Arsenic is widely spread in our living environment and imposes a big challenge on human health worldwide. Arsenic damages biological systems through multiple mechanisms including the generation of reactive oxygen species. The transcription factor Nrf2 regulates the cellular antioxidant response that protects cells from various insults. In this study, the protective role of Nrf2 in arsenic toxicity was investigated in a human bladder urothelial cell line, UROtsa. Using an UROtsa cell line stably infected with Nrf2-siRNA, we clearly demonstrate that compromised Nrf2 expression sensitized the cells to As(III)- and MMA(III)-induced toxicity. On the other hand, the activation of the Nrf2 pathway by tert-butylhydroquinone (tBHQ) and sulforaphane (SF), the known Nrf2-inducers, rendered UROtsa cells more resistant to As(III)- and MMA(III). Furthermore, the wild type mouse embryo fibroblast (WT-MEF) cells were protected from As(III)- and MMA(III)-induced toxicity following Nrf2 activation by tBHQ or SF whereas neither tBHQ nor SF conferred protection in the Nrf2−/−-MEF cells, demonstrating that tBHQ- or SF-mediated protection against As(III)- and MMA(III)-induced toxicity depends on Nrf2 activation. These results, obtained by both loss of function and gain of function analyses, clearly demonstrate the protective role of Nrf2 in arsenic-induced toxicity. The current work lays the groundwork for using Nrf2 activators for therapeutic and dietary interventions against adverse effects of arsenic.
Nrf2; Keap1; arsenic; arsenite; MMA(III); UROtsa
Arsenic is present in the environment and has become a worldwide health concern due to its toxicity and carcinogenicity. However, the specific mechanism(s) by which arsenic elicits its toxic effects has yet to be fully elucidated. The transcription factor nuclear factor (erythroid-derived 2)-like 2 (Nrf2) has been recognized as the master regulator of a cellular defense mechanism against toxic insults. This review highlights studies demonstrating that arsenic activates the Nrf2-Keap1 antioxidant pathway by a distinct mechanism from that of natural compounds such as sulforaphane (SF) found in broccoli sprouts or tert-butylhyrdoquinone (tBHQ), a natural antioxidant commonly used as a food preservative. Evidence also suggests that arsenic prolongs Nrf2 activation and may mimic constitutive activation of Nrf2, which has been found in several human cancers due to disruption of the Nrf2-Keap1 axis. The current literature strongly suggests that activation of Nrf2 by arsenic potentially contributes to, rather than protects against, arsenic toxicity and carcinogenicity. The mechanism(s) by which known Nrf2 activators, such as the natural chemopreventive compounds SF and lipoic acid, protect against the deleterious effects caused by arsenic will also be discussed. These findings will provide insight to further understand how arsenic promotes a prolonged Nrf2 response, which will lead to the identification of novel molecular markers and development of rational therapies for the prevention or intervention of arsenic-induced diseases. The National Institute of Environmental Health Science (NIEHS) Outstanding New Environmental Scientist (ONES) award has provided the opportunity to review the progress both in the fields of arsenic toxicology and Nrf2 biology. Much of the funding has led to (1) the novel discovery that arsenic activates the Nrf2 pathway by a mechanism different to that of other Nrf2 activators, such as sulforaphane and tert-butylhydroquinone, (2) activation of Nrf2 by chemopreventive compounds protects against arsenic toxicity and carcinogenicity both in vitro and in vivo, (3) constitutive activation of Nrf2 by disrupting Keap1-mediated negative regulation contributes to cancer and chemoresistance, (4) p62-mediated sequestration of Keap1 activates the Nrf2 pathway, and (5) arsenic-mediated Nrf2 activation may be through a p62-dependent mechanism. All of these findings have been published and are discussed in this review. This award has laid the foundation for my laboratory to further investigate the molecular mechanism(s) that regulate the Nrf2 pathway and how it may play an integral role in arsenic toxicity. Moreover, understanding the biology behind arsenic toxicity and carcinogenicity will help in the discovery of potential strategies to prevent or control arsenic-mediated adverse effects.
Nrf2; Arsenic; Keap1; Oxidative stress; p62; Autophagy; Chemoprevention
Groundwater contaminated with arsenic imposes a big challenge to human health worldwide. Using natural compounds to subvert the detrimental effects of arsenic represents an attractive strategy. The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) is a critical regulator of the cellular antioxidant response and xenobiotic metabolism. Recently, activation of the Nrf2 signaling pathway has been reported to confer protection against arsenic-induced toxicity in a cell culture model.
The goal of the present work was to identify a potent Nrf2 activator from plants as a chemopreventive compound and to demonstrate the efficacy of the compound in battling arsenic-induced toxicity.
Oridonin activated the Nrf2 signaling pathway at a low subtoxic dose and was able to stabilize Nrf2 by blocking Nrf2 ubiquitination and degradation, leading to accumulation of the Nrf2 protein and activation of the Nrf2-dependent cytoprotective response. Pretreatment of UROtsa cells with 1.4 μM oridonin significantly enhanced the cellular redox capacity, reduced formation of reactive oxygen species (ROS), and improved cell survival after arsenic challenge.
