Inducible carboxypeptidase activity. A role in clot lysis in vivo

Elucidating the molecular mechanisms of fibrinolytic shutdown after severe injury: The role of thrombin-activatable fibrinolysis inhibitor

BACKGROUND

The mechanisms underlying trauma-induced coagulopathy remain elusive. Hyperfibrinolysis has been linked to increased plasminogen activation and antiprotease consumption; however, the mechanistic players in its counterpart, fibrinolysis shutdown, remain unclear. We hypothesize that thrombin-activatable fibrinolysis inhibitor (TAFI) plays a major role in fibrinolytic shutdown after injury.

METHODS

As part of this observational cohort study, whole blood was collected from trauma activation patients at a single, level 1 trauma center. Citrated rapid thrombelastography and the following enzyme-linked immunosorbent assays were conducted: thrombin, antithrombin, thrombin-antithrombin complex, TAFI, plasminogen, antiplasmin, plasmin-antiplasmin (PAP), tissue plasminogen activator, plasminogen activator inhibitor 1, and tissue plasminogen activator-plasminogen activator inhibitor 1 complex. Univariate and cluster analysis were performed.

RESULTS

Overall, 56 patients (median age, 33.5 years; 70% male) were included. The majority (57%) presented after blunt mechanism and with severe injury (median New Injury Severity Score, 27). Two clusters of patients were identified: Group 1 (normal fibrinolysis, n = 21) and Group 2 (fibrinolysis shutdown, n = 35). Group 2 had significantly lower fibrinolysis with a median LY30 of 1.1% (interquartile range [IQR], 0.1-1.9%) versus 2.1% (IQR, 0.5-2.8%) in Group 1; while the median LY30 was within physiologic range, 45% of patients in Group 2 were in shutdown (vs. 24% in Group 1, p = 0.09). Compared with Group 1, Group 2 had significantly higher PAP (median, 4.7 [IQR, 1.7-9.3] vs. 1.4 [1.0-2.1] μg/mL in Group 1; p = 0.002) and higher TAFI (median, 152.5% [IQR, 110.3-190.7%] vs. 121.9% [IQR, 93.2-155.6%]; p = 0.04). There was a strong correlation between PAP and TAFI ( R2 = 0.5, p = 0.0002).

CONCLUSION

The presented data characterize fibrinolytic shutdown, indicating an initial plasmin burst followed by diminished fibrinolysis, which is distinct from hypofibrinolysis (inadequate plasmin burst and fibrinolysis). After an initial thrombin and plasmin burst (increased PAP), fibrinolysis is inhibited, mediated in part by increased TAFI.

Hypofibrinolytic state and high thrombin generation may play a major role in SARS-COV2 associated thrombosis

BACKGROUND

Thirty percent of Covid-19 patients admitted to intensive care units present with thrombotic complications despite thromboprophylaxis. Bed rest, obesity, hypoxia, coagulopathy, and acute excessive inflammation are potential mechanisms reported by previous studies. Better understanding of the underlying mechanisms leading to thrombosis is crucial for developing more appropriate prophylaxis and treatment strategies.

OBJECTIVE

We aimed to assess fibrinolytic activity and thrombin generation in 78 Covid-19 patients.

PATIENTS AND METHODS

Forty-eight patients admitted to the intensive care unit and 30 patients admitted to the internal medicine department were included in the study. All patients received thromboprophylaxis. We measured fibrinolytic parameters (tissue plasminogen activator, PAI-1, thrombin activatable fibrinolysis inhibitor, alpha2 anti-plasmin, and tissue plasminogen activator-modified ROTEM device), thrombin generation, and other coagulation tests (D-dimer, fibrinogen, factor VIII, antithrombin).

RESULTS AND CONCLUSIONS

We observed two key findings: a high thrombin generation capacity that remained within normal values despite heparin therapy and a hypofibrinolysis mainly associated with increased PAI-1 levels. A modified ROTEM is able to detect both hypercoagulability and hypofibrinolysis simultaneously in Covid-19 patients with thrombosis.

TAFI deficiency causes maladaptive vascular remodeling after hemophilic joint bleeding

Excessive vascular remodeling is characteristic of hemophilic arthropathy (HA) and may contribute to joint bleeding and the progression of HA. Mechanisms for pathological vascular remodeling after hemophilic joint bleeding are unknown. In hemophilia, activation of thrombin-activatable fibrinolysis inhibitor (TAFI) is impaired, which contributes to joint bleeding and may also underlie the aberrant vascular remodeling. Here, hemophilia A (factor VIII-deficient; FVIII-deficient) mice or TAFI-deficient mice with transient (antibody-induced) hemophilia A were used to determine the role of FVIII and TAFI in vascular remodeling after joint bleeding. Excessive vascular remodeling and vessel enlargement persisted in FVIII-deficient and TAFI-deficient mice, but not in transient hemophilia WT mice, after similar joint bleeding. TAFI-overexpression in FVIII-deficient mice prevented abnormal vessel enlargement and vascular leakage. Age-related vascular changes were observed with FVIII or TAFI deficiency and correlated positively with bleeding severity after injury, supporting increased vascularity as a major contributor to joint bleeding. Antibody-mediated inhibition of uPA also prevented abnormal vascular remodeling, suggesting that TAFI's protective effects include inhibition of uPA-mediated plasminogen activation. In conclusion, the functional TAFI deficiency in hemophilia drives maladaptive vascular remodeling in the joints after bleeding. These mechanistic insights allow targeted development of potentially new strategies to normalize vascularity and control rebleeding in HA.

Current understanding of the pathophysiology of chronic thromboembolic pulmonary hypertension

Chronic thromboembolic pulmonary hypertension (CTEPH) is a unique form of pulmonary hypertension arising from fibrotic obliteration of major pulmonary arteries. Pro-thrombotic states, large clot burden and impaired dissolution are believed to contribute to the occurrence and progression of thrombosis after an acute pulmonary embolic event. Recent data utilizing several models have facilitated the understanding of clot resolution. This review summarizes current knowledge on pathophysiological mechanisms of major vessel occlusion in CTEPH.

Pathogenesis of disseminated intravascular coagulation in patients with acute promyelocytic leukemia, and its treatment using recombinant human soluble thrombomodulin

Acute promyelocytic leukemia (APL) is an uncommon subtype of acute myelogenous leukemia characterized by the proliferation of blasts with distinct morphology, a specific balanced reciprocal translocation t(15;17), and life-threatening hemorrhage caused mainly by enhanced fibrinolytic-type disseminated intravascular coagulation (DIC). The introduction of all-trans retinoic acid (ATRA) into anthracycline-based induction chemotherapy regimens has dramatically improved overall survival of individuals with APL, although hemorrhage-related death during the early phase of therapy remains a serious problem. Moreover, population-based studies have shown that the incidence of early death during induction chemotherapy is nearly 30 %, and the most common cause of death is associated with hemorrhage. Thus, development of a novel treatment strategy to alleviate abnormal coagulation in APL patients is urgently required. Recombinant human soluble thrombomodulin (rTM) comprises the active extracellular domain of TM, and has been used for treatment of DIC since 2008 in Japan. Use of rTM in combination with remission induction chemotherapy, including ATRA, produces potent resolution of DIC without exacerbation of bleeding tendency in individuals with APL. This review article discusses the pathogenesis and features of DIC caused by APL, as well as the possible anticoagulant and anti-leukemic action of rTM in APL patients.

