Inhibition of thrombin generation by protein S at low procoagulant stimuli: implications for maintenance of the hemostatic balance

Characterization of thrombophilia-related plasmas evaluated by anticoagulants-mediated thrombin and plasmin generation assays

Disturbances in the balance between coagulation, anticoagulation and fibrinolysis may lead to thrombosis or haemorrhage. Simultaneous assessments of thrombin and plasmin facilitate overall understandings of pathological haemostasis, especially for thrombophilia. Here, we characterized coagulation-fibrinolysis potentials in plasmas with thrombophilia using anticoagulants-mediated thrombin-plasmin generation assay (T/P-GA). T/P-GA was initiated by adding tissue factor, tissue-type plasminogen activator and anticoagulants [recombinant-thrombomodulin (rTM), activated protein (P)C (APC) and antithrombin (AT)], followed by simultaneous thrombin generation and plasma generation monitoring. Patients' plasmas with PC-deficiency (PC-def), PS-deficiency (PS-def), AT-deficiency (AT-def), factor VLeiden (FVL) and antiphospholipid syndrome (APS) were evaluated. A ratio of peak-thrombin (or peak-plasmin) with and without anticoagulants was calculated as anticoagulants (+)/anticoagulants (-). First, TG, in rTM-mediated, PC-def, PS-def and FVL showed higher peak-thrombin ratios than the controls, whereas AT-def and APS exhibited no differences from the controls. In APC-mediated, PC-def, PS-def and AT-def showed low peak-thrombin ratios, similar to the controls, but immune-depleted PS-def (<1%) showed the higher ratio than the controls. FVL and APS showed higher peak-thrombin ratios than the controls. In AT-mediated, peak-thrombin ratios in PS-def, PC-def and APS were lower than in controls, but those in AT-def and FVL was not significantly different from the controls. Second, PG, in rTM-mediated, all thrombophilia plasmas showed low peak-plasmin ratios (∼0.5), but no significant difference was observed, relative to the controls. In APC and AT-mediated, peak-plasmin ratios in thrombophilia-related plasmas were similar to the controls (∼1.0). Anticoagulants-mediated T/P-GA may classify thrombin generation characteristics in thrombophilia-related plasmas upon adding anticoagulants.

Pleiotropic anticoagulant functions of protein S, consequences for the clinical laboratory. Communication from the SSC of the ISTH

Hereditary deficiencies of protein S (PS) increase the risk of thrombosis. However, assessing the plasma levels of PS is complicated by its manifold physiological interactions, while the large inter-individual variability makes it problematic to establish reliable cut-off values. PS has multiple physiological functions, with only two appearing to have significant anticoagulant properties: the activated protein C (APC) and tissue factor pathway inhibitor alpha (TFPIα) cofactor activities. Current clinical laboratory investigations for deficiency in PS function rely only on the APC-dependent activity. This communication presents an argument for reclassifying the qualitative PS deficiencies to differentiate the two major anticoagulant functions of PS. Reliable assays are necessary for accurate evaluation of PS function when making a specific diagnosis of PS deficiency based on the anticoagulant phenotype alone. This report emphasizes the pleiotropic anticoagulant functions of PS and presents evidence-based recommendations for their implementation in the clinical laboratory.

Identification of a Plasma MicroRNA Profile Associated With Venous Thrombosis

OBJECTIVE

Venous thrombosis (VT) is a complex condition with a highly heritable genetic component that predisposes one to its development. Certain microRNAs (miRNAs) might be used as biomarkers of VT, but few studies have examined miRNA expression in this respect. The aim of the present work was to identify a plasma miRNA profile associated with VT. Approach and Results: miRNAs were analyzed by quantitative polymerase chain reaction in plasma samples from members of the GAIT-2 (Genetic Analysis of Idiopathic Thrombophilia 2) population (n=935). A discovery phase involving the screening of 752 miRNAs from a subset of 104 GAIT-2 subjects was followed by an internal validation phase in which the selected miRNAs were quantified in the whole GAIT-2 population. In the discovery phase, 16 miRNAs were selected, including 9 associated with VT and 7 that correlated with an intermediate phenotype of VT. In the next phase, 4 miRNAs were validated as differentially expressed (false discovery rate, <0.1) in VT: hsa-miR-126-3p, hsa-miR-885-5p, hsa-miR-194-5p, and hsa-miR-192-5p. The 4 miRNAs each returned a significant (P<0.05) odds ratio for VT (range of 1.3-1.8). A risk model including the 4 miRNAs, age, and sex returned an area under the receiver operating characteristic curve of 0.77. Moreover, all 4 miRNAs showed significant correlations with intermediate phenotypes of VT (eg, protein S and factor VII). The targets of the miRNAs in the blood coagulation pathway and their interactions are also discussed.

CONCLUSIONS

The present results suggest a 4-miRNA plasma profile associated with VT is of potential use in predicting the risk of this condition.

High protein S activity due to C4b-binding protein deficiency in a 34-year-old Surinamese female with ischemic retinopathy

In this study, we present the first case of a 34‐year‐old Surinamese female with ischemic retinopathy and increased free protein S due to C4BP deficiency. Possibly, the low PS/C4BP complex level has increased the risk of arterial thrombosis in our patient.

