Hepatitis C virus core protein interacts with the cytoplasmic tail of lymphotoxin-beta receptor

Induction of p38- and gC1qR-dependent IL-8 expression in pulmonary fibroblasts by soluble hepatitis C core protein

Background

Recent studies suggest that HCV infection is associated with progressive declines in pulmonary function in patients with underlying pulmonary diseases such as asthma and chronic obstructive pulmonary disease. Few molecular studies have addressed the inflammatory aspects of HCV-associated pulmonary disease. Because IL-8 plays a fundamental role in reactive airway diseases, we examined IL-8 signaling in normal human lung fibroblasts (NHLF) in response to the HCV nucleocapsid core protein, a viral antigen shown to modulate intracellular signaling pathways involved in cell proliferation, apoptosis and inflammation.

Methods

NHLF were treated with HCV core protein and assayed for IL-8 expression, phosphorylation of the p38 MAPK pathway, and for the effect of p38 inhibition.

Results

Our studies demonstrate that soluble HCV core protein induces significant increases in both IL-8 mRNA and protein expression in a dose- and time-dependent manner. Treatment with HCV core led to phosphorylation of p38 MAPK, and expression of IL-8 was dependent upon p38 activation. Using TNFα as a co-stimulant, we observed additive increases in IL-8 expression. HCV core-mediated expression of IL-8 was inhibited by blocking gC1qR, a known receptor for soluble HCV core linked to MAPK signaling.

Conclusion

These studies suggest that HCV core protein can lead to enhanced p38- and gC1qR-dependent IL-8 expression. Such a pro-inflammatory role may contribute to the progressive deterioration in pulmonary function recently recognized in individuals chronically infected with HCV.

Screening of hepatocyte proteins binding to complete S protein of hepatitis B virus by yeast-two hybrid system

AIM: To investigate the biological function of complete S protein and to look for proteins interacting with complete S protein in hepatocytes.

METHODS: We constructed bait plasmid expressing complete S protein of HBV by cloning the gene of complete S protein into pGBKT7, then the recombinant plasmid DNA was transformed into yeast AH109 (a type). The transformed yeast AH109 was mated with yeast Y187 (α type) containing liver cDNA library plasmid in 2 ×YPDA medium. Diploid yeast was plated on synthetic dropout nutrient medium (SD/-Trp-Leu-His-Ade) containing X-α-gal for selection and screening. After extracting and sequencing of plasmids from positive (blue) colonies, we underwent sequence analysis by bioinformatics.

RESULTS: Nineteen colonies were selected and sequenced. Among them, five colonies were Homo sapiens solute carrier family 25, member 23 (SLC25A23), one was Homo sapiens calreticulin, one was human serum albumin (ALB) gene, one was Homo sapiens metallothionein 2A, two were Homo sapiens betaine-homocysteine methyltransferase, three were Homo sapiens Na⁺ and H⁺ coupled amino acid transport system N, one was Homo sapiens CD81 antigen (target of anti-proliferative antibody 1) (CD81), three were Homo sapiens diazepam binding inhibitor, two colonies were new genes with unknown function.

CONCLUSION: The yeast-two hybrid system is an effective method for identifying hepatocyte proteins interacting with complete S protein of HBV. The complete S protein may bind to different proteins i.e., its multiple functions in vivo.

Hepatitis C virus core protein suppresses NF-kappaB activation and cyclooxygenase-2 expression by direct interaction with IkappaB kinase beta

In addition to hepatocytes, hepatitis C virus (HCV) infects immune cells, including macrophages. However, little is known concerning the impact of HCV infection on cellular functions of these immune effector cells. Lipopolysaccharide (LPS) activates IkappaB kinase (IKK) signalsome and NF-kappaB, which leads to the expression of cyclooxygenase-2 (COX-2), which catalyzes production of prostaglandins, potent effectors on inflammation and possibly hepatitis. Here, we examined whether expression of HCV core interferes with IKK signalsome activity and COX-2 expression in activated macrophages. In reporter assays, HCV core inhibited NF-kappaB activation in RAW 264.7 and MH-S murine macrophage cell lines treated with bacterial LPS. HCV core inhibited IKK signalsome and IKKbeta kinase activities induced by tumor necrosis factor alpha in HeLa cells and coexpressed IKKgamma in 293 cells, respectively. HCV core was coprecipitated with IKappaKappabeta and prevented nuclear translocation of IKKbeta. NF-kappaB activation by either LPS or overexpression of IKKbeta was sufficient to induce robust expression of COX-2, which was markedly suppressed by ectopic expression of HCV core. Together, these data indicate that HCV core suppresses IKK signalsome activity, which blunts COX-2 expression in macrophages. Additional studies are necessary to determine whether interrupted COX-2 expression by HCV core contributes to HCV pathogenesis.

Identification of residues in the hepatitis C virus core protein that are critical for capsid assembly in a cell-free system

Significant advances have been made in understanding hepatitis C virus (HCV) replication through development of replicon systems. However, neither replicon systems nor standard cell culture systems support significant assembly of HCV capsids, leaving a large gap in our knowledge of HCV virion formation. Recently, we established a cell-free system in which over 60% of full-length HCV core protein synthesized de novo in cell extracts assembles into HCV capsids by biochemical and morphological criteria. Here we used mutational analysis to identify residues in HCV core that are important for capsid assembly in this highly reproducible cell-free system. We found that basic residues present in two clusters within the N-terminal 68 amino acids of HCV core played a critical role, while the uncharged linker domain between them was not. Furthermore, the aspartate at position 111, the region spanning amino acids 82 to 102, and three serines that are thought to be sites of phosphorylation do not appear to be critical for HCV capsid formation in this system. Mutation of prolines important for targeting of core to lipid droplets also failed to alter HCV capsid assembly in the cell-free system. In addition, wild-type HCV core did not rescue assembly-defective mutants. These data constitute the first systematic and quantitative analysis of the roles of specific residues and domains of HCV core in capsid formation.

Expression of bcl-2 protein in chronic hepatitis C: Effect of interferon alpha 2b with ribavirin therapy

AIM: Mechanisms responsible for persistence of HCV infection and liver damage in chronic hepatitis C are not clear. Apoptosis is an important form of host immune response against viral infections. Anti-apoptotic protein bcl-2 expression on liver tissue as well as the influence of interferon alpha 2b (IFNα2b) and ribavirin (RBV) were analyzed in patients with chronic hepatitis C.

METHODS: In 30 patients with chronic hepatitis C (responders - R and non-responders - NR) treated with IFNα2b+RBV, protein bcl-2 was determined in hepatocytes and in liver associated lymphocytes before and after the treatment.

