Thrombin Cofactor Research Articles

77 Complement regulator factor H is a cofactor for thrombin in both pro- and anticoagulant roles

77 Complement regulator factor H is a cofactor for thrombin in both pro- and anticoagulant roles

Thrombin cleaves recombinant soluble thrombomodulin into a lectin-like domain fragment and a fragment with protein C-activating cofactor activity.

Thrombomodulin (TM) is a transmembrane protein that plays an important role in regulating the coagulation system by acting as a cofactor for thrombin in protein C activation. Additionally, TM is involved in inflammation. Previous studies have shown that soluble fragments of TM of varying sizes, which are derived from membrane-bound TM, are present in plasma and urine. Soluble fragments of TM are speculated to exhibit biological activity. Among these, a lectin-like domain fragment (TMD1) is of particular importance. Recombinant TMD1 has previously been shown to attenuate lipopolysaccharide-induced inflammation. Here, we report that thrombin cleaves recombinant soluble TM, which is used for the treatment of disseminated intravascular coagulation associated with sepsis, into TMD1 and a fragment comprising the C-terminal portion of TM (TMD23), the latter of which retains the cofactor activity for activating protein C. Our findings suggest that thrombin not only activates protein C on membrane-bound TM but may also cleave TM to generate TMD1.

Evaluation of Serum level of thrombomodulin in cases with preeclampsia

Abstract Background Preeclampsia is a multisystmic disorder of unknown cause. Endothelial cell damage has recently been suggested to underlie the pathologic change in preeclamptic pregnancy. Thrombomodulin an endothelial cell surface glycoprotein act as a co-factor for thrombin catalyzed activation of protein C. activated protein C inhibits coagulation by inactivation the coagulation factor Va and VIIIa. Aim of the Work to assess the changes in thrombomodulin level in women with preeclampsia. Patients and Methods This prospective case-control study was conducted on 123 women at Ain Shams University Maternity Hospital. Results Regarding clinic-pathological features of pre-eclampsia patients and healthy control groups, our study found that there was high significant difference (p ≤ 0.01) between hypertensive and normal patients regarding (hypertension, obesity and history of PET). Our study found that there was high significant difference (p ≤ 0.01) between pre-eclampsia patients and healthy control group regarding serum thrombomodulin protein level and serum thrombomodulin protein increases significantly with mild and severe preeclampsia and HELLP syndrome and considered a good marker for evaluation of hypertensive patients with pregnancy. Conclusion serum thrombomodulin protein level is considered a good marker for evaluation of hypertensive patients with pregnancy.

Angiopoietins bind thrombomodulin and inhibit its function as a thrombin cofactor

Angiopoietin-1 (Ang1) and Angiopoietin-2 (Ang2) are ligands for Tie2, an endothelial-specific receptor tyrosine kinase that is an essential regulator of angiogenesis. Here we report the identification, via expression cloning, of thrombomodulin (TM) as another receptor for Ang1 and Ang2. Thrombomodulin is an endothelial cell surface molecule that plays an essential role as a coagulation inhibitor via its function as a cofactor in the thrombin-mediated activation of protein C, an anticoagulant protein, as well as thrombin-activatable fibrinolysis inhibitor (TAFI). Ang1 and Ang2 inhibited the thrombin/TM-mediated generation of activated protein C and TAFI in cultured endothelial cells, and inhibited the binding of thrombin to TM in vitro. Ang2 appears to bind TM with higher affinity than Ang1 and is a more potent inhibitor of TM function. Consistent with a potential role for angiopoietins in coagulation, administration of thrombin to mice rapidly increased plasma Ang1 levels, presumably reflecting release from activated platelets (previously shown to contain high levels of Ang1). In addition, Ang1 levels were significantly elevated in plasma prepared from wound blood, suggesting that Ang1 is released from activated platelets at sites of vessel injury. Our results imply a previously undescribed role for angiopoietins in the regulation of hemostasis.

