Activation of ATF6 and an ATF6 DNA binding site by the endoplasmic reticulum stress response

β-Arrestin-2 enhances endoplasmic reticulum stress-induced glomerular endothelial cell injury by activating transcription factor 6 in diabetic nephropathy

BACKGROUND

Glomerular endothelial cell (GENC) injury is a characteristic of early-stage diabetic nephropathy (DN), and the investigation of potential therapeutic targets for preventing GENC injury is of clinical importance.

AIM

To investigate the role of β-arrestin-2 in GENCs under DN conditions.

METHODS

Eight-week-old C57BL/6J mice were intraperitoneally injected with streptozotocin to induce DN. GENCs were transfected with plasmids containing siRNA-β-arrestin-2, shRNA-activating transcription factor 6 (ATF6), pCDNA-β-arrestin-2, or pCDNA-ATF6. Additionally, adeno-associated virus (AAV) containing shRNA-β-arrestin-2 was administered via a tail vein injection in DN mice.

RESULTS

The upregulation of β-arrestin-2 was observed in patients with DN as well as in GENCs from DN mice. Knockdown of β-arrestin-2 reduced apoptosis in high glucose-treated GENCs, which was reversed by the overexpression of ATF6. Moreover, overexpression of β-arrestin-2 Led to the activation of endoplasmic reticulum (ER) stress and the apoptosis of GENCs which could be mitigated by silencing of ATF6. Furthermore, knockdown of β-arrestin-2 by the administration of AAV-shRNA-β-arrestin-2 alleviated renal injury in DN mice.

CONCLUSION

Knockdown of β-arrestin-2 prevents GENC apoptosis by inhibiting ATF6-mediated ER stress in vivo and in vitro. Consequently, β-arrestin-2 may represent a promising therapeutic target for the clinical management of patients with DN.

sUPRa is a dual-color reporter for unbiased quantification of the unfolded protein response with cellular resolution

The unfolded protein response (UPR) maintains proteostasis upon endoplasmic reticulum (ER) stress, and is initiated by a range of physiological and pathological processes. While there have been advances in developing fluorescent reporters for monitoring individual signaling pathways of the UPR, this approach may not capture a cell's overall UPR activity. Here we describe a novel sensor of UPR activity, sUPRa, which is designed to report the global UPR. sUPRa displays excellent response characteristics, outperforms reporters of individual UPR pathways in terms of sensitivity and kinetics, and responds to a range of different ER stress stimuli. Furthermore, sUPRa's dual promoter and fluorescent protein design ensures that both UPR-active and inactive cells are detected, and controls for reporter copy number. Using sUPRa, we reveal UPR activation in layer 2/3 pyramidal neurons of mouse cerebral cortex following a period of sleep deprivation. sUPRa affords new opportunities for quantifying physiological UPR activity with cellular resolution.

Quantitative proteomics reveals cellular responses to individual mAb expression and tunicamycin in CHO cells

Chinese hamster ovary (CHO) cells are popular in the pharmaceutical industry for their ability to produce high concentrations of antibodies and their resemblance to human cells in terms of protein glycosylation patterns. Current data indicate the relevance of CHO cells in the biopharmaceutical industry, with a high number of product commendations and a significant market share for monoclonal antibodies. To enhance the production capabilities of CHO cells, a deep understanding of their cellular and molecular composition is crucial. Genome sequencing and proteomic analysis have provided valuable insights into the impact of the bioprocessing conditions, productivity, and product quality. In our investigation, we conducted a comparative analysis of proteomic profiles in high and low monoclonal antibody-producing cell lines and studied the impact of tunicamycin (TM)-induced endoplasmic reticulum (ER) stress. We examined the expression levels of different proteins including unfolded protein response (UPR) target genes by using label-free quantification techniques for protein abundance. Our results show the upregulation of proteins associated with protein folding mechanisms in low producer vs. high producer cell line suggesting a form of ER stress related to specific protein production. Further, Hspa9 and Dnaja3 are notable candidates activated by the mitochondria UPR and play important roles in protein folding processes in mitochondria. We identified significant upregulation of Nedd8 and Lgmn proteins in similar levels which may contribute to UPR stress. Interestingly, the downregulation of Hspa5/Bip and Pdia4 in response to tunicamycin treatment suggests a low-level UPR activation. KEY POINTS: • Proteome profiling of recombinant CHO cells under mild TM treatment. • Identified protein clusters are associated with the unfolded protein response (UPR). • The compared cell lines revealed noticeable disparities in protein expression levels.

Irisin Ameliorate Acute Pancreatitis and Acinar Cell Viability through Modulation of the Unfolded Protein Response (UPR) and PPARγ-PGC1α-FNDC5 Pathways

Acute pancreatitis (AP) entails pancreatic inflammation, tissue damage and dysregulated enzyme secretion, including pancreatic lipase (PL). The role of irisin, an anti-inflammatory and anti-apoptotic cytokine, in AP and exocrine pancreatic stress is unclear. We have previously shown that irisin regulates PL through the PPARγ-PGC1α-FNDC5 pathway. In this study, we investigated irisin and irisin's pathway on AP in in vitro (AR42J-B13) and ex vivo (rat primary acinar) models using molecular, biochemical and immunohistochemistry methodology. Pancreatitis induction (cerulein (cer)) resulted in a significant up-regulation of the PPARγ-PGC1α-FNDC5 axis, PL expression and secretion and endoplasmic reticulum (ER) stress unfolded protein response (UPR) signal-transduction markers (CHOP, XBP-1 and ATF6). Irisin addition in the cer-pancreatitis state resulted in a significant down-regulation of the PPARγ-PGC1α-FNDC5 axis, PPARγ nucleus-translocation and inflammatory state (TNFα and IL-6) in parallel to diminished PL expression and secretion (in vitro and ex vivo models). Irisin addition up-regulated the expression of pro-survival UPR markers (ATF6 and XBP-1) and reduced UPR pro-apoptotic markers (CHOP) under cer-pancreatitis and induced ER stress (tunicamycin), consequently increasing cells viability. Irisin's pro-survival effect under cer-pancreatitis state was abolished under PPARγ inhibition. Our findings suggest irisin as a potential therapeutic option for AP via its ability to up-regulate pro-survival UPR signals and activate the PPARγ-PGC1α-FNDC5 pathway.

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.

Curcumin affects apoptosis of colorectal cancer cells through ATF6-mediated endoplasmic reticulum stress

Colorectal cancer (CRC) is the main cause of cancer-associated death. Herein, we treated SW620 and HT-29 CRC cells with different curcumin concentrations, followed by treatment with the half maximal inhibitory concentration (IC50) curcumin/endoplasmic reticulum stress (ERS) inhibitor 4-phenyl butyric acid (4-PBA)/activating transcription factor 6 (ATF6) interference plasmid (si-ATF6). We detected cell proliferation/apoptosis, ATF6 cellular localization/nuclear translocation, ion concentration, ATF6 protein/apoptotic protein (Bax/Bcl-2/Cleaved Caspase-3) levels, and ERS-related proteins (glucose-regulated protein 78 [Grp78]/C/EBP homologous protein [CHOP]). We discovered inhibited cell proliferation/growth, enhanced cell apoptosis/(Bax/Bcl-2) ratio/Cleaved Caspase-3 levels/Ca2+ concentration in the cytoplasm/ERS-related protein (Grp78/CHOP) levels, and activated ERS following treatment with IC50 curcumin. 4-PBA partially reversed the inhibitory effect of curcumin on SW620 cells by restraining ERS. Curcumin stimulated ATF6 expression and its nuclear translocation to activate ERS. ATF6 silencing partly annulled the inhibitory effect of curcumin on SW620 cells. Our study explored the molecular mechanism of curcumin affecting CRC cell apoptosis through ATF6.

