Endoplasmic Reticulum Stress Interacts With Inflammation in Human Diseases

The endoplasmic reticulum (ER) is a critical organelle for normal cell function and homeostasis. Disturbance in the protein folding process in the ER, termed ER stress, leads to the activation of unfolded protein response (UPR) that encompasses a complex network of intracellular signaling pathways. The UPR can either restore ER homeostasis or activate pro‐apoptotic pathways depending on the type of insults, intensity and duration of the stress, and cell types. ER stress and the UPR have recently been linked to inflammation in a variety of human pathologies including autoimmune, infectious, neurodegenerative, and metabolic disorders. In the cell, ER stress and inflammatory signaling share extensive regulators and effectors in a broad spectrum of biological processes. In spite of different etiologies, the two signaling pathways have been shown to form a vicious cycle in exacerbating cellular dysfunction and causing apoptosis in many cells and tissues. However, the interaction between ER stress and inflammation in many of these diseases remains poorly understood. Further understanding of the biochemistry, cell biology, and physiology may enable the development of novel therapies that spontaneously target these pathogenic pathways. J. Cell. Physiol. 231: 288–294, 2016. © 2015 Wiley Periodicals, Inc.

     

细胞内质网应激与糖脂代谢紊乱的机制关联

在真核细胞中,内质网是蛋白质合成、折叠、加工及其质量监控的重要场所。当内质网难以承担蛋白折叠的高负荷时则引发内质网应激(ER stress),激活细胞的未折叠蛋白响应(unfoldedprotein response,UPR)。细胞通过内质网跨膜蛋白ATF6、PERK和IRE1介导的三条极为关键的UPR信号通路,调控下游相关基因的表达,以增强内质网对蛋白折叠的处理能力。因此,UPR通路在细胞的稳态平衡中具有举足轻重的作用,而这一动态过程的调控对于维持机体的正常生理功能至关重要。近来大量研究表明,在哺乳动物中内质网应激与机体的营养感应和糖脂代谢的调控过程密切相关。在肝脏、脂肪、胰岛以及下丘脑等不同的组织器官中,内质网应激均影响代谢通路的调节机制,因此在糖脂代谢紊乱的发生发展中扮演重要的角色。综上所述,进一步深入了解内质网应激引发代谢异常的生理学机制,可以为肥胖、脂肪肝及2型糖尿病等相关代谢性疾病的防治提供新的潜在药物靶点和重要的理论线索。     

ER stress-induced cell death mechanisms

The endoplasmic-reticulum (ER) stress response constitutes a cellular process that is triggered by a variety of conditions that disturb folding of proteins in the ER. Eukaryotic cells have developed an evolutionarily conserved adaptive mechanism, the unfolded protein response (UPR), which aims to clear unfolded proteins and restore ER homeostasis. In cases where ER stress cannot be reversed, cellular functions deteriorate, often leading to cell death. Accumulating evidence implicates ER stress-induced cellular dysfunction and cell death as major contributors to many diseases, making modulators of ER stress pathways potentially attractive targets for therapeutics discovery. Here, we summarize recent advances in understanding the diversity of molecular mechanisms that govern ER stress signaling in health and disease. This article is part of a Special Section entitled: Cell Death Pathways. Guest Editors: Frank Madeo and Slaven Stekovic.     

ER stress and the unfolded protein response

Conformational diseases are caused by mutations altering the folding pathway or final conformation of a protein. Many conformational diseases are caused by mutations in secretory proteins and reach from metabolic diseases, e.g. diabetes, to developmental and neurological diseases, e.g. Alzheimer's disease. Expression of mutant proteins disrupts protein folding in the endoplasmic reticulum (ER), causes ER stress, and activates a signaling network called the unfolded protein response (UPR). The UPR increases the biosynthetic capacity of the secretory pathway through upregulation of ER chaperone and foldase expression. In addition, the UPR decreases the biosynthetic burden of the secretory pathway by downregulating expression of genes encoding secreted proteins. Here we review our current understanding of how an unfolded protein signal is generated, sensed, transmitted across the ER membrane, and how downstream events in this stress response are regulated. We propose a model in which the activity of UPR signaling pathways reflects the biosynthetic activity of the ER. We summarize data that shows that this information is integrated into control of cellular events, which were previously not considered to be under control of ER signaling pathways, e.g. execution of differentiation and starvation programs.     

内质网应激响应在脂肪组织中的代谢调节作用

在真核细胞中,内质网是蛋白合成、加工及质量监控的关键细胞器,也是Ca2+储存及脂质合成的重要场所.细胞通过未折叠蛋白响应(unfolded protein response,UPR)感应外界不同刺激引发的内质网应激,在维持细胞功能稳态中发挥至关重要的作用.在哺乳动物中,三个位于内质网的跨膜蛋白——肌醇依赖酶la(ino...     

