ER stress signaling by regulated proteolysis of ATF6

Endoplasmic reticulum stress-a key guardian in cancer

Endoplasmic reticulum stress (ERS) is a cellular stress response characterized by excessive contraction of the endoplasmic reticulum (ER). It is a pathological hallmark of many diseases, such as diabetes, obesity, and neurodegenerative diseases. In the unique growth characteristic and varied microenvironment of cancer, high levels of stress are necessary to maintain the rapid proliferation and metastasis of tumor cells. This process is closely related to ERS, which enhances the ability of tumor cells to adapt to unfavorable environments and promotes the malignant progression of cancer. In this paper, we review the roles and mechanisms of ERS in tumor cell proliferation, apoptosis, metastasis, angiogenesis, drug resistance, cellular metabolism, and immune response. We found that ERS can modulate tumor progression via the unfolded protein response (UPR) signaling of IRE1, PERK, and ATF6. Targeting the ERS may be a new strategy to attenuate the protective effects of ERS on cancer. This manuscript explores the potential of ERS-targeted therapies, detailing the mechanisms through which ERS influences cancer progression and highlighting experimental and clinical evidence supporting these strategies. Through this review, we aim to deepen our understanding of the role of ER stress in cancer development and provide new insights for cancer therapy.

Downregulating miRNA-199a-5p exacerbates fluorouracil-induced cardiotoxicity by activating the ATF6 signaling pathway

BACKGROUND

Fluorouracil (5-FU) might produce serious cardiac toxic reactions. miRNA-199a-5p is a miRNA primarily expressed in myocardial cells and has a protective effect on vascular endothelium. Under hypoxia stress, the expression level of miRNA-199a-5p was significantly downregulated and is closely related to cardiovascular events such as coronary heart disease, heart failure, and hypertension. We explored whether 5-FU activates the endoplasmic reticulum stress ATF6 pathway by regulating the expression of miRNA-199a-5p in cardiac toxicity.

METHODS

This project established a model of primary cardiomyocytes derived from neonatal rats and treated them with 5-FU in vitro. The expression of miRNA-199a-5p and its regulation were explored in vitro and in vivo.

RESULTS

5-FU decreases the expression of miRNA-199a-5p in cardiomyocytes, activates the endoplasmic reticulum stress ATF6 pathway, and increases the expression of GRP78 and ATF6, affecting the function of cardiomyocytes, and induces cardiac toxicity. The rescue assay further confirmed that miRNA-199a-5p supplementation can reduce the cardiotoxicity caused by 5-FU, and its protective effect on cardiomyocytes depends on the downregulation of the endoplasmic reticulum ATF6 signaling pathway.

CONCLUSIONS

5-FU can down-regulate expression of miRNA-199a-5p, then activate the endoplasmic reticulum stress ATF6 pathway, increase the expression of GRP78 and ATF6, affect the function of cardiomyocytes, and induce cardiac toxicity.

KCNH2A561V Heterozygous Mutation Inhibits KCNH2 Protein Expression via The Activation of UPR Mediated by ATF6

The potassium channel protein KCNH2 is encoded by KCNH2 gene, and there are more than 300 mutations of KCNH2. Unfolded protein response (UPR) is typically initiated in response to an accumulation of unfolded and/or misfolded proteins in the endoplasmic reticulum (ER). The present study aimed to explore the UPR process and the role of activating transcription factor 6 (ATF6) in the abnormal expression of potassium voltage-gated channel subfamily H member 2 (KCNH2)A561V. The wild-type (wt) KCNH2 and A561V mutant KCNH2 was constructed with his-tag. The 293 cells were used and divided into KCNH2wt+KCNH2A561V, KCNH2wt and KCNH2A561V groups. The expression levels of ATF6 and KCNH2 in different groups were detected by Western blotting, reverse transcription-quantitative PCR, immunofluorescence and immuno-coprecipitation assays. The protein types and abundance of immuno-coprecipitation samples were analyzed by mass spectrometry. The proteomic analysis of the mass spectrometry results was carried out by using the reactome database and GO (Gene Ontology) tool. The mRNA expression levels of KCNH2 and ATF6 in the KCNH2wt+KCNH2A561V group were higher compared with the KCNH2A561V group. However, the full-length protein expression of ATF6 was inhibited, indicating that ATF6 was highly activated and a substantial number of ATF6 was sheared in KCNH2wt+KCNH2A561V group compared with control group. Furthermore, A561V-KCNH2 mutation leading to the accumulation of the immature form of KCNH2 (135 kDa bands) in ER, resulting in the reduction of the ratio of 155 kDa/135 kDa. In addition, the abundance of UPR-related proteins in the KCNH2A561V group was higher compared with the KCNH2wt+KCNH2A561V group. The 'cysteine biosynthetic activity' of GO:0019344 process and the 'positive regulation of cytoplasmic translation activity' of GO:2000767 process in the KCNH2A561V group were higher compared with the KCNH2wt+KCNH2A561V group. Hence, co-expression of wild-type and A561V mutant KCNH2 in 293 cells activated the UPR process, which led to the inhibition of protein translation and synthesis, in turn inhibiting the expression of KCNH2. These results provided a theoretical basis for clinical treatment of Long QT syndrome.

Essential requirement for IER3IP1 in B cell development

In a forward genetic screen of mice with N-ethyl-N-nitrosourea-induced mutations for aberrant immune function, we identified animals with low percentages of B220+ cells in the peripheral blood. The causative mutation was in Ier3ip1, encoding immediate early response 3 interacting protein 1 (IER3IP1), an endoplasmic reticulum membrane protein mutated in an autosomal recessive neurodevelopmental disorder termed Microcephaly with simplified gyration, Epilepsy and permanent neonatal Diabetes Syndrome (MEDS) in humans. However, no immune function for IER3IP1 had previously been reported. The viable hypomorphic Ier3ip1 allele uncovered in this study, identical to a reported IER3IP1 variant in a MEDS patient, reveals an essential hematopoietic-intrinsic role for IER3IP1 in B cell development and function. We show that IER3IP1 forms a complex with the Golgi transmembrane protein 167A and limits activation of the unfolded protein response mediated by inositol-requiring enzyme-1α and X-box binding protein 1 in B cells. Our findings suggest that B cell deficiency may be a feature of MEDS.

The potential roles of ATF family in the treatment of Alzheimer's disease

Activating transcription factors, ATFs, is a family of transcription factors that activate gene expression and transcription by recognizing and combining the cAMP response element binding proteins (CREB). It is present in various viruses as a cellular gene promoter. ATFs is involved in regulating the mammalian gene expression that is associated with various cell physiological processes. Therefore, ATFs play an important role in maintaining the intracellular homeostasis. ATF2 and ATF3 is mostly involved in mediating stress responses. ATF4 regulates the oxidative metabolism, which is associated with the survival of cells. ATF5 is presumed to regulate apoptosis, and ATF6 is involved in the regulation of endoplasmic reticulum stress (ERS). ATFs is actively studied in oncology. At present, there has been an increasing amount of research on ATFs for the treatment of neurological diseases. Here, we have focused on the different types of ATFs and their association with Alzheimer's disease (AD). The level of expression of different ATFs have a significant difference in AD patients when compared to healthy control. Recent studies have suggested that ATFs are implicated in the pathogenesis of AD, such as neuronal repair, maintenance of synaptic activity, maintenance of cell survival, inhibition of apoptosis, and regulation of stress responses. In this review, the potential role of ATFs for the treatment of AD has been highlighted. In addition, we have systematically reviewed the progress of research on ATFs in AD. This review will provide a basic and innovative understanding on the pathogenesis and treatment of AD.

Activation of the UPR sensor ATF6α is regulated by its redox-dependent dimerization and ER retention by ERp18

Membrane and secretory proteins are synthesized in the endoplasmic reticulum (ER). Perturbations to ER function disrupts protein folding, causing misfolded proteins to accumulate, a condition known as ER stress. Cells adapt to stress by activating the unfolded protein response (UPR), which ultimately restores proteostasis. A key player in the UPR response is ATF6α, which requires release from ER retention and modulation of its redox status during activation. Here, we report that ER stress promotes formation of a specific ATF6α dimer, which is preferentially trafficked to the Golgi for processing. We show that ERp18 regulates ATF6α by mitigating its dimerization and trafficking to the Golgi and identify redox-dependent oligomerization of ATF6α as a key mechanism regulating its function during the UPR.

