ER Stress-mediated Cell Death Research Articles

Paeoniflorin Induces ER Stress-Mediated Apoptotic Cell Death by Generating Nox4-Derived ROS under Radiation in Gastric Cancer.

Gastric cancer is one of the most prevalent cancer types worldwide, and its resistance to cancer therapies, such as chemotherapy and radiotherapy, has made treating it a major challenge. Paeoniflorin (PF) is one potential pharmacological treatment derived from paeony root. However, in cancer, the molecular mechanisms and biological functions of PF are still unclear. In the present study, we found that PF exerts anti-tumor effects in vivo and in vitro and induces apoptotic cell death through ER stress, calcium (Ca2+), and reactive oxygen species (ROS) release in gastric cancer cells. However, ROS inhibition by DPI and NAC blocks cell death and the PERK signaling pathway via the reduction of Nox4. Moreover, PF triggers a synergistic inhibitory effect of the epithelial-mesenchymal transition (EMT) process under radiation exposure in radiation-resistant gastric cancer cells. These findings indicate that PF-induced Ca2+ and ROS release overcomes radioresistance via ER stress and induces cell death under radiation in gastric cancer cells. Therefore, PF, in combination with radiation, may be a powerful strategy for gastric cancer therapy.

Necrosis-Inducing High-Valent Oxo-Rhenium(V) Complexes with Potent Antitumor Activity: Synthesis, Aquation Chemistry, Cisplatin Cross-Resistance Profile, and Mechanism of Action.

Chemotherapy with the cytotoxic platinum (Pt) drugs cisplatin, carboplatin, and oxaliplatin is the mainstay of anticancer therapy in the clinic. The antitumor activity of Pt drugs originates from their ability to induce apoptosis via covalent adduct formation with nuclear DNA. While the phenomenal clinical success is highly encouraging, resistance and adverse toxic side effects limit the wider applicability of Pt drugs. To circumvent these limitations, we embarked on an effort to explore the antitumor potential of a new class of oxo-rhenium(V) complexes of the type [(N∧N)(EG)Re(O)Cl] (where EG = ethylene glycolate and N∧N = bipyridine, Bpy (1); phenanthroline, Phen (2); 3,4,7,8-tetramethyl-phenanthroline, Me4Phen (3)). Investigation of speciation chemistry in aqueous media revealed the formation of [(N∧N)Re(O)(OH)3] as the biologically active species. Complex 3 was found to be the most potent among the three, with IC50 values ranging from 0.1 to 0.4 μM against a panel of cancer cells, which is 5-70-fold lower when compared with cisplatin. The higher potency of 3 is attributed to its higher lipophilicity, which enhanced cellular uptake. Importantly, complex 3 efficiently overcomes cisplatin resistance in ovarian, lung, and prostate cancer cells. In addition to reporting the aquation chemistry and identifying the active species in aqueous media, we performed in-depth in vitro mechanistic studies, which revealed that complex 3 preferentially accumulates in mitochondria, depletes mitochondrial membrane potential, and upregulates intracellular reactive oxygen species (ROS), leading to ER stress-mediated necrosis-mediated cancer cell death.

HBV with precore and basal core promoter mutations exhibits a high replication phenotype and causes ER stress-mediated cell death in humanized liver chimeric mice.

Mutations within the precore (PC) and basal core promoter (BCP) regions of the HBV genome are associated with fulminant hepatitis and HBV reactivation. These mutations may enhance viral replication, but little is known about whether they directly induce damage to the liver. We investigated mechanisms of direct cytopathic effects induced by the infection with PC/BCP mutants in the absence of immune response in vitro and in vivo . Mice with humanized livers and hepatocytes derived from humanized mice were infected with either wild-type or mutant-type PC/BCP HBV, and the HBV replication and human hepatocyte damage were evaluated. HBV proliferated vigorously in mice with PC/BCP-mutant infection, and the severe loss of human hepatocytes with a slight human ALT elevation subsequently occurred only in PC/BCP mutant mice. In PC/BCP mutant infection, the accumulation of HBsAg in humanized livers colocalized with the endoplasmic reticulum, leading to apoptosis through unfolded protein response in HBV-infected hepatocytes. RNA-sequencing revealed the molecular characteristics of the phenotype of PC/BCP mutant infection in a humanized mouse model. Reduced ALT elevation and higher HBV DNA levels in this model are consistent with characteristics of HBV reactivation, indicating that the hepatocyte damage in this model might mimic HBV reactivation followed by hepatocyte damage under immunosuppressive conditions. PC and BCP mutations were associated with enhanced viral replication and cell death induced by ER stress using HBV infection models. These mutations might be associated with liver damage in patients with fulminant hepatitis or HBV reactivation.

