ATF6 activated by proteolysis binds in the presence of NF-Y (CBF) directly to the cis-acting element responsible for the mammalian unfolded protein response

The expression of Transmembrane Protein 100 is regulated by alterations in calcium signaling rather than endoplasmic reticulum stress

Transmembrane protein 100 (TMEM100) comprises 134 amino acid residues and is highly conserved among vertebrates. Tmem100 has been recently reported as a key factor in angiogenesis, pain transmission, and tumor suppression. Although the importance of TMEM100 function is well supported, few studies have elucidated its expression mechanism. In the current study, we found that activating transcription factor 6α, a transcription factor activated by endoplasmic reticulum (ER) stress, enhanced Tmem100 promoter activity. Two ER stress response element-like motifs were identified in the mouse Tmem100 promoter region. However, additional experiments using another type of ER stress inducer demonstrated that calcium signaling was more important than ER stress in the regulation of TMEM100 expression. Intracellular calcium signaling controls biological processes such as cell proliferation and embryonic development. This study suggested that TMEM100 performs various functions in response to alterations in calcium signaling in addition to those in response to ER stress.

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.

A data-entrained computational model for testing the regulatory logic of the vertebrate unfolded protein response

The vertebrate unfolded protein response (UPR) is characterized by multiple interacting nodes among its three pathways, yet the logic underlying this regulatory complexity is unclear. To begin to address this issue, we created a computational model of the vertebrate UPR that was entrained upon and then validated against experimental data. As part of this validation, the model successfully predicted the phenotypes of cells with lesions in UPR signaling, including a surprising and previously unreported differential role for the eIF2α phosphatase GADD34 in exacerbating severe stress but ameliorating mild stress. We then used the model to test the functional importance of a feedforward circuit within the PERK/CHOP axis and of cross-regulatory control of BiP and CHOP expression. We found that the wiring structure of the UPR appears to balance the ability of the response to remain sensitive to endoplasmic reticulum stress and to be deactivated rapidly by improved protein-folding conditions. This model should serve as a valuable resource for further exploring the regulatory logic of the UPR.

Identification of ASB7 as ER stress responsive gene through a genome wide in silico screening for genes with ERSE

The endoplasmic reticulum (ER) not only performs its basic function of regulating calcium homeostasis, lipid biosynthesis, folding, modifying and transporting proteins but also plays a decisive role in regulating multiple cellular processes ranging from cell growth and differentiation to apoptosis and autophagy. Disturbances in ER homeostasis initiate the unfolded protein response (UPR) implicated in the pathogenesis of many human diseases. Drugging the UPR components for therapeutic interventions has received considerable attention. The purpose of this study is to identify genes that are previously unsuspected to be regulated under ER stress. Because ER stress-inducible gene expression is majorly regulated under ERSE elements, we screened human genome by adopting an in silico approach using ERSE elements (I, II, III) as probes and identified 337 candidate genes. Having knowledge of the importance of E3 ubiquitin ligase in the ERAD machinery; we validated our preliminary search by focusing on one of the hits i.e. ASB7 gene that encodes E3 ubiquitin ligase. In HeLa cells, we found that pharmacological induction of ER stress led to an increase in the expression of ASB7 with simultaneous activation of UPR pathways. Although knockdown of ASB7 expression leads to significant reduction in GRP78 and CHOP mRNA levels, it did not protect cells from ER stress-induced cell death. Also, an up-regulation in the expression of pro-inflammatory genes like TNF-α and IL-1β in ASB7 knockdown cells was observed under ER stress. Collectively, our findings suggest that ASB7 is regulated under ER stress and this study also identifies several other genes that could apparently be regulated under ER stress.

Tannic Acid Induces Endoplasmic Reticulum Stress-Mediated Apoptosis in Prostate Cancer

Endoplasmic reticulum (ER) stress is an intriguing target with significant clinical importance in chemotherapy. Interference with ER functions can lead to the accumulation of unfolded proteins, as detected by transmembrane sensors that instigate the unfolded protein response (UPR). Therefore, controlling induced UPR via ER stress with natural compounds could be a novel therapeutic strategy for the management of prostate cancer. Tannic acid (a naturally occurring polyphenol) was used to examine the ER stress mediated UPR pathway in prostate cancer cells. Tannic acid treatment inhibited the growth, clonogenic, invasive, and migratory potential of prostate cancer cells. Tannic acid demonstrated activation of ER stress response (Protein kinase R-like endoplasmic reticulum kinase (PERK) and inositol requiring enzyme 1 (IRE1)) and altered its regulatory proteins (ATF4, Bip, and PDI) expression. Tannic acid treatment affirmed upregulation of apoptosis-associated markers (Bak, Bim, cleaved caspase 3, and cleaved PARP), while downregulation of pro-survival proteins (Bcl-2 and Bcl-xL). Tannic acid exhibited elevated G₁ population, due to increase in p18INK4C and p21WAF1/CIP1 expression, while cyclin D1 expression was inhibited. Reduction of MMP2 and MMP9, and reinstated E-cadherin signifies the anti-metastatic potential of this compound. Altogether, these results demonstrate that tannic acid can promote apoptosis via the ER stress mediated UPR pathway, indicating a potential candidate for cancer treatment.

Silencing calreticulin gene might protect cardiomyocytes from angiotensin II-induced apoptosis

AIMS

Calreticulin (CRT), as a chaperone, contributes to protein folding and quality control cycle. CRT is an important factor regulating Ca²⁺ that participates in cell apoptosis. However, the function of CRT in the heart is still controversial. Therefore, we aimed to investigate the potential role of CRT in angiotensin II-induced cardiomyocytes apoptosis.

MAIN METHODS

Primary cultured neonatal cardiomyocytes were stimulated with angiotensin II to induce the apoptosis. Expression of CRT and endoplasmic reticulum (ER) stress associated protein was detected by western blotting after angiotensin II stimulation for 24 h. The reactive oxygen species (ROS) production and mitochondrial membrane potential (MMP) were also detected. Additionally, the function of CRT on cardiomyocytes apoptosis and ER stress/unfolded protein response signaling pathway was investigated by transfecting specific CRT-targeting siRNA.

KEY FINDINGS

Cardiomyocytes apoptosis was induced by angiotensin II. The protein level of CRT was elevated after angiotensin -II stimulation for 24 h. Additionally, the protein levels of GRP78, ATF4, C-ATF6, CHOP and the ROS production were elevated, but the Bcl-2 expression and the level of MMP were down-regulated. After silencing CRT gene in the process of angiotensin II-induced cardiomyocytes apoptosis, cardiomyocytes apoptosis rate decreased, meanwhile the protein expression of CRT, GRP78, ATF4, C-ATF6 and CHOP were down-regulated. However, the Bcl-2 expression was up-regulated, and the increase of ROS and the loss of MMP were alleviated.

SIGNIFICANCE

Our study demonstrated that CRT might protect cardiomyocytes from apoptosis induced by angiotensin II, in which ER stress and mitochondria function were identified as possible underlying molecular bases.

