Identification of the G13 (cAMP-response-element-binding protein-related protein) gene product related to activating transcription factor 6 as a transcriptional activator of the mammalian unfolded protein response

Regulatory crosstalk within the mammalian unfolded protein response

Increased demands on the protein folding capacity of the endoplasmic reticulum (ER) trigger the unfolded protein response (UPR). Comprised of a tripartite signaling system, the UPR regulates translation and gene transcription to manifest pro-adaptive and, if necessary, pro-apoptotic outcomes. The three UPR pathways, initiated by activating transcription factor 6, inositol requiring enzyme 1, and protein kinase RNA-activated-like ER kinase (PERK), direct distinct downstream signaling events. However, it is becoming increasingly clear that interplay between the cascades is vital in shaping the UPR. In particular, recent discoveries have revealed that PERK-dependent signals mediate both inter- and intra-pathway regulation within the UPR, underscoring the critical role of the PERK pathway in the cellular response to ER stress.

Stress and the inflammatory process: a major cause of pancreatic cell death in type 2 diabetes

Type 2 diabetes (T2D) is a complex metabolic disorder characterized by hyperglycemia in the context of insulin resistance, which precedes insulin deficiency as a result of β-cell failure. Accumulating evidence indicates that β-cell loss in T2D results as a response to the combination of oxidative stress and endoplasmic reticulum (ER) stress. Failure of the ER's adaptive capacity and further activation of the unfolded protein response may trigger macroautophagy (hereafter referred as autophagy) as a process of self-protection and inflammation. Many studies have shown that inflammation plays a very important role in the pathogenesis of T2D. Inflammatory mechanisms and cytokine production activated by stress via the inflammasome may further alter the normal structure of β-cells by inducing pancreatic islet cell apoptosis. Thus, the combination of oxidative and ER stress, together with autophagy insufficiency and inflammation, may contribute to β-cell death or dysfunction in T2D. Therapeutic approaches aimed at ameliorating stress and inflammation may therefore prove to be promising targets for the development of new diabetes treatment methods. Here, we discuss different mechanisms involved in stress and inflammation, and the role of antioxidants, endogenous and chemical chaperones, and autophagic pathways, which may shift the tendency from ER stress and apoptosis toward cell survival. Strategies targeting cell survival can be essential for relieving ER stress and reestablishing homeostasis, which may diminish inflammation and prevent pancreatic β-cell death associated with T2D.

Role of the unfolded protein response in β cell compensation and failure during diabetes

Pancreatic β cell failure leads to diabetes development. During disease progression, β cells adapt their secretory capacity to compensate the elevated glycaemia and the peripheral insulin resistance. This compensatory mechanism involves a fine-tuned regulation to modulate the endoplasmic reticulum (ER) capacity and quality control to prevent unfolded proinsulin accumulation, a major protein synthetized within the β cell. These signalling pathways are collectively termed unfolded protein response (UPR). The UPR machinery is required to preserve ER homeostasis and β cell integrity. Moreover, UPR actors play a key role by regulating ER folding capacity, increasing the degradation of misfolded proteins, and limiting the mRNA translation rate. Recent genetic and biochemical studies on mouse models and human UPR sensor mutations demonstrate a clear requirement of the UPR machinery to prevent β cell failure and increase β cell mass and adaptation throughout the progression of diabetes. In this review we will highlight the specific role of UPR actors in β cell compensation and failure during diabetes.

Regulation of the transcriptome by ER stress: non-canonical mechanisms and physiological consequences

The mammalian unfolded protein response (UPR) is propagated by three ER-resident transmembrane proteins, each of which initiates a signaling cascade that ultimately culminates in production of a transcriptional activator. The UPR was originally characterized as a pathway for upregulating ER chaperones, and a comprehensive body of subsequent work has shown that protein synthesis, folding, oxidation, trafficking, and degradation are all transcriptionally enhanced by the UPR. However, the global reach of the UPR extends to genes involved in diverse physiological processes having seemingly little to do with ER protein folding, and this includes a substantial number of mRNAs that are suppressed by stress rather than stimulated. Through multiple non-canonical mechanisms emanating from each of the UPR pathways, the cell dynamically regulates transcription and mRNA degradation. Here we highlight these mechanisms and their increasingly appreciated impact on physiological processes.

The essential biology of the endoplasmic reticulum stress response for structural and computational biologists

The endoplasmic reticulum (ER) stress response is a cytoprotective mechanism that maintains homeostasis of the ER by upregulating the capacity of the ER in accordance with cellular demands. If the ER stress response cannot function correctly, because of reasons such as aging, genetic mutation or environmental stress, unfolded proteins accumulate in the ER and cause ER stress-induced apoptosis, resulting in the onset of folding diseases, including Alzheimer's disease and diabetes mellitus. Although the mechanism of the ER stress response has been analyzed extensively by biochemists, cell biologists and molecular biologists, many aspects remain to be elucidated. For example, it is unclear how sensor molecules detect ER stress, or how cells choose the two opposite cell fates (survival or apoptosis) during the ER stress response. To resolve these critical issues, structural and computational approaches will be indispensable, although the mechanism of the ER stress response is complicated and difficult to understand holistically at a glance. Here, we provide a concise introduction to the mammalian ER stress response for structural and computational biologists.

Arsenic trioxide induces unfolded protein response in vascular endothelial cells

Chronic arsenic exposure has been linked to endothelial dysfunction and apoptosis. We investigate the involvement of unfolded protein response (UPR) signaling in the arsenic-mediated cytotoxicity of the SVEC4-10 mouse endothelial cells. The SVEC4-10 cells underwent apoptosis in response to As2O3 dose- and time-dependently, accompanied by increased accumulation of calcium, and activation of caspase-3. These phenomena were completely inhibited by α-lipoic acid (LA), which did not scavenge ROS over-production, but were only partially or not ameliorated by tiron, a potent superoxide scavenger. Moreover, arsenic activated UPR, leading to phosphorylation of eukaryotic translation initiation factor 2 subunit α (eIF2α), induction of ATF4, and processing of ATF6. Treatment with arsenic also triggered the expression of endoplasmic reticulum (ER) stress markers, GRP78 (glucose-regulated protein), and CHOP (C/EBP homologous protein). The activation of eIF2α, ATF4 and ATF6 and expression of GRP78 and CHOP are repressed by both LA and tiron, indicating arsenic-induced UPR is mediated through ROS-dependent and ROS-independent pathways. Arsenic also induced ER stress-inducible genes, BAX, PUMA (p53 upregulated modulator of apoptosis), TRB3 (tribbles-related protein 3), and SNAT2 (sodium-dependent neutral amino acid transporter 2). Consistent with intracellular calcium and cell viability data, ROS may not be important in arsenic-induced death, because tiron did not affect the expression of these pro-apoptotic genes. In addition, pretreatment with salubrinal, a selective inhibitor of eIF2α dephosphorylation, enhanced arsenic-induced GRP78 and CHOP expression and partially prevented arsenic cytotoxicity in SVEC4-10 cells. Taken together, these results suggest that arsenic-induced endothelial cytotoxicity is associated with ER stress, which is mediated by ROS-dependent and ROS-independent signaling.

