Regulation of estrogen and progesterone receptor levels in mouse mammary epithelial cells grown in serum-free collagen gel cultures.

The effect of collagenase dissociation of virgin mouse mammary glands on the level of mammary epithelial cytosolic estrogen receptors (ER) and progesterone receptors (PR) was assessed. After cell dissociation, ER was present in mammary epithelial cells at concentrations similar to those found in the whole gland. However, PR appeared to be affected by the collagenase treatment. The regulation of ER and PR in mouse mammary epithelial cells isolated by collagenase dissociation and grown within collagen gels was then determined. After 7 days in culture under serum-free conditions inside a collagen gel, PR and, to a lesser extent ER, as characterized by high affinity binding and specificity, were present in the epithelial cells. Although at a low level, the ER were determined to be functional, since estradiol (E2) was able to promote nuclear accumulation of ER and to induce PR. PRL was able to increase cytosolic ER and PR concentrations. The combination of progesterone (P) and PRL was more effective than PRL or P alone in increasing PR. The induction of PR by P and PRL was inhibited when epidermal growth factor was present in the culture medium. Previous studies have shown that P, PRL, and epidermal growth factor, but not E2 (either alone or in combination with these factors) are able to stimulate cell proliferation in vitro. We conclude that the effects of E2 on protein synthesis and proliferation are dissociated in vitro. The difference between the effect of E2 and PRL or P on growth may be related either to the initial concentrations of their respective receptors or estrogen may stimulate growth indirectly.