Mechanism of action of ribavirin in a novel hepatitis C virus replication cell system

Broad-spectrum antiviral strategy: Host-targeting antivirals against emerging and re-emerging viruses

Viral infections are amongst the most prevalent diseases that pose a significant threat to human health. Targeting viral proteins or host factors represents two primary strategies for the development of antiviral drugs. In contrast to virus-targeting antivirals (VTAs), host-targeting antivirals (HTAs) offer advantages in terms of overcoming drug resistance and effectively combating a wide range of viruses, including newly emerging ones. Therefore, targeting host factors emerges as an extremely promising strategy with the potential to address critical challenges faced by VTAs. In recent years, extensive research has been conducted on the discovery and development of HTAs, leading to the approval of maraviroc, a chemokine receptor type 5 (CCR5) antagonist used for the treatment of HIV-1 infected individuals, with several other potential treatments in various stages of development for different viral infections. This review systematically summarizes advancements made in medicinal chemistry regarding various host targets and classifies them into four distinct catagories based on their involvement in the viral life cycle: virus attachment and entry, biosynthesis, nuclear import and export, and viral release.

Controversies’ clarification regarding ribavirin efficacy in measles and coronaviruses: Comprehensive therapeutic approach strictly tailored to COVID-19 disease stages

BACKGROUND

Ribavirin is a broad-spectrum nucleoside antiviral drug with multimodal mechanisms of action, which supports its longevity and quality as a clinical resource. It has been widely administered for measles and coronavirus infections. Despite the large amount of data concerning the use of ribavirin alone or in combination for measles, severe acute respiratory syndrome, Middle East respiratory syndrome, and coronavirus disease 2019 (COVID-19) outbreaks, the conclusions of these studies have been contradictory. Underlying reasons for these discrepancies include possible study design inaccuracies and failures and misinterpretations of data, and these potential confounds should be addressed.

AIM

To determine the confounding factors of ribavirin treatment studies and propose a therapeutic scheme for COVID-19.

METHODS

PubMed database was searched over a period of five decades utilizing the terms “ribavirin” alone or combined with other compounds in measles, severe acute respiratory syndrome, Middle East respiratory syndrome, and COVID-19 infections. The literature search was performed and described according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Articles were considered eligible when they reported on ribavirin dose regimens and/or specified outcomes concerning its efficacy and/or possible adverse-effects. In vitro and animal studies were also retrieved. A chapter on ribavirin’s pharmacology was included as well.

RESULTS

In addition to the difficulties and pressures of an emerging pandemic, there is the burden of designing and conducting well-organized, double-blind, randomized controlled trials. Many studies have succumbed to specific pitfalls, one of which was identified in naturally ribavirin-resistant Vero cell lines in in vitro studies. Other pitfalls include study design inconsistent with the well-established clinical course of disease; inappropriate pharmacology of applied treatments; and the misinterpretation of study results with misconceived generalizations. A comprehensive treatment for COVID-19 is proposed, documented by thorough, long-term investigation of ribavirin regimens in coronavirus infections.

CONCLUSION

A comprehensive treatment strictly tailored to distinct disease stages was proposed based upon studies on ribavirin and coronavirus infections.

Identification of ribavirin-responsive cis-elements for GPAM suppression in the GPAM genome

Glycerol-3-phosphate acyltransferase, mitochondrial (GPAM) is a rate-limiting enzyme catalyzing triglyceride synthesis. Recently, we demonstrated that the anti-viral drug ribavirin (RBV) reduces GPAM expression by downregulating CCAAT/enhancer-binding protein α (C/EBPα). However, the precise mechanisms of GPAM suppression have remained unclear. Here, we found that RBV suppressed GPAM expression by downregulating not only C/EBPα, but also sterol regulatory element-binding protein-1c (SREBP-1c). We also found that cis-elements regulated by C/EBPα and SREBP-1c functioned as distal and proximal enhancers, respectively, to express hepatocyte- and adipocytes-specific GPAM variants. These results imply that RBV disrupts formation of the enhancer machineries on the GPAM genome by downregulating both transcription factors. Our findings may contribute to the development of treatments for fatty liver diseases caused by aberrant triglyceride synthesis.

Mutations Identified in the Hepatitis C Virus (HCV) Polymerase of Patients with Chronic HCV Treated with Ribavirin Cause Resistance and Affect Viral Replication Fidelity

Ribavirin has been used for 25 years to treat patients with chronic hepatitis C virus (HCV) infection; however, its antiviral mechanism of action remains unclear. Here, we studied virus evolution in a subset of samples from a randomized 24-week trial of ribavirin monotherapy versus placebo in chronic HCV patients, as well as the viral resistance mechanisms of the observed ribavirin-associated mutations in cell culture. Thus, we performed next-generation sequencing of the full-length coding sequences of HCV recovered from patients at weeks 0, 12, 20, 32 and 40 and analyzed novel single nucleotide polymorphisms (SNPs), diversity, and mutation-linkage. At week 20, increased genetic diversity was observed in 5 ribavirin-treated compared to 4 placebo-treated HCV patients due to new synonymous SNPs, particularly G-to-A and C-to-U ribavirin-associated transitions. Moreover, emergence of 14 nonsynonymous SNPs in HCV nonstructural 5B (NS5B) occurred in treated patients, but not in placebo controls. Most substitutions located close to the NS5B polymerase nucleotide entry site. Linkage analysis showed that putative resistance mutations were found in the majority of genomes in ribavirin-treated patients. Identified NS5B mutations from genotype 3a patients were further introduced into the genotype 3a cell-culture-adapted DBN strain for studies in Huh7.5 cells. Specific NS5B substitutions, including DBN-D148N+I363V, DBN-A150V+I363V, and DBN-T227S+S183P, conferred resistance to ribavirin in long-term cell culture treatment, possibly by reducing the HCV polymerase error rate. In conclusion, prolonged exposure of HCV to ribavirin in chronic hepatitis C patients induces NS5B resistance mutations leading to increased polymerase fidelity, which could be one mechanism for ribavirin resistance.

