Global proteome analysis of hepatitis B virus expressing human hepatoblastoma cell line HepG2

Small Hepatitis B Virus Surface Antigen Promotes Hepatic Gluconeogenesis via Enhancing Glucagon/cAMP/Protein Kinase A/CREB Signaling

Hepatitis B virus (HBV) is a major risk factor for serious liver diseases. The liver plays a unique role in controlling carbohydrate metabolism to maintain the glucose level within the normal range. Chronic HBV infection has been reported to associate with a high prevalence of diabetes. However, the detailed molecular mechanism underlying the potential association remains largely unknown. Here, we report that liver-targeted delivery of small HBV surface antigen (SHBs), the most abundant viral protein of HBV, could elevate blood glucose levels and impair glucose and insulin tolerance in mice by promoting hepatic gluconeogenesis. Hepatocytes with SHB expression also exhibited increased glucose production and expression of gluconeogenic genes glucose-6-phosphatase (G6pc) and phosphoenolpyruvate carboxykinase (PEPCK) in response to glucagon stimulation. Mechanistically, SHBs increased cellular levels of cyclic AMP (cAMP) and consequently activated protein kinase A (PKA) and its downstream effector cAMP-responsive element binding protein (CREB). SHBs-induced activation of CREB enhanced transcripts of gluconeogenic genes, thus promoting hepatic gluconeogenesis. The elevated cAMP level resulted from increased transcription activity and expression of adenylyl cyclase 1 (AC1) by SHBs through a binary E-box factor binding site (BEF). Taken together, we unveiled a novel pathogenic role and mechanism of SHBs in hepatic gluconeogenesis, and these results might highlight a potential target for preventive and therapeutic intervention in the development and progression of HBV-associated diabetes. IMPORTANCE Chronic HBV infection causes progressive liver damage and is found to be a risk factor for diabetes. However, the mechanism in the regulation of glucose metabolism by HBV remains to be established. In the current study, we demonstrate for the first time that the small hepatitis B virus surface antigen (SHBs) of HBV elevates AC1 transcription and expression to activate cAMP/PKA/CREB signaling and subsequently induces the expression of gluconeogenic genes and promotes hepatic gluconeogenesis both in vivo and in vitro. This study provides a direct link between HBV infection and diabetes and implicates that SHBs may represent a potential target for the treatment of HBV-induced metabolic disorders.

Hepatitis B virus small envelope protein promotes HCC angiogenesis via ER stress signaling to upregulate VEGFA expression

Hepatocellular carcinoma (HCC) is a hypervascular tumor and accumulating evidence has indicated that stimulation of angiogenesis by HBV may contribute to HCC malignancy. The small protein of hepatitis B virus surface antigen (HBsAg), SHBs, is the most abundant HBV viral protein and has a close clinical association with HCC, however, whether SHBs contributes to HCC angiogenesis remains unknown. This study reports that forced expression of SHBs in HCC cells promoted xenograft tumor growth and increased the microvessel density (MVD) within the tumors. Consistently, HBsAg was also positively correlated with MVD count in HCC patients' specimens. The conditioned media from the SHBs-transfected HCC cells increased the capillary tube formation and migration of human umbilical vein endothelial cells (HUVECs). Intriguingly, overexpression of SHBs increased VEGFA expression at both mRNA and protein levels. A higher VEGFA expression level was also observed in the xenograft tumors transplanted with SHBs-expressing HCC cells and in HBsAg-positive HCC tumor tissues as compared to their negative controls. As expected, in the culture supernatants, the secretion of VEGFA was also significantly enhanced from HCC cells expressing SHBs, which promoted HUVECs migration and vessel formation. Furthermore, all the three unfolded protein response (UPR) sensors IRE1α, PERK and ATF6 associated with endoplasmic reticulum (ER) stress were found activated in the SHBs-expressing cells and correlated with VEGFA protein expression and secretion. Taken together, these results suggest an important role of SHBs in HCC angiogenesis and may highlight a potential target for preventive and therapeutic intervention of HBV-related HCC and its malignant progression. IMPORTANCE Chronic hepatitis B virus infection is one of the important risk factors for the development and progression of hepatocellular carcinoma (HCC). HCC is characteristic of hypervascularization even at early phases of the disease due to overexpression of angiogenic factors like vascular endothelial growth factor-A (VEGFA). However, a detailed mechanism in the HBV-induced angiogenesis remains to be established. In this study, we demonstrate for the first time that the most abundant HBV viral protein, i.e. small surface antigens (SHBs) can enhance the angiogenic capacity of HCC cells by upregulation of VEGFA expression both in vitro and in vivo. Mechanistically, SHBs induced endoplasmic reticulum (ER) stress which consequently activated unfolded protein response (UPR) signaling to increase VEGFA expression and secretion. This study suggests that SHBs plays an important pro-angiogenic role in HBV-associated HCC and may represent a potential target for anti-angiogenic therapy in the HCC.