Signaling plasticity in the integrated stress response

The Integrated Stress Response (ISR) is an essential homeostatic signaling network that controls the cell's biosynthetic capacity. Four ISR sensor kinases detect multiple stressors and relay this information to downstream effectors by phosphorylating a common node: the alpha subunit of the eukaryotic initiation factor eIF2. As a result, general protein synthesis is repressed while select transcripts are preferentially translated, thus remodeling the proteome and transcriptome. Mounting evidence supports a view of the ISR as a dynamic signaling network with multiple modulators and feedback regulatory features that vary across cell and tissue types. Here, we discuss updated views on ISR sensor kinase mechanisms, how the subcellular localization of ISR components impacts signaling, and highlight ISR signaling differences across cells and tissues. Finally, we consider crosstalk between the ISR and other signaling pathways as a determinant of cell health.

A review on the mechanisms underlying the antitumor effects of natural products by targeting the endoplasmic reticulum stress apoptosis pathway

Cancer poses a substantial risk to human life and wellbeing as a result of its elevated incidence and fatality rates. Endoplasmic reticulum stress (ERS) is an important pathway that regulates cellular homeostasis. When ERS is under- or overexpressed, it activates the protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK)-, inositol-requiring enzyme 1 (IRE1)- and activating transcription Factor 6 (ATF6)-related apoptotic pathways to induce apoptosis. Tumor cells and microenvironment are susceptible to ERS, making the modulation of ERS a potential therapeutic approach for treating tumors. The use of natural products to treat tumors has substantially progressed, with various extracts demonstrating antitumor effects. Nevertheless, there are few reports on the effectiveness of natural products in inducing apoptosis by specifically targeting and regulating the ERS pathway. Further investigation and elaboration of its mechanism of action are still needed. This paper examines the antitumor mechanism of action by which natural products exert antitumor effects from the perspective of ERS regulation to provide a theoretical basis and new research directions for tumor therapy.

Strategies targeting endoplasmic reticulum stress to improve Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder with motor symptoms, which is caused by the progressive death of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). Accumulating evidence shows that endoplasmic reticulum (ER) stress occurring in the SNpc DA neurons is an early event in the development of PD. ER stress triggers the activation of unfolded protein response (UPR) to reduce stress and restore ER function. However, excessive and continuous ER stress and UPR exacerbate the risk of DA neuron death through crosstalk with other PD events. Thus, ER stress is considered a promising therapeutic target for the treatment of PD. Various strategies targeting ER stress through the modulation of UPR signaling, the increase of ER's protein folding ability, and the enhancement of protein degradation are developed to alleviate neuronal death in PD models. In this review, we summarize the pathological role of ER stress in PD and update the strategies targeting ER stress to improve ER protein homeostasis and PD-related events.

A motor neuron disease-associated mutation produces non-glycosylated Seipin that induces ER stress and apoptosis by inactivating SERCA2b

A causal relationship between endoplasmic reticulum (ER) stress and the development of neurodegenerative diseases remains controversial. Here, we focused on Seipinopathy, a dominant motor neuron disease, based on the finding that its causal gene product, Seipin, is a protein that spans the ER membrane twice. Gain-of-function mutations of Seipin produce non-glycosylated Seipin (ngSeipin), which was previously shown to induce ER stress and apoptosis at both cell and mouse levels albeit with no clarified mechanism. We found that aggregation-prone ngSeipin dominantly inactivated SERCA2b, the major calcium pump in the ER, and decreased the calcium concentration in the ER, leading to ER stress and apoptosis in human colorectal carcinoma-derived cells (HCT116). This inactivation required oligomerization of ngSeipin and direct interaction of the C-terminus of ngSeipin with SERCA2b, and was observed in Seipin-deficient neuroblastoma (SH-SY5Y) cells expressing ngSeipin at an endogenous protein level. Our results thus provide a new direction to the controversy noted above.

UPRmt and coordinated UPRER in type 2 diabetes

The mitochondrial unfolded protein response (UPR^mt) is a molecular mechanism that maintains mitochondrial proteostasis under stress and is closely related to various metabolic diseases, such as type 2 diabetes (T2D). Similarly, the unfolded protein response of the endoplasmic reticulum (UPR^ER) is responsible for maintaining proteomic stability in the endoplasmic reticulum (ER). Since the mitochondria and endoplasmic reticulum are the primary centers of energy metabolism and protein synthesis in cells, respectively, a synergistic mechanism must exist between UPR^mt and UPR^ER to cooperatively resist stresses such as hyperglycemia in T2D. Increasing evidence suggests that the protein kinase RNA (PKR)-like endoplasmic reticulum kinase (PERK) signaling pathway is likely an important node for coordinating UPR^mt and UPR^ER. The PERK pathway is activated in both UPR^mt and UPR^ER, and its downstream molecules perform important functions. In this review, we discuss the mechanisms of UPR^mt, UPR^ER and their crosstalk in T2D.

Impact of ER Stress and ER-Mitochondrial Crosstalk in Huntington's Disease

Accumulation of misfolded proteins is a common phenomenon of several neurodegenerative diseases. The misfolding of proteins due to abnormal polyglutamine (PolyQ) expansions are linked to the development of PolyQ diseases including Huntington's disease (HD). Though the genetic basis of PolyQ repeats in HD remains prominent, the primary molecular basis mediated by PolyQ toxicity remains elusive. Accumulation of misfolded proteins in the ER or disruption of ER homeostasis causes ER stress and activates an evolutionarily conserved pathway called Unfolded protein response (UPR). Protein homeostasis disruption at organelle level involving UPR or ER stress response pathways are found to be linked to HD. Due to dynamic intricate connections between ER and mitochondria, proteins at ER-mitochondria contact sites (mitochondria associated ER membranes or MAMs) play a significant role in HD development. The current review aims at highlighting the most updated information about different UPR pathways and their involvement in HD disease progression. Moreover, the role of MAMs in HD progression has also been discussed. In the end, the review has focused on the therapeutic interventions responsible for ameliorating diseased states via modulating either ER stress response proteins or modulating the expression of ER-mitochondrial contact proteins.

Regulation of Endoplasmic Reticulum Stress-Autophagy: A Potential Therapeutic Target for Ulcerative Colitis

Ulcerative colitis (UC) is a chronic nonspecific inflammation that mainly affects the mucosa and submucosa of the rectum and colon. Numerous studies have shown that endoplasmic reticulum stress (ERS)-induced autophagy plays a vital role in the pathogenesis of UC. ERS is the imbalance of internal balance caused by misfolded or unfolded proteins accumulated in the endoplasmic reticulum (ER).Excessive ERS triggers the unfolded protein response (UPR), an increase in inositol-requiring enzyme 1, and a Ca²⁺ overload, which activates the autophagy pathway. Autophagy is an evolutionarily conserved method of cellular self-degradation. Dysregulated autophagy causes inflammation, disruption of the intestinal barrier, and imbalance of intestinal homeostasis, therefore increasing the risk of colonic diseases. This review summarizes the pathogenesis of ERS, UPR, and ERS-related autophagy in UC, providing potential new targets and more effective treatment options for UC.

Role of UPR Sensor Activation in Cell Death-Survival Decision of Colon Cancer Cells Stressed by DPE Treatment

Polyphenols have been shown to possess several beneficial properties, including properties involved in the prevention or treatment of cancer. Among these polyphenols, a leading role is played by dihydroxyphenylethanol (DPE), the most powerful antioxidant compound contained in the olive oil. DPE has been previously reported to induce endoplasmic reticulum (ER) stress and to reduce cell survival in colon cancer, one of the most common and aggressive cancers in developed countries. In this study, we further investigated the activation of UPR by DPE and explored the roles of the three UPR sensors, inositol-requiring enzyme (IRE) 1 alpha, protein kinase RNA-like endoplasmic reticulum kinase (PERK), and activating transcription factor (ATF6), in the cell death-survival decision of wt and mutp53 colon cancer cells and the underlying mechanisms involved. We also unveiled a new interplay between ATF6 and wt, as well as mutp53, which may have important implications in cancer therapy.

