Control of mammary epithelial cell DNA synthesis by epidermal growth factor, cholera toxin, and IGF-1: specific inhibitory effect of prolactin on EGF-stimulated cell growth.

Epidermal growth factor (EGF) stimulates mouse mammary cell proliferation in vivo, but its maximal level in the mammary gland is at midlactation, when little mammary growth takes place. The present studies use a normal murine mammary gland epithelial cell line (NMuMG) to determine the effects of the mammary mitogens EGF, cholera toxin (CT), and insulin-like growth factor-1 (IGF-1) on cell proliferation and how cell growth is altered by addition of the lactogenic hormone prolactin (PRL). EGF and CT stimulated over a fourfold increase of DNA synthesis in NMuMG cells when compared to basal levels. Only a twofold stimulation of DNA synthesis was observed when cells were treated with IGF-1. There was a slight increase in the percentage of cells in S-phase when these agents were added in combination with each other. Physiological levels of PRL had no significant effect on CT- or IGF-1-induced DNA synthesis but reduced EGF-stimulated cell proliferation to basal levels. Furthermore, PRL reduced the percentage of cells in S-phase to IGF-1- and CT-induced levels in the presence of EGF. Interestingly, PRL increased cellular levels of EGF mRNA after 2 h of treatment, which is similar to the response of mouse mammary glands cultured in lactogenic hormones. We conclude that even though PRL can increase the amount of EGF mRNA in mammary epithelial cells, it also eliminates EGF-induced mitogenesis.