Thrombin activatable fibrinolysis inhibitor : its role in slow coronary flow

BACKGROUND

The slow coronary flow (SCF) phenomenon is characterized by slow progression of angiographic contrast medium in the coronary arteries in the absence of stenosis in the epicardial vessels. The pathophysiological mechanisms of SCF phenomenon remain uncertain. Several hypotheses, however, have been suggested for SCF phenomenon, including an early form of atherosclerosis, small vessel dysfunction, dilatation of coronary vessels, imbalance between vasoconstrictor and vasodilatory factors, platelet function disorder, and inflammation. Atherosclerosis and inflammation are the most accepted mechanisms for the pathogenesis of SCF. Thrombin activatable fibrinolysis inhibitor (TAFI) was described as a new inhibitor of fibrinolysis recently and plays an important role in coagulation and fibrinolysis. In previous studies, the role of TAFI was associated with inflammation and evolution of atherosclerosis in coronary artery disease. There are no data available about TAFI levels in patients with SCF phenomenon investigated by angiography. Our goal was to evaluate TAFI antigen (Ag) levels in patients with SCF and to determine the association of the TAFI Ag level with traditional cardiovascular risk factors in our study.

METHODS

The study group constituted 41 patients with angiographically confirmed SCF and 46 patients with normal coronary flow as the control group. The TAFI Ag levels of each patient were determined.

RESULTS

Between the control and study group, a statistical difference in the levels of TAFI Ag (p < 0.05) was observed. The TAFI Ag level was significantly higher in the SCF group than the control group (132.21 ± 21.14 versus 122.15 ± 21.59).

CONCLUSION

We have demonstrated that TAFI might be a risk factor for the development of SCF independently of conventional cardiovascular risk factors. In addition, TAFI Ag levels were positively correlated with C-reactive protein (CRP) known as an acute phase reactant. Our findings support the reports of previous studies that increased TAFI levels may be associated with inflammation. Further large studies are required to evaluate the importance of TAFI antigen levels in relation to the development of SCF.

Thrombin-activatable fibrinolysis inhibitor (TAFI) is enhanced in major trauma patients without infectious complications

BACKGROUND

Infectious complications frequently occur after major trauma, leading to increased morbidity and mortality. Thrombin-activatable fibrinolysis inhibitor (TAFI), a procarboxypeptidase in plasma, plays a dual role in regulating both coagulation and inflammation. Activated TAFI (TAFIa) has broad anti-inflammatory properties due to its inactivation of active inflammatory mediators (anaphylatoxins C3a and C5a, bradykinin, osteopontin).

OBJECTIVES

The purpose of this study was to determine if TAFI plays a role in the development of inflammatory complications after major trauma.

PATIENTS/METHODS

Upon arrival at the emergency department (ED), plasma levels of TAFI and TAFIa were measured in 26 multiple traumatized patients for 10 consecutive days. Systemic levels of inflammatory mediators, including interleukin-6 (IL-6), procalcitonin (PCT), C-reactive protein (CRP) and leukocytes were determined.

RESULTS

Fifteen patients developed pneumonia and/or sepsis (compl) and 11 had no complications (wo compl). Overall injury severity and age were comparable in both groups. Complications occurred approximately 5 days after trauma. IL-6 increased on day 5, whereas CRP, PCT and leukocytes started to increase on day 6 in the compl-group. Upon arrival at the ED and on days 1 and 4, TAFI levels were significantly lower in the compl-group compared to the wo compl-group (p=0.0215). Similarly, TAFIa was significantly lower on day 4 in the compl-group than in the wo compl-group (p=0.049).

CONCLUSIONS

This pilot study shows that TAFI levels are inversely correlated with inflammation-associated development of complications after major trauma.

New antithrombotic drugs: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

This article focuses on new antithrombotic drugs that are in or are entering phase 3 clinical testing. Development of these new agents was prompted by the limitations of existing antiplatelet, anticoagulant, or fibrinolytic drugs. Addressing these unmet needs, this article (1) outlines the rationale for development of new antithrombotic agents; (2) describes the new antiplatelet, anticoagulant, and fibrinolytic drugs; and (3) provides clinical perspectives on the opportunities and challenges faced by these novel agents.

Plasma carboxypeptidase B downregulates inflammatory responses in autoimmune arthritis

The immune and coagulation systems are both implicated in the pathogenesis of rheumatoid arthritis (RA). Plasma carboxypeptidase B (CPB), which is activated by the thrombin/thrombomodulin complex, plays a procoagulant role during fibrin clot formation. However, an antiinflammatory role for CPB is suggested by the recent observation that CPB can cleave proinflammatory mediators, such as C5a, bradykinin, and osteopontin. Here, we show that CPB plays a central role in downregulating C5a-mediated inflammatory responses in autoimmune arthritis. CPB deficiency exacerbated inflammatory arthritis in a mouse model of RA, and cleavage of C5a by CPB suppressed the ability of C5a to recruit immune cells in vivo. In human patients with RA, genotyping of nonsynonymous SNPs in the CPB-encoding gene revealed that the allele encoding a CPB variant with longer half-life was associated with a lower risk of developing radiographically severe RA. Functionally, this CPB variant was more effective at abrogating the proinflammatory properties of C5a. Additionally, expression of both CPB and C5a in synovial fluid was higher in patients with RA than in those with osteoarthritis. These findings suggest that CPB plays a critical role in dampening local, C5a-mediated inflammation and represents a molecular link between inflammation and coagulation in autoimmune arthritis.

Flexibility of the thrombin-activatable fibrinolysis inhibitor pro-domain enables productive binding of protein substrates

We have previously reported that thrombin-activatable fibrinolysis inhibitor (TAFI) exhibits intrinsic proteolytic activity toward large peptides. The structural basis for this observation was clarified by the crystal structures of human and bovine TAFI. These structures evinced a significant rotation of the pro-domain away from the catalytic moiety when compared with other pro-carboxypeptidases, thus enabling access of large peptide substrates to the active site cleft. Here, we further investigated the flexible nature of the pro-domain and demonstrated that TAFI forms productive complexes with protein carboxypeptidase inhibitors from potato, leech, and tick (PCI, LCI, and TCI, respectively). We determined the crystal structure of the bovine TAFI-TCI complex, revealing that the pro-domain was completely displaced from the position observed in the TAFI structure. It protruded into the bulk solvent and was disordered, whereas TCI occupied the position previously held by the pro-domain. The authentic nature of the presently studied TAFI-inhibitor complexes was supported by the trimming of the C-terminal residues from the three inhibitors upon complex formation. This finding suggests that the inhibitors interact with the active site of TAFI in a substrate-like manner. Taken together, these data show for the first time that TAFI is able to form a bona fide complex with protein carboxypeptidase inhibitors. This underlines the unusually flexible nature of the pro-domain and implies a possible mechanism for regulation of TAFI intrinsic proteolytic activity in vivo.

