Quantitative measurement of events in the mammalian unfolded protein response

Evaluation of aminopyrrolidine amide to improve chloride transport in CFTR-defective cells

The aminopyrrolidine amide PF-429242 is a specific inhibitor of the Site-1 Protease which is responsible for the cleavage, and thus the activation of the Activating Transcription Factor6 that down regulates many genes, during the Unfolded Protein Response. We hypothesized that PF-429242 could be used to prevent the ATF6-dependent down regulation of some genes. We chose the CFTR gene encoding the CFTR chloride channel as a model because it is down-regulated by ATF6 in Cystic Fibrosis. We evaluated the action of PF-429242 in human bronchial cells expressing the most frequent mutation of CFTR (p.Phe508del) found in patients. We observed that PF-429242 increases the synthesis of the mRNA and the protein encoded by the CFTR gene harbouring the mutation. We also observed that PF-429242 alleviates the defects of the p.Phe508del-CFTR channel in human Cystic Fibrosis cells. Our results suggest that aminopyrrolidine amide is a potential therapeutic target for Cystic Fibrosis that could also have beneficial effects in other diseases involving CFTR, such as the Chronic Obstructive Pulmonary Disease.

Impact of polymer-modified gold nanoparticles on brain endothelial cells: exclusion of endoplasmic reticulum stress as a potential risk factor

A library of polymer-coated gold nanoparticles (AuNPs) differing in size and surface modifications was examined for uptake and induction of cellular stress responses in the endoplasmic reticulum (ER stress) in human brain endothelial cells (hCMEC/D3). ER stress is known to affect the physiology of endothelial cells (ECs) and may lead to inflammation or apoptosis. Thus, even if applied at non-cytotoxic concentrations ER stress caused by nanoparticles should be prevented to reduce the risk of vascular diseases and negative effects on the integrity of barriers (e.g. blood-brain barrier). We exposed hCMEC/D3 to twelve different AuNPs (three sizes: 18, 35, and 65 nm, each with four surface-modifications) for various times and evaluated their effects on cytotoxicity, proinflammatory mediators, barrier functions and factors involved in ER stress. We demonstrated a time-dependent uptake of all AuNPs and no cytotoxicity for up to 72 h of exposure. Exposure to certain AuNPs resulted in a time-dependent increase in the proinflammatory markers IL-8, MCP-1, sVCAM, sICAM. However, none of the AuNPs induced an increase in expression of the chaperones and stress sensor proteins BiP and GRP94, respectively, or the transcription factors ATF4 and ATF6. Furthermore, no upregulation of the UPR stress sensor receptor PERK, no active splicing product of the transcription factor XBP1 and no upregulation of the transcription factor CHOP were detectable. In conclusion, the results of the present study indicate that effects of different-sized gold nanoparticles modified with various polymers were not related to the induction of ER stress in brain microvascular endothelial cells or led to apoptosis.

Anti-diabetic effect of 3-hydroxy-2-naphthoic acid, an endoplasmic reticulum stress-reducing chemical chaperone

Lots of experimental and clinical evidences indicate that chronic exposure to saturated fatty acids and high level of glucose is implicated in insulin resistance, beta cell failure and ultimately type 2 diabetes. In this study, we set up cell-based experimental conditions to induce endoplasmic reticulum (ER) stress and insulin resistance using high concentration of palmitate (PA). Hydroxynaphthoic acids (HNAs) were formerly identified as novel chemical chaperones to resolve ER stress induced by tunicamycin. In this study, we found the compounds have the same suppressive effect on PA-induced ER stress in HepG2 cells. The representing compound, 3-HNA reduced PA-induced phosphorylation of JNK, IKKβ and IRS1 (S307) and restored insulin signaling cascade which involves insulin receptor β, IRS1 and Akt. The insulin sensitizing effect of 3-HNA was confirmed in 3T3-L1 adipocytes, where the compound augmented insulin signaling and glucose transporter 4 (GLUT4) membrane translocation. 3-HNA also protected the pancreatic beta cells from PA-induced apoptosis by reducing ER stress. Upon 3-HNA treatment to ob/ob mice at 150mg/kg/day dosage, the diabetic parameters including glucose tolerance and systemic insulin sensitivity were significantly improved. Postmortem examination showed that 3-HNA markedly reduced ER stress and insulin resistance in the liver tissues and it sensitized insulin signaling in the liver and the skeletal muscle. Our results demonstrated that 3-HNA can sensitize insulin signaling by coping with lipotoxicity-induced ER stress as a chemical chaperone and suggested it holds therapeutic potential for insulin resistance and type 2 diabetes.

Low intensity focused ultrasound (LOFU) modulates unfolded protein response and sensitizes prostate cancer to 17AAG

The hypoxic tumor microenvironment generates oxidative Endoplasmic Reticulum (ER) stress, resulting in protein misfolding and unfolded protein response (UPR). UPR induces several molecular chaperones including heat-shock protein 90 (HSP90), which corrects protein misfolding and improves survival of cancer cells and resistance to tumoricidal therapy although prolonged activation of UPR induces cell death. The HSP90 inhibitor, 17AAG, has shown promise against various solid tumors, including prostate cancer (PC). However, therapeutic doses of 17AAG elicit systemic toxicity. In this manuscript, we describe a new paradigm where the combination therapy of a non-ablative and non-invasive low energy focused ultrasound (LOFU) and a non-toxic, low dose 17AAG causes synthetic lethality and significant tumoricidal effects in mouse and human PC xenografts. LOFU induces ER stress and UPR in tumor cells without inducing cell death. Treatment with a non-toxic dose of 17AAG further increased ER stress in LOFU treated PC and switch UPR from a cytoprotective to an apoptotic response in tumors resulting significant induction of apoptosis and tumor growth retardation. These observations suggest that LOFU-induced ER stress makes the ultrasound-treated tumors more susceptible to chemotherapeutic agents, such as 17AAG. Thus, a novel therapy of LOFU-induced chemosensitization may be designed for locally advanced and recurrent tumors.