Transcriptional regulation of mouse mesencephalic astrocyte-derived neurotrophic factor in Neuro2a cells

Mesencephalic astrocyte-derived neurotrophic factor (MANF) is a novel type of trophic factor. Recent studies indicate that the MANF gene is induced in response to endoplasmic reticulum (ER) stress through ER stress response element II (ERSE-II) in its 5'-flanking region. In this study, we evaluated the roles of six ER stress response transcription factors in the regulation of the promoter activities of the mouse MANF gene via ERSE-II using various types of mutant MANF luciferase reporter constructs. Treatment with thapsigargin (Tg) induced MANF mRNA generation in parallel with the elevation of ATF6α, sXBP and Luman mRNA levels in Neuro2a cells. Of the six transcription factors, ATF6β most strongly increased the MANF promoter activity via ERSE-II, while the effects of ATF6β and sXBP1 were moderate. However, overexpression of Luman or OASIS did not enhance ERSE-II-dependent MANF promoter activity in Neuro2a cells. To evaluate the relationships between transcription factors in the regulation of ERSE-II-dependent MANF promoter activity, we transfected two effective transcription factor constructs chosen from ATF6α, ATF6β, uXBP1 and sXBP1 into Neuro2a cells with the MANF reporter construct. The MANF promoter activity induced by co-transfection of ATF6α with ATF6β was significantly lower than that induced by ATF6α alone, while other combinations did not show any effect on the ERSE-II-dependent MANF promoter activity in Neuro2a cells. Our study is the first to show the efficiency of ER stress-related transcription factors for ERSE-II in activating the transcription of the mouse MANF gene in Neuro2a cells.

Signaling pathways from the endoplasmic reticulum and their roles in disease

The endoplasmic reticulum (ER) is an organelle in which newly synthesized secretory and transmembrane proteins are assembled and folded into their correct tertiary structures. However, many of these ER proteins are misfolded as a result of various stimuli and gene mutations. The accumulation of misfolded proteins disrupts the function of the ER and induces ER stress. Eukaryotic cells possess a highly conserved signaling pathway, termed the unfolded protein response (UPR), to adapt and respond to ER stress conditions, thereby promoting cell survival. However, in the case of prolonged ER stress or UPR malfunction, apoptosis signaling is activated. Dysfunction of the UPR causes numerous conformational diseases, including neurodegenerative disease, metabolic disease, inflammatory disease, diabetes mellitus, cancer, and cardiovascular disease. Thus, ER stress-induced signaling pathways may serve as potent therapeutic targets of ER stress-related diseases. In this review, we will discuss the molecular mechanisms of the UPR and ER stress-induced apoptosis, as well as the possible roles of ER stress in several diseases.

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.

The role of the unfolded protein response in diabetes mellitus

The endoplasmic reticulum (ER) plays a key role in the synthesis and modification of secretory and membrane proteins in all eukaryotic cells. Under normal conditions, these proteins are correctly folded and assembled in the ER. However, when cells are exposed to environmental factors such as overproduction of ER proteins, viral infections, or glucose deprivation, the secretory and membrane proteins can accumulate in unfolded or misfolded forms in the lumen of the ER, and consequently, cause stress in the ER. To maintain cellular homeostasis, cells induce several responses to ER stress. In mammalian cells, ER stress responses are induced by a diversity of signal pathways. There are three ER-located transmembrane proteins that play important roles in mammalian ER stress responses: activating transcription factor 6, inositol-requiring protein 1, and protein kinase RNA-like endoplasmic reticulum kinase. ER stress is linked to various diseases, including diabetes. This review highlights the particular importance of ER stress-responsive molecules in insulin biosynthesis, glyconeogenesis, insulin resistance, glucose intolerance, and pancreatic β-cell apoptosis. An understanding of the pathogenic mechanism of diabetes from the aspect of ER stress is crucial in formulating therapeutic strategies.

ATF6α/β-mediated adjustment of ER chaperone levels is essential for development of the notochord in medaka fish

ATF6α and ATF6β are membrane-bound transcription factors activated by regulated intramembrane proteolysis in response to endoplasmic reticulum (ER) stress to induce various ER quality control proteins. ATF6α- and ATF6β single-knockout mice develop normally, but ATF6α/β double knockout causes embryonic lethality, the reason for which is unknown. Here we show in medaka fish that ATF6α is primarily responsible for transcriptional induction of the major ER chaperone BiP and that ATF6α/β double knockout, but not ATF6α- or ATF6β single knockout, causes embryonic lethality, as in mice. Analyses of ER stress reporters reveal that ER stress occurs physiologically during medaka early embryonic development, particularly in the brain, otic vesicle, and notochord, resulting in ATF6α- and ATF6β-mediated induction of BiP, and that knockdown of the α1 chain of type VIII collagen reduces such ER stress. The absence of transcriptional induction of several ER chaperones in ATF6α/β double knockout causes more profound ER stress and impaired notochord development, which is partially rescued by overexpression of BiP. Thus ATF6α/β-mediated adjustment of chaperone levels to increased demands in the ER is essential for development of the notochord, which synthesizes and secretes large amounts of extracellular matrix proteins to serve as the body axis before formation of the vertebra.

Endoplasmic reticulum and the unfolded protein response: dynamics and metabolic integration

The endoplasmic reticulum (ER) is a dynamic intracellular organelle with multiple functions essential for cellular homeostasis, development, and stress responsiveness. In response to cellular stress, a well-established signaling cascade, the unfolded protein response (UPR), is activated. This intricate mechanism is an important means of re-establishing cellular homeostasis and alleviating the inciting stress. Now, emerging evidence has demonstrated that the UPR influences cellular metabolism through diverse mechanisms, including calcium and lipid transfer, raising the prospect of involvement of these processes in the pathogenesis of disease, including neurodegeneration, cancer, diabetes mellitus and cardiovascular disease. Here, we review the distinct functions of the ER and UPR from a metabolic point of view, highlighting their association with prevalent pathologies.

C/EBP homologous protein (CHOP) contributes to suppression of metabolic genes during endoplasmic reticulum stress in the liver

The unfolded protein response (UPR) senses stress in the endoplasmic reticulum (ER) and initiates signal transduction cascades that culminate in changes to gene regulation. Long recognized as a means for improving ER protein folding through up-regulation of ER chaperones, the UPR is increasingly recognized to play a role in the regulation of metabolic pathways. ER stress is clearly connected to altered metabolism in tissues such as the liver, but the mechanisms underlying this connection are only beginning to be elucidated. Here, working exclusively in vivo, we tested the hypothesis that the UPR-regulated CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP) participates in the transcriptional regulation of metabolism during hepatic ER stress. We found that metabolic dysregulation was associated with induction of eIF2α signaling and CHOP up-regulation during challenge with tunicamycin or Velcade. CHOP was necessary for suppression of genes encoding the transcriptional master regulators of lipid metabolism: Cebpa, Ppara, and Srebf1. This action of CHOP required a contemporaneous CHOP-independent stress signal. CHOP bound directly to C/EBP-binding regions in the promoters of target genes, whereas binding of C/EBPα and C/EBPβ to the same regions was diminished during ER stress. Our results thus highlight a role for CHOP in the transcriptional regulation of metabolism.

