Transcriptional regulation of hepatitis B virus by nuclear hormone receptors is a critical determinant of viral tropism

Inhibition of alpha interferon (IFN-α)-induced microRNA-122 negatively affects the anti-hepatitis B virus efficiency of IFN-α

Alpha interferon (IFN-α)-based therapy can effectively treat chronic hepatitis B virus (HBV) infection, which causes life-threatening complications. Responses to IFN-α therapy vary greatly in chronic hepatitis B (CHB) patients, but underlying mechanisms are almost unknown. In this study, we found that IFN-α treatment induced a marked decrease of microRNA-122 (miR-122) expression in hepatocytes. We next showed that IFN-α-induced miR-122 downregulation was only partly due to transcriptional suppression. One IFN-stimulated gene (ISG), NT5C3, which was identified as a miR-122 target, efficiently inhibited miR-122 by binding and sequestering miR-122 with its mRNA 3'-untranslated region (3'-UTR), indicating that this ISG is involved in IFN-α-mediated miR-122 suppression. Notably, the inhibitory effect of IFN-α on miR-122 was completely abolished by blocking IFN-α-induced upregulation of NT5C3 mRNA expression by RNA interference (RNAi). Meanwhile, we observed that miR-122 dramatically inhibited HBV expression and replication. Finally, we showed that IFN-α-mediated HBV-inhibitory effects could be enhanced significantly by blocking IFN-α-induced downregulation of miR-122. We therefore concluded that IFN-α-induced inhibition of miR-122 may negatively affect the anti-HBV function of IFN-α. These data provide valuable insights for a better understanding of the antiviral mechanism of IFN-α and raise further potential interest in enhancing its anti-HBV efficacy.

Mucroporin-M1 inhibits hepatitis B virus replication by activating the mitogen-activated protein kinase (MAPK) pathway and down-regulating HNF4α in vitro and in vivo

Hepatitis B virus (HBV) is a noncytopathic human hepadnavirus that causes acute, chronic hepatitis and hepatocellular carcinoma (HCC). As the clinical utility of current therapies is limited, new anti-HBV agents and sources for such agents are still highly sought after. Here, we report that Mucroporin-M1, a scorpion venom-derived peptide, reduces the amount of extracellular HBsAg, HBeAg, and HBV DNA productions of HepG2.2.15 cells in a dose-dependent manner and inhibits HBV capsid DNA, HBV intracellular RNA replication intermediates and the HBV Core protein in the cytoplasm of HepG2.2.15 cells. Using a mouse model of HBV infection, we found that HBV replication was significantly inhibited by intravenous injection of the Mucroporin-M1 peptide. This inhibitory activity was due to a reduction in HBV promoter activity caused by a decrease in the binding of HNF4α to the precore/core promoter region. Furthermore, we confirmed that Mucroporin-M1 could selectively activate mitogen-activated protein kinases (MAPKs) and lead to the down-regulation of HNF4α expression, which explains the decreased binding of HNF4α to the HBV promoter. Moreover, when the protein phosphorylation activity of the MAPK pathway was inhibited, both HNF4α expression and HBV replication recovered. Finally, we proved that treatment with the Mucroporin-M1 peptide increased phosphorylation of the MAPK proteins in HBV-harboring mice. These results implicate Mucroporin-M1 peptide can activate the MAPK pathway and then reduce the expression of HNF4α, resulting in the inhibition of HBV replication in vitro and in vivo. Our work also opens new doors to discovering novel anti-HBV agents or sources.

Inhibition of hepatitis B Virus replication by hepatocyte nuclear factor 4-alpha specific short hairpin RNA

BACKGROUND

Previous studies showed that hepatocyte nuclear factor 4α (HNF4α) may play a critical role in hepatitis B virus (HBV) replication.

AIMS

This study aimed to investigate the effect of knocking down of HNF4α with RNA interference technique on HBV replication in a HBV replication mouse model.

METHODS

Four HNF4α, specific short hairpin RNA (shRNA)-producing plasmids were constructed. HBV mRNA and DNA replication intermediates were analysed using Northern and Southern blot respectively. The expression of HNF4α and HBV core antigen (HBcAg) was detected using immunohistochemistry technique.

RESULTS

One of the HNF4α shRNAs, HNF4α shRNA1, efficiently inhibited the expression of HNF4α in HepG2 cells and mice liver. HBV RNA transcripts and DNA replication intermediates in HNF4α shRNA1 group were decreased 67.3 and 76%, respectively, in HepG2 cells, and 68.1 and 70.6% in mice liver respectively. The expression level of HBcAg in the liver was also decreased with the inhibition of HNF4α expression.

CONCLUSIONS

These results suggested that decreasing of HNF4α expression was associated with the reduced level of HBV replication in HepG2 cells and mice liver. These data indicated that HNF4α played a critical role in HBV replication in vivo, and HNF4α shRNA could inhibit HBV replication in vivo.

MicroRNA-141 represses HBV replication by targeting PPARA

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression primarily at the post-transcriptional level and play critical roles in a variety of physiological and pathological processes. In this report, miR-141 was identified to repress HBV expression by screening a small miRNA expressing library and synthetic miR-141 mimics could also significantly suppress HBV expression and replication in HepG2 cells. Bioinformatic analysis and experiment assays indicate that peroxisome proliferator-activated receptor alpha (PPARA) was the target of hsa-miR-141 during this process. Furthermore, knockdown of PPARA by small interfering RNA (siRNA) inhibited HBV replication similar to levels observed for miR-141. Promoter functional analysis indicated that repression of HBV replication by miR-141 mimics or siRNA was mediated by interfering with the HBV promoter functions, consistent with previous studies demonstrating that PPARA regulated HBV gene expression through interactions with HBV promoter regulatory elements. Our results suggest that miR-141 suppressed HBV replication by reducing HBV promoter activities by down-regulating PPARA. This study provides new insights into the molecular mechanisms associated with HBV-host interactions. Furthermore, this information may facilitate the development of novel anti-HBV therapeutic strategies.

Loss of microRNA 122 expression in patients with hepatitis B enhances hepatitis B virus replication through cyclin G(1) -modulated P53 activity

Hepatitis B virus (HBV) causes chronic infection in about 350 million people worldwide. Given the important role of the most abundant liver-specific microRNA, miR-122, in hepatic function and liver pathology, here we investigated the potential role and mechanism of miR-122 in regulating HBV replication. We found that miR-122 expression in liver was significantly down-regulated in patients with HBV infection compared with healthy controls, and the miR-122 levels were negatively correlated with intrahepatic viral load and hepatic necroinflammation. The depletion of endogenous miR-122 by its antisense inhibitor led to enhanced HBV replication, whereas overexpression of miR-122 by transfection of mimic or its expression vector inhibited viral production. We next identified cyclin G(1) as an miR-122 target from multiple candidate target genes that are involved in the regulation of HBV replication. Overexpression and knockdown studies both showed that cyclin G(1) regulated viral replication in HBV transfected cells. We also observed that cyclin G(1) expression was up-regulated in HBV-infected patients, and cyclin G(1) levels were inversely associated with miR-122 expression in liver tissues. Using coimmunoprecipitation, a luciferase reporter system, and electrophoretic mobility shift assay, we further demonstrated that cyclin G(1) specifically interacted with p53, and this interaction blocked the specific binding of p53 to HBV enhancer elements and simultaneously abrogated p53-mediated inhibition of HBV transcription. Finally, we show that miR-122 suppressed HBV replication in p53 wildtype cells but not in null isogenic cells.

CONCLUSION

miR-122 down-regulates its target cyclin G(1) , and thus interrupts the interaction between cyclin G(1) and p53 and abrogates p53-mediated inhibition of HBV replication. Our work shows that miR-122 down-regulation induced by HBV infection can impact HBV replication and possibly contribute to viral persistence and carcinogenesis.

