Lisbeth F. A. Maurissen

Regulation of TFPI function by protein S

Summary.  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.

Similar hypercoagulable state and thrombosis risk in type I and type III protein S-deficient individuals from families with mixed type I/III protein S deficiency

Background Protein S, which circulates in plasma in both free and bound forms, is an anticoagulant protein that stimulates activated protein C and tissue factor pathway inhibitor. Hereditary type I protein S deficiency (low total and low free protein S) is a well-established risk factor for venous thrombosis, whereas the thrombosis risk associated with type III deficiency (normal total and low free protein S) has been questioned. Design and Methods Kaplan-Meier analysis was performed on 242 individuals from 30 families with protein S deficiency. Subjects were classified as normal, or having type I or type III deficiency according to their total and free protein S levels. Genetic and functional studies were performed in 23 families (132 individuals). Results Thrombosis-free survival was not different between type I and type III protein S-deficient individuals. Type III deficient individuals were older and had higher protein S, tissue factor pathway inhibitor and prothrombin levels than type I deficient individuals. Thrombin generation assays sensitive to the activated protein C- and tissue factor pathway inhibitor-cofactor activities of protein S revealed similar hypercoagulable states in type I and type III protein S-deficient plasma. Twelve PROS1 mutations and two large deletions were identified in the genetically characterized families. Conclusions Not only type I, but also type III protein S deficiency is associated with a hypercoagulable state and increased risk of thrombosis. These findings may, however, be restricted to type III deficient individuals from families with mixed type I/III protein S deficiency, as these represented 80% of type III deficient individuals in our cohort.

Protein S is a cofactor for tissue factor pathway inhibitor

Protein S is a vitamin K-dependent protein that acts as a cofactor of the anticoagulant protein APC. However, protein S also exhibits anticoagulant activity in the absence of APC. Thrombin generation experiments in normal plasma and in plasma deficient in tissue factor pathway inhibitor (TFPI) and/or protein S demonstrated that protein S stimulates the inhibition of TF by TFPI. Kinetic analysis in model systems containing purified proteins showed that protein S enhances the formation of the binary FXa:TFPI complex by reducing the Ki of TFPI from approximately 4 nM to approximately 0.5 nM. Enhancement of inhibitory activity of TFPI by protein S is only observed with full-length TFPI and in the presence of a negatively charged phospholipid surface. The Ki decrease brings the TFPI concentration necessary for FXa:TFPI complex formation within range of the plasma TFPI concentration which increases FXa:TFPI complex formation and accelerates feedback inhibition of the TF pathway by enhancing the formation of the quaternary TFPI:FXa:TF:FVIIa complex. Thus, protein S is not only a cofactor of APC, but also of TFPI. A reduced TFPI cofactor activity may contribute to the increased risk of venous thrombosis in protein-S deficient individuals. Using calibrated automated thrombography we have developed two assays that enable quantification of the functional activity of the TFPI/protein S system in plasma. These assays show that the activity of the TFPI/protein S system is greatly impaired in oral contraceptive users.

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

Summary.  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.

A positive family history for premature cardiovascular disease identifies patients prone to recurrent arterial thrombotic events.

Background: Cardiovascular disease (CVD) is characterized by slow progressive atherosclerosis and arterial thrombotic events, leading to occlusions. Whether either of these presentations is more likely in patients with a genetic predisposition for CVD is still unknown. We suggest that a genetic predisposition for CVD is related to recurrent events of the same nature. Methods: We retrospectively investigated 275 patients with premature CVD and divided them in two groups according to their first event: an arterial thrombotic event or stable atherosclerosis. We used a Cox proportional-hazards model to estimate the effect of a positive family history for CVD on recurrent events of the same nature. This was tested in the entire cohort and in patients with coronary artery disease only. Results: Patients with a first arterial thrombotic event and a positive family history had a threefold increased risk for a recurrent event of the same nature, compared to patients with a negative family history (hazard ratio 3.00, 95% confidence interval 1.32–6.81); p < 0.05). In contrast, a positive family history was not associated with an increased risk for a recurrent stable atherosclerosis (hazard ratio 0.98 (95% confidence interval 0.59–1.63). These findings were similar analysing the patients with coronary artery disease only. Additional adjustments for other risk factors did not change these associations. Conclusions: Patients with a first premature arterial thrombotic event and a positive family history for CVD have an increased risk for a second event of the same nature. This might be due to unknown hereditary mechanisms leading to recurrent acute events.

Similar hypercoagulable state and thrombosis risk in type I and type III protein S-deficient individuals from mixed type I/III families

BACKGROUND Protein S, which circulates in plasma in a free and bound form, is an anticoagulant protein that stimulates both activated protein C (APC) and tissue factor pathway inhibitor (TFPI). Hereditary type I protein S deficiency (low total and low free protein S) is a well-established risk factor for venous thrombosis, whereas the thrombosis risk associated with type III deficiency (normal total and low free protein S) has been questioned. DESIGN AND METHODS Kaplan-Meier analysis was performed on 242 individuals from 30 families with protein S deficiency. Subjects were classified as normal, type I deficient or type III deficient according to their total and free protein S levels. Genetic and functional studies were performed in 23 families (132 individuals). RESULTS Thrombosis-free survival was not different between type I and type III protein S-deficient individuals. Type III deficient individuals were older and had higher protein S, TFPI and prothrombin levels than type I deficient individuals. Thrombin generation assays sensitive to the APC- and TFPI-cofactor activities of protein S revealed similar hypercoagulable states in type I and type III protein S-deficient plasma. Twelve PROS1 mutations and 2 large deletions were identified in the genetically characterized families. CONCLUSIONS Not only type I, but also type III protein S deficiency is associated with a hypercoagulable state and increased thrombosis risk. However, these findings may be restricted to type III deficient individuals from families with mixed type I/III protein S deficiency, as these represented 80% of type III deficient individuals in our cohort.