Open access peer-reviewed chapter

Multiple Modes of Nrf2 Regulation and Transcriptional Response

Written By

Sherin T. Mathew and Ola Hammarsten

Submitted: 12 April 2016 Reviewed: 22 August 2016 Published: 21 December 2016

DOI: 10.5772/65295

From the Edited Volume

A Master Regulator of Oxidative Stress - The Transcription Factor Nrf2

Edited by Jose Antonio Morales-Gonzalez, Angel Morales-Gonzalez and Eduardo Osiris Madrigal-Santillan

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Cells have defense systems to deal with chemical insults from the environment. Some examples are chemical scavengers like glutathione and enzymes such as superoxide dismutase that inactivate radicals and other reactive chemicals in the cytoplasm. It is perhaps surprising that these protective systems are not maximally expressed in an unstressed cell. Rather, the ability to inactivate toxic chemicals is tightly regulated and only induced when needed. As a consequence, unstressed cells are usually very sensitive to radicals, but become more resistant as the cellular defense system has been appropriately upregulated after a few hours. The transcription factor Nrf2 is known to be a master regulator of many cytoprotective enzymes and proteins. Chemical inducers of Nrf2 inactivate its repressor, Keap1, when they react with critical cysteine residues in Keap1. The release of Nrf2 from Keap1 results in enhanced expression of genes involved in detoxification. This generates a feedback loop where Nrf2 induces protective enzymes capable of inactivating the chemical that reacted with Keap1. An unproven, but likely, scenario is that Nrf2 transcriptional response can vary depending on the nature of the chemical insult. The aim of this chapter is to examine the mechanisms by which the cell can sense different reactive chemicals and modulate protective responses. It is likely that this knowledge is of vital importance in the development of clinical Nrf2 activators in preventive medicine.


  • Keap1
  • Nrf2
  • cysteine residues
  • transcriptional response
  • Nrf2 activator

1. Introduction

Cells in our bodies are constantly challenged by reactive substances such as radicals, in part due to the high oxygen content in the surrounding. To prevent unwanted chemical reactions, cells have evolved sophisticated defense mechanisms. These systems allow cellular adaptation to toxins in our environment by regulating the expression of an elaborate network of cytoprotective proteins and chemical scavengers.

To a large extent, this cytoprotective system is regulated by the transcription factor nuclear E2‐factor‐related factor 2 (Nrf2) and its primary suppressor kelch‐like ECH‐associated protein 1 (Keap1). Under basal conditions, Keap1 decreases the Nrf2 protein levels through ubiquitination and proteasomal degradation [1, 2]. Upon activation, Nrf2 escapes Keap1 repression and Nrf2 protein levels get stabilized and translocate to the nucleus. In the nucleus, Nrf2‐Maf heterodimer binds to the antioxidant response element (ARE) in the regulatory region of target genes and promotes transcription [3]. A wide array of genes including antioxidant proteins and detoxifying enzymes, transporter and metabolic enzymes, enzymes for glutathione biosynthesis, proteases, and chaperone are transcriptionally activated by Nrf2 [4]. It is likely that the cellular response to a chemical challenge will be different if the toxic substance is an oxidative or a reductive toxin.

An example of this diversification in response can be found in the bacteria Escherichia coli where the redox system is mainly regulated by transcription factors OxyR and SoxRS. The cysteine residues of OxyR sense elevated levels of hydrogen peroxide [5], whereas SoxRS iron‐sulfur (2Fe‐2S) clusters act as sensors of superoxide. When activated in this way, OxyR and SoxRS become active transcription factors that induce distinct but partially overlapping set of cytoprotective genes adapting the cell to deal with hydrogen peroxide or superoxide.

In a similar way, it is thought that our cells diversify the redox response depending on the nature of the chemical insult. As mentioned above, electrophiles and oxidants react with cysteine residues in Keap1 which blocks Nrf2 proteasomal degradation and thus mediates transcriptional activation of many genes [6]. However, little is known about how the Nrf2 system can differentiate the transcriptional response. Here, we review the most recent literature on how Nrf2 cross talks with multiple signaling pathways and evokes different signaling response including inflammation, metabolism, apoptosis, proliferation, and differentiation. This knowledge is likely of great importance when Nrf2‐activating drugs are developed to boost our radical defense systems.


2. Redox regulation in bacteria

The bacterial redox system in E. coli is coordinately regulated by two transcription factors—SoxRS and OxyR. SoxRS is activated in response to stress induced by superoxide anion and OxyR responds to stress caused by hydrogen peroxide.

2.1. OxyR‐hydrogen peroxide sensor

The transcription factor OxyR belongs to LysR family of transcriptional activators. In response to hydrogen peroxide stress, two cysteine residues, Cys‐199 and Cys‐208 oxidize leading to the formation of intramolecular disulfide bond [7]. Oxidized OxyR promotes transcription of genes including katG (a hydrogen peroxidase I), ahpCF (an alkylhydroperoxide reductase), oxyS (a regulatory RNA involved in DNA repair), gorA (a glutathione reductase), and glutaredoxin 1 (grxA). Enzymatic reduction of the disulfide bond switches off the OxyR function and the OxyR transcription factor therefore functions as an “on/off switch” with disulfide bond formation in response to oxidative stress [810]. Studies suggest that Cys‐199 thiol activates OxyR through several redox‐related modifications including S‐OH, S‐nitrosylation, and S‐glutathione and the resulting OxyR differs in structure, properties, and genes activated [5].

