Identification and characterization of pancreatic eukaryotic initiation factor 2 alpha-subunit kinase, PEK, involved in translational control

The PERK Arm of the Unfolded Protein Response Negatively Regulates Transmissible Gastroenteritis Virus Replication by Suppressing Protein Translation and Promoting Type I Interferon Production

Coronavirus replication is closely associated with the endoplasmic reticulum (ER), the primary cellular organelle for protein synthesis, folding, and modification. ER stress is a common consequence in coronavirus-infected cells. However, how the virus-induced ER stress influences coronavirus replication and pathogenesis remains controversial. Here, we demonstrated that infection with the alphacoronavirus transmissible gastroenteritis virus (TGEV) induced ER stress and triggered the unfolded protein response (UPR) in vitro and in vivo, and ER stress negatively regulated TGEV replication in vitro Although TGEV infection activated all three UPR pathways (activating transcription factor 6 [ATF6], inositol-requiring enzyme 1 [IRE1], and protein kinase R-like ER kinase [PERK]), the virus-triggered UPR suppressed TGEV replication in both swine testicular (ST) and IPEC-J2 cells primarily through activation of the PERK-eukaryotic initiation factor 2α (eIF2α) axis, as shown by functional studies with overexpression, small interfering RNA (siRNA), or specific chemical inhibitors. Moreover, we demonstrated that PERK-eIF2α axis-mediated inhibition of TGEV replication occurs through phosphorylated eIF2α-induced overall attenuation of protein translation. In addition to direct inhibition of viral production, the PERK-eIF2α pathway activated NF-κB and then facilitated type I IFN production, resulting in TGEV suppression. Taken together, our results suggest that the TGEV-triggered PERK-eIF2α pathway negatively regulates TGEV replication and represents a vital aspect of host innate responses to invading pathogens.IMPORTANCE The induction of ER stress is a common outcome in cells infected with coronaviruses. The UPR initiated by ER stress is actively involved in viral replication and modulates the host innate responses to the invading viruses, but these underlying mechanisms remain incompletely understood. We show here that infection with the alphacoronavirus TGEV elicited ER stress in vitro and in vivo, and the UPR PERK-eIF2α branch was predominantly responsible for the suppression of TGEV replication by ER stress. Furthermore, the PERK-eIF2α axis inhibited TGEV replication through direct inhibition of viral proteins due to global translation inhibition and type I IFN induction. These findings highlight a critical role of the UPR PERK-eIF2α pathway in modulating host innate immunity and coronavirus replication.

Two distinct nodes of translational inhibition in the Integrated Stress Response

The Integrated Stress Response (ISR) refers to a signaling pathway initiated by stress-activated eIF2α kinases. Once activated, the pathway causes attenuation of global mRNA translation while also paradoxically inducing stress response gene expression. A detailed analysis of this pathway has helped us better understand how stressed cells coordinate gene expression at translational and transcriptional levels. The translational attenuation associated with this pathway has been largely attributed to the phosphorylation of the translational initiation factor eIF2α. However, independent studies are now pointing to a second translational regulation step involving a downstream ISR target, 4E-BP, in the inhibition of eIF4E and specifically cap-dependent translation. The activation of 4E-BP is consistent with previous reports implicating the roles of 4E-BP resistant, Internal Ribosome Entry Site (IRES) dependent translation in ISR active cells. In this review, we provide an overview of the translation inhibition mechanisms engaged by the ISR and how they impact the translation of stress response genes.

Role of eIF2α Kinases in Translational Control and Adaptation to Cellular Stress

A central mechanism regulating translation initiation in response to environmental stress involves phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α). Phosphorylation of eIF2α causes inhibition of global translation, which conserves energy and facilitates reprogramming of gene expression and signaling pathways that help to restore protein homeostasis. Coincident with repression of protein synthesis, many gene transcripts involved in the stress response are not affected or are even preferentially translated in response to increased eIF2α phosphorylation by mechanisms involving upstream open reading frames (uORFs). This review highlights the mechanisms regulating eIF2α kinases, the role that uORFs play in translational control, and the impact that alteration of eIF2α phosphorylation by gene mutations or small molecule inhibitors can have on health and disease.

Chrysin induces cell growth arrest, apoptosis, and ER stress and inhibits the activation of STAT3 through the generation of ROS in bladder cancer cells

Chrysin is a natural flavone that has various biological activities, including antitumor effects. However, the effect of chrysin on bladder cancer cells remains elusive. The present study investigated the effects of chrysin on bladder cancer cells and its underlying mechanisms. The results demonstrated that chrysin induced apoptosis via the intrinsic pathway, as evidenced by activation of caspase-9 and caspase-3, however not caspase-8. In addition, chrysin reduced the expression of anti-apoptotic B cell lymphoma (Bcl) proteins including Bcl-2, Mcl-1, Bcl-xl, and promoted the protein expression of pro-apoptotic Bcl-2 associated X, apoptosis regulator. Chrysin also induced endoplasmic reticulum stress via activation of the unfolded protein response of PRKR-like endoplasmic reticulum kinase, eIF2α and activating transcription factor 4 in bladder cancer cells. Additionally, chrysin inhibited the signal transducer and activator of transcription 3 pathway. Furthermore, the generation of reactive oxygen species (ROS) was detected following treatment with chrysin. The ROS scavenger N-acetylcysteine inhibited the antitumor effect of chrysin. Collectively, these results indicate chrysin may act as a promising therapeutic candidate for targeting bladder cancer.

A lifetime of stress: ATF6 in development and homeostasis

BACKGROUND

Activating transcription factor 6 (ATF6) is an endoplasmic reticulum (ER)-localised protein and member of the leucine zipper family of transcription factors. Best known for its role in transducing signals linked to stress to the endoplasmic reticulum, the 50 kDa activated form of ATF6 is now emerging as a major regulator of organogenesis and tissue homeostasis. Responsible for the correct folding, secretion and membrane insertion of a third of the proteome in eukaryotic cells, the ER encompasses a dynamic, labyrinthine network of regulators, chaperones, foldases and cofactors. Such structures are crucial to the extensive protein synthesis required to undergo normal development and maintenance of tissue homeostasis. When an additional protein synthesis burden is placed on the ER, ATF6, in tandem with ER stress transducers inositol requiring enzyme 1 (IRE1) and PKR-like endoplasmic reticulum kinase (PERK), slows the pace of protein translation and induces the production of stress-reducing chaperones and foldases.

MAIN TEXT

In the context of development and tissue homeostasis, however, distinct cellular impacts have been attributed to ATF6. Drawing on data published from human, rodent, fish, goat and bovine research, this review first focuses on ATF6-mediated regulation of osteo- and chondrogenesis, ocular development as well as neuro- and myelinogenesis. The purported role of ATF6 in development of the muscular and reproductive systems as well as adipo- and lipogenesis is then described. With relevance to cardiac disease, cancer and brain disorders, the importance of ATF6 in maintaining tissue homeostasis is the subject of the final section.

CONCLUSION

In conclusion, the review encourages further elucidation of ATF6 regulatory operations during organogenesis and tissue homeostasis, to spawn the development of ATF6-targeted therapeutic strategies.