FK506-Binding Protein 11 Is a Novel Plasma Cell-Specific Antibody Folding Catalyst with Increased Expression in Idiopathic Pulmonary Fibrosis.

Antibodies are central effectors of the adaptive immune response, widespread used therapeutics, but also potentially disease-causing biomolecules. Antibody folding catalysts in the plasma cell are incompletely defined. Idiopathic pulmonary fibrosis (IPF) is a fatal chronic lung disease with increasingly recognized autoimmune features. We found elevated expression of FK506-binding protein 11 (FKBP11) in IPF lungs where FKBP11 specifically localized to antibody-producing plasma cells. Suggesting a general role in plasma cells, plasma cell-specific FKBP11 expression was equally observed in lymphatic tissues, and in vitro B cell to plasma cell differentiation was accompanied by induction of FKBP11 expression. Recombinant human FKBP11 was able to refold IgG antibody in vitro and inhibited by FK506, strongly supporting a function as antibody peptidyl-prolyl cis-trans isomerase. Induction of ER stress in cell lines demonstrated induction of FKBP11 in the context of the unfolded protein response in an X-box-binding protein 1 (XBP1)-dependent manner. While deficiency of FKBP11 increased susceptibility to ER stress-mediated cell death in an alveolar epithelial cell line, FKBP11 knockdown in an antibody-producing hybridoma cell line neither induced cell death nor decreased expression or secretion of IgG antibody. Similarly, antibody secretion by the same hybridoma cell line was not affected by knockdown of the established antibody peptidyl-prolyl isomerase cyclophilin B. The results are consistent with FKBP11 as a novel XBP1-regulated antibody peptidyl-prolyl cis-trans isomerase and indicate significant redundancy in the ER-resident folding machinery of antibody-producing hybridoma cells.

TREX1 Deficiency Induces ER Stress-Mediated Neuronal Cell Death by Disrupting Ca2+ Homeostasis

TREX1 is an exonuclease that degrades extranuclear DNA species in mammalian cells. Herein, we show a novel mechanism by which TREX1 interacts with the BiP/GRP78 and TREX1 deficiency triggers ER stress through the accumulation of single-stranded DNA and activates unfolded protein response (UPR) signaling via the disruption of the TREX1-BiP/GRP78 interaction. In TREX1 knockdown cells, the activation of ER stress signaling disrupted ER Ca2+ homeostasis via the ERO1α-IP3R1-CaMKII pathway, leading to neuronal cell death. Moreover, TREX1 knockdown dysregulated the Golgi-microtubule network through Golgi fragmentation and decreased Ac-α-tubulin levels, contributing to neuronal injury. These alterations were also observed in neuronal cells harboring a TREX1 mutation (V91M) that has been identified in hereditary spastic paraplegia (HSP) patients in Korea. Notably, this mutation leads to defects in the TREX1-BiP/GRP78 interaction and mislocalization of TREX1 from the ER and possible disruption of the Golgi-microtubule network. In summary, the current study reveals TREX1 as a novel regulator of the BiP/GRP78 interaction and shows that TREX1 deficiency promotes ER stress-mediated neuronal cell death, which indicates that TREX1 may hold promise as a therapeutic target for neurodegenerative diseases such as HSP.

Effects of Ginsenoside Rg3 on Inhibiting Differentiation, Adipogenesis, and ER Stress-Mediated Cell Death in Brown Adipocytes.

