The Nrf2 cell defence pathway: Keap1-dependent and -independent mechanisms of regulation

BRCA1 and Oxidative Stress

The breast cancer susceptibility gene 1 (BRCA1) has been well established as a tumor suppressor and functions primarily by maintaining genome integrity. Genome stability is compromised when cells are exposed to oxidative stress. Increasing evidence suggests that BRCA1 regulates oxidative stress and this may be another mechanism in preventing carcinogenesis in normal cells. Oxidative stress caused by reactive oxygen species (ROS) is implicated in carcinogenesis and is used strategically to treat human cancer. Thus, it is essential to understand the function of BRCA1 in oxidative stress regulation. In this review, we briefly summarize BRCA1's many binding partners and mechanisms, and discuss data supporting the function of BRCA1 in oxidative stress regulation. Finally, we consider its significance in prevention and/or treatment of BRCA1-related cancers.

Identification of chromomoric acid C-I as an Nrf2 activator in Chromolaena odorata

Activation of nuclear factor-erythroid 2-related factor 2 (Nrf2) contributes to several beneficial bioactivities of natural products, including induction of an increased cellular stress resistance and prevention or resolution of inflammation. In this study, the potential of a crude leaf extract of Chromolaena odorata, traditionally used against inflammation and skin lesions, was examined for Nrf2 activation. Guided by an Nrf2-dependent luciferase reporter gene assay, the phytoprostane chromomoric acid C-I (1) was identified as a potent Nrf2 activator from C. odorata with a CD (concentration doubling the response of vehicle-treated cells) of 5.2 μM. When tested at 1-10 μM, 1 was able to induce the endogenous Nrf2 target gene heme oxygenase 1 (HO-1) in fibroblasts. Between 2 and 5 μM, compound 1 induced HO-1 in vascular smooth muscle cells (VSMC) and inhibited their proliferation in a HO-1-dependent manner, without eliciting signs of cytotoxicity.

The clinical potential of influencing Nrf2 signaling in degenerative and immunological disorders

Nuclear factor (erythroid-derived 2)-like 2 (Nrf2; encoded in humans by the NFE2L2 gene) is a transcription factor that regulates the gene expression of a wide variety of cytoprotective phase II detoxification and antioxidant enzymes through a promoter sequence known as the antioxidant-responsive element (ARE). The ARE is a promoter element found in many cytoprotective genes; therefore, Nrf2 plays a pivotal role in the ARE-driven cellular defense system against environmental stresses. Agents that target the ARE/Nrf2 pathway have been tested in a wide variety of disorders, with at least one new Nrf2-activating drug now approved by the US Food and Drug Administration. Examination of in vitro and in vivo experimental results, and taking into account recent human clinical trial results, has led to an opinion that Nrf2-activating strategies – which can include drugs, foods, dietary supplements, and exercise – are likely best targeted at disease prevention, disease recurrence prevention, or slowing of disease progression in early stage illnesses; they may also be useful as an interventional strategy. However, this rubric may be viewed even more conservatively in the pathophysiology of cancer. The activation of the Nrf2 pathway has been widely accepted as offering chemoprevention benefit, but it may be unhelpful or even harmful in the setting of established cancers. For example, Nrf2 activation might interfere with chemotherapies or radiotherapies or otherwise give tumor cells additional growth and survival advantages, unless they already possess mutations that fully activate their Nrf2 pathway constitutively. With all this in mind, the ARE/Nrf2 pathway remains of great interest as a possible target for the pharmacological control of degenerative and immunological diseases, both by activation and by inhibition, and its regulation remains a promising biological target for the development of new therapies.

