The roles of endoplasmic reticulum overload response induced by HCV and NS4B protein in human hepatocyte viability and virus replication

Hepatitis Delta Virus Antigens Trigger Oxidative Stress, Activate Antioxidant Nrf2/ARE Pathway, and Induce Unfolded Protein Response

Hepatitis delta virus (HDV) is a viroid-like satellite that may co-infect individuals together with hepatitis B virus (HBV), as well as cause superinfection by infecting patients with chronic hepatitis B (CHB). Being a defective virus, HDV requires HBV structural proteins for virion production. Although the virus encodes just two forms of its single antigen, it enhances the progression of liver disease to cirrhosis in CHB patients and increases the incidence of hepatocellular carcinoma. HDV pathogenesis so far has been attributed to virus-induced humoral and cellular immune responses, while other factors have been neglected. Here, we evaluated the impact of the virus on the redox status of hepatocytes, as oxidative stress is believed to contribute to the pathogenesis of various viruses, including HBV and hepatitis C virus (HCV). We show that the overexpression of large HDV antigen (L-HDAg) or autonomous replication of the viral genome in cells leads to increased production of reactive oxygen species (ROS). It also leads to the upregulated expression of NADPH oxidases 1 and 4, cytochrome P450 2E1, and ER oxidoreductin 1α, which have previously been shown to mediate oxidative stress induced by HCV. Both HDV antigens also activated the Nrf2/ARE pathway, which controls the expression of a spectrum of antioxidant enzymes. Finally, HDV and its large antigen also induced endoplasmic reticulum (ER) stress and the concomitant unfolded protein response (UPR). In conclusion, HDV may enhance oxidative and ER stress induced by HBV, thus aggravating HBV-associated pathologies, including inflammation, liver fibrosis, and the development of cirrhosis and hepatocellular carcinoma.

Endoplasmic Reticulum Stress in Elderly Patients with COVID-19: Potential of Melatonin Treatment

Aging processes, including immunosenescence, inflammation, inflammasome formation, genomic instability, telomeric attrition, and altered autophagy, are involved in viral infections and they may contribute to increased pathophysiological responses to the SARS-CoV-2 infection in the elderly; this poses additional risks of accelerated aging, which could be found even after recovery. Aging is associated with oxidative damage. Moreover, SARS-CoV-2 infections may increase the production of reactive oxygen species and such infections will disturb the Ca++ balance via an endoplasmic reticulum (ER) stress-mediated unfolded protein response. Although vaccine development and anti-inflammation therapy lower the severity of COVID-19, the prevalence and mortality rates are still alarming in some countries worldwide. In this review, we describe the involvement of viral proteins in activating ER stress transducers and their downstream signals and in inducing inflammation and inflammasome formation. Furthermore, we propose the potential of melatonin as an ER stress modulator, owing to its antioxidant, anti-inflammatory, and immunoregulatory effects in viral infections. Considering its strong safety profile, we suggest that additive melatonin supplementation in the elderly could be beneficial in treating COVID-19.

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 interaction of SIRT4 and Calreticulin during ER stress in glia cells

Endoplasmic Reticulum (ER) stress is the response that occurs after the dysfunction of ER and its structure. Activated UPR triggers a stress response using ER membrane proteins such as PERK, IRE-1, GRP78, ATF5 ve ATF6. Sirtuins are enzymes that carry out post-translational modifications such as deacetylation and ADP-ribosylation. In our previous study, we identified Calreticulin as a SIRT4-interacting protein via mass spectrometry. Calreticulin binds to misfolded proteins, prevents them from leaving ER, which results in the reduction of ER stress. In this study, we aimed to investigate the interaction between SIRT4 and Calreticulin during ER stress in glia cells (IHA-immortalized human astrocytes). To trigger ER stress in glia cells, we first optimized the dose and the duration of the Tunicamycin which is 2.5 μg/ml concentration for 16 h. SIRT4 gene was silenced with lentiviral particles using 4 MOI (Multiplicity of Infection). In SIRT4-silenced cells, when treated with 2.5 μg/ml Tunicamycin for 16 h, the increase in the expressions of ATF6, GRP78 and the ratio of spliced/unspliced XBP1 mRNA were reduced. This shows that silencing SIRT4 may decrease ER stress. SIRT4-Calreticulin interaction was shown both in control and ER-stress induced glia cells. Additionally, this interaction did not change with the ER stress. SIRT4 only ADP-ribosylates Calreticulin during ER stress. Normally, SIRT4 ADP-ribosylates and deactivates Calreticulin during ER stress condition. When SIRT4 is silenced, the ADP-ribosylation level of Calreticulin decreases resulting in the activation of Calreticulin and the reduction of ER stress. In summary, SIRT4 inhibitors may be investigated as protective agents or drug candidates in neurodegenerative diseases where ER stress mostly underlies as one of the molecular mechanisms.

