Potential conflict of interest: Nothing to report.
Oxidative stress has been identified as a key mechanism of hepatitis C virus (HCV)–induced pathogenesis. Studies have suggested that HCV increases the generation of hydroxyl radical and peroxynitrite close to the cell nucleus, inflicting DNA damage, but the source of reactive oxygen species (ROS) remains incompletely characterized. We hypothesized that HCV increases the generation of superoxide and hydrogen peroxide close to the hepatocyte nucleus and that this source of ROS is reduced nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase 4 (Nox4). Huh7 human hepatoma cells and telomerase-reconstituted primary human hepatocytes, transfected or infected with virus-producing HCV strains of genotypes 2a and 1b, were examined for messenger RNA (mRNA), protein, and subcellular localization of Nox proteins along with the human liver. We found that genotype 2a HCV induced persistent elevations of Nox1 and Nox4 mRNA and proteins in Huh7 cells. HCV genotype 1b likewise elevated the levels of Nox1 and Nox4 in telomerase-reconstituted primary human hepatocytes. Furthermore, Nox1 and Nox4 proteins were increased in HCV-infected human liver versus uninfected liver samples. Unlike Nox1, Nox4 was prominent in the nuclear compartment of these cells as well as the human liver, particularly in the presence of HCV. HCV-induced ROS and nuclear nitrotyrosine could be decreased with small interfering RNAs to Nox1 and Nox4. Finally, HCV increased the level of transforming growth factor beta 1 (TGFβ1). TGFβ1 could elevate Nox4 expression in the presence of infectious HCV, and HCV increased Nox4 at least in part through TGFβ1. Conclusion: HCV induced a persistent elevation of Nox1 and Nox4 and increased nuclear localization of Nox4 in hepatocytes in vitro and in the human liver. Hepatocyte Nox proteins are likely to act as a persistent, endogenous source of ROS during HCV-induced pathogenesis. Hepatology 2010
Hepatitis C virus (HCV) is a blood-borne pathogen that can cause serious liver diseases such as cirrhosis and hepatocellular carcinoma. The mechanism by which HCV induces pathogenesis remains unclear. However, HCV infection is associated with significant oxidative/nitrosative stress with increased lipid peroxidation and oxidative DNA damage, and oxidative/nitrosative stress has been identified as a potential key player in the pathogenesis induced by HCV. In terms of chemistry, HCV infection has been associated with iron overload, and phlebotomy improves oxidative stress markers and liver pathology; this suggests a role for Fenton chemistry.1 In addition, oxidative DNA damage and mutations to p53 that occur with HCV can be decreased by inhibition of the synthesis of nitric oxide, and nitrotyrosine is elevated in the liver of hepatitis C patients; this indicates that peroxynitrite is also likely to be involved.2 Peroxynitrite is generated in a nonenzymatic reaction between nitric oxide and superoxide anion.
In terms of the source, nitric oxide is likely to derive from inducible nitric oxide synthase, which is induced by HCV. The source of reactive oxygen species (ROS) during HCV infection, however, has not been completely characterized. Several studies have identified mitochondria as the source of ROS in various cell culture models of HCV, and mitochondrial dysfunction is likely to be important in HCV-induced pathogenesis. However, the core protein of the Japanese fulminant hepatitis 1 (JFH1) strain, which generates infectious virus particles in cell culture, does not localize to the mitochondria, and whether HCV elements are sufficient to induce mitochondrial ROS production, permeability transition, and apoptosis in a consistent manner is unclear.3, 4 In addition, reactive species tend to be compartmentalized in the cell and form a concentration gradient that originates from their sites of generation.5 In particular, hydroxyl radical is highly reactive and tends to react with whatever molecule is nearby; the chemical reactivity of nitric oxide is also markedly increased by conversion to peroxynitrite. Thus, for HCV to induce peroxynitrite and hydroxyl radical–dependent DNA damage directly, these molecules would need to be generated close to the DNA. Furthermore, unlike nitric oxide and hydrogen peroxide (H2O2), which can diffuse across membranes, whether superoxide anion can escape the mitochondria to react with nitric oxide in the nucleus is questioned. The nitric oxide–dependent nuclear DNA damage that occurs during HCV infection, therefore, suggests that there might be another source of superoxide anion closer to the cell nucleus.