Chromatin remodelling factor BAF155 protects hepatitis B virus X protein (HBx) from ubiquitin-independent proteasomal degradation

HBx is a short-lived protein whose rapid turnover is mainly regulated by ubiquitin-dependent proteasomal degradation pathways. Our prior work identified BAF155 to be one of the HBx binding partners. Since BAF155 has been shown to stabilize other members of the SWI/SNF chromatin remodelling complex by attenuating their proteasomal degradation, we proposed that BAF155 might also contribute to stabilizing HBx protein in a proteasome-dependent manner. Here we report that BAF155 protected hepatitis B virus X protein (HBx) from ubiquitin-independent proteasomal degradation by competing with the 20S proteasome subunit PSMA7 to bind to HBx. BAF155 was found to directly interact with HBx via binding of its SANT domain to the HBx region between amino acid residues 81 and 120. Expression of either full-length BAF155 or SANT domain increased HBx protein levels whereas siRNA-mediated knockdown of endogenous BAF155 reduced HBx protein levels. Increased HBx stability and steady-state level by BAF155 were attributable to inhibition of ubiquitin-independent and PSMA7-mediated protein degradation. Consequently, overexpression of BAF155 enhanced the transcriptional transactivation function of HBx, activated protooncogene expression and inhibited hepatoma cell clonogenicity. These results suggest that BAF155 plays important roles in ubiquitin-independent degradation of HBx, which may be related to the pathogenesis and carcinogenesis of HBV-associated HCC.

Repression of Death Receptor-Mediated Apoptosis of Hepatocytes by Hepatitis B Virus e Antigen

Hepatitis B virus (HBV) e antigen (HBeAg) is associated with viral persistence and pathogenesis. Resistance of HBV-infected hepatocytes to apoptosis is seen as one of the primary promotors for HBV chronicity and malignancy. Fas receptor/ligand (Fas/FasL) and the tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) system plays a key role in hepatic death during HBV infection. We found that HBeAg mediates resistance of hepatocytes to FasL or TRAIL-induced apoptosis. Introduction of HBeAg into human hepatocytes rendered resistance to FasL or TRAIL cytotoxicity in a p53-dependent manner. HBeAg further inhibited the expression of p53, total Fas, membrane-bound Fas, TNF receptor superfamily member 10a, and TNF receptor superfamily member 10b at both mRNA and protein levels. In contrast, HBeAg enhanced the expression of soluble forms of Fas through facilitation of Fas alternative mRNA splicing. In a mouse model, expression of HBeAg in mice injected with recombinant adenovirus-associated virus 8 inhibited agonistic anti-Fas antibody-induced hepatic apoptosis. Xenograft tumorigenicity assay also found that HBeAg-induced carcinogenesis was resistant to the proapoptotic effect of TRAIL and chemotherapeutic drugs. These results indicate that HBeAg may prevent hepatocytes from FasL and TRAIL-induced apoptosis by regulating the expression of the proapoptotic and antiapoptotic forms of death receptors, which may contribute to the survival and persistence of infected hepatocytes during HBV infection.

Comprehending a Killer: The Akt/mTOR Signaling Pathways Are Temporally High-Jacked by the Highly Pathogenic 1918 Influenza Virus

