Regulation of TFPI function by protein S

Thrombophilia Screening: Not So Straightforward

Although inherited thrombophilias are lifelong risk factors for a first thrombotic episode, progression to thrombosis is multifactorial and not all individuals with inherited thrombophilia develop thrombosis in their lifetimes. Consequently, indiscriminate screening in patients with idiopathic thrombosis is not recommended, since presence of a thrombophilia does not necessarily predict recurrence or influence management, and testing should be selective. It follows that a decision to undertake laboratory detection of thrombophilia should be aligned with a concerted effort to identify any significant abnormalities, because it will inform patient management. Deficiencies of antithrombin and protein C are rare and usually determined using phenotypic assays assessing biological activities, whereas protein S deficiency (also rare) is commonly detected with antigenic assays for the free form of protein S since available activity assays are considered to lack specificity. In each case, no single phenotypic assay is capable of detecting every deficiency, because the various mutations express different molecular characteristics, rendering thrombophilia screening repertoires employing one assay per potential deficiency, of limited effectiveness. Activated protein C resistance (APCR) is more common than discrete deficiencies of antithrombin, protein C, and protein S and also often detected initially with phenotypic assays; however, some centres perform only genetic analysis for factor V Leiden, as this is responsible for most cases of hereditary APCR, accepting that acquired APCR and rare F5 mutations conferring APCR will go undetected if only factor V Leiden is evaluated. All phenotypic assays have interferences and limitations, which must be factored into decisions about if, and when, to test, and be given consideration in the laboratory during assay performance and interpretation. This review looks in detail at performance and limitations of routine phenotypic thrombophilia assays.

Role of tissue factor pathway inhibitor in hormone-induced venous thromboembolism

ABSTRACT

Exposure to higher levels of steroid hormones, like that in pregnancy or during combined hormonal contraception, increases the risk of venous thromboembolism. Development of resistance to activated protein C (APC) thought to be the underlying pathomechanism of this prothrombotic state. This coagulation phenomena is largely to be explained by the hormone-induced impairment of the protein S/ tissue factor pathway inhibitor (TFPI) leading to a less efficient inactivation of factor Va and factor VIIIa by APC. APC resistance and decreased protein S/TFPI function were associated with the risk of first as well as recurrent venous thromboembolism. Preexisting disturbances in these pathways are likely to predispose to thrombosis during hormone exposure and can persist over years after the thrombosis event.Further studies are necessary to investigate the predictive value of forgoing APC resistance and decreased protein S/TFPI function or an excessive alteration in these parameters during hormone intake on the development of hormone-induced venous thromboembolism.

Natural anticoagulant discovery, the gift that keeps on giving: finding FV-Short

The complex reactions of blood coagulation are balanced by several natural anticoagulants resulting in tuned hemostasis. During several decades, the knowledge base of the natural anticoagulants has greatly increased and we have also learned about antiinflammatory and cytoprotective activities expressed by antithrombin and activated protein C (APC). Some coagulation proteins have also been found to function as anticoagulants; e.g., thrombin when bound to thrombomodulin activates protein C. Another example is factor V (FV), which in addition to being a procofactor to FVa has emerged as an anticoagulant. The discovery of APC resistance, caused by FVLeiden, as a thrombosis risk factor resulted in the identification of FV as an APC cofactor working in synergy with protein S in the regulation of FVIIIa in the Xase complex. More recently, a natural anticoagulant FV splice isoform (FV-Short) was discovered when investigating the East Texas bleeding disorder. In FV-Short, the truncated B domain exposes a high-affinity binding site for tissue factor pathway inhibitor alpha (TFPIα), and together with protein S a high-affinity trimolecular complex is generated. The FXa-inhibitory activity of TFPIα is synergistically stimulated by FV-Short and protein S. The circulating FV-Short/protein S/TFPIα complex concentration is normally low (≈0.2 nM) but provides an anticoagulant threshold. In the East Texas bleeding, the concentration of the complex, and thus the threshold, is increased 10-fold, which results in bleeding manifestations. The anticoagulant properties of FV were discovered during investigations of individual patients and follow the great tradition of bed-to-bench and bench-to-bed research in the coagulation field.

Haemostasis and Inflammatory Parameters as Potential Diagnostic Biomarkers for VTE in Trauma-Immobilized Patients

Venous thromboembolism (VTE), which encompasses deep venous thrombosis (DVT) and pulmonary embolism (PE), is a major public health concern due to its high incidences of morbidity and mortality. Patients who have experienced trauma with prolonged immobilization are at an increased risk of developing VTE. Plasma D-dimer levels have been known to be elevated in trauma patients, and they were closely correlated with the number of fractures. In other words, plasma D-dimer levels cannot be used as the only indicator of VTE in trauma cases. Given the limitations, further study is needed to explore other potential biomarkers for diagnosing VTE. To date, various established and novel VTE biomarkers have been studied in terms of their potential for predicting VTE, diagnostic performance, and improving clinical therapy for VTE. Therefore, this review aims to provide information regarding classic and essential haemostasis (including prothrombin time (PT), activated partial thromboplastin time (aPTT), D-dimer, fibrinogen, thrombin generation, protein C, protein S, antithrombin, tissue factor pathway inhibitor, and platelet count) and inflammatory biomarkers (C-reactive protein, erythrocyte sedimentation rate, and soluble P-selectin) as potential diagnostic biomarkers that can predict the risk of VTE development among trauma patients with prolonged immobilization. Thus, further advancement in risk stratification using these biomarkers would allow for a better diagnosis of patients with VTE, especially in areas with limited resources.

Factor V Leiden-independent activated protein C resistance: Communication from the plasma coagulation inhibitors subcommittee of the International Society on Thrombosis and Haemostasis Scientific and Standardisation Committee

Activated protein C resistance (APC-R) due to the single-nucleotide polymorphism factor V Leiden (FVL) is the most common cause of hereditary thrombophilia. It is found predominantly in Caucasians and is uncommon or absent in other populations. Although FVL is responsible for >90% of cases of hereditary APC-R, a number of other F5 variants that also confer various degrees of APC-R and thrombotic risk have been described. Acquired APC-R due to increased levels of coagulation factors, reduced levels of inhibitors, or the presence of autoantibodies occurs in a variety of conditions and is an independent risk factor for thrombosis. It is common for thrombophilia screening protocols to restrict assessment for APC-R to demonstrating the presence or absence of FVL. The aim of this Scientific and Standardisation Committee communication is to detail the causes of FVL-independent APC-R to widen the diagnostic net, particularly in situations in which in vitro APC-R is encountered in the absence of FVL. Predilution clotting assays are not FVL specific and are used to detect clinically significant F5 variants conferring APC-R, whereas different forms of acquired APC-R are preferentially detected using the classical activated partial thromboplastin time-based APC-R assay without predilution and/or endogenous thrombin potential APC-R assays. Resource-specific recommendations are given to guide the detection of FVL-independent APC-R.

A hydrophobic patch (PLVIVG; 1481-1486) in the B-domain of factor V-short is crucial for its synergistic TFPIα-cofactor activity with protein S and for the formation of the FXa-inhibitory complex comprising FV-short, TFPIα, and protein S

BACKGROUND

Factor V-short (FV756-1458) is a natural splice variant functioning in synergy with protein S as tissue factor pathway inhibitor alpha (TFPIα)-cofactor in inhibition of factor Xa (FXa). An exposed acid region (AR2; 1493-1537) in the B domain binds TFPIα. The preAR2 (1458-1492) is crucial for the synergistic TFPIα-cofactor activity between FV-short and protein S and for assembly of a trimolecular FXa-inhibitory complex among FV-short, protein S, and TFPIα.

