XBP-1 is required for biogenesis of cellular secretory machinery of exocrine glands

The role of the unfolded protein response in diabetes mellitus

The endoplasmic reticulum (ER) plays a key role in the synthesis and modification of secretory and membrane proteins in all eukaryotic cells. Under normal conditions, these proteins are correctly folded and assembled in the ER. However, when cells are exposed to environmental factors such as overproduction of ER proteins, viral infections, or glucose deprivation, the secretory and membrane proteins can accumulate in unfolded or misfolded forms in the lumen of the ER, and consequently, cause stress in the ER. To maintain cellular homeostasis, cells induce several responses to ER stress. In mammalian cells, ER stress responses are induced by a diversity of signal pathways. There are three ER-located transmembrane proteins that play important roles in mammalian ER stress responses: activating transcription factor 6, inositol-requiring protein 1, and protein kinase RNA-like endoplasmic reticulum kinase. ER stress is linked to various diseases, including diabetes. This review highlights the particular importance of ER stress-responsive molecules in insulin biosynthesis, glyconeogenesis, insulin resistance, glucose intolerance, and pancreatic β-cell apoptosis. An understanding of the pathogenic mechanism of diabetes from the aspect of ER stress is crucial in formulating therapeutic strategies.

XBP1 promoter polymorphism modulates platinum-based chemotherapy gastrointestinal toxicity for advanced non-small cell lung cancer patients

BACKGROUND

The X-box binding protein 1 (XBP1) is a critical transcription factor in the endoplasmic reticulum stress response, which is essential for the maintenance of cellular homeostasis. Here, we investigated whether the regulatory variant rs2269577 of the XBP1 gene influences clinical outcome in advanced non-small cell lung cancer (NSCLC) patients undergoing platinum-based chemotherapy.

PATIENTS AND METHODS

We recruited 663 Chinese patients with advanced NSCLC treated with platinum-based regimens and assessed the association between rs2269577 and clinical outcome. Subsequent functional analyses, including real-time quantitative PCR and dual-luciferase assays, were performed to explore possible molecular mechanisms.

RESULTS

The G/G genotype of rs2269577 was significantly associated with severe gastrointestinal toxicity compared with the homozygous C/C genotype (P=0.012, odds ratio=2.755), particularly in the female, performance status 0-1, and adenocarcinoma subgroups. No significant relevance was found between rs2269577 and treatment efficacy. In gastric epithelial cells, in vitro molecular analyses demonstrated that XBP1 mRNA expression levels decreased after treatment with cisplatin and the G allele of rs2269577 weakened the transcriptional activity of the XBP1 promoter.

CONCLUSION

This is the first study to evaluate the effect of XBP1 polymorphism on severe chemotherapy-related adverse outcomes in platinum-treated advanced NSCLC patients using both pharmacogenomics and functional molecular analyses.

The ubiquitin ligase Mindbomb 1 coordinates gastrointestinal secretory cell maturation

After cell fate specification, differentiating cells must amplify the specific subcellular features required for their specialized function. How cells regulate such subcellular scaling is a fundamental unanswered question. Here, we show that the E3 ubiquitin ligase Mindbomb 1 (MIB1) is required for the apical secretory apparatus established by gastric zymogenic cells as they differentiate from their progenitors. When Mib1 was deleted, death-associated protein kinase-1 (DAPK1) was rerouted to the cell base, microtubule-associated protein 1B (MAP1B) was dephosphorylated, and the apical vesicles that normally support mature secretory granules were dispersed. Consequently, secretory granules did not mature. The transcription factor MIST1 bound the first intron of Mib1 and regulated its expression. We further showed that loss of MIB1 and dismantling of the apical secretory apparatus was the earliest quantifiable aberration in zymogenic cells undergoing transition to a precancerous metaplastic state in mouse and human stomach. Our results reveal a mechanistic pathway by which cells can scale up a specific, specialized subcellular compartment to alter function during differentiation and scale it down during disease.

Phospholipids: "greasing the wheels" of humoral immunity

Phospholipids are major structural components of all cellular membranes. In addition, certain phospholipids execute regulatory activities that affect cell behavior, function and fate in critically important physiological settings. The influence of phospholipids is especially obvious in the adaptive immune system, where these macromolecules mediate both intrinsic and extrinsic effects on B and T lymphocytes. This review article highlights the action of lysophospholipid sphingosine-1-phosphate as a lymphocyte chemoattractant, the function of phosphatidylinositol phosphates as signaling conduits in lymphocytes and the role of phospholipids as raw materials for membrane assembly and organelle biogenesis in activated B lymphocytes. Special emphasis is placed on the means by which these three processes push humoral immune responses forward. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

ATF6α/β-mediated adjustment of ER chaperone levels is essential for development of the notochord in medaka fish

ATF6α and ATF6β are membrane-bound transcription factors activated by regulated intramembrane proteolysis in response to endoplasmic reticulum (ER) stress to induce various ER quality control proteins. ATF6α- and ATF6β single-knockout mice develop normally, but ATF6α/β double knockout causes embryonic lethality, the reason for which is unknown. Here we show in medaka fish that ATF6α is primarily responsible for transcriptional induction of the major ER chaperone BiP and that ATF6α/β double knockout, but not ATF6α- or ATF6β single knockout, causes embryonic lethality, as in mice. Analyses of ER stress reporters reveal that ER stress occurs physiologically during medaka early embryonic development, particularly in the brain, otic vesicle, and notochord, resulting in ATF6α- and ATF6β-mediated induction of BiP, and that knockdown of the α1 chain of type VIII collagen reduces such ER stress. The absence of transcriptional induction of several ER chaperones in ATF6α/β double knockout causes more profound ER stress and impaired notochord development, which is partially rescued by overexpression of BiP. Thus ATF6α/β-mediated adjustment of chaperone levels to increased demands in the ER is essential for development of the notochord, which synthesizes and secretes large amounts of extracellular matrix proteins to serve as the body axis before formation of the vertebra.

Suppression of XBP1S mediates high glucose-induced oxidative stress and extracellular matrix synthesis in renal mesangial cell and kidney of diabetic rats

Recent evidences suggest that endoplasmic reticulum (ER) stress was involved in multi pathological conditions, including diabetic nephropathy (DN). X-box binding protein 1(XBP1), as a key mediator of ER stress, has been proved having the capability of preventing oxidative stress. In this study, we investigated the effects of spliced XBP1 (XBP1S), the dominant active form of XBP1, on high glucose (HG)-induced reactive oxygen species (ROS) production and extracellular matrix (ECM) synthesis in cultured renal mesangial cells (MCs) and renal cortex of STZ-induced diabetic rats. Real time PCR and Western blot were used to evaluate the mRNA and protein levels respectively. Transfection of recombinant adenovirus vector carrying XBP1S gene (Ad-XBP1S) was used to upregulate XBP1S expression. XBP1S siRNA was used to knockdown XBP1S expression. ROS level was detected by dihydroethidium (DHE) fluorescent probe assay. The results showed that HG treatment significantly reduced XBP1S protein and mRNA level in the cultured MCs while no obvious change was observed in unspliced XBP1 (XBP1U). In the mean time, the ROS production, collagen IV and fibronectin expressions were increased. Diphenylene-chloride iodonium (DPI), a NADPH oxidase inhibtor, prevented HG-induced increases in ROS as well as collagen IV and fibronectin expressions. Transfection of Ad-XBP1S reversed HG-induced ROS production and ECM expressions. Knockdown intrinsic XBP1S expression induced increases in ROS production and ECM expressions. Supplementation of supreoxide reversed the inhibitory effect of Ad-XBP1S transfection on ECM synthesis. P47phox was increased in HG-treated MCs. Ad-XBP1S transfection reversed HG-induced p47phox increase while XBP1S knockdown upregulated p47phox expression. In the renal cortex of diabetic rats, the expression of XBP1S was reduced while p47phox, collagen IV and fibronectin expression were elevated. These results suggested that XBP1S pathway of ER stress was involved in HG-induced oxidative stress and ECM synthesis. A downstream target of XBP1S in regulating ROS formation might be NADPH oxidase.

