Evidence that activation of platelets and endothelium by thrombin involves distinct sites of interaction. Studies with the dysthrombin, Thrombin Quick I

PAR-4 agonist AYPGKF stimulates thromboxane production by human platelets

Previous reports have indicated that thrombin-induced thromboxane production by human platelets occurs through two types of interaction between thrombin and the platelet surface. One of these interactions is with protease activated receptor(PAR)-1, the first identified thrombin receptor. These studies were undertaken to determine whether stimulation of PAR-4 also results in thromboxane production. The results show that treatment of washed human platelets with the PAR-4 agonist AYPGKF stimulates a maximum of 40% to 60% of the thromboxane produced by 100 nmol/L thrombin. Maximal thromboxane production requires approximately 1.0 mmol/L AYPGKF, despite the observation that maximal aggregation is produced by 45 micromol/L AYPGKF. Thromboxane produced by the combined stimulation of PAR-1 and PAR-4 is additive. Pretreatment of platelets with 45 micromol/L AYPGKF partially desensitizes thromboxane production in response to higher concentrations of AYPGKF and thrombin but not to stimulation by SFLLRN. PAR-4-induced stimulation is also significantly inhibited by 60 micromol/L genistein. It is concluded that activation through either PAR-1 or PAR-4 results in thromboxane production, but that stimulation of neither receptor alone produces thromboxane equivalent to that produced by 100 nmol/L thrombin. Thus, these findings demonstrate the presence of two pathways for thrombin-induced thromboxane production by platelets as proposed previously.

Effects of protein kinase C inhibitors on thromboxane production by thrombin-stimulated platelets

The purpose of these studies was to identify a possible role for protein kinase C in thromboxane production. The effects of four putative protein kinase C inhibitors were studied with platelet stimulation by thrombin (0.5-150 nM), Thrombin Quick I (1.5-500 nM) or a thrombin receptor (protease activated receptor-1) agonist peptide (TRAP) (5-120 microM). Thromboxane production was increased by the bisindolylmaleimide derivative, 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimi de (GF 109203X), unchanged by the inhibitors 12-(2-cyanoethyl)-6,7, 12,13-tetrahydro-13-methyl-5-oxo-5H-indolo (2,3-a) pyrrolo (3, 4-c)-carbazole (Gö 6976) and 5,21:12,17-dimetheno-18H-dibenzo[i, o]pyrrolo[3,4-l][1,8]diazacyclohexadecine-18,20(19H)-dione, 8-[(dimethylamino)methyl]-6,7,8,9,10,11-hexahydro-, monomethanesulfonate (379196), the latter of which is protein kinase C beta-selective, and decreased by 1-[6-[(3-acetyl-2,4, 6-trihydroxy-5-methylphenyl)methyl]-5,7-dihydroxy-2, 2-dimethyl-2H-1-benzopyran-8-yl]-3-phenyl-2-propen-1-one (rottlerin), an inhibitor selective for protein kinase C delta. These results indicate complex regulation of thromboxane synthesis in human platelets including a probable role for protein kinase C delta. The results taken together further suggest that GF 109203X may suppress negative feedback resulting from an unidentified kinase and that the classical protein kinase C isoforms alpha and beta do not have a significant role in regulating thromboxane production by platelets.

Existence of endogenous inhibitor(s) of prostaglandin endoperoxide H synthase activities in murine NIH3T3 fibroblasts

The production of prostaglandins (PGs) is regulated by several processes, i.e. arachidonic acid release, prostaglandin endoperoxide H synthase (PGHS) activity and its induction. In the present study, we investigated the possibility that inhibitor(s) of PGHS activities exist endogenously and regulate PG production. NIH3T3 cell, a cloned murine fibroblast, expressed PGHS-1 under unstimulated conditions and induced PGHS-2 protein under stimulation by serum. When PGHS activity was measured by individual intact cell assay, there was heterogeneity in PGHS activities in individual cells after serum treatment. We isolated and cultured many cloned cells which showed different PGHS activities after serum treatment. In these cells, however, there was no difference in the expression of PGHS-1 and PGHS-2 mRNA. The protein level of PGHS-1 and PGHS-2 was also not different in these cloned cells. The lysate obtained from the cells that showed lower PGHS activity after the serum treatment suppressed PGHS-1 activities of sheep seminal vesicle and PGHS-2 activity in purified ovine placenta. The suppression by this lysate was stronger than that by the lysate obtained from the cells which showed higher PGHS activity after the serum treatment. The production of the inhibitor(s) was up-regulated by the serum treatment and abolished by actinomycin D and cycloheximide during the serum treatment. From these results, we suggest that there is some endogenous inhibitor(s) of PGHS-1 and -2 activities in NIH3T3 cells, and that the inhibitor(s) is induced by serum.

