Effects of alcohol-induced increase in CYP2E1 content in human liver microsomes on the activity and cooperativity of CYP3A4

We investigate the effect of the alcohol-induced increase in the content of CYP2E1 in human liver microsomes (HLM) on the function of CYP3A4. Membrane incorporation of the purified CYP2E1 into HLM considerably increases the rate of metabolism of 7-benzyloxyquinoline (BQ) and attenuates the homotropic cooperativity observed with this CYP3A4-specific substrate. It also eliminates the activating effect of α-naphthoflavone (ANF) seen in some HLM samples. To probe the physiological relevance of these effects, we compared three pooled preparations of HLM from normal donors (HLM-N) with a pooled preparation from ten heavy alcohol consumers (HLM-A). The composition of the P450 pool in all samples was characterized by the mass-spectrometric determination of 11 cytochrome P450 species. The fractional content of CYP2E1 in HLM-A was from 2.0 to 3.4 times higher than in HLM-N. In contrast, the content of CYP3A4 in HLM-A was the lowest among all samples. Despite that, HLM-A exhibited a much higher metabolism rate and a lower homotropic cooperativity with BQ, similar to CYP2E1-enriched HLM-N. To substantiate the involvement of interactions between CYP2E1 and CYP3A4 in these effects, we probed hetero-association of these proteins in CYP3A4-containing Supersomes™ with a technique employing CYP2E1 labeled with BODIPY-618 maleimide. These experiments evinced the interactions between the two enzymes and revealed an inhibitory effect of ANF on their association. Our results demonstrate that the functional properties of CYP3A4 are fundamentally dependent on the composition of the cytochrome P450 ensemble and suggest a possible impact of chronic alcohol exposure on the pharmacokinetics of drugs metabolized by CYP3A4.

The Aryl hydrocarbon receptor mediates tobacco-induced PD-L1 expression and is associated with response to immunotherapy

Whether tobacco carcinogens enable exposed cells immune escape resulting in carcinogenesis, and why patients who smoke respond better to immunotherapies than non-smokers, remains poorly understood. Here we report that cigarette smoke and the carcinogen benzo(a)pyrene (BaP) induce PD-L1 expression on lung epithelial cells in vitro and in vivo, which is mediated by aryl hydrocarbon receptor (AhR). Anti-PD-L1 antibody or deficiency in AhR significantly suppresses BaP-induced lung cancer. In 37 patients treated with anti-PD-1 antibody pembrolizumab, 13/16 (81.3%) patients who achieve partial response or stable disease express high levels of AhR, whereas 12/16 (75%) patients with progression disease exhibit low levels of AhR in tumor tissues. AhR inhibitors exert significant antitumor activity and synergize with anti-PD-L1 antibody in lung cancer mouse models. These results demonstrate that tobacco smoke enables lung epithelial cells to escape from adaptive immunity to promote tumorigenesis, and AhR predicts the response to immunotherapy and represents an attractive therapeutic target.

Changes in the expression of genes involved in the ovarian function of rats caused by daily exposure to 3-methylcholanthrene and their prevention by α-naphthoflavone

Daily exposure to low doses of 3-methylcholanthrene (3MC) during the pubertal period in rats disrupts both follicular growth and ovulation. Thus, to provide new insights into the toxicity mechanism of 3MC in the ovary, here we investigated the effect of daily exposure to 3MC on selected ovarian genes, the role of the aryl hydrocarbon receptor (AhR) and the level of epigenetic remodeling of histone post-transcriptional modifications. Immature rats were daily injected with 3MC (0.1 or 1 mg/kg) and mRNA expression of genes involved in different ovarian processes were evaluated. Of the 29 genes studied, 18 were up-regulated, five were down-regulated and six were not altered. To assess whether AhR was involved in these changes, we used the chromatin immunoprecipitation assay. 3MC increased AhR binding to promoter regions of genes involved in Notch signaling (Hes1, Jag1), activation of primordial follicles (Cdk2), cell adhesion (Icam1), stress and tumor progression (Dnajb6), apoptosis (Bax, Caspase-9) and expression of growth and transcription factors (Igf2, Sp1). Studying the trimethylation and acetylation of histone 3 (H3K4me3 and H3K9Ac, respectively) of these genes, we found that 3MC increased H3K4me3 in Cyp1a1, Jag1, Dnajb6, Igf2, Notch2, Adamts1, Bax and Caspase-9, and H3K9Ac in Cyp1a1, Jag1, Cdk2, Dnajb6, Igf2, Icam1, and Sp1. Co-treatment with α-naphthoflavone (αNF), a specific antagonist of AhR, prevented almost every 3MC-induced changes. Despite the low dose used in these experiments, daily exposure to 3MC induced changes in both gene expression and epigenomic remodeling, which may lead to premature ovarian failure.

The effects of CYP1A inhibition on alkyl-phenanthrene metabolism and embryotoxicity in marine medaka (Oryzias melastigma)

Alkylated polycyclic aromatic hydrocarbons (alkyl-PAHs) are the predominant form of PAHs in crude oils, of which, 3-5 ring alkyl-PAH may cause dioxin-like toxicity to early life stages of fish. Retene (7-isopropyl-1-methylphenanthrene), a typical alkyl-phenanthrene compound, can be more toxic than phenanthrene, and the mechanism of retene toxicity is likely related to its rapid biotransformation by cytochrome P450 (CYP) enzymes to metabolites with a wide array of structures and potential toxicities. Here, we investigated how α-naphthoflavone (ANF), a cytochrome P450 1A (CYP1A) inhibitor, affected the embryotoxicity of retene and the role that CYP1A inhibition may play in the interactions. Marine medaka (Oryzias melastigma) embryos were exposed, separately or together, to 200 μg/L retene with 0, 5, 10, 100, and 200 μg/L ANF for 14 days. The results showed that ANF significantly inhibited the induction of CYP1A activity by retene; however, ANF interacted with retene to induce significant developmental toxicity and genotoxicity at 10, 100, and 200 μg/L (p < 0.01). Tissue concentrations of retene and its metabolites and lipid hydroperoxide (LPO) activity also increased, whereas the inhibition of the glutathione S-transferase (GST) activity and the alteration in metabolic profiles of retene were observed. The interactions of retene with ANF indicate that CYP1A inhibition was possibly act through different mechanisms to produce similar developmental effects and genotoxicity. Retene metabolites and altered metabolic profile were likely responsible for retene embryotoxicity to marine medaka. Therefore, elevated toxicity of alkyl-phenanthrene under CYP1A inhibitor suggested that the ecotoxicity of PAHs in coastal water may have underestimated the threat of PAHs to fish or ecosystem.

