NS5A, a nonstructural protein of hepatitis C virus, binds growth factor receptor-bound protein 2 adaptor protein in a Src homology 3 domain/ligand-dependent manner and perturbs mitogenic signaling

Efficient rescue of hepatitis C virus RNA replication by trans-complementation with nonstructural protein 5A

Studies of Hepatitis C virus (HCV) RNA replication have become possible with the development of subgenomic replicons. This system allows the functional analysis of the essential components of the viral replication complex, which so far are poorly defined. In the present study we wanted to investigate whether lethal mutations in HCV nonstructural genes can be rescued by trans-complementation. Therefore, a series of replicon RNAs carrying mutations in NS3, NS4B, NS5A, and NS5B that abolish replication were transfected into Huh-7 hepatoma cells harboring autonomously replicating helper RNAs. Similar to data described for the Bovine viral diarrhea virus (C. W. Grassmann, O. Isken, N. Tautz, and S. E. Behrens, J. Virol. 75:7791-7802, 2001), we found that only NS5A mutants could be efficiently rescued. There was no evidence for RNA recombination between helper and mutant RNAs, and we did not observe reversions in the transfected mutants. Furthermore, we established a transient complementation assay based on the cotransfection of helper and mutant RNAs. Using this assay, we extended our results and demonstrated that (i) inactivating NS5A mutations affecting the amino-terminal amphipathic helix cannot be complemented in trans; (ii) replication of the helper RNA is not necessary to allow efficient trans-complementation; and (iii) the minimal sequence required for trans-complementation of lethal NS5A mutations is NS3 to -5A, whereas NS5A expressed alone does not restore RNA replication. In summary, our results provide the first insight into the functional organization of the HCV replication complex.

Hepatitis C virus replication in stably transfected HepG2 cells promotes hepatocellular growth and tumorigenesis

HepG2 cells stably transfected with a full-length, infectious hepatitis C virus (HCV) cDNA demonstrated consistent replication of HCV for more than 3 years. Intracellular minus strand HCV RNA was present. Minus strand synthesis was NS5B dependent, and was sensitive to interferon alpha (IFN alpha) treatment. NS5B and HCV core protein were detectable. HCV stimulated HepG2 cell growth and survival in culture, in soft agar, and accelerated tumor growth in SCID mice. These mice became HCV RNA positive in blood, where the virus was also sensitive to IFN alpha. The RNA banded at the density of HCV, and was resistant to RNase prior to extraction. Hence, HCV stably replicates in HepG2 cells, stimulates hepatocellular growth and tumorigenesis, and is susceptible to IFN alpha both in vitro and in vivo.

Genetic diversity and evolution of hepatitis C virus--15 years on

In the 15 years since the discovery of hepatitis C virus (HCV), much has been learned about its role as a major causative agent of human liver disease and its ability to persist in the face of host-cell defences and the immune system. This review describes what is known about the diversity of HCV, the current classification of HCV genotypes within the family Flaviviridae and how this genetic diversity contributes to its pathogenesis. On one hand, diversification of HCV has been constrained by its intimate adaptation to its host. Despite the >30 % nucleotide sequence divergence between genotypes, HCV variants nevertheless remain remarkably similar in their transmission dynamics, persistence and disease development. Nowhere is this more evident than in the evolutionary conservation of numerous evasion methods to counteract the cell's innate antiviral defence pathways; this series of highly complex virus-host interactions may represent key components in establishing its 'ecological niche' in the human liver. On the other hand, the mutability and large population size of HCV enables it to respond very rapidly to new selection pressures, manifested by immune-driven changes in T- and B-cell epitopes that are encountered on transmission between individuals with different antigen-recognition repertoires. If human immunodeficiency virus type 1 is a precedent, future therapies that target virus protease or polymerase enzymes may also select very rapidly for antiviral-resistant mutants. These contrasting aspects of conservatism and adaptability provide a fascinating paradigm in which to explore the complex selection pressures that underlie the evolution of HCV and other persistent viruses.

Calcium-dependent calpain proteases are implicated in processing of the hepatitis C virus NS5A protein

The nonstructural 5A (NS5A) protein of the hepatitis C virus (HCV) is a multifunctional phosphoprotein that is implicated in viral replication and HCV-mediated pathogenesis. We report here that the NS5A protein from the HCV genotype 1a is processed into shorter distinct forms when expressed in mammalian cells (Vero, HepG2, HuH-7, and WRL68) infected with an NS5A-expressing HSV-1-based amplicon vector or when transiently transfected with NS5A-expressing plasmids in the absence of exogenous apoptotic stimuli. Inhibitor studies combined with cell-free cleavage assays suggest that calcium-dependent calpain proteases, in addition to caspase-like proteases, are involved in NS5A processing. Interestingly, His-tagging experiments indicated that all the detectable NS5A-cleaved products are N-terminal forms of the protein. Additionally, immunofluorescence studies showed that, despite proteolytic cleavage, the NS5A protein exhibits a cytoplasm-perinuclear localization similar to that of the full-length protein. Thus, our results are consistent with recent data that demonstrated that NS5A is capable of perturbing intracellular calcium homeostasis and suggest that NS5A is both an inducer and a substrate of the calcium-dependent calpain protease(s). This may imply that cleavage of NS5A by calpain(s) could play a role in the modulation of NS5A function.

Hepatitis C virus NS5A: tales of a promiscuous protein

The non-structural 5A (NS5A) protein of hepatitis C virus (HCV) has been the subject of intensive research over the last decade. It is generally accepted that NS5A is a pleiotropic protein with key roles in both viral RNA replication and modulation of the physiology of the host cell. Our understanding of the role of NS5A in the virus life cycle has been hampered by the lack of a robust in vitro system for the study of HCV replication, although the recent development of the subgenomic replicon has at least allowed us to begin to dissect the involvement of NS5A in the process of viral RNA replication. Early studies into the effects of NS5A on cell physiology relied on expression of NS5A either alone or in the context of other non-structural proteins; the advent of the replicon system has allowed the extrapolation of these studies to a more physiologically relevant cellular context. Despite recent progress, this field is controversial, and there is much work to be accomplished before we fully understand the many functions of this protein. In this article, the current state of our knowledge of NS5A, discussing in detail its direct involvement in virus replication, together with its role in modulating the cellular environment to favour virus replication and persistence, are reviewed. The effects of NS5A on interferon signalling, and the regulation of cell growth and apoptosis are highlighted, demonstrating that this protein is indeed of critical importance for HCV and is worthy of further investigation.

Introduction of NS5A mutations enables subgenomic HCV replicon derived from chimpanzee-infectious HC-J4 isolate to replicate efficiently in Huh-7 cells

Hepatitis C virus (HCV) subgenomic replicon has been reported to replicate efficiently and continuously in human hepatoma Huh-7 cells. To extend the previous results to other isolated HCV clones, we constructed another HCV replicon from HC-J4, one of chimpanzee-infectious HCV clones. An HCV replicon derived from HC-J4 (RpJ4) consists of HCV-5' untranslated region, neomycin phosphotransferase gene, the encephalomyocarditis virus internal ribosomal entry site, HCV nonstructural region, NS3 to NS5B, and HCV-3' untranslated region. The adaptive mutations known to be required for HCV-Con1 replicon were introduced in RpJ4 replicon, aa.(amino acids number according to HC-J4) 2197 serine to proline, deletion of serine at aa.2201, and aa.2204 serine to isoleucine (RpJ4-S2197P, RpJ4-S22001del, and RpJ4-S2204I). RpJ4/ISDR mutant and RpJ4-S2201del/ISDR mutant were also constructed by introducing six amino acid mutations into the interferon sensitivity determining region (ISDR). After transfection into Huh-7 cells and G418 selection, RpJ4 and RpJ4/ISDR mutants did not produce any colony. In contrast, G418-resistant cells were transduced efficiently by RpJ4-S2197P, RpJ4-S2204I, RpJ4-S2201del and RpJ4-S2201del/ISDR mutant, with the RpJ4-S2201del/ISDR mutant being most efficient. Hence the HCV replicon derived from HC-J4 can replicate efficiently following the introduction of adaptive mutations into the upstream region of ISDR. Moreover, additional introduction of mutations into ISDR further enhanced its replication. These findings demonstrate that the genetic structure of the NS5A domain is critical in HCV replications.