Stimulation of thrombin- and plasmin-mediated activation of thrombin-activatable fibrinolysis inhibitor by anionic molecules

BackgroundThrombin-activatable fibrinolysis inhibitor (TAFI) is a proenzyme that, once activated, attenuates fibrinolysis by removing C-terminal lysine residues from partially degraded fibrin. TAFI can be activated by thrombin or plasmin via a cleavage at Arg92 that removes the activation peptide from the enzyme, TAFIa. Thrombomodulin enhances thrombin-mediated TAFI activation and glycosaminoglycans enhance plasmin-mediated TAFI activation. The aim of this study was to investigate whether there are other anionic molecules that function as a cofactor for thrombin- or plasmin-mediated TAFI activation. MethodsTAFI activation by thrombin or plasmin was studied in the presence of physiological anionic molecules (polyphosphate, heparin, hyaluronan, DNA and dermatan sulfate) and the non-physiological sodium dodecyl sulfate (SDS). Additionally, the effect of these molecules on TAFIa stability and on thrombin-mediated protein C activation was determined. ResultsUnfractioned heparin, calcium-saturated polyphosphate with an average chain length of 100 monomers (Ca-PolyP100) and SDS significantly enhanced TAFI activation by thrombin and plasmin. Dermatan sulfate and polyphosphates with sodium as counter ion (Na-PolyP700, Na-PolyP100 and Na-PolyP70) enhanced plasmin-mediated but not thrombin-mediated TAFI activation. Additionally, unfractioned heparin, Ca-PolyP100 and SDS enhanced thrombin-mediated protein C activation. The different nature of anionic molecules capable of enhancing TAFI and protein C activation suggests a general mechanism. ConclusionsSeveral anionic molecules function as (potent) cofactors for thrombin- and plasmin-mediated TAFI activation and thrombin-mediated protein C activation. This may imply that thrombin and plasmin activity is regulated in the vasculature by more cofactors than currently appreciated.

Thrombomodulin Gene Mutations Associated with Thromboembolic Diseases

BackgroundThrombomodulin (TM), encoded by the THBD gene, is an endothelial transmembrane glycoprotein that acts as a cofactor for thrombin in the activation of protein C (PC). It has been clearly demonstrated that the anticoagulant and profibrinolytic functions of the thrombin-TM-PC system play critical roles in the prevention of thromboembolic diseases.Methods and ResultsUsing direct sequencing, six novel mutations were identified in THBD gene of eight patients with either arterial or venous thrombosis: c.376G>T (p.Asp126Tyr), c.569C>G (p.Ser190Trp), c.659T>G (p.Leu220X), c.1208G>A (p.Arg403Lys), c.1288G>A (p.Gly430Ser), c.*6_*23del and c.*23_*40del. Of the mutations, c.376G>T and c.*6_*23del were detected in a 10-year old child with venous thrombosis (VTE), who died one year later. The c.376G>T point mutation predicting p.Asp126Tyr in thrombomodulin protein was also detected in a 38-year old VTE patient. The nonsense mutation c.659T>G recurred in a 55-year old patient with coronary artery disease (CAD) and a 46-year old VTE patient, who died from repeated venous thrombosis two years later. The wild type and the mutant thrombomodulin were expressed in HEK-293t cells. Two mutations were found to have reduced TM protein expression compared to wild-type (100%), Ser190Trp (69.1% ±5%, P < .05), p.Leu220X (19.1% ± 7.6%, P < .001). Comparing to wild type, four point mutations diminished the cofactor activity of TM according to the reduced level of activation of PC in cultured cells. As for the two 18-bp deletions, a psiCHECKTM-2 vector containing wild type and mutant 3'UTR of TM was constructed. Compared with the wild-type, the deletions significantly reduced the reporter gene-expression level. The antigen level and activity in cultured cells with c.376G>T have no significant difference with the wild type, indicating that the mutant may cause a thrombophilic state through other unknown pathological mechanisms.ConclusionThe results suggest that gene mutation of thrombomodulin may be important in the pathogenesis of thrombotic diseases. Further study on genetics of thrombosis should focus on resequencing of THBD gene in different populations. DisclosuresNo relevant conflicts of interest to declare.

