miR-217 Regulates Normal and Tumor Cell Fate Following Induction of Endoplasmic Reticulum Stress

Rapidly proliferating cancer cells require a microenvironment where essential metabolic nutrients like glucose, oxygen, and growth factors become scarce as the tumor volume surpasses the established vascular capacity of the tissue. Limits in nutrient availability typically trigger growth arrest and/or apoptosis to prevent cellular expansion. However, tumor cells frequently co-opt cellular survival pathways thereby favoring cell survival under this environmental stress. The unfolded protein response (UPR) pathway is typically engaged by tumor cells to favor adaptation to stress. PERK, an endoplasmic reticulum (ER) protein kinase and UPR effector is activated in tumor cells and contributes tumor cell adaptation by limiting protein translation and balancing redox stress. PERK also induces miRNAs that contribute to tumor adaptation. miR-211 and miR-216b were previously identified as PERK-ATF4-regulated miRNAs that regulate cell survival. We have identified another PERK-responsive miRNA, miR-217, with increased expression under prolonged ER stress. Key targets of miR-217 are identified as TRPM1, the host gene for miR-211 and EZH2. Evidence is provided that miR-217 expression is essential for the rapid loss of miR-211 in prolonged ER stress and provides a functional link for determining whether cells adapt to stress or commit to apoptosis.

IMPLICATIONS

PERK-dependent induction of miR-217 limits accumulation and function of the prosurvival miRNA, miR-211, to establish cell fate and promote cell commitment to apoptosis.

A Pilot Study on Blood Components in COVID-19 Affected Subjects: A Correlation to UPR Signalling and ER-Stress

Abstract

The endoplasmic reticulum (ER) is the site for protein synthesis, its folding and secretion. An intricate set of signalling pathways, called UPR pathways, have been evolved by ER in mammalian cells, to allow the cell to respond the presence of misfolded proteins within the ER. Breaching of these signalling systems by disease oriented accumulation of unfolded proteins may develop cellular stress. The aim of this study is to explore whether COVID-19 infection is responsible for developing this kind of endoplasmic reticulum related stress (ER-stress). ER-stress was evaluated by checking the expression of ER-stress markers e.g. PERK (adapting) and TRAF2 (alarming). ER-stress was correlated to several blood parameters viz. IgG, pro- and anti-inflammatory cytokines, leukocytes, lymphocytes, RBC, haemoglobin and PaO₂/FiO₂ ratio (ratio of arterial oxygen partial pressure to fractional inspired oxygen) in COVID-19 affected subjects. COVID-19 infection was found to be a state of protein homeostasis (proteostasis) collapse. Changes in IgG levels showed very poor immune response by the infected subjects. At the initial phase of the disease, pro-inflammatory cytokine levels were high and anti-inflammatory cytokines levels were low; though they were partly compromised at later phase of the disease. Total leukocyte concentration increased over the period of time; while percentage of lymphocytes were dropped. No significant changes were observed in cases of RBC counts and haemoglobin (Hb) levels. Both RBC and Hb were maintained at their normal range. In mildly stressed group, PaO₂/FiO₂ ratio (oxygenation status) was in the higher side of normal range; whereas in other two groups the ratio was in respiratory distress syndrome mode. Virus could induce mild to severe ER-stress, which could be the cause of cellular death and systemic dysfunction introducing fatal consequences.

Graphical Abstract

Schematic representation of SARS-CoV-2 infection and related consequences.

