XBP1 controls diverse cell type- and condition-specific transcriptional regulatory networks

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.

Dissociation of inositol-requiring enzyme (IRE1α)-mediated c-Jun N-terminal kinase activation from hepatic insulin resistance in conditional X-box-binding protein-1 (XBP1) knock-out mice

Hepatic insulin resistance has been attributed to both increased endoplasmic reticulum (ER) stress and accumulation of intracellular lipids, specifically diacylglycerol (DAG). The ER stress response protein, X-box-binding protein-1 (XBP1), was recently shown to regulate hepatic lipogenesis, suggesting that hepatic insulin resistance in models of ER stress may result from defective lipid storage, as opposed to ER-specific stress signals. Studies were designed to dissociate liver lipid accumulation and activation of ER stress signaling pathways, which would allow us to delineate the individual contributions of ER stress and hepatic lipid content to the pathogenesis of hepatic insulin resistance. Conditional XBP1 knock-out (XBP1Δ) and control mice were fed fructose chow for 1 week. Determinants of whole-body energy balance, weight, and composition were determined. Hepatic lipids including triglyceride, DAGs, and ceramide were measured, alongside markers of ER stress. Whole-body and tissue-specific insulin sensitivity were determined by hyperinsulinemic-euglycemic clamp studies. Hepatic ER stress signaling was increased in fructose chow-fed XBP1Δ mice as reflected by increased phosphorylated eIF2α, HSPA5 mRNA, and a 2-fold increase in hepatic JNK activity. Despite JNK activation, XBP1Δ displayed increased hepatic insulin sensitivity during hyperinsulinemic-euglycemic clamp studies, which was associated with increased insulin-stimulated IRS2 tyrosine phosphorylation, reduced hepatic DAG content, and reduced PKCε activity. These studies demonstrate that ER stress and IRE1α-mediated JNK activation can be disassociated from hepatic insulin resistance and support the hypothesis that hepatic insulin resistance in models of ER stress may be secondary to ER stress modulation of hepatic lipogenesis.

Identification of genes for bone mineral density variation by computational disease gene identification strategy

We previously used five freely available bioinformatics tools (Prioritizer, Geneseeker, PROSPECTR and SUSPECTS, Disease Gene Prediction, and Endeavour) to analyze the thirteen well-replicated osteoporosis susceptibility loci and identify a subset of most likely candidate osteoporosis susceptibility genes (Huang et al. in J Hum Genet 53:644-655, 2008). In the current study, we experimentally tested the association between bone mineral density (BMD) and the 9 most likely candidate genes [LAMC2(1q25-q31), MATN3(2p24-p23), ITGAV(2q31-q32), ACVR1(2q23-q24), TDGF1(3p21.31), EGF(4q25), IGF1(12q22-q23), ZIC2(13q32), BMP2(20p12)] which were pinpointed by 4 or more bioinformatics tools. Forty tag SNPs in nine candidate genes were genotyped in a southern Chinese female case-control cohort consisting of 1643 subjects. Single- and multi-marker association analyses were performed using logistic regression analysis implemented by PLINK. Potential transcription factor binding sites were predicted by MatInspector. The strongest association was observed between rs10178256 (MATN3) and trochanter (P < 0.001) and total hip BMD (P = 0.002). The SNP rs6214 (IGF1) showed consistent association with BMD at all the four measured skeletal sites (P = 0.005-0.044). Prediction of transcription factor binding suggested that the minor allele G of rs10178256 might abolish the binding of MESP1 and MESP2 which play vital roles in bone homeostasis, whereas the minor allele G of rs6214 might create an additional binding site for XBP1, a constitutive regulator of endoplasmic reticulum stress response. Our data suggested that variants in MATN3 and IGF1 were involved in BMD regulation in southern Chinese women.

Inactivation of CBF/NF-Y in postnatal liver causes hepatocellular degeneration, lipid deposition, and endoplasmic reticulum stress

We previously demonstrated that CBF activity is needed for cell proliferation and early embryonic development. To examine the in vivo function of CBF in differentiated hepatocytes, we conditionally deleted CBF-B in hepatocytes after birth. Deletion of CBF-B resulted in progressive liver injury and severe hepatocellular degeneration 4 weeks after birth. Electron microscopic examination demonstrated pleiotropic changes of hepatocytes including enlarged cell and nuclear size, intracellular lipid deposition, disorganized endoplasmic reticulum, and mitochondrial abnormalities. Gene expression analyses showed that deletion of CBF-B activated expression of specific endoplasmic reticulum (ER) stress-regulated genes. Inactivation of CBF-B also inhibited expression of C/EBP alpha, an important transcription factor controlling various metabolic processes in adult hepatocytes. Altogether, our study reveals for the first time that CBF is a key transcription factor controlling ER function and metabolic processes in mature hepatocytes.

Scaling factors: transcription factors regulating subcellular domains

Developing cells acquire mature fates in part by selective (i.e. qualitatively different) expression of a few cell-specific genes. However, all cells share the same basic repertoire of molecular and subcellular building blocks. Therefore, cells must also specialize according to quantitative differences in cell-specific distributions of those common molecular resources. Here we propose the novel hypothesis that evolutionarily-conserved transcription factors called scaling factors (SFs) regulate quantitative differences among mature cell types. SFs: (1) are induced during late stages of cell maturation; (2) are dedicated to specific subcellular domains; and, thus, (3) allow cells to emphasize specific subcellular features. We identify candidate SFs and discuss one in detail: MIST1 (BHLHA15, vertebrates)/DIMM (CG8667, Drosophila); professional secretory cells use this SF to scale up regulated secretion. Because cells use SFs to develop their mature properties and also to adapt them to ever-changing environmental conditions, SF aberrations likely contribute to diseases of adult onset.

Unfolded proteins and endoplasmic reticulum stress in neurodegenerative disorders

The stimuli for neuronal cell death in neurodegenerative disorders are multi-factorial and may include genetic predisposition, environmental factors, cellular stressors such as oxidative stress and free radical production, bioenergy failure, glutamate-induced excitotoxicity, neuroinflammation, disruption of Ca(2+) -regulating systems, mitochondrial dysfunction and misfolded protein accumulation. Cellular stress disrupts functioning of the endoplasmic reticulum (ER), a critical organelle for protein quality control, leading to induction of the unfolded protein response (UPR). ER stress may contribute to neurodegeneration in a range of neurodegenerative disorders. This review summarizes the molecular events occurring during ER stress and the unfolded protein response and it specifically evaluates the evidence suggesting the ER stress response plays a role in neurodegenerative disorders.

