Hepatitis C virus NS5A protein interacts with and negatively regulates the non-receptor protein tyrosine kinase Syk

The spleen tyrosine kinase (SYK): A crucial therapeutic target for diverse liver diseases

Spleen tyrosine kinase (SYK) is an enigmatic protein tyrosine kinase, and involved in signal transduction related with lots of cellular processes. It's highly expressed in the cells of hematopoietic origin and acts as an important therapeutic target in the treatment of autoimmune diseases and allergic disorders. In recent years, more and more evidences indicate that SYK is expressed in non-hematopoietic cells and effectively regulates various non-immune biological responses as well. In this review, we mainly summary the role of SYK in different liver diseases. Robust SYK expression has been discovered in hepatocytes, hepatic stellate cells, as well as Kupffer cells, which participates in the regulation of numerous signal transduction in various liver diseases (e.g. hepatitis, liver fibrosis and hepatocellular carcinoma). In addition, the blockage of SYK activity using small molecule modulators is considered as a significant therapeutic strategy against liver diseases, and both hepatic SYK and non-hepatic SYK could become highly promising therapeutic targets. Totally, even though some critical points about the significance of SYK in liver diseases treatment still need further elaboration, more reliable biotechnical or pharmacological therapy modes will be established based on the better understanding of the relationship between SYK and liver diseases.

Role of spleen tyrosine kinase in liver diseases

Spleen tyrosine kinase (SYK) is a non-receptor tyrosine kinase expressed in most hematopoietic cells and non-hematopoietic cells and play a crucial role in both immune and non-immune biological responses. SYK mediate diverse cellular responses via an immune-receptor tyrosine-based activation motifs (ITAMs)-dependent signalling pathways, ITAMs-independent and ITAMs-semi-dependent signalling pathways. In liver, SYK expression has been observed in parenchymal (hepatocytes) and non-parenchymal cells (hepatic stellate cells and Kupffer cells), and found to be positively correlated with the disease severity. The implication of SYK pathway has been reported in different liver diseases including liver fibrosis, viral hepatitis, alcoholic liver disease, non-alcoholic steatohepatitis and hepatocellular carcinoma. Antagonism of SYK pathway using kinase inhibitors have shown to attenuate the progression of liver diseases thereby suggesting SYK as a highly promising therapeutic target. This review summarizes the current understanding of SYK and its therapeutic implication in liver diseases.

Hepatitis C Virus NS5A Protein Promotes the Lysosomal Degradation of Hepatocyte Nuclear Factor 1α via Chaperone-Mediated Autophagy

Hepatitis C virus (HCV) infection is closely associated with type 2 diabetes. We reported that HCV infection induces the lysosomal degradation of hepatocyte nuclear factor 1 alpha (HNF-1α) via interaction with HCV nonstructural protein 5A (NS5A) protein, thereby suppressing GLUT2 gene expression. The molecular mechanisms of selective degradation of HNF-1α caused by NS5A are largely unknown. Chaperone-mediated autophagy (CMA) is a selective lysosomal degradation pathway. Here, we investigated whether CMA is involved in the selective degradation of HNF-1α in HCV-infected cells and observed that the pentapeptide spanning from amino acid (aa) 130 to aa 134 of HNF-1α matches the rule for the CMA-targeting motif, also known as KFERQ motif. A cytosolic chaperone protein, heat shock cognate protein of 70 kDa (HSC70), and a lysosomal membrane protein, lysosome-associated membrane protein type 2A (LAMP-2A), are key components of CMA. Immunoprecipitation analysis revealed that HNF-1α was coimmunoprecipitated with HSC70, whereas the Q130A mutation (mutation of Q to A at position 130) of HNF-1α disrupted the interaction with HSC70, indicating that the CMA-targeting motif of HNF-1α is important for the association with HSC70. Immunoprecipitation analysis revealed that increasing amounts of NS5A enhanced the association of HNF-1α with HSC70. To determine whether LAMP-2A plays a role in the degradation of HNF-1α protein, we knocked down LAMP-2A mRNA by RNA interference; this knockdown by small interfering RNA (siRNA) recovered the level of HNF-1α protein in HCV J6/JFH1-infected cells. This result suggests that LAMP-2A is required for the degradation of HNF-1α. Immunofluorescence study revealed colocalization of NS5A and HNF-1α in the lysosome. Based on our findings, we propose that HCV NS5A interacts with HSC70 and recruits HSC70 to HNF-1α, thereby promoting the lysosomal degradation of HNF-1α via CMA.IMPORTANCE Many viruses use a protein degradation system, such as the ubiquitin-proteasome pathway or the autophagy pathway, for facilitating viral propagation and viral pathogenesis. We investigated the mechanistic details of the selective lysosomal degradation of hepatocyte nuclear factor 1 alpha (HNF-1α) induced by hepatitis C virus (HCV) NS5A protein. Using site-directed mutagenesis, we demonstrated that HNF-1α contains a pentapeptide chaperone-mediated autophagy (CMA)-targeting motif within the POU-specific domain of HNF-1α. The CMA-targeting motif is important for the association with HSC70. LAMP-2A is required for degradation of HNF-1α caused by NS5A. We propose that HCV NS5A interacts with HSC70, a key component of the CMA machinery, and recruits HSC70 to HNF-1α to target HNF-1α for CMA-mediated lysosomal degradation, thereby facilitating HCV pathogenesis. We discovered a role of HCV NS5A in CMA-dependent degradation of HNF-1α. Our results may lead to a better understanding of the role of CMA in the pathogenesis of HCV.

