Activation of hepatic Nrf2 in vivo by acetaminophen in CD-1 mice

Human pluripotent stem cells for modeling toxicity

The development of xenobiotics, driven by the demand for therapeutic, domestic and industrial uses continues to grow. However, along with this increasing demand is the risk of xenobiotic-induced toxicity. Currently, safety screening of xenobiotics uses a plethora of animal and in vitro model systems which have over the decades proven useful during compound development and for application in mechanistic studies of xenobiotic-induced toxicity. However, these assessments have proven to be animal-intensive and costly. More importantly, the prevalence of xenobiotic-induced toxicity is still significantly high, causing patient morbidity and mortality, and a costly impediment during drug development. This suggests that the current models for drug safety screening are not reliable in toxicity prediction, and the results not easily translatable to the clinic due to insensitive assays that do not recapitulate fully the complex phenotype of a functional cell type in vivo. Recent advances in the field of stem cell research have potentially allowed for a readily available source of metabolically competent cells for toxicity studies, derived using human pluripotent stem cells harnessed from embryos or reprogrammed from mature somatic cells. Pluripotent stem cell-derived cell types also allow for potential disease modeling in vitro for the purposes of drug toxicology and safety pharmacology, making this model possibly more predictive of drug toxicity compared with existing models. This article will review the advances and challenges of using human pluripotent stem cells for modeling metabolism and toxicity, and offer some perspectives as to where its future may lie.

The Keap1-Nrf2 cell defense pathway--a promising therapeutic target?

By regulating the basal and inducible expression of an abundance of detoxification enzymes, antioxidant proteins, xenobiotic transporters and other stress response proteins, the Keap1-Nrf2 pathway plays a crucial role in determining the sensitivity of mammalian cells to chemical and oxidative insults that have the capacity to provoke cellular harm. This review highlights historical and recent advances in our understanding of the molecular mechanisms that regulate the activity of the Keap1-Nrf2 pathway. The important role of Nrf2 in protecting against the onset of specific diseases and drug-induced toxicities is also examined, alongside the emerging role of Nrf2 in promoting oncogenesis and chemotherapeutic drug resistance. A particular emphasis is placed on the potential for translation of this mechanistic understanding into clinical strategies that can improve human health, with consideration of the potential applications of targeting Nrf2 therapeutically.

Quercetin modulates Nrf2 and glutathione-related defenses in HepG2 cells: Involvement of p38

Dietary flavonoid quercetin has been suggested as a cancer chemopreventive agent, but the mechanisms of action remain unclear. This study investigated the influence of quercetin on p38-MAPK and the potential regulation of the nuclear transcription factor erythroid-2p45-related factor (Nrf2) and the cellular antioxidant/detoxifying defense system related to glutathione (GSH) by p38 in HepG2 cells. Incubation of HepG2 cells with quercetin at a range of concentrations (5-50μM) for 4 or 18h induced a differential effect on the modulation of p38 and Nrf2 in HepG2 cells, 50μM quercetin showed the highest activation of p38 at 4h of treatment and values of p38 similar to those of control cells after 18 h of incubation, together with the inhibition of Nrf2 at both incubation times. Quercetin (50μM) induced a time-dependent activation of p38, which was in concert with a transient stimulation of Nrf2 to provoke its inhibition afterward. Quercetin also increased GSH content, mRNA levels of glutamylcysteine-synthetase (GCS) and expression and/or activity of glutathione-peroxidase, glutathione-reductase and GCS after 4h of incubation, and glutathione-S-transferase after 18h of exposure. Further studies with the p38 specific inhibitor SB203580 showed that the p38 blockage restored the inhibited Nrf2 transcription factor and the enzymatic expression and activity of antioxidant/detoxificant enzymes after 4h exposure. In conclusion, p38-MAPK is involved in the mechanisms of the cell response to quercetin through the modulation of Nrf2 and glutathione-related enzymes in HepG2 cells.

Endogenous interleukin-4 regulates glutathione synthesis following acetaminophen-induced liver injury in mice

In a recent study, we reported that interleukin (IL)-4 had a protective role against acetaminophen (APAP)-induced liver injury (AILI), although the mechanism of protection was unclear. Here, we carried out more detailed investigations and have shown that one way IL-4 may control the severity of AILI is by regulating glutathione (GSH) synthesis. In the present studies, the protective role of IL-4 in AILI was established definitively by showing that C57BL/6J mice made deficient in IL-4 genetically (IL-4(-/-)) or by depletion with an antibody, were more susceptible to AILI than mice not depleted of IL-4. The increased susceptibility of IL-4(-/-) mice was not due to elevated levels of hepatic APAP-protein adducts but was associated with a prolonged reduction in hepatic GSH that was attributed to decreased gene expression of γ-glutamylcysteine ligase (γ-GCL). Moreover, administration of recombinant IL-4 to IL-4(-/-) mice postacetaminophen treatment diminished the severity of liver injury and increased γ-GCL and GSH levels. We also report that the prolonged reduction of GSH in APAP-treated IL-4(-/-) mice appeared to contribute toward increased liver injury by causing a sustained activation of c-Jun-N-terminal kinase (JNK) since levels of phosphorylated JNK remained significantly higher in the IL-4(-/-) mice up to 24 h after APAP treatment. Overall, these results show for the first time that IL-4 has a role in regulating the synthesis of GSH in the liver under conditions of cellular stress. This mechanism appears to be responsible at least in part for the protective role of IL-4 against AILI in mice and may have a similar role not only in AILI in humans but also in pathologies of the liver caused by other drugs and etiologies.

Changes in mouse liver protein glutathionylation after acetaminophen exposure

The role of protein glutathionylation in acetaminophen (APAP)-induced liver injury was investigated in this study. A single oral gavage dose of 150 or 300 mg/kg APAP in B6C3F1 mice produced increased serum alanine aminotransferase and aspartate aminotransferase levels and liver necrosis in a dose-dependent manner. The ratio of GSH to GSSG was decreased in a dose-dependent manner, suggesting that APAP produced a more oxidizing environment within the liver. Despite the increased oxidation state, the level of global protein glutathionylation was decreased at 1 h and continued to decline through 24 h. Immunohistochemical localization of glutathionylated proteins showed a complex dynamic change in the lobule zonation of glutathionylated proteins. At 1 h after APAP exposure, the level of glutathionylation decreased in the single layer of hepatocytes around the central veins but increased mildly in the remaining centrilobular hepatocytes. This increase correlated with the immunohistochemical localization of APAP covalently bound to protein. Thereafter, the level of glutathionylation decreased dramatically over time in the centrilobular regions with major decreases observed at 6 and 24 h. Despite the overall decreased glutathionylation, a layer of cells lying between the undamaged periportal region and the damaged centrilobular hepatocytes exhibited high levels of glutathionylation at 3 and 6 h in all samples and in some 24-h samples that had milder injury. These temporal and zonal pattern changes in protein glutathionylation after APAP exposure indicate that protein glutathionylation may play a role in protein homeostasis during APAP-induced hepatocellular injury.

Aldo-keto reductase-7A protects liver cells and tissues from acetaminophen-induced oxidative stress and hepatotoxicity

Aldo-keto reductase-7A (AKR7A) is an enzyme important for bioactivation and biodetoxification. Previous studies suggested that Akr7a might be transcriptionally regulated by oxidative stress-responsive transcription factor nuclear factor erythroid 2 p45-related factor 2 (Nrf2), a protein highly responsive to acetaminophen (APAP) or its intermediate metabolite, N-acetyl-p-benzoquinoneimine (NAPQI). This study was, therefore, carried out to investigate whether Akr7a is involved in the protection against APAP-induced oxidative stress and hepatotoxicity. We found that in response to APAP or NAPQI exposure, Akr7a3 mRNA and protein were significantly up-regulated in vitro in human HepG2 and LO2 cells. Similarly, strong induction was observed for Akr7a5 in mouse AML12 hepatocytes exposed to APAP. In vivo in wild-type rats, significant up-regulation of hepatic AKR7A1 protein was observed after administration of APAP. On the other hand, depletion of Nrf2 reduced the expression of Akr7a3, suggesting that Nrf2, indeed, contributes significantly to the induction of Akr7a. Moreover, loss of cell viability in Nrf2-depleted cells was significantly rescued by coexpression of AKR7A3. Furthermore, increased AKR7A3 in HepG2 cells was associated with the up-regulation of oxidative stress-related enzymes to enhance cellular antioxidant defense, which appeared to contribute significantly to protection against APAP-induced toxicity. In a line of transgenic rats overexpressing AKR7A1, increased AKR7A1 stimulated the expression of Nrf2 and other Nrf2-regulated genes, but did not better protect rats from APAP insults. In contrast, depletion of Akr7a5 in vitro in cultured AML12 cells or depletion of Akr7a1 in vivo in rat liver greatly increased APAP-induced hepatotoxicity.