Endoplasmic reticulum stress in the β-cell pathogenesis of type 2 diabetes

Type 2 diabetes is a complex metabolic disorder characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency by β-cell failure. Even if the mechanisms underlying the pathogenesis of β-cell failure are still under investigation, recent increasing genetic, experimental, and clinical evidence indicate that hyperactivation of the unfolded protein response (UPR) to counteract metabolic stresses is closely related to β-cell dysfunction and apoptosis. Signaling pathways of the UPR are “a double-edged sword” that can promote adaptation or apoptosis depending on the nature of the ER stress condition. In this paper, we summarized our current understanding of the mechanisms and components related to ER stress in the β-cell pathogenesis of type 2 diabetes.

Heightened induction of proapoptotic signals in response to endoplasmic reticulum stress in primary fibroblasts from a mouse model of longevity

Previous work from our laboratory has shown that primary fibroblasts from long-lived Snell dwarf mice display a higher sensitivity to the lethal effects of endoplasmic reticulum (ER) stressors, such as thapsigargin, than cells from normal mice. Here we show that thapsigargin induces higher expression of CHOP, enhanced cleavage of caspase-12, higher caspase-3 activity, and increased phosphorylation of c-JUN, all indicators of enhanced apoptosis, in dwarf fibroblasts. Dwarf and normal fibroblasts show no genotypic difference in up-regulating BiP, GRP94, and ERp72 proteins after exposure to thapsigargin. However, dwarf fibroblasts express lower basal levels of a number of putative XBP1 target genes including Armet, Edem1, Erdj3, p58(IPK) and Sec61a1, as well as Ire1α itself. Furthermore, when exposed to thapsigargin, dwarf fibroblasts display attenuated splicing of Xbp1, but similar phosphorylation of eIF2α, in comparison to normal fibroblasts. These data support the notion that IRE1/XBP1 signaling is set at a lower level in dwarf fibroblasts. Diminished Xbp1 splicing in dwarf-derived fibroblasts may tilt the balance between prosurvival and proapoptotic signals in favor of apoptosis, thereby leading to higher induction of proapoptotic signals in these cells and ultimately their increased sensitivity to ER stressors. These results, together with recent findings in Caenorhabditis elegans daf-2 mutants, point to a potential interplay between insulin/IGF-1 signals and unfolded protein response signaling.

PPARα–LXR as a novel metabolostatic signalling axis in skeletal muscle that acts to optimize substrate selection in response to nutrient status

LXR (liver X receptor) and PPARα (peroxisome-proliferator-activated receptor α) are nuclear receptors that control the expression of genes involved in glucose and lipid homoeostasis. Using wild-type and PPARα-null mice fed on an LXR-agonist-supplemented diet, the present study analysed the impact of pharmacological LXR activation on the expression of metabolically important genes in skeletal muscle, testing the hypothesis that LXR activation can modulate PPAR action in skeletal muscle in a manner dependent on nutritional status. In the fed state, LXR activation promoted a gene profile favouring lipid storage and glucose oxidation, increasing SCD1 (stearoyl-CoA desaturase 1) expression and down-regulating PGC-1α (PPARγ co-activator-1α) and PDK4 (pyruvate dehydrogenase kinase 4) expression. PPARα deficiency enhanced LXR stimulation of SCD1 expression, and facilitated elevated SREBP-1 (sterol-regulatory-element-binding protein-1) expression. However, LXR-mediated down-regulation of PGC-1α and PDK4 was opposed and reversed by PPARα deficiency. During fasting, prior LXR activation augmented PPARα signalling to heighten FA (fatty acid) oxidation and decrease glucose oxidation by augmenting fasting-induced up-regulation of PGC-1α and PDK4 expression, effects opposed by PPARα deficiency. Starvation-induced down-regulation of SCD1 expression was opposed by antecedent LXR activation in wild-type mice, an effect enhanced further by PPARα deficiency, which may elicit increased channelling of FA into triacylglycerol to limit lipotoxicity. Our results also identified potential regulatory links between the protein deacetylases SIRT1 (sirtuin 1) and SIRT3 and PDK4 expression in muscle from fasted mice, with a requirement for PPARα. In summary, we therefore propose that a LXR–PPARα signalling axis acts as a metabolostatic regulatory mechanism to optimize substrate selection and disposition in skeletal muscle according to metabolic requirement.

Transcription factor TEAD4 regulates expression of myogenin and the unfolded protein response genes during C2C12 cell differentiation

The TEAD (1-4) transcription factors comprise the conserved TEA/ATTS DNA-binding domain recognising the MCAT element in the promoters of muscle-specific genes. Despite extensive genetic analysis, the function of TEAD factors in muscle differentiation has proved elusive due to redundancy among the family members. Expression of the TEA/ATTS DNA-binding domain that acts as a dominant negative repressor of TEAD factors in C2C12 myoblasts inhibits their differentiation, whereas selective shRNA knockdown of TEAD4 results in abnormal differentiation characterised by the formation of shortened myotubes. Chromatin immunoprecipitation coupled to array hybridisation shows that TEAD4 occupies 867 promoters including those of myogenic miRNAs. We show that TEAD factors directly induce Myogenin, CDKN1A and Caveolin 3 expression to promote myoblast differentiation. RNA-seq identifies a set of genes whose expression is strongly reduced upon TEAD4 knockdown among which are structural and regulatory proteins and those required for the unfolded protein response. In contrast, TEAD4 represses expression of the growth factor CTGF (connective tissue growth factor) to promote differentiation. Together these results show that TEAD factor activity is essential for normal C2C12 cell differentiation and suggest a role for TEAD4 in regulating expression of the unfolded protein response genes.

