Chronic hepatitis B virus (HBV) infection is a worldwide health problem, and it has become one of the major causes of end-stage liver disease, including cirrhosis and hepatocellular carcinoma (HCC). In the past decade, the crucial role of HBV in hepatocarcinogenesis has been well established, but the mechanisms underlying how HBV induces malignant transformation of hepatocytes remains unclear. HBV X (HBx) is a 154-amino-acid (154-aa) multifunctional protein that has roles in gene transcription, cell proliferation, and apoptosis. For a long time, HBx has been suspected of playing positive roles in hepatocarcinogenesis, possibly by affecting viral replication and viral proliferation directly or indirectly.
In an attempt to fully understand the role of environmental factors on HBx protein in regulating HBV transcription and replication, researchers used two different cell culture systems in vitro and an immunocompetent HBV replication mouse model in vivo. Moreover, we sought to verify roles of the transcriptional transactivation regions in the C-terminal transactivation domain of HBx in modulating the levels of HBV transcription and replication under physiological conditions in vivo.
HBx was found to be required for wild-type levels of HBV replication in HepG2 cells, but was not essential for the establishment of HBV replication in Huh7 cells, indicating that the effects of HBx on HBV replication and transcription vary depending on the experimental system. No wonder there is confusion about the role(s) of the X protein. In contrast to in vivo experiments, HBx has an important role in stimulating HBV transcription and replication in hepatocytes in vivo. The transcriptional transactivation function of HBx may be crucial for its stimulatory effect on HBV transcription and replication.
Role and Functional Domain of Hepatitis B Virus X Protein in Regulating HBV Transcription and Replication in Vitro and in Vivo. Viruses 2013, 5(5), 1261-1271; doi:10.3390/v5051261
The role of hepatitis B virus (HBV) X protein (HBx) in the regulation of HBV replication remains controversial. In the present study, the role of HBx in regulating HBV replication was initially investigated in both HepG2 and Huh7 in vitro cell lines with a transient transfection system. Next, the regions of HBx responsible for transcriptional transactivation and promotion of HBV replication were mapped in an HBV replication mouse model by in vivo transfection of a series of HBx expression plasmids. In an in vitro setting, HBx deﬁciency had little effect on HBV replication in Huh7 cells, but impaired HBV replication in HepG2 cells. In an in vivo setting, HBx had a strong enhancing effect on HBV transcription and replication. For the C-terminal two-thirds of the protein (amino acids [aa] 51 to 154) was required for this function of HBx, and the regions spanning aa 52 to 72 and 88 to 154 were found to be important for the stimulatory function of HBx on HBV replication. In conclusion, the role of HBx in HBV replication regulation is affected by host cell type, and HBx has an important role in stimulating HBV transcription and replication in hepatocytes in vivo. Further, the transcriptional transactivation function of HBx may be crucial for its stimulatory effect on HBV transcription and replication.