Dietary Choline Mitigates High-Fat Diet-Impaired Chylomicrons Assembly via UPRer Modulated by perk DNA Methylation

High-fat diets (HFD) lead to impairment of chylomicrons (CMs) assembly and adversely influence intestinal lipid homeostasis. However, the mechanisms of HFD impairing CMs assembly have yet to be fully understood. Additionally, although choline, as a lipid-lowering agent, has been widely used and its deficiency has been closely linked to non-alcoholic steatohepatitis (NASH), the contribution of choline by functioning as a methyl donor in alleviating HFD-induced intestinal lipid deposition is unknown. Thus, this study was conducted to determine the mechanism of HFD impairing CMs assembly and also tested the effect of choline acting as a methyl donor in this process. To this end, in this study, four diets (control, HFD, choline and HFD + choline diet) were fed to yellow catfish for 10 weeks in vivo and their intestinal epithelial cells were isolated and incubated for 36 h in fatty acids (FA) with or without choline solution combining si-perk transfection in vitro. The key findings from this study as follows: (1) HFD caused impairment of CMs assembly main by unfolded protein response (UPRer). HFD activated perk and then induced UPRer, which led to endoplasmic reticulum dysfunction and further impaired CMs assembly via protein-protein interactions between Perk and Apob48. (2) Choline inhibited the transcriptional expression level of perk via activating the -211 CpG methylation site, which initiated the subsequent ameliorating effect on HFD-impaired CMs assembly and intestinal lipid dysfunction. These results provide a new insight into direct crosstalk between UPRer and CMs assembly, and also emphasize the critical contribution of choline acting as a methyl donor and shed new light on choline-deficient diet-induced NASH.

Thiopurines activate an antiviral unfolded protein response that blocks influenza A virus glycoprotein accumulation

Influenza A viruses (IAVs) utilize host shutoff mechanisms to limit antiviral gene expression and redirect translation machinery to the synthesis of viral proteins. Previously, we showed that IAV replication is sensitive to protein synthesis inhibitors that block translation initiation and induce formation of cytoplasmic condensates of untranslated messenger ribonucleoprotein complexes called stress granules (SGs). In this study, using an image-based high-content screen, we identified two thiopurines, 6-thioguanine (6-TG) and 6-thioguanosine (6-TGo), that triggered SG formation in IAV-infected cells and blocked IAV replication in a dose-dependent manner without eliciting SG formation in uninfected cells. 6-TG and 6-TGo selectively disrupted the synthesis and maturation of IAV glycoproteins hemagglutinin (HA) and neuraminidase (NA) without affecting the levels of the viral RNAs that encode them. By contrast, these thiopurines had minimal effect on other IAV proteins or the global host protein synthesis. Disruption of IAV glycoprotein accumulation by 6-TG and 6-TGo correlated with activation of unfolded protein response (UPR) sensors activating transcription factor-6 (ATF6), inositol requiring enzyme-1 (IRE1) and PKR-like endoplasmic reticulum kinase (PERK), leading to downstream UPR gene expression. Treatment of infected cells with the chemical chaperone 4-phenylbutyric acid diminished thiopurine-induced UPR activation and partially restored the processing and accumulation of HA and NA. By contrast, chemical inhibition of the integrated stress response downstream of PERK restored accumulation of NA monomers but did not restore processing of viral glycoproteins. Genetic deletion of PERK enhanced the antiviral effect of 6-TG without causing overt cytotoxicity, suggesting that while UPR activation correlates with diminished viral glycoprotein accumulation, PERK could limit the antiviral effects of drug-induced ER stress. Taken together, these data indicate that 6-TG and 6-TGo are effective host-targeted antivirals that trigger the UPR and selectively disrupt accumulation of viral glycoproteins.IMPORTANCESecreted and transmembrane proteins are synthesized in the endoplasmic reticulum (ER), where they are folded and modified prior to transport. Many viruses rely on the ER for the synthesis and processing of viral glycoproteins that will ultimately be incorporated into viral envelopes. Viral burden on the ER can trigger the unfolded protein response (UPR). Much remains to be learned about how viruses co-opt the UPR to ensure efficient synthesis of viral glycoproteins. Here, we show that two FDA-approved thiopurine drugs, 6-TG and 6-TGo, induce the UPR, which represents a previously unrecognized effect of these drugs on cell physiology. This thiopurine-mediated UPR activation blocks influenza virus replication by impeding viral glycoprotein accumulation. Our findings suggest that 6-TG and 6-TGo may have broad antiviral effect against enveloped viruses that require precise tuning of the UPR to support viral glycoprotein synthesis.

