Cell survival responses to environmental stresses via the Keap1-Nrf2-ARE pathway

Cancer related mutations in NRF2 impair its recognition by Keap1-Cul3 E3 ligase and promote malignancy

The nuclear factor E2-related factor 2 (Nrf2) is a master transcriptional activator of genes encoding numerous cytoprotective enzymes that are induced in response to environmental and endogenously derived oxidative/electrophilic agents. Under normal, nonstressed circumstances, low cellular concentrations of Nrf2 are maintained by proteasomal degradation through a Keap1-Cul3-Roc1-dependent mechanism. A model for Nrf2 activation has been proposed in which two amino-terminal motifs, DLG and ETGE, promote efficient ubiquitination and rapid turnover; known as the two-site substrate recognition/hinge and latch model. Here, we show that in human cancer, somatic mutations occur in the coding region of NRF2, especially among patients with a history of smoking or suffering from squamous cell carcinoma; in the latter case, this leads to poor prognosis. These mutations specifically alter amino acids in the DLG or ETGE motifs, resulting in aberrant cellular accumulation of Nrf2. Mutant Nrf2 cells display constitutive induction of cytoprotective enzymes and drug efflux pumps, which are insensitive to Keap1-mediated regulation. Suppression of Nrf2 protein levels by siRNA knockdown sensitized cancer cells to oxidative stress and chemotherapeutic reagents. Our results strongly support the contention that constitutive Nrf2 activation affords cancer cells with undue protection from their inherently stressed microenvironment and anti-cancer treatments. Hence, inactivation of the Nrf2 pathway may represent a therapeutic strategy to reinforce current treatments for malignancy. Congruously, the present study also provides in vivo validation of the two-site substrate recognition model for Nrf2 activation by the Keap1-Cul3-based E3 ligase.

Stress-activated genomic expression changes serve a preparative role for impending stress in yeast

Yeast cells respond to stress by mediating condition-specific gene expression changes and by mounting a common response to many stresses, called the environmental stress response (ESR). Giaever et al. previously revealed poor correlation between genes whose expression changes in response to acute stress and genes required to survive that stress, raising question about the role of stress-activated gene expression. Here we show that gene expression changes triggered by a single dose of stress are not required to survive that stimulus but rather serve a protective role against future stress. We characterized the increased resistance to severe stress in yeast preexposed to mild stress. This acquired stress resistance is dependent on protein synthesis during mild-stress treatment and requires the "general-stress" transcription factors Msn2p and/or Msn4p that regulate induction of many ESR genes. However, neither protein synthesis nor Msn2/4p is required for basal tolerance of a single dose of stress, despite the substantial expression changes triggered by each condition. Using microarrays, we show that Msn2p and Msn4p play nonredundant and condition-specific roles in gene-expression regulation, arguing against a generic general-stress function. This work highlights the importance of condition-specific responses in acquired stress resistance and provides new insights into the role of the ESR.

Prevention of methylmercury-induced mitochondrial depolarization, glutathione depletion and cell death by 15-deoxy-delta-12,14-prostaglandin J(2)

Methylmercury (MeHg) is an environmental toxin that causes severe neurological complications in humans and experimental animals. In addition to neurons, glia in the central nervous system are very susceptible to MeHg toxicity. Pretreatment of glia with the prostaglandin derivative, 15-deoxy-delta-12,14-prostaglandin J(2) (15d-PGJ(2)), caused a significant protection against MeHg cytotoxicity. Results with the C6 glioma cells demonstrated that the protection was dependent on the duration of pretreatment, suggesting that time was required for the up-regulation of cellular defenses. Subsequent experiments indicated that 15d-PGJ(2) prevented MeHg induced mitochondrial depolarization. Similar protection against MeHg cytotoxicity was observed in primary cultures of mouse glia. Analysis of cellular glutathione (GSH) levels indicated that 15d-PGJ(2) caused an up-regulation of GSH and prevented MeHg-induced GSH depletion. Buthionine sulfoximine (BSO), a GSH synthesis inhibitor, completely inhibited the GSH induction by 15d-PGJ(2). However, BSO did not prevent the stabilization of mitochondrial potential and only partially prevented the protection caused by 15d-PGJ(2). While induction of heme oxygenase-1 was implicated in the cytoprotection by 15d-PGJ(2) under some experimental conditions, additional experiments indicated that this enzyme was not involved in the cytoprotection observed in this system. Together, these results suggested that while up-regulation of GSH by 15d-PGJ(2) might help cells to defend against MeHg toxicity, there may be other yet unidentified mechanism(s) initiated by 15d-PGJ(2) treatment that contributed to its protection against MeHg cytotoxicity.

Xenobiotic-activated receptors: from transcription to drug metabolism to disease

Xenobiotic-activated receptors (XARs) are a group of ligand-activated transcription factors that are evolutionally specialized to regulate genomic programs to protect the body against innumerable chemicals from the environment. XARs share unique properties, such as promiscuous ligand binding, conserved structural motifs, common protein partners, and overlapping target genes. These unique features of XARs clearly distinguish them from receptors that are activated by endogenous chemicals to regulate energy metabolism, reproduction, and growth and differentiation. XARs regulate xenobiotic metabolism and disposition by controlling the expression and induction of drug-metabolizing enzymes and transporters. Furthermore, XARs integrate a broad range of protective mechanisms, such as antioxidative response and immune/inflammatory functions, to antagonize foreign chemicals. As the primary means of xenobiotic sensing and defense, XARs are intimately involved in drug disposition, polymorphic drug clearance, drug-drug interaction, and pathogenesis of some chemically induced cancers and chronic diseases. As a consequence, some XAR characteristics have been exploited in drug development and safety evaluation of drugs and environmental carcinogens and toxicants. In this perspective, common features and recent advances in the structures, modes of action, and implications in disease and drug development of XARs are discussed.

Cellular stress response: a novel target for chemoprevention and nutritional neuroprotection in aging, neurodegenerative disorders and longevity

The predominant molecular symptom of aging is the accumulation of altered gene products. Moreover, several conditions including protein, lipid or glucose oxidation disrupt redox homeostasis and lead to accumulation of unfolded or misfolded proteins in the aging brain. Alzheimer's and Parkinson's diseases or Friedreich ataxia are neurological diseases sharing, as a common denominator, production of abnormal proteins, mitochondrial dysfunction and oxidative stress, which contribute to the pathogenesis of these so called "protein conformational diseases". The central nervous system has evolved the conserved mechanism of unfolded protein response to cope with the accumulation of misfolded proteins. As one of the main intracellular redox systems involved in neuroprotection, the vitagene system is emerging as a neurohormetic potential target for novel cytoprotective interventions. Vitagenes encode for cytoprotective heat shock proteins (Hsp) Hsp70 and heme oxygenase-1, as well as thioredoxin reductase and sirtuins. Nutritional studies show that ageing in animals can be significantly influenced by dietary restriction. Thus, the impact of dietary factors on health and longevity is an increasingly appreciated area of research. Reducing energy intake by controlled caloric restriction or intermittent fasting increases lifespan and protects various tissues against disease. Genetics has revealed that ageing may be controlled by changes in intracellular NAD/NADH ratio regulating sirtuin, a group of proteins linked to aging, metabolism and stress tolerance in several organisms. Recent findings suggest that several phytochemicals exhibit biphasic dose responses on cells with low doses activating signaling pathways that result in increased expression of vitagenes encoding survival proteins, as in the case of the Keap1/Nrf2/ARE pathway activated by curcumin and NAD/NADH-sirtuin-1 activated by resveratrol. Consistently, the neuroprotective roles of dietary antioxidants including curcumin, acetyl-L-carnitine and carnosine have been demonstrated through the activation of these redox-sensitive intracellular pathways. Although the notion that stress proteins are neuroprotective is broadly accepted, still much work needs to be done in order to associate neuroprotection with specific pattern of stress responses. In this review the importance of vitagenes in the cellular stress response and the potential use of dietary antioxidants in the prevention and treatment of neurodegenerative disorders is discussed.

