Characterization of human tissue factor pathway inhibitor variants expressed in Saccharomyces cerevisiae

New insights into the biology of tissue factor pathway inhibitor

Tissue factor pathway inhibitor (TFPI) dampens the initiation of blood coagulation by inhibiting two potent procoagulant complexes, tissue factor-factor VIIa (TF-FVIIa) and early forms of prothrombinase. TFPI isoforms, TFPIα and TFPIβ, result from alternative splicing of mRNA, producing distinct C-terminal ends of the two proteins. Both isoforms inhibit TF-FVIIa, but only TFPIα can inhibit early forms of prothrombinase by binding of its positively charged C-terminus with high affinity to the acidic B-domain exosite of FVa, which is generated upon activation by FXa. TFPIα and TFPIβ are produced in cultured human endothelial cells, while platelets contain only TFPIα. Knowledge of the anticoagulant mechanisms and tissue expression patterns of TFPIα and TFPIβ have improved our understanding of the phenotypes observed in different mouse models of TFPI deficiency, the east Texas bleeding disorder, and the development of pharmaceutical agents that block TFPI function to treat hemophilia.

Cellular expression and biological activities of alternatively spliced forms of tissue factor pathway inhibitor

PURPOSE OF REVIEW

Tissue factor pathway inhibitor (TFPI) is an anticoagulant protein that inhibits tissue factor-factor VIIa (TF-fVIIa) and factor Xa (fXa). Recent studies revealed distinct cellular expression patterns for TFPIα and TFPIβ and spurred additional experiments to define unique functions for these alternatively spliced TFPI isoforms.

RECENT FINDINGS

TFPIα is produced by endothelial cells, localizes to an intracellular granule, and is released following cellular stimulation with thrombin or heparin. TFPIα also is produced by megakaryocytes and released from activated platelets. Platelet TFPIα limits clot growth following vessel injury and alters bleeding in hemophilia, suggesting that its primary physiological role is modulation of clot development. TFPIβ is made by endothelial cells, localizes to the endothelium surface, and is not in platelets. TFPIβ is an effective inhibitor of TF-mediated cellular migration and may act to dampen the adverse effects of intravascular TF expressed during inflammation.

SUMMARY

Knowledge of TFPI isoform expression and activity provides new insights into the biochemical regulation of TF-mediated thrombotic and inflammatory disease. Recent findings have therapeutic implications for use of recombinant TFPI to treat severe sepsis in community-acquired pneumonia or to achieve improved engraftment of hematopoietic stem cells, and for development of TFPI-blocking pharmaceuticals to treat hemophilia.

Hemostatic effect of a monoclonal antibody mAb 2021 blocking the interaction between FXa and TFPI in a rabbit hemophilia model

Hemophilia is treated by IV replacement therapy with Factor VIII (FVIII) or Factor IX (FIX), either on demand to resolve bleeding, or as prophylaxis. Improved treatment may be provided by drugs designed for subcutaneous and less frequent administration with a reduced risk of inhibitor formation. Tissue factor pathway inhibitor (TFPI) down-regulates the initiation of coagulation by inhibition of Factor VIIa (FVIIa)/tissue factor/Factor Xa (FVIIa/TF/FXa). Blockage of TFPI inhibition may facilitate thrombin generation in a hemophilic setting. A high-affinity (KD = 25pM) mAb, mAb 2021, against TFPI was investigated. Binding of mAb 2021 to TFPI effectively prevented inhibition of FVIIa/TF/FXa and improved clot formation in hemophilia blood and plasma. The binding epitope on the Kunitz-type protease inhibitor domain 2 of TFPI was mapped by crystallography, and showed an extensive overlap with the FXa contact region highlighting a structural basis for its mechanism of action. In a rabbit hemophilia model, an intravenous or subcutaneous dose significantly reduced cuticle bleeding. mAb 2021 showed an effect comparable with that of rFVIIa. Cuticle bleeding in the model was reduced for at least 7 days by a single intravenous dose of mAb 2021. This study suggests that neutralization of TFPI by mAb 2021 may constitute a novel treatment option in hemophilia.