The Role of the UPR Pathway in the Pathophysiology and Treatment of Bipolar Disorder

Bipolar disorder (BD) is a mood disorder that affects millions worldwide and is associated with severe mood swings between mania and depression. The mood stabilizers valproate (VPA) and lithium (Li) are among the main drugs that are used to treat BD patients. However, these drugs are not effective for all patients and cause serious side effects. Therefore, better drugs are needed to treat BD patients. The main barrier to developing new drugs is the lack of knowledge about the therapeutic mechanism of currently available drugs. Several hypotheses have been proposed for the mechanism of action of mood stabilizers. However, it is still not known how they act to alleviate both mania and depression. The pathology of BD is characterized by mitochondrial dysfunction, oxidative stress, and abnormalities in calcium signaling. A deficiency in the unfolded protein response (UPR) pathway may be a shared mechanism that leads to these cellular dysfunctions. This is supported by reported abnormalities in the UPR pathway in lymphoblasts from BD patients. Additionally, studies have demonstrated that mood stabilizers alter the expression of several UPR target genes in mouse and human neuronal cells. In this review, we outline a new perspective wherein mood stabilizers exert their therapeutic mechanism by activating the UPR. Furthermore, we discuss UPR abnormalities in BD patients and suggest future research directions to resolve discrepancies in the literature.

Remodeling of Ion Channel Trafficking and Cardiac Arrhythmias

Both inherited and acquired cardiac arrhythmias are often associated with the abnormal functional expression of ion channels at the cellular level. The complex machinery that continuously traffics, anchors, organizes, and recycles ion channels at the plasma membrane of a cardiomyocyte appears to be a major source of channel dysfunction during cardiac arrhythmias. This has been well established with the discovery of mutations in the genes encoding several ion channels and ion channel partners during inherited cardiac arrhythmias. Fibrosis, altered myocyte contacts, and post-transcriptional protein changes are common factors that disorganize normal channel trafficking during acquired cardiac arrhythmias. Channel availability, described notably for hERG and K_V1.5 channels, could be another potent arrhythmogenic mechanism. From this molecular knowledge on cardiac arrhythmias will emerge novel antiarrhythmic strategies.

Unfolded protein response during cardiovascular disorders: a tilt towards pro-survival and cellular homeostasis

The endoplasmic reticulum (ER) is an organelle that orchestrates the production and proper assembly of an extensive types of secretory and membrane proteins. Endoplasmic reticulum stress is conventionally related to prolonged disruption in the protein folding machinery resulting in the accumulation of unfolded proteins in the ER. This disruption is often manifested due to oxidative stress, Ca²⁺ leakage, iron imbalance, disease conditions which in turn hampers the cellular homeostasis and induces cellular apoptosis. A mild ER stress is often reverted back to normal. However, cells retaliate to acute ER stress by activating the unfolded protein response (UPR) which comprises three signaling pathways, Activating transcription factor 6 (ATF6), inositol requiring enzyme 1 alpha (IRE1α), and protein kinase RNA-activated-like ER kinase (PERK). The UPR response participates in both protective and pro-apoptotic responses and not much is known about the mechanistic aspects of the switch from pro-survival to pro-apoptosis. When ER stress outpaces UPR response then cell apoptosis prevails which often leads to the development of various diseases including cardiomyopathies. Therefore, it is important to identify molecules that modulate the UPR that may serve as promising tools towards effective treatment of cardiovascular diseases. In this review, we elucidated the latest advances in construing the contribution imparted by the three arms of UPR to combat the adverse environment in the ER to restore cellular homeostasis during cardiomyopathies. We also summarized the various therapeutic agents that plays crucial role in tilting the UPR response towards pro-survival.

The Unfolded Protein Response: An Overview

Simple Summary

The unfolded protein response (UPR) is the cells’ way of maintaining the balance of protein folding in the endoplasmic reticulum, which is the section of the cell designated for folding proteins with specific destinations such as other organelles or to be secreted by the cell. The UPR is activated when unfolded proteins accumulate in the endoplasmic reticulum. This accumulation puts a greater load on the molecules in charge of folding the proteins, and therefore the UPR works to balance this by lowering the number of unfolded proteins present in the cell. This is done in multiple ways, such as lowering the number of proteins that need to be folded; increasing the folding ability of the endoplasmic reticulum and by removing some of the unfolded proteins which take longer to fold. If the UPR is successful at reducing the number of unfolded proteins, the UPR is inactivated and the cells protein folding balance is returned to normal. However, if the UPR is unsuccessful, then this can lead to cell death.

Abstract

The unfolded protein response is the mechanism by which cells control endoplasmic reticulum (ER) protein homeostasis. Under normal conditions, the UPR is not activated; however, under certain stresses, such as hypoxia or altered glycosylation, the UPR can be activated due to an accumulation of unfolded proteins. The activation of the UPR involves three signaling pathways, IRE1, PERK and ATF6, which all play vital roles in returning protein homeostasis to levels seen in non-stressed cells. IRE1 is the best studied of the three pathways, as it is the only pathway present in Saccharomyces cerevisiae. This pathway involves spliceosome independent splicing of HAC1 or XBP1 in yeast and mammalians cells, respectively. PERK limits protein synthesis, therefore reducing the number of new proteins requiring folding. ATF6 is translocated and proteolytically cleaved, releasing a NH₂ domain fragment which is transported to the nucleus and which affects gene expression. If the UPR is unsuccessful at reducing the load of unfolded proteins in the ER and the UPR signals remain activated, this can lead to programmed cell death.

Endoplasmic reticulum acetyltransferases Atase1 and Atase2 differentially regulate reticulophagy, macroautophagy and cellular acetyl-CoA metabolism

Nε-lysine acetylation in the ER lumen is a recently discovered quality control mechanism that ensures proteostasis within the secretory pathway. The acetyltransferase reaction is carried out by two type-II membrane proteins, ATase1/NAT8B and ATase2/NAT8. Prior studies have shown that reducing ER acetylation can induce reticulophagy, increase ER turnover, and alleviate proteotoxic states. Here, we report the generation of Atase1-/- and Atase2-/- mice and show that these two ER-based acetyltransferases play different roles in the regulation of reticulophagy and macroautophagy. Importantly, knockout of Atase1 alone results in activation of reticulophagy and rescue of the proteotoxic state associated with Alzheimer's disease. Furthermore, loss of Atase1 or Atase2 results in widespread adaptive changes in the cell acetylome and acetyl-CoA metabolism. Overall, our study supports a divergent role of Atase1 and Atase2 in cellular biology, emphasizing ATase1 as a valid translational target for diseases characterized by toxic protein aggregation in the secretory pathway.

Crosstalk between endoplasmic reticulum stress and oxidative stress: a dynamic duo in multiple myeloma

Under physiological and pathological conditions, cells activate the unfolded protein response (UPR) to deal with the accumulation of unfolded or misfolded proteins in the endoplasmic reticulum. Multiple myeloma (MM) is a hematological malignancy arising from immunoglobulin-secreting plasma cells. MM cells are subject to continual ER stress and highly dependent on the UPR signaling activation due to overproduction of paraproteins. Mounting evidence suggests the close linkage between ER stress and oxidative stress, demonstrated by overlapping signaling pathways and inter-organelle communication pivotal to cell fate decision. Imbalance of intracellular homeostasis can lead to deranged control of cellular functions and engage apoptosis due to mutual activation between ER stress and reactive oxygen species generation through a self-perpetuating cycle. Here, we present accumulating evidence showing the interactive roles of redox homeostasis and proteostasis in MM pathogenesis and drug resistance, which would be helpful in elucidating the still underdefined molecular pathways linking ER stress and oxidative stress in MM. Lastly, we highlight future research directions in the development of anti-myeloma therapy, focusing particularly on targeting redox signaling and ER stress responses.

Oxidative Stress and Endoplasmic Reticulum Stress in Rare Respiratory Diseases

Several studies have shown that some rare respiratory diseases, such as alpha-1 antitrypsin deficiency (AATD), idiopathic pulmonary fibrosis (IPF), cystic fibrosis (CF), and primary ciliary dyskinesia (PCD) present oxidative stress (OS) and endoplasmic reticulum (ER) stress. Their involvement in these pathologies and the use of antioxidants as therapeutic agents to minimize the effects of OS are discussed in this review.

PERK Pathway and Neurodegenerative Disease: To Inhibit or to Activate?