Renal protection by 3H-1,2-dithiole-3-thione against cisplatin through the Nrf2-antioxidant pathway

Cisplatin is commonly used for the treatment of several solid tumors. However, its clinical use is often limited by renal toxicity. The indirect antioxidant 3H-1,2-dithiole-3-thione (D3T) has been known to protect cells from oxidative damage by up-regulating the expression of antioxidative genes through the transcription factor NF-E2-related factor 2 (Nrf2) pathway. We hypothesized that D3T treatment may be protective against cisplatin-induced nephrotoxicity by enhancing the antioxidative capacity of renal cells. In cultured murine tubular epithelial cells, D3T facilitates the nuclear accumulation of Nrf2 and the subsequent expression of its target genes such as glutamate cysteine ligase (GCL). Increased GSH pool in D3T-treated renal cells appears to be associated with amelioration of cisplatin-mediated cell death. Protective effects of D3T were also observed in mice. Oral administration of D3T (0.25mmol/kg) increased the expression of GCL in mouse kidney, which resulted in suppression of cisplatin-mediated increases in blood urea nitrogen and serum creatinine. Histopathological changes representing cisplatin-induced acute renal failure were also effectively ameliorated by D3T treatment. Collectively, these results indicate that pharmacological activation of the Nrf2 pathway might have a beneficial effect on reducing chemotherapy-associated cytotoxic adverse effects.

KLF2 Primes the Antioxidant Transcription Factor Nrf2 for Activation in Endothelial Cells

Objective— Atheroprotective blood flow induces expression of anti-inflammatory Krüppel-like factor 2 (KLF2) and activates antioxidant transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) in vascular endothelium. Previously, we obtained KLF2-induced gene expression profiles in ECs, containing several Nrf2 target genes. Our aim was to investigate the role of KLF2 in shear stress–mediated activation of Nrf2 in human umbilical vein endothelial cells (HUVECs).

Methods and Results— Expression of Nrf2 and its targets NAD(P)H dehydrogenase quinone 1 (NQO1) and heme oxygenase (HO-1) was elevated by shear and KLF2. KLF2 knockdown showed that shear-induced expression of NQO1 but not Nrf2 was dependent on KLF2. KLF2 overexpression in absence of flow resulted in more efficient activation of Nrf2 by tert-butyl hydroquinone (tBHQ) through enhanced nuclear localization, and promoted expression of a large panel of Nrf2-dependent genes resulting in superior protection against oxidative stress. Comparison of shear-, KLF2-, and Nrf2-induced transcriptomes showed that the majority of shear-modulated gene sets is influenced by KLF2 or Nrf2.

Conclusions— We report that KLF2 substantially enhances antioxidant activity of Nrf2 by increasing its nuclear localization and activation. The synergistic activity of these two transcription factors forms a major contribution to the shear stress–elicited transcriptome in endothelial cells.

Adaptive response to GSH depletion and resistance to L-buthionine-(S,R)-sulfoximine: involvement of Nrf2 activation

Pharmacological depletion of L-gamma-glutamyl-L-cysteinyl-glycine (GSH) has been implicated in the sensitization of cancer cells to alkylating agents and apoptosis. However, some types of cells do not induce apoptotic response following chemical depletion of GSH. In the present study, we report that murine embryonic fibroblasts (MEFs) can survive in the presence of GSH inhibitor L-buthionine-(S,R)-sulfoximine (BSO), even though most intracellular GSH was depleted. As a cellular adaptive mechanism, BSO treatment effectively activated the NF-E2-related factor 2 (Nrf2) pathway, which led to up-regulation of antioxidant enzymes in these cells through the extracellular signal-regulated kinase cascade. While nrf2-deficient MEFs lost the inducibility of antioxidant genes, which resulted in higher levels of reactive oxygen species accumulation, caspase-3 activation, and cell death than wild-type cells. Finally, nrf2-deficient cells can be more sensitized to doxorubicin-induced cell death by BSO pre-incubation, while wild-type cells were not. In addition, BSO-mediated cell death was facilitated by administering Nrf2 siRNA to chemoresistant human ovarian cancer cells. These results indicate that Nrf2 is the primary factor inducing the cell survival system under GSH depletion and that the effect of BSO as a chemosensitizer might be enhanced by inhibition of Nrf2.

Essential roles of the PI3 kinase/Akt pathway in regulating Nrf2-dependent antioxidant functions in the RPE

PURPOSE

To investigate functional interactions between the phosphatidylinositol 3-kinase (PI3K)/Akt pathway and the nuclear factor erythroid 2-related factor 2 (Nrf2)-dependent antioxidant system in cultured human retinal pigment epithelium (RPE) cells.

METHODS

Cultured ARPE-19 cells were treated with different concentrations of PI3K inhibitors, followed by exposure to sulforaphane, an Nrf2 inducer. Akt phosphorylation was detected by Western blot analysis. Intracellular glutathione (GSH) content was measured by HPLC. Expression of genes downstream of Nrf2, including glutamate-cysteine ligase (GCL) and glutathione S-transferase, was measured by quantitative RT-PCR. Nrf2 activity was measured by a dual luciferase assay after transfection of a reporter plasmid containing the antioxidant response element (ARE). The small interference RNA approach was used to knock down Nrf2 in the RPE. Nrf2 localization was determined by subcellular fractionation and Western blot analyses.

RESULTS

PI3K inhibitors wortmannin and LY294002 caused dose-dependent cellular and mitochondrial GSH depletion and downregulation of the modulatory subunit of GCL in cultured RPE cells. Both the basal and the induced Nrf2 activities were inhibited by wortmannin and LY294002. Overexpression of a constitutively active form of Akt potentiated Nrf2 activation, and the effect of Akt was blocked by siRNA that knocked down Nrf2. LY294002 also inhibited sulforaphane-induced Nrf2 nuclear translocation.

CONCLUSIONS

The PI3K/Akt pathway plays key roles in regulating Nrf2-ARE-dependent protection against oxidative stress in the RPE.

Functional interference between glycogen synthase kinase-3 beta and the transcription factor Nrf2 in protection against kainate-induced hippocampal cell death

Excitotoxicity mediated by glutamate receptors may underlay the pathology of several neurologic diseases. Considering that oxidative stress is central to excitotoxic damage, in this study we sought to analyze if the transcription factor Nrf2, guardian of redox homeostasis, might be targeted to prevent kainate-induced neuron death. Hippocampal slices from Nrf2 knockout mice exhibited increased oxidative stress and cell death compared to those of control mice in response to kainate, as determined with the redox sensitive probes 2,7-dichlorodihydrofluorescein diacetate (H(2)DCFAC) and propidium iodide and lactate dehydrogenase release, respectively, therefore demonstrating a role of Nrf2 in antioxidant protection against excitotoxicity. In the hippocampus of mice intraperitoneally injected with kainate we observed a rapid activation of Akt, inhibition of GSK-3beta and translocation of Nrf2 to the nucleus, but after 4 h Akt was inactive, GSK-3beta was active and Nrf2 was mostly cytosolic, therefore extending our previous studies which indicate that GSK-3beta excludes Nrf2 from the nucleus. Lithium, a GSK-3beta inhibitor, promoted Nrf2 transcriptional activity towards an Antioxidant-Response-Element (ARE) luciferase reporter and cooperated with sulforaphane (SFN) to induce this reporter and to increase the protein levels of heme oxygenase-1 (HO-1), coded by a representative ARE-containing gene. Conversely, ARE activation by SFN was attenuated by over-expression of active GSK-3beta. Finally, combined treatment with SFN and lithium attenuated oxidative stress and cell death in kainate-treated hippocampal slices of wild type mice but not Nrf2 null littermates. Our findings identify the axis GSK-3beta/Nrf2 as a pharmacological target in prevention of excitotoxic neuronal death.

