Mammary fibroblast-derived hepatocyte growth factor and mammogenic hormones stimulate the growth of mouse mammary epithelial cells in primary culture

We recently isolated a mammary growth factor from mouse mammary fibroblast-conditioned medium and identified it as a homologue of human HGF (hepatocyte growth factor) (Sasaki et al. Biochem Biophys Res Commun 199: 772-779, 1994). To elucidate the role of mammary fibroblast-derived HGF in mammary epithelial cell growth in vivo, we used recombinant mouse HGF and examined its effect on the growth of primary cultures of mouse mammary epithelial cells in this study. HGF stimulated the growth of mammary epithelial cells not only in monolayer culture but also in three-dimensional collagen gel matrix culture. In contrast, mammogenic hormones, i.e., prolactin and progesterone, stimulated the growth of mammary epithelial cells only when the cells were cultured in a collagen gel matrix, suggesting that the three-dimensional culture conditions are necessary to observe the growth-stimulatory effect of mammogenic hormones seen in vivo. Under these three-dimensional culture conditions, an additive effect of HGF and mammogenic hormones was also seen. Northern blot analysis revealed that HGF mRNA was expressed in the mammary gland as well as in cultured mammary fibroblasts but not in cultured mammary epithelial cells, indicating that HGF is produced preferentially by the stromal fibroblasts of the mammary gland. These observations thus suggest a role for HGF as a mammary stromal fibroblast-derived growth factor which, in cooperation with mammogenic hormones, stimulates the growth of mammary epithelial cells in vivo.