Abstract 213: RNF126 E3 ubiquitin ligase targets p21cip for ubiquitin-mediated degradation

Abstract Many E3 ubiquitin ligases, such as Mdm2 and Skp2, are potential therapeutic targets for cancers. To identify novel oncogenic E3 ubiquitin ligases as anti-cancer targets, we screened an E3 ubiquitin ligase siRNA library containing siRNA pools against 555 individual E3s using the SRB assay in the MDA-MB-231 breast cancer cell line and the PC3 prostate cancer cell line. RNF126 was identified and validated as a promising candidate from this screening. Depletion of RNF126 dramatically decreased cell viability in these cancer cell lines. Consistently, RNF126 depletion delayed cell cycle G1/S progression and decreased cell proliferation. By a protein array analysis, we found that RNF126 depletion increased the cell cycle dependent kinase inhibitor p21cip protein levels in both MDA-MB-231 and PC3. Knockdown of RNF126 stabilized the p21 protein rather than increased the p21 mRNA levels. We demonstrated that RNF126 interacts with p21 in cells and in vitro, and the protein-protein interaction depended on the N-terminal and middle parts of RNF126. RNF126 over-expression increased the p21 protein ubiquitination in an E3 ligase activity dependent manner. RNF126 knockdown induced loss of cell viability in MDA-MB-231 and PC-3 can be partially rescued by depletion of p21. RNF126 stable knockdown in SW527 inhibited tumor growth in SCID mice. Finally, we found that RNF126 is highly expressed in ER negative breast cancer cell lines and negatively correlated with the p21 expression levels. These findings suggest that RNF126 promotes cancer cell proliferation through targeting p21 for ubiquitin-mediated degradation. RNF126 could be a novel therapeutic target in breast and prostate cancers. This study was partially supported by a grant from the American Cancer Society (RSG-08-199-01), grants from National Nature Science Foundation of China (81072162, 81120108019, and U1132605), and a grant from Yunnan Province High-Profile Talents Program (2010CI114). Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 213. doi:1538-7445.AM2012-213