Resolution of monomeric and heterodimeric forms of tissue factor, the high-affinity cellular receptor for factor VII

Extracellular haemoglobin upregulates and binds to tissue factor on macrophages: Implications for coagulation and oxidative stress

The mechanisms of crosstalk between haemolysis, coagulation and innate immunity are evolutionarily conserved from the invertebrate haemocyanin to the vertebrate haemoglobin (Hb). In vertebrates, extracellular Hb resulting from haemolytic infections binds bacterial lipopolysaccharide (LPS) to unleash the antimicrobial redox activity of Hb. Because bacterial invasion also upregulates tissue factor (TF), the vertebrate coagulation initiator, we asked whether there may be functional interplay between the redox activity of Hb and the procoagulant activity of TF. Using real-time PCR, TF-specific ELISA, flow cytometry and TF activity assay, we found that Hb upregulated the expression of functional TF in macrophages. ELISA, flow cytometry and immunofluorescence microscopy showed binding between Hb and TF, in isolation and in situ. Bioinformatic analysis of Hb and TF protein sequences showed co-evolution across species, suggesting that Hbβ binds TF. Empirically, TF suppressed the LPS-induced activation of Hb redox activity. Furthermore, Hb desensitised TF to the effects of antioxidants like glutathione or serum. This bi-directional regulation between Hb and TF constitutes a novel link between coagulation and innate immunity. In addition, induction of TF by Hb is a potentially central mechanism for haemolysis to trigger coagulation.

Human platelets synthesize and express functional tissue factor

The source and significance of bloodborne tissue factor (TF) are controversial. TF mRNA, protein, and TF-dependent procoagulant activity (PCA) have been detected in human platelets, but direct evidence of TF synthesis is missing. Nonstimulated monocyte-free platelets from most patients expressed TF mRNA, which was enhanced or induced in all of them after platelet activation. Immunoprecipitation assays revealed TF protein (mainly of a molecular weight [Mr] of approximately 47 kDa, with other bands of approximately 35 and approximately 60 kDa) in nonstimulated platelet membranes, which also increased after activation. This enhancement was concomitant with TF translocation to the plasma membrane, as demonstrated by immunofluorescence–confocal microscopy and biotinylation of membrane proteins. Platelet PCA, assessed by factor Xa (FXa) generation, was induced after activation and was inhibited by 48% and 76% with anti-TF and anti-FVIIa, respectively, but not by intrinsic pathway inhibitors. Platelets incorporated [35S]-methionine into TF proteins with Mr of approximately 47 kDa, approximately 35 kDa, and approximately 60 kDa, more intensely after activation. Puromycin but not actinomycin D or DRB (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) inhibited TF neosynthesis. Thus, human platelets not only assemble the clotting reactions on their membrane, but also supply their own TF for thrombin generation in a timely and spatially circumscribed process. These observations simplify, unify, and provide a more coherent formulation of the current cell-based model of hemostasis.

PI3K-Akt pathway suppresses coagulation and inflammation in endotoxemic mice

OBJECTIVE

In endotoxemia, lipopolysaccharide (LPS) induces a systemic inflammatory response and intravascular coagulation. Monocytes orchestrate the innate immune response to LPS by expressing a variety of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha), and the procoagulant molecule, tissue factor (TF). In this study, we analyzed the role of the phosphoinositide 3-kinase (PI3K)-Akt pathway in the activation of coagulation and the innate immune response in a mouse model of endotoxemia.

METHODS AND RESULTS

Wortmannin and LY294002 were used to inhibit the PI3K-Akt pathway. We found that wortmannin inhibited LPS-induced Akt phosphorylation in blood cells. Inhibition of the PI3K-Akt pathway significantly increased TF mRNA expression in blood cells, TF antigen, and thrombin-antithrombin III levels in the plasma, and fibrin deposition in the liver of endotoxemic mice. Inhibition of the PI3K-Akt pathway also strongly enhanced LPS-induced cytokine expression and the levels of soluble E-selectin in the plasma, suggesting enhanced activation of both monocytes and endothelial cells. Wortmannin treatment also increased the number of macrophages in the liver and kidney of endotoxemic mice. Finally, wortmannin and LY294002 dramatically reduced the survival time of endotoxemic mice.

