Human glutamate cysteine ligase gene regulation through the electrophile response element

Abstract

Glutathione (GSH) is the primary nonprotein thiol in the cell. It has many important roles in cell function, including regulating redox-dependent signal transduction pathways. The content of GSH within the cell varies with stress. In many cases, a process involving GSH synthesis results in adaptation to subsequent stressors. Sustained increases in GSH content are controlled primarily through induction of two genes, Gclc and Gclm, leading to the synthesis of the rate-limiting enzyme for GSH synthesis, glutamate cysteine ligase. Each of these genes in humans has a number of putative enhancer elements in their promoters. Overall, the most important element in both Gclc and Gclm expression is the electrophile response element. We review the evidence that has led to this conclusion and the implications for the redox-dependent regulation of this critical intracellular antioxidant.

Introduction

Glutathione (GSH) is found in the millimolar range in most cells, making it the most abundant nonprotein thiol. GSH is well known to be essential to antioxidant defense, regulation of the cell cycle, and gene expression [1], [2], [3], [4], [5]. The involvement of GSH in the protection of the cell against exposure to toxicants, and in the metabolism of xenobiotic compounds through formation of conjugates, is well established. GSH also interacts with glutaredoxin and protein disulfide isomerases to modulate the tertiary structure of proteins through thiol–disulfide exchange. In addition to the essential role in defense and metabolism, it has become clear in the past few years that GSH contributes to redox signaling mediated by reactive oxygen and nitrogen species. Mechanisms include the acute regulation of specific enzymes, such as the nitric oxide synthases, cyclooxygenases, and lipoxygenases that produce redox-active signaling molecules, and the signal transduction pathways that control the transcriptional regulation of proteins that adapt the cell to environmental or pathological stress.

The response of a cell to a stress often involves changes in GSH content, which may first be consumed in reactions that protect the cell by removing the deleterious compound and then restored to levels which often exceed those found before exposure to the stressor. GSH is depleted as it forms conjugates with a great variety of electrophilic compounds, primarily through the action of glutathione S-transferases (GST) [6]. Conjugation with GSH is a frequent, although not universal, aspect of both xenobiotic and normal physiological metabolism, as mentioned above, and has been thoroughly reviewed [7]. When glutathione conjugates are formed with small molecules they are then excreted from cells [8], which is generally considered an important detoxification mechanism, including the removal of electrophiles [9]. Glutathione peroxidase uses GSH as a cofactor to remove peroxides from the cell, leading to the formation of glutathione disulfide, GSSG. GSH must then be replaced through either enzymatic reduction of GSSG by glutathione reductase or de novo synthesis. Enzymatic synthesis is primarily controlled at the level of transcription of two genes, Gclc and Gclm. The regulation of these genes is predominantly mediated by the electrophile response element, or EpRE. This review summarizes our current understanding of the EpRE-dependent regulation of these genes in humans and examines the redox-depending signaling pathways that mediate this expression.