Molecular chaperones and proteostasis regulation during redox imbalance

Free radicals originate from both exogenous environmental sources and as by-products of the respiratory chain and cellular oxygen metabolism. Sustained accumulation of free radicals, beyond a physiological level, induces oxidative stress that is harmful for the cellular homeodynamics as it promotes the oxidative damage and stochastic modification of all cellular biomolecules including proteins. In relation to proteome stability and maintenance, the increased concentration of oxidants disrupts the functionality of cellular protein machines resulting eventually in proteotoxic stress and the deregulation of the proteostasis (homeostasis of the proteome) network (PN). PN curates the proteome in the various cellular compartments and the extracellular milieu by modulating protein synthesis and protein machines assembly, protein recycling and stress responses, as well as refolding or degradation of damaged proteins. Molecular chaperones are key players of the PN since they facilitate folding of nascent polypeptides, as well as holding, folding, and/or degradation of unfolded, misfolded, or non-native proteins. Therefore, the expression and the activity of the molecular chaperones are tightly regulated at both the transcriptional and post-translational level at organismal states of increased oxidative and, consequently, proteotoxic stress, including ageing and various age-related diseases (e.g. degenerative diseases and cancer). In the current review we present a synopsis of the various classes of intra- and extracellular chaperones, the effects of oxidants on cellular homeodynamics and diseases and the redox regulation of chaperones.

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Free radicals originate from various sources and at physiological concentrations are essential for the modulation of cell signalling pathways.

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Abnormally high levels of free radicals induce oxidative stress and damage all cellular biomolecules, including proteins.

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Molecular chaperones facilitate folding of nascent polypeptides, as well as holding, folding, and/or degradation of damaged proteins.

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The expression and the activity of chaperones during oxidative stress are regulated at both the transcriptional and post-translational level.

Nrf2 protects against TWEAK-mediated skeletal muscle wasting

Skeletal muscle (SM) regeneration after injury is impaired by excessive inflammation. Particularly, the inflammatory cytokine tumour necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) is a potent inducer of skeletal muscle wasting and fibrosis. In this study we investigated the role of Nrf2, a major regulator of oxidative stress defence, in SM ischemia/reperfusion (I/R) injury and TWEAK induced atrophy. We explored the time-dependent expression of TWEAK after I/R in SM of Nrf2-wildtype (WT) and knockout (KO) mice. Nrf2-KO mice expressed significant higher levels of TWEAK as compared to WT mice. Consequently, Nrf2-KO mice present an insufficient regeneration as compared to Nrf2-WT mice. Moreover, TWEAK stimulation activates Nrf2 in the mouse myoblast cell line C2C12. This Nrf2 activation inhibits TWEAK induced atrophy in C2C12 differentiated myotubes. In summary, we show that Nrf2 protects SM from TWEAK-induced cell death in vitro and that Nrf2-deficient mice therefore have poorer muscle regeneration.

New insight into the molecular drug target of diabetic nephropathy

Diabetic nephropathy (DN) lowered quality of life and shortened life expectancy amongst those affected. Evidence indicates interaction between advanced glycation end products (AGEs), activated protein kinase C (PKC) and angiotensin II exacerbate the progression of DN. Inhibitors of angiotensin-converting enzyme (ACEIs), renin angiotensin aldosterone system (RAAS), AGEs, and PKC have been tested for slowing down the progression of DN. The exact molecular drug targets that lead to the amelioration of renal injury in DN are not well understood. This review summarizes the potential therapeutic targets, based on putative mechanism in the progression of the disease.

Inhibition of NRF2 by PIK-75 augments sensitivity of pancreatic cancer cells to gemcitabine

We describe the potential benefit of PIK-75 in combination of gemcitabine to treat pancreatic cancer in a preclinical mouse model. The effect of PIK-75 on the level and activity of NRF2 was characterized using various assays including reporter gene, quantitative PCR, DNA-binding and western blot analyses. Additionally, the combinatorial effect of PIK-75 and gemcitabine was evaluated in human pancreatic cancer cell lines and a xenograft model. PIK-75 reduced NRF2 protein levels and activity to regulate its target gene expression through proteasome-mediated degradation of NRF2 in human pancreatic cancer cell lines. PIK-75 also reduced the gemcitabine-induced NRF2 levels and the expression of its downstream target MRP5. Co-treatment of PIK-75 augmented the antitumor effect of gemcitabine both in vitro and in vivo. Our present study provides a strong mechanistic rationale to evaluate NRF2 targeting agents in combination with gemcitabine to treat pancreatic cancers.

