The woodchuck hepatitis virus X gene is important for establishment of virus infection in woodchucks

Hepatitis B Viral Protein HBx: Roles in Viral Replication and Hepatocarcinogenesis

Hepatitis B virus (HBV) infection remains a major public health concern worldwide, with approximately 296 million individuals chronically infected. The HBV-encoded X protein (HBx) is a regulatory protein of 17 kDa, reportedly responsible for a broad range of functions, including viral replication and oncogenic processes. In this review, we summarize the state of knowledge on the mechanisms underlying HBx functions in viral replication, the antiviral effect of therapeutics directed against HBx, and the role of HBx in liver cancer development (including a hypothetical model of hepatocarcinogenesis). We conclude by highlighting major unanswered questions in the field and the implications of their answers.

Hepatitis B x (HBx) as a Component of a Functional Cure for Chronic Hepatitis B

Patients who are carriers of the hepatitis B virus (HBV) are at high risk of chronic liver disease (CLD) which proceeds from hepatitis, to fibrosis, cirrhosis and to hepatocellular carcinoma (HCC). The hepatitis B-encoded X antigen, HBx, promotes virus gene expression and replication, protects infected hepatocytes from immunological destruction, and promotes the development of CLD and HCC. For virus replication, HBx regulates covalently closed circular (ccc) HBV DNA transcription, while for CLD, HBx triggers cellular oxidative stress, in part, by triggering mitochondrial damage that stimulates innate immunity. Constitutive activation of NF-κB by HBx transcriptionally activates pro-inflammatory genes, resulting in hepatocellular destruction, regeneration, and increased integration of the HBx gene into the host genome. NF-κB is also hepatoprotective, which sustains the survival of infected cells. Multiple therapeutic approaches include direct-acting anti-viral compounds and immune-stimulating drugs, but functional cures were not achieved, in part, because none were yet devised to target HBx. In addition, many patients with cirrhosis or HCC have little or no virus replication, but continue to express HBx from integrated templates, suggesting that HBx contributes to the pathogenesis of CLD. Blocking HBx activity will, therefore, impact multiple aspects of the host–virus relationship that are relevant to achieving a functional cure.

The association between hepatocarcinogenesis and intracellular alterations due to hepatitis B virus infection

Chronic hepatitis B virus (HBV) infection is a worldwide health problem leading to severe liver dysfunction, including liver cirrhosis and hepatocellular carcinoma. Although current antiviral therapies for chronic HBV infection have been improved and can lead to a strong suppression of viral replication, it is difficult to completely eliminate the virus with these therapies once chronic HBV infection is established in the host. Furthermore, chronic HBV infection alters intracellular metabolism and signalling pathways, resulting in the activation of carcinogenesis in the liver. HBV produces four viral proteins: hepatitis B surface-, hepatitis B core-, hepatitis B x protein, and polymerase; each plays an important role in HBV replication and the intracellular signalling pathways associated with hepatocarcinogenesis. In vitro and in vivo experimental models for analyzing HBV infection and replication have been established, and gene expression analyses using microarrays or next-generation sequencing have also been developed. Thus, it is possible to clarify the molecular mechanisms for intracellular alterations, such as endoplasmic reticulum stress, oxidative stress, and epigenetic modifications. In this review, the impact of HBV viral proteins and intracellular alterations in HBV-associated hepatocarcinogenesis are discussed.

Hepatitis B x antigen (HBx) is an important therapeutic target in the pathogenesis of hepatocellular carcinoma

Hepatitis B virus (HBV) is a human pathogen that has infected an estimated two billion people worldwide. Despite the availability of highly efficacious vaccines, universal screening of the blood supply for virus, and potent direct acting anti-viral drugs, there are more than 250 million carriers of HBV who are at risk for the sequential development of hepatitis, fibrosis, cirrhosis and hepatocellular carcinoma (HCC). More than 800,000 deaths per year are attributed to chronic hepatitis B. Many different therapeutic approaches have been developed to block virus replication, and although effective, none are curative. These treatments have little or no impact upon the portions of integrated HBV DNA, which often encode the virus regulatory protein, HBx. Although given little attention, HBx is an important therapeutic target because it contributes importantly to (a) HBV replication, (b) in protecting infected cells from immune mediated destruction during chronic infection, and (c) in the development of HCC. Thus, the development of therapies targeting HBx, combined with other established therapies, will provide a functional cure that will target virus replication and further reduce or eliminate both the morbidity and mortality associated with chronic liver disease and HCC. Simultaneous targeting of all these characteristics underscores the importance of developing therapies against HBx.

Canonical and Divergent N-Terminal HBx Isoform Proteins Unveiled: Characteristics and Roles during HBV Replication

Hepatitis B virus (HBV) X protein (HBx) is a viral regulatory and multifunctional protein. It is well-known that the canonical HBx reading frame bears two phylogenetically conserved internal in-frame translational initiation codons at Met2 and Met3, thus possibly generating divergent N-terminal smaller isoforms during translation. Here, we demonstrate that the three distinct HBx isoforms are generated from the ectopically expressed HBV HBx gene, named XF (full-length), XM (medium-length), and XS (short-length); they display different subcellular localizations when expressed individually in cultured hepatoma cells. Particularly, the smallest HBx isoform, XS, displayed a predominantly cytoplasmic localization. To study HBx proteins during viral replication, we performed site-directed mutagenesis to target the individual or combinatorial expression of the HBx isoforms within the HBV viral backbone (full viral genome). Our results indicate that of all HBx isoforms, only the smallest HBx isoform, XS, can restore WT levels of HBV replication, and bind to the viral mini chromosome, thereby establishing an active chromatin state, highlighting its crucial activities during HBV replication. Intriguingly, we found that sequences of HBV HBx genotype H are devoid of the conserved Met3 position, and therefore HBV genotype H infection is naturally silent for the expression of the HBx XS isoform. Finally, we found that the HBx XM (medium-length) isoform shares significant sequence similarity with the N-terminus domain of the COMMD8 protein, a member of the copper metabolism MURR1 domain-containing (COMMD) protein family. This novel finding might facilitate studies on the phylogenetic origin of the HBV X protein. The identification and functional characterization of its isoforms will shift the paradigm by changing the concept of HBx from being a unique, canonical, and multifunctional protein toward the occurrence of different HBx isoforms, carrying out different overlapping functions at different subcellular localizations during HBV genome replication. Significantly, our current work unveils new crucial HBV targets to study for potential antiviral research, and human virus pathogenesis.

Retroviral Antisense Transcripts and Genes: 33 Years after First Predicted, a Silent Retroviral Revolution?

Paradigm shifts throughout the history of microbiology have typically been ignored, or met with skepticism and resistance, by the scientific community. This has been especially true in the field of virology, where the discovery of a "contagium vivum fluidum", or infectious fluid remaining after excluding bacteria by filtration, was initially ignored because it did not coincide with the established view of microorganisms. Subsequent studies on such infectious agents, eventually termed "viruses", were met with skepticism. However, after an abundance of proof accumulated, viruses were eventually acknowledged as defined microbiological entities. Next, the proposed role of viruses in oncogenesis in animals was disputed, as was the unique mechanism of genome replication by reverse transcription of RNA by the retroviruses. This same pattern of skepticism holds true for the prediction of the existence of retroviral "antisense" transcripts and genes. From the time of their discovery, it was thought that retroviruses encoded proteins on only one strand of proviral DNA. However, in 1988, it was predicted that human immunodeficiency virus type 1 (HIV-1), and other retroviruses, express an antisense protein encoded on the DNA strand opposite that encoding the known viral proteins. Confirmation came quickly with the characterization of the antisense protein, HBZ, of the human T-cell leukemia virus type 1 (HTLV-1), and the finding that both the protein and its antisense mRNA transcript play key roles in viral replication and pathogenesis. However, acceptance of the existence, and potential importance, of a corresponding antisense transcript and protein (ASP) in HIV-1 infection and pathogenesis has lagged, despite gradually accumulating theoretical and experimental evidence. The most striking theoretical evidence is the finding that asp is highly conserved in group M viruses and correlates exclusively with subtypes, or clades, responsible for the AIDS pandemic. This review outlines the history of the major shifts in thought pertaining to the nature and characteristics of viruses, and in particular retroviruses, and details the development of the hypothesis that retroviral antisense transcripts and genes exist. We conclude that there is a need to accelerate studies on ASP, and its transcript(s), with the view that both may be important, and overlooked, targets in anti-HIV therapeutic and vaccine strategies.

