The transcriptional function of the c-Myc oncoprotein and its regulation by the ubiquitin/proteasome pathway

The c-Myc proto-oncogene encodes a short lived-transcription factor that plays an important role in cellular proliferation, growth and apoptosis. c-myc is often rearranged in tumours resulting in deregulated expression. c-Myc both activates and represses transcription of a number of target genes. This thesis focuses firstly on the mechanism by which c-Myc represses differentiation-induced genes and secondly on the regulation of c-Myc by ubiquitin/proteasome mediated turnover and its consequences for c-Myc function in transcription. Our results show that differentiation-induced expression of the cyclin-dependent kinase inhibitor (CKI) p21Cip1 is repressed by Myc at the level of transcription. Myc was shown to repress the p21Cip1 core promoter by direct interaction with the initiator binding protein Miz-1. The rapid turnover of c-Myc is shown to be mediated by the ubiquitin/proteasome pathway and we have identified the phosphorylation site Thr58, which is frequently mutated in Burkitt’s lymphoma, as an important recognition site for this process. As a result of Thr58 mutation, c-Myc escapes this regulation which results in Myc protein accumulation. We further show that the E3 ubiquitin ligase SCFSkp2 interacts with Myc during G1-S phase transition of the cell cycle and promotes its ubiquitylation and proteasomal degradation. Surprisingly, Skp2 promotes c-Myc-induced S-phase transition and is required for transcriptional activation by Myc. Moreover our data suggest that Skp2 and components of the proteasome is recruited by c-Myc target gene promoters in conjunction with protein ubiquitylation. These results suggest that Skp2 is a transcriptional cofactor for c-Myc. The thesis suggests an important role for c-Myc at the G1/S transition by transcriptional repression of the CKI p21Cip1 and by stimulation of cell cycle genes via Skp2 coactivator function. The thesis also sheds light on the regulation of c-Myc turnover and suggests an important interdependence between transcription and ubiquitylation.