Improvement of Chicken Primordial Germ Cell Maintenance In Vitro by Blockade of the Aryl Hydrocarbon Receptor Endogenous Activity

Primordial germ cells (PGCs) are the undifferentiated progenitors of gametes. Germline competent PGCs can be developed as a cell-based system for genetic modification in chickens, which provides a valuable tool for transgenic technology with both research and industrial applications. This implies manipulation of PGCs, which, in recent years, encouraged a lot of research focused on the study of PGCs and the way of improving their culture. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that besides mediating toxic responses to environmental contaminants plays pivotal physiological roles in various biological processes. Since a novel compound that acts as an antagonist of this receptor has been reported to promote expansion of hematopoietic stem cells, we conducted the present study with the aim of determining whether addition of an established AHR antagonist to the standard culture medium used nowadays for in vitro chicken PGCs culture improves ex vivo expansion. We have found that addition of α-naphthoflavone in culture medium promotes the amplification of undifferentiated cells and that this effect is exerted by the blockade of AHR action. Our results constitute the first report of the successful use of a readily available AHR antagonist to improve avian PGCs expansion, and they further extend the knowledge of the effects of AHR modulation in undifferentiated cells.

Sex hormone modulation of both induction and inhibition of CYP1A by genistein in HepG2/C3A cells

Genistein is a widely consumed phytoestrogen in dietary supplements and has been reported to play roles in both cancer prevention and promotion. These conflicting effects may be complicated by sex differences. Cytochrome P450 1A (CYP1A) participates in carcinogen activation and detoxification, and the enzyme may interact with genistein. Therefore, modulation of CYP1A by a combination of genistein and sex hormones could be responsible for sex differences related to cancer prevention and promotion. In the current study, a human liver cell line, HepG2/C3A, cultured in sex hormone-supplemented media was used to investigate the modulatory effect of genistein on CYP1A gene expression and activity. Genistein exerted both long-term (72 h) induction and short-term (immediate) inhibition of CYP1A activity in HepG2/C3A cells. In the long-term study, CYP1A gene expression and enzyme activity were induced to a greater extent in male hormone-supplemented cells than female ones. In the short-term study, CYP1A activity was inhibited more strongly by genistein in the male hormone-supplemented cells than in the female hormone-supplemented cells. These significant differences suggest that male hormones can modulate the effects of genistein on CYP1A gene expression and activity.

α-Naphthoflavone modulates inflammatory response in adipocytes-macrophages interaction through NFκB signaling

OBJECTIVE

Our previous study demonstrated that α-naphthoflavone (α-NF) inhibits mouse 3T3-L1 pre-adipocytes differentiation via PPARγ, a key transcription factor in adipogenesis. Due to the critical role of inflammation in adipogenesis, we speculated that the suppression role of α-NF in adipogenesis might involve in modulation of cytokines secretion raised by adipocyte differentiation cocktail. Therefore, the present study aims to investigate the role of α-NF in modulating of inflammatory response during adipocytes differentiation and adipocyte-macrophage interaction.

METHODS

Conditioned medium from different doses of α-NF treated 10-day differentiated 3T3-L1 adipocytes were collected to culture RAW264.7 macrophages. Conditioned medium from activated macrophages and α-NF pre-treated macrophage were used to investigate the effects of α-NF in adipocytes differentiation. Cultured cells and medium were harvested for RT-PCR, Western blot and ELISA.

RESULTS

α-NF dose-dependently decreased TNF-α and IL-6 and increased IL-10 expression induced by IDM (Insulin, dexamethasone, isobutylmethylxanthine) in 3T3-L1 pre-adipocytes. Conditioned medium from α-NF treated 3T3-L1 differentiated cells inhibited inflammatory response in mouse macrophage cell line RAW264.7 in contrast to IDM control medium. NFĸB activation elicited by IDM was suppressed by α-NF in a dose-response manner. Consequently, decreased TNF-α and increased IL-10 secretion, downstream targets of NFĸB signaling pathway, were observed with α-NF in macrophages. Finally, Conditioned medium from α-NF pre-treated, LPS-activated macrophages ameliorated the suppression of 3T3-L1 adipogenesis by LPS activated macrophages.

CONCLUSION

Our results suggest that α-NF regulates inflammation response in both adipocytes and macrophages and adipocyte-macrophage interaction which contributes to pre-adipocyte differentiation.

Isomer-nonspecific action of dichlorodiphenyltrichloroethane on aryl hydrocarbon receptor and G-protein-coupled receptor 30 intracellular signaling in apoptotic neuronal cells

Extended residual persistence of the pesticide dichlorodiphenyltrichloroethane (DDT) raises concerns about its long-term neurotoxic effects. Little is known, however, about DDT toxicity during the early stages of neural development. This study demonstrated that DDT-induced apoptosis of mouse embryonic neuronal cells is a caspase-9-, caspase-3-, and GSK-3β-dependent process, which involves p,p'-DDT-specific impairment of classical ERs. It also provided evidence for DDT-isomer-nonspecific alterations of AhR- and GPR30-mediated intracellular signaling, including changes in the levels of the receptor and receptor-regulated mRNAs, and also changes in the protein levels of the receptors. DDT-induced stimulation of AhR-signaling and reduction of GPR30-signaling were verified using selective ligands and specific siRNAs. Co-localization of the receptors was demonstrated with confocal microscopy, and the presence of functional GPR30 was detected by electrophysiology. This study demonstrates that stimulation of AhR-signaling and impairment of GPR30-signaling play important roles in the propagation of DDT-induced apoptosis during the early stages of neural development.

Metabolism of chamaechromone in vitro with human liver microsomes and recombinant human drug-metabolizing enzymes

Chamaechromone is a major component in the dried roots of Stellera chamaejasme with antihepatitis B virus and insecticidal activity. In this study, metabolic profiles of chamaechromone were investigated in human liver microsomes. One monohydroxide and two monoglucuronides of chamaechromone were identified. The enzyme kinetics for both hydroxylation and glucuronidation were fitted to the Michaelis-Menten equation. The hydroxylation of chamaechromone was inhibited by α-naphthoflavone, and predominantly catalyzed by recombinant human cytochrome P450 1A2, whereas the glucuronidation was inhibited by quercetin, 1-naphthol, and fluconazole, and mainly catalyzed by recombinant human UDP-glucuronosyltransferase 1A3, 1A7, 1A9, and 2B7.

Classification of polycyclic aromatic hydrocarbons based on mutagenicity in lung tissue through DNA microarray

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental pollutants produced in the combustion of organic matter. Exposure to PAHs raises the risk of lung cancer and inflammatory and allergic disorders such as asthma. DNA microarray technologies have been applied to research on toxicogenomics in the recent years. To evaluate the mutagenicity of PAHs and constituents of environmental pollutants in lung tissue, including metabolic activation, human alveolar epithelial type II cells (A549) were treated with nonmutagenic PAH pyrene and with the mutagenic PAHs benzo-[a]-pyrene, 1-nitropyrene, or 1,8-dinitropyrene. Comparison of genome-wide microarray expression profiles between a nonmutagenic and a mutagenic PAH-treated group revealed that xenobiotic response genes such as CYP1B1 were commonly upregulated in two groups and that DNA damage induced genes, especially p53-downstream genes such as p21 (CDKN1A) were upregulated only in the mutagenic PAH-treated group. Pretreatment with cytochrome P450 inhibitor α-naphthoflavone or p53 inhibitor pifithrin-α inhibited the benzo-[a]-pyrene-induced p21 expression. These data suggest that when PAHs enter the cells, lung epithelium induces PAH metabolic activating enzymes, and then the DNA damages-recognition signal is converged with p53 downstream genes. This metabolic activation and DNA damage is induced in lung epithelium, and the mutagenicity of PAHs can be classified by DNA microarray expression profiles.

