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

---

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

ATP and thrombin both induced Ca2+ mobilization from intracellular Ca2+ store site of megakaryocyte, the progenitor cell of platelet (Uneyama C., Uneyama H. and Akaike N. (1993) J. Physiol. (Lond.), 470, 73-749). Since in platelet, thrombin is known as a strong agonist and ADP is known as a weak agonist, we further investigated the effect of these agonists on megakaryocyte. Thrombin induced Ca2+ mobilization, 5-hydroxy tryptamine (5-HT) release and aggregatory morphological changes in megakaryocyte, but ATP induced only Ca2+ mobilization. Thrombin-induced 5-HT release was inhibited by adenylate cyclase-activating drugs, and the morphological changes could be induced by H-8, an inhibitor of cAMP-dependent protein kinase. These results suggest that the Ca2+ mobilization is not sufficient to induce morphological changes, and the signal to cause morphological changes in megakaryocyte may be cAMP.
...
PMID:Not Ca2+ but CAMP is the second messenger for morphological changes in rat megakaryocyte. 777 97

The multifunctional cytokine interleukin 6 (IL-6) is a potent promoter of megakaryocytic maturation in vitro. In vivo, IL-6 has similar effects on the maturation of megakaryocytes, as shown by enhancing size, ploidy and platelet production. IL-6 is capable of augmenting the platelet count in both normal animals and those with reduced megakaryocyte mass; ongoing clinical trials suggest a similar thrombocytopoietic effect in man. Moreover, IL-6 alters platelet function, rendering them more sensitive to activation by thrombin and platelet activating factor. Finally, IL-6 promotes increases in plasma fibrinogen and von Willebrand factor, and a decrease in free protein S concentration. These modifications of the platelet and coagulant phases of the clotting mechanism may result in an overall prohemostatic tendency, which may prove beneficial for the amelioration of bleeding propensity following chemotherapy. However, additional investigation will be required to determine if IL-6-mediated alterations of hemostasis may lead to pathologic thrombosis.
...
PMID:Effects of interleukin 6 on megakaryocytes and on canine platelet function. 795 Oct 5

Human erythroleukemia (HEL) cells express megakaryocyte/platelet membrane markers and thus have been used as a model for studying platelet membrane receptors and their coupling to cell signaling pathways. Our previous studies, however, indicated that platelets and HEL cells possess different subtypes of adenosine A2 receptors. Furthermore, we now report that, whereas adenosine inhibits intracellular Ca2+ increases in platelets, it potentiates the rise in intracellular Ca2+ produced by thrombin, prostaglandin E1, thapsigargin, and the calcium ionophore A23187 in HEL cells. Stable adenosine analogs potentiated intracellular Ca2+ increases with a rank order of potencies of 5'-N-ethylcarboxamidoadenosine (NECA) > (R)-(-)-N6-(2-phenylisopropyl)adenosine (R-PIA) >> CGS 21680, suggesting that this effect is mediated by A2b receptors. EC50 values for NECA and R-PIA were 0.8 and 42 microM, respectively. NECA (100 microM) potentiated by 2-3-fold the increase in intracellular Ca2+ produced by 0.3 unit/ml thrombin. This effect was mimicked by cholera toxin and was shared by other Gs-coupled receptors, such as those activated by the prostacyclin analog iloprost and prostaglandin E1, indicating the involvement of Gs proteins. Adenosine analogs also increased intracellular cAMP with the same rank order of potencies. The membrane-permeable analog 8-bromo-cAMP, however, had no effect on intracellular Ca2+ levels, indicating that the potentiation of intracellular Ca2+ increases and the activation of adenylate cyclase are parallel but independent events. The increase in intracellular Ca2+ produced by adenosine is due not to an increase in phosphoinositide hydrolysis but, rather, to an increase in calcium influx, and it is lost if cells are studied in the absence of extracellular Ca2+. We conclude, therefore, that adenosine A2b receptors in HEL cells are coupled to Gs proteins and their activation leads to stimulation of adenylate cyclase and, independently, to potentiation of the rise in intracellular Ca2+. We speculate that A2b receptors in HEL cells activate a calcium channel through a cholera toxin-sensitive mechanism that requires an initial increase in intracellular Ca2+.
...
PMID:Positive modulation of intracellular Ca2+ levels by adenosine A2b receptors, prostacyclin, and prostaglandin E1 via a cholera toxin-sensitive mechanism in human erythroleukemia cells. 802 9

