Molecular determinants of TRIF proteolysis mediated by the hepatitis C virus NS3/4A protease

From viruses to cancer: exploring the role of the hepatitis C virus NS3 protein in carcinogenesis

Hepatitis C virus (HCV) chronically infects approximately 170 million people worldwide and is a known etiological agent of hepatocellular carcinoma (HCC). The molecular mechanisms of HCV-mediated carcinogenesis are not fully understood. This review article focuses on the oncogenic potential of NS3, a viral protein with transformative effects on cells, although the precise mechanisms remain elusive. Unlike the more extensively studied Core and NS5A proteins, NS3's roles in cancer development are less defined but critical. Research indicates that NS3 is implicated in several carcinogenic processes such as proliferative signaling, cell death resistance, genomic instability and mutations, invasion and metastasis, tumor-related inflammation, immune evasion, and replicative immortality. Understanding the direct impact of viral proteins such as NS3 on cellular transformation is crucial for elucidating HCV's role in HCC development. Overall, this review sheds light on the molecular mechanisms used by NS3 to contribute to hepatocarcinogenesis, and highlights its significance in the context of HCV-associated HCC, underscoring the need for further investigation into its specific molecular and cellular actions.

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

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

Role of hepcidin upregulation and proteolytic cleavage of ferroportin 1 in hepatitis C virus-induced iron accumulation

Hepatitis C virus (HCV) is a pathogen characterized not only by its persistent infection leading to the development of cirrhosis and hepatocellular carcinoma (HCC), but also by metabolic disorders such as lipid and iron dysregulation. Elevated iron load is commonly observed in the livers of patients with chronic hepatitis C, and hepatic iron overload is a highly profibrogenic and carcinogenic factor that increases the risk of HCC. However, the underlying mechanisms of elevated iron accumulation in HCV-infected livers remain to be fully elucidated. Here, we observed iron accumulation in cells and liver tissues under HCV infection and in mice expressing viral proteins from recombinant adenoviruses. We established two molecular mechanisms that contribute to increased iron load in cells caused by HCV infection. One is the transcriptional induction of hepcidin, the key hormone for modulating iron homeostasis. The transcription factor cAMP-responsive element-binding protein hepatocyte specific (CREBH), which was activated by HCV infection, not only directly recognizes the hepcidin promoter but also induces bone morphogenetic protein 6 (BMP6) expression, resulting in an activated BMP-SMAD pathway that enhances hepcidin promoter activity. The other is post-translational regulation of the iron-exporting membrane protein ferroportin 1 (FPN1), which is cleaved between residues Cys284 and Ala285 in the intracytoplasmic loop region of the central portion mediated by HCV NS3-4A serine protease. We propose that host transcriptional activation triggered by endoplasmic reticulum stress and FPN1 cleavage by viral protease work in concert to impair iron efflux, leading to iron accumulation in HCV-infected cells.

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

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

Progress, evolving therapeutic/diagnostic approaches, and challenges in the management of hepatitis C virus infections

Hepatitis C virus (HCV) infections are emerging as one of the foremost challenges in healthcare owing to its chronicity and the virus's quasispecies nature. Worldwide, over 170 million people are chronically infected with HCV, with an annual mortality of over 500,000 people across the world. The emerging pathophysiological evidence links HCV infections to a risk of developing liver diseases such as cirrhosis and hepatocellular carcinoma. Despite the great strides that have been made towards understanding the pathophysiology of disease progression, the tailored treatments of HCV infection remain to be established. The present review provides an update of the literature pertaining to evolving therapeutic approaches and prophylactic measures for the effective management of HCV infections. An extensive discussion of established and experimental immune prophylactic measures also sheds light on current developments in the design of vaccination strategies against HCV infection. We have also attempted to address the application of nanotechnology in formulating effective therapeutic interventions against HCV. Pointing out the limitations of the existing diagnostic methods and therapeutic approaches against HCV might inspire the design and development of novel, efficient, reliable, and cost-effective diagnostic technologies as well as novel therapeutic and immune prophylactic interventions for the effective management of HCV.

Innate Immunity in Hepatitis C Virus Infection

Activation and viral control of the innate immune response are hallmarks of hepatitis C virus (HCV) infection and are major determinants of spontaneous clearance or progression to chronic infection and liver disease. In this review, we provide a contemporary overview of how HCV is sensed by the host cell to trigger innate immune activation and the mechanisms deployed by the virus to evade this response. Type I and III interferons (IFNs) are crucial mediators of antiviral innate immunity against HCV, and we specifically highlight the importance of IFN-λ host genetics for the outcome of HCV infection. Last, we focus on the proinflammatory responses elicited by HCV infection and describe our current understanding of how interleukin (IL)-1β signaling and cross talk between the IL-1β and IFN signaling pathways lead to sustained inflammation and increased risk of liver pathology.

[Hepatitis C virus (Flaviviridae: Hepacivirus: Hepacivirus C): regulation of signaling reactions of innate immunity]

Studying the regulation of signaling reactions of innate immunity by the hepatitis C virus (HCV) will help to reveal the causes of the transition of the acute form of the disease to a chronic course. The molecular mechanisms of activation by HCV RNA of innate immunity receptors TLR and RLR and signal transduction processes leading to the synthesis of IFN and inflammatory cytokines are considered. The inhibitory effects of non-structural and structural HCV proteins on immune signaling reactions are analyzed in detail. The information presented is the result of an analysis of literature data published in international databases mainly over the past 5 years. In conclusion, signaling receptors are proposed as targets for the development of new antiviral drugs with immunotherapeutic activity.

