Human apolipoprotein e is required for infectivity and production of hepatitis C virus in cell culture

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.

Impact of the autophagy machinery on hepatitis C virus infection

Autophagy is a cellular process that catabolizes cytoplasmic components and maintains energy homeostasis. As a stress response, the autophagy machinery interconnects a wide range of cellular pathways, enhancing the spread of certain pathogens while limiting others, and has become a highly active research area over the past several years. Independent laboratories have recently reported that autophagy vesicles accumulate in hepatitis C virus (HCV) infected cells and that autophagy proteins can function as proviral factors required for HCV replication. In this review, we summarize what is currently known about the interplay between autophagy and HCV and the possible mechanisms whereby autophagy proteins might favor HCV propagation.

HCV replication and assembly: a play in one act

HCV represents a serious public health problem worldwide. The current therapy for this virus is only partially effective and new antiviral therapies are urgently needed. Therefore, HCV assembly emerges as a potential therapeutic target. The HCV morphogenesis process presents the peculiarity of the double role of the nonstructural proteins in both the replication and assembly processes. Recently, the cross-talk between structural and nonstructural proteins for virion morphogenesis has been under investigation. We aim to review genetic, cell biology and biochemical data in order to reach a working model for the collaboration of all HCV proteins in the assembly process.

Cholesterol and chronic hepatitis C virus infection

Cholesterol is an essential molecule for the life cycle of the hepatitis C virus (HCV). This review focuses on the roles of cholesterol in HCV infection and introduces HCV events related to cholesterol metabolism and applications for cholesterol metabolism as a therapeutic target. HCV appears to alter host lipid metabolism into its preferable state, which is clinically recognized as steatosis and hypocholesterolemia. While hepatic fatty acid and triglyceride syntheses are upregulated in chronic hepatitis C patients, no direct evidence of increased hepatic de novo cholesterol biosynthesis has been obtained. Impaired VLDL secretion from hepatocytes is suggested to increase intracellular cholesterol concentrations, which may lead to hypocholesterolemia. Clinically, lower serum cholesterol levels are associated with lower rates of sustained virological responses (SVR) to pegylated-interferon plus ribavirin therapy, but the reason remains unclear. Clinical trials targeting HMG-CoA reductase, the rate-limiting enzyme in the cholesterol biosynthetic pathway, are being conducted using statins. Anti-HCV actions by statins appear to be caused by the inhibition of geranylgeranyl pyrophosphate synthesis rather than their cholesterol lowering effects. Other compounds that block various steps of cholesterol metabolic pathways have also been studied to develop new strategies for the complete eradication of this virus.

Interplay among cellular polarization, lipoprotein metabolism and hepatitis C virus entry

Hepatitis C virus (HCV) infects more than three million new individuals worldwide each year. In a high percentage of patients, acute infections become chronic, eventually progressing to fibrosis, cirrhosis, and hepatocellular carcinoma. Given the lack of effective prophylactic or therapeutic vaccines, and the limited sustained virological response rates to current therapies, new approaches are needed to prevent, control, and clear HCV infection. Entry into the host cell, being the first step of the viral cycle, is a potential target for the design of new antiviral compounds. Despite the recent discovery of the tight junction-associated proteins claudin-1 and occludin as HCV co-receptors, which is an important step towards the understanding of HCV entry, the precise mechanisms are still largely unknown. In addition, increasing evidence indicates that tools that are broadly employed to study HCV infection do not accurately reflect the real process in terms of viral particle composition and host cell phenotype. Thus, systems that more closely mimic natural infection are urgently required to elucidate the mechanisms of HCV entry, which will in turn help to design antiviral strategies against this part of the infection process.

Lipoprotein component associated with hepatitis C virus is essential for virus infectivity

Many chronic hepatitis patients with hepatitis C virus (HCV) are observed to have a degree of steatosis which is a factor in the progression of liver diseases. Transgenic mice expressing HCV core protein develop liver steatosis before the onset of hepatocellular carcinoma, suggesting active involvement of HCV in the de-regulation of lipid metabolism in host cells. However, the role of lipid metabolism in HCV life cycle has not been fully understood until the establishment of in vitro HCV infection and replication system. In this review we focus on HCV production with regard to modification of lipid metabolism observed in an in vitro HCV infection and replication system. The importance of lipid droplet to HCV production has been recognized, possibly at the stage of virus assembly, although the precise mechanism of lipid droplet for virus production remains elusive. Association of lipoprotein with HCV in circulating blood in chronic hepatitis C patients is observed. In fact, HCV released from culture medium is also associated with lipoprotein. The fact that treatment of HCV fraction with lipoprotein lipase (LPL) abolished infectivity indicates the essential role of lipoprotein's association with virus particle in the virus life cycle. In particular, apolipoprotein E (ApoE), a component of lipoprotein associated with HCV plays a pivotal role in HCV infectivity by functioning as a virus ligand to lipoprotein receptor that also functions as HCV receptor. These results strongly suggest the direct involvement of lipid metabolism in the regulation of the HCV life cycle.

Unique ties between hepatitis C virus replication and intracellular lipids

Hepatitis C virus (HCV) infects approximately 3% of the world's population, establishing a lifelong infection in the majority of cases. The life cycle of HCV is closely tied to the lipid metabolism of liver cells, and lipid droplets have emerged as crucial intracellular organelles that support persistent propagation of viral infection. In this review, we examine recent advances in our understanding of how HCV usurps intracellular lipids to propagate, and highlight unique opportunities for therapeutic intervention.

Structural requirements of virion-associated cholesterol for infectivity, buoyant density and apolipoprotein association of hepatitis C virus

Our earlier study has demonstrated that hepatitis C virus (HCV)-associated cholesterol plays a key role in virus infectivity. In this study, the structural requirement of sterols for infectivity, buoyant density and apolipoprotein association of HCV was investigated further. We removed cholesterol from virions with methyl β-cyclodextrin, followed by replenishment with 10 exogenous cholesterol analogues. Among the sterols tested, dihydrocholesterol and coprostanol maintained the buoyant density of HCV and its infectivity, and 7-dehydrocholesterol restored the physical appearance of HCV, but suppressed its infectivity. Other sterol variants with a 3β-hydroxyl group or with an aliphatic side chain did not restore density or infectivity. We also provide evidence that virion-associated cholesterol contributes to the interaction between HCV particles and apolipoprotein E. The molecular basis for the effects of different sterols on HCV infectivity is discussed.

