RAS MEDIATES SRC BUT NOT EPIDERMAL GROWTH FACTOR-RECEPTOR TYROSINE KINASE SIGNALING PATHWAYS IN GH(4) NEUROENDOCRINE CELLS

p21Ras has been implicated as a critical signaling component in mediat ing the effects of many growth factor receptor/tyrosine kinases on cel l growth and differentiation. However, the precise functional role of Ras in establishing a cell-specific transcriptional response to a ubiq uitous growth factor remains unclear. We have utilized a transient cot ransfection model system in epidermal growth factor (EGF)-responsive c ultured GH(4) rat pituitary neuroendocrine cells to investigate the ro le of Ras in coupling EGF receptor (EGF-R) and v-Src tyrosine kinase s ignals to the activation of a cell-specific promoter for the rat (r) p rolactin (PRL) gene. A significant dose- and time-dependent EGF stimul ation of the transfected rPRL promoter was obtained. A similar degree of activation of the rPRL promoter was obtained by cotransfection of a plasmid encoding v-Src. Cotransfection of a construct encoding the do minant-negative Ras, N17Ras, produced almost complete inhibition of v- Src-induced rPRL promoter activity, while EGF-stimulated rPRL promoter activity was unaffected. Similarly, EGF activation of a c-Fos promote r was unaffected by N17Ras, while v-Src activation was blocked. Hence, using transcription regulation as a functional assay, we show that Ra s is not required for the EGF-mediated control of the rPRL and c-Fos p romoters, whereas Ras is critical in mediating the v-Src effects to th ese two promoters. These observations emphasize that, despite current biochemical data linking the EGF-R and Ras pathways, the functional si gnificance of such an interaction should be analyzed in a biologically relevant manner and may differ as a function of cell type.