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.

Activated thrombin activatable fibrinolysis inhibitor levels are associated with the risk of cardiovascular death in patients with coronary artery disease: the AtheroGene study

BACKGROUND

Thrombin activatable fibrinolysis inhibitor (TAFI) attenuates fibrinolysis. Results on the association between TAFI levels and the risk of coronary artery disease (CAD) are inconsistent.

OBJECTIVES

We investigated the association between TAFI levels and the risk of cardiovascular events in CAD.

PATIENTS/METHODS

1668 individuals with angiographically proven CAD at baseline were followed for a median of 2.3 years, as part of the prospective AtheroGene cohort. Fifty-six deaths from cardiovascular (CV) causes and 35 non-fatal CV events were observed.

RESULTS

At baseline, three TAFI measurements were available: one evaluating the total amount of TAFI (t-TAFI), one measuring the TAFIa/TAFIai amount, and the last the released activated peptide (TAFI-AP). TAFIa/TAFIai levels were associated with increased risk of CV death [hazard ratio (HR) for one tertile increase, 2.38 (1.56-3.63); P < 10(-4)]. This association remained significant after adjustment for conventional risk factors, CRP levels, white blood count and markers of thrombin generation and fibrinolysis [HR = 1.69 (1.07-2.67); P = 0.01]. In addition, CPB2 gene polymorphisms explained 12%, 6%, and 3% of t-TAFI, TAFIa/TAFIai and TAFI-AP levels, respectively, but none was associated with CV events.

CONCLUSIONS

The amount of activated TAFI, measured by TAFIa/TAFIai ELISA, but not of the t-TAFI is independently associated with the risk of CV death.

The crystal structure of thrombin-activable fibrinolysis inhibitor (TAFI) provides the structural basis for its intrinsic activity and the short half-life of TAFIa

Mature thrombin-activable fibrinolysis inhibitor (TAFIa) is a highly unstable metallocarboxypeptidase that stabilizes blood clots by clipping C-terminal lysine residues from partially degraded fibrin. In accordance with its in vitro antifibrinolytic activity, animal studies have reported that inhibition of mature TAFI aids in the prevention of thrombosis. The level of TAFI activity is stringently regulated through (i) controlled proteolytic truncation of the zymogen (TAFI), generating the mature enzyme, TAFIa, and (ii) the short half-life of TAFIa. TAFI itself exhibits an intrinsic enzymatic activity, which is likely required to provide a baseline level of antifibrinolytic activity. The novel crystal structure presented here reveals that the active site of TAFI is accessible, providing the structural explanation for the its intrinsic activity. It also supports the notion that an "instability region" exists, in agreement with site-directed mutagenesis studies. Sulfate ions, bound to this region, point toward a potential heparin-binding site and could explain how heparin stabilizes TAFIa.

New antithrombotic drugs: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition)

This chapter focuses on new antithrombotic drugs that are in phase II or III clinical testing. Development of these new agents was prompted by limitations of existing antiplatelet, anticoagulant, or fibrinolytic drugs. Addressing these unmet needs, this chapter (1) outlines the rationale for development of new antithrombotic agents, (2) describes the new antiplatelet, anticoagulant, and fibrinolytic drugs, and (3) provides clinical perspectives on the opportunities and challenges faced by these novel agents.

Fenofibrate improves endothelial function and decreases thrombin-activatable fibrinolysis inhibitor concentration in metabolic syndrome

Procoagulant state, inflammation, and endothelial dysfunction have been documented in metabolic syndrome. Endothelial dysfunction is a strong predictor of cardiovascular events. Studies on the association of thrombin-activatable fibrinolysis inhibitor and thrombosis are still controversial, but substantial evidence suggests that increased thrombin-activatable fibrinolysis inhibitor or thrombin-activatable fibrinolysis inhibits or protects against arterial thrombosis. This study aimed to assess concomitantly the effects of fenofibrate therapy on thrombin-activatable fibrinolysis inhibitor concentrations and endothelial functions in patients with metabolic syndrome. Twenty-five patients (16 women; mean age 50.4 +/- 7.0) were enrolled in the study. Plasma thrombin-activatable fibrinolysis inhibitor, C-reactive protein, and fibrinogen levels were measured before fenofibrate administration and after 8 weeks of fenofibrate treatment. Endothelial function was assessed by endothelial-dependent flow-mediated dilatation from brachial artery. Pretreatment (baseline) thrombin-activatable fibrinolysis inhibitor level was 52.3 (1.2-119.7) decreasing to 7.7 (0.9-51.2; P < 0.001) after 8 weeks of fibrate treatment. Endothelial functions, which were measured with flow-mediated dilatation, were significantly improved after treatment (mean flow-mediated dilatation was 6.76 +/- 2.21 at baseline and 10.66 +/- 1.17% after 8 week of fenofibrate treatment, P < 0.001). Fenofibrate decreases thrombin-activatable fibrinolysis inhibitor levels and improves endothelial function in metabolic syndrome and, thus, suggests a potential for protection against cardiovascular effects. Further studies are warranted to confirm the effects of fibrates on thrombin-activatable fibrinolysis inhibitor and for conclusive evidence on the association between thrombin-activatable fibrinolysis inhibitor and thrombosis.

Proteolytic cleavage of carboxypeptidase N markedly increases its antifibrinolytic activity

BACKGROUND

Carboxypeptidase N (CPN) is a constitutively active basic carboxypeptidase sharing specificity with activated thrombin-activable fibrinolysis inhibitor (TAFIa). Generally, CPN is regarded as being non-antifibrinolytic. However, this assumption has not been thoroughly investigated, particularly with respect to long-term antifibrinolysis. In addition, a recent report has shown that plasmin cleavage increases the catalytic activity of CPN. Therefore, we investigated the antifibrinolytic properties of CPN and plasmin-cleaved CPN (CPNc).

METHODS

CPN was incubated with plasmin for various periods of time and the prolongation of clot lysis at various concentrations of CPN/CPNc mixture was investigated in TAFI-depleted plasma. CPN cleavage was analyzed by electrophoresis and catalytic activity was determined by monitoring cleavage of the small substrate, FA-Ala-Lys.