Zinc protects endothelial cells from hydrogen peroxide via Nrf2-dependent stimulation of glutathione biosynthesis

Oxidative stress is one of the main causes of vascular disease. This study aims to investigate the antioxidant activity exerted by zinc in primary rat endothelial cells (EC). Using a 24-h treatment with hydrogen peroxide as a model for oxidative stress, we found that zinc supplementation protects from peroxide-induced cell death via increasing the transcription of the catalytic subunit (heavy chain) of glutamate-cysteine ligase (GCLC) and the concentrations of glutathione (GSH). Conversely, zinc depletion significantly decreased the expression of GCLC and the cellular GSH levels, resulting in an increased susceptibility of EC to oxidative stress. Using confocal microscopy and the RNA silencing technique, we found that zinc upregulates the expression of GCLC by activating the transcription factor Nrf2. Surprisingly, the intracellular zinc sensor, metal-responsive transcription factor-1, is not involved in the zinc-induced expression of GCLC. The present study shows that zinc controls the redox state of EC by regulating the de novo synthesis of GSH. This molecular mechanism may contribute to the elaboration of new nutritional and/or pharmaceutical approaches for protecting the endothelium against oxidative stress.

Regulation of glutathione synthesis

Glutathione (GSH) is a ubiquitous intracellular peptide with diverse functions that include detoxification, antioxidant defense, maintenance of thiol status, and modulation of cell proliferation. GSH is synthesized in the cytosol of all mammalian cells in a tightly regulated manner. The major determinants of GSH synthesis are the availability of cysteine, the sulfur amino acid precursor, and the activity of the rate-limiting enzyme, glutamate cysteine ligase (GCL). GCL is composed for a catalytic (GCLC) and modifier (GCLM) subunit and they are regulated at multiple levels and at times differentially. The second enzyme of GSH synthesis, GSH synthase (GS) is also regulated in a coordinated manner as GCL subunits and its up-regulation can further enhance the capacity of the cell to synthesize GSH. Oxidative stress is well known to induce the expression of GSH synthetic enzymes. Key transcription factors identified thus far include Nrf2/Nrf1 via the antioxidant response element (ARE), activator protein-1 (AP-1) and nuclear factor kappa B (NFkappaB). Dysregulation of GSH synthesis is increasingly being recognized as contributing to the pathogenesis of many pathological conditions. These include diabetes mellitus, pulmonary fibrosis, cholestatic liver injury, endotoxemia and drug-resistant tumor cells. Manipulation of the GSH synthetic capacity is an important target in the treatment of many of these disorders.

Nutrients and phytochemicals: from bioavailability to bioefficacy beyond antioxidants

The effect of any dietary compound is influenced by the active bioavailable dose rather than the dose ingested. Depending on the individual predisposition, including genetics and medication, a bioavailable dose may cause different magnitudes of effects in different people. Age might affect the predisposition and thus the requirements for nutrients including phytonutrients (e.g. phytochemicals such as flavonoids, phenolic acids and glucosinolates). These are not essential for growth and development but to maintain body functions and health throughout the adult and later phases of life; they are 'lifespan essentials'. Major mechanisms involved in chronic, age-related diseases include the oxidant/antioxidant balance, but the latest research indicates indirect effects of dietary bioactives in vivo and adaptive responses in addition to direct radical scavenging.

Impaired lung homeostasis in neonatal mice exposed to cigarette smoke

In infants, smoke exposure is associated with more respiratory illnesses and decreased lung function. We hypothesized that perinatal lung is particularly susceptible to the damaging effects of cigarette smoke (CS) and that exposure to CS during this period may alter expression of immune response genes and adversely affect lung growth. To test this, we exposed neonatal mice to 14 days of CS. Immediately after exposure to CS, pulmonary gene expression profiling was performed on 2-week-old CS-exposed lung and age-matched control lung. Nitrotyrosine, TUNEL, MAC3, and phospho-SMAD-2 (p-SMAD2) staining was also performed. At 8 weeks of age, lung volume measurements were determined and mean linear intercept measurements were calculated. Pulmonary gene expression profiling revealed that CS exposure significantly inhibited type 1 and type 2 interferon pathway genes in neonatal lung, compared with age-matched control lung. Neonatal CS-exposed lung also had a significant increase in n-tyrosine, TUNEL, and p-SMAD2 staining when compared with adult CS-exposed lung and age-matched control lung. Lung volumes at 8 weeks of age were modestly but significantly decreased in mice exposed to CS in the neonatal period compared with age-matched controls, consistent with impaired lung growth. The results of this study indicate that exposure to CS during the neonatal period inhibits expression of genes involved in innate immunity and mildly impairs postnatal lung growth. These findings may in part explain the increased incidence of respiratory symptoms in infants and children exposed to CS.

Phosphorylation of Nrf2 in the transcription activation domain by casein kinase 2 (CK2) is critical for the nuclear translocation and transcription activation function of Nrf2 in IMR-32 neuroblastoma cells

The antioxidant-activated transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) regulates the induction of cytoprotective genes against chemical toxicity and oxidative injuries. The role of phosphorylation in Nrf2 activation has been suggested but remains elusive. We report that phenolic antioxidant/pro-oxidant tert-butylhydroquinone (tBHQ) induced two forms of the Nrf2 protein in neuroblastoma cells (IMR-32), which migrated as distinctive bands on SDS-PAGE. In vitro treatment with lambda phosphatase eliminated the slower migrating form and increased the amount of the faster migrating form of Nrf2. In vivo (32)Pi-phosphorylation resulted in (32)Pi-labeling of the Nrf2 protein in the presence of tBHQ that can be dephosphorylated by lambda phosphotase, indicating that the slower migrating form is a phosphorylated Nrf2 protein and the faster form an unphosphorylated Nrf2. Unphosphorylated Nrf2 predominated in the cytoplasm, whereas the phosphorylated form preferentially localized in the nucleus. Nuclear Nrf2 can be dephosphorylated by lambda phosphotase in vitro and be converted to the faster migrating form, implicating phosphorylation of Nrf2 in the cytoplasmic-nuclear translocation of the protein. Deletional analyses from both the carboxyl- and amino-ends revealed the transcription activation (TA) domains Neh4 (Nrf2-ECH homology 4) and Neh5 (Nrf2-ECH homology 5) as a major region necessary for the phosphorylation. The TA domains are characterized by the presence of multiple phosphorylation sites of casein kinase 2 (CK2). Moreover, CK2 phosphorylated the TA domains in vitro. Treatment with CK2 inhibitor 2-dimethylamino-4,5,6,7,-tetrabromo-1H-benzimidazole (DMAT) blocked the induction of endogenous target genes of Nrf2 in cells and inhibited the TA activities of both the full length and the TA domains of Nrf2 to a large extent. Finally, phosphorylation of the TA domains correlated with the nuclear translocation of Nrf2 that was inhibited by DMAT in a concentration-dependent manner. The findings demonstrated that phosphorylation of Nrf2 at the TA domains by CK2 is an integral component of Nrf2 activation necessary for the nuclear localization and transcription activation function of Nrf2 in neuroblastoma cells.

