Endoplasmic reticulum stress-mediated autophagy contributes to bluetongue virus infection via the PERK-eIF2α pathway

CXCR3 antagonist rescues ER stress and reduces inflammation and JEV infection in mice brain

The endoplasmic reticulum (ER) is crucial for maintaining cellular homeostasis, and synthesis and folding of proteins and lipids. The ER is sensitive to stresses including viral infection that perturb the intracellular energy level and redox state, and accumulating unfolded/misfolded proteins. Viruses including Japanese encephalitis virus (JEV) activates unfolded protein response (UPR) causing ER stress in host immune cells and promotes inflammation and apoptotic cell death. The chemokine receptor CXCR3 has been reported to play important role in the accumulation of inflammatory immune cells and neuronal cell death in several disease conditions. Recently we described the involvement of CXCR3 in regulating inflammation and JEV infection in mice brain. Supplementation with a CXCR3 antagonist AMG487 significantly reduced JEV infection in the mice brain in conjunction with the downregulation of UPR pathway via PERK:eIF2α:CHOP, and decreased mitochondrial ROS generation, inflammation and apoptotic cell death. Alongside, AMG487 treatment improved interferon (IFN)-α/β synthesis in JEV-infected mice brain. Thus, suggesting a potential therapeutic role of CXCR3 antagonist against JEV infection.

The Multi-Faceted Role of Autophagy During Animal Virus Infection

Autophagy is a process of degradation to maintain cellular homeostatic by lysosomes, which ensures cellular survival under various stress conditions, including nutrient deficiency, hypoxia, high temperature, and pathogenic infection. Xenophagy, a form of selective autophagy, serves as a defense mechanism against multiple intracellular pathogen types, such as viruses, bacteria, and parasites. Recent years have seen a growing list of animal viruses with autophagy machinery. Although the relationship between autophagy and human viruses has been widely summarized, little attention has been paid to the role of this cellular function in the veterinary field, especially today, with the growth of serious zoonotic diseases. The mechanisms of the same virus inducing autophagy in different species, or different viruses inducing autophagy in the same species have not been clarified. In this review, we examine the role of autophagy in important animal viral infectious diseases and discuss the regulation mechanisms of different animal viruses to provide a potential theoretical basis for therapeutic strategies, such as targets of new vaccine development or drugs, to improve industrial production in farming.

Chemotherapy Resistance: Role of Mitochondrial and Autophagic Components

Simple Summary

Chemotherapy resistance is a common occurrence during cancer treatment that cancer researchers are attempting to understand and overcome. Mitochondria are a crucial intracellular signaling core that are becoming important determinants of numerous aspects of cancer genesis and progression, such as metabolic reprogramming, metastatic capability, and chemotherapeutic resistance. Mitophagy, or selective autophagy of mitochondria, can influence both the efficacy of tumor chemotherapy and the degree of drug resistance. Regardless of the fact that mitochondria are well-known for coordinating ATP synthesis from cellular respiration in cellular bioenergetics, little is known its mitophagy regulation in chemoresistance. Recent advancements in mitochondrial research, mitophagy regulatory mechanisms, and their implications for our understanding of chemotherapy resistance are discussed in this review.

Abstract

Cancer chemotherapy resistance is one of the most critical obstacles in cancer therapy. One of the well-known mechanisms of chemotherapy resistance is the change in the mitochondrial death pathways which occur when cells are under stressful situations, such as chemotherapy. Mitophagy, or mitochondrial selective autophagy, is critical for cell quality control because it can efficiently break down, remove, and recycle defective or damaged mitochondria. As cancer cells use mitophagy to rapidly sweep away damaged mitochondria in order to mediate their own drug resistance, it influences the efficacy of tumor chemotherapy as well as the degree of drug resistance. Yet despite the importance of mitochondria and mitophagy in chemotherapy resistance, little is known about the precise mechanisms involved. As a consequence, identifying potential therapeutic targets by analyzing the signal pathways that govern mitophagy has become a vital research goal. In this paper, we review recent advances in mitochondrial research, mitophagy control mechanisms, and their implications for our understanding of chemotherapy resistance.