2.2. SoxRS‐superoxide sensor

The superoxide response system in E. coli regulates transcription of targets involved in detoxification (superoxide dismutase), DNA repair (endonuclease IV), and glucose‐6‐phosphate dehydrogenase. The iron‐sulfur (2Fe‐2S) clusters of the SoxR act as sensors of superoxide and undergo one‐electron oxidation/reduction and induce the transcriptional activity of SoxR [11]. SoxR protein binds DNA to activate the expression of SoxS which in turn activates cell protection genes in response to superoxide and nitric oxide [12]. Site‐specific mutation studies have shown that four conserved cysteine residues at the C‐terminal domain of the SoxR polypeptide act as the ligands for the [2Fe‐2S] clusters which has crucial role in transcriptional activity of SoxR [13].

2.3. OxyR‐SoxR interaction

Both OxyR and SoxR proteins exist in oxidized and reduced forms but only the oxidized form of these proteins induces the expression of antioxidant defense system. Although the SoxR and SoxS proteins are mainly involved in response to superoxide, several studies have reported that SoxRS regulon may be activated by hydrogen peroxide indicating the overlapping between the specific response systems. SoxR protein senses the increased levels of hydrogen peroxide and activates the SoxRS system [1416].


3. Keap1‐mediated Nrf2 regulation

The critical importance of Keap1 as a negative regulator of Nrf2 is supported by the observation that the deletion of Keap1 gene in mice causes constitutive activation of Nrf2. Keap1-/- knockout mice died shortly after birth due to hyperkeratosis in the upper digestive tract but the phenotype conditions were reversed when both Nrf2 and Keap1 were disrupted [17, 18]. The cysteine‐rich protein Keap1 regulates active degradation of Nrf2 under basal conditions by functioning as an adaptor to cullin3 (Cul3)‐ringbox1 (Rbx1) containing E3 ubiquitin ligase complex (Figure 1) [19]. The Neh2 domain of Nrf2 binds to Kelch domain of Keap1 through the “hinge‐and‐latch” mechanism [20]. Under basal conditions, Nrf2 ETGE motif acts as a hinge and forms an “open” conformation by binding to Kelch subunit of Keap1 and the DLG motif which acts as the latch binds to Keap1 subunit to form the “closed” conformation and targets Nrf2 for proteasomal degradation. Cysteine residues of Keap1 sense reactive oxygen species (ROS) or electrophiles in the cellular environment causing conformation changes in Keap1. The modified Keap1 can disrupt its interaction with the low‐affinity DLG motif, whereas the high‐affinity ETGE motif remains associated with Keap1. As the DLG motif fails to bind to Keap1, it affects the orientation of lysine residues within the Neh2 domain of Nrf2 preventing its ubiquitination and degradation [21]. After redox homeostasis is restored, Keap1 moves into the nucleus and controls nuclear export of Nrf2 for subsequent proteasomal degradation in the cytoplasm (Figure 1) [22].

Figure 1.

Keap1‐mediated Nrf2 regulatory pathway. Under basal conditions, Nrf2 is bound to Keap1 and undergoes rapid degradation. Upon induction, cysteine residues in Keap1 are modified, the E3 ubiquitin ligase activity is suppressed, and Nrf2 levels increase. Activated Nrf2 enters the nucleus and dimerizes with Maf to promote transcription of ARE‐dependent genes. Finally, Nrf2 is transported out of the nucleus by Keap1 for subsequent proteasomal degradation.

3.1. Distinct Keap1 cysteine modifications

Various synthetic and plant‐derived phytochemicals including isothiocyanates, Michael acceptors, and coumarins are shown to activate Nrf2 system [23]. Many of these substances protect human cells and animals from a diverse array of toxins and radiation [24]. These structurally diverse Nrf2‐inducing agents share a common property of reacting with sulfhydryl groups and cysteine residues in Keap1 [6, 25]. ROS or electrophilic reaction with specific cysteine residues causes conformational changes in Keap1 and prevents proteasomal degradation of Nrf2 [26]. Site‐directed mutagenic studies have identified critical cysteine residues as important factors involved in Nrf2 regulation. Mutation in Cys273 or Cys288, located at intervening region of Keap1, blocked the Keap1‐dependent ubiquitination and degradation of Nrf2 under basal conditions. Mutation in Cys151 at the BTB domain of Keap1 blocked Nrf2 release from Keap1 in response to sulforaphane and maintained Keap1 repression of Nrf2. Cys273 or Cys288 is required for Keap1 repression of Nrf2 under basal conditions and Cys151 is important for Nrf2 activation in response to electrophilic stress [2729].

