Characterization of the association of tissue factor pathway inhibitor with human placenta

Intersection of regulatory pathways controlling hemostasis and hemochorial placentation

Hemochorial placentation is characterized by the development of trophoblast cells specialized to interact with the uterine vascular bed. We utilized trophoblast stem (TS) cell and mutant rat models to investigate regulatory mechanisms controlling trophoblast cell development. TS cell differentiation was characterized by acquisition of transcript signatures indicative of an endothelial cell-like phenotype, which was highlighted by the expression of anticoagulation factors including tissue factor pathway inhibitor (TFPI). TFPI localized to invasive endovascular trophoblast cells of the rat placentation site. Disruption of TFPI in rat TS cells interfered with development of the endothelial cell-like endovascular trophoblast cell phenotype. Similarly, TFPI was expressed in human invasive/extravillous trophoblast (EVT) cells situated within first-trimester human placental tissues and following differentiation of human TS cells. TFPI was required for human TS cell differentiation to EVT cells. We next investigated the physiological relevance of TFPI at the placentation site. Genome-edited global TFPI loss-of-function rat models revealed critical roles for TFPI in embryonic development, resulting in homogeneous midgestation lethality prohibiting analysis of the role of TFPI as a regulator of the late-gestation wave of intrauterine trophoblast cell invasion. In vivo trophoblast-specific TFPI knockdown was compatible with pregnancy but had profound effects at the uterine-placental interface, including restriction of the depth of intrauterine trophoblast cell invasion while leading to the accumulation of natural killer cells and increased fibrin deposition. Collectively, the experimentation implicates TFPI as a conserved regulator of invasive/EVT cell development, uterine spiral artery remodeling, and hemostasis at the maternal-fetal interface.

Intrauterine lethality in Tfpi gene disrupted mice is differentially suppressed during mid- and late-gestation by platelet TFPIα overexpression

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is an anticoagulant protein required for murine embryonic development. Intrauterine lethality of Tfpi-/- mice occurs at mid- and late gestation, the latter of which is associated with severe cerebrovascular defects. Megakaryocytes produce only the TFPIα isoform, which is stored within platelets and released upon activation.

OBJECTIVES

To examine biological activities of platelet TFPIα (pTFPIα) by characterizing effects of pTFPIα overexpression in Tfpi-/- mice.

METHODS

Transgenic mice overexpressing pTFPIα were generated and crossed onto the Tfpi-/- background. Genetic and histological analyses of embryos were performed to investigate the function of pTFPIα during embryogenesis.

RESULTS

The transgene (Tg) increased pTFPIα 4- to 5-fold without altering plasma TFPI in adult Tfpi+/+ and Tfpi+/- mice but did not rescue Tfpi-/- mice to wean. Analyses of the impact of pTFPIα overexpression on Tfpi-/- survival, however, were complicated by linkage between the Tg integration site and the endogenous Tfpi locus on chromosome 2. Strain-specific genetic interactions also modulated Tfpi-/- embryonic survival. After accounting for these underlying genetic factors, pTFPIα overexpression completely suppressed mid-gestational lethality of Tfpi-/- embryos but had no effect on development of cerebrovascular defects during late gestation resulting in their lack of survival to wean.

CONCLUSIONS

pTFPIα overexpression rescued Tfpi-/- embryos from mid-gestational but not late gestational lethality. The prevalence of underlying genetic factors complicating analyses within our study illustrates the importance of meticulously characterizing transgenic mouse models to avoid spurious interpretation of results.

Maintaining extraembryonic expression allows generation of mice with severe tissue factor pathway inhibitor deficiency

Tissue factor pathway inhibitor (TFPI) is a serine protease with multiple anticoagulant activities. The Kunitz1 (K1) domain of TFPI binds the active site of factor VIIa and is required for inhibition of tissue factor (TF)/factor VIIa catalytic activity. Mice lacking TFPI K1 domain die in utero. TFPI is highly expressed on trophoblast cells of the placenta. We used genetic strategies to selectively ablate exon 4 encoding TFPI K1 domain in the embryo, while maintaining expression in trophoblast cells. This approach resulted in expected Mendelian frequency of TFPI K1 domain-deficient mice. Real-time polymerase chain reaction confirmed 95% to 99% genetic deletion and a similar reduction in transcript expression. Western blotting confirmed the presence of a truncated protein instead of full-length TFPI. Mice with severe TFPI K1 deficiency exhibited elevated thrombin-antithrombin (TAT) levels, frequent fibrin deposition in renal medulla, and increased susceptibility to TF-induced pulmonary embolism. They were fertile, and most lived normal life spans without any overt thrombotic events. Of 43 mice observed, 2 displayed extensive brain ischemia and infarction. We conclude that in contrast to complete absence of TFPI K1 domain, severe deficiency is compatible with in utero development, adult survival, and reproductive functions in mice. Inhibition of TFPI activity is being evaluated as a means of boosting thrombin generation in hemophilia patients. Our results show that in mice severe reduction of TFPI K1 activity is associated with a prothrombotic state without overt developmental outcomes. We note fibrin deposits in the kidney and rare cases of brain ischemia.

Re-evaluation of mouse tissue factor pathway inhibitor and comparison of mouse and human tissue factor pathway inhibitor physiology

Essentials Mouse models are often used to define roles of tissue factor pathway inhibitor (TFPI) in man. TFPI isoform-specific KOs reveal unexpected differences between mouse and human TFPI physiology. Mouse plasma contains 20 times more TFPI than man, derived from TFPIγ, a form not found in man. TFPIγ null mice, expressing only TFPI isoforms α and β, may better reflect the human situation. SUMMARY: Background Mouse models can provide insight into the pathophysiology of human thrombosis and hemostasis. Tissue factor pathway inhibitor (TFPI) regulates coagulation through protein S (PS)-enhanced factor (F) Xa inhibition and FXa-dependent inhibition of FVIIa/tissue factor (TF) activity. TFPI is expressed as isoforms α and β in man, and α, β and γ in the mouse. Objective Assess the reliability of extending TFPI-related studies in mice to humans. Method Compare mouse and human TFPI physiology using a variety of methods. Results Mouse TFPI and human TFPI are similar in regard to: (i) the mechanisms for FVIIa/TF and FXa inhibition; (ii) TFPIα is a soluble form and TFPIβ is glycosyl phosphatidyl inositol (GPI) membrane anchored; (iii) the predominant circulating form of TFPI in plasma is lipoprotein-associated; (iv) low levels of TFPIα circulate in plasma and increase following heparin treatment; and (v) TFPIα is the isoform in platelets. They differ in that: (i) mouse TFPI circulates at a ~20-fold higher concentration; (ii) mouse lines with isolated isoform deletions show this circulating mouse TFPI is derived from TFPIγ; (iii) sequences homologous to the mouse TFPIγ exon are present in many species, including man, but in primates are unfavorable for splicing; and (iv) tandem mass spectrometry (MS/MS) detects sequences for TFPI isoforms α and β in human plasma and α and γ in mouse plasma. Conclusion To dissect the pathophysiological roles of human TFPIα and TFPIβ, studies in TFPIγ null mice, expressing only α and β, only α or only β should better reflect the human situation.

