EGFR and EphA2 are host factors for hepatitis C virus entry and possible targets for antiviral therapy

Lipids: a key for hepatitis C virus entry and a potential target for antiviral strategies

Viruses have evolved to complex relationship with their host cells. Many viruses modulate the lipid composition, lipid synthesis and signaling of their host cell. Lipids are also an essential part of the life cycle of the hepatitis C virus (HCV). HCV is a major human pathogen, persistently infecting 170 million people worldwide, with no currently effective treatment available for all patients. HCV appears to make use of the host lipid metabolism and one common feature of chronic hepatitis C is the steatosis, characterized by excessive accumulation of triglycerides and lipid content in the liver. Thus, HCV lifecycle appears to be closely connected to host cell lipid metabolism, from cell entry, through viral RNA replication to viral particle production and formation/assembly. HCV particles have a unique lipid composition, certainly distinct from other viruses. In the blood of chronically-infected patients, viral particles are bound to serum lipoproteins and are thus called lipo-viro-particles. The density of these circulating viral particles is heterogeneous. Specific infectivity and fusion of low density particles are greater than those of high density particles. Lipids and association to lipoproteins therefore play a key role in HCV life cycle. The purpose of this review is to make a state of the art on recent findings on the contribution of lipids in cell entry and membrane fusion of HCV. The influence of lipids as chemically-defined entities will be analyzed, as well as the role played by cholesterol transporters and lipoprotein receptors in HCV entry and fusion. Since viral entry would constitute a key target for antiviral strategies, inhibitor molecules interacting with viral and/or cellular membranes or interfering with the function of lipid metabolism regulators of HCV entry could offer strong antiviral potential. This will be lastly discussed in this review.

Virology of hepatitis C virus infection

The multiple steps of the HCV lifecycle have been recently unravelled, thanks to the development of a number of cell-free or cell culture-based model systems. The HCV lifecycle offers a very large number of potential targets of intervention for specific anti-HCV drugs, including direct-acting antiviral drugs and host-targeted agents. HCV virological tools include enzyme immunoassays (EIAs) that detect anti-HCV antibodies and detect/quantify HCV core antigen, and molecular biology-based assays that are used to detect and quantify HCV RNA and to determine the HCV genotype. Recently, point-of-care tests and alternatives to laboratory tests that require whole-blood samples have been developed to improve access to screening, diagnosis and care. Virological tools are used in clinical practice to diagnose acute and chronic HCV infection, to decide who should be treated, to select the optimal therapy and to monitor treatment responses in order to tailor treatment duration.

Hepatitis C virus induces epidermal growth factor receptor activation via CD81 binding for viral internalization and entry

While epidermal growth factor receptor (EGFR) has been shown to be important in the entry process for multiple viruses, including hepatitis C virus (HCV), the molecular mechanisms by which EGFR facilitates HCV entry are not well understood. Using the infectious cell culture HCV model (HCVcc), we demonstrate that the binding of HCVcc particles to human hepatocyte cells induces EGFR activation that is dependent on interactions between HCV and CD81 but not claudin 1. EGFR activation can also be induced by antibody mediated cross-linking of CD81. In addition, EGFR ligands that enhance the kinetics of HCV entry induce EGFR internalization and colocalization with CD81. While EGFR kinase inhibitors inhibit HCV infection primarily by preventing EGFR endocytosis, antibodies that block EGFR ligand binding or inhibitors of EGFR downstream signaling have no effect on HCV entry. These data demonstrate that EGFR internalization is critical for HCV entry and identify a hitherto-unknown association between CD81 and EGFR.

Liver kidney microsomal type 1 antibodies reduce the CYP2D6 activity in patients with chronic hepatitis C virus infection

Liver kidney microsomal type 1 (LKM-1) antibodies have been shown to decrease the CYP2D6 activity in vitro and are present in a minority of patients with chronic hepatitis C infection. We investigated whether LKM-1 antibodies might reduce the CYP2D6 activity in vivo. All patients enrolled in the Swiss Hepatitis C Cohort Study and tested for LKM-1 antibodies were assessed (n = 1723): 10 eligible patients were matched with patients without LKM-1 antibodies. Patients were genotyped for CYP2D6 variants to exclude individuals with a poor metabolizer genotype. CYP2D6 activity was measured by a specific substrate using the dextromethorphan/dextrorphan metabolic ratio to classify patients into four activity phenotypes. All patients had a CYP2D6 extensive metabolizer genotype. The observed phenotype was concordant with the CYP2D6 genotype in most LKM-negative patients, whereas only three LKM-1 positive patients had a concordant phenotype (six presented an intermediate and one a poor metabolizer phenotype). The median DEM/DOR ratio was sixfold higher in LKM-1 positive than in LKM-1 negative patients (0.096 vs. 0.016, P = 0.004), indicating that CYP2D6 metabolic function was significantly reduced in the presence of LKM-1 antibodies. In chronic hepatitis C patients with LKM-1 antibodies, the CYP2D6 metabolic activity was on average reduced by 80%. The impact of LKM-1 antibodies on CYP2D6-mediated drug metabolism pathways warrants further translational studies.

Mutations that alter use of hepatitis C virus cell entry factors mediate escape from neutralizing antibodies

BACKGROUND & AIMS

The development of vaccines and other strategies to prevent hepatitis C virus (HCV) infection is limited by rapid viral evasion. HCV entry is the first step of infection; this process involves several viral and host factors and is targeted by host-neutralizing responses. Although the roles of host factors in HCV entry have been well characterized, their involvement in evasion of immune responses is poorly understood. We used acute infection of liver graft as a model to investigate the molecular mechanisms of viral evasion.

METHODS

We studied factors that contribute to evasion of host immune responses using patient-derived antibodies, HCV pseudoparticles, and cell culture-derived HCV that express viral envelopes from patients who have undergone liver transplantation. These viruses were used to infect hepatoma cell lines that express different levels of HCV entry factors.

RESULTS

By using reverse genetic analyses, we identified altered use of host-cell entry factors as a mechanism by which HCV evades host immune responses. Mutations that alter use of the CD81 receptor also allowed the virus to escape neutralizing antibodies. Kinetic studies showed that these mutations affect virus-antibody interactions during postbinding steps of the HCV entry process. Functional studies with a large panel of patient-derived antibodies showed that this mechanism mediates viral escape, leading to persistent infection in general.

CONCLUSIONS

We identified a mechanism by which HCV evades host immune responses, in which use of cell entry factors evolves with escape from neutralizing antibodies. These findings advance our understanding of the pathogenesis of HCV infection and might be used to develop antiviral strategies and vaccines.