Combination of CYP inhibitor with MEK/ERK inhibitor enhances the inhibitory effect on ERK in BRAF mutant colon cancer cells

BACKGROUND/AIM

To investigate mechanisms of discrepancy in response to a MEK/ERK inhibitor, U0126, in KRAS- and BRAF-mutant colorectal cancer cells.

MATERIALS AND METHODS

Multiparametric flow cytometry was performed on two colon cancer cell lines, HCT116 and HT29. Cells were treated with U0126, and phospho-specific antibodies were used to monitor ERK signaling.

RESULTS

HCT116 and HT29 cells were treated with increasing amounts of U0126. The western blot analysis revealed that by increasing the amount of U0126 resulted in inhibition of phospho-ERK, in HCT116 and to a lesser degree in HT29 cells. Microarray profiling identified CYP1A1 and 1A2 overexpression in HT29 cells and that inhibition of CYP1A1 with α-naphthoflavone and furanfylline restored sensitivity to U0126 in HT29 cells.

CONCLUSION

Combination of a CYP inhibitor with MEK/ERK inhibitor enhances the inhibitory effect on ERK in BRAF-mutant colon cancer cells.

α-Naphthoflavone inhibits 3T3-L1 pre-adipocytes differentiation via modulating p38MAPK signaling

α-Naphthoflavone (α-NF) is a synthetic flavonone derivative and is well known as a potent inhibitor of aromatase in a variety of systems. However, its role in lipid metabolism remains far from understood. The aim of current study was to investigate the effects of α-NF on 3T3-L1 pre-adipocytes differentiation and the mechanism through which it acts. Treatment of 3T3-L1 cells with α-NF in conjunction with a hormone cocktail resulted in α-NF mediated suppression of adipocyte differentiation in a dose dependent manner. At the molecular level, our findings demonstrated that α-NF inhibited the mid and late phase, but not the early phase of adipogenic markers expression during 3T3-L1 adipogenesis. The phosphorylation of p38 was activated upon adipogenic stimulation, yet was substantially suppressed by α-NF treatment. α-NF also synergistically inhibited expression of the adipogenic marker peroxisome proliferator-activated receptor gamma (PPARγ) expression together with p38 selective inhibitor, SB203580. Our study demonstrated for the first time that α-NF is capable of suppressing 3T3-L1 adipocyte differentiation and that this effect likely occurs through repression of the p38MAPK signaling pathway.

Dll1/Notch activation contributes to bortezomib resistance by upregulating CYP1A1 in multiple myeloma

One of the greatest challenges in multiple myeloma (MM) treatment is to overcome drug resistance. Many pathways are involved including Notch signaling. Notch receptors are expressed by MM cells and Notch ligand Dll1 is present on bone marrow (BM) stromal cells. In this study, we demonstrate that Dll1 can activate Notch signaling mostly through Notch2 receptor and can contribute to drug resistance to bortezomib, both in murine and human MM cells. Blocking the Notch pathway by DAPT (gamma secretase inhibitor) could reverse this effect and increased sensitivity to bortezomib. We describe the upregulation of CYP1A1, a Cytochrome P450 enzyme involved in drug metabolism, as a possible mechanism of Dll1/Notch induced bortezomib resistance. This was confirmed by inhibition experiments using α-Naphthoflavone or CYP1A1-siRNA that resulted in an increased sensitivity to bortezomib. In addition, in vivo data showed that combination treatment of DAPT with bortezomib was able to increase bortezomib sensitivity and prolonged overall survival in the 5T33MM mouse model. Our data provide a potential strategy to overcome bortezomib resistance by Notch inhibition in MM therapy.

Aryl hydrocarbon receptor-independent up-regulation of intracellular calcium concentration by environmental polycyclic aromatic hydrocarbons in human endothelial HMEC-1 cells

Polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene (B(a)P) constitute a major family of widely-distributed environmental toxic contaminants, known as potent ligands of the aryl hydrocarbon receptor (AhR). B(a)P has been recently shown to trigger an early and transient increase of intracellular calcium concentration ([Ca(2+)](i)), involved in AhR-related up-regulation of target genes by B(a)P. This study was designed to determine whether AhR may play a role in [Ca(2+)](i) induction provoked by B(a)P. We demonstrated that, in addition to B(a)P, various PAHs, including pyrene and benzo(e)pyrene, known to not or only very poorly interact with AhR, similarly up-regulated [Ca(2+)](i) in human endothelial HMEC-1 cells. Moreover, α-naphthoflavone, a flavonoïd antagonist of AhR, was also able to induce [Ca(2+)](i). Knocking-down AhR expression in HMEC-1 cells through transfection of siRNAs, was finally demonstrated to not prevent B(a)P-mediated induction of [Ca(2+)](i), whereas it efficiently counteracted B(a)P-mediated induction of the referent AhR target gene cytochrome P-450 1B1. Taken together, these data demonstrate that environmental PAHs trigger [Ca(2+)](i) induction in an AhR-independent manner.

[Research progress of the atypical kinetic profiles of cytochrome P450 enzymes]

Cytochrome P450 enzymes are composed of many isozymes and involved in the biotransformation of both exogenous and endogenous substances. A growing number of studies have found that the P450 enzymes do not always follow the classical Michaelis-Menten kinetics, but show atypical kinetic behavior, which is also the current research hotspot. In this paper, the category and mechanisms of atypical kinetics of the P450 enzyme were reviewed, providing theoretical basis for the research of enzyme kinetics.

Effects of cytochrome P450 inhibitors on peroxidase activity

OBJECTIVES

Of several enzymes metabolizing xenobiotics, cytochrome P450 (CYP) and peroxidase enzymes seem to be most important. One of the major challenges in studies investigating metabolism of xenobiotics is to resolve which of these two groups of enzymes is predominant to metabolize individual xenobiotic compounds. Utilization of selective inhibitors of CYP and peroxidase enzymes might be a useful tool to identify the contribution of these enzymes to metabolism of xenobiotics in samples, where both types of enzymes are present. The aim of this study was to investigate specificities of several known CYP inhibitors to these enzymes; whether they inhibit only the CYP enzymes and do not inhibit peroxidases.

METHODS

Since the oxidation of o-anisidine catalyzed by a model peroxidase used, horseradish peroxidase (HRP), is a two-substrate reaction, the inhibition potential of tested chemicals was studied with respect to both peroxidase substrates, o-anisidine and hydrogen peroxide. Initial velocities of o-anisidine oxidation by HRP under various conditions were determined spectrophotometrically.