Hepatitis C virus NS5A protein interacts with lysine methyltransferase SET and MYND domain-containing 3 and induces activator protein 1 activation

Hepatitis C virus (HCV) non-structural protein 5A (NS5A) is a multifunctional protein that is involved in the HCV life cycle and pathogenesis. In this study, a host protein(s) interacting with NS5A by tandem affinity purification were searched for with the aim of elucidating the role of NS5A. An NS5A-interacting protein, SET and MYND domain-containing 3 (SMYD3), a lysine methyltransferase reportedly involved in the development of cancer, was identified. The interaction between NS5A and SMYD3 was confirmed in ectopically expressing, HCV RNA replicon-harboring and HCV-infected cells. The other HCV proteins did not bind to SMYD3. SMYD3 bound to NS5A of HCV genotypes 1b and 2a. Deletion mutational analysis revealed that domains II and III of NS5A (amino acids [aa] 250 to 447) and the MYND and N-SET domains of SMYD3 (aa 1 to 87) are involved in the full extent of NS5A-SMYD3 interaction. NS5A co-localized with SMYD3 exclusively in the cytoplasm, thereby inhibiting nuclear localization of SMYD3. Moreover, NS5A formed a complex with SMYD3 and heat shock protein 90 (HSP90), which is a positive regulator of SMYD3. The intensity of binding between SMYD3 and HSP90 was enhanced by NS5A. Luciferase reporter assay demonstrated that NS5A significantly induces activator protein 1 (AP-1) activity, this being potentiated by co-expression of SMYD3 with NS5A. Taken together, the present results suggest that NS5A interacts with SMYD3 and induces AP-1 activation, possibly by facilitating binding between HSP90 and SMYD3. This may be a novel mechanism of AP-1 activation in HCV-infected cells.

STAT1 is essential for the inhibition of hepatitis C virus replication by interferon-λ but not by interferon-α

Interferon-α (IFN-α) and IFN-λ are structurally distinct cytokines that bind to different receptors, but induce expression of similar sets of genes through Janus kinase (JAK)-signal transducers and activators of transcription (STAT) pathways. The difference between IFN-α and IFN-λ signaling remains poorly understood. Here, using the CRISPR/Cas9 system, we examine the role of STAT1 and STAT2 in the inhibition of hepatitis C virus (HCV) replication by IFN-α and IFN-λ. Treatment with IFN-α increases expression of IFN-stimulated genes (ISGs) such as double-stranded RNA-activated protein kinase (PKR) and decreases viral RNA and protein levels in HCV-infected Huh-7.5 human hepatoma cells. These responses are only partially attenuated by knockout of STAT1 but are abolished by knockout of STAT2. In contrast, the inhibition of HCV replication by IFN-λ is abolished by knockout of STAT1 or STAT2. Microarray analysis reveals that IFN-α but not IFN-λ can induce expression of the majority of ISGs in STAT1 knockout cells. These findings suggest that IFN-α can inhibit HCV replication through a STAT2-dependent but STAT1-independent pathway, whereas IFN-λ induces ISG expression and inhibits HCV replication exclusively through a STAT1- and STAT2-dependent pathway.