Phosphorylation of hepatitis C virus nonstructural protein 5A modulates its protein interactions and viral RNA replication

The study of the hepatitis C virus (HCV) has been hindered by the lack of in vitro model systems. The recent development of HCV subgenomic RNA replicons has permitted the study of viral RNA replication in cell culture; however, the requirements for efficient replication of replicons in this system are poorly understood. Many viral isolates do not function as replicons and most require conserved changes, termed adaptive mutations, to replicate efficiently. In this report, we focus on the HCV nonstructural protein 5A (NS5A), a frequent locus for adaptive mutation. We found the interaction between NS5A and human vesicle-associated membrane protein-associated protein A (hVAP-A), a cellular target N-ethylmaleimide-sensitive factor attachment protein receptor, to be required for efficient RNA replication: NS5A mutations that blocked interaction with hVAP-A strongly reduced HCV RNA replication. Further analyses revealed an inverse correlation between NS5A phosphorylation and hVAP-A interaction. A subset of the previously identified adaptive mutations suppressed NS5A hyperphosphorylation and promoted hVAP-A binding. Our results support a model in which NS5A hyperphosphorylation disrupts interaction with hVAP-A and negatively regulates viral RNA replication, suggesting that replicon-adaptive mutations act by preventing the phosphorylation-dependent dissociation of the RNA replication complex.

The NS5A protein of hepatitis C virus is a zinc metalloprotein

The NS5A protein of hepatitis C virus is believed to be an integral part of the viral replicase. Despite extensive investigation, the role of this protein remains elusive. Only limited biochemical characterization of NS5A has been performed, with most research to date involving the myriad of host proteins and signaling cascades that interact with NS5A. The need for better characterization of NS5A is paramount for elucidating the role of this protein in the virus life cycle. Examination of NS5A using bioinformatics tools suggested the protein consisted of three domains and contained an unconventional zinc binding motif within the N-terminal domain. We have developed a method to produce NS5A and performed limited proteolysis to confirm the domain organization model. The zinc content of purified NS5A and the N-terminal domain of NS5A was determined, and each of these proteins was found to coordinate one zinc atom per protein. The predicted zinc binding motif consists of four cysteine residues, conserved among the Hepacivirus and Pestivirus genera, fitting the formula of CX17CXCX20C. Mutation of any of the four cysteine components of this motif reduced NS5A zinc coordination and led to a lethal phenotype for HCV RNA replication, whereas mutation of other potential metal coordination residues in the N-terminal domain of NS5A, but outside the zinc binding motif, had little effect on zinc binding and, aside from one exception, were tolerated for replication. Collectively, these results indicate that NS5A is a zinc metalloprotein and that zinc coordination is likely required for NS5A function in the hepatitis C replicase.

Phosphorylation of hepatitis C virus nonstructural protein 5A modulates its protein interactions and viral RNA replication

The study of the hepatitis C virus (HCV) has been hindered by the lack of in vitro model systems. The recent development of HCV subgenomic RNA replicons has permitted the study of viral RNA replication in cell culture; however, the requirements for efficient replication of replicons in this system are poorly understood. Many viral isolates do not function as replicons and most require conserved changes, termed adaptive mutations, to replicate efficiently. In this report, we focus on the HCV nonstructural protein 5A (NS5A), a frequent locus for adaptive mutation. We found the interaction between NS5A and human vesicle-associated membrane protein-associated protein A (hVAP-A), a cellular target N-ethylmaleimide-sensitive factor attachment protein receptor, to be required for efficient RNA replication: NS5A mutations that blocked interaction with hVAP-A strongly reduced HCV RNA replication. Further analyses revealed an inverse correlation between NS5A phosphorylation and hVAP-A interaction. A subset of the previously identified adaptive mutations suppressed NS5A hyperphosphorylation and promoted hVAP-A binding. Our results support a model in which NS5A hyperphosphorylation disrupts interaction with hVAP-A and negatively regulates viral RNA replication, suggesting that replicon-adaptive mutations act by preventing the phosphorylation-dependent dissociation of the RNA replication complex.

Dominant negative effect of wild-type NS5A on NS5A-adapted subgenomic hepatitis C virus RNA replicon

An efficient model is currently used to study hepatitis C virus (HCV) replication in cell culture. It involves transfection in Huh7, a hepatoma-derived cell line, of an antibiotic (neomycin) selectable HCV subgenomic replicon encoding the non-structural (NS) proteins from NS3 to NS5B. However, strong and sustained replication is achieved only on the appearance of adaptive mutations in viral proteins. The most effective of these adaptive mutations are concentrated mainly in NS5A, not only into the original Con1 but also in the recently established HCV-BK and HCV-H77 isolate-derived replicons. This suggests that the expression of wild-type (wt) NS5A may not allow efficient HCV RNA replication in cell culture. With the use of a beta-lactamase reporter gene as a marker for HCV replication and TaqMan RNA analysis, the replication of different HCV replicons in cotransfection experiments was investigated. Comparing wt with NS5A-adapted replicons, the strong evidence accumulated showed that the expression of wt NS5A was actually able to inhibit the replication of NS5A-adapted replicons. This feature was characterized as a dominant negative effect. Interestingly, an NS5B (R2884G)-adapted replicon, containing a wt NS5A, was dominant negative on an NS5A-adapted replicon but was not inhibited by the original Con1 replicon. In conclusion, these studies revealed that the original wt Con1 replicon is not only incompetent for replication in cell culture, but is also able to interfere with NS5A-adapted replicons.

HCV NS5A abrogates p53 protein function by interfering with p53-DNA binding

AIM

To evaluate the inhibition effect of HCV NS5A on p53 transactivation on p21 promoter and explore its possible mechanism for influencing p53 function.

METHODS

p53 function of transactivation on p21 promoter was studied with a luciferase reporter system in which the luciferase gene is driven by p21 promoter, and the p53-DNA binding ability was observed with the use of electrophoretic mobility-shift assay (EMSA). Lipofectin mediated p53 or HCV NS5A expression vectors were used to transfect hepatoma cell lines to observe whether HCV NS5A could abrogate the binding ability of p53 to its specific DNA sequence and p53 transactivation on p21 promoter. Western blot experiment was used for detection of HCV NS5A and p53 proteins expression.

RESULTS

Relative luciferase activity driven by p21 promoter increased significantly in the presence of endogenous p53 protein. Compared to the control group, exogenous p53 protein also stimulated p21 promoter driven luciferase gene expression in a dose-dependent way. HCV NS5A protein gradually inhibited both endogenous and exogenous p53 transactivation on p21 promoter with increase of the dose of HCV NS5A expression plasmid. By the experiment of EMSA, we could find p53 binding to its specific DNA sequence and, when co-transfected with increased dose of HCV NS5A expression vector, the p53 binding affinity to its DNA gradually decreased and finally disappeared. Between the Huh 7 cells transfected with p53 expression vector alone or co-transfected with HCV NS5A expression vector, there was no difference in the p53 protein expression.

CONCLUSION

HCV NS5A inhibits p53 transactivation on p21 promoter through abrogating p53 binding affinity to its specific DNA sequence. It does not affect p53 protein expression.