Objectives Ginsenoside Rg3 (Rg3), a main active component of Panax ginseng, has various therapeutic properties in literatures, and it has been studied for its potential use in obesity control due to its antiadipogenic effects in white adipocytes. However, little is known about its effects on brown adipocytes. Methods The mechanisms through which Rg3 inhibits differentiation, adipogenesis, and ER stress-mediated cell death in mouse primary brown adipocytes (MPBAs) are explored. Results Rg3 significantly induced cytotoxicity in differentiated MPBAs but not in undifferentiated MPBAs. Rg3 treatment downregulated the expression of differentiation and adipogenesis markers and the level of perilipin in MPBAs while upregulating the expression of lipolytic Kruppel-like factor genes. Rg3 also induced lipolysis and efflux of triglycerides from MPBAs and subsequently increased proinflammatory cytokine levels. Notably, Rg3 treatment resulted in elevation of ER stress and proapoptotic markers in MPBAs. Conclusions Our results demonstrate that Rg3 is able to selectively exert cytotoxicity in differentiated MPBAs while leaving undifferentiated MPBAs intact, resulting in the induction of ER stress and subsequent cell death in MPBAs via regulation of various genes related to adipocyte differentiation, adipogenesis, lipolysis, and inflammation. These results indicate that further studies on the potential therapeutic applications of Rg3 are warranted.

RSK2 protects human breast cancer cells under endoplasmic reticulum stress through activating AMPKα2-mediated autophagy.

Autophagy can protect stressed cancer cell by degradation of damaged proteins and organelles. However, the regulatory mechanisms behind this cellular process remain incompletely understood. Here, we demonstrate that RSK2 (p90 ribosomal S6 kinase 2) plays a critical role in ER stress-induced autophagy in breast cancer cells. We demonstrated that the promotive effect of RSK2 on autophagy resulted from directly binding of AMPKα2 in nucleus and phosphorylating it at Thr172 residue. IRE1α, an ER membrane-associated protein mediating unfolded protein response (UPR), is required for transducing the signal for activation of ERK1/2-RSK2 under ER stress. Suppression of autophagy by knockdown of RSK2 enhanced the sensitivity of breast cancer cells to ER stress both in vitro and in vivo. Furthermore, we demonstrated that inhibition of RSK2-mediated autophagy rendered breast cancer cells more sensitive to paclitaxel, a chemotherapeutic agent that induces ER stress-mediated cell death. This study identifies RSK2 as a novel controller of autophagy in tumor cells and suggests that targeting RSK2 can be exploited as an approach to reinforce the efficacy of ER stress-inducing agents against cancer.

Abstract 257: Calreticulin Regulates Acetylation in Cardiac Cells During Endoplasmic Reticulum Stress

Background: Myocardial infarction (MI) is exacerbated by endoplasmic reticulum (ER) stress, which contributes to apoptosis signaling and tissue death in cardiomyocytes. Although the calcium-handing protein calreticulin is upregulated during MI, the role of calreticulin in the progression of ER stress remains controversial. We hypothesized that calreticulin expression accelerates ER stress via regulation of the acetylation status of ER proteins, and knockdown reduces pro-apoptotic ER stress. Methods: We examined the expression of calreticulin in human cardiac cells in response to ischemia or thapsigargin-mediated ER stress. To test the effect of calreticulin modulation on cell mortality, we generated a stable partial calreticulin knockdown line in proliferating human cardiomyocyte-derived cells. We further examined the induction and acetylation of ER stress signaling proteins, including proteins in each of the three major pathways that account for ER-stress mediated cell death. Results and Conclusions: Calreticulin is upregulated in human ischemic heart failure cardiac tissue, as well as simulated hypoxia and reoxygenation and thapsigargin-mediated ER stress. It was found that partial knockdown protects against the expression of caspase 12, CHOP, and reduces thapsigargin-driven TUNEL staining. In addition, significant changes in the acetylation of ER stress proteins and the expression of the deacetylase SIRT1 were observed. These data shed light on the role that calreticulin plays in apoptosis signaling during ER stress in cardiac cells.