Approaches to imaging unfolded secretory protein stress in living cells

The endoplasmic reticulum (ER) is the point of entry of proteins into the secretory pathway. Nascent peptides interact with the ER quality control machinery that ensures correct folding of the nascent proteins. Failure to properly fold proteins can lead to loss of protein function and cytotoxic aggregation of misfolded proteins that can lead to cell death. To cope with increases in the ER unfolded secretory protein burden, cells have evolved the Unfolded Protein Response (UPR). The UPR is the primary signaling pathway that monitors the state of the ER folding environment. When the unfolded protein burden overwhelms the capacity of the ER quality control machinery, a state termed ER stress, sensor proteins detect accumulation of misfolded peptides and trigger the UPR transcriptional response. The UPR, which is conserved from yeast to mammals, consists of an ensemble of complex signaling pathways that aims at adapting the ER to the new misfolded protein load. To determine how different factors impact the ER folding environment, various tools and assays have been developed. In this review, we discuss recent advances in live cell imaging reporters and model systems that enable researchers to monitor changes in the unfolded secretory protein burden and activation of the UPR and its associated signaling pathways.

Dengue 2 infection of HepG2 liver cells results in endoplasmic reticulum stress and induction of multiple pathways of cell death

BACKGROUND

A number of studies have implicated the direct involvement of the liver in dengue virus (DENV) infection, and it has been widely shown that liver cells subsequently undergo apoptosis. The mechanism by which liver cells undergo apoptosis in response to DENV infection remains unclear. To provide further information on the mechanism of apoptosis in DENV infected liver cells, HepG2 cells were infected with DENV 2 and analyzed for the induction of ER stress, apoptosis and autophagy.

RESULTS

In response to DENV infection, HepG2 cells showed the induction of both the ER resident unfolded protein response as well as the Noxa/PUMA stress response pathways. Proteolytic activation of caspases 4, 7, 8 and 9 was observed as well as changes in mitochondrial transmembrane potential. Increased monodansylcadaverine staining was observed in DENV infected cells, consistent with the previously reported induction of autophagy.

CONCLUSIONS

These results are consistent with a model in which the induction of multiple ER stress pathways is coupled with the induction of multiple cell death pathways as a mechanism to ensure the removal of infected liver cells from the system.

[Alveolar epithelial cell injury as an etiopathogenic factor in pulmonary fibrosis]

Idiopathic pulmonary fibrosis (IPF) is characterized by a progressive accumulation of extracellular matrix and an imbalance between profibrotic and antifibrotic mediators. In the last few years, understanding of the mechanisms of the biology of IPF has increased. One of the most significant discoveries is the finding that alveolar epithelial cell injury plays an important role in the pathogenesis of this disease. In this review, we describe some of the mechanisms involved in alveolar cell injury and their contribution to the development of IPF.

Emerging evidence for endoplasmic reticulum stress in the pathogenesis of idiopathic pulmonary fibrosis

While the factors that regulate the onset and progression of idiopathic pulmonary fibrosis (IPF) are incompletely understood, recent investigations have revealed that endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR) are prominent in alveolar epithelial cells in this disease. Initial observations linking ER stress and IPF were made in cases of familial interstitial pneumonia (FIP), the familial form of IPF, in a family with a mutation in surfactant protein C (SFTPC). Subsequent studies involving lung biopsy specimens revealed that ER stress markers are highly expressed in the alveolar epithelium in IPF and FIP. Recent mouse modeling has revealed that induction of ER stress in the alveolar epithelium predisposed to enhanced lung fibrosis after treatment with bleomycin, which is mediated at least in part by increased alveolar epithelial cell (AEC) apoptosis. Emerging data also indicate that ER stress in AECs could impact fibrotic remodeling by altering inflammatory responses and inducing epithelial-mesenchymal transition. Although the cause of ER stress in IPF remains unknown, common environmental exposures such as herpesviruses, inhaled particulates, and cigarette smoke induce ER stress and are candidates for contributing to AEC dysfunction by this mechanism. Together, investigations to date suggest that ER stress predisposes to AEC dysfunction and subsequent lung fibrosis. However, many questions remain regarding the role of ER stress in initiation and progression of lung fibrosis, including whether ER stress or the UPR could be targeted for therapeutic benefit.

Crimean-Congo hemorrhagic fever virus-infected hepatocytes induce ER-stress and apoptosis crosstalk

Crimean-Congo hemorrhagic fever virus (CCHFV) is a widely distributed tick-borne member of the Nairovirus genus (Bunyaviridae) with a high mortality rate in humans. CCHFV induces a severe disease in infected patients that includes, among other symptoms, massive liver necrosis and failure. The interaction between liver cells and CCHFV is therefore important for understanding the pathogenesis of this disease. Here, we described the in vitro CCHFV-infection and -replication in the hepatocyte cell line, Huh7, and the induced cellular and molecular response modulation. We found that CCHFV was able to infect and replicate to high titres and to induce a cytopathic effect (CPE). We also observed by flow cytometry and real time quantitative RT-PCR evidence of apoptosis, with the participation of the mitochondrial pathway. On the other hand, we showed that the replication of CCHFV in hepatocytes was able to interfere with the death receptor pathway of apoptosis. Furthermore, we found in CCHFV-infected cells the over-expression of PUMA, Noxa and CHOP suggesting the crosstalk between the ER-stress and mitochondrial apoptosis. By ELISA, we observed an increase of IL-8 in response to viral replication; however apoptosis was shown to be independent from IL-8 secretion. When we compared the induced cellular response between CCHFV and DUGV, a mild or non-pathogenic Nairovirus for humans, we found that the most striking difference was the absence of CPE and apoptosis. Despite the XBP1 splicing and PERK gene expression induced by DUGV, no ER-stress and apoptosis crosstalk was observed. Overall, these results suggest that CCHFV is able to induce ER-stress, activate inflammatory mediators and modulate both mitochondrial and death receptor pathways of apoptosis in hepatocyte cells, which may, in part, explain the role of the liver in the pathogenesis of CCHFV.

