Cellular responses to endoplasmic reticulum stress and apoptosis

The endoplasmic reticulum (ER) is the cell organelle where secretory and membrane proteins are synthesized and folded. Correctly folded proteins exit the ER and are transported to the Golgi and other destinations within the cell, but proteins that fail to fold properly—misfolded proteins—are retained in the ER and their accumulation may constitute a form of stress to the cell—ER stress. Several signaling pathways, collectively known as unfolded protein response (UPR), have evolved to detect the accumulation of misfolded proteins in the ER and activate a cellular response that attempts to maintain homeostasis and a normal flux of proteins in the ER. In certain severe situations of ER stress, however, the protective mechanisms activated by the UPR are not sufficient to restore normal ER function and cells die by apoptosis. Most research on the UPR used yeast or mammalian model systems and only recently Drosophila has emerged as a system to study the molecular and cellular mechanisms of the UPR. Here, we review recent advances in Drosophila UPR research, in the broad context of mammalian and yeast literature.     

未折叠蛋白响应的激活机制

未折叠蛋白在内质网（endoplasmic reticulum,ER）腔中累积造成ER应激，此时细胞启动未折叠蛋白响应（unfolded protein response,UPR）以恢复蛋白质稳态。目前已知有三种UPR感受器，即IRE1、PERK和ATF6，它们均为ER跨膜蛋白，在ER应激时被激活并启动下游UPR信号通路。虽然UPR感受器最早是在研究细胞如何应对ER应激时发现的，但它们如何感知ER应激至今未得到完满的回答。随着研究的深入，人们发现UPR的功能不仅限于维持蛋白质稳态，而UPR感受器也不是只对未折叠蛋白累积作出响应。本文对UPR的发现及其经典通路作一介绍，着重阐述目前已知的UPR感受器的激活机制，并就UPR和ER应激关系以及该领域存在的问题进行讨论。     

The endoplasmic reticulum-associated degradation is necessary for plant salt tolerance

Eukaryotic organisms have quality-control mechanisms that allow misfolded or unassembled proteins to be retained in the endoplasmic reticulum (ER) and subsequently degraded by ER-associated degradation (ERAD). The ERAD pathway is well studied in yeast and mammals; however, the biological functions of plant ERAD have not been reported. Through molecular and cellular biological approaches, we found that ERAD is necessary for plants to overcome salt stress. Upon salt treatment ubiquitinated proteins increased in plant cells, especially unfolded proteins that quickly accumulated in the ER and subsequently induced ER stress responses. Defect in HRD3A of the HRD1/HRD3 complex of the ERAD pathway resulted in alteration of the unfolded protein response (UPR), increased plant sensitivity to salt, and retention of ERAD substrates in plant cells. Furthermore, we demonstrated that Ca(2+) release from the ER is involved in the elevation of UPR and reactive oxygen species (ROS) participates the ERAD-related plant salt response pathway.     

The impact of the unfolded protein response on human disease

A central function of the endoplasmic reticulum (ER) is to coordinate protein biosynthetic and secretory activities in the cell. Alterations in ER homeostasis cause accumulation of misfolded/unfolded proteins in the ER. To maintain ER homeostasis, eukaryotic cells have evolved the unfolded protein response (UPR), an essential adaptive intracellular signaling pathway that responds to metabolic, oxidative stress, and inflammatory response pathways. The UPR has been implicated in a variety of diseases including metabolic disease, neurodegenerative disease, inflammatory disease, and cancer. Signaling components of the UPR are emerging as potential targets for intervention and treatment of human disease.     