A balance between TFPI and thrombin-mediated platelet activation is required for murine embryonic development

Tissue factor pathway inhibitor (TFPI) is a critical anticoagulant protein present in endothelium and platelets. Mice lacking TFPI (Tfpi(-/-)) die in utero from disseminated intravascular coagulation. They are rescued by concomitant tissue factor (TF) deficiency, demonstrating that TFPI modulates TF function in vivo. Recent studies have found TFPI inhibits prothrombinase activity during the initiation of coagulation and limits platelet accumulation during thrombus formation, implicating TFPI in modulating platelet procoagulant activity. To examine whether altered platelet function would compensate for the lack of TFPI and rescue TFPI-null embryonic lethality, Tfpi(+/-) mice lacking the platelet thrombin receptor, protease activated receptor 4 (PAR4; Par4(-/-)), or its coreceptor, PAR3, were mated. PAR3 deficiency did not rescue Tfpi(-/-) embryos, but >40% of expected Tfpi(-/-):Par4(-/-) offspring survived to adulthood. Adult Tfpi(-/-):Par4(-/-) mice did not exhibit overt thrombosis. However, they had focal sterile inflammation with fibrin(ogen) deposition in the liver and elevated plasma thrombin-antithrombin complexes, indicating activation of coagulation at baseline. Tfpi(-/-):Par4(-/-) mice have platelet and fibrin accumulation similar to Par4(-/-) mice following venous electrolytic injury but were more susceptible than Par4(-/-) mice to TF-induced pulmonary embolism. In addition, ∼30% of the Tfpi(-/-):Par4(-/-) mice were born with short tails. Tfpi(-/-):Par4(-/-) mice are the first adult mice described that lack TFPI with unaltered TF. They demonstrate that TFPI physiologically modulates thrombin-dependent platelet activation in a manner that is required for successful embryonic development and identify a role for TFPI in dampening intravascular procoagulant stimuli that lead to thrombin generation, even in the absence of thrombin-mediated platelet activation.

Syndecan-3 and TFPI colocalize on the surface of endothelial-, smooth muscle-, and cancer cells

Background

Tissue factor (TF) pathway inhibitor (TFPI) exists in two isoforms; TFPIα and TFPIβ. Both isoforms are cell surface attached mainly through glycosylphosphatidylinositol (GPI) anchors. TFPIα has also been proposed to bind other surface molecules, like glycosaminoglycans (GAGs). Cell surface TFPIβ has been shown to exert higher anticoagulant activity than TFPIα, suggesting alternative functions for TFPIα. Further characterization and search for novel TFPI binding partners is crucial to completely understand the biological functions of cell associated TFPI.

Methods and Results

Potential association of TFPI to heparan sulphate (HS) proteoglycans in the syndecan family were evaluated by knock down studies and flow cytometry analysis. Cell surface colocalization was assessed by confocal microscopy, and native PAGE or immunoprecipitation followed by Western blotting was used to test for protein interaction. Heparanase was used to enzymatically degrade cell surface HS GAGs. Anticoagulant potential was evaluated using a factor Xa (FXa) activity assay. Knock down of syndecan-3 in endothelial,- smooth muscle- and breast cancer cells reduced the TFPI surface levels by 20-50%, and an association of TFPIα to syndecan-3 on the cell surface was demonstrated. Western blotting indicated that TFPIα was found in complex with syndecan-3. The TFPI bound to syndecan-3 did not inhibit the FXa generation. Removal of HS GAGs did not release TFPI antigen from the cells.

Conclusions

We demonstrated an association between TFPIα and syndecan-3 in vascular cells and in cancer cells, which did not appear to depend on HS GAGs. No anticoagulant activity was detected for the TFPI associated with syndecan-3, which may indicate coagulation independent functions for this cell associated TFPI pool. This will, however, require further investigation.

Biology of tissue factor pathway inhibitor

Recent studies of the anticoagulant activities of the tissue factor (TF) pathway inhibitor (TFPI) isoforms, TFPIα and TFPIβ, have provided new insight into the biochemical and physiological mechanisms that underlie bleeding and clotting disorders. TFPIα and TFPIβ have tissue-specific expression patterns and anticoagulant activities. An alternative splicing event in the 5' untranslated region allows for translational regulation of TFPIβ expression. TFPIα has 3 Kunitz-type inhibitor domains (K1, K2, K3) and a basic C terminus, whereas TFPIβ has the K1 and K2 domains attached to a glycosylphosphatidyl inositol-anchored C terminus. TFPIα is the only isoform present in platelets, whereas endothelial cells produce both isoforms, secreting TFPIα and expressing TFPIβ on the cell surface. TFPIα and TFPIβ inhibit both TF-factor VIIa-dependent factor Xa (FXa) generation and free FXa. Protein S enhances FXa inhibition by TFPIα. TFPIα produces isoform-specific inhibition of prothrombinase during the initiation of coagulation, an anticoagulant activity that requires an exosite interaction between its basic C terminus and an acidic region in the factor Va B domain. Platelet TFPIα may be optimally localized to dampen initial thrombin generation. Similarly, endothelial TFPIβ may be optimally localized to inhibit processes that occur when endothelial TF is present, such as during the inflammatory response.

Translation of human tissue factor pathway inhibitor-β mRNA is controlled by alternative splicing within the 5' untranslated region

OBJECTIVE

Tissue factor pathway inhibitor (TFPI) blocks the initiation of coagulation by inhibiting TF-activated factor VII, activated factor X, and early prothrombinase. Humans produce two 3' splice variants, TFPIα and TFPIβ, which are differentially expressed in endothelial cells and platelets and possess distinct structural features affecting their inhibitory function. TFPI also undergoes alternative splicing of exon 2 within its 5' untranslated region. The role of exon 2 splicing in translational regulation of human TFPI isoform expression is investigated.

APPROACH AND RESULTS

Exon 2 splicing occurs in TFPIα and TFPIβ transcripts. Human tissue mRNA analysis uncovered a wide variability of exon 2 expression. Polysome analysis revealed a repressive effect of exon 2 on TFPIβ translation but not on TFPIα. Luciferase reporter assays further exposed strong translational repression of TFPIβ (90%) but not TFPIα. Use of a Morpholino to remove exon 2 from TFPI mRNA increased cell surface expression of endogenous TFPIβ. Exon 2 also repressed luciferase production (80% to 90%) when paired with the β-actin 3' untranslated region, suggesting that it is a general translational negative element whose effects are overcome by the TFPIα 3' untranslated region.

