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Previous studies have indicated that the protein kinase C (PKC) enzyme system plays an important role in maintenance of intestinal epithelial homeostasis and that alterations in the expression/function of PKC isozymes are critical for the development of intestinal neoplasia. The current study investigated the role of PKC δ signaling in intestinal epithelial cells. Immunoblot analysis revealed that, while IEC-18 non-transformed intestinal crypt cells and mucosal cells in situ express PKC δ, colon adenocarcinoma cells uniformly exhibit markedly reduced levels of the enzyme. To gain insight into the function(s) of PKC δ in intestinal epithelial cells, levels of the protein were manipulated using adenoviral infection and RNA interference technology. Overexpression of PKC δ in IEC-18 cells promoted apoptosis within 6 h of treatment with the PKC agonist phorbol 12-myristate 13-acetate (PMA), while overexpression of PKC α or silencing of PKC δ had little effect on the viability of these cells. Notably, restoration of PKC δ expression in a panel of colon adenocarcinoma cell lines including DLD-1, FETDNR/RII, RCA, GEO, and Moser resulted in massive cell death, even in the absence of PKC agonists, as confirmed by flow cytometry, PARP cleavage, and caspase activation. Restoration of PKC α, which is also expressed at markedly low levels in these cells, did not affect cell viability. Consistent with studies in other systems, PKC δ-induced apoptosis in colon adenocarcinoma cells was associated with proteolytic cleavage of the enzyme. Collectively, these data establish PKC δ as a mediator of apoptosis signaling in intestinal epithelial cells and may provide an explanation for the consistent absence of PKC δ expression in human colon adenocarcinoma cell lines. Supported by NIH grants DK54909, DK60632, and CA16056, and by a grant from the Roswell Park Alliance Foundation.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]