OBJECTIVE

To identify which part of preAR2 is required for the synergistic TFPIα-cofactor activity between FV-short and protein S.

METHODS

A FXa-inhibition assay was used to test the synergistic TFPIα cofactor activity between protein S and new FV-short variants FV709-1476, FV712-1478, FV712-1481, FV712-1484, FV712-1487, and FV712-1490. A microtiter-based assay analyzed binding among FV-short variants, protein S, and TFPIα.

RESULTS

FV709-1476, FV712-1478, and FV712-1481 were fully active as synergistic TFPIα cofactors with protein S; FV712-1484 showed intermediate activity; and FV712-1487 and FV712-1490 were inactive. TFPIα interacted with all variants in the absence of protein S but FV712-1478 and FV712-1481 bound TFPIα with highest affinity. None of the FV-short variants bound directly to protein S in the absence of TFPIα. In the presence of TFPIα, efficient cooperative binding was demonstrated between protein S, TFPIα, and FV709-1476, FV712-1478, or FV712-1481. In contrast, no cooperativity among TFPIα, protein S, and FV712-1484, FV712-1487, or FV712-1490 was seen.

CONCLUSION

A short hydrophobic patch in preAR2 (PLVIVG, 1481-1486) in FV-short is crucial for the synergistic TFPIα-cofactor activity between FV-short and protein S and for the assembly of a trimolecular FXa-inhibitory complex among FV-short, protein S, and TFPIα.

Non-severe COVID-19 is associated with endothelial damage and hypercoagulability despite pharmacological thromboprophylaxis

BACKGROUND

Hypercoagulability and endothelial dysfunction are hallmarks of coronavirus disease 2019 (COVID-19) and appear to predict disease severity. A high incidence of thrombosis despite thromboprophylaxis is reported in patients with moderate to severe COVID-19. Recent randomized clinical trials suggest that therapeutic-intensity heparin confers a survival benefit in moderate-severity COVID-19 compared to standard-intensity heparin, potentially by harnessing heparin-mediated endothelial-stabilizing and anti-inflammatory effects.

OBJECTIVE

We hypothesized that patients with moderate-severity COVID-19 exhibit enhanced hypercoagulability despite standard-intensity thromboprophylaxis with low molecular weight heparin (LMWH) compared to non-COVID-19 hospitalized patients.

METHODS

Patients with moderate COVID-19 and a control group (severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]-negative hospitalized patients) receiving LMWH thromboprophylaxis were recruited. Markers of endothelial damage and plasma thrombin generation parameters were assessed.

RESULTS

Tissue plasminogen activator levels were significantly increased in the COVID-19 group (8.3 ± 4.4 vs. 4.9 ± 2.4 ng/ml; P = .02) compared to non-COVID-19-hospitalized patients. Despite thromboprophylaxis, mean endogenous thrombin potential was significantly increased among COVID-19 patients (1929 ± 448 vs. 1528 ± 460.8 nM*min; P = .04) but lag time to thrombin generation was significantly prolonged (8.1 ± 1.8 vs. 6.2 ± 1.8 mins; P = .02). While tissue factor pathway inhibitor (TFPI) levels were similar in both groups, in the presence of an inhibitory anti-TFPI antibody, the difference in lag time between the groups was abrogated.

CONCLUSIONS

Collectively, these data demonstrate that COVID-19 of moderate severity is associated with increased plasma thrombin generation and endothelial damage, and that hypercoagulability persists despite standard LMWH thromboprophylaxis. These findings may be of clinical interest given recent clinical trial data which suggest escalated heparin dosing in non-severe COVID-19 may be associated with improved clinical outcomes.

The Impact of Anticoagulant Activity of Tissue Factor Pathway Inhibitor Measured by a Novel Functional Assay for Predicting Deep Venous Thrombosis in Trauma Patients: A Prospective Nested Case-Control Study

Deep venous thrombosis (DVT) is a common complication in patients with traumatic injury. Tissue factor pathway inhibitor (TFPI) is a natural anticoagulant protein in the extrinsic coagulation pathway. However, the relationship between DVT after trauma and the anticoagulant activity of TFPI remains unclear. In this prospective study, we investigated the role of TFPI in trauma patients with DVT to evaluate whether the anticoagulant activity of TFPI measured by a new functional assay can be used to help predict the risk of DVT. Patients and methods: This prospective nested case-control study enrolled trauma patients and healthy volunteers. Forty-eight trauma patients diagnosed with DVT and forty-eight matched trauma patients without DVT were included in the study. 120 healthy volunteers were also included as controls. Blood samples and case information were collected at admission. Patients accepted angiography before surgery to diagnose DVT. The parameters examined included TFPI anticoagulant activity, free-TFPI antigen, blood cell counts, and routine clinical coagulation tests. Results: For the parameters of TFPI anticoagulant activity, three were markedly increased in the DVT group compared to the non-DVT group (TFPI initial anticoagulant time ratio, P  =  .022; TFPI whole anticoagulant time ratio, P  =  .048; and TFPI anticoagulant rate, P  =  .034). The free-TFPI antigen concentration also showed a significant increasing trend in trauma patients with DVT compared with trauma patients without DVT (P  =  .035). Multivariate logistic regression analysis identified four independent factors for the development of DVT (TFPI initial anticoagulant time ratio, free-TFPI antigen, prothrombin time, and red blood cell count). We calculated the TFPI correlation coefficient and found that the area under the receiver operating characteristic curve was .821. Conclusions: A novel functional assay was developed to measure the anticoagulant activity of TFPI. The anticoagulant activity of TFPI can be used as a potential biomarker for diagnosing DVT in trauma patients.

Lung Epithelial Cell Transcriptional Regulation as a Factor in COVID-19-associated Coagulopathies

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has rapidly become a global pandemic. In addition to the acute pulmonary symptoms of coronavirus disease (COVID-19) (the disease associated with SARS-CoV-2 infection), pulmonary and distal coagulopathies have caused morbidity and mortality in many patients. Currently, the molecular pathogenesis underlying COVID-19-associated coagulopathies are unknown. Identifying the molecular basis of how SARS-CoV-2 drives coagulation is essential to mitigating short- and long-term thrombotic risks of sick and recovered patients with COVID-19. We aimed to perform coagulation-focused transcriptome analysis of in vitro infected primary respiratory epithelial cells, patient-derived bronchial alveolar lavage cells, and circulating immune cells during SARS-CoV-2 infection. Our objective was to identify transcription-mediated signaling networks driving coagulopathies associated with COVID-19. We analyzed recently published experimentally and clinically derived bulk or single-cell RNA sequencing datasets of SARS-CoV-2 infection to identify changes in transcriptional regulation of blood coagulation. We also confirmed that the transcriptional expression of a key coagulation regulator was recapitulated at the protein level. We specifically focused our analysis on lung tissue-expressed genes regulating the extrinsic coagulation cascade and the plasminogen activation system. Analyzing transcriptomic data of in vitro infected normal human bronchial epithelial cells and patient-derived bronchial alveolar lavage samples revealed that SARS-CoV-2 infection induces the extrinsic blood coagulation cascade and suppresses the plasminogen activation system. We also performed in vitro SARS-CoV-2 infection experiments on primary human lung epithelial cells to confirm that transcriptional upregulation of tissue factor, the extrinsic coagulation cascade master regulator, manifested at the protein level. Furthermore, infection of normal human bronchial epithelial cells with influenza A virus did not drive key regulators of blood coagulation in a similar manner as SARS-CoV-2. In addition, peripheral blood mononuclear cells did not differentially express genes regulating the extrinsic coagulation cascade or plasminogen activation system during SARS-CoV-2 infection, suggesting that they are not directly inducing coagulopathy through these pathways. The hyperactivation of the extrinsic blood coagulation cascade and the suppression of the plasminogen activation system in SARS-CoV-2-infected epithelial cells may drive diverse coagulopathies in the lung and distal organ systems. Understanding how hosts drive such transcriptional changes with SARS-CoV-2 infection may enable the design of host-directed therapeutic strategies to treat COVID-19 and other coronaviruses inducing hypercoagulation.