A Novel Role of Ascorbic Acid in Anti-Inflammatory Pathway and ROS Generation in HEMA Treated Dental Pulp Stem Cells

Resin (co)monomers issued from restorative dental materials are able to distribute in the dental pulp or the gingiva, to get to the saliva and to the flowing blood. Many authors have recently shown that methacrylate-based resins, in particular 2-hydroxyethylmethacrylate (HEMA), are responsible of inflammatory and autophagic processes in human dental pulp stem cells (hDPSCs) while ascorbic acid (AS), an antioxidant molecule, can assume a protective role in cell homeostasis. The purpose of the current work was to study if 50 µg/mL AS can affect the inflammatory status induced by 2 mM HEMA in hDPSCs, a tissue–specific cell population. Cell proliferation, cytokine release, morphological arrangement and reactive oxygen species (ROS) formation were determined respectively by MTT, ELISA, morphological analysis and dichlorofluorescein assay. The hDPSCs exposed to HEMA let to an increment of ROS formation and in the expression of high levels of inflammatory mediators such as nuclear factor-κB (NFkB), inflammatory cytokines such as interleukin IL6, IL8, interferon (IFN)ɣ and monocyte chemoattractant protein (MCP)1. Moreover, HEMA induced the up-regulation of pospho-extracellular signal–regulated kinases (pERK)/ERK signaling pathway associated to the nuclear translocation. AS treatment significantly down-regulated the levels of pro-inflammatory mediators. Then, the natural product AS reduced the detrimental result promoted by methacrylates in clinical dentistry, in fact restore cell proliferation, reduce the pro-inflammatory cytokine, downregulate ROS production and of NFkB/pERK/ERK signaling path. In synthesis, AS, could improve the quality of dental care and play a strategic role as innovative endodontic compound easy to use and with reasonable cost.

Expression and Clinical Significance of Protein Kinase RNA-Like Endoplasmic Reticulum Kinase and Phosphorylated Eukaryotic Initiation Factor 2α in Pancreatic Ductal Adenocarcinoma

OBJECTIVES

Endoplasmic reticulum stress and subsequent phosphorylation of eukaryotic initiation factor 2α (eIF2α) by protein kinase R-like endoplasmic reticulum kinase (PERK) plays an important role in the development and chemoresistance of pancreatic ductal adenocarcinoma (PDAC). However, the expression and significance of phosphorylated eIF2α (p-eIF2α) and PERK in PDAC have not been examined.

METHODS

We examined p-eIF2α and PERK expression in 84 PDAC and paired normal pancreas samples by immunohistochemistry and Western blotting and correlated the results with clinicopathologic parameters and survival.

RESULTS

Mean PERK H score was 140.8 in PDAC compared with 82.1 in normal pancreas (P < 0.001). High p-eIF2α expression was present in 56% of PDACs versus 7.6% of normal pancreases (P < 0.001). High PERK and p-eIF2α expression correlated with shorter overall survival (P = 0.048 and P = 0.03, respectively). By multivariate analysis, high p-eIF2α (P = 0.01), positive margin (P = 0.002), and lymph node metastasis (P = 0.01) were independent prognosticators for survival.

CONCLUSIONS

The expression levels of PERK and p-eIF2α are higher in PDAC than those in normal pancreas. High levels of PERK and p-eIF2α are predictors of shorter survival in PDAC patients, suggesting that PERK and eIF2α could be promising targets in PDAC.