The aging stress response

Aging is the outcome of a balance between damage and repair. The rate of aging and the appearance of age-related pathology are modulated by stress response and repair pathways that gradually decline, including the proteostasis and DNA damage repair networks and mitochondrial respiratory metabolism. Highly conserved insulin/IGF-1, TOR, and sirtuin signaling pathways in turn control these critical cellular responses. The coordinated action of these signaling pathways maintains cellular and organismal homeostasis in the face of external perturbations, such as changes in nutrient availability, temperature, and oxygen level, as well as internal perturbations, such as protein misfolding and DNA damage. Studies in model organisms suggest that changes in signaling can augment these critical stress response systems, increasing life span and reducing age-related pathology. The systems biology of stress response signaling thus provides a new approach to the understanding and potential treatment of age-related diseases.     

ER stress and its functional link to mitochondria: role in cell survival and death

The endoplasmic reticulum (ER) is the primary site for synthesis and folding of secreted and membrane-bound proteins. Proteins are translocated into ER lumen in an unfolded state and require protein chaperones and catalysts of protein folding to assist in proper folding. Properly folded proteins traffic from the ER to the Golgi apparatus; misfolded proteins are targeted to degradation. Unfolded protein response (UPR) is a highly regulated intracellular signaling pathway that prevents accumulation of misfolded proteins in the ER lumen. UPR provides an adaptive mechanism by which cells can augment protein folding and processing capacities of the ER. If protein misfolding is not resolved, the UPR triggers apoptotic cascades. Although the molecular mechanisms underlying ER stress-induced apoptosis are not completely understood, increasing evidence suggests that ER and mitochondria cooperate to signal cell death. Mitochondria and ER form structural and functional networks (mitochondria-associated ER membranes [MAMs]) essential to maintain cellular homeostasis and determine cell fate under various pathophysiological conditions. Regulated Ca(2+) transfer from the ER to the mitochondria is important in maintaining control of prosurvival/prodeath pathways. We discuss the signaling/communication between the ER and mitochondria and focus on the role of the mitochondrial permeability transition pore in these complex processes.     

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

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

Unfolded Protein Response Signaling and Metabolic Diseases

The endoplasmic reticulum (ER) is a central organelle for protein biosynthesis, folding, and traffic. Perturbations in ER homeostasis create a condition termed ER stress and lead to activation of the complex signaling cascade called the unfolded protein response (UPR). Recent studies have documented that the UPR coordinates multiple signaling pathways and controls various physiologies in cells and the whole organism. Furthermore, unresolved ER stress has been implicated in a variety of metabolic disorders, such as obesity and type 2 diabetes. Therefore, intervening in ER stress and modulating signaling components of the UPR would provide promising therapeutics for the treatment of human metabolic diseases.     

Regulation of ER stress-induced macroautophagy by protein kinase C

The endoplasmic reticulum (ER) is the primary site for folding and quality control for proteins destined to the cell surface and intracellular organelles. A variety of cellular insults alter ER homeostasis to disrupt protein folding, cause the accumulation of misfolded protein and activate an autophagic response. However, the molecular signaling pathways required for ER stress-induced autophagy are largely unknown. Recently, we discovered that a novel-type protein kinase C family member (PKCθ) is required for ER stress-induced autophagy. We shown that ER stress, in a Ca²⁺-dependent manner, induces PKCθ phosphorylation within the activation loop and localization with LC3-II in punctate cytoplasmic structures. Pharmacological inhibition, siRNA-mediated knockdown, or transdominant-negative mutant expression of PKCθ block the ER stress-induced autophagic response. PKCθ activation is not required for autophagy induced by amino acid starvation, and PKCθ activation in response to ER stress does not require either the mTOR kinase or the unfolded protein response signaling pathways. Herein, we review and discuss the significance of these findings with respect to regulation of autophagy in response to ER stress.

Addendum to: Sakaki K, Wu J, Kaufman RJ. Protein kinase C-θ is required for autophagy in response to stress in the endoplasmic reticulum. J Biol Chem 2008; 283:15370-80.     

