Cyclophilin B is a functional regulator of hepatitis C virus RNA polymerase

Regulation of de novo-initiated RNA synthesis in hepatitis C virus RNA-dependent RNA polymerase by intermolecular interactions

The hepatitis C virus (HCV) RNA-dependent RNA polymerase (RdRp) has been proposed to change conformations in association with RNA synthesis and to interact with cellular proteins. In vitro, the RdRp can initiate de novo from the ends of single-stranded RNA or extend a primed RNA template. The interactions between the Delta1 loop and thumb domain in NS5B are required for de novo initiation, although it is unclear whether these interactions are within an NS5B monomer or are part of a higher-order NS5B oligomeric complex. This work seeks to address how polymerase conformation and/or oligomerization affects de novo initiation. We have shown that an increasing enzyme concentration increases de novo initiation by the genotype 1b and 2a RdRps while primer extension reactions are not affected or inhibited under similar conditions. Initiation-defective mutants of the HCV polymerase can increase de novo initiation by the wild-type (WT) polymerase. GTP was also found to stimulate de novo initiation. Our results support a model in which the de novo initiation-competent conformation of the RdRp is stimulated by oligomeric contacts between individual subunits. Using electron microscopy and single-molecule reconstruction, we attempted to visualize the low-resolution conformations of a dimer of a de novo initiation-competent HCV RdRp.

[Suppression of hepatitis C virus (HCV) replication with serine palmitoyltransferase inhibitor]

Hepatitis C virus (HCV) persists chronically in most infected patients, eventually causing chronic hepatitis, liver cirrhosis, and in some cases hepatocellular carcinoma. The combination therapy of PEG-IFN and ribavirin improves efficacy in many patients, although it does not lead to sufficient achievements in genotype 1b patients. To aid in invention of new anti-HCV reagents, we focused on host factors that contributed to HCV lifecycle. We identified serine palmitoyltransferase inhibitor as an anti-HCV reagent through high-throughput screening using HCV replicon cells. We investigated the mechanism of anti-HCV effect of SPT inhibitor. It has been reported that sphingolipids and cholesterol compose the lipid raft where replication of HCV occurs. We investigated the influence of SPT inhibitor to lipid rafts by analyzing the detergent-resistant membrane (DRM). The analysis showed that SPT inhibitor moved HCV RNA-dependent RNA polymerase (NS5B) to detergent-soluble fraction from DRM, and Biacore analysis indicated binding of sphingomyelin to NS5B. These results suggest that SPT inhibitor disrupts the interaction between NS5B and sphingomyelin. Moreover, we evaluated the anti-HCV effect of SPT inhibitor in vivo with humanized chimeric mice. SPT inhibitor led to rapid decline in serum HCV-RNA of about 1-2 log within 8 days. Furthermore, combination therapy of SPT inhibitor and PEG-IFN achieved about 3 log reduction in serum HCV-RNA.

Antiviral therapy for hepatitis C virus: beyond the standard of care

Hepatitis C virus (HCV) represents a major health burden, with an estimated 180 million chronically infected individuals worldwide. These patients are at increased risk of developing liver cirrhosis and hepatocellular carcinoma. Infection with HCV is the leading cause of liver transplantation in the Western world. Currently, the standard of care (SoC) consists of pegylated interferon alpha (pegIFN-α) and ribavirin (RBV). However this therapy has a limited efficacy and is associated with serious side effects. Therefore more tolerable, highly potent inhibitors of HCV replication are urgently needed. Both Specifically Targeted Antiviral Therapy for HCV (STAT-C) and inhibitors that are believed to interfere with the host-viral interaction are discussed.

Calcineurin inhibitor tacrolimus does not interfere with the suppression of hepatitis C virus infection by interferon-alpha

Immunosuppression considerably affects hepatitis C virus (HCV) recurrence and the outcome of antiviral treatment after liver transplantation. Recent findings have suggested that the calcineurin inhibitor tacrolimus (Tac), unlike cyclosporine A (CsA), interferes with the antiviral activity of interferon-alpha (IFN-alpha) in vitro. The aim of this study was to more extensively investigate the effects of calcineurin inhibitors on IFN-alpha signaling and antiviral activity in subgenomic and infectious HCV models. Treatment with Tac and CsA did not affect Huh7 cell proliferation at doses of 10 to 500 ng/mL; however, it completely inhibited T cell proliferation. In contrast to previous reports, Tac had no effect on IFN-alpha-stimulated reporter gene expression, even at the dose of 5 microg/mL. Furthermore, in Huh7 subgenomic HCV replicon cells, treatment with Tac had no significant effect on the suppression of viral replication by IFN-alpha. In the infectious HCV model, treatment with IFN-alpha effectively inhibited both viral RNA replication and de novo production of virus particles, and neither was attenuated at any concentration of Tac. CsA had no significant effect on IFN-alpha-stimulated reporter gene expression; however, as shown previously, a combination of CsA (at 500 ng/mL and higher) and IFN-alpha resulted in enhanced inhibition of viral replication in both the subgenomic and infectious HCV models. In conclusion, our study shows no evidence that Tac or CsA interferes with IFN-alpha-mediated inhibition of HCV replication and virion production in vitro. Therefore, no further mechanistic arguments have been found to break the clinical controversy about the choice of calcineurin inhibitors during posttransplantation antiviral therapy.

Cyclosporine inhibits a direct interaction between cyclophilins and hepatitis C NS5A

BACKGROUND

Hepatitis C Virus (HCV) infection is a leading indication for liver transplantation. HCV infection reoccurs almost universally post transplant, decreasing both graft longevity and patient survival. The immunosuppressant, cyclosporine A (CsA) has potent anti-HCV activity towards both HCV replicons and the genotype 2a cell culture infectious virus. Previously, we isolated mutations in the 1bN replicon with less sensitivity to CsA that mapped to both NS5A and NS5B regions of the virus. Mutations in NS5A alone conferred decreased CsA susceptibility regardless of NS5B mutations.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the mechanisms by which NS5A mutations contribute to CsA resistance and if they are strain dependent. Using in vitro mutagenesis, the amino acid position 321 mutation of NS5A was restored to the wild-type tyrosine residue conferring partial CsA susceptibility on the mutant replicon. The 321 mutation also alters CsA susceptibility of the JFH cell culture virus. Additionally, we demonstrated a novel CsA-sensitive interaction between NS5A and both cyclophilin A and B. Both the mutant NS5A and wild type NS5A bind cyclophilin in vitro. The NS5A: cyclophilin interaction requires both the NS5A region identified by the resistance mutants and cyclophilin catalytic residues. In cell culture, NS5A from CsA resistant mutant has an enhanced interaction with cyclophilin B. Additionally; NS5B facilitates a stronger binding of mutant NS5A to endogenous cyclophilin B than wild-type in cell culture.

CONCLUSIONS/SIGNIFICANCE

Collectively, this data suggests direct interactions between cyclophilins and NS5A are critical to understand for optimal use of cyclophilin inhibitors in anti-HCV therapy.

