HCV NS5A inhibits interferon-alpha signaling through suppression of STAT1 phosphorylation in hepatocyte-derived cell lines

Suppression of Innate Immunity by the Hepatitis C Virus (HCV): Revisiting the Specificity of Host-Virus Interactive Pathways

The hepatitis C virus (HCV) is a major causative agent of hepatitis that may also lead to liver cancer and lymphomas. Chronic hepatitis C affects an estimated 2.4 million people in the USA alone. As the sole member of the genus Hepacivirus within the Flaviviridae family, HCV encodes a single-stranded positive-sense RNA genome that is translated into a single large polypeptide, which is then proteolytically processed to yield the individual viral proteins, all of which are necessary for optimal viral infection. However, cellular innate immunity, such as type-I interferon (IFN), promptly thwarts the replication of viruses and other pathogens, which forms the basis of the use of conjugated IFN-alpha in chronic hepatitis C management. As a countermeasure, HCV suppresses this form of immunity by enlisting diverse gene products, such as HCV protease(s), whose primary role is to process the large viral polyprotein into individual proteins of specific function. The exact number of HCV immune suppressors and the specificity and molecular mechanism of their action have remained unclear. Nonetheless, the evasion of host immunity promotes HCV pathogenesis, chronic infection, and carcinogenesis. Here, the known and putative HCV-encoded suppressors of innate immunity have been reviewed and analyzed, with a predominant emphasis on the molecular mechanisms. Clinically, the knowledge should aid in rational interventions and the management of HCV infection, particularly in chronic hepatitis.

Hepatitis C Virus Nonstructural Protein 5A Interacts with Immunomodulatory Kinase IKKε to Negatively Regulate Innate Antiviral Immunity

Hepatitis C virus (HCV) infection can lead to chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. HCV employs diverse strategies to evade host antiviral innate immune responses to mediate a persistent infection. In the present study, we show that nonstructural protein 5A (NS5A) interacts with an NF-κB inhibitor immunomodulatory kinase, IKKε, and subsequently downregulats beta interferon (IFN-β) promoter activity. We further demonstrate that NS5A inhibits DDX3-mediated IKKε and interferon regulatory factor 3 (IRF3) phosphorylation. We also note that hyperphosphorylation of NS5A mediats protein interplay between NS5A and IKKε, thereby contributing to NS5A-mediated modulation of IFN-β signaling. Lastly, NS5A inhibits IKKε-dependent p65 phosphorylation and NF-κB activation. Based on these findings, we propose NS5A as a novel regulator of IFN signaling events, specifically by inhibiting IKKε downstream signaling cascades through its interaction with IKKε. Taken together, these data suggest an additional mechanistic means by which HCV modulates host antiviral innate immune responses to promote persistent viral infection.

STAT1 and Its Crucial Role in the Control of Viral Infections

The signal transducer and activator of transcription (STAT) 1 protein plays a key role in the immune response against viruses and other pathogens by transducing, in the nucleus, the signal from type I, type II and type III IFNs. STAT1 activates the transcription of hundreds of genes, some of which have been well characterized for their antiviral properties. STAT1 gene deletion in mice and complete STAT1 deficiency in humans both cause rapid death from severe infections. STAT1 plays a key role in the immunoglobulin class-switch recombination through the upregulation of T-bet; it also plays a key role in the production of T-bet+ memory B cells that contribute to tissue-resident humoral memory by mounting an IgG response during re-infection. Considering the key role of STAT1 in the antiviral immune response, many viruses, including dangerous viruses such as Ebola and SARS-CoV-2, have developed different mechanisms to inhibit this transcription factor. The search for drugs capable of targeting the viral proteins implicated in both viral replication and IFN/STAT1 inhibition is important for the treatment of the most dangerous viral infections and for future viral pandemics, as shown by the clinical results obtained with Paxlovid in patients infected with SARS-CoV-2.

Viral Manipulations of the Cullin-RING Ubiquitin Ligases

Cullin-RING ubiquitin ligases (CRLs) are efficient and diverse toolsets of the cells to regulate almost every biological process. However, these characteristics have also been usurped by many viruses to optimize for their replication. CRLs are often at the forefront of the arms races in the coevolution of viruses and hosts. Here we review the modes of actions and functional consequences of viral manipulations of host cell CRLs. We also discuss the therapeutic applications to target these viral manipulations for treating viral infections.

Strategies to Circumvent Host Innate Immune Response by Hepatitis C Virus

Innate immune responses generate interferons, proinflammatory cytokines, complement activation, and natural killer (NK) cell response. Ultimately, this leads to the induction of a robust virus-specific adaptive immunity. Although the host innate immune system senses and responds to eliminate virus infection, hepatitis C virus (HCV) evades immune attack and establishes persistent infection within the liver. Spontaneous clearance of HCV infection is associated with a prompt induction of innate immunity generated in an infected host. In this review, we have highlighted the current knowledge of our understanding of host⁻HCV interactions, especially for endogenous interferon production, proinflammatory response, NK cell response, and complement activation, which may impair the generation of a strong adaptive immune response for establishment of chronicity. The information may provide novel strategies in augmenting therapeutic intervention against HCV.

Myxovirus resistance protein A inhibits hepatitis C virus replication through JAK-STAT pathway activation

The interferon-inducible dynamin-like GTPase myxovirus resistance protein A (MxA) exhibits activity against multiple viruses. However, its role in the life cycle of hepatitis C virus (HCV) is unclear, and the mechanisms underlying the anti-HCV activity of MxA require further investigation. In this study, we demonstrated that exogenous MxA expression in the Huh7 and Huh7.5.1 hepatoma cell lines significantly decreased the levels of HCV RNA and core proteins, whereas MxA knockdown exerted the opposite effect. MxA-mediated inhibition of HCV replication was found to involve the JAK-STAT pathway: STAT1 phosphorylation and the expression of IFN-stimulated genes (ISGs) such as guanylate-binding protein 1 and 2'-5'-oligoadenylate synthetase 1 were augmented by MxA overexpression and reduced by endogenous MxA silencing. Treatment with the JAK inhibitor ruxolitinib abrogated the MxA-mediated suppression of HCV replication and activation of the JAK-STAT pathway. Additionally, transfection with an MxA mutant with disrupted GTP-binding consensus motifs abrogated activation of the JAK-STAT pathway and resistance to HCV replication. This study shows that MxA inhibits HCV replication by activating the JAK-STAT signaling pathway through a mechanism involving its GTPase function.

Interferon sensitivity-determining region of hepatitis C virus influences virus production and interferon signaling

The number of amino acid substitutions in the interferon (IFN) sensitivity-determining region (ISDR) of hepatitis C virus (HCV) NS5A is a strong predictor for the outcome of IFN-based treatment. To assess the involvement of ISDR in the HCV life cycle and to clarify the molecular mechanisms influencing IFN susceptibility, we used recombinant JFH-1 viruses with NS5A of the genotype 1b Con1 strain (JFH1/5ACon1) and with NS5A ISDR containing 7 amino acid substitutions (JFH1/5ACon1/i-7mut), and compared the virus propagation and the induction of interferon-stimulated genes (ISGs). By transfecting RNAs of these strains into HuH-7-derived cells, we found that the efficiency of infectious virus production of JFH1/5ACon1/i-7mut was attenuated compared with JFH1/5ACon1. After transfecting full-length HCV RNA into HepaRG cells, the mRNA expression of ISGs was sufficiently induced by IFN treatment in JFH1/5ACon1/i-7mut-transfected but not in JFH1/5ACon1-transfected cells. These data suggested that the NS5A-mediated inhibition of ISG induction was deteriorated by amino acid substitutions in the ISDR. In conclusion, using recombinant JFH-1 viruses, we demonstrated that HCV NS5A is associated with infectious virus production and the inhibition of IFN signaling, and amino acid substitutions in the NS5A ISDR deteriorate these functions. These observations explain the strain-specific evasion of IFN signaling by HCV.

