Protein kinase Cε is oncogenic in colon epithelial cells by interaction with the ras signal transduction pathway

We have shown previously that overexpression of the ε isoform of protein kinase C (PKCε) in rat colonic epithelial cells causes malignant transformation, possibly by interacting with the ras signal transduction pathway (Oncogene 12: 847, 1996). We have now performed experiments to examine certain early steps in the ras signaling pathway. A marked increase of Raf-1 phosphorylation was detected in tumorigenic ras-transformed D/ras as well as in D/ε cells (overexpressing PKCε), compared to the nontumorigenic D/WT parental line. Moreover, in the PKCε-transformed D/ε cell line, stable transfection with a dominant-negative raf-1 (DNraf) sequence caused complete regression of the neoplastic phenotype. These results suggested that PKCε-induced transformation was associated with increased Raf-1 activation, and that DNraf could block the oncogenic effect of PKCε. Furthermore, transfection of D/WT cells with dominant-negative ras induced arrest of cell growth, and subsequent transfection with PKCε cDNA enhanced cell proliferation and induced neoplastic transformation. These results suggest that ras acts upstream of PKCε, and that overexpression of PKCε circumvents the block in cell proliferation caused by dominant-negative ras. We conclude that PKCε exerts its oncogenic activity in rat colonic cells by affecting the ras signaling cascade at the level of Raf-1 activation.