Activation of the ER stress gene gadd153 by hepatitis C virus sensitizes cells to oxidant injury

ATF4 Signaling in HIV-1 Infection: Viral Subversion of a Stress Response Transcription Factor

Cellular integrated stress response (ISR), the mitochondrial unfolded protein response (UPRmt), and IFN signaling are associated with viral infections. Activating transcription factor 4 (ATF4) plays a pivotal role in these pathways and controls the expression of many genes involved in redox processes, amino acid metabolism, protein misfolding, autophagy, and apoptosis. The precise role of ATF4 during viral infection is unclear and depends on cell hosts, viral agents, and models. Furthermore, ATF4 signaling can be hijacked by pathogens to favor viral infection and replication. In this review, we summarize the ATF4-mediated signaling pathways in response to viral infections, focusing on human immunodeficiency virus 1 (HIV-1). We examine the consequences of ATF4 activation for HIV-1 replication and reactivation. The role of ATF4 in autophagy and apoptosis is explored as in the context of HIV-1 infection programmed cell deaths contribute to the depletion of CD4 T cells. Furthermore, ATF4 can also participate in the establishment of innate and adaptive immunity that is essential for the host to control viral infections. We finally discuss the putative role of the ATF4 paralogue, named ATF5, in HIV-1 infection. This review underlines the role of ATF4 at the crossroads of multiple processes reflecting host-pathogen interactions.

Endoplasmic reticulum stress in liver diseases

The endoplasmic reticulum (ER) is an intracellular organelle that fosters the correct folding of linear polypeptides and proteins, a process tightly governed by the ER-resident enzymes and chaperones. Failure to shape the proper 3-dimensional architecture of proteins culminates in the accumulation of misfolded or unfolded proteins within the ER, disturbs ER homeostasis, and leads to canonically defined ER stress. Recent studies have elucidated that cellular perturbations, such as lipotoxicity, can also lead to ER stress. In response to ER stress, the unfolded protein response (UPR) is activated to reestablish ER homeostasis ("adaptive UPR"), or, conversely, to provoke cell death when ER stress is overwhelmed and sustained ("maladaptive UPR"). It is well documented that ER stress contributes to the onset and progression of multiple hepatic pathologies including NAFLD, alcohol-associated liver disease, viral hepatitis, liver ischemia, drug toxicity, and liver cancers. Here, we review key studies dealing with the emerging role of ER stress and the UPR in the pathophysiology of liver diseases from cellular, murine, and human models. Specifically, we will summarize current available knowledge on pharmacological and non-pharmacological interventions that may be used to target maladaptive UPR for the treatment of nonmalignant liver diseases.

Endoplasmic Reticulum Stress in Hepatitis B Virus and Hepatitis C Virus Infection

Endoplasmic reticulum (ER) stress, a type of cellular stress, always occurs when unfolded or misfolded proteins accumulating in the ER exceed the protein folding capacity. Because of the demand for rapid viral protein synthesis after viral infection, viral infections become a risk factor for ER stress. The hepatocyte is a cell with large and well-developed ER, and hepatitis virus infection is widespread in the population, indicating the interaction between hepatitis viruses and ER stress may have significance for managing liver diseases. In this paper, we review the process that is initiated by the hepatocyte through ER stress against HBV and HCV infection and explain how this information can be helpful in the treatment of HBV/HCV-related diseases.

The Regulation of Integrated Stress Response Signaling Pathway on Viral Infection and Viral Antagonism

The integrated stress response (ISR) is an adaptational signaling pathway induced in response to different stimuli, such as accumulation of unfolded and misfolded proteins, hypoxia, amino acid deprivation, viral infection, and ultraviolet light. It has been known that viral infection can activate the ISR, but the role of the ISR during viral infection is still unclear. In some cases, the ISR is a protective mechanism of host cells against viral infection, while viruses may hijack the ISR for facilitating their replication. This review highlighted recent advances on the induction of the ISR upon viral infection and the downstream responses, such as autophagy, apoptosis, formation of stress granules, and innate immunity response. We then discussed the molecular mechanism of the ISR regulating viral replication and how viruses antagonize this cellular stress response resulting from the ISR.

Hesperidin-CAMKIV interaction and its impact on cell proliferation and apoptosis in the human hepatic carcinoma and neuroblastoma cells

Calcium/calmodulin-dependent protein kinase IV (CAMKIV) is a key regulatory molecule of cell signaling, and thereby controls its growth and proliferation, including expression of certain genes. The overexpression of CAMKIV is directly associated with the development of different types of cancers. Hesperidin is abundantly found in citrus fruits and exhibits wide range of pharmacological activities including anti-inflammatory, antibacterial and anticancerous effects. We have investigated binding mechanism of hesperidin with the CAMKIV using molecular docking methods followed by fluorescence quenching and isothermal titration calorimetric assays. An appreciable binding affinity of hesperidin was observed with CAMKIV during fluorescence quenching and isothermal titration calorimetric studies. Efficacy of hesperidin to inhibit the growth of human hepatic carcinoma (HepG2) and neuroblastoma (SH-SY5Y) cancer cell lines were investigated. Hesperidin has significantly reduced the proliferation of HepG2 and SH-SY5Y cells and induces apoptosis by activating the caspase-3-dependent intrinsic pathway through the upregulation of proapoptotic Bax protein. Hesperidin treatment reduces the mitochondrial membrane potential of HepG2 and SH-SY5Y cells. All these observations clearly anticipated hesperidin a potent inhibitor of CAMKIV which may be further exploited a newer therapeutic approach for the management of different cancer types.