Unique transcriptional changes in coagulation cascade genes in SARS-CoV-2-infected lung epithelial cells: A potential factor in COVID-19 coagulopathies

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has rapidly become a global pandemic. In addition to the acute pulmonary symptoms of COVID-19 (the disease associated with SARS-CoV-2 infection), pulmonary and distal coagulopathies have caused morbidity and mortality in many patients. Currently, the molecular pathogenesis underlying COVID-19 associated coagulopathies are unknown. While there are many theories for the cause of this pathology, including hyper inflammation and excess tissue damage, the cellular and molecular underpinnings are not yet clear. By analyzing transcriptomic data sets from experimental and clinical research teams, we determined that changes in the gene expression of genes important in the extrinsic coagulation cascade in the lung epithelium may be important triggers for COVID-19 coagulopathy. This regulation of the extrinsic blood coagulation cascade is not seen with influenza A virus (IAV)-infected NHBEs suggesting that the lung epithelial derived coagulopathies are specific to SARS-Cov-2 infection. This study is the first to identify potential lung epithelial cell derived factors contributing to COVID-19 associated coagulopathy.

Investigation of patients with unclassified bleeding disorder and abnormal thrombin generation for physiological coagulation inhibitors reveals multiple abnormalities and a subset of patients with increased tissue factor pathway inhibitor activity

INTRODUCTION

We have routinely used thrombin generation to investigate patients with unclassified bleeding disorder (UBD).

AIMS

To investigate haemostatic abnormalities in patients with UBD that had abnormal thrombin generation on at least one occasion.

METHODS

Investigation of 13 known UBD patients with thrombin generation and detailed haemostatic testing was undertaken including TFPI assays but also thrombomodulin and fibrinogen-γ.

RESULTS

12 females and 1 male were included. No patient had a platelet function disorder or coagulation factor deficiency that explained the bleeding phenotype, though 2 patients had factor deficiencies; a factor X of 0.41 IU/mL and a factor XI of 0.51 IU/mL. ThromboGenomics revealed variants for these factors but no other abnormalities. Patients were included who previously had either prolonged lag time or decreased endogenous thrombin potential (ETP) via high dose tissue factor (5 pmol/L) or low dose tissue factor (1.5 pmol/L) with corn trypsin inhibitor (CTI). Tissue factor pathway inhibitor (TFPI) activity was significantly increased (P < .001; increased in 8 patients) compared with controls and abnormalities in soluble thrombomodulin (2 patients), fibrinogen-γ (1 patient) and tPA (4 patients for each) were seen. Total and free TFPI levels were not increased. Mixing studies of patient plasma with 50:50 normal plasma for thrombin generation via low dose tissue factor failed to correct the ETP consistent with ongoing inhibition. Addition of an anti-TFPI antibody partially corrected thrombin generation to normal levels. TFPI sequencing was unremarkable.

CONCLUSION

TFPI activity may be increased in a subset of UBD patients. Further research studies are warranted in UBD patients for coagulation inhibitor abnormalities.

Protein Kinase Cθ Via Activating Transcription Factor 2-Mediated CD36 Expression and Foam Cell Formation of Ly6Chi Cells Contributes to Atherosclerosis

BACKGROUND

Although the role of thrombin in atherothrombosis is well studied, its role in the pathogenesis of diet-induced atherosclerosis is not known.

METHODS

Using a mouse model of diet-induced atherosclerosis and molecular biological approaches, here we have explored the role of thrombin and its G protein-coupled receptor signaling in diet-induced atherosclerosis.

RESULTS

In exploring the role of G protein-coupled receptor signaling in atherogenesis, we found that thrombin triggers foam cell formation via inducing CD36 expression, and these events require Par1-mediated Gα12-Pyk2-Gab1-protein kinase C (PKC)θ-dependent ATF2 activation. Genetic deletion of PKCθ in apolipoprotein E (ApoE)^-/- mice reduced Western diet-induced plaque formation. Furthermore, thrombin induced Pyk2, Gab1, PKCθ, and ATF2 phosphorylation, CD36 expression, and foam cell formation in peritoneal macrophages of ApoE^-/- mice. In contrast, thrombin only stimulated Pyk2 and Gab1 but not ATF2 phosphorylation or its target gene CD36 expression in the peritoneal macrophages of ApoE^-/-:PKCθ^-/- mice, and it had no effect on foam cell formation. In addition, the aortic root cross-sections of Western diet-fed ApoE^-/- mice showed increased Pyk2, Gab1, PKCθ, and ATF2 phosphorylation and CD36 expression as compared with ApoE^-/-:PKCθ^-/- mice. Furthermore, although the monocytes from peripheral blood and the aorta of Western diet-fed ApoE^-/- mice were found to contain more of Ly6C^hi cells than Ly6C^lo cells, the monocytes from Western diet-fed ApoE^-/-:PKCθ^-/- mice were found to contain more Ly6C^lo cells than Ly6C^hi cells. It is interesting to note that the Ly6C^hi cells showed higher CD36 expression with enhanced capacity to form foam cells as compared with Ly6C^lo cells.

CONCLUSIONS

These findings reveal for the first time that thrombin-mediated Par1-Gα12 signaling via targeting Pyk2-Gab1-PKCθ-ATF2-dependent CD36 expression might be playing a crucial role in diet-induced atherogenesis.

New functional test for the TFPIα cofactor activity of Protein S working in synergy with FV-Short

Essentials Protein S and FV-Short are synergistic cofactors to Tissue Factor Pathway Inhibitor α (TFPIα). An assay for the TFPIα synergistic cofactor activity of protein S with FV-Short was developed. The assay was specific for the synergistic TFPIα-cofactor activity of free protein S. Protein S deficient individuals with known mutations were correctly distinguished from controls. SUMMARY: Background Protein S is an anticoagulant cofactor to both activated protein C and tissue factor pathway inhibitor (TFPIα). The TFPIα-cofactor activity of protein S is stimulated by a short isoform of factor V (FV-Short), the two proteins functioning in synergy. Objective Using the synergistic TFPIα-cofactor activity between protein S and FV-Short to develop a functional test for plasma protein S. Patients/Methods TFPIα-mediated inhibition of FXa in the presence of FV-Short, protein S and negatively charged phospholipid vesicles was monitored in time by synthetic substrate S2765. TFPIα, FXa and FV-Short were purified proteins, whereas diluted plasma from protein S deficient patients or controls were used as source for protein S. Results The assay was specific for free protein S demonstrating good correlation to free protein S plasma levels (r = 0.92) with a Y-axis intercept of -5%. Correlation to concentrations of total protein S (free and C4BPβ+-bound) was lower (r = 0.88) and the Y-axis intercept was +46%, which is consistent with the specificity for free protein S. The test distinguished protein S-deficient individuals from 6 families with known ProS1 mutations from family members having no mutation. Protein S levels of warfarin-treated protein S deficient cases were lower than protein S in cases treated with warfarin for other causes. Conclusions We describe a new assay measuring the TFPIα-cofactor activity of plasma protein S. The test identifies type I/III protein S deficiencies and will be a useful tool to detect type II protein S deficiency having defective TFPIα-cofactor activity.

