OPEN Endoplasmic reticulum stress SUBJECT AREAS: preconditioning attenuates RNA DECAY ENDOPLASMIC RETICULUM methylmercury-induced cellular damage STRESS SIGNALLING RNAI by inducing favorable stress responses Fusako Usuki1, Masatake Fujimura2 & Akio Yamashita3 Received 6 March 2013 1Department of Clinical Medicine, 2Basic Medical Sciences, National Institute for Minamata Disease, 4058-18 Hama, Minamata Accepted 867-0008, Japan, 3Department of molecular biology, Yokohama City University School of Medicine, 3-9 Fuku-ura, Kanazawa, 17 July 2013 Yokohama 236-0004, Japan. Published 2 August 2013 We demonstrate that methylmercury (MeHg)-susceptible cells preconditioned with an inhibitor of endoplasmic reticulum (ER) Ca21-ATPase, thapsigargin, showed resistance to MeHg cytotoxicity through favorable stress responses, which included phosphorylation of eukaryotic initiation factor 2 alpha (Eif2a), accumulation of activating transcription factor 4 (Atf4), upregulation of stress-related proteins, and Correspondence and activation of extracellular signal regulated kinase pathway. In addition, ER stress preconditioning induced requests for materials suppression of nonsense-mediated mRNA decay (NMD) mainly through the phospho-Eif2a-mediated should be addressed to general suppression of translation initiation and possible combined effects of decreased several NMD F.U. ([email protected]. components expression. Atf4 accumulation was not mediated by NMD inhibition but translation inhibition jp) of its upstream open reading frame (uORF) and translation facilitation of its protein-coding ORF by the phospho-Eif2a. These results suggested that ER stress plays an important role in MeHg cytotoxicity and that the modulation of ER stress has therapeutic potential to attenuate MeHg cytotoxicity, the underlying mechanism being the induction of integrated stress responses. ells and tissues can be protected against a potentially lethal stress by the pre-exposure to the same or different milder stress. Preconditioning cytoprotection has been described in ischemic preconditioning against myocardial infarction1,2 or delayed neuronal cell death3, and endoplasmic reticulum (ER) stress C 4 5 preconditioning against renal epithelial cell oxidative injury or cardiomyocyte oxidative injury . ER stress preconditioning can be promoted by the treatment of thapsigargin (TPG), a specific inhibitor of ER Ca21- ATPase6.ERCa21-ATPase maintains the ER Ca21 pool by pumping Ca21 into the ER lumen from the cytoplasm under physiological conditions. Therefore, TPG induces a dose-dependent release of the ER-stored Ca21 pool and promotes ER stress. Methylmercury (MeHg) is a major environmental toxicant which affects various cellular functions depending on cellular context and developmental phase. MeHg triggers the activation or suppression of several cellular signalling pathways that determine the subsequent cellular fate. Accidental MeHg poisonings involving humans have been recorded, including in Japan7, Iraq8, and the USA9. MeHg toxicity currently continues to be an environmental risk to human health, especially in susceptible populations who frequently eat substantial amounts of fish or fish predators. The critical role of oxidative stress in the pathogenesis of MeHg toxicity has been demonstrated both in vitro10–16 and in vivo17–19. Failure to protect cells against MeHg-induced early oxidative stress triggers subsequent ER stress and apoptosis, suggesting that protective factors against ER stress may be important in combating MeHg cytotoxicity, especially in its later stages14. Furthermore it has been reported that the overexpression of the ubiquitin-conjugating enzyme cdc34 protects cells against MeHg cytotoxicity20,21, suggesting that ER stress may play an important role. Environmental stresses cause various cellular stress responses including initiation of synthesis of stress-related proteins and termination of synthesis of non-stress-related proteins. Some stress-related proteins are predicted to depend on the execution of nonsense-mediated mRNA decay (NMD) due to the presence of upstream open reading frame (uORF) in its 59-untranslated region (59 UTR)22. NMD is an mRNA surveillance mechanism that eliminates mRNAs containing premature termination codons (PTCs) and PTC-harbored mRNAs are produced SCIENTIFIC REPORTS | 3 : 2346 | DOI: 10.1038/srep02346 1 www.nature.com/scientificreports in genetic diseases, cancer, and also normal subjects through alterna- pretreatment with TPG effectively protects against MeHg-induced tive splicing22. Environmental stresses such as hypoxia23 and amino apoptotic processes. acid starvation22 are known to suppress NMD activity. However, the mechanism of NMD suppression and its role on environmental Effect of pretreatment with TPG