41 patients with combined hyperlipemia and serum triacylglycerols between 2.0 and 15.0 mmol/L and serum total cholesterol >5.3 mmol/L at the end of a 3-month dietary run-in period were treated with simvastatin at 20 mg/d for at least 10 weeks; patients were then randomized into 2 groups receiving simvastatin+omega-3 fatty acids at 3.36 g/d or placebo (corn oil) for an additional 5 weeks. Hemostatic variables that have been associated with increased thrombotic tendency were evaluated with subjects in the fasting state and during postprandial hyperlipemia before and after combined treatment. Supplementation of omega-3 fatty acid reduced tissue factor pathway inhibitor antigen (P<0.05) in the fasting state, reduced the degree of postprandial hyperlipemia (P<0.005), and reduced activated factor VII concentration appearing during postprandial hyperlipemia. In conclusion, omega-3 fatty acids given in addition to simvastatin to patients with combined hyperlipemia reduced the free tissue factor pathway inhibitor fraction in the fasting state and inhibited the activation of factor VII occurring during postprandial lipemia, thus representing a potential beneficial effect on the hemostatic risk profile in this patient group.

Proteolysis of blood coagulation factor VIII by the factor VIIa-tissue factor complex: generation of an inactive factor VIII cofactor

Activation of factor VIII by thrombin occurs via limited proteolysis at R372, R740, and R1689. The resultant active factor VIIIa molecule consists of three noncovalently associated subunits: A1-a1, A2-a2, and A3-C1-C2 (50, 45, and 73 kDa respectively). Further proteolysis of factor VIIIa at R336 and R562 by activated protein C subsequently inactivates this cofactor. We now find that the factor VIIa-tissue factor complex (VIIa-TF/PL), the trigger of blood coagulation with restricted substrate specificity, can also catalyze limited proteolysis of factor VIII. Proteolysis of factor VIII was observed at 10 sites, producing 2 major fragments (47 and 45 kDa) recognized by an anti-factor VIII A2 domain antibody. Time courses indicated the slow conversion of the large fragment to 45 kDa, followed by further degradation into at least two smaller fragments. N-Terminal sequencing along with time courses of proteolysis indicated that VIIa-TF/PL cleaved factor VIII first at R740, followed by concomitant cleavage at R336 and R372. Although cleavage of the light chain at R1689 was observed, the majority remained uncleaved after 17 h. Consistent with this, only a transient 2-fold increase in factor VIII clotting activity was observed. Thus, heavy chain cleavage of factor VIII by VIIa-TF/PL produces an inactive factor VIII cofactor no longer capable of activation by thrombin. In addition, VIIa-TF/PL was found to inactivate thrombin-activated factor VIII. We hypothesize that these proteolyses may constitute an alternative pathway to regulate coagulation under certain conditions. In addition, the ability of VIIa-TF/PL to cleave factor VIII at 10 sites greatly expands the known protein substrate sequences recognized by this enzyme-cofactor complex.

Down-regulation of monocyte tissue factor mediated by tissue factor pathway inhibitor and the low density lipoprotein receptor-related protein

Inflammatory mediators like bacterial lipopolysaccharide induce monocytes to express tissue factor (TF), the cell-surface protein that triggers the blood clotting cascade in hemostasis and thrombotic disease. The physiologic ligand for TF is the serine protease, factor VIIa (FVIIa), and the resulting bimolecular enzyme, TF/FVIIa, can be reversibly inhibited by tissue factor pathway inhibitor (TFPI). Culturing monocytic cells in the presence of both FVIIa and TFPI caused down-regulation of TF expression via reducing its half-life. To exert this effect, FVIIa had to be competent to bind both TF and TFPI, and TFPI had to contain the C-terminal domain required for binding to other cell-surface receptors, including the low density lipoprotein receptor-related protein (LRP). TF down-regulation by FVIIa plus TFPI was abrogated by the 39-kDa receptor-associated protein, which blocks binding of all known ligands to LRP. Furthermore, treatment with FVIIa plus TFPI caused monocyte TF to colocalize with alpha-adaptin, a component of clathrin-coated pits. Thus, in addition to reversibly inhibiting TF/FVIIa catalytic activity, TFPI also mediates the permanent down-regulation of cell-surface TF in monocytic cells via LRP-dependent internalization and degradation. This represents an unusual mechanism for receptor internalization, requiring ligand-dependent bridging of one cell-surface receptor (TF) to a second cell-surface receptor (LRP), the latter being capable of clathrin-mediated internalization.