CONCLUSIONS

These data suggest that the PI3K-Akt pathway suppresses LPS-induced inflammation and coagulation in endotoxemic mice.

Properties of proteins in cancer procoagulant preparations that are detected by anti-tissue factor antibodies

Cancer procoagulant (CP) and tissue factor (TF; only in complex with Factor VIIa (FVIIa)) can activate FX to FXa. Controversy still exists whether or not CP is an entity different from TF, or whether CP activity is due to contamination of CP preparations with TF/FVIIa complex. We therefore looked for proteins in CP preparations that were detected by anti-TF antibodies and then sequenced these proteins. One- and two-dimensional gels of CP and TF were used to identify proteins immunoreactive to monoclonal anti-CP and anti-TF antibodies (Mabs). Those proteins in the CP preparation recognized by anti-TF antibodies were sequenced. Angiotensinogen precursor, alpha-1-antitrypsin precursor, and vitamin D-binding protein were identified along with one so far unidentified sequence; however, no TF-sequences were identified. Also, no proteins with the correct molecular weight for TF were identified using anti-TF antibodies. It seems possible that CP preparations contain proteins that have some epitopes similar to the epitopes recognized in TF by anti-TF Mab. However, these proteins do neither have the molecular weight nor the amino acid sequence of TF.

Hemoglobin induces the production and release of matrix metalloproteinase-9 from human malignant cells

Matrix metalloproteinase-9 (MMP-9) plays a crucial role in both angiogenesis and tumor invasion. Vascular endothelial growth factor (VEGF) has been shown to up-regulate the expression of MMP-9 in vascular smooth muscle cells. We recently reported that hemoglobin (Hb) enhances the expression of tissue factor (TF) and VEGF on TF-positive human malignant cells. Therefore, to explore the relationship between tumor cell angiogenic protein VEGF and MMP-9, we studied the effect of Hb on MMP-9 production in human A375 malignant melanoma and J82 bladder carcinoma (TF+) cells and in KG1 myeloid leukemia (TF-) cells. Malignant cells were incubated with varying concentrations (0-1.0 mg/ml) of Hb and analyzed for released MMP-9 by gelatin zymography, dot immunoblotting, enzyme-linked immunosorbent assay, and Western blotting. Hb (0.50 mg/ml) induced an almost two-fold increase of MMP-9 in both A375 malignant melanoma (398 +/- 62 versus 233 +/- 61.0 ng/ml, P = 0.027) and J82 bladder carcinoma cells (1.55 +/- 0.12 versus 0.80 +/- 0.004 ng/ml, P = 0.004), compared with cells incubated without Hb. This release of MMP-9 was significantly inhibited by cycloheximide (95%) and by the specific inhibitors of protein tyrosine kinase, genistein (70 +/- 3.0%, P = 0.00027 and 67 +/- 1.0%, P = 0.00005) and mitogen-activated protein (MAP)-kinase, PD98059 (56 +/- 2.0%, P = 0.0001 and 62 +/- 1.0%, P = 0.00003) in A375 and J82 cells, respectively. In contrast, Hb (2.0 mg/ml) did not increase MMP-9 in KG1 cells. We conclude that Hb-induced synthesis of active MMP-9 in TF-bearing malignant cells is due to de novo synthesis of newly formed protein and is mediated by protein tyrosine kinase and by mitogen-activated protein kinase pathways.

