Transcriptional induction of the human prolactin gene by cAMP requires two cis-acting elements and at least the pituitary-specific factor Pit-1

To identify the cis-acting elements responsible for cAMP stimulation of human prolactin (hPRL) promoter activity, pituitary GC cells were transfected with 5'-deleted hPRL promoters fused to the chloramphenicol acetyltransferase reporter gene. The proximal regulatory region (coordinates -250 to -42) was sufficient to confer strong cAMP stimulation (+/- 25 fold). Further 5' and 3' deletions performed within this proximal region demonstrated that two types of cis-acting elements are involved in the cAMP regulation: (i) the binding sites of the pituitary-specific factor Pit-1, and (ii) the sequence between coordinates -115 and -85 (named fragment A), which contains a TGACG motif. We show by gel-shift and Southwestern experiments that fragment A binds Pit-1 monomer and also a ubiquitous factor that is neither cAMP-responsive element-binding protein nor activator protein-1. Strong cAMP induction was observed when fragment A was juxtaposed to a Pit-1 binding site. That Pit-1 plays an important role was supported further by the finding that the hPRL proximal region conferred cAMP regulation when linked to the herpes simplex virus thymidine kinase promoter only in pituitary GC cells and not in other heterologous cells, which do not express Pit-1. Furthermore, we observed that concatenated Pit-1 binding sites were able to confer cAMP responsiveness to the thymidine kinase promoter in GC cells.