Essential role of phosphatidylinositol 3-kinase-dependent CCAAT/enhancer binding protein beta activation in the induction of glutathione S-transferase by oltipraz

Potential Mechanisms by which Glucocorticoids Induce Breast Carcinogenesis through Nrf2 Inhibition

Breast cancer is the most common malignancy among women worldwide. Several studies indicate that, in addition to established risk factors for breast cancer, other factors such as cortisol release related to psychological stress and drug treatment with high levels of glucocorticoids may also contribute significantly to the initiation of breast cancer. There are several possible mechanisms by which glucocorticoids might promote neoplastic transformation of breast tissue. Among these, the least known and studied is the inhibition of the nuclear erythroid factor 2-related (Nrf2)-antioxidant/electrophile response element (ARE/EpRE) pathway by high levels of glucocorticoids. Specifically, Nrf2 is a potent transcriptional activator that plays a central role in the basal and inducible expression of many cytoprotective genes that effectively protect mammalian cells from various forms of stress and reduce the propensity of tissues and organisms to develop disease or malignancy including breast cancer. Consequently, a loss of Nrf2 in response to high levels of gluco-corticoids may lead to a decrease in cellular defense against oxidative stress, which plays an important role in the initiation of human mammary carcinogenesis. In the present review, we provide a comprehensive overview of the current state of knowledge of the cellular mechanisms by which both glucocorticoid pharmacotherapy and endogenous GCs (cortisol in humans and corticosterone in rodents) may contribute to breast cancer development through inhibition of the Nrf2-ARE/EpRE pathway and the protective role of melatonin against glucocorticoid-induced apoptosis in the immune system.

Pinocembrin-7-Methylether Protects SH-SY5Y Cells Against 6-Hydroxydopamine-Induced Neurotoxicity via Modulating Nrf2 Induction Through AKT and ERK Pathways

The present study aimed to evaluate the neuroprotective effects and underlying mechanisms of pinocembrin-7-methylether (PME), a natural bioflavonoid, in 6-hydroxydopamine (6-OHDA)-induced models of Parkinson's disease in vivo and in vitro. First, we found that PME decreased apoptosis in 6-OHDA-intoxicated SH-SY5Y cells. PME also blocked several 6-OHDA-induced mitochondrial apoptotic cascades, including loss of mitochondrial membrane potential, caspase 3 and PARP activation, and a decrease in the Bcl-2/Bax ratio. Also, PME suppressed 6-OHDA-induced oxidative stress while increasing antioxidant enzymatic activity. Further investigations indicated that PME significantly enhanced nuclear accumulation of Nrf2, improved ARE promoter activity, and upregulated HO-1 and NQO1 expression levels. In addition, siRNA-mediated Nrf2 knockdown abolished PME-induced anti-oxidative and anti-apoptotic effects. Interestingly, we found that PME promoted phosphorylation of AKT and ERK, whereas pharmacological inhibition of AKT or ERK pathways diminished PME-induced Nrf2 activation and protective actions. Moreover, PME attenuated 6-OHDA-induced loss of dopaminergic neurons and ameliorated locomotor deficiency in zebrafish, supporting the neuroprotective actions of PME in vivo. In summary, we found that PME conferred neuroprotection against 6-OHDA-induced neurotoxicity in PD models in vivo and in vitro. Taken together, our findings suggest that activation of Nrf2/ARE/HO-1 signaling cascades contributes to PME-induced anti-oxidative and neuroprotective actions, which are at least partially mediated by AKT and ERK pathways.

Reactive oxygen species as potential antiviral targets

Reactive oxygen species (ROS) are by-products of cellular metabolism and can be either beneficial, at low levels, or deleterious, at high levels, to the cell. It is known that several viral infections can increase oxidative stress, which is mainly facilitated by viral-induced imbalances in the antioxidant defence mechanisms of the cell. While the exact role of ROS in certain viral infections (adenovirus and dengue virus) remains unknown, other viruses can use ROS for enhancement of pathogenesis (SARS coronavirus and rabies virus) or replication (rhinovirus, West Nile virus and vesicular stomatitis virus) or both (hepatitis C virus, human immunodeficiency virus and influenza virus). While several viral proteins (mainly for hepatitis C and human immunodeficiency virus) have been identified to play a role in ROS formation, most mediators of viral ROS modulation are yet to be elucidated. Treatment of viral infections, including hepatitis C virus, human immunodeficiency virus and influenza virus, with ROS inhibitors has shown a decrease in both pathogenesis and viral replication both in vitro and in animal models. Clinical studies indicating the potential for targeting ROS-producing pathways as possible broad-spectrum antiviral targets should be evaluated in randomized controlled trials.

Effects of C2-Ceramide and Oltipraz on Hepatocyte Nuclear Factor-1 and Glutathione S-Transferase A1 in Acetaminophen-Mediated Acute Mice Liver Injury

In this study, acetaminophen (APAP)-induced acute liver injury mice model was used to investigate the effects of C2-ceramide and oltipraz on hepatocyte nuclear factor 1 (HNF-1) and glutathione S-transferase A1 (GSTA1). Notably, C2-ceramide caused alteration in mice serum transaminases and liver tissue indexes, and aggravated hepatic injury, while oltipraz alleviated hepatic injury. By screening, the optimal concentrations of C2-ceramide and oltipraz were confirmed to be 120 and 150 μmol/L, respectively. In histopathology, karyolysis and more necrotic cells and bleeding spots were appeared on administration of C2-ceramide, but only a small amount of inflammatory cells infiltration was seen after oltipraz treatment. In addition, RT-PCR and western blot results revealed that the mRNA and protein expression levels of HNF-1 and GSTA1 in liver were significantly decreased (p < 0.01) with the administration of 120 μmol/L C2-ceramide. Meanwhile, GSTA1 content in serum increased up to 1.27-fold. In contrast, 150 μmol/L oltipraz incorporation to APAP model mice resulted in obvious elevation (p < 0.01) in the mRNA and protein expression levels of HNF-1 and GSTA1 in liver, and serum GSTA1 content decreased up to 0.77-fold. In conclusion, C2-ceramide could down-regulate the expression of HNF-1 and GSTA1 which exacerbated hepatic injury, while oltipraz could up-regulate the expression of HNF-1 and GSTA1 which mitigated hepatic injury.

Pinostrobin Exerts Neuroprotective Actions in Neurotoxin-Induced Parkinson's Disease Models through Nrf2 Induction

The aim of the present study was to assess the neuroprotective effects of pinostrobin (PSB), a dietary bioflavonoid, and its underlying mechanisms in neurotoxin-induced Parkinson's disease (PD) models. First, PSB could attenuate 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced loss of dopaminergic neurons and improve behavior deficiency in zebrafish, supporting its potential neuroprotective actions in vivo. Next, PSB could decreased apoptosis and death in the 1-methyl-4-phenylpyridinium (MPP⁺)-intoxicated SH-SY5Y cells, evidenced by MTT, LDH, Annexin V-FITC/PI, and DNA fragmentation assay. PSB also blocked MPP⁺-induced apoptotic cascades, including loss of mitochondrial membrane potential, activation of caspase 3, and reduced ratio of Bcl-2/Bax. In addition, PSB suppressed MPP⁺-induced oxidative stress but increased antioxidant enzymes, evidenced by decrease of reactive oxygen species generation and lipid peroxidation and up-regulation of GSH-Px, SOD, CAT, GSH/GSSG, and NAD/NADH. Further investigations showed that PSB significantly enhanced Nrf2 expression and nuclear accumulation, improved ARE promoter activity and up-regulated expression of HO-1 and GCLC. Furthermore, Nrf2 knockdown via specific Nrf2 siRNA abolished PSB-induced antioxidative and antiapoptotic effects against MPP⁺ insults. Interestingly, we then found that PSB promoted phosphorylation of PI3K/AKT and ERK, and pharmacological inhibition of PI3K/AKT or ERK signaling diminished PSB-induced Nrf2/ARE activation and protective actions. In summary, PSB confers neuroprotection against MPTP/MPP⁺-induced neurotoxicity in PD models. Promoting activation of Nrf2/ARE signaling contributes to PSB-mediated antioxidative and neuroprotective actions, which, in part, is mediated by PI3K/AKT and ERK.

