The stress response protein REDD1 promotes diabetes-induced oxidative stress in the retina by Keap1-independent Nrf2 degradation

The transcription factor nuclear factor erythroid-2-related factor 2 (Nrf2) plays a critical role in reducing oxidative stress by promoting the expression of antioxidant genes. Both individuals with diabetes and preclinical diabetes models exhibit evidence of a defect in retinal Nrf2 activation. We recently demonstrated that increased expression of the stress response protein regulated in development and DNA damage 1 (REDD1) is necessary for the development of oxidative stress in the retina of streptozotocin-induced diabetic mice. In the present study, we tested the hypothesis that REDD1 suppresses the retinal antioxidant response to diabetes by repressing Nrf2 function. We found that REDD1 ablation enhances Nrf2 DNA-binding activity in the retina and that the suppressive effect of diabetes on Nrf2 activity is absent in the retina of REDD1-deficient mice compared with WT. In human MIO-M1 Müller cell cultures, REDD1 deletion prevented oxidative stress in response to hyperglycemic conditions, and this protective effect required Nrf2. REDD1 suppressed Nrf2 stability by promoting its proteasomal degradation independently of Nrf2's interaction with Kelch-like ECH-associated protein 1 (Keap1), but REDD1-mediated Nrf2 degradation required glycogen synthase kinase 3 (GSK3) activity and Ser-351/Ser-356 of Nrf2. Diabetes diminished inhibitory phosphorylation of glycogen synthase kinase 3β (GSK3β) at Ser-9 in the retina of WT mice but not in REDD1-deficient mice. Pharmacological inhibition of GSK3 enhanced Nrf2 activity and prevented oxidative stress in the retina of diabetic mice. The findings support a model wherein hyperglycemia-induced REDD1 blunts the Nrf2 antioxidant response to diabetes by activating GSK3, which, in turn, phosphorylates Nrf2 to promote its degradation.

The GTPase KRAS suppresses the p53 tumor suppressor by activating the NRF2-regulated antioxidant defense system in cancer cells

In human cancer cells that harbor mutant KRAS and WT p53 (p53), KRAS contributes to the maintenance of low p53 levels. Moreover, KRAS depletion stabilizes and reactivates p53 and thereby inhibits malignant transformation. However, the mechanism by which KRAS regulates p53 is largely unknown. Recently, we showed that KRAS depletion leads to p53 Ser-15 phosphorylation (P-p53) and increases the levels of p53 and its target p21/WT p53-activated fragment 1 (WAF1)/CIP1. Here, using several human lung cancer cell lines, siRNA-mediated gene silencing, immunoblotting, quantitative RT-PCR, promoter-reporter assays, and reactive oxygen species (ROS) assays, we demonstrate that KRAS maintains low p53 levels by activating the NRF2 (NFE2-related factor 2)-regulated antioxidant defense system. We found that KRAS depletion led to down-regulation of NRF2 and its targets NQO1 (NAD(P)H quinone dehydrogenase 1) and SLC7A11 (solute carrier family 7 member 11), decreased the GSH/GSSG ratio, and increased ROS levels. We noted that the increase in ROS is required for increased P-p53, p53, and p21^Waf1/cip1 levels following KRAS depletion. Downstream of KRAS, depletion of RalB (RAS-like proto-oncogene B) and IκB kinase-related TANK-binding kinase 1 (TBK1) activated p53 in a ROS- and NRF2-dependent manner. Consistent with this, the IκB kinase inhibitor BAY11-7085 and dominant-negative mutant IκBαM inhibited NF-κB activity and increased P-p53, p53, and p21^Waf1/cip1 levels in a ROS-dependent manner. In conclusion, our findings uncover an important role for the NRF2-regulated antioxidant system in KRAS-mediated p53 suppression.