RESULTS

CPN exhibited antifibrinolytic properties in plasma clot lysis assays when present at supraphysiological concentrations. Depletion of CPN from plasma decreased the lysis time of clots formed from minimally diluted plasma at low tissue-type plasminogen activator (t-PA) concentrations. Plasmin cleavage of CPN markedly increased the antifibrinolytic properties. CPN and CPNc prolonged lysis in a non-saturable, dose-dependent, and t-PA-dependent manner. At sufficient concentration, CPN and CPNc prolonged lysis at least forty-fivefold. CPNc was 700% more antifibrinolytic than CPN but only 7% more active toward FA-Ala-Lys. The active site inhibitor GEMSA eliminated the antifibrinolytic effects of CPN and CPNc. Antifibrinolytic activity correlated with cleavage of active and/or regulatory subunits, presumably generating heterodimeric CPNc.

CONCLUSIONS

Limited proteolysis of CPN by plasmin generates an enzyme with greatly increased antifibrinolytic properties. We speculate that (patho)physiological proteolysis of CPN may generate a long-term antifibrinolytic enzyme.

Curiouser and curiouser: recent advances in measurement of thrombin-activatable fibrinolysis inhibitor (TAFI) and in understanding its molecular genetics, gene regulation, and biological roles

The thrombin-activatable fibrinolysis inhibitor (TAFI) pathway defines a novel molecular connection between blood coagulation and both fibrinolysis and inflammation. TAFI is a plasma zymogen that can be activated by thrombin, the thrombin-thrombomodulin complex, or plasmin. The activated form of TAFI (TAFIa) attenuates fibrinolysis by removing the carboxyl-terminal lysine residues from partially degraded fibrin that mediate positive feedback in the fibrinolytic cascade. A role for TAFIa in modulating inflammation is suggested by the ability of this enzyme to down-regulate pericellular plasminogen activation and to inactivate the inflammatory peptides bradykinin and the anaphylatoxins C3a and C5a. The focus of this review is on recent advances in the clinical measurement of the TAFI pathway in human subjects and what this has revealed in terms of the molecular genetics of TAFI, the biological variation in plasma TAFI antigen levels, potential regulators of expression of the gene encoding TAFI, and the TAFI pathway as a risk factor for the development of vascular diseases. Although this field is in its infancy, much recent progress has been made and the available data suggest that the TAFI pathway is an intriguing new player in a variety of physiological and pathophysiological contexts.

Demonstration of enhanced endogenous fibrinolysis in thrombin activatable fibrinolysis inhibitor-deficient mice

To investigate the importance of thrombin activatable fibrinolysis inhibitor (TAFI) in the stabilization of plasma clots, we have compared fibrinolysis in TAFI-deficient (KO) and wild-type (WT) littermate mice. TAFI-deficient mice were previously generated by targeted gene disruption. The level of TAFI activity generated in plasma from WT mice in the presence of added thrombin and thrombomodulin (activatable TAFI) is twice that of plasma from TAFI heterozygous mice (HET); no activatable TAFI is detected in TAFI KO plasma. In vitro, TAFI KO plasma clots lysed faster than WT plasma clots, and HET plasma clots lysed at an intermediate rate. The rate of clot lysis for KO mice is not changed in the presence of potato carboxypeptidase inhibitor, a specific inhibitor of TAFIa, whereas the WT and HET clot lysis rates are increased in the presence of potato carboxypeptidase inhibitor. C-terminal lysine residues are preserved on partially degraded clots from KO mice, but are absent from partially degraded WT clots. In vivo, in a batroxobin-induced pulmonary embolism model, KO mice displayed a lower retention of fibrin in the lungs than did WT mice. These results are the first demonstration of enhanced endogenous fibrinolysis in an in vivo model without the addition of exogenous thrombolytic.

Thrombin activatable fibrinolysis inhibitor (TAFI)--how does thrombin regulate fibrinolysis?

The thrombin-catalysed conversion of plasma fibrinogen into fibrin and the development of an insoluble fibrin clot are the final steps of the coagulation cascade during haemostasis. A delicate balance between coagulation and fibrinolysis determines the stability of the fibrin clot. Thrombin plays a central role in this process, it not only forms the clot but it is also involved in stabilizing the clot by activating thrombin activatable fibrinolysis inhibitor (TAFI). Activated TAFI protects the fibrin clot against lysis. Here we will discuss the mechanisms for regulation of fibrinolysis by thrombin. The role of the coagulation system for the generation of thrombin and for the activation of TAFI implies that defects in thrombin generation will directly affect the protection of clots against lysis. Thus, defects in activation of TAFI might contribute to the severity of bleeding disorders. Vice versa an increased activation of TAFI due to an increased rate of thrombin generation might lead to thrombotic disorders. Specific inhibitors of activated TAFI or inhibitors that interfere with the generation of thrombin might provide novel therapeutic strategies for thrombolytic therapy. Besides having a role in the regulation of fibrinolysis, TAFI may also have an important function in the regulation of inflammation, wound healing and blood pressure.

Plasma TAFI and soluble CD40 ligand do not predict reperfusion following thrombolysis for acute myocardial infarction

INTRODUCTION

Thrombolytic therapy fails to achieve reperfusion in almost a third of patients with acute myocardial infarction. Thrombin activatable fibrinolysis inhibitor (TAFI) and soluble CD40 ligand (sCD40L) are novel endogenous fibrinolytic and atherothrombotic factors that determine clot stability. We investigated whether admission plasma thrombin activatable fibrinolysis inhibitor (TAFI) and soluble CD40 ligand (sCD40L) concentrations predicted reperfusion following thrombolytic therapy in patients with acute myocardial infarction.

MATERIALS AND METHODS

Prior to administration of thrombolytic therapy, venous blood was collected from 110 patients presenting with acute ST segment elevation myocardial infarction and plasma assayed for tissue plasminogen activator (t-PA) antigen and activity, plasminogen activator inhibitor type-1 antigen (PAI-1), TAFI antigen and activity, C-reactive protein (CRP) and sCD40L concentrations. Reperfusion was determined using continuous ST segment monitoring.

RESULTS

Reperfusion occurred in 77 (70%) patients with a mean treatment to reperfusion time of 83 +/- 46 min. Peak creatine kinase was significantly lower in patients who reperfused (1578 +/- 1199 versus 2200 +/- 1744 U/L; P < 0.05) and correlated with time to reperfusion (r = 0.44 [95% CI: 0.23 - 0.61], P = 0.0001). There was a modest correlation between plasma TAFI antigen and activity (r = 0.3 [95% CI: 0.04 - 0.53]; P < 0.05). There were no significant associations between coronary reperfusion and plasma concentrations of t-PA, PAI-1, TAFI, CRP or sCD40L.

CONCLUSIONS

Systemic plasma TAFI, sCD40L and CRP concentrations do not predict reperfusion in patients receiving thrombolytic therapy for acute ST elevation myocardial infarction.

