Oligomer synthesis by priming deficient polymerase in hepatitis B virus core particle

Decoding the multifaceted interventions between human sirtuin 2 and dynamic hepatitis B viral proteins to confirm their roles in HBV replication

The human sirtuin 2 gene (SIRT2) encodes a full-length Sirt2 protein (i.e., the Sirt2 isoform 1), which primarily functions as a cytoplasmic α-tubulin deacetylase, and which promotes the growth of hepatocellular carcinoma (HCC). Hepatitis B virus (HBV) replication itself, or HBV X (HBx) protein-mediated transcriptional transactivation, enhances Sirt2.1 expression; therefore, Sirt2.1 itself is capable of positively increasing HBV transcription and replication. Sirt2.1 is linked to liver fibrosis and epithelial-to-mesenchymal transition and, consequently, augments the risk of HCC. The Sirt2.1 protein enhances the HBV replication cycle by activating the AKT/glycogen synthase kinase 3 beta (GSK3β)/β-catenin pathway. It also activates the transcription of the viral enhancer I/HBx promoter (EnI/Xp) and enhancer II/HBc promoter (EnII/Cp) by targeting the transcription factor p53. The Sirt2 isoform 2 (Sirt2.2) is mainly localized in the cytoplasm, and the N-terminus is shorter by 37 amino acids than that of Sirt2.1. Despite the truncation of the N-terminal region, Sirt2.2 is still capable of enhancing HBV replication and activating the AKT/GSK3β/β-catenin signaling pathway. The Sirt2 isoform 5 (Sirt2.5) is primarily localized to the nucleus, it lacks a nuclear export signal (NES), and the catalytic domain (CD) is truncated. Upon HBV replication, expression of the Sirt2 isoforms is also enhanced, which further upregulates the HBV replication, and, therefore, supports the vicious cycle of viral replication and progression of the disease. Sirt2 diversely affects HBV replication such that its isoform 1 intensely augments HBV replication and isoform 2 (despite of the truncated N-terminal region) moderately enhances HBV replication. Isoform 5, on the other hand, tends to protect the cell (for smooth long-term continued viral replication) from HBV-induced extreme damage or death via a discrete set of regulatory mechanisms impeding viral mRNAs, the hepatitis B core/capsid protein (HBc), core particles, replicative intermediate (RI) DNAs, and covalently closed circular DNA (cccDNA) levels, and, consequently, limiting HBV replication. In contrast to Sirt2.1 and Sirt 2.2, the Sirt2.5-mediated HBV replication is independent of the AKT/GSK3β/β-catenin signaling cascade. Sirt2.5 is recruited more at cccDNA than the recruitment of Sirt2.1 onto the cccDNA. This recruitment causes the deposition of more histone lysine methyltransferases (HKMTs), including SETDB1, SUV39H1, EZH2, and PR-Set7, along with the respective corresponding transcriptional repressive markers such as H3K9me3, H3K27me3, and H4K20me1 onto the HBV cccDNA. In HBV-replicating cells, Sirt2.5 can also make complexes with PR-Set7 and SETDB1. In addition, Sirt2.5 has the ability to turn off transcription from cccDNA through epigenetic modification via either direct or indirect interaction with HKMTs.

Peptidyl-prolyl cis/trans isomerase Pin1 interacts with hepatitis B virus core particle, but not with HBc protein, to promote HBV replication

Here, we demonstrate that the peptidyl-prolyl cis/trans isomerase Pin1 interacts noncovalently with the hepatitis B virus (HBV) core particle through phosphorylated serine/threonine-proline (pS/TP) motifs in the carboxyl-terminal domain (CTD) but not with particle-defective, dimer-positive mutants of HBc. This suggests that neither dimers nor monomers of HBc are Pin1-binding partners. The ¹⁶²TP, ¹⁶⁴SP, and ¹⁷²SP motifs within the HBc CTD are important for the Pin1/core particle interaction. Although Pin1 dissociated from core particle upon heat treatment, it was detected as an opened-up core particle, demonstrating that Pin1 binds both to the outside and the inside of the core particle. Although the amino-terminal domain S/TP motifs of HBc are not involved in the interaction, ⁴⁹SP contributes to core particle stability, and ¹²⁸TP might be involved in core particle assembly, as shown by the decreased core particle level of S49A mutant through repeated freeze and thaw and low-level assembly of the T128A mutant, respectively. Overexpression of Pin1 increased core particle stability through their interactions, HBV DNA synthesis, and virion secretion without concomitant increases in HBV RNA levels, indicating that Pin1 may be involved in core particle assembly and maturation, thereby promoting the later stages of the HBV life cycle. By contrast, parvulin inhibitors and PIN1 knockdown reduced HBV replication. Since more Pin1 proteins bound to immature core particles than to mature core particles, the interaction appears to depend on the stage of virus replication. Taken together, the data suggest that physical association between Pin1 and phosphorylated core particles may induce structural alterations through isomerization by Pin1, induce dephosphorylation by unidentified host phosphatases, and promote completion of virus life cycle.

PPIases Par14/Par17 Affect HBV Replication in Multiple Ways

Human parvulin 14 (Par14) and parvulin 17 (Par17) are peptidyl-prolyl cis/trans isomerases that upregulate hepatitis B virus (HBV) replication by binding to the conserved ¹³³Arg-Pro¹³⁴ (RP) motif of HBc and core particles, and ¹⁹RP²⁰-²⁸RP²⁹ motifs of HBx. In the absence of HBx, Par14/Par17 have no effect on HBV replication. Interaction with Par14/Par17 enhances the stability of HBx, core particles, and HBc. Par14/Par17 binds outside and inside core particles and is involved in HBc dimer-dimer interaction to facilitate core particle assembly. Although HBc RP motif is important for HBV replication, R133 residue is solely important for its interaction with Par14/Par17. Interaction of Par14 and Par17 with HBx involves two substrate-binding residues, Glu46/Asp74 (E46/D74) and E71/D99, respectively, and promotes HBx translocation to the nucleus and mitochondria. In the presence of HBx, Par14/Par17 are efficiently recruited to cccDNA and promote transcriptional activation via specific DNA-binding residues Ser19/44 (S19/44). S19 and E46/D74 of Par14, and S44 and E71/D99 of Par17, are also involved in the recruitment of HBc onto cccDNA. Par14/Par17 upregulate HBV replication via various effects that are mediated in part through the HBx-Par14/Par17-cccDNA complex and triple HBc, Par14/Par17, and cccDNA interactions in the nucleus, as well as via core particle-Par14/Par17 interactions in the cytoplasm.

