p21 super(WAF1/CIP1) Selectively Controls the Transcriptional Activity of Estrogen Receptor alpha

Estrogen receptors (ER) are ligand-dependent transcription factors that regulate growth, differentiation, and maintenance of cellular functions in a wide variety of tissues. We report here that p21 super(WAF1/CIP1), a cyclin- dependent kinase (Cdk) inhibitor, cooperates with CBP to regulate the ER alpha - mediated transcription of endogenous target genes in a promoter-specific manner. The estrogen-induced expression of the progesterone receptor and WISP-2 mRNA transcripts in MCF-7 cells was enhanced by p21 super(WAF1/CIP1), whereas that of the cyclin D1 mRNA was reduced and the pS2 mRNA was not affected. Chromatin immunoprecipitation assays revealed that p21 super(WAF1/CIP1) was recruited simultaneously with ER alpha and CBP to the endogenous progesterone receptor gene promoter in an estrogen-dependent manner. Experiments in which the p21 super(WAF1/CIP1) protein was knocked down by RNA interference showed that the induction of the expression of the gene encoding the progesterone receptor required p21 super(WAF1/CIP1), in contrast with that of the cyclin D1 and pS2 genes. p21 super(WAF1/CIP1) induced not only cell cycle arrest in breast cancer cells but also milk fat globule protein and lipid droplets, indicators of the differentiated phenotype, as well as cell flattening and increase of the volume of the cytoplasm. These results indicate that p21 super(WAF1/CIP1), in addition to its Cdk-regulatory role, behaves as a transcriptional coactivator in a gene- specific manner implicated in cell differentiation.