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.

Targeting the Integrated Stress Response in Cancer Therapy

The integrated stress response (ISR) is an evolutionarily conserved intra-cellular signaling network which is activated in response to intrinsic and extrinsic stresses. Various stresses are sensed by four specialized kinases, PKR-like ER kinase (PERK), general control non-derepressible 2 (GCN2), double-stranded RNA-dependent protein kinase (PKR) and heme-regulated eIF2α kinase (HRI) that converge on phosphorylation of serine 51 of eIF2α. eIF2α phosphorylation causes a global reduction of protein synthesis and triggers the translation of specific mRNAs, including activating transcription factor 4 (ATF4). Although the ISR promotes cell survival and homeostasis, when stress is severe or prolonged the ISR signaling will shift to regulate cellular apoptosis. We review the ISR signaling pathway, regulation and importance in cancer therapy.

Sec62 Regulates Endoplasmic Reticulum Stress and Autophagy Balance to Affect Foot-and-Mouth Disease Virus Replication

Endoplasmic reticulum (ER) stress-induced autophagy is closely associated with viral infection and propagation. However, the intrinsic link between ER stress, autophagy, and viral replication during foot-and-mouth disease virus (FMDV) infection is not fully elucidated. Our previous studies demonstrated that FMDV infection activated the ER stress-associated UPR of the PERK-eIF2a and ATF6 signaling pathway, whereas the IRE1a signaling was suppressed. We found that the activated-ATF6 pathway participated in FMDV-induced autophagy and FMDV replication, while the IRE1α pathway only affected FMDV replication. Further studies indicated that Sec62 was greatly reduced in the later stages of FMDV infection and blocked the activation of the autophagy-related IRE1α-JNK pathway. Moreover, it was also found that Sec62 promoted IRE1a phosphorylation and negatively regulated FMDV proliferation. Importantly, Sec62 may interact with LC3 to regulate ER stress and autophagy balance and eventually contribute to FMDV clearance via fusing with lysosomes. Altogether, these results suggest that Sec62 is a critical molecule in maintaining and recovering ER homeostasis by activating the IRE1α-JNK pathway and delivering autophagosome into the lysosome, thus providing new insights on FMDV-host interactions and novel antiviral therapies.

Partial glycosylation of the Ibaraki virus NS3 protein is sufficient to support virus propagation

Ibaraki virus (IBAV) causes Ibaraki disease. We have previously shown that IBAV NS3 protein is highly glycosylated and that tunicamycin, an inhibitor of N-linked glycosylation, suppressed NS3 glycosylation and viral propagation. Since tunicamycin is known to cause endoplasmic reticulum (ER) stress, we explored the effects of ER stress and NS3 glycosylation on IBAV infection using tunicamycin and thapsigargin. These reagents both induced ER stress and NS3 glycosylation inhibition in a concentration-dependent manner, and as in our previous report, high concentrations of tunicamycin and thapsigargin suppressed IBAV propagation. However, lower concentrations of these reagents produced limited differences in IBAV propagation, despite their ability to suppress NS3 glycosylation and induce ER stress. These findings suggest that a considerable degree of NS3 glycosylation inhibition and ER stress induction does not suppress IBAV propagation. Conversely, lower concentrations of thapsigargin enhanced IBAV propagation, suggesting that moderate ER stress could benefit IBAV.

