HCV NS5A and IRF9 compete for CypA binding

Inhibitors of Cyclophilin A: Current and Anticipated Pharmaceutical Agents for Inflammatory Diseases and Cancers

Cyclophilin A, a widely prevalent cellular protein, exhibits peptidyl-prolyl cis-trans isomerase activity. This protein is predominantly located in the cytosol; additionally, it can be secreted by the cells in response to inflammatory stimuli. Cyclophilin A has been identified to be a key player in many of the biological events and is therefore involved in several diseases, including vascular and inflammatory diseases, immune disorders, aging, and cancers. It represents an attractive target for therapeutic intervention with small molecule inhibitors such as cyclosporin A. Recently, a number of novel inhibitors of cyclophilin A have emerged. However, it remains elusive whether and how many cyclophilin A inhibitors function in the inflammatory diseases and cancers. In this review, we discuss current available data about cyclophilin A inhibitors, including cyclosporin A and its derivatives, quinoxaline derivatives, and peptide analogues, and outline the most recent advances in clinical trials of these agents. Inhibitors of cyclophilin A are poised to enhance our comprehension of the molecular mechanisms that underpin inflammatory diseases and cancers associated with cyclophilin A. This advancement will aid in the development of innovative pharmaceutical treatments in the future.

The Cyclophilin Inhibitor Rencofilstat Decreases HCV-Induced Hepatocellular Carcinoma Independently of Its Antiviral Activity

There is an urgent need for the identification of new drugs that inhibit HCV-induced hepatocellular carcinoma (HCC). Our work demonstrates that cyclophilin inhibitors (CypIs) represent such new drugs. We demonstrate that the nonimmunosuppressive cyclosporine A (CsA) analog (CsAa) rencofilstat possesses dual therapeutic activities for the treatment of HCV infection and HCV-induced HCC. Specifically, we show that the HCV infection of humanized mice results in the progressive development of HCC. This is true for the four genotypes tested (1 to 4). Remarkably, we demonstrate that rencofilstat inhibits the development of HCV-induced HCC in mice even when added 16 weeks after infection when HCC is well established. Importantly, we show that rencofilstat drastically reduces HCC progression independently of its anti-HCV activity. Indeed, the CypI rencofilstat inhibits HCC, while other anti-HCV agents such as NS5A (NS5Ai) and NS5B (NS5Bi) fail to reduce HCC. In conclusion, this study shows for the first time that the CypI rencofilstat represents a potent therapeutic agent for the treatment of HCV-induced HCC.

The Cyclophilin Inhibitor Rencofilstat Decreases HCV-induced Hepatocellular Carcinoma Independently of Its Antiviral Activity

There is an urgent need for the identification of new drugs that inhibit HCV-induced hepatocellular carcinoma (HCC). Our work demonstrates that cyclophilin inhibitors (CypI) represent such new drugs. We demonstrated that the non-immunosuppressive cyclosporine A (CsA) analog (CsAa) rencofilstat possesses dual therapeutic activities for the treatment of HCV infection and HCV-induced HCC. Specifically, we showed that HCV infection of humanized mice results in the progressive development of HCC. This was true for four genotypes tested (1 to 4). Remarkably, we demonstrated that rencofilstat inhibits the development of HCV-induced HCC in mice even when added 16 weeks post-infection when HCC is well established. Importantly, we showed that rencofilstat drastically reduces HCC progression independently of its anti-HCV activity. Indeed, the CypI rencofilstat inhibits HCC while other anti-HCV agents such as NS5A (NS5Ai) and NS5B (NS5Bi) fail to reduce HCC. In conclusion, this study shows for the first time that the CypI rencofilstat represents a potent therapeutic agent for the treatment of HCV-induced HCC.