Physical and functional interaction between hepatitis C virus NS5A protein and ovarian tumor protein deubiquitinase 7B

Hepatitis C virus (HCV) NS5A protein plays crucial roles in viral RNA replication, virus assembly, and viral pathogenesis. Although NS5A has no known enzymatic activity, it modulates various cellular pathways through interaction with cellular proteins. HCV NS5A (and other HCV proteins) are reportedly degraded through the ubiquitin-proteasome pathway; however, the physiological roles of ubiquitylation and deubiquitylation in HCV infection are largely unknown. To elucidate the role of deubiquitylation in HCV infection, an attempt was made to identify a deubiquitinase (DUB) that can interact with NS5A protein. An ovarian tumor protein (OTU), deubiquitinase 7B (OTUD7B), was identified as a novel NS5A-binding protein. Co-immunoprecipitation analyses showed that NS5A interacts with OTUD7B in both Huh-7 and HCV RNA replicon cells. Immunofluorescence staining revealed that HCV NS5A protein colocalizes with OTUD7B in the cytoplasm. Moreover, HCV infection was found to enhance the nuclear localization of OTUD7B. The OTUD7B-binding domain on NS5A was mapped using a series of NS5A deletion mutants. The present findings suggest that the domain I of NS5A is important and the region from amino acid 121 to 126 of NS5A essential for the interaction. Either V121A or V124A mutation in NS5A disrupts the NS5A-OTUD7B interaction. The results of this in vivo ubiquitylation assay suggest that HCV NS5A enhances OTUD7B DUB activity. Taken together, these results suggest that HCV NS5A protein interacts with OTUD7B, thereby modulating its DUB activity.

Interactome of the hepatitis C virus: Literature mining with ANDSystem

A study of the molecular genetics mechanisms of host-pathogen interactions is of paramount importance in developing drugs against viral diseases. Currently, the literature contains a huge amount of information that describes interactions between HCV and human proteins. In addition, there are many factual databases that contain experimentally verified data on HCV-host interactions. The sources of such data are the original data along with the data manually extracted from the literature. However, the manual analysis of scientific publications is time consuming and, because of this, databases created with such an approach often do not have complete information. One of the most promising methods to provide actualisation and completeness of information is text mining. Here, with the use of a previously developed method by the authors using ANDSystem, an automated extraction of information on the interactions between HCV and human proteins was conducted. As a data source for the text mining approach, PubMed abstracts and full text articles were used. Additionally, external factual databases were analyzed. On the basis of this analysis, a special version of ANDSystem, extended with the HCV interactome, was created. The HCV interactome contains information about the interactions between 969 human and 11 HCV proteins. Among the 969 proteins, 153 'new' proteins were found not previously referred to in any external databases of protein-protein interactions for HCV-host interactions. Thus, the extended ANDSystem possesses a more comprehensive detailing of HCV-host interactions versus other existing databases. It was interesting that HCV proteins more preferably interact with human proteins that were already involved in a large number of protein-protein interactions as well as those associated with many diseases. Among human proteins of the HCV interactome, there were a large number of proteins regulated by microRNAs. It turned out that the results obtained for protein-protein interactions and microRNA-regulation did not depend on how well the proteins were studied, while protein-disease interactions appeared to be dependent on the level of study. In particular, the mean number of diseases linked to well-studied proteins (proteins were considered well-studied if they were mentioned in 50 or more PubMed publications) from the HCV interactome was 20.8, significantly exceeding the mean number of associations with diseases (10.1) for the total set of well-studied human proteins present in ANDSystem. For proteins not highly poorly-studied investigated, proteins from the HCV interactome (each protein was referred to in less than 50 publications) distribution of the number of diseases associated with them had no statistically significant differences from the distribution of the number of diseases associated with poorly-studied proteins based on the total set of human proteins stored in ANDSystem. With this, the average number of associations with diseases for the HCV interactome and the total set of human proteins were 0.3 and 0.2, respectively. Thus, ANDSystem, extended with the HCV interactome, can be helpful in a wide range of issues related to analyzing HCV-host interactions in the search for anti-HCV drug targets. The demo version of the extended ANDSystem covered here containing only interactions between human proteins, genes, metabolites, diseases, miRNAs and molecular-genetic pathways, as well as interactions between human proteins/genes and HCV proteins, is freely available at the following web address: http://www-bionet.sscc.ru/psd/andhcv/.

