Suppression of mRNA accumulation by the duck hepatitis B virus reverse transcriptase

The hepatitis B virus polymerase

Hepatitis B virus (HBV) is a hepatotropic, partially double-stranded DNA virus that replicates by reverse transcription and is a major cause of chronic liver disease and hepatocellular carcinoma. Reverse transcription is catalyzed by the four-domain multifunctional HBV polymerase (P) protein that has protein-priming, RNA- and DNA-dependent DNA synthesis (i.e., reverse transcriptase), and ribonuclease H activities. P also likely promotes the three strand transfers that occur during reverse transcription, and it may participate in immune evasion by HBV. Reverse transcription is primed by a tyrosine residue in the amino-terminal domain of P, and P remains covalently attached to the product DNA throughout reverse transcription. The reverse transcriptase activity of P is the target for the nucleos(t)ide analog drugs that dominate HBV treatment, and P is the target of ongoing efforts to develop new drugs against both the reverse transcriptase and ribonuclease H activities. Despite the unusual reverse transcription pathway catalyzed by P and the importance of P to HBV therapy, understanding the enzymology and structure of HBV P severely lags that of the retroviral reverse transcriptases due to substantial technical challenges to studying the enzyme. Obtaining a better understanding of P will broaden our appreciation of the diversity among reverse transcribing elements in nature, and will help improve treatment for people chronically infected with HBV.

Anti-Hepatitis B Virus Effects of Dehydrocheilanthifoline fromCorydalis saxicola

In this report, the anti-hepatitis B virus (HBV) activity of dehydrocheilanthifoline (DHCH), a quaternary ammonium alkaloid isolated from the traditional Chinese medicine Corydalis saxicola Bunting (Papaveraceae), was determined in vitro. Following six days of treatment, DHCH efficiently suppressed the secretions of HBsAg and HBeAg in HepG2.2.15 cell cultures, with a half-maximal inhibitory concentration (IC50) of 15.84 and 17.12 μM, and with a therapeutic index (TI) of 7.32 and 6.77, respectively. Further studies revealed that DHCH reduced the levels of extracellular DNA, intracellular DNA and covalently closed circular DNA (cccDNA) of HBV in a dose-dependent and time-dependent manner, with IC50values of 15.08, 7.62 and 8.25 μM, respectively after six days of treatment. In contrast, the level of viral pre-genomic RNA (pgRNA) increased 6.13-fold after treatment with DHCH. Together, it was demonstrated for the first time that DHCH could significantly inhibit the replication of HBV, which warrants further studies on the antiviral mechanisms of DHCH, and suggests that it may be a promising candidate in the therapy of HBV infection.

Cryptic protein priming sites in two different domains of duck hepatitis B virus reverse transcriptase for initiating DNA synthesis in vitro

Initiation of reverse transcription in hepadnaviruses is accomplished by a unique protein-priming mechanism whereby a specific Y residue in the terminal protein (TP) domain of the viral reverse transcriptase (RT) acts as a primer to initiate DNA synthesis, which is carried out by the RT domain of the same protein. When separate TP and RT domains from the duck hepatitis B virus (DHBV) RT protein were tested in a trans-complementation assay in vitro, the RT domain could also serve, unexpectedly, as a protein primer for DNA synthesis, as could a TP mutant lacking the authentic primer Y (Y96) residue. Priming at these other, so-called cryptic, priming sites in both the RT and TP domains shared the same requirements as those at Y96. A mini RT protein with both the TP and RT domains linked in cis, as well as the full-length RT protein, could also initiate DNA synthesis using cryptic priming sites. The cryptic priming site(s) in TP was found to be S/T, while those in the RT domain were Y and S/T. As with the authentic TP Y96 priming site, the cryptic priming sites in the TP and RT domains could support DNA polymerization subsequent to the initial covalent linkage of the first nucleotide to the priming amino acid residue. These results provide new insights into the complex mechanisms of protein priming in hepadnaviruses, including the selection of the primer residue and the interactions between the TP and RT domains that is essential for protein priming.

Effect of oxymatrine on the replication cycle of hepatitis B virus in vitro

AIM: To determine the antiviral mechanism or target of oxymatrine against hepatitis B virus (HBV).

METHODS: HepG2.2.15 cells were incubated with culture medium containing 500 μg/mL of oxymatrine for 2 and 5 d. The surface antigen of HBV (HBsAg) and e antigen of HBV (HBeAg) in supernatant were determined by ELISA. HBV DNA in supernatant, and intracellular covalently closed circular DNA (cccDNA), relaxed circular DNA (rcDNA) and pregenomic RNA (pgRNA) were quantified by specific real-time polymerase chain reaction (PCR) or reverse transcription (RT)-PCR.