M2 macrophages exhibit higher sensitivity to oxLDL-induced lipotoxicity than other monocyte/macrophage subtypes

BACKGROUND

In obesity, phenotypic switches occur in macrophage populations such that the predominantly M2-polarised anti-inflammatory state seen in lean individuals changes to a predominantly M1-polarised pro-inflammatory state in those who are obese. However, the mechanisms by which these phenotypic shifts occur have not yet been fully elucidated.

RESULTS

The effects of oxLDL (1-40 μg/ml; 24 h) on several parameters relevant to the Unfolded Protein Response (UPR)-mediated lipotoxic effects of oxLDL (disruption of ER Ca²⁺ handling; activation of the UPR transcription factor XBP-1; upregulation of the UPR target genes BiP and CHOP; apoptosis; cell viability) were investigated in human primary monocyte-derived macrophages, and also in monocyte-macrophages derived from the THP-1 monocytic cell line. A consistent pattern was observed: M2-polarised macrophages were more sensitive to the lipotoxic effects of oxLDL than either non-polarised macrophages or non-differentiated monocytic cells. Specifically, M2-polarised macrophages were the only cell type to undergo significantly increased apoptosis (Primary cells: 1.23 ± 0.01 basal; THP-1-derived: 1.97 ± 0.12 basal; P < 0.05 in both cases) and decreased cell viability (Primary cells: 0.79 ± 0.04 basal; THP-1-derived: 0.67 ± 0.02 basal; P < 0.05 in both cases) when exposed to oxLDL levels similar to those seen in overweight individuals (ie. 1 μg/ml).

CONCLUSIONS

We propose that the enhanced susceptibility of M2-polarised macrophages to lipotoxicity seen in the present in vitro study could, over time, contribute to the phenotypic shift seen in obese individuals in vivo. This is because a higher degree of oxLDL-induced lipotoxic cell death within M2 macrophages could contribute to a decrease in numbers of M2 cells, and thus a relative increase in proportion of non-M2 cells, within macrophage populations. Given the pro-inflammatory characteristics of a predominantly M1-polarised state, the data presented here may constitute a useful contribution to our understanding of the origin of the pro-inflammatory nature of obesity, and of the pathogenesis of obesity-associated inflammatory disorders such as Type 2 diabetes and atherosclerosis.

Dysregulation of the unfolded protein response in db/db mice with diet-induced steatohepatitis

In humans with nonalcoholic fatty liver, diabetes is associated with more advanced disease. We have previously shown that diabetic db/db mice are highly susceptible to methionine choline-deficient diet (MCD)-induced hepatic injury. Because activation of the unfolded protein response (UPR) is an important adaptive cellular mechanism in diabetes, obesity, and fatty liver, we hypothesized that dysregulation of the UPR may partially explain how diabetes could promote liver injury. Db/db and db/m mice were fed the MCD or control diet for 4 weeks to characterize differences in UPR activation and downstream injury. Wildtype mice (C57BLKS/J) fed the MCD or control diet were treated with SP600125; a c-Jun N-terminal kinase (JNK) inhibitor and its effect on liver injury and UPR activation was measured. The MCD diet resulted in global up-regulation of the UPR in both diabetic db/db and nondiabetic db/m mice. db/db mice had an inadequate activation of recovery pathways (GADD34, XBP-1(s)) and accentuated activation of injury pathways related to persistent eif2-α phosphorylation (activating transcription factor 4 [ATF-4], C/EBP homologous transcription factor [CHOP], oxireductase endoplasmic reticulum oxidoreductin-1 [ERO-1α], JNK, nuclear factor kappaB [NF-κB]) compared to db/m mice. This led to increased expression of inflammatory mediators such as tumor necrosis factor alpha (TNF-α), ICAM-1, and MCP-1 compared to db/m mice. Interestingly, whereas pharmacologic JNK inhibition did not prevent the development of MCD diet-induced steatohepatitis, it did attenuate UPR and downstream inflammatory signaling.

CONCLUSION

MCD-fed db/db mice develop a more proinflammatory milieu than db/m mice associated with an impaired ability to dephosphorylate eif2-α through GADD34, impairing cellular recovery. These data may enhance our understanding of why diabetics with nonalcoholic steatohepatitis are prone to develop more severe liver injury than nondiabetic patients.

Classical and neonatal Marfan syndrome mutations in fibrillin-1 cause differential protease susceptibilities and protein function

Mutations in fibrillin-1 give rise to Marfan syndrome (MFS) characterized by vascular, skeletal, and ocular abnormalities. Fibrillins form the backbone of extracellular matrix microfibrils in tissues including blood vessels, bone, and skin. They are crucial for regulating elastic fiber biogenesis and growth factor bioavailability. To compare the molecular consequences of mutations causing the severe neonatal MFS with mutations causing the milder classical MFS, we introduced representative point mutations from each group in a recombinant human fibrillin-1 fragment. Structural effects were analyzed by circular dichroism spectroscopy and analytical gel filtration chromatography. Proteolytic susceptibility was probed with non-physiological and physiological proteases, including plasmin, thrombin, matrix metalloproteinases, and cathepsins. All mutant proteins showed a similar gross secondary structure and no differences in heat stability as compared with the wild-type protein. Proteins harboring neonatal mutations were typically more susceptible to proteolytic cleavage compared with those with classical mutations and the wild-type protein. Proteolytic neo-cleavage sites were found both in close proximity and distant to the mutations, indicating small but significant structural changes exposing cryptic cleavage sites. We also report for the first time that cathepsin K and V cleave non-mutated fibrillin-1 at several domain boundaries. Compared with the classical mutations and the wild type, the group of neonatal mutations more severely affected the ability of fibrillin-1 to interact with heparin/heparan sulfate, which plays a role in microfibril assembly. These results suggest differential molecular pathogenetic concepts for neonatal and classical MFS including enhanced proteolytic susceptibility for physiologically relevant enzymes and loss of function for heparin binding.