A new functional assay of thrombin activatable fibrinolysis inhibitor

New thrombin activatable fibrinolysis inhibitor (TAFI) assays are necessary for studying the role of this fibrinolysis inhibitor in cardiovascular disease. The identification of a functional single nucleotide polymorphism (SNP) (1040C/T) leading to a TAFI-variant with increased stability but lower antigen levels has made the determination of functional activity even more essential. Therefore, we developed a new assay for the functional activity of TAFI in citrated plasma samples. This assay is based on the retardation of plasma clot lysis by TAFIa. TAFI activation was induced simultaneously with fibrin formation and lysis was mediated by rt-PA. The variability of other plasma components was minimized by a 20-fold dilution of the samples in TAFI-depleted plasma. Lysis times (-/+ potato carboxypeptidase inhibitor) and the TAFI-related retardation of clot lysis, the functional parameter of the assay, were determined in a group of 92 healthy volunteers, as well as TAFI antigen levels (electroimmunoassay) and two TAFI SNPs (-438A/G and 1040C/T). TAFI-related retardation was 19.8 +/- 5.6 min (mean +/- SD) and was correlated with the antigen level. The specific antifibrinolytic activity of TAFI was associated with the -438A/G and 1040C/T genotypes. Individuals with the 325Ile-variant had on average a 34% higher TAFI-specific antifibrinolytic activity than individuals with the 325Thr-isoform. The TAFI-related retardation in the two groups of individuals did not differ, as a lower level compensated for the higher specific antifibrinolytic activity of the 325Ile-isoform. This assay provides valuable information about the performance of different TAFI isoforms and constitutes a new method for studying the role of TAFI in cardiovascular disease.

Role of isoleucine residues 182 and 183 in thrombin-activatable fibrinolysis inhibitor

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a procarboxypeptidase that, once activated, can attenuate fibrinolysis. The active form, TAFIa, is a labile enzyme, with a half-life of a few minutes at 37 degrees C. Understanding the molecular mechanisms of TAFIa inactivation will allow the development of compounds that modulate TAFIa activity. Based on their three-dimensional model of TAFI, Barbosa Pereira et al. [J Mol Biol (2002), vol. 321, pp. 537-547] suggested that Ile182 and Ile183 were involved in the instability of TAFIa. However, these carboxypeptidases are, unlike TAFIa, stable proteases. Therefore, we constructed, expressed and characterized a TAFI mutant in which Ile182 and Ile183 were changed into the residues found in pancreas carboxypeptidase B at corresponding positions, Arg and Glu. The active form of the mutant, TAFIa-I182R-I183E, had a similar half-life as wild-type TAFIa, showing that Ile182 and Ile183 were not involved in the regulation of TAFIa stability. Remarkably, however, TAFI-I182R-I183E was activated at a lower rate by thrombin-thrombomodulin (mutant: 45 +/- 2 U L(-1) s(-1) and wild type: 103 +/- 3 U L(-1) s(-1)), thrombin (mutant: 1 +/-0.1 U L(-1) s(-1) and wild type 3 +/- 0.2 U L(-1) s(-1)) and plasmin (mutant: 0.8 +/- 0.04 U L(-1) s(-1) and wild type: 5.0 +/-0.2 U L(-1) s(-1)) compared with wild-type TAFI. Accordingly, it had a sixfold reduced antifibrinolytic potential. In conclusion, analysis of TAFI-I182R-I183E showed that I182 and I183 are not involved in TAFIa inactivation by conformational instability but that these residues may be involved in the activation of TAFI and stabilization of the fibrin clot.

Understanding the enzymology of fibrinolysis and improving thrombolytic therapy

Cardiovascular disease is responsible for 17 million deaths per year but acute myocardial infarction and stroke can be treated with thrombolytics ("clot busters"), which are plasminogen activators. However, despite many years of study and huge investment from the pharmaceutical industry, clinical trials of new drugs have often been disappointing. Part of the problem may be our incomplete understanding of the regulation of plasminogen activation in vivo. We have developed precise in vitro methods and with the application of computer simulations, we hope to improve our understanding of plasminogen activation to facilitate improvements in thrombolytic therapy.

Antithrombotic agents in the treatment of severe sepsis

Sepsis, a systemic inflammatory syndrome, is a response to infection and when associated with multiple organ dysfunction is termed severe sepsis. It remains a leading cause of mortality in the critically ill. The response to the invading microorganisms may be considered as a balance between a pro-inflammatory and an anti-inflammatory reaction. While an inadequate pro-inflammatory reaction and a strong anti-inflammatory response could lead to overwhelming infection and the death of the patient, a strong and uncontrolled pro-inflammatory response, manifested by the release of pro-inflammatory mediators may lead to microvascular thrombosis and multiple organ failure. Endotoxin triggers sepsis via the release of various mediators such as tumour necrosis factor-alpha and interleukin-1 (IL-1). These cytokines activate the complement and coagulation systems, release adhesion molecules, prostaglandins, leukotrienes, reactive oxygen species and nitric oxide. Other mediators involved in the sepsis syndrome include IL-1, -6 and -8; arachidonic acid metabolites; platelet activating factor; histamine; bradykinin; angiotensin; complement components and vasoactive intestinal peptide. These pro-inflammatory responses are counteracted by IL-10. Most of the trials targeting the different mediators of the pro-inflammatory response have failed due to a lack of correct definition of sepsis. Understanding the exact pathophysiology of the disease will enable more advanced treatment options. Targeting the coagulation system with various anticoagulant agents including, activated protein C, and tissue factor pathway inhibitor (TFPI) is a rational approach. Many clinical trials have been conducted to evaluate these agents in severe sepsis. While trials on antithrombin and TFPI were not so successful, the double-blind, placebo-controlled, Phase III trial of recombinant human activated Protein C Worldwide Evaluation in Severe Sepsis (PROWESS) was successful, creating a significant decrease in mortality when compared to the placebo group. A better understanding of the pathophysiologic mechanism of severe sepsis will provide better treatment options, and combination antithrombotic treatment may provide a multipronged approach for the treatment of severe sepsis.

