Glucagon treatment interferes with an early step of duck hepatitis B virus infection

Host functions used by hepatitis B virus to complete its life cycle: Implications for developing host-targeting agents to treat chronic hepatitis B

Similar to other mammalian viruses, the life cycle of hepatitis B virus (HBV) is heavily dependent upon and regulated by cellular (host) functions. These cellular functions can be generally placed in to two categories: (a) intrinsic host restriction factors and innate defenses, which must be evaded or repressed by the virus; and (b) gene products that provide functions necessary for the virus to complete its life cycle. Some of these functions may apply to all viruses, but some may be specific to HBV. In certain cases, the virus may depend upon the host function much more than does the host itself. Knowing which host functions regulate the different steps of a virus' life cycle, can lead to new antiviral targets and help in developing novel treatment strategies, in addition to improving a fundamental understanding of viral pathogenesis. Therefore, in this review we will discuss known host factors which influence key steps of HBV life cycle, and further elucidate therapeutic interventions targeting host-HBV interactions.

Requirement of the cyclic adenosine monophosphate response element-binding protein for hepatitis B virus replication

The cyclic adenosine monophosphate-response element (CRE)-transcription factor complex participates in the regulation of viral gene expression and pathologic processes caused by various viruses. The hepatitis B virus (HBV) enhancer I directs liver-specific transcription of viral genes and contains a CRE sequence (HBV-CRE); however, whether the HBV-CRE and CRE-binding protein (CREB) are required for the HBV life cycle remains to be determined. This study was designed to investigate the role of CREB in HBV replication and gene expression. Sequence-comparison analysis of 984 HBVs reported worldwide showed that the HBV-CRE sequence is highly conserved, indicating the possibility that it plays an important role in the HBV life cycle. The binding of CREB to the HBV-CRE site was markedly inhibited by oligonucleotides containing HBV-CRE and consensus CRE sequences in vitro and in vivo. The HBV promoter activity was demonstrated to be dependent upon the transactivation activity of CREB. Treatment with CRE decoy oligonucleotides reduced HBV promoter activity, and this was reversed by CREB overexpression. The levels of viral transcripts, DNA, and antigens were remarkably decreased in response to the overexpression of CREB mutants or treatment with the CRE decoy oligonucleotides, whereas enhancing CREB activity increased the levels of viral transcripts. In addition, introduction of a three-base mutation into the HBV-CRE led to a marked reduction in HBV messenger RNA synthesis.

CONCLUSION

Taken together, our results demonstrate that both replication and gene expression of HBV require a functional CREB and HBV-CRE. We have also demonstrated that CRE decoy oligonucleotides and the overexpression of CREB mutants can effectively block the HBV life cycle, suggesting that interventions against CREB activity could provide a new avenue to treat HBV infection.

Regulation of hepatitis B virus replication by the phosphatidylinositol 3-kinase-akt signal transduction pathway

The phosphatidylinositol 3-kinase (PI3K)-protein kinase B (Akt) signaling pathway is one of the major oncogenic pathways and is activated in many types of human cancers, including hepatocellular carcinoma. It can also be activated by the hepatitis C virus (HCV) nonstructural 5A (NS5A) protein. In the present study, we set out to determine the regulatory effects of this pathway on the replication of hepatitis B virus (HBV). Our results demonstrate that the expression of a constitutively active Akt1 profoundly inhibited HBV RNA transcription and consequently reduced HBV DNA replication in HepG2 cells. This suppression of HBV gene transcription was apparently mediated by the activation of mTOR, as it was abolished by the mTOR inhibitor rapamycin. Moreover, treatment of HBV-expressing HepG2.2.15 cells with inhibitors of PI3K, Akt, and mTOR increased the transcription of 3.5-kb and 2.4-kb viral RNA as well as the replication of HBV DNA. This observation implies that the basal level activation of this pathway in HepG2 cells regulated HBV replication. Consistent with previous reports showing that the HCV NS5A protein could bind to the p85 subunit of PI3K and activate the PI3K-Akt signal transduction pathway, our results showed that expression of this protein could inhibit HBV RNA transcription and reduce HBV DNA replication in HepG2 cells. Taken together, our results suggest that the activation of the PI3K-Akt pathway during liver oncogenesis may be at least partially responsible for the elimination of HBV replication from tumor cells and may also provide an explanation for the observed suppression of HBV replication by HCV coinfection.