Anthraquinone and naphthopyrone glycosides from Cassia obtusifolia seeds mediate hepatoprotection via Nrf2-mediated HO-1 activation and MAPK modulation

Cassia obtusifolia L. seed is one of the most popular traditional Chinese medicine for mutagenicity, genotoxicity, hepatotoxicity, and acute inflammatory diseases. We evaluated the hepatoprotective activity of anthraquinone and naphthopyrone glycosides isolated from the butanol fraction of C. obtusifolia seeds and explored their effects on cell signaling pathways. Continuous chromatographic separation led to the isolation of 1-desmethylaurantio-obtusin 2-O-β-D-glucopyranoside (1), rubrofusarin 6-O-β-D-apiofuranosyl-(1 → 6)-O-β-D-glucopyranoside (2) and rubrofusarin 6-O-β-gentiobioside (3). All glycosides were non-toxic at concentrations up to 80 µM. The increased intracellular reactive oxygen species (ROS) and decreased glutathione levels observed after tert-butylhydroperoxide (t-BHP) intoxication were ameliorated by all three glycosides, with compound 3 being the most active. Pretreatment with the three glycosides increased nuclear factor erythroid-2-related factor 2 (Nrf2)-mediated heme oxidase-1 (HO-1) expression. All the glycosides enhanced the phosphorylation of c-Jun N-terminal kinase (JNK), and extracellular signal-regulated kinase (ERK), and the dephosphorylation of p38. The protective effects of the anthraquinone and naphthopyrone glycosides against t-BHP-induced oxidative damage in human liver-derived HepG2 cells were due to the prevention of ROS generation and up-regulated activity of HO-1 via Nrf2 activation and modulation of the JNK/ERK/MAPK signaling pathway. The data indicate the potential of these compounds as hepatoprotective agents in pharmaceuticals and/or nutraceuticals.

The trypanocidal benznidazole promotes adaptive response to oxidative injury: Involvement of the nuclear factor-erythroid 2-related factor-2 (Nrf2) and multidrug resistance associated protein 2 (MRP2)

Oxidative stress is a frequent cause underlying drug-induced hepatotoxicity. Benznidazole (BZL) is the only trypanocidal agent available for treatment of Chagas disease in endemic areas. Its use is associated with side effects, including increases in biomarkers of hepatotoxicity. However, BZL potential to cause oxidative stress has been poorly investigated. Here, we evaluated the effect of a pharmacologically relevant BZL concentration (200μM) at different time points on redox status and the counteracting mechanisms in the human hepatic cell line HepG2. BZL increased reactive oxygen species (ROS) after 1 and 3h of exposure, returning to normality at 24h. Additionally, BZL increased glutathione peroxidase activity at 12h and the oxidized glutathione/total glutathione (GSSG/GSSG+GSH) ratio that reached a peak at 24h. Thus, an enhanced detoxification of peroxide and GSSG formation could account for ROS normalization. GSSG/GSSG+GSH returned to control values at 48h. Expression of the multidrug resistance-associated protein 2 (MRP2) and GSSG efflux via MRP2 were induced by BZL at 24 and 48h, explaining normalization of GSSG/GSSG+GSH. BZL activated the nuclear erythroid 2-related factor 2 (Nrf2), already shown to modulate MRP2 expression in response to oxidative stress. Nrf2 participation was confirmed using Nrf2-knockout mice in which MRP2 mRNA expression was not affected by BZL. In summary, we demonstrated a ROS increase by BZL in HepG2 cells and a glutathione peroxidase- and MRP2 driven counteracting mechanism, being Nrf2 a key modulator of this response. Our results could explain hepatic alterations associated with BZL therapy.

The Transcription Factor MEF2 Is a Novel Regulator of Gsta Gene Class in Mouse MA-10 Leydig Cells

Testosterone is essential for spermatogenesis and the development of male sexual characteristics. However, steroidogenesis produces a significant amount of reactive oxygen species (ROS), which can disrupt testosterone production. The myocyte enhancer factor 2 (MEF2) is an important regulator of organogenesis and cell differentiation in various tissues. In the testis, MEF2 is present in Sertoli and Leydig cells throughout fetal and adult life. MEF2-deficient MA-10 Leydig cells exhibit a significant decrease in steroidogenesis concomitant with a reduction in glutathione S-transferase (GST) activity and in the expression of the 4 Gsta members (GST) that encode ROS inactivating enzymes. Here, we report a novel role for MEF2 in ROS detoxification by directly regulating Gsta expression in Leydig cells. Endogenous Gsta1-4 mRNA levels were decreased in MEF2-deficient MA-10 Leydig cells. Conversely, overexpression of MEF2 increased endogenous Gsta1 levels. MEF2 recruitment to the proximal Gsta1 promoter and direct binding on the -506-bp MEF2 element were confirmed by chromatin immunoprecipitation and DNA precipitation assays. In MA-10 Leydig cells, MEF2 activates the Gsta1 promoter and cooperates with Ca(2+)/calmodulin-dependent kinases I to further enhance Gsta1 promoter activity. These effects were lost when the -506-bp MEF2 element was mutated or when a MEF2-Engrailed dominant negative protein was used. Similar results were obtained on the Gsta2, Gsta3, and Gsta4 promoters, suggesting a global role for MEF2 factors in the regulation of all 4 Gsta genes. Altogether, our results identify a novel role for MEF2 in the expression of genes involved in ROS detoxification, a process essential for adequate testosterone production in Leydig cells.

Tryptanthrin protects hepatocytes against oxidative stress via activation of the extracellular signal-regulated kinase/NF-E2-related factor 2 pathway

Tryptanthrin [6,12-dihydro-6,12-dioxoindolo-(2,1-b)-quinazoline], originally isolated from Isatidis radix, has been characterized as having anti-microbial and anti-tumor activities. It is well-known that excess oxidative stress is one of the major factors causing cell damage in the liver. This study investigated the cytoprotective effects and molecular mechanism of tryptanthrin against tert-butyl hydroperoxide (tBHP)-induced oxidative stress in human hepatocyte-derived HepG2 cells. Tryptanthrin pre-treatment blocked the reactive oxygen species production, mitochondrial dysfunction, and cell death induced by tBHP. Moreover, tryptanthrin reversed tBHP-induced GSH reduction. This study also confirmed the activation of nuclear factor erythroid 2-related factor 2 (Nrf2) by tryptanthrin as a plausible molecular mechanism for its cytoprotective effects. Specifically, tryptanthrin treatment induced nuclear translocation and transactivation of Nrf2 as well as phosphorylation of extracellular signal-regulated kinase (ERK), a potential upstream kinase of Nrf2. Tryptanthrin also up-regulated the expression of the heme oxygenase 1 and glutamate-cysteine ligase catalytic subunits, which are representative target genes of Nrf2. Moreover, inhibitor of ERK was used to verify the important role of the ERK-Nrf2 pathway in the hepatoprotective effects of tryptanthrin. In conclusion, this study demonstrated that tryptanthrin protects hepatocytes against oxidative stress through the activation of the ERK/Nrf2 pathway in HepG2 cells.