Regulation of Integrated Stress Response Sensitizes U87MG Glioblastoma Cells to Temozolomide Through the Mitochondrial Apoptosis Pathway

Glioblastomas are the most frequently diagnosed and worst primary malignancy of the central nervous system, with very poor prognosis. The first-line antiglioma drug temozolomide shows decreasing therapeutic efficacy as treatment progresses. As the integrated stress response (ISR) may be a resistance factor and severe stress might transform the protective effect of the ISR into a damage effect, pharmacological regulation of ISR may be an effective way to sensitize glioma to temozolomide. The aim of the present study was to investigate the mechanisms of the ISR in regulating the therapeutic effect of temozolomide in the human glioblastoma multiforme cell line U87MG. Cultured U87MG cells were treated with temozolomide and PCR array was used to screen key factors in the response to treatment. Cells were co-treated with temozolomide and the eIF2α phosphatase inhibitor salubrinal, and cell apoptosis was measured. Combination treatment with temozolomide and salubrinal had a synergistic effect on cell viability. Salubrinal could upregulate the expression of ATF4, a key factor in the ISR, and enhance temozolomide-induced apoptosis. ATF4 transcriptionally regulated expression of the BH3-ONLY protein NOXA, thus inducing mitochondrial apoptosis. These findings suggest that ISR and ATF4 are involved in the death crosstalk between the endoplasmic reticulum and mitochondria and might be a potential target to enhance the therapeutic effect of temozolomide in patients with glioblastoma multiforme. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc.

α‑synuclein induces apoptosis of astrocytes by causing dysfunction of the endoplasmic reticulum‑Golgi compartment

Although previous work has demonstrated that the overexpression of wild-type or mutant α-synuclein (α-syn) can induce cell death via a number of different mechanisms, including oxidative stress, dysfunction of the ubiquitin-proteasome degradation system, mitochondrial damage and endoplasmic reticulum (ER) stress, research interest has primarily focused on neurons. However, there is accumulating evidence that suggests that astrocytes may be involved in the earliest changes, as well as the progression of Parkinson's disease (PD), though the role of α-syn in astrocytes has not been widely studied. In the present study, it was revealed that the mutant α-syn (A53T and A30P) in astrocytes triggered ER stress via the protein kinase RNA-like ER kinase/eukaryotic translation initiation factor 2α signaling pathway. Astrocyte apoptosis was induced through a CCAAT-enhancer-binding protein homologous protein-mediated pathway. In addition, Golgi fragmentation was observed in the process. On the other hand, it was also demonstrated, in a primary neuronal-astroglial co-culture system, that the overexpression of α-syn significantly decreased the levels of glia-derived neurotrophic factor (GDNF) and partly inhibited neurite outgrowth. Although direct evidence is currently lacking, it was proposed that dysfunction of the ER-Golgi compartment in astrocytes overexpressing α-syn may lead to a decline of GDNF levels, which in turn would suppress neurite outgrowth. Taken together, the results of the present study offer further insights into the pathogenesis of PD from the perspective of astrocytes, which may provide novel strategies for the diagnosis and treatment of PD in the future.

Natural products targeting ER stress pathway for the treatment of cardiovascular diseases

Endoplasmic reticulum (ER) is the main organelle for the synthesis, folding, and processing of secretory and transmembrane proteins. Pathological stimuli including hypoxia, ischaemia, inflammation and oxidative stress interrupt the homeostatic function of ER, leading to accumulation of unfolded proteins, a condition referred to as ER stress. ER stress triggers a complex signalling network referred as the unfolded protein response (UPR). Extensive studies have demonstrated that ER stress plays an important role in the pathogenesis of various cardiovascular diseases such as heart failure, ischemic heart disease and atherosclerosis. The importance of natural products in modern medicine are well recognized and continues to be of interests as a source of novel lead compounds. Natural products targeting components of UPR and reducing ER stress offers an innovative strategic approach to treat cardiovascular diseases. In this review, we discussed several therapeutic interventions using natural products with potential cardiovascular protective properties targeting ER stress signalling pathways.

Importance of the subcellular location of protein deposits in neurodegenerative diseases

Alzheimer's disease, Parkinson's, Huntington's, amyotrophic lateral sclerosis (ALS) and prion disorders are devastating neurodegenerative diseases of increasing prevalence in aging populations. Although clinically different, they share similar molecular features: the accumulation of one or two proteins in abnormal conformations inside or outside neurons. Enhancing protein quality control systems could be a useful strategy to neutralize the abnormal proteins causing neurodegenerative diseases. This review emphasizes the subcellular location of protein deposits in neurodegenerative diseases and the need to tailor strategies aimed at boosting protein quality control systems to the affected subcellular compartment. Inhibition of a protein phosphatase terminating the unfolded protein response will be discussed as a strategy to protect from diseases associated with misfolded proteins in the endoplasmic reticulum.

Selection of growth-related genes and dominant genotypes in transgenic Yellow River carp Cyprinus carpio L

Transgenic Yellow River carp is characterized by rapid growth rate and high feed-conversion efficiency and exhibits a great application prospect. However, there is still a significant separation of growth traits in the transgenic Yellow River carp family; as such, growth-related genotypes must be screened for molecular marker-assisted selection. In this study, 23 growth-related candidate genes containing 48 SNP markers were screened through bulked segregant analysis (BSA) among transgenic Yellow River carp family members showing significant separation of growth traits. Then, two growth-related genes (Nos. 17 and 14 genes) were identified through combined genome-wide association study (GWAS) of candidate genes and validation of the full-sibling family approach. Nos. 17 and 14 genes encode BR serine/threonine-protein kinase 2 (BRSK2) and eukaryotic translation-initiation factor 2-alpha kinase 3 (Eif2ak3), respectively. The average body weight of three subgroups carrying the genotypes 17GG, 17GG + 14CC, and 17GG + 14TT of these two genes increased by 27.96, 38.28, and 33.72%, respectively, compared with the controls. The proportion of individuals with body weight > 500 g in these subgroups increased by 19.22, 26.82, and 30.92%, respectively. The results showed that appropriate genotype carriers can be selected from the progeny population through BSA sequencing combined with simplified GWAS analysis. Hence, basic population for breeding can be constructed and transgenic Yellow River carp strains with stable production performance and uniform phenotypic properties can be bred.

Unfolded protein response is activated in Krabbe disease in a manner dependent on the mutation type

Krabbe disease, one of the autosomal-recessive lysosomal storage disorders (LSDs), is caused by a deficiency of galactocerebrosidase (GALC) activity, resulting in the intracellular accumulation of psychosine, which is cytotoxic for neuronal cells. Genetically pathogenic mutations result in conformational changes in GALC and disrupt the lysosmal trafficking of cargos, which subsequently accumulate in the endoplasmic reticulum (ER). Recently, ER stress together with the activation of the unfolded protein response (UPR) has been suggested to play a key role in the pathogenesis of LSDs. In this study, we hence investigated whether the UPR is activated in Krabbe disease using COS-7 cells expressing pathogenic GALC mutants and skin fibroblasts (SFs) from Krabbe disease patients with various phenotypes, using a combination of semiquantitative and quantitative real-time polymerase chain reactions. We found that UPR activation in Krabbe disease depends on the mutations and cell types, and there is the possibility that multiple pathways, involving ER chaperones, inositol-requiring kinase 1, and protein kinase regulated by RNA-like ER kinase are activated by mutations associated with the infantile form. These results indicate that in Krabbe disease, each misfolded/unfolded protein evokes different UPR activation depending on the mutation, and that the activated pathways affect the phenotypes.

