Equilibrium binding of thrombin to recombinant human thrombomodulin: effect of hirudin, fibrinogen, factor Va, and peptide analogues

Recombinant thrombomodulin of different domains for pharmaceutical, biomedical, and cell transplantation applications

Thrombomodulin (TM) is a membrane glycoprotein mainly expressed by vascular endothelial cells and is involved in many physiological and pathological processes, such as coagulation, inflammation, cancer development, and embryogenesis. Human TM consists of 557 amino acids divided into five distinct domains: N-terminal lectin-like domain (designated as TMD1); six epidermal growth factor (EGF)-like domain (TMD2); Ser/Thr-rich domain (TMD3); transmembrane domain (TMD4); and cytoplasmic tail domain (TMD5). The different domains are responsible for different biological functions of TM. In the past decades, various domains of TM have been cloned and expressed for TM structural and functional study. Further, recombinant TMs of different domains show promising antithrombotic and anti-inflammatory activity in both rodents and primates and a recombinant soluble TM has been approved for therapeutic application. This review highlights recombinant TMs of diverse structures and their biological functions, as well as the complex interactions of TM with factors involved in the related biological processes. Particularly, recent advances in exploring recombinant TM of different domains for pharmaceutical, biomedical, and cell transplantation applications are summarized.

Successful Administration of Recombinant Human Soluble Thrombomodulin α (Recomodulin) for Disseminated Intravascular Coagulation during Induction Chemotherapy in an Elderly Patient with Acute Monoblastic Leukemia Involving the t(9;11)(p22;q23) MLL/AF9 Translocation

Patients with acute myelogenous leukemia complicate with disseminated intravascular coagulation (DIC), not only at the time of the initially leukemia diagnosis, but also during induction chemotherapy. In Japan, recently, a recombinant human soluble thrombomodulin alpha (Recomodulin) has been introduced as a new type of anti-DIC agent for clinical use in patients with hematological cancer or infectious disease. We describe a 67-year-old female case in which 25,600 units of Recomodulin for 6 days were successfully administered for both initially complicating and therapy-induced DIC without any troubles of bleeding in an acute monoblastic leukemia (AML-M5a) patient with the MLL gene translocation. Furthermore, the levels of DIC biomarkers recovered rapidly after the Recomodulin treatment. Our case suggests that DIC control using Recomodulin is one of the crucial support-therapies during remission induction chemotherapy in patients with acute leukemia of which type tends to complicate extramedullary or extranodal infiltration having potential to onset DIC.

Polyphosphate binds with high affinity to exosite II of thrombin

BACKGROUND

Polyphosphate (a linear polymer of inorganic phosphate) is secreted from platelet dense granules, and we recently showed that it accelerates factor V activation by thrombin.

OBJECTIVE

To examine the interaction of polyphosphate with thrombin.

METHODS AND RESULTS

Thrombin, but not prothrombin, altered the electrophoretic migration of polyphosphate in gel mobility assays. Thrombin binding to polyphosphate was influenced by ionic strength, and was evident even in plasma. Two positively charged exosites on thrombin mediate its interactions with other proteins and accessory molecules: exosite I (mainly with thrombin substrates), and exosite II (mainly with certain anionic polymers). Free thrombin, thrombin in complex with hirudin's C-terminal dodecapeptide and gamma-thrombin all bound polyphosphate similarly, excluding exosite I involvement. Mutations within exosite II, but not within exosite I, the Na(+)-binding site or hydrophobic pocket, weakened thrombin binding to polyphosphate as revealed by NaCl dependence. Surface plasmon resonance demonstrated tight interaction of polyphosphate with thrombin (K(d) approximately 5 nm) but reduced interaction with a thrombin exosite II mutant. Certain glycosaminoglycans, including heparin, only partially competed with polyphosphate for binding to thrombin, and polyphosphate did not reduce heparin-catalyzed inactivation of thrombin by antithrombin.

CONCLUSION

Polyphosphate interacts with thrombin's exosite II at a site that partially overlaps with, but is not identical to, the heparin-binding site. Polyphosphate interactions with thrombin may be physiologically relevant, as the polyphosphate concentrations achievable following platelet activation are far above the approximately 5 nM K(d) for the polyphosphate-thrombin interaction.

The anticoagulant fraction from the leaves ofDiospyros kaki L. has an antithrombotic activity

The leaves of Persimmon (Diospyros kaki L.) has long been used for tea in Korea since it was thought to be effective against hypertension. An anticoagulant fraction was purified through gel filtration G-100, hydrophobic, gel filtration G-150, and FPLC, Phenyl superpose column chromatographies. The purified fraction was homogenous and its Mr was estimated 10,000 Da by gel filtration and SDS-PAGE. The purified fraction was sensitive to treatment of subtilisin B, but not to heat and its activity was not changed after periodate oxidation, indicating that the activity was not due to carbohydrates. It delayed thrombin time (TT), activated partial thromboplastin time (APTT), and prothrombin time (PT) using human plasma. TT was more sensitive than APTT and PT, suggesting that the anticoagulant activity may be caused by a degradation or a defect of fibrin or thrombin. It did not cause the hydrolysis of fibrin after incubation. However, it inhibited thrombin-catalyzed fibrin formation with a competitive inhibition pattern. These results indicate that it may be an antithrombotic agent and that it is bound to fibrinogen binding sites of thrombin.

Phase I study of a novel recombinant human soluble thrombomodulin, ART-123

BACKGROUND

Anticoagulants are often given for extended periods of time to patients at high risk for venous thromboembolism, such as after orthopedic surgery. Daily subcutaneous (sc) injections can be inconvenient to the patient. A long-acting anticoagulant requiring less frequent dosing could make treatment more acceptable. Thrombomodulin is a natural anticoagulant that activates protein C, which leads to inactivation of factor (F)Va and FVIIIa and decreased thrombin formation. Recombinant human thrombomodulin is a novel anticoagulant with a long half-life in animal models.

METHODS AND RESULTS

This phase I study examined pharmacokinetics, pharmacodynamics, and safety of recombinant human soluble thrombomodulin (ART-123) after administration of doses between 0.02 and 0.06 mg kg(-1) body weight intravenously (iv), and between 0.02 and 0.45 mg kg(-1) sc in 55 healthy volunteers. The plasma half-life was 2-3 days after sc injection of various single doses. Plasma ART-123 levels estimated to be needed for prevention of thrombus formation in humans were maintained for at least 6 days after single sc injection of 0.30 and 0.45 mg kg(-1) ART-123. Antithrombotic activity with these doses was demonstrated by achieving prothrombinase inhibition of more than 80% for more than 6 days after administration. No major bleeding occurred. Pharmacodynamic modeling revealed that adequate antithrombotic ART-123 levels can be achieved for 6 days with one dose of 0.45 mg kg(-1) ART-123, and for 12 days with 2 doses of 0.30 mg kg(-1), given 5 days apart.