Oxyresveratrol abrogates oxidative stress by activating ERK-Nrf2 pathway in the liver

Oxyresveratrol is a polyphenolic phytoalexin produced by plants as an antioxidant. This study investigated the hepatoprotective effects of oxyresveratrol as well as its underlying mechanism of action. Here, we evaluated the protective effects of oxyresveratrol against tert-butyl hydroperoxide (tBHP)-induced severe oxidative stress in HepG2 cells as well as acute liver injury caused by carbon tetrachloride (CCl4) in mice. tBHP-induced reactive oxygen species production and cell death in hepatocytes were blocked by oxyresveratrol, as indicated by MTT, TUNEL, and FACS analyses. Moreover, pretreatment with oxyresveratrol increased nuclear translocation and transactivation of NF-E2-related factor 2 (Nrf2), as assessed by antioxidant response element reporter gene expression and immunofluorescence staining, and transactivated expression of both hemeoxygenase-1 and glutamate-cysteine ligase catalytic subunit. More importantly, oxyresveratrol induced phosphorylation of Nrf2 mediated through activation of extracellular signal-regulated kinase 1/2 (ERK1/2). Further, ERK inhibitors such as PD98059 and U0126 blocked phosphorylation of Nrf2 as well as the protective effect of oxyresveratrol in mitochondria. In mice, oral administration of oxyresveratrol significantly prevented hepatocyte degeneration, inflammatory cell infiltration, as well as elevation of plasma markers such as ALT and AST induced by CCl4 injection. In conclusion, this study confirmed that oxyresveratrol protected hepatocytes against oxidative stress and mitochondrial dysfunction, which might be associated with activation of Nrf2.

Cardiac energy dependence on glucose increases metabolites related to glutathione and activates metabolic genes controlled by mechanistic target of rapamycin

Background

Long chain acyl‐CoA synthetases (ACSL) catalyze long‐chain fatty acids (FA) conversion to acyl‐CoAs. Temporal ACSL1 inactivation in mouse hearts (Acsl1^H−/−) impaired FA oxidation and dramatically increased glucose uptake, glucose oxidation, and mTOR activation, resulting in cardiac hypertrophy. We used unbiased metabolomics and gene expression analyses to elucidate the cardiac cellular response to increased glucose use in a genetic model of inactivated FA oxidation.

Methods and Results

Metabolomics analysis identified 60 metabolites altered in Acsl1^H−/− hearts, including 6 related to glucose metabolism and 11 to cysteine and glutathione pathways. Concurrently, global cardiac transcriptional analysis revealed differential expression of 568 genes in Acsl1^H−/− hearts, a subset of which we hypothesized were targets of mTOR; subsequently, we measured the transcriptional response of several genes after chronic mTOR inhibition via rapamycin treatment during the period in which cardiac hypertrophy develops. Hearts from Acsl1^H−/− mice increased expression of several Hif1α‐responsive glycolytic genes regulated by mTOR; additionally, expression of Scl7a5, Gsta1/2, Gdf15, and amino acid‐responsive genes, Fgf21, Asns, Trib3, Mthfd2, were strikingly increased by mTOR activation.

Conclusions

The switch from FA to glucose use causes mTOR‐dependent alterations in cardiac metabolism. We identified cardiac mTOR‐regulated genes not previously identified in other cellular models, suggesting heart‐specific mTOR signaling. Increased glucose use also changed glutathione‐related pathways and compensation by mTOR. The hypertrophy, oxidative stress, and metabolic changes that occur within the heart when glucose supplants FA as a major energy source suggest that substrate switching to glucose is not entirely benign.

Identification of heme oxygenase-1 stimulators by a convenient ELISA-based bilirubin quantification assay

The upregulation of heme oxygenase-1 (HO-1) has proven to be a useful tool for fighting inflammation. In order to identify new HO-1 inducers, an efficient screening method was developed which can provide new lead structures for drug research. We designed a simple ELISA-based HO-1 enzyme activity assay, which allows for the screening of 12 compounds in parallel in the setting of a 96-well plate. The well-established murine macrophage cell line RAW264.7 is used and only about 26µg of protein from whole cell lysates is needed for the analysis of HO-1 activity. The quantification of HO-1 activity is based on an indirect ELISA using the specific anti-bilirubin antibody 24G7 to quantify directly bilirubin in the whole cell lysate, applying a horseradish peroxidase-tagged antibody together with ortho-phenylenediamine and H2O2 for detection. The bilirubin is produced on the action of HO enzymes by converting their substrate heme to biliverdin and additional recombinant biliverdin reductase together with NADPH at pH 7.4 in buffer. This sensitive assay allows for the detection of 0.57-82pmol bilirubin per sample in whole cell lysates. Twenty-three small molecules, mainly natural products with an α,β-unsaturated carbonyl unit such as polyphenols, including flavonoids and chalcones, terpenes, an isothiocyanate, and the drug oltipraz were tested at typically 6 or 24h incubation with RAW264.7 cells. The activity of known HO-1 inducers was confirmed, while the chalcones cardamonin, flavokawain A, calythropsin, 2',3,4'-trihydroxy-4-methoxychalcone (THMC), and 2',4'-dihydroxy-3,4-dimethoxychalcone (DHDMC) were identified as new potent HO-1 inducers. The highest inductive power after 6h incubation was found at 10µM for DHDMC (6.1-fold), carnosol (3.9-fold), butein (3.1-fold), THMC (2.9-fold), and zerumbone (2.5-fold). Moreover, the time dependence of HO-1 protein production for DHDMC was compared to its enzyme activity, which was further evaluated in the presence of lipopolysaccharide and the specific HO-1 inhibitor tin protoporphyrin IX. Taken together, we developed a convenient and highly sensitive ELISA-based HO-1 enzyme activity assay, allowing the identification and characterization of molecules potentially useful for the treatment of inflammatory and autoimmune diseases.

Identification of noninvasive biomarkers for nephrotoxicity using HK-2 human kidney epithelial cells

The kidney is an important site of xenobiotic-induced toxicity. Because the traditional markers of renal injury indicate only severe renal damage, new biomarkers are needed for a more sensitive and reliable evaluation of renal toxicity. This study was designed to identify in vitro noninvasive biomarkers for efficient assessment of nephrotoxicity by using cisplatin as a model of nephrotoxic compounds. To this end, a comparative proteomic analysis of conditioned media from HK-2 human kidney epithelial cells treated with cisplatin was performed. Here, we identified pyruvate kinase M1/M2 isoform M2 (PKM2) and eukaryotic translation elongation factor 1 gamma (EF-1γ) as potential biomarker candidates for evaluation of nephrotoxicity. PKM2 and EF-1γ were increased by cisplatin in a kidney cell-specific manner, most likely due to cisplatin-induced apoptosis. The increase of PKM2 and EF-1γ levels in conditioned media was also observed in the presence of other nephrotoxic agents with different cytotoxic mechanisms such as CdCl2, HgCl2, and cyclosporine A. Rats treated with cisplatin, CdCl2, or HgCl2 presented increased levels of PKM2 and EF-1γ in the urine and kidney tissue. Taken together, this study identified two noninvasive biomarker candidates, PKM2 and EF-1γ, by comparative proteomic analysis. These new biomarkers may offer an alternative to traditional renal markers for efficient evaluation of nephrotoxicity.

An active metabolite of oltipraz (M2) increases mitochondrial fuel oxidation and inhibits lipogenesis in the liver by dually activating AMPK

BACKGROUND AND PURPOSE

Oltipraz, a cancer chemopreventive agent, has an anti-steatotic effect via liver X receptor-α (LXRα) inhibition. Here we have assessed the biological activity of a major metabolite of oltipraz (M2) against liver steatosis and steatohepatitis and the underlying mechanism(s).

EXPERIMENTAL APPROACH

Blood biochemistry and histopathology were assessed in high-fat diet (HFD)-fed mice treated with M2. An in vitroHepG2 cell model was used to study the mechanism of action. Immunoblotting, real-time PCR and luciferase reporter assays were performed to measure target protein or gene expression levels.