Discovery of ITX 4520: a highly potent orally bioavailable hepatitis C virus entry inhibitor

The manuscript reports an identification of a highly potent, orally bioavailable hepatitis C virus entry inhibitor through optimization of a previously reported class of molecules (1) that were not stable in the rat plasma. Compound 39 (ITX 4520) exhibited an excellent PK profile in both rats and dogs with good oral exposure, half-life and oral bioavailability. The compound is also well-tolerated in the preliminary in vivo toxicity studies and has been selected as a pre-clinical candidate for our HCV clinical pipeline.

Liver X receptors agonists impede hepatitis C virus infection in an Idol-dependent manner

Hepatitis C virus (HCV) is a major human pathogen that causes many serious diseases, including acute and chronic hepatitis, cirrhosis and hepatocellular carcinoma. Treatments for this virus are inadequate, and improved antiviral therapies are necessary. Although the precise mechanisms regulating HCV entry into hepatic cells are still unknown, the low-density lipoprotein receptor (LDLR) has been shown to be essential for entry of infectious HCV particles. Liver X receptors (LXR) were recently reported to control LDLR expression through the regulation of the expression of the Idol (inducible degrader of the LDLR) protein, which could trigger the ubiquitination and degradation of LDLR. In this study, we analyzed the antiviral effect of Idol in vitro. The results demonstrated that Huh7.5.1 cells that exogenously expressed Idol were resistant to HCV infection. Next, the treatment of HCV-infected Huh7.5.1 cells with either synthetic LXR agonists (GW3965 or T0901317) or the natural LXR ligand 24(S),25-epoxycholesterol inhibited HCV infection in a dose-dependent manner. Furthermore, a combination of LXR agonists and HCV RNA replication inhibitors exerted additive effects against HCV, as revealed by isobologram analysis. In conclusion, our data indicate that molecules such as LXR agonists, which could stimulate the expression of Idol, represent a new class of potential anti-HCV compounds, and these compounds could be developed for therapeutic use against HCV infection, either as a monotherapy, or in combination with other anti-HCV drugs.

Study of hepatitis C virus entry in genetically humanized mice

Approximately 2% of the world's population is chronically infected with hepatitis C virus (HCV). Chronic hepatitis C can culminate in end stage liver disease and liver cancer if the infection is untreated. Current therapy is only partially effective and a vaccine for HCV does not exist. Since the discovery of HCV as the etiologic agent causing hepatitis C several experimental tools have been developed which have improved our understanding of the viral life cycle and the interaction of HCV with human cells. However, it remains challenging to study HCV infection in its native liver environment given its narrow species tropism, limited to humans and chimpanzees. Mice can be rendered susceptible to HCV infection by transplanting human hepatocytes into immunocompromized liver injury strains. Such human liver chimeric mice are useful as a challenge model for human hepatotropic pathogens but their utility is hampered by their inability to mount functional immune responses and practical aspects including high costs, low throughput, and donor-to-donor variability. The barriers that restrict HCV species tropism are incompletely understood. We have previously shown that expression of human CD81 and human OCLN is required for HCV uptake into mouse cells. This led to the construction of a genetically humanized mouse model for HCV infection. Here, we provide a detailed protocol for the generation of these animals and highlight some of its applications for studying HCV biology and preclinical testing of drug and vaccine candidates.

Treating hepatitis C infection by targeting the host

More than 130 million people worldwide are chronically infected with the hepatitis C virus (HCV), which can lead to cirrhosis, liver failure, and hepatocellular carcinoma. Although recently approved HCV NS3-4A protease inhibitors significantly improve treatment response rates, current HCV treatment is still frequently limited by side effects and by the low genetic barrier to viral resistance against direct-acting antiviral agents. A complementary strategy is to target the host cellular factors that support the HCV life cycle. Several studies, including RNA interference screens, demonstrated that HCV depends on dozens, if not hundreds, of cellular proteins to complete its life cycle. A better understanding of the interactions between HCV proteins and host factors may help to identify host targets for antiviral therapy. In this review, we highlight some of the host factors that are particularly attractive targets for the treatment of HCV.

Eph-dependent cell-cell adhesion and segregation in development and cancer

Numerous studies attest to essential roles for Eph receptors and their ephrin ligands in controlling cell positioning and tissue patterning during normal and oncogenic development. These studies suggest multiple, sometimes contradictory, functions of Eph-ephrin signalling, which under different conditions can promote either spreading and cell-cell adhesion or cytoskeletal collapse, cell rounding, de-adhesion and cell-cell segregation. A principle determinant of the balance between these two opposing responses is the degree of receptor/ligand clustering and activation. This equilibrium is likely altered in cancers and modulated by somatic mutations of key Eph family members that have emerged as candidate cancer markers in recent profiling studies. In addition, cross-talk amongst Ephs and with other signalling pathways significantly modulates cell-cell adhesion, both between and within Eph- and ephrin-expressing cell populations. This review summarises our current understanding of how Eph receptors control cell adhesion and morphology, and presents examples demonstrating the importance of these events in normal development and cancer.

Animal models for the study of hepatitis C virus infection and related liver disease

Hepatitis C virus (HCV) causes liver-related death in more than 300,000 people annually. Treatments for patients with chronic HCV are suboptimal, despite the introduction of directly acting antiviral agents. There is no vaccine that prevents HCV infection. Relevant animal models are important for HCV research and development of drugs and vaccines. Chimpanzees are the best model for studies of HCV infection and related innate and adaptive host immune responses. They can be used in immunogenicity and efficacy studies of HCV vaccines. The only small animal models of robust HCV infection are T- and B- cell deficient mice with human chimeric livers. Although these mice cannot be used in studies of adaptive immunity, they have provided new insights into HCV neutralization, interactions between virus and receptors, innate host responses, and therapeutic approaches. Recent progress in developing genetically humanized mice is exciting, but these models only permit studies of specific steps in the HCV life cycle and have limited or no viral replication.

Occult infection with hepatitis C virus: friend or foe?

Hepatitis C virus (HCV) infection is a global pandemic associated with a growing disease burden due to cirrhosis and the consequent morbidity and mortality. Transmission is largely via blood-to-blood contact. Following primary infection, a minority of individuals clear the infection predominantly via cellular immune mechanisms, whereas the majority become chronically infected. Recent data suggest that a third outcome may also be possible, termed 'occult' infection in which subjects who are known, or suspected to have previously been infected with HCV, no longer have viral RNA in their serum at levels detectable by sensitive commercial assays, but do have virus detected by ultra-sensitive techniques. Occult infection has also been detected in peripheral blood mononuclear cells, which may indicate an extra-hepatic reservoir of the virus. Although the clinical significance of occult infection remains unknown, most authors have raised concerns of recrudescent infection. Here we critically review the published literature, suggest further avenues of investigation and propose that occult infection may be beneficial to the host by maintaining immunological memory to protect against reinfection.