Hepatitis C virus NS3-4A protease regulates the lipid environment for RNA replication by cleaving host enzyme 24-dehydrocholesterol reductase

Many RNA viruses create specialized membranes for genome replication by manipulating host lipid metabolism and trafficking, but in most cases, we do not know the molecular mechanisms responsible or how specific lipids may impact the associated membrane and viral process. For example, hepatitis C virus (HCV) causes a specific, large-fold increase in the steady-state abundance of intracellular desmosterol, an immediate precursor of cholesterol, resulting in increased fluidity of the membrane where HCV RNA replication occurs. Here, we establish the mechanism responsible for HCV's effect on intracellular desmosterol, whereby the HCV NS3-4A protease controls activity of 24-dehydrocholesterol reductase (DHCR24), the enzyme that catalyzes conversion of desmosterol to cholesterol. Our cumulative evidence for the proposed mechanism includes immunofluorescence microscopy experiments showing co-occurrence of DHCR24 and HCV NS3-4A protease; formation of an additional, faster-migrating DHCR24 species (DHCR24*) in cells harboring a HCV subgenomic replicon RNA or ectopically expressing NS3-4A; and biochemical evidence that NS3-4A cleaves DHCR24 to produce DHCR24* in vitro and in vivo. We further demonstrate that NS3-4A cleaves DHCR24 between residues Cys⁹¹ and Thr⁹² and show that this reduces the intracellular conversion of desmosterol to cholesterol. Together, these studies demonstrate that NS3-4A directly cleaves DHCR24 and that this results in the enrichment of desmosterol in the membranes where NS3-4A and DHCR24 co-occur. Overall, this suggests a model in which HCV directly regulates the lipid environment for RNA replication through direct effects on the host lipid metabolism.

Potent HCV NS3 Protease Inhibition by a Water-Soluble Phyllanthin Congener

NS3/4A protease of hepatitis C virus (HCV) plays an important role in viral RNA replication. A 1,4-diphenylbutanedicarboxylic acid derivative, namely, phyllanthin, extracted from the leaf of a herbal plant, Phyllanthus amarus, inhibits HCV NS3/4A protease and replication activities. However, the reduced aqueous solubility, high toxicity, and poor oral bioavailability are major impediments with phyllanthin. We herein present a design approach to generate phyllanthin congeners in order to potentiate inhibition activity against protease. The phyllanthin congeners were synthesized by chemical methods and subjected to systematic biological studies. One of the congeners, annotated as D8, is identified as a novel and potent inhibitor of the HCV-NS3/4Aprotease activity in vitro and the viral RNA replication in cell culture. Structural analysis using the computational-based docking approach demonstrated important noncovalent interactions between D8 and the catalytic residues of the viral protease. Furthermore, D8 was found to be significantly nontoxic in cell culture. More importantly, oral administration of D8 in BALB/c mice proved its better tolerability and bioavailability, as compared to native phyllanthin. Taken together, this study reveals a promising candidate for developing anti-HCV therapeutics to control HCV-induced liver diseases.

Near-Neighbor Interactions in the NS3-4A Protease of HCV Impact Replicative Fitness of Drug-Resistant Viral Variants

A variety of amino acid substitutions in the NS3-4A protease of the hepatitis C virus lead to protease inhibitor (PI) resistance. Many of these significantly impair the replication fitness of the resistant variants in a genotype- and subtype-dependent manner, a critical factor in determining the probability with which resistant variants will persist. However, the underlying molecular mechanisms are unknown. Here, we present a novel residue-interaction network approach to determine how near-neighbor interactions of PI resistance mutations in NS3-4A can impact protease functional sites dependent on their genomic background. We constructed subtype-specific consensus residue networks for subtypes 1a and 1b from protease structure ensembles combined with biological properties of protein residues and evolutionary amino acid conservation. By applying local and global network topology analysis and visual exploration, we characterize PI resistance-associated sites and outline differences in near-neighbor interactions. We find local residue-interaction patterns and features at protease functional sites that are subtype specific. The noncovalent bonding patterns indicate higher fitness costs conferred by PI resistance mutations in a subtype 1b genomic background and explain the prevalence of Q80K and R155K in subtype 1a. Based on local residue interactions, we predict a subtype-specific role for the protease residue NS3-Q80 in molecular mechanisms related to the assembly of infectious virus particles that is supported by experimental data on the capacity of Q80K variants to replicate and produce infectious virus in subtype 1a and 1b cell culture.

The cellular stress response in hepatitis C virus infection: A balancing act to promote viral persistence and host cell survival

Oxidative- and endoplasmic reticulum (ER)-stress are common events during hepatitis C virus (HCV) infection and both regulate cell survival and determine clinical outcome. In response to intrinsic and extrinsic cellular stress, different adaptive mechanisms have evolved in hepatocytes to restore cellular homeostasis like the anti-oxidant response, the unfolded protein response (UPR) and the integrated stress response (ISR). In this review, we focus on the cellular stress response in the context of acute and chronic HCV infection. The mechanisms of induction and modulation of oxidative- and ER-stress are reviewed and analyzed from both perspectives: viral persistence and cell survival. Besides, we delve into the activation of the eIF2α/ATF4 pathway and selective autophagy induction; pathways involved in the elimination of harmful viral proteins after oxidative stress induction. For this, the negative role of autophagy upon HCV infection or negative regulation of viral replication is analyzed. Finally, we hypothesize that the cellular stress response in hepatocytes plays a major role for HCV control thus acting as an important host-factor for virus clearance during the early stages of HCV infection.

Mononuclear phagocyte system in hepatitis C virus infection

The mononuclear phagocyte system (MPS), which consists of monocytes, dendritic cells (DCs), and macrophages, plays a vital role in the innate immune defense against pathogens. Hepatitis C virus (HCV) is efficient in evading the host immunity, thereby facilitating its development into chronic infection. Chronic HCV infection is the leading cause of end-stage liver diseases, liver cirrhosis, and hepatocellular carcinoma. Acquired immune response was regarded as the key factor to eradicate HCV. However, innate immunity can regulate the acquired immune response. Innate immunity-derived cytokines shape the adaptive immunity by regulating T-cell differentiation, which determines the outcome of acute HCV infection. Inhibition of HCV-specific T-cell responses is one of the most important strategies for immune system evasion. It is meaningful to illustrate the role of innate immune response in HCV infection. With the MPS being the important factor in innate immunity, therefore, understanding the role of the MPS in HCV infection will shed light on the pathophysiology of chronic HCV infection. In this review, we outline the impact of HCV infection on the MPS and cytokine production. We discuss how HCV is detected by the MPS and describe the function and impairment of MPS components in HCV infection.

Hepatitis C virus NS4B induces the degradation of TRIF to inhibit TLR3-mediated interferon signaling pathway

Toll-like receptor 3 (TLR3) senses dsRNA intermediates produced during RNA virus replication to activate innate immune signaling pathways through adaptor protein TRIF. Many viruses have evolved strategies to block TLR3-mediated interferon signaling via targeting TRIF. Here we studied how hepatitis C virus (HCV) antagonizes the TLR3-mediated interferon signaling. We found that HCV-encoded NS4B protein inhibited TLR3-mediated interferon signaling by down-regulating TRIF protein level. Mechanism studies indicated that the downregulation of TRIF by NS4B was dependent on caspase8. NS4B transfection or HCV infection can activate caspase8 to promote TRIF degradation, leading to suppression of TLR3-mediated interferon signaling. Knockout of caspase8 can prevent TRIF degradation triggered by NS4B, thereby enhancing the TLR3-mediated interferon signaling activation in response to HCV infection. In conclusion, our work revealed a new mechanism for HCV to evade innate immune response by blocking the TLR3-mediated interferon signaling via NS4B-induced TRIF degradation.