Post-transcriptional, post-translational and pharmacological regulation of tissue factor pathway inhibitor

: Tissue factor (TF) pathway inhibitor (TFPI) is an endogenous natural anticoagulant that readily inhibits the extrinsic coagulation initiation complex (TF-FVIIa-Xa) and prothrombinase (FXa, FVa and calcium ions). Alternatively, spliced TFPI isoforms (α, β and δ) are expressed by vascular and extravascular cells and regulate thrombosis and haemostasis, as well as cell signalling functions of TF complexes via protease-activated receptors (PARs). Proteolysis of TFPI plays an important role in regulating physiological roles of the TF pathway in host defense and possibly haemostasis. Elimination of TFPI inhibition has therefore been proposed as an approach to improve haemostasis in haemophilia patients. In this review, we focus on posttranscription and translational modification of TFPI and its function in thrombosis and how pharmacological inhibitors and endogenous proteases interfere with TFPI and alter haemostasis.

Factor V-short and protein S as synergistic tissue factor pathway inhibitor (TFPIα) cofactors

BACKGROUND

FV-Short is a normal splice variant of Factor V (FV) having a short B domain, which exposes a high affinity-binding site for tissue factor pathway inhibitor α (TFPIα). FV-Short and TFPIα circulate in complex in plasma.

OBJECTIVES

The aim was to elucidate whether FV-Short affects TFPIα as inhibitor of coagulation FXa and to test whether the TFPIα-cofactor activity of protein S is influenced by FV-Short.

METHODS

Recombinant FV, wild-type FV-Short and a FV-Short thrombin-cleavage resistant variant were expressed and purified. The influence of FV and FV-Short variants and/or protein S on the FXa inhibitory activity of TFPIα was monitored both in a purified system and in a plasma-based thrombin generation assay.

RESULTS

FV-Short had intrinsically weak TFPIα-cofactor activity but with protein S present, FV-Short yielded efficient inactivation of FXa. Protein S alone did not promote full TFPIα-activity. Intact FV was inefficient at low protein S concentrations and had 10-fold lower activity compared to FV-Short at physiological protein S levels. Activation of FV-Short by thrombin resulted in the loss of the TFPIα-cofactor activity. The synergistic TFPIα-cofactor activity of FV-Short and protein S was also demonstrated in plasma using a thrombin generation assay.

CONCLUSIONS

FV-Short and protein S are highly efficient, synergistic cofactors to TFPIα in the regulation of FXa activity, whereas full length FV has lower activity. Our results suggest the formation of an efficient FXa-inhibitory complex between FV-Short, TFPIα and protein S on the surface of negatively charged phospholipids.

Zinc: An important cofactor in haemostasis and thrombosis

There is mounting evidence that zinc, the second most abundant transition metal in blood, is an important mediator of haemostasis and thrombosis. Prompted by the observation that zinc deficiency is associated with bleeding and clotting abnormalities, there now is evidence that zinc serves as an effector of coagulation, anticoagulation and fibrinolysis. Zinc binds numerous plasma proteins and modulates their structure and function. Because activated platelets secrete zinc into the local microenvironment, the concentration of zinc increases in the vicinity of a thrombus. Consequently, the role of zinc varies depending on the microenvironment; a feature that endows zinc with the capacity to spatially and temporally regulate haemostasis and thrombosis. This paper reviews the mechanisms by which zinc regulates coagulation, platelet aggregation, anticoagulation and fibrinolysis and outlines how zinc serves as a ubiquitous modulator of haemostasis and thrombosis.

The neuropeptide galanin promotes an anti-thrombotic phenotype on endocardial endothelial cells from heart failure patients

Thromboembolic complications are a significant cause of mortality and re-hospitalization in heart failure (HF) patients. One source of thrombi is the ventricular endocardial surface that becomes increasingly pro-thrombotic as HF progresses. Anticoagulation comes with bleeding risks so identifying therapeutic agents for improving cardiac endothelial health are of critical clinical importance. Endocardial endothelial cells are closely apposed to cardiac sympathetic nerves. In HF, cardiac sympathetic nerves are dysregulated and promote disease progression. Whether endocardial endothelial health and function is impacted by sympathetic dysregulation in HF is unknown. Also unexplored is the impact of neuropeptides, such as galanin and neuropeptide Y (NPY), co-released from sympathetic nerve terminals, on endothelial health. In this study we examined the effect of sympathetic nerve-released neurotransmitters and neuropeptides on the procoagulant phenotype of cultured human endocardial endothelial cells from HF patients. As a functional readout of procoagulant state we examined thrombin-mediated von Willebrand factor (vWF) extrusion and multimer expression. We demonstrate that vWF extrusion and multimer expression is promoted by thrombin, that isoproterenol (a beta-adrenergic receptor agonist) augments this effect, whereas co-treatment with the beta-blockers propranolol and carvedilol blocks this effect. We also show that vWF extrusion and multimer expression is attenuated by treatment with the neuropeptide galanin, but not with NPY. Our results are consistent with a protective role of beta-blockers and galanin on endocardial endothelial health in heart failure. Improving endothelial health through galanin therapy is a future clinical application of this study.

Novel insights into the regulation of coagulation by factor V isoforms, tissue factor pathway inhibitorα, and protein S

Factor V (FV) is a regulator of both pro- and anticoagulant pathways. It circulates as a single-chain procofactor, which is activated by thrombin or FXa to FVa that serves as cofactor for FXa in prothrombin activation. The cofactor function of FVa is regulated by activated protein C (APC) and protein S. FV can also function as an anticoagulant APC cofactor in the inhibition of FVIIIa in the membrane-bound tenase complex (FIXa/FVIIIa). In recent years, it has become clear that FV also functions in multiple ways in the tissue factor pathway inhibitor (TFPI) anticoagulant pathway. Of particular importance is a FV splice variant (FV-Short) that serves as a carrier and cofactor to TFPIα in the inhibition of FXa. FV-Short is generated through alternative splicing of exon 13 that encodes the large activation B domain. A highly negatively charged binding site for TFPIα is exposed in the C-terminus of the FV-Short B domain, which binds the positively charged C-terminus of TFPIα, thus keeping TFPIα in circulation. The binding of TFPIα to FV-Short is also instrumental in localizing the inhibitor to the surface of negatively charged phospholipids, where TFPIα inhibits FXa in process that is stimulated by protein S. Plasma FV activation intermediates and partially proteolyzed platelet FV similarly bind TFPIα with high affinity and regulate formation of prothrombinase. The novel insights gained into the interaction between FV isoforms, TFPIα, and protein S have opened a new avenue for research about the mechanisms of coagulation regulation and also for future development of therapeutics aimed at modulating coagulation.