Arthropods Under Pressure: Stress Responses and Immunity at the Pathogen-Vector Interface

Understanding what influences the ability of some arthropods to harbor and transmit pathogens may be key for controlling the spread of vector-borne diseases. Arthropod immunity has a central role in dictating vector competence for pathogen acquisition and transmission. Microbial infection elicits immune responses and imparts stress on the host by causing physical damage and nutrient deprivation, which triggers evolutionarily conserved stress response pathways aimed at restoring cellular homeostasis. Recent studies increasingly recognize that eukaryotic stress responses and innate immunity are closely intertwined. Herein, we describe two well-characterized and evolutionarily conserved mechanisms, the Unfolded Protein Response (UPR) and the Integrated Stress Response (ISR), and examine evidence that these stress responses impact immune signaling. We then describe how multiple pathogens, including vector-borne microbes, interface with stress responses in mammals. Owing to the well-conserved nature of the UPR and ISR, we speculate that similar mechanisms may be occurring in arthropod vectors and ultimately impacting vector competence. We conclude this Perspective by positing that novel insights into vector competence will emerge when considering that stress-signaling pathways may be influencing the arthropod immune network.

The role of host eIF2α in viral infection

Background

eIF2α is a regulatory node that controls protein synthesis initiation by its phosphorylation or dephosphorylation. General control nonderepressible-2 (GCN2), protein kinase R-like endoplasmic reticulum kinase (PERK), double-stranded RNA (dsRNA)-dependent protein kinase (PKR) and heme-regulated inhibitor (HRI) are four kinases that regulate eIF2α phosphorylation.

Main body

In the viral infection process, dsRNA or viral proteins produced by viral proliferation activate different eIF2α kinases, resulting in eIF2α phosphorylation, which hinders ternary tRNA^Met-GTP-eIF2 complex formation and inhibits host or viral protein synthesis. The stalled messenger ribonucleoprotein (mRNP) complex aggregates under viral infection stress to form stress granules (SGs), which encapsulate viral RNA and transcription- and translation-related proteins, thereby limiting virus proliferation. However, many viruses have evolved a corresponding escape mechanism to synthesize their own proteins in the event of host protein synthesis shutdown and SG formation caused by eIF2α phosphorylation, and viruses can block the cell replication cycle through the PERK-eIF2α pathway, providing a favorable environment for their own replication. Subsequently, viruses can induce host cell autophagy or apoptosis through the eIF2α-ATF4-CHOP pathway.

Conclusions

This review summarizes the role of eIF2α in viral infection to provide a reference for studying the interactions between viruses and hosts.

Effect of endoplasmic reticulum stress-mediated excessive autophagy on apoptosis and formation of kidney stones

AIMS

This study was designed to reveal the role and underlying mechanism of excessive autophagy mediated by ERS via the PERK-eIF2α pathway in the apoptosis and formation of CaOx kidney stones.

MAIN METHODS

Ethylene glycol (EG) was used to establish a rat model of CaOx kidney stones, and 100 mg/kg of ERS inhibitor 4-phenylbutyric acid (4-PBA) or 60 mg/kg of autophagy inhibitor chloroquine (CQ) was administered daily to the rats. Four weeks after administration, we collected blood and kidney tissues to analyze the occurrence of ERS and autophagy, apoptosis, renal function, renal tubular crystal deposition, and kidney damage, respectively.

KEY FINDINGS

We observed that both 4-PBA and CQ treatment significantly inhibited the excessive autophagy and reduced apoptosis as well as decreasing p-PERK and p-eIF2α expressions. Meanwhile, the proportion of kidney weight, contents of creatinine and blood urea nitrogen, excretion of neutrophil gelatinase-associated lipocalin and kidney injury molecule 1, and renal tubular deposition were markedly down-regulated.

SIGNIFICANCE

The findings in this study suggested that ERS induced excessive autophagy via the PERK-eIF2α pathway, regulating cell damage and apoptosis. ERS-mediated inhibition of excessive autophagy effectively protected kidney function and prevented the apoptosis and formation of kidney stones.

