Studies on the mechanism by which thyroid hormones enhance alpha-lactalbumin activity in explants from mouse mammary glands.

Addition of 3,5,3'-triiodo-L-thyronine to cultures of mammary gland explants in serumfree medium containing insulin, hydrocortisone and prolactin results in a 3 to 5-fold increase in the activity of the milk-protein alpha-lactalbumin over that seen in the presence of the latter three hormones alone. The thyroid hormone does not act by substituting for any of the other hormones. It need not be present throughout the culture period but can act if added along with prolactin after insulin and hydrocortisone have induced formation of the rough endoplasmic reticulum. Delayed addition of the thyroid hormone also results in further stimulation of cells already responding maximally to insulin, hydrocortisone and prolactin. These effects of triiodothyronine are not blocked by progesterone at 1 microng per ml. They are, however, blocked by the addition of inhibitors of RNA (actinomycin D) or protein (cycloheximide or puromycin) synthesis, suggesting that the thyroid hormone increases the synthesis of the alpha-lactalbumin molecule itself. The thyroid hormone appears to act by altering the responsiveness of the mammary gland explants to prolactin, but not to insulin or hydrocortisone. In the presence of 10-9M triiodothyronine, enhanced alpha-lactalbumin activity is consistently obtained at prolactin concentrations as low as 4.5 x 10(-12)M whereas, in the absence of the thyroid hormone, ten times more prolactin (4.5 x 10(-11)M) is needed to obtain an increase in alpha-lactalbumin activity.