The role of cells refractory to productive infection in acute hepatitis B viral dynamics

During acute hepatitis B virus (HBV) infection viral loads reach high levels ( approximately 10(10) HBV DNA per ml), and nearly every hepatocyte becomes infected. Nonetheless, approximately 85-95% of infected adults clear the infection. Although the immune response has been implicated in mediating clearance, the precise mechanisms remain to be elucidated. As infection clears, infected cells are replaced by uninfected ones. During much of this process the virus remains plentiful but nonetheless does not rekindle infection. Here, we analyze data from a set of individuals identified during acute HBV infection and develop mathematical models to test the role of immune responses in various stages of early HBV infection. Fitting the models to data we are able to separate the kinetics of the noncytolytic and the cytolytic immune responses, thus explaining the relative contribution of these two processes. We further show that we need to hypothesize that newly generated uninfected cells are refractory to productive infection. Without this assumption, viral resurgence is observed as uninfected cells are regenerated. Such protection, possibly mediated by cytokines, may also be important in resolving other acute viral infections.

Regulation of hepatitis B virus replication by the ras-mitogen-activated protein kinase signaling pathway

The replication of hepatitis B virus (HBV) can be regulated by a variety of factors, including hormones, growth factors, and cytokines. However, the molecular mechanisms of these regulations are largely unknown. Ras is a small GTPase that responds to many of these external stimuli. In this study, we investigated the possible effect of Ras on the replication of HBV. Our results indicated that activated Ras could suppress the replication of HBV in both Huh7 and HepG2 cells. This suppression was independent of the X protein and most likely occurred at the transcriptional level. Deletion-mapping analysis of the HBV core promoter and its upstream ENI and ENII enhancers revealed multiple elements responsive to activated Ras. This suppression of HBV replication by activated Ras was apparently mediated by the mitogen-activated protein (MAP) kinase pathway, as it was accompanied by activation of ERK1/2 and abolished by the MEK1/2 inhibitor U0126. Our results thus indicate that external stimuli may suppress HBV replication through the Ras-MAP kinase pathway.

Nuclear localization of the duck hepatitis B virus capsid protein: detection and functional implications of distinct subnuclear bodies in a compartment associated with RNA synthesis and maturation

We have reinvestigated the subcellular distribution of the duck hepatitis B virus (DHBV) core protein in infected duck hepatocytes and in transfected cells. By using indirect immunofluorescence, the protein was found to be localized not only in the cytoplasm, as described previously, but also within the cell nucleus, being concentrated in distinct, brightly staining nuclear core bodies (NCBs). In colocalization studies using confocal microscopy, the NCBs were found exclusively in the periphery of nuclear subdomains characterized as splicing factor compartments and distal to other subnuclear domains. Also relevant for their functional significance is that the NCBs formed during the establishment of virus infection, i.e., at very low overall concentrations of newly synthesized core protein, and persisted throughout all stages of infection. Moreover, a subset of NCBs colocalized with foci of pregenomic DHBV RNA present at concentrations detectable by fluorescence in situ hybridization. Taken together, these findings indicate that a minor fraction of the DHBV core protein molecules escapes the major cytoplasmic assembly pathway to accumulate in specific subnuclear domains, and they furthermore suggest that these NCBs serve a role in the synthesis and/or maturation of the DHBV RNA pregenome.

Woodchuck hepatitis virus replication and antigen expression gradually decrease in preneoplastic hepatocellular lineages

BACKGROUND/AIMS

Hepatocellular carcinomas elicited in woodchucks by the woodchuck hepatitis virus (WHV) emerge gradually from parenchymal areas of minimal structural deviation via two predominant preneoplastic hepatocellular lineages, composed of either glycogenotic/basophilic or amphophilic/basophilic cell foci. In this study we analyzed WHV replication during neoplastic development in both lineages.