Interplay Between Non-Canonical NF-κB Signaling and Hepatitis B Virus Infection

The non-canonical nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) signaling pathway is an important component of NF-κB transcription complex. Activation of this pathway mediates the development and function of host immune system involved in inflammation and viral infection. During hepatitis B virus (HBV) infection, there is a complex interaction between infected hepatocytes and the immune cells, which can hinder antiviral immune responses and is associated with pathological changes in liver tissue. Consistently, the host immune system is closely related to the severity of liver damage and the level of viral replication. Previous studies indicated that the non-canonical NF-κB signaling pathway was affected by HBV and might play an important regulatory role in the antiviral immunity. Therefore, systematically elucidating the interplay between HBV and non-canonical NF-κB signaling will contribute the discovery of more potential therapeutic targets and novel drugs to treat HBV infection.

Molecular Mechanisms and Animal Models of HBV-Related Hepatocellular Carcinoma: With Emphasis on Metastatic Tumor Antigen 1

Hepatocellular carcinoma (HCC) is an important cause of cancer death worldwide, and hepatitis B virus (HBV) infection is a major etiology, particularly in the Asia-Pacific region. Lack of sensitive biomarkers for early diagnosis of HCC and lack of effective therapeutics for patients with advanced HCC are the main reasons for high HCC mortality; these clinical needs are linked to the molecular heterogeneity of hepatocarcinogenesis. Animal models are the basis of preclinical and translational research in HBV-related HCC (HBV-HCC). Recent advances in methodology have allowed the development of several animal models to address various aspects of chronic liver disease, including HCC, which HBV causes in humans. Currently, multiple HBV-HCC animal models, including conventional, hydrodynamics-transfection-based, viral vector-mediated transgenic, and xenograft mice models, as well as the hepadnavirus-infected tree shrew and woodchuck models, are available. This review provides an overview of molecular mechanisms and animal models of HBV-HCC. Additionally, the metastatic tumor antigen 1 (MTA1), a cancer-promoting molecule, was introduced as an example to address the importance of a suitable animal model for studying HBV-related hepatocarcinogenesis.

Screening of the HBx transactivation domain interacting proteins and the function of interactor Pin1 in HBV replication

Hepatitis B virus (HBV) X protein (HBx) has been determined to play a crucial role in the replication and transcription of HBV, and its biological functions mainly depend on the interaction with other host proteins. This study aims at screening the proteins that bind to the key functional domain of HBx by integrated proteomics. Proteins that specifically bind to the transactivation domain of HBx were selected by comparing interactors of full-length HBx and HBx-D5 truncation determined by glutathione-S-transferase (GST) pull-down assay combined with mass spectrometry (MS). The function of HBx interactor Pin1 in HBV replication was further investigated by in vitro experiments. In this study, a total of 189 proteins were identified from HepG2 cells that specifically bind to the transactivation domain of HBx by GST pull-down and subsequent MS. After gene ontology (GO) analysis, Pin1 was selected as the protein with the highest score in the largest cluster functioning in protein binding, and also classified into the cluster of proteins with the function of structural molecule activity, which is of great potential to be involved in HBV life cycle. The interaction between Pin1 and HBx has been further confirmed by Ni²⁺-NTA pulldown assay, co-immunoprecipitation, and immunofluorescence microscopy. HBsAg and HBeAg levels significantly decreased in Pin1 expression inhibited HepG2.2.15 cells. Besides, the inhibition of Pin1 expression in HepG2 cells impeded the restored replication of HBx-deficient HBV repaired by ectopic HBx expression. In conclusion, our study identified Pin1 as an interactor binds to the transactivation domain of HBx, and suggested the potential association between Pin1 and the function of HBx in HBV replication.

Application of the woodchuck animal model for the treatment of hepatitis B virus-induced liver cancer

This review describes woodchucks chronically infected with the woodchuck hepatitis virus (WHV) as an animal model for hepatocarcinogenesis and treatment of primary liver cancer or hepatocellular carcinoma (HCC) induced by the hepatitis B virus (HBV). Since laboratory animal models susceptible to HBV infection are limited, woodchucks experimentally infected with WHV, a hepatitis virus closely related to HBV, are increasingly used to enhance our understanding of virus-host interactions, immune response, and liver disease progression. A correlation of severe liver pathogenesis with high-level viral replication and deficient antiviral immunity has been established, which are present during chronic infection after WHV inoculation of neonatal woodchucks for modeling vertical HBV transmission in humans. HCC in chronic carrier woodchucks develops 17 to 36 mo after neonatal WHV infection and involves liver tumors that are comparable in size, morphology, and molecular gene signature to those of HBV-infected patients. Accordingly, woodchucks with WHV-induced liver tumors have been used for the improvement of imaging and ablation techniques of human HCC. In addition, drug efficacy studies in woodchucks with chronic WHV infection have revealed that prolonged treatment with nucleos(t)ide analogs, alone or in combination with other compounds, minimizes the risk of liver disease progression to HCC. More recently, woodchucks have been utilized in the delineation of mechanisms involved in innate and adaptive immune responses against WHV during acute, self-limited and chronic infections. Therapeutic interventions based on modulating the deficient host antiviral immunity have been explored in woodchucks for inducing functional cure in HBV-infected patients and for reducing or even delaying associated liver disease sequelae, including the onset of HCC. Therefore, woodchucks with chronic WHV infection constitute a well-characterized, fully immunocompetent animal model for HBV-induced liver cancer and for preclinical evaluation of the safety and efficacy of new modalities, which are based on chemo, gene, and immune therapy, for the prevention and treatment of HCC in patients for which current treatment options are dismal.

Harnessing adipose‑derived stem cells to release specialized secretome for the treatment of hepatitis B

Mesenchymal stem cells (MSCs) have the function of repairing damaged tissue, which is known to be mediated by the secretome, the collection of secretory materials shed from MSCs. Adjusting the culture conditions of MSCs can lead to a significant difference in the composition of the secretome. It was hypothesized that pre‑sensitization of MSCs with specific disease‑causing agents could harness MSCs to release the therapeutic materials specialized for the disease. To validate this hypothesis, the present study aimed to generate a 'disease‑specific secretome' for hepatitis caused by hepatitis B virus using hepatitis BX antigen (HBx) as a disease‑causing material. Secretary materials (HBx‑IS) were collected following the stimulation of adipose‑derived stem cells (ASCs) with 100‑fold diluted culture media of AML12 hepatocytes that had been transfected with pcDNA‑HBx for 24 h. An animal model of hepatitis B was generated by injecting HBx into mice, and the mice were subsequently intravenously administered a control secretome (CS) or HBx‑IS. Compared with the CS injection, the HBx‑IS injection significantly reduced the serum levels of interleukin‑6 and tumor necrosis factor‑α (pro‑inflammatory cytokines). Western blot analysis and immunohistochemistry of the liver specimens revealed that the HBx‑IS injection led to a higher expression of liver regeneration‑related markers, including hepatocyte growth factor and proliferating cell nuclear antigen, a lower expression of pro‑apoptotic markers, such as cleaved caspase 3 and Bim in mouse livers, and a lower expression of pro‑inflammatory markers (F4/80 and CD68) compared to the CS injection. HBx‑IS exhibited higher liver regenerative, anti‑inflammatory and anti‑apoptotic properties, particularly in the mouse model of hepatitis B compared to CS. This suggests that the secretome obtained by stimulating ASCs with disease‑causing agents may have a more prominent therapeutic effect on the specific disease than the naïve secretome.