Increased incidence of cytomegalovirus coinfection in HCV-infected patients with late liver fibrosis is associated with dysregulation of JAK-STAT pathway

Herein, we examined the association between cytomegalovirus (CMV) coinfection and the progression of liver fibrosis in hepatitis C virus (HCV) infection, and investigated the effect of CMV coinfection on JAK-STAT pathway. CMV DNAemia was detected by PCR in DNA from controls (n = 120), and HCV patients with early (F0-F1, n = 131) and late (F2-F4, n = 179) liver fibrosis. By quantitative real time PCR (qRT-PCR), we examined the profile of 8 JAK-STAT transcripts in PBMCs RNA from 90 HCV patients (39 CMV positive and 51 CMV negative), 4 CMV mono-infected patients, and 15 controls. Our results demonstrated higher incidence of CMV in F2-F4 group than in control (OR 5.479, 95% CI 3.033–9.895, p < 0.0001) or F0-F1 groups (OR 2, 95% CI 1.238–3.181, p = 0.005). qRT-PCR showed downregulation of STAT2 (p = 0.006) and IRF7 (p = 0.02) in CMV positive group compared to CMV negative one. The downregulation of STAT2 and IRF7 was mainly in CMV positive patients with late fibrosis compared to CMV negative patients (p = 0.0007 for IRF7 and p = 0.01 for STAT2). Our results are the first to report that CMV coinfection is a possible risk factor for the progression of HCV-induced liver fibrosis, and thereby CMV screening and treatment are important for HCV patients.

Nonstructural 5A Protein of Hepatitis C Virus Interferes with Toll-Like Receptor Signaling and Suppresses the Interferon Response in Mouse Liver

The hepatitis C virus nonstructural protein NS5A is involved in resistance to the host immune response, as well as the viral lifecycle such as replication and maturation. Here, we established transgenic mice expressing NS5A protein in the liver and examined innate immune responses against lipopolysaccharide (LPS) in vivo. Intrahepatic gene expression levels of cytokines such as interleukin-6, tumor necrosis factor-α, and interferon-γ were significantly suppressed after LPS injection in the transgenic mouse liver. Induction of the C-C motif chemokine ligand 2, 4, and 5 was also suppressed. Phosphorylation of the signal transducer and activator of transcription 3, which is activated by cytokines, was also reduced, and expression levels of interferon-stimulated genes, 2’-5’ oligoadenylate synthase, interferon-inducible double-stranded RNA-activated protein kinase, and myxovirus resistance 1 were similarly suppressed. Since LPS binds to toll-like receptor 4 and stimulates the downstream pathway leading to induction of these genes, we examined the extracellular signal-regulated kinase and IκB-α. The phosphorylation levels of these molecules were reduced in transgenic mouse liver, indicating that the pathway upstream of the molecules was disrupted by NS5A. Further analyses revealed that the interaction between interleukin-1 receptor-associated kinase-1 and tumor necrosis factor receptor associated factor-6 was dispersed in transgenic mice, suggesting that NS5A may interfere with this interaction via myeloid differentiation primary response gene 88, which was shown to interact with NS5A. Since the gut microbiota, a source of LPS, is known to be associated with pathological conditions in liver diseases, our results suggest the involvement of NS5A in the pathogenesis of HCV infected-liver via the suppression of innate immunity.

Acetaldehyde Disrupts Interferon Alpha Signaling in Hepatitis C Virus-Infected Liver Cells by Up-Regulating USP18

BACKGROUND

Alcohol consumption exacerbates the pathogenesis of hepatitis C virus (HCV) infection and worsens disease outcomes. The exact reasons are not clear yet, but they might be partially attributed to the ability of alcohol to further suppress the innate immunity. Innate immunity is known to be already decreased by HCV in liver cells.

METHODS

In this study, we aimed to explore the mechanisms of how alcohol metabolism dysregulates IFNα signaling (STAT1 phosphorylation) in HCV⁺ hepatoma cells. To this end, CYP2E1⁺ Huh7.5 cells were infected with HCV and exposed to the acetaldehyde (Ach) generating system (AGS).

RESULTS

Continuously produced Ach suppressed IFNα-induced STAT1 phosphorylation, but increased the level of a protease, USP18 (both measured by Western blot), which interferes with IFNα signaling. Induction of USP18 by Ach was confirmed in primary human hepatocyte cultures and in livers of ethanol-fed HCV transgenic mice. Silencing of USP18 by specific siRNA attenuated the pSTAT1 suppression by Ach. The mechanism by which Ach down-regulates pSTAT1 is related to an enhanced interaction between IFNαR2 and USP18 that finally dysregulates the cross talk between the IFN receptor on the cell surface and STAT1. Furthermore, Ach decreases ISGylation of STAT1 (protein conjugation of a small ubiquitin-like modifier, ISG15, Western blot), which preserves STAT1 activation. Suppressed ISGylation leads to an increase in STAT1 K48 polyubiquitination which allows pSTAT1 degrading by proteasome.

CONCLUSIONS

We conclude that Ach disrupts IFNα-induced STAT1 phosphorylation by the up-regulation of USP18 to block the innate immunity protection in HCV-infected liver cells, thereby contributing to HCV-alcohol pathogenesis. This, in part, may explain the mechanism of HCV-infection exacerbation/progression in alcohol-abusing patients.

Classical swine fever virus NS5A protein changed inflammatory cytokine secretion in porcine alveolar macrophages by inhibiting the NF-κB signaling pathway

Background

Classical swine fever (CSF) caused by CSF virus (CSFV) is a highly contagious disease of the pigs. A number of studies have suggested that CSFV non-structural (NS) 5A protein is involved in CSFV-associated pathogenesis, but its mechanism is still uncertain. The aim of this study was to investigate the roles of NS5A protein in CSFV-associated pathogenesis in cultured porcine alveolar macrophages (PAMs).

Methods

After PAMs cultured in vitro were transfected with CSFV NS5A, the alterations in IL-1β, IL-6 and TNF-α expression were determined by ELISA, the RIG-I signaling activity related to inflammatory cytokine secretion was investigated by Western blot and Immunofluorescent staining.

Results

It was suggested that, the stable expressed CSFV NS5A solely had no influence on the expressions of inflammatory cytokines IL-1β, IL-6 and TNF-α in PAMs Moreover, NS5A protein could suppressed IL-1β, IL-6 and TNF-α expression induced by poly(I:C). It was also showed that NS5A protein did not impair the expressions of RIG-I, MDA5, IPS-1, NF-κB and IkBα in cells without poly(I:C) stimulation. Protein expressions of RIG-I, MDA5, IPS-1, NF-κB were not disrupted by NS5A protein in poly(I:C)-stimulated cells, while poly(I:C)-induced NF-κB nuclear translocation and activity was obviously suppressed by this protein. A suppression in poly(I:C)-induced IkBα degradation in NS5A-expressing cells was also observed.