Intricately Regulated: A Cellular Toolbox for Fine-Tuning XBP1 Expression and Activity

Stress in the endoplasmic reticulum (ER) triggers the unfolded protein response (UPR), a signaling mechanism that allows cellular adaptation to ER stress by engaging pro-adaptive transcription factors and alleviating protein folding demand. One such transcription factor, X-box binding protein (XBP1), originates from the inositol-requiring transmembrane kinase/endoribonuclease 1 (IRE1) UPR stress sensor. XBP1 up-regulates a pool of genes involved in ER protein translocation, protein folding, vesicular trafficking and ER- associated protein degradation. Recent data suggest that the regulation of XBP1 expression and transcriptional activity may be a tissue- and stress-dependent phenomenon. Moreover, the intricacies involved in “fine-tuning” XBP1 activity in various settings are now coming to light. Here, we provide an overview of recent developments in understanding the regulatory mechanisms underlying XBP1 expression and activity and discuss the significance of these new insights.

Protein disulfide isomerase-associated 6 is an ATF6-inducible ER stress response protein that protects cardiac myocytes from ischemia/reperfusion-mediated cell death

Proper folding of secreted and transmembrane proteins made in the rough endoplasmic reticulum (ER) requires oxygen for disulfide bond formation. Accordingly, ischemia can impair ER protein folding and initiate the ER stress response, which we previously showed is activated in the ischemic heart and in culture cardiac myocytes subjected to simulated ischemia. ER stress and ischemia activate the transcription factor, activating transcription factor 6 (ATF6), which induces numerous genes, many of which have not been identified, or examined in the heart. Using an ATF6 transgenic mouse model, we previously showed that ATF6 protected the heart from ischemic damage; however, the mechanism of this protection remains to be determined. In this study, we showed that, in the mouse heart, and in cultured cardiac myocytes, ATF6 induced the protein disulfide isomerase associated 6 (PDIA6) gene, which encodes an ER enzyme that catalyzes protein disulfide bond formation. Moreover, in cultured cardiac myocytes, ER stress-mediated PDIA6 promoter activation was ATF6-dependent, and required an ER stress response element (ERSE) and a nearby CCAAT box element. Electromobility shift assays and chromatin immunoprecipitation showed that ATF6 bound to the ERSE in the PDIA6 promoter, in vitro, and in the mouse heart, in vivo. Gain- and loss-of-function studies showed that PDIA6 protected cardiac myocytes against simulated ischemia/reperfusion-induced death in a manner that was dependent on the catalytic activity of PDIA6. Thus, by facilitating disulfide bond formation, and enhanced ER protein folding, PDIA6 may contribute to the protective effects of ATF6 in the ischemic mouse heart.

The specialized unfolded protein response of B lymphocytes: ATF6α-independent development of antibody-secreting B cells

B lymphocytes, like all mammalian cells, are equipped with the unfolded protein response (UPR), a complex signaling system allowing for both pro- and mal-adaptive responses to increased demands on the endoplasmic reticulum (ER). The UPR is comprised of three signaling pathways initiated by the ER transmembrane stress sensors, IRE1α/β, PERK and ATF6α/β. Activation of IRE1 yields XBP1(S), a transcription factor that directs expansion of the ER and enhances protein biosynthetic and secretory machinery. XBP1(S) is essential for the differentiation of B lymphocytes into antibody-secreting cells. In contrast, the PERK pathway, a regulator of translation and transcription, is dispensable for the generation of antibody-secreting cells. Functioning as a transcription factor, ATF6α can augment ER quality control processes and drive ER expansion, but the potential role of this UPR pathway in activated B cells has not been investigated. Here, we report studies of ATF6α-deficient B cells demonstrating that ATF6α is not required for the development of antibody-secreting cells. Thus, when B cells are stimulated to secrete antibody, a specialized UPR relies exclusively on the IRE1-XBP1 pathway to remodel the ER and expand cellular secretory capacity.

Endoplasmic reticulum stress and type 2 diabetes

Given the functional importance of the endoplasmic reticulum (ER), an organelle that performs folding, modification, and trafficking of secretory and membrane proteins to the Golgi compartment, the maintenance of ER homeostasis in insulin-secreting β-cells is very important. When ER homeostasis is disrupted, the ER generates adaptive signaling pathways, called the unfolded protein response (UPR), to maintain homeostasis of this organelle. However, if homeostasis fails to be restored, the ER initiates death signaling pathways. New observations suggest that both chronic hyperglycemia and hyperlipidemia, known as important causative factors of type 2 diabetes (T2D), disrupt ER homeostasis to induce unresolvable UPR activation and β-cell death. This review examines how the UPR pathways, induced by high glucose and free fatty acids (FFAs), interact to disrupt ER function and cause β-cell dysfunction and death.

The unfolded protein response at the crossroads of cellular life and death during endoplasmic reticulum stress

One of the early cellular responses to endoplasmic reticulum (ER) stress is the activation of the unfolded protein response (UPR). ER stress and the UPR are both implicated in numerous human diseases and pathologies. In spite of this, our knowledge of the molecular mechanisms that regulate cell fate following ER stress is limited. The UPR is initiated by three ER transmembrane receptors: PKR-like ER kinase (PERK), activating transcription factor (ATF) 6 and inositol-requiring enzyme 1 (IRE1). These proteins sense the accumulation of unfolded proteins and their activation triggers specific adaptive responses to resolve the stress. Intriguingly, the very same receptors can initiate signalling pathways that lead to apoptosis when the attempts to resolve the ER stress fail. In this review, we describe the known pro-apoptotic signalling pathways emanating from activated PERK, ATF6 and IRE1 and discuss how their signalling switches from an adaptive to a pro-apoptotic response.

Endoplasmic reticulum stress and inflammation: mechanisms and implications in diabetic retinopathy

The endoplasmic reticulum (ER) is the primary cellular compartment where proteins are synthesized and modified before they can be transported to their destination. Dysfunction of the ER impairs protein homeostasis and leads to the accumulation of misfolded/unfolded proteins in the ER, or ER stress. While it has long been recognized that ER stress is a major cause of conformational disorders, such as Alzheimer's disease, Huntington's disease, certain types of cancer, and type 2 diabetes, recent evidence suggests that ER stress is also implicated in many chronic inflammatory diseases. These diseases include irritable bowel syndrome, atherosclerosis, diabetic complications, and many others. Diabetic retinopathy is a common microvascular complication of diabetes, characterized by chronic inflammation, progressive damage to retinal vascular and neuronal cells, vascular leakage, and abnormal blood vessel growth (neovascularization). In this review, we discuss the role and mechanisms of ER stress in retinal inflammation and vascular damage in diabetic retinopathy.

Sensing endoplasmic reticulum stress

This chapter provides an overview of our present understanding of mechanisms of sensing protein folding status and endoplasmic reticulum (ER) stress in eukaryotic cells. The ER folds and matures most secretory and transmembrane proteins. Mis- or unfolded proteins are sensed by specialized ER stress sensors, such as IRE1, PERK and ATF6, which initiate several cellular responses and signaling pathways to restore ER homeostasis. These intracellular signaling events are called the unfolded protein response (UPR). Here we focus on how ER stress and protein folding status in the ER are sensed by the ER stress sensors by summarizing results from recent structural, biochemical and genetic approaches.