The New Anticoagulants

Currently, the anticoagulants available are limited to warfarin, heparin compounds, and direct thrombin inhibitors. Warfarin, the most commonly used outpatient anticoagulant, has significant shortcomings. There are numerous drug/drug and drug/food interactions, and there is difficulty in dosing for one-third of patients. Numerous new anticoagulants are undergoing testing. The meta-pentasaccharides use the core molecule of heparin and replace one or more of the sulphated groups. They are administered by subcutaneous injection, but they require only once per week dosing. Numerous studies have shown equivalents or superiority to warfarin in treatment for thromboembolic events. Direct thrombin inhibitors block thrombin (IIa) and are used mostly for heparin-induced thrombosis or during coronary interventions. However, there is an orally administered direct thrombin inhibitor, Dabigatran, which is undergoing phase III testing. It can be given without regard to weight, age, or gender with minimal drug interactions. This drug has been proven to be equivalent to low molecular weight heparin (LMWH) in deep venous thrombosis (DVT) prophylaxis and showed no excess bleeding. Another promising group of anticoagulants with numerous investigations underway are the direct X inhibitors. Most are excreted renally as opposed to hepatic clearance with intravenous or oral dosing. Numerous phase II studies have shown them to have equivalent or superior prophylaxis for DVT when compared with LMWH. Oral IXa inhibitors and an oral thrombin cofactor inhibitor are also under development. It is clear that in the near future, anticoagulants will be available that offer significant advantages when compared to those currently in use.

Remodeling of carotid arteries is associated with increased expression of thrombomodulin in a mouse transverse aortic constriction model

Thrombomodulin (TM) is an endothelial glycoprotein that functions as a thrombin cofactor in the activation of protein C. Recent evidence has revealed that TM has unique effects on cellular proliferation, adhesion, and inflammation. We examined TM expression in the arterial remodeling process with different shear conditions. Quantitative real-time reverse transcription-PCR (Q-PCR) revealed that shear stress (25 dyne/cm(2) for 6 hours) induced a 2.6 +/- 0.4-fold increase in TM mRNA levels in endothelial cell culture. Adult FVB (Friend leukemia virus B strain) mice underwent transverse aortic constriction (TAC) between the right (RCA) and left carotid artery (LCA). Doppler (n = 8), morphometric (n = 8), and Q-PCR (n = 8 or 10) studies were performed on carotid arteries at different time points. The RCA lumen and media area increased. The LCA wall shear stress decreased after TAC. RCA wall shear stress increased at day 7 followed by a decrease to the baseline at day 28. TM mRNA level in the LCA was decreased by 61% at day 7 after TAC (0.39 +/- 0.04; p < 0.05 vs. baseline). It progressively returned to the baseline at day 14 (0.85 +/- 0.12) and day 28 (1.48 +/- 0.05; all p = NS). TM appeared in the media of the RCA; TM mRNA level in the RCA was increased by 11-fold at day 14 after TAC (11.0 +/- 0.22) and progressively decreased at day 28 (5.34 +/- 0.25, all p < 0.05 vs. baseline). Our studies suggested that altered shear stress induced significant TM gene expression changes during the arterial remodeling process.

Cleavage of Factor Va Heavy Chain at Arg643 by Thrombin Partially Inactivates the Cofactor.