Significance of hepatitis virus infection in the oncogenic initiation of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common causes of cancer-related death worldwide. Chronic infection of hepatitis B virus (HBV) and/or hepatitis C virus (HCV) is a major risk factor in the development of the HCC, independently from excessive alcohol abuse and metabolic disease. Since the biology of HBV and HCV is different, their oncogenic effect may go through different mechanisms, direct and/or indirect. Viral hepatitis infection is associated with cellular inflammation, oxidative stress, and DNA damage, that may lead to subsequent hepatic injuries such as chronic hepatitis, fibrosis, cirrhosis, and finally HCC. Direct oncogenic properties of these viruses are related with their genotypic characteristics and the ability of viral proteins to interact with host proteins, thus altering the molecular pathways balance of the cells. In addition, the integration of HBV DNA, especially the gene S and X, in a particular site of the host genome can disrupt chromosomal stability and may activate various oncogenic mechanisms, including those in hematopoietic cells. Recently, several studies also had demonstrated that viral hepatitis could trigger the population of hepatic cancer stem cells. This review summarize available pre-clinical and clinical data in literature regarding oncogenic properties of HBV and HCV in the early initiation of HCC.

Chaperones in hepatitis C virus infection

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

Hepatitis B and C virus-induced hepatitis: Apoptosis, autophagy, and unfolded protein response

AIM

To investigate the co-incidence of apoptosis, autophagy, and unfolded protein response (UPR) in hepatitis B (HBV) and C (HCV) infected hepatocytes.

METHODS

We performed immunofluorescence confocal microscopy on 10 liver biopsies from HBV and HCV patients and tissue microarrays of HBV positive liver samples. We used specific antibodies for LC3β, cleaved caspase-3, BIP (GRP78), and XBP1 to detect autophagy, apoptosis and UPR, respectively. Anti-HCV NS3 and anti-HBs antibodies were also used to confirm infection. We performed triple blind counting of events to determine the co-incidence of autophagy (LC3β punctuate), apoptosis (cleaved caspase-3), and unfolded protein response (GRP78) with HBV and HCV infection in hepatocytes. All statistical analyses were performed using SPSS software for Windows (Version 16 SPSS Inc, Chicago, IL, United States). P-values < 0.05 were considered statistically significant. Statistical analyses were performed with Mann-Whitney test to compare incidence rates for autophagy, apoptosis, and UPR in HBV- and HCV-infected cells and adjacent non-infected cells.

RESULTS

Our results showed that infection of hepatocytes with either HBV and HCV induces significant increase (P < 0.001) in apoptosis (cleavage of caspase-3), autophagy (LC3β punctate), and UPR (increase in GRP78 expression) in the HCV- and HBV-infected cells, as compared to non-infected cells of the same biopsy sections. Our tissue microarray immunohistochemical expression analysis of LC3β in HBV(Neg) and HBV(Pos) revealed that majority of HBV-infected hepatocytes display strong positive staining for LC3β. Interestingly, although XBP splicing in HBV-infected cells was significantly higher (P < 0.05), our analyses show a slight increase of XBP splicing was in HCV-infected cells (P > 0.05). Furthermore, our evaluation of patients with HBV and HCV infection based on stage and grade of the liver diseases revealed no correlation between these pathological findings and induction of apoptosis, autophagy, and UPR.

CONCLUSION

The results of this study indicate that HCV and HBV infection activates apoptosis, autophagy and UPR, but slightly differently by each virus. Further studies are warranted to elucidate the interconnections between these pathways in relation to pathology of HCV and HBV in the liver tissue.

Expression of endoplasmic reticulum stress markers GRP78 and CHOP induced by oxidative stress in blue light-mediated damage of A2E-containing retinal pigment epithelium cells

AIMS

Age-related lipofuscin N-retinylidene-N-retinylethanolamine (A2E) accumulated in human retinal pigment epithelium (RPE) cells confers susceptibility to blue light-mediated damage, which represents one pathogenesis of age-related macular degeneration. This study investigated the expression of 2 best-characterized endoplasmic reticulum (ER) stress markers, glucose-related protein 78 (GRP78) and C/EBP homologous protein (CHOP), as well as their regulation by oxidative stress after blue light-mediated damage of A2E-containing RPE cells.

METHODS

ARPE-19 cells were incubated with A2E (10, 25, 50 μM) for 2 h and exposed to blue light for 20 min. A2E distributions in RPE cells were assessed via laser scanning confocal microscope and liquid chromatography-mass spectrometry. Cell viability was measured by a Cell Titer 96 Aqueous One Solution cell proliferation assay. The quantity of intracellular reactive oxygen species (ROS) was detected by dihydroethidium fluorescence using flow cytometry. Expressions of GRP78 and CHOP were measured at both mRNA and protein levels. To examine the role of oxidative stress in regulating GRP78 and CHOP expression, RPE cells were pretreated with the antioxidant N-acetylcysteine (NAC) for 2 h. RNA interference of GRP78 performed by short hairpin RNA was used to evaluate the effect of GRP78 in blue light-mediated damage of RPE cells.