An updated literature review: how HBV X protein regulates the propagation of the HBV

Chronic HBV infection constitutes a burden on human beings and is closely associated with hepatocellular carcinoma. The propagation of the HBV is determined by many factors, and the HBV X protein (HBx) could have a significant influence on this. HBx is a regulatory protein that can directly or indirectly interact with many cellular proteins to affect both the propagation of the HBV and the activity of the host cells. In this review, we summarized the possible mechanisms by which HBx regulates HBV replication at transcriptional and post-transcriptional levels in various experimental systems.

Hepatitis B Virus-X Downregulates Expression of Selenium Binding Protein 1

Selenium binding protein 1 (SELENBP1) has been known to be reduced in various types cancer, and epigenetic change is shown to be likely to account for the reduction of SELNEBP1 expression. With cDNA microarray comparative analysis, we found that SELENBP1 is markedly decreased in hepatitis B virus-X (HBx)-expressing cells. To clarify the effect of HBx on SELENBP1 expression, we compared the expression levels of SELENBP1 mRNA and protein by semi-quantitative RT-PCR, Northern blot, and Western blot. As expected, SELENBP1 expression was shown to be reduced in cells expressing HBx, and reporter gene analysis showed that the SELENBP1 promoter is repressed by HBx. In addition, the stepwise deletion of 5′ flanking promoter sequences resulted in a gradual decrease in basal promoter activity and inhibition of SELENBP1 expression by HBx. Moreover, immunohistochemistry on tissue microarrays containing 60 pairs of human liver tissue showed decreased intensity of SELENBP1 in tumor tissues as compared with their matched non-tumor liver tissues. Taken together, our findings suggest that inhibition of SELENBP1 expression by HBx might act as one of the causes in the development of hepatocellular carcinoma caused by HBV infection.

The Regulation of HBV Transcription and Replication

Hepatitis B virus (HBV) is a major human pathogen lacking a reliable curative therapy. Current therapeutics target the viral reverse transcriptase/DNA polymerase to inhibit viral replication but generally fail to resolve chronic HBV infections. Due to the limited coding potential of the HBV genome, alternative approaches for the treatment of chronic infections are desperately needed. An alternative approach to the development of antiviral therapeutics is to target cellular gene products that are critical to the viral life cycle. As transcription of the viral genome is an essential step in the viral life cycle, the selective inhibition of viral RNA synthesis is a possible approach for the development of additional therapeutic modalities that might be used in combination with currently available therapies. To address this possibility, a molecular understanding of the relationship between viral transcription and replication is required. The first step is to identify the transcription factors that are the most critical in controlling the levels of HBV RNA synthesis and to determine their in vivo role in viral biosynthesis. Mapping studies in cell culture utilizing reporter gene constructs permitted the identification of both ubiquitous and liver-enriched transcription factors capable of modulating transcription from the four HBV promoters. However, it was challenging to determine their relative importance for viral biosynthesis in the available human hepatoma replication systems. This technical limitation was addressed, in part, by the development of non-hepatoma HBV replication systems where viral biosynthesis was dependent on complementation with exogenously expressed transcription factors. These systems revealed the importance of specific nuclear receptors and hepatocyte nuclear factor 3 (HNF3)/forkhead box A (FoxA) transcription factors for HBV biosynthesis. Furthermore, using the HBV transgenic mouse model of chronic viral infection, the importance of various nuclear receptors and FoxA isoforms could be established in vivo. The availability of this combination of systems now permits a rational approach toward the development of selective host transcription factor inhibitors. This might permit the development of a new class of therapeutics to aid in the treatment and resolution of chronic HBV infections, which currently affects approximately 1 in 30 individuals worldwide and kills up to a million people annually.

Contribution of hepatitis B virus X protein-induced aberrant microRNA expression to hepatocellular carcinoma pathogenesis

The hepatitis B virus-encoded X (HBX) protein plays important roles in Hepatocellular carcinoma (HCC). Previous studies have demonstrated that HBX can induce alterations in the expression of numerous microRNAs (miRNAs) involved in the carcinogenesis of various tumors. However, the global profile of liver miRNA changes induced by HBX has not been characterized. In this study, we conducted a miRNA microarray analysis to investigate the influence of HBX on the expression of total miRNAs in liver in relation to HCC. Comparative analysis of the data from human normal liver cells (L02) and human HCC cells (HepG2), with or without HBX, identified 19 differentially expressed miRNAs, including 5 with known association to HBX. Target gene prediction for the aberrantly expressed miRNAs identified a total of 304 potential target genes, involved in sundry pathways. Finally, pathway analysis of the HBXinduced miRNAs pathway showed that 5 of the total miRNAs formed an internetwork, suggesting that HBX might exert its pathological effects on hepatic cells through functional synergy with miRNAs that regulated common pathways in liver cells. Therefore, this work provides new insights into the mechanisms of HCC as well as potential diagnostic markers or therapeutic targets for use in clinical management of HCC.

Multiple Functions of Cellular FLIP Are Essential for Replication of Hepatitis B Virus

Hepatitis B virus (HBV) infection is a leading cause of liver diseases; however, the host factors which facilitate the replication and persistence of HBV are largely unidentified. Cellular FLICE inhibitory protein (c-FLIP) is a typical antiapoptotic protein. In many cases of liver diseases, the expression level of c-FLIP is altered, which affects the fate of hepatocytes. We previously found that c-FLIP and its cleaved form interact with HBV X protein (HBx), which is essential for HBV replication, and regulate diverse cellular signals. In this study, we investigated the role of endogenous c-FLIP in HBV replication and its underlying mechanisms. The knockdown of endogenous c-FLIP revealed that this protein regulates HBV replication through two different mechanisms. (i) c-FLIP interacts with HBx and protects it from ubiquitin-dependent degradation. The N-terminal DED1 domain of c-FLIP is required for HBx stabilization. (ii) c-FLIP regulates the expression or stability of hepatocyte nuclear factors (HNFs), which have critical roles in HBV transcription and maintenance of hepatocytes. c-FLIP regulates the stability of HNFs through physical interactions. We verified our findings in three HBV infection systems: HepG2-NTCP cells, differentiated HepaRG cells, and primary human hepatocytes. In conclusion, our results identify c-FLIP as an essential factor in HBV replication. c-FLIP regulates viral replication through its multiple effects on viral and host proteins that have critical roles in HBV replication.IMPORTANCE Although the chronic hepatitis B virus (HBV) infection still poses a major health concern, the host factors which are required for the replication of HBV are largely uncharacterized. Our studies identify cellular FLICE inhibitory protein (c-FLIP) as an essential factor in HBV replication. We found the dual roles of c-FLIP in regulation of HBV replication: c-FLIP interacts with HBx and enhances its stability and regulates the expression or stability of hepatocyte nuclear factors which are essential for transcription of HBV genome. Our findings may provide a new target for intervention in persistent HBV infection.

Identification of p90 Ribosomal S6 Kinase 2 as a Novel Host Protein in HBx Augmenting HBV Replication by iTRAQ-Based Quantitative Comparative Proteomics

PURPOSE

The aim of this study was to screen for novel host proteins that play a role in HBx augmenting Hepatitis B virus (HBV) replication.