CONCLUSIONS

Exon 2 is a molecular switch that prevents translation of TFPIβ. This is the first demonstration of a 5' untranslated region alternative splicing event that alters translation of isoforms produced via independent 3' splicing events within the same gene. Therefore, it represents a previously unrecognized mechanism for translational control of protein expression. Differential expression of exon 2 denotes a mechanism to provide temporal and tissue-specific regulation of TFPIβ-mediated anticoagulant activity.

Comparison of the inhibitory activities of human tissue factor pathway inhibitor (TFPI)α and TFPIβ

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is an alternatively spliced protein with two isoforms, TFPIα and TFPIβ, which differ in their C-terminal structure and cellular localization. Detailed characterization of their inhibitory activity is needed to define potentially unique inhibitory roles in tissue factor (TF)-mediated thrombotic and inflammatory disease, and to understand how pharmaceuticals targeted to different structural regions of the TFPI isoforms alter hemostasis in hemophilia patients.

METHODS

The TF inhibitory activity of TFPIβ localized to the surface of CHO cells was compared with that of soluble TFPIα by the use of in vitro and in vivo assays.

RESULTS

In TF-factor VIIa-mediated FXa generation assays, TFPIβ was a slightly better inhibitor than TFPIα, which was approximately three-fold better than TFPI-160, a soluble, altered form of TFPI similar to TFPIβ. In direct FXa inhibitory assays, TFPIβ had an IC50 2.5-fold lower than that of TFPIα and 56-fold lower than that of TFPI-160. TFPIβ inhibited TF-mediated CHO cell migration though Matrigel, whereas TFPIα and TFPI-160 were poor inhibitors, demonstrating that TFPIβ effectively blocks TF-initiated signaling events during cellular migration through matrices that are not permeable to soluble forms of TFPI. Furthermore, TFPIβ inhibited TF-dependent CHO cell infiltration into lung tissue following tail vein injection into SCID mice, and blocked the development of consumptive coagulopathy.

CONCLUSIONS

TFPIβ is a slightly better inhibitor of TF procoagulant activity than TFPIα. As a surface-associated protein, TFPIβ is a much better inhibitor of TF-mediated cellular migration than soluble TFPIα, and may specifically act in the inhibition of TF-mediated signaling events on inflamed endothelium and/or monocytes.

TFPIα and TFPIβ are expressed at the surface of breast cancer cells and inhibit TF-FVIIa activity

BACKGROUND

Tissue factor (TF) pathway inhibitor-1 (TFPI) is expressed in several malignant tissues- and cell lines and we recently reported that it possesses anti-tumor effects in breast cancer cells, indicating a biological role of TFPI in cancer. The two main splice variants of TFPI; TFPIα and TFPIβ, are both able to inhibit TF-factor VIIa (FVIIa) activity in normal cells, but only TFPIα circulates in plasma. The functional importance of TFPIβ is therefore largely unknown, especially in cancer cells. We aimed to characterize the expression and function of TFPIα, TFPIβ, and TF in a panel of tumor derived breast cancer cell lines in comparison to normal endothelial cells.

METHODS

TFPIα, TFPIβ, and TF mRNA and protein measurements were conducted using qRT-PCR and ELISA, respectively. Cell-associated TFPI was detected after phosphatidylinositol-phospholipase C (PI-PLC) and heparin treatment by flow cytometry, immunofluorescence, and Western blotting. The potential anticoagulant activity of cell surface TFPI was determined in a factor Xa activity assay.

RESULTS

The expression of both isoforms of TFPI varied considerably among the breast cancer cell lines tested, from no expression in Sum149 cells to levels above or in the same range as normal endothelial cells in Sum102 and MDA-MB-231 cells. PI-PLC treatment released both TFPIα and TFPIβ from the breast cancer cell membrane and increased TF activity on the cell surface, showing TF-FVIIa inhibitory activity of the glycosylphosphatidylinositol- (GPI-) anchored TFPI. Heparin treatment released TFPIα without decreasing the cell surface levels, thus indicating the presence of intracellular storage pools of TFPIα in the breast cancer cells.

CONCLUSION

GPI-attached TFPI located at the surface of breast cancer cells inhibited TF activity and could possibly reduce TF signaling and breast cancer cell growth locally, indicating a therapeutic potential of the TFPIβ isoform.

Tissue factor pathway inhibitor is an inhibitor of factor VII-activating protease

BACKGROUND

Factor VII-activating protease (FSAP) is a serine protease that circulates in plasma in its inactive single-chain form and can be activated upon contact with dead cells. When activated by apoptotic cells, FSAP leads to the release of nucleosomes. The serpins C1-inhibitor and α(2) -antiplasmin are reported to be the major inhibitors of FSAP. However, regulation of FSAP activity by Kunitz-type inhibitors is not well studied.

OBJECTIVES

To compare the inhibition of FSAP activity and FSAP-induced nucleosome release from apoptotic cells by tissue factor pathway inhibitor (TFPI) with that of C1-inhibitor and α(2) -antiplasmin.

METHODS

Apoptotic cells were incubated with plasma or FSAP in presence of the inhibitor, and nucleosome release was analyzed with flow cytometry. Monoclonal antibodies against TFPI and altered forms of TFPI were used to investigate which domains of TFPI contribute to FSAP inhibition.

RESULTS AND CONCLUSIONS

We show that TFPI abrogates FSAP activity and nucleosome release from apoptotic cells. TFPI is a much more efficient inhibitor than C1-inhibitor or α(2) -antiplasmin. The active site of K2 is required for inhibition of FSAP. A direct binding interaction between FSAP and the C-terminal domain of TFPI is also required for efficient inhibition. Inhibition of FSAP-induced nucleosome release by recombinant TFPI might, in part, explain the anti-inflammatory effects of recombinant TFPI infusion observed in animal and human sepsis.

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.

Tissue factor pathway inhibitor: structure-function

TFPI is a multivalent, Kunitz-type proteinase inhibitor, which, due to alternative mRNA splicing, is transcribed in three isoforms: TFPIalpha, TFPIdelta, and glycosyl phosphatidyl inositol (GPI)-anchored TFPIbeta. The microvascular endothelium is thought to be the principal source of TFPI and TFPIalpha is the predominant isoform expressed in humans. TFPIalpha, apparently attached to the surface of the endothelium in an indirect GPI-anchor-dependent fashion, represents the greatest in vivo reservoir of TFPI. The Kunitz-2 domain of TFPI is responsible for factor Xa inhibition and the Kunitz-1 domain is responsible for factor Xa-dependent inhibition of the factor VIIa/tissue factor catalytic complex. The anticoagulant activity of TFPI in one-stage coagulation assays is due mainly to its inhibition of factor Xa through a process that is enhanced by protein S and dependent upon the Kunitz-3 and carboxyterminal domains of full-length TFPIalpha. Carboxyterminal truncated forms of TFPI as well as TFPIalpha in plasma, however, inhibit factor VIIa/tissue factor in two-stage assay systems. Studies in gene-disrupted mice demonstrate the physiological importance of TFPI.