METHODS

In minimal deviation areas, preneoplastic hepatocellular foci, and hepatocellular neoplasms, developing in 16 WHV-carriers 31-38 months after WHV-inoculation, the proportion of hepatocytes containing WHV replicative intermediates (as detected by in situ hybridization for WHV DNA) and immunoreactive for WHV core and surface antigens was assessed.

RESULTS

Appearance of WHV replicative intermediates and expression of antigens were limited to the cytoplasm of hepatocytes and were strongly correlated (P<0.0001), both showing high levels in minimal deviation areas, but markedly reduced amounts in all types of preneoplastic hepatic focus (P<0.0001), and in hepatocellular adenomas. Most hepatocellular carcinomas were negative for WHV replicative intermediates and antigens.

CONCLUSIONS

In both the glycogenotic-basophilic and the amphophilic-basophilic preneoplastic hepatocellular lineage, WHV replication and antigen expression gradually decrease early during the preneoplastic phase. The close correlation of these changes with metabolic aberrations characterizing preneoplastic hepatocellular lineages suggests that oncogenic effects mimicking insulin/glucagon imbalances may be responsible for the repression of hepadnaviral replication.

A dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN)-related protein is highly expressed on human liver sinusoidal endothelial cells and promotes HIV-1 infection

The discovery of dendritic cell (DC)-specific intercellular adhesion molecule (ICAM)-3-grabbing nonintegrin (DC-SIGN) as a DC-specific ICAM-3 binding receptor that enhances HIV-1 infection of T cells in trans has indicated a potentially important role for adhesion molecules in AIDS pathogenesis. A related molecule called DC-SIGNR exhibits 77% amino acid sequence identity with DC-SIGN. The DC-SIGN and DC-SIGNR genes map within a 30-kb region on chromosome 19p13.2-3. Their strong homology and close physical location indicate a recent duplication of the original gene. Messenger RNA and protein expression patterns demonstrate that the DC-SIGN-related molecule is highly expressed on liver sinusoidal cells and in the lymph node but not on DCs, in contrast to DC-SIGN. Therefore, we suggest that a more appropriate name for the DC-SIGN-related molecule is L-SIGN, liver/lymph node-specific ICAM-3-grabbing nonintegrin. We show that in the liver, L-SIGN is expressed by sinusoidal endothelial cells. Functional studies indicate that L-SIGN behaves similarly to DC-SIGN in that it has a high affinity for ICAM-3, captures HIV-1 through gp120 binding, and enhances HIV-1 infection of T cells in trans. We propose that L-SIGN may play an important role in the interaction between liver sinusoidal endothelium and trafficking lymphocytes, as well as function in the pathogenesis of HIV-1.

Signals for bidirectional nucleocytoplasmic transport in the duck hepatitis B virus capsid protein

Hepadnavirus genome replication involves cytoplasmic and nuclear stages, requiring balanced targeting of cytoplasmic nucleocapsids to the nuclear compartment. In this study, we analyze the signals determining capsid compartmentalization in the duck hepatitis B virus (DHBV) animal model, as this system also allows us to study hepadnavirus infection of cultured primary hepatocytes. Using fusions to the green fluorescent protein as a functional assay, we have identified a nuclear localization signal (NLS) that mediates nuclear pore association of the DHBV nucleocapsid and nuclear import of DHBV core protein (DHBc)-derived polypeptides. The DHBc NLS mapped is unique. It bears homology to repetitive NLS elements previously identified near the carboxy terminus of the capsid protein of hepatitis B virus, the human prototype of the hepadnavirus family, but it maps to a more internal position. In further contrast to the hepatitis B virus core protein NLS, the DHBc NLS is not positioned near phosphorylation target sites that are generally assumed to modulate nucleocytoplasmic transport. In functional assays with a knockout mutant, the DHBc NLS was found to be essential for nuclear pore association of the nucleocapsid. The NLS was found to be also essential for virus production from the full-length DHBV genome in transfected cells and from hepatocytes infected with transcomplemented mutant virus. Finally, the DHBc additionally displayed activity indicative of a nuclear export signal, presumably counterbalancing NLS function in the productive state of the infected cell and thereby preventing nucleoplasmic accumulation of nucleocapsids.