A “cysteine code” has been proposed for the Keap1‐dependent Nrf2 regulation as cysteine modifications play crucial role in mediating Nrf2 activation [30]. Studies suggest that Nrf2‐inducing agents such as diethylmaleate (DEM), dimethylfumarate (DMF), and sulforaphane prefer Cys151 residue for Nrf2 induction, whereas 2‐cyano‐3, 12 dioxooleana‐1, 9d iene‐28‐imidazolide (CDDO‐Im) and heavy metals such as cadmium chloride (CdCl2) and arsenic activate Nrf2 in a Cys151‐independent manner. Another well‐known Nrf2 inducer, tert‐butyl hydroquinone (tBHQ), gets oxidized to the electrophilic metabolite tert‐butyl benzoquinone and modifies the Cys151 cysteine residues [31, 32]. The differential reactivity of cysteines in Keap1, “the cysteine code,” does not, however, explain how this translates into differential toxin‐dependent activation of genes by Nrf2.


4. Nrf2 network

Recent advances have revealed that Nrf2 cross talks with different signaling pathways and influences the transcriptional response. Beyond cellular response against oxidative stress, Nrf2 is reported to be involved in inflammation, metabolism, apoptosis, proliferation, and differentiation.

4.1. Cross talk between Nrf2 and NF‐κB pathway

Nuclear factor‐κB (NF‐κB) was first identified in David Baltimore's laboratory around 30 years ago as a transcription factor that activated the kB immunoglobulin promoter in B‐cells [33]. Since then, NF‐κB has been implemented in a diverse array of conditions mostly linked to acute and chronic inflammation.

NF‐κB is normally sequestered in the cytoplasm by its negative regulator IkB-alpha;. Upon T‐cell or B‐cell receptor activation in response to infection or tumor necrosis factor (TNF)α receptor stimulation IkBα is phosphorylated by the IkB kinase (IKK) complex. Phosphorylation of IkBα is followed by ubiquitination and proteasomal degradation of IkBα and releases NF‐κB. NF‐κB translocates to the nucleus and activates target genes having the kB elements in their promoters [34]. It has been proposed that after simultaneous activation, NF‐κB antagonizes Nrf2‐mediated gene transcription. Conversely, some Nrf2 inducers suppress NF‐κB signaling. Therefore, it seems that the inflammatory induction of NF‐κB can be suppressed by Nrf2 activation and vice versa. It is possible that the inflammatory response that generates radicals to defeat bacteria must downregulate the radical scavenger function of the Nrf2 to function optimally [35] (Figure 2). Similarly, the radicals produced by the inflammatory response could activate the Nrf2 system in neighboring normal cells.

Therefore, the anti‐inflammatory effects of Nrf2 can be due, in part, to its ability to act as a feedback regulator of NF‐κB and the absence of Nrf2 can create a situation where NF‐κB lacks a controller to turn off the inflammatory signal, resulting in chronic inflammatory conditions such as observed in arteries damaged by ionizing radiation [36]. For example, Nrf2 knockout mice are more susceptible to lipopolysaccharide (LPS)‐induced neuroinflammation. Activation of the Nrf2 pathway in normal cells with sulforaphane decreased the production of inflammatory markers [37]. The Nrf2 target gene hemeoxygenase‐1 (HO‐1) inhibits NF‐κB‐mediated transcription of cellular adhesion molecules and could thereby block accumulation of inflammatory cells [38]. In addition, Keap1 downregulates NF‐κB by promoting proteasomal degradation of its activator IKKβ [39].

Similarly, there are several examples of how NF‐κB downregulates the Nrf2 response. Keap1 interacts with NF‐κB, and thereby represses the Nrf2 transcriptional activity [40]. NF‐κB blocks Nrf2 transcriptional activation of target genes [41]. This may result in increased oxidative stress which in turn further activates NF‐κB [42]. In addition, NF‐κB can also promote HDAC3 association with MafK and thus compete with Nrf2 heterodimer formation and transcription of Nrf2‐dependent genes [43].

There are also examples where NF‐κB promoting activation of Nrf2 and Nrf2‐regulated genes. Functional NF‐κB‐binding sites have been found in the promoter of the NRF2 gene resulting in overexpression of Nrf2 in acute myeloid leukemia cells [44]. In another study, the activation of small GTPase protein RAC1 (Ras‐related C3 botulinum toxin substrate 1) induced the expression of Nrf2 target gene HO‐1 and caused the inhibition of NF‐κB function [45].

Figure 2.

Different factors involved in the functional network between Nrf2 and NF‐κB network.

Several other proteins are known to interact with both pathways during the signaling process. One example is p62 protein which accumulates due to autophagy deficiency, and activates Nrf2 through direct interaction with Keap1. P62 sequesters Keap1 into aggregates and inhibits Keap1‐mediated ubiquitylation and degradation of Nrf2. This resulted in increased Nrf2 stabilization and activation of target genes [46, 47]. Similarly, p62 protein oligomerizes and promotes nerve growth factor (NGF)‐mediated NF‐κB signaling [48].

Another protein that interacts with both Nrf2 and NF‐κB pathway is glycogen synthase kinase‐3β (GSK‐3β), a Ser/Thr kinase involved in glycogen metabolism and apoptosis. GSK‐3β phosphorylates the Neh6 domain of Nrf2 and targets subsequent proteasomal degradation by β‐TrCP (β‐transducing repeat‐containing protein)‐Skp1(S‐phase kinase‐associated protein1)‐Cul1‐Rbx1 E3 ubiquitin ligase complex [49]. In NF‐κB system, GSK‐3β phosphorylates p65 subunit and increases its DNA‐binding affinity and subsequent transcriptional response [50]. Moreover, β‐TrCP mediates proteasomal degradation of the inhibitory protein, IkBα, and allows NF‐κB release. Thus, β‐TrCP functions as positive regulator of NF‐κB activity and negative regulator of Nrf2‐ARE activity (Figure 2).