KEY RESULTS

M2 treatment inhibited HFD-induced steatohepatitis and diminished oxidative stress in liver. It increased expression of genes encoding proteins involved in mitochondrial fuel oxidation. Mitochondrial DNA content and oxygen consumption rate were enhanced. Moreover, M2 treatment repressed activity of LXRα and induction of its target genes, indicating anti-lipogenic effects. M2 activated AMP-activated protein kinase (AMPK). Inhibition of AMPK by over-expression of dominant negative AMPK (DN-AMPK) or by Compound C prevented M2 from inducing genes for fatty acid oxidation and repressed sterol regulatory element binding protein-1c (SREBP-1c) expression. M2 activated liver kinase B1 (LKB1) and increased the AMP/ATP ratio. LKB1 knockdown failed to reverse target protein modulations or AMPK activation by M2, supporting the proposal that both LKB1 and increased AMP/ATP ratio contribute to its anti-steatotic effect.

CONCLUSION AND IMPLICATIONS

M2 inhibited liver steatosis and steatohepatitis by enhancing mitochondrial fuel oxidation and inhibiting lipogenesis. These effects reflected activation of AMPK elicited by increases in LKB1 activity and AMP/ATP ratio.

miR-125b transcriptionally increased by Nrf2 inhibits AhR repressor, which protects kidney from cisplatin-induced injury

MicroRNAs (miRNAs) have a role in the cellular defense mechanism. Nuclear factor erythroid-2-related factor 2 (Nrf2) increases antioxidant enzyme capacity. However, miRNA transcriptionally controlled by Nrf2 had been uncharacterized. Here we report that miR-125b is transactivated by Nrf2 and inhibits aryl hydrocarbon receptor (AhR) repressor (AhRR). Bioinformatic approaches enabled us to extract six candidate miRNAs. Of them, only miR-125b was increased in the kidney of mice treated with oltipraz. Nrf2 overexpression enhanced primary, precursor and mature miR-125b levels. Functional assays revealed MIR125B1 is a bona fide target gene of Nrf2. Oltipraz treatment protected the kidney from cisplatin toxicity with increase of miR-125b. Consistently, Nrf2 knockout abrogated an adaptive increase of miR-125b elicited by cisplatin, augmenting kidney injury. An integrative network of miRNA and messenger RNA changes enabled us to predict miR-125b as an inhibitor of AhRR for the control of AhR activity and cell survival. In our molecular study, miR-125b inhibited AhRR and thereby activated AhR, leading to the induction of mdm2. Consistently, p53 activation by cisplatin was diminished by either miR-125b or oltipraz treatment. The results of experiments using miR-125b mimic or small interfering RNA of AhRR verified the role of miR-125b in AhRR regulation for kidney protection. In conclusion, miR-125b is transcriptionally activated by Nrf2 and serves as an inhibitor of AhRR, which contributes to protecting kidney from acute injury.

Korean red ginseng extract prevents APAP-induced hepatotoxicity through metabolic enzyme regulation: the role of ginsenoside Rg3, a protopanaxadiol

BACKGROUND

Inappropriate use of acetaminophen (APAP) can lead to morbidity and mortality secondary to hepatic necrosis.

AIMS

We evaluated the beneficial effect and molecular mechanism of Korean red ginseng (KRG) on the APAP-mediated hepatotoxicity and identified a major component of KRG for hepatoprotection.

METHODS

Survival test, liver function test, histopathological study, APAP-metabolic profiling and gene expression were examined in mice. We determined the enzyme expression and upstream signalling in H4IIE cells analysed by RT-PCR, immunoblotting, siRNA gene knockdown and promoter-luciferase assay.

RESULTS

High doses of KRG reduced mortality at the LD50 of APAP. APAP increased AST and ALT activities, which were abrogated by low doses of KRG. These protective effects were consistent with the results from histopathological examinations. KRG altered APAP metabolic profiles through inhibition of cytochrome P450 2E1 and induction of glutathione S-transferase A2 (GSTA2). Knockdown of GSTA2 catalyses the conjugation of glutathione reversed KRG-mediated protection against N-acetyl-p-benzoquinone imine in H4IIE cells. The nuclear Nrf2 and C/EBPβ, which are essential transcriptional factors for GSTA2 were increased by KRG. These effects were downstream of multiple signalling, including PI3K, JNK or PKA. Ginsenoside Rg3 but not Rb1, Rc and Rg1 significantly increased GSTA2 protein expression. Rg3 resulted in the transcriptional activation of GSTA2 downstream of the multiple cellular signalling.

CONCLUSIONS

These results demonstrate that KRG is efficacious in protection against APAP-induced hepatotoxicity and mortality through metabolic regulation and that Rg3 is a major component of KRG for the GST induction, implying that Rg3 should be considered to be a potential hepatoprotective agent.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

The role of Nrf2 and apoptotic signaling pathways in oroxylin A-mediated responses in HCT-116 colorectal adenocarcinoma cells and xenograft tumors

Oroxylin A is a flavonoid found in the roots of Scutellaria baicalensis Georgi, a herbal medicine commonly used as an antipyretic, analgesic, antitumor, and anti-in&fllig;ammatory agent. It has recently been investigated for its anticancer activities in hepatoma, gastric, and breast tumors. Here, we investigated the antitumor effects of oroxylin A in human colon carcinoma HCT-116 cells in vitro and in vivo. We characterized the proapoptotic effect of oroxylin A using diamidino-phenyl-indole (DAPI) and annexin V/PI staining. We then found that both caspase-3 and caspase-9 were activated, the expression of Bcl-2 protein decreased, and the expression of Bax protein increased after treatment with oroxylin A. In addition, oroxylin A increased nuclear transcription factor erythroid-related factor 2 (Nrf2) expression and induced Nrf2 translocation into the nucleus. Furthermore, we found that oroxylin A treatment elevated intracellular reactive oxygen species levels and increased the protein expression level of two of the Nrf2 target genes heme oxygenase-1 and NADP(H):quinone oxidoreductase-1 in HCT-116 cells. Finally, our study demonstrated that oral administration of oroxylin A significantly decreased tumor volume and weight in immunodeficient mice that were inoculated with HCT-116 cells. The in-vivo chemopreventive efficacy of oroxylin A against HCT-116 human colon cancer was accompanied by its proapoptotic and Nrf2-inducing activities, which correlates with the in-vitro study. This is the first demonstration of oroxylin A-dependent chemoprevention in colon cancer and may offer a potential mechanism for its anticancer action in vivo.

Fyn inhibition by cycloalkane-fused 1,2-dithiole-3-thiones enhances antioxidant capacity and protects mitochondria from oxidative injury

Fyn kinase has emerged as a regulator of diverse pathological processes. However, therapeutic Fyn inhibitors are not available. This study investigated the potential of a series of cycloalkane-fused dithiolethiones (CDTs) or other congeners to increase antioxidant capacity in association with Fyn inhibition, as well as the molecular basis for this effect. Treatment of HepG2 cells with each agent protected the mitochondria from oxidative injury elicited by arachidonic acid and iron, which increased cell viability; 4,5,6,7-tetrahydrobenzo-1,2-dithiole-3-thione (SNU1A) and 5,6-dihydro-4H-cyclopenta-1,2-dithiole-3-thione (SNU2A) were the most effective, whereas 5-methyl-1,2-dithiole-3-thione (SNU3A) was less active. 5-(Quinolin-2-yl)-1,2-dithiole-3-thione (SNU3E) had a minimal effect. SNU1A treatment decreased mitochondrial superoxide production and enabled cells to restore mitochondrial membrane permeability. Oxidative injury caused by arachidonic acid and iron enhanced Fyn phosphorylation at a tyrosine residue, which was decreased by SNU1A treatment. 2,3-Dihydro-N,N-dimethyl-2-oxo-3-[(4,5,6,7-tetrahydro-1H-indol-2-yl)methylene]-1H-indole-5-sulfonamide (SU6656), a known Fyn inhibitor, had a similar effect. Fyn inhibition contributed to protecting mitochondria from injury through AMP-activated protein kinase (AMPK), as supported by reversal of this effect with Fyn overexpression. Consistently, Fyn overexpression attenuated AMPK activation by SNU1A, which strengthens the inhibitory role of Fyn in AMPK activity. CDTs had antioxidant effects, as shown by increases in GSH contents and inhibition of H(2)O(2) production. They also had the ability to activate nuclear factor E2-related factor 2 (Nrf2), a key antioxidant transcription factor. Fyn overexpression decreased the Nrf2 activation induced by SNU1A. Our results demonstrate that CDTs exert cytoprotective effects by protecting mitochondria and increasing the cellular antioxidant capacity, which may result not only from Fyn inhibition leading to AMPK activation but also from Nrf2 activation.