Production and characterization of HCV particles from serum-free culture

Hepatitis C virus (HCV) is a major cause of liver cancer, and it is therefore important to develop a prophylactic strategy for HCV infection. In recent years, a system for cell culture of the infectious HCV particle has been established, and the inactivated particle has potential as an antigen for vaccine development. In this study, we aimed to establish highly efficient HCV particle purification procedures using the following serum-free culture of HCV particles. First, naïve human hepatoma Huh7 cells were grown in serum-free medium that was supplemented with human-derived insulin, transferrin and sodium selenite. Then, in vitro transcribed JFH-1 or J6/JFH-1 chimeric HCV-RNA was transfected into the serum-free conditioned Huh7 cells. Infectious HCV was secreted into the culture supernatant with the same efficiency as that from cells cultured in FBS-containing medium. The HCV-core protein and RNA continued to be detected in the culture supernatant when the infected cells were subcultured in serum-free medium. Sucrose gradient centrifugation analyses indicated that the profiles of HCV-core, HCV-RNA and the infectivity of HCV particles were almost identical between HCV from FBS-supplemented and serum-free cultures. We further determined that anti-CD81, anti-SR-BI and anti-E2 antibodies inhibited infection by serum-free cultured HCV to a greater extent than infection by HCV from FBS-supplemented cultures. These HCV particles also differed in the level of associated apoplipoproteins: the ApoE level was lower in serum-free cultured HCV. ApoB and ApoE antibody-depletion assays suggested that infection of serum-free cultured HCV was independent of ApoB and ApoE proteins. These data suggest that lipids conjugated with HCV affect infection and neutralization.

Multifaceted roles for lipids in viral infection

Viruses have evolved complex and dynamic interactions with their host cell. In recent years we have gained insight into the expanding roles for host lipids in the virus life cycle. In particular, viruses target lipid signaling, synthesis, and metabolism to remodel their host cells into an optimal environment for their replication. This review highlights examples from different viruses that illustrate the importance of these diverse virus–lipid interactions.

Scavenger receptor class B type I and the hypervariable region-1 of hepatitis C virus in cell entry and neutralisation

Hepatitis C virus (HCV) infection is a leading cause of chronic liver disease worldwide and represents a major public health problem. Viral attachment and entry - the first encounter of the virus with the host cell - are major targets of neutralising immune responses. Thus, a detailed understanding of the HCV entry process offers interesting opportunities for the development of novel therapeutic strategies. Different cellular or soluble host factors mediate HCV entry, and considerable progress has been made in recent years to decipher how they induce HCV attachment, internalisation and membrane fusion. Among these factors, the scavenger receptor class B type I (SR-BI/SCARB1) is essential for HCV replication in vitro, through its interaction with the HCV E1E2 surface glycoproteins and, more particularly, the HVR1 segment located in the E2 protein. SR-BI is an interesting receptor because HCV, whose replication cycle intersects with lipoprotein metabolism, seems to exploit some aspects of its physiological functions, such as cholesterol transfer from high-density lipoprotein (HDL), during cell entry. SR-BI is also involved in neutralisation attenuation and therefore could be an important target for therapeutic intervention. Recent results suggest that it should be possible to identify inhibitors of the interaction of HCV with SR-BI that do not impair its important physiological properties, as discussed in this review.

Antiviral stilbene 1,2-diamines prevent initiation of hepatitis C virus RNA replication at the outset of infection

The recent development of a cell culture model of hepatitis C virus (HCV) infection based on the JFH-1 molecular clone has enabled discovery of new antiviral agents. Using a cell-based colorimetric screening assay to interrogate a 1,200-compound chemical library for anti-HCV activity, we identified a family of 1,2-diamines derived from trans-stilbene oxide that prevent HCV infection at nontoxic, low micromolar concentrations in cell culture. Structure-activity relationship analysis of ~ 300 derivatives synthesized using click chemistry yielded compounds with greatly enhanced low nanomolar potency and a > 1,000:1 therapeutic ratio. Using surrogate models of HCV infection, we showed that the compounds selectively block the initiation of replication of incoming HCV RNA but have no impact on viral entry, primary translation, or ongoing HCV RNA replication, nor do they suppress persistent HCV infection. Selection of an escape variant revealed that NS5A is directly or indirectly targeted by this compound. In summary, we have identified a family of HCV inhibitors that target a critical step in the establishment of HCV infection in which NS5A translated de novo from an incoming genomic HCV RNA template is required to initiate the replication of this important human pathogen.

Assembly of hepatitis C virus particles

Infection with hepatitis C virus (HCV) is a major risk factor for chronic hepatitis, cirrhosis and hepatocellular carcinoma. Once robust cell culture systems for production of recombinant infectious HCV became available, evidence on molecular mechanisms underlying assembly and release of the virus particles began to accumulate. Recent studies have demonstrated that lipid droplets and viral nonstructural proteins play key roles in HCV morphogenesis. This review considers the current knowledge about maturation of HCV structural proteins and production of viral infectious particles.

Dengue virus-induced autophagy regulates lipid metabolism

Autophagy influences numerous cellular processes, including innate and adaptive immunity against intracellular pathogens. However, some viruses, including dengue virus (DENV), usurp autophagy to enhance their replication. The mechanism for a positive role of autophagy in DENV infection is unclear. We present data that DENV induction of autophagy regulates cellular lipid metabolism. DENV infection leads to an autophagy-dependent processing of lipid droplets and triglycerides to release free fatty acids. This results in an increase in cellular β-oxidation, which generates ATP. These processes are required for efficient DENV replication. Importantly, exogenous fatty acids can supplant the requirement of autophagy in DENV replication. These results define a role for autophagy in DENV infection and provide a mechanism by which viruses can alter cellular lipid metabolism to promote their replication.

