The mechanism of progesterone-glucocorticoid interaction in regulation of casein gene expression.

The mechanism of progesterone inhibition of casein gene expression was analyzed in a two-step culture model of the whole mammary organ of the mouse. Cellular accumulation of casein mRNA (mRNA,.,) was used as a measure of casein gene expression. A specific mouse casein cDNA (cDNA,.,) was used as a hybridization probe to monitor mRN&,,, in the glands. Six days of step I incubation in medium containing the “mammogenic hormone mixture” (MHM), insulin, prolactin, estradiol-l7P, progesterone, and growth hormone, showed only a basal level of mRNAcsn in the lobuloalveolar glands. Step I1 incubation of the lobuloalveolar glands in medium with MHM plus cortisol stimulated a 2-fold increase of mRNA,., over the basal level, indicating that addition of cortisol in MHM permits casein gene expression in presence of progesterone. Step I1 incubation with cortisol and progesteronefree MHM stimulated a 17to 19-fold increase of mRNAcsn after 6 days and addition of progesterone in the medium blocked this rise. Step I1 incubation with MHM containing both progesterone and cortisol (1pg/ ml each) produced only 3.3-fold stimulation after 6 days, whereas at a higher (5 pg/ml) cortisol concentration, mRNAfsn in the gland increased 10-fold, indicating that, a t higher concentration, glucocorticoid can limit the antagonistic action of progesterone. MHM plus different increased molar ratios of progesterone to cortisol in the medium caused a rapid decrease of mRN&,, in the glands. Receptor assays done in vitro showed that progesterone competes for the [3Hldexamethasone binding sites in the cytosol from the mammary gland in culture. [3H]Dexamethasone binding to cytosol receptors also declined in the glands cultured in the medium with increased progesterone/cortisol molar ratio. Thus, occurrence of the reduced levels of mRNAsn and the corresponding depletion of glucocorticoid binding to its cytosol receptors caused by increased progesterone/cortisol in the culture medium suggests that one of the mechanisms of progesterone antagonism on casein gene expression may relate to progesterone-mediated inhibition of glucocorticoid binding to cytosol receptors in the mammary cells.