4.2. Nrf2 and other signaling pathways

Phosphorylation of Nrf2 by several protein kinases can lead to stabilization and activation of Nrf2. For example, several studies have demonstrated the involvement of P13 kinase/AKT pathway in regulating Nrf2 nuclear translocation and ARE‐dependent gene expression [51, 52]. P13K phosphorylation of PKB/Akt suppresses proteasomal degradation of Nrf2 by GSK‐3β [53]. In another study, the biotinylated derivative of the triterpenoid CDDO (2‐cyano‐3,12‐dioxooleana‐1,9‐dien‐28‐oic acid) activated PI3K‐PKB/Akt signaling through modification of Cys‐124 in the active site of PTEN, causing inhibition of the lipid phosphatase function of PTEN [54]. Similarly, increased Nrf2 activity was observed in PTEN‐mutant cells with increased activation of the PI3K‐Akt signaling which in turn suppressed the GSK‐3β‐mediated Nrf2 repression [55]. But in Keap1‐deficient cells, the deletion of PTEN increased Nrf2 accumulation to a greater extent and this supports the regulatory role of PTEN‐GSK‐3‐β‐TrCP signaling [56]. Under conditions of autophagy dysregulation, Nrf2 is activated by p62. In a recent study, Nrf2 inducer sulforaphane activated p62 through SPBP (stromelysin‐1 platelet‐derived growth factor‐responsive element‐binding protein), which acts as a coactivator of Nrf2 [57]. Another Nrf2 activator, arsenic, activates Nrf2 through p62‐mediated Keap1 sequestration and this persistent activation of Nrf2 can be the reason for arsenic‐induced toxicity [58]. The mitogen‐activated protein kinase (MAPK)‐signaling pathway is activated by Nrf2 inducers like tBHQ and sulforaphane [59, 60]. P38, a member of MAPK family, has been shown to influence Nrf2 activation both in positive and negative manner and this suggests the complex nature of Nrf2 regulation by MAPK [60]. During endoplasmic reticulum stress, protein kinase PERK activates Nrf2 and provides cell survival benefits. PERK‐induced phosphorylation of Nrf2 allows its release from Keap1 and translocation into the nucleus for subsequent gene transcription [61].

Another signaling pathway involving protein kinase C has been shown to increase Nrf2‐target gene expression while its inhibition caused significant decrease in tBHQ‐induced Nrf2 nuclear translocation [62]. PKC phosphorylates serine 40 of Nrf2 resulting in the Nrf2 release from Keap1 for subsequent gene expressions and mutation of specific residue reduced the gene expression level by 50% indicating that PKC functions along with Keap1 [63, 64].


5. Differential Nrf2 response

Based on the evidences, different inducers target specific or combination of different cysteine residues to activate Nrf2, suggesting the function of “cysteine codes” which converts the preferential target cysteine modifications into distinct biological effects. Understanding the cysteine code for each Nrf2‐activating compound will help to increase the biological effects of different inducers. However, Nrf2 activation could not be solely responsible for the diverse biological effects caused by Nrf2 inducers. Most of the Nrf2‐inducing agents have the inherent ability to react with cysteine residues and there is a possibility of interaction with other cellular proteins and thereby generating distinct cellular response. For example, a proteomics study of sulforaphane‐derived sulfoxythiocarbamate analogs has identified different protein targets other than Keap1 [65]. Many Nrf2 inducers are well known for the prevention and treatment of several human disorders and some of them have been clinically investigated. For example, the methyl ester derivative of CDDO triterpenoid is a potent Nrf2 inducer and as low nanomolar concentrations of CDDO‐Me (bardoxolone methyl) stimulate Nrf2‐dependent gene expressions. The phase II clinical trials for the treatment of chronic kidney disease (BEAM study) in patient with Type 2 diabetes indicated that CDDO-Me could improve kidney function. However, the phase III trial (BEACON study) was terminated due to serious side effects and mortality observed in treated group [66]. However, the exact mechanism behind the adverse effects are not clear, the long‐term drug exposure as well as administration of fixed dose of drug not adjusted for kidney function might have influenced the response. Nrf2 is overexpressed in many types of cancers and several oncogenes are reported to evoke Nrf2 expression in cancer cells [67]. As Nrf2 plays a key role in cytoprotection, cancer cells benefit the protective effect of Nrf2 to create a favorable microenvironment for tumor growth and drug resistance. Studies found that two different antioxidants, N‐acetylcysteine (NAC) or vitamin E supplementation in mice with lung tumors substantially increased the number, size, and stage of the tumors. NAC and vitamin E reduced the ROS levels, DNA damage, and expression of p53 tumor‐suppressor gene [68]. Similarly, NAC and vitamin E supplementation increased metastasis in mice with malignant melanoma [69]. Consistent results were observed in another independent study where NAC administration increased metastasis of melanoma in mice [70]. It is notable that ROS plays both negative and positive roles in cellular signaling. In normal conditions, low levels of ROS may function as messengers in cell signaling, while excess ROS levels have adverse effect on cellular macromolecules and lead to cell death. Antioxidant agents protect cells against ROS by increasing the antioxidant potential through Nrf2 activation. However, suppressing normal physiological ROS may affect the cell communication and signal transduction. For example, exercise generates ROS but promotes health benefits, especially in increasing insulin sensitivity. Transient production of ROS during exercise induced signaling systems that activated molecular targets for insulin sensitivity; however, antioxidant supplementation blocked the molecular signaling for cellular defense and insulin sensitivity mediated by exercise‐induced ROS formation and thereby abrogates the health‐promoting effects of exercise [71].