Characterization of antibody-mediated neutralization directed against the hypervariable region 1 of hepatitis C virus E2 glycoprotein

The hypervariable region 1 (HVR1) comprising the first 27 aa of E2 glycoprotein is a target for neutralizing antibodies against hepatitis C virus (HCV), but the mechanisms of this neutralization in the cell-culture-infectious genotype 2a strain JFH1 HCV virus (HCVcc) system are unknown. Two rabbit polyclonal sera, R1020 and R140, recognizing the HVR1 of the genotype 1a isolates H77c and Glasgow (Gla), respectively, and a Gla HVR1-specific mouse mAb AP213 have been described previously. However, attempts to generate of antibodies to the JFH1 HVR1 were unsuccessful. Therefore, this study produced chimeric JFH1 HCVcc viruses harbouring the H77c or Gla HVR1 to assess the reactivity of antibodies to this region and their effects on virus infectivity. The inter-genotypic HVR1 swap did not significantly affect virus infectivity. The genotype 1a HVR1-specific antibodies neutralized chimeric viruses in an isolate-dependent manner, underlining the role of HVR1 in HCV infection. The neutralizing antibodies reacted mainly with the C-terminal portion of HVR1, and detailed mapping identified A17, F20 and Q21 in the Gla HVR1 sequence and T21 (and possibly L20) in the corresponding H77c sequence as key epitope residues for AP213 and R140, and R1020, respectively. Importantly, none of the antibodies inhibited in vitro binding of viral envelope glycoproteins to the best-characterized HCV receptor, CD81, or to the glycosaminoglycan attachment factors. However, the HVR1 antibodies were capable of post-attachment neutralization. Overall, this study emphasizes the role of HVR1 in HCVcc entry and provides new tools to study this region further in the context of complete virions.

Hepatitis C virus entry into hepatocytes: molecular mechanisms and targets for antiviral therapies

Hepatitis C virus (HCV) is a major cause of liver cirrhosis and hepatocellular carcinoma. Preventive modalities are absent and the current antiviral treatment is limited by resistance, toxicity, and high costs. Viral entry is required for initiation, spread, and maintenance of infection, and thus is a promising target for antiviral therapy. HCV entry is a highly orchestrated process involving viral and host cell factors. These include the viral envelope glycoproteins E1 and E2, CD81, scavenger receptor BI, and tight junction proteins claudin-1 and occludin. Recent studies in preclinical models and HCV-infected patients have demonstrated that the virus has developed multiple strategies to escape host immune responses during viral entry. These include evasion from neutralizing antibodies and viral spread by cell-cell transmission. These challenges have to be taken into account for the design of efficient antiviral strategies. Thus, a detailed understanding of the mechanisms of viral entry and escape is a prerequisite to define viral and cellular targets and develop novel preventive and therapeutic antivirals. This review summarizes the current knowledge about the molecular mechanisms of HCV entry into hepatocytes, highlights novel targets and reviews the current preclinical and clinical development of compounds targeting entry. Proof-of-concept studies suggest that HCV entry inhibitors are a novel and promising class of antivirals widening the preventive and therapeutic arsenal against HCV infection.

Immunotherapeutic potential of neutralizing antibodies targeting conserved regions of the HCV envelope glycoprotein E2

HCV is a major cause of chronic liver disease worldwide. There is no vaccine available and the current antiviral therapies fail to cure approximately half of treated patients. Liver disease caused by HCV infection is the most common indication for orthotopic liver transplantation. Unfortunately, reinfection of the new liver is universal and often results in an aggressive form of the disease leading to graft loss and the need for retransplantation. Immunotherapies using antibodies that potently inhibit HCV infection have the potential to control or even prevent graft reinfection. The virion envelope glycoproteins E1 and E2, which are involved in HCV entry into host cells, are the targets of neutralizing antibodies. To date, a number of monoclonal antibodies targeting conserved regions of E2 have been described that display outstanding neutralizing capabilities against HCV infection in both in vitro and in vivo systems. This article will summarize the current literature on these neutralizing anti-E2 antibodies and discuss their potential immunotherapeutic efficacy.

Protein kinase D negatively regulates hepatitis C virus secretion through phosphorylation of oxysterol-binding protein and ceramide transfer protein

Hepatitis C virus (HCV) RNA replicates its genome on specialized endoplasmic reticulum modified membranes termed membranous web and utilizes lipid droplets for initiating the viral nucleocapsid assembly. HCV maturation and/or the egress pathway requires host sphingolipid synthesis, which occur in the Golgi. Ceramide transfer protein (CERT) and oxysterol-binding protein (OSBP) play a crucial role in sphingolipid biosynthesis. Protein kinase D (PKD), a serine/threonine kinase, is recruited to the trans-Golgi network where it influences vesicular trafficking to the plasma membrane by regulation of several important mediators via phosphorylation. PKD attenuates the function of both CERT and OSBP by phosphorylation at their respective Ser(132) and Ser(240) residues (phosphorylation inhibition). Here, we investigated the functional role of PKD in HCV secretion. Our studies show that HCV gene expression down-regulated PKD activation. PKD depletion by shRNA or inhibition by pharmacological inhibitor Gö6976 enhanced HCV secretion. Overexpression of a constitutively active form of PKD suppressed HCV secretion. The suppression by PKD was subverted by the ectopic expression of nonphosphorylatable serine mutant CERT S132A or OSBP S240A. These observations imply that PKD negatively regulates HCV secretion/release by attenuating OSBP and CERT functions by phosphorylation inhibition. This study identifies the key role of the Golgi components in the HCV maturation process.