Role of exosite binding modulators in the inhibition of Fxa by TFPI

Tissue factor pathway inhibitor (TFPI) down-regulates the extrinsic coagulation pathway by inhibiting FXa and FVIIa. Both TFPI and FXa interact with several plasma proteins (e. g. prothrombin, FV/FVa, protein S) and non-proteinaceous compounds (e. g. phospholipids, heparin). It was our aim to investigate effects of ligands that bind to FXa and TFPI on FXa inhibition by full-length TFPI (designated TFPI) and truncated TFPI (TFPI1-150). Inhibition of FXa by TFPI and TFPI1-150 and effects of phospholipids, heparin, prothrombin, FV, FVa, and protein S thereon was quantified from progress curves of conversion of the FXa-specific chromogenic substrate CS11-(65). Low concentrations negatively charged phospholipids (~10 μM) already maximally stimulated (up to 5- to 6-fold) FXa inhibition by TFPI. Unfractionated heparin at concentrations (0.2–1 U/ml) enhanced FXa inhibition by TFPI ~8-fold, but impaired inhibition at concentrations > 1 U/ml. Physiological protein S and FV concentrations both enhanced FXa inhibition by TFPI 2- to 3-fold. In contrast, thrombin-activated FV (FVa) impaired the ability of TFPI to inhibit FXa. FXa inhibition by TFPI1–150 was not affected by FV, FVa, protein S, phospholipids and heparin. TFPI potently inhibited FXa-catalysed prothrombin activation in the absence of FVa, but hardly inhibited prothrombin activation in the presence of thrombin-activated FVa. In conclusion, physiological concentrations TFPI (0.25–0.5 nM TFPI) inhibit FXa with a t1/2 between 3–15 minutes. Direct FXa inhibition by TFPI is modulated by physiological concentrations prothrombin, FV, FVa, protein S, phospholipids and heparin indicating the importance of these modulators for the in vivo anticoagulant activity of TFPI.

Platelet-mediated proteolytic down regulation of the anticoagulant activity of protein S in individuals with haematological malignancies

The natural anticoagulant protein S contains a so-called thrombin-sensitive region (TSR), which is susceptible to proteolytic cleavage. We have previously shown that a platelet-associated protease is able to cleave protein S under physiological plasma conditions in vitro. The aim of the present study was to investigate the relation between platelet-associated protein S cleaving activity and in vivo protein S cleavage, and to evaluate the impact of in vivo protein S cleavage on its anticoagulant activity. Protein S cleavage in healthy subjects and in thrombocytopenic and thrombocythaemic patients was evaluated by immunological techniques. Concentration of cleaved and intact protein S was correlated to levels of activated protein C (APC)-dependent and APC-independent protein S anticoagulant activity. In plasma from healthy volunteers 25% of protein S is cleaved in the TSR. While in plasma there was a clear positive correlation between levels of intact protein S and both APC-dependent and APC-independent protein S anticoagulant activities, these correlations were absent for cleaved protein S. Protein S cleavage was significantly increased in patients with essential thrombocythaemia (ET) and significantly reduced in patients with chemotherapy-induced thrombocytopenia. In ET patients on cytoreductive therapy, both platelet count and protein S cleavage returned to normal values. Accordingly, platelet transfusion restored cleavage of protein S to normal values in patients with chemotherapy-induced thrombocytopenia. In conclusion, proteases from platelets seem to contribute to the presence of cleaved protein S in the circulation and may enhance the coagulation response in vivo by down regulating the anticoagulant activity of protein S.

Endogenous thrombin potential changes during the first cycle of oral contraceptive use

OBJECTIVES

Venous thromboembolism (VTE) risk increases within months of combination oral contraceptive (COC) initiation. Because elevated endogenous thrombin potential (ETP) has been found in several studies to be a VTE risk factor, we evaluated the extent of ETP changes during the initial cycle of an ethinyl estradiol (EE) and levonorgestrel (LNG) COC. We also assessed the relationship between ETP changes and systemic EE and LNG concentrations.

STUDY DESIGN

Participants provided multiple blood samples during a first 21-day cycle of a 30-mcg EE/150-mcg LNG COC and after a further 7 days without an active COC. Thrombin generation measured with and without addition of activated protein C (APC) yielded ETP+APC and ETP-APC and the normalized APC sensitivity ratio (nAPCsr). EE and LNG pharmacokinetic analyses were conducted over 24 h after the first COC tablet and again at steady state.

RESULTS

Thrombin generation was determined in 16 of the 17 women who completed the study. Mean ETP-APC increased steadily to 21% above baseline at 24 h after the 6th COC tablet (COC624; p<.001) and to 28% above baseline at steady state (COC21; p<.001). The percentage increase in mean ETP+APC was considerably more - 54% at COC624 and 79% at steady state. Mean nAPCsr increased by 28% at COC624 and by 41% at steady state. Higher concentrations of EE or LNG were not correlated with greater increases in ETP.

CONCLUSIONS

ETP increases during the first COC cycle were substantial.

IMPLICATIONS

The early increases in ETP may provide biological support for the rapid increase in VTE risk during initial COC use. The lack of association between this clotting system perturbation and the systemic EE concentration is surprising and deserves further study.

New functional assays to selectively quantify the activated protein C- and tissue factor pathway inhibitor-cofactor activities of protein S in plasma

Essentials Protein S is a cofactor of activated protein C (APC) and tissue factor pathway inhibitor (TFPI). There are no assays to quantify separate APC and TFPI cofactor activities of protein S in plasma. We developed assays to measure the APC- and TFPI-cofactor activities of protein S in plasma. The assays were sensitive to protein S deficiency, and not affected by the Factor V Leiden mutation.