Mechanism of resistance of hepatitis C virus replicons to structurally distinct cyclophilin inhibitors

The current standard of care for hepatitis C virus (HCV) infection, pegylated alpha interferon in combination with ribavirin, has a limited response rate and adverse side effects. Drugs targeting viral proteins are in clinical development, but they suffer from the development of high viral resistance. The inhibition of cellular proteins that are essential for viral amplification is thought to have a higher barrier to the emergence of resistance. Three cyclophilin inhibitors, the cyclosporine analogs DEBIO-025, SCY635, and NIM811, have shown promising results for the treatment of HCV infection in early clinical trials. In this study, we investigated the frequency and mechanism of resistance to cyclosporine (CsA), NIM811, and a structurally unrelated cyclophilin inhibitor, SFA-1, in replicon-containing Huh7 cells. Cross-resistance between all clones was observed. NIM811-resistant clones were selected only after obtaining initial resistance to either CsA or SFA-1. The time required to select resistance against cyclophilin inhibitors was significantly longer than that required for resistance selection against viral protein inhibitors, and the achievable resistance level was substantially lower. Resistance to cyclophilin inhibitors was mediated by amino acid substitutions in NS3, NS5A, and NS5B, with NS5A mutations conferring the majority of resistance. Mutation D320E in NS5A mediated most of the resistance conferred by NS5A. Taken together, the results indicate that there is a very low frequency and level of resistance to cyclophilin-binding drugs mediated by amino acid substitutions in three viral proteins. The interaction of cyclophilin with NS5A seems to be the most critical, since the NS5A mutations have the largest impact on resistance.

CD147/EMMPRIN acts as a functional entry receptor for measles virus on epithelial cells

Measles is a highly contagious human disease caused by measles virus (MeV) and remains the leading cause of death in children, particularly in developing countries. Wild-type MeV preferentially infects lymphocytes by using signaling lymphocytic activation molecule (SLAM), whose expression is restricted to hematopoietic cells, as a receptor. MeV also infects other epithelial and neuronal cells that do not express SLAM and causes pneumonia and diarrhea and, sometimes, serious symptoms such as measles encephalitis and subacute sclerosing panencephalitis. The discrepancy between the tissue tropism of MeV and the distribution of SLAM-positive cells suggests that there are unknown receptors other than SLAM for MeV. Here we identified CD147/EMMPRIN (extracellular matrix metalloproteinase inducer), a transmembrane glycoprotein, which acts as a receptor for MeV on epithelial cells. Furthermore, we found the incorporation of cyclophilin B (CypB), a cellular ligand for CD147, in MeV virions, and showed that inhibition of CypB incorporation significantly attenuated SLAM-independent infection on epithelial cells, while it had no effect on SLAM-dependent infection. To date, MeV infection was considered to be triggered by binding of its hemagglutinin (H) protein and cellular receptors. Our present study, however, indicates that MeV infection also occurs via CD147 and virion-associated CypB, independently of MeV H. Since CD147 is expressed in a variety of cells, including epithelial and neuronal cells, this molecule possibly functions as an entry receptor for MeV in SLAM-negative cells. This is the first report among members of the Mononegavirales that CD147 is used as a virus entry receptor via incorporated CypB in the virions.

The use of AlphaLISA technology to detect interaction between hepatitis C virus-encoded NS5A and cyclophilin A

Cyclosporine A (CsA) is an immunosuppressive molecule that also impedes replication of hepatitis C virus (HCV). CsA inhibits isomerase activity of cellular-encoded cyclophilin proteins, of which cyclophilin A (CypA) in particular is required for HCV replication. Evidence suggests that the HCV-encoded NS5A and NS5B proteins may govern dependence of the virus on CypA-mediated isomerase activity, although the molecular mechanisms involved are unclear. However, association of NS5A and NS5B, with CypA has been reported, raising the possibility that direct interaction between these proteins facilitates HCV replication. In the present study, mammalian two-hybrid and AlphaLISA technologies were utilized to detect interactions between NS5A and NS5B, with CypA. AlphaLISA analysis revealed associations between NS5A and CypA using purified proteins, and in cell lysates prepared from co-transfected cells. Importantly, the NS5A-CypA interactions were sensitive to CsA in a dose-responsive manner and an isomerase mutant of CypA interacted with NS5A less efficiently than wild-type CypA. These findings correlate the anti-HCV properties of CsA with an ability of the compound to disrupt NS5A-CypA interactions in vitro and in vivo, whilst providing the basis for development of assay platforms suitable to screen compound libraries for novel inhibitors of the NS5A-CypA interaction.

Know your enemy: translating insights about the molecular biology of hepatitis C virus into novel therapeutic approaches

Identified in 1989 as the cause of what was then known as hepatitis non-A non-B, the hepatitis C virus (HCV) continues to be a significant global public health threat, given that an estimated 123 million individuals are chronically infected and, thus, at risk for cirrhosis and hepatocellular carcinoma. After 20 years of basic and clinical research into HCV infection, the backbone of therapy has remained interferon, a drug that - in a different formulation - was already being employed before HCV was even identified. Nonetheless, research has overcome many obstacles that stood in the way of studying this pre-eminent human pathogen. Hard-won insights into its molecular biology have identified promising therapeutic targets, and we are now on the verge of an era where rationally designed therapeutics, also referred to as specifically targeted antiviral therapy for HCV, will reshape the treatment of hepatitis C. This article describes recent insights on the molecular biology of HCV and the efforts to translate them into clinical applications.

Cyclophilin A-independent recruitment of NS5A and NS5B into hepatitis C virus replication complexes

The mechanisms by which cyclophilin A (CypA) governs hepatitis C virus (HCV) replication remain unknown. Since CypA binds two essential components of the HCV replication complex (RC)--the polymerase NS5B and the phosphoprotein NS5A--we asked in this study whether CypA regulates their RC association. We found that CypA, via its isomerase pocket, locates in a protease-resistant compartment similar to that where HCV replicates. CypA association with this compartment is not mediated by HCV. Moreover, CypA depletion of RC does not influence NS5A and NS5B RC association, arguing against a model where CypA governs HCV replication by recruiting NS5A or NS5B into RC.

RNA-dependent RNA polymerase of hepatitis C virus binds to its coding region RNA stem-loop structure, 5BSL3.2, and its negative strand

The hepatitis C virus NS5B RNA-dependent RNA polymerase (RdRp) is a key enzyme involved in viral replication. Interaction between NS5B RdRp and the viral RNA sequence is likely to be an important step in viral RNA replication. The C-terminal half of the NS5B-coding sequence, which contains the important cis-acting replication element, has been identified as an NS5B-binding sequence. In the present study, we confirm the specific binding of NS5B to one of the RNA stem-loop structures in the region, 5BSL3.2. In addition, we show that NS5B binds to the complementary strand of 5BSL3.2 (5BSL3.2N). The bulge structure of 5BSL3.2N was shown to be indispensable for tight binding to NS5B. In vitro RdRp activity was inhibited by 5BSL3.2N, indicating the importance of the RNA element in the polymerization by RdRp. These results suggest the involvement of the RNA stem-loop structure of the negative strand in the replication process.