CONCLUSIONS

Recombinant human soluble thrombomodulin (ART-123) has a long half-life after sc injection and is well tolerated, making it a suitable agent to be tested in clinical thromboprophylaxis trials.

A recombinant murine meizothrombin precursor, prothrombin R157A/R268A, inhibits thrombosis in a model of acute carotid artery injury

Mutations in human prothrombin that generate a stable form of meizothrombin or meizothrombin(desF1) cause dysprothrombinemia in both the homozygous and heterozygous state, suggesting that meizothrombin has dominant anticoagulant effects in vivo. The enzymatic characterization of recombinant mouse meizothrombin, meizothrombin(desF1), and thrombin indicates that all 3 enzymes have similar activity toward the chromogenic substrate S-2238, that meizothrombin and meizothrombin(desF1) have less than 10% of the fibrinogen-clotting activity of thrombin, and that meizothrombin is more active than thrombin or meizothrombin(desF1) for thrombomodulin-dependent protein C activation. Thus, activated mouse prothrombin R157A/R268A is similar to human meizothrombin in activity toward S-2238, fibrinogen, and protein C. The time to occlusion after FeCl3-induced carotid artery injury was delayed (11.8 ± 3.6 minutes, n = 5) in Cf2+/- mice infused with prothrombin R157A/R268A compared with control mice infused with wild-type prothrombin (5.3 ± 1.5 minutes, n = 3; P = .006). In this model, prothrombin R157A/R268A has anticoagulant activity that reflects its decreased fibrinogen-clotting activity and preserved protein C-activating activity and is consistent with dominant inhibition of fibrinogen clotting. (Blood. 2004;104:415-419)

Minimally oxidized low-density lipoprotein regulates hemostasis factors of brain capillary endothelial cells

Minimally oxidized low-density lipoprotein (MM-LDL) is a potent atherogenic lipoprotein. We analyzed the effects of MM-LDL on brain capillary endothelial expression of plasminogen activator inhibitor-1 (PAI-1), tissue-type plasminogen activator (tPA), and thrombomodulin (TM). Cultured bovine brain capillary endothelial cells (BEC) incubated with MM-LDL (25 microg/ml) for 24 h showed increased PAI-1 mRNA levels by approximately seven-fold, while tPA and TM mRNA levels were reduced by 84% and 75%, respectively. Moreover, PAI-1 protein levels increased two-fold (16.8+/-7.6 vs. 7.6+/-2.1 ng/ml, p<0.05), whereas tPA protein levels decreased by 45% (1.3+/-0.5 ng/ml vs. 2.3+/-0.7 ng/ml, p<0.05), and TM protein level decreased by 40%. Following incubation with MM-LDL, PAI-1 activity was increased 35% (18.4+/-5.0 vs. 24.8+/-5.2 AU/ml, p<0.05), while TM activity was decreased by 30%. MM-LDL therefore has substantial pro-thrombotic effects on brain capillary endothelial cells, reducing both endothelial fibrinolytic capacity (downregulating tPA while upregulating PAI-1) and anticoagulant function (downregulating TM). These results suggest that MM-LDL may contribute to thrombus formation in the brain.

Identification and characterization of novel salivary thrombin inhibitors from the ixodidae tick, Haemaphysalis longicornis

Novel antithrombin molecules were identified from the ixodidae tick, Haemaphysalis longicornis. These molecules, named madanin 1 and 2, are 7-kDa proteins and show no significant similarities to any previously identified proteins. Assays using human plasma showed that madanin 1 and 2 dose-dependently prolonged both activated partial thromboplastin time and prothrombin time, indicating that they inhibit both the intrinsic and extrinsic pathways. Direct binding assay by surface plasmon resonance measurement demonstrated that madanin 1 and 2 specifically interacted with thrombin. Furthermore, it was clearly shown that madanin 1 and 2 inhibited conversion of fibrinogen into fibrin by thrombin, thrombin-catalyzed activation of factor V and factor VIII, and thrombin-induced aggregation of platelets without affecting thrombin amidolytic activity. These results suggest that madanin 1 and 2 bind to the anion-binding exosite 1 on the thrombin molecule, but not to the active cleft, and interfere with the association of fibrinogen, factor V, factor VIII and thrombin receptor on platelets with an anion-binding exosite 1. They appear to be exosite 1-directed competitive inhibitors.

Identification of the epidermal growth factor-like domains of thrombomodulin essential for the acceleration of thrombin-mediated inactivation of single-chain urokinase-type plasminogen activator

Single-chain urokinase-type plasminogen activator (scu-PA) can be cleaved by thrombin into a virtually inactive form called thrombin-cleaved two-chain urokinase-type plasminogen activator (tcu-PA/T), a process accelerated by thrombomodulin, which contains six epidermal growth factor (EGF)-like domains. In this study, we identified the EGF-like domains of thrombomodulin required for the acceleration of the inactivation of scu-PA by thrombin using various forms of thrombomodulin (TM). scu-PA was treated with thrombin in the absence and presence of full-length rabbit TM (containing EGF1-6), recombinant TM comprising all of the extracellular domains including EGF1-6 (TMLEO) and recombinant TM comprising EGF4-6 plus the interconnecting region between EGF3 and EGF4 (TMEi4-6), and the tcu-PA/T generated was quantitated in each case. Rabbit TM accelerated the inactivation of scu-PA approximately 35-fold, while both recombinant forms accelerated it only threefold due to the absence of a critical chondroitin sulfate moiety. Subsequently, TME5-6 was prepared by cyanogen bromide digestion of TMEi4-6. TME5-6 bound to thrombin but did not accelerate the activation of protein C. In contrast, the inactivation of scu-PA by thrombin was accelerated to the same extent as that induced by TMLEO and TMEi4-6. This study demonstrates that, in addition to the chondroitin sulfate moiety, only EGF-like domains 5 and 6 are essential for the acceleration of the inactivation of scu-PA by thrombin. This differs from the domains that are critical for activation of protein C (EGF-like domains i4-6) and thrombin activatable fibrinolysis inhibitor (EGF-like domains 3-6).