Previous transcriptomic analyses suggested that the 1918 influenza A virus (IAV1918), one of the most devastating pandemic viruses of the 20th century, induces a dysfunctional cytokine storm and affects other innate immune response patterns. Because all viruses are obligate parasites that require host cells for replication, we globally assessed how IAV1918 induces host protein dysregulation. We performed quantitative mass spectrometry of IAV1918-infected cells to measure host protein dysregulation. Selected proteins were validated by immunoblotting and phosphorylation levels of members of the PI3K/AKT/mTOR pathway were assessed. Compared to mock-infected controls, >170 proteins in the IAV1918-infected cells were dysregulated. Proteins mapped to amino sugar metabolism, purine metabolism, steroid biosynthesis, transmembrane receptors, phosphatases and transcription regulation. Immunoblotting demonstrated that IAV1918 induced a slight up-regulation of the lamin B receptor whereas all other tested virus strains induced a significant down-regulation. IAV1918 also strongly induced Rab5b expression whereas all other tested viruses induced minor up-regulation or down-regulation. IAV1918 showed early reduced phosphorylation of PI3K/AKT/mTOR pathway members and was especially sensitive to rapamycin. These results suggest the 1918 strain requires mTORC1 activity in early replication events, and may explain the unique pathogenicity of this virus.

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Proteomic analyses of influenza 1918 virus-infected cells identified >170 dysregulated host proteins.

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Dysregulated proteins mapped to numerous important cellular pathways.

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1918 virus infection showed prominent early reduced phosphorylation of PI3K/Akt/mTOR.

The 1918 influenza pandemic was one of the most devastating infectious disease events of the 20th century, resulting in 20–100 million deaths. Gene-based assays showed severe dysregulation of the host's cytokine responses, but little was known about global protein responses to virus infection. This work identifies unique and temporal alterations in phosphorylation of the PI3K/AKT/mTOR signaling pathway, which is important in determining cell death. This work paves the way for further research on how this pathway influences host mechanisms responsible for aiding virus replication and in determining levels and severity of influenza virus-induced patho

Proteomic analysis of A-549 cells infected with human adenovirus 40 by LC-MS

Human Adenoviruses (HAdVs) are etiological agents of different syndromes such as gastroenteritis, cystitis, ocular, and respiratory diseases, and infection by these viruses may cause alterations in cellular homeostasis. The objective of the study was the proteomic analysis of A-549 cells infected with HAdV-40 using LC-MS. At 30 h of infection, the quantitative analysis revealed 336 differentially expressed proteins. From them, 206 were induced (up-regulated) and 130 were suppressed (down-regulated). The majority of up-regulated proteins were related to energy, cellular organization, stress response, and apoptosis pathways. It was observed alteration of cell metabolism with increase of the glycolytic pathway, β-oxidation, and respiratory chain. Also, the results suggest cytoskeleton reorganization and apoptosis induction. The data can improve knowledge about the replication of HAdV-40 in cell culture considering the proteins related to distinct metabolic pathways induced by viral infection in A-549 cells.

Sendai virus recruits cellular villin to remodel actin cytoskeleton during fusion with hepatocytes

Reconstituted Sendai viral envelopes (virosomes) are well recognized for their promising potential in membrane fusion-mediated delivery of bioactive molecules to liver cells. Despite the known function of viral envelope glycoproteins in catalyzing fusion with cellular membrane, the role of host cell proteins remains elusive. Here, we used two-dimensional differential in-gel electrophoresis to analyze hepatic cells in early response to virosome-induced membrane fusion. Quantitative mass spectrometry together with biochemical analysis revealed that villin, an actin-modifying protein, is differentially up-regulated and phosphorylated at threonine 206-an early molecular event during membrane fusion. We found that villin influences actin dynamics and that this influence, in turn, promotes membrane mixing through active participation of Sendai viral envelope glycoproteins. Modulation of villin in host cells also resulted in a discernible effect on the entry and egress of progeny Sendai virus. Taken together, these results suggest a novel mechanism of regulated viral entry in animal cells mediated by host factor villin.

Insulin receptor substrate-4 interacts with ubiquitin-specific protease 18 to activate the Jak/STAT signaling pathway

Ubiquitin-specific protease 18 (USP18) as a negative regulator of the Jak/STAT signaling pathway plays an important role in the host innate immune response. USP18 has been shown to bind to the type I interferon receptor subunit 2 (IFNAR2) to down-regulate the Jak/STAT signaling. In this study, we showed that insulin receptor substrate (IRS)-4 functioned as a novel USP18-binding protein. Co-precipitation assays revealed that two regions (amino acids 335–400 and 1094-1257) of IRS4 were related to bind to the C- terminal region of USP18. IRS4 binding to USP18 diminished the inhibitory effect of USP18 on Jak/STAT signaling. IRS4 over-expression enhanced while IRS4 knock-down suppressed the Jak/STAT signaling in the presence of IFN-a stimulation. As such, IRS4 increased IFN-a-mediated anti-HCV activity. Mechanistically, IRS4 promoted the IFN-a-induced Jak/STAT signaling by interact with USP18. These results suggested that IRS4 binds to USP18 to diminish the blunting effect of USP18 on IFN-a-induced Jak/STAT signaling. Our findings indicated that IRS4 is a novel USP18-binding protein that can be used to boost the host innate immunity to control HCV, and potentially other viruses that are sensitive to IFN-a.

