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      Nrf2, the Master Regulator of Anti-Oxidative Responses

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          Abstract

          Tight regulation of inflammation is very important to guarantee a balanced immune response without developing chronic inflammation. One of the major mediators of the resolution of inflammation is the transcription factor: the nuclear factor erythroid 2-like 2 (Nrf2). Stabilized following oxidative stress, Nrf2 induces the expression of antioxidants as well as cytoprotective genes, which provoke an anti-inflammatory expression profile, and is crucial for the initiation of healing. In view of this fundamental modulatory role, it is clear that both hyper- or hypoactivation of Nrf2 contribute to the onset of chronic diseases. Understanding the tight regulation of Nrf2 expression/activation and its interaction with signaling pathways, known to affect inflammatory processes, will facilitate development of therapeutic approaches to prevent Nrf2 dysregulation and ameliorate chronic inflammatory diseases. We discuss in this review the principle mechanisms of Nrf2 regulation with a focus on inflammation and autophagy, extending the role of dysregulated Nrf2 to chronic diseases and tumor development.

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          Molecular mechanisms of the Keap1–Nrf2 pathway in stress response and cancer evolution.

          The Keap1–Nrf2 regulatory pathway plays a central role in the protection of cells against oxidative and xenobiotic damage. Under unstressed conditions, Nrf2 is constantly ubiquitinated by the Cul3–Keap1 ubiquitin E3 ligase complex and rapidly degraded in proteasomes. Upon exposure to electrophilic and oxidative stresses, reactive cysteine residues of Keap1 become modified, leading to a decline in the E3 ligase activity, stabilization of Nrf2 and robust induction of a battery of cytoprotective genes. Biochemical and structural analyses have revealed that the intact Keap1 homodimer forms a cherry-bob structure in which one molecule of Nrf2 associates with two molecules of Keap1 by using two binding sites within the Neh2 domain of Nrf2. This two-site binding appears critical for Nrf2 ubiquitination. In many human cancers, missense mutations in KEAP1 and NRF2 genes have been identified. These mutations disrupt the Keap1–Nrf2 complex activity involved in ubiquitination and degradation of Nrf2 and result in constitutive activation of Nrf2. Elevated expression of Nrf2 target genes confers advantages in terms of stress resistance and cell proliferation in normal and cancer cells. Discovery and development of selective Nrf2 inhibitors should make a critical contribution to improved cancer therapy.
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            Decoding cell death signals in liver inflammation.

            Inflammation can be either beneficial or detrimental to the liver, depending on multiple factors. Mild (i.e., limited in intensity and destined to resolve) inflammatory responses have indeed been shown to exert consistent hepatoprotective effects, contributing to tissue repair and promoting the re-establishment of homeostasis. Conversely, excessive (i.e., disproportionate in intensity and permanent) inflammation may induce a massive loss of hepatocytes and hence exacerbate the severity of various hepatic conditions, including ischemia-reperfusion injury, systemic metabolic alterations (e.g., obesity, diabetes, non-alcoholic fatty liver disorders), alcoholic hepatitis, intoxication by xenobiotics and infection, de facto being associated with irreversible liver damage, fibrosis, and carcinogenesis. Both liver-resident cells (e.g., Kupffer cells, hepatic stellate cells, sinusoidal endothelial cells) and cells that are recruited in response to injury (e.g., monocytes, macrophages, dendritic cells, natural killer cells) emit pro-inflammatory signals including - but not limited to - cytokines, chemokines, lipid messengers, and reactive oxygen species that contribute to the apoptotic or necrotic demise of hepatocytes. In turn, dying hepatocytes release damage-associated molecular patterns that-upon binding to evolutionary conserved pattern recognition receptors-activate cells of the innate immune system to further stimulate inflammatory responses, hence establishing a highly hepatotoxic feedforward cycle of inflammation and cell death. In this review, we discuss the cellular and molecular mechanisms that account for the most deleterious effect of hepatic inflammation at the cellular level, that is, the initiation of a massive cell death response among hepatocytes. Copyright © 2013 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved.
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              A noncanonical mechanism of Nrf2 activation by autophagy deficiency: direct interaction between Keap1 and p62.

              In response to stress, cells can utilize several cellular processes, such as autophagy, which is a bulk-lysosomal degradation pathway, to mitigate damages and increase the chances of cell survival. Deregulation of autophagy causes upregulation of p62 and the formation of p62-containing aggregates, which are associated with neurodegenerative diseases and cancer. The Nrf2-Keap1 pathway functions as a critical regulator of the cell's defense mechanism against oxidative stress by controlling the expression of many cellular protective proteins. Under basal conditions, Nrf2 is ubiquitinated by the Keap1-Cul3-E3 ubiquitin ligase complex and targeted to the 26S proteasome for degradation. Upon induction, the activity of the E3 ubiquitin ligase is inhibited through the modification of cysteine residues in Keap1, resulting in the stabilization and activation of Nrf2. In this current study, we identified the direct interaction between p62 and Keap1 and the residues required for the interaction have been mapped to 349-DPSTGE-354 in p62 and three arginines in the Kelch domain of Keap1. Accumulation of endogenous p62 or ectopic expression of p62 sequesters Keap1 into aggregates, resulting in the inhibition of Keap1-mediated Nrf2 ubiquitination and its subsequent degradation by the proteasome. In contrast, overexpression of mutated p62, which loses its ability to interact with Keap1, had no effect on Nrf2 stability, demonstrating that p62-mediated Nrf2 upregulation is Keap1 dependent. These findings demonstrate that autophagy deficiency activates the Nrf2 pathway in a noncanonical cysteine-independent mechanism.
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                Author and article information

                Journal
                Int J Mol Sci
                Int J Mol Sci
                ijms
                International Journal of Molecular Sciences
                MDPI
                1422-0067
                20 December 2017
                December 2017
                : 18
                : 12
                : 2772
                Affiliations
                [1 ]Fraunhofer Institute for Molecular Biology and Applied Ecology IME, Project Group Translational Medicine & Pharmacology TMP, Theodor-Stern-Kai 7, 60590 Frankfurt, Germany; sandra.vomund@ 123456ime.fraunhofer.de (S.V.); Michael.Parnham@ 123456ime.fraunhofer.de (M.J.P.); b.bruene@ 123456biochem.uni-frankfurt.de (B.B.)
                [2 ]Institute of Biochemistry I-Pathobiochemistry, Faculty of Medicine, Goethe-University Frankfurt, Theodor-Stern-Kai 7, 60590 Frankfurt, Germany; schaefer@ 123456biochem.uni-frankfurt.de
                Author notes
                [* ]Correspondence: vonknethen@ 123456biochem.uni-frankfurt.de ; Tel.: +49-69-6301-6989; Fax: +49-69-6301-4203
                [†]

                These authors contributed equally to this work.

                Article
                ijms-18-02772
                10.3390/ijms18122772
                5751370
                29261130
                dffe1b6f-97f7-4429-9def-374dee24edfd
                © 2017 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 17 November 2017
                : 16 December 2017
                Categories
                Review

                Molecular biology
                nrf2,electrophiles,reactive oxygen species,transcription factor,antioxidants
                Molecular biology
                nrf2, electrophiles, reactive oxygen species, transcription factor, antioxidants

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