Differential and overlapping targets of the transcriptional regulators NRF1, NRF2, and NRF3 in human cells

The nuclear factor (erythroid 2)-like (NRF) transcription factors are a subset of cap'n'collar transcriptional regulators. They consist of three members, NRF1, NRF2, and NRF3, that regulate the expression of genes containing antioxidant-response elements (AREs) in their promoter regions. Although all NRF members regulate ARE-containing genes, each is associated with distinct roles. A comprehensive study of differential and overlapping DNA-binding and transcriptional activities of the NRFs has not yet been conducted. Here, we performed chromatin immunoprecipitation (ChIP)-exo sequencing, an approach that combines ChIP with exonuclease treatment to pinpoint regulatory elements in DNA with high precision, in conjunction with RNA-sequencing to define the transcriptional targets of each NRF member. Our approach, done in three U2OS cell lines, identified 31 genes that were regulated by all three NRF members, 27 that were regulated similarly by all three, and four genes that were differentially regulated by at least one NRF member. We also found genes that were up- or down-regulated by only one NRF member, with 84, 84, and 22 genes that were regulated by NRF1, NRF2, and NRF3, respectively. Analysis of the ARE motifs identified in ChIP peaks revealed that NRF2 prefers binding to AREs flanked by GC-rich regions and that NRF1 prefers AT-rich flanking regions. Thus, sequence preference, likely in combination with upstream signaling events, determines NRF member activation under specific cellular contexts. Our analysis provides a comprehensive description of differential and overlapping gene regulation by the transcriptional regulators NRF1, NRF2, and NRF3.

Angiogenin activates the astrocytic Nrf2/antioxidant-response element pathway and thereby protects murine neurons from oxidative stress

The angiogenin (ANG) gene is mutated frequently in individuals with amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease characterized by the progressive loss of motor neurons. Delivering human ANG to mice that display ALS-like symptoms extends their lifespan and improves motor function. ANG is a secretory vertebrate RNase that enters neuronal cells and cleaves a subset of tRNAs, leading to the inhibition of translation initiation and the assembly of stress granules. Here, using murine neuronal and astrocytic cell lines, we find that ANG triggers the activation of the Nrf2 (nuclear factor erythroid 2-related factor 2) pathway, which provides a critical cellular defense against oxidative stress. This activation, which occurred in astrocytes but not in neurons, promoted the survival of proximal neurons that had oxidative injury. These findings extend the role of ANG as a neuroprotective agent and underscore its potential utility in ALS management.

Mini-GAGR, an intranasally applied polysaccharide, activates the neuronal Nrf2-mediated antioxidant defense system

Oxidative stress triggers and exacerbates neurodegeneration in Alzheimer's disease (AD). Various antioxidants reduce oxidative stress, but these agents have little efficacy due to poor blood-brain barrier (BBB) permeability. Additionally, single-modal antioxidants are easily overwhelmed by global oxidative stress. Activating nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2) and its downstream antioxidant system are considered very effective for reducing global oxidative stress. Thus far, only a few BBB-permeable agents activate the Nrf2-dependent antioxidant system. Here, we discovered a BBB-bypassing Nrf2-activating polysaccharide that may attenuate AD pathogenesis. Mini-GAGR, a 0.7-kDa cleavage product of low-acyl gellan gum, increased the levels and activities of Nrf2-dependent antioxidant enzymes, decreased reactive oxygen species (ROS) under oxidative stress in mouse cortical neurons, and robustly protected mitochondria from oxidative insults. Moreover, mini-GAGR increased the nuclear localization and transcriptional activity of Nrf2 similarly to known Nrf2 activators. Mechanistically, mini-GAGR increased the dissociation of Nrf2 from its inhibitor, Kelch-like ECH-associated protein 1 (Keap1), and induced phosphorylation and nuclear translocation of Nrf2 in a protein kinase C (PKC)- and fibroblast growth factor receptor (FGFR1)-dependent manner. Finally, 20-day intranasal treatment of 3xTg-AD mice with 100 nmol of mini-GAGR increased nuclear p-Nrf2 and growth-associated protein 43 (GAP43) levels in hippocampal neurons, reduced p-tau and β-amyloid (Aβ) peptide-stained neurons, and improved memory. The BBB-bypassing Nrf2-activating polysaccharide reported here may be effective in reducing oxidative stress and neurodegeneration in AD.