Emerging anticoagulant and thrombolytic drugs

Since its discovery, heparin has been used intensely as an anticoagulant for several medical and surgical indications. However, efforts are in progress to replace heparin because of its serious complications, such as intraoperative and postoperative bleeding, osteoporosis, alopecia, heparin resistance, heparin rebound, heparin-induced thrombocytopenia (HIT) and thrombosis syndrome (HITTS), and other disadvantages. Significant developments in the field of new anticoagulants have resulted in the evaluation and introduction of low molecular weight heparins (LMWHs) and heparinoids, hirudin, ancrod, synthetic peptides and peptidomimetics. However, despite significant progress in the development of these new anticoagulants, a better or an ideal anticoagulant for cardiovascular patients is not yet available and heparin still continues to amaze both basic scientists and the clinicians. To minimise the adverse effects of heparin, newer approaches to optimise its use in combination with the new anticoagulants may provide better clinical outcome. In our experience, the off-label use of argatroban at a dose of 300 microg/kg iv. bolus followed by 10 microg/kg/minute infusion in combination with aggrastat (a glycoprotein [GP] IIb/IIIa inhibitor) at a dose of 10 microg/kg iv. bolus followed by an infusion of 0.15 microg/kg/minute in patients with HIT undergoing percutaneous coronary interventions resulted in elevation of celite activated clotting time (ACT) to 300 seconds followed by a gradual decline and the ACT remained above 200 seconds even after 200 min of drug administration. A bewildering array of newer anticoagulants now exist, such as LMWHs and heparinoids, indirect or direct thrombin inhibitors, oral thrombin inhibitors, such as melagatran (AstraZeneca) and HC-977 (Mitsubishi Pharmaceuticals), Factor IXa inhibitors, indirect or direct Factor Xa inhibitors, Factor VIIa/tissue factor (TF) pathway inhibitor, newer antiplatelet agents, such as GPIIb/IIIa inhibitors, fibrin specific thrombolytic agent, such as tenecteplase and modulation of the endogenous fibrinolytic activity by thrombin activatable fibrinolytic inhibitor (TAFI), Factor XIIIa inhibitors and PAI-1 inhibitors. The quest for newer anticoagulant, antiplatelet and fibrinolytic agents will continue until ideal agents are found.

Inhibition of carboxypeptidase U (TAFIa) activity improves rt-PA induced thrombolysis in a dog model of coronary artery thrombosis

The objective of this study was to test the hypothesis if thrombolysis induced by recombinant tissue-type plasminogen activator, (rt-PA) could be facilitated by inhibiting carboxypeptidase U (CPU, active Thrombin Activatable Fibrinolysis Inhibitor, TAFIa) activity. The efficacy of rt-PA alone, or in combination with the carboxypeptidase inhibitor MERGETPA, was compared in a dog model of coronary artery thrombosis. Twenty dogs were randomised in two groups, one received rt-PA, 1 mg kg(-1), as intravenous infusion over 20 min starting 30 min after thrombus formation, and the other group received rt-PA, 1 mg kg(-1), as group one with the addition of MERGEPTA 5 mg kg(-1) starting 25 min prior to coronary artery occlusion and followed by infusion of 5 mg kg(-1) h(-1) until the end of experiment. Efficacy was assessed by determination of time to lysis, duration of patency and blood flow during patency. Both groups had similar baseline characteristics with respect to haemodynamic parameters, i.e., heart rate, blood pressure and coronary artery blood flow. Coadministration of rt-PA and MERGETPA resulted in significant decrease in time to lysis (15+/-1.5 min vs. 20+/-1.7 min, p=0.03), increased patency time (87+/-16 min vs. 46+/-12 min, p=0.047) and increased coronary blood flow during patency (1131 mL h(-1) vs. 405 mL h(-1), p=0.015), compared to rt-PA alone. These results indicate that an inhibitor of CPU activity may have a beneficial effect in patients undergoing thrombolytic therapy by attaining shorter time to reperfusion and improved coronary patency.

Properties of recombinant human plasma procarboxypeptidase U produced in mammalian and insect cells

BACKGROUND

Carboxypeptidase U (EC 3.4.17.20, TAFIa) is a new member of the metallocarboxypeptidase family circulating in human plasma as a zymogen. It is activated during coagulation and is considered as an important player in the regulation of fibrinolysis.

METHODS

Heterologous expression of human plasma procarboxypeptidase U (proCPU, TAFI) was obtained in mammalian cells (C127 and DON) and in insect cells (Sf21 and H5 cells). Conditioned media were purified by cation-exchange chromatography and plasminogen affinity chromatography to yield an essentially pure protein.

RESULTS

All systems gave high expression levels (6-20 mg/l). Due to differences in glycosylation of the activation peptide, the recombinant variants of proCPU migrated differently on SDS-PAGE (52-65 kDa). However, after activation, all active recombinant enzymes migrated at 35 kDa, similar to native CPU and no evidence for post-translational modification of the catalytic domains could be detected. For the mammalian cell produced variants, activation was more efficient after desialylation. After activation, CPU showed low solubility (0.2 mg/ml) but was inhibited similarly as native CPU.

CONCLUSIONS

Mammalian cell systems were the most efficient for the production of human plasma recombinant proCPU. The obtained zymogen differs with respect to the extent and the heterogeneity of glycosylation but, after activation, the experiments did not reveal any alteration between the recombinant and native protein.

New anticoagulant drugs: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy

This article about new anticoagulant drugs is part of the seventh American College of Chest Physicians Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. The limitations of existing oral and parenteral anticoagulant agents have prompted a search for novel agents. Focusing on new anticoagulant drugs for the prevention and treatment of arterial and venous thrombosis, this article (1) reviews arterial and venous thrombogenesis, (2) discusses the regulation of coagulation, (3) describes the pathways for testing new anticoagulant agents, (4) describes new anticoagulant strategies focusing primarily on agents in phase II or III clinical testing, and (5) provides clinical perspective as to which of these new strategies is most likely to succeed.

Design and synthesis of potent, orally active, inhibitors of carboxypeptidase U (TAFIa)

A series of 3-mercapto-propionic acid derivatives that function as reversible inhibitors of carboxypeptidase U have been prepared. We present a successful design strategy using cyclic, low basicity guanidine mimetics resulting in potent, selective and bioavailable inhibitors of carboxypeptidase U (TAFIa).

Factor XII-dependent increases in thrombin activity induce carboxypeptidase-mediated attenuation of pharmacological fibrinolysis

Activation of the contact system in patients treated with fibrinolytic agents may be an important source of thrombin that activates thrombin-activated fibrinolysis inhibitor (TAFI) and attenuates fibrinolysis. Factor (F)XIIa in plasma increased 2-fold over 60 min in patients given either tissue plasminogen activator (t-PA) or streptokinase (SK). To determine whether FXIIa-mediated generation of thrombin and activated TAFI (TAFIa) attenuates fibrinolysis in vitro, plasma clots were incubated with SK (250 U mL-1) or t-PA (2.5 g mL-1) and the rate of lysis was measured. Plasma FXIIa impaired lysis judging from marked acceleration when 2.5 micro m corn trypsin inhibitor were added (lysis increased by 172 +/- 144% for SK and 40 +/- 31% for t-PA vs. no inhibitor, n = 16, P < 0.01). Moreover, inhibition of thrombin with hirudin and TAFIa with carboxypeptidase inhibitor accelerated lysis. We conclude that activation of FXII increases thrombin generation, which promotes TAFIa-mediated attenuation of fibrinolysis.

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

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.