Adaptive immunity to the hepatitis C virus

The hepatitis C virus (HCV) is a global public health problem affecting approximately 2% of the human population. The majority of HCV infections (more than 70%) result in life-long persistence of the virus that substantially increases the risk of serious liver diseases, including cirrhosis and hepatocellular carcinoma. The remainder (less than 30%) resolves spontaneously, often resulting in long-lived protection from persistence upon reexposure to the virus. To persist, the virus must replicate and this requires effective evasion of adaptive immune responses. In this review, the role of humoral and cellular immunity in preventing HCV persistence, and the mechanisms used by the virus to subvert protective host responses, are considered.

Neutralizing antibody-resistant hepatitis C virus cell-to-cell transmission

Hepatitis C virus (HCV) can initiate infection by cell-free particle and cell-cell contact-dependent transmission. In this study we use a novel infectious coculture system to examine these alternative modes of infection. Cell-to-cell transmission is relatively resistant to anti-HCV glycoprotein monoclonal antibodies and polyclonal immunoglobulin isolated from infected individuals, providing an effective strategy for escaping host humoral immune responses. Chimeric viruses expressing the structural proteins representing the seven major HCV genotypes demonstrate neutralizing antibody-resistant cell-to-cell transmission. HCV entry is a multistep process involving numerous receptors. In this study we demonstrate that, in contrast to earlier reports, CD81 and the tight-junction components claudin-1 and occludin are all essential for both cell-free and cell-to-cell viral transmission. However, scavenger receptor BI (SR-BI) has a more prominent role in cell-to-cell transmission of the virus, with SR-BI-specific antibodies and small-molecule inhibitors showing preferential inhibition of this infection route. These observations highlight the importance of targeting host cell receptors, in particular SR-BI, to control viral infection and spread in the liver.

Production of hepatitis C virus lacking the envelope-encoding genes for single-cycle infection by providing homologous envelope proteins or vesicular stomatitis virus glycoproteins in trans

Hepatitis C virus (HCV) infection is a major worldwide health problem. The envelope glycoproteins are the major components of viral particles. Here we developed a trans-complementation system that allows the production of infectious HCV particles in whose genome the regions encoding envelope proteins are deleted (HCVΔE). The lack of envelope proteins could be efficiently complemented by the expression of homologous envelope proteins in trans. HCVΔE production could be enhanced significantly by previously described adaptive mutations in NS3 and NS5A. Moreover, HCVΔE could be propagated and passaged in packaging cells stably expressing HCV envelope proteins, resulting in only single-round infection in wild-type cells. Interestingly, we found that vesicular stomatitis virus (VSV) glycoproteins could efficiently rescue the production of HCV lacking endogenous envelope proteins, which no longer required apolipoprotein E for virus production. VSV glycoprotein-mediated viral entry could allow for the bypass of the natural HCV entry process and the delivery of HCV replicon RNA into HCV receptor-deficient cells. Our development provides a new tool for the production of single-cycle infectious HCV particles, which should be useful for studying individual steps of the HCV life cycle and may also provide a new strategy for HCV vaccine development.

Assembly of infectious hepatitis C virus particles

A hallmark of the hepatitis C virus (HCV) replication cycle is its tight link with host cell lipid synthesis. This is best illustrated by the peculiar pathway used for the assembly of infectious HCV particles. Research in the past few years has shown that formation of HC-virions is closely connected to lipid droplets that could serve as an assembly platform. Moreover, HCV particle production appears to be strictly linked to very-low-density lipoproteins. In this review, we focus on new insights into the molecular aspects of the architecture and assembly of this unique type of virus particle.

Association between phospholipids and free cholesterol in high-density lipoprotein and the response to hepatitis C treatment in Japanese with genotype 1b

Pegylated interferon and ribavirin combination therapy is the standard treatment for patients with chronic hepatitis C (CHC), but treatment failure can be difficult to predict. We and others have reported a relation between lipid values and sustained viral responses in patients with CHC. However, the relationship between lipid values and treatment failure has not been previously reported. The present study investigated the association between the profiles of phospholipids and free cholesterol (FC), the main constitutive ingredients of the surface of lipoprotein, classified according to particle size and hepatitis C treatment, and determined the usefulness of these parameters for predicting the outcome of treatment. Fifty-five patients with CHC (33 men and 22 women) were included in the study. The serum total cholesterol, triglyceride, phospholipids, and FC levels in the lipoprotein subclasses were determined using high-performance liquid chromatography with gel permeation columns, enabling the lipoproteins to be classified into 13 subclasses according to particle size. According to a univariate analysis, the treatment failure group had a significantly higher serum phospholipid level overall in the high-density lipoprotein (HDL) and medium HDL fractions as well as a higher serum FC level in the HDL fraction and all HDL subclass fractions compared with the corresponding values in the non-nonvirological response group. Higher serum phospholipid and FC concentrations in the HDL subclasses were predictive of a failure to respond in patients with genotype 1b.

Biochemical and morphological properties of hepatitis C virus particles and determination of their lipidome

A hallmark of hepatitis C virus (HCV) particles is their association with host cell lipids, most notably lipoprotein components. It is thought that this property accounts for the low density of virus particles and their large heterogeneity. However, the composition of infectious virions and their biochemical and morphological properties are largely unknown. We developed a system in which the envelope glycoprotein E2 was N-terminally tagged with a FLAG epitope. This virus, designated Jc1E2(FLAG), produced infectivity titers to wild type levels and allowed affinity purification of virus particles that were analyzed for their protein and lipid composition. By using mass spectrometry, we found the lipid composition of Jc1E2(FLAG) particles to resemble the one very low- and low density-lipoprotein with cholesteryl esters accounting for almost half of the total HCV lipids. Thus, HCV particles possess a unique lipid composition that is very distinct from all other viruses analyzed so far and from the human liver cells in which HCV was produced. By electron microscopy (EM), we found purified Jc1E2(FLAG) particles to be heterogeneous, mostly spherical structures, with an average diameter of about 73 nm. Importantly, the majority of E2-containing particles also contained apoE on their surface as assessed by immuno-EM. Taken together, we describe a rapid and efficient system for the production of large quantities of affinity-purified HCV allowing a comprehensive analysis of the infectious virion, including the determination of its lipid composition.