Hemoglobin binds melanoma cell tissue factor and enhances its procoagulant activity

Tissue factor (TF), the membrane-bound glycoprotein that normally initiates the coagulation pathway, is expressed on the surface of various cells including endothelial cells, fibroblasts, monocytes and tumor cells. We recently reported that hemoglobin (Hb) enhances TF expression and procoagulant activity on TF-bearing human A375 malignant melanoma cells. To elucidate the mechanism of Hb-induced TF expression, we studied the interaction between purified TF from human A375 malignant melanoma cells and Hb. Selective binding of highly purified melanoma cell TF-apoprotein to Hb was demonstrated under native conditions using a dot-immunobinding assay and under denaturing conditions by Western blotting. The complex formation between purified melanoma cell TF-apoprotein and Hb was also demonstrated by the binding of fluid-phase Hb to immobilized TF-apoprotein (0-2.0 microg/ml) in an enzyme-linked immunosorbent assay. The binding was specific, concentration-dependent, saturable and inhibited significantly (60%) by Concanavalin-A. Hb enhanced the factor X-activating procoagulant activity of melanoma cell TF in a concentration-dependent manner, but had no effect on recombinant human TF. Concanavalin-A and wheat germ agglutinin significantly (60%) inhibited the Hb-induced procoagulant activity of malignant cell TF. We conclude that TF-apoprotein selectively binds Hb, most probably via the carbohydrate moieties (alpha-d-glucosyl; alpha-d-mannosyl and N-acetyl-beta-d-glucosaminyl residues) of TF, and enhances its procoagulant activity. The physiological significance of this interaction remains to be established.

Purification and properties of human melanoma cell tissue factor

Tissue factor (TF) is a transmembrane glycoprotein that acts as a receptor for nonactivated and activated factor VII (FVII) and triggers the coagulation cascade. TF plays an important role in hemostasis, but may also have noncoagulation functions in vascular development, angiogenesis, and tumor cell metastasis. In tumor cells, analysis of the role of TF has been hampered by the lack of purified TF. In this study, TF antigen was identified on human A375 malignant melanoma cells using flow cytometry. We further purified TF apoprotein 2,000-fold to homogeneity from A375 melanoma cells using immunoaffinity chromatography. On SDS-polyacrylamide gel electrophoresis under reduction, purified TF apoprotein gave two major protein bands corresponding to molecular weights of 53 and 34 to 36 KD. The identity of these forms of TF was confirmed by Western blotting using a polyclonal antibody against human brain TF. Under reduction, the TF antibody bound with a monomeric form of TF (53 KD), and without reduction, to several forms of TF (34 to 128 KD). Preliminary carbohydrate analysis suggested that TF is a glycoprotein and contains about 22% total carbohydrates. The coagulant activity of the purified apoprotein was reconstituted by the addition of phospholipids. The effects of varying concentrations (0 to 8 microg) of polyclonal antibodies to TF and FVII on TF procoagulant activity were studied. Both antibodies inhibited more than 70% of the procoagulant activity of TF in an FX activation assay. The complex formation between purified TF apoprotein and FVIIa was demonstrated by using an enzyme-linked immunosorbent assay. TF formed a complex with FVIIa in a concentration-dependent and saturable manner. We conclude that in human melanoma cells, TF occurs in monomeric and heterodimeric forms and appears to have similar properties as reported for TF from other sources.

Hemoglobin enhances tissue factor expression on human malignant cells

Tissue Factor (TF) is a transmembrane glycoprotein that complexes with factor VII/activated factor VII to initiate blood coagulation. TF may be expressed on the surface of various cells including monocytes and endothelial cells. Over-expression of TF in human tumor cell lines promotes metastasis. We recently showed that hemoglobin (Hb) forms a specific complex with TF purified from human malignant melanoma cells and enhances its procoagulant activity (PCA). To further study this interaction, we examined the effect of Hb on the expression of TF on human malignant (TF+) cells and KG1 myeloid leukemia (TF-) cells. Human melanoma A375 and J82 bladder carcinoma cells, which express TF at moderate and relatively high levels, respectively, were incubated with varying concentrations (0-1.5 mg/ml) of Hb. After washing, cells were analyzed for Hb binding and TF expression using flow cytometry and confocal microscopy. Hb bound to the cells in a concentration-dependent manner, and increased both TF expression and PCA. The human A375 malignant melanoma cells incubated with Hb (1 mg/ml) expressed up to six times more TF antigen than cells without Hb. This increase in TF expression and PCA of intact cells incubated with Hb was significantly inhibited by cycloheximide at a concentration of 10 microg/ml (P < 0.01). An increase in total cellular TF antigen content was demonstrated by specific immunoassay. In contrast, Hb (5 mg/ml) did not induce TF expression and PCA on KG1 cells as determined by flow cytometry and TF (FXAA) activity. We conclude that Hb specifically binds to TF-bearing malignant cells and increases their PCA. This effect seems to be at least partly due to de novo synthesis of TF and increased surface expression. However, the exact mechanism by which Hb binds and upregulates TF expression remains to be determined.

