Protein translation and folding are coupled by an endoplasmic-reticulum-resident kinase

Normal and defective pathways in biogenesis and maintenance of the insulin storage pool

Both basal and glucose-stimulated insulin release occur primarily by insulin secretory granule exocytosis from pancreatic β cells, and both are needed to maintain normoglycemia. Loss of insulin-secreting β cells, accompanied by abnormal glucose tolerance, may involve simple exhaustion of insulin reserves (which, by immunostaining, appears as a loss of β cell identity), or β cell dedifferentiation, or β cell death. While various sensing and signaling defects can result in diminished insulin secretion, somewhat less attention has been paid to diabetes risk caused by insufficiency in the biosynthetic generation and maintenance of the total insulin granule storage pool. This Review offers an overview of insulin biosynthesis, beginning with the preproinsulin mRNA (translation and translocation into the ER), proinsulin folding and export from the ER, and delivery via the Golgi complex to secretory granules for conversion to insulin and ultimate hormone storage. All of these steps are needed for generation and maintenance of the total insulin granule pool, and defects in any of these steps may, weakly or strongly, perturb glycemic control. The foregoing considerations have obvious potential relevance to the pathogenesis of type 2 diabetes and some forms of monogenic diabetes; conceivably, several of these concepts might also have implications for β cell failure in type 1 diabetes.

Stress Granules in the Post-transcriptional Regulation of Immune Cells

Immune cell activation triggers transcriptional and translational programs eliciting cellular processes, such as differentiation or proliferation, essential for an efficient immune response. These dynamic processes require an intricate orchestration of regulatory mechanisms to control the precise spatiotemporal expression of proteins. Post-transcriptional regulation ensures the control of messenger RNA metabolism and appropriate translation. Among these post-transcriptional regulatory mechanisms, stress granules participate in the control of protein synthesis. Stress granules are ribonucleoprotein complexes that form upon stress, typically under control of the integrated stress response. Such structures assemble upon stimulation of immune cells where they control selective translational programs ensuring the establishment of accurate effector functions. In this review, we summarize the current knowledge about post-transcriptional regulation in immune cells and highlight the role of stress sensors and stress granules in such regulation.

Export Control: Post-transcriptional Regulation of the COPII Trafficking Pathway

The coat protein complex II (COPII) mediates forward trafficking of protein and lipid cargoes from the endoplasmic reticulum. COPII is an ancient and essential pathway in all eukaryotes and COPII dysfunction underlies a range of human diseases. Despite this broad significance, major aspects of COPII trafficking remain incompletely understood. For example, while the biochemical features of COPII vesicle formation are relatively well characterized, much less is known about how the COPII system dynamically adjusts its activity to changing physiologic cues or stresses. Recently, post-transcriptional mechanisms have emerged as a major mode of COPII regulation. Here, we review the current literature on how post-transcriptional events, and especially post-translational modifications, govern the COPII pathway.

Hypoxia, endoplasmic reticulum stress and chemoresistance: dangerous liaisons

Solid tumors often grow in a micro-environment characterized by < 2% O₂ tension. This condition, together with the aberrant activation of specific oncogenic patwhays, increases the amount and activity of the hypoxia-inducible factor-1α (HIF-1α), a transcription factor that controls up to 200 genes involved in neoangiogenesis, metabolic rewiring, invasion and drug resistance. Hypoxia also induces endoplasmic reticulum (ER) stress, a condition that triggers cell death, if cells are irreversibly damaged, or cell survival, if the stress is mild.Hypoxia and chronic ER stress both induce chemoresistance. In this review we discuss the multiple and interconnected circuitries that link hypoxic environment, chronic ER stress and chemoresistance. We suggest that hypoxia and ER stress train and select the cells more adapted to survive in unfavorable conditions, by activating pleiotropic mechanisms including apoptosis inhibition, metabolic rewiring, anti-oxidant defences, drugs efflux. This adaptative process unequivocally expands clones that acquire resistance to chemotherapy.We believe that pharmacological inhibitors of HIF-1α and modulators of ER stress, although characterized by low specificty and anti-cancer efficacy when used as single agents, may be repurposed as chemosensitizers against hypoxic and chemorefractory tumors in the next future.

Prion-Associated Neurodegeneration Causes Both Endoplasmic Reticulum Stress and Proteasome Impairment in a Murine Model of Spontaneous Disease

Prion diseases are a group of neurodegenerative disorders that can be spontaneous, familial or acquired by infection. The conversion of the prion protein PrP^C to its abnormal and misfolded isoform PrP^Sc is the main event in the pathogenesis of prion diseases of all origins. In spontaneous prion diseases, the mechanisms that trigger the formation of PrP^Sc in the central nervous system remain unknown. Several reports have demonstrated that the accumulation of PrP^Sc can induce endoplasmic reticulum (ER) stress and proteasome impairment from the early stages of the prion disease. Both mechanisms lead to an increment of PrP aggregates in the secretory pathway, which could explain the pathogenesis of spontaneous prion diseases. Here, we investigate the role of ER stress and proteasome impairment during prion disorders in a murine model of spontaneous prion disease (TgVole) co-expressing the Ub^G76V-GFP reporter, which allows measuring the proteasome activity in vivo. Spontaneously prion-affected mice showed a significantly higher accumulation of the PKR-like ER kinase (PERK), the ER chaperone binding immunoglobulin protein (BiP/Grp78), the ER protein disulfide isomerase (PDI) and the Ub^G76V-GFP reporter than age-matched controls in certain brain areas. The upregulation of PERK, BiP, PDI and ubiquitin was detected from the preclinical stage of the disease, indicating that ER stress and proteasome impairment begin at early stages of the spontaneous disease. Strong correlations were found between the deposition of these markers and neuropathological markers of prion disease in both preclinical and clinical mice. Our results suggest that both ER stress and proteasome impairment occur during the pathogenesis of spontaneous prion diseases.

The Ubiquitin-Proteasome System in Immune Cells

The ubiquitin–proteasome system (UPS) is the major intracellular and non-lysosomal protein degradation system. Thanks to its unique capacity of eliminating old, damaged, misfolded, and/or regulatory proteins in a highly specific manner, the UPS is virtually involved in almost all aspects of eukaryotic life. The critical importance of the UPS is particularly visible in immune cells which undergo a rapid and profound functional remodelling upon pathogen recognition. Innate and/or adaptive immune activation is indeed characterized by a number of substantial changes impacting various cellular processes including protein homeostasis, signal transduction, cell proliferation, and antigen processing which are all tightly regulated by the UPS. In this review, we summarize and discuss recent progress in our understanding of the molecular mechanisms by which the UPS contributes to the generation of an adequate immune response. In this regard, we also discuss the consequences of UPS dysfunction and its role in the pathogenesis of recently described immune disorders including cancer and auto-inflammatory diseases.

Identification of Two Novel Compound Heterozygous EIF2AK3 Mutations Underlying Wolcott-Rallison Syndrome in a Chinese Family

Objective: Wolcott-Rallison syndrome is a rare autosomal recessive inheritance disorder caused by the defectiveness of eukaryotic translation initiation factor 2 alpha kinase 3 (EIF2AK3), which encodes the PKR-like endoplasmic reticulum kinase (PERK). Defect in EIF2AK3 results in a permanent diabetes in early infancy or newborn period, a tendency to develop skeletal fractures and other associated disorders such as severe liver and renal dysfunction, and central hypothyroidism. Two patients with Wolcott-Rallison syndrome-like manifestations in a Chinese family and family members were genetically analyzed to identify if any variations that occurred in EIF2AK3, which may cause Wolcott-Rallison syndrome. Methods: Whole-exome sequencing (WES) was performed to identify genetic variations, and Sanger sequencing was conducted to verify the identified variations in the family members with Wolcott-Rallison syndrome (WRS) clinical manifestations. Several bioinformatics tools were employed to predict the effect of EIF2AK3 variations on the protein function. The impact on PERK protein was analyzed by sequential analysis and evolution conservation study. Results: Two novel EIF2AK3 heterozygous single base variations (c.2818C>T and c.2980G>C) were detected in the proband. PERK has two functional domains: one is regulatory domain (aa 1-576), and the other is catalytic domain (aa 577-1,115). Both variations are missense mutations and locate in catalytic domain of PERK; c.2818C>T resulted in a residue substitution of proline for serine at amino acid site 940 (p.Pro940Ser), and variation c.2980G>C caused an amino acid change at position 994 from glutamic acid to glutamine (p.Glu994Gln). These novel missense variations may affect the physiological functions of PERK protein. Conclusions: Two novel compound heterozygous EIF2AK3 variations (c.2818C>T, p.Pro940Ser and c.2980G>C, p.Glu994Gln) were found in a Chinese family. The identification of the variations and verification of their pathogenicity extended the variation spectrum of EIF2AK3 variations causing Wolcott-Rallison syndrome and enriched valuable information for precise medical intervention for Wolcott-Rallison syndrome in China.