With the extension of life span in recent decades, there is an increasing burden of late-onset neurodegenerative diseases, for which effective treatments are lacking. Neurodegenerative diseases include the widespread Alzheimer’s disease (AD) and Parkinson’s disease (PD), the less frequent Huntington’s disease (HD) and Amyotrophic Lateral Sclerosis (ALS) and also rare early-onset diseases linked to mutations that cause protein aggregation or loss of function in genes that maintain protein homeostasis. The difficulties in applying gene therapy approaches to tackle these diseases is drawing increasing attention to strategies that aim to inhibit cellular toxicity and restore homeostasis by intervening in cellular pathways. These include the unfolded protein response (UPR), activated in response to endoplasmic reticulum (ER) stress, a cellular affliction that is shared by these diseases. Special focus is turned to the PKR-like ER kinase (PERK) pathway of the UPR as a target for intervention. However, the complexity of the pathway and its ability to promote cell survival or death, depending on ER stress resolution, has led to some confusion in conflicting studies. Both inhibition and activation of the PERK pathway have been reported to be beneficial in disease models, although there are also some reports where they are counterproductive. Although with the current knowledge a definitive answer cannot be given on whether it is better to activate or to inhibit the pathway, the most encouraging strategies appear to rely on boosting some steps without compromising downstream recovery.

cAMP-Independent Activation of the Unfolded Protein Response by Cholera Toxin

Cholera toxin (CT) is an AB₅ protein toxin that activates the stimulatory alpha subunit of the heterotrimeric G protein (Gsα) through ADP-ribosylation. Activation of Gsα produces a cytopathic effect by stimulating adenylate cyclase and the production of cAMP. To reach its cytosolic Gsα target, CT binds to the plasma membrane of a host cell and travels by vesicle carriers to the endoplasmic reticulum (ER). The catalytic CTA1 subunit then exploits the quality control mechanism of ER-associated degradation to move from the ER to the cytosol. ER-associated degradation is functionally linked to another quality control system, the unfolded protein response (UPR). However, the role of the UPR in cholera intoxication is unclear. We report here that CT triggers the UPR after 4 h of toxin exposure. A functional toxin was required to induce the UPR, but, surprisingly, activation of the adenylate cyclase signaling pathway was not sufficient to trigger the process. Toxin-induced activation of the UPR coincided with increased toxin accumulation in the cytosol. Chemical activation of the heterotrimeric G protein or the UPR also enhanced the onset of CTA1 delivery to the cytosol, thus producing a toxin-sensitive phenotype. These results indicate there is a cAMP-independent response to CT that activates the UPR and thereby enhances the efficiency of intoxication.

Structure guided design and synthesis of furyl thiazolidinedione derivatives as inhibitors of GLUT 1 and GLUT 4, and evaluation of their anti-leukemic potential

Cancer cells increase their glucose uptake and glycolytic activity to meet the high energy requirements of proliferation. Glucose transporters (GLUTs), which facilitate the transport of glucose and related hexoses across the cell membrane, play a vital role in tumor cell survival and are overexpressed in various cancers. GLUT1, the most overexpressed GLUT in many cancers, is emerging as a promising anti-cancer target. To develop GLUT1 inhibitors, we rationally designed, synthesized, structurally characterized, and biologically evaluated in-vitro and in-vivo a novel series of furyl-2-methylene thiazolidinediones (TZDs). Among 25 TZDs tested, F18 and F19 inhibited GLUT1 most potently (IC50 11.4 and 14.7 μM, respectively). F18 was equally selective for GLUT4 (IC50 6.8 μM), while F19 was specific for GLUT1 (IC50 152 μM in GLUT4). In-silico ligand docking studies showed that F18 interacted with conserved residues in GLUT1 and GLUT4, while F19 had slightly different interactions with the transporters. In in-vitro antiproliferative screening of leukemic/lymphoid cells, F18 was most lethal to CEM cells (CC50 of 1.7 μM). Flow cytometry analysis indicated that F18 arrested cell cycle growth in the subG0-G1 phase and lead to cell death due to necrosis and apoptosis. Western blot analysis exhibited alterations in cell signaling proteins, consistent with cell growth arrest and death. In-vivo xenograft study in a CEM model showed that F18 impaired tumor growth significantly.

Impact of Mixture Effects between Emerging Organic Contaminants on Cytotoxicity: A Systems Biological Understanding of Synergism between Tris(1,3-dichloro-2-propyl)phosphate and Triphenyl Phosphate

Humans are exposed to many xenobiotics simultaneously, but little is known about the toxic effects based on chemical-chemical interactions. This study aims at evaluating the binary interactions between 13 common environmental organic compounds (resulting in 78 pairs) by observing their cytotoxicity on HepG2 cells. Among all of the tested pairs, the combination of flame-retardant triphenyl phosphate (TPP) and tris(1,3-dichloro-2-propyl)phosphate (TDCPP) exhibited one of the most significant synergistic effects. We further characterized the transcriptome and metabolome after combined exposure to TPP and TDCPP and individual exposure. The results suggested that the coexposure caused many more changes in gene expressions and cellular activities. The transcriptome data showed that the coexposure triggered significant pathway changes including "cholesterol biosynthesis" and "ATF6-Alpha activated chaperone genes", together with distinct gene ontology (GO) terms such as the "negative regulation of the ERK1 and ERK2 cascade". Additionally, coexposure enhanced the biological activity of liver X receptors and nuclear factor erythroid 2-related factor 2 (Nrf2). The metabolome data showed that coexposure significantly elevated oxidative stress and affected the purine and pyrimidine metabolism. Overall, this study showed that interactions, which may enhance or suppress the biological processes, are common among environmental chemicals, although their environmental relevance should be studied in the future.

Selective Modulation of Dynamic Protein Complexes

Dynamic proteins perform critical roles in cellular machines, including those that control proteostasis, transcription, translation, and signaling. Thus, dynamic proteins are prime candidates for chemical probe and drug discovery but difficult targets because they do not conform to classical rules of design and screening. Selectivity is pivotal for candidate probe molecules due to the extensive interaction network of these dynamic hubs. Recognition that the traditional rules of probe discovery are not necessarily applicable to dynamic proteins and their complexes, as well as technological advances in screening, have produced remarkable results in the last 2-4 years. Particularly notable are the improvements in target selectivity for small-molecule modulators of dynamic proteins, especially with techniques that increase the discovery likelihood of allosteric regulatory mechanisms. We focus on approaches to small-molecule screening that appear to be more suitable for highly dynamic targets and have the potential to streamline identification of selective modulators.

The UPR in Neurodegenerative Disease: Not Just an Inside Job

Neurons are highly specialized cells that continuously and extensively communicate with other neurons, as well as glia cells. During their long lifetime, the post-mitotic neurons encounter many stressful situations that can disrupt protein homeostasis (proteostasis). The importance of tight protein quality control is illustrated by neurodegenerative disorders where disturbed neuronal proteostasis causes neuronal dysfunction and loss. For their unique function, neurons require regulated and long-distance transport of membrane-bound cargo and organelles. This highlights the importance of protein quality control in the neuronal endomembrane system, to which the unfolded protein response (UPR) is instrumental. The UPR is a highly conserved stress response that is present in all eukaryotes. However, recent studies demonstrate the existence of cell-type-specific aspects of the UPR, as well as cell non-autonomous UPR signaling. Here we discuss these novel insights in view of the complex cellular architecture of the brain and the implications for neurodegenerative diseases.

The bZIP Proteins of Oncogenic Viruses

Basic leucine zipper (bZIP) transcription factors (TFs) govern diverse cellular processes and cell fate decisions. The hallmark of the leucine zipper domain is the heptad repeat, with leucine residues at every seventh position in the domain. These leucine residues enable homo- and heterodimerization between ZIP domain α-helices, generating coiled-coil structures that stabilize interactions between adjacent DNA-binding domains and target DNA substrates. Several cancer-causing viruses encode viral bZIP TFs, including human T-cell leukemia virus (HTLV), hepatitis C virus (HCV) and the herpesviruses Marek’s disease virus (MDV), Epstein–Barr virus (EBV) and Kaposi’s sarcoma-associated herpesvirus (KSHV). Here, we provide a comprehensive review of these viral bZIP TFs and their impact on viral replication, host cell responses and cell fate.