Transforming growth factor-β1 suppresses hepatitis B virus replication by the reduction of hepatocyte nuclear factor-4α expression

Several studies have demonstrated that cytokine-mediated noncytopathic suppression of hepatitis B virus (HBV) replication may provide an alternative therapeutic strategy for the treatment of chronic hepatitis B infection. In our previous study, we showed that transforming growth factor-beta1 (TGF-β1) could effectively suppress HBV replication at physiological concentrations. Here, we provide more evidence that TGF-β1 specifically diminishes HBV core promoter activity, which subsequently results in a reduction in the level of viral pregenomic RNA (pgRNA), core protein (HBc), nucleocapsid, and consequently suppresses HBV replication. The hepatocyte nuclear factor 4alpha (HNF-4α) binding element(s) within the HBV core promoter region was characterized to be responsive for the inhibitory effect of TGF-β1 on HBV regulation. Furthermore, we found that TGF-β1 treatment significantly repressed HNF-4α expression at both mRNA and protein levels. We demonstrated that RNAi-mediated depletion of HNF-4α was sufficient to reduce HBc synthesis as TGF-β1 did. Prevention of HNF-4α degradation by treating with proteasome inhibitor MG132 also prevented the inhibitory effect of TGF-β1. Finally, we confirmed that HBV replication could be rescued by ectopic expression of HNF-4α in TGF-β1-treated cells. Our data clarify the mechanism by which TGF-β1 suppresses HBV replication, primarily through modulating the expression of HNF-4α gene.

Study of the expression levels of Hepatocyte nuclear factor 4 alpha and 3 beta in patients with different outcome of HBV infection

Hepatocyte nuclear factors 4 alpha (HNF4α) and 3 beta (HNF3β) are members of a group of liver-enriched transcription factors (LETFs) that play important roles in regulating the replication of hepatitis B virus (HBV) and liver inflammation. However, the relationship of the level of HNF4α and HNF3β with the severity of HBV-infected liver diseases is unclear. In this study, liver tissue samples from different types of HBV patients were collected, and HNF4α and HNF3β expression were detected by immunohistochemistry. The expression of HNF4α was significant higher in patients with severe hepatitis B(SHB) than those with chronic hepatitis B(CHB) and liver cirrhosis(LC) (both P < 0.05), but similar between patients with CHB and LC (P > 0.05). And the expression of HNF3β was similar among patients with CHB, LC and SHB (P > 0.05 for all pairwise comparison). This suggests that the expression level of HNF4α was different in patients with different outcome of HBV infection, high expression level of HNF4α may correlate with occurrence of SHB

Dynamics of HBV cccDNA expression and transcription in different cell growth phase

Background

The covalently closed-circular DNA (cccDNA) of hepatitis B virus (HBV) is associated with viral persistence in HBV-infected hepatocytes. However, the regulation of cccDNA and its transcription in the host cells at different growth stages is not well understood.

Methods

We took advantages of a stably HBV-producing cell line, 1.3ES2, and examine the dynamic changes of HBV cccDNA, viral transcripts, and viral replication intermediates in different cellular growth stages.

Results

In this study, we showed that cccDNA increased suddenly in the initial proliferation phase of cell growth, probably attributable to its nuclear replenishment by intracellular nucleocapsids. The amount of cccDNA then decreased dramatically in the cells during their exponential proliferation similar to the loss of extrachromosomal plasmid DNA during cell division, after which it accumulated gradually while the host cells grew to confluency. We found that cccDNA was reduced in dividing cells and could be removed when proliferating cells were subjected to long term of lamivudine (3TC) treatment. The amounts of viral replicative intermediates were rapidly reduced in these proliferating cells and were significantly increased after cells reaching confluency. The expression levels of viral transcripts were increased in parallel with the elevated expression of hepatic transcription factors (HNF4α, CEBPα, PPARα, etc.) during cell growth confluency. The HBV transcripts were transcribed from both integrated viral genome and cccDNA, however the transcriptional abilities of cccDNA was less efficient then that from integrated viral genome in all cell growth stages. We also noted increases in the accumulation of intracellular viral particles and the secretion of mature virions as the cells reached confluency and ceased to grow.

Conclusions

Based on the dynamics of HBV replication, we propose that HBV replication is modulated differently in the different stages of cell growth, and can be divided into three phases (initial proliferation phase, exponential proliferation phase and growth confluency phase) according to the cell growth curve. The regulation of cccDNA in different cell growth phase and its importance regarding HBV replication are discussed.

Limited effects of bile acids and small heterodimer partner on hepatitis B virus biosynthesis in vivo

Multiple nuclear receptors, including hepatocyte nuclear factor 4α (HNF4α), retinoid X receptor α (RXRα) plus peroxisome proliferator-activated receptor α (PPARα), RXRα plus farnesoid X receptor α (FXRα), liver receptor homolog 1 (LRH1), and estrogen-related receptors (ERRs), have been shown to support efficient viral biosynthesis in nonhepatoma cells in the absence of additional liver-enriched transcription factors. Although HNF4α has been shown to be critical for the developmental expression of hepatitis B virus (HBV) biosynthesis in the liver, the relative importance of the various nuclear receptors capable of supporting viral transcription and replication in the adult in vivo has not been clearly established. To investigate the role of the nuclear receptor FXR and the corepressor small heterodimer partner (SHP) in viral biosynthesis in vivo, SHP-expressing and SHP-null HBV transgenic mice were fed a bile acid-supplemented diet. The increased FXR activity and SHP expression levels resulting from bile acid treatment did not greatly modulate HBV RNA and DNA synthesis. Therefore, FXR and SHP appear to play a limited role in modulating HBV biosynthesis, suggesting that alternative nuclear receptors are more critical determinants of viral transcription in the HBV transgenic mouse model of chronic viral infection. These observations suggest that hepatic bile acid levels or therapeutic agents targeting FXR may not greatly modulate viremia during natural infection.

Role of peroxisome proliferator-activated receptor gamma coactivator 1alpha in AKT/PKB-mediated inhibition of hepatitis B virus biosynthesis

Hepatitis B virus (HBV) transcription and replication are essentially restricted to hepatocytes because liver-enriched transcription factors govern viral RNA synthesis. The level of transcription from the HBV promoters depends on both the transcription factors binding to these regulatory sequence elements and their ability to recruit coactivators capable of mediating assembly of the transcription preinitiation complex containing RNA polymerase II. Nuclear receptors are a primary determinant of HBV pregenomic RNA synthesis and, hence, viral replication. Peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α) enhances the activity of nuclear receptors and, consequently, HBV biosynthesis. PGC1α is also an important target of signal transduction pathways involved in hepatic glucose and lipid homeostasis, suggesting that this coactivator may have an important role in modulating HBV biosynthesis under various physiological conditions. Consistent with this suggestion, v-akt murine thymoma viral oncogene homolog/protein kinase B (AKT/PKB) is shown to modulate PGC1α activity and, hence, HBV transcription and replication in a cell line-specific manner. In addition, AKT can modulate HBV replication in some but not all cell lines at a posttranscriptional step in the viral life cycle. These observations demonstrate that growth and nutritional signals have the capacity to influence viral production, but the magnitude of these effects will depend on the precise cellular context in which they occur.

