Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:3.4.21.5 (thrombin)
33,306 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

We have evaluated the mechanism by which crosslinking human platelet Fc receptor (FcR) for IgG triggers platelet aggregation and the platelet release reaction. Platelet FcR was crosslinked by incubating purified human platelets with anti-FcRII monoclonal antibody and F(ab')2 anti-mouse Ig. The resultant [Ca2+]i increase, monitored by Fura-2 and measured in the absence of extracellular Ca2+, reached a peak of 750 +/- 50 nmol/L. The effects of cyclooxygenase inhibitors, aspirin and indomethacin, and a phospholipase A2 inhibitor, dibromoacetophenone, were examined. Regardless of the inhibitor, at least 25% of the [Ca2+]i increase remained. Thrombin (0.2 U/mL) stimulated an immediate [Ca2+]i increase that reached 1.95 +/- 0.8 mumol/L. The [Ca2+]i increase generated by thrombin was only slightly reduced by these inhibitors. Crosslinking the FcRII of platelets resulted in a fivefold increase in the production of [3H]inositol phosphates, (IP) which, in the absence of extracellular Ca2+ was insensitive to aspirin. The activation of a [Ca2+]i increase along with the measured increases in IP indicate that FcRII crosslinking leads to the activation of phospholipase C (PLC). In contrast to thrombin, platelet activation via FcRII depends to a large extent on arachidonic acid metabolites. However, neither cyclooxygenase nor phospholipase A2 inhibitors completely blocked FcRII-stimulated [Ca2+]i increase. These observations led us to propose that crosslinking of platelet FcRII initially activates PLC.
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PMID:Signal transduction by the platelet Fc receptor. 214 75

The tumor-promoting phorbol ester 4 beta-phorbol 12-myristate 13-acetate (PMA) inhibited thrombin-stimulated arachidonic acid (AA) release in rabbit and human platelets. PMA was effective over the same concentration range that activates protein kinase C in intact rabbit platelets: IC50 vs thrombin = 0.5 nM, greater than 90% inhibition at 10 nM. Suppression of thrombin-stimulated AA release was evident within 5 min of pretreatment with 1 nM PMA. A non-tumor-promoting phorbol ester, 4-O-methyl PMA, showed a very weak ability to inhibit AA release. Thrombin-stimulated serotonin secretion was progressively inhibited by PMA pretreatment in platelets, while PMA was a stimulus for secretion at higher concentrations. 1-(5-Isoquinolinylsulfonyl)-2-methyl-piperazine (H-7), a selective inhibitor of protein kinase C, blocked PMA-induced inhibition of AA release. Furthermore, H-7 enhanced the effect of thrombin on AA release. PMA pretreatment reduced the inhibitory effect of thrombin on forskolin-stimulated cAMP accumulation, but had no effect on nonstimulated cAMP metabolism in the presence of thrombin. PMA did not inhibit AA release caused by A23187 or melittin. In digitonin-permeabilized platelets, thrombin plus guanosine 5'-(3-O-thio)triphosphate (GTP gamma S)-stimulated AA release, but not GTP gamma S- and AIF4(-)-stimulated AA release, was abolished by PMA pretreatment. These results suggest that activation of protein kinase C may exert negative feedback on the receptor-mediated activation of phospholipase A2. A possible uncoupling of thrombin receptor to GTP-binding protein leading to activation of phospholipase A2 by PMA pretreatment is discussed.
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PMID:Modes of inhibitory action of 4 beta-phorbol 12-myristate 13-acetate in thrombin-stimulated arachidonic acid release in intact and permeabilized platelets. 215 60

