The ubiquitin–proteasome system regulates p53-mediated transcription at p21waf1 promoter

A high-content chemical screen identifies ellipticine as a modulator of p53 nuclear localization

p53 regulates apoptosis and the cell cycle through actions in the nucleus and cytoplasm. Altering the subcellular localization of p53 can alter its biological function. Therefore, small molecules that change the localization of p53 would be useful chemical probes to understand the influence of subcellular localization on the function of p53. To identify such molecules, a high-content screen for compounds that increased the localization of p53 to the nucleus or cytoplasm was developed, automated, and conducted. With this image-based assay, we identified ellipticine that increased the nuclear localization of GFP-mutant p53 protein but not GFP alone in Saos-2 osteosarcoma cells. In addition, ellipticine increased the nuclear localization of endogenous p53 in HCT116 colon cancer cells with a resultant increase in the transactivation of the p21 promoter. Increased nuclear p53 after ellipticine treatment was not associated with an increase in DNA double stranded breaks, indicating that ellipticine shifts p53 to the nucleus through a mechanism independent of DNA damage. Thus, a chemical biology approach has identified a molecule that shifts the localization of p53 and enhances its nuclear activity.

MDM2 chaperones the p53 tumor suppressor

The murine double minute (mdm2) gene encodes an E3 ubiquitin ligase that plays a key role in the degradation of p53 tumor suppressor protein. Nevertheless recent data highlight other p53-independent functions of MDM2. Given that MDM2 protein binds ATP, can interact with the Hsp90 chaperone, plays a role in the modulation of transcription factors and protection and activation of DNA polymerases, and is involved in ribosome assembly and nascent p53 protein biosynthesis, we have evaluated and found MDM2 protein to possess an intrinsic molecular chaperone activity. MDM2 can substitute for the Hsp90 molecular chaperone in promoting binding of p53 to the p21-derived promoter sequence. This reaction is driven by recycling of MDM2 from the p53 complex, triggered by binding of ATP to MDM2. The ATP binding mutant MDM2 protein (K454A) lacks the chaperone activity both in vivo and in vitro. Mdm2 cotransfected in the H1299 cell line with wild-type p53 stimulates efficient p53 folding in vivo but at the same time accelerates the degradation of p53. MDM2 in which one of the Zn(2+) coordinating residues is mutated (C478S or C464A) blocks degradation but enhances folding of p53. This is the first demonstration that MDM2 possesses an intrinsic molecular chaperone activity, indicating that the ATP binding function of MDM2 can mediate its chaperone function toward the p53 tumor suppressor.

Ubiquitin- and ATP-independent proteolytic turnover of p21 by the REGgamma-proteasome pathway

We previously demonstrated that the proteasome activator REGgamma directs degradation of the steroid receptor coactivator SRC-3 by the 20S proteasome in an ATP- and ubiquitin-independent manner. Our efforts to identify additional endogenous direct targets of the REGgamma proteasome revealed that p21(Waf/Cip1), a central cyclin-dependent kinase inhibitor, is another endogenous target. Gain-of-function analysis, RNAi knockdown, REGgamma-deficient MEF analysis, and pulse-chase experiments substantiate that REGgamma promotes degradation of unbound p21. Cell-free proteasome proteolysis assays using purified REGgamma, p21, and the 20S proteasome confirm that REGgamma directly mediates degradation of free p21 in an ATP- and ubiquitin-independent manner. Depletion of REGgamma in a thyroid carcinoma cell line results in cell-cycle and proliferative alterations. Our study reveals that, in addition to degrading the SRC-3 growth coactivator, REGgamma also has a role in the regulation of the cell cycle through its ability to influence the level of a cell-cycle regulator(s).