The interaction between HCV and nuclear receptor-mediated pathways

Hepatitis C virus (HCV) is presently the leading indication for liver transplantation in Western countries. Treatment for HCV infection includes a combination of pegylated interferon and ribavirin, which produces highly variable response rates. This reflects the lack of information regarding the roles of host and viral components during viral pathogenesis. Vital processes regulated by the liver, including metabolism, lipid homeostasis, cellular proliferation, and the immune response, are known to be systematically dysregulated as a result of persistent HCV infection. Nuclear receptors and their ligands are recognized as indispensable regulators of liver homeostasis. Pathways mediated by the nuclear receptor superfamily have been shown to be profoundly disrupted during HCV infection, leading to an increased importance in elucidating the exact nature of this complex relationship. Expanded understanding of the role of nuclear receptors in HCV infection may therefore be an essential step in the search for a more universally effective treatment.

Hepatocyte metabolic signalling pathways and regulation of hepatitis B virus expression

Hepatitis B virus (HBV) is a small DNA virus responsible for significant morbidity and mortality worldwide. The liver, which is the main target organ for HBV infection, provides the virus with the machinery necessary for persistent infection and propagation, a process that might ultimately lead to severe liver pathologies such as chronic hepatitis, cirrhosis and liver cancer. HBV gene expression is regulated mainly at the transcriptional level by recruitment of a whole set of cellular transcription factors (TFs) and co-activators to support transcription. Over the years, many of these TFs were identified and interestingly enough most are associated with the body's nutritional state. These include the hepatocyte nuclear factors, forkhead Box O1, Farnesoid X receptor, cyclic-AMP response element-binding (CREB), CCAAT/enhancer-binding protein (C/EBP) and glucocorticoid receptor TFs and the transcription coactivator PPARγ coactivator-1α. Consequently, HBV gene expression is linked to hepatic metabolic processes such as glucose and fat production and utilization as well as bile acids' production and secretion. Furthermore, recent evidence indicates that HBV actively interferes with some of these hepatic metabolic processes by manipulating key TFs, such as CREB and C/EBP, to meet its requirements. The discovery of the mechanisms by which HBV is controlled by the hepatic metabolic milieu may broaden our understanding of the unique regulation of HBV expression and may also explain the mechanisms by which HBV induces liver pathologies. The emerging principle of the intimate link between HBV and liver metabolism can be further exploited for host-targeted therapeutic strategies.

Application of hepatitis B virus replication mouse model

AIM: To evaluate the value of the hepatitis B virus (HBV) replication mouse model with regard to several aspects of the study of HBV biology.

METHODS: To evaluate the HBV replication mouse model in detecting the efficacy of anti-HBV agents, the interferon inducer polyinosinic-polytidylin acid (polyIC) and nucleotide analogues adefovir and entecavir were administered to mice injected with wild type pHBV4.1, and the inhibiting effect of these agents on HBV DNA replication was evaluated. To identify the model’s value in a replication ability study of HBV drug-resistant mutants and a HBx-minus mutant, telbivudine resistance mutants (rtM204I, ayw subtype), adefovir resistance mutants (rtA181V + rtN236T, ayw subtype) and HBx-minus mutants were injected respectively, and their corresponding HBV DNA replication intermediates in mouse liver were assessed.

RESULTS: Compared with the wild type HBV replication mouse model without antiviral agent treatment, the HBV DNA replication intermediates of the polyIC-treated group were decreased 1-fold; while in the entecavir- and adefovir-treated groups, the levels of HBV DNA replication intermediates were inhibited 13.6-fold and 1.4-fold, respectively. For the mouse models injected with telbivudine resistance mutant, adefovir resistance mutant and HBx-minus mutant, HBV DNA replication intermediates could still be detected, but the levels of HBV DNA replication intermediates of these mutants decreased 4.5-fold, 5.6-fold and 2.9-fold respectively, compared with the mouse model with wild type HBV plasmid.

CONCLUSION: The HBV replication mouse model we established was a useful and convenient tool to detect the efficacy of antiviral agents and to study the replication ability of HBV mutants in vivo.

A look behind closed doors: interaction of persistent viruses with dendritic cells

Persistent infections with HIV, hepatitis B virus and hepatitis C virus are major causes of morbidity and mortality worldwide. As sentinels of the immune system, dendritic cells (DCs) are crucial for the generation of protective antiviral immunity. Recent advances in our understanding of the role of DCs during infection with these viruses provide insights into the mechanisms used by these viruses to exploit DC function and evade innate and adaptive immunity. In this Review we highlight the current knowledge about the interaction between DCs and these viruses and the underlying mechanisms that might influence the outcome of viral infections.

Initiation of duck hepatitis B virus infection requires cleavage by a furin-like protease

The entry mechanism of hepatitis B virus (HBV) has not been defined, and this impedes development of antiviral therapies aimed at an early step in the viral life cycle. HBV infection has both host and tissue specificities. For the related duck hepatitis B virus (DHBV), duck carboxypeptidase D (DCPD) has been proposed as the species-specific docking receptor, while glycine decarboxylase (DGD) may serve as a tissue-specific cofactor or secondary receptor. DGD binds to several truncated versions of the viral large envelope protein but not to the full-length protein, suggesting a need for proteolytic cleavage of the envelope protein by a furin-like proprotein convertase. In the present study, we found that transfected DCPD could confer DHBV binding to non-duck cell lines but that this was followed by rapid virus release from cells. Coexpression of furin led to DCPD cleavage and increased virus retention. Treatment of DHBV particles with endosome prepared from duck liver led to cleavage of the large envelope protein, and such viral preparation could generate a small amount of covalently closed circular DNA in LMH cells, a chicken hepatoma cell line resistant to DHBV infection. A furin inhibitor composed of decanoyl-RVKR-chloromethylketone blocked endosomal cleavage of the large envelope protein in vitro and suppressed DHBV infection of primary duck hepatocytes in vivo. These findings suggest that furin or a furin-like proprotein convertase facilitates DHBV infection by cleaving both the docking receptor and the viral large envelope protein.

The HepaRG cell line: biological properties and relevance as a tool for cell biology, drug metabolism, and virology studies

Liver progenitor cells may play an important role in carcinogenesis in vivo and represent therefore useful cellular materials for in vitro studies. The HepaRG cell line, which is a human bipotent progenitor cell line capable to differentiate toward two different cell phenotypes (i.e., biliary-like and hepatocyte-like cells), has been established from a liver tumor associated with chronic hepatitis C. This cell line represents a valuable alternative to ex vivo cultivated primary human hepatocytes (PHH), as HepaRG cells share some features and properties with adult hepatocytes. The cell line is particularly useful to evaluate drugs and perform drug metabolism studies, as many detoxifying enzymes are expressed and functional. It is also an interesting tool to study some aspect of progenitor biology (e.g., differentiation process), carcinogenesis, and the infection by some pathogens for which the cell line is permissive (e.g., HBV infection). Overall, this chapter gives a concise overview of the biological properties and potential applications of this cell line.

Multiple nuclear receptors may regulate hepatitis B virus biosynthesis during development

Hepatitis B virus (HBV) replicates by the reverse transcription of the viral 3.5 kb pregenomic RNA. Therefore the level of expression of this transcript in the liver is a primary determinant of HBV biosynthesis. In vivo neonatal transcription of the HBV 3.5 kb pregenomic RNA is developmental regulated by hepatocyte nuclear factor 4α (HNF4α). In addition, viral biosynthesis in non-hepatoma cells can be supported directly by this nuclear receptor. However HBV transcription and replication can be supported by additional nuclear receptors including the retinoid X receptor α/peroxisome proliferator-activated receptor α (RXRα/PPARα), retinoid X receptor α/farnesoid X receptor α (RXRα/FXRα), liver receptor homolog 1 (LRH1) and estrogen-related receptors (ERR) in non-hepatoma cells. Therefore during neonatal liver development, HNF4α may progressively activate viral transcription and replication by binding directly to the proximal HNF4α recognition sequence within the nucleocapsid promoter. Alternatively, HNF4α may support viral biosynthesis in vivo indirectly by activating a network of liver-enriched nuclear receptors that, in combination, direct HBV 3.5 kb pregenomic RNA transcription and replication.