The unfolded protein response (UPR) maintains cellular proteostasis during stress by activating sensors located to the endoplasmic reticulum (ER) membrane. A major sensor for this response, ATF6α, is activated by release from ER retention and trafficking to the Golgi, where it is cleaved to generate a bZIP transactivator to initiate a transcriptional response. The reduction of a disulfide in monomeric ATF6α is thought to be necessary for release from retention, trafficking, and proteolysis. Here we show that, following ER stress, ATF6α undergoes a redox switch to form a disulfide bonded dimer, which traffics to the Golgi for cleavage by the S1P protease. Additionally, we find that overexpression of ERp18 attenuates dimer formation thereby limiting Golgi trafficking. Our results provide mechanistic insight into activation of the ATF6α pathway, revealing an unexpected role for redox-dependent oligomerization prior to Golgi trafficking.

An in vitro vesicle formation assay reveals cargo clients and factors that mediate vesicular trafficking

Protein sorting in the secretory pathway is a fundamentally important cellular process, but the clients of a specific cargo sorting machinery remains largely underinvestigated. Here, utilizing a vesicle formation assay to profile proteins associated with vesicles, we identified cytosolic proteins that are associated with vesicle membranes in a GTP-dependent manner or that interact with GTP-bound Sar1A. We found that two of them, FAM84B and PRRC1, regulate anterograde trafficking. Moreover, we revealed specific clients of two export adaptors, SURF4 and ERGIC53. These analyses demonstrate that our approach is powerful to identify factors that regulate vesicular trafficking and to uncover clients of specific cargo receptors, providing a robust method to reveal insights into the secretory pathway.

The fidelity of protein transport in the secretory pathway relies on the accurate sorting of proteins to their correct destinations. To deepen our understanding of the underlying molecular mechanisms, it is important to develop a robust approach to systematically reveal cargo proteins that depend on specific sorting machinery to be enriched into transport vesicles. Here, we used an in vitro assay that reconstitutes packaging of human cargo proteins into vesicles to quantify cargo capture. Quantitative mass spectrometry (MS) analyses of the isolated vesicles revealed cytosolic proteins that are associated with vesicle membranes in a GTP-dependent manner. We found that two of them, FAM84B (also known as LRAT domain containing 2 or LRATD2) and PRRC1, contain proline-rich domains and regulate anterograde trafficking. Further analyses revealed that PRRC1 is recruited to endoplasmic reticulum (ER) exit sites, interacts with the inner COPII coat, and its absence increases membrane association of COPII. In addition, we uncovered cargo proteins that depend on GTP hydrolysis to be captured into vesicles. Comparing control cells with cells depleted of the cargo receptors, SURF4 or ERGIC53, we revealed specific clients of each of these two export adaptors. Our results indicate that the vesicle formation assay in combination with quantitative MS analysis is a robust and powerful tool to uncover novel factors that mediate vesicular trafficking and to uncover cargo clients of specific cellular factors.

The Ca2+ -binding protein sorcin stimulates transcriptional activity of the unfolded protein response mediator ATF6

Sorcin is a calcium-binding protein involved in maintaining endoplasmic reticulum (ER) Ca²⁺ stores. We have previously shown that overexpressing sorcin under the rat insulin promoter was protective against high-fat diet-induced pancreatic beta-cell dysfunction in vivo. Activating transcription factor 6 (ATF6) is a key mediator of the unfolded protein response (UPR) that provides cellular protection during the progression of ER stress. Here, using nonexcitable HEK293 cells, we show that sorcin overexpression increased ATF6 signalling, whereas sorcin knock out caused a reduction in ATF6 transcriptional activity and increased ER stress. Altogether, our data suggest that sorcin downregulation during lipotoxic stress may prevent full ATF6 activation and a normal UPR during the progression of obesity and insulin resistance.

The role of ATF6 in Cr(VI)-induced apoptosis in DF-1 cells

Hexavalent chromium (Cr(VI)) is a common heavy metal pollutant in environment and has been proved possessing the cytotoxicity. In this study, we aimed to investigate the role of activating transcription factor 6 (ATF-6) in apoptosis of chicken embryo fibroblasts cell line (DF-1) induced by Cr(VI). Firstly, DF-1 cells were exposed to Cr(VI) to establish the cytotoxicity model, then the cell apoptosis and ATF-6 protein level were analyzed. By silencing ATF-6 gene, changes of the apoptosis rate and apoptotic proteins were examined. To further explore the regulatory mechanism of ATF-6, endoplasmic reticulum (ER) stress, mitochondrial function, reactive oxygen species (ROS) level, as well as the related pathway were evaluated. Results showed that Cr(VI) can result in DF-1 cell apoptosis, along with mitochondrial membrane potential (MMP) reducing and ER stress. Meanwhile, ATF-6 silencing lowered the apoptosis rate and ER stress level, showing with the decrease of XBP-1, PERK, GRP78, Caspase-12, Cleaved Caspase-3 and the increase of Bcl-2. Further analysis found that ATF-6 silencing down-regulated ROS and caused MMP loss, suggesting that ATF-6 silencing inhibited Cr(VI)-induced mitochondrial damage. In conclusion, this study indicate that ATF-6 plays an important regulatory role in Cr(VI)-induced DF-1 cell apoptosis through the ER stress and mitochondrial pathway.

Induction of the Unfolded Protein Response during Bovine Alphaherpesvirus 1 Infection

Bovine herpesvirus 1 (BoHV-1) is an alphaherpesvirus that causes great economic losses in the cattle industry. Herpesvirus infection generally induces endoplasmic reticulum (ER) stress, and the unfolded protein response (UPR) in infected cells. However, it is not clear whether ER stress and UPR can be induced by BoHV-1 infection. Here, we found that ER stress induced by BoHV-1 infection could activate all three UPR sensors (the activating transcription factor 6 (ATF6), the inositol-requiring enzyme 1 (IRE1), and the protein kinase RNA-like ER kinase (PERK)) in MDBK cells. During BoHV-1 infection, the ATF6 pathway of UPR did not affect viral replication. However, both knockdown and specific chemical inhibition of PERK attenuated the BoHV-1 proliferation, and chemical inhibition of PERK significantly reduced the viral replication at the post-entry step of the BoHV-1 life cycle. Furthermore, knockdown of IRE1 inhibits BoHV-1 replication, indicating that the IRE1 pathway may promote viral replication. Further study revealed that BoHV-1 replication was enhanced by IRE1 RNase activity inhibition at the stage of virus post-entry in MDBK cells. Furthermore, IRE1 kinase activity inhibition and RNase activity enhancement decrease BoHV1 replication via affecting the virus post-entry step. Our study revealed that BoHV-1 infection activated all three UPR signaling pathways in MDBK cells, and BoHV-1-induced PERK and IRE1 pathways may promote viral replication. This study provides a new perspective for the interactions of BoHV-1 and UPR, which is helpful to further elucidate the mechanism of BoHV-1 pathogenesis.

ALA protects against ERS-mediated apoptosis in a cochlear cell model with low citrate synthase expression

A/J mouse is a model of age-related hearing loss (AHL). Mutation in the citrate synthase (Cs) gene of the mouse plays an important role in the hearing loss and degeneration of cochlear cells. To investigate the pathogenesis of cochlear cell damage in A/J mice resulted from Cs mutation, we downregulated the expression level of CS in HEI-OC1, a cell line of mouse cochlea, by shRNA. The results showed that low CS expression led to low ability of cell proliferation. Further study revealed an increase level of reactive oxygen species (ROS), activation of ATF6 mediated endoplasmic reticulum stress (ERS) and high expression levels of caspase12 and Bax in the cells. Moreover, the AEBSF, an ATF6 inhibitor, could reduce the expression levels of caspase-12 and Bax by inhibiting the hydrolysis of ATF6 in the cells. Finally, antioxidant alpha-lipoic acid (ALA) reduced the ROS levels and the apoptotic signals in the cell model with low CS expression. We therefore conclude that the ERS mediated apoptosis, which is triggered by ROS, may be involved in the cell degeneration in the cochleae of A/J mice.