Nuclear factor erythroid-derived factor 2-related factor 2 regulates transcription of CCAAT/enhancer-binding protein β during adipogenesis

Nuclear factor erythroid-derived factor 2-related factor 2 (Nrf2) is a cap-n-collar basic leucine zipper transcription factor that is involved in the cellular adaptive response to oxidative stress. Our previous study reported that targeted disruption of the Nrf2 gene in mice decreases adipose tissue mass and protects against obesity induced by a high-fat diet. Deficiency of Nrf2 in preadipocytes and mouse embryonic fibroblasts led to impaired adipogenesis. Consistent with these findings, the current study found that lack of Nrf2 in primary cultured mouse preadipocytes and 3T3-L1 cells hampered adipogenic differentiation induced by hormonal cocktails. Stable knockdown of Nrf2 in 3T3-L1 cells blocked the enhanced adipogenesis caused by deficiency of kelch-like ECH-associated protein 1 (Keap1), a Cul3-adapter protein that allows for Nrf2 to be ubiquinated and degraded by the 26S protesome complex. In addition, increased production of reactive oxygen species (ROS) and activation of Nrf2 occurred at the very early stage upon adipogenic hormonal challenge in 3T3-L1 cells, followed by an immediate induction of CCAAT/enhancer-binding protein β (C/EBPβ). Knockdown of Nrf2 led to reduced expression of C/EBPβ induced by adipogenic hormonal cocktails, chemical Nrf2 activators or Keap1 silencing. Cebpβ promoter-driven reporter assays and chromatin immunoprecipitation suggested that Nrf2 associates with a consensus antioxidant response element (ARE) binding site in the promoter of the Cebpβ gene during adipogenesis and upregulates its expression. These findings demonstrate a novel role of Nrf2 beyond xenobiotic detoxification and antioxidant response, and suggest that Nrf2 is one of the transcription factors that control the early events of adipogenesis by regulating expression of Cebpβ.

Mechanisms of induction of cytosolic and microsomal glutathione transferase (GST) genes by xenobiotics and pro-inflammatory agents

Glutathione transferase (GST) isoezymes are encoded by three separate families of genes (designated cytosolic, microsomal and mitochondrial transferases), with distinct evolutionary origins, that provide mammalian species with protection against electrophiles and oxidative stressors in the environment. Members of the cytosolic class Alpha, Mu, Pi and Theta GST, and also certain microsomal transferases (MGST2 and MGST3), are up-regulated by a diverse spectrum of foreign compounds typified by phenobarbital, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, pregnenolone-16α-carbonitrile, 3-methylcholanthrene, 2,3,7,8-tetrachloro-dibenzo-p-dioxin, β-naphthoflavone, butylated hydroxyanisole, ethoxyquin, oltipraz, fumaric acid, sulforaphane, coumarin, 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole, 12-O-tetradecanoylphorbol-13-acetate, dexamethasone and thiazolidinediones. Collectively, these compounds induce gene expression through the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), the aryl hydrocarbon receptor (AhR), NF-E2-related factor 2 (Nrf2), peroxisome proliferator-activated receptor-γ (PPARγ) and CAATT/enhancer binding protein (C/EBP) β. The microsomal T family includes 5-lipoxygenase activating protein (FLAP), leukotriene C(4) synthase (LTC4S) and prostaglandin E(2) synthase (PGES-1), and these are up-regulated by tumour necrosis factor-α, lipopolysaccharide and transforming growth factor-β. Induction of genes encoding FLAP, LTC4S and PGES-1 is mediated by the transcription factors C/EBPα, C/EBPδ, C/EBPϵ, nuclear factor-κB and early growth response-1. In this article we have reviewed the literature describing the mechanisms by which cytosolic and microsomal GST are up-regulated by xenobiotics, drugs, cytokines and endotoxin. We discuss cross-talk between the different induction mechanisms, and have employed bioinformatics to identify cis-elements in the upstream regions of GST genes to which the various transcription factors mentioned above may be recruited.

Farnesoid X receptor activation by chenodeoxycholic acid induces detoxifying enzymes through AMP-activated protein kinase and extracellular signal-regulated kinase 1/2-mediated phosphorylation of CCAAT/enhancer binding protein β

Farnesoid X receptor (FXR) regulates redox homeostasis and elicits a cytoprotective effect. CCAAT/enhancer binding protein-β (C/EBPβ) plays a role in regulating the expression of hepatocyte-specific genes and contributes to hepatocyte protection and liver regeneration. In view of the role of FXR in xenobiotic metabolism and hepatocyte survival, this study investigated the potential of FXR to activate C/EBPβ for the induction of detoxifying enzymes and the responsible regulatory pathway. Chenodeoxycholic acid (CDCA), a major component in bile acids, activates FXR. In HepG2 cells, CDCA treatment activated C/EBPβ, as shown by increases in its phosphorylation, nuclear accumulation, and expression. 3-(2,6-Dichlorophenyl)-4-(3'-carboxy-2-chlorostilben-4-yl-)oxymethyl-5-isopropyl-isoxazole (GW4064), a synthetic FXR ligand, had similar effects. In addition, CDCA enhanced luciferase gene transcription from the construct containing -1.65-kb GSTA2 promoter, which contained C/EBP response element (pGL-1651). Moreover, CDCA treatment activated AMP-activated protein kinase (AMPK), which led to extracellular signal-regulated kinase 1/2 (ERK1/2) activation, as evidenced by the results of experiments using a dominant-negative mutant of AMPKα and chemical inhibitor. The activation of ERK1/2 was responsible for the activating phosphorylation of C/EBPβ. FXR knockdown attenuated the ability of CDCA to activate AMPK and ERK1/2 and phosphorylate C/EBPβ. Consistently, enforced expression of FXR promoted the phosphorylation of AMPKα, ERK1/2, and C/EBPβ, verifying that C/EBPβ phosphorylation elicited by CDCA results from the activation of AMPK and ERK1/2 by FXR. In mice, CDCA treatment activated C/EBPβ with the induction of detoxifying enzymes in the liver. Our results demonstrate that CDCA induces antioxidant and xenobiotic-metabolizing enzymes by activating C/EBPβ through AMPK-dependent ERK1/2 pathway downstream of FXR.

A Novel Mechanism of PPARgamma Regulation of TGFbeta1: Implication in Cancer Biology

Peroxisome proliferator-activated receptor-γ (PPARγ) and retinoic acid X-receptor (RXR) heterodimer, which regulates cell growth and differentiation, represses the TGFβ1 gene that encodes for the protein involved in cancer biology. This review will introduce the novel mechanism associated with the inhibition of the TGFβ1 gene by PPARγ activation, which regulates the dephosphorylation of Zf9 transcription factor. Pharmacological manipulation of TGFβ1 by PPARγ activators can be applied for treating TGFβ1-induced pathophysiologic disorders such as cancer metastasis and fibrosis. In this article, we will discuss the opposing effects of TGFβ on tumor growth and metastasis, and address the signaling pathways regulated by PPARγ for tumor progression and suppression.