Identification of a novel negative regulatory element on the hepatitis B virus S-(+)-strand

In this study, we scanned the whole hepatitis B virus (HBV) genome for the identification of potential regulatory elements located on the S-(+)-strand. With pCDNA3.1-HBV1.3 as template which contains 1.3-fold HBV whole genome, HBV fragments were amplified by PCR methods, and then inserted into the upstream of a heterologous luciferase reporter vector (pGL3control) in antisense orientation, allowing the HBV expression from the S-(+)-strand. We found that the reporter plasmid containing nt 509-1(3182)-2639 of HBV inhibited luciferase gene transcription and expression in HepG2 cells. Our results strongly suggested that nt 453-250 of HBV may act as a novel negative regulatory element, which has not been reported before. Serial deletion analyses further indicated that nt 453-250 sequence of HBV genome would be the minimal sequence essential for the inhibitory effect of the novel negative regulatory element.

Distinct regulation of hepatitis B virus biosynthesis by peroxisome proliferator-activated receptor gamma coactivator 1alpha and small heterodimer partner in human hepatoma cell lines

The human hepatoma cell lines HepG2 and Huh7 have been used extensively to study hepatitis B virus (HBV) transcription and replication. Both cell lines support transcription of the 3.5-kb viral pregenomic RNA and subsequent viral DNA synthesis by reverse transcription. The effects of the coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC1alpha) and corepressor small heterodimer partner (SHP) on HBV transcription and replication mediated by nuclear receptors were examined in the context of individual nuclear receptors in nonhepatoma cells and in hepatoma cells in an attempt to determine the relative contribution of the various nuclear receptors to viral biosynthesis in the hepatoma cells. PGC1alpha and SHP modulated viral biosynthesis differently in the human hepatoma cell lines HepG2 and Huh7, indicating distinct modes of transcriptional regulation. Consistent with this suggestion, it appears that retinoid X receptor alpha/farnesoid X receptor alpha and liver receptor homolog 1 or estrogen-related receptor beta (ERRbeta) may contribute to the majority of the viral replication observed in HepG2 cells, whereas ERRalpha and ERRgamma are probably responsible for the majority of viral biosynthesis in Huh7 cells. Therefore, this approach indicates that the transcriptional regulation of HBV biosynthesis in HepG2 and Huh7 cells is primarily controlled by different transcription factors.

Peroxisome proliferator-activated receptor gamma Coactivator 1alpha and small heterodimer partner differentially regulate nuclear receptor-dependent hepatitis B virus biosynthesis

Hepatitis B virus (HBV) biosynthesis involves the transcription of the 3.5-kb viral pregenomic RNA, followed by its reverse transcription into viral DNA. Consequently, the modulation of viral transcription influences the level of virus production. Nuclear receptors are the only transcription factors known to support viral pregenomic RNA transcription and replication. The coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC1alpha) and corepressor small heterodimer partner (SHP) have central roles in regulating energy homeostasis in the liver by modulating the transcriptional activities of nuclear receptors. Therefore, the effect of PGC1alpha and SHP on HBV transcription and replication mediated by nuclear receptors was examined in the context of individual nuclear receptors in nonhepatoma cells and in hepatoma cells. This analysis indicated that viral replication mediated by hepatocyte nuclear factor 4alpha, retinoid X receptor alpha (RXRalpha) plus peroxisome proliferator-activated receptor alpha (PPARalpha), and estrogen-related receptor (ERR) displayed differential sensitivity to PGC1alpha activation and SHP inhibition. The effects of PGC1alpha and SHP on viral biosynthesis in the human hepatoma cell line Huh7 were similar to those observed in the nonhepatoma cells expressing ERRalpha and ERRgamma. This suggests that these nuclear receptors, potentially in combination with RXRalpha plus PPARalpha, may have a major role in governing HBV transcription and replication in this cell line. Additionally, this functional approach may help to distinguish the transcription factors in various liver cells governing viral biosynthesis under a variety of physiologically relevant conditions.

Developmental regulation of hepatitis B virus biosynthesis by hepatocyte nuclear factor 4alpha

The host cellular factors that promote persistent viral infections in vivo are, in general, poorly understood. Utilizing the hepatitis B virus (HBV) transgenic mouse model of chronic infection, we demonstrate that the nuclear receptor, hepatocyte nuclear factor 4alpha (HNF4alpha, NR2A1), is essential for viral biosynthesis in the liver. The dependency of HBV transcription on HNF4alpha links viral biosynthesis and persistence to a developmentally regulated transcription factor essential for host viability.

Peroxisome proliferator-activated receptors in HBV-related infection

Thirty years after its discovery, the hepatitis B virus (HBV) still remains a major global public health problem. Worldwide, two billion subjects have been infected, 350 million have a chronic infection and more than 600 000 die annually of HBV-related liver disease or hepatocellular carcinoma; new infections occur because of the presence of a large reservoir of chronic carriers of the virus. Since a decade several studies describe the interrelations between HBV and nuclear receptors and more particularly the peroxisome proliferator-activated receptors (PPARs). After a brief introduction, this review will make a rapid description of HBV incidence and biology. Then a report of the literature on the role of PPARs on viral transcription and replication will be developed. Finally, the role of HBV on PPARγ expression and activity will be discussed. Concluding remarks and perspectives will close this review.

Persistence of the hepatitis B virus covalently closed circular DNA in HepaRG human hepatocyte-like cells

The recently described hepatic cell line HepaRG is the sole hepatoma cell line susceptible to hepatitis B virus (HBV) infection. It provides a unique tool for investigating some unresolved issues of the virus' biology, particularly the formation of the viral mini-chromosome believed to be responsible for the persistence of infection. In this study, we characterized the main features of HBV infection: it is restricted to a subpopulation of differentiated hepatocyte-like cells that express albumin as a functional marker and represents around 10 % of all differentiated HepaRG cells. Infection may persist for more than 100 days in cells maintained at the differentiated state. Even though infected cells continued to produce infectious viral particles, very limited or no spreading of infection was observed. Low genetic variation was also observed in the viral DNA from viruses found in the supernatant of infected cells, although this cannot explain the lack of reinfection. HBV infection of HepaRG cells appears to be a very slow process: viral replication starts at around day 8 post-infection and reaches a maximum at day 13. Analysis of viral DNA showed slow and inefficient conversion of the input relaxed circular DNA into covalently closed circular (CCC) DNA, but no further amplification. Continuous lamivudine treatment inhibited viral replication, but neither prevented viral infection nor initial formation of CCC DNA. In conclusion, HBV infection in differentiated HepaRG cells is characterized by long-term persistence without a key feature of hepadnaviruses, the so-called 'CCC DNA amplification' described in the duck hepatitis B model.

Limited effects of fasting on hepatitis B virus (HBV) biosynthesis in HBV transgenic mice

Nuclear receptors have a unique role in governing hepatitis B virus (HBV) transcription and replication. Hepatocyte nuclear factor 4alpha (HNF4alpha) and retinoid X receptor alpha (RXRalpha) plus peroxisome proliferator-activated receptor alpha (PPARalpha) have been shown to support viral biosynthesis in nonhepatoma cells in the absence of additional liver-enriched transcription factors. However, the in vivo importance of these nuclear receptors in HBV biosynthesis has been investigated only to a limited extent. Fasting has been shown to activate gluconeogenesis, in part, by activating PPARgamma coactivator 1 alpha, which in turn leads to activation of HNF4alpha- and RXRalpha/PPARalpha-mediated transcription. As HBV pregenomic RNA synthesis is primarily believed to be regulated by HNF4alpha under normal physiological conditions, it was of interest to determine the effect of fasting on the levels of HBV RNA and DNA synthesis. Fasting was shown to rather modestly increase the levels of viral proteins, transcripts, and replication intermediates in the HBV transgenic mouse model of chronic viral infection, suggesting that caloric restriction may modulate viremia to some extent during natural infection.

