Expression of the alternative reading frame protein of Hepatitis C virus induces cytokines involved in hepatic injuries

A Proteomic Approach to Study the Biological Role of Hepatitis C Virus Protein Core+1/ARFP

Hepatitis C virus is the major cause of chronic liver diseases and the only cytoplasmic RNA virus known to be oncogenic in humans. The viral genome gives rise to ten mature proteins and to additional proteins, which are the products of alternative translation initiation mechanisms. A protein-known as ARFP (alternative reading frame protein) or Core+1 protein-is synthesized by an open reading frame overlapping the HCV Core coding region in the (+1) frame of genotype 1a. Almost 20 years after its discovery, we still know little of the biological role of the ARFP/Core+1 protein. Here, our differential proteomic analysis of stable hepatoma cell lines expressing the Core+1/Long isoform of HCV-1a relates the expression of the Core+1/Long isoform with the progression of the pathology of HCV liver disease to cancer.

Hepatitis C virus alternative reading frame protein (ARFP): Production, features, and pathogenesis

Earlier observation suggests that hepatitis C virus (HCV) is a single-stranded RNA virus which encodes at least 10 viral proteins. F protein is a novel protein which has been discovered recently. These studies suggest three mechanisms for the production of this protein concerning ribosomal frameshift at codon 10, initial translation at codons 26 and 85 or 87. In this study, the association between protein F and chronicity of hepatocellular carcinoma (HCC) has been reviewed. Evidence suggests that humoral immune system can recognize this protein and produce antibodies against it. By detecting antibodies in infected people, investigators found that F protein might have a role in HCV infection causing chronic cirrhosis and HCC as higher prevalence was found in patients with mentioned complications. The increment of CD4+, CD25+, and FoxP3+ T cells, along with CD8+ T cells with low expression of granzyme B, also leads to weaker responses of the immune system which helps the infection to become chronic. Moreover, it contributes to the survival of the virus in the body through affecting the production of interferon. F protein also might play roles in the disease development, resulting in HCC. The existence of F protein affects cellular pathways through upregulating p53, c-myc, cyclin D1, and phosphorylating Rb. This review will summarize these effects on immune system and related mechanisms in cellular pathways.

Hepatitis C Virus core+1/ARF Protein Modulates the Cyclin D1/pRb Pathway and Promotes Carcinogenesis

Viruses often encompass overlapping reading frames and unconventional translation mechanisms in order to maximize the output from a minimum genome and to orchestrate their timely gene expression. Hepatitis C virus (HCV) possesses such an unconventional open reading frame (ORF) within the core-coding region, encoding an additional protein, initially designated ARFP, F, or core+1. Two predominant isoforms of core+1/ARFP have been reported, core+1/L, initiating from codon 26, and core+1/S, initiating from codons 85/87 of the polyprotein coding region. The biological significance of core+1/ARFP expression remains elusive. The aim of the present study was to gain insight into the functional and pathological properties of core+1/ARFP through its interaction with the host cell, combining in vitro and in vivo approaches. Our data provide strong evidence that the core+1/ARFP of HCV-1a stimulates cell proliferation in Huh7-based cell lines expressing either core+1/S or core+1/L isoforms and in transgenic liver disease mouse models expressing core+1/S protein in a liver-specific manner. Both isoforms of core+1/ARFP increase the levels of cyclin D1 and phosphorylated Rb, thus promoting the cell cycle. In addition, core+1/S was found to enhance liver regeneration and oncogenesis in transgenic mice. The induction of the cell cycle together with increased mRNA levels of cell proliferation-related oncogenes in cells expressing the core+1/ARFP proteins argue for an oncogenic potential of these proteins and an important role in HCV-associated pathogenesis.IMPORTANCE This study sheds light on the biological importance of a unique HCV protein. We show here that core+1/ARFP of HCV-1a interacts with the host machinery, leading to acceleration of the cell cycle and enhancement of liver carcinogenesis. This pathological mechanism(s) may complement the action of other viral proteins with oncogenic properties, leading to the development of hepatocellular carcinoma. In addition, given that immunological responses to core+1/ARFP have been correlated with liver disease severity in chronic HCV patients, we expect that the present work will assist in clarifying the pathophysiological relevance of this protein as a biomarker of disease progression.