Conclusion

These data indicated that CSFV NS5A protein could inhibit the secretion of inflammatory cytokine induced by poly(I:C) through the suppression of the NF-κB signaling pathway, indicating the participation of CSFV NS5A protein in the pathogenesis of CSFV.

Intrinsic Viral Factors Are the Dominant Determinants of the Hepatitis C Virus Response to Interferon Alpha Treatment in Chimeric Mice

BACKGROUND

Hepatitis C virus infection is a global health problem. New direct-acting antiviral agents have been recently approved. However, due to their high cost and some genotypes remaining difficult to treat, interferon-based therapy with pegylated interferon and ribavirin likely may remain a component of hepatitis C virus treatment for some patients. Unfortunately, pegylated interferon / ribavirin treatment achieved favorable outcomes in less than 50% of patients. Factors determining the outcome to pegylated interferon/ribavirin include both host and viral factors. It has been a major challenge to separate the host and viral factors in most in vivo systems.

AIMS & METHODS

We used two hepatitis C virus strains from patients with different interferon-sensitivities and three hepatocyte donors, each with distinct interleukin 28B and interferon lambda 4 single nucleotide polymorphisms to investigate the contributions of viral and host factors to the response of hepatitis C virus to interferon treatment in chimeric mice.

RESULTS AND CONCLUSIONS

We found that viral factors were the dominant factors in determining the interferon treatment outcomes in chimeric mice. Host factors, such as pre-treatment liver interferon-stimulated gene expression and single nucleotide polymorphisms near interleukin 28B and interferon lambda 4 coding regions, were less important determinants of the response to interferon in the chimeric mice than they were in patients. Our results also suggest that a complete immune system as seen in patients may be required for host factors such as single nucleotide polymorphisms near interleukin 28B/interferon lambda 4 and pre-treatment liver interferon-stimulated gene upregulation to have an effect on the interferon response.

The role of HCV proteins on treatment outcomes

For many years, the standard of treatment for hepatitis C virus (HCV) infection was a combination of pegylated interferon alpha (Peg-IFN-α) and ribavirin for 24–48 weeks. This treatment regimen results in a sustained virologic response (SVR) rate in about 50 % of cases. The failure of IFN-α-based therapy to eliminate HCV is a result of multiple factors including a suboptimal treatment regimen, severity of HCV-related diseases, host factors and viral factors. In recent years, advances in HCV cell culture have contributed to a better understanding of the viral life cycle, which has led to the development of a number of direct-acting antiviral agents (DAAs) that target specific key components of viral replication, such as HCV NS3/4A, HCV NS5A, and HCV NS5B proteins. To date, several new drugs have been approved for the treatment of HCV infection. Application of DAAs with IFN-based or IFN-free regimens has increased the SVR rate up to >90 % and has allowed treatment duration to be shortened to 12–24 weeks. The impact of HCV proteins in response to IFN-based and IFN-free therapies has been described in many reports. This review summarizes and updates knowledge on molecular mechanisms of HCV proteins involved in anti-IFN activity as well as examining amino acid variations and mutations in several regions of HCV proteins associated with the response to IFN-based therapy and pattern of resistance associated amino acid variants (RAV) to antiviral agents.

Early IFN type I response: Learning from microbial evasion strategies

Type I interferon (IFN) comprises a class of cytokines first discovered more than 50 years ago and initially characterized for their ability to interfere with viral replication and restrict locally viral propagation. As such, their induction downstream of germ-line encoded pattern recognition receptors (PRRs) upon recognition of pathogen-associated molecular patterns (PAMPs) is a hallmark of the host antiviral response. The acknowledgment that several PAMPs, not just of viral origin, may induce IFN, pinpoints at these molecules as a first line of host defense against a number of invading pathogens. Acting in both autocrine and paracrine manner, IFN interferes with viral replication by inducing hundreds of different IFN-stimulated genes with both direct anti-pathogenic as well as immunomodulatory activities, therefore functioning as a bridge between innate and adaptive immunity. On the other hand an inverse interference to escape the IFN system is largely exploited by pathogens through a number of tactics and tricks aimed at evading, inhibiting or manipulating the IFN pathway, that result in progression of infection or establishment of chronic disease. In this review we discuss the interplay between the IFN system and some selected clinically important and challenging viruses and bacteria, highlighting the wide array of pathogen-triggered molecular mechanisms involved in evasion strategies.

Tricistronic hepatitis C virus subgenomic replicon expressing double transgenes

AIM: To construct a tricistronic hepatitis C virus (HCV) replicon with double internal ribosome entry sites (IRESes) of only 22 nucleotides for each, substituting the encephalomyocarditis virus (EMCV) IRESes, which are most often used as the translation initiation element to form HCV replicons.

METHODS: The alternative 22-nucleotide IRES, RNA-binding motif protein 3 IRES (Rbm3 IRES), was used to form a tricistronic HCV replicon, to facilitate constructing HCV-harboring stable cell lines and successive antiviral screening using a luciferase marker. Briefly, two sequential Rbm3 IRESes were inserted into bicistronic pUC19-HCV plasmid, consequently forming a tricistronic HCV replicon (pHCV-rep-NeoR-hRluc), initiating the translation of humanized Renilla luciferase and HCV non-structural gene, along with HCV authentic IRES initiating the translation of neomycin resistance gene. The sH7 cell lines, in which the novel replicon RNA stably replicated, were constructed by neomycin and luciferase activity screening. The intracellular HCV replicon RNA, expression of inserted foreign genes and HCV non-structural gene, as well as response to anti-HCV agents, were measured in sH7 cells and cells transiently transfected with tricistronic replicon RNA.

RESULTS: The intracellular HCV replicon RNA and expression of inserted foreign genes and HCV non-structural gene in sH7 cells and cells transiently transfected with tricistronic replicon RNA were comparable to those in cells stably or transiently transfected with traditional bicistronic HCV replicons. The average relative light unit in pHCV-rep-NeoR-hRluc group was approximately 2-fold of those in the pUC19-HCV-hRLuc and Tri-JFH1 groups (1.049 × 10⁸± 2.747 × 10⁷vs 5.368 × 10⁷± 1.016 × 10⁷, P < 0.05; 1.049 × 10⁸± 2.747 × 10⁷vs 5.243 × 10⁷± 1.194 × 10⁷, P < 0.05), suggesting that the translation initiation efficiency of the first Rbm3 IRES in the two sequential IRESes was stronger than the HCV authentic IRES and EMCV IRES. The fold changes of 72 h/4 h relative light units in the pHCV-rep-NeoR-hRluc and pUC19-HCV-hRLuc groups were similar (159.619 ± 9.083 vs 163.536 ± 24.031, P = 0.7707), and were both higher than the fold change in the Tri-JFH1 group 159.619± 9.083 vs 140.811 ± 9.882, P < 0.05; 163.536 ± 24.031 vs 140.811 ± 9.882, P < 0.05), suggesting that the replication potency of the Rbm3 IRES tricistronic replicon matched the replication of bicistronic replicon and exceeded the potency of EMCV IRES replicon. Replication of tricistronic replicons was suppressed by ribavirin, simvastatin, atorvastatin, telaprevir and boceprevir. Interferon-alpha 2b could not block replication of the novel replicon RNA in sH7 cells. After interferon stimulation, MxA mRNA and protein levels were lower in sH7 than in parental cells.

