Methylmercury, an environmental electrophile capable of activation and disruption of the Akt/CREB/Bcl-2 signal transduction pathway in SH-SY5Y cells

Takamitsu Unoki,Yoshito Kumagai,Koji Tsuboi,Toshiyuki Kaji,Motohiro Nishida,Yumi Abiko,Takashi Uehara,Takashi Toyama

doi:10.1038/srep28944

Abstract

Methylmercury (MeHg) modifies cellular proteins via their thiol groups in a process referred to as “S-mercuration”, potentially resulting in modulation of the cellular signal transduction pathway. We examined whether low-dose MeHg could affect Akt signaling involved in cell survival. Exposure of human neuroblastoma SH-SY5Y cells of up to 2 μM MeHg phosphorylated Akt and its downstream signal molecule CREB, presumably due to inactivation of PTEN through S-mercuration. As a result, the anti-apoptotic protein Bcl-2 was up-regulated by MeHg. The activation of Akt/CREB/Bcl-2 signaling mediated by MeHg was, at least in part, linked to cellular defence because either pretreatment with wortmannin to block PI3K/Akt signaling or knockdown of Bcl-2 enhanced MeHg-mediated cytotoxicity. In contrast, increasing concentrations of MeHg disrupted Akt/CREB/Bcl-2 signaling. This phenomenon was attributed to S-mercuration of CREB through Cys286 rather than Akt. These results suggest that although MeHg is an apoptosis-inducing toxicant, this environmental electrophile is able to activate the cell survival signal transduction pathway at lower concentrations prior to apoptotic cell death.

Highlights

Methylmercury (MeHg) exists as a naturally occurring substance or industrially produced environmental contaminant and is known to be a neurotoxicant that affects the central nervous system resulting in disorders such as Minamata disease in Japan and MeHg poisoning in Iraq[1,2]
Knockdown of B-cell lymphoma 2 (Bcl-2) significantly accelerated MeHg-mediated cytotoxicity. These results suggest that activating the Akt/cAMP response element-binding protein (CREB) signal transduction pathway caused by treatment with MeHg certainly diminishes late apoptosis and its cytotoxicity during MeHg exposure, indicative of a hormetic response
The present study indicates that activation of the Akt/CREB/Bcl-2 signal transduction pathway in response to MeHg in SH-SY5Y cells does not increase linearly but rather follows a bell-shaped curve

Summary

Introduction

Methylmercury (MeHg) exists as a naturally occurring substance or industrially produced environmental contaminant and is known to be a neurotoxicant that affects the central nervous system resulting in disorders such as Minamata disease in Japan and MeHg poisoning in Iraq[1,2]. The environmental electrophile 1,2-naphthoquinone activates protein tyrosine phosphatase (PTP) 1B/epidermal growth factor receptor (EGFR) signalling, which is involved in cell survival, and kelch-like ECH-associated protein 1 (Keap1)/nuclear factor (erythroid-derived 2)-like 2 (Nrf2) signalling, which regulates phase-II xenobiotic detoxification enzymes and phase-III transporters associated with cellular protection against this electrophile through S-arylation of PTP1B and Keap[110,11] These observations suggest that MeHg may be associated with disruption of cellular homeostasis, and activation of the cellular defence system at low doses through induction of electrophilic signal transduction pathways resulting. We hypothesized that activation of such different signalling pathways associated with cell survival and cell death could be dependent on the examined concentration and covalent modification of cellular proteins by MeHg seems to play a role in the observed phenomena To address this issue, we explored MeHg-mediated non-linear Akt/CREB/ Bcl-2 signal transduction and contribution of S-mercuration to modulation of the signaling

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Conclusion