Steatosis and intrahepatic lymphocyte recruitment in hepatitis C virus transgenic mice

To assess the effects of constitutive hepatitis C virus (HCV) gene expression on liver, transgenic mice carrying the entire HCV open reading frame inserted in the alpha1 antitrypsin (A1AT) gene were generated. Expression of A1AT/HCV mRNA was found to be mainly limited to perivascular areas of the liver as indicated by in situ hybridization analysis. HCV core protein was detected in Western blots of liver extracts, whereas the expression of E2, NS3 and NS5 proteins was revealed by immunostaining of liver samples using HCV-specific antisera. Histological analysis of HCV transgenic mice showed that these animals develop extensive steatosis, but very little necrosis of liver tissue. Moreover, a consistent T cell infiltrate and a slight hepatocyte proliferation were observed. Phenotypic analysis of cells infiltrating the liver indicated that recruitment and/or expansion of residing CD8(+), NK, NKT and gammadelta T cells occurred in transgenic animals. Among these cells, a large fraction of CD8(+) T lymphocytes released mainly IL-10 and, to a lesser extent, IFN-gamma upon mitogenic stimulation in vitro. Furthermore, both intrahepatic lymphocytes and splenocytes did not produce cytokines in response to HCV antigens. Thus, these data indicate that constitutive expression of HCV proteins may be responsible for intrahepatic lymphocyte recruitment in absence of viral antigen recognition. This response is likely to be driven by virus-induced cellular factors and may play a significant role in the immunopathology of chronic HCV infection and liver disease.

Hepatitis C virus NS5A mediated STAT3 activation requires co-operation of Jak1 kinase

Hepatitis C virus (HCV) is a major etiologic agent for chronic hepatitis worldwide and often leads to cirrhosis and hepatocellular carcinoma. However, the mechanism for development of chronic hepatitis or hepatocarcinogenesis by HCV remains unclear. Signal transducers and activators of transcription (STATs) family proteins function as the downstream effectors of cytokine signaling and play a critical role in cell growth regulation. In many cancers including liver, STAT3 is often constitutively activated, although the mechanism of persistent activation of STAT3 is unknown. The nonstructural protein 5A (NS5A) encoded from the HCV genome has shown cell growth regulatory properties. In this study, we have observed that HCV NS5A activates STAT3 phosphorylation, which in turn translocates into the nucleus. In vivo activation of STAT3 was also observed in the liver of transgenic mice expressing HCV NS5A. Introduction of NS5A in hepatoma cells modulated STAT3 downstream molecules Bcl-xL and p21 expression. To determine if STAT3 activation by NS5A could induce STAT3 mediated gene expression, a luciferase reporter construct based on a synthetic promoter was used to transfect hepatoma cells. Activation of endogenous cellular STAT3 by HCV NS5A induced luciferase gene expression through STAT3 specific binding elements. Our subsequent studies suggested that NS5A forms a complex with Jak1 and recruits STAT3 for activation. Taken together, our results suggested that NS5A activates STAT3 through co-operation of Jak1 kinase and activated STAT3 may contribute to HCV-mediated pathogenesis.

The hepatitis C virus NS5A protein binds to members of the Src family of tyrosine kinases and regulates kinase activity

The hepatitis C virus (HCV) non-structural NS5A protein has been shown to associate with a variety of cellular signalling proteins. Of particular interest is the observation that a highly conserved C-terminal polyproline motif in NS5A was able to interact with the Src-homology 3 (SH3) domains of the adaptor protein Grb2. As it has previously been shown that specific polyproline motifs can interact with a range of SH3 domains, we investigated whether NS5A was capable of interacting with other SH3 domain-containing proteins. We show here that NS5A interacts with the SH3 domains of members of the Src family of tyrosine kinases: a combination of in vitro binding assays and co-immunoprecipitation experiments revealed an interaction between NS5A and Hck, Lck, Lyn and Fyn, but interestingly not Src itself. Mutational analysis confirmed that the polyproline motif responsible for binding to Grb2 also bound to the SH3 domains of Hck, Lck, Lyn and Fyn. Furthermore, a previously unidentified polyproline motif, adjacent to the first motif, was also able to mediate binding to the SH3 domain of Lyn. Using transient transfections and Huh-7 cells harbouring a persistently replicating subgenomic HCV replicon we demonstrate that NS5A bound to native Src-family kinases in vivo and differentially modulated kinase activity, inhibiting Hck, Lck and Lyn but activating Fyn. Lastly, we show that signalling pathways controlled by Src-family kinases are modulated in replicon cells. We conclude that the interactions between NS5A and Src-family kinases are physiologically relevant and may play a role in either virus replication or pathogenesis.

Effect of interaction between hepatitis C virus NS5A and NS5B on hepatitis C virus RNA replication with the hepatitis C virus replicon

Hepatitis C virus (HCV) NS5A has been reported to be important for the establishment of replication by adaptive mutations or localization, although its role in viral replication remains unclear. It was previously reported that NS5A interacts with NS5B via two regions of NS5A in the isolate JK-1 and modulates the activity of NS5B RdRp (Y. Shirota et al., J. Biol. Chem., 277:11149-11155, 2002), but the biological significance of this interaction has not been determined. In this study, we addressed the effect of this interaction on HCV RNA replication with an HCV replicon system derived from the isolate M1LE (H. Kishine et al., Biochem. Biophys. Res. Commun., 293:993-999, 2002). We constructed three internal deletion mutants, M1LE/5Adel-1 and M1LE/5Adel-2, each encoding NS5A which cannot bind NS5B, and M1LE/5Adel-3, encoding NS5A that can bind NS5B. After transfection into Huh-7 cells, M1LE/5Adel-3 was replication competent, but both M1LE/5Adel-1 and M1LE/5Adel-2 were not. Next we prepared 20 alanine-substituted clustered mutants within both NS5B-binding regions and examined the effect of these mutants on HCV RNA replication. Only 5 of the 20 mutants were replication competent. Subsequently, we introduced a point mutation, S225P, a deletion of S229, or S232I into NS5A and prepared cured Huh-7 cells that were cured of RNA replication by alpha interferon. Finally, with these point mutations and cured cells, we established a highly improved replicon system. In this system, only the same five mutants were replication competent. These results strongly suggest that the interaction between NS5A and NS5B is critical for HCV RNA replication in the HCV replicon system.

Characterization of emerging European-like porcine reproductive and respiratory syndrome virus isolates in the United States

European-like field isolates of porcine reproductive and respiratory syndrome virus (PRRSV) have recently emerged in North America. The full-length genomic sequence of an index isolate characterized in 1999, strain EuroPRRSV, served as the reference strain for further studies of the evolution and epidemiology of European-like isolates (type 1) in the United States. Strain EuroPRRSV shared 90.1 to 100% amino acid identity with the prototype European strain, Lelystad, within the structural and nonstructural open reading frames (ORFs) and 95.3% overall nucleotide identity. The 5' untranslated region and two nonstructural regions within ORF 1 were closely examined due to significant divergence from strain Lelystad. A 51-bp deletion in a region within ORF 1a, coding for nonstructural protein 2 (NSP2), was observed. Sequence analysis of the structural ORFs 2 to 7 of additional European-like isolates indicated that these isolates share 93% nucleotide identity with one another and 95 to 96% identity with the Lelystad strain but only 70% identity with the North American reference strain VR-2332. Phylogenetic analysis with published PRRSV ORF 3, 5, and 7 nucleotide sequences indicated that these newly emerging isolates form a clade with the Lelystad and United Kingdom PRRSV isolates. Detailed analysis of four of these isolates with a panel of 60 monoclonal antibodies directed against the structural proteins confirmed a recognition pattern that was more consistent with strain Lelystad than with other North American isolates.