Viral kinetics suggests a reconciliation of the disparate observations of the modulation of claudin-1 expression on cells exposed to hepatitis C virus

The tight junction protein claudin-1 (CLDN1) is necessary for hepatitis C virus (HCV) entry into target cells. Recent studies have made disparate observations of the modulation of the expression of CLDN1 on cells following infection by HCV. In one study, the mean CLDN1 expression on cells exposed to HCV declined, whereas in another study HCV infected cells showed increased CLDN1 expression compared to uninfected cells. Consequently, the role of HCV in modulating CLDN1 expression, and hence the frequency of cellular superinfection, remains unclear. Here, we present a possible reconciliation of these disparate observations. We hypothesized that viral kinetics and not necessarily HCV-induced receptor modulation underlies these disparate observations. To test this hypothesis, we constructed a mathematical model of viral kinetics in vitro that mimicked the above experiments. Model predictions provided good fits to the observed evolution of the distribution of CLDN1 expression on cells following exposure to HCV. Cells with higher CLDN1 expression were preferentially infected and outgrown by cells with lower CLDN1 expression, resulting in a decline of the mean CLDN1 expression with time. At the same time, because the susceptibility of cells to infection increased with CLDN1 expression, infected cells tended to have higher CLDN1 expression on average than uninfected cells. Our study thus presents an explanation of the disparate observations of CLDN1 expression following HCV infection and points to the importance of considering viral kinetics in future studies of receptor expression on cells exposed to HCV.

Novel small-molecule inhibitors of hepatitis C virus entry block viral spread and promote viral clearance in cell culture

Combinations of direct-acting anti-virals offer the potential to improve the efficacy, tolerability and duration of the current treatment regimen for hepatitis C virus (HCV) infection. Viral entry represents a distinct therapeutic target that has been validated clinically for a number of pathogenic viruses. To discover novel inhibitors of HCV entry, we conducted a high throughput screen of a proprietary small-molecule compound library using HCV pseudoviral particle (HCVpp) technology. We independently discovered and optimized a series of 1,3,5-triazine compounds that are potent, selective and non-cytotoxic inhibitors of HCV entry. Representative compounds fully suppress both cell-free virus and cell-to-cell spread of HCV in vitro. We demonstrate, for the first time, that long term treatment of an HCV cell culture with a potent entry inhibitor promotes sustained viral clearance in vitro. We have confirmed that a single amino acid variant, V719G, in the transmembrane domain of E2 is sufficient to confer resistance to multiple compounds from the triazine series. Resistance studies were extended by evaluating both the fusogenic properties and growth kinetics of drug-induced and natural amino acid variants in the HCVpp and HCV cell culture assays. Our results indicate that amino acid variations at position 719 incur a significant fitness penalty. Introduction of I719 into a genotype 1b envelope sequence did not affect HCV entry; however, the overall level of HCV replication was reduced compared to the parental genotype 1b/2a HCV strain. Consistent with these findings, I719 represents a significant fraction of the naturally occurring genotype 1b sequences. Importantly, I719, the most relevant natural polymorphism, did not significantly alter the susceptibility of HCV to the triazine compounds. The preclinical properties of these triazine compounds support further investigation of entry inhibitors as a potential novel therapy for HCV infection.

Kaposi's sarcoma-associated herpesvirus interacts with EphrinA2 receptor to amplify signaling essential for productive infection

Kaposi's sarcoma-associated herpesvirus (KSHV), etiologically associated with Kaposi's sarcoma, uses integrins (α3β1, αVβ3, and αVβ5) and associated signaling to enter human dermal microvascular endothelial cells (HMVEC-d), an in vivo target of infection. KSHV infection activated c-Cbl, which induced the selective translocation of KSHV into lipid rafts (LRs) along with the α3β1, αVβ3, and xCT receptors, but not αVβ5. LR-translocated receptors were monoubiquitinated, leading to productive macropinocytic entry, whereas non-LR-associated αVβ5 was polyubiquitinated, leading to clathrin-mediated entry that was targeted to lysosomes. Because the molecule(s) that integrate signal pathways and productive KSHV macropinocytosis were unknown, we immunoprecipitated KSHV-infected LR fractions with anti-α3β1 antibodies and analyzed them by mass spectrometry. The tyrosine kinase EphrinA2 (EphA2), implicated in many cancers, was identified in this analysis. EphA2 was activated by KSHV. EphA2 was also associated with KSHV and integrins (α3β1 and αVβ3) in LRs early during infection. Preincubation of virus with soluble EphA2, knockdown of EphA2 by shRNAs, or pretreatment of cells with anti-EphA2 monoclonal antibodies or tyrosine kinase inhibitor dasatinib significantly reduced KSHV entry and gene expression. EphA2 associates with c-Cbl-myosin IIA and augmented KSHV-induced Src and PI3-K signals in LRs, leading to bleb formation and macropinocytosis of KSHV. EphA2 shRNA ablated macropinocytosis-associated signaling events, virus internalization, and productive nuclear trafficking of KSHV DNA. Taken together, these studies demonstrate that the EphA2 receptor acts as a master assembly regulator of KSHV-induced signal molecules and KSHV entry in endothelial cells and suggest that the EphA2 receptor is an attractive target for controlling KSHV infection.

Productive hepatitis C virus infection of stem cell-derived hepatocytes reveals a critical transition to viral permissiveness during differentiation

Primary human hepatocytes isolated from patient biopsies represent the most physiologically relevant cell culture model for hepatitis C virus (HCV) infection, but these primary cells are not readily accessible, display individual variability, and are largely refractory to genetic manipulation. Hepatocyte-like cells differentiated from pluripotent stem cells provide an attractive alternative as they not only overcome these shortcomings but can also provide an unlimited source of noncancer cells for both research and cell therapy. Despite its promise, the permissiveness to HCV infection of differentiated human hepatocyte-like cells (DHHs) has not been explored. Here we report a novel infection model based on DHHs derived from human embryonic (hESCs) and induced pluripotent stem cells (iPSCs). DHHs generated in chemically defined media under feeder-free conditions were subjected to infection by both HCV derived in cell culture (HCVcc) and patient-derived virus (HCVser). Pluripotent stem cells and definitive endoderm were not permissive for HCV infection whereas hepatic progenitor cells were persistently infected and secreted infectious particles into culture medium. Permissiveness to infection was correlated with induction of the liver-specific microRNA-122 and modulation of cellular factors that affect HCV replication. RNA interference directed toward essential cellular cofactors in stem cells resulted in HCV-resistant hepatocyte-like cells after differentiation. The ability to infect cultured cells directly with HCV patient serum, to study defined stages of viral permissiveness, and to produce genetically modified cells with desired phenotypes all have broad significance for host-pathogen interactions and cell therapy.