Hepatitis C Virus Frameshift/Alternate Reading Frame Protein Suppresses Interferon Responses Mediated by Pattern Recognition Receptor Retinoic-Acid-Inducible Gene-I

Hepatitis C virus (HCV) actively evades host interferon (IFN) responses but the mechanisms of how it does so are not completely understood. In this study, we present evidence for an HCV factor that contributes to the suppression of retinoic-acid-inducible gene-I (RIG-I)-mediated IFN induction. Expression of frameshift/alternate reading frame protein (F/ARFP) from HCV -2/+1 frame in Huh7 hepatoma cells suppressed type I IFN responses stimulated by HCV RNA pathogen-associated molecular pattern (PAMP) and poly(IC). The suppression occurred independently of other HCV factors; and activation of interferon stimulated genes, TNFα, IFN-λ1, and IFN-λ2/3 was likewise suppressed by HCV F/ARFP. Point mutations in the full-length HCV sequence (JFH1 genotype 2a strain) were made to introduce premature termination codons in the -2/+1 reading frame coding for F/ARFP while preserving the original reading frame, which enhanced IFNα and IFNβ induction by HCV. The potentiation of IFN response by the F/ARFP mutations was diminished in Huh7.5 cells, which already have a defective RIG-I, and by decreasing RIG-I expression in Huh7 cells. Furthermore, adding F/ARFP back via trans-complementation suppressed IFN induction in the F/ARFP mutant. The F/ARFP mutants, on the other hand, were not resistant to exogenous IFNα. Finally, HCV-infected human liver samples showed significant F/ARFP antibody reactivity, compared to HCV-uninfected control livers. Therefore, HCV F/ARFP likely cooperates with other viral factors to suppress type I and III IFN induction occurring through the RIG-I signaling pathway. This study identifies a novel mechanism of pattern recognition receptor modulation by HCV and suggests a biological function of the HCV alternate reading frame in the modulation of host innate immunity.

PD-1/PD-L1 signal pathway participates in HCV F protein-induced T cell dysfunction in chronic HCV infection

Programmed cell death-1/programmed cell death-1 ligand 1 (PD-1/PD-L1) inhibitory signal pathway has been verified to be involved in the establishment of persistent viral infections. Blockade of PD-1/PD-L1 engagement to reinvigorate T cell activity is supposed to be a potential therapeutic scheme. Studies have verified the participation of PD-1/PD-L1 in hepatitis C virus (HCV) core protein-regulated immune response. To determine the roles of PD-1/PD-L1 signal pathway in HCV F protein-induced immunoreaction in chronic HCV infection, variations in T cells were examined. The results showed that PD-1 expression on CD8(+) and CD4(+) T cells was increased with HCV F stimulation in both chronic HCV patients and healthy controls, and could be reduced partly by PD-1/PD-L1 blocking. Additionally, by PD-1/PD-L1 blocking, HCV F-induced inhibition of T cell proliferation and promotion of cellular apoptosis were partly or even totally recovered. Furthermore, levels of both Th1 and Th2 cytokines were elevated in the presence of anti-PD-L1 antibody. All these results indicated that PD-1/PD-L1 signal pathway also participates in HCV F protein-induced immunoregulation. PD-1/PD-L1 blocking plays important roles in the restoration of effective functionality of the impaired T cells in chronic HCV patients.