Endoplasmic reticulum stress enhances fibrotic remodeling in the lungs

Evidence of endoplasmic reticulum (ER) stress has been found in lungs of patients with familial and sporadic idiopathic pulmonary fibrosis. We tested whether ER stress causes or exacerbates lung fibrosis by (i) conditional expression of a mutant form of surfactant protein C (L188Q SFTPC) found in familial interstitial pneumonia and (ii) intratracheal treatment with the protein misfolding agent tunicamycin. We developed transgenic mice expressing L188Q SFTPC exclusively in type II alveolar epithelium by using the Tet-On system. Expression of L188Q SFTPC induced ER stress, as determined by increased expression of heavy-chain Ig binding protein (BiP) and splicing of X-box binding protein 1 (XBP1) mRNA, but no lung fibrosis was identified in the absence of a second profibrotic stimulus. After intratracheal bleomycin, L188Q SFTPC-expressing mice developed exaggerated lung fibrosis and reduced static lung compliance compared with controls. Bleomycin-treated L188Q SFTPC mice also demonstrated increased apoptosis of alveolar epithelial cells and greater numbers of fibroblasts in the lungs. With a complementary model, intratracheal tunicamycin treatment failed to induce lung remodeling yet resulted in augmentation of bleomycin-induced fibrosis. These data support the concept that ER stress produces a dysfunctional epithelial cell phenotype that facilitates fibrotic remodeling. ER stress pathways may serve as important therapeutic targets in idiopathic pulmonary fibrosis.

Dengue infection of monocytic cells activates ER stress pathways, but apoptosis is induced through both extrinsic and intrinsic pathways

Monocytic cells are believed to be an important mediator of the pathology of dengue disease in cases of secondary infection where pre-existing antibodies from a first infection facilitate virus entry to Fc receptor bearing cells. In this study we assessed the induction of the ER stress in response to infection of monocytic U937 cells with all four DENV serotypes as well as the induction of apoptosis. Clear evidence of ER stress and the production of pro-apoptotic signals were observed, together with activation of caspase 9. Surprisingly, caspase 8 was also activated, independently of caspase 9, and this was accompanied by an increase in the expression of TNF-α, suggesting the simultaneous but independent activation of both extrinsic and intrinsic apoptosis pathways. Both the induction of ER stress and apoptosis were shown to be serotype independent.

Rosiglitazone induces the unfolded protein response, but has no significant effect on cell viability, in monocytic and vascular smooth muscle cells

Given the safety concerns expressed over negative cardiovascular outcomes resulting from the clinical use of rosiglitazone, and the view that rosiglitazone exerts PPARγ-independent effects alongside its insulin-sensitising PPARγ-dependent effects, we hypothesised that rosiglitazone may trigger Unfolded Protein Responses (UPRs) due to disruptions in [Ca(2+)](i) homeostasis within two cardiovascular cell types: monocytic (MM6) and vascular smooth muscle (A7r5) cells. In microsomal samples derived from both cell types, pre-incubation with rosiglitazone rapidly (30min) brought about concentration-dependent PPARγ-independent inhibition of Ca(2+)ATPase activity (IC(50) ∼2μM). Fluo-3 fluorimetric data demonstrated in intact cells that 1h treatment with 1 or 10μM rosiglitazone caused Ca(2+) ions to leak into the cytoplasm. Gene expression analysis showed that within 4h of rosiglitazone exposure, the UPR transcription factor XBP-1 was activated (likely due to corresponding ER Ca(2+) depletion), and the UPR target genes BiP and SERCA2b were subsequently upregulated within 24-72h. After 72h 1 or 10μM rosiglitazone treatment, microsomal Ca(2+)ATPase activity increased to >2-fold of that seen in control microsomes, while [Ca(2+)](i) returned to basal, indicating that UPR-triggered SERCA2b upregulation was responsible for enhanced enzymatic Ca(2+) sequestration within the ER. This appeared to be sufficient to replenish ER Ca(2+) stores and restore normal cell physiology, as cell viability levels were not decreased due to rosiglitazone treatment throughout a 2-week study. Thus, incubation with 1-10μM rosiglitazone triggers the UPR, but does not prove cytotoxic, in cells of the cardiovascular system. This observation provides an important contribution to the current debate over the use of rosiglitazone in the clinical treatment of Type-2 Diabetes.

IRE1alpha controls cyclin A1 expression and promotes cell proliferation through XBP-1

IRE1 is a conserved dual endoribonuclease/protein kinase that is indispensable for directing the endoplasmic reticulum (ER) stress response in yeast, flies, and worms. In mammalian systems, however, the precise biological activities carried out by IRE1alpha are unclear. Here, molecular and chemical genetic approaches were used to control IRE1 activity in a number of prostate cancer cell lines and the resulting impact on gene transcription, cell survival, and proliferation was examined. Modulating IRE1alpha activity had no transcriptional effect on the induction of genes classically associated with the ER stress response (Grp78 and CHOP) or cell survival when confronted with ER stress agents. Rather, IRE1alpha activity was positively correlated to proliferation. Since Xbp-1 mRNA is the sole known substrate for IRE1 endoribonuclease activity, the role of this transcription factor in mediating proliferation was examined. Repressing total Xbp-1 levels by siRNA techniques effectively slowed proliferation. In an effort to identify IRE1/XBP-1 targets responsible for the cell cycle response, genome-wide differential mRNA expression analysis was performed. Consistent with its ability to sense ER stress, IRE1alpha induction led to an enrichment of ER-Golgi, plasma membrane, and secretory gene products. An increase in cyclin A1 expression was the only differentially expressed cell cycle regulatory gene found. Greater cyclin A protein levels were consistently observed in cells with active IRE1alpha and were dependent on XBP-1. We conclude that IRE1alpha activity controls a subset of the ER stress response and mediates proliferation through tight control of Xbp-1 splicing.