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.

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.

Activated platelets signal chemokine synthesis by human monocytes

Human blood monocytes adhere rapidly and for prolonged periods to activated platelets that display P-selectin, an adhesion protein that recognizes a specific ligand on leukocytes, P-selectin glycoprotein-1. We previously demonstrated that P-selectin regulates expression and secretion of cytokines by stimulated monocytes when it is presented in a purified, immobilized form or by transfected cells. Here we show that thrombin-activated platelets induce the expression and secretion of monocyte chemotactic protein-1 and IL-8 by monocytes. Enhanced monokine synthesis requires engagement of P-selectin glycoprotein-1 on the leukocyte by P-selectin on the platelet. Secretion of the chemokines is not, however, directly signaled by P-selectin; instead, tethering of the monocytes by P-selectin is required for their activation by RANTES (regulated upon activation normal T cell expressed presumed secreted), a platelet chemokine not previously known to induce immediate-early gene products in monocytes. Adhesion of monocytes to activated platelets results in nuclear translocation of p65 (RelA), a component of the NF-kappaB family of transcription factors that binds kappaB sequences in the regulatory regions of monocyte chemotactic protein-1, IL-8, and other immediate-early genes. However, expression of tissue factor, a coagulation protein that also has a kappaB sequence in the 5' regulatory region of its gene, is not induced in monocytes adherent to activated platelets. Thus, contact of monocytes with activated platelets differentially affects the expression of monocyte products. These experiments suggest that activated platelets regulate chemokine secretion by monocytes in inflammatory lesions in vivo and provide a model for the study of gene regulation in cell-cell interactions.

Tissue factor expression in human leukemic cells

Patients with acute leukemia are at increased risk for thrombotic and hemorrhagic complications, particularly those patients with acute promyelocytic leukemia (APL) undergoing induction chemotherapy. These serious complications have been attributed by some authors to the release of tissue factor (TF) procoagulant activity (PCA), particularly during cytotoxic chemotherapy. In previous studies of normal peripheral blood cells, only cells of the monocyte lineage have been found to express TF PCA. Therefore, several questions remain regarding the origin and characterization of the PCA in malignant leukemic cells, particularly those thought to be derived from granulocyte progenitor cells. We utilized a full-length cDNA probe, several monoclonal antibodies (MAbs) and a sensitive one-stage PCA assay to study the expression of TF in the human cell line, HL-60, in human peripheral blood monocytes/macrophages (Mo/Mø) and in highly purified populations of human polymorphonuclear leukocytes (PMN). In the HL-60 cells we detected low but significant levels of TF mRNA and TF antigen (TF:Ag). In unstimulated cells, coordinate increased levels of TF mRNA, TF:Ag and TF PCA expression were noted following phorbol-ester-induced macrophage differentiation of the cells, but a decreased level of TF mRNA with no change in the basal level of TF:Ag expression occurred following retinoic acid-induced granulocyte differentiation of this cell line. Long-term cultures of stimulated mature Mo/Mø demonstrated initial coordinate expression of TF mRNA, TF:Ag and TF PCA, but TF:Ag expression persisted even after 7 days (when TF PCA was undetectable). No TF PCA, TF:Ag or TF mRNA was demonstrated in highly purified populations of human PMN, regardless of culture conditions. Discordant expression of TF mRNA, TF:Ag and TF PCA in HL-60 cells suggests the possibility of novel, post-synthetic mechanisms for the regulation of TF PCA expression, which might be dependent on the phenotypic differentiation level of the cell. Such mechanisms (yet to be defined) might account for the ability of some leukemic cells, which frequently express characteristics of more than one cell line (e.g. monocytes and granulocytes), to express a TF gene product capable of activating blood coagulation.