Interferon-alpha responsible EPN3 regulates hepatitis B virus replication

Hepatitis B virus (HBV) infection remains a major health problem worldwide, and the current antiviral therapy, including nucleoside analogs, cannot achieve life-long cure, and clarification of antiviral host immunity is necessary for eradication. Here, we found that a clathrin-binding membrane protein epsin3 (EPN3) negatively regulates the expression of HBV RNA. EPN3 expression was induced by transfection of an HBV replicon plasmid, and reduced HBV-RNA level in hepatic cell lines and murine livers hydrodynamically injected with the HBV replicon plasmid. Viral RNA reduction by EPN3 was dependent on transcription, and independent from epsilon structure of viral RNA. Viral RNA reduction by overexpression of p53 or IFN-α treatment, was attenuated by knockdown of EPN3, suggesting its role downstream of IFN-α and p53. Taken together, this study demonstrates the anti-HBV role of EPN3. The mechanism how it decreases HBV transcription is discussed.

Ca2+/Calmodulin-Dependent Protein Kinase II Inhibits Hepatitis B Virus Replication from cccDNA via AMPK Activation and AKT/mTOR Suppression

Ca2+/calmodulin-dependent protein kinase II (CaMKII), which is involved in the calcium signaling pathway, is an important regulator of cancer cell proliferation, motility, growth, and metastasis. The effects of CaMKII on hepatitis B virus (HBV) replication have never been evaluated. Here, we found that phosphorylated, active CaMKII is reduced during HBV replication. Similar to other members of the AMPK/AKT/mTOR signaling pathway associated with HBV replication, CaMKII, which is associated with this pathway, was found to be a novel regulator of HBV replication. Overexpression of CaMKII reduced the expression of covalently closed circular DNA (cccDNA), HBV RNAs, and replicative intermediate (RI) DNAs while activating AMPK and inhibiting the AKT/mTOR signaling pathway. Findings in HBx-deficient mutant-transfected HepG2 cells showed that the CaMKII-mediated AMPK/AKT/mTOR signaling pathway was independent of HBx. Moreover, AMPK overexpression reduced HBV cccDNA, RNAs, and RI DNAs through CaMKII activation. Although AMPK acts downstream of CaMKII, AMPK overexpression altered CaMKII phosphorylation, suggesting that CaMKII and AMPK form a positive feedback loop. These results demonstrate that HBV replication suppresses CaMKII activity, and that CaMKII upregulation suppresses HBV replication from cccDNA via AMPK and the AKT/mTOR signaling pathway. Thus, activation or overexpression of CaMKII may be a new therapeutic target against HBV infection.

The HBV Core Protein and Core Particle Both Bind to the PPiase Par14 and Par17 to Enhance Their Stabilities and HBV Replication

We recently reported that the PPIase Par14 and Par17 encoded by PIN4 upregulate HBV replication in an HBx-dependent manner by binding to conserved arginine–proline (RP) motifs of HBx. HBV core protein (HBc) has a conserved ¹³³RP¹³⁴ motif; therefore, we investigated whether Par14/Par17 bind to HBc and/or core particles. Native agarose gel electrophoresis (NAGE) and immunoblotting and co-immunoprecipitation were used. Chromatin immunoprecipitation from HBV-infected HepG2-hNTCP-C9 cells was performed. NAGE and immunoblotting revealed that Par14/Par17 bound to core particles and co-immunoprecipitation revealed that Par14/Par17 interacted with core particle assembly-defective, and dimer-positive HBc-Y132A. Thus, core particles and HBc interact with Par14/Par17. Par14/Par17 interacted with the HBc ¹³³RP¹³⁴ motif possibly via substrate-binding E46/D74 and E71/D99 motifs. Although Par14/Par17 dissociated from core particles upon heat treatment, they were detected in 0.2 N NaOH-treated opened-up core particles, demonstrating that Par14/Par17 bind outside and inside core particles. Furthermore, these interactions enhanced the stabilities of HBc and core particles. Like HBc-Y132A, HBc-R133D and HBc-R133E were core particle assembly-defective and dimer-positive, demonstrating that a negatively charged residue at position 133 cannot be tolerated for particle assembly. Although positively charged R133 is solely important for Par14/17 interactions, the ¹³³RP¹³⁴ motif is important for efficient HBV replication. Chromatin immunoprecipitation from HBV-infected cells revealed that the S19 and E46/D74 residues of Par14 and S44 and E71/D99 residues of Par17 were involved in recruitment of ¹³³RP¹³⁴ motif-containing HBc into cccDNA. Our results demonstrate that interactions of HBc, Par14/Par17, and cccDNA in the nucleus and core particle–Par14/Par17 interactions in the cytoplasm are important for HBV replication.

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.

An Alternatively Spliced Sirtuin 2 Isoform 5 Inhibits Hepatitis B Virus Replication from cccDNA by Repressing Epigenetic Modifications Made by Histone Lysine Methyltransferases

Sirtuin 2 (Sirt2), an NAD⁺-dependent protein deacetylase, deacetylates tubulin, AKT, and other proteins. Previously, we showed that Sirt2 isoform 1 (Sirt2.1) increased replication of hepatitis B virus (HBV). Here, we show that HBV replication upregulates the expression of Sirt2 primary and alternatively spliced transcripts and their respective isoforms, 1, 2, and 5. Since Sirt2 isoform 5 (Sirt2.5) is a catalytically inactive nuclear protein with a spliced-out nuclear export signal (NES), we speculated that its different localization affects its activity. The overexpression of Sirt2.5 reduced expression of HBV mRNAs, replicative intermediate DNAs, and covalently closed circular DNA (cccDNA), an activity opposite that of Sirt2.1 and Sirt2.2. Unlike the Sirt2.1-AKT interaction, the Sirt2.5-AKT interaction was weakened by HBV replication. Unlike Sirt2.1, Sirt2.5 activated the AKT/GSK-3β/β-catenin signaling pathway very weakly and independently of HBV replication. When the NES and an N-terminal truncated catalytic domain were added to the Sirt2.5 construct, it localized in the cytoplasm and increased HBV replication (like Sirt2.1 and Sirt2.2). Chromatin immunoprecipitation assays revealed that more Sirt2.5 was recruited to cccDNA than Sirt2.1. The recruitment of histone lysine methyltransferases (HKMTs), such as SETDB1, SUV39H1, EZH2, and PR-Set7, and their respective transcriptional repressive markers, H3K9me3, H3K27me3, and H4K20me1, to cccDNA also increased in Sirt2.5-overexpressing cells. Among these, the Sirt2.5-PR-Set7 and -SETDB1 interactions increased upon HBV replication. These results demonstrate that Sirt2.5 reduces cccDNA levels and viral transcription through epigenetic modification of cccDNA via direct and/or indirect association with HKMTs, thereby exhibiting anti-HBV activity.IMPORTANCE Sirt2, a predominant cytoplasmic α-tubulin deacetylase, promotes the growth of hepatocellular carcinoma; indeed, HBV replication increases Sirt2 expression, and overexpression of Sirt2 is associated with hepatic fibrosis and epithelial-to-mesenchymal transition. Increased amounts of Sirt2 isoforms 1, 2, and 5 upon HBV replication might further upregulate HBV replication, leading to a vicious cycle of virus replication/disease progression. However, we show here that catalytically inactive nuclear Sirt2.5 antagonizes the effects of Sirt2.1 and Sirt2.2 on HBV replication, thereby inhibiting cccDNA level, transcription of cccDNA, and subsequent synthesis of replicative intermediate DNA. More Sirt2.5 was recruited to cccDNA than Sirt2.1, thereby increasing epigenetic modification by depositing transcriptional repressive markers, possibly through direct and/or indirect association with histone lysine methyltransferases, such as SETDB1, SUV39H1, EZH2, and/or PR-Set7, which represses HBV transcription. Thus, Sirt2.5 might provide a functional cure for HBV by silencing the transcription of HBV.