Platelet responses to compound interactions with thrombin

Catalytic and noncatalytic interactions of thrombin with platelets are investigated with use of thrombin variants with altered specificities and with ligands of thrombin receptors on platelets. Both alpha-thrombin and weakly coagulant meizothrombin-des-fragment-1 (mu-thrombin) hydrolyze proteolytically activated receptor 1 for thrombin (rPAR1(T), recombinant) with catalytic efficiencies of >10(7) M(-)(1) s(-)(1), whereas rPAR1(T) is not a substrate for weakly coagulant beta-thrombin. In contrast, both mu-thrombin and beta-thrombin are weak agonists of platelet dense body (ATP) secretion. Antibodies that block rPAR1(T) cleavage strongly inhibit the secretory reaction to alpha- and mu-thrombins but not to beta-thrombin or to thrombin receptor activating peptide (TRAP). However, catalytically inactive FPR-thrombin, which binds glycoprotein Ib but does not inhibit rPAR1(T) cleavage, inhibits responses to TRAP as well as those to alpha- and mu-thrombins, which indicates that binding of the inactive enzyme to platelets influences the function of PAR1(T). An antibody that inhibits binding of thrombin to platelet glycoprotein Ib inhibits secretory responses to thrombin but not to TRAP, so occupancy of glycoprotein Ib per se accounts for only part of the attenuation. All three thrombins stimulate a rise in cytosolic Ca(II), and the dose response to beta-thrombin is congruent with that for ATP secretion. However, the response of cytosolic Ca(II) is 10-100 times more sensitive to mu-thrombin and alpha-thrombin than ATP secretion is, and is inhibited by neither anti-PAR1(T) Ig nor FPR-thrombin. Thus, alpha-thrombin appears to have an activity not shared by either mu- or beta-thrombins. This activity is owed to more than coupling of independent signals from cleavage of two proteolytically activated receptors, as there is no synergism when mu-thrombin and beta-thrombin costimulate secretion. It is concluded either that alpha-thrombin has a third interaction site on platelets with which neither mu-thrombin nor beta-thrombin interacts or that dual receptors are coordinately cleaved. In either case, the strong secretory response to thrombin appears to be moderated, independently of cytosolic Ca(II), by occupancy of a noncatalytic interaction site such as glycoprotein Ib.

The platelet high affinity binding site for thrombin mimics hirudin, modulates thrombin-induced platelet activation, and is distinct from the glycoprotein Ib-IX-V complex

The platelet high affinity binding site for thrombin appears to be described by a classical receptor-ligand interaction that is distinct from the platelet thrombin receptor/substrate, PAR-1. However, the identification and function of the high affinity binding site with respect to its physiological importance have continued to elude investigators. Prior studies using two mutant thrombins suggested that thrombin interaction with the platelet high affinity binding site is mediated through an extensive portion of the thrombin molecule involving residues within the substrate binding pocket and the anion binding exosite (Leong, L., Henriksen, R. A., Kermode, J. C., Rittenhouse, S. E., and Tracy, P. B. (1992) Biochemistry 31, 2567-2575) and may mimic a thrombin-hirudin interaction. To test this hypothesis, an anti-hirudin peptide antibody (anti-hirpeptide Ab) was raised against a peptide mimicking the COOH terminus of hirudin. The Ab recognized adherent platelets and those in suspension as determined by enzyme-linked immunosorbent assay and immunofluorescence microscopy, respectively. 125I-Thrombin binding to platelets was inhibited in the presence of the anti-hirpeptide Ab in a dose-dependent manner with maximal inhibition >90%. Analyses of data from binding studies of 125I-thrombin to platelets at a fixed Ab concentration indicated that the anti-hirpeptide Ab inhibited the high affinity binding interaction exclusively. In addition, thrombin-induced increases in platelet [Ca2+]i were enhanced by blocking the high affinity binding site with the Ab due to redistribution of the agonist to PAR-1. Thrombin Quick I-induced platelet calcium mobilization was unaffected by the presence of the Ab, consistent with the inability of thrombin Quick I to bind to the high affinity site. Even though glycoprotein (GP) Ib contains a hirudin-like region within the alpha subunit, the postulated high affinity binding site, direct binding of 125I-thrombin could not be demonstrated to transfected Chinese hamster ovary and L cells expressing the GP Ib-IX-V complex. Furthermore, an anti-GP Ib Ab, raised to the peptide region proposed as the thrombin high affinity site, did not enhance thrombin-induced platelet calcium mobilization. The anti-hirpeptide Ab recognized a population of platelet membrane proteins distinct from PAR-1 and GP Ib by three-color immunofluorescence using confocal microscopy. These combined studies demonstrate that the high affinity binding site for thrombin is a unique platelet protein distinct from GP Ib which modulates the effective thrombin concentration localized at the human platelet surface.