Endoplasmic Reticulum Stress and Unfolded Protein Response Pathways: Potential for Treating Age-related Retinal Degeneration

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) and their aggregation impair normal cellular function and can be toxic, leading to cell death. Prolonged expression of misfolded proteins triggers ER stress, which initiates a cascade of reactions called the unfolded protein response (UPR). Protein misfolding is the basis for a variety of disorders known as ER storage or conformational diseases. There are an increasing number of eye disorders associated with misfolded proteins and pathologic ER responses, including retinitis pigmentosa (RP). Herein we review the basic cellular and molecular biology of UPR with focus on pathways that could be potential targets for treating retinal degenerative diseases.

ER stress and apoptosis: a new mechanism for retinal cell death

The endoplasmic reticulum (ER) is the primary subcellular organelle where proteins are synthesized and folded. When the homeostasis of the ER is disturbed, unfolded or misfolded proteins accumulate in the ER lumen, resulting in ER stress. In response to ER stress, cells activate a set of tightly controlled regulatory programs, known as the unfolded protein response (UPR), to restore the normal function of the ER. However, if ER stress is sustained and the adaptive UPR fails to eliminate unfolded/misfolded proteins, apoptosis will occur to remove the stressed cells. In recent years, a large body of studies has shown that ER stress-induced apoptosis is implicated in numerous human diseases, such as diabetes and neurogenerative diseases. Moreover, emerging evidence supports a role of ER stress in retinal apoptosis and cell death in blinding disorders such as age-related macular degeneration and diabetic retinopathy. In the present review, we summarize recent progress on ER stress and apoptosis in retinal diseases, focusing on various proapoptotic and antiapoptotic pathways that are activated by the UPR, and discuss how these pathways contribute to ER stress-induced apoptosis in retinal cells.

Binding of activating transcription factor 6 to the A5/Core of the rat insulin II gene promoter does not mediate its transcriptional repression

Pancreatic β-cells have a well-developed endoplasmic reticulum due to their highly specialized secretory function to produce insulin in response to glucose and nutrients. It has been previously reported that overexpression of activating transcription factor 6 (ATF6) reduces insulin gene expression in part via upregulation of small heterodimer partner. In this study, we investigated whether ATF6 directly binds to the insulin gene promoter, and whether its direct binding represses insulin gene promoter activity. A bioinformatics analysis identified a putative ATF6 binding site in the A5/Core region of the rat insulin II gene promoter. Direct binding of ATF6 was confirmed using several approaches. Electrophoretic mobility shift assays in nuclear extracts from MCF7 cells, isolated rat islets and insulin-secreting HIT-T15 cells showed ATF6 binding to the native A5/Core of the rat insulin II gene promoter. Antibody-mediated supershift analyses revealed the presence of both ATF6 isoforms, ATF6α and ATF6β, in the complex. Chromatin immunoprecipitation assays confirmed the binding of ATF6α and ATF6β to a region encompassing the A5/Core of the rat insulin II gene promoter in isolated rat islets. Overexpression of the active (cleaved) fragment of ATF6α, but not ATF6β, inhibited the activity of an insulin promoter-reporter by 50%. However, the inhibitory effect of ATF6α was insensitive to mutational inactivation or deletion of the A5/Core. Therefore, although ATF6 binds directly to the A5/Core of the rat insulin II gene promoter, this direct binding does not appear to contribute to its repressive activity.

A review of the mammalian unfolded protein response

Proteins requiring post-translational modifications such as N-linked glycosylation are processed in the endoplasmic reticulum (ER). A diverse array of cellular stresses can lead to dysfunction of the ER and ultimately to an imbalance between protein-folding capacity and protein-folding load. Cells monitor protein folding by an inbuilt quality control system involving both the ER and the Golgi apparatus. Unfolded or misfolded proteins are tagged for degradation via ER-associated degradation (ERAD) or sent back through the folding cycle. Continued accumulation of incorrectly folded proteins can also trigger the unfolded protein response (UPR). In mammalian cells, UPR is a complex signaling program mediated by three ER transmembrane receptors: activating transcription factor 6 (ATF6), inositol requiring kinase 1 (IRE1) and double-stranded RNA-activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK). UPR performs three functions, adaptation, alarm, and apoptosis. During adaptation, the UPR tries to reestablish folding homeostasis by inducing the expression of chaperones that enhance protein folding. Simultaneously, global translation is attenuated to reduce the ER folding load while the degradation rate of unfolded proteins is increased. If these steps fail, the UPR induces a cellular alarm and mitochondrial mediated apoptosis program. UPR malfunctions have been associated with a wide range of disease states including tumor progression, diabetes, as well as immune and inflammatory disorders. This review describes recent advances in understanding the molecular structure of UPR in mammalian cells, its functional role in cellular stress, and its pathophysiology.

UPR-Mediated Membrane Biogenesis in B Cells

The unfolded protein response (UPR) can coordinate the regulation of gene transcription and protein translation to balance the load of client proteins with the protein folding and degradative capacities of the ER. Increasing evidence also implicates the UPR in the regulation of lipid synthesis and membrane biogenesis. The differentiation of B lymphocytes into antibody-secreting cells is marked by significant expansion of the ER, the site for antibody synthesis and assembly. In activated B cells, the demand for membrane protein and lipid components leads to activation of the UPR transcriptional activator XBP1(S) which, in turn, initiates a cascade of biochemical events that enhance supplies of phospholipid precursors and build machinery for the synthesis, maturation, and transport of secretory proteins. The alterations in lipid metabolism that occur during this developmental transition and the impact of membrane phospholipid restriction on B cell secretory characteristics are discussed in this paper.

Alteration of ceramide synthase 6/C16-ceramide induces activating transcription factor 6-mediated endoplasmic reticulum (ER) stress and apoptosis via perturbation of cellular Ca2+ and ER/Golgi membrane network

Mechanisms that regulate endoplasmic reticulum (ER) stress-induced apoptosis in cancer cells remain enigmatic. Recent data suggest that ceramide synthase1-6 (CerS1-6)-generated ceramides, containing different fatty acid chain lengths, might exhibit distinct and opposing functions, such as apoptosis versus survival in a context-dependent manner. Here, we investigated the mechanisms involved in the activation of one of the major ER stress response proteins, ATF-6, and subsequent apoptosis by alterations of CerS6/C(16)-ceramide. Induction of wild type (WT), but not the catalytically inactive mutant CerS6, increased tumor growth in SCID mice, whereas siRNA-mediated knockdown of CerS6 induced ATF-6 activation and apoptosis in multiple human cancer cells. Down-regulation of CerS6/C(16)-ceramide, and not its further metabolism to glucosylceramide or sphingomyelin, activated ATF-6 upon treatment with ER stress inducers tunicamycin or SAHA (suberoylanilide hydroxamic acid). Induction of WT-CerS6 expression, but not its mutant, or ectopic expression of the dominant-negative mutant form of ATF-6 protected cells from apoptosis in response to CerS6 knockdown and tunicamycin or SAHA treatment. Mechanistically, ATF-6 activation was regulated by a concerted two-step process involving the release of Ca(2+) from the ER stores ([Ca(2+)](ER)), which resulted in the fragmentation of Golgi membranes in response to CerS6/C(16)-ceramide alteration. This resulted in the accumulation of pro-ATF-6 in the disrupted ER/Golgi membrane network, where pro-ATF6 is activated. Accordingly, ectopic expression of a Ca(2+) chelator calbindin prevented the Golgi fragmentation, ATF-6 activation, and apoptosis in response to CerS6/C(16)-ceramide down-regulation. Overall, these data suggest a novel mechanism of how CerS6/C(16)-ceramide alteration activates ATF6 and induces ER-stress-mediated apoptosis in squamous cell carcinomas.