Physiologically relevant cell-culture models for infection with hepatitis C virus (HCV) are scarce, and infection by viruses derived from patient serum has been inefficient. Differentiated human hepatocyte-like cells derived from pluripotent stem cells demonstrate hepatic functions but have not been explored for HCV infection studies. Here we report a novel infection model based on these hepatocyte-like cells. Stem cells and definitive endoderm successfully resisted HCV infection, whereas hepatic progenitor cells derived from the stem cells were productively infected by both human- and cell-culture-derived HCV. We determined the point of transition from resistance to susceptibility and, by comparative gene profiling, identified the host factors that were correlated with susceptibility. Genetic modification of human embryonic stem cells, coupled with hepatic differentiation, generated hepatocyte-like cells that were resistant to HCV infection. Our study establishes a new noncancerous and renewable cell-culture system for HCV infection, permits direct infection of cells by patient sera in vitro, identifies a defined transition to HCV susceptibility during hepatocyte differentiation, and demonstrates the feasibility of generating virus-resistant human hepatocyte-like cells in vitro.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Hepatitis C virus entry: beyond receptors

HCV is a blood-borne pathogen that affects approximately 3% of the global population and leads to progressive liver disease. Recent advances have identified an essential role for host cell molecules: tetraspanin CD81, scavenger receptor B1 and the tight junction proteins claudin-1 and occludin in HCV entry, suggesting a complex multi-step process. The conserved nature of this receptor-dependent step in the viral life cycle offers an attractive target for therapeutic intervention. Evidence is emerging that additional factors other than classical receptors, such as inflammatory mediators regulate the ability of hepatocytes to support HCV entry, and as such may provide potential avenues for drug design and development. In this review, we summarise the recent literature on HCV entry mechanisms with a view to realising the future potential of therapeutically targeting this process.

Solution structure of the first Sam domain of Odin and binding studies with the EphA2 receptor

The EphA2 receptor plays key roles in many physiological and pathological events, including cancer. The process of receptor endocytosis and the consequent degradation have attracted attention as possible means of overcoming the negative outcomes of EphA2 in cancer cells and decreasing tumor malignancy. A recent study indicates that Sam (sterile alpha motif) domains of Odin, a member of the ANKS (ankyrin repeat and sterile alpha motif domain-containing) family of proteins, are important for the regulation of EphA2 endocytosis. Odin contains two tandem Sam domains (Odin-Sam1 and -Sam2). Herein, we report on the nuclear magnetic resonance (NMR) solution structure of Odin-Sam1; through a variety of assays (employing NMR, surface plasmon resonance, and isothermal titration calorimetry techniques), we clearly demonstrate that Odin-Sam1 binds to the Sam domain of EphA2 in the low micromolar range. NMR chemical shift perturbation experiments and molecular modeling studies point out that the two Sam domains interact with a head-to-tail topology characteristic of several Sam-Sam complexes. This binding mode is similar to that we have previously proposed for the association between the Sam domains of the lipid phosphatase Ship2 and EphA2. This work further validates structural elements relevant for the heterotypic Sam-Sam interactions of EphA2 and provides novel insights for the design of potential therapeutic compounds that can modulate receptor endocytosis.

Living in the liver: hepatic infections

The liver has vital metabolic and clearance functions that involve the uptake of nutrients, waste products and pathogens from the blood. In addition, its unique immunoregulatory functions mediated by local expression of co-inhibitory receptors and immunosuppressive mediators help to prevent inadvertent organ damage. However, these tolerogenic properties render the liver an attractive target site for pathogens. Although most pathogens that reach the liver via the blood are eliminated or controlled by local innate and adaptive immune responses, some pathogens (such as hepatitis viruses) can escape immune control and persist in hepatocytes, causing substantial morbidity and mortality worldwide. Here, we review our current knowledge of the mechanisms of liver targeting by pathogens and describe the interplay between pathogens and host factors that promote pathogen elimination and maintain organ integrity or that allow pathogen persistence.

Essential roles for soluble virion-associated heparan sulfonated proteoglycans and growth factors in human papillomavirus infections

A subset of human papillomavirus (HPV) infections is causally related to the development of human epithelial tumors and cancers. Like a number of pathogens, HPV entry into target cells is initiated by first binding to heparan sulfonated proteoglycan (HSPG) cell surface attachment factors. The virus must then move to distinct secondary receptors, which are responsible for particle internalization. Despite intensive investigation, the mechanism of HPV movement to and the nature of the secondary receptors have been unclear. We report that HPV16 particles are not liberated from bound HSPG attachment factors by dissociation, but rather are released by a process previously unreported for pathogen-host cell interactions. Virus particles reside in infectious soluble high molecular weight complexes with HSPG, including syndecan-1 and bioactive compounds, like growth factors. Matrix mellatoproteinase inhibitors that block HSPG and virus release from cells interfere with virus infection. Employing a co-culture assay, we demonstrate HPV associated with soluble HSPG-growth factor complexes can infect cells lacking HSPG. Interaction of HPV-HSPG-growth factor complexes with growth factor receptors leads to rapid activation of signaling pathways important for infection, whereas a variety of growth factor receptor inhibitors impede virus-induced signaling and infection. Depletion of syndecan-1 or epidermal growth factor and removal of serum factors reduce infection, while replenishment of growth factors restores infection. Our findings support an infection model whereby HPV usurps normal host mechanisms for presenting growth factors to cells via soluble HSPG complexes as a novel method for interacting with entry receptors independent of direct virus-cell receptor interactions.

A subset of the >120 different types of human papillomaviruses (HPVs) are the most common cause of sexually transmitted infections. Certain HPVs are also associated with approximately 5% of all cancers worldwide. Like many pathogens, HPVs bind first to heparan sulfate proteoglycans (HSPGs) on cells before moving to more specific uptake receptors. However, relatively little is known about the mechanism(s) that triggers the translocation of HPV from HSPGs to the receptors that facilitate entry. As obligate parasites, viruses have evolved numerous means to hijack host cell functions to cause infection. We report two novel mechanisms of pathogen-host interactions. First, bound HPV particles are liberated from cells in an active complex with HSPGs and growth factors rather than dissociating from the sugars to engage secondary receptors. Second, HPV uses the specificity of the associated growth factors to bridge to their cognate receptors as opposed to direct binding to a cell internalization receptor. Signals transduced during these interactions are important for HPV infection. Our study provides new insights into the transmission of a significant viral pathogen and reveals novel means whereby microbes may repurpose normal cell functions during infection of their hosts. Likewise, this work uncovers new targets for HPV prophylaxis.

