Targeting of Miz-1 is essential for Myc-mediated apoptosis

The c-Myc oncoprotein plays a central role in human cancer via its ability to either activate or repress the transcription of essential downstream targets. For many of the repressed target genes, down-regulation by c-Myc relies on its ability to bind and inactivate the transcription factor Miz-1. Although Miz-1 inactivation is suspected to be essential for at least some of the biological activities of c-Myc, it has been difficult to demonstrate this requirement experimentally. Using a combination of short hairpin RNA-mediated knockdown and a previously characterized mutant of c-Myc that is defective for Miz-1 inactivation, we examined whether this inactivation is critical for three of the most central biological functions of c-Myc, cell cycle progression, transformation, and apoptosis. The results of this analysis demonstrated that in the in vitro assays utilized here, Miz-1 inactivation is dispensable for c-Myc-induced cell cycle progression and transformation. In marked contrast, the ability of c-Myc to induce apoptosis in primary diploid human fibroblasts in response to growth factor withdrawal is entirely dependent on its ability to inactivate Miz-1. These data have a significant impact on our understanding of the biochemical mechanisms dictating how c-Myc mediates opposing biological functions, such as transformation and apoptosis, and demonstrate the first requirement for Miz-1 inactivation in any of the biological functions of c-Myc.