SUMMARY

Background Protein S plays an important role in the down-regulation of coagulation as cofactor for activated protein C (APC) and tissue factor pathway inhibitor (TFPI). Aim To develop functional assays to quantify the APC- and TFPI-cofactor activities of protein S in plasma. Methods APC- and TFPI-cofactor activities of protein S in plasma were measured using calibrated automated thrombography in protein S-depleted plasma supplemented with a small amount of sample plasma either in the presence of anti-TFPI antibodies and APC (APC-cofactor activity) or at excess full-length TFPI without APC (TFPI-cofactor activity). Total and free protein S levels in plasma were measured by ELISAs. Results Average APC-cofactor activities of protein S were 113%, 108% and 89% in plasma from normal individuals (n = 15), FV Leiden heterozygotes (n = 14) and FV Leiden homozygotes (n = 7), respectively, whereas the average APC-cofactor activity of protein S in plasma from heterozygous protein S-deficient individuals (n = 21) was significantly lower (55%). Similar trends were observed for the TFPI-cofactor activity of protein S, with averages of 109%, 115% and 124% in plasma from individuals with normal protein S levels and different FV Leiden genotypes, and 64% in plasma from protein S-deficient patients. APC-cofactor activities of protein S correlated significantly with free and total protein S antigen levels, whereas TFPI-cofactor activities correlated less with protein S antigen levels. Conclusion We have developed functional protein S assays that measure both the APC- and TFPI-cofactor activities of protein S in plasma, which are hardly if at all affected by the FV Leiden mutation.

Serum omentin and vaspin levels in cirrhotic patients with and without portal vein thrombosis

AIM

To investigate serum omentin and vaspin levels in cirrhotic patients; and to assess the relationship of these levels with hemostatic parameters, metabolic abnormalities, cirrhosis severity and etiology.

METHODS

Fifty-one cirrhotic patients (17 with portal vein thrombosis) were analyzed. Serum omentin and vaspin levels were measured with commercially available direct enzyme-linked immunosorbent assays (ELISAs). To assess platelet activity, the following tests were performed using a MULTIPLATE^®PLATELET FUNCTION ANALYZER: (1) an ADP-induced platelet activation test; (2) a cyclooxygenase dependent aggregation test (ASPI test); (3) a von Willebrand factor and glycoprotein Ib-dependent aggregation (using ristocetin) test (RISTO test); and (4) a test for thrombin receptor-activating peptide-6 induced activation of the thrombin receptor, which is sensitive to IIb/IIIa receptor antagonists.

RESULTS

Omentin, but not vaspin, serum concentrations were significantly decreased in patients with portal vein thrombosis (PVT) (P = 0.01). Prothrombin levels were significantly increased in patients with PVT (P = 0.01). The thrombin receptor activating peptide (TRAP) test results were significantly lower in the PVT group (P = 0.03). No significant differences in adipokines serum levels were found regarding the etiology or severity of liver cirrhosis assessed according to the Child-Pugh or Model of End-Stage Liver Disease (MELD) scores. There was a significant increase in the TRAP (P = 0.03), ASPI (P = 0.001) and RISTO high-test (P = 0.02) results in patients with lower MELD scores. Serum omentin and vaspin levels were significantly down-regulated in patients without insulin resistance (P = 0.03, P = 0.02, respectively). A positive relationship between omentin and vaspin levels were found both when all of the patients were analyzed (r = 0.41, P = 0.01) and among those with PVT (r = 0.94, P < 0.001).

CONCLUSION

Serum omentin levels are increased in patients without PVT. Cirrhosis origin and grade do not affect omentin and vaspin levels. The analyzed adipokines do not influence platelet activity.

Portal vein thrombosis in cirrhosis is not associated with intestinal barrier disruption or increased platelet aggregability

OBJECTIVE

Portal vein thrombosis (PVT) is a common complication of cirrhosis, but its pathogenesis is unclear. We tested the hypotheses that PVT is the result of platelet hyperactivity or intestinal barrier disruption.

METHODS

This study included 49 patients with cirrhosis (15 females) of mixed etiology. Based on spiral computed-tomography, the patients were divided into two groups: with PVT (n=16) and without PVT (n=33). Serum biomarkers of intestinal barrier integrity were endotoxins and zonulin, and platelet activity was assessed with multiple electrode aggregometry.

RESULTS

The levels of endotoxin (43.5±18.3ng/ml vs. 36.9±7.5ng/ml; P=0.19) and zonulin (56.3±31.1ng/ml vs. 69.3±63.1ng/ml; P=0.69) were not different between the patients with and without PVT. Moreover, endotoxin and zonulin did not correlate with the coagulation and platelet parameters. The platelet aggregability measured with the TRAP and the ADP tests was decreased in PVT patients. In the logistic regression analysis the PVT incidence was related to the levels of D-dimer and bilirubin as well as the TRAP test results. Patients with PVT presented with significantly higher levels of D-dimer (4.45±2.59 vs. 3.03±2.97mg/l; P<0.05) and prothrombin levels (175±98.8μg/ml vs. 115±72.9μg/ml; P<0.05) than patients without thrombosis. PVT could be excluded with a 90% negative predictive value when the D-dimer level was below 1.82mg/l.

CONCLUSIONS

Endotoxemia and platelet activity are not determinants of PVT in patients with cirrhosis. The D-dimer measurement has diagnostic significance for PVT in patients with liver cirrhosis.

Progestin isoforms provide different levels of protein S expression in HepG2 cells

INTRODUCTION

Use of combined oral contraceptives (COCs) results in acquired protein S (PS) deficiency, a well-established risk factor for venous thromboembolism (VTE). The risk of VTE due to COCs containing newer-generation progestins is double compared with COCs containing older-generation progestins, although there is little difference in estrogen contents between the generations. In contrast, progestin-only contraceptives do not confer an increased risk of VTE. In this study, we aimed to investigate how different isoforms of progestin in COCs affect the risk of VTE by measuring PS expression.

MATERIALS AND METHODS

The effect of progestin, levonorgestrel (LNG) or drospirenone (DRSP), on PS mRNA expression in HepG2 cells was measured using reverse transcription-quantitative PCR; PS level was determined using Western blot analysis. PROS1 promoter activity, PS mRNA stability, and de novo synthesis of PS mRNA were examined in HepG2 cells after treatment with progestin.