Thrombin-activatable fibrinolysis inhibitor in young patients with myocardial infarction and its relationship with the fibrinolytic function and the protein C system

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a fibrinolytic inhibitor. Studies in coronary artery disease have reported increased TAFI activity (TAFI Act) and low TAFI antigen (TAFI Ag) levels. This controversy might be explained by the polymorphisms of its gene. Only the Thr325Ile polymorphism modulates both TAFI Ag and Act levels in vitro. This study assessed TAFI Ag and Act levels, TAFI Thr325Ile polymorphism, the fibrinolytic and protein C systems and some prothrombotic mutations in a young patient group (n = 127, aged < 51 years, with myocardial infarction) and a control group (n = 99) with similar characteristics. Patients exhibited hypofibrinolysis and higher plasminogen activator inhibitor-1 (PAI-1) levels. Although TAFI Ag was lower, TAFI Act level was significantly higher in patients and positively correlated with PAI-1, protein C inhibitor and the euglobulin lysis time. No differences between groups were found according to the Thr325Ile polymorphism. Irrespective of the genotype, patients had higher TAFI Act levels. The Ile-325 variant exhibited lower TAFI Ag levels. We suggest that the hypofibrinolysis observed in these patients results from an increase in both PAI-1 and TAFI Act, which is not related to the Thr325Ile polymorphism. Patients have high TAFI Act with low TAFI Ag levels, probably because of an increased stability of TAFI related to a fibrinolytic hypofunction.

Thrombin and fibrinolysis

When the activities of the coagulation and fibrinolytic cascades are properly regulated, so that fibrin (FN) deposition and removal are properly balanced, the vascular system is protected from catastrophic blood loss at the site of an injury, while its fluidity is ensured elsewhere. When these activities are not properly regulated, however, the organism is subjected to either excessive bleeding or thrombosis. Thrombomodulin on the endothelial cell is very important in this regulation because it converts thrombin to an anticoagulant enzyme by directing it toward the activation of protein C. It also converts thrombin to an antifibrinolytic enzyme by directing it toward the activation of thrombin-activatable fibrinolysis inhibitor (TAFI). By doing so, it creates a direct molecular connection between the coagulation and fibrinolytic cascades, such that activation of the former suppresses the activity of the latter. Recent studies indicate that the TAFI pathway functions in vivo and is likely relevant in maintaining the proper balance between FN deposition and removal. Whether it will be a target for pharmaceutical manipulation of this balance remains to be determined.

Thrombin-activatable fibrinolysis inhibitor (TAFI, plasma procarboxypeptidase B, procarboxypeptidase R, procarboxypeptidase U)

Recently, a new inhibitor of fibrinolysis was described, which downregulated fibrinolysis after it was activated by thrombin, and was therefore named TAFI (thrombin-activatable fibrinolysis inhibitor; EC 3.4.17.20). TAFI turned out to be identical to the previously described proteins, procarboxypeptidase U, procarboxypeptidase R, and plasma procarboxypeptidase B. Activated TAFI (TAFIa) downregulates fibrinolysis by the removal of carboxy-terminal lysines from fibrin. These carboxy-terminal lysines are exposed upon limited proteolysis of fibrin by plasmin and act as ligands for the lysine-binding sites of plasminogen and tissue-type plasminogen activator (t-PA). Elimination of these lysines by TAFIa abrogates the fibrin cofactor function of t-PA-mediated plasminogen activation, resulting in a decreased rate of plasmin generation and thus downregulation of fibrinolysis. In this review, the characteristics of TAFI are summarized, with an emphasis on the pathways leading to activation of TAFI and the role of TAFIa in the inhibition of fibrinolysis. However, it cannot be ruled out that TAFI has other, as yet undefined, functions in biology.

In vivo regulation of plasminogen function by plasma carboxypeptidase B

The major functions of plasminogen (Plg) in fibrinolysis and cell migration depend on its binding to carboxy-terminal lysyl residues. The ability of plasma carboxypeptidase B (pCPB) to remove these residues suggests that it may act as a suppressor of these Plg functions. To evaluate this role of pCPB in vivo, homozygote pCPB-deficient mice were generated by homologous recombination, and the resulting pCPB(-/-) mice, which were viable and healthy, were mated to Plg(-/-) mice. Plg(+/-) mice show intermediate levels of fibrinolysis and cell migration compared with Plg wild-type and deficient mice, reflecting the intermediate levels of the Plg antigen in their plasma. Differences in Plg-dependent functions between pCPB(+/+), pCPB(+/-), and pCPB(-/-) mice were then analyzed in a Plg(+/-) background. In a pulmonary clot lysis model, fibrinolysis was significantly increased in mice with partial (pCPB(+/-)) or total absence (pCPB(-/-)) of pCPB compared with their wild-type counterparts (pCPB(+/+)). In a thioglycollate model of peritoneal inflammation, leukocyte migration at 72 hours increased significantly in Plg(+/-)/pCPB(+/-) and Plg(+/-)/pCPB(-/-) compared with their wild-type counterparts. These studies demonstrate a definitive role of pCPB as a modulator of the pivotal functions of Plg in fibrinolysis and cell migration in vivo.

Enhanced endogenous thrombolysis induced by a specific factor Xa inhibitor, DX-9065a, evaluated in a rat arterial thrombolysis model in vivo

We have previously established an animal model to investigate mechanisms of arterial thrombolysis in vivo and have demonstrated that endogenous thrombolysis, mediated by thrombin-activatable fibrinolysis inhibitor, is enhanced by administration of specific thrombin inhibitors. The aim of the present study was to evaluate the effects of a synthetic and specific factor Xa inhibitor, DX-9065a, on endogenous fibrinolysis. Mural thrombi were formed in rat mesenteric arterioles by helium-neon laser irradiation in the presence of Evans blue. Thrombolysis was continuously monitored by video microscopy and was quantified using image analysis software. Oral and intravenous administration of DX-9065a enhanced endogenous thrombolysis in vivo. The mechanisms require additional investigation using other experimental systems, but nevertheless, the present results extended our previous findings and further suggested that the enhanced fibrinolysis might be due to depressed activity thrombin-activatable fibrinolysis inhibitor. The synthetic factor Xa inhibitor could provide the basis for a useful thrombolytic agent.

Basic principles in thrombolysis: regulatory role of plasminogen

During thrombolytic therapy, patients are treated with a plasminogen activator in order to stimulate the fibrinolytic system by converting the precursor plasminogen into the active enzyme plasmin. The fibrinolytic process can be divided into two phases. In the first phase, plasminogen binds to intact fibrin and initial fibrinolysis takes place. As a result, carboxyterminal lysine residues are generated, which represent new binding sites for plasminogen. In the second phase, plasminogen binds to these sites and fibrinolysis is accelerated because the local plasminogen concentration is strongly enhanced and because this plasminogen has a higher reactivity. For instance, both single-chain urokinase-type plasminogen activator (scu-PA) and staphylokinase have a high preference for this type of plasminogen, which explains their fibrin-selective action. A recently discovered thrombin-activatable fibrinolysis inhibitor (TAFI) eliminates carboxyterminal lysine residues from partially degraded fibrin and, thus, inhibits the second phase of fibrinolysis. These mechanisms show that plasminogen plays an important regulatory role in fibrinolysis and thrombolysis.