Xenobiotic perturbation of ER stress and the unfolded protein response

The proper folding, assembly, and maintenance of cellular proteins is a highly regulated process and is critical for cellular homeostasis. Multiple cellular compartments have adapted their own systems to ensure proper protein folding, and quality control mechanisms are in place to manage stress due to the accumulation of unfolded proteins. When the accumulation of unfolded proteins exceeds the capacity to restore homeostasis, these systems can result in a cell death response. Unfolded protein accumulation in the endoplasmic reticulum (ER) leads to ER stress with activation of the unfolded protein response (UPR) governed by the activating transcription factor 6 (ATF6), inositol requiring enzyme-1 (IRE1), and PKR-like endoplasmic reticulum kinase (PERK) signaling pathways. Many xenobiotics have been shown to influence ER stress and UPR signaling with either pro-survival or pro-death features. The ultimate outcome is dependent on many factors including the mechanism of action of the xenobiotic, concentration of xenobiotic, duration of exposure (acute vs. chronic), cell type affected, nutrient levels, oxidative stress, state of differentiation, and others. Assessing perturbations in activation or inhibition of ER stress and UPR signaling pathways are likely to be informative parameters to measure when analyzing mechanisms of action of xenobiotic-induced toxicity.

Lysine acetylation in the lumen of the ER: a novel and essential function under the control of the UPR

The N(ε)-amino group of lysine residues can be transiently modified by the addition of an acetyl group. Recognized functions of N(ε)-lysine acetylation include regulation of activity, molecular stabilization and conformational assembly of a protein. For more than forty years lysine acetylation was thought to occur only in the cytosol and nucleus. Targets included cytoskeletal-associated proteins as well as transcription factors, histone proteins and proteins involved in DNA recombination and repair. However, in 2007 we reported that a type I membrane protein involved in the pathogenesis of Alzheimer's disease was transiently acetylated on the ε amino group of seven lysine residues while transiting along the secretory pathway. Surprisingly, the acetylation occurred in the lumen of the endoplasmic reticulum (ER) forcing us to reconsider old paradigms. Indeed, if lysine acetylation can occur in the lumen of the ER, then all the essential biochemical elements of the reaction must be available in the lumen of the organelle. Follow-up studies revealed the existence of ER-based acetyl-CoA:lysine acetyltransferases as well as a membrane transporter that translocates acetyl-CoA from the cytosol into the ER lumen. Large-scale proteomics showed that the list of substrates of the ER-based acetylation machinery includes both transiting and resident proteins. Finally, genetic studies revealed that this machinery is tightly linked to human diseases. Here, we describe these exciting findings as well as recent biochemical and cellular advances, and discuss possible impact on both human physiology and pathology.

Transcriptional Regulation of the Ufm1 Conjugation System in Response to Disturbance of the Endoplasmic Reticulum Homeostasis and Inhibition of Vesicle Trafficking

Homeostasis of the endoplasmic reticulum (ER) is essential for normal cellular functions. Disturbance of this homeostasis causes ER stress and activates the Unfolded Protein Response (UPR). The Ufm1 conjugation system is a novel Ubiquitin-like (Ubl) system whose physiological target(s) and biological functions remain largely undefined. Genetic study has demonstrated that the Ufm1-activating enzyme Uba5 is indispensible for erythroid differentiation in mice, highlighting the importance of this novel system in animal development. In this report we present the evidence for involvement of RCAD/Ufl1, a putative Ufm1-specific E3 ligase, and its binding partner C53/LZAP protein in ufmylation of endogenous Ufm1 targets. Moreover, we found that the Ufm1 system was transcriptionally up-regulated by disturbance of the ER homeostasis and inhibition of vesicle trafficking. Using luciferase reporter and ChIP assays, we dissected the Ufm1 promoter and found that Ufm1 was a potential target of Xbp-1, one of crucial transcription factors in UPR. We further examined the effect of Xbp-1 deficiency on the expression of the Ufm1 components. Interestingly, the expression of Ufm1, Uba5, RCAD/Ufl1 and C53/LZAP in wild-type mouse embryonic fibroblasts (MEFs) was significantly induced by inhibition of vesicle trafficking, but the induction was negated by Xbp-1 deficiency. Finally, we found that knockdown of the Ufm1 system in U2OS cells triggered UPR and amplification of the ER network. Taken together, our study provided critical insight into the regulatory mechanism of the Ufm1 system and established a direct link between this novel Ubl system and the ER network.

Stress of endoplasmic reticulum: the cytological "scenario" of pathogenesis of human diseases

The phenomenon of stress of endoplasmic reticulum (ER) attracts an increasingly higher attention of the researchers. The available data suggest that ER dysfunction is a common component of many human pathologies including oncological diseases and neurodegenerative viral diseases. Stress of ER is of special significance in the cell populations actively secreting proteins because protein folding disturbances play an important role in its development. This fact opens up prospects for the better understanding of pathogenesis of diabetes mellitus and some other diseases. The present work summarizes the current concepts of biochemical mechanisms underlying stress of endoplasmic reticulum and elucidates the key signal pathways for its compensation and proapoptosis.

Intricately Regulated: A Cellular Toolbox for Fine-Tuning XBP1 Expression and Activity

Stress in the endoplasmic reticulum (ER) triggers the unfolded protein response (UPR), a signaling mechanism that allows cellular adaptation to ER stress by engaging pro-adaptive transcription factors and alleviating protein folding demand. One such transcription factor, X-box binding protein (XBP1), originates from the inositol-requiring transmembrane kinase/endoribonuclease 1 (IRE1) UPR stress sensor. XBP1 up-regulates a pool of genes involved in ER protein translocation, protein folding, vesicular trafficking and ER- associated protein degradation. Recent data suggest that the regulation of XBP1 expression and transcriptional activity may be a tissue- and stress-dependent phenomenon. Moreover, the intricacies involved in “fine-tuning” XBP1 activity in various settings are now coming to light. Here, we provide an overview of recent developments in understanding the regulatory mechanisms underlying XBP1 expression and activity and discuss the significance of these new insights.

