MDM2 promotes p21<sup>waf1/cip1</sup> proteasomal turnover independently of ubiquitylation
Abstract
The CDK inhibitor p21 is degraded by a ubiquitin-independent proteolytic pathway. Here, we show that MDM2 mediates this degradation process. Overexpression of wild-type or ring finger-deleted, but not nuclear localization signal (NLS)-deleted, MDM2 decreased p21 levels without ubiquitylating this protein and affecting its mRNA level in p53 cells. This decrease was reversed by the proteasome inhibitors MG132 and lactacystin, by p19, and by small interfering RNA (siRNA) against MDM2. p21 bound to MDM2 in vitro and in cells. The p21-binding-defective mutant of MDM2 was unable to degrade p21. MDM2 shortened the half-life of both exogenous and endogenous p21 by 50% and led to the degradation of its lysine-free mutant. Consequently, MDM2 suppressed p21-induced cell growth arrest of human p53 and p53/Rbcells. These results demonstrate that MDM2 directly inhibits p21 function by reducing p21 stability in a ubiquitin-independent fashion.
Acknowledgements
We thank David M.Keller and Jayme Gallegos for proofreading this manuscript. We also thank Guillinao Lozano, Wafik S.el-Deiry, Jiandong Chen, Charles J.Sherr, Yue Xiong, Bruce E.Clurman and Anindya Dutta for generously providing us with cell lines, antibodies and plasmids. This work is supported by grants to H.L. from the NIH ({"type":"entrez-nucleotide","attrs":{"text":"CA095441","term_id":"34948748","term_text":"CA095441"}}CA095441, CA93614 and {"type":"entrez-nucleotide","attrs":{"text":"CA079721","term_id":"34931993","term_text":"CA079721"}}CA079721).