The eIF2 kinase PERK and the integrated stress response facilitate activation of ATF6 during endoplasmic reticulum stress

This study shows that the eIF2 kinase PERK is required not only for translational control but also for activation of ATF6 and its target genes in the unfolded protein response. The PERK pathway facilitates both the synthesis of ATF6 and trafficking of ATF6 from the endoplasmic reticulum to the Golgi for intramembrane proteolysis and activation of ATF6.

Disruptions of the endoplasmic reticulum (ER) that perturb protein folding cause ER stress and elicit an unfolded protein response (UPR) that involves translational and transcriptional changes in gene expression aimed at expanding the ER processing capacity and alleviating cellular injury. Three ER stress sensors (PERK, ATF6, and IRE1) implement the UPR. PERK phosphorylation of the α subunit of eIF2 during ER stress represses protein synthesis, which prevents further influx of ER client proteins. Phosphorylation of eIF2α (eIF2α∼P) also induces preferential translation of ATF4, a transcription activator of the integrated stress response. In this study we show that the PERK/eIF2α∼P/ATF4 pathway is required not only for translational control, but also for activation of ATF6 and its target genes. The PERK pathway facilitates both the synthesis of ATF6 and trafficking of ATF6 from the ER to the Golgi for intramembrane proteolysis and activation of ATF6. As a consequence, liver-specific depletion of PERK significantly reduces both the translational and transcriptional phases of the UPR, leading to reduced protein chaperone expression, disruptions of lipid metabolism, and enhanced apoptosis. These findings show that the regulatory networks of the UPR are fully integrated and help explain the diverse biological defects associated with loss of PERK.

Animal models in the study of the unfolded protein response

The endoplasmic reticulum, a highly dynamic and complex organelle, is the site for synthesis, folding, and modification of transmembrane and secretory proteins. Any disruptions to the endoplasmic reticulum such as an accumulation of misfolded or unfolded proteins results in activation of the unfolded protein response (UPR). The UPR is comprised of three distinct signal transduction pathways that work to restore homeostasis to the endoplasmic reticulum. This review summarizes select mouse models available to study the UPR and the information learned from the analyses of these models.

Luminal domain of ATF6 alone is sufficient for sensing endoplasmic reticulum stress and subsequent transport to the Golgi apparatus

The transcription factor ATF6 is constitutively synthesized as a type II transmembrane protein embedded in the endoplasmic reticulum (ER). When unfolded proteins accumulate in the ER, ATF6 senses such ER stress via an as yet undetermined mechanism and relocates to the Golgi apparatus where it is cleaved by sequential action of Site-1 and Site-2 proteases, allowing liberated N-terminal fragments to translocate into the nucleus. This ATF6-mediated transcriptional induction of ER-localized molecular chaperones and folding enzymes together with components of ER-associated degradation leads to the maintenance of ER homeostasis in mammals. Here, we demonstrated that the luminal domain of ATF6 alone is sufficient for sensing ER stress and subsequent transportation to the Golgi apparatus. This domain of ATF6 was inserted between the N-terminal signal sequence and C-terminal tandem affinity purification tag. The resulting ATF6(C)-TAP translocated into the ER, where it was glycosylated and disulfide bonded. ATF6(C)-TAP occurred as monomer and dimer, and exhibited a relatively short half-life, similar to full-length ATF6. On application of dithiothreitol- or thapsigargin-induced ER stress, the ER chaperone BiP dissociated from ATF6(C)-TAP, and ATF6(C)-TAP was transported to the Golgi apparatus and then secreted into medium. Calnexin and protein disulfide isomerase were identified as cellular proteins capable of binding to ATF6(C)-TAP in addition to BiP, and subsequent analysis revealed that protein disulfide isomerase was bound to ATF6(C)-TAP with chaperone activity. These findings indicate that ATF6(C)-TAP can be used as a tool to isolate protein(s) that escort ATF6 from the ER to the Golgi apparatus in response to ER stress.

The endoplasmic reticulum stress sensor, ATF6α, protects against neurotoxin-induced dopaminergic neuronal death

Oxidative stress and endoplasmic reticulum (ER) stress are thought to contribute to the pathogenesis of various neurodegenerative diseases including Parkinson disease (PD), however, the relationship between these stresses remains unclear. ATF6α is an ER-membrane-bound transcription factor that is activated by protein misfolding in the ER and functions as a critical regulator of ER quality control proteins in mammalian cells. The goal of this study was to explore the cause-effect relationship between oxidative stress and ER stress in the pathogenesis of neurotoxin-induced model of PD. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a dopaminergic neurotoxin known to produce oxidative stress, activated ATF6α and increased ER chaperones and ER-associated degradation (ERAD) component in dopaminergic neurons. Importantly, MPTP induced formation of ubiquitin- immunopositive inclusions and loss of dopaminergic neurons more prominently in mice deficient in ATF6α than in wild-type mice. Cultured cell experiments revealed that 1-methyl-4-phenylpyridinium (MPP(+))-induced oxidative stress not only promoted phosphorylation of p38 mitogen-activated protein kinase (p38MAPK) but also enhanced interaction between phosphorylated p38MAPK and ATF6α, leading to increment in transcriptional activator activity of ATF6α. Thus, our results revealed a link between oxidative stress and ER stress by showing the importance of ATF6α in the protection of the dopaminergic neurons from MPTP that occurs through oxidative stress-induced activation of ATF6α and p38MAPK-mediated enhancement of ATF6α transcriptional activity.

Mammalian endoplasmic reticulum stress sensor IRE1 signals by dynamic clustering

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response (UPR), an intracellular signaling pathway that adjusts the protein folding capacity of the ER according to need. If homeostasis in the ER protein folding environment cannot be reestablished, cells commit to apoptosis. The ER-resident transmembrane kinase-endoribonuclease inositol-requiring enzyme 1 (IRE1) is the best characterized UPR signal transduction molecule. In yeast, Ire1 oligomerizes upon activation in response to an accumulation of misfolded proteins in the ER. Here we show that the salient mechanistic features of IRE1 activation are conserved: mammalian IRE1 oligomerizes in the ER membrane and oligomerization correlates with the onset of IRE1 phosphorylation and RNase activity. Moreover, the kinase/RNase module of human IRE1 activates cooperatively in vitro, indicating that formation of oligomers larger than four IRE1 molecules takes place upon activation. High-order IRE1 oligomerization thus emerges as a conserved mechanism of IRE1 signaling. IRE1 signaling attenuates after prolonged ER stress. IRE1 then enters a refractive state even if ER stress remains unmitigated. Attenuation includes dissolution of IRE1 clusters, IRE1 dephosphorylation, and decline in endoribonuclease activity. Thus IRE1 activity is governed by a timer that may be important in switching the UPR from the initially cytoprotective phase to the apoptotic mode.

