Hepatitis C virus NS5A protein--a master regulator?

Hepatitis C Virus Non-Structural Protein 5A (NS5A) Disrupts Mitochondrial Dynamics and Induces Mitophagy

Mitophagy is a selective form of autophagy, targeting damaged mitochondria for lysosomal degradation. Although HCV infection has been shown to induce mitophagy, the precise underlying mechanism and the effector protein responsible remain unclear. Herein, we demonstrated that the HCV non-structural protein 5A (NS5A) plays a key role in regulating cellular mitophagy. Specifically, the expression of HCV NS5A in the hepatoma cells triggered hallmarks of mitophagy including mitochondrial fragmentation, loss of mitochondrial membrane potential, and Parkin translocation to the mitochondria. Furthermore, mitophagy induction through the expression of NS5A led to an increase in autophagic flux as demonstrated by an accumulation of LC3II in the presence of bafilomycin and a time-dependent decrease in p62 protein level. Intriguingly, the expression of NS5A concomitantly enhanced reactive oxygen species (ROS) production, and treatment with an antioxidant attenuated the NS5A-induced mitophagy event. These phenomena are similarly recapitulated in the NS5A-expressing HCV subgenomic replicon cells. Finally, we demonstrated that expression of HCV core, which has been documented to inhibit mitophagy, blocked the mitophagy induction both in cells harboring HCV replicating subgenomes or expressing NS5A alone. Our results, therefore, identified a new role for NS5A as an important regulator of HCV-induced mitophagy and have implications to broadening our understanding of the HCV-mitophagy interplay.

Small-molecule inhibitors of hepatitis C virus (HCV) non-structural protein 5A (NS5A): a patent review (2010-2015)

INTRODUCTION

Non-structural 5A (NS5A) protein has achieved a considerable attention as an attractive target for the treatment of hepatitis C (HCV). A number of novel NS5A inhibitors have been reported to date. Several drugs having favorable ADME properties and mild side effects were launched into the pharmaceutical market. For instance, daclatasvir was launched in 2014, elbasvir is currently undergoing registration, ledipasvir was launched in 2014 as a fixed-dose combination with sofosbuvir (NS5B inhibitor). Areas covered: Thomson integrity database and SciFinder database were used as a valuable source to collect the patents on small-molecule NS5A inhibitors. All the structures were ranked by the date of priority. Patent holder and antiviral activity for each scaffold claimed were summarized and presented in a convenient manner. A particular focus was placed on the best-in-class bis-pyrrolidine-containing NS5A inhibitors. Expert opinion: Several first generation NS5A inhibitors have recently progressed into advanced clinical trials and showed superior efficacy in reducing viral load in infected subjects. Therapy schemes of using these agents in combination with other established antiviral drugs with complementary mechanisms of action can address the emergence of resistance and poor therapeutic outcome frequently attributed to antiviral drugs.

Update on hepatitis C: Direct-acting antivirals

Hepatitis C virus (HCV) was discovered 26 years ago. For decades, interferon-based therapy has been the mainstay of treatment for HCV. Recently, several direct-acting antivirals (DAAs) have been approved for treatment of HCV-infected patients and to help combat the virus. These drugs have revolutionized the management of HCV as all-oral regimens with favorable side effect profiles and superior rates of sustained virological response. Emerging real-world data are demonstrating results comparable to registration trials for DAA agents. Suddenly, the potential for eradicating HCV is on the horizon.

Discovery of daclatasvir, a pan-genotypic hepatitis C virus NS5A replication complex inhibitor with potent clinical effect

The discovery and development of the first-in-class hepatitis C virus (HCV) NS5A replication complex inhibitor daclatasvir (6) provides a compelling example of the power of phenotypic screening to identify leads engaging novel targets in mechanistically unique ways. HCV NS5A replication complex inhibitors are pan-genotypic in spectrum, and this mechanistic class provides the most potent HCV inhibitors in vitro that have been described to date. Clinical trials with 6 demonstrated a potent effect on reducing plasma viral load and, in combination with mechanistically orthogonal HCV inhibitors, established the ability to cure even the most difficult infections without the need for immune stimulation. In this Drug Annotation, we describe the discovery of the original high-throughput screening lead 7 and the chemical conundrum and challenges resolved in optimizing to 6 as a clinical candidate and finally we summarize the results of select clinical studies.