TFPIβ is the GPI-anchored TFPI isoform on human endothelial cells and placental microsomes

Tissue factor pathway inhibitor (TFPI) produces factor Xa-dependent feedback inhibition of factor VIIa/tissue factor-induced coagulation. Messages for 2 isoforms of TFPI have been identified. TFPIα mRNA encodes a protein with an acidic N-terminus, 3 Kunitz-type protease inhibitor domains and a basic C-terminus that has been purified from plasma and culture media. TFPIβ mRNA encodes a form in which the Kunitz-3 and C-terminal domains of TFPIα are replaced with an alternative C-terminus that directs the attachment of a glycosylphosphatidylinositol (GPI) anchor, but whether TFPIβ protein is actually expressed is not clear. Moreover, previous studies have suggested that the predominant form of TFPI released from cells by phosphatidylinositol-specific phospholipase C (PIPLC) treatment is TFPIα, implying it is bound at cell surfaces to a separate GPI-anchored coreceptor. Our studies show that the form of TFPI released by PIPLC treatment of cultured endothelial cells and placental microsomes is actually TFPIβ based on (1) migration on SDS-PAGE before and after deglycosylation, (2) the lack of a Kunitz-3 domain, and (3) it contains a GPI anchor. Immunoassays demonstrate that, although endothelial cells secrete TFPIα, greater than 95% of the TFPI released by PIPLC treatment from the surface of endothelial cells and from placental microsomes is TFPIβ.

Is heparin a placental anticoagulant in high-risk pregnancies?

Randomized control trials show beneficial effects of heparin in high-risk pregnancies to prevent preeclampsia and intrauterine growth restriction. However, the lack of placental pathology data in these trials challenges the assumption that heparin is a placental anticoagulant. Recent data show that placental infarction is probably associated with abnormalities in development of the placenta, characterized by poor maternal perfusion and an abnormal villous trophoblast compartment in contact with maternal blood, than with maternal thrombophilia. At-risk pregnancies may therefore be predicted by noninvasive prenatal testing of placental function in mid-pregnancy. Heparin has diverse cellular functions that include direct actions on the trophoblast. Dissecting the non-anticoagulant actions of heparin may indicate novel and safer therapeutic targets to prevent the major placental complications of pregnancy.

Alternatively spliced tissue factor pathway inhibitor: functional implications

Tissue factor pathway inhibitor (TFPI) is a factor Xa dependent inhibitor of tissue factor initiated blood coagulation. In recent years several alternatively spliced forms of TFPI have been identified. These alternatively spliced forms have different C-terminal regions and have different mechanisms for association with cell surfaces. They are differentially expressed in human and mouse tissues and may have distinct physiological functions.

Association of Asn221Ser mutation in tissue factor pathway inhibitor-beta with plasma total tissue factor pathway inhibitor level

Tissue factor pathway inhibitor (TFPI) is an anticoagulant protease inhibitor that inhibits the tissue factor-initiated blood coagulation cascade reactions. Based on these anticoagulant functions of TFPI, we hypothesized that genetic variations in TFPI may alter the TFPI expression or impair the anticoagulant function and could predispose persons to deep vein thrombosis (DVT). This study was undertaken to examine whether the genetic polymorphisms in TFPI are associated with the plasma TFPI levels and risk for DVT. We sequenced the entire coding regions of TFPI in 175 Japanese DVT patients and identified 12 genetic variants, including one missense mutation, Asn221Ser. The missense mutation occurred at the site presumably attached to the glycosylphosphatidylinositol anchor in the TFPI-beta form. The allele frequency of the mutant Ser-coding allele of the Asn221Ser mutation was 8% in the Japanese general population consisting of 1684 individuals. The Asn221Ser mutation was significantly associated with the total TFPI levels (Asn/Asn, n = 108, total TFPI = 56.57 +/- 0.88 ng/ml (mean +/- SD) vs. Asn/Ser + Ser/Ser, n = 16, total TFPI = 63.44 +/- 2.28 ng/ml, P = 0.0058). The genotype was not associated with the free TFPI level. This Asn221Ser mutation was not associated with DVT. Thus, the Asn221Ser mutation occurring in the TFPI-beta form was associated with the total TFPI level, but not a risk for DVT. The absence of the putative glycosylphosphatidylinositol anchor in TFPI-beta under pathological conditions remains to be studied.

Alternatively spliced isoforms of tissue factor pathway inhibitor

Tissue factor pathway inhibitor (TFPI) is the major regulator of tissue factor (TF)-induced coagulation. It down regulates coagulation by binding to the TF/fVIIa complex in a fXa dependent manner. It is predominantly produced by microvascular endothelial cells, though it is also found in platelets, monocytes, smooth muscle cells, and plasma. Its physiological importance is demonstrated by the embryonic lethality observed in TFPI knockout mice and by the increase in thrombotic burden that occurs when heterozygous TFPI mice are bred with mice carrying genetic risk factors for thrombotic disease, such as factor V Leiden. Multiple TFPI isoforms, termed TFPIalpha, TFPIbeta, and TFPIdelta in humans and TFPIalpha, TFPIbeta, and TFPIgamma in mice, have been described, which differ in their domain structure and method for cell surface attachment. A significant functional difference between these isoforms has yet to be described in vivo. Both human and mouse tissues produce, on average, approximately 10 times more TFPIalpha message when compared to that of TFPIbeta. Consistent with this finding, several lines of evidence suggest that TFPIalpha is the predominant protein isoform in humans. In contrast, recent work from our laboratory demonstrates that TFPIbeta is the major protein isoform produced in adult mice, suggesting that TFPI isoform production is translationally regulated.

Temporal expression of alternatively spliced forms of tissue factor pathway inhibitor in mice

BACKGROUND

Mouse tissue factor pathway inhibitor (TFPI) is produced in three alternatively spliced isoforms that differ in domain structure and mechanism for cell surface binding. Tissue expression of TFPI isoforms in mice was characterized as an initial step for identification of their physiological functions.

METHODS AND RESULTS

Sequence homology demonstrates that TFPIalpha existed over 430 Ma while TFPIbeta and TFPIgamma evolved more recently. In situ hybridization studies of heart and lung did not reveal any cells exclusively expressing a single isoform. Although our previous studies have demonstrated that TFPIalpha mRNA is more prevalent than TFPIbeta or TFPIgamma mRNA in mouse tissues, western blot studies demonstrated that TFPIbeta is the primary protein isoform produced in adult tissues, while TFPIalpha is expressed during embryonic development and in placenta. Consistent with TFPIbeta as the primary isoform produced within adult vascular beds, the TFPI isoform in mouse plasma migrates like TFPIbeta in SDS-PAGE and mice have a much smaller heparin-releasable pool of plasma TFPIalpha than humans.