RESULTS

After blue light exposure, A2E-treated RPE cells showed a gradual decrease in cell viability and a particular increase in ROS levels. Meanwhile, the expressions of GRP78 and CHOP in A2E-treated RPE cells were significantly increased at different time points after illumination. Pretreatment with NAC attenuated the expression of 2 ER stress markers, especially CHOP in A2E and blue light-treated RPE cells. Silencing of GRP78 by RNA interference upregulated CHOP and caspase-12 expression as well as aggravated the blue light-mediated damage of A2E-laden RPE cells.

CONCLUSION

RPE cells exhibited ROS accumulation and subsequent elevation of GRP78 and CHOP expression after A2E and blue light-induced damage. The ROS scavenger NAC diminished ER stress protein expression, suggesting a connection between ER and oxidative stress in blue light-mediated damage of A2E-containing RPE cells. Besides, GRP78 may play a protective role in it.

Unfolded protein response in hepatitis C virus infection

Hepatitis C virus (HCV) is a single-stranded, positive-sense RNA virus of clinical importance. The virus establishes a chronic infection and can progress from chronic hepatitis, steatosis to fibrosis, cirrhosis, and hepatocellular carcinoma (HCC). The mechanisms of viral persistence and pathogenesis are poorly understood. Recently the unfolded protein response (UPR), a cellular homeostatic response to endoplasmic reticulum (ER) stress, has emerged to be a major contributing factor in many human diseases. It is also evident that viruses interact with the host UPR in many different ways and the outcome could be pro-viral, anti-viral or pathogenic, depending on the particular type of infection. Here we present evidence for the elicitation of chronic ER stress in HCV infection. We analyze the UPR signaling pathways involved in HCV infection, the various levels of UPR regulation by different viral proteins and finally, we propose several mechanisms by which the virus provokes the UPR.

The ER quality control and ER associated degradation machineries are vital for viral pathogenesis

The endoplasmic reticulum (ER) is central to protein production and membrane lipid synthesis. The unfolded protein response (UPR) supports cellular metabolism by ensuring protein quality control in the ER. Most positive strand RNA viruses cause extensive remodeling of membranes and require active membrane synthesis to promote infection. How viruses interact with the cellular machinery controlling membrane metabolism is largely unknown. Furthermore, there is mounting data pointing to the importance of the UPR and ER associated degradation (ERAD) machineries in viral pathogenesis in eukaryotes emerging topic. For many viruses, the UPR is an early event that is essential for persistent infection and benefits virus replication. In addition, many viruses are reported to commandeer ER resident chaperones to contribute to virus replication and intercellular movement. In particular, calreticulin, the ubiquitin machinery, and the 26S proteasome are most commonly identified components of the UPR and ERAD machinery that also regulate virus infection. In addition, researchers have noted a link between UPR and autophagy. It is well accepted that positive strand RNA viruses use autophagic membranes as scaffolds to support replication and assembly. However this topic has yet to be explored using plant viruses. The goal of research on this topic is to uncover how viruses interact with this ER-related machinery and to use this information for designing novel strategies to boost immune responses to virus infection.

Investigation of biomarkers in peripheral blood cells of Pakistani chronic HCV patients with genotype 3a: an assessment of HCV-induced pathogenesis

ABSTRACT: 

Aim: The main objective of the current study was to measure the mRNA expression of potent genes involved in oxidative stress, fibrosis and insulin resistance in chronic HCV patients using peripheral white blood cells. Materials & methods: A total of 50 individuals were selected for this study, including 40 chronic HCV patients and ten healthy individuals. RNA was extracted from human peripheral white blood cells of chronic HCV-infected patients and healthy individuals. Transcript analysis of selected genes was performed using real-time PCR. Results: A significant increase was observed in the mRNA expression of Nox4, TGF-β, TNF-α and collagen, while a significant decrease was observed in mRNA expression of Cu/Zn SOD and IL-10. However, there was no significant change found in mRNA expression of SOCS3 and SOCS7 level. Conclusion: The current study confirms the significant expression of different genes in peripheral white blood cells of chronic HCV patients. This could be an easy, cost-efficient approach for the assessment of HCV-induced pathologies at extrahepatic sites, which could be further validated in comparison with liver tissue biopsies.

The ER quality control and ER associated degradation machineries are vital for viral pathogenesis

The endoplasmic reticulum (ER) is central to protein production and membrane lipid synthesis. The unfolded protein response (UPR) supports cellular metabolism by ensuring protein quality control in the ER. Most positive strand RNA viruses cause extensive remodeling of membranes and require active membrane synthesis to promote infection. How viruses interact with the cellular machinery controlling membrane metabolism is largely unknown. Furthermore, there is mounting data pointing to the importance of the UPR and ER associated degradation (ERAD) machineries in viral pathogenesis in eukaryotes emerging topic. For many viruses, the UPR is an early event that is essential for persistent infection and benefits virus replication. In addition, many viruses are reported to commandeer ER resident chaperones to contribute to virus replication and intercellular movement. In particular, calreticulin, the ubiquitin machinery, and the 26S proteasome are most commonly identified components of the UPR and ERAD machinery that also regulate virus infection. In addition, researchers have noted a link between UPR and autophagy. It is well accepted that positive strand RNA viruses use autophagic membranes as scaffolds to support replication and assembly. However this topic has yet to be explored using plant viruses. The goal of research on this topic is to uncover how viruses interact with this ER-related machinery and to use this information for designing novel strategies to boost immune responses to virus infection.