Hormesis and adaptive cellular control systems

Hormetic dose response occurs for many endpoints associated with exposures of biological organisms to environmental stressors. Cell-based U- or inverted U-shaped responses may derive from common processes involved in activation of adaptive responses required to protect cells from stressful environments. These adaptive pathways extend the region of cellular homeostasis and are protective against ultimate cell, organ, and system toxicity. However, the activation of stress responses carries a significant energetic cost to the cell, leading to alterations of a variety of basal cellular functions in adapted or stressed cells. This tradeoff of resources between the unstressed and adapted states may lead to U-or inverted U-shaped dose response curves for some precursor endpoints. We examine this general hypothesis with chlorine, a prototype oxidative stressor, using a combination of cellular studies with gene expression analysis of response pathways and with computational modeling of activation of control networks. Discrete cellular states are expected as a function of exposure concentration and duration. These cellular states include normal functioning state, adaptive and stressed states at mild to intermediate exposures, and overt toxicity in the presence of an overwhelming concentration of stressors. These transitions can be used to refine default risk assessment practices that do not currently accommodate adaptive responses.

Activation of Nrf2 by arsenite and monomethylarsonous acid is independent of Keap1-C151: enhanced Keap1-Cul3 interaction

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.

PGAM5 tethers a ternary complex containing Keap1 and Nrf2 to mitochondria

Eukaryote cells balance production of reactive oxygen species (ROS) with levels of anti-oxidant enzyme activity to maintain cellular redox homeostasis. Mitochondria are a major source of ROS, while many anti-oxidant genes are regulated by the Nrf2 transcription factor. Keap1, a redox-regulated substrate adaptor for a cullin-based ubiquitin ligase, targets Nrf2 for proteosome-mediated degradation and represses Nrf2-dependent gene expression. We have previously identified a member of the phosphoglycerate mutase family, PGAM5, as a Keap1-binding protein. In this report, we demonstrate that PGAM5 is targeted to the outer membrane of mitochondria by an N-terminal mitochondrial-localization sequence. Furthermore, we provide evidence that PGAM5 forms a ternary complex containing both Keap1 and Nrf2, in which the dimeric Keap1 protein simultaneously binds both PGAM5 and Nrf2 through their conserved E(S/T)GE motifs. Knockdown of either Keap1 or PGAM5 activates Nrf2-dependent gene expression. We suggest that this ternary complex provides a molecular framework for understanding how nuclear anti-oxidant gene expression is regulated in response to changes in mitochondrial function(s).

Activating transcription factor 3 is a novel repressor of the nuclear factor erythroid-derived 2-related factor 2 (Nrf2)-regulated stress pathway

The transcription factor nuclear factor erythroid-derived 2-related factor 2 (Nrf2) regulates induction of an extensive cellular stress response network when complexed with the cAMP-responsive element binding protein (CBP) at antioxidant response elements (ARE) located in the promoter region of target genes. Activating transcription factor 3 (ATF3) can repress Nrf2-mediated signaling in a manner that is not well understood. Here, we show that ATF3-mediated suppression is a consequence of direct ATF3-Nrf2 protein-protein interactions that result in displacement of CBP from the ARE. This work establishes ATF3 as a novel repressor of the Nrf2-directed stress response pathway.

Acrolein: sources, metabolism, and biomolecular interactions relevant to human health and disease

Acrolein (2-propenal) is ubiquitously present in (cooked) foods and in the environment. It is formed from carbohydrates, vegetable oils and animal fats, amino acids during heating of foods, and by combustion of petroleum fuels and biodiesel. Chemical reactions responsible for release of acrolein include heat-induced dehydration of glycerol, retro-aldol cleavage of dehydrated carbohydrates, lipid peroxidation of polyunsaturated fatty acids, and Strecker degradation of methionine and threonine. Smoking of tobacco products equals or exceeds the total human exposure to acrolein from all other sources. The main endogenous sources of acrolein are myeloperoxidase-mediated degradation of threonine and amine oxidase-mediated degradation of spermine and spermidine, which may constitute a significant source of acrolein in situations of oxidative stress and inflammation. Acrolein is metabolized by conjugation with glutathione and excreted in the urine as mercapturic acid metabolites. Acrolein forms Michael adducts with ascorbic acid in vitro, but the biological relevance of this reaction is not clear. The biological effects of acrolein are a consequence of its reactivity towards biological nucleophiles such as guanine in DNA and cysteine, lysine, histidine, and arginine residues in critical regions of nuclear factors, proteases, and other proteins. Acrolein adduction disrupts the function of these biomacromolecules which may result in mutations, altered gene transcription, and modulation of apoptosis.

Resveratrol induces glutathione synthesis by activation of Nrf2 and protects against cigarette smoke-mediated oxidative stress in human lung epithelial cells

Nuclear erythroid-related factor 2 (Nrf2), a redox-sensitive transcription factor, is involved in transcriptional regulation of many antioxidant genes, including glutamate-cysteine ligase (GCL). Cigarette smoke (CS) is known to cause oxidative stress and deplete glutathione (GSH) levels in alveolar epithelial cells. We hypothesized that resveratrol, a polyphenolic phytoalexin, has antioxidant signaling properties by inducing GSH biosynthesis via the activation of Nrf2 and protects lung epithelial cells against CS-mediated oxidative stress. Treatment of human primary small airway epithelial and human alveolar epithelial (A549) cells with CS extract (CSE) dose dependently decreased GSH levels and GCL activity, effects that were associated with enhanced production of reactive oxygen species. Resveratrol restored CSE-depleted GSH levels by upregulation of GCL via activation of Nrf2 and also quenched CSE-induced release of reactive oxygen species. Interestingly, CSE failed to induce nuclear translocation of Nrf2 in A549 and small airway epithelial cells. On the contrary, Nrf2 was localized in the cytosol of alveolar and airway epithelial cells due to CSE-mediated posttranslational modifications such as aldehyde/carbonyl adduct formation and nitration. On the other hand, resveratrol attenuated CSE-mediated Nrf2 modifications, thereby inducing its nuclear translocation associated with GCL gene transcription, as demonstrated by GCL-promoter reporter and Nrf2 small interfering RNA approaches. Thus resveratrol attenuates CSE-mediated GSH depletion by inducing GSH synthesis and protects epithelial cells by reversing CSE-induced posttranslational modifications of Nrf2. These data may have implications in dietary modulation of antioxidants in treatment of chronic obstructive pulmonary disease.

Idiosyncratic drug reactions: past, present, and future

Although the major working hypothesis for the mechanism of idiosyncratic drug reactions (IDRs), the hapten hypothesis, has not changed since 1987, several hypotheses have been added, for example, the danger hypothesis and the pharmaceutical interaction hypothesis. Genetic studies have found that several IDRs are linked to specific HLA genes, providing additional evidence that they are immune-mediated. Evidence that most IDRs are caused by reactive metabolites has led pharmaceutical companies to avoid drug candidates that form significant amounts of reactive metabolites; however, at least one IDR, ximelagatran-induced liver toxicity, does not appear to be caused by a reactive metabolite. It is possible that there are biomarkers such as those related to cell stress that would predict that a drug candidate would cause a significant incidence of IDRs; however, there has been no systematic study of the changes in gene expression induced by drugs known to cause IDRs. A major impediment to the study of the mechanisms of IDRs is the paucity of valid animal models, and if we had a better mechanistic understanding, it should be easier to develop such models. There is growing evidence that these adverse reactions are more varied and complex than previously recognized, and it is unlikely that a quick fix will be achieved. However, IDRs are an important cause of patient morbidity and mortality and markedly increase the uncertainty of drug development; therefore, continued basic research in this area is essential.