A new hepatoma cell line exhibiting high susceptibility to hepatitis B virus infection

Hepatitis B virus (HBV) infection, which increases the risk of cirrhosis and hepatocellular carcinoma and requires lifelong treatment, has become a major global health problem. However, host factors essential to the HBV life cycle are still unclear, and the development of new drugs is needed. Cells derived from the human hepatoma cell line HepG2 and engineered to overexpress sodium taurocholate cotransporting polypeptide (NTCP: a receptor for HBV), termed HepG2/NTCP cells, are widely used as the cell-based HBV infection and replication systems for HBV research. We recently found that human hepatoma cell line Li23-derived cells overexpressing NTCP (A8 cells subcloned from Li23 cells), whose gene expression profile was distinct from that of HepG2/NTCP cells, were also sensitive to HBV infection. However, the HBV susceptibility of A8 cells was around 1/100 that of HepG2/NTCP cells. Since we considered that plural cell assay systems will be needed for the objective evaluation of anti-HBV reagents, as we previously demonstrated in hepatitis C virus research, we here attempted to develop a new Li23 cell-derived assay system equivalent to that using HepG2/NTCP cells. By repeated subcloning of A8 cells, we successfully established a new cell line (A8.15.78.10) exhibiting high HBV susceptibility equal to that of HepG2/NTCP cells. Characterization of A8.15.78.10 cells revealed that the increase of HBV susceptibility was correlated with increases in the protein and glycosylation levels of NTCP, and with decreased expression of STING, a factor contributing to innate immunity. Finally, we performed a comparative evaluation of HBV entry inhibitors (cyclosporin A and rosiglitazone) by an HBV/secNL reporter assay using A8.15.78.10 cells or HepG2/NTCP cells. The results confirmed that cyclosporin A exhibited anti-HBV activity in both cell lines, as previously reported. However, we found that rosiglitazone did not show the anti-HBV activity in A8.15.78.10 cells, although it worked in HepG2/NTCP cells as previously reported. This suggested that the difference in anti-HBV activity between cyclosporin A and rosiglitazone was due to the different types of cells used for the assay. In conclusion, plural assay systems using different types of cells are required for the objective and impartial evaluation of anti-HBV reagents.

Ribavirin-induced down-regulation of CCAAT/enhancer-binding protein α leads to suppression of lipogenesis

Recently, we demonstrated that the anti-viral drug ribavirin (RBV) had the ability to suppress lipogenesis through down-regulation of retinoid X receptor α (RXRα) under the control of the intracellular GTP-level and AMP-activated protein kinase-related kinases, especially microtubule affinity regulating kinase 4 (MARK4). RXRα-overexpression attenuated but did not abolish lipogenesis suppression by RBV, implying that additional factor(s) were involved in this suppressive effect. In the present study, we found that the protein level, but not the mRNA level, of CCAAT/enhancer-binding protein α (C/EBPα) was down-regulated by RBV in hepatic cells. Treatment with proteasome inhibitor attenuated RBV-induced down-regulation of C/EBPα, suggesting that RBV promoted degradation of C/EBPα protein via the ubiquitin-proteasome pathway. Depletion of intracellular GTP through inosine monophosphate dehydrogenase inhibition by RBV led to down-regulation of C/EBPα. In contrast, down-regulation of C/EBPα by RBV was independent of RXRα and MARK4. Knockdown of C/EBPα reduced the intracellular neutral lipid levels and the expression of genes related to the triglyceride (TG) synthesis pathway, especially glycerol-3-phosphate acyltransferase, mitochondrial (GPAM), which encodes the first rate-limiting TG enzyme. Overexpression of C/EBPα yielded the opposite results. We also observed that RBV decreased GPAM expression. Moreover, overexpression of GPAM attenuated RBV-induced reduction in the intracellular neutral lipid levels. These data suggest that down-regulation of C/EBPα by RBV leads to the reduction in GPAM expression, which contributes to the suppression of lipogenesis. Our findings about the mechanism of RBV action in lipogenesis suppression will provide new insights for therapy against the active lipogenesis involved in hepatic steatosis and hepatocellular carcinomas.

Multiple antiviral activities of the antimalarial and anti-hepatitis C drug candidates N-89 and N-251

The chemically synthesized endoperoxide compound N-89 and its derivative N-251 were shown to have potent antimalarial activity. We previously demonstrated that N-89 and N-251 potently inhibited the RNA replication of hepatitis C virus (HCV), which belongs to the Flaviviridae family. Since antimalarial and anti-HCV mechanisms have not been clarified, we were interested whether N-89 and N-251 possessed the activity against viruses other than HCV. In this study, we examined the effects of N-89 and N-251 on other flaviviruses (dengue virus and Japanese encephalitis virus) and hepatitis viruses (hepatitis B virus and hepatitis E virus). Our findings revealed that N-89 and N-251 moderately inhibited the RNA replication of Japanese encephalitis virus and hepatitis E virus, although we could not detect those anti-dengue virus activities. We also observed that N-89 and N-251 moderately inhibited the replication of hepatitis B virus at the step after viral translation. These results suggest the possibility that N-89 and N-251 act on some common host factor(s) that are necessary for viral replications, rather than the possibility that N-89 and N-251 directly act on the viral proteins except for HCV. We describe a new type of antiviral reagents, N-89 and N-251, which are applicable to multiple different viruses.

Antiviral Effect of Ribavirin against HCV Associated with Increased Frequency of G-to-A and C-to-U Transitions in Infectious Cell Culture Model

Ribavirin (RBV) is a broad-spectrum antiviral active against a wide range of RNA viruses. Despite having been used for decades in the treatment of chronic hepatitis C virus (HCV) infection, the precise mechanism of action of RBV is unknown. In other viruses, it inhibits propagation by increasing the rate of G-to-A and C-to-U transitions. Here, we utilized the J6/JFH1 HCV cell-culture system to investigate whether RBV inhibits HCV through the same mechanism. Infected Huh7.5 cells were treated with increasing concentrations of RBV or its phosphorylated forms. A fragment of the HCV NS5B-polymerase gene was amplified, cloned, and sequenced to estimate genetic distances. We confirm that the antiviral effect of all three RBV-drug forms on HCV relies on induction of specific transitions (G-to-A and C-to-U). These mutations lead to generation of non-infectious virions, reflected by decreased spread of HCV in cell culture despite relatively limited effect on virus genome titers. Moreover, treatment experiments conducted on a novel Huh7.5 cell line stably overexpressing adenosine kinase, a key enzyme for RBV activation, yielded comparable results. This study indicates that RBV action on HCV in hepatoma cell-culture is exerted through increase in mutagenesis, mediated by RBV triphosphate, and leading to production of non-infectious viruses.

Antiviral Effect of Ribavirin against HCV Associated with Increased Frequency of G-to-A and C-to-U Transitions in Infectious Cell Culture Model

Ribavirin (RBV) is a broad-spectrum antiviral active against a wide range of RNA viruses. Despite having been used for decades in the treatment of chronic hepatitis C virus (HCV) infection, the precise mechanism of action of RBV is unknown. In other viruses, it inhibits propagation by increasing the rate of G-to-A and C-to-U transitions. Here, we utilized the J6/JFH1 HCV cell-culture system to investigate whether RBV inhibits HCV through the same mechanism. Infected Huh7.5 cells were treated with increasing concentrations of RBV or its phosphorylated forms. A fragment of the HCV NS5B-polymerase gene was amplified, cloned, and sequenced to estimate genetic distances. We confirm that the antiviral effect of all three RBV-drug forms on HCV relies on induction of specific transitions (G-to-A and C-to-U). These mutations lead to generation of non-infectious virions, reflected by decreased spread of HCV in cell culture despite relatively limited effect on virus genome titers. Moreover, treatment experiments conducted on a novel Huh7.5 cell line stably overexpressing adenosine kinase, a key enzyme for RBV activation, yielded comparable results. This study indicates that RBV action on HCV in hepatoma cell-culture is exerted through increase in mutagenesis, mediated by RBV triphosphate, and leading to production of non-infectious viruses.