RESULTS AND CONCLUSIONS

In the presence of progestins, PS mRNA and protein expressions were significantly upregulated in HepG2 cells due to the augmentation of de novo PS mRNA expression modulated by RNA polymerase II (Pol II), thereby facilitating PS transcription elongation. Moreover, the transcription elongation inhibitor blocked progestin-mediated de novo PS mRNA expression. Conversely, progestin did not affect PROS1 promoter activity and PS mRNA stability. Pol II elongation efficiency in the newer-generation progestin (DRSP) treatment was not as strong compared with older-generation progestin (LNG), suggesting the difference in VTE risk between COC generations.

Tissue factor-independent inhibition of thrombin generation by tissue factor pathway inhibitor-α

BACKGROUND

Tissue factor pathway inhibitor-α (TFPIα) inhibits factor Xa by forming a binary TFPI-FXa complex in a reaction that is stimulated by protein S. TF-FVIIa forms a quaternary complex with TFPIα and FXa, which shuts off the initiation of coagulation via the extrinsic pathway.

AIM

To investigate whether direct inhibition of FXa by TFPIα independently of TF plays a role in downregulating coagulation.

METHODS

Inhibition of FXa by TFPIα in plasma was determined by measuring thrombin generation triggered with FXa, the FX activator from Russell's viper venom (RVV-X), FXIa, or FIXa. TF-independent anticoagulant activities of TFPIα and its cofactor, protein S, were quantified: (i) after neutralization of TFPIα and protein S with anti-TFPI or anti-protein S antibodies; and (ii) in TFPI-depleted or protein S-depleted plasmas supplemented with varying amounts of TFPIα or protein S.

RESULTS

Both anti-TFPI and anti-protein S antibodies enhanced thrombin generation in plasma triggered with RVV-X, FXa, FIXa, or FXIa. Anti-TFPI and anti-protein S antibodies decreased the lag time and increased the peak height of thrombin generation to the same extent, indicating that inhibition of FXa by TFPIα requires the presence of protein S. TFPIα and protein S titrations in TFPI-depleted or protein S-depleted plasma in which thrombin formation was initiated with triggers other than TF also revealed TF-independent anticoagulant activity of TFPIα, which was completely dependent on the presence of protein S.

CONCLUSION

Direct inhibition of FXa by TFPIα contributes to the downregulation of coagulation.

Inhibition of tissue factor:factor VIIa-catalyzed factor IX and factor X activation by TFPI and TFPI constructs

BACKGROUND

TFPI is a Kunitz-type protease inhibitor that downregulates the extrinsic coagulation pathway by inhibiting factor Xa (FXa) and FVIIa. All three Kunitz domains (KD1, KD2, and KD3) and protein S are required for optimal inhibition of FXa and FVIIa. There is limited information on Kunitz domain requirements of the inhibition of TF:FVIIa-catalyzed FIX and FX activation by TFPI.

AIM

To investigate the role of the Kunitz domains of TFPI and protein S in the inhibition of FX and FIX activation.

METHODS

Inhibition of TF:FVIIa-catalyzed FX and FIX activation by full-length TFPI (TFPIFL ) and TFPI constructs was quantified from progress curves of FXa and FIXa generation measured with chromogenic substrates.

RESULTS AND CONCLUSIONS

TFPIFL inhibited TF:FVIIa-catalyzed FIX activation with a Ki of 16.7 nmol L(-1) . Protein S reduced the Ki to 1.0 nmol L(-1) . TFPI1-150 and KD1-KD2 had 10-fold higher Ki values and were not stimulated by protein S. Single Kunitz domains were poor inhibitors of TF:FVIIa-catalyzed FIX activation (Ki >800 nm). FX activation was measured at limiting FVIIa and excess TF or vice versa. At both conditions, TFPIFL , TFPI1-150 , and KD1-KD2 showed similar inhibition of FX activation. However, at low phospholipid concentrations, TFPIFL was ~ 15-fold more active than TFPI1-150 or KD1-KD2. Apparently, excess phospholipids act as a kind of sink for TFPIFL , limiting its availability for TF:FVIIa inhibition. Preformed FXa:TFPIFL/1-150 complexes rapidly and stoichiometrically inhibited FIX and FX activation by TF:FVIIa, indicating that binary TFPI:FXa complex formation is the limiting step in TF:FVIIa inhibition. Protein S also enhanced inhibition of TF:FVIIa-catalyzed FX activation by TFPI.

Protein S and factor V in regulation of coagulation on platelet microparticles by activated protein C

INTRODUCTION

Platelets are the main source of microparticles in plasma and the concentration of microparticles is increased in many diseases. As microparticles expose negatively charged phospholipids, they can bind and assemble the procoagulant enzyme-cofactor complexes. Our aim was to elucidate possible regulation of these complexes on microparticles by the anticoagulant protein C system.

MATERIALS AND METHODS

Platelets were activated with thrombin ± collagen or the calcium ionophore A23187 ± thrombin to generate microparticles. The microparticles were analyzed using flow cytometry and functional coagulation assays to characterize parameters with importance for the activated protein C system.

RESULTS

Activation with A23187+thrombin was most efficient, fully converting the platelets to microparticle-like vesicles, characterized by high lactadherin and protein S binding capacity. Suppression of thrombin generation by activated protein C in plasma spiked with these microparticles was dependent on the presence of plasma protein S. Experiments with purified components showed that activated protein C inhibited both factor Va and factor VIIIa on the microparticle surface. Inhibition of factor Va was stimulated by, but not fully dependent on, the presence of protein S. In the factor VIIIa-degradation, activated protein C was dependent on the addition of protein S, and exogenous factor V further increased the efficiency.