Identification of novel myeloma-specific XBP1 peptides able to generate cytotoxic T lymphocytes: a potential therapeutic application in multiple myeloma

The purpose of these studies was to identify HLA-A2⁺ immunogenic peptides derived from XBP1 antigens to induce a multiple myeloma (MM)-specific immune response. Six native peptides from non-spliced XBP1 antigen and three native peptides from spliced XBP1 antigen were selected and evaluated for their HLA-A2 specificity. Among them, XBP1_184–192, XBP1 SP_196–204 and XBP1 SP_367–375 peptides showed the highest level of binding affinity, but not stability to HLA-A2 molecules. Novel heteroclitic XBP1 peptides, YISPWILAV or YLFPQLISV, demonstrated a significant improvement in HLA-A2 stability from their native XBP1_184–192 or XBP1 SP_367–375 peptide, respectively. Cytotoxic T lymphocytes generated by repeated stimulation of CD3⁺ T cells with each HLA-A2-specific heteroclitic peptide showed an increased percentage of CD8⁺ (cytotoxic) and CD69⁺/CD45RO⁺ (activated memory) T cells and a lower percentage of CD4⁺ (helper) and CD45RA⁺/CCR7⁺ (naïve) T cells, which were distinct from the control T cells. Functionally, the CTLs demonstrated MM-specific and HLA-A2-restricted proliferation, IFN-γ secretion and cytotoxic acivity in response to MM cell lines and importantly, cytotoxicty against primary MM cells. These data demonstrate the distinct immunogenic characteristics of unique heteroclitic XBP1 peptides which induce MM-specific CTLs and highlights their potential application for immunotherapy to treat the patients with MM or its pre-malignant condition.

Inflammation and cellular stress: a mechanistic link between immune-mediated and metabolically driven pathologies

BACKGROUND

Multiple cellular stress responses have been implicated in chronic diseases such as obesity, diabetes, cardiovascular, and inflammatory bowel diseases. Even though phenotypically different, chronic diseases share cellular stress signaling pathways, in particular endoplasmic reticulum (ER) unfolded protein response (UPR).

RESULTS AND METHODS

The purpose of the ER UPR is to restore ER homeostasis after challenges of the ER function. Among the triggers of ER UPR are changes in the redox status, elevated protein synthesis, accumulation of unfolded or misfolded proteins, energy deficiency and glucose deprivation, cholesterol depletion, and microbial signals. Numerous mouse models have been used to characterize the contribution of ER UPR to several pathologies, and ER UPR-associated signaling has also been demonstrated to be relevant in humans. Additionally, recent evidence suggests that the ER UPR is interrelated with metabolic and inflammatory pathways, autophagy, apoptosis, and mitochondrial stress signaling. Furthermore, microbial as well as nutrient sensing is integrated into the ER-associated signaling network.

CONCLUSION

The data discussed in the present review highlight the interaction of ER UPR with inflammatory pathways, metabolic processes and mitochondrial function, and their interrelation in the context of chronic diseases.

Innate antimicrobial immunity in inflammatory bowel diseases

Inflammatory bowel diseases are characterized by chronic intestinal inflammation at different sites. Data from animal models as well as human patients including gene-association studies suggest that different components of the innate barrier function are primarily defective. These recent advances support the evolving hypothesis that intestinal bacteria induce inflammation predominantly as a result of a weakened innate mucosal barrier in genetically predisposed individuals. This article discusses our current understanding of the primary events of disease. Together, these findings should result in new therapeutic avenues aimed at restoring antimicrobial barrier function to prevent a bacterial-triggered inflammatory response.

Signalling to and from the secretory pathway

For growth, survival, communication and homeostasis, cells transport a large number of proteins to the plasma membrane and the extracellular medium by using the secretory pathway. Consequently, to adapt to the surrounding environment and the different intracellular contexts, the secretory pathway needs to accommodate and respond to a plethora of endogenous and exogenous stimuli. It is now well established that several kinases, known to be activated by environmental stimuli, signal from the plasma membrane to the secretory pathway in order to remodel its architecture and modulate the cellular secretion capacity. By contrast, membranes of the early secretory pathway, similar to the endosomal system, can also initiate and modulate signalling cascades, thereby spatially organising cellular signalling and eliciting a different cellular outcome than when signalling is localised to the plasma membrane. This Commentary highlights recent contributions to our understanding of the mutual regulation of the secretory pathway and cellular signalling.

The IRE1α-XBP1 pathway is essential for osteoblast differentiation through promoting transcription of Osterix

During skeletal development, osteoblasts produce large amounts of extracellular matrix proteins and must therefore increase their secretory machinery to handle the deposition. The accumulation of unfolded protein in the endoplasmic reticulum induces an adoptive mechanism called the unfolded protein response (UPR). We show that one of the most crucial UPR mediators, inositol-requiring protein 1α (IRE1α), and its target transcription factor X-box binding protein 1 (XBP1), are essential for bone morphogenic protein 2-induced osteoblast differentiation. Furthermore, we identify Osterix (Osx, a transcription factor that is indispensible for bone formation) as a target gene of XBP1. The promoter region of the Osx gene encodes two potential binding motifs for XBP1, and we show that XBP1 binds to these regions. Thus, the IRE1α-XBP1 pathway is involved in osteoblast differentiation through promoting Osx transcription.

The ER stress factor XBP1s prevents amyloid-beta neurotoxicity

Alzheimer's disease (AD) is an incurable neurodegenerative disorder clinically characterized by progressive cognitive impairment. A prominent pathologic hallmark in the AD brain is the abnormal accumulation of the amyloid-β 1-42 peptide (Aβ), but the exact pathways mediating Aβ neurotoxicity remain enigmatic. Endoplasmic reticulum (ER) stress is induced during AD, and has been indirectly implicated as a mediator of Aβ neurotoxicity. We report here that Aβ activates the ER stress response factor X-box binding protein 1 (XBP1) in transgenic flies and in mammalian cultured neurons, yielding its active form, the transcription factor XBP1s. XBP1s shows neuroprotective activity in two different AD models, flies expressing Aβ and mammalian cultured neurons treated with Aβ oligomers. Trying to identify the mechanisms mediating XBP1s neuroprotection, we found that in PC12 cells treated with Aβ oligomers, XBP1s prevents the accumulation of free calcium (Ca(2+)) in the cytosol. This protective activity can be mediated by the downregulation of a specific isoform of the ryanodine Ca(2+) channel, RyR3. In support of this observation, a mutation in the only ryanodine receptor (RyR) in flies also suppresses Aβ neurotoxicity, indicating the conserved mechanisms between the two AD models. These results underscore the functional relevance of XBP1s in Aβ toxicity, and uncover the potential of XBP1 and RyR as targets for AD therapeutics.

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.