Chaperones in hepatitis C virus infection

The hepatitis C virus (HCV) infects approximately 3% of the world population or more than 185 million people worldwide. Each year, an estimated 350000-500000 deaths occur worldwide due to HCV-associated diseases including cirrhosis and hepatocellular carcinoma. HCV is the most common indication for liver transplantation in patients with cirrhosis worldwide. HCV is an enveloped RNA virus classified in the genus Hepacivirus in the Flaviviridae family. The HCV viral life cycle in a cell can be divided into six phases: (1) binding and internalization; (2) cytoplasmic release and uncoating; (3) viral polyprotein translation and processing; (4) RNA genome replication; (5) encapsidation (packaging) and assembly; and (6) virus morphogenesis (maturation) and secretion. Many host factors are involved in the HCV life cycle. Chaperones are an important group of host cytoprotective molecules that coordinate numerous cellular processes including protein folding, multimeric protein assembly, protein trafficking, and protein degradation. All phases of the viral life cycle require chaperone activity and the interaction of viral proteins with chaperones. This review will present our current knowledge and understanding of the role of chaperones in the HCV life cycle. Analysis of chaperones in HCV infection will provide further insights into viral/host interactions and potential therapeutic targets for both HCV and other viruses.

Th1 and Th2 cytokine profiles induced by hepatitis C virus F protein in peripheral blood mononuclear cells from chronic hepatitis C patients

Th1 and Th2 cytokine response has been confirmed to be correlated with the pathogenesis of HCV infection. The aim of the study is to investigate the Th1 and Th2 cytokine profiles induced by HCV alternate reading frame protein (F protein) in chronic hepatitis C patients. We assessed the immune responses specific to HCV F protein in 55 chronic HCV patients. IFN-γ, IL-2, IL-4 and IL-5 secretion by peripheral blood mononuclear cells (PBMC) post F protein stimulation were compared among HCV patients and healthy donors. Finally, the associations between HCV F protein and HLA class II alleles were explored. We found that the seroprevalence of anti-F antibodies in HCV-related hepatocellular carcinoma (HCC) patients was significantly higher than that of patients without HCC, but such a significant difference in humoral immune responses to F protein was not observed in HCV 1b-infected- and non-HCV 1b-infected-patients. Additionally, the PBMC proliferation of HCC patients was significantly lower than that of patients without HCC. Furthermore, F protein stimulation of PBMCs from F-seropositive patients resulted in Th2 biased cytokine responses (significantly decreased IFN-γ and/or IL-2 and significantly increased IL-4 and/or IL-5 levels) that reportedly may contribute to HCC progression and pathogenesis. However, no significant difference in the association between HCV F protein and HLA-DRB1*0201, 0301, 0405, 1001 and HLA-DQB1*0201, 0401, 0502, 0602 was observed in this study. These findings suggest that F protein may contribute to the HCV-associated bias in Th1/Th2 responses of chronic hepatitis C patients including the progress of HCC pathogenesis.

The ubiquitin-proteasome system in positive-strand RNA virus infection

Positive-stranded RNA viruses, like many other viruses, have evolved to exploit the host cellular machinery to their own advantage. In eukaryotic cells, the ubiquitin-proteasome system (UPS) that serves as the major intracellular pathway for protein degradation and modification plays a crucial role in the regulation of many fundamental cellular functions. A growing amount of evidence has suggested that the UPS can be utilized by positive-sense RNA viruses. The UPS eliminates excess viral proteins that prevent viral replication and modulates the function of viral proteins through post-translational modification mediated by ubiquitin or ubiquitin-like proteins. This review will discuss the current understanding of how positive RNA viruses have evolved various mechanisms to usurp the host UPS to modulate the function and stability of viral proteins. In addition to the pro-viral function, UPS-mediated viral protein degradation may also constitute a host defense process against some positive-stranded RNA viral infections. This issue will also be discussed in the current review.