Structural basis of the unfolded protein response

The unfolded protein response (UPR) is a network of intracellular signaling pathways that maintain the protein-folding capacity of the endoplasmic reticulum (ER) in eukaryotic cells. Dedicated molecular sensors embedded in the ER membrane detect incompletely folded or unfolded proteins in the ER lumen and activate a transcriptional program that increases the abundance of the ER according to need. In metazoans the UPR additionally regulates translation and thus relieves unfolded protein load by globally reducing protein synthesis. If homeostasis in the ER cannot be reestablished, the metazoan UPR switches from the prosurvival to the apoptotic mode. The UPR involves a complex, coordinated action of many genes that is controlled by one ER-embedded sensor, Ire1, in yeasts, and three sensors, Ire1, PERK, and ATF6, in higher eukaryotes, including human. We discuss the emerging molecular understanding of the UPR and focus on the structural biology of Ire1 and PERK, the two recently crystallized UPR sensors.

Silencing of lipid metabolism genes through IRE1α-mediated mRNA decay lowers plasma lipids in mice

XBP1 is a key regulator of the unfolded protein response (UPR), which is involved in a wide range of physiological and pathological processes. XBP1 ablation in liver causes profound hypolipidemia in mice, highlighting its critical role in lipid metabolism. XBP1 deficiency triggers feedback activation of its upstream enzyme IRE1α, instigating regulated IRE1-dependent decay (RIDD) of cytosolic mRNAs. Here, we identify RIDD as a crucial control mechanism of lipid homeostasis. Suppression of RIDD by RNA interference or genetic ablation of IRE1α reversed hypolipidemia in XBP1-deficient mice. Comprehensive microarray analysis of XBP1 and/or IRE1α-deficient liver identified genes involved in lipogenesis and lipoprotein metabolism as RIDD substrates, which might contribute to the suppression of plasma lipid levels by activated IRE1α. Ablation of XBP1 ameliorated hepatosteatosis, liver damage, and hypercholesterolemia in dyslipidemic animal models, suggesting that direct targeting of either IRE1α or XBP1 might be a feasible strategy to treat dyslipidemias.

Endoplasmic reticulum stress induces a caspase-dependent N-terminal cleavage of RBX1 protein in B cells

Endoplasmic reticulum (ER) stress develops when the ER is overloaded with too many proteins to fold. This elicits a signaling pathway called the unfolded protein response. The unfolded protein response is physiologically required for the terminal development of B cells into antibody-secreting plasma cells. Ring Box Protein 1 (RBX1) is a 14-kDa protein necessary for ubiquitin ligation activity of the multimeric cullin ring ubiquitin ligases (CRLs). As RBX1 is shared by a large number of CRLs, alterations in its activity may lead to global changes in protein stability. We discovered that RBX1 is cleaved in the course of LPS-induced plasma cell differentiation and in multiple myeloma cell lines upon induction of pharmacological ER stress. The cleavage is executed by several caspase proteases that cleave RBX1 eight amino acids from the N terminus. To address the possible implication of RBX1 cleavage for CRL activity, we replaced the endogenous RBX1 homolog of the yeast Saccharomyces cerevisiae, Roc1, with the wild type or the N-terminal Δ8 mutant human RBX1. We show that yeast expressing the cleaved RBX1 are hypersensitive to ER stress and are impaired in CRL-mediated ubiquitination and degradation. We propose a model by which N-terminal cleavage of RBX1 impairs its activity and promotes susceptibility to ER stress induction.

The unfolded protein response in secretory cell function

The endoplasmic reticulum (ER) controls many important aspects of cellular function, including processing of secreted and membrane proteins, synthesis of membranes, and calcium storage. Maintenance of ER function is controlled through a network of signaling pathways collectively known as the unfolded protein response (UPR). The UPR balances the load of incoming proteins with the folding capacity of the ER and allows cells to adapt to situations that disrupt this balance. This disruption is referred to as ER stress. Although ER stress often arises in pathological situations, the UPR plays a central role in the normal development and function of cells specializing in secretion. Many aspects of this response are conserved broadly across eukaryotes; most organisms use some subset of a group of ER transmembrane proteins to signal to the nucleus and induce a broad transcriptional upregulation of genes involved in ER function. However, new developments in metazoans, plants, and fungi illustrate interesting variations on this theme. Here, we summarize mechanisms for detecting and counteracting ER stress, the role of the UPR in normal secretory cell function, and how these pathways vary across organisms and among different tissues and cell types.

XBP1S associates with RUNX2 and regulates chondrocyte hypertrophy

BMP2 (bone morphogenetic protein 2) is known to activate unfolded protein response signaling molecules, including XBP1S and ATF6. However, the influence on XBP1S and ATF6 in BMP2-induced chondrocyte differentiation has not yet been elucidated. In this study, we demonstrate that BMP2 mediates mild endoplasmic reticulum stress-activated ATF6 and directly regulates XBP1S splicing in the course of chondrogenesis. XBP1S is differentially expressed during BMP2-stimulated chondrocyte differentiation and exhibits prominent expression in growth plate chondrocytes. This expression is probably due to the activation of the XBP1 gene by ATF6 and splicing by IRE1a. ATF6 directly binds to the 5'-flanking regulatory region of the XBP1 gene at its consensus binding elements. Overexpression of XBP1S accelerates chondrocyte hypertrophy, as revealed by enhanced expression of type II collagen, type X collagen, and RUNX2; however, knockdown of XBP1S via the RNAi approach abolishes hypertrophic chondrocyte differentiation. In addition, XBP1S associates with RUNX2 and enhances RUNX2-induced chondrocyte hypertrophy. Altered expression of XBP1S in chondrocyte hypertrophy was accompanied by altered levels of IHH (Indian hedgehog) and PTHrP (parathyroid hormone-related peptide). Collectively, XBP1S may be a novel regulator of hypertrophic chondrocyte differentiation by 1) acting as a cofactor of RUNX2 and 2) affecting IHH/PTHrP signaling.

SEC23B is required for the maintenance of murine professional secretory tissues

In eukaryotic cells, newly synthesized secretory proteins require COPII (coat protein complex II) to exit the endoplasmic reticulum (ER). COPII contains five core components: SAR1, SEC23, SEC24, SEC13, and SEC31. SEC23 is a GTPase-activating protein that activates the SAR1 GTPase and also plays a role in cargo recognition. Missense mutations in the human COPII paralogues SEC23A and SEC23B result in craniolenticulosutural dysplasia and congenital dyserythropoietic anemia type II, respectively. We now report that mice completely deficient for SEC23B are born with no apparent anemia phenotype, but die shortly after birth, with degeneration of professional secretory tissues. In SEC23B-deficient embryonic pancreas, defects occur in exocrine and endocrine tissues shortly after differentiation. Pancreatic acini are completely devoid of zymogen granules, and the ER is severely distended. Similar ultrastructural alterations are also observed in salivary glands, but not in liver. Accumulation of proteins in the ER lumen activates the proapoptotic pathway of the unfolded protein response, suggesting a central role for apoptosis in the degeneration of these tissues in SEC23B-deficient embryos. Although maintenance of the secretory pathway should be required by all cells, our findings reveal a surprising tissue-specific dependence on SEC23B for the ER exit of highly abundant cargo, with high levels of SEC23B expression observed in professional secretory tissues. The disparate phenotypes in mouse and human could result from residual SEC23B function associated with the hypomorphic mutations observed in humans, or alternatively, might be explained by a species-specific shift in function between the closely related SEC23 paralogues.