The Kunitz 1 and Kunitz 3 domains of tissue factor pathway inhibitor are required for efficient inhibition of factor Xa

Tissue factor pathway inhibitor (TFPI) is a slow tight-binding inhibitor that inhibits factor (F)Xa in a biphasic fashion: a rapid formation of loose FXa·TFPI encounter complex is followed by slow rearrangement into a tight FXa·TFPI* complex in which the Kunitz-2 (K2) domain of TFPI binds and inhibits FXa. In the current study, full-length TFPI (TFPIfl) and various truncated TFPI constructs were used to assess the importance of TFPI domains other than K2 in the inhibition of FXa. In the absence of Ca2+ ions, FXa was more effectively inhibited by TFPIfl than Gla-domain less FXa. In turn, Ca2+ ions impaired FXa inhibition by TFPIfl but not by TFPI constructs that lack the C-terminus. This suggests that, in absence of Ca2+ ions, interactions between the C-terminus of TFPI and the Gla-domain of FXa promote FXa-inhibition. TFPIfl and K2K3 had similar efficiencies for encounter complex formation. However, K2K3 showed monophasic inhibition instead of biphasic inhibition, indicating absence of rearrangement into a tight complex. K1K2 and TFPI1-161 showed biphasic inhibition, but had less efficient encounter complex formation than TFPIfl. Finally, K2K3 was a 10-fold more efficient FXa- inhibitor than K2. These results indicate that K3-C-terminus enhances the formation of encounter complex and that K1 is required for isomerisation of the encounter- into tight complex. Since TFPIfl has a 10-fold higher Ki than K2K3-C-terminus, we propose that K1 is not only required for the transition of the loose to the tight FXa·TFPI* complex, but also inhibits FXa·TFPI encounter complex formation. This inhibitory activity is counteracted by K3 and C-terminus.

Caveolae optimize tissue factor-Factor VIIa inhibitory activity of cell-surface-associated tissue factor pathway inhibitor

TFPI (tissue factor pathway inhibitor) is an anticoagulant protein that prevents intravascular coagulation through inhibition of fXa (Factor Xa) and the TF (tissue factor)-fVIIa (Factor VIIa) complex. Localization of TFPI within caveolae enhances its anticoagulant activity. To define further how caveolae contribute to TFPI anticoagulant activity, CHO (Chinese-hamster ovary) cells were co-transfected with TF and membrane-associated TFPI targeted to either caveolae [TFPI-GPI (TFPI-glycosylphosphatidylinositol anchor chimaera)] or to bulk plasma membrane [TFPI-TM (TFPI-transmembrane anchor chimaera)]. Stable clones had equal expression of surface TF and TFPI. TX-114 cellular lysis confirmed localization of TFPI-GPI to detergent-insoluble membrane fractions, whereas TFPI-TM localized to the aqueous phase. TFPI-GPI and TFPI-TM were equally effective direct inhibitors of fXa in amidolytic assays. However, TFPI-GPI was a significantly better inhibitor of TF-fVIIa than TFPI-TM, as measured in both amidolytic and plasma-clotting assays. Disrupting caveolae by removing membrane cholesterol from EA.hy926 cells, which make TFPIα, CHO cells transfected with TFPIβ and HUVECs (human umbilical vein endothelial cells) did not affect their fXa inhibition, but significantly decreased their inhibition of TF-fVIIa. These studies confirm and quantify the enhanced anticoagulant activity of TFPI localized within caveolae, demonstrate that caveolae enhance the inhibitory activity of both TFPI isoforms and define the effect of caveolae as specifically enhancing the anti-TF activity of TFPI.

The anticoagulant and antithrombotic mechanisms of heparin

The molecular basis for the anticoagulant action of heparin lies in its ability to bind to and enhance the inhibitory activity of the plasma protein antithrombin against several serine proteases of the coagulation system, most importantly factors IIa (thrombin), Xa and IXa. Two major mechanisms underlie heparin's potentiation of antithrombin. The conformational changes induced by heparin binding cause both expulsion of the reactive loop and exposure of exosites of the surface of antithrombin, which bind directly to the enzyme target; and a template mechanism exists in which both inhibitor and enzyme bind to the same heparin molecule. The relative importance of these two modes of action varies between enzymes. In addition, heparin can act through other serine protease inhibitors such as heparin co-factor II, protein C inhibitor and tissue factor plasminogen inhibitor. The antithrombotic action of heparin in vivo, though dominated by anticoagulant mechanisms, is more complex, and interactions with other plasma proteins and cells play significant roles in the living vasculature.