EXPERIMENTAL DESIGN

Three HepG2 cell lines stably harboring different functional domains of HBx (HBx, HBx-Cm6, and HBx-Cm16) were cultured. ITRAQ technology integrated with LC-MS/MS analysis was applied to identify the proteome differences among these three cell lines.

RESULTS

In brief, a total of 70 different proteins were identified among HepG2-HBx, HepG2-HBx-Cm6, and HepG2-HBx-Cm16 by double repetition. Several differentially expressed proteins, including p90 ribosomal S6 kinase 2 (RSK2), were further validated. RSK2 was expressed at higher levels in HepG2-HBx and HepG2-HBx-Cm6 compared with HepG2-HBx-Cm16. Furthermore, levels of HBV replication intermediates were decreased after silencing RSK2 in HepG2.2.15. An HBx-minus HBV mutant genome led to decreased levels of HBV replication intermediates and these decreases were restored to levels similar to wild-type HBV by transient ectopic expression of HBx. After silencing RSK2 expression, the levels of HBV replication intermediates synthesized from the HBx-minus HBV mutant genome were not restored to levels that were observed with wild-type HBV by transient HBx expression.

CONCLUSION AND CLINICAL RELEVANCE

Based on iTRAQ quantitative comparative proteomics, RSK2 was identified as a novel host protein that plays a role in HBx augmenting HBV replication.

Role of HBx in hepatitis B virus persistence and its therapeutic implications

Chronic hepatitis B virus infection is a significant risk factor for cirrhosis and hepatocellular carcinoma. The HBx protein is required for virus replication, but the lack of robust infection models has hindered our understanding of HBx functions that could be targeted for antiviral purposes. We briefly review three properties of HBx: its binding to DDB1 and its regulation of cell survival and metabolism, to illustrate how a single viral protein can have multiple effects in a cell. We propose that different functions of HBx are needed, depending on the changing hepatocyte environment encountered during a chronic virus infection, and that these functions might serve as novel therapeutic targets for inhibiting hepatitis B virus replication and the development of associated diseases.

An RNAi-based high-throughput screening assay to identify small molecule inhibitors of hepatitis B virus replication

Persistent or chronic infection with the hepatitis B virus (HBV) represents one of the most common viral diseases in humans. The hepatitis B virus deploys the hepatitis B virus X protein (HBx) as a suppressor of host defenses consisting of RNAi-based silencing of viral genes. Because of its critical role in countering host defenses, HBx represents an attractive target for antiviral drugs. Here, we developed and optimized a loss-of-function screening procedure, which identified a potential pharmacophore that abrogated HBx RNAi suppression activity. In a survey of 14,400 compounds in the Maybridge Screening Collection, we prioritized candidate compounds via high-throughput screening based on reversal of green fluorescent protein (GFP)-reported, RNAi-mediated silencing in a HepG2/GFP-shRNA RNAi sensor line. The screening yielded a pharmacologically active compound, N-(2,4-difluorophenyl)-N'-[3-(1H-imidazol-1-yl) propyl] thiourea (IR415), which blocked HBx-mediated RNAi suppression indicated by the GFP reporter assay. We also found that IR415 reversed the inhibitory effect of HBx protein on activity of the Dicer endoribonuclease. We further confirmed the results of the primary screen in IR415-treated, HBV-infected HepG2 cells, which exhibited a marked depletion of HBV core protein synthesis and down-regulation of pre-genomic HBV RNA. Using a molecular interaction analysis system, we confirmed that IR415 selectively targets HBx in a concentration-dependent manner. The screening assay presented here allows rapid and improved detection of small-molecule inhibitors of HBx and related viral proteins. The assay may therefore potentiate the development of next-generation RNAi pathway-based therapeutics and promises to accelerate our search for novel and effective drugs in antiviral research.

A Role for the Host DNA Damage Response in Hepatitis B Virus cccDNA Formation-and Beyond?

Chronic hepatitis B virus (HBV) infection puts more than 250 million people at a greatly increased risk to develop end-stage liver disease. Like all hepadnaviruses, HBV replicates via protein-primed reverse transcription of a pregenomic (pg) RNA, yielding an unusually structured, viral polymerase-linked relaxed-circular (RC) DNA as genome in infectious particles. Upon infection, RC-DNA is converted into nuclear covalently closed circular (ccc) DNA. Associating with cellular proteins into an episomal minichromosome, cccDNA acts as template for new viral RNAs, ensuring formation of progeny virions. Hence, cccDNA represents the viral persistence reservoir that is not directly targeted by current anti-HBV therapeutics. Eliminating cccDNA will thus be at the heart of a cure for chronic hepatitis B. The low production of HBV cccDNA in most experimental models and the associated problems in reliable cccDNA quantitation have long hampered a deeper understanding of cccDNA molecular biology. Recent advancements including cccDNA-dependent cell culture systems have begun to identify select host DNA repair enzymes that HBV usurps for RC-DNA to cccDNA conversion. While this list is bound to grow, it may represent just one facet of a broader interaction with the cellular DNA damage response (DDR), a network of pathways that sense and repair aberrant DNA structures and in the process profoundly affect the cell cycle, up to inducing cell death if repair fails. Given the divergent interactions between other viruses and the DDR it will be intriguing to see how HBV copes with this multipronged host system.

Hepatitis B virus modulates store-operated calcium entry to enhance viral replication in primary hepatocytes

Many viruses modulate calcium (Ca²⁺) signaling to create a cellular environment that is more permissive to viral replication, but for most viruses that regulate Ca²⁺ signaling, the mechanism underlying this regulation is not well understood. The hepatitis B virus (HBV) HBx protein modulates cytosolic Ca²⁺ levels to stimulate HBV replication in some liver cell lines. A chronic HBV infection is associated with life-threatening liver diseases, including hepatocellular carcinoma (HCC), and HBx modulation of cytosolic Ca²⁺ levels could have an important role in HBV pathogenesis. Whether HBx affects cytosolic Ca²⁺ in a normal hepatocyte, the natural site of an HBV infection, has not been addressed. Here, we report that HBx alters cytosolic Ca²⁺ signaling in cultured primary hepatocytes. We used single cell Ca²⁺ imaging of cultured primary rat hepatocytes to demonstrate that HBx elevates the cytosolic Ca²⁺ level in hepatocytes following an IP₃-linked Ca²⁺ response; HBx effects were similar when expressed alone or in the context of replicating HBV. HBx elevation of the cytosolic Ca²⁺ level required extracellular Ca²⁺ influx and store-operated Ca²⁺ (SOC) entry and stimulated HBV replication in hepatocytes. We used both targeted RT-qPCR and transcriptome-wide RNAseq analyses to compare levels of SOC channel components and other Ca²⁺ signaling regulators in HBV-expressing and control hepatocytes and show that the transcript levels of these various proteins are not affected by HBV. We also show that HBx regulation of SOC-regulated Ca²⁺ accumulation is likely the consequence of HBV modulation of a SOC channel regulatory mechanism. In support of this, we link HBx enhancement of SOC-regulated Ca²⁺ accumulation to Ca²⁺ uptake by mitochondria and demonstrate that HBx stimulates mitochondrial Ca²⁺ uptake in primary hepatocytes. The results of our study may provide insights into viral mechanisms that affect Ca²⁺ signaling to regulate viral replication and virus-associated diseases.