[Advances in research on HCV replication and virion formation]

Hepatitis C virus (HCV) establishes a persistent infection and is recognized as a major cause of chronic liver diseases worldwide. Although much work remains to be done regarding the viral life cycle, significant progress has been made with respect to the molecular biology of HCV, especially the viral genome replication and virion formation. A variety of host cell factors, which play roles in replication of the viral genome RNA, have been identified. Involvement of lipid droplet, lipid metabolism and the viral nonstructural proteins in the production of the infectious particles has also been revealed.

Efficient hepatitis C virus particle formation requires diacylglycerol acyltransferase-1

Hepatitis C virus (HCV) infection is closely tied to the lipid metabolism of liver cells. Here we identify the triglyceride-synthesizing enzyme diacylglycerol acyltransferase-1 (DGAT1) as a key host factor for HCV infection. DGAT1 interacts with the viral nucleocapsid core and is required for the trafficking of core to lipid droplets. Inhibition of DGAT1 activity or RNAi-mediated knockdown of DGAT1 severely impairs infectious virion production, implicating DGAT1 as a new target for antiviral therapy.

Ultrastructural and biophysical characterization of hepatitis C virus particles produced in cell culture

We analyzed the biochemical and ultrastructural properties of hepatitis C virus (HCV) particles produced in cell culture. Negative-stain electron microscopy revealed that the particles were spherical (∼40- to 75-nm diameter) and pleomorphic and that some of them contain HCV E2 protein and apolipoprotein E on their surfaces. Electron cryomicroscopy revealed two major particle populations of ∼60 and ∼45 nm in diameter. The ∼60-nm particles were characterized by a membrane bilayer (presumably an envelope) that is spatially separated from an internal structure (presumably a capsid), and they were enriched in fractions that displayed a high infectivity-to-HCV RNA ratio. The ∼45-nm particles lacked a membrane bilayer and displayed a higher buoyant density and a lower infectivity-to-HCV RNA ratio. We also observed a minor population of very-low-density, >100-nm-diameter vesicular particles that resemble exosomes. This study provides low-resolution ultrastructural information of particle populations displaying differential biophysical properties and specific infectivity. Correlative analysis of the abundance of the different particle populations with infectivity, HCV RNA, and viral antigens suggests that infectious particles are likely to be present in the large ∼60-nm HCV particle populations displaying a visible bilayer. Our study constitutes an initial approach toward understanding the structural characteristics of infectious HCV particles.

Intravascular transfer contributes to postprandial increase in numbers of very-low-density hepatitis C virus particles

BACKGROUND & AIMS

The physical association of hepatitis C virus (HCV) particles with lipoproteins in plasma results in distribution of HCV in a broad range of buoyant densities. This association is thought to increase virion infectivity by mediating cell entry via lipoprotein receptors. We sought to determine if factors that affect triglyceride-rich lipoprotein (TRL) metabolism alter the density and dynamics of HCV particles in the plasma of patients with chronic HCV infection.

METHODS

Fasting patients (n = 10) consumed a high-fat milkshake; plasma was collected and fractionated by density gradients. HCV- RNA was measured in the very-low-density fraction (VLDF, d < 1.025 g/mL) before and at 7 serial time points postprandially.

RESULTS

The amount of HCV RNA in the VLDF (HCV(VLDF)) increased a mean of 26-fold, peaking 180 minutes after the meal (P < .01). Quantification of HCV RNA throughout the density gradient fractions revealed that HCV(VLDF) rapidly disappeared, rather than migrating into the adjacent density fraction. Immuno-affinity separation of the VLDF, using antibodies that recognize apolipoprotein B-100 and not apolipoprotein B-48, showed that HCV(VLDF) is composed of chylomicron- and VLDL-associated HCV particles; peaking 120 and 180 minutes after the meal, respectively. Plasma from fasting HCV-infected patients mixed with uninfected plasma increased the quantity of HCV(VLDF), compared with that mixed with phosphate-buffered saline, showing extracellular assembly of HCV(VLDF).

CONCLUSIONS

Dietary triglyceride alters the density and dynamics of HCV in plasma. The rapid clearance rate of HCV(VLDF) indicates that association with TRL is important for HCV infectivity. HCV particles, such as exchangeable apolipoproteins, appear to reassociate with TRLs in the vascular compartment.

Hepatitis C virus p7-a viroporin crucial for virus assembly and an emerging target for antiviral therapy

The hepatitis C virus (HCV), a hepatotropic plus-strand RNA virus of the family Flaviviridae, encodes a set of 10 viral proteins. These viral factors act in concert with host proteins to mediate virus entry, and to coordinate RNA replication and virus production. Recent evidence has highlighted the complexity of HCV assembly, which not only involves viral structural proteins but also relies on host factors important for lipoprotein synthesis, and a number of viral assembly co-factors. The latter include the integral membrane protein p7, which oligomerizes and forms cation-selective pores. Based on these properties, p7 was included into the family of viroporins comprising viral proteins from multiple virus families which share the ability to manipulate membrane permeability for ions and to facilitate virus production. Although the precise mechanism as to how p7 and its ion channel function contributes to virus production is still elusive, recent structural and functional studies have revealed a number of intriguing new facets that should guide future efforts to dissect the role and function of p7 in the viral replication cycle. Moreover, a number of small molecules that inhibit production of HCV particles, presumably via interference with p7 function, have been reported. These compounds should not only be instrumental in increasing our understanding of p7 function, but may, in the future, merit further clinical development to ultimately optimize HCV-specific antiviral treatments.

Cellular lipid droplets and hepatitis C virus life cycle

Lipid droplets (LDs) are cellular lipid storage organelles involved not only in lipid homeostasis but also in a variety of diseases. Chronic hepatitis C virus (HCV) infection affects host lipid metabolism, and thus induces LD accumulation in the liver. Recent studies have suggested that cellular LDs also play a crucial role in the HCV life cycle. Interactions between HCV proteins, especially the core protein, and LDs are required for the morphogenesis of infectious HCV. The present minireview will summarize the recent research progress about this unique relationship between LDs and the HCV life cycle.