CONCLUSIONS

Protein S is crucial for activated protein C-mediated inhibition of thrombin generation on platelet-derived microparticles in plasma. Moreover, protein S and factor V are synergistic cofactors in the inhibition of factor VIIIa. The results demonstrate that the activated protein C system has the capacity to counterbalance the procoagulant ability of microparticles.

Small peptides blocking inhibition of factor Xa and tissue factor-factor VIIa by tissue factor pathway inhibitor (TFPI)

Tissue factor pathway inhibitor (TFPI) is a Kunitz-type protease inhibitor that inhibits activated factor X (FXa) via a slow-tight binding mechanism and tissue factor-activated FVII (TF-FVIIa) via formation of a quaternary FXa-TFPI-TF-FVIIa complex. Inhibition of TFPI enhances coagulation in hemophilia models. Using a library approach, we selected and subsequently optimized peptides that bind TFPI and block its anticoagulant activity. One peptide (termed compound 3), bound with high affinity to the Kunitz-1 (K1) domain of TFPI (Kd ∼1 nM). We solved the crystal structure of this peptide in complex with the K1 of TFPI at 2.55-Å resolution. The structure of compound 3 can be segmented into a N-terminal anchor; an Ω-shaped loop; an intermediate segment; a tight glycine-loop; and a C-terminal α-helix that is anchored to K1 at its reactive center loop and two-stranded β-sheet. The contact surface has an overall hydrophobic character with some charged hot spots. In a model system, compound 3 blocked FXa inhibition by TFPI (EC50 = 11 nM) and inhibition of TF-FVIIa-catalyzed FX activation by TFPI (EC50 = 2 nM). The peptide prevented transition from the loose to the tight FXa-TFPI complex, but did not affect formation of the loose FXa-TFPI complex. The K1 domain of TFPI binds and inhibits FVIIa and the K2 domain similarly inhibits FXa. Because compound 3 binds to K1, our data show that K1 is not only important for FVIIa inhibition but also for FXa inhibition, i.e. for the transition of the loose to the tight FXa-TFPI complex. This mode of action translates into normalization of coagulation of hemophilia plasmas. Compound 3 thus bears potential to prevent bleeding in hemophilia patients.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Decreased tissue factor pathway inhibitor (TFPI)-dependent anticoagulant capacity in patients with cirrhosis who have decreased protein S but normal TFPI plasma levels

Protein S acts as a cofactor for tissue factor pathway inhibitor (TFPI) in the down regulation of thrombin generation, and acquired and congenital protein S deficiencies are associated with a concomitant TFPI deficiency. In contrast, in patients with liver diseases, decreased protein S, but normal or increased levels of TFPI have been reported. We compared TFPI and protein S plasma levels between 26 patients with cirrhosis and 20 healthy controls and found that TFPI levels were comparable between patients (111 ± 38%) and controls (108 ± 27%), despite reduced protein S levels (74 ± 23% in patients vs. 98 ± 10% in controls). Subsequently, we quantified the activity of the TFPI-protein S system by measuring thrombin generation in the absence and presence of neutralizing antibodies to protein S or TFPI. Ratios of peak thrombin generation in the absence and presence of these antibodies were calculated. Both the protein S and the TFPI ratios were increased in patients with cirrhosis compared to controls. Protein S ratios were (0·62 [0·08-0·93] in patients vs. 0·32 [0·20-0·54] in controls; TFPI ratios were 0·50 [0·05-0·90] in patients vs. 0·18 [0·11-0·49] in controls). Thus, although the acquired protein S deficiency in patients with cirrhosis is not associated with decreased TFPI levels, the TFPI/protein S anticoagulant system is functionally impaired.

TFPI resistance related to inherited or acquired protein S deficiency

BACKGROUND

Protein S (PS) is an essential component of the protein C pathway and PS deficiency can explain a poor response to activated protein C. It has recently been shown that PS also acts as a cofactor of Tissue Factor Pathway Inhibitor (TFPI).

OBJECTIVES

In the present study, we investigated whether PS deficiency could be responsible for a poor response to TFPI.

PATIENTS/METHODS

Thirty-one patients with inherited PS deficiency, seven pregnant women and 36 controls were enrolled in the study. We measured the plasma response to added TFPI using a two-step diluted prothrombin time (dPT) assay. The response of the different plasmas to the anticoagulant activity of TFPI was expressed as TFPI Normalised Ratio (TFPI NR).

RESULTS

The median TFPI NR was statistically significantly lower in patients with inherited PS deficiency (0.5) than in controls (1.0) (p<0.0001). It was statistically significantly lower in patients with type I inherited PS deficiency (0.47) compared to patients with type III inherited PS deficiency (0.58) (p=0.018). In contrast, it did not differ between patients with and without thrombosis. Median TFPI NR values were statistically significantly lower during pregnancy (0.54) than 3 months after delivery (0.71) (p=0.016). TFPI NR values correlated well with PS activity values (R(2)=0.681) whatever the nature of the PS deficiency.

CONCLUSIONS

Our findings confirm that PS deficiency results in a poor anticoagulant response to TFPI, demonstrating again the cofactor role of PS in TFPI activity.

Inhibition of Intrinsic Xase by Protein S

Objective—

Protein S is a vitamin K–dependent plasma protein that functions in the feedback regulation of thrombin generation. Our goal was to determine how protein S regulates the intrinsic pathway of blood coagulation.