Curcumin pretreatment induces Nrf2 and an antioxidant response and prevents hemin-induced toxicity in primary cultures of cerebellar granule neurons of rats

Curcumin is a bifunctional antioxidant derived from Curcuma longa. This study identifies curcumin as a neuroprotectant against hemin-induced damage in primary cultures of cerebellar granule neurons (CGNs) of rats. Hemin, the oxidized form of heme, is a highly reactive compound that induces cellular injury. Pretreatment of CGNs with 5–30 μM curcumin effectively increased by 2.3–4.9 fold heme oxygenase-1 (HO-1) expression and by 5.6–14.3-fold glutathione (GSH) levels. Moreover, 15 μM curcumin attenuated by 55% the increase in reactive oxygen species (ROS) production, by 94% the reduction of GSH/glutathione disulfide (GSSG) ratio, and by 49% the cell death induced by hemin. The inhibition of heme oxygenase system or GSH synthesis with tin mesoporphyrin and buthionine sulfoximine, respectively, suppressed the protective effect of curcumin against hemin-induced toxicity. These data strongly suggest that HO-1 and GSH play a major role in the protective effect of curcumin. Furthermore, it was found that 24 h of incubation with curcumin increases by 1.4-, 2.3-, and 5.2-fold the activity of glutathione reductase, glutathione S-transferase and superoxide dismutase, respectively. Additionally, it was found that curcumin was capable of inducing nuclear factor (erythroid-derived 2)-like 2 (Nrf2) translocation into the nucleus. These data suggest that the pretreatment with curcumin induces Nrf2 and an antioxidant response that may play an important role in the protective effect of this antioxidant against hemin-induced neuronal death.

The emerging role of the Nrf2-Keap1 signaling pathway in cancer

The Nrf2–Keap1 signaling pathway is key to cell defense and survival pathways. Nrf2 can protect cells and tissues from toxicants and carcinogens, and several Nrf2 activators are currently being tested as chemopreventive compounds. However, several studies also suggest that Nrf2 may protect cancer cells from chemotherapeutic agents and promote cancer cell proliferation. Here, Jaramillo and Zhang provide an overview of the Nrf2–Keap1 signaling pathway in cancer. They discuss the dual role of Nrf2 in cancer and the challenges in developing Nrf2-based drugs for chemoprevention and therapy.

The Nrf2 (nuclear factor erythroid 2 [NF-E2]-related factor 2 [Nrf2])–Keap1 (Kelch-like erythroid cell-derived protein with CNC homology [ECH]-associated protein 1) signaling pathway is one of the most important cell defense and survival pathways. Nrf2 can protect cells and tissues from a variety of toxicants and carcinogens by increasing the expression of a number of cytoprotective genes. As a result, several Nrf2 activators are currently being tested as chemopreventive compounds in clinical trials. Just as Nrf2 protects normal cells, studies have shown that Nrf2 may also protect cancer cells from chemotherapeutic agents and facilitate cancer progression. Nrf2 is aberrantly accumulated in many types of cancer, and its expression is associated with a poor prognosis in patients. In addition, Nrf2 expression is induced during the course of drug resistance. Collectively, these studies suggest that Nrf2 contributes to both intrinsic and acquired chemoresistance. This discovery has opened up a broad spectrum of research geared toward a better understanding of the role of Nrf2 in cancer. This review provides an overview of (1) the Nrf2–Keap1 signaling pathway, (2) the dual role of Nrf2 in cancer, (3) the molecular basis of Nrf2 activation in cancer cells, and (4) the challenges in the development of Nrf2-based drugs for chemoprevention and chemotherapy.

