Role of caveolin-1 in hepatitis B virus-induced hepatocarcinogenesis

Abstract

Introduction and Project Objectives: Chronic hepatitis B virus (HBV) infection has been well established to be a major risk factor of hepatocellualr carcinoma (HCC). Accumulating evidence have shown that the smallest protein, hepatitis B virus x prtoein (HBx), among the four proteins encoded by HBV plays a crucial role in HCC pathogenesis. HBx has been implicated in various aspects of liver diseases, for instance, chronic hepatitis, cirrhosis and hepatocarcinogenesis. HBV genome integration generates 3’-deletion in HBV X gene and results in C-terminal truncated HBx. Indeed, C-terminal truncated HBx is frequently detected in tumors of HBV-positive patients with HCC. Functionally, truncated HBx possesses more potent oncogenic capacity than full length form. However, the mechanism underlying the actions of truncated HBx in HCC remains unresolved. The overall goal of this study is to investigate the molecular mechanism and functional effect of truncated HBx-mediated caveolin-1 (Cav1) expression in HCC. We hypothesize that truncated HBx protein acts as a positive regulator of Cav1 leading to the acquisition of aggressive behavior of cancer cells. Methods: The expression of Cav1 and presence of truncated HBx were analyzed and correlated in HCC cell lines and clinical samples. The activation of Cav1 promoter by truncated HBx was studied using luciferase reporter assay. The expression of truncated HBx was induced by doxycycline in SMMC7721 cells and subjected to various functional assays in vitro and in vivo. The expression and role of downstream effector of Cav1 in truncated HBx overexpressing cells was also investigated. Results: Higher expression of Cav1 was found to be significantly correlated with the presence of C-terminal truncated HBx in HCC tissues when compared to cases without truncated HBx. Expression of two naturally occurring C-terminal deletion mutants, HBx1-130 and HBx1-119 were able to upregulate the expression and activate the promoter of Cav1. Conversely, knockdown of HBx expression resulted in reduced expression of Cav1. SMMC7721 cells established with doxycycline inducible expression of HBx1-119 displayed augmented cell migration, invasiveness and tumor development. However, such enhancement in growth and motility was abrogated when Cav1 expression was suppressed in HBx1-119 cells. Our findings also revealed that FERM domain containing protein 5 (FRMD5), acts downstream of Cav1, was also upregulated by HBx1-119. Knockdown of FRMD5 abrogated the oncogenic capacity of HBx1-119. Conclusions: C-terminal truncated HBx mediates the upregulation of Cav1-FRMD5 resulted in the enhanced aggressiveness of HCC cells. Our findings have deciphered novel molecular pathways contributing to the understanding of HBV-induced hepatocarcinogenesis.