Regulation of XBP-1 signaling during transient and stable recombinant protein production in CHO cells

X-box binding protein 1 (XBP-1) is a key regulator of cellular unfolded protein response (UPR). The spliced isoform of XBP-1, XBP-1S, is a transcription activator, which is expressed only when UPR is induced. However, the impact of recombinant protein production on the regulation of XBP-1 signaling in CHO cells is not well understood. In this report, we cloned the Chinese hamster XBP-1 homolog to aid the investigation of the interplay between protein productivity, culture conditions, and endogenous XBP-1 signaling in CHO cells. Interestingly, expression of XBP-1S is detected in the non-producing and unstressed CHO-K1 cells. Transient expression of recombinant erythropoietin reveals a positive correlation between XBP-1 mRNA abundance and protein production level. However, such a correlation is not observed in batch cultivation of stable producing cell lines. The increased XBP-1 splicing is detected in late-phase cultures, suggesting that induction of XBP-1S may be a result of nutrient limitations or other environmental stresses rather than that of increased intracellular accumulation of recombinant proteins. Our data suggest that XBP-1 is a key determinant for the secretory capacity of CHO cells. Understanding its dynamic regulation hence provides a rational basis for cellular engineering strategies to improve recombinant protein secretion.

Implications of endoplasmic reticulum stress, the unfolded protein response and apoptosis for molecular cancer therapy. Part I: targeting p53, Mdm2, GADD153/CHOP, GRP78/BiP and heat shock proteins

BACKGROUND

In eukaryotes, endoplasmic reticulum stress (ERS) and the unfolded protein response (UPR) are coordinately regulated to maintain steady-state levels and activities of various cellular proteins to ensure cell survival.

OBJECTIVE

This review (Part I of II) focuses on specific ERS and UPR signalling regulators, their expression in the cancer phenotype and apoptosis, and proposes how their implication in these processes can be rationalised into proteasome inhibition, apoptosis induction and the development of more efficacious targeted molecular cancer therapies.

METHOD

In this review, we contextualise many ERS and UPR client proteins that are deregulated or mutated in cancers and show links between ERS and the UPR, their implication in oncogenic transformation, tumour progression and escape from immune surveillance, apoptosis inhibition, angiogenesis, metastasis, acquired drug resistance and poor cancer prognosis.

CONCLUSION

Evasion of programmed cell death or apoptosis is a hallmark of cancer that enables tumour cells to proliferate uncontrollably. Successful eradication of cancer cells through targeting ERS- and UPR-associated proteins to induce apoptosis is currently being pursued as a central tenet of anticancer drug discovery.

Endoplasmic reticulum stress is a target for therapy in Waldenstrom macroglobulinemia

Waldenstrom macroglobulinemia (WM) is an incurable low-grade lymphoma characterized by bone marrow (BM) involvement of IgM secreting lymphoplasmacytic cells. The induction of unfolded protein response (UPR) genes ("physiologic" UPR) enables cells to differentiate into professional secretory cells capable of production of high amounts of endoplasmic reticulum (ER)-processed proteins, such as immunoglobulins. Ultimately, the initially cytoprotective UPR triggers an apoptotic cascade if ER stress is not corrected, called proapoptotic/terminal UPR. We show that WM cells inherently express the physiologic UPR machinery compared with normal BM cells, and that increased ER stress leads to proapoptotic/terminal UPR in WM cells. We therefore examined tunicamycin, ER stress inducer, for potential antitumor effects in WM. Tunicamycin induced significant cytotoxicity, apoptosis and cell-cycle arrest, and inhibited DNA synthesis in WM cell lines and primary BM CD19(+) cells from patients with WM with an inhibitory concentration (IC(50)) of 0.5 microg/mL to 1 microg/mL, but not in healthy donor cells. Importantly, coculture of WM cells in the context of the BM microenvironment did not inhibit tunicamycin-induced cytotoxicity. Finally, we demonstrate that ER stress inducer synergizes with other agents used in the treatment of WM. These preclinical studies provide a framework for further evaluation of ER stress inducing agents as therapeutic agents in WM.

Subtilase cytotoxin activates PERK, IRE1 and ATF6 endoplasmic reticulum stress-signalling pathways

Subtilase cytotoxin (SubAB) is the prototype of a new family of AB(5) cytotoxins produced by Shiga toxigenic Escherichia coli. Its cytotoxic activity is due to its capacity to enter cells and specifically cleave the essential endoplasmic reticulum (ER) chaperone BiP (GRP78). In the present study, we have examined its capacity to trigger the three ER stress-signalling pathways in Vero cells. Activation of PKR-like ER kinase was demonstrated by phosphorylation of eIF2alpha, which occurred within 30 min of toxin treatment, and correlated with inhibition of global protein synthesis. Activation of inositol-requiring enzyme 1 was demonstrated by splicing of X-box-binding protein 1 mRNA, while activating transcription factor 6 activation was demonstrated by depletion of the 90 kDa uncleaved form, and appearance of the 50 kDa cleaved form. The rapidity with which ER stress-signalling responses are triggered by exposure of cells to SubAB is consistent with the hypothesis that cleavage by the toxin causes BiP to dissociate from the signalling molecules.

Absence of direct effect of low-power millimeter-wave radiation at 60.4 GHz on endoplasmic reticulum stress

Millimeter waves (MMW) at frequencies around 60 GHz will be used in the very near future in the emerging local wireless communication systems and the potential health hazards of artificially induced environmental exposures represent a major public concern. The main aim of this study was to investigate the potential effects of low-power MMW radiations on cellular physiology. To this end, the human glial cell line, U-251 MG, was exposed to 60.4 GHz radiation at a power density of 0.14 mW/cm(2) and potential effect of MMW radiations on endoplasmic reticulum (ER) stress was investigated. ER is very sensitive to environmental insults and its homeostasis is altered in various pathologies. Through several assay systems, we found that exposure to 60.4 GHz does not modify ER protein folding and secretion, nor induces XBP1 or ATF6 transcription factors maturation. Moreover, expression of ER-stress sensor, BiP/GRP78 was examined by real-time PCR, in exposed or non-exposed cells to MMW radiations. Our data demonstrated the absence of significant changes in mRNA levels for BiP/GRP78. Our results showed that ER homeostasis does not undergo any modification at molecular level after exposure to low-power MMW radiation at 60.4 GHz. This report is the first study of ER-stress induction by MMW radiations.