RESULTS

The CYP inhibitors metyrapone, troleandomycine, disulfiram, sulfaphenazole, quinidine and 1-aminobenzotriazole do not inhibit o-anisidine oxidation catalyzed by HRP. In contrast, ketoconazole, diethyldithiocarbamate, ellipticine, α-naphtoflavone, proadifen SKF525A, piperonylbutoxide, were found to inhibit not only the CYPs, but also the HRP-mediated oxidation of o-anisidine. Interestingly, α-naphtoflavone inhibits oxidation of o-anisidine by HRP with respect to H2O2, but not with respect to o-anisidine. Diethyldithiocarbamate is the most potent peroxidase inhibitor of o-anisidine oxidation with Ki with respect to o-anisidine of 10 μM and Ki with respect to H2O2 of 60 μM, being even the better peroxidase inhibitor than the classical "peroxidase inhibitor" - propyl gallate (Ki with respect to o-anisidine of 60 μM and Ki with respect to H2O2 of 750 μM).

CONCLUSIONS

The results of the present study demonstrate that 1-aminobenzotriazole, a potent inhibitor of various CYP enzymes, seems to be the best candidate suitable for utilization in studies evaluating participation of CYP enzymes in metabolism of xenobiotics in various complex biological materials containing both CYP and peroxidase enzymes. Moreover, precaution to prevent misinterpretation of results is necessary in cases when proadifen SKF525A, piperonylbutoxide, diethyldithiocarbamate, ketoconazole, α-naphtoflavone and ellipticine are used in similar studies (as CYP inhibitors in various complex biological materials containing both CYP and peroxidase enzymes), since these chemicals can except of CYP enzymes inhibit also peroxidase-mediated reactions.

The arylhydrocarbon receptor is only marginally involved in the antileukemic effects of its ligand curcumin

BACKGROUND

Acute myeloid leukaemia (AML) continues to present demanding treatment challenges, as in general the prognosis for long-term survival remains dire for the patients. Natural plant-derived substances with antileukemic properties offer new treatment possibilities or may act as by-stander therapy. Their molecular mechanisms of action are often not entirely clear, limiting theory-directed screening and application strategies. The plant substance curcumin is a known activator of the transcription factor aryl hydrocarbon receptor (AhR), and has well-documented antileukemic effects. The AhR regulates cell processes, including cell cycle and apoptosis. We ask here whether direct AhR-activation by curcumin contributes to its antileukemic/apoptotic potential.

MATERIALS AND METHODS

The induction of caspases 3/7, 8, and 9, the breakdown of mitochondrial transmembrane potential, the BCL-2/BAX ratio, and the DNA content of cells were measured as indicators of apoptosis. In addition, the induction of cell cycle inhibitors p21 and p27 were assessed.

RESULTS

While triggering of AhR signalling by curcumin in HL-60 cells was confirmed, induction of the above apoptosis parameters was not blocked by two AhR antagonists, alpha-naphtoflavone (alphaNF) and 3'-methoxy-4'nitroflavone (MNF). Only a moderate (20%) AhR-dependent induction of caspases 3/7 was detectable. Interestingly, transcriptional changes induced by curcumin and by anticarcinogenic 1,25-dihydroxy vitamin D3 overlapped by one third.

CONCLUSION

We conclude that AhR is only marginally involved in the antileukemic effects of its ligand curcumin.

Lack of constitutive and inducible ethoxyresorufin-O-deethylase activity in the liver of suckermouth armored catfish (Hypostomus affinis and Hypostomus auroguttatus, Loricariidae)

We investigated the presence and inducibility of CYP1A in suckermouth catfish (Hypostomus affinis and Hypostomus auroguttatus, Loricariidae), tilapia (Oreochromis niloticus, Cichlidae) and mice (Mus musculus, Muridae). Alkoxyresorufin-O-dealkylases (EROD, MROD, PROD and BROD) were detected and proved to be inducible (beta-naphthoflavone, BNF or dimethylbenz[a]anthracene, DMBA, 50 mg/kg bw ip) in liver microsomes from tilapia and mice. In loricariids, alkoxyresorufin-O-dealkylases were either undetectable (MROD/EROD) or very low (PROD/BROD), and so they remained after treatment with BNF or DMBA. Ethoxycoumarin-O-deethylase (ECOD) was recorded in all species and proved not to be inducible by BNF or DMBA. In loricariids and tilapia, ECOD was not depressed by a concentration of alpha-naphthoflavone (CYP1A-inhibitor) that markedly depressed EROD in tilapia. A CYP1A-like protein was detected by a monoclonal antibody in rats, mice and tilapia, but not in loricariids. A polyclonal antibody, however, detected a CYP1A-like protein in liver microsomes of loricariids. Suckermouth catfish, rats, mice and tilapia express a protein reactive with a polyclonal antibody against trout CYP3A. Loricariids and tilapia exhibited marked genotoxic responses (enhanced incidence of micronucleated erythrocytes) following treatment DMBA (50 mg/kg bw ip), a promutagen activated by CYP1A/1B. Therefore, although not exhibiting EROD, a CYP1A-mediated activity, loricariids converted DMBA into its genotoxic metabolites. Our findings suggest that the CYP1A-like protein of locariid catfish recognizes DMBA, but not ethoxyresorufin, as a substrate.

[Enzyme kinetics of ligustilide metabolism in rat liver microsomes]

To study the enzyme kinetics of ligustilide metabolism and the effects of selective CYP450 inhibitors on the metabolism of ligustilide in liver microsomes of rat, a LC-MS method was established for quantitative analysis of ligustilide in liver microsomes incubation system with nitrendipine as internal standard. The determination m/z for ligustilide was 173, and for nitrendipine, 315. An optimum incubation system was found and various selective CYP inhibitors were used to investigate their inhibitory effects on the metabolism of ligustilide. The results showed that enzyme kinetics of ligustilide could be significantly inhibited by ketoconazole, trimethoprim and a-naphthoflavon but scarcely inhibited by omeprazole, 4-methylpyrazole and quinidine. Therefore, CYP3A4, CYP2C9 and CYP1A2 are the major isoenzyme participated in in vitro metabolism of ligustilide.

A low halogenated biphenyl (PCB3) increases CYP1A1 expression and activity via the estrogen receptor beta in the porcine ovary

Polychlorinated biphenyls (PCBs) have been detected at high levels, up to hundreds of pg/ml, in human ovarian follicle fluid. The effect of PCBs on the ovary and the consequences of exposure are largely unknown. We have previously shown that PCB3 (4-chlorobiphenyl) increases the secretion of estradiol and the activity of cytochrome P450s (CYPs) in ovarian follicle cells. Our goal here is to elucidate the mechanism of CYP induction by this congener. Exposure of porcine follicle cells, a co-culture of theca and granulosa cells, to 6 ng/ml of PCB3 caused an increase in CYP1A1 protein and enzymatic activity, in the same manner as exposure to exogenous 17beta-estradiol. No changes were seen in the protein level of the aryl hydrocarbon receptor (AhR), which mediates the first step in the signaling pathway of CYP1A1 induction. However, a strong reduction was seen in the protein level of estrogen receptor beta (ERbeta), while no effect was seen on ERalpha protein levels. These result suggest that: 1) PCB3 acts as an agonist of ERbeta but not the Ah receptor in the ovarian follicles, 2) PCB3 is not only an efficacious inducer of CYP1A1 expression and activity, but also a substrate for this enzyme. Changes in the expression level of CYP1A1 not only alter the intensity of the activity of PCB3, but also the activity of estrogen in the ovary.

