Involvement of the ubiquitous Oct-1 transcription factor in hormonal induction of β-casein gene expression

Transcription of the milk protein beta-case in gene is induced by the lactogenic hormones Prl (prolactin) and glucocorticoids. Multiple transcription factors involved in this induction have been identified, including the STAT5 (signal transducer and activator of transcription 5) and the GR (glucocorticoid receptor). Our previous studies have identified a binding site for the ubiquitous Oct-1 (octamer-binding transcription factor 1) protein in the lactogenic hormonal regulatory region of the mouse beta-casein promoter. In the present study, we report that Oct-1 is indeed expressed and binds to the beta-casein promoter in mammary epithelial cells. Oct-1 activates hormonally induced beta-casein promoter activity in a dose-dependent manner. Hormonal induction of promoter activity was decreased not only by mutating the Oct-1 binding site from ATTAGCAT to GCTAGCAT, which abolishes Oct-1 binding (50% decrease, P < 0.01), but also by changing the site to the consensus Oct-1-binding motif ATTTGCAT (40% decrease, P < 0.01). Reversing the Oct-1-binding site reduced hormonal induction by 70% (P < 0.01), showing that orientation of Oct-1 binding is also critical in hormonal action. In transient transfection experiments, Oct-1 cottaboratively transactivated the beta-casein gene promoter with STAT5 and/or GR in the presence of Prl receptor in cells treated with the lactogernic hormones. The C-terminus of Oct-1 was not essential to its function. The results of the present study provide biochemical evidence that the ubiquitous Oct-1 transcription factor may be involved in hormonally regulated, tissue-specific beta-casein gene expression.