ER stress and cancer: The FOXO forkhead transcription factor link

The endoplasmic reticulum (ER) is a cellular organelle with central roles in maintaining proteostasis due to its involvement in protein synthesis, folding, quality control, distribution and degradation. The accumulation of misfolded proteins in the ER lumen causes 'ER stress' and threatens overall cellular proteostasis. To restore ER homeostasis, cells evoke an evolutionarily conserved adaptive signalling and gene expression network collectively called the 'unfolded protein response (UPR)', a complex biological process which aims to restore proteostasis. When ER stress is overwhelming and beyond rectification, the normally pro-survival UPR can shift to induce cell termination. Emerging evidence from mammalian, fly and nematode worm systems reveals that the FOXO Forkhead proteins integrate upstream ER stress and UPR signals with the transcriptional machinery to decrease translation, promote cell survival/termination and increase the levels of ER-resident chaperones and of ER-associated degradation (ERAD) components to restore ER homeostasis. The high rates of protein synthesis/translation associated with cancer cell proliferation and metabolism, as well as mutations resulting in aberrant proteins, also induce ER stress and the UPR. While the pro-survival side of the UPR underlies its ability to sustain and promote cancers, its apoptotic functions can be exploited for cancer therapies by offering the chance to 'flick the proteostatic switch'. To this end, further studies are required to fully reevaluate the roles and regulation of these UPR signalling molecules, including FOXO proteins and their targets, in cancer initiation and progression as well as the effects on inhibiting their functions in cancer cells. This information will help to establish these UPR signalling molecules as possible therapeutic targets and putative biomarkers in cancers.

The protein kinase PERK/EIF2AK3 regulates proinsulin processing not via protein synthesis but by controlling endoplasmic reticulum chaperones

Loss-of-function mutations of the protein kinase PERK (EIF2AK3) in humans and mice cause permanent neonatal diabetes and severe proinsulin aggregation in the endoplasmic reticulum (ER), highlighting the essential role of PERK in insulin production in pancreatic β cells. As PERK is generally known as a translational regulator of the unfolded protein response (UPR), the underlying cause of these β cell defects has often been attributed to derepression of proinsulin synthesis, resulting in proinsulin overload in the ER. Using high-resolution imaging and standard protein fractionation and immunological methods we have examined the PERK-dependent phenotype more closely. We found that whereas proinsulin aggregation requires new protein synthesis, global protein and proinsulin synthesis are down-regulated in PERK-inhibited cells, strongly arguing against proinsulin overproduction being the root cause of their aberrant ER phenotype. Furthermore, we show that PERK regulates proinsulin proteostasis by modulating ER chaperones, including BiP and ERp72. Transgenic overexpression of BiP and BiP knockdown (KD) both promoted proinsulin aggregation, whereas ERp72 overexpression and knockdown rescued it. These findings underscore the importance of ER chaperones working in concert to achieve control of insulin production and identify a role for PERK in maintaining a functional balance among these chaperones.

Inhibition of autophagy by chloroquine induces apoptosis in primary effusion lymphoma in vitro and in vivo through induction of endoplasmic reticulum stress

Autophagy plays a crucial role in cancer cell survival and the inhibition of autophagy is attracting attention as an emerging strategy for the treatment of cancer. Chloroquine (CQ) is an anti-malarial drug, and is also known as an inhibitor of autophagy. Recently, it has been found that CQ induces cancer cell death through the inhibition of autophagy; however, the underlying mechanism is not entirely understood. In this study, we identified the role of CQ-induced cancer cell death using Primary Effusion Lymphoma (PEL) cells. We found that a CQ treatment induced caspase-dependent apoptosis in vitro. CQ also suppressed PEL cell growth in a PEL xenograft mouse model. We showed that CQ activated endoplasmic reticulum (ER) stress signal pathways and induced CHOP, which is an inducer of apoptosis. CQ-induced cell death was significantly decreased by salbrinal, an ER stress inhibitor, indicating that CQ-induced apoptosis in PEL cells depended on ER stress. We show here for the first time that the inhibition of autophagy induces ER stress-mediated apoptosis in PEL cells. Thus, the inhibition of autophagy is a novel strategy for cancer chemotherapy.

Axonal Activation of the Unfolded Protein Response Promotes Axonal Regeneration Following Peripheral Nerve Injury

Adult mammalian peripheral neurons have an intrinsic regrowth capacity in response to axonal injury. The induction of calcium ion (Ca²⁺) oscillations at an injured site is critical for the regulation of regenerative responses. In polarized neurons, distal axonal segments contain a well-developed endoplasmic reticulum (ER) network that is responsible for Ca²⁺ homeostasis. Although these characteristics implicate the relevance among injury-induced Ca²⁺ dynamics, axonal ER-derived signaling, and regenerative responses propagated along the axons, the details are not fully understood. In the present study, we found that Ca²⁺ release from the axonal ER was accelerated in response to injury. Additionally, axonal injury-dependent Ca²⁺ release from the ER activated unfolded protein response (UPR) signaling at injured sites. Inhibition of axonal UPR signaling led to fragmentation of the axonal ER and disrupted growth cone formation, suggesting that activation of axonal UPR branches following axonal injury promotes regeneration via regulation of ER reconstruction and formation of growth cones. Our studies revealed that local activation of axonal UPR signaling by injury-induced Ca²⁺ release from the ER is critical for regeneration. These findings provide a new concept for the link between injury-induced signaling at a distant location and regulation of organelle and cytoskeletal formation in the orchestration of axonal regeneration.

Methamphetamine-mediated endoplasmic reticulum (ER) stress induces type-1 programmed cell death in astrocytes via ATF6, IRE1α and PERK pathways

Methamphetamine (MA), a psychostimulant drug has been associated with a variety of neurotoxic effects which are thought to be mediated by induction of pro-inflammatory cytokines/chemokines, oxidative stress and damage to blood-brain-barrier. Conversely, the ER stress-mediated apoptosis has been implicated in several neurodegenerative diseases. However, its involvement in MA-mediated neurodegenerative effects remains largely unexplored. The present study was undertaken to assess the effect of MA on ER stress and its possible involvement in apoptosis. For this purpose, SVGA astrocytes were treated with MA, which induced the expressions of BiP and CHOP at both, mRNA and protein levels. This phenomenon was also confirmed in HFA and various regions of mouse brain. Assessment of IRE1α, ATF6 and PERK pathways further elucidated the mechanistic details underlying MA-mediated ER stress. Knockdown of various intermediate molecules in ER stress pathways using siRNA demonstrated reduction in MA-mediated CHOP. Finally, MA-mediated apoptosis was demonstrated via MTT assay and TUNEL staining. The involvement of ER stress in the apoptosis was demonstrated with the help of MTT and TUNEL assays in the presence of siRNA against various ER stress proteins. The apoptosis also involved activation of caspase-3 and caspase-9, which was reversed by knockdown with various siRNAs. Altogether, this is the first report demonstrating mechanistic details responsible for MA-mediated ER stress and its role in apoptosis. This study provides a novel group of targets that can be explored in future for management of MA-mediated cell death and MA-associated neurodegenerative disorders.