Hepatitis C Virus Particle Assembly Involves Phosphorylation of NS5A by the c-Abl Tyrosine Kinase

Hepatitis C virus (HCV) nonstructural protein 5A (NS5A) is thought to regulate the replication of viral RNA and the assembly of virus particles in a serine/threonine phosphorylation-dependent manner. However, the host kinases that phosphorylate NS5A have not been fully identified. Here, we show that HCV particle assembly involves the phosphorylation of NS5A by the c-Abl tyrosine kinase. Pharmacological inhibition or knockdown of c-Abl reduces the production of infectious HCV (J6/JFH1) particles in Huh-7.5 cells without markedly affecting viral RNA translation and replication. NS5A is tyrosine-phosphorylated in HCV-infected cells, and this phosphorylation is also reduced by the knockdown of c-Abl. Mutational analysis reveals that NS5A tyrosine phosphorylation is dependent, at least in part, on Tyr(330) (Tyr(2306) in polyprotein numbering). Mutation of this residue to phenylalanine reduces the production of infectious HCV particles but does not affect the replication of the JFH1 subgenomic replicon. These findings suggest that c-Abl promotes HCV particle assembly by phosphorylating NS5A at Tyr(330).

HCV upregulates Bim through the ROS/JNK signalling pathway, leading to Bax-mediated apoptosis

We previously reported that hepatitis C virus (HCV) infection induces Bax-triggered, mitochondrion-mediated apoptosis by using the HCV J6/JFH1 strain and Huh-7.5 cells. However, it was still unclear how HCV-induced Bax activation. In this study, we showed that the HCV-induced activation and mitochondrial accumulation of Bax were significantly attenuated by treatment with a general antioxidant, N-acetyl cysteine (NAC), or a specific c-Jun N-terminal kinase (JNK) inhibitor, SP600125, with the result suggesting that the reactive oxygen species (ROS)/JNK signalling pathway is upstream of Bax activation in HCV-induced apoptosis. We also demonstrated that HCV infection transcriptionally activated the gene for the pro-apoptotic protein Bim and the protein expression of three major splice variants of Bim (BimEL, BimL and BimS). The HCV-induced increase in the Bim mRNA and protein levels was significantly counteracted by treatment with NAC or SP600125, suggesting that the ROS/JNK signalling pathway is involved in Bim upregulation. Moreover, HCV infection led to a marked accumulation of Bim on the mitochondria to facilitate its interaction with Bax. On the other hand, downregulation of Bim by siRNA (small interfering RNA) significantly prevented HCV-mediated activation of Bax and caspase 3. Taken together, these observations suggest that HCV-induced ROS/JNK signalling transcriptionally activates Bim expression, which leads to Bax activation and apoptosis induction.

Methylation of the PTPRO gene in human hepatocellular carcinoma and identification of VCP as its substrate

We have previously reported that the gene encoding protein tyrosine phosphatase receptor type-O (PTPRO) is suppressed by promoter methylation in a rat model of hepatocellular carcinoma (HCC) and it functions as tumor suppressor in leukemia and lung cancer. Here, we explored the methylation and expression of PTPRO as well as its function in human HCC. MassARRAY analysis of primary human HCC and matching liver samples (n = 24) revealed significantly higher (P = 0.004) methylation density at the promoter CGI in tumors. Combined bisulfite restriction analysis (COBRA) of another set of human HCC samples (n = 17) demonstrated that the CGI was methylated in 29% of tumors where expression of PTPRO was lower than that in corresponding matching livers. A substrate-trapping mutant of PTPRO that stabilizes the bound substrates was used to identify its novel substrate(s). VCP/p97 was found to be a PTPRO substrate by mass spectrometry of the peptides pulled down by the substrate-trapping mutant of PTPRO. Tyrosyl dephosphorylation of VCP following ectopic expression of wild-type PTPRO in H293T and HepG2 cells confirmed that it is a bona fide substrate of PTPRO. Treatment of PTPRO overexpressing HepG2 cells with Doxorubicin, a DNA damaging drug commonly used in therapy of primary HCC, sensitized these cells to this potent anticancer drug that correlated with dephosphorylation of VCP. Taken together, these results demonstrate methylation and downregulation of PTPRO in a subset of primary human HCC and establish VCP as a novel functionally important substrate of this tyrosine phosphatase that could be a potential molecular target for HCC therapy.