Hepatitis C virus induces CD81 and claudin-1 endocytosis

Hepatitis C virus (HCV) leads to progressive liver disease and hepatocellular carcinoma. Current treatments are only partially effective, and new therapies targeting viral and host pathways are required. Virus entry into a host cell provides a conserved target for therapeutic intervention. Tetraspanin CD81, scavenger receptor class B member I, and the tight-junction proteins claudin-1 and occludin have been identified as essential entry receptors. Limited information is available on the role of receptor trafficking in HCV entry. We demonstrate here that anti-CD81 antibodies inhibit HCV infection at late times after virus internalization, suggesting a role for intracellular CD81 in HCV infection. Several tetraspanins have been reported to internalize via motifs in their C-terminal cytoplasmic domains; however, CD81 lacks such motifs, leading several laboratories to suggest a limited role for CD81 endocytosis in HCV entry. We demonstrate CD81 internalization via a clathrin- and dynamin-dependent process, independent of its cytoplasmic domain, suggesting a role for associated partner proteins in regulating CD81 trafficking. Live cell imaging demonstrates CD81 and claudin-1 coendocytosis and fusion with Rab5 expressing endosomes, supporting a role for this receptor complex in HCV internalization. Receptor-specific antibodies and HCV particles increase CD81 and claudin-1 endocytosis, supporting a model wherein HCV stimulates receptor trafficking to promote particle internalization.

Treatment of patients with genotype 3 chronic hepatitis C--current and future therapies

Genotype 3 is a common type of HCV infection, and standard therapy using pegylated interferon (PEG-IFN) and ribavirin (RBV) is quite effective in these patients. While a short course of 16 weeks may result in comparable end of therapy responses, relapse rates are often high. A 24-week course is therefore preferable, and is expected to result in sustained virological response (SVR) rates of more than 70%. The 24-week course is especially recommended in the presence of steatosis (often associated with Genotype 3 infection), fibrosis stage two or more, high BMI and high viral load. In patients who do not achieve a rapid viral response (RVR) with combination therapy, an extended course up to 48 weeks should be considered. While not as definite as for genotype 1 patients, the presence of the CC variant of IL28b could help in the initial prognosis and the need for additional treatment, if an RVR is not achieved. The role of directly acting antiviral agents (DAA) has not been fully evaluated in treatment naïve, non-responders and relapsers in genotype 3 patients. Initial results with the cyclophilin inhibitor Debio-025 are quite encouraging. There is an urgent need for large clinical trials using DAA and host modulators in patients with G3 infection.

A human monoclonal antibody targeting scavenger receptor class B type I precludes hepatitis C virus infection and viral spread in vitro and in vivo

Endstage liver disease caused by chronic hepatitis C virus (HCV) infection is the leading indication for liver transplantation in the Western world. However, immediate reinfection of the grafted donor liver by circulating virus is inevitable and liver disease progresses much faster than the original disease. Standard antiviral therapy is not well tolerated and usually ineffective in liver transplant patients, whereas anti-HCV immunotherapy is hampered by the extreme genetic diversity of the virus and its ability to spread by way of cell-cell contacts. We generated a human monoclonal antibody against scavenger receptor class B type I (SR-BI), monoclonal antibody (mAb)16-71, which can efficiently prevent infection of Huh-7.5 hepatoma cells and primary hepatocytes by cell-culture-derived HCV (HCVcc). Using an Huh7.5 coculture system we demonstrated that mAb16-71 interferes with direct cell-to-cell transmission of HCV. Finally we evaluated the in vivo efficacy of mAb16-71 in "human liver urokinase-type plasminogen activator, severe combined immune deficiency (uPA-SCID) mice" (chimeric mice). A 2-week anti-SR-BI therapy that was initiated 1 day before viral inoculation completely protected all chimeric mice from infection with serum-derived HCV of different genotypes. Moreover, a 9-day postexposure therapy that was initiated 3 days after viral inoculation (when viremia was already observed in the animals) suppressed the rapid viral spread observed in untreated control animals. After cessation of anti-SR-BI-specific antibody therapy, a rise of the viral load was observed.

CONCLUSION

Using in vitro cell culture and human liver-chimeric mouse models, we show that a human mAb targeting the HCV coreceptor SR-BI completely prevents infection and intrahepatic spread of multiple HCV genotypes. This strategy may be an efficacious way to prevent infection of allografts following liver transplantation in chronic HCV patients, and may even hold promise for the prevention of virus rebound during or following antiviral therapy.

Permissiveness of human hepatoma cell lines for HCV infection

Background

Although primary and established human hepatoma cell lines have been evaluated for hepatitis C virus (HCV) infection in vitro, thus far only Huh7 cells have been found to be highly permissive for infectious HCV. Since our understanding of the HCV lifecycle would benefit from the identification of additional permissive cell lines, we assembled a panel of hepatic and non-hepatic cell lines and assessed their ability to support HCV infection. Here we show infection of the human hepatoma cell lines PLC/PRF/5 and Hep3B with cell culture-derived HCV (HCVcc), albeit to lower levels than that achieved in Huh7 cells. To better understand the reduced permissiveness of PLC and Hep3B cells for HCVcc infection, we performed studies to evaluate the ability of each cell line to support specific steps of the viral lifecycle (i.e. entry, replication, egress and spread).

Results

We found that while the early events in HCV infection (i.e. entry plus replication initiation) are cumulatively equivalent or only marginally reduced in PLC and Hep3B cells, later steps of the viral life cycle such as steady-state replication, de novo virus production and/or spread are impaired to different degrees in PLC and Hep3B cultures compared to Huh7 cell cultures. Interestingly, we also observed that interferon stimulated gene (i.e. ISG56) expression was significantly and differentially up-regulated in PLC and Hep3B cells following viral infection.

Conclusions

We conclude that the restrictions observed later during HCV infection in these cell lines could in part be attributed to HCV-induced innate signaling. Nevertheless, the identification of two new cell lines capable of supporting authentic HCVcc infection, even at reduced levels, expands the current repertoire of cell lines amendable for the study of HCV in vitro and should aid in further elucidating HCV biology and the cellular determinants that modulate HCV infection.