Oxidative stress, a trigger of hepatitis C and B virus-induced liver carcinogenesis

Virally induced liver cancer usually evolves over long periods of time in the context of a strongly oxidative microenvironment, characterized by chronic liver inflammation and regeneration processes. They ultimately lead to oncogenic mutations in many cellular signaling cascades that drive cell growth and proliferation. Oxidative stress, induced by hepatitis viruses, therefore is one of the factors that drives the neoplastic transformation process in the liver. This review summarizes current knowledge on oxidative stress and oxidative stress responses induced by human hepatitis B and C viruses. It focuses on the molecular mechanisms by which these viruses activate cellular enzymes/systems that generate or scavenge reactive oxygen species (ROS) and control cellular redox homeostasis. The impact of an altered cellular redox homeostasis on the initiation and establishment of chronic viral infection, as well as on the course and outcome of liver fibrosis and hepatocarcinogenesis will be discussed The review neither discusses reactive nitrogen species, although their metabolism is interferes with that of ROS, nor antioxidants as potential therapeutic remedies against viral infections, both subjects meriting an independent review.

Hepatitis B Virus X Protein Induces Hepatic Steatosis by Enhancing the Expression of Liver Fatty Acid Binding Protein

Hepatitis B virus (HBV) has been implicated as a potential trigger of hepatic steatosis although molecular mechanisms involved in the pathogenesis of HBV-associated hepatic steatosis still remain elusive. Our prior work has revealed that the expression level of liver fatty acid binding protein 1 (FABP1), a key regulator of hepatic lipid metabolism, was elevated in HBV-producing hepatoma cells. In this study, the effects of HBV X protein (HBx) mediated FABP1 regulation on hepatic steatosis and the underlying mechanism were determined. mRNA and protein levels of FABP1 were measured by quantitative RT-PCR (qPCR) and Western blotting. HBx-mediated FABP1 regulation was evaluated by luciferase assay, coimmunoprecipitation, and chromatin immunoprecipitation. Hepatic lipid accumulation was measured by using Oil-Red-O staining and the triglyceride level. It was found that expression of FABP1 was increased in HBV-producing hepatoma cells, the sera of HBV-infected patients, and the sera and liver tissues of HBV-transgenic mice. Ectopic overexpression of HBx resulted in upregulation of FABP1 in HBx-expressing hepatoma cells, whereas HBx abolishment reduced FABP1 expression. Mechanistically, HBx activated the FABP1 promoter in an HNF3β-, C/EBPα-, and PPARα-dependent manner, in which HBx increased the gene expression of HNF3β and physically interacted with C/EBPα and PPARα. On the other hand, knockdown of FABP1 remarkably blocked lipid accumulation both in long-chain free fatty acids treated HBx-expressing HepG2 cells and in a high-fat diet-fed HBx-transgenic mice. Therefore, FABP1 is a key driver gene in HBx-induced hepatic lipid accumulation via regulation of HNF3β, C/EBPα, and PPARα. FABP1 may represent a novel target for treatment of HBV-associated hepatic steatosis.

IMPORTANCE

Accumulating evidence from epidemiological and experimental studies has indicated that chronic HBV infection is associated with hepatic steatosis. However, the molecular mechanism underlying HBV-induced pathogenesis of hepatic steatosis still remains to be elucidated. In this study, we found that expression of liver fatty acid binding protein (FABP1) was dramatically increased in the sera of HBV-infected patients and in both sera and liver tissues of HBV-transgenic mice. Forced expression of HBx led to FABP1 upregulation, whereas knockdown of FABP1 remarkably diminished lipid accumulation in both in vitro and in vivo models. It is possible that HBx promotes hepatic lipid accumulation through upregulating FABP1 in the development of HBV-induced nonalcoholic fatty liver disease. Therefore, inhibition of FABP1 might have therapeutic value in steatosis-associated chronic HBV infection.