RESULTS: Treatment with oxymatrine for 2 d and 5 d reduced the production of HBV by the cell line, as indicated by the decline of HBsAg (22.67%, t = 5.439, P = 0.0322 and 22.39%, t = 5.376, P = 0.0329, respectively), HBeAg (55.34%, t = 9.859, P = 0.0101 and 43.97%, t = 14.080, P = 0.0050) and HBV DNA (40.75%, t = 4.570, P = 0.0447 and 75.32%, t = 14.460, P = 0.0047) in the supernatant. Intracellular cccDNA was also markedly reduced by 63.98% (t = 6.152, P = 0.0254) and 80.83% (t = 10.270, P = 0.0093), and intracellular rcDNA by 34.35% (t = 4.776, P = 0.0413) and 39.24% (t = 10.050, P = 0.0097). In contrast, intracellular pgRNA increased by 6.90-fold (t = 8.941, P = 0.0123) and 3.18-fold (t = 7.432, P = 0.0176) after 500 μg/mL of oxymatrine treatment for 2 d and 5 d, respectively.

CONCLUSION: Oxymatrine may inhibit the replication of HBV by interfering with the process of packaging pgRNA into the nucleocapsid, or inhibiting the activity of the viral DNA polymerase.

Chimeric constructs between two hepatitis B virus genomes confirm transcriptional impact of core promoter mutations and reveal multiple effects of core gene mutations

Hepatitis B virus (HBV) clone 4B replicated much more efficiently than clone 2A of the same genotype. Introduction of its T1753C, A1762T, G1764A, and C1766T core promoter mutations into the 2A genome greatly enhanced genome replication and suppressed HBeAg expression. Here we show that these effects are mediated by transcriptional up regulation of pregenomic RNA and suppression of precore RNA. Analysis of chimeric constructs suggested that the 5' end of the 2A core gene conferred higher level of pregenomic RNA, but less core protein and genome replication relative to the 4B sequence. Genome maturity of secreted virions was reduced by mutations present in the core protein of the 2A genome but enhanced by mutations found in the 4B core protein. The 4B core protein migrated faster than that of clone 2A. The possible links among the various phenotypes and the responsible mutations remain to be established.

Pre-P is a secreted glycoprotein encoded as an N-terminal extension of the duck hepatitis B virus polymerase gene

The duck hepatitis B virus (DHBV) pregenomic RNA is a bicistronic mRNA encoding the core and polymerase proteins. Thirteen AUGs (C2 to C14) and 10 stop codons (S1 to S10) are located between the C1 AUG for the core protein and the P1 AUG that initiates polymerase translation. We previously found that the translation of the DHBV polymerase is initiated by ribosomal shunting. Here, we assessed the biosynthetic events after shunting. Translation of the polymerase open reading frame was found to initiate at the C13, C14, and P1 AUGs. Initiation at the C13 AUG occurred through ribosomal shunting because translation from this codon was cap dependent but was insensitive to blocking ribosomal scanning internally in the message. C13 and C14 are in frame with P1, and translation from these upstream start codons led to the production of larger isoforms of P. We named these isoforms "pre-P" by analogy to the pre-C and pre-S regions of the core and surface antigen open reading frames. Pre-P was produced in DHBV16 and AusDHBV-infected duck liver and was predicted to exist in 80% of avian hepadnavirus strains. Pre-P was not encapsidated into DHBV core particles, and the viable strain DHBV3 cannot make pre-P, so it is not essential for viral replication. Surprisingly, we found that pre-P is an N-linked glycoprotein that is secreted into the medium of cultured cells. These data indicate that DHBV produces an additional protein that has not been previously reported. Identifying the role of pre-P may improve our understanding of the biology of DHBV infection.

