Respiratory epithelial cell responses to cigarette smoke: The unfolded protein response

Cigarette smoking exposes the respiratory epithelium to highly toxic, reactive oxygen nitrogen species which damage lung proteins in the endoplasmic reticulum (ER), the cell organelle in which all secreted and membrane proteins are processed. Accumulation of damaged or misfolded proteins in the ER, a condition termed ER stress, activates a complex cellular process termed the unfolded protein responses (UPR). The UPR acts to restore cellular protein homeostasis by regulating all aspects of protein metabolism including: protein translation and syntheses; protein folding; and protein degradation. However, activation of the UPR may also induce signaling pathways which induce inflammation and cell apoptosis. This review discusses the role of UPR in the respiratory epithelial cell response to cigarette smoke and the pathogenesis of lung diseases like COPD.     

Evidence of RedOX Imbalance during Zika Virus Infection Promoting the Formation of Disulfide-Bond-Dependent Oligomers of the Envelope Protein

Flaviviruses replicate in membrane factories associated with the endoplasmic reticulum (ER). Significant levels of flavivirus viral protein accumulation contribute to ER stress. As a consequence, the host cell exhibits an Unfolded Protein Response (UPR), subsequently stimulating appropriate cellular responses such as adaptation, autophagy or apoptosis. The correct redox conditions of this compartment are essential to forming native disulfide bonds in proteins. Zika virus (ZIKV) has the ability to induce persistent ER stress leading to the activation of UPR pathways. In this study, we wondered whether ZIKV affects the redox balance and consequently the oxidative protein folding in the ER. We found that ZIKV replication influences the redox state, leading to the aggregation of the viral envelope protein as amyloid-like structures in the infected cells.     

Endoplasmic reticulum stress as a therapeutic target in cardiovascular disease

Cardiovascular disease constitutes a major and increasing health burden in developed countries. Although treatments have progressed, the development of novel treatments for patients with cardiovascular diseases remains a major research goal. The endoplasmic reticulum (ER) is the cellular organelle in which protein folding, calcium homeostasis, and lipid biosynthesis occur. Stimuli such as oxidative stress, ischemic insult, disturbances in calcium homeostasis, and enhanced expression of normal and/or folding-defective proteins lead to the accumulation of unfolded proteins, a condition referred to as ER stress. ER stress triggers the unfolded protein response (UPR) to maintain ER homeostasis. The UPR involves a group of signal transduction pathways that ameliorate the accumulation of unfolded protein by increasing ER-resident chaperones, inhibiting protein translation and accelerating the degradation of unfolded proteins. The UPR is initially an adaptive response but, if unresolved, can lead to apoptotic cell death. Thus, the ER is now recognized as an important organelle in deciding cell life and death. There is compelling evidence that the adaptive and proapoptotic pathways of UPR play fundamental roles in the development and progression of cardiovascular diseases, including heart failure, ischemic heart diseases, and atherosclerosis. Thus, therapeutic interventions that target molecules of the UPR component and reduce ER stress will be promising strategies to treat cardiovascular diseases. In this review, we summarize the recent progress in understanding UPR signaling in cardiovascular disease and its related therapeutic potential. Future studies may clarify the most promising molecules to be investigated as targets for cardiovascular diseases.     

The role of endoplasmic reticulum stress in the progression of atherosclerosis

Prolonged activation of the endoplasmic reticulum (ER) stress pathway known as the unfolded protein response (UPR) can lead to cell pathology and subsequent tissue dysfunction. There is now ample evidence that the UPR is chronically activated in atherosclerotic lesional cells, particularly advanced lesional macrophages and endothelial cells. The stressors in advanced lesions that can lead to prolonged activation of the UPR include oxidative stress, oxysterols, and high levels of intracellular cholesterol and saturated fatty acids. Importantly, these arterial wall stressors may be especially prominent in the settings of obesity, insulin resistance, and diabetes, all of which promote the clinical progression of atherosclerosis. In the case of macrophages, prolonged ER stress triggers apoptosis, which in turn leads to plaque necrosis if the apoptotic cells are not rapidly cleared. ER stress-induced endothelial cell apoptosis may also contribute to plaque progression. Another potentially important proatherogenic effect of prolonged ER stress is activation of inflammatory pathways in macrophages and, perhaps in response to atheroprone shear stress, endothelial cells. Although exciting work over the last decade has begun to shed light on the mechanisms and in vivo relevance of ER stress-driven atherosclerosis, much more work is needed to fully understand this area and to enable an informed approach to therapeutic translation.     

