Raf-1 Protein Kinase Is an Integral Component of the Oncogenic Signal Cascade Shared by Epidermal Growth Factor and Platelet-Derived Growth Factor

Our recent studies with cell mutants indicate that a cascade shared by the epidermal growth factor (EGF) and platelet-derived growth factor (PDGF) signals exists in NRK cells and mediates oncogenic signals induced by many oncogenes (A. Masuda, S. Kizaka-Kondoh, H. Miwatani, Y. Terada, H. Nojima, and H. Okayama, New Biol. 4:489-503, 1992). We have employed the antisense RNA technique to investigate possible involvement of Raf-1 kinase in this signal transduction cascade. NRK cell clones highly reduced in the Raf-1 production are generated by the expression of a c-raf-1 antisense RNA. They have no apparent growth defects and retain proper mitotic responses to growth factors but are refractory to transformation by EGF or PDGF plus transforming growth factor beta, v-erbB, v-fms, v-K-ras, v-mos, v-fos, v-src, simian virus 40 large T, and polyomavirus middle T but not by v-raf or adenovirus E1A. These results not only support our model for the oncogenic signal cascade but also lead to the conclusion that Raf-1 protein kinase is a downstream component of this oncogenic signal cascade shared by EGF and PDGF.