RESULTS: The treatment diminished bcl-2 protein accumulation in liver cells in patients with hepatitis C (P<0.05). Before and after the therapy, we detected bcl-2 protein in R in 87±15% and 83±20% of hepatocytes and in 28±18% and 26±10% of liver-associated lymphocytes, respectively. In NR, the values before treatment decreased from 94±32% to 88±21% of hepatocytes and 39±29% to 28±12% of lymphocytes with bcl-2 expression. There was no statistical correlation between bcl-2 expression on liver tissue with inflammatory activity, fibrosis and biochemical parameters before and after the treatment.

CONCLUSION: IFNα2b+RBV treatment, by bcl-2 protein expression decrease, enables apoptosis of hepatocytes and associated liver lymphocytes, which in turn eliminate hepatitis C viruses.

Proteomic profiling of cellular proteins interacting with the hepatitis C virus core protein

Hepatitis C virus (HCV) is a causative agent of chronic hepatitis and hepatocellular carcinoma. The core protein of HCV packages the viral RNA genome to form a nucleocapsid. In addition to its function as a structural protein, core protein is involved in regulation of cellular transcription, virus-induced transformation, and pathogenesis. To gain insights into cellular functions of the core protein by identification of cellular proteins interacting with the core protein, we employed a proteomic approach. Hepatocytes soluble cytoplasmic proteins were applied to the core proteins immobilized on Ni-nitrilotriacetic resin and total bound cellular proteins were resolved by 2-DE. Analyses of interacting proteins by matrix-assisted laser desorption/ionization-time of flight mass spectrometry allowed identification of 14 cellular proteins binding to the core protein. These proteins include DEAD-box polypeptide 5, similar in function to a known protein identified previously by yeast two-hybrid screening and 13 newly identified cellular proteins. Interestingly, nine protein spots were identified as intermediate microfilament proteins, including cytokeratins (five spots for cytokeratin 8, two for cytokeratin 19, and one for cytokeratin 18) and vimentin. Cytokeratin 8 and vimentin, which were previously shown to be involved in the infection processes of other viruses, were further analyzed to confirm their in vivo interactions with the core protein by immunoblotting and immunofluorescence microscopy. We discuss the functional implications of the interactions of the core protein with newly identified cellular proteins in HCV infection and pathogenesis.

Activation of RB/E2F signaling pathway is required for the modulation of hepatitis C virus core protein-induced cell growth in liver and non-liver cells

Hepatitis C virus (HCV) core protein is a multifunctional protein that affects transcription and cell growth in vitro and in vivo. Here, we confirm the proliferative activities of core protein in liver and non-liver cells and delineate part of the mechanism whereby core protein promotes cell growth. We show that core protein suppresses the expression of tumor suppressor protein p53 and cyclin-dependent kinase (CDK) inhibitor p21 and enhances the activation of cyclin-dependent kinase 2 (CDK2), the phosphorylation of retinoblastoma (Rb), the activation of the transcription factor E2F-1, and the expression of E2F-1 and S phase kinase-interacting protein 2 (SKP2) genes. Pretreatment of core protein-expressing cells with the inhibitor of CDK2, Butyrolactone I, abolished the phosphorylation of Rb, the activation of E2F-1, and inhibited the expression of E2F-1 gene and cell growth induced. Consistent with these findings, we define a new signaling pathway whereby the HCV core protein mediates cell growth in infected cells.

T cell response in hepatitis C virus infection

Hepatitis C virus (HCV) is a hepatotropic RNA virus that causes acute and chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. It is widely accepted that cellular immune responses play an important role in viral clearance and disease pathogenesis. However, HCV often evades effective immune recognition and has a propensity to persist in the majority of acutely infected individuals (ca. 80%). The immunological and virological basis for the inefficiency of the cellular immune response to clear or control the virus is not known. Recent studies, however, have provided new insights into the mechanisms of viral clearance and persistence that will be discussed in detail.

Effects of the hepatitis C virus core protein on innate cellular defense pathways

The hepatitis C virus (HCV) core protein is thought to contribute to HCV pathogenesis through its interaction with various signal transduction pathways. In this study, we explored the interaction of the core protein with innate defense pathways (interferon [IFN] regulatory factor [IRF], Jak-Stat, and inducible nitric oxide synthase [iNOS]) in HeLa and Huh7 human cell lines. Expression of a patient-derived genotype 1b core protein activated human IRF-1 and guanylate-binding protein-2 (GBP-2) promoters, induced IRF-1 mRNA, but failed to induce IRF-3 phosphorylation. HCV core protein caused dose-dependent induction of the IFN-beta promoter and IFN-beta mRNA but not the IFN-alpha1 and IFN-alpha4 promoters. In the presence of IFN-alpha, core expression was associated with increased IFN-stimulated gene factor 3 (ISGF3) binding to the IFN-stimulated response element (ISRE) and tyrosine phosphorylation of Stat1. Core expression resulted in dose-dependent activation of the ISRE and gamma activated sequence (GAS) promoters, in both the absence and the presence of either IFN-alpha or IFN-gamma. Core stimulated the human iNOS promoter and induced iNOS protein. The data indicate that HCV core can modulate IRF, Jak-Stat, and iNOS pathways and suggest mechanisms by which core could affect HCV persistence and pathogenesis.

Targeting of hepatitis C virus core protein to mitochondria through a novel C-terminal localization motif

The hepatitis C virus (HCV) core protein represents the first 191 amino acids of the viral precursor polyprotein and is cotranslationally inserted into the membrane of the endoplasmic reticulum (ER). Processing at position 179 by a recently identified intramembrane signal peptide peptidase leads to the generation and potential cytosolic release of a 179-amino-acid matured form of the core protein. Using confocal microscopy, we observed that a fraction of the mature core protein colocalized with mitochondrial markers in core-expressing HeLa cells and in Huh-7 cells containing the full-length HCV replicon. Subcellular fractionation confirmed this observation and showed that the core protein associates with purified mitochondrial fractions devoid of ER contaminants. The core protein also fractionated with mitochondrion-associated membranes, a site of physical contact between the ER and mitochondria. Using immunoelectron microscopy and in vitro mitochondrial import assays, we showed that the core protein is located on the mitochondrial outer membrane. A stretch of 10 amino acids within the hydrophobic C terminus of the processed core protein conferred mitochondrial localization when it was fused to green fluorescent protein. The location of the core protein in the outer mitochondrial membrane suggests that it could modulate apoptosis or lipid transfer, both of which are associated with this subcellular compartment, during HCV infection.