African Swine Fever Virus MGF505-7R Interacts with Interferon Regulatory Factor 9 to Evade the Type I Interferon Signaling Pathway and Promote Viral Replication

African swine fever (ASF) is an acute and severe infectious disease caused by the ASF virus (ASFV). The mortality rate of ASF in pigs can reach 100%, causing huge economic losses to the pig industry. Here, we found that ASFV protein MGF505-7R inhibited the beta interferon (IFN-β)-mediated Janus-activated kinase-signal transducer and activation of transcription (JAK-STAT) signaling. Our results demonstrate that MGF505-7R inhibited interferon-stimulated gene factor 3 (ISGF3)-mediated IFN-stimulated response element (ISRE) promoter activity. Importantly, we observed that MGF505-7R inhibits ISGF3 heterotrimer formation by interacting with interferon regulatory factor 9 (IRF9) and inhibits the nuclear translocation of ISGF3. Moreover, to demonstrate the role of MGF505-7R in IFN-I signal transduction during ASFV infection, we constructed and evaluated ASFV-ΔMGF505-7R recombinant viruses. ASFV-ΔMGF505-7R restored STAT2 and STAT1 phosphorylation, alleviated the inhibition of ISGF3 nuclear translocation, and showed increased susceptibility to IFN-β, unlike the parental GZ201801 strain. In conclusion, our study shows that ASFV protein MGF505-7R plays a key role in evading IFN-I-mediated innate immunity, revealing a new mode of evasion for ASFV. IMPORTANCE ASF, caused by ASFV, is currently prevalent in Eurasia, with mortality rates reaching 100% in pigs. At present, there are no safe or effective vaccines against ASFV. In this study, we found that the ASFV protein MGF505-7R hinders IFN-β signaling by interacting with IRF9 and inhibiting the formation of ISGF3 heterotrimers. Of note, we demonstrated that MGF505-7R plays a role in the immune evasion of ASFV in infected hosts and that recombinant viruses alleviated the effect on type I IFN (IFN-I) signaling and exhibited increased susceptibility to IFN-β. This study provides a theoretical basis for developing vaccines against ASFV using strains with MGF505-7R gene deletions.

Cyclophilin A Inhibits Human Respiratory Syncytial Virus (RSV) Replication by Binding to RSV-N through Its PPIase Activity

Human respiratory syncytial virus (hRSV) is the most common pathogen which causes acute lower respiratory infection (ALRI) in infants. Recently, virus-host interaction has become a hot spot of virus-related research, and it needs to be further elaborated for RSV infection. In this study, we found that RSV infection significantly increased the expression of cyclophilin A (cypA) in clinical patients, mice, and epithelial cells. Therefore, we evaluated the function of cypA in RSV replication and demonstrated that virus proliferation was accelerated in cypA knockdown host cells but restrained in cypA-overexpressing host cells. Furthermore, we proved that cypA limited RSV replication depending on its PPIase activity. Moreover, we performed liquid chromatography-mass spectrometry, and the results showed that cypA could interact with several viral proteins, such as RSV-N, RSV-P, and RSV-M2-1. Finally, the interaction between cypA and RSV-N was certified by coimmunoprecipitation and immunofluorescence. Those results provided strong evidence that cypA may play an inhibitory role in RSV replication through interaction with RSV-N via its PPIase activity.

IMPORTANCE RSV-N, packed in the viral genome to form the ribonucleoprotein (RNP) complex, which is recognized by the RSV RNA-dependent RNA polymerase (RdRp) complex to initiate viral replication and transcription, plays an indispensable role in the viral biosynthesis process. cypA, binding to RSV-N, may impair this function by weakening the interaction between RSV-N and RSV-P, thus leading to decreased viral production. Our research provides novel insight into cypA antiviral function, including binding to viral capsid protein to inhibit viral replication, which may be helpful for new antiviral drug exploration.

Advances in Foot-and-Mouth Disease Virus Proteins Regulating Host Innate Immunity

Foot-and-mouth disease (FMD) is a highly contagious disease that affects cloven-hoofed animals such as pigs, cattle, and sheep. The disease is caused by the foot-and-mouth disease virus (FMDV) which has a non-enveloped virion with icosahedral symmetry that encapsulates a positive-sense, single-stranded RNA genome of ∼8.4 kb. FMDV infection causes obvious immunosuppressive effects on the host. In recent years, studies on the immunosuppressive mechanism of FMDV have become a popular topic. In addition, studies have shown that many FMDV proteins are involved in the regulation of host innate immunity and have revealed mechanisms by which FMDV proteins mediate host innate immunity. In this review, advances in studies on the mechanisms of interaction between FMDV proteins and host innate immunity are summarized to provide a comprehensive understanding of FMDV pathogenesis and the theoretical basis for FMD prevention and control.