Haemostatic factors do not account for worse outcomes from ischaemic stroke in patients with higher C-reactive protein concentrations

Background Although the role of microparticles was recently implicated in stroke pathophysiology, the association between microparticles and inflammation is still not fully understood. The aim of this cohort study of 66 patients was to assess a relation between haemostatic factors, C-reactive protein and clinical outcome of ischaemic stroke. Methods Plasma microparticles procoagulant activity, concentrations of tissue factor-bearing microparticles, tissue factor and tissue factor pathway inhibitor in ischaemic stroke patients were determined with enzyme-linked immunosorbent assays at the time of initial diagnosis, along with serum C-reactive protein concentrations. Patients were divided into two groups depending on their C-reactive protein concentrations (C-reactive protein <3 mg/L; n = 28 vs. C-reactive protein ≥3 mg/L; n = 38). The analysed clinical outcome measures included the National Institutes of Health Stroke Scale and the Barthel Index. Results The two C-reactive protein groups did not differ significantly in terms of microparticles procoagulant activities, tissue factor-bearing microparticles, tissue factor and tissue factor pathway inhibitor concentrations. A significant correlation was observed between tissue factor pathway inhibitor and National Institutes of Health Stroke Scale score at admission ( R = 0.3, P = 0.03). Patients with C-reactive protein ≥3 mg/L presented with significantly higher National Institutes of Health Stroke Scale scores (median, 9.00 vs. 5.50, P = 0.002) and lower Barthel Index scores (median, 20.00 vs. 65.00, P = 0.002) than individuals with C-reactive protein <3 mg/L. The C-reactive protein concentrations correlated positively with National Institutes of Health Stroke Scale scores ( R = 0.3, P = 0.02) and inversely with Barthel Index scores ( R = - 0.4, P = 0.002). Conclusions Altogether, these findings imply that haemostatic parameters (microparticles, tissue factor-bearing microparticles, tissue factor, tissue factor pathway inhibitor) do not account for elevated C-reactive protein concentrations in ischaemic stroke patients.

The role of microRNA-27a/b and microRNA-494 in estrogen-mediated downregulation of tissue factor pathway inhibitor α

Essentials Estrogens are known to influence the expression of microRNAs in breast cancer cells. We looked at microRNAs in estrogenic regulation of tissue factor pathway inhibitor α (TFPIα). Estrogen upregulated microRNA-27a/b and microRNA-494 through the estrogen receptor α. MicroRNA-27a/b and microRNA-494 are partly involved in estrogenic downregulation of TFPIα.

SUMMARY

Background Tissue factor pathway inhibitor (TFPI) has been linked to breast cancer pathogenesis. We have recently reported TFPI mRNA levels to be downregulated by estrogens in a breast cancer cell line (MCF7) through the estrogen receptor α (ERα). Accumulating evidence also indicates that activation of ERα signaling by estrogens may modulate the expression of target genes indirectly through microRNAs (miRNAs). Objectives To examine if miRNAs are involved in the estrogenic downregulation of TFPIα. Methods Computational analysis of the TFPI 3'-untranslated region (UTR) identified potential binding sites for miR-19a/b, miR-27a/b, miR-494, and miR-24. Transient overexpression or inhibition of the respective miRNAs was achieved by transfection of miRNA mimics or inhibitors. Direct targeting of TFPI 3'-UTR by miR-27a/b and miR-494 was determined by luciferase reporter assay in HEK293T cells. Effects of 17α-ethinylestradiol (EE2) and fulvestrant on relative miR-27a/b, miR-494, and TFPI mRNA levels in MCF7 cells were determined by qRT-PCR and secreted TFPIα protein by ELISA. Transient knockdown of ERα was achieved by siRNA transfection. Results EE2 treatment lead to a significant increase in miR-19a, miR-27a/b, miR-494, and miR-24 mRNA levels in MCF7 cells through ERα. miR-27a/b and miR-494 mimics lead to reduced TFPI mRNA and protein levels. Luciferase assay showed direct targeting of miR-27a/b and miR-494 on TFPI mRNA. Impaired estrogen-mediated downregulation of TFPI mRNA was detected in anti-miR-27a/b and anti-miR-494 transfected cells. Conclusions Our results provide evidence that miR-27a/b and miR-494 regulate TFPIα expression and suggest a possible role of these miRNAs in the estrogen-mediated downregulation of TFPIα.

Tissue Factor Pathway Inhibitor-Coated Stents Inhibit Restenosis in a Rabbit Carotid Artery Model

INTRODUCTION

Our aim was to study the efficacy and safety of tissue factor pathway inhibitor (TFPI)-coated stents in inhibiting restenosis in a rabbit carotid artery model.

METHODS

Subculture was conducted in aorta smooth muscle cell, which was taken from male Wistar rat, and the 3-5-generation cells were taken for plasmid transfection and cytotoxicity experiment. TFPI microspheres were made of a TFPI plasmid which was enwrapped by poly-l-glutamic acid (PLGA). TFPI-coated stents (n = 7) and bare metal stents (n = 6) were implanted into prepared carotid artery stenosis model of New Zealand white rabbits. The transfection efficiency of TFPI gene and its influence on animal tissue, restenosis inhibition, and biochemical indicator were observed.

RESULT

Tissue factor pathway inhibitor microspheres can transfect successfully into cells, and present no cytotoxicity. Autopsy results showed no pathological changes in liver and spleen of rabbits after implanting TFPI-coated stents. TFPI gene could transfect and express successfully in vessel wall cells, and thrombus was found in some lumens of bare metal stents group after 7 day, while no such thrombus was observed in coated stents group. Degree of hyperplasia of coronary endarterectomy in bare metal stents group was evidently higher than those in coated stents group. Obvious stent restenosis was discovered only in one case in bare metal stents group (diameter stenosis ≥50%). However, no case in coated stents group showed with stent restenosis.

CONCLUSION

Tissue factor pathway inhibitor-coated stents could successfully transfect TFPI gene into vessel wall cells, thereby inhibiting restenosis without obvious side effect in the rabbit carotid artery model.

Progress in research into the genes associated with venous thromboembolism

BACKGROUND

Venous thromboembolism (VTE), including both deep vein thrombosis (DVT) and pulmonary embolism (PE), is a common, lethal disorder that affects hospitalized and non-hospitalized patients. This study aimed to review the progress in the research into VTE.

DATA SOURCES

We reviewed the studies about VTE and verified different genetic polymoriphisms of VTE.

RESULTS

The pathogenesis of VTE involves hereditary and acquired factors. Many studies indicated that the disorder of coagulation and fibirnolytic system is of utmost importance to this disease. Genetic polymoriphism-related VTE demonstrated significant differences among geographies and ethnicities.

CONCLUSION

VTE has many risk factors, but genetic factors play an important role.

Molecular basis of the clotting defect in a bleeding patient missing the Asp-185 codon in the factor X gene

Factor X (FX) is a vitamin K-dependent plasma zymogen, which following activation to factor Xa (FXa), converts prothrombin to thrombin in the blood clotting cascade. It was recently demonstrated that a natural variant of FX carrying the Asp-185 deletion (FX-D185del, chymotrypsinogen numbering) was associated with mild bleeding in a patient with severe FX deficiency. In this study, we expressed FX-D185del in mammalian cells and characterized its properties in appropriate kinetic assays in purified systems. We discovered that while the FX variant can be normally activated by physiological activators; both amidolytic and proteolytic activities of the mutant are dramatically impaired. Interestingly, factor Va (FVa) significantly improved the proteolytic defect when the mutant protease was assembled into the prothrombinase complex. Thus, in contrast to >50-fold catalytic defect in the absence of FVa, the variant activated prothrombin with only ~2.5-fold decreased catalytic efficiency in the presence of the cofactor. The FXa variant dramatically lost its susceptibility to inhibition by antithrombin and tissue factor pathway inhibitor, thus exhibiting ~2-3 orders of magnitude lower reactivity with the plasma inhibitors. Further studies revealed that Na(+) no longer activates the variant protease, suggesting that the functionally important allosteric linkage between the Na(+)-binding and the P1-binding sites of the protease has been eliminated. These results suggest that the lower catalytic efficiency of FXa-D185del in the bleeding patient may be partially compensated by the loss of its reactivity with plasma inhibitors, possibly explaining the basis for the paradoxical severe FX deficiency with only mild bleeding tendency for this mutation.