CONCLUSIONS

The data demonstrate that alternatively spliced isoforms of TFPI are temporally expressed in mouse tissues at the level of protein production. TFPIalpha and TFPIbeta are produced in embryonic tissues and in placenta while adult tissues produce almost exclusively TFPIbeta.

Expression of tissue factor pathway inhibitor by endothelial cells and platelets

Tissue factor pathway inhibitor (TFPI) is a potent anticoagulant protein that abrogates the activity of the tissue factor-factor VIIa catalytic complex that activates blood coagulation in vivo. The importance of TFPI in the regulation of blood coagulation is emphasized by how its activity is modulated in human disease. Decreased TFPI activity contributes to the development of both arterial and venous thrombosis and has been implicated in the thrombotic events occurring in women using oral contraceptives and in patients with paroxysmal nocturnal hemoglobinuria. Both endothelial cells and platelets produce TFPI. Our laboratory is interested in the mechanisms for expression of TFPI on the surface of these cells to better understand how TFPI prevents intravascular thrombosis. Studies of cultured endothelial cells and human placenta have demonstrated that TFPI associates with the cell surface through a glycosyl phosphatidyinositol (GPI)-anchor in a manner that is not dependent on GAGs or altered by heparin. TFPI is not directly bound to the GPI-anchor; instead it appears to bind tightly to a GPI-anchored protein. This GPI-anchored protein appears to be necessary for proper trafficking of TFPI to the cell surface. An alternatively spliced form of TFPI, TFPIbeta, is a truncated form of TFPI that is directly attached to a GPI-anchor. However, it is not clear that human endothelial cells produce TFPIbeta. Platelets produce TFPI but not TFPIbeta. TFPI is expressed on the platelet surface following dual activation with collagen plus thrombin, but not through a GPI-anchor. Studies using mouse models of TFPI deficiency are currently being conducted in our laboratory to determine if distinct physiological functions of endothelial and platelet TFPI exist in vivo.

Further insight into the heparin-releasable and glycosylphosphatidylinositol-lipid--anchored forms of tissue factor pathway inhibitor

The release of tissue factor pathway inhibitor (TFPI) from human umbilical vein endothelial cells (HUVECs) was investigated using heparin and phospholipase C. The experiment included incubating HUVECs with 0, 1, or 10 U/mL heparin diluted in Dulbecco Modified Eagle's Medium plus 5% fetal calf serum for 1 or 24 hours. A statistically significant increase in TFPI activity levels was seen at 1 hour, but not at 24 hours. A 20-fold increase in the release of TFPI after phospholipase C treatment of HUVECs was demonstrated, confirming that it is glycosylphosphatidylinositol-lipid (GPI) anchored. Sequential treatment of HUVECs with phospholipase C and heparin was performed, and a trend was observed where GPI-anchored TFPI levels were increased after 1 hour of pretreatment with heparin but were decreased after 24 hours. Serum is a requirement for the heparin-dependent release of TFPI from HUVECs. Heparin pretreatment of HUVECs may affect levels of GPI anchored TFPI in a time and dose-dependent manner.

Sepsis-induced coagulation in the baboon lung is associated with decreased tissue factor pathway inhibitor

Increased tissue factor (TF)-dependent procoagulant activity in sepsis may be partly due to decreased expression or function of tissue factor pathway inhibitor (TFPI). To test this hypothesis, baboons were infused with live Escherichia coli and sacrificed after 2, 8, or 24 hours. Confocal and electron microscopy revealed increased leukocyte infiltration and fibrin deposition in the intravascular and interstitial compartments. Large amounts of TF were detected by immunostaining in leukocytes and platelet-rich microthrombi. TF induction was documented by quantitative reverse transcriptase-polymerase chain reaction, enzyme-linked immunosorbent assay, and coagulation assays. Lung-associated TFPI antigen and mRNA decreased during sepsis, and TFPI activity diminished abruptly at 2 hours. Blocking antibodies against TFPI increased fibrin deposition in septic baboon lungs, suggesting that TF-dependent coagulation might be aggravated by reduced endothelial TFPI. Decreased TFPI activity coincided with the release of tissue plasminogen activator and the peak of plasmin generation, suggesting that TFPI could undergo proteolytic inactivation by plasmin. Enhanced plasmin produced in septic baboons by infusion of blocking antibodies against plasminogen activator inhibitor-1 led to decreased lung-associated TFPI and unforeseen massive fibrin deposition. We conclude that activation of TF-driven coagulation not adequately countered by TFPI may underlie the widespread thrombotic complications of sepsis.

Active tissue factor pathway inhibitor is expressed on the surface of coated platelets

The incorporation of blood-borne forms of tissue factor (TF) into a growing blood clot is necessary for normal fibrin generation and stabilization of the blood clot. Tissue factor pathway inhibitor (TFPI) is the primary physiologic inhibitor of tissue factor and is present within platelets. Expression of TFPI on the platelet surface may be the optimal location for it to abrogate blood-borne TF activity that incorporates within the blood clot, balancing the need for adequate hemostasis while preventing development of occlusive thrombosis. TFPI is produced by megakaryocytes but is not expressed on the platelet surface. Activation of platelets with thrombin receptor activation peptide does not cause release or surface expression of TFPI, demonstrating that TFPI is not stored within platelet alpha granules. TFPI is expressed on the platelet surface following dual-agonist activation with convulxin plus thrombin to produce coated platelets. In association with its expression on the surface of coated platelets TFPI is also released in microvesicles or as a soluble protein.

Postprandial thrombin activatable fibrinolysis inhibitor and markers of endothelial dysfunction in type 2 diabetic patients

The aim of this study was to assess postprandial changes in thrombin activatable fibrinolysis inhibitor (TAFI) antigen, a thrombin-dependent fibrinolysis inhibitor with anti-inflammatory properties, and soluble markers of endothelial dysfunction in normotriglyceridemic type 2 diabetic patients. Fasting and postprandial TAFI antigen, thrombomodulin, tissue factor pathway inhibitor (TFPI), and plasminogen activator inhibitor 1 were assessed in 12 normotriglyceridemic type 2 diabetic patients treated with diet (hemoglobin A1c, 6.80% +/- 0.67%) and 14 controls. Fasting low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, free fatty acids and apolipoprotein B, and fasting and postprandial triglyceride, glucose, and insulin were also measured. Fasting TAFI was higher in the control group (102% +/- 16.9% vs 72.9% +/- 15.9%; P < .0005) and was inversely correlated with glycemic control. It decreased 4 hours after the meal (31.8% reduction [P < .005] for controls and 12.6% [P < .05] for diabetic patients) and returned to fasting levels after 8 hours. This decrement was correlated with fasting TAFI, glucose and hemoglobin A1c, and the area under the curve of glucose. Thrombomodulin, TFPI, and plasminogen activator inhibitor 1 were similar in both groups, with thrombomodulin and TFPI showing a transient postprandial increase. A fat-rich meal produces a transient increase in markers of endothelial dysfunction and a temporary reduction in TAFI, an anti-inflammatory molecule whose concentration is low in type 2 diabetes mellitus.