Inhibition of the key metabolic pathways, glycolysis and lipogenesis, of oral cancer by bitter melon extract

BackgroundMetabolic reprogramming is one of the hallmarks of cancer which favours rapid energy production, biosynthetic capabilities and therapy resistance. In our previous study, we showed bitter melon extract (BME) prevents carcinogen induced mouse oral cancer. RNA sequence analysis from mouse tongue revealed a significant modulation in “Metabolic Process” by altering glycolysis and lipid metabolic pathways in BME fed group as compared to cancer group. In present study, we evaluated the effect of BME on glycolysis and lipid metabolism pathways in human oral cancer cells.MethodsCal27 and JHU022 cells were treated with BME. RNA and protein expression were analysed for modulation of glycolytic and lipogenesis genes by quantitative real-time PCR, western blot analyses and immunofluorescence. Lactate and pyruvate level was determined by GC/MS. Extracellular acidification and glycolytic rate were measured using the Seahorse XF analyser. Shotgun lipidomics in Cal27 and JHU022 cell lines following BME treatment was performed by ESI/ MS. ROS was measured by FACS.ResultsTreatment with BME on oral cancer cell lines significantly reduced mRNA and protein expression levels of key glycolytic genes SLC2A1 (GLUT-1), PFKP, LDHA, PKM and PDK3. Pyruvate and lactate levels and glycolysis rate were reduced in oral cancer cells following BME treatment. In lipogenesis pathway, we observed a significant reduction of genes involves in fatty acid biogenesis, ACLY, ACC1 and FASN, at the mRNA and protein levels following BME treatment. Further, BME treatment significantly reduced phosphatidylcholine, phosphatidylethanolamine, and plasmenylethanolamine, and reduced iPLA2 activity. Additionally, BME treatment inhibited lipid raft marker flotillin expression and altered its subcellular localization. ER-stress associated CHOP expression and generation of mitochondrial reactive oxygen species were induced by BME, which facilitated apoptosis.ConclusionOur study revealed that bitter melon extract inhibits glycolysis and lipid metabolism and induces ER and oxidative stress-mediated cell death in oral cancer. Thus, BME-mediated metabolic reprogramming of oral cancer cells will have important preventive and therapeutic implications along with conventional therapies.Graphical abstract

2135-P: Genetic Ablation of CB1R Protects against Streptozotocin-Induced ß-Cell Destruction in Mice

Approximately 1.25 million Americans have type 1 diabetes mellitus (T1DM), resulting from β-cell destruction. There is no known way to prevent it or to stop destruction of any remaining β-cells. Although the cannabinoid system, specifically cannabinoid 1 receptor (CB1R), is implicated in obesity and T2DM, its role, if any, in T1DM has not been studied. To address this, we induced β-cell destruction in wild type mice (WT: n=8) and mice in which CB1R was genetically nullified in β-cells only (β-CB1R-/-: n=8) in adulthood by five injections of streptozotocin (50 mg/kg intraperitoneally [IP]), a toxin selective for β-cells. We found that β-CB1R-/- mice had random blood glucose (BG) levels in the range of 137-289 mg/dL throughout the study period (28 days) as well as preserved insulin secretion with superior glucose tolerance after IP glucose (2g/kg) at day 8, 15 or 27, relative to their (random BG 500-600 mg/dL) WT littermates. CB1R knockout drove significant increases in pancreatic islet autophagic activity based on increased accumulation of autophagosome hallmarks (LC3B, Beclin-1, ATG5 and ATG7) and reduced p62 expression. Insulin and LC3B were colocalized in CB1R-deficient β-cells. Remarkably, the islets did not show the upregulation of ER unfolded protein response, which was present in β-cells of WT mice, while exerting the induction of protective chaperones. These anti-stress functions were accompanied by increased phosphorylation of AKT (ser) 473 and ERK in β-cells, which reveals a previously unrecognized route for PI3K-AKT pathway to activate the anti-apoptotic NFkB/p65 signaling that coordinately is permissive to β-cell survival. We conclude that CB1R nullification restores autophagy machinery as a novel signaling pathway to counteract ER stress-mediated cell death and confers resistance to β-cell destruction. We propose that CB1R is a potential therapeutic target during the honeymoon phase of T1DM. Disclosure K. Aseer: None. J.M. Egan: None.