Interferon Regulatory Factor 9 Structure and Regulation

Interferon regulatory factor 9 (IRF9) is an integral transcription factor in mediating the type I interferon antiviral response, as part of the interferon-stimulated gene factor 3. However, the role of IRF9 in many important non-communicable diseases has just begun to emerge. The duality of IRF9’s role in conferring protection but at the same time exacerbates diseases is certainly puzzling. The regulation of IRF9 during these conditions is not well understood. The high homology of IRF9 DNA-binding domain to other IRFs, as well as the recently resolved IRF9 IRF-associated domain structure can provide the necessary insights for progressive inroads on understanding the regulatory mechanism of IRF9. This review sought to outline the structural basis of IRF9 that guides its regulation and interaction in antiviral immunity and other diseases.

Foot-and-mouth disease virus nonstructural protein 2B interacts with cyclophilin A, modulating virus replication

Foot-and-mouth disease (FMD) is a highly contagious virus that affects cloven-hoofed animals. To understand better the role of nonstructural protein 2B of the causative agent FMD virus (FMDV) in the process of virus replication, we identified a porcine host protein, cyclophilin A (CypA), which interacts with FMDV 2B. The 2B-CypA interaction was confirmed by coimmunoprecipitation and GST pull-down assays. CypA showed antiviral functions during FMDV infection. Overexpression of CypA decreased FMDV leader protein (Lpro) and 3A at protein levels. CypA-induced reduction of Lpro enhanced the synthesis of host proteins and increased the integrality of host eukaryotic translation initiation factor (eIF)-4γ (eIF4G). The reduction of Lpro and 3A was dependent on the proteasome pathway. No interaction was identified between CypA and Lpro or 3A. However, CypA-induced reduction of Lpro and 3A was suppressed by 2B, and disruption of 2B-CypA interaction impaired this inhibitive effect induced by 2B. In summary, our findings identify the antiviral role of CypA against FMDV and provide key insights into how FMDV antagonizes host antiviral response by 2B protein.-Liu, H., Xue, Q., Cao, W., Yang, F., Ma, L., Liu, W., Zhang, K., Liu, X., Zhu, Z., Zheng, H. Foot-and-mouth disease virus nonstructural protein 2B interacts with cyclophilin A, modulating virus replication.

Immune Evasion Strategies during Chronic Hepatitis B and C Virus Infection

Both hepatitis B virus (HBV) and hepatitis C virus (HCV) infections are a major global healthcare problem with more than 240 million and 70 million infected, respectively. Both viruses persist within the liver and result in progressive liver disease, resulting in liver fibrosis, cirrhosis and hepatocellular carcinoma. Strikingly, this pathogenesis is largely driven by immune responses, unable to clear an established infection, rather than by the viral pathogens themselves. Even though disease progression is very similar in both infections, HBV and HCV have evolved distinct mechanisms, by which they ensure persistence within the host. Whereas HCV utilizes a cloak-and-dagger approach, disguising itself as a lipid-like particle and immediately crippling essential pattern-recognition pathways, HBV has long been considered a "stealth" virus, due to the complete absence of innate immune responses during infection. Recent developments and access to improved model systems, however, revealed that even though it is among the smallest human-tropic viruses, HBV may, in addition to evading host responses, employ subtle immune evasion mechanisms directed at ensuring viral persistence in the absence of host responses. In this review, we compare the different strategies of both viruses to ensure viral persistence by actively interfering with viral recognition and innate immune responses.

