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)

All integrin alpha subunits contain a highly conserved KXGFFKR motif in their cytoplasmic domains that plays a crucial role in the regulation of integrin affinity for their ligands. We show that a lipid-modified peptide corresponding to the cytoplasmic region, 989-995, of the platelet integrin subunit glycoprotein GpIIb (alphaIIb), palmitoyl-KVGFFKR (Ppep; 10 microM), but not a similarly modified scrambled peptide (palmitoyl-FKFVRGK), can specifically induce platelet activation and aggregation equivalent to that of strong agonists such as thrombin. Ppep-induced aggregation is also associated with indices of platelet activation including thromboxane A2 (TXA2) synthesis (EC50 = 45 +/- 5 microM), secretion of alpha-granules detected as enhanced surface expression of P-selectin (EC50 = 52 +/- 8 microM), and conformational changes in GpIIb/IIIa measured by the monoclonal antibody, PAC-1 (EC50 = 3.7 +/- 1 microM). The TXA2 receptor antagonist, SQ29548, PGE1, and the ADP scavenger, apyrase, differentially inhibit the aggregation response and TXA2 synthesis in response to Ppep. Similarly, GpIIb/IIIa antagonists (RO-449883 and integrelin), which inhibit aggregation by greater than 90%, have little effect on peptide-induced TXA2 synthesis, suggesting that this event is independent of fibrinogen binding to GpIIb/IIIa. Alanine-stepping of the Ppep sequence identifies GFFK(991-994) as the critical residues in all peptide-mediated events. We conclude that this peptide can imitate the cytoplasmic domain of GpIIb and initiate parallel but independent signaling pathways, one leading to ligand binding and platelet aggregation and the other to intracellular signaling events such as TXA2 synthesis and secretion.
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PMID:A sequence within the cytoplasmic tail of GpIIb independently activates platelet aggregation and thromboxane synthesis. 968 82

Human placental ecto-ATP diphosphohydrolase (ATPDase), an 82 kDa single-chain glycoprotein, was purified to high specific activity using a specific murine monoclonal antibody MK33 (IgG1-kappa). Structurally, protein-based analysis showed this enzyme to be almost identical to that of CD39 lymphoid cell activation antigen deduced by cDNA sequencing (Maliszewski CR, et al, J Immunol 1994; 153:3574); but differing in the NH2-terminal amino acid sequence, suggesting that placental ecto-ATPDase is most likely an isoform of CD39 generated by alternative splicing of the pre-mRNA. Functionally, placental ecto-ATPDase totally inhibits the secondary platelet aggregation induced by agonists at a final concentration (f.c.) of 1 microgram/ml. The purified enzyme (1 microgram/ml, final), pre-incubated with washed platelets prior to alpha-thrombin stimulation, completely inhibits the activation of platelet glycoprotein (GP) IIb/IIIa, thereby blocking the binding of fibrinogen or von Willebrand factor to platelets. Further, under different shear stresses, the enzyme modulates platelet aggregation differently. Low shear stress-induced platelet aggregation is blocked by this enzyme in a dose-dependent manner and is totally blocked at f.c. 0.5 microgram/ml. Under high shear stress, however, this protein at a f.c. of 0.5 microgram/ml mediates almost complete disaggregation of platelets without affecting the initial aggregation. Using immunohistochemical analysis, this enzyme was observed to be localized at the syncytiotrophoblasts of placental microvilli and the endothelial cells (ECs) of the umbilical vein obtained at full-term normal delivery, but scarcely at the ECs of the umbilical artery.
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PMID:Placental ecto-ATP diphosphohydrolase: its structural feature distinct from CD39, localization and inhibition on shear-induced platelet aggregation. 984 14