Extracellular vesicles secreted by HBV-infected cells modulate HBV persistence in hydrodynamic HBV transfection mouse model

Hepatitis B, a viral infection that affects the liver, is thought to affect over 257 million people worldwide, and long-term infection can lead to life-threatening issues such as cirrhosis or liver cancer. Chronic hepatitis B develops by the interaction between hepatitis B virus (HBV) and host immune response. However, questions of how HBV-infected cells thwart immune system defenses remain unanswered. Extracellular vesicles (EVs) are used for cellular communication, carrying cargoes such as RNAs, proteins, and lipids and delivering them intracellularly after being endocytosed by target cells. HBV-infected liver cells secrete several types of EVs into body fluids such as complete and incomplete virions, and exosomes. We previously demonstrated that monocytes that incorporated EVs moved to immunoregulatory phenotypes via up-regulation of PD-L1, an immunocheckpoint molecule, and down-regulation of CD69, a leukocyte activation molecule. In this study, we transfected mice with HBV using hydrodynamic injection and studied the effects of EVs secreted by HBV-infected liver cells. EVs secreted from cells with HBV replication strongly suppressed the immune response, inhibiting the eradication of HBV-replicating cells in the mice transfected with HBV. EVs were systemically incorporated in multiple organs, including liver, bone marrow (BM), and intestine. Intriguingly, the BM cells that incorporated EVs acquired intestinal tropism and the dendritic cell populations in the intestine increased. These findings suggest that the EVs secreted by HBV-infected liver cells exert immunosuppressive functions, and that an association between the liver, bone marrow, and intestinal tract exists through EVs secreted from HBV-infected cells.

HBV Genome and Life Cycle

Chronic hepatitis B virus (HBV) infection remains to be a serious threat to public health and is associated with many liver diseases including chronic hepatitis B (CHB), liver cirrhosis, and hepatocellular carcinoma. Although nucleos(t)ide analogues (NA) and pegylated interferon-α (Peg-IFNα) have been confirmed to be efficient in inhibiting HBV replication, it is difficult to eradicate HBV and achieve the clinical cure of CHB. Therefore, long-term therapy has been recommended to CHB treatment under the current antiviral therapy. In this context, the new antiviral therapy targeting one or multiple critical steps of viral life cycle may be an alternative approach in future. In the last decade, the functional receptor [sodium-taurocholate cotransporting polypeptide (NTCP)] of HBV entry into hepatocytes has been discovered, and the immature nucleocapsids containing the non- or partially reverse-transcribed pregenomic RNA, the nucleocapsids containing double-strand linear DNA (dslDNA), and the empty particles devoid of any HBV nucleic acid have been found to be released into circulation, which have supplemented the life cycle of HBV. The understanding of HBV life cycle may offer a new instruction for searching the potential antiviral targets, and the new viral markers used to monitor the efficacy of antiviral therapy for CHB patients in the future.

Different antiviral activities of natural APOBEC3C, APOBEC3G, and APOBEC3H variants against hepatitis B virus

Some APOBEC3 family members have antiviral activity against retroviruses and DNA viruses. Hepatitis B virus (HBV) is a DNA virus that is the major causative factor of severe liver diseases such as cirrhosis and hepatocellular carcinoma. To determine whether APOBEC3 variants in humans have different anti-HBV activities, we evaluated natural variants of APOBEC3C, APOBEC3G, and APOBEC3H using an HBV-replicating cell culture model. Our data demonstrate that the APOBEC3C variant S188I had increased restriction activity and hypermutation frequency against HBV DNA. In contrast, the APOBEC3G variant H186R did not alter the anti-HBV and hypermutation activities. Among APOBEC3H polymorphisms (hap I-VII) and splicing variants (SV-200, SV-183, SV-182, and SV-154), hap II SV-183 showed the strongest restriction activity. These data suggest that the genetic variations in APOBEC3 genes may affect the efficiency of HBV elimination in humans.

Adenosine deaminase acting on RNA-1 (ADAR1) inhibits hepatitis B virus (HBV) replication by enhancing microRNA-122 processing

Adenosine deaminases acting on RNA-1 (ADAR1) involves adenosine to inosine RNA editing and microRNA processing. ADAR1 is known to be involved in the replication of various viruses, including hepatitis C and D. However, the role of ADAR1 in hepatitis B virus (HBV) infection has not yet been elucidated. Here, for the first time, we demonstrated ADAR1 antiviral activity against HBV. ADAR1 has two splicing isoforms in human hepatocytes: constitutive p110 protein and interferon-α (IFN-α)-responsive p150 protein. We found that overexpression of ADAR1 decreased HBV RNA in an HBV culture model. A catalytic-site mutant ADAR1 also decreased HBV RNA levels, whereas another adenosine deaminases that act on the RNA (ADAR) family protein, ADAR2, did not. Moreover, the induction of ADAR1 by stimulation with IFN-α also reduced HBV RNA levels. Decreases in endogenous ADAR1 expression by knock-down or knock-out increased HBV RNA levels. A major hepatocyte-specific microRNA, miRNA-122, was found to be positively correlated with ADAR1 expression, and exogenous miRNA-122 decreased both HBV RNA and DNA, whereas, conversely, transfection with a miRNA-122 inhibitor increased them. The reduction of HBV RNA by ADAR1 expression was abrogated by p53 knock-down, suggesting the involvement of p53 in the ADAR1-mediated reduction of HBV RNA. This study demonstrated, for the first time, that ADAR1 plays an antiviral role against HBV infection by increasing the level of miRNA-122 in hepatocytes.