Increased HMGB1 expression correlates with higher expression of c-IAP2 and pERK in colorectal cancer

The aim of this study was to investigate the relationship between high-mobility group box 1 (HMGB1) and colorectal cancer (CRC).In this prospective study, patients with CRC undergoing primary surgery and healthy subjects (control group) were enrolled from July 2013 to December 2014. The serum HMGB1 concentration and HMGB1 mRNA expression were determined using enzyme-linked immunosorbent assay reverse transcription-polymerase chain reaction, respectively. Immunohistochemical analysis was performed to determine HMGB1, pERK, and c-inhibitor of apoptosis protein 2 (c-IAP2) protein expression levels in the cancer tissues.A total 144 patients with CRC and 50 healthy subjects underwent serum HMGB1 testing. Resected specimens of 50 patients were used for HMGB1 mRNA and protein expression analyses. Mean serum HMGB1 level in the patients with CRC was higher than that of the control group (8.42 μg/L vs 1.79 μg/L, P < .05). Mean serum HMGB1 level in the patients with CRC with distant metastasis was significantly higher than that of the controls (13.32 μg/L vs 7.37 μg/L, P < .05). The HMGB1 mRNA and protein expression levels in the CRC tissues were significantly higher than those in the adjacent normal mucosa. HMGB1 protein expression positively correlated with the lymph node metastasis. There were positive correlations between HMGB1 and c-IAP2 (r = 0.457, P < .05), HMGB1 and pERK (r = 0.461, P < .05), as well as pERK and c-IAP2 (r = 0.399, P < .05).HMGB1 expression in CRC correlates with distant and lymph node metastasis. It may inhibit apoptosis by inducing activation of pERK and c-IAP2.

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

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

PERK-opathies: An Endoplasmic Reticulum Stress Mechanism Underlying Neurodegeneration

The unfolded protein response (UPR) plays a vital role in maintaining cell homeostasis as a consequence of endoplasmic reticulum (ER) stress. However, prolonged UPR activity leads to cell death. This time-dependent dual functionality of the UPR represents the adaptive and cytotoxic pathways that result from ER stress. Chronic UPR activation in systemic and neurodegenerative diseases has been identified as an early sign of cellular dyshomeostasis. The Protein Kinase R-like ER Kinase (PERK) pathway is one of three major branches in the UPR, and it is the only one to modulate protein synthesis as an adaptive response. The specific identification of prolonged PERK activity has been correlated with the progression of disorders such as diabetes, Alzheimer's disease, and cancer, suggesting that PERK plays a role in the pathology of these disorders. For the first time, the term "PERK-opathies" is used to group these diseases in which PERK mediates detriment to the cell culminating in chronic disorders. This article reviews the literature documenting links between systemic disorders with the UPR, but with a specific emphasis on the PERK pathway. Then, articles reporting links between the UPR, and more specifically PERK, and neurodegenerative disorders are presented. Finally, a therapeutic perspective is discussed, where PERK interventions could be potential remedies for cellular dysfunction in chronic neurodegenerative disorders.

Differential expression of endoplasmic reticulum stress-response proteins in different renal tubule subtypes of OVE26 diabetic mice

Regulation of the endoplasmic reticulum (ER) stress-response pathway during the course of diabetes specifically in renal tubules is unclear. Since tubule cell dysfunction is critical to progression of diabetic nephropathy, this study analyzed markers of ER stress response and ER chaperones at different stages of diabetes and in different renal tubule subtypes of OVE26 type-1 diabetic mice. ER stress-responseinduced chaperones GRP78, GRP94, and protein disulfide isomerase (PDI) were increased in isolated cortical tubules of older diabetic mice, while PDI was decreased in tubules of young diabetic mice. Immunofluorescence staining of kidneys from older mice showed GRP78 and PDI upregulation in all cortical tubule segments, with substantial induction of PDI in distal tubules. Protein kinase RNA-like endoplasmic reticulum kinase (PERK) phosphorylation was increased in cortical tubules of young diabetic mice, with no differences between older diabetic and control mice. Expression of ER stress-induced PERK inhibitor p58IPK was decreased and then increased in all tubule subtypes of young and older mice, respectively. Knockdown of PERK by small interfering RNA (siRNA) increased fibronectin secretion in cultured proximal tubule cells. Tubules of older diabetic mice had significantly more apoptotic cells, and ER stress-induced proapoptotic transcription factor C/EBP homologous protein (CHOP) was increased in proximal and distal tubules of diabetic mice and diabetic humans. CHOP induction in OVE26 mice was not altered by severity of proteinuria. Overexpression of CHOP in cultured proximal tubule cells increased expression of fibronectin. These findings demonstrate differential ER stress-response signaling in tubule subtypes of diabetic mice and implicate a role for PERK and CHOP in tubule cell matrix protein production.