Cytochrome P450 2C11 5'-flanking region and promoter: regulation by aromatic hydrocarbons in vitro

Aromatic hydrocarbons elicit toxic and adaptive responses via the aryl hydrocarbon receptor (AHR). Aromatic hydrocarbons suppress the transcription of the growth hormone-regulated, male-specific rat hepatic cytochrome P450 2C11 gene (CYP2C11) in vivo via an unknown mechanism. We hypothesize that the suppression of CYP2C11 by aromatic hydrocarbons is mediated by the gene's promoter and 5'-flanking region. Following bioinformatic analysis of putative transcription factor (TF) binding sites, we cloned extended lengths of the CYP2C11 5'-flanking region into a promoterless luciferase plasmid. Suppression of CYP2C11 constructs was not observed upon treatment of transfected rat 5L, BP8 or mouse Hepa-1 cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or 3-methylcholanthrene. In human HepG2 cells, the 10.1-kb construct displayed a pronounced 6- to 8-fold induction by TCDD. Deletion analysis localized the paradoxical induction response to a region between -1.8 kb and -1.3 kb, which contains a dioxin-responsive element (DRE) previously shown by us to be capable of binding activated AHR. This was confirmed by site-directed mutagenesis of the DRE. Induction of the 10.1-kb construct by TCDD in HepG2 cells was blocked by alpha-naphthoflavone, an AHR antagonist/partial agonist. The AHR is likely involved in the induction of CYP2C11-luciferase activity by TCDD in HepG2 cells and this response is at least partly DRE-mediated. Although CYP2C11 is suppressed by aromatic hydrocarbons in vivo, CYP2C11-luciferase constructs display a potentially misleading paradoxical induction in vitro that is cell-specific. Regulation of CYP2C11-luciferase plasmids is being studied in vivo in rat liver, where an intact endocrine system and the full complement of TFs needed for CYP2C11 suppression are present.

Dioxin affects glucose transport via the arylhydrocarbon receptor signal cascade in pluripotent embryonic carcinoma cells

Intoxication by dioxins such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) leads, among other damages, to early embryo loss, fetal malformations, and cardiovascular toxicity. Apart from binding to the arylhydrocarbon receptor (AhR), the mechanism of TCDD-mediated embryo toxicity is still unclear. We investigated possible modes of a TCDD-mediated toxicity, particularly in glucose metabolism, in pluripotent P19 mouse embryonic carcinoma cells. Undifferentiated P19 cells were exposed to 1-100 nM TCDD and characterized for AhR signaling. For studying cell differentiation, P19 cells were exposed to 10 nM TCDD at stage of embryoid body formation, and analyzed on glucose metabolism and cardiac differentiation during the next 3 wk. TCDD treatment activated the AhR-signaling cascade within 1 h, confirmed by AhR translocation, induction of cytochrome P450 1A1 expression, and activation of the xenobiotic response element. Although cell viability and transcription of the cardiac marker protein alpha-myosin heavy chain were affected, TCDD did not inhibit the differentiation of P19 cells to pulsating cardiomyocytes. TCDD significantly down-regulated the expression levels of the glucose transporter (GLUT) isoforms 1 and 3. After 24-h TCDD treatment, GLUT1 was no longer localized in the plasma membrane of P19 cells. The impaired GLUT expression correlated with a lower glucose uptake in 5-d-old embryoid bodies. The TCDD effects were mediated by AhR, as shown by preculture with the AhR antagonist alpha-naphthoflavone. Our data demonstrate that an AhR-mediated disturbance in GLUT expression and insufficient glucose uptake may be major mechanisms in TCDD embryo toxicity.

Antiteratogenic effects of α-naphthoflavone on 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposed mice in utero

The effects of alpha-naphthoflavone, an aryl hydrocarbon receptor (AhR) antagonist, on the reproductive toxicity and teratogenicity induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were investigated. Pregnant C57BL/6J mice were orally administered alpha-naphthoflavone either once on gestational day 12 (GD12; 50 microg/kg) or for 6 days (GD8-GD13; 5 mg/kg/day) followed by an oral challenge with TCDD (14 microg/kg) on GD12. Cesarean section was performed on GD18 for the evaluation of maternal and fetal toxicities. TCDD caused severe fetal malformations including cleft palate (43.7%) and renal pelvic and ureteric dilatations (100%). The administration of alpha-naphthoflavone either in a single treatment or 6-days remarkably reduced the incidence of cleft palate to 27.6% and 26.5%, respectively. In addition, the degree of renal pelvic and ureteric dilatations caused by TCDD were significantly attenuated by repeated treatment of alpha-naphthoflavone. These results suggest that AhR antagonists such as alpha-naphthoflavone could be promising candidates for reducing the incidence and severity of fetal malformations caused by TCDD exposure in utero.

Characterization of a 7,8-benzoflavone double effect on CFTR Cl(-) channel activity

The human cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the superfamily of adenosine triphosphate (ATP)-binding cassette (ABC) transporter ATPases. This protein forms a Cl(-) channel with a complex regulation; gene mutations cause cystic fibrosis disease. We investigated the interaction between the protein and the flavone UCCF-029 using the patch-clamp technique in the excised inside-out configuration in order to study the molecular mechanism of action for this potentiator on completely phosphorylated channel (25 U/ml protein kinase A) and a relatively low level of ATP (0.3 mM: ). Low concentrations of UCCF-029 (<50 nM: ) increase the open probability (p (o)), favoring the channel transition to an activated state, while high UCCF-029 (>50 nM: ) levels determine inhibition of the CFTR by a reduction of the total open time. Our data suggest that this drug can potentiate CFTR by binding to a specific site on the nucleotide binding domain, promoting dimer formation. The response of CFTR to variable concentrations of ATP is not modified by application of the potentiator UCCF-029 at either low, activatory, concentration or high, inhibitory, levels. Hence, we conclude that the potentiator may not interfere with binding of ATP but probably acts at an independent site in the protein, interacting directly with CFTR to modulate channel activity.

