Unconventional Secretion of PKCd Exerts Tumorigenic Function via Stimulation of ERK1/2 Signaling in Liver Cancer

Abstract

Expression of human protein kinase C delta (PKCd) protein has been linked to many types of cancers. PKCd is known to be a multifunctional PKC family member and has been rigorously studied as an intracellular signaling molecule. Here we show that PKCd is a secretory protein that regulates cell growth of liver cancer. Full-length PKCd was secreted to the extracellular space in living liver cancer cells under normal cell culture conditions and in xenograft mouse models. Patients with liver cancer showed higher levels of serum PKCd than patients with chronic hepatitis or liver cirrhosis or healthy individuals. In liver cancer cells, PKCd secretion was executed in an endoplasmic reticulum (ER)-Golgi–independent manner, and the inactivation status of cytosolic PKCd was required for its secretion. Furthermore, colocalization studies showed that extracellular PKCd was anchored on the cell surface of liver cancer cells via association with glypican 3, a liver cancer–related heparan sulfate proteoglycan. Addition of exogenous PKCd activated IGF-1 receptor (IGF1R) activation and subsequently enhanced activation of ERK1/2, which led to accelerated cell growth in liver cancer cells. Conversely, treatment with anti-PKCd antibody attenuated activation of both IGF1R and ERK1/2 and reduced cell proliferation and spheroid formation of liver cancer cells and tumor growth in xenograft mouse models. This study demonstrates the presence of PKCd at the extracellular space and the function of PKCd as a growth factor and provides a rationale for the extracellular PKCd-targeting therapy of liver cancer. Significance: PKCd secretion from liver cancer cells behaves as a humoral growth factor that contributes to cell growth via activation of proliferative signaling molecules, which may be potential diagnostic or therapeutic targets.