ATF6 safeguards organelle homeostasis and cellular aging in human mesenchymal stem cells

Loss of organelle homeostasis is a hallmark of aging. However, it remains elusive how this occurs at gene expression level. Here, we report that human mesenchymal stem cell (hMSC) aging is associated with dysfunction of double-membrane organelles and downregulation of transcription factor ATF6. CRISPR/Cas9-mediated inactivation of ATF6 in hMSCs, not in human embryonic stem cells and human adipocytes, results in premature cellular aging, characteristic of loss of endomembrane homeostasis. Transcriptomic analyses uncover cell type-specific constitutive and stress-induced ATF6-regulated genes implicated in various layers of organelles’ homeostasis regulation. FOS was characterized as a constitutive ATF6 responsive gene, downregulation of which contributes to hMSC aging. Our study unravels the first ATF6-regulated gene expression network related to homeostatic regulation of membrane organelles, and provides novel mechanistic insights into aging-associated attrition of human stem cells.

Infliction of proteotoxic stresses by impairment of the unfolded protein response or proteasomal inhibition as a therapeutic strategy for mast cell leukemia

The intensity and duration of endoplasmic reticulum (ER) stress converts the unfolded protein response (UPR) from an adaptive into a terminal response. The first regulates homeostasis, the latter triggers apoptosis. Cells that rapidly proliferate and possess developed secretory capabilities, such as leukemia cells, depend on an efficiently operating UPR to maintain proteostasis. Activation of terminal UPR by either blockade of adaptive UPR or exaggeration of ER stress has been explored as a novel approach in cancer therapy. For mast cell leukemia (MCL) the efficacy of both approaches, by utilizing the KIT^V560G,D816V-positive MCL cell line HMC-1.2, was investigated. We show that HMC-1.2 cells display a tonic activation of the IRE1α arm of the UPR, which constitutively generates spliced XBP1. Inhibition of IRE1α by different types of inhibitors (MKC-8866, STF-083010, and KIRA6) suppressed proliferation at concentrations needed for blockade of IRE1α-mediated XBP1 splicing. At higher concentrations, these inhibitors triggered an apoptotic response. Blocking the proteasome by bortezomib, which confers an exaggerated UPR, resulted in a marked cytotoxic response. Bortezomib treatment also caused activation of the kinase JNK, which played a pro-proliferative and anti-apoptotic role. Hence, the combination of bortezomib with a JNK inhibitor synergized to induce cell death. In summary, the UPR can be addressed as an effective therapeutic target against KIT^D816V-positive MCL.

Emerging roles of ER stress in the etiology and pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by synaptic dysfunction and accumulation of abnormal aggregates formed by amyloid-β peptides or phosphorylated tau proteins. Accumulating evidence suggests that alterations in the buffering capacity of the proteostasis network are a salient feature of AD. The endoplasmic reticulum (ER) is the main compartment involved in protein folding and secretion and is drastically affected in AD neurons. ER stress triggers the activation of the unfolded protein response (UPR), a signal transduction pathway that enforces adaptive programs to recover homeostasis or trigger apoptosis of irreversibly damaged cells. Experimental manipulation of specific UPR signaling modules in preclinical models of AD has revealed a key role of this pathway in regulating protein misfolding and neurodegeneration. Recent studies suggest that the UPR also influences synaptic plasticity and memory through ER stress-independent mechanisms. Consequently, targeting of the UPR in AD is emerging as an interesting therapeutic approach to modify the two pillars of AD, protein misfolding and synaptic failure. Here, we review the functional role of ER stress signaling in AD, discussing the complex involvement of the pathway in controlling neuronal survival, the amyloid cascade, neurodegeneration and synaptic function. Recent intervention efforts to target the UPR with pharmacological and gene therapy strategies are also discussed.

Protein Localization at Mitochondria-ER Contact Sites in Basal and Stress Conditions

Mitochondria-endoplasmic reticulum (ER) contacts (MERCs) are sites at which the outer mitochondria membrane and the Endoplasmic Reticulum surface run in parallel at a constant distance. The juxtaposition between these organelles determines several intracellular processes such as to name a few, Ca²⁺ and lipid homeostasis or autophagy. These specific tasks can be exploited thanks to the enrichment (or re-localization) of dedicated proteins at these interfaces. Recent proteomic studies highlight the tissue specific composition of MERCs, but the overall mechanisms that control MERCs plasticity remains unclear. Understanding how proteins are targeted at these sites seems pivotal to clarify such contextual function of MERCs. This review aims to summarize the current knowledge on protein localization at MERCs and the possible contribution of the mislocalization of MERCs components to human disorders.

Curcumin induces osteoblast differentiation through mild-endoplasmic reticulum stress-mediated such as BMP2 on osteoblast cells

AIMS

Curcumin (diferuloylmethane or [1E,6E]-1,7-bis[4-hydroxy-3-methoxyphenyl]-1,6heptadiene-3,5-dione) is a phenolic natural product derived from the rhizomes of the turmeric plant, Curcuma longa. It is reported to have various biological actions such as anti-oxidative, anti-inflammatory, and anti-cancer effects. However, the molecular mechanism of osteoblast differentiation by curcumin has not yet been reported.

MAIN METHODS

The cytotoxicity of curcumin was identified using the 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Expression of osteogenic markers and endoplasmic reticulum (ER) stress markers in C3H1-T1/2 cells were measured using reverse-transcriptase polymerase chain reaction (RT-PCR) and Western blotting. Alkaline phosphatase (ALP) staining was performed to assess ALP activity in C3H10T1/2 cells. Transcriptional activity was detected using a luciferase reporter assay.

KEY FINDINGS

Curcumin increased the expression of genes such as distal-less homeobox 5 (Dlx5), runt-related transcription factor 2 (Runx2), ALP, and osteocalcin (OC), which subsequently induced osteoblast differentiation in C3H10T1/2 cells. In addition, ALP activity and mineralization was found to be increased by curcumin treatment. Curcumin also induced mild ER stress similar to bone morphogenetic protein 2 (BMP2) function in osteoblast cells. Next, we confirmed that curcumin increased mild ER stress and osteoblast differentiation similar to BMP2 in C3H10T1/2 mesenchymal stem cells. Transient transfection studies also showed that curcumin increased ATF6-Luc activity, while decreasing the activities of CREBH-Luc and SMILE-Luc. In addition, similar to BMP2, curcumin induced the phosphorylation of Smad 1/5/9.

SIGNIFICANCE

Overall, these results demonstrate that curcumin-induced mild ER stress increases osteoblast differentiation via ATF6 expression in C3H10T1/2 cells.

Transcription Factor C/EBP Homologous Protein in Health and Diseases

C/EBP homologous protein (CHOP), known also as DNA damage-inducible transcript 3 and as growth arrest and DNA damage-inducible protein 153 (GADD153), is induced in response to certain stressors. CHOP is universally acknowledged as a main conduit to endoplasmic reticulum stress-induced apoptosis. Ongoing research established the existence of CHOP-mediated apoptosis signaling networks, for which novel downstream targets are still being determined. However, there are studies that contradict this notion and assert that apoptosis is not the only mechanism by which CHOP plays in the development of pathologies. In this review, insights into the roles of CHOP in pathophysiology are summarized at the molecular and cellular levels. We further focus on the newest advances that implicate CHOP in human diseases including cancer, diabetes, neurodegenerative disorders, and notably, fibrosis.