Cadmium regulation of apoptotic and stress response genes in tumoral and immortalized epithelial cells of the human breast

Cadmium (Cd) is a widely-disseminated metal which can be imported and accumulated in living cells thereby drastically interfering with their biological mechanisms. Increasing interest has been recently focused on the elucidation of the cellular and molecular aspects of Cd-dependent regulation of gene expression and signal transduction pathways in different model system. Concerning breast cancer, very limited studies have been produced so far on the role played by Cd on estrogen receptor-negative human breast cancer cells, that are expected to be insensitive to the already-proven metallo-estrogenic effect exerted by Cd on the estrogen receptor-positive cell counterparts. Here, we have examined the effects of long-term (96 h) exposure of estrogen receptor-negative MDA-MB231 malignant adenocarcinoma cells to CdCl(2) at 5 microM concentration, corresponding to the IC(50) for this time of incubation, by evaluating the expression levels of genes coding for stress response factors (e.g. heat shock proteins and metallothioneins), and for apoptosis-related factors and enzymes. In parallel, we tested the gene expression pattern of immortalized HB2 breast epithelial cells, taken as non-tumoral counterpart, after the same exposure to the metal which instead did not exert any change in their cell number with respect to controls. Our cumulative results indicate that, whilst HB2 cells appear to activate defense mechanisms against metal stress principally via metallothionein massive up-regulation and appearance of the spliced form of XBP-1 message, MDA-MB231 cells seem to couple the onset of a protective reaction (e.g. up-regulation of hsp27 and metallothioneins) to the switching-on of new intracellular pathways directing cells to a kind of death which shares several aspects with the apoptotic program, such as down-regulation of Bcl-2 and over-expression of Dap kinase and several caspases.

Endoplasmic reticulum stress in alveolar epithelial cells is prominent in IPF: association with altered surfactant protein processing and herpesvirus infection

Recent evidence suggests that dysfunctional type II alveolar epithelial cells (AECs) contribute to the pathogenesis of idiopathic pulmonary fibrosis (IPF). Based on the hypothesis that disease-causing mutations in surfactant protein C (SFTPC) provide an important paradigm for studying IPF, we investigated a potential mechanism of AEC dysfunction suggested to result from mutant SFTPC expression: induction of endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). We evaluated biopsies from 23 IPF patients (including 3 family members with L188Q SFTPC mutations, 10 individuals with familial interstitial pneumonia without SFTPC mutations, and 10 individuals with sporadic IPF) and sections from 10 control lungs. After demonstrating UPR activation in cultured A549 cells expressing mutant SFTPC, we identified prominent expression of UPR markers in AECs in the lungs of patients with SFTPC mutation-associated fibrosis. In individuals with familial interstitial pneumonia without SFTPC mutations and patients with sporadic IPF, we also found UPR activation selectively in AECs lining areas of fibrotic remodeling. Because herpesviruses are found frequently in IPF lungs and can induce ER stress, we investigated expression of viral proteins in lung biopsies. Herpesvirus protein expression was found in AECs from 15/23 IPF patients and colocalized with UPR markers in AECs from these patients. ER stress and UPR activation are found in the alveolar epithelium in patients with IPF and could contribute to disease progression. Activation of these pathways may result from altered surfactant protein processing or chronic herpesvirus infection.

Coupling cystic fibrosis to endoplasmic reticulum stress: Differential role of Grp78 and ATF6

Cystic fibrosis (CF) is the most common Caucasian autosomal recessive disease. It is due to mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene encoding the CFTR protein, which is a chloride (Cl(-)) channel. The most common mutation leads to a missing phenylalanine at position 508 (DeltaF508). The DeltaF508-CFTR protein is misfolded and retained in the endoplasmic reticulum and may trigger the unfolded protein response (UPR). Furthermore, CF is accompanied by inflammation and infection, which are also involved in the UPR. To date, the UPR transducer ATF6 and ER stress sensor Grp78 have been used as UPR markers. Therefore, our aim was to study the activation of ATF6 and Grp78 in transfected human epithelial cells expressing the DeltaF508-CFTR protein, and we showed that they are activated in these cells. We investigated the effect of exogenous UPR inducers thapsigargin (Tg) and tunicamycin (Tu) on Grp78 and ATF6 expression. Whereas the cells reacted to the UPR induction, we show a difference in the electrophoretic pattern of ATF6. The Grp78/ATF6 complex was previously described, but its stability during UPR is controversial. Using co-immunoprecipitation we show that it is stable in DeltaF508-CFTR-expressing cells and is maintained under UPR conditions. Finally, using siRNA, we show that decreased ATF6 expression induces increased cAMP-dependent halide flux through DeltaF508-CFTR due to its increased membrane localization. Therefore, our results suggest that UPR may be triggered in CF and that ATF6 may be a therapeutic target.

Dengue virus serotype infection specifies the activation of the unfolded protein response

Background

Dengue and Dengue hemorrhagic fever have emerged as some of the most important mosquito-borne viral diseases in the tropics. The mechanisms of pathogenesis of Dengue remain elusive. Recently, virus-induced apoptosis mediated by the Unfolded Protein Response (UPR) has been hypothesised to represent a crucial pathogenic event in viral infection. In an attempt to evaluate the contribution of the UPR to virus replication, we have characterized each component of this signalling pathway following Dengue virus infection.

Results

We find that upon Dengue virus infection, A549 cells elicit an UPR which is observed at the level of translation attenuation (as visualized by the phosphorylation of eIF2alpha) and activation of specific pathways such as nuclear translocation of ATF-6 and splicing of XBP-1. Interestingly, we find that specific serotype of virus modulate the UPR with different selectivity. In addition, we demonstrate that perturbation of the UPR by preventing the dephosphorylation of the translation initiation factor eIF2alpha using Salubrinal considerably alters virus infectivity.

