title: Thrombopoietin (TPO) induces c-myc expression through a PI3K- and MAPK-dependent pathway that is not mediated by Akt, PKCzeta or mTOR in TPO-dependent cell lines and primary megakaryocytes.
abstract: Thrombopoietin (TPO) and its receptor (c-Mpl) are the major regulators of megakaryocyte and platelet production and serve a critical and non-redundant role in hematopoietic stem cell (HSC) biology. TPO signals through the Jak-STAT, Ras-Raf-MAPK, and PI3K pathways, and promotes survival, proliferation, and polyploidization in megakaryocytes. The proto-oncogene c-myc also plays an important role in many of these same processes. In this work we studied the regulated expression of c-myc in megakaryocytic cell lines and primary cells by quantitative real-time RT-PCR. We found that TPO induced expression of c-myc in 1 h in both hematopoietic cell lines (UT-7 and BaF3/Mpl) and mature murine megakaryocytes. The TPO-induced expression of c-myc was blocked by a phosphatidylinositol 3-kinase (PI3K) inhibitor, suggesting that TPO stimulated c-myc expression through a PI3K-dependent pathway. Of interest, our study showed that overexpression of active Akt did not rescue the effect of PI3K blockade on c-myc expression, rather, enhanced it. In addition, inhibitors of protein kinase C (PKC)zeta and the target of rapamycin (mTOR) also failed to affect c-myc mRNA expression, while c-myc mRNA expression was reduced by inhibition of the mitogen activated protein kinase (MAPK) pathway. Therefore, we conclude that TPO stimulates c-myc expression in primary megakaryocytes through a PI3K- and MAPK-dependent pathway that is not mediated by Akt, PKCzeta or mTOR.
authors.affiliation: Department of Clinical Microscopy, Faculty of Medical Technology, Mahidol University, Bangkok, Thailand.
authors.forename: Supantitra
authors.initials: S
authors.lastname: Chanprasert
authors.forename: Amy E
authors.initials: AE
authors.lastname: Geddis
authors.forename: Charlene
authors.initials: C
authors.lastname: Barroga
authors.forename: Norma E
authors.initials: NE
authors.lastname: Fox
authors.forename: Kenneth
authors.initials: K
authors.lastname: Kaushansky
mesh_terms: D000818:Animals; D002460:Cell Line; D006224:Cricetinae; D005260:Female; D005786:Gene Expression Regulation / Q000187:drug effects*; D006801:Humans; D020935:MAP Kinase Signaling System / Q000187:drug effects*; D008533:Megakaryocytes / Q000187:drug effects*; D051379:Mice; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D011493:Protein Kinase C / Q000037:antagonists & inhibitors; D011494:Protein Kinases / Q000494:pharmacology; D000090063:Proto-Oncogene Mas; D051057:Proto-Oncogene Proteins c-akt / Q000037:antagonists & inhibitors / Q000378:metabolism*; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases; D013926:Thrombopoietin / Q000494:pharmacology*; D000099221:Protein Kinase C zeta
chemical_list: D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D011493:Protein Kinase C; D011494:Protein Kinases; D000090063:Proto-Oncogene Mas; D051057:Proto-Oncogene Proteins c-akt; D016271:Proto-Oncogene Proteins c-myc; D012333:RNA, Messenger; D058570:TOR Serine-Threonine Kinases; D013926:Thrombopoietin; D000099221:Protein Kinase C zeta; C000715470:MAS1 protein, human; C546842:MTOR protein, human; C546843:mTOR protein, mouse