In this respect, another potential source of ROS during HCV infection is the reduced nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase (Nox) proteins, which consist of Nox1, Nox2, Nox3, Nox4, Nox5, dual oxidase 1 (Duox1), and Duox2.6 Nox proteins catalyze the transfer of electrons from NAD(P)H to O2 to produce superoxide and, secondarily, H2O2 through dismutation of superoxide. Recently, hepatocytes and Huh7 human hepatoma cells have been found to express Nox family enzymes.7 Furthermore, Nox4 has been reported to localize to the nuclei of various cell types.8, 9 Diphenylene iodonium (DPI), which was used to decrease ROS generation by HCV core protein in the initial study by Okuda et al.,4 is also an inhibitor of flavoproteins, which is commonly used to inhibit Nox.4 There is also a precedent for the activation of Nox2 in phagocytes during HCV infection in response to HCV NS3 protein.1
The goal of this study, therefore, was to examine the role of hepatocyte Nox protein(s) in ROS and peroxynitrite generation, which is increased by HCV. We hypothesize that HCV increases the generation of peroxynitrite and ROS close to the cell nucleus and that this source of ROS is Nox4. Using state-of-the-art in vitro replication models of HCV as well as the human liver, we present evidence that (1) hepatocyte Nox1 and Nox4 are persistently elevated with HCV genotypes 2a and 1b, (2) HCV increases the nuclear localization of Nox4, (3) hepatocyte Nox enzymes are prominent sources of ROS and peroxynitrite generation in the nucleus during HCV infection, and (4) transforming growth factor beta 1 (TGFβ1) likely plays an important role in the modulation of Nox4 by HCV.
pJFH1 (which generates infectious virus particles of genotype 2a), its replicative-null mutant (pJFH1-GND), subgenomic pSgJFH1-Luc (which supports viral RNA genome replication without generating virus particles), and genotype 1b pEF-CG1bRbz/Neo plus its replicative-null mutant (pEF-CG1b GNDRbz/Neo) were used (Sg and Luc indicate subgenomic and luciferase, respectively. EF denotes elongation factor 1α promotor. Rbz and Neo indicate ribozyme and neomycin phosphotransferase, respectively).10, 11
Cells, Transfection/Infection, and Tissues.
Huh7 human hepatoma cells were transfected with in vitro transcribed HCV RNA and cultured, as previously described.12 Telomerase-reconstituted primary human hepatocytes were transfected with pEF-CG1bRbz/Neo, pEF-CG1bGNDRbz/Neo, or control EF-driven vector alone as described for Huh7, and cell clones that were stably transfected with these constructs were selected and maintained in a G418-containing medium (Invitrogen).13 For virus infection, 2 mL of the extracellular medium from JFH1-transfected cells was used to inoculate naïve Huh7 cells with 3 mL of fresh medium, as described.10 Then, the cells were cultured and harvested at various time points, as indicated in the Results section.
Huh7-Nox4 cells, which constitutively overexpress Nox4 enzyme, were generated by the transfection of Huh7 cells with human Nox4 complementary DNA (cDNA) with Lipofectamine 2000 (Invitrogen) and by the selection of stable cell clones with 0.5 mg/mL G418 (Invitrogen). Control cell clones (Huh7-pcDNA), transfected with an empty plasmid vector alone, were also generated. These cells were maintained in a 0.25 mg/mL G418-containing medium. For experiments involving these or other stable cell clones, G418 was removed from the cell culture medium 1 day prior to the experiment.
HCV-infected and uninfected human liver tissues were acquired through the National Disease Research Interchange. Tissues were snap-frozen and were human immunodeficiency virus–negative and hepatitis B virus surface antigen–negative. All tissues were from females between the ages of 49 and 56 years who suffered non–liver-related deaths. This study was approved by the institutional review boards at Lawrence Livermore National Laboratory and the University of California, Merced.
Determination of Nox Messenger RNA (mRNA) and Protein Levels.
Total intracellular RNA was extracted with TRIzol (Invitrogen). Nox/Duox mRNA levels were quantified by real-time quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) with Power SYBR green PCR master mix. Primer sequences for Nox1, Nox2, Nox3, Nox4, Nox5, Duox1, and Duox2 are summarized in Supporting Table 1. qRT-PCR results for Nox4 were confirmed by standard reverse-transcriptase polymerase chain reaction with three different primers sets (Supporting Table 1). qRT-PCR reactions without an RNA template and without reverse transcriptase served as negative controls. The RNA levels were normalized by 18S ribosomal RNA (rRNA) or glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNA. Western blot analyses of proteins were carried out, as previously described.12 Data were analyzed by densitometry with IS2000R (Kodak).