Streamlined, automated protocols for the production of milligram quantities of untagged recombinant human cyclophilin-A (hCypA) and untagged human proliferating cell nuclear antigen (hPCNA) using AKTAxpress

We developed streamlined, automated purification protocols for the production of milligram quantities of untagged recombinant human cyclophilin-A (hCypA) and untagged human proliferating cell nuclear antigen (hPCNA) from Escherichia coli, using the AKTAxpress chromatography system. The automated 2-step (cation exchange and size exclusion) purification protocol for untagged hCypA results in final purity and yields of 93% and approximately 5 mg L(-1) of original cell culture, respectively, in under 12h, including all primary sample processing and column equilibration steps. The novel automated 4-step (anion exchange, desalt, heparin-affinity and size exclusion, in linear sequence) purification protocol for untagged hPCNA results in final purity and yields of 87% and approximately 4 mg L(-1) of original cell culture, respectively, in under 24h, including all primary sample processing and column equilibration steps. This saves in excess of four full working days when compared to the traditional protocol, producing protein with similar final yield, purity and activity. Furthermore, it limits a time-dependent protein aggregation, a problem with the traditional protocol that results in a loss of final yield. Both automated protocols were developed to use generic commercially available pre-packed columns and automatically prepared minimal buffers, designed to eliminate user and system variations, maximize run reproducibility, standardize yield and purity between batches, increase throughput and reduce user input to a minimum. Both protocols represent robust generic methods for the automated production of untagged hCypA and hPCNA.

Severe osteogenesis imperfecta in cyclophilin B-deficient mice

Osteogenesis Imperfecta (OI) is a human syndrome characterized by exquisitely fragile bones due to osteoporosis. The majority of autosomal dominant OI cases result from point or splice site mutations in the type I collagen genes, which are thought to lead to aberrant osteoid within developing bones. OI also occurs in humans with homozygous mutations in Prolyl-3-Hydroxylase-1 (LEPRE1). Although P3H1 is known to hydroxylate a single residue (pro-986) in type I collagen chains, it is unclear how this modification acts to facilitate collagen fibril formation. P3H1 exists in a complex with CRTAP and the peptidyl-prolyl isomerase cyclophilin B (CypB), encoded by the Ppib gene. Mutations in CRTAP cause OI in mice and humans, through an unknown mechanism, while the role of CypB in this complex has been a complete mystery. To study the role of mammalian CypB, we generated mice lacking this protein. Early in life, Ppib-/- mice developed kyphosis and severe osteoporosis. Collagen fibrils in Ppib-/- mice had abnormal morphology, further consistent with an OI phenotype. In vitro studies revealed that in CypB–deficient fibroblasts, procollagen did not localize properly to the golgi. We found that levels of P3H1 were substantially reduced in Ppib-/- cells, while CRTAP was unaffected by loss of CypB. Conversely, knockdown of either P3H1 or CRTAP did not affect cellular levels of CypB, but prevented its interaction with collagen in vitro. Furthermore, knockdown of CRTAP also caused depletion of cellular P3H1. Consistent with these changes, post translational prolyl-3-hydroxylation of type I collagen by P3H1 was essentially absent in CypB–deficient cells and tissues from CypB–knockout mice. These data provide significant new mechanistic insight into the pathophysiology of OI and reveal how the members of the P3H1/CRTAP/CypB complex interact to direct proper formation of collagen and bone.

Osteogenesis Imperfecta (OI), also known as “brittle bone disease,” is an inherited condition with multiple defects in collagen-containing structures, including the bones, skin, and other connective tissues. Patients with OI suffer from short stature, scoliosis, thin skin, hearing loss, and, most notably, fragile bones that break with little or no trauma. Although many cases are due to dominantly inherited point mutations in the collagen genes, autosomal recessive forms have been described due to defects in the genes for Prolyl-3-Hydroxylase-1 (LEPRE1) and Cartilage-Associated Protein (CRTAP), proteins that modify newly synthesized procollagen. Some patients with OI do not have mutations in any of the known disease-related genes. Here, through the use of newly generated knockout mice, we identify the endoplasmic-reticulum resident prolyl-isomerase cyclophilin B (CypB) as a new autosomal recessive OI gene in mice. CypB, P3H1, and CRTAP were shown to have interrelated effects in maintaining their respective protein levels and ability to bind to collagen. These studies enhance our understanding about how collagen, the most abundant protein in the body, becomes properly assembled to form bones with adequate strength.

Multiple cyclophilins involved in different cellular pathways mediate HCV replication

Three cyclophilin inhibitors (DEBIO-025, SCY635, and NIM811) are currently in clinical trials for hepatitis C therapy. The mechanism of action of these, however, is not completely understood. There are at least 16 cyclophilins expressed in human cells which are involved in a diverse set of cellular processes. Large-scale siRNA experiments, chemoproteomic assays with cyclophilin binding compounds, and mRNA profiling of HCV replicon containing cells were used to identify the cyclophilins that are instrumental to HCV replication. The previously reported cyclophilin A was confirmed and additional cyclophilin containing pathways were identified. Together, the experiments provide strong evidence that NIM811 reduces viral replication by inhibition of multiple cyclophilins and pathways with protein trafficking as the most strongly and persistently affected pathway.

The long and winding road leading to the identification of the hepatitis C virus

This review describes work conducted largely in my laboratory at the Chiron Corporation between 1982 and 1989 that led to the identification of the hepatitis C virus (HCV). Key colleagues included Dr. Qui-Lim Choo in my laboratory and Dr. George Kuo also of Chiron as well as my collaborator Dr. Daniel Bradley at the CDC who provided many biological samples from the NANBH chimpanzee model. Numerous molecular approaches were explored including the screening of tens of millions of bacterial cDNA clones derived from these materials. While this early genomics approach resulted in the identification of many host gene activities associated with NANBH, no genes of proven infectious etiology could be identified. A separate avenue of our research led to the molecular characterization of the complete hepatitis delta viral genome but unfortunately, this could not be used as a molecular handle for HCV. Largely following input from Dr. Kuo, I initiated a blind cDNA immunoscreening approach involving the large-scale screening of bacterial proteomic cDNA libraries derived from NANBH-infectious chimpanzee materials (prior to the development of PCR technology) using sera from NANBH patients as a presumptive source of viral antibodies. Eventually, this novel approach to identifying agents of infectious etiology led to the isolation of a single small cDNA clone that was proven to be derived from the HCV genome using various molecular and serological criteria. This discovery has facilitated the development of effective diagnostics, blood screening tests and the elucidation of promising drug and vaccine targets to control this global pathogen.

Cyclosporine A inhibits hepatitis C virus nonstructural protein 2 through cyclophilin A

Numerous anti-hepatitis C virus (HCV) drugs targeting either the viral nonstructural 3 (NS3) protease or NS5B polymerase are currently in clinical testing. However, rapid resistance development is a major problem and optimal therapy will clearly require a combination of multiple mechanisms of action. Cyclosporine A (CsA) and its nonimmunosuppressant derivatives are among the more promising drugs under development. Based on work with subgenomic HCV replicons it has been thought that they act as NS5B-inhibitors. In this study we show that CsA inhibits replication of full-length HCV Japanese Fulminant Hepatitis (JFH1) genomes about 10-fold more efficiently than subgenomic replicons. This effect is dependent on the presence of NS2 in the viral polyprotein and mediated through cellular cyclophilin A. NS2 is either an additional target for CsA-dependent inhibition or modulates the antiviral activity against NS3 to NS5B proteins. CsA is thus the first anti-HCV drug shown to act through NS2.

CONCLUSION

CsA inhibits replication of JFH1 full-length genomes much more efficiently than subgenomic replicons by targeting cleavage at the NS2/NS3 junction and possibly other nonreplication lifecycle steps.