Several complex transcriptional and posttranslational networks are involved in mediating Nrf2 activation and thereby enabling diverse functional response. Moreover, networking with other signaling pathways expands the function of Nrf2 as a potent regulator of differential biological processes such as cell proliferation, apoptosis, angiogenesis, and metastasis. The cross-talks between different transcription factors may influence the outcome of therapeutic interventions. Understanding the molecular mechanisms involved in regulating Nrf2 can therefore provide insights that may benefit novel therapeutic manipulation of this pathway.


  1. 1. Itoh K, Wakabayashi N, Katoh Y, Ishii T, Igarashi K, Engel JD, et al. Keap1 represses nuclear activation of antioxidant responsive elements by Nrf2 through binding to the amino‐terminal Neh2 domain. Genes and Development. 1999;13(1):76–86.
  2. 2. McMahon M, Itoh K, Yamamoto M, Hayes JD. Keap1‐dependent proteasomal degradation of transcription factor Nrf2 contributes to the negative regulation of antioxidant response element‐driven gene expression. The Journal of Biological Chemistry. 2003;278(24):21592–600.
  3. 3. Itoh K, Chiba T, Takahashi S, Ishii T, Igarashi K, Katoh Y, et al. An Nrf2/small Maf heterodimer mediates the induction of phase II detoxifying enzyme genes through antioxidant response elements. Biochemical and Biophysical Research Communications. 1997;236(2):313–22.
  4. 4. Kobayashi M, Yamamoto M. Molecular mechanisms activating the Nrf2‐Keap1 pathway of antioxidant gene regulation. Antioxidants and Redox Signaling. 2005;7(3–4):385–94.
  5. 5. Kim SO, Merchant K, Nudelman R, Beyer WF, Jr., Keng T, DeAngelo J, et al. OxyR: a molecular code for redox‐related signaling. Cell. 2002;109(3):383–96.
  6. 6. Dinkova‐Kostova AT, Holtzclaw WD, Cole RN, Itoh K, Wakabayashi N, Katoh Y, et al. Direct evidence that sulfhydryl groups of Keap1 are the sensors regulating induction of phase 2 enzymes that protect against carcinogens and oxidants. Proceedings of the National Academy of Sciences of the United States of America. 2002;99(18):11908–13.
  7. 7. Zheng M, Aslund F, Storz G. Activation of the OxyR transcription factor by reversible disulfide bond formation. Science. 1998;279(5357):1718–21.
  8. 8. Lee C, Lee SM, Mukhopadhyay P, Kim SJ, Lee SC, Ahn WS, et al. Redox regulation of OxyR requires specific disulfide bond formation involving a rapid kinetic reaction path. Nature Structural and Molecular Biology. 2004;11(12):1179–85.
  9. 9. Helmann JD. OxyR: a molecular code for redox sensing? Science's STKE. 2002;2002(157):pe46.
  10. 10. Choi H, Kim S, Mukhopadhyay P, Cho S, Woo J, Storz G, et al. Structural basis of the redox switch in the OxyR transcription factor. Cell. 2001;105(1):103–13.
  11. 11. Ding H, Hidalgo E, Demple B. The redox state of the [2Fe‐2S] clusters in SoxR protein regulates its activity as a transcription factor. The Journal of Biological Chemistry. 1996;271(52):33173–5.
  12. 12. Demple B, Amabile‐Cuevas CF. Redox redux: the control of oxidative stress responses. Cell. 1991;67(5):837–9.
  13. 13. Bradley TM, Hidalgo E, Leautaud V, Ding H, Demple B. Cysteine‐to‐alanine replacements in the Escherichia coli SoxR protein and the role of the [2Fe‐2S] centers in transcriptional activation. Nucleic Acids Research. 1997;25(8):1469–75.
  14. 14. Manchado M, Michan C, Pueyo C. Hydrogen peroxide activates the SoxRS regulon in vivo. Journal of Bacteriology. 2000;182(23):6842–4.
  15. 15. Semchyshyn H, Bagnyukova T, Storey K, Lushchak V. Hydrogen peroxide increases the activities of soxRS regulon enzymes and the levels of oxidized proteins and lipids in Escherichia coli. Cell Biology International. 2005;29(11):898–902.
  16. 16. Zheng M, Wang X, Templeton LJ, Smulski DR, LaRossa RA, Storz G. DNA microarray‐mediated transcriptional profiling of the Escherichia coli response to hydrogen peroxide. Journal of Bacteriology. 2001;183(15):4562–70.
  17. 17. Wakabayashi N, Itoh K, Wakabayashi J, Motohashi H, Noda S, Takahashi S, et al. Keap1‐null mutation leads to postnatal lethality due to constitutive Nrf2 activation. Nature Genetics. 2003;35(3):238–45.
