Regulation of Myc oncoprotein function by E3 ubiquitin ligases

The Myc oncoprotein/transcription factor plays an important role in controlling cell proliferation, and is deregulated in many human cancers. Myc is a short-lived protein that is turned over via the ubiquitin-proteasome pathway. The major aim of this thesis was to identify E3 ubiquitin ligases that regulate Myc turnover and/or function. Further we aimed to understand the mechanisms behind IFN-γ-induced inactivation of Myc. Through this work we identified the first E3 ligase, SCFSkp2, interacting with Myc and promoting its ubiquitylation and proteasomal turnover. Surprisingly, Skp2 promotes Myc-induced S-phase entry and is required for transcriptional activation by Myc. We show that Myc recruits Skp2 and the proteasome to target promoters and that the proteasome subunit Sug1 is required for Myc transactivation, suggesting that both Skp2 and Sug1 act as coactivators for Myc-induced transcription. We further found an interaction between Myc and the tumour suppressor/E3 ligase von Hippel Lindau (VHL) that targets hypoxia-inducible factor (HIF) for degradation. Unlike HIF, Myc binds VHL also during hypoxia through a distinct binding site. VHL is shown to play a role in ubiquitylation of Myc, but surprisingly not in degradation, thus displaying a new non-proteasomal VHL function. Further, VHL interacts with a subset of Myc target promoters and is possibly involved in regulation of certain Myc target genes. We also present results showing that Cyclin E/Cdk2 regulates the stability of Myc by phosphorylating Ser-62 and functions as a Myc cofactor, increasing Myc transactivation and stability. We further show that this phosphorylation is inhibited by the growth inhibitory cytokine IFN-γ via upregulation of the Cdk inhibitor p27, resulting in increased Myc turnover via the ubiquitin/proteasome pathway. Taken together, this thesis presents three examples of regulation of Myc ubiquitylation with different consequences for Myc function and stability. These findings emphasise the importance of Myc regulation by E3 ligases at different biological levels and could potentially be of importance for the development of novel cancer therapeutics