Noncanonical binding of BiP ATPase domain to Ire1 and Perk is dissociated by unfolded protein CH1 to initiate ER stress signaling

The unfolded protein response (UPR) is an essential cell signaling system that detects the accumulation of misfolded proteins within the endoplasmic reticulum (ER) and initiates a cellular response in order to maintain homeostasis. How cells detect the accumulation of misfolded proteins remains unclear. In this study, we identify a noncanonical interaction between the ATPase domain of the ER chaperone BiP and the luminal domains of the UPR sensors Ire1 and Perk that dissociates when authentic ER unfolded protein C_H1 binds to the canonical substrate binding domain of BiP. Unlike the interaction between chaperone and substrates, we found that the interaction between BiP and UPR sensors was unaffected by nucleotides. Thus, we discover that BiP is dual functional UPR sensor, sensing unfolded proteins by canonical binding to substrates and transducing this event to noncanonical, signaling interaction to Ire1 and Perk. Our observations implicate BiP as the key component for detecting ER stress and suggest an allosteric mechanism for UPR induction.

DOI:http://dx.doi.org/10.7554/eLife.03522.001

Proteins perform many essential tasks in cells, but to be able to work they first have to correctly fold into a specific three-dimensional shape. Within the cell, many proteins are folded with the help of ‘chaperone’ proteins. If any proteins fold incorrectly, the normal workings of the cell can be disturbed, which may damage the cell. This is more likely to happen if a cell suddenly requires a large number of proteins to be made, which can overwhelm the chaperone proteins.

In humans and other eukaryotic organisms, many proteins are folded in a compartment within the cell called the endoplasmic reticulum. Inside this compartment there is a system called the unfolded protein response that detects misfolded proteins and boosts the cell's capacity to re-fold them. As part of this system, two sensor proteins detect when misfolded proteins are present, but it is not clear how they do so. It has been suggested that a chaperone protein called BiP may be able to activate these sensor proteins in order to turn on the unfolded protein response.

In this study, Carrara et al. studied the sensor proteins and BiP using an artificial set-up in the laboratory. The experiments show that both of the sensor proteins can bind to a section of the BiP chaperone called the ATPase domain. However, in the presence of an unfolded protein, BiP stopped interacting with the sensor proteins, which could allow the sensor proteins to activate the unfolded protein response. The experiments also show that BiP must bind to the unfolded protein to activate the unfolded protein response.

Carrara et al.'s findings suggest that BiP has a dual role in cells: to sense unfolded proteins by binding to them, and then to activate the sensor proteins that trigger the unfolded protein response.

Together, these results suggest a new model for how cells detect and respond to misfolded proteins within the endoplasmic reticulum, and may provide new targets for therapies to treat diseases caused by defects in protein folding.

DOI:http://dx.doi.org/10.7554/eLife.03522.002

Immunogenic calreticulin exposure occurs through a phylogenetically conserved stress pathway involving the chemokine CXCL8