Fast homogeneous assay for plasma procarboxypeptidase U

Carboxypeptidase U (EC 3.4.17.20, CPU, TAFIa) is a novel determinant of the fibrinolytic rate. It circulates as an inactive zymogen, procarboxypeptidase U, which becomes active during the process of coagulation. We developed a high throughput method on microtiter plates for the determination of the procarboxypeptidase U concentration in human plasma samples. Following activation of procarboxypeptidase U by thrombin-thrombomodulin, the resulting enzyme activity cleaves p-OH-Hip-Arg and the generated p-OH-hippuric acid is converted by hippuricase to p-hydroxybenzoic acid and glycine. Finally, oxidative coupling of p-hydroxybenzoic acid with 4-aminoantipyrine by NaIO4 forms the quinoneimine dye. The absorbance of the latter dye is determined at 506 nm in a microtiter plate reader. A mean value of 620 U/l was found, with a CV of 3.0% within-run and 4.3% between-run. The assay showed a good correlation with the activities observed using a HPLC assay as reference method (n = 25, r = 0.979). The presented method enables the routine analysis of large sample pools in clinical setting.

New anticoagulant drugs

Arterial and venous thrombosis are major causes of morbidity and mortality. Arterial thrombosis is the most common cause of myocardial infarction, stroke, and limb gangrene. Venous thrombosis leads to potentially fatal pulmonary embolism and to post-phlebitic syndrome. Because arterial thrombi consist of platelet aggregates held together by small amounts of fibrin, strategies to inhibit arterial thrombogenesis focus mainly on blocking platelet function, but often include anticoagulants to prevent fibrin deposition. Anticoagulants are used for the prevention and treatment of venous thrombosis because venous thrombi consist mainly of fibrin and red blood cells. This paper (1) reviews arterial and venous thrombogenesis, (2) outlines new anticoagulant strategies, and (3) provides clinical perspectives on these new strategies.

Carboxypeptidase U at the interface between coagulation and fibrinolysis

In 1988, Hendricks et al. first reported on the presence of carboxypeptidase U (U refers to the unstable nature of the enzyme) in human serum. One decade later, the importance of carboxypeptidase U (CPU) in the regulation of fibrin clot dissolution is well documented. CPU circulates in plasma as an inactive zymogen, proCPU, that is converted to its active form during coagulation and fibrinolysis. CPU cleaves off C-terminal lysine residues exposed on fibrin partially degraded by the action of plasmin. Because these C-terminal lysine residues are important for upregulating the fibrinolytic rate, CPU thus slows down fibrinolysis.

Thrombin-activatable fibrinolysis inhibitor (TAFI, plasma procarboxypeptidase B, procarboxypeptidase R, procarboxypeptidase U)

Recently, a new inhibitor of fibrinolysis was described. This inhibitor downregulated fibrinolysis after it was activated by thrombin, and was therefore named TAFI (thrombin-activatable fibrinolysis inhibitor; EC 3.4.17.20). TAFI turned out to be identical to previously described proteins, procarboxypeptidase U, procarboxypeptidase R, and plasma procarboxypeptidase B. In this overview, the protein will be referred to as TAFI. TAFI is a procarboxypeptidase and a member of the family of metallocarboxypeptidases. These enzymes are circulating in plasma and are present in several tissues such as pancreas. In this review, we will describe the properties of basic carboxypeptidases with the emphasis on the role of TAFI in coagulation and fibrinolysis. It cannot be ruled out, however, that TAFI has other, yet undefined, functions in biology.

Roles of thermal instability and proteolytic cleavage in regulation of activated thrombin-activable fibrinolysis inhibitor

We have used site-directed mutagenesis and a recombinant expression system for thrombin-activable fibrinolysis inhibitor (TAFI) in order to identify the thrombin cleavage site in activated TAFI (TAFIa) and to determine the relative contribution of proteolytic cleavage and thermal instability in regulation of TAFIa activity in clots. Arg-330 of TAFIa had been proposed to be the thrombin cleavage site based on studies with trypsin, but mutation of this residue to Gln did not prevent thrombin-mediated cleavage nor did mutation to Gln of the nearby Arg-320 residue. However, mutation of Arg-302 to Gln abolished thrombin-mediated cleavage of TAFIa. All TAFIa variants were susceptible to plasmin cleavage. Interestingly, all Arg to Gln substitutions decreased the thermal stability of TAFIa. The antifibrinolytic potential of the TAFI mutants in vitro correlates with the thermal stability of their respective TAFIa species, indicating that this property plays a key role in regulating the activity if TAFIa. Incubation of TAFIa under conditions that result in complete thermal inactivation of the enzyme accelerates subsequent thrombin- and plasmin-mediated cleavage of TAFIa. Moreover, the extent of cleavage of TAFIa by thrombin does not affect the rate of decay of TAFIa activity. Collectively, these studies point to a role for the thermal instability, but not for proteolytic cleavage, of TAFIa in regulation of its activity and, thus, of its antifibrinolytic potential. Finally, we propose a model for the thermal instability of TAFIa.

Reduced activity of TAFI (thrombin-activatable fibrinolysis inhibitor) in acute promyelocytic leukaemia

Acute promyelocytic leukaemia (APL) is a disease that is distinguished from other leukaemias by the high potential for early haemorrhagic death. Several processes are involved, such as disseminated intravascular coagulation and hyperfibrinolysis. Recently, TAFI (thrombin-activatable fibrinolysis inhibitor) was identified as a link between coagulation and fibrinolysis. TAFI can be activated by thrombin, and in its activated form potently attenuates fibrinolysis by removing C-terminal lysine and arginine residues that are important for the binding and activation of plasminogen. Activation of TAFI by the coagulation system results in a down-regulation of fibrinolytic activity and, thereby, prevents a rapid dissolution of the fibrin clot. To establish whether TAFI was involved in the severity of the bleeding complications in APL, the TAFI antigen and activity levels were determined in a group of 15 patients. The TAFI antigen concentration was normal, but the activity of TAFI was severely reduced in APL by approximately 60%. The reduction of TAFI activity was most probably caused by the action of plasmin on TAFI because in vitro experiments revealed that plasmin slightly reduced antigen levels but severely reduced TAFI activity. The acquired functional TAFI deficiency in APL may contribute to the severity of the haemorrhagic diathesis because of the impaired capacity of the coagulation system to protect the fibrin clot from fibrinolysis.