Towards a small animal model for hepatitis C

Hepatitis C virus (HCV) causes chronic liver disease and affects an estimated 3% of the world's population. Options for the prevention or therapy of HCV infection are limited; there is no vaccine and the nonspecific, interferon-based treatments now in use are frequently ineffective and have significant side effects. A small-animal model for HCV infection would significantly expedite antiviral compound development and preclinical testing, as well as open new avenues to decipher the mechanisms that underlie viral pathogenesis. The natural species tropism of HCV is, however, limited to humans and chimpanzees. Here, we discuss the prospects of developing a mouse model for HCV infection, taking into consideration recent results on HCV entry and replication, and new prospects in xenotransplantation biology. We highlight three independent, but possibly complementary, approaches towards overcoming current species barriers and generating a small-animal model for HCV pathogenesis.

Identification of cellular and viral factors related to anti-hepatitis C virus activity of cyclophilin inhibitor

We have so far reported that an immunosuppressant cyclosporin A (CsA), a well-known cyclophilin (CyP) inhibitor (CPI), strongly suppressed hepatitis C virus (HCV) replication in cell culture, and that CyPB was a cellular cofactor for viral replication. To further investigate antiviral mechanisms of CPI, we here developed cells carrying CsA-resistant HCV replicons, by culturing the HCV subgenomic replicon cells for 4 weeks in the presence of CsA with G418. Transfection of total RNA from the isolated CsA-resistant cells to naïve Huh7 cells conferred CsA resistance, suggesting that the replicon RNA itself was responsible for the resistant phenotype. Of the identified amino acid mutations, D320E in NS5A conferred the CsA resistance. The replicon carrying the D320E mutation was sensitive to interferon-alpha, but was resistant to CsA and other CPIs including NIM811 and sanglifehrin A. Knockdown of individual CyP subtypes revealed CyP40, in addition to CyPA and CyPB, contributed to viral replication, and CsA-resistant replicons acquired independence from CyPA for efficient replication. These data provide important evidence on the mechanisms underlying the regulation of HCV replication by CyP and for designing novel and specific anti-HCV strategies with CPIs.

Suppression of hepatitis C virus replication by protein kinase C-related kinase 2 inhibitors that block phosphorylation of viral RNA polymerase

Hepatitis C virus (HCV) infection is a serious threat to human health worldwide. In spite of the continued search for specific and effective anti-HCV therapies, the rapid emergence of drug-resistance variants has been hampering the development of anti-HCV drugs designed to target viral enzymes. Targeting host factors has therefore emerged as an alternative strategy offering the potential to circumvent the ever-present complication of drug resistance. We previously identified protein kinase C-related kinase 2 (PRK2) as a cellular kinase that phosphorylates the HCV RNA-dependent RNA polymerase (RdRp). Here, we report the anti-HCV activity of HA1077, also known as fasudil, and Y27632, which blocks HCV RdRp phosphorylation by suppressing PRK2 activation. Treatment of a Huh7 cell line, stably expressing a genotype 1b HCV subgenomic replicon RNA, with 20 microm each of HA1077 and Y27632 reduced the HCV RNA level by 55% and 30%, respectively. A combination of the inhibitors with 100 IU/mL interferon alpha (IFN-alpha) significantly potentiated the anti-HCV drug activities resulting in approximately a 2-log(10) viral RNA reduction. We also found that IFN-alpha does not activate PRK2 as well as its upstream kinase PDK1 in HCV-replicating cells. Furthermore, treatment of HCV-infected cells with 20 microm each of HA1077 and Y27632 reduced the levels of intracellular viral RNA by 70% and 92%, respectively. Taken together, the results identify PRK2 inhibitors as potential antiviral drugs that act by suppressing HCV replication via inhibition of viral RNA polymerase phosphorylation.

HCV genotype 1b chimeric replicon with NS5B of JFH-1 exhibited resistance to cyclosporine A

Cyclosporine A (CsA) is a well-characterized anti-HCV reagent. Recently it was reported that the genotype 2a JFH-1 strain was more resistant than genotype 1 HCV strains to CsA in a cell culture system. However, the JFH-1 responsible region for the resistance to CsA remains unclear. It was also demonstrated that in genotype 1b HCVs, NS5B interacts with cyclophilin (CyP). To clarify whether or not NS5B of JFH-1 is significant for CsA resistance, we developed a chimeric replicon with NS5B of JFH-1 in the genotype 1b backbone. The chimeric replicon was more resistant to CsA than the parental genotype 1b replicon. Furthermore, reduction of CyPA had a greater effect on HCV RNA replication and sensitivity to CsA than reduction of CyPB. Here, we demonstrated that NS5B of JFH-1 contributed to this strain's CsA-resistant phenotype. NS5B and CyPA are significant for determining HCV's sensitivity to CsA.

Oxidative stress induces anti-hepatitis C virus status via the activation of extracellular signal-regulated kinase

Recently, we reported that beta-carotene, vitamin D(2), and linoleic acid inhibited hepatitis C virus (HCV) RNA replication in hepatoma cells. Interestingly, in the course of the study, we found that the antioxidant vitamin E negated the anti-HCV activities of these nutrients. These results suggest that the oxidative stress caused by the three nutrients is involved in their anti-HCV activities. However, the molecular mechanism by which oxidative stress induces anti-HCV status remains unknown. Oxidative stress is also known to activate extracellular signal-regulated kinase (ERK). Therefore, we hypothesized that oxidative stress induces anti-HCV status via the mitogen activated protein kinase (MAPK)/ERK kinase (MEK)-ERK1/2 signaling pathway. In this study, we found that the MEK1/2-specific inhibitor U0126 abolished the anti-HCV activities of the three nutrients in a dose-dependent manner. Moreover, U0126 significantly attenuated the anti-HCV activities of polyunsaturated fatty acids, interferon-gamma, and cyclosporine A, but not statins. We further demonstrated that, with the exception of the statins, all of these anti-HCV nutrients and reagents actually induced activation of the MEK-ERK1/2 signaling pathway, which was inhibited or reduced by treatment not only with U0126 but also with vitamin E. We also demonstrated that phosphorylation of ERK1/2 by cyclosporine A was attenuated with N-acetylcysteine treatment and led to the negation of inhibition of HCV RNA replication. We propose that a cellular process that follows ERK1/2 phosphorylation and is specific to oxidative stimulation might lead to down-regulation of HCV RNA replication.

CONCLUSION

Our results demonstrate the involvement of the MEK-ERK1/2 signaling pathway in the anti-HCV status induced by oxidative stress in a broad range of anti-HCV reagents. This intracellular modulation is expected to be a therapeutic target for the suppression of HCV RNA replication.

Investigation of the hepatitis C virus replication complex

Formation of a membrane-associated replication complex, composed of viral proteins, replicating RNA, altered cellular membranes, and other host factors, is a hallmark of all positive-strand RNA viruses. In the case of HCV, RNA replication takes place in a likely endoplasmic reticulum-derived membrane alteration referred to as the "membranous web." In vitro transcription-translation, membrane extraction and flotation analyses, immunofluorescence microscopy, fluorescent in situ hybridization, and RNA metabolic labeling followed by confocal laser scanning microscopy have yielded insights into the structure and function of the HCV replication complex. We describe these techniques and highlight selected results.