Identification of the Niemann-Pick C1-like 1 cholesterol absorption receptor as a new hepatitis C virus entry factor

Hepatitis C virus (HCV) is a leading cause of liver disease worldwide. With ∼170 million individuals infected and current interferon-based treatment having toxic side effects and marginal efficacy, more effective antivirals are crucially needed. Although HCV protease inhibitors were just approved by the US Food and Drug Administration (FDA), optimal HCV therapy, analogous to HIV therapy, will probably require a combination of antivirals targeting multiple aspects of the viral lifecycle. Viral entry represents a potential multifaceted target for antiviral intervention; however, to date, FDA-approved inhibitors of HCV cell entry are unavailable. Here we show that the cellular Niemann-Pick C1-like 1 (NPC1L1) cholesterol uptake receptor is an HCV entry factor amendable to therapeutic intervention. Specifically, NPC1L1 expression is necessary for HCV infection, as silencing or antibody-mediated blocking of NPC1L1 impairs cell culture-derived HCV (HCVcc) infection initiation. In addition, the clinically available FDA-approved NPC1L1 antagonist ezetimibe potently blocks HCV uptake in vitro via a virion cholesterol-dependent step before virion-cell membrane fusion. Moreover, ezetimibe inhibits infection by all major HCV genotypes in vitro and in vivo delays the establishment of HCV genotype 1b infection in mice with human liver grafts. Thus, we have not only identified NPC1L1 as an HCV cell entry factor but also discovered a new antiviral target and potential therapeutic agent.

Hepatitis C virus host cell entry

The hepatitis C virus (HCV) is a major medical problem with at least 130 million infected individuals worldwide. Over the last decade multiple host factors required for HCV cell entry have been identified, but a detailed understanding of their mechanistic interplay remains elusive. Nonetheless, recent advances in defining species-specific barriers of HCV transmission have allowed the identification of a minimal set of entry factors that are required for HCV infection of rodent cells and has culminated in an animal model that recapitulates HCV entry in vivo. A detailed understanding of the viral uptake pathway is imperative to define new drug targets allowing for more effective intervention against this devastating disease.

Immunosuppression, liver injury and post-transplant HCV recurrence

Hepatitis C virus (HCV) infection is a major cause for liver transplantation worldwide. Still, HCV re-infection of the graft occurs in almost all cases. Most liver transplant recipients experience episodes of graft hepatitis associated with fibrosis progression and graft failure. Clinical management of graft hepatitis can be challenging as in addition to rejection and HCV-induced hepatitis various other factors might be involved including toxic liver injury, steatohepatitis, ischaemic bile duct lesions or infections with other pathogens. Treatment options are often contradictory for different causes of graft hepatitis, and the role of distinct immunosuppressive drugs has been discussed controversially. Corticosteroids increase the infectivity of HCV by altering expression levels of entry factors and other immunosuppressive agents may have diverse effects on HCV replication and fibrosis progression. Interferon alpha-therapy of hepatitis C shows limited efficacy and tolerability in liver transplant recipients and may also cause rejection. In this review we summarize the current knowledge on mechanisms of liver injury in post-transplant hepatitis C, discuss the pros and cons of immunosuppressive agents in this specific setting and describe potential novel approaches to prevent HCV reinfection.

Curcumin attenuates Concanavalin A-induced liver injury in mice by inhibition of Toll-like receptor (TLR) 2, TLR4 and TLR9 expression

Curcumin has antiviral, antioxidant, and anti-inflammatory properties. However, the hepatoprotective effects and molecular mechanisms of curcumin on acute liver injury have not been carefully examined. The aims of this study were to examine the anti-inflammatory effect of curcumin on Concanavalin A (Con A) induced hepatitis, and to elucidate its underlying molecular mechanisms in mice. Mice received curcumin (200 mg/kg body weight) by gavage before Con A intravenous administration. We found that curcumin pretreatment was able to significantly reduce the elevated plasma aminotransferase levels and liver necrosis in Con A-induced hepatitis. Also, curcumin pretreatment reduced intrahepatic expression of genes encoding pro-inflammatory molecules such as tumor necrosis factor α (TNF-α) and interferon γ (IFN-γ) as compared with the vehicle controls, but augmented anti-inflammatory cytokine interleukin 10 (IL-10) by enzyme linked immunosorbent assay (ELISA). Furthermore, the expression levels of Toll-like receptor (TLR) 2, TLR4 and TLR9 mRNA or protein in liver tissues were significantly lowered by curcumin treatment. Curcumin pretreatment did not affect hepatic Kupffer cell numbers after Con A injection. These results suggest that curcumin pretreatment protects against T cell-mediated hepatitis in mice. The beneficial effect of curcumin may be partly mediated by inhibiting the expression levels of TLR2, TLR4 and TLR9 in the liver.

The green tea polyphenol, epigallocatechin-3-gallate, inhibits hepatitis C virus entry

Hepatitis C virus (HCV) is a major cause of liver cirrhosis and hepatocellular carcinoma. Current antiviral therapy fails to clear infection in a substantial proportion of cases. Drug development is focused on nonstructural proteins required for RNA replication. Individuals undergoing orthotopic liver transplantation face rapid, universal reinfection of the graft. Therefore, antiviral strategies targeting the early stages of infection are urgently needed for the prevention of HCV infection. In this study, we identified the polyphenol, epigallocatechin-3-gallate (EGCG), as an inhibitor of HCV entry. Green tea catechins, such as EGCG and its derivatives, epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC), have been previously found to exert antiviral and antioncogenic properties. EGCG had no effect on HCV RNA replication, assembly, or release of progeny virions. However, it potently inhibited Cell-culture-derived HCV (HCVcc) entry into hepatoma cell lines as well as primary human hepatocytes. The effect was independent of the HCV genotype, and both infection of cells by extracellular virions and cell-to-cell spread were blocked. Pretreatment of cells with EGCG before HCV inoculation did not reduce HCV infection, whereas the application of EGCG during inoculation strongly inhibited HCV infectivity. Moreover, treatment with EGCG directly during inoculation strongly inhibited HCV infectivity. Expression levels of all known HCV (co-)receptors were unaltered by EGCG. Finally, we showed that EGCG inhibits viral attachment to the cell, thus disrupting the initial step of HCV cell entry.

CONCLUSION

The green tea molecule, EGCG, potently inhibits HCV entry and could be part of an antiviral strategy aimed at the prevention of HCV reinfection after liver transplantation.