This study has explored the sulfation of proteins by guinea pig megakaryocytes and platelets and by human platelets. Guinea pig megakaryocytes were incubated in vitro with [35S]sulfate, and the sulfated proteins were separated from proteoglycans by DEAE-Sephacel chromatography and analyzed by SDS-PAGE. The megakaryocytes esterified sulfate to a number of proteins, with the most extensive label migrating at M(r) 42,000, and a second heavily labeled band at M(r) 103,000 in the 0.1 M NaCl DEAE eluate, and 50 and 180 kDa in the 0.23 M NaCl eluate. [35S]-Labeled GPlb alpha was immunoprecipitated from megakaryocyte Triton X-100 extracts. Guinea pig platelet proteins were labeled in vivo by injection of the animals with a single dose of H2(35)SO4. The platelets were activated with thrombin, and cytoskeletal proteins were isolated after treatment of the activated platelets with Triton X-100. About 20% of the platelet macromolecule-associated [35S]sulfate was incorporated into sulfated proteins, which were recovered primarily in the cytoskeleton. The cytoskeleton-associated sulfate radiolabel migrated on SDS-PAGE primarily with actin and additionally with several higher molecular weight proteins. A M(r) 42,000 [35S]-labeled protein was immunoprecipitated by a monoclonal anti-actin antibody, along with molecules of M(r) 160,000 and 180,000 and some higher M(r) material, from the megakaryocytes labeled in vitro with [35S]sulfate. Actin was labeled on 2D isoelectric focusing/SDS-PAGE gels. In addition, there was a very acidic series of heavily [35S]-labeled 42 kDa proteins with about eight components of different isoelectric points with a pattern identical to the M(r) 40,000 cytoskeletal-associated glycoprotein Pltpg40 isolated by Hildreth et al. (1991, Blood 77:121). We hypothesize that sulfation of the cytoskeletal proteins might be involved in cytoskeletal protein interactions and function.
...
PMID:Sulfation of guinea pig megakaryocyte and platelet proteins. 816 74

In the present study we measured membrane fluidity as the lateral mobility of the lipid probe 1,1'-ditetradecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate by fluorescence recovery after photobleaching in the plasma membrane of a single megakaryocyte, the progenitor cell of platelets. Megakaryocytes after 13 days in culture (maturation stage III) had a lateral diffusion coefficient (D) of (4.56 +/- 0.10) x 10(-9) cm2/s and a mobile fraction of 65 +/- 2% (means +/- SEM, n = 140). Megakaryocytes isolated from rib had a similar D and mobile fraction. Stimulation with alpha-thrombin (1-10 U/ml) induced a dose-dependent decrease in D to (3.40 +/- 0.22) x 10(-9) cm2/s between 1-5 min after stimulation (P < 0.001). The mobile fraction did not change. A similar decrease in D was found following stimulation with ADP (20 microM) and ionomycin (100 nM). Modulation of calpain I activity with calpain I inhibitor or tetracain had no effect. Pretreatment with cytochalasin B or colchicine decreased D to (3.64 +/- 0.29) x 10(-9) cm2/s (P < 0.003) and (3.96 +/- 0.18) x 10(-9) cm2/s (P < 0.013) respectively. After stimulation D decreased further in cytochalasin-treated cells (3.37 +/- 0.16) x 10(-9) cm2/s (P < 0.020) but remained at the same level in colchicine-treated cells. Both treatments increased the mobile fraction to 73-75% in stimulated megakaryocytes (P < 0.03). These data indicate that the diffusion velocity of lipids in megakaryocytes is low and decreases further after stimulation. These changes are independent of calpain I. Treatments that decrease the cytoskeletal mass and thereby increase the mobility of proteins in the plasma membrane increase the number of lipids that participate in this process.
...
PMID:Rapid alterations in lateral mobility of lipids in the plasma membrane of activated human megakaryocytes. 816 23