Monogenic and syndromic diabetes due to endoplasmic reticulum stress

The endoplasmic reticulum (ER) lies at the crossroads of protein folding, calcium storage, lipid metabolism, and the regulation of autophagy and apoptosis. Accordingly, dysregulation of ER homeostasis leads to β-cell dysfunction in type 1 and type 2 diabetes that ultimately culminates in cell death. The ER is therefore an emerging target for understanding the mechanisms of diabetes mellitus that captures the complex etiologies of this multifactorial class of metabolic disorders. Our strategy for developing ER-targeted diagnostics and therapeutics is to focus on monogenic forms of diabetes related to ER dysregulation in an effort to understand the exact contribution of ER stress to β-cell death. In this manner, we can develop personalized genetic medicine for ERstress-related diabetic disorders, such as Wolfram syndrome. In this article, we describe the phenotypes and molecular pathogenesis of ERstress-related monogenic forms of diabetes.

Inonotus obliquus extract alleviates myocardial ischemia/reperfusion injury by suppressing endoplasmic reticulum stress

Inonotus obliquus (IO) is an edible fungus that exerts various biological functions, including anti‑inflammatory, antitumor and immunomodulatory effects. The present study was designed to investigate the role of IO extract (IOE) in myocardial ischemia/reperfusion (MI/R) and determine the exact molecular mechanisms. The left anterior descending coronary artery was ligated to establish the MI/R injury model in rats. IOE exhibited a novel cardioprotective effect, as shown by improvement in cardiac function and decrease in infarct size. Pretreatment with IOE activated antioxidant enzymes in cardiomyocytes, including glutathione peroxidase, superoxide dismutase and catalase. IOE pretreatment also induced the upregulation of NAD‑dependent protein deacetylase sirtuin‑1 (SIRT1) and downregulation of glucose‑regulated protein 78, phosphorylated (p‑) protein kinase R‑like endoplasmic reticulum kinase, p‑eukaryotic translation initiation factor 2 subunit α, C/EBP homologous protein and caspase‑12. Furthermore, IOE alleviated endoplasmic reticulum (ER) stress‑induced apoptosis in cardiomyocytes by decreasing the mRNA levels of caspase‑12. IOE inhibited apoptosis induced by overexpression of pro‑caspase‑9 and pro‑caspase‑3. In summary, IOE pretreatment protects the heart against MI/R injury through attenuating oxidative damage and suppressing ER stress‑induced apoptosis, which may be primarily due to SIRT1 activation.

Protein Kinases Signaling in Pancreatic Beta-cells Death and Type 2 Diabetes

Type 2 diabetes (T2D) is a worldwide serious public health problem. Insulin resistance and β-cell failure are the two major components of T2D pathology. In addition to defective endoplasmic reticulum (ER) stress signaling due to glucolipotoxicity, β-cell dysfunction or β-cell death initiates the deleterious vicious cycle observed in T2D. Although the primary cause is still unknown, overnutrition that contributes to the induction of the state of low-grade inflammation, and the activation of various protein kinases-related metabolic pathways are main factors leading to T2D. In this chapter following subjects, which have critical checkpoints regarding β-cell fate and protein kinases pathways are discussed; hyperglycemia-induced β-cell failure, chronic accumulation of unfolded protein in β-cells, the effect of intracellular reactive oxygen species (ROS) signaling to insulin secretion, excessive saturated free fatty acid-induced β-cell apoptosis, mitophagy dysfunction, proinflammatory responses and insulin resistance, and the reprogramming of β-cell for differentiation or dedifferentiation in T2D. There is much debate about selecting proposed therapeutic strategies to maintain or enhance optimal β-cell viability for adequate insulin secretion in T2D. However, in order to achieve an effective solution in the treatment of T2D, more intensive clinical trials are required on newer therapeutic options based on protein kinases signaling pathways.

Up-regulation of thioredoxin system by puerarin inhibits lipid uptake in macrophages

Cellular oxidative stress promotes lipid accumulation in macrophages during atherogenesis. Puerarin is a natural isoflavone with beneficial effects against oxidation and atherosclerosis. In this study, we investigated the effects of puerarin on lipid uptake and explored the underlying molecular regulation. We found puerarin up-regulated thioredoxin-1 (Trx1) and Trx reductase-1 (TrxR1) expression; it increased TrxR1 activity, cellular thiols contents and decreased oxidized form of Trx1, thus inhibiting cellular ROS generation. Confocal microscope and flow cytometry analysis showed fluorescence labeled Dil-oxLDL uptake was dramatically inhibited by puerarin in RAW264.7 cells as well as in primary bone marrow derived macrophages and peritoneal macrophages. The effects were reversed when Trx1 was inhibited by treatment with Trx1 inhibitor PX-12 or Trx1 siRNA. We also found scavenger receptors such as SR-A and Lox-1, but not CD36 were involved in the Trx1-mediated lipid uptake inhibition. Moreover, measurements of foam cell accumulation and ROS production in sections of aortic roots showed those were reduced by puerarin but raised when additional treatment with PX-12 or Trx1 siRNA in apoE-/- mice, which demonstrates the lipid uptake reduction by puerarin requires Trx1 inhibition in vivo. In addition, we analyzed the upstream regulation and found puerarin induced Nrf2 activity; cooperation between Nrf2 and ATF4 facilitated the puerarin effects. PERK phosphorylation was detected to be increased by puerarin, while PERK inhibition reduced cellular Trx1, TrxR1, nuclear Nrf2 and ATF4. Altogether, puerarin modulates PERK/Nrf2 that coordinates with ATF4 to active Trx1, which causes SR-A and Lox-1 reduction and lipid uptake inhibition in macrophages. This suggests Trx1 could be an effective target by puerarin in the prevention of atherosclerosis.

Cholesterol-Induced Phenotypic Modulation of Smooth Muscle Cells to Macrophage/Fibroblast-like Cells Is Driven by an Unfolded Protein Response

OBJECTIVE

Vascular smooth muscle cells (SMCs) dedifferentiate and initiate expression of macrophage markers with cholesterol exposure. This phenotypic switching is dependent on the transcription factor Klf4 (Krüppel-like factor 4). We investigated the molecular pathway by which cholesterol induces SMC phenotypic switching. Approach and Results: With exposure to free cholesterol, SMCs decrease expression of contractile markers, activate Klf4, and upregulate a subset of macrophage and fibroblast markers characteristic of modulated SMCs that appear with atherosclerotic plaque formation. These phenotypic changes are associated with activation of all 3 pathways of the endoplasmic reticulum unfolded protein response (UPR), Perk (protein kinase RNA-like endoplasmic reticulum kinase), Ire (inositol-requiring enzyme) 1α, and Atf (activating transcription factor) 6. Blocking the movement of cholesterol from the plasma membrane to the endoplasmic reticulum prevents free cholesterol-induced UPR, Klf4 activation, and upregulation of the majority of macrophage and fibroblast markers. Cholesterol-induced phenotypic switching is also prevented by global UPR inhibition or specific inhibition of Perk signaling. Exposure to chemical UPR inducers, tunicamycin and thapsigargin, is sufficient to induce these same phenotypic transitions. Finally, analysis of published single-cell RNA sequencing data during atherosclerotic plaque formation in hyperlipidemic mice provides preliminary in vivo evidence of a role of UPR activation in modulated SMCs.