Endoplasmic reticulum stress and lipids in health and diseases

The endoplasmic reticulum (ER) is a complex and dynamic organelle that regulates many cellular pathways, including protein synthesis, protein quality control, and lipid synthesis. When one or multiple ER roles are dysregulated and saturated, the ER enters a stress state, which, in turn, activates the highly conserved unfolded protein response (UPR). By sensing the accumulation of unfolded proteins or lipid bilayer stress (LBS) at the ER, the UPR triggers pathways to restore ER homeostasis and eventually induces apoptosis if the stress remains unresolved. In recent years, it has emerged that the UPR works intimately with other cellular pathways to maintain lipid homeostasis at the ER, and so does at cellular levels. Lipid distribution, along with lipid anabolism and catabolism, are tightly regulated, in part, by the ER. Dysfunctional and overwhelmed lipid-related pathways, independently or in combination with ER stress, can have reciprocal effects on other cellular functions, contributing to the development of diseases. In this review, we summarize the current understanding of the UPR in response to proteotoxic stress and LBS and the breadth of the functions mitigated by the UPR in different tissues and in the context of diseases.     

Endoplasmic reticulum stress response in yeast and humans

Stress pathways monitor intracellular systems and deploy a range of regulatory mechanisms in response to stress. One of the best-characterized pathways, the UPR (unfolded protein response), is an intracellular signal transduction pathway that monitors ER (endoplasmic reticulum) homoeostasis. Its activation is required to alleviate the effects of ER stress and is highly conserved from yeast to human. Although metazoans have three UPR outputs, yeast cells rely exclusively on the Ire1 (inositol-requiring enzyme-1) pathway, which is conserved in all Eukaryotes. In general, the UPR program activates hundreds of genes to alleviate ER stress but it can lead to apoptosis if the system fails to restore homoeostasis. In this review, we summarize the major advances in understanding the response to ER stress in Sc (Saccharomyces cerevisiae), Sp (Schizosaccharomyces pombe) and humans. The contribution of solved protein structures to a better understanding of the UPR pathway is discussed. Finally, we cover the interplay of ER stress in the development of diseases.     

Identification of mitogen-activated protein kinase signaling pathways that confer resistance to endoplasmic reticulum stress in Saccharomyces cerevisiae

Hypoxia activates all components of the unfolded protein response (UPR), a stress response initiated by the accumulation of unfolded proteins within the endoplasmic reticulum (ER). Our group and others have shown previously that the UPR, a hypoxia-inducible factor-independent signaling pathway, mediates cell survival during hypoxia and is required for tumor growth. Identifying new genes and pathways that are important for survival during ER stress may lead to the discovery of new targets in cancer therapy. Using the set of 4,728 homozygous diploid deletion mutants in budding yeast, Saccharomyces cerevisiae, we did a functional screen for genes that conferred resistance to ER stress-inducing agents. Deletion mutants in 56 genes showed increased sensitivity under ER stress conditions. Besides the classic UPR pathway and genes related to calcium homeostasis, we report that two additional pathways, including the SLT2 mitogen-activated protein kinase (MAPK) pathway and the osmosensing MAPK pathway, were also required for survival during ER stress. We further show that the SLT2 MAPK pathway was activated during ER stress, was responsible for increased resistance to ER stress, and functioned independently of the classic IRE1/HAC1 pathway. We propose that the SLT2 MAPK pathway is an important cell survival signaling pathway during ER stress. This study shows the feasibility of using the yeast deletion pool to identify relevant mammalian orthologues of the UPR.     

That which does not kill me makes me stronger: adapting to chronic ER stress

Cells respond to the accumulation of unfolded proteins by activating signal transduction cascades that improve protein folding. One example of such a cascade is the unfolded protein response (UPR), which senses protein folding stress in the endoplasmic reticulum (ER) and leads to improvement in the protein folding and processing capacity of the organelle. A central paradox of the UPR, and indeed of all such stress pathways, is that the response is designed to facilitate both adaptation to stress and apoptosis, depending upon the nature and severity of the stressor. Understanding how the UPR can allow for adaptation, instead of apoptosis, is of tremendous physiological importance. Recent advances have improved our understanding of ER stress and the vertebrate UPR, which suggest possible mechanisms by which cells adapt to chronic stress.     

