Charakterisierung der Myc-Funktion durch Miz-1 interaktionsdefiziente Myc-Mutanten

C-myc is a transcription factor of the helix-loop-helix/leuzinzipper-family. It can both activate as well as repress gene expression. More over, it plays a key role in the induction and repression of proliferation, apoptosis and differentiation. For activation of gene transcription a dimerization with its partner protein Max is required. This allows the binding of the Myc-Max-heterodimer or -tetramer to the so called E-box element in the promoter. Various Myc-binding proteins have been found, which can form a complex with Myc for gene repression. One of these proteins is the formally described Myc interacting zinc finger protein 1, abbreviated as Miz-1. This transcription factor activates the cell cycle inhibitor p15ink4b via a TGF-ß driven pathway. Myc on the other side represses Miz-1 by using the cofactor p300 the same binding site on which Miz-1 competes. Miz-1 as well as Max bind to the HLH domain of Myc and is located in a tertiary complex with Myc and Max. The purpose of this dissertation was to contribute to the clarification of the repression of Myc by the Myc-Miz-1-complex in the biology of the cell. Therefor two forms of the HLH domain mutated alleles of c-myc were applied: MycV394D and MycS405F, which have been biologically characterized in this work. The biochemically Miz-1-interaction deficient mutants were analyzed in terms of their activating function with Max. They showed the same effect on cell division, cell cycle distribution and growth behavior in the biological assay as Mycwt (wild type). In the next step the mutations were compared in Mefp53+/- cells due to differences in the influence to cellular aging (senescence). MycV394D showed a delay of cell aging, which could not be explained by a different induction of p19ARF or p53. To identify potential target genes of Miz-1, Mycwt and MycV394D infected Mefp53+/- have been compared by a microarray analysis. While the genes inducted by Myc were regulated by Mycwt and MycV394D in the same way, a number of genes, which play a role in proliferation, adhesion and transcription, showed a higher expression in MycV394D-infected than in Mycwt-infected cells. Three of these genes identified, namely p21Cip1, p57Kip2 and C/EBP-alpha, were later considered as a direct target genes of Miz-1. In the work of Herold et al. (2002), in which the results of 77 this dissertation were published, Miz-1 could be identified by the use of MycV394D also as an essential protein of UV-induced p21Cip1 expression. After all it can be suggest, that Myc proteins have integrated and conserved at least two distinct biochemical properties: the ability to activate through Max and the ability to repress through Miz-1. The precise importance of the complex formation between Myc and Miz-1 in tumorigenesis can in future be addressed with appropriate transgenic and knockin models making use of the MycV394D mutant.