CONCLUSION: Tricistronic HCV replicon with double Rbm3 IRESes could be applied to evaluate the replication inhibition efficacy of anti-HCV agents.

Isolation and characterization of interferon lambda-resistant hepatitis C virus replicon cell lines

Pegylated interferon lambda-1a (Lambda) is currently in clinical development for the treatment of chronic hepatitis C virus (HCV) infection. To gain insight into the potential mechanisms of non-responsiveness that may occur in patients treated with Lambda, HCV subgenomic replicon cell-lines with impaired susceptibility to the unpegylated recombinant (r) form of interferon (IFN) lambda-1 (rIFNλ) were isolated and characterized. The selected replicon cell populations showed a defect in the activation of the IFN-dependent JAK-STAT signaling pathway. Reduced phosphorylation of STAT proteins and lower expression levels of the cellular janus kinases Jak1 and Tyk2 were observed in these cell populations, which may account for the impaired JAK-STAT signaling and reduced antiviral responses to rIFNλ. Overall, this in vitro study provides molecular insights into the possible mechanism of viral evasion to rIFNλ in the HCV replicon cell system.

Impact of host and virus genome variability on HCV replication and response to interferon

Since the discovery of hepatitis C virus (HCV), treatment has proven difficult and the regimen of pegylated interferon-α and ribavirin is only effective for half of patients. Evidence suggests that host and viral genome variations play a role in either viral clearance or persistence. Powerful genomic technologies have made it possible to study genome-wide associations with treatment response, which yielded critical genetic polymorphisms that predict treatment response. This has important implications for treatment of HCV infection and opened the door to the possibility of genetic marker-guided treatment (personalized medicine). This review will focus on the recent advances in understanding host and viral genetic variations with regards to treatment and the importance for future therapeutic intervention.

Response of hepatitis C virus to long-term passage in the presence of alpha interferon: multiple mutations and a common phenotype

Cell culture-produced hepatitis C virus (HCV) has been subjected to up to 100 serial passages in human hepatoma cells in the absence or presence of different doses of alpha interferon (IFN-α). Virus survival, genetic changes, fitness levels, and phenotypic traits have been examined. While high initial IFN-α doses (increasing from 1 to 4 IU/ml) did not allow HCV survival beyond passage 40, a gradual exposure (from 0.25 to 10 IU/ml) allowed the virus to survive for at least 100 passages. The virus passaged in the presence of IFN-α acquired IFN-α resistance as evidenced by enhanced progeny production and viral protein expression in an IFN-α environment. A partial IFN-α resistance was also noted in populations passaged in the absence of IFN-α. All lineages acquired adaptative mutations, and multiple, nonsynonymous mutations scattered throughout the genome were present in IFN-α-selected populations. Comparison of consensus sequences indicates a dominance of synonymous versus nonsynonymous substitutions. IFN-α-resistant populations displayed decreased sensitivity to a combination of IFN-α and ribavirin. A phenotypic trait common to all assayed viral populations is the ability to increase shutoff host cell protein synthesis, accentuated in infections with IFN-α-selected populations carried out in the presence of IFN-α. The trait was associated with enhanced phosphorylation of protein kinase R (PKR) and eIF2α, although other contributing factors are likely. The results suggest that multiple, independent mutational pathways can confer IFN-α resistance to HCV and might explain why no unified picture has been obtained regarding IFN-α resistance in vivo.

Future classes of hepatitis C virus therapeutic agents

Recent advances in understanding of the molecular characteristics of the hepatitis C virus have led to the development of novel antiviral therapeutics. Direct-acting antivirals are designed to inhibit viral targets, whereas host-targeted antivirals block host factors that are used by the virus for its own life cycle. The rapid development of agents in multiple classes has led to the promise of shorter therapy duration, an improved side effect profile, and eventually interferon-sparing regimens. This article reviews novel hepatitis C virus therapeutics in development, including mechanism of action, efficacy, and adverse effects.

HCV NS5A and IRF9 compete for CypA binding

BACKGROUND & AIMS

Cyclophilin A (CypA) is vital for HCV replication. Cyp inhibitors successfully decrease viral loads in HCV-infected patients. However, their mechanisms of action remain unknown. Since interferon (IFN) can also suppress HCV replication, we asked whether a link between CypA and the IFN response exists.

METHODS

We used cellular and recombinant pulldown approaches to investigate the possibility of a specific association of CypA with host ligands.

RESULTS

We found for the first time that CypA binds to a major component of the IFN response - the IFN regulatory factor 9 (IRF9). IRF9 is the DNA-binding component of the transcriptional IFN-stimulated gene factor 3 (ISGF3). CypA binds directly to IRF9 via its peptidyl-prolyl isomerase (PPIase) pocket. Cyp inhibitors such as cyclosporine A (CsA) or non-immunosuppressive derivates such as alisporivir and SCY-635, prevent IRF9-CypA complex formation. CypA binds to the C-terminal IRF-association-domain (IAD), but not to the DNA-binding or linker domains of IRF9. Remarkably, CypA associates with the multimeric ISGF3 complex. We also obtained evidence that CypA neutralization enhances IFN-induced transcription. Interestingly, the hepatitis C virus (HCV) non-structural 5A (NS5A) protein, which is known to modulate the IFN response, competes with IRF9 for CypA binding and can prevent the formation of IRF9-CypA complexes.

CONCLUSIONS

This study demonstrates for the first time that CypA binds specifically to a component of the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, IRF9. This study also reveals a novel opportunity of HCV to modulate the IFN response via NS5A.

Direct acting antivirals for the treatment of chronic viral hepatitis

The development and evaluation of antiviral agents through carefully designed clinical trials over the last 25 years have heralded a new dawn in the treatment of patients chronically infected with the hepatitis B and C viruses, but not so for the D virus (HBV, HCV, and HDV). The introduction of direct acting antivirals (DDAs) for the treatment of HBV carriers has permitted the long-term use of these compounds for the continuous suppression of viral replication, whilst in the case of HCV in combination with the standard of care [SOC, pegylated interferon (PegIFN), and ribavirin] sustained virological responses (SVRs) have been achieved with increasing frequency. Progress in the case of HDV has been slow and lacking in significant breakthroughs.This paper aims to summarise the current state of play in treatment approaches for chonic viral hepatitis patients and future perspectives.

Innate immune responses in hepatitis C virus infection

Hepatitis C virus (HCV) is a major causative agent of chronic hepatitis and hepatocellular carcinoma worldwide and thus poses a significant public health threat. A hallmark of HCV infection is the extraordinary ability of the virus to persist in a majority of infected people. Innate immune responses represent the front line of defense of the human body against HCV immediately after infection. They also play a crucial role in orchestrating subsequent HCV-specific adaptive immunity that is pivotal for viral clearance. Accumulating evidence suggests that the host has evolved multifaceted innate immune mechanisms to sense HCV infection and elicit defense responses, while HCV has developed elaborate strategies to circumvent many of these. Defining the interplay of HCV with host innate immunity reveals mechanistic insights into hepatitis C pathogenesis and informs approaches to therapy. In this review, we summarize recent advances in understanding innate immune responses to HCV infection, focusing on induction and effector mechanisms of the interferon antiviral response as well as the evasion strategies of HCV.

