Overexpression of protein kinase C epsilon is oncogenic in rat colonic epithelial cells

We have analysed the expression of five protein kinase C [PKC] isoforms in an in vitro model using nontumorigenic rat colonic epithelial cells FRC/TEX CL D [D/WT] and in the related tumorigenic Ha-ras-transformed FRC/TEX CL D/H-ras line [D/ras]. The PKC subspecies α, δ, ε and ζ were expressed at the protein level in both D/WT and D/ras cells, while βPKC was undetectable in both lines. The levels of expression of the δ and ζ isoforms were similar in D/WT and D/ras cells. αPKC expression was decreased and εPKC was increased in D/ras cells compared to the D/WT line. To assess whether overexpression of εPKC was linked to the transformed phenotype, we have generated from D/WT cells two clones (D/ε-5 and D/ε-9) which stably overexpress εPKC about fivefold. Overexpression of εPKC caused marked morphological changes in both transfected clones, which were accompanied by increased saturation densities and anchorage-independent colony formation in semisolid agar. These growth effects were attenuated or reversed by chronic incubation with phorbol 12-myristate 13-acetate. Furthermore, D/ε-5 and D/ε-9 cells formed tumors in athymic nude mice with 100% incidence while the parental D/WT or vector alone (D/MV12) controls produced no tumors. We conclude that εPKC can act as an oncoprotein when modestly overproduced in nontumorigenic D/WT colonic cells, and that this isoform of PKC may be linked to ras-modulated signal transduction leading to neoplastic transformation in colonic epithelium.