Systems Biology Reveals NS4B-Cyclophilin A Interaction: A New Target to Inhibit YFV Replication

Yellow fever virus (YFV) replication is highly dependent on host cell factors. YFV NS4B is reported to be involved in viral replication and immune evasion. Here interactions between NS4B and human proteins were determined using a GST pull-down assay and analyzed using 1-DE and LC-MS/MS. We present a total of 207 proteins confirmed using Scaffold 3 Software. Cyclophilin A (CypA), a protein that has been shown to be necessary for the positive regulation of flavivirus replication, was identified as a possible NS4B partner. 59 proteins were found to be significantly increased when compared with a negative control, and CypA exhibited the greatest difference, with a 22-fold change. Fisher's exact test was significant for 58 proteins, and the p value of CypA was the most significant (0.000000019). The Ingenuity Systems software identified 16 pathways, and this analysis indicated sirolimus, an mTOR pathway inhibitor, as a potential inhibitor of CypA. Immunofluorescence and viral plaque assays showed a significant reduction in YFV replication using sirolimus and cyclosporine A (CsA) as inhibitors. Furthermore, YFV replication was strongly inhibited in cells treated with both inhibitors using reporter BHK-21-rep-YFV17D-LucNeoIres cells. Taken together, these data suggest that CypA-NS4B interaction regulates YFV replication. Finally, we present the first evidence that YFV inhibition may depend on NS4B-CypA interaction.

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

Background

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

Methods

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

Results

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

Conclusion

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

Functional differences in hepatitis C virus nonstructural (NS) 3/4A- and 5A-specific T cell responses

The hepatitis C virus nonstructural (NS) 3/4A and NS5A proteins are major targets for the new direct-acting antiviral compounds. Both viral proteins have been suggested as modulators of the response to the host cell. We have shown that NS3/4A- and NS5A-specific T cell receptors confer different effector functions, and that killing of NS3/4A-expressing hepatocytes is highly dependent on IFN-γ. We here characterize the functional differences in the T cell responses to NS3/4A and NS5A. NS3/4A- and NS5A-specific T cells could be induced at various frequencies in wild-type-, NS3/4A-, and NS5A-transgenic mice. Priming of NS5A-specific T cells required a high DNA dose, and was unlike NS3/4A dependent on both CD4⁺ and CD8⁺ T cells, but less influenced by CD25⁺/GITR⁺ regulatory T cells. The presence of IL-12 greatly improved specific CD8⁺ T cell priming by NS3/4A but not by NS5A, suggesting a less dependence of IFN-γ for NS5A. This notion was supported by the observation that NS5A-specific T cells could eliminate NS5A-expressing hepatocytes also in the absence of IFN-γ-receptor-2. This supports that NS3/4A- and NS5A-specific T cells become activated and eliminate antigen expressing, or infected hepatocytes, by distinct mechanisms, and that NS5A-specific T cells show an overall less dependence of IFN-γ.

Chaperones in hepatitis C virus infection

The hepatitis C virus (HCV) infects approximately 3% of the world population or more than 185 million people worldwide. Each year, an estimated 350000-500000 deaths occur worldwide due to HCV-associated diseases including cirrhosis and hepatocellular carcinoma. HCV is the most common indication for liver transplantation in patients with cirrhosis worldwide. HCV is an enveloped RNA virus classified in the genus Hepacivirus in the Flaviviridae family. The HCV viral life cycle in a cell can be divided into six phases: (1) binding and internalization; (2) cytoplasmic release and uncoating; (3) viral polyprotein translation and processing; (4) RNA genome replication; (5) encapsidation (packaging) and assembly; and (6) virus morphogenesis (maturation) and secretion. Many host factors are involved in the HCV life cycle. Chaperones are an important group of host cytoprotective molecules that coordinate numerous cellular processes including protein folding, multimeric protein assembly, protein trafficking, and protein degradation. All phases of the viral life cycle require chaperone activity and the interaction of viral proteins with chaperones. This review will present our current knowledge and understanding of the role of chaperones in the HCV life cycle. Analysis of chaperones in HCV infection will provide further insights into viral/host interactions and potential therapeutic targets for both HCV and other viruses.

