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Yeast As a Chassis for Developing Functional Assays to Study Human P53
Published on: August 4, 2019
Regulation of the p53 tumor suppressor pathway: the problems and promises of studying Mdm2's E3 ligase function
Hilary V Clegg1, Yanping Zhang
1Lineberger Comprehensive Cancer Center, Curriculum in Genetics and Molecular Biology, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, NC 27514, USA.
Abstract:
Mdm2 is a major negative regulator of the tumor suppressor p53 and has long been thought to inhibit p53 in two ways: by ubiquitinating p53 to signal for its degradation, and by binding to p53, masking its transactivation domain. Mdm2 is also believed to control its own levels by autoubiquitination. Despite the widespread acceptance of these hypotheses, the supporting data were drawn primarily from in vitro and ectopic expression studies, which have not always been corroborated when tested in the more physiologically relevant setting of a knock-in or knock-out mouse model. Recently, a mouse model was generated in which a single point mutation (C462A) in Mdm2s RING domain abrogated Mdm2s E3 activity while leaving Mdm2-p53 binding intact. This study called into question two major dogmas about Mdm2 by suggesting that when endogenously expressed, (1) Mdm2 cannot inhibit p53 sufficiently by binding without ubiquitination, and (2) Mdm2 may not be regulated by autoubiquitination. Two years later, we are still without definitive answers for why these results conflict with previous findings, but we have gained new insights from subsequent studies. Here, we discuss potential reasons for the discrepancies concerning Mdm2s functions and how they might be resolved, taking into account new research in the field.
Insights
The E3 ligase activity of Mdm2 is crucial for inhibiting the tumor suppressor p53. New research suggests Mdm2 may not regulate p53 or itself through ubiquitination as previously believed.
Area of Science:
- Molecular Biology
- Cancer Biology
- Biochemistry
Background:
- Mdm2 is a key negative regulator of the tumor suppressor p53.
- Mdm2 is thought to inhibit p53 via ubiquitination and by masking its transactivation domain.
- Mdm2 is also believed to regulate its own levels through autoubiquitination.
Purpose of the Study:
- To investigate the role of Mdm2's E3 ligase activity in p53 regulation.
- To re-evaluate established models of Mdm2 function in a physiological context.
- To reconcile conflicting data regarding Mdm2's mechanisms of action.
Main Methods:
- Utilized a knock-in mouse model with a mutated Mdm2 RING domain (C462A) to abrogate E3 activity.
- Assessed Mdm2-p53 binding and p53 ubiquitination in the absence of Mdm2's E3 ligase function.
- Analyzed Mdm2 autoubiquitination in the context of impaired E3 activity.
Main Results:
- Ablation of Mdm2's E3 activity via the C462A mutation did not abolish Mdm2-p53 binding.
- Endogenous Mdm2 with abrogated E3 activity showed reduced capacity to inhibit p53.
- Evidence suggests Mdm2 may not be regulated by autoubiquitination.
Conclusions:
- Mdm2's E3 ligase activity is essential for its robust inhibition of p53.
- The canonical model of Mdm2 function requires re-evaluation in light of endogenous studies.
- Discrepancies highlight the importance of physiological context in understanding Mdm2's role in cancer.
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