Histamine is known to be a mediator of inflammation. In order to understand the role of histamine in platelets, we have examined the effects of histamine on arachidonic acid (AA) release, cAMP accumulation, inositol trisphosphate production, and serotonin secretion. Incubation of rabbit (and human) platelets with histamine resulted in rapid increase of [3H]AA release from the platelets prelabeled with [3H]AA. The effect of histamine was blocked by the addition of H1 receptor antagonist mepyramine. Histamine did not substantially affect the cAMP content and inositol trisphosphate production. Histamine-stimulated AA release was not observed in digitonin-permeabilized platelets, whereas histamine acted synergistically with GTP or GTP analog, guanosine 5'-(3-O-thio)triphosphate. Histamine-stimulated, and GTP analog-dependent AA release was inhibited by guanosine 5'-(2-O-thio) diphosphate. The effects of three receptor stimulants, thrombin, norepinephrine, and histamine were both diminished by 1 microgram/ml of pertussis toxin treatment and by the antiserum against GTP-binding proteins (G proteins) treatment. However, the antiserum against beta gamma subunits of G proteins inhibited the histamine effect, not thrombin effect. 4 beta-Phorbol 12-myristate 13-acetate (PMA) treatment enhanced histamine-stimulated AA release and serotonin secretion but inhibited thrombin-stimulated reactions. The effect of PMA was dose dependent and was due to enhance the coupling of histamine receptors and G proteins. The results show the existence of H1 histamine receptors which couple phospholipase A2 activation via pertussis toxin-sensitive G proteins. Histamine actions differ in sensitivities to anti-beta gamma antiserum treatment and PMA treatment from thrombin actions.
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PMID:Histamine-stimulated and GTP-binding proteins-mediated phospholipase A2 activation in rabbit platelets. 215 20

Mechanisms of stimulus-response coupling in platelets are as complex and varied as the compounds that elicit the responses. The complexities are compounded by feedback mechanisms from substances released or synthesized by platelets as well as by "cross-talk" between signal transduction pathways. Examples of cross-talk include the ability of epinephrine to inhibit platelet adenylate cyclase through a G protein-mediated mechanism while causing platelet aggregation by some other mechanism and the ability of cAMP to inhibit thrombin-stimulated diacylglycerol formation. Despite the complexities, certain common threads are beginning to emerge, such as the involvement of G proteins in transducing many receptor-mediated processes, the involvement of relatively few second messenger pathways and the role of calcium in many of events leading to platelet responses, and the common involvement of protein kinases in carrying out second messenger function. The latter offers a useful assay for the effect of many agonists because they lead to the phosphorylation of specific proteins that can readily be detected by radioautography. Indeed, the emphasis has shifted in the past 10 years from relatively crude measurements of platelet function such as aggregation to precise, quantifiable measurement of processes such as protein phosphorylation and calcium release, which are indicators of the fundamental mechanisms involved in platelet function and thus serve as assays of these processes. On the other hand, there are other pathways and regulators yet to be discovered, notably regarding the action of epinephrine and the regulation of phospholipase A2. In addition, certain receptors remain elusive, including those for ADP and eicosanoids. The mechanisms of action of thrombin and cathepsin G, which involve their proteolytic activities, also remain an enigma. The combination of new insights into second messenger function and the techniques of molecular biology will allow many of these problems to be resolved, providing new approaches to therapy of thromboembolic disorders.
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PMID:Mechanisms of platelet activation and inhibition. 215 2

The stimulation of phospholipase A2 by thrombin and type 2 (P2)-purinergic receptor agonists in Chinese hamster ovary cells is mediated by the G protein Gi. To delineate alpha chain regulatory regions responsible for control of phospholipase A2, chimeric cDNAs were constructed in which different lengths of the alpha subunit of Gs (alpha s) were replaced with the corresponding sequence of the Gi alpha subunit (alpha i2). When a carboxyl-terminal chimera alpha s-i(38), which has the last 38 amino acids of alpha s substituted with the last 36 residues of alpha i2, was expressed in Chinese hamster ovary cells, the receptor-stimulated phospholipase A2 activity was inhibited, although the chimera could still activate adenylyl cyclase. Thus, alpha s-i(38) is an active alpha s, but also a dominant negative alpha i molecule, indicating that the last 36 amino acids of alpha i2 are a critical domain for G protein regulation of phospholipase A2 activity.
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PMID:A G protein mutant that inhibits thrombin and purinergic receptor activation of phospholipase A2. 216 41