The involvement of platelets in neovascularization was investigated in the matrigel tube formation assay, an in vitro model of angiogenesis. Platelets promoted the formation of capillary-like structures (expressed as relative tube area) number- and time-dependently. Relative tube area increased from 0.98+/-0.02 (n = 8) in the presence of 6.25 x 10(4), to 3.21+/-0.12 (n=8) in the presence of 10(6) platelets/well compared to 0.54+/-0.04 (n=8) in their absence. This increase was unaffected by acetyl salicylic acid (ASA), apyrase, and hirudin. Photographs from representative experiments, showed that platelets adhered along the differentiating endothelium. Addition of alpha-thrombin (0.1-1 i.u. ml(-1)), the nitric oxide (NO) donor sodium nitroprusside (SNP; 1-100 microM) or the NO synthase inhibitor, L-NG-arginine-methylester (L-NAME, 30-300 microM) to the assay, had no effect on tube formation compared to that seen with platelets alone. Neuraminidase (0.01 i.u./10(7) platelets), which strips sialic acid residues from membrane glycoproteins, abolished the promoting effect of platelets on tube formation. The relative tube area in the presence of neuraminidase-treated platelets was 0.81+/-0.03 (n = 8), in the presence of untreated platelets 1.69+/-0.09, P<0.001 (n=8) and in the absence of platelets, 0.80+/-0.04 (n=8). The tetrapeptide Arg-Gly-Asp-Ser (RGDS; 20-200 microM) which inhibits von Willebrand factor, fibrinogen and fibronectin-mediated adhesion, had no effect on the promoting effect of platelets on tube formation. These results indicate that platelets promote angiogenesis in vitro. This effect is largely independent from activation by alpha-thrombin, is not modified by manipulating NO and prostaglandin metabolism and proceeds possibly through adhesion of the platelets to the differentiating endothelium.
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PMID:Evidence that platelets promote tube formation by endothelial cells on matrigel. 986 54

Thrombin and other agonists that induce secretion and aggregation in human platelets also activate phospholipase D (PLD), but the signaling cascade leading to activation of PLD in human platelets is not yet clear. We have determined that apyrase, which scavenges ADP secreted during platelet activation, is able to block or reduce the PLD activation stimulated by low (0.1 U/ml or less) or high (0.3- 1.0 U/ml) concentrations of thrombin, respectively. Neither ADP (up to 100 microM) nor its more potent analogue 2-methylthio-ADP (up to 100 microM), however, are able to stimulate PLD alone, and even the addition of fibrinogen, which results in platelet aggregation, is not sufficient for PLD activation. In contrast, ADP is able to stimulate PLD in the presence of low concentrations of thrombin that alone have little or no effect, suggesting ADP may play an amplifying role in platelet PLD activation. This hypothesis is supported by the finding that the purinergic receptor antagonist ARL 66096, an ATP analogue, reduces in a concentration-dependent fashion the PLD response to thrombin (IC50=28 nM with 0.1 U/ml thrombin). ARL 66096 also abolishes the PLD activation by ADP observed in the presence of low concentrations of thrombin, confirming that the antagonist inhibits an ADP-dependent component of the response. In addition, the thromboxane A2 receptor agonist U46619 activates PLD, and this response is markedly reduced by ARL 66096. Concomitantly, phosphorylation of the protein kinase C substrate pleckstrin in response to thrombin or U46619 is partially or totally inhibited by ARL 66096, respectively, consistent with ADP stimulation of protein kinase C being involved in the PLD response to these agonists. Based on these findings, we conclude that ADP secretion and activation of purinergic ADP receptors is an important amplification mechanism in the signal transduction pathways leading to PLD activation in human platelets.
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PMID:Secreted ADP plays a central role in thrombin-induced phospholipase D activation in human platelets. 986 70

Evidence that secreted dense granule adenine nucleotides mediate part of the agonist-induced cytosolic calcium ([Ca2+]i) responses in human platelets was obtained from comparisons of fura-2-loaded platelets from normal subjects and from patients with a form of platelet storage pool deficiency (SPD) in which the secretory dense granules and their contents are virtually absent. SPD platelets had normal initial [Ca2+]i increases induced by thrombin and the endoperoxide analog U46619, but a significantly enhanced decay of elevated [Ca2+]i levels following the initial increases. With thrombin, this enhanced [Ca2+]i decay was associated with decreased Ca2+ influx, as measured by Mn2+ quench of fura-2 fluorescence. Addition of micromolar concentrations of ADP, alone or together with ATP, after stimulation reversed the enhanced [Ca2+]i decay and increased Mn2+ quench in SPD platelets, but had no effect on these responses in normal platelets, while addition of 100-fold higher concentrations of ATP or apyrase before stimulation increased [Ca2+]i decay and decreased Mn2+ quench in normal platelets, but had little effect in SPD platelets. ATP and alpha,beta-methylene ATP, a specific agonist for P2X1 receptors, at micromolar concentrations also increased Mn2+ quench, but to lesser extents than did ADP, in SPD platelets isolated and loaded with fura-2 in the presence of apyrase. Similar effects of ADP and excess ATP were seen in U46619-stimulated platelets, but decreased Ca2+ influx could not be measured directly in SPD platelets, presumably due to the very transient influx response seen with U46619. These results suggest that secreted dense granule ADP and ATP contribute to the maintenance of elevated [Ca2+]i levels, but not to the initial [Ca2+]i increases, in stimulated human platelets, most likely via a nucleotide-specific component of Ca2+ influx which may be mediated by interactions with both P2X1 and P2Y1 purinoceptors.
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PMID:Secreted dense granule adenine nucleotides promote calcium influx and the maintenance of elevated cytosolic calcium levels in stimulated human platelets. 1006 8