Anthocyanin complex exerts anti-cholangiocarcinoma activities and improves the efficacy of drug treatment in a gemcitabine-resistant cell line

Cholangiocarcinoma (CCA) is a deleterious bile duct tumor with poor prognosis and is relatively resistant to chemotherapy. Therefore, alternative or supplementary agents with anticancer and chemosensitizing activities may be useful for the treatment of CCA. A novel anthocyanin complex (AC) nanoparticle, developed from extracts of cobs of purple waxy corn and petals of blue butterfly pea, has exhibited chemopreventive potential in vivo. In the present study, the anti-CCA activities of AC and their underlying molecular mechanisms were investigated further in vitro using a CCA cell line (KKU213). The potential use of AC as a chemosensitizer was also evaluated in a gemcitabine-resistant CCA cell line (KKU214GemR). It was demonstrated that AC treatment suppressed proliferation of KKU213 CCA cells in dose- and time-dependent manners. AC treatment also induced apoptosis and mitochondrial superoxide production, decreased clonogenicity of CCA cells, and downregulated forkhead box protein M1 (FOXM1), nuclear factor-κB (NF-κB) and pro-survival protein B-cell lymphoma-2 (Bcl-2). The expression of endoplasmic reticulum (ER) stress-response proteins, including protein kinase RNA-like ER kinase, phosphorylated eIF2α, eukaryotic initiation factor 2α and activating transcription factor 4, also decreased following AC treatment. It was also identified that AC treatment inhibited KKU214GemR cell proliferation in dose- and time-dependent manners. Co-treatment of KKU214GemR cells with low doses of AC together with gemcitabine significantly enhanced efficacy of the latter against this cell line. Therefore, it is suggested that AC treatment is cytotoxic to KKU213 cells, possibly via downregulation of FOXM1, NF-κB, Bcl-2 and the ER stress response, and by induction of mitochondrial superoxide production. AC also sensitizes KKU214GemR to gemcitabine treatment, which may have potential for overcoming drug resistance of CCA.

Endoplasmic reticulum chaperone glucose-regulated protein 78 in gastric cancer: An emerging biomarker

The endoplasmic reticulum (ER) is the principal organelle responsible for the synthesis, initial post-translational modification, folding, export and secretion of proteins. It is also responsible for the maintenance of cellular homeostasis. In response to cellular stress conditions including glucose deprivation, hypoxia and changes in calcium homeostasis, ER stress machinery is activated and triggers the unfolded protein response, resulting in the restoration of homeostasis or activation of cell death. Glucose-regulated protein 78 (GRP78), a molecular chaperone, may be induced by ER stress at the transcriptional and translational level. A number of studies have demonstrated that GRP78 serves an important role in tumor cell proliferation, metastasis, angiogenesis and drug-resistance. The present review systematically describes the association between GRP78 expression and gastric cancer pathogenesis, and emphasizes that GRP78 is a novel diagnostic and therapeutic biomarker of gastric cancer.

Hepatic loss of Lissencephaly 1 (Lis1) induces fatty liver and accelerates liver tumorigenesis in mice

The liver is a major organ in lipid metabolism, and its malfunction leads to various diseases. Nonalcoholic fatty liver disease, the most common chronic liver disorder in developed countries, is characterized by the abnormal retention of excess lipid within hepatocytes and predisposes individuals to liver cancer. We previously reported that the levels of Lissencephaly 1 (LIS1, also known as PAFAH1B1) are down-regulated in human hepatocellular carcinoma. Following up on this observation, we found that genetic deletion of Lis1 in the mouse liver increases lipid accumulation and inflammation in this organ. Further analysis revealed that loss of Lis1 triggers endoplasmic reticulum (ER) stress and reduces triglyceride secretion. Attenuation of ER stress by addition of tauroursodeoxycholic acid (TUDCA) diminished lipid accumulation in the Lis1-deficient hepatocytes. Moreover, the Golgi stacks were disorganized in Lis1-deficient liver cells. Of note, the Lis1 liver-knockout mice exhibited increased hepatocyte ploidy and accelerated development of liver cancer after exposure to the liver carcinogen diethylnitrosamine (DEN). Taken together, these findings suggest that reduced Lis1 levels can spur the development of liver diseases from steatosis to liver cancer and provide a useful model for delineating the molecular pathways that lead to these diseases.

The unfolded protein response in ischemic heart disease

Ischemic heart disease is a severe stress condition that causes extensive pathological alterations and triggers cardiac cell death. Accumulating evidence suggests that the unfolded protein response (UPR) is strongly induced by myocardial ischemia. The UPR is an evolutionarily conserved cellular response to cope with protein-folding stress, from yeast to mammals. Endoplasmic reticulum (ER) transmembrane sensors detect the accumulation of unfolded proteins and stimulate a signaling network to accommodate unfolded and misfolded proteins. Distinct mechanisms participate in the activation of three major signal pathways, viz. protein kinase RNA-like ER kinase, inositol-requiring protein 1, and activating transcription factor 6, to transiently suppress protein translation, enhance protein folding capacity of the ER, and augment ER-associated degradation to refold denatured proteins and restore cellular homeostasis. However, if the stress is severe and persistent, the UPR elicits inflammatory and apoptotic pathways to eliminate terminally affected cells. The ER is therefore recognized as a vitally important organelle that determines cell survival or death. Recent studies indicate the UPR plays critical roles in the pathophysiology of ischemic heart disease. The three signaling branches may elicit distinct but overlapping effects in cardiac response to ischemia. Here, we outline the findings and discuss the mechanisms of action and therapeutic potentials of the UPR in the treatment of ischemic heart disease.

Nrf2 protects human lens epithelial cells against H2O2-induced oxidative and ER stress: The ATF4 may be involved

Our previous study has shown heme oxygenase-1 (HO-1) protects human lens epithelial cells (LECs) against H₂O₂-induced oxidative stress and apoptosis. Nrf2, the major regulator of HO-1, is triggered during the mutual induction of oxidative stress and ER stress. In response to ER stress, unfolded protein response (UPR) serves as a program of transcriptional and translational regulation mechanism with PERK involved. Both Nrf2 and ATF4 are activated as the downstream effect of PERK signaling coordinating the convergence of dual stresses. However, the ways in which Nrf2 interacting with ATF4 regulates deteriorated redox state have not yet been fully explored. Here, the transfected LECs with Nrf2 overexpression illustrated enhanced resistance in morphology and viability upon H₂O₂ treatment condition. Intracellular ROS accumulation arouses ER stress, initiating PERK dependent UPR and inducing the downstream signal Nrf2 and ATF4 auto-phosphorylation. Further, converging at target promoters, ATF4 facilitates Nrf2 with the expression of ARE-dependent phase II antioxidant and detoxification enzymes. According to either Nrf2 or ATF4 gene modification, our data suggests a novel interaction between Nrf2 and ATF4 under oxidative and ER stress, thus drives specific enzymatic and non-enzymatic reactions of antioxidant mechanisms maintaining redox homeostasis. Therapies that restoring Nrf2 or ATF4 expression might help to postpone LECs aging and age-related cataract formation.

