Growth responsiveness to prolactin and its loss in normal rat mammary cells in culture.

Growth response of mammary epithelial cells to hormones, particularly to prolactin, was studied by using a primary culture of rat mammary gland organoids. After allowing the cells to attach and spread on the surface of plastic culture dishes, the effect of hormones was tested by means of [3H]-thymidine incorporation and autoradiography in a medium containing 1% fetal calf serum. In this system, prolactin showed a modest but significant growth-stimulatory activity (50% over control), and addition of insulin or hydrocortisone or both enhanced the growth stimulation of prolactin to a large extent. Growth stimulation caused by these hormones without prolactin was always significantly lower than that caused with prolactin. A dose-response study indicated that prolactin can stimulate growth at physiological concentrations. The maximum stimulation was observed at 1-5 micrograms/ml. The growth-stimulatory effect of prolactin was decreased as the culture period was prolonged, and by the 4th day in culture the effect was no longer observed. In contrast, the stimulatory effect of insulin was constant over the 5-day culture period. Phosphoethanolamine, which has been shown to be a growth factor for some rat and human mammary carcinoma cells, showed 2-fold growth stimulation when added with prolactin, insulin or hydrocortisone. The stimulatory effect was again not observed in older cultures, as in the case of prolactin.