CONCLUSION

AKR7A proteins are significantly up-regulated in response to APAP/NAPQI exposure to contribute significantly to protection against APAP-induced hepatotoxicity. AKR7A mediates this protection, in part, through enhancing hepatocellular antioxidant defense.

A possible mechanism for hepatotoxicity induced by BIRB-796, an orally active p38 mitogen-activated protein kinase inhibitor

BIRB-796, a selective inhibitor of p38 mitogen-activated protein kinase, has entered clinical trials for the treatment of autoimmune diseases. Levels of alanine transaminase, a biomarker of hepatic toxicity in clinical pathology, were found to be increased in Crohn's disease patients treated with BIRB-796. The purpose of the present study was to clarify the molecular mechanism(s) of this hepatotoxicity. A toxicogenomic analysis using a highly sensitive DNA chip, 3D-Gene™ Mouse Oligo chip 24k, indicated that BIRB-796 treatment activated the nuclear factor (erythroid-derived 2)-like 2 signaling pathway, which plays a key role in the response to oxidative stress. A reactive intermediate of BIRB-796 was detected by the glutathione-trapping method using mouse and human liver microsomes. The production of this reactive metabolite in the liver may be one of the causes of BIRB-796's hepatotoxicity.

Acetaminophen-induced hepatotoxicity in mice occurs with inhibition of activity and nitration of mitochondrial manganese superoxide dismutase

In overdose the analgesic/antipyretic acetaminophen (APAP) is hepatotoxic. Toxicity is mediated by initial hepatic metabolism to N-acetyl-p-benzoquinone imine (NAPQI). After low doses NAPQI is efficiently detoxified by GSH. However, in overdose GSH is depleted, NAPQI covalently binds to proteins as APAP adducts, and oxygen/nitrogen stress occurs. Toxicity is believed to occur by mitochondrial dysfunction. Manganese superoxide dismutase (MnSOD) inactivation by protein nitration has been reported to occur during other oxidant stress-mediated diseases. MnSOD is a critical mitochondrial antioxidant enzyme that prevents peroxynitrite formation within the mitochondria. To examine the role of MnSOD in APAP toxicity, mice were treated with 300 mg/kg APAP. GSH was significantly reduced by 65% at 0.5 h and remained reduced from 1 to 4 h. Serum alanine aminotransferase did not significantly increase until 4 h and was 2290 IU/liter at 6 h. MnSOD activity was significantly reduced by 50% at 1 and 2 h. At 1 h, GSH was significantly depleted by 62 and 80% at nontoxic doses of 50 and 100 mg/kg, respectively. No further GSH depletion occurred with hepatotoxic doses of 200 and 300 mg/kg APAP. A dose response decrease in MnSOD activity was observed for APAP at 100, 200, and 300 mg/kg. Immunoprecipitation of MnSOD from livers of APAP-treated mice followed by Western blot analysis revealed nitrated MnSOD. APAP-MnSOD adducts were not detected. Treatment of recombinant MnSOD with NAPQI did not produce APAP protein adducts. The data indicate that MnSOD inactivation by nitration is an early event in APAP-induced hepatic toxicity.

Altered cardiac repolarization in association with air pollution and air temperature among myocardial infarction survivors

BACKGROUND

Epidemiological studies have shown that ambient particulate matter (PM) and changes in air temperature are associated with increased cardiopulmonary events.

OBJECTIVE

We hypothesized that patients with previous myocardial infarction (MI) experience changes in heart rate (HR) and repolarization parameters, such as Bazett-corrected QT interval (QTc), and T-wave amplitude (Tamp), in association with increases in air pollution and temperature changes.

METHODS

Between May 2003 and February 2004, 67 MI survivors from the Augsburg KORA-MI registry repeatedly sent 16 sec electrocardiograms (ECGs) with a personal transmitter (Viapac) via telephone to the Philips Monitoring Center, where ECG parameters were immediately analyzed. Meteorological data and air pollutants were acquired from fixed monitoring sites on an hourly basis. Additive mixed models were used for analysis. Effect modification by patient characteristics was investigated.

RESULTS

The analysis of the 1,745 ECGs revealed an increased HR associated with interquartile range (IQR) increases in PM levels among participants not using beta-adrenergic receptor blockers and among those with body mass index ≥ 30 kg/m². We observed a 24- to 47-hr lagged QTc prolongation [0.5% change (95% confidence interval, 0.0-1.0%)] in association with IQR increases in levels of PM ≤ 2.5 µm in aerodynamic diameter, especially in patients with one [0.6% (0.1-1.0%)] or two [1.2% (0.4-2.1%)] minor alleles of the nuclear factor (erythroid-derived 2)-like 2 (NFE2L2) single-nucleotide polymorphism rs2364725. Positive immediate (0-23 hr) and inverse delayed (48-71 hr up to 96-119 hr) associations were evident between PM and Tamp. We detected an inverse U-shaped association between temperature and Tamp, with a maximum Tamp at 5°C.

CONCLUSIONS

Increased air pollution levels and temperature changes may lead to changes in HR and repolarization parameters that may be precursors of cardiac problems.

Modification in oxidative stress, inflammation, and lipoprotein assembly in response to hepatocyte nuclear factor 4alpha knockdown in intestinal epithelial cells

Hepatocyte nuclear factor 4α (HNF4α) is a nuclear transcription factor mainly expressed in the liver, intestine, kidney, and pancreas. Many of its hepatic and pancreatic functions have been described, but limited information is available on its role in the gastrointestinal tract. The objectives of this study were to evaluate the anti-inflammatory and antioxidant functions of HNF4α as well as its implication in intestinal lipid transport and metabolism. To this end, the HNF4A gene was knocked down by transfecting Caco-2 cells with a pGFP-V-RS lentiviral vector containing an shRNA against HNF4α. Inactivation of HNF4α in Caco-2 cells resulted in the following: (a) an increase in oxidative stress as demonstrated by the levels of malondialdehyde and conjugated dienes; (b) a reduction in secondary endogenous antioxidants (catalase, glutathione peroxidase, and heme oxygenase-1); (c) a lower protein expression of nuclear factor erythroid 2-related factor that controls the antioxidant response elements-regulated antioxidant enzymes; (d) an accentuation of cellular inflammatory activation as shown by levels of nuclear factor-κB, interleukin-6, interleukin-8, and leukotriene B4; (e) a decrease in the output of high density lipoproteins and of their anti-inflammatory and anti-oxidative components apolipoproteins (apo) A-I and A-IV; (f) a diminution in cellular lipid transport revealed by a lower cellular secretion of chylomicrons and their apoB-48 moiety; and (g) alterations in the transcription factors sterol regulatory element-binding protein 2, peroxisome proliferator-activated receptor α, and liver X receptor α and β. In conclusion, HNF4α appears to play a key role in intestinal lipid metabolism as well as intestinal anti-oxidative and anti-inflammatory defense mechanisms.

Drug bioactivation and protein adduct formation in the pathogenesis of drug-induced toxicity

Adverse drug reactions (ADRs) remain a major complication of drug therapy and can be classified as 'on-target' or 'off-target' (idiosyncratic) reactions. On-target reactions can be predicted from the known primary or secondary pharmacology of the drug and often represent an exaggeration of the pharmacological effect of the drug. In contrast, off-target adverse reactions cannot be predicted from knowledge of the basic pharmacology of the drug. The exact mechanisms of idiosyncratic drug reactions are still unclear; however it is believed that they can be initiated by chemically reactive drug metabolites. It is well known that xenobiotics can undergo metabolic bioactivation reactions which have the potential to cause cellular stress and damage. Bioactivation of drugs is thought to have the potential of initiating covalent linkages between cellular protein and drugs which can be recognised by the adaptive immune system in the absence of detectable cellular stress. This process cannot yet be predicted in pre-clinical models or discovered in clinical trials. Because of this hazard perception, the formation of chemically reactive metabolites in early drug discovery remains a serious impediment to the development of new medicines and can lead to withdrawal of an otherwise effective therapeutic agent. The fear of such reactions occurring at the post-licensing stage - when such problems first become evident - is a major contribution to drug attrition. The first step towards such methodology has been the development of chemically reactive metabolite screens. The chemical basis of drug bioactivation can usually be rationalised and synthetic strategies put in place to prevent such bioactivation. However, there is no simple correlation between drug bioactivation in vitro and adverse drug reactions in the clinic. Such a chemical approach is clearly limited by the facts that (a) not all drugs that can undergo bioactivation by human drug-metabolising enzymes are associated with hypersensitivity in the clinic and (b) drug bioactivation may not always be a mandatory step in drug hypersensitivity. To predict such reactions in early drug development, it will require an integrated understanding of the chemical, immunological and genetic basis of adverse drug reactions in patients, which in turn will depend on the development of novel in vitro experimental systems.