The cell biology of receptor-mediated virus entry

The cell imposes multiple barriers to virus entry. However, viruses exploit fundamental cellular processes to gain entry to cells and deliver their genetic cargo. Virus entry pathways are largely defined by the interactions between virus particles and their receptors at the cell surface. These interactions determine the mechanisms of virus attachment, uptake, intracellular trafficking, and, ultimately, penetration to the cytosol. Elucidating the complex interplay between viruses and their receptors is necessary for a full understanding of how these remarkable agents invade their cellular hosts.

Association between the epidermal growth factor rs4444903 G/G genotype and advanced fibrosis at a young age in chronic hepatitis C

BACKGROUND

The epidermal growth factor (EGF) rs4444903 A>G polymorphism has been associated with the development of liver cancer, which commonly complicates cirrhosis of viral origin; however, whether this polymorphism might be associated with fibrosis progression in chronic viral hepatitis is unknown. The present study was performed to assess the allelic and genotypic frequencies of the rs4444903 A>G polymorphism in patients with chronic hepatitis C virus HCV infection and to ascertain whether this polymorphism might be an independent predictor of the degree of fibrosis.

METHODS

An RFLP-PCR technique was used to genotype 645 patients (211 with cirrhosis); 528 were referred for the diagnosis and treatment of chronic hepatitis C, and 117 were transplanted for HCV-related end stage liver disease. A group of 428 healthy subjects served as a control. All the subjects were of Caucasian ethnicity.

RESULTS

The EGF rs4444903 A>G polymorphism genotype frequencies in HCV chronic infected patients were as follows: A/A=227 (35.3%), A/G=328 (50.9%), and G/G=90 (14.8%). Genotype frequencies were found to differ between patients with an Ishak staging score⩽2 (A/A=117, A/G=157, G/G=34) and patients with a score>2 (A/A=110, A/G=171, G/G=56, p=0.038). A highly significant linear relationship between increasing stage scores and EGF genotype was detected in younger patients (A/A: 2.02±0.18, A/G: 2.55±0.17, G/G: 3.00±0.32, p=0.008). However, no significant association was detected between the stage score and EGF genotype in older patients (A/A: 3.79±0.19, A/G: 3.64±0.15, G/G: 3.98±0.30 p=0.579).

CONCLUSIONS

The EGF rs4444903 A>G polymorphism may facilitate liver fibrosis progression in Caucasian patients with chronic hepatitis C, especially in younger patients.

Use of human hepatocyte-like cells derived from induced pluripotent stem cells as a model for hepatocytes in hepatitis C virus infection

Host tropism of hepatitis C virus (HCV) is limited to human and chimpanzee. HCV infection has never been fully understood because there are few conventional models for HCV infection. Human induced pluripotent stem cell-derived hepatocyte-like (iPS-Hep) cells have been expected to use for drug discovery to predict therapeutic activities and side effects of compounds during the drug discovery process. However, the suitability of iPS-Hep cells as an experimental model for HCV research is not known. Here, we investigated the entry and genomic replication of HCV in iPS-Hep cells by using HCV pseudotype virus (HCVpv) and HCV subgenomic replicons, respectively. We showed that iPS-Hep cells, but not iPS cells, were susceptible to infection with HCVpv. The iPS-Hep cells expressed HCV receptors, including CD81, scavenger receptor class B type I (SR-BI), claudin-1, and occludin; in contrast, the iPS cells showed no expression of SR-BI or claudin-1. HCV RNA genome replication occurred in the iPS-Hep cells. Anti-CD81 antibody, an inhibitor of HCV entry, and interferon, an inhibitor of HCV genomic replication, dose-dependently attenuated HCVpv entry and HCV subgenomic replication in iPS-Hep cells, respectively. These findings suggest that iPS-Hep cells are an appropriate model for HCV infection.

Hepatitis C virus entry into the hepatocyte

Hepatitis C virus (HCV) is a small enveloped virus with a positive stranded RNA genome belonging to the Flaviviridae family. The virion has the unique ability of forming a complex with lipoproteins, which is known as the lipoviroparticle. Lipoprotein components as well as the envelope proteins, E1 and E2, play a key role in virus entry into the hepatocyte. HCV entry is a complex multistep process involving sequential interactions with several cell surface proteins. The virus relies on glycosaminoglycans and possibly the low-density lipoprotein receptors to attach to cells. Furthermore, four specific entry factors are involved in the following steps which lead to virus internalization and fusion in early endosomes. These molecules are the scavenger receptor SRB1, tetraspanin CD81 and two tight junction proteins, Claudin-1 and Occludin. Although they are essential to HCV entry, the precise role of these molecules is not completely understood. Finally, hepatocytes are highly polarized cells and which likely affects the entry process. Our current knowledge on HCV entry is summarized in this review.

Targeting Eph receptors with peptides and small molecules: progress and challenges

The Eph receptors are a large family of receptor tyrosine kinases. Their kinase activity and downstream signaling ability are stimulated by the binding of cell surface-associated ligands, the ephrins. The ensuing signals are bidirectional because the ephrins can also transduce signals (known as reverse signals) following their interaction with Eph receptors. The ephrin-binding pocket in the extracellular N-terminal domain of the Eph receptors and the ATP-binding pocket in the intracellular kinase domain represent potential binding sites for peptides and small molecules. Indeed, a number of peptides and chemical compounds that target Eph receptors and inhibit ephrin binding or kinase activity have been identified. These molecules show promise as probes to study Eph receptor/ephrin biology, as lead compounds for drug development, and as targeting agents to deliver drugs or imaging agents to tumors. Current challenges are to find (1) small molecules that inhibit Eph receptor-ephrin interactions with high binding affinity and good lead-like properties and (2) selective kinase inhibitors that preferentially target the Eph receptor family or subsets of Eph receptors. Strategies that could also be explored include targeting additional Eph receptor interfaces and the ephrin ligands.

The hepatitis C virus glycan shield and evasion of the humoral immune response

Despite the induction of effective immune responses, 80% of hepatitis C virus (HCV)-infected individuals progress from acute to chronic hepatitis. In contrast to the cellular immune response, the role of the humoral immune response in HCV clearance is still subject to debate. Indeed, HCV escapes neutralizing antibodies in chronically infected patients and reinfection has been described in human and chimpanzee. Studies of antibody-mediated HCV neutralization have long been hampered by the lack of cell-culture-derived virus and the absence of a small animal model. However, the development of surrogate models and recent progress in HCV propagation in vitro now enable robust neutralization assays to be performed. These advances are beginning to shed some light on the mechanisms of HCV neutralization. This review summarizes the current state of knowledge of the viral targets of anti-HCV-neutralizing antibodies and the mechanisms that enable HCV to evade the humoral immune response. The recent description of the HCV glycan shield that reduces the immunogenicity of envelope proteins and masks conserved neutralizing epitopes at their surface constitutes the major focus of this review.