Relative quantification of glycosaminoglycan-induced upregulation of TFPI-mRNA expression in vitro

OBJECTIVE

Tissue factor pathway inhibitor (TFPI) is a multivalent Kunitz-type serine proteinase inhibitor that plays a central role in the extrinsic pathway of blood coagulation and is mainly expressed by endothelial cells. In this study we examined the in vitro effects of heparin and other glycosaminoglycans on TFPI mRNA-expression in cultivated human endothelial (Ea.hy 926) and in chondrosarcoma (SW 1353) cells.

METHODS

We used a LightCycler-based method for relative quantification of the TFPI-mRNA expression before and after stimulation. The cells were stimulated with different concentrations of heparin (with and without addition of protamin), heparan sulfate (HS) and chondroitin-6-sulfate (CS). Cells were harvested after incubation times of 4, 8 and 24h, total RNA was isolated, and cDNA was synthesized and quantified relatively to a constantly expressed housekeeping gene.

RESULTS

Stimulation of Ea.hy 926 cells with unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) caused a time- and dose-dependent upregulation of TFPI-mRNA expression with LMWH showing the stronger effect. In contrast to this, HS led to a strongly and CS to a slightly decreased TFPI-mRNA expression. SW 1353 cells which were stimulated with LMWH/UFH and HS/CS did not show a significant up- or downregulative effect.

CONCLUSION

Our results show that we have developed a versatile method for the relative quantification of TFPI-mRNA expression. As a conclusion, the determined heparin-induced upregulation of TFPI-mRNA expression can be considered a major component of the modulation of the anticoagulant properties of the endothelium.

Disposition of tissue factor pathway inhibitor during cardiopulmonary bypass

BACKGROUND

The tissue factor (TF) factor (F) VIIa complex activates coagulation FIX and FX to initiate coagulation, and also cleaves protease activated receptors (PARs) to initiate inflammatory processes in vascular cells. Tissue factor pathway inhibitor (TFPI) is the only specific inhibitor of the TF-FVIIa complex, regulating both its procoagulant and pro-inflammatory properties. Upon heparin infusion during cardiopulmonary bypass (CPB), a heparin releasable pool of endothelial associated TFPI circulates in plasma. Following protamine neutralization of heparin, the plasma TFPI level decreases, but does not return completely to baseline, suggesting that during CPB a fraction of the plasma TFPI becomes heparin-independent. We have investigated the structural and functional properties of plasma TFPI during CPB to further characterize how TFPI is altered during this procedure.

METHODS

We enrolled 17 patients undergoing first-time cardiac surgery involving CPB. Plasma samples were obtained at baseline, 5 min and 1 h after start of CPB (receiving heparin), 10 min after protamine administration (off CPB) and 24 h following surgery. Samples were analyzed for full-length and free (non-lipoprotein bound) TFPI antigen by enzyme-linked immunosorbent assay (ELISA) and for TFPI anticoagulant activity using an amidolytic assay. Western blot analysis was used to identify TFPI species of varying molecular weights in three additional patients. Dunnett's test for post hoc comparisons was used for statistical analysis.

RESULTS

The ELISA and Western blot data indicated that an increase in full-length TFPI accounted for most of the heparin releasable TFPI. Following heparin neutralization with protamine, the full-length TFPI antigen returned to baseline levels while the free TFPI antigen and the total plasma TFPI activity remained elevated. This was associated with the appearance of a new 38 kDa form of plasma TFPI identified by Western blot analysis. The 38 kDa form of TFPI did not react with an antibody directed against the C-terminal region of TFPI indicating it has undergone proteolysis within this region. All TFPI measurements returned to baseline 24 h following CPB.

CONCLUSIONS

During CPB the full-length form of TFPI is the predominant form in plasma because of its prompt release from the endothelial surface following heparin administration. Upon heparin neutralization with protamine, full-length TFPI redistributes back to the endothelial surface. However, a new 38 kDa TFPI fragment is generated during CPB and remains circulating in plasma, indicating that TFPI undergoes proteolytic degradation during CPB. This degradation may result in a decrease in endothelium-associated TFPI immediately post-CPB, and may contribute to the procoagulant and proinflammatory state that often complicates CPB.

A GPI-anchored co-receptor for tissue factor pathway inhibitor controls its intracellular trafficking and cell surface expression

BACKGROUND

Tissue factor pathway inhibitor (TFPI) lacks a membrane attachment signal but it remains associated with the endothelial surface via its association with an, as yet, unidentified glycosyl phosphatidylinositol (GPI)-anchored co-receptor.

OBJECTIVES/METHODS

Cellular trafficking of TFPI within aerolysin-resistant ECV304 and EA.hy926 cells, which do not express GPI-anchored proteins on their surface, was compared with their wild-type counterparts.

RESULTS AND CONCLUSIONS

Although aerolysin-resistant cells produce normal amounts of TFPI mRNA, TFPI is not expressed on the cell surface and total cellular TFPI is greatly decreased compared with wild-type cells. Additionally, normal, not increased, amounts of TFPI are secreted into conditioned media indicating that TFPI is degraded within the aerolysin-resistant cells. Confocal microscopy and studies using metabolic inhibitors demonstrate that aerolysin-resistant cells produce TFPI and transport it into the Golgi with subsequent degradation in lysosomes. The experimental results provide no evidence that cell surface TFPI originates from secreted TFPI that binds back to a GPI-anchored protein. Instead, the data suggest that TFPI tightly, but reversibly, binds to a GPI anchored co-receptor in the ER/Golgi. The co-receptor then acts as a molecular chaperone for TFPI by trafficking it to the cell surface of wild-type cells or to lysosomes of aerolysin-resistant cells. TFPI that escapes co-receptor binding is secreted through the same pathway in both wild-type and aerolysin-resistant cells. The data provide a framework for understanding how TFPI is expressed on endothelium.