Fibrinolytic and antithrombotic protease from Spirodela polyrhiza

A fibrinolytic protease was purified from a Chinese herb (Spirodela polyrhiza). The protease has a molecular mass of 145 kDa and 70 kDa in gel filtration and SDS-polyacrlamide gel electrophoresis (PAGE), respectively, implying it is a dimer. Its optimum pH was 4.5-5.0. The enzyme was stable below 42 degrees C and after lyophilization. The enzyme activity was inhibited significantly by leupeptin and aprotinin. The protease hydrolyzed not only fibrin but also fibrinogen, cleaving Aalpha and Bbeta without affecting the gamma chain of fibrinogen. It preferentially cleaved the peptide bond of Arg or Lys of synthetic substrates (P1 position). The enzyme had an anticoagulating activity measured with activated partial thromboplastin time (APTT), thrombin time (TT), and prothrombin time (PT) tests. It delayed APTT, TT, and PT two times at the concentration of 36, 39, and 128 nM, respectively and this was drastically reduced after heat treatment.

Transforming growth factor-beta mediates astrocyte-specific regulation of brain endothelial anticoagulant factors

BACKGROUND AND PURPOSE

Astrocytes are potent regulators of brain capillary endothelial cell function. Recently, astrocytes were shown to regulate brain capillary endothelial expression of the fibrinolytic enzyme tissue plasminogen activator (tPA) and the anticoagulant thrombomodulin (TM). To study the mechanism of this process, we examined the hypothesis that astrocyte regulation of endothelial tPA and TM is mediated by transforming growth factor-beta (TGF-beta).

METHODS

Brain capillary endothelial cells were grown in blood-brain barrier models. We examined astrocyte-endothelial cocultures, endothelial monocultures, and astrocyte-conditioned media (ACM) for the expression of TGF-beta. We also incubated endothelial cells with ACM to determine the role of TGF-beta. Following 24 hours of incubation, we assayed for tPA and TM mRNA, as well as tPA and TM activity.

RESULTS

Astrocyte-endothelial cocultures and ACM exhibited significantly higher levels of active TGF-beta than brain endothelial monocultures and endothelial cells grown in nonconditioned media, respectively. Brain endothelial cells incubated with ACM exhibited reduced tPA and TM mRNA and activity. Treatment with exogenous TGF-beta produced dose-dependent reductions in tPA and TM. The effects of ACM on both tPA and TM were blocked by TGF-beta neutralizing antibody.

CONCLUSIONS

These data indicate that TGF-beta mediates astrocyte regulation of brain capillary endothelial expression of tPA and TM.

Evidence for multiple enzyme site involvement in the modulation of thrombin activity by products of prothrombin proteolysis

Kinetic evidence is presented for the interaction of prothrombin with several distinctive topological regions of the thrombin molecule. Modulations of thrombin catalytic activity on the protein substrates prothrombin and prethrombin 1 are demonstrated that involve the fragment 1 and fragment 2 portions. The inhibitory effects are demonstrably non-competitive. In addition to exhibiting non-competitive inhibition, fragment 2 is capable of enhancing proteolysis by thrombin; and therefore to react with a second region of the enzyme. On the basis of the crystallographic studies of the complex between fragment 2 and thrombin (Arni et al., Biochemistry 32 (1992) 4727), this activating site is proposed to be associated with exosite II. The allosteric switch between procoagulant and anticoagulant activities identified from studies by Di Cera (Dang et al., Proc. Natl. Acad. Sci USA 92 (1995) 5977) could be 'thrown' by a macromolecular effector that is generated during thrombin formation--a plausible mechanism for switching that deserves further investigation.

Activation of thrombin-activable fibrinolysis inhibitor requires epidermal growth factor-like domain 3 of thrombomodulin and is inhibited competitively by protein C

Thrombomodulin is a cofactor protein on vascular endothelial cells that inhibits the procoagulant functions of thrombin and enhances thrombin-catalyzed activation of anticoagulant protein C. Thrombomodulin also accelerates the proteolytic activation of a plasma procarboxypeptidase referred to as thrombin-activable fibrinolysis inhibitor (TAFI). In this study, we describe structures on recombinant membrane-bound thrombomodulin that are required for human TAFI activation. Deletion of the N-terminal lectin-like domain and epidermal growth factor (EGF)-like domains 1 and 2 had no effect on TAFI or protein C activation, whereas deletions including EGF-like domain 3 selectively abolished thrombomodulin cofactor activity for TAFI activation. Provided that thrombomodulin EGF-like domain 3 was present, TAFI competitively inhibited protein C activation catalyzed by the thrombin-thrombomodulin complex. A thrombomodulin construct lacking EGF-like domain 3 functioned normally as a cofactor for protein C activation but was insensitive to inhibition by TAFI. Thus, the anticoagulant and antifibrinolytic cofactor activities of thrombomodulin have distinct structural requirements: protein C binding to the thrombin-thrombomodulin complex requires EGF-like domain 4, whereas TAFI binding also requires EGF-like domain 3.

Enhancement of the functional repertoire of the rat parietal peritoneal mesothelium in vivo: directed expression of the anticoagulant and antiinflammatory molecule thrombomodulin

We have used our previously described ex vivo mesothelial cell (MC)-mediated gene therapy strategy (Gene Ther. 2:393-401, 1995) to modify the functional properties of the rat parietal peritoneal mesothelium in vivo by expression of a membrane-bound recombinant protein on the MC surface. Rat primary MCs were stably transfected (using strontium phosphate DNA coprecipitation) with a plasmid containing the gene for rat thrombomodulin (TM), a transmembrane glycoprotein that functions as an essential cofactor for the physiological activation of the anticoagulant protein C by the enzyme thrombin. As demonstrated by immunohistochemistry and by direct equilibrium binding with radiolabeled thrombin, genetically modified MCs expressed high levels of TM antigen on their surface in vitro. As judged by a thrombin-dependent protein C activation assay, such MC membrane-bound TM was biologically active. Once reseeded on the denuded parietal peritoneal surface of syngeneic recipients, these TM-transfected MCs continued to express TM antigen in vivo for at least 90 days. Moreover, the recombinant TM expressed on the reconstituted parietal mesothelium retained its ability to activate protein C in a thrombin-dependent manner. Our data indicate that MC-mediated expression of TM can be used to augment the anticoagulant properties of the parietal peritoneal surface. In general, our results suggest that ex vivo MC-mediated gene therapy can be used to deliver other therapeutic transmembrane proteins to the MC surface to enhance the functional repertoire of the parietal mesothelium in vivo.

