Role of nuclear Met receptor in hepatocellular carcinoma

Abstract

Met is a receptor tyrosine kinase encoded by c-Met proto-oncogene which triggers a wide range of normal physiological signaling cascades upon activation by hepatocyte growth factor (HGF). However, aberrant Met expression and activity are commonly found in human cancers. Emerging evidence has shown the presence of nuclear Met (nMet) in some cancerous tissues and cell lines, suggesting that nMet could have unexplored functions in the nucleus. Met is an attractive therapeutic target for human cancers. Indeed, therapeutic antibodies and inhibitors that antagonize dysregulated Met are currently in clinical trials. However, the classic therapeutic strategies against the Met surface receptor will unequivocally exhibit limitations in acting against nMet. Despite the oncogenic role of Met in hepatocellular carcinoma (HCC), the existence and functions of nMet in HCC has never been reported. In this study, we aim to evaluate the clinical relevance and characterize the functions of nMet in HCC. We studied nMet expression in 103 human HCC paired samples by immunohistochemistry. Our data showed that nMet was overexpressed in 89.3% of HCC tissues and its expression was progressively increased along HCC development. Nonetheless, nMet overexpression was significantly associated with venous invasion and poorer overall survival. We found that nMet, which has a lower molecular weight than Met, could only be detected using an antibody against the carboxyl terminus of Met (C28) in tumorous tissues. This finding suggests that nMet only comprises the carboxyl cytoplasmic region of full length Met. Immunofluorescence microscopy showed that juxtamembrane deletion of Met accumulates in the nucleus whereas truncated tyrosine kinase domain accumulates in the cytoplasm, implying a region important for nuclear localization of the cytoplasmic fragment. Cellular fractionation also confirmed the existence of nMet in the nuclear fraction of HCC cells. To explore the functional role of nMet, expression vector was constructed to express the cytoplasmic protein of Met in cells. In vitro functional study showed that overexpression of nMet augments the invasiveness of HCC cells suggesting the unexplored functional effect of nMet exerted within nucleus in promoting HCC invasiveness. Furthermore, we showed that nMet activates NF-kappaB reporter activity which might enhance the pro-metastatic gene expression to promote tumor invasiveness and aggressiveness.