Ligand specificity of human thrombomodulin. Equilibrium binding of human thrombin, meizothrombin, and factor Xa to recombinant thrombomodulin

Autoantibodies to thrombin directed against both of its cryptic exosites

Immunoglobulin G (IgG) anti-thrombin autoantibodies (ATA) were purified from the plasma of a patient referred for haematological investigation because of bleeding for 24 h following a dental extraction. The ATA did not inhibit the catalytic activity of thrombin against a chromogenic substrate, suggesting that they did not interact with the active site of thrombin. The ATA did, however, prolong the time required to generate thrombin in plasma, suggesting that they inhibited factor V and factor VIII activation. Surface plasmon resonance (SPR) was used to demonstrate that ATA bound to thrombin with high affinity. Competition of thrombin-ATA binding, using known thrombin exosite I and II ligands (hirudin, thrombomodulin and heparin), demonstrated that ATA bound to both thrombin exosites. Thrombin residues that are important for ATA binding were identified using a library of 53 recombinant thrombin variants encompassing alanine substitutions of 78 surface-exposed residues. They were H66, R68, R70 and Y71 in exosite I, and R89, R93, E94, R98, R245 and K248 in exosite II. ATA bound predominantly to exosite II. They did not bind to prothrombin, illustrating the cryptic nature of both exosites exposed and presented as potential antigens following prothrombin conversion to thrombin.

Inhibition of invasion and experimental metastasis of murine melanoma cells by human soluble thrombomodulin

Thrombomodulin (TM) is an anticoagulant molecule expressed on the endothelial cell surface and soluble TM antigen, which is present in human plasma and urine, represents the products of limited proteolytic cleavage of cell-surface TM. Recently, it was demonstrated that TM is also expressed on the surface of several tumor cells and the expression level of TM negatively correlated with malignancy in cancer. We investigated the effect of soluble TM isolated from human urine (uTM) on the invasion and metastasis of murine melanoma cells (B16F10 cells) through a reconstituted basement membrane (Matrigel) and in a murine model of experimental lung metastasis. Matrigel reconstituted with uTM inhibited the invasion of B16F10 cells in a dose-dependent manner in a range from 10 to 1000 ng/ml uTM as compared with the control Matrigel without uTM. The inhibitory action of uTM was not altered in the presence of an excess amount of hirudin, an inhibitor of thrombin proteolytic activity, but abolished in the presence of anti-human TM IgG. Matrigel reconstituted with thrombin (1 NIH unit/ml) enhanced the invasion level of cells by 1.5-fold relative to the control Matrigel without thrombin. The thrombin-enhanced invasion of B16F10 cells was repressed by addition of hirudin (10 units/ml) or uTM (100 ng/ml) into the Matrigel. Matrigel reconstituted with hirudin (10 units/ml) and uTM (100 ng/ml) additionally accelerated the inhibitory activity of hirudin or uTM on the thrombin-enhanced invasion of B16F10 cells. Moreover, metastatic colonies formed in the lungs of mice injected intravenously with B16F10 cells were significantly reduced by injection of uTM once a day up to 2 days after co-injection of uTM with the cells. These results suggested that Matrigel reconstituted with uTM inhibited the invasion of B16F10 cells in vitro through a thrombin-independent mechanism and the injection of uTM suppressed experimental lung metastasis of the cells in mice.

Vascular protease receptors: integrating haemostasis and endothelial cell functions

The endothelium plays a crucial dynamic role as a protective interface between blood and the underlying tissues during the haemostatic process, which maintains blood flow in the circulation and prevents life-threatening blood loss. Following vessel wall injury with initial platelet adhesion and aggregation to exposed subendothelial extracellular matrix, the initiation, amplification, and control of haemostasis depend on structurally unrelated membrane-associated receptors for blood coagulation proteases including tissue factor, G-protein-coupled protease-activatable receptors, thrombomodulin, and protein C receptor, respectively. In addition to their regulatory role in haemostasis, the respective (pro-)enzyme ligands such as Factors VIIa and Xa, thrombin or protein C mediate specific signalling pathways in vascular cells related to migration, proliferation or adhesion. The functional importance of these receptors beyond haemostasis has been manifested by various lethal and pathological phenotypes in knock-out mice. These protease receptors thereby provide important molecular links in the vascular system and serve to integrate haemostasis with endothelial cell functions which are relevant for the (patho-)physiological responses to injury or inflammatory challenges.

Inhibition of meizothrombin and meizothrombin(desF1) by heparin cofactor II

Meizothrombin and meizothrombin(desF1) are intermediates formed during the conversion of prothrombin to thrombin by factor Xa, factor Va, phospholipids, and Ca2+ (prothrombinase). These intermediates are active toward synthetic peptide substrates but have limited ability to interact with platelets or macromolecular substrates such as fibrinogen. Meizothrombin and meizothrombin(desF1) activate protein C, however, and may exert primarily an anticoagulant effect. In this study, we investigated the inhibition of meizothrombin and meizothrombin(desF1) by two glycosaminoglycan-dependent protease inhibitors, heparin cofactor II (HCII) and antithrombin (AT). Purified recombinant meizothrombin and meizothrombin(desF1) were inhibited by HCII in the presence of dermatan sulfate with maximal second-order rate constants of 8 x 10(6) M-1.min-1 and 1.8 x 10(7) M-1.min-1, respectively, but were inhibited less than one-tenth as fast by AT in the presence of heparin. Similarly, the products of the prothrombinase reaction were inhibited in situ more effectively by HCII than by AT. When HCII and dermatan sulfate were present continuously during the prothrombinase reaction, meizothrombin was trapped as a sodium dodecyl sulfate-stable complex with HCII and no amidolytic activity could be detected with a thrombin substrate. Our findings indicate that HCII is an effective inhibitor of meizothrombin and meizothrombin(desF1) and, therefore, might regulate the anticoagulant activity of these proteases.