Parvulin 14 and Parvulin 17 Bind to HBx and cccDNA and Upregulate Hepatitis B Virus Replication from cccDNA to Virion in an HBx-Dependent Manner

The parvulin 14 (Par14) and parvulin 17 (Par17) proteins, which are both encoded by the PIN4 gene, play roles in protein folding, chromatin remodeling, DNA binding, ribosome biogenesis, and cell cycle progression. However, the effects of Par14 and Par17 on viral replication have never been explored. In this study, we found that, in the presence of HBx, either Par14 or Par17 could upregulate hepatitis B virus (HBV) replication, whereas in the absence of HBx, neither Par14 nor Par17 had any effect on replication. Overexpression of Par14/Par17 markedly increased the formation of covalently closed circular DNA (cccDNA), synthesis of HBV RNA and DNA, and virion secretion. Conversely, PIN4 knockdown significantly decreased HBV replication in HBV-transfected and -infected cells. Coimmunoprecipitation revealed that Par14/Par17 engaged in direct physical interactions with HBx in the cytoplasm, nucleus, and mitochondria, possibly mediated through substrate-binding residues on Par14/Par17 (E46/D74 and E71/D99, respectively) and conserved ¹⁹R²⁰P-²⁸R²⁹P motifs on HBx. Furthermore, these interactions enhanced HBx stability, promoted HBx translocation to the nuclear and mitochondrial fractions, and increased HBV replication. Chromatin immunoprecipitation assays revealed that, in the presence of HBx, Par14/Par17 were efficiently recruited to cccDNA and promoted transcriptional activation via specific DNA-binding residues (S19/44). In contrast, in the absence of HBx, Par14/Par17 bound cccDNA only at the basal level and did not promote transcriptional activation. Taken together, our results demonstrate that Par14 and Par17 upregulate HBV RNA transcription and DNA synthesis, thereby increasing the HBV cccDNA level, through formation of the cccDNA-Par14/17-HBx complex.IMPORTANCE The HBx protein plays an essential regulatory role in HBV replication. We found that substrate-binding residues on the human parvulin peptidylprolyl cis/trans isomerase proteins Par14 and Par17 bound to conserved arginine-proline (RP) motifs on HBx in the cytoplasm, nucleus, and mitochondria. The HBx-Par14/Par17 interaction stabilized HBx; promoted its translocation to the nucleus and mitochondria; and stimulated multiple steps of HBV replication, including cccDNA formation, HBV RNA and DNA synthesis, and virion secretion. In addition, in the presence of HBx, the Par14 and Par17 proteins bound to cccDNA and promoted its transcriptional activation. Our results suggest that inhibition or knockdown of Par14 and Par17 may represent a novel therapeutic option against HBV infection.

Sirtuin 2 Isoform 1 Enhances Hepatitis B Virus RNA Transcription and DNA Synthesis through the AKT/GSK-3β/β-Catenin Signaling Pathway

Sirtuin 2 (Sirt2), a NAD⁺-dependent protein deacetylase, is overexpressed in many hepatocellular carcinomas (HCCs) and can deacetylate many proteins, including tubulins and AKT, prior to AKT activation. Here, we found that endogenous Sirt2 was upregulated in wild-type hepatitis B virus (HBV WT)-replicating cells, leading to tubulin deacetylation; however, this was not the case in HBV replication-deficient-mutant-transfected cells and 1.3-mer HBV WT-transfected and reverse transcriptase inhibitor (entecavir or lamivudine)-treated cells, but all HBV proteins were expressed. In HBV WT-replicating cells, upregulation of Sirt2 induced AKT activation, which consequently downregulated glycogen synthase kinase 3β (GSK-3β) and increased β-catenin levels; however, downregulation of Sirt2 in HBV-nonreplicating cells impaired AKT/GSK-3β/β-catenin signaling. Overexpression of Sirt2 isoform 1 stimulated HBV transcription and consequently HBV DNA synthesis, which in turn activated AKT and consequently increased β-catenin levels, possibly through physical interactions with Sirt2 and AKT. Knockdown of Sirt2 by short hairpin RNAs (shRNAs), inhibition by 2-cyano-3-[5-(2,5-dichlorophenyl)-2-furanyl]-N-5-quinolinyl-2-propenamide (AGK2), or dominant negative mutant expression inhibited HBV replication, reduced AKT activation, and decreased β-catenin levels. Through HBV infection, we demonstrated that Sirt2 knockdown inhibited HBV replication from transcription. Although HBx itself activates AKT and upregulates β-catenin, Sirt2-mediated signaling and upregulated HBV replication were HBx independent. Since constitutively active AKT inhibits HBV replication, the results suggest that upregulated Sirt2 and activated AKT may balance HBV replication to prolong viral replication, eventually leading to the development of HCC. Also, the results indicate that Sirt2 inhibition may be a new therapeutic option for controlling HBV infection and preventing HCC.IMPORTANCE Even though Sirt2, a NAD⁺-dependent protein deacetylase, is overexpressed in many HCCs, and overexpressed Sirt2 promotes hepatic fibrosis and associates positively with vascular invasion by primary HCCs through AKT/GSK-3β/β-catenin signaling, the relationship between Sirt2, HBV, HBx, and/or HBV-associated hepatocarcinogenesis is unclear. Here, we show that HBV DNA replication, not HBV expression, correlates positively with Sirt2 upregulation and AKT activation. We demonstrate that overexpression of Sirt2 further increases HBV replication, increases AKT activation, downregulates GSK-3β, and increases β-catenin levels. Conversely, inhibiting Sirt2 decreases HBV replication, reduces AKT activation, and decreases β-catenin levels. Although HBx activates AKT to upregulate β-catenin, Sirt2-mediated effects were not dependent on HBx. The results also indicate that a Sirt2 inhibitor may control HBV infection and prevent the development of hepatic fibrosis and HCC.

The Structural Biology of Hepatitis B Virus: Form and Function

Hepatitis B virus is one of the smallest human pathogens, encoded by a 3,200-bp genome with only four open reading frames. Yet the virus shows a remarkable diversity in structural features, often with the same proteins adopting several conformations. In part, this is the parsimony of viruses, where a minimal number of proteins perform a wide variety of functions. However, a more important theme is that weak interactions between components as well as components with multiple conformations that have similar stabilities lead to a highly dynamic system. In hepatitis B virus, this is manifested as a virion where the envelope proteins have multiple structures, the envelope-capsid interaction is irregular, and the capsid is a dynamic compartment that actively participates in metabolism of the encapsidated genome and carries regulated signals for intracellular trafficking.

Inhibitory effect of Phyllanthus urinaria L. extract on the replication of lamivudine-resistant hepatitis B virus in vitro

BACKGROUND

Long-term treatment of chronic hepatitis B (CHB) with nucleos(t)ide analogs results in the emergence of drug-resistant hepatitis B virus (HBV) harboring mutations in the polymerase (P) gene. The Phyllanthus extract has anti-HBV activity; however, its antiviral activity against lamivudine (LMV)-resistant mutants has not been examined.