We identified oridonin as representing a novel class of Nrf2 activators and illustrated the mechanism by which the Nrf2 pathway is activated. Furthermore, we demonstrated the feasibility of using natural compounds targeting Nrf2 as a therapeutic approach to protect humans from various environmental insults that may occur daily.
antioxidant responsive element; antitumor; ARE; arsenic; chemoprevention; diterpenoid; Keap1; Nrf2; oridonin; oxidative stress; rubescensin
Nrf2 is a major cytoprotective gene and is a key chemopreventive target against cancer and other diseases. Here we show that Nrf2 faces a dilemma in defense against 4-aminobiphenyl (ABP), a major human bladder carcinogen from tobacco smoke and other environmental sources. While Nrf2 protected mouse liver against ABP (which is metabolically activated in liver), the bladder level of N-(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-ABP), the predominant ABP-DNA adduct formed in bladder cells and tissues, was markedly higher in Nrf2+/+ mice than in Nrf2−/− mice after ABP exposure. Notably, Nrf2 protected bladder cells against ABP in vitro. Mechanistic investigations showed that the dichotomous effects of Nrf2 could be explained at least partly by upregulation of UDP-glucuronosyltransferase (UGT). Nrf2 promoted conjugation of ABP with glucuronic acid in the liver, increasing urinary excretion of the conjugate. While glucuronidation of ABP and its metabolites is a detoxification process, these conjugates, which are excreted in urine, are known to be unstable in acidic urine, leading to delivery of the parent compounds to bladder. Hence, while higher liver UGT activity may protect the liver against ABP it increases bladder exposure to ABP. These findings raise concerns of potential bladder toxicity when Nrf2-activating chemopreventive agents are used in humans exposed to ABP, especially in smokers. We further demonstrate that 5,6-dihydrocyclopenta[c][1,2]-dithiole-3(4H)-thione (CPDT) significantly inhibits dG-C8-ABP formation in bladder cells and tissues, but does not appear to significantly modulate ABP-catalyzing UGT in liver. Thus, CPDT exemplifies a counteracting solution to the dilemma posed by Nrf2.
Exposure to arsenic is associated with an increased risk of lung disease. Novel strategies are needed to reduce the adverse health effects associated with arsenic exposure in the lung. Nrf2, a transcription factor that mediates an adaptive cellular defense response, is effective in detoxifying environmental insults and prevents a broad spectrum of diseases induced by environmental exposure to harmful substances. In this report, we tested whether Nrf2 activation protects mice from arsenic-induced toxicity. We used an in vivo arsenic inhalation model that is highly relevant to low environmental human exposure to arsenic-containing dusts. Two-week exposure to arsenic-containing dust resulted in pathological alterations, oxidative DNA damage, and mild apoptotic cell death in the lung; all of which were blocked by sulforaphane (SF) in an Nrf2-dependent manner. Mechanistically, SF-mediated activation of Nrf2 alleviated inflammatory responses by modulating cytokine production. This study provides strong evidence that dietary intervention targeting Nrf2 activation is a feasible approach to reduce adverse health effects associated with arsenic exposure.
Nrf2; Keap1; Arsenic; Antioxidant response
The transcription factor Nrf2 is a master regulator of cellular defence: Nrf2 null mice (Nrf2(−/−)) are highly susceptible to chemically induced toxicities. We report a comparative iTRAQ-based study in Nrf2(−/−) mice treated with a potent inducer, methyl-2-cyano-3,12-dioxooleana-1,9(11)dien-28-oate (CDDO-me; bardoxolone -methyl), to define both the Nrf2-dependent basal and inducible hepatoproteomes. One thousand five hundred twenty-one proteins were fully quantified (FDR < 1%). One hundred sixty-one were significantly different (P < 0.05) between WT and Nrf2(−/−) mice, confirming extensive constitutive regulation by Nrf2. Treatment with CDDO-me (3 mg/kg; i.p.) resulted in significantly altered expression of 43 proteins at 24 h in WT animals. Six proteins were regulated at both basal and inducible levels exhibiting the largest dynamic range of Nrf2 regulation: cytochrome P4502A5 (CYP2A5; 17.2-fold), glutathione-S-transferase-Mu 3 (GSTM3; 6.4-fold), glutathione-S-transferase Mu 1 (GSTM1; 5.9-fold), ectonucleoside-triphosphate diphosphohydrolase (ENTPD5; 4.6-fold), UDP-glucose-6-dehydrogenase (UDPGDH; 4.1-fold) and epoxide hydrolase (EPHX1; 3.0-fold). These proteins, or their products, thus provide a potential source of biomarkers for Nrf2 activity. ENTPD5 is of interest due to its emerging role in AKT signalling and, to our knowledge, this protein has not been previously shown to be Nrf2-dependent. Only two proteins altered by CDDO-me in WT animals were similarly affected in Nrf2(−/−) mice, demonstrating the high degree of selectivity of CDDO-me for the Nrf2:Keap1 signalling pathway.
The Nrf2:Keap1 signalling pathway is attracting considerable interest as a therapeutic target for different disease conditions. For example, CDDO-me (bardoxolone methyl) was investigated in clinical trials for the treatment of acute kidney disease, and dimethyl fumarate, recently approved for reducing relapse rate in multiple sclerosis, is a potent Nrf2 inducer. Such compounds have been suggested to act through multiple mechanisms; therefore, it is important to define the selectivity of Nrf2 inducers to assess the potential for off-target effects that may lead to adverse drug reactions, and to provide biomarkers with which to assess therapeutic efficacy. Whilst there is considerable information on the global action of such inducers at the mRNA level, this is the first study to catalogue the hepatic protein expression profile following acute exposure to CDDO-me in mice. At a dose shown to evoke maximal Nrf2 induction in the liver, CDDO-me appeared highly selective for known Nrf2-regulated proteins. Using the transgenic Nrf2(−/−) mouse model, it could be shown that 97% of proteins induced in wild type mice were associated with a functioning Nrf2 signalling pathway. This analysis allowed us to identify a panel of proteins that were regulated both basally and following Nrf2 induction. Identification of these proteins, which display a large magnitude of variation in their expression, provides a rich source of potential biomarkers for Nrf2 activity for use in experimental animals, and which may be translatable to man to define individual susceptibility to chemical stress, including that associated with drugs, and also to monitor the pharmacological response to Nrf2 inducers.