Kelch-like ECH-associated protein 1 (KEAP1) differentially regulates nuclear factor erythroid-2-related factors 1 and 2 (NRF1 and NRF2)

Nuclear factor erythroid-2-related factor 1 (NRF1) and NRF2 are essential for maintaining redox homeostasis and coordinating cellular stress responses. They are highly homologous transcription factors that regulate the expression of genes bearing antioxidant-response elements (AREs). Genetic ablation of NRF1 or NRF2 results in vastly different phenotypic outcomes, implying that they play different roles and may be differentially regulated. Kelch-like ECH-associated protein 1 (KEAP1) is the main negative regulator of NRF2 and mediates ubiquitylation and degradation of NRF2 through its NRF2-ECH homology-like domain 2 (Neh2). Here, we report that KEAP1 binds to the Neh2-like (Neh2L) domain of NRF1 and stabilizes it. Consistently, NRF1 is more stable in KEAP1^+/+ than in KEAP1^-/- isogenic cell lines, whereas NRF2 is dramatically stabilized in KEAP1^-/- cells. Replacing NRF1's Neh2L domain with NRF2's Neh2 domain renders NRF1 sensitive to KEAP1-mediated degradation, indicating that the amino acids between the DLG and ETGE motifs, not just the motifs themselves, are essential for KEAP1-mediated degradation. Systematic site-directed mutagenesis identified the core amino acid residues required for KEAP1-mediated degradation and further indicated that the DLG and ETGE motifs with correct spacing are insufficient as a KEAP1 degron. Our results offer critical insights into our understanding of the differential regulation of NRF1 and NRF2 by KEAP1 and their different physiological roles.

Hexokinase 2 and nuclear factor erythroid 2-related factor 2 transcriptionally coactivate xanthine oxidoreductase expression in stressed glioma cells

A dynamic network of metabolic adaptations, inflammatory responses, and redox homeostasis is known to drive tumor progression. A considerable overlap among these processes exists, but several of their key regulators remain unknown. To this end, here we investigated the role of the proinflammatory cytokine IL-1β in connecting these processes in glioma cells. We found that glucose starvation sensitizes glioma cells to IL-1β-induced apoptosis in a manner that depended on reactive oxygen species (ROS). Although IL-1β-induced JNK had no effect on cell viability under glucose deprivation, it mediated nuclear translocation of hexokinase 2 (HK2). This event was accompanied by increases in the levels of sirtuin 6 (SIRT6), nuclear factor erythroid 2-related factor 2 (Nrf2), and xanthine oxidoreductase (XOR). SIRT6 not only induced ROS-mediated cell death but also facilitated nuclear Nrf2-HK2 interaction. Recruitment of the Nrf2-HK2 complex to the ARE site on XOR promoter regulated its expression. Importantly, HK2 served as transcriptional coactivator of Nrf2 to regulate XOR expression, indicated by decreased XOR levels in siRNA-mediated Nrf2 and HK2 knockdown experiments. Our results highlight a non-metabolic role of HK2 as transcriptional coactivator of Nrf2 to regulate XOR expression under conditions of proinflammatory and metabolic stresses. Our insights also underscore the importance of nuclear activities of HK2 in the regulation of genes involved in redox homeostasis.