Factor VIIa/tissue factor generates a form of factor V with unchanged specific activity, resistance to activation by thrombin, and increased sensitivity to activated protein C

Factor VIIa, in complex with tissue factor (TF), is the serine protease responsible for initiating the clotting cascade. This enzyme complex (TF/VIIa) has extremely restricted substrate specificity, recognizing only three previously known macromolecular substrates (serine protease zymogens, factors VII, IX, and X). In this study, we found that TF/VIIa was able to cleave multiple peptide bonds in the coagulation cofactor, factor V. SDS-PAGE analysis and sequencing indicated the factor V was cleaved at Arg679, Arg709, Arg1018, and Arg1192, resulting in a molecule with a truncated heavy chain and an extended light chain. This product (FVTF/VIIa) had essentially unchanged activity in clotting assays when compared to the starting material. TF reconstituted into phosphatidylcholine vesicles was ineffective as a cofactor for the factor VIIa cleavage of factor V. However, incorporation of phosphatidylethanolamine in the vesicles had little effect over the presence of 20% phosphatidylserine. FVTF/VIIa was as sensitive to inactivation by activated protein C (APC) as thrombin activated factor V as measured in clotting assays or by the appearance of the expected heavy chain cleavage products. The FVTF/VIIa could be further cleaved by thrombin to release the normal light chain, albeit at a significantly slower rate than native factor V, to yield a fully functional product. These studies thus reveal an additional substrate for the TF/VIIa complex. They also indicate a new potential regulatory pathway of the coagulation cascade, i.e., the production of a form of factor V that can be destroyed by APC without the requirement for full activation of the cofactor precursor.

Two-chain factor VIIa generated in the pericardium during surgery with cardiopulmonary bypass : relationship to increased thrombin generation and heparin concentration

Several recent studies have proposed that coagulation is triggered during cardiopulmonary bypass surgery by extrinsic pathway activation involving factor VIIa generation, but the methodology was indirect. Therefore, 12 patients were studied during routine cardiac and cardiopulmonary bypass surgery. Samples were taken before, during, and after bypass from the perfusate, from the aorta (retrograde cardiac drainage), pericardium, and collected suction fluid originating from the whole operative field. These samples were analyzed by enzyme-linked immunosorbent assay for 2-chain factor VIIa, by prothrombin F1+2 assay, by thrombin-antithrombin (TAT) assay, and for heparin concentration. Factor VIIa, F1+2, and TAT levels in samples from the pericardium were greatly elevated (mean, 0.92 to 1.01, 227 to 334, and 399 to 526 microg/L, respectively; preoperative mean, 0.33, 32.3, and 1.90 microg/L, respectively; P<0. 05 for all), whereas levels in suction fluid were less consistently high. Factor VIIa and both F1+2 and thrombin-antithrombin levels in samples from the aorta, pericardium, and suction fluid were significantly correlated (r=0.57, P<0.001, n=111; and r=0.51, P<0. 001, n=105, respectively), and all were inversely correlated with heparin levels (r>-0.35, P<0.001, n>92). There was no evidence of factor VIIa generation in the circuit during bypass surgery, and both F1+2 and thrombin-antithrombin levels rose only approximately 2-fold, probably because heparin levels were higher than they were in the pericardium (P<0.05). We concluded that appreciable activation of factor VII occurs on the pericardium and that this is associated with increased thrombin generation. Ineffective local heparinization may be partly responsible. These results suggest that pericardium-induced activation of factor VII should be the target of anticoagulant strategies during cardiopulmonary bypass surgery.

New concepts in coagulation

The process of blood coagulation is a complex and incompletely understood process. In the last decade or so critical steps have been taken toward better understanding this process. It is now widely agreed that the principal initiating pathway of coagulation is the so-called extrinsic pathway due to the action of tissue factor and Factor VII. Concomitant with this appreciation has come an understanding and elucidation of the roles of tissue factor pathway inhibitor. Although the "intrinsic pathway" is no longer felt to be the initiator of coagulation, recent evidence suggests that Factor XIa may constitute an important amplification pathway of the coagulation system in vivo. Refinement of flow cytometry has enabled the detection of novel platelet antigens on activated platelet surfaces. It is hoped that detection and characterization of these antigens, including adhesion molecules such as P-selectin, will enable further understanding of the platelet's role in pathological coagulation and inflammation. The endothelium is also intricately involved and recent work has determined the importance of endothelial produced factors such as endothelium-derived relaxation factor, endothelin, and thrombomodulin. Finally, with the meteoric rise in molecular genetic technology, specific genetic abnormalities in a number of plasma proteins has been elucidated, with marked implications on the understanding of the coagulation process. For example, the mutation on the gene for Factor V, leading to Arg506 replacement with Gln, produces activated protein C resistance with a concomitant increased risk of venous thrombosis. Thus, significant advances in knowledge of the endothelium, platelets, and plasma factors involved in coagulation have been made and now the challenge of the future is to better elucidate the interactions of these components.