Hepatitis C virus NS5A disrupts STAT1 phosphorylation and suppresses type I interferon signaling

Responses to alpha interferon (IFN-α)-based treatment are dependent on both host and viral factors and vary markedly among patients infected with different hepatitis C virus (HCV) genotypes (GTs). Patients infected with GT3 viruses consistently respond better to IFN treatment than do patients infected with GT1 viruses. The mechanisms underlying this difference are not well understood. In this study, we sought to determine the effects of HCV NS5A proteins from different genotypes on IFN signaling. We found that the overexpression of either GT1 or GT3 NS5A proteins significantly inhibited IFN-induced IFN-stimulated response element (ISRE) signaling, phosphorylated STAT1 (P-STAT1) levels, and IFN-stimulated gene (ISG) expression compared to controls. GT1 NS5A protein expression exhibited stronger inhibitory effects on IFN signaling than did GT3 NS5A protein expression. Furthermore, GT1 NS5A bound to STAT1 with a higher affinity than did GT3 NS5A. Domain mapping revealed that the C-terminal region of NS5A conferred these inhibitory effects on IFN signaling. The overexpression of HCV NS5A increased HCV replication levels in JFH1-infected cells through the further reduction of levels of P-STAT1, ISRE signaling, and downstream ISG responses. We demonstrated that the overexpression of GT1 NS5A proteins resulted in less IFN responsiveness than did the expression of GT3 NS5A proteins through stronger binding to STAT1. We confirmed that GT1 NS5A proteins exerted stronger IFN signaling inhibition than did GT3 NS5A proteins in an infectious recombinant JFH1 virus. The potent antiviral NS5A inhibitor BMS-790052 did not block NS5A-mediated IFN signaling suppression in an overexpression model, suggesting that NS5A's contributions to replication are independent of its subversive action on IFN. We propose a model in which the binding of the C-terminal region of NS5A to STAT1 leads to decreased levels of P-STAT1, ISRE signaling, and ISG transcription and, ultimately, to preferential GT1 resistance to IFN treatment.

Anti-hepatitis C virus drugs in development

Development of robust cell culture models for hepatitis C viral infection has greatly increased our understanding of this virus and its life cycle. This knowledge has led to the development of many drugs that target specific elements of viral replication, including viral proteins and host factors required for replication. The NS3/4A serine protease inhibitors were the first of these to be used in the clinic, and reagents that target other elements of the viral lifecycle are in advanced stages of clinical development. These include new NS3/4A protease inhibitors, NS5B RNA-dependent RNA polymerase inhibitors, NS5A inhibitors, and host-directed antivirals, such as cyclophilin inhibitors. Alternative interferons with possibly improved tolerability, specifically interferon-λ1 (interleukin-29), are also under development. These new reagents against hepatitis C virus should lead to highly effective, well-tolerated, and likely interferon-sparing therapies in the next several years.

Suppression of interferon-α signaling by hepatitis E virus

The interferon (IFN) system is integral to the host response against viruses, and many viruses have developed strategies to overcome its antiviral effects. The effects of hepatitis E virus (HEV), the causative agent of hepatitis E, on IFN signaling have not been investigated primarily because of the nonavailability of an efficient in vitro culture system or small animal models of infection. We report here the generation of A549 cell lines persistently infected with genotype 3 HEV, designated as HEV-A549 cells and the effects HEV has on IFN-α-mediated Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling. Treatment of HEV-A549 cells with 250, 500, and 1000 U/mL of IFN-α for 72 hours showed a dose-dependent reduction in HEV RNA levels by 10%, 20%, and 50%, respectively. IFN-α-stimulated genes coding for the antiviral proteins dsRNA-activated protein kinase (PKR) and 2',5'-oligoadenylate synthetase (2',5'-OAS) were down-regulated in IFN-α-treated HEV-A549 cells. HEV infection also prevented IFN-α-induced phosphorylation of STAT1. Regulation of STAT1 by HEV was specific, as phosphorylation of STAT2, tyrosine kinase (Tyk) 2, and Jak1 by IFN-α was unaltered. Additionally, STAT1 levels were markedly increased in HEV-A549 cells compared with naive A549 cells. Furthermore, binding of HEV open reading frame (ORF)3 protein to STAT1 in HEV-A549 cells was observed. HEV ORF3 protein alone inhibited IFN-α-induced phosphorylation of STAT1 and down-regulated the IFN-α-stimulated genes encoding PKR, 2',5'-OAS, and myxovirus resistance A.

CONCLUSION

HEV inhibits IFN-α signaling through the regulation of STAT1 phosphorylation in A549 cells. These findings have implications for the development of new strategies against hepatitis E.

Geranylgeranylacetone has anti-hepatitis C virus activity via activation of mTOR in human hepatoma cells

BACKGROUND

Geranylgeranylacetone (GGA), an isoprenoid compound which includes retinoids, has been used orally as an anti-ulcer drug in Japan. GGA acts as a potent inducer of anti-viral gene expression by stimulating ISGF3 formation in human hepatoma cells. This drug has few side effects and reinforces the effect of IFN when administered in combination with peg-IFN and ribavirin. This study verified the anti-HCV activity of GGA in a replicon system. In addition, mechanisms of anti-HCV activity were examined in the replicon cells.

METHODS

OR6 cells stably harboring the full-length genotype 1 replicon containing the Renilla luciferase gene, ORN/C-5B/KE, were used to examine the influence of the anti-HCV effect of GGA. After treatment, the cells were harvested with Renilla lysis reagent and then subjected to a luciferase assay according to the manufacturer's protocol.

RESULT

The results showed that GGA had anti-HCV activity. GGA induced anti-HCV replicon activity in a time- and dose-dependent manner. GGA did not activate the tyrosine 701 and serine 727 on STAT-1, and did not induce HSP-70 in OR6 cells. The anti-HCV effect depended on the GGA induced mTOR activity, not STAT-1 activity and PKR. An additive effect was observed with a combination of IFN and GGA.

CONCLUSIONS

GGA has mTOR dependent anti-HCV activity. There is a possibility that the GGA anti-HCV activity can be complimented by IFN. It will be necessary to examine the clinical effectiveness of the combination of GGA and IFN for HCV patients in the future.