Conclusion

This report provides evidence that Dengue infection induces and regulates the three branches of the UPR signaling cascades. This is a basis for our understanding of the viral regulation and conditions beneficial to the viral infection. Furthermore, modulators of UPR such as Salubrinal that inhibit Dengue replication may open up an avenue toward cell-protective agents that target the endoplasmic reticulum for anti-viral therapy.

Mutations of the ELA2 gene found in patients with severe congenital neutropenia induce the unfolded protein response and cellular apoptosis

Severe congenital neutropenia (SCN) is an inborn disorder of granulopoiesis. Mutations of the ELA2 gene encoding neutrophil elastase (NE) are responsible for most cases of SCN and cyclic neutropenia (CN), a related but milder disorder of granulopoiesis. However, the mechanisms by which these mutations disrupt granulopoiesis are unclear. We hypothesize that the ELA2 mutations result in the production of misfolded NE protein, activation of the unfolded protein response (UPR), and ultimately apoptosis of granulocytic precursors. Expression of mutant NE but not wild-type NE strongly induced BiP/GRP78 mRNA expression and XBP1 mRNA splicing, 2 classic markers of the UPR. The magnitude of UPR activation by a specific ELA2 mutation correlated with its associated clinical phenotype. Consistent with the UPR model, expression of mutant NE in primary human granulocytic precursors increased expression of CHOP (DDITS) and induced apoptosis in a protease-independent fashion. Most strikingly, UPR activation and decreased NE protein expression were detected in primary granulocytic precursors from SCN patients. Collectively, these data provide strong support for a UPR model of SCN disease pathogenesis and place SCN in a growing list of human diseases caused by misfolded proteins.

A highly sensitive assay for monitoring the secretory pathway and ER stress

BACKGROUND

The secretory pathway is a critical index of the capacity of cells to incorporate proteins into cellular membranes and secrete proteins into the extracellular space. Importantly it is disrupted in response to stress to the endoplasmic reticulum that can be induced by a variety of factors, including expression of mutant proteins and physiologic stress. Activation of the ER stress response is critical in the etiology of a number of diseases, such as diabetes and neurodegeneration, as well as cancer. We have developed a highly sensitive assay to monitor processing of proteins through the secretory pathway and endoplasmic reticulum (ER) stress in real-time based on the naturally secreted Gaussia luciferase (Gluc).

METHODOLOGY/PRINCIPLE FINDINGS

An expression cassette for Gluc was delivered to cells, and its secretion was monitored by measuring luciferase activity in the conditioned medium. Gluc secretion was decreased down to 90% when these cells were treated with drugs that interfere with the secretory pathway at different steps. Fusing Gluc to a fluorescent protein allowed quantitation and visualization of the secretory pathway in real-time. Expression of this reporter protein did not itself elicit an ER stress response in cells; however, Gluc proved very sensitive at sensing this type of stress, which is associated with a temporary decrease in processing of proteins through the secretory pathway. The Gluc secretion assay was over 20,000-fold more sensitive as compared to the secreted alkaline phosphatase (SEAP), a well established assay for monitoring of protein processing and ER stress in mammalian cells.

CONCLUSIONS/SIGNIFICANCE

The Gluc assay provides a fast, quantitative and sensitive technique to monitor the secretory pathway and ER stress and its compatibility with high throughput screening will allow discovery of drugs for treatment of conditions in which the ER stress is generally induced.

Spinal cord injury induces endoplasmic reticulum stress with different cell-type dependent response

The mechanisms of injury-induced apoptosis of neurons within the spinal cord are poorly understood. In this study, we show that spinal cord injury (SCI) induces endoplasmic reticulum stress revealed by the activation of an unbalanced unfolded protein response (UPR). Using a weight-drop contusion model of SCI, the UPR activation was characterized by a quick transient phosphorylation of alpha subunit of eukaryotic initiation factor 2 soon restored by the up-regulation of its regulator Gadd34; an effective cleavage/activation of the ATF6alpha transcription factor leading to up-regulation of the canonical UPR target genes Chop, Xbp1 and Grp78; the presence of the processing of Xbp1 mRNA indicative of inositol requiring kinase 1 activation, and a gradual accumulation of C/EBP homologous transcription factor protein (CHOP) with concomitant caspase-12 activation. Interestingly, the subcellular distribution of CHOP was found in the nucleus of neurons and oligodendrocytes but in the cytoplasm of astrocytes. Considering the pro-apoptotic action attributed to this transcription factor, this phenomenon might account for the different susceptibility of cell types to dye after SCI.

Double de novo mutations ofELA2 in cyclic and severe congenital neutropenia

Heterozygous mutations of ELA2, encoding the protease neutrophil elastase (NE), cause either autosomal dominant cyclic neutropenia or severe congenital neutropenia (SCN). Three hypotheses have been proposed for how allelic mutations produce these different disorders: 1) disruption of proteolytic activity; 2) mislocalization of the protein; or 3) destabilization of the protein resulting in induction of the unfolded protein response. As with other dominant diseases with reduced reproductive fitness, sporadic cases can result from new mutations not inherited from either parent. Here we report an exceptional genetic phenomenon in which both a cyclic neutropenia patient and an SCN patient each possess two new ELA2 mutations. Because of the rarity of the phenomenon, we investigated the origins of the mutations and found that both arise nonmosaically and in cis from the paternally-inherited allele. Moreover, these cases offer a unique opportunity to investigate molecular pathways distinguishing these two forms of hereditary neutropenia. We have characterized the mutants separately and in combination, with respect to their effects on proteolysis, subcellular trafficking, and induction of the unfolded protein response. Each pair of mutations acts more or less additively to produce equivalent net effects on reducing proteolytic activity and induction of the unfolded protein response, yet each has different and somewhat opposing effects on disturbing subcellular localization, thus offering support for a role for protein mistrafficking as a disease mechanism.