Subcellular fractionation of nuclear and cytoplasmic fractions was performed as described by Vayalil et al.14 or with the NE-PER kit (Pierce). Then, the fractions were analyzed with western blots. Calnexin, pan-cadherin, and GAPDH were analyzed as cytoplasmic markers, and lamin A/C and histone deacetylase 1 (HDAC1) were analyzed as nuclear markers with antibodies from Santa Cruz Biotechnologies.
Immunofluorescence Staining and Confocal Laser Scanning Microscopy.
Cells were fixed with 3.5% formaldehyde for 5 minutes and incubated with phosphate-buffered saline containing 1% (wt/vol) bovine serum albumin, 0.05% (wt/vol) NaN3, and 0.02% (wt/vol) saponin. Samples were subsequently incubated with primary and then fluorophore-conjugated secondary antibodies, mounted onto microscope slides, and imaged via confocal laser scanning microscopy (C1, Nikon). When propidium iodide (PI) was used, 100 μg/mL RNase A was added during primary antibody incubation to remove the RNA. Tissue samples were fixed for 15 minutes with 100% acetone and blocked with 0.4% Triton-X containing 5% bovine serum albumin for 30 minutes prior to incubation with the antibodies.
Small Interfering RNA (siRNA).
Control and HCV-replicating cells were transfected with Nox1, Nox4, or nontargeting control siRNAs (100 nM; Smartpool siRNAs, Dharmacon) with RNAiMax (Invitrogen) per the manufacturer's protocol.
H2O2, Superoxide, Nitrotyrosine, and Nox Enzyme Activity Measurements.
Cells were incubated with 10 μM dihydroethidium (HE) for 30 minutes and analyzed for intracellular superoxide via the monitoring of the level of 2-hydroxyethidium (2-OH-E+) by high-performance liquid chromatography (HPLC).15 Extracellular H2O2 was measured by horse radish peroxidase–catalyzed p-hydroxyphenylaminoacetic acid dimerization assay.16 Data were normalized by the total protein, which was determined by bicinchoninic acid assay (Pierce). Nitrotyrosine levels were analyzed by confocal microscopy with antibodies to nitrotyrosine (Santa Cruz Biotechnologies). The total cellular adenosine triphosphate (ATP) content was determined with an ATP assay kit (Sigma-Aldrich). Nox enzyme activities were determined by cytochrome c assay17 (see the supporting information for details).
The TGFβ1 level in the medium samples was determined with the TGF-Beta 1 Ready-Set-Go! kit (eBioscience) and normalized by the cell number.
Data were analyzed with the Student t test or one-way analysis of variance with SigmaStat 3.1 (Jandel Scientific). A P value ≤ 0.05 was considered significant. Data are presented as means ± standard errors of the mean. Experiments were repeated three to eight times.
HCV Induces a Pro-Oxidative Environment with Increased Levels of ROS.
Huh7 human hepatoma cells were transfected with JFH1 RNA of genotype 2a, which generates infectious HCV particles in cell culture, and were evaluated first for viral replication. Mock transfection, transfection with replicative-null GND RNA, or both were performed as negative controls. Replication of JFH1 but not the GND RNA was readily demonstrated by the continued detection of HCV RNAs and proteins by western blotting (Supporting Fig. 1).
To determine whether HCV increased the level of ROS, we measured the H2O2 concentration. As H2O2 diffuses across biomembranes, the H2O2 concentration was assessed by the extracellular measurement of H2O2. As shown in Fig. 1A, the H2O2 concentration increased significantly with HCV, and this increase was almost completely removed by DPI, an inhibitor of flavoproteins. HCV also increased the fluorescence of H2-dichlorofluorescein diacetate, which measures nonspecific intracellular oxidation, and altered the intracellular glutathione (GSH) concentration in a DPI-sensitive manner (Supporting Fig. 2).
Next, we analyzed the intracellular superoxide concentration by monitoring the generation of 2-OH-E+, a specific product of superoxide, from HE with HPLC.15 Menadione, which generated ROS via redox cycling, was used as a positive control. Both menadione and HCV increased the level of 2-OH-E+ (Fig. 1B). In contrast, extracellular generation of superoxide, measured by the nitroblue tetrazolium reduction assay, did not increase significantly with HCV (P > 0.05; data not shown). Therefore, the infectious virus-generating JFH1 strain induced a pro-oxidative environment with increased levels of ROS in Huh7 cells.