New insights into modulation of thrombin formation

Thrombin is a pleiotropic enzyme that regulates hemostasis and nonhemostatic functions, including an array of actions within and on the vasculature. Physiologically, thrombin generation serves mainly to protect against thrombosis, but also to maintain vascular endothelial integrity. This protective effect is mediated in part through generation of anticoagulant enzymes, including activated protein C, formed on the action of thrombin on the endothelial receptor thrombomodulin. Partly, thrombin's vascular effects are effectuated through interaction with protease-activated receptors on various cell types. Pathophysiologically, downregulation and shedding of anticoagulant-acting receptors such as thrombomodulin and endothelial protein C receptor may contribute to a shift in activities of thrombin towards thrombogenic and proinflammatory actions. This shift may typically occur in the process of atherosclerosis, leading to a proatherogenic direction of the effects of thrombin. Therapeutically, the long-term inhibition of thrombin may create new ways of reducing atherosclerosis burden, altering the plaque phenotype.

Tissue factor and tissue factor pathway inhibitor in chronically inflamed gallbladder mucosa

We characterised a tissue factor (TF) and tissue factor pathway inhibitor (TFPI) expression in relation to severity of inflammatory infiltration of the gallbladder mucosa in a chronic cholecystitis. We prospectively studied the gallbladder specimens obtained from 54 patients who had undergone cholecystectomy due to chronic calculous cholecystitis and 16 calculosis-free gallbladder specimens obtained from patients who underwent cholecystectomy due to the polyp/polyps as well as in cases of gallbladder injury. To assess TF and TFPI immunoreactivity by immunohistochemistry, the monoclonal anti-human TF and TFPI antibodies were used. The inflammatory infiltration of the gallbladder mucosa was reflected by the number of CD3 and CD68 positive cells. The expression of TF and TFPI differed significantly between the cholecystitis and the control group. Most capillary endothelial cells of the cholecystitis group presented weak expression for TFPI. The mean number of CD3 positive lymphocytes in the cholecystitis group was 18.6 ± 12.2, but the mean number of CD68 positive cells was 29.7 ± 13.9. In the control sections, it was 3.1 ± 1.9 and 8.8 ± 3.9, respectively (P < 0.001). The results of the current study suggest that the tissue procoagulant state found may be engaged in the etiopathogenesis of the cholecystitis.

The effect of different hormonal contraceptives on plasma levels of free protein S and free TFPI

Use of combined oral contraceptives is associated with a three- to six-fold increased risk of venous thrombosis. Hormonal contraceptives induce acquired resistance to activated protein C (APC), which predicts the risk of venous thrombosis. The biological basis of the acquired APC resistance is unknown. Free protein S (PS) and free tissue factor pathway inhibitor (TFPI) are the two main determinants of APC. Our objective was to assess the effect of both hormonal and non-hormonal contraceptives with different routes of administration on free TFPI and free PS levels. We conducted an observational study in 243 users of different contraceptives and measured APC sensitivity ratios (nAPCsr), free TFPI and free PS levels. Users of contraceptives with the highest risk of venous thrombosis as reported in recent literature, had the lowest free TFPI and free PS levels, and vice versa, women who used contraceptives with the lowest risk of venous thrombosis had the highest free TFPI and free PS levels. An association was observed between levels of free TFPI and nAPCsr, and between free PS and nAPCsr. The effect of oral contraceptives on TFPI and PS is a possible explanation for the increased risk of venous thrombosis associated with oral contraceptives.

Tissue hemostasis and chronic inflammation in colon biopsies of patients with inflammatory bowel disease

Inflammatory bowel disease (IBD) is characterized by a chronic inflammation accompanied by procoagulation settings. However, tissue hemostasis in IBD patients was only incidentally reported. Accordingly, the current study characterizes changes in tissue hemostasis components in a colon inflammatory setting. Serial cryostat sections of endoscopic mucosal biopsy specimens taken from 26 consecutive IBD patients diagnosed de novo and normal colon resection specimens taken from 6 patients were immunohistochemically stained with monoclonal anti-human tissue factor (TF), tissue factor pathway inhibitor (TFPI), thrombomodulin (TM), as well as CD3 and CD68 positive cells. The hemostatic components studied differed significantly from the control subjects. Up-regulation predominated in the case of TF while down-regulation was mainly found in TM and TFPI in IBD. In the control sections, TF was observed in a few fibroblast-shaped cells in the lamina propria, while in the majority of IBD sections, TF positively stained small microvessels, infiltrating mononuclear cells and fibroblast-shaped cells tightly surrounding the colon crypts. Thrombomodulin intensively stained the endothelium of the small capillary vessels in the control, whereas such staining mainly accompanied infiltrating mononuclear cells of the IBD subjects. Tissue factor pathway inhibitor positively stained the endothelium of the small capillary vessels in the control group, whereas in the IBD group endothelial cells presented only weak TFPI staining. The mean number of CD3-positive lymphocytes in IBD was 23.3 ± 14.3, but the mean number of CD68-positive cells was 114.5 ± 55.8. In the control sections, it was 4.1 ± 2.4 and 39.6 ± 17.9, respectively. There was no relationship between CD3 and CD68 (+) cells and the hemostasis markers studied. The results of the current study indicate a shift of tissue hemostasis toward the procoagulant state irrespective of the severity of inflammatory infiltration. In addition, TF distribution in the colon sections of IBD patients may indicate a role in the restoration of the barrier function in injured intestinal mucosa.

Protein C anticoagulant and cytoprotective pathways

Plasma protein C is a serine protease zymogen that is transformed into the active, trypsin-like protease, activated protein C (APC), which can exert multiple activities. For its anticoagulant action, APC causes inactivation of the procoagulant cofactors, factors Va and VIIIa, by limited proteolysis, and APC's anticoagulant activity is promoted by protein S, various lipids, high-density lipoprotein, and factor V. Hereditary heterozygous deficiency of protein C or protein S is linked to moderately increased risk for venous thrombosis, while a severe or total deficiency of either protein is linked to neonatal purpura fulminans. In recent years, the beneficial direct effects of APC on cells which are mediated by several specific receptors have become the focus of much attention. APC-induced signaling can promote multiple cytoprotective actions which can minimize injuries in various preclinical animal injury models. Remarkably, pharmacologic therapy using APC demonstrates substantial neuroprotective effects in various murine injury models, including ischemic stroke. This review summarizes the molecules that are central to the protein C pathways, the relationship of pathway deficiencies to venous thrombosis risk, and mechanisms for the beneficial effects of APC.

Tissue factor pathway inhibitor blocks angiogenesis via its carboxyl terminus

OBJECTIVE

Tissue factor pathway inhibitor (TFPI) is the primary regulator of the tissue factor (TF) coagulation pathway. As such, TFPI may regulate the proangiogenic effects of TF. TFPI may also affect angiogenesis independently of TF, through sequences within its polybasic carboxyl terminus (TFPI C terminus [TFPIct]). We aimed to determine the effects of TFPI on angiogenesis and the role of TFPIct.