The ATF6-EGF Pathway Mediates the Awakening of Slow-Cycling Chemoresistant Cells and Tumor Recurrence by Stimulating Tumor Angiogenesis

Slow-cycling cancer cells (SCCs) with a quiescence-like phenotype are believed to perpetrate cancer relapse and progression. However, the mechanisms that mediate SCC-derived tumor recurrence are poorly understood. Here, we investigated the mechanisms underlying cancer recurrence after chemotherapy, focusing on the interplay between SCCs and the tumor microenvironment. We established a preclinical model of SCCs by exposing non-small-cell lung cancer (NSCLC) cells to either the proliferation-dependent dye carboxyfluorescein diacetate succinimidyl ester (CFSE) or chemotherapeutic drugs. An RNA sequencing analysis revealed that the established SCCs exhibited the upregulation of a group of genes, especially epidermal growth factor (EGF). Increases in the number of vascular endothelial growth factor receptor (VEGFR)-positive vascular endothelial cells and epidermal growth factor receptor (EGFR) activation were found in NSCLC cell line- and patient-derived xenograft tumors that progressed upon chemotherapy. EGFR tyrosine kinase inhibitors effectively suppressed the migration and tube formation of vascular endothelial cells. Furthermore, activating transcription factor 6 (ATF6) induced the upregulation of EGF, and its antagonism effectively suppressed these SCC-mediated events and inhibited tumor recurrence after chemotherapy. These results suggest that the ATF6-EGF signaling axis in SCCs functions to trigger the angiogenesis switch in residual tumors after chemotherapy and is thus a driving force for the switch from SCCs to actively cycling cancer cells, leading to tumor recurrence.

Fine tuning of the unfolded protein response by ISRIB improves neuronal survival in a model of amyotrophic lateral sclerosis

Loss of protein folding homeostasis features many of the most prevalent neurodegenerative disorders. As coping mechanism to folding stress within the endoplasmic reticulum (ER), the unfolded protein response (UPR) comprises a set of signaling mechanisms that initiate a gene expression program to restore proteostasis, or when stress is chronic or overwhelming promote neuronal death. This fate-defining capacity of the UPR has been proposed to play a key role in amyotrophic lateral sclerosis (ALS). However, the several genetic or pharmacological attempts to explore the therapeutic potential of UPR modulation have produced conflicting observations. In order to establish the precise relationship between UPR signaling and neuronal death in ALS, we have developed a neuronal model where the toxicity of a familial ALS-causing allele (mutant G93A SOD1) and UPR activation can be longitudinally monitored in single neurons over the process of neurodegeneration by automated microscopy. Using fluorescent UPR reporters we established the temporal and causal relationship between UPR and neuronal death by Cox regression models. Pharmacological inhibition of discrete UPR processes allowed us to establish the contribution of PERK (PKR-like ER kinase) and IRE1 (inositol-requiring enzyme-1) mechanisms to neuronal fate. Importantly, inhibition of PERK signaling with its downstream inhibitor ISRIB, but not with the direct PERK kinase inhibitor GSK2606414, significantly enhanced the survival of G93A SOD1-expressing neurons. Characterization of the inhibitory properties of both drugs under ER stress revealed that in neurons (but not in glial cells) ISRIB overruled only part of the translational program imposed by PERK, relieving the general inhibition of translation, but maintaining the privileged translation of ATF4 (activating transcription factor 4) messenger RNA. Surprisingly, the fine-tuning of the PERK output in G93A SOD1-expressing neurons led to a reduction of IRE1-dependent signaling. Together, our findings identify ISRIB-mediated translational reprogramming as a new potential ALS therapy.

Role of Endoplasmic Reticulum Stress Sensor IRE1α in Cellular Physiology, Calcium, ROS Signaling, and Metaflammation

Inositol-requiring transmembrane kinase endoribonuclease-1α (IRE1α) is the most prominent and evolutionarily conserved unfolded protein response (UPR) signal transducer during endoplasmic reticulum functional upset (ER stress). A IRE1α signal pathway arbitrates yin and yang of cellular fate in objectionable conditions. It plays several roles in fundamental cellular physiology as well as in several pathological conditions such as diabetes, obesity, inflammation, cancer, neurodegeneration, and in many other diseases. Thus, further understanding of its molecular structure and mechanism of action during different cell insults helps in designing and developing better therapeutic strategies for the above-mentioned chronic diseases. In this review, recent insights into structure and mechanism of activation of IRE1α along with its complex regulating network were discussed in relation to their basic cellular physiological function. Addressing different binding partners that can modulate IRE1α function, UPRosome triggers different downstream pathways depending on the cellular backdrop. Furthermore, IRE1α are in normal cell activities outside the dominion of ER stress and activities under the weather of inflammation, diabetes, and obesity-related metaflammation. Thus, IRE1 as an ER stress sensor needs to be understood from a wider perspective for comprehensive functional meaning, which facilitates us with assembling future needs and therapeutic benefits.

Multiexon deletion alleles of ATF6 linked to achromatopsia

Achromatopsia (ACHM) is an autosomal recessive disease that results in severe visual loss. Symptoms of ACHM include impaired visual acuity, nystagmus, and photoaversion starting from infancy; furthermore, ACHM is associated with bilateral foveal hypoplasia and absent or severely reduced cone photoreceptor function on electroretinography. Here, we performed genetic sequencing in 3 patients from 2 families with ACHM, identifying and functionally characterizing 2 mutations in the activating transcription factor 6 (ATF6) gene. We identified a homozygous deletion covering exons 8-14 of the ATF6 gene from 2 siblings from the same family. In another patient from a different family, we identified a heterozygous deletion covering exons 2 and 3 of the ATF6 gene found in trans with a previously identified ATF6 c.970C>T (p.Arg324Cys) ACHM disease allele. Recombinant ATF6 proteins bearing these exon deletions showed markedly impaired transcriptional activity by qPCR and RNA-Seq analysis compared with WT-ATF6. Finally, RNAscope revealed that ATF6 and the related ATF6B transcripts were expressed in cones as well as in all retinal layers in normal human retina. Overall, our data identify loss-of-function ATF6 disease alleles that cause human foveal disease.

ATF6 as a Nodal Regulator of Proteostasis in the Heart

Proteostasis encompasses a homeostatic cellular network in all cells that maintains the integrity of the proteome, which is critical for optimal cellular function. The components of the proteostasis network include protein synthesis, folding, trafficking, and degradation. Cardiac myocytes have a specialized endoplasmic reticulum (ER) called the sarcoplasmic reticulum that is well known for its role in contractile calcium handling. However, less studied is the proteostasis network associated with the ER, which is of particular importance in cardiac myocytes because it ensures the integrity of proteins that are critical for cardiac contraction, e.g., ion channels, as well as proteins necessary for maintaining myocyte viability and interaction with other cell types, e.g., secreted hormones and growth factors. A major aspect of the ER proteostasis network is the ER unfolded protein response (UPR), which is initiated when misfolded proteins in the ER activate a group of three ER transmembrane proteins, one of which is the transcription factor, ATF6. Prior to studies in the heart, ATF6 had been shown in model cell lines to be primarily adaptive, exerting protective effects by inducing genes that encode ER proteins that fortify protein-folding in this organelle, thus establishing the canonical role for ATF6. Subsequent studies in isolated cardiac myocytes and in the myocardium, in vivo, have expanded roles for ATF6 beyond the canonical functions to include the induction of genes that encode proteins outside of the ER that do not have known functions that are obviously related to ER protein-folding. The identification of such non-canonical roles for ATF6, as well as findings that the gene programs induced by ATF6 differ depending on the stimulus, have piqued interest in further research on ATF6 as an adaptive effector in cardiac myocytes, underscoring the therapeutic potential of activating ATF6 in the heart. Moreover, discoveries of small molecule activators of ATF6 that adaptively affect the heart, as well as other organs, in vivo, have expanded the potential for development of ATF6-based therapeutics. This review focuses on the ATF6 arm of the ER UPR and its effects on the proteostasis network in the myocardium.

