Extracellular matrix regulates ovarian hormone-dependent proliferation of mouse mammary epithelial cells

Mammary stromal cells can modulate steroid hormone responsiveness both in vivo and in vitro. One of the mechanisms by which stromal cells can influence epithelial cell behavior is by modifying the composition of the extracellular matrix (ECM). In this report, we have investigated the effects of five ECM molecules on control of epithelial cell proliferation by estrogen (E2) and progestin (R5020) under serum-free culture conditions. To assess the contribution of mammary gland differentiation in determining epithelial cell interactions with ECM, the behavior of mammary epithelial cells derived from nulliparous and pregnant mice was compared. We report the novel finding that the proliferative responses of mammary epithelial cells to progestin is influenced by specific ECM molecules. However, the primary determinant of hormonal responsiveness is the developmental state of the gland from which the epithelial cells were derived. Nulliparous-derived epithelial cells, proliferated in response to R5020 only on fibronectin (FN) and collagen IV (Col IV). The more highly differentiated, pregnancy-derived epithelial cells were not responsive to E2 or R5020 on any ECM. To determine if steroid hormone receptors were targets of ECM-mediated effects, ER and PR levels were analyzed. In both nulliparous and pregnancy-derived cultures, PR binding levels were maintained at similar levels on all ECMs. However, ER levels were not maintained in nulliparous-derived cultures, and this may have contributed to the lack of a significant response to E2. Alternatively or in addition, E2-induced responses may require additional signals or growth factors that are provided by stromal cells in vivo or by serum supplementation in vitro. These results demonstrate the ECM molecules, fibronectin and collagen IV, can modulate responsiveness of mammary epithelial cells to R5020 in vitro, and may be the mediators of stromal influences on hormone responsiveness in vivo. However, the specific effects of ECM and hormones are also determined by the developmental state of the mammary gland from which the cells are derived. Thus, mammary gland differentiation, ovarian hormones, and ECM composition may act in concert to determine the outcome of hormone treatment on cell proliferation.