Protein Kinase Cδ Negatively Regulates Hedgehog Signaling by Inhibition of Gli1 Activity

Constitutive activation of the hedgehog pathway is implicated in the development of many human malignancies; hedgehog targets, PTCH1 and Gli1, are markers of hedgehog signaling activation and are expressed in most hedgehog-associated tumors. Protein kinase C delta (PKC delta) generally slows proliferation and induces cell cycle arrest of various cell lines. In this study, we show that activated PKC delta (wild-type PKC delta stimulated by phorbol 12-myristate 13-acetate or constitutively active PKC delta) decreased Gli-luciferase reporter activity in NIH/3T3 cells, as well as the endogenous hedgehog-responsive gene PTCH1. In human hepatoma (i.e. Hep3B) cells, wild-type PKC delta and constitutively active PKC delta decreased the expression levels of endogenous Gli1 and PTCH1. In contrast, PKC delta siRNA increased the expression levels of these target genes. Silencing of PKC delta by siRNA rescued the inhibition of cell growth by KAAD-cyclopamine, an antagonist of hedgehog signaling element Smoothened, suggesting that PKC delta acts downstream of Smoothened. The biological relevance of our study is shown in hepatocellular carcinoma where we found that hepatocellular carcinoma with detectable hedgehog signaling had weak or no detectable expression of PKC delta, whereas PKC delta highly expressing tumors had no detectable hedgehog signaling. Our results demonstrate that PKC delta alters hedgehog signaling by inhibition of Gli protein transcriptional activity. Furthermore, our findings suggest that, in certain cancers, PKC delta plays a role as a negative regulator of tumorigenesis by regulating hedgehog signaling.