Endoplasmic Reticulum Stress in Diabetic Nephrology: Regulation, Pathological Role, and Therapeutic Potential

Recent progress has been made in understanding the roles and mechanisms of endoplasmic reticulum (ER) stress in the development and pathogenesis of diabetic nephropathy (DN). Hyperglycemia induces ER stress and apoptosis in renal cells. The induction of ER stress can be cytoprotective or cytotoxic. Experimental treatment of animals with ER stress inhibitors alleviated renal damage. Considering these findings, the normalization of ER stress by pharmacological agents is a promising approach to prevent or arrest DN progression. The current article reviews the mechanisms, roles, and therapeutic aspects of these findings.

[Hepatitis C virus (Flaviviridae: Hepacivirus: Hepacivirus C): regulation of signaling reactions of innate immunity]

Studying the regulation of signaling reactions of innate immunity by the hepatitis C virus (HCV) will help to reveal the causes of the transition of the acute form of the disease to a chronic course. The molecular mechanisms of activation by HCV RNA of innate immunity receptors TLR and RLR and signal transduction processes leading to the synthesis of IFN and inflammatory cytokines are considered. The inhibitory effects of non-structural and structural HCV proteins on immune signaling reactions are analyzed in detail. The information presented is the result of an analysis of literature data published in international databases mainly over the past 5 years. In conclusion, signaling receptors are proposed as targets for the development of new antiviral drugs with immunotherapeutic activity.

Naringenin ameliorates insulin resistance by modulating endoplasmic reticulum stress in hepatitis C virus-infected liver

Hepatitis C virus (HCV) infection may lead to hepatic insulin resistance (IR), and endoplasmic reticulum (ER) stress has been found to induce IR. In our previous study, naringenin (NGEN) had an insulin sensitization effect on the HCV core protein (HCVCP) infected mouse livers. In the present study, we examined the effects of NGEN on HCVCP infection-induced ER stress and investigated the insulin sensitization mechanism involved. We found that XBP1s was up-regulated in the livers of HCV-infected patients, in hepatocytes with HCV infection, and in HCVCP-infected mice. HCVCP induces ER stress in the mouse liver and hepatocytes by increasing XBP1s and downstream gene expression. Pre-treatment with NGEN inhibited the ER stress and downstream gene expression both in vivo and in vitro. Similar to the HCVCP infection results, NGEN also inhibited the ER stress in tunicamycin-treated Huh-7.5.1 cells. In addition, the role of IRE1α in HCVCP-induced IR was detected, and knockdown of IRE1α abolished HCVCP-stimulated IR. Overexpression induced IR but could be abolished by NGEN. NGEN also blocked the HCVCP-induced IRE1α expression levels that were up-regulated in vivo. Our data reveal that ER stress may be associated with HCV-induced IR, and NGEN treatment inhibited ER stress activity and increased insulin sensitivity by decreasing the expression of IRE1α.