Cathepsin G, a proteolytic enzyme from activated leukocytes, can interact with platelets during inflammation and thrombosis. Platelets that have been exposed to cathepsin G in thrombi may recirculate if they are freed during fibrinolysis. To determine whether some of the subsequent functions of such platelets would be impaired, we investigated the responses of cathepsin G-pretreated platelets to agonists that they would encounter in the circulation. Suspensions of washed human platelets were labeled with [14C]serotonin and resuspended in Tyrode-albumin solution (with 2 mM Ca2+ and apyrase). After 15 minute incubation with 400 nM cathepsin G at 37 degrees C, 52+/-3% of [14C]serotonin had been released, and glycoprotein Ib was degraded. The platelets were washed and resuspended in fresh medium to remove cathepsin G and released materials. Ristocetin-induced agglutination was abolished, indicating that the binding site for von Willebrand Factor on glycoprotein Ib had been removed. Aggregation and release of residual [14C]serotonin in response to 0.1-1.0 U/mL thrombin was blocked or greatly reduced by the cathepsin G pretreatment. This inhibition is probably largely due to cleavage by cathepsin G of some of the protease-activated receptors at the C-terminal side of Ser42 so that the tethered ligand is lost. Pretreatment with cathepsin G did not affect responses to ADP or a low concentration of platelet-activating factor in the presence of fibrinogen, indicating that receptors for these agonists were unaffected and that the function of the fibrinogen receptor, GPIIb/IIIa was unchanged. Responses to cathepsin G, the thrombin receptor-activating peptide SFLLRN, collagen, or the thromboxane A2 mimetic U46619 were partially inhibited, even in the presence of added fibrinogen. Platelet adhesion to a collagen-coated surface was 51+/-7% inhibited, which may indicate cleavage of a collagen receptor or receptors; this may partly account for strong inhibition of collagen-induced aggregation and release of granule contents; additionally, as shown by inhibition of responses to U46619, the function of the thromboxane A2 receptor may be compromised. Thus, although cathepsin G activates platelets, if they recirculate after interaction with it, their subsequent adhesion to damaged vessel walls, aggregation, and release of granule contents induced by thrombin and collagen will be diminished.
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PMID:Effects of cathepsin G pretreatment of platelets on their subsequent responses to aggregating agents. 1052 9

Evidence was obtained about the mechanism responsible for platelet integrin alpha(2)beta activation by determining effects of various inhibitors on soluble collagen binding, a parameter to assess integrin alpha(2)beta(1) activation, in stimulated platelets. Agonists that can also activate platelet glycoprotein IIb/IIIa are able to activate integrin alpha(2)beta(1), but those operating via glycoprotein Ib cannot. Activation of alpha(2)beta(1) induced by low thrombin or collagen-related peptide concentrations was almost completely inhibited by apyrase, and the inhibitors wortmannin, 4-amino-5-(chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, bisindolylmaleimide I, and SQ29548 significantly inhibited it. Activation induced by high thrombin or collagen-related peptide concentrations was far less sensitive to these inhibitors. However, only wortmannin markedly inhibited ADP-induced integrin alpha(2)beta(1) activation, and this was not ADP concentration-dependent. These results suggest that at the low agonist concentrations, the released ADP would be a primary inducer of integrin alpha(2)beta(1) activation, while at the high agonist concentrations, there would be several pathways through which integrin alpha(2)beta(1) activation can be induced. Kinetic analyses revealed that ADP-induced platelets had about the same number of binding sites (B(max)) as thrombin-induced platelets, but their affinity (K(d)) for soluble collagen was 3.7-12.7-fold lower, suggesting that activated integrin alpha(2)beta(1) induced by ADP is different from that induced by thrombin. The data are consistent with an activation mechanism involving released ADP and in which there exists two different states of activated integrin alpha(2)beta(1); these activated forms of integrin alpha(2)beta(1) would have different conformations that determine their ligand affinity.
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PMID:Signal-transducing mechanisms involved in activation of the platelet collagen receptor integrin alpha(2)beta(1). 1071 21

