Thrombus lysis by uPA, scuPA and tPA is regulated by plasma TAFI

Summary.  The carboxypeptidase, TAFIa or CPU, is known to prolong plasma clot lysis by tissue plasminogen activator (tPA) and to have a role in thrombus stability in vivo. This current study examined lysis by urokinase (uPA) and single chain urokinase (scuPA) in addition to tPA. Further, we investigated the role of TAFIa in a model thrombus system, in which thrombi are formed under conditions of flow. We show that human thrombi, formed in vivo , and model thrombi both contain TAFI . No effect of thrombus TAFIa was observed in thrombus lysis assays, except when thrombi were bathed in plasma, in which case addition of potato tuber carboxypeptidase inhibitor (CPI) resulted in doubling of the rate of lysis. TAFIa inhibited lysis of model thrombi and plasma clots by uPA, scuPA in addition to lysis by tPA. The effect of TAFIa was more evident at high concentrations of plasminogen activator such as those used in thrombolytic therapy. Addition of plasminogen increased lysis and, in its presence, the enhancement by CPI was smaller. Thus the action of TAFIa could be partially overcome by plasminogen, whether lysis was by tPA, uPA or scuPA. This is consistent with TAFIa exerting its effect primarily through modifying the binding of plasminogen to fibrin and to a lesser extent through modification of the binding of tPA to fibrin.     

Activation of coagulation factor XI, without detectable contact activation in dengue haemorrhagic fever

A prospective cohort study was performed in 50 patients with dengue haemorrhagic fever (DHF) to determine the potential role of the contact activation system and factor XI activation (intrinsic pathway) in the coagulation disorders in DHF. To establish whether TAFI (thrombin-activatable fibrinolysis inhibitor) was involved in the severity of the coagulation disorders, the TAFI antigen and activity levels were also determined. Markers of contact activation (kallikrein--C1-inhibitor complexes), the intrinsic pathway of coagulation (factor XIa--C1-inhibitor complexes) and TAFI were measured and correlated to thrombin generation markers (thrombin--anti-thrombin complexes (TAT), prothrombin fragment 1+2 (F1+2)) and a marker for fibrinolysis [plasmin--alpha 2--anti-plasmin complexes (PAP)]. Activation of the intrinsic pathway of coagulation was clearly demonstrated by elevated levels of factor XIa--C1-inhibitor complexes, without evidence of contact activation, reflected by undetectable kallikrein--C1-inhibitor complexes. Both TAFI antigen and activity levels were decreased in all patients, which may contribute to the severity of bleeding complications in DHF because of the impaired capacity of the coagulation system to protect the fibrin clot from fibrinolysis. These findings in a human viral infection model are in accordance with earlier findings in bacterial sepsis.     

Deficiency in thrombin-activatable fibrinolysis inhibitor (TAFI) protected mice from ferric chloride-induced vena cava thrombosis

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a plasma carboxypeptidase that renders a fibrin-containing thrombus less sensitive to lysis. Since the role of TAFI in thrombus formation is still controversial in mice, our present study was designed to evaluate mice deficient in TAFI (TAFI^−/−) on FeCl₃-induced vena cava and carotid artery thrombosis. Parallel studies were carried out in wild-type mice using a potato carboxypeptidase inhibitor (PCI), a selective inhibitor of activated TAFI (TAFIa). Significant reduction in thrombus formation was observed in TAFI^−/− mice (n = 8, P < 0.05 compared to wild-type littermates) but not in heterozygous (TAFI^+/−) mice in 3.5% FeCl₃-induced vena cava thrombosis. A similar effect was observed following treatment with 5 mg/kg bolus plus 5 mg/kg/h PCI in the same venous thrombosis model in C57BL/6 mice (n = 8, P < 0.01 compared to vehicle). No compositional difference was observed for the venous thrombi in TAFI^−/− and wild-type littermates with or without PCI treatment using histological assessment. In contrast, neither TAFI deficiency nor treatment with PCI showed antithrombotic efficacy in the 3.5% FeCl₃-induced carotid artery thrombosis model. In a tail transection bleeding time model, both TAFI deficiency and PCI treatment increased bleeding time up to 4.5 and 3.5 times, respectively, over controls (P < 0.05, n = 8). Similar ex vivo fibrinolytic activities were demonstrated for both TAFI deficiency and PCI treatment as enhanced lysis of thrombin-induced plasma clots and lysis of whole blood clot in a thrombelastograph. These data provide direct evidence for the role of TAFIa in vena cava thrombosis without the addition of exogenous thrombolytic in mice. The strong ex vivo fibrinolytic activity of TAFI deficiency or TAFIa inhibition by PCI provides a biomarker of TAFIa inhibition that tracks in vivo antithrombotic efficacy.     

