MDM2 promotes p21 waf1/cip1 proteasomal turnover independently of ubiquitylation

The CDK inhibitor p21 waf1/cip1 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 waf1/cip1 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 arf , and by small interfering RNA (siRNA) against MDM2. p21 waf1/cip1 bound to MDM2 in vitro and in cells. The p21 waf1/cip1 ‐binding‐defective mutant of MDM2 was unable to degrade p21 waf1/cip1 . MDM2 shortened the half‐life of both exogenous and endogenous p21 waf1/cip1 by 50% and led to the degradation of its lysine‐free mutant. Consequently, MDM2 suppressed p21 waf1/cip1 ‐induced cell growth arrest of human p53 −/− and p53 −/− /Rb −/− cells. These results demonstrate that MDM2 directly inhibits p21 waf1/cip1 function by reducing p21 waf1/cip1 stability in a ubiquitin‐independent fashion.