RAS, BUT NOT SRC, TRANSFORMATION OF RIE-1 EPITHELIAL-CELLS IS DEPENDENT ON ACTIVATION OF THE MITOGEN-ACTIVATED PROTEIN-KINASE CASCADE

Src transformation of NIH3T3 mouse fibroblasts has been shown to be de pendent on Ras function. Since we recently showed that the signaling p athways that mediate Ras transformation of RIE-1 rat intestinal epithe lial cells are distinct from those that cause Ras transformation of fi broblasts, we utilized three approaches to determine if Src transforma tion of RIE-1 cells is dependent on Ras, First, although both Ras and Src cause upregulation of an epidermal growth factor (EGF) receptor-de pendent autocrine growth loop, only Ras transformation required this a ctivity, Second, whereas both Src and Ras caused upregulation of the p 42 and p44 mitogen-activated protein kinases (MAPKs), only Ras transfo rmation was blocked by the inhibition of MAPK activation by treatment with the PD 98059 MEK inhibitor, Third, treatment with the farnesyltra nsferase inhibitor FTI-277 blocked Ras, but not Src, transformation. T aken together, these observations suggest that Src transformation of R IE-1 cells is not dependent on Ras, Finally, we determined that Ras ac tivation of Raf-independent pathways alone is sufficient to cause grow th transformation of RIE-1 cells. Thus, both Ras and Src cause transfo rmation of RIE-1 cells via pathways distinct from those required to ca use transformation of NIH3T3 cells.