Immune responses in hepatitis C: is virus or host the problem?

PURPOSE OF REVIEW

Hepatitis C virus is an RNA virus that usually establishes persistent infection in its host. As an important cause of cirrhosis and hepatocellular carcinoma worldwide, hepatitis C is a growing public health concern. Despite recent advances in therapy, most people infected with the virus can expect lifelong infection. In the minority of those exposed and who spontaneously clear virus, a robust hepatitis C virus-specific T cell response of T helper 1 type correlates with resolution. The longevity of this response in the recovered state and the potential for hepatitis C virus-specific T cells to protect against future infection are critical parameters for vaccine design.

RECENT FINDINGS

The literature of the past year dissected components of protective immunity to hepatitis C and emphasized the importance of the CD4 helper response in both the expansion and maintenance of hepatitis C virus-specific CD8(+) T cells. Other important studies examined how the virus interacts with immune cells to subvert both innate and adaptive immune responses in acute and chronic infection.

SUMMARY

Defining the essential components of protective immunity against a highly mutable virus like hepatitis C underpins successful vaccine design. By understanding viral and host factors which influence hepatitis C virus-specific T cell maintenance and function, we are better equipped to devise immunomodulatory therapies and vaccines which induce robust and lasting immunity.

The molecular basis of HCV-mediated immune dysregulation

Chronic hepatitis C virus (HCV) infection, which occurs in over 85% of patients and causes mild to severe liver disease, is a growing burden to health systems worldwide. The propensity of HCV to establish persistent infection suggests that the virus, which is non-cytopathic, has evolved one or more mechanisms aimed at evading host immunity. In addition to the appearance of quasispecies, which may arise under selective pressure during B and T cell responses, HCV gene products interact with host proteins in order to subvert immune surveillance. Gaining insight into these interactions may provide the basis for novel therapies aimed at preventing chronic HCV infection.

Increased prooxidant production and enhanced susceptibility to glutathione depletion in HepG2 cells co-expressing HCV core protein and CYP2E1

Hepatitis C virus (HCV) and HCV core protein are hypothesized to induce hepatic oxidative stress and exacerbate injury caused by other toxins such as ethanol that induce the cytochrome P450 enzyme, CYP2E1. In the current study, the effects of HCV core protein [sequence genotype 1b, (nt 342-915)] on parameters indicative of oxidative stress were evaluated in HepG2 cells stably over expressing CYP2E1 (E47), or vector controls (C34). Stable (>10 passages) expression of HCV core protein and CYP2E1 was confirmed in clonal cell lines at the level of mRNA and immunoreactive protein. Prooxidant production, as determined by cellular oxidation of dichlorodihydrofluorescin and dihydroethidium (HE), was increased by expression of HCV core protein in the presence or absence of CYP2E1. Depletion of glutathione (GSH) with buthionine sulfoximine (BSO) enhanced prooxidant production in both C34 and E47 cells. In addition, prooxidant production was greater in BSO-treated cells expressing HCV core protein, and this effect was further enhanced in cells expressing both HCV core and CYP2E1. The CYP2E1 inhibitor, 4-methylpyrazole, could suppress increased prooxidant production in E47 cells. Finally, cells co-expressing both CYP2E1 and HCV core protein showed significantly decreased viability following GSH depletion. These studies show simultaneous expression of HCV core protein and CYP2E1 increases parameters indicative of oxidative stress as well as sensitization to cell injury induced by GSH depletion. These results support the hypothesis that enhanced injury in hepatocytes over expressing both HCV core protein and CYP2E1 is mediated by increases in oxidative stress.

Activation of the CKI-CDK-Rb-E2F Pathway in Full Genome Hepatitis C Virus-expressing Cells

Hepatitis C virus (HCV) causes persistent infection in hepatocytes, and this infection is, in turn, strongly associated with the development of hepatocellular carcinoma. To clarify the mechanisms underlying these effects, we established a Cre/loxP conditional expression system for the precisely self-trimmed HCV genome in human liver cells. Passage of hepatocytes expressing replicable full-length HCV (HCR6-Rz) RNA caused up-regulation of anchorage-independent growth after 44 days. In contrast, hepatocytes expressing HCV structural, nonstructural, or all viral proteins showed no significant changes after passage for 44 days. Only cells expressing HCR6-Rz passaged for 44 days displayed acceleration of CDK activity, hyperphosphorylation of Rb, and E2F activation. These results demonstrate that full genome HCV expression up-regulates the CDK-Rb-E2F pathway much more effectively than HCV proteins during passage.

Hepatitis C virus core protein upregulates transforming growth factor-beta 1 transcription

The majority of persons with chronic hepatitis C virus (HCV) infection develop liver fibrosis. Transforming growth factor (TGF)-beta 1 plays a pivotal role in the pathogenesis of post-inflammatory liver scarring. To clarify the influence of HCV infection on liver fibrosis, a reporter assay was used to investigate the effect of viral proteins on TGF-beta 1 expression in human hepatoma cells. Of all HCV proteins investigated (core, E1/E2/p7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B), only the core protein activated the TGF-beta 1 promoter and upregulated TGF-beta 1 expression measured by an RNase protection assay. Bases -376 to -331 bp in the promoter region of TGF-beta 1 are responsible for upregulation by HCV core protein, and the nuclear protein that binds to this region increased with the stimulation of HCV core protein. Blocking the mitogen-activated protein kinase pathway prevented upregulation of TGF-beta 1 by HCV core protein. The immunological response is supposed to be a major factor to cause the secretion of TGF-beta 1 from non-parenchymal cells, but the results suggest that the HCV core protein expression may upregulate directly TGF-beta 1 transcription in parenchymal cells and suggest a new paradigm for exacerbation of liver fibrosis by HCV infection.

Induction of FAS ligand expression in a human hepatoblastoma cell line by HCV core protein

Tumour cells and virus infected cells expressing Fas ligand (FasL) can evade immune surveillance by inducing apoptosis in T cells expressing Fas. In order to characterise a possible role of hepatitis C virus (HCV) core protein in similar mechanisms during HCV infection, we investigated Fas ligand expression and activity in a human hepatoblastoma cell line (HepG2) constitutively expressing this protein. Strong FasL induction was detected by immunoblotting and flow cytometry analysis in the core expressing cell lines Hep39. In contrast, vector transfected cells or cell lines expressing HCV E1-E2 proteins did not show FasL expression. Co-cultivation experiments of Hep39 cells with a Fas-sensitive T cell line indicated that FasL induced by the core protein had apoptotic activity toward target cells. Effect of the core protein on induction of FasL promoter was further examined by co-transfection of HepG2 cells with core-bearing plasmid and a vector in which luciferase gene expression is driven by human FasL promoter. Results of the luciferase assay indicated a positive regulation of FasL promoter by the core protein. In conclusion, HCV core protein plays a role in the induction of functional FasL in hepatoblastoma cell line and apoptosis in a target T cell line expressing Fas. Similar mechanisms may contribute, in vivo, to establishment of chronic infection and development of hepatocellular carcinoma (HCC).