IRE1α deficiency promotes tumor cell death and eIF2α degradation through PERK dipendent autophagy

Sensors of endoplasmic reticulum (ER) stress function in a co-ordinated manner. In the present study we investigated the relationship between IRE1α and PERK pathways and survival of ER stressed U937 cells and BC3 cells. To this end, we investigated the effects of a subcytotoxic concentration of Tunicamycin in IRE1α-proficient and in IRE1α-deficient cells, by pharmacological inhibition with 4μ8 C or down-regulation by specific siRNA. We show that either type of IRE1α deficiency affects eIF2α expression and causes cell death increase. GSK2606414, a PERK inhibitor, and PERK specific siRNA prevent eIF2α down-regulation and restore cell survival. Degradation of this protein is due to autophagy, as it is prevented by bafilomycin and not by proteasome inhibition. Furthermore, activation of the autophagy flux is PERK dependent. Also the Cathepsin B inhibitor CA074 prevents eIF2α from degradation and reduces cell death. Altogether, these results show that IRE1α deficiency in ER stressed cells leads to an unexpected decrease of eIF2α, an important molecule for protein translation, through PERK dependent autophagy. Thus, IRE1/XBP1 inhibitors may represent a feasible strategy for tumor therapy, while PERK inhibitors may vanish the goal.

Cognitive Impairment and Endoplasmic Reticulum Stress Induced by Repeated Short-Term Sevoflurane Exposure in Early Life of Rats

Sevoflurane is one of the most commonly used volatile anaesthetics for children, but the safety of prolonged or repeated clinical use of sevoflurane in infants or children is controversial. Here, we investigated the effects of sevoflurane on rats in early life and the time scale of those effects. Our behavioral results indicated that repeated short-term exposure of new-born rats to sevoflurane caused learning and memory impairment, while a single exposure of rats to sevoflurane was relatively safe. Further mechanistic investigation revealed that repeated sevoflurane exposure impaired long-term potentiation (LTP), downregulated the expression of certain synaptogenesis-related proteins (GluR1, PSD95) and upregulated proteins related to endoplasmic reticulum (ER) stress in the hippocampus. An ER stress inhibitor, tauroursodeoxycholic acid (TUDCA), reversed the changes in the levels of synaptic plasticity proteins. Our results provide new evidence for the clinical concerns regarding repeated sevoflurane anesthesia.

A PERK-miR-211 axis suppresses circadian regulators and protein synthesis to promote cancer cell survival

The unfolded protein response (UPR) is a stress-activated signalling pathway that regulates cell proliferation, metabolism and survival. The circadian clock coordinates metabolism and signal transduction with light/dark cycles. We explore how UPR signalling interfaces with the circadian clock. UPR activation induces a 10 h phase shift in circadian oscillations through induction of miR-211, a PERK-inducible microRNA that transiently suppresses both Bmal1 and Clock, core circadian regulators. Molecular investigation reveals that miR-211 directly regulates Bmal1 and Clock via distinct mechanisms. Suppression of Bmal1 and Clock has the anticipated impact on expression of select circadian genes, but we also find that repression of Bmal1 is essential for UPR-dependent inhibition of protein synthesis and cell adaptation to stresses that disrupt endoplasmic reticulum homeostasis. Our data demonstrate that c-Myc-dependent activation of the UPR inhibits Bmal1 in Burkitt's lymphoma, thereby suppressing both circadian oscillation and ongoing protein synthesis to facilitate tumour progression.

A Complex Relationship between Immunity and Metabolism in Drosophila Diet-Induced Insulin Resistance

Both systemic insulin resistance and tissue-specific insulin resistance have been described in Drosophila and are accompanied by many indicators of metabolic disease. The downstream mediators of insulin-resistant pathophysiology remain unclear. We analyzed insulin signaling in the fat body studying loss and gain of function. When expression of the sole Drosophila insulin receptor (InR) was reduced in larval fat bodies, animals exhibited developmental delay and reduced size in a diet-dependent manner. Fat body InR knockdown also led to reduced survival on high-sugar diets. To look downstream of InR at potential mediators of insulin resistance, transcriptome sequencing (RNA-seq) studies in insulin-resistant fat bodies revealed differential expression of genes, including those involved in innate immunity. Obesity-associated insulin resistance led to increased susceptibility of flies to infection, as in humans. Reduced innate immunity was dependent on fat body InR expression. The peptidoglycan recognition proteins (PGRPs) PGRP-SB2 and PGRP-SC2 were selected for further study based on differential expression studies. Downregulating PGRP-SB2 selectively in the fat body protected animals from the deleterious effects of overnutrition, whereas downregulating PGRP-SC2 produced InR-like phenotypes. These studies extend earlier work linking the immune and insulin signaling pathways and identify new targets of insulin signaling that could serve as potential drug targets to treat type 2 diabetes.

Link between endoplasmic reticulum stress and autophagy in neurodegenerative diseases

Increasing evidence suggests that endoplasmic reticulum (ER) stress and autophagy play an important role in regulating brain function. ER stress activates three major branches of the unfolded protein response (UPR) pathways, namely inositol-requiring enzyme-1 (IRE1), double stranded RNA-activated protein kinase (PKR)-like ER kinase (PERK) and activating transcription factor 6 (ATF6)-mediated pathways. Recent studies have suggested that these UPR signals may be linked to autophagy. In this review article, we summarize recent evidence and discuss a possible link between ER stress and autophagy with regard to neurodegenerative diseases. Furthermore, possible pharmacological strategies targeting UPR and autophagy are discussed.

Protective effect of autophagy on endoplasmic reticulum stress induced apoptosis of alveolar epithelial cells in rat models of COPD

During the present study, we explored the protective effects of autophagy on endoplasmic reticulum (ER) stress (ERS) induced apoptosis belonging to alveolar epithelial cells (AECs) in rat models with chronic obstructive pulmonary disease (COPD). Fifty-six 12-week-old male Sprague–Dawley (SD) rats were randomly assigned into the COPD group (rats exposed to cigarette smoke (CS)), the 3-methyladenine (3-MA) intervention group (COPD rats were administrated with 10 mg/kg autophagy inhibitors), the chloroquine (CQ)-intervention group (COPD rats were administrated 40 mg/kg CQ), and the control group (rats breathed in normal saline). The forced expiratory volume in 0.3 s/forced vital capacity (FEV_0.3/FVC%), inspiratory resistance (RI), and dynamic lung compliance (Cdyn) were measured and recorded. The expressions of PKR-like ER kinase (PERK) and CCAAT/enhancer-binding protein-homologous protein (CHOP) were detected by immunohistochemistry. The cell apoptotic rates of AECs were analyzed by terminal deoxynucleotidyl transferase (TdT) mediated dUTP-biotin nick end-labeling (TUNEL) staining. The expression levels of light chain 3 (LC3-II), p62, Beclin-1, ATG5, ATG7, Caspase-12, and Caspase-3 were detected by Western blotting. Results showed that the COPD group exhibited a lower FEV_0.3/FVC% and Cdyn, and a higher RI than the control group. Compared with the control group, the integrated optical density (IOD) values of PERK and CHOP, the apoptotic rate of AECs, and expressions of LC3-II, Beclin-1, ATG5, ATG7, Caspase-3, and Caspase-12 expressions were significantly higher, whereas p62 expression was significantly lower in the COPD group. Based on the results obtained during the present study, it became clear that the inhibition of autophagy could attenuate the ERS-induced apoptosis of AECs in rats with COPD.

