Protein Kinase C Zeta Regulates Human Pancreatic Cancer Cell Transformed Growth and Invasion through a STAT3-Dependent Mechanism

Pancreatic cancer is a very aggressive disease with few therapeutic options. In this study, we investigate the role of protein kinase C zeta (PKC zeta) in pancreatic cancer cells. PKC zeta has been shown to act as either a tumor suppressor or tumor promoter depending upon the cellular context. We find that PKC zeta expression is either maintained or elevated in primary human pancreatic tumors, but is never lost, consistent with PKC zeta playing a promotive role in the pancreatic cancer phenotype. Genetic inhibition of PKC zeta reduced adherent growth, cell survival and anchorage-independent growth of human pancreatic cancer cells in vitro. Furthermore, PKC zeta inhibition reduced orthotopic tumor size in vivo by inhibiting tumor cell proliferation and increasing tumor necrosis. In addition, PKC zeta inhibition reduced tumor metastases in vivo, and caused a corresponding reduction in pancreatic cancer cell invasion in vitro. Signal transducer and activator of transcription 3 (STAT3) is often constitutively active in pancreatic cancer, and plays an important role in pancreatic cancer cell survival and metastasis. Interestingly, inhibition of PKC zeta significantly reduced constitutive STAT3 activation in pancreatic cancer cells in vitro and in vivo. Pharmacologic inhibition of STAT3 mimicked the phenotype of PKC zeta inhibition, and expression of a constitutively active STAT3 construct rescued the transformed phenotype in PKC zeta-deficient cells. We conclude that PKC zeta is required for pancreatic cancer cell transformed growth and invasion in vitro and tumorigenesis in vivo, and that STAT3 is an important downstream mediator of the pro-carcinogenic effects of PKC zeta in pancreatic cancer cells.