Kontrolle der Myc-abhängigen Tumorgenese durch das Ubiquitin-Proteasom-System: Regulation von Myc durch die Ubiquitinligase Trim33
Zusammenfassung der Projektergebnisse
The MYC proto-oncogene encodes a short-lived transcription factor, which regulates multiple biological processes by modulating expression of a broad range of genes. In tumor cells, Myc protein levels are often upregulated by alterations in the upstream regulatory pathways or mutations in the MYC gene itself. In this project, we have studied ubiquitindependent mechanisms, which regulate proteolysis and transcriptional function of Myc. First, we have shown that Fbw7, a tumor suppressor F-box protein, which targets Myc for degradation, is regulated by PI3K-dependent phosphorylation. Phosphorylation of Fbw7 is required for its full activity towards Myc and other substrates, but inhibits autocatalytic ubiquitin transfer, suggesting that the balance between these antagonistic activities is important for Fbw7 function. Using gene targeting in mice, we found that both autocatalytic and substrate-directed ubiquitination by Fbw7 are antagonized by the Usp28 deubiquitinase. Usp28 preferentially counteracts ubiquitination of Fbw7, and loss of Usp28 triggers Fbw7 turnover, leading to the accumulation of Fbw7 substrates. This effect is found in several tissues, including lung, liver and pancreas. In contrast, in the intestine Fbw7 remains stable upon loss of Usp28, promoting Fbw7 substrate degradation. Consistently, Usp28 deletion attenuates Apcmindependent intestinal tumorigenesis. Usp28 is overexpressed in human colorectal cancers, but is deleted or mutated in tumors of other tissues, including skin and prostate. Our findings predict that deletions of Usp28 in these tissues may functionally mimic Fbw7 lossof-function and promote tumorigenesis, and that Usp28 may, therefore, function as a tissue-specific tumor promoter or suppressor. Further, we found that Fbw7-mediated degradation of Myc in S phase of the cell cycle is attenuated via alternative ubiquitination by the SCF(β-Trcp) ubiqutiin ligase. SCF(β-Trcp) assembles degradation-incompetent Lys63-linked chains on Myc and stabilizes it during recovery from replication stress, allowing efficient mitotic entry. Finally, we are studying the direct role of ubiquitination in the transcriptional function of Myc. Ubiquitination is not required for association of Myc with chromatin and does not affect polymerase recruitment to the promoters of target genes. However, ubiquitinationdeficient Myc mutants fail to promote the release of elongation-competent polymerase. Whereas inactivation of individual ligases does not strongly affect Myc-dependent transcription, degradation by the proteasome is essential for efficient gene regulation by Myc. We propose that redundant ubiquitin ligase activities drive proteasomal turnover of Myc, which is specifically required to promote elongation by PolII. Together, our data reveal a surprising bifunctional role of proteasomal degradation in controlling cellular Myc function − it maintains Myc levels below an oncogenic threshold, but is also essential for its transcriptional activity.
Projektbezogene Publikationen (Auswahl)
- (2010). Ubiquitination of the Myc amino-terminus by beta-TrCP antagonizes Fbw7-mediated degradation. Nature Cell Biology, 12, 973-81
Popov, N., Schülein, C., Jaenicke, L. and Eilers, M.
(Siehe online unter https://doi.org/10.1038/ncb2104) - (2011) PI3K-dependent phosphorylation of Fbw7 modulates substrate degradation and activity. FEBS Letters, 585: 2151–2157
Schuelein, C., Eilers, M., Popov, N.
(Siehe online unter https://doi.org/10.1016/j.febslet.2011.05.036) - (2013). Stabilization of Myc through heterotypic poly-ubiquitination by mLANA is critical for γ-herpesvirus lymphoproliferation. PLoS Pathog. 9, e1003554
Rodrigues, L., Popov, N., Kaye, K.M., Simas, J.P.
(Siehe online unter https://doi.org/10.1371/journal.ppat.1003554) - (2014) Dual regulation of Fbw7 function and oncogenic transformation by Usp28. Cell Rep, 9: 1099–1109
Schülein-Völk, C., Wolf, E., Zhu, J., Xu, W., Taranets, L., Hellmann, A., Diefenbacher, M., Behrens, A., Eilers, M., Popov, N.
(Siehe online unter https://doi.org/10.1016/j.celrep.2014.09.057) - (2014) The deubiquitinase USP28 controls intestinal homeostasis and promotes colorectal cancer. J Clin Invest, 124: 3407-18
Diefenbacher, M., Popov, N., Blake, S.M, Schülein-Völk, C., Nye, E., Spencer-Dene, B., Jaenicke, L., Eilers, M., Behrens, A.
(Siehe online unter https://doi.org/10.1172/JCI73733)