High-throughput screening of the yeast kinome: identification of human serine/threonine protein kinases that phosphorylate the hepatitis C virus NS5A protein

The hepatitis C virus NS5A protein plays a critical role in virus replication, conferring interferon resistance to the virus through perturbation of multiple intracellular signaling pathways. Since NS5A is a phosphoprotein, it is of considerable interest to understand the role of phosphorylation in NS5A function. In this report, we investigated the phosphorylation of NS5A by taking advantage of 119 glutathione S-transferase-tagged protein kinases purified from Saccharomyces cerevisiae to perform a global screening of yeast kinases capable of phosphorylating NS5A in vitro. A database BLAST search was subsequently performed by using the sequences of the yeast kinases that phosphorylated NS5A in order to identify human kinases with the highest sequence homologies. Subsequent in vitro kinase assays and phosphopeptide mapping studies confirmed that several of the homologous human protein kinases were capable of phosphorylating NS5A. In vivo phosphopeptide mapping revealed phosphopeptides common to those generated in vitro by AKT, p70S6K, MEK1, and MKK6, suggesting that these kinases may phosphorylate NS5A in mammalian cells. Significantly, rapamycin, an inhibitor commonly used to investigate the mTOR/p70S6K pathway, reduced the in vivo phosphorylation of specific NS5A phosphopeptides, strongly suggesting that p70S6 kinase and potentially related members of this group phosphorylate NS5A inside the cell. Curiously, certain of these kinases also play a major role in mRNA translation and antiapoptotic pathways, some of which are already known to be regulated by NS5A. The findings presented here demonstrate the use of high-throughput screening of the yeast kinome to facilitate the major task of identifying human NS5A protein kinases for further characterization of phosphorylation events in vivo. Our results suggest that this novel approach may be generally applicable to the screening of other protein biochemical activities by mechanistic class.

Induced oxidative stress and activated expression of manganese superoxide dismutase during hepatitis C virus replication: role of JNK, p38 MAPK and AP-1

Activation of cellular kinases and transcription factors mediates the early phase of the cellular response to chemically or biologically induced stress. In the present study we investigated the oxidant/antioxidant balance in Huh-7 cells expressing the HCV (hepatitis C virus) subgenomic replicon, and observed a 5-fold increase in oxidative stress during HCV replication. We used MnSOD (manganese-superoxide dismutase) as an indicator of the cellular antioxidant response, and found that its activity, protein levels and promoter activity were significantly increased, whereas Cu/ZnSOD was not affected. The oxidative stress-induced protein kinases p38 MAPK (mitogen-activated protein kinase) and JNK (c-Jun N-terminal kinase) were activated in the HCV repliconcontaining cells and in Huh-7 cells transduced with Ad-NS5A [a recombinant adenovirus encoding NS5A (non-structural protein 5A)], coupled with a 4-5-fold increase in AP-1 (activator protein-1) DNA binding. Ava.1 cells, which encode a replication-defective HCV replicon, showed no significant changes in MnSOD, p38 MAPK or JNK activity. The AP-1 inhibitors dithiothreitol and N -acetylcysteine, as well as a dominant negative AP-1 mutant, significantly reduced AP-1 activation, demonstrating that this activation is oxidative stress-related. Exogenous NS5A had no effect on AP-1 activation in vitro, suggesting that NS5A acts at the upstream targets of AP-1 involving p38 MAPK and JNK signalling cascades. AP-1-dependent gene expression was increased in HCV subgenomic replicon-expressing Huh-7 cells. MnSOD activation was blocked by inhibitors of JNK (JNKI1) and p38 MAPK (SB203580), but not by an ERK (extracellular-signal-regulated kinase) inhibitor (U0126), in HCV-replicating and Ad-NS5A-transduced cells. Our results demonstrate that cellular responses to oxidative stress in HCV subgenomic replicon-expressing and Ad-NS5A-transduced cells are regulated by two distinct signalling pathways involving p38 MAPK and JNK via AP-1 that is linked to increased oxidative stress and therefore to an increased antioxidant MnSOD response.

The Hepatitis C virus NS5A protein activates a phosphoinositide 3-kinase-dependent survival signaling cascade

Hepatitis C virus (HCV) establishes a persistent infection, with up to 80% of infected individuals proceeding to chronic hepatitis, which in many cases may result in liver cirrhosis and hepatocellular carcinoma (HCC); indeed HCV infection is increasingly associated with the development of HCC. The long time period (up to 30 years) between primary infection and the onset of HCC implies that HCV is not directly oncogenic but in some way predisposes patients to develop tumors, though the mechanism for this is unclear as yet. We report here that NS5A binds directly to the Src homology 3 domain of the p85 regulatory subunit of phosphoinositide 3-kinase (PI3K), and this interaction is mediated by a novel (non-proline-rich) motif within NS5A. Coimmunoprecipitation analysis revealed that NS5A bound native heterodimeric PI3K and enhanced the phosphotransferase activity of the catalytic (p110) subunit both in vitro and in human cell lines harboring a subgenomic HCV replicon or expressing NS5A alone. NS5A-mediated activation of PI3K resulted in increased phosphorylation and activity of Akt/protein kinase B and concomitantly provided protection against the induction of apoptosis in both replicon-harboring cells and cells stably expressing NS5A alone. These data suggest that stimulation of PI3K by NS5A may represent an indirect mechanism for development of HCC in HCV-infected patients and further suggests potential therapeutic strategies to counteract the occurrence of HCV-related HCC.

Hepatitis C virus NS5A as a potential viral Bcl-2 homologue interacts with Bax and inhibits apoptosis in hepatocellular carcinoma

Treatment of hepatocellular carcinoma (HCC) cells with butyrate can induce apoptosis irrespective of hepatitis B virus integration. No information is available, however, regarding the effect of butyrate on HCC in the presence of hepatitis C virus (HCV) because some HCV proteins can regulate cell survival. By gene transfer, we found that HCV core enhances but HCV NS5A antagonizes sodium phenylbutyrate (NaPB)-induced apoptosis in HCC cells, which is independent of p53. We then chose the p53-negative Hep3B HCC cell to investigate the mechanism of anti-apoptosis mediated by NS5A. In the NaPB-treated Hep3B cells without NS5A expression, induction of apoptosis was associated with Bax redistribution from the cytosol to the nucleus interior and subsequently, to a nuclear membrane-bound form. In the NS5A expressing Hep3B cells, NaPB treatment also triggered relocalization of both Bax and NS5A from the cytosol to the nucleus interior but Bax retained inside the nucleus and did not finally move to the nuclear membrane. Using double immunofluorescence and coimmunoprecipitation, we demonstrated that NS5A co-localizes and interacts with Bax in the nucleus. The HCV NS5A protein was further found to contain Bcl-2 homology domains (BH3, BH1 and BH2). Additional studies using deleted NS5A constructs were carried out to determine whether the BH2 domain or nuclear localization signal (NLS) in NS5A is required for interaction with Bax in the nucleus or inhibition of apoptosis. NS5A with deletion of both BH2 domain and NLS localized in the cytoplasm, dissociated with Bax, and lost anti-apoptosis activity during NaPB treatment. In contrast, NS5A with intact BH domains except NLS still bound directly to Bax in the perinuclear region or the nucleus, but showed less association with Bax in the nucleus and lower effect in apoptosis inhibition than full-length NS5A. These results suggest that HCV NS5A as a Bcl-2 homologue interacts with Bax to protect p53-negative HCC cells from NaPB-induced apoptosis.

RNA amplification, fidelity and reproducibility of expression profiling

We quantitatively address the effect of T7 RNA amplification on expression profiling data, and answer the following questions: (1) What fraction of genes sampled is amplified non-linearly? (2) What is the effect of RNA amplification on comparative expression measurements? (3) If there is amplification bias, is the bias dependent on the degree of amplification or the amount of starting material and (4) Does amplification increase the overall variability of the results? We show that while there is significant amplification bias, the bias is consistent and generally has little effect on array comparisons between amplified samples.