Envelope protein-mediated down-regulation of hepatitis B virus receptor in infected hepatocytes

Entry of duck hepatitis B virus (DHBV) is initiated by specific interaction of its large envelope protein (L) with a cellular entry receptor, recently identified as carboxypeptidase D (CPD; historically gp180). In this report, we present evidence demonstrating that this receptor is down-regulated as a result of DHBV infection: (i) receptor levels determined by Western blot were much reduced in DHBV-infected duck livers and undetectable by immunostaining in infected cultured hepatocytes; (ii) results from metabolic labeling experiments indicate enhanced receptor protein turnover; (iii) the kinetics of receptor loss from newly infected cells correlated with the accumulation of newly synthesized viral protein; (iv) expression of DHBV L protein, transduced from a recombinant adenovirus, was sufficient to eliminate gp180/CPD from the Golgi compartment, its normal predominant location; (v) gp180/CPD remained absent from the Golgi compartment in infected hepatocytes, even after overexpression from a recombinant adenovirus, while residual amounts subsequently became detectable in a perinuclear compartment, containing DHBV L protein; (vi) expression of DHBV L protein in a HepG2 cell line, stably expressing gp180/CPD, leads to incomplete receptor maturation and induces its degradation. Taken together, these data are consistent with a model in which the virus receptor interacts early in the biosynthetic pathway with the viral L protein, leading to its retention in a pre-Golgi compartment and to subsequent degradation, thus preventing receptor interference with the export of DHBV via the secretory pathway which it shares with its receptor. Accordingly, and analogously with receptor down-regulation in retroviral systems, DHBV receptor down-modulation may account for the much-reduced efficiency of DHBV superinfection of preinfected hepatocytes.

Intracellular hepadnavirus nucleocapsids are selected for secretion by envelope protein-independent membrane binding

Hepadnaviruses are DNA viruses but, as pararetroviruses, their morphogenesis initiates with the encapsidation of an RNA pregenome, and these viruses have therefore evolved mechanisms to exclude nucleocapsids that contain incompletely matured genomes from participating in budding and secretion. We provide here evidence that binding of hepadnavirus core particles from the cytosol to their target membranes is a distinct step in morphogenesis, discriminating among different populations of intracellular capsids. Using the duck hepatitis B virus (DHBV) and a flotation assay, we found about half of the intracellular capsids to be membrane associated due to an intrinsic membrane-binding affinity. In contrast to free cytosolic capsids, this subpopulation contained largely mature, double-stranded DNA genomes and lacked core protein hyperphosphorylation, both features characteristic for secreted virions. Against expectation, however, the selective membrane attachment observed did not require the presence of the large DHBV envelope protein, which has been considered to be crucial for nucleocapsid-membrane interaction. Furthermore, removal of surface-exposed phosphate residues from nonfloating capsids by itself did not suffice to confer membrane affinity and, finally, hyperphosphorylation was absent from nonenveloped nucleocapsids that were released from DHBV-transfected cells. Collectively, these observations argue for a model in which nucleocapsid maturation, involving the viral genome, capsid structure, and capsid dephosphorylation, leads to the exposure of a membrane-binding signal as a step crucial for selecting the matured nucleocapsid to be incorporated into the capsid-independent budding of virus particles.

A metastable form of the large envelope protein of duck hepatitis B virus: low-pH release results in a transition to a hydrophobic, potentially fusogenic conformation

We have examined the structure and fusion potential of the duck hepatitis B virus (DHBV) envelope proteins by treating subviral particles with deforming agents known to release envelope proteins of viruses from a metastable to a fusion-active state. Exposure of DHBV particles to low pH triggered a major structural change in the large envelope protein (L), resulting in exposure of trypsin sites within its S domain but without affecting the same region in the small surface protein (S) subunits. This conformational change was associated with increased hydrophobicity of the particle surface, most likely arising from surface exposure of the hydrophobic first transmembrane domain (TM1). In the hydrophobic conformation, DHBV particles were able to bind to liposomes and intact cells, while in their absence these particles aggregated, resulting in viral inactivation. These results suggests that some L molecules are in a spring-loaded metastable state which, when released, exposes a previously hidden hydrophobic domain, a transition potentially representing the fusion-active state of the envelope.