Human ezrin-moesin-radixin proteins modulate hepatitis C virus infection

Host cytoskeletal proteins of the ezrin-moesin-radixin (EMR) family have been shown to modulate single-stranded RNA virus infection through regulating stable microtubule formation. Antibody engagement of CD81, a key receptor for hepatitis C virus (HCV) entry, induces ezrin phosphorylation. Here we tested the role of EMR proteins in regulating HCV infection and explored potential therapeutic targets. We show that HCV E2 protein induces rapid ezrin phosphorylation and its cellular redistribution with F-actin by way of spleen tyrosine kinase (SYK). Therapeutically blocking the functional roles of SYK or F-actin reorganization significantly reduced Huh7.5 cell susceptibility to HCV J6/JFH-1 infection. Using gene regulation, real-time quantitative polymerase chain reaction, western blot, and fluorescent microscopy analysis, we found that proteins of the EMR family differentially regulate HCV infection in the J6/JFH-1/Huh7.5 cell system. Moesin and radixin, but not ezrin, expression were significantly decreased in chronic HCV J6/JFH-1-infected Huh7.5 cells and HCV-infected patient liver biopsies compared to controls. The decreases in moesin and radixin in HCV J6/JFH-1-infected Huh7.5 cells were associated with a significant increase in stable microtubules. Ezrin knockdown inhibited immediate postentry events in HCV infection. Overexpression of moesin or radixin significantly reduced HCV protein expression. In contrast, transient knockdown of moesin or radixin augmented HCV infection. Making use of the Con1 HCV replicon system, we tested the effect of EMR proteins on HCV replication. We found that transient knockdown of moesin increased HCV RNA expression while overexpression of EMR showed no significant effect on HCV replication.

CONCLUSION

Our findings demonstrate the important role of EMR proteins during HCV infection at the postentry level and highlight possible novel targets for HCV treatment.

Role of herpes simplex virus VP11/12 tyrosine-based motifs in binding and activation of the Src family kinase Lck and recruitment of p85, Grb2, and Shc

Previous studies have shown that the abundant herpes simplex virus 1 (HSV-1) tegument protein VP11/12, encoded by gene UL46, stimulates phosphatidylinositol 3-kinase (PI3-kinase)/Akt signaling: it binds the Src family kinase (SFK) Lck, is tyrosine phosphorylated, recruits the p85 subunit of PI3-kinase, and is essential for the activation of Akt during HSV-1 infection. The C-terminal region of VP11/12 contains tyrosine-based motifs predicted to bind the SH2 domains of SFKs (YETV and YEEI), p85 (YTHM), and Grb2 (YENV) and the phosphotyrosine-binding (PTB) domain of Shc (NPLY). We inactivated each of these motifs in the context of the intact viral genome and examined effects on binding and activation of Lck and recruitment of p85, Grb2, and Shc. Inactivating the p85, Grb2, or Shc motif reduced (p85) or eliminated (Grb2 and Shc) the interaction with the cognate signaling molecule without greatly affecting the other interactions or activation of Lck. Inactivating either SFK motif had only a minor effect on Lck binding and little or no effect on recruitment of p85, Grb2, or Shc. In contrast, inactivation of both SFK motifs severely reduced Lck binding and activation and tyrosine phosphorylation of VP11/12 and reduced (p85) or eliminated (Grb2 and Shc) binding of other signaling proteins. Overall, these data demonstrate the key redundant roles of the VP11/12 SFK-binding motifs in the recruitment and activation of SFKs and indicate that activated SFKs then lead (directly or indirectly) to phosphorylation of the additional motifs involved in recruiting p85, Grb2, and Shc. Thus, VP11/12 appears to mimic an activated growth factor receptor.