Ctenopharyngodon idella PERK (EIF2AK3) decreases cell viability by phosphorylating eIF2α under ER stress

As an upstream kinase of eIF2α, protein kinase RNA-like ER (endoplasmic reticulum) kinase (PERK) is a type I transmembrane protein located in ER in eukaryotic cells. PERK is mainly composed of two domains, the intracavitary domain for BIP protein combination and the dissociative C-terminal region containing a typical serine/threonine kinase domain which promotes the phosphorylation of eIF2α. In this study, we cloned a PERK (also known as EIF2AK3) gene from grass carp (Ctenopharyngodon idella). The full-length cDNA of grass carp PERK (CiPERK) is 5192 bp including a 176 bp of 5' untranslated region, a 1719 bp of 3' untranslated region and a 3297 bp of the longest open reading frame (ORF) encoding 1098 amino acids. Phylogenetic analysis exhibits that CiPERK shares a high degree of sequence homology to the counterparts in other teleosts. RT-PCR indicated that CiPERK expression was significantly up-regulated following the stimulation with TM (tunicamycin). To study the function of CiPERK, the N-terminal sequence of CiPERK and CiGRP78 sequence were separately subcloned into the expression vectors pCMV-HA and pCMV-Flag for co-immunoprecipitation and GST-Pulldown assays. The assays indicated that CiPERK and CiGRP78 can combine with each other in normal conditions. However, under ER stress (TM stimulation) CiPERK can improve the eIF2α phosphorylation level. In addition, CCK assay showed the overexpression of CiPERK in CIK cells decreases the cell viability.

Intestinal Epithelial Cell Endoplasmic Reticulum Stress and Inflammatory Bowel Disease Pathogenesis: An Update Review

The intestinal epithelial cells serve essential roles in maintaining intestinal homeostasis, which relies on appropriate endoplasmic reticulum (ER) function for proper protein folding, modification, and secretion. Exogenous or endogenous risk factors with an ability to disturb the ER function can impair the intestinal barrier function and activate inflammatory responses in the host. The last decade has witnessed considerable progress in the understanding of the functional role of ER stress and unfolded protein response (UPR) in the gut homeostasis and its significant contribution to the pathogenesis of inflammatory bowel disease (IBD). Herein, we review recent evidence supporting the viewpoint that deregulation of ER stress and UPR signaling in the intestinal epithelium, including the absorptive cells, Paneth cells, goblet cells, and enteroendocrine cells, mediates the action of genetic or environmental factors driving colitis in experimental animals and IBD patients. In addition, we highlight pharmacologic application of chaperones or small molecules that enhance protein folding and modification capacity or improve the function of the ER. These molecules represent potential therapeutic strategies in the prevention or treatment of IBD through restoring ER homeostasis in intestinal epithelial cells.

Novel allylated monocarbonyl analogs of curcumin induce mitotic arrest and apoptosis by reactive oxygen species-mediated endoplasmic reticulum stress and inhibition of STAT3

Curcumin is a promising active compound from a natural source and is extensively being tested in clinical trials because of its bio-functional properties. However, poor bioavailability has hampered its clinical application. Numerous attempts have been made in our laboratory to discover analogs of curcumin with enhanced bioavailability and superior pharmacological activity. In this study, we have investigated a new series of allylated monocarbonyl analogs of curcumin (MAC) and tested their effect on gastric cancer cells. Our results show six MAC that selectively targeted cancer cell lines to inhibit growth and induce apoptosis. This activity was achieved by generation of reactive oxygen species (ROS) by MAC. We selected one effective MAC (CA10) for further investigation and show that CA10 inhibits cell growth by causing G2/M cell cycle arrest and induction of apoptotic death. CA10 induced ROS generation and subsequent activation of endoplasmic reticulum (ER) stress and inhibition of signal transducer and activator of transcription 3 (STAT3) phosphorylation, inhibits cancer cell proliferation. These anti-tumor activities of CA10 were confirmed in gastric cancer xenografts. CA10 induced ROS, activated the ER stress pathway and inhibited STAT3 phosphorylation and gastric xenografts tumor growth in mice. Our studies provide experimental evidence that MAC CA10 effectively targets gastric cancer in preclinical models by enhancing ROS and ROS-mediated signaling.

Age-related cataracts: Role of unfolded protein response, Ca2+ mobilization, epigenetic DNA modifications, and loss of Nrf2/Keap1 dependent cytoprotection

Age-related cataracts are closely associated with lens chronological aging, oxidation, calcium imbalance, hydration and crystallin modifications. Accumulating evidence indicates that misfolded proteins are generated in the endoplasmic reticulum (ER) by most cataractogenic stresses. To eliminate misfolded proteins from cells before they can induce senescence, the cells activate a clean-up machinery called the ER stress/unfolded protein response (UPR). The UPR also activates the nuclear factor-erythroid-2-related factor 2 (Nrf2), a central transcriptional factor for cytoprotection against stress. Nrf2 activates nearly 600 cytoprotective target genes. However, if ER stress reaches critically high levels, the UPR activates destructive outputs to trigger programmed cell death. The UPR activates mobilization of ER-Ca2+ to the cytoplasm and results in activation of Ca2+-dependent proteases to cleave various enzymes and proteins which cause the loss of normal lens function. The UPR also enhances the overproduction of reactive oxygen species (ROS), which damage lens constituents and induce failure of the Nrf2 dependent cytoprotection. Kelch-like ECH-associated protein 1 (Keap1) is an oxygen sensor protein and regulates the levels of Nrf2 by the proteasomal degradation. A significant loss of DNA methylation in diabetic cataracts was found in the Keap1 promoter, which overexpresses the Keap1 protein. Overexpressed Keap1 significantly decreases the levels of Nrf2. Lower levels of Nrf2 induces loss of the redox balance toward to oxidative stress thereby leading to failure of lens cytoprotection. Here, this review summarizes the overall view of ER stress, increases in Ca2+ levels, protein cleavage, and loss of the well-established stress protection in somatic lens cells.

Modulation of mitochondrial dysfunction and endoplasmic reticulum stress are key mechanisms for the wide-ranging actions of epoxy fatty acids and soluble epoxide hydrolase inhibitors

The arachidonic acid cascade is arguably the most widely known biologic regulatory pathway. Decades after the seminal discoveries involving its cyclooxygenase and lipoxygenase branches, studies of this cascade remain an active area of research. The third and less widely known branch, the cytochrome P450 pathway leads to highly active oxygenated lipid mediators, epoxy fatty acids (EpFAs) and hydroxyeicosatetraenoic acids (HETEs), which are of similar potency to prostanoids and leukotrienes. Unlike the COX and LOX branches, no pharmaceuticals currently are marketed targeting the P450 branch. However, data support therapeutic benefits from modulating these regulatory lipid mediators. This is being approached by stabilizing or mimicking the EpFAs or even by altering the diet. These approaches lead to predominantly beneficial effects on a wide range of apparently unrelated states resulting in an enigma of how this small group of natural chemical mediators can have such diverse effects. EpFAs are degraded by soluble epoxide hydrolase (sEH) and stabilized by inhibiting this enzyme. In this review, we focus on interconnected aspects of reported mechanisms of action of EpFAs and inhibitors of soluble epoxide hydrolase (sEHI). The sEHI and EpFAs are commonly reported to maintain homeostasis under pathological conditions while remaining neutral under normal physiological conditions. Here we provide a conceptual framework for the unique and broad range of biological activities ascribed to epoxy fatty acids. We argue that their mechanism of action pivots on their ability to prevent mitochondrial dysfunction, to reduce subsequent ROS formation and to block resulting cellular signaling cascades, primarily the endoplasmic reticulum stress. By stabilizing the mitochondrial - ROS - ER stress axis, the range of activity of EpFAs and sEHI display an overlap with the disease conditions including diabetes, fibrosis, chronic pain, cardiovascular and neurodegenerative diseases, for which the above outlined mechanisms play key roles.

