Proteasomal degradation of core protein variants from chronic hepatitis B patients

TRIM26 positively affects hepatitis B virus replication by inhibiting proteasome-dependent degradation of viral core protein

Chronic hepatitis B virus (HBV) infection is a major medical concern worldwide. Current treatments for HBV infection effectively inhibit virus replication; however, these treatments cannot cure HBV and novel treatment-strategies should be necessary. In this study, we identified tripartite motif-containing protein 26 (TRIM26) could be a supportive factor for HBV replication. Small interfering RNA-mediated TRIM26 knockdown (KD) modestly attenuated HBV replication in human hepatocytes. Endogenous TRIM26 physically interacted with HBV core protein (HBc), but not polymerase and HBx, through the TRIM26 SPRY domain. Unexpectedly, TRIM26 inhibited HBc ubiquitination even though TRIM26 is an E3 ligase. HBc was degraded by TRIM26 KD in Huh-7 cells, whereas the reduction was restored by a proteasome inhibitor. RING domain-deleted TRIM26 mutant (TRIM26ΔR), a dominant negative form of TRIM26, sequestered TRIM26 from HBc, resulting in promoting HBc degradation. Taking together, this study demonstrated that HBV utilizes TRIM26 to avoid the proteasome-dependent HBc degradation. The interaction between TRIM26 and HBc might be a novel therapeutic target against HBV infection.

Design and Synthesis of Hepatitis B Virus (HBV) Capsid Assembly Modulators and Evaluation of Their Activity in Mammalian Cell Model

Capsid assembly modulators (CAMs) have emerged as a promising class of antiviral agents. We studied the effects of twenty-one newly designed and synthesized CAMs including heteroaryldihydropyrimidine compounds (HAPs), their analogs and standard compounds on hepatitis B virus (HBV) capsid assembly. Cytoplasmic expression of the HBV core (HBc) gene driven by the exogenously delivered recombinant alphavirus RNA replicon was used for high level production of the full-length HBc protein in mammalian cells. HBV capsid assembly was assessed by native agarose gel immunoblot analysis, electron microscopy and inhibition of virion secretion in HepG2.2.15 HBV producing cell line. Induced fit docking simulation was applied for modelling the structural relationships of the synthesized compounds and HBc. The most efficient were the HAP class compounds—dihydropyrimidine 5-carboxylic acid n-alkoxyalkyl esters, which induced the formation of incorrectly assembled capsid products and their accumulation within the cells. HBc product accumulation in the cells was not detected with the reference HAP compound Bay 41-4109, suggesting different modes of action. A significant antiviral effect and substantially reduced toxicity were revealed for two of the synthesized compounds. Two new HAP compounds revealed a significant antiviral effect and a favorable toxicity profile that allows these compounds to be considered promising leads and drug candidates for the treatment of HBV infection. The established alphavirus based HBc expression approach allows for the specific selection of capsid assembly modulators directly in the natural cell environment.

The interaction of hepatitis B virus with the ubiquitin proteasome system in viral replication and associated pathogenesis

Background

The ubiquitin proteasome system (UPS) regulates the expression levels of cellular proteins by ubiquitination of protein substrates followed by their degradation via the proteasome. As a highly conserved cellular degradation mechanism, the UPS affects a variety of biological processes and participates in viral propagation.

Main body

During hepatitis B virus (HBV) infection, the UPS is shown to act as a double-edged sword in viral pathogenesis. On the one hand, the UPS acts as a host defense mechanism to selectively recognize HBV proteins as well as special cellular proteins that favor the viral life cycle and induces their ubiquitin-dependent proteasomal degradation to limit HBV infection. On the other hand, the HBV has evolved to subvert the UPS function for its own advantage. Moreover, in the infected hepatocytes, certain cellular proteins that are dependent on the UPS are involved in abnormal biological processes which are mediated by HBV.

Conclusion

The molecular interaction of HBV with the UPS to modulate viral propagation and pathogenesis is summarized in the review. Considering the important role of the UPS in HBV infection, a better understanding of the HBV-UPS interaction could provide novel insight into the mechanisms that are involved in viral replication and pathogenesis and help to develop potential treatment strategies targeting the UPS.

Proteasomes regulate hepatitis B virus replication by degradation of viral core-related proteins in a two-step manner

The cellular proteasomes presumably inhibit the replication of hepatitis B virus (HBV) due to degradation of the viral core protein (HBcAg). Common proteasome inhibitors, however, either enhance or inhibit HBV replication. In this study, the exact degradation process of HBcAg and its influences on HBV replication were further studied using bioinformatic analysis, protease digestion assays of recombinant HBcAg, and proteasome inhibitor treatments of HBV-producing cell line HepG2.2.15. Besides HBcAg and hepatitis B e antigen precursor, common hepatitis B core-related antigens (HBcrAgs), the small and the large degradation intermediates of these HBcrAgs (HBcrDIs), were regularly found in cytosol of HepG2.2.15 cells. Further, the results of investigation reveal that the degradation process of cytosolic HBcrAgs in proteasomes consists of two steps: the limited proteolysis into HBcrDIs by the trypsin-like (TL) activity and the complete degradation of HBcrDIs by the chymotrypsin-like (chTL) activity. Concordantly, HBcrAgs and the large HBcrDI or HBcrDIs (including the small HBcrDI) were accumulated when the TL or chTL activity was inhibited, which generally correlated with enhancement and inhibition of HBV replication, respectively. The small HBcrDI inhibited HBV replication by assembling into the nucleocapsids and preventing the victim particles from being mature enough for envelopment. The two-step degradation manner may highlight some new anti-HBV strategies.