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

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

Expression of tissue factor by melanoma cells promotes efficient hematogenous metastasis

Metastasis is a multistep process which requires highly adapted interactions of tumor cells with host target organs. Compared with nonmetastatic cells, metastatic human melanoma cells express 1000-fold higher level of tissue factor (TF), the major cellular initiator of the plasma coagulation protease cascades. To explore whether TF may contribute to metastatic tumor dissemination, we analyzed the effect of specific inhibition of TF function on human melanoma metastasis in severe combined immunodeficient (SCID) mice. Using species-specific antibodies to TF, we demonstrate that initial adherence in insufficient for successful tumor cell implantation in a target organ. Rapid arrest of human tumor cells in the lungs of mice was not diminished by inhibition of TF. However, inhibition of TF receptor function and consequent reduction in local protease generation abolished prolonged adherence of tumor cells, resulting in significantly reduced numbers of tumor cells retained in the vasculature of the lungs. The growth of pulmonary metastases was also significantly inhibited by a blocking anti-TF monoclonal antibody and Fab fragments thereof, whereas a noninhibitory antibody lacked antimetastatic effects. Cell surface expression of functional TF thus contributes to melanoma progression by allowing metastatic cells to provide requisite signals for prolonged adhesive interactions and/or transmigration of tumor cells across the endothelium, resulting in successful metastatic tumor implantation.

Expression and purification of a soluble tissue factor fusion protein with an epitope for an unusual calcium-dependent antibody

The use of bacterial signal peptides to target recombinant mammalian proteins to the periplasmic space of Escherichia coli (to promote proper disulfide bond formation) has met with variable success. We report the design and use of a bacterial expression vector to direct recombinant fusion proteins to the periplasmic space of E. coli: it contains the signal peptide from the pelB gene of Erwinia carotovora linked to a small peptide epitope for an unusual calcium-dependent antibody (HPC4). HPC4 binds to the epitope in a Ca(2+)-dependent manner, but the epitope itself does not bind Ca2+. We have used this system to express a biologically active, soluble form of tissue factor, the protein responsible for triggering the blood clotting cascade. Soluble tissue factor was secreted into the culture medium at 1-2 mg/liter, from which it could be readily purified using immobilized HPC4 antibody. The HPC4 epitope could be removed by digestion with thrombin or factor Xa, although a free amino terminus was not required for function since soluble tissue factor was equally active with the epitope still in place. This vector/epitope system permits large-scale expression and purification of recombinant soluble tissue factor and should be generally applicable to the isolation of other recombinant proteins. Furthermore, the epitope confers Ca(2+)-dependent binding of the fusion protein to HPC4 antibody while avoiding the creation of a new metal binding site on the fusion protein itself. Tb3+ can bind in this Ca2+ site near Trp, allowing this site to serve as a means of attaching a fluorescent probe to tissue factor.

Two sites in the tissue factor extracellular domain mediate the recognition of the ligand factor VIIa