Inhibition of endoplasmic reticulum stress-induced autophagy promotes the killing effect of X-rays on sarcoma in mice

Endoplasmic reticulum (ER) stress is a conserved cellular process for cells to clear unfolded or misfolded proteins and maintain cell homeostasis under stress conditions. Autophagy may act as a pro-survival strategy to cope with multiple stress conditions in tumor progression and distant metastasis. Although many studies have demonstrated that there is a close correlation between radiation-induced ER stress and autophagy, the molecular mechanisms currently remain unclear. In the present study, we performed an in vivo study concerning the effect of autophagy induced by ER stress on the radiosensitivity of mouse sarcoma using X-rays. Our results documented that X-rays could induce ER stress in sarcoma and then autophagy was activated by unfolded protein response (UPR) through the IRE1-JNK-pBcl2-Beclin1 signaling axis. The induction of autophagy caused a decline in cell apoptosis while inhibiting the autophagy resulted in increased apoptosis and inhibition of tumor progression. Combined treatment of X-ray exposure and chloroquine increased ER stress-related apoptosis and enhanced the radiosensitivity of mouse sarcoma that was not sensitive to X-ray irradiation alone. Thus, our study indicates that inhibition of ER stress-induced autophagy might be a novel strategy to improve the efficacy of radiotherapy against radioresistant sarcoma.

Classical Swine Fever Virus Infection Induces Endoplasmic Reticulum Stress-Mediated Autophagy to Sustain Viral Replication in vivo and in vitro

Endoplasmic reticulum (ER) stress-mediated autophagy plays significant roles in replication and pathogenesis of many animal viruses. However, the relationship between ER stress, autophagy, and viral replication during in vivo and in vitro infection of classical swine fever virus (CSFV) remains unclear. In this study, we established a pig model for CSFV infection and found that viral loads of CSFV differed in 10 kinds of infected organs, and that the degree of tissue lesions was to some extent positively correlated with CSFV replication in vivo. Next, we found that CSFV infection not only induced ER stress and subsequently activated three unfolded protein responses (UPR) pathways including protein kinase R-like ER kinase (PERK), inositol requiring enzyme 1 (IRE1), and activating transcription factor-6 (ATF-6) pathways, but also triggered complete autophagy in main immune organs and partial nonimmune organs exhibiting severer pathological injuries and higher viral loads. However, only the IRE1 pathway and no autophagy were activated in some other nonimmune organs with slighter pathologies and lower viral loads. These results indicate a potential link between CSFV-induced ER stress and autophagy, both of which are associated with the CSFV replication in vivo. We further performed in vitro experiments and found that CSFV infection activates the PERK and IRE1 pathways and autophagy in cultured porcine kidney cell lines (PK-15) and macrophage cell lines (3D4/2), and pharmacological regulation of ER stress remarkably changed autophagic activities induced by CSFV, suggesting that CSFV-induced autophagy can be mediated by ER stress possibly via the PERK and IRE1 pathway. Furthermore, treatment with ER stress regulators significantly altered copy numbers of NS5B genes, expression of Npro proteins, and viral titers in CSFV-infected cells or in cells treated with autophagy regulators prior to CSFV infection, suggesting the requirement of ER stress-mediated autophagy for CSFV replication in vitro. Collectively, our data demonstrate that CSFV induces ER stress-mediated autophagy to sustain its replication in vivo and in vitro, which may be one of the potential strategies exploited by CSFV for immune evasion. This finding will provide new insights into mechanisms of replication and pathogenesis of CSFV, and development of new strategies for controlling CSF.

Japanese Encephalitis Virus Induces Apoptosis and Encephalitis by Activating the PERK Pathway