Hepatitis B virus X protein increases the IL-1β-induced NF-κB activation via interaction with evolutionarily conserved signaling intermediate in Toll pathways (ECSIT)

Hepatitis B virus X protein (HBx) transactivates multiple transcription factors including nuclear factor-kappa B (NF-κB) that regulates inflammatory-related genes. However, the regulatory mechanism of HBx in NF-κB activation remains largely unknown. This study reports that HBx augments the interleukin-1β (IL-1β)-induced NF-κB activation via interaction with a Toll-like receptor (TLR) adapter protein, ECSIT (evolutionarily conserved signaling intermediate in Toll pathways). GST pull-down and co-immunoprecipitation analyses showed that HBx interacted with ECSIT. Deletion analysis of HBx in a CytoTrap two-hybrid system revealed that the interaction region of HBx for ECSIT was attributed to aa 51-80. Co-transfection of HBx and ECSIT in IL-1β-stimulated cells appeared to activate IKK and IκB signaling pathway as phosphorylation of both IKK α/β and IκBα was increased whereas knockdown of ECSIT or HBxΔ51-80 mutant attenuated the phosphorylation. As a consequence of IκBα degradation, NF-κB was activated as evidenced by increases in NF-κB transcriptional activity and the nuclear translocation of p65 and p50 that resulted in the induction of IL-10. In contrast, knockdown of ECSIT by siRNA or treatment with an NF-κB selective inhibitor (helenalin) abolished the NF-κB activation and IL-10 expression. We conclude that ECSIT appears to be a novel HBx-interacting signal molecule and their interaction is mechanistically important in IL-1β induction of NF-κB activation.

Epigenetic silencing of NAD(P)H:quinone oxidoreductase 1 by hepatitis B virus X protein increases mitochondrial injury and cellular susceptibility to oxidative stress in hepatoma cells

NAD(P)H:quinone oxidoreductase 1 (NQO1) is a phase II enzyme that participates in the detoxification of dopamine-derived quinone molecules and reactive oxygen species. Our prior work using a proteomic approach found that NQO1 protein levels were significantly decreased in stable hepatitis B virus (HBV)-producing hepatoma cells relative to the empty-vector-transfected controls. However, the mechanism and biological significance of the NQO1 suppression remain elusive. In this study we demonstrate that HBV X protein (HBx) induces epigenetic silencing of NQO1 in hepatoma cells through promoter hypermethylation via recruitment of DNA methyltransferase DNMT3A to the promoter region of the NQO1 gene. In HBV-related hepatocellular carcinoma (HCC) specimens, HBx expression was correlated negatively to NQO1 transcripts but positively to NQO1 promoter hypermethylation. Downregulation of NQO1 by HBx reduced intracellular glutathione levels, impaired mitochondrial function, and increased susceptibility of hepatoma cells to oxidative stress-induced cell injury. These results suggest a novel mechanism for HBV-mediated pathogenesis of chronic liver diseases, including HCC.

Identification of cellular proteome using two-dimensional difference gel electrophoresis in ST cells infected with transmissible gastroenteritis coronavirus

Background

Transmissible gastroenteritis coronavirus (TGEV) is an enteropathogenic coronavirus that causes diarrhea in pigs, which is correlated with high morbidity and mortality in suckling piglets. Information remains limited about the comparative protein expression of host cells in response to TGEV infection. In this study, cellular protein response to TGEV infection in swine testes (ST) cells was analyzed, using the proteomic method of two-dimensional difference gel electrophoresis (2D DIGE) coupled with MALDI-TOF-TOF/MS identification.

Results

33 differentially expressed protein spots, of which 23 were up-regulated and 10 were down-regulated were identified. All the protein spots were successfully identified. The identified proteins were involved in the regulation of essential processes such as cellular structure and integrity, RNA processing, protein biosynthesis and modification, vesicle transport, signal transduction, and the mitochondrial pathway. Western blot analysis was used to validate the changes of alpha tubulin, keratin 19, and prohibitin during TGEV infection.

Conclusions

To our knowledge, we have performed the first analysis of the proteomic changes in host cell during TGEV infection. 17 altered cellular proteins that differentially expressed in TGEV infection were identified. The present study provides protein-related information that should be useful for understanding the host cell response to TGEV infection and the underlying mechanism of TGEV replication and pathogenicity.