Tissue factor (TF) binds the serine protease coagulation factor VIIa and initiates the coagulation protease cascade by forming a catalytic cofactor-enzyme complex. Using a photoactivatable crosslinking reagent coupled to factor VIIa, we have identified interactive sites in the amino-terminal (residues 44-84) and the carboxyl-terminal (residues 129-169) aspect of the extracellular domain of TF. Epitopes of inhibitory antibodies have previously indicated participation of these regions in TF function. The presence of the gamma-carboxyglutamic acid domain in factor VIIa appears to facilitate the interaction with the negatively charged, amino-proximate site, whereas crosslinking of TF with VIIa or des-(1-38)-VIIa at the positively charged carboxyl-proximate site was similar. Lack of alpha-helical secondary structure in the TF extracellular domain is consistent with the proposed structural similarity of TF with the cytokine receptor family. The interactive sites identified for TF are located in sequence spans that demonstrate a low degree of sequence conservation among the members of this receptor family. Regions with highly conserved residues, such as sequences encoded by exon 2 and 5 in TF, were not implicated in ligand recognition, suggesting that conserved residues in the receptor family may maintain the common beta-strand architecture, and variable regions provide a pair of nonidentical motifs for oriented ligand recognition.

Antibody mapping of tissue factor implicates two different exon-encoded regions in function

Tissue Factor (TF), a small transmembrane glycoprotein, is the cellular receptor for the zymogen Factor VII and the serine protease Factor VIIa (VIIa). TF provides cofactor function for VIIa in the catalytically active (TF: VIIa) binary complex. To explore the structural loci of TF that are responsible for binding of VII and VIIa, monoclonal antibodies (MAbs) and sequence-specific polyclonal antibodies to the native TF protein were analysed for inhibition of VII binding. Two independent epitopes of MAbs were localized by reciprocal competition and by binding of the MAbs to different proteolytic fragments of TF. The epitopes were also characterized in part by progressive C-terminal deletional mutation of the TF protein. Reactivity of the anti-(locus II) MAb TF9-6G4 is consistent with epitope localization in residues Thr40-Val83, encoded by exon 3. In contrast, the anti-(locus I) MAb TF9-5G9 was reactive with fragments encompassing exon 4 (Thr106-Lys165). Antibodies to linear sequences encoded by the same two exons also inhibited VII binding. These data suggest a minimum requirement for two of the four exon-encoded regions of TF for the functional integrity of this receptor cofactor with respect to ligand recognition and high-affinity binding.

Formation of tissue factor activity following incubation of recombinant human tissue factor apoprotein with plasma lipoproteins

Incubation of recombinant human tissue factor apoprotein (Apo-TF) with human plasma decreased the recalcified clotting time of this plasma in a time-and dose-dependent manner suggesting relipidation of the Apo-TF by plasma lipoproteins. Incubation of Apo-TF with purified preparations of human very low density, low density and high density lipoproteins resulted in tissue factor activity in a clotting assay. The order of effectiveness was VLDL greater than LDL much greater than HDL. Tissue factor activity generated by incubation of a fixed amount of Apo-TF with plasma lipoproteins was lipoprotein concentration-dependent and saturable. The association of Apo-TF with lipoprotein particles was supported by gel filtration studies in which 125I-Apo-TF coeluted with the plasma lipoprotein in the void volume of a Superose 6 column in the presence and absence of calcium ions. In addition, void-volume Apo-TF-lipoprotein fractions exhibited tissue factor activity. These results suggest that the factor VIII-bypassing activity of bovine Apo-TF observed in a canine hemophilic model may be due, in part, to its association with plasma lipoproteins and expression of functional tissue factor activity.

Purification of recombinant human tissue factor

Tissue factor (TF) is a 263 amino acid membrane-bound procoagulant protein that serves as a cofactor for the serine protease factor VII (fVII). Recombinant human TF (rTF) produced in both human kidney 293 cells and Escherichia coli has been immunoaffinity purified by using a TF-specific monoclonal antibody. Recombinant TF produced in 293 cells is glycosylated and migrates on reducing SDS-PAGE with an apparent molecular weight (Mr) of 45K. Some interchain disulfide-bonded rTF dimers are observed under nonreducing conditions. The E. coli produced rTF has a molecular weight of 33K and 35K, with the 33K band missing nine amino acids at the carboxy terminus. Although the E. coli produced rTF does not contain any carbohydrate, it is fully functional in both a chromogenic assay and a one-stage prothrombin time assay. A variant has been constructed wherein the cytoplasmic cysteine (residue 245) has been mutagenized to a serine residue. The amount of disulfide-linked aggregates is dramatically reduced following immunoaffinity purification of this four-cysteine variant (C2455), which is active in the chromogenic and prothrombin time assays.