Stress-sensing mechanisms and the physiological roles of the Keap1-Nrf2 system during cellular stress

Transcription factor Nrf2 (NF-E2-related factor 2) is a master regulator of cellular responses against environmental stresses. Nrf2 induces the expression of detoxification and antioxidant enzymes and suppresses the induction of pro-inflammatory cytokine genes. Keap1 (Kelch-like ECH-associated protein 1) is an adaptor subunit of Cullin 3-based E3 ubiquitin ligase. Keap1 regulates the activity of Nrf2 and acts as a sensor for oxidative and electrophilic stresses. In this review, we discuss the molecular mechanisms by which the Keap1-Nrf2 system senses and regulates the cellular response to environmental stresses. In particular, we focus on the multiple stress-sensing mechanisms of Keap1 and novel regulatory functions of Nrf2.

Sustained O-GlcNAcylation reprograms mitochondrial function to regulate energy metabolism

Dysfunctional mitochondria and generation of reactive oxygen species (ROS) promote chronic diseases, which have spurred interest in the molecular mechanisms underlying these conditions. Previously, we have demonstrated that disruption of post-translational modification of proteins with β-linked N-acetylglucosamine (O-GlcNAcylation) via overexpression of the O-GlcNAc-regulating enzymes O-GlcNAc transferase (OGT) or O-GlcNAcase (OGA) impairs mitochondrial function. Here, we report that sustained alterations in O-GlcNAcylation either by pharmacological or genetic manipulation also alter metabolic function. Sustained O-GlcNAc elevation in SH-SY5Y neuroblastoma cells increased OGA expression and reduced cellular respiration and ROS generation. Cells with elevated O-GlcNAc levels had elongated mitochondria and increased mitochondrial membrane potential, and RNA-sequencing analysis indicated transcriptome reprogramming and down-regulation of the NRF2-mediated antioxidant response. Sustained O-GlcNAcylation in mouse brain and liver validated the metabolic phenotypes observed in the cells, and OGT knockdown in the liver elevated ROS levels, impaired respiration, and increased the NRF2 antioxidant response. Moreover, elevated O-GlcNAc levels promoted weight loss and lowered respiration in mice and skewed the mice toward carbohydrate-dependent metabolism as determined by indirect calorimetry. In summary, sustained elevation in O-GlcNAcylation coupled with increased OGA expression reprograms energy metabolism, a finding that has potential implications for the etiology, development, and management of metabolic diseases.

Pharmacological stimulation of nuclear factor (erythroid-derived 2)-like 2 translation activates antioxidant responses

Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is the master regulator of the antioxidant response, and its function is tightly regulated at the transcriptional, translational, and post-translational levels. It is well-known that Nrf2 is regulated at the protein level by proteasomal degradation via Kelch-like ECH-associated protein 1 (Keap1), but how Nrf2 is regulated at the translational level is less clear. Here, we show that pharmacological stimulation increases Nrf2 levels by overcoming basal translational repression. We developed a novel reporter assay that enabled identification of natural compounds that induce Nrf2 translation by a mechanism independent of Keap1-mediated degradation. Apigenin, resveratrol, and piceatannol all induced Nrf2 translation. More importantly, the pharmacologically induced Nrf2 overcomes Keap1 regulation, translocates to the nucleus, and activates the antioxidant response. We conclude that translational regulation controls physiological levels of Nrf2, and this can be modulated by apigenin, resveratrol, and piceatannol. Also, targeting this mechanism with novel compounds could provide new insights into prevention and treatment of multiple diseases in which oxidative stress plays a significant role.

Nuclear factor erythroid 2-related factor 2 enhances carcinogenesis by suppressing apoptosis and promoting autophagy in nickel-transformed cells