Nox4 and Nox1 mRNAs Are Persistently Increased with Genomic but Not Subgenomic JFH1 RNA.
The data in Fig. 1 also suggested that flavoproteins were involved in the increased generation of ROS in the JFH1 cells. Therefore, we examined whether the mitochondria served as the source of ROS by incubating cells with MitoSOX Red, which measures mitochondrial superoxide, and monitoring its fluorescence by confocal microscopy. Antimycin increased the detection of mitochondrial superoxide anions as expected (Supporting Fig. 2C). However, we did not find any significant increase in the mitochondrial superoxide with JFH1 (Supporting Fig. 2C). In addition, the total cellular ATP content was not significantly altered by JFH1 (92.2% ± 4.1% of the control, P > 0.05). These data suggested that flavoproteins other than those in the mitochondria were responsible for the increased level of ROS in JFH1-transfected cells.
Next, we evaluated whether hepatocyte Nox proteins played a role in the increased detection of ROS with HCV. Huh7 cells were transfected with JFH1 RNA or mock-transfected and analyzed for Nox mRNA levels by qRT-PCR.7 Cells were also transfected with subgenomic JFH1 RNA for comparison. All seven Nox mRNAs could be detected in these cells (Supporting Table 2). Most of all, we found that Nox4 mRNA began to be significantly elevated in the JFH1 cells at 48 hours, and the increase persisted at least to day 17, at which point the increase was more than 10-fold (Fig. 2A). In addition, Nox1 mRNA increased significantly with JFH1, and the increase persisted at least to days 14 to 17 (Fig. 2B; some data not shown). In contrast, Duox2 mRNA increased between 48 and 96 hours with JFH1, but this increase was not sustained (Fig. 2C). Nox2, Nox3, Nox5, and Duox1 mRNAs did not increase with JFH1 (data not shown). Subgenomic JFH1SgLuc RNA, which supports viral RNA genome replication without producing virus particles, replicated in these cells as expected (Supporting Fig. 3) but did not elevate Nox1, Nox4, or Duox2 mRNAs (Fig. 2C,D). Thus, Nox1 and Nox4 mRNAs showed prolonged elevation with genotype 2a HCV in cell culture, and the structural genes of HCV and/or generation of infectious virions appeared to be necessary for the increases. HCV also increased p22phox, NOXA1, NOXO1, and p67phox mRNAs (Supporting Fig. 4).
Nox4 and Nox1 Protein Levels Are Also Increased by HCV.
Next, Huh7 cells that were either transfected with JFH1 RNA or infected with a virus-containing cell culture medium from JFH1 RNA-transfected cells (Supporting Fig. 5) were analyzed for the levels of Nox1 and Nox4 proteins by western blotting. Nox1 and Nox4 proteins increased with HCV RNA transfection as well as infection (Fig. 3A,B,D). Higher molecular weight bands (>65 kDa) were also detected, particularly in the presence of HCV. Furthermore, Nox1 and Nox4 proteins were significantly elevated in HCV-infected human liver versus uninfected liver samples (Fig. 3C). Therefore, Nox1 and Nox4 proteins were significantly elevated in vitro and during natural infection in vivo in the presence of HCV.
HCV, Nox Enzymes, and ROS.
To examine whether Nox1 and Nox4 played a role in the virus-induced ROS elevation, we used siRNAs to specifically knock down Nox1 and Nox4 gene expression in these cells. Nox1 siRNA decreased the Nox1 protein level to 27.3% ± 19.2% of the level of the controls transfected with nontargeting siRNAs at 72 hours (P < 0.05); Nox4 siRNA decreased the Nox4 protein level to 45.2% ± 12.3% of the level of the controls at 72 hours (P < 0.05; Fig. 4A). In addition, Nox1 and Nox4 siRNAs significantly decreased H2O2 and intracellular superoxide concentrations in the JFH1 cells (Fig. 4B,C). Nox1 and Nox4 siRNAs did not decrease other Nox mRNAs and selectively decreased the target protein without affecting Nox4 and Nox1 proteins, respectively (Supporting Fig. 6; some data not shown). We also performed Nox activity assays with permeabilized Huh7 cells and found increased Nox enzyme activity with HCV that could be inhibited by DPI as well as siRNAs to Nox1 and Nox4 (Fig. 4D). Therefore, Nox1 and Nox4 proteins, which were increased by HCV in these cells, were functionally active in the generation of ROS. Likewise, the HCV-infected liver showed an increase in the NADPH–dependent generation of superoxide that was DPI-sensitive (Fig. 4E).