Functional interaction of endoplasmic reticulum stress and hepatitis B virus in the pathogenesis of liver diseases

Hepatitis B virus (HBV) is a non-cytopathic virus that causes acute and chronic inflammatory liver diseases, often leading to the pathogenesis of hepatocellular carcinoma (HCC). Although many studies for the roles of HBV on pathogenesis of the liver diseases, such as non-alcoholic fatty liver disease (NAFLD), hepatic inflammation, cirrhosis, and HCC, have been reported, the mechanisms are not fully understood. Endoplasmic reticulum (ER) and mitochondria have the protective mechanisms to restore their damaged function by intrinsic or extrinsic stresses, but their chronic dysfunctions are associated with the pathogenesis of the various diseases. Furthermore, HBV can affect intra- or extracellular homeostasis through induction of ER and mitochondrial dysfunctions, leading to liver injury. Therefore, the mechanism by which HBV induces ER or mitochondrial stresses may be a therapeutic target for treatment of liver diseases.

Activating Transcription Factor-6α Deletion Modulates the Endoplasmic Reticulum Stress Response after Spinal Cord Injury but Does Not Affect Locomotor Recovery

The endoplasmic reticulum stress response (ERSR) is activated in a variety of neurodegenerative diseases and/or traumatic injuries. Subsequent restoration of ER homeostasis may contribute to improvement in the functional outcome of these diseases. We recently demonstrated improvements in hindlimb locomotion after thoracic spinal cord injury (SCI) and implicated oligodendrocyte survival as a potential mechanism using genetic and pharmacological inhibition of the protein kinase ribonucleic acid-like ER kinase- CCAAT/enhancer binding homologous protein (PERK-CHOP) arm of the ERSR. Here, we investigated the contribution of activating transcription factor-6 (ATF6), an ERSR signaling effector comprising the second arm of ERSR, in the pathogenesis of SCI. In contrast to what was seen after attenuation of PERK-CHOP signaling, genetic ablation of ATF6 results in modulation of ERSR and decreased survival in oligodendrocyte precursor cells against ER stress. Further, ATF6 loss delays the ERSR after SCI, potentiates PERK-ATF4-CHOP signaling and fails to improve locomotor deficits. These data suggest that deleting ATF6 levels is unlikely to be a viable therapeutic target to improve functional recovery after SCI.

The ins and outs of endoplasmic reticulum-controlled lipid biosynthesis

Endoplasmic reticulum (ER)-localized enzymes synthesize the vast majority of cellular lipids. The ER therefore has a major influence on cellular lipid biomass and balances the production of different lipid categories, classes, and species. Signals from outside and inside the cell are directed to ER-localized enzymes, and lipid enzyme activities are defined by the integration of internal, homeostatic, and external information. This allows ER-localized lipid synthesis to provide the cell with membrane lipids for growth, proliferation, and differentiation-based changes in morphology and structure, and to maintain membrane homeostasis across the cell. ER enzymes also respond to physiological signals to drive carbohydrates and nutritionally derived lipids into energy-storing triglycerides. In this review, we highlight some key regulatory mechanisms that control ER-localized enzyme activities in animal cells. We also discuss how they act in concert to maintain cellular lipid homeostasis, as well as how their dysregulation contributes to human disease.

Neuronal activity-dependent local activation of dendritic unfolded protein response promotes expression of brain-derived neurotrophic factor in cell soma

Unfolded protein response (UPR) has roles not only in resolving the accumulation of unfolded proteins owing to endoplasmic reticulum (ER) stress, but also in regulation of cellular physiological functions. ER stress transducers providing the branches of UPR signaling are known to localize in distal dendritic ER of neurons. These reports suggest that local activation of UPR branches may produce integrated outputs for distant communication, and allow regulation of local events in highly polarized neurons. Here, we demonstrated that synaptic activity‐ and brain‐derived neurotrophic factor (BDNF)‐dependent local activation of UPR signaling could be associated with dendritic functions through retrograde signal propagation by using murine neuroblastoma cell line, Neuro‐2A and primary cultured hippocampal neurons derived from postnatal day 0 litter C57BL/6 mice. ER stress transducer, inositol‐requiring kinase 1 (IRE1), was activated at postsynapses in response to excitatory synaptic activation. Activated dendritic IRE1 accelerated accumulation of the downstream transcription factor, x‐box‐binding protein 1 (XBP1), in the nucleus. Interestingly, excitatory synaptic activation‐dependent up‐regulation of XBP1 directly facilitated transcriptional activation of BDNF. BDNF in turn drove its own expression via IRE1‐XBP1 pathway in a protein kinase A‐dependent manner. Exogenous treatment with BDNF promoted extension and branching of dendrites through the protein kinase A‐IRE1‐XBP1 cascade. Taken together, our findings indicate novel mechanisms for communication between soma and distal sites of polarized neurons that are coordinated by local activation of IRE1‐XBP1 signaling. Synaptic activity‐ and BDNF‐dependent distinct activation of dendritic IRE1‐XBP1 cascade drives BDNF expression in cell soma and may be involved in dendritic extension.

Cover Image for this issue: doi. 10.1111/jnc.14159.

Inhibition of IRE1α-mediated XBP1 mRNA cleavage by XBP1 reveals a novel regulatory process during the unfolded protein response

Background: The mammalian endoplasmic reticulum (ER) continuously adapts to the cellular secretory load by the activation of an unfolded protein response (UPR).  This stress response results in expansion of the ER, upregulation of proteins involved in protein folding and degradation, and attenuation of protein synthesis.  The response is orchestrated by three signalling pathways each activated by a specific signal transducer, either inositol requiring enzyme α (IRE1α), double-stranded RNA-activated protein kinase-like ER kinase (PERK) or activating transcription factor 6 (ATF6).  Activation of IRE1α results in its oligomerisation, autophosphorylation and stimulation of its ribonuclease activity.  The ribonuclease initiates the splicing of an intron from mRNA encoding the transcription factor, X-box binding protein 1 (XBP1), as well as degradation of specific mRNAs and microRNAs.

Methods: To investigate the consequence of expression of exogenous XBP1, we generated a stable cell-line expressing spliced XBP1 mRNA under the control of an inducible promotor.

Results: Following induction of expression, high levels of XBP1 protein were detected, which allowed upregulation of target genes in the absence of induction of the UPR.  Remarkably under stress conditions, the expression of exogenous XBP1 repressed splicing of endogenous XBP1 mRNA without repressing the activation of PERK.

Conclusions: These results illustrate that a feedback mechanism exists to attenuate Ire1α ribonuclease activity in the presence of XBP1.

Inhibition of IRE1α-mediated XBP1 mRNA cleavage by XBP1 reveals a novel regulatory process during the unfolded protein response

Background: The mammalian endoplasmic reticulum (ER) continuously adapts to the cellular secretory load by the activation of an unfolded protein response (UPR).  This stress response results in expansion of the ER, upregulation of proteins involved in protein folding and degradation, and attenuation of protein synthesis.  The response is orchestrated by three signalling pathways each activated by a specific signal transducer, either inositol requiring enzyme α (IRE1α), double-stranded RNA-activated protein kinase-like ER kinase (PERK) or activating transcription factor 6 (ATF6).  Activation of IRE1α results in its oligomerisation, autophosphorylation and stimulation of its ribonuclease activity.  The ribonuclease initiates the splicing of an intron from mRNA encoding the transcription factor, X-box binding protein 1 (XBP1), as well as degradation of specific mRNAs and microRNAs. Methods: To investigate the consequence of expression of exogenous XBP1, we generated a stable cell-line expressing spliced XBP1 mRNA under the control of an inducible promotor. Results: Following induction of expression, high levels of XBP1 protein were detected, which allowed upregulation of target genes in the absence of induction of the UPR.  Remarkably under stress conditions, the expression of exogenous XBP1 repressed splicing of endogenous XBP1 mRNA without repressing the activation of PERK. Conclusions: These results illustrate that a feedback mechanism exists to attenuate Ire1α ribonuclease activity in the presence of XBP1.