The endoplasmic reticulum stress response and diabetic kidney disease

The endoplasmic reticulum (ER) folds and modifies proteins; however, during conditions of cellular stress, unfolded proteins accumulate in the ER and activate the unfolded protein response (UPR). The UPR, also referred to as the ER stress response, activates three distinct signaling cascades that are designed to globally reduce transcription and translation. The three major arms of the mammalian UPR include 1) protein kinase RNA (PKR)-like ER kinase (PERK), 2) inositol-requiring protein-1 (IRE1α), and 3) activating transcription factor-6 (ATF6) pathways. The PERK pathway rapidly attenuates protein translation, whereas the ATF6 and IRE1α cascades transcriptionally upregulate ER chaperone genes that promote proper folding and ER-associated degradation (ERAD) of proteins. This integrated response in turn allows the folding machinery of the ER to catch up with the backlog of unfolded proteins. The ER stress response plays a role in a number of pathophysiological processes, including pancreatic β-cell failure and apoptosis. The goals of the current review are to familiarize investigators with cellular and tissue activation of this response in the rodent and human diabetic kidney. Additionally, we will review therapeutic modulators of the ER stress response and discuss their efficacy in models of diabetic kidney disease. The ER stress response has both protective and deleterious features. A better understanding of the molecular pathways regulated during this process in a cell- and disease-specific manner could reveal novel therapeutic strategies in chronic renal diseases, including diabetic kidney disease.

Transcription factor MIST1 in terminal differentiation of mouse and human plasma cells

Despite their divergent developmental ancestry, plasma cells and gastric zymogenic (chief) cells share a common function: high-capacity secretion of protein. Here we show that both cell lineages share increased expression of a cassette of 269 genes, most of which regulate endoplasmic reticulum (ER) and Golgi function, and they both induce expression of the transcription factors X-box binding protein 1 (Xbp1) and Mist1 during terminal differentiation. XBP1 is known to augment plasma cell function by establishing rough ER, and MIST1 regulates secretory vesicle trafficking in zymogenic cells. We examined morphology and function of plasma cells in wild-type and Mist1(-/-) mice and found subtle differences in ER structure but no overall defect in plasma cell function, suggesting that Mist1 may function redundantly in plasma cells. We next reasoned that MIST1 might be useful as a novel and reliable marker of plasma cells. We found that MIST1 specifically labeled normal plasma cells in mouse and human tissues, and, moreover, its expression was also characteristic of plasma cell differentiation in a cohort of 12 human plasma cell neoplasms. Overall, our results show that MIST1 is enriched upon plasma cell differentiation as a part of a genetic program facilitating secretory cell function and also that MIST1 is a novel marker of normal and neoplastic plasma cells in mouse and human tissues.

Anti-apoptotic function of Xbp1 as an IL-3 signaling molecule in hematopoietic cells

Cytokine signaling is critical for proliferation, survival and differentiation of hematopoietic cell, and interleukin-3 (IL-3) is required for maintenance of many hematopoietic cell lines, such as BaF3. We have isolated apoptosis-resistant clones of BaF3 using retroviral insertional mutagenesis and the Xbp1 locus was identified as a retroviral integration site. Expression and splicing of the Xbp1 transcript was conserved in the resistant clone but was promptly disappeared on IL-3 withdrawal in parental BaF3. IL-3 stimulation of BaF3 cells enhanced Xbp1 promoter activity and induced phosphorylation of the endoplasmic reticulum stress sensor protein IRE1, resulting in the increase in Xbp1S that activates unfolded protein response. When downstream signaling from IL-3 was blocked by LY294002 and/or dn-Stat5, Xbp1 expression was downregulated and IRE1 phosphorylation was suppressed. Inhibition of IL-3 signaling as well as knockdown of Xbp1-induced apoptosis in BaF3 cells. In contrast, constitutive expression of Xbp1S protected BaF3 from apoptosis during IL-3 depletion. However, cell cycle arrest at the G1 stage was observed in BaF3 and myeloid differentiation was induced in IL-3-dependent 32Dcl3 cells. Expression of apoptosis-, cell cycle- and differentiation-related genes was modulated by Xbp1S expression. These results indicate that the proper transcriptional and splicing regulation of Xbp1 by IL-3 signaling is important in homeostasis of hematopoietic cells.

The unfolded protein response mediates adaptation to exercise in skeletal muscle through a PGC-1α/ATF6α complex

Exercise has been shown to be effective for treating obesity and type 2 diabetes. However, the molecular mechanisms for adaptation to exercise training are not fully understood. Endoplasmic reticulum (ER) stress has been linked to metabolic dysfunction. Here we show that the unfolded protein response (UPR), an adaptive response pathway that maintains ER homeostasis upon luminal stress, is activated in skeletal muscle during exercise and adapts skeletal muscle to exercise training. The transcriptional coactivator PGC-1α, which regulates several exercise-associated aspects of skeletal muscle function, mediates the UPR in myotubes and skeletal muscle through coactivation of ATF6α. Efficient recovery from acute exercise is compromised in ATF6α(-/-) mice. Blocking ER-stress-related cell death via deletion of CHOP partially rescues the exercise intolerance phenotype in muscle-specific PGC-1α KO mice. These findings suggest that modulation of the UPR through PGC1α represents an alternative avenue to improve skeletal muscle function and achieve metabolic benefits.

Proteasome inhibition represses unfolded protein response and Nox4, sensitizing vascular cells to endoplasmic reticulum stress-induced death

Background

Endoplasmic reticulum (ER) stress has pathophysiological relevance in vascular diseases and merges with proteasome function. Proteasome inhibition induces cell stress and may have therapeutic implications. However, whether proteasome inhibition potentiates ER stress-induced apoptosis and the possible mechanisms involved in this process are unclear.

Methodology/Principal Findings

Here we show that proteasome inhibition with MG132, per se at non-lethal levels, sensitized vascular smooth muscle cells to caspase-3 activation and cell death during ER stress induced by tunicamycin (Tn). This effect was accompanied by suppression of both proadaptive (KDEL chaperones) and proapoptotic (CHOP/GADD153) unfolded protein response markers, although, intriguingly, the splicing of XBP1 was markedly enhanced and sustained. In parallel, proteasome inhibition completely prevented ER stress-induced increase in NADPH oxidase activity, as well as increases in Nox4 isoform and protein disulfide isomerase mRNA expression. Increased Akt phosphorylation due to proteasome inhibition partially offset the proapoptotic effect of Tn or MG132. Although proteasome inhibition enhanced oxidative stress, reactive oxygen species scavenging had no net effect on sensitization to Tn or MG132-induced cell death.