To examine whether Nox4 overexpression was sufficient to increase the generation of ROS and to ascertain whether Nox4 could generate superoxide anion in our system, we also generated Huh7-Nox4 cells that were stably transfected with human Nox4 cDNA. As shown in Fig. 4F, Huh7-Nox4 cells showed increased expression of Nox4 protein in comparison with the control cell clones stably transfected with an empty plasmid vector instead. Also, both H2O2 and intracellular superoxide concentrations were elevated in the Nox4-overexpressing cells (Fig. 4F).
Subcellular Localization of Nox1 and Nox4.
Next, we determined the subcellular localization of Nox1 and Nox4 proteins by confocal laser scanning microscopy. The nucleus was counterstained with PI. Nox4 was found in the cytoplasm as well as nucleus in control Huh7 cells (Fig. 5A). In addition, the amount of Nox4 in the nucleus increased significantly with HCV. Conversely, Nox1 showed primarily cytoplasmic, extranuclear localization in both control and JFH1 cells (Fig. 5B). HCV core protein was readily detected in the JFH1 cells, and this indicated that the viral proteins were being expressed as expected (Fig. 5C). Additional immunofluorescence studies showed a colocalization of Nox4 and calnexin, an endoplasmic reticulum marker, as well as an overlap between Nox4 and lamin A/C, a nuclear membrane protein; nuclear Nox4 and colocalization of Nox4 with lamin A/C again increased with HCV (Supporting Fig. 7). Cell fractionation studies further confirmed the presence of Nox4 in both cytoplasmic and nuclear fractions from control and JFH1 cells, and the amount of Nox4 protein increased in both fractions with HCV (Fig. 5D). Again, Nox1 was predominantly located in the cytoplasmic fraction, and its location did not change significantly with HCV (Fig. 5E). Therefore, Nox4 showed at least partial nuclear localization in Huh7 cells, and the amount of Nox4 in the nucleus increased with HCV.
Hepatocyte Nox1 and Nox4, Genotype 1b HCV, and Human Liver.
The prevalence of HCV genotype 2a can be as high as 20% and depends on the geographical region, but the most prevalent genotype is genotype 1. Therefore, we examined whether HCV genotype 1b also increased the nuclear localization of Nox4 with a CG1bRbz construct that generated HCV genotype 1b.11 Telomerase-reconstituted primary human fetal hepatocytes that were stably transfected with CG1bRbz/Neo, replicative-null CG1bRbz GND/Neo, or an empty vector alone were selected with G418. Both positive-sense and negative-sense HCV RNAs and core protein expression were demonstrated in the CG1bRbz cells by qRT-PCR and immunofluorescence staining (Supporting Fig. 8).
We found that Nox4 protein was increased in CG1bRbz-transfected fetal hepatocytes versus cells transfected with CG1bRbz GND or the control plasmid alone (Fig. 6A). In addition, Nox4 was prominent in both the nucleus and cytoplasm of CG1bRbz-transfected fetal cells (Fig. 6A). Nox1 was also elevated in the hepatocytes with CG1bRbz (Fig. 6B). The genotype 1b subgenomic replicon (Con1), like the JFH1 replicon, did not induce Nox1 or Nox4 (data not shown). Therefore, hepatocyte Nox4 and Nox1 were modulated similarly by HCV genotypes 1b and 2a.
Then, we evaluated human liver samples to test whether Nox1 and Nox4 showed similar subcellular localization during natural HCV infection. HCV core protein in the HCV+ liver sample was readily detected by immunofluorescence (Fig. 6C). Consistent with the data in Fig. 3C, increased levels of both Nox1 and Nox4 proteins could be detected in the HCV-infected human liver compared to the uninfected liver (Fig. 6C,D). Furthermore, Nox4 colocalized with lamin A/C in the HCV-infected liver (Fig. 6C). As a control, Duox1 did not increase in these tissues with HCV or show an overlap with lamin A/C (Fig. 6E). Therefore, HCV also increased the nuclear localization of Nox4 during natural HCV infection.
HCV Increases the Level of Nitrotyrosine in the Nucleus: The Role of Nox4.
