An early effect of prolactin on the formation of -lactalbumin by mouse mammary epithelial cells.

Epithelial cells from freshly isolated mammary glands of mature virgin mice make minute amounts of casein and exhibit low levels of lactose synthetase A-protein (a UDPgalactosyltransferase) activity. They have barely detectable lactose synthetase B-protein (oc-lactalbumin) activity. When cultured in the presence of insulin, hydrocortisone, and prolactin, mammary explants from mature virgin mice develop the capacity to synthesize these milk proteins asynchronously. Casein synthesis and A-protein activity reach their maxima after 72 hours of exposure to the hormones and 24 hours before the emergence of B-protein activity. Estradiol-17P, or prolactin, priming of mature virgin mice reduces the time required for the development of B-protein activity in the mammary explants, while the time course of casein synthesis and appearance of A-protein activity is not appreciably affected. As a result, the secretory proteins, casein and ol-lactalbumin, develop synchronously. This represents a heretofore undiscovered manifestation of the influence of prolactin in this system.