HCV NS5A protein containing potential ligands for both Src homology 2 and 3 domains enhances autophosphorylation of Src family kinase Fyn in B cells

Hepatitis C virus (HCV) infects B lymphocytes and induces mixed cryoglobulinemia and B cell non-Hodgkin's lymphoma. The molecular mechanism for the pathogenesis of HCV infection-mediated B cell disorders remains obscure. To identify the possible role for HCV nonstructural 5A (NS5A) protein in B cells, we generated the stable B cell lines expressing Myc-His tagged NS5A. Immunoprecipitation study in the presence or absence of pervanadate (PV) implied that NS5A was tyrosine phosphorylated by pervanadate (PV) treatment of the cells. Therefore we examined pull-down assay by using glutathione S-transferase (GST)-fusion proteins of various Src homology 2 (SH2) domains, which associates with phosphotyrosine within a specific amino acid sequence. The results showed that NS5A specifically bound to SH2 domain of Fyn from PV-treated B cells in addition to Src homology 3 (SH3) domain. Substitution of Arg¹⁷⁶ to Lys in the SH2 domain of Fyn abrogated this interaction. Deletion mutational analysis demonstrated that N-terminal region of NS5A was not required for the interaction with the SH2 domain of Fyn. Tyr³³⁴ was identified as a tyrosine phosphorylation site in NS5A. Far-western analysis revealed that SH2 domain of Fyn directly bound to NS5A. Fyn and NS5A were colocalized in the lipid raft. These results suggest that NS5A directly binds to the SH2 domain of Fyn in a tyrosine phosphorylation-dependent manner. Lastly, we showed that the expression of NS5A in B cells increased phosphorylation of activation loop tyrosine in the kinase domain of Fyn. NS5A containing ligand for both SH2 and SH3 domains enhances an aberrant autophosphorylation and kinase activity of Fyn in B cells.

Hepatitis C virus infection suppresses GLUT2 gene expression via downregulation of hepatocyte nuclear factor 1α

Hepatitis C virus (HCV) infection causes not only intrahepatic diseases but also extrahepatic manifestations, including type 2 diabetes. We previously reported that HCV replication suppresses cellular glucose uptake by downregulation of cell surface expression of glucose transporter 2 (GLUT2) (D. Kasai et al., J. Hepatol. 50:883-894, 2009). GLUT2 mRNA levels were decreased in both HCV RNA replicon cells and HCV J6/JFH1-infected cells. To elucidate molecular mechanisms of HCV-induced suppression of GLUT2 gene expression, we analyzed transcriptional regulation of the GLUT2 promoter using a series of GLUT2 promoter-luciferase reporter plasmids. HCV-induced suppression of GLUT2 promoter activity was abrogated when the hepatocyte nuclear factor 1α (HNF-1α)-binding motif was deleted from the GLUT2 promoter. HNF-1α mRNA levels were significantly reduced in HCV J6/JFH1-infected cells. Furthermore, HCV infection remarkably decreased HNF-1α protein levels. We assessed the effects of proteasome inhibitor or lysosomal protease inhibitors on the HCV-induced reduction of HNF-1α protein levels. Treatment of HCV-infected cells with a lysosomal protease inhibitor, but not with a proteasome inhibitor, restored HNF-1α protein levels, suggesting that HCV infection promotes lysosomal degradation of HNF-1α protein. Overexpression of NS5A protein enhanced lysosomal degradation of HNF-1α protein and suppressed GLUT2 promoter activity. Immunoprecipitation analyses revealed that the region from amino acids 1 to 126 of the NS5A domain I physically interacts with HNF-1α protein. Taken together, our results suggest that HCV infection suppresses GLUT2 gene expression via downregulation of HNF-1α expression at transcriptional and posttranslational levels. HCV-induced downregulation of HNF-1α expression may play a crucial role in glucose metabolic disorders caused by HCV.

