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

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 PKC delta expression levels are decreased in colon tumor cell lines with respect to non-malignant cells. Reciprocal co-immunoprecipitation and immunofluorescence studies revealed that PKC delta 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 PKC delta in cancer SW480 cells, which do not possess a functional beta-catenin destruction complex, did not affect beta-catenin-mediated transcriptional activity. However, in human colon carcinoma RKO cells, which have a normal beta-catenin destruction complex, negatively affected beta-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 PKC delta and by the expression of a dominant negative form of PKC delta in RKO cells. Remarkably, the PKC delta stably depleted cells exhibited augmented tumorigenic activity in grafted mice. We show that PKC delta functions in a mechanism that involves regulation of beta-catenin degradation, because PKC delta inhibition induces beta-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 PKC delta diminished APC phosphorylation in intact cells, suggesting that PKC delta may modulate canonical Wnt activation negatively through APC phosphorylation.