Accumulated evidence demonstrates that Japanese encephalitis virus (JEV) infection triggers endoplasmic reticulum (ER) stress and neuron apoptosis. ER stress sensor protein kinase R-like endoplasmic reticulum kinase (PERK) has been reported to induce apoptosis under acute or prolonged ER stress. However, the precise role of PERK in JEV-induced apoptosis and encephalitis remains unknown. Here, we report that JEV infection activates the PERK-ATF4-CHOP apoptosis pathway both in vitro and in vivo PERK activation also promotes the formation of stress granule, which in turn represses JEV-induced apoptosis. However, PERK inhibitor reduces apoptosis, indicating that JEV-activated PERK predominantly induces apoptosis via the PERK-ATF4-CHOP apoptosis pathway. Among JEV proteins that have been reported to induce ER stress, only JEV NS4B can induce PERK activation. PERK has been reported to form an active molecule by dimerization. The coimmunoprecipitation assay shows that NS4B interacts with PERK. Moreover, glycerol gradient centrifugation shows that NS4B induces PERK dimerization. Both the LIG-FHA and the LIG-WD40 domains within NS4B are required to induce PERK dimerization, suggesting that JEV NS4B pulls two PERK molecules together by simultaneously interacting with them via different motifs. PERK deactivation reduces brain cell damage and encephalitis during JEV infection. Furthermore, expression of JEV NS4B is sufficient to induce encephalitis via PERK in mice, indicating that JEV activates PERK primarily via its NS4B to cause encephalitis. Taken together, our findings provide a novel insight into JEV-caused encephalitis.IMPORTANCE Japanese encephalitis virus (JEV) infection triggers endoplasmic reticulum (ER) stress and neuron apoptosis. ER stress sensor protein kinase R-like endoplasmic reticulum kinase (PERK) has been reported to induce apoptosis under acute or prolonged ER stress. However, whether the PERK pathway of ER stress response plays important roles in JEV-induced apoptosis and encephalitis remains unknown. Here, we found that JEV infection activates ER stress sensor PERK in neuronal cells and mouse brains. PERK activation induces apoptosis via the PERK-ATF4-CHOP apoptosis pathway upon JEV infection. Among the JEV proteins prM, E, NS1, NS2A, NS2B, and NS4B, only NS4B activates PERK. Moreover, activated PERK participates in apoptosis and encephalitis induced by JEV and NS4B. These findings provide a novel therapeutic approach for JEV-caused encephalitis.

VEGF-A promotes angiogenesis after acute myocardial infarction through increasing ROS production and enhancing ER stress-mediated autophagy

Proangiogenesis is generally regarded as an effective approach for treating ischemic heart disease. Vascular endothelial growth factor (VEGF)-A is a strong and essential proangiogenic factor. Reactive oxygen species (ROS), endoplasmic reticulum (ER) stress, and autophagy are implicated in the process of angiogenesis. This study is designed to clarify the regulatory mechanisms underlying VEGF-A, ROS, ER stress, autophagy, and angiogenesis in acute myocardial infarction (AMI). A mouse model of AMI was successfully established by occluding the left anterior descending coronary artery. Compared with the sham-operated mice, the microvessel density, VEGF-A content, ROS production, expression of vascular endothelial cadherin, positive expression of 78 kDa glucose-regulated protein/binding immunoglobulin protein (GRP78/Bip), and LC3 puncta in CD31-positive endothelial cells of the ischemic myocardium were overtly elevated. Moreover, VEGF-A exposure predominantly increased the expression of beclin-1, autophagy-related gene (ATG) 4, ATG5, inositol-requiring enzyme-1 (IRE-1), GRP78/Bip, and LC3-II/LC3-I as well as ROS production in the human umbilical vein endothelial cells (HUVECs) in a dose and time-dependent manner. Both beclin-1 small interfering RNA and 3-methyladenine treatment predominantly mitigated VEGF-A-induced tube formation and migration of HUVECs, but they failed to elicit any notable effect on VEGF-A-increased expression of GRP78/Bip. Tauroursodeoxycholic acid not only obviously abolished VEGF-A-induced increase of IRE-1, GRP78/Bip, beclin-1 expression, and LC3-II/LC3-I, but also negated VEGF-A-induced tube formation and migration of HUVECs. Furthermore, N-acetyl- l-cysteine markedly abrogated VEGF-A-increased ROS production, IRE-1, GRP78/Bip, beclin-1 expression, and LC3-II/LC3-I in the HUVECs. Taken together, our data demonstrated that increased spontaneous production of VEGF-A may induce angiogenesis after AMI through initiating ROS-ER stress-autophagy axis in the vascular endothelial cells.