Keap1 redox-dependent regulation of doxorubicin-induced oxidative stress response in cardiac myoblasts

Doxorubicin (DOX) is a widely prescribed treatment for a broad scope of cancers, but clinical utility is limited by the cumulative, dose-dependent cardiomyopathy that occurs with repeated administration. DOX-induced cardiotoxicity is associated with the production of reactive oxygen species (ROS) and oxidation of lipids, DNA and proteins. A major cellular defense mechanism against such oxidative stress is activation of the Keap1/Nrf2-antioxidant response element (ARE) signaling pathway, which transcriptionally regulates expression of antioxidant genes such as Nqo1 and Gstp1. In the present study, we address the hypothesis that an initial event associated with DOX-induced oxidative stress is activation of the Keap1/Nrf2-dependent expression of antioxidant genes and that this is regulated through drug-induced changes in redox status of the Keap1 protein. Incubation of H9c2 rat cardiac myoblasts with DOX resulted in a time- and dose-dependent decrease in non-protein sulfhydryl groups. Associated with this was a near 2-fold increase in Nrf2 protein content and enhanced transcription of several of the Nrf2-regulated down-stream genes, including Gstp1, Ugt1a1, and Nqo1; the expression of Nfe2l2 (Nrf2) itself was unaltered. Furthermore, both the redox status and the total amount of Keap1 protein were significantly decreased by DOX, with the loss of Keap1 being due to both inhibited gene expression and increased autophagic, but not proteasomal, degradation. These findings identify the Keap1/Nrf2 pathway as a potentially important initial response to acute DOX-induced oxidative injury, with the primary regulatory events being the oxidation and autophagic degradation of the redox sensor Keap1 protein.

Systems analysis of oxidant stress in the vasculature

Systems biology and network analysis are emerging as valuable tools for the discovery of novel relationships, the identification of key regulatory factors, and the prediction of phenotypic changes in complex biological systems. Redox homeostasis in the vasculature is maintained by an intricate balance between oxidant-generating and antioxidant systems. When these systems are perturbed, conditions are permissive for oxidant stress, which, in turn, promotes vascular dysfunction and structural remodeling. Owing to the number of elements involved in redox regulation and the different vascular pathophenotypes associated with oxidant stress, vascular oxidant stress represents an ideal system to study by network analysis. Networks offer a method to organize experimentally derived factors, including proteins, metabolites, and DNA, that are represented as nodes into an unbiased comprehensive platform for study. Through analysis of the network, it is possible to determine essential or regulatory nodes, identify previously unknown connections between nodes, and locate modules, which are groups of nodes located within the same neighborhood that function together and have implications for phenotype. Investigators have only recently begun to construct oxidant stress-related networks to examine vascular structure and function; however, these early studies have provided mechanistic insight to further our understanding of this complicated biological system.

Chronic treatment with the mood-stabilizing drug lithium up-regulates nuclear factor E2-related factor 2 in rat pheochromocytoma PC12 cells in vitro

The mood-stabilizing drug lithium is the most commonly used treatment for bipolar disorder. Previous studies have shown that chronic treatment with lithium produces a protective effect against oxidative stress. Nuclear factor E2-related factor 2 (Nrf2) is a gene transcription factor that binds to the electrophile response element (EpRE) and triggers expression of various genes with antioxidant properties. Nrf2 contributes significantly to cytoprotection against oxidative stress. The purpose of this study is to determine the role of Nrf2 in the protective effect of lithium against oxidative stress. We found, using immunoblotting analysis, that chronic, but not acute treatment with lithium increased nuclear levels of Nrf2 in rat pheochromocytoma PC12 cells. DNA pull-down assay has shown that Nrf2 can bind to a double-strained oligonucleotide containing an EpRE site from glutathione s-transferase A4. Electrophorectic gel shift analysis further showed that chronic treatment with lithium increased Nrf-2-EpRE binding activity. We also found that knocking down Nrf2 with its short hairpin RNA inhibited lithium-increased expression of Nrf2 and suppressed the protective effect of lithium against hydrogen peroxide (H₂O₂)-reduced cell viability and H₂O₂-increased DNA fragmentation. Because Nrf2 can induce expression of various genes that play important roles in cytoprotection, the current findings suggest that Nrf2 may mediate the neuroprotective effect of lithium against oxidative stress.

Involvement of redox state in the aging of Drosophila melanogaster

SIGNIFICANCE

The main objective of this review was to provide an exposition of investigations, conducted in Drosophila melanogaster, on the role of reactive oxygen species and redox state in the aging process. While early transgenic studies did not clearly support the validity of the oxidative stress hypothesis of aging, predicated on the accumulation of structural damage, they spawned a broader search for redox-related effects that might impact the aging process.

