Nrf2 signaling and cell survival

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Abstract

Nrf2:INrf2 acts as a sensor for oxidative/electrophilic stress. INrf2 serves as an adaptor to link Nrf2 to the ubiquitin ligase Cul3–Rbx1 complex that ubiquitinate and degrade Nrf2. Under basal conditions, cytosolic INrf2/Cul3–Rbx1 is constantly degrading Nrf2. When a cell encounters stress Nrf2 dissociates from the INrf2 and translocates into the nucleus. Oxidative/electrophilic stress induced modification of INrf2Cysteine151 and/or protein kinase C (PKC)-mediated phosphorylation of Nrf2Serine40 controls Nrf2 release from INrf2 followed by stabilization and nuclear translocation of Nrf2. Nrf2 binds to the antioxidant response element (ARE) and activates a myriad of genes that protect cells against oxidative/electrophilic stress and neoplasia. A delayed response of oxidative/electrophilic stress activates GSK-3β that phosphorylates Fyn at unknown threonine residue(s). Phosphorylated Fyn translocates to the nucleus and phosphorylates Nrf2Tyrosine568 that leads to nuclear export and degradation of Nrf2. Prothymosin-α mediated nuclear translocation of INrf2 also degrades nuclear Nrf2. The degradation of Nrf2 both in cytosol and nuclear compartments rapidly brings down its levels to normal resulting in suppression of Nrf2 downstream gene expression. An auto-regulatory loop between Nrf2 and INrf2 controls their cellular abundance. Nrf2 regulates INrf2 by controlling its transcription, and INrf2 controls Nrf2 by degrading it. In conclusion, switching on and off of Nrf2 combined with promoting an auto-regulatory loop between them regulates activation/deactivation of defensive genes leading to protection of cells against adverse effects of oxidative and electrophilic stress and promote cell survival.

Introduction

Reactive oxygen species (ROS) cause oxidative stress and have a profound impact on the survival and evolution of all living organisms (Breimer, 1990, Meneghini, 1997). Endogenous and exogenous reactions/pathways generate ROS (Grisham and McCord, 1986, Helen et al., 1993, Kerr et al., 1996, Wei, 1998, Kasprzak, 1995, Suzuki et al., 1997, Kim et al., 2008, Breen and Murphy, 1995, Last et al., 1994, Goetz and Luch, 2008, Ward, 1994). Therefore, it is clear that all cells must continuously strive to keep the levels of ROS in check. ROS attack DNA and other cellular macromolecules causing oxidative stress and many other physiological and pathological conditions. These conditions include aging, neurodegenerative diseases, arthritis, arteriosclerosis, inflammatory responses, and tumor induction and promotion (Grisham and McCord, 1986, Ward, 1994, Kim et al., 2008, Rosen et al., 1995). Many of these conditions, including cancer, are often preceded by damage or mutation of genomic DNA (Ames et al., 1995). Much of the research on ROS has been centered on the damaging effects of oxidative stress. However, it is now apparent that ROS activate a battery of cellular enzymes that either prevent the generation of ROS or detoxify ROS and thereby protect the cell against damage caused by oxidative stress. Prokaryotic cells utilize transcription factors OxyR and SoxRS to sense the redox state of the cell, and in times of oxidative stress these factors induce the expression of about eighty defensive genes (Bauer et al., 1999, Zheng and Storz, 2000). Similar mechanisms of protection against oxidative stress have been found in eukaryotic cells that activate at least one hundred genes (Dhakshinamoorthy et al., 2000, Jaiswal, 2004, Zhang, 2006, Kobayashi and Yamamoto, 2006, Copple et al., 2008). Their products regulate a wide variety of cellular activities including signal transduction, proliferation, and immunologic defense reactions. There is a great variety of factors involved in the cellular response to oxidative stress. For instance, activation of heat shock response activator protein 1, NF-E2 related factor 2 (Nrf2), extracellular signal related kinases (ERK1/2), protein kinase B (Akt), and NF-κB promote cell survival, whereas prolonged activation of c-jun N-terminal kinases (JNK), p38 kinase, and TP53 may lead to cell cycle arrest and apoptosis (Halliwell and Gutteridge, 2007). The Nrf2 pathway is presumably the most important for the cell to deal with oxidative/electrophilic stress generated from exposure to exogenous and endogenous chemicals, metals and radiation (Dhakshinamoorthy et al., 2000).