Discovery of highly potent small molecule Hepatitis C Virus entry inhibitors

Novel, highly potent small molecule HCV entry inhibitors are reported. The SAR exploration of a hit molecule identified from screening of a compound library led to the identification of highly potent compounds with IC(50) values of <5 nM in the tissue culture HCV infectious assay.

Hepatitis C virus (HCV) envelope glycoproteins E1 and E2 contain reduced cysteine residues essential for virus entry

The HCV envelope glycoproteins E1 and E2 contain eight and 18 highly conserved cysteine residues, respectively. Here, we examined the oxidation state of E1E2 heterodimers incorporated into retroviral pseudotyped particles (HCVpp) and investigated the significance of free sulfhydryl groups in cell culture-derived HCV (HCVcc) and HCVpp entry. Alkylation of free sulfhydryl groups on HCVcc/pp with a membrane-impermeable sulfhydryl-alkylating reagent 4-(N-maleimido)benzyl-α-trimethylammonium iodide (M135) prior to virus attachment to cells abolished infectivity in a dose-dependent manner. Labeling of HCVpp envelope proteins with EZ-Link maleimide-PEG2-biotin (maleimide-biotin) detected free thiol groups in both E1 and E2. Unlike retroviruses that employ disulfide reduction to facilitate virus entry, the infectivity of alkylated HCVcc could not be rescued by addition of exogenous reducing agents. Furthermore, the infectivity of HCVcc bound to target cells was not affected by addition of M135 indicative of a change in glycoprotein oxidation state from reduced to oxidized following virus attachment to cells. By contrast, HCVpp entry was reduced by 61% when treated with M135 immediately following attachment to cells, suggesting that the two model systems might demonstrate variations in oxidation kinetics. Glycoprotein oxidation was not altered following binding of HCVpp incorporated E1E2 to soluble heparin or recombinant CD81. These results suggest that HCV entry is dependent on the presence of free thiol groups in E1 and E2 prior to cellular attachment and reveals a new essential component of the HCV entry process.

A weak neutralizing antibody response to hepatitis C virus envelope glycoprotein enhances virus infection

We have completed a phase 1 safety and immunogenicity trial with hepatitis C virus (HCV) envelope glycoproteins, E1 and E2, with MF59 adjuvant as a candidate vaccine. Neutralizing activity to HCV genotype 1a was detected in approximately 25% of the vaccinee sera. In this study, we evaluated vaccinee sera from poor responders as a potential source of antibody dependent enhancement (ADE) of HCV infection. Sera with poor neutralizing activity enhanced cell culture grown HCV genotype 1a or 2a, and surrogate VSV/HCV pseudotype infection titer, in a dilution dependent manner. Surrogate pseudotypes generated from individual HCV glycoproteins suggested that antibody to the E2 glycoprotein; but not the E1 glycoprotein, was the principle target for enhancing infection. Antibody specific to FcRII expressed on the hepatic cell surface or to the Fc portion of Ig blocked enhancement of HCV infection by vaccinee sera. Together, the results from in vitro studies suggested that enhancement of viral infectivity may occur in the absence of a strong antibody response to HCV envelope glycoproteins.

Hepatitis C virus is primed by CD81 protein for low pH-dependent fusion

Hepatitis C virus (HCV) entry into permissive cells is a complex process that involves interactions with at least four co-factors followed by endocytosis and low pH-dependent fusion with endosomes. The precise sequence of receptor engagement and their roles in promoting HCV E1E2 glycoprotein-mediated fusion are poorly characterized. Because cell-free HCV tolerates an acidic environment, we hypothesized that binding to one or more receptors on the cell surface renders E1E2 competent to undergo low pH-induced conformational changes and promote fusion with endosomes. To test this hypothesis, we examined the effects of low pH and of the second extracellular loop (ECL2) of CD81, one of the four entry factors, on HCV infectivity. Pretreatment with an acidic buffer or with ECL2 enhanced infection through changing the E1E2 conformation, as evidenced by the altered reactivity of these proteins with conformation-specific antibodies and stable association with liposomes. However, neither of the two treatments alone permitted direct fusion with the cell plasma membrane. Sequential HCV preincubation with ECL2 and acidic buffer in the absence of target cells resulted in a marked loss of infectivity, implying that the receptor-bound HCV is primed for low pH-dependent conformational changes. Indeed, soluble receptor-pretreated HCV fused with the cell plasma membrane at low pH under conditions blocking an endocytic entry pathway. These findings suggest that CD81 primes HCV for low pH-dependent fusion early in the entry process. The simple triggering paradigm and intermediate conformations of E1E2 identified in this study could help guide future vaccine and therapeutic efforts to block HCV infection.

Hepatitis C virus infection induces the expression of amphiregulin, a factor related to the activation of cellular survival pathways and required for efficient viral assembly

Amphiregulin (AREG) is a ligand of the epidermal growth factor (EGF) receptor and may play a role in the development of cirrhosis and hepatocellular carcinoma in patients infected with hepatitis C virus (HCV). AREG showed an enhanced expression in HCV-infected human hepatoma cells according to gene array analysis. Therefore, we addressed the question about the role of AREG in HCV infection. AREG expression level was elevated in hepatoma cells containing a subgenomic HCV replicon or infected by HCV. Using a reporter assay, AREG promoter activity was found to be upregulated upon HCV infection. The enhanced AREG expression in hepatoma cells was partly caused by dsRNAs, HCV NS3 protein and autocrine stimulation. AREG was able to activate cellular signalling pathways including ERK, Akt and p38, promote cell proliferation, and protect cells from HCV-induced cell death. Further, knockdown of AREG expression increased the efficiency of HCV entry, as proven by HCV pseudoparticles reporter assay. However, the formation and release of infectious HCV particles were reduced by AREG silencing with a concomitant accumulation of intracellular HCV RNA pool, indicating that the assembly and release of HCV progeny may require AREG expression. Blocking the MAPK-ERK pathway by U0126 in Huh7.5.1 cells had a similar effect on HCV replication. In conclusion, HCV infection leads to an increase in AREG expression in hepatocytes. AREG expression is essential for efficient HCV assembly and virion release. Due to the activation of the cellular survival pathways, AREG may counteract HCV-induced apoptosis of infected hepatocytes and facilitate the development of liver cirrhosis and hepatocellular carcinoma.