Prothrombinase, the enzyme complex required to activate prothrombin, is composed of the serine protease factor Xa and the cofactor factor Va, associated in 1:1 stoichiometry on a phospholipid surface in the presence of Ca2+. Incorporation of factor Va in prothrombinase is required for any meaningful rate of thrombin generation and the arrest of hemorrhage. Factor Va inactivation down-regulates thrombin production resulting in the termination of the hemostatic response. The principal enzyme involved in this down-regulation is activated protein C (APC). Factor Va is formed following enzymatic cleavage of the single chain procofactor, factor V (Mr 330,000) by thrombin. Thrombin cleaves and activates the procofactor sequentially at Arg709, Arg1018, and Arg1545. The active cofactor, factor Va, is composed of heavy (HC105, Mr 105,000) and light (Mr 74,000) chains non-covalently associated in the presence of divalent ions. Previous studies of factor Va inactivation on human umbilical vein endothelial cells (HUVEC) have shown that thrombin cleaves the heavy chain at the COOH-terminus to produce a Mr 97,000 fragment containing the NH2-terminal portion of the heavy chain and a Mr 8,000 peptide representing the COOH-terminus of the molecule which remains attached to the heavy chain by a disulfide bond. The thrombin cleavage appeared to occur between residues 586 and 654. This region contains a consensus sequence for cleavage by thrombin located between residues 640–643 (S-P-R). To evaluate the functional importance of thrombin cleavage at Arg643 for factor Va inactivation, site-directed mutagenesis was used to create recombinant factor V molecules with mutations R643→Q (factor VR643Q) and R643→A (factor VR643A). All recombinant molecules were purified to homogeneity and assayed for activity following extended activation with thrombin. Under similar experimental conditions, cleavage of HC105 and appearance of the Mr 97,000 heavy chain fragment in the wild type molecule correlated with partial loss of cofactor activity, while following incubation of factor VR643Q and factor VR643A with thrombin no cleavage of HC105 at Arg643 was observed and no presence of the Mr 97,000 heavy chain fragment was noticed. Further, no loss in cofactor activity was observed using these mutant recombinant factor Va molecules following extended incubation with thrombin. The endothelial cell surface has been presumed to be the site of PC activation and factor Va inactivation in vivo. The relative phospholipid composition of endothelial membranes has been suggested to be consistent with their ability to support factor Va inactivation in a manner analogous to the commonly used phospholipid system composed of 25% phosphatidylserine and 75% phosphatidylcholine. In the experiments conducted on the HUVEC surface incubation of 20 nM plasma factor V with 0.1 nM thrombin resulted in almost complete cleavage of HC105 over a 60 minute thrombin treatment. In the experiments presented herein much higher concentrations of thrombin were necessary to obtain a similar effect. The combined data suggest the presence of a cofactor for thrombin on the surface of endothelial cells that would facilitate cleavage of factor Va heavy chain at Arg643. Collectively, the data demonstrate that cleavage of HC105 at Arg643 by thrombin results in a partially inactive cofactor molecule and provide for an APC-independent anticoagulant effect of thrombin.

Thrombomodulin plays an important role in arterial remodeling and neointima formation in mouse carotid ligation model

Thrombomodulin (TM) is a cell membrane-bound glycoprotein that functions as a thrombin cofactor in the activation of protein C. Its protein structure includes a N-terminal lectin-like domain (D1), 6 epidermal growth factor repeats (D2), a serine-threonine-rich region (D3), a transmembrane domain (D4) and a short cytoplasmic tail (D5). Recent studies have demonstrated the direct effect of TM on cellular proliferation, adhesion and inflammation. In the study, we investigated the role of TM in vascular remodeling and neointima formation in a mouse carotid ligation model. TM expressions on the endothelium, neointima and media were examined in the ligated carotid artery by immunohistochemistry and quantitative real-time reverse transcription PCR. Endothelial TM expression decreased after ligation and appeared later in the media and neointima, which is quite similar to the appearance of TM in the human atherosclerotic process. Recombinant TMD123 was prepared. It was effective for thrombin-dependent protein C activation and the inhibition of leukocyte adhesion to the vessel wall after carotid ligation. Recombinant TMD123 and saline was administered immediately before and after carotid ligation. The TM-treated arteries demonstrated significantly less arterial dilatation (30279 +/- 12605 vs 73789 +/- 15073 microm(2), p < 0.05) in response to less neointima formation (14179 +/- 6538 vs 42227 +/- 8754 microm(2), p < 0.05) at 4 weeks after ligation. Our data indicated that there was a compensatory increase in TM expression in the media and neointima in relation to the reduced endothelial TM after carotid ligation. Early recombinant TM treatment in mice undergoing carotid ligation altered vascular remodeling and decreased the severity of neointima formation.