Thrombomodulin expression in bovine brain capillaries. Anticoagulant function of the blood-brain barrier, regional differences, and regulatory mechanisms

Thrombomodulin (TM), a key cofactor of the TM-protein C pathway, is of major biologic significance for the antithrombotic properties of endothelial cells. Yet, there is uncertainty whether TM is expressed in brain and what mechanisms govern brain endothelial anticoagulant activity. In this study, bovine brain capillaries were used as an in vitro model of the blood-brain barrier to determine factors involved in the regulation of TM expression in cerebral vasculature. Quantitative competitive-polymerase chain reaction assay revealed significant regional differences in the amount of brain capillary TM mRNA, i.e., cortical > cerebellar > pontine, consistent with the reverse transcription-polymerase chain reaction findings in which the abundance of TM mRNA was analyzed relative to beta-actin mRNA. Regional differences in TM mRNA brain capillary level correlated well with differences in protein C activation. The TM mRNA and activity were not detectable in brain parenchyma. Pathogenic mediators of ischemic stroke, interleukin 1 beta (10 U/mL), and tumor necrosis factor alpha (10 U/mL), produced a time-dependent decrease in brain capillary TM mRNA (t1/2 of 2.1 and 3.9 hours, respectively) and reduced endothelial TM activity. Incubation of brain capillaries with retinoic acid (10 mumol/L) and dibutyryl cAMP (3 mmol/L) resulted in a 4-fold increase in TM mRNA at 4 and 8 hours, respectively, followed by an increase in protein C activation. We conclude that TM at the blood-brain barrier is likely to be an important physiologic anticoagulant in brain microcirculation. Its downregulation by cytokines may contribute to ischemic brain damage and potentially could be counteracted by retinoic acid and cAMP.

Functional Interactions Between the Thrombin Receptor and the T-Cell Antigen Receptor in Human T-Cell Lines

The proteolytically activated thrombin receptor (TR) is expressed by T lymphocytes, which suggests that thrombin may modulate T-cell activation at sites of hemostatic stress. We examined the relationship between TR function and T-cell activation in the Jurkat human T-cell line and in T-cell lines with defined defects in T-cell antigen receptor (TCR) function. Stimulation with thrombin or the synthetic TR peptide SFLLRN produced intracellular Ca2+ transients in Jurkat cells. As the concentration of TR agonist was increased, peak Ca2+ mobilization increased, but influx of extracellular Ca2+ decreased. TR signaling was enhanced in a TCR-negative Jurkat line and in T-cell lines deficient in the tyrosine kinase lck or the tyrosine phosphatase CD45, both of which are essential for normal TCR function. TCR cross-linking with anti-CD3 IgM desensitized TR signaling in Jurkat cells, but not in CD45-deficient cells. A proteinase-activated receptor (PAR-2)–specific agonist peptide, SLIGKV, produced small Ca2+ transients in both MEG-01 human megakaryocytic cells and Jurkat cells, but was less potent than the TR-specific agonist TFRIFD in both cell types. Like TR signaling, PAR-2 signaling was enhanced in TCR-negative or lck-deficient Jurkat clones. These findings provide evidence for functional cross-talk between proteolytically activated receptors and the TCR.

Inactivation of thrombin by antithrombin is accompanied by inactivation of regulatory exosite I

Exosite I of the blood clotting proteinase, thrombin, mediates interactions of the enzyme with certain inhibitors, physiological substrates and regulatory proteins. Specific binding of a fluorescein-labeled derivative of the COOH-terminal dodecapeptide of hirudin ([5F] Hir54-65) to exosite I was used to probe changes in the function of the regulatory site accompanying inactivation of thrombin by its physiological serpin inhibitor, antithrombin. Fluorescence-monitored equilibrium binding studies showed that [5F]Hir54-65 and Hir54-65 bound to human alpha-thrombin with dissociation constants of 26 +/- 2 nM and 38 +/- 5 nM, respectively, while the affinity of the peptides for the stable thrombin-antithrombin complex was undetectable (>/=200-fold weaker). Kinetic studies showed that the loss of binding sites for [5F]Hir54-65 occurred with the same time-course as the loss of thrombin catalytic activity. Binding of [5F] Hir54-65 and Hir54-65 to thrombin was correlated quantitatively with partial inhibition of the rate of the thrombin-antithrombin reaction, maximally decreasing the bimolecular rate constants 1.7- and 2.1-fold, respectively. These results support a mechanism in which thrombin and the thrombin-Hir54-65 complex can associate with antithrombin and undergo formation of the covalent thrombin-antithrombin complex at modestly different rates, with inactivation of exosite I leading to dissociation of the peptide occurring subsequent to the rate-limiting inactivation of thrombin. This mechanism may function physiologically in localizing the activity of thrombin by allowing inactivation of thrombin that is bound in exosite I-mediated complexes with regulatory proteins, such as thrombomodulin and fibrin, without prior dissociation of these complexes. Concomitant with inactivation of thrombin, the thrombin-antithrombin complex may be irreversibly released due to exosite I inactivation.

Identification and characterization of the thrombin binding sites on fibrin

Thrombin binds to fibrin at two classes of non-substrate sites, one of high affinity and the other of low affinity. We investigated the location of these thrombin binding sites by assessing the binding of thrombin to fibrin lacking or containing gamma' chains, which are fibrinogen gamma chain variants that contain a highly anionic carboxyl-terminal sequence. We found the high affinity thrombin binding site to be located exclusively in D domains on gamma' chains (Ka, 4.9 x 10(6) M-1; n, 1.05 per gamma' chain), whereas the low affinity thrombin binding site was in the fibrin E domain (Ka, 0.29 x 10(6) M-1; n, 1.69 per molecule). The amino-terminal beta15-42 fibrin sequence is an important constituent of low affinity binding, since thrombin binding at this site is greatly diminished in fibrin molecules lacking this sequence. The tyrosine-sulfated, thrombin exosite-binding hirudin peptide, S-Hir53-64 (hirugen), inhibited both low and high affinity thrombin binding to fibrin (IC50 1.4 and 3.0 microM respectively). The presence of the high affinity gamma' chain site on fibrinogen molecules did not inhibit fibrinogen conversion to fibrin as assessed by thrombin time measurements, and thrombin exosite binding to fibrin at either site did not inhibit its catalytic activity toward a small thrombin substrate, S-2238. We infer from these findings that there are two low affinity non-substrate thrombin binding sites, one in each half of the dimeric fibrin E domain, and that they may represent a residual aspect of thrombin binding and cleavage of its substrate fibrinogen. The high affinity thrombin binding site on gamma' chains is a constitutive feature of fibrin as well as fibrinogen.

