The hepatitis B virus polymerase

The hepatitis B virus surface antigen: An evolved perfection and its unresolved mysteries

The Hepatitis B Virus has long afflicted the human race, with a widespread impact on the global health system and profound medical implications for those who are chronically infected. Despite its relatively recent discovery, over the last 50 years great advancements have been made towards the characterisation of this complex etiological agent. The virus itself has a highly evolved genome which encodes for seven viral proteins, three of which (the surface antigens) were consequential in the initial discovery and isolation of the virus. These surface antigens are ubiquitously important throughout the viral lifecycle, from capsid envelopment through to receptor-mediated invasion into the hepatocytes. The hepatitis B surface antigens (in particular, the large protein) adopt complex topological folds and tertiary structures, and it is this topological intricacy which facilitates the diverse roles the three surface antigens play in HBV maturation and infection. Here, the biochemical and topological attributes of the three surface antigens are reviewed in detail, with particular focus on their relevance to the establishment of infection. Further research is still required to elucidate the coordinates of the antigen loop and the dynamic topological changes of key motifs during entry and viral morphogenesis; these in turn may provide new leads for therapeutics which may potentiate a functional cure for chronic hepatitis B.

Discovery of bimodal hepatitis B virus ribonuclease H and capsid assembly inhibitors

Hepatitis B virus (HBV) ribonuclease H (RNaseH) inhibitors are a potent class of antivirals that prevent degradation of the viral pregenomic RNA during reverse transcription and block formation of mature HBV DNAs. Development of HBV RNaseH inhibitors is entering advanced preclinical analyses. To ensure the mechanism of action was fully understood, we defined the effects of RNaseH inhibitors on other steps of HBV replication. Some N-hydroxypyridinedione (HPD) HBV RNaseH inhibitors significantly reduced accumulation of capsids in HBV-replicating cells. A representative HPD 1466, with a 50% effective concentration against HBV replication of 0.25 µM, decreased capsid and core protein accumulation by 50-90% in HepDES19 and HepG2.2.15 cells. Surprisingly, 1466 did not affect pregenomic RNA encapsidation, demonstrating a specific effect on empty capsids. HBV genomic replication was not necessary for 1466's inhibitory effect as it decreased capsid accumulation in cells transfected with replication-deficient mutants blocking pgRNA encapsidation (Δ-bulge), DNA synthesis (YMHA), and RNaseH (D702A) activities. 1466 also decreased capsid and core protein accumulation in cells transfected with a core protein expression plasmid, indicating that other HBV products are unneeded. 1466 reduced initial capsid assembly rates in biochemical assembly reactions employing purified core protein (Cp149), demonstrating a specific effect on HBV core protein. We conclude that the bimodal HPD HBV RNaseH inhibitor 1466 is the prototypic member of a new class of capsid assembly modulators (CAM) that inhibits capsid assembly rather than accelerating it, as all other CAM classes do. We propose that this class be called CAM-I, for CAM-inhibitor. These results lay the foundation for identifying bimodal HBV antivirals targeting the RNaseH and capsid assembly.

From the Cytoplasm into the Nucleus-Hepatitis B Virus Travel and Genome Repair

Hepatitis B virus (HBV) is a major global health concern, affecting millions of people worldwide. HBV is part of the hepadnaviridae family and one of the primary causes of acute and chronic liver infections, leading to conditions such as cirrhosis and hepatocellular carcinoma (HCC). Understanding the intracellular transport and genome repair mechanisms of HBV is crucial for developing new drugs, which-in combination with immune modulators-may contribute to potential cures. This review will explore the current knowledge of HBV intracytoplasmic and nuclear transport, as well as genome repair processes, while drawing comparisons to other viruses with nuclear replication.

HBV DNA polymerase regulates tumor cell glycogen to enhance the malignancy of HCC cells

BACKGROUND

The essential function of HBV DNA polymerase (HBV-DNA-Pol) is to initiate viral replication by reverse transcription; however, the role of HBV-DNA-Pol in HBV-associated HCC has not been clarified. Glycogen phosphorylase L (PYGL) is a critical regulator of glycogenolysis and is involved in tumorigenesis, including HCC. However, it is unknown whether HBV-DNA-Pol regulates PYGL to contribute to HCC tumorigenesis.

METHODS

Bioinformatic analysis, real-time quantitative PCR, western blotting, and oncology functional assays were performed to determine the contribution of HBV-DNA-Pol and PYGL to HCC development and glycolysis. The mechanisms of co-immunoprecipitation and ubiquitination were employed to ascertain how HBV-DNA-Pol upregulated PYGL.

RESULTS

Overexpression of HBV-DNA-Pol enhanced HCC progression in vitro and in vivo. Mechanistically, HBV-DNA-Pol interacted with PYGL and increased PYGL protein levels by inhibiting PYGL ubiquitination, which was mediated by the E3 ligase TRIM21. HBV-DNA-Pol competitively impaired the binding of PYGL to TRIM21 due to its stronger binding affinity to TRIM21, suppressing the ubiquitination of PYGL. Moreover, HBV-DNA-Pol promoted glycogen decomposition by upregulating PYGL, which led to an increased flow of glucose into glycolysis, thereby promoting HCC development.