CONCLUSIONS

Our data demonstrate that UPR is necessary and sufficient to drive phenotypic switching of SMCs to cells that resemble modulated SMCs found in atherosclerotic plaques. Preventing a UPR in hyperlipidemic mice diminishes atherosclerotic burden, and our data suggest that preventing SMC transition to dedifferentiated cells expressing macrophage and fibroblast markers contributes to this decreased plaque burden.

The Role of Mitochondrial Dysfunction and ER Stress in TDP-43 and C9ORF72 ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease of the motor system with complex determinants, including genetic and non-genetic factors. Despite this heterogeneity, a key pathological signature is the mislocalization and aggregation of specific proteins in the cytoplasm, suggesting that convergent pathogenic mechanisms focusing on disturbances in proteostasis are important in ALS. In addition, many cellular processes have been identified as potentially contributing to disease initiation and progression, such as defects in axonal transport, autophagy, nucleocytoplasmic transport, ER stress, calcium metabolism, the unfolded protein response and mitochondrial function. Here we review the evidence from in vitro and in vivo models of C9ORF72 and TDP-43-related ALS supporting a central role in pathogenesis for endoplasmic reticulum stress, which activates an unfolded protein response (UPR), and mitochondrial dysfunction. Disruption in the finely tuned signaling between the ER and mitochondria through calcium ions may be a crucial trigger of mitochondrial deficits and initiate an apoptotic signaling cascade, thus acting as a point of convergence for multiple upstream disturbances of cellular homeostasis and constituting a potentially important therapeutic target.

Endoplasmic Reticulum Homeostasis and Stress Responses in Caenorhabditis elegans

The unfolded protein response (UPR) is an evolutionarily conserved adaptive regulatory pathway that alleviates protein-folding defects in the endoplasmic reticulum (ER). Physiological demands, environmental perturbations and pathological conditions can cause accumulation of unfolded proteins in the ER and the stress signal is transmitted to the nucleus to turn on a series of genes to respond the challenge. In metazoan, the UPR pathways consisted of IRE1/XBP1, PEK-1 and ATF6, which function in parallel and downstream transcriptional activation triggers the proteostasis networks consisting of molecular chaperones, protein degradation machinery and other stress response pathways ((Labbadia J, Morimoto RI, F1000Prime Rep 6:7, 2014); (Shen X, Ellis RE, Lee K, Annu Rev Biochem 28:893-903, 2014)). The integrated responses act on to resolve the ER stress by increasing protein folding capacity, attenuating ER-loading translation, activating ER-associated proteasomal degradation (ERAD), and regulating IRE1-dependent decay of mRNA (RIDD). Therefore, the effective UPR to internal and external causes is linked to the multiple pathophysiological conditions such as aging, immunity, and neurodegenerative diseases. Recent development in the research of the UPR includes cell-nonautonomous features of the UPR, interplay between the UPR and other stress response pathways, unconventional UPR inducers, and noncanonical UPR independent of the three major branches, originated from multiple cellular and molecular machineries in addition to ER. Caenorhabditis elegans model system has critically contributed to these unprecedented aspects of the ER UPR and broadens the possible therapeutic targets to treat the ER-stress associated human disorders and time-dependent physiological deterioration of aging.

Post-Translational Modification of Cysteines: A Key Determinant of Endoplasmic Reticulum-Mitochondria Contacts (MERCs)

Cells must adjust their redox state to an ever-changing environment that could otherwise result in compromised homeostasis. An obvious way to adapt to changing redox conditions depends on cysteine post-translational modifications (PTMs) to adapt conformation, localization, interactions and catalytic activation of proteins. Such PTMs should occur preferentially in the proximity of oxidative stress sources. A particular concentration of these sources is found near membranes where the endoplasmic reticulum (ER) and the mitochondria interact on domains called MERCs (Mitochondria-Endoplasmic Reticulum Contacts). Here, fine inter-organelle communication controls metabolic homeostasis. MERCs achieve this goal through fluxes of Ca²⁺ ions and inter-organellar lipid exchange. Reactive oxygen species (ROS) that cause PTMs of mitochondria-associated membrane (MAM) proteins determine these intertwined MERC functions. Chronic changes of the pattern of these PTMs not only control physiological processes such as the circadian clock but could also lead to or worsen many human disorders such as cancer and neurodegenerative diseases.

Transcriptome Analysis Reveals Unfolded Protein Response Was Induced During the Early Stage of Burkholderia pseudomallei Infection in A549 Cells

Burkholderia pseudomallei is a zoonotic pathogen that usually affects patients' lungs and causes serious melioidosis. The interaction of B. pseudomallei with its hosts is complex, and cellular response to B. pseudomallei infection in humans still remains to be elucidated. In this study, transcriptomic profiling of B. pseudomallei-infected human lung epithelial A549 cells was performed to characterize the cellular response dynamics during the early infection (EI) stage. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were performed by using the online databases DAVID 6.8 and KOBAS 3.0. Real-time quantitative PCR and western blot were used for validation experiments. Compared with the negative control group (NC), a set of 36 common genes varied over time with a cut-off level of 1.5-fold change, and a P-value < 0.05 was identified. Bioinformatics analysis indicated that the PERK-mediated unfolded protein response (UPR) was enriched as the most noteworthy biological process category, which was enriched as a branch of UPR in the signaling pathway of protein processing in the endoplasmic reticulum. Other categories, such as inflammatory responses, cell migration, and apoptosis, were also focused. The molecular chaperone Bip (GRP78), PERK, and PERK sensor-dependent phosphorylation of eIF2α (p-eIF2α) and ATF4 were verified to be increasing over time during the EI stage, suggesting that B. pseudomallei infection activated the PERK-mediated UPR in A549 cells. Collectively, these results provide important initial insights into the intimate interaction between B. pseudomallei and lung epithelial cells, which can be further explored toward the elucidation of the cellular mechanisms of B. pseudomallei infections in humans.

A Molecular Mechanism for Turning Off IRE1α Signaling during Endoplasmic Reticulum Stress

Misfolded proteins in the endoplasmic reticulum (ER) activate IRE1α endoribonuclease in mammalian cells, which mediates XBP1 mRNA splicing to produce an active transcription factor. This promotes the expression of specific genes to alleviate ER stress, thereby attenuating IRE1α. Although sustained activation of IRE1α is linked to human diseases, it is not clear how IRE1α is attenuated during ER stress. Here, we identify that Sec63 is a subunit of the previously identified IRE1α/Sec61 translocon complex. We find that Sec63 recruits and activates BiP ATPase through its luminal J-domain to bind onto IRE1α. This leads to inhibition of higher-order oligomerization and attenuation of IRE1α RNase activity during prolonged ER stress. In Sec63-deficient cells, IRE1α remains activated for a long period of time despite the presence of excess BiP in the ER. Thus, our data suggest that the Sec61 translocon bridges IRE1α with Sec63/BiP to regulate the dynamics of IRE1α signaling in cells.

The stress sensor IRE1α is attenuated during prolonged ER stress by a poorly understood mechanism. Li et al. show that IRE1α forms a complex with the Sec61/Sec63 translocon in cells. Sec63 mediates BiP binding to IRE1α and thereby inhibits IRE1α oligomerization and attenuates IRE1α signaling during prolonged ER stress.

miR-106b suppresses pathological retinal angiogenesis

MicroRNAs are small non-coding RNAs that post-transcriptionally regulate gene expression. We recently demonstrated that levels of miR-106b were significantly decreased in the vitreous and plasma of patients with neovascular age-related macular degeneration (AMD). Here we show that expression of the miR-106b-25 cluster is negatively regulated by the unfolded protein response pathway of protein kinase RNA-like ER kinase (PERK) in a mouse model of neovascular AMD. A reduction in levels of miR-106b triggers vascular growth both in vivo and in vitro by inducing production of pro-angiogenic factors. We demonstrate that therapeutic delivery of miR-106b to the retina with lentiviral vectors protects against aberrant retinal angiogenesis in two distinct mouse models of pathological retinal neovascularization. Results from this study suggest that miRNAs such as miR-106b have the potential to be used as multitarget therapeutics for conditions characterized by pathological retinal angiogenesis.