Human enteric microsomal CYP4F enzymes O-demethylate the antiparasitic prodrug pafuramidine

CYP4F enzymes, including CYP4F2 and CYP4F3B, were recently shown to be the major enzymes catalyzing the initial oxidative O-demethylation of the antiparasitic prodrug pafuramidine (DB289) by human liver microsomes. As suggested by a low oral bioavailability, DB289 could undergo first-pass biotransformation in the intestine, as well as in the liver. Using human intestinal microsomes (HIM), we characterized the enteric enzymes that catalyze the initial O-demethylation of DB289 to the intermediate metabolite, M1. M1 formation in HIM was catalyzed by cytochrome P450 (P450) enzymes, as evidenced by potent inhibition by 1-aminobenzotriazole and the requirement for NADPH. Apparent K(m) and V(max) values ranged from 0.6 to 2.4 microM and from 0.02 to 0.89 nmol/min/mg protein, respectively (n = 9). Of the P450 chemical inhibitors evaluated, ketoconazole was the most potent, inhibiting M1 formation by 66%. Two inhibitors of P450-mediated arachidonic acid metabolism, HET0016 (N-hydroxy-N'-(4-n-butyl-2-methylphenyl)formamidine) and 17-octadecynoic acid, inhibited M1 formation in a concentration-dependent manner (up to 95%). Immunoinhibition with an antibody raised against CYP4F2 showed concentration-dependent inhibition of M1 formation (up to 92%), whereas antibodies against CYP3A4/5 and CYP2J2 had negligible to modest effects. M1 formation rates correlated strongly with arachidonic acid omega-hydroxylation rates (r(2) = 0.94, P < 0.0001, n = 12) in a panel of HIM that lacked detectable CYP4A11 protein expression. Quantitative Western blot analysis revealed appreciable CYP4F expression in these HIM, with a mean (range) of 7 (3-18) pmol/mg protein. We conclude that enteric CYP4F enzymes could play a role in the first-pass biotransformation of DB289 and other xenobiotics.

In Vitro Metabolism of Isoline, a Pyrrolizidine Alkaloid fromLigularia duciformis, by Rodent Liver Microsomal Esterase and Enhanced Hepatotoxicity by Esterase Inhibitors

Isoline, a major retronecine-type pyrrolizidine alkaloid (PA) from the Chinese medicinal herb Ligularia duciformis, was suggested to be the most toxic known PA. Its in vitro metabolism was thus examined in rat and mouse liver microsomes, and its toxicity was compared with that of clivorine and monocrotaline after i.p. injection in mice. Isoline was more rapidly metabolized by both microsomes than clivorine and monocrotaline and converted to two polar metabolites M1 and M2, which were spectroscopically determined to be bisline (a deacetylated metabolite of isoline) and bisline lactone, respectively. Both metabolites were formed in the presence or absence of an NADPH-generating system with liver microsomes but not cytosol. Their formation was completely inhibited by the esterase inhibitors, triorthocresyl phosphate (TOCP) and phenylmethylsulfonyl fluoride, but not at all or partially by cytochrome P450 (P450) inhibitors, alpha-naphthoflavone and proadifen (SKF 525A), respectively. These results demonstrated that both metabolites were produced by microsomal esterase(s) but not P450 isozymes. The esterase(s) involved showed not only quite different activities but also responses to different inhibitors in rat and mouse liver microsomes, suggesting that different key isozyme(s) or combinations might be responsible for the deacetylation of isoline. Isoline injected i.p. into mice induced liver-specific toxicity that was much greater than that with either clivorine or monocrotaline, as judged by histopathology as well as serum alanine aminotransferase and aspartate aminotransferase levels. Isoline-induced hepatotoxicity was remarkably enhanced by the esterase inhibitor TOCP but was reduced by the P450 inhibitor SKF 525A, indicating that rodent hepatic esterase(s) played a principal role in the detoxification of isoline via rapid deacetylation in vivo.

2,3,7,8-Tetrachlorodibenzo-p-dioxin modulates functional differentiation of mouse bone marrow-derived dendritic cells Downregulation of RelB by 2,3,7,8-tetrachlorodibenzo-p-dioxin

We have previously shown that benzo(a)pyrene inhibits the growth and functional differentiation of mouse bone marrow (BM)-derived dendritic cells (DCs) [Hwang, J.A., Lee, J.A., Cheong, S.W., Youn, H.J., Park, J.H., 2007. Benzo(a)pyrene inhibits growth and functional differentiation of mouse bone marrow-derived dendritic cells. Downregulation of RelB and eIF3 p170 by benzo(a)pyrene. Toxicol. Lett. 169, 82-90]. Since the toxic effects of benzo(a)pyrene are aryl hydrocarbon receptor (AhR)-dependent, we examined the effects of the very potent AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the growth and functional differentiation of mouse BM-derived DCs. Ten nanomolars of TCDD had significant effects on functional differentiation of mouse DCs derived from BM cultured in the presence of GM-CSF and IL-4. The yields of DCs, flow-cytometrically analyzed for co-expression of CD11c/MHCII or CD11c/CD86, were reduced for TCDD-treated cultures, but TCDD itself had no effect on the growth of BM. DCs from TCDD-treated cultures expressed higher levels of MHCII and CD86, whereas expression of CD11c was reduced, compared with vehicle-treated cultures. Production of IL-10, but not IL-12, by the DCs from TCDD-treated cultures was decreased. Allogeneic T-cell stimulating ability of TCDD-treated DCs was increased compared to control DCs. The effects of TCDD were dependent on aryl hydrocarbon receptor (AhR), because alpha-naphthoflavone, an AhR antagonist, suppressed the effects of TCDD on IL-10 production and T-cell stimulating ability. RT-PCR revealed the downregulation of RelB, a transcription factor necessary for DCs differentiation and function. Taken together, although benzo(a)pyrene and TCDD exert their effects via binding to AhR, their effects on the growth and functional differentiation of bone marrow-derived DCs are different.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces oxidative stress, DNA strand breaks, and poly(ADP-ribose) polymerase-1 activation in human breast carcinoma cell lines

The formation of reactive oxygen species (ROS) plays a critical role in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced toxicities in mammalian cells since it promotes cell proliferation, growth arrest, and apoptosis. In this study, we investigated whether TCDD induces oxidative stress and DNA damage in human ERalpha(+)/MCF-7 and ERalpha(-)/MDA-MB-231 breast cancer cells and whether this is accompanied by the initiation of DNA repair events. Results indicated that viability of MCF-7 and MDA-MB-231 cells was concentration- and time-dependently reduced by TCDD. Further, we observed significant increases in ROS formation and decreases in intracellular glutathione (GSH) in these two cell lines after TCDD treatment. Overall, the extent of cell death was greater in MCF-7 cells than in MDA-MB-231 cells whereas the magnitude of ROS formation and GSH depletion was greater in MDA-MB-231 cells than in MCF-7 cells. In addition, we observed that at non-cytotoxic concentration (1nM for 5h), TCDD induced decreases in intracellular NAD(P)H and NAD(+) in MCF-7 and MDA-MB-231 cells. These decreases were completely blocked by three types of poly(ADP-ribose) polymerase-1 (PARP-1) inhibitors. The catalytic activation of PARP-1 in cells treated with TCDD was confirmed by detection of the presence of polymers of ADP-ribose-modified PARP-1 using Western blotting. Moreover, we demonstrated increases in the number of DNA strand breaks in MCF-7 and MDA-MB-231 cells exposed to TCDD as measured by the single-cell gel electrophoresis (Comet) assay. Overall, this evidence confirms that TCDD induces decreases in intracellular NAD(P)H and NAD(+) through PARP-1 activation mediated by formation of DNA strand breaks. In addition, we demonstrated that the extent of oxidative stress and DNA damage was greater in MDA-MB-231 cells than in MCF-7 cells, with a strong correlation to estrogen receptor (ER) status. In conclusions, our findings add further support to the theme that ROS formation is a significant determinant factor in mediating the induction of oxidative DNA damage and repair in human breast cancer cells exposed to TCDD and that the TCDD-induced oxidative stress and DNA damage may, in part, contribute to TCDD-induced carcinogenesis.