SAT-LB064 Mitotane Induces Autophagy: A Mechanism to Promote Chemoresistance in Adrenocortical Carcinoma

Adrenocortical carcinoma (ACC) is an orphan malignancy with dismal survival rates. Although recent large genomic studies have shed insight into underlying molecular mechanisms, there are currently no targeted therapies available. Autophagy is an important cellular process to promote survival in a low nutrient tumor microenvironment or in the setting of physiologic stimuli such as endoplasmic reticulum stress (ER stress). In some settings, autophagy can result in drug resistance to cytotoxic chemotherapies and tyrosine kinase inhibitors. The role of autophagy in ACC has not been well characterized. Previous research in H295R ACC cells has shown that mitotane, the only FDA approved drug for ACC, promotes cell death in part through ER stress. Since autophagy has been shown to promote resistance to ER stress-mediated cell death, we hypothesized that autophagy may be activated in ACC cells as a compensatory mechanism, promoting cell survival in the face of mitotane treatment. CU-ACC2 cells demonstrated functional autophagy in response to Earle’s Balanced Salt Solution (EBSS), a potent stimulant of autophagy, indicated by a significant increase in the autophagy mediator LC3-II, as detected by immunoblot and enhanced autophagic flux detected by flow cytometry in cells expressing a tandem LC3-mCherry-GFP construct. To test if mitotane induces ER stress in our novel cell lines, qPCR was performed examining the expression of several ER stress pathway components including CHOP and XBP1 cleavage. In agreement with previous data in H295R ACC cells, there was significant cleavage of XBP1 and increased expression of CHOP in response to mitotane treatment in CU-ACC2, confirming ER stress induction. Mitotane also induced autophagy, as measured by LC3 detection by immunoblot as well as flow cytometry. ER stress induction was detected as early as 6 hours, while autophagy induction occurred later and peaked at 24 hours suggesting that ER stress precedes induction of autophagy in ACC cells. A functional effect of autophagy induction after mitotane treatment was demonstrated by treatment with mitotane with or without the autophagy inhibitor, chloroquine. Significantly more cleaved PARP, suggesting greater apoptosis, was detected after 24 and 48 hours of treatment with combination therapy than with than with either treatment alone. Together, these data suggest that mitotane activates ER stress to trigger apoptosis, but this treatment also activates autophagy, which abrogates the toxic effects. Thus, inhibition of the autophagy pathway may be a potential therapeutic adjunct to mitotane therapy in patients with ACC. Funding: VA Merit, Cancer League of Colorado, NIH K08, NIH R01 to AT. Unless otherwise noted, all abstracts presented at ENDO are embargoed until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. s presented at a news conference are embargoed until the date and time of the news conference. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.

SILAC-based quantitative proteomics using mass spectrometry quantifies endoplasmic reticulum stress in whole HeLa cells.

ABSTRACTThe unfolded protein response (UPR) involves extensive proteome remodeling in many cellular compartments. To date, a comprehensive analysis of the UPR has not been possible because of technological limitations. Here, we employ stable isotope labeling with amino acids in cell culture (SILAC)-based proteomics to quantify the response of over 6200 proteins to increasing concentrations of tunicamycin in HeLa cells. We further compare the effects of tunicamycin (5 µg/ml) to those of thapsigargin (1 µM) and DTT (2 mM), both activating the UPR through different mechanisms. This systematic quantification of the proteome-wide expression changes that follow proteostatic stress is a resource for the scientific community, enabling the discovery of novel players involved in the pathophysiology of the broad range of disorders linked to proteostasis. We identified increased expression in 38 proteins not previously linked to the UPR, of which 15 likely remediate ER stress, and the remainder may contribute to pathological outcomes. Unexpectedly, there are few strongly downregulated proteins, despite expression of the pro-apoptotic transcription factor CHOP, suggesting that IRE1-dependent mRNA decay (RIDD) has a limited contribution to ER stress-mediated cell death in our system.