Nickel-containing compounds are widely used in industry. Nickel is a known human carcinogen that primarily affects the lungs. Proposed mechanisms of nickel-induced carcinogenesis include disruption of cellular iron homeostasis, generation of reactive oxygen species (ROS), and induction of hypoxia signaling. However, the precise molecular mechanisms of nickel-induced malignant transformation and tumor development remain unclear. This study shows that the transcription factor Nrf2 is highly expressed in lung tumor tissue and in nickel-transformed human lung bronchial epithelial BEAS-2B cells (NiT cells). Additionally, constitutively high levels of Nrf2 play a critical role in apoptosis resistance in NiT cells. Basal ROS levels were extremely low in NiT cells and were correlated with elevated expression levels of both antioxidant enzymes (e.g. catalase and superoxide dismutases) and antiapoptotic proteins (e.g. Bcl-2 and Bcl-xL). These processes are tightly controlled by Nrf2. Autophagy inhibition, induced pharmacologically or genetically, enhanced Ni²⁺-induced apoptosis, indicating that the induction of autophagy is the cause of apoptosis resistance in NiT cells. Using similar approaches, we show that in NiT cells the inhibition of apoptosis decreases autophagy. We have shown that Stat3, which is up-regulated by Nrf2, controls autophagy induction in NiT cells. Colony formation and tumor growth were significantly attenuated by knockdown of Nrf2 or Bcl-2. Taken together, this study demonstrates that in NiT cells constitutively high Nrf2 expression inhibits apoptosis by up-regulating antioxidant enzymes and antiapoptotic proteins to increase autophagy via Stat3 signaling. These findings indicate that the Nrf2-mediated suppression of apoptosis and promotion of autophagy contribute to nickel-induced transformation and tumorigenesis.

Critical Contribution of Nuclear Factor Erythroid 2-related Factor 2 (NRF2) to Electrophile-induced Interleukin-11 Production

Nuclear factor erythroid 2-related factor 2 (NRF2) is a transcription factor that plays a crucial role in protection of cells from electrophile-induced toxicity through up-regulating phase II detoxifying enzymes and phase III transporters. We previously reported that oxidative stress induces up-regulation of interleukin-11 (IL-11), a member of the IL-6 family that ameliorates acetaminophen-induced liver toxicity. However, a role for IL-11 in protection of cells from electrophile-induced toxicity remains unclear. Here we show that an environmental electrophile, 1,2-naphthoquinone (1,2-NQ), but not 15d-prostaglandin J₂ (PGJ₂) or tert-butylhydroxyquinone (tBHQ), induced IL-11 production. Consistent with a crucial role for prolonged ERK activation in H₂O₂-induced IL-11 production, 1,2-NQ, but not 15d-PGJ₂ or tBHQ, elicited prolonged ERK activation. Conversely, inhibition of the ERK pathway by a MEK inhibitor completely blocked 1,2-NQ-induced IL-11 production at both protein and mRNA levels, further substantiating an intimate cross-talk between ERK activation and 1,2-NQ-induced IL-11 production. Promoter analysis of the Il11 gene revealed that two AP-1 sites were essential for 1,2-NQ-induced promoter activities. Among various members of the AP-1 family, Fra-1 was up-regulated by 1,2-NQ, and its up-regulation was blocked by a MEK inhibitor. Although NRF2 was not required for H₂O₂-induced IL11 up-regulation, NRF2 was essential for 1,2-NQ-induced IL11 up-regulation by increasing Fra-1 proteins possibly through promoting mRNA translation of FOSL1 Finally, intraperitoneal administration of 1,2-NQ induced body weight loss in wild-type mice, which was further exacerbated in Il11ra1^-/- mice compared with Il11ra1^+/- mice. Together, both Fra-1 and NRF2 play crucial roles in IL-11 production that protects cells from 1,2-NQ intestinal toxicity.