Antiviral activity of a small molecule deubiquitinase inhibitor occurs via induction of the unfolded protein response

Ubiquitin (Ub) is a vital regulatory component in various cellular processes, including cellular responses to viral infection. As obligate intracellular pathogens, viruses have the capacity to manipulate the ubiquitin (Ub) cycle to their advantage by encoding Ub-modifying proteins including deubiquitinases (DUBs). However, how cellular DUBs modulate specific viral infections, such as norovirus, is poorly understood. To examine the role of DUBs during norovirus infection, we used WP1130, a small molecule inhibitor of a subset of cellular DUBs. Replication of murine norovirus in murine macrophages and the human norovirus Norwalk virus in a replicon system were significantly inhibited by WP1130. Chemical proteomics identified the cellular DUB USP14 as a target of WP1130 in murine macrophages, and pharmacologic inhibition or siRNA-mediated knockdown of USP14 inhibited murine norovirus infection. USP14 is a proteasome-associated DUB that also binds to inositol-requiring enzyme 1 (IRE1), a critical mediator of the unfolded protein response (UPR). WP1130 treatment of murine macrophages did not alter proteasome activity but activated the X-box binding protein-1 (XBP-1) through an IRE1-dependent mechanism. In addition, WP1130 treatment or induction of the UPR also reduced infection of other RNA viruses including encephalomyocarditis virus, Sindbis virus, and La Crosse virus but not vesicular stomatitis virus. Pharmacologic inhibition of the IRE1 endonuclease activity partially rescued the antiviral effect of WP1130. Taken together, our studies support a model whereby induction of the UPR through cellular DUB inhibition blocks specific viral infections, and suggest that cellular DUBs and the UPR represent novel targets for future development of broad spectrum antiviral therapies.

Deubiquitinases (DUBs) are enzymes, which are implicated in many cellular processes but their functions during virus infection are not well understood. We used WP1130, a small molecule inhibitor of a subset of DUBs, as a probe to unravel the functions of DUBs during norovirus infections. We identified USP14 as a cellular DUB target of WP1130 that is required for optimal norovirus infection. Furthermore, we demonstrated that chemical induction of the unfolded protein response can significantly inhibit viral progeny production of several RNA viruses, including noroviruses. These results suggest that chemical inhibition of cellular DUBs and/or modulation of the unfolded protein response could represent novel targets for therapy against a variety of viral pathogens.

The specialized unfolded protein response of B lymphocytes: ATF6α-independent development of antibody-secreting B cells

B lymphocytes, like all mammalian cells, are equipped with the unfolded protein response (UPR), a complex signaling system allowing for both pro- and mal-adaptive responses to increased demands on the endoplasmic reticulum (ER). The UPR is comprised of three signaling pathways initiated by the ER transmembrane stress sensors, IRE1α/β, PERK and ATF6α/β. Activation of IRE1 yields XBP1(S), a transcription factor that directs expansion of the ER and enhances protein biosynthetic and secretory machinery. XBP1(S) is essential for the differentiation of B lymphocytes into antibody-secreting cells. In contrast, the PERK pathway, a regulator of translation and transcription, is dispensable for the generation of antibody-secreting cells. Functioning as a transcription factor, ATF6α can augment ER quality control processes and drive ER expansion, but the potential role of this UPR pathway in activated B cells has not been investigated. Here, we report studies of ATF6α-deficient B cells demonstrating that ATF6α is not required for the development of antibody-secreting cells. Thus, when B cells are stimulated to secrete antibody, a specialized UPR relies exclusively on the IRE1-XBP1 pathway to remodel the ER and expand cellular secretory capacity.

SENP1 deficiency promotes ER stress-induced apoptosis by increasing XBP1 SUMOylation

The transcription factor X box-binding protein 1 (XBP1) is a key component of the endoplasmic reticulum (ER) stress response. Recently, it has been reported that the spliced XBP1 (XBP1s), an activated XBP1 during ER stress, can be SUMOylated. Here, we identify Sentrin/SUMO-specific protease 1 (SENP1) as a specific de-SUMOylation protease for XBP1. SENP1 can increase the transcriptional activity of XBP1. In Senp1 (-/-) cells, the SUMOylated XBP1 is accumulated, and the expression of XBP1 target genes is downregulated in response to ER stress. Moreover, SENP1 deficiency significantly increases ER stress-induced apoptosis through accumulating XBP1 SUMOylation. These results reveal an essential function of SENP1 in ER stress response through regulating XBP1 SUMOylation.

The unfolded protein response at the crossroads of cellular life and death during endoplasmic reticulum stress

One of the early cellular responses to endoplasmic reticulum (ER) stress is the activation of the unfolded protein response (UPR). ER stress and the UPR are both implicated in numerous human diseases and pathologies. In spite of this, our knowledge of the molecular mechanisms that regulate cell fate following ER stress is limited. The UPR is initiated by three ER transmembrane receptors: PKR-like ER kinase (PERK), activating transcription factor (ATF) 6 and inositol-requiring enzyme 1 (IRE1). These proteins sense the accumulation of unfolded proteins and their activation triggers specific adaptive responses to resolve the stress. Intriguingly, the very same receptors can initiate signalling pathways that lead to apoptosis when the attempts to resolve the ER stress fail. In this review, we describe the known pro-apoptotic signalling pathways emanating from activated PERK, ATF6 and IRE1 and discuss how their signalling switches from an adaptive to a pro-apoptotic response.

ER Stress and Lipid Metabolism in Adipocytes

The role of endoplasmic reticulum (ER) stress is a rapidly emerging field of interest in the pathogenesis of metabolic diseases. Recent studies have shown that chronic activation of ER stress is closely linked to dysregulation of lipid metabolism in several metabolically important cells including hepatocytes, macrophages, β-cells, and adipocytes. Adipocytes are one of the major cell types involved in the pathogenesis of the metabolic syndrome. Recent advances in dissecting the cellular and molecular mechanisms involved in the regulation of adipogenesis and lipid metabolism indicate that activation of ER stress plays a central role in regulating adipocyte function. In this paper, we discuss the current understanding of the potential role of ER stress in lipid metabolism in adipocytes. In addition, we touch upon the interaction of ER stress and autophagy as well as inflammation. Inhibition of ER stress has the potential of decreasing the pathology in adipose tissue that is seen with energy overbalance.

The molecular basis for selective inhibition of unconventional mRNA splicing by an IRE1-binding small molecule

IRE1 couples endoplasmic reticulum unfolded protein load to RNA cleavage events that culminate in the sequence-specific splicing of the Xbp1 mRNA and in the regulated degradation of diverse membrane-bound mRNAs. We report on the identification of a small molecule inhibitor that attains its selectivity by forming an unusually stable Schiff base with lysine 907 in the IRE1 endonuclease domain, explained by solvent inaccessibility of the imine bond in the enzyme-inhibitor complex. The inhibitor (abbreviated 4μ8C) blocks substrate access to the active site of IRE1 and selectively inactivates both Xbp1 splicing and IRE1-mediated mRNA degradation. Surprisingly, inhibition of IRE1 endonuclease activity does not sensitize cells to the consequences of acute endoplasmic reticulum stress, but rather interferes with the expansion of secretory capacity. Thus, the chemical reactivity and sterics of a unique residue in the endonuclease active site of IRE1 can be exploited by selective inhibitors to interfere with protein secretion in pathological settings.