Tissue factor pathway inhibitor-gamma is an active alternatively spliced form of tissue factor pathway inhibitor present in mice but not in humans

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is a potent inhibitor of tissue factor procoagulant activity produced as two alternatively spliced isoforms, TFPIalpha and TFPIbeta, which differ in domain structure and mechanism for cell surface association. 3' Rapid amplification of cDNA ends was used to search for new TFPI isoforms. TFPIgamma, a new alternatively spliced form of TFPI, was identified and characterized.

METHODS

The tissue expression, cell surface association and anticoagulant activity of TFPIgamma were characterized and compared to those of TFPIalpha and TFPIbeta through studies of mouse and human tissues and expression of recombinant proteins in Chinese hamster ovary (CHO) cells.

RESULTS

TFPIgamma is produced by alternative splicing using the same 5'-splice donor site as TFPIbeta and a 3'-splice acceptor site 276 nucleotides beyond the stop codon of TFPIbeta in exon 8. The resulting protein has the first two Kunitz domains connected to an 18 amino acid C-terminal region specific to TFPIgamma. TFPIgamma mRNA is differentially produced in mouse tissues but is not encoded within the human TFPI gene. When expressed in CHO cells, TFPIgamma is secreted into conditioned media and effectively inhibits tissue factor procoagulant activity.

CONCLUSIONS

TFPIgamma is a third alternatively spliced form of TFPI that is widely expressed in mouse tissues but not made by human tissues. It contains the first two Kunitz domains and is a secreted, rather than a cell surface-associated, protein. It is a functional anticoagulant and may partially explain the resistance of mice to coagulopathy in tissue factor-mediated models of disease.

Protein S stimulates inhibition of the tissue factor pathway by tissue factor pathway inhibitor

Tissue factor (TF) plays an important role in hemostasis, inflammation, angiogenesis, and the pathophysiology of atherosclerosis and cancer. In this article we uncover a mechanism in which protein S, which is well known as the cofactor of activated protein C, specifically inhibits TF activity by promoting the interaction between full-length TF pathway inhibitor (TFPI) and factor Xa (FXa). The stimulatory effect of protein S on FXa inhibition by TFPI is caused by a 10-fold reduction of the K(i) of the FXa/TFPI complex, which decreased from 4.4 nM in the absence of protein S to 0.5 nM in the presence of protein S. This decrease in K(i) not only results in an acceleration of the feedback inhibition of the TF-mediated coagulation pathway, but it also brings the TFPI concentration necessary for effective FXa inhibition well within range of the concentration of TFPI in plasma. This mechanism changes the concept of regulation of TF-induced thrombin formation in plasma and demonstrates that protein S and TFPI act in concert in the inhibition of TF activity. Our data suggest that protein S deficiency not only increases the risk of thrombosis by impairing the protein C system but also by reducing the ability of TFPI to down-regulate the extrinsic coagulation pathway.

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.

Probing the interface between factor Xa and tissue factor in the quaternary complex tissue factor-factor VIIa-factor Xa-tissue factor pathway inhibitor

Blood coagulation is triggered by the formation of a complex between factor VIIa (FVIIa) and its cofactor, tissue factor (TF). TF-FVIIa is inhibited by tissue factor pathway inhibitor (TFPI) in two steps: first TFPI is bound to the active site of factor Xa (FXa), and subsequently FXa-TFPI exerts feedback inhibition of TF-FVIIa. The FXa-dependent inhibition of TF-FVIIa activity by TFPI leads to formation of the quaternary complex TF-FVIIa-FXa-TFPI. We used site-directed fluorescence probing to map part of the region of soluble TF (sTF) that interacts with FXa in sTF-FVIIa-FXa-TFPI. We found that the C-terminal region of sTF, including positions 163, 166, 200 and 201, is involved in binding to FXa in the complex, and FXa, most likely via its Gla domain, is also in contact with the Gla domain of FVIIa in this part of the binding region. Furthermore, a region that includes the N-terminal part of the TF2 domain and the C-terminal part of the TF1 domain, i.e. the residues 104 and 197, participates in the interaction with FXa in the quaternary complex. Moreover, comparisons of the interaction areas between sTF and FX(a) in the quaternary complex sTF-FVIIa-FXa-TFPI and in the ternary complexes sTF-FVII-FXa or sTF-FVIIa-FX demonstrated large similarities.