Interaction of hepatitis C virus NS5A with La protein revealed by T7 phage display

Although the hepatitis C virus (HCV) genome is synthesized by the virus-encoded RNA-dependent RNA polymerase NS5B, other viral and cellular factors are assumed to be required for template-specific initiation and regulation of RNA-synthesis. The cellular protein La, which normally associates with RNA polymerase III transcripts, also interacts with the 5'- and 3'-untranslated regions of several RNA viruses, including HCV. To investigate whether other viral gene products may be involved in this interaction, we constructed an HCV cDNA expression library in bacteriophage T7 allowing portions of the HCV polyprotein to be displayed on the phage surface. Screening of the phage library against La resulted in selection of clones displaying the N-terminal region of HCV NS5A. Co-precipitation of full-length and truncated forms of recombinant NS5A with La revealed that the N-terminal region of NS5A was both necessary and sufficient for binding to La. Although this region of NS5A is essential for HCV replication, the role of the NS5A-La interaction in the infected cell remains to be established.

1Hepatitis C virus NS5A protein modulates c-Jun N-terminal kinase through interaction with tumor necrosis factor receptor-associated factor 2

The nonstructural 5A (NS5A) protein of hepatitis C virus (HCV) is a phosphoprotein possessing various functions. We have previously reported that the HCV NS5A protein interacts with tumor necrosis factor (TNF) receptor-associated factor (TRAF) domain of TRAF2 (Park, K.-J., Choi, S.-H., Lee, S. Y., Hwang, S. B., and Lai, M. M. C. (2002) J. Biol. Chem. 277, 13122-13128). Both TNF-alpha- and TRAF2-mediated nuclear factor-kappaB (NF-kappaB) activations were inhibited by NS5A-TRAF2 interaction. Because TRAF2 is required for the activation of both NF-kappaB and c-Jun N-terminal kinase (JNK), we investigated HCV NS5A protein for its potential capacity to modulate TRAF2-mediated JNK activity. Using in vitro kinase assay, we have found that NS5A protein synergistically activated both TNF-alpha- and TRAF2-mediated JNK in human embryonic kidney 293T cells. Furthermore, synergism of NS5A-mediated JNK activation was inhibited by dominant-negative form of MEK kinase 1. Our in vivo binding data show that NS5A does not inhibit interaction between TNF receptor-associated death domain and TRAF2 protein, indicating that NS5A and TRAF2 may form a ternary complex with TNF receptor-associated death domain. These results indicate that HCV NS5A protein modulates TNF signaling of the host cells and may play a role in HCV pathogenesis.

Gene expression profiling of the cellular transcriptional network regulated by alpha/beta interferon and its partial attenuation by the hepatitis C virus nonstructural 5A protein

Alpha/beta interferons (IFN-alpha/beta) induce potent antiviral and antiproliferative responses and are used to treat a wide range of human diseases, including chronic hepatitis C virus (HCV) infection. However, for reasons that remain poorly understood, many HCV isolates are resistant to IFN therapy. To better understand the nature of the cellular IFN response, we examined the effects of IFN treatment on global gene expression by using several types of human cells, including HeLa cells, liver cell lines, and primary fetal hepatocytes. In response to IFN, 50 of the approximately 4,600 genes examined were consistently induced in each of these cell types and another 60 were induced in a cell type-specific manner. A search for IFN-stimulated response elements (ISREs) in genomic DNA located upstream of IFN-stimulated genes revealed both previously identified and novel putative ISREs. To determine whether HCV can alter IFN-regulated gene expression, we performed microarray analyses on IFN-treated HeLa cells expressing the HCV nonstructural 5A (NS5A) protein and on IFN-treated Huh7 cells containing an HCV subgenomic replicon. NS5A partially blocked the IFN-mediated induction of 14 IFN-stimulated genes, an effect that may play a role in HCV resistance to IFN. This block may occur through repression of ISRE-mediated transcription, since NS5A also inhibited the IFN-mediated induction of a reporter gene driven from an ISRE-containing promoter. In contrast, the HCV replicon had very little effect on IFN-regulated gene expression. These differences highlight the importance of comparing results from multiple model systems when investigating complex phenomena such as the cellular response to IFN and viral mechanisms of IFN resistance.

The hepatitis C virus non-structural NS5A protein inhibits activating protein-1 function by perturbing ras-ERK pathway signaling

The hepatitis C virus nonstructural 5A (NS5A) protein is a pleiotropic phosphoprotein that has been shown to associate with a wide variety of cellular signaling proteins. Of particular interest is the observation that a highly conserved C-terminal Class II polyproline motif within NS5A mediated association with the Src homology 3 domains of members of the Src family of tyrosine kinases and the mitogenic adaptor protein Grb2 (A. Macdonald, K. Crowder, A. Street, C. McCormick, and M. Harris, submitted for publication). In this study, we analyzed the consequences of NS5A expression on mitogenic signaling pathways within a variety of cell lines. Utilizing a transient luciferase reporter system, we observed that NS5A inhibited the activity of the mitogenic and stress-activated transcription factor activating protein-1 (AP1). This inhibition was dependent upon a Class II polyproline motif within NS5A. Using a combination of dominant active and negative mutants of components of the MAPK signaling pathways, selective inhibitors, together with immunoblotting with phospho-specific and phosphorylation-independent antibodies, we determined the signaling pathways targeted by NS5A to inhibit AP1. These studies demonstrated that in both stable NS5A-expressing cells and Huh-7-derived cells harboring subgenomic hepatitis C virus (HCV) replicons, this inhibition was mediated through the ERK signaling pathway. Importantly, a comparable inhibition of AP1 reporter activity was observed in hepatocyte-derived cell lines transduced with a baculovirus vector driving expression of full-length HCV polyprotein. In conclusion, these data strongly suggest a role for the NS5A protein in the perturbation of mitogenic signaling pathways in HCV-infected hepatocytes.

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Hepatitis viruses and the MAPK pathway: is this a survival strategy?

The viruses that cause hepatitis comprise of at least five different agents, which share the ability to cause inflammation and necrosis of the liver. The disease spectrum is quite diverse and the outcome of infection by the different hepatitis viruses can be rationalized based on virus-host cell interactions. New insights into the molecular basis of viral hepatitis reveal that three of these agents - the hepatitis B, C and E viruses (HBV, HCV and HEV) modulate the mitogen-activated protein kinase (MAPK) signaling pathway. In this review we briefly describe the structural organization of the MAPK cascade and emphasize its importance as a central pathway in the signaling network. Selected mechanisms through which HBV, HCV and HEV proteins target various steps in the MAPK pathway are discussed and used to propose a pro-survival outcome for the host cell. In addition, we offer an insight into how the common theme of MAPK activation and its downstream effects may be used to rationalize the different outcomes of hepatitis B, C and E.

The hepatitis C virus persistence: how to evade the immune system?

Hepatitis C virus (HCV) is an emerging virus of medical importance. A majority of HCV infections become chronic and lead to chronic hepatitis, liver cirrhosis and hepatocellular carcinoma. HCV usually induces robust immune responses, but it frequently escapes the immune defense to establish persistent infection. The fact that HCV exists as an evolving quasispecies plays an important role in the selection of escape mutants. Furthermore, several viral proteins interfere with cellular functions, in particular, those involved in the immune response of the host. Several HCV proteins also modulate cell signalling through interaction with different effectors involved in cell proliferation and apoptosis, or in the interferon-signalling pathway. In addition, HCV infects immune cells such as B and T cells, and thus affects their normal functions. These various strategies used by HCV to counter the immune response of the host are reviewed here. A better understanding of these mechanisms would help design new therapeutic targets.

