MDM2 Suppresses p73 Function without Promoting p73 Degradation

Abstract

The newly identified p53 homolog p73 can mimic the transcriptional activation function of p53. We investigated whether p73, like p53, participates in an autoregulatory feedback loop with MDM2. p73 bound to MDM2 both in vivo and in vitro. Wild-type but not mutant MDM2, expressed in human p53 null osteosarcoma Saos-2 cells, inhibited p73- and p53-dependent transcription driven by the MDM2 promoter-derived p53RE motif as measured in transient-transfection and chloramphenicol acetyltransferase assays and also inhibited p73-induced apoptosis in p53-null human lung adenocarcinoma H1299 cells. MDM2 did not promote the degradation of p73 but instead disrupted the interaction of p73, but not of p53, with p300/CBP by competing with p73 for binding to the p300/CBP N terminus. Both p73α and p73β stimulated the expression of the endogenous MDM2 protein. Hence, MDM2 is transcriptionally activated by p73 and, in turn, negatively regulates the function of this activator through a mechanism distinct from that used for p53 inactivation.

ACKNOWLEDGMENTS

We thank Deborah Freedman, Lin Wu, Angie Tereskey, Arnold J. Levine, Kristen Walker, Hongwu Chen, Ronald Evans, Roland Kwok, James Lundblad, Richard Goodman, David Livingston, Zhi-Xiong Xiao, and David Lane for anti-MDM2 antibodies, MDM2 mutants, CBP plasmids, anti-Flag antibodies, Flag-p300-expressing baculovirus, p300 plasmids, MDM2 plasmids, and the Pab421 cell clone. We thank Richard Goodman, Matt Thayer, and Buddy Ullman for stimulating discussion and critically reading the manuscript.

W. G. Kaelin, Jr., is an Assistant Investigator of the Howard Hughes Medical Institute. M. Oren was supported by the Israel Science Foundation, Israel Academy of Sciences and Humanities—Centers of Excellence Program. J. Chen was supported by an NIH grant. This work was supported partly by grants to H. Lu from the American Cancer Society (RPG-98-191-01-CBE), NIH (RO1 CA 79721-01), Oregon Cancer Center, Medical Research Foundation of Oregon, and Oregon Division of American Cancer Society.