Transactivation of genes encoding for phase II enzymes and phase III transporters by phytochemical antioxidants

The induction of phase II enzymes and phase III transporters contributes to the metabolism, detoxification of xenobiotics, antioxidant capacity, redox homeostasis and cell viability. Transactivation of the genes that encode for phase II enzymes and phase III transporters is coordinatively regulated by activating transcription factors in response to external stimuli. Comprehensive studies indicate that antioxidant phytochemicals promote the induction of phase II enzymes and/or phase III transporters through various signaling pathways, including phosphoinositide 3-kinase, protein kinase C, and mitogen-activated protein kinases. This paper focuses on the molecular mechanisms and signaling pathways responsible for the transactivation of genes encoding for these proteins, as orchestrated by a series of transcription factors and related signaling components.

Nrf2-ARE signaling pathway and natural products for cancer chemoprevention

BACKGROUND

One of the potential strategies for preventing cancers is using food-based natural products to induce cytoprotective enzymes including phase II and antioxidative enzymes that act in concert to detoxify and eliminate harmful reactive intermediates formed from carcinogens. The antioxidant response element (ARE), which is activated upon binding of the nuclear factor E2-related protein 2 (Nrf2) transcription factor protein, has been identified in the regulatory regions of numerous genes encoding cytoprotective enzymes. Herein, we summarized the current body of knowledge regarding Nrf2 regulation as well as highlighted the Nrf2/ARE activators from natural products, which will potentially be used as chemopreventive agents for cancer patients.

METHODS

Via reviewing Pubmed, we summarized the current progress in the molecular mechanisms of Nrf2 regulation and the major classes of dietary components that act as promising chemopreventive agents through evoking Nrf2-ARE core signaling pathway.

RESULTS

Under basal condition, Nrf2 is at low level, sequestered in the cytoplasm by being tethered to an actin binding Kelch-like ECH associating protein 1 (Keap1). Pharmacological and putative chemopreventive agents trigger the release of Nrf2 from Keap1, allowing it to translocate into the nucleus and drive the gene expression of detoxifying enzymes to perform cancer chemopreventive effect.

CONCLUSION

Augmenting both expression and activity of phase II detoxification and antioxidant enzymes via Nrf2-ARE core signaling pathway would be a rational approach for cancer chemoprevention and the number of novel Nrf2/ARE activators from dietary sources is growing.

Antioxidant and mitochondrial protective effects of oxidized metabolites of oltipraz

IMPORTANCE OF THE FIELD

Comprehensive studies indicate that oltipraz exerts cancer chemopreventive effects. Oltipraz has other therapeutic potentials, which include anti-fibrotic effect, inhibition of insulin resistance, mitochondrial protection and cytoprotective effect against oxidative stress. Although antioxidant mechanisms may account for its cancer chemopreventive effect, details on the molecular mechanism still remain to be clarified.

AREAS COVERED IN THIS REVIEW

Two major metabolic pathways of oltipraz include oxidative desulfuration of the thione to yield 4-methyl-5-(pyrazin-2-yl)-3H-1,2-dithiol-3-one and molecular rearrangement to 7-methyl-6,8-bis(methylthio)H-pyrrolo[1,2-a]pyrazine. In addition to the diverse pharmacological effects of oltipraz, the oxidized metabolites may have distinct biological effects on cell survival. The AMP-activated protein kinase pathway has been recognized as a key cascade for mitochondrial protection and cell survival events, which can be activated by the oxidized metabolites of oltipraz.

WHAT THE READER WILL GAIN

In this review, the metabolic activation of oltipraz and the role of the cell signaling pathways in regulating the expression of Phase II genes and antioxidant activity are discussed with particular reference to their effects on mitochondrial protection and cell survival.

TAKE HOME MESSAGE

In terms of therapeutic potential, the findings reviewed here demonstrate a therapeutic potential for oxidized metabolite of oltipraz and offer comparison of antioxidant capacity between metabolites and parent compound.

Inhibition of liver fibrosis by solubilized coenzyme Q10: Role of Nrf2 activation in inhibiting transforming growth factor-beta1 expression

Coenzyme Q10 (CoQ10), an endogenous antioxidant, is important in oxidative phosphorylation in mitochondria. It has anti-diabetic and anti-cardiovascular disease effects, but its ability to protect against liver fibrosis has not been studied. Here, we assessed the ability of solubilized CoQ10 to improve dimethylnitrosamine (DMN)-induced liver fibrogenesis in mice. DMN treatments for 3 weeks produced a marked liver fibrosis as assessed by histopathological examination and tissue 4-hydroxyproline content. Solubilized CoQ10 (10 and 30 mg/kg) significantly inhibited both the increases in fibrosis score and 4-hydroxyproline content induced by DMN. Reverse transcription-polymerase chain reaction and Western blot analyses revealed that solubilized CoQ10 inhibited increases in the transforming growth factor-beta1 (TGF-beta1) mRNA and alpha-smooth muscle actin (alpha-SMA) protein by DMN. Interestingly, hepatic glutamate-cysteine ligase (GCL) and glutathione S-transferase A2 (GSTA2) were up-regulated in mice treated with CoQ10. Solubilized CoQ10 also up-regulated antioxidant enzymes such as catalytic subunits of GCL and GSTA2 via activating NF-E2 related factor2 (Nrf2)/antioxidant response element (ARE) in H4IIE hepatoma cells. Moreover, CoQ10's inhibition of alpha-SMA and TGF-beta1 expressions disappeared in Nrf2-null MEF cells. In contrast, Nrf2 overexpression significantly decreased the basal expression levels of alpha-SMA and TGF-beta1 in Nrf2-null MEF cells. These results demonstrated that solubilized CoQ10 inhibited DMN-induced liver fibrosis through suppression of TGF-beta1 expression via Nrf2/ARE activation.

Nrf2:INrf2 (Keap1) signaling in oxidative stress

Nrf2:INrf2 (Keap1) are cellular sensors of chemical- and radiation-induced oxidative and electrophilic stress. Nrf2 is a nuclear transcription factor that controls the expression and coordinated induction of a battery of defensive genes encoding detoxifying enzymes and antioxidant proteins. This is a mechanism of critical importance for cellular protection and cell survival. Nrf2 is retained in the cytoplasm by an inhibitor, INrf2 which functions as an adapter for Cul3/Rbx1-mediated degradation of Nrf2. In response to oxidative/electrophilic stress, Nrf2 is switched on and then off by distinct early and delayed mechanisms. Oxidative/electrophilic modification of INrf2 cysteine 151 and/or protein kinase C phosphorylation of Nrf2 serine 40 results in the escape or release of Nrf2 from INrf2. Nrf2 is stabilized and translocates to the nucleus, forms heterodimers with unknown proteins, and binds the antioxidant response element, which leads to coordinated activation of gene expression. It takes less than 15 min from the time of exposure to switch on nuclear import of Nrf2. This is followed by activation of a delayed mechanism that controls the switching off of Nrf2 activation of gene expression. GSK3beta phosphorylates Fyn at an unknown threonine residue(s), leading to the nuclear localization of Fyn. Fyn phosphorylates Nrf2 tyrosine 568, resulting in the nuclear export of Nrf2, binding with INrf2, and degradation of Nrf2. The switching on and off of Nrf2 protects cells against free radical damage, prevents apoptosis, and promotes cell survival.

Nrf2:INrf2 (Keap1) signaling in oxidative stress

Nrf2:INrf2 (Keap1) are cellular sensors of chemical- and radiation-induced oxidative and electrophilic stress. Nrf2 is a nuclear transcription factor that controls the expression and coordinated induction of a battery of defensive genes encoding detoxifying enzymes and antioxidant proteins. This is a mechanism of critical importance for cellular protection and cell survival. Nrf2 is retained in the cytoplasm by an inhibitor, INrf2 which functions as an adapter for Cul3/Rbx1-mediated degradation of Nrf2. In response to oxidative/electrophilic stress, Nrf2 is switched on and then off by distinct early and delayed mechanisms. Oxidative/electrophilic modification of INrf2 cysteine 151 and/or protein kinase C phosphorylation of Nrf2 serine 40 results in the escape or release of Nrf2 from INrf2. Nrf2 is stabilized and translocates to the nucleus, forms heterodimers with unknown proteins, and binds the antioxidant response element, which leads to coordinated activation of gene expression. It takes less than 15 min from the time of exposure to switch on nuclear import of Nrf2. This is followed by activation of a delayed mechanism that controls the switching off of Nrf2 activation of gene expression. GSK3beta phosphorylates Fyn at an unknown threonine residue(s), leading to the nuclear localization of Fyn. Fyn phosphorylates Nrf2 tyrosine 568, resulting in the nuclear export of Nrf2, binding with INrf2, and degradation of Nrf2. The switching on and off of Nrf2 protects cells against free radical damage, prevents apoptosis, and promotes cell survival.

