Involvement of Oct-1 in transcriptional regulation of beta-casein gene expression in mouse mammary gland.

Mouse beta-casein gene promoter contains a region termed block C which is crucial for its gene transcription induced by lactogenic hormones. Nuclear extracts from mouse mammary glands contain at least two binding complexes (DS1 and DS2) which specifically bind to double-stranded block C region DNA. The binding sequence of these complexes was identified to be 5'-AAATTAGCATGT-3' which contains a sequence element related to the consensus octamer motif's complement ATTTGCAT. In the present study, we demonstrate that this sequence element indeed is the binding site for octamer-binding transcription factors (Octs) and Octs represent the double-stranded DNA binding proteins specifically binding to the block C region. Formation of the specific double-stranded binding complexes can be completely blocked by Oct binding motif oligonucleotides and anti-rOct-1 antiserum. We also show that Oct-1B represents at least partial, if not all, double-stranded binding protein, DS1, in mammary nuclear extract. Oct-1B may function as a transcriptional activator on casein gene promoter. The Oct binding activity to beta-casein gene promoter in the mammary gland is affected under influence of hormones both in vitro and in vivo. The DS1 binding activity can be induced by the combination of lactogenic hormones insulin, hydrocortisone and prolactin in organ culture of virgin mouse mammary gland. The binding activity in vivo can be induced by injection of progesterone or its combination with estradiol in virgin mice.