title: Thrombopoietin upregulates the promoter conformation of p53 in a proliferation-independent manner coincident with a decreased expression of Bax: potential mechanisms for survival enhancing effects.
abstract: Thrombopoietin (Tpo) has proliferative and maturational effects on immature and more committed cells, respectively. We previously reported a role for Tpo as a survival factor in the factor-dependent human cell line M07e by demonstrating that Tpo suppresses apoptosis in the absence of induced proliferation. Wild-type p53 is a tumor suppressor gene that can play a vital role in mediating growth factor withdrawal-induced apoptosis in factor-dependent hematopoietic cells. Wild-type p53 can switch from a suppressor conformation, with an antiproliferative, pro-apoptotic phenotype, to a promoter conformation that has a diminished ability to mediate cell cycle arrest and apoptosis. In an effort to elucidate the mechanisms through which Tpo suppresses apoptosis, we investigated the effects of Tpo treatment on p53-mediated apoptosis in M07e cells. Tpo upregulated the expression of the promoter conformation of p53 in M07e cells coincident with a downregulation of Bax and Mdm2 protein levels. Protein levels of Bcl-2 and Bcl-xL did not significantly vary as a function of growth-factor stimulation. Conversely, the levels of suppressor conformation p53 were maximal when M07e was in a growth arrested state and decreased during factor stimulation. Furthermore, Tpo treatment induced an extranuclear buildup and greatly weakened the DNA binding capacity of p53. p53-specific antisense oligonucleotide treatment recapitulated the effects of Tpo treatment on the levels of Bax, Mdm-2, and Bcl-2. These results suggest that Tpo is suppressing growth factor withdrawal induced-apoptosis, at least in part, by downregulating the expression of pro-apoptotic Bax protein levels, through modulating the conformation of p53, which results in a functional inactivation of its pro-apoptotic abilities.
authors.affiliation: Departments of Microbiology/Immunology, Medicine (Hematology/Oncology), and the Walther Oncology Center, Indiana University School of Medicine, Indianapolis, IN, USA.
authors.forename: A
authors.initials: A
authors.lastname: Ritchie
authors.forename: A
authors.initials: A
authors.lastname: Gotoh
authors.forename: J
authors.initials: J
authors.lastname: Gaddy
authors.forename: S E
authors.initials: SE
authors.lastname: Braun
authors.forename: H E
authors.initials: HE
authors.lastname: Broxmeyer
mesh_terms: D015153:Blotting, Western; D002455:Cell Division; D002470:Cell Survival; D002478:Cells, Cultured; D006801:Humans; D010641:Phenotype; D011401:Promoter Regions, Genetic; D011518:Proto-Oncogene Proteins / Q000096:biosynthesis*; D019253:Proto-Oncogene Proteins c-bcl-2; D011994:Recombinant Proteins / Q000378:metabolism; D013926:Thrombopoietin / Q000494:pharmacology*; D016159:Tumor Suppressor Protein p53 / Q000235:genetics*; D015854:Up-Regulation / Q000187:drug effects*; D051028:bcl-2-Associated X Protein
chemical_list: C494826:BAX protein, human; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D011994:Recombinant Proteins; D016159:Tumor Suppressor Protein p53; D051028:bcl-2-Associated X Protein; D013926:Thrombopoietin