Ribavirin suppresses hepatic lipogenesis through inosine monophosphate dehydrogenase inhibition: Involvement of adenosine monophosphate-activated protein kinase-related kinases and retinoid X receptor α

Ribavirin (RBV) has been widely used as an antiviral reagent, specifically for patients with chronic hepatitis C. We previously demonstrated that adenosine kinase, which monophosphorylates RBV into the metabolically active form, is a key determinant for RBV sensitivity against hepatitis C virus RNA replication. However, the precise mechanism of RBV action and whether RBV affects cellular metabolism remain unclear. Analysis of liver gene expression profiles obtained from patients with advanced chronic hepatitis C treated with the combination of pegylated interferon and RBV showed that the adenosine kinase expression level tends to be lower in patients who are overweight and significantly decreases with progression to advanced fibrosis stages. In our effort to investigate whether RBV affects cellular metabolism, we found that RBV treatment under clinically achievable concentrations suppressed lipogenesis in hepatic cells. In this process, guanosine triphosphate depletion through inosine monophosphate dehydrogenase inhibition by RBV and adenosine monophosphate-activated protein kinase-related kinases, especially microtubule affinity regulating kinase 4, were required. In addition, RBV treatment led to the down-regulation of retinoid X receptor α (RXRα), a key nuclear receptor in various metabolic processes, including lipogenesis. Moreover, we found that guanosine triphosphate depletion in cells induced the down-regulation of RXRα, which was mediated by microtubule affinity regulating kinase 4. Overexpression of RXRα attenuated the RBV action for suppression of lipogenic genes and intracellular neutral lipids, suggesting that down-regulation of RXRα was required for the suppression of lipogenesis in RBV action. Conclusion: We provide novel insights about RBV action in lipogenesis and its mechanisms involving inosine monophosphate dehydrogenase inhibition, adenosine monophosphate-activated protein kinase-related kinases, and down-regulation of RXRα. RBV may be a potential reagent for anticancer therapy against the active lipogenesis involved in hepatocarcinogenesis. (Hepatology Communications 2017;1:550-563).

Evaluation of preclinical antimalarial drugs, which can overcome direct-acting antivirals-resistant hepatitis C viruses, using the viral reporter assay systems

Persistent hepatitis C virus (HCV) infection causes chronic liver diseases and is a major global health problem. Recently developed treatments with direct-acting antivirals (DAAs) have largely improved the sustained virologic response rate of patients with chronic hepatitis C. However, this approach is still hindered by its great expense and the problem of drug resistance. Using our cell-based HCV assay systems, we reported that the preclinical antimalarial drugs N-89 and N-251 exhibited potent anti-HCV activities. In this study we used our assay systems to evaluate the anti-HCV activities of six kinds of DAAs individually or in combination with N-89 or N-251. The results showed that the DAAs had potent anti-HCV activities and N-89 or N-251 contributed additive or synergistic effect. Using DAA-resistant HCV-RNA-replicating cells, which were prepared by continuous treatment with each DAA, we demonstrated that N-89 and N-251 could overcome all of the DAA-resistant HCVs. These preclinical drugs would have been potential as components of a therapeutic regimen that also included combinations of various DAAs. In addition, sequence analysis of the NS3-NS5B regions of the DAA-resistant HCV genomes newly found several amino acid (aa) substitutions that were suggested to contribute to DAA-resistance in addition to the aa substitutions already known to cause DAA-resistance. Among these new aa substitutions, we found that two substitutions in the NS3 region (D79G and S174Y) contributed to simeprevir- and/or asunaprevir-resistance.

A novel benzo-heterocyclic amine derivative N30 inhibits influenza virus replication by depression of Inosine-5'-Monophospate Dehydrogenase activity

BACKGROUD

Influenza virus is still a huge threat to the world-wide public health. Host inosine-5'- monophosphate dehydrogenase (IMPDH) involved in the synthesis of guanine nucleotides, is known to be a potential target to inhibit the replication of viruses. Herein, we evaluated antiviral activity of a benzo-heterocyclic amine derivative N30, which was designed to inhibit IMPDH.

RESULTS

The results demonstrated that N30 inhibited the replication of H1N1, H3N2, influenza B viruses, including oseltamivir and amantadine resistant strains in vitro. Mechanistically, neuraminidase inhibition assay and hemagglutination inhibition assay suggested that N30 did not directly target the two envelope glycoproteins required for viral adsorption or release. Instead, the compound could depress the activity of IMPDH type II. Based on these findings, we further confirmed that N30 provided a strong inhibition on the replication of respiratory syncytial virus, coronavirus, enterovirus 71 and a diverse strains of coxsackie B virus.

CONCLUSIONS

We identified the small molecule N30, as an inhibitor of IMPDH, might be a potential candidate to inhibit the replication of various viruses.

Suppression of autophagy by mycophenolic acid contributes to inhibition of HCV replication in human hepatoma cells

Previous studies have shown that mycophenolic acid (MPA) has an anti-HCV activity. However, the mechanism of MPA-mediated inhibition of HCV replication remains to be determined. This study investigated whether MPA has an effect on autophagy, a cellular machinery required for HCV replication, thereby, inhibits HCV replication in Huh7 cells. MPA treatment of Huh7 cells could suppress autophagy, evidenced by decreased LC3B-II level and conversion of LC3B-I to LC3B-II, decreased autophagosome formation, and increased p62 level compared to MPA-untreated cells. Tunicamycin treatment or HCV infection could induce cellular autophagy, however, MPA also exhibited its inhibitory effect on tunicamycin- or HCV infection-induced autophagy. The expression of three autophagy-related genes, Atg3, Atg5, and Atg7 were identified to be inhibited by MPA treatment. Over-expression of these genes could partly recover HCV replication inhibited by MPA; however, silencing their expression by siRNAs could enhance the inhibitory effect of MPA on HCV. Collectively, these results reveal that suppression of autophagy by MPA plays a role in its anti-HCV activity. Down-regulating the expression of three autophagy-related genes by MPA involves in its antiviral mechanism.