Plasma antigen levels of thrombin-activatable fibrinolysis inhibitor did not differ in patients with or without disseminated intravascular coagulation

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a carboxypeptidase that downregulates fibrinolysis and might play some roles in the pathogenesis of disseminated intravascular coagulation (DIC). We prospectively examined the plasma TAFI antigen levels in patients highly suspected to be suffering from DIC. Patients were subdivided into overt DIC and non-DIC groups according to a DIC scoring system. The Sepsis-related Organ Failure Assessment (SOFA) scores were concurrently calculated on patients with sepsis. Overall, there were 23 non-DIC patients and 20 patients with overt DIC. Their baseline characteristics were similar, but patients with overt DIC had much more aberrant coagulation tests and higher lactate dehydrogenase levels. However, there was no significant difference between overt DIC and non-DIC patients regarding their TAFI antigen levels [median/interquartile range (IQR) 74.41/13.98 and 75.29/15.16, respectively, p=0.543]. On regression analysis, TAFI antigen levels were not correlated with either C-reactive protein levels or various coagulation test results. In patients with sepsis (n=31), TAFI levels among three risk groups stratified by low (5), intermediate (6–10), and high (11) SOFA scores were not statistically disparate (median/IQR 65.24/15.14, 74.63/13.79, and 75.29/21.51, respectively, p=0.684), either. Our result indicated that plasma TAFI antigen levels did not vary significantly between patients with or without DIC. Further, they did not possess any correlation with the severity of organ injury in patients with sepsis. The role of TAFI antigen in the pathogenesis of DIC needs further elucidation by future studies.     

Low thrombin activatable fibrinolysis inhibitor activity levels are associated with an increased risk of a first myocardial infarction in men

Background

Studies on the relation between thrombin activatable fibrinolysis inhibitor (TAFI) and arterial thrombosis have produced conflicting results.TAFI regulates fibrinolysis, but other roles of this inhibitor, including anti-inflammatory properties, have also been demonstrated.

Design and Methods

We investigated the association between TAFI activity and the risk of myocardial infarction. Additionally, we studied the association of common single nucleotide polymorphisms in the TAFI gene with levels of the TAFI protein and risk of myocardial infarction.We included 554 men under 70 years old with a first myocardial infarction and 643 controls participating in the Study of Myocardial Infarctions Leiden (SMILE), a case-control study.

Results

We found odds ratios (95% confidence intervals) of a first myocardial infarction of 2.4 (1.6–3.6), 3.2 (2.1–4.7) and 3.4 (2.3–5.1) for subjects whose TAFI levels were in the third, second and first quartiles (lowest TAFI levels), respectively, compared with the fourth quartile, after adjusting for arterial disease risk factors. The rare -438A and 1040T alleles were associated with lower, and the rare 505G allele with higher TAFI levels than the common alleles. Carriers of the -438A allele had an increased risk of myocardial infarction (odds ratio 1.6 (1.0–2.5) for AA; odds ratio 1.2 (0.9–1.5) for AG compared with GG). The other single nucleotide polymorphisms were not associated with myocardial infarction.

