Protein kinase C ζ is a positive modulator of canonical Wnt signaling pathway in tumoral colon cell lines

The colonic epithelium is a continuously renewing tissue with a dynamic turnover of cells. Wnt pathway is a key regulator of its homeostasis and is altered in a large proportion of colon cancers. Protein kinase C (PKC) family of serine/threonine kinases are also involved in colon tumor formation and progression; however, the molecular role played by them in the Wnt pathway, is poorly understood. Reciprocal coimmunoprecipitation and immunofluorescence studies revealed that PKC zeta interacts with beta-catenin mainly in tumoral colon cells, which overexpressed this PKC isoform. The pharmacological inhibition, the small interference RNA-mediated knockdown of PKC zeta or the expression of a dominant-negative form of it in tumoral SW480 cells, blocked in a dose-dependent manner the constitutive transcriptional activity mediated by beta-catenin, the cell proliferation and the expression of the Wnt target gene c-myc. Remarkably, the PKC zeta stably depleted cells exhibited diminished tumorigenic activity in grafted mice. We show that PKC zeta functions in a mechanism that does not involve beta-catenin degradation since the effects produced by PKC zeta inhibition were also obtained in the presence of proteosome inhibitor and in cells expressing a beta-catenin degradation-resistant mutant. It was found that PKC zeta activity regulates the nuclear localization of beta-catenin since PKC zeta inhibition induces a rapid export of beta-catenin from the nucleus to the cytoplasm in a Leptomycin B sensitive manner. Taken together, our results indicate that the atypical PKC zeta plays an important role in the positive regulation of canonical Wnt pathway.