•Liver proteomes from WT, Nrf2-null and Nrf2-induced mice were compared by iTRAQ•Of 1521 proteins quantified, 161 were regulated basally and 43 following induction•Six proteins were both basally and inducibly regulated, with high dynamic ranges•In order of fold change, these proteins were CYP2A5, GSTM3, GSTM1, ENTPD5, G6PD, EPHX1•These proteins may yield translatable biomarkers for clinical development
Nrf2; iTRAQ; ENTPD5; CYP2A5; Hepatoproteome; CDDO
Chronic human exposure to inorganic arsenic (iAs), a potent environmental oxidative stressor, is associated with increased prevalence of Type 2 diabetes, where impairment of pancreatic β-cell function is a key pathogenic factor. Nuclear factor E2-related factor 2 (Nrf2) is a central transcription factor regulating cellular adaptive response to oxidative stress. However, persistent activation of Nrf2 in response to chronic oxidative stress, including inorganic arsenite (iAs3+) exposure, blunts glucose-triggered reactive oxygen species (ROS) signaling and impairs glucose-stimulated insulin secretion (GSIS). In the current study, we found that MIN6 pancreatic β-cells with stable knockdown of Nrf2 (Nrf2-KD) by lentiviral shRNA and pancreatic islets isolated from Nrf2-knockout (Nrf2−/−) mice exhibited reduced expression of several antioxidant and detoxification enzymes in response to acute iAs3+ exposure. As a result, Nrf2-KD MIN6 cells and Nrf2−/− islets were more susceptible to iAs3+ and monomethylarsonous acid (MMA3+)-induced cell damage, as measured by decreased cell viability, augmented apoptosis and morphological change. Pretreatment of MIN6 cells with Nrf2 activator tert-butylhydroquinone protected the cells from iAs3+-induced cell damage in an Nrf2-dependent fashion. In contrast, antioxidant N-acetyl cysteine protected Nrf2-KD MIN6 cells against acute cytotoxicity of iAs3+. The present study demonstrates that Nrf2-mediated antioxidant response is critical in the pancreatic β-cell defense mechanism against acute cytotoxicity by arsenic. The findings here, combined with our previous results on the inhibitory effect of antioxidants on ROS signaling and GSIS, suggest that Nrf2 plays paradoxical roles in pancreatic β-cell dysfunction induced by environmental arsenic exposure.
arsenic; diabetes; pancreatic β-cell; islets; Nrf2; oxidative stress; cytotoxicity
Drinking water contaminated with arsenic, a human carcinogen, is a worldwide health issue. An understanding of cellular signaling events in response to arsenic exposure and rational designing of strategies to reduce arsenic damages by modulating signaling events are important to fight against arsenic-induced diseases. Previously, we reported that activation of the Nrf2-mediated cellular defense pathway confers protection against toxic effects induced by sodium arsenite [As(III)] or monomethylarsonous acid [MMA(III)]. Paradoxically, arsenic has been reported to induce the Nrf2-dependent signaling pathway. Here, we report the unique mechanism of Nrf2 induction by arsenic. Similar to tert-butylhydroquinone (tBHQ) or sulforaphane (SF), arsenic induced the Nrf2-dependent response through enhancing Nrf2 protein levels by inhibiting Nrf2 ubiquitination and degradation. However, the detailed action of arsenic in Nrf2 induction is different from that of tBHQ or SF. Arsenic markedly enhanced the interaction between Keap1 and Cul3, subunits of the E3 ubiquitin ligase for Nrf2, which led to impaired dynamic assembly/disassembly of the E3 ubiquitin ligase and thus decreased its ligase activity. Furthermore, induction of Nrf2 by arsenic is independent of the previously identified C151 residue in Keap1 that is required for Nrf2 activation by tBHQ or SF. Distinct mechanisms of Nrf2 activation by seemingly harmful and beneficial reagents provide a molecular basis to design Nrf2-activating agents for therapeutic intervention.
Background: Nuclear factor E2-related factors (NRFs), including NRF2 and NRF1, play critical roles in mediating the cellular adaptive response to oxidative stress. Human exposure to inorganic arsenic, a potent oxidative stressor, causes various dermal disorders, including hyperkeratosis and skin cancer.
Objective: We investigated the cross-regulations among NRF2, NRF1, and KEAP1, a cullin-3–adapter protein that allows NRF2 to be ubiquinated and degraded by the proteasome complex, in arsenic-induced antioxidant responses.
Results: In human keratinocyte HaCaT cells, selective knockdown (KD) of NRF2 by lentiviral short hairpin RNAs (shRNAs) significantly reduced the expression of many antioxidant enzymes and sensitized the cells to acute cytotoxicity of inorganic arsenite (iAs3+). In contrast, silencing KEAP1 led to a dramatic resistance to iAs3+-induced apoptosis. Pretreatment of HaCaT cells with NRF2 activators, such as tert-butylhydroquinone, protects the cells against acute iAs3+ toxicity in an NRF2-dependent fashion. Consistent with the negative regulatory role of KEAP1 in NRF2 activation, KEAP1-KD cells exhibited enhanced transcriptional activity of NRF2 under nonstressed conditions. However, deficiency in KEAP1 did not facilitate induction of NRF2-target genes by iAs3+. In addition, NRF2 silencing reduced the expression of KEAP1 at transcription and protein levels but increased the protein expression of NRF1 under the iAs3+-exposed condition. In contrast, silencing KEAP1 augmented protein accumulation of NRF2 under basal and iAs3+-exposed conditions, whereas the iAs3+-induced protein accumulation of NRF1 was attenuated in KEAP1-KD cells.