Effects of Resistance-Associated NS5A Mutations in Hepatitis C Virus on Viral Production and Susceptibility to Antiviral Reagents

Direct-acting antivirals (DAAs) for hepatitis C virus (HCV) have potent anti-HCV effects but may provoke resistance-associated variants (RAVs). In this study, we assessed the characteristics of these RAVs and explored efficacious anti-HCV reagents using recombinant HCV with NS5A from a genotype 1b strain. We replaced the NS5A of JFH1 with that of Con1 (JFH1/5ACon1) and introduced known NS5A inhibitor resistance mutations (L31M, L31V, L31I and Y93H) individually or in combination. Susceptibilities against anti-HCV reagents were also investigated. RAVs with Y93H exhibited high extracellular core antigen levels and infectivity titers. Variants with any single mutation showed mild to moderate resistance against NS5A inhibitors, whereas variants with double mutations at both L31 and Y93 showed severe resistance. The variants with mutations exhibited similar levels of susceptibility to interferon (IFN)-α, IFN-λ1, IFN-λ3 and Ribavirin. Variants with the Y93H mutation were more sensitive to protease inhibitors compared with JFH1/5ACon1. In conclusion, the in vitro analysis indicated that the Y93H mutation enhanced infectious virus production, suggesting advantages in the propagation of RAVs with this mutation. However, these RAVs were susceptible to protease inhibitors. Thus, a therapeutic regimen that includes these reagents is a promising means to eradicate these RAVs.

Synergistic antiviral activity of ribavirin and interferon-α against parrot bornaviruses in avian cells

Avian bornaviruses are the causative agents of proventricular dilatation disease (PDD), a widely distributed and often fatal disease in captive psittacines. Because neither specific prevention measures nor therapies against PDD and bornavirus infections are currently available, new antiviral strategies are required to improve animal health. We show here that the nucleoside analogue ribavirin inhibited bornavirus activity in a polymerase reconstitution assay and reduced viral load in avian cell lines infected with two different parrot bornaviruses. Furthermore, we observed that ribavirin enhanced type I IFN signalling in avian cells. Combined treatment of avian bornavirus-infected cells with ribavirin and recombinant IFN-α strongly enhanced the antiviral efficiency compared to either drug alone. The combined use of ribavirin and type I IFN might represent a promising new strategy for therapeutic treatment of captive parrots persistently infected with avian bornaviruses.

Trends in Antiviral Strategies

Viral populations are true moving targets regarding the genomic sequences to be targeted in antiviral designs. Experts from different fields have expressed the need of new paradigms for antiviral interventions and viral disease control. This chapter reviews several strategies that aim at counteracting the adaptive capacity of viral quasispecies. The proposed designs are based on combinations of different antiviral drugs and immune modulators, or in the administration of virus-specific mutagenic agents, in an approach termed lethal mutagenesis of viruses. It consists of decreasing viral fitness by an excess of mutations that render viral proteins sub-optimal or non-functional. Viral extinction by lethal mutagenesis involves several sequential, overlapping steps that recapitulate the major concepts of intra-population interactions and genetic information stability discussed in preceding chapters. Despite the magnitude of the challenge, the chapter closes with some optimistic prospects for an effective control of viruses displaying error-prone replication, based on the combined targeting of replication fidelity and the induction of the innate immune response.

Ribavirin contributes to eradicate hepatitis C virus through polarization of T helper 1/2 cell balance into T helper 1 dominance

The mechanism of action of ribavirin (RBV) as an immunomodulatory and antiviral agent and its clinical significance in the future treatment of patients with hepatitis C virus (HCV) infection are reviewed. RBV up-regulates type 1 and/or 2 cytokines to modulate the T helper (Th) 1/2 cell balance to Th1 dominance. Examination of co-stimulatory signaling indicated that RBV down-modulates inducible co-stimulator on Th cells, which contributes to differentiating naïve Th cells into Th2 cells while reducing their interleukin-10 production. The effects on T-regulatory (Treg) cells were also investigated, and RBV inhibited the differentiation of naïve Th cells into adaptive Treg cells by down-modulating forkhead box-P3. These findings indicate that RBV mainly down-regulates the activity of Th2 cells, resulting in the maintenance of Th1 activity that contributes to abrogating HCV-infected hepatocytes. Although an interferon-free treatment regimen exhibits almost the same efficacy without serious complications, regimens with RBV will be still be used because of their ability to facilitate the cellular immune response, which may contribute to reducing the development of hepatocellular carcinogenesis in patients infected with HCV.

Ribavirin Inhibits Parrot Bornavirus 4 Replication in Cell Culture

Parrot bornavirus 4 is an etiological agent of proventricular dilatation disease, a fatal neurologic and gastrointestinal disease of psittacines and other birds. We tested the ability of ribavirin, an antiviral nucleoside analog with antiviral activity against a range of RNA and DNA viruses, to inhibit parrot bornavirus 4 replication in duck embryonic fibroblast cells. Two analytical methods that evaluate different products of viral replication, indirect immunocytochemistry for viral specific nucleoprotein and qRT-PCR for viral specific phosphoprotein gene mRNA, were used. Ribavirin at concentrations between 2.5 and 25 μg/mL inhibited parrot bornavirus 4 replication, decreasing viral mRNA and viral protein load, in infected duck embryonic fibroblast cells. The addition of guanosine diminished the antiviral activity of ribavirin suggesting that one possible mechanism of action against parrot bornavirus 4 may likely be through inosine monophosphate dehydrogenase inhibition. This study demonstrates parrot bornavirus 4 susceptibility to ribavirin in cell culture.

Establishment of hepatitis C virus RNA-replicating cell lines possessing ribavirin-resistant phenotype

BACKGROUND

Ribavirin (RBV) is a potential partner of interferon-based therapy and recently approved therapy using direct acting antivirals for patients with chronic hepatitis C. However, the precise mechanisms underlying RBV action against hepatitis C virus (HCV) replication are not yet understood. To clarify this point, we attempted to develop RBV-resistant cells from RBV-sensitive HCV RNA-replicating cells.

METHODOLOGY/PRINCIPAL FINDINGS

By repetitive RBV (100 μM) treatment (10 weeks) of 3.5-year-cultured OL8 cells, in which genome-length HCV RNA (O strain of genotype 1b) efficiently replicates, dozens of colonies that survived RBV treatment were obtained. These colonies were mixed together and further treated with high doses of RBV (up to 200 μM). By such RBV treatment, we successfully established 12 RBV-survived genome-length HCV RNA-replicating cell lines. Among them, three representative cell lines were characterized. HCV RNA replication in these cells resisted RBV significantly more than that in the parental OL8 cells. Genetic analysis of HCV found several common and conserved amino acid substitutions in HCV proteins among the three RBV-resistant cell species. Furthermore, using cDNA microarray and quantitative RT-PCR analyses, we identified 5 host genes whose expression levels were commonly altered by more than four-fold among these RBV-resistant cells compared with the parental cells. Moreover, to determine whether viral or host factor contributes to RBV resistance, we developed newly HCV RNA-replicating cells by introducing total RNAs isolated from RBV-sensitive parental cells or RBV-resistant cells into the HCV RNA-cured-parental or -RBV-resistant cells using an electroporation method, and evaluated the degrees of RBV resistance of these developed cells. Consequently, we found that RBV-resistant phenotype was conferred mainly by host factor and partially by viral factor.