Activation of protein kinase R in the manganese-induced apoptosis of PC12 cells

Manganese neurotoxicity leads to Parkinson-like symptoms associated with the apoptotic cell death of dopaminergic neurons. Protein kinase R (PKR) is a serine/threonine-specific protein kinase that has been implicated in several cellular signal transduction pathways, including the induction of apoptosis. Here, we investigated the role of PKR in the manganese-induced apoptosis of dopamine-producing pheochromocytoma PC12 cells. Manganese (0.5 mM) induced the proteolytic cleavage of PKR and caspase-3, DNA fragmentation, and cell death, which were prevented by the co-treatment of PC12 cells with a PKR specific inhibitor, C16 in a concentration-dependent manner. C16 did not affect the manganese-induced activation of the c-Jun N-terminal kinase (JNK)/p38 mitogen-activated protein kinase (MAPK) pathway, indicating that PKR functions downstream of JNK and p38 MAPK. In contrast, C16 triggered the activation of the p44/42 MAPK (ERK1/2) pathway and induced hemoxygenase-1, both in the absence and presence of manganese. PKR is reportedly involved in endoplasmic reticulum (ER) stress-induced apoptosis. Manganese activated all three branches of the unfolded protein response in PC12 cells; however, this effect was very weak compared with the ER stress induced by the well-known ER stress inducers thapsigargin and tunicamycin. Moreover, C16 did not affect manganese-induced ER stress at concentrations that almost prevented caspase-3 activation and DNA fragmentation. These results suggest that PKR is involved in manganese-induced apoptotic cell death and stress response, such as the activation of the p44/42 MAPK pathway and the induction of hemoxygenase-1. Although manganese induced a faint, but typical, ER stress, these events contributed little to manganese-induced apoptosis.

The ER Unfolded Protein Response Effector, ATF6, Reduces Cardiac Fibrosis and Decreases Activation of Cardiac Fibroblasts

Activating transcription factor-6 α (ATF6) is one of the three main sensors and effectors of the endoplasmic reticulum (ER) stress response and, as such, it is critical for protecting the heart and other tissues from a variety of environmental insults and disease states. In the heart, ATF6 has been shown to protect cardiac myocytes. However, its roles in other cell types in the heart are unknown. Here we show that ATF6 decreases the activation of cardiac fibroblasts in response to the cytokine, transforming growth factor β (TGFβ), which can induce fibroblast trans-differentiation into a myofibroblast phenotype through signaling via the TGFβ–Smad pathway. ATF6 activation suppressed fibroblast contraction and the induction of α smooth muscle actin (αSMA). Conversely, fibroblasts were hyperactivated when ATF6 was silenced or deleted. ATF6 thus represents a novel inhibitor of the TGFβ–Smad axis of cardiac fibroblast activation.

Endoplasmic reticulum stress signaling in cancer and neurodegenerative disorders: Tools and strategies to understand its complexity

The endoplasmic reticulum (ER) comprises a network of tubules and vesicles that constitutes the largest organelle of the eukaryotic cell. Being the location where most proteins are synthesized and folded, it is crucial for the upkeep of cellular homeostasis. Disturbed ER homeostasis triggers the activation of a conserved molecular machinery, termed the unfolded protein response (UPR), that comprises three major signaling branches, initiated by the protein kinase RNA-like endoplasmic reticulum kinase (PERK), inositol-requiring enzyme 1 (IRE1) and the activating transcription factor 6 (ATF6). Given the impact of this intricate signaling network upon an extensive list of cellular processes, including protein turnover and autophagy, ER stress is involved in the onset and progression of multiple diseases, including cancer and neurodegenerative disorders. There is, for this reason, an increasing number of publications focused on characterizing and/or modulating ER stress, which have resulted in a wide array of techniques employed to study ER-related molecular events. This review aims to sum up the essentials on the current knowledge of the molecular biology of endoplasmic reticulum stress, while highlighting the available tools used in studies of this nature.

Sledgehammer to Scalpel: Broad Challenges to the Heart and Other Tissues Yield Specific Cellular Responses via Transcriptional Regulation of the ER-Stress Master Regulator ATF6α

There are more than 2000 transcription factors in eukaryotes, many of which are subject to complex mechanisms fine-tuning their activity and their transcriptional programs to meet the vast array of conditions under which cells must adapt to thrive and survive. For example, conditions that impair protein folding in the endoplasmic reticulum (ER), sometimes called ER stress, elicit the relocation of the ER-transmembrane protein, activating transcription factor 6α (ATF6α), to the Golgi, where it is proteolytically cleaved. This generates a fragment of ATF6α that translocates to the nucleus, where it regulates numerous genes that restore ER protein-folding capacity but is degraded soon after. Thus, upon ER stress, ATF6α is converted from a stable, transmembrane protein, to a rapidly degraded, nuclear protein that is a potent transcription factor. This review focuses on the molecular mechanisms governing ATF6α location, activity, and stability, as well as the transcriptional programs ATF6α regulates, whether canonical genes that restore ER protein-folding or unexpected, non-canonical genes affecting cellular functions beyond the ER. Moreover, we will review fascinating roles for an ATF6α isoform, ATF6β, which has a similar mode of activation but, unlike ATF6α, is a long-lived, weak transcription factor that may moderate the genetic effects of ATF6α.

Type I interferons and endoplasmic reticulum stress in health and disease

Type I interferons (IFNs) comprise of pro-inflammatory cytokines created, as well as sensed, by all nucleated cells with the main objective of blocking pathogens-driven infections. Owing to this broad range of influence, type I IFNs also exhibit critical functions in many sterile inflammatory diseases and immunopathologies, especially those associated with endoplasmic reticulum (ER) stress-driven signaling pathways. Indeed, over the years accumulating evidence has indicated that the presence of ER stress can influence the production, or sensing of, type I IFNs induced by perturbations like pattern recognition receptor (PRR) agonists, infections (bacterial, viral or parasitic) or autoimmunity. In this article we discuss the link between type I IFNs and ER stress in various diseased contexts. We describe how ER stress regulates type I IFNs production or sensing, or how type I IFNs may induce ER stress, in various circumstances like microbial infections, autoimmunity, diabetes, cancer and other ER stress-related contexts.

Control of Protein Homeostasis in the Early Secretory Pathway: Current Status and Challenges

: Discrimination between properly folded proteins and those that do not reach this state is necessary for cells to achieve functionality. Eukaryotic cells have evolved several mechanisms to ensure secretory protein quality control, which allows efficiency and fidelity in protein production. Among the actors involved in such process, both endoplasmic reticulum (ER) and the Golgi complex play prominent roles in protein synthesis, biogenesis and secretion. ER and Golgi functions ensure that only properly folded proteins are allowed to flow through the secretory pathway while improperly folded proteins have to be eliminated to not impinge on cellular functions. Thus, complex quality control and degradation machineries are crucial to prevent the toxic accumulation of improperly folded proteins. However, in some instances, improperly folded proteins can escape the quality control systems thereby contributing to several human diseases. Herein, we summarize how the early secretory pathways copes with the accumulation of improperly folded proteins, and how insufficient handling can cause the development of several human diseases. Finally, we detail the genetic and pharmacologic approaches that could be used as potential therapeutic tools to treat these diseases.

ERp18 regulates activation of ATF6α during unfolded protein response

Activation of the ATF6α signaling pathway is initiated by trafficking of ATF6α from the ER to the Golgi apparatus. Its subsequent proteolysis releases a transcription factor that translocates to the nucleus causing downstream gene activation. How ER retention, Golgi trafficking, and proteolysis of ATF6α are regulated and whether additional protein partners are required for its localization and processing remain unresolved. Here, we show that ER-resident oxidoreductase ERp18 associates with ATF6α following ER stress and plays a key role in both trafficking and activation of ATF6α. We find that ERp18 depletion attenuates the ATF6α stress response. Paradoxically, ER stress accelerates trafficking of ATF6α to the Golgi in ERp18-depleted cells. However, the translocated ATF6α becomes aberrantly processed preventing release of the soluble transcription factor. Hence, we demonstrate that ERp18 monitors ATF6α ER quality control to ensure optimal processing following trafficking to the Golgi.