Activation of mammalian unfolded protein response is compatible with the quality control system operating in the endoplasmic reticulum

Newly synthesized secretory and transmembrane proteins are folded and assembled in the endoplasmic reticulum (ER) where an efficient quality control system operates so that only correctly folded molecules are allowed to move along the secretory pathway. The productive folding process in the ER has been thought to be supported by the unfolded protein response (UPR), which is activated by the accumulation of unfolded proteins in the ER. However, a dilemma has emerged; activation of ATF6, a key regulator of mammalian UPR, requires intracellular transport from the ER to the Golgi apparatus. This suggests that unfolded proteins might be leaked from the ER together with ATF6 in response to ER stress, exhibiting proteotoxicity in the secretory pathway. We show here that ATF6 and correctly folded proteins are transported to the Golgi apparatus via the same route and by the same mechanism under conditions of ER stress, whereas unfolded proteins are retained in the ER. Thus, activation of the UPR is compatible with the quality control in the ER and the ER possesses a remarkable ability to select proteins to be transported in mammalian cells in marked contrast to yeast cells, which actively utilize intracellular traffic to deal with unfolded proteins accumulated in the ER.     

A trip to the ER: coping with stress

The accumulation of unfolded proteins in the lumen of the endoplasmic reticulum (ER) induces a coordinated adaptive program called the unfolded protein response (UPR). The UPR alleviates stress by upregulating protein folding and degradation pathways in the ER and inhibiting protein synthesis. With a basic conceptual framework for the UPR, including the identification of key mediators of the response, now in place, recent work has turned towards investigating how the response is regulated and how its effects radiate beyond the immediate realm of protein secretion. This review highlights advances in these areas and attempts to forecast important issues that must be addressed soon.     

Interplay of endoplasmic reticulum stress and autophagy in neurodegenerative disorders

The common underlying feature of most neurodegenerative diseases such as Alzheimer disease (AD), prion diseases, Parkinson disease (PD), and amyotrophic lateral sclerosis (ALS) involves accumulation of misfolded proteins leading to initiation of endoplasmic reticulum (ER) stress and stimulation of the unfolded protein response (UPR). Additionally, ER stress more recently has been implicated in the pathogenesis of HIV-associated neurocognitive disorders (HAND). Autophagy plays an essential role in the clearance of aggregated toxic proteins and degradation of the damaged organelles. There is evidence that autophagy ameliorates ER stress by eliminating accumulated misfolded proteins. Both abnormal UPR and impaired autophagy have been implicated as a causative mechanism in the development of various neurodegenerative diseases. This review highlights recent advances in the field on the role of ER stress and autophagy in AD, prion diseases, PD, ALS and HAND with the involvement of key signaling pathways in these processes and implications for future development of therapeutic strategies.     

Light enhances the unfolded protein response as measured by BiP2 gene expression and the secretory GFP-2SC marker in Arabidopsis

Disruption of the protein-folding capacity in the ER induces the accumulation of unfolded proteins and ER stress, which activate the unfolded protein response (UPR). Although UPR has been extensively studied in yeast and mammals, much less is known about UPR and its relationship with light in plants. Here, we examined the effects of chemically induced UPR and light on a molecular marker of UPR (binding protein, BiP2, gene expression) and a secretory green fluorescent protein marker (GFP-2SC) that is trafficked from the ER to vacuole in Arabidopsis thaliana (L). UPR, which was induced by DTT and tunicamycin (TM), increased Bip2 mRNA levels and decreased the levels of microsomal and vacuolar forms of GFP-2SC. Treatment with protease inhibitors lessened the effects of DTT and TM on GFP-2SC, indicating the decrease in GFP levels partially involved protein degradation. Light treatments synergistically enhanced the decrease in GFP levels in both the ER and vacuole and induced the expression of UPR marker genes for BiP2 and protein disulfide isomerase (PDI, EC 5.3.4.1). DTT and TM treatments required light for maximal induction of the UPR. Light-induced UPR occurred during the daily dark to light cycle and when dark-adapted plants were exposed to light. We propose that light activates the UPR to increase the protein folding capacity in the ER to accommodate an increase in translation during dark to light transitions.     

