Opposing Roles for ERK1/2 in Neuronal Oxidative Toxicity: DISTINCT MECHANISMS OF ERK1/2 ACTION AT EARLY VERSUS LATE PHASES OF OXIDATIVE STRESS

Yue Luo,Donald B Defranco

doi:10.1074/jbc.m512430200

Abstract

Glutamate-induced oxidative toxicity is mediated by glutathione depletion in the HT22 mouse hippocampal cell line. Previous results with pharmacological agents implicated the extracellular signal-regulated kinases-1/2 (ERK1/2) in glutamate toxicity in HT22 cells and immature embryonic rat cortical neurons. In this report, we definitively establish a role for ERK1/2 in oxidative toxicity using dominant negative MEK1 expression in transiently transfected HT22 cells to block glutamate-induced cell death. In contrast, chronic activation of ERK (i.e. brought about by transfection of constitutively active ERK2 chimera) is not sufficient to trigger HT22 cell death demonstrating that ERK1/2 activation is not sufficient for toxicity. Activation of ERK1/2 in HT22 cells has a distinct kinetic profile with an initial peak occurring between 30 min and 1 h of glutamate treatment and a second peak typically emerging after 6 h. We demonstrate here that the initial phase of ERK1/2 induction is because of activation of metabotropic glutamate receptor type I (mGluRI). ERK1/2 activation by mGluRI contributes to an HT22 cell adaptive response to oxidative stress as glutamate-induced toxicity is enhanced upon pharmacological inhibition of mGluRI. The protective effect of ERK1/2 activation at early times after glutamate treatment is mediated by a restoration of glutathione (GSH) levels that are reduced because of depletion of intracellular cysteine pools. Thus, ERK1/2 appears to play dual roles in HT22 cells acting as part of a cellular adaptive response during the initial phases of glutamate-induced oxidative stress and contributing to toxicity during later stages of stress.

Highlights

Oxidative stress can contribute to neuronal toxicity and has been implicated in both acute injury and chronic neuropathological conditions [1, 2]
HT22 cells were cotransfected with dominant negative MEK1 (DN-MEK1) and mitochondrial-targeted enhanced yellow fluorescent protein expression plasmids
A mechanistic basis for diverse effects of Extracellular signal-regulated kinases-1/2 (ERK1/2) in neurons is beginning to emerge, it is often difficult to make meaningful comparisons of results obtained in different cell lines or neuronal cell types

Summary

Introduction

Oxidative stress can contribute to neuronal toxicity and has been implicated in both acute injury and chronic neuropathological conditions [1, 2]. Oxidative toxicity can be induced by glutamate treatment in the HT22 mouse hippocampal cell line [3,4,5] and immature primary embryonic rat cortical neuron cultures [4, 6, 7] In these models, glutamate treatment leads to glutathione (GSH) depletion and subsequent accumulation of reactive oxygen species (ROS) [8]. Signaling kinases, such as mitogen-activated protein kinases (MAPK), are activated upon glutamate-induced oxidative stress in HT22 cells and primary neurons and are likely to affect targets that either limit or promote oxidative toxicity. MGluRI activation of ERK1/2 represents a cellular defense response that attempts to limit glutathione depletion resulting from glutamate-induced cysteine depletion This protective response mediated by ERK1/2 is unable to overcome an overwhelming and chronic oxidative stress that utilizes ERK1/2 in its final stages to promote neuronal cell death. ERK1/2 activation may serve opposing roles in neuronal oxidative toxicity acting initially through effects on glutathione metabolism to limit oxidative stress but

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