METHODS

HBV harboring LMV-resistant mutations (rtM204I, rtM204V, and rtM204S) in the P gene at the YMDD ((203)tyrosine-methionine-aspartate-aspartate(206)) reverse transcriptase (RT) active site were generated and their sensitivity to Phyllanthus urinaria koreanis extract examined. Southern blotting and real-time PCR were used to determine the concentration of plant extract required to inhibit HBV DNA synthesis by 50 and 90% (EC50 and EC90, respectively). An enzyme-linked immunosorbent assay was used to measure the EC50 of HBV surface antigen (HBsAg) and HBV core antigen (HBcAg) secretion, and the 50% cytotoxic concentration of the extract was measured in a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Real-time RT-PCR was used to measure mRNA expression levels.

RESULTS

The expression of intracellular HBV DNAs in HBV WT- or mutant-transfected HepG2 cells decreased upon treatment with Phyllanthus extract. The secretion of HBsAg and HBcAg also fell in a dose-dependent manner. Phyllanthus extract induced interferon-beta (IFN-β), cyclooxygenase-2 (COX-2), and interleukin-6 (IL-6) mRNA expression in HBV WT-transfected HepG2 cells, possibly via activation of extracellular signal-regulated kinases and c-jun N-terminal kinases and the induction of retinoic acid inducible gene-I, toll-like receptor 3, myeloid differentiation primary response gene 88, and/or tumor necrosis factor receptor-associated factor 6 gene expression. HBV transfection in the absence of extract or exposure of cells to extract alone did not trigger these signaling cascades.

CONCLUSIONS

Phyllanthus extract inhibited HBV DNA synthesis and HBsAg and HBcAg secretion by replicating cells harboring HBV wild-type and LMV-resistant mutants, likely by inducing the expression of IFN-β, COX-2, and IL-6. These data indicate that Phyllanthus extract may be useful as an alternative therapeutic agent for the treatment of drug-resistant CHB patients.

TGF-β suppression of HBV RNA through AID-dependent recruitment of an RNA exosome complex

Transforming growth factor (TGF)-β inhibits hepatitis B virus (HBV) replication although the intracellular effectors involved are not determined. Here, we report that reduction of HBV transcripts by TGF-β is dependent on AID expression, which significantly decreases both HBV transcripts and viral DNA, resulting in inhibition of viral replication. Immunoprecipitation reveals that AID physically associates with viral P protein that binds to specific virus RNA sequence called epsilon. AID also binds to an RNA degradation complex (RNA exosome proteins), indicating that AID, RNA exosome, and P protein form an RNP complex. Suppression of HBV transcripts by TGF-β was abrogated by depletion of either AID or RNA exosome components, suggesting that AID and the RNA exosome involve in TGF-β mediated suppression of HBV RNA. Moreover, AID-mediated HBV reduction does not occur when P protein is disrupted or when viral transcription is inhibited. These results suggest that induced expression of AID by TGF-β causes recruitment of the RNA exosome to viral RNP complex and the RNA exosome degrades HBV RNA in a transcription-coupled manner.

Encapsidated hepatitis B virus reverse transcriptase is poised on an ordered RNA lattice

Assembly of a hepatitis B virus (HBV) virion begins with the formation of an RNA-filled core composed of a symmetrical capsid (built of core protein), viral pregenomic RNA, and viral reverse transcriptase. To generate the circular dsDNA genome of HBV, reverse transcription requires multiple template switches within the confines of the capsid. To date, most anti-HBV therapeutics target this reverse transcription process. The detailed molecular mechanisms of this crucial process are poorly understood because of the lack of structural information. We hypothesized that capsid, RNA, and viral reverse transcriptase would need a precise geometric organization to accomplish reverse transcription. Here we present the asymmetric structure of authentic RNA-filled cores, determined to 14.5-Å resolution from cryo-EM data. Capsid and RNA are concentric. On the interior of the RNA, we see a distinct donut-like density, assigned to viral reverse transcriptase, which pins the viral pregenomic RNA to the capsid inner surface. The observation of a unique ordered structure inside the core suggests that assembly and the first steps of reverse transcription follow a single, determinate pathway and strongly suggests that all subsequent steps in DNA synthesis do as well.

Phosphoacceptors threonine 162 and serines 170 and 178 within the carboxyl-terminal RRRS/T motif of the hepatitis B virus core protein make multiple contributions to hepatitis B virus replication

Phosphorylation of serines 157, 164, and 172 within the carboxyl-terminal SPRRR motif of the hepatitis B virus (HBV) core (C) protein modulates HBV replication at multiple stages. Threonine 162 and serines 170 and 178, located within the carboxyl-terminal conserved RRRS/T motif of HBV C protein, have been proposed to be protein kinase A phosphorylation sites. However, in vivo phosphorylation of these residues has never been observed, and their contribution to HBV replication remains unknown. In this study, [(32)P]orthophosphate labeling of cells expressing C proteins followed by immunoprecipitation with anti-HBc antibody revealed that threonine 162 and serines 170 and 178 are phosphoacceptor residues. A triple-alanine-substituted mutant, mimicking dephosphorylation of all three residues, drastically decreased pregenomic RNA (pgRNA) encapsidation, thereby decreasing HBV DNA synthesis. In contrast, a triple-glutamate-substituted mutant, mimicking phosphorylation of these residues, decreased DNA synthesis without significantly decreasing encapsidation. Neither triple mutant affected C protein expression or core particle assembly. Individual alanine substitution of threonine 162 significantly decreased minus-strand, plus-strand, and relaxed-circular DNA synthesis, demonstrating that this residue plays multiple roles in HBV DNA synthesis. Double-alanine substitution of serines 170 and 178 reduced HBV replication at multiple stages, indicating that these residues also contribute to HBV replication. Thus, in addition to serines 157, 164, and 172, threonine 162 and serines 170 and 178 of HBV C protein are also phosphorylated in cells, and phosphorylation and dephosphorylation of these residues play multiple roles in modulation of HBV replication.

IMPORTANCE

Threonine 162, within the carboxyl-terminal end of the hepatitis B virus (HBV adw) core (C) protein, has long been ignored as a phosphoacceptor, even though it is highly conserved among mammalian hepadnaviruses and in the overlapping consensus RxxS/T, RRxS/T, and TP motifs. Here we show, for the first time, that in addition to the well-known phosphoacceptor serines 157, 164, and 172 in SPRRR motifs, threonine 162 and serines 170 and 178 in the RRRS/T motif are phosphorylated in cells. We also show that, like serines 157, 164, and 172, phosphorylated and dephosphorylated threonine 162 and serines 170 and 178 contribute to multiple steps of HBV replication, including pgRNA encapsidation, minus-strand and plus-strand DNA synthesis, and relaxed-circular DNA synthesis. Of these residues, threonine 162 is the most important. Furthermore, we show that phosphorylation of C protein is required for efficient completion of HBV replication.