Nrf2 Transcription Factor Can Directly Regulate mTOR: LINKING CYTOPROTECTIVE GENE EXPRESSION TO A MAJOR METABOLIC REGULATOR THAT GENERATES REDOX ACTIVITY

Nrf2 is a master transcription factor that regulates a wide variety of cellular proteins by recognizing and binding to antioxidant response elements (AREs) in their gene promoter regions. In this study we show that increasing cellular Nrf2 results in transcriptional activation of the gene for mTOR, which is central to the PI3K signaling pathway. This is the case in cells with normal physiological PI3K. However, in cells with abnormally active PI3K increased cellular Nrf2 levels have no effect on mTOR. ChIP assays results show that increased Nrf2 binding is associated with decreased p65 binding and H3-K27me3 signal (marker of gene repression) as well as increased H3-K4me3 signal (marker of gene activation). However, in cells with PI3K activation, no effect of cellular Nrf2 increase on mTOR transcription was observed. In these cells, increasing Nrf2 levels increases Nrf2 promoter binding marginally, whereas p65 binding and H3-K27me3 mark were significantly increased, and H3-K4me3 signal is reduced. Together, these data show for the first time that Nrf2 directly regulates mTOR transcription when the PI3K pathway is intact, whereas this function is lost when PI3K is activated. We have identified a link between the Nrf2 system of sensing environmental stress and mTOR, which is a key cellular protein in metabolism. Studies in cells with activating mutations in the PI3K pathway suggest that Nrf2 transcriptional regulation of mTOR is related to promoter binding of p65 and of methylation of histone residues permissive of transcription.

The Nuclear Factor (Erythroid-derived 2)-like 2 and Proteasome Maturation Protein Axis Mediate Bortezomib Resistance in Multiple Myeloma

Resistance to the proteasome inhibitor bortezomib is an emerging clinical problem whose mechanisms have not been fully elucidated. We considered the possibility that this could be associated with enhanced proteasome activity in part through the action of the proteasome maturation protein (POMP). Bortezomib-resistant myeloma models were used to examine the correlation between POMP expression and bortezomib sensitivity. POMP expression was then modulated using genetic and pharmacologic approaches to determine the effects on proteasome inhibitor sensitivity in cell lines and in vivo models. Resistant cell lines were found to overexpress POMP, and while its suppression in cell lines enhanced bortezomib sensitivity, POMP overexpression in drug-naive cells conferred resistance. Overexpression of POMP was associated with increased levels of nuclear factor (erythroid-derived 2)-like (NRF2), and NRF2 was found to bind to and activate the POMP promoter. Knockdown of NRF2 in bortezomib-resistant cells reduced POMP levels and proteasome activity, whereas its overexpression in drug-naive cells increased POMP and proteasome activity. The NRF2 inhibitor all-trans-retinoic acid reduced cellular NRF2 levels and increased the anti-proliferative and pro-apoptotic activities of bortezomib in resistant cells, while decreasing proteasome capacity. Finally, the combination of all-trans-retinoic acid with bortezomib showed enhanced activity against primary patient samples and in a murine model of bortezomib-resistant myeloma. Taken together, these studies validate a role for the NRF2/POMP axis in bortezomib resistance and identify NRF2 and POMP as potentially attractive targets for chemosensitization to this proteasome inhibitor.

Characterization of RA839, a Noncovalent Small Molecule Binder to Keap1 and Selective Activator of Nrf2 Signaling

The activation of the transcription factor NF-E2-related factor 2 (Nrf2) maintains cellular homeostasis in response to oxidative stress by the regulation of multiple cytoprotective genes. Without stressors, the activity of Nrf2 is inhibited by its interaction with the Keap1 (kelch-like ECH-associated protein 1). Here, we describe (3S)-1-[4-[(2,3,5,6-tetramethylphenyl) sulfonylamino]-1-naphthyl]pyrrolidine-3-carboxylic acid (RA839), a small molecule that binds noncovalently to the Nrf2-interacting kelch domain of Keap1 with a Kd of ∼6 μM, as demonstrated by x-ray co-crystallization and isothermal titration calorimetry. Whole genome DNA arrays showed that at 10 μM RA839 significantly regulated 105 probe sets in bone marrow-derived macrophages. Canonical pathway mapping of these probe sets revealed an activation of pathways linked with Nrf2 signaling. These pathways were also activated after the activation of Nrf2 by the silencing of Keap1 expression. RA839 regulated only two genes in Nrf2 knock-out macrophages. Similar to the activation of Nrf2 by either silencing of Keap1 expression or by the reactive compound 2-cyano-3,12-dioxooleana-1,9-dien-28-oic acid methyl ester (CDDO-Me), RA839 prevented the induction of both inducible nitric-oxide synthase expression and nitric oxide release in response to lipopolysaccharides in macrophages. In mice, RA839 acutely induced Nrf2 target gene expression in liver. RA839 is a selective inhibitor of the Keap1/Nrf2 interaction and a useful tool compound to study the biology of Nrf2.

