Growth of normal and neoplastic mouse mammary epithelial cells in primary culture: stimulation by conditioned medium from mouse mammary fibroblasts.

Conditioned medium obtained from mouse mammary fibroblasts markedly stimulated both DNA synthesis and cell proliferation in primary monolayer cultures of mouse mammary tumor cells. Growth of normal mammary epithelial cells from virgin female mice was likewise stimulated by the conditioned medium. The growth-stimulating activity was absent in conditioned media from 3T3 cells, 3T6 cells, mouse embryonic and lung fibroblasts, mammary tumor cells and normal mammary epithelial cells. Stimulation of DNA synthesis in mammary tumor cells was also observed with epidermal growth factor (EGF), although the effect was less than that observed with the conditioned medium. Receptor studies using 125I-labeled EGF further showed that the specific binding of EGF to mammary tumor cells was not inhibited by the conditioned medium. The molecular weight of the growth-stimulating activity in mammary fibroblast-conditioned medium was estimated to be approximately 100,000 daltons by Sephadex G-200 column chromatography. These observations suggest that mammary fibroblasts, which constitute the stroma of the mammary gland, produce a mammary epithelial cell growth factor(s) distinct from EGF.