Unfolded protein response transducer IRE1-mediated signaling independent of XBP1 mRNA splicing is not required for growth and development of medaka fish

When activated by the accumulation of unfolded proteins in the endoplasmic reticulum, metazoan IRE1, the most evolutionarily conserved unfolded protein response (UPR) transducer, initiates unconventional splicing of XBP1 mRNA. Unspliced and spliced mRNA are translated to produce pXBP1(U) and pXBP1(S), respectively. pXBP1(S) functions as a potent transcription factor, whereas pXBP1(U) targets pXBP1(S) to degradation. In addition, activated IRE1 transmits two signaling outputs independent of XBP1, namely activation of the JNK pathway, which is initiated by binding of the adaptor TRAF2 to phosphorylated IRE1, and regulated IRE1-dependent decay (RIDD) of various mRNAs in a relatively nonspecific manner. Here, we conducted comprehensive and systematic genetic analyses of the IRE1-XBP1 branch of the UPR using medaka fish and found that the defects observed in XBP1-knockout or IRE1-knockout medaka were fully rescued by constitutive expression of pXBP1(S). Thus, the JNK and RIDD pathways are not required for the normal growth and development of medaka. The unfolded protein response sensor/transducer IRE1-mediated splicing of XBP1 mRNA encoding its active downstream transcription factor to maintain the homeostasis of the endoplasmic reticulum is sufficient for growth and development of medaka fish.

Stressed Out - Therapeutic Implications of ER Stress Related Cancer Research

The unfolded protein response (UPR) is an established and well-studied cellular response to the stress and serves to relieve the stress and reinstate cellular homeostasis. It occurs in the endoplasmic reticulum (ER), responsible of properly folding and processing of secretory and transmembrane proteins. It is extremely sensitive to alteration in homeostasis caused by various internal or external stressors which leads to accumulation of misfolded or unfolded proteins in the ER lumen. The UPR works by restoring protein homeostasis in the ER, either through the boosting of protein-folding and degradation capability or by assuaging the demands for such effects, and can cause the activation of cell death if unable to do so. Cancer cells have adapted to gain advantage from the UPR and keeping the cell away from apoptosis and promoting survival, including survival of the cancer stem cells and evading the immune system. Several components of the UPR are overexpressed in a malignant cell and are responsible for resistance from various chemotherapy options and radiotherapy, which are also responsible for causing ER stress and activating the UPR. In this review, we discuss the various ways in which UPR can aid different cancers to survive and evade therapy and highlight recent research, which exploits the UPR to confer sensitivity to these cancer cells against various drugs and radiation.

Neuronal activity-dependent local activation of dendritic unfolded protein response promotes expression of brain-derived neurotrophic factor in cell soma

Unfolded protein response (UPR) has roles not only in resolving the accumulation of unfolded proteins owing to endoplasmic reticulum (ER) stress, but also in regulation of cellular physiological functions. ER stress transducers providing the branches of UPR signaling are known to localize in distal dendritic ER of neurons. These reports suggest that local activation of UPR branches may produce integrated outputs for distant communication, and allow regulation of local events in highly polarized neurons. Here, we demonstrated that synaptic activity- and brain-derived neurotrophic factor (BDNF)-dependent local activation of UPR signaling could be associated with dendritic functions through retrograde signal propagation by using murine neuroblastoma cell line, Neuro-2A and primary cultured hippocampal neurons derived from postnatal day 0 litter C57BL/6 mice. ER stress transducer, inositol-requiring kinase 1 (IRE1), was activated at postsynapses in response to excitatory synaptic activation. Activated dendritic IRE1 accelerated accumulation of the downstream transcription factor, x-box-binding protein 1 (XBP1), in the nucleus. Interestingly, excitatory synaptic activation-dependent up-regulation of XBP1 directly facilitated transcriptional activation of BDNF. BDNF in turn drove its own expression via IRE1-XBP1 pathway in a protein kinase A-dependent manner. Exogenous treatment with BDNF promoted extension and branching of dendrites through the protein kinase A-IRE1-XBP1 cascade. Taken together, our findings indicate novel mechanisms for communication between soma and distal sites of polarized neurons that are coordinated by local activation of IRE1-XBP1 signaling. Synaptic activity- and BDNF-dependent distinct activation of dendritic IRE1-XBP1 cascade drives BDNF expression in cell soma and may be involved in dendritic extension. Cover Image for this issue: doi. 10.1111/jnc.14159.

Molecular analysis of oxalate-induced endoplasmic reticulum stress mediated apoptosis in the pathogenesis of kidney stone disease

Oxalate, a non-essential end product of metabolism, causes hyperoxaluria and eventually calcium oxalate (CaOx) stone disease. Kidney cells exposed to oxalate stress results in generation of reactive oxygen species (ROS) and progression of stone formation. Perturbations in endoplasmic reticulum (ER) result in accumulation of misfolded proteins and Ca²⁺ ions homeostasis imbalance and serve as a common pathway for various diseases, including kidney disorders. ER stress induces up-regulation of pro-survival protein glucose-regulated protein 78 (GRP78) and pro-apoptotic signaling protein C/EBP homologous protein (CHOP). Since the association of oxalate toxicity and ER stress on renal cell damage is uncertain, the present study is an attempt to elucidate the interaction of GRP78 with oxalate by computational analysis and study the role of ER stress in oxalate-mediated apoptosis in vitro and in vivo. Molecular docking results showed that GRP78-oxalate/CaOx interaction takes place. Oxalate stress significantly up-regulated expression of ER stress markers GRP78 and CHOP both in vitro and in vivo. Exposure of oxalate increased ROS generation and altered antioxidant enzyme activities. N-Acetyl cysteine treatment significantly ameliorated oxalate-mediated oxidative stress and moderately attenuated ER stress marker expression. The result indicates oxalate toxicity initiated oxidative stress-induced ER stress and also activating ER stress mediated apoptosis directly. In addition, the up-regulation of transforming growth factor β-₁ revealed oxalate may induce kidney fibrosis through ER stress-mediated mechanisms. The present study provide insights into the pathogenic role of oxidative and ER stress by oxalate exposure in the formation of calcium oxalate stone.

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.