Essential role of cyclophilin A for hepatitis C virus replication and virus production and possible link to polyprotein cleavage kinetics

Viruses are obligate intracellular parasites and therefore their replication completely depends on host cell factors. In case of the hepatitis C virus (HCV), a positive-strand RNA virus that in the majority of infections establishes persistence, cyclophilins are considered to play an important role in RNA replication. Subsequent to the observation that cyclosporines, known to sequester cyclophilins by direct binding, profoundly block HCV replication in cultured human hepatoma cells, conflicting results were obtained as to the particular cyclophilin (Cyp) required for viral RNA replication and the underlying possible mode of action. By using a set of cell lines with stable knock-down of CypA or CypB, we demonstrate in the present work that replication of subgenomic HCV replicons of different genotypes is reduced by CypA depletion up to 1,000-fold whereas knock-down of CypB had no effect. Inhibition of replication was rescued by over-expression of wild type CypA, but not by a mutant lacking isomerase activity. Replication of JFH1-derived full length genomes was even more sensitive to CypA depletion as compared to subgenomic replicons and virus production was completely blocked. These results argue that CypA may target an additional viral factor outside of the minimal replicase contributing to RNA amplification and assembly, presumably nonstructural protein 2. By selecting for resistance against the cyclosporine analogue DEBIO-025 that targets CypA in a dose-dependent manner, we identified two mutations (V2440A and V2440L) close to the cleavage site between nonstructural protein 5A and the RNA-dependent RNA polymerase in nonstructural protein 5B that slow down cleavage kinetics at this site and reduce CypA dependence of viral replication. Further amino acid substitutions at the same cleavage site accelerating processing increase CypA dependence. Our results thus identify an unexpected correlation between HCV polyprotein processing and CypA dependence of HCV replication.

Cochaperone activity of human butyrate-induced transcript 1 facilitates hepatitis C virus replication through an Hsp90-dependent pathway

Hepatitis C virus (HCV) nonstructural protein 5A (NS5A) is a component of the replication complex consisting of several host and viral proteins. We have previously reported that human butyrate-induced transcript 1 (hB-ind1) recruits heat shock protein 90 (Hsp90) and FK506-binding protein 8 (FKBP8) to the replication complex through interaction with NS5A. To gain more insights into the biological functions of hB-ind1 in HCV replication, we assessed the potential cochaperone-like activity of hB-ind1, because it has significant homology with cochaperone p23, which regulates Hsp90 chaperone activity. The chimeric p23 in which the cochaperone domain was replaced with the p23-like domain of hB-ind1 exhibited cochaperone activity comparable to that of the authentic p23, inhibiting the glucocorticoid receptor signaling in an Hsp90-dependent manner. Conversely, the chimeric hB-ind1 in which the p23-like domain was replaced with the cochaperone domain of p23 resulted in the same level of recovery of HCV propagation as seen in the authentic hB-ind1 in cells with knockdown of the endogenous hB-ind1. Immunofluorescence analyses revealed that hB-ind1 was colocalized with NS5A, FKBP8, and double-stranded RNA in the HCV replicon cells. HCV replicon cells exhibited a more potent unfolded-protein response (UPR) than the parental and the cured cells upon treatment with an inhibitor for Hsp90. These results suggest that an Hsp90-dependent chaperone pathway incorporating hB-ind1 is involved in protein folding in the membranous web for the circumvention of the UPR and that it facilitates HCV replication.

Cyclophilin inhibitors

The percentage of patients chronically infected with hepatitis C virus (HCV) who have reached sustained antiviral response has increased since the introduction of the pegylated interferon-alpha (pIFNa) and ribavirin (RBV) treatment. However, the current standard pIFNa/RBV therapy not only has a low success rate (about 50%) but is often associated with serious side effects. Thus, there is an urgent need for the development of new anti-HCV agents. Cyclophilin (Cyp) inhibitors are among the most promising of the new anti-HCV agents under development. Recent clinical studies demonstrate that Cyp inhibitors are potent anti-HCV drugs, with a novel mechanism of action and efficacy profiles that make them attractive candidates for combination with current and future HCV treatments.

Cyclosporine inhibits flavivirus replication through blocking the interaction between host cyclophilins and viral NS5 protein

Although flaviviruses cause significant human diseases, no effective therapy is currently available. Host factors essential for viral replication are potential targets for antiviral development. Here we report that cyclophilins (CyPs), a family of cellular peptidyl-prolyl isomerases (PPIases), play a role in flavivirus replication. Huh-7.5 cells with knockdown of different isoforms of CyP were less efficient than parental cells in supporting flavivirus replication, including West Nile virus (WNV), dengue virus, and yellow fever virus. The low viral replication in CyP A (CyPA) knockdown cells could be rescued by trans supplying of a wild-type CyPA but not by trans supplying of a mutant CyPA (defective in the PPIase activity), indicating that the isomerase activity of CyPA is critical for viral replication. Immunoprecipitation and biochemical pulldown analyses showed that CyPA interacts with WNV genomic RNA and viral NS5 protein in the replication complex. Furthermore, antiviral experiments demonstrated that cyclosporine (Cs; an 11-amino-acid cyclic peptide known to block the PPIase activity of CyPA) inhibits flavivirus replication in cell culture at nontoxic concentrations. Time-of-addition and transient replicon results indicated that Cs inhibits flavivirus at the step of viral RNA synthesis. Biochemical analysis showed that Cs directly blocks the interaction between CyPA and WNV NS5 protein. Our results suggest that host CyPA is a component of flavivirus replication complex and could be targeted for potential antiviral development.

Targeting host factors: a novel rationale for the management of hepatitis C virus

Hepatitis C is recognized as a major threat to global public health. The current treatment of patients with chronic hepatitis C is the addition of ribavirin to interferon-based therapy which has limited efficacy, poor tolerability, and significant expense. New treatment options that are more potent and less toxic are much needed. Moreover, more effective treatment is an urgent priority for those who relapse or do not respond to current regimens. A major obstacle in combating hepatitis C virus (HCV) infection is that the fidelity of the viral replication machinery is notoriously low, thus enabling the virus to quickly develop mutations that resist compounds targeting viral enzymes. Therefore, an approach targeting the host cofactors, which are indispensable for the propagation of viruses, may be an ideal target for the development of antiviral agents because they have a lower rate of mutation than that of the viral genome, as long as they have no side effects to patients. Drugs targeting, for example, receptors of viral entry, host metabolism or nuclear receptors, which are factors required to complete the HCV life cycle, may be more effective in combating the viral infection. Targeting host cofactors of the HCV life cycle is an attractive concept because it imposes a higher genetic barrier for resistance than direct antiviral compounds. However the principle drawback of this strategy is the greater potential for cellular toxicity.

Target cell cyclophilins facilitate human papillomavirus type 16 infection

Following attachment to primary receptor heparan sulfate proteoglycans (HSPG), human papillomavirus type 16 (HPV16) particles undergo conformational changes affecting the major and minor capsid proteins, L1 and L2, respectively. This results in exposure of the L2 N-terminus, transfer to uptake receptors, and infectious internalization. Here, we report that target cell cyclophilins, peptidyl-prolyl cis/trans isomerases, are required for efficient HPV16 infection. Cell surface cyclophilin B (CyPB) facilitates conformational changes in capsid proteins, resulting in exposure of the L2 N-terminus. Inhibition of CyPB blocked HPV16 infection by inducing noninfectious internalization. Mutation of a putative CyP binding site present in HPV16 L2 yielded exposed L2 N-terminus in the absence of active CyP and bypassed the need for cell surface CyPB. However, this mutant was still sensitive to CyP inhibition and required CyP for completion of infection, probably after internalization. Taken together, these data suggest that CyP is required during two distinct steps of HPV16 infection. Identification of cell surface CyPB will facilitate the study of the complex events preceding internalization and adds a putative drug target for prevention of HPV–induced diseases.

