Prolactin-induced alpha-lactalbumin activity in mammary explants from pregnant rabbits. A role for epidermal growth factor and glucocorticoids.

Exogenous prolactin alone can induce alpha-lactalbumin activity in rabbit mammary explants. Under these conditions, exogenous corticosol has no effect. However, low levels of epidermal growth factor (EGF) can markedly inhibit the induction by prolactin, and this inhibitory effect, in turn, can be prevented by cortisol. The steroid can, in fact, convert EGF from a potent inhibitor into an agent which enhances the induction. None of the other growth factors tested inhibits induction of alpha-lactalbumin activity by prolactin, and no other type of steroid tested overcomes the effect of EGF. The results suggest that, under physiological conditions, glucocorticoid may be essential for induction of alpha-lactalbumin activity in the rabbit in order to override the effect of serum EGF.