CONCLUSIONS/SIGNIFICANCE

These newly established HCV RNA-replicating cell lines should become useful tools for further understanding the anti-HCV mechanisms of RBV.

Induction of cytoplasmic pattern in the form of "rods and rings" through the treatment of hepatitis C: a case report

Female patient, complaining of weakness and pain in hypogastric, was admitted to the emergency department of the University Hospital of the West of Paraná (HUOP). During the interview reported treatment of chronic infection with hepatitis C virus (HCV) with peginterferon and ribavirin. Among the laboratory tests ordered, the search for self-antibodies against cellular antigens, traditionally known as antinuclear factor, showed fluorescence shaped like rods and/or rings in the cytoplasm of cells. This study attempts to clarify the relationship between this pattern not yet completely understood and the clinical picture of the patient. This pattern is characterized by 3-10 μm rods or rings with 2-5 μm in diameter scattered throughout the cytoplasm of the cell. Therefore, this new standard has been designated as "rods and rings" (RR). The antigenic target of this reaction was identified as inosine-5'-monophosphate dehydrogenase type 2 (IMPDH2) which is a key enzyme in the synthesis of purine nucleotides. The IMPDH2 enzyme aggregated or modified shaped RR in those patients treated with ribavirin may become antigenic and induce an autoimmune response. It is possible that interferon alpha stimulates the occurrence of anti-RR reactivity apparently induced by ribavirin. So far it is not known why the standard RR in HEp2 cells occurs only in a fraction of patients with HCV. Previous studies presented in this paper allow affirming that these antibodies associated with the standard RR are strongly related to hepatitis C. Moreover, it can be stated that the occurrence of anti-RR reactivity is promoted by combination therapy with interferon and ribavirin.

Ribavirin enhances myeloid-derived suppressor cell differentiation through CXCL9/10 downregulation

Elevation of myeloid-derived suppressor cells (MDSCs) was observed in some viral infectious diseases. In this study, we studied whether ribavirin, a widely used clinical antiviral drug, could impact the differentiation of human MDSCs in vitro. Flow cytometric analysis showed that ribavirin treatment (5-20 µg/ml) significantly enhanced the differentiation of monocytic MDSCs in a dose-dependent manner. The ribavirin-generated MDSCs were immune-suppressive toward autologous T cells. The mRNA expression of some cytokines was further examined by quantitative reverse transcription polymerase chain reaction. We observed a significant down-regulation of chemokine (C-X-C motif) ligand 9 (CXCL9) and CXCL10 mRNA in ribavirin-generated MDSCs, when compared with control. Peripheral blood mononuclear cells from clinical chronic hepatitis C patients subjected to ribavirin therapy also displayed a similar suppression in CXCL9/10 mRNA expression. Administration of recombinant CXCL9/10 proteins clearly counteracted the effect of ribavirin on MDSCs. In summary, this study showed that ribavirin enhanced human MDSCs differentiation in vitro, which may be attribute to the down-regulation of CXCL9/10 expression.

Ribavirin: a drug active against many viruses with multiple effects on virus replication and propagation. Molecular basis of ribavirin resistance

Ribavirin has proven to be effective against several viruses in the clinical setting and a multitude of viruses in vitro. With up to five different proposed mechanisms of action, recent advances have begun to discern the hierarchy of antiviral effects at play depending on the virus and the host conditions under scrutiny. Studies reveal that for many viruses, antiviral mechanisms may differ depending on cell type in vitro and in vivo. Further analyses are thus required to accurately identify mechanisms to more optimally determine clinical treatments. In recent years, a growing number of ribavirin resistant and sensitive variants have been identified. These variants not only inform on the specific mechanisms by which ribavirin enfeebles the virus, but also can themselves be tools to identify new antiviral compounds.

Genetic characterization of hepatitis C virus in long-term RNA replication using Li23 cell culture systems

BACKGROUND

The most distinguishing genetic feature of hepatitis C virus (HCV) is its remarkable diversity and variation. To understand this feature, we previously performed genetic analysis of HCV in the long-term culture of human hepatoma HuH-7-derived HCV RNA-replicating cell lines. On the other hand, we newly established HCV RNA-replicating cell lines using human hepatoma Li23 cells, which were distinct from HuH-7 cells.

METHODOLOGY/PRINCIPAL FINDINGS

Li23-derived HCV RNA-replicating cells were cultured for 4 years. We performed genetic analysis of HCVs recovered from these cells at 0, 2, and 4 years in culture. Most analysis was performed in two separate parts: one part covered from the 5'-terminus to NS2, which is mostly nonessential for RNA replication, and the other part covered from NS3 to NS5B, which is essential for RNA replication. Genetic mutations in both regions accumulated in a time-dependent manner, and the mutation rates in the 5'-terminus-NS2 and NS3-NS5B regions were 4.0-9.0×10(-3) and 2.7-4.0×10(-3) base substitutions/site/year, respectively. These results suggest that the variation in the NS3-NS5B regions is affected by the pressure of RNA replication. Several in-frame deletions (3-105 nucleotides) were detected in the structural regions of HCV RNAs obtained from 2-year or 4-year cultured cells. Phylogenetic tree analyses clearly showed that the genetic diversity of HCV was expanded in a time-dependent manner. The GC content of HCV RNA was significantly increased in a time-dependent manner, as previously observed in HuH-7-derived cell systems. This phenomenon was partially due to the alterations in codon usages for codon optimization in human cells. Furthermore, we demonstrated that these long-term cultured cells were useful as a source for the selection of HCV clones showing resistance to anti-HCV agents.

CONCLUSIONS/SIGNIFICANCE

Long-term cultured HCV RNA-replicating cells are useful for the analysis of evolutionary dynamics and variations of HCV and for drug-resistance analysis.

Novel permissive cell lines for complete propagation of hepatitis C virus

Hepatitis C virus (HCV) is a major etiologic agent of chronic liver diseases. Although the HCV life cycle has been clarified by studying laboratory strains of HCV derived from the genotype 2a JFH-1 strain (cell culture-adapted HCV [HCVcc]), the mechanisms of particle formation have not been elucidated. Recently, we showed that exogenous expression of a liver-specific microRNA, miR-122, in nonhepatic cell lines facilitates efficient replication but not particle production of HCVcc, suggesting that liver-specific host factors are required for infectious particle formation. In this study, we screened human cancer cell lines for expression of the liver-specific α-fetoprotein by using a cDNA array database and identified liver-derived JHH-4 cells and stomach-derived FU97 cells, which express liver-specific host factors comparable to Huh7 cells. These cell lines permit not only replication of HCV RNA but also particle formation upon infection with HCVcc, suggesting that hepatic differentiation participates in the expression of liver-specific host factors required for HCV propagation. HCV inhibitors targeting host and viral factors exhibited different antiviral efficacies between Huh7 and FU97 cells. Furthermore, FU97 cells exhibited higher susceptibility for propagation of HCVcc derived from the JFH-2 strain than Huh7 cells. These results suggest that hepatic differentiation participates in the expression of liver-specific host factors required for complete propagation of HCV.