IRE1α activation protects mice against acetaminophen-induced hepatotoxicity

The mammalian stress sensor IRE1α plays a central role in the unfolded protein, or endoplasmic reticulum (ER), stress response by activating its downstream transcription factor XBP1 via an unconventional splicing mechanism. IRE1α can also induce the degradation of a subset of mRNAs in a process termed regulated IRE1-dependent decay (RIDD). Although diverse mRNA species can be degraded by IRE1α in vitro, the pathophysiological functions of RIDD are only beginning to be explored. Acetaminophen (APAP) overdose is the most frequent cause of acute liver failure in young adults in the United States and is primarily caused by CYP1A2-, CYP2E1-, and CYP3A4-driven conversion of APAP into hepatotoxic metabolites. We demonstrate here that genetic ablation of XBP1 results in constitutive IRE1α activation in the liver, leading to RIDD of Cyp1a2 and Cyp2e1 mRNAs, reduced JNK activation, and protection of mice from APAP-induced hepatotoxicity. A pharmacological ER stress inducer that activated IRE1α suppressed the expression of Cyp1a2 and Cyp2e1 in WT, but not IRE1α-deficient mouse liver, indicating the essential role of IRE1α in the down-regulation of these mRNAs upon ER stress. Our study reveals an unexpected function of RIDD in drug metabolism.

The unfolded protein response: controlling cell fate decisions under ER stress and beyond

Protein-folding stress at the endoplasmic reticulum (ER) is a salient feature of specialized secretory cells and is also involved in the pathogenesis of many human diseases. ER stress is buffered by the activation of the unfolded protein response (UPR), a homeostatic signalling network that orchestrates the recovery of ER function, and failure to adapt to ER stress results in apoptosis. Progress in the field has provided insight into the regulatory mechanisms and signalling crosstalk of the three branches of the UPR, which are initiated by the stress sensors protein kinase RNA-like ER kinase (PERK), inositol-requiring protein 1α (IRE1α) and activating transcription factor 6 (ATF6). In addition, novel physiological outcomes of the UPR that are not directly related to protein-folding stress, such as innate immunity, metabolism and cell differentiation, have been revealed.

The adaptor protein CRK is a pro-apoptotic transducer of endoplasmic reticulum stress

Excessive demands on the protein folding capacity of the endoplasmic reticulum (ER) cause irremediable ER stress and contribute to cell loss in a number of cell degenerative diseases, including type 2 diabetes and neurodegeneration^1,2. The signals communicating catastrophic ER damage to the mitochondrial apoptotic machinery remain poorly understood^3-6. We used a biochemical approach to purify a cytosolic activity induced by ER stress that causes release of cytochrome c from isolated mitochondria. We discovered that the principal component of the purified pro-apoptotic activity is proto-oncogene CT10-regulated kinase (CRK), an adaptor protein with no known catalytic activity⁷. Crk^-/- cells are strongly resistant to ER stress-induced apoptosis. Moreover, CRK is cleaved in response to ER stress to generate an N-terminal ~14kD fragment with greatly enhanced cytotoxic potential. We identified a putative BCL2 homology-3 (BH3) domain within this N-terminal CRK fragment, which sensitizes isolated mitochondria to cytochrome c release and when mutated significantly reduces CRK's apoptotic activity in vivo. Together these results identify CRK as a pro-apoptotic protein that signals irremediable ER stress to the mitochondrial execution machinery.

The unfolded protein response: integrating stress signals through the stress sensor IRE1α

Stress induced by accumulation of unfolded proteins at the endoplasmic reticulum (ER) is a classic feature of secretory cells and is observed in many tissues in human diseases including cancer, diabetes, obesity, and neurodegeneration. Cellular adaptation to ER stress is achieved by the activation of the unfolded protein response (UPR), an integrated signal transduction pathway that transmits information about the protein folding status at the ER to the nucleus and cytosol to restore ER homeostasis. Inositol-requiring transmembrane kinase/endonuclease-1 (IRE1α), the most conserved UPR stress sensor, functions as an endoribonuclease that processes the mRNA of the transcription factor X-box binding protein-1 (XBP1). IRE1α signaling is a highly regulated process, controlled by the formation of a dynamic scaffold onto which many regulatory components assemble, here referred to as the UPRosome. Here we provide an overview of the signaling and regulatory mechanisms underlying IRE1α function and discuss the emerging role of the UPR in adaptation to protein folding stress in specialized secretory cells and in pathological conditions associated with alterations in ER homeostasis.

The unfolded protein response and its role in intestinal homeostasis and inflammation

The unfolded protein response (UPR) is a signaling pathway from the endoplasmic reticulum (ER) to the nucleus that protects cells from the stress caused by misfolded or unfolded proteins [1, 2]. As such, ER stress is an ongoing challenge for all cells given the central biologic importance of secretion as part of normal physiologic functions. This is especially the case for cells that are highly dependent upon secretory function as part of their major duties. Within mucosal tissues, the intestinal epithelium is especially dependent upon an intact UPR for its normal activities [3]. This review will discuss the UPR and the special role that it provides in the functioning of the intestinal epithelium and, when dysfunctional, its implications for understanding mucosal homeostasis and intestinal inflammation, as occurs in inflammatory bowel disease (IBD).

UPR-Mediated Membrane Biogenesis in B Cells

The unfolded protein response (UPR) can coordinate the regulation of gene transcription and protein translation to balance the load of client proteins with the protein folding and degradative capacities of the ER. Increasing evidence also implicates the UPR in the regulation of lipid synthesis and membrane biogenesis. The differentiation of B lymphocytes into antibody-secreting cells is marked by significant expansion of the ER, the site for antibody synthesis and assembly. In activated B cells, the demand for membrane protein and lipid components leads to activation of the UPR transcriptional activator XBP1(S) which, in turn, initiates a cascade of biochemical events that enhance supplies of phospholipid precursors and build machinery for the synthesis, maturation, and transport of secretory proteins. The alterations in lipid metabolism that occur during this developmental transition and the impact of membrane phospholipid restriction on B cell secretory characteristics are discussed in this paper.

Allele-specific siRNA knockdown as a personalized treatment strategy for vascular Ehlers-Danlos syndrome in human fibroblasts

The vascular type of the Ehlers-Danlos syndrome (vEDS) is caused by dominant-negative mutations in the procollagen type III (COL3A1) gene. Patients with this autosomal dominant disorder have a shortened life expectancy due to complications from ruptured vessels or hollow organs. We tested the effectiveness of allele-specific RNA interference (RNAi) to reduce the mutated phenotype in fibroblasts. Small-interfering RNAs (siRNAs) discriminating between wild-type and mutant COL3A1 allele were identified by a luciferase reporter gene assay and in primary fibroblasts from a normal donor and a patient with vEDS. The best discriminative siRNA with the mutation at position 10 resulted in >90% silencing of the mutant allele without affecting the wild-type allele. Transmission and immunogold electron microscopy of extracted extracellular matrices from untreated fibroblasts of the patient with vEDS revealed structurally abnormal fibrils. After siRNA treatment, collagen fibrils became similar to fibrils from fibroblasts of normal and COL3A1 haploinsufficient donors. In addition, it was shown that expression of mutated COL3A1 activates the unfolded protein response and that reduction of the amount of mutated protein by siRNA reduces cellular stress. Taken together, the results provide evidence that allele-specific siRNAs are able to reduce negative effects of mutated COL3A1 proteins. Thus, the application of allele-specific RNAi may be a promising direction for future personalized therapies to reduce the severity of vEDS.