The Smc5/6 Complex Restricts HBV when Localized to ND10 without Inducing an Innate Immune Response and Is Counteracted by the HBV X Protein Shortly after Infection

The structural maintenance of chromosome 5/6 complex (Smc5/6) is a restriction factor that represses hepatitis B virus (HBV) transcription. HBV counters this restriction by expressing HBV X protein (HBx), which targets Smc5/6 for degradation. However, the mechanism by which Smc5/6 suppresses HBV transcription and how HBx is initially expressed is not known. In this study we characterized viral kinetics and the host response during HBV infection of primary human hepatocytes (PHH) to address these unresolved questions. We determined that Smc5/6 localizes with Nuclear Domain 10 (ND10) in PHH. Co-localization has functional implications since depletion of ND10 structural components alters the nuclear distribution of Smc6 and induces HBV gene expression in the absence of HBx. We also found that HBV infection and replication does not induce a prominent global host transcriptional response in PHH, either shortly after infection when Smc5/6 is present, or at later times post-infection when Smc5/6 has been degraded. Notably, HBV and an HBx-negative virus establish high level infection in PHH without inducing expression of interferon-stimulated genes or production of interferons or other cytokines. Our study also revealed that Smc5/6 is degraded in the majority of infected PHH by the time cccDNA transcription could be detected and that HBx RNA is present in cell culture-derived virus preparations as well as HBV patient plasma. Collectively, these data indicate that Smc5/6 is an intrinsic antiviral restriction factor that suppresses HBV transcription when localized to ND10 without inducing a detectable innate immune response. Our data also suggest that HBx protein may be initially expressed by delivery of extracellular HBx RNA into HBV-infected cells.

Hepatitis B Virus Protein X Induces Degradation of Talin-1

In the infected human hepatocyte, expression of the hepatitis B virus (HBV) accessory protein X (HBx) is essential to maintain viral replication in vivo. HBx critically interacts with the host damaged DNA binding protein 1 (DDB1) and the associated ubiquitin ligase machinery, suggesting that HBx functions by inducing the degradation of host proteins. To identify such host proteins, we systematically analyzed the HBx interactome. One HBx interacting protein, talin-1 (TLN1), was proteasomally degraded upon HBx expression. Further analysis showed that TLN1 levels indeed modulate HBV transcriptional activity in an HBx-dependent manner. This indicates that HBx-mediated TLN1 degradation is essential and sufficient to stimulate HBV replication. Our data show that TLN1 can act as a viral restriction factor that suppresses HBV replication, and suggest that the HBx relieves this restriction by inducing TLN1 degradation.

HBX Protein-Induced Downregulation of microRNA-18a is Responsible for Upregulation of Connective Tissue Growth Factor in HBV Infection-Associated Hepatocarcinoma

Background

This study was designed to improve our understanding of the role of miR-18a and its target (connective tissue growth factor (CTGF), which are mediators in HBX-induced hepatocellular carcinoma (HCC).

Material/Methods

We first investigated the expression of several candidate microRNAs (miRNAs) reported to have been aberrantly expressed between HepG2 and HepG2.2.15, which is characterized by stable HBV infection, while the CTGF is identified as a target of miR-18a. Furthermore, the expression of CTGF evaluated in HepG2 was transfected with HBX, while the HepG2.2.15 was transfected with miR-18a and CTGF siRNA. We examined the cell cycle at the same time.

Results

We found that the expression of miR-18a was abnormally reduced in the HBV-positive HCC tissue samples compared with HBV-negative HCC samples. Through the use of a luciferase reporter system, we also identified CTGF 3′UTR (1046–1052 bp) as the exact binding site for miR-18a. We also observed a clear increase in CTGF mRNA and protein expression levels in HBV-positive HCC human tissue samples in comparison with the HBV-negative controls, indicating a possible negatively associated relationship between miR-18a and CTGF. Furthermore, we investigated the effect of HBX overexpression on miR-18a and CTGF, as well as the viability and cell cycle status of HepG2 cells. In addition, we found that HBX introduction downregulated miR-18a, upregulated CTGF, elevated the viability, and promoted cell cycle progression. We transfected HepG2.2.15 with miR-18a mimics and CTGF siRNA, finding that upregulated miR-18a and downregulated CTGF suppress the viability and cause cell cycle arrest.

Conclusions

Our study shows the role of the CTGF gene as a target of miR-18a, and identifies the function of HBV/HBX/miR-18a/CTGF as a key signaling pathway mediating HBV infection-induced HCC.

Hepatitis B virus molecular biology and pathogenesis

As obligate intracellular parasites, viruses need a host cell to provide a milieu favorable to viral replication. Consequently, viruses often adopt mechanisms to subvert host cellular signaling processes. While beneficial for the viral replication cycle, virus-induced deregulation of host cellular signaling processes can be detrimental to host cell physiology and can lead to virus-associated pathogenesis, including, for oncogenic viruses, cell transformation and cancer progression. Included among these oncogenic viruses is the hepatitis B virus (HBV). Despite the availability of an HBV vaccine, 350-500 million people worldwide are chronically infected with HBV, and a significant number of these chronically infected individuals will develop hepatocellular carcinoma (HCC). Epidemiological studies indicate that chronic infection with HBV is the leading risk factor for the development of HCC. Globally, HCC is the second highest cause of cancer-associated deaths, underscoring the need for understanding mechanisms that regulate HBV replication and the development of HBV-associated HCC. HBV is the prototype member of the Hepadnaviridae family; members of this family of viruses have a narrow host range and predominately infect hepatocytes in their respective hosts. The extremely small and compact hepadnaviral genome, the unique arrangement of open reading frames, and a replication strategy utilizing reverse transcription of an RNA intermediate to generate the DNA genome are distinguishing features of the Hepadnaviridae. In this review, we provide a comprehensive description of HBV biology, summarize the model systems used for studying HBV infections, and highlight potential mechanisms that link a chronic HBV-infection to the development of HCC. For example, the HBV X protein (HBx), a key regulatory HBV protein that is important for HBV replication, is thought to play a cofactor role in the development of HBV-induced HCC, and we highlight the functions of HBx that may contribute to the development of HBV-associated HCC.

Pharmacokinetics of (-)-β-D-2,6-Diaminopurine Dioxolane and its Metabolite, Dioxolane Guanosine, in Woodchucks (Marmota Monax)

The woodchuck ( Marmota monax) is a useful animal model for evaluating the in-vivo efficacy of antiviral agents against hepatitis B viral infection (HBV). The pharmacokinetics of a newly synthesized antiviral agent (-)-β-D-2,6-diaminopurine dioxolane (DAPD) in woodchucks is reported. DAPD is a nucleoside analogue, having potent and selective activity against human immunodeficiency virus and HBV in vitro. DAPD is susceptible to deamination in vivo by the ubiquitously present enzyme adenosine deaminase yielding the active metabolite dioxolane guanosine (DXG). The pharmacokinetics of DAPD and DXG were characterized following intravenous (i.v.) and oral (p.o.) administration of 20 mg kg−1 of DAPD to woodchucks. Plasma and urine samples were collected, and nucleoside concentrations were determined by HPLC. Following intravenous administration, the half-life of DAPD averaged 6.7 ± 4.3 h, and that of DXG averaged 17.6 ± 14.5 h. The mean total clearance and steady state volume of distribution of DAPD were 0.33 ± 0.14 L h kg−1 and 1.76 ± 0.65 L kg−1, respectively. The oral bioavailability of DAPD ranged from 3.7-8.2%; however, the apparent availability of DXG following oral administration of DAPD was 10.5-53%.