Effect of heparin chain length on the interaction with tissue factor pathway inhibitor (TFPI)

Tissue factor pathway inhibitor (TFPI) is a heparin-binding protein involved in the extrinsic blood coagulation system. In order to elucidate the minimal size of heparin chain required for the interaction with TFPI, we prepared a series of heparin-derived oligosaccharides with tailored chain length ranged from disaccharide to eicosasaccharide after the successive treatments of heparin, including partial N-desulphation, deaminative cleavage with nitrous acid and gel-filtration. Affinity chromatography study of each oligosaccharide fraction using TFPI as the ligand indicated that increasing the degree of polymerisation causes increased affinity, and that a remarkable change in the affinity occurs between the decamers and dodecamers. Measurement of factor Xa inhibitory activity of TFPI in the presence of each oligosaccharide fraction indicated that the fractions shorter than dodecamers only slightly enhanced the TFPI activity for factor Xa inhibition, while the fractions larger than octadecamers had an effect comparable to full-length heparin. These were compatible to the results from the kinetic analyses of the interaction between TFPI and heparin-derived oligosaccharide with an evanescent wave-based biosensor system, IAsys, using a TFPI C-terminal peptide as the ligand.

The analysis of heparin-protein interactions using evanescent wave biosensor with regioselectively desulfated heparins as the ligands

Evanescent wave biosensor has been recently employed as a powerful tool for analyses of macromolecular interactions. In the present study, evanescent wave biosensor analysis was developed to analyze the heparin-protein interaction using as ligands a series of heparin derivatives regioselectively desulfated by chemical methods, particularly to evaluate the effect of each sulfate group of heparin. The method for immobilizing heparin on the cuvette of the evanescent wave biosensor equipment was optimized to obtain the high response required for accurate measurement. The best result was achieved when the amino group introduced at the reducing end of heparin was coupled with carboxymethyl dextran on the surface of the cuvette using glycolchitosan as a multivalent linker. The established system appeared to describe well the interactions of heparin with such proteins as acidic and basic fibroblast growth factors and tissue factor pathway inhibitor.

Thermodynamic linkage between the S1 site, the Na+ site, and the Ca2+ site in the protease domain of human coagulation factor xa. Studies on catalytic efficiency and inhibitor binding

The serine protease domain of factor Xa (FXa) contains a sodium as well as a calcium-binding site. Here, we investigated the functional significance of these two cation-binding sites and their thermodynamic links to the S1 site. Kinetic data reveal that Na(+) binds to the substrate bound FXa with K(d) approximately 39 mm in the absence and approximately 9.5 mm in the presence of Ca(2+). Sodium-bound FXa (sodium-Xa) has approximately 18-fold increased catalytic efficiency ( approximately 4.5-fold decrease in K(m) and approximately 4-fold increase in k(cat)) in hydrolyzing S-2222 (benzoyl-Ile-Glu-Gly-Arg-p-nitroanilide), and Ca(2+) further increases this k(cat) approximately 1.4-fold. Ca(2+) binds to the protease domain of substrate bound FXa with K(d) approximately 705 microm in the absence and approximately 175 microm in the presence of Na(+). Ca(2+) binding to the protease domain of FXa (Xa-calcium) has no effect on the K(m) but increases the k(cat) approximately 4-fold in hydrolyzing S-2222, and Na(+) further increases this k(cat) approximately 1.4-fold. In agreement with the K(m) data, sodium-Xa has approximately 5-fold increased affinity in its interaction with p-aminobenzamidine (S1 site probe) and approximately 4-fold increased rate in binding to the two-domain tissue factor pathway inhibitor; Ca(2+) (+/-Na(+)) has no effect on these interactions. Antithrombin binds to Xa-calcium with a approximately 4-fold faster rate, to sodium-Xa with a approximately 24-fold faster rate and to sodium-Xa-calcium with a approximately 28-fold faster rate. Thus, Ca(2+) and Na(+) together increase the catalytic efficiency of FXa approximately 28-fold. Na(+) enhances Ca(2+) binding, and Ca(2+) enhances Na(+) binding. Further, Na(+) enhances S1 site occupancy, and S1 site occupancy enhances Na(+) binding. Therefore, Na(+) site is thermodynamically linked to the S1 site as well as to the protease domain Ca(2+) site, whereas Ca(2+) site is only linked to the Na(+) site. The significance of these findings is that during physiologic coagulation, most of the FXa formed will exist as sodium-Xa-calcium, which has maximum biologic activity.