Methods and Results—

We used plasma, including platelet-rich plasma, and in vitro methods to determine how the intrinsic pathway of blood coagulation is regulated by protein S. We obtained the following results: (1) activated partial thromboplastin time assays with protein S–supplemented plasma confirmed that protein S prolongs clotting time; (2) a modified activated partial thromboplastin time assay with factor IX (fIX)–deficient plasma confirmed that protein S affects fIX-initiated clotting; (3) a fIXa/factor VIIIa (fVIIIa)–mediated thrombin generation assay with either platelet-rich plasma or factor-deficient plasma, initiated with a limiting amount of tissue factor, was regulated by protein S; (4) in the presence of phosphatidylserine vesicles, protein S inhibited fIXa in the absence and presence of fVIIIa; and (5) protein S altered only the K M for factor X activation by fIXa in the absence of fVIIIa and both k cat and K M in the presence of fVIIIa.

Conclusion—

From our findings, it can be concluded that protein S inhibits fIXa in the presence or absence of fVIIIa in an activated protein C–independent way.

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

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

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.

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.

Activated protein C cofactor function of protein S: a critical role for Asp95 in the EGF1-like domain

Protein S has an established role in the protein C anticoagulant pathway, where it enhances the factor Va (FVa) and factor VIIIa (FVIIIa) inactivating property of activated protein C (APC). Despite its physiological role and clinical importance, the molecular basis of its action is not fully understood. To clarify the mechanism of the protein S interaction with APC, we have constructed and expressed a library of composite or point variants of human protein S, with residue substitutions introduced into the Gla, thrombin-sensitive region (TSR), epidermal growth factor 1 (EGF1), and EGF2 domains. Cofactor activity for APC was evaluated by calibrated automated thrombography (CAT) using protein S-deficient plasma. Of 27 variants tested initially, only one, protein S D95A (within the EGF1 domain), was largely devoid of functional APC cofactor activity. Protein S D95A was, however, gamma-carboxylated and bound phospholipids with an apparent dissociation constant (Kd(app)) similar to that of wild-type (WT) protein S. In a purified assay using FVa R506Q/R679Q, purified protein S D95A was shown to have greatly reduced ability to enhance APC-induced cleavage of FVa Arg306. It is concluded that residue Asp95 within EGF1 is critical for APC cofactor function of protein S and could define a principal functional interaction site for APC.

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.

C4BPB/C4BPA is a new susceptibility locus for venous thrombosis with unknown protein S-independent mechanism: results from genome-wide association and gene expression analyses followed by case-control studies

Through its binding with protein S (PS), a key element of the coagulation/fibrinolysis cascade, the C4b-binding protein (C4BP) has been hypothesized to be involved in the susceptibility to venous thrombosis (VT). To identify genetic factors that may influence the plasma levels of the 3 C4BP existing isoforms, alpha(7)beta(1), alpha(6)beta(1), and alpha(7)beta(0), we conducted a genome-wide association study by analyzing 283 437 single nucleotide polymorphisms (SNPs) in the Genetic Analysis of Idiopathic Thrombophilia (GAIT) study composed of 352 persons. Three SNPs at the C4BPB/C4BPA locus were found genome-wide significantly associated with alpha(7)beta(0) levels. One of these SNPs was further found to explain approximately 11% of the variability of mRNA C4BPA expression in the Gutenberg Heart Study composed of 1490 persons, with no effect on C4BPB mRNA expression. The allele associated with increased alpha(7)beta(0) plasma levels and increased C4BPA expression was further found associated with increased risk of VT (odds ratio [OR] = 1.24 [1.03-1.53]) in 2 independent case-control studies (MARseille THrombosis Association study [MARTHA] and FActeurs de RIsque et de récidives de la maladie thromboembolique VEineuse [FARIVE]) gathering 1706 cases and 1379 controls. This SNP was not associated with free PS or total PS. In conclusion, we observed strong evidence that the C4BPB/C4BPA locus is a new susceptibility locus for VT through a PS-independent mechanism that remains to be elucidated.

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.

Regulation of TFPI function by protein S

Protein S is an anticoagulant cofactor of full-length tissue factor pathway inhibitor (TFPI) that facilitates optimal factor Xa-inhibition and efficient down-regulation of thrombin generation in plasma. Protein S and TFPI are constitutively active in plasma and therefore provide an effective anticoagulant barrier against unwanted procoagulant activity in the circulation. In this review, we describe the current status on how TFPI-activity depends on protein S, and show that TFPI and protein S are major regulators of thrombin generation both in the absence and presence of activated protein C (APC). As there is covariation of plasma TFPI and protein S levels both in health and in disease, these findings suggest that the risk of venous thrombosis associated with protein S deficiency states might be in part explained by the accompanying low plasma TFPI levels.

Platelet factor 4 impairs the anticoagulant activity of activated protein C

Platelet factor 4 (PF4) is an abundant platelet alpha-granule chemokine released following platelet activation. PF4 interacts with thrombomodulin and the gamma-carboxyglutamic acid (Gla) domain of protein C, thereby enhancing activated protein C (APC) generation by the thrombin-thrombomodulin complex. However, the protein C Gla domain not only mediates protein C activation in vivo, but also plays a critical role in modulating the diverse functional properties of APC once generated. In this study we demonstrate that PF4 significantly inhibits APC anti-coagulant activity. PF4 inhibited both protein S-dependent APC anticoagulant function in plasma and protein S-dependent factor Va (FVa) proteolysis 3- to 5-fold, demonstrating that PF4 impairs protein S cofactor enhancement of APC anticoagulant function. Using recombinant factor Va variants FVa-R506Q/R679Q and FVa-R306Q/R679Q, PF4 was shown to impair APC proteolysis of FVa at position Arg(306) by 3-fold both in the presence and absence of protein S. These data suggest that PF4 contributes to the poorly understood APC resistance phenotype associated with activated platelets. Finally, despite PF4 binding to the APC Gla domain, we show that APC in the presence of PF4 retains its ability to initiate PAR-1-mediated cytoprotective signaling. In summary, we propose that PF4 acts as a critical regulator of APC generation, but also differentially targets APC toward cytoprotective, rather than anticoagulant function at sites of vascular injury with concurrent platelet activation.