Conclusions: Our studies suggest that NRF2, KEAP1, and NRF1 are coordinately involved in the regulation of the cellular adaptive response to iAs3+-induced oxidative stress.
antioxidant response; arsenic; cytotoxicity; KEAP1; keratinocyte; NRF1; NRF2
Aims: The NF-E2 p45-related factor 2 (Nrf2) signaling pathway regulates the cellular antioxidant response and activation of Nrf2 has recently been shown to limit tissue damage from exposure to environmental toxicants, including As(III). In an attempt to identify improved molecular agents for systemic protection against environmental insults, we have focused on the identification of novel medicinal plant-derived Nrf2 activators. Results: Tanshinones [tanshinone I (T-I), tanshinone IIA, dihydrotanshinone, cryptotanshinone], phenanthrenequinone-based redox therapeutics derived from the medicinal herb Salvia miltiorrhiza, have been tested as experimental therapeutics for Nrf2-dependent cytoprotection. Using a dual luciferase reporter assay overexpressing wild-type or mutant Kelch-like ECH-associated protein-1 (Keap1), we demonstrate that T-I is a potent Keap1-C151-dependent Nrf2 activator that stabilizes Nrf2 by hindering its ubiquitination. In human bronchial epithelial cells exposed to As(III), T-I displays pronounced cytoprotective activity with upregulation of Nrf2-orchestrated gene expression. In Nrf2 wild-type mice, systemic administration of T-I attenuates As(III) induced inflammatory lung damage, a protective effect not observed in Nrf2 knockout mice. Innovation: Tanshinones have been identified as a novel class of Nrf2-inducers for antioxidant tissue protection in an in vivo As(III) inhalation model, that is relevant to low doses of environmental exposure. Conclusion: T-I represents a prototype Nrf2-activator that displays cytoprotective activity upon systemic administration targeting lung damage originating from environmental insults. T-I based Nrf2-directed systemic intervention may provide therapeutic benefit in protecting other organs against environmental insults. Antioxid. Redox Signal. 19, 1647–1661.
Mouse cytochrome P450 2A5 (CYP2A5) is upregulated in various liver diseases and a putative common feature for all of these conditions is altered cellular redox status. Nuclear factor erythroid 2-like 2 (Nrf2) is a transcription factor that is post-translationally regulated by oxidative stress and controls the transcription of protective target genes. In the present study, we have characterized the regulation of CYP2A5 by Nrf2 and evaluated gene expression, protein content and activity of anti-oxidant enzymes in the Nrf2+/+ and Nrf2−/− mice model of non-alcoholic fatty liver (NAFLD). After eight weeks of feeding on a high-fat diet, livers from Nrf2−/− mice showed a substantial increase in macro and microvesicular steatosis and a massive increase in the number of neutrophil polymorphs, compared to livers from wild-type mice treated similarly. Livers of Nrf2−/− mice on the high-fat diet exhibited more oxidative stress than their wild-type counterparts as assessed by a significant depletion of reduced glutathione that was coupled with increases in malondialdehyde. Furthermore, results in Nrf2-deficient mice showed that CYP2A5 expression was significantly attenuated in the absence of Nrf2, as was found with the conventional target genes of Nrf2. The treatment of wild-type mice with high-fat diet leaded to nuclear accumulation of Nrf2, and co-immunoprecipitation experiments showed that Nrf2 was bound to Cyp2a5. These findings suggest that the high-fat diet induced alteration in cellular redox status and induction of CYP2A5 was modulated through the redox-sensitive transcription Nrf2.
•CYP2A5 up-regulation in response to NAFLD was Nrf2 dependent.•NAFLD induces oxidant stress.•A protective role for Nrf2 against hepatic damage by NAFLD was demonstrated.•NAFLD induces translocation of Nrf2 from the cytoplasm to the nucleus.•Nrf2 binding to CYP2a5 was shown.
ARE, Antioxidant response element; Nrf2, Nuclear factor erythroid 2-like 2; KO, Knockout; WT, Wild-type; P450, Cytochrome P450; NASH, Nonalcoholic steatohepatitis; NAFLD, Nonalcoholic fatty liver disease; co-IP, Co-immunoprecipitation; Coumarin 7-hydroxylase; Oxidative stress; glutathione S-transferases; High-fat diet; Knockout mice
Diabetic nephropathy is one of the major causes of renal failure, which is accompanied by the production of reactive oxygen species (ROS). Nrf2 is the primary transcription factor that controls the antioxidant response essential for maintaining cellular redox homeostasis. Here, we report our findings demonstrating a protective role of Nrf2 against diabetic nephropathy.
RESEARCH DESIGN AND METHODS
We explore the protective role of Nrf2 against diabetic nephropathy using human kidney biopsy tissues from diabetic nephropathy patients, a streptozotocin-induced diabetic nephropathy model in Nrf2−/− mice, and cultured human mesangial cells.
The glomeruli of human diabetic nephropathy patients were under oxidative stress and had elevated Nrf2 levels. In the animal study, Nrf2 was demonstrated to be crucial in ameliorating streptozotocin-induced renal damage. This is evident by Nrf2−/− mice having higher ROS production and suffering from greater oxidative DNA damage and renal injury compared with Nrf2+/+ mice. Mechanistic studies in both in vivo and in vitro systems showed that the Nrf2-mediated protection against diabetic nephropathy is, at least, partially through inhibition of transforming growth factor-β1 (TGF-β1) and reduction of extracellular matrix production. In human renal mesangial cells, high glucose induced ROS production and activated expression of Nrf2 and its downstream genes. Furthermore, activation or overexpression of Nrf2 inhibited the promoter activity of TGF-β1 in a dose-dependent manner, whereas knockdown of Nrf2 by siRNA enhanced TGF-β1 transcription and fibronectin production.
This work clearly indicates a protective role of Nrf2 in diabetic nephropathy, suggesting that dietary or therapeutic activation of Nrf2 could be used as a strategy to prevent or slow down the progression of diabetic nephropathy.