Kaempferol suppresses cisplatin-induced apoptosis via inductions of heme oxygenase-1 and glutamate-cysteine ligase catalytic subunit in HEI-OC1 cell

PURPOSE

The present study was undertaken to elucidate the chemoprotective mechanism of kaempferol, which possesses anti-oxidative and anti-apoptotic properties.

METHODS

House Ear Institute-Organ of Corti 1 (HEI-OC1) cells were treated with kaempferol in the presence or absence of cisplatin. Cisplatin-induced oxidative stress was assessed by analysis of Comet assay, DNA-laddering assay and activation of caspases. Heme oxygenase-1 (HO-1), mitogen-activated protein kinase (MAPK) pathway and nuclear factor-E2-related factor 2 (Nrf2) were measured by Western blot analysis. Transfection of small interfering RNAs (siRNA), glutathione (GSH) assay and RT-PCR were performed in this study.

RESULTS

Kaempferol protected cells against cisplatin-induced apoptosis in a dose-dependent manner in HEI-OC1 cells. Kaempferol-induced HO-1 expression protected against cell death though the c-Jun N-terminal kinase (JNK) pathway and by the aid of Nrf2 translocation. Kaempferol increased the cellular level of GSH and the expression of GCLC time-dependently. siRNA GCLC blocked the increase of GSH level by kaempferol and the protective effect of kaempferol against cisplatin-induced cell death.

CONCLUSION

The expression of HO-1 by kaempferol inhibits cisplatin-induced apoptosis in HEI-OC1 cells, and the mechanism of protective effect is also associated with its inductive effect of GCLC expression.

Diet restriction inhibits apoptosis and HMGB1 oxidation and promotes inflammatory cell recruitment during acetaminophen hepatotoxicity

Acetaminophen (APAP) overdose is a major cause of acute liver failure and serves as a paradigm to elucidate mechanisms, predisposing factors and therapeutic interventions. The roles of apoptosis and inflammation during APAP hepatotoxicity remain controversial. We investigated whether fasting of mice for 24 h can inhibit APAP-induced caspase activation and apoptosis through the depletion of basal ATP. We also investigated in fasted mice the critical role played by inhibition of caspase-dependent cysteine 106 oxidation within high mobility group box-1 protein (HMGB1) released by ATP depletion in dying cells as a mechanism of immune activation. In fed mice treated with APAP, necrosis was the dominant form of hepatocyte death. However, apoptosis was also observed, indicated by K18 cleavage, DNA laddering and procaspase-3 processing. In fasted mice treated with APAP, only necrosis was observed. Inflammatory cell recruitment as a consequence of hepatocyte death was observed only in fasted mice treated with APAP or fed mice cotreated with a caspase inhibitor. Hepatic inflammation was also associated with loss in detection of serum oxidized-HMGB1. A significant role of HMGB1 in the induction of inflammation was confirmed with an HMGB1-neutralizing antibody. The differential response between fasted and fed mice was a consequence of a significant reduction in basal hepatic ATP, which prevented caspase processing, rather than glutathione depletion or altered APAP metabolism. Thus, the inhibition of caspase-driven apoptosis and HMGB1 oxidation by ATP depletion from fasting promotes an inflammatory response during drug-induced hepatotoxicity/liver pathology.

Pharmacogenetics of drug-induced liver injury

Recent progress in research on drug-induced liver injury (DILI) has been determined by key developments in two areas. First, new technologies allow the identification of genetic risk factors with improved sensitivity, specificity, and efficiency. Second, new mechanistic concepts of DILI emphasize the importance of unspecific "downstream" events following drug-specific initial "upstream" hepatocyte injury and of complex interactions between environmental and genetic risk factors. The integration of genetic and mechanistic concepts is essential for current research approaches, and genetic studies of DILI now focus on targets that affect the function and transcriptional regulation of genes relating not only to drug metabolism but also to human leukocyte antigens (HLAs), cytokines, oxidative stress, and hepatobiliary transporters. Risk factors affecting unspecific downstream mechanisms may be identified using pooled DILI cases caused by various drugs. The power to detect variants that confer a low risk can be increased by recruitment of strictly selected cases through large networks, whereas controls may also be obtained from genotyped reference populations. The first genomewide studies of DILI identified HLA variants as risk factors for hepatotoxicity associated with flucloxacillin and ximelagatran, and their design has defined a new standard for pharmacogenetic studies. From a clinical and regulatory point of view, there is a need for genetic tests that identify patients at increased hepatotoxic risk. However, DILI is a rare complex disease, and pharmacogenetic studies have so far not been able to identify interactions of several risk factors defining a high population-attributable risk and clinically relevant absolute risk for DILI.

Generating hepatic cell lineages from pluripotent stem cells for drug toxicity screening

Hepatotoxicity is an enormous and increasing problem for the pharmaceutical industry. Early detection of problems during the drug discovery pathway is advantageous to minimize costs and improve patient safety. However, current cellular models are sub-optimal. This review addresses the potential use of pluripotent stem cells in the generation of hepatic cell lineages. It begins by highlighting the scale of the problem faced by the pharmaceutical industry, the precise nature of drug-induced liver injury and where in the drug discovery pathway the need for additional cell models arises. Current research is discussed, mainly for generating hepatocyte-like cells rather than other liver cell-types. In addition, an effort is made to identify where some of the major barriers remain in translating what is currently hypothesis-driven laboratory research into meaningful platform technologies for the pharmaceutical industry.

Proteomic analysis of Nrf2 deficient transgenic mice reveals cellular defence and lipid metabolism as primary Nrf2-dependent pathways in the liver

The transcription factor Nrf2 regulates expression of multiple cellular defence proteins through the antioxidant response element (ARE). Nrf2-deficient mice (Nrf2(-/-)) are highly susceptible to xenobiotic-mediated toxicity, but the precise molecular basis of enhanced toxicity is unknown. Oligonucleotide array studies suggest that a wide range of gene products is altered constitutively, however no equivalent proteomics analyses have been conducted. To define the range of Nrf2-regulated proteins at the constitutive level, protein expression profiling of livers from Nrf2(-/-) and wild type mice was conducted using both stable isotope labelling (iTRAQ) and gel electrophoresis methods. To establish a robust reproducible list of Nrf2-dependent proteins, three independent groups of mice were analysed. Correlative network analysis (MetaCore) identified two predominant groups of Nrf2-regulated proteins. As expected, one group comprised proteins involved in phase II drug metabolism, which were down-regulated in the absence of Nrf2. Surprisingly, the most profound changes were observed amongst proteins involved in the synthesis and metabolism of fatty acids and other lipids. Importantly, we show here for the first time, that the enzyme ATP-citrate lyase, responsible for acetyl-CoA production, is negatively regulated by Nrf2. This latter finding suggests that Nrf2 is a major regulator of cellular lipid disposition in the liver.

Acute oxidative stress and systemic Nrf2 activation by the ketogenic diet

The mechanisms underlying the efficacy of the ketogenic diet (KD) remain unknown. Recently, we showed that the KD increased glutathione (GSH) biosynthesis. Since the NF E2-related factor 2 (Nrf2) transcription factor is a primary responder to cellular stress and can upregulate GSH biosynthesis, we asked whether the KD activates the Nrf2 pathway. Here we report that rats consuming a KD show acute production of H(2)O(2) from hippocampal mitochondria, which decreases below control levels by 3 weeks, suggestive of an adaptive response. 4-Hydroxy-2-nonenal (4-HNE), an electrophilic lipid peroxidation end product known to activate the Nrf2 detoxification pathway, was also acutely increased by the KD. Nrf2 nuclear accumulation was evident in both the hippocampus and liver, and the Nrf2 target, NAD(P)H:quinone oxidoreductase (NQO1), exhibited increased activity in both the hippocampus and liver after 3 weeks. We also found chronic depletion of liver tissue GSH, while liver mitochondrial antioxidant capacity was preserved. These data suggest that the KD initially produces mild oxidative and electrophilic stress, which may systemically activate the Nrf2 pathway via redox signaling, leading to chronic cellular adaptation, induction of protective proteins, and improvement of the mitochondrial redox state.

Differential effect of covalent protein modification and glutathione depletion on the transcriptional response of Nrf2 and NF-kappaB

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.

