NEDD8 is an ubiquitin-like protein sharing approximately 60% amino acid identity with ubiquitin and has biological roles in cell cycle progression, viability and development. Recently, a number of oncoproteins and tumor suppressors have been identified as NEDD8 substrates, including MDM2 and p53. MDM2 is an oncogenic E3 ligase that promotes NEDD8 modification and ubiquitin-mediated degradation of the tumor suppressor transcription factor, p53. Cellular stresses such as DNA damage lead to p53 activation due, in part, to MDM2 destabilization by mechanisms that are not completely understood. Studies in mice demonstrate the biological role of MDM2 is to negatively regulate p53 function, however, when overexpressed or amplified, MDM2 has p53-independent oncogenic functions presumably due to the regulation of additional substrates. One such substrate may be the p53 family member, p73. p73 exists as multiple isoforms and accumulating evidence suggests that the N-terminal isoforms dictate its role in tumorigenesis. The full-length pro-apoptotic TAp73 isoforms are induced by chemotherapies and are able to transactivate p53-target genes to initiate cell cycle arrest and apoptosis. Conversely, the N-terminally truncated ΔNp73 isoforms lack the transactivation domain (TAD) and consequently act as dominant-negative inhibitors for all TA isoforms of the p53 family, and are overexpressed in human tumors. Here, we report that TAp73, but not ΔNp73, is covalently modified by NEDD8 in an MDM2-dependent manner, attenuating its transactivation function and promoting cytoplasmic localization of neddylated TAp73. These results provide the first evidence of a covalent post-translational modification exclusively targeting the TA isoforms of p73, and identify the MDM2-TAp73 interaction as a promising therapeutic target. We also demonstrate that the stability of MDM2 is regulated by the NEDD8 pathway and identify NEDP1 as a chemotherapy-induced isopeptidase that deneddylates MDM2, resulting in MDM2 destabilization, concomitant with p53 activation. This study identifies a novel p53 activating mechanism in response to chemotherapy. In conclusion, the work presented herein has helped characterize the function of NEDD8 modification of MDM2 and the p53 family, and identify mechanisms by which MDM2 and the NEDD8 pathway may be targeted in the development of anti-cancer therapeutics.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/26258 |
| Date | 17 February 2011 |
| Creators | Watson, Ian |
| Contributors | Irwin, Meredith, Ohh, Michael |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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