The fibrinogen anion-binding exosite of thrombin is necessary for induction of rises in intracellular calcium and prostacyclin production in endothelial cells

Thrombin: An Approach to Developing a Higher-Order Reference Material and Reference Measurement Procedure for Substance Identity, Amount, and Biological Activities

Thrombin, the proteolytic enzyme that catalyzes the transformation of soluble fibrinogen to the polymerized fibrin clot, participates in multiple reactions in blood coagulation in addition to the clotting reaction. Although reference materials have existed for many years, structural characterization and measurement of biological activity have never been sufficient to permit claims of clear metrological traceability for the thrombin preparations. Our current state-of-the-art methods for protein characterization and determination of the catalytic properties of thrombin now make it practical to develop and characterize a metrologically acceptable reference material and reference measurement procedure for thrombin. Specifically, α-thrombin, the biologically produced protease formed during prothrombin activation, is readily available and has been extensively characterized. Dependences of thrombin proteolytic and peptide hydrolytic activities on a variety of substrates, pH, specific ions, and temperature are established, although variability remains for the kinetic parameters that describe thrombin enzymatic action. The roles of specific areas on the surface of the thrombin molecule (exosites) in substrate recognition and catalytic efficiency are described and characterized. It is opportune to develop reference materials of high metrological order and technical feasibility. In this article, we review the properties of α-thrombin important for its preparation and suggest an approach suitable for producing a reference material and a reference measurement procedure that is sensitive to thrombin’s catalytic competency on a variety of substrates.

Receptor tyrosine kinase EphA2 mediates thrombin-induced upregulation of ICAM-1 in endothelial cells in vitro

Thrombin potently induces endothelial inflammation. One of the responses is upregulation of adhesion molecules such as ICAM-1, resulting in enhanced leukocyte attachment to the endothelium. In this report, we examine the contribution of EphA2 in thrombin-induced expression of ICAM-1 in human umbilical vein endothelial cells (HUVECs). We showed that thrombin transiently induced tyrosine- phosphorylation of EphA2 in a Src-kinase dependent manner. This transactivation was mediated through PAR-1, because a PAR-1 specific agonistic peptide also transactivated EphA2. Expression knockdown of endogenous EphA2 by siRNAs blocked ICAM-1 upregulation and leukocyte/endothelium attachment induced by thrombin. Overexpression of exogenous mouse EphA2 rescued both ICAM-1 expression and leukocyte attachment induced by thrombin in endogenous EphA2-knockdown HUVECs. Mechanistically, we showed EphA2 knockdown suppressed thrombin-induced serine 536 phosphorylation of NFkappaB, an event critical of ICAM-1 transcriptional upregulation. Collectively, our results strongly suggest EphA2 is a necessary component for thrombin-induced ICAM-1 upregulation.

Antiangiogenic property of human thrombin

Using protein chromatography, we purified and identified human prothrombin from human plasma as antiangiogenic. Prothrombin significantly inhibited endothelial cell tube formation in vitro at 10 microg/ml. Importantly, it also inhibited bFGF-induced angiogenesis in Matrigel-plug assays performed in mice. The proteolytic activity of thrombin appeared to be critical for the antiangiogenic activity of prothrombin. For example, thrombin exhibited inhibitory effects on endothelial cell tube formation in vitro at 10 U/ml. Addition of lepirudin, a specific inhibitor of thrombin, completely blocked prothrombin's and thrombin's antiangiogenic effects in vitro. We also assessed the importance of thrombin receptors in angiogenesis. Using small peptides that activate different protease-activated receptors (PARs), we showed that activation of PAR-1 led to inhibition of endothelial cell tube formation in vitro and bFGF-induced angiogenesis in vivo. Collectively, our data suggest that thrombin's proteolytic activity can be antiangiogenic.