Platelets contribute to arterial thrombosis by multiple mechanisms that promote blood clotting, favor vasoconstriction, activate the procoagulant capacity of endothelium, and stimulate inflammation. These activities are augmented by turbulent blood flow. Classic antiplatelet therapy with aspirin to prevent occlusive stroke offers significant clinical benefit (20-25% risk reduction), yet is less effective than in prevention of coronary artery occlusion (up to 50% risk reduction of myocardial infarction in unstable angina). Since aspirin's antiplatelet effects are limited to blocking a single metabolic pathway - namely inhibition of thromboxane A(2) formation -, and aspirin fails to alter platelet adhesion, other antiplatelet agents that target ADP receptors, platelet surface glycoproteins (such as the GPIIb/IIIa complex), or platelet-dependent thrombin generation offer additional clinical benefits by blocking additional separate pathways or the final common pathway of platelet activation. Combinations of antiplatelet agents, such as aspirin/dipyridamole, aspirin/clopidogrel, or aspirin/GPIIb/IIIa inhibitors, have recently been tested for improved efficacy in clinical trials. Soluble recombinant CD39, an ecto-ADPase, protects against stroke in animal models by metabolizing released ADP/ATP to antiplatelet derivatives. In general, combinations of antiplatelet agents promise greater efficacy than single drugs in preventing stroke, since interactions among different antiplatelet mechanisms can be synergistic. However, such combinations may also increase the risk of bleeding, so that precise understanding of risk/benefit ratios that address the possibility of intracranial as well as gastrointestinal bleeding will require careful monitoring in large clinical trials of patients at risk of stroke, with particular attention to the elderly.
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PMID:Antiplatelet agents in stroke prevention. combination therapy: present and future. 1109 82

Apyrase, secreted by ticks during feeding, is a platelet aggregation inhibitor that functions as a regulator of the host's hemostatic system. This present study concerns the disaggregation effect of salivary gland apyrase from the tick Ornithodoros savignyi. Secondarily aggregated platelets, disaggregated by apyrase, exhibited a reversal of shape from a spherical (aggregated) form to a discoid form, reminiscent of reversible aggregation at low ADP concentrations in citrated platelet-rich plasma. However, they showed a dilatory open canaliculary system and an absence of granules indicating disaggregation after degranulation had taken place. In contrast, disaggregation by the fibrin(ogen)olytic enzyme, plasmin, showed that platelets degranulated, but retained a spherical form with numerous extended pseudopods. While thrombin had no effect on aggregation or clotting of platelets disaggregated with plasmin, it did activate those platelets disaggregated with apyrase and clotted the plasma. This is the first study to describe the disaggregating effects of tick derived apyrase on aggregated platelets. It also shows that apyrase can disaggregate platelets even after secondary aggregation and degranulation of platelets has taken place. Platelet aggregation is one of the main barriers encountered by ticks during feeding and counteraction of this process by ticks is an important factor for successful feeding.
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PMID:Disaggregation of aggregated platelets by apyrase from the tick, Ornithodoros savignyi (Acari: Argasidae). 1111 Feb 38

Platelet activation is normally induced by primary agonists such as adenosine diphosphate (ADP), thrombin, and collagen, whereas other agonists, such as epinephrine, can play important accessory roles. It is now reported that the macrophage-derived chemokine (MDC), thymus activation-regulated chemokine (TARC), and stromal cell-derived factor one (SDF-1) are highly effective activators of platelet function under a variety of conditions, stimulating platelet shape change, aggregation, and adhesion to collagen or fibrinogen. Chemokine-mediated platelet activation was rapid and maximal (less than 5 seconds) under arterial flow conditions and depended strongly on the presence of low levels of primary agonists such as ADP or thrombin. Concentrations of ADP (0.05-0.25 microM) or thrombin (0.005-0.02 U/mL) that induced minimal aggregation caused major aggregation acting in combination with the chemokines. The ability of apyrase to block chemokine-dependent aggregation or adhesion was consistent with an important role for ADP. Chemokine-stimulated aggregation was also insensitive to indomethacin, suggesting that the activation of cyclo-oxygenase is not involved. TARC, MDC, and SDF-1 increased intracellular calcium concentrations [Ca(2+)](i) when combined with low levels of ADP. The MDC and TARC receptor CCR4 was expressed on platelets, and an anti-CCR4 antibody blocked aggregation induced by TARC or MDC. Treatment of platelets with SDF-1 and MDC rapidly exposed P-selectin (CD62P) on the cell surface but did not induce the secretion of serotonin. These findings suggest that the chemokines MDC, TARC, and SDF-1, which may be produced during inflammatory responses, coupled with low levels of ADP or thrombin, can serve as strong stimuli for activating platelet function.
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PMID:Adenosine diphosphate strongly potentiates the ability of the chemokines MDC, TARC, and SDF-1 to stimulate platelet function. 1115 20


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