IMPORTANCE

Previous studies have shown that liver-specific host factors are required for efficient replication of HCV RNA and formation of infectious particles. In this study, we screened human cancer cell lines for expression of the liver-specific α-fetoprotein by using a cDNA array database and identified novel permissive cell lines for complete propagation of HCVcc without any artificial manipulation. In particular, gastric cancer-derived FU97 cells exhibited a much higher susceptibility to HCVcc/JFH-2 infection than observed in Huh7 cells, suggesting that FU97 cells would be useful for further investigation of the HCV life cycle, as well as the development of therapeutic agents for chronic hepatitis C.

Adenosine kinase is a key determinant for the anti-HCV activity of ribavirin

Ribavirin (RBV) is often used in conjunction with interferon-based therapy for patients with chronic hepatitis C. There is a drastic difference in the anti-hepatitis C virus (HCV) activity of RBV between the HuH-7-derived assay system, OR6, possessing the RBV-resistant phenotype (50% effective concentration [EC50 ]: >100 µM) and the recently discovered Li23-derived assay system, ORL8, possessing the RBV-sensitive phenotype (EC50 : 8 µM; clinically achievable concentration). This is because the anti-HCV activity of RBV was mediated by the inhibition of inosine monophosphate dehydrogenase in RBV-sensitive ORL8 cells harboring HCV RNA. By means of comparative analyses using RBV-resistant OR6 cells and RBV-sensitive ORL8 cells, we tried to identify host factor(s) determining the anti-HCV activity of RBV. We found that the expression of adenosine kinase (ADK) in ORL8 cells was significantly higher than that in RBV-resistant OR6 cells harboring HCV RNA. Ectopic ADK expression in OR6 cells converted them from an RBV-resistant to an RBV-sensitive phenotype, and inhibition of ADK abolished the activity of RBV. We showed that the differential ADK expression between ORL8 and OR6 cells was not the result of genetic polymorphisms in the ADK gene promoter region and was not mediated by a microRNA control mechanism. We found that the 5' untranslated region (UTR) of ADK messenger RNA in ORL8 cells was longer than that in OR6 cells, and that only a long 5' UTR possessed internal ribosome entry site (IRES) activity. Finally, we demonstrated that the long 5' UTR functioned as an IRES in primary human hepatocytes.

CONCLUSION

These results indicate that ADK acts as a determinant for the activity of RBV and provide new insight into the molecular mechanism underlying differential drug sensitivity.

New preclinical antimalarial drugs potently inhibit hepatitis C virus genotype 1b RNA replication

BACKGROUND

Persistent hepatitis C virus (HCV) infection causes chronic liver diseases and is a global health problem. Although new triple therapy (pegylated-interferon, ribavirin, and telaprevir/boceprevir) has recently been started and is expected to achieve a sustained virologic response of more than 70% in HCV genotype 1 patients, there are several problems to be resolved, including skin rash/ageusia and advanced anemia. Thus a new type of anti-HCV drug is still needed.

METHODOLOGY/PRINCIPAL FINDINGS

Recently developed HCV drug assay systems using HCV-RNA-replicating cells (e.g., HuH-7-derived OR6 and Li23-derived ORL8) were used to evaluate the anti-HCV activity of drug candidates. During the course of the evaluation of anti-HCV candidates, we unexpectedly found that two preclinical antimalarial drugs (N-89 and its derivative N-251) showed potent anti-HCV activities at tens of nanomolar concentrations irrespective of the cell lines and HCV strains of genotype 1b. We confirmed that replication of authentic HCV-RNA was inhibited by these drugs. Interestingly, however, this anti-HCV activity did not work for JFH-1 strain of genotype 2a. We demonstrated that HCV-RNA-replicating cells were cured by treatment with only N-89. A comparative time course assay using N-89 and interferon-α demonstrated that N-89-treated ORL8 cells had more rapid anti-HCV kinetics than did interferon-α-treated cells. This anti-HCV activity was largely canceled by vitamin E. In combination with interferon-α and/or ribavirin, N-89 or N-251 exhibited a synergistic inhibitory effect.

CONCLUSIONS/SIGNIFICANCE

We found that the preclinical antimalarial drugs N-89 and N-251 exhibited very fast and potent anti-HCV activities using cell-based HCV-RNA-replication assay systems. N-89 and N-251 may be useful as a new type of anti-HCV reagents when used singly or in combination with interferon and/or ribavirin.

Interferon and ribavirin combination treatment synergistically inhibit HCV internal ribosome entry site mediated translation at the level of polyribosome formation

PURPOSE

Although chronic hepatitis C virus (HCV) infection has been treated with the combination of interferon alpha (IFN-α) and ribavirin (RBV) for over a decade, the mechanism of antiviral synergy is not well understood. We aimed to determine the synergistic antiviral mechanisms of IFN-α and RBV combination treatment using HCV cell culture.

METHODS

The antiviral efficacy of IFN-α, RBV alone and in combination was quantitatively measured using HCV infected and replicon cell culture. Direct antiviral activity of these two drugs at the level of HCV internal ribosome entry site (IRES) mediated translation in Huh-7 cell culture was investigated. The synergistic antiviral effect of IFN-α and RBV combination treatment was verified using both the CalcuSyn Software and MacSynergy Software.

RESULTS

RBV combination with IFN-α efficiently inhibits HCV replication cell culture. Our results demonstrate that IFN-α, interferon lambda (IFN-λ) and RBV each inhibit the expression of HCV IRES-GFP and that they have a minimal effect on the expression of GFP in which the translation is not IRES dependent. The combination treatments of RBV along with IFN-α or IFN-λ were highly synergistic with combination indexes <1. We show that IFN-α treatment induce levels of PKR and eIF2α phosphorylation that prevented ribosome loading of the HCV IRES-GFP mRNA. Silencing of PKR expression in Huh-7 cells prevented the inhibitory effect of IFN-α on HCV IRES-GFP expression. RBV also blocked polyribosome loading of HCV-IRES mRNA through the inhibition of cellular IMPDH activity, and induced PKR and eIF2α phosphorylation. Knockdown of PKR or IMPDH prevented RBV induced HCV IRES-GFP translation.

