Prolactin as a mitogen in mammary cells.

Prolactin (PRL) acts as both a mitogen and a differentiating agent in the breast. The decision to respond to PRL as a mitogen by breast cells depends on the hormonal milieu in which the epithelial cell resides. In addition, PRL's action on the breast is regulated (1) at the level of the hormone itself; (2) at the receptor level; (3) at the level of selection of signaling pathway; and, (4) by combinations of these aspects. The development of cell lines containing only one class of the PRL receptors and showing qualitative differences in response and signaling pathways will help in understanding the pleiotropic nature of PRL action.