Protease activated receptors: theme and variations

The four PAR family members are G protein coupled receptors that are normally activated by proteolytic exposure of an occult tethered ligand. Three of the family members are thrombin receptors. The fourth (PAR2) is not activated by thrombin, but can be activated by other proteases, including trypsin, tryptase and Factor Xa. This review focuses on recent information about the manner in which signaling through these receptors is initiated and terminated, including evidence for inter- as well as intramolecular modes of activation, and continuing efforts to identify additional, biologically-relevant proteases that can activate PAR family members.

alpha-Thrombin upregulates G alpha i3 in human vascular endothelial cells

BACKGROUND AND PURPOSE

During thrombosis, alpha-thrombin becomes sequestered by fibrin and the subendothelial basement membrane, and it is available to interact with the vasculature over prolonged periods. In this study, we investigated the long-term effect of alpha-thrombin on G alpha i3 and G alpha s levels in human vascular endothelial cells (EC). Because obesity is associated with changes in receptor signaling in many animal models, we also explored the influence of this clinical risk factor.

METHODS

Primary cultures of human EC were exposed to alpha-thrombin for 16 hours, and immunologically detectable G alpha i3 and G alpha s levels were measured.

RESULTS

alpha-Thrombin (100 nmol/L) increased G alpha i3 levels in EC derived from the cerebral microvasculature and superficial temporal artery (4.2 +/- 1.2-fold and 2.8 +/- 0.32-fold, respectively) but had no significant effect on EC derived from omental artery (P > .6) or from the superficial temporal artery of obese (body mass index > or = 28 kg/m2) patients (P > .4). The expression of G alpha s was unchanged in all cell types (P > or = .1). Two other circulating peptides, vasoactive intestinal peptide and endothelin-1, failed to alter the expression of either G protein in EC from the cerebral microvasculature, further demonstrating the specificity of the alpha-thrombin effect. However, thrombin receptor activating protein-14 mimicked the alpha-thrombin response and increased G alpha i3 in EC derived from the cerebral microvasculature and superficial temporal artery.

CONCLUSIONS

We conclude that alpha-thrombin increases G alpha i3 expression in some EC through activation of its tethered liganded receptor. Obesity appears to suppress this action of alpha-thrombin.

Inhibition of tumor necrosis factor signal transduction in endothelial cells by dimethylaminopurine

Tumor necrosis factor (TNF) promotes diverse responses in endothelial cells that are important to the host response to infections and malignancies; however, less is known of the postreceptor events important to TNF action in endothelial cells than in many other cell types. Since phosphorylation cascades are implicated in cytokine signaling, the effects of the protein kinase inhibitor dimethylaminopurine (DMAP) on TNF action in bovine aortic endothelial cells (BAEC) were investigated. In BAEC, TNF promotes phosphorylation of eukaryotic initiation factor 4E (eIF-4E), c-Jun N-terminal kinase (JNK) and ceramide-activated protein kinase activities, Jun-b expression, prostacyclin production, and, when protein synthesis is inhibited, cytotoxicity. DMAP abrogated or significantly attenuated each of these responses to TNF, without affecting the specific binding of TNF to its receptors. Histamine, another agent active in the endothelium, promotes phosphorylation of elongation factor-2 (EF-2) and prostacyclin production, but not phosphorylation of eIF-4E in BAEC. Histamine-stimulated EF-2 phosphorylation was not inhibited and prostacyclin production was unaffected by DMAP. These observations demonstrate that a distinct signal transduction cascade, which can be selectively inhibited by DMAP, promotes the response of BAEC to TNF. Thus, we have identified a reagent, DMAP, that may be useful for characterizing the TNF signal transduction pathway.

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.