Therapeutic potential of dithiolethiones for hepatic diseases

Comprehensive studies support the notion that oltipraz [4-methyl-5-(2-pyrazynyl)-1,2-dithiole-3-thione] and its congeners exert cancer chemopreventive effects by the prevention, inhibition or reversal of carcinogenic processes. Recently, it was found that dithiolethione compounds had the activities to prevent or treat fibrosis, insulin resistance, and mitochondrial protective effects in the liver by a mechanism involving AMP-activated protein kinase (AMPK) and/or 70-kDa ribosomal protein S6 kinase 1 (S6K1). Moreover, chemical regulation of the AMPK-S6K1 pathway was found to affect Liver X receptor (LXR) activity and lipogenesis, leading to the identification of AMPK and S6K1 as targets for treating hepatic steatosis. These biological activities of dithiolethiones may offer a novel approach to pharmaceutical intervention. This review focuses on the interaction between oltipraz and the AMPK-mTOR-S6K1 pathway, which regulates genes that confer hepatocyte protection from intoxication, disrupted energy metabolism, and inflammation. In terms of therapeutic potential, the findings reviewed here demonstrate a new therapeutic potential for dithiolethiones, which function in a unique manner, and offer the possibility of new treatments for hepatic diseases.

Role of hepatitis B virus X repression of C/EBPbeta activity in the down-regulation of glutathione S-transferase A2 gene: implications in other phase II detoxifying enzyme expression

1. A genome-wide in silico screening rendered the genes of phase II enzymes in the rat genome whose promoters contain the putative DNA elements interacting with CCAAT/enhancer binding protein (C/EBP) and NF-E2-related factor (Nrf2). The hepatitis B virus X (HBx) protein strongly modulates the transactivation and/or the repression of genes regulated by some bZIP transcription factors. 2. This study investigated the effects of HBx on the induction of phase II enzymes with the aim of elucidating the role of HBx interaction with C/EBPbeta or Nrf2 bZIP transcription factors in hepatocyte-derived cells. 3. Immunoblot and reporter gene analyses revealed that transfection of HBx interfered with the constitutive and inducible GSTA2 transactivation promoted by oltipraz (C/EBPbeta activator), but not that by tert-butylhydroquinone (t-BHQ, Nrf2 activator). Moreover, HBx transfection completely inhibited GSTA2 reporter gene activity induced by C/EBPbeta, but failed to inhibit that by Nrf2. 4. Gel shift assays identified that HBx inhibited the increase in C/EBPbeta-DNA complex formation by oltipraz, but not the increase in Nrf2-DNA complex by t-BHQ. Immunoprecipitation and immunoblot assays verified the direct interaction between HBx and C/EBPbeta. Moreover, chromatin immunoprecipitation assays confirmed HBx inhibition of C/EBPbeta binding to its binding site in the GSTA2 gene promoter. HBx repressed the induction of other phase II enzymes including GSTP, UDP-glucuronyltransferase 1A, microsomal epoxide hydrolase, GSTM1, GSTM2, and gamma-glutamylcysteine synthase. 5. These results demonstrate that HBx inhibits the induction of phase II detoxifying enzymes, which is mediated by its interaction with C/EBPbeta, but not Nrf2, substantiating the specific role of HBx in phase II detoxifying capacity.

Oxidized metabolites of oltipraz exert cytoprotective effects against arachidonic acid through AMP-activated protein kinase-dependent cellular antioxidant effect and mitochondrial protection

Oltipraz protects cells from chemical-induced carcinogenesis partly because of phase 2 enzyme induction. Certain oltipraz metabolites also induce phase 2 enzymes. This study investigated the cytoprotective effects of the oxidized metabolites of oltipraz against arachidonic acid (AA), a proinflammatory fatty acid that causes cellular reactive oxygen species (ROS) production and mitochondrial impairment, and the mechanistic basis of their action in HepG2 cells. Treatment with 4-methyl-5-(pyrazin-2-yl)-3H-1,2-dithiol-3-one (M1) or 7-methyl-6,8-bis(methylthio)H-pyrrolo[1,2-a]-pyrazine (M2), but not 7-methyl-8-(methylsulfinyl)-6-(methylthio)H-pyrrolo[1,2-a]pyrazine (M3) or 7-methyl-6,8-bis(methylsulfinyl)H-pyrrolo[1,2-a]pyrazine (M4), enabled cells to protect against AA-induced apoptosis. M1 and M2 treatment protected cells from ROS produced by AA and inhibited AA-induced glutathione depletion. Moreover, both M1 and M2 effectively inhibited mitochondrial dysfunction induced by AA, although M2 alone slightly elicited it at a relatively high concentration. M1 and M2 activated AMP-activated protein kinase (AMPK), but M3 and M4 failed to do so. AMPK activation by M1 and M2 contributed to cell survival against AA through a decrease in cellular ROS production and prevention of mitochondrial dysfunction, as shown by the reversal of the metabolites' restoration of mitochondrial membrane potential by compound C treatment or overexpression of a dominant-negative mutant AMPK. Consistently, 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside, an AMPK activator, also had a cytoprotective and antioxidant effect against AA. Our results demonstrate that, of the major metabolites of oltipraz, M1 and M2 are capable of protecting cells from AA-induced ROS production and mitochondrial dysfunction, which may be associated with AMPK activation.

Oltipraz promotion of liver regeneration after partial hepatectomy: The role of PI3-kinase-dependent C/EBPbeta and cyclin E regulation

Oltipraz, a representative cancer chemopreventive agent, regenerates cirrhotic liver via CCAAT/enhancer binding protein beta (C/EBPbeta). This study examined the effect of oltipraz on liver regeneration after partial hepatectomy (PH) and explored the role of phosphatidylinositol 3-kinase (PI3K) pathway responsible in liver regeneration. Oltipraz treatment (30 mg/kg/day, for 3 days) promoted liver regeneration in PH rats, but did not increase hepatocyte growth factor production. Subcellular fractionation and electrophoretic mobility shift assays showed that oltipraz treatment increased C/EBPbeta-DNA binding activity in the liver of sham control rats and further enhanced PH-mediated nuclear translocation of C/EBPbeta. The expression of cyclin E and the activity of cyclin E-dependent kinase were both enhanced by oltipraz treatment of PH rats. The signaling pathway that controls C/EBPbeta and cyclin E were studied in H4IIE cells, a rat-derived hepatocyte cell line. Oltipraz potentiated the nuclear accumulation of C/ EBPbeta and C/EBPbeta-DNA binding activity in cells incubated in a medium containing serum. PI3K and its downstream kinase, p70S6 kinase, were both required for C/EBPbeta-dependent induction of cyclin E by oltipraz, as shown by chemical inhibition and plasmid transfection experiments. The results of this study demonstrate that oltipraz treatment enhances liver regeneration after PH, which involves activation of C/EBPbeta and C/EBPbeta-dependent cyclin E expression via the PI3K-p70S6 kinase pathway.