  18. 18. Taguchi K, Maher JM, Suzuki T, Kawatani Y, Motohashi H, Yamamoto M. Genetic analysis of cytoprotective functions supported by graded expression of Keap1. Molecular and Cellular Biology. 2010;30(12):3016–26.
  19. 19. Zhang DD, Lo SC, Cross JV, Templeton DJ, Hannink M. Keap1 is a redox‐regulated substrate adaptor protein for a Cul3‐dependent ubiquitin ligase complex. Molecular and Cellular Biology. 2004;24(24):10941–53.
  20. 20. Tong KI, Kobayashi A, Katsuoka F, Yamamoto M. Two‐site substrate recognition model for the Keap1‐Nrf2 system: a hinge and latch mechanism. Biological Chemistry. 2006;387(10–11):1311–20.
  21. 21. Tong KI, Katoh Y, Kusunoki H, Itoh K, Tanaka T, Yamamoto M. Keap1 recruits Neh2 through binding to ETGE and DLG motifs: characterization of the two‐site molecular recognition model. Molecular and Cellular Biology. 2006;26(8):2887–900.
  22. 22. Sun Z, Zhang S, Chan JY, Zhang DD. Keap1 controls postinduction repression of the Nrf2‐mediated antioxidant response by escorting nuclear export of Nrf2. Molecular and Cellular Biology. 2007;27(18):6334–49.
  23. 23. Kensler TW, Wakabayashi N. Nrf2: friend or foe for chemoprevention? Carcinogenesis. 2010;31(1):90–9.
  24. 24. Kim SB, Pandita RK, Eskiocak U, Ly P, Kaisani A, Kumar R, et al. Targeting of Nrf2 induces DNA damage signaling and protects colonic epithelial cells from ionizing radiation. Proceedings of the National Academy of Sciences of the United States of America. 2012;109(43):E2949–55.
  25. 25. Dinkova‐Kostova AT, Talalay P. Direct and indirect antioxidant properties of inducers of cytoprotective proteins. Molecular Nutrition and Food Research. 2008;52(Suppl 1):S128–38.
  26. 26. Li Y, Paonessa JD, Zhang Y. Mechanism of chemical activation of Nrf2. PLoS One. 2012;7(4):e35122.
  27. 27. Zhang DD, Hannink M. Distinct cysteine residues in Keap1 are required for Keap1‐dependent ubiquitination of Nrf2 and for stabilization of Nrf2 by chemopreventive agents and oxidative stress. Molecular and Cellular Biology. 2003;23(22):8137–51.
  28. 28. Wakabayashi N, Dinkova‐Kostova AT, Holtzclaw WD, Kang MI, Kobayashi A, Yamamoto M, et al. Protection against electrophile and oxidant stress by induction of the phase 2 response: fate of cysteines of the Keap1 sensor modified by inducers. Proceedings of the National Academy of Sciences of the United States of America. 2004;101(7):2040–5.
  29. 29. Yamamoto T, Suzuki T, Kobayashi A, Wakabayashi J, Maher J, Motohashi H, et al. Physiological significance of reactive cysteine residues of Keap1 in determining Nrf2 activity. Molecular and Cellular Biology. 2008;28(8):2758–70.
  30. 30. He X, Ma Q. NRF2 cysteine residues are critical for oxidant/electrophile‐sensing, Kelch‐like ECH‐associated protein‐1‐dependent ubiquitination‐proteasomal degradation, and transcription activation. Molecular Pharmacology. 2009;76(6):1265–78.
  31. 31. Takaya K, Suzuki T, Motohashi H, Onodera K, Satomi S, Kensler TW, et al. Validation of the multiple sensor mechanism of the Keap1‐Nrf2 system. Free Radical Biology and Medicine. 2012;53(4):817–27.
  32. 32. Wang XJ, Sun Z, Chen W, Li Y, Villeneuve NF, Zhang DD. Activation of Nrf2 by arsenite and monomethylarsonous acid is independent of Keap1‐C151: enhanced Keap1‐Cul3 interaction. Toxicology and Applied Pharmacology. 2008;230(3):383–9.
  33. 33. Sen R, Baltimore D. Multiple nuclear factors interact with the immunoglobulin enhancer sequences. Cell. 1986;46(5):705–16.
  34. 34. Winston JT, Strack P, Beer‐Romero P, Chu CY, Elledge SJ, Harper JW. The SCF beta‐TRCP‐ubiquitin ligase complex associates specifically with phosphorylated destruction motifs in IkappaBalpha and beta‐catenin and stimulates IkappaBalpha ubiquitination in vitro. Genes and Development. 1999;13(3):270–83.
  35. 35. Buelna‐Chontal M, Zazueta C. Redox activation of Nrf2 & NF‐kappaB: a double end sword? Cell Signal. 2013;25(12):2548–57.