Enhanced thrombin sensitivity of a factor VIII-heparin cofactor II hybrid

Generation of thrombin at a site of vascular injury is a key event in the arrest of bleeding. In addition to the conversion of fibrinogen into the insoluble fibrin, thrombin can initiate a number of positive and negative feedback mechanisms that either sustain or down-regulate clot formation. We have modulated the thrombin sensitivity of human blood coagulation factor VIII, an essential cofactor in the intrinsic pathway of blood coagulation. We have substituted an acidic region of factor VIII corresponding to amino acid sequence Asp712-Ala736 by amino acid sequence Ile51-Leu80 of the thrombin inhibitor heparin cofactor II. Functional analysis of the resulting factor VIII-heparin cofactor II hybrid, termed des-(868-1562)-factor VIII-HCII, revealed an increase in procoagulant activity as measured in a one-stage clotting assay. Incubation of purified des-(868-1562)-factor VIII-HCII with different amounts of thrombin showed that this protein was more readily activated by thrombin when compared with des-(868-1562)-factor VIII, a control protein lacking amino acid sequence Ile51-Leu80 of heparin cofactor II. This was manifested by an increase in the second order rate constant of activation by thrombin for des-(868-1562)-factor VIII-HCII (12.0 +/- 0.48 x 10(6) M-1 s-1) compared with des-(868-1562)-factor VIII (1.77 +/- 0.21 x 10(6) M-1 s-1). Our data suggest that amino acid sequence Ile51-Leu80 of heparin cofactor II endows factor VIII with increased sensitivity towards thrombin which results in accelerated clot formation.

The endothelial cell protein C receptor. Inhibition of activated protein C anticoagulant function without modulation of reaction with proteinase inhibitors

A soluble form of the endothelial cell protein C receptor (EPCR) was analyzed for the ability to modulate the functional properties of protein C and activated protein C (APC). In a plasma clotting system initiated with factor Xa, EPCR blocked the anticoagulant activity of APC in a dose-dependent fashion. EPCR had no influence on clotting in the absence of APC. Consistent with the plasma results, EPCR slowed the proteolytic inactivation of factor Va by slowing both of the key proteolytic cleavages in the heavy chain of factor Va. EPCR did not prevent protein C activation by the soluble thrombin-thrombomodulin complex, did not alter the inactivation of APC by alpha1-antitrypsin or protein C inhibitor, and did not influence the kinetics of peptide paranitroanilide substrate cleavage significantly. We conclude that EPCR binds to an exosite on APC that selectively modulates the enzyme specificity in a manner reminiscent of the influence of thrombomodulin on thrombin.

Structural resiliency of an EGF-like subdomain bound to its target protein, thrombin

The thrombin-bound structures of native peptide fragments from the fifth EGF-like domain of thrombomodulin were determined by use of NMR and transferred NOE spectroscopy. The bound peptides assume an EGF-like structure of an antiparallel beta-sheet, a novel structural motif observed for a bound peptide in protein-peptide complexes. There is a remarkable structural resiliency of this structure motif manifested in its ability to accommodate a different number of residues within the disulfide loop. Docking experiments revealed that the key contacts with thrombin are hydrophobic interactions between the side chains of residues Ile 414 and Ile 424 of thrombomodulin and a hydrophobic pocket on the thrombin surface. Residues Leu 415, Phe 419, and Ile 420, which would have been buried in intact EGF-like domains, are unfavorably exposed in the complex of thrombin with the EGF-like thrombomodulin fragment, thus providing a rationale for the enhancement of binding affinity upon the deletion of Ile 420. The unique beta-sheet structures of the bound peptides are specified by the presence of disulfide bridges in the peptides because a corresponding linear thrombomodulin fragment folds into a sheet structure with a different backbone topology. The different bound conformations for the linear and the cyclized peptides indicate that side-chain interactions within a specific environment may dictate the folding of bound peptides in protein-peptide complexes.

Selection of a suppressor mutation that restores affinity of an oligonucleotide inhibitor for thrombin using in vitro genetics

The thrombin aptamer is a single-stranded DNA of 15 nucleotides that was identified by the selection of thrombin-binding molecules from a large combinatorial library of oligonucleotides. This prototype aptamer of thrombin has a unique double G-tetrad structure capable of inhibiting thrombin at nanomolar concentrations through binding to a specific region within thrombin exosite I. Substitution of arginine 70 in thrombin exosite I with glutamic acid effectively eliminated binding of the prototype thrombin aptamer. In contrast, aptamers selected against R70E thrombin were able to bind and inhibit both wild-type and R70E thrombins, and displayed potassium-independent inhibition. Aptamers selected against R70-E thrombin bound to sites identical or overlapping with that of the prototype thrombin aptamer. These aptamers retained the potential to form double G-tetrad structures; however, these structures would be destabilized by a T-->A substitution, disrupting the T4-T13 base pairing found in the prototype. This destabilization appeared to be partially compensated by newly recruited structural elements. Thus, selection against R70E thrombin did not lead to aptamers that bound to alternative sites, but instead to ssDNA structures with a suppressor mutation that accommodated the mutation in thrombin within a double G-tetrad context. These results provide insight into the aptamer-thrombin interaction and suggest that the binding site for the prototype is the dominant aptamorigenic site on thrombin.

Functional mapping of the surface residues of human thrombin

Utilizing site-directed mutagenesis, 77 charged and polar residues that are highly exposed on the surface of human thrombin were systematically substituted with alanine. Functional assays using thrombin mutants identified residues that were required for the recognition and cleavage of the procoagulant substrate fibrinogen (Lys21, Trp50, Lys52, Asn53 + Thr55, Lys65, His66, Arg68, Tyr71, Arg73, Lys77, Lys106 + Lys107, Asp193 + Lys196, Glu202, Glu229, Arg233, Asp234) and the anticoagulant substrate protein C (Lys21, Trp50, Lys65, His66, Arg68, Tyr71, Arg73, Lys77, Lys106 + Lys107, Glu229, Arg233), interactions with the cofactor thrombomodulin (Gln24, Arg70) and inhibition by the thrombin aptamer, an oligonucleotide-based thrombin inhibitor (Lys65, His66, Arg70, Tyr71, Arg73). Although there is considerable overlap between the functional epitopes, distinct and specific residues with unique functions were identified. When the functional residues were mapped on the surface of thrombin, they were located on a single hemisphere of thrombin that included both the active site cleft and the highly basic exosite 1. No functional residues were located on the opposite face of thrombin. Residues with procoagulant or anticoagulant functions were not spatially separated but interdigitated with residues of opposite or shared function. Thus thrombin utilizes the same general surface for substrate recognition regardless of substrate function although the critical contact residues may vary.