Inhibition of double-stranded RNA-dependent protein kinase prevents oxytosis and ferroptosis in mouse hippocampal HT22 cells

Double-stranded RNA-dependent protein kinase (PKR) is a component of signal transduction pathways mediating various stress signals including oxidative stress and endoplasmic reticulum (ER) stress and is suggested to be implicated in several neurodegenerative diseases. Cell death in neurodegenerative conditions has been linked to oxidative stress; however, the involvement of PKR in endogenous oxidative stress such as oxytosis and ferroptosis which is quite distinct from classical apoptosis remains unknown. We investigated here the effect of a PKR inhibitor C16 (an imidazole-oxindole derivative) on oxytosis and ferroptosis in cultured HT22 mouse hippocampal cells. C16 prevented glutamate- and erastin-induced cell death, reactive oxygen species accumulation, Ca²⁺ influx, phosphorylation of inositol-requiring enzyme 1 (IRE1), one of the three branches of ER stress signaling and its downstream signaling components. On the other hand, C16 did not prevent oxidative stress-induced heme oxygenase-1 expression; instead, C16 activated the extracellular signal-regulated kinase pathway. The protective effect of C16 is diminished in PKR knockout HT22 cells. Real time measurements of the oxygen consumption rate and extracellular acidification rate over a long period of time leading to cell death showed that C16 partially prevented erastin-induced mitochondrial and glycolytic dysfunction. These results suggest that PKR is an important component of oxytosis and ferroptosis and the inhibition of PKR is neuroprotective against endogenous oxidative stress-induced cell death and provide an effective strategy for neuroprotection.

The unfolded protein response induced by Tembusu virus infection

Background

Tembusu virus (TMUV), classified in the genus Flavivirus, causes reduced egg production and neurological problems in poultry. Flavivirus replication depends on the host endoplasmic reticulum (ER) and induces ER stress that leads to activation of the cellular unfolded protein response (UPR), an important signalling pathway that regulates many biological functions involved in viral pathogenesis and innate immunity. However, the mechanism of TMUV-induced UPR activation remains unclear.

Results

In this study, we systematically investigated the three UPR pathways in TMUV-infected BHK-21 cells. Our results showed that expression of glucose-related protein 78 (GRP78) and GRP94 was upregulated during the course of TMUV infection. We then demonstrated that TMUV activated the PERK pathway in the early stage of infection, resulting in upregulation of ATF4, GADD34 and CHOP, with CHOP induction leading to caspase-3 activation. We also found the IRE1 pathway to be activated, leading to splicing of X box binding protein 1 (XBP1) mRNA and enhanced expression of p58^IPK. Finally, we observed increased expression of ATF6 and activity of ER stress-response elements, suggesting stimulation of the ATF6 pathway. In addition, ATF6 pathway activation correlated with the induction of downstream chaperones calnexin, calreticulin, ERp57 and PDI. UPR activity was also observed by the marked elevation in GRP78 and sXBP1 levels in TMUV-infected DF-1 cells.

Conclusions

This is the first report that TMUV infection-induced ER stress activates three branches of the UPR, and these results lay the foundation for elucidating the pathogenesis of TMUV and understanding the inherent mechanism of TMUV infection as well as the host response.

Emerging role of Unfolded Protein Response (UPR) mediated proteotoxic apoptosis in diabetes

Endoplasmic reticulum (ER) is a crucial single membrane organelle that acts as a quality control system for cellular proteins as it is intricately involved in their synthesis, folding and trafficking to the respective targets. Type 2 diabetes is characterized by enhanced blood glucose level that promotes insulin resistance and hampers cellular glucose metabolism. Hyperglycemia provokes mitochondrial ROS production and glycation of proteins which exert a tremendous load on ER for conventional refolding of misfolded/unfolded and nascent proteins that perturb ER homeostasis resulting in apoptotic cell death. Impairment in ER functions is suspected to be through specific ER membrane-bound proteins known as Unfolded Protein Response (UPR) sensor proteins. Conformational changes in these proteins induce oligomerization and cross-autophosphorylation which facilitate processes required for the restoration of ER homeostatic imbalance. Multiple studies have reported the involvement of UPR mediated autophagy and apoptotic pathways in the progression of metabolic disorders including diabetes, cardiac ischemia/reperfusion injury and hypoxia-mediated cell death. In this review, the involvement of UPR pathways in the progression of diabetes associated complications have been addressed, which underscores molecular crosstalks during neuropathy, nephropathy, hepatic injury and retinopathy. A better understanding of these molecular interventions may reveal advanced therapeutic approaches for preventing diabetic comorbidities. The article also highlights the importance of phytochemicals that are emerging as novel ER stress inhibitors and are being explored for targeted interaction in preventing cell death responses during diabetes.

Fluorescence Activated Cell Sorting (FACS) in Genome-Wide Genetic Screening of Membrane Trafficking

About one-third of cellular proteins in eukaryotic cells are localized to membrane-enclosed organelles in the endomembrane system. Trafficking of these membrane proteins (including soluble lumenal proteins) among the organelles is mediated by small sac-like vesicles. Vesicle-mediated membrane trafficking regulates a broad range of biological processes, many of which are still poorly understood at the molecular level. A powerful approach to dissect a vesicle-mediated membrane trafficking pathway is unbiased genome-wide genetic screening, which only recently became possible in mammalian cells with the isolation of haploid human cell lines and the development of CRISPR-Cas9 genome editing. Here, we describe a FACS-based method to select populations of live mutant cells based on the surface levels of endogenous proteins or engineered reporters. Collection of these mutant populations enables subsequent deep sequencing and bioinformatics analysis to identify genes that regulate the trafficking pathway. This method can be readily adapted to genetically dissect a broad range of mammalian membrane trafficking processes using haploid genetics or CRISPR-Cas9 screens. © 2018 by John Wiley & Sons, Inc.

A novel role of ER stress signal transducer ATF6 in regulating enterovirus A71 viral protein stability

Background

Due to limited coding capacity of viral genome, enterovirus A71 (EV-A71) co-opts host nuclear proteins for its replication. Upon ER stress, the ER-localized 90 kDa activating transcription factor 6 (p90ATF6) is proteolytically cleaved to produce the transcriptionally active amino-terminal 50 kDa (p50ATF6) product where it enters the nucleus to activate a subset of unfolded protein response and ER-associated degradation (also known as ERAD) genes. During EV-A71 infection, however, this p50ATF6 product was not detected in the nucleus, and its downstream target genes were not activated.

Methods

We examined the role of ATF6 during EV-A71 infection, including its cleavage process and its role in viral life cycle by silencing or overexpressing ATF6.

Results

We showed that a potential cleavage in the middle of p90ATF6 produced an amino-terminal ~ 45 kDa fragment in a viral protease-independent but EV-A71-dependent manner. The disappearance of ATF6 was not restricted to a specific strain of EV-A71 or cell type, and was not simply caused by picornavirus-mediated global translational shutoff. This cleavage of ATF6, which was most likely mediated by the host response, was nevertheless independent of both cellular caspases and XBP1-associated proteasomes. The silencing of ATF6 expression by small interfering RNA suppressed viral titers due to reduced viral protein stability. This effect was markedly restored by the ectopic expression of p90ATF6.

Conclusion

Our findings indicate that ATF6 plays a distinct role in viral protein stability and that the host uses different cleavage strategies, rather than conventional cleavage by generating p50ATF6, to combat viral infection.

Electronic supplementary material

The online version of this article (10.1186/s12929-018-0412-x) contains supplementary material, which is available to authorized users.