Endoplasmic reticulum stress in diethylnitrosamine-induced rat liver cancer

To analyze the significance of endoplasmic reticulum stress (ERS) in the development of diethylnitrosamine (DEN)-induced liver cancer in rats, critical regulatory factors in ERS signaling pathways were investigated in the present study. The results showed that the expression of ERS-related proteins gradually increased in the early and mid-term stages of carcinogenesis, while in the later stages, the expression of these proteins did not change significantly after reaching a peak. ERS is involved in DEN-induced rat liver injury, the proliferation of liver cells and the occurrence and development of liver cirrhosis. However, ERS did not affect hepatoma cell growth following the formation of rat liver cancer in the current study.

Upregulation of CHOP/GADD153 during coronavirus infectious bronchitis virus infection modulates apoptosis by restricting activation of the extracellular signal-regulated kinase pathway

Induction of the unfolded protein response (UPR) is an adaptive cellular response to endoplasmic reticulum (ER) stress that allows a cell to reestablish ER homeostasis. However, under severe and persistent ER stress, prolonged UPR may activate unique pathways that lead to cell death. In this study, we investigated the activation of the protein kinase R-like ER kinase (PERK) pathway of UPR in cells infected with the coronavirus infectious bronchitis virus (IBV) and its relationship with IBV-induced apoptosis. The results showed moderate induction of PERK phosphorylation in IBV-infected cells. Meanwhile, activating transcription factor 4 (ATF4) was upregulated at the protein level in the infected cells, resulting in the induction in trans of the transcription factor ATF3 and the proapoptotic growth arrest and DNA damage-inducible protein GADD153. Knockdown of PERK by small interfering RNA (siRNA) suppressed the activation of GADD153 and the IBV-induced apoptosis. Interestingly, knockdown of protein kinase R (PKR) by siRNA and inhibition of the PKR kinase activity by 2-aminopurine (2-AP) also reduced the IBV-induced upregulation of GADD153 and apoptosis induction. In GADD153-knockdown cells, IBV-induced apoptosis was suppressed and virus replication inhibited, revealing a key role of GADD153 in IBV-induced cell death and virus replication. Analysis of the pathways downstream of GADD153 revealed much more activation of the extracellular signal-related kinase (ERK) pathway in GADD153-knockdown cells during IBV infection, indicating that GADD153 may modulate apoptosis through suppression of the pathway. This study provides solid evidence that induction of GADD153 by PERK and PKR plays an important regulatory role in the apoptotic process triggered by IBV infection.

Hepatitis C virus induces the mitochondrial translocation of Parkin and subsequent mitophagy

Hepatitis C Virus (HCV) induces intracellular events that trigger mitochondrial dysfunction and promote host metabolic alterations. Here, we investigated selective autophagic degradation of mitochondria (mitophagy) in HCV-infected cells. HCV infection stimulated Parkin and PINK1 gene expression, induced perinuclear clustering of mitochondria, and promoted mitochondrial translocation of Parkin, an initial event in mitophagy. Liver tissues from chronic HCV patients also exhibited notable levels of Parkin induction. Using multiple strategies involving confocal and electron microscopy, we demonstrated that HCV-infected cells display greater number of mitophagosomes and mitophagolysosomes compared to uninfected cells. HCV-induced mitophagy was evidenced by the colocalization of LC3 puncta with Parkin-associated mitochondria and lysosomes. Ultrastructural analysis by electron microscopy and immunoelectron microscopy also displayed engulfment of damaged mitochondria in double membrane vesicles in HCV-infected cells. The HCV-induced mitophagy occurred irrespective of genotypic differences. Silencing Parkin and PINK1 hindered HCV replication suggesting the functional relevance of mitophagy in HCV propagation. HCV-mediated decline of mitochondrial complex I enzyme activity was rescued by chemical inhibition of mitophagy or by Parkin silencing. Overall our results suggest that HCV induces Parkin-dependent mitophagy, which may have significant contribution in mitochondrial liver injury associated with chronic hepatitis C.

Hepatitis C virus (HCV) infection alters host lipid metabolism. HCV-induced mitochondrial dysfunction may promote the metabolic alterations by affecting mitochondrial β-oxidation and oxidative phosphorylation. Dysfunctional mitochondria are detrimental to cell survival and require rapid clearance to sustain cell viability. Here, we investigated the effect of HCV gene expression in promoting selective autophagy of dysfunctional mitochondria, also termed mitophagy. HCV infection stimulated the gene expression of Parkin and PINK1, the two key mediators of mitophagy. Parkin stimulation was also observed in liver biopsies of chronic hepatitis C patients. HCV infection induced the perinuclear clustering of mitochondria and triggered Parkin translocation to mitochondria, a hallmark of mitophagy. Concomitant with the mitochondrial translocation of Parkin, we observed ubiquitination of Parkin and its substrates in HCV-infected cells. We also demonstrate the formation of mitophagosomes and their subsequent delivery to lysosomes in HCV-infected cells. Silencing both Parkin and PINK1 hindered HCV replication, suggesting the functional significance of mitophagy in HCV life cycle. Furthermore, we demonstrate that Parkin-dependent mitophagy is directly associated with HCV-mediated decline in oxidative phosphorylation. Our results implicate the functional significance of Parkin and mitophagy in the persistence of HCV infection and mitochondrial injury commonly seen in patients with chronic hepatitis C.