Thrombin inhibition by cyclic peptides from thrombomodulin

Peptides corresponding to the loop regions of the fourth, fifth, and sixth epidermal growth factor (EGF)-like domains of thrombomodulin (TM) have been synthesized and assayed for thrombin inhibition, as indicated by both inhibition of thrombin-mediated fibrinogen clotting and inhibition of the association of thrombin with TM that results in protein C activation. Peptides from the fifth EGF-like domain showed significant inhibition of fibrinogen clotting and protein C activation, whereas peptides from the fourth and sixth EGF-like domains were weak inhibitors in both assays. Two structural features were important for inhibitory potency of the peptides from the fifth EGF-like domain: cyclization by a disulfide bond and attachment of the "tail" amino acids C-terminal to the disulfide loop. Linear control peptides did not significantly inhibit clotting or protein C activation. The C-terminal loop alone, the "tail" peptide, or a mixture of the two were at least 10-fold less potent inhibitors of clotting or protein C activation. A more constrained peptide analog was designed by deletion of an isoleucine within the C5-C6 disulfide loop, TM52-1 + 5C. This analog was a better inhibitor in both assay systems, having a Ki for protein C activation of 26 microM.

Absence of the blood-clotting regulator thrombomodulin causes embryonic lethality in mice before development of a functional cardiovascular system

We have targeted the murine thrombomodulin (TM) gene in embryonic stem cells and generated embryos as well as mice with TM deficiency. The heterozygous TM-deficient (TM+/-) mice as compared to wild-type (TM+/+) littermates exhibit 50% reductions in the levels of TM mRNA and TM protein. However, TM+/- mice appear normal and are free of thrombotic complications. The homozygous TM-deficient (TM-/-) embryos die before embryonic day 9.5. An overall retardation in growth and development of TM-/- embryos is first evident on embryonic day 8.5 (8-12 somite pairs). However, no specific pathologic abnormalities are observed. These initial changes take place at a time when TM is normally expressed in the parietal yolk sac. The removal of embryonic day 7.5 TM-/- embryos from maternal decidua and their subsequent culture in vitro allow development to proceed to stages not observed in vivo (13-20 somite pairs) with the appearance of normal blood vessels in the visceral yolk sac and embryo. The results of our studies suggest that the failure of TM-/- embryos to survive at mid-gestation is a consequence of dysfunctional maternal-embryonic interactions caused by the absence of TM in the parietal yolk sac and demonstrate that the receptor is necessary for normal embryonic development in utero.

Thrombin interaction with a recombinant N-terminal extracellular domain of the thrombin receptor in an acellular system

The cDNA of the human endothelial cell thrombin receptor has been cloned and a chimeric fusion protein consisting of glutathione-S-transferase (GST) and the portion 25-97 corresponding to the N-terminal first extracellular domain of the thrombin receptor (TRE) has been expressed in Escherichia coli. Introduction of a factor Xa cleavage site in the fusion protein allowed purification of TRE after removal from the GST carrier protein. Purified GST-TRE or TRE have been tested in solution for their ability to interact with thrombin. alpha-Thrombin cleaved the fusion protein at position Arg-41-Ser-42 of TRE in a time- and concentration-dependent manner and GST-TRE competed with the tripeptidic substrate S-2238 for hydrolysis by thrombin (Ki = 0.5 microM). gamma-Thrombin that lacks the anion-binding exosite was 100-fold less potent than alpha-thrombin at cleaving GST-TRE. TRE competed with polymerizing fibrin monomers for binding to thrombin (Ki = 7.5 microM). The cleavage of GST-TRE by alpha-thrombin was inhibited by several alpha-thrombin exosite ligands such as the C-terminal peptide of hirudin, thrombomodulin and fibrin(ogen) fragment E. In contrast, platelet glycocalicin did not inhibit GST-TRE cleavage. In conclusion, the use of purified soluble GST-TRE allowed us to derive an affinity constant for thrombin interaction with the N-terminal domain of the receptor and to confirm the location of the cleavage site at Arg41-Ser-42 of the receptor. The importance of the thrombin anion-binding exosite for thrombin receptor recognition is highlighted by the low reactivity of gamma-thrombin for GST-TRE and by competition experiments, which in addition indicate that binding sites for fibrin(ogen), thrombomodulin and GST-TRE are overlapping. In contrast, binding of thrombin to GST-TRE and glycocalicin are not mutually exclusive, indicating that glycocalicin and TRE interact with discrete subsites within the large groove that constitutes the anion-binding exosite.

Persistent thrombin generation in humans during specific thrombin inhibition with hirudin

BACKGROUND

The degree to which antithrombotic drugs suppress thrombin generation is unknown. Because hirudin, unlike antithrombin III, binds intravascular thrombin rapidly and selectively to yield a circulating inactive complex of 3- to 4-hour half-life, we used intravenous hirudin in humans to investigate the course of thrombin generation during and early after anticoagulation with this potent, direct antithrombin.

METHODS AND RESULTS

Intravascular thrombin was measured with an ELISA for the thrombin-hirudin complex formed during and for 18 hours after stopping a 6-hour infusion of hirudin at 0.1, 0.2, and 0.3 mg.kg-1.h-1 in three groups of six patients each. With free hirudin in 20- to 10,000-fold molar excess of thrombin and peak activated partial thromboplastin times of 2.3 to 3.0 times baseline, mean plasma thrombin-hirudin complex increased from 794 +/- 85 pg/mL (mean +/- SEM) 15 minutes after the start of the infusion to 1617 +/- 151 pg/mL at 6 hours of infusion to 2667 +/- 654 pg/mL at 24 hours. During the 24-hour observation period, plasma concentration of fragment 1.2 (the peptide released during conversion of prothrombin to thrombin) never fell below baseline but rather increased transiently during the hirudin infusion. Plasma concentrations of thrombin-antithrombin III complex (in ng/mL) decreased from 4.34 +/- 0.40 at baseline to 1.64 +/- 0.13 at 6 hours (P < .001) and gradually increased after stopping the infusion to 5.7 +/- 0.87 at 24 hours (nonsignificant compared with baseline).

CONCLUSIONS

Measurement of thrombin-hirudin complex may be used as a marker of thrombin generation in humans. Persistent accumulation of thrombin-hirudin complex and generation of fragment 1.2 during and after completion of potent anticoagulation with hirudin suggest thrombin generation is not blocked by high-affinity thrombin inhibition. The persistent formation of thrombin during declining plasma levels of hirudin may contribute to the pathogenesis of rethrombosis early after antithrombin therapy or during inadequate anticoagulation.