Lactogens Prevent ER Stress-Induced ß-Cell Death and Diabetes

ER stress is highly relevant in the context of diabetes, as inducers of β cell death and dysfunction implicated in the disease - including proinflammatory cytokines and glucolipotoxicity - are known to impair ER function. Previous work has shown that prolactin (Prl) and placental lactogen (PL), which bind to the same receptor (Prl-R), are important for β cell function and proliferation, and enhance survival against cytokines and glucolipotoxicity. Here, we examine the effect of Prl/PL directly on ER stress and the associated unfolded protein response (UPR) in the β cell. ER stress was induced in vitro via tunicamycin in INS1, primary mouse, and primary human β cells, concurrent with Prl/veh treatment. Expression of ER stress and UPR associated genes was measured via qRT-PCR and preliminarily via WB, and insulin-TUNEL co-staining was used to assay for β cell death. As an in vivo model, heterozygous Akita mice - a well-established β cell ER stress model which becomes diabetic by 4 weeks of age - were bred to single-transgenic mice overexpressing PL in the β cell. Prl increases survival of INS1, primary mouse and human β cells against ER stress-mediated cell death in vitro (2.34±0.19% TM vs. 0.73±0.19% TM+Prl, human). Differences in mRNA expression point towards modulation of the PERK/IRE1 UPR arms. Non-Tg Akita mice quickly become and remain hyperglycemic/diabetic, while overexpression of PL in the β cells of these mice significantly improves glycemia in males (663±29mg/dL non-Tg vs. 327±37mg/dL Tg, 12 weeks) and results in normoglycemia in females. In vivo GSIS at 12 weeks shows a trend towards restoration of β cell function in Tg compared to non-Tg Akita mice. Males at this age exhibit improved β cell mass (0.28±0.12mg non-Tg vs. 2.29±0.44mg Tg), with similar trends in β cell proliferation. Taken together, these data demonstrate that Prl/PL can prevent and protect against ER stress in the β cell, in vitro and in vivo. As ER stress is highly relevant to diabetes, modulation of the lactogenic pathway could present a novel therapy for this disease. Disclosure R. Li: None. R.F. Hampton: None. N. Guthalu Kondegowda: None. R. Fenutria: None. R.C. Vasavada: None.

ATP Maintenance via Two Types of ATP Regulators Mitigates Pathological Phenotypes in Mouse Models of Parkinson's Disease

Parkinson's disease is assumed to be caused by mitochondrial dysfunction in the affected dopaminergic neurons in the brain. We have recently created small chemicals, KUSs (Kyoto University Substances), which can reduce cellular ATP consumption. By contrast, agonistic ligands of ERRs (estrogen receptor-related receptors) are expected to raise cellular ATP levels via enhancing ATP production. Here, we show that esculetin functions as an ERR agonist, and its addition to culture media enhances glycolysis and mitochondrial respiration, leading to elevated cellular ATP levels. Subsequently, we show the neuroprotective efficacies of KUSs, esculetin, and GSK4716 (an ERRγ agonist) against cell death in Parkinson's disease models. In the surviving neurons, ATP levels and expression levels of α-synuclein and CHOP (an ER stress-mediated cell death executor) were all rectified. We propose that maintenance of ATP levels, by inhibiting ATP consumption or enhancing ATP production, or both, would be a promising therapeutic strategy for Parkinson's disease.

Targeting multiple pro-apoptotic signaling pathways with curcumin in prostate cancer cells.

Curcumin, an extract from the turmeric rhizome (Curcuma longa), is known to exhibit anti-inflammatory, antioxidant, chemopreventive and antitumoral activities against aggressive and recurrent cancers. Accumulative data indicate that curcumin may induce cancer cell death. However, the detailed mechanism underlying its pro-apoptotic and anti-cancer effects remains to be elucidated. In the present study, we examined the signaling pathways triggered by curcumin, specifically, the exact molecular mechanisms of curcumin-induced apoptosis in highly metastatic human prostate cancer cells. The effect of curcumin was evaluated using for the first time in prostate cancer, a gel-free shotgun quantitative proteomic analysis coupled with Tandem Mass Tag isobaric labeling-based-signaling networks. Results were confirmed at the gene expression level by qRT-PCR and at the protein expression level by western blot and flow cytometry. Our findings revealed that curcumin induced an Endoplasmic Reticulum stress-mediated apoptosis in PC3. The mechanisms by which curcumin promoted cell death in these cells were associated with cell cycle arrest, increased reactive oxygen species, autophagy and the Unfolded Protein Response. Furthermore, the upregulation of ER stress was measured using key indicators of ER stress: Glucose-Regulated Protein 78, Inositol-Requiring Enzyme 1 alpha, Protein Disulfide isomerase and Calreticulin. Chronic ER stress induction was concomitant with the upregulation of pro-apoptotic markers (caspases 3,9,12) and Poly (ADP-ribose) polymerase. The downregulated proteins include anti-apoptotic and anti-tumor markers, supporting their curcumin-induced pro-apoptotic role in prostate cancer cells. Taken together, these data suggest that curcumin may serve as a promising anticancer agent by inducing a chronic ER stress mediated cell death and activation of cell cycle arrest, UPR, autophagy and oxidative stress responses.