DEV induce autophagy via the endoplasmic reticulum stress related unfolded protein response

Duck enteritis virus (DEV) can infect ducks, geese, and many other poultry species and leads to acute, septic and highly fatal infectious disease. Autophagy is an evolutionarily ancient pathway that plays an important role in many viral infections. We previously reported that DEV infection induces autophagy for its own benefit, but how this occurs remains unclear. In this study, endoplasmic reticulum (ER) stress was triggered by DEV infection, as demonstrated by the increased expression of the ER stress marker glucose-regulated protein 78 (GRP78) and the dilated morphology of the ER. Pathways associated with the unfolded protein response (UPR), including the PKR-like ER protein kinase (PERK) and inositol-requiring enzyme 1 (IRE1) pathways, but not the activating transcription factor 6 (ATF6) pathway, were activated in DEV-infected duck embryo fibroblast (DEF) cells. In addition, the knockdown of both PERK and IRE1 by small interfering RNAs (siRNAs) reduced the level of LC3-II and viral yields, which suggested that the PERK-eukaryotic initiation factor 2α (eIF2α) and IRE1-x-box protein1 (XBP1) pathways may contribute to DEV-induced autophagy. Collectively, these data offer new insight into the mechanisms of DEV -induced autophagy through activation of the ER stress-related UPR pathway.

The endoplasmic reticulum stress-autophagy pathway is involved in apelin-13-induced cardiomyocyte hypertrophy in vitro

Apelin is the endogenous ligand for the G protein-coupled receptor APJ, and plays important roles in the cardiovascular system. Our previous studies showed that apelin-13 promotes the hypertrophy of H9c2 rat cardiomyocytes through the PI3K-autophagy pathway. The aim of this study was to explore what roles ER stress and autophagy played in apelin-13-induced hypertrophy of cardiomyocytes in vitro. Treatment of H9c2 cells with apelin-13 (0.001-2 μmol/L) dose-dependently increased the production of ROS and the expression levels of NADPH oxidase 4 (NOX4). Knockdown of Nox4 with siRNAs effectively prevented the reduction of GSH/GSSG ratio in apelin-13-treated cells. Furthermore, apelin-13 treatment dose-dependently increased the expression of Bip and CHOP, two ER stress markers, in the cells. Knockdown of APJ or Nox4 with the corresponding siRNAs, or application of NADPH inhibitor DPI blocked apelin-13-induced increases in Bip and CHOP expression. Moreover, apelin-13 treatment increased the formation of autophagosome and ER fragments and the LC3 puncta in the ER of the cells. Knockdown of APJ, Nox4, Bip or CHOP with the corresponding siRNAs, or application of DPI or salubrinal attenuated apelin-13-induced overexpression of LC3-II/I and beclin 1. Finally, knockdown of Nox4, Bip or CHOP with the corresponding siRNAs, or application of salubrinal significantly suppressed apelin-13-induced increases in the cell diameter, volume and protein contents. Our results demonstrate that ER stress-autophagy is involved in apelin-13-induced H9c2 cell hypertrophy.

Effects of calcium-dependent molecular chaperones and endoplasmic reticulum in the amygdala in rats under single‑prolonged stress