Two high-rate pentose-phosphate pathways in cancer cells

The relevant role of pentose phosphate pathway (PPP) in cancer metabolic reprogramming has been usually outlined by studying glucose-6-phosphate dehydrogenase (G6PD). However, recent evidence suggests an unexpected role for a less characterized PPP, triggered by hexose-6-phosphate dehydrogenase (H6PD) within the endoplasmic reticulum (ER). Studying H6PD biological role in breast and lung cancer, here we show that gene silencing of this reticular enzyme decreases cell content of PPP intermediates and d-ribose, to a similar extent as G6PD silencing. Decrease in overall NADPH content and increase in cell oxidative status are also comparable. Finally, either gene silencing impairs at a similar degree cell proliferating activity. This unexpected response occurs despite the absence of any cross-interference between the expression of both G6PD and H6PD. Thus, overall cancer PPP reflects the contribution of two different pathways located in the cytosol and ER, respectively. Disregarding the reticular pathway might hamper our comprehension of PPP role in cancer cell biology.

Tissue-specific roles of GCN2 in aging and autosomal dominant retinitis pigmentosa

The organisms have the capacity to sense and adapt to their surroundings for their life in a dynamic environment. In response to amino acid starvation, cells activate a rectifying physiological program, termed the integrated stress response (ISR), to restore cellular homeostasis. General controlled non-repressed (GCN2) kinase is a master regulator of the ISR and modulates protein synthesis in response to amino acid starvation. We previously established the GCN2/ATF4/4E-BP pathway in development and aging. Here, we investigated the tissue-specific roles of GCN2 upon dietary restriction of amino acid in a Drosophila model. The knockdown of GCN2 in the gut and fat body, an energy sensing organ in Drosophila, abolished the beneficial effect of GCN2 in lifespan extension upon dietary restriction of amino acids. Proteome analysis in an autosomal dominant retinitis pigmentosa (ADRP) model showed that dietary restriction of amino acids regulates the synthesis of proteins in several pathways, including mitochondrial translation, mitochondrial gene expression, and regulation of biological quality, and that gcn2-mutant flies have reduced levels of these mitochondria-associated proteins, which may contribute to retinal degeneration in ADRP. These results indicate that the tissue-specific regulation of GCN2 contributes to normal physiology and ADRP progression.

Melatonin Inhibits Glucose-Induced Apoptosis in Osteoblastic Cell Line Through PERK-eIF2α-ATF4 Pathway

Osteoporosis is a common disease resulting in deteriorated microarchitecture and decreased bone mass. In type 2 diabetes patients, the incidence of osteoporosis is significantly higher accompanied by increased apoptosis of osteoblasts. In this study, using the osteoblastic cell line MC3T3-E1, we show that high glucose reduces cell viability and induces apoptosis. Also, high glucose leads to endoplasmic reticulum (ER) stress (ERS) via an increase in calcium flux and upregulation of the ER chaperone binding immunoglobulin protein (BiP). Moreover, it induces post-translational activation of eukaryotic initiation factor 2 alpha (eIF2α) which functions downstream of PKR-like ER kinase (PERK). This subsequently leads to post-translational activation of the transcription factor 4 (ATF4) and upregulation of C/EBP-homologous protein (CHOP) which is an ER stress-induced regulator of apoptosis, as well as downstream effectors DNAJC3, HYOU1, and CALR. Interestingly, melatonin treatment significantly alleviates the high-glucose induced changes in cell growth, apoptosis, and calcium influx by inhibiting the PERK-eIF2α-ATF4-CHOP signaling pathway. Additionally, the MC3T3-E1 cells engineered to express a phosphodead eIF2α mutant did not show high glucose induced ER stress, confirming that melatonin protects osteoblasts against high-glucose induced changes by decreasing ER-stress induced apoptosis by impacting the PERK-eIF2α-ATF4-CHOP signaling pathway. The protective of melatonin against high glucose-induced ER stress and apoptosis was attenuated when the cells were pre-treated with a melatonin receptor antagonist, indicating that the effect of melatonin was mediated via the melatonin receptors in this context. These findings lay the provide mechanistic insights of melatonin’s protective action on osteoblasts and will be potentially be useful in ongoing pre-clinical and clinical studies to evaluate melatonin as a therapeutic option for diabetic osteoporosis.

Furosemide as a Probe Molecule for the Treatment of Neuroinflammation in Alzheimer's Disease

The accumulation and deposition of β-amyloid (Aβ) is one postulated cause of Alzheimer's disease (AD). In addition to its direct toxicity on neurons, Aβ may induce neuroinflammation through the concomitant activation of microglia. Emerging evidence suggests that microglia-mediated neuroinflammation plays an important role in the pathogenesis of AD. As brain macrophages, microglia engulf misfolded-Aβ by phagocytosis. However, the accumulated toxic Aβ may paradoxically "hyper-activate" microglia into a neurotoxic proinflammatory and less phagocytotic phenotype, contributing to neuronal death. This study reports that the known drug furosemide is a potential probe molecule for reducing AD-neuroinflammation. Our data demonstrate that furosemide inhibits the secretion of proinflammatory TNF-α, IL-6, and nitric oxide; downregulates the mRNA level of Cd86 and the protein expression of COX-2, iNOS; promotes phagocytic activity; and enhances the expression of anti-inflammatory IL-1RA and arginase. Our mechanism of action studies further demonstrate that furosemide reduces LPS-induced upregulation of endoplasmic reticulum (ER) stress marker genes, including Grp78, Atf4, Chop, tXbp1, and sXbp1. These data support the observation that furosemide is a known drug with the capacity to downregulate the proinflammatory microglial M1 phenotype and upregulate the anti-inflammatory M2 phenotype, a potentially powerful and beneficial pharmacologic effect for inflammatory diseases such as AD.

Calcium depletion challenges endoplasmic reticulum proteostasis by destabilising BiP-substrate complexes

The metazoan endoplasmic reticulum (ER) serves both as a hub for maturation of secreted proteins and as an intracellular calcium storage compartment, facilitating calcium-release-dependent cellular processes. ER calcium depletion robustly activates the unfolded protein response (UPR). However, it is unclear how fluctuations in ER calcium impact organellar proteostasis. Here, we report that calcium selectively affects the dynamics of the abundant metazoan ER Hsp70 chaperone BiP, by enhancing its affinity for ADP. In the calcium-replete ER, ADP rebinding to post-ATP hydrolysis BiP-substrate complexes competes with ATP binding during both spontaneous and co-chaperone-assisted nucleotide exchange, favouring substrate retention. Conversely, in the calcium-depleted ER, relative acceleration of ADP-to-ATP exchange favours substrate release. These findings explain the rapid dissociation of certain substrates from BiP observed in the calcium-depleted ER and suggest a mechanism for tuning ER quality control and coupling UPR activity to signals that mobilise ER calcium in secretory cells.

COG5 variants lead to complex early onset retinal degeneration, upregulation of PERK and DNA damage

Leber congenital amaurosis (LCA), a form of autosomal recessive severe early-onset retinal degeneration, is an important cause of childhood blindness. This may be associated with systemic features or not. Here we identified COG5 compound-heterozygous variants in patients affected with a complex LCA phenotype associated with microcephaly and skeletal dysplasia. COG5 is a component of the COG complex, which facilitates retrograde Golgi trafficking; if disrupted this can result in protein misfolding. To date, variants in COG5 have been associated with a distinct congenital disorder of glycosylation (type IIi) and with a variant of Friedreich’s ataxia. We show that COG5 variants can also result in fragmentation of the Golgi apparatus and upregulation of the UPR modulator, PKR-like endoplasmic reticulum kinase (PERK). In addition, upregulation of PERK induces DNA damage in cultured cells and in murine retina. This study identifies a novel role for COG5 in maintaining ER protein homeostasis and that disruption of that role results in activation of PERK and early-onset retinal degeneration, microcephaly and skeletal dysplasia. These results also highlight the importance of the UPR pathway in early-onset retinal dystrophy and as potential therapeutic targets for patients.

