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So far, the oncogenic potential of Myc that contributes to the genesis of a broad spectrum of malignant tumors has been mainly ascribed to the activation of target gene expression in a complex with Max. However, current studies suggest an important functional role of the c-Myc/Miz1 interaction mediating transcripional repression. Therefore, my thesis aimed at investigating the interference of the c-Myc/Miz1 complex with the UVB induced DNA damage response and the oncogene-induced apoptosis via the tumor suppressor Arf. Initially, I used murine, embryonic fibroblasts expressing a truncated version of the transcription factor Miz1, Miz1ΔPOZ, and a RNAi approach to study the importance of Miz1 as an activator of Cdkn1a expression. These experiments showed that Miz1 is not essential for the expression of the cell cycle inhibitor p21Cip1 in the cellular system used. However, Miz1 exerts a non-transcriptional function during the ATR-dependent signaling cascade in a human cell system. This is due to the stabilization of the mediator protein TopBP1 und the subsequent activation of this pathway. This function of Miz1 depends on the recruitment of a fraction of TopBP1 to chromatin resulting in the protection of TopBP1 from HectH9-mediated ubiquitination and subsequent proteasomal degradation. c-Myc antagonizes this stabilizing interaction of Miz1 and TopBP1. Overexpression of c-Myc results in the dissociation of TopBP1 from chromatin and its subsequent degradation. Consequently, increased levels of c-Myc block the activation of the UVB induced signal transduction and, therefore, possibly the repair of DNA damage. Furthermore, I was able to demonstrate a role of Miz1, c-Myc and the tumor suppressor Arf in preventing oncogenic transformation. Miz1, c-Myc and Arf assemble into a DNA-binding complex that silences genes by disrupting the interaction of Miz1 with its coactivator nucleophosmin and by the formation of heterochromatin at the promotors of target genes. In addition, association with Arf and Myc triggers the post-translational modification of Miz1 by the ubiquitin-like molecule SUMO. As c-Myc harbors a potential SUMO-binding motif and binds SUMO moieties in vitro, the SUMOylation of Miz1 as well as the association of the SUMOylated Miz1 protein with c-Myc may be involved in the maintenance of the repressive chromatin state. Beside the cell cycle inhibitor P15INK4B, this complex also switches off many genes associated with cell-cell and cell-matrix adhesions, causing cells expressing Miz1, c-Myc and Arf to lose their attachment and undergo anoikis. The assembly of this repressive complex relies on the interaction of both Arf and Miz1 with the transcription factor c-Myc, suggesting that c-Myc levels may be crucial in switching Arf’s function from irreversible cell cycle arrest to cell death. Therefore, this way of action of the c-Myc/Miz1 complex points to a tumor protective mechansim that facilitates the eliminiation of cells harboring an oncogenic mutation.