METHODS AND RESULTS

Transgenic overexpression of TFPI attenuated angiogenesis in the murine hindlimb ischemia model and an aortic sprout assay. In vitro, TFPI inhibited endothelial cell migration. Peptides within the human TFPIct inhibited endothelial cell cord formation and migration in response to vascular endothelial growth factor (VEGF) 165 but not VEGF121. Furthermore, exposure to human TFPIct inhibited the phosphorylation of VEGF receptor 2 at residue Lys951, a residue known to be critical for endothelial cell migration. Finally, systemic delivery of a murine TFPIct peptide inhibited angiogenesis in the hindlimb model.

CONCLUSION

These data demonstrate an inhibitory role for TFPI in angiogenesis that is, in part, mediated through peptides within its carboxyl terminus. In addition to its known role as a TF antagonist, TFPI, via its carboxyl terminus, may regulate angiogenesis by directly blocking VEGF receptor 2 activation and attenuating the migratory capacity of endothelial cells.

Evaluation of the procoagulant activity of endogenous phospholipids in the platelet-free plasma of children with sickle cell disease using functional assays

BACKGROUND

The mechanisms of hypercoagulability in sickle cell disease (SCD) are poorly understood.

OBJECTIVE

We aimed to explore the procoagulant activity of endogenous phospholipids (ePL) in the platelet-free plasma of SCD children.

METHODS

A factor Xa clotting time (XACT), thrombin generation (TG) and a capture-based assay for the detection of procoagulant microparticles (PMP) were used. Forty three SCD children (35 SS, 6 SC and 2 Sβ+) were evaluated at steady-state and compared to 20 controls. Fourteen patients were also evaluated during vaso-occlusive crisis. TG was performed using 10 pM tissue factor without addition of exogenous phospholipids. A control condition was also performed using 10 pM tissue factor and 4 μM phospholipids. Percentages of the test/control conditions were calculated for the peak height (% peak), endogenous thrombin potential (% ETP) and velocity index (% VI).

RESULTS

XACT times were shorter, PMP levels, peak height and velocity index of thrombin generation were higher in SCD patients than controls. Lag time and ETP were not different between the two groups. % peak, % ETP and % VI were higher in patients than controls. Significant correlations were found between PMP levels and XACT, also between PMP levels and peak height, velocity index, ETP and their respective percentages to the control condition, but not with lag time. Double heterozygous patients showed intermediate values for XACT and TG parameters. No significant difference was observed when comparing patients at steady-state versus vaso-occlusive crisis.

CONCLUSION

High procoagulant activity of ePL was observed in the platelet-free plasma of SCD children, probably borne by procoagulant microparticles. This may contribute to a high hemostatic potential and predisposition to thrombotic complications in these patients.

Increased tissue factor pathway inhibitor activity is associated with myocardial infarction in young women: results from the RATIO study

BACKGROUND

The tissue factor pathway inhibitor (TFPI)/protein S anticoagulant system is a potent inhibitor of blood coagulation. TFPI and protein S are major determinants of thrombin generation (TG) tests determined at low tissue factor (TF) and at high TF concentrations in the presence of activated protein C (APC). Both TFPI and protein S protect against venous thrombosis, but the importance of the TFPI/protein S system in arterial thrombosis remains unclear.

OBJECTIVES

To investigate the influence of the TFPI/protein S anticoagulant system on the risk of myocardial infarction (MI) in young women.

METHODS

The RATIO study is a case-control study in women under 50 years of age, including 205 patients and 638 controls. TFPI and protein S were quantified using ELISA. The TFPI/protein S activity (nTFPIr) and the APC sensitivity ratio (nAPCsr) were determined using TG tests. Odds ratios (ORs) adjusted for putative confounders and corresponding 95% confidence intervals (95% CI) were determined.

RESULTS

Women with MI had higher TFPI levels than controls (135.9 ± 40% vs. 124.2 ± 41%), resulting in increased TFPI/protein S activities and increased APC sensitivity. Furthermore, an increased TFPI activity was associated with MI [nTFPIr: adjusted OR Q1 vs. Q4 = 2.1 (95%CI 1.1-4.1)]. Additionally, an increased APC sensitivity was associated with MI [nAPCsr: adjusted OR Q1 vs. Q4 = 1.7 (95% CI 0.9-3.2)]

CONCLUSION

Women with MI had increased TFPI levels compared with controls. Consequently, the TFPI/protein S activity and APC sensitivity are increased in women with MI. Whether this increase in TFPI activity acts as a compensating mechanism for an increased procoagulant state or is a marker of endothelial damage remains to be investigated.

Complex assemblies of factors IX and X regulate the initiation, maintenance, and shutdown of blood coagulation

Blood hemostasis is accomplished by a complex network of (anti-)coagulatory and fibrinolytic processes. These physiological processes are implemented by the assembly of multiprotein complexes involving both humoral and cellular components. Coagulation factor X, and particularly, factor IX, exemplify the dramatic enhancement that is obtained by the synergistic interaction of cell surface, inorganic and protein cofactors, protease, and substrate. With a focus on structure-function relationship, we review the current knowledge of activity modulation principles in the coagulation proteases factors IX and X and indicate future challenges for hemostasis research. This chapter is organized by describing the principles of hierarchical activation of blood coagulation proteases, including endogenous and exogenous protease activators, cofactor binding, substrate specificities, and protein inhibitors. We conclude by outlining pharmaceutical opportunities for unmet needs in hemophilia and thrombosis.

Protein S inherited qualitative deficiency: novel mutations and phenotypic influence

BACKGROUND

Only a few mutations associated with qualitative protein S deficiency have already been described. Sensitivity and specificity for type II PROS1 mutations of commercially available reagents for measuring Protein S (PS) activity are not well established. Whether these mutations are significant risk factors for thrombosis remains an unresolved question.

METHODS

In order to address the first point, we present and discuss the results of PROS1 analysis performed in the 30 probands with type II PS-inherited deficiency suspicion and 35 relatives, studied in our laboratory between 2000 and 2008. In order to investigate the influence of type II mutations on the coagulability level, thrombin generation tests were performed on plasma from 102 PROS1 type II, type I/III or PS Herleen mutation heterozygous carriers and controls.

RESULTS

Mutations (12 novel, six already described) which probably explain the qualitative phenotype, were found in 27 (90%) out of the 30 probands studied. In relatives, 78% of heterozygotes presented with a type II phenotype. An APC resistance phenotype was documented in type II and type I/III defects heterozygous carriers; however, the effect of type II was milder than the effect of type I/III PS mutations.

CONCLUSIONS

A PS functional assay (Staclot PS, Stago) was efficient in screening for PROS1 type II defects, particularly in probands. A significant positive influence of type II mutations on ex vivo thrombin generation was demonstrated. However, whether these mutations increase the risk of venous thromboembolism requires further investigation.