Endoplasmic reticulum stress in liver disease

Endoplasmic reticulum stress (ERS) is an important self-defense mechanism of the cell. ERS initially activates survival pathway, but sustained ERS will induce apoptosis. ERS and apoptosis induced by ERS are involved in the pathogenesis of many liver diseases, including viral hepatitis, alcohol-induced liver injury, nonalcoholic fatty liver disease, drug-induced liver disease, acute hepatic failure, and hepatocellular carcinoma. It is of important theoretical and practical significance for curing liver diseases to find some new drugs targeting ERS-induced apoptosis. The present review will discuss the survival and death pathways mediated by ERS, the role of ERS in the pathogenesis of hepatic diseases, and therapeutic interventions for these diseases.

Hepatitis E virus ORF2 protein activates the pro-apoptotic gene CHOP and anti-apoptotic heat shock proteins

Background

Hepatitis E virus (HEV) is a non-enveloped plus-strand RNA virus that causes acute hepatitis. The capsid protein open reading frame 2 (ORF2) is known to induce endoplasmic reticulum stress in ORF2 expressing cells.

Methodology/Principal Findings

In this study we found that HEV ORF2 activates the expression of the pro-apoptotic gene C/EBP homologous protein (CHOP). ORF2 stimulates the CHOP promoter mainly through AARE (amino acid response elements) and to a minor extent the ERSE (endoplasmic reticulum stress response elements). Activating transcription factor 4 (ATF4) protein binds and activates the AARE regulatory sites of the CHOP promoter. ORF2 expression also leads to increased phosphorylation of eukaryotic initiation factor 2 alpha (eIF2α) that in turn initiates the translation of ATF4 mRNA. The pro-apoptotic gene CHOP is an important trigger to initiate endoplasmic reticulum stress induced apoptosis. However, the activation of CHOP by ORF2 in this study did not induce apoptosis, nor did BCL2-associated X protein (Bax) translocate to mitochondria. Microarray analysis revealed an ORF2 specific increased expression of chaperones Hsp72, Hsp70B', and co-chaperone Hsp40. Co-immunoprecipitation (Co-IP) and in silico molecular docking analysis suggests that HEV ORF2 interacts with Hsp72. In addition, Hsp72 shows nuclear accumulation in ORF2 expressing cells.

Conclusions/Significance

These data provide new insight into simultaneously occurring counter-acting effects of HEV ORF2 that may be part of a strategy to prevent host suicide before completion of the viral replication cycle.

ATF4 and the integrated stress response are induced by ethanol and cytochrome P450 2E1 in human hepatocytes

BACKGROUND & AIMS

Molecular mechanisms underlying alcoholic liver disease (ALD) are still not fully understood. Activating transcription factor-4 (ATF4) is the master coordinator of the integrated stress response (ISR), an adaptive pathway triggered by multiple stressors. which can promote cell death and induce metabolic dysregulation if the stress is intense or prolonged. The aim of this study was to assess the effect of alcohol on the ISR signaling pathway in human liver cells and to define the role of cytochrome P450 2E1 (CYP2E1) in this response.

METHODS

Primary cultured human hepatocytes and human HepG2 cells over-expressing CYP2E1 by adenoviral infection were exposed to ethanol (25-100mM) for 8-48h.

RESULTS

Ethanol treatment of both liver cells up-regulated ATF4 as well as the pro-survival and the pro-apoptotic transcriptional program of the ISR. Indeed, in CYP2E1-expressing HepG2 cells exposed to ethanol, the expression of ISR target genes (HMOX-1, GCLC, AsnS, IGFBP-1, GADD34,CHOP, ATF3, CHAC1) was induced. Up-regulation of ATF4 and the ISR transcriptional program was decreased by addition of the anti-oxidant glutathione. Several mechanisms mediated ATF4 protein induction, including, at early times, the phosphorylation of eIF2α which controls ATF4 translation, and, at later times, increased mRNA level and increased stability of the protein. A decrease in cell survival was also observed.

CONCLUSIONS

This study demonstrates that both CYP2E1 and ethanol induce ATF4 and the integrated stress response, a pathway which coordinates signals from multiple stresses, as well as established risk factors for ALD, and can display detrimental cellular effects upon prolonged activation.