Hepatitis C virus core functions as a suppressor of cyclin-dependent kinase-activating kinase and impairs cell cycle progression

We investigated how the hepatitis C virus (HCV) core protein affects the cell cycle profile and cell cycle-related molecules by using the HCV core-expressing stable transfectant. Analysis of the cell cycle profile showed that HCV core impaired G(1) to S transition. The E2F-mediated transcription, phosphorylation of the retinoblastoma protein, and cyclin-dependent kinase (CDK) 4 and CDK2 activities were suppressed in HCV core-expressing cells. The expression levels of G(1) phase-related CDKs/cyclins and various CDK inhibitors were not substantially affected by expression of HCV core. When influences of HCV core on CDK-activating kinase (CAK) were examined, the expression levels of the CAK components, CDK7, cyclin H, and MAT1, were not affected. However, formation of the ternary CAK complex, CAK activity, and the CDK2 level with activating phosphorylation were inhibited by expression of the HCV core. The direct effect of HCV core on CAK was further assessed in the cell-free system by adding the in vitro translated HCV core protein to the anti-CDK7 immunoprecipitate from the cell. The results showed that HCV core led to dissociation of MAT1 from the CAK complex and suppressed the CAK activity. Furthermore, the binding assay revealed that the HCV core was directed against CDK7. Their interaction occurred mainly in the nucleus by the immunostaining. In conclusion, the HCV core protein interacts with CAK and functions as an extrinsic suppressor of CAK. This may be the molecular basis of HCV core-mediated suppression of cell cycle progression. Our findings suggest a novel mechanism concerning HCV core-mediated alteration in the cell cycle machinery.

Fcgamma receptor-like activity of hepatitis C virus core protein

We have previously demonstrated that viral particles with the properties of nonenveloped hepatitis C virus (HCV) nucleocapsids occur in the serum of HCV-infected individuals (1). We show here that nucleocapsids purified directly from serum or isolated from HCV virions have FcgammaR-like activity and bind "nonimmune" IgG via its Fcgamma domain. HCV core proteins produced in Escherichia coli and in the baculovirus expression system also bound "nonimmune" IgG and their Fcgamma fragments. Folded conformation was required for IgG binding because the FcgammaR-like site of the core protein was inactive in denaturing conditions. Studies with synthetic core peptides showed that the region spanning amino acids 3-75 was essential for formation of the IgG-binding site. The interaction between the HCV core and human IgG is more efficient in acidic (pH 6.0) than in neutral conditions. The core protein-binding site on the IgG molecule differs from those for C1q, FcgammaRII (CD32), and FcgammaRIII (CD16) but overlaps with that for soluble protein A from Staphylococcus aureus (SpA), which is located in the CH2-CH3 interface of IgG. These characteristics of the core-IgG interaction are very similar to those of the neonatal FcRn. Surface plasmon resonance studies suggested that the binding of an anti-core antibody to HCV core protein might be "bipolar" through its paratope to the corresponding epitope and by its Fcgamma region to the FcgammaR-like motif on this protein. These features of HCV nucleocapsids and HCV core protein may confer an advantage for HCV in terms of survival by interfering with host defense mechanisms mediated by the Fcgamma part of IgG.

Hepatitis B virus X protein activates a survival signaling by linking SRC to phosphatidylinositol 3-kinase

We have previously shown that transactivation-proficient hepatitis virus B X protein (HBx) protects Hep 3B cells from transforming growth factor-beta (TGF-beta)-induced apoptosis via activation of the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt signaling pathway. This work further investigated how HBx activates PI 3-kinase. Src activity was elevated in Hep 3B cells following expression of transactivation-proficient HBx or HBx-GFP fusion proteins. The Src family kinase inhibitor PP2 and C-terminal Src kinase (Csk) both alleviated HBx-mediated PI 3-kinase activation and protection from TGF-beta-induced apoptosis. Therefore, HBx activated a survival signal by linking Src to PI 3-kinase. Systemic subcellular fractionation and membrane flotation assays indicated that approximately 1.5% of ectopically expressed HBxGFP was associated with periplasmic membrane where Src was located. However, neither nucleus-targeted nor periplasmic membrane-targeted HBxGFP was able to upregulate Src activity or to augment PI 3-kinase survival signaling pathway.

Molecular viral oncology of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the fifth most common cancer, but the third leading cause of cancer death, in the world, with more than 500,000 fatalities annually. The major etiology of HCC/liver cancer in people is hepatitis B virus (HBV), followed by hepatitis C virus infection (HCV), although nonviral causes also play a role in a minority of cases. Recent molecular studies confirm what was suspected: that HCC tissue from different individuals have many phenotypic differences. However, there are clearly features that unify HCC occurring in a background of viral hepatitis B and C. HCC due to HBV and HCV may be an indirect result of enhanced hepatocyte turnover that occurs in an effort to replace infected cells that have been immunologically attacked. Viral functions may also play a more direct role in mediating oncogenesis. This review considers the molecular and cellular mechanisms involved in primary hepatocellular carcinoma, using a viral perspective.

Subversion of immune responses by hepatitis C virus: immunomodulatory strategies beyond evasion?

Hepatitis C virus (HCV) is an important human pathogen that causes mild to severe liver disease worldwide. This positive-strand RNA virus is remarkably efficient at establishing persistent infection. In order for a non-cytopathic virus such as HCV to persist, the virus must escape immune recognition or inhibit the host immune responses. Immune escape via mutations in antigenic sites may occur under selective pressure during B-cell or T-cell responses to HCV infection, and may serve as a mechanism for the establishment HCV persistence. In addition to antigenic variation, HCV is able to subvert the host immune response by encoding specific viral gene product(s). An understanding of the mechanisms behind HCV persistence will provide a basis for the rational design of vaccines and novel therapeutic agents targeting human HCV infection.