Thrombomodulin (TM) is an essential cofactor for the physiologic activation of the anticoagulant protein C by thrombin. We have observed that the expression of TM mRNA in response to retinoic acid was markedly increased in human U937 monoblast-like cells, and human MEG01 megakaryocyte-like cells, but not in human umbilical vein cells, murine hemangioma cells, human K562 erythroblast-like cells, and murine HSD fibroblast-like cells. TM activity in U937 cells and MEG01 cells was not detectable in untreated cells, but developed rapidly after treatment with retinoic acid. In endothelial cells there was minimal change in TM activity in response to retinoic acid treatment. We have isolated clones for the genes for murine and human TM and have identified potential retinoic acid response elements in the 5'-flanking region of the human gene. In U937 cells the increase in mRNA levels was associated with increased transcription, and transient transfection studies with reporter plasmids demonstrate functional retinoic acid response elements present in the 5'-flanking region of the gene. Deletion of, and mutations introduced into, the potential retinoic acid response element confirm the functional response in transient transfections.
...
PMID:Characterization of thrombomodulin expression in response to retinoic acid and identification of a retinoic acid response element in the human thrombomodulin gene. 820 15

Thrombin is a potent physiological platelet agonist, critical for haemostasis and thrombosis. Several studies have recently demonstrated that thrombin also displays functional effects on bone marrow platelet progenitors, i.e. the megakaryocytes, and possibly on megakaryocyte precursors. Following a brief description of the different cell systems used for these investigations, this paper focuses on data related to thrombin receptors, their coupling mechanisms, and biological responses due to their activation on cells of the megakaryocytic lineage. In spite of some differences, whose significance remains to be established, most data reported give evidence of considerable homologies in thrombin receptors and signal transduction mechanisms between platelets and cells belonging to the megakaryocytic lineage. In addition, a potential functional role for thrombin in the regulation of megakaryocytopoiesis is emerging. The physiological relevance of these observations are discussed.
...
PMID:Thrombin interactions with cells of the megakaryocytic lineage. 829 26

1. The responses of megakaryocytes isolated from rat bone marrow to externally applied adenosine triphosphate (ATP) were investigated in the whole-cell mode by the use of nystatin perforated patch-clamp technique. 2. ATP at 1-100 microM evoked periodic outward currents at a holding potential of -40 mV. The reversal potential of the currents was close to K+ equilibrium potential (EK) and the K+ channel blockers such as quinine and quinidine suppressed the currents, indicating that the outward currents are predominantly carried by K+. 3. Since it has been reported that adenosine diphosphate (ADP) evoked monophasic K+ current using a conventional whole-cell recording, we compared the results obtained by perforated and conventional patch-clamp techniques. The crucial difference between our results and previous results was due to the intracellular perfusion with internal solution containing a high concentration of EGTA by which both current shape and concentration response were modified. 4. The membrane permeable Ca2+ chelator, 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxy methyl ester; BAPTA AM), inhibited the K+ current concentration dependently, suggesting that ATP-induced oscillatory K+ currents are caused by changes in cytoplasmic free Ca2+ concentration ([Ca2+]i). 5. With increasing ATP concentration, the frequency and the maximum amplitude of K+ current oscillation increased and the latency of current, which is the period required to activate the first K+ current after ATP application, decreased. 6. ADP, 2-methylthio-ATP and ATP-gamma-S could also evoke the periodic K+ currents, but adenosine, uridine triphosphate (UTP) and alpha-beta-methylene adenosine 5'-triphosphate (AMP-CPP) failed. 2-Methylthio-ATP was the most potent agonist; next was ADP which showed a 10-30 times stronger effect than ATP. Cross-desensitization was observed between ATP and ADP, but not between ATP or ADP and thrombin. 7. Extracellular Ca2+ was not required for the ATP-induced K+ current activation, indicating that Ca2+ released from intracellular pools induced the oscillatory response. In addition, the agonist potency increased when extracellular Ca2+ concentration ([Ca2+]o) decreased, suggesting that the principal agonists might be ATP4- and ADP3-. 8. The results suggest the presence of a novel subtype of purinoceptor in the megakaryocyte plasma membrane which induces cytoplasmic Ca2+ oscillation and evokes periodic K+ current flux.
...
PMID:Cytoplasmic Ca2+ oscillation in rat megakaryocytes evoked by a novel type of purinoceptor. 830 53