Conclusion/Relevance

These data indicate unfolded protein response-independent pathways whereby proteasome inhibition sensitizes vascular smooth muscle to ER stress-mediated cell death. This may be relevant to understand the therapeutic potential of such compounds in vascular disease associated with increased neointimal hyperplasia.

XBP1 controls maturation of gastric zymogenic cells by induction of MIST1 and expansion of the rough endoplasmic reticulum

BACKGROUND & AIMS

The transition of gastric epithelial mucous neck cells (NCs) to digestive enzyme-secreting zymogenic cells (ZCs) involves an increase in rough endoplasmic reticulum (ER) and formation of many large secretory vesicles. The transcription factor MIST1 is required for granulogenesis of ZCs. The transcription factor XBP1 binds the Mist1 promoter and induces its expression in vitro and expands the ER in other cell types. We investigated whether XBP1 activates Mist1 to regulate ZC differentiation.

METHODS

Xbp1 was inducibly deleted in mice using a tamoxifen/Cre-loxP system; effects on ZC size and structure (ER and granule formation) and gastric differentiation were studied and quantified for up to 13 months after deletion using morphologic, immunofluorescence, quantitative reverse-transcriptase polymerase chain reaction, and immunoblot analyses. Interactions between XBP1 and the Mist1 promoter were studied by chromatin immunoprecipitation from mouse stomach and in XBP1-transfected gastric cell lines.

RESULTS

Tamoxifen-induced deletion of Xbp1 (Xbp1Δ) did not affect survival of ZCs but prevented formation of their structure. Xbp1Δ ZCs shrank 4-fold, compared with those of wild-type mice, with granulogenesis and cell shape abnormalities and disrupted rough ER. XBP1 was required and sufficient for transcriptional activation of MIST1. ZCs that developed in the absence of XBP1 induced ZC markers (intrinsic factor, pepsinogen C) but showed abnormal retention of progenitor NC markers.

CONCLUSIONS

XBP1 controls the transcriptional regulation of ZC structural development; it expands the lamellar rough ER and induces MIST1 expression to regulate formation of large granules. XBP1 is also required for loss of mucous NC markers as ZCs form.

Different early ER-stress responses in the CLN8(mnd) mouse model of neuronal ceroid lipofuscinosis

Neuronal ceroid lipofuscinoses (NCLs) are a group of inherited neurodegenerative disorders characterized by epilepsy, progressive motor and cognitive decline, blindness, and by the accumulation of autofluorescent lipopigment. Late-infantile onset forms (LINCL) include those linked to mutations in CLN8 gene, encoding a transmembrane protein at the endoplasmic reticulum (ER). In the motor neuron degeneration (mnd) mouse model of the CLN8-LINCL (CLN8(mnd)), we carried out an analysis of ER stress-related molecules in CNS structures that exhibit a variable rate of disease progression (early retinal degeneration and delayed brain and motoneuron dysfunction). At the presymptomatic state of 1-month-old CLN8(mnd) mice, we found an upregulation of GRP78 and activation of the transcription factor-6 (ATF6) in all structures examined, an activation of a CHOP-dependent pathway in the cerebellum, hippocampus and retina, a caspase-12-dependent pathway in the retina and no activation of these two pathways in the cerebral cortex and spinal cord. An increased CHOP expression was detected in the cortex and spinal cord at the early symptomatic state (4 months). Caspase-3 cleavage occurred presymptomatically in the cerebellum, hippocampus and retina, and symptomatically in the cerebral cortex and spinal cord. We also monitored activation of NF-κB, which is engaged in the alarming phase of ER stress, together with increased levels of TRAF2, TNF-α and TNFR1, and no activation of ASK-1/JNK signalling pathway, all over mnd structures. The results suggest that early ER-stress responses distinctly combined and ER-stress pathways integrated with inflammatory responses may contribute to the progression of the CLN8(mnd) disease in CNS structures.

The endoplasmic reticulum stress response in the pancreatic β-cell

Eukaryotic cells respond to stress in the endoplasmic reticulum (ER) resulting from insufficient protein folding capacity or altered ER homeostasis by activating the unfolded protein response (UPR). In mammalian cells the UPR is mediated by at least three ER-localized sensors/transducers, and the cellular response and susceptibility to ER stress is likely to be cell-type specific to some degree. Here, we review the response of pancreatic β-cells or islets to ER stress induced by pharmacological agents, misfolded insulin expression, excessive nutrient exposure and in animal models of type 2 diabetes. This review highlights the particular importance of PERK-mediated translational control and the transcriptional response in pancreatic β-cells and how these relate to the highly specialized function of β-cells, namely glucose-regulated insulin secretion and production. We examine how chronic ER stress may prematurely 'age' the β-cell or cause its genetic reprogramming to either reduce its ability to mount a cell survival response to ER stress, or impair normal function. Both could contribute to β-cell failure in diabetes. We also explore the therapeutic potential of targeting the UPR to preserve β-cell function.

High levels of Hsp90 cochaperone p23 promote tumor progression and poor prognosis in breast cancer by increasing lymph node metastases and drug resistance

p23 is a heat shock protein 90 (Hsp90) cochaperone located in both the cytoplasm and nucleus that stabilizes unliganded steroid receptors, controls the catalytic activity of certain kinases, regulates protein-DNA dynamics, and is upregulated in several cancers. We had previously shown that p23-overexpressing MCF-7 cells (MCF-7+p23) exhibit increased invasion without affecting the estrogen-dependent proliferative response, which suggests that p23 differentially regulates genes controlling processes linked to breast tumor metastasis. To gain a comprehensive view of the effects of p23 on estrogen receptor (ER)-dependent and -independent gene expression, we profiled mRNA expression from control versus MCF-7+p23 cells in the absence and presence of estrogen. A number of p23-sensitive target genes involved in metastasis and drug resistance were identified. Most striking is that many of these genes are also misregulated in invasive breast cancers, including PMP22, ABCC3, AGR2, Sox3, TM4SF1, and p8 (NUPR1). Upregulation of the ATP-dependent transporter ABCC3 by p23 conferred resistance to the chemotherapeutic agents etoposide and doxorubicin in MCF-7+p23 cells. MCF-7+p23 cells also displayed higher levels of activated Akt and an expanded phosphoproteome relative to control cells, suggesting that elevated p23 also enhances cytoplasmic signaling pathways. For breast cancer patients, tumor stage together with high cytoplasmic p23 expression more accurately predicted disease recurrence and mortality than did stage alone. High nuclear p23 was found to be associated with high cytoplasmic p23, therefore both may promote tumor progression and poor prognosis by increasing metastatic potential and drug resistance in breast cancer patients.