Abundance of aryl hydrocarbon receptor potentiates benzo[a]pyrene-induced apoptosis in Hepa1c1c7 cells via CYP1A1 activation

Although B[a]P-induced apoptosis has been demonstrated in Hepa1c1c7 cells, the cellular signaling pathway(s) by which benzo[a]pyrene (B[a]P) elicits a cytotoxicity-mediated apoptogenic role remains to be elucidated. In this study, we showed that B[a]P induces apoptosis in a p53-mediated and caspase-3-dependent manner, which relates to the accumulation of the S phase of the cell cycle. Importantly, we have shown for the first time that Hepa1c1c7 cells retain a considerably high content of aryl hydrocarbon receptor (AhR) protein before B[a]P exposure, assuming that this status enables the cells to respond to B[a]P more readily as well as more efficiently. B[a]P treatment resulted in the downregulation of AhR and induced cytochrome P450 1A1 (CYP1A1) (but not cytochrome P450 1B1 (CYP1B1)) expression and activity. While alpha-naphtoflavone (alpha-NF) and ellipticine suppressed B[a]P-induced CYP1A1 activation as well as apoptosis, the 2,3',4,5'-tetramethoxystilbene (TMS) and pyrene, known CYP1B1 inhibitors, failed to inhibit apoptosis. However, alpha-NF alone significantly increased CYP1A1 protein expression but not its activity, suggesting that alpha-NF more likely works as an AhR agonist in Hepa1c1c7 cells after B[a]P, rather than a direct inhibitor of CYP1A1 activity. In conclusion, it is suggested that the abundance of endogenous AhR level is an indispensable condition for an efficient cellular signaling of B[a]P and that control of AhR activity in Hepa1c1c7 cells might be important to cell fate resulting from CYP1A1 activation after B[a]P.

Effect of cytochrome P450 (CYP) inducers on caffeine metabolism in the rat

Our previous studies, carried out using rat cDNA-expressed cytochrome P450 (CYP) isoforms, liver microsomes and specific CYP inhibitors, showed that the 1-N- and 3-N-demethylation of caffeine at a therapeutic concentration was predominantly catalyzed by CYP1A2 and CYP2C, its 7-N-demethylation was governed by P450s of the CYP2C subfamily, while its 8-hydroxylation was specifically mediated by CYP1A2. The present study was aimed at corroborating the above-described results using another experimental model, i.e. a study of caffeine metabolism in the liver microsomes and specific CYP inducers. Animals received one of the following inducers: beta-naphthoflavone (100 mg/kg i.p. for 4 days), phenobarbital (10 mg/kg for 6 days or 100 mg/kg i.p. for 4 days), pregnenolone 16alpha-carbonitrile (100 mg/kg i.p. for 4 days) or 15% ethanol ( approximately 11 g/kg in drinking water for 6 days). Sixteen hours after the last dose of an inducer liver microsomes were prepared and the caffeine metabolism and CYP isoform activities (testosterone 2alpha-, 2beta-, 6beta-, 7alpha-, 16beta-hydroxylation and warfarin 7-hydroxylation) were investigated. beta-Naphthoflavone (mainly a CYP1A inducer and CYP2C11 inhibitor) potently accelerated the metabolism of caffeine, the effect on 7-N-demethylation being the weakest. Moreover, the influence of beta-naphthoflavone on caffeine metabolism was more potent at the substrate concentration of 100 microM than 800 microM, in particular in the case of 7-N-demethylation and 8-hydroxylation. Pregnenolone-16alpha-carbonitrile (mainly a CYP3A inducer and CYP2C11 inhibitor) moderately induced 8-hydroxylation only. Phenobarbital (an inducer of CYP2B and other CYPs and a CYP2C11 inhibitor) moderately stimulated the metabolism of caffeine, but practically did not affect 7-N-demethylation. Ethanol (mainly a CYP2E1 inducer) modestly increased the rates of the N-demethylation reactions. The presently obtained data confirm the pivotal role of CYP1A2 in the metabolism of caffeine, as well as the involvement of CYP3A in the 8-hydroxylation of caffeine and that of CYP2C11 in its 7-N-demethylation.

Inhibition of CYP1A enzymes by alpha-naphthoflavone causes both synergism and antagonism of retene toxicity to rainbow trout (Oncorhynchus mykiss)

Retene (7-isopropyl-1-methyl phenanthrene) is a polycyclic aromatic hydrocarbon (PAH), that causes dioxin-like toxicity to early life stages of fish, including increased rates of mortality, developmental defects characterized as blue sac disease (BSD), and induction of CYP1A enzymes. This study determined whether toxicity is associated with retene, or with its metabolism by CYP1A enzymes to hydroxylated derivatives. Larval rainbow trout (Oncorhynchus mykiss) were co-exposed to four concentrations of waterborne retene and four concentrations of waterborne alpha-naphthoflavone (ANF), a compound that antagonizes CYP1A induction and inhibits oxygenation reactions. The prevalence of mortality and BSD increased in an exposure-dependent manner for larvae exposed to retene alone. Tissue concentrations of CYP1A protein and retene metabolites also increased, but no un-metabolized retene (i.e., the parent compound) was measurable. At low concentrations of ANF, toxicity increased dramatically, while tissue concentrations of polar hydroxylated metabolites of retene decreased, and concentrations of less polar metabolites, and of parent retene, increased. At the highest concentration of ANF, retene toxicity was eliminated, and parent retene was the predominant form in tissue; no concentration of ANF was toxic by itself. The inhibition of retene hydroxylation and toxicity by ANF suggests that toxicity was caused by specific retene metabolites, and not by parent retene. The potentiation of retene toxicity at low concentrations of ANF, and the antagonism at high concentrations is a unique, non-linear interaction based on modulating CYP1A enzyme activity and retene metabolism. It demonstrates that effects on fish of different complex mixtures of hydrocarbons may not be easily predicted.