Infectivity of hepatitis C virus is influenced by association with apolipoprotein E isoforms

Hepatitis C virus (HCV) is a causative agent of chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. HCV in circulating blood associates with lipoproteins such as very low density lipoprotein (VLDL) and low-density lipoprotein (LDL). Although these associations suggest that lipoproteins are important for HCV infectivity, the roles of lipoproteins in HCV production and infectivity are not fully understood. To clarify the roles of lipoprotein in the HCV life cycle, we analyzed the effect of apolipoprotein E (ApoE), a component of lipoprotein, on virus production and infectivity. The production of infectious HCV was significantly reduced by the knockdown of ApoE. When an ApoE mutant that fails to be secreted into the culture medium was used, the amount of infectious HCV in the culture medium was dramatically reduced; the infectious HCV accumulated inside these cells, suggesting that infectious HCV must associate with ApoE prior to virus release. We performed rescue experiments in which ApoE isoforms were ectopically expressed in cells depleted of endogenous ApoE. The ectopic expression of the ApoE2 isoform, which has low affinity for the LDL receptor (LDLR), resulted in poor recovery of infectious HCV, whereas the expression of other isoforms, ApoE3 and ApoE4, rescued the production of infectious virus, raising it to an almost normal level. Furthermore, we found that the infectivity of HCV required both the LDLR and scavenger receptor class B, member I (SR-BI), ligands for ApoE. These findings indicate that ApoE is an essential apolipoprotein for HCV infectivity.

Lipoprotein lipase and hepatic triglyceride lipase reduce the infectivity of hepatitis C virus (HCV) through their catalytic activities on HCV-associated lipoproteins

The effect of lipolysis by lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) on hepatitis C virus (HCV) infection was evaluated. First, medium from HuH7.5 cells bearing HCV genome replication was treated with LPL. LPL treatment led to reduced HCV infectivity, shifted HCV to higher densities, and lowered the amount of apolipoprotein E-associated HCV. The effect of endogenous HTGL secreted from HuH7.5 on HCV infectivity was next examined. Neutralization of HTGL by an anti-HTGL antibody resulted in suppression of LPL-induced reduction in infectivity of HCV-bearing medium, while knockdown of HTGL by siRNA led to increased HCV infectivity irrespective of LPL. HCV in medium from HTGL knockdown cells was found in fractions with a lower density. These results indicate that changes in the nature of HCV-associated lipoproteins by LPL and/or HTGL affect HCV infectivity, suggesting that association of HCV with specific lipoproteins is important for HCV infectivity.

Last stop before exit - hepatitis C assembly and release as antiviral drug targets

Chronic Hepatitis C infection is a global health problem. While primary infection is often inapparent, it becomes chronic in most cases. Chronic infection with Hepatitis C virus (HCV) frequently leads to liver cirrhosis or liver cancer. Consequently, HCV infection is one of the leading causes for liver transplantation in industrialized countries. Current treatment is not HCV specific and is only effective in about half of the infected patients. This situation underlines the need for new antivirals against HCV. To develop new and more efficient drugs, it is essential to specifically target the different steps of the viral life cycle. Of those steps, the targeting of HCV assembly has the potential to abolish virus production. This review summarizes the advances in our understanding of HCV particle assembly and the identification of new antiviral targets of potential interest in this late step of the HCV life cycle.

The C-terminal alpha-helix domain of apolipoprotein E is required for interaction with nonstructural protein 5A and assembly of hepatitis C virus

We have recently demonstrated that human apolipoprotein E (apoE) is required for the infectivity and assembly of hepatitis C virus (HCV) (K. S. Chang, J. Jiang, Z. Cai, and G. Luo, J. Virol. 81:13783-13793, 2007; J. Jiang and G. Luo, J. Virol. 83:12680-12691, 2009). In the present study, we have determined the molecular basis underlying the importance of apoE in HCV assembly. Results derived from mammalian two-hybrid studies demonstrate a specific interaction between apoE and HCV nonstructural protein 5A (NS5A). The C-terminal third of apoE per se is sufficient for interaction with NS5A. Progressive deletion mutagenesis analysis identified that the C-terminal α-helix domain of apoE is important for NS5A binding. The N-terminal receptor-binding domain and the C-terminal 20 amino acids of apoE are dispensable for the apoE-NS5A interaction. The NS5A-binding domain of apoE was mapped to the middle of the C-terminal α-helix domain between amino acids 205 and 280. Likewise, deletion mutations disrupting the apoE-NS5A interaction resulted in blockade of HCV production. These findings demonstrate that the specific apoE-NS5A interaction is required for assembly of infectious HCV. Additionally, we have determined that using different major isoforms of apoE (E2, E3, and E4) made no significant difference in the apoE-NS5A interaction. Likewise, these three major isoforms of apoE are equally compatible with infectivity and assembly of infectious HCV, suggesting that apoE isoforms do not differentially modulate the infectivity and/or assembly of HCV in cell culture.

Morphological characterization and fusion properties of triglyceride-rich lipoproteins obtained from cells transduced with hepatitis C virus glycoproteins

The density of hepatitis C virus (HCV) particles circulating in the blood of chronically infected patients and of cell-culture produced HCV is heterogeneous. Specific infectivity and fusion of low density particles are higher than those of high density particles. We recently characterized hybrid particles produced by Caco-2 colon or Huh-7.5 liver cells transduced with HCV E1 and E2 envelope glycoproteins. Caco-2-derived particles, called empty lipo-viral particles (eLVP), are composed of triglyceride-rich lipoproteins positive for apolipoproteins B (i.e. apoB100 and apoB48) and contain HCV E1 and E2. Here we aimed at characterizing the morphology and in vitro fusion properties of eLVP using electron microscopy and fluorescence spectroscopy. They displayed the aspect of beta-lipoproteins, and immunogold labeling confirmed the presence of apoB and HCV E1 and E2 at their surface. These particles are able to fuse with lipid bilayers (liposomes) in a fusion process leading to the coalescence of internal contents of triglyceride-rich lipoproteins particles and liposomes. Fusion was pH-dependent and could be inhibited by either Z-fFG, a peptide known to inhibit viral fusion, or by monoclonal antibodies directed against HCV E2 or the apolipoprotein moiety of the hybrid particle. Interestingly, particles derived from Huh-7.5 cells failed to display equivalent efficient fusion. Optimal fusion activity is, thus, observed when HCV envelope proteins are associated to apoB-positive hybrid particles. Our results, therefore, point to a crucial role of the E1 and E2 proteins in HCV fusion with a subtle interplay with the apolipoprotein part of eLVP.

