1998 Fiscal Year Final Research Report Summary
Cytokine receptor-mediated regulation of MAP kinases in human platelets
| Project/Area Number |
09671109
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| Research Category |
Grant-in-Aid for Scientific Research (C)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Hematology
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| Research Institution | KYOTO UNIVERSITY |
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Principal Investigator |
TAKAYAMA Hiroshi Kyoto University, Assistant Professor, 医学研究科, 助手 (10197220)
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| Project Period (FY) |
1997 – 1998
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| Keywords | Thrombopoietin / platelets / MAP kinase / protein kinase C / Cytoskeleton |
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| Research Abstract |
The thrombopoietin (TPO) receptor is expressed in the megakaryocytic lineage from late progenitors to platelets. We investigated the effect of TPO on the extracellular signal-regulated kinase (ERK) activation pathway in human platelets. TPO by itself did not activate ERK1, ERK2 and protein kinase C(PKC), whereas TPO directly enhanced the PKC-dependent activation of ERKs induced by other agonists including thrombin and phorbol esters, without affecting the PKC activation by those agonists. TPO did not activate the mitogen-activated protein kinase/ERK kinases, MEK1 and MEK2, but activated Raf-1 and directly augmented the PKC-mediated MEK activation, suggesting that TPO primarily potentiates the ERK pathway through regulating MEKs or upstream steps of MEKs including Raf-1. The MEK inhibitor PD098059 failed to affect not only thrombin-induced or phorbol ester-induced aggregation, but also potentiation of aggregation by TPO, denying the primary involvement of ERKs and MEKs in those events.
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ERKs and MEKs were located mainly in the detergent-soluble/non-cytoskeletal fractions. ERKs but not MEKs were relocated to the cytoskeleton following platelet aggregation and actin polymerization. These data indicate that TPO synergizes with other agonists in the ERK activation pathway of platelets and that this synergy might affect functions of the cytoskeleton possibly regulated by ERKs.
TPO does not induce aggregation by itself but potentiates other-agonist-induced aggregation in aspirin-treated or -untreated platelets in vivo. Since we found that both ERKs and MEKs were not primarily involved in platelet aggregation as described above, we investigated effects of TPO on activation of p38 mitogen-activated protein kinase to study how TPO affects platelet functions. Thrombin but not phorbol 12, 13-dibutyrate(PDBu) activated p38 irrespective of aspirin pretreatment TPO did not activate p38 by itself, whereas TPO pretreatment potentiated thrombin-induced activation of p38, whether platelet were aspirinized or not. TPO also potentiated p38 activation induced by a thrombin receptor agonist peptide, a thromboxane A2 analogue, collagen, crosslinking the glycoprotein VI, ADP, and epinephrine. TPO did not promote phosphorylation of Hsp27 and cytosolic phospholipase A2 by itself but enhanced thrombin-induced phosphorylation of them. The specific p38 inhibitor SB203580 strongly inhibited such phosphorylation, confirming that it is mediated via the p38 pathway. SB203580 inhibited, but not completely, ADP- or thrombin-induced aggregation and its enhancement by TPO, whether platelets were aspirinized or not In contrast, although TPO also potentiated PDBu-induced aggregation in aspirinized platelets, SB203580 did not inhibit it irrespective of TPO pretreatment. The p38 pathway could be one of the mechanisms by which TPO potentiates agonist-induced aggregation in both aspirin-sensitive and -insensitive manners. Less
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Research Products
(2 results)