Non-Alcoholic Steatohepatitis: A Review of Its Mechanism, Models and Medical Treatments

Non-alcoholic steatohepatitis (NASH) develops from non-alcoholic fatty liver disease (NAFLD). Currently, around 25% of the population is estimated to have NAFLD, and 25% of NAFLD patients are estimated to have NASH. NASH is typically characterized by liver steatosis inflammation, and fibrosis driven by metabolic disruptions such as obesity, diabetes, and dyslipidemia. NASH patients with significant fibrosis have increased risk of developing cirrhosis and liver failure. Currently, NASH is the second leading cause for liver transplant in the United States. More importantly, the risk of developing hepatocellular carcinoma from NASH has also been highlighted in recent studies. Patients may have NAFLD for years before progressing into NASH. Although the pathogenesis of NASH is not completely understood, the current “multiple-hits” hypothesis suggests that in addition to fat accumulation, elevated oxidative and ER stress may also drive liver inflammation and fibrosis. The development of clinically relevant animal models and pharmacological treatments for NASH have been hampered by the limited understanding of the disease mechanism and a lack of sensitive, non-invasive diagnostic tools. Currently, most pre-clinical animal models are divided into three main groups which includes: genetic models, diet-induced, and toxin + diet-induced animal models. Although dietary models mimic the natural course of NASH in humans, the models often only induce mild liver injury. Many genetic and toxin + diet-induced models rapidly induce the development of metabolic disruption and serious liver injury, but not without their own shortcomings. This review provides an overview of the “multiple-hits” hypothesis and an evaluation of the currently existing animal models of NASH. This review also provides an update on the available interventions for managing NASH as well as pharmacological agents that are currently undergoing clinical trials for the treatment of NASH.

Sestrin2 protects against bavachin induced ER stress through AMPK/mTORC1 signaling pathway in HepG2 cells

Bavachin (BV), a natural flavonoid compound derived from Psoralea corylifolia L, has been reported to be a potential hepatotoxin. Our previous studies have found that BV can induce endoplasmic reticulum (ER) stress-related cell apoptosis, but the molecular mechanism underlying BV-induced ER stress remains obscure. Sestrin2, a highly conserved stress-inducible protein, is involved in the cellular responses of various stress conditions and homeostatic regulation. However, whether Sestrin2 participated in the ER stress related hepatotoxicity against BV is still elusive. In the present study, we aim to investigate the role of BV on liver injury of mice and the impact of Sestrin2 on BV-induced ER stress in HepG2 cells. The results in mice showed that BV induced ER stress related liver injury with increased Sestrin2 expression involvement. Knockdown of Sestrin2 with siRNA aggravated BV-induced ER stress significantly in HepG2 cells. Further mechanistic study uncovered that inhibition of mTORC1 with rapamycin blocked BV-induced ER stress, and treatment with Sestrin2 siRNA blocked the inhibition effect of AMPK to mTORC1. Therefore, constant mTORC1 would lead to accumulation of misfolded or unfolded proteins and aggravated ER stress. Collectively, our study indicates that Sestrin2 confers protection against BV-induced ER stress via activating of the AMPK/mTORC1 pathway.

Detection of immunogenic cell death and its relevance for cancer therapy

Chemotherapy, radiation therapy, as well as targeted anticancer agents can induce clinically relevant tumor-targeting immune responses, which critically rely on the antigenicity of malignant cells and their capacity to generate adjuvant signals. In particular, immunogenic cell death (ICD) is accompanied by the exposure and release of numerous damage-associated molecular patterns (DAMPs), which altogether confer a robust adjuvanticity to dying cancer cells, as they favor the recruitment and activation of antigen-presenting cells. ICD-associated DAMPs include surface-exposed calreticulin (CALR) as well as secreted ATP, annexin A1 (ANXA1), type I interferon, and high-mobility group box 1 (HMGB1). Additional hallmarks of ICD encompass the phosphorylation of eukaryotic translation initiation factor 2 subunit-α (EIF2S1, better known as eIF2α), the activation of autophagy, and a global arrest in transcription and translation. Here, we outline methodological approaches for measuring ICD markers in vitro and ex vivo for the discovery of next-generation antineoplastic agents, the development of personalized anticancer regimens, and the identification of optimal therapeutic combinations for the clinical management of cancer.

Tauopathies: Deciphering Disease Mechanisms to Develop Effective Therapies

Tauopathies are neurodegenerative diseases characterized by the pathological accumulation of microtubule-associated protein tau (MAPT) in the form of neurofibrillary tangles and paired helical filaments in neurons and glia, leading to brain cell death. These diseases include frontotemporal dementia (FTD) and Alzheimer's disease (AD) and can be sporadic or inherited when caused by mutations in the MAPT gene. Despite an incredibly high socio-economic burden worldwide, there are still no effective disease-modifying therapies, and few tau-focused experimental drugs have reached clinical trials. One major hindrance for therapeutic development is the knowledge gap in molecular mechanisms of tau-mediated neuronal toxicity and death. For the promise of precision medicine for brain disorders to be fulfilled, it is necessary to integrate known genetic causes of disease, i.e., MAPT mutations, with an understanding of the dysregulated molecular pathways that constitute potential therapeutic targets. Here, the growing understanding of known and proposed mechanisms of disease etiology will be reviewed, together with promising experimental tau-directed therapeutics, such as recently developed tau degraders. Current challenges faced by the fields of tau research and drug discovery will also be addressed.

Annexin A2 regulates unfolded protein response via IRE1-XBP1 axis in macrophages during P. aeruginosa infection

Pseudomonas aeruginosa is a severe Gram-negative opportunistic bacterium that causes a spectrum of organ system diseases, particularly in immunocompromised patients. This bacterium has been shown to induce unfolded protein response (UPR) during mammalian infection. Annexin A2 (AnxA2) is a multicompartmental protein relating to a number of cellular processes; however, it remains unknown whether AnxA2 coordinates a UPR pathway under bacterial infection conditions. Here, we report that the endoplasmic reticulum stress inositol-requiring enzyme 1 (IRE1)-X-box binding protein 1 (XBP1) pathway was up-regulated by AnxA2 through p38 MAPK signaling following P. aeruginosa infection in macrophages, whereas ATF4 and ATF6 not. In addition, XBP1 was found as a positive regulator of innate immunity to tame P. aeruginosa challenges by enhancing autophagy and bacterial clearance. XBP1 also facilitated NF-κB activation to elicit the release of proinflammatory cytokines predominantly in macrophages. Together, our findings identify AnxA2 as a regulator for XBP1-mediated UPR pathway.

The Endoplasmic Reticulum Stress/Unfolded Protein Response and Their Contributions to Parkinson's Disease Physiopathology

Parkinson’s disease (PD) is a multifactorial age-related movement disorder in which defects of both mitochondria and the endoplasmic reticulum (ER) have been reported. The unfolded protein response (UPR) has emerged as a key cellular dysfunction associated with the etiology of the disease. The UPR involves a coordinated response initiated in the endoplasmic reticulum that grants the correct folding of proteins. This review gives insights on the ER and its functioning; the UPR signaling cascades; and the link between ER stress, UPR activation, and physiopathology of PD. Thus, post-mortem studies and data obtained by either in vitro and in vivo pharmacological approaches or by genetic modulation of PD causative genes are described. Further, we discuss the relevance and impact of the UPR to sporadic and genetic PD pathology.

Small molecule strategies to harness the unfolded protein response: where do we go from here?

The unfolded protein response (UPR) plays a central role in regulating endoplasmic reticulum (ER) and global cellular physiology in response to pathologic ER stress. The UPR is comprised of three signaling pathways activated downstream of the ER membrane proteins IRE1, ATF6, and PERK. Once activated, these proteins initiate transcriptional and translational signaling that functions to alleviate ER stress, adapt cellular physiology, and dictate cell fate. Imbalances in UPR signaling are implicated in the pathogenesis of numerous, etiologically-diverse diseases, including many neurodegenerative diseases, protein misfolding diseases, diabetes, ischemic disorders, and cancer. This has led to significant interest in establishing pharmacologic strategies to selectively modulate IRE1, ATF6, or PERK signaling to both ameliorate pathologic imbalances in UPR signaling implicated in these different diseases and define the importance of the UPR in diverse cellular and organismal contexts. Recently, there has been significant progress in the identification and characterization of UPR modulating compounds, providing new opportunities to probe the pathologic and potentially therapeutic implications of UPR signaling in human disease. Here, we describe currently available UPR modulating compounds, specifically highlighting the strategies used for their discovery and specific advantages and disadvantages in their application for probing UPR function. Furthermore, we discuss lessons learned from the application of these compounds in cellular and in vivo models to identify favorable compound properties that can help drive the further translational development of selective UPR modulators for human disease.