Novel functions of thrombomodulin in inflammation

The objective of this study was to review the mechanisms by which thrombomodulin (TM) may modulate inflammation. The data were taken from published research performed by other laboratories and our own experimental results. TM is a transmembrane glycoprotein receptor and cofactor for thrombin in the protein C anticoagulant system. Recent studies have revealed that TM has activities, both dependent and independent of either protein C or thrombin, that affect biological systems beyond the coagulation pathway. This review highlights recent insights, provided by in vitro and in vivo analyses, into how the unique structural domains of TM effectively modify coagulation, fibrinolysis, and inflammation in health and disease. A paradigm is presented to describe how these apparently distinct functions are integrated to maintain homeostasis under stress conditions. Finally, we explore the potential diagnostic and therapeutic utility of dissecting out the structure-function correlates of TM. We conclude that TM plays a central role in regulating not only hemostasis but also inflammation, thus providing a close link between these processes. Elucidation of the molecular mechanisms by which TM functions will likely provide novel targets for therapeutic intervention.

The inactivation of single-chain urokinase-type plasminogen activator by thrombin on cultured human endothelial cells

Single-chain urokinase-type plasminogen activator (scu-PA) is cleaved by thrombin, resulting in an inactive molecule called thrombin-cleaved two-chain urokinase-type plasminogen activator (tcu-PA/T). There is no knowledge about cell-mediated inactivation of scu-PA. We have studied whether scu-PA bound to cultured human umbilical vein endothelial cells (HUVEC) could be inactivated by thrombin. High molecular weight scu-PA was bound to HUVEC and incubated with increasing amounts of thrombin for 30 min at 37°C. Cell-bound urokinase-type plasminogen activator (u-PA) was released and levels of scu-PA, tcu-PA/T and active two-chain u-PA were measured using sensitive bioimmunoassays. Cell-bound scu-PA was efficiently inactivated by thrombin. Fifty percent inactivation of scu-PA occurred at about 0.2 nM thrombin. In the presence of monoclonal anti-urokinase receptor IgG, at least 50% of the binding of scu-PA to HUVEC was inhibited. The relative amount of tcu-PA/T that was generated by thrombin was not affected by the monoclonal antibody. These results indicated that scu-PA bound to HUVEC via the urokinase receptor can be inactivated by thrombin. The efficient inactivation of cell-bound scu-PA suggests that a cofactor for thrombin may be involved, like thrombomodulin or glycosaminoglycans. It is concluded that scu-PA bound to the urokinase receptor on a cell surface can be inactivated by thrombin, which may have profound effects on u-PA-mediated local fibrinolysis and extracellular proteolysis during processes in which thrombin is also involved.

Homocysteine, atherogenesis and thrombosis

Clinical studies indicate that moderately elevated levels of homocysteine are associated with both atherosclerosis and venous thrombosis. We review new studies that shed light on potential mechanisms for the vascular toxicity of homocysteine. Homocysteine stimulates vascular smooth muscle cells to proliferate and to synthesize collagen, hallmarks of atherogenesis. Homocysteine also adversely effects the anticoagulant systems that are mediated by the vascular endothelium. Homocysteine inhibits the expression and activity of endothelial cell-surface thrombomodulin, the thrombin cofactor responsible for protein C activation. Homocysteine inhibits the antithrombin III binding activity of endothelial heparan sulfate proteoglycan, thereby suppressing the anticoagulant effect of AT III. Homocysteine has been found to inhibit the fibrinolytic properties of the endothelial surface by inhibiting the binding of tissue plasminogen activator to a cell-surface receptor. Homocysteine also stimulates HUVEC tissue factor activity. The binding of lipoprotein(a) (Lp(a)) to fibrin may explain, in part, its atherogenicity. Homocysteine dramatically enhances the affinity between Lp(a), and fibrin providing a link between elevated levels of homocysteine, Lp(a) and atherosclerosis.