Extensive pancreas regeneration following acinar-specific disruption of Xbp1 in mice

BACKGROUND & AIMS

Progression of diseases of the exocrine pancreas, which include pancreatitis and cancer, is associated with increased levels of cell stress. Pancreatic acinar cells are involved in development of these diseases and, because of their high level of protein output, they require an efficient, unfolded protein response (UPR) that mediates recovery from endoplasmic reticulum (ER) stress following the accumulation of misfolded proteins.

METHODS

To study recovery from ER stress in the exocrine organ, we generated mice with conditional disruption of Xbp1 (a principal component of the UPR) in most adult pancreatic acinar cells (Xbp1fl/fl). We monitored the effects of constitutive ER stress in the exocrine pancreas of these mice.

RESULTS

Xbp1-null acinar cells underwent extensive apoptosis, followed by a rapid phase of recovery in the pancreas that included expansion of the centroacinar cell compartment, formation of tubular complexes that contained Hes1- and Sox9-expressing cells, and regeneration of acinar cells that expressed Mist1 from the residual, surviving Xbp1+ cell population.

CONCLUSIONS

XBP1 is required for homeostasis of acinar cells in mice; ER stress induces a regenerative response in the pancreas that involves acinar and centroacinar cells, providing the needed capacity for organ recovery from exocrine pancreas disease.

Trachealess (Trh) regulates all tracheal genes during Drosophila embryogenesis

The Drosophila trachea is a branched tubular epithelia that transports oxygen and other gases. trachealess (trh), which encodes a bHLH-PAS transcription factor, is among the first genes to be expressed in the cells that will form the trachea. In the absence of trh, tracheal cells fail to invaginate to form tubes and remain on the embryo surface. Expression of many tracheal-specific genes depends on trh, but all of the known targets have relatively minor phenotypes compared to loss of trh, suggesting that there are additional targets. To identify uncharacterized transcriptional targets of Trh and to further understand the role of Trh in embryonic tracheal formation, we performed an in situ hybridization screen using a library of ~100 tracheal-expressed genes identified by the Berkeley Drosophila Genome Project (BDGP). Surprisingly, expression of every tracheal gene we tested was dependent on Trh, suggesting a major role for Trh in activation and maintenance of tracheal gene expression. A re-examination of the interdependence of the known early-expressed transcription factors, including trh, ventral veinless (vvl) and knirps/knirps-related (kni/knrl), suggests a new model for how gene expression is controlled in the trachea, with trh regulating expression of vvl and kni, but not vice versa. A pilot screen for the targets of Vvl and Kni/Knrl revealed that Vvl and Kni have only minor roles compared to Trh. Finally, genome-wide microarray experiments identified additional Trh targets and revealed that a variety of biological processes are affected by the loss of trh.

Stressed out about obesity: IRE1α-XBP1 in metabolic disorders

The global obesity epidemic is associated with a series of health-threatening diseases including type 2 diabetes. Accumulating evidence suggest that the physiology and homeostasis of the endoplasmic reticulum (ER) is intimately involved in the underlying mechanisms linking obesity and diabetes. Specifically, recent studies indicate a crucial role for the inositol-requiring enzyme 1α (IRE1α)/X-box binding protein 1 (XBP1) pathway, the most conserved branch of the unfolded protein response (UPR), in glucose and lipid metabolism as well as in insulin function. Focusing on the IRE1α-XBP1 pathway, we review recent advances in our understanding of the role of UPR in obesity and obesity-associated metabolic disorders.

ALS genetic modifiers that increase survival of SOD1 mice and are suitable for therapeutic development

Amyotrophic lateral sclerosis (ALS) is a frequently fatal motor neuron disease without any cure. To find molecular therapeutic targets, several studies crossed transgenic ALS murine models with animals transgenic for some ALS target genes. We aimed to revise the new discoveries and new works in this field. We selected the 10 most promising genes, according to their capability when down-regulated or up-regulated in ALS animal models, for increasing life span and mitigating disease progression: XBP-1, NogoA and NogoB, dynein, heavy and medium neurofilament, NOX1 and NOX2, MLC-mIGF-1, NSE-VEGF, and MMP-9. Interestingly, some crucial modifier genes have been described as being involved in common pathways, the most significant of which are inflammation and cytoskeletal activities. The endoplasmic reticulum also seems to play an important role in ALS pathogenesis, as it is involved in different selected gene pathways. In addition, these genes have evident links to each other, introducing the hypothesis of a single unknown, common pathway involving all of these identified genes and others to be discovered.

Isolation of genes involved in pancreas regeneration by subtractive hybridization

The deterioration of β cells in the pancreas is a crucial factor in the progression of diabetes mellitus; therefore, the recovery of β cells is of vital importance for effective diabetic therapeutic strategies. Partially pancreatectomized rats have been used for the investigation of pancreatic regeneration. Because it was determined that tissue extract from the partially-dissected pancreas induces pancreatic differentiation in embryonic stem cells, paracrine factors were thought to be involved in the regeneration. In this study, we screened for genes that had higher mRNA levels 2 days after 60%-pancreatectomy. The genes were isolated using subtractive hybridization and DNA sequencing. Twelve genes (adipsin, Aplp2, Clu, Col1a2, Glul, Krt8, Lgmn, LOC299907, LOC502894, Pla2g1b, Reg3α and Xbp1) were identified, and RT-PCR and real-time PCR analyses were performed to validate their expression levels. Among the genes identified, three genes (Glul, Lgmn and Reg3a) were selected for further analyses. Assays revealed that Glul and Reg3α enhance cell growth. Glul, Lgmn and Reg3α change the expression level of islet marker genes, where NEUROD, NKX2.2, PAX4 and PAX6 are up-regulated and somatostatin is down-regulated. Thus, we believe that Glul, Lgmn and Reg3a can serve as novel targets in diabetes mellitus genetic therapy.

Functional characterization of the ER stress induced X-box-binding protein-1 (Xbp-1) in the porcine system

Background

The unfolded protein response (UPR) is an evolutionary conserved adaptive reaction for increasing cell survival under endoplasmic reticulum (ER) stress conditions. X-box-binding protein-1 (Xbp1) is a key transcription factor of UPR that activates genes involved in protein folding, secretion, and degradation to restore ER function. The UPR induced by ER stress was extensively studied in diseases linked to protein misfolding and aggregations. However, in the porcine system, genes in the UPR pathway were not investigated. In this study, we isolated and characterized the porcine Xbp1 (pXbp1) gene in ER stress using porcine embryonic fibroblast (PEF) cells and porcine organs. ER stress was induced by the treatment of tunicamycin and cell viability was investigated by the MTT assay. For cloning and analyzing the expression pattern of pXbp1, RT-PCR analysis and Western blot were used. Knock-down of pXbp1 was performed by the siRNA-mediated gene silencing.

Results

We found that the pXbp1 mRNA was the subject of the IRE1α-mediated unconventional splicing by ER stress. Knock-down of pXbp1 enhanced ER stress-mediated cell death in PEF cells. In adult organs, pXbp1 mRNA and protein were expressed and the spliced forms were detected.

Conclusions

It was first found that the UPR mechanisms and the function of pXbp1 in the porcine system. These results indicate that pXbp1 plays an important role during the ER stress response like other animal systems and open a new opportunity for examining the UPR pathway in the porcine model system.