Expression and characterization of wild-type TFPI and the [P151L]TFPI mutant in insect cells

Tissue factor pathway inhibitor (TFPI) is a multivalent Kunitz-type serine proteinase inhibitor that plays a central role in the extrinsic pathway of blood coagulation. In earlier studies we could identify the [P151L]TFPI mutant, and we could also demonstrate that heterozygous carriers of this mutant show a nine-fold increased risk for deep venous thrombosis (DVT). To express greater amounts of both proteins and to enable their characterization, we expressed wild-type TFPI as well as [P151L]TFPI in High Five insect cells with expression rates of up to 215 ng/ml for wild-type TFPI and 214 ng/ml for [P151L]TFPI. The specific inhibitory activities for the recombinant proteins were determined as 11.3 and 11.5 mU/ng, respectively. Both proteins were detected via Western blot analysis and ELISA. The recombinant proteins' inhibitory activities were characterized by a chromogenic assay and by the determination of a modified activated thromboplastin time (aPTT) in which both of them proved to be inhibitorily active. We also examined both recombinant proteins' binding properties to glycoproteins, glycosaminoglycans, lipoproteins and tissue factor. Our results show that we have developed an efficient model system for the recombinant expression of inhibitorily active wild-type TFPI as well as [P151L]TFPI in insect cells, and we were able to characterize both proteins' inhibitory properties by determination of their influence on the aPTT and also their binding properties. Although both recombinant proteins did not show a significant difference in their effect on the aPTT, their binding properties differed significantly between the wild type and mutant protein.

Fetomaternal cross talk in the placental vascular bed: control of coagulation by trophoblast cells

Humans and rodents exhibit a peculiar type of placentation in which zygote-derived trophoblast cells, rather than endothelial cells, line the terminal maternal vascular space. This peculiar aspect of the placental vasculature raises important questions about the relative contribution of fetal and maternal factors in the local control of hemostasis in the placenta and how these might determine the phenotypic expression of thrombophilia-associated complications of pregnancy. Using genomewide expression analysis, we identify a panel of genes that determine the ability of fetal trophoblast cells to regulate hemostasis at the fetomaternal interface. We show that spontaneous differentiation of trophoblast stem cells is associated with the acquisition of an endothelial cell-like thromboregulatory gene expression program. This program is developmentally regulated and conserved between mice and humans. We further show that trophoblast cells sense, via the expression of protease activated receptors, the presence of activated coagulation factors. Engagement of these receptors results in cell-type specific changes in gene expression. Our observations define candidate fetal genes that are potential risk modifiers of maternal thrombophilia-associated pregnancy complications and provide evidence that coagulation activation at the fetomaternal interface can affect trophoblast physiology altering placental function in the absence of frank thrombosis.

Comparison of cell-surface TFPIalpha and beta

BACKGROUND

Tissue factor pathway inhibitor (TFPI) is mainly produced by endothelial cells and alternative mRNA splicing generates two forms, TFPIalpha and TFPIbeta. A portion of expressed TFPI remains associated with the cell surface through both direct (TFPIbeta) and indirect (TFPIalpha) glycosylphosphatidyl-inositol (GPT)-mediated anchorage.

OBJECTIVE

Compare the structure and properties of TFPIalpha and TFPIbeta.

METHODS

TFPIalpha and TFPIbeta, with protein molecular masses of 36 and 28 kDa, respectively, migrate similarly (46 kDa) on SDS-PAGE. Experiments using specific glycosidases were carried out to determine the different glycosylation pattern of the two forms. ECV304 cells, a cell line with some endothelial properties, were stimulated with IL-lbeta, LPS, and TNFalpha for up to 24 hrs and mRNA levels and protein synthesis were determined. Stable clones of ECV304 cells that express reduced levels of TFPIalpha, TFPIbeta or both were produced using a plasmid-based small-interfering RNA technique. Surface TFPI activity was determined by a two-stage chromogenic assay based on the ability of each form to inhibit FXa activation by FVIIa on cells with comparable amount of tissue factor (TF).

RESULTS AND CONCLUSIONS

The deglycosylation studies show that the difference in molecular masses is due to a greater degree of sialylation in O-linked carbohydrate in TFPIbeta. The mRNA and protein levels of neither form of TFPI were affected by stimulation of cells with inflammatory stimuli. Although TFPIalpha comprises 80% of the surface-TFPI, TFPIbeta was responsible for the bulk of the cellular FVIIa/TF inhibitory activity, suggesting a potential alternative role for cell surface TFPIalpha.

Caveolin-1 enhances tissue factor pathway inhibitor exposure and function on the cell surface

Tissue factor pathway inhibitor (TFPI) blocks tissue factor-factor VIIa (TF-FVIIa) activation of factors X and IX through the formation of the TF-FVIIa-FXa-TFPI complex. Most TFPI in vivo associates with caveolae in endothelial cells (EC). The mechanism of this association and the anticoagulant role of caveolar TFPI are not yet known. Here we show that expression of caveolin-1 (Cav-1) in 293 cells keeps TFPI exposed on the plasmalemma surface, decreases the membrane lateral mobility of TFPI, and increases the TFPI-dependent inhibition of TF-FVIIa. Caveolae-associated TFPI supports the co-localization of the quaternary complex with caveolae. To investigate the significance of these observations for EC we used RNA interference to deplete the cells of Cav-1. Functional assays and fluorescence microscopy revealed that the inhibitory properties of TFPI were diminished in EC lacking Cav-1, apparently through deficient assembly of the quaternary complex. These findings demonstrate that caveolae regulate the inhibition by cell-bound TFPI of the active protease production by the extrinsic pathway of coagulation.

Detached endothelial cells and microparticles as sources of tissue factor activity

INTRODUCTION

Cytokine activation of endothelial cell monolayers is associated with cell detachment, microparticle shedding from plasma membranes, and phosphatidylserine appearance in the plasma membrane outer leaflets. While tissue factor expression on activated endothelial cells and microparticles is well documented, the contribution of detached endothelial cells to tissue factor activity is less clear. We studied tissue factor expression and the role of tissue factor pathway inhibitor on adherent and detached endothelial cells and on microparticles following endothelial cell activation with TNF-alpha.

MATERIALS AND METHODS

Detached endothelial cells and microparticles were obtained from cultures of human umbilical vein endothelial cells by differential centrifugation of cell culture supernatant. For microparticle capture, an antibody directed against CD146 was used. Functional tissue factor activity was measured by chromogenic assay and tissue factor antigen by ELISA. Endothelial cell and microparticle morphology was examined by light and transmission electron microscopy.

RESULTS

After cell activation for 22 h, functional tissue factor activity was distributed as follows: 60%, adherent endothelial cells; 35%, detached cells; and 5%, microparticles. Tissue factor protein followed a similar distribution. Cell detachment was 47%. Electron microscopy demonstrated shedding of microparticles with a diameter of 0.1-0.6 mum. Cy3-annexin V revealed increased phosphatidylserine on activated adherent endothelial cells and microparticles. Pre-incubation of adherent and detached endothelial cells and microparticles with anti-tissue factor antibody blocked factor Xa production. Pre-incubation with anti-tissue factor pathway inhibitor antibody increased tissue factor activity of adherent endothelial cells 2.8-fold, detached cells 1.4-fold, and microparticles 45-fold.

