Interactions of growth factors and retroviral oncogenes with mitogenic signal transduction pathways of Balb/MK keratinocytes.

Balb/MK epidermal keratinocytes require epidermal growth factor (EGF) for growth in serum-containing medium and terminally differentiate in response to high Ca++ concentration. Several oncogenic retroviruses have been shown to relieve the EGF requirement and to block calcium-induced terminal differentiation. We developed a chemically defined medium to investigate the minimum growth factor requirements for Balb/MK cells, as well as how such requirements might be altered by retroviral oncogenes. In this medium insulin, apparently acting as IGF-1, and EGF supported cell growth in a manner comparable to serum and EGF. Acidic as well as basic fibroblast growth factor (FGF) substituted for EGF but not insulin in supporting Balb/MK proliferation. Infection with retroviruses containing v-ras oncogenes (v-H-ras, v-Ki-ras), oncogenes derived from growth factor receptors (v-erbB, v-fms) or the v-mos oncogene permitted growth in defined medium containing insulin but lacking EGF. The v-fgr oncogene, a member of the src subfamily, was unique in conferring independence from both insulin and EGF. Our findings establish the applicability of this system for biologic assay of epithelial cell growth factors as well as identification of specific growth factor requirements that can be altered or complemented by the actions of specific oncogenes.