The SAT Protein of Porcine Parvovirus Accelerates Viral Spreading through Induction of Irreversible Endoplasmic Reticulum Stress

The SAT protein (SATp) of porcine parvovirus (PPV) accumulates in the endoplasmic reticulum (ER), and SAT deletion induces the slow-spreading phenotype. The in vitro comparison of the wild-type Kresse strain and its SAT knockout (SAT^-) mutant revealed that prolonged cell integrity and late viral release are responsible for the slower spreading of the SAT^- virus. During PPV infection, regardless of the presence or absence of SATp, the expression of downstream ER stress response proteins (Xbp1 and CHOP) was induced. However, in the absence of SATp, significant differences in the quantity and the localization of CHOP were detected, suggesting a role of SATp in the induction of irreversible ER stress in infected cells. The involvement of the induction of irreversible ER stress in porcine testis (PT) cell necrosis and viral egress was confirmed by treatment of infected cells by ER stress-inducing chemicals (MG132, dithiothreitol, and thapsigargin), which accelerated the egress and spreading of both the wild-type and the SAT^- viruses. UV stress induction had no beneficial effect on PPV infection, underscoring the specificity of ER stress pathways in the process. However, induction of CHOP and its nuclear translocation cannot alone be responsible for the biological effect of SAT, since nuclear CHOP could not complement the lack of SAT in a coexpression experiment.IMPORTANCE SATp is encoded by an alternative open reading frame of the PPV genome. Earlier we showed that SATp of the attenuated PPV NADL-2 strain accumulates in the ER and accelerates virus release and spreading. Our present work revealed that slow spreading is a general feature of SAT^- PPVs and is the consequence of prolonged cell integrity. PPV infection induced ER stress in infected cells regardless of the presence of SATp, as demonstrated by the morphological changes of the ER and expression of the stress response proteins Xbp1 and CHOP. However, the presence of SATp made the ER stress more severe and accelerated cell death during infection, as shown by the higher rate of expression of CHOP and alteration of the localization of CHOP. The beneficial effect of irreversible ER stress on PPV spread was confirmed by treatment of infected cells with ER stress-inducing chemicals.

Thinking Outside the Box

A broad definition of preconditioning is "the preparation for a subsequent action." Mounting evidence demonstrates that novel remote preconditioning paradigms, in which protective stimuli experienced locally can capacitate systemic tolerance and enhanced cell viability upon exposure to ensuing cellular insults, have been largely successful in the field of cardiovascular ischemia/reperfusion injury. To ensure successful protective preconditioning, some models (including the uterus) have been demonstrated to activate the unfolded protein response (UPR), which is a cellular stress response controlled at the level of the endoplasmic reticulum. However, in the context of remote preconditioning, activation of these intracellular molecular pathways must result in the extracellular transmission of adaptive signals to remote targets. In our recently published manuscript, we have described the activation of the UPR in the pregnant uterine myocyte to be associated with increased uterine myocyte quiescence and normal gestational length. We hypothesize that ubiquitous uterine gestational stresses experienced in every pregnancy, which have been demonstrated in other systems to activate the UPR, may induce a robust paracrine dissemination of a uterine secretome, for example, glucose-regulated protein 78, with preconditioning-like properties. Furthermore, we speculate that the gestational stress-induced uterine secretome acts to promote both local and systemic tolerance to the ensuing gestational insults, allowing for the maintenance of uterine quiescence. In this context, preterm labor may be the result of a pregnant uterus experiencing a stress it cannot accommodate or when it is unable to host an appropriate UPR resulting in insufficient preconditioning and a diminished local and systemic capacity to tolerate pregnancy-dependent increases in normal gestational stress. This is highly attractive from a clinical viewpoint as we ultimately aim to identify local and systemic adaptations that may serve as preconditioning stimuli for use as a strategy to restore appropriate preconditioning profiles to prolong uterine quiescence in pregnancy.

Fibroblast growth factor 21 and its novel association with oxidative stress

Fibroblast growth factor 21 (FGF21) is an endocrine-member of the FGF family. It is synthesized mainly in the liver, but it is also expressed in adipose tissue, skeletal muscle, and many other organs. It has a key role in glucose and lipid metabolism, as well as in energy balance. FGF21 concentration in plasma is increased in patients with obesity, insulin resistance, and metabolic syndrome. Recent findings suggest that such increment protects tissue from an increased oxidative stress environment. Different types of physical stress, such as strenuous exercising, lactation, diabetic nephropathy, cardiovascular disease, and critical illnesses, also increase FGF21 circulating concentration. FGF21 is now considered a stress-responsive hormone in humans. The discovery of an essential response element in the FGF21 gene, for the activating transcription factor 4 (ATF4), involved in the regulation of oxidative stress, and its relation with genes such as NRF2, TBP-2, UCP3, SOD2, ERK, and p38, places FGF21 as a key regulator of the oxidative stress cell response. Its role in chronic diseases and its involvement in the treatment and follow-up of these diseases has been recently the target of new studies. The diminished oxidative stress through FGF21 pathways observed with anti-diabetic therapy is another clue of the new insights of this hormone.

Curcumin analog L48H37 induces apoptosis through ROS-mediated endoplasmic reticulum stress and STAT3 pathways in human lung cancer cells

Lung cancer is the leading cause of cancer-related deaths. Curcumin is a well-known natural product with anticancer ability, however, its poor bioavailability and pharmacokinetic profiles have limited its application in anticancer therapy. Previously, we reported that L48H37, a novel analog of curcumin with higher bioavailability, ameliorated LPS-induced inflammation, but the anticancer effect of L48H37 is still unknown. In the present study, we have investigated the effects of L48H37 in human lung cancer cells. Our results show that L48H37 decreases lung cancer cell growth and colony formation. These alterations were mediated through induction of G2/M cell cycle arrest and apoptosis in lung cancer cells. After L48H37 treatment, ER stress-related proteins were increased, and the expression of p-STAT3 was decreased in a dose-dependent manner. L48H37 also induced the accumulation of ROS in lung cancer cells, and pretreatment with NAC could fully reverse L48H37-induced reactive oxygen species (ROS) increase. Blocking ROS was able to reverse L48H37-induced endoplasmic reticulum (ER) stress, cell cycle arrest, and apoptosis. Finally, we show that L48H37 inhibits the growth of lung cancer xenografts without exhibiting toxicity. Treatment of mice bearing human lung cancer xenografts with L48H37 was also associated with indices of ER stress activation. In summary, our results provide evidence for a novel anti-tumor candidate for the treatment of lung cancer.

Ire1α in Pomc Neurons Is Required for Thermogenesis and Glycemia

Whether neuronal inositol-requiring enzyme 1 (Ire1) is required for the proper regulation of energy balance and glucose homeostasis is unclear. We found that pro-opiomelanocortin (Pomc)-specific deficiency of Ire1α accelerated diet-induced obesity concomitant with a decrease in energy expenditure. This hypometabolic phenotype included deficits in thermogenic responses to diet and cold exposure as well as "beiging" of white adipose tissue. We also demonstrate that loss of Ire1α in Pomc neurons impaired whole-body glucose and insulin tolerance as well as hepatic insulin sensitivity. At the cellular level, deletion of Ire1α in Pomc neurons elevated hypothalamic endoplasmic reticulum (ER) stress and predisposed Pomc neurons to leptin and insulin resistance. Together, the current studies extend and confirm conclusions that Ire1α-Xbp1s and associated molecular targets link ER stress in arcuate Pomc neurons to aspects of normal energy and glucose homeostasis.

