Epsilon Open Archive

Abstract

The Myc/Max/Mad transcription factor network regulates a large number of genes involved in cell growth, differentiation and apoptosis. Myc activates and Mad represses and overlapping set of growth-promoting genes, and the network therefore seems to act as a molecular switch between growth and differentiation. Deregulation of myc–family genes occurs frequently in human cancers and is often associated with poor prognosis. One aim of this thesis was to identify negative growth signals affecting the expression and/or activity of N-Myc- and Mad-family in neuroblastomas. Our results show that Mad1 was upregulated during phorbol ester (TPA)-induced neuronal differentiation of SH-SY5Y human neuroblastoma cells lacking N-myc amplification, whereas this response was largely inhibited in N-myc-amplified neuroblastoma cells. However, interferon-γ (IFN-γ) in combination with retinoic acid (RA) or TPA strongly enhanced differentiation and growth arrest in the latter. This was accompanied by reduced N-Myc and/or upregulated Mad1 expression, resulting in changed occupancy at the ODC target gene promoter in vivo towards Mad1/Max predominance, and repression of N-Myc/Mad1 target genes. This suggests that combined RA+IFN-γ “differentiation therapy” may potentially be of clinical interest in neuroblastoma treatment. Addressing the role of Mad1 during hematopoietic differentiation, we show that enforced Mad1 expression in U-937 monoblasts by an inducible expression system led to inhibited cell growth and proliferation. However, Mad1 did not induce or enhance differentiation in the absence or presence of differentiation signals. Surprisingly, Mad1 stimulated TNF-α-, but not Fas-induced apoptosis in U-937 cells. Finally, addressing the still unclear mechanism by which c-Myc represses transcription, our results showed that differentiation-induced transcription of the cyclin-dependent kinase inhibitor p21Cip1 gene was repressed by c-Myc through interaction with the Zn-finger protein Miz-1 in vivo at the p21 core promoter. c-Myc seems to switch Miz-1 function from an activator to a repressor, thereby inhibiting cell cycle exit and differentiation.