Hepatitis C viral protein translation: mechanisms and implications in developing antivirals

Hepatitis C viral protein translation occurs in a cap-independent manner through the use of an internal ribosomal entry site (IRES) present within the viral 5'-untranslated region. The IRES is composed of highly conserved structural domains that directly recruit the 40S ribosomal subunit to the viral genomic RNA. This frees the virus from relying on a large number of translation initiation factors that are required for cap-dependent translation, conferring a selective advantage to the virus especially in times when the availability of such factors is low. Although the mechanism of translation initiation on the Hepatitis C virus (HCV) IRES is well established, modulation of the HCV IRES activity by both cellular and viral factors is not well understood. As the IRES is essential in the HCV life cycle and as such remains well conserved in an otherwise highly heterogenic virus, the process of HCV protein translation represents an attractive target in the development of novel antivirals. This review will focus on the mechanisms of HCV protein translation and how this process is postulated to be modulated by cis-acting viral factors, as well as trans-acting viral and cellular factors. Numerous therapeutic approaches investigated in targeting HCV protein translation for the development of novel antivirals will also be discussed.

High levels of HCV core+1 antibodies in HCV patients with hepatocellular carcinoma

The core region of the hepatitis C virus (HCV) genome possesses an overlapping ORF that has been shown to encode a protein, known as the alternate reading frame protein (ARFP), F or core+1. The biological role of this protein remains elusive, as it appears to be non-essential for virus replication. However, a number of independent studies have shown that the ARFP/F/core+1 protein elicits humoral and cellular immune responses in HCV-infected individuals and interacts with important cellular proteins. To assess the significance of the core+1 humoral response in HCV-infected patients, we examined the prevalence of anti-core+1 antibodies in sera from patients with hepatocellular carcinoma (HCC) in comparison with chronically HCV-infected individuals without HCC. We produced two HCV core+1 histidine-tagged recombinant proteins for genotypes 1a (aa 11–160) and 1b (aa 11–144), as well as a non-tagged highly purified recombinant core+1/S protein (aa 85–144) of HCV-1b. Using an in-house ELISA, we tested the prevalence of core+1 antibodies in 45 patients with HCC in comparison with 47 chronically HCV-infected patients without HCC and 77 negative-control sera. More than 50 % of the serum samples from HCC patients reacted with all core+1 antigens, whereas <26 % of the sera from the non-HCC HCV-infected individuals tested positive. No core+1-specific reactivity was detected in any of the control samples. In conclusion, the high occurrence of anti-core+1 antibodies in the serum of HCC patients suggests a role for the ARFP/F/core+1 protein in the pathogenesis of HCC.

High baseline interleukine-8 level is an independent risk factor for the achievement of sustained virological response in chronic HCV patients

Hepatitis C virus (HCV), a major cause of liver disease throughout the world, is difficult to treat with interferon (IFN) (and various formulations and combinations thereof) being the only approved molecule available. It has been investigated recently that proinflammatory chemokine interleukin-8 (IL-8) induced by HCV partially inhibits the antiviral IFN-α therapy. Therefore, the current study was aimed to prospectively utilize the baseline IL-8 levels in the HCV infected serum and predicts its role in sustained virological response (SVR) to IFN-α+ribavirin therapy, in chronic HCV patients in Pakistan. One hundred and ten hepatitis C patients without any other infections underwent IFN-α+ribavirin combination treatment. Baseline IL-8 levels were determined before starting of the therapy for all these patients. Fifteen normal volunteers negative for HCV were kept as control. The baseline IL-8 levels were found significantly higher in all HCV positive patients as compared to normal healthy volunteers (1083.54 ± 85.72 pg/ml versus 6.99 ± 1.05 pg/ml [mean ± SEM], p<0.01) and were also significantly higher in non-responders than responders (p<0.05). Comparatively higher mean baseline IL-8 levels were observed in non-responders (2442.02 ± 159.92 pg/ml), than late (1009.31 ± 45.31) and rapid (540.91 ± 27.06 pg/ml) responders. Significant relation was observed between baseline IL-8 level and response to IFN therapy (p<0.01). Results of this study suggest that increased levels of IL-8 in HCV infection might be involved in pathogenesis, persistence and resistance to IFN-α+ribavirin combination therapy.

Dendritic cells in hepatitis C infection: can they (help) win the battle?