Integrating the UPRmt into the mitochondrial maintenance network

Mitochondrial function is central to many different processes in the cell, from oxidative phosphorylation to the synthesis of iron-sulfur clusters. Therefore, mitochondrial dysfunction underlies a diverse array of diseases, from neurodegenerative diseases to cancer. Stress can be communicated to the cytosol and nucleus from the mitochondria through many different signals, and in response the cell can effect everything from transcriptional to post-transcriptional responses to protect the mitochondrial network. How these responses are coordinated have only recently begun to be understood. In this review, we explore how the cell maintains mitochondrial function, focusing on the mitochondrial unfolded protein response (UPR^mt), a transcriptional response that can activate a wide array of programs to repair and restore mitochondrial function.

Akkermansia muciniphila improves metabolic profiles by reducing inflammation in chow diet-fed mice

Abnormal shifts in the composition of gut microbiota contribute to the pathogenesis of metabolic diseases, including obesity and type 2 diabetes (T2DM). The crosstalk between gut microbes and the host affects the inflammatory status and glucose tolerance of the individuals, but the underlying mechanisms have not been elucidated completely. In this study, we treated the lean chow diet-fed mice with Akkermansia muciniphila, which is thought to be inversely correlated with inflammation status and body weight in rodents and humans, and we found that A. muciniphila supplementation by daily gavage for five weeks significantly alleviated body weight gain and reduced fat mass. Glucose tolerance and insulin sensitivity were also improved by A. muciniphila supplementation compared with the vehicle. Furthermore, A. muciniphila supplementation reduced gene expression related to fatty acid synthesis and transport in liver and muscle; meanwhile, endoplasmic reticulum (ER) stress in liver and muscle was also alleviated by A. muciniphila. More importantly, A. muciniphila supplementation reduced chronic low-grade inflammation, as reflected by decreased plasma levels of lipopolysaccharide (LPS)-binding protein (LBP) and leptin, as well as inactivated LPS/LBP downstream signaling (e.g. decreased phospho-JNK and increased IKBA expression) in liver and muscle. Moreover, metabolomics profiling in plasma also revealed an increase in anti-inflammatory factors such as α-tocopherol, β-sitosterol and a decrease of representative amino acids. In summary, our study demonstrated that A. muciniphila supplementation relieved metabolic inflammation, providing underlying mechanisms for the interaction of A. muciniphila and host health, pointing to possibilities for metabolic benefits using specific probiotics supplementation in metabolic healthy individuals.

Driving Cancer Tumorigenesis and Metastasis Through UPR Signaling

In the tumor microenvironment, cancer cells encounter both external and internal factors that can lead to the accumulation of improperly folded proteins in the Endoplasmic Reticulum (ER) lumen, thus causing ER stress. When this happens, an adaptive mechanism named the Unfolded Protein Response (UPR) is triggered to help the cell cope with this change and restore protein homeostasis in the ER. Sequentially, one would expect that the activation of the three UPR branches, driven namely by IRE1, PERK, and ATF6, are crucial for the adaptation of cancer cells to the changing environment and thus for their survival and further propagation. Indeed, in the last few years, an increasing amount of studies has shown the implication of UPR signaling in different aspects of carcinogenesis and tumor progression. Features such as sustaining proliferation and resistance to cell death, genomic instability, altered metabolism, increased inflammation and tumor-immune infiltration, invasion and metastasis, and angiogenesis, defined as "the hallmarks of cancer", can be regulated by the UPR machinery. At the same time, new potential therapeutic interventions applicable to different kinds of cancers are being revealed. In order to describe the emerging role of UPR in cancer biology, these are the points that will be discussed in this chapter.

PERK Is a Haploinsufficient Tumor Suppressor: Gene Dose Determines Tumor-Suppressive Versus Tumor Promoting Properties of PERK in Melanoma

The unfolded protein response (UPR) regulates cell fate following exposure of cells to endoplasmic reticulum stresses. PERK, a UPR protein kinase, regulates protein synthesis and while linked with cell survival, exhibits activities associated with both tumor progression and tumor suppression. For example, while cells lacking PERK are sensitive to UPR-dependent cell death, acute activation of PERK triggers both apoptosis and cell cycle arrest, which would be expected to contribute tumor suppressive activity. We have evaluated these activities in the BRAF-dependent melanoma and provide evidence revealing a complex role for PERK in melanoma where a 50% reduction is permissive for Braf^V600E-dependent transformation, while complete inhibition is tumor suppressive. Consistently, PERK mutants identified in human melanoma are hypomorphic with dominant inhibitory function. Strikingly, we demonstrate that small molecule PERK inhibitors exhibit single agent efficacy against Braf^V600E-dependent tumors highlighting the clinical value of targeting PERK.

PERK is critical for progression of specific cancers and has provided stimulus for the generation of small molecule PERK inhibitors. Paradoxically, the anti-proliferative and pro-death functions of PERK have potential tumor suppressive qualities. We demonstrate that PERK can function as either a tumor suppressor or a pro-adaptive tumor promoter and the nature of its function is determined by gene dose. Preclinical studies suggest a therapeutic threshold exists for PERK inhibitors.

Experimental reconstitution of chronic ER stress in the liver reveals feedback suppression of BiP mRNA expression

Endoplasmic reticulum (ER) stress is implicated in many chronic diseases, but very little is known about how the unfolded protein response (UPR) responds to persistent ER stress in vivo. Here, we experimentally reconstituted chronic ER stress in the mouse liver, using repeated injection of a low dose of the ER stressor tunicamycin. Paradoxically, this treatment led to feedback-mediated suppression of a select group of mRNAs, including those encoding the ER chaperones BiP and GRP94. This suppression was due to both silencing of the ATF6α pathway of UPR-dependent transcription and enhancement of mRNA degradation, possibly via regulated IRE1-dependent decay (RIDD). The suppression of mRNA encoding BiP was phenocopied by ectopic overexpression of BiP protein, and was also observed in obese mice. Our findings suggest that persistent cycles of UPR activation and deactivation create an altered, quasi-stable setpoint for UPR-dependent transcriptional regulation—an outcome that could be relevant to conditions such as metabolic syndrome.

DOI:http://dx.doi.org/10.7554/eLife.20390.001

Toxic chemicals, extreme temperatures and other abnormal environmental conditions can cause the cells in our bodies to become stressed. Several kinds of stresses overwhelm a compartment in the cell called the endoplasmic reticulum, which is critical for processing new proteins so that they can work correctly. Endoplasmic reticulum stress has been linked to long-term diseases such as diabetes, cancer and neurodegenerative diseases.

Most of what is known about how cells sense and respond to endoplasmic reticulum stress comes from studies on isolated cells that were subjected to harsh conditions that cells cannot tolerate for longer than a day or two. By contrast, little is known about how cells within whole organisms respond to milder but longer-lasting endoplasmic reticulum stress, which is closer to what occurs during disease.

To investigate this issue, Gomez and Rutkowski treated mice repeatedly with a chemical that causes mild endoplasmic reticulum stress in the liver. The cells exposed to this persistent stress responded differently to those exposed to severe short-term stress. Whereas short-term stress causes liver cells to turn on genes that help the endoplasmic reticulum to process proteins more efficiently, long-term stress causes cells to turn off some of those genes.