Human Liver Infection in a Dish: Easy-To-Build 3D Liver Models for Studying Microbial Infection

Human liver infection is a major cause of death worldwide, but fundamental studies on infectious diseases affecting humans have been hampered by the lack of robust experimental models that accurately reproduce pathogen-host interactions in an environment relevant for the human disease. In the case of liver infection, one consequence of this absence of relevant models is a lack of understanding of how pathogens cross the sinusoidal endothelial barrier and parenchyma. To fill that gap we elaborated human 3D liver in vitro models, composed of human liver sinusoidal endothelial cells (LSEC) and Huh-7 hepatoma cells as hepatocyte model, layered in a structure mimicking the hepatic sinusoid, which enable studies of key features of early steps of hepatic infection. Built with established cell lines and scaffold, these models provide a reproducible and easy-to-build cell culture approach of reduced complexity compared to animal models, while preserving higher physiological relevance compared to standard 2D systems. For proof-of-principle we challenged the models with two hepatotropic pathogens: the parasitic amoeba Entamoeba histolytica and hepatitis B virus (HBV). We constructed four distinct setups dedicated to investigating specific aspects of hepatic invasion: 1) pathogen 3D migration towards hepatocytes, 2) hepatocyte barrier crossing, 3) LSEC and subsequent hepatocyte crossing, and 4) quantification of human hepatic virus replication (HBV). Our methods comprise automated quantification of E. histolytica migration and hepatic cells layer crossing in the 3D liver models. Moreover, replication of HBV virus occurs in our virus infection 3D liver model, indicating that routine in vitro assays using HBV or others viruses can be performed in this easy-to-build but more physiological hepatic environment. These results illustrate that our new 3D liver infection models are simple but effective, enabling new investigations on infectious disease mechanisms. The better understanding of these mechanisms in a human-relevant environment could aid the discovery of drugs against pathogenic liver infection.

Oxethazaine inhibits hepatitis B virus capsid assembly by blocking the cytosolic calcium-signalling pathway

Chronic hepatitis B virus (HBV) infection is a serious public health problem and may progress to liver fibrosis, cirrhosis and hepatocellular carcinoma. It is currently treated with PEGylated IFN-α2a and nucleoside/nucleotide analogues (NAs). However, PEGylated IFN treatment has problems of high cost, low efficiency and side effects. Long-term administration of NAs is necessary to avoid virus relapse, which can cause drug resistance and side effects. New efforts are now being directed to develop novel anti-HBV drugs targeting either additional viral targets other than viral DNA polymerase or host targets to improve the treatment of chronic hepatitis B. In this study, we discovered that oxethazaine, approved for clinic use in a few countries such as Japan, India, South Africa and Brazil, can dose-dependently reduce the levels of HBV envelope antigen, extracellular HBV DNA in supernatants and intracellular HBV total DNA. However, the levels of HBV cccDNA and HBV RNAs were not affected by oxethazaine treatment. Further study confirmed that oxethazaine acts on the virus assembly stage of the HBV life cycle. A study of the mechanisms of oxethazaine suggested that this drug inhibits HBV replication and capsid assembly by blocking the cytosolic calcium-signalling pathway. Moreover, oxethazaine could inhibit the replication of lamivudine/entecavir-dual-resistant and adefovir-resistant HBV mutants. In conclusion, our study suggests that oxethazaine may serve as a promising drug, or could be used as a starting point for anti-HBV drug discovery.

Hepatitis B Virus X and Regulation of Viral Gene Expression

The efficient replication of hepatitis B virus (HBV) requires the HBV regulatory hepatitis B virus X (HBx) protein. The exact contributions of HBx are not fully understood, in part because of the limitations of the assays used for its study. When HBV replication is driven from a plasmid DNA, the contribution of HBx is modest. However, there is an absolute requirement for HBx in assays that recapitulate the infectious virus life cycle. There is much evidence that HBx can contribute directly to HBV replication by acting on viral promoters embedded within protein coding sequences. In addition, HBx may also contribute indirectly by modulating cellular pathways to benefit virus replication. Understanding the mechanism(s) of HBx action during virus replication may provide insight into novel ways to disrupt chronic HBV replication.

RPB5-Mediating Protein Suppresses Hepatitis B Virus (HBV) Transcription and Replication by Counteracting the Transcriptional Activation of Hepatitis B virus X Protein in HBV Replication Mouse Model

Background:

RPB5-Mediating protein (RMP) is associated with the RNA polymerase II subunit RPB5. This protein functionally counteracts the transcriptional activation of Hepatitis B Virus X protein (HBx) by competitively binding to the RPB5; however, the effects of RMP on Hepatitis B virus (HBV) transcription and replication remain unknown.

Objectives:

The purpose of this study was to investigate the effect of RMP on viral transcription and replication in vivo by using the hydrodynamic-based HBV replication mouse model.

Materials and Methods:

Male balb/c mice were transfected with wild type (1.2 wt) or the HBx minus HBV plasmids (1.2x (-)) with or without HBx and RMP, to establish an HBV replication mouse model by hydrodynamic injection through the tail vein. The HBV RNA and HBV DNA replication intermediates (RI) were analyzed in the liver.

Results:

RPB5-Mediating protein could inhibit HBV transcription and replication in groups transfected with the 1.2 wt and HBx. The inhibitory effect disappeared in the 1.2x (-) groups, yet it reappeared in the groups co-transfected with 1.2x (-) and HBx. An inhibitory effect was indicated at a low dose of RMP (0.3 ug, 0.5 ug and 0.7 ug) compared to the control group and groups that had received high doses of RMP.

Conclusions:

Our study demonstrated that a low dose of RMP could inhibit HBV transcription and replication, which is dependent on the appearance of HBx in vivo.

Animal models and the molecular biology of hepadnavirus infection

Australian antigen, the envelope protein of hepatitis B virus (HBV), was discovered in 1967 as a prevalent serum antigen in hepatitis B patients. Early electron microscopy (EM) studies showed that this antigen was present in 22-nm particles in patient sera, which were believed to be incomplete virus. Complete virus, much less abundant than the 22-nm particles, was finally visualized in 1970. HBV was soon found to infect chimpanzees, gorillas, orangutans, gibbon apes, and, more recently, tree shrews (Tupaia belangeri) and cynomolgus macaques (Macaca fascicularis). This restricted host range placed limits on the kinds of studies that might be performed to better understand the biology and molecular biology of HBV and to develop antiviral therapies to treat chronic infections. About 10 years after the discovery of HBV, this problem was bypassed with the discovery of viruses related to HBV in woodchucks, ground squirrels, and ducks. Although unlikely animal models, their use revealed the key steps in hepadnavirus replication and in the host response to infection, including the fact that the viral nuclear episome is the ultimate target for immune clearance of transient infections and antiviral therapy of chronic infections. Studies with these and other animal models have also suggested interesting clues into the link between chronic HBV infection and hepatocellular carcinoma.

Technical standards for hepatitis B virus X protein (HBx) research

Chronic infection with hepatitis B virus (HBV) is a risk factor for developing hepatocellular carcinoma (HCC). The life cycle of HBV is complex and has been difficult to study because HBV does not infect cultured cells. The HBV regulatory X protein (HBx) controls the level of HBV replication and possesses an HCC cofactor role. Attempts to understand the mechanism(s) that underlie HBx effects on HBV replication and HBV-associated carcinogenesis have led to many reported HBx activities that are likely influenced by the assays used. This review summarizes experimental systems commonly used to study HBx functions, describes limitations of these experimental systems that should be considered, and suggests approaches for ensuring the biological relevance of HBx studies.

A simple method for estimating the strength of natural selection on overlapping genes

Overlapping genes, where one DNA sequence codes for two proteins with different reading frames, are not uncommon in viruses and cellular organisms. Estimating the direction and strength of natural selection acting on overlapping genes is important for understanding their functionality, origin, evolution, maintenance, and potential interaction. However, the standard methods for estimating synonymous (d_S) and nonsynonymous (d_N) nucleotide substitution rates are inapplicable here because a nucleotide change can be simultaneously synonymous and nonsynonymous when both reading frames involved are considered. We have developed a simple method that can estimate d_N/d_S and test for the action of natural selection in each relevant reading frame of the overlapping genes. Our method is an extension of the modified Nei-Gojobori method previously developed for nonoverlapping genes. We confirmed the reliability of our method using extensive computer simulation. Applying this method, we studied the longest human sense–antisense overlapping gene pair, LRRC8E and ENSG00000214248. Although LRRC8E (leucine-rich repeat containing eight family, member E) is known to regulate cell size, the function of ENSG00000214248 is unknown. Our analysis revealed purifying selection on ENSG00000214248 and suggested that it originated in the common ancestor of bony vertebrates.

