Hepatitis B virus x protein in the pathogenesis of hepatitis B virus‐induced hepatocellular carcinoma

Abstract

Currently available evidence supports a role for the hepatitis B virus (HBV) x gene and protein in the pathogenesis of HBV‐induced hepatocellular carcinoma (HCC). HBx gene is often included, and remains functionally active, in the HBV DNA that is frequently integrated into cellular DNA during hepatocellular carcinogenesis. HBx protein promotes cell cycle progression, inactivates negative growth regulators, and binds to and inhibits the expression of p53 tumour suppressor gene and other tumour suppressor genes and senescence‐related factors. However, the molecular mechanisms responsible for HBx protein‐induced HCC remain uncertain. Only some of the more fully documented or more recently recognised mechanisms are reviewed. During recent years evidence has accumulated that HBx protein modulates transcription of methyl transferases, causing regional hypermethylation of DNA that results in silencing of tumour suppressor genes, or global hypomethylation that results in chromosomal instability, thereby playing a role in hepatocarcinogenesis. HBx protein has both anti‐apoptotic and pro‐apoptotic actions, apparently contradictory effects that have yet to be explained. Particularly important among the anti‐apoptotic properties is inhibition of p53. Recent experimental observations suggest that HBx protein may increase the expression of TERT and telomerase activity, prolonging the life‐span of hepatocytes and contributing to malignant transformation. The protein also interferes with nucleotide excision repair through both p53‐dependent and p53‐ independent mechanisms. Carboxy‐terminal truncated HBx protein loses its inhibitory effects on cell proliferation and pro‐apoptotic properties, and it may enhance the protein's ability to transform oncogenes. Dysregulation of IGF‐II enhances proliferation and anti‐apoptotic effects of oncogenes, resulting in uncontrolled cell growth.