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 epsilon isoform of protein kinase C (PKC epsilon) 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/epsilon cells (overexpressing PKC epsilon), compared to the nontumorigenic D/WT parental Line. Moreover, in the PKC epsilon-transformed D/epsilon cell line, stable transfection with a dominant-negative raf-1 (DNraf) sequence caused complete regression of the neoplastic phenotype. These results suggested that PKC epsilon-induced transformation was associated with increased Raf-1 activation, and that DNraf could block the oncogenic effect of PKC epsilon, Furthermore, transfection of D/WT cells with dominant-negative ras induced arrest of cell growth, and subsequent transfection,vith PKC epsilon cDNA enhanced cell proliferation and induced neoplastic transformation. These results suggest that ras acts upstream of PKC epsilon, and that overexpression of PKC epsilon circumvents the block in cell proliferation caused by dominant-negative ras. We conclude that PKC epsilon exerts its oncogenic activity in rat colonic cells by affecting the ras signaling cascade at the level of Raf-1 activation.