Neuregulin 1 discovered as a cleavage target for the HCV NS3/4A protease by a microfluidic membrane protein array

The hepatitis C virus (HCV) non-structural protein 3 (NS3) is essential for HCV maturation. The NS3/4A protease is a target for several HCV treatments and is a well-known target for HCV drug discovery. The protein is membrane associated and thus probably interacts with other membrane proteins. However, the vast majority of known NS3 host partners are soluble proteins rather than membrane proteins, most likely due to lack of appropriate platforms for their discovery. Utilization of an integrated microfluidics platform enables analysis of membrane proteins in their native form. We screened over 2800 membrane proteins for interaction with NS3 and 90 previously unknown interactions were identified. Of these, several proteins were selected for validation by co-immunoprecipitation and for NS3 proteolytic activity. Bearing in mind the considerable number of interactions formed, together with the popularity of NS3/4A protease as a drug target, it was striking to note its lack of proteolytic activity. Only a single protein, Neuregulin1, was observed to be cleaved, adding to the 3 known NS3/4A cleavage targets. Neuregulin1 participates in neural proliferation. Recent studies have shown its involvement in HCV infection and hepatocellular carcinoma. We showed that NS3/4A triggers an increase in neuregulin1 mRNA levels in HCV infected cells. Despite this increase, its protein concentration is decreased due to proteolytic cleavage. Additionally, its EGF-like domain levels were increased, possibly explaining the ErbB2 and EGFR upregulation in HCV infected cells. The newly discovered protein interactions may provide insights into HCV infection mechanisms and potentially provide new therapeutic targets against HCV.

Suppression of Host Innate Immune Response by Hepatitis C Virus via Induction of Autophagic Degradation of TRAF6

Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) is an important adaptor molecule that mediates the TNFR family and interleukin-1 (IL-1)/Toll-like receptor (TLR) signaling cascades. These pathways are important for the host to control viral infections. In this report, we demonstrated that hepatitis C virus (HCV) depleted TRAF6 from its host cells through a posttranslational mechanism. This depletion was independent of proteasomes, as it was not affected by the proteasome inhibitor MG132, but it was suppressed by bafilomycin A1, which led to the association of TRAF6 with autophagosomes. As bafilomycin A1 is a vacuolar ATPase inhibitor that inhibits autophagic protein degradation, these results suggested that HCV depleted TRAF6 via autophagy. The degradation of TRAF6 was apparently mediated by the p62 sequestosome protein, which is a factor important for selective autophagy, as it could bind to TRAF6 and its silencing stabilized TRAF6. The depletion of TRAF6 suppressed activation of NF-κB and induction of proinflammatory cytokines and enhanced HCV replication. In contrast, the overexpression of TRAF6 suppressed HCV replication. These results revealed a novel mechanism that was used by HCV to disrupt the host innate immune responses for viral replication and persistence.

IMPORTANCE

HCV can cause severe liver diseases and is one of the most important human pathogens. It establishes chronic infections in the great majority of patients that it infects, indicating that it has evolved sophisticated mechanisms to evade host immunity. TRAF6 is an important signaling molecule that mediates activation of NF-κB and expression of proinflammatory cytokines and interferons. In this study, we found that HCV infection suppressed the host innate immune response through the induction of autophagic degradation of TRAF6. This finding provided important information for further understanding how HCV evades host immunity to establish persistence.

Protease inhibitors partially overcome the interferon nonresponse phenotype in patients with chronic hepatitis C

The outcome of triple therapy with protease inhibitors (PI) depends on the intrinsic response to interferon. Interferon-stimulated gene (ISG) expression differs by cell type in the liver and is a strong predictor of interferon responsiveness. Patients who respond well to interferon have low/absent ISG expression in hepatocytes but significant ISG expression in macrophages. Nonresponders (NRs) show the opposite pattern. We aimed to determine the association between cell-type-specific ISG staining and treatment outcome with PI-based triple therapy. Liver biopsy tissue from consecutive patients treated with boceprevir or telaprevir with peginterferon and ribavirin was stained for myxovirus A (MxA). Staining was scored 0-3 in macrophages (M-MxA) and hepatocytes (H-MxA), and IL28B genotyping was performed. Of 56 patients included 41 achieved SVR (73%) (sustained virological response), 2 (4%) relapsed, 10 (18%) were NRs, and 3 (5%) were lost to follow-up. Median M-MxA staining was stronger and H-MxA staining was weaker in patients who achieved SVR. MxA staining correlated with IL28B genotype and with the HCV RNA decline during lead-in phase. However, unlike with dual therapy, the negative predictive value (NPV) of absent or weak M-MxA staining was poor (42%), while the positive predictive value improved (93%). Although by multivariable logistic regression M-MxA staining was significantly associated with SVR (OR 4.35, 1.32-14.28, P = 0.012), the predictive ability was inadequate to withhold therapy. The interaction between macrophages and hepatocytes plays a critical role in interferon responsiveness; however, the addition of a PI at least partially overcomes the interferon nonresponse phenotype making the predictive ability of ISG staining less clinically useful.

Hepatocytes as Immunological Agents

Hepatocytes are targeted for infection by a number of major human pathogens, including hepatitis B virus, hepatitis C virus, and malaria. However, hepatocytes are also immunological agents in their own right. In systemic immunity, they are central in the acute-phase response, which floods the circulation with defensive proteins during diverse stresses, including ischemia, physical trauma, and sepsis. Hepatocytes express a variety of innate immune receptors and, when challenged with pathogen- or damage-associated molecular patterns, can deliver cell-autonomous innate immune responses that may result in host defense or in immunopathology. Important human pathogens have evolved mechanisms to subvert these responses. Finally, hepatocytes talk directly to T cells, resulting in a bias toward immune tolerance.

