Methylmercury exposure induces ROS/Akt inactivation-triggered endoplasmic reticulum stress-regulated neuronal cell apoptosis

Yao-Pang Chung; Cheng-Chieh Yen; Feng-Cheng Tang; Kuan-I Lee; Shing-Hwa Liu; Chin-Ching Wu; Shang-Shu Hsieh; Chin-Chuan Su; Chun-Ying Kuo; Ya-Wen Chen

doi:10.1016/j.tox.2019.152245

Methylmercury exposure induces ROS/Akt inactivation-triggered endoplasmic reticulum stress-regulated neuronal cell apoptosis

Toxicology. 2019 Sep 1:425:152245. doi: 10.1016/j.tox.2019.152245. Epub 2019 Jul 19.

Authors

Affiliations

¹ Institute of Toxicology, College of Medicine, National Taiwan University, Taipei 100, Taiwan; Department of Occupational Safety and Health, College of Health Care and Management, Chung Shan Medical University, Taichung, 402, Taiwan.
² Department of Occupational Safety and Health, College of Health Care and Management, Chung Shan Medical University, Taichung, 402, Taiwan; Department of Occupational Medicine, Chung Shan Medical University Hospital, Taichung, 402, Taiwan.
³ Department of Occupational Medicine, Changhua Christian Hospital, Changhua County, 500, Taiwan; Department of Leisure Services Management, Chaoyang University of Technology, Taichung, 413, Taiwan.
⁴ Department of Emergency, Taichung Tzuchi Hospital, The Buddhist Tzuchi Medical Foundation, Taichung, 427, Taiwan.
⁵ Institute of Toxicology, College of Medicine, National Taiwan University, Taipei 100, Taiwan.
⁶ Department of Public Health, China Medical University, Taichung, 404, Taiwan.
⁷ Department of Otorhinolaryngology, Head and Neck Surgery, Changhua Christian Hospital, Changhua County 500, Taiwan.
⁸ Department of Physiology and Graduate Institute of Basic Medical Science, School of Medicine, College of Medicine, China Medical University, Taichung 404, Taiwan. Electronic address: ywc@mail.cmu.edu.tw.

PMID: 31330229
DOI: 10.1016/j.tox.2019.152245

Abstract

Epidemiological studies have positively linked mercury exposure and neurodegenerative diseases (ND). Methylmercury (MeHg), an organic form of mercury, is a ubiquitous and potent environmental neurotoxicant that easily crosses the blood-brain barrier and causes irreversible injury to the central nervous system (CNS). However, the molecular mechanisms underlying MeHg-induced neurotoxicity remain unclear. Here, the present study found that Neuro-2a cells underwent apoptosis in response to MeHg (1-5 μM), which was accompanied by increased phosphatidylserine (PS) exposure on the outer cellular membrane leaflets, caspase-3 activity, and the activation of caspase cascades and poly (ADP-ribose) polymerase (PARP). Exposure of Neuro-2a cells to MeHg also triggered endoplasmic reticulum (ER) stress, which was identified via several key molecules (including: glucose-regulated protein (GRP)78, GRP94, C/EBP homologous protein (CHOP) X-box binding protein(XBP)-1, protein kinase R-like ER kinase (PERK), eukaryotic initiation factor 2α (eIF2α), inositol-requiring enzyme(IRE)-1, activation transcription factor(AFT)4, and ATF6. Transfection with GRP78-, GRP94-, CHOP-, and XBP-1-specific small interfering (si)RNA significantly suppressed the expression of these proteins, and attenuated cytotoxicity and caspase-12, -7, and -3 activation in MeHg-exposed cells. Furthermore, MeHg dramatically decreased Akt phosphorylation, and the overexpression of activation of Akt1 (myr-Akt1) could significantly prevent MeHg-induced Akt inactivation, as well as apoptotic and ER stress-related signals. Pretreatment with the antioxidant N-acetylcysteine (NAC) effectively prevented MeHg-induced neuronal cell reactive oxygen species (ROS) generation, apoptotic and ER stress-related signals, and Akt inactivation. Collectively, these results indicate that MeHg exerts its cytotoxicity in neurons by inducing ROS-mediated Akt inactivation up-regulated ER stress, which induces apoptosis and ultimately leads to cell death.

Keywords: Akt; Apoptosis; Endoplasmic reticulum stress; Methylmercury; Neurotoxicity; Reactive oxygen species.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Animals
Apoptosis / drug effects*
Blotting, Western
Caspase 3 / metabolism
Cell Line, Tumor
Endoplasmic Reticulum Chaperone BiP
Endoplasmic Reticulum Stress / drug effects*
Methylmercury Compounds / toxicity*
Mice
Neurons / drug effects*
Proto-Oncogene Proteins c-akt / metabolism*
Reactive Oxygen Species / metabolism*
Real-Time Polymerase Chain Reaction

Substances

Endoplasmic Reticulum Chaperone BiP
Hspa5 protein, mouse
Methylmercury Compounds
Reactive Oxygen Species
Proto-Oncogene Proteins c-akt
Caspase 3