Hepatitis B virus HBx protein interactions with the ubiquitin proteasome system

The hepatitis B virus (HBV) causes acute and chronic hepatitis, and the latter is a major risk factor for the development of hepatocellular carcinoma (HCC). HBV encodes a 17-kDa regulatory protein, HBx, which is required for virus replication. Although the precise contribution(s) of HBx to virus replication is unknown, many viruses target cellular pathways to create an environment favorable for virus replication. The ubiquitin proteasome system (UPS) is a major conserved cellular pathway that controls several critical processes in the cell by regulating the levels of proteins involved in cell cycle, DNA repair, innate immunity, and other processes. We summarize here the interactions of HBx with components of the UPS, including the CUL4 adaptor DDB1, the cullin regulatory complex CSN, and the 26S proteasome. Understanding how these protein interactions benefit virus replication remains a challenge due to limited models in which to study HBV replication. However, studies from other viral systems that similarly target the UPS provide insight into possible strategies used by HBV.

Susceptibility of different hepatitis B virus isolates to interferon-alpha in a mouse model based on hydrodynamic injection

Interferon alpha (IFN-α) is commonly used for the treatment of chronic hepatitis B (CHB) patients. Many factors including viral genetics may determine the outcome of IFN-α therapy. In this study, we tested whether the expression of IFN-α directly in the liver inhibits HBV gene expression and replication using a HBV hydrodynamic injection (HI) mouse model. Two replication-competent clones from different HBV isolates that belonging to HBV genotype A and B based on a pAAV vector (pAAV-HBV-A and pAAV-HBV-B) were compared for their susceptibility to IFN-α. HBV clones were injected into mice either alone or in combination with a murine (m) IFN-α expression plasmid (pmIFN-α). HBsAg and HBeAg concentrations and HBV DNA levels in mice differed after injection of these two HBV clones. Co-application of pmIFN-α together with the two distinct isolates resulted in markedly different kinetics of decline of HBsAg, HBeAg, and HBV DNA levels in the mice. Immunohistochemical staining of liver sections with anti-HBc showed that mIFN-α application completely inhibited the expression of HBcAg in mice inoculated with pAAV-HBV-B, whereas the expression of HBcAg was only reduced in mice with pAAV-HBV-A. Consistently, mice injected with pAAV-HBV-B and pmIFN-α showed higher expression levels of the IFN-stimulated genes (ISGs) ISG15, OAS, PKR as well as proinflammatory cytokine IL-6 in the liver. In addition, expression levels of anti-inflammatory cytokine IL-10 was down-regulated significantly in liver of the mice injected with pAAV-HBV-B and pmIFN-α. Our data demonstrate that IFN-α exerts antiviral activity in HBV mouse model, but different HBV isolates may have diverse susceptibility to IFN-α.

The roles of hepatitis B virus-encoded X protein in virus replication and the pathogenesis of chronic liver disease

INTRODUCTION

Hepatitis B virus (HBV) is a major cause of chronic liver disease (CLD) and hepatocellular carcinoma (HCC) worldwide. More than 350 million people are at risk for HCC, and with few treatment options available, therapeutic approaches to targets other than the virus polymerase will be needed. This review suggests that the HBV-encoded X protein, HBx, would be an outstanding target because it contributes to the biology and pathogenesis of HBV in three fundamental ways.

AREAS COVERED

First, HBx is a trans-activating protein that stimulates virus gene expression and replication, thereby promoting the development and persistence of the carrier state. Second, HBx partially blocks the development of immune responses that would otherwise clear the virus, and protects infected hepatocytes from immune-mediated destruction. Thus, HBx contributes to the development of CLD without virus clearance. Third, HBx alters patterns of host gene expression that make possible the emergence of HCC. The selected literature cited is from the National Library of Medicine (Pubmed and Medline).

EXPERT OPINION

Understanding the mechanisms, whereby HBx supports virus replication and promotes pathogenesis, suggests that HBx will be an important therapeutic target against both virus replication and CLD aimed at the chemoprevention of HCC.

Tackling hepatitis B virus-associated hepatocellular carcinoma--the future is now

Hepatocellular carcinoma (HCC) is one of the most lethal and prevalent cancers in many developing countries including India. Among the various etiological factors being implicated in the cause of HCC, the most important cause, however, is hepatitis B virus (HBV) infection. Among all HBV genes, HBx is the most critical carcinogenic component, the molecular mechanisms of which have not been completely elucidated. Despite its clinical significance, there exists a very elemental understanding of the molecular, cellular, and environmental mechanisms that drive disease pathogenesis in HCC infected with HBV. Furthermore, there are only limited therapeutic options, the clinical benefits of which are insignificant. Therefore, the quest for novel and effective therapeutic regimen against HBV-related HCC is of paramount importance. This review attempts to epitomize the current state of knowledge of this most common and dreaded liver neoplasm, highlighting the putative treatment avenues and therapeutic research strategies that need to be implemented with immediate effect for tackling HBV-related HCC that has plagued the medical and scientific fraternity for decades. Additionally, this review proposes a novel "five-point" management algorithm for HBV-related HCC apart from portraying the unmet needs, principal challenges, and scientific perspectives that are relevant to controlling this accelerating global health crisis.

Calcium signaling in the liver

Intracellular free Ca(2+) ([Ca(2+)]i) is a highly versatile second messenger that regulates a wide range of functions in every type of cell and tissue. To achieve this versatility, the Ca(2+) signaling system operates in a variety of ways to regulate cellular processes that function over a wide dynamic range. This is particularly well exemplified for Ca(2+) signals in the liver, which modulate diverse and specialized functions such as bile secretion, glucose metabolism, cell proliferation, and apoptosis. These Ca(2+) signals are organized to control distinct cellular processes through tight spatial and temporal coordination of [Ca(2+)]i signals, both within and between cells. This article will review the machinery responsible for the formation of Ca(2+) signals in the liver, the types of subcellular, cellular, and intercellular signals that occur, the physiological role of Ca(2+) signaling in the liver, and the role of Ca(2+) signaling in liver disease.

HBV induced HCC: major risk factors from genetic to molecular level

Hepatocellular carcinoma (HCC) is a deadly and emerging disease leading to death in Asian countries. High hepatitis B virus (HBV) load and chronic hepatitis B (CHB) infection increase the risk of developing HCC. HBV is a DNA virus that can integrate DNA into host genome thereby increase the yield of transactivator protein HBxAg that may deregulate many pathways involving in metabolism of cells. Several monogenic and polygenic risk factors are also involved in HCC development. This review summarizes the mechanism involved in HCC development and discusses some promising therapies to make HCC curative.