TLR3 Plays Significant Roles against HBV-Associated HCC

Toll-like receptor 3 (TLR3) is a pattern-recognizing receptor that is involved in immune signaling and plays a crucial role in survival by being able to recognize various viral components including double-stranded RNA (dsRNA). The role of TLR3 in hepatocellular carcinoma (HCC) with hepatitis B virus (HBV) infections is not well understood. To investigate the ability of TLR3 in regulating HBV replication in HCC, 80 cases of human HCC were collected and their tissue microarray was made. In HCC cells, the expression and location of TLR3, hepatitis-associated virus, and interstitial immunoreactive cells were assayed with immunohistochemical staining. The apoptosis of tumor cells was also detected by TUNEL stain. Correlations between TLR3 expression and HBV infection, interstitial immunoreactive cells, and cells apoptosis in HCC were investigated. In addition, we explored whether TLR3 agonist dsRNA can inhibit HepG2.2.15 cells secreting HBV. We found that the cytoplasmic expression of TLR3 in HCC is positively related to HBsAg infection and HCC with cirrhosis and promotes interstitial immunoreactive cells infiltration and cancer cells apoptosis. In HepG2.2.15 cells, dsRNA inhibited the secretion of HBV and induced apoptosis. These results indicate that TLR3 signaling activity may be involved in immune responses against HBV in HCC.

Immune tolerance in liver disease

Liver tolerance is manifest as a bias toward immune unresponsiveness, both in the context of a major histocompatibility complex-mismatched liver transplant and in the context of liver infection. Two broad classes of mechanisms account for liver tolerance. The presentation of antigens by different liver cell types results in incomplete activation of CD8(+) T cells, usually leading to initial proliferation followed by either clonal exhaustion or premature death of the T cell. Many liver infections result in relatively poor CD4(+) T-cell activation, which may be because liver antigen-presenting cells express a variety of inhibitory cytokines and coinhibitor ligands. Poor CD4(+) T-cell activation by liver antigens likely contributes to abortive activation, exhaustion, and early death of CD8(+) T cells. In addition, a network of active immunosuppressive pathways in the liver is mediated mostly by myeloid cells. Kupffer cells, myeloid-derived suppressor cells, and liver dendritic cells both promote activation of regulatory T cells and suppress CD8(+) and CD4(+) effector T cells. This suppressive network responds to diverse inputs, including signals from hepatocytes, sinusoidal endothelial cells, and hepatic stellate cells.

CONCLUSION

Though liver tolerance may be exploited by pathogens, it serves a valuable purpose. Hepatitis A and B infections occasionally elicit a powerful immune response sufficient to cause fatal massive liver necrosis. More commonly, the mechanisms of liver tolerance limit the magnitude of intrahepatic immune responses, allowing the liver to recover. The cost of this adaptive mechanism may be incomplete pathogen eradication, leading to chronic infection.

Autophagy in HCV infection: keeping fat and inflammation at bay

Hepatitis C virus (HCV) infection is one of the main causes of chronic liver disease. Viral persistence and pathogenesis rely mainly on the ability of HCV to deregulate specific host processes, including lipid metabolism and innate immunity. Recently, autophagy has emerged as a cellular pathway, playing a role in several aspects of HCV infection. This review summarizes current knowledge on the molecular mechanisms that link the HCV life cycle with autophagy machinery. In particular, we discuss the role of HCV/autophagy interaction in dysregulating inflammation and lipid homeostasis and its potential for translational applications in the treatment of HCV-infected patients.

Type I and III interferon production in response to RNA viruses

The biology of RNA viruses is closely linked to the type I and type III interferon (IFN) response of the host. These viruses display a range of molecular patterns that may be detected by host cells resulting in the induction of IFNs. Consequently, there are many examples of mechanisms employed by RNA viruses to block or delay IFN induction and reduce the expression of IFN-stimulated genes (ISGs), a necessary step in the virus lifecycle because of the capacity of IFNs to block virus replication. Efficient transmission of viruses depends, in part, on maintaining a balance between virus replication and host survival; specialized host cells, such as plasmacytoid dendritic cells, can sense viral molecular patterns and produce IFNs to help maintain this balance. There are now many examples of RNA viruses inducing type I and type III IFNs, and although these IFNs act through different receptors, in many systems studied, they induce a similar spectrum of genes. However, there may be a difference in the temporal expression pattern, with more prolonged expression of ISGs in response to type III IFN compared with type I IFN. There are also examples of synergy between type I and type III IFNs to induce antiviral responses. Clearly, it is important to understand the different roles of these IFNs in the antiviral response in vivo. One of the most striking differences between these 2 IFN systems is the distribution of the receptors: type I IFN receptors are expressed on most cells, yet type III receptor expression is restricted primarily to epithelial cells but has also been demonstrated on other cells, including dendritic cells. There is increasing evidence that type III IFNs are a key control mechanism against RNA viruses that infect respiratory and enteric epithelia.

Toll-like receptors and liver disease

Toll-like receptors (TLRs) are pattern recognition receptors that play an important role in host defence by recognizing pathogen-associated molecular patterns (PAMP). Recent studies indicate that TLR signalling plays an important role in progression of chronic liver diseases. Ongoing clinical trials suggest that therapeutic manipulation of TLR pathways may offer novel means of reversing chronic liver diseases. Upon activation by their respective ligands, TLRs initiate an intracellular pro-inflammatory/anti-inflammatory signalling cascade via recruitment of various adaptor proteins. TLR associated signalling pathways are tightly regulated to keep a check on inappropriate production of pro-inflammatory cytokines and interferons thereby preventing various autoimmune and inflammatory processes. Herein, we review the current state of knowledge of hepatic distribution, signalling pathways and therapeutic modulation of TLRs in chronic liver diseases.

Genetic background for development of resistance mutations within the HCV NS3 protease-helicase in direct acting antiviral naive patients

BACKGROUND

Subtype-specific response to ketoamide NS3 protease inhibitors is observed in patients with genotype 1 HCV infection. Whether the genetic diversity in the molecular target site of ketoamide compounds prior to treatment plays a role for resistance development and lower treatment response in subtype 1a is poorly understood.

METHODS

Using a public database, we retrieved worldwide NS3-sequence information of 581 dominant HCV variants from patients chronically infected with genotype 1 that were naive to direct-acting antivirals. We applied measures from phylogeny to study the pretreatment genetic diversity and complexity in NS3 full-length as well as the protease-helicase interface for subtype 1a and 1b, respectively.

