Repression of p15(INK4b) expression by Myc through association with Miz-1

Deregulated expression of c-myc can induce cell proliferation in establishe d cell lines and in primary mouse embryonic fibroblasts (MEFs), through a c ombination of both transcriptional activation and repression by Myc. Here w e show that a Myc-associated transcription factor, Miz-1, arrests cells in G1 phase and inhibits cyclin D-associated kinase activity. Miz-1 upregulate s expression of the cyclin-dependent kinases (CDK) inhibitor p15(INK4b) by binding to the initiator element of the p15(INK4b) promoter. Myc and Max fo rm a complex with Miz-1 at the p15 initiator and inhibit transcriptional ac tivation by Miz-1. Expression of Myc in primary cells inhibits the accumula tion of p15(INK4b) that is associated with cellular senescence; conversely deletion of c-myc in an established cell line activates p15(INK4b) expressi on. Alleles of c-myc that are unable to bind to Miz-1 fail to inhibit accum ulation of p15(INK4b) messenger RNA in primary cells and are, as a conseque nce, deficient in immortalization.