RECENT ADVANCES

Initial evidence implicating the thiol redox state as a possible causative factor in aging has been obtained in Drosophila. Overexpression of genes, such as GCL, G6PD, Prx2, and Prx5, which are involved in the maintenance of thiol redox homeostasis, has strong positive effects on longevity. Further, the depletion of peroxiredoxin activity in the mitochondria through the double knockdown of Prx5 and Prx3 not only results in a redox crisis but also elicits a rapid aging phenotype.

CRITICAL ISSUES

Herein, we summarize the present status of knowledge about the main components of the machinery controlling thiol redox homeostasis and describe how age-related redox fluctuations might impact aging more acutely through disruption of the redox-sensitive signaling mechanisms rather than via the simple accumulation of structural damage.

FUTURE DIRECTIONS

Based on these initial insights into the plausible impact of redox fluctuations on redox signaling, future studies should focus on the pathways that have been explicitly implicated in aging, such as insulin signaling, TOR, and JNK/FOXO, with particular attention to elements that are redox sensitive.

Regulation of the Nrf2 antioxidant pathway by microRNAs: New players in micromanaging redox homeostasis

MicroRNAs are now thought to play a central role in the regulation of many diverse aspects of cell biology; however, it remains to be fully elucidated how microRNAs can orchestrate cellular redox homeostasis, which plays a central role in a multitude of physiological and pathophysiological processes. The redox-sensitive transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) serves as a "master regulator" of cell survival through the coordinated induction of phase II and antioxidant defense enzymes to counteract oxidative stress and modulate redox signaling events. MicroRNAs are able to "fine-tune" the regulation of processes including those directly interacting with the Nrf2 pathway and the generation of reactive oxygen species (ROS). This review highlights that cellular redox homeostasis can be regulated by microRNAs through their modulation of Nrf2-driven antioxidant gene expression as well as key enzymes that generate ROS, which in turn can alter the biogenesis and processing of microRNAs. Therefore redox sensitive microRNAs or "redoximiRs" add an important regulatory mechanism for redox signaling beyond the well-characterized actions of Nrf2. The potential exists for microRNA-based therapies where diminished antioxidant defenses and dysregulated redox signaling can lead to cardiovascular diseases, cancers, neurodegeneration, and accelerated aging.

Redox activation of Nrf2 & NF-κB: a double end sword?

Moderate concentrations of reactive oxygen species (ROS) are produced by diverse sources under physiological conditions. At such low levels, these molecules may act as upstream mediators of relevant signaling pathways; however an increase in their concentration with respect to the antioxidant system activity, changes their redox signaling function into a deleterious role. Thus, cell health depends, at least in part, on redox balance. This review includes global aspects of oxygen chemistry, ROS generation, antioxidant system, and redox signaling. It is also focused on the description of two relevant redox-sensitive transcription factors: nuclear factor erythroid 2-related factor 2 (Nrf2), which may be a potential target to confer cell protection, and nuclear factor κB (NF-κB), which is involved in deleterious effects in the cell. Finally, recent findings on the interplay between both factors for the development of different pathologies are discussed.

Glutathione imbalance in patients with X-linked adrenoleukodystrophy

BACKGROUND

X-linked adrenoleukodystrophy (X-ALD) is a genetic disorder of X-linked inheritance caused by a mutation in the ABCD1 gene which determines an accumulation of long-chain fatty acids in plasma and tissues. Recent evidence shows that oxidative stress may be a hallmark in the pathogenesis of X-ALD and glutathione plays an important role in the defense against free radicals. In this study we have analyzed glutathione homeostasis in lymphocytes of 14 patients with X-ALD and evaluated the balance between oxidized and reduced forms of glutathione, in order to define the role of this crucial redox marker in this condition.

METHODS

Lymphocytes, plasma and erythrocytes were obtained from the whole blood of 14 subjects with X-ALD and in 30 healthy subjects. Total, reduced and protein-bound glutathione levels were measured in lymphocytes by HPLC analysis. Erythrocyte free glutathione and antioxidant enzyme activities, plasma thiols and carbonyl content were determined by spectrophotometric assays.