Porcine bocavirus NP1 negatively regulates interferon signaling pathway by targeting the DNA-binding domain of IRF9

To subvert host antiviral immune responses, many viruses have evolved countermeasures to inhibit IFN signaling pathway. Porcine bocavirus (PBoV), a newly identified porcine parvovirus, has received attention because it shows clinically high co-infection prevalence with other pathogens in post-weaning multisystemic wasting syndrome (PWMS) and diarrheic piglets. In this study, we screened the structural and non-structural proteins encoded by PBoV and found that the non-structural protein NP1 significantly suppressed IFN-stimulated response element (ISRE) activity and subsequent IFN-stimulated gene (ISG) expression. However, NP1 affected neither the activation and translocation of STAT1/STAT2, nor the formation of the heterotrimeric transcription factor complex ISGF3 (STAT1/STAT2/IRF9). Detailed analysis demonstrated that PBoV NP1 blocked the ISGF3 DNA-binding activity by combining with the DNA-binding domain (DBD) of IRF9. In summary, these results indicate that PBoV NP1 interferes with type I IFN signaling pathway by blocking DNA binding of ISGF3 to attenuate innate immune responses.

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Porcine bocavirus (PBoV) NP1 interferes with the IFN α/β signaling pathway.

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PBoV NP1 does not prevent STAT1/STAT2 phosphorylation and nuclear translocation.

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PBoV NP1 inhibits the DNA-binding activity of ISGF3.

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PBoV NP1 interacts with the DNA-binding domain of IRF9.

Bioengineering and semisynthesis of an optimized cyclophilin inhibitor for treatment of chronic viral infection

Inhibition of host-encoded targets, such as the cyclophilins, provides an opportunity to generate potent high barrier to resistance antivirals for the treatment of a broad range of viral diseases. However, many host-targeted agents are natural products, which can be difficult to optimize using synthetic chemistry alone. We describe the orthogonal combination of bioengineering and semisynthetic chemistry to optimize the drug-like properties of sanglifehrin A, a known cyclophilin inhibitor of mixed nonribosomal peptide/polyketide origin, to generate the drug candidate NVP018 (formerly BC556). NVP018 is a potent inhibitor of hepatitis B virus, hepatitis C virus (HCV), and HIV-1 replication, shows minimal inhibition of major drug transporters, and has a high barrier to generation of both HCV and HIV-1 resistance.

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Optimization and preclinical analysis of a bacterial natural product

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Combination of bioengineering and semisynthetic chemistry

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Preclinical analysis revealing potent antiviral activity

Discovery of functionalized bisimidazoles bearing cyclic aliphatic-phenyl motifs as HCV NS5A inhibitors

This Letter describes the discovery of a number of functionalized bisimidazoles bearing a cyclohexylphenyl, piperidylphenyl, or bicyclo[2,2,2]octylphenyl motif as HCV NS5A inhibitors. Compounds 2c, 4b and 6 have demonstrated low single-digit nM potency in gt-1a replicon and double-digit pM potency in gt-1b replicon, respectively. Moreover, both 4b and 6 have, respectively, exhibited good oral bioavailability in rats with a favorable liver/plasma ratio of the drug concentration.

Kinetic analyses reveal potent and early blockade of hepatitis C virus assembly by NS5A inhibitors

BACKGROUND & AIMS

All-oral regimens combining different classes of direct-acting antivirals (DAA) are highly effective for treatment of patients with chronic hepatitis C. NS5A inhibitors will likely form a component of future interferon-sparing treatment regimens. However, despite their potential, the detailed mechanism of action of NS5A inhibitors is unclear. To study their mechanisms, we compared their kinetics of antiviral suppression with those of other classes of DAA, using the hepatitis C virus genotype 1a cell culture-infectious virus H77S.3.

METHODS

We performed detailed kinetic analyses of specific steps in the hepatitis C virus life cycle using cell cultures incubated with protease inhibitors, polymerase inhibitors, or NS5A inhibitors. Assays were designed to measure active viral RNA synthesis and steady-state RNA abundance, polyprotein synthesis, virion assembly, and infectious virus production.

RESULTS

Despite their high potency, NS5A inhibitors were slow to inhibit viral RNA synthesis compared with protease or polymerase inhibitors. By 24 hours after addition of an NS5A inhibitor, polyprotein synthesis was reduced <50%, even at micromolar concentrations. In contrast, inhibition of virus release by NS5A inhibitors was potent and rapid, with onset of inhibition as early as 2 hours. Cells incubated with NS5A inhibitors were rapidly depleted of intracellular infectious virus and RNA-containing hepatitis C virus particles, indicating a block in virus assembly.