Specific gene expression in type 1 diabetic patients with and without cardiac autonomic neuropathy

We hypothesized that some molecular pathways might interact to initiate the process of nervous tissue destruction, promoting cardiac autonomic neuropathy (CAN) in the course of diabetes type 1 (T1D). The study group consisted of 60 T1D patients (58.33% women/41.67% men), on standard therapy. The control group consisted of twenty healthy volunteers recruited in accordance with age, gender and body weight. The presence of CAN was documented by the Ewing test method (ProSciCard apparatus). A microarray data analysis was performed using Gene Spring version 13. The microarray results for selected genes were confirmed by real-time PCR (qRT-PCR), using specific TaqMan Gene Expression Assays. Plasma IL-6 content was measured by an enzyme-linked immunosorbent assay (ELISA). The p < 0.05 value was considered as statistically significant. The microarray analysis, confirmed by qRTPCR, showed significant up-regulation of autophagy, quantity of mitochondria, quality regulatory genes (mTOR, GABARAPL2) apoptosis, ER-stress and inflammation (NFKB1, IL1b, IL1R1, SOD1), in T1D when compared to the control group. A significantly higher IL-6 protein level was observed in T1D patients, in comparison to the control group. We concluded that the observed changes in gene expression and activation of intracellular pathways give a coherent picture of the important role of oxidative stress in inflammation and the activation of apoptosis in the pathomechanism of DM. The significance of the inflammatory process, confirmed by the increased level of the inflammation biomarker IL-6 in the pathomechanisms of CAN was shown even in patients with properly treated T1D.

The PERK Inhibitor GSK2606414 Enhances Reovirus Infection in Head and Neck Squamous Cell Carcinoma via an ATF4-Dependent Mechanism

Reovirus type 3 Dearing (reovirus) is a tumor-selective oncolytic virus currently under evaluation in clinical trials. Here, we report that the therapeutic efficacy of reovirus in head and neck squamous cell cancer can be enhanced by targeting the unfolded protein response (UPR) kinase, protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK). PERK inhibition by GSK2606414 increased reovirus efficacy in both 2D and 3D models in vitro, while perturbing the normal host cell response to reovirus-induced endoplasmic reticulum (ER) stress. UPR reporter constructs were used for live-cell 3D spheroid imaging. Profiling of eIF2a-ATF4, IRE1a-XBP1, and ATF6 pathway activity revealed a context-dependent increase in eIF2a-ATF4 signaling due to GSK2606414. GSK2606414 blocked eIF2a-ATF4 signaling because of the canonical ER stress agent thapsigargin. In the context of reovirus infection, GSK2606414 induced eIF2a-ATF4 signaling. Knockdown of eIF2a kinases PERK, GCN2, and PKR revealed eIF2a-ATF4 reporter activity was dependent on either PERK or GCN2. Knockdown of ATF4 abrogated the GSK2606414-induced increase in reovirus protein levels, confirming eIF2a-ATF signaling as key to the observed phenotype. Our work identifies a novel approach to enhance the efficacy and replication of reovirus in a therapeutic setting.

Involvement of Metabolic Lipid Mediators in the Regulation of Apoptosis

Apoptosis is the physiological mechanism of cell death and can be modulated by endogenous and exogenous factors, including stress and metabolic alterations. Reactive oxygen species (ROS), as well as ROS-dependent lipid peroxidation products (including isoprostanes and reactive aldehydes including 4-hydroxynonenal) are proapoptotic factors. These mediators can activate apoptosis via mitochondrial-, receptor-, or ER stress-dependent pathways. Phospholipid metabolism is also an essential regulator of apoptosis, producing the proapoptotic prostaglandins of the PGD and PGJ series, as well as the antiapoptotic prostaglandins of the PGE series, but also 12-HETE and 20-HETE. The effect of endocannabinoids and phytocannabinoids on apoptosis depends on cell type-specific differences. Cells where cannabinoid receptor type 1 (CB1) is the dominant cannabinoid receptor, as well as cells with high cyclooxygenase (COX) activity, undergo apoptosis after the administration of cannabinoids. In contrast, in cells where CB2 receptors dominate, and cells with low COX activity, cannabinoids act in a cytoprotective manner. Therefore, cell type-specific differences in the pro- and antiapoptotic effects of lipids and their (oxidative) products might reveal new options for differential bioanalysis between normal, functional, and degenerating or malignant cells, and better integrative biomedical treatments of major stress-associated diseases.

Repression of viral gene expression and replication by the unfolded protein response effector XBP1u

The unfolded protein response (UPR) is a cellular homeostatic circuit regulating protein synthesis and processing in the ER by three ER-to-nucleus signaling pathways. One pathway is triggered by the inositol-requiring enzyme 1 (IRE1), which splices the X-box binding protein 1 (Xbp1) mRNA, thereby enabling expression of XBP1s. Another UPR pathway activates the activating transcription factor 6 (ATF6). Here we show that murine cytomegalovirus (MCMV), a prototypic β-herpesvirus, harnesses the UPR to regulate its own life cycle. MCMV activates the IRE1-XBP1 pathway early post infection to relieve repression by XBP1u, the product of the unspliced Xbp1 mRNA. XBP1u inhibits viral gene expression and replication by blocking the activation of the viral major immediate-early promoter by XBP1s and ATF6. These findings reveal a redundant function of XBP1s and ATF6 as activators of the viral life cycle, and an unexpected role of XBP1u as a potent repressor of both XBP1s and ATF6-mediated activation.

Cells survive by making many different proteins that each carry out specific tasks. To work correctly, each protein must be made and then folded into the right shape. Cells carefully monitor protein folding because unfolded proteins can compromise their viability. A protein called XBP1 is important in controlling how cells respond to unfolded proteins. Normally, cells contain a form of this protein called XBP1u, while increasing numbers of unfolded proteins trigger production of a form called XBP1s. The change from one form to the other is activated by a protein called IRE1.

Viruses often manipulate stress responses like the unfolded protein response to help take control of the cell and produce more copies of the virus. Murine cytomegalovirus, which is known as MCMV for short, is a herpes-like virus that infects mice; it stops IRE1 activation and XBP1s production during the later stages of infection. However, research had shown that the unfolded protein response was triggered for a short time at an early stage of infection with MCMV, and it was unclear why this might be.

Hinte et al. studied the effect of MCMV on cells grown in the laboratory. The experiments showed that a small dose of cell stress, namely activating the unfolded protein response briefly during early infection, helps to activate genes from the virus that allow it to take over the cell. Together, XBP1s and another protein called ATF6 help to switch on the viral genes. The virus also triggers IRE1 helping to reduce the levels of XBP1u, which could slow down the infection. Later, suppressing the unfolded protein response allows copies of the virus to be made faster to help spread the infection.

These findings reveal new details of how viruses precisely manipulate their host cells at different stages of infection. These insights could lead to new ways to manage or prevent viral infections.

IRE1β negatively regulates IRE1α signaling in response to endoplasmic reticulum stress

IRE1β is an ER stress sensor uniquely expressed in epithelial cells lining mucosal surfaces. Here, we show that intestinal epithelial cells expressing IRE1β have an attenuated unfolded protein response to ER stress. When modeled in HEK293 cells and with purified protein, IRE1β diminishes expression and inhibits signaling by the closely related stress sensor IRE1α. IRE1β can assemble with and inhibit IRE1α to suppress stress-induced XBP1 splicing, a key mediator of the unfolded protein response. In comparison to IRE1α, IRE1β has relatively weak XBP1 splicing activity, largely explained by a nonconserved amino acid in the kinase domain active site that impairs its phosphorylation and restricts oligomerization. This enables IRE1β to act as a dominant-negative suppressor of IRE1α and affect how barrier epithelial cells manage the response to stress at the host-environment interface.

