Thyroid hormone regulation of alpha-lactalbumin: differential glycosylation and messenger ribonucleic acid synthesis in mouse mammary glands.

Mouse mammary tissue, when cultured in the presence of insulin, corticoids, PRL, and physiological levels of T3, shows increased synthesis and secretion of alpha-lactalbumin. Tissue cultured in the presence of insulin, hydrocortisone, PRL, and T3 synthesizes two distinct forms of alpha-lactalbumin, but secretes only one form. Tissue cultured in the absence of T3 synthesizes and secretes only one form. To address the question of whether these two electrophoretically distinct forms arose by differential glycosylation of the same polypeptide or by synthesis of two different polypeptide precursor chains, mammary tissue was cultured in the presence of insulin, corticoids, and PRL with or without T3, and the mRNA and alpha-lactalbumins were isolated. Northern blot analyses indicated that mammary gland tissue cultured in the presence of T3 contained 2.46 times more alpha-lactalbumin mRNA than tissue cultured only in the presence of insulin, hydrocortisone, and PRL. This enhanced mRNA level was confirmed by in vitro translation experiments where tissue cultured in the presence of insulin, hydrocortisone, PRL, and T3 produced mRNA that resulted in 2.1 times as much radiolabeled alpha-lactalbumin as tissue cultured in the absence of T3. Sodium dodecyl sulfate-polyacrylamide gel analysis of the in vitro translation products revealed only one band, suggesting the presence of only one message. Endoglycosidase digestion of the two forms of alpha-lactalbumin produced in the presence of T3 resolved them into a single band on sodium dodecyl sulfate-polyacrylamide gels. Thus, the electrophoretic differences between the two forms synthesized in the presence of T3 appear to be due to differential N-linked glycosylation of the same polypeptide chain and not to synthesis of two different polypeptide precursor chains.