IRS1 deficiency protects β-cells against ER stress-induced apoptosis by modulating sXBP-1 stability and protein translation.

Endoplasmic reticulum (ER) stress is among several pathological features that underlie β-cell failure in the development of type 1 and type 2 diabetes. Adaptor proteins in the insulin/insulin-like-growth factor-1 signaling pathways, such as insulin receptor substrate-1 (IRS1) and IRS2, differentially impact β-cell survival but the underlying mechanisms remain unclear. Here we report that β-cells deficient in IRS1 (IRS1KO) are resistant, while IRS2 deficiency (IRS2KO) makes them susceptible to ER stress-mediated apoptosis. IRS1KOs exhibited low nuclear accumulation of spliced XBP-1 due to its poor stability, in contrast to elevated accumulation in IRS2KO. The reduced nuclear accumulation in IRS1KO was due to protein instability of Xbp1 secondary to proteasomal degradation. IRS1KO also demonstrated an attenuation in their general translation status in response to ER stress revealed by polyribosomal profiling. Phosphorylation of eEF2 was dramatically increased in IRS1KO enabling the β-cells to adapt to ER stress by blocking translation. Furthermore, significantly high ER calcium (Ca2+) was detected in IRS1KO β-cells even upon induction of ER stress. These observations suggest that IRS1 could be a therapeutic target for β-cell protection against ER stress-mediated cell death by modulating XBP-1 stability, protein synthesis, and Ca2+ storage in the ER.

Mitochondrial oxidative phosphorylation controls cancer cell's life and death decisions upon exposure to MAPK inhibitors.

Although MAPK pathway inhibitors are becoming a promising anticancer strategy, they are insufficient to fully eliminate cancer cells and their long-term efficacy is strikingly limited in patients with BRAF-mutant melanomas. It is well established that BRAF inhibitors (BRAFi) hamper glucose uptake before the apparition of cell death. Here, we show that BRAFi induce an extensive restructuring of mitochondria including an increase in mitochondrial activity and biogenesis associated with mitochondrial network remodeling. Furthermore, we report a close interaction between ER and mitochondria in melanoma exposed to BRAFi. This physical connection facilitates mitochondrial Ca2+ uptake after its release from the ER. Interestingly, Mfn2 silencing disrupts the ER–mitochondria interface, intensifies ER stress and exacerbates ER stress-induced apoptosis in cells exposed to BRAFi in vitro and in vivo. This mitochondrial control of ER stress-mediated cell death is similar in both BRAF- and NRAS-mutant melanoma cells exposed to MEK inhibitors. This evidence reinforces the relevance in combining MAPK pathway inhibitors with mitochondriotropic drugs to improve targeted therapies.

Sphingosine Kinase 1 Protects Hepatocytes from Lipotoxicity via Down-regulation of IRE1α Protein Expression

Aberrant deposition of fat including free fatty acids in the liver often causes damage to hepatocytes, namely lipotoxicity, which is a key pathogenic event in the development and progression of fatty liver diseases. This study demonstrates a pivotal role of sphingosine kinase 1 (SphK1) in protecting hepatocytes from lipotoxicity. Exposure of primary murine hepatocytes to palmitate resulted in dose-dependent cell death, which was enhanced significantly in Sphk1-deficient cells. In keeping with this, expression of dominant-negative mutant SphK1 also markedly promoted palmitate-induced cell death. In contrast, overexpression of wild-type SphK1 profoundly protected hepatocytes from lipotoxicity. Mechanistically, the protective effect of SphK1 is attributable to suppression of ER stress-mediated pro-apoptotic pathways, as reflected in the inhibition of IRE1α activation, XBP1 splicing, JNK phosphorylation, and CHOP induction. Of note, SphK1 inhibited the IRE1α pathway by reducing IRE1α expression at the transcriptional level. Moreover, S1P mimicked the effect of SphK1, suppressing IRE1α expression in a receptor-dependent manner. Furthermore, enforced overexpression of IRE1α significantly blocked the protective effect of SphK1 against lipotoxicity. Therefore, this study provides new insights into the role of SphK1 in hepatocyte survival and uncovers a novel mechanism for protection against ER stress-mediated cell death.