Establishment and validation of an endoplasmic reticulum stress reporter to monitor zebrafish ATF6 activity in development and disease

Induction of endoplasmic reticulum (ER) stress is associated with diverse developmental and degenerative diseases. Modified ER homeostasis causes activation of conserved stress pathways at the ER called the unfolded protein response (UPR). ATF6 is a transcription factor activated during ER stress as part of a coordinated UPR. ATF6 resides at the ER and, upon activation, is transported to the Golgi apparatus, where it is cleaved by proteases to create an amino-terminal cytoplasmic fragment (ATF6f). ATF6f translocates to the nucleus to activate transcriptional targets. Here, we describe the establishment and validation of zebrafish reporter lines for ATF6 activity. These transgenic lines are based on a defined and multimerized ATF6 consensus site, which drives either eGFP or destabilized eGFP, enabling dynamic study of ATF6 activity during development and disease. The results show that the reporter is specific for the ATF6 pathway, active during development and induced in disease models known to engage UPR. Specifically, during development, ATF6 activity is highest in the lens, skeletal muscle, fins and gills. The reporter is also activated by common chemical inducers of ER stress, including tunicamycin, thapsigargin and brefeldin A, as well as by heat shock. In models for amyotrophic lateral sclerosis and cone dystrophy, ATF6 reporter expression is induced in spinal cord interneurons or photoreceptors, respectively, suggesting a role for ATF6 response in multiple neurodegenerative diseases. Collectively our results show that these ATF6 reporters can be used to monitor ATF6 activity changes throughout development and in zebrafish models of disease.

This article has an associated First Person interview with the first author of the paper.

Summary: In this study, we validate transgenic zebrafish generated to specifically report the activity of ATF6, representing a major branch of the endoplasmic reticulum stress pathway with functions in development and disease.

Marburg virus regulates the IRE1/XBP1-dependent unfolded protein response to ensure efficient viral replication

Viruses regulate cellular signalling pathways to ensure optimal viral replication. During Marburg virus (MARV) infection, large quantities of the viral glycoprotein GP are produced in the ER; this may result in the activation of the unfolded protein response (UPR). The most conserved pathway to trigger UPR is initiated by IRE1. Activation of IRE1 results in auto-phosphorylation, splicing of the XBP1 mRNA and translation of the XBP1s protein. XBP1s binds cis-acting UPR elements (UPRE) which leads to the enhanced expression of genes which should restore ER homeostasis. XBP1u protein is translated, if IRE1 is not activated. Here we show that ectopic expression of MARV GP activated the IRE1-XBP1 axis of UPR as monitored by UPRE luciferase assays. However, while at 24 h of infection with MARV IRE1 was phosphorylated, expression of XBP1s was only slightly enhanced and UPRE activity was not detected. The IRE1-XBP1 axis was not active at 48 h p.i. Co-expression studies of MARV proteins demonstrated that the MARV protein VP30 suppressed UPRE activation. Co-immunoprecipitation analyses revealed an RNA-dependent interaction of VP30 with XBP1u. Knock-out of IRE1 supported MARV infection at late time points. Taken together, these results suggest that efficient MARV propagation requires specific regulation of IRE1 activity.

KSHV activates unfolded protein response sensors but suppresses downstream transcriptional responses to support lytic replication

Herpesviruses usurp host cell protein synthesis machinery to convert viral mRNAs into proteins, and the endoplasmic reticulum (ER) to ensure proper folding, post-translational modification and trafficking of secreted and transmembrane viral proteins. Overloading ER folding capacity activates the unfolded protein response (UPR), whereby sensor proteins ATF6, PERK and IRE1 initiate a stress-mitigating transcription program that accelerates catabolism of misfolded proteins while increasing ER folding capacity. Kaposi’s sarcoma-associated herpesvirus (KSHV) can be reactivated from latency by chemical induction of ER stress, which causes accumulation of the XBP1s transcription factor that transactivates the viral RTA lytic switch gene. The presence of XBP1s-responsive elements in the RTA promoter suggests that KSHV evolved a mechanism to respond to ER stress. Here, we report that ATF6, PERK and IRE1 were activated upon reactivation from latency and required for efficient KSHV lytic replication; genetic or pharmacologic inhibition of each UPR sensor diminished virion production. Despite UPR sensor activation during KSHV lytic replication, downstream UPR transcriptional responses were restricted; 1) ATF6 was cleaved to activate the ATF6(N) transcription factor but ATF6(N)-responsive genes were not transcribed; 2) PERK phosphorylated eIF2α but ATF4 did not accumulate; 3) IRE1 caused XBP1 mRNA splicing, but XBP1s protein did not accumulate and XBP1s-responsive genes were not transcribed. Ectopic expression of the KSHV host shutoff protein SOX did not affect UPR gene expression, suggesting that alternative viral mechanisms likely mediate UPR suppression during lytic replication. Complementation of XBP1s deficiency during KSHV lytic replication inhibited virion production in a dose-dependent manner in iSLK.219 cells but not in TREx-BCBL1-RTA cells. However, genetically distinct KSHV virions harvested from these two cell lines were equally susceptible to XBP1s restriction following infection of naïve iSLK cells. This suggests that cell-intrinsic properties of BCBL1 cells may circumvent the antiviral effect of ectopic XBP1s expression. Taken together, these findings indicate that while XBP1s plays an important role in reactivation from latency, it can inhibit virus replication at a later step, which the virus overcomes by preventing its synthesis. These findings suggest that KSHV hijacks UPR sensors to promote efficient viral replication while sustaining ER stress.

Like all viruses, Kaposi’s sarcoma-associated herpesvirus (KSHV) uses cellular machinery to create viral proteins. Some of these proteins are folded and modified in the endoplasmic reticulum (ER) and traverse the cellular secretory apparatus. Exceeding ER protein folding capacity activates the unfolded protein response (UPR), which resolves ER stress by putting the brakes on protein synthesis and turning on stress-mitigating genes. We show that KSHV replication activates the three cellular proteins that sense ER stress, which are each required to support efficient viral replication. By contrast, KSHV blocks the UPR gene expression program downstream from each of these activated sensor proteins. The failure to resolve ER stress might normally be expected to put the virus at a disadvantage, but we demonstrate that reversal of this scenario is worse; when we supplement infected epithelial cells with the UPR transcription factor XBP1s to artificially stimulate the production of UPR-responsive gene products, virus replication is blocked at a late stage and very few viruses are released from infected cells. Taken together, these observations suggest that KSHV requires UPR sensor protein activation to replicate but has dramatically altered the outcome to prevent the synthesis of new UPR proteins and sustain stress in the ER compartment.

Development of a Reporter System Monitoring Regulated Intramembrane Proteolysis of the Transmembrane bZIP Transcription Factor ATF6α

When endoplasmic reticulum (ER) functions are perturbed, the ER induces several signaling pathways called unfolded protein response to reestablish ER homeostasis through three ER transmembrane proteins: inositol-requiring enzyme 1 (IRE1), PKR-like ER kinase (PERK), and activating transcription factor 6 (ATF6). Although it is important to measure the activity of ATF6 that can indicate the status of the ER, no specific cell-based reporter assay is currently available. Here, we report a new cell-based method for monitoring ER stress based on the cleavage of ATF6α by sequential actions of proteases at the Golgi apparatus during ER stress. A new expressing vector was constructed by using fusion gene of GAL4 DNA binding domain (GAL4DBD) and activation domain derived from herpes simplex virus VP16 protein (VP16AD) followed by a human ATF6α N-terminal deletion variant. During ER stress, the GAL4DBD-VP16AD(GV)-hATF6α deletion variant was cleaved to liberate active transcription activator encompassing GV-hATF6α fragment which could translocate into the nucleus. The translocated GV-hATF6α fragment strongly induced the expression of firefly luciferase in HeLa Luciferase Reporter cell line containing a stably integrated 5X GAL4 site-luciferase gene. The established double stable reporter cell line HLR-GV-hATF6α(333) represents an innovative tool to investigate regulated intramembrane proteolysis of ATF6α. It can substitute active pATF6(N) binding motif-based reporter cell lines.

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.

