PKCδ acts as a growth and tumor suppressor in rat colonic epithelial cells

We have analysed the expression of three calcium-independent isoforms of protein kinase C (PKC), PKCδ, PKCε and PKCζ, in an in vitro model of colon carcinogenesis consisting of the nontumorigenic rat colonic epithelial cell line D/WT, and a derivative src-transformed line D/src. While PKCζ and PKCε showed similar protein levels, PKCδ was markedly decreased in D/src cells when compared to the D/WT line. To assess whether down-regulation of PKCδ was causally involved in the neoplastic phenotype in D/src cells, we prepared a kinase-defective mutant of PKCδ. Stable transfection of this sequence caused morphological and growth changes characteristic of partial transformation in D/WT cells. Moreover, to test whether PKCδ was involved in growth control and transformation in this model, we overexpressed PKCδ in D/src cells. Transfected cells underwent marked growth and morphological modifications toward the D/WT phenotype. In a late stage in culture, transfected cells ceased to proliferate, rounded up and degenerated into multinucleated, giant-like cells. We conclude that PKCδ can reverse the transformed phenotype and act as a suppressor of cell growth in D/src cells. Moreover, our data show that downregulation of this isoenzyme of PKC may cooperate in the neoplastic transformation induced by the src oncogene in D/WT cells.