Fine-tuning PERK signaling for neuroprotection

Protein translation and folding are tightly controlled processes in all cells, by proteostasis, an important component of which is the unfolded protein response (UPR). During periods of endoplasmic reticulum stress because of protein misfolding, the UPR activates a coordinated response in which the PERK branch activation restricts translation, while a variety of genes involved with protein folding, degradation, chaperone expression and stress responses are induced through signaling of the other branches. Chronic overactivation of the UPR, particularly the PERK branch, is observed in the brains of patients in a number of protein misfolding neurodegenerative diseases, including Alzheimer's, and Parkinson's diseases and the tauopathies. Recently, numerous genetic and pharmacological studies in mice have demonstrated the effectiveness of inhibiting the UPR for eliciting therapeutic benefit and boosting memory. In particular, fine-tuning the level of PERK inhibition to provide neuroprotection without adverse side effects has emerged as a safe, effective approach. This includes the recent discovery of licensed drugs that can now be repurposed in clinical trials for new human treatments for dementia. This review provides an overview of the links between UPR overactivation and neurodegeneration in protein misfolding disorders. It discusses recent therapeutic approaches targeting this pathway, with a focus on treatments that fine-tune PERK signaling.

Development of a fluorescent reporter system for monitoring ER stress in Chinese hamster ovary cells and its application for therapeutic protein production

Mammalian cell expression systems have become a workhorse for the production of biotherapeutic proteins. As such, there is an ever increasing demand for higher productivity from these expression platforms to reduce manufacturing costs. While great advances have been made in the optimization of culture conditions and cell line selection to improve productivity, protein mis-folding remains a common limitation to high levels of production of therapeutic proteins. Accumulation of mis- and unfolded protein in the endoplasmic reticulum (ER) causes ER stress and initiates the unfolded protein response (UPR) that results in an activation of protein folding machinery, translation attenuation in an effort to proper folding of the newly synthesized peptides or may even lead to apoptosis if the correct folding is not restored. As a result, UPR associated apoptosis often results in lower protein expression. To better understand the molecular mechanisms in these pathways, we developed a reporter construct that detects Inositol-requiring enzyme 1 (IRE1)-alpha mediated splicing of X-box binding protein 1 (XBP1) to monitor the course of UPR activation in cell lines expressing monoclonal antibodies. Using this reporter we observed a clear activation of UPR in cells treated with known ER stress causing pharmacological agents, such as Tunicamycin (Tm) and Thapsigargin (Tg), as well as in stable IgG expressing cells during fed-batch cultures. Furthermore, we developed a stress metric that we term as ER stress index (ERSI) to gauge basal ER stress in cells which we used as a predictive tool for isolation of high IgG expressing cell lines. This reporter system, with its ability to monitor the stress involved in recombinant protein expression, has utility to assist in devising engineering strategies for improved production of biotherapeutic drugs.

Endothelial cells, endoplasmic reticulum stress and oxysterols

Oxysterols are bioactive lipids that act as regulators of lipid metabolism, inflammation, cell viability and are involved in several diseases, including atherosclerosis. Mounting evidence linked the atherosclerosis to endothelium dysfunction; in fact, the endothelium regulates the vascular system with roles in processes such as hemostasis, cell cholesterol, hormone trafficking, signal transduction and inflammation. Several papers shed light the ability of oxysterols to induce apoptosis in different cell lines including endothelial cells. Apoptotic endothelial cell and endothelial denudation may constitute a critical step in the transition to plaque erosion and vessel thrombosis, so preventing the endothelial damaged has garnered considerable attention as a novel means of treating atherosclerosis. Endoplasmic reticulum (ER) is the site where the proteins are synthetized and folded and is necessary for most cellular activity; perturbations of ER homeostasis leads to a condition known as endoplasmic reticulum stress. This condition evokes the unfolded protein response (UPR) an adaptive pathway that aims to restore ER homeostasis. Mounting evidence suggests that chronic activation of UPR leads to cell dysfunction and death and recently has been implicated in pathogenesis of endothelial dysfunction. Autophagy is an essential catabolic mechanism that delivers misfolded proteins and damaged organelles to the lysosome for degradation, maintaining basal levels of autophagic activity it is critical for cell survival. Several evidence suggests that persistent ER stress often results in stimulation of autophagic activities, likely as a compensatory mechanism to relieve ER stress and consequently cell death. In this review, we summarize evidence for the effect of oxysterols on endothelial cells, especially focusing on oxysterols-mediated induction of endoplasmic reticulum stress.

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Endothelial cells dysfunction is critical in the process of atherothrombosis.

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Endoplasmic reticulum stress is a key component in endothelial cell dysfunction.

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Oxysterols are oxidation products of cholesterol found in atherosclerosis lesions.

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Oxysterols are potential modulators of endoplasmic reticulum stress.

Altered Expression of Endoplasmic Reticulum Stress-Related Genes in the Middle Frontal Cortex of Subjects with Autism Spectrum Disorder

The endoplasmic reticulum (ER) is an important organelle responsible for the folding and sorting of proteins. Disturbances in ER homeostasis can trigger a cellular response known as the unfolded protein response, leading to accumulation of unfolded or misfolded proteins in the ER lumen called ER stress. A number of recent studies suggest that mutations in autism spectrum disorder (ASD)-susceptible synaptic genes induce ER stress. However, it is not known whether ER stress-related genes are altered in the brain of ASD subjects. In the present study, we investigated the mRNA expression of ER stress-related genes (ATF4, ATF6, PERK, XBP1, sXBP1, CHOP, and IRE1) in the postmortem middle frontal gyrus of ASD and control subjects. RT-PCR analysis showed significant increases in the mRNA levels of ATF4, ATF6, PERK, XBP1, CHOP, and IRE1 in the middle frontal gyrus of ASD subjects. In addition, we found a significant positive association of mRNA levels of ER stress genes with the diagnostic score for stereotyped behavior in ASD subjects. These results, for the first time, provide the evidence of the dysregulation of ER stress genes in the brain of subjects with ASD.

Ablation of Perk in Schwann Cells Improves Myelination in the S63del Charcot-Marie-Tooth 1B Mouse

In factory cells, the accumulation of misfolded protein provokes the unfolded protein response (UPR). For example, deletion of serine 63 (S63del) in myelin protein zero (P0) induces P0 accumulation in the endoplasmic reticulum (ER) of Schwann cells and a persistent UPR associated with Charcot-Marie-Tooth 1B (CMT1B) demyelinating peripheral neuropathy in human and mouse. PERK (protein kinase RNA-like ER kinase) is the ER stress sensor that attenuates global translation by phosphorylating eIF2α. Inhibition of the eIF2α holophosphatase GADD34:PP1, increases the phosphorylation of eIF2α in Schwann cells and largely rescues S63del neuropathy. Nonetheless, reducing phosphorylation of eIF2α, by Perk haploinsufficiency, also ameliorates the myelin defects of S63del nerves. This contradictory finding prompted us to investigate whether the beneficial effect of Perk deficiency on myelination could derive from neurons. To test this hypothesis, we generated and compared Schwann cell- and neuron-specific ablation of Perk in S63del nerves. Our data suggest that the detrimental effect of Perk in CMT1B derives primarily from Schwann cells. Furthermore, we show that Perk loss of function in Schwann cells restores myelination without diminishing accumulation of P0 or markers of ER stress, suggesting that Perk may modulate myelination through a pathway independent of the UPR.