Chemical genomics of cancer chemopreventive dithiolethiones

3H-1,2-dithiole-3-thione (D3T) and its analogues 4-methyl-5-pyrazinyl-3H-1,2-dithiole-3-thione (OLT) and 5-tert-butyl-3H-1,2-dithiole-3-thione (TBD) are chemopreventive agents that block or diminish early stages of carcinogenesis by inducing activities of detoxication enzymes. While OLT has been used in clinical trials, TBD has been shown to be more efficacious and possibly less toxic than OLT in animals. Here, we utilize a robust and high-resolution chemical genomics procedure to examine the pharmacological structure-activity relationships of these compounds in livers of male rats by microarray analyses. We identified 226 differentially expressed genes that were common to all treatments. Functional analysis identified the relation of these genes to glutathione metabolism and the nuclear factor, erythroid derived 2-related factor 2 pathway (Nrf2) that is known to regulate many of the protective actions of dithiolethiones. OLT and TBD were shown to have similar efficacies and both were weaker than D3T. In addition, we identified 40 genes whose responses were common to OLT and TBD, yet distinct from D3T. As inhibition of cytochrome P450 (CYP) has been associated with the effects of OLT on CYP expression, we determined the half maximal inhibitory concentration (IC(50)) values for inhibition of CYP1A2. The rank order of inhibitor potency was OLT >> TBD >> D3T, with IC(50) values estimated as 0.2, 12.8 and >100 microM, respectively. Functional analysis revealed that OLT and TBD, in addition to their effects on CYP, modulate liver lipid metabolism, especially fatty acids. Together, these findings provide new insight into the actions of clinically relevant and lead dithiolethione analogues.

Liquiritigenin, a flavonoid aglycone from licorice, has a choleretic effect and the ability to induce hepatic transporters and phase-II enzymes

Liquiritigenin (LQ), an active component of licorice, has an inhibitory effect on LPS-induced inhibitory nitric oxide synthase expression. This study investigated the effects of LQ on choleresis, the expression of hepatic transporters and phase-II enzymes, and fulminant hepatitis. The choleretic effect and the pharmacokinetics of LQ and its glucuronides were monitored in rats. After intravenous administration of LQ, the total area under the plasma concentration-time curve of glucuronyl metabolites was greater than that of LQ in plasma, which accompanied elevations in bile flow rate and biliary excretion of bile acid, glutathione, and bilirubin. The expressions of hepatocellular transporters and phase-II enzymes were assessed by immunoblots, real-time PCR, and immunohistochemistry. In the livers of rats treated with LQ, the protein and mRNA levels of multidrug resistance protein 2 and bile salt export pump were increased in the liver, which was verified by their increased localizations in canalicular membrane. In addition, LQ treatment enhanced the expression levels of major hepatic phase-II enzymes. Consistent with these results, LQ treatments attenuated galactosamine/LPS-induced hepatitis in rats, as supported by decreases in the plasma alanine aminotransferase, liver necrosis, and plasma TNF-alpha. These results demonstrate that LQ has a choleretic effect and the ability to induce transporters and phase-II enzymes in the liver, which may be associated with a hepatoprotective effect against galactosamine/LPS. Our findings may provide insight into understanding the action of LQ and its therapeutic use for liver disease.

Dietary chemoprevention strategies for induction of phase II xenobiotic-metabolizing enzymes in lung carcinogenesis: A review

Lung cancer is the leading cause of cancer mortality for men and women in the United States and is a growing worldwide problem. Protection against lung cancer is associated with higher dietary intake of fruits and vegetables, according to recent large epidemiologic studies. One strategy for lung cancer chemoprevention focuses on the use of agents to modulate the metabolism and disposition of tobacco, environmental and endogenous carcinogens through upregulation of detoxifying phase II enzymes. We summarize the substantial evidence that suggests that induction of phase II enzymes, particularly the glutathione S-transferases, plays a direct role in chemoprotection against lung carcinogenesis. The engagement of the Keap1-Nrf2 complex regulating the antioxidant response element (ARE) signaling pathway has been identified as a key molecular target of chemopreventive phase II inducers in several systems. Monitoring of phase II enzyme induction has led to identification of novel chemopreventive agents such as the isothiocyanate sulforaphane, and the 1,2-dithiole-3-thiones. However, no agents have yet demonstrated clear benefit in human cell systems, or in clinical trials. Alternative strategies include: (a) using intermediate cancer biomarkers for the endpoint in human trials; (b) high-throughput small molecule discovery approaches for induced expression of human phase II genes; and (c) integrative approaches that consider pharmacogenetics, along with pharmacokinetics and pharmacodynamics in target lung tissue. These approaches may lead to a more effective strategy of tailored chemoprevention efforts using compounds with proven human activity.

Inhibition of arachidonic acid and iron-induced mitochondrial dysfunction and apoptosis by oltipraz and novel 1,2-dithiole-3-thione congeners

4-Methyl-5-(2-pyrazinyl)-1,2-dithiole-3-thione (oltipraz), a prototype drug candidate containing a 1,2-dithiole-3-thione moiety, has been widely studied as a cancer chemopreventive agent. Oltipraz and other novel 1,2-dithiole-3-thione congeners have the capability to prevent insulin resistance via AMP-activated protein kinase (AMPK) activation. Arachidonic acid (AA, a proinflammatory fatty acid) exerts a deleterious effect on mitochondria and promotes reactive oxygen species (ROS) production. This study investigated whether AA alone or in combination with iron (catalyst of autooxidation) causes ROS-mediated mitochondrial impairment, and if so, whether oltipraz and synthetic 1,2-dithiole-3-thiones protect mitochondria and cells against excess ROS produced by AA + iron. Oltipraz treatment effectively inhibited mitochondrial permeability transition promoted by AA + iron in HepG2 cells, thereby protecting cells from ROS-induced apoptosis. Oltipraz was found to attenuate apoptosis induced by rotenone (complex I inhibitor), but not that by antimycin A (complex III inhibitor), suggesting that the inhibition of AA-induced apoptosis by oltipraz might be associated with the electron transport system. AMPK activation by oltipraz contributed to cell survival, which was supported by the reversal of oltipraz's restoration of mitochondrial membrane potential by concomitant treatment of compound C. By the same token, an AMPK activator inhibited AA + iron-induced mitochondrial permeability transition with an increase in cell viability. Moreover, new 1,2-dithiole-3-thiones with the capability of AMPK activation protected cells from mitochondrial permeability transition and ROS overproduction induced by AA + iron. Our results demonstrate that oltipraz and new 1,2-dithiole-3-thiones are capable of protecting cells from AA + iron-induced ROS production and mitochondrial dysfunction, which may be associated with AMPK activation.

The Nrf2 activator oltipraz also activates the constitutive androstane receptor

Oltipraz (OPZ) is a well known inducer of NAD(P)H:quinone oxidoreductase (NQO1) along with other enzymes that comprise the nuclear factor E2-related factor 2 (Nrf2) battery of detoxification genes. However, OPZ treatment also induces expression of CYP2B, a gene regulated by the constitutive androstane receptor (CAR). Therefore, this study was designed to determine whether OPZ induces gene expression in the mouse liver through activation of CAR in addition to Nrf2. OPZ increased the mRNA expression of both Cyp2b10 and Nqo1 in C57BL/6 mouse livers. As expected, in livers from Nrf2-/- mice, OPZ induction of Nqo1 was reduced, indicating Nqo1 induction is dependent on Nrf2 activation, whereas Cyp2b10 induction was unchanged. The robust induction of Cyp2b10 by OPZ in wild-type mice was completely absent in CAR-/- mice, revealing a CAR-dependent induction by OPZ. OPZ also induced transcription of the human CYP2B6 promoter-reporter containing the phenobarbital (PB) responsive element in mouse liver using an in vivo transcription assay. Additionally, OPZ induced in vivo nuclear accumulation of CAR at 3 h but, as with PB, was unable to reverse androstanol repression of mouse CAR constitutive activity in transiently transfected HepG2 cells. In summary, OPZ induces expression of Cyp2b10 and Nqo1 via the activation of CAR and Nrf2, respectively.