Homocysteine and Hemostasis: Pathogenetic Mechanisms Predisposing to Thrombosis

Growing evidence suggests that moderately elevated levels of homocyteine are associated not only with arterial thrombosis and atherosclerosis but also with venous thrombosis as well. We have reviewed recent studies that indicate that homocysteine inhibits several different anticoagulant mechanisms that are mediated by the vascular endothelium. The protein C enzyme system appears to be one of the most important anticoagulant pathways in the blood. Homocysteine inhibits the expression and activity of endothelial cell surface thrombomodulin, the thrombin cofactor responsible for protein C activation. Homocysteine inhibits the antithrombin III binding activity of endothelial heparan sulfate proteoglycan, thereby suppressing the anticoagulant effect of antithrombin III. Homocysteine also inhibits the ecto-ADPase activity of human umbilical vein endothelial cells (HUVECS). Because ADP is a potent platelet aggregatory agent, this action of homocysteine is prothrombotic. Homocysteine also interferes with the fibrinolytic properties of the endothelial surface because it inhibits the binding of tissue plasminogen activator. Homocysteine stimulates HUVEC tissue factor activity. We have found that lipoprotein(a) [Lp(a)] also stimulates HUVEC tissue factor activity. The combination of Lp(a) plus homocysteine induced more tissue factor activity than either agent alone. These disruptions in several different vessel wall-related anticoagulant functions provide plausable mechanisms for the occurrence of thrombosis in hyperhomocysteinemia.

Gene Structure of Human Thrombomodulin, a Cofactor for Thrombin Catalyzed Activation of Protein C1

The gene coding for human thrombomodulin, a thrombin receptor on endothelial cells and a cofactor for the activation of anticoagulant protein C zymogen, was isolated from a human genomic library by employing human thrombomodulin cDNA as a probe. The nucleotide sequences of the gene and the adjacent 5' and 3' flanking regions were then determined. The nucleotide sequence of this gene with approximately 3.7 kilobase pairs was identical to that of the cDNA, indicating that the gene for human thrombomodulin is free of introns. Hybridization data showed that there is only a single thrombomodulin gene in the human genome.

The light chain of factor Va contains the activity of factor Va that accelerates protein C activation by thrombin.

Protein C, a vitamin K-dependent protein, circulates in plasma as an inactive precursor. Once activated, it possesses potent anticoagulant activity through the inactivation of factors Va and VIIIa. Thrombin, the only known physiologic activator of this protein, is catalytically inefficient. Thrombomodulin, a protein purified from rabbit lungs, has been reported to enhance protein C activation by thrombin. We have previously demonstrated that factor Va, a substrate for activated protein C, is also a thrombin cofactor in the activation of protein C (Salem, H.H., Broze, G.J., Miletich, J. P., and Majerus, P.W. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 1584-1588). When factor Va is fractionated to its individual components, only the light chain (Mr 78,000) has thrombin cofactor activity. Although factor Va and thrombomodulin can both stimulate thrombin-catalyzed protein C activation, the physiological relationship between these two proteins remains to be determined.

Enhancement of migration inhibitory factor activity by plasma esterase inhibitors

The plasma esterase inhibitors alpha2-macroglobulin, alpha1-antitrypsin, C1-inhibitor, antithrombin-heparin cofactor, and, as previously described, soybean trypsin inhibitor (Kunitz) and diisopropylphosphorofluoridate (9) enhance the response of guinea pig macrophages to migration inhibitory factor. To obtain this effect, macrophages are incubated with inhibitors prior to assay. The data suggest that (a) the enhancement of migration inhibitory factor response is due to the inhibition of esterases associated with the macrophage through a distinct active site on the inhibitors, and (b) that the active sites of antithrombin-heparin cofactor and soybean trypsin inhibitor, which interact with the macrophage enzymes, are different from the active sites of these inhibitors which interact with thrombin and trypsin respectively. Chemical modification of the active site of antithrombin-heparin cofactor for thrombin and of soybean trypsin inhibitor for trypsin does not affect their capacity to enhance the migration inhibitory factor response. From studies with thrombin, it was known that antithrombin-heparin cofactor has a heparin binding site. Addition of heparin was found to prevent the migration inhibitory factor-enhancing effect of antithrombin-heparin cofactor. The present results suggest that plasma esterase inhibitors may play a regulatory role in the response of macrophages to mediators of cellular immunity.