Increased susceptibility of Nrf2-null mice to 1-bromopropane-induced hepatotoxicity

1-Bromopropane (1-BP) was introduced as an alternative to ozone-depleting solvents. However, it was found to exhibit neurotoxicity, reproductive toxicity, and hepatotoxicity in rodents and neurotoxicity in human. However, the mechanisms underlying the toxicities of 1-BP remain elusive. The present study investigated the role of oxidative stress in 1-BP-induced hepatotoxicity using nuclear factor erythroid 2-related factor 2 (Nrf2)-null mice. Groups of 24 male Nrf2-null mice and 24 male wild-type (WT) C57BL/6J mice were each divided into three groups of eight and exposed to 1-BP at 0, 100, or 300 ppm for 8 h/day for 28 days by inhalation. Liver histopathology showed significantly larger area of necrosis in Nrf2-null mice relative to WT mice at the same exposure level. Nrf2-null mice also had greater malondialdehyde (MDA) levels, higher ratio of oxidized glutathione/reduced form of glutathione, and lower total glutathione content. The constitutive level and the increase in ratio per exposure level of glutathione S-transferase (GST) activity were lower in the liver of Nrf2-null mice than WT mice. Exposure to 1-BP at 300 ppm increased the messenger RNA levels of heme oxygenase-1 (HO-1), glutamate-cysteine ligase modifier subunit (GcLm), glutamate-cysteine synthetase (GcLc), glutathione reductase, and NAD(P)H: quinone oxidoreductase 1 (NQO1) in WT mice but not in Nrf2-null mice except for GST Yc2. Nrf2-null mice were more susceptible to 1-BP-induced hepatotoxicity. That oxidative stress plays a role in 1-BP hepatotoxicity is deduced from the low expression levels and activities of antioxidant enzymes and high MDA levels in Nrf2-null mice.

Silencing glycogen synthase kinase-3beta inhibits acetaminophen hepatotoxicity and attenuates JNK activation and loss of glutamate cysteine ligase and myeloid cell leukemia sequence 1

Previously we demonstrated that c-Jun N-terminal kinase (JNK) plays a central role in acetaminophen (APAP)-induced liver injury. In the current work, we examined other possible signaling pathways that may also contribute to APAP hepatotoxicity. APAP treatment to mice caused glycogen synthase kinase-3beta (GSK-3beta) activation and translocation to mitochondria during the initial phase of APAP-induced liver injury ( approximately 1 h). The silencing of GSK-3beta, but not Akt-2 (protein kinase B) or glycogen synthase kinase-3alpha (GSK-3alpha), using antisense significantly protected mice from APAP-induced liver injury. The silencing of GSK-3beta affected several key pathways important in conferring protection against APAP-induced liver injury. APAP treatment was observed to promote the loss of glutamate cysteine ligase (GCL, rate-limiting enzyme in GSH synthesis) in liver. The silencing of GSK-3beta decreased the loss of hepatic GCL, and promoted greater GSH recovery in liver following APAP treatment. Silencing JNK1 and -2 also prevented the loss of GCL. APAP treatment also resulted in GSK-3beta translocation to mitochondria and the degradation of myeloid cell leukemia sequence 1 (Mcl-1) in mitochondrial membranes in liver. The silencing of GSK-3beta reduced Mcl-1 degradation caused by APAP treatment. The silencing of GSK-3beta also resulted in an inhibition of the early phase (0-2 h), and blunted the late phase (after 4 h) of JNK activation and translocation to mitochondria in liver following APAP treatment. Taken together our results suggest that activation of GSK-3beta is a key mediator of the initial phase of APAP-induced liver injury through modulating GCL and Mcl-1 degradation, as well as JNK activation in liver.

Signal transduction pathways involved in drug-induced liver injury

Hepatocyte death following drug intake is the critical event in the clinical manifestation of drug-induced liver injury (DILI). Traditionally, hepatocyte death caused by drugs had been attributed to overwhelming oxidative stress and mitochondria dysfunction caused by reactive metabolites formed during drug metabolism. However, recent studies have also shown that signal transduction pathways activated/inhibited during oxidative stress play a key role in DILI. In acetaminophen (APAP)-induced liver injury, hepatocyte death requires the sustained activation of c-Jun kinase (JNK), a kinase important in mediating apoptotic and necrotic death. Inhibition of JNK using chemical inhibitors or knocking down JNK can prevent hepatocyte death even in the presence of extensive glutathione (GSH) depletion, covalent binding, and oxidative stress. Once activated, JNK translocates to mitochondria, to induce mitochondria permeability transition and trigger hepatocyte death. Mitochondria are central targets where prodeath kinases such as JNK, prosurvival death proteins such as bcl-xl, and oxidative damage converge to determine hepatocyte survival. The importance of mitochondria in DILI is also observed in the Mn-SOD heterozygous (+/-) model, where mice with less mitochondrial Mn-SOD are sensitized to liver injury caused by certain drugs. An extensive body of research is accumulating suggesting a central role of mitochondria in DILI. Drugs can also cause redox changes that inhibit important prosurvival pathways such as NF-kappaB. The inhibition of NF-kappaB by subtoxic doses of APAP sensitizes hepatocyte to the cytotoxic actions of tumor necrosis factor (TNF). Many drugs will induce liver injury if simultaneously treated with LPS, which promotes inflammation and cytokine release. Drugs may be sensitizing hepatocytes to the cytotoxic effects of cytokines such as TNF, or vice versa. Overall many signaling pathways are important in regulating DILI, and represent potential therapeutic targets to reduce liver injury caused by drugs.

Monocyclic aromatic amines as potential human carcinogens: old is new again

Alkylanilines are a group of chemicals whose ubiquitous presence in the environment is a result of the multitude of sources from which they originate. Exposure assessments indicate that most individuals experience lifelong exposure to these compounds. Many alkylanilines have biological activity similar to that of the carcinogenic multi-ring aromatic amines. This review provides an overview of human exposure and biological effects. It also describes recent investigations into the biochemical mechanisms of action that lead to the assessment that they are most probably more complex than those of the more extensively investigated multi-ring aromatic amines. Not only is nitrenium ion chemistry implicated in DNA damage by alkylanilines but also reactions involving quinone imines and perhaps reactive oxygen species. Recent results described here indicate that alkylanilines can be potent genotoxins for cultured mammalian cells when activated by exogenous or endogenous phase I and phase II xenobiotic-metabolizing enzymes. The nature of specific DNA damage products responsible for mutagenicity remains to be identified but evidence to date supports mechanisms of activation through obligatory N-hydroxylation as well as subsequent conjugation by sulfation and/or acetylation. A fuller understanding of the mechanisms of alkylaniline genotoxicity is expected to provide important insights into the environmental and genetic origins of one or more human cancers and may reveal a substantial role for this group of compounds as potential human chemical carcinogens.

Role of reactive metabolites in drug-induced hepatotoxicity

Drugs are generally converted to biologically inactive forms and eliminated from the body, principally by hepatic metabolism. However, certain drugs undergo biotransformation to metabolites that can interfere with cellular functions through their intrinsic chemical reactivity towards glutathione, leading to thiol depletion, and functionally critical macromolecules, resulting in reversible modification, irreversible adduct formation, and irreversible loss of activity. There is now a great deal of evidence which shows that reactive metabolites are formed from drugs known to cause hepatotoxicity, such as acetaminophen, tamoxifen, isoniazid, and amodiaquine. The main theme of this article is to review the evidence for chemically reactive metabolites being initiating factors for the multiple downstream biological events culminating in toxicity. The major objectives are to understand those idiosyncratic hepatotoxicities thought to be caused by chemically reactive metabolites and to define the role of toxic metabolites.

The keap1-nrf2 cellular defense pathway: mechanisms of regulation and role in protection against drug-induced toxicity

Adverse drug reactions pose a significant public health problem. In some cases, the process of drug metabolism can contribute to the onset of toxicity through the bioactivation of a parent molecule to a chemically reactive intermediate. In order to maintain a favorable balance between bioactivation and detoxification, mammalian cells have evolved an inducible cell defense system known as the antioxidant response pathway. The activity of this cytoprotective pathway is largely regulated by the transcription factor Nrf2, which governs the expression of many phase II detoxification and antioxidant enzymes. In turn, the activity of Nrf2 is regulated by the cysteine-rich cytosolic inhibitor Keap1, which acts as a "sensor" for chemical/oxidative stress. This article summarizes our current understanding of the molecular mechanisms that regulate the function of the Keap1-Nrf2 pathway and highlights the importance of Nrf2 in the protection against drug-induced toxicity.