RESULTS

We found polymorphic sites more frequently in variants of subtype 1b than subtype 1a. Moreover, a significantly higher number of synonymous and non-synonymous substitutions were found in subtype 1b (P<0.001). Transitions were more frequent than transversions, most notably in subtype 1a, whereas the higher average number of nucleotide differences per site was found in subtype 1b. A comparison of NS3 full-length versus domain interface residues for both subtypes revealed a significant difference only for synonymous substitutions (P<0.001).

CONCLUSIONS

Our study suggests that the nature of a mismatch nucleotide exchange in NS3 may constitute an important viral genetic factor for response to ketoamide protease inhibitors. Our analysis further suggests that the subtype-specific pace of resistance development seen in clinical trials is not primarily related to differences in genetic diversity in the direct acting antiviral naive population, but rather appears to correlate with the natural frequency of transition mutations characteristic of each subtype.

A synthetic dsRNA, as a TLR3 pathwaysynergist, combined with sorafenib suppresses HCC in vitro and in vivo

Background

Recent studies have demonstrated that synthetic dsRNAs may produce therapeutic effects in a target-independent manner through stimulation of the toll-like receptor-3 (TLR3)/interferon pathway; as a result, angiogenesis and proliferation of tumor cells are inhibited. Thus, this pathway may become a potential target of dsRNA in tumor suppression. In this study, we evaluated the role of synthetic dsRNA as a TLR3 synergist and by combining with sorafenib in anti-hepatocellular carcinoma (HCC) in vitro and in vivo.

Methods

Four dsRNAs were designed and synthesized. One of them that was capable of activating TLR3 most effectively in human HCC cell line (HepG2.2.15) was selected as a TLR3 synergist (called BM-06). Subsequently, the expression of proteins relating to TLR3 signaling pathway, such as NF-κB, caspase 8 survivin, bcl-2 and PCNA affected by BM-06, sorafenib alone or in combination, was compared. The migration, proliferation and apoptosis of HepG2.2.15 cells were evaluated in presence of BM-06, sorafenib alone or in combination of both. The similar treatments were also applied in an SD rat primary HCC model.

Results

qRT-PCR data showed that the expression of TLR3 and NF-κB in HepG2.2.15 cells was enhanced. BM-06 was selected as a TLR3 synergist capable of activating the TLR3/interferon pathway most effective among 4 synthetic dsRNAs. The migration and proliferation were significantly inhibited in treated HepG2.2.15 cells with BM-06 or Sorafenib alone as compared with PBS-sham control (P < 0.01). However, the role of combination BM-06 with Sorafenib was the most prominent. Tumor cell apoptotic rate was increased by BM-06 or combination when compared to PBS or poly(I:C) (P < 0.05). Similarly, in orthotopic HCC SD rats, the effect of the combination was superior to either agent alone on the inhibition of tumor growth and induction of HCC cell apoptosis (P < 0.05).

Conclusions

dsRNA alone was capable of inhibiting the proliferation of HepG2.2.15 cells and tumor growth of orthotopic HCC SD rats, but the effect of combination of dsRNA with sorafenib was more prominent. These findings implicate the potential role of combined use of a dsRNA, a TLR3 synergist, and sorafenib in inhibition of HCC.

Development and characterization of a replicon-based phenotypic assay for assessing HCV NS4B from clinical isolates

The hepatitis C virus (HCV) NS4B inhibitors have shown potent inhibition of HCV replication in vitro. To assess the effect of viral diversity on the susceptibility to NS4B inhibitors, genotype (GT)-specific GT1a and GT1b replicon shuttle vectors were designed and created for cloning HCV NS4B genes from clinical isolates. For the GT1b NS4B shuttle vector, the S2204I adaptive mutation was introduced in NS5A to improve replication due to the replacement of the K1846T adaptive mutation in NS4B with NS4B from the clinical isolates. In addition to the adaptive mutations, a newly identified Huh-7 cell line, Huh-7-1C, which is highly permissive for both GT1a and GT1b replication, was used to further enhance the replication levels. HCV NS4B gene from clinical isolates was amplified and inserted into the corresponding GT1a and GT1b modified lab strain chimeric replicons. GT1a and GT1b chimeric replicons expressing diverse NS4B genes from corresponding subtypes of clinical isolates replicated at highly efficient levels for phenotypic analysis. Due to natural variation in their amino acid residues in NS4B, these isolates displayed varying drug susceptibilities to an NS4B inhibitor. In mixed populations with wild-type, the sensitivity of resistance detection of NS4B resistant mutants H94R and V105M was between 20% and 80%. The chimeric shuttle vectors can be used to characterize the activity of antiviral drugs targeting NS4B from diverse natural clinical isolates and aid in the development of novel compounds against HCV NS4B.

Inhibitory effect of dsRNA TLR3 agonist in a rat hepatocellular carcinoma model

Hepatocellular carcinoma (HCC) is one of the most common types of malignant tumor. Studies have demonstrated that the toll‑like receptor 3 (TLR3)/interferon pathway is inhibitory in cancer cell proliferation, suggesting that the activation of this pathway may have therapeutic potential. In the present study, the inhibitory effects of BM‑06, a double‑stranded (ds)RNA TLR3 agonist, against HCC were studied in vivo. Using a 2‑acetylaminofluorene-induced HCC rat model, histological examination and analysis of corresponding biomarkers following treatment with BM-06, showed a decrease in tumor growth and cell proliferation, and an increase in apoptosis compared with that in a phosphate‑buffered saline control group. In addition, the observed antitumor effect of BM‑06 in the HCC rat model was demonstrated to be superior to the known TLR3 agonist, polyinosinic-polycytidylic acid.

