Mechanisms of c-myc-mediated transcriptional repression of growth arrest genes.

Constitutive expression of the proto-oncogene c-myc results in oncogenic activation and contributes to progression of a wide range of human and animal tumors. Myc executes its multiple activities mostly through transcriptional regulation of the target genes. The special interest of this review is the mechanism of transcriptional repression of cell cycle inhibitors by Myc. Myc suppresses expression of cell cycle/growth arrest genes gas1, p15, p21, p27, and gadd34, -45, and -153. It appears that Myc represses growth arrest gene transcription by at least two distinct mechanisms. One mechanism is limited to the binding of Myc-Max heterodimers to the Inr element in their promoters and inhibition of Miz-1 or other transcriptional activators via the C-terminal domain of c-Myc. This mechanism requires DNA binding of the Myc-Max complex to Inr sequences. The other mechanism is dependent on c-Myc binding to the Sp1 transcription factor via the c-Myc central region and inhibition of Sp1 transcriptional activity. At this time it is not entirely clear which Sp1-containing promoters will be repressed by c-Myc and what other modes of c-Myc transcriptional repression may exist. The ability of c-Myc to repress transcription of growth arrest genes may contribute to its potential to promote proliferation and oncogenesis.