on the MeHg-induced increase in stress-induced expression of stress-related proteins remain to be intracellular reactive oxygen species (ROS). Because MeHg- elucidated. induced apoptosis is triggered by an increase in intracellular ROS In the present study, we adopted ER stress preconditioning levels13,14, we investigated the effect of ER stress preconditioning on approach to investigate cellular stress responses in the context of MeHg-induced oxidative stress using flow cytometry. Results the mechanism and of protective factors for MeHg toxicity. Our demonstrated that a MeHg-mediated increase in ROS occurred 7 h results demonstrate that ER stress preconditioning by the pretreat- after exposure in cells pretreated with TPG, which was later than in ment with TPG alleviates MeHg-induced cytoxicity through the non-preconditioned cells (Fig. 2A). The results suggest that ER stress induction of favorable stress responses. We also showed that the preconditioning alleviates MeHg-induced oxidative stress. ER stress preconditioning induced suppression of NMD mainly Since the MeHg-mediated increase in ROS induces the activation through the phospho-eukaryotic initiation factor 2 alpha (Eif2a)- of mitogen-activated protein kinase (MAPK) pathways, which are mediated general suppression of translation initiation. involved in MeHg-induced apoptosis in C2C12-DMPK16013,14,we investigated the effect of prior ER stress on MAPK pathway activa- Results tion. As shown in Fig. 2B, the apoptosis–related stress–activated Effect of pretreatment with TPG on MeHg cytotoxicity. The MeHg- protein kinases (SAPK)/c-Jun NH2-terminal kinases (JNK) pathway susceptible C2C12 myogenic cell line (C2C12-DMPK160) showed was activated by MeHg exposure, while pretreatment with TPG sup- signs of apoptosis within 24 h after exposure to low levels of pressed this effect. In contrast, the cell survival–related extracellular MeHg13,14. Since the cytotoxic effect of MeHg was mediated by the signal–regulated kinase (ERK) pathway was upregulated in TPG- early oxidative stress and subsequent ER stress response14,20,21,we pretreated cells compared to non-pretreated cells. hypothesized that ER stress preconditioning could provide protec- tion against MeHg cytotoxicity. To address this, we first examined Pretreatment with TPG upregulates antioxidant protein–coding the effect of prior ER stress on MeHg-induced cell damage. Based on mRNAs. Because ER stress preconditioning suppressed early MeHg- 14 the previous cell viability data , two points of higher dose than EC50 induced increases in ROS, basal changes in antioxidant mRNAs were value for MeHg were examined. As shown in Fig. 1A, pretreatment examined to determine the antioxidant capacity of preconditioned with 0.1–0.3 mg/ml TPG protected C2C12-DMPK160 cells against cells. As shown in Fig. 3A–C, pretreatment with TPG induced both 0.4 and 0.5 mM MeHg-induced cytotoxicity. significant upregulation of mRNAs encoding the anti-oxidant We next analyzed the protective effect of ER stress precondition- enzymes, glutathione peroxidase 1 (Gpx1), thioredoxin reductase 1 ing on MeHg-induced apoptosis by flow cytometry (Fig. 1B). C2C12- (Txnrd1), and manganese superoxide dismutase (Mn-Sod) com- DMPK160 treated with 0.4 mM MeHg underwent apoptosis to a pared to non-preconditioned cells. Western blot analyses of cell greater extent than untreated cells. Pretreatment with 0.3 mg/ml lysates demonstrated that nuclear factor erythroid 2-related factor TPG attenuated MeHg-induced apoptosis. The results indicate that 2 (Nrf2), Txnrd1, and Mn-Sod antioxidant proteins were induced in Figure 1 | Effect of pretreatment with TPG on MeHg cytotoxicity. (A) Cell viability of C2C12-DMPK 160 cells pretreated with TPG 16 h before exposure to 0.4 or 0.5 mM MeHg was determined. Pretreatment with TPG (100–300 ng/ml) attenuated MeHg cytotoxicity. The viability of untreated cells was regarded as 100%. Values represent means 6 SE (n 5 6). *, **Significantly different from TPG-untreated and MeHg-treated cells by a one-way ANOVA (*p , 0.05, **p , 0.01). (B) Apoptosis analysis. The upper panel shows flow cytometry analysis of C2C12-DMPK160 stained with propidium iodide (PI) and FITC-Annexin V. The vertical axis indicates PI fluorescence intensity and horizontal axis Annexin V fluorescence. Exposure to 0.4 mM MeHg for 16 h increased the number of cells undergoing apoptosis (Annexin V-FITC-positive and PI-negative). A minor population of cells was observed to be Annexin V-FITC- and PI-positive, indicating that they were in end-stage apoptosis or already dead. The lower panel shows the profile of frequency of viable cells (Annexin V-FITC- and PI-negative) and cells undergoing apoptosis (Annexin V-FITC-positive and PI-negative). Pretreatment with TPG decreased