Multi-step regulation of interferon induction by hepatitis C virus

Acute hepatitis C virus (HCV) infection evokes several distinct innate immune responses in host, but the virus usually propagates by circumventing these responses. Although a replication intermediate double-stranded RNA is produced in infected cells, type I interferon (IFN) induction and immediate cell death are largely blocked in infected cells. In vitro studies suggested that type I and III IFNs are mainly produced in HCV-infected hepatocytes if the MAVS pathway is functional, and dysfunction of this pathway may lead to cellular permissiveness to HCV replication and production. Cellular immunity, including natural killer cell activation and antigen-specific CD8 T-cell proliferation, occurs following innate immune activation in response to HCV, but is often ineffective for eradication of HCV. Constitutive dsRNA stimulation differs in output from type I IFN therapy, which has been an authentic therapy for patients with HCV. Host innate immune responses to HCV RNA/proteins may be associated with progressive hepatic fibrosis and carcinogenesis once persistent HCV infection is established in opposition to the IFN system. Hence, innate RNA sensing exerts pivotal functions against HCV genome replication and host pathogenesis through modulation of the IFN system. Molecules participating in the RIG-I and Toll-like receptor 3 pathways are the main targets for HCV, disabling the anti-viral functions of these IFN-inducing molecules. We discuss the mechanisms that abolish type I and type III IFN production in HCV-infected cells, which may contribute to understanding the mechanism of virus persistence and resistance to the IFN therapy.

dsRNA-dependent protein kinase PKR and its role in stress, signaling and HCV infection

The double-stranded RNA-dependent protein kinase PKR plays multiple roles in cells, in response to different stress situations. As a member of the interferon (IFN)‑Stimulated Genes, PKR was initially recognized as an actor in the antiviral action of IFN, due to its ability to control translation, through phosphorylation, of the alpha subunit of eukaryotic initiation factor 2 (eIF2α). As such, PKR participates in the generation of stress granules, or autophagy and a number of viruses have designed strategies to inhibit its action. However, PKR deficient mice resist most viral infections, indicating that PKR may play other roles in the cell other than just acting as an antiviral agent. Indeed, PKR regulates several signaling pathways, either as an adapter protein and/or using its kinase activity. Here we review the role of PKR as an eIF2α kinase, its participation in the regulation of the NF-κB, p38MAPK and insulin pathways, and we focus on its role during infection with the hepatitis C virus (HCV). PKR binds the HCV IRES RNA, cooperates with some functions of the HCV core protein and may represent a target for NS5A or E2. Novel data points out for a role of PKR as a pro-HCV agent, both as an adapter protein and as an eIF2α-kinase, and in cooperation with the di-ubiquitin-like protein ISG15. Developing pharmaceutical inhibitors of PKR may help in resolving some viral infections as well as stress-related damages.

Innate immune responses in hepatitis C virus infection

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

Interaction of Hepatitis C virus proteins with pattern recognition receptors

Hepatitis C virus (HCV) is an important human pathogen that causes acute and chronic hepatitis, cirrhosis and hepatocellular carcinoma worldwide. This positive stranded RNA virus is extremely efficient in establishing persistent infection by escaping immune detection or hindering the host immune responses. Recent studies have discovered two important signaling pathways that activate the host innate immunity against viral infection. One of these pathways utilizes members of Toll-like receptor (TLR) family and the other uses the RNA helicase retinoic acid inducible gene I (RIG-I) as the receptors for intracellular viral double stranded RNA (dsRNA), and activation of transcription factors. In this review article, we summarize the interaction of HCV proteins with various host receptors/sensors through one of these two pathways or both, and how they exploit these interactions to escape from host defense mechanisms. For this purpose, we searched data from Pubmed and Google Scholar. We found that three HCV proteins; Core (C), non structural 3/4 A (NS3/4A) and non structural 5A (NS5A) have direct interactions with these two pathways. Core protein only in the monomeric form stimulates TLR2 pathway assisting the virus to evade from the innate immune system. NS3/4A disrupts TLR3 and RIG-1 signaling pathways by cleaving Toll/IL-1 receptor domain-containing adapter inducing IFN-beta (TRIF) and Cardif, the two important adapter proteins of these signaling cascades respectively, thus halting the defense against HCV. NS5A downmodulates the expressions of NKG2D on natural killer cells (NK cells) via TLR4 pathway and impairs the functional ability of these cells. TLRs and RIG-1 pathways have a central role in innate immunity and despite their opposing natures to HCV proteins, when exploited together, HCV as an ever developing virus against host immunity is able to accumulate these mechanisms for near unbeatable survival.

Exploitation of lipid components by viral and host proteins for hepatitis C virus infection

Hepatitis C virus (HCV), which is a major causative agent of blood-borne hepatitis, has chronically infected about 170 million individuals worldwide and leads to chronic infection, resulting in development of steatosis, cirrhosis, and eventually hepatocellular carcinoma. Hepatocellular carcinoma associated with HCV infection is not only caused by chronic inflammation, but also by the biological activity of HCV proteins. HCV core protein is known as a main component of the viral nucleocapsid. It cooperates with host factors and possesses biological activity causing lipid alteration, oxidative stress, and progression of cell growth, while other viral proteins also interact with host proteins including molecular chaperones, membrane-anchoring proteins, and enzymes associated with lipid metabolism to maintain the efficiency of viral replication and production. HCV core protein is localized on the surface of lipid droplets in infected cells. However, the role of lipid droplets in HCV infection has not yet been elucidated. Several groups recently reported that other viral proteins also support viral infection by regulation of lipid droplets and core localization in infected cells. Furthermore, lipid components are required for modification of host factors and the intracellular membrane to maintain or up-regulate viral replication. In this review, we summarize the current status of knowledge regarding the exploitation of lipid components by viral and host proteins in HCV infection.

Antiviral applications of Toll-like receptor agonists

In the past, antiviral research has focused mainly on viral targets. As the search for effective and differentiated antiviral therapies continues, cellular targets are becoming more common, bringing with them a variety of challenges and concerns. Toll-like receptors (TLRs) provide a unique mechanism to induce an antiviral state in the host. In this review we introduce TLRs as targets for the pharmaceutical industry, including how they signal and thereby induce an antiviral state through the production of type I interferons. We examine how TLRs are being therapeutically targeted and discuss several clinically precedented agents for which efficacy and safety data are available. We describe some of the chemistries that have been applied to both small molecule and large molecule leads to tune agonist potency, and offer a differentiated safety profile through targeting certain compartments such as the gut or the lung, thereby limiting systemic drug exposure and affecting systemic cytokine levels. The application of low-dose agonists of TLRs as vaccine adjuvants or immunoprotective agents is also presented. Some of the challenges presented by this approach are then discussed, including viral evasion strategies and mechanism-linked inflammatory cytokine induction.