The purpose of the present study was to investigate the role of endoplasmic reticulum (ER)‑resident molecular chaperone proteins to identify whether these proteins were involved in post‑traumatic stress disorder (PTSD). The present study detected changes of calreticulin (CRT), calnexin (CNX) and ERp57 in the amygdala of rats, which may with aim of providing a novel insight into the modulation effect of amygdala in PTSD. Single‑prolonged stress (SPS) was applied to create the models of PTSD in rats. The expression levels of CRT, CNX and ERp57 were examined using immunohistochemistry or immunofluorescence, western blot analysis and reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR). The results showed that SPS induced significant changes in CRT, CNX and ERp57 expression levels. Furthermore, the expression levels of CRT, CNX and ERp57 were significantly upregulated when compared to that in the control group after SPS exposure by western blot analysis (P<0.05). RT‑qPCR analysis supported these results, indicating an upregulation of mRNA expression level. Taken together, the present findings suggest that SPS may induce changes to the expression of CRT, CNX and ERp57 in the amygdala of rats. The present study provides an insight into the effects of ER‑resident molecular chaperones in the amygdala participating in PTSD, and provides the experimental basis and a mechanism for the pathophysiology of PTSD.

Cartilage-Specific Autophagy Deficiency Promotes ER Stress and Impairs Chondrogenesis in PERK-ATF4-CHOP-Dependent Manner

Autophagy is activated during nutritionally depleted or hypoxic conditions to facilitate cell survival. Because growth plate is an avascular and hypoxic tissue, autophagy may have a crucial role during chondrogenesis; however, the functional role and underlying mechanism of autophagy in regulation of growth plate remains elusive. In this study, we generated ^TamCart Atg7^-/- (Atg7cKO) mice to explore the role of autophagy during endochondral ossification. Atg7cKO mice exhibited growth retardation associated with reduced chondrocyte proliferation and differentiation, and increased chondrocyte apoptosis. Meanwhile, we observed that Atg7 ablation mainly induced the PERK-ATF4-CHOP axis of the endoplasmic reticulum (ER) stress response in growth plate chondrocytes. Although Atg7 ablation induced ER stress in growth plate chondrocytes, the addition of phenylbutyric acid (PBA), a chemical chaperone known to attenuate ER stress, partly neutralized such effects of Atg7 ablation on longitudinal bone growth, indicating the causative interaction between autophagy and ER stress in growth plate. Consistent with these findings in vivo, we also observed that Atg7 ablation in cultured chondrocytes resulted in defective autophagy, elevated ER stress, decreased chondrocytes proliferation, impaired expression of col10a1, MMP-13, and VEGFA for chondrocyte differentiation, and increased chondrocyte apoptosis, while such effects were partly nullified by reduction of ER stress with PBA. In addition, Atg7 ablation-mediated impaired chondrocyte function (chondrocyte proliferation, differentiation, and apoptosis) was partly reversed in CHOP^-/- cells, indicating the causative role of the PERK-ATF4-CHOP axis of the ER stress response in the action of autophagy deficiency in chondrocytes. In conclusion, our findings indicate that autophagy deficiency may trigger ER stress in growth plate chondrocytes and contribute to growth retardation, thus implicating autophagy as an important regulator during chondrogenesis and providing new insights into the clinical potential of autophagy in cartilage homeostasis. © 2017 American Society for Bone and Mineral Research.

Inhibiting autophagy with chloroquine enhances the anti-tumor effect of high-LET carbon ions via ER stress-related apoptosis

Energetic carbon ions (CI) offer great advantages over conventional radiations such as X- or γ-rays in cancer radiotherapy. High linear energy transfer (LET) CI can induce both endoplasmic reticulum (ER) stress and autophagy in tumor cells under certain circumstances. The molecular connection between ER stress and autophagy in tumor exposed to high-LET radiation and how these two pathways influence the therapeutic effect against tumor remain poorly understood. In this work, we studied the impact of autophagy and apoptosis induced by ER stress following high-LET CI radiation on the radiosensitivity of S180 cells both in vitro and in vivo. In the in vitro experiment, X-rays were also used as a reference radiation. Our results documented that the combination of CI radiation with chloroquine (CQ), a special autophagy inhibitor, produced more pronounced proliferation suppression in S180 cells and xenograft tumors. Co-treatment with CI radiation and CQ could block autophagy through the IRE1/JNK/Beclin-1 axis and enhance apoptotic cell death via the activation of C/EBP homologous protein (CHOP) by the IRE1 pathway rather than PERK in vitro and in vivo. Thus, our study indicates that inhibiting autophagy might be a promising therapeutic strategy in CI radiotherapy via aggravating the ER stress-related apoptosis.