Discovery and development of hepatitis C virus NS5A replication complex inhibitors

Lead inhibitors that target the function of the hepatitis C virus (HCV) nonstructural 5A (NS5A) protein have been identified by phenotypic screening campaigns using HCV subgenomic replicons. The demonstration of antiviral activity in HCV-infected subjects by the HCV NS5A replication complex inhibitor (RCI) daclatasvir (1) spawned considerable interest in this mechanistic approach. In this Perspective, we summarize the medicinal chemistry studies that led to the discovery of 1 and other chemotypes for which resistance maps to the NS5A protein and provide synopses of the profiles of many of the compounds currently in clinical trials. We also summarize what is currently known about the NS5A protein and the studies using NS5A RCIs and labeled analogues that are helping to illuminate aspects of both protein function and inhibitor interaction. We conclude with a synopsis of the results of notable clinical trials with HCV NS5A RCIs.

Zinc mesoporphyrin induces rapid proteasomal degradation of hepatitis C nonstructural 5A protein in human hepatoma cells

BACKGROUND & AIMS

The nonstructural 5A (NS5A) protein of hepatitis C virus (HCV) plays a critical role in HCV replication and is an attractive target for the therapy of HCV infection. So far, little is known about the posttranslational regulation of NS5A protein and its precise role in HCV RNA replication. Our objectives were to elucidate the down-regulation of NS5A protein and HCV RNA replication by zinc mesoporphyrin (ZnMP) and the mechanism by which this process occurs.

METHODS

Human hepatoma cells expressing HCV proteins were used to investigate the posttranslational regulation of ZnMP on NS5A protein by Western blots and immunoprecipitation. Real-time quantitative reverse transcription polymerase chain reaction was used to determine the effects of ZnMP on HCV RNA replication.

RESULTS

ZnMP selectively and markedly down-regulated NS5A protein levels by increasing degradation of NS5A protein (half-life fell from 18.7 hours to 2.7 hours). The proteasome inhibitors epoxomicin and MG132 significantly abrogated degradation of NS5A protein by ZnMP without affecting levels of NS5A in the absence of ZnMP. Analysis of immunoprecipitates with an antiubiquitin antibody revealed polyubiquitination of NS5A, suggesting that ZnMP induces ubiquitination of NS5A protein. In addition, 10 micromol/L of ZnMP reduced HCV replication by approximately 63% in the Con1 replicon cells, approximately 70% in J6/Japanese fulminant hepatitis 1 HCV-transfected cells, and approximately 90% in J6/Japanese fulminant hepatitis 1 HCV-infected cells without affecting cell viability.

CONCLUSIONS

ZnMP produces a rapid and profound down-regulation of the NS5A protein by enhancing its polyubiquitination and proteasome-dependent catabolism. ZnMP may hold promise as a novel agent to treat HCV infection.

Natural genetic engineering of hepatitis C virus NS5A for immune system counterattack

The Hepatitis C virus nonstructural 5A (NS5A) protein is a hydrophilic phosphoprotein with diverse functions. The domain assignment of NS5A had been refined using a systematic in silico bioinformatics approach using DOMAC, the protein is divided into three domains and domain III is subdivided into two subdomains using ProDom and SSEP servers. The fold structure for domains II and III were predicted using the meta-server 3D-Jury. Scanning motif databases (SMART, BLOCKS, and PROSITE) gave new motifs. Two important motifs, the interleukins 1 and 8 interaction motifs, relating to NS5A function in inducing the interleukin 8 promoter, were discovered from the BLOCKS scan. Protein-protein interaction motifs were predicted as hot loops and disordered regions, corresponding to binding regions with the ds-protein kinase R, viral polymerase, and Src homology 3 signaling proteins binding motif. Other hot loops were predicted in the V3 region and in the single-stranded DNA-binding protein motif. The different mechanisms by which the NS5A protein leads to immune system signaling dysfunction points to the natural genetic engineering of this protein.

Hepatitis C virus infection of T cells inhibits proliferation and enhances fas-mediated apoptosis by down-regulating the expression of CD44 splicing variant 6

BACKGROUND

A lymphotropic hepatitis C virus strain (HCV, SB strain, hereafter "SB-HCV") has been shown to infect established T cell lines (Molt-4 and Jurkat) and primary human naive CD4(+) T cells. During T cell development and activation, transient expression of CD44 splicing variant 6 (CD44v6) plays a significant role.