Current concepts of mechanisms in drug-induced hepatotoxicity

Drug-induced liver injury (DILI) has become a leading cause of severe liver disease in Western countries and therefore poses a major clinical and regulatory challenge. Whereas previously drug-specific pathways leading to initial injury of liver cells were the main focus of mechanistic research and classifications, current concepts see these as initial upstream events and appreciate that subsequent common downstream pathways and their attenuation by drugs and other environmental and genetic factors also have a profound impact on the risk of an individual patient to develop overt liver disease. This review summarizes current mechanistic concepts of DILI in a 3-step model that limits its principle mechanisms to three main ways of initial injury, i.e. direct cell stress, direct mitochondrial impairment, and specific immune reactions. Subsequently, initial injury initiates further downstream events, i.e. direct and death receptor-mediated pathways leading to mitochondrial permeability transition, which then results in apoptotic or necrotic cell death. For all mechanisms, mitochondria play a central role in events leading to apoptotic vs. necrotic cell death. New treatment targets consequently focus on interference with downstream pathways that mediate injury and therefore determine the ultimate outcome of DILI. Genome wide and targeted pharmacogenetic as well as metabonomic approaches are now used in order to reach the key goals of a better understanding of mechanisms in hepatotoxicity, and to develop new strategies for its prediction and treatment. However, the complexity of interactions between genetic and environmental risk factors is considerable, and DILI therefore currently remains unpredictable for most hepatotoxins.

Disruption of the mGsta4 gene increases life span of C57BL mice

The lipid peroxidation product 4-hydroxynonenal (4-HNE) forms as a consequence of oxidative stress. By electrophilic attack on biological macromolecules, 4-HNE mediates signaling or may cause toxicity. A major route of 4-HNE disposal is via glutathione conjugation, in the mouse catalyzed primarily by glutathione transferase mGSTA4-4. Unexpectedly, mGsta4-null mice, in which 4-HNE detoxification is impaired, have an extended life span. This finding could be explained by the observed activation of the transcription factor Nrf2 in the knockout mice, which in turn leads to an induction of antioxidant and antielectrophilic defenses. Especially, the latter could provide a detoxification mechanism that contributes to enhanced longevity. We propose that disruption of 4-HNE conjugation elicits a hormetic response in which an initially increased supply of 4-HNE is translated into activation of Nrf2, leading to a new steady state in which the rise of 4-HNE concentrations is dampened, but life-extending detoxification mechanisms are concomitantly induced.

Regulation of tissue-specific carboxylesterase expression by pregnane x receptor and constitutive androstane receptor

The liver- and intestine-enriched carboxylesterase 2 (CES2) enzyme catalyzes the hydrolysis of several clinically important anticancer agents administered as prodrugs. For example, irinotecan, a carbamate prodrug used in the treatment of colorectal cancer, is biotransformed in vivo by CES2 in intestine and liver, thereby producing a potent topoisomerase I inhibitor. Pregnane X receptor (PXR) and constitutive androstane receptor (CAR), two members of the nuclear receptor superfamily of ligand-activated transcription factors, mediate gene activation in response to xenobiotic stress. Together, these receptors comprise a protective response in mammals that coordinately regulate hepatic transport, metabolism, and elimination of numerous xenobiotic compounds. In the present study, microarray analysis was used to identify PXR target genes in duodenum in mice. Here, we show that a gene encoding a member of the CES2 subtype of liver- and intestine-enriched CES enzymes, called Ces6, is induced after treatment with pregnenolone 16alpha-carbonitrile in a PXR-dependent manner in duodenum and liver in mice. Treatment of mice with the CAR activator 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene also induced expression of Ces6 in duodenum and liver in a CAR-dependent manner, whereas treatment with phenobarbital produced induction of Ces6 exclusively in liver. These data identify a key role for PXR and CAR in regulating the drug-inducible expression and activity of an important CES enzyme in vivo. Future studies should focus on determining whether these signaling pathways governing drug-inducible CES expression in intestine and liver are conserved in humans.

Effect of repeated administration with subtoxic doses of acetaminophen to rats on enterohepatic recirculation of a subsequent toxic dose

Development of resistance to toxic effects of acetaminophen (APAP) was reported in rodents and humans, though the mechanism is only partially understood. We examined in rats the effect of administration with subtoxic daily doses (0.2, 0.3, and 0.6g/kg, i.p.) of APAP on enterohepatic recirculation and liver toxicity of a subsequent i.p. toxic dose of 1g/kg, given 24h after APAP pre-treatment. APAP and its major metabolite APAP-glucuronide (APAP-Glu) were determined in bile, urine, serum and liver homogenate. APAP pre-treatment was not toxic, as determined by serum markers of liver damage and neither induced oxidative stress as demonstrated by assessment of ROS generation in liver or glutathione species in liver and bile. APAP pre-treatment induced a partial shift from biliary to urinary elimination of APAP-Glu after administration with the toxic dose, and decreased hepatic content and increased serum content of this conjugate, consistent with a marked up-regulation of its basolateral transporter Mrp3 relative to apical Mrp2. Preferential secretion of APAP-glu into blood decreased enterohepatic recirculation of APAP, thus attenuating liver exposition to the intact drug, as demonstrated 6h after administration with the toxic dose. The beneficial effect of interfering the enterohepatic recirculation was alternatively tested in animals receiving activated charcoal by gavage to adsorb APAP of biliary origin. The data indicated decreased liver APAP content and glutathione consumption. We conclude that selective up-regulation of Mrp3 expression by APAP pre-treatment may contribute to development of resistance to APAP hepatotoxicity, at least in part by decreasing its enterohepatic recirculation.

Understanding the role of reactive metabolites in drug-induced hepatotoxicity: state of the science

Drug-induced liver injury (DILI) represents a major impediment to the development of new drugs and is a leading cause of drug withdrawal. The occurrence of hepatotoxicity has been closely associated with the formation of chemically reactive metabolites. Huge investment has focused on the screening of chemically reactive metabolites to offer a pragmatic approach to produce safer drugs and also reduce drug attrition and prevent market place withdrawal. However, questions surrounding the importance of chemically reactive metabolites still remain. Increasing evidence now exists for the multi-factorial nature of DILI, in particular the role played by the host immune system or disease state in the pathogenesis of DILI. This review aims to evaluate the current measures for the prediction and diagnosis of DILI and to highlight investigations being made to understand the multidimensional nature. Some of the steps being made to generate improved physiological systems to identify more sensitive, reflective mechanism-based biomarkers to aid the earlier identification of DILI and develop safer medicines are also discussed.

Reduced acetaminophen-induced liver injury in mice by genetic disruption of IL-1 receptor antagonist

Acetaminophen (APAP) induced increases in intrahepatic expression of interleukin (IL)-1 alpha, IL-1 beta, and IL-1 receptor antagonist (IL-1ra), when administered intraperitoneally. These observations prompted us to define the pathophysiological roles of IL-1ra in APAP-induced liver injury. Compared with wild-type (WT) mouse-derived hepatocytes, IL-1ra-deficient (IL-1ra KO)-derived hepatocytes exhibited more resistance against APAP but not APAP-derived major toxic metabolite, N-acetyl-p-benzoquinone imine (NAPQI). Moreover, the amounts of a major APAP adduct (selenium-binding protein), an indicator of NAPQI generation from APAP, was significantly lower in IL-1ra KO mice than WT mice with depressed intrahepatic expression of CYP1A2, CYP2E1, and CYP3A11, the enzymes crucially involved in NAPQI generation from APAP. These observations would indicate that IL-1ra deficiency impaired APAP metabolism. IL-1 alpha and IL-1 beta were expressed to similar extents in livers of untreated IL-1ra KO and WT mice. By contrast, the intranuclear amount of p65 of NF-kappaB, which can suppress the gene expression of CYP1A2, CYP2E1, and CYP3A11, was higher in untreated IL-1ra KO than WT mice. Moreover, when mice were intraperitoneally administered APAP (200 mg/kg), IL-1ra KO mice exhibited attenuated APAP-induced liver injury as evidenced by reductions in serum alanine transferase levels and histopathological changes such as centrilobular necrosis, hemorrhages, and leukocyte infiltration. Finally, when given 12 h before APAP challenge, IL-1 alpha repressed the intrahepatic expression of CYP1A2, CYP2E1, and CYP3A11, eventually reducing APAP-induced liver injury, along with reduction in APAP adducts. Collectively, NF-kappaB was activated without any stimuli by the genetic disruption of IL-1ra, and suppressed cytochrome P450 enzyme expression, thereby reducing APAP-induced liver injury.