HeLa cell response proteome alterations induced by mammalian reovirus T3D infection

Background

Cells are exposed to multiple stressors that induce significant alterations in signaling pathways and in the cellular state. As obligate parasites, all viruses require host cell material and machinery for replication. Virus infection is a major stressor leading to numerous induced modifications. Previous gene array studies have measured infected cellular transcriptomes. More recently, mass spectrometry-based quantitative and comparative assays have been used to complement such studies by examining virus-induced alterations in the cellular proteome.

Methods

We used SILAC (stable isotope labeling with amino acids in cell culture), a non-biased quantitative proteomic labeling technique, combined with 2-D HPLC/mass spectrometry and reciprocal labeling to identify and measure relative quantitative differences in HeLa cell proteins in purified cytosolic and nuclear fractions after reovirus serotype 3 Dearing infection. Protein regulation was determined by z-score analysis of each protein’s label distribution.

Results

A total of 2856 cellular proteins were identified in cytosolic fractions by 2 or more peptides at >99% confidence and 884 proteins were identified in nuclear fractions. Gene ontology analyses indicated up-regulated host proteins were associated with defense responses, immune responses, macromolecular binding, regulation of immune effector processes, and responses to virus, whereas down-regulated proteins were involved in cell death, macromolecular catabolic processes, and tissue development.

Conclusions

These analyses identified numerous host proteins significantly affected by reovirus T3D infection. These proteins map to numerous inflammatory and innate immune pathways, and provide the starting point for more detailed kinetic studies and delineation of virus-modulated host signaling pathways.

Proteomic changes in HEK-293 cells induced by hepatitis delta virus replication

Hepatitis delta virus (HDV) infection greatly increases the risk of hepatocellular carcinoma in hepatitis B virus chronically infected patients. HDV is highly dependent on host factors for accomplishment of the replication cycle. However, these factors are largely unknown and the mechanisms involved in the pathogenicity of the virus still remain elusive. Here, we made use of the HEK-293 cell line, which was engineered in order to mimic HDV replication. Five different proteomes were analyzed and compared using a MS-based quantitative proteomics approach by (18)O/(16)O stable isotope labeling. About 3000 proteins were quantified and 89 found to be differentially expressed as a consequence HDV RNA replication. The down-regulation of p53 , HSPE, and ELAV as well as the up-regulation of Transportin 1 , EIF3D, and Cofilin 1 were validated by Western blot. A systems biology approach was additionally used to analyze altered pathways and networks. The G2/M DNA damage checkpoint and pyruvate metabolism were among the most affected pathways, and Cancer was the most likely disease associated to HDV replication. Western blot analysis allowed identifying 14-3-3 σ interactor as down-regulated protein acting in the G2/M cell cycle control checkpoint. This evidence supports deregulation of G2/M checkpoint as a possible mechanism involved in the promotion of HDV associated hepatocellular carcinoma.

BIOLOGICAL SIGNIFICANCE

This manuscript provides a description of changes observed in the cellular proteome that arise as result of expression of the hepatitis delta virus (HDV) antigen as well as virus genome replication. Using a systems biology approach cancer was found to be the most probable disease associated with HDV replication. Additionally, results show that HDV alters the regulation of G2/M cell cycle control checkpoint. Taken together, our data provide new insights into probable mechanisms associated with the increased incidence of hepatocellular carcinoma observed in HDV infected patients.

Non-Biased Enrichment Does Not Improve Quantitative Proteomic Delineation of Reovirus T3D-Infected HeLa Cell Protein Alterations