Genetic dissection of the Nrf2-dependent redox signaling-regulated transcriptional programs of cell proliferation and cytoprotection

The beta zipper (bZip) transcription factor, nuclear factor erythroid 2, like 2 (Nrf2), acting via an antioxidant/electrophile response element, regulates the expression of several antioxidant enzymes and maintains cellular redox homeostasis. Nrf2 deficiency diminishes pulmonary expression of several antioxidant enzymes, rendering them highly susceptible to various mouse models of prooxidant-induced lung injury. We recently demonstrated that Nrf2 deficiency impairs primary cultured pulmonary epithelial cell proliferation and greatly enhances sensitivity to prooxidant-induced cell death. Glutathione (GSH) supplementation rescued cells from these defects associated with Nrf2 deficiency. To further delineate the mechanisms by which Nrf2, via redox signaling, regulates cellular protection and proliferation, we compared the global expression profiling of Nrf2-deficient cells with and without GSH supplementation. We found that GSH regulates the expression of various networks of transcriptional programs including 1) several antioxidant enzymes involved in cellular detoxification of reactive oxygen species and recycling of thiol status and 2) several growth factors, growth factor receptors, and integrins that are critical for cell growth and proliferation. We also found that Nrf2 deficiency enhances the expression levels of several genes encoding proinflammatory cytokines; however, GSH supplementation markedly suppressed their expression. Collectively, these findings uncover an important insight into the nature of genes regulated by Nrf2-dependent redox signaling through GSH that are involved in cellular detoxification and proliferation.

Role of the Nrf2-antioxidant system in cytotoxicity mediated by anticancer cisplatin: implication to cancer cell resistance

The treatment of alkylating cytotoxic drug cisplatin is often limited by high incidence rate of resistance. In the present study, the potential involvement of the transcription factor Nrf2 in determination of cisplatin cytotoxicity has been investigated. Nrf2-deficient murine embryonic fibroblasts showed increased cell death, cytotoxicity, and apoptosis in response to cisplatin treatment compared to wild-type cells. Cisplatin-resistant human ovarian cancer SK-OV cells, which are retaining 25-fold higher levels of GSH than murine fibroblasts, could be sensitized by inhibition of Nrf2. Transfection with Nrf2 siRNA into SK-OV cells resulted in severe degree of GSH depletion and exacerbated cytotoxicity following cisplatin treatment compared to scrambled RNA control. In conclusion, we propose that the Nrf2 pathway, which plays a protective role in normal cells, can be a potential target to control cancer cell resistance to oxidants, cytotoxic chemicals, and radiation.

GSK-3beta down-regulates the transcription factor Nrf2 after oxidant damage: relevance to exposure of neuronal cells to oxidative stress

Oxidant injury activates the neuroprotective pathway represented by phosphatidylinositol 3 kinase (PI3K) and Akt. However, the final outcome of oxidant exposure is often associated with neuronal death. This study was aimed to identify the molecular mechanism responsible for loss of tolerance to an oxidative environment. In N2A neuroblasts, serum and H2O2 exhibited different kinetics of regulation for the Ser/Thr kinases Akt and glycogen synthase kinase 3beta (GSK-3beta) and for the transcription factor Nrf2, which governs redox homeostasis. Thus, H2O2 rapidly activated Akt, inhibited GSK-3beta, and directed the transcription factor Nrf2 to the nucleus, but after 4 h Akt was inactive, GSK-3beta was active and Nrf2 was more cytosolic than nuclear. Inhibition of the PI3K/Akt pathway by LY294002, impeded the short-term effect of H2O2 on nuclear translocation of Nrf2. GSK-3beta activation (inhibiting PI3K/Akt) or direct GSK-3beta inhibition in cerebellar granule neurons resulted in respective nuclear exclusion and nuclear accumulation of Nrf2. Moreover, in these neurons, nuclear accumulation of Nrf2 correlated with increased heme oxygenase-1 expression. Over-expression of the kinase active GSK-3beta (Delta9) mutant, induced Nrf2 cytoplasmic localization and inhibited Nrf2 transcriptional activity towards an antioxidant-response-element luciferase reporter. Moreover, GSK-3beta (Delta9) sensitized N2A neuroblasts to H2O2-induced oxidative stress and cell death. This study identifies GSK-3beta, a kinase known to participate in neurodegeneration, as a fundamental element in the down-regulation of the antioxidant cell defense elicited by Nrf2 after oxidant injury and provides a mechanism to explain the loss of oxidant tolerance that happens under persistent oxidant exposure such as those found in several neuropathologies.

Glutathione production is regulated via distinct pathways in stressed and non-stressed cortical neurons

Peroxynitrite-mediated damage has been linked to numerous neurological and neurodegenerative diseases, including stroke, Alzheimer's and Parkinson's Diseases, amyotrophic lateral sclerosis and multiple sclerosis. Studies on the toxic effects of peroxynitrite in neurons have focused primarily on adverse effects resulting from the nitration of cellular proteins as the principal mode of toxicity while the consequences of the modulation of kinase pathways by peroxynitrite have received relatively less attention. Our results show that treatment of primary rat neurons with the peroxynitrite donor, SIN-1, leads to decreases in glutathione (GSH) levels and cell viability via a novel extracellular-signal-related kinase (ERK)/c-Myc phosphorylation pathway and a reduction in the nuclear expression of NF-E2-related factor-2 (Nrf2) that down-regulate the expression of glutamate cysteine ligase, the rate limiting enzyme for GSH synthesis. The flavonoid fisetin protects against the SIN-1-mediated alterations in ERK/c-Myc phosphorylation, nuclear Nrf2 levels, glutamate cysteine ligase levels, GSH concentration and cell viability. We also show that inhibition of mitogen-activated protein kinase kinase or Raf kinase can increase GSH levels in unstressed primary rat neurons through the same ERK/c-Myc phosphorylation pathway. Together, these results demonstrate that distinct signaling pathways modulate GSH metabolism in unstressed and stressed cortical neurons.

Preclinical evaluation of targeting the Nrf2 pathway by triterpenoids (CDDO-Im and CDDO-Me) for protection from LPS-induced inflammatory response and reactive oxygen species in human peripheral blood mononuclear cells and neutrophils

Sepsis is characterized by an inappropriate host immune-inflammatory response and sustained oxidative damage. Nrf2, a bZIP oxidant-responsive transcription factor, regulates a battery of cytoprotective genes including antioxidants and maintains cellular redox homeostasis. Mouse studies have demonstrated a critical role of Nrf2 in improving survival during sepsis. This preclinical ex vivo study using neutrophils and peripheral blood mononuclear cells (PBMCs) as a surrogate cells evaluates the efficacy of CDDO-Im and CDDO-Me [imidazole and methyl ester derivative of 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid (CDDO)] to activate the Nrf2 pathway and protect from lipopolysaccharide (LPS)-induced inflammatory response in humans. CDDO-Im treatment significantly induced Nrf2-dependent antioxidative genes (HO-1, GCLC, GCLM, and NQO1) in PBMCs isolated from six normal subjects. CDDO-Im increased nuclear accumulation of Nrf2 protein. Pretreatment of PBMC by CDDO-Im significantly attenuated LPS-induced cytokine expression. Similar increases in levels of antioxidant genes and suppression of LPS-induced cytokine expression was observed after CDDO-Me pretreatment. CDDO-Im also greatly inhibited LPS, fMLP, TNF-alpha, and TPA-induced ROS generation in neutrophils. In conclusion, these results demonstrate that activation of the Nrf2-dependent antioxidative pathway by CDDO-Im or CDDO-Me protects against the LPS-induced inflammatory response and suggest that they can be potential therapeutic candidates for intervening sepsis syndrome.