Probing the structure of hirudin from Hirudinaria manillensis by limited proteolysis. Isolation, characterization and thrombin-inhibitory properties of N-terminal fragments

Hirudin is the most potent and specific inhibitor of the blood-clotting enzyme thrombin so far known. Several hirudin variants were isolated mostly from Hirudo medicinalis and shown to be polypeptide chains of approximately 7 kDa with three internal disulfide bridges. In this study, limited proteolysis has been used to probe aspects of the structure and dynamics of a hirudin variant HM2 isolated from Hirudinaria manillensis. Proteolysis of the polypeptide chain of 64-amino-acid residues of hirudin HM2 by protease from Staphylococcus aureus V8, trypsin, thermolysin and subtilisin occurs at region 41-49 of the chain. The N-terminal fragments 1-41 and 1-47 were isolated to homogeneity and shown to maintain inhibitory action on thrombin, though much lower than the intact protein. The results were interpreted on the basis of a proposed three-dimensional structure of hirudin HM2 deduced by protein modelling the known structure of hirudin variant HV1 from Hirudo medicinalis (75% sequence similarity between HM2 and HV1). Both proteolysis experiments and protein modelling provide evidence for the existence in hirudin HM2 of a N-terminal well-structured domain (core) and a C-terminal flexible polypeptide segment. Determination of the accessible surface area of the three-dimensional model of hirudin HM2 showed that the sites of preferential cleavages are at the surface of the polypeptide molecule.

Thrombin-bound conformation of a cyclic anticoagulant peptide using transferred nuclear Overhauser effect (NOE), distance geometry, and NOE simulations

A cyclic hirudin C-terminal fragment, even truncated to eight residues, retains higher activity in binding to thrombin compared with its linear analogue of the same peptide length. In this paper, we report the results of a conformational study of a cyclic hirudin peptide 55-62 ([sequence: see text]) bound to the exosite of thrombin. Based on transferred NOE data and molecular modeling, conformation of the 18-membered ring was well defined with little mobility in the loop region. The backbone conformations of residues Glu (58) to Lys (61) are very similar to those in a native 11-residue linear peptide in the thrombin-bound state [Ni et al. (1990) Biochemistry, vol. 29, pp. 4479-4489; Skrzypczak-Jankun et al. (1991) Journal of Molecular Biology, vol. 221, pp. 1379-1393]. The side chain of Ile (59), the most critical residue, assumes a gauche- conformation, as observed in linear hirudin peptides. The bound conformations of residues Phe(56) and Glu (57) are defined by an iterative procedure involving the matching of computed and experimental transferred NOE intensities. The exosite of thrombin appears to be very specific to both the backbone and the side-chain conformations of the hirudin C-terminal peptides, especially those of residues Phe(56) and Ile(59).

Enhancement of thrombin-thrombomodulin-catalysed protein C activation by phosphatidylethanolamine containing unsaturated fatty acids: possible physiological significance of phosphatidylethanolamine in anticoagulant activity of thrombomodulin

The effects of phospholipid vesicles and their fatty acid compositions on the acceleration of Protein C activation by thrombin-thrombomodulin was studied in vitro. Four main phospholipid fractions were prepared from cultured human umbilical vein endothelial cells, and purified thrombomodulin from human placenta was reconstituted into vesicles consisting of phosphatidylcholine (PtdCho) alone, PtdCho plus phosphatidylethanolamine (PtdEtn), PtdCho plus phosphatidylserine (PtdSer) and PtdCho plus PtdIns (1:1, w/w in each case). Vesicles of PtdCho, PtdIns/PtdCho, PtdSer/PtdCho and PtdEtn/PtdCho increased thrombin-thrombomodulin-catalysed protein C activation by 1.2-, 1.9-, 4.3- and 8.4-fold respectively compared with that in the absence of phospholipid. This Protein C activation was not affected by distearoyl PtdEtn/distearoyl PtdCho, whereas it was markedly increased with increasing content of unsaturated fatty acid in PtdEtn. The thrombin-dependent Protein C activation by thrombomodulin reconstituted into dilinolenoyl PtdEtn/distearoyl PtdCho was 14.6 times that by thrombomodulin reconstituted into distearoyl PtdEtn/distearoyl PtdCho, as a result of a decrease in the dissociation constant (Kd) for thrombin and the Michaelis constant (Km) for Protein C of thrombomodulin. Binding of Protein C to PtdEtn/PtdCho fixed to a microwell plate required the presence of CaCl2 and increased with increasing degree of unsaturation of fatty acid in PtdEtn. As PtdEtn appeared on the outside of the plasma membrane in cultured human umbilical vein endothelial cells after thrombin stimulation, it was presumed that Protein C activation could be elevated by PtdEtn at the outer surface of the plasma membrane via an increased affinity between thrombomodulin, thrombin and Protein C, resulting from both increased formation of the thrombin-thrombomodulin complex via a conformational change in thrombomodulin and increased binding of Protein C to the membrane phospholipid in a Ca(2+)-dependent manner.

Design and validation of a new immunoassay for soluble forms of thrombomodulin and studies on plasma

Thrombomodulin (TM), purified from human placental homogeneate by affinity chromatography on DIP-Thrombin agarose, was used to develop monoclonal antibodies (MAbs). Two of them, 3E2 and 24FM (both IgG1, K), which were not calcium-dependent, were found convenient for developing a two-site enzyme immunoassay. Testing of recombinant and truncated forms of TM26 demonstrated that the species containing the amino terminus including the lectin-like domain and the epidermal growth factor (EGF)-like domains 1-4 were fully measured. The working range was from 2 to 100 ng/ml with a detection threshold of 2 ng/ml. Intraassay and interassay reproducibilities were, respectively, below 7.4% and 8.6%, whereas recovery of purified TM was between 88 and 114% in plasma. Mean plasma concentration was 42.1 (+/- 11.3) ng/ml (males 51.8 +/- 7.9 ng/ml, females 34.8 +/- 7.8 ng/ml) and it was established on 62 normal individuals between the ages of 21 and 55 (28 males and 34 females). This new assay is a convenient tool for measuring plasma TM and establishing its diagnostic and predictive value in diseases associated to endothelial damage.