Human papillomaviruses (HPV), especially HPV types 16 and 18, are a major cause of cancer in women worldwide. HPV16, like most genital HPV types, relies on heparan sulfate proteoglycans (HSPGs) to attach to host cells and to the extracellular matrix. Attachment is mediated by surface-exposed basic residues of the major capsid protein, L1. This triggers conformational changes affecting L1 and the minor capsid protein, L2. However, it is not known what interaction triggers these structural changes and if any host cell protein is involved. Now we have identified a host cell chaperone, Cyclophilin B (CyPB), as essential for efficient HPV16 and HPV18 infection. CyPB, which is present on the cell surface in association with specific forms of O-sulfated HSPG as well as in the lumen of intracellular membrane structures, is an energy-independent enzyme, which catalyzes cis/trans isomerization of peptidyl-prolyl bonds. We demonstrate that CyPB facilitates conformational changes resulting in exposure of the L2 N-terminus, which is required for infectious entry. In addition, we present some evidence suggesting that members of the cyclophilin family are required for a second, probably intracellular, step of HPV16 infection. This is the first report implicating cell surface chaperones as essential host factors for viral infection.

Class III phosphatidylinositol 4-kinase alpha and beta are novel host factor regulators of hepatitis C virus replication

Host factor pathways are known to be essential for hepatitis C virus (HCV) infection and replication in human liver cells. To search for novel host factor proteins required for HCV replication, we screened a subgenomic genotype 1b replicon cell line (Luc-1b) with a kinome and druggable collection of 20,779 siRNAs. We identified and validated several enzymes required for HCV replication, including class III phosphatidylinositol 4-kinases (PI4KA and PI4KB), carbamoyl-phosphate synthetase 2, aspartate transcarbamylase, and dihydroorotase (CAD), and mevalonate (diphospho) decarboxylase. Knockdown of PI4KA could inhibit the replication and/or HCV RNA levels of the two subgenomic genotype 1b clones (SG-1b and Luc-1b), two subgenomic genotype 1a clones (SG-1a and Luc-1a), JFH-1 genotype 2a infectious virus (JFH1-2a), and the genomic genotype 1a (FL-1a) replicon. In contrast, PI4KB knockdown inhibited replication and/or HCV RNA levels of Luc-1b, SG-1b, and Luc-1a replicons. The small molecule inhibitor, PIK93, was found to block subgenomic genotype 1b (Luc-1b), subgenomic genotype 1a (Luc-1a), and genomic genotype 2a (JFH1-2a) infectious virus replication in the nanomolar range. PIK93 was characterized by using quantitative chemical proteomics and in vitro biochemical assays to demonstrate PIK93 is a bone fide PI4KA and PI4KB inhibitor. Our data demonstrate that genetic or pharmacological modulation of PI4KA and PI4KB inhibits multiple genotypes of HCV and represents a novel druggable class of therapeutic targets for HCV infection.

Isoform-specific inhibition of cyclophilins

Cyclophilins belong to the enzyme class of peptidyl prolyl cis-trans isomerases which catalyze the cis-trans isomerization of prolyl bonds in peptides and proteins in different folding states. Cyclophilins have been shown to be involved in a multitude of cellular functions like cell growth, proliferation, and motility. Among the 20 human cyclophilin isoenzymes, the two most abundant members of the cyclophilin family, CypA and CypB, exhibit specific cellular functions in several inflammatory diseases, cancer development, and HCV replication. A small-molecule inhibitor on the basis of aryl 1-indanylketones has now been shown to discriminate between CypA and CypB in vitro. CypA binding of this inhibitor has been characterized by fluorescence anisotropy- and isothermal titration calorimetry-based cyclosporin competition assays. Inhibition of CypA- but not CypB-mediated chemotaxis of mouse CD4(+) T cells by the inhibitor provided biological proof of discrimination in vivo.

Naturally occurring substitutions conferring resistance to hepatitis C virus polymerase inhibitors in treatment-naive patients infected with genotypes 1–5

Background

The hepatitis C virus (HCV) RNA-dependent RNA polymerase, NS5B, is essential for virus RNA replication. It is thus an attractive therapeutic target. Several compound nucleoside analogues, non-nucleoside inhibitors and cyclosporine analogues are being developed to inhibit NS5B activity. However, nucleotide changes in the NS5B gene can confer resistance to them.

Methods

We investigated the prevalence of known substitutions conferring resistance in HCV polymerase in 124 treatment-naive French patients infected with HCV genotypes 1, 2, 3, 4 or 5 by sequencing the NS5B gene.

Results

None of the 124 HCV NS5B sequences analysed contained substitutions conferring resistance to nucleoside analogues; however, NS5B polymerases containing substitutions conferring resistance to non-nucleoside inhibitors were frequent within genotype 1 strains (17%) and very common in non-genotype 1 strains. Similarly, substitutions conferring resistance to cyclosporine analogues were more prevalent within the various genotypes.

Conclusions

Naturally occurring substitutions conferring resistance to NS5B inhibitors are common in treatment-naive patients infected with HCV genotype 1, 2, 3, 4 or 5. Their influence on treatment outcome should be assessed.

Critical role of cyclophilin A and its prolyl-peptidyl isomerase activity in the structure and function of the hepatitis C virus replication complex

Replication of hepatitis C virus (HCV) RNA occurs on intracellular membranes, and the replication complex (RC) contains viral RNA, nonstructural proteins, and cellular cofactors. We previously demonstrated that cyclophilin A (CyPA) is an essential cofactor for HCV infection and the intracellular target of cyclosporine's anti-HCV effect. Here we investigate the mechanism by which CyPA facilitates HCV replication. Cyclosporine treatment specifically blocked the incorporation of NS5B into the RC without affecting either the total protein level or the membrane association of the protein. Other nonstructural proteins or viral RNAs in the RC were not affected. NS5B from the cyclosporine-resistant replicon was resistant to this disruption of RC incorporation. We also isolated membrane fractions from both naïve and HCV-positive cells and found that CyPA is recruited into membrane fractions in HCV-replicating cells via an interaction with RC-associated NS5B, which is sensitive to cyclosporine treatment. Finally, we introduced point mutations in the prolyl-peptidyl isomerase (PPIase) motif of CyPA and demonstrated a critical role of this motif in HCV replication in cDNA rescue experiments. We propose a model in which the incorporation of the HCV polymerase into the RC depends on its interaction with a cellular chaperone protein and in which cyclosporine inhibits HCV replication by blocking this critical interaction and the PPIase activity of CyPA. Our results provide a mechanism of action for the cyclosporine-mediated inhibition of HCV and identify a critical role of CyPA's PPIase activity in the proper assembly and function of the HCV RC.