Inhibition of Tissue Factor-Factor VIIa-catalyzed Factor X Activation by Factor Xa-Tissue Factor Pathway Inhibitor

The physiological inhibitor of tissue factor (TF).factor VIIa (FVIIa), full-length tissue factor pathway inhibitor (TFPI(FL)) in complex with factor Xa (FXa), has a high affinity for anionic phospholipid membranes. The role of anionic phospholipids in the inhibition of TF.FVIIa-catalyzed FX activation was investigated. FXa generation at a rotating disc coated with TF embedded in a membrane composed of pure phosphatidylcholine (TF.PC) or 25% phosphatidylserine and 75% phosphatidylcholine (TF.PSPC) was measured in the presence of preformed complexes of FXa.TFPI(FL) or FXa.TFPI(1-161) (TFPI lacking the third Kunitz domain and C terminus). At TF.PC, FXa.TFPI(FL) and FXa.TFPI(1-161) showed similar rate constants of inhibition (0.07 x 10(8) M(-1) s(-1) and 0.1 x 10(8) M(-1) s(-1), respectively). With phosphatidylserine present, the rate constant of inhibition for FXa.TFPI(FL) increased 3-fold compared with a 9-fold increase in the rate constant for FXa. TFPI(1-161). Incubation of TF.PSPC with FXa.TFPI(FL) in the absence of FVIIa followed by depletion of solution FXa.TFPI(FL) showed that FXa.TFPI(FL) remained bound at the membrane and pursued its inhibitory activity. This was not observed with FXa.TFPI(1-161) or at TF.PC membranes. These data suggest that the membrane-bound pool of FXa.TFPI(FL) may be of physiological importance in an on-site regulation of TF.FVIIa activity.

Structural requirements of human tissue factor pathway inhibitor (TFPI) and heparin for TFPI-heparin interaction

Heparin affinity chromatography of synthetic peptide fragments mimicking tissue factor pathway inhibitor (TFPI) indicated that the minimal heparin binding sequence consists of 12 amino acid residues located at the C-terminal tail. Within this minimal sequence, Arg-257 and Arg-259 appeared to contribute most significantly to interaction with heparin. Affinity chromatography of TFPI using immobilized heparin derivatives regiospecifically desulfated at O-6 of the glucosamine residue, N-2 of the glucosamine residue, and/or O-2 of the iduronic acid residue indicated that all the sulfate groups in heparin appeared to be required for TFPI-heparin interaction. Among them, however, the 6-O-sulfate groups appeared to make the largest contribution to the interaction, while the 2-O-sulfate groups contributed the least. In vitro experiments on the inhibition of factor Xa by TFPI enhanced with native and chemically modified heparins afforded similar results.

Association of Tissue Factor Pathway Inhibitor With Human Umbilical Vein Endothelial Cells