Abstract 1769: Rottlerin induces ER stress-mediated cell death in pancreatic stellate cells

Abstract Pancreatic ductal adenocarcinoma (PDAC) is a deadly disease with a 5-year survival rate under 5%. Whereas activated pancreatic stellate cells (PaSC) are important players in PDAC stroma, simultaneously targeting both cancer and stromal cells can be beneficial for cancer therapy. We previously reported that the phytochemical rottlerin dose-dependently reduces PaSC viability through dysregulation of autophagy and induction of apoptosis, and attenuates tumor progression in PDAC experimental models. Interestingly, both rottlerin and metformin have been shown to affect mitochondrial function, and metformin has been demonstrated to induce AMPK-dependent, CHOP-mediated cell death. Here we further studied the mechanisms underlying cell death and autophagy in rottlerin-treated mouse PaSC. Rottlerin rapidly depolarized mitochondrial membrane, increased oxygen consumption and decreased cellular ATP levels and the expression of the mitochondrial transmembrane protein TOM20, suggesting mitochondrial damage. Rottlerin dysregulated autophagy as indicated by rapid and prolonged increasing in LC3I-LC3II conversion and p62/SQSTM1 accumulation. However, rottlerin-induced LC3II did not further change in the presence of inhibitors of autophagic flux, suggesting that rottlerin blocks autophagy progression in PaSC. In addition, rottlerin induced marked AMPK activation and inhibited mTOR signaling. In contrast to metformin, AMPKα1/2 knockdown had no effect on rottlerin-induced inhibition of mTOR signaling and cell viability. Furthermore, rottlerin induced ER stress as manifested by persistent activation of the PERK pathway and upregulation of proapoptotic transcription factor CHOP. Importantly, rottlerin-induced apoptosis was greatly reduced in PaSC isolated from CHOP null mice. In summary, these data indicate that rottlerin reduces PaSC viability through impairing mitochondrial function, inhibiting mTOR signaling and inducing prolonged ER stress. Specifically, CHOP induction plays an important role in determining cell-fate in mitochondria-targeting agents, such as rottlerin and metformin. Citation Format: Hsin-Yuan Su, Richard Waldron, Raymond Gong, Stephen Pandol, Aurelia Lugea. Rottlerin induces ER stress-mediated cell death in pancreatic stellate cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1769. doi:10.1158/1538-7445.AM2015-1769

Inhibition of GADD34, the stress-inducible regulatory subunit of the endoplasmic reticulum stress response, does not enhance functional recovery after spinal cord injury.

Activation of the endoplasmic reticulum stress response (ERSR) is a hallmark of various pathological diseases and/or traumatic injuries. Restoration of ER homeostasis can contribute to improvement in the functional outcome of these diseases. Using genetic and pharmacological inhibition of the PERK-CHOP arm of the ERSR, we recently demonstrated improvements in hindlimb locomotion after spinal cord injury (SCI) and implicated oligodendrocyte survival as a potential mechanism. Here, we investigated the contribution of stress-inducible PPP1R15A/GADD34, an ERSR signaling effector downstream of CHOP that dephosphorylates eIF2α, in the pathogenesis of SCI. We show that although genetic ablation of GADD34 protects oligodendrocyte precursor cells (OPCs) against ER stress-mediated cell death in vitro and results in differential ERSR attenuation in vivo after SCI, there is no improvement in hindlimb locomotor function. Guanabenz, a FDA approved antihypertensive drug, was recently shown to reduce the burden of misfolded proteins in the ER by directly targeting GADD34. Guanabenz protected OPCs from ER stress-mediated cell death in vitro and attenuated the ERSR in vivo after SCI. However, guanabenz administration failed to rescue the locomotor deficits after SCI. These data suggest that deletion of GADD34 alone is not sufficient to improve functional recovery after SCI.