Functional tissue factor is entirely cell surface expressed on lipopolysaccharide-stimulated human blood monocytes and a constitutively tissue factor-producing neoplastic cell line

Tissue factor (TF) is an integral membrane glycoprotein which, as the receptor and essential cofactor for coagulation factors VII and VIIa (FVII and FVIIa, respectively), is the primary cellular activator of the coagulation protease cascade. Previous studies on the procoagulant activity of a variety of cell types (either lysed or in the intact state) have variously been interpreted as showing that TF is either stored intracellularly or is present in a cryptic form in the surface membrane. Using mAbs to TF, we have directly investigated the subcellular localization and functional activity of TF in lipopolysaccharide-stimulated blood monocytes and J82 bladder carcinoma cells. Blocking of surface TF of viable cells with inhibitory anti-TF mAbs abolished greater than 90% of TF activity of the intact cells as well as of lysed cells. Furthermore, quantitative analysis of the binding of FVII and anti-TF mAb to J82 cells demonstrated that all surface-expressed TF molecules were capable of binding the ligand, FVII. By immunoelectron microscopy, TF was present only in the surface membrane of monocytes and J82 cells, although the latter also contained apparently inactive TF antigen in multivesicular bodies. On the intact cell surface the catalytic activity of the TF-FVIIa complex was investigated and found to be markedly less relative to cell lysates. Membrane alterations that affect the cofactor activity of TF may be a means of regulating the extent of initiation of the coagulation protease cascade in various cellular settings.

Studies on leukemic cell tissue factor

Apoprotein part of tissue factor of human placenta was purified 871 fold from the starting material with 4.2% yield by concanavalin A-Sepharose affinity chromatography and SDS-PAGE. The molecular weight of purified apoprotein was 45,000 in non-reduced condition and 49,000 in reduced condition. Tissue factor of human leukemia cells (FAB classification:M2 and M3) and cultured leukemia cell lines (HL-60 and Molt-4) was analyzed using specific rabbit anti-tissue factor IgG raised against purified material. Endotoxin stimulated HL-60 and Molt-4 also expressed procoagulant activity which was inhibited by tissue factor immune IgG. By immunostaining of the purified material, the lysate of leukemia cells (M2 and M3) and cultured leukemia cells (HL-60 and MOLT-4) revealed a major band of the same apparent molecular weight. Immuno-electron microscopic study on tissue factor of HL-60 cells produced the following findings: stimulation by endotoxin resulted in the formation of pseudopods of the cell membrane, and immunogold particles accumulated mainly on these pseudopods and cisternal spaces of rough endoplasmic reticulum, indicating exposure of the tissue factor to the surface of perturbed cell membrane with concurrent increase in tissue factor synthesis.

Monoclonal antibody analysis of purified and cell-associated tissue factor

Tissue factor (TF), one of the cell-surface initiators of blood coagulation, has been implicated as the major molecule of this type and as a critical controlling molecule in hemostasis, thrombosis and inflammation. Analysis of the expression of human TF by cells has been hampered by the lack of suitable molecular probes. We have prepared a library of twenty-four murine hybridomas which stably secrete monoclonal antibodies to human TF. Based on their characteristics, these monoclonals can be categorized into a minimum of five distinct groups. Twenty-three of the hybridoma antibodies strongly inhibited TF activity, which was attributable to blocking of formation of the bimolecular complex of TF and factor VII. We have used these antibodies to demonstrate directly that TF is the sole high affinity factor VII receptor on an intact cell. We have also demonstrated the immunologic relationship between constitutive and induced expression of the protein responsible for TF-like activity by several cells and tissues. Most of the antibodies were found to inhibit TF activity expressed by other primate species, and the potential in vivo therapeutic use of monoclonal antibodies of differing intramolecular specificity is discussed.