Inhibition of hepatitis C virus replication through adenosine monophosphate-activated protein kinase-dependent and -independent pathways

Persistent infection with hepatitis C virus (HCV) is closely correlated with type 2 diabetes. In this study, replication of HCV at different glucose concentrations was investigated by using J6/JFH1-derived cell-adapted HCV in Huh-7.5 cells and the mechanism of regulation of HCV replication by AMP-activated protein kinase (AMPK) as an energy sensor of the cell analyzed. Reducing the glucose concentration in the cell culture medium from 4.5 to 1.0 g/L resulted in suppression of HCV replication, along with activation of AMPK. Whereas treatment of cells with AMPK activator 5-aminoimidazole-4-carboxamide 1-β-D-ribofuranoside (AICAR) suppressed HCV replication, compound C, a specific AMPK inhibitor, prevented AICAR's effect, suggesting that AICAR suppresses the replication of HCV by activating AMPK in Huh-7.5 cells. In contrast, compound C induced further suppression of HCV replication when the cells were cultured in low glucose concentrations or with metformin. These results suggest that low glucose concentrations and metformin have anti-HCV effects independently of AMPK activation.

Hepatitis C virus infection promotes hepatic gluconeogenesis through an NS5A-mediated, FoxO1-dependent pathway

Chronic hepatitis C virus (HCV) infection is often associated with type 2 diabetes. However, the precise mechanism underlying this association is still unclear. Here, using Huh-7.5 cells either harboring HCV-1b RNA replicons or infected with HCV-2a, we showed that HCV transcriptionally upregulated the genes for phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase), the rate-limiting enzymes for hepatic gluconeogenesis. In this way, HCV enhanced the cellular production of glucose 6-phosphate (G6P) and glucose. PEPCK and G6Pase gene expressions are controlled by the transcription factor forkhead box O1 (FoxO1). We observed that although neither the mRNA levels nor the protein levels of FoxO1 expression were affected by HCV, the level of phosphorylation of FoxO1 at Ser319 was markedly diminished in HCV-infected cells compared to the control cells, resulting in an increased nuclear accumulation of FoxO1, which is essential for sustaining its transcriptional activity. It was unlikely that the decreased level of FoxO1 phosphorylation was mediated through Akt inactivation, as we observed an increased phosphorylation of Akt at Ser473 in HCV-infected cells compared to control cells. By using specific inhibitors of c-Jun N-terminal kinase (JNK) and reactive oxygen species (ROS), we demonstrated that HCV infection induced JNK activation via increased mitochondrial ROS production, resulting in decreased FoxO1 phosphorylation, FoxO1 nuclear accumulation, and, eventually, increased glucose production. We also found that HCV NS5A mediated increased ROS production and JNK activation, which is directly linked with the FoxO1-dependent increased gluconeogenesis. Taken together, these observations suggest that HCV promotes hepatic gluconeogenesis through an NS5A-mediated, FoxO1-dependent pathway.

Advances in research of interaction between hepatitis C virus nonstructural proteins and host proteins

Hepatitis C virus (HCV) is another common cause of chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma after hepatitis B virus (HBV). Up to now, the mechanisms by which HCV promotes persistent infection and cancer remain unclear, and there are neither effective drugs nor vaccines against HCV available. Interaction between virus proteins and host proteins is a hot topic in research of the pathogenesis of viral hepatitis. Recent research shows that interaction between HCV nonstructural proteins and host proteins has an important impact on viral replication, carcinogenesis, interferon resistance, and disorders of glycometabolism and lipid metabolism. This paper summarizes the recent advances in research of interaction between HCV nonstructural proteins and host proteins.

Heat shock protein 72 is associated with the hepatitis C virus replicase complex and enhances viral RNA replication

The NS5A protein of the hepatitis C virus (HCV) is an integral component of the viral replicase. It also modulates cellular signaling and perturbs host interferon responses. The multifunctional characteristics of NS5A are mostly attributed to its ability to interact with various cellular proteins. This study aimed to identify the novel cellular factors that interact with NS5A and decipher the significance of this interaction in viral replication. The NS5A-interacting proteins were purified by the tandem affinity purification (TAP) procedure from cells expressing NS5A and identified by mass spectrometry. The chaperone protein Hsp72 was identified herein. In vivo protein-protein interaction was verified by co-immunoprecipitation and an in situ proximity ligation assay. In addition to NS5A, Hsp72 was also associated with other members of the replicase complex, NS3 and NS5B, suggesting that it might be directly involved in the HCV replication complex. Hsp72 plays a positive regulatory role in HCV RNA replication by increasing levels of the replicase complex, which was attributed either to the increased stability of the viral proteins in the replicase complex or to the enhanced translational activity of the internal ribosome entry site of HCV. The fact that the host chaperone protein Hsp72 is involved in HCV RNA replication may represent a therapeutic target for controlling virus production.