The C-terminal region of hepatitis C core protein is required for Fas-ligand independent apoptosis in Jurkat cells by facilitating Fas oligomerization

Hepatitis C virus (HCV) is remarkable for its ability to establish persistent infection. Studies suggest that HCV core protein modulates immune responses to viral infection and can bind Fas receptor in vitro. To further examine the role of HCV core protein in Fas signaling, full-length (aa 1-192) and truncated (aa 1-152) HCV core proteins were expressed in Jurkat lymphocytes and cells were assayed for apoptotic response, caspase activation, and Fas activation. Jurkat expressing full-length but not truncated core protein exhibited ligand-independent apoptosis. Cytoplasmic targeting of truncated core protein recapitulated its ability to induce apoptosis. Activation of caspases 8 and 3 was necessary and sufficient for full-length core to induce apoptosis. Jurkat cells expressing full-length but not truncated core protein induced Fas receptor aggregation. HCV core activates apoptotic pathways in Jurkat via Fas and requires cytoplasmic localization of core. Infection of host lymphocytes by HCV may alter apoptotic signaling and skew host responses to acute infection.

Acute hepatitis C in a chronically HIV-infected patient: evolution of different viral genomic regions

AIM

To analyze the molecular evolution of different viral genomic regions of HCV in an acute HCV infected patient chronically infected with HIV through a 42-month follow-up.

METHODS

Serum samples of a chronically HIV infected patient that seroconverted to anti HCV antibodies were sequenced, from the event of superinfection through a period of 17 months and in a late sample (42nd month). Hypervariable genomic regions of HIV (V3 loop of the gp120) and HCV (HVR-1 on the E2 glycoprotein gene) were studied. In order to analyze genomic regions involved in different biological functions and with the cellular immune response, HCV core and NS5A were also chosen to be sequenced. Amplification of the different regions was done by RT-PCR and directly sequenced. Confirmation of sequences was done on reamplified material. Nucleotide sequences of the different time points were aligned with CLUSTAL W 1.5, and the corresponding amino acid ones were deduced.

RESULTS

Hypervariable genomic regions of both viruses (HVR1 and gp120 V3 loop) presented several nonsynonymous changes but, while in the gp120 V3 loop mutations were detected in the sample obtained right after HCV superinfection and maintained throughout, they occurred following a sequential and cumulative pattern in the HVR1. In the NS5A region of HCV, two amino acid changes were detected during the follow-up period, whereas the core region presented several amino acid replacements, once the HCV chronic infection had been established.

CONCLUSION

During the HIV-HCV superinfection, each genomic region analyzed shows a different evolutionary pattern. Most of the nucleotide substitutions observed are non-synonymous and clustered in previously described epitopes, thus suggesting an immune-driven evolutionary process.

The hepatitis C virus core protein modulates T cell responses by inducing spontaneous and altering T-cell receptor-triggered Ca2+ oscillations

Alterations of cytokine responses are thought to favor the establishment of persistent hepatitis C virus (HCV) infection, enhancing the risk of liver cirrhosis and hepatocellular carcinoma. Expression of the HCV core (C) protein modulates transcription of the IL-2 promoter in T lymphocytes by activating the nuclear factor of activated T lymphocyte (NFAT) pathway. Here we report on the effect of HCV C on Ca2+ signaling, which is essential for activation of NFAT. Expression of HCV C correlated with increased levels of cytosolic Ca2+ and spontaneous Ca2+ oscillations in transfected Jurkat cells. Triggering of the T-cell receptor induced a prolonged Ca2+ response characterized by vigorous high frequent oscillations in a high proportion of the responding cells. This was associated with decreased sizes and accelerated emptying of the intracellular calcium stores. The effect of HCV C on calcium mobilization was not dependent on phospholipase C-gamma 1 (PLC-gamma) activity or increased inositol 1,4,5-trisphosphate (IP3) production and did not require functional IP3 receptors, suggesting that insertion of the viral protein in the endoplasmic reticulum membrane may be sufficient to promote Ca2+ leakage with dramatic downstream consequences on the magnitude and duration of the response. Our data suggest that expression of HCV C in infected T lymphocytes may contribute to the establishment of persistent infections by inducing Ca2+ oscillations that regulate both the efficacy and information content of Ca2+ signals and are ultimately responsible for induction of gene expression and functional differentiation.

The hepatitis C virus NS2 protein is an inhibitor of CIDE-B-induced apoptosis

Chronic hepatitis C virus (HCV) infection frequently leads to liver cancer. To determine the viral factor(s) potentially involved in viral persistence, we focused our work on NS2, a viral protein of unknown function. To assign a role for NS2, we searched for cellular proteins that interact with NS2. Performing a two-hybrid screen on a human liver cDNA library, we found that NS2 interacted with the liver-specific pro-apoptotic CIDE-B protein. Binding specificity of NS2 for CIDE-B was confirmed by cell-free assays associated with colocalization studies and coprecipitation experiments on human endogenous CIDE-B. CIDE-B, a member of the novel CIDE family of apoptosis-inducing factors, has been reported to show strong cell death-inducing activity in its C-terminal domain. We show that this CIDE-B killing domain is involved in the NS2 interaction. NS2 binding was sufficient to inhibit CIDE-B-induced apoptosis because an NS2 deletion mutant unable to interact with CIDE-B in vitro lost its capacity to interfere with CIDE-B cell death activity. Although it has been reported that CIDE-B-induced apoptosis is characterized by mitochondrial localization, the precise apoptotic mechanism remained unknown. Here, we show that CIDE-B induced cell death in a caspase-dependent manner through cytochrome c release from mitochondria. Furthermore, we found that NS2 counteracted the cytochrome c release induced by CIDE-B. In vivo, the CIDE-B protein level was extremely low in adenovirus-infected transgenic mice expressing the HCV polyprotein compared with that in wild-type mice. We suggest that NS2 interferes with the CIDE-B-induced death pathway and participates in HCV strategies to subvert host cell defense.

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.

Interaction between hepatitis C virus core protein and translin protein- a possible molecular mechanism for hepatocellular carcinoma and lymphoma caused by hepatitis C virus

AIM: To investigate the interaction between hepatitis C virus core protein and translin protein and its role in the pathogenensis of hepatocellular carcinoma and lymphoma.

METHODS: With the components of the yeast two hybrid system 3,“ bait” plasmids of HCV core the gene was constructed. After proving that hepatitis C virus core protein could be firmly expressed in AH109 yeast strains, yeast two- hybrid screening was performed by mating AH109 with Y187 that transformed with liver cDNA library plasmids - pACT2 and then plated on quadrople dropout (QDO) medium and then assayed for α-gal activity. Sequencing analysis of the genes of library plasmids in yeast colonies that could grow on QDO with α-gal activity was performed. The interaction between HCV core protein and the protein we obtained from positive colony was further confirmed by repeating yeast two - hybrid analysis and coimmunoprecipitation in vitro.