CONCLUSIONS

DAAs that target NS5A rapidly inhibit intracellular assembly of genotype 1a virions. They also inhibit formation of functional replicase complexes, but have no activity against preformed replicase, thereby resulting in slow shut-off of viral RNA synthesis.

The HCV life cycle: in vitro tissue culture systems and therapeutic targets

Hepatitis C virus (HCV) is a highly variable plus-strand RNA virus of the family Flaviviridae. Viral strains are grouped into six epidemiologically relevant genotypes that differ from each other by more than 30% at the nucleotide level. The variability of HCV allows immune evasion and facilitates persistence. It is also a substantial challenge for the development of specific antiviral therapies effective across all HCV genotypes and for prevention of drug resistance. Novel HCV cell culture models were instrumental for identification and profiling of therapeutic strategies. Concurrently, these models revealed numerous host factors critical for HCV propagation, some of which have emerged as targets for antiviral therapy. It is generally assumed that the use of host factors is conserved among HCV isolates and genotypes. Additionally, the barrier to viral resistance is thought to be high when interfering with host factors. Therefore, current drug development includes both targeting of viral factors but also of host factors essential for virus replication. In fact, some of these host-targeting agents, for instance inhibitors of cyclophilin A, have advanced to late stage clinical trials. Here, we highlight currently available cell culture systems for HCV, review the most prominent host-targeting strategies against hepatitis C and critically discuss opportunities and risks associated with host-targeting antiviral strategies.

From chemical tools to clinical medicines: nonimmunosuppressive cyclophilin inhibitors derived from the cyclosporin and sanglifehrin scaffolds

The cyclophilins are widely expressed enzymes that catalyze the interconversion of the cis and trans peptide bonds of prolines. The immunosuppressive natural products cyclosporine A and sanglifehrin A inhibit the enzymatic activity of the cyclophilins. Chemical modification of both the cyclosporine and sanglifehrin scaffolds has produced many analogues that inhibit cyclophilins in vitro but have reduced immunosuppressive properties. Three nonimmunosuppressive cyclophilin inhibitors (alisporivir, SCY-635, and NIM811) have demonstrated clinical efficacy for the treatment of hepatitis C infection. Additional candidates are in various stages of preclinical development for the treatment of hepatitis C or myocardial reperfusion injury. Recent publications suggest that cyclophilin inhibitors may have utility for the treatment of diverse viral infections, inflammatory indications, and cancer. In this review, we document the structure-activity relationships of the nonimmunosuppressive cyclosporins and sanglifehrins in clinical and preclinical development. Aspects of the pharmacokinetic behavior and chemical biology of these drug candidates are also described.

Interferon regulatory factor 9 protects against cardiac hypertrophy by targeting myocardin

Pathological cardiac hypertrophy is a major risk factor for heart failure. In this study, we identified interferon regulatory factor 9 (IRF9), a member of the IRF family, as a previously unidentified negative regulator of cardiac hypertrophy. The level of IRF9 expression was remarkably elevated in the hearts from animals with aortic banding-induced cardiac hypertrophy. IRF9-deficient mice exhibited pronounced cardiac hypertrophy after pressure overload, as demonstrated by increased cardiomyocyte size, extensive fibrosis, reduced cardiac function, and enhanced expression of hypertrophy markers, whereas transgenic mice with cardiac-specific overexpression of murine IRF9 exhibited a significant reduction in the hypertrophic response. Mechanistically, IRF9 competes with p300 for binding to the transcription activation domain of myocardin, a coactivator of serum response factor (SRF). This interaction markedly suppresses the transcriptional activity of myocardin because IRF9 overexpression strongly inhibits the ability of myocardin to activate CArG box-dependent reporters. These results provide compelling evidence that IRF9 inhibits the development of cardiac hypertrophy by suppressing the transcriptional activity of myocardin in the heart.

