Repression of transcription of the p27(Kip1) cyclin-dependent kinase inhibitor gene by c-Myc

Upon engagement of the B Cell Receptor (BCR) of WEHI 231 immature B cells, a drop in c-Myc expression is followed by activation of the cyclin-dependen t kinase inhibitor (CKI) p27(kip1), which induces growth arrest and apoptos is, Here, we report inverse patterns of p27 and c-Myc protein expression fo llow BCR engagement, We present evidence demonstrating, for the first time, that the p27(Kip1) gene is a target of transcriptional repression by c-Myc , Specifically, the changes in p27 protein levels correlated with changes i n p27 mRNA levels, and gene transcription, Induction of p27 promoter activi ty followed BCR engagement of WEHI 231 cells, and this induction could be r epressed upon co-transfection of a c-Myc expression vector. Inhibition of t he TATA-less p27 promoter by c-Myr was also observed in Jurkat T cells, vas cular smooth muscle, and Hs578T breast canter cells, extending the observat ion beyond immune cells, Consistent with putative Inr element CC (A) under bar GACC (where +1 is underlined) at the start site of transcription in the p27 promoter, deletion of Myc homology box II reduced the extent of repres sion, Furthermore, enhanced repression was observed upon transfection of th e c-Myc 'super-repressor', with mutation of Phe115 to Leu, The sequences me diating transcriptional activity and c-Myc repression were mapped to bp -20 to +20 of the p27 gene, Finally, binding of Max was shown to facilitate c- Myc binding and repression of p27 promoter activity, Overall, these studies identify the p27 CKI gene as a new target whereby c-Myc can control cell p roliferation, survival and neoplastic transformation.