RESULTS: A gene from a positive colony was the gene of translin, a recombination hotspot binding protein. The interaction between HCV core protein and translin protein could be proved not only in yeast, but also in vitro.

CONCLUSION: The core protein of HCV can interact with translin protein. This can partly explain the molecular mechanism for hepatocellular carcinoma and lymphoma caused by HCV.

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.

Mechanisms for inhibition of hepatitis B virus gene expression and replication by hepatitis C virus core protein

We have demonstrated previously that the core protein of hepatitis C virus (HCV) exhibits suppression activity on gene expression and replication of hepatitis B virus (HBV). Here we further elucidated the suppression mechanism of HCV core protein. We demonstrated that HCV core protein retained the inhibitory effect on HBV gene expression and replication when expressed as part of the full length of HCV polyprotein. Based on the substitution mutational analysis, our results suggested that mutation introduced into the bipartite nuclear localization signal of the HCV core protein resulted in the cytoplasmic localization of core protein but did not affect its suppression ability on HBV gene expression. Mutational studies also indicated that almost all dibasic residue mutations within the N-terminal 101-amino acid segment of the HCV core protein (except Arg(39)-Arg(40)) impaired the suppression activity on HBV replication but not HBV gene expression. The integrity of Arg residues at positions 101, 113, 114, and 115 was found to be essential for both suppressive effects, whereas the Arg residue at position 104 was important only in the suppression of HBV gene expression. Moreover, our results indicated that the suppression on HBV gene expression was mediated through the direct interaction of HCV core protein with the trans-activator HBx protein, whereas the suppression of HBV replication involved the complex formation between HBV polymerase (pol) and the HCV core protein, resulting in the structural incompetence for the HBV pol to bind the package signal and consequently abolished the formation of the HBV virion. Altogether, this study suggests that these two suppression effects on HBV elicited by the HCV core protein likely depend on different structural context but not on nuclear localization of the core protein, and the two effects can be decoupled as revealed by its differential targets (HBx or HBV pol) on these two processes of the HBV life cycle.

Alteration of intrahepatic cytokine expression and AP-1 activation in transgenic mice expressing hepatitis C virus core protein

Hepatitis C virus (HCV) infection often leads to the development of hepatocellular carcinoma (HCC), but its molecular mechanism has not been clearly elucidated. Previously, transgenic mice constitutively expressing HCV core protein have been shown to develop HCC, suggesting a pivotal role of the core protein in hepatocarcinogenesis. Here, we analyzed the expression of cytokines associated with a variety of cellular processes, including cell proliferation, in the mouse model for HCV-associated HCC to define the molecular events prior to oncogenesis. The expression of tumor necrosis factor-alpha and interleukin-1beta was increased at both protein and mRNA levels. In addition, the activities of c-Jun N-terminal kinase and activator protein-1 (AP-1), downstream effectors, were enhanced, while IkappaB kinase or nuclear factor-kappaB activities were not enhanced. Thus, the altered in vivo expression of cytokines with AP-1 activation in consequence to the core protein expression may contribute to hepatocarcinogenesis in persistent HCV infection.

Virus-specific CD8+ lymphocytes share the same effector-memory phenotype but exhibit functional differences in acute hepatitis B and C

Hepatitis B and hepatitis C viruses (HBV and HCV) are both noncytopathic and can cause acute and chronic infections of the liver. Although they share tropism for the same organ, development of chronic hepatitis is much more frequent following HCV infection, suggesting different mechanisms of viral persistence. In this study, we show that circulating HBV- and HCV-specific tetramer-positive CD8 cells during the acute phase of hepatitis B and C belong almost entirely to an effector-memory subset (CCR7(-) CD45RA(-)). Despite this phenotypic similarity, HBV- and HCV-specific CD8 cells show striking functional differences. HBV-specific tetramer-positive CD8 cells express high perforin content ex vivo, expand vigorously, and display efficient cytotoxic activity and gamma interferon (IFN-gamma) production upon peptide stimulation. A comparable degree of functional efficiency is maintained after the resolution of hepatitis B. In contrast, HCV-specific CD8 cells in the acute phase of hepatitis C express significantly lower levels of perforin molecules ex vivo and show depressed CD8 function in terms of proliferation, lytic activity, and IFN-gamma production, irrespective of the final outcome of the disease. This defect is transient, because HCV-specific CD8 cells can progressively improve their function in patients with self-limited hepatitis C, while the CD8 function remains persistently depressed in subjects with a chronic evolution.

Hepatitis C virus core protein leads to immune suppression and liver damage in a transgenic murine model

Hepatitis C virus (HCV) is remarkably efficient in establishing persistent infection, possibly mediated by an impaired immune response to HCV infection. There is compelling evidence that HCV can infect immune cells, such as macrophages, B cells, and T cells. It has been previously reported that HCV core, the first protein expressed during the early phase of viral infection, contains the immunomodulatory function of suppressing host immune responses. This altered function of immune cells caused by HCV infection may explain the ineffective immune response to HCV. To further characterize the immunomodulatory role of HCV core in vivo, we generated transgenic (TG) mice by directing the expression of core protein to T lymphocytes by using the CD2 promoter. T-lymphocyte responses, including the production of gamma interferon and interleukin-2, were significantly diminished in these mice compared to their non-TG littermates. The inhibition of T-lymphocyte responsiveness may be due to the increased susceptibility of peripheral T lymphocytes to Fas-mediated apoptosis. Surprisingly, significant lymphocyte infiltration was observed in the portal tracts of livers isolated from core TG mice, associated with increasing serum alanine aminotransferase levels. Moreover, no intrahepatic lymphocytes or liver damage was found in non-TG littermates and core TG mice bred to Fas-deficient lpr mice. These results suggest that HCV core drives liver injury by increasing Fas-mediated apoptosis and liver infiltration of peripheral T cells.

Hepatitis C virus: immunosuppression by complement regulatory pathway

Hepatitis C virus (HCV) infection in humans is almost invariably associated with viral persistence and chronic hepatitis. HCV-induced chronic hepatitis is a major risk factor for the development of hepatocellular carcinoma. The high incidence of HCV persistence suggests that this virus has evolved one or more mechanisms to evade and possibly suppress host immune responses. To understand the mechanism(s) involved in the establishment of HCV persistence, we have identified an HCV core protein as an immunomodulatory molecule to suppress host immune response. We have further determined a molecular mechanism of HCV core-mediated immune suppression by searching for a potential host protein(s) capable of associating with the HCV core protein. Interestingly, the Clq complement receptor, gC1qR, can bind to the HCV core. Clq is a ligand of gClqR and is involved in the early defense against viral infection as well as regulation of adaptive immune response. Similar to Clq, the HCV core can inhibit human T-lymphocyte proliferative response through its interaction with the gC1qR. It implicates that HCV core/gClqR-induced immune suppression may play a critical role in the establishment of persistent infection.