Liver protein profiling in chronic hepatitis C: identification of potential predictive markers for interferon therapy outcome

The current anti-hepatitis C virus (HCV) therapy, based on pegylated-interferon alpha and ribavirin, has limited success rate and is accompanied by several side effects. The aim of this study was to identify protein profiles in pretreatment liver biopsies of HCV patients correlating with the outcome of antiviral therapy. Cytosolic or membrane/organelle-enriched protein extracts from liver biopsies of eight HCV patients were analyzed by two-dimensional fluorescence difference gel electrophoresis and mass spectrometry. Overall, this analysis identified 21 proteins whose expression levels correlate with therapy response. These factors are involved in interferon-mediated antiviral activity, stress response, and energy metabolism. Moreover, we found that post-translational modifications of dihydroxyacetone kinase were also associated with therapy outcome. Differential expression of the five best performing markers (STAT1, Mx1, DD4, DAK, and PD-ECGF) was confirmed by immunoblotting assays in an independent group of HCV patients. Finally, we showed that a prediction model based on the expression levels of these markers classifies responder and nonresponder patients with an accuracy of 85.7%. These results provide evidence that the analysis of pretreatment liver protein profiles is valuable for discriminating between responder and nonresponder HCV patients, and may contribute to reduce the number of nonresponder patients exposed to therapy-associated risks.

PCBP2 enhances the antiviral activity of IFN-α against HCV by stabilizing the mRNA of STAT1 and STAT2

Interferon-α (IFN-α) is a natural choice for the treatment of hepatitis C, but half of the chronically infected individuals do not achieve sustained clearance of hepatitis C virus (HCV) during treatment with IFN-α alone. The virus can impair IFN-α signaling and cellular factors that have an effect on the viral life cycles. We found that the protein PCBP2 is down-regulated in HCV-replicon containing cells (R1b). However, the effects and mechanisms of PCBP2 on HCV are unclear. To determine the effect of PCBP2 on HCV, overexpression and knockdown of PCBP2 were performed in R1b cells. Interestingly, we found that PCBP2 can facilitate the antiviral activity of IFN-α against HCV, although the RNA level of HCV was unaffected by either the overexpression or absence of PCBP2 in R1b cells. RIP-qRT-PCR and RNA half-life further revealed that PCBP2 stabilizes the mRNA of STAT1 and STAT2 through binding the 3′Untranslated Region (UTR) of these two molecules, which are pivotal for the IFN-α anti-HCV effect. RNA pull-down assay confirmed that there were binding sites located in the C-rich tracts in the 3′UTR of their mRNAs. Stabilization of mRNA by PCBP2 leads to the increased protein expression of STAT1 and STAT2 and a consistent increase of phosphorylated STAT1 and STAT2. These effects, in turn, enhance the antiviral effect of IFN-α. These findings indicate that PCBP2 may play an important role in the IFN-α response against HCV and may benefit the HCV clinical therapy.

Interferon alpha regulates MAPK and STAT1 pathways in human hepatoma cells

BACKGROUND

Signaling events triggered by interferon (IFN) account for the molecular mechanisms of antiviral effect. JAK-STAT pathway plays a critical role in IFN signaling, and other pathways are also implicated in IFN-mediated antiviral effect. Changes in mitogen-activated protein kinase (MAPK) and STAT1 pathways were evaluated in human hepatoma cells Huh7 and HepG2 upon IFN alpha treatment.

RESULTS

Phosphorylation of ERK was significantly and specifically up-regulated, whereas enhanced phosphorylation of upstream kinase MEK was unobservable upon IFN alpha treatment. A mild increase in p38 MAPK, SAPK/JNK and downstream target ATF-2 phosphorylation was detectable after exposure to IFN alpha, indicating differential up-regulation of the MAPK signaling cascades. Moreover, STAT1 phosphorylation was strongly enhanced by IFN alpha.

CONCLUSION

IFN alpha up-regulates MAPK and STAT1 pathways in human hepatoma cells, and may provide useful information for understanding the IFN signaling.

Transcriptional repression of C4 complement by hepatitis C virus proteins

The fourth component of human complement (C4) plays an important role in innate immune function. C4 activity has been observed to be significantly lower in patients with chronic hepatitis C virus (HCV) infections, although the mechanism remains unknown. In this study, we have examined the mechanisms of C4 regulation by HCV. Liver biopsy specimens from patients with chronic HCV infections displayed significantly lower C4 mRNA levels than liver tissue samples from patients with unrelated liver disease. Further, C4 mRNA levels of the two isoforms (C4A and C4B) were significantly reduced in hepatocytes transfected with RNA from HCV genotype 1a or 2a. Subsequently, a significant C4 regulatory role of HCV core or NS5A upon C4 promoter activity was observed. HCV core or NS5A transgenic mice displayed a reduction in C4 mRNA. Gamma interferon (IFN-γ)-induced C4 promoter activation was also impaired in the presence of HCV proteins. We further demonstrated that HCV core reduced the expression of upstream stimulating factor 1 (USF-1), a transcription factor important for basal C4 expression. On the other hand, the expression of interferon regulatory factor 1 (IRF-1), which is important for IFN-γ-induced C4 expression, was inhibited by hepatocytes expressing HCV NS5A. These results underscore the roles of HCV proteins in innate immune regulation in establishing a chronic infection.

Advances in research of interaction between hepatitis C virus nonstructural proteins and host proteins

Hepatitis C virus (HCV) is another common cause of chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma after hepatitis B virus (HBV). Up to now, the mechanisms by which HCV promotes persistent infection and cancer remain unclear, and there are neither effective drugs nor vaccines against HCV available. Interaction between virus proteins and host proteins is a hot topic in research of the pathogenesis of viral hepatitis. Recent research shows that interaction between HCV nonstructural proteins and host proteins has an important impact on viral replication, carcinogenesis, interferon resistance, and disorders of glycometabolism and lipid metabolism. This paper summarizes the recent advances in research of interaction between HCV nonstructural proteins and host proteins.

Hepatitis C virus infection impairs IRF-7 translocation and Alpha interferon synthesis in immortalized human hepatocytes

Hepatitis C virus (HCV) establishes chronic infection in a significant number of infected humans, although the mechanisms for chronicity remain largely unknown. We have previously shown that HCV infection in immortalized human hepatocytes (IHH) induces beta interferon (IFN-β) expression (T. Kanda, R. Steele, R. Ray, and R. B. Ray, J. Virol. 81:12375-12381, 2007). However, the regulation of the downstream signaling pathway for IFN-α production by HCV is not clearly understood. In this study, the regulation of the IFN signaling pathway following HCV genotype 1a (clone H77) or genotype 2a (clone JFH1) infection of IHH was examined. HCV infection upregulated expression of total STAT1 but failed to induce phosphorylation and efficient nuclear translocation. Subsequent study revealed that HCV infection induces IFN-stimulated response element activation, as evidenced by upregulation of 2',5'-oligoadenylate synthetase 1. However, nuclear translocation of IRF-7 was impaired following HCV infection. In HCV-infected IHH, IFN-α expression initially increased (up to 24 h) and then decreased at later time points, and IFN-α-inducible protein 27 was not induced. Interestingly, HCV infection blocked IRF-7 nuclear translocation upon poly(I-C) or IFN-α treatment of IHH. Together, our data suggest that HCV infection enhances STAT1 expression but impairs nuclear translocation of IRF-7 and its downstream molecules. These impairments in the IFN-α signaling pathway may, in part, be responsible for establishment of chronic HCV infection.