Alcohol disrupts endoplasmic reticulum function and protein secretion in hepatocytes

Background: Many alcoholic patients have serum protein deficiency that contributes to their systemic problems. The unfolded protein response (UPR) is induced in response to disequilibrium in the protein folding capability of the endoplasmic reticulum (ER) and is implicated in hepatocyte lipid accumulation and apoptosis, which are associated with alcoholic liver disease (ALD). We investigated whether alcohol affects ER structure, function, and UPR activation in hepatocytes in vitro and in vivo. Methods: HepG2 cells expressing human cytochrome P450 2E1 and mouse alcohol dehydrogenase (VL‐17A) were treated for up to 48 hours with 50 and 100 mM ethanol. Zebrafish larvae at 4 days postfertilization were exposed to 350 mM ethanol for 32 hours. ER morphology was visualized by fluorescence in cells and transmission electron microscopy in zebrafish. UPR target gene activation was assessed using quantitative PCR, in situ hybridization, and Western blotting. Mobility of the major ER chaperone, BIP, was monitored in cells by fluorescence recovery after photobleaching (FRAP). Results: VL‐17A cells metabolized alcohol yet only had slight activation of some UPR target genes following ethanol treatment. However, ER fragmentation, crowding, and accumulation of unfolded proteins as detected by immunofluorescence and FRAP demonstrate that alcohol induced some ER dysfunction despite the lack of UPR activation. Zebrafish treated with alcohol, however, showed modest ER dilation, and several UPR targets were significantly induced. Conclusions: Ethanol metabolism directly impairs ER structure and function in hepatocytes. Zebrafish are a novel in vivo system for studying ALD.     

Mammalian endoplasmic reticulum stress sensor IRE1 signals by dynamic clustering

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

Mitochondria at the interface between danger signaling and metabolism: role of unfolded protein responses in chronic inflammation

Inflammatory bowel diseases (IBDs), like many other chronic diseases, feature multiple cellular stress responses including endoplasmic reticulum (ER) unfolded protein response (UPR). Maintaining protein homeostasis is indispensable for cell survival and, consequently, distinct signaling pathways have evolved to transmit organelle stress. While the ER UPR, aiming to restore ER homeostasis after challenges to ER function, has been extensively studied in the context of chronic diseases, only recently the related mitochondrial UPR (mtUPR), induced by disturbances of mitochondrial proteostasis, has drawn some attention. ER and mitochondria are in close contact and interact physically and functionally. Accumulating data have placed mitochondria at the center of diverse cellular functions and suggest mitochondria as integrators of signaling pathways such as autophagy and inflammation. Consequently, it is likely that mitochondrial stress and ER stress cannot be regarded separately and that mitochondrial stress, as well as ER stress, participates in the pathology of IBD. Protein homeostasis is particularly sensitive toward infections, oxidative stress, and energy deficiency. Thus, environmental disturbances impacting organelle function lead to the concerted activation of distinct UPRs. The metabolic status might therefore serve as an innate mechanism to sense the epithelial environment, including luminal-derived and host-derived factors. This review highlights mtUPR and its interrelation with ER UPR, focuses on recent studies identifying mitochondria as integrators of cellular danger signaling, and, furthermore, illustrates the importance ER UPR and mitochondrial dysfunction in IBD.     