METHODS

SB-HCV was used to infect Molt-4 cells, and their cellular proliferation and CD44 expression was examined.

RESULTS

SB-HCV-infected Molt-4 cells expressed a significantly lower level of the CD44v6 isoform. The infected cells could be divided into 2 carboxyfluorescein succinimidyl ester (CFSE) groups, CFSE-high (indicating low proliferation activity; 34.2% of the cells) and CFSE-low (indicating high proliferation activity; 62.5% of the cells), whereas uninfected cells consisted of only a CFSE-low population. Of the CFSE-high cells, 82.4% were positive for the HCV protein NS5A, whereas only 1.2% of the CFSE-low cells were positive for this protein. Among the HCV proteins, NS5A alone caused the down-regulation of CD44v6 expression. After cells were stimulated with phorbol myristate acetate, the amount of phosphorylated mitogen-activated protein (MAP) kinase was significantly reduced in CFSE-high, SB-HCV-infected Molt-4 cells. After Fas ligand stimulation, SB-HCV-infected Molt-4 cells had increased cleavage of caspase 8 and 3 and enhanced apoptosis, compared with the rates of cleavage and apoptosis in control groups, indicating that SB-HCV infection increased Fas-mediated apoptosis.

CONCLUSION

HCV replication in T cells suppresses cellular proliferation and enhances susceptibility to Fas signaling by inhibiting CD44v6 signaling and expression.

Intragenotypic JFH1 based recombinant hepatitis C virus produces high levels of infectious particles but causes increased cell death

The full-length hepatitis C virus (HCV) JFH1 genome (genotype 2a) produces moderate titers of infectious particles in cell culture but the optimal determinants required for virion production are unclear. It has been shown that intragenotypic recombinants encoding core to NS2 from J6CF in the context of JFH1 are more robust in the release of viral particles. To understand the contributions of structural and nonstructural genes to HCV replication potential and infectivity, we have characterized intragenotypic recombinant genotype 2a viruses with different portions of the J6 isolate engineered into the JFH1 infectious clone. All genomes produced high levels of intracellular HCV RNA and NS3 protein in Huh-7.5 transfected cells. However, JFH1 genomes containing J6 sequences from C to E2 (CE2) or C to p7 (Cp7) secreted up to 100-fold more infectious HCV particles than the parental JFH1 clone. Subsequent infection of naive Huh-7.5 cells with each of the J6/JFH1 recombinants at a multiplicity of infection of 0.0003 resulted in high viral titers only for CE2 and Cp7 viruses. Comparison of virion production by the Cp7 J6/JFH1 recombinant to previously described J6/JFH1 recombinants showed flexibility of the chimeric junction. Moreover, NTRNS2 a chimeric virus equivalent to the previously reported FL-J6/JFH chimera, showed a 10-fold enhancement of virus titers compared to CNS2. NTRNS2 differs from CNS2 by three nucleotide differences residing in the 5' NTR and core coding sequence and all three nucleotide changes were necessary for increased virion production. Importantly, cells producing Cp7 virus showed increased apoptosis compared with JFH1, an effect correlating with virion production. These studies begin to unravel requirements for robust virus replication and the relationship between increased virion production and host cell viability.

Hepatitis C virus genetic variability in patients undergoing antiviral therapy

Hepatitis C virus (HCV) has been the subject of intense research and clinical investigations due to its worldwide prevalence and major role in chronic liver disease. Like most RNA viruses, HCV circulates in vivo as a complex population of different but closely related viral variants, commonly referred to as a quasispecies. Recent studies suggest that ribavirin might exert an antiviral effect against HCV through both mutagenic effect and an impairment of RNA replication. The introduction of alpha interferon (IFN-alpha) plus ribavirin combination therapy was an important breakthrough in the treatment of chronic HCV infection. However, the rate of sustained virological response is still unsatisfactory, particularly in patients infected with HCV genotype 1. Viral persistence, a hallmark of HCV, may result from a dynamic control of the host response by the virus. In children with chronic HCV infection, the viral population is initially highly homogeneous, but diversifies during prolonged infection which seems to be a common event during chronic hepatitis C in childhood. Coinfection of human immunodeficiency virus 1 (HIV-1) patients by HCV can complicate the treatment of these patients with highly active antiretroviral therapy (HAART). HIV coinfection is associated with a decrease of HCV quasispecies variability, which appears to be reversed by effective HAART.