To evaluate whether currently popular aspirin regimens have an effect on the prostaglandin synthesis in human megakaryocytes we measured thromboxane B2 (TXB2) synthesis in response to thrombin stimulation in human megakaryocytes ex vivo. Human megakaryocytes were purified by Counterflow Centrifugal Elutriation from bone marrow punctures, taken from volunteers before and 2 hours after ingestion of one dose of 500 mg (n = 4), 80 mg (n = 4) or 40 mg (n = 2) aspirin. Subsequently, megakaryocytes were purified before and 12 h after ingestion of 80 mg (n = 3) aspirin twice daily for 1 week and 12 h after 500 mg (n = 3) aspirin. On average, 140 +/- 102 x 10(3) (mean +/- 1 SD) megakaryocytes were recovered. We found that aspirin inhibits megakaryocyte cyclooxygenase in a dose-dependent manner. Two hours after 500 mg of aspirin, TXB2 synthesis in megakaryocytes was inhibited by 96.8 +/- 2%, whereas one dose of 80 and 40 mg aspirin showed an inhibition of 79.4 +/- 13.7% and 80 +/- 6.2% respectively. However, the inhibition of TXB2 synthesis seems not to be long-lasting since, 12 h after the ingestion of aspirin, an increase of megakaryocyte TXB2 production could be observed which reached significance after the 500 mg aspirin dosage (p < 0.048). We conclude that human megakaryocyte cyclooxygenase is sensitive to aspirin inhibition and that low doses of aspirin (40 and 80 mg) enter the systemic circulation and are able to inhibit megakaryocyte cyclooxygenase, but this inhibition is incomplete and megakaryocyte cyclooxygenase seems to recover within 12 h after ingestion of aspirin.
...
PMID:Influence of aspirin on human megakaryocyte prostaglandin synthesis. 831 88

Clusterin, a 70-Kd disulfide-linked two-chain plasma glycoprotein circulates in blood as a high-density lipoprotein particle and is highly induced after tissue injury and tissue remodeling. In this study, peripheral blood leukocytes were assayed for clusterin expression. The protein was predominantly detectable in human platelets by immune cytochemistry. The content of clusterin was determined and amounts to 2.5 +/- 1.3 micrograms/10(9) platelets, thus representing about 2% of the blood pool. Clusterin purified from human platelets had the same molecular weight as plasma clusterin under nonreducing conditions and was composed of two disulfide-linked nonidentical subunits of the same size. Both preparations were sensitive to reduction yielding the two subunits of 35 Kd. In contrast to plasma clusterin, the platelet form was not complexed to apolipoprotein A-I. By immunogold labeling, alpha-granule localization of clusterin was observed. Complete release of platelet clusterin occurred at optimal doses of A23187, phorbol myristate acetate (PMA), and thrombin. Because clusterin mRNA was detected by hybridization in situ in bone marrow-derived megakaryocytes, platelet clusterin is most likely produced and packaged into alpha-granules during megakaryocyte development.
...
PMID:Human megakaryocytes express clusterin and package it without apolipoprotein A-1 into alpha-granules. 832 15


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>

---