Opportunities and Risks of Host-targeting Antiviral Strategies for Hepatitis C

Hepatitis C virus (HCV) infects more than 2 % of the world population with highest prevalence in parts of Africa and Asia. Past standard of care using interferon α and ribavirin had adverse effects and showed modest efficacy for some HCV genotypes spurring the development of direct acting antivirals (DAAs). Such DAAs target viral proteins and are thus better tolerated but they suffer from emergence of vial resistance. Furthermore, DAAs are often HCV genotype specific. Novel drug candidates targeting host factors required for HCV propagation, so called host-targeting antivirals (HTAs), promise to overcome both caveats. The genetic barrier to resistance is usually considered to be high for HTAs and all HCV genotypes presumably use the same host factors. Recent data, however, challenge these assumptions, at least for some HTAs. Here, we highlight the most important host-targeting strategies against hepatitis C and critically discuss their opportunities and risks.

Cyclophilins as modulators of viral replication

Cyclophilins are peptidyl‐prolyl cis/trans isomerases important in the proper folding of certain proteins. Mounting evidence supports varied roles of cyclophilins, either positive or negative, in the life cycles of diverse viruses, but the nature and mechanisms of these roles are yet to be defined. The potential for cyclophilins to serve as a drug target for antiviral therapy is evidenced by the success of non-immunosuppressive cyclophilin inhibitors (CPIs), including Alisporivir, in clinical trials targeting hepatitis C virus infection. In addition, as cyclophilins are implicated in the predisposition to, or severity of, various diseases, the ability to specifically and effectively modulate their function will prove increasingly useful for disease intervention. In this review, we will summarize the evidence of cyclophilins as key mediators of viral infection and prospective drug targets.

Optimization of Cyclophilin Inhibitors for Use in Antiviral Therapy

Cyclophilins are members of the Propyl Peptidase Isomerase (PPIase) family of proteins and have recently been found to be required for efficient replication and/or infectivity of several viruses. Cyclosporine A (CsA), the prototypical inhibitor of cyclophilins has shown good activity against several key viruses, including HIV‐1 and HCV, however the immunosuppressive activity of CsA precludes its use as an effective anti‐viral agent. Structural information derived from the ternary complex formed by CsA, cyclophilin A and calcineurin has allowed the design of non‐immunosuppressive derivatives of CsA that retain, and in some cases improve, antiviral activity toward hepatitis C. Chemical modification of CsA has led to compounds with improved pharmacokinetic properties and with reduced drug‐drug interaction potential. Non‐CsA derived inhibitors of cyclophilin A have recently been identified and hold promise as synthetically more tractable leads for cyclophilin‐based discovery projects.

Profile of alisporivir and its potential in the treatment of hepatitis C

Two classes of hepatitis C antiviral agents currently exist, ie, direct-acting antivirals and host-targeting antivirals. Direct-acting antivirals target viral proteins including NS3/NS4A protease, NS5B polymerase and NS5A protein, while host-targeting antivirals target various host proteins critical for replication of the hepatitis C virus (HCV). Alisporivir is the most advanced host-targeting antiviral in clinical development. Alisporivir blocks HCV replication by neutralizing the peptidyl-prolyl isomerase activity of the abundant host cytosolic protein, cyclophilin A. Due to its unique mechanism of antiviral action, alisporivir is pangenotypic, provides a high barrier for development of viral resistance, and does not permit cross-resistance to direct-acting antivirals. Alisporivir has an excellent pharmacokinetic and safety profile. Phase I and II clinical studies have demonstrated that alisporivir causes a dramatic reduction in viral loads in HCV-infected patients. Alisporivir was shown to be highly potent in treatment-naïve and treatment-experienced patients with genotype 1 as well as in those with genotypes 2 or 3. Low viral breakthrough rates were observed and the most frequent clinical and laboratory adverse events associated with alisporivir in combination with pegylated interferon-alpha and ribavirin were similar to those associated with pegylated interferon-alpha and ribavirin used alone. A laboratory abnormality observed in some patients receiving alisporivir is hyperbilirubinemia, which is related to transporter inhibition and not to liver toxicity. The most recent clinical results suggest that alisporivir plus other direct-acting antivirals should provide a successful treatment option for difficult-to-treat populations, such as nonresponders to prior interferon-alpha therapy and patients with cirrhosis. In conclusion, alisporivir represents an attractive candidate component of future interferon-free regimens.