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.

Lack of in vivo blockade of Fas- and TNFR1-mediated hepatocyte apoptosis by the hepatitis C virus

In vitro data have shown that the hepatitis C virus (HCV) core protein binds to protein members of the tumour necrosis factor receptor (TNFR) superfamily. Since this interaction could be relevant to HCV persistence and oncogenesis, this study assessed whether HCV may interfere with the apoptotic cascade in vivo. Apoptosis (by TUNEL) and Fas and TNFR1 expression (by immunohistochemistry) were scored in the liver of 60 chronic hepatitis C patients. Results were compared with the liver disease grading and staging scores and the HCV replication level in serum and liver. Apoptotic hepatocytes were stained in 29 cases. Fas was expressed in 35 cases and TNFR1 in 21, 15 patients (25%) being negative for both receptors. Overall, the numbers of TUNEL-, Fas- and TNFR-positive hepatocytes did not correlate with the extent of intrahepatic CD8+ T-lymphocyte infiltration, the grading and staging of liver disease, or the serum or liver HCV RNA levels. Furthermore, when patients expressing either Fas or TNFR1 were stratified according to serum HCV RNA levels, cases with detectable hepatocyte apoptosis had higher HCV viraemias. In conclusion, an HCV-mediated, in vivo blockade of hepatocyte apoptosis via the Fas- or TNFR1-dependent pathways seems unlikely.

Human eukaryotic initiation factor 4AII associates with hepatitis C virus NS5B protein in vitro

Hepatitis C virus (HCV) NS5B protein has been shown to have RNA-dependent RNA polymerase (RdRp) activity by itself and is a key enzyme involved in viral replication. Using analyses with the yeast two-hybrid system and in vitro binding assay, we found that human eukaryotic initiation factor 4AII (heIF4AII), which is a component of the eIF4F complex and RNA-dependent ATPase/helicase, interacted with NS5B protein. These two proteins were shown to be partially colocalized in the perinuclear region. The binding site in HCV NS5B protein was localized within amino acid residues 495 to 537 near the C terminus. Since eIF4A has a helicase activity and functions in a bidirectional manner, the binding of HCV NS5B protein to heIF4AII raises the possibility that heIF4AII facilitates the genomic RNA synthesis of NS5B protein by unwinding the secondary structure of the HCV genome and is a host component of viral replication complex.

Detection of a novel unglycosylated form of hepatitis C virus E2 envelope protein that is located in the cytosol and interacts with PKR

The hepatitis C virus (HCV) envelope protein E2 has been shown to accumulate in the lumen of the endoplasmic reticulum (ER) as a properly folded glycoprotein as well as large aggregates of misfolded proteins. In the present study, we have identified an additional unglycosylated species, with an apparent molecular mass of 38 kDa (E2-p38). In contrast to the glycosylated E2, E2-p38 is significantly less stable and is degraded through the proteasome pathway. Correspondingly, E2-p38 is found to be ubiquitinated. E2-p38 is localized mostly in the cytosol, in contrast to the glycosylated form, which is exclusively membrane associated. Alpha interferon (IFN-alpha) treatment or overexpression of the double-stranded RNA-activated protein kinase (PKR) significantly increased the stability of E2-p38, consistent with a previous report (D. R. Taylor, S. T. Shi, P. R. Romano, G. N. Barber, and M. M. Lai, Science 285:107-110, 1999) that E2 interacts with PKR and inhibits its kinase activity. Direct interaction between PKR and E2-p38, but not the glycosylated form of E2, was also observed. These results show that E2-p38 is the form of E2 that interacts with PKR in the cytosol and may contribute to the resistance of HCV to IFN-alpha. Thus, an ER protein can exist in the cytosol as an unglycosylated species and impair cellular functions.

Mitochondrial injury, oxidative stress, and antioxidant gene expression are induced by hepatitis C virus core protein

BACKGROUND & AIMS

The mechanisms of liver injury in chronic hepatitis C virus (HCV) infection are poorly understood. Indirect evidence suggests that oxidative stress and mitochondrial injury play a role. The aim of this study was to determine if the HCV core protein itself alters mitochondrial function and contributes to oxidative stress.

METHODS

HCV core protein was expressed in 3 different cell lines, and reactive oxygen species (ROS) and lipid peroxidation products were measured.

RESULTS

Core expression uniformly increased ROS. In 2 inducible expression systems, core protein also increased lipid peroxidation products and induced antioxidant gene expression as well. A mitochondrial electron transport inhibitor prevented the core-induced increase in ROS. A fraction of the expressed core protein localized to the mitochondria and was associated with redistribution of cytochrome c from mitochondrial to cytosolic fractions. Sensitivity to oxidative stress was also seen in HCV transgenic mice in which increased intrahepatic lipid peroxidation products occurred in response to carbon tetrachloride.

CONCLUSIONS

Oxidative injury occurs as a direct result of HCV core protein expression both in vitro and in vivo and may involve a direct effect of core protein on mitochondria. These results provide new insight into the pathogenesis of hepatitis C and provide an experimental rationale for investigation of antioxidant therapy.

Hepatitis C virus core protein induces cell proliferation and activates ERK, JNK, and p38 MAP kinases together with the MAP kinase phosphatase MKP-1 in a HepG2 Tet-Off cell line

Hepatitis C virus (HCV) core protein is a multifunctional protein interacting with cellular and viral proteins and promoters. A tetracycline-regulated system was used to generate a HepG2 Tet-Off cell line allowing regulated expression of a full-length (191 aa) and an N(c)-truncated core protein (160 aa). In this system HCV core protein expression activates extracellular signal-regulated kinase (ERK), c-jun N-terminal kinase (JNK), and p38 mitogen-activated protein (MAP) kinase, induces MAP kinase phosphatase MKP-1 expression, and increases cell proliferation. This was accompanied by an activation of c-Jun and ATF-2, but not Elk-1 and c-Fos. Furthermore, AP-1 activation was independent of c-Fos. Full-length and N(c)-truncated HCV core proteins exerted similar effects.