Arsenic is a well-established human carcinogen of poorly understood mechanism of genotoxicity. It is generally accepted that arsenic acts indirectly by generating oxidative DNA damage that can be converted to replication-dependent DNA double-strand breaks (DSBs), as well as by interfering with DNA repair pathways and DNA methylation. Here we show that in budding yeast arsenic also causes replication and transcription-independent DSBs in all phases of the cell cycle, suggesting a direct genotoxic mode of arsenic action. This is accompanied by DNA damage checkpoint activation resulting in cell cycle delays in S and G2/M phases in wild type cells. In G1 phase, arsenic activates DNA damage response only in the absence of the Yku70–Yku80 complex which normally binds to DNA ends and inhibits resection of DSBs. This strongly indicates that DSBs are produced by arsenic in G1 but DNA ends are protected by Yku70–Yku80 and thus invisible for the checkpoint response. Arsenic-induced DSBs are processed by homologous recombination (HR), as shown by Rfa1 and Rad52 nuclear foci formation and requirement of HR proteins for cell survival during arsenic exposure. We show further that arsenic greatly sensitizes yeast to phleomycin as simultaneous treatment results in profound accumulation of DSBs. Importantly, we observed a similar response in fission yeast Schizosaccharomyces pombe, suggesting that the mechanisms of As(III) genotoxicity may be conserved in other organisms.
Arsenic is a highly toxic compound which causes several types of cancer in humans. However, precise mechanisms of arsenic carcinogenesis remain elusive and are still a matter of debate. For example, the oxidative stress theory of arsenic proposes that arsenic generates reactive oxygen species producing oxidative DNA damage that can be converted to DNA double-strand breaks (DSBs) during replication. Using budding yeast as a model organism, we show that arsenic is able to induce DSBs in the absence of transcription, replication and pronounced oxidative stress. Importantly, we also demonstrate that arsenic greatly enhances cytotoxic activity of antitumor drug phleomycin, as evidenced by increased sensitivity and DNA fragmentation visible upon co-treatment. Our work suggests that arsenic acts as a direct inducer of DNA breaks and could be potentially used with other anticancer drugs, like phleomycin-related bleomycin, as a new combinatory therapy to treat cancers that poorly respond to these drugs. Additionally, since in many countries millions of people are exposed to high doses of arsenic in drinking water, we believe that our findings about genotoxicity of arsenic are important not only to geneticists but also to the general public.
CRMs activate Nrf2, but inhibit NF-κB, and GSH depletion without covalent modification activates both Nrf2 and NF-κB. This leads to cellular protection against the potentially harmful effects of redox perturbation.
Liver injury associated with exposure to therapeutic agents that undergo hepatic metabolism can involve the formation of reactive metabolites. These may cause redox perturbation which can result in oxidative stress as well as protein modification leading to activation or inhibition of cellular transcriptional responses. Nevertheless, the effects of these challenges on more than one transcriptional pathway simultaneously remain unclear. We have investigated two transcription factors known to be sensitive to electrophilic stress and redox perturbation, Nrf2 and NF-κB, in mouse liver cells. Cellular stress was induced by the probes: N-acetyl-p-benzoquinineimine (NAPQI), the reactive metabolite of acetaminophen; dinitrochlorobenzene (DNCB), a model electrophile; and buthionine (S,R)-sulfoximine (BSO), an inhibitor of glutamate-cysteine ligase. NAPQI, DNCB and BSO can all cause glutathione (GSH) depletion; however only NAPQI and DNCB can covalently bind proteins. We also employed RNAi to manipulate Keap1 (the inhibitor of Nrf2), Nrf2 itself and NF-κB-p65, to understand their roles in the response to drug stress. All three chemicals induced Nrf2, but NF-κB binding activity was only increased after BSO treatment. In fact, NF-κB binding activity decreased after exposure to NAPQI and DNCB. While RNAi depletion of Keap1 led to reduced toxicity following exposure to DNCB, depletion of Nrf2 and NF-κB augmented toxicity. Interestingly, increased Nrf2 caused by Keap1 depletion was reversed by co-depletion of NF-κB. We demonstrate that Keap1/Nrf2 and NF-κB respond differently to electrophiles that bind proteins covalently and the redox perturbation associated with glutathione depletion, and that crosstalk may enable NF-κB to partly influence Nrf2 expression during cellular stress.
DILI, drug-induced liver injury; CRMs, chemically reactive metabolites; GSH, glutathione; Nrf2, nuclear factor-erythroid 2 (NF-E2)-related factor; NF-κB, NF-kappa B; ARE, anti-oxidant response element; NAPQI, N-acetyl-p-benzoquinineimine; DNCB, dinitrochlorobenzene; BSO, buthionine (S,R)-sulfoximine; RNAi, RNA interference; Nrf2; Keap1; NF-κB; RNAi; Cellular stress; Transcriptional response
Previous research demonstrated that 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment increased the number of skin papillomas in v-Ha-ras transgenic (Tg.AC) mice that had received sodium arsenite [(As(III)] in drinking water, indicating that this model is useful for studying the toxic effects of arsenic in vivo. Because the liver is a known target of arsenic, we examined the pathophysiologic and molecular effects of inorganic and organic arsenical exposure on Tg.AC mouse liver in this study. Tg.AC mice were provided drinking water containing As(III), sodium arsenate [As(V)], monomethylarsonic acid [(MMA(V)], and 1,000 ppm dimethylarsinic acid [DMA(V)] at dosages of 150, 200, 1,500, or 1,000 ppm as arsenic, respectively, for 17 weeks. Control mice received unaltered water. Four weeks after initiation of arsenic treatment, TPA at a dose of 1.25 μg/200 μL acetone was applied twice a week for 2 weeks to the shaved dorsal skin of all mice, including the controls not receiving arsenic. In some cases arsenic exposure reduced body weight gain and caused mortality (including moribundity). Arsenical exposure resulted in a dose-dependent accumulation of arsenic in the liver that was unexpectedly independent of chemical species and produced hepatic global DNA hypomethylation. cDNA microarray and reverse transcriptase–polymerase chain reaction analysis revealed that all arsenicals altered the expression of numerous genes associated with toxicity and cancer. However, organic arsenicals [MMA(V) and DMA(V)] induced a pattern of gene expression dissimilar to that of inorganic arsenicals. In summary, subchronic exposure of Tg.AC mice to inorganic or organic arsenicals resulted in toxic manifestations, hepatic arsenic accumulation, global DNA hypomethylation, and numerous gene expression changes. These effects may play a role in arsenic-induced hepatotoxicity and carcinogenesis and may be of particular toxicologic relevance.
arsenicals (arsenic forms); gene expression; mouse liver; subchronic toxicity; toxicokinetics
Drinking water contaminated with inorganic arsenic is associated with increased risk for different types of cancer. Paradoxically, arsenic trioxide can also be used to induce remission in patients with acute promyelocytic leukemia (APL) with a success rate of approximately 80%. A comprehensive study examining the mechanisms and potential signaling pathways contributing to the anti-tumor properties of arsenic trioxide has not been carried out.
