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 ubiquity-lating 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^-/- R^b-/-cells. These results demonstrate that MDM2 directly inhibits p21^waf1/cip1 function by reducing p21 ^waf1/cip1 stability in a ubiquitin-independent fashion.