Identification of glycyrrhizin as a thrombin inhibitor

Glycyrrhizin (GL), an anti-inflammatory compound isolated from Glycyrrhiza glabra, was identified as a new thrombin inhibitor: (a) It prolonged plasma recalcification and thrombin and fibrinogen clotting times, and (b) it inhibited thrombin-induced, but not collagen-, PAF- or convulxin-induced platelet aggregation. On the other hand, GL did not block thrombin's amidolytic activity upon S-2238. Furthermore, the fluorescence emission intensity of dansyl-thrombin was increased upon GL binding. Moreover, GL displaced hirudin as an inhibitor of thrombin-catalyzed hydrolysis of S-2238. Our data provide evidence that GL is a selective inhibitor of thrombin (the first one isolated from plants) that is able to exert its anti-thrombin action by interacting with the enzyme's anion binding exosite 1. A pharmacophoric search identified GL as a sialyl Lewis X (SLe[X]) mimetic compound able to inhibit selectin binding to SLe(X). However, SLe(X) did not affect thrombin clotting activities, which indicates a lack of its interaction with thrombin and distinguishes both molecules. It is suggested that the anti-inflammatory effect of GL may be due to its effective anti-thrombin action.

Functional characterization of recombinant human meizothrombin and Meizothrombin(desF1). Thrombomodulin-dependent activation of protein C and thrombin-activatable fibrinolysis inhibitor (TAFI), platelet aggregation, antithrombin-III inhibition

Recombinant human prothrombin (rII) and two mutant forms (R155A, R271A,R284A (rMZ) and R271A,R284A (rMZdesF1)) were expressed in mammalian cells. Following activation and purification, recombinant thrombin (rIIa) and stable analogues of meizothrombin (rMZa) and meizothrombin(desF1) (rMZdesF1a) were obtained. Studies of the activation of protein C in the presence of recombinant soluble thrombomodulin (TM) show TM-dependent stimulation of protein C activation by all three enzymes and, in the presence of phosphatidylserine/phosphatidylcholine phospholipid vesicles, rMZa is 6-fold more potent than rIIa. In the presence of TM, rMZa was also shown to be an effective activator of TAFI (thrombin-activatable fibrinolysis inhibitor) (Bajzar, L., Manuel, R., and Nesheim, M. E. (1995) J. Biol. Chem. 270, 14477-14484). All three enzymes were capable of inducing platelet aggregation, but 60-fold higher concentrations of rMZa and rMZdesF1a were required to achieve the effects obtained with rIIa. Second order rate constants (M-1.min-1) for inhibition by antithrombin III (AT-III) were 2.44 x 10(5) (rIIa), 6.10 x 10(4) (rMZa), and 1.05 x 10(5) (rMZdesF1a). The inhibition of rMZa and rMZdesF1a by AT-III is not affected by heparin. All three enzymes bound similarly to hirudin. The results of this and previous studies imply that full-length meizothrombin has marginal procoagulant properties compared to thrombin. However, meizothrombin has potent anticoagulant properties, expressed through TM-dependent activation of protein C, and can contribute to down-regulation of fibrinolysis through the TM-dependent activation of TAFI.

Evidence that meizothrombin is an intermediate product in the clotting of whole blood

Meizothrombin is an intermediate that is produced during the conversion of prothrombin to thrombin in systems composed of purified factor Xa and factor Va that are quantitatively assembled on an anionic phospholipid surface. The biological significance of this intermediate has recently been challenged by the apparent absence of meizothrombin during clotting of sodium citrate-anticoagulated plasma. We analyzed the formation of thrombin during coagulation of nonanticoagulated, unchilled, minimally manipulated whole blood in glass tubes. The process was stopped at 0, 3, 5, and 7 minutes by the addition of biotinylated peptidyl chloromethyl-ketone active-site labeling reagents. Plasma/serum was separated by centrifugation, and labeled species were extracted by immunoadsorption with a polyclonal anti-prothrombin antibody. The purified prothrombin-derived species were separated by SDS-polyacrylamide gradient gel electrophoresis and visualized on a chemiluminescent avidin blot. Meizothrombin appeared as an intermediate product of this reaction and persisted with some increase through the 7-minute time point. We also observed incorporation of the active-site label into a species of lower molecular weight consistent with the B1 chain of beta- and/or gamma-thrombin. These degraded forms of thrombin have not been previously demonstrated in a biologically relevant preparation. Our data clearly establish the generation of meizothrombin as an intermediate product of thrombin generation during whole-blood clotting. The data also represent the first experimental evidence for the generation of beta- and gamma-thrombin in a biologically relevant environment and time scale.