Then, we examined whether Nox4 served as a source of ROS for increased generation of peroxynitrite close to the cell nucleus. Control and HCV-replicating cells were analyzed for nitrotyrosine by confocal microscopy with and without knockdown of Nox1 and Nox4 gene expression with the siRNAs. HCV increased the level of nitrotyrosine in the nucleus (Fig. 7A). In addition, Nox4 siRNA decreased the level of nitrotyrosine in the nucleus, as did NG-methyl-L-arginine acetate (L-NMA), an inhibitor of nitric oxide synthase (Fig. 7B). Nox1 siRNA also led to an overall decrease in the level of nitrotyrosine in these cells, but in contrast to Nox4 siRNA, some nuclear nitrotyrosine remained (see the arrows, Fig. 7B). In addition, we performed a Nox activity assay using nuclear fractions from JFH1 and mock-transfected cells, and we found increased generation of superoxide with HCV that was DPI-sensitive (Fig. 7C); nuclear Nox activity could also be partly attenuated with Nox4 siRNA by 24.7% ± 1.3% (P < 0.05). Therefore, hepatocyte Nox enzymes could act as a prominent source of ROS for the generation of peroxynitrite in and around the nucleus during complete HCV replication.
TGFβ1 Is Involved in the Elevation of Nox4 by HCV.
TGFβ has been shown to induce Nox4, and the TGFβ concentration is elevated in hepatitis C patients.6, 18 Thus, we evaluated whether HCV increased the level of TGFβ in our system and whether HCV elevated Nox4 through TGFβ. TGFβ1 increased Nox4 mRNA in the HCV-replicating cells (Fig. 8A). Furthermore, HCV increased the level of TGFβ1, and the HCV-induced increase in Nox4 could be attenuated with antibodies to TGFβ1 (Fig. 8B,C). These data suggest that TGFβ1 is involved in the elevation of Nox4 by HCV.
Oxidative/nitrosative stress has been increasingly implicated in viral infections, including HCV, but the sources of ROS during HCV infection have not been completely characterized. Here, using state-of-the-art HCV cell culture systems and human liver samples, we present evidence that hepatocyte Nox1 and Nox4 are prominent sources of ROS during complete HCV replication. In agreement with a recent report that JFH1 core does not localize to the mitochondria, we did not find a significant elevation of mitochondrial ROS or ATP depletion with JFH1.3 However, it is possible that the role of mitochondria in HCV-induced oxidative stress is more pronounced with certain viral genotypes or cell types. Previously, HCV core protein was suggested to reduce the cell's ability to up-regulate its antioxidant defenses.1 However, hepatitis C patients have elevated levels of antioxidant genes, and JFH1 increased the GSH concentration in our study (Supporting Fig. 2B)1; thus, to what extent HCV interferes with the antioxidant defense mechanisms during complete viral replication remains to be further examined.
In this study, our objective was not only to find the source of ROS during complete HCV replication but also to find the source of superoxide for peroxynitrite generation that we predicted would occur near the cell nucleus. In agreement with this hypothesis, nitrotyrosine and Nox activity were increased in the JFH1-transfected cell nucleus, and this increase was attenuated with siRNAs to Nox. Also, although the relative amount of nuclear Nox4 versus cytoplasmic Nox4 tended to vary from one experiment to another, Nox4 was always at least partly nuclear and colocalized with lamin A/C, particularly in the presence of HCV. Furthermore, HCV elevated the intracellular superoxide concentration, and Huh7 cells overexpressing Nox4 showed an increased superoxide level. These data do not completely rule out the possibility that Nox4 generates superoxide indirectly through another source (or other sources) of superoxide in the cell, and the significant effect that Nox1 siRNA had on nuclear nitrotyrosine could at least in part be due to the uncoupling of nitric oxide synthase by peroxynitrite. Nevertheless, our data strongly indicate that Nox enzymes can elevate the intracellular superoxide concentration either directly or indirectly in the cell and lead to increased generation of peroxynitrite in the hepatocyte nucleus during HCV infection. Indeed, although Nox4 has recently been suggested to generate H2O2 rather than superoxide by virtue of the chemical mechanism involving a terminal electron transfer from the one electron–carrying heme B, Nox family proteins must generate superoxide first before the formation of secondary products.6 Thus, the reported inability to detect superoxide with some Nox/Duox enzymes is likely due to rapid dismutation of superoxide to form H2O2, which under some circumstances occurs more rapidly than the reaction with the superoxide-detecting probe. Also, if anything can outcompete superoxide dismutase (SOD) for superoxide, it will be nitric oxide reacting with superoxide to generate peroxynitrite.5 In fact, we were able to detect a larger amount of 2-OH-E+ by inhibiting SOD in our cells, and this suggests a significant competition between the probe and SOD for the reaction with superoxide (unpublished observations, Reyes de Mochel and Choi, 2009). ROS/reactive nitrogen species thus generated would then cooperate with other Nox/Duox enzymes and other potential sources of ROS outside the nucleus to induce a chronic state of oxidative/nitrosative stress during HCV infection. In this scheme, ROS generated by nuclear Nox4 and other extranuclear sources of ROS would form concentration gradients, the probability of their reacting with target molecules diminishing with increasing distance from their respective origin (Fig. 8D).