The exposure of calreticulin (CRT) on the surface of stressed and dying cancer cells facilitates their uptake by dendritic cells and the subsequent presentation of tumor-associated antigens to T lymphocytes, hence stimulating an anticancer immune response. The chemotherapeutic agent mitoxantrone (MTX) can stimulate the peripheral relocation of CRT in both human and yeast cells, suggesting that the CRT exposure pathway is phylogenetically conserved. Here, we show that pheromones can act as physiological inducers of CRT exposure in yeast cells, thereby facilitating the formation of mating conjugates, and that a large-spectrum inhibitor of G protein-coupled receptors (which resemble the yeast pheromone receptor) prevents CRT exposure in human cancer cells exposed to MTX. An RNA interference screen as well as transcriptome analyses revealed that chemokines, in particular human CXCL8 (best known as interleukin-8) and its mouse ortholog Cxcl2, are involved in the immunogenic translocation of CRT to the outer leaflet of the plasma membrane. MTX stimulated the production of CXCL8 by human cancer cells in vitro and that of Cxcl2 by murine tumors in vivo. The knockdown of CXCL8/Cxcl2 receptors (CXCR1/Cxcr1 and Cxcr2) reduced MTX-induced CRT exposure in both human and murine cancer cells, as well as the capacity of the latter-on exposure to MTX-to elicit an anticancer immune response in vivo. Conversely, the addition of exogenous Cxcl2 increased the immunogenicity of dying cells in a CRT-dependent manner. Altogether, these results identify autocrine and paracrine chemokine signaling circuitries that modulate CRT exposure and the immunogenicity of cell death.

Mechanisms that lessen benefits of β-secretase reduction in a mouse model of Alzheimer's disease

The β-secretase enzyme BACE1 (β-site amyloid precursor protein-cleaving enzyme 1), which initiates amyloid-β (Aβ) production, is an excellent therapeutic target for Alzheimer's disease (AD). However, recent evidence raises concern that BACE1-inhibiting approaches may encounter dramatic declines in their abilities to ameliorate AD-like pathology and memory deficits during disease progression. Here, we used BACE1 haploinsufficiency as a therapeutic relevant model to evaluate the efficacy of partial inhibition of this enzyme. Specifically, we crossed BACE1(+/-) mice with 5XFAD transgenic mice and investigated the mechanisms by which Aβ accumulation and related memory impairments become less sensitive to rescue by BACE1(+/-) reduction. Haploinsufficiency lowered BACE1 expression by ∼50% in 5XFAD mice regardless of age in concordance with reduction in gene copy number. However, profound Aβ plaque pathology and memory deficits concomitant with BACE1 equivalent to wild-type control levels remained in BACE1(+/-)·5XFAD mice with advanced age (15-18 months old). Therefore, BACE1 haploinsufficiency is not sufficient to block the elevation of BACE1 expression (approximately twofold), which is also reported to occur during human AD progression, in 5XFAD mice. Our investigation revealed that PERK (PKR-endoplasmic reticulum-related kinase)-dependent activation of eIF2α (eukaryotic translation initiation factor-2α) accounts for the persistent BACE1 upregulation in BACE1(+/-)·5XFAD mouse brains at 15-18 months of age. Moreover, BACE1 haploinsufficiency was also no longer able to prevent reduction in the expression of neprilysin, a crucial Aβ-degrading enzyme, in 5XFAD mice with advanced age. These findings demonstrate that partial BACE1 suppression cannot attenuate deleterious BACE1-elevating or neprilysin-reducing mechanisms, limiting its capabilities to reduce cerebral Aβ accumulation and rescue memory defects during the course of AD development.

Proteostasis regulation at the endoplasmic reticulum: a new perturbation site for targeted cancer therapy

To deal with the constant challenge of protein misfolding in the endoplasmic reticulum (ER), eukaryotic cells have evolved an ER protein quality control (ERQC) mechanism that is integrated with an adaptive stress response. The ERQC pathway is comprised of factors residing in the ER lumen that function in the identification and retention of aberrantly folded proteins, factors in the ER membrane for retrotranslocation of misfolded polypeptides, and enzymes in the cytosol that degrade retrotranslocated proteins. The integrated stress response (termed ER stress or unfolded protein response, UPR) contains several signaling branches elicited from the ER membrane, which fine-tune the rate of protein synthesis and entry into the ER to match the ER folding capacity. The fitness of the cell, particularly those bearing a high secretory burden, is critically dependent on functional integrity of the ER, which in turn relies on these stress-attenuating mechanisms to maintain protein homeostasis, or proteostasis. Aberrant proteostasis can trigger cellular apoptosis, making these adaptive stress response systems attractive targets for perturbation in treatment of cell malignancies. Here, we review our current understanding of how the cell preserves ER proteostasis and discuss how we may harness the mechanistic information on this process to develop new cancer therapeutics.

