THROMBOPOIETIN UP-REGULATES THE PROMOTER CONFORMATION OF P53 IN A PROLIFERATION-INDEPENDENT MANNER COINCIDENT WITH A DECREASED EXPRESSION OF BAX - POTENTIAL MECHANISMS FOR SURVIVAL ENHANCING EFFECTS

Thrombopoietin (Tpo) has proliferative and maturational effects on imm ature 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 absenc e of induced proliferation. Wild-type p53 is a tumor suppressor gene t hat can play a vital role in mediating growth factor withdrawal-induce d 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 e lucidate the mechanisms through which Tpo suppresses apoptosis, we inv estigated the effects of Tpo treatment on p53-mediated apoptosis in M0 7e cells. Tpo upregulated the expression of the promoter conformation of p53 in M07e cells coincident with a downregulation of Bar and Mdm2 protein levels. Protein levels of Bcl-2 and Bcl-x(L) did not significa ntly 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. Further more, Tpo treatment induced an extranuclear buildup and greatly weaken ed the DNA binding capacity of p53. p53-specific antisense oligonucleo tide treatment recapitulated the effects of Tpo treatment on the level s of Bar, Mdm-2, and Bcl-2. These results suggest that Tpo is suppress ing growth factor withdrawal induced-apoptosis, at least in part, by d ownregulating the expression of pro-apoptotic Bar protein levels, thro ugh modulating the conformation of p53, which results in a functional inactivation of its pro-apoptotic abilities. (C) 1997 by The American Society of Hematology.