未折叠蛋白反应与2型糖尿病胰岛β细胞凋亡

内质网(ER)是细胞内蛋白质合成、折叠的重要场所,对应激极为敏感.多种因素均可导致ER功能发生改变,统称为内质网应激(ERS).ERS可以启动细胞内一系列适应性反应,即未折叠蛋白反应(UPR),以恢复细胞内环境的稳态.但持久和(或)剧烈的ERS将启动细胞凋亡程序.胰岛β细胞具有高度发达的ER,使其对2型糖尿病长期高血糖和游离脂肪酸所致的应激更为敏感.大量的研究表明,作为ERS组成部分之一的UPR在β细胞凋亡中起重要作用.现综述UPR导致2型糖尿病胰岛β细胞凋亡的具体机制.     

内质网应激与心血管疾病的研究进展

内质网(ER)是真核细胞最主要的膜性结构,是细胞内重要生理过程发生的关键细胞器。在多种内外因素的作用下,ER的稳态受到破坏,导致蛋白质加工运输受阻,未折叠蛋白或错误折叠蛋白在ER腔内聚集,形成内质网应激(ERS),并触发未折叠蛋白反应(UPR)。适度的ERS通过UPR信号通路减少蛋白质合成、促进蛋白质降解、增加协助蛋白质折叠的分子伴侣,最终缓解ER压力。但是,如果ERS过强或持续时间过长,超过细胞的自身调节能力时,UPR可启动细胞凋亡,亦可导致疾病。大量研究表明,ERS与多种心血管疾病(CVD)的发生发展密切相关。该综述主要阐述UPR在几种常见CVD中的研究进展和靶向UPR作为CVD的潜在治疗方法。     

Biology of endoplasmic reticulum stress in the heart

The endoplasmic reticulum (ER) is a multifunctional intracellular organelle supporting many processes required by virtually every mammalian cell, including cardiomyocytes. It performs diverse functions, including protein synthesis, translocation across the membrane, integration into the membrane, folding, posttranslational modification including N-linked glycosylation, and synthesis of phospholipids and steroids on the cytoplasmic side of the ER membrane, and regulation of Ca(2+) homeostasis. Perturbation of ER-associated functions results in ER stress via the activation of complex cytoplasmic and nuclear signaling pathways, collectively termed the unfolded protein response (UPR) (also known as misfolded protein response), leading to upregulation of expression of ER resident chaperones, inhibition of protein synthesis and activation of protein degradation. The UPR has been associated with numerous human pathologies, and it may play an important role in the pathophysiology of the heart. ER stress responses, ER Ca(2+) buffering, and protein and lipid turnover impact many cardiac functions, including energy metabolism, cardiogenesis, ischemic/reperfusion, cardiomyopathies, and heart failure. ER proteins and ER stress-associated pathways may play a role in the development of novel UPR-targeted therapies for cardiovascular diseases.     

Melatonin and endoplasmic reticulum stress: relation to autophagy and apoptosis

Endoplasmic reticulum (ER) is a dynamic organelle that participates in a number of cellular functions by controlling lipid metabolism, calcium stores, and proteostasis. Under stressful situations, the ER environment is compromised, and protein maturation is impaired; this causes misfolded proteins to accumulate and a characteristic stress response named unfolded protein response (UPR). UPR protects cells from stress and contributes to cellular homeostasis re‐establishment; however, during prolonged ER stress, UPR activation promotes cell death. ER stressors can modulate autophagy which in turn, depending of the situation, induces cell survival or death. Interactions of different autophagy‐ and apoptosis‐related proteins and also common signaling pathways have been found, suggesting an interplay between these cellular processes, although their dynamic features are still unknown. A number of pathologies including metabolic, neurodegenerative and cardiovascular diseases, cancer, inflammation, and viral infections are associated with ER stress, leading to a growing interest in targeting components of the UPR as a therapeutic strategy. Melatonin has a variety of antioxidant, anti‐inflammatory, and antitumor effects. As such, it modulates apoptosis and autophagy in cancer cells, neurodegeneration and the development of liver diseases as well as other pathologies. Here, we review the effects of melatonin on the main ER stress mechanisms, focusing on its ability to regulate the autophagic and apoptotic processes. As the number of studies that have analyzed ER stress modulation by this indole remains limited, further research is necessary for a better understanding of the crosstalk between ER stress, autophagy, and apoptosis and to clearly delineate the mechanisms by which melatonin modulates these responses.