Identification and characterization of amphiphysin II as a novel cellular interaction partner of the hepatitis C virus NS5A protein

The hepatitis C virus (HCV) NS5A protein is highly phosphorylated by cellular protein kinases. To study how NS5A might be integrated in cellular kinase signalling, we isolated phosphoproteins from HuH-7 hepatoma cells that specifically interacted with recombinant NS5A protein. Subsequent mass spectrometry identified the adaptor protein amphiphysin II as a novel interaction partner of NS5A. Mutational analysis revealed that complex formation is primarily mediated by a proline-rich region in the C-terminal part of NS5A, which interacts with the amphiphysin II Src homology 3 domain. Importantly, we could further demonstrate specific co-precipitation and cellular co-localization of endogenous amphiphysin II with NS5A in HuH-7 cells carrying a persistently replicating subgenomic HCV replicon. Although the NS5A-amphiphysin II interaction appeared to be dispensable for replication of these HCV RNAs in cell culture, our results indicate that NS5A-amphiphysin II complex formation might be of physiological relevance for the HCV life cycle.

Hepatitis C virus--cell interactions and their role in pathogenesis

In summary, HCV-cell interactions include those directly involved with the HCV life cycle such as virus attachment, entry, and replication. Included within this broad area of research are the interactions of HCV proteins with the IFN system, cytokine and chemokine pathways such as IL-8, and various other cellular proteins and pathways. The plethora of contradictory and sometimes confusing accessory HCV-host interactions defies precise predictions of their role in HCV biology. It is clear that these virus-cell interactions affect HCV replication, antiviral resistance, persistence, and pathogenesis. Because HCV-host interactions are initiated immediately on infection, they are operative during acute HCV infection, whereby HCV interacts with innate cellular antiviral and immune systems. The magnitude and duration of these HCV-host interactions therefore may influence the development of acquired immunity. Because HCV exists as a quasispecies in all infected individuals, heterogeneity in biological responses to HCV-host interactions is predicted, revealing opportunities for the development of various genotypic and phenotypic prognostic indicators. With the model systems in place, these hypotheses can be tested. The challenge for the future is to determine if there is a hierarchical importance to these interactions, to delineate how these virus-cell interactions affect the patient infected with HCV, and to determine whether any of these interactions represents a target for therapeutic intervention.

Antiviral effect and virus-host interactions in response to alpha interferon, gamma interferon, poly(i)-poly(c), tumor necrosis factor alpha, and ribavirin in hepatitis C virus subgenomic replicons

The recently developed hepatitis C virus (HCV) subgenomic replicon system was utilized to evaluate the efficacy of several known antiviral agents. Cell lines that persistently maintained a genotype 1b replicon were selected. The replicon resident in each cell line had acquired adaptive mutations in the NS5A region that increased colony-forming efficiency, and some replicons had acquired NS3 mutations that alone did not enhance colony-forming efficiency but were synergistic with NS5A mutations. A replicon constructed from the infectious clone of the HCV-1 strain (genotype 1a) was not capable of inducing colony formation even after the introduction of adaptive mutations identified in the genotype 1b replicon. Alpha interferon (IFN-alpha), IFN-gamma, and ribavirin exhibited antiviral activity, while double-stranded RNA (dsRNA) and tumor necrosis factor alpha did not. Analysis of transcript levels for a series of genes stimulated by IFN (ISGs) or dsRNA following treatment with IFN-alpha, IFN-gamma, and dsRNA revealed that both IFNs increased ISG transcript levels, but that some aspect of the dsRNA response pathway was defective in Huh7 cells and replicon cell lines in comparison to primary chimpanzee and tamarin hepatocytes. The colony-forming efficiency of the replicon was reduced or eliminated following replication in the presence of ribavirin, implicating the induction of error-prone replication. The potential role of error-prone replication in the synergy observed between IFN-alpha and ribavirin in attaining sustained viral clearance is discussed. These studies reveal characteristics of Huh7 cells that may contribute to their unique capacity to support HCV RNA synthesis and demonstrate the utility of the replicon system for mechanistic studies on antiviral agents.

Hepatitis C virus biology

Hepatitis C virus infection represents a major problem of public health with around 350 millions of chronically infected individuals worldwide. The frequent evolution towards severe liver disease and cancer are the main features of HCV chronic infection. Antiviral therapies, mainly based on the combination of IFN and ribavirin can only assure a long term eradication of the virus in less than half of treated patients. The mechanisms underlying HCV pathogenesis and persistence in the host are still largely unknown and the efforts made by researchers in the understanding the viral biology have been hampered by the absence of a reliable in vitro and in vivo system reproducing HCV infection. The present review will mainly focus on viral pathogenetic mechanisms based on the interaction of HCV proteins (especially core, NS3 and NS5A) with host cellular signaling transduction pathways regulating cell growth and viability and on the strategies developed by the virus to persist in the host and escape to antiviral therapy. Past and recent data obtained in this field with different experimental approaches will be discussed.

Unlocking the mysteries of virus-host interactions: does functional genomics hold the key?

The interactions between viruses and the cells they infect are complex and multifaceted. While viruses strive to usurp cellular functions to their advantage, the cell strives to thwart these efforts by mounting a variety of defensive responses. These responses may include the induction of interferon, stress response, or apoptotic pathways, all of which are accompanied by changes in gene expression. Some viruses consistently win this tug of war, whereas others succumb to cellular defense mechanisms. The viral and cellular factors that determine the outcome are for the most part still unknown. With the advent of functional genomics, potent new technologies are now available to probe the complexities of virus-host interactions in ever increasing depth and detail. We describe here our efforts to use microarrays, proteomics, and bioinformatics to focus in on the changes in gene expression and protein production that occur in a virus-infected cell and to use these technologies to unlock the mysteries of virus-host interactions.

Hepatitis C virus NS5A protein binds TBP and p53, inhibiting their DNA binding and p53 interactions with TBP and ERCC3

Among the hepatotropic viruses, hepatitis C virus (HCV) is considered to be the leading cause of liver disease in humans, affecting approximately 2% of the world population. HCV-encoded nonstructural protein 5A (NS5A) is a 56-58-kDa phosphoprotein, which is produced from the processing of viral polyprotein. The potential mechanism(s) by which NS5A is able to influence key cellular processes are largely unknown. In this study, we investigated the functional properties of NS5A. In vivo co-immunoprecipitation and pull-down assays demonstrated that NS5A forms a heteromeric complex with TATA box binding protein (TBP) and tumor suppressor protein p53. Mutants of TBP and p53 showed reduced binding to NS5A. To determine the functional relevance of these associations, we found that NS5A inhibits the binding of both p53 and TBP to their DNA consensus binding sequences in vitro. NS5A also inhibited the p53-TBP and p53-excision repair cross complementing factor 3 (ERCC3) protein-protein complex formation. Furthermore, NS5A repressed the p53 regulated p21 (WAF1) promoter and a synthetic promoter containing multiple p53 responsive DNA elements binding sites in HCT116 p53(+/+) cell line. p53-mediated transcriptional activation from both promoters was reduced approximately 3-5-fold following expression of NS5A. Taken together, these results suggest that NS5A may exert its influence on key cellular processes by functional associations with p53 and TBP. This could explain one of the possible mechanism(s) by which NS5A is able to exert its effect on cellular gene expression and cell growth regulation.

To interfere and to anti-interfere: the interplay between hepatitis C virus and interferon

As popular strategies used by numerous viruses, interception of interferon (IFN) signaling and inhibition of IFN-induced antiviral functions allow viruses to evade the host immune response and set up successful infections. Hepatitis C virus (HCV), the leading cause of chronic liver disease worldwide and a major public health hazard, causes persistent infection in the majority of infected individuals. IFN-based therapies, currently the only ones available for HCV infection, have been unable to eliminate viral infection in the majority of patients, and many studies suggest that HCV possesses mechanisms to antagonize the IFN-induced antiviral response. Multiple viral, host, and IFN-associated factors have been implicated in the interplay between HCV and IFN. Two viral proteins, NS5A and E2, became the focus of much attention and extensive study because of their abilities to inhibit IFN-induced, double-stranded RNA-activated protein kinase (PKR), a major mediator of the IFN-induced biologic response, and to perturb the IFN signaling pathway. In this review, we discuss the significance of the interferon sensitivity determining region (ISDR) within NS5A, which has been the subject of intense debates. In addition, we discuss the potential mechanisms by which NS5A interferes with IFN signaling and the current working models. Further understanding of the molecular mechanisms underlying the interaction between HCV and IFN will likely facilitate improvement of current IFN-based therapies and development of novel treatments for the HCV pandemic. Future HCV research will benefit from both the development of efficient, convenient model systems for viral propagation, and the utilization of high throughput, genomic-scale approaches.