  36. 36. Halle M, Gabrielsen A, Paulsson‐Berne G, Gahm C, Agardh HE, Farnebo F, et al. Sustained inflammation due to nuclear factor‐kappa B activation in irradiated human arteries. Journal of the American College of Cardiology. 2010;55(12):1227–36.
  37. 37. Innamorato NG, Rojo AI, Garcia‐Yague AJ, Yamamoto M, de Ceballos ML, Cuadrado A. The transcription factor Nrf2 is a therapeutic target against brain inflammation. Journal of Immunology. 2008;181(1):680–9.
  38. 38. Soares MP, Seldon MP, Gregoire IP, Vassilevskaia T, Berberat PO, Yu J, et al. Heme oxygenase‐1 modulates the expression of adhesion molecules associated with endothelial cell activation. Journal of Immunology. 2004;172(6):3553–63.
  39. 39. Lee DF, Kuo HP, Liu M, Chou CK, Xia W, Du Y, et al. KEAP1 E3 ligase‐mediated downregulation of NF‐kappaB signaling by targeting IKKbeta. Molecular Cell. 2009;36(1):131–40.
  40. 40. Yu M, Li H, Liu Q, Liu F, Tang L, Li C, et al. Nuclear factor p65 interacts with Keap1 to repress the Nrf2‐ARE pathway. Cellular Signalling. 2011;23(5):883–92.
  41. 41. Liu GH, Qu J, Shen X. NF‐kappaB/p65 antagonizes Nrf2‐ARE pathway by depriving CBP from Nrf2 and facilitating recruitment of HDAC3 to MafK. Biochim et Biophysica Acta. 2008;1783(5):713‐27.
  42. 42. Gloire G, Legrand‐Poels S, Piette J. NF‐kappaB activation by reactive oxygen species: fifteen years later. Biochemical Pharmacology. 2006;72(11):1493–505.
  43. 43. Hwang YJ, Lee EW, Song J, Kim HR, Jun YC, Hwang KA. MafK positively regulates NF‐kappaB activity by enhancing CBP‐mediated p65 acetylation. Scientific Reports. 2013;3:3242.
  44. 44. Rushworth SA, Zaitseva L, Murray MY, Shah NM, Bowles KM, MacEwan DJ. The high Nrf2 expression in human acute myeloid leukemia is driven by NF‐kappaB and underlies its chemo‐resistance. Blood. 2012;120(26):5188–98.
  45. 45. Cuadrado A, Martin‐Moldes Z, Ye J, Lastres‐Becker I. Transcription factors NRF2 and NF‐kappaB are coordinated effectors of the Rho family, GTP‐binding protein RAC1 during inflammation. The Journal of Biological Chemistry. 2014;289(22):15244–58.
  46. 46. Lau A, Wang XJ, Zhao F, Villeneuve NF, Wu T, Jiang T, et al. A noncanonical mechanism of Nrf2 activation by autophagy deficiency: direct interaction between Keap1 and p62. Molecular and Cellular Biology. 2010;30(13):3275–85.
  47. 47. Komatsu M, Kurokawa H, Waguri S, Taguchi K, Kobayashi A, Ichimura Y, et al. The selective autophagy substrate p62 activates the stress responsive transcription factor Nrf2 through inactivation of Keap1. Nature Cell Biology. 2010;12(3):213–23.
  48. 48. Wooten MW, Geetha T, Seibenhener ML, Babu JR, Diaz‐Meco MT, Moscat J. The p62 scaffold regulates nerve growth factor‐induced NF‐kappaB activation by influencing TRAF6 polyubiquitination. The Journal of Biological Chemistry. 2005;280(42):35625‐9.
  49. 49. Rada P, Rojo AI, Chowdhry S, McMahon M, Hayes JD, Cuadrado A. SCF/{beta}‐TrCP promotes glycogen synthase kinase 3‐dependent degradation of the Nrf2 transcription factor in a Keap1‐independent manner. Molecular and Cellular Biology. 2011;31(6):1121–33.
  50. 50. Park SH, Park‐Min KH, Chen J, Hu X, Ivashkiv LB. Tumor necrosis factor induces GSK3 kinase‐mediated cross‐tolerance to endotoxin in macrophages. Nature Immunology. 2011;12(7):607–15.
  51. 51. Kang KW, Cho MK, Lee CH, Kim SG. Activation of phosphatidylinositol 3‐kinase and Akt by tert‐butylhydroquinone is responsible for antioxidant response element‐mediated rGSTA2 induction in H4IIE cells. Molecular Pharmacology. 2001;59(5):1147–56.
  52. 52. Li MH, Cha YN, Surh YJ. Peroxynitrite induces HO‐1 expression via PI3K/Akt‐dependent activation of NF‐E2‐related factor 2 in PC12 cells. Free Radical Biology and Medicine. 2006;41(7):1079–91.
  53. 53. Hayes JD, Chowdhry S, Dinkova‐Kostova AT, Sutherland C. Dual regulation of transcription factor Nrf2 by Keap1 and by the combined actions of beta‐TrCP and GSK‐3. Biochemical Society Transactions. 2015;43(4):611–20.
  54. 54. Pitha‐Rowe I, Liby K, Royce D, Sporn M. Synthetic triterpenoids attenuate cytotoxic retinal injury: cross‐talk between Nrf2 and PI3K/AKT signaling through inhibition of the lipid phosphatase PTEN. Investigative Ophthalmology and Visual Science. 2009;50(11):5339–47.