Zinc and the Msc2 zinc transporter protein are required for endoplasmic reticulum function

In this report, we show that zinc is required for endoplasmic reticulum function in Saccharomyces cerevisiae. Zinc deficiency in this yeast induces the unfolded protein response (UPR), a system normally activated by unfolded ER proteins. Msc2, a member of the cation diffusion facilitator (CDF) family of metal ion transporters, was previously implicated in zinc homeostasis. Our results indicate that Msc2 is one route of zinc entry into the ER. Msc2 localizes to the ER when expressed at normal levels. UPR induction in low zinc is exacerbated in an msc2 mutant. Genetic and biochemical evidence indicates that this UPR induction is due to genuine ER dysfunction. Notably, we found that ER-associated protein degradation is defective in zinc-limited msc2 mutants. We also show that the vacuolar CDF proteins Zrc1 and Cot1 are other pathways of ER zinc acquisition. Finally, zinc deficiency up-regulates the mammalian ER stress response indicating a conserved requirement for zinc in ER function among eukaryotes.     

Role of the unfolded protein response in determining the fate of tumor cells and the promise of multi-targeted therapies

Although there have been advances in our understanding of carcinogenesis and development of new treatments, cancer remains a common cause of death. Many regulatory pathways are incompletely understood in cancer development and progression, with a prime example being those related to the endoplasmic reticulum (ER). The pathological sequelae that arise from disruption of ER homeostasis are not well defined. The ER is an organelle that is responsible for secretory protein biosynthesis and the quality control of protein folding. The ER triggers an unfolded protein response (UPR) when misfolded proteins accumulate, and while the UPR acts to restore protein folding and ER homeostasis, this response can work as a switch to determine the death or survival of cells. The treatment of cancer with agents that target the UPR has shown promising outcomes. The UPR has wide crosstalk with other signaling pathways. Multi-targeted cancer therapies which target the intersections within signaling networks have shown synergistic tumoricidal effects. In the present review, the basic cellular and signaling pathways of the ER and UPR are introduced; then the crosstalk between the ER and other signaling pathways is summarized; and ultimately, the evidence that the UPR is a potential target for cancer therapy is discussed. Regulation of the UPR downstream signaling is a common therapeutic target for different tumor types. Tumoricidal effects achieved from modulating the UPR downstream signaling could be enhanced by phosphodiesterase 5 (PDE5) inhibitors. Largely untapped by Western medicine for cancer therapies are Chinese herbal medicines. This review explores and discusses the value of some Chinese herbal extracts as PDE5 inhibitors.     

疱疹病毒与内质网应激

内质网是分泌型蛋白和膜蛋白折叠及翻译后修饰的主要场所.病毒感染所引起的宿主细胞内环境的改变可使细胞或病毒的未折叠和/或错误折叠蛋白在内质网中大量聚集,使内质网处于生理功能紊乱的应激状态.为了缓解这种应激压力,细胞会启动未折叠蛋白反应(UPR),并通过一系列分子的信号转导维持内质网稳态;同时病毒也会通过对UPR的精密调控...     

Cross‐compartment proteostasis regulation during redox imbalance induced ER stress

Imbalance in protein homeostasis in specific subcellular organelles is alleviated through organelle‐specific stress response pathways. As a canonical example of stress activated pathway, accumulation of misfolded proteins in ER activates unfolded protein response (UPR) in almost all eukaryotic organisms. However, very little is known about the involvement of proteins of other organelles that help to maintain the cellular protein homeostasis during ER stress. In this study, using iTRAQ‐based LC–MS approach, we identified organelle enriched proteins that are differentially expressed in yeast (Saccharomyces cerevisiae) during ER stress in the absence of UPR sensor Ire1p. We have identified about 750 proteins from enriched organelle fraction in three independent iTRAQ experiments. Induction of ER stress resulted in the differential expression of 93 proteins in WT strains, 40 of which were found to be dependent on IRE1. Our study reveals a cross‐talk between ER‐ and mitochondrial proteostasis exemplified by an Ire1p‐dependent induction of Hsp60p, a mitochondrial chaperone. Thus, in this study, we show changes in protein levels in various organelles in response to ER stress. A large fraction of these changes were dependent on canonical UPR signalling through Ire1, highlighting the importance of interorganellar cross‐talk during stress.