Endotoxin stimulates liver macrophages to release mediators that inhibit an early step in hepadnavirus replication

Hepadnaviruses are known to be sensitive to various extracellular mediators. Therefore, bacterial endotoxin, which induces the secretion of proinflammatory mediators in the liver, was studied for its effect on hepadnavirus infection in vitro using the duck hepatitis B virus (DHBV) model. In initial experiments, endotoxin was shown to inhibit DHBV replication in primary duck hepatocyte cultures prepared by standard collagenase perfusion. As a primary endotoxin target, hepatic nonparenchymal cells (NPC) contaminating primary hepatocyte cultures, and among these probably macrophages (Kupffer cells), were identified to secrete polypeptide mediators into the cell culture medium. When added during DHBV infection, these mediators elicited the principal antiviral effect in a dose-dependent fashion. On the molecular level, they inhibited accumulation of viral proteins as well as amplification of the nuclear extrachromosomal DHBV DNA templates. In hepatocytes with an established DHBV infection, DHBV protein and progeny virus production was inhibited while the levels of established nuclear DHBV DNA templates and viral transcripts remained unaffected. Finally, in hepatocytes infected with a replication-deficient recombinant DHBV-green fluorescent protein (GFP) virus, the endotoxin-induced mediators markedly reduced GFP expression from chimeric DHBV-GFP transcripts, indicating that the major effect is at a level of translation of viral RNAs. Taken together, the data obtained demonstrate that antiviral mediators, and among these the cytokines alpha interferon (IFN-alpha) and IFN-gamma, are released from hepatic NPC, most probably liver macrophages, upon endotoxin stimulation; furthermore, these mediators act at a posttranscriptional step of hepadnavirus replication.

Cellular receptor traffic is essential for productive duck hepatitis B virus infection

We have investigated the mechanism of duck hepatitis B virus (DHBV) entry into susceptible primary duck hepatocytes (PDHs), using mutants of carboxypeptidase D (gp180), a transmembrane protein shown to act as the primary cellular receptor for avian hepatitis B virus uptake. The variant proteins were abundantly produced from recombinant adenoviruses and tested for the potential to functionally outcompete the endogenous wild-type receptor. Overexpression of wild-type gp180 significantly enhanced the efficiency of DHBV infection in PDHs but did not affect ongoing DHBV replication, an observation further supporting gp180 receptor function. A gp180 mutant deficient for endocytosis abolished DHBV infection, indicating endocytosis to be the route of hepadnaviral entry. With further gp180 variants, carrying mutations in the cytoplasmic domain and characterized by an accelerated turnover, the ability of gp180 to function as a DHBV receptor was found to depend on a wild-type-like sorting phenotype which largely avoids transport toward the endolysosomal compartment. Based on these data, we propose a model in which a distinct intracellular DHBV traffic to the endosome, but not beyond, is a prerequisite for completion of viral entry, i.e., for fusion and capsid release. Furthermore, the deletion of the two enzymatically active carboxypeptidase domains of gp180 did not lead to a loss of receptor function.

Hepatitis B virus biology

Hepadnaviruses (hepatitis B viruses) cause transient and chronic infections of the liver. Transient infections run a course of several months, and chronic infections are often lifelong. Chronic infections can lead to liver failure with cirrhosis and hepatocellular carcinoma. The replication strategy of these viruses has been described in great detail, but virus-host interactions leading to acute and chronic disease are still poorly understood. Studies on how the virus evades the immune response to cause prolonged transient infections with high-titer viremia and lifelong infections with an ongoing inflammation of the liver are still at an early stage, and the role of the virus in liver cancer is still elusive. The state of knowledge in this very active field is therefore reviewed with an emphasis on past accomplishments as well as goals for the future.