Here we applied a systems biology approach to identify gene biomarkers that underlie tumor cell responses to arsenic-induced cytotoxicity. The baseline gene expression levels of 14,500 well characterized human genes were associated with the GI50 data of the NCI-60 tumor cell line panel from the developmental therapeutics program (DTP) database. Selected biomarkers were tested in vitro for the ability to influence tumor susceptibility to arsenic trioxide.
A significant association was found between the baseline expression levels of 209 human genes and the sensitivity of the tumor cell line panel upon exposure to arsenic trioxide. These genes were overlayed onto protein-protein network maps to identify transcriptional networks that modulate tumor cell responses to arsenic trioxide. The analysis revealed a significant enrichment for the oxidative stress response pathway mediated by nuclear factor erythroid 2-related factor 2 (NRF2) with high expression in arsenic resistant tumor cell lines. The role of the NRF2 pathway in protecting cells against arsenic-induced cell killing was validated in tumor cells using shRNA-mediated knock-down.
In this study, we show that the expression level of genes in the NRF2 pathway serve as potential gene biomarkers of tumor cell responses to arsenic trioxide. Importantly, we demonstrate that tumor cells that are deficient for NRF2 display increased sensitivity to arsenic trioxide. The results of our study will be useful in understanding the mechanism of arsenic-induced cytotoxicity in cells, as well as the increased applicability of arsenic trioxide as a chemotherapeutic agent in cancer treatment.
Aims: Nrf2 is an essential transcription factor for protection against oxidant disorders. However, its role in organ development and neonatal disease has received little attention. Therapeutically administered oxygen has been considered to contribute to bronchopulmonary dysplasia (BPD) in prematurity. The current study was performed to determine Nrf2-mediated molecular events during saccular-to-alveolar lung maturation, and the role of Nrf2 in the pathogenesis of hyperoxic lung injury using newborn Nrf2-deficient (Nrf2−/−) and wild-type (Nrf2+/+) mice. Results: Pulmonary basal expression of cell cycle, redox balance, and lipid/carbohydrate metabolism genes was lower while lymphocyte immunity genes were more highly expressed in Nrf2−/− neonates than in Nrf2+/+ neonates. Hyperoxia-induced phenotypes, including mortality, arrest of saccular-to-alveolar transition, and lung edema, and inflammation accompanying DNA damage and tissue oxidation were significantly more severe in Nrf2−/− neonates than in Nrf2+/+ neonates. During lung injury pathogenesis, Nrf2 orchestrated expression of lung genes involved in organ injury and morphology, cellular growth/proliferation, vasculature development, immune response, and cell–cell interaction. Bioinformatic identification of Nrf2 binding motifs and augmented hyperoxia-induced inflammation in genetically deficient neonates supported Gpx2 and Marco as Nrf2 effectors. Innovation: This investigation used lung transcriptomics and gene targeted mice to identify novel molecular events during saccular-to-alveolar stage transition and to elucidate Nrf2 downstream mechanisms in protection from hyperoxia-induced injury in neonate mouse lungs. Conclusion:
Nrf2 deficiency augmented lung injury and arrest of alveolarization caused by hyperoxia during the newborn period. Results suggest a therapeutic potential of specific Nrf2 activators for oxidative stress-associated neonatal disorders including BPD. Antioxid. Redox Signal. 00, 000–000.
The Nrf2-Keap1 signaling pathway is a protective mechanism promoting cell survival. Activation of the Nrf2 pathway by natural compounds has been proven to be an effective strategy for chemoprevention. Interestingly, a cancer-promoting function of Nrf2 has recently been observed in many types of tumors due to deregulation of the Nrf2-Keap1 axis, which leads to constitutive activation of Nrf2. Here, we report a novel mechanism of Nrf2 activation by arsenic that is distinct from that of chemopreventive compounds. Arsenic deregulates the autophagic pathway through blockage of autophagic flux, resulting in accumulation of autophagosomes and sequestration of p62, Keap1, and LC3. Thus, arsenic activates Nrf2 through a noncanonical mechanism (p62 dependent), leading to a chronic, sustained activation of Nrf2. In contrast, activation of Nrf2 by sulforaphane (SF) and tert-butylhydroquinone (tBHQ) depends upon Keap1-C151 and not p62 (the canonical mechanism). More importantly, SF and tBHQ do not have any effect on autophagy. In fact, SF and tBHQ alleviate arsenic-mediated deregulation of autophagy. Collectively, these findings provide evidence that arsenic causes prolonged activation of Nrf2 through autophagy dysfunction, possibly providing a scenario similar to that of constitutive activation of Nrf2 found in certain human cancers. This may represent a previously unrecognized mechanism underlying arsenic toxicity and carcinogenicity in humans.
Human exposure to inorganic arsenic (iAs), a potent oxidative stressor, causes various dermal disorders, including hyperkeratosis and skin cancer. Nuclear factor–erythroid 2–related factor 1 (NRF1, also called NFE2L1) plays a critical role in regulating the expression of many antioxidant response element (ARE)-dependent genes.
