Cooperative effects of STAT5 (signal transducer and activator of transcription 5) and C/EBP β (CCAAT/enhancer-binding protein-β) on β-casein gene transcription are mediated by the glucocorticoid receptor

beta -Casein gene transcription is controlled primarily by a composite response element (CoRE) that integrates signaling from the lactogenic hormones, PRL, insulin, and hydrocortisone, in mammary epithelial cells. This CoRE contains binding sites for STAT5 (signal transducer and activator of transcription 5) and C/EBP beta (CCAAT/enhancer-binding protein-beta) and several half-sites for glucocorticoid receptor (GR). To examine how interactions among these three transcription factors might regulate beta -casein gene transcription, a COS cell reconstitution system was employed. Cooperative transactivation was observed when all three factors were expressed, but unexpectedly was not seen between STATE and C/EBP beta in the absence of full-length, transcriptionally active GR. Cooperativity required the amino-terminal transactivation domain of C/EBP beta, and neither C/EBP alpha nor C/EBP delta was able to substitute for C/EBP beta when cotransfected with STAT5 and GR. Different GR determinants were needed for transcriptional cooperation between STAT5 and GR as compared with those required for all three transcription factors. These studies provide some new insights into the mechanisms responsible for high level, tissue-specific expression conferred by the beta -casein CoRE.