Different substrate utilization between prostaglandin endoperoxide H synthase-1 and -2 in NIH3T3 fibroblasts

Recent studies suggested that prostaglandin endoperoxide H synthase- and prostaglandin endoperoxide H synthase-2 (PGHS-1 and PGHS-2) utilize different pools of arachidonic acid for synthesizing prostanoids. Using cultured murine NIH3T3 fibroblasts, we investigated the mechanism for the different utilization of arachidonic acid between PGHS-1 and -2. Histofluorescence staining for PGHS activity in intact cells demonstrated that quiescent 3T3 cells expressed only PGHS-1 activity and serum-activated 3T3 cells pretreated with aspirin expressed only PGHS-2 activity. Endogenous arachidonic acid released by calcium ionophore A23187 was not converted by PGHS-1 but exclusively converted by PGHS-2. In the cell free system, the kinetics of PGHS-1 were not so much different from those of PGHS-2. However, in intact cells, arachidonic acid at concentrations lower than 2.5 microM was converted by PGHS-2 alone but not by PGHS-1. Our findings indicated that this small amount of arachidonic acid as released by some stimuli is converted exclusively by PGHS-2. Furthermore, treating the PGHS-2-expressing cells with sodium selenite or ebselen, reducing agents of intracellular peroxides, only decreased PGHS-2 activity. We speculate that only PGHS-2 has been activated by intracellular peroxides and subsequently, it can convert the arachidonic acid released endogenously.

Thrombin-induced thromboxane synthesis by human platelets. Properties of anion binding exosite I-independent receptor

These studies have examined the effects of thrombin-related agonists in stimulating thromboxane production by human platelets. The results presented show that (1) the maximal response elicited by thrombin receptor agonist peptide (TRAP) stimulation was 40% to 50% of that seen with thrombin or the thrombin mutant Thrombin Quick I; (2) pretreatment of platelets with prostaglandin E1 or genistein resulted in differential inhibition of thromboxane production in response to TRAP compared with either enzyme agonist; (3) an antibody to the thrombin receptor cleavage site that inhibits increases in intracellular [Ca2+] only partially reduced thromboxane production in response to 5 nmol+L thrombin and 15 nmol/L Thrombin Quick I; (4) preincubation with 20 mumol/L TRAP resulted in desensitization to further stimulation by 100 mumol/L TRAP, but not by 100 nmol/L thrombin; and (5) the response to thrombin after TRAP desensitization was completely inhibited by the tyrosine kinase inhibitor genistein and was independent of an intracellular [Ca2+] flux, The cumulative results may be explained by the existence of two proteolytically activated receptors that result in thromboxane production in response to thrombin. One is the thrombin receptor/substrate, PAR-1. Stimulation through the second receptor/substrate depends on a genistein-sensitive step, is independent of an intracellular Ca2+ flux, and is initiated by a thrombin-activated receptor that does not depend on interaction with anion-binding exosite I, as previously indicated by the relative activity of Thrombin Quick I in stimulating platelet aggregation and thromboxane production. The proposed second thrombin receptor on platelets represents an additional member of the class of proteolytically activated receptors.