STAT1 and pathogens, not a friendly relationship

STAT1 belongs to the STAT family of transcription factors, which comprises seven factors: STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B and STAT6. STAT1 is a 91 kDa protein originally identified as the mediator of the cellular response to interferon (IFN) α, and thereafter found to be a major component of the cellular response to IFNγ. STAT1 is, in fact, involved in the response to several cytokines and to growth factors. It is activated by cytokine receptors via kinases of the JAK family. STAT1 becomes phosphorylated and forms a dimer which enters the nucleus and triggers the transcription of its targets. Although not lethal at birth, selective gene deletion of STAT1 in mice leads to rapid death from severe infections, demonstrating its major role in the response to pathogens. Similarly, in humans who do not express STAT1, there is a lack of resistance to pathogens leading to premature death. This indicates a key, non-redundant function of STAT1 in the defence against pathogens. Thus, to successfully infect organisms, bacterial, viral or parasitic pathogens must overcome the activity of STAT1, and almost all the steps of this pathway can be blocked or inhibited by proteins produced in infected cells. Interestingly, some pathogens, like the oncogenic Epstein–Barr virus, have evolved a strategy which uses STAT1 activation.

The N-terminal region of severe acute respiratory syndrome coronavirus protein 6 induces membrane rearrangement and enhances virus replication

The severe acute respiratory syndrome coronavirus (SARS-CoV) accessory protein 6 (p6) is a 63-amino-acid multifunctional Golgi-endoplasmic reticulum (ER) membrane-associated protein, with roles in enhancing virus replication and in evading the innate immune response to infection by inhibiting STAT1 (signal transducer and activator of transcription factor 1) translocation to the nucleus. Here, we demonstrate that p6 has an N-terminal region-cytoplasm-C-terminal region-cytoplasm configuration with residues 2 to 37 likely membrane embedded. Expression of p6, or of its N-terminal 41-amino-acid region, in the absence of other viral proteins, induced the formation of membranous structures, some of which were similar to double membrane vesicles involved in virus replication. Consistent with a role in virus replication, p6 partially colocalized with nonstructural protein 3 (nsp3), a marker for virus replication complexes. Further, while the C-terminal region is required for preventing STAT1 translocation to the nucleus, our results also indicated that the N-terminal 18 amino acids were necessary for maximal inhibition. Collectively, these results support the notion that p6 is a two-domain protein, although the function of each is not completely independent of the other.

Hepatitis B virus overexpresses suppressor of cytokine signaling-3 (SOCS3) thereby contributing to severity of inflammation in the liver

The mechanism by which hepatitis B virus (HBV) infection causes severe inflammatory liver diseases is multifactorial and related to interactions with cell signaling pathways and the ensuing inflammatory response. Activation of JAK/STAT/SOCS signaling is essential for the induction of cellular antiviral responses, contributes to apoptosis and is negatively regulated by SOCS proteins. Recent reports have shown that SOCS3 activation interferes with viral protein expression and treatment response and thereby plays a major role in hepatitis virus infections. We analyzed the expression of SOCS3 in liver specimens from HBV-infected patients using immunohistochemistry (IHC) and determined the effect of HBV on STAT/SOCS signaling in functional cell culture experiments (HuH-7) using HBV-expressing adenoviral constructs (AdHBV). Increased expression of SOCS3 protein was identified in liver specimens from patients with chronic HBV-infection and this correlated with the severity of liver inflammation. In accordance with the IHC-findings, in vitro analyses demonstrated that HBV infection of HuH7 cells was associated with increased expression of SOCS3 protein. In spite of the over expression of its negative regulator SOCS3 we observed a constitutive activation of STAT3. SOCS1 levels were not increased while pSTAT1 was suppressed in HBV-infected HuH7 cells. Our results demonstrate that STAT/SOCS-signaling is dysregulated in HBV-infected hepatocytes both in vivo and in vitro and this correlated with the severity of liver inflammatory changes. This interference of STAT/SOCS signaling by HBV may result in an ineffective immune response against HBV and potentially contributes to viral pathogenesis, malignant transformation and may represent an important mechanism of viral persistence.

Interferon-alpha-induced mTOR activation is an anti-hepatitis C virus signal via the phosphatidylinositol 3-kinase-Akt-independent pathway

OBJECT

The interferon-induced Jak-STAT signal alone is not sufficient to explain all the biological effects of IFN. The PI3-K pathways have emerged as a critical additional component of IFN-induced signaling. This study attempted to clarify that relationship between IFN-induced PI3-K-Akt-mTOR activity and anti-viral action.

RESULT

When the human normal hepatocyte derived cell line was treated with rapamycin (rapa) before accretion of IFN-alpha, tyrosine phosphorylation of STAT-1 was diminished. Pretreatment of rapa had an inhibitory effect on the IFN-alpha-induced expression of PKR and p48 in a dose dependent manner. Rapa inhibited the IFN-alpha inducible IFN-stimulated regulatory element luciferase activity in a dose-dependent manner. However, wortmannin, LY294002 and Akt inhibitor did not influence IFN-alpha inducible luciferase activity. To examine the effect of PI3-K-Akt-mTOR on the anti-HCV action of IFN-alpha, the full-length HCV replication system, OR6 cells were used. The pretreatment of rapa attenuated its anti-HCV replication effect in comparison to IFN-alpha alone, whereas the pretreatment with PI3-K inhibitors, wortmannin and LY294002 and Akt inhibitor did not influence IFN-induced anti-HCV replication.

CONCLUSION

IFN-induced mTOR activity, independent of PI3K and Akt, is the critical factor for its anti-HCV activity. Jak independent mTOR activity involved STAT-1 phosphorylation and nuclear location, and then PKR is expressed in hepatocytes.

Hepatitis C virus (HCV) employs multiple strategies to subvert the host innate antiviral response

Hepatitis C virus (HCV) is a serious global health problem which accounts for approximately 40% of chronic liver diseases worldwide. HCV frequently establishes a persistent infection, although it is recognized and targeted by innate immunity as well as cellular and humoral immune mechanisms. This suggests that HCV has developed powerful strategies to escape elimination by innate and adaptive immunity. HCV-induced liver injury is thought to be mainly immune-mediated rather than due to direct cytopathic effects of the virus. Hence, therapeutic strategies should target those mechanisms favoring viral persistence since unspecific enhancement of host antiviral immunity may theoretically also promote liver injury. The present review summarizes our current understanding of how the hepatitis C virus interferes with the innate antiviral host-response to establish persistent infection.

Cutting the gordian knot-development and biological relevance of hepatitis C virus cell culture systems

Worldwide approximately 180 million people are chronically infected with hepatitis C virus (HCV). HCV isolates exhibit extensive genetic heterogeneity and have been grouped in six genotypes and various subtypes. Additionally, several naturally occurring intergenotypic recombinants have been described. Research on the viral life cycle, efficient therapeutics, and a vaccine has been hampered by the absence of suitable cell culture systems. The first system permitting studies of the full viral life cycle was intrahepatic transfection of RNA transcripts of HCV consensus complementary DNA (cDNA) clones into chimpanzees. However, such full-length clones were not infectious in vitro. The development of the replicon system and HCV pseudo-particles allowed in vitro studies of certain aspects of the viral life cycle, RNA replication, and viral entry, respectively. Identification of the genotype 2 isolate JFH1, which for unknown reasons showed an exceptional replication capability and resulted in formation of infectious viral particles in the human hepatoma cell line Huh7, led in 2005 to the development of the first full viral life cycle in vitro systems. JFH1-based systems now enable in vitro studies of the function of viral proteins, their interaction with each other and host proteins, new antivirals, and neutralizing antibodies in the context of the full viral life cycle. However, several challenges remain, including development of cell culture systems for all major HCV genotypes and identification of other susceptible cell lines.