Mutations in the hepatitis C virus polymerase that increase RNA binding can confer resistance to cyclosporine A

Hepatitis C virus (HCV) infection leads to acute and chronic liver diseases, and new classes of anti-HCV therapeutics are needed. Cyclosporine A (CsA) inhibits HCV replication and CsA derivatives that lack the immunosuppressive function are currently in clinical trials as candidate anti-HCV drugs. Here we characterize several independently derived HCV replicons with varying levels of CsA resistance due to mutations in nonstructural protein 5B (NS5B), the HCV-encoded polymerase. Mutant HCV replicons engineered with these mutations showed resistance to CsA. The mutations reside in two distinct patches in the polymerase: the template channel and one face of a concave surface behind the template channel. Mutant NS5B made by cells expressing the HCV replicon had increased ability to bind to RNA in the presence of CsA. Purified recombinant NS5B proteins containing the mutations were better at de novo initiated RNA synthesis than the wild-type control. Furthermore, the mutant proteins were able to bind RNA with approximately 8-fold higher affinity. Last, mutation near the template channel alleviated the lethal phenotype of a mutation in the concave patch, P540A. This intramolecular compensation for the HCV replicase function by amino acid changes in different domains was further confirmed in an infectious cell culture-derived virus system.

CONCLUSION

An increased level of CsA resistance is associated with distinct mutations in the NS5B gene that increase RNA binding in the presence of CsA, and the intramolecular communications between residues of the thumb and the C-terminal domains are important for HCV replicase function.

New and experimental therapies for HCV

Despite improvements to treatments for HCV infection, almost half of patients cannot be cured with standard combination therapy (pegylated interferon alpha and ribavirin). The HCV life cycle offers a number of potential targets for molecular therapy, and several specifically targeted antiviral therapies for HCV (STAT-Cs) are in preclinical and clinical stages of development. Evidence to date suggests that monotherapy with any antiviral drug is unlikely to eradicate HCV infection. Combination therapy with interferon and ribavirin is necessary for the augmentation of antiviral drug activity and/or prevention of drug resistance. Results from clinical trials carried out in the past few years on STAT-C agents in combination with standard therapy with peginterferon and ribavirin provide great promise of higher rates of sustained virological response and, potentially, shorter duration of therapy than standard therapy alone achieves. Although pegylated interferon and ribavirin are likely to remain a cornerstone of therapeutic regimens in the short term, combinations of antiviral drugs of different classes, possibly along with novel agents that target host factors and modulate viral replication or augment antiviral defenses, offer the eventual possibility of interferon-free regimens.

Hepatitis C virus virology and new treatment targets

Hepatitis C virus (HCV) infection is the leading cause of chronic liver disease. An estimated 130 million people worldwide are persistently infected with HCV. Almost half of patients who have chronic HCV infection cannot be cured with the standard treatment consisting of pegylated IFN-alpha and ribavirin. For those patients who do not respond to this standard antiviral therapy, there is currently no approved treatment option available. Recent progress in structure determination of HCV proteins and development of a subgenomic replicon system enables the development of a specifically targeted antiviral therapy for hepatitis C. Many HCV-specific compounds are now under investigation in preclinical and clinical trials.

Cellular and molecular biology of HCV infection and hepatitis

HCV (hepatitis C virus) infects nearly 3% of the population worldwide and has emerged as a major causative agent of liver disease, resulting in acute and chronic infections that can lead to fibrosis, cirrhosis and hepatocellular carcinoma. Hepatitis C represents the leading cause of liver transplantation in the United States and Europe. A positive-strand RNA virus of the Flaviviridae family, HCV contains a single-stranded RNA genome of approx. 9600 nucleotides. The genome RNA serves as both mRNA for translation of viral proteins and the template for RNA replication. Cis-acting RNA elements within the genome regulate RNA replication by forming secondary structures that interact with each other and trans-acting factors. Although structural proteins are clearly dispensable for RNA replication, recent evidence points to an important role of several non-structural proteins in particle assembly and release, turning their designation on its head. HCV enters host cells through receptor-mediated endocytosis, and the process requires the co-ordination of multiple cellular receptors and co-receptors. RNA replication takes place at specialized intracellular membrane structures called 'membranous webs' or 'membrane-associated foci', whereas viral assembly probably occurs on lipid droplets and endoplasmic reticulum. Liver inflammation plays a central role in the liver damage seen in hepatitis C, but many HCV proteins also directly contribute to HCV pathogenesis. In the present review, the molecular and cellular aspects of the HCV life cycle and the role of viral proteins in pathological liver conditions caused by HCV infection are described.

Human VAP-C negatively regulates hepatitis C virus propagation

Human vesicle-associated membrane protein-associated protein (VAP) subtype A (VAP-A) and subtype B (VAP-B) are involved in the regulation of membrane trafficking, lipid transport and metabolism, and the unfolded protein response. VAP-A and VAP-B consist of the major sperm protein (MSP) domain, the coiled-coil motif, and the C-terminal transmembrane anchor and form homo- and heterodimers through the transmembrane domain. VAP-A and VAP-B interact with NS5B and NS5A of hepatitis C virus (HCV) through the MSP domain and the coiled-coil motif, respectively, and participate in the replication of HCV. VAP-C is a splicing variant of VAP-B consisting of the N-terminal half of the MSP domain of VAP-B followed by the subtype-specific frameshift sequences, and its biological function has not been well characterized. In this study, we have examined the biological functions of VAP-C in the propagation of HCV. VAP-C interacted with NS5B but not with VAP-A, VAP-B, or NS5A in immunoprecipitation analyses, and the expression of VAP-C inhibited the interaction of NS5B with VAP-A or VAP-B. Overexpression of VAP-C impaired the RNA replication of the HCV replicon and the propagation of the HCV JFH1 strain, whereas overexpression of VAP-A and VAP-B enhanced the replication. Furthermore, the expression of VAP-C was observed in various tissues, whereas it was barely detected in the liver. These results suggest that VAP-C acts as a negative regulator of HCV propagation and that the expression of VAP-C may participate in the determination of tissue tropism of HCV propagation.

Cyclophilins contribute to Stat3 signaling and survival of multiple myeloma cells

Signal transducer and activator of transcription 3 (Stat3) is the major mediator of interleukin-6 (IL-6) family cytokines. In addition, Stat3 is known to be involved in the pathophysiology of many malignancies. Here, we show that the cis-trans peptidyl-prolyl isomerase cyclophilin (Cyp) B specifically interacts with Stat3, whereas the highly related CypA does not. CypB knockdown inhibited the IL-6-induced transactivation potential but not the tyrosine phosphorylation of Stat3. Binding of CypB to Stat3 target promoters and alteration of the intranuclear localization of Stat3 on CypB depletion suggested a nuclear function of Stat3/CypB interaction. By contrast, CypA knockdown inhibited Stat3 IL-6-induced tyrosine phosphorylation and nuclear translocation. The Cyp inhibitor cyclosporine A (CsA) caused similar effects. However, Stat1 activation in response to IL-6 or interferon-gamma was not affected by Cyp silencing or CsA treatment. As a result, Cyp knockdown shifted IL-6 signaling to a Stat1-dominated pathway. Furthermore, Cyp depletion or treatment with CsA induced apoptosis in IL-6-dependent multiple myeloma cells, whereas an IL-6-independent line was not affected. Thus, Cyps support the anti-apoptotic action of Stat3. Taken together, CypA and CypB both play pivotal roles, yet at different signaling levels, for Stat3 activation and function. These data also suggest a novel mechanism of CsA action.