Tissue factor pathway inhibitor (TFPI) is a serine protease inhibitor of the extrinsic coagulation system, synthesized in endothelial cells, which has recently been shown to play an important role in the regulation of activated coagulation factors at the endothelial cell surface. In the present study we investigated the subcellular localization and metabolism of TFPI in human umbilical vein endothelial cells (HUVEC). Immunocytochemical labeling of HUVEC with anti-TFPI showed specific labeling associated with the cell surface and with many intracellular organelles including the Golgi complex. Further characterization of these organelles was performed by colocalizing the anti-TFPI with 3-(2,4-dinitroanilino)′-amino-N-methyldipropylamine (DAMP; to demonstrate low pH), mannose phosphate receptor (endosomes), and LAMP 1 (late endocytic compartments). TFPI also colocalized with antibodies to the human transferrin receptor, a marker for early endocytic, recycling compartment. Endogenous TFPI colocalized with biotin in intracellular vesicles during endocytosis after biotinylation of the cell surface, which indicated that TFPI was being co-internalized with the surface biotin. The binding of exogenously added 125I-TFPI increased linearly to HUVEC over the concentration range of 0 to 32 nmol/L without saturation, the binding was not affected by up to a thousand-fold molar excess of unlabeled TFPI, and heparin inhibited the binding dose dependently. An intact C-terminal domain was important for the interaction between TFPI and the cell surface of HUVEC, because less than 10% of a C-terminal truncated form of TFPI (TFPI1-161 ) was bound after addition of equimolar concentrations of full-length TFPI. Exogenously added 125I-TFPI was not degraded in HUVEC during 4 hours at 37°C. The presence of TFPI in endocytic and recycling compartments support the hypothesis that endogenous, membrane-anchored TFPI could be internalized for subsequent recycling back to the cell surface.

An enzyme linked immunosorption assay for tissue factor pathway inhibitor

An assay for the quantification of full-length and carboxy-terminus truncated tissue factor pathway inhibitor (TFPI) has been developed. The assay is a classical two-antibody sandwich assay with a monoclonal capture antibody directed against the third Kunitz-type domain of human TFPI and a polyclonal rabbit peroxidase-labelled anti-human TFPI detecting antibody. The assay is sensitive to full-length and carboxy-terminus truncated TFPI with intact third Kunitz-type domain, but not to two-domain TFPI. TFPI associated with lipoproteins is not or only sparsely detected and TFPI in complex with factor Xa only partially measured. The assays gives linear reference curves in the dose range of 5 to 100 ng/ml in a double logarithmic plot. The normal range assessed from analyses on citrated plasma from 81 normal human donors is 7.8 to 26.0 ng/ml (average +/- 2 SD, log-normal distribution). There is no statistically significant difference between TFPI levels measured in 10 fasting and 10 non-fasting individuals. The reproducibility of the assay is about 5.6-5.9% (relative standard error) and the within-days and between-day reproducibilities are 4.7-5.1% and 5.9-8.5%, respectively. The assay is in very good agreement with a commercial ELISA assay recently marketed. A robust, reproducible and convenient ELISA assay for the determination of full-length and three-domain TFPI has been developed.

Tissue factor pathway inhibitor in complex with low density lipoprotein isolated from human plasma does not possess anticoagulant function in tissue factor-induced coagulation in vitro

Tissue factor pathway inhibitor (TFPI) is a potent inhibitor of the extrinsic coagulation system. In human plasma 70-85% is associated with apoB-containing lipoproteins whereas 10-20% exists in a carrier free form. The purpose of the present study was to assess the anticoagulant function of TFPI in complex with low density lipoproteins (LDL) on tissue factor (TF)-induced coagulation in vitro. LDL-TFPI complexes were isolated by preparative density gradient ultracentrifugation, LDL-free TFPI by preparative gel filtration and the anticoagulant properties were assessed by a diluted prothrombin time assay (dPT). LDL-free TFPI (0-0.46 U/ml) added to the dPT mixture, caused a prominent dose-dependent prolongation of dPT (0-42.2 sec.) which could be abolished by the addition of blocking anti-TFPI IgG. Contrary, increasing amounts of LDL-bound TFPI (0-4.0 U/ml) shortened dPT by 11.4 sec at the highest concentration. LDL-bound TFPI was not immunodetected by anti-TFPI IgG directed against the distal portion of the C-terminus, and appeared on Western blotting with a major band at 67 kDa and a weak band at 34 kDa which suggest that LDL-bound TFPI lack anticoagulant function due to carboxy terminal truncation. Our data provide evidence for the hypothesis that the anticoagulant function of TFPI is restricted to its carrier free form in human plasma.

