Insulin like growth factor-I independence in rat mammary carcinoma cells: a dominant phenotype in somatic cell hybrid experiments.

Normal rat mammary epithelial cells have an absolute requirement for insulin (IN) or insulin-like growth factor-I (IGF-I) for proliferation in serum-free medium. By contrast, serially transplantable rat mammary carcinoma (RMC) cells are IGF-I-independent for continuous growth in vitro. Previously, we demonstrated that IGF-I independence is not mediated by an autocrine loop. Therefore, experiments were carried out to determine if IGF-I independence behaves as a dominant or recessive phenotype in somatic cell hybridization experiments. IGF-I-independent 1-9RMT cells were rendered hygromycin-resistant and IGF-I-dependent MCF-10A cells were rendered G418 resistant by infection with retroviral expression vectors. Cells of each line were co-incubated in 60 mm dishes and fused with polyethylene glycol. Hybrid cells were selected with media containing hygromycin and G418 in the presence or absence of IN. In three experiments, approximately the same number of colonies emerged in double selection media in the presence or absence of IN. Furthermore, colonies that emerged in IN-containing media, when subcultured, grew equally well in the presence or absence of IN. Thus, fusion of IGF-I-independent RMC cells with IGF-I-dependent human mammary epithelial cells yields hybrids that are IGF-I-independent for growth in serum-free medium.