Section snippets

Antioxidant response element and NF-E2 related factors

Exposure of cells to antioxidants and xenobiotics leads to the induction of a battery of defensive genes encoding detoxifying enzymes [NAD(P)H:quinone oxidoreductase 1 (NQO1), glutathione S-transferases (GST), heme oxygenase 1 (HO-1)], antioxidant and related proteins [thioredoxins, γ-glutamate cysteine ligase (γ-GCS), glutathione peroxidase, glutathione reductase], ubiquitination enzymes and proteasomes, and drug transporters (MRPs) (reviewed in Dhakshinamoorthy et al., 2000, Jaiswal, 2004,

INrf2, a cytosolic Inhibitor of Nrf2

INrf2 (Inhibitor of Nrf2) or KEAP1 (Kelch-like ECH-associated protein1), protein retains Nrf2 in the cytoplasm (Dhakshinamoorthy and Jaiswal, 2001, Itoh et al., 1999) (Fig. 2). Analysis of the INrf2 amino acid sequence revealed a protein–protein interaction domain BTB/POZ BTB (broad complex, tramtrack, bric-a-brac)/POZ (poxvirus, zinc finger) and a Kelch domain (Itoh et al., 1999). In the Drosophila Kelch protein, and in PIP, the Kelch domain binds to actin (Albagli et al., 1995, Kim et al.,

Mechanism of signal transduction and regulation of Nrf2 activation and degradation

A hypothetical model illustrating the role of INrf2, Nrf2 and other ARE-binding factors in activation of antioxidant genes by antioxidants and xenobiotics is shown in Fig. 3. Nrf2:INrf2 acts as a sensor for oxidative/electrophilic stress. In response to oxidative/electrophilic stress, Nrf2 is switched on (separation from INrf2 and stabilization of Nrf2) and then off (ubiquitination and degradation of Nrf2) by distinct early and delayed mechanisms. Oxidative/electrophilic modification of

Auto-regulatory loop between INrf2 and Nrf2

An auto-regulatory loop between stress sensors INrf2 and Nrf2 controls their cellular abundance and ARE-mediated gene expression and induction (Lee et al., 2007). An ARE in the reverse strand of the proximal INrf2 promoter that binds to Nrf2 regulates expression and antioxidant induction of the INrf2 gene expression. The induction of INrf2 followed ubiquitination and degradation of Nrf2 and suppression of INrf2 gene expression. In other words, Nrf2 regulates INrf2 by controlling its

Nrf2:INrf2 in cell survival, cancer and chemoprevention, and drug resistance

Nrf2 is considered a major mechanism of protection against chemical and radiation capable of damaging DNA integrity and initiating carcinogenesis (Giudice and Montella, 2006). It protects cells against chemical and radiation stress and promotes cell survival. The Nrf2-knockout mouse was prone to acute damages induced by acetaminophen, ovalbumin, cigarette smoke, pentachlorophenol and 4-vinylcyclohexene diepoxide and had increased tumor formation when they were exposed to carcinogens such as

Future perspectives

Further studies are required to completely understand the mechanism of signal transduction from chemicals and radiation to Nrf2:INrf2 complex leading to the release of Nrf2 from INrf2 and activation of a battery of defensive genes. Preliminary studies demonstrate that deactivation of Nrf2 is equally important as activation of Nrf2. Therefore, studies must also explore negative regulation of Nrf2. Future studies should also address the role of Nrf2 and INrf2 in cell survival and drug resistance

Acknowledgments

This work was supported by NIGMS grant RO1 GM047466 and NIEHS grant RO1 ES012265.

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