Infection with hepatitis C virus (HCV) is a public health problem; it establishes a chronic course in ~85% of infected patients and increases their risk for developing liver cirrhosis, hepatocellular carcinoma, and significant extrahepatic manifestations. The mechanisms of HCV persistence remain elusive and are largely related to inefficient clearance of the virus by the host immune system. Dendritic cells (DCs) are the most efficient inducers of immune responses; they are capable of triggering productive immunity and maintaining the state of tolerance to self- and non-self antigens. During the past decade, multiple research groups have focused on DCs, in hopes of unraveling an HCV-specific DC signature or DC-dependent mechanisms of antiviral immunity which would lead to a successful HCV elimination strategy. This review incorporates the latest update in the current status of knowledge on the role of DCs in anti-HCV immunity as it relates to several challenging questions: (a) the phenotype and function of diverse DC subsets in HCV-infected patients; (b) the characteristics of non-human HCV infection models from the DCs' point of view; (c) how can in vitro systems, ranging from HCV protein- or peptide-exposed DC to HCV protein-expressing DCs, and in vivo systems, ranging from HCV protein-expressing transgenic mice to HCV-infected non-human primates, be employed to dissect the role of DCs in triggering/maintaining a robust antiviral response; and (d) the prospect of DC-based strategy for managing and finding a cure for HCV infection.

Synonymous mutations in the core gene are linked to unusual serological profile in hepatitis C virus infection

The biological role of the protein encoded by the alternative open reading frame (core+1/ARF) of the Hepatitis C virus (HCV) genome remains elusive, as does the significance of the production of corresponding antibodies in HCV infection. We investigated the prevalence of anti-core and anti-core+1/ARFP antibodies in HCV-positive blood donors from Cambodia, using peptide and recombinant protein-based ELISAs. We detected unusual serological profiles in 3 out of 58 HCV positive plasma of genotype 1a. These patients were negative for anti-core antibodies by commercial and peptide-based assays using C-terminal fragments of core but reacted by Western Blot with full-length core protein. All three patients had high levels of anti-core+1/ARFP antibodies. Cloning of the cDNA that corresponds to the core-coding region from these sera resulted in the expression of both core and core+1/ARFP in mammalian cells. The core protein exhibited high amino-acid homology with a consensus HCV1a sequence. However, 10 identical synonymous mutations were found, and 7 were located in the aa(99–124) region of core. All mutations concerned the third base of a codon, and 5/10 represented a T>C mutation. Prediction analyses of the RNA secondary structure revealed conformational changes within the stem-loop region that contains the core+1/ARFP internal AUG initiator at position 85/87. Using the luciferase tagging approach, we showed that core+1/ARFP expression is more efficient from such a sequence than from the prototype HCV1a RNA. We provide additional evidence of the existence of core+1/ARFP in vivo and new data concerning expression of HCV core protein. We show that HCV patients who do not produce normal anti-core antibodies have unusually high levels of antit-core+1/ARFP and harbour several identical synonymous mutations in the core and core+1/ARFP coding region that result in major changes in predicted RNA structure. Such HCV variants may favour core+1/ARFP production during HCV infection.

The HCV ARFP/F/core+1 protein: production and functional analysis of an unconventional viral product

Hepatitis C virus (HCV) is an enveloped positive-strand RNA virus of the Flaviviridae family. It has a genome of about 9,600 nucleotides encoding a large polyprotein (about 3,000 amino acids) that is processed by cellular and viral proteases into at least 10 structural and nonstructural viral proteins. A novel HCV protein has also been identified by our laboratory and others. This protein--known as ARFP (alternative reading frame protein), F (for frameshift) or core+1 (to indicate the position) protein--is synthesized by an open reading frame overlapping the core gene at nucleotide +1 (core+1 ORF). However, almost 10 years after its discovery, we still know little of the biological role of the ARFP/F/core+1 protein. Abolishing core+1 protein production has no affect on HCV replication in cell culture or uPA-SCID mice, suggesting that core+1 protein is probably not important for the HCV reproductive cycle. However, the detection of specific anti-core+1 antibodies and T-cell responses in HCV-infected patients, as reported by many independent laboratories, provides strong evidence that this protein is produced in vivo. Furthermore, analyses of the HCV sequences isolated from patients with hepatocellular carcinoma and in vitro studies have provided strong preliminary evidence to suggest that core+1 protein plays a role in advanced liver disease and liver cancer. The available in vitro data also suggest that certain core function proteins may depend on production of the core+1 protein. We describe here the discovery of the various forms of the core+1 protein and what is currently known about the mechanisms of their production and their biochemical and functional properties. We also provide a detailed summary of the results of patient-based research.