Transactivation of the hepatitis B virus core promoter by the nuclear receptor FXRalpha

Hepatitis B virus (HBV) core promoter activity is positively and negatively regulated by nuclear receptors, a superfamily of ligand-activated transcription factors, via cis-acting sequences located in the viral genome. In this study, we investigated the role of farnesoid X receptor alpha (FXRalpha) in modulating transcription from the HBV core promoter. FXRalpha is a liver-enriched nuclear receptor activated by bile acids recognizing hormone response elements by forming heterodimers with retinoid X receptor alpha (RXRalpha). Electrophoretic mobility shift assays demonstrated that FXRalpha-RXRalpha heterodimers can bind two motifs on the HBV enhancer II and core promoter regions, presenting high homology to the consensus (AGGTCA) inverted repeat FXRalpha response elements. In transient transfection of the human hepatoma cell line Huh-7, bile acids enhanced the activity of a luciferase reporter containing the HBV enhancer II and core promoter sequences through FXRalpha. Moreover, using a greater-than-genome-length HBV construct, we showed that FXRalpha also increased synthesis of the viral pregenomic RNA and DNA replication intermediates. The data strongly suggest that FXRalpha is another member of the nuclear receptor superfamily implicated in the regulation of HBV core promoter activity and that bile acids could play an important role in the natural history of HBV infection.

The "metabolovirus" model of hepatitis B virus suggests nutritional therapy as an effective anti-viral weapon

Hepatitis B virus (HBV) is a small DNA virus that targets the liver almost exclusively. Chronic infection with HBV might lead to severe liver-related pathologies including chronic hepatitis, cirrhosis and hepatocellular carcinoma. Based on its enhancer composition, which links nutritional signals that control hepatic glucose and fat metabolism in the liver to HBV gene expression and replication, it appears that the virus has adopted a regulatory system that is unique to the major hepatic metabolic genes. This unique virus-host interaction, mediated by metabolic events in the liver, is designated by us the "metabolovirus model". We hypothesize that by mimicking the expression of key genes implicated in glucose homeostasis, HBV sophisticatedly exploits the host resources to ensure its persistence. Specifically, by recruiting transcription factors and coactivators common to essential hepatic metabolic genes the virus avoids a possible resistance by its host, on the one hand, and ensures a timely and proper response to changes in its environment in terms of metabolic milieu, on the other hand. Furthermore, by coupling its gene expression to the expression of hepatic metabolic genes that fluctuate during the day, we predict a fluctuating nature of HBV gene expression. This can serve the virus in its attempts to escape the host immune system in addition to other immune evading strategies adopted by the virus, such as the secretion of the e antigen. Based on our "metabolovirus model", we suggest new mechanisms to previously unexplained clinical phenomena, such as the observed diversity in disease severity between different geographical areas that differ in nutritional habits. Furthermore, given the up-regulatory effect of food deprivation on HBV gene expression and replication, we suggest that conditions of short-term starvation should be completely avoided by HBV-infected individuals, and dietary recommendations such as the ingestion of complex carbohydrates before sleep should be adopted. Thus, our hypothesis sets the stage for viral manipulation by controlling food intake, and opens additional avenues towards food or nutritional therapy as an effective anti-HBV weapon.

Differential inhibition of nuclear hormone receptor-dependent hepatitis B virus replication by the small heterodimer partner

The nuclear hormone receptors hepatocyte nuclear factor 4 (HNF4) and retinoid X receptor alpha (RXRalpha) plus peroxisome proliferator-activated receptor alpha (PPARalpha) heterodimer support hepatitis B virus (HBV) pregenomic RNA synthesis and viral replication in nonhepatoma cells. Small heterodimer partner (SHP), an orphan nuclear hormone receptor lacking a DNA binding domain, inhibits nuclear hormone receptor-mediated viral transcription and replication. The inhibition of HBV replication by SHP is dependent on the presence of nuclear hormone receptors. HBV replication that is dependent on HNF4 is considerably more sensitive to SHP-mediated inhibition than RXRalpha/PPARalpha-directed viral biosynthesis. SHP inhibition of HBV biosynthesis in HepG2 cells suggests that multiple nuclear hormone receptors mediate viral replication in this human hepatoma cell line. These observations suggest that the physiological regulation of HBV biosynthesis by SHP in the liver will depend on both the level of SHP expression and the relative contribution of HNF4 and RXRalpha/PPARalpha, plus potentially additional nuclear hormone receptors, to HBV RNA synthesis and replication.

Collective repression of the hepatitis B virus enhancer II by human TR4 and TR2 orphan receptors

AIM

The human testicular receptor 4 and 2 (TR4 and TR2) orphan receptors are members of the nuclear receptor superfamily that regulate target gene expression via binding to the consensus AGGTCA direct repeats of the hormone response elements. Previous studies have reported that TR4 and other nuclear receptors could bind to the direct repeat 1 element of the hepatitis B virus (HBV) core promoter.

METHODS

Differential gene expression of HBV caused by TR4 and TR2 was determined by gel retardation and functional assays.

RESULTS

Electrophoretic mobility shift assay demonstrated that TR4 and TR2 might bind to the direct repeat 6 element of the HBV enhancer II region. RESULTS of the dual-luciferasereporter gene assay showed that TR4 and TR2 might significantly suppress HBV gene expression through this direct repeat 6 element in the enhancer II.

CONCLUSION

These results implied that TR4 together with its heterodimer partner TR2 could collectively play a significant role in the transcriptional suppression of HBV gene expression via the direct repeat 6 element in the enhancer II. Therefore, the application of nuclear receptors potentially may be antiviral agents in chronic HBV infection.

Establishment and primary application of a mouse model with hepatitis B virus replication

AIM: To establish a rapid and convenient animal model with hepatitis B virus (HBV) replication.

METHODS: A naked DNA solution of HBV-replication-competent plasmid was transferred to BALB/C mice via the tail vein, using a hydrodynamic in vivo transfection procedure. After injection, these mice were sacrificed on d 1, 3, 4, 5, 7 and 10. HBV DNA replication intermediates in the liver were analyzed by Southern blot hybridization. The expression of hepatitis B core antigen (HBcAg) and hepatitis B surface antigen (HBsAg) in the liver was checked by immunohistochemistry. Serum HBsAg and hepatitis B e antigen (HBeAg) was detected by enzyme-linked immunosorbent assay (ELISA). Inhibition of HBV replication was compared in HBV replication model mice treated intraperitoneally with polyinosinic-polytidylin acid (polyIC) or phosphate-buffered saline (PBS).

RESULTS: After hydrodynamic in vivo transfection, HBV DNA replication intermediates in the mouse liver were detectable on d 1 and abundant on d 3 and 4, the levels were slightly decreased and remained relatively stable between d 5 and 7, and were almost undetectable on d 10. The expression patterns of HBcAg and HBsAg were similar to that of HBV replication intermediate DNA, except that they reached a peak on d 1 after injection. No obvious differences in HBV DNA replication intermediates were observed in the left, right and middle lobes of the liver. After treatment with polyIC, the level of HBV intermediate DNA in the liver was lower than that in the control mice injected with PBS.

CONCLUSION: A rapid and convenient mouse model with a high level of HBV replication was developed and used to investigate the inhibitory effect of polyIC on HBV replication, which provides a useful tool for future functional studies of the HBV genome.