Preparation by alkaline treatment and detailed characterisation of empty hepatitis B virus core particles for vaccine and gene therapy applications

Hepatitis B virus (HBV) core (HBc) virus-like particles (VLPs) are one of the most powerful protein engineering tools utilised to expose immunological epitopes and/or cell-targeting signals and for the packaging of genetic material and immune stimulatory sequences. Although HBc VLPs and their numerous derivatives are produced in highly efficient bacterial and yeast expression systems, the existing purification and packaging protocols are not sufficiently optimised and standardised. Here, a simple alkaline treatment method was employed for the complete removal of internal RNA from bacteria- and yeast-produced HBc VLPs and for the conversion of these VLPs into empty particles, without any damage to the VLP structure. The empty HBc VLPs were able to effectively package the added DNA and RNA sequences. Furthermore, the alkaline hydrolysis technology appeared efficient for the purification and packaging of four different HBc variants carrying lysine residues on the HBc VLP spikes. Utilising the introduced lysine residues and the intrinsic aspartic and glutamic acid residues exposed on the tips of the HBc spikes for chemical coupling of the chosen peptide and/or nucleic acid sequences ensured a standard and easy protocol for the further development of versatile HBc VLP-based vaccine and gene therapy applications.

Entecavir combined with furin inhibitor simultaneously reduces hepatitis B virus replication and e antigen secretion

BACKGROUND

The antiviral therapy of chronic hepatitis B virus (HBV) infection pursues the dual goals, virological response (undetectable serum HBV DNA) and hepatitis B e antigen (HBeAg) serological response (serum HBeAg loss/seroconversion). It is relatively difficult, however, to realize the serological response, especially for nucleotide/nucleoside analogs. Furin, a proprotein convertase, is involved in HBeAg maturation. The suppression of furin using inhibitors accordingly reduces HBeAg secretion, but possibly enhances HBV replication. For these reasons, the strategy based on the combination of nucleoside analog entecavir (ETV) and furin inhibitors to inhibit HBV replication and HBeAg secretion simultaneously were studied here.

METHODS

The suppression of furin was performed using inhibitors decanoyl-RVKR-chloromethylketone (CMK) and hexa-D-arginine (D6R) or the expression of furin inhibitory prosegment. The influence of furin suppression on HBV replication and the effect of CMK combined with nucleoside analog entecavir (ETV) on HBV replication and HBeAg secretion was investigated in HepG2.2.15 cells. HBeAg level in media was detected using enzyme-linked immunosorbent assay. Intracellular viral antigens and HBV DNA were detected using Western and Southern blotting analyses, respectively.

RESULTS

CMK, D6R and the expression of inhibitory prosegment all significantly reduced HBeAg secretion, but only CMK enhance HBV replication. Concordantly, only CMK post-transcriptionally accumulated cytosolic HBV replication-essential hepatitis B core antigen (HBcAg). The HBcAg-accumulating effect of CMK was further found to be resulted from its redundant inhibitory effect on the trypsin-like activity of cellular proteasomes that are responsible for HBcAg degradation. Moreover, the viral replication-enhancing effect of CMK was abrogated by ETV and ETV combined with CMK reduced HBV replication and HBeAg secretion simultaneously.

CONCLUSION

The suppression of furin itself does not enhance HBV replication. Nucleotide/nucleoside analogs combined with furin inhibitors may be a potential easy way to realize the dual goals of the antiviral therapy for chronic hepatitis B in the future.

A novel human protein is able to interact with hepatitis B virus core deletion mutant but not with the wild-type protein

Hepatitis B virus mutants with in-frame deletions in the central part of the core gene are associated with a severe course of infection in long-term immunosuppressed renal transplant recipients. In this study, yeast two-hybrid system was employed to investigate interaction capabilities of two core mutants with deleted 77-93 and 86-93 amino acids. The same mutant and wild-type (WT) protein pairs which form core-like particles inside bacterial cells were able to interact also in two-hybrid system. To find host proteins possibly involved in enhanced pathogenesis of the mutant variants, a human hepatocyte cDNA library was screened for proteins interacting with the mutant but not with the WT core protein. A human protein of unknown function FLJ20850 interacted specifically with the mutant proteins. An attempt to determine interacting regions revealed that FLJ20850 was unable to interact without significant parts of its C- or N-end, and introduced deletion in the central region conferred interaction capability to the WT core protein.

GTPase activity is not essential for the interferon-inducible MxA protein to inhibit the replication of hepatitis B virus

Multiple studies have established that GTPase activity is critical for MxA to act against RNA viruses. Recently, it was shown that MxA can also restrict the replication of hepatitis B virus (HBV), a DNA virus, but the requirements for GTPase activity in inhibition of HBV by MxA remain unknown. Here, we report that GTPase-defective mutants (K83A, T103A, and L612K) can downregulate extracellular HBsAg and HBeAg and reduce the expression of extra- and intracellular HBV DNA in HepG2 cells to levels similar to that achieved by wild-type MxA. Furthermore, TMxA and T103, two nuclear forms of wild-type MxA and a GTPase-defective mutant (T103A) could only slightly decrease the expression of extra- and intracellular HBV DNA in HepG2 cells. In conclusion, GTPase activity is not essential for MxA protein to inhibit HBV replication, and MxA may have only a minimal effect on the replicative cycle of HBV in the nucleus.