Gene expression profile of Huh-7 cells expressing hepatitis C virus genotype 1b or 3a core proteins

BACKGROUND

The liver disease expression in chronic hepatitis C patients is variable and may partially depend on the sequence of the infecting viral genotype.

AIM

To identify some hepatitis C virus (HCV) genotype-specific virus-host interactions potentially leading to clinically significant consequences.

METHODS

We compared the gene expression profile of Huh-7 cells transiently expressing the core protein of HCV genotype 1b and 3a using microarray technology.

RESULTS

Thirty-two genes were overexpressed in Huh-7 transfected with the HCV genotype 1b core protein and 57 genes in cells transfected with the genotype 3a core protein. On the other hand, we found 20 genes downregulated by core 1b and 31 genes by core 3a. These included genes involved in lipid transport and metabolism, cell cycle, immune response and insulin signalling.

CONCLUSION

The expression of HCV core proteins of different genotypes leads to a specific gene expression profile. This may account for the variable disease expression associated with HCV infection.

Seroconversion to hepatitis C virus alternate reading frame protein during acute infection

The existence of hepatitis C virus (HCV) proteins encoded by alternate reading frames overlapping the core-encoding region has been suggested. Several mechanisms of production have been postulated, and the functions of these proteins in the HCV life cycle remain unknown. We analyzed cases of seroconversion to an alternate reading frame protein in a group of 17 patients infected by one of the two HCV genotype 1b strains during an outbreak in a hemodialysis unit. Three patients seroconverted, and antibodies were transiently detected in another patient. Three of these patients were infected by one of the two HCV strains, whereas the strain infecting the remaining patient could not be identified. Quasispecies sequence analysis of the core-coding region showed no differences in the core or +1 reading frame sequences that could explain alternate reading frame protein seroconversion in some but not all of the patients infected by one of the HCV strains, and no such difference was found between the two strains. Because differences in the structure of RNA elements could play a role in frameshift events, we conducted a predictive analysis of RNA folding. No difference was found between the patients who did and did not seroconvert to alternate reading frame protein.

CONCLUSION

Our findings prove that alternate reading frame proteins can be produced during acute HCV infection. However, seroconversion does not occur in all patients for unknown reasons. Alternate reading frame protein could be generated by minority quasispecies variants or variants that occur transiently.

Internal initiation stimulates production of p8 minicore, a member of a newly discovered family of hepatitis C virus core protein isoforms

The hepatitis C virus (HCV) core gene is more conserved at the nucleic acid level than is necessary to preserve the sequence of the core protein, suggesting that it contains information for additional functions. We used a battery of anticore antibodies to test the hypothesis that the core gene directs the synthesis of core protein isoforms. Infectious viruses, replicons, and RNA transcripts expressed a p8 minicore containing the C-terminal portion of the p21 core protein and lacking the N-terminal portion. An interferon resistance mutation, U271A, which creates an AUG at codon 91, upregulated p8 expression in Con1 replicons, suggesting that p8 is produced by an internal initiation event and that 91-AUG is the preferred, but not the required, initiation codon. Synthesis of p8 was independent of p21, as shown by the abundant production of p8 from transcripts containing an UAG stop codon that blocked p21 production. Three infectious viruses, JFH-1 (2a core), J6/JFH (2a core), and H77/JFH (1a core), and a bicistronic construct, Bi-H77/JFH, all expressed both p8 and larger isoforms. The family of minicores ranges in size from 8 to 14 kDa. All lack the N-terminal portion of the p21 core. In conclusion, the core gene contains an internal signal that stimulates the initiation of protein synthesis at or near codon 91, leading to the production of p8. Infectious viruses of both genotype 1 and 2 HCV express a family of larger isoforms, in addition to p8. Minicores lack significant portions of the RNA binding domain of p21 core. Studies are under way to determine their functions.