Complementarity between epsilon and phi sequences in pregenomic RNA influences hepatitis B virus replication efficiency

Hepatitis B virus (HBV) replication requires the viral polymerase to reverse transcribe the 3.5-kb pregenomic viral RNA within the nucleocapsid. It has been proposed that a sequence element designated phi (phi), which is located 32 nucleotides upstream of the 3' DR1 pregenomic RNA sequence and is complementary to epsilon, is required for efficient minus-strand synthesis because it may mediate the translocation of the viral polymerase plus the three nucleotide primer from epsilon to DR1. A mutation in phi has been identified which can be compensated for with a complementary mutation in epsilon. This observation supports the suggestion that epsilon and phi base pair during the process of polymerase translocation from epsilon to DR1. However, additional mutations in phi were not complemented by the corresponding mutations in epsilon indicating that the functional recognition of epsilon and epsilon/phi stem-loop structures by polymerase probably requires both sequence- and structure-specific information.

PGC-1alpha controls hepatitis B virus through nutritional signals

Hepatitis B virus (HBV) is a 3.2-kb DNA virus that replicates preferentially in the liver. Liver-enriched nuclear receptors (NRs) play a major role in the HBV life cycle, operating as essential transcription factors for viral gene expression. Notably, these NRs are also key players in metabolic processes that occur in the liver, serving as central transcription factors for key enzymes of gluconeogenesis, fatty acid beta-oxidation, and ketogenesis. However, the association between these metabolic events and HBV gene expression is poorly understood. Here we show that peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha), a major metabolic regulator and a coactivator of key gluconeogenic genes, robustly coactivates HBV transcription. We further demonstrate that the liver-enriched NR hepatocyte nuclear factor 4alpha that binds HBV plays an important role in this process. Physiologically, we show that a short-term fast that turns on the gluconeogenic program robustly induces HBV gene expression in vivo. This induction is completely reversible by refeeding and depends on PGC-1alpha. We conclude that HBV is tightly regulated by changes in the body's nutritional state through the metabolic regulator PGC-1alpha. Our data provide evidence for nutrition signaling to control viral gene expression and life cycle and thus ascribe to metabolism an important role in virus-host interaction.

Hepatocyte-like cells transdifferentiated from a pancreatic origin can support replication of hepatitis B virus

Recently, a rat pancreatic cell line (AR42J-B13) was shown to transdifferentiate to hepatocyte-like cells upon induction with dexamethasone (Dex). The aim of this study is to determine whether transdifferentiated hepatocytes can indeed function like bona fide liver cells and support replication of hepatotropic hepatitis B virus (HBV). We stably transfected AR42J-B13 cells with HBV DNA and examined the expression of hepatocyte markers and viral activities in control and transdifferentiated cells. A full spectrum of HBV replicative intermediates, including covalently closed circular DNA (cccDNA) and Dane particles, were detected only after induction with Dex and oncostatin M. Strikingly, the small envelope protein and RNA of HBV were increased by 40- to 100-fold upon induction. When HBV RNAs were examined by primer extension analysis, novel core- and precore-specific transcripts were induced by Dex which initiated at nucleotide (nt) 1820 and nt 1789, respectively. Most surprisingly, another species of core-specific RNA, which initiates at nt 1825, is always present at almost equal intensity before and after Dex treatment, a result consistent with Northern blot analysis. The fact that HBV core protein is dramatically produced only after transdifferentiation suggests the possibility of both transcriptional and translational regulation of HBV core antigen in HBV-transfected AR42J-B13 cells. Upon withdrawal of Dex, HBV replication and gene expression decreased rapidly-less than 50% of the cccDNA remained detectable in 1.5 days. Our studies demonstrate that the transdifferentiated AR42J-B13 cells can function like bona fide hepatocytes. This system offers a new opportunity for basic research of virus-host interactions and pancreatic transdifferentiation.

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.

The transcriptional transactivation function of HBx protein is important for its augmentation role in hepatitis B virus replication

The role and functional domain of hepatitis B virus (HBV) X protein (HBx) in regulating HBV transcription and replication were investigated with a transient transfection system in the human hepatoma cell line HepG2 using wild-type or HBx-minus HBV genome constructs and a series of deletion or mutation HBx expression plasmids. We show here that HBx has augmentation effects on HBV transcription and replication as a HBV mutant genome with defective X gene led to decreased levels of 3.5-kb HBV RNA and HBV replication intermediates and that these decreases can be restored by either transient ectopic expression of HBx or a stable HBx expression cell line. The C-terminal two-thirds (amino acids [aa] 51 to 154), which contain the transactivation domain, is required for this function of HBx; the N-terminal one-third (aa 1 to 50) is not required. Using the alanine scanning mutagenesis strategy, we demonstrated that the regions between aa 52 to 65 and 88 to 154 are important for the augmentation function of HBx in HBV replication. By the luciferase reporter gene analysis, we found that the transactivation and coactivation activities of HBx coincide well with its augmentation function in HBV transcription and replication. These results suggest that HBx has an important role in stimulating HBV transcription and replication and that the transcriptional transactivation function of HBx may be critical for its augmentation effect on HBV replication.

Presence and integration of HBV DNA in mouse oocytes

AIM: Hepatitis B is a worldwide public health problem. To explore the feasibility of hepatitis B virus (HBV) vertical transmission via oocytes, the presence and integration of HBV DNA in mouse oocytes were studied.

METHODS: Genomic DNA was isolated and metaphases were prepared, respectively from mouse oocytes cocultured with pBR322-HBV DNA plasmids. PCR, Southern blot, dot hybridization and fluorescence in situ hybridization (FISH) were performed to explore the existence and integration of HBV DNA in oocytes.

RESULTS: PCR detected positive bands in the tested samples, and then Southern blot revealed clear hybridization signals in PCR products. Final washing solutions were collected for dot hybridization and no signal for HBV DNA was observed, which excluded the possibility that contamination of washing solutions gave rise to positive results of PCR and Southern blot. FISH demonstrated that 36 of 1000 metaphases presented positive signals.

CONCLUSION: HBV DNA sequences are able to pass through the zona and oolemma to enter into oocytes and to integrate into their chromosomes. HBV DNA sequences might be brought into embryo via oocytes as vectors when they are fertilized with normal spermatozoa.

Impaired expression of the peroxisome proliferator-activated receptor alpha during hepatitis C virus infection

BACKGROUND AND AIMS

Liver inflammation, fibrosis, and dyslipidemia are common features in patients with chronic hepatitis C virus (HCV) infection. Because peroxisome proliferator-activated receptor alpha (PPARalpha) is highly expressed in the liver and is involved in the regulation of lipid metabolism and inflammation, we sought to determine whether HCV infection may locally impair PPARalpha expression and activity.

METHODS

PPARalpha expression was investigated in liver biopsy specimens of 86 untreated patients with HCV infection and controls, by using real-time polymerase chain reaction (PCR), Western blot analysis, and immunohistochemistry. PPARalpha activity was assessed by quantification of the key gene target carnitine palmitoyl acyl-CoA transferase 1 (CPT1A) messenger RNA (mRNA). The influence of HCV core protein on PPARalpha mRNA expression was analyzed in vitro by real-time PCR in HCV core-expressing HepG2 cells activated with the PPARalpha ligand fenofibric acid.

RESULTS

Hepatic concentrations of PPARalpha and CPT1A expressed by hepatocytes were impaired profoundly in the livers of untreated patients with HCV infection compared with controls. A mean decrease of 85% in PPARalpha mRNA expression paralleled with a lack of CPT1A mRNA induction also were observed in HCV core-expressing HepG2 cells compared with controls.

