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A TSG101/MDM2 regulatory loop modulates MDM2 degradation and MDM2/p53 feedback control
1Department of Genetics, Stanford University School of Medicine, Stanford, CA 94305, USA.
Abstract:
The p53 tumor suppressor protein and the MDM2 oncoprotein form a feedback-control loop that up-regulates cellular MDM2 production, blocks p53 activity, and promotes p53 decay. tsg101 was discovered as a gene whose deficiency results in neoplastic transformation of NIH 3T3 cells and the ability to generate metastatic tumors in nude mice. Its protein product contains a domain, Ubc, characteristic of the catalytic domain of ubiquitin conjugase (E2) enzymes but lacking an active-site cysteine crucial for ubiquitin conjugase activity. Here we report that TSG101 participates with MDM2 in an autoregulatory loop that modulates the cellular levels of both proteins, and also of p53, by affecting protein decay. We show that the Ubc domain of TSG101 interferes with ubiquitination of MDM2, that TSG101 inhibits MDM2 decay and elevates its steady-state level, and that these events are associated with down-regulation of p53 protein. Conversely, pulse-chase and Western blot experiments in wild-type and mutant fibroblasts indicate that elevation of MDM2 by overexpression of wild-type p53, by amplification of the endogenous MDM2 gene, or by transfection of MDM2-expressing constructs promotes TSG101 loss, which we show occurs by 26S proteasome-dependent decay. Our results identify TSG101 as both a regulator of, and target of, MDM2/p53 circuitry.
Insights
The tumor suppressor TSG101 protein regulates MDM2 and p53 levels by affecting protein decay. TSG101 interferes with MDM2 ubiquitination, stabilizing MDM2 and decreasing p53, while MDM2 promotes TSG101 decay.
Area of Science:
- Molecular and Cellular Biology
- Oncology
- Protein Regulation
Background:
- The p53 tumor suppressor and MDM2 oncoprotein form a feedback loop regulating p53 activity and decay.
- TSG101 deficiency leads to neoplastic transformation and metastasis.
- TSG101 contains a Ubc domain characteristic of ubiquitin conjugase enzymes.
Purpose of the Study:
- To investigate the role of TSG101 in the MDM2/p53 regulatory circuitry.
- To elucidate the mechanism by which TSG101 modulates MDM2 and p53 protein levels.
Main Methods:
- Analysis of protein decay using pulse-chase experiments.
- Western blot analysis to quantify protein levels.
- Investigation of ubiquitination processes and proteasomal degradation.
Main Results:
- TSG101's Ubc domain inhibits MDM2 ubiquitination, stabilizing MDM2 and reducing p53 levels.
- Elevated MDM2 levels promote TSG101 degradation via the 26S proteasome.
- TSG101 acts as both a regulator and a target within the MDM2/p53 pathway.
Conclusions:
- TSG101 participates in an autoregulatory loop with MDM2, influencing the stability of both proteins and p53.
- This interaction highlights a novel mechanism of protein decay regulation in the context of cancer-related proteins.
- TSG101 is a key component in the complex interplay governing MDM2 and p53 homeostasis.