Hepatitis C virus NS4B induces the degradation of TRIF to inhibit TLR3-mediated interferon signaling pathway

Toll-like receptor 3 (TLR3) senses dsRNA intermediates produced during RNA virus replication to activate innate immune signaling pathways through adaptor protein TRIF. Many viruses have evolved strategies to block TLR3-mediated interferon signaling via targeting TRIF. Here we studied how hepatitis C virus (HCV) antagonizes the TLR3-mediated interferon signaling. We found that HCV-encoded NS4B protein inhibited TLR3-mediated interferon signaling by down-regulating TRIF protein level. Mechanism studies indicated that the downregulation of TRIF by NS4B was dependent on caspase8. NS4B transfection or HCV infection can activate caspase8 to promote TRIF degradation, leading to suppression of TLR3-mediated interferon signaling. Knockout of caspase8 can prevent TRIF degradation triggered by NS4B, thereby enhancing the TLR3-mediated interferon signaling activation in response to HCV infection. In conclusion, our work revealed a new mechanism for HCV to evade innate immune response by blocking the TLR3-mediated interferon signaling via NS4B-induced TRIF degradation.

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.

ER homeostasis and autophagy

The endoplasmic reticulum (ER) is a key site for lipid biosynthesis and folding of nascent transmembrane and secretory proteins. These processes are maintained by careful homeostatic control of the environment within the ER lumen. Signalling sensors within the ER detect perturbations within the lumen (ER stress) and employ downstream signalling cascades that engage effector mechanisms to restore homeostasis. The most studied signalling mechanism that the ER employs is the unfolded protein response (UPR), which is known to increase a number of effector mechanisms, including autophagy. In this chapter, we will discuss the emerging role of autophagy as a UPR effector pathway. We will focus on the recently discovered selective autophagy pathway for ER, ER-phagy, with particular emphasis on the structure and function of known mammalian ER-phagy receptors, namely FAM134B, SEC62, RTN3 and CCPG1. Finally, we conclude with our view of where the future of this field can lead our understanding of the involvement of ER-phagy in ER homeostasis.

Modulation of Cell Death Pathways by Hepatitis C Virus Proteins in Huh7.5 Hepatoma Cells

The hepatitis C virus (HCV) causes chronic liver disease leading to fibrosis, cirrhosis, and hepatocellular carcinoma. HCV infection triggers various types of cell death which contribute to hepatitis C pathogenesis. However, much is still unknown about the impact of viral proteins on them. Here we present the results of simultaneous immunocytochemical analysis of markers of apoptosis, autophagy, and necrosis in Huh7.5 cells expressing individual HCV proteins or their combinations, or harboring the virus replicon. Stable replication of the full-length HCV genome or transient expression of its core, Е1/Е2, NS3 and NS5B led to the death of 20-47% cells, 72 h posttransfection, whereas the expression of the NS4A/B, NS5A or NS3-NS5B polyprotein did not affect cell viability. HCV proteins caused different impacts on the activation of caspases-3, -8 and -9 and on DNA fragmentation. The structural core and E1/E2 proteins promoted apoptosis, whereas non-structural NS4A/B, NS5A, NS5B suppressed apoptosis by blocking various members of the caspase cascade. The majority of HCV proteins also enhanced autophagy, while NS5A also induced necrosis. As a result, the death of Huh7.5 cells expressing the HCV core was induced via apoptosis, the cells expressing NS3 and NS5B via autophagy-associated death, and the cells expressing E1/E2 glycoproteins or harboring HCV the replicon via both apoptosis and autophagy.

Anastasis and the ER stress response: Solving the paradox of the unfolded protein response in cancer

In recent years, studies have suggested a novel pathway for cell survival, which faces scientific skepticism and interest in its concept of cell 'resurrection' - that is, the anastasis of cells at late-stage apoptosis. While biomarkers have been discovered, many of these are related to the endoplasmic reticulum (ER) stress response - acting also to promote cell survival in the presence of perturbation. The promises of anastasis, if accepted, will greatly impact translational medicine especially in the treatment of cancer, since apoptosis is generally irreversible in the late stages, and chemotherapy is performed to maximize tumor death and minimize off-target effects. As with all new concepts, there is a need to demarcate anastasis from a well-studied survival mechanism - the ER stress response - if the concept is to progress any further. In this article, it is proposed that anastasis and the ER stress response are one and the same mechanism, demarcated only by the presence of persistent stress. Further, anastasis solves the paradox of the unfolded protein response (UPR) in cancer by providing rationale in C/EBP homologous protein (CHOP)-induced tumor survival, such that CHOP-mediated apoptosis initiates genetic alterations in favor of its survival. After which, the cell regenerates through an enhanced ER stress response. Hence, anastatic cell recovery is the ER stress response post-apoptosis.