Evidence for multiple distinct interactions between hepatitis B virus P protein and its cognate RNA encapsidation signal during initiation of reverse transcription

Replication of hepatitis B virus (HBV) via protein-primed reverse transcription is initiated by binding of the viral P protein to the conserved ε stem-loop on the pregenomic (pg) RNA. This triggers encapsidation of the complex and the ε-templated synthesis of a short P protein-linked DNA oligonucleotide (priming) for subsequent minus-strand DNA extension. ε consists of a lower and upper stem, a bulge containing the priming template, and an apical loop. The nonhelical subelements are considered important for DNA synthesis and pgRNA packaging whereas the role of the upper stem is not well characterized. Priming itself could until recently not be addressed because in vitro generated HBV P - ε complexes showed no activity. Focussing on the four A residues at the base and tip of the upper ε stem and the two U residues in the loop we first investigated the impact of 24 mutations on viral DNA accumulation in transfected cells. While surprisingly many mutations were tolerated, further analyzing the negatively acting mutations, including in a new cell-free priming system, revealed divergent position-related impacts on pgRNA packaging, priming activity and possibly initiation site selection. This genetic separability implies that the ε RNA undergoes multiple distinct interactions with P protein as pgRNA encapsidation and replication initiation progress, and that the strict conservation of ε in nature may reflect its optimal adaptation to comply with all of them. The data further define the most attractive mutants for future studies, including as decoys for interference with HBV replication.

Uracil DNA glycosylase counteracts APOBEC3G-induced hypermutation of hepatitis B viral genomes: excision repair of covalently closed circular DNA

The covalently closed circular DNA (cccDNA) of the hepatitis B virus (HBV) plays an essential role in chronic hepatitis. The cellular repair system is proposed to convert cytoplasmic nucleocapsid (NC) DNA (partially double-stranded DNA) into cccDNA in the nucleus. Recently, antiviral cytidine deaminases, AID/APOBEC proteins, were shown to generate uracil residues in the NC-DNA through deamination, resulting in cytidine-to-uracil (C-to-U) hypermutation of the viral genome. We investigated whether uracil residues in hepadnavirus DNA were excised by uracil-DNA glycosylase (UNG), a host factor for base excision repair (BER). When UNG activity was inhibited by the expression of the UNG inhibitory protein (UGI), hypermutation of NC-DNA induced by either APOBEC3G or interferon treatment was enhanced in a human hepatocyte cell line. To assess the effect of UNG on the cccDNA viral intermediate, we used the duck HBV (DHBV) replication model. Sequence analyses of DHBV DNAs showed that cccDNA accumulated G-to-A or C-to-T mutations in APOBEC3G-expressing cells, and this was extensively enhanced by UNG inhibition. The cccDNA hypermutation generated many premature stop codons in the P gene. UNG inhibition also enhanced the APOBEC3G-mediated suppression of viral replication, including reduction of NC-DNA, pre-C mRNA, and secreted viral particle-associated DNA in prolonged culture. Enhancement of APOBEC3G-mediated suppression by UNG inhibition was not observed when the catalytic site of APOBEC3G was mutated. Transfection experiments of recloned cccDNAs revealed that the combination of UNG inhibition and APOBEC3G expression reduced the replication ability of cccDNA. Taken together, these data indicate that UNG excises uracil residues from the viral genome during or after cccDNA formation in the nucleus and imply that BER pathway activities decrease the antiviral effect of APOBEC3-mediated hypermutation.

RNA editing of hepatitis B virus transcripts by activation-induced cytidine deaminase

Activation-induced cytidine deaminase (AID) is essential for the somatic hypermutation (SHM) and class-switch recombination (CSR) of Ig genes. The mechanism by which AID triggers SHM and CSR has been explained by two distinct models. In the DNA deamination model, AID converts cytidine bases in DNA into uridine. The uridine is recognized by the DNA repair system, which produces DNA strand breakages and point mutations. In the alternative model, RNA edited by AID is responsible for triggering CSR and SHM. However, RNA deamination by AID has not been demonstrated. Here we found that C-to-T and G-to-A mutations accumulated in hepatitis B virus (HBV) nucleocapsid DNA when AID was expressed in HBV-replicating hepatic cell lines. AID expression caused C-to-T mutations in the nucleocapsid DNA of RNase H-defective HBV, which does not produce plus-strand viral DNA. Furthermore, the RT-PCR products of nucleocapsid viral RNA from AID-expressing cells exhibited significant C-to-T mutations, whereas viral RNAs outside the nucleocapsid did not accumulate C-to-U mutations. Moreover, AID was packaged within the nucleocapsid by forming a ribonucleoprotein complex with HBV RNA and the HBV polymerase protein. The encapsidation of the AID protein with viral RNA and DNA provides an efficient environment for evaluating AID's RNA and DNA deamination activities. A bona fide RNA-editing enzyme, apolipoprotein B mRNA editing catalytic polypeptide 1, induced a similar level of C-to-U mutations in nucleocapsid RNA as AID. Taken together, the results indicate that AID can deaminate the nucleocapsid RNA of HBV.

Protein-primed terminal transferase activity of hepatitis B virus polymerase

Hepatitis B virus (HBV) replication requires reverse transcription of an RNA pregenome (pgRNA) by a multifunctional polymerase (HP). HP initiates viral DNA synthesis by using itself as a protein primer and an RNA signal on pgRNA, termed epsilon (Hε), as the obligatory template. We discovered a Mn(2+)-dependent transferase activity of HP in vitro that was independent of Hε but also used HP as a protein primer. This protein-primed transferase activity was completely dependent on the HP polymerase active site. The DNA products of the transferase reaction were linked to HP via a phosphotyrosyl bond, and replacement of the Y63 residue of HP, the priming site for templated DNA synthesis, almost completely eliminated DNA synthesis by the transferase activity, suggesting that Y63 also serves as the predominant priming site for the transferase reaction. For this transferase activity, HP could use all four deoxynucleotide substrates, but TTP was clearly favored for extensive polymerization. The transferase activity was highly distributive, leading to the synthesis of DNA homo- and hetero-oligomeric and -polymeric ladders ranging from 1 nucleotide (nt) to >100 nt in length, with single-nt increments. As with Hε-templated DNA synthesis, the protein-primed transferase reaction was characterized by an initial stage that was resistant to the pyrophosphate analog phosphonoformic acid (PFA) followed by PFA-sensitive DNA synthesis, suggestive of an HP conformational change upon the synthesis of a nascent DNA oligomer. These findings have important implications for HBV replication, pathogenesis, and therapy.