Phenylbutyric acid suppresses protein accumulation-mediated ER stress in retrovirus-infected astrocytes and delays onset of paralysis in infected mice

Many neurodegenerative diseases are associated with accumulation of misfolded proteins in cells of the central nervous system (CNS). We have previously reported that accumulation of the precursor envelope protein gPr80(env) of ts1, a mutant of Moloney murine leukemia virus (MoMuLV), in the endoplasmic reticulum (ER) of infected astrocytes, results in ER stress, oxidative stress and cell death, subsequently leading to ts1-mediated neurodegeneration in infected mice. In the present study, we assessed whether treatments that reduce the accumulation of gPr80(env) in the ER of ts1-infected astrocytes provided a protective effect against ER stress and cell death. We show that treatment with phenylbutyric acid (PBA) can prevent the unfolded protein response (UPR), ER stress and cell death in cultured ts1-infected astrocytes. The protective effect of PBA is associated with its ability to reduce gPr80(env) accumulation and to increase the expression of proteins involved in protein folding in the ER, such as protein disulfide isomerase (PDI) and ERp44, rather than by decrease mRNA levels of gPr80(env) or alter the proteasomal degradation process for gPr80(env). In infected mice treated with PBA we also noted a reduction in the severity of the neuropathology in brainstem tissues and a delayed onset of paralysis. These results show that PBA is a potentially effective drug for the treatment of neurodegeneration caused by protein accumulation in cells of the CNS.

Endoplasmic reticulum stress contributes to vitamin E succinate-induced apoptosis in human gastric cancer SGC-7901 cells

Vitamin E succinate (RRR-alpha-tocopheryl succinate, VES), an efficient inducer of apoptosis, acts as a potent agent for cancer therapy. However, the mechanism by which VES mediates the effects are not yet fully understood. Here we studied the effect of endoplasmic reticulum (ER) stress and unfolded protein response (UPR) on VES-induced apoptosis of SGC-7901 human gastric cancer cells. VES caused cytological changes typical of apoptosis, increased ER dilation and cytosolic Ca(2+) concentration. And endogenous ER stress markers, GRP78 and GRP94 were transcriptionally and translationally altered. In response to VES, induction of CHOP, activation of caspase-4 and JNK were observed. Furthermore, VES also triggered activation of UPR components, including RNA-dependent protein kinase (PKR)-like ER kinase (PERK), activating transcription factor 6 (ATF6), X-box-binding protein 1 (XBP1), and ATF4 in a concentration- and time-dependent manner. Consequently, our results suggest that VES-induced apoptosis is coupled to ER stress and UPR activation in SGC-7901 human gastric cancer cells.

Induction of liver steatosis and lipid droplet formation in ATF6alpha-knockout mice burdened with pharmacological endoplasmic reticulum stress

We burdened mice with intraperitoneal injection of the endoplasmic reticulum stress-inducing reagent tunicamycin, and found that wild-type mice were able to recover from the insult, whereas ATF6α-knockout mice exhibited liver dysfunction and steatosis. Our results establish links between endoplasmic reticulum stress, lipid metabolism and steatosis

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) activates homeostatic responses collectively termed the unfolded protein response. Among the three principal signaling pathways operating in mammals, activating transcription factor (ATF)6α plays a pivotal role in transcriptional induction of ER-localized molecular chaperones and folding enzymes as well as components of ER-associated degradation, and thereby mouse embryonic fibroblasts deficient in ATF6α are sensitive to ER stress. However, ATF6α-knockout mice show no apparent phenotype under normal growing conditions. In this report, we burdened mice with intraperitoneal injection of the ER stress-inducing reagent tunicamycin and found that wild-type mice were able to recover from the insult, whereas ATF6α-knockout mice exhibited liver dysfunction and steatosis. Thus, ATF6α-knockout mice accumulated neutral lipids in the liver such as triacylglycerol and cholesterol, which was ascribable to blockage of β-oxidation of fatty acids caused by decreased mRNA levels of the enzymes involved in the process, suppression of very-low-density lipoprotein formation due to destabilized apolipoprotein B-100, and stimulation of lipid droplet formation resulting from transcriptional induction of adipose differentiation-related protein. Accordingly, the hepatocytes of tunicamycin-injected knockout mice were filled with many lipid droplets. These results establish links among ER stress, lipid metabolism, and steatosis.

ATF6 is important under both pathological and physiological states in the heart

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) evokes the ER stress response, including activating transcription factor 6 (ATF6), a key transcriptional activator to maintain cellular homeostasis. The ER stress has recently been reported to cause various diseases, but the role of ATF6 in the heart remains unknown. We clarified the role of ATF6 in the heart. The ATF6 activity was increased in the murine heart after myocardial infarction (MI). Treatment of mice with 4-(2-aminoethyl) benzenesulfonyl fluoride, an inhibitor of ATF6, further reduced cardiac function and increased the mortality rate at 14days after MI. Pharmacological inhibition of ATF6 induced dilatation of left ventricle and depression of cardiac function even in sham-operated murine hearts. The transgenic mice that expressed dominant negative mutant of ATF6 showed larger left ventricular dimension and reduced fractional shortening compared with wild-type littermates, resulting in death of heart failure at approximately 8weeks of age. In contrast, cardiac function after MI was better in transgenic mice that expressed constitutively active mutant of ATF6, compared with wild-type littermates. These results suggest that activation of the ER stress response factor ATF6 plays a critical role in not only protecting hearts under the pathological state but also maintaining cardiac function under the physiological state.

Endoplasmic reticulum stress and the unfolded protein response

The endoplasmic reticulum (ER) is the key cellular organelle involved in protein homoeostasis. The unfolded protein response (UPR) is a fundamental cellular process triggered by ER stress because of lack of ATP or primary ER dysfunction. The UPR is activated and dysregulated in non-alcoholic fatty liver disease (NAFLD). The UPR has been shown to be involved in both normal physiologic functions and the cellular response to a host of pathologic states. This article reviews the pathways by which the UPR unfolds and its potential role in the development and progression of NAFLD.

Signalling pathways in the unfolded protein response: development from yeast to mammals

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) under ER stress conditions activates a series of homoeostatic responses collectively termed the unfolded protein response (UPR). The UPR is unique in which the molecular mechanisms it uses to transmit signals from the ER lumen to the nucleus are completely different to those used for signalling from the plasma membrane. An ER stress signal is sensed and transmitted across the membrane by a transmembrane protein(s) in the ER. Interestingly, the number of such functional sensors/transducers, ubiquitously expressed, has increased with evolution, for example, one in Saccharomyces cerevisiae, two in Caenorhabditis elegans and Drosophila melanogaster, and three in mammals. Accordingly, mammalian cells are able to cope with ER stress in a more sophisticated manner. Here, I summarize the mechanisms and activation consequences of UPR signalling pathways in yeast, worm, fly and mammalian cells. I also discuss how they have evolved to counteract ER stress effectively.

Ischemia activates the ATF6 branch of the endoplasmic reticulum stress response

Stresses that perturb the folding of nascent endoplasmic reticulum (ER) proteins activate the ER stress response. Upon ER stress, ER-associated ATF6 is cleaved; the resulting active cytosolic fragment of ATF6 translocates to the nucleus, binds to ER stress response elements (ERSEs), and induces genes, including the ER-targeted chaperone, GRP78. Recent studies showed that nutrient and oxygen starvation during tissue ischemia induce certain ER stress response genes, including GRP78; however, the role of ATF6 in mediating this induction has not been examined. In the current study, simulating ischemia (sI) in a primary cardiac myocyte model system caused a reduction in the level of ER-associated ATF6 with a coordinate increase of ATF6 in nuclear fractions. An ERSE in the GRP78 gene not previously shown to be required for induction by other ER stresses was found to bind ATF6 and to be critical for maximal ischemia-mediated GRP78 promoter induction. Activation of ATF6 and the GRP78 promoter, as well as grp78 mRNA accumulation during sI, were reversed upon simulated reperfusion (sI/R). Moreover, dominant-negative ATF6, or ATF6-targeted miRNA blocked sI-mediated grp78 induction, and the latter increased cardiac myocyte death upon simulated reperfusion, demonstrating critical roles for endogenous ATF6 in ischemia-mediated ER stress activation and cell survival. This is the first study to show that ATF6 is activated by ischemia but inactivated upon reperfusion, suggesting that it may play a role in the induction of ER stress response genes during ischemia that could have a preconditioning effect on cell survival during reperfusion.