The C terminus of foamy retrovirus Gag contains determinants for encapsidation of Pol protein into virions

Foamy viruses (FV) differ from orthoretroviruses in many aspects of their replication cycle. A major difference is in the mode of Pol expression, regulation, and encapsidation into virions. Orthoretroviruses synthesize Pol as a Gag-Pol fusion protein so that Pol is encapsidated into virus particles through Gag assembly domains. However, as FV express Pol independently of Gag from a spliced mRNA, packaging occurs through a distinct mechanism. FV genomic RNA contains cis-acting sequences that are required for Pol packaging, suggesting that Pol binds to RNA for its encapsidation. However, it is not known whether Gag is directly involved in Pol packaging. Previously our laboratory showed that sequences flanking the three glycine-arginine-rich (GR) boxes at the C terminus of FV Gag contain domains important for RNA packaging and Pol expression, cleavage, and packaging. We have now shown that both deletion and substitution mutations in the first GR box (GR1) prevented neither the assembly of particles with wild-type density nor packaging of RNA genomes but led to a defect in Pol packaging. Site-directed mutagenesis of GR1 indicated that the clustered positively charged amino acids in GR1 play important roles in Pol packaging. Our results suggest that GR1 contains a Pol interaction domain and that a Gag-Pol complex is formed and binds to RNA for incorporation into virions.

Hepatitis B virus polymerase inhibits the interferon-inducible MyD88 promoter by blocking nuclear translocation of Stat1

Previous studies have suggested that hepatitis B virus (HBV) blocks expression of the alpha interferon (IFN-alpha)-inducible myeloid differential primary response protein (MyD88) gene. To study the molecular mechanism(s) of the inhibition of MyD88 expression by HBV, MyD88 promoter reporter plasmids and vectors expressing different HBV viral proteins were constructed. Co-transfection experiments showed that IFN-induced MyD88 promoter activity was inhibited by HBV polymerase expression in a dose-dependent manner and that the terminal protein (TP) domain of HBV polymerase was responsible for this antagonistic activity. Analysis of site mutants showed that the region targeted by the polymerase protein contained the signal transducer and activator of transcription (Stat) binding site. Chromatin immunoprecipitation analysis showed that the IFN-induced DNA-binding activity of Stat1 was affected. Further study demonstrated that the HBV polymerase protein inhibited the Stat1 nuclear translocation induced by IFN-alpha, but did not induce Stat1 degradation nor interfere with its phosphorylation. In addition, HBV polymerase could inhibit the transcriptional activity of other IFN-stimulated response element-driven promoters and the expression of interferon-stimulated genes (ISGs), such as Stat1 and ISG15. In summary, these results indicate that HBV polymerase is a general inhibitor of IFN signalling and can inhibit IFN-inducible MyD88 expression by inhibiting the activity of the MyD88 promoter through blocking the nuclear translocation of Stat1.

Hepatitis B virus polymerase suppresses translation of pregenomic RNA via a mechanism involving its interaction with 5' stem-loop structure

The pregenomic RNA (pgRNA) of hepadnaviruses serves a dual role: as mRNA for the core (C) and polymerase (P) synthesis and as an RNA template for viral genome replication. A question arises as to how these two roles are regulated. We hypothesized that the P protein could suppress translation of the pgRNA via its interaction with 5' stem-loop structure (epsilon or encapsidation signal). Consistent with the hypothesis, we observed up-regulation of the C protein level in the absence of the P protein expression in a physiological context. Importantly, translational suppression depended on the 5' epsilon sequence. Furthermore, the impact of the P protein on ongoing translation of the C ORF was directly demonstrated by polysome distribution analysis. We conclude that the P protein suppresses translation of the pgRNA via a mechanism involving its interaction with the 5' epsilon sequence, a finding that implicates the coordinated switch from translation to genome replication.

Hepatitis B virus replication

Hepadnaviruses, including human hepatitis B virus (HBV), replicate through reverse transcription of an RNA intermediate, the pregenomic RNA (pgRNA). Despite this kinship to retroviruses, there are fundamental differences beyond the fact that hepadnavirions contain DNA instead of RNA. Most peculiar is the initiation of reverse transcription: it occurs by protein-priming, is strictly committed to using an RNA hairpin on the pgRNA, ε, as template, and depends on cellular chaperones; moreover, proper replication can apparently occur only in the specialized environment of intact nucleocapsids. This complexity has hampered an in-depth mechanistic understanding. The recent successful reconstitution in the test tube of active replication initiation complexes from purified components, for duck HBV (DHBV), now allows for the analysis of the biochemistry of hepadnaviral replication at the molecular level. Here we review the current state of knowledge at all steps of the hepadnaviral genome replication cycle, with emphasis on new insights that turned up by the use of such cell-free systems. At this time, they can, unfortunately, not be complemented by three-dimensional structural information on the involved components. However, at least for the ε RNA element such information is emerging, raising expectations that combining biophysics with biochemistry and genetics will soon provide a powerful integrated approach for solving the many outstanding questions. The ultimate, though most challenging goal, will be to visualize the hepadnaviral reverse transcriptase in the act of synthesizing DNA, which will also have strong implications for drug development.