Dual and opposing roles of the unfolded protein response regulated by IRE1alpha and XBP1 in proinsulin processing and insulin secretion

As a key regulator of the unfolded protein response, the transcription factor XBP1 activates genes in protein secretory pathways and is required for the development of certain secretory cells. To elucidate the function of XBP1 in pancreatic β-cells, we generated β-cell-specific XBP1 mutant mice. Xbp1(f/f);RIP-cre mice displayed modest hyperglycemia and glucose intolerance resulting from decreased insulin secretion from β-cells. Ablation of XBP1 markedly decreased the number of insulin granules in β-cells, impaired proinsulin processing, increased the serum proinsulin:insulin ratio, blunted glucose-stimulated insulin secretion, and inhibited cell proliferation. Notably, XBP1 deficiency not only compromised the endoplasmic reticulum stress response in β-cells but also caused constitutive hyperactivation of its upstream activator, IRE1α, which could degrade a subset of mRNAs encoding proinsulin-processing enzymes. Hence, the combined effects of XBP1 deficiency on the canonical unfolded protein response and its negative feedback activation of IRE1α caused β-cell dysfunction in XBP1 mutant mice. These results demonstrate that IRE1α has dual and opposing roles in β-cells, and that a precisely regulated feedback circuit involving IRE1α and its product XBP1s is required to achieve optimal insulin secretion and glucose control.

Autophagy, microbial sensing, endoplasmic reticulum stress, and epithelial function in inflammatory bowel disease

Increasing evidence has emerged that supports an important intersection between 3 fundamental cell biologic pathways in the pathogenesis of inflammatory bowel disease. These include the intersection between autophagy, as revealed by the original identification of ATG16L1 and IRGM as major genetic risk factors for Crohn's disease, and intracellular bacterial sensing, as shown by the importance of NOD2 in autophagy induction upon bacterial entry into the cell. A pathway closely linked to autophagy and innate immunity is the unfolded protein response, initiated by endoplasmic reticulum stress due to the accumulation of misfolded proteins, which is genetically related to ulcerative colitis and Crohn's disease (XBP1 and ORMDL3). Hypomorphic ATG16L1, NOD2, and X box binding protein-1 possess the common attribute of profoundly affecting Paneth cells, specialized epithelial cells at the bottom of intestinal crypts involved in antimicrobial function. Together with their functional juxtaposition in the environmentally exposed intestinal epithelial cell, their remarkable functional convergence on Paneth cells and their behavior in response to environmental factors, including microbes, these 3 pathways are of increasing importance to understanding the pathogenesis of inflammatory bowel disease. Moreover, in conjunction with studies that model deficient nuclear factor-κB function, these studies suggest a central role for altered intestinal epithelial cell function as one of the earliest events in the development of inflammatory bowel disease.

Secretory pathway stress responses as possible mechanisms of disease involving Golgi Ca2+ pump dysfunction

In mammalian tissues, uptake of Ca(2+) and Mn(2+) by Golgi membranes is mediated by the secretory pathway Ca(2+) -ATPases, SPCA1 and SPCA2, encoded by the ATP2C1 and ATP2C2 genes. Loss of one copy of the ATP2C1 gene, which causes SPCA1 haploinsufficiency, leads to squamous cell tumors of keratinized epithelia in mice and to Hailey-Hailey disease, an acantholytic skin disease, in humans. Although the disease phenotypes resulting from SPCA1 haploinsufficiency in mice and humans are quite different, each species-specific phenotype is remarkably similar to those arising as a result of null mutations in one copy of the ATP2A2 gene, encoding SERCA2, the endoplasmic reticulum (ER) Ca(2+) pump. SERCA2 haploinsufficiency, like SPCA1 haploinsufficiency, causes squamous cell tumors in mice and Darier's disease, also an acantholytic skin disease, in humans. The phenotypic similarities between SPCA1 and SERCA2 haploinsufficiency in the two species, and the general functions of the two pumps in consecutive compartments of the secretory pathway, suggest that the underlying disease mechanisms are similar. In this review, we discuss evidence supporting the view that chronic Golgi stress and/or ER stress resulting from Ca(2+) pump haploinsufficiencies leads to activation of cellular stress responses in keratinocytes, with the predominance of proapoptotic pathways (although not necessarily apoptosis itself) leading to acantholytic skin disease in humans and the predominance of prosurvival pathways leading to tumors in mice.

Uncovering the transcriptional circuitry in skeletal muscle regeneration

Skeletal muscle has a remarkable ability to regenerate after repeated and complete destruction of the tissue. The healing phases for an injured muscle undergo an activation program controlled by a dynamically inducible transcriptional regulatory network. Mapping a complex mammalian transcriptional network is confronted by significant challenges and requires the integration of multiple experimental data types. In this work we present a system approach to describe the transcriptional circuitry during skeletal muscle regeneration using time-course expression data and motif scanning information. Time-lagged correlation analysis was utilized to evaluate the transcription factor (TF) → target associations. Our analysis identified six TFs that potentially play a central role throughout the regeneration process. Four of them have previously been described to be important for muscle regeneration and differentiation. The remaining two TFs are identified as novel regulators that may have a role in the regeneration process. We hope that our work may provide useful clues to help accelerate the recovery process in injured skeletal muscle.

Endoplasmic reticulum stress in liver disease

The unfolded protein response (UPR) is activated upon the accumulation of misfolded proteins in the endoplasmic reticulum (ER) that are sensed by the binding immunoglobulin protein (BiP)/glucose-regulated protein 78 (GRP78). The accumulation of unfolded proteins sequesters BiP so it dissociates from three ER-transmembrane transducers leading to their activation. These transducers are inositol requiring (IRE) 1α, PKR-like ER kinase (PERK), and activating transcription factor (ATF) 6α. PERK phosphorylates eukaryotic initiation factor 2 alpha (eIF2α) resulting in global mRNA translation attenuation, and concurrently selectively increases the translation of several mRNAs, including the transcription factor ATF4, and its downstream target CHOP. IRE1α has kinase and endoribonuclease (RNase) activities. IRE1α autophosphorylation activates the RNase activity to splice XBP1 mRNA, to produce the active transcription factor sXBP1. IRE1α activation also recruits and activates the stress kinase JNK. ATF6α transits to the Golgi compartment where it is cleaved by intramembrane proteolysis to generate a soluble active transcription factor. These UPR pathways act in concert to increase ER content, expand the ER protein folding capacity, degrade misfolded proteins, and reduce the load of new proteins entering the ER. All of these are geared toward adaptation to resolve the protein folding defect. Faced with persistent ER stress, adaptation starts to fail and apoptosis occurs, possibly mediated through calcium perturbations, reactive oxygen species, and the proapoptotic transcription factor CHOP. The UPR is activated in several liver diseases; including obesity associated fatty liver disease, viral hepatitis, and alcohol-induced liver injury, all of which are associated with steatosis, raising the possibility that ER stress-dependent alteration in lipid homeostasis is the mechanism that underlies the steatosis. Hepatocyte apoptosis is a pathogenic event in several liver diseases, and may be linked to unresolved ER stress. If this is true, restoration of ER homeostasis prior to ER stress-induced cell death may provide a therapeutic rationale in these diseases. Herein we discuss each branch of the UPR and how they may impact hepatocyte function in different pathologic states.

