The Hormonal Control of Lactose Synthetase in the Developing Mouse Mammary Gland

The levels of the A and B proteins of lactose synthetase were measured in organ cultures of mouse mammary glands treated with different hormone combinations. Both the A and B proteins were stimulated specifically by insulin, hydrocortisone, and prolactin. Prolactin, however, induced the synthesis of each protein when the organ cultures were pretreated with insulin and hydrocortisone. Under these conditions there was a 10-fold increase in the A protein activity and a 2-fold increase in B protein activity. Specific inhibitors of protein synthesis prevented the induction of both proteins by prolactin. When the levels of the proteins were measured in mammary glands from mice at different stages of pregnancy, it was found that the level of the A protein increased to a maximum towards the end of pregnancy. In contrast, the B protein remained at a low level during pregnancy, but rose dramatically on the initiation of lactation. The relative levels of the A and B proteins in glands from late pregnant mice are similar to those observed after prolactin stimulation of organ cultures pretreated with insulin and hydrocortisone. The asynchronous expression of the A and B proteins may represent a unique type of hormonal control of a biosynthetic pathway, and appears to reflect the specific functional requirements of the mammary gland during development and lactation.