Noncanonical Fungal Autophagy Inhibits Inflammation in Response to IFN-γ via DAPK1

Defects in a form of noncanonical autophagy, known as LC3-associated phagocytosis (LAP), lead to increased inflammatory pathology during fungal infection. Although LAP contributes to fungal degradation, the molecular mechanisms underlying LAP-mediated modulation of inflammation are unknown. We describe a mechanism by which inflammation is regulated during LAP through the death-associated protein kinase 1 (DAPK1). The ATF6/C/EBP-β/DAPK1 axis activated by IFN-γ not only mediates LAP to Aspergillus fumigatus but also concomitantly inhibits Nod-like receptor protein 3 (NLRP3) activation and restrains pathogenic inflammation. In mouse models and patient samples of chronic granulomatous disease, which exhibit defective autophagy and increased inflammasome activity, IFN-γ restores reduced DAPK1 activity and dampens fungal growth. Additionally, in a cohort of hematopoietic stem cell-transplanted patients, a genetic DAPK1 deficiency is associated with increased inflammation and heightened aspergillosis susceptibility. Thus, DAPK1 is a potential drugable player in regulating the inflammatory response during fungal clearance initiated by IFN-γ.

TM7SF3, a novel p53-regulated homeostatic factor, attenuates cellular stress and the subsequent induction of the unfolded protein response

Earlier reported small interfering RNA (siRNA) high-throughput screens, identified seven-transmembrane superfamily member 3 (TM7SF3) as a novel inhibitor of pancreatic β-cell death. Here we show that TM7SF3 maintains protein homeostasis and promotes cell survival through attenuation of ER stress. Overexpression of TM7SF3 inhibits caspase 3/7 activation. In contrast, siRNA-mediated silencing of TM7SF3 accelerates ER stress and activation of the unfolded protein response (UPR). This involves inhibitory phosphorylation of eukaryotic translation initiation factor 2α activity and increased expression of activating transcription factor-3 (ATF3), ATF4 and C/EBP homologous protein, followed by induction of apoptosis. This process is observed both in human pancreatic islets and in a number of cell lines. Some of the effects of TM7SF3 silencing are evident both under basal conditions, in otherwise untreated cells, as well as under different stress conditions induced by thapsigargin, tunicamycin or a mixture of pro-inflammatory cytokines (tumor necrosis factor alpha, interleukin-1 beta and interferon gamma). Notably, TM7SF3 is a downstream target of p53: activation of p53 by Nutlin increases TM7SF3 expression in a time-dependent manner, although silencing of p53 abrogates this effect. Furthermore, p53 is found in physical association with the TM7SF3 promoter. Interestingly, silencing of TM7SF3 promotes p53 activity, suggesting the existence of a negative-feedback loop, whereby p53 promotes expression of TM7SF3 that acts to restrict p53 activity. Our findings implicate TM7SF3 as a novel p53-regulated pro-survival homeostatic factor that attenuates the development of cellular stress and the subsequent induction of the UPR.

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.

Hsp70 May Be a Molecular Regulator of Schistosome Host Invasion

Schistosomiasis is a debilitating disease that affects over 240 million people worldwide and is considered the most important neglected tropical disease following malaria. Free-swimming freshwater cercariae, one of the six morphologically distinct schistosome life stages, infect humans by directly penetrating through the skin. Cercariae identify and seek the host by sensing chemicals released from human skin. When they reach the host, they burrow into the skin with the help of proteases and other contents released from their acetabular glands and transform into schistosomula, the subsequent larval worm stage upon skin infection. Relative to host invasion, studies have primarily focused on the nature of the acetabular gland secretions, immune response of the host upon exposure to cercariae, and cercaria-schistosomulum transformation methods. However, the molecular signaling pathways involved from host-seeking through the decision to penetrate skin are not well understood. We recently observed that heat shock factor 1 (Hsf1) is localized to the acetabular glands of infectious schistosome cercariae, prompting us to investigate a potential role for heat shock proteins (HSPs) in cercarial invasion. In this study, we report that cercarial invasion behavior, similar to the behavior of cercariae exposed to human skin lipid, is regulated through an Hsp70-dependent process, which we show by using chemical agents that target Hsp70. The observation that biologically active protein activity modulators can elicit a direct and clear behavioral change in parasitic schistosome larvae is itself interesting and has not been previously observed. This finding suggests a novel role for Hsp70 to act as a switch in the cercaria-schistosomulum transformation, and it allows us to begin elucidating the pathways associated with cercarial host invasion. In addition, because the Hsp70 protein and its structure/function is highly conserved, the model that Hsp70 acts as a behavior transitional switch could be relevant to other parasites that also undergo an invasion process and can apply more broadly to other organisms during morphological transitions. Finally, it points to a new function for HSPs in parasite/host interactions.

Parasitic schistosome worms cause morbid disease in over 240 million individuals worldwide. Acute infections with these worms can lead to Katayama fever, while chronic infections can lead to portal hypertension, enlarged abdomen, and liver damage. The infective larval stage, called cercariae, are free-swimming and can detect, seek, and penetrate human skin to enter the human host circulatory system, eventually developing into egg-laying adult worms that cause schistosomiasis. Molecular pathways associated with the initial cercarial invasion of the host, however, are largely unknown, especially with respect to the parasite-specific signals involved in host detection and subsequent decision to invade. Here, we describe a role for Hsp70 in cercarial invasion behavior. To date, only generic stimulation with skin lipid, linoleic acid or L-arginine are known to induce cercarial invasion behavior; thus, we can begin an initial investigation of molecular requirements for host invasion and environment transition for schistosomes and possibly other parasitic organisms.

Identification of Novel SCIRR69-Interacting Proteins During ER Stress Using SILAC-Immunoprecipitation Quantitative Proteomics Approach

Spinal cord injury and regeneration-related protein #69 (SCIRR69),also known as cAMP-responsive element-binding protein 3-like 2, belongs to the CREB/ATF family, some members of which play significant roles in ER stress. However, it is still not fully elucidated whether SCIRR69 involves in ER stress and its biochemical and functional roles during ER stress. In this study, we firstly treated fetal rat spinal cord neuron cells (SCN) and PC12 cells with ER stress activator thapsigargin (TG) or tunicamycin (TM) and then detected the expression pattern of SCIRR69 in response to ER stress at mRNA and protein levels using real-time PCR assay and immunoblotting. Results showed that the expression pattern of SCIRR69 was largely consistent with those of ER stress marker (ATF6, BIP and CHOP) at either mRNA level or protein level, implying that SCIRR69 may play important roles in ER stress. Subsequently, we used stable isotope labeling by amino acids in cell culture (SILAC)-immunoprecipitation quantitative proteomics to identify interaction partners of SCIRR69 during TG-induced ER stress in PC12 cells and found that transitional endoplasmic reticulum ATPase (TERA) and sideroflexin-1 (SFXN1) were potential SCIRR69-interacting proteins. The interaction between SCIRR69 and TERA or SFXN1 was validated using co-immunoprecipitation. Those results provide some clues for novel signaling nexuses that made by interactions between SCIRR69 and TERA or SFXN1. Our findings may facilitate a better understanding of the fundamental functions of SCIRR69 during ER stress.

Melanoma and the Unfolded Protein Response

The UPR (unfolded protein response) has been identified as a key factor in the progression and metastasis of cancers, notably melanoma. Several mediators of the UPR are upregulated in cancers, e.g., high levels of GRP78 (glucose-regulator protein 78 kDa) correlate with progression and poor outcome in melanoma patients. The proliferative burden of cancer induces stress and activates several cellular stress responses. The UPR is a tightly orchestrated stress response that is activated upon the accumulation of unfolded proteins within the ER (endoplasmic reticulum). The UPR is designed to mediate two conflicting outcomtes, recovery and apoptosis. As a result, the UPR initiates a widespread signaling cascade to return the cell to homeostasis and failing to achieve cellular recovery, initiates UPR-induced apoptosis. There is evidence that ER stress and subsequently the UPR promote tumourigenesis and metastasis. The complete role of the UPR has yet to be defined. Understanding how the UPR allows for adaption to stress and thereby assists in cancer progression is important in defining an archetype of melanoma pathology. In addition, elucidation of the mechanisms of the UPR may lead to development of effective treatments of metastatic melanoma.