Effects of chemical chaperones on oxidative stress and detergent-insoluble species formation following conditional expression of amyloid precursor protein carboxy-terminal fragment

Oxidative stress, protein misfolding, protein complex formation, and detergent insolubility are biochemical features of Alzheimer's disease (AD). We tested the cause-and-effect relationships among these using MC65 human neuroblastoma cells that exhibit toxicity upon conditional expression of carboxy-terminal fragments (CTFs) of the human amyloid precursor protein (APP). Treatments with three different antioxidants (alpha-tocopherol, N-acetyl cysteine, and alpha-lipoic acid) or three different compounds (glycerol, trimethylamine-N-oxide, and 4-phenylbutyric acid) that have been described to have a "chemical chaperone" function in promoting protein folding all had a protective effect on MC65 cells and decreased markers of oxidative damage and accumulation of high molecular weight amyloid (A) beta-immunoreactive (IR) species. However, chaperones partially reduced detergent insolubility of the remaining Abeta-IR species, while antioxidants did not. These results suggest that protein misfolding associated with overexpression of APP CTFs promotes oxidative stress and cytotoxicity and contributes to formation of detergent-insoluble species that appear unrelated to cytotoxicity.

Methoxyflavones protect cells against endoplasmic reticulum stress and neurotoxin

Enhanced endoplasmic reticulum (ER) stress leads to cell death in various pathophysiological situations. During a search for compounds that regulate ER stress, we identified methoxyflavones, a group of flavonoids, as strong protective agents against ER stress. Analysis in mouse insulinoma MIN6 cells revealed that methoxyflavones mildly activated the eukaryotic initiation factor 2alpha and nuclear factor erythroid 2-related factor pathways, but not the XBP1 pathway, and induced downstream genes, including glucose-regulated protein (GRP) 78, a molecular chaperone in the ER. The protective effect of methoxyflavones was enhanced by agents that increase intracellular cAMP levels such as forskolin, dibutyryl-cAMP and IBMX, but suppressed by the protein kinase A (PKA) inhibitor H-89, suggesting involvement of the PKA pathway in the regulation of ER stress by methoxyflavones. Consistent with the results in cultured cells, pretreatment of mice with tangeretin, a methoxyflavone, enhanced expression of GRP78 and HO-1 without causing ER stress in renal tubular epithelium and prevented tunicamycin-induced cell death. Furthermore, preadministration of tangeretin in mice enhanced expression of GRP78 in the substantia nigra pars compacta and protected dopaminergic neurons against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, a neurotoxin that induces both oxidative and ER stress. These results suggest that methoxyflavones play an important role in the regulation of ER stress and could be a therapeutic target for the ER stress-related diseases.

Genetic background influences UPR but not PLP processing in the rumpshaker model of PMD/SPG2

Mutations of the proteolipid protein gene (PLP1) cause Pelizaeus-Merzbacher disease (PMD) and Spastic paraplegia type 2 (SPG2). The rumpshaker mutation is associated with mild forms of PMD or SPG2 in man and the identical mutation occurs in mice, the phenotype depending on genetic background. The mild phenotype in C3H mice becomes a lethal disease when expressed on the C57BL/6 background. rumpshaker PLP is synthesised at a similar rate to wild type but is rapidly degraded by the proteasome. We show that the rates of synthesis, degradation and myelin incorporation of PLP/DM20 are similar in mutants on both backgrounds and therefore differences in PLP processing are unlikely to be the basis of the phenotypic variation. An unfolded protein response (UPR) is activated in rumpshaker. Whereas activation of CHOP correlates with phenotypic severity, we find no difference in the response of BiP and X-box protein1 (Xbp1) between the two strains.

Quantitative measurement of spliced XBP1 mRNA as an indicator of endoplasmic reticulum stress

The unfolded protein response (UPR) events triggered by the accumulation of unfolded protein in endoplasmic reticulum (ER) activate the three UPR signaling pathways mediated by IRE1, ATF6 and PERK. Spliced XBP1 mRNA induced by activated IRE1 is translated to the protein, a potent transcription factor that induces BiP expression. XBP1 is also induced by activated ATF6. It is thus thought to be an important marker reflecting both IRE1 and ATF6 signaling in response to ER stress. For quantitative measurement of XBP1 gene expression, it is important to distinguish between the spliced and non-spliced form of XBP1 mRNA. We have developed a new method to detect the spliced XBP1 mRNA by means of real-time PCR and we compared the result with measurements of the expression of the ER stress inducible gene BiP. A good correlation was found between spliced XBP1 expression and BiP expression. Thus, our method may be useful for simple and quantitative evaluation of ER stress.

Activation of the unfolded protein response in infarcted mouse heart and hypoxic cultured cardiac myocytes

Endoplasmic reticulum (ER) stresses that reduce ER protein folding activate the unfolded protein response (UPR). One effector of the UPR is the transcription factor X-box binding protein-1 (XBP1), which is expressed on ER stress-mediated splicing of the XBP1 mRNA. XBP1 induces certain ER-targeted proteins, eg, glucose-regulated protein 78 (GRP78), that help resolve the ER stress and foster cell survival. In this study, we determined whether hypoxia can activate the UPR in the cardiac context. Neonatal rat ventricular myocyte cultures subjected to hypoxia (16 hours) exhibited increased XBP1 mRNA splicing, XBP1 protein expression, GRP78 promoter activation, and GRP78 protein levels; however, the levels of these UPR markers declined during reoxygenation, suggesting that the UPR is activated during hypoxia but not during reoxygenation. When cells were infected with a recombinant adenovirus (AdV) encoding dominant-negative XBP1 (AdV-XBP1dn), UPR markers were reduced; however, hypoxia/reoxygenation-induced apoptosis increased. Confocal immunocytofluorescence demonstrated that hypoxia induced GRP78 in neonatal rat and isolated adult mouse ventricular myocytes. Moreover, mouse hearts subjected to in vivo myocardial infarction exhibited increased GRP78 expression in cardiac myocytes near the infarct, but not in healthy cells distal to the infarct. These results indicate that hypoxia activates the UPR in cardiac myocytes and that XBP1-inducible proteins may contribute to protecting the myocardium during hypoxic stress.