The hepatotoxic metabolite of acetaminophen directly activates the Keap1-Nrf2 cell defense system

The transcription factor Nrf2 regulates the expression of numerous cytoprotective genes in mammalian cells. We have demonstrated previously that acetaminophen activates Nrf2 in mouse liver following administration of non-hepatotoxic and hepatotoxic doses in vivo, implying that Nrf2 may have an important role in the protection against drug-induced liver injury. Nrf2 activation has been proposed to occur through the modification of cysteine residues within Keap1, the cytosolic repressor of Nrf2. We hypothesized that acetaminophen activates Nrf2 via the formation of its reactive metabolite N-acetyl-p-benzoquinoneimine (NAPQI), which may disrupt the repression of Nrf2 through the modification of cysteine residues within Keap1. Here, we show that NAPQI can directly activate the Nrf2 pathway in mouse liver cells, inducing an adaptive defense response that is antagonized by RNA interference targeted against Nrf2. Furthermore, mass spectrometric analysis shows that NAPQI selectively modifies cysteine residues in Keap1, both in recombinant protein in vitro and in cells ectopically expressing Keap1. Using this cell-based model, we demonstrate that activation of Nrf2 by NAPQI and a panel of probe molecules [dexamethasone 21-mesylate, 15-deoxy-Delta-((12,14))-prostaglandin J(2), 2,4-dinitrochlorobenzene, and iodoacetamide] correlates with the selective modification of cysteine residues located within the intervening region of Keap1. However, substantial depletion of glutathione (to less than 15% of basal levels) by buthionine sulfoximine, which does not directly modify Keap1, is also sufficient to activate Nrf2.

CONCLUSION

Nrf2 can be activated via the direct modification of cysteine residues located within the intervening region of Keap1, but also via the substantial depletion of glutathione without the requirement for direct modification of Keap1. It is possible that both of these mechanisms contribute to the activation of Nrf2 by acetaminophen.

Protective role of c-Jun N-terminal kinase 2 in acetaminophen-induced liver injury

Recent studies in mice suggest that stress-activated c-Jun N-terminal protein kinase 2 (JNK2) plays a pathologic role in acetaminophen (APAP)-induced liver injury (AILI), a major cause of acute liver failure (ALF). In contrast, we present evidence that JNK2 can have a protective role against AILI. When male C57BL/6J wild type (WT) and JNK2(-/-) mice were treated with 300mg APAP/kg, 90% of JNK2(-/-) mice died of ALF compared to 20% of WT mice within 48h. The high susceptibility of JNK2(-/-) mice to AILI appears to be due in part to deficiencies in hepatocyte proliferation and repair. Therefore, our findings are consistent with JNK2 signaling playing a protective role in AILI and further suggest that the use of JNK inhibitors as a potential treatment for AILI, as has been recommended by other investigators, should be reconsidered.

Up-regulation of hepatic heme oxygenase-1 expression by locally induced interleukin-6 in rats administered carbon tetrachloride intraperitoneally

We previously reported that interleukin-6 (IL-6) was locally produced in the early period after intraperitoneal (i.p.) or subcutaneous carbon tetrachloride (CCl4) administration, but not after oral (p.o.) administration. In the present study, we focused on the up-regulation of stress-inducible proteins induced by IL-6 after i.p. CCl4 administration. The expression of heme oxygenase-1 (HO-1) (EC 1.14.99.3) mRNA and protein were induced more in rats administered CCl4 via the i.p. route, compared with the p.o. route; however, expression of heat shock protein (HSP) 72 and HSP90 mRNA were increased to similar extents in both experimental groups. The induction of HO-1 mRNA and protein after i.p. CCl4 administration were significantly reduced after pretreatment with anti-rat IL-6 antibody. Activation of the signal transducer and activator of transcription factor 3 (STAT3), which promotes HO-1 expression, peaked together with plasma levels of IL-6 after i.p. CCl4 administration, suggesting that hepatic HO-1 expression was increased by IL-6 via the Janus kinase/STAT3 pathway. The present data indicate that hepatic HO-1 is up-regulated by endogenously produced IL-6, in addition to its up-regulation by heme derived from cytochrome P450 which has already been reported in rats administered i.p. CCl4. The up-regulation of hepatic HO-1 expression may reduce the tissue injury to livers caused by CCl4.

alpha(1)-Adrenoceptor antagonists prevent paracetamol-induced hepatotoxicity in mice

BACKGROUND AND PURPOSE

Paracetamol, a major cause of acute liver failure (ALF) represents a significant clinical problem. Adrenoceptor stimulation or antagonism can modulate chemical-induced hepatotoxicity. We investigated the role of endogenous catecholamines and alpha(1)-adrenoceptors in the development of paracetamol- induced hepatotoxicity.

EXPERIMENTAL APPROACH

Paracetamol (3.5 mmol kg(-1)) was administered to male CD-1 mice, with and without alpha(1)-adrenoceptor antagonists (prazosin, doxazosin, terazosin and tamsulosin; 35.7 micromol kg(-1)). Serum transaminases and hepatic glutathione (GSH) levels were assessed as markers of hepatic damage. Paracetamol bioactivation was assessed by covalent binding, hepatic and urinary conjugate formation and uridine glucuronosyltransferase activity. Plasma catecholamines levels and hepatic congestion were also analysed.

KEY RESULTS

Plasma catecholamine levels were significantly elevated 5 h post paracetamol administration. Prazosin prevented hepatotoxicity when administered 1 h before a toxic paracetamol insult and importantly, when administered up to 1 h post paracetamol injection. Prazosin had no effect on paracetamol-induced depletion of hepatic GSH, paracetamol bioactivation or paracetamol-induced transcription of defence genes. Paracetamol toxicity is associated with marked accumulation of erythrocytes within hepatic sinusoids and prazosin completely prevented this accumulation.

CONCLUSION AND IMPLICATIONS

Paracetamol-induced hepatocellular damage is associated with increased circulating catecholamines. alpha(1)-Adrenoceptor antagonists conferred complete protection from paracetamol -induced hepatotoxicity. Protection was associated with absence of hepatic erythrocyte accumulation. Increased catecholamine levels may contribute to the pathophysiology of paracetamol-induced hepatotoxicity by compromising hepatic perfusion. Protection against paracetamol toxicity by alpha(1) antagonists in mice has implications for therapeutic management of patients presenting with paracetamol overdose and ALF.

Nrf2 signaling: an adaptive response pathway for protection against environmental toxic insults

Human exposures to environmental toxicants have been associated with development of a number of diseases. Animal experiments have identified a number of cytoprotective enzymes under the transcriptional control of NF-E2-related factor 2 (Nrf2) including electrophile conjugation and antioxidative enzymes and enzymes responsible for the production of antioxidants, reducing equivalents and cofactors. The up-regulation of these enzymes represents an adaptive response which occurs in the face of exposure to electrophilic or oxidative compounds thereby leading to enhanced metabolism of these molecules or their reactive metabolites. This adaptive response is regulated by an interaction between Keap1 and Nrf2 in which the exposure to reactive molecules is sensed either directly by Keap1 or indirectly by cellular signaling cascades resulting in activation of Nrf2 transcriptional regulation. The Nrf2-mediated adaptive response has been shown to attenuate toxicity and carcinogenesis during electrophile or oxidative stress as well as inflammation in rodent models. The cytoprotective attributes of the Nrf2 signaling pathway have been targeted for chemoprevention as administration of Nrf2-inducing agents has been shown to result in decreased carcinogenesis in animal models and altered carcinogen metabolism in humans. On the other hand, polymorphisms in the Nrf2 signaling pathway can lead to differential susceptibility to disease while mutations in the Nrf2 signaling pathway have been shown to an effective mechanism for cancer cells to evade chemotherapy. Overall, the Nrf2 cytoprotective adaptive response has evolved to be a powerful protective strategy for organisms against exposure to environmental toxicants and may provide insight into differential disease susceptibilities across populations and responses to therapies designed to alleviate these conditions.

The Nrf2-Keap1 defence pathway: role in protection against drug-induced toxicity

The metabolic biotransformation of xenobiotics to chemically reactive metabolites can, in some instances, underlie the pathogenesis of certain adverse drug reactions, due to the development of chemical or oxidative stress. In order to guard against such stresses, mammalian cells have evolved multi-faceted, highly-regulated defence systems, one of the most important being that which is regulated by the transcription factor Nrf2. Through regulating the expression of numerous cytoprotective genes, Nrf2 serves as a critical determinant of a cell's capacity to survive, or succumb, to a toxic insult. The aim of this review is to summarise our current understanding of the biochemistry that underlies the Nrf2 defence pathway, and highlight the important role of this transcription factor in the protection against drug-induced toxicity, primarily through the examination of recent investigations that have demonstrated an increased vulnerability to various toxins in animals lacking Nrf2.