Thrombin generation testing in routine clinical practice: are we there yet?

Thrombin is the central enzyme in the coagulation cascade. Estimation of an individual's potential to generate thrombin may correlate more closely with a hyper- or hypo-coagulable phenotype, compared to traditional coagulation tests. The possible correlation and recent technical advances in thrombin generation measurement has caused a significant interest in the method and the development of commercial assays. Several variations of the assay exist depending on the defect to be investigated. Fluorogenic thrombin generation assays have acceptable intra-laboratory variation but a higher inter-laboratory variation. Variation in preanalytical variables makes comparisons between studies difficult. Thrombin generation is highly variable between individuals and there are suggestions that this may allow individualized treatment based on global haemostatic response in patients with bleeding disorders or on anticoagulant therapy. In patients with thrombotic disorders it may be possible to identify those at higher risk of recurrent thrombosis. For both scenarios, however, data from large prospective studies are lacking or inconclusive and a good relationship between thrombin generation and phenotype remains to be established. Further standardization of the assay is needed before large multicentre studies can be conducted and until then thrombin generation in routine clinical practice is not yet a reality.

Vitamin K-induced modification of coagulation phenotype in VKORC1 homozygous deficiency

BACKGROUND

Combined vitamin K-dependent clotting factor (VKCF) deficiency type 2 (VKCFD2) is a rare bleeding disorder caused by mutated vitamin K 2,3-epoxide reductase complex subunit 1 (VKORC1) gene.

METHODS AND RESULTS

An Italian patient with moderate to severe bleeding tendency was genotyped, and found to be homozygous for the unique VKORC1 mutation (Arg98Trp) so far detected in VKCFD2. The activity levels of VKCFs were differentially reduced, and inversely related to the previously estimated affinity of procoagulant factor propeptides for the gamma-carboxylase. The normal (factor IX) or reduced antigen levels (other VKCFs) produced a gradient in specific activities. Vitamin K supplementations resulted in reproducible, fast and sustained normalization of PT and APTT. At 24 h the activity/antigen ratios of VKCFs were close to normal, and activity levels were completely (factor VII and IX), virtually (prothrombin, factor X and protein C) or partially (protein S) restored. Thrombin generation assays showed a markedly shortened lag time. The time to peak observed at low tissue factor concentration, potentially mimicking the physiological trigger and able to highlight the effect of reduced protein S levels, was shorter than that in pooled normal plasma. At 72 h the thrombin generation times were normal, and the decrease in activity of procoagulant VKCFs was inversely related to their half-life in plasma. The improved coagulation phenotype permitted the uneventful clinical course after invasive diagnostic procedures.

CONCLUSIONS

Modification of coagulation phenotypes in VKCFD2 after vitamin K supplementation was clinically beneficial, and provided valuable patterns of factor specific biosynthesis, half-life and decay.

Re-evaluation of the role of the protein S-C4b binding protein complex in activated protein C-catalyzed factor Va-inactivation

Protein S expresses cofactor activity for activated protein C (APC) by enhancing the APC-catalyzed proteolysis at R306 in factor Va. It is generally accepted that only free protein S is active and that complex formation with C4b-binding protein (C4BP) inhibits the APC-cofactor activity of protein S. However, the present study shows that protein S-C4BP expresses APC-cofactor activity and stimulates APC-catalyzed proteolysis at R306 more than 10-fold, but instead inhibits proteolysis at R506 by APC 3- to 4-fold. Free protein S stimulates APC-catalyzed cleavage at R306 approximately 20-fold and has no effect on cleavage at R506. The resulting net effect of protein S-C4BP complex formation on APC-catalyzed factor Va inactivation is a 6- to 8-fold reduction in factor Va inactivation when compared with free protein S, which is not explained by inhibition of APC-cofactor activity of protein S at R306, but by generation of a specific inhibitor for APCcatalyzed proteolysis at R506 of factor Va. These results are of interest for carriers of the factor VLeiden mutation (R506Q), as protein S-C4BP effectively enhances APC-catalyzed factor Va (R306) inactivation in plasma containing factor VLeiden.

The effect of tissue factor concentration on calibrated automated thrombography in the presence of inhibitor bypass agents

Thrombin generation has been suggested as a method to monitor treatment with factor eight inhibitor bypassing activity (FEIBA) or recombinant FVIIa (rFVIIa). The sensitivity of the assay for individual coagulation factors is dependent on the tissue factor (TF) concentration. An inverse relation between the rFVIIa concentration needed to shorten the clotting time and TF concentration has been shown but the data on thrombin generation are inconsistent. Information on TF concentration in measurements with FEIBA is limited. We studied the influence of TF concentration (1 and 5 pM) on thrombin generation through spiking experiments with rFVIIa and/or FEIBA in the plasma of severe haemophilia A patients and after four and three treatment episodes, respectively, using the calibrated automated thrombin generation assay (CAT) in platelet poor plasma. Spiking with FEIBA showed a linear relation with the endogenous thrombin potential (ETP)/peak at 1 pM but substrate depletion at 5 pM. Spiking with rFVIIa showed a near linear dose-response relation with the ETP/peak at 1 pm but only a shortening of the initiation phase at 5 pM. Similar effects were present in post-treatment samples. FEIBA acted synergistically with rFVIIa. This suggest a role for CAT in monitoring inhibitor bypass treatment but low TF concentrations are required to avoid substrate depletion with FEIBA and to demonstrate the effect of rFVIIa.