SIGNIFICANCE STATEMENT

In many endoplasmic reticulum (ER) stress-related disorders, activation of the unfolded protein sensor protein kinase RNA-like ER kinase (PERK) kinase is beneficial. Nonetheless, in Charcot-Marie-Tooth 1B neuropathy mice, we show that activation of PERK in Schwann cells, but not in neurons, is detrimental for myelination. PERK may interfere with myelination, independent of its role in ER stress.

Reactive oxygen species-mediated unfolded protein response pathways in preimplantation embryos

Excessive production of reactive oxygen species (ROS) and endoplasmic reticulum (ER) stress-mediated responses are critical to embryonic development in the challenging in vitro environment. ROS production increases during early embryonic development with the increase in protein requirements for cell survival and growth. The ER is a multifunctional cellular organelle responsible for protein folding, modification, and cellular homeostasis. ER stress is activated by a variety of factors including ROS. Such stress leads to activation of the adaptive unfolded protein response (UPR), which restores homeostasis. However, chronic stress can exceed the toleration level of the ER, resulting in cellular apoptosis. In this review, we briefly describe the generation and impact of ROS in preimplantation embryo development, the ROS-mediated activation mechanism of the UPR via the ER, and the subsequent activation of signaling pathways following ER stress in preimplantation embryos.

Effect of Hypoxic Injury in Mood Disorder

Hypoxemia is a common complication of the diseases associated with the central nervous system, and neurons are highly sensitive to the availability of oxygen. Neuroplasticity is an important property of the neural system controlling breathing, memory, and cognitive ability. However, the underlying mechanism has not yet been clearly elucidated. In recent years, several pieces of evidence have highlighted the effect of hypoxic injury on neuronal plasticity in the pathogenesis and treatment of mood disorder. Therefore, the present study reviewed the relevant articles regarding hypoxic injury and neuronal plasticity and discussed the pathological changes and physiological functions of neurons in hypoxemia in order to provide a translational perspective to the relevance of hypoxic injury and mood disorder.

Endoplasmic reticulum stress and inflammation in the central nervous system

Persistent endoplasmic reticulum (ER) stress is thought to drive the pathology of many chronic disorders due to its potential to elicit aberrant inflammatory signaling and facilitate cell death. In neurodegenerative diseases, the accumulation of misfolded proteins and concomitant induction of ER stress in neurons contributes to neuronal dysfunction. In addition, ER stress responses induced in the surrounding neuroglia may promote disease progression by coordinating damaging inflammatory responses, which help fuel a neurotoxic milieu. Nevertheless, there still remains a gap in knowledge regarding the cell-specific mechanisms by which ER stress mediates neuroinflammation. In this review, we will discuss recently uncovered inflammatory pathways linked to the ER stress response. Moreover, we will summarize the present literature delineating how ER stress is generated in Alzheimer’s disease, Parkinson’s disease, Amyotrophic Lateral Sclerosis, and Multiple Sclerosis, and highlight how ER stress and neuroinflammation intersect mechanistically within the central nervous system. The mechanisms by which stress-induced inflammation contributes to the pathogenesis and progression of neurodegenerative diseases remain poorly understood. Further examination of this interplay could present unappreciated insights into the development of neurodegenerative diseases, and reveal new therapeutic targets.

Sodium 4-phenylbutyric acid prevents murine acetaminophen hepatotoxicity by minimizing endoplasmic reticulum stress

BACKGROUND

Acetaminophen (APAP) overdose induces severe oxidative stress followed by hepatocyte apoptosis/necrosis. Previous studies have indicated that endoplasmic reticulum (ER) stress is involved in the cell death process. Therefore, we investigated the effect of the chemical chaperone 4-phenyl butyric acid (PBA) on APAP-induced liver injury in mice.

METHODS

Eight-week-old male C57Bl6/J mice were given a single intraperitoneal (i.p.) injection of APAP (450 mg/kg body weight), following which some were repeatedly injected with PBA (120 mg/kg body weight, i.p.) every 3 h starting at 0.5 h after the APAP challenge. All mice were then serially euthanized up to 12 h later.

RESULTS

PBA treatment dramatically ameliorated the massive hepatocyte apoptosis/necrosis that was observed 6 h after APAP administration. PBA also significantly prevented the APAP-induced increases in cleaved activating transcription factor 6 and phosphorylation of c-Jun N-terminal protein kinase and significantly blunted the increases in mRNA levels for binding immunoglobulin protein, spliced X-box binding protein-1, and C/EBP homologous protein. Moreover, PBA significantly prevented APAP-induced Bax translocation to the mitochondria, and the expression of heme oxygenase-1 mRNA and 4-hydroxynonenal. By contrast, PBA did not affect hepatic glutathione depletion following APAP administration, reflecting APAP metabolism.

CONCLUSIONS

PBA prevents APAP-induced liver injury even when an APAP challenge precedes its administration. The underlying mechanism of action most likely involves the prevention of ER stress-induced apoptosis/necrosis in the hepatocytes during APAP intoxication.

PPP1R15A-mediated dephosphorylation of eIF2α is unaffected by Sephin1 or Guanabenz

Dephosphorylation of translation initiation factor 2 (eIF2α) terminates signalling in the mammalian integrated stress response (ISR) and has emerged as a promising target for modifying the course of protein misfolding diseases. The [(o-chlorobenzylidene)amino]guanidines (Guanabenz and Sephin1) have been proposed to exert protective effects against misfolding by interfering with eIF2α-P dephosphorylation through selective disruption of a PP1-PPP1R15A holophosphatase complex. Surprisingly, they proved inert in vitro affecting neither stability of the PP1-PPP1R15A complex nor substrate-specific dephosphorylation. Furthermore, eIF2α-P dephosphorylation, assessed by a kinase shut-off experiment, progressed normally in Sephin1-treated cells. Consistent with its role in defending proteostasis, Sephin1 attenuated the IRE1 branch of the endoplasmic reticulum unfolded protein response. However, repression was noted in both wildtype and Ppp1r15a deleted cells and in cells rendered ISR-deficient by CRISPR editing of the Eif2s1 locus to encode a non-phosphorylatable eIF2α (eIF2α^S51A). These findings challenge the view that [(o-chlorobenzylidene)amino]guanidines restore proteostasis by interfering with eIF2α-P dephosphorylation.

NCLs and ER: A stressful relationship

The Neuronal Ceroid Lipofuscinoses (NCLs, Batten disease) are a group of inherited neurodegenerative disorders with variable age of onset, characterized by the lysosomal accumulation of autofluorescent ceroid lipopigments. The endoplasmic reticulum (ER) is a critical organelle for normal cell function. Alteration of ER homeostasis leads to accumulation of misfolded protein in the ER and to activation of the unfolded protein response. ER stress and the UPR have recently been linked to the NCLs. In this review, we will discuss the evidence for UPR activation in the NCLs, and address its connection to disease pathogenesis. Further understanding of ER-stress response involvement in the NCLs may encourage development of novel therapeutical agents targeting these pathogenic pathways.

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ER-stress activation has been linked to various neurodegenerative diseases.

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ER-stress is a common patho-mechanism in four forms of NCL.

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Pharmacological modulation of UPR could provide new treatment for NCL.