The unfolded protein response is not necessary for the G1/S transition, but it is required for chromosome maintenance in Saccharomyces cerevisiae

BACKGROUND

The unfolded protein response (UPR) is a eukaryotic signaling pathway, from the endoplasmic reticulum (ER) to the nucleus. Protein misfolding in the ER triggers the UPR. Accumulating evidence links the UPR in diverse aspects of cellular homeostasis. The UPR responds to the overall protein synthesis capacity and metabolic fluxes of the cell. Because the coupling of metabolism with cell division governs when cells start dividing, here we examined the role of UPR signaling in the timing of initiation of cell division and cell cycle progression, in the yeast Saccharomyces cerevisiae.

METHODOLOGY/PRINCIPAL FINDINGS

We report that cells lacking the ER-resident stress sensor Ire1p, which cannot trigger the UPR, nonetheless completed the G1/S transition on time. Furthermore, loss of UPR signaling neither affected the nutrient and growth rate dependence of the G1/S transition, nor the metabolic oscillations that yeast cells display in defined steady-state conditions. Remarkably, however, loss of UPR signaling led to hypersensitivity to genotoxic stress and a ten-fold increase in chromosome loss.

CONCLUSIONS/SIGNIFICANCE

Taken together, our results strongly suggest that UPR signaling is not necessary for the normal coupling of metabolism with cell division, but it has a role in genome maintenance. These results add to previous work that linked the UPR with cytokinesis in yeast. UPR signaling is conserved in all eukaryotes, and it malfunctions in a variety of diseases, including cancer. Therefore, our findings may be relevant to other systems, including humans.

Mammalian endoplasmic reticulum stress sensor IRE1 signals by dynamic clustering

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

An extended set of PRDM1/BLIMP1 target genes links binding motif type to dynamic repression

The transcriptional repressor B lymphocyte-induced maturation protein-1 (BLIMP1) regulates gene expression and cell fate. The DNA motif bound by BLIMP1 in vitro overlaps with that of interferon regulatory factors (IRFs), which respond to inflammatory/immune signals. At such sites, BLIMP1 and IRFs can antagonistically regulate promoter activity. In vitro motif selection predicts that only a subset of BLIMP1 or IRF sites is subject to antagonistic regulation, but the extent to which antagonism occurs is unknown, since an unbiased assessment of BLIMP1 occupancy in vivo is lacking. To address this, we identified an extended set of promoters occupied by BLIMP1. Motif discovery and enrichment analysis demonstrate that multiple motif variants are required to capture BLIMP1 binding specificity. These are differentially associated with CpG content, leading to the observation that BLIMP1 DNA-binding is methylation sensitive. In occupied promoters, only a subset of BLIMP1 motifs overlap with IRF motifs. Conversely, a distinct subset of IRF motifs is not enriched amongst occupied promoters. Genes linked to occupied promoters containing overlapping BLIMP1/IRF motifs (e.g. AIM2, SP110, BTN3A3) are shown to constitute a dynamic target set which is preferentially activated by BLIMP1 knock-down. These data confirm and extend the competitive model of BLIMP1 and IRF interaction.

Transcriptional regulation by CHIP/LDB complexes

It is increasingly clear that transcription factors play versatile roles in turning genes "on" or "off" depending on cellular context via the various transcription complexes they form. This poses a major challenge in unraveling combinatorial transcription complex codes. Here we use the powerful genetics of Drosophila combined with microarray and bioinformatics analyses to tackle this challenge. The nuclear adaptor CHIP/LDB is a major developmental regulator capable of forming tissue-specific transcription complexes with various types of transcription factors and cofactors, making it a valuable model to study the intricacies of gene regulation. To date only few CHIP/LDB complexes target genes have been identified, and possible tissue-dependent crosstalk between these complexes has not been rigorously explored. SSDP proteins protect CHIP/LDB complexes from proteasome dependent degradation and are rate-limiting cofactors for these complexes. By using mutations in SSDP, we identified 189 down-stream targets of CHIP/LDB and show that these genes are enriched for the binding sites of APTEROUS (AP) and PANNIER (PNR), two well studied transcription factors associated with CHIP/LDB complexes. We performed extensive genetic screens and identified target genes that genetically interact with components of CHIP/LDB complexes in directing the development of the wings (28 genes) and thoracic bristles (23 genes). Moreover, by in vivo RNAi silencing we uncovered novel roles for two of the target genes, xbp1 and Gs-alpha, in early development of these structures. Taken together, our results suggest that loss of SSDP disrupts the normal balance between the CHIP-AP and the CHIP-PNR transcription complexes, resulting in down-regulation of CHIP-AP target genes and the concomitant up-regulation of CHIP-PNR target genes. Understanding the combinatorial nature of transcription complexes as presented here is crucial to the study of transcription regulation of gene batteries required for development.

Inositol-requiring enzyme 1alpha is a key regulator of angiogenesis and invasion in malignant glioma

Inositol-requiring enzyme 1 (IRE1) is a proximal endoplasmic reticulum (ER) stress sensor and a central mediator of the unfolded protein response. In a human glioma model, inhibition of IRE1alpha correlated with down-regulation of prevalent proangiogenic factors such as VEGF-A, IL-1beta, IL-6, and IL-8. Significant up-regulation of antiangiogenic gene transcripts was also apparent. These transcripts encode SPARC, decorin, thrombospondin-1, and other matrix proteins functionally linked to mesenchymal differentiation and glioma invasiveness. In vivo, using both the chick chorio-allantoic membrane assay and a mouse orthotopic brain model, we observed in tumors underexpressing IRE1: (i) reduction of angiogenesis and blood perfusion, (ii) a decreased growth rate, and (iii) extensive invasiveness and blood vessel cooption. This phenotypic change was consistently associated with increased overall survival in glioma-implanted recipient mice. Ectopic expression of IL-6 in IRE1-deficient tumors restored angiogenesis and neutralized vessel cooption but did not reverse the mesenchymal/infiltrative cell phenotype. The ischemia-responsive IRE1 protein is thus identified as a key regulator of tumor neovascularization and invasiveness.