We investigated the role of NRF1 in arsenic-induced antioxidant response and cytotoxicity in human keratinocytes.
In cultured human keratinocyte HaCaT cells, inorganic arsenite (iAs3+) enhanced the protein accumulation of long isoforms (120–140 kDa) of NRF1 in a dose- and time-dependent fashion. These isoforms accumulated mainly in the nuclei of HaCaT cells. Selective deficiency of NRF1 by lentiviral short-hairpin RNAs in HaCaT cells [NRF1-knockdown (KD)] led to decreased expression of γ-glutamate cysteine ligase catalytic subunit (GCLC) and regulatory subunit (GCLM) and a reduced level of intracellular glutathione. In response to acute iAs3+ exposure, induction of some ARE-dependent genes, including NAD(P)H:quinone oxidoreductase 1 (NQO1), GCLC, and GCLM, was significantly attenuated in NRF1-KD cells. However, the iAs3-induced expression of heme oxygenase 1 (HMOX-1) was unaltered by silencing NRF1, suggesting that HMOX-1 is not regulated by NRF1. In addition, the lack of NRF1 in HaCaT cells did not disturb iAs3+-induced NRF2 accumulation but noticeably decreased Kelch-like ECH-associated protein 1 (KEAP1) levels under basal and iAs3+-exposed conditions, suggesting a potential interaction between NRF1 and KEAP1. Consistent with the critical role of NRF1 in the transcriptional regulation of some ARE-bearing genes, knockdown of NRF1 significantly increased iAs3+-induced cytotoxicity and apoptosis.
Here, we demonstrate for the first time that long isoforms of NRF1 contribute to arsenic-induced antioxidant response in human keratinocytes and protect the cells from acute arsenic cytotoxicity.
apoptosis; arsenic; cytotoxicity; KEAP1; keratinocyte; NRF1; NRF2; oxidative stress
As a major cellular defense mechanism, the Nrf2/Keap1 pathway regulates expression of genes involved in detoxification and stress response. Our previous study revealed activation of the Nrf2/Keap1 pathway at the maturation phase during mouse esophageal development, suggesting a potential function in epithelial defense. Here we hypothesize that Nrf2 is involved in the barrier function of esophageal epithelium, and plays a protective role against gastroesophageal reflux disease (GERD).
Human esophageal biopsy samples, mouse surgical models and Nrf2-/- mice were used to assess the role of the Nrf2/Keap1 pathway in esophageal mucosal barrier function. Trans-epithelial electrical resistance (TEER) was measured with mini-Ussing chambers. Hematoxylin and eosin (HE) staining and transmission electron microscopy were used to examine cell morphology, while gene microarray, immunohistochemistry, Western blotting and ChIP analysis were used to assess the expression of pathway genes.
Nrf2 was expressed in normal esophageal epithelium and activated in GERD of both humans and mice. Nrf2 deficiency and gastroesophageal reflux in mice, either alone or in combination, reduced TEER and increased intercellular space diameter in esophageal epithelium. Nrf2 target genes and gene sets associated with oxidoreductase activity, mitochondrial biogenesis and energy production were down-regulated in the esophageal epithelium of Nrf2-/- mice. Consistent with the antioxidative function of Nrf2, a DNA oxidative damage marker (8OHdG) dramatically increased in esophageal epithelial cells of Nrf2-/- mice compared with those of wild-type mice. Interestingly, ATP biogenesis, Cox IV (a mitochondrial protein) and Claudin-4 (Cldn4) expression were down-regulated in the esophageal epithelium of Nrf2-/- mice, suggesting that energy-dependent tight junction integrity was subject to Nrf2 regulation. ChIP analysis confirmed the binding of Nrf2 to Cldn4 promoter.
Nrf2 deficiency impairs esophageal barrier function through disrupting energy-dependent tight junction. Elucidating the role of this pathway in GERD has potential implications for the pathogenesis and therapy of the disease.
Nrf2; esophagus; TEER; GERD
Previous studies have proved that the environmental toxicant, inorganic arsenic, activates nuclear factor erythroid 2-related factor 2 (Nrf2) pathway in many different cell types. This study tried to explore the hepatic Nrf2 pathway upon arsenic treatment comprehensively, since liver is one of the major target organs of arsenical toxicity. Our results showed that inorganic arsenic significantly induced Nrf2 protein and mRNA expression in Chang human hepatocytes. We also observed a dose-dependent increase of antioxidant response element- (ARE-) luciferase activity. Both the mRNA and protein levels of NAD(P)H:quinone oxidoreductase 1 (NQO1) and heme oxygenase-1 (HO-1) were all upregulated dramatically. On the other hand, entry and accumulation of Nrf2 protein in the nucleus, while exportting the transcriptional repressor BTB and CNC homology 1 (Bach1) from nucleus to cytoplasm, were also confirmed by western blot and immunofluorescence assay. Our results therefore confirmed the arsenic-induced Nrf2 pathway activation in hepatocytes and also suggested that the translocation of Bach1 was associated with the regulation of Nrf2 pathway by arsenic. Hepatic Nrf2 pathway plays indispensable roles for cellular defenses against arsenic hepatotoxicity, and the interplay of Bach1 and Nrf2 may be helpful to understand the self-defensive responses and the diverse biological effects of arsenicals.