Endoplasmic reticulum stress in liver disease

The unfolded protein response (UPR) is activated upon the accumulation of misfolded proteins in the endoplasmic reticulum (ER) that are sensed by the binding immunoglobulin protein (BiP)/glucose-regulated protein 78 (GRP78). The accumulation of unfolded proteins sequesters BiP so it dissociates from three ER-transmembrane transducers leading to their activation. These transducers are inositol requiring (IRE) 1α, PKR-like ER kinase (PERK), and activating transcription factor (ATF) 6α. PERK phosphorylates eukaryotic initiation factor 2 alpha (eIF2α) resulting in global mRNA translation attenuation, and concurrently selectively increases the translation of several mRNAs, including the transcription factor ATF4, and its downstream target CHOP. IRE1α has kinase and endoribonuclease (RNase) activities. IRE1α autophosphorylation activates the RNase activity to splice XBP1 mRNA, to produce the active transcription factor sXBP1. IRE1α activation also recruits and activates the stress kinase JNK. ATF6α transits to the Golgi compartment where it is cleaved by intramembrane proteolysis to generate a soluble active transcription factor. These UPR pathways act in concert to increase ER content, expand the ER protein folding capacity, degrade misfolded proteins, and reduce the load of new proteins entering the ER. All of these are geared toward adaptation to resolve the protein folding defect. Faced with persistent ER stress, adaptation starts to fail and apoptosis occurs, possibly mediated through calcium perturbations, reactive oxygen species, and the proapoptotic transcription factor CHOP. The UPR is activated in several liver diseases; including obesity associated fatty liver disease, viral hepatitis, and alcohol-induced liver injury, all of which are associated with steatosis, raising the possibility that ER stress-dependent alteration in lipid homeostasis is the mechanism that underlies the steatosis. Hepatocyte apoptosis is a pathogenic event in several liver diseases, and may be linked to unresolved ER stress. If this is true, restoration of ER homeostasis prior to ER stress-induced cell death may provide a therapeutic rationale in these diseases. Herein we discuss each branch of the UPR and how they may impact hepatocyte function in different pathologic states.

Hepatitis C virus hijacks host lipid metabolism

Hepatitis C virus (HCV) enhances its replication by modulating host cell lipid metabolism. HCV circulates in the blood in association with lipoproteins. HCV infection is associated with enhanced lipogenesis, reduced secretion, and beta-oxidation of lipids. HCV-induced imbalance in lipid homeostasis leads to steatosis. Many lipids are crucial for the virus life cycle, and inhibitors of cholesterol/fatty acid biosynthetic pathways inhibit virus replication, maturation and secretion. HCV negatively modulates the synthesis and secretion of very low-density lipoproteins (VLDL). Components involved in VLDL assembly are also required for HCV morphogenesis/secretion, suggesting that HCV co-opts the VLDL secretory pathway for its own secretion. This review highlights HCV-altered lipid metabolic events that aid the virus life cycle and ultimately promote liver disease.

The SARS Coronavirus 3a protein causes endoplasmic reticulum stress and induces ligand-independent downregulation of the type 1 interferon receptor

The Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) is reported to cause apoptosis of infected cells and several of its proteins including the 3a accessory protein, are pro-apoptotic. Since the 3a protein localizes to the endoplasmic reticulum (ER)-Golgi compartment, its role in causing ER stress was investigated in transiently transfected cells. Cells expressing the 3a proteins showed ER stress based on activation of genes for the ER chaperones GRP78 and GRP94. Since ER stress can cause differential modulation of the unfolded protein response (UPR), which includes the inositol-requiring enzyme 1 (IRE-1), activating transcription factor 6 (ATF6) and PKR-like ER kinase (PERK) pathways, these were individually tested in 3a-expressing cells. Only the PERK pathway was found to be activated in 3a-expressing cells based on (1) increased phosphorylation of eukaryotic initiation factor 2 alpha (eIF2α) and inhibitory effects of a dominant-negative form of eIF2α on GRP78 promoter activity, (2) increased translation of activating transcription factor 4 (ATF4) mRNA, and (3) ATF4-dependent activation of the C/EBP homologous protein (CHOP) gene promoter. Activation of PERK affects innate immunity by suppression of type 1 interferon (IFN) signaling. The 3a protein was found to induce serine phosphorylation within the IFN alpha-receptor subunit 1 (IFNAR1) degradation motif and to increase IFNAR1 ubiquitination. Confocal microscopic analysis showed increased translocation of IFNAR1 into the lysosomal compartment and flow cytometry showed reduced levels of IFNAR1 in 3a-expressing cells. These results provide further mechanistic details of the pro-apoptotic effects of the SARS-CoV 3a protein, and suggest a potential role for it in attenuating interferon responses and innate immunity.

Elevated hepatic iron: a confounding factor in chronic hepatitis C

Historically, iron overload in the liver has been associated with the genetic disorders hereditary hemochromatosis and thalassemia and with unusual dietary habits. More recently, elevated hepatic iron levels also have been observed in chronic hepatitis C virus (HCV) infection. Iron overload in the liver causes many changes including induction of oxidative stress, damage to lysosomes and mitochondria, altered oxidant defense systems and stimulation of hepatocyte proliferation. Chronic HCV infection causes numerous pathogenic changes in the liver including induction of endoplasmic reticulum stress, the unfolded protein response, oxidative stress, mitochondrial dysfunction and altered growth control. Understanding the molecular and cellular changes that could occur in a liver which has elevated hepatic iron levels and in which HCV replication and gene expression are ongoing has clinical relevance and represents an area of research in need of further investigation.