Hepatitis C virus-host interactions: the NS5A protein and the interferon/chemokine systems

The interactions that occur between viral proteins and host factors, such as cellular proteins and signal transduction machinery, have a significant influence on the replication, persistence, and pathogenesis of all viruses. This is exemplified by hepatitis C virus (HCV), which infects an estimated 3% of the world's population and is a significant cause of liver disease. HCV-host interactions also affect the outcome of interferon (IFN) antiviral therapy, which is effective only in certain patients. In this review, we focus on the HCV nonstructural 5A (NS5A) protein, a model for diverse virus-host interactions, and highlight the interaction of viruses, including HCV, with the chemokine system.

Subversion of cell signaling pathways by hepatitis C virus nonstructural 5A protein via interaction with Grb2 and P85 phosphatidylinositol 3-kinase

Hepatitis C virus (HCV) sets up a persistent infection in patients that likely involves a complex virus-host interaction. We previously found that the HCV nonstructural 5A (NS5A) protein interacts with growth factor receptor-binding protein 2 (Grb2) adaptor protein and inhibits the activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) by epidermal growth factor (EGF). In the present study, we extended this analysis and investigated the specificity of the Grb2-NS5A interaction and whether the subversion of mitogenic signaling involves additional pathways. NS5A containing mutations within the C-terminal proline-rich motif neither bound Grb2 nor inhibited ERK1/2 activation by EGF, demonstrating that NS5A-Grb2 binding and downstream effects were due to direct interactions. Interestingly, NS5A could also form a complex with the Grb2-associated binder 1 (Gab1) protein in an EGF treatment-dependent manner. However, the NS5A-Gab1 association, which appeared indirect, was not mediated by direct NS5A-Grb2 interaction but was likely dependent on direct NS5A interaction with the p85 subunit of phosphatidylinositol 3-kinase (PI3K). The in vivo association of NS5A with p85 PI3K required the N-terminal, but not the C-terminal, region of NS5A. The downstream effects of the NS5A-p85 PI3K interaction included increased tyrosine phosphorylation of p85 PI3K in response to EGF. Consistent with this observation and the antiapoptotic properties of NS5A, we also detected enhanced tyrosine phosphorylation of the downstream AKT protein kinase and increased serine phosphorylation of BAD, a proapoptotic factor and an AKT substrate, in the presence of NS5A. These results collectively suggest a model in which NS5A interacts with Grb2 to inhibit mitogenic signaling while simultaneously promoting the PI3K-AKT cell survival pathway by interaction with p85 PI3K, which may represent a crucial step in HCV persistence and pathogenesis.

Interaction between hepatitis C virus proteins and host cell factors

Since the discovery of the hepatitis C virus (HCV) as the causative agent of non-A, non-B hepatitis, significant effort has been devoted to understanding this important pathogen. Despite the difficulty in culturing this virus efficiently, much is known about the organization of the viral genome and the functions of many of the viral proteins. Through the use of surrogate expression systems combined with cellular fractionation, pull-down experiments and yeast two-hybrid screens, numerous interactions between hepatitis C virus proteins and cellular components have been identified. The relevance of many of these interactions to hepatitis C biology remains to be demonstrated. This review discusses recent developments in this area of HCV research.

Viruses and the type I interferon antiviral system: induction and evasion

The type I interferon (IFN) system responds to viral infection and induces an "antiviral state" in cells, providing an important first line of defense against virus infection. Interaction of type I IFNs (IFN alpha and IFN beta) with their receptor induces hundreds of cellular genes. Of the proteins induced by IFN, the antiviral function of only a few is known, and their mechanisms of action are only partly understood. Additionally, although viral-encoded mechanisms that counteract specific components of the type I IFN response have been known for some time, it has recently become clear that many (if not most) viruses encode some form of IFN-antagonist. Understanding the interplay between viral-encoded IFN antagonists and the interferon response will be essential if the therapeutic potential of IFNs is to be fully exploited.

The nonstructural NS5A protein of hepatitis C virus: an expanding, multifunctional role in enhancing hepatitis C virus pathogenesis

The hepatitis C virus (HCV) NS5A gene product is a phosphorylated 56- to 58-kD nonstructural protein that displays a multitude of activities related to enhancement of viral pathogenesis. Although associated with other viral encoded proteins as part of the viral replicase complex positioned on the cytoplasmic side of the endoplasmic reticulum, a role for NS5A in viral replication has not been defined. Post-translational modifications of NS5A include phosphorylation and potential proteolytic processing to smaller molecular weight forms able to translocate to the nucleus. Both the identification of a putative interferon (IFN) sensitivity-determining region within NS5A, as well as the direct interaction with and inhibition of the IFN-induced double-stranded RNA-dependent protein kinase (PKR) by NS5A remain controversial. Truncated versions of NS5A can act as transcriptional activators, while other recently characterized interactions of NS5A with cellular proteins indicate its pleiotropic role in HCV-host interactions. NS5A itself has no direct effect on IFN-alpha signaling or activation, but other abundant interactions with members of the cellular signaling apparatus, transcription activation machinery and cell cycle-regulatory kinases have been described (e.g. growth factor receptor-bound protein 2, p53, p21/waf and cyclins). Many of these interactions block the apoptotic cellular response to persistent HCV infection. More recently, another altogether different mechanism attenuating the IFN-alpha response was reported based on induction of interleukin (IL)-8. IL-8, in model systems, potentiates viral replication and mutes the nonspecific intracellular IFN antiviral response. Evidence supporting a complex multimechanistic role of NS5A in promoting viral persistence, pathogenesis and, indirectly, viral-related hepatocarcinogenesis indicates its key role in HCV pathobiology.

Nonstructural 5A protein of hepatitis C virus modulates tumor necrosis factor alpha-stimulated nuclear factor kappa B activation

The hepatitis C virus nonstructural protein 5A (NS5A) is a multifunctional phosphoprotein that leads to pleiotropic responses, in part by regulating cell growth and cellular signaling pathways. Here we show that overexpression of NS5A inhibits tumor necrosis factor (TNF)-alpha-induced nuclear factor kappaB (NF-kappaB) activation in HEK293 cells, as determined by luciferase reporter gene expression and by electrophoretic mobility shift assay. When overexpressed, NS5A cannot inhibit the recruitment of TNF receptor-associated factor 2 (TRAF2) and IkappaB kinase (IKK)beta into the TNF receptor 1-TNF receptor-associated death domain complex. In contrast, NS5A is a part of the TNF receptor 1 signaling complex. NF-kappaB activation by TNF receptor-associated death domain and TRAF2 was inhibited by NS5A, whereas MEKK1 and IKKbeta-dependent NF-kappaB activation was not affected, suggesting that NS5A may inhibit NF-kappaB activation signaled by TRAF2. Coimmunoprecipitation and colocalization of NS5A and TRAF2 expressed in vivo provide compelling evidence that NS5A directly interacts with TRAF2. This interaction was mapped to the middle one-third (amino acids 148-301) of NS5A and the TRAF domain of TRAF2. Our findings suggest a possible molecular mechanism that could explain the ability of NS5A to negatively regulate TNF-alpha-induced NF-kappaB activation.