Bakuchiol Is a Phenolic Isoprenoid with Novel Enantiomer-selective Anti-influenza A Virus Activity Involving Nrf2 Activation

Influenza represents a substantial threat to human health and requires novel therapeutic approaches. Bakuchiol is a phenolic isoprenoid compound present in Babchi (Psoralea corylifolia L.) seeds. We examined the anti-influenza viral activity of synthetic bakuchiol using Madin-Darby canine kidney cells. We found that the naturally occurring form, (+)-(S)-bakuchiol, and its enantiomer, (-)-(R)-bakuchiol, inhibited influenza A viral infection and growth and reduced the expression of viral mRNAs and proteins in these cells. Furthermore, these compounds markedly reduced the mRNA expression of the host cell influenza A virus-induced immune response genes, interferon-β and myxovirus-resistant protein 1. Interestingly, (+)-(S)-bakuchiol had greater efficacy than (-)-(R)-bakuchiol, indicating that chirality influenced anti-influenza virus activity. In vitro studies indicated that bakuchiol did not strongly inhibit the activities of influenza surface proteins or the M2 ion channel, expressed in Chinese hamster ovary cells. Analysis of luciferase reporter assay data unexpectedly indicated that bakuchiol may induce some host cell factor(s) that inhibited firefly and Renilla luciferases. Next generation sequencing and KeyMolnet analysis of influenza A virus-infected and non-infected cells exposed to bakuchiol revealed activation of transcriptional regulation by nuclear factor erythroid 2-related factor (Nrf), and an Nrf2 reporter assay showed that (+)-(S)-bakuchiol activated Nrf2. Additionally, (+)-(S)-bakuchiol up-regulated the mRNA levels of two Nrf2-induced genes, NAD(P)H quinone oxidoreductase 1 and glutathione S-transferase A3. These findings demonstrated that bakuchiol had enantiomer-selective anti-influenza viral activity involving a novel effect on the host cell oxidative stress response.

Antioncogenic and Oncogenic Properties of Nrf2 in Arsenic-induced Carcinogenesis

Arsenic (As(3+)) is a carcinogen with considerable environmental and occupational relevancy. The present study shows that As(3+)-transformed human lung bronchial epithelial BEAS-2B cells (AsT cells) exhibit the property of apoptosis resistance. The level of basal reactive oxygen species (ROS) is very low in AsT cells in correlation with elevated expressions of both antioxidant enzymes and antiapoptotic proteins. Nuclear factor erythroid 2-related factor (Nrf2) and p62 are constitutively expressed. These two proteins up-regulate antioxidant enzymes and antiapoptotic proteins. The knockdown of Nrf2 or p62 by small interfering RNA (siRNA) enhanced both ROS levels and As(3+)-induced apoptosis in transformed cells. AsT cells have autophagy deficiency as evidenced by reduced formation of microtubule-associated protein 1 light chain 3 (LC3)-II, GFP-LC3 puncta, and autophagy flux. Results obtained using a soft agar assay and shRNA Nrf2-transfected cells show that Nrf2 plays an antioncogenic role before transformation, whereas this transcription factor plays an oncogenic role after transformation. In addition, depletion of Nrf2 by shRNA dramatically inhibited growth and proliferation of transformed cells. Furthermore, the Nrf2 protein levels and antiapoptotic and antioxidant enzyme levels are higher in lung adenocarcinoma than in normal tissues. Collectively, this study demonstrates that a constitutively high level of Nrf2 in AsT cells up-regulates the antioxidant proteins catalase and superoxide dismutase as well as the antiapoptotic proteins Bcl-2 and Bcl-xL. The final consequences are decreased ROS generation and increased apoptotic resistance, cell survival and proliferation, and tumorigenesis.