Intersection of the unfolded protein response and hepatic lipid metabolism

The liver plays a central role in whole-body lipid metabolism by governing the synthesis, oxidization, transport and excretion of lipids. The unfolded protein response (UPR) was identified as a signal transduction system that is activated by ER stress. Recent studies revealed a critical role of the UPR in hepatic lipid metabolism. The IRE1/XBP1 branch of the UPR is activated by high dietary carbohydrates and controls the expression of genes involved in fatty acid and cholesterol biosynthesis. PERK mediated eIF2alpha phosphorylation is also required for the expression of lipogenic genes and the development of hepatic steatosis, likely by activating C/EBP and PPARgamma transcription factors. Further studies to define the molecular pathways that lead to the activation of the UPR by nutritional cues in the liver, and their contribution to human metabolic disorders such as hepatic steatosis, atherosclerosis and type 2 diabetes that are associated with dysregulation of lipid homeostasis, are warranted.

Expression of two endoplasmic reticulum stress markers, GRP78 and GADD153, in rat retinal detachment model and its implication

PURPOSE

To investigate the expression of endoplasmic reticulum (ER) stress-related genes, glucose-regulated protein 78 (GRP78) and growth arrest DNA damage-inducible gene 153 (GADD153)/CPEBP homologous protein (CHOP), in rat retinal detachment (RD) model.

MATERIALS AND METHODS

At various time points after RD, the apoptosis of retinal cells was detected by TdT-mediated fluorescein-16-dUTP nick-end labelling (TUNEL) assay; GRP78 and GADD153 mRNA levels were detected by reverse transcription (RT)-PCR; proteins were detected by western blotting analysis; protein distributions in the retinal cells were observed by immunofluorescence using laser-scanning confocal microscope.

RESULTS

After RD, the apoptosis was peaked on 2-4 d and then dropped down. The GRP78 mRNA and GADD153 mRNA levels in RD groups on 0.5, 1, 2, and 4 d were all significantly higher than those in the control group (P<0.05). The expression of GRP78 mRNA peaked on 1-2 d after RD. Expression of GRP78 protein was significantly higher than that in the normal control group on 0.5, 1, 2, 4, 8, 16, and 32 d after RD (P<0.05). The expression of GRP78 protein was observed in all the layers of retina in the RD groups, and peaked on 8, 16, and 32 d. The expression of GADD153 protein, mostly in photoreceptor layers, was significantly higher than that in the control group on 0.5, 1, 2, and 4 d after RD (P<0.05).

CONCLUSIONS

ER stress-related markers, GRP78 and GADD153, are elevated after RD. The elevation of GADD153 is in parallel with the post-RD apoptosis of retinal cells, suggesting that ER stress-mediated death is likely to be activated after RD and involved in post-RD vision loss.

Virus-induced unfolded protein response attenuates antiviral defenses via phosphorylation-dependent degradation of the type I interferon receptor

Phosphorylation-dependent ubiquitination and degradation of the IFNAR1 chain of the type I interferon (IFN) receptor is regulated by two different pathways, one of which is ligand independent. We report that this ligand-independent pathway is activated by inducers of unfolded protein responses (UPR), including viral infection, and that such activation requires the endoplasmic reticulum-resident protein kinase PERK. Upon viral infection, activation of this pathway promotes phosphorylation-dependent ubiquitination and degradation of IFNAR1, specifically inhibiting type I IFN signaling and antiviral defenses. Knockin of an IFNAR1 mutant insensitive to virus-induced turnover or conditional knockout of PERK prevented IFNAR1 degradation, whether UPR-induced or virus-induced, and restored cellular responses to type I IFN and resistance to viruses. These data suggest that specific activation of the PERK component of UPR can favor viral replication. Interfering with PERK-dependent IFNAR1 degradation could therefore contribute to therapeutic strategies against viral infections.

Redox mechanisms in hepatic chronic wound healing and fibrogenesis

Reactive oxygen species (ROS) generated within cells or, more generally, in a tissue environment, may easily turn into a source of cell and tissue injury. Aerobic organisms have developed evolutionarily conserved mechanisms and strategies to carefully control the generation of ROS and other oxidative stress-related radical or non-radical reactive intermediates (that is, to maintain redox homeostasis), as well as to 'make use' of these molecules under physiological conditions as tools to modulate signal transduction, gene expression and cellular functional responses (that is, redox signalling). However, a derangement in redox homeostasis, resulting in sustained levels of oxidative stress and related mediators, can play a significant role in the pathogenesis of major human diseases characterized by chronic inflammation, chronic activation of wound healing and tissue fibrogenesis. This review has been designed to first offer a critical introduction to current knowledge in the field of redox research in order to introduce readers to the complexity of redox signalling and redox homeostasis. This will include ready-to-use key information and concepts on ROS, free radicals and oxidative stress-related reactive intermediates and reactions, sources of ROS in mammalian cells and tissues, antioxidant defences, redox sensors and, more generally, the major principles of redox signalling and redox-dependent transcriptional regulation of mammalian cells. This information will serve as a basis of knowledge to introduce the role of ROS and other oxidative stress-related intermediates in contributing to essential events, such as the induction of cell death, the perpetuation of chronic inflammatory responses, fibrogenesis and much more, with a major focus on hepatic chronic wound healing and liver fibrogenesis.