The responses to alpha- and gamma-thrombin were studied in normal and Bernard-Soulier platelets labelled with [32P]phosphate, to investigate the relationship between thrombin binding to the platelet membrane glycoprotein Ib (GPIb) and thrombin-induced platelet activation. For this purpose we conducted parallel studies of the kinetics of platelet aggregation, granule secretion, hydrolysis of polyphosphoinositides, formation of phosphatidic acid, phosphorylation of the myosin light chain (p20) and of the 43 kDa protein (p43), and thromboxane B2 formation. Like alpha-thrombin, gamma-thrombin activated control platelets via all the above metabolic responses, but only after a prolonged lag. In Bernard-Soulier platelets, alpha-thrombin induced polyphosphoinositide hydrolysis and phosphatidic acid formation, p20 and p43 phosphorylation, thromboxane B2 formation, secretion and to a lesser extent aggregation, but only after a prolonged lag. The metabolic responses of Bernard-Soulier platelets to gamma-thrombin were very similar to those of control platelets. We have previously showed that GPIb which is not present in Bernard-Soulier platelets binds alpha- but not gamma-thrombin. The present results indicate that thrombin binding to GPIb is not directly coupled either with the activation of phospholipase C specific to polyphosphoinositides, or with the activation of protein kinase C and phospholipase A2. However, thrombin binding to GPIb appears to promote an early mechanism which accelerates all the platelet responses.
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PMID:The common pathway for alpha- and gamma-thrombin-induced platelet activation is independent of GPIb: a study of Bernard-Soulier platelets. 216 23

High levels of soluble phospholipase A2 (PLA2) activity have been detected in tissues fluids associated with inflammatory diseases. However, the cellular origin for PLA2 has not been demonstrated. Several groups of investigators have proposed that platelets, macrophages and chondrocytes may be the cellular source of this enzyme. In fact, soluble PLA2 is secreted extracellularly from rabbit and rat chondrocytes and from human synovial cells in response to cytokine stimulation (1). PLA2 activity has been shown to be increased upon stimulation by the chemotactic peptide (f-met-leu-phe) and thrombin in neutrophils and platelets (2). PLA2 has been found to have pro-inflammatory effects and causes a dose dependent infiltration of leukocytes and increased vascular permeability (3). The vascular actions of PLA2 have been proposed to be mediated through the release of prostaglandin E2 and thromboxane (4). We have reported that purified PLA2 from snake venom stimulated the release of leukotrienes and lipoxins from endogenous sources in porcine leukocytes. However, there is no information regarding the mechanism of action of human PLA2 on inflammatory cells and the generation of leukotrienes. In this report, we present evidence that PLA2 isolated from human platelets can stimulate the production of leukotriene B4 from human polymorphonuclear leukocytes. These results suggest that soluble PLA2 may function as a secretagogue of LTB4 in inflammatory sites and further amplify the inflammatory processes by inducing chemotaxis of circulating leukocytes.
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PMID:Phospholipase A2 as leukotriene B4 secretagogue for human polymorphonuclear leukocytes. 217 67