Our discovery of nuclear Nox4 raises the question of exactly where in the nucleus Nox4 is located and how HCV changes the location of Nox4 without affecting the location of Nox1. Nox family enzymes have multiple transmembrane domains and are membrane-bound.6 In this respect, it may be important to note that the endoplasmic reticulum membrane is contiguous with the nuclear membrane. Also, the nucleoplasm is generally membrane-free, but intranuclear membrane structures have been reported,19 and Nox4 might be located within the inner or outer nuclear membrane or intranuclear cisternae of hepatocytes. If Nox4 is responsible for peroxynitrite-dependent DNA damage, it is most likely located on the inner nuclear membrane or intranuclear membrane, with its active site facing the nucleoplasm. Notice that in our nuclear Nox activity assays, the nuclear pore was likely to allow NADPH, cytochrome c, and SOD to enter the nucleus. A detailed analysis of the subcellular location of Nox4 by electron microscopy is underway. With respect to the mechanism of increased nuclear localization of Nox4, HCV is known to induce severe membrane and nuclear alterations.20 Thus, the increased nuclear location of Nox4 might be a result of the virus modifying the cell for its replication. In addition, as some Nox4 could be found in the nucleus even without HCV (Fig. 5), nuclear Nox4 likely represents a normal cellular process that is enhanced by HCV. In this study, we focused primarily on 50- to 65-kDa Nox1/4 protein bands, which corresponded to the expected sizes of Nox1 and Nox4 proteins. The higher molecular weight bands, however, also increased with HCV and could be partly decreased with siRNAs (Figs. 3 and 4), and they might represent Nox protein complexes or posttranslationally modified Nox.9
Nox enzymes have been implicated in antimicrobial defense, toll-like receptor signaling, lung fibrosis, and cancers. H2O2 and Nox enzymes can also increase iron uptake and mediate many biological effects of TGFβ.14 Tumor necrosis factor alpha and TGFβ, which are proinflammatory and fibrogenic cytokines, are elevated during HCV infection, and these cytokines are known inducers of Nox4.6, 18 We also found that TGFβ played a major role in the HCV-induced elevation of Nox4 in hepatocytes, although other factors might also be involved (Fig. 8; unpublished findings, Wang, Ito, and Choi, 2010). Some of these findings were corroborated by a recent study by Boudreau et al.21 Importantly, hepatocellular carcinoma, associated with chronic hepatitis C, is typically preceded by cirrhosis. Therefore, hepatocyte Nox protein(s) may provide a link between inflammation, fibrogenesis, and hepatocarcinogenesis during chronic hepatitis C. In particular, nuclear Nox4 is likely to be highly significant in the HCV-induced DNA damage in the initiation of cancer as well as reversible and/or irreversible protein modifications in the modulation of cell signaling and host gene expression. The ability to regulate Nox proteins also makes them potential targets for therapy. Therefore, our study provides new insights into a possible mechanism of HCV-induced pathogenesis and points to potential targets for therapy directed at the source of ROS.
The authors thank Takaji Wakita for JFH1 constructs, Mark Zern for telomerase-reconstituted primary human hepatocytes, Balaraman Kalyanaraman for the 2-OH-E+ standard, and Henry Jay Forman for the HPLC systems and discussion. They also thank Muhammad Sheikh, Michael David, Matthew Meyer, David Ojcius, Rui-Ming Liu, and the late T. S. Benedict Yen for discussion and Anna Nandipati, Simrita Kaur (McNair Scholars Program), Sam Chung, and Armand McGee (Basic and Advanced Science and Technology Academies of Research program) for technical assistance.