Translocase of outer mitochondrial membrane 70 induces interferon response and is impaired by hepatitis C virus NS3

Hepatitis C virus (HCV) elevated expression of the translocase of outer mitochondrial membrane 70 (Tom70). Interestingly, overexpression of Tom70 induces interferon (IFN) synthesis in hepatocytes, and it was impaired by HCV. Here, we addressed the mechanism of this impairment. The HCV NS3/4A protein induced Tom70 expression. The HCV NS3 protein interacted in cells, and cleaved the adapter protein mitochondrial anti-viral signaling (MAVS). Ectopic overexpression of Tom70 could not inhibit this cleavage. As a result, IRF-3 phosphorylation was impaired and IFN-β induction was suppressed. These results indicate that MAVS works upstream of Tom70 and the cleavage of MAVS by HCV NS3 protease suppresses signaling of IFN induction.

Y-box-binding protein 1 interacts with hepatitis C virus NS3/4A and influences the equilibrium between viral RNA replication and infectious particle production

The hepatitis C virus (HCV) NS3/4A protein has several essential roles in the virus life cycle, most probably through dynamic interactions with host factors. To discover cellular cofactors that are co-opted by HCV for its replication, we elucidated the NS3/4A interactome using mass spectrometry and identified Y-box-binding protein 1 (YB-1) as an interacting partner of NS3/4A protein and HCV genomic RNA. Importantly, silencing YB-1 expression decreased viral RNA replication and severely impaired the propagation of the infectious HCV molecular clone JFH-1. Immunofluorescence studies further revealed a drastic HCV-dependent redistribution of YB-1 to the surface of the lipid droplets, an important organelle for HCV assembly. Core and NS3 protein-dependent polyprotein maturation were shown to be required for YB-1 relocalization. Unexpectedly, YB-1 knockdown cells showed the increased production of viral infectious particles while HCV RNA replication was impaired. Our data support that HCV hijacks YB-1-containing ribonucleoparticles and that YB-1-NS3/4A-HCV RNA complexes regulate the equilibrium between HCV RNA replication and viral particle production.

In-cell selectivity profiling of membrane-anchored and replicase-associated hepatitis C virus NS3-4A protease reveals a common, stringent substrate recognition profile

The need to identify anti-Flaviviridae agents has resulted in intensive biochemical study of recombinant nonstructural (NS) viral proteases; however, experimentation on viral protease-associated replication complexes in host cells is extremely challenging and therefore limited. It remains to be determined if membrane anchoring and/or association to replicase-membrane complexes of proteases, such as hepatitis C virus (HCV) NS3-4A, plays a regulatory role in the substrate selectivity of the protease. In this study, we examined trans-endoproteolytic cleavage activities of membrane-anchored and replicase-associated NS3-4A using an internally consistent set of membrane-anchored protein substrates mimicking all known HCV NS3-4A polyprotein cleavage sequences. Interestingly, we detected cleavage of substrates encoding for the NS4B/NS5A and NS5A/NS5B junctions, but not for the NS3/NS4A and NS4A/NS4B substrates. This stringent substrate recognition profile was also observed for the replicase-associated NS3-4A and is not genotype-specific. Our study also reveals that ER-anchoring of the substrate is critical for its cleavage by NS3-4A. Importantly, we demonstrate that in HCV-infected cells, the NS4B/NS5A substrate was cleaved efficiently. The unique ability of our membrane-anchored substrates to detect NS3-4A activity alone, in replication complexes, or within the course of infection, shows them to be powerful tools for drug discovery and for the study of HCV biology.

Molecular mechanisms of viral and host cell substrate recognition by hepatitis C virus NS3/4A protease

Hepatitis C NS3/4A protease is a prime therapeutic target that is responsible for cleaving the viral polyprotein at junctions 3-4A, 4A4B, 4B5A, and 5A5B and two host cell adaptor proteins of the innate immune response, TRIF and MAVS. In this study, NS3/4A crystal structures of both host cell cleavage sites were determined and compared to the crystal structures of viral substrates. Two distinct protease conformations were observed and correlated with substrate specificity: (i) 3-4A, 4A4B, 5A5B, and MAVS, which are processed more efficiently by the protease, form extensive electrostatic networks when in complex with the protease, and (ii) TRIF and 4B5A, which contain polyproline motifs in their full-length sequences, do not form electrostatic networks in their crystal complexes. These findings provide mechanistic insights into NS3/4A substrate recognition, which may assist in a more rational approach to inhibitor design in the face of the rapid acquisition of resistance.

Hepatitis C Virus Evasion from RIG-I-Dependent Hepatic Innate Immunity

Exposure to hepatitis C virus (HCV) usually results in persistent infection that often develops into chronic liver disease. Interferon-alpha (IFN) treatment comprises the foundation of current approved therapy for chronic HCV infection but is limited in overall efficacy. IFN is a major effector of innate antiviral immunity and is naturally produced in response to viral infection when viral pathogen-associated molecular patterns (PAMPs) are recognized as nonself and are bound by cellular pathogen recognition receptors (PRRs), including Toll-like receptors (TLRs) and the RIG-I-like receptors (RLRs). Within hepatocytes, RIG-I is a major PRR of HCV infection wherein PAMP interactions serve to trigger intracellular signaling cascades in the infected hepatocyte to drive IFN production and the expression of interferon-stimulated genes (ISGs). ISGs function to limit virus replication, modulate the immune system, and to suppress virus spread. However, studies of HCV-host interactions have revealed several mechanisms of innate immune regulation and evasion that feature virus control of PRR signaling and regulation of hepatic innate immune programs that may provide a molecular basis for viral persistence.

Potential treatment options and future research to increase hepatitis C virus treatment response rate

Hepatitis C virus (HCV) is a liver-tropic blood-borne pathogen that affects more than 170 million people worldwide. Although acute infections are usually asymptomatic, up to 90% of HCV infections persist with the possibility of long-term consequences such as liver fibrosis, cirrhosis, steatosis, insulin resistance, or hepatocellular carcinoma. As such, HCV-associated liver disease is a major public health concern. Although the currently available standard of care therapy of pegylated interferon α plus ribavirin successfully treats infection in a subset of patients, the development of more effective, less toxic HCV antivirals is a health care imperative. This review not only discusses the limitations of the current HCV standard of care but also evaluates upcoming HCV treatment options and how current research elucidating the viral life cycle is facilitating the development of HCV-specific therapeutics that promise to greatly improve treatment response rates both before and after liver transplantation.