Dependence on vitamin K-dependent protein S for eukaryotic cell secretion of the beta-chain of C4b-binding protein

The anticoagulant vitamin K-dependent protein S (PS) circulates in plasma in two forms, 30% free and 70% being bound to the complement regulatory protein C4b-binding protein (C4BP). The major C4BP isoform consists of 7 α-chains and 1 β-chain (C4BPβ(+)), the chains being linked by disulfide bridges. PS binds to the β-chain with high affinity. In plasma, PS is in molar excess over C4BPβ(+) and due to the high affinity, all C4BPβ(+) molecules contain a bound PS. Taken together with the observation that PS-deficient patients have decreased levels of C4BPβ(+), this raises the question of whether PS is important for secretion of the β-chain from the cell. To test this hypothesis, HEK293 cells were stably and transiently transfected with β-chain cDNA in combinations with cDNAs for PS and/or the α-chain. The concentration of β-chains in the medium increased after co-transfection with PS cDNA, but not by α-chain cDNA, suggesting secretion of the β-chains from the cells to be dependent on concomitant synthesis of PS, but not of the α-chains. Thus, β-chains that were not disulfide-linked to the α-chains were secreted in complex with PS, either as monomers or dimers. Pulse-chase demonstrated that the complexes between PS and β-chain were formed intracellularly, in the endoplasmic reticulum. In conclusion, our results demonstrate that successful secretion of β-chains depends on intracellular complex formation with PS, but not on the α-chains. This provides an explanation for the decreased β-chain levels observed in PS-deficient patients.

Thrombin generation-based assays to measure the activity of the TFPI-protein S pathway in plasma from normal and protein S-deficient individuals

BACKGROUND

Protein S acts as a cofactor for full-length tissue factor pathway inhibitor (TFPI) in the downregulation of thrombin formation.

OBJECTIVE

To develop a functional test to measure the activity of the TFPI-protein S system in plasma.

METHODS/PATIENTS

Using calibrated automated thrombography, we quantified the activity of the TFPI-protein S system in plasma by measuring thrombin generation in the absence and presence of neutralizing antibodies against protein S or TFPI. Moreover, we designed an enzyme-linked immunosorbent assay (ELISA) to determine the level of full-length TFPI in plasma. The performance of these assays was examined in plasma from 85 normal individuals and from 35 members of protein S-deficient families.

RESULTS

The ratio of thrombin peaks determined in the absence and presence of anti-protein S antibodies (protein S ratio = 0.5 in normal plasma) is a measure of the TFPI cofactor activity of protein S, whereas the ratio of thrombin peaks determined in the absence and presence of anti-TFPI antibodies (TFPI ratio = 0.25 in normal plasma) is a measure of the overall activity of the TFPI-protein S system. Protein S and TFPI ratios were elevated in protein S-deficient individuals, indicating an impairment of the TFPI-protein S system. Both ratios correlated well with full-length TFPI levels, which were significantly lower in protein S-deficient patients than in normal family members.

CONCLUSIONS

Functional assays for the TFPI-protein S system and an ELISA for full-length TFPI were developed. These assays show that the activity of the TFPI-protein S anticoagulant pathway is impaired in individuals with congenital protein S deficiency.

APC resistance: biological basis and acquired influences

Proteolytic inactivation of factors Va (FVa) and VIIIa (FVIIIa) by activated protein C (APC) and its cofactors protein S and factor V (FV) is a key process in the physiological down-regulation of blood coagulation. Functional abnormalities of this pathway, which manifest themselves in vitro as a poor anticoagulant response of plasma to added APC (APC resistance), are prevalent in the general population and are associated with an increased risk of venous thrombosis. APC resistance was originally discovered in thrombophilic families and later shown to be associated with the common FV Arg506Gln (FV(Leiden)) mutation, which abolishes one of the APC-cleavage sites in FV. Although FV(Leiden) is the major cause of hereditary APC resistance, it is becoming increasingly clear that several other genetic and acquired conditions contribute to APC resistance and thereby increase the risk of venous thrombosis. This paper reviews the multifactorial etiology of APC resistance and discusses its clinical implications.

Mechanisms of estrogen-induced venous thromboembolism

The use of oral contraceptives (OC) is a well established risk factor for venous thrombosis. It has been known for many years that almost all haemostatic parameters i.e. plasma levels of coagulation factors, anticoagulant proteins and proteins involved in the fibrinolytic pathway change during OC use. The discovery of several risk factors of venous thrombosis in the 1990s shed new light on the association between the effects of OC on the haemostatic system and the increased risk of venous thrombosis. In this review, we summarize the current knowledge on the effects of different kinds of hormonal contraceptives (OC, transdermal contraceptives, vaginal ring and levonorgestrel-releasing intrauterine device) on haemostatic variables and the relationship between the changes of these variables and the risk of venous thrombosis.

Hereditary and acquired protein S deficiencies are associated with low TFPI levels in plasma

BACKGROUND

Protein S and tissue factor pathway inhibitor (TFPI) act together in down-regulating coagulation.

OBJECTIVE

To investigate the TFPI/protein S system in hereditary and acquired protein S deficiency.

METHODS

Plasma antigen levels of protein S and full-length TFPI were determined in heterozygous type I protein S-deficient individuals (n=35), patients on oral anticoagulant treatment (OAT) (n=29), oral contraceptive (OC) users (n=10) and matched controls. Thrombin generation was determined using calibrated automated thrombography.

RESULTS

Full-length TFPI levels were lower in type I protein S-deficient individuals (76.8+/-33.8%) than in age- and sex-matched controls (128.0+/-59.4%, P<0.001). Among protein S-deficient individuals with thrombosis, those on OAT had not only lower total protein S levels (25.7+/-8.2% vs. 54.7+/-8.2%, P<0.001), but also lower full-length TFPI levels (52.6+/-15.0% vs. 75.4+/-22.9%, P=0.009) than those not on OAT. Similarly, OC users had lower protein S (73.8+/-11.5% vs. 87.9+/-10.8%, P=0.005) and full-length TFPI levels (73.7+/-27.7% vs. 106.4+/-29.2%, P=0.007) than non-users. When triggered with tissue factor, plasma from protein S-deficient individuals generated 3-5-fold more thrombin than control plasma. The difference was only partially corrected by normalization of the protein S level, full correction requiring additional normalization of the TFPI level. Protein S-immunodepletion experiments indicated that free protein S and full-length TFPI form a complex in plasma, and the protein S/TFPI interaction was confirmed by surface plasmon resonance analysis.

CONCLUSIONS

Full-length TFPI binds to protein S in plasma and is reduced in genetic and acquired protein S deficiency. The concomitant TFPI deficiency substantially contributes to the hypercoagulable state associated with protein S deficiency.

Protein S as cofactor for TFPI

In the last decades evidence was obtained that protein S not only acts as cofactor of activated protein C (APC) in the downregulation of coagulation, but also expresses anticoagulant activity in the absence of APC. The search for the mechanism(s) underlying the APC-independent anticoagulant activity of protein S was hampered by the fact that protein S exhibited 2 seemingly identical anticoagulant activities in model systems and in plasma. Later it was shown that the anticoagulant activity of purified protein S in model systems was dependent on the concentration of phospholipid vesicles and was explained by low amounts of protein S multimers generated during purification that effectively inhibited phospholipid-dependent coagulation reactions via competition for phospholipid binding sites. Plasma does not contain multimers, and the anticoagulant activity of protein S in plasma was not affected by the phospholipid concentration but was dependent on the amount of tissue factor (TF) used for initiation of thrombin generation. This led to the discovery that protein S acts as cofactor of tissue factor pathway inhibitor (TFPI) which stimulates the inhibition of factor Xa by TFPI approximately 10-fold. The current review describes the background of the TFPI-cofactor activity of protein S as well as the rationale for the observation that the TFPI/protein S system particularly inhibits the TF pathway at low procoagulant stimuli.