Ribosomal protein S7 ubiquitination during ER stress in yeast is associated with selective mRNA translation and stress outcome

eIF2α phosphorylation-mediated translational regulation is crucial for global translation repression by various stresses, including the unfolded protein response (UPR). However, translational control during UPR has not been demonstrated in yeast. This study investigated ribosome ubiquitination-mediated translational controls during UPR. Tunicamycin-induced ER stress enhanced the levels of ubiquitination of the ribosomal proteins uS10, uS3 and eS7. Not4-mediated monoubiquitination of eS7A was required for resistance to tunicamycin, whereas E3 ligase Hel2-mediated ubiquitination of uS10 was not. Ribosome profiling showed that the monoubiquitination of eS7A was crucial for translational regulation, including the upregulation of the spliced form of HAC1 (HAC1i) mRNA and the downregulation of Histidine triad NucleoTide-binding 1 (HNT1) mRNA. Downregulation of the deubiquitinating enzyme complex Upb3-Bre5 increased the levels of ubiquitinated eS7A during UPR in an Ire1-independent manner. These findings suggest that the monoubiquitination of ribosomal protein eS7A plays a crucial role in translational controls during the ER stress response in yeast.

Modulation of de Novo Lipogenesis Improves Response to Enzalutamide Treatment in Prostate Cancer

Simple Summary

Prostate cancer cells produce lipids via the activation of a specific pathway called fatty acid synthesis, also known as De novo lipogenesis. This pathway is essential for the survival and growth of most types of cancer cells, including prostate cancer. In our study, we showed that prostate cancer cells activate this lipid synthesis pathway to become more aggressive and develop resistance to commonly used therapeutic agents for advanced prostate cancer such as enzalutamide, an effective and commonly used androgen receptor (AR) targeted agent. Interestingly, by combining enzalutamide with a lipid synthesis pathway inhibitor, we were able to show that growth of prostate cancer tumors was more effectively reduced than with either agent alone. We also showed that this combination led to cell stress and death by changing the lipid content in the cell. These important findings could lead to new therapeutic strategies combining effective AR targeted therapies with lipid synthesis inhibitors for the treatment of advanced prostate cancer.

Abstract

De novo lipogenesis (DNL) is now considered as a hallmark of cancer. The overexpression of key enzymes of DNL is characteristic of both primary and advanced disease and may play an important role in resistance to therapies. Here, we showed that DNL is highly enhanced in castrate resistant prostate cancer (CRPC) cells compared to hormone sensitive and enzalutamide resistant cells. This observation suggests that this pathway plays an important role in the initiation of aggressive prostate cancer and in the development of enzalutamide resistance. Importantly, here we show that both prostate cancer cells sensitive and resistant to enzalutamide are dependent on DNL to proliferate. We next combined enzalutamide with an inhibitor of Stearoyl CoA Desaturase 1 (SCD1), an important enzyme in DNL, and observed significantly reduced tumor growth caused by the important change in tumoral lipid desaturation. Our findings suggest that the equilibrium between monounsaturated fatty acids and saturated fatty acids is essential in the establishment of the more aggressive prostate cancer phenotype and that the combination therapy induces a disruption of this equilibrium leading to an important decrease of cell proliferation. These findings provide new insights into the role of DNL in the progression of prostate cancer cells. The study also provides the rationale for the use of an inhibitor of SCD1 in combination with enzalutamide to improve response, delay enzalutamide resistance and improve disease free progression.

Mitochondrial stress and GDF15 in the pathophysiology of sepsis

Mitochondria are multifunctional organelles that regulate diverse cellular processes. Mitochondrial stress, including stress generated by electron transport chain defects and impaired mitochondrial proteostasis, is intimately involved in various diseases and pathological conditions. Sepsis is a life-threatening condition that occurs when an imbalanced host response to infection leads to organ dysfunction. Metabolic disturbances and impaired immune responses are implicated in the pathogenesis and development of sepsis. Given that mitochondria play central roles in cellular metabolism, mitochondrial stress is predicted to be involved in the pathological mechanism of sepsis. Under mitochondrial stress, cells activate stress response systems to maintain homeostasis. This mitochondrial stress response transcriptionally activates genes involved in cell survival and death. Mitochondrial stress also induces the release of distinctive secretory proteins from cells. Recently, we showed that growth differentiation factor 15 (GDF15) is a major secretory protein induced by mitochondrial dysfunction. In this article, we provide a brief overview of mitochondrial stress response and GDF15, and discuss the potential role of GDF15 in the pathophysiology of sepsis.

Limonin ameliorates dextran sulfate sodium-induced chronic colitis in mice by inhibiting PERK-ATF4-CHOP pathway of ER stress and NF-κB signaling

Inflammatory bowel disease (IBD) is a chronic gastrointestinal inflammation regulated by intricate mechanisms. Limonin, a natural tetracyclic triterpenoid compound, possesses multiple bioactivities including anti-inflammation, anti-cancer and so on. However, the therapeutic potential and the underlying mechanism of limonin on IBD remain unclear. Here, we probe into the effect of limonin on chronic colitis induced by dextran sulfate sodium (DSS) and illustrated the potential mechanisms. We found that limonin relieved the risk and severity of DSS-induced chronic colitis in mice through various aspects including increasing body weight and colon length, decreasing the mortality rate, inhibiting MPO activity and improving colon pathology. Limonin also decreased the production of proinflammatory cytokines TNF-α, IL-1β, IL-6 and the expression of inflammatory proteins COX-2, iNOS in colon tissues from DSS-induced colitis mice. Moreover, limonin attenuated DSS-induced chronic colitis by inhibiting PERK-ATF4-CHOP pathway of endoplasmic reticulum (ER) stress and NF-κB signaling. In vitro, limonin not only decreased LPS-induced higher production of pro-inflammatory cytokines and inflammatory proteins mentioned above by inhibiting NF-κB signaling in macrophage cells RAW264.7, but also suppressed PERK-ATF4-CHOP pathway of ER stress. In summary, our study demonstrated that limonin mitigated DSS-induced chronic colitis via inhibiting PERK-ATF4-CHOP pathway of ER stress and NF-κB signaling. All of this study provides the possibility for limonin as an effective drug for chronic colitis of IBD in the future.

Endoplasmic reticulum stress causes insulin resistance by inhibiting delivery of newly synthesized insulin receptors to the cell surface

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER stress and activates a signaling network known as the unfolded protein response (UPR). Here we characterize how ER stress and the UPR inhibit insulin signaling. We find that ER stress inhibits insulin signaling by depleting the cell surface population of the insulin receptor. ER stress inhibits proteolytic maturation of insulin proreceptors by interfering with transport of newly synthesized insulin proreceptors from the ER to the plasma membrane. Activation of AKT, a major target of the insulin signaling pathway, by a cytosolic, membrane-bound chimera between the AP20187-inducible FV2E dimerization domain and the cytosolic protein tyrosine kinase domain of the insulin receptor was not affected by ER stress. Hence, signaling events in the UPR, such as activation of the JNK mitogen-activated protein (MAP) kinases or the pseudokinase TRB3 by the ER stress sensors IRE1α and PERK, do not contribute to inhibition of signal transduction in the insulin signaling pathway. Indeed, pharmacologic inhibition and genetic ablation of JNKs, as well as silencing of expression of TRB3, did not restore insulin sensitivity or rescue processing of newly synthesized insulin receptors in ER-stressed cells. [Media: see text].