Monoclonal antibodies against human thrombomodulin whose epitope is located in epidermal growth factor-like domains

Thrombomodulin (TM) on endothelial cells is a glycoprotein that functions as a cofactor for thrombin-catalyzed activation of protein C. The structural requirement for thrombin binding and cofactor activity were investigated using monoclonal antibodies (moAbs) against TM and site-directed mutagenesis of recombinant human soluble TM (rsTM). Results showed that moAb 2A2 inhibited thrombin binding to rsTM and also abolished its functions as a cofactor in thrombin-catalyzed activation of protein C and as an anticoagulant by modifying thrombin-induced fibrinogen clotting and platelet aggregation, moAb 1F2 did not affect its activity as an anticoagulant, but inhibited its cofactor activity, and moAb 10A3 did not inhibit either activity. Epitope analysis was carried out by site directed mutagenesis of rsTM expressed in CHO cells. Some proteins with mutations within the second disulfide loop of the fourth EGF-like domain showed reduced affinity for moAb 1F2, but retained cofactor activity. These results suggest that the epitope of moAb 1F2 includes the second disulfide loop of the fourth EGF-like domain, which is close to a region required for cofactor activity. Mutant proteins of the third disulfide loop of the fifth EGF-like domain showed loss of interaction with moAb 2A2. Thus the epitope of moAb 2A2 may include the third disulfide loop of the fifth EGF-like domain. Furthermore, replacement of Asn-439 by Gln decreased the cofactor activity and anticoagulant activity, and resulted in low affinity for either moAb 1F2 or 2A2, suggesting that Asn-439, which is located in the second disulfide loop of the sixth EGF-like domain, is critical for determining the functional conformation of the EGF-like domains 4-6.

Thrombomodulin expression by human keratinocytes. Induction of cofactor activity during epidermal differentiation

Thrombomodulin is an endothelial cell surface glycoprotein that inhibits the procoagulant activities of thrombin and accelerates activation of the anticoagulant protein C. Because protein C deficiency is associated with cutaneous thrombosis, we investigated the expression of thrombomodulin in human skin. Thrombomodulin was detected by immunohistochemical staining both in dermal endothelial cells and in epidermal keratinocytes. Within the epidermis, thrombomodulin staining was limited to keratinocytes of the spinous layer, suggesting that thrombomodulin is induced when basal keratinocytes begin to terminally differentiate. Thrombomodulin expression also correlated with squamous differentiation in epidermal malignancies; little or no thrombomodulin staining was seen in five basal cell carcinomas, whereas strong thrombomodulin staining was observed in each of five squamous cell carcinomas. Human foreskin keratinocytes cultured in medium containing 0.07 mM calcium chloride synthesized functional thrombomodulin with cofactor activity comparable to thrombomodulin in human umbilical vein endothelial cells. Stimulation of keratinocyte differentiation with 1.4 mM calcium chloride for 48 h produced 3.5-, 3.2-, and 5.6-fold increases in thrombomodulin cofactor activity, antigen, and mRNA, respectively. These observations suggest that thrombin is regulated by keratinocyte thrombomodulin at sites of cutaneous injury, and indicate a potential role for thrombomodulin in epidermal differentiation.

Late-fibrin(ogen) fragment E modulates human alpha-thrombin specificity

Fibrinogen contains at least two independent sites having demonstrable affinity for alpha-thrombin. One of these two sites, located in the fibrin E domain, binds to structures within the anion-binding exosite of alpha-thrombin. Taking advantage of its solubility, we have used late-fibrin(ogen) fragment E in competition experiments to examine its effect on alpha-thrombin specificity. We show that fragment E modulates alpha-thrombin enzymic activity towards small synthetic substrates, suggesting that fibrin-thrombin interaction might induce subtle changes in the conformation near the catalytic center of the enzyme. In addition, fragment E behaved as a competitive inhibitor of alpha-thrombin-catalyzed fibrinopeptide-A cleavage (Ki = 5.2 +/- 1.3 microM), indicating that alpha-thrombin interaction with the fibrin moiety of fibrinogen makes a major contribution to the efficacy of fibrinogen hydrolysis. Fragment E inhibited alpha-thrombin-induced serotonin release by platelets (concentration required to obtain 50% inhibition, IC50 = 10 microM) and alpha-thrombin binding to GPIb. Fragment E competitively inhibited alpha-thrombin binding to thrombomodulin (Ki = 18.3 +/- 0.8 microM) but did not inhibit protein-C activation in the absence of thrombomodulin. The data are consistent with the proposal that fibrin, platelet GPIb and thrombomodulin bind to overlapping, but probably non-identical sites, while protein C binds to an independent site on alpha-thrombin.

Recombinant human thrombomodulin attenuates human endothelial cell activation by human thrombin

Two glycoforms of recombinant human thrombomodulin (TM; TMD1-105 and TMD1-75), an endothelial cell membrane protein, were tested for their ability to alter thrombin-induced activation of cultured human umbilical vein endothelial cells (HUVECs). After stimulation with 10 nmol/L thrombin, HUVEC generation of inositol-1,4,5-trisphosphate (IP3), a potent Ca(2+)-mobilizing second messenger, was dose-dependently blocked by TMD1-105. Both TMD1-105 (IC50 = 10 nmol/L) and TMD1-75 (IC50 = 100 nmol/L) blocked the enhanced prostacyclin synthesis by HUVEC monolayers treated with 10 nmol/L thrombin. HUVEC monolayer permeability to Evans blue dye-labeled albumin increased from 0.125 +/- 0.06 microL/min in control experiments to 0.380 +/- 0.09 microL/min after treatment with 100 nmol/L thrombin (P < .05). Incubation of HUVECs with TMD1-105 alone (600 nmol/L) had no effect (0.114 +/- 0.04 microL/min) on basal permeability. In contrast, incubation of 100 nmol/L thrombin with 600 nmol/L TMD1-105 reduced this increase in HUVEC permeability to almost control levels (0.142 +/- 0.06 microL/min). These results demonstrate that recombinant human TM, a potent in vitro anticoagulant, also functions as an antagonist of thrombin receptor-mediated HUVEC activation. In addition to its anticoagulant functions, the high-affinity endothelial cell receptor TM may play a role in modulating endothelial cell activation by thrombin.

Structure-function studies of the epidermal growth factor domains of human thrombomodulin

Structure-function relationships in the 6 epidermal growth factor-like domains of human thrombomodulin (TME, residues 227-462) were studied by deletion mutagenesis. Purified and characterised proteins were used for kinetic studies. Deletion of EGF1, EGF2 and residues 310-332 in EGF3 had no effect on thrombin binding (Kd) or on kcat/KM for protein C activation by the thrombin-thrombomodulin complex. Deletion of the rest of EGF3 and the interdomain loop between EGF3 and EGF4 had no effect on Kd but decreased kcat/KM to 10% of TME. Deletion of residues 447-462 of EGF6 had no effect on kcat/KM but increased Kd for thrombin approximately 6-fold. Thus, the region 333-350 in EGF3-4 is critical for protein C activation by the thrombin-thrombomodulin complex and the region 447-462 in EGF6 is critical for thrombin binding.