Mass spectrometry-based methods have allowed elucidation of alterations in complex proteomes, such as eukaryotic cells. Such studies have identified and measured relative abundances of thousands of host proteins after cells are infected with a virus. One of the potential limitations in such studies is that generally only the most abundant proteins are identified, leaving the deep richness of the cellular proteome largely unexplored. We differentially labeled HeLa cells with light and heavy stable isotopic forms of lysine and arginine and infected cells with reovirus strain T3D. Cells were harvested at 24 h post-infection. Heavy-labeled infected and light-labeled mock-infected cells were mixed together 1:1. Cells were then divided into cytosol and nuclear fractions and each fraction analyzed, both by standard 2D-HPLC/MS, and also after each fraction had been reacted with a random hexapeptide library (Proteominer^® beads) to attempt to enrich for low-abundance cellular proteins. A total of 2,736 proteins were identified by two or more peptides at >99% confidence, of which 66 were significantly up-regulated and 67 were significantly down-regulated. Up-regulated proteins included those involved in antimicrobial and antiviral responses, GTPase activity, nucleotide binding, interferon signaling, and enzymes associated with energy generation. Down-regulated proteins included those involved in cell and biological adhesion, regulation of cell proliferation, structural molecule activity, and numerous molecular binding activities. Comparisons of the r² correlations, degree of dataset overlap, and numbers of peptides detected suggest that non-biased enrichment approaches may not provide additional data to allow deeper quantitative and comparative mining of complex proteomes.

Hepatitis B virus X protein blocks filamentous actin bundles by interaction with eukaryotic translation elongat ion factor 1 alpha 1

Hepatitis B virus (HBV)-encoded X protein (HBx protein) is a multi-functional regulatory protein. It functions by protein-protein interaction and plays a pivotal role in the pathogenesis of HBV-related diseases. However, the partners in hepatocytes interacting with HBx protein are far from understood fully. In this study, immunoprecipitation was employed to screen for binding partners for the HBx protein from huh-7 hepatoma cells infected with recombinant adenovirus expressing HBx protein, and five cellular proteins including eukaryotic translation elongation factor 1 alpha 1 (eEF1A1), were identified. The interaction between HBx protein and eEF1A1 was confirmed further using a GST pull-down assay and co-immunoprecipitation, respectively. In Huh-7 hepatoma cells, the HBx protein inhibits dimer formation of eEF1A1, hence blocks filamentous actin bundling. These findings provide new insights into the molecular mechanisms involved in the functions of the HBx protein.

Hepatitis B virus X protein enhances activation of nuclear factor κB through interaction with valosin-containing protein

Hepatitis B virus X protein (HBx protein) is a multifunctional regulatory protein. The transactivation of nuclear factor kappa B (NF-κB) by HBx protein has been shown to be of importance in the pathogenesis of HBV-related diseases. However, the mechanism involved remains largely unclear. In this study, a CytoTrap yeast two-hybrid system was employed to screen binding partners of the HBx protein; 29 cellular proteins, including valosin-containing protein (VCP), were identified. The interaction between HBx protein and VCP was further confirmed in vitro and in vivo using a glutathione S-transferase pull-down assay and co-immunoprecipitation, respectively. It was also shown that this interaction is mediated by amino acid residues 51-120 of the HBx protein. In Huh-7 hepatoma cells, HBx protein enhanced the VCP-mediated activation of NF-κB. Our findings provide new insights into the molecular mechanisms that lead to the activation of NF-κB by HBx protein.

Hepatitis B doubly spliced protein, generated by a 2.2 kb doubly spliced hepatitis B virus RNA, is a pleiotropic activator protein mediating its effects via activator protein-1- and CCAAT/enhancer-binding protein-binding sites

The 2.2 kb doubly spliced defective hepatitis B virus (HBV) genome is frequently detected in the serum of patients with chronic hepatitis B. However, the biological significance of this type of defective genome is not well understood. In this study, expression of the hepatitis B doubly spliced protein (HBDSP) was confirmed from the 2.2 kb doubly spliced defective HBV genome, which was isolated and transfected into Huh-7 hepatoma cells. To explore the potential pathogenicity of HBDSP, hepatocellular proteins interacting with HBDSP were screened by a yeast two-hybrid assay. Unexpectedly, HBDSP could transactivate the GAL4-responsive element, and deletion mapping revealed that the fragment located between residues Leu-48 and Gln-75 of HBDSP was crucial for transactivation activity. In Huh-7 hepatoma cells, HBDSP localized predominantly to the cytoplasm and showed transactivating effects on the cytomegalovirus immediate-early promoter, simian virus 40 enhancer/promoter and HBV regulatory elements including the S1 promoter, S2 promoter, Enhancer I and core upstream regulatory sequences. Further studies revealed that the transactivating activities were mediated by activator protein-1- and CCAAT/enhancer-binding protein-binding sites. These findings suggest that HBDSP is a pleiotropic activator protein that can potentially serve as an HBV virulence factor.