C-terminal substitution of HBV core proteins with those from DHBV reveals that arginine-rich 167RRRSQSPRR175 domain is critical for HBV replication

To investigate the contributions of carboxyl-terminal nucleic acid binding domain of HBV core (C) protein for hepatitis B virus (HBV) replication, chimeric HBV C proteins were generated by substituting varying lengths of the carboxyl-terminus of duck hepatitis B virus (DHBV) C protein for the corresponding regions of HBV C protein. All chimeric C proteins formed core particles. A chimeric C protein with 221–262 amino acids of DHBV C protein, in place of 146–185 amino acids of the HBV C protein, supported HBV pregenomic RNA (pgRNA) encapsidation and DNA synthesis: 40% amino acid sequence identity or 45% homology in the nucleic-acid binding domain of HBV C protein was sufficient for pgRNA encapsidation and DNA synthesis, although we predominantly detected spliced DNA. A chimeric C protein with 221–241 and 251–262 amino acids of DHBV C, in place of HBV C 146–166 and 176–185 amino acids, respectively, could rescue full-length DNA synthesis. However, a reciprocal C chimera with 242–250 of DHBV C (²⁴²RAGSPLPRS²⁵⁰) introduced in place of 167–175 of HBV C (¹⁶⁷RRRSQSPRR¹⁷⁵) significantly decreased pgRNA encapsidation and DNA synthesis, and full-length DNA was not detected, demonstrating that the arginine-rich ¹⁶⁷RRRSQSPRR¹⁷⁵ domain may be critical for efficient viral replication. Five amino acids differing between viral species (underlined above) were tested for replication rescue; R169 and R175 were found to be important.

Relevance of interleukin-10RB to chronic hepatitis B virus infection and biological activities of interferon-λ and interleukin-22

Purpose

The association of a single nucleotide polymorphism of interleukin (IL)-10RB codon 47 with the chronic hepatitis B virus (HBV) infection has been reported in two ethnic populations with different results, but not in a Korean population. IL-10RB is a subunit of receptor complexes for interferon-lambda (IFN-λ) and IL-22, which have antiviral and hepatocyte-protective activity, respectively. This study examined the association of IL-10RB K47E with the outcomes of HBV infection in Korean subjects and the cellular response to these cytokines.

Methods

Genotypes of IL-10RB and the outcomes of HBV infection were analyzed in 1,000 Korean patients. The effect of IFN-λ or IL-22 on HBV replication and cell viability was assessed in hepatoma cells expressing IL-10RB K47 or E47. The transcript level of IL-10RB was examined in Epstein Barr virus-transformed B cells and hepatoma cells.

Results

IL-10RB K47E was associated with chronic HBV infection but not with hepatoma in the Korean population. IL-10RB K47E was associated with the transcript level of IL-10RB in transformed B cells but not with the responses in hepatoma cells to IFN-λ or IL-22. HBV replication or 5-fluorouracil-induced cell death was suppressed by treatment of IFN-λ or IL-22 in an IL-10RB K47E-independent manner.

Conclusions

IL-10RB K47E is related to chronic HBV infection in a Korean population, but not to cellular responsiveness to IFN-λ and IL-22.

Electronic supplementary material

The online version of this article (doi:10.1007/s12072-012-9361-8) contains supplementary material, which is available to authorized users.

In vitro epsilon RNA-dependent protein priming activity of human hepatitis B virus polymerase

Hepatitis B virus (HBV) replicates its DNA genome through reverse transcription of a pregenomic RNA (pgRNA) by using a multifunctional polymerase (HP). A critical function of HP is its specific recognition of a viral RNA signal termed ε (Hε) located on pgRNA, which is required for specific packaging of pgRNA into viral nucleocapsids and initiation of viral reverse transcription. HP initiates reverse transcription by using itself as a protein primer (protein priming) and Hε as the obligatory template. We have purified HP from human cells that retained Hε binding activity in vitro. Furthermore, HP purified as a complex with Hε, but not HP alone, displayed in vitro protein priming activity. While the HP-Hε interaction in vitro and in vivo required the Hε internal bulge, but not its apical loop, and was not significantly affected by the cap-Hε distance, protein priming required both the Hε apical loop and internal bulge, as well as a short distance between the cap and Hε, mirroring the requirements for RNA packaging. These studies have thus established new HBV protein priming and RNA binding assays that should greatly facilitate the dissection of the requirements and molecular mechanisms of HP-Hε interactions, RNA packaging, and protein priming.

A SELEX-screened aptamer of human hepatitis B virus RNA encapsidation signal suppresses viral replication

BACKGROUND

The specific interaction between hepatitis B virus (HBV) polymerase (P protein) and the ε RNA stem-loop on pregenomic (pg) RNA is crucial for viral replication. It triggers both pgRNA packaging and reverse transcription and thus represents an attractive antiviral target. RNA decoys mimicking ε in P protein binding but not supporting replication might represent novel HBV inhibitors. However, because generation of recombinant enzymatically active HBV polymerase is notoriously difficult, such decoys have as yet not been identified.

METHODOLOGY/PRINCIPAL FINDINGS

Here we used a SELEX approach, based on a new in vitro reconstitution system exploiting a recombinant truncated HBV P protein (miniP), to identify potential ε decoys in two large ε RNA pools with randomized upper stem. Selection of strongly P protein binding RNAs correlated with an unexpected strong enrichment of A residues. Two aptamers, S6 and S9, displayed particularly high affinity and specificity for miniP in vitro, yet did not support viral replication when part of a complete HBV genome. Introducing S9 RNA into transiently HBV producing HepG2 cells strongly suppressed pgRNA packaging and DNA synthesis, indicating the S9 RNA can indeed act as an ε decoy that competitively inhibits P protein binding to the authentic ε signal on pgRNA.

CONCLUSIONS/SIGNIFICANCE

This study demonstrates the first successful identification of human HBV ε aptamers by an in vitro SELEX approach. Effective suppression of HBV replication by the S9 aptamer provides proof-of-principle for the ability of ε decoy RNAs to interfere with viral P-ε complex formation and suggests that S9-like RNAs may further be developed into useful therapeutics against chronic hepatitis B.

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.

Adiponectin, a downstream target gene of peroxisome proliferator-activated receptor γ, controls hepatitis B virus replication

In this study, HepG2-hepatitis B virus (HBV)-stable cells that did not overexpress HBx and HBx-deficient mutant-transfected cells were analyzed for their expression of HBV-induced, upregulated adipogenic and lipogenic genes. The mRNAs of CCAAT enhancer binding protein α (C/EBPα), peroxisome proliferator-activated receptor γ (PPARγ), adiponectin, liver X receptor α (LXRα), sterol regulatory element binding protein 1c (SREBP1c), and fatty acid synthase (FAS) were expressed at higher levels in HepG2-HBV and lamivudine-treated stable cells and HBx-deficient mutant-transfected cells than in the HepG2 cells. Lamivudine treatment reduced the mRNA levels of PPARγ and C/EBPα. Conversely, HBV replication was upregulated by adiponectin and PPARγ agonist rosiglitazone treatments and was downregulated by adiponectin siRNAs. Collectively, our results demonstrate that HBV replication and/or protein expression, even in the absence of HBx, upregulated adipogenic or lipogenic genes, and that the control of adiponectin might prove useful as a therapeutic modality for the treatment of chronic hepatitis B.