Subcellular localizations of the hepatitis C virus alternate reading frame proteins

Alternate reading frame proteins (ARFPs) resulting either from frameshifting, from transcriptional slippage or from internal initiation in the +1 open reading frame (ORF) of hepatitis C virus (HCV) core protein coding sequence have been described in vitro. As an approach to study the roles of these proteins, we investigate the subcellular localization of ARFPs fused with the green fluorescent protein (GFP) either at their N- or C-terminus. Most GFP fusion products have a diffuse localization, as revealed by confocal microscopy. One GFP chimeric protein, arising from internal initiation at codon 26 in the +1 ORF (ARFP(26-161)), is specifically targeted to mitochondria. Mitochondrial localization was confirmed by immunoblot with an anti-ARFP antibody of a mitochondria-enriched cellular fraction. Mitochondrial targeting of ARFP(26-161) mostly involved the N-terminal portion of the protein as revealed by the cellular localization of truncated mutants. Interestingly, ARFP(26-161) from both genotypes 1a and 1b, but not the protein from the genotype 2a JFH1 infectious sequence, exhibit mitochondrial localization. These results are the first concerning the cellular localization and the role of this HCV ARFP; they may serve as a platform for further studies on its mitochondrial effects and their role in the virus life cycle and pathogenesis.

Ubiquitin-independent degradation of hepatitis C virus F protein

Hepatitis C virus (HCV) F protein is encoded by the +1 reading frame of the viral genome. It overlaps with the core protein coding sequence, and multiple mechanisms for its expression have been proposed. The full-length F protein that is synthesized by translational ribosomal frameshift at codons 9 to 11 of the core protein sequence is a labile protein. By using a combination of genetic, biochemical, and cell biological approaches, we demonstrate that this HCV F protein can bind to the proteasome subunit protein alpha3, which reduces the F-protein level in cells in a dose-dependent manner. Deletion-mapping analysis identified amino acids 40 to 60 of the F protein as the alpha3-binding domain. This alpha3-binding domain of the F protein together with its upstream sequence could significantly destabilize the green fluorescent protein, an otherwise stable protein. Further analyses using an F-protein mutant lacking lysine and a cell line that contained a temperature-sensitive E1 ubiquitin-activating enzyme indicated that the degradation of the F protein was ubiquitin independent. Based on these observations as well as the observation that the F protein could be degraded directly by the 20S proteasome in vitro, we propose that the full-length HCV F protein as well as the F protein initiating from codon 26 is degraded by an ubiquitin-independent pathway that is mediated by the proteasome subunit alpha3. The ability of the F protein to bind to alpha3 raises the possibility that the HCV F protein may regulate protein degradation in cells.

Differential reactivity of putative genotype 2 hepatitis C virus F protein between chronic and recovered infections

To date, all studies regarding hepatitis C virus (HCV) F protein have been based on expression in vitro/in vivo of recombinant protein or monoclonal antibodies derived from genotype 1a or 1b sequences, but not from other genotypes. The objective of this study was to prepare a putative genotype 2 recombinant F protein and evaluate its reactivity in plasma from individuals with chronic HCV infection or who had recovered from infection. One genotype 2 strain was selected for F protein (F-2) and core expression in bacterial culture. An ELISA was developed and applied to samples from patients with chronic infection or recovered infection of various genotypes. The anti-F-2 response in 117 samples showed a significantly higher reactivity in chronic than in recovered HCV-infected blood donors (P<0.001), but no difference was found among genotypes. However, the correlation between anti-F and anti-core was more significant in genotypes 1 and 2 than in genotype 3. Anti-F-2 titres were also significantly higher in chronic than in recovered individuals (P<0.0001). Antibody titres to recombinant genotype 2 core protein or to genotype 1 multiple proteins used in commercial anti-HCV assays paralleled the anti-F-2 end-point antibody titre. This study thus demonstrated the antigenicity of genotype 2 HCV F protein, although the exact location of the natural frameshift position remains unknown. The difference in anti-F-2 response between chronic and recovered infection, the cross-reactivity irrespective of genotype and the correlation of antibody response with structural and non-structural antigens suggest that the immune response to F protein is an integral part of the natural HCV infection.