CONCLUSIONS

HCV infection is related to altered expression and function of the anti-inflammatory nuclear receptor PPARalpha. These results identify hepatic PPARalpha as one mechanism underlying the pathogenesis of HCV infection, and as a new therapeutic target in traditional treatment of HCV-induced liver injury.

Enhancer I predominance in hepatitis B virus gene expression

Previous studies of human hepatitis B virus (HBV) transcription revealed the requirement of two enhancer elements. Enhancer I (EnhI) is located upstream of the X promoter and is targeted by multiple activators, including basic leucine zipper proteins, and enhancer II (EnhII) is located upstream to the PreCore promoter and is targeted mainly by nuclear receptors (NRs). The mode of interplay between these enhancers and their unique contributions in regulating HBV transcription remained obscure. By using time course analysis we revealed that the HBV transcripts are categorized into early and late groups. Chang (CCL-13) cells are impaired in expression of the late transcripts. This could be corrected by overexpressing EnhII activators, such as hepatocyte nuclear factor 4 alpha, the retinoid X receptor alpha, and the peroxisome proliferator-activated receptor alpha, suggesting that in Chang cells EnhI but not EnhII is active. Replacing the 5'-end EnhI sequence with a synthetic Gal4 response (UAS) DNA fragment ceased the production of the early transcripts. Under this condition NR overexpression poorly activated EnhII. However, activation of the UAS by Gal4-p53 restored both the expression of the early transcripts and the EnhII response to NRs. Thus, a functional EnhI is required for activation of EnhII. We found a major difference between Gal4-p53 and Gal4-VP16 behavior. Gal4-p53 activated the early transcripts, while Gal4-VP16 inhibited the early transcripts but activated the late transcripts. These findings indicate that the composition of the EnhI binding proteins may play a role in early to late switching. Our data provides strong evidence for the role of EnhI in regulating global and temporal HBV gene expression.

Glycine decarboxylase mediates a postbinding step in duck hepatitis B virus infection

Envelope protein precursors of many viruses are processed by a basic endopeptidase to generate two molecules, one for receptor binding and the other for membrane fusion. Such a cleavage event has not been demonstrated for the hepatitis B virus family. Two binding partners for duck hepatitis B virus (DHBV) pre-S envelope protein have been identified. Duck carboxypeptidase D (DCPD) interacts with the full-length pre-S protein and is the DHBV docking receptor, while duck glycine decarboxylase (DGD) has the potential to bind several deletion constructs of the pre-S protein in vitro. Interestingly, DGD but not DCPD expression was diminished following prolonged culture of primary duck hepatocytes (PDH), which impaired productive DHBV infection. Introduction of exogenous DGD promoted formation of protein-free viral genome, suggesting restoration of several early events in viral life cycle. Conversely, blocking DGD expression in fresh PDH by antisense RNA abolished DHBV infection. Moreover, addition of DGD antibodies soon after virus binding reduced endogenous DGD protein levels and impaired production of covalently closed circular DNA, the template for DHBV gene expression and genome replication. Our findings implicate this second pre-S binding protein as a critical cellular factor for productive DHBV infection. We hypothesize that DCPD, a molecule cycling between the cell surface and the trans-Golgi network, targets DHBV particles to the secretary pathway for proteolytic cleavage of viral envelope protein. DGD represents the functional equivalent of other virus receptors in its interaction with processed viral particles.

Expansion and contraction of the hepatitis B virus transcriptional template in infected chimpanzees

We have previously shown that hepatitis B virus (HBV) replication is controlled by noncytolytic mechanisms that depend primarily on the effector functions of the CD8(+) T cell response, especially the production of IFN-gamma in the liver. The mechanisms that control the nuclear pool of viral covalently closed circular DNA (cccDNA) transcriptional template of HBV, which must be eliminated to eradicate infection, have been difficult to resolve. To examine those mechanisms, we quantitated intrahepatic HBV cccDNA levels in acutely infected chimpanzees whose virological, immunological, and pathological features were previously described. Our results demonstrate that the elimination kinetics of the cccDNA are more rapid than the elimination of HBV antigen-positive hepatocytes during the early phase of viral clearance, and they coincide with the influx of small numbers of IFN-gamma producing CD8(+) T cells into the liver. In contrast, terminal clearance of the cccDNA is associated with the peak of liver disease and hepatocellular turnover and with a surge of IFN-gamma producing CD8(+) T cells in the liver. Collectively, these results suggest that cccDNA clearance is a two-step process mediated by the cellular immune response. The first step reduces the pool of cccDNA molecules noncytolytically, probably by eliminating their relaxed circular DNA precursors and perhaps by destabilizing them. The second step enhances this process by destroying infected hepatocytes and triggering their turnover. Surprisingly, despite this multipronged response, traces of cccDNA persist indefinitely in the liver, likely providing a continuous antigenic stimulus that confers lifelong immunity.

Transcriptional and posttranscriptional control of hepatitis B virus gene expression

Hepatitis B virus (HBV) infects humans and certain nonhuman primates. Viral clearance and acute disease are associated with a strong, polyclonal, multispecific cytotoxic T lymphocyte response. Infiltrating T cells, as well as other activated inflammatory cells, produce cytokines that can regulate hepatocellular gene expression. Using an HBV transgenic mouse model, our laboratory has previously demonstrated that adoptive transfer of HBV-specific cytotoxic T lymphocytes or injection of IL-2 can noncytopathically inhibit HBV gene expression by a posttranscriptional IFN-gamma- and/or tumor necrosis factor alpha-dependent mechanism. Here, we report that HBV gene expression can also be controlled at the posttranscriptional level during persistent lymphocytic choriomeningitis virus infection. In contrast, it is controlled at the transcriptional level during acute murine cytomegalovirus infection or after repetitive polyinosinic-polycytidylic acid injection. Finally, we show that transcriptional inhibition of HBV is associated with changes in liver-specific gene expression. These results elucidate pathways that regulate the viral life cycle and suggest additional approaches for the treatment of chronic HBV infection.

Distinct modes of regulation of transcription of hepatitis B virus by the nuclear receptors HNF4alpha and COUP-TF1

To study the effects of the nuclear receptors (NRs) HNF4alpha and COUP-TF1 on the life cycle of hepatitis B virus (HBV), the human hepatoma cell line Huh7 was transiently cotransfected with plasmids containing the HBV genome and encoding these two NRs. Overexpression of HNF4alpha and COUP-TF1 led to a 9-fold increase and a 7- to 10-fold decrease, respectively, in viral DNA synthesis. These two NRs also exhibited distinct modes of regulation of viral transcription. Overexpression of HNF4alpha led to a more-than-10-fold increase in synthesis of the pregenomic RNA but to only a 2- to 3-fold increase in synthesis of the pre-C and S RNAs. Moreover, the NR response element within the pre-C promoter, NRRE(preC,) played the major role in activation of pregenomic RNA synthesis by HNF4alpha. On the other hand, overexpression of COUP-TF1 led to an over-10-fold repression of synthesis of both pre-C and pregenomic RNAs mediated through either NRRE(preC) or NRRE(enhI). HNF4alpha and COUP-TF1 antagonized each other's effects on synthesis of pregenomic RNA and viral DNA when they were co-overexpressed. A naturally occurring HBV variant which allows for binding by HNF4alpha but not COUP-TF1 in its NRRE(preC) exhibited significantly higher levels of synthesis of pregenomic RNA and viral DNA than wild-type HBV in coexpression experiments. Last, deletion analysis revealed that non-NRRE sequences located within both the C and pre-S1 regions are also essential for maximum activation of the pregenomic promoter by HNF4alpha but not for repression by COUP-TF1. Thus, HNF4alpha and COUP-TF1 function through different mechanisms to regulate expression of the HBV genes.