Regulation of basal cellular physiology by the homeostatic unfolded protein response

The extensive membrane network of the endoplasmic reticulum (ER) is physically juxtaposed to and functionally entwined with essentially all other cellular compartments. Therefore, the ER must sense diverse and constantly changing physiological inputs so it can adjust its numerous functions to maintain cellular homeostasis. A growing body of new work suggests that the unfolded protein response (UPR), traditionally charged with signaling protein misfolding stress from the ER, has been co-opted for the maintenance of basal cellular homeostasis. Thus, the UPR can be activated, and its output modulated, by signals far outside the realm of protein misfolding. These findings are revealing that the UPR causally contributes to disease not just by its role in protein folding but also through its broad influence on cellular physiology.

Temporal regulation of Cat-1 (cationic amino acid transporter-1) gene transcription during endoplasmic reticulum stress

Expression of the Cat-1 gene (cationic amino acid transporter-1) is induced in proliferating cells and in response to a variety of stress conditions. The expression of the gene is mediated via a TATA-less promoter. In the present study we show that an Sp1 (specificity protein 1)-binding site within a GC-rich region of the Cat-1 gene controls its basal expression and is important for induction of the gene during the UPR (unfolded protein response). We have shown previously that induction of Cat-1 gene expression during the UPR requires phosphorylation of the translation initiation factor eIF2alpha (eukaryotic initiation factor 2alpha) by PERK (protein-kinase-receptor-like endoplasmic reticulum kinase), one of the signalling pathways activated during the UPR. This leads to increased translation of the transcription factor ATF4 (activating transcription factor 4). We also show that a second signalling pathway is required for sustained transcriptional induction of the Cat-1 gene during the UPR, namely activation of IRE1 (inositol-requiring enzyme 1) leading to alternative splicing of the mRNA for the transcription factor XBP1 (X-box-binding protein 1). The resulting XBP1s (spliced XBP1) can bind to an ERSE (endoplasmic-reticulum-stress-response-element), ERSE-II-like, that was identified within the Cat-1 promoter. Surprisingly, eIF2alpha phosphorylation is required for accumulation of XBP1s. We propose that the signalling via phosphorylated eIF2alpha is required for maximum induction of Cat-1 transcription during the UPR by inducing the accumulation of both ATF4 and XBP1s.

Airborne particulate matter selectively activates endoplasmic reticulum stress response in the lung and liver tissues

Recent studies have suggested a link between inhaled particulate matter (PM) exposure and increased mortality and morbidity associated with pulmonary and cardiovascular diseases. However, a precise understanding of the biological mechanism underlying PM-associated toxicity and pathogenesis remains elusive. Here, we investigated the impact of PM exposure in intracellular stress signaling pathways with animal models and cultured cells. Inhalation exposure of the mice to environmentally relevant fine particulate matter (aerodynamic diameter < 2.5 μm, PM(2.5)) induces endoplasmic reticulum (ER) stress and activation of unfolded protein response (UPR) in the lung and liver tissues as well as in the mouse macrophage cell line RAW264.7. Ambient PM(2.5) exposure activates double-strand RNA-activated protein kinase-like ER kinase (PERK), leading to phosphorylation of translation initiation factor eIF2α and induction of C/EBP homologous transcription factor CHOP/GADD153. Activation of PERK-mediated UPR pathway relies on the production of reactive oxygen species (ROS) and is critical for PM(2.5)-induced apoptosis. Furthermore, PM(2.5) exposure can activate ER stress sensor IRE1α, but it decreases the activity of IRE1α in splicing the mRNA encoding the UPR trans-activator X-box binding protein 1 (XBP1). Together, our study suggests that PM(2.5) exposure differentially activates the UPR branches, leading to ER stress-induced apoptosis through the PERK-eIF2α-CHOP UPR branch. This work provides novel insights into the cellular and molecular basis by which ambient PM(2.5) exposure elicits its cytotoxic effects that may be related to air pollution-associated pathogenesis.

Insulin/IGF-1 signaling mutants reprogram ER stress response regulators to promote longevity

When unfolded proteins accumulate in the endoplasmic reticulum (ER), the unfolded protein response is activated. This ER stress response restores ER homeostasis by coordinating processes that decrease translation, degrade misfolded proteins, and increase the levels of ER-resident chaperones. Ribonuclease inositol-requiring protein-1 (IRE-1), an endoribonuclease that mediates unconventional splicing, and its target, the XBP-1 transcription factor, are key mediators of the unfolded protein response. In this study, we show that in Caenorhabditis elegans insulin/IGF-1 pathway mutants, IRE-1 and XBP-1 promote lifespan extension and enhance resistance to ER stress. We show that these effects are not achieved simply by increasing the level of spliced xbp-1 mRNA and expression of XBP-1's normal target genes. Instead, in insulin/IGF-1 pathway mutants, XBP-1 collaborates with DAF-16, a FOXO-transcription factor that is activated in these mutants, to enhance ER stress resistance and to activate new genes that promote longevity.

Rapamycin inhibits postprandial-mediated X-box-binding protein-1 splicing in rat liver

Recent studies have linked the unfolded protein response (UPR), in particular the inositol-requiring, endoplasmic reticulum-to-nucleus signaling protein 1alpha (IRE1alpha)-X-box-binding protein-1 (XBP1) branch of the UPR, to the regulation of lipogenesis and hepatic steatosis. In this study, we examined the hypothesis that the postprandial environment can activate the IRE1alpha-XBP1 branch of the UPR in the liver via a mammalian target of rapamycin complex 1 (mTORC1)-dependent mechanism. Toward this end, rats were fed a high-carbohydrate diet (68% of energy from corn starch) for 3 h in the absence or presence of rapamycin (intraperitoneal injection of 1 mg/kg) and liver tissue was taken 1 or 7 h following the feeding period. Feeding activated the mTORC1 pathway and IRE1alpha, induced XBP1 splicing, and increased the expression of XBP1 target genes and lipogenic genes in the liver. The presence of rapamycin prevented the activation of mTORC1 and IRE1alpha, XBP1 splicing, and the increased expression of XBP1 target genes and lipogenic genes. Rapamycin also prevented the feeding-induced increase in nuclear sterol regulatory element binding protein 1c. These data suggest that the postprandial environment promotes activation of the IRE1-XBP1 branch of the UPR in the liver. This activation appears to be mediated in part by mTORC1.