Apoptosis and regeneration of hepatocytes during recovery from transient hepadnavirus infections

It is well known that hepatitis B virus infections can be transient or chronic, but the basis for this dichotomy is not known. To gain insight into the mechanism responsible for the clearance of hepadnavirus infections, we have performed a molecular and histologic analysis of liver tissues obtained from transiently infected woodchucks during the critical phase of the recovery period. We found as expected that clearance from transient infections occurred subsequent to the appearance of CD4(+) and CD8(+) T cells and the production of interferon gamma and tumor necrosis factor alpha in the infected liver. These events were accompanied by a significant increase in apoptosis and regeneration of hepatocytes. Surprisingly, however, accumulation of virus-free hepatocytes was delayed for several weeks following this initial influx of lymphocytes. In addition, we observed that chronically infected animals can exhibit levels of T-cell accumulation, cytokine expression, and apoptosis that are comparable with those observed during the initial phase of transient infections. Our results are most consistent with a model for recovery predicting replacement of infected hepatocytes with regenerated cells, which by unknown mechanisms remain protected from reinfection in animals that can be cured.

Interferon gene transfer by a hepatitis B virus vector efficiently suppresses wild-type virus infection

Hepatitis B viruses specifically target the liver, where they efficiently infect quiescent hepatocytes. Here we show that human and avian hepatitis B viruses can be converted into vectors for liver-directed gene transfer. These vectors allow hepatocyte-specific expression of a green fluorescent protein in vitro and in vivo. Moreover, when used to transduce a type I interferon gene, expression of interferon efficiently suppresses wild-type virus replication in the duck model of hepatitis B virus infection. These data suggest local cytokine production after hepatitis-B-virus-mediated gene transfer as a promising concept for the treatment of acquired liver diseases, including chronic hepatitis B.

Host cell-virus cross talk: phosphorylation of a hepatitis B virus envelope protein mediates intracellular signaling

Phosphorylation of cytosolic pre-S domains of the duck hepatitis B virus (DHBV) large envelope protein (L) was identified as a regulatory modification involved in intracellular signaling. By using biochemical and mass spectrometric analyses of phosphopeptides obtained from metabolically radiolabeled L protein, a single phosphorylation site was identified at serine 118 as part of a PX(S/T)P motif, which is strongly preferred by ERK-type mitogen-activated protein kinases (MAP kinases). ERK2 specifically phosphorylated L at serine 118 in vitro, and L phosphorylation was inhibited by a coexpressed MAP kinase-specific phosphatase. Furthermore, L phosphorylation and ERK activation were shown to be induced in parallel by various stimuli. Functional analysis with transfected cells showed that DHBV L possesses the ability to activate gene expression in trans and, by using mutations eliminating (S-->A) or mimicking (S-->D) serine phosphorylation, that this function correlates with L phosphorylation. These mutations had, however, no major effects on virus production in cell culture and in vivo, indicating that L phosphorylation and transactivation are not essential for hepadnavirus replication and morphogenesis. Together, these data suggest a role of the L protein in intracellular host-virus cross talk by varying the levels of pre-S phosphorylation in response to the state of the cell.

Avian hepatitis B virus infection is initiated by the interaction of a distinct pre-S subdomain with the cellular receptor gp180

Functionally relevant hepadnavirus-cell surface interactions were investigated with the duck hepatitis B virus (DHBV) animal model by using an in vitro infection competition assay. Recombinant DHBV pre-S polypeptides, produced in Escherichia coli, were shown to inhibit DHBV infection in a dose-dependent manner, indicating that monomeric pre-S chains were capable of interfering with virus-receptor interaction. Particle-associated pre-S was, however, 30-fold more active, suggesting that cooperative interactions enhance particle binding. An 85-amino-acid pre-S sequence, spanning about half of the DHBV pre-S chain, was characterized by deletion analysis as essential for maximal inhibition. Pre-S polypeptides from heron hepatitis B virus (HHBV) competed DHBV infection equally well despite a 50% difference in amino acid sequence and a much-reduced infectivity of HHBV for duck hepatocytes. These observations are taken to indicate (i) that the functionality of the DHBV pre-S subdomain, which interacts with the cellular receptor, is determined predominantly by a defined three-dimensional structure rather than by primary sequence elements; (ii) that cellular uptake of hepadnaviruses is a multistep process involving more than a single cellular receptor component; and (iii) that gp180, a cellular receptor candidate unable to discriminate between DHBV and HHBV, is a common component of the cellular receptor complex for avian hepadnaviruses.