Legionella suppresses the host unfolded protein response via multiple mechanisms

The intracellular pathogen, Legionella pneumophila, secretes ∼300 effector proteins to modulate the host environment. Given the intimate interaction between L. pneumophila and the endoplasmic reticulum, we investigated the role of the host unfolded protein response (UPR) during L. pneumophila infection. Interestingly, we show that the host identifies L. pneumophila infection as a form of endoplasmic reticulum stress and the sensor pATF6 is processed to generate pATF6(N), a transcriptional activator of downstream UPR genes. However, L. pneumophila is able to suppress the UPR and block the translation of prototypical UPR genes, BiP and CHOP. Furthermore, biochemical studies reveal that L. pneumophila uses two effectors (Lgt1 and Lgt2) to inhibit the splicing of XBP1u mRNA to spliced XBP1 (XBP1s), an UPR response regulator. Thus, we demonstrate that L. pneumophila is able to inhibit the UPR by multiple mechanisms including blocking XBP1u splicing and causing translational repression. This observation highlights the utility of L. pneumophila as a powerful tool for studying a critical protein homeostasis regulator.

The bacterium Legionella pneumophila, a causative agent of severe pneumonia, replicates inside an endoplasmic reticulum-like organelle in the host cells. Here, Treacy-Abarca and Mukherjee show that the pathogen dampens the host's unfolded protein response (UPR) pathway by multiple mechanisms.

Endoplasmic reticulum stress impairment in the spinal dorsal horn of a neuropathic pain model

Endoplasmic reticulum (ER) stress has been implicated in neurodegenerative diseases, but its role in neuropathic pain remains unclear. In this study, we examined the ER stress and the unfolded protein response (UPR) activation in a L5 spinal nerve ligation (SNL)-induced rat neuropathic pain model. SNL-induced neuropathic pain was assessed behaviorally using the CatWalk system, and histologically with microglial activation in the dorsal spinal horn. L5 SNL induced BIP upregulation in the neuron of superficial laminae of dorsal spinal horn. It also increased the level of ATF6 and intracellular localization into the nuclei in the neurons. Moreover, spliced XBP1 was also markedly elevated in the ipsilateral spinal dorsal horn. The PERK-elF2 pathway was activated in astrocytes of the spinal dorsal horn in the SNL model. In addition, electron microscopy revealed the presence of swollen cisternae in the dorsal spinal cord after SNL. Additionally, inhibition of the ATF6 pathway by intrathecal treatment with ATF6 siRNA reduced pain behaviors and BIP expression in the dorsal horn. The results suggest that ER stress might be involved in the induction and maintenance of neuropathic pain. Furthermore, a disturbance in UPR signaling may render the spinal neurons vulnerable to peripheral nerve injury or neuropathic pain stimuli.

Aberrant lipid metabolism in anaplastic thyroid carcinoma reveals stearoyl CoA desaturase 1 as a novel therapeutic target

CONTEXT

Currently there are no efficacious therapies for patients with anaplastic thyroid carcinoma (ATC) that result in long-term disease stabilization or regression.

OBJECTIVE

We sought to identify pathways critical for ATC cell progression and viability in an effort to develop new therapeutic strategies. We investigated the effects of targeted inhibition of stearoyl-CoA desaturase 1 (SCD1), a constituent of fatty acid metabolism overexpressed in ATC.

DESIGN

A gene array of ATC and normal thyroid tissue was performed to identify gene transcripts demonstrating altered expression in tumor samples. Effects of pharmacological and the genetic inhibition of SCD1 on tumor cell viability as well as cell signaling responses to therapy were evaluated in in vitro and in vivo models of this rare, lethal malignancy.

RESULTS

The gene array analysis revealed consistent distortion of fatty acid metabolism and overexpression of SCD1 in ATC and well-differentiated thyroid carcinomas. SCD1 is critical for ATC cell survival and proliferation, the inhibition of which induced endoplasmic reticulum stress, activation of the unfolded protein response, and apoptosis. Combined suppression of endoplasmic reticulum-associated degradation, a prosurvival component of the unfolded protein response, using proteasome inhibitors resulted in a synergistic decrease in tumor cell proliferation and increased cell death.

CONCLUSIONS

SCD1 is a novel oncogenic factor specifically required for tumor cell viability in ATC. Furthermore, the expression of SCD1 appears to be correlated with thyroid tumor aggressiveness and may serve as a prognostic biomarker. These findings substantiate SCD1 as a novel tumor-specific target for therapy in patients with ATC and should be further investigated in a clinical setting.

A stressful microenvironment: opposing effects of the endoplasmic reticulum stress response in the suppression and enhancement of adaptive tumor immunity

The recent clinical success of immunotherapy in the treatment of certain types of cancer has demonstrated the powerful ability of the immune system to control tumor growth, leading to significantly improved patient survival. However, despite these promising results current immunotherapeutic strategies are still limited and have not yet achieved broad acceptance outside the context of metastatic melanoma. The limitations of current immunotherapeutic approaches can be attributed in part to suppressive mechanisms present in the tumor microenvironment that hamper the generation of robust antitumor immune responses thus allowing tumor cells to escape immune-mediated destruction. The endoplasmic reticulum (ER) stress response has recently emerged as a potent regulator of tumor immunity. The ER stress response is an adaptive mechanism that allows tumor cells to survive in the harsh growth conditions inherent to the tumor milieu such as low oxygen (hypoxia), low pH and low levels of glucose. Activation of ER stress can also alter the cancer cell response to therapies. In addition, the ER stress response promotes tumor immune evasion by inducing the production of protumorigenic inflammatory cytokines and impairing tumor antigen presentation. However, the ER stress response can boost antitumor immunity in some situations by enhancing the processing and presentation of tumor antigens and by inducing the release of immunogenic factors from stressed tumor cells. Here, we discuss the dualistic role of the ER stress response in the modulation of tumor immunity and highlight how strategies to either induce or block ER stress can be employed to improve the clinical efficacy of tumor immunotherapy.

ER stress-induced clearance of misfolded GPI-anchored proteins via the secretory pathway

Proteins destined for the cell surface are first assessed in the endoplasmic reticulum (ER) for proper folding before release into the secretory pathway. This ensures that defective proteins are normally prevented from entering the extracellular environment, where they could be disruptive. Here, we report that, when ER folding capacity is saturated during stress, misfolded glycosylphosphatidylinositol-anchored proteins dissociate from resident ER chaperones, engage export receptors, and quantitatively leave the ER via vesicular transport to the Golgi. Clearance from the ER commences within minutes of acute ER stress, before the transcriptional component of the unfolded protein response is activated. These aberrant proteins then access the cell surface transiently before destruction in lysosomes. Inhibiting this stress-induced pathway by depleting the ER-export receptors leads to aggregation of the ER-retained misfolded protein. Thus, this rapid response alleviates the elevated burden of misfolded proteins in the ER at the onset of ER stress, promoting protein homeostasis in the ER.

Arachidonoyl-ethanolamide activates endoplasmic reticulum stress-apoptosis in tumorigenic keratinocytes: Role of cyclooxygenase-2 and novel J-series prostamides

Non-melanoma skin cancer and other epithelial tumors overexpress cyclooxygenase-2 (COX-2), differentiating them from normal cells. COX-2 metabolizes arachidonic acid to prostaglandins including, the J-series prostaglandins, which induce apoptosis by mechanisms including endoplasmic reticulum (ER) stress. Arachidonoyl-ethanolamide (AEA) is a cannabinoid that causes apoptosis in diverse tumor types. Previous studies from our group demonstrated that AEA was metabolized by COX-2 to J-series prostaglandins. Thus, the current study examines the role of COX-2, J-series prostaglandins, and ER stress in AEA-induced apoptosis. In tumorigenic keratinocytes that overexpress COX-2, AEA activated the PKR-like ER kinase (PERK), inositol requiring kinase-1 (IRE1), and activating transcription factor-6 (ATF6) ER stress pathways and the ER stress apoptosis-associated proteins, C/EBP homologous protein-10 (CHOP10), caspase-12, and caspase-3. Using an ER stress inhibitor, it was determined that ER stress was required for AEA-induced apoptosis. To evaluate the role of COX-2 in ER stress-apoptosis, HaCaT keratinocytes with low endogenous COX-2 expression were transfected with COX-2 cDNA or an empty vector and AEA-induced ER stress-apoptosis occurred only in the presence of COX-2. Moreover, LC-MS analysis showed that the novel prostaglandins, 15-deoxyΔ(12,14) PGJ2 -EA and Δ(12) PGJ2 /PGJ2-EA, were synthesized from AEA. These findings suggest that AEA will be selectively toxic in tumor cells that overexpress COX-2 due to the metabolism of AEA by COX-2 to J-series prostaglandin-ethanolamides (prostamides). Hence, AEA may be an ideal topical agent for the elimination of malignancies that overexpress COX-2.

