CCAAT/enhancer-binding protein alpha is a physiological regulator of prolactin gene expression

The sequence -101/-92 of the PRL promoter has been shown to be essential for both basal and hormone-increased PRL gene transcription. It is important to identify transcription factors that bind to this sequence if we are to understand the regulation of the PRL gene. Nuclear proteins, metabolically labeled with 35S were used in gel mobility shift experiments to examine which protein(s) binds to this region of the PRL promoter. An abundant 43-kDa protein binds to the PRL promoter at -106/-87. Two 43-kDa transcription factors were identified in cytosolic extracts of GH4 cells, CCAAT enhancer-binding protein alpha (C/EBP alpha) and cAMP response element-binding protein. Both of these bind to the PRL promoter, and both were present in GH4 cell nuclear extract, but only C/EBP alpha was definitively identified in complexes with PRL promoter DNA. Expression of C/EBP alpha increased basal PRL gene expression almost 6-fold, whereas expression of Chop10 that can act as an inhibitor of C/EBP alpha reduced the basal activity of the PRL promoter 60-75%. Mutational analysis demonstrated that the ability of C/EBP alpha to increase basal expression of the PRL promoter was dependent on the sequence -101/-92. These data suggest that C/EBP alpha is an important transcription factor that regulates PRL gene expression.