Protein Kinase C Delta Negatively Modulates Canonical Wnt Pathway and Cell Proliferation in Colon Tumor Cell

The tumor suppressor Adenomatous Polyposis coli (APC) gene is mutated or lost in most colon cancers. Alterations in Protein kinase C (PKC) isozyme expression and aberrant regulation also comprise early events in intestinal carcinomas. Here we show that PKCd expression levels are decreased in colon tumor cell lines with respect to non-malignant cells. Reciprocal co-immunoprecipitation and immunofluorescence studies revealed that PKCd interacts specifically with both full-length (from non-malignant cells) and truncated APC protein (from cancerous cells) at the cytoplasm and at the cell nucleus. Selective inhibition of PKCd in cancer SW480 cells, which do not possess a functional b-catenin destruction complex, did not affect b-catenin-mediated transcriptional activity. However, in human colon carcinoma RKO cells, which have a normal b-catenin destruction complex, negatively affected b-catenin-mediated transcriptional activity, cell proliferation, and the expression of Wnt target genes C-MYC and CYCLIN D1. These negative effects were confirmed by siRNA-mediated knockdown of PKCd and by the expression of a dominant negative form of PKCd in RKO cells. Remarkably, the PKCd stably depleted cells exhibited augmented tumorigenic activity in grafted mice. We show that PKCd functions in a mechanism that involves regulation of b-catenin degradation, because PKCd inhibition induces b-catenin stabilization at the cytoplasm and its nuclear presence at the C-MYC enhancer even without Wnt3a stimulation. In addition, expression of a dominant form of PKCd diminished APC phosphorylation in intact cells, suggesting that PKCd may modulate canonical Wnt activatio