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

Abstract

Expression of human protein kinase C delta (PKCδ) protein has been linked to many types of cancers. PKCδ is known to be a multifunctional PKC family member and has been rigorously studied as an intracellular signaling molecule. Here we show that PKCδ is a secretory protein that regulates cell growth of liver cancer. Full-length PKCδ 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 PKCδ than patients with chronic hepatitis or liver cirrhosis or healthy individuals. In liver cancer cells, PKCδ secretion was executed in an endoplasmic reticulum (ER)-Golgi–independent manner, and the inactivation status of cytosolic PKCδ was required for its secretion. Furthermore, colocalization studies showed that extracellular PKCδ 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 PKCδ 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-PKCδ 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 PKCδ at the extracellular space and the function of PKCδ as a growth factor and provides a rationale for the extracellular PKCδ-targeting therapy of liver cancer. Significance: PKCδ 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.

Multiple subcellular localizations and functions of protein kinase Cδ in liver cancer

“…The cationic amino acid clusters closely resemble the NLS of intracellular proteins[ 68 ]. In fact, extracellular NLS-containing proteins, such as importin α1, huRNP-K, and PKCδ are detected at the cell surface of human cells[ 9 , 69 , 70 ]. These extracellular NLS-containing proteins are more likely to be located at the cell surface by binding to HSPGs.…”

“…Furthermore, glypican3, a liver cancer-specific HSPG, was identified as a receptor for cell surface PKCδ[ 71 ]. Both Cheng et al [ 72 , 73 ] and we showed that GPC3 regulates the activation of insulin-like growth factor 1 receptor (IGF1R)[ 9 ]. In fact, we found that extracellular PKCδ induces activation of IGF1R via association with GPC3 and its downstream signaling molecules, such as ERK1/2 and STAT3.…”

“…We also found that PKCδ is secreted into the extracellular space in liver cancer[ 9 ]. Extracellular accumulation of PKCδ was detected in different liver cancer cell lines but not in hepatocytes, suggesting that PKCδ secretion may be specific to liver cancer cells.…”

“…Phosphorylation also affects the subcellular localization of PKCδ and its activation. Our recent study revealed that cytosolic PKCδ translocates to the extracellular space and acts as a growth factor for liver cancer cells or tumors[ 9 ]. In this review, we summarize studies reported to date regarding the intracellular function of PKCδ in cancerous phenotypes of liver cancer.…”

Co-administration of tyrosine kinase inhibitors with rottlerin in metastatic prostate cancer cells

“…Regulating several intracellular signaling pathways, PKC has been linked to the carcinogenesis of many malignancies, including prostate cancer (Ratnayake et al, 2021[ 35 ]). Within the family of PKC isoforms, PKCδ plays a critical role in regulating apoptosis and tumor progression (Yamada et al, 2021[ 40 ]). PKCδ is a serine-threonine kinase with opposing functions that depend on its localization, tyrosine phosphorylation and the presence of other pro- and anti-apoptotic signaling molecules.…”