Activation of the ATF6 branch of the unfolded protein response in neurons improves stroke outcome

Impaired function of the endoplasmic reticulum (ER stress) is a hallmark of many human diseases including stroke. To restore ER function in stressed cells, the unfolded protein response (UPR) is induced, which activates 3 ER stress sensor proteins including activating transcription factor 6 (ATF6). ATF6 is then cleaved by proteases to form the short-form ATF6 (sATF6), a transcription factor. To determine the extent to which activation of the ATF6 UPR branch defines the fate and function of neurons after stroke, we generated a conditional and tamoxifen-inducible sATF6 knock-in mouse. To express sATF6 in forebrain neurons, we crossed our sATF6 knock-in mouse line with Emx1-Cre mice to generate ATF6-KI mice. After the ATF6 branch was activated in ATF6-KI mice with tamoxifen, mice were subjected to transient middle cerebral artery occlusion. Forced activation of the ATF6 UPR branch reduced infarct volume and improved functional outcome at 24 h after stroke. Increased autophagic activity at early reperfusion time after stroke may contribute to the ATF6-mediated neuroprotection. We concluded that the ATF6 UPR branch is crucial to ischemic stroke outcome. Therefore, boosting UPR pro-survival pathways may be a promising therapeutic strategy for stroke.

Endoplasmic Reticulum (ER) Stress and Endocrine Disorders

The endoplasmic reticulum (ER) is the organelle where secretory and membrane proteins are synthesized and folded. Unfolded proteins that are retained within the ER can cause ER stress. Eukaryotic cells have a defense system called the “unfolded protein response” (UPR), which protects cells from ER stress. Cells undergo apoptosis when ER stress exceeds the capacity of the UPR, which has been revealed to cause human diseases. Although neurodegenerative diseases are well-known ER stress-related diseases, it has been discovered that endocrine diseases are also related to ER stress. In this review, we focus on ER stress-related human endocrine disorders. In addition to diabetes mellitus, which is well characterized, several relatively rare genetic disorders such as familial neurohypophyseal diabetes insipidus (FNDI), Wolfram syndrome, and isolated growth hormone deficiency type II (IGHD2) are discussed in this article.

Role of IRE1α/XBP-1 in Cystic Fibrosis Airway Inflammation

Cystic fibrosis (CF) pulmonary disease is characterized by chronic airway infection and inflammation. The infectious and inflamed CF airway environment impacts on the innate defense of airway epithelia and airway macrophages. The CF airway milieu induces an adaptation in these cells characterized by increased basal inflammation and a robust inflammatory response to inflammatory mediators. Recent studies have indicated that these responses depend on activation of the unfolded protein response (UPR). This review discusses the contribution of airway epithelia and airway macrophages to CF airway inflammatory responses and specifically highlights the functional importance of the UPR pathway mediated by IRE1/XBP-1 in these processes. These findings suggest that targeting the IRE1/XBP-1 UPR pathway may be a therapeutic strategy for CF airway disease.

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.

Involvement of Nrf2 in proteasome inhibition-mediated induction of ORP150 in thyroid cancer cells

Oxygen-regulated protein 150 (ORP150) is an inducible ER chaperone by numerous cellular insults and sustains cellular viability. We have previously reported that ORP150 is differentially induced in a panel thyroid cancer cells and represents as an unwanted molecular consequence during exposure to proteasome inhibition. However, the molecular basis for induction of ORP150 by proteasome inhibitors in thyroid cancer cells remains unclear. In the current study, we found that −421/−307 and −243/+53 regions at the ORP150 gene were responsible for its transactivation by MG132 in thyroid cancer cells. Nrf2 directly transactivated the ORP150 gene by direct binding with the −421/−307 region. Nrf2 also indirectly activated OPR150 transcription via facilitating recruitment of ATF4 to the −243/+53 region. Collectively, this study highlights the molecular mechanism by which proteasome inhibition stimulates ORP150 expression via Nrf2 in thyroid cancer cells.

Caspase-12 is involved in stretch-induced apoptosis mediated endoplasmic reticulum stress

It is well recognized that mandibular growth, which is caused by a variety of functional appliances, is considered to be the result of both neuromuscular and skeletal adaptations. Accumulating evidence has demonstrated that apoptosis plays an important role in the adaptation of skeletal muscle function. However, the underlying mechanism of apoptosis that is induced by stretch continues to be incompletely understood. Endoplasmic reticulum stress (ERS), a newly defined signaling pathway, initiates apoptosis. This study seeks to determine if caspase-12 is involved in stretch-induced apoptosis mediated endoplasmic reticulum stress in myoblast and its underlying mechanism. Apoptosis was assessed by Hochest staining, DAPI staining and annexin V binding and PI staining. ER chaperones, such as GRP78, CHOP and caspase-12, were determined by reverse transcription polymerase chain reaction (RT-PCR) and Western blot. Furthermore, caspase-12 inhibitor was used to value the mechanism of the caspase-12 pathway. Apoptosis of myoblast, which is subjected to cyclic stretch, was observed in a time-dependent manner. We found that GRP78 mRNA and protein were significantly increased and CHOP and caspase-12 were activated in myoblast that was exposed to cyclic stretch. Caspase-12 inhibition reduced stretch-induced apoptosis, and caspase-12 activated caspase-3 to induce apoptosis. We concluded that caspase-12 played an important role in stretch-induced apoptosis that is associated by endoplasmic reticulum stress by activating caspase-3.

ATF6 Decreases Myocardial Ischemia/Reperfusion Damage and Links ER Stress and Oxidative Stress Signaling Pathways in the Heart

RATIONALE

Endoplasmic reticulum (ER) stress causes the accumulation of misfolded proteins in the ER, activating the transcription factor, ATF6 (activating transcription factor 6 alpha), which induces ER stress response genes. Myocardial ischemia induces the ER stress response; however, neither the function of this response nor whether it is mediated by ATF6 is known.

OBJECTIVE

Here, we examined the effects of blocking the ATF6-mediated ER stress response on ischemia/reperfusion (I/R) in cardiac myocytes and mouse hearts.

METHODS AND RESULTS

Knockdown of ATF6 in cardiac myocytes subjected to I/R increased reactive oxygen species and necrotic cell death, both of which were mitigated by ATF6 overexpression. Under nonstressed conditions, wild-type and ATF6 knockout mouse hearts were similar. However, compared with wild-type, ATF6 knockout hearts showed increased damage and decreased function after I/R. Mechanistically, gene array analysis showed that ATF6, which is known to induce genes encoding ER proteins that augment ER protein folding, induced numerous oxidative stress response genes not previously known to be ATF6-inducible. Many of the proteins encoded by the ATF6-induced oxidative stress genes identified here reside outside the ER, including catalase, which is known to decrease damaging reactive oxygen species in the heart. Catalase was induced by the canonical ER stressor, tunicamycin, and by I/R in cardiac myocytes from wild-type but not in cardiac myocytes from ATF6 knockout mice. ER stress response elements were identified in the catalase gene and were shown to bind ATF6 in cardiac myocytes, which increased catalase promoter activity. Overexpression of catalase, in vivo, restored ATF6 knockout mouse heart function to wild-type levels in a mouse model of I/R, as did adeno-associated virus 9-mediated ATF6 overexpression.

CONCLUSIONS

ATF6 serves an important role as a previously unappreciated link between the ER stress and oxidative stress gene programs, supporting a novel mechanism by which ATF6 decreases myocardial I/R damage.