Oxidative and electrophilic stress induces multidrug resistance-associated protein transporters via the nuclear factor-E2-related factor-2 transcriptional pathway

Multidrug resistance-associated proteins (Mrps) are adenosine triphosphate-dependent transporters that efflux chemicals out of cells. In the liver, Mrp2 transports bilirubin-glucuronide, glutathione (GSH), and drug conjugates into bile, whereas Mrp3 and Mrp4 efflux these entities into blood. The purpose of this study was to determine whether oxidative conditions (that is, the disruption of hepatic GSH synthesis) or the administration of nuclear factor-E2-related factor-2 (Nrf2) activators (oltipraz and butylated hydroxyanisole) can induce hepatic Mrp transporters and whether that induction is through the Nrf2 transcriptional pathway. Livers from hepatocyte-specific glutamate-cysteine ligase catalytic subunit-null mice had increased nuclear Nrf2 levels, marked gene and protein induction of the Nrf2 target gene NAD(P)H:quinone oxidoreductase 1, as well as Mrp2, Mrp3, and Mrp4 expression. The treatment of wild-type and Nrf2-null mice with oltipraz and butylated hydroxyanisole demonstrated that the induction of Mrp2, Mrp3, and Mrp4 is Nrf2-dependent. In Hepa1c1c7 cells treated with the Nrf2 activator tert-butyl hydroquinone, chromatin immunoprecipitation with Nrf2 antibodies revealed the binding of Nrf2 to antioxidant response elements in the promoter regions of mouse Mrp2 [-185 base pairs (bp)], Mrp3 (-9919 bp), and Mrp4 (-3767 bp).

CONCLUSION

The activation of the Nrf2 regulatory pathway stimulates the coordinated induction of hepatic Mrps.

Investigation of the effect of a panel of model hepatotoxins on the Nrf2-Keap1 defence response pathway in CD-1 mice

The Keap1-Nrf2-ARE signalling pathway has emerged as an important regulator of the mammalian defence system to enable detoxification and clearance of foreign chemicals. Recent studies by our group using paracetamol (APAP), diethylmaleate and buthionine sulphoximine have shown that for a given xenobiotic molecule, Nrf2 induction in the murine liver is associated with protein reactivity and glutathione depletion. Here, we have investigated, in vivo, whether the ability of four murine hepatotoxins, paracetamol, bromobenzene (BB), carbon tetrachloride (CCl4) and furosemide (FS) to deplete hepatic glutathione (GSH) is related to induction of hepatic Nrf2 nuclear translocation and Nrf2-dependent gene expression. Additionally, we studied whether hepatic Nrf2 nuclear translocation is a general response during the early stages of acute hepatic chemical stress in vivo. Male CD-1 mice were administered APAP (3.5 mmol/kg), FS (1.21 mmol/kg), BB (4.8 mmol/kg) and CCl4 (1 mmol/kg) for 1, 5 and 24h. Each compound elicited significant serum ALT increases after 24h (ALT U/L: APAP, 3036+/-1462; BB, 5308+/-2210; CCl4, 5089+/-1665; FS, 2301+/-1053), accompanied by centrilobular damage as assessed by histopathology. Treatment with APAP also elicited toxicity at a much earlier time point (5h) than the other hepatotoxins (ALT U/L: APAP, 1780+/-661; BB, 161+/-15; CCl4, 90+/-23; FS, 136+/-27). Significant GSH depletion was seen with APAP (9.6+/-1.7% of control levels) and BB (52.8+/-6.2% of control levels) 1h after administration, but not with FS and CCl4. Western Blot analysis revealed an increase in nuclear Nrf2, 1h after administration of BB (209+/-10% control), CCl4 (146+/-3% control) and FS (254+/-41% control), however this was significantly lower than the levels observed in the APAP-treated mice (462+/-36% control). The levels of Nrf2-dependent gene induction were also analysed by quantitative real-time PCR and Western blotting. Treatment with APAP for 1h caused a significant increase in the levels of haem oxygenase-1 (HO-1; 2.85-fold) and glutamate cysteine ligase (GCLC; 1.62-fold) mRNA. BB and FS did not affect the mRNA levels of either gene after 1h of treatment; however CCl4 significantly increased HO-1 mRNA at this time point. After 24h treatment with the hepatotoxins, there was evidence for the initiation of a late defence response. BB significantly increased both HO-1 and GCLC protein at this time point, CCl4 increased GCLC protein alone, although FS did not alter either of these proteins. In summary, we have demonstrated that the hepatotoxins BB, CCl4 and FS can induce a small but significant increase in Nrf2 accumulation in hepatic nuclei. However, this was associated with modest changes in hepatic GSH, a delayed development of toxicity and was insufficient to activate an early functional adaptive response to these hepatotoxins.

Emerging role of Nrf2 in protecting against hepatic and gastrointestinal disease

Transcription factor NF-E2-related factor 2 (Nrf2) belongs to the basic region-leucine zipper family and is activated in response to electrophiles and reactive oxygen species. Nrf2 coordinately regulates the constitutive and inducible transcription of a wide array of genes involved in drug metabolism, detoxification, and antioxidant defenses. During periods of oxidative stress, Nrf2 is released from sequestration in the cytoplasm and translocates to the nucleus. Nrf2 binds antioxidant response elements (AREs) in the regulatory regions of target genes and activates transcription. Genetically modified mice lacking Nrf2 serve as a useful tool for identifying new ARE-regulated genes and assessing the ability of Nrf2 to confer protection against a variety of pathologies in numerous organs including the liver, intestine, lung, skin, and nervous system. With regards to the liver and gastrointestinal tract, Nrf2 knockout mice are more susceptible to acetaminophen-induced hepatocellular injury, benzo[a]pyrene-induced tumor formation and Fas- and TNFalpha -mediated hepatocellular apoptosis. The higher sensitivity of Nrf2 knockout mice to chemical toxicity is due in part to reduced basal and inducible expression of detoxification enzymes. Nrf2 may also be important in protecting against liver fibrosis, gallstone development, and formation of aberrant crypt foci. Research of Nrf2 has opened up new opportunities in understanding how antioxidant defense pathways are regulated, how oxidative stress contributes to disease progression and may serve as a novel target for designing therapies to prevent and treat diseases in which oxidative stress is implicated.

Induction of Mrp3 and Mrp4 transporters during acetaminophen hepatotoxicity is dependent on Nrf2

The transcription factor NFE2-related factor 2 (Nrf2) mediates detoxification and antioxidant gene transcription following electrophile exposure and oxidative stress. Mice deficient in Nrf2 (Nrf2-null) are highly susceptible to acetaminophen (APAP) hepatotoxicity and exhibit lower basal and inducible expression of cytoprotective genes, including NADPH quinone oxidoreductase 1 (Nqo1) and glutamate cysteine ligase (catalytic subunit, or Gclc). Administration of toxic APAP doses to C57BL/6J mice generates electrophilic stress and subsequently increases levels of hepatic Nqo1, Gclc and the efflux multidrug resistance-associated protein transporters 1-4 (Mrp1-4). It was hypothesized that induction of hepatic Mrp1-4 expression following APAP is Nrf2 dependent. Plasma and livers from wild-type (WT) and Nrf2-null mice were collected 4, 24 and 48 h after APAP. As expected, hepatotoxicity was greater in Nrf2-null compared to WT mice. Gene and protein expression of Mrp1-4 and the Nrf2 targets, Nqo1 and Gclc, was measured. Induction of Nqo1 and Gclc mRNA and protein after APAP was dependent on Nrf2 expression. Similarly, APAP treatment increased hepatic Mrp3 and Mrp4 mRNA and protein in WT, but not Nrf2-null mice. Mrp1 was induced in both genotypes after APAP, suggesting that elevated expression of this transporter was independent of Nrf2. Mrp2 was not induced in either genotype at the mRNA or protein levels. These results show that Nrf2 mediates induction of Mrp3 and Mrp4 after APAP but does not affect Mrp1 or Mrp2. Thus coordinated regulation of detoxification enzymes and transporters by Nrf2 during APAP hepatotoxicity is a mechanism by which hepatocytes may limit intracellular accumulation of potentially toxic chemicals.

Mechanisms of drug-induced liver disease

Drug-induced liver injury depends initially on development of hepatocyte stress and cell death, which can be induced directly by parent drugs or by toxic metabolites. Hepatocyte stress can lead to activation of built-in death programs for apoptosis or necrosis. Subsequently, the innate immune system's participation is recruited. The interplay between proinflammatory and anti-inflammatory components of innate immune system determines the outcome of drug-induced liver injury. Both environmental factors and genetic differences in cellular responses to stress and the innate immune response may account for different susceptibilities between individuals to drug-induced liver injury.