CONCLUSIONS

Our study reveals a novel mechanism by which HBV-DNA-Pol promotes HCC by controlling glycogen metabolism in HCC, establishing a direct link between HBV-DNA-Pol and the Warburg effect, thereby providing novel targets for HCC treatment and drug development.

In Vitro Enzymatic and Cell Culture-Based Assays for Measuring Activity of HBV Ribonuclease H Inhibitors

HBV is a small, enveloped DNA virus that replicates by reverse transcription of an RNA intermediate. Current anti-HBV treatment regiments employ interferon α or nucleos(t)ide analogs, but they are not curative, are of long duration, and can be accompanied by systemic side-effects. The HBV ribonuclease H (RNaseH) is essential for viral replication; however, it is unexploited as a drug target. RNaseH inhibitors that actively block viral replication would represent an important addition to the potential new drugs for treating HBV infection. Here, we describe two methods to measure the activity of RNaseH inhibitors. The DNA oligonucleotide-directed RNA cleavage assay allows mechanistic analysis of compounds for anti-HBV RNaseH activity. Analysis of preferential inhibition of plus-polarity DNA strand synthesis by HBV RNaseH inhibitors in a cell culture model of HBV replication can be used to measure the ability of RNaseH inhibitors to block viral replication.

Biology of the hepatitis B virus (HBV) core and capsid assembly modulators (CAMs) for chronic hepatitis B (CHB) cure

Hepatitis B virus (HBV) is a hepadnavirus, a small DNA virus that infects liver tissue, with some unusual replication steps that share similarities to retroviruses. HBV infection can lead to chronic hepatitis B (CHB), a life-long infection associated with significant risks of liver disease, especially if untreated. HBV is a significant global health problem, with hundreds of millions currently living with CHB. Currently approved strategies to prevent or inhibit HBV are highly effective, however, a cure for CHB has remained elusive. To achieve a cure, elimination of the functionally integrated HBV covalently closed chromosomal DNA (cccDNA) genome is required. The capsid core is an essential component of HBV replication, serving roles when establishing infection and in creating new virions. Over the last two and a half decades, significant efforts have been made to find and characterize antivirals that target the capsid, specifically the HBV core protein (Cp). The antivirals that interfere with the kinetics and morphology of the capsid, termed capsid assembly modulators (CAMs), are extremely potent, and clinical investigations indicate they are well tolerated and highly effective. Several CAMs offer the potential to cure CHB by decreasing the cccDNA pools. Here, we review the biology of the HBV capsid, focused on Cp, and the development of inhibitors that target it.

Global Prevalence of Domestic Cat Hepadnavirus: An Emerging Threat to Cats’ Health?

Hepatitis B is an infectious hepatocellular disease of global concern caused by hepatitis B virus (HBV), which belongs to Hepadnaviridae. Recently, a novel HBV-like virus, domestic cat hepadnavirus (DCH), was detected from an immunocompromised cat with a hepatic disease in Australia. Subsequent molecular investigation by independent research groups revealed that its prevalence rates were 6.5% in Australia, 10.8% in Italy, 12.4% in Thailand, 12.3% in Malaysia, 3.08% in the United Kingdom, and 0.78% in Japan. Although the correlation between DCH infection and hepatic diseases remains to be elucidated, understanding the diversity of circulating DCH will contribute to its prevention and control in domestic cats. Herein, we summarize the current epidemiological data of DCH in these countries.

Efficient Inhibition of Hepatitis B Virus (HBV) Replication and cccDNA Formation by HBV Ribonuclease H Inhibitors during Infection

The hepatitis B virus (HBV) ribonuclease H (RNase H) is an attractive but unexploited drug target. Here, we addressed three limitations to the current state of RNase H inhibitor development: (a) Efficacy has been assessed only in transfected cell lines. (b) Cytotoxicity data are from transformed cell lines rather than primary cells. (c) It is unknown how the compounds work against nucleos(t)ide analog resistant HBV strains. Three RNase H inhibitors from different chemotypes, 110 (α-hydroxytropolone), 1133 (N-hydroxypyridinedione), and 1073 (N-hydroxynapthyridinone), were tested in HBV-infected HepG2-NTCP cells for inhibition of cccDNA accumulation and HBV product formation. 50% effective concentrations (EC₅₀s) were 0.049-0.078 μM in the infection studies compared to 0.29-1.6 μM in transfected cells. All compounds suppressed cccDNA formation by >98% at 5 μM when added shortly after infection. HBV RNA, intracellular and extracellular DNA, and HBsAg secretion were all robustly suppressed. The greater efficacy of the inhibitors when added shortly after infection is presumably due to blocking amplification of the HBV cccDNA, which suppresses events downstream of cccDNA formation. The compounds had 50% cytotoxic concentrations (CC₅₀s) of 16-100 μM in HepG2-derived cell lines but were nontoxic in primary human hepatocytes, possibly due to the quiescent state of the hepatocytes. The compounds had similar EC₅₀s against replication of wild-type, lamivudine-resistant, and adefovir/lamivudine-resistant HBV, as expected because the RNase H inhibitors do not target the viral reverse transcriptase active site. These studies expand confidence in inhibiting the HBV RNase H as a drug strategy and support inclusion of RNase H inhibitors in novel curative drug combinations for HBV.