Binding of tissue factor pathway inhibitor to cultured endothelial cells-influence of glycosaminoglycans

Tissue factor pathway inhibitor (TFPI) is mainly bound to the vessel wall and is released to circulating blood after injections of heparin. It has been suggested that the highly positively charged carboxy terminal end of heparin releasable TFPI is bound to negatively charged binding molecule(s), presumably glycosaminoglycans (GAGs), on the luminal surface of endothelial cells. The aim of the present study was to characterize this binding. Confluent monolayers of human umbilical vein endothelial cells (HUVECs) and Ea.hy926 cells were incubated with 125I-labelled recombinant TFPI (rTFPI). Two different rTFPI preparations were used in the experiments; one preparation was full-length rTFPI and one preparation was truncated at the C-terminal end (rTFPI1-161). Binding of 125I-rTFPI reached equilibrium conditions after 2 hours incubation at room temperature. Scatchard plots indicated a single class of binding sites with a mean Kd value of 164 +/- 16 nmol/L for HUVECs and a Kd value of 296 +/- 10 nmol/L for Ea.hy926 cells. The number of rTFPI binding sites per cell were approximately 1.10(7). Binding of 125I-rTFPI1-161 was non-specific. GAGs reduced binding of 125I-rTFPI in a dose-dependent manner by 50-75%. The potency of different GAGs to displace bound rTFPI was in the following order: Unfractionated heparin (UF) > low-molecular weight (LMW) heparin > hexadecasaccharides/octasaccharides/dodecasaccharides > heparan sulfate > dermatan sulfate. Treatment of the cells with heparinase III, with chondroitinase ABC lyase, or with sodium chlorate (to prevent sulfation) did not influence the binding of TFPI. We conclude that the C-terminal end is necessary for binding of TFPI to endothelial cells, but the binding is weak and does not involve GAGs.

Pharmacokinetics and delayed experimental anti-thrombotic effect of two domain non-glycosylated tissue factor pathway inhibitor

Tissue Factor Pathway Inhibitor (TFPI) is a naturally occurring inhibitor of the TF-FVIIa induced coagulation in the presence of FXa. Recombinant two domain TFPI, where Asn 117 on the FXa-inhibitory domain was exchanged to a Gln yielding non-glycosylated TFPI (117QTFPI1-161), was evaluated regarding pharmacokinetics and delayed antithrombotic potential in the rabbit. Pharmacokinetic study; 117QTFPI1-161 vs glycosylated TFPI1-161. Three rabbits/group were used and received 1,0 mg/kg a bolus iv injection. Plasma-TFPI was measured for three hours. The alpha-phase half-life was similar, the beta-phase half-life was close to four times longer for 117QTFPI1-161 (37 vs 10 min). Clearance of 117QTFPI1-161 was nearly two times lower (45 vs 21 ml/kg/min). Delayed anti-thrombotic study; 10 rabbits/group were used. 5 Groups; placebo + placebo, placebo + LMWH60 anti-Xa IU/kg, placebo + 117QTFPI1-161 0,25 mg/kg, 117QTFPI1-161 1,0 and 4,0 mg/kg + placebo. First injection 60 min prior to the second one, which coincided with the thrombus induction. The experimental thrombosis used combines a chemical destruction of the endothelium with a partial restriction of the bloodflow in the jugular veins. The thrombusweight was significantly reduced in LMWH and 117QTFPI1-161 1,0 and 4,0 mg/kg groups (0,6-2,6 vs 11,8 mg). Frequency of occlusive thrombosis was significantly reduced in the LMWH and 117QTFPI1-161 4,0 mg groups. All groups significantly effected the aXa-assay, the LMWH-group the most (0,85 IU/ml). LMWH was the only substance to prolong the dilute-PT-assay at the different timepoints. Absence of glycosylation increases the beta-phase half-life and decreases clearance of two domain TFPI. 117QTFPI1-161 (1,0 and 4,0 mg) has an antithrombotic effect indistinguishable from LMWH even though given 60 min before the thrombusinduction but with a considerable less effect on anti-Xa, APTT and no effect on dilute-PT. Glycosylation of TFPI influences the pharmacokinetics but not the antithrombotic capacity in this experimental setting.

