Role of growth factor synthesis in the acquisition of insulin/insulin-like growth factor I independence in rat mammary carcinoma cells.

In previous work, we demonstrated that a subset of carcinogen-induced rat mammary carcinomas consists of cells that are independent of growth factors strictly required by normal rat mammary epithelial (RME) cells for long-term growth in serum-free medium. Furthermore, only those tumors that expressed growth factor independence in vitro were serially transplantable in vivo. The present studies were aimed at determining if the independence of insulin (IN)/insulin-like growth factor I (IGF-I) is mediated by autocrine factors synthesized by the rat mammary tumor (RMT) cells. The results of these experiments indicate that IN/IGF-I-independent RMT cells do not synthesize IGF-I that is detectable at the message or peptide level. Both normal and neoplastic cells do, however, secrete IGF-I-binding activity. Conditioned medium, cell lysates, and cell extracts obtained from growth factor-independent cells do not contain growth factor activity that can substitute for IN for growth of IN-dependent RMT cells. Growth factor-independent cells do not express a density dependence for growth in IN-free medium nor do they respond to exogenous IN or IGF-I in low density growth assays. By contrast, growth factor-dependent cells that were rendered IN/IGF-I independent by transfection with an expression vector containing the IGF-I gene secrete IGF-I-like biological activity that is readily detectable and maintain responsiveness to exogenous IN at low densities. Taken together, these results suggest that growth factor-independent RMT cells are truly autonomous of IN/IGF-I for growth and are not synthesizing a growth factor that satisfies their IN/IGF-I requirement in an autocrine manner.