Epsilon Open Archive

Abstract

The myc-family of proto-oncogenes (c-, N- and L-myc) encodes transcription factors of the basic region helix-loop-helix leucine zipper (bHLHZip) family and plays important roles in cell proliferation, metabolism, apoptosis and many other fundamental cellular processes. myc-family genes are frequently deregulated in cancer, often correlating with poor prognosis. c-Myc is estimated to regulate 15% of all genes. This thesis focuses firstly on the mechanisms behind posttranslational negative regulation of Myc activities induced by growth inhibitory signals, secondly on the regulation of transcription of genes involved in G1/S transition during cell cycle by Myc, and thirdly on characterization of myc-related bHLH genes in Arabidopsis. Our results show that the cytokine interferon (IFN)-γ induces ubiquitylation and degradation of Myc-family proteins in N-myc-amplified neuroblastoma cells and in Myc-transformed monocytic cells and induces expression of the Myc antagonist Mad1. IFN-γ-induced turnover of Myc involves reduced phosphorylation of Ser-62 through inhibition of cyclin E/ cyclin-dependent kinase 2 (Cdk2), which was identified as a Ser-62 kinase. The mechanism behind this inhibition was shown to be IFN-γ-induced expression of the Cdk-inhibitor p27Kip1. p27Kip1 was further demonstrated to target the Myc/cyclin E/Cdk2-complex directly at Myc target promoters, resulting in transcriptional repression of Myc target genes. This study also shows that during cell cycle progression, Myc associates with S-phase genes already in early G1 but cannot activate those genes until in late G1/S. This correlated with binding of the E3 ubiquitin ligase and cofactor Skp2 to these promoters. We propose that Skp2 and cyclin E/Cdk2 cooperate in this regulation. Taken together, this thesis suggests that Myc intimately interact with cyclinE/Cdk2, p27Kip1, and Skp2 in the control of the cell cycle genes, which might provide potential new strategies for cancer therapy. Our study in the plant Arabidopsis thaliana (At)characterized 84 myc-related At bHLH genes. Expression profiling of these genes in proliferative tissues showed that 15 of the bHLH genes were expressed in a cell cycle regulated manner, suggesting that they may play a role in cell division in plants.