Nrf2 plays a critical role in defending against oxidative stress and inflammation. We previously reported that Nrf2 confers protection against ultraviolet-B (UVB)-induced inflammation, sunburn reaction, and is involved in sulforaphane-mediated photo-protective effects in the skin. In this study, we aimed to demonstrate the protective role of Nrf2 against inflammation-mediated extracellular matrix (ECM) damage induced by UVB irradiation. Ear biopsy weights were significantly increased in both Nrf2 wild-type (Nrf2 WT) and knockout (Nrf2 KO) mice one week after UVB irradiation. However, these weights increased more significantly in KO mice compared to WT mice, suggesting a greater inflammatory response in KO mice. In addition, we analyzed the protein expression of numerous markers, including macrophage inflammatory protein-2 (MIP-2), pro-matrix metalloproteinase-9 (MMP-9), and p53. p53, a regulator of DNA repair, was overexpressed in Nrf2 KO mice, indicating that the absence of Nrf2 led to more sustained DNA damage. There was also more substantial ECM degradation and increased inflammation in UVB-irradiated Nrf2 KO mice compared to UVB-irradiated WT mice. Furthermore, the protective effects of Nrf2 in response to UVB irradiation were mediated by increased HO-1 protein expression. Collectively, our results show that Nrf2 plays a key role in protecting against UVB irradiation and that the photo-protective effect of Nrf2 is closely related to the inhibition of ECM degradation and inflammation.
Nuclear factor (erythroid-derived 2)-like 2 (NFE2L2 or Nrf2); Ultraviolet; Inflammation; p53; pro-MMP-9; MIP-2; ECM
To investigate the role of Nrf2 as a master defense against the hepatotoxicity produced by various chemicals, Nrf2-null, wild-type, Keap1-knock down (Keap1-Kd) and Keap1-hepatocyte knockout (Keap1-HKO) mice were used as a “graded Nrf2 activation” model. Mice were treated with 14 hepatotoxicants at appropriate doses, and blood and liver samples were collected thereafter (6 h to 7 days depending on the hepatotoxicant). Graded activation of Nrf2 offered a Nrf2-dependent protection against the hepatotoxicity produced by carbon tetrachloride, acetaminophen, microcystin, phalloidin, furosemide, cadmium, and lithocholic acid, as evidenced by serum alanine aminotransferase (ALT) activities and by histopathology. Nrf2 activation also offered moderate protection against liver injury produced by ethanol, arsenic, bromobenzene, and allyl alcohol but had no effects on the hepatotoxicity produced by D-galactosamine/endotoxin and the Fas ligand antibody Jo-2. Graded Nrf2 activation reduced the expression of inflammatory genes (MIP-2, mKC, IL-1β, IL-6, and TNFα), oxidative stress genes (Ho-1, Egr1), ER stress genes (Gadd45 and Gadd153), and genes encoding cell death (Noxa, Bax, Bad, and caspase3). Thus, this study demonstrates that Nrf2 prevents the liver from many, but not all, hepatotoxicants. The Nrf2-mediated protection is accompanied by induction of antioxidant genes, suppression of inflammatory responses, and attenuation of oxidative stress.
A systems engineering approach is presented for describing the kinetics and dynamics that are elicited upon arsenic exposure of human hepatocytes. The mathematical model proposed here tracks the cellular reaction network of inorganic and organic arsenic compounds present in the hepatocyte and analyzes the production of toxicologically potent by-products and the signaling they induce in hepatocytes.
Methods and results
The present modeling effort integrates for the first time a cellular-level semi-mechanistic toxicokinetic (TK) model of arsenic in human hepatocytes with a cellular-level toxicodynamic (TD) model describing the arsenic-induced reactive oxygen species (ROS) burst, the antioxidant response, and the oxidative DNA damage repair process. The antioxidant response mechanism is described based on the Keap1-independent Nuclear Factor-erythroid 2-related factor 2 (Nrf2) signaling cascade and accounts for the upregulation of detoxifying enzymes. The ROS-induced DNA damage is simulated by coupling the TK/TD formulation with a model describing the multistep pathway of oxidative DNA repair. The predictions of the model are assessed against experimental data of arsenite-induced genotoxic damage to human hepatocytes; thereby capturing in silico the mode of the experimental dose–response curve.
The integrated cellular-level TK/TD model presented here provides significant insight into the underlying regulatory mechanism of Nrf2-regulated antioxidant response due to arsenic exposure. While computational simulations are in a fair good agreement with relevant experimental data, further analysis of the system unravels the role of a dynamic interplay among the feedback loops of the system in controlling the ROS upregulation and DNA damage response. This TK/TD framework that uses arsenic as an example can be further extended to other toxic or pharmaceutical agents.
Arsenic; Nrf2; Anti-oxidant response; Human hepatocytes; Indirect response modeling
The iron chelate, ferric nitrilotriacetate (FeNTA), induces acute proximal tubular necrosis as a consequence of lipid peroxidation and oxidative tissue damage. Chronic exposure of FeNTA leads to a high incidence of renal adenocarcinomas in rodents. NF-e2-related factor 2 (Nrf2) is a transcription factor that is activated by oxidative stress and electrophiles, and regulates the basal and inducible expression of numerous detoxifying and antioxidant genes. To determine the roles of Nrf2 in regulating renal gene expression and protecting against oxidative stress-induced kidney damage, wild-type and Nrf2-null mice were administered FeNTA. Renal Nrf2 protein translocated to the nucleus at 6 h after FeNTA treatment. FeNTA increased mRNA levels of Nrf2 target genes, including NQO1, GCLC, GSTpi1/2, Mrp1, 2, and 4 in kidneys from wild-type mice, but not Nrf2-null mice. Protein expression of NQO1, a prototypical Nrf2 target gene, was increased in wild-type mice, with no change in Nrf2-null mice. FeNTA produced more nephrotoxicity in Nrf2-null mice than wild-type mice as indicated by higher serum urea nitrogen and creatinine levels, as more urinary NAG, stronger 4-hydroxynonenal protein adduct staining, and more extensive proximal tubule damage. Furthermore, pretreatment with CDDO-Im, a potent small molecule Nrf2 activator, protected mice against FeNTA-induced renal toxicity. Collectively, these results suggest that activation of Nrf2 protects mouse kidneys from FeNTA-induced oxidative stress damage by coordinately up-regulating the expression of cytoprotective genes.
Nrf2; FeNTA; NQO1; oxidative stress; Mrp; kidney