IRE1 directs proteasomal and lysosomal degradation of misfolded rhodopsin

Endoplasmic reticulum (ER) is responsible for folding of secreted and membrane proteins in eukaryotic cells. Disruption of ER protein folding leads to ER stress. Chronic ER stress can cause cell death and is proposed to underlie the pathogenesis of many human diseases. Inositol-requiring enzyme 1 (IRE1) directs a key unfolded protein response signaling pathway that controls the fidelity of ER protein folding. IRE1 signaling may be particularly helpful in preventing chronic ER stress and cell injury by alleviating protein misfolding in the ER. To examine this, we used a chemical-genetic approach to selectively activate IRE1 in mammalian cells and tested how artificial IRE1 signaling affected the fate of misfolded P23H rhodopsin linked to photoreceptor cell death. We found that IRE1 signaling robustly promoted the degradation of misfolded P23H rhodopsin without affecting its wild-type counterpart. We also found that IRE1 used both proteasomal and lysosomal degradation pathways to remove P23H rhodopsin. Surprisingly, when one degradation pathway was compromised, IRE1 signaling could still promote misfolded rhodopsin degradation using the remaining pathway. Last, we showed that IRE1 signaling also reduced levels of several other misfolded rhodopsins with lesser effects on misfolded cystic fibrosis transmembrane conductance regulator. Our findings reveal the diversity of proteolytic mechanisms used by IRE1 to eliminate misfolded rhodopsin.     

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.     

Coupling endoplasmic reticulum stress to the cell death program

The endoplasmic reticulum (ER) regulates protein synthesis, protein folding and trafficking, cellular responses to stress and intracellular calcium (Ca(2+)) levels. Alterations in Ca(2+) homeostasis and accumulation of misfolded proteins in the ER cause ER stress that ultimately leads to apoptosis. Prolonged ER stress is linked to the pathogenesis of several different neurodegenerative disorders. Apoptosis is a form of cell death that involves the concerted action of a number of intracellular signaling pathways including members of the caspase family of cysteine proteases. The two main apoptotic pathways, the death receptor ('extrinsic') and mitochondrial ('intrinsic') pathways, are activated by caspase-8 and -9, respectively, both of which are found in the cytoplasm. Recent studies point to the ER as a third subcellular compartment implicated in apoptotic execution. Here, we review evidence for the contribution of various cellular molecules that contribute to ER stress and subsequent cellular death. It is hoped that dissection of the molecular components and pathways that alter ER structure and function and ultimately promote cellular death will provide a framework for understanding degenerative disorders that feature misfolded proteins.     

HCV causes chronic endoplasmic reticulum stress leading to adaptation and interference with the unfolded protein response

Background

The endoplasmic reticulum (ER) is the cellular site for protein folding. ER stress occurs when protein folding capacity is exceeded. This stress induces a cyto-protective signaling cascades termed the unfolded protein response (UPR) aimed at restoring homeostasis. While acute ER stress is lethal, chronic sub-lethal ER stress causes cells to adapt by attenuation of UPR activation. Hepatitis C virus (HCV), a major human pathogen, was shown to cause ER stress, however it is unclear whether HCV induces chronic ER stress, and if so whether adaptation mechanisms are initiated. We wanted to characterize the kinetics of HCV-induced ER stress during infection and assess adaptation mechanisms and their significance.

Methods and Findings

The HuH7.5.1 cellular system and HCV-transgenic (HCV-Tg) mice were used to characterize HCV-induced ER stress/UPR pathway activation and adaptation. HCV induced a wave of acute ER stress peaking 2–5 days post-infection, which rapidly subsided thereafter. UPR pathways were activated including IRE1 and EIF2α phosphorylation, ATF6 cleavage and XBP-1 splicing. Downstream target genes including GADD34, ERdj4, p58ipk, ATF3 and ATF4 were upregulated. CHOP, a UPR regulated protein was activated and translocated to the nucleus. Remarkably, UPR activity did not return to baseline but remained elevated for up to 14 days post infection suggesting that chronic ER stress is induced. At this time, cells adapted to ER stress and were less responsive to further drug-induced ER stress. Similar results were obtained in HCV-Tg mice. Suppression of HCV by Interferon-α 2a treatment, restored UPR responsiveness to ER stress tolerant cells.

Conclusions

Our study shows, for the first time, that HCV induces adaptation to chronic ER stress which was reversed upon viral suppression. These finding represent a novel viral mechanism to manipulate cellular response pathways.     

Calcium signaling at the endoplasmic reticulum: fine-tuning stress responses

Endoplasmic reticulum (ER) calcium signaling is implicated in a myriad of coordinated cellular processes. The ER calcium content is tightly regulated as it allows a favorable environment for protein folding, in addition to operate as a major reservoir for fast and specific release of calcium. Altered ER homeostasis impacts protein folding, activating the unfolded protein response (UPR) as a rescue mechanism to restore proteostasis. ER calcium release impacts mitochondrial metabolism and also fine-tunes the threshold to undergo apoptosis under chronic stress. The global coordination between UPR signaling and energetic demands takes place at mitochondrial associated membranes (MAMs), specialized subdomains mediating interorganelle communication. Here we discuss current models explaining the functional relationship between ER homeostasis and various cellular responses to coordinate proteostasis and metabolic maintenance.