RESULTS

A significant decrease of total and reduced glutathione was found in lymphocytes of patients, associated to high levels of all oxidized glutathione forms. A decline of free glutathione was particularly significant in erythrocytes. The increased oxidative stress in X-ALD was additionally confirmed by the decrease of plasma thiols and the high level of carbonyls.

CONCLUSION

Our results strongly support a role for oxidative stress in the pathophysiology of X-ALD and strengthen the importance of the balance among glutathione forms as a hallmark and a potential biomarker of the disease.

Translational control of Nrf2 within the open reading frame

Nuclear Factor Erythroid 2-Related Factor 2 (Nrf2) is a transcription factor that is essential for the regulation of an effective antioxidant and detoxifying response. The regulation of its activity can occur at transcription, translation and post-translational levels. Evidence suggests that under environmental stress conditions, new synthesis of Nrf2 is required - a process that is regulated by translational control and is not fully understood. Here we described the identification of a novel molecular process that under basal conditions strongly represses the translation of Nrf2 within the open reading frame (ORF). This mechanism is dependent on the mRNA sequence within the 3' portion of the ORF of Nrf2 but not in the encoded amino acid sequence. The Nrf2 translational repression can be reversed with the use of synonymous codon substitutions. This discovery suggests an additional layer of control to explain the reason for the low Nrf2 concentration under quiescent state.

Glucose availability is a decisive factor for Nrf2-mediated gene expression

Activation of the transcription factor Nrf2 (nuclear factor-erythroid 2-related factor 2) is one of the major cellular defense lines against oxidative and xenobiotic stress, but also influences genes involved in lipid and glucose metabolism. It is unresolved whether the cytoprotective and metabolic responses mediated by Nrf2 are connected or separable events in non-malignant cells. In this study we show that activation of Nrf2, either by the small molecule sulforaphane or knockout of the Nrf2 inhibitor Keap1, leads to increased cellular glucose uptake and increased glucose addiction in fibroblasts. Upon Nrf2 activation glucose is preferentially metabolized through the pentose phosphate pathway with increased production of NADPH. Interference with the supply of glucose or the pentose phosphate pathway and NADPH generation not only hampers Nrf2-mediated detoxification of reactive oxygen species on the enzyme level but also Nrf2-initiated expression of antioxidant defense proteins, such as glutathione reductase and heme-oxygenase1. We conclude that the Nrf2-dependent protection against oxidative stress relies on an intact pentose phosphate pathway and that there is crosstalk between metabolism and detoxification already at the level of gene expression in mammalian cells.

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Activation of Nrf2 results in increased cellular glucose uptake.

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Upon activation of Nrf2 glucose is preferentially metabolized through the PPP.

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The resulting increase in NADPH is not only pivotal for functional detoxification of ROS, but also for Nrf2-dependent gene expression in mammalian cells.

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These data complement our understanding of the metabolic shade of Nrf2 action.

Frataxin deficiency leads to reduced expression and impaired translocation of NF-E2-related factor (Nrf2) in cultured motor neurons

Oxidative stress has been implicated in the pathogenesis of Friedreich's Ataxia (FRDA), a neurodegenerative disease caused by the decreased expression of frataxin, a mitochondrial protein responsible of iron homeostasis. Under conditions of oxidative stress, the activation of the transcription factor NF-E2-related factor (Nrf2) triggers the antioxidant cellular response by inducing antioxidant response element (ARE) driven genes. Increasing evidence supports a role for the Nrf2-ARE pathway in neurodegenerative diseases. In this study, we analyzed the expression and the distribution of Nrf2 in silenced neurons for frataxin gene. Decreased Nrf2 mRNA content and a defective activation after treatment with pro-oxidants have been evidenced in frataxin-silenced neurons by RT-PCR and confocal microscopy. The loss of Nrf2 in FRDA may greatly enhance the cellular susceptibility to oxidative stress and make FRDA neurons more vulnerable to injury. Our findings may help to focus on this promising target, especially in its emerging role in the neuroprotective response.