Conclusions

Low TAFI activity levels are associated with increased risk of a first myocardial infarction in men. The results on the association between TAFI single nucleotide polymorphisms and myocardial infarction were inconsistent.     

Overall haemostatic potential can be used for estimation of thrombin-activatable fibrinolysis inhibitor-dependent fibrinolysis in vivo and for possible follow-up of recombinant factor VIIa treatment in patients with inhibitors to factor VIII.

Thrombin generation induced by recombinant factor VIIa (rFVIIa) in patients with haemophilia and/or inhibitors to factor VIII/IX could enhance generation of thrombin-activatable fibrinolysis inhibitor (TAFI), a recently described link between coagulation and fibrinolysis. TAFI is unstable and it is not easy to measure its active form in vivo. Overall haemostatic potential (OHP) is a novel method for haemostasis estimation, based on determination of the fibrin aggregation curve in which tiny amounts of thrombin are used for activation of clotting. We measured OHP in six patients with inhibitors to factor VIII before injection of rFVIIa and 10 and 120 min thereafter. Overall fibrinolytic potential (OFP) and clot lysis time (CLT) analysed by this method could be used for indirect estimation of TAFI generation. We found no change in pro-TAFI and total TAFI antigen before and after treatment with rFVIIa. OHP was almost undetectable before treatment but increased into the range of normal pooled plasma 10 and 120 min after rFVIIa treatment, as did CLT. However, after addition of potato tuber carboxypeptidase inhibitor, a specific inhibitor of TAFI, the shortening of CLT was lower than that in NPP. OFP was increased in patient plasma both 10 and 120 min after treatment compared with NPP. There was a strong positive correlation between pro-TAFI concentration and shortening of CLT after PTCI addition and a negative correlation between pro-TAFI concentration and OFP 10 min after rFVIIa injection. Thus, rFVIIa normalizes OHP and CLT 10 min after injection. While this improvement slightly decreases, but still exists after 2 hours, it suggests efficacy in bleeding prevention using a protocol based on rFVIIa administration every 2 hours.     

The activation peptide of thrombin‐activatable fibrinolysis inhibitor: a role in activity and stability of the enzyme?

Summary.  Background:  Thrombin-activatable fibrinolysis inhibitor (TAFI) is a 56-kDa procarboxypeptidase. Proteolytic enzymes activate TAFI into TAFIa, an inhibitor of fibrinolysis, by cleaving off the N-terminal activation peptide (amino acids 1–92), from the enzyme moiety. Activated TAFI is unstable, with a half-life of approximately 10 min at 37 °C. So far, it is unknown whether the activation peptide is released or remains attached to the catalytic domain, and whether it influences TAFIa's properties. The current study was performed to clarify these issues. Methods:  TAFI was activated, and the activity and half-life of the enzyme were determined in the presence and absence of the activation peptide. Results:  TAFIa was active both before and after removal of the activation peptide, and the half-life of TAFIa was identical in the two preparations. Furthermore, we observed that intrinsically inactivated TAFIa (TAFIai) aggregated into large, insoluble complexes that could be removed by centrifugation. Conclusions:  The data presented in this article show that the activation peptide of TAFI is not required for TAFIa activity and that the activation peptide has no effect on the stability of the enzyme. These results are in favour of a model in which the activation peptide solely stabilizes the structure of the proenzyme. After activation of TAFI and subsequent breakage of interactions between the activation peptide and the catalytic domain, the activation peptide is no longer capable of performing this stabilizing task, and the integrity of the catalytic domain is lost rapidly. The resulting TAFIai is more prone to proteolysis and aggregation.     

Tissue factor pathway inhibitor and thrombin-activatable carboxypeptidase B for prediction of early atherosclerosis in gouty arthritis

Background

Gouty arthritis (GA) is a chronic inflammatory arthritis in which both clinical and subclinical atherosclerosis are more frequent. The dynamic equilibrium between coagulation and fibrinolysis is impaired in inflammatory diseases. We determined TFPI and TAFI antigen levels in GA patients and evaluated their association with subclinical atherosclerosis.