CONCLUSIONS

Detached endothelial cells as well as microparticles from activated endothelial cell monolayers express tissue factor activity, and this activity on microparticles is markedly inhibited by microparticle-associated tissue factor pathway inhibitor.

Role for the Kunitz-3 domain of tissue factor pathway inhibitor-alpha in cell surface binding

BACKGROUND

Tissue factor pathway inhibitor (TFPI)-alpha, a key regulator of tissue factor-induced coagulation, contains 3 tandem Kunitz-type inhibitory domains. Kunitz-1 binds and inhibits factor VIIa in the factor VIIa/tissue factor complex, and Kunitz-2 binds and inhibits factor Xa. The role of the Kunitz-3 domain of TFPI-alpha, however, has remained an enigma.

METHODS AND RESULTS

To determine the structures within TFPI-alpha involved in its binding to cell surface, altered forms of TFPI-alpha were expressed in C127 (mouse mammary) cells: C-terminal truncated forms TFPI-alpha (252), TFPI-alpha (242), and TFPI-alpha (181), which also lacks the third Kunitz domain (K3); TFPI-alpha (desK3), which lacks only the K3 domain; and TFPI-alpha (R199L), in which the putative P1 site in K3 is changed from arginine to leucine. By flow cytometry (fluorescence-activated cell sorting), the altered forms 252, 242, and R199L showed significantly reduced binding, whereas the forms 181 and desK3 completely failed to bind to the cell surface. Transient expression of WT-, desK3-, and K3/K2-TFPI-alpha (in which K3 is replaced with K2) in another cell line (b-end3, mouse endothelial) produced comparable results. Exogenously added C-terminal truncated and R199L forms of TFPI-alpha bound poorly and desK3 did not bind at all to the surface of ECV304 cells in which TFPI-alpha expression had been "knocked down" by RNA interference.

CONCLUSIONS

Optimal cell binding of endogenously expressed TFPI-alpha requires its K3 and C-terminal domains, and within the K3 domain, the P1 (R199) residue plays an important role. Thus, one role of the K3 domain involves the cell surface localization of TFPI-alpha.

Regulation of tissue factor-induced signaling by endogenous and recombinant tissue factor pathway inhibitor 1

Tissue factor (TF) triggers upstream coagulation signaling via the activation of protease-activated receptors (PARs) of relevance for inflammation and angiogenesis. TF pathway inhibitor 1 (TFPI-1) is the physiologic inhibitor of TF-initiated coagulation, but its role in regulating TF signaling is poorly understood. Here, we demonstrate that endogenous, endothelial cell-expressed TFPI-1 controls TF-mediated signaling through PARs. In endothelial cells transduced with TF to mimic exacerbated TF expression in vascular cells, TF-VIIa-Xa ternary complex-dependent activation of PAR1 remained intact when TF-mediated Xa generation was blocked with 2.5 to 5 nM recombinant TFPI-1 (rTFPI-1). Concordantly, inhibition of signaling in PAR1-expressing Chinese hamster ovary (CHO) cells required about 30-fold higher rTFPI-1 concentrations than necessary for anticoagulation. Studies with proteoglycan-deficient CHO cells document a crucial role of accessory receptors in supporting the anticoagulant and antisignaling activities of rTFPI-1. Coexpression of PAR2 with TF enhanced rTFPI-mediated inhibition of TF-VIIa-Xa-mediated PAR1 signaling, suggesting an unexpected role of PAR2 in the inhibitory control of TF signaling. These experiments are of potential significance for the limited therapeutic benefit of rTFPI-1 in systemic inflammation and recommend caution in using anticoagulant potency as a measure to predict how efficacious TF-directed inhibitors block cell signaling during initiation of coagulation.

Formation of factors IXa and Xa by the extrinsic pathway: differential regulation by tissue factor pathway inhibitor and antithrombin III

The activation of factor X by VIIa/TF and the Xa-dependent inhibition of the enzyme complex by tissue factor pathway inhibitor (TFPI) are considered primary steps in the initiation of coagulation. IX activation by VIIa/TF is considered to contribute catalyst necessary for further Xa production in the ensuing amplification phase. We have investigated Xa and IXabeta production by VIIa-TF in a system reconstituted with both X and IX and the principal physiologic inhibitors of this pathway TFPI and antithrombin III (AT). Kinetic studies without inhibitors established that IX and X functioned as competitive alternate substrates for VIIa/TF with similar kinetic constants. When both IX and X were present, TFPI significantly inhibited the extent of formation of either IXabeta or Xa. In contrast, AT rapidly depleted active Xa with a small effect on IXabeta formation. When both AT and TFPI were present, active IXabeta formation significantly exceeded the formation of active Xa regardless of the VIIa/TF concentration. These findings could be quantitatively accounted for by a model encompassing the kinetics of the individual activation and inhibition steps. Active Xa formation by this pathway is regulated in a principal way by its rapid inactivation by AT. In contrast, the Xa-dependent inhibitory reactions of TFPI play a primary role in limiting zymogen consumption and the formation of active IXabeta. These regulatory phenomena yield active IXabeta as a major rather than secondary product of VIIa/TF. Our findings raise the possibility that IXabeta produced by the extrinsic pathway, and its ability to function within the intrinsic Xase complex to activate X may play a significant role in producing Xa necessary for both the initiation and sustained phases of the procoagulant response following vascular damage.

Glycosyl phosphatidylinositol anchorage of tissue factor pathway inhibitor

BACKGROUND

The endothelium is a major source of tissue factor pathway inhibitor (TFPI), the endogenous regulator of TF-induced coagulation, and a significant proportion of the expressed TFPI remains associated with the endothelial surface.

METHODS AND RESULTS

Phosphatidylinositol-specific phospholipase C (PI-PLC) treatment reduced TFPI at the surface of cultured endothelial cells by approximately 80%, and at least a portion of the TFPI released by PI-PLC contained an intrinsic glycosylphosphatidylinositol (GPI) anchor that is recognized by anti-crossreactive determinant antibodies. Endothelial cells express both of the alternatively spliced forms of TFPI mRNA at a ratio of TFPIbeta/TFPIalpha mRNA of approximately 0.1 to 0.2. In Chinese hamster ovary (CHO) cells, TFPIalpha is predominantly secreted, whereas TFPIbeta is a GPI-anchored membrane protein. Like TFPIbeta, the small proportion of the TFPIalpha expressed by CHO cells that remains surface associated is also released by PI-PLC treatment, suggesting that it is bound to a separate GPI-anchored protein(s) at the surface of the cells.

CONCLUSIONS

Both direct (TFPIbeta) and indirect (TFPIalpha) GPI-mediated membrane anchorage is involved in localizing TFPI to the surface of cells.