Molecular mechanism of human Nrf2 activation and degradation: role of sequential phosphorylation by protein kinase CK2

Nrf2 is a key transcription factor in the cellular response to oxidative stress. In this study we identify two phosphorylated forms of endogenous human Nrf2 after chemically induced oxidative stress and provide evidence that protein kinase CK2-mediated sequential phosphorylation plays potential roles in Nrf2 activation and degradation. Human Nrf2 has a predicted molecular mass of 66 kDa. However, immunoblots showed that two bands at 98 and 118 kDa, which are identified as phosphorylated forms, are increased in response to Nrf2 inducers. In addition, human Nrf2 was found to be a substrate for CK2 which mediated two steps of phosphorylation, resulting in two forms of Nrf2 migrating with differing M(r) at 98 kDa (Nrf2-98) and 118 kDa (Nrf2-118). Our results support a role in which calmodulin binding regulates CK2 activity, in that cold (25 degrees C) Ca(2+)-free media (cold/Ca(2+)-free) decreased both cellular calcium levels and CK2-calmodulin binding and induced Nrf2-118 formation, the latter of which was prevented by CK2-specific inhibitors. Gel shift assays showed that the Nrf2-118 generated under cold/Ca(2+)-free conditions does not bind to the antioxidant response element, indicating that Nrf2-98 has transcriptional activity. In contrast, Nrf2-118 is more susceptible to degradation. These results provide evidence for phosphorylation by CK2 as a critical controlling factor in Nrf2-mediated cellular antioxidant response.

The Metabolism and Toxicity of Furosemide in the Wistar Rat and CD-1 Mouse: a Chemical and Biochemical Definition of the Toxicophore

Furosemide, a loop diuretic, causes hepatic necrosis in mice. Previous evidence suggested hepatotoxicity arises from metabolic bioactivation to a chemically reactive metabolite that binds to hepatic proteins. To define the nature of the toxic metabolite, we examined the relationship between furosemide metabolism in CD-1 mice and Wistar rats. Furosemide (1.21 mmol/kg) was shown to cause toxicity in mice, but not rats, at 24 h, without resulting in glutathione depletion. In vivo covalent binding to hepatic protein was 6-fold higher in the mouse (1.57 +/- 0.98 nmol equivalent bound/mg protein) than rat (0.26 +/- 0.13 nmol equivalent bound/mg protein). In vivo covalent binding to mouse hepatic protein was reduced 14-fold by a predose of the cytochrome P450 (P450) inhibitor, 1-aminobenzotriazole (ABT; 0.11 +/- 0.04 nmol equivalent bound/mg protein), which also reduced hepatotoxicity. Administration of [(14)C]furosemide to bile duct-cannulated rats demonstrated turnover to glutathione conjugate (8.8 +/- 2.8%), gamma-ketocarboxylic acid metabolite (22.1 +/- 3.3%), N-dealkylated metabolite (21.1 +/- 2.9%), and furosemide glucuronide (12.8 +/- 1.8%). Furosemide-glutathione conjugate was not observed in bile from mice dosed with [(14)C]furosemide. The novel gamma-ketocarboxylic acid, identified by nuclear magnetic resonance spectroscopy, indicates bioactivation of the furan ring. Formation of gamma-ketocarboxylic acid was P450-dependent. In mouse liver microsomes, a gamma-ketoenal furosemide metabolite was trapped, forming an N-acetylcysteine/N-acetyl lysine furosemide adduct. Furosemide (1 mM, 6 h) became irreversibly bound to primary mouse and rat hepatocytes, 0.73 +/- 0.1 and 2.44 +/- 0.3 nmol equivalent bound/mg protein, respectively, which was significantly reduced in the presence of ABT, 0.11 +/- 0.03 and 0.21 +/- 0.1 nmol equivalent bound/mg protein, respectively. Furan rings are part of new chemical entities, and mechanisms underlying species differences in toxicity are important to understand to decrease the drug attrition rate.

Extract of Ginkgo biloba induces phase 2 genes through Keap1-Nrf2-ARE signaling pathway

The standard extract of Ginkgo biloba (EGb) has been demonstrated to possess remarkable antioxidant activity in both cell lines and animals. However, the molecular mechanism underlying this effect is not fully understood. Phase 2 enzymes play important roles in the antioxidant system by reducing electrophiles and reactive oxygen species (ROS). We demonstrated that EGb induced typical phase 2 genes: glutamate cysteine ligase catalytic subunit (GCLC) and glutathione-S-transferase subunit-P1 (GST-P1), by real-time PCR. To investigate the molecular mechanism of this induction, we used quinone oxidoreductase 1 (NQO1) -- Antioxidant response element (ARE) reporter assay and found that EGb activated the activity of the wild type but not the one with ARE mutated. It indicated that EGb induced these genes through ARE, a cis-acting motif located in the promoter region of nearly all phase 2 genes. Since nuclear factor erythroid 2-related factor 2 (Nrf2) binds ARE to enhance the expression of phase 2 genes, we detected the Nrf2 content in nucleus and found an accumulation of Nrf2 stimulated by EGb. In a further test of Kelch-like ECH-associated protein 1 (Keap1), the repression protein of Nrf2 in the cytosol under resting condition, we found that Keap1 content was inhibited by EGb and then more Nrf2 would be released to translocate into nucleus. Thus, EGb was testified for the first time to induce the phase 2 genes through the Keap1-Nrf2-ARE signaling pathway, which is (or part of) the antioxidant mechanism of EGb.

Heme oxygenase-1 as a potential therapeutic target for hepatoprotection

Heme oxygenase (HO), the rate limiting enzyme in the breakdown of heme into carbon monoxide (CO), iron and bilirubin, has recently received overwhelming research attention. To date three mammalian HO isozymes have been identified, and the only inducible form is HO-1 while HO-2 and HO-3 are constitutively expressed. Advances in unveiling signal transduction network indicate that a battery of redox-sensitive transcription factors, such as activator protein-1 (AP-1), nuclear factor-kappa B (NF-kappaB) and nuclear factor E2-related factor-2 (Nrf2), and their upstream kinases including mitogen-activated protein kinases play an important regulatory role in HO-1 gene induction. The products of the HO-catalyzed reaction, particularly CO and biliverdin/bilirubin have been shown to exert protective effects in several organs against oxidative and other noxious stimuli. In this context, it is interesting to note that induction of HO-1 expression contributes to protection against liver damage induced by several chemical compounds such as acetaminophen, carbon tetrachloride and heavy metals, suggesting HO-1 induction as an important cellular endeavor for hepatoprotection. The focus of this review is on the significance of targeted induction of HO-1 as a potential therapeutic strategy to protect against chemically-induced liver injury as well as hepatocarcinogenesis.

Selection of new chemical entities with decreased potential for adverse drug reactions

Adverse drug reactions, such as hepatotoxicity, blood dyscrasias and hypersensitivity are a major obstacle for the use and the development of new medicines. Many forms of organ-directed toxicity can arise from the bioactivation of drugs to so-called chemically reactive metabolites, which can modify tissue macromolecules. It is well established that the toxicities of model hepatotoxins, such as acetaminophen, furosemide, bromobenzene and methapyrilene can be correlated with the generation of chemically reactive metabolites, which can be detected by measurement of the irreversible binding of radiolabelled material to hepatic protein and/or the detection of stable phase II metabolites such as glutathione conjugates. The basic chemistry of the reaction of such metabolites with model nucleophiles is relatively well understood. A major challenge is to define how certain reactive intermediates may chemically modify critical proteins and how modification of specific amino acids may alter protein function which in turn may affect cell signalling, regulation, defence, function and viability. This in turn will determine whether or not bioactivation will result in a particular form of drug-induced injury. It is now clear that even relatively simple reactive intermediates can react in a discriminative manner with particular cellular proteins and even with specific amino acids within those proteins. Therefore both non-covalent, as well as covalent bonds will be important determinants of the target protein for a particular reactive metabolite. Mammalian cells have evolved numerous defence systems against reactive intermediates. Sensitive redox proteins such as Nrf-2 recognize oxidative stress and electrophilic agents. This is achieved by chemical modification of cysteine groups within keap-1, which normally forms an inactive heterodimer with Nrf-2. Modification of keap-1 releases Nrf-2 that translocates to the nucleus and effects gene transcription of a number of genes involved in the detoxication of chemically reactive metabolites. Diminution of protein function can occur by either covalent modification of nucleophilic amino acids (e.g. cysteine, lysine, histidine etc.) or oxidation of thiols, which can be reversible or irreversible. In the case of acetaminophen, more than 30 target proteins have been identified and for several of them, corresponding alterations in protein function have been defined in the context of tissue necrosis. Alternatively, protein modification may induce signalling systems which initiate cell death, an immune response or to an altered tissue genotype.