The cyclophilin inhibitor Debio 025 combined with PEG IFNalpha2a significantly reduces viral load in treatment-naïve hepatitis C patients

The anti-hepatitis C virus (HCV) effect and safety of three different oral doses of the cyclophilin inhibitor Debio 025 in combination with pegylated interferon-alpha2a (PEG IFN-alpha2a) were investigated in a multicenter, randomized, double-blind, placebo-controlled escalating dose-ranging phase II study in treatment-naïve patients with chronic hepatitis C. Doses of 200, 600, and 1,000 mg/day Debio 025 in combination with PEG IFN-alpha2a 180 microg/week for 4 weeks were compared with monotherapy with either 1,000 mg/day Debio 025 or 180 microg/week PEG IFN-alpha2a. In patients with genotypes 1 and 4, the 600- and 1,000-mg combination treatments induced a continuous decay in viral load that reached -4.61 +/- 1.88 and -4.75 +/- 2.19 log(10) IU/mL at week 4, respectively. In patients with genotypes 2 and 3, HCV RNA levels at week 4 were reduced by -5.91 +/- 1.11 and -5.89 +/- 0.43 log(10) IU/mL, respectively, with the same treatment regimens. Adverse events were comparable between treatment groups apart from a higher incidence of neutropenia associated with PEG IFN-alpha2a and an increased incidence of isolated hyperbilirubinemia at the highest dose of Debio 025 (1,000 mg/day).

CONCLUSION

These results confirm that Debio 025 has a potent activity and an additive effect on HCV RNA reduction in genotype 1 and 4 patients at 600 and 1,000 mg/day when combined with PEG IFN-alpha2a.

The isomerase active site of cyclophilin A is critical for hepatitis C virus replication

Cyclosporine A and nonimmunosuppressive cyclophilin (Cyp) inhibitors such as Debio 025, NIM811, and SCY-635 block hepatitis C virus (HCV) replication in vitro. This effect was recently confirmed in HCV-infected patients where Debio 025 treatment dramatically decreased HCV viral load, suggesting that Cyps inhibitors represent a novel class of anti-HCV agents. However, it remains unclear how these compounds control HCV replication. Recent studies suggest that Cyps are important for HCV replication. However, a profound disagreement currently exists as to the respective roles of Cyp members in HCV replication. In this study, we analyzed the respective contribution of Cyp members to HCV replication by specifically knocking down their expression by both transient and stable small RNA interference. Only the CypA knockdown drastically decreased HCV replication. The re-expression of an exogenous CypA escape protein, which contains escape mutations at the small RNA interference recognition site, restored HCV replication, demonstrating the specificity for the CypA requirement. We then mutated residues that reside in the hydrophobic pocket of CypA where proline-containing peptide substrates and cyclosporine A bind and that are vital for the enzymatic or the hydrophobic pocket binding activity of CypA. Remarkably, these CypA mutants fail to restore HCV replication, suggesting for the first time that HCV exploits either the isomerase or the chaperone activity of CypA to replicate in hepatocytes and that CypA is the principal mediator of the Cyp inhibitor anti-HCV activity. Moreover, we demonstrated that the HCV NS5B polymerase associates with CypA via its enzymatic pocket. The study of the roles of Cyps in HCV replication should lead to the identification of new targets for the development of alternate anti-HCV therapies.

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.

Review article: investigational agents for chronic hepatitis C

BACKGROUND

The need for effective treatment for chronic hepatitis C infection has driven the development of novel antiviral agents that target specific steps in the viral replication cycle.

AIM

To evaluate the current literature concerning investigational agents for chronic hepatitis C virus infection.

METHODS

Resources used included PubMed, conference proceedings from the American and European Liver Associations' meetings 2005-2008 and the National Institute of Health's clinical trials website (http://www.clinicaltrials.gov). The focus was restricted to investigational agents that have progressed beyond preclinical development.

RESULTS

Over 50 investigational agents for chronic hepatitis C infection are currently in clinical development. Specifically targeted anti-viral therapy for HCV (STAT-C) shows great promise with NS3/4a protease inhibitors now entering phase 3 programmes. New interferon-alpha and ribavirin formulations aim to optimize anti-viral efficacy yet limit toxicity. Other candidates include novel immunomodulators and therapeutic vaccines.

CONCLUSIONS

A new era of therapy for chronic hepatitis C beckons, promising increased cure rates with shortened duration of therapy. However, the era will not be without challenges including viral resistance, drug toxicity and the need to optimize combination therapy in the face of a rapidly evolving therapeutic arsenal.

Cyclophilin B stimulates RNA synthesis by the HCV RNA dependent RNA polymerase

Cyclophilins are cellular peptidyl isomerases that have been implicated in regulating hepatitis C virus (HCV) replication. Cyclophilin B (CypB) is a target of cyclosporin A (CsA), an immunosuppressive drug recently shown to suppress HCV replication in cell culture. Watashi et al. recently demonstrated that CypB is important for efficient HCV replication, and proposed that it mediates the anti-HCV effects of CsA through an interaction with NS5B [Watashi K, Ishii N, Hijikata M, Inoue D, Murata T, Miyanari Y, et al. Cyclophilin B is a functional regulator of hepatitis C virus RNA polymerase. Mol Cell 2005;19:111-22]. We examined the effects of purified CypB proteins on the enzymatic activity of NS5B. Recombinant CypB purified from insect cells directly stimulated NS5B-catalyzed RNA synthesis. CypB increased RNA synthesis by NS5B derived from genotype 1a, 1b, and 2a HCV strains. Stimulation appears to arise from an increase in productive RNA binding. NS5B residue Pro540, a previously proposed target of CypB peptidyl-prolyl isomerase activity, is not required for stimulation of RNA synthesis.

In vitro selection of RNA aptamers that bind the RNA-dependent RNA polymerase of hepatitis C virus: a possible role of GC-rich RNA motifs in NS5B binding

We employed SELEX (systematic evolution of ligands by exponential enrichment) and identified high affinity RNA aptamers to the hepatitis C virus NS5B RNA-dependent RNA polymerase (RdRp). GC-rich stretches were identified in many of the aptamers. Deletion of the 5'-end single-stranded GC-stretch (CGGG) of the highest binding RNA impaired the binding and the inhibitory activity of the RNA to NS5B RdRp. The majority of the mutants with a single base substitution on the CGGG motif exhibited weaker binding to NS5B. Interestingly, the CGGG motif is present on the stem structure of the NS5B coding RNA (5BSL3.2), which is considered to be an important cis-acting replication element. The 5BSL3.2 RNA showed substantial binding to NS5B, while a point mutation on the CGGG motif reduced the binding of RNA to NS5B. These results suggest a GC-stretch to be the RNA element recognized by NS5B.

A functional genomic screen identifies cellular cofactors of hepatitis C virus replication

Hepatitis C virus (HCV) chronically infects 3% of the world's population, and complications from HCV are the leading indication for liver transplantation. Given the need for better anti-HCV therapies, one strategy is to identify and target cellular cofactors of the virus lifecycle. Using a genome-wide siRNA library, we identified 96 human genes that support HCV replication, with a significant number of them being involved in vesicle organization and biogenesis. Phosphatidylinositol 4-kinase PI4KA and multiple subunits of the COPI vesicle coat complex were among the genes identified. Consistent with this, pharmacologic inhibitors of COPI and PI4KA blocked HCV replication. Targeting hepcidin, a peptide critical for iron homeostasis, also affected HCV replication, which may explain the known dysregulation of iron homeostasis in HCV infection. The host cofactors for HCV replication identified in this study should serve as a useful resource in delineating new targets for anti-HCV therapies.