In the previous study, we generated a monoclonal antibody, 8F11, against NL-17, a high metastatic clone derived from a metastatic variant of murine colon adenocarcinoma 26. 8F11 inhibited platelet aggregation induced by NL-17 and recognized a Mr 44,000 membrane protein as antigen. In the present study, the reactivity of 8F11 to murine B16 melanoma and its metastatic variants was examined, and the antigen recognized by 8F11 on the cell surface was characterized. 8F11 was found to strongly react with 3 metastatic variants of B16 melanoma. In contrast, only slight reactivity was observed with parent B16 melanoma. The reactivity of the antibody to these cells was in the order B16F10 greater than B16BL-6 greater than B16F1 much greater than B16. Western blot analysis showed a Mr 41,000 protein as the antigen recognized by 8F11 on the cell surface of B16F10 cells. The Mr 41,000 antigen appeared to be a glycoprotein that bound to wheat germ agglutinin as has been observed for the Mr 44,000 antigen of NL-17. To elucidate the functional role of the Mr 41,000 antigen in B16 melanoma, platelet aggregation induced by B16 and B16F10 was compared. B16 was reported to stimulate platelet aggregation by the generation of thrombin, whereas B16F10 was found to activate platelet by at least 2 mechanisms: one dependent on thrombin and the other independent on thrombin. The activity of B16 and its metastatic variants to induce platelet aggregation in the presence of MD805, a synthetic antagonist of thrombin, well correlated with the reactivity of 8F11 to these cells. 8F11 blocked platelet activation by B16F10 under conditions preventing thrombin activity such as enzymatic formation of lysolecithin through the treatment of the cell surface with phospholipase A2 or in the presence of MD805. These data indicate that Mr 41,000 glycoprotein recognized by 8F11 on metastatic variants of B16 melanoma is involved in the thrombin-independent platelet aggregation. A positive correlation was observed between the levels of Mr 41,000 glycoprotein expression of B16 and its metastatic variants and their pulmonary metastasis after i.v. injection, suggesting Mr 41,000 glycoprotein, as well as other factors reported previously, may play an important role in the hematogenous spread of B16 melanoma.
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PMID:Expression of a Mr 41,000 glycoprotein associated with thrombin-independent platelet aggregation in high metastatic variants of murine B16 melanoma. 220 29

The steady-state fluorescence anisotropy of membranes labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH) or its 4'-trimethylammonio derivative, TMA-DPH, is generally considered a measure for the lipid order and, hence, inversely related to membrane fluidity. We now report that anisotropy values of DPH- and TMA-DPH-labeled human platelets are considerably influenced by experimental conditions like the platelet concentration, which do not affect membrane fluidity. Activation of platelets with thrombin increases, but activation with ionomycin decreases anisotropy values with both labels. Such anisotropy changes are not detected in platelet membranes or platelet lipids, when isolated after activation of the intact platelets. We present evidence that the anisotropy changes of intact platelets are not a consequence of modified lipid composition (e.g., as would be induced by phospholipase A2 activity) but are, at least partially, caused by changed optical properties of the cell suspension. Measurement of membrane fluidity of platelets by fluorescence polarization is severely hindered by a high turbidity of the platelet suspension and also by changes in the turbidity and platelet morphology during the activation process.
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PMID:Membrane fluidity of non-activated and activated human blood platelets. 236 76

Acute pancreatitis (AP) is believed to result from intraparenchymal activation of trypsin and other digestive enzymes within the pancreas followed by autodigestion of the gland. Gabexate mesilate (FOY), a synthetic guanidino acid ester exhibiting potent and versatile inhibitory actions on a number of proteinases (e.g., trypsin, kallikrein, C1-r, C1 esterase, plasmin, thrombin, phospholipase A2), was examined for its ability to protect the rat pancreas against development of AP induced by pharmacological doses of ceruletide (CRT). Rats were i.v. infused for 6 h with either CRT (5 micrograms/kg/h) or CRT + FOY (50 mg/kg/h). In FOY-treated rats the serum amylase and trypsinogen concentrations were reduced by 60 and 80%, respectively, compared to rats infused with CRT alone. Histologically, the extent of acinar cell vacuolization in the pancreas was significantly reduced and interstitial edema, although not assessed by quantitative morphometric techniques, appeared to be qualitatively lessened in the FOY-treated rats. The ability of FOY to inhibit significantly AP produced by supramaximal doses of CRT, coupled with its inhibitory properties on components of the coagulation and complement cascades, stress the importance of continued research on this compound as a potential therapeutic agent for treatment of AP and its systemic sequelae.
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PMID:Gabexate mesilate (FOY) protects against ceruletide-induced acute pancreatitis in the rat. 244 41


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