A comparative analysis of the substrate permissiveness of HCV and GBV-B NS3/4A proteases reveals genetic evidence for an interaction with NS4B protein during genome replication

The hepatitis C virus (HCV) serine protease (NS3/4A) processes the NS3-NS5B segment of the viral polyprotein and also cleaves host proteins involved in interferon signaling, making it an important target for antiviral drug discovery and suggesting a wide breadth of substrate specificity. We compared substrate specificities of the HCV protease with that of the GB virus B (GBV-B), a distantly related nonhuman primate hepacivirus, by exchanging amino acid sequences at the NS4B/5A and/or NS5A/5B cleavage junctions between these viruses within the backbone of subgenomic replicons. This mutagenesis study demonstrated that the GBV-B protease had a broader substrate tolerance, a feature corroborated by structural homology modeling. However, despite efficient polyprotein processing, GBV-B RNAs containing HCV sequences at the C-terminus of NS4B had a pseudo-lethal replication phenotype. Replication-competent revertants contained second-site substitutions within the NS3 protease or NS4B N-terminus, providing genetic evidence for an essential interaction between NS3 and NS4B during genome replication.

Hepatitis B virus polymerase inhibits RIG-I- and Toll-like receptor 3-mediated beta interferon induction in human hepatocytes through interference with interferon regulatory factor 3 activation and dampening of the interaction between TBK1/IKKepsilon and DDX3

Hepatitis B virus (HBV) infection remains one of the most serious health problems worldwide. Whilst studies have shown that HBV impairs interferon (IFN) production from dendritic cells in chronic hepatitis B patients, it remains unknown whether HBV inhibits IFN production in human hepatocytes. Using transient transfection assays in a primary human hepatocyte cell line (PH5CH8), this study demonstrated that HBV polymerase inhibits IFN-beta promoter activity induced by Newcastle disease virus, Sendai virus or poly(I : C) in a dose-dependent manner, whilst ectopic expression of the HBV core and X proteins had no effect on IFN-beta promoter activity. In addition, HBV polymerase blocked cellular IFN-beta expression and consequent antiviral immunity revealed by an infection protection assay. Furthermore, overexpression of key molecules on the IFN-beta induction axis, together with HBV polymerase, resulted in a block of IFN-beta promoter activity triggered by RIG-I, IPS-1, TRIF, TBK1 and IKKepsilon, but not by an IFN regulatory factor 3 dominant-positive mutant (IRF3-5D), suggesting that HBV polymerase prevents IFN-beta expression at the TBK1/IKKepsilon level. Further studies showed that HBV polymerase inhibited phosphorylation, dimerization and nuclear translocation of IRF3, in response to Sendai virus infection. Finally, it was shown that HBV polymerase-mediated dampening of the interaction between TBK1/IKKepsilon and DDX3 may be involved in the inhibitory effect on IFN-beta induction. Taken together, these findings reveal a novel role of HBV polymerase in HBV counteraction of IFN-beta production in human hepatocytes.

Hepatitis C virus experimental model systems and antiviral drug research

An estimated 130 million people worldwide are chronically infected with hepatitis C virus (HCV) making it a leading cause of liver disease worldwide. Because the currently available therapy of pegylated interferon-alpha and ribavirin is only effective in a subset of patients, the development of new HCV antivirals is a healthcare imperative. This review discusses the experimental models available for HCV antiviral drug research, recent advances in HCV antiviral drug development, as well as active research being pursued to facilitate development of new HCV-specific therapeutics.

Induction and evasion of innate antiviral responses by hepatitis C virus

Persistent hepatitis C virus infection is associated with progressive hepatic fibrosis and liver cancer. Acute infection evokes several distinct innate immune responses, but these are partially or completely countered by the virus. Hepatitis C virus proteins serve dual functions in replication and immune evasion, acting to disrupt cellular signaling pathways leading to interferon synthesis, subvert Jak-STAT signaling to limit expression of interferon-stimulated genes, and block antiviral activities of interferon-stimulated genes. The net effect is a multilayered evasion of innate immunity, which negatively influences the subsequent development of antigen-specific adaptive immunity, thereby contributing to virus persistence and resistance to therapy.

Hepatitis C virus non-structural protein 3 (HCV NS3): a multifunctional antiviral target

Hepatitis C virus non-structural protein 3 contains a serine protease and an RNA helicase. Protease cleaves the genome-encoded polyprotein and inactivates cellular proteins required for innate immunity. Protease has emerged as an important target for the development of antiviral therapeutics, but drug resistance has turned out to be an obstacle in the clinic. Helicase is required for both genome replication and virus assembly. Mechanistic and structural studies of helicase have hurled this enzyme into a prominent position in the field of helicase enzymology. Nevertheless, studies of helicase as an antiviral target remain in their infancy.

A new insight into hepatitis C vaccine development

Chronic hepatitis C virus (HCV) infection remains a serious burden to public health worldwide. Currently, HCV-infected patients could undergo antiviral therapy by giving pegylated IFN-α with ribavirin. However, this therapy is only effective in around 50% of patients with HCV genotype 1, which accounts for more than 70% of all HCV infection, and it is not well tolerated for most patients. Moreover, there is no vaccine available. The efforts on identifying protective immunity against HCV have progressed recently. Neutralizing antibodies and robust T cell responses including both CD4⁺ and CD8⁺ have been shown to be related to the clearance of HCV, which have shed lights on the potential success of HCV vaccines. There are many vaccines developed and tested before entering clinical trials. Here, we would first discuss strategies of viral immune evasion and correlates of protective host immunity and finally review some prospective vaccine approaches against chronic HCV infection.

Hijacking hepatitis C viral replication with a non-coding replicative RNA

The current treatments used against RNA viruses have a limited efficacy and are often hampered by the induction of side-effects. The specific delivery of antiviral proteins in infected cells should increase their efficiency and reduce their impact on healthy cells. Here, we describe the development of a new approach which takes advantage of the viral replication machinery to specifically target the antiviral protein expression to the infected cells. The strategy is based on the delivery of a non-coding (-)RNA carrying the structures required for the binding of the viral replication complex and the complementary sequence of an antiviral gene. The viral replication complex replicates the (-)RNA similarly to the viral genome to give a coding (+)RNA from which the antiviral protein will be expressed. As non-infected cells do not express the replication complex, this specific machinery can be used to target virus-infected cells without affecting healthy cells. We show that this approach can be successfully applied to the hepatitis C virus. In both replicon-harboring cells (genotype 1b) and JFH-1 infected cells (genotype 2a), nrRNAs induced a strong decrease in genomic RNA and viral protein NS5A. These effects were correlated with a strong activation of several interferon-stimulating genes.