Further investigation revealed that cells in the livers of obese mice show similar patterns of gene activity as cells exposed to long-term endoplasmic reticulum stress. The findings presented by Gomez and Rutkowski could therefore also help us to understand more about the liver problems that often occur during obesity and diabetes.

Further studies are now needed to examine exactly how long-lasting stress can shut off the cells’ protective mechanisms. Future experiments could also investigate whether other types of cells and organs respond to long-term endoplasmic reticulum stress in the same way as cells in the liver.

DOI:http://dx.doi.org/10.7554/eLife.20390.002

Identification of critical paralog groups with indispensable roles in the regulation of signaling flow

Extensive cross-talk between signaling pathways is required to integrate the myriad of extracellular signal combinations at the cellular level. Gene duplication events may lead to the emergence of novel functions, leaving groups of similar genes - termed paralogs - in the genome. To distinguish critical paralog groups (CPGs) from other paralogs in human signaling networks, we developed a signaling network-based method using cross-talk annotation and tissue-specific signaling flow analysis. 75 CPGs were found with higher degree, betweenness centrality, closeness, and ‘bowtieness’ when compared to other paralogs or other proteins in the signaling network. CPGs had higher diversity in all these measures, with more varied biological functions and more specific post-transcriptional regulation than non-critical paralog groups (non-CPG). Using TGF-beta, Notch and MAPK pathways as examples, SMAD2/3, NOTCH1/2/3 and MEK3/6-p38 CPGs were found to regulate the signaling flow of their respective pathways. Additionally, CPGs showed a higher mutation rate in both inherited diseases and cancer, and were enriched in drug targets. In conclusion, the results revealed two distinct types of paralog groups in the signaling network: CPGs and non-CPGs. Thus highlighting the importance of CPGs as compared to non-CPGs in drug discovery and disease pathogenesis.

Cyclin D1, cancer progression, and opportunities in cancer treatment

Mammalian cells encode three D cyclins (D1, D2, and D3) that coordinately function as allosteric regulators of cyclin-dependent kinase 4 (CDK4) and CDK6 to regulate cell cycle transition from G1 to S phase. Cyclin expression, accumulation, and degradation, as well as assembly and activation of CDK4/CDK6 are governed by growth factor stimulation. Cyclin D1 is more frequently dysregulated than cyclin D2 or D3 in human cancers, and as such, it has been more extensively characterized. Overexpression of cyclin D1 results in dysregulated CDK activity, rapid cell growth under conditions of restricted mitogenic signaling, bypass of key cellular checkpoints, and ultimately, neoplastic growth. This review discusses cyclin D1 transcriptional, translational, and post-translational regulations and its biological function with a particular focus on the mechanisms that result in its dysregulation in human cancers.

Cell-type specific variation in the induction of ER stress and downstream events in chikungunya virus infection

Over the last decade infections with the mosquito transmitted chikungunya virus (CHIKV) have become a major worldwide concern, and considerable efforts have been made in understanding the interaction of this virus with the host cell machinery. Studies have documented the induction of the unfolded protein response (UPR), as well as the induction of apoptosis and autophagy in response to CHIKV infection. This study comparatively analysed these three processes in two cell lines, Hela and HepG2. Infection of Hela cells was characterized by activation of the PERK/eIF2α branch of the UPR, the induction of autophagy and early apoptosis, while infection of HepG2 cells was characterized by activation of the IRE/XBP1 branch of the UPR, limited or no activation of autophagy and comparatively later apoptosis. These results show that the specific cell context is an important mediator of the host cell response to CHIKV infection.

Simvastatin exerts anti-hepatitis B virus activity by inhibiting expression of minichromosome maintenance protein 7 in HepG2.2.15 cells

Simvastatin (SIM), a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, has been reported to inhibit the activity of hepatitis B virus (HBV), however, the mechanism underlying its antiviral function remains unknown. Minichromosome maintenance (MCM) 7, a component of the MCM complex, has been reported to act as an important host factor aiding virus genome replication in host cells. The present study demonstrated that downregulation of MCM7 inhibited the expression of proteins transferred by adenoviral vectors. This suggests an association between MCM7 and viral DNA expression. Thus, the current study aimed to investigate whether SIM affected MCM7 expression. Notably, the results of the present study indicated that following exposure to SIM the protein expression levels of MCM7 in HepG2.2.15, a human HBV‑transfected liver cell line, was decreased. In addition, the HBV DNA replication in the cell line was suppressed. As quantitative polymerase chain reaction experiments demonstrated that SIM did not downregulate the mRNA expression level of MCM7, the current study further investigated whether SIM affects the translation of MCM7. Western blot experiments indicated that SIM improved the activation of eukaryotic initiation factor‑2α (eIF2α), a protein synthesis initiation factor, and upregulated the upstream factors of eIF2α, protein kinase RNA‑like endoplasmic reticulum kinase, which is regulated by the liver kinase B1 (LKB1)‑AMP‑activated protein kinase (AMPK) signaling pathway. These results indicated that SIM induced HBV downregulation via an MCM‑dependent mechanism, and SIM may inhibit MCM7 expression by increasing the phosphorylation of eIF2α, which is mediated by the LKB1-AMPK signaling pathway.

Cell Signaling and Stress Responses

Stress-signaling pathways are evolutionarily conserved and play an important role in the maintenance of homeostasis. These pathways are also critical for adaptation to new cellular environments. The endoplasmic reticulum (ER) unfolded protein response (UPR) is activated by biosynthetic stress and leads to a compensatory increase in ER function. The JNK and p38 MAPK signaling pathways control adaptive responses to intracellular and extracellular stresses, including environmental changes such as UV light, heat, and hyperosmotic conditions, and exposure to inflammatory cytokines. Metabolic stress caused by a high-fat diet represents an example of a stimulus that coordinately activates both the UPR and JNK/p38 signaling pathways. Chronic activation of these stress-response pathways ultimately causes metabolic changes associated with obesity and altered insulin sensitivity. Stress-signaling pathways, therefore, represent potential targets for therapeutic intervention in the metabolic stress response and other disease processes.

ER Stress-Mediated Signaling: Action Potential and Ca(2+) as Key Players

The proper functioning of the endoplasmic reticulum (ER) is crucial for multiple cellular activities and survival. Disturbances in the normal ER functions lead to the accumulation and aggregation of unfolded proteins, which initiates an adaptive response, the unfolded protein response (UPR), in order to regain normal ER functions. Failure to activate the adaptive response initiates the process of programmed cell death or apoptosis. Apoptosis plays an important role in cell elimination, which is essential for embryogenesis, development, and tissue homeostasis. Impaired apoptosis can lead to the development of various pathological conditions, such as neurodegenerative and autoimmune diseases, cancer, or acquired immune deficiency syndrome (AIDS). Calcium (Ca(2+)) is one of the key regulators of cell survival and it can induce ER stress-mediated apoptosis in response to various conditions. Ca(2+) regulates cell death both at the early and late stages of apoptosis. Severe Ca(2+) dysregulation can promote cell death through apoptosis. Action potential, an electrical signal transmitted along the neurons and muscle fibers, is important for conveying information to, from, and within the brain. Upon the initiation of the action potential, increased levels of cytosolic Ca(2+) (depolarization) lead to the activation of the ER stress response involved in the initiation of apoptosis. In this review, we discuss the involvement of Ca(2+) and action potential in ER stress-mediated apoptosis.