Progesterone regulation of a pregnancy-specific transcription repressor to beta-casein gene promoter in mouse mammary gland.

Progesterone inhibits casein gene expression in the mammary gland during pregnancy. A pregnancy-specific mammary nuclear factor (PMF) is a transcriptional repressor that binds to the two separate sites of the beta-casein promoter region and mediates the inhibition of mouse beta-casein gene expression by progesterone. PMF has been detected in mouse mammary tissue during pregnancy, but not during the virginal or lactational periods. Southwestern hybridization showed that the PMF-binding sequence (TGAT/ATCA) bound a protein with a mol wt of 65 kilodaltons from mammary nuclear extract from pregnant mice, but not that from lactating animals. In this study the hormonal regulation of binding activity of PMF and its relationship to casein gene expression were also examined in vivo using ovariectomy of pregnant mice. Ovariectomy rapidly reduces the circulating progesterone and estrogen levels and induces lactogenesis in the mammary tissue of pregnant mice. After ovariectomy, the binding activity of PMF in the mammary gland was reduced to a very low level (< 5% of the sham-operated level) at 24 h, whereas the binding activity of another nuclear protein, octamer-like binding factor, was not altered 24 h after the operation. Progesterone injection at the time of ovariectomy inhibited the loss of PMF-binding activity, and the level of its activity was maintained at about 66% the level of sham-operated pregnant animals 24 h after steroid administration. The ability of progesterone to prevent the ovariectomy-mediated decrease in PMF-binding activity was specific, since other steroid hormones, such as estrogen, hydrocortisone, or testosterone, were incapable of doing so. In contrast to PMF, ovariectomy increased the level of beta-casein mRNA in mammary tissue, and this increase was inhibited by concomitant administration of progesterone, suggesting that the extent of beta-casein gene expression was inversely related to PMF-binding activity in mammary tissue. These results are consistent with the view that PMF plays a role in the regulation of casein gene expression by mediating the inhibitory action of progesterone.