Zhu, JidongBlenis, JohnYuan, Junying2019-10-052008Zhu, J., J. Blenis, and J. Yuan. 2008. “Activation of PI3K/Akt and MAPK Pathways Regulates Myc-Mediated Transcription by Phosphorylating and Promoting the Degradation of Mad1.” Proceedings of the National Academy of Sciences 105 (18): 6584–89. https://doi.org/10.1073/pnas.0802785105.0027-84240744-28311091-6490http://nrs.harvard.edu/urn-3:HUL.InstRepos:41483478Mad1, a member of the Myc/Max/Mad family, suppresses Myc-mediated transcriptional activity by competing with Myc for heterodimerization with its obligatory partner, Max. The expression of Mad1 suppresses Myc-mediated cell proliferation and transformation. The levels of Mad1 protein are generally low in many human cancers, and Mad1 protein has a very short half-life. However, the mechanism that regulates the turnover of Mad1 protein is poorly understood. In this study, we showed that Mad1 is a substrate of p90 ribosomal kinase (RSK) and p70 S6 kinase (S6K). Both RSK and S6K phosphorylate serine 145 of Mad1 upon serum or insulin stimulation. Ser-145 phosphorylation of Mad1 accelerates the ubiquitination and degradation of Mad1 through the 26S proteasome pathway, which in turn promotes the transcriptional activity of Myc. Our study provides a direct link between the growth factor signaling pathways regulated by PI3 kinase/Akt and MAP kinases with Myc-mediated transcription.en-USJournal Article2019-10-0510.1073/pnas.0802785105