Incorporation of deoxyribonucleotides and ribonucleotides by a dNTP-binding cleft mutated reverse transcriptase in hepatitis B virus core particles

Our recent observation that hepatitis B virus (HBV) DNA polymerase (P) might initiate minus-strand DNA synthesis without primer [Kim et al., (2004) Virology 322, 22-30], raised a possibility that HBV P protein may have the potential to function as an RNA polymerase. Thus, we mutated Phe 436, a bulky amino acid with aromatic side chain, at the putative dNTP-binding cleft in reverse transcriptase (RT) domain of P protein to smaller amino acids (Gly or Val), and examined RNA polymerase activity. HBV core particles containing RT dNTP-binding cleft mutant P protein were able to incorporate (32)P-ribonucleotides, but not HBV core particles containing wild type (wt), priming-deficient mutant, or RT-deficient mutant P proteins. Since all the experiments were conducted with core particles isolated from transfected cells, our results indicate that the HBV RT mutant core particles containing RT dNTP-binding cleft mutant P protein could incorporate both deoxyribonucleotides and ribonucleotides in replicating systems.

Effect of interferon-lambda on replication of hepatitis B virus in human hepatoma cells

The antiviral activity of interferon-lambda (IFN-lambda) against several viruses has been reported, but it has not been investigated whether human IFN-lambda can affect replication of the hepatitis B virus (HBV) in human hepatocyte-derived cells. We found that replication of HBV in a human hepatoma cell line was reduced by approximately 30% following treatment with a high concentration of IFN-lambda. Furthermore, we showed that replication of HBV in another human hepatoma cell line was not suppressed by IFN-lambda. However, these cell lines showed a similar level of transcription for the heterodimeric IFN-lambda receptor and the IFN-lambda-induced antiviral proteins. Our results suggest that the antiviral activity of IFN-lambda against HBV may be limited in human cells.

Hepatitis B virus x protein induces perinuclear mitochondrial clustering in microtubule- and Dynein-dependent manners

The hepatitis B virus (HBV) X protein (HBx) is thought to play a key role in HBV replication and the development of liver cancer. It became apparent that HBx induces mitochondrial clustering at the nuclear periphery, but the molecular basis for mitochondrial clustering is not understood. Since mitochondria move along the cytoskeleton as a cargo of motor proteins, we hypothesized that mitochondrial clustering induced by HBx occurs by an altered intracellular motility. Here, we demonstrated that the treatment of HBx-expressing cells with a microtubule-disrupting drug (nocodazole) abrogated mitochondrial clustering, while the removal of nocodazole restored clustering within 30 to 60 min, indicating that mitochondrial transport is occurring in a microtubule-dependent manner. The addition of a cytochalasin D-disrupting actin filament, however, did not measurably affect mitochondrial clustering. Mitochondrial clustering was further studied by observations of HBV-related hepatoma cells and HBV-replicating cells. Importantly, the abrogation of the dynein activity in HBx-expressing cells by microinjection of a neutralizing anti-dynein intermediate-chain antibody, dynamitin overexpression, or the addition of a dynein ATPase inhibitor significantly suppressed the mitochondrial clustering. In addition, HBx induced the activation of the p38 mitogen-activated protein kinase (MAPK) and inhibition of the p38 kinase activity by SB203580-attenuated HBx-induced mitochondrial clustering. Taken together, HBx activation of the p38 MAPK contributed to the increase in the microtubule-dependent dynein activity. The data suggest that HBx plays a novel regulatory role in subcellular transport systems, perhaps facilitating the process of maturation and/or assembly of progeny particles during HBV replication. Furthermore, mitochondrion aggregation induced by HBx may represent a cellular process that underlies disease progression during chronic viral infection.

Intracellular antibody fragment against hepatitis B virus X protein does not inhibit viral replication

Replication of the hepatitis B virus is suppressed by deficiency of the X protein. Although several molecules that block cellular targets of X protein reduce the production of hepatitis B virus progeny, the effect of a specific inhibitor of X protein on viral replication has not been investigated. To block X protein specifically, we adopted an intracellular expression approach using H7 single chain variable fragment (H7scFv), an antibody fragment against X protein. We previously demonstrated that cytoplasmic expression of H7scFv inhibits X protein-induced tumorigenicity and transactivation. In this study, intracellular H7scFv expression inhibits reporter gene transactivation but not viral replication determined by endogenous hepatitis B virus polymerase activity assay and real-time PCR. Our findings imply that intracellular expression of antibody fragment against X protein may not be an alternative therapeutic modality for inhibition of hepatitis B virus replication.

Infection of human hepatocyte chimeric mouse with genetically engineered hepatitis B virus

Studies of hepatitis B virus (HBV) mutants have been hampered by the lack of a small animal model with long-term infection of cloned HBV. Using a mouse model in which liver cells were highly replaced with human hepatocytes that survived over a long time with mature human hepatocyte function, we performed transmission experiments of HBV. Human serum containing HBV and the virus produced in HepG2 cell lines that transiently or stably transfected with 1.4 genome length HBV DNA were inoculated. Genetically modified e-antigen-negative mutant strain also was produced and inoculated into the mouse model. A high-level (approximately 10(10) copies/mL) viremia was observed in mice inoculated with HBV-positive human serum samples. The level of viremia tended to be high in mice with a continuously high human hepatocyte replacement index. High levels and long-lasting viremia also were observed in mice injected with the in vitro generated HBV. The viremia continued up to 22 weeks until death or killing. Passage experiments showed that the serum of these mice contained infectious HBV. Genetically engineered hepatitis B e antigen-negative mutant clone also was shown to be infectious. Lamivudine effectively reduced the level of viremia in these infected mice. In conclusion, this mouse model of HBV infection is a useful tool for the study of HBV virology and evaluation of anti-HBV drugs. Our results indicate that HBeAg is dispensable for active viral production and transmission.

Hepatitis B virus DNA replication is coordinated by core protein serine phosphorylation and HBx expression

The hepatitis B virus (HBV) core protein forms the capsid of viral particles and is essential for viral genome DNA replication and maturation. The C terminus of core protein contains three serines at positions 155, 162, and 170, phosphorylation of which is important for viral DNA replication. We demonstrate that the phosphorylation of these serines is stimulated by the viral HBx protein, a regulatory protein that activates signal transduction pathways and viral replication. HBx is therefore shown to stimulate HBV replication by increasing core serine phosphorylation. Mutational, biochemical, and mixing studies of C-terminal core serine mutants demonstrate that multiple serine phosphorylations occur on the same core protein. Mutation of individual core protein serines is shown to inhibit HBV replication at distinct stages corresponding to encapsidation of viral pregenomic RNA, reverse transcription, and restriction to synthesis of specific DNA replicative intermediates. We therefore demonstrate that a primary target of HBV replication that is regulated by HBx protein corresponds to increased phosphorylation of the viral core protein. We also demonstrate that core phosphorylation mediated by HBx promotes sequential progression of viral replication through the assembly of capsids primed for different stages of DNA synthesis.