Hepatitis C virus core and envelope proteins do not suppress the host's ability to clear a hepatic viral infection

Several hepatitis C virus (HCV) proteins have been shown in vitro to interact with host cellular components that are involved in immune regulation. However, there is a paucity of data supporting the relevance of these observations to the in vivo situation. To test the hypothesis that such an interaction suppresses immune responses, we studied a line of transgenic C57BL/6 mice that express the HCV core and envelope proteins in the liver. The potential effects of these proteins on the hepatic immune response were evaluated by challenging these mice with a hepatotropic adenovirus. Both transgenic and nontransgenic mice developed similar courses of infection and cleared the virus from the liver by 28 days postinfection. Both groups of mice mounted similar immunoglobulin G (IgG), IgG2a, interleukin-2, and tumor necrosis factor alpha responses against the virus. Additionally, BALB/c mice were able to clear infection with recombinant adenovirus that does or does not express the HCV core and envelope 1 proteins in the same manner. These data suggest that HCV core and envelope proteins do not inhibit the hepatic antiviral mechanisms in these murine experimental systems and thus favor a model in which HCV circumvents host responses through a mechanism that does not involve general suppression of intrahepatic immune responses.

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.

GERp95 belongs to a family of signal-transducing proteins and requires Hsp90 activity for stability and Golgi localization

GERp95 (Golgi-endoplasmic reticulum protein 95 kDa) is part of a large family of highly conserved proteins found in all metazoans and the fission yeast Schizosaccharomyces pombe. Genetic studies suggest that homologs of GERp95 are components of signaling pathways that regulate cellular differentiation, development, and RNA interference. However, the precise molecular functions of these proteins remain unknown. Genetic analysis of GERp95 homologs has been complicated by the presence of multiple genes with overlapping functions in most organisms. Binding partners for members of this protein family have not been identified. The purpose of this study was to identify proteins that associate with GERp95. Glutathione S-transferase-GERp95 fusions were expressed in transfected cells, and proteins that bound to GERp95 were co-purified using glutathione-agarose beads. The amino-terminal region of GERp95 was found to interact with the specialized chaperone Hsp90 and a number of its cognate binding proteins. Inhibition of Hsp90 activity with geldanamycin or radicicol resulted in rapid degradation of newly synthesized GERp95. The membrane-associated pool of GERp95 was not bound to Hsp90, although activity of this chaperone was required for stable association of GERp95 with the Golgi in normal rat kidney cells. These results indicate that GERp95 engages an Hsp90 chaperone complex prior to association with intracellular membranes.

Virus targeting of the tumor necrosis factor superfamily

Herpesviruses appear to peacefully coexist with their natural hosts, with infection typically manifested as a benign, but lifelong process. However, coexistence depends on active resistance by innate and specific immune defenses as revealed in the striking virulence of herpesviruses when immunity fails. This pattern of infection is characteristic of a viral pathogen, such as cytomegalovirus, that has evolved efficient strategies targeted at host defense systems. Targeting members of the tumor necrosis factor (TNF)/lymphotoxin (LT) superfamily of cytokines is a strategy found in all herpesviruses, which suggests the existence of an intimate evolutionary link in their host-parasite relationship. Here we examine some of the strategies used by herpesvirus that target members of the TNF superfamily and discuss a recent study that revealed a novel mechanism that links LT-related ligands and interferons (IFN) to the establishment of coexistence between herpesvirus and its host cell.

Nonenveloped nucleocapsids of hepatitis C virus in the serum of infected patients

One of the characteristics of hepatitis C virus (HCV) is the high incidence of persistent infection. HCV core protein, in addition to forming the viral nucleocapsid, has multiple regulatory functions in host-cell transcription, apoptosis, cell transformation, and lipid metabolism and may play a role in suppressing host immune response. This protein is thought to be present in the bloodstream of the infected host as the nucleocapsid of infectious, enveloped virions. This study provides evidence that viral particles with the physicochemical, morphological, and antigenic properties of nonenveloped HCV nucleocapsids are present in the plasma of HCV-infected individuals. These particles have a buoyant density of 1.32 to 1.34 g/ml in CsCl, are heterogeneous in size (with predominance of particles 38 to 43 or 54 to 62 nm in diameter on electron microscopy), and express on their surface epitopes located in amino acids 24 to 68 of the core protein. Similar nucleocapsid-like particles are also produced in insect cells infected with recombinant baculovirus bearing cDNA for structural HCV proteins. HCV core particles isolated from plasma were used to generate anti-core monoclonal antibodies (MAbs). These MAbs stained HCV core in the cytoplasm of hepatocytes from experimentally infected chimpanzees in the acute phase of the infection. These chimpanzees had concomitantly HCV core antigen in serum. These findings suggest that overproduction of nonenveloped nucleocapsids and their release into the bloodstream are properties of HCV morphogenesis. The presence of circulating cores in serum and accumulation of the core protein in liver cells during the early phase of infection may contribute to the persistence of HCV and its many immunopathological effects in the infected host.

Hepatitis C virus core protein: intriguing properties and functional relevance

Hepatitis C virus (HCV) often causes a prolonged and persistent infection, and an association between hepatocellular carcinoma (HCC) and HCV infection has been noted. The pathogenesis of liver damage is at least in part related to virus-mediated factors. Understanding the molecular basis of pathogenesis is a major challenge in gaining insight into HCV-associated disease progression. Recent experimental evidence using HCV cloned genomic regions suggests that the core protein has numerous functional activities. These include its likely role in encapsidation of viral RNA, a regulatory effect on cellular and unrelated viral promoters, interactions with a number of cellular proteins, an modulatory role in programmed cell death or apoptosis under certain conditions, involvement in cell growth promotion and immortalization, induction of HCC in transgenic mice, and a possible immunoregulatory role. These intriguing properties suggest that the core protein, in concert with cellular factors, may contribute to pathogenesis during persistent HCV infection.

Cytoplasmic localization is important for transcription factor nuclear factor-kappa B activation by hepatitis C virus core protein through its amino terminal region

We previously reported that hepatitis C virus core protein (core) activates the transcription factor nuclear factor-kappa B (NF-kappa B) when expressed transiently. In the present study, we investigated the relationship between the NF-kappa B activation capacity and subcellular localization of the core. By changing the subcellular localization of the C-terminally truncated core from the nucleus to the cytoplasm, NF-kappa B was activated. In addition, NF-kappa B activity was augmented by forcing the mutated core to move to the endoplasmic reticulum. It was also suggested that the region from aa 21 to 80 of the core is involved in the activation of NF-kappa B.