Hormonal modulation of the casein gene expression in a mammogenesis-lactogenesis culture model of the whole mammary gland of the mouse.

Whole mammary glands from immature virgin female BALB/c mice, primed in vivo with estradiol-17g and progesterone, were incubated in a serum-free medium, containing combinations of different hormones. The initial incubation for 6 days (Step I) in medium with the mammogenic hormones (insulin, prolactin, estradiol, progesterone, and growth hormone) stimulated the development of pregnancy-like lobuloalveolar structures. The lobuloalveolar morphogenesis of the gland in vitro also resulted into a pronounced increase in epithelial cell number. Hybridization assays using a cDNA probe to 15 S casein mRNA (cDNA,,,) showed that casein mRNA (mRNA,.,) is virtually nondetectable in the ductal glands from unprimed and primed animals. Concentration of the mRNAcsn was 0.00067%, with 147 molecules/cell in the lobuloalveolar glands obtained after the 6-day Step I incubation in the corticosteroid-free mammogenic medium. This low level of mRNAeen in the gland was unaltered after additional 3 days of culture in prolactinor progesterone-deficient mammogenic hormone medium. After the lobuloalveolar glands were transferred to the Step I1 culture medium containing the lactogenic hormone combination (insulin, prolactin, and cortisol) an increase in mRNA,,, accumulation was measurable by the cDN&., probe. This stimulation of mRNAcsn concentration in total RNA of the gland was progressive. Between 0 to 9 days in the lactogenic medium, mRN&,., level rose from 0.00067% to 0.09%, a 134-fold increase. During the same period, the number of epithelial cells per g of tissue remained essentially unaltered, while the cellular accumulation of the mRNA,., increased 255-fold, from 147 to 37,524 molecules/cell. Thus, the results of the present two-step, mammogenesis-lactogenesis culture model show that the presence of cortisol in a medium containing prolactin is essential for the stimulation of casein gene expression in murine mammary gland.