Alpha-thrombin and trypsin use different receptors to stimulate arachidonic acid metabolism

Rat liver cells (the C-9 cell line) are stimulated to metabolize arachidonic acid by alpha-thrombin, its receptor polypeptide, gamma-thrombin, and trypsin. Prostaglandin (PG) I2 synthesis stimulated by alpha-thrombin is inhibited by dansylarginine N-(3-ethyl-1,5-pentanediyl) amide (DAPA), by hirudin, by the synthetic tyrosine-sulfated dodecapeptide corresponding to residues 53-64 of hirudin (hirugen), by the Tyr(SO3H)63-hirudin fragment 54-65 and by rabbit lung thrombomodulin. Stimulation of arachidonic acid metabolism by the receptor octapeptide, SFLLRNPN, is not affected by DAPA or hirudin. gamma-Thrombin stimulates arachidonic acid metabolism but at 300 to 400-fold higher concentrations. Trypsin stimulates arachidonic acid metabolism. Trypsin's proteolytic activity is required--its ability to stimulate is abolished if it is incubated with Na-p-tosyl-L-lysine chloromethyl ketone (TLCK) or bovine pancreatic trypsin inhibitor. Prior treatment of the rat liver cells with alpha-thrombin blocks subsequent stimulation by alpha-thrombin, but not by trypsin, whereas prior treatment with trypsin blocks subsequent stimulation by trypsin, but not the activity stimulated by alpha-thrombin. Prior treatment of the cells with the serine-proteases, chymotrypsin, pancreatic or neutrophil elastase and thrombocytin from Bothrups atrox venom, block alpha-thrombin's activation of PGI2 production, but not the activity stimulated by trypsin. These findings indicate that alpha-thrombin and trypsin stimulate PGI2 production via different receptors.

Activation of the cloned platelet thrombin receptor decreases the pertussis-toxin-dependent ADP-ribosylation of the membrane and soluble inhibitory guanine-nucleotide-binding-alpha proteins. Inhibition by the prostacyclin analog, iloprost

Thrombin binds at least to two sites of the platelet surface; to the recently cloned thrombin receptor [Vu, T. K., Hung, D. T., Wheaton, V. I. & Coughlin, S. R. (1991) Cell 64, 1057-1068] and to glycoprotein Ib. In the present study, the decrease of pertussis-toxin-dependent ADP-ribosylation of membrane and soluble inhibitory guanine-nucleotide-binding alpha (Gi alpha) proteins was measured after platelet stimulation with a thrombin-receptor-activating peptide (TRAP), and compared to stimulation with thrombin. Stimulation of intact platelets with TRAP decreased the pertussis-toxin-dependent ADP-ribosylation of the major membrane 41-kDa Gi alpha protein and the minor soluble 40 kDa Gi alpha protein recently described in platelets [Gennity, J. M. & Siess, W. (1991) Biochem. J. 279, 643-650]. The kinetics and extent of the decrease of pertussis-toxin-dependent ADP-ribosylation after stimulation of TRAP were similar to the effect of thrombin. The decrease of pertussis-toxin-dependent ADP-ribosylation of the soluble Gi alpha protein was more pronounced and observed at lower agonist concentrations than the decrease of the membrane Gi alpha protein. Desensitization of the thrombin receptor by incubating platelets with a low concentration of TRAP reduced the subsequent decrease of pertussis-toxin-dependent ADP-ribosylation of Gi alpha proteins, evoked by TRAP or thrombin. Platelet stimulation with gamma-thrombin that does not bind to glycoprotein Ib also showed a decrease in the pertussis-toxin-dependent ADP-ribosylation of the soluble and membrane Gi alpha proteins. Treatment of platelets with the stable prostacyclin analog, iloprost, reduced the decrease of pertussis-toxin-dependent ADP-ribosylation of Gi alpha proteins induced by TRAP or thrombin. Among other platelet stimuli tested (endoperoxide/thromboxane analog U44619, collagen, ADP, vasopressin), only U44619 decreased the pertussis-toxin-dependent ADP-ribosylation of the soluble and membrane Gi alpha proteins to a degree comparable to TRAP. It is concluded that the thrombin-induced activation of both the membrane and soluble Gi alpha proteins in platelets occurs via stimulation of the recently cloned thrombin receptor and is independent of the binding of thrombin to glycoprotein Ib. Furthermore, the coupling thrombin receptor/Gi protein is reduced by intracellular cAMP.