Hepatitis B virus X protein (HBx)
Hepatitis B virus (HBV) infection is a major health problem, with 400 million people chronically infected worldwide who are at high risk of developing liver cirrhosis and hepatocellular carcinoma (HCC). The epidemiological evidence linking HBV infection to HCC is very strong, and despite the mechanisms underlying HBV-associated carcinogenesis remain to be fully defined, a growing number of studies support a direct role of HBV in the process. The HBV-encoded regulatory protein hepatitis B virus X protein (HBx) is thought to contribute to HBV oncogenicity. HBx transforms SV40-immortalized mouse hepatocytes, induces cell cycle progression within the regenerating liver, causes liver cancer in some transgenic mice, and acts as a cofactor to accelerate cancer development in other mouse models. HBx is a 154-amino acid protein with an N-terminal negative regulatory domain and C-terminal transactivation or coactivation domain that has been detected both in the cytoplasm and in the nuclei of infected hepatocytes. Studies in transfected cells have shown that HBx expression affects cellular functions such as cytoplasmic calcium regulation, cell signaling, transcription, cell proliferation, DNA repair, and apoptosis.
The lack of homology of the X ORF to host protein and its high conservation to other mammalian hepadnaviruses genomes strongly suggest that HBx play a role in virus life cycle. Although initial studies suggested that HBx was not required for virus replication in cell culture, experiments with the highly related woodchuck hepatitis virus (WHV) system indicate that the WHV X protein (WHx) is required for virus replication in vivo. Studies performed using a plasmid-based replication assays that use greater-than-unit-length HBV genomes transfected in HCC cell lines or injected via the mouse tail vein have repeatedly confirmed that HBx potentiate HBV replication. The HBx protein behaves as a promiscuous transactivator of viral and cellular promoters. Although the subcellular localization of HBx seems to be mainly cytoplasmic, a small variable fraction of the protein can be found in the nucleus, and the ability of HBx to activate transcription of host genes is thought to occur indirectly by interaction with nuclear transcription factors or by activation of different signal transduction pathways in the cytoplasm.
Nuclear HBx binds the HBV minichromosome and modifies the epigenetic regulation of cccDNA function. PNAS USA November 11 2009. doi: 10.1073/pnas.0908365106
HBV cccDNA, the template for transcription of all viral mRNAs, accumulates in the nucleus of infected cells as a stable episome organized into minichromosomes by histones and non-histone viral and cellular proteins. Using a cccDNA-specific chromatin immunoprecipitation (ChIP)-based quantitative assay, we have previously shown that transcription of the HBV minichromosome is regulated by epigenetic changes of cccDNA-bound histones and that modulation of the acetylation status of cccDNA-bound H3/H4 histones impacts on HBV replication. We now show that the cellular histone acetyltransferases CBP, p300, and PCAF/GCN5, and the histone deacetylases HDAC1 and hSirt1 are all recruited in vivo onto the cccDNA. We also found that the HBx regulatory protein produced in HBV replicating cells is recruited onto the cccDNA minichromosome, and the kinetics of HBx recruitment on the cccDNA parallels the HBV replication. As expected, an HBV mutant that does not express HBx is impaired in its replication, and exogenously expressed HBx transcomplements the replication defects. p300 recruitment is severely impaired, and cccDNA-bound histones are rapidly hypoacetylated in cells replicating the HBx mutant, whereas the recruitment of the histone deacetylases hSirt1 and HDAC1 is increased and occurs at earlier times. Finally, HBx mutant cccDNA transcribes significantly less pgRNA. Altogether our results further support the existence of a complex network of epigenetic events that influence cccDNA function and HBV replication and identify an epigenetic mechanism (i.e., to prevent cccDNA deacetylation) by which HBx controls HBV replication.
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Tags: Biology, cancer, HBV, Medicine, Microbiology, Science, Virology, virus
