Abstract 3107: Novel molecular mechanisms of MDM2-induced p21Waf1 degradation

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC The MDM2 oncogene is overexpressed in many human cancers, and the tumorigenicity of MDM2 has mainly been attributed to the auto-regulatory loop it forms with p53. However, there is increasing evidence that the tumorigenicity of MDM2 is also dependent upon its p53-independent activities. One of the p53-independent functions of MDM2 is its induction of the ubiquitin-independent proteasomal degradation of p21Waf1. The present study was designed to characterize the mechanism(s) by which MDM2 induces p21Waf1 degradation in the p53-independent setting. We determined the regions of MDM2 required for p21Waf1 degradation using pull-down assays and Western blotting, and then examined the mechanisms using limited proteolysis and fluorescence resonance energy transfer (FRET). We demonstrated that the MDM2-p21Waf1 interaction depends on MDM2's central domain, and that nuclear localization of both proteins is necessary for p21Waf1 degradation. Specifically, amino acids 226-250 of MDM2 are required for p21Waf1 binding and degradation, and amino acids 251-260 are necessary for p21Waf1 degradation. The latter region induces a conformation change in p21Waf1, increasing its binding to the C8 subunit of the proteasome, leading to its degradation. When MDM2 lacked either segment (aa 226-250 or aa 251-260), it lost its capacity to promote p21Waf1 degradation and cell cycle progression. In summary, our results demonstrate a novel mechanism by which MDM2 induces the ubiquitin-independent proteasomal degradation of an intact protein. This previously unknown chaperone-like function of MDM2 induces a conformational change in p21Waf1, leading to its increased binding to the C8 subunit of the proteasome to induce its subsequent degradation. It is possible that this mechanism may also be active for other substrates. Thus, the present study provides further evidence supporting the importance of the p53-independent activities of MDM2, confirming the significance of MDM2 as a target for cancer therapy. (Supported by NIH/NCI grants R01 CA 112029 and R01CA121211.) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3107.