Transcriptional slippage prompts recoding in alternate reading frames in the hepatitis C virus (HCV) core sequence from strain HCV-1

Since the first report of frameshifting in HCV-1, its sequence has been the paradigm for examining the mechanism that directs alternative translation of the hepatitis C virus (HCV) genome. The region encoding the core protein from this strain contains a cluster of 10 adenines at codons 8-11, which is thought to direct programmed ribosomal frameshifting (PRF), but formal evidence for this process has not been established unequivocally. To identify the mechanisms of frameshifting, this study used a bicistronic dual luciferase reporter system in a coupled transcription/translation in vitro assay. This approach revealed +1 as well as -1 frameshifting, whereas point mutations, selectively introduced between codons 8 and 11, demonstrated that PRF did not readily account for frameshifting in strain HCV-1. Sequence analysis of cDNAs derived from RNA transcribed by T7 RNA polymerase in the dual luciferase reporter system, as well as in both a subgenomic replicon and an infectious clone derived from strain JFH1, identified additions and deletions of adenines between codons 8 and 11 due to transcriptional slippage (TS). Moreover, RNA isolated from cells infected with virus generated by JFH1 containing the A-rich tract also contained heterogeneity in the adenine sequence, strongly suggesting TS by the NS5B viral polymerase. These findings have important implications for insight into frameshifting events in HCV-1 and demonstrate for the first time the involvement of transcriptional slippage in this recoding event.

Hepatitis C virus non-structural protein-2 activates CXCL-8 transcription through NF-kappaB

Hepatitis C is a devastating disease worldwide. Proteins encoded by the etiologic agent, hepatitis C virus (HCV), are believed to play important roles in HCV-associated pathogenesis. However, the biological functions of the non-structural protein-2 (NS2) encoded by HCV are not well characterized. Here, we show that HCV NS2 protein activates CXCL-8 (interleukin-8, IL-8) transcription in HepG2 cells as measured by reverse transcription-polymerase chain reaction and IL-8 promoter-luciferase reporter assays. Furthermore, when the kappaB site on the IL-8 promoter was eliminated by mutagenesis or when intracellular NF-kappaB activity was suppressed by an inhibitor, NS2 did not activate the IL-8 promoter, suggesting a role of NF-kappaB in this process. These results prompted us to hypothesize that HCV NS2 might be able to activate NF-kappaB. This hypothesis was tested by determination of NF-kappaB-driven reporter gene expression and NF-kappaB p65 subunit subcellular localization after HCV NS2 expression. Indeed, NS2 could up-regulate NF-kappaB-driven luciferase activity and was associated with p65 nuclear localization. These results demonstrate that HCV NS2 up-regulates IL-8 transcription through NF-kappaB. This newly identified function increases our understanding of the role of HCV NS2 protein in virus-host interactions.

New lane in the information highway: alternative reading frame peptides elicit T cells with potent antiretrovirus activity

CD8(+) T cells rapidly recognize virus-infected cells due to the generation of antigenic peptides from defective ribosomal products (DRiPs) that are encoded by standard open reading frames (ORFs). New data now show that alternative reading frame (ARF) DRiPs can also induce robust CD8(+) T cell responses. ARF-specific T cells control retroviral replication and select for viral escape in monkeys, providing the most compelling evidence to date for the biological relevance of ARF immunosurveillance.