Methods

We included 45 GA patients (41 males, 4 females; mean age: 51.6 years) and 25 asymptomatic hyperuricemic (AHU) subjects (19 males, 6 females; mean age: 48.1 years). Cardiovascular risk factors were determined. TAFI and TFPI levels were determined by ELISA. B-mode ultrasonography was used to detect subclinical atherosclerosis.

Results

Cardiovascular risk factors were similar in both groups. The carotid IMT was significantly higher in GA group than in AHU group (0.74 ± 0.23 mm vs. 0.61 ± 0.13 mm, p = 0.009). TFPI level was significantly higher in GA group than in AHU group (86.2 ± 48.9 ng/mL vs. 25.8 ± 21.4 ng/mL, p < 0.001); TAFI antigen was significantly higher in AHU group (22.6 ± 3.6 ng/mL vs. 25.7 ± 5.3 ng/mL, p = 0.006) than in GA patients. Atherosclerotic plaque formation was more frequent in GA group (p = 0.041). When GA patients with and without plaques were compared, the first group had significantly higher mean age (p = 0.01) and TFPI level (p = 0.028). TFPI level correlated with carotid IMT (r = 0.302; p = 0.028). Logistic regression analysis showed that age (OR: 1.236, 95%CI: 1.059-1.443, p = 0.007) and TFPI (OR: 1.031, 95%CI: 1.008-1.054, p = 0.008) were independent risk factors for the presence of plaques.

Conclusions

GA patients had more frequent subclinical atherosclerosis than subjects with AHU. Higher TFPI levels in GA patients –probably associated with enhanced endothelial damage- were related to subclinical atherosclerosis. Lower TAFI levels in GA pointed to impaired fibrinolysis.     

Active PAI-1 as marker for venous thromboembolism: Case–control study using a comprehensive panel of PAI-1 and TAFI assays

Background

Both activated Thrombin Activatable Fibrinolysis Inhibitor (TAFI) and active Plasminogen Activator Inhibitor-1 (PAI-1) attenuate fibrinolysis and may therefore contribute to the pathophysiology of Venous ThromboEmbolism (VTE). Whether increased TAFI and/or PAI-1 concentrations are associated with VTE is unclear.

Objective

To study an association of impaired fibrinolysis and VTE using a comprehensive panel of in-house developed assays measuring intact TAFI, activation peptide of TAFI (AP-TAFI), PAI-1 antigen, endogenous PAI-1:t-PA complex (PAI-1:t-PA) and active PAI-1 levels in 102 VTE patients and in 113 healthy controls (HC).

Results

Active PAI-1 was significantly higher in VTE patients compared to HC (20.9 [9.6-37.8] ng/ml vs. 6.2 [3.5-9.7] ng/ml, respectively). Active PAI-1 was the best discriminator with an area under the ROC curve and 95% confidence interval (AUROC [95%CI]) of 0.84 [0.79-0.90] compared to 0.75 [0.68-0.72] for PAI-1:t-PA, 0.65 [0.58-0.73] for PAI-1 antigen, 0.62 [0.54-0.69] for AP-TAFI and 0.51 [0.44-0.59] for intact TAFI. Using ROC analysis, we defined an optimal cut-off of 12.8 ng/ml for active PAI-1, with corresponding sensitivity of 71 [61–79] % and specificity of 89 [82–94] %. A lack of association with the time between VTE event and sample collection or with the intake of anticoagulant treatment suggests that active PAI-1 levels are sustainable high in VTE patients.

Conclusions

This case–control study emphasizes the clinical importance of measuring active PAI-1 instead of PAI-1 antigen and identifies active PAI-1 as a potential marker of VTE. Prognostic studies will need to address the clinical significance of active PAI-1 as biomarker.