Thrombin receptor on rat primary hippocampal neurons: coupled calcium and cAMP responses

We have tested the hypothesis that hippocampal neurons respond to thrombin via a neuronal thrombin receptor. A human neuroblastoma cell line, SK-N-SH, known to be thrombin responsive morphologically, responded both to thrombin and thrombin receptor agonist peptide (TRAP 42-55) with elevation of intracellular calcium. In Western blots of membranes from SK-N-SH cells and cultured rat hippocampal neurons using an antibody against the N-terminal peptide of the human thrombin receptor, putative receptor proteins of 66 and 47 kDa were detected in both cells. Neurons were treated with thrombin and TRAP 42-55 (TRAP-14) to determine their effects on intracellular levels of calcium and cAMP. Only 10% of the neurons showed a rapid response to thrombin, but most responded rapidly to agonist peptide with a prolonged elevation of intracellular free calcium. Neuronal cAMP levels were decreased by 40% after 24 h thrombin treatment. This decrease in cAMP level could be blocked by both the Gi-protein inhibitor, pertussis toxin, and the thrombin inhibitor, hirudin, suggesting a possible involvement of Gi-protein-coupled receptor activation. Furthermore, rapid calcium and cAMP responses were apparently induced by pre-treatment of neurons with thrombin for 24 h and subsequent washout. In summary, these data indicate that rat primary hippocampal neurons have thrombin receptors whose responses to thrombin apparently are up-regulated by 24 h thrombin pre-treatment. These results may have implications for synaptic remodeling, learning and memory.

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

Thrombin stimulation of prostacyclin (PGI2) synthesis by cultured human umbilical vein endothelial cells (HUVEC) requires the active site of thrombin and involves rapid and transient rises in cytoplasmic free calcium [Ca2+]i. In this study, we investigated whether or not the anion-binding exosite for fibrinogen recognition of thrombin (which confers certain substrate specificities) is also necessary for the induction of rises in [Ca2+]i and PGI2 production. Thrombin variants which lack either the catalytic site (DIP-alpha-thrombin) or anion-binding exosite (gamma-thrombin) either alone or in combination failed to induce rises in [Ca2+]i or PGI2 production in HUVEC. To further study the role of the anion-binding exosite of thrombin in the activation of HUVEC, COOH-terminal fragments of hirudin were used. This portion of hirudin interacts with the anion-binding exosite of thrombin and inhibits thrombin-induced fibrinogen coagulation while leaving the catalytic activity of thrombin intact. A 21-amino acid COOH-terminal peptide of hirudin (N alpha-acetyldesulfato-hirudin45-65 or Hir45-65) inhibited thrombin-induced (0.5 U/ml) rises in [Ca2+]i and PGI2 production with IC50 of 0.13 and 0.71 microM, respectively. Similar results were obtained using shorter hirudin-derived peptides. Thus, the fibrinogen anion-binding exosite of thrombin is required for alpha-thrombin-induced rises in [Ca2+]i and PGI2 production in HUVEC.

Identification of the primary structural defect in the dysthrombin thrombin Quick I: substitution of cysteine for arginine-382

A congenitally dysfunctional form of prothrombin, prothrombin Quick, was isolated from the plasma of an individual with less than 2% of normal prothrombin activity. Following activation of prothrombin Quick, two dysfunctional thrombins, thrombin Quick I and thrombin Quick II, were isolated. Functional characterization of thrombin Quick I indicated an increase in KM and a decrease in kcat, relative to thrombin, for release of fibrinopeptide A. Comparison of kcat/KM for thrombin Quick I to the value obtained for thrombin yielded a relative catalytic efficiency of 0.012 for thrombin Quick I [Henriksen, R. A., & Owen, W. G. (1987) J. Biol. Chem. 262, 4664-4669]. Lysyl endopeptidase digestor of reduced and S-carboxymethylated thrombin and thrombin Quick I has resulted in the identification of an altered peptide in this dysthrombin. Edman degradation of the isolated peptide has shown that the altered residue in this protein is Arg-382 which is replaced by Cys. This could result from a point mutation in the Arg codon, CGC, to yield TGC. Together, these results indicate that Arg-382 is a critical residue in determining the specificity of thrombin toward fibrinogen. Similar relative activities for thrombin Quick I in stimulating platelet aggregation, in the release of prostacyclin from human umbilical vein endothelium, and in the release of fibrinopeptide A suggest that these activities of thrombin share the same specificity determinants.