Inactivation of hepatitis B virus replication in cultured cells and in vivo with engineered transcription activator-like effector nucleases

Chronic hepatitis B virus (HBV) infection remains an important global health problem. Stability of the episomal covalently closed circular HBV DNA (cccDNA) is largely responsible for the modest curative efficacy of available therapy. Since licensed anti-HBV drugs have a post-transcriptional mechanism of action, disabling cccDNA is potentially of therapeutic benefit. To develop this approach, we engineered mutagenic transcription activator-like effector nucleases (TALENs) that target four HBV-specific sites within the viral genome. TALENs with cognate sequences in the S or C open-reading frames (ORFs) efficiently disrupted sequences at the intended sites and suppressed markers of viral replication. Following triple transfection of cultured HepG2.2.15 cells under mildly hypothermic conditions, the S TALEN caused targeted mutation in ~35% of cccDNA molecules. Markers of viral replication were also inhibited in vivo in a murine hydrodynamic injection model of HBV replication. HBV target sites within S and C ORFs of the injected HBV DNA were mutated without evidence of toxicity. These findings are the first to demonstrate a targeted nuclease-mediated disruption of HBV cccDNA. Efficacy in vivo also indicates that these engineered nucleases have potential for use in treatment of chronic HBV infection.

Hepatitis B virus X protein-induced aberrant epigenetic modifications contributing to human hepatocellular carcinoma pathogenesis

Hepatocellular carcinoma (HCC) remains one of the most prevalent malignant diseases worldwide, and the majority of cases are related to hepatitis B virus (HBV) infection. Interactions between the HBV-encoded X (HBx) protein and host factors are known to play major roles in the onset and progression of HBV-related HCC. These dynamic molecular mechanisms are extremely complex and lead to prominent changes in the host genetic and epigenetic architecture. This review summarizes the current knowledge about HBx-induced epigenetic changes, including aberrations in DNA methylation, histone modifications, and microRNA expression, and their roles in HBV-infected liver cells and HBV-related HCC. Moreover, the HBx-mediated epigenetic control of HBV covalently closed circular DNA (cccDNA) is also discussed. Although this field of study is relatively new, the accumulated evidence has indicated that the epigenetic events induced by HBx play important roles in the development of HBV-related HCC. Ongoing research will help to identify practical applications of the HBV-related epigenetic signatures as biomarkers for early HCC detection or as potential targets to prevent and treat HBV-related HCC.

Role and functional domain of hepatitis B virus X protein in regulating HBV transcription and replication in vitro and in vivo

The role of hepatitis B virus (HBV) X protein (HBx) in the regulation of HBV replication remains controversial. In the present study, the role of HBx in regulating HBV replication was initially investigated in both HepG2 and Huh7 in vitro cell lines with a transient transfection system. Next, the regions of HBx responsible for transcriptional transactivation and promotion of HBV replication were mapped in an HBV replication mouse model by in vivo transfection of a series of HBx expression plasmids. In an in vitro setting, HBx deficiency had little effect on HBV replication in Huh7 cells, but impaired HBV replication in HepG2 cells. In an in vivo setting, HBx had a strong enhancing effect on HBV transcription and replication. For the C-terminal two-thirds of the protein (amino acids [aa] 51 to 154) was required for this function of HBx, and the regions spanning aa 52 to 72 and 88 to 154 were found to be important for the stimulatory function of HBx on HBV replication. In conclusion, the role of HBx in HBV replication regulation is affected by host cell type, and HBx has an important role in stimulating HBV transcription and replication in hepatocytes in vivo. Further, the transcriptional transactivation function of HBx may be crucial for its stimulatory effect on HBV transcription and replication.

Molecular virology of chronic hepatitis B and C: parallels, contrasts and impact on drug development and treatment outcome

Chronic infections with hepatitis B virus (HBV) and hepatitis C virus (HCV) are highly prevalent worldwide, causing significant liver disease and thus representing high unmet medical needs. Accordingly, substantial pharmaceutical and clinical research efforts have been made to develop and improve treatments for these viruses. While HBV and HCV are both hepatotropic viruses that can cause similar disease in chronically infected patients, they belong to different viral families. There are substantial differences in the molecular virology of HBV and HCV that have profound implications for therapeutic strategy. In particular, HBV has a long-lived nuclear form of its genome (covalently closed circular DNA) that is able to persist in the face of potent inhibition of viral replication. In contrast, HCV does not have a long-lived genome form and depends on active replication to maintain infection; HCV is therefore much more susceptible to eradication by potent antiviral agents. Additional differences between HBV and HCV with therapeutic implications include the size, structure and heterogeneity of their respective viral genomes. These factors influence the number of targets available for therapeutic intervention, response to therapy among viral genotypes and the emergence of viral resistance. Substantial progress has been made in treating each infection, but unique challenges remain. In this review, key differences in the molecular virology of hepatitis B and C will be presented, highlighting their impact on antiviral therapy (particularly with respect to direct-acting antivirals) and the challenges they present to the cure of each disease.

Hepatitis B virus X protein stimulates gene expression selectively from extrachromosomal DNA templates

Chronic hepatitis B virus (HBV) infection is a major risk factor for liver cancer development. HBV encodes the hepatitis B virus X (HBx) protein that promotes transcription of the viral episomal DNA genome by the host cell RNA polymerase II. Here we provide evidence that HBx accomplishes this task by a conserved and unusual mechanism. Thus, HBx strongly stimulates expression of transiently transfected reporter constructs, regardless of the enhancer and promoter sequences. This activity invariably requires HBx binding to the cellular UV-damaged DDB1 E3 ubiquitin ligase, suggesting a common mechanism. Unexpectedly, none of the reporters tested is stimulated by HBx when integrated into the chromosome, despite remaining responsive to their cognate activators. Likewise, HBx promotes gene expression from the natural HBV episomal template but not from a chromosomally integrated HBV construct. The same was observed with the HBx protein of woodchuck HBV. HBx does not affect nuclear plasmid copy number and functions independently of CpG dinucleotide methylation.

CONCLUSION

We propose that HBx supports HBV gene expression by a conserved mechanism that acts specifically on episomal DNA templates independently of the nature of the cis-regulatory sequences. Because of its uncommon property and key role in viral transcription, HBx represents an attractive target for new antiviral therapies.

Experimental models and therapeutic approaches for HBV

Liver disease associated to persistent infection with the hepatitis B virus (HBV) continues to be a major health problem of global impact. In spite of the existence of an effective vaccine, approximately 360 million people are chronically infected worldwide, who are at high risk of developing liver cirrhosis and hepatocellular carcinoma. Although current therapeutic regimens can efficiently suppress viral replication, the unique replication strategies employed by HBV permit the virus to persist within the infected hepatocytes. As a consequence, relapse of viral activity is commonly observed after cessation of treatment with polymerase inhibitors. The narrow host range of HBV has hindered progresses in understanding specific steps of HBV replication and the development of more effective therapeutic strategies aiming at achieving sustained viral control and, eventually, virus eradication. This review will focus on summarizing recent advances obtained with well-established and more innovative experimental models, giving emphasis on the strength of the different systems as tools for elucidating distinct aspects of HBV persistence and for the development of new therapeutic approaches.

The C-terminal region of the hepatitis B virus X protein is required for its stimulation of HBV replication in primary mouse hepatocytes

Hepatitis B virus (HBV) infection is an important risk factor for hepatocellular carcinoma (HCC). The hepatitis B virus X protein (HBx), a multifunctional regulatory protein encoded by HBV, is known to be involved in stimulation of viral replication by modulating cell cycle status. HBx is required for maximal virus replication in plasmid-based replication assays in immortalized human liver HepG2 cells and in primary rat hepatocytes. Moreover, the C-terminal region of HBx is important for HBV replication in HepG2 cells. However, in normal hepatocytes, the region of HBx that is responsible for its effect on cell cycle regulation and HBV replication is unclear. We have demonstrated that HBx is similarly required for maximal HBV replication in primary mouse hepatocytes and that the C-terminus of HBx is essential for its ability to stimulate HBV replication by inducing quiescent hepatocytes to exit G0 phase of the cell cycle but stall in G1 phase. Our studies establish that primary mouse hepatocytes support HBx-dependent HBV replication, and provide further evidence for the effect of the C-terminal region of HBx on HBV infection and replication.