Role of annexin A2 in the production of infectious hepatitis C virus particles

Hepatitis C virus (HCV) is an important human pathogen affecting 170 million chronically infected individuals. In search for cellular proteins involved in HCV replication, we have developed a purification strategy for viral replication complexes and identified annexin A2 (ANXA2) as an associated host factor. ANXA2 colocalized with viral nonstructural proteins in cells harboring genotype 1 or 2 replicons as well as in infected cells. In contrast, we found no obvious colocalization of ANXA2 with replication sites of other positive-strand RNA viruses. The silencing of ANXA2 expression showed no effect on viral RNA replication but resulted in a significant reduction of extra- and intracellular virus titers. Therefore, it seems likely that ANXA2 plays a role in HCV assembly rather than in genome replication or virion release. Colocalization studies with individually expressed HCV nonstructural proteins indicated that NS5A specifically recruits ANXA2, probably by an indirect mechanism. By the deletion of individual NS5A subdomains, we identified domain III (DIII) as being responsible for ANXA2 recruitment. These data identify ANXA2 as a novel host factor contributing, with NS5A, to the formation of infectious HCV particles.

Lipid metabolism and HCV infection

Chronic infection by hepatitis C virus (HCV) can lead to severe liver disease and is a global healthcare problem. The liver is highly metabolically active and one of its key functions is to control the balance of lipid throughout the body. A number of pathologies have been linked to the impact of HCV infection on liver metabolism. However, there is also growing evidence that hepatic metabolic processes contribute to the HCV life cycle. This review summarizes the relationship between lipid metabolism and key stages in the production of infectious HCV.

Hepatitis C virus: assembly and release of virus particles

Hepatitis C virus is a blood-borne virus that typically establishes a chronic infection in the liver, which often results in cirrhosis and hepatocellular carcinoma. Progress in understanding the complete virus life cycle has been greatly enhanced by the recent availability of a tissue culture system that produces infectious virus progeny. Thus, it is now possible to gain insight into the roles played by viral components in assembly and egress and the cellular pathways that contribute to virion formation. This minireview describes the key determining viral and host factors that are needed to produce infectious virus.

Hepatitis C virus hypervariable region 1 modulates receptor interactions, conceals the CD81 binding site, and protects conserved neutralizing epitopes

The variability of the hepatitis C virus (HCV), which likely contributes to immune escape, is most pronounced in hypervariable region 1 (HVR1) of viral envelope protein 2. This domain is the target for neutralizing antibodies, and its deletion attenuates replication in vivo. Here we characterized the relevance of HVR1 for virus replication in vitro using cell culture-derived HCV. We show that HVR1 is dispensable for RNA replication. However, viruses lacking HVR1 (Delta HVR1) are less infectious, and separation by density gradients revealed that the population of Delta HVR1 virions comprises fewer particles with low density. Strikingly, Delta HVR1 particles with intermediate density (1.12 g/ml) are as infectious as wild-type virions, while those with low density (1.02 to 1.08 g/ml) are poorly infectious, despite quantities of RNA and core similar to those in wild-type particles. Moreover, Delta HVR1 particles exhibited impaired fusion, a defect that was partially restored by an E1 mutation (I347L), which also rescues infectivity and which was selected during long-term culture. Finally, Delta HVR1 particles were no longer neutralized by SR-B1-specific immunoglobulins but were more prone to neutralization and precipitation by soluble CD81, E2-specific monoclonal antibodies, and patient sera. These results suggest that HVR1 influences the biophysical properties of released viruses and that this domain is particularly important for infectivity of low-density particles. Moreover, they indicate that HVR1 obstructs the viral CD81 binding site and conserved neutralizing epitopes. These functions likely optimize virus replication, facilitate immune escape, and thus foster establishment and maintenance of a chronic infection.

Hepatitis C virus differentially modulates activation of forkhead transcription factors and insulin-induced metabolic gene expression

Chronic hepatitis C virus (HCV) infection is often associated with insulin resistance and hepatic steatosis. Insulin regulates gene expression of key enzymes in glucose and lipid metabolism by modulating the activity of specific Forkhead box transcriptional regulators (FoxO1 and FoxA2) via the phosphatidylinositol 3-kinase (PI3K)-Akt signaling pathway in the liver. In this study, we observed that HCV infection of human hepatocytes impaired insulin-induced FoxO1 translocation from the nucleus to the cytoplasm and significantly reduced accumulation of FoxA2 in the nucleus. Phosphorylation of FoxO1 at Ser(256), a downstream target for Akt, was inhibited in hepatocytes infected with HCV or expressing the core protein or full-length (FL) genome of HCV. Further, an interaction between FoxO1 and 14-3-3 protein, important for FoxO1 translocation, was inhibited in HCV core-expressing cells. Hepatocytes infected with HCV, expressing the core protein alone or polyprotein displayed an increased level of glucose-6-phosphatase (G6P) mRNA. On the other hand, microsomal triglycerol transfer protein (MTP) activity and apolipoprotein B (ApoB) secretion were significantly reduced in hepatocytes expressing HCV proteins. Together, these observations suggest that HCV infection or ectopic expression of the core protein either alone or together with other viral proteins from an FL gene construct differentially modulates FoxO1 and FoxA2 activation and affects insulin-induced metabolic gene regulation in human hepatocytes.