Hepatocyte nuclear factor 3beta inhibits hepatitis B virus replication in vivo

Hepatitis B virus (HBV) transgenic mice expressing rat hepatocyte nuclear factor 3beta (HNF3beta) were generated by breeding HBV transgenic mice with transgenic mice that constitutively overexpress the rat HNF3beta polypeptide in the liver. HBV 3.5-, 2.4- and 2.1-kb transcripts were reduced 2- to 4-fold in these mice relative to the HBV transgenic mouse controls. In contrast, the abundance of viral replication intermediates was profoundly reduced in HBV transgenic mice by overexpression of HNF3beta. This results, in part, from the preferential reduction in the level of the pregenomic 3.5-kb RNA relative to the precore 3.5-kb RNA. Therefore, it is apparent that increased expression of HNF3beta modestly reduces viral transcription and dramatically inhibits replication in vivo in the HBV transgenic mouse. This suggests that altering the activity of this transcription factor in vivo in chronic HBV carriers might be therapeutically beneficial.

Mechanisms of inhibition of nuclear hormone receptor-dependent hepatitis B virus replication by hepatocyte nuclear factor 3beta

The nuclear hormone receptors hepatocyte nuclear factor 4 (HNF4) and the retinoid X alpha (RXRalpha) plus the peroxisome proliferator-activated receptor alpha (PPARalpha) heterodimer support hepatitis B virus (HBV) replication in nonhepatoma cells. Hepatocyte nuclear factor 3 (HNF3) inhibits nuclear hormone receptor-mediated viral replication. Inhibition of HBV replication by HNF3beta is associated with the preferential reduction in the level of the pregenomic RNA compared with that of precore RNA. Hepatitis B e antigen (HBeAg), encoded by the precore RNA, mediates part of the inhibition of viral replication by HNF3beta. The amino-terminal transcriptional activation domain of HNF3beta is essential for the inhibition of HBV replication. The activation of transcription by HNF3 from HBV promoters downstream from the nucleocapsid promoter appears to contribute indirectly to the reduction in the steady-state level of 3.5-kb HBV RNA, possibly by interfering with the elongation rate of these transcripts. Therefore, transcriptional interference mediated by HNF3 may also regulate HBV RNA synthesis and viral replication.

Avian and Mammalian hepadnaviruses have distinct transcription factor requirements for viral replication

Hepadnavirus replication occurs in hepatocytes in vivo and in hepatoma cell lines in cell culture. Hepatitis B virus (HBV) replication can occur in nonhepatoma cells when pregenomic RNA synthesis from viral DNA is activated by the expression of the nuclear hormone receptors hepatocyte nuclear factor 4 (HNF4) and the retinoid X receptor alpha (RXR alpha) plus peroxisome proliferator-activated receptor alpha (PPAR alpha) heterodimer. Nuclear hormone receptor-dependent HBV replication is inhibited by hepatocyte nuclear factor 3 (HNF3). In contrast, HNF3 and HNF4 support duck hepatitis B virus (DHBV) replication in nonhepatoma cells, whereas the RXR alpha-PPAR alpha heterodimer inhibits HNF4-dependent DHBV replication. HNF3 and HNF4 synergistically activate DHBV pregenomic RNA synthesis and viral replication. The conditions that support HBV or DHBV replication in nonhepatoma cells are not able to support woodchuck hepatitis virus replication. These observations indicate that avian and mammalian hepadnaviruses have distinct transcription factor requirements for viral replication.

Cytokine-sensitive replication of hepatitis B virus in immortalized mouse hepatocyte cultures

We have previously shown that alpha/beta interferon (IFN-alpha/beta) and gamma interferon (IFN-gamma) inhibit hepatitis B virus (HBV) replication by eliminating pregenomic RNA containing viral capsids from the hepatocyte. We have also shown that HBV-specific cytotoxic T lymphocytes that induce IFN-gamma and tumor necrosis factor alpha (TNF-alpha) in the liver can inhibit HBV gene expression by destabilizing preformed viral mRNA. In order to further study the antiviral activity of IFN-alpha/beta, IFN-gamma, and TNF-alpha at the molecular level, we sought to reproduce these observations in an in vitro system. Accordingly, hepatocytes were derived from the livers of HBV-transgenic mice that also expressed the constitutively active cytoplasmic domain of the human hepatocyte growth factor receptor (c-Met). Here, we show that the resultant well-differentiated, continuous hepatocyte cell lines (HBV-Met) replicate HBV and that viral replication in these cells is efficiently controlled by IFN-alpha/beta or IFN-gamma, which eliminate pregenomic RNA-containing capsids from the cells as they do in the liver. Furthermore, we demonstrate that IFN-gamma, but not IFN-alpha/beta, is capable of inhibiting HBV gene expression in this system, especially when it acts synergistically with TNF-alpha. These cells should facilitate the analysis of the intracellular signaling pathways and effector mechanisms responsible for these antiviral effects.

Functional characterization of the interferon regulatory element in the enhancer 1 region of the hepatitis B virus genome

An interferon-stimulated response element (ISRE)/interferon regulatory element (IRE) spanning nucleotide coordinates 1091-1100 is present in the enhancer 1/X gene promoter region of the hepatitis B virus (HBV) genome. In the context of a minimal promoter element, the enhancer 1/X gene promoter ISRE/IRE was shown to be a functional regulatory site capable of mediating interferon alpha- (IFNalpha) and interferon-stimulated gene factor 3 (ISGF3)-specific transcriptional activation in transient transfection analysis. The enhancer 1/X gene promoter ISRE/IRE was also shown to mediate interferon regulatory factor (IRF) 1 and IRF7 activation of transcription from a minimal promoter construct. In contrast, IFNalpha and the IRFs had minimal effect on HBV transcription and replication in the context of the viral genome in cell culture.

Critical roles of nuclear receptor response elements in replication of hepatitis B virus

Functional analysis of the roles of the nuclear receptor response elements (NRREs) in the transcription and replication of hepatitis B virus (HBV) in the context of its whole genome has been hampered by the extensive overlapping of the NRREs with the regions encoding viral proteins. We introduced point mutations that inactivate the NRREs individually without altering the open reading frames of viral proteins. These mutations in the context of a plasmid containing 1.2 copies of the HBV genome were transiently transfected into the human hepatoma cell line Huh7. Inactivation of the NRRE in either the preC promoter (NRRE(preC)) or enhancer I (NRRE(enhI)) led to moderate reductions in synthesis of viral RNAs. Concurrent inactivation of both NRREs led to 7- to 8-fold reductions in synthesis of the preC, pregenomic, and preS RNAs and a 15-fold reduction in synthesis of the S RNA. The accumulation of viral DNA in the cytoplasmic nucleocapsids and virion particles in the culture medium was also reduced seven- to eightfold. These results suggest that these NRREs are critical for the efficient propagation of HBV in hepatocytes. In cotransfection experiments we also found that overexpression of PPARalpha-RXRalpha in the presence of their respective ligands led to a fourfold increase in pregenomic RNA synthesis and a four- to fivefold increase in viral DNA synthesis, while it had little or no effect on synthesis of the other viral RNAs. Similar effects were observed with overexpression of PPARgamma-RXRalpha in the presence of their respective ligands. This activation was dependent on NRRE(preC), because the increase in synthesis of viral RNA and DNA was not observed when this site was mutated. Likewise, no activation of synthesis of pregenomic RNA and viral DNA by PPARalpha-RXRalpha was observed in a naturally occurring NRRE(preC)(-) mutant of HBV. Our results suggest that interactions between nuclear receptors and NRREs present in the HBV genome may play critical roles in regulating its transcription and replication during HBV infection of hepatocytes.