Regulation of the ER stress response by a mitochondrial microprotein

Cellular homeostasis relies on having dedicated and coordinated responses to a variety of stresses. The accumulation of unfolded proteins in the endoplasmic reticulum (ER) is a common stress that triggers a conserved pathway called the unfolded protein response (UPR) that mitigates damage, and dysregulation of UPR underlies several debilitating diseases. Here, we discover that a previously uncharacterized 54-amino acid microprotein PIGBOS regulates UPR. PIGBOS localizes to the mitochondrial outer membrane where it interacts with the ER protein CLCC1 at ER–mitochondria contact sites. Functional studies reveal that the loss of PIGBOS leads to heightened UPR and increased cell death. The characterization of PIGBOS reveals an undiscovered role for a mitochondrial protein, in this case a microprotein, in the regulation of UPR originating in the ER. This study demonstrates microproteins to be an unappreciated class of genes that are critical for inter-organelle communication, homeostasis, and cell survival.

Cells trigger an unfolded protein response (UPR) in the endoplasmic reticulum, but its regulation by mitochondria is unclear. Here, the authors report a 54-residue microprotein PIGBOS that participates in inter-organelle contact between the endoplasmic reticulum and the mitochondria and may regulate UPR.

The Unfolded Protein Response: Detecting and Responding to Fluctuations in the Protein-Folding Capacity of the Endoplasmic Reticulum

Most of the secreted and plasma membrane proteins are synthesized on membrane-bound ribosomes on the endoplasmic reticulum (ER). They require engagement of ER-resident chaperones and foldases that assist in their folding and maturation. Since protein homeostasis in the ER is crucial for cellular function, the protein-folding status in the organelle's lumen is continually surveyed by a network of signaling pathways, collectively called the unfolded protein response (UPR). Protein-folding imbalances, or "ER stress," are detected by highly conserved sensors that adjust the ER's protein-folding capacity according to the physiological needs of the cell. We review recent developments in the field that have provided new insights into the ER stress-sensing mechanisms used by UPR sensors and the mechanisms by which they integrate various cellular inputs to adjust the folding capacity of the organelle to accommodate to fluctuations in ER protein-folding demands.

Autosomal Dominant Tubulointerstitial Kidney Disease with Adult Onset due to a Novel Renin Mutation Mapping in the Mature Protein

Autosomal dominant tubulointerstitial kidney disease (ADTKD) is a genetically heterogeneous renal disorder leading to progressive loss of renal function. ADTKD-REN is due to rare mutations in renin, all localized in the protein leader peptide and affecting its co-translational insertion in the endoplasmic reticulum (ER). Through exome sequencing in an adult-onset ADTKD family we identified a new renin variant, p.L381P, mapping in the mature protein. To assess its pathogenicity, we combined genetic data, computational and predictive analysis and functional studies. The L381P substitution affects an evolutionary conserved residue, co-segregates with renal disease, is not found in population databases and is predicted to be deleterious by in silico tools and by structural modelling. Expression of the L381P variant leads to its ER retention and induction of the Unfolded Protein Response in cell models and to defective pronephros development in zebrafish. Our work shows that REN mutations outside of renin leader peptide can cause ADTKD and delineates an adult form of ADTKD-REN, a condition which has usually its onset in childhood. This has implications for the molecular diagnosis and the estimated prevalence of the disease and points at ER homeostasis as a common pathway affected in ADTKD-REN, and possibly more generally in ADTKD.

Mutant p53 improves cancer cells' resistance to endoplasmic reticulum stress by sustaining activation of the UPR regulator ATF6

Missense mutations in the TP53 gene are frequent in human cancers, giving rise to mutant p53 proteins that can acquire oncogenic properties. Gain of function mutant p53 proteins can enhance tumour aggressiveness by promoting cell invasion, metastasis and chemoresistance. Accumulating evidences indicate that mutant p53 proteins can also modulate cell homeostatic processes, suggesting that missense p53 mutation may increase resistance of tumour cells to intrinsic and extrinsic cancer-related stress conditions, thus offering a selective advantage. Here we provide evidence that mutant p53 proteins can modulate the Unfolded Protein Response (UPR) to increase cell survival upon Endoplasmic Reticulum (ER) stress, a condition to which cancer cells are exposed during tumour formation and progression, as well as during therapy. Mechanistically, this action of mutant p53 is due to enhanced activation of the pro-survival UPR effector ATF6, coordinated with inhibition of the pro-apoptotic UPR effectors JNK and CHOP. In a triple-negative breast cancer cell model with missense TP53 mutation, we found that ATF6 activity is necessary for viability and invasion phenotypes. Together, these findings suggest that ATF6 inhibitors might be combined with mutant p53-targeting drugs to specifically sensitise cancer cells to endogenous or chemotherapy-induced ER stress.

MicroRNA-33-3p Regulates Vein Endothelial Cell Apoptosis in Selenium-Deficient Broilers by Targeting E4F1

Selenium (Se) is a type of nutrient element. The tissues of organisms can have pathological damage, including apoptosis, due to Se deficiency. Apoptosis is an important cell process and plays a key role in vascular disease and Se-deficient symptoms. In this study, the Se-deficient broiler model was duplicated, miR-33-3p in the vein was overexpressed in response to Se-deficiency, and miR-33-3p target gene E4F transcription factor 1 (E4F1) expression was also confirmed. We utilized ectopic miR-33-3p expression to validate its function for apoptosis. The results showed that miR-33-3p-targeted E4F1 are involved in the glucose-regulated protein 78- (GRP78-) induced endoplasmic reticulum stress (ERS) apoptosis pathway. We presumed that Se deficiency might trigger apoptosis via downregulating miR-33-3p. Interestingly, the miR-33-3p inhibitor and VER-155008 (GRP78 inhibitor) partly hindered the apoptosis caused by Se deficiency. Thus, the above information provides a new avenue toward understanding the mechanism of Se deficiency and reveals a novel apoptotic injury regulation model in vascular disease.

Endoplasmic Reticulum Stress in Metabolic Liver Diseases and Hepatic Fibrosis

Endoplasmic reticulum (ER) stress is a major contributor to liver disease and hepatic fibrosis, but the role it plays varies depending on the cause and progression of the disease. Furthermore, ER stress plays a distinct role in hepatocytes versus hepatic stellate cells (HSCs), which adds to the complexity of understanding ER stress and its downstream signaling through the unfolded protein response (UPR) in liver disease. Here, the authors focus on the current literature of ER stress in nonalcoholic and alcoholic fatty liver diseases, how ER stress impacts hepatocyte injury, and the role of ER stress in HSC activation and hepatic fibrosis. This review provides insight into the complex signaling and regulation of the UPR, parallels and distinctions between different liver diseases, and how ER stress may be targeted as an antisteatotic or antifibrotic therapy to limit the progression of liver disease.

ATF6α Activation Enhances Survival against Chemotherapy and Serves as a Prognostic Indicator in Osteosarcoma

Patients with metastatic or relapsed/refractory osteosarcoma (OS) have a 5-year survival rate of <30%. This has remained unchanged over several decades. One of the factors contributing to lack of improvement in survival is the development of chemoresistance. Hence, elucidating and targeting the mechanisms that promote survival against chemotherapy and lead to chemoresistance is pivotal to improving outcomes for these patients. We identified that endoplasmic reticulum (ER) stress-activated transcription factor, ATF6α, is essential for the survival of OS cells against chemotherapy induced cell death. ATF6α cleavage and activity were enhanced in OS cells compared to normal osteoblasts and knockdown of ATF6α expression enhanced sensitivity of OS cells against chemotherapy induced cell death. This was in part due to increased Bax activation. Pharmacologic inhibition or knock-down of downstream targets of ATF6α, protein disulfide isomerases (PDI) and ERO1β, a thiol oxidase that is involved in the re-oxidation of PDIs also independently induced pronounced killing of OS cells following chemotherapy. Analysis of primary tumors from OS patients reveals that patients with high levels of nuclear ATF6α: (1) also had increased expression of its downstream targets the chaperone BiP and enzyme PDI, (2) had a significant likelihood of developing metastasis at diagnosis, (3) had significantly poorer overall and progression free survival, and (4) had poorer response to chemotherapy. These findings suggest that targeting survival signaling by the ATF6α pathway in OS cells may favor eradication of refractory OS tumor cells and ATF6α could be a useful predictor for chemo-responsiveness and prognosis.