Regulation of the death-associated protein kinase 1 expression and autophagy via ATF6 requires apoptosis signal-regulating kinase 1

The death-associated protein kinase 1 (DAPK1) is an important regulator of cell death and autophagy. Recently, we have identified that ATF6, an endoplasmic reticulum-resident transcription factor, in association with the transcription factor CEBP-β, regulates the gamma interferon (IFN-γ)-induced expression of Dapk1 (P. Gade et al., Proc. Natl. Acad. Sci. U. S. A. 109:10316-10321, 2012, doi.org/10.1073/pnas.1119273109). IFN-γ-induced proteolytic processing of ATF6 and phosphorylation of C/EBP-β were essential for the formation of a novel transcriptional complex that regulates DAPK1. Here, we report that IFN-γ activates the ASK1-MKK3/MKK6-p38 mitogen-activated protein kinase (MAPK) pathway for controlling the activity of ATF6. The terminal enzyme in this pathway, p38 MAPK, phosphorylates a critical threonine residue in ATF6 upstream of its DNA binding domain. ATF6 mutants defective for p38 MAPK phosphorylation fail to undergo proteolytic processing in the Golgi apparatus and drive IFN-γ-induced gene expression and autophagy. We also show that mice lacking Ask1 are highly susceptible to lethal bacterial infection owing to defective autophagy. Together, these results identify a novel host defense pathway controlled by IFN-γ signaling.

Activation of ER stress by hydrogen peroxide in C2C12 myotubes

The purpose of this study was to examine the link between oxidative stress and endoplasmic reticulum (ER) stress in myogenic cells. C2C12 myotubes were incubated with hydrogen peroxide (H2O2, 200 μM) and harvested 4h or 17 h after the induction of this oxidative stress. A massive upregulation of binding immunoglobulin protein (BiP) was found, indicating the presence of ER stress. Nevertheless, the three branches of the unfolded protein response (UPR) were not activated to the same extent. The double-stranded RNA-dependent protein kinase (PKR)-like ER kinase (PERK) branch was the most activated as shown by the increase of phospho-eukaryotic translation-initiation factor 2α (eIF2α, Ser51) and the mRNA levels of activating transcription factor 4 (ATF4), C/EBP homologous (CHOP) and tribbles homolog 3 (TRB3). The slight increase in the spliced form of X-box binding protein 1 (XBP1s) together with the decrease of the unspliced form (XBP1u) indicated a higher endoribonuclease activity of inositol-requiring 1α (IRE1α). The transcriptional activity of activating transcription factor 6 (ATF6) remained unchanged after incubation with H2O2. The mechanisms by which the three branches of UPR can be specifically regulated by oxidative stress are currently unresolved and need further investigations.

Inhibition of the prohormone convertase subtilisin-kexin isoenzyme-1 induces apoptosis in human melanoma cells

Prohormone convertases (PCs) are endoproteases that process many substrates in addition to hormone precursors. Although overexpression of PCs is linked to carcinogenesis in some solid tumors, the role of subtilisin-kexin isoenzyme-1 (SKI-1) in this context is unknown. We show that SKI-1 is constitutively expressed in human pigment cells with higher SKI activity in seven out of eight melanoma cell lines compared with normal melanocytes. SKI-1 immunoreactivity is also detectable in tumor cells of melanoma metastases. Moreover, tissue samples of the latter display higher SKI-1 mRNA levels and activity than normal skin. From various stimuli tested, 12-O-tetradecanoylphorbol-13-acetate and tunicamycin affected SKI-1 expression. Importantly, SKI-1 inhibition by the cell-permeable enzyme inhibitor decanoyl-RRLL-chloromethylketone (dec-RRLL-CMK) not only suppressed proliferation and metabolic activity of melanoma cells in vitro but also reduced tumor growth of melanoma cells injected intracutaneously into immunodeficient mice. Mechanistic studies revealed that dec-RRLL-CMK induces classical apoptosis of melanoma cells in vitro and affects expression of several SKI-1 target genes including activating transcription factor 6 (ATF6). However, ATF6 gene silencing does not result in apoptosis of melanoma cells, suggesting that dec-RRLL-CMK induces cell death in an ATF6-independent manner. Our findings encourage further studies on SKI-1 as a potential target for melanoma therapy.

Transcription factor SCIRR69 involved in the activation of brain-derived neurotrophic factor gene promoter II in mechanically injured neurons

The spinal cord injury and regeneration-related gene #69 (SCIRR69), which was identified in our screen for genes upregulated after spinal cord injury, encode a protein belonging to the cAMP response element-binding protein (CREB)/ATF family of transcription factors. Our previous study showed that SCIRR69 functions as a transcriptional activator. However, the target gene regulated by SCIRR69 and its roles in injured neurons remain unknown. In this study, we showed that SCIRR69 is widely distributed in the central nervous system. Full-length SCIRR69 is an endoplasmic reticulum-bound protein. Following mechanical injury to neurons, SCIRR69 was induced and proteolytically cleaved by site-1 and site-2 proteases, and the proteolytically cleaved SCIRR69 (p60-SCIRR69) was translocated to the nucleus where it bound to brain-derived neurotrophic factor (BDNF) gene promoter II. In addition, loss- and gain-of-function studies confirmed that SCIRR69 is involved in the regulation of BDNF expression in injured neurons. As expected, the culture supernatants of PC12 cells stably expressing p60-SCIRR69 contained higher levels of BDNF, and more remarkably promoted neurite outgrowth in a spinal cord slice culture model in vitro than the supernatants of control cells. These results suggest that SCIRR69 is a novel regulator of the BDNF gene and may play an important role in the repair and/or regeneration of damaged neural tissues by specifically activating BDNF promoter II.

Transcriptional cross talk between orphan nuclear receptor ERRγ and transmembrane transcription factor ATF6α coordinates endoplasmic reticulum stress response

Orphan nuclear receptor ERRγ is a member of nuclear receptor superfamily that regulates several important cellular processes including hepatic glucose and alcohol metabolism. However, mechanistic understanding of transcriptional regulation of the ERRγ gene remains to be elucidated. Here, we report that activating transcription factor 6α (ATF6α), an endoplasmic reticulum (ER)-membrane–bound basic leucine zipper (bZip) transcription factor, directly regulates ERRγ gene expression in response to ER stress. ATF6α binds to ATF6α responsive element in the ERRγ promoter. The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α) is required for this transactivation. Chromatin immunoprecipitation (ChIP) assay confirmed the binding of both ATF6α and PGC1α on the ERRγ promoter. ChIP assay demonstrated histone H3 and H4 acetylation occurs at the ATF6α and PGC1α binding site. Of interest, ERRγ along with PGC1α induce ATF6α gene transcription upon ER stress. ERRγ binds to an ERRγ responsive element in the ATF6α promoter. ChIP assay confirmed that both ERRγ and PGC1α bind to a site in the ATF6α promoter that exhibits histone H3 and H4 acetylation. Overall, for the first time our data show a novel pathway of cross talk between nuclear receptors and ER-membrane–bound transcription factors and suggest a positive feed-forward loop regulates ERRγ and ATF6α gene transcription.