The integrated stress response

In response to diverse stress stimuli, eukaryotic cells activate a common adaptive pathway, termed the integrated stress response (ISR), to restore cellular homeostasis. The core event in this pathway is the phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) by one of four members of the eIF2α kinase family, which leads to a decrease in global protein synthesis and the induction of selected genes, including the transcription factor ATF4, that together promote cellular recovery. The gene expression program activated by the ISR optimizes the cellular response to stress and is dependent on the cellular context, as well as on the nature and intensity of the stress stimuli. Although the ISR is primarily a pro-survival, homeostatic program, exposure to severe stress can drive signaling toward cell death. Here, we review current understanding of the ISR signaling and how it regulates cell fate under diverse types of stress.

GSK2656157, a PERK inhibitor, reduced LPS-induced IL-1β production through inhibiting Caspase 1 activation in macrophage-like J774.1 cells

IL-1β is one of the inflammatory cytokines and is cleaved from pro-IL-1β proteolytically by activated Caspase 1. For the activation of Caspase 1, inflammasome was formed by two signals, what is called, priming and triggering signals. In this study, it was found that mouse macrophage J774.1 cells, when treated by single large amount of lipopolysaccharide (LPS), produced a significant amount of IL-1β. On the other hand, IL-1β production was not detected when treated by a single, small amount of LPS. Then, focusing on endoplasmic reticulum (ER) stress response among stress responses induced by a large amount of LPS, when GSK2656157, a PERK inhibitor, was used for inhibition of ER stress, GSK2656157 reduced IL-1β production dose-dependently. Next, when Thapsigargin, an ER stress reagent, was added with LPS, IL-1β production increased more than by LPS alone. Thus, these results suggested that ER stress was involved in LPS-induced IL-1β production. When the activation of Caspase 1 was examined by fluorescence activated cell sorter analysis, it was found that GSK2656157 inhibited LPS-induced Caspase 1 activation. Further, it was confirmed that GSK2656157 did not affect LPS-induced TNF-α production and activation of NF-κB and specifically inhibited the PERK/eIF-2α pathway. Therefore, it was found that GSK2656157 specifically inhibited ER stress induced by large amount of LPS and reduced LPS-induced IL-1β production through inhibition of Caspase 1 activation.

Dissection and integration of the autophagy signaling network initiated by bluetongue virus infection: crucial candidates ERK1/2, Akt and AMPK

Bluetongue virus (BTV), a complex double-stranded segmented RNA virus, has been found to initiate cellular autophagy for its own benefit. Here, with a view to understanding the underlying mechanisms, we first systematically dissected the exact signaling network in BTV-induced autophagy. We found that the activity of mTOR, a crucial pivot, was inhibited by BTV1 infection, subsequently leading to downstream p70S6K suppression and autophagy initiation. We then explored the upstream regulators of mTOR and analyzed their activities via a series of assays. We found BTV1-induced autophagy to be independent of the ERK1/2 signaling pathway. However, the BTV1-induced inhibition of PI3K/Akt was found to be partially responsible for mTOR inactivation and subsequent autophagy initiation. Furthermore, we found unexpectedly that AMPK seemed to play a more important role in BTV1-induced autophagy. Elevated [Ca²⁺]_cyto-mediated activation of CaMKKβ exactly managed the activation of AMPK, which then positively regulated autophagy through suppressing mTOR. We must emphasize that TSC2 is a fatal mediator between upstream Akt or AMPK and downstream mTOR through its phosphorylation. Taken together, our data suggested that the BTV1-induced inhibition of the Akt-TSC2-mTOR pathway and the upregulation of the AMPK-TSC2-mTOR pathway both contributed to autophagy initiation and further favored virus replication.