Recent advances in hepatitis C virus cell entry

More than 170 million patients worldwide are chronically infected with hepatitis C virus (HCV). Prevalence rates range from 0.5% in Northern European countries to 28% in some areas of Egypt. HCV is hepatotropic, and in many countries chronic hepatitis C is a leading cause of liver disease including fibrosis, cirrhosis and hepatocellular carcinoma. HCV persists in 50-85% of infected patients, and once chronic infection is established, spontaneous clearance is rare. HCV is a member of the Flaviviridae family, in which it forms its own genus. Many lines of evidence suggest that the HCV life cycle displays many differences to that of other Flaviviridae family members. Some of these differences may be due to the close interaction of HCV with its host's lipid and particular triglyceride metabolism in the liver, which may explain why the virus can be found in association with lipoproteins in serum of infected patients. This review focuses on the molecular events underlying the HCV cell entry process and the respective roles of cellular co-factors that have been implied in these events. These include, among others, the lipoprotein receptors low density lipoprotein receptor and scavenger receptor BI, the tight junction factors occludin and claudin-1 as well as the tetraspanin CD81. We discuss the roles of these cellular factors in HCV cell entry and how association of HCV with lipoproteins may modulate the cell entry process.

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

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

Apolipoprotein E genotype and hepatitis C, HIV and herpes simplex disease risk: a literature review

Apolipoprotein E is a polymorphic and multifunctional protein with numerous roles in lipoprotein metabolism. The three common isoforms apoE2, apoE3 and apoE4 show isoform-specific functional properties including different susceptibilities to diseases. ApoE4 is an accepted risk factor for Alzheimer's disease and cardiovascular disorders. Recently, associations between apoE4 and infectious diseases have been demonstrated. This review summarises how apoE4 may be involved in the infection incidence and associated pathologies of specific infectious diseases, namely hepatitis C, human immunodeficiency virus disease and herpes simplex.ApoE4 seems to be protective against chronic hepatitis C virus infection and retards fibrosis progression. In contrast apoE4 enhances the fusion rate of human immunodeficiency virus with target cell membranes, resulting in accelerated cell entry and faster disease progression. Its association with human immunodeficiency virus-associated dementia remains controversial. Regarding herpes simplex virus infection, apoE4 intensifies virus latency and is associated with increased oxidative damage of the central nervous system, and there is some evidence that herpes simplex virus infection in combination with the apoE4 genotype may be associated with an increased risk of Alzheimer's disease. In addition to reviewing available data from human trials, evidence derived from a variety of cell culture and animal models are considered in this review in order to provide mechanistic insights into observed association between apoE4 genotype and viral disease infection and pathology.

Apolipoprotein E interacts with hepatitis C virus nonstructural protein 5A and determines assembly of infectious particles

Chronic hepatitis C virus (HCV) infection is a major cause of liver disease worldwide. Restriction of HCV infection to human hepatocytes suggests that liver-specific host factors play a role in the viral life cycle. Using a yeast-two-hybrid system, we identified apolipoprotein E (apoE) as a liver-derived host factor specifically interacting with HCV nonstructural protein 5A (NS5A) but not with other viral proteins. The relevance of apoE-NS5A interaction for viral infection was confirmed by co-immunoprecipitation and co-localization studies of apoE and NS5A in an infectious HCV cell culture model system. Silencing apoE expression resulted in marked inhibition of infectious particle production without affecting viral entry and replication. Analysis of particle production in liver-derived cells with silenced apoE expression showed impairment of infectious particle assembly and release. The functional relevance of the apoE-NS5A interaction for production of viral particles was supported by loss or decrease of apoE-NS5A binding in assembly-defective viral mutants.

CONCLUSION

These results suggest that recruitment of apoE by NS5A is important for viral assembly and release of infectious viral particles. These findings have important implications for understanding the HCV life cycle and the development of novel antiviral strategies targeting HCV-lipoprotein interaction.

Hepatitis C virus hijacks host lipid metabolism

Hepatitis C virus (HCV) enhances its replication by modulating host cell lipid metabolism. HCV circulates in the blood in association with lipoproteins. HCV infection is associated with enhanced lipogenesis, reduced secretion, and beta-oxidation of lipids. HCV-induced imbalance in lipid homeostasis leads to steatosis. Many lipids are crucial for the virus life cycle, and inhibitors of cholesterol/fatty acid biosynthetic pathways inhibit virus replication, maturation and secretion. HCV negatively modulates the synthesis and secretion of very low-density lipoproteins (VLDL). Components involved in VLDL assembly are also required for HCV morphogenesis/secretion, suggesting that HCV co-opts the VLDL secretory pathway for its own secretion. This review highlights HCV-altered lipid metabolic events that aid the virus life cycle and ultimately promote liver disease.

Hepatitis C virus: viral proteins on the move

There is now increasing evidence that LDs (lipid droplets) play a central role in the production of infectious HCV (hepatitis C virus) and participate in virus assembly. Two viral proteins, namely core, which forms the capsid, and NS5A (non-structural 5A protein), a component of complexes engaged in viral RNA synthesis, are detected at LD surfaces in infected cells. Interactions between the two proteins may be critical for anchoring RNA replication sites to droplets for initiating virus assembly. The requirements for targeting of core in particular has received considerable attention since the nature of its interaction with LDs could play a key role in determining the efficiency of virion production. As well as attaching to droplets, core is able to alter their intracellular distribution and direct them towards the microtubule organizing centre. Inhibitors that disrupt microtubules block this redistribution by core and there is a concomitant decrease in virus production. Therefore altered dynamics of LDs may contribute to HCV assembly and release. The purpose of targeting LDs by HCV may be linked to their contribution to the formation of VLDLs (very-low-density lipoproteins) in hepatocytes since virus circulating in infected patients is associated with lipoprotein. Thus HCV may utilize the role played by LDs in the formation of lipoprotein particles as part of its life cycle and access this pathway by direct interaction of viral components with these intracellular storage organelles.