Natural genetic engineering of hepatitis C virus NS5A for immune system counterattack

The Hepatitis C virus nonstructural 5A (NS5A) protein is a hydrophilic phosphoprotein with diverse functions. The domain assignment of NS5A had been refined using a systematic in silico bioinformatics approach using DOMAC, the protein is divided into three domains and domain III is subdivided into two subdomains using ProDom and SSEP servers. The fold structure for domains II and III were predicted using the meta-server 3D-Jury. Scanning motif databases (SMART, BLOCKS, and PROSITE) gave new motifs. Two important motifs, the interleukins 1 and 8 interaction motifs, relating to NS5A function in inducing the interleukin 8 promoter, were discovered from the BLOCKS scan. Protein-protein interaction motifs were predicted as hot loops and disordered regions, corresponding to binding regions with the ds-protein kinase R, viral polymerase, and Src homology 3 signaling proteins binding motif. Other hot loops were predicted in the V3 region and in the single-stranded DNA-binding protein motif. The different mechanisms by which the NS5A protein leads to immune system signaling dysfunction points to the natural genetic engineering of this protein.

Role of oxysterol binding protein in hepatitis C virus infection

Hepatitis C virus (HCV) RNA genome replicates within the ribonucleoprotein (RNP) complex in the modified membranous structures extended from endoplasmic reticulum. A proteomic analysis of HCV RNP complexes revealed the association of oxysterol binding protein (OSBP) as one of the components of these complexes. OSBP interacted with the N-terminal domain I of the HCV NS5A protein and colocalized to the Golgi compartment with NS5A. An OSBP-specific short hairpin RNA that partially downregulated OSBP expression resulted in a decrease of the HCV particle release in culture supernatant with little effect on viral RNA replication. The pleckstrin homology (PH) domain located in the N-terminal region of OSBP targeted this protein to the Golgi apparatus. OSBP deletion mutation in the PH (DeltaPH) domain failed to localize to the Golgi apparatus and inhibited the HCV particle release. These studies suggest a possible functional role of OSBP in the HCV maturation process.

HCV replication suppresses cellular glucose uptake through down-regulation of cell surface expression of glucose transporters

BACKGROUND/AIMS

Persistent infection with hepatitis C virus (HCV) causes extrahepatic diseases, including diabetes. We investigated the possible effect(s) of HCV replication on cellular glucose uptake and expression of the facilitative glucose transporter (GLUT) 2 and 1.

METHODS

We used Huh-7.5 cells harboring either an HCV subgenomic RNA replicon (SGR) or an HCV full-genomic RNA replicon (FGR), HCV-infected cells, and the respective cells treated with interferon (IFN). We also used liver tissue samples obtained from patients with or without HCV infection.

RESULTS

Glucose uptake and surface expression of GLUT2 and GLUT1 were suppressed in SGR, FGR and HCV-infected cells compared to the control cells. Expression levels of GLUT2 mRNA, but not GLUT1 mRNA, were lower in SGR, FGR and HCV-infected cells than in the control. Luciferase reporter assay demonstrated decreased GLUT2 promoter activities in SGR, FGR and HCV-infected cells. IFN treatment restored glucose uptake, GLUT2 surface expression, GLUT2 mRNA expression and GLUT2 promoter activities. Also, GLUT2 expression was reduced in hepatocytes of liver tissues obtained from HCV-infected patients.

CONCLUSIONS

HCV replication down-regulates cell surface expression of GLUT2 partly at the transcriptional level, and possibly at the intracellular trafficking level as suggested for GLUT1, thereby lowering glucose uptake by hepatocytes.