The impact of major surgery on blood coagulation factors and thrombin generation

We studied the blood coagulation system of 14 patients with metastatic malignancies before and after they had undergone major surgery. In addition to measuring a battery of coagulation factors, we assessed the function of the system with assays of whole blood thrombin generation. With the exceptions of factor VIII (fVIII), which increased, and fibrinogen and fIX, which did not change, the activities of all the pro- and anticoagulant proteins were significantly lower postoperatively. However, the thrombin generating capacity of the system was relatively preserved. Although the integral of thrombin activity over time was lower after surgery, the mean peak thrombin concentration was unchanged and the time to clot formation was shortened. Similar changes could be reproduced by lowering the concentrations of pro- and anticoagulant factors together in control blood samples. Therefore, simultaneous reductions in pro- and anticoagulant proteins postoperatively worked to maintain the functional integrity of the blood coagulation system.

Differential effects of high prothrombin levels on thrombin generation depending on the cause of the hyperprothrombinemia

BACKGROUND

Hyperprothrombinemia, resulting from the prothrombin G20210A mutation or other causes, is associated with activated protein C (APC) resistance and increased thrombosis risk. When high prothrombin levels are a result of increased hepatic biosynthesis, these effects may be counteracted by concomitantly increased levels of the anticoagulant factors (particularly protein S). Differently, in prothrombin G20210A carriers only prothrombin levels are elevated.

OBJECTIVE

To investigate whether prothrombin G20210A carriers have a more severe hypercoagulable state than non-carriers with comparable prothrombin levels.

PATIENTS/METHODS

Coagulation factor levels, thrombin generation (Calibrated Automated Thrombogram in the presence and absence of APC) and APC resistance were measured in normal (n = 132), heterozygous (n = 167) and homozygous (n = 3) individuals.

RESULTS

Prothrombin levels, thrombin generation and APC resistance were higher in carriers of the prothrombin G20210A mutation (especially those who had experienced venous thrombosis) than in non-carriers, whereas protein S and antithrombin levels were similar among genotype groups. Because individuals with high prothrombin levels in the absence of the prothrombin G20210A mutation tend to have all liver-synthesized factors elevated, carriers of the mutation had lower protein S and antithrombin levels than non-carriers with equally high prothrombin levels. Accordingly, they also generated more thrombin and showed a tendency toward higher APC resistance. Analogous effects, but less pronounced, were observed in homozygotes for the prothrombin A19911G polymorphism, which also upregulates prothrombin levels.

CONCLUSIONS

Individuals with hyperprothrombinemia as a result of prothrombin gene mutations generate more thrombin and tend to be more APC-resistant than individuals with comparable prothrombin levels because of other causes.

Plasma contains protein S monomers and multimers with similar direct anticoagulant activity

BACKGROUND

Protein S (PS) has activated protein C-independent, direct anticoagulant activity (PS-direct). We reported that both multimers and monomers of affinity-purified PS have PS-direct similar to that in plasma, in contrast to another report.

OBJECTIVE

We extended our studies to establish the molecular forms and activity of plasma PS.

METHODS

Novel ELISAs were developed that could detect only multimeric, not monomeric, PS because they employed the same monoclonal antibody for capture and detection. PS forms were also examined on native PAGE immunoblots. A new activity assay for PS-direct was applied to plasma and gel-filtered plasma fractions.

RESULTS

Plasma PS multimers were clearly demonstrated using the ELISAs; 30-60% of free plasma PS appeared to be multimeric, a proportion similar to that of affinity-purified PS. On immunoblots, plasma PS multimers were more easily detected after gel filtration; plasma PS monomers and several apparent multimers comigrated with respective forms of affinity-purified PS. Antigen elution profiles after gel filtration of plasma revealed at least one major peak of apparent PS multimers (40-55% of free PS appeared multimeric). Biotin-factor Xa could bind to both plasma PS monomers and multimers. Strong plasma PS-direct was demonstrated, and plasma PS monomers, multimers, and PS-C4b-binding protein complexes each reconstituted PS-depleted plasma to similar levels of PS-direct.

CONCLUSION

Our data are in disagreement with a report that monomeric purified PS has little PS-direct and that only monomeric PS exists in plasma. We find that both affinity-purified and plasma PS exist as monomers and multimers with similar PS-direct.

Multifunctional specificity of the protein C/activated protein C Gla domain

Activated protein C (APC) has potent anticoagulant and anti-inflammatory properties that are mediated in part by its interactions with its cofactor protein S and the endothelial cell protein C receptor (EPCR). The protein C/APC Gla domain is implicated in both interactions. We sought to identify how the protein C Gla domain enables specific protein-protein interactions in addition to its conserved role in phospholipid binding. The human prothrombin Gla domain, which cannot bind EPCR or support protein S cofactor activity, has 22/45 residues that are not shared with the human protein C Gla domain. We hypothesized that the unique protein C/APC Gla domain residues were responsible for mediating the specific interactions. To assess this, we generated 13 recombinant protein C/APC variants incorporating the prothrombin residue substitutions. Despite anticoagulant activity similar to wild-type APC in the absence of protein S, APC variants APC(PT33-39) (N33S/V34S/D35T/D36A/L38D/A39V) and APC(PT36/38/39) (D36A/L38D/A39V) were not stimulated by protein S, whereas APC(PT35/36) (D35T/D36A) exhibited reduced protein S sensitivity. Moreover, PC(PT8/10) (L8V/H10K) displayed negligible EPCR affinity, despite normal binding to anionic phospholipid vesicles and factor Va proteolysis in the presence and absence of protein S. A single residue variant, PC(PT8), also failed to bind EPCR. Factor VIIa, which also possesses Leu-8, bound soluble EPCR with similar affinity to wild-type protein C, collectively confirming Leu-8 as the critical residue for EPCR recognition. These results reveal the specific Gla domain residues responsible for mediating protein C/APC molecular recognition with both its cofactor and receptor and further illustrate the multifunctional potential of Gla domains.