Deficiency of suppressor enhancer Lin12 1 like (SEL1L) in mice leads to systemic endoplasmic reticulum stress and embryonic lethality

Stress in the endoplasmic reticulum (ER) plays an important causal role in the pathogenesis of several chronic diseases such as Alzheimer, Parkinson, and diabetes mellitus. Insight into the genetic determinants responsible for ER homeostasis will greatly facilitate the development of therapeutic strategies for the treatment of these debilitating diseases. Suppressor enhancer Lin12 1 like (SEL1L) is an ER membrane protein and was thought to be involved in the quality control of secreted proteins. Here we show that the mice homozygous mutant for SEL1L were embryonic lethal. Electron microscopy studies revealed a severely dilated ER in the fetal liver of mutant embryos, indicative of alteration in ER homeostasis. Consistent with this, several ER stress responsive genes were significantly up-regulated in the mutant embryos. Mouse embryonic fibroblast cells deficient in SEL1L exhibited activated unfolded protein response at the basal state, impaired ER-associated protein degradation, and reduced protein secretion. Furthermore, markedly increased apoptosis was observed in the forebrain and dorsal root ganglions of mutant embryos. Taken together, our results demonstrate an essential role for SEL1L in protein quality control during mouse embryonic development.

The unfolded protein response in melanocytes: activation in response to chemical stressors of the endoplasmic reticulum and tyrosinase misfolding

Accumulation of proteins in the endoplasmic reticulum (ER) triggers the unfolded protein response (UPR), comprising three signaling pathways initiated by Ire1, Perk and Atf6 respectively. Unfolded protein response activation was compared in chemically stressed murine wildtype melanocytes and mutant melanocytes that retain tyrosinase in the ER. Thapsigargin, an ER stressor, activated all pathways in wildtype melanocytes, triggering Caspase 12-mediated apoptosis at toxic doses. Albino melanocytes expressing mutant tyrosinase showed evidence of ER stress with increased Ire1 expression, but the downstream effector, Xbp1, was not activated even following thapsigargin treatment. Attenuation of Ire1 signaling was recapitulated in wildtype melanocytes treated with thapsigargin for 8 days, with diminished Xbp1 activation observed after 4 days. Atf6 was also activated in albino melanocytes, with no response to thapsigargin, while the Perk pathway was not activated and thapsigargin treatment elicited robust expression of the downstream effector CCAAT-enhancer-binding protein homologous protein. Thus, melanocytes adapt to ER stress by attenuating two UPR pathways.

Endoplasmic reticulum stress and the inflammatory basis of metabolic disease

The endoplasmic reticulum (ER) is the major site in the cell for protein folding and trafficking and is central to many cellular functions. Failure of the ER's adaptive capacity results in activation of the unfolded protein response (UPR), which intersects with many different inflammatory and stress signaling pathways. These pathways are also critical in chronic metabolic diseases such as obesity, insulin resistance, and type 2 diabetes. The ER and related signaling networks are emerging as a potential site for the intersection of inflammation and metabolic disease.

X-box binding protein 1 regulates brain natriuretic peptide through a novel AP1/CRE-like element in cardiomyocytes

The unfolded protein response (UPR) is triggered to assist protein folding when endoplasmic reticulum (ER) function is impaired. Recent studies demonstrated that ER stress can also induce cell-specific genes. In this study, we examined whether X-box binding protein 1 (XBP1), a major UPR-linked transcriptional factor, regulates the expression of brain natriuretic peptide (BNP) in cardiomyocytes. In samples from failing human hearts, extensive splicing of XBP1 was observed along with increased expression of glucose-regulated protein of 78 kDa (GRP78), a target of spliced XBP1 (sXBP1), suggesting that the UPR was induced in heart failure in humans. Interestingly, quantitative real-time PCR revealed a positive correlation between cardiac expression of GRP78 and BNP, leading us to test the hypothesis that sXBP1 regulates BNP as well as GRP78 in cardiomyocytes. A pharmacological ER stressor caused a dose-dependent increase in the expression of sXBP1 and BNP by cultured cardiomyocytes. Short interfering RNA targeting XBP1 suppressed the induction of BNP expression by a pharmacological ER stressor or norepinephrine, which was rescued by the adenovirus-mediated overexpression of sXBP1. The promoter assay with overexpression of sXBP1 or norepinephrine showed that the proximal AP1/CRE-like element in the promoter region of BNP was critical for transcriptional regulation of BNP by sXBP1. Direct binding of sXBP1 to this element was confirmed by the chromatin immunoprecipitation assay. These findings suggest that ER stress observed in failing hearts regulates cardiac BNP expression through a novel promoter region of the AP1/CRE-like element.

Role of autophagy and autophagy genes in inflammatory bowel disease

Polymorphisms associated with two genes in the autophagy pathway, ATG16L1 and IRGM1, have been implicated in susceptibility to Crohn's disease, an idiopathic inflammatory disease typically involving the gastrointestinal tract. The intestinal mucosa is a site of careful immune regulation where the epithelium and immune cells encounter pathogens as well as a robust and diverse population of indigenous microbes that are predominately bacteria. Since the role of autophagy in immunity is broad and expanding, it is unclear which downstream functions of autophagy and which cell types are the key factors in Crohn's disease susceptibility. This chapter reviews the recent literature on the roles of ATG16L1 and IRGM1 in the autophagy pathway, inflammation, antimicrobial immunity, and the biology of the intestine, and discusses how these genes may contribute to Crohn's disease pathogenesis.

Linking ER Stress to Autophagy: Potential Implications for Cancer Therapy

Different physiological and pathological conditions can perturb protein folding in the endoplasmic reticulum, leading to a condition known as ER stress. ER stress activates a complex intracellular signal transduction pathway, called unfolded protein response (UPR). The UPR is tailored essentially to reestablish ER homeostasis also through adaptive mechanisms involving the stimulation of autophagy. However, when persistent, ER stress can switch the cytoprotective functions of UPR and autophagy into cell death promoting mechanisms. Recently, a variety of anticancer therapies have been linked to the induction of ER stress in cancer cells, suggesting that strategies devised to stimulate its prodeath function or block its prosurvival function, could be envisaged to improve their tumoricidial action. A better understanding of the molecular mechanisms that determine the final outcome of UPR and autophagy activation by chemotherapeutic agents, will offer new opportunities to improve existing cancer therapies as well as unravel novel targets for cancer treatment.

XBP1: the last two decades

The transcription factor X box-binding protein 1 (XBP1) was isolated two decades ago in a search for regulators of major histocompatibility complex (MHC) class II gene expression. Many years of research finally revealed a protein with many functions, none of them related to the MHC. This paper provided an overview of what this multifunctional transcription factor actually does.