Hepatitis C virus infection induces apoptosis through a Bax-triggered, mitochondrion-mediated, caspase 3-dependent pathway

We previously reported that cells harboring the hepatitis C virus (HCV) RNA replicon as well as those expressing HCV NS3/4A exhibited increased sensitivity to suboptimal doses of apoptotic stimuli to undergo mitochondrion-mediated apoptosis (Y. Nomura-Takigawa, et al., J. Gen. Virol. 87:1935-1945, 2006). Little is known, however, about whether or not HCV infection induces apoptosis of the virus-infected cells. In this study, by using the chimeric J6/JFH1 strain of HCV genotype 2a, we demonstrated that HCV infection induced cell death in Huh7.5 cells. The cell death was associated with activation of caspase 3, nuclear translocation of activated caspase 3, and cleavage of DNA repair enzyme poly(ADP-ribose) polymerase, which is known to be an important substrate for activated caspase 3. These results suggest that HCV-induced cell death is, in fact, apoptosis. Moreover, HCV infection activated Bax, a proapoptotic member of the Bcl-2 family, as revealed by its conformational change and its increased accumulation on mitochondrial membranes. Concomitantly, HCV infection induced disruption of mitochondrial transmembrane potential, followed by mitochondrial swelling and release of cytochrome c from mitochondria. HCV infection also caused oxidative stress via increased production of mitochondrial superoxide. On the other hand, HCV infection did not mediate increased expression of glucose-regulated protein 78 (GRP78) or GRP94, which are known as endoplasmic reticulum (ER) stress-induced proteins; this result suggests that ER stress is not primarily involved in HCV-induced apoptosis in our experimental system. Taken together, our present results suggest that HCV infection induces apoptosis of the host cell through a Bax-triggered, mitochondrion-mediated, caspase 3-dependent pathway(s).

Microarray analysis identifies a common set of cellular genes modulated by different HCV replicon clones

Background

Hepatitis C virus (HCV) RNA synthesis and protein expression affect cell homeostasis by modulation of gene expression. The impact of HCV replication on global cell transcription has not been fully evaluated. Thus, we analysed the expression profiles of different clones of human hepatoma-derived Huh-7 cells carrying a self-replicating HCV RNA which express all viral proteins (HCV replicon system).

Results

First, we compared the expression profile of HCV replicon clone 21-5 with both the Huh-7 parental cells and the 21-5 cured (21-5c) cells. In these latter, the HCV RNA has been eliminated by IFN-α treatment. To confirm data, we also analyzed microarray results from both the 21-5 and two other HCV replicon clones, 22-6 and 21-7, compared to the Huh-7 cells. The study was carried out by using the Applied Biosystems (AB) Human Genome Survey Microarray v1.0 which provides 31,700 probes that correspond to 27,868 human genes. Microarray analysis revealed a specific transcriptional program induced by HCV in replicon cells respect to both IFN-α-cured and Huh-7 cells. From the original datasets of differentially expressed genes, we selected by Venn diagrams a final list of 38 genes modulated by HCV in all clones. Most of the 38 genes have never been described before and showed high fold-change associated with significant p-value, strongly supporting data reliability. Classification of the 38 genes by Panther System identified functional categories that were significantly enriched in this gene set, such as histones and ribosomal proteins as well as extracellular matrix and intracellular protein traffic. The dataset also included new genes involved in lipid metabolism, extracellular matrix and cytoskeletal network, which may be critical for HCV replication and pathogenesis.

Conclusion

Our data provide a comprehensive analysis of alterations in gene expression induced by HCV replication and reveal modulation of new genes potentially useful for selection of antiviral targets.

The role for endoplasmic reticulum stress in diabetes mellitus

Accumulating evidence suggests that endoplasmic reticulum (ER) stress plays a role in the pathogenesis of diabetes, contributing to pancreatic beta-cell loss and insulin resistance. Components of the unfolded protein response (UPR) play a dual role in beta-cells, acting as beneficial regulators under physiological conditions or as triggers of beta-cell dysfunction and apoptosis under situations of chronic stress. Novel findings suggest that "what makes a beta-cell a beta-cell", i.e., its enormous capacity to synthesize and secrete insulin, is also its Achilles heel, rendering it vulnerable to chronic high glucose and fatty acid exposure, agents that contribute to beta-cell failure in type 2 diabetes. In this review, we address the transition from physiology to pathology, namely how and why the physiological UPR evolves to a proapoptotic ER stress response and which defenses are triggered by beta-cells against these challenges. ER stress may also link obesity and insulin resistance in type 2 diabetes. High fat feeding and obesity induce ER stress in liver, which suppresses insulin signaling via c-Jun N-terminal kinase activation. In vitro data suggest that ER stress may also contribute to cytokine-induced beta-cell death. Thus, the cytokines IL-1beta and interferon-gamma, putative mediators of beta-cell loss in type 1 diabetes, induce severe ER stress through, respectively, NO-mediated depletion of ER calcium and inhibition of ER chaperones, thus hampering beta-cell defenses and amplifying the proapoptotic pathways. A better understanding of the pathways regulating ER stress in beta-cells may be instrumental for the design of novel therapies to prevent beta-cell loss in diabetes.