Stimulation of Mammary Epithelial Cell Growthin Vitro: Interaction of Epidermal Growth Factor and Mammogenic Hormones*

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Abstract
A serum-free primary cell culture system was used to examine the direct effects and interactions of mammogenic hormones and epidermal growth factor (EGF) on the growth of mouse mammary epithelial cells. Epithelial cells were isolated by collagenase dissociation followed by Percoll gradient centrifugation and cultured within collagen gels in a mixture of Ham''s F-12-Dulbecco''s Minimum Essential Medium (1:1) containing insulin (10 .mu.g/ml), crude soybean lecithin, trace elements, trypsin inhibitor, and antioxidants. Progesterone (P; 10-6-10-8 M) or ovine PRL (prolactin; 1 .mu.g/ml), in the absence of EGF, stimulated the growth of cells from mature virgin mice 2- to 4-fold over that of controls cultured in basal medium only. P and PRL synergized in stimulating growth 3- to 17-fold. 17.beta.-Estradiol (10-7-10-10 M) alone did not stimulate growth or synergize with P and/or PRL. This lack of growth stimulation by 17.beta.-estradiol was also observed in medium containing a low concentration of insulin (0.1 .mu.g/ml). EGF (10 ng/ml) alone stimulated growth to the same extent as the combination of P and PRL. EGF at 1, but not 10, ng/ml when combined with P and PRL could additively stimulate growth. Cells from midpregnant mice were less responsive than cells from virgin mice to the growth-stimulating effects of the combination of P and PRL (2-fold stimulation at most), but not to EGF (3- to 6-fold stimulation). Corticosterone, deoxycorticosterone, and aldosterone, but not cortisol, could synergize with PRL in stimulating the growth of cells from mature virgin mice. However, only deoxycorticosterone could stimulate growth in the absence of PRL. These results suggest that PRL, P, and adrenal corticoids may directly stimulate the growth of mouse mammary epithelial cells. The physiologically relevant adrenal corticoids, corticosterone and aldosterone, only potentiate the stimulatory effect of PRL. The hormonal stimulation of growth in vitro can be obscured by an optimum concentration (10 ng/ml) of EGF. The relative growth responses to mammogenic hormones and EGF may depend on the degree of differentiation of the cells.