CONCLUSIONS

We demonstrated both IFN-α and RBV inhibit HCV IRES through prevention of polyribosome formation. The combination of IFN-α and RBV treatment synergistically inhibits HCV IRES translation via using two different mechanisms involving PKR activation and depletion of intracellular guanosine pool through inhibition of IMPDH.

Extinction of hepatitis C virus by ribavirin in hepatoma cells involves lethal mutagenesis

Lethal mutagenesis, or virus extinction produced by enhanced mutation rates, is under investigation as an antiviral strategy that aims at counteracting the adaptive capacity of viral quasispecies, and avoiding selection of antiviral-escape mutants. To explore lethal mutagenesis of hepatitis C virus (HCV), it is important to establish whether ribavirin, the purine nucleoside analogue used in anti-HCV therapy, acts as a mutagenic agent during virus replication in cell culture. Here we report the effect of ribavirin during serial passages of HCV in human hepatoma Huh-7.5 cells, regarding viral progeny production and complexity of mutant spectra. Ribavirin produced an increase of mutant spectrum complexity and of the transition types associated with ribavirin mutagenesis, resulting in HCV extinction. Ribavirin-mediated depletion of intracellular GTP was not the major contributory factor to mutagenesis since mycophenolic acid evoked a similar decrease in GTP without an increase in mutant spectrum complexity. The intracellular concentration of the other nucleoside-triphosphates was elevated as a result of ribavirin treatment. Mycophenolic acid extinguished HCV without an intervening mutagenic activity. Ribavirin-mediated, but not mycophenolic acid-mediated, extinction of HCV occurred via a decrease of specific infectivity, a feature typical of lethal mutagenesis. We discuss some possibilities to explain disparate results on ribavirin mutagenesis of HCV.

Ribavirin Does Not Impair the Suppressive Activity of Foxp3(+)CD4(+)CD25(+) Regulatory T Cells

Ribavirin is an antiviral drug used in combination with pegylated interferon-α (IFN-α) for the treatment of hepatitis C virus (HCV) infection. Recently, ribavirin was reported to inhibit the suppressive activity of regulatory T (Treg) cells. In the present study, we re-evaluated the effect of ribavirin on Foxp3⁺CD4⁺CD25⁺ Treg cells from normal donors. First, we examined the expression of CTLA-4 and CD39, which are known to play a role in the suppressive function of Treg cells. We found that ribavirin treatment did not modulate the expression of CTLA-4 and CD39 in Treg cells. We also studied the effect of ribavirin on Treg cells in the presence of IFN-α; however, the expression of CTLA-4 and CD39 in Treg cells was not changed by ribavirin in the presence of IFN-α. Next, we directly evaluated the effect of ribavirin on the suppressive activity of Treg cells in the standard Treg suppression assay, by co-culturing CFSE-labeled non-Treg CD4⁺ T cells with purified Treg cells. We found that ribavirin did not attenuate the suppressive activity of Treg cells. Taken together, while ribavirin reversed Treg cell-mediated suppression of effector T cells in the previous study, we herein demonstrate that ribavirin does not impair the suppressive activity of Treg cells.

Unfolded protein response pathways regulate Hepatitis C virus replication via modulation of autophagy

Background

Hepatitis C virus (HCV) induces endoplasmic reticulum (ER) stress which, in turn, activates the unfolding protein response (UPR). UPR activates three distinct signalling pathways. Additionally, UPR induces autophagy (UPR-autophagy pathways). On the other hand, it has become clear that some positive-single-strand RNA viruses utilize autophagy. Some groups have used the siRNA silencing approach to show that autophagy is required for HCV RNA replication. However, the mechanism of induction of the UPR-autophagy pathways remain unclear in the cells with HCV.

Method and results: we used a genome-length HCV RNA (strain O of genotype 1b) replication system (OR6) in hepatoma cells (HuH-7-derived OR6 cells). As control, we used OR6c cells from which the HCV genome had been removed by treatment with interferon-α. The UPR-autophagy pathways were activated to a greater degree in the OR6 cells as compared to the OR6c cells. Rapamycin, mTOR-independent autophagy inducer, activated HCV replication in the OR6 cells. On the other hand, HCV replication in the cells was inhibited by 3-methyladenine (3-MA), which is an inhibitor of autophagy. Salubrinal (Eukaryotic Initiation Factor 2(eIF2)-alpha phosphatase inhibitor), 3-ethoxy-5, 6-dibromosalicylaldehyde (X-box binding protein-1 (XBP-1) splicing inhibitor) and sp600125 (c-Jun N-terminal kinases (JNK) inhibitor) inhibited HCV replication and autophagy. Additionally, HCV replication and autophagy were inhibited more strongly by combination of these inhibitors.

Conclusion

Our results suggest that UPR-autophagy pathways exert an influence on HCV replication. Therefore, control these pathways may serve as a novel therapeutic strategy against replication of HCV.

Interference with ERK and STAT signaling pathways and inhibition of hepatitis C virus replication by ribavirin

Ribavirin in combination with interferon (IFN)-α is the approved treatment for hepatitis C virus (HCV) infection. Interference of ribavirin with signaling events is involved in its biological activities. However, little is known of signaling pathways induced by ribavirin following HCV infection. In human hepatoma cells, effects of ribavirin on ERK and signal transducers and activators of transcription (STAT) pathways, HCV replication, and antiviral gene expression were evaluated before and after cell culture-derived HCV infection. Ribavirin reduced phosphorylation of Raf, MEK, ERK, Tyk2, and STAT1, but selectively increased STAT3 phosphorylation. IFN-α synergistically regulated ERK and STAT3 phosphorylation with ribavirin, and up-regulated expression and phosphorylation of STAT1. Ribavirin dose-dependently decreased HCV RNA replication and HCV protein expression, with slight induction of IFN regulatory factor 9 and IFN-stimulated gene 15. Ribavirin and IFN-α exerted a synergetic inhibitory effect on HCV. ERK and STAT pathways were down-regulated by ribavirin following HCV infection. These results suggest that ribavirin may mediate anti-HCV activity through interference with ERK and STAT pathways.

Eukaryotic initiation factor 5B: a new player for the anti-hepatitis C virus effect of ribavirin?