Organelle Stress and Crosstalk in Kidney Disease

Organelles play important roles in maintaining cellular homeostasis. Organelle stress responses, especially in mitochondria, endoplasmic reticula (ER), and primary cilia, are deeply involved in kidney disease pathophysiology. Mitochondria are the center of energy production in most eukaryotic cells. Renal proximal tubular cells are highly energy demanding and abundant in mitochondria. Mitochondrial dysfunctions in association with energy metabolism alterations produce reactive oxygen species and promote inflammation in proximal tubular cells, resulting in progression of kidney disease. The ER play critical roles in controlling protein quality. Unfolded protein response (UPR) pathways are the adaptive response to ER stress for maintaining protein homeostasis. UPR pathway dysregulation under pathogenic ER stress often occurs in glomerular and tubulointerstitial cells and promotes progression of kidney disease. The primary cilia sense extracellular signals and maintain calcium homeostasis in cells. Dysfunction of the primary cilia in autosomal dominant polycystic kidney disease reduces the calcium concentration in proximal tubular cells, leading to increased cell proliferation and retention of cyst fluid. In recent years, the direct interaction at membrane contact sites has received increased attention in association with the development of imaging technologies. The part of the ER that is directly connected to mitochondria is termed the mitochondria-associated ER membrane (MAM), which regulates calcium homeostasis and phospholipid metabolism in cells. Disruption of MAM integrity collapses cellular homeostasis and leads to diseases such as diabetes and Alzheimer disease. This review summarizes recent research on organelle stress and crosstalk, and their involvement in kidney disease pathophysiology. In addition, potential treatment options that target organelle stress responses are discussed.

Stress relief for cancer immunotherapy: implications for the ER stress response in tumor immunity

The solid tumor microenvironment is replete with factors that present a stress to infiltrating immune cells. Endoplasmic reticulum (ER) stress sensor PKR-like ER kinase (PERK) is primed to sense and respond to the burden of misfolded proteins in the ER lumen induced by cell stressors. PERK has documented roles as a master regulator of acute and chronic responses to cell stress as well as in the regulation of cell metabolism. Here, we provide an overview of the roles of PERK based on what is known and remains to be tested in immune cells in tumors and impacts on tumor control. PERK is one of several ER kinases able to preferentially induce activating transcription factor 4 (ATF4) as a response to cell stress. ATF4 orchestrates the oxidative stress response and governs amino acid metabolism. We discuss the tested role of ATF4 in tumor immunity and provide insight on the dueling protective and deleterious roles that ATF4 may play in the stress of solid tumors.

Autophagy, Unfolded Protein Response and Lung Disease

Acute Respiratory Distress Syndrome is a severe disorder affecting thousands of individuals worldwide. The available medical countermeasures do not sufficiently suppress the unacceptable high mortality rates associated with those in need. Thus, intense efforts aim to delineate the function of the lung endothelium, so to deliver new therapeutic approaches against this disease. The present manuscript attempts to shed light on the interrelations between the unfolded protein response and autophagy towards lung disease, to deliver a new line of possible therapeutic approaches against the ferocious Acute Respiratory Distress Syndrome.

Comparative analysis of developing grain transcriptome reveals candidate genes and pathways improving GPC in wheat lines derived from wild emmer

The grain protein content (GPC) in modern wheat is inherently low. Wild emmer wheat (Triticum turgidum ssp. dicoccoides, 2n = 4x = 28, AABB) gene pool harbors wide genotypic variations in GPC. However, the characterization of candidate genes associated with high GPC is a challenge due to the complex characteristic of this trait. In the current study, we performed RNA-seq analysis on developing grains of wild emmer genotype D1, common wheat CN16, and their hexaploid wide hybrid BAd107-4 with contrasting GPC. We have found a total of 39,795 expressed genes on chromosomes A and B, of which 24,152 were shared between D1, CN16, and BAd107-4. From 1744 differentially expressed genes (DEGs), 1203 were downregulated and 541 were upregulated in the high GPC (D1+BAd107-4) compared with low GPC (CN16) groups. The majority of DEGs were associated with protein processing in endoplasmic reticulum, starch and sucrose metabolism, galactose metabolism, and protein export pathways. Expression levels of nine randomly selected genes were verified by qRT-PCR, which was consistent with the transcriptome data. The present database will help us to understand the potential regulation networks underlying wheat grain protein accumulation and provide the foundation for simultaneous improvement of grain protein content and yield in wheat breeding programs.

Characterization of a PERK Kinase Inhibitor with Anti-Myeloma Activity

Due to increased immunoglobulin production and uncontrolled proliferation, multiple myeloma (MM) plasma cells develop a phenotype of deregulated unfolded protein response (UPR). The eIF2-alpha kinase 3 [EIF2αK3, protein kinase R (PKR)-like ER kinase (PERK)], the third known sensor of endoplasmic reticulum (ER) stress, is a serine-threonine kinase and, like the other two UPR-related proteins, i.e., IRE1 and ATF6, it is bound to the ER membrane. MM, like other tumors showing uncontrolled protein secretion, is highly dependent to UPR for survival; thus, inhibition of PERK can be an effective strategy to suppress growth of malignant plasma cells. Here, we have used GSK2606414, an ATP-competitive potent PERK inhibitor, and found significant anti-proliferative and apoptotic effects in a panel of MM cell lines. These effects were accompanied by the downregulation of key components of the PERK pathway as well as of other UPR elements. Consistently, PERK gene expression silencing significantly increased cell death in MM cells, highlighting the importance of PERK signaling in MM biology. Moreover, GSK2606414, in combination with the proteasome inhibitor bortezomib, exerted an additive toxic effect in MM cells. Overall, our data suggest that PERK inhibition could represent a novel combinatorial therapeutic approach in MM.

Toxic effects of endoplasmic reticulum stress transducer BBF2H7-derived small peptide fragments on neuronal cells

Aggregation, fibril formation, and deposition of amyloid β (Aβ) protein are believed to be the central pathogeneses of Alzheimer's disease (AD). Numerous studies have shown that fibril formation is promoted by preformed seeds at the beginning of the aggregation process. Therefore, aggregated molecules that promote fibrillization of Aβ protein as seeds could affect the pathology. We recently found that approximately 40 amino acid hydrophobic peptides, BBF2H7-derived small peptide (BSP) fragments, are generated via intramembranous cleavage under endoplasmic reticulum (ER) stress conditions. Interestingly, similar to Aβ protein, the fragments exhibit a high aggregation propensity and form fibril structures. It has been noted that ER stress is involved in the pathogenesis of AD. In this study, we examined the effect of BSP fragments on aggregation and cytotoxicity of Aβ_1-40 protein, which is generated as a major species of Aβ protein, but has a lower aggregative property than Aβ_1-42 protein. We demonstrated that BSP fragments promote aggregation of Aβ_1-40 protein. Aggregates of Aβ_1-40 protein mediated by BSP fragments also exhibited potent neurotoxicity. Our findings suggest the possibility that BSP fragments affect accumulation of Aβ proteins and are involved in the pathogenesis of AD.

Endoplasmic reticulum stress remodels alveolar bone formation after tooth extraction

Bone healing in tooth extraction sockets occurs in a complex environment containing saliva and many microorganisms and is affected by many factors. Endoplasmic reticulum (ER) stress affects bone metabolism, but the role of ER stress in bone healing after tooth extraction remains unclear. We utilized a rat tooth extraction model, in which we promoted wound healing by using salubrinal to regulate the ER stress response. Western blot analysis showed increased expression of p‐eIF2α/eIF2α, Runx2 and alkaline phosphatase (ALP) in bone tissue, and histological assays showed irregularly arranged and new bone with more collagen fibres 14 days after tooth extraction and after modulating the degree of ER stress. Micro‐CT showed that modulating ER stress to an appropriate degree increases bone filling in regards to the density in the bottom and the surrounding bone wall of the tooth extraction wounds. Transmission electron microscopy showed rough ER expansion and newly formed collagen fibrils in osteoblasts after modulating ER stress to an appropriate degree. We also used different concentrations of salubrinal to evaluate the resistance to tunicamycin‐induced ER stress in an osteogenic induction environment. Salubrinal restored the tunicamycin‐induced decrease in the viability of primary calvarial osteoblasts and increased the expression of Runx2 and ALP, and decreased p‐eIF2α/eIF2α in a dose‐dependent manner. Taken together, the results demonstrate that ER stress occurred after tooth extraction, and regulating the degree of ER stress can promote bone healing in tooth extraction sockets, providing clinical evidence for bone healing.