Sulforaphane mobilizes cellular defenses that protect skin against damage by UV radiation

UV radiation (UVR) is a complete carcinogen that elicits a constellation of pathological events, including direct DNA damage, generation of reactive oxidants that peroxidize lipids and damage other cellular components, initiation of inflammation, and suppression of the immune response. Recent dramatic increases in the incidence of nonmelanoma skin cancers are largely attributable to higher exposure of an aging population to UVR. Therefore, the development of cellular strategies for intrinsic protection of the skin against the deleterious effects of UVR is imperative. Here we show that erythema resulting from UVR is a comprehensive and noninvasive biomarker for assessing UVR damage and can be precisely and easily quantified in human skin. Topical application of sulforaphane-rich extracts of 3-day-old broccoli sprouts up-regulated phase 2 enzymes in the mouse and human skin, protected against UVR-induced inflammation and edema in mice, and reduced susceptibility to erythema arising from narrow-band 311-nm UVR in humans. In six human subjects (three males and three females, 28-53 years of age), the mean reduction in erythema across six doses of UVR (300-800 mJ/cm(2) in 100 mJ/cm(2) increments) was 37.7% (range 8.37-78.1%; P = 0.025). This protection against a carcinogen in humans is catalytic and long lasting.

Tocopherol transfer protein deficiency modifies nuclear receptor transcriptional networks in lungs: Modulation by cigarette smoke in vivo

Dietary factors and environmental pollutants initiate signaling cascades that converge on AhR:Nrf2:NF-kappaB transcription factor (TF) networks and, in turn, affect the health of the organism through its effects on the expression of numerous genes. Reactive oxygen metabolites (ROMs) have been hypothesized to be common mediators in these pathways. alpha-Tocopherol (AT) is a potent, lipophilic, scavenger of ROMs in vitro and has been hypothesized to be a major chain-breaking anti-oxidant in lipoproteins and biological membranes in vivo. The lung offers a vital organ to test the various postulated actions of AT in vivo. Lung AT concentrations can be manipulated by several methods that include dietary and genetic techniques. In this study we have used mice with severe AT deficiency inflicted at birth by the deletion of AT transfer protein (ATTP) which is abundantly expressed in the liver and regulates systemic concentrations of AT. Mice and humans deficient in ATTP are AT deficient. Female ATTP-deficient (ATTP-KO) mice and their congenic ATTP normal (WT) mice fed a diet containing 35 IU AT/kg diet were used to test our hypothesis. The mice (n=5/group) were exposed to either air or cigarette smoke (CS, total suspended particles 60 mg/m(3), 6h/day), a source of ROM, for 3 or 10 days. Post-exposure lung tissue was dissected, RNA extracted from each lung and it was pooled group-wise and processed for GeneChip analysis (Affymetrix 430A 2.0). Differential analysis of the transcriptomes ( approximately 16,000 mRNAs) identified CS sensitive genes that were modulated by lung AT-concentration. CS activated AhR driven genes such as cyp1b1 whose induction was augmented in CS-exposed, AT-deficient lungs. However, CS-induced expression of some of the Nrf2 driven genes was not potentiated in the AT-deficient lungs. Largest clusters of CS-AT sensitive genes were lymphocyte and leukocyte specific genes. These gene-clusters included those encoding cytokines and immunoglobulins, which were repressed by CS and were modulated by lung AT concentrations. Our genome-wide analysis suggests reciprocal regulation of xenobiotic and immune response genes by CS and a modulatory role of lung AT concentration on the expression of these clusters of genes. These data suggest that in vivo network of AT, AT-metabolites and ATTP affects the transcription of genes driven by AhR, Nrf2 and NF-kappaB, transcription factor networks that transduce cellular metabolic signals and orchestrate adaptive responses of lungs to inhaled environmental pollutants.

Role of increased expression of the proteasome in the protective effects of sulforaphane against hydrogen peroxide-mediated cytotoxicity in murine neuroblastoma cells

The 26S proteasome is responsible for degradation of abnormal proteins and may play a role in cell survival upon oxidative stress. The indirect antioxidant sulforaphane (SFN) protects animal tissues from chemical toxicants by increasing the expression of several families of Nrf2-regulated genes. The role of induction of the 26S proteasome in cytoprotection by SFN was investigated in murine neuroblastoma Neuro2A cells. SFN enhanced the expression of the catalytic subunits of the proteasome, as well as proteasomal peptidase activities in these cells. Such treatment with SFN protected cells from hydrogen peroxide-mediated cytotoxicity in a manner dependent on proteasomal function. Inhibition of proteasome activities using pharmacological interventions significantly attenuated the protective effects of SFN against hydrogen peroxide cytotoxicity, as well as protein oxidation. Moreover, overexpression of the catalytic subunit PSMB5 enhanced proteasome function and led to elevated resistance against hydrogen peroxide toxicity and extent of protein oxidation compared to blank-plasmid-transfected cells. Pretreatment of PSMB5-overexpressing cells with SFN did not further enhance this resistance. Collectively, these results suggest that the cytoprotective effects of SFN against oxidative stress are in part due to up-regulation of the proteasome system. Therefore, inducers of proteasome expression may ameliorate the accumulation of damaged proteins associated with neurodegeneration and other diseases in whose etiologies protein oxidation plays a role.

Activation of nuclear factor (erythroid-2 like) factor 2 by toxic bile acids provokes adaptive defense responses to enhance cell survival at the emergence of oxidative stress

Oxidative stress, causing necrotic and apoptotic cell death, is associated with bile acid toxicity. Using liver (HepG2, Hepa1c1c7, and primary human hepatocytes) and intestinal (C2bbe1, a Caco-2 subclone) cells, we demonstrated that toxic bile acids, such as lithocholic acid (LCA) and chenodeoxycholic acid, induced the nuclear factor (erythroid-2 like) factor 2 (Nrf2) target genes, especially the rate-limiting enzyme in glutathione (GSH) biosynthesis [glutamate cysteine ligase modulatory subunit (GCLM) and glutamate cysteine ligase catalytic subunit (GCLC)] and thioredoxin reductase 1. Nrf2 activation and induction of Nrf2 target genes were also evident in vivo in the liver of CD-1 mice treated 7 to 8 h or 4 days with LCA. Silencing of Nrf2 via small-interfering RNA suppressed basal and bile acid-induced mRNA levels of the above-mentioned genes. Consistent with this, overexpression of Nrf2 enhanced, but dominant-negative Nrf2 attenuated, Nrf2 target gene induction by bile acids. The activation of Nrf2-antioxidant responsive element (ARE) transcription machinery by bile acids was confirmed by increased nuclear accumulation of Nrf2, enhanced ARE-reporter activity, and increased Nrf2 binding to ARE. It is noteworthy that Nrf2 silencing increased cell susceptibility to LCA toxicity, as evidenced by reduced cell viability and increased necrosis and apoptosis. Concomitant with GCLC/GCLM induction, cellular GSH was significantly increased in bile acid-treated cells. Cotreatment with N-acetyl-l-cysteine, a GSH precursor, ameliorated LCA toxicity, whereas cotreatment with buthionine sulfoximine, a GSH synthesis blocker, exacerbated it. In summary, this study provides molecular evidence linking bile acid toxicity to oxidative stress. Nrf2 is centrally involved in counteracting such oxidative stress by enhancing adaptive antioxidative response, particularly GSH biosynthesis, and hence cell survival.