The structure of the PERK kinase domain suggests the mechanism for its activation

The endoplasmic reticulum (ER) unfolded protein response (UPR) is comprised of several intracellular signaling pathways that alleviate ER stress. The ER-localized transmembrane kinase PERK is one of three major ER stress transducers. Oligomerization of PERK's N-terminal ER luminal domain by ER stress promotes PERK trans-autophosphorylation of the C-terminal cytoplasmic kinase domain at multiple residues including Thr980 on the kinase activation loop. Activated PERK phosphorylates Ser51 of the α-subunit of translation initiation factor 2 (eIF2α), which inhibits initiation of protein synthesis and reduces the load of unfolded proteins entering the ER. The crystal structure of PERK's kinase domain has been determined to 2.8 Å resolution. The structure resembles the back-to-back dimer observed in the related eIF2α kinase PKR. Phosphorylation of Thr980 stabilizes both the activation loop and helix αG in the C-terminal lobe, preparing the latter for eIF2α binding. The structure suggests conservation in the mode of activation of eIF2α kinases and is consistent with a `line-up' model for PERK activation triggered by oligomerization of its luminal domain.

The unfolded protein response is required to maintain the integrity of the endoplasmic reticulum, prevent oxidative stress and preserve differentiation in β-cells

Diabetes is an epidemic of worldwide proportions caused by β-cell failure. Nutrient fluctuations and insulin resistance drive β-cells to synthesize insulin beyond their capacity for protein folding and secretion and thereby activate the unfolded protein response (UPR), an adaptive signalling pathway to promote cell survival upon accumulation of unfolded protein in the endoplasmic reticulum (ER). Protein kinase-like endoplasmic reticulum kinase (PERK) signals one component of the UPR through phosphorylation of eukaryotic initiation factor 2 on the α-subunit (eIF2α) to attenuate protein synthesis, thereby reducing the biosynthetic burden. β-Cells uniquely require PERK-mediated phosphorylation of eIF2α to preserve cell function. Unabated protein synthesis in β-cells is sufficient to initiate a cascade of events, including oxidative stress, that are characteristic of β-cell failure observed in type 2 diabetes. In contrast to acute adaptive UPR activation, chronic activation increases expression of the proapoptotic transcription factor CAAT/enhancer-binding protein homologous protein (CHOP). Chop deletion in insulin-resistant mice profoundly increases β-cell mass and prevents β-cell failure to forestall the progression of diabetes. The findings suggest an unprecedented link by which protein synthesis and/or misfolding in the ER causes oxidative stress and should encourage the development of novel strategies to treat diabetes.

Viral strategies to subvert the mammalian translation machinery

Viruses do not carry their own protein biosynthesis machinery and the translation of viral proteins therefore requires that the virus usurps the machinery of the host cell. To allow optimal translation of viral proteins at the expense of cellular proteins, virus families have evolved a variety of methods to repress the host translation machinery, while allowing effective viral protein synthesis. Many viruses use noncanonical mechanisms that permit translation of their own RNAs under these conditions. Viruses have also developed mechanisms to evade host innate immune responses that would repress translation under conditions of viral infection, in particular PKR activation in response to double-stranded RNA (dsRNA). Importantly, the study of viral translation mechanisms has enormously enhanced our understanding of many aspects of the cellular protein biosynthesis pathway and its components. A number of unusual mechanisms of translation initiation that were first discovered in viruses have since been observed in cellular mRNAs, and it has become apparent that a diverse range of translation mechanisms operates in eukaryotes, allowing subtle regulation of this essential process.