The integrated stress response in pulmonary disease

The respiratory tract and its resident immune cells face daily exposure to stress, both from without and from within. Inhaled pathogens, including severe acute respiratory syndrome coronavirus 2, and toxins from pollution trigger a cellular defence system that reduces protein synthesis to minimise viral replication or the accumulation of misfolded proteins. Simultaneously, a gene expression programme enhances antioxidant and protein folding machineries in the lung. Four kinases (PERK, PKR, GCN2 and HRI) sense a diverse range of stresses to trigger this “integrated stress response”. Here we review recent advances identifying the integrated stress response as a critical pathway in the pathogenesis of pulmonary diseases, including pneumonias, thoracic malignancy, pulmonary fibrosis and pulmonary hypertension. Understanding the integrated stress response provides novel targets for the development of therapies.

Activating Transcription Factor 6 Mediates Inflammatory Signals in Intestinal Epithelial Cells Upon Endoplasmic Reticulum Stress

BACKGROUND & AIMS

Excess and unresolved endoplasmic reticulum (ER) stress in intestinal epithelial cells (IECs) promotes intestinal inflammation. Activating transcription factor 6 (ATF6) is one of the signaling mediators of ER stress. We studied the pathways that regulate ATF6 and its role for inflammation in IECs.

METHODS

We performed an RNA interference screen, using 23,349 unique small interfering RNAs targeting 7783 genes and a luciferase reporter controlled by an ATF6-dependent ERSE (ER stress-response element) promoter, to identify proteins that activate or inhibit the ATF6 signaling pathway in HEK293 cells. To validate the screening results, intestinal epithelial cell lines (Caco-2 cells) were transfected with small interfering RNAs or with a plasmid overexpressing a constitutively active form of ATF6. Caco-2 cells with a CRISPR-mediated disruption of autophagy related 16 like 1 gene (ATG16L1) were used to study the effect of ATF6 on ER stress in autophagy-deficient cells. We also studied intestinal organoids derived from mice that overexpress constitutively active ATF6, from mice with deletion of the autophagy related 16 like 1 or X-Box binding protein 1 gene in IECs (Atg16l1^ΔIEC or Xbp1^ΔIEC, which both develop spontaneous ileitis), from patients with Crohn's disease (CD) and healthy individuals (controls). Cells and organoids were incubated with tunicamycin to induce ER stress and/or chemical inhibitors of newly identified activator proteins of ATF6 signaling, and analyzed by real-time polymerase chain reaction and immunoblots. Atg16l1^ΔIEC and control (Atg16l1^fl/fl) mice were given intraperitoneal injections of tunicamycin and were treated with chemical inhibitors of ATF6 activating proteins.

RESULTS

We identified and validated 15 suppressors and 7 activators of the ATF6 signaling pathway; activators included the regulatory subunit of casein kinase 2 (CSNK2B) and acyl-CoA synthetase long chain family member 1 (ACSL1). Knockdown or chemical inhibition of CSNK2B and ACSL1 in Caco-2 cells reduced activity of the ATF6-dependent ERSE reporter gene, diminished transcription of the ATF6 target genes HSP90B1 and HSPA5 and reduced NF-κB reporter gene activation on tunicamycin stimulation. Atg16l1^ΔIEC and or Xbp1^ΔIEC organoids showed increased expression of ATF6 and its target genes. Inhibitors of ACSL1 or CSNK2B prevented activation of ATF6 and reduced CXCL1 and tumor necrosis factor (TNF) expression in these organoids on induction of ER stress with tunicamycin. Injection of mice with inhibitors of ACSL1 or CSNK2B significantly reduced tunicamycin-mediated intestinal inflammation and IEC death and expression of CXCL1 and TNF in Atg16l1^ΔIEC mice. Purified ileal IECs from patients with CD had higher levels of ATF6, CSNK2B, and HSPA5 messenger RNAs than controls; early-passage organoids from patients with active CD show increased levels of activated ATF6 protein, incubation of these organoids with inhibitors of ACSL1 or CSNK2B reduced transcription of ATF6 target genes, including TNF.

CONCLUSIONS

Ileal IECs from patients with CD have higher levels of activated ATF6, which is regulated by CSNK2B and HSPA5. ATF6 increases expression of TNF and other inflammatory cytokines in response to ER stress in these cells and in organoids from Atg16l1^ΔIEC and Xbp1^ΔIEC mice. Strategies to inhibit the ATF6 signaling pathway might be developed for treatment of inflammatory bowel diseases.

COPII mitigates ER stress by promoting formation of ER whorls

Cells mitigate ER stress through the unfolded protein response (UPR). Here, we report formation of ER whorls as an effector mechanism of the ER stress response. We found that strong ER stress induces formation of ER whorls, which contain ER-resident proteins such as the Sec61 complex and PKR-like ER kinase (PERK). ER whorl formation is dependent on PERK kinase activity and is mediated by COPII machinery, which facilitates ER membrane budding to form tubular-vesicular ER whorl precursors. ER whorl precursors then go through Sec22b-mediated fusion to form ER whorls. We further show that ER whorls contribute to ER stress-induced translational inhibition by possibly modulating PERK activity and by sequestering translocons in a ribosome-free environment. We propose that formation of ER whorls reflects a new type of ER stress response that controls inhibition of protein translation.

Regulation of cellular immunity by activating transcription factor 4

Activating transcription factor 4 (ATF4) is a DNA binding transcription factor belonging to the family of basic Leucine zipper proteins. ATF4 can be activated in response to multiple cellular stress signals including endoplasmic reticulum stress in the event of improper protein folding or oxidative stress because of mitochondrial dysfunction as well as hypoxia. There are multiple downstream targets of ATF4 that can coordinate the regulation between survival and apoptosis of a cell based on time and exposure to stress. ATF4, therefore, has a broad range of control that results in the modulation of immune cells of the innate and adaptive responses leading to regulation of the cellular immunity. Studies provide evidence that ATF4 can regulate immune cells such as macrophages, T cells, B cells, NK cells and dendritic cells contributing to progression of disease. Immune cells can be exposed to stressed environment in the event of a pathogen attack, infection, inflammation, or in the tumor microenvironment leading to increased ATF4 activity to regulate these responses. ATF4 can further control differentiation and maturation of different immune cell types becoming a determinant of effective immune regulation. Additionally, ATF4 has been heavily implicated in rendering effector immune cells dysfunctional that are used to target tumorigenesis. Therefore, there is a need to evaluate where the literature stands in understanding the overall role of ATF4 in regulating cellular immunity to identify therapeutic targets and generalized mechanisms for different disease progressions.

Reactive oxygen species mediate anlotinib-induced apoptosis via activation of endoplasmic reticulum stress in pancreatic cancer

Anlotinib (AL3818), a novel multi-targeted receptor tyrosine kinase inhibitor, has recently been proven to be an antitumour drug. This study aimed to explore the antitumour effect of anlotinib and its underlying molecular mechanisms in human pancreatic cancer (PC) cells. The anti-proliferative effect of anlotinib for three PC cell lines was validated using CCK-8, colony formation and EdU detection assays. Cell cycle, cell apoptosis, and reactive oxygen species (ROS) detection assays, a PC xenograft model and immunohistochemistry were performed to elucidate the mechanisms by which anlotinib induced tumour lethality in vitro and in vivo. These results demonstrated that anlotinib inhibited proliferation, induced G2/M phase arrest and triggered apoptosis in PC cell lines. Anlotinib induced PC’s apoptosis through the accumulation of ROS which activated the endoplasmic reticulum (ER) stress via PERK/p-eIF2α/ATF4 pathway. Furthermore, we demonstrated that the expression level of Nrf2, an antioxidant protein, increased with anlotinib treatment. Nrf2 knockdown enhanced the pro-apoptotic effect of anlotinib and the expression of the PERK/p-eIF2α/ATF4 pathway. The in vivo results suggested that suppressing Nrf2 improved the antitumour effect of anlotinib on PC cells. These data indicated that the apoptotic effect of anlotinib on PC cells was induced by ER stress via the accumulation of ROS. In the future, anlotinib combined with an Nrf2 inhibitor may provide a new therapeutic strategy for the treatment of human PC.