  55. 55. Rojo AI, Rada P, Mendiola M, Ortega‐Molina A, Wojdyla K, Rogowska‐Wrzesinska A, et al. The PTEN/NRF2 axis promotes human carcinogenesis. Antioxidants and Redox Signaling. 2014;21(18):2498–514.
  56. 56. Taguchi K, Hirano I, Itoh T, Tanaka M, Miyajima A, Suzuki A, et al. Nrf2 enhances cholangiocyte expansion in PTEN‐deficient livers. Molecular and Cellular Biology. 2014;34(5):900–13.
  57. 57. Darvekar SR, Elvenes J, Brenne HB, Johansen T, Sjottem E. SPBP is a sulforaphane induced transcriptional coactivator of NRF2 regulating expression of the autophagy receptor p62/SQSTM1. PLoS One. 2014;9(1):e85262.
  58. 58. Lau A, Zheng Y, Tao S, Wang H, Whitman SA, White E, et al. Arsenic inhibits autophagic flux, activating the Nrf2‐Keap1 pathway in a p62‐dependent manner. Molecular and Cellular Biology. 2013;33(12):2436–46.
  59. 59. Yu R, Lei W, Mandlekar S, Weber MJ, Der CJ, Wu J, et al. Role of a mitogen‐activated protein kinase pathway in the induction of phase II detoxifying enzymes by chemicals. The Journal of Biological Chemistry. 1999;274(39):27545–52.
  60. 60. Yu R, Mandlekar S, Lei W, Fahl WE, Tan TH, Kong AN. p38 mitogen‐activated protein kinase negatively regulates the induction of phase II drug‐metabolizing enzymes that detoxify carcinogens. The Journal of Biological Chemistry. 2000;275(4):2322–7.
  61. 61. Cullinan SB, Zhang D, Hannink M, Arvisais E, Kaufman RJ, Diehl JA. Nrf2 is a direct PERK substrate and effector of PERK‐dependent cell survival. Molecular and Cellular Biology. 2003;23(20):7198–209.
  62. 62. 6262. Huang HC, Nguyen T, Pickett CB. Regulation of the antioxidant response element by protein kinase C‐mediated phosphorylation of NF‐E2‐related factor 2. Proceedings of the National Academy of Sciences of the United States of America. 2000;97(23):12475–80.
  63. 63. Huang HC, Nguyen T, Pickett CB. Phosphorylation of Nrf2 at Ser‐40 by protein kinase C regulates antioxidant response element‐mediated transcription. The Journal of Biological Chemistry. 2002;277(45):42769–74.
  64. 64. Niture SK, Jain AK, Jaiswal AK. Antioxidant‐induced modification of INrf2 cysteine 151 and PKC‐delta‐mediated phosphorylation of Nrf2 serine 40 are both required for stabilization and nuclear translocation of Nrf2 and increased drug resistance. Journal of Cell Science. 2009;122(Pt 24):4452–64.
  65. 65. Ahn YH, Hwang Y, Liu H, Wang XJ, Zhang Y, Stephenson KK, et al. Electrophilic tuning of the chemoprotective natural product sulforaphane. Proceedings of the National Academy of Sciences of the United States of America. 2010;107(21):9590‐5.
  66. 66. de Zeeuw D, Akizawa T, Audhya P, Bakris GL, Chin M, Christ‐Schmidt H, et al. Bardoxolone methyl in type 2 diabetes and stage 4 chronic kidney disease. The New England Journal of Medicine. 2013;369(26):2492–503.
  67. 67. DeNicola GM, Karreth FA, Humpton TJ, Gopinathan A, Wei C, Frese K, et al. Oncogene‐induced Nrf2 transcription promotes ROS detoxification and tumorigenesis. Nature. 2011;475(7354):106–9.
  68. 68. Sayin VI, Ibrahim MX, Larsson E, Nilsson JA, Lindahl P, Bergo MO. Antioxidants accelerate lung cancer progression in mice. Science Translational Medicine. 2014;6(221):221ra15.
  69. 69. Le Gal K, Ibrahim MX, Wiel C, Sayin VI, Akula MK, Karlsson C, et al. Antioxidants can increase melanoma metastasis in mice. Science Translational Medicine. 2015;7(308):308re8.
  70. 70. Piskounova E, Agathocleous M, Murphy MM, Hu Z, Huddlestun SE, Zhao Z, et al. Oxidative stress inhibits distant metastasis by human melanoma cells. Nature. 2015;527(7577):186–91.
  71. 71. Ristow M, Zarse K, Oberbach A, Kloting N, Birringer M, Kiehntopf M, et al. Antioxidants prevent health‐promoting effects of physical exercise in humans. Proceedings of the National Academy of Sciences of the United States of America. 2009;106(21):8665–70.

Written By

Sherin T. Mathew and Ola Hammarsten

Submitted: 12 April 2016 Reviewed: 22 August 2016 Published: 21 December 2016