Physiological unfolded protein response regulated by OASIS family members, transmembrane bZIP transcription factors

The endoplasmic reticulum (ER) plays role in the maintenance of numerous aspects of cellular and organismal homeostasis by folding, modifying, and exporting nascent secretory and transmembrane proteins. Failure of the ER's adaptive capacity results in accumulation of unfolded or malfolded proteins in the ER lumen (ER stress). To avoid cellular damage, mammalian cells activate the specific signals from the ER to the cytosol or nucleus to enhance the capacity for protein folding, attenuate the synthesis of proteins, and degrade unfolded proteins. These signaling pathways are collectively known as the unfolded protein response (UPR). The canonical branches of the UPR are mediated by three ER membrane-bound proteins, PERK, IRE1, and ATF6. These ER stress transducers basically play important roles in cell survival after ER stress. Recently, novel types of ER stress transducers, OASIS family members that share a region of high sequence similarity with ATF6 have been identified. They have a transmembrane domain, which allows them to associate with the ER, and possess a transcription-activation domain and a bZIP domain. OASIS family proteins include OASIS, BBF2H7, CREBH, AIbZIP, and Luman. Despite the structural similarities among OASIS family proteins and ATF6, differences in activating stimuli, tissue distribution, and response element binding indicate specialized functions of each member on regulating the UPR in the specific organs and tissues. Here, we summarize our current understanding of biochemical characteristics and in vivo functions of OASIS family proteins, particularly focusing on OASIS and BBF2H7. A growing body of new works suggests that the UPR branches regulated by OASIS family members play essential roles in cell differentiation and maturation or maintenance of basal cellular homeostasis in mammals.

Role of endoplasmic reticulum stress in cystic fibrosis-related airway inflammatory responses

Chronic airway infection and inflammation are hallmarks of cystic fibrosis (CF) pulmonary disease. The altered airway environment resulting from infection and inflammation can affect the innate defense of the airway epithelia. Luminal bacterial and inflammatory stimuli trigger an adaptation in human airway epithelia, characterized by a hyperinflammatory response to inflammatory mediators, which is mediated by an expansion of the endoplasmic reticulum (ER) and its Ca(2+) stores. Recent studies demonstrated that a form of ER stress, the unfolded protein response (UPR), is activated in airway epithelia by bacterial infection-induced airway inflammation. UPR-dependent signaling is responsible for the ER Ca(2+) store expansion-mediated amplification of airway inflammatory responses. These studies highlight the functional importance of the UPR in airway inflammation and suggest that targeting the UPR may be a therapeutic strategy for airway diseases typified by chronic inflammation. This article reviews the contribution of airway epithelia to airway inflammatory responses, discusses how expansion of the ER Ca(2+) stores in inflamed airway epithelia contributes to airway inflammation, describes the functional role of the UPR in these processes, and discusses how UPR activation might be relevant for CF airways inflammatory disease.

Environmental and genetic stressors and the unfolded protein response in exocrine pancreatic function - a hypothesis

The exocrine pancreas has the greatest protein synthetic capacity of any mammalian organ and is challenged with the synthesis, processing and transporting a large load of digestive enzymes. Based on recent findings we present a hypothesis proposing that mutations in the digestive enzymes and environmental risks impacting the pancreas (i.e., alcohol abuse, smoking, metabolic disorders, and drugs) cause endoplasmic reticulum (ER) stress. We review recent findings showing that in normal pancreas the ER stress resulting from alcohol abuse leads to an adaptive unfolded protein response (UPR) allowing for maintenance of protein synthesis, processing, and transport. However, when key pathways necessary for the adaptive UPR are altered, the exocrine cell of the pancreas is unable to maintain these processes and cellular pathology results. These findings may explain why some individuals with alcohol abuse disorders develop organ injury and disease while most do not. Further, the findings allow us to hypothesize that the UPR in the exocrine pancreas adapts the protein synthetic machinery of the ER stress resulting from mutational and environmental stressors. When the ability of the UPR to adapt to the stressors is exceeded, pathologic pathways and disease develop.

Adaptive unfolded protein response attenuates alcohol-induced pancreatic damage

BACKGROUND & AIMS

Endoplasmic reticulum (ER) stress responses (collectively known the unfolded protein response [UPR]) have important roles in several human disorders, but their contribution to alcoholic pancreatitis is not known. We investigated the role of X-box binding protein 1 (XBP1), a UPR regulator, in prevention of alcohol-induced ER stress in the exocrine pancreas.

METHODS

Wild-type and Xbp1(+/-) mice were fed control or ethanol diets for 4 weeks. Pancreatic tissue samples were then examined by light and electron microscopy to determine pancreatic alterations; UPR regulators were analyzed biochemically.

RESULTS

In wild-type mice, ethanol activated a UPR, increasing pancreatic levels of XBP1 and XBP1 targets such as protein disulfide isomerase (PDI). In these mice, pancreatic damage was minor. In ethanol-fed Xbp1(+/-) mice, XBP1 and PDI levels were significantly lower than in ethanol-fed wild-type mice. The combination of XBP1 deficiency and ethanol feeding reduced expression of regulators of ER function and the up-regulation of proapoptotic signals. Moreover, ethanol feeding induced oxidation of PDI, which might compromise PDI-mediated disulfide bond formation during ER protein folding. In ethanol-fed Xbp1(+/-) mice, ER stress was associated with disorganized and dilated ER, loss of zymogen granules, accumulation of autophagic vacuoles, and increased acinar cell death.

CONCLUSIONS

Long-term ethanol feeding causes oxidative ER stress, which activates a UPR and increases XBP1 levels and activity. A defective UPR due to XBP1 deficiency results in ER dysfunction and acinar cell pathology.

Integrating the mechanisms of apoptosis induced by endoplasmic reticulum stress

The ability to respond to perturbations in endoplasmic reticulum (ER) function is a fundamentally important property of all cells, but ER stress can also lead to apoptosis. In settings of chronic ER stress, the associated apoptosis may contribute to pathophysiological processes involved in a number of prevalent diseases, including neurodegenerative diseases, diabetes, atherosclerosis and renal disease. The molecular mechanisms linking ER stress to apoptosis are the topic of this review, with emphases on relevance to pathophysiology and integration and complementation among the various apoptotic pathways induced by ER stress.

Investigating the pathobiology of alcoholic pancreatitis

Alcohol abuse is one of the most common causes of pancreatitis. The risk of developing alcohol-induced pancreatitis is related to the amount and duration of drinking. However, only a small portion of heavy drinkers develop disease, indicating that other factors (genetic, environmental, or dietary) contribute to disease initiation. Epidemiologic studies suggest roles for cigarette smoking and dietary factors in the development of alcoholic pancreatitis. The mechanisms underlying alcoholic pancreatitis are starting to be understood. Studies from animal models reveal that alcohol sensitizes the pancreas to key pathobiologic processes that are involved in pancreatitis. Current studies are focussed on the mechanisms responsible for the sensitizing effect of alcohol; recent findings reveal disordering of key cellular organelles including endoplasmic reticulum, mitochondria, and lysosomes. As our understanding of alcohol's effects continue to advance to the level of molecular mechanisms, insights into potential therapeutic strategies will emerge providing opportunities for clinical benefit.