An amino-terminal amphipathic alpha-helix mediates membrane association of the hepatitis C virus nonstructural protein 5A

Hepatitis C virus (HCV) nonstructural protein 5A (NS5A), a phosphoprotein of unknown function, is believed to be a component of a membrane-associated viral replication complex. The determinants for membrane association of NS5A, however, have not been defined. By double label immunofluorescence analyses, NS5A was found to be associated with the endoplasmic reticulum (ER) or an ER-derived modified compartment both when expressed alone or in the context of the entire HCV polyprotein. Systematic deletion and green fluorescent protein fusion analyses allowed us to map the membrane anchor to the amino-terminal 30 amino acid residues of NS5A. Membrane association occurred by a posttranslational mechanism and resulted in properties of an integral membrane protein. Circular dichroism structural studies of a synthetic peptide corresponding to the NS5A membrane anchor, designated NS5A(1-31), demonstrated the presence of an amphipathic alpha-helix that was found to be highly conserved among 280 HCV isolates of various genotypes. The detergent-binding properties of this helical peptide together with the nature and location of its amino acids suggest a mechanism of membrane insertion via the helix hydrophobic side, yielding a topology parallel to the lipid bilayer in the cytoplasmic leaflet of the ER membrane. These findings have important implications for the structural and functional organization of the HCV replication complex and may define novel targets for antiviral intervention.

Interferon, PKR, virology, and genomics: what is past and what is next in the new millennium?

This paper provides an opportunity to reflect on my work in the interferon and cytokine field for the past almost 20 years and to look forward to the future. Winning the Milstein Award in 1999 was a great thrill. I briefly trace the history of my career from New York City to Seattle, leading up to the Paris award, and then look forward to the future that is full of promise because of the near-infinite power of genomics, computers, and other new technologies.

Hepatitis C virus NS5A colocalizes with the core protein on lipid droplets and interacts with apolipoproteins

The nonstructural protein 5A (NS5A) of the hepatitis C virus (HCV) has been shown to interact with a variety of cellular proteins and implicated in the regulation of cell growth, interferon resistance, and other cellular signaling pathways, but the role of NS5A in HCV pathogenesis has not been firmly established. To further characterize this multifunctional protein, we instigated the studies of the subcellular localization of NS5A in a hepatoma cell line. NS5A was localized to the perinuclear membrane structures, including the endoplasmic reticulum (ER) and the Golgi apparatus, by immunofluorescence staining and confocal microscopy. In addition, it was also associated with the surface of cytoplasmic globular structures when expressed alone or as a part of the NS3-5B polyprotein. Oil red O staining revealed that these globular structures were lipid droplets, where the HCV core protein was also localized. The association of NS5A with intracellular membrane was further confirmed by membrane flotation analysis. To determine whether NS5A interacts with any cellular lipid-binding protein, we performed yeast two-hybrid screening in both HepG2 and human liver cDNA libraries. Apolipoprotein A1 (apoA1), one of the protein components of high-density lipoprotein (HDL) particles, was identified by two independent screening processes. The interaction between NS5A and apoA1 was confirmed by both in vitro pull-down and in vivo coimmunoprecipitation experiments. Immunofluorescence staining revealed a significant colocalization of NS5A and apoA1 in the Golgi apparatus. Our results established an association of NS5A with lipid droplets and apoA1, suggesting that NS5A, together with the core protein, may play a role in the pathogenesis of the derangement of lipid metabolism, contributing to liver steatosis commonly observed in hepatitis C.

Prevalence of wild-type in NS5A-PKR protein kinase binding domain in HCV-related hepatocellular carcinoma

BACKGROUND/AIMS

Experimental studies have demonstrated that the wild-type PKR-NS5A strain of hepatitis C virus (HCV) may have oncogenic potential through the binding and functional repression of PKR protein kinase. To assess whether the NS5A-PKR-binding domain may be involved in HCV-related liver carcinogenesis, its sequence was analyzed in the sera of 85 patients with hepatocellular carcinoma (HCC) and in 51 patients with chronic active hepatitis (CAH). In 13 HCC cases sequence analysis was also performed in tumor and nontumor liver tissues.

METHODS

The nucleotide sequences of the PKR-binding domain were inferred by direct sequencing of the amplified HCV products and deduced amino acids were compared with the sequence of HCV-J.

RESULTS

A wild-type or single mutated strain which retains PKR-binding activity was found in 88% of HCC and 69% of CAH cases (P=0.0096). All but three HCC cases showed no divergences in amino acid changes between serum and liver tissues. The wild-type strains were equally distributed between the HCC with or without cirrhosis.

CONCLUSIONS

The prevalance of the wild-type NS5A-PKR strain is significantly higher in HCC than in CAH. These data suggest that inhibition of PKR activity by HCV might represent a potential mechanism of HCV-related carcinogenesis.

The small 11-kDa protein from B19 parvovirus binds growth factor receptor-binding protein 2 in vitro in a Src homology 3 domain/ligand-dependent manner

The small 11-kDa proteins of B19 parvovirus contain three proline-rich regions which conform to consensus Src homology 3 (SH3) ligand sequences present in signaling molecules within the cell. We have shown that the B19 11-kDa proteins specifically interact with the growth factor receptor-binding protein 2 (Grb2) in vitro. Mutation of prolines within one of the three SH3 ligand-like sequences decreases the binding of B19 11-kDa proteins to Grb2, suggesting that the proline-rich region is involved in the B19 11-kDa/Grb2 interaction. Therefore, the B19 11-kDa proteins may function to alter Grb2-mediated signaling by disrupting SH3 domain/ligand interactions. These results implicate the 11-kDa proteins in B19 pathogenesis through perturbation of normal cellular signaling pathways.

The hepatitis C virus core protein interacts with NS5A and activates its caspase-mediated proteolytic cleavage

Viral proteins interact with one another during viral replication, assembly, and maturation. Systematic interaction assays of the hepatitis C virus (HCV) proteins using the yeast two-hybrid method have uncovered a novel interaction between core and NS5A. This interaction was confirmed by in vitro binding assays, and coimmunoprecipitation in mammalian cells. Core and NS5A are also colocalized in COS-7 cells. Interestingly, NS5A is cleaved to give specific-size fragments, when core is coexpressed in mammalian cells. Overexpression of core produced many dying and rounded cells and effects such as DNA laddering and the truncation of poly(ADP-ribose) polymerase 1 (PARP1), both indicators of apoptosis. These observations led us to investigate the link between the induction of apoptosis by core and the cleavage of NS5A. The proteolysis of NS5A and these apoptotic events can be inhibited by caspase inhibitor, Z-VAD, indicating that core induces apoptosis and the cleavage of NS5A by caspases. In cells infected by the HCV, core may provide the intrinsic apoptotic signal, which produces truncated forms of NS5A. The biological function of core-NS5A interaction and the downstream effect of NS5A cleavage are discussed.

The ORF3 protein of hepatitis E virus binds to Src homology 3 domains and activates MAPK

The hepatitis E virus (HEV) is the causative agent of hepatitis E, an acute form of viral hepatitis. The biology and pathogenesis of HEV remain poorly understood. We have used in vitro binding assays to show that the HEV ORF3 protein (pORF3) binds to a number of cellular signal transduction pathway proteins. This includes the protein tyrosine kinases Src, Hck, and Fyn, the p85alpha regulatory subunit of phosphatidylinositol 3-kinase, phospholipase Cgamma, and the adaptor protein Grb2. A yeast two-hybrid assay was used to further confirm the pORF3-Grb2 interaction. The binding involves a proline-rich region in pORF3 and the src homology 3 (SH3) domains in the cellular proteins. Competition assays and computer-assisted modeling was used to evaluate the binding surfaces and interaction energies of the pORF3.SH3 complex. In pORF3-expressing cells, pp60(src) was found to associate with an 80-kDa protein, but no activation of the Src kinase was observed in these cells. However, there was increased activity and nuclear localization of ERK in the pORF3-expressing cells. These studies suggest that pORF3 is a viral regulatory protein involved in the modulation of cell signaling. The ORF3 protein of HEV appears to be the first example of a SH3 domain-binding protein encoded by a virus that causes an acute and primarily self-limited infection.