Epsilon Open Archive

Abstract

The c-Myc proto-oncogene encodes a transcription factor that controls genes involved in cell cycle regulation, cell growth, apoptosis and other cellular processes together with its partner Max. c-Myc has been shown to be altered in a wide variety of human tumors. This thesis focuses on post-translational regulation of Myc function. The aims was firstly to elucidate mechanisms of regulation of Myc degradation and components involved, and secondly to understand the mechanism(s) behind IFN-γ-induced inactivation of Myc. The c-Myc oncoprotein is short–lived phosphoprotein. We have found that Myc is tightly regulated via ubiquitin/proteasome-mediated turnover and that mutation of the Thr-58 phosphorylation site, which is frequently mutated in both in Burkitt’s lymphoma leads to stabilization of c-Myc. We have further established that the E3-ubiquitin ligase SCFSkp2 associates with c-Myc in late G1 and S-phase of the cell cycle, inducing not only ubiquitin-mediated degradation of c-Myc but surprisingly transcriptional activation of c-Myc target genes. Skp2 as well as proteasomal subunits were shown to be recruited to a Myc target promoter in vivo in a Myc-dependent manner. The second part of the thesis shows that the cytokine IFN-γ restore differentiation and cell cycle arrest in v-Myc expressing monocytic cells through inhibition of Myc-induced transcription, Myc DNA-binding and destabilization of Myc:Max heterodimers, correlating with dephosphorylation of Myc. We further found that IFN-γ reduces phosphorylation of Ser-62 and increases ubiquitin/proteasome-mediated degradation of Myc in a Ser-62 dependent manner.CycE/CDK2 was identified as a Ser-62 kinase in vivo and in vitro. IFN-γ-induced degradation and Ser-62 dephosphorylation correlated with inactivation of CDK2 through p27Kip1, which was shown to be required for this process. In conclusion, the thesis suggests that ubiquitin/proteasome-mediated turnover is an essential level of regulation of Myc function that may have important future implications for treatment of tumors with deregulated Myc expression.