Translational repression during chronic hypoxia is dependent on glucose levels

Translation is often repressed in cell lines that are exposed to hypoxic conditions (0.5% - 1.5% O2) but this repression requires prolonged exposure (> 16 h). We report here that prolonged exposure to hypoxia results in the depletion of glucose from the media and that the loss of glucose correlates with the shut down in translation. Furthermore, we show that the addition of glucose or reoxygenation restores translation in hypoxic PC3 cells. This indicates that both glucose depletion and hypoxia are required for translational repression. We also show that eIF2alpha phosphorylation is reversed by glucose addition. Moreover, we present data that strongly indicate that eIF2alpha phosphorylation as well as the translational inhibition that occurs when cells are grown under conditions of glucose depletion and hypoxia is pancreatic eIF2alpha kinase (PERK) independent. We believe this is the first report to show that glucose depletion is required for translational repression under hypoxic conditions and that this explains why prolonged exposure to hypoxia is required for this inhibition. Since the physiological conditions that lead to tumor hypoxia would also likely lead to reduced glucose levels, understanding the interplay of glucose and hypoxia in regulating tumor metabolism will provide important information on the growth and development of solid tumors.

Cellular interplay between neurons and glia: toward a comprehensive mechanism for excitotoxic neuronal loss in neurodegeneration

Astrocytes perform vital maintenance, functional enhancement, and protective roles for their associated neurons; however these same mechanisms may become deleterious for neurons under some conditions. In this review, we highlight two normally protective pathways, the endoplasmic reticulum (ER) stress response and an endogenous antioxidant response, which may become neurotoxic when activated in astrocytes during the inflammation associated with neurodegeneration. Stimulation of these multifaceted pathways affects a panoply of cellular processes. Of particular importance is the effect these pathways have on the homeostasis of the excitatory amino acid neurotransmitter, glutamate. The endogenous antioxidant response increases extracellular glutamate in the pursuit of making the cellular antioxidant, glutathione, by increasing expression of the xCT subunit of the cystine/glutamate antiporter. Meanwhile, inflammatory mediators such as TNFα reduce levels of membrane-bound glutamate scavenging proteins such as the excitatory amino acid transporters. Together, these cellular activities may result in a net increase in extracellular glutamate that could alter neuronal function and lead to excitotoxicity. Here we discuss the role of N-methyl-D-aspartate receptors, which, when excessively stimulated by glutamate, can cause neuronal dysfunction and loss via activation of calpains. While there are other pathways acting in concert or parallel to those we describe here, this review explores a rationale to explain how two protective mechanisms may result in neuronal loss during neurodegeneration.

Endoplasmic reticulum stress and the unfolded protein response in cellular models of Parkinson's disease

6-hydroxydopamine, 1-methyl-4-phenyl-pyridinium (MPP+), and rotenone cause the death of dopaminergic neurons in vitro and in vivo and are widely used to model Parkinson's disease. To identify regulated genes in such models, we performed serial analysis of gene expression on neuronal PC12 cells exposed to 6-hydroxydopamine. This revealed a striking increase in transcripts associated with the unfolded protein response. Immunoblotting confirmed phosphorylation of the key endoplasmic reticulum stress kinases IRE1alpha and PERK (PKR-like ER kinase) and induction of their downstream targets. There was a similar response to MPP+ and rotenone, but not to other apoptotic initiators. As evidence that endoplasmic reticulum stress contributes to neuronal death, sympathetic neurons from PERK null mice in which the capacity to respond to endoplasmic reticulum stress is compromised were more sensitive to 6-hydroxydopamine. Our findings, coupled with evidence from familial forms of Parkinson's disease, raise the possibility of widespread involvement of endoplasmic reticulum stress and the unfolded protein response in the pathophysiology of this disease.