Induction of Nrf2-regulated genes by 3H-1, 2-dithiole-3-thione through the ERK signaling pathway in murine keratinocytes

Electrophile and free radical detoxifying enzymes including NAD(P)H:quinine oxidoreductase 1 (Nqo1) play an important role in the defense system by enhancing cellular antioxidant capacity. Chemopreventive efficacy of 3H-1,2-dithiole-3-thione (D3T) is mediated through activation of the transcription factor Nrf2 and subsequent elevation of detoxifying enzymes. In the present study, we have investigated the potential role of extracellular signal-regulated kinase (ERK) in regulation of D3T-induced and Nrf2-dependent gene expression in murine keratinocytes. Expression levels of Nqo1 were highly inducible by D3T treatment and increased nuclear levels of Nrf2 were observed in these cells. Treatment with pharmacological inhibitor of ERK1/2 largely blocked nuclear accumulation of Nrf2, ARE-driven reporter gene expression, and induction of Nqo1, as well as other phase 2 genes. Activation of ERK1/2 has been demonstrated following treatment with D3T. While, inhibitors of p38, PKC and PI3K did not affect ARE-driven gene expression. Involvement of the ERK1/2 cascade in inducible ARE-transcription activities was also observed in cells treated with other types of inducers oltipraz, sulforaphane and hydrogen peroxide. Collectively, current study suggests that phosphorylation cascade via ERK1/2 is associated with the activation process of Nrf2 and subsequent transactivation of its target gene Nqo1 following treatment with dithiolethione in murine keratinocyte.

NRF2 modulates aryl hydrocarbon receptor signaling: influence on adipogenesis

The NF-E2 p45-related factor 2 (NRF2) and the aryl hydrocarbon receptor (AHR) are transcription factors controlling pathways modulating xenobiotic metabolism. AHR has recently been shown to affect Nrf2 expression. Conversely, this study demonstrates that NRF2 regulates expression of Ahr and subsequently modulates several downstream events of the AHR signaling cascade, including (i) transcriptional control of the xenobiotic metabolism genes Cyp1a1 and Cyp1b1 and (ii) inhibition of adipogenesis in mouse embryonic fibroblasts (MEFs). Constitutive expression of AHR was affected by Nrf2 genotype. Moreover, a pharmacological activator of NRF2 signaling, CDDO-IM {1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole}, induced Ahr, Cyp1a1, and Cyp1b1 transcription in Nrf2+/+ MEFs but not in Nrf2-/- MEFs. Reporter analysis and chromatin immunoprecipitation assay revealed that NRF2 directly binds to one antioxidant response element (ARE) found in the -230-bp region of the promoter of Ahr. Since AHR negatively controls adipocyte differentiation, we postulated that NRF2 would inhibit adipogenesis through the interaction with the AHR pathway. Nrf2-/- MEFs showed markedly accelerated adipogenesis upon stimulation, while Keap1-/- MEFs (which exhibit higher NRF2 signaling) differentiated slowly compared to their congenic wild-type MEFs. Ectopic expression of Ahr and dominant-positive Nrf2 in Nrf2-/- MEFs also substantially delayed differentiation. Thus, NRF2 directly modulates AHR signaling, highlighting bidirectional interactions of these pathways.

Gene expression of transporters and phase I/II metabolic enzymes in murine small intestine during fasting

Background

Fasting has dramatic effects on small intestinal transport function. However, little is known on expression of intestinal transport and phase I/II metabolism genes during fasting and the role the fatty acid-activated transcription factor PPARα may play herein. We therefore investigated the effects of fasting on expression of these genes using Affymetrix GeneChip MOE430A arrays and quantitative RT-PCR.

Results

After 24 hours of fasting, expression levels of 33 of the 253 analyzed transporter and phase I/II metabolism genes were changed. Upregulated genes were involved in transport of energy-yielding molecules in processes such as glycogenolysis (G6pt1) and mitochondrial and peroxisomal oxidation of fatty acids (Cact, Mrs3/4, Fatp2, Cyp4a10, Cyp4b1). Other induced genes were responsible for the inactivation of the neurotransmitter serotonin (Sert, Sult1d1, Dtd, Papst2), formation of eicosanoids (Cyp2j6, Cyp4a10, Cyp4b1), or for secretion of cholesterol (Abca1 and Abcg8). Cyp3a11, typically known because of its drug metabolizing capacity, was also increased. Fasting had no pronounced effect on expression of phase II metabolic enzymes, except for glutathione S-transferases which were down-regulated. Time course studies revealed that some genes were acutely regulated, whereas expression of other genes was only affected after prolonged fasting. Finally, we identified 8 genes that were PPARα-dependently upregulated upon fasting.

Conclusion

We have characterized the response to fasting on expression of transporters and phase I/II metabolic enzymes in murine small intestine. Differentially expressed genes are involved in a variety of processes, which functionally can be summarized as a) increased oxidation of fat and xenobiotics, b) increased cholesterol secretion, c) increased susceptibility to electrophilic stressors, and d) reduced intestinal motility. This knowledge increases our understanding of gut physiology, and may be of relevance for e.g. pre-surgery regimen of patients.

Polyamine-modulated factor 1 represses glucocorticoid receptor activity

Polyamine-modulated factor 1 (PMF-1) has been reported to interact with NF-E2 related factor 2 (Nrf-2) and activate the polyamine-induced transcription of spermidine/spermine N(1)-acetyltransferase (SSAT) gene. Polyamines are important regulators of cell growth and cell death and have been implicated in glucocorticoid-induced apoptosis. In the present study, we have identified and characterized new functional binding partners for PMF-1. Our results demonstrate that PMF-1 binds to the glucocorticoid receptor (GR). PMF-1 also represses glucocorticoid-induced transcription. Furthermore, we show that PMF-1 has an intrinsic repression activity, which could contribute to the repressive effect on GR. PMF-1 can also interact with the GR corepressor, receptor-interacting protein 140 (RIP140), but does not further enhance the repressive effect of RIP140. Our results suggest that PMF-1 has a broader function in regulation of genes and can contribute to glucocorticoid signaling.

Novel semisynthetic analogues of betulinic acid with diverse cytoprotective, antiproliferative, and proapoptotic activities

Betulinic acid (BA), a pentacyclic triterpene isolated from birch bark and other plants, selectively inhibits the growth of human cancer cell lines. However, the poor potency of BA hinders its clinical development, despite a lack of toxicity in animal studies even at high concentrations. Here, we describe six BA derivatives that are markedly more potent than BA for inhibiting inducible nitric oxide synthase, activating phase 2 cytoprotective enzymes, and inducing apoptosis in cancer cells and in Bax/Bak(-/-) fibroblasts, which lack two key proteins involved in the intrinsic, mitochondrial-dependent apoptotic pathway. Notably, adding a cyano-enone functionality in the A ring of BA enhanced its cytoprotective properties, but replacing the cyano group with a methoxycarbonyl strikingly increased potency in the apoptosis assays. Higher plasma and tissue levels were obtained with the new BA analogues, especially CBA-Im [1-(2-cyano-3-oxolupa-1,20(29)-dien-28-oyl)imidazole], compared with BA itself and at concentrations that were active in vitro. These results suggest that BA is a useful platform for drug development, and the enhanced potency and varied biological activities of CBA-Im make it a promising candidate for further chemoprevention or chemotherapeutic studies.

Induction of the phase 2 response in mouse and human skin by sulforaphane-containing broccoli sprout extracts

The isothiocyanate sulforaphane was isolated from broccoli extracts in a bioactivity-guided fractionation as the principal and very potent inducer of cytoprotective phase 2 enzymes and subsequently shown to inhibit tumor development in animal models that involve various carcinogens and target organs. Because broccoli and broccoli sprouts are widely consumed, extracts obtained from them are viewed as convenient vehicles for sulforaphane delivery to humans. In relation to our current interest in devising strategies for protection against UV light-induced skin cancer, it was necessary to examine the safety and efficacy of topical application of sulforaphane-containing broccoli sprout extracts as single and multiple doses in both mice and humans. Topical application of an extract delivering 100 nmol sulforaphane/cm(2) increased the protein levels of NAD(P)H:quinone oxidoreductase 1 (NQO1), glutathione S-transferase A1, and heme oxygenase 1, three representative phase 2 enzymes, in mouse skin epidermis. Quantitative assessment of the activity of NQO1 24 h after dosing showed increases of 1.5- and 2.7-fold after application of single and multiple (thrice, every 24 h) doses, respectively. A dose-escalation safety study in healthy human subjects revealed no adverse reactions when doses as high as 340 nmol of sulforaphane in the form of broccoli sprout extracts were applied topically to the center of a 1-cm-diameter circle drawn on the volar forearm. A subsequent efficacy study showed that despite the interindividual differences in basal levels, the enzyme activity of NQO1 in homogenates of 3-mm full thickness skin punch biopsies increased in a dose-dependent manner, with maximum increases of 1.5- and 4.5-fold after application of 150 nmol doses, once or three times (at 24 h-intervals), respectively, thus providing direct evidence for induction of the phase 2 response in humans.