BiP negatively affects ricin transport

The AB plant toxin ricin binds both glycoproteins and glycolipids at the cell surface via its B subunit. After binding, ricin is endocytosed and then transported retrogradely through the Golgi to the endoplasmic reticulum (ER). In the ER, the A subunit is retrotranslocated to the cytosol in a chaperone-dependent process, which is not fully explored. Recently two separate siRNA screens have demonstrated that ER chaperones have implications for ricin toxicity. ER associated degradation (ERAD) involves translocation of misfolded proteins from ER to cytosol and it is conceivable that protein toxins exploit this pathway. The ER chaperone BiP is an important ER regulator and has been implicated in toxicity mediated by cholera and Shiga toxin. In this study, we have investigated the role of BiP in ricin translocation to the cytosol. We first show that overexpression of BiP inhibited ricin translocation and protected cells against the toxin. Furthermore, shRNA-mediated depletion of BiP enhanced toxin translocation resulting in increased cytotoxicity. BiP-dependent inhibition of ricin toxicity was independent of ER stress. Our findings suggest that in contrast to what was shown with the Shiga toxin, the presence of BiP does not facilitate, but rather inhibits the entry of ricin into the cytosol.

Potential for therapeutic manipulation of the UPR in disease

Increased endoplasmic reticulum (ER) stress and the activated unfolded protein response (UPR) signaling associated with it play key roles in physiological processes as well as under pathological conditions. The UPR normally protects cells and re-establishes cellular homeostasis, but prolonged UPR activation can lead to the development of various pathologies. These features make the UPR signaling pathway an attractive target for the treatment of diseases whose pathogenesis is characterized by chronic activation of this pathway. Here, we focus on the molecular signaling pathways of the UPR and suggest possible ways to target this response for therapeutic purposes.

Decreased vitamin B12 availability induces ER stress through impaired SIRT1-deacetylation of HSF1

Vitamin B12 (cobalamin) is a key determinant of S-adenosyl methionine (SAM)-dependent epigenomic cellular regulations related to methylation/acetylation and its deficiency produces neurodegenerative disorders by elusive mechanisms. Sirtuin 1 deacetylase (SIRT1) triggers cell response to nutritional stress through endoplasmic reticulum (ER) stress. Recently, we have established a N1E115 dopaminergic cell model by stable expression of a transcobalamin–oleosin chimera (TO), which impairs cellular availability of vitamin B12, decreases methionine synthase activity and SAM level, and reduces cell proliferation. In contrast, oleosin-transcobalamin chimera (OT) does not modify the phenotype of transfected cells. Presently, the impaired cellular availability of vitamin B12 in TO cells activated irreversible ER stress pathways, with increased P-eIF-2α, P-PERK, P-IRE1α, ATF6, ATF4, decreased chaperon proteins and increased pro-apoptotic markers, CHOP and cleaved caspase 3, through reduced SIRT1 expression and consequently greater acetylation of heat-shock factor protein 1 (HSF1). Adding either B12, SIRT1, or HSF1 activators as well as overexpressing SIRT1 or HSF1 dramatically reduced the activation of ER stress pathways in TO cells. Conversely, impairing SIRT1 and HSF1 by siRNA, expressing a dominant negative form of HSF1, or adding a SIRT1 inhibitor led to B12-dependent ER stress in OT cells. Addition of B12 abolished the activation of stress transducers and apoptosis, and increased the expression of protein chaperons in OT cells subjected to thapsigargin, a strong ER stress stimulator. AdoX, an inhibitor of methyltransferase activities, produced similar effects than decreased B12 availability on SIRT1 and ER stress by a mechanism related to increased expression of hypermethylated in cancer 1 (HIC1). Taken together, these data show that cellular vitamin B12 has a strong modulating influence on ER stress in N1E115 dopaminergic cells. The impaired cellular availability in vitamin B12 induces irreversible ER stress by greater acetylation of HSF1 through decreased SIRT1 expression, whereas adding vitamin B12 produces protective effects in cells subjected to ER stress stimulation.

Activation of the unfolded protein response is associated with pulmonary hypertension

Pulmonary hypertension remains an important cause of morbidity and mortality. Although there is currently no cure, descriptions of defective intracellular trafficking and protein misfolding in vascular cell models of pulmonary hypertension have been recently reported. We tested the hypothesis that activation of the unfolded protein response (UPR) would be associated with the development of severe PH. We investigated activation of the UPR in archival tissues from patients with severe PH, and in the monocrotaline-induced rat model of severe PH. We tested the ability of a pharmacologic agent capable of modulating the UPR to prevent and reverse pulmonary hypertension. We found evidence of an active UPR in archival tissue from humans with PH, but not in control lungs. Similarly, monocrotaline-treated rats demonstrated a significant difference in expression of each of the major arms of the UPR compared to controls. Interestingly, the UPR preceded the appearance of macrophages and the development of lung vascular remodeling in the rats. Treatment of monocrotaline rats with salubrinal, a modulator of the PERK arm of the UPR, attenuated PH and was associated with a decrease in lung macrophages. In culture, pulmonary artery smooth muscle cells with UPR induction produced IL-6 and CCL-2/MCP-1, and stimulated macrophage migration. These effects were abolished by pretreatment of cells with salubrinal. These data support the hypothesis that the UPR may play a role in the pathogenesis of inflammatory vascular remodeling and PH. As such, understanding the functional contributions of the UPR in the setting of PH may have important therapeutic implications.

Early dengue virus protein synthesis induces extensive rearrangement of the endoplasmic reticulum independent of the UPR and SREBP-2 pathway

The rearrangement of intracellular membranes has been long reported to be a common feature in diseased cells. In this study, we used dengue virus (DENV) to study the role of the unfolded protein response (UPR) and sterol-regulatory-element-binding-protein-2 (SREBP-2) pathway in the rearrangement and expansion of the endoplasmic reticulum (ER) early after infection. Using laser scanning confocal and differential interference contrast microscopy, we demonstrate that rearrangement and expansion of the ER occurs early after DENV-2 infection. Through the use of mouse embryonic fibroblast cells deficient in XBP1 and ATF6, we show that ER rearrangement early after DENV infection is independent of the UPR. We then demonstrate that enlargement of the ER is independent of the SREBP-2 activation and upregulation of 3-hydroxy-3-methylglutaryl-Coenzyme-A reductase, the rate-limiting enzyme in the cholesterol biosynthesis pathway. We further show that this ER rearrangement is not inhibited by the treatment of DENV-infected cells with the cholesterol-inhibiting drug lovastatin. Using the transcription inhibitor actinomycin D and the translation elongation inhibitor cycloheximide, we show that de novo viral protein synthesis but not host transcription is necessary for expansion and rearrangement of the ER. Lastly, we demonstrate that viral infection induces the reabsorption of lipid droplets into the ER. Together, these results demonstrate that modulation of intracellular membrane architecture of the cell early after DENV-2 infection is driven by viral protein expression and does not require the induction of the UPR and SREBP-2 pathways. This work paves the way for further study of virally-induced membrane rearrangements and formation of cubic membranes.

Cloning and characterization of SCIRR69: a novel transcriptional factor belonging to the CREB/ATF family

The complete cDNA sequence of a novel gene, SCIRR69 (spinal cord injury and regeneration related no. 69 gene), was obtained by RACE technique. It codes for a protein of 521 amino acid residues homologous to human CREB3l2 (also known as BBF2H7) and mouse CREB3l2. The protein contains a basic DNA binding and leucine zipper dimerization (B-ZIP) motif and a hydrophobic region representing a putative transmembrane domain, similar to the structure of other CREB/ATF transcription factors. Monoclonal antibody against SCIRR69 was developed and could recognize the SCIRR69 protein in both native and denatured forms. Constructing of SCIRR69 fusion proteins with the GAL4 DNA-binding domain disclosed that SCIRR69 functioned as a transcriptional activator and its N-terminal 60 amino acids accounted for the activation ability. SCIRR69 resides in the cytoplasm of primary neurons, whereas neuron damage by incision led to the cleavage and translocation from the cytoplasm to the nucleus. These results suggest that SCIRR69 is activated by proteolytic cleavage at the transmembrane domain in response to neuron damage and its amino-terminal cytoplasmic domain translocates into the nucleus to activate the transcription of target genes.