Inhibitory properties of separate recombinant Kunitz-type-protease-inhibitor domains from tissue-factor-pathway inhibitor

Tissue-factor-pathway inhibitor (TFPI) is a multivalent inhibitor with three tandemly arranged Kunitz- type-protease-inhibitor (KPI) domains. Previous studies [Girard, Y. J., Warren, L. A., Novotny , W. F., Likert, K. M., Brown, S. G., Miletich, J. R & Broze, G. J. (1989) Nature 338, 518-520] by means of site-directed mutagenesis indicated that KPI domain 1 interacts with factor VIIa, that KPI domain 2 interacts with factor Xa, and that KPI domain 3 is apparently without inhibitory function. To elucidate the reaction mechanism of this complex inhibitor, we followed a different approach and studied the inhibitory properties of fragments of TFPI obtained by expression in yeast. Results obtained with TFPI-(1-161)-peptide and separate recombinant TFPI-KPI domains 1, 2 and 3 showed that KPI domain 1 inhibited factor VIIa/tissue factor (Ki = 250 nM), KPI domain 2 inhibited factor Xa (Ki = 90 nM), and that KPI domain 3 was without detectable inhibitory function. Studies with separate KPI domains also showed that KPI domain 2 was mainly responsible for inhibition of trypsin (Ki = 0.1 nM) and chymotrypsin (Ki = 0.75 nM), whereas KPI domain 1 inhibited plasmin (Ki = 26 nM) and cathepsin G (Ki = 200 nM). The structural basis for the interaction between serine proteases and KPI domains is discussed in terms of putative three-dimensional models of the proteins derived by comparative molecular-modelling methods. Studies of factor Xa inhibition by intact TFPI (Ki approximately 0.02 nM) suggested that regions other than the contact area of the KPI domain, interacted strongly with factor Xa. Secondary-site interactions were crucial for TFPI inhibition of factor Xa but was of little or no importance for its inhibition of trypsin.

Full-length human tissue factor pathway inhibitor inhibits human activated protein C in the presence of heparin

Previous studies have demonstrated that tissue factor pathway inhibitor (TFPI) purified from a hepatoma cell line failed to inhibit human activated protein C (APC) or human thrombin. In the present study, we have examined the ability of full-length TFPI and a truncated form of TFPI lacking the third Kunitz-type domain and C-terminal tail (TFPI1-161) to inhibit the amidolytic activity of human APC in the presence and absence of heparin. TFPI readily inhibited APC amidolytic activity only in the presence of heparin, whereas TFPI1-161 failed to inhibit APC amidolytic activity in the presence or absence of heparin. Optimal inhibition of APC by TFPI was observed at 1 U/ml heparin. The results of competition studies between factor Xa and APC for inhibition by TFPI in the presence of heparin suggested that the second Kunitz-type domain in TFPI was responsible for the inhibition of APC.

Characterization of the binding between tissue factor pathway inhibitor and glycosaminoglycans

Tissue Factor Pathway Inhibitor (TFPI) is a heparin binding protein and injection of heparin causes a release of TFPI to plasma. In order to understand the binding between TFPI and heparin in more detail we have in this study looked into some of the heparin characteristics and their importance for the TFPI-heparin interaction. We have developed an assay based on the use of heparin-Sepharose micro columns in order to compare small quantities of heparin fractions as well as different glycosaminoglycans on a weight basis for their TFPI binding. In this assay a glycosaminoglycan in solution compete with heparin-Sepharose for TFPI binding. Size fractionated heparin was analyzed for binding to TFPI, and a clear dependency on the molecular weight was observed. The highest TFPI binding capacity was found for fractions with a molecular weight above 10,000 Da, while no binding was measured below 2,000 Da. No difference in TFPI binding appeared after fractionation of heparin according to its affinity towards antithrombin, thus indicating that TFPI binding does not require the specific antithrombin binding site. A heparin fraction of 10,000 Da was fractionated on a mono Q column, resulting in four fractions with different charge densities. The charge density turned out to be a very important parameter for the binding of TFPI. A number of different glycosaminoglycans were tested and the following order of TFPI affinity was found: heparin >> dermatan sulphate > heparan sulphate > chondroitin sulphate C. No binding was observed for chondroitin sulphate A or hyaluronic acid.