The addition of the broad-spectrum antiviral agent ribavirin (RBV), a synthetic guanosine analog, to interferon-alpha (IFNα) monotherapy has been a major breakthrough in the treatment of patients with hepatitis C virus (HCV), as it greatly improved treatment response rates. Although several mechanisms of action have been proposed for RBV's antiviral activity, each with some experimental evidence, the precise mechanism by which it acts synergistically with IFNα has remained elusive. A cornerstone of the antiviral IFNα response is phosphorylation of the α subunit of eukaryotic initiation factor (eIF)2. This limits the availability of eIF2⋅GTP⋅Met-tRNA(i)(Met) ternary complexes, reduces formation of the 43S preinitiation complexes, ultimately blocking viral (and most cellular) mRNA translation. However recent studies indicated that translation driven by the HCV internal ribosome entry site (IRES) is insensitive to eIF2α phosphorylation. Particularly, in addition to the general eIF2-dependent pathway of translation, the HCV IRES makes use of a bacterial-like, eIF2-independent pathway requiring as initiation factors only eIF5B (an analog of bacterial IF2) and eIF3. Together, these observations support a model in which cellular stresses that induce eIF2α phosphorylation (e.g. treatment with IFNα) cause HCV IRES-directed translation to switch from an eIF2-dependent mode to an eIF5B-dependent mode, defining a tactic used by HCV to evade the INFα response. Eukaryotic eIF5B is a ribosome-dependent GTPase that is responsible for 80S complex formation in translation initiation but shows much lower affinities for GTP than to other GTPases, thus suggesting that it may mis-incorporate the RBV triphosphate (RTP) in place of GTP even at the RBV concentrations achieved in clinical use. Consequently, we theorize that RTP bound to eIF5B lowering its affinity for ribosome, blocks the 80S complex formation on HCV IRES inhibiting the eIF5B-dependent translation used by HCV to elude IFNα response. In conclusion, our hypothesis provides a mechanistic explanation for the phenomenon of RBV enhancement in INFα-based therapy.

Development of hepatitis C virus production reporter-assay systems using two different hepatoma cell lines

A hepatitis C virus (HCV) infection system was developed previously using the HCV JFH-1 strain (genotype 2a) and HuH-7 cells, and this cell culture is so far the only robust production system for HCV. In patients with chronic hepatitis C, the virological effects of pegylated interferon and ribavirin therapy differ depending on the HCV strain and the genetic background of the host. Recently, we reported the hepatoma-derived Li23 cell line, in which the JFH-1 life cycle is reproduced at a level almost equal to that in HuH-7-derived RSc cells. To monitor the HCV life cycle more easily, we here developed JFH-1 reporter-assay systems using both HuH-7- and Li23-derived cell lines. To identify any genetic mutations by long-term cell culture, HCV RNAs in HuH-7 cells were amplified 130 days after infection and subjected to sequence analysis to find adaptive mutation(s) for robust virus replication. We identified two mutations, H2505Q and V2995L, in the NS5B region. V2995L but not H2505Q enhanced JFH-1 RNA replication. However, we found that H2505Q but not V2995L enhanced HCV RNA replication of strain O (genotype 1b). We also selected highly permissive D7 cells by serial subcloning of Li23 cells. The expression levels of claudin-1 and Niemann–Pick C1-like 1 in D7 cells are higher than those in parental Li23 cells. In this study, we developed HCV JFH-1 reporter-assay systems using two distinct hepatoma cell lines, HuH-7 and Li23. The mutations in NS5B resulted in different effects on strains O and JFH-1 HCV RNA replication.

Mycophenolic acid augments interferon-stimulated gene expression and inhibits hepatitis C Virus infection in vitro and in vivo

Mycophenolic acid (MPA) is a highly effective immunosuppressant that has broad antiviral activity against different viruses and can act in synergy with interferon-α (IFN-α) on hepatitis C virus (HCV) replication. MPA is a potent inosine monophosphate dehydrogenase (IMPDH) inhibitor but the antiviral mechanisms are less understood. The aim of this study was to investigate the inhibition of HCV infection by MPA and the molecular basis for its synergy with IFN-α. The role of IMPDH and interferon-stimulated genes (ISGs) was investigated in two HCV models using gain- or loss-of-function approaches. The in vivo effect of MPA treatment was studied in NOD/SCID mice engrafted with HCV replicon cells. Potent antiviral effects of MPA at clinically relevant concentrations were observed with both the subgenomic and JFH1-derived infectious HCV models. MPA treatment in mice resulted in a specific and robust inhibition of HCV replication. Ectopic expression of an MPA-resistant IMPDH2 mutant in HCV host cells completely reversed the antiproliferative effect of MPA but only partially affected the antiviral potency. However, similar to ribavirin, MPA induced expression of multiple antiviral ISGs, including interferon regulatory factor 1 (IRF1). Cotreatment of MPA with IFN-α resulted in additive effects on ISG expression and enhanced IFN-induced luciferase reporter activity. Knockdown of IRF1, but not IFITM3, significantly attenuated the inhibition of HCV replication by MPA.

CONCLUSION

MPA exerts a potent anti-HCV effect in vitro and in mice and acts in synergy with IFN-α. MPA's antiviral activity partially depends on IMPDH but also involves stimulation of ISGs, providing a molecular basis for its synergy with IFN-α.

Virus-drug interactions--molecular insight into immunosuppression and HCV

Liver transplantation is an effective treatment for end-stage liver disease that is attributable to chronic HCV infection. However, long-term outcomes are compromised by universal virological recurrence in the graft. Reinfection that occurs after transplantation has increased resistance to current interferon-based antiviral therapy and often leads to accelerated development of cirrhosis. Important risk factors for severe HCV recurrence are linked to immunosuppression. Owing to the lack of good randomized, controlled trials, the optimal choice of immunosuppressants is still debated. By contrast, much progress has been made in the understanding of HCV biology and the antiviral action of interferons. These new insights have greatly expanded our knowledge of the molecular interplay between HCV and immunosuppressive drugs. In this article, we explore the effect of different immunosuppressants on the complex cellular events involved in HCV infection and interferon signalling. Potential implications for clinical practice and future drug development are discussed.

Oxidative stress, endogenous antioxidants, alcohol, and hepatitis C: pathogenic interactions and therapeutic considerations

Hepatitis C virus (HCV) is a blood-borne pathogen that was identified as an etiologic agent of non-A, non-B hepatitis in 1989. HCV is estimated to have infected at least 170 million people worldwide. The majority of patients infected with HCV do not clear the virus and become chronically infected, and chronic HCV infection increases the risk for hepatic steatosis, cirrhosis, and hepatocellular carcinoma. HCV induces oxidative/nitrosative stress from multiple sources, including inducible nitric oxide synthase, the mitochondrial electron transport chain, hepatocyte NAD(P)H oxidases, and inflammation, while decreasing glutathione. The cumulative oxidative burden is likely to promote both hepatic and extrahepatic conditions precipitated by HCV through a combination of local and more distal effects of reactive species, and clinical, animal, and in vitro studies strongly point to a role of oxidative/nitrosative stress in HCV-induced pathogenesis. Oxidative stress and hepatopathogenesis induced by HCV are exacerbated by even low doses of alcohol. Alcohol and reactive species may have other effects on hepatitis C patients such as modulation of the host immune system, viral replication, and positive selection of HCV sequence variants that contribute to antiviral resistance. This review summarizes the current understanding of redox interactions of HCV, outlining key experimental findings, directions for future research, and potential applications to therapy.