Contribution of factor VII genotype to activated FVII levels. Differences in genotype frequencies between northern and southern European populations

The relationship between coagulation factor VII (FVII) levels in plasma and FVII genotypes, determined by three polymorphisms (5'F7, IVS7, and 353R/Q), were studied in 500 control subjects enrolled in European multicenter study. The selection of particular FVII genotypes and the analysis of variance clearly indicated the independent contribution of a single 5'F7 insertion (A2) or 353Q (M2) allele to lowering plasma levels of activated FVII (FVIIa) (by a mean 25%). The M2 allele alone was found to make a major contribution to the genetically determined component of the FVIIa levels. Genotypes associated with low FVII levels were significantly rarer in the northern part of Europe (Oslo) than in the southern part (Rome, Murcia). The contribution made by the FVII genotype to the total variance of FVIIa levels was higher (30%) than that made to either FVII activity (25%) or FVII antigen (12%). Subjects with different FVII genotypes showed up to fivefold differences in mean FVIIa values, thus allowing attribution of a substantial part of the considerable interindividual variation to genetic variation, which may be of assistance in the interpretation of FVIIa levels on an individual basis. When FVII levels were adjusted by age and by triglyceride levels, the contribution of FVII genotypes to the FVII phenotypic variance was virtually unchanged. Taken together, these data indicate that in healthy control subjects the FVII genotype is a major predictor of plasma FVIIa levels and would support further study on the role of FVII genetic components in the development of cardiovascular disease.

Tissue factor expression in mesothelial cells: induction both in vivo and in vitro

Exudative pleural effusions are characterized by a high protein content and frequently progress to loculation and fibrosis. To test the hypothesis that tissue factor (TF) plays an integral role in this process, we investigated the expression of TF by human mesothelial cells (HMC) both in vivo and in vitro, and measured the effect of serum on HMC expression of TF in vitro. In vivo TF expression was not detected in HMC of normal pleura, but was detected in HMC of pleura overlying inflamed lung. In vitro, quiescent HMC demonstrated negligible levels of TF expression; however, upon serum stimulation there was a marked induction in both TF protein level and activity, peaking at 8-9 h. In contrast, treating quiescent HMC with plasma resulted in a further small, but significant, decrease in TF expression. This serum-induced rise in TF was also reflected in TF mRNA levels and did not require de novo protein synthesis. These results suggest that induction of HMC TF expression may be important in triggering both the intrapleural activation of prothrombin and the deposition of fibrin characteristic of inflammatory effusions.

Tissue factor cytoplasmic domain peptide is multiply phosphorylated in vitro

Human tissue factor was phosphorylated when incubated with lysates of U87-MG cells or fractions from preparative isoelectric focusing of the lysates. The cytoplasmic domain peptide, isolated following chemical cleavage at cysteine 245, focused on PhastGel IEF near pH 3.4, indicating the presence of three phosphate groups. A peptide corresponding to the carboxyl-terminal cytoplasmic domain (residues 245-263) was synthesized and shown to be a protein kinase substrate when incubated with lysates of U87-MG cells and radiolabeled ATP. As found with full-length tissue factor, the TF(245-263) peptide was phosphorylated at all three serines, but a diphosphate form was also identified. TF(245-263) was phosphorylated in the absence of calcium as well as in the presence of calphostin C, indicating that phosphorylation can be independent of protein kinase C. These results reveal that tissue factor can be multiply phosphorylated in vitro, and that the synthetic TF(245-263) cytoplasmic domain peptide serves as a model substrate.

A Soluble Tissue Factor Mutant Is a Selective Anticoagulant and Antithrombotic Agent

One approach to developing safer and more efficacious agents for the treatment of thrombotic disease involves the design and testing of inhibitors that block specific steps in the coagulation cascade. We describe here the development of a mutant of human tissue factor (TF ) as a specific antagonist of the extrinsic pathway of blood coagulation and the testing of this mutant in a rabbit model of arterial thrombosis. Alanine substitutions of Lys residues 165 and 166 in human TF have been shown previously to diminish the cofactor function of TF in support of factor X (FX) activation catalyzed by factor VIIa (FVIIa). The K165A:K166A mutations have been incorporated into soluble TF (sTF; residues 1-219) to generate the molecule “hTFAA.” hTFAA binds FVIIa with kinetics and affinity equivalent to wild-type sTF, but the hTFAA⋅FVIIa complex shows a 34-fold reduction in catalytic efficiency for FX activation relative to the activity measured for sTF⋅FVIIa. hTFAA inhibits the activation of FX catalyzed by the complex formed between FVIIa and relipidated TF(1-243). hTFAA prolongs prothrombin time (PT) determined with human plasma and relipidated TF(1-243) or membrane bound TF, and has no effect on activated partial thromboplastin time, but is 70-fold less potent as an inhibitor of PT with rabbit plasma. The rabbit homologue of this mutant (“rTFAA”) was produced and shown to have greater potency with rabbit plasma. Both hTFAA and rTFAA display an antithrombotic effect in a rabbit model of arterial thrombosis with rTFAA giving full efficacy at a lower dose than hTFAA. Compared to heparin doses of equal antithrombotic potential, hTFAA and rTFAA cause less bleeding as judged by measurements of the cuticle bleeding time. These results indicate that TF⋅FVIIa is a good target for the development of new anticoagulant drugs for the treatment of thrombotic disease.