The proximal milk protein binding factor binding site is required for the prolactin responsiveness of the sheep beta-lactoglobulin promoter in Chinese hamster ovary cells

To identify cis-acting prolactin (PRL) response elements within the sheep beta-lactoglobulin (BLG) promoter, CHO cells were co-transfected with a rabbit PRL-receptor (PRL-R) expression plasmid and a number of BLG-CAT constructs. Resection through the 4200 bp BLG promoter diminished the PRL response. Mutation of the proximal binding site for milk protein binding factor (MPBF), a previously described mammary gland transcription factor, abolished the PRL inducibility of full length and shorter forms of the promoter. MPBF was shown to be similar to the Stat protein mammary gland factor (MGF) which has been shown to mediate PRL responsiveness of the rat beta-casein gene in mammary cells. MPBF binding activity was detected in the nucleus of CHO cells and was increased 2-6-fold in cells stably transfected with the PRL-R. The lactating mammary gland has high levels of MPBF binding activity and it is likely that this has an important role in the PRL induction of a variety of milk protein genes.