ATF6alpha induces XBP1-independent expansion of the endoplasmic reticulum

A link exists between endoplasmic reticulum (ER) biogenesis and the unfolded protein response (UPR), a complex set of signaling mechanisms triggered by increased demands on the protein folding capacity of the ER. The UPR transcriptional activator X-box binding protein 1 (XBP1) regulates the expression of proteins that function throughout the secretory pathway and is necessary for development of an expansive ER network. We previously demonstrated that overexpression of XBP1(S), the active form of XBP1 generated by UPR-mediated splicing of Xbp1 mRNA, augments the activity of the cytidine diphosphocholine (CDP-choline) pathway for biosynthesis of phosphatidylcholine (PtdCho) and induces ER biogenesis. Another UPR transcriptional activator, activating transcription factor 6alpha (ATF6alpha), primarily regulates expression of ER resident proteins involved in the maturation and degradation of ER client proteins. Here, we demonstrate that enforced expression of a constitutively active form of ATF6alpha drives ER expansion and can do so in the absence of XBP1(S). Overexpression of active ATF6alpha induces PtdCho biosynthesis and modulates the CDP-choline pathway differently than does enforced expression of XBP1(S). These data indicate that ATF6alpha and XBP1(S) have the ability to regulate lipid biosynthesis and ER expansion by mechanisms that are at least partially distinct. These studies reveal further complexity in the potential relationships between UPR pathways, lipid production and ER biogenesis.

Intersection of the unfolded protein response and hepatic lipid metabolism

The liver plays a central role in whole-body lipid metabolism by governing the synthesis, oxidization, transport and excretion of lipids. The unfolded protein response (UPR) was identified as a signal transduction system that is activated by ER stress. Recent studies revealed a critical role of the UPR in hepatic lipid metabolism. The IRE1/XBP1 branch of the UPR is activated by high dietary carbohydrates and controls the expression of genes involved in fatty acid and cholesterol biosynthesis. PERK mediated eIF2alpha phosphorylation is also required for the expression of lipogenic genes and the development of hepatic steatosis, likely by activating C/EBP and PPARgamma transcription factors. Further studies to define the molecular pathways that lead to the activation of the UPR by nutritional cues in the liver, and their contribution to human metabolic disorders such as hepatic steatosis, atherosclerosis and type 2 diabetes that are associated with dysregulation of lipid homeostasis, are warranted.

Activation of the endoplasmic reticulum stress-associated transcription factor x box-binding protein-1 occurs in a subset of normal germinal-center B cells and in aggressive B-cell lymphomas with prognostic implications

X box-binding protein 1 (Xbp-1) is a transcription factor that is required for the terminal differentiation of B lymphocytes into plasma cells. The Xbp-1 gene is activated in response to endoplasmic reticulum stress signals, which generate a 50-kDa nuclear protein that acts as a potent transactivator and regulates the expression of genes related to the unfolded protein response. Activated Xbp-1 is essential for cell survival in plasma-cell tumors but its role in B-cell lymphomas is unknown. We analyzed the expression of activated Xbp-1 in reactive lymphoid tissues, 411 lymphomas and plasma-cell neoplasms, and 24 B-cell lines. In reactive tissues, Xbp-1 was only found in nuclear extracts. Nuclear expression of Xbp-1 was observed in occasional reactive plasma cells and in a subpopulation of Irf-4(+)/Bcl-6(-)/Pax-5(-) B cells in the light zones of reactive germinal centers, probably representing cells committed to plasma-cell differentiation. None of the low-grade lymphomas showed evidence of Xbp-1 activation; however, Xbp-1 activation was found in 28% of diffuse large B-cell lymphomas, independent of germinal or postgerminal center phenotype, as well as in 48% of plasmablastic lymphomas and 69% of plasma-cell neoplasms. Diffuse large B-cell lymphomas with nuclear Xbp-1 expression had a significantly worse response to therapy and shorter overall survival compared with negative tumors. These findings suggest that Xbp-1 activation may play a role in the pathogenesis of aggressive B-cell lymphomas.

pXBP1(U), a negative regulator of the unfolded protein response activator pXBP1(S), targets ATF6 but not ATF4 in proteasome-mediated degradation

Cells from yeast to humans activate unconventional mRNA splicing when unfolded proteins accumulate in the endoplasmic reticulum (ER) under ER stress conditions. The substrate of this splicing in mammalian cells is XBP1 mRNA, which encodes the unfolded protein response (UPR)-specific transcription factor XBP1. The C-terminal region of XBP1 is switched as a result of the splicing. Thus, unspliced and spliced mRNAs produce pXBP1(U) of 261 aa and pXBP1(S) of 376 aa, respectively, with the N-terminal region containing the DNA-binding domain shared. As the pXBP1(S)-specific C-terminal region functions as an activation domain, pXBP1(S) can activate transcription efficiently. We recently found that pXBP1(U) shuttles between the nucleus and cytoplasm, owing to the presence of a nuclear exclusion signal in the pXBP1(U)-specific C-terminal region, in marked contrast to the exclusively nuclear localization of pXBP1(S). pXBP1(U) can associate with pXBP1(S), and pXBP1(U)-pXBP1(S) complex is rapidly degraded by the proteasome. Two other transcription factors are activated in response to ER stress, namely ATF6 and ATF4. ATF6 is a UPR-specific transcription factor, whereas ATF4 is activated by not only ER stress but also various other stimuli. In this study, we show that pXBP1(U) targets the active form of ATF6 but not ATF4 for destruction by the proteasome via direct association. This enhanced degradation is mediated by the degradation domain located at the pXBP1(U)-specific C-terminal end. We conclude that pXBP1(U) functions as a negative regulator of the UPR-specific transcription factors ATF6 and pXBP1(S).

Transcriptional repression of the prosurvival endoplasmic reticulum chaperone GRP78/BIP by E2F1

The endoplasmic reticulum chaperone GRP78/BIP plays a central role in the prosurvival machinery, and its enhanced expression has been implicated in drug resistance, carcinogenesis, and metastasis. E2F1, as part of an antitumor safeguard mechanism, promotes apoptosis regardless of functional p53. Using cells that are defective in p53, we show that E2F1 represses GRP78/BIP at the transcriptional level, and this requires its DNA binding domain. Analysis of human GRP78/BIP promoter reporter constructs revealed that the region between -371 and -109 of the proximal promoter contains major E2F1-responsive elements. Toward understanding the underlying mechanism of this regulation, we performed chromatin immunoprecipitation and gel shift assays, demonstrating that E2F1 directly binds to GC-rich regions in the distal GC-box and endoplasmic reticulum stress response element -126 by interfering with the binding of positive regulatory proteins Sp1 and TFII-I of the ER stress response element-binding factor complex. We further show that TFII-I, which is required for optimal stress induction of GRP78/BIP, is suppressed by E2F1 on the protein level. Finally, our studies suggest a molecular link between the inhibition of GRP78/BIP and E2F1-mediated chemosensitization of tumor cells, underscoring its relevance for cancer treatment. Together, the data provide a new mechanism for the incompletely understood tumor suppressor function of E2F1.