Human parvovirus B19 NS1 protein modulates inflammatory signaling by activation of STAT3/PIAS3 in human endothelial cells

The pathogenic mechanism by which parvovirus B19 may induce inflammatory cardiomyopathy (iCMP) is complex but is known to involve inflammatory processes, possibly including activation of JAK/STAT signaling. The nonstructural B19 protein NS1 acts as a transactivator triggering signaling cascades that eventually lead to activation of interleukin 6 (IL-6). We examined the impact of NS1 on modulation of STAT signaling in human endothelial cells (HMEC-1). The NS1 sequences were identified from B19 DNA isolated from the myocardia of patients with fatal iCMP. B19 infection as well as NS1 overexpression in HMEC-1 cells produced a significant upregulation in the phosphorylation of both tyrosine(705) and serine(727) STAT3 (P < 0.05). The increased STAT3 phosphorylation was accompanied by dimerization, nuclear translocation, and DNA binding of pSTAT3. In contrast, NS1 expression did not result in increased STAT1 activation. Notably, the expression levels of the negative regulators of STAT activation, SOCS1 and SOCS3, were not altered by NS1. However, the level of PIAS3 was upregulated in NS1-expressing HMEC-1 cells. Analysis of the transcriptional activation of target genes revealed that NS1-induced STAT3 signaling was associated with upregulation of genes involved in immune response (e.g., the IFNAR1 and IL-2 genes) and downregulation of genes associated with viral defense (e.g., the OAS1 and TYK2 genes). Our results demonstrate that B19 NS1 modulates the STAT/PIAS pathway. The NS1-induced upregulation of STAT3/PIAS3 in the absence of STAT1 phosphorylation and the lack of SOCS1/SOCS3 activation may contribute to the mechanisms by which B19 evades the immune response and establishes persistent infection in human endothelial cells. Thus, NS1 may play a critical role in the mechanism of viral pathogenesis in B19-associated iCMP.

Isolation and gene analysis of interferon alpha-resistant cell clones of the hepatitis C virus subgenome

Hepatitis C virus (HCV) proteins appear to play an important role in IFN-resistance, but the molecular mechanism remains unclear. To clarify the mechanism in HCV replicon RNA harboring Huh-7 cells (Huh-9-13), we isolated cellular clones with impaired IFNalpha-sensitivity. Huh-9-13 was cultured for approximately 2 months in the presence of IFNalpha, and 4 IFNalpha-resistant cell clones showing significant resistances were obtained. When total RNA from clones was introduced into Huh-7 cells, the transfected cells also exhibited IFNalpha-resistance. Although no common mutations were present, mutations in NS3 and NS5A regions were accumulated. Transactivation of IFNalpha and IFNalpha-stimulated Stat-1 phosphorylation were reduced, and the elimination of HCV replicon RNA from the clones restored the IFNalpha signaling. These results suggest that the mutations in the HCV replicon RNA, at least in part, cause an inhibition of IFN signaling and are important for acquisition of IFNalpha resistance in Huh-9-13.

Prevention of cancer through immunization: Prospects and challenges for the 21st century

Persistent infection by several microbial agents is responsible for at least 15% of cancer globally, including most cancers of the liver, stomach, and cervix. The recent development of vaccines that can prevent infection and premalignant disease caused by human papillomaviruses (HPV), which cause virtually all cases of cervical cancer as well as some other cancers, has focused renewed attention on infection control as a means of reducing the global cancer burden. For vaccines to prevent cancer-causing infection with hepatitis C virus, Helicobacter pylori, or Epstein Barr virus, new vaccine technologies to induce more effective protective responses are required. For the two available cancer control vaccines, designed to prevent infection with HPV and hepatitis B virus, the major challenge is to promote effective vaccine deployment through education programs and increased affordability/accessibility for underserved populations, particularly in the developing world, where the cancer burden attributable to infection by these two viruses is greatest.

Human butyrate-induced transcript 1 interacts with hepatitis C virus NS5A and regulates viral replication

Hepatitis C virus (HCV) nonstructural protein 5A (NS5A) is required for the replication of the viral genome and is involved in several host signaling pathways. To gain further insight into the functional role of NS5A in HCV replication, we screened human cDNA libraries by a yeast two-hybrid system using NS5A as the bait and identified human butyrate-induced transcript 1 (hB-ind1) as a novel NS5A-binding protein. Endogenously and exogenously expressed hB-ind1 was coimmunoprecipitated with NS5A of various genotypes through the coiled-coil domain of hB-ind1. The small interfering RNA (siRNA)-mediated knockdown of hB-ind1 in human hepatoma cell lines suppressed the replication of HCV RNA replicons and the production of infectious particles of HCV genotype 2a strain JFH1. Furthermore, these reductions were canceled by the expression of an siRNA-resistant hB-ind1 mutant. Among the NS5A-binding host proteins involved in HCV replication, hB-ind1 exhibited binding with FKBP8, and hB-ind1 interacted with Hsp90 through the FxxW motif in its N-terminal p23 homology domain. The impairment of the replication of HCV RNA replicons and of the production of infectious particles of JFH1 virus in the hB-ind1 knockdown cell lines was not reversed by the expression of an siRNA-resistant hB-ind1 mutant in which the FxxW motif was replaced by AxxA. These results suggest that hB-ind1 plays a crucial role in HCV RNA replication and the propagation of JFH1 virus through interaction with viral and host proteins.

Hepatitis C Virus non-structural 5A abrogates signal transducer and activator of transcription-1 nucleartranslocation induced by IFN-α through dephosphorylation

AIM: To study the effect of Hepatitis C virus non-structural 5A (HCV NS5A) on IFNα induced signal transducer and activator of transcription-1 (STAT1) phosphorylation and nuclear translocation.

METHODS: Expression of STAT1 Tyr701 phosphorylation at different time points was confirmed by Western blot, and the time point when p-STAT1 expressed most, was taken as the IFN induction time for further studies. Immunocytochemistry was used to confirm the successful transient transfection of NS5A expression plasmid. Immunofluorescene was performed to observe if there was any difference in IFNα-induced STAT1 phosphorylation and nuclear translocation between HCV NS5A-expressed and non-HCV NS5A-expressed cells. Western blot was used to compare the phosphorylated STAT1 protein of the cells.

RESULTS: Expression of HCV NS5A was found in the cytoplasm of P^{CNS5A-transfected} Huh7 cells, but not in the PRC/CMV transfected or non-transfected cells. STAT1 Tyr701 phosphorylation was found strongest in 30 min of IFN induction. STAT1 phosphorylation and nuclear import were much less in the presence of HCV NS5A protein in contrast to P^{RC/CMV-transfected} and non-transfected cells under fluorescent microscopy, which was further confirmed by Western blot.

CONCLUSION: HCV NS5A expression plasmid is successfully transfected into Huh7 cells and HCV NS5A protein is expressed in the cytoplasm of the cells. IFN-α is able to induce STAT1 phosphrylation and nuclear translocation, and this effect is inhibited by HCV NS5A protein, which might be another possible resistance mechanism to interferon alpha therapy.