Reactive carbonyls and oxidative stress: potential for therapeutic intervention

Reactive aldehydes and ketones are produced as a result of oxidative stress in several disease processes. Considerable evidence is now accumulating that these reactive carbonyl products are also involved in the progression of diseases, including neurodegenerative disorders, diabetes, atherosclerosis, diabetic complications, reperfusion after ischemic injury, hypertension, and inflammation. To counter carbonyl stress, cells possess enzymes that can decrease aldehyde load. These enzymes include aldehyde dehydrogenases (ALDH), aldo-keto reductases (AKR), carbonyl reductase (CBR), and glutathione S-transferases (GST). Some of these enzymes are inducible by chemoprotective compounds via Nrf2/ARE- or AhR/XRE-dependent mechanisms. This review describes the metabolism of reactive carbonyls and discusses the potential for manipulating levels of carbonyl-metabolizing enzymes through chemical intervention.

In vitro immunotoxicity of bis(tri-n-butyltin)oxide (TBTO) studied by toxicogenomics

The biocide and environmental pollutant bis(tri-n-butyltin)oxide (TBTO) causes thymus atrophy in rodents. Whether the depletion of thymic lymphocytes by tributyltin compounds may be the result of inhibition of cell proliferation or induction of apoptosis is subject of debate. We examined gene expression profiles in primary rat thymocytes exposed to TBTO in vitro at dose levels of 0, 0.1, 0.3, 0.5, and 1.0microM. By measuring cell viability and apoptosis, exposure conditions were selected that would provide information on changes in gene expression preceding or accompanying functional effects of TBTO. Several processes related to TBTO-induced toxicity were detected at the transcriptome level. Effects on lipid metabolisms appeared to be the first indication of disruption of cellular function. Many transcriptional effects of TBTO at higher dose levels were related to apoptotic processes, which corresponded to present or subsequent thymocyte apoptosis observed phenotypically. The gene expression profile was, however, not unambiguous since expression of apoptosis-related genes was both increased and decreased. Stimulation of glucocorticoid receptor signaling appeared to be a relevant underlying mechanism of action. These findings suggest that TBTO exerts its toxic effects on the thymus primarily by affecting apoptotic processes, but the possibility is discussed that this may in fact represent an early effect that precedes inhibition of cell proliferation. At the highest dose level tested, TBTO additionally repressed mitochondrial function and immune cell activation. Our in vitro toxicogenomics approach thus identified several cellular and molecular targets of TBTO that may mediate the toxicity towards thymocytes and thereby its immunosuppressive effects.

Nitro-linoleic acid inhibits vascular smooth muscle cell proliferation via the Keap1/Nrf2 signaling pathway

Nitroalkenes, the nitration products of unsaturated fatty acids formed via NO-dependent oxidative reactions, have been demonstrated to exert strong biological actions in endothelial cells and monocytes/macrophages; however, little is known about their effects on vascular smooth muscle cells (VSMCs). The present study examined the role of nitro-linoleic acid (LNO(2)) in the regulation of VSMC proliferation. We observed that LNO(2) inhibited VSMC proliferation in a dose-dependent manner. In addition, LNO(2) induced growth arrest of VSMCs in the G(1)/S phase of the cell cycle with an upregulation of the cyclin-dependent kinase inhibitor p27(kip1). Furthermore, LNO(2) triggered nuclear factor-erythroid 2-related factor 2 (Nrf2) nuclear translocation and activation of the antioxidant-responsive element-driven transcriptional activity via impairing Kelch-like ECH-associating protein 1 (Keap1)-mediated negative control of Nrf2 activity in VSMCs. LNO(2) upregulated the expression of Nrf2 protein levels, but not mRNA levels, in VSMCs. A forced activation of Nrf2 led to an upregulation of p27(kip1) and growth inhibition of VSMCs. In contrast, knock down of Nrf2 using an Nrf2 siRNA approach reversed the LNO(2)-induced upregulation of p27(kip1) and inhibition of cellular proliferation in VSMCs. These studies provide the first evidence that nitroalkene LNO(2) inhibits VSMC proliferation through activation of the Keap1/Nrf2 signaling pathway, suggesting an important role of nitroalkenes in vascular biology.

Deficiency in Nrf2-GSH signaling impairs type II cell growth and enhances sensitivity to oxidants

Redox imbalance has been implicated in the pathogenesis of many acute and chronic lung diseases. The b-Zip transcription factor Nrf2 acts via an antioxidant/electrophilic response element to regulate antioxidants and maintain cellular redox homeostasis. Our previous studies have shown that Nrf2-deficient mice (Nrf2(-/-)) show reduced pulmonary expression of several antioxidant enzymes, which renders them highly susceptible to hyperoxia-induced lung injury. To better understand the physiologic significance of Nrf2-induced redox signaling, we have used primary cells isolated from the lungs of Nrf2(+/+) and Nrf2(-/-) mice. Our studies were focused on type II cells because these cells are constantly exposed to the oxidant environment and play key roles in host defense, injury, and repair processes. Using this system, we now report that an Nrf2 deficiency leads to defects in type II cell proliferation and greatly enhances the cells' sensitivity to oxidant-induced cell death. These defects were closely associated with high levels of reactive oxygen species (ROS) and redox imbalance in Nrf2(-/-) cells. Glutathione (GSH) supplementation rescued these phenotypic defects associated with the Nrf2 deficiency. Intriguingly, although the antioxidant N-acetyl-cysteine drastically squelched ROS levels, it was unable to counteract growth arrest in Nrf2(-/-) cells. Moreover, despite their elevated levels of ROS, Nrf2(-/-) type II cells were viable and, like their wild-type counterparts, exhibited normal differentiation characteristics. Our data suggest that dysfunctional Nrf2-regulated GSH-induced signaling is associated with deregulation of type II cell proliferation, which contributes to abnormal injury and repair and leads to respiratory impairment.

Applying Mechanisms of Chemical Toxicity to Predict Drug Safety

Toxicology can no longer be used only as a science that reacts to problems but must be more proactive in predicting potential human safety issues with new drug candidates. Success in this area must be based on an understanding of the mechanisms of toxicity. This review summarizes and extends some of the concepts of an American Chemical Society ProSpectives meeting on the title subject held in June 2006. One important area is the discernment of the exact nature of the most common problems in drug toxicity. Knowledge of chemical structure alerts and relevant biological pathways are important. Biological activation to reactive products and off-target pharmacology are considered to be major contexts of drug toxicity, although defining exactly what the contributions are is not trivial. Some newer approaches to screening for both have been developed. A goal in predictive toxicology is the use of in vitro methods and database development to make predictions concerning potential modes of toxicity and to stratify drug candidates for further development. Such predictions are desirable for several economic and other reasons but are certainly not routine yet. However, progress has been made using several approaches. Some examples of the application of studies of wide-scale biological responses are now available, with incorporation into development paradigms.