Human HRD1 promoter carries a functional unfolded protein response element to which XBP1 but not ATF6 directly binds

Quality control of proteins in the endoplasmic reticulum (ER) is achieved by two mechanisms, the productive folding mechanism, which is assisted by a number of ER-localized molecular chaperones and folding enzymes (collectively termed ER chaperones), and the ER-associated degradation (ERAD) mechanism, by which misfolded proteins are degraded by the ubiquitin-dependent proteasome system in the cytosol. Accumulation of unfolded proteins in the ER activates the unfolded protein response (UPR), resulting in transcriptional induction of ER chaperones and ERAD components. In mammals, three signalling pathways operate for the UPR, namely the IRE1-XBP1, PERK-ATF4 and ATF6 pathways. Analysis of mouse embryonic fibroblasts deficient in UPR signalling molecule indicates that transcriptional induction of ERAD components depends on the IRE1-XBP1 pathway. However, the molecular basis of this finding remains unclear. Here, we analysed the promoter of human HRD1, which encodes an E3 ubiquitin ligase, an important component of ERAD. We found that induction of HRD1 is mediated by two cis-acting elements, a canonical ER stress response element and a novel element we designate as UPR element II. The presence of UPR element II to which XBP1 but not ATF6 directly binds explains at least in part the dependency of HRD1 induction on the IRE1-XBP1 pathway.

A novel protein, Luman/CREB3 recruitment factor, inhibits Luman activation of the unfolded protein response

Luman/CREB3 (also called LZIP) is an endoplasmic reticulum (ER)-bound cellular transcription factor. It has been implicated in the mammalian unfolded protein response (UPR), as well as herpes simplex virus reactivation from latency in sensory neurons. Here, we report the identification of a novel Luman recruitment factor (LRF). Like Luman, LRF is a UPR-responsive basic-region leucine zipper protein that is prone to proteasomal degradation. Being a highly unstable protein, LRF interacts with Luman through the leucine zipper region and promotes Luman degradation. LRF was found to recruit the nuclear form of Luman to discrete nuclear foci, which overlap with the nuclear receptor coactivator GRIP1 bodies, and repress the transactivation activity of Luman. Compared to LRF+/+ mouse embryonic fibroblast (MEF) cells, the levels of CHOP, EDEM, and Herp were elevated in LRF-/- MEF cells. We propose that LRF is a negative regulator of the UPR. For Luman, it may represent another level of regulation following Luman proteolytic cleavage on the ER and nuclear translocation. In addition to inducing rapid Luman turnover, LRF may repress the transactivation potential of Luman by sequestering it in the LRF nuclear bodies away from key cofactors (such as HCF-1) that are required for transcriptional activation.

ATF6 is a transcription factor specializing in the regulation of quality control proteins in the endoplasmic reticulum

Eukaryotic cells cope with endoplasmic reticulum (ER) stress by activating the unfolded protein response (UPR), a coordinated system of transcriptional and translational controls, which ensures the integrity of synthesized proteins. Mammalian cells express three UPR transducers in the ER, namely IRE1, PERK and ATF6. The IRE1 pathway, which is conserved from yeast to humans, mediates transcriptional induction of not only ER quality control proteins (molecular chaperones, folding enzymes and components of ER-associated degradation) but also proteins working at various stages of secretion. The PERK pathway, conserved in metazoan cells, is responsible for translational control and also participates in transcriptional control in mammals. ATF6 is an ER-membrane-bound transcription factor activated by ER stress-induced proteolysis which consists of two closely related factors, ATF6alpha and ATF6beta, in mammals. ATF6alpha but not ATF6beta plays an important role in transcriptional control. In this study, we performed a genome-wide search for ATF6alpha-target genes in mice. Only 30 of the 14,729 analyzable genes were identified as specific targets, of which 40% were ER quality control proteins, 20% were ER proteins, while the rest had miscellaneous functions. The negative effects of the absence of PERK on transcriptional induction of ER quality control proteins could be explained by its inhibitory effect on ATF6alpha activation. Further, proteins involved in transport from the ER are not regulated by ATF6alpha, and transport of folded cargo molecules from the ER was not affected by the absence of ATF6alpha. Based on these results, we propose that ATF6 is a transcription factor specialized in the regulation of ER quality control proteins.

The role of the unfolded protein response in the heart

The misfolding of nascent proteins, or the unfolding of proteins after synthesis is complete, can occur in response to numerous environmental stresses, or as a result of mutations that de-stabilize protein structure. Cells have developed elaborate protein quality control systems that recognize improperly folded proteins and either refold them or facilitate their degradation. One such quality control system is the unfolded protein response, or the UPR. The UPR is a highly conserved signal transduction system that is activated when cells are subjected to conditions that alter the endoplasmic reticulum (ER) in ways that impair the folding of nascent proteins in this organelle. Recent observations indicate that in the heart, the UPR is activated during acute stresses, including ischemia/reperfusion, as well as upon longer term stresses that lead to cardiac hypertrophy and heart failure. Moreover, certain aspects of the UPR are activated during, and are required for proper heart development. This review summarizes recent studies of the UPR in the heart, focusing on the possible roles of the UPR in contributing to, or protecting from ischemia/reperfusion damage.

Transmembrane bZIP transcription factors in ER stress signaling and the unfolded protein response

Regulated intramembrane proteolysis (RIP) of the transmembrane transcription factor ATF6 represents a key step in effecting adaptive response to the presence of unfolded or malfolded protein in the endoplasmic reticulum. Recent studies have highlighted new ATF6-related transmembrane transcription factors. It is likely that current models for ER stress signaling are incomplete and that the expansion of the bZIP transmembrane family reflects selectivity in many aspects of these responses, including the type and duration of any particular stress, the cell type in which it occurs, and the integration with other aspects of cell-type-specific organization or additional intrinsic pathways, and the integration and communication between these pathways, not only in a cell-type-specific manner, but also between different tissues and organs. This review summarizes current information on the bZIP-transmembrane proteins and discusses outstanding questions on the elucidation of the stress signals, the repertoire of components involved in regulating different aspects of the forward transport, cleavage, nuclear import, transcriptional activity, and turnover of each of these factors, and dissection of the integration of the various outputs into broad coordinated responses.

How transmembrane proteins sense endoplasmic reticulum stress

The unfolded protein response (UPR) is an adaptive stress response in which cells recover from the accumulation of unfolded or misfolded proteins in the endoplasmic reticulum (ER) by increasing its protein-folding capacity. The IRE1 pathway in the UPR is evolutionarily conserved from yeast to human, and two other pathways involving PERK and ATF6 have also evolved in higher eukaryotes. These three intracellular signaling pathways originate in the ER lumen, where unfolded or misfolded proteins are recognized by the three transmembrane ER stress sensors IRE1, PERK, and ATF6. This review focuses on current progress with efforts to elucidate how stress sensors recognize the accumulation of unfolded proteins.