p62/Sequestosome-1, Autophagy-related Gene 8, and Autophagy in Drosophila Are Regulated by Nuclear Factor Erythroid 2-related Factor 2 (NRF2), Independent of Transcription Factor TFEB

The selective autophagy receptor p62/sequestosome 1 (SQSTM1) interacts directly with LC3 and is involved in oxidative stress signaling in two ways in mammals. First, p62 is transcriptionally induced upon oxidative stress by the NF-E2-related factor 2 (NRF2) by direct binding to an antioxidant response element in the p62 promoter. Second, p62 accumulation, occurring when autophagy is impaired, leads to increased p62 binding to the NRF2 inhibitor KEAP1, resulting in reduced proteasomal turnover of NRF2. This gives chronic oxidative stress signaling through a feed forward loop. Here, we show that the Drosophila p62/SQSTM1 orthologue, Ref(2)P, interacts directly with DmAtg8a via an LC3-interacting region motif, supporting a role for Ref(2)P in selective autophagy. The ref(2)P promoter also contains a functional antioxidant response element that is directly bound by the NRF2 orthologue, CncC, which can induce ref(2)P expression along with the oxidative stress-associated gene gstD1. However, distinct from the situation in mammals, Ref(2)P does not interact directly with DmKeap1 via a KEAP1-interacting region motif; nor does ectopically expressed Ref(2)P or autophagy deficiency activate the oxidative stress response. Instead, DmAtg8a interacts directly with DmKeap1, and DmKeap1 is removed upon programmed autophagy in Drosophila gut cells. Strikingly, CncC induced increased Atg8a levels and autophagy independent of TFEB/MitF in fat body and larval gut tissues. Thus, these results extend the intimate relationship between oxidative stress-sensing NRF2/CncC transcription factors and autophagy and suggest that NRF2/CncC may regulate autophagic activity in other organisms too.

Activation of the UPR protects against cigarette smoke-induced RPE apoptosis through up-regulation of Nrf2

Recent studies have revealed a role of endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR) in the regulation of RPE cell activity and survival. Herein, we examined the mechanisms by which the UPR modulates apoptotic signaling in human RPE cells challenged with cigarette smoking extract (CSE). Our results show that CSE exposure induced a dose- and time-dependent increase in ER stress markers, enhanced reactive oxygen species (ROS), mitochondrial fragmentation, and apoptosis of RPE cells. These changes were prevented by the anti-oxidant NAC or chemical chaperone TMAO, suggesting a close interaction between oxidative and ER stress in CSE-induced apoptosis. To decipher the role of the UPR, overexpression or down-regulation of XBP1 and CHOP genes was manipulated by adenovirus or siRNA. Overexpressing XBP1 protected against CSE-induced apoptosis by reducing CHOP, p-p38, and caspase-3 activation. In contrast, XBP1 knockdown sensitized the cells to CSE-induced apoptosis, which is likely through a CHOP-independent pathway. Surprisingly, knockdown of CHOP reduced p-eIF2α and Nrf2 resulting in a marked increase in caspase-3 activation and apoptosis. Furthermore, Nrf2 inhibition increased ER stress and exacerbated cell apoptosis, while Nrf2 overexpression reduced CHOP and protected RPE cells. Our data suggest that although CHOP may function as a pro-apoptotic gene during ER stress, it is also required for Nrf2 up-regulation and RPE cell survival. In addition, enhancing Nrf2 and XBP1 activity may help reduce oxidative and ER stress and protect RPE cells from cigarette smoke-induced damage.