HCV NS4B targets Scribble for proteasome-mediated degradation to facilitate cell transformation

Hepatitis C virus (HCV) nonstructural protein 4B (NS4B) is a multi-transmembrane protein, but little is known about how NS4B contributes to HCV replication and tumorigenesis. Its C-terminal domain (CTD) has been shown to associate with intracellular membrane, and we have previously shown that NS4B CTD contains a class I PDZ-binding motif (PBM). Here, we demonstrated that NS4B PBM interacts with the PDZ-containing tumor suppressor protein, Scribble, using immunofluorescence and co-immunoprecipitation assays, and this interaction requires at least three contiguous PDZ domains of Scribble. In addition, NS4B PBM specifically induced Scribble degradation by activating the proteasome-ubiquitin pathway. Similar Scribble degradation was also observed in HCV-infected cells, suggesting NS4B could work in the context of HCV. Finally, NS4B PBM mutants showed reduced colony formation capacity compared with its wild-type counterpart, indicating that NS4B PBM plays important roles in NS4B-mediated cell transformation. Altogether, we provide a mechanism by which NS4B induces cell transformation through its PBM, which specifically interacts with the PDZ domains of Scribble and targets Scribble for degradation.

Hepatitis C virus and its protein NS4B activate the cancer-related STAT3 pathway via the endoplasmic reticulum overload response

Oxidative stress induces the activation of signal transducer and activator of transcription 3 (STAT3), which plays an important role in hepatocellular carcinoma (HCC). We have previously reported that hepatitis C virus (HCV) and its protein NS4B induce the production of reactive oxygen species (ROS) via the endoplasmic reticulum overload response (EOR) in human hepatocytes. Here, we found that NS4B and HCV induce STAT3 activation and stimulate the expression of cancer-related STAT3 target genes, including VEGF, c-myc, MMP-9 and Mcl-1, by EOR in human hepatocytes. Moreover, the cancer-related STAT3 pathway activated by NS4B and HCV via EOR were found to promote human hepatocyte viability. Taken together, these findings revealed that HCV NS4B might contribute to HCC by activating the EOR-mediated cancer-related STAT3 pathway, and this could provide novel insights into HCV-induced HCC.

Oxidative Stress in the Healthy and Wounded Hepatocyte: A Cellular Organelles Perspective

Accurate control of the cell redox state is mandatory for maintaining the structural integrity and physiological functions. This control is achieved both by a fine-tuned balance between prooxidant and anti-oxidant molecules and by spatial and temporal confinement of the oxidative species. The diverse cellular compartments each, although structurally and functionally related, actively maintain their own redox balance, which is necessary to fulfill specialized tasks. Many fundamental cellular processes such as insulin signaling, cell proliferation and differentiation and cell migration and adhesion, rely on localized changes in the redox state of signal transducers, which is mainly mediated by hydrogen peroxide (H₂O₂). Therefore, oxidative stress can also occur long before direct structural damage to cellular components, by disruption of the redox circuits that regulate the cellular organelles homeostasis. The hepatocyte is a systemic hub integrating the whole body metabolic demand, iron homeostasis and detoxification processes, all of which are redox-regulated processes. Imbalance of the hepatocyte's organelles redox homeostasis underlies virtually any liver disease and is a field of intense research activity. This review recapitulates the evolving concept of oxidative stress in the diverse cellular compartments, highlighting the principle mechanisms of oxidative stress occurring in the healthy and wounded hepatocyte.