Thyroid hormones enhance the synthesis and secretion of alpha-lactalbumin by mouse mammary tissue in vitro.

The syntheses and secretion of casein and alpha-lactalbumin were examined in organ culture of midpregnancy mouse mammary glands using serum-free medium supplemented with various combinations of insulin, hydrocortisone, PRL, and L-T3. Using highly specific antibodies raised against mouse caseins and alpha-lactalbumin, we demonstrate a selective enhancement of alpha-lactalbumin and lactose synthesis and secretion when all four hormones are present in the culture medium. Production of casein was not modified by the presence of L-T3. Hydrocortisone at concentrations of 10(-9)-10(-6) M inhibited the secretion of both casein and alpha-lactalbumin into the culture medium. The addition of L-T3 to the medium selectively overcame the inhibition of alpha-lactalbumin secretion by hydrocortisone. Extracts of tissue cultured in the presence of L-T3 contained two distinct forms of alpha-lactalbumin, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the absence of L-T3, synthesis of a single form of alpha-lactalbumin prevailed. In the culture medium, predominantly one form of alpha-lactalbumin was detected regardless of the presence of L-T3 during culture. These data clearly indicate that thyroid hormones play an important regulatory role in functional differentiation of the mouse mammary gland.