Targeted salmon gene array (SalArray): a toxicogenomic tool for gene expression profiling of interactions between estrogen and aryl hydrocarbon receptor signalling pathways

In toxicogenomics, gene arrays are valuable tools in the identification of differentially expressed genes and potentially identify new gene biomarkers altered by exposure of organisms to xenobiotic compounds, either singly or as complex mixtures. In this study, we investigated the mechanisms of interaction between estrogen receptor (ER) and aryl hydrocarbon receptor (Ah receptor or AhR) signalling pathways using toxicogenomic approaches. First, we generated cDNA libraries using suppressive subtractive hybridization (SSH) of clones containing differentially expressed genes from Atlantic salmon (Salmo salar) separately exposed to ER and AhR agonists. Second, a targeted gene array (SalArray) was developed based on true-positive differentially expressed genes. In the experimental setup, primary cultures of salmon hepatocytes isolated by a two-step perfusion method were exposed for 48 h to nonylphenol (NP; 5 microM) and 3,3',4,4'-tetrachlorobiphenyl (TCB; 1 microM), singly and combined, in the absence or presence of antagonists. Using a targeted SalArray, we demonstrate that exposure of salmon to NP singly or in combination with TCB produced differential gene expression patterns in salmon liver. Array analysis showed that exposure of hepatocytes to NP mainly altered genes involved in the estrogenic pathway, including genes for steroid hormone synthesis and metabolism. The anti-estrogenic properties of TCB were demonstrated in the array analysis as genes induced by NP were decreased by TCB. To study the effects of TCB on ER-mediated transcription, hepatocytes were treated for 48 h with tamoxifen (Tam; 1 microM) and ICI182,780 (ICI; 1 microM). The effect of AhR on ER-mediated transcription was investigated by blocking AhR activity with alpha-naphthoflavone (ANF; 0.1 and 1 microM). Quantitative real-time polymerase chain reactions confirmed the changes in expression of ERalpha, ERbeta, vitellogenin (Vtg), zona radiata protein (Zr-protein), and vigilin for the ER pathway and AhRalpha, AhRbeta, AhRR, ARNT, CYP1A1, UDPGT, and a 20S proteasome beta-subunit for the AhR pathway. We found that exposure to NP and TCB both singly and in combination produced gene expression patterns that were negatively influenced by individual receptor antagonists. TCB caused decreased ER-mediated gene expression, and NP caused decreased AhR-mediated responses. Inhibition of AhR with ANF did not reverse the effect of TCB on ER-mediated transcription suggesting that AhRs do not have a direct role on TCB-mediated decreases of ER-mediated responses. In contrast, the inhibition of ER with Tam and ICI reversed the transcription of AhR-mediated responses (except AhRR). Taken together, the findings in the present study demonstrate a complex mode of ER-AhR interaction, possibly involving competition for common cofactors. This complex mode of interaction is further supported by the observation that the presence of ER antagonists potentiated the transcription of AhR isoforms and their mediated responses when TCB was given alone (more so for AhRbeta). Thus, the inhibitory ER-AhR interactions can be used to further investigate specific genes found to be affected in our targeted SalArray chip that are important for the reproductive effects of endocrine disruptors.

Expression of zebra fish aromatase cyp19a and cyp19b genes in response to the ligands of estrogen receptor and aryl hydrocarbon receptor

Many endocrine-disrupting chemicals act via estrogen receptor (ER) or aryl hydrocarbon receptor (AhR). To investigate the interference between ER and AhR, we studied the effects of 17beta-estradiol (E2) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the expression of zebra fish cyp19a (zfcyp19a) and cyp19b (zfcyp19b) genes, encoding aromatase P450, an important steroidogenic enzyme. In vivo (mRNA quantification in exposed zebra fish larvae) and in vitro (activity of zfcyp19-luciferase reporter genes in cell cultures in response to chemicals and zebra fish transcription factors) assays were used. None of the treatments affected zfcyp19a, excluding the slight upregulation by E2 observed in vitro. Strong upregulation of zfcyp19b by E2 in both assays was downregulated by TCDD. This effect could be rescued by the addition of an AhR antagonist. Antiestrogenic effect of TCDD on the zfcyp19b expression in the brain was also observed on the protein level, assessed by immunohistochemistry. TCDD alone did not affect zfcyp19b expression in vivo or promoter activity in the presence of zebra fish AhR2 and AhR nuclear translocator 2b (ARNT2b) in vitro. However, in the presence of zebra fish ERalpha, AhR2, and ARNT2b, TCDD led to a slight upregulation of promoter activity, which was eliminated by either an ER or AhR antagonist. Studies with mutated reporter gene constructs indicated that both mechanisms of TCDD action in vitro were independent of dioxin-responsive elements (DREs) predicted in the promoter. This study shows the usefulness of in vivo zebra fish larvae and in vitro zfcyp19b reporter gene assays for evaluation of estrogenic chemical actions, provides data on the functionality of DREs predicted in zfcyp19 promoters and shows the effects of cross talk between ER and AhR on zfcyp19b expression. The antiestrogenic effect of TCDD demonstrated raises further concerns about the neuroendocrine effects of AhR ligands.

CYP3A4 mediated in vitro metabolism of vinflunine in human liver microsomes

AIM

To study the metabolism of vinflunine and the effects of selective cytochrome P-450 (CYP450) inhibitors on the metabolism of vinflunine in human liver microsomes.

METHODS

Individual selective CYP450 inhibitors were used to investigate their effects on the metabolism of vinflunine and the principal CYP450 isoform involved in the formation of metabolites M(1) and M(2) in human liver microsomes.

RESULTS

Vinflunine was rapidly metabolized to 2 metabolites: M(1) and M(2) in human liver microsomes. M(1) and M(2) were tentatively presumed to be the N-oxide metabolite or hydroxylated metabolite and epoxide metabolite of vinflunine, respectively. Ketoconazole uncompetitively inhibited the formation of M(1), and competitively inhibited the formation of M(2), while alpha-naphthoflavone, sulfaphenazole, diethyl dithiocarbamate, tranylcypromine and quinidine had little or no inhibitory effect on the formation of M(1) and M(2).

CONCLUSION

Vinflunine is rapidly metabolized in human liver microsomes, and CYP3A4 is the major human CYP450 involved in the metabolism of vinflunine.

Estrogen receptor α as a target for indole-3-carbinol

A wealth of preclinical evidence supports the antitumorigenic properties of indole-3-carbinol (I3C), which is a major bioactive food component in cruciferous vegetables. However, the underlying molecular mechanism(s) accounting for these effects remain unresolved. In the present study, estrogen receptor alpha (ER-alpha) was identified as a potential molecular target for I3C. Treating MCF-7 cells with 100 microM I3C reduced ER-alpha mRNA expression by approximately 60% compared to controls. This reduction in ER-alpha transcript levels was confirmed using real-time polymerase chain reaction. The I3C dimer, 3,3'-diindolylmethane (DIM), was considerably more effective in depressing ER-alpha mRNA in MCF-7 cells than the monomeric unit. The suppressive effects of 5 microM DIM on ER-alpha mRNA was comparable to that caused by 100 microM I3C. DIM is known to accumulate in the nucleus and is a preferred ligand for aryl hydrocarbon receptor (AhR) to I3C. The addition of other AhR ligands, alpha-naphthoflavone (alpha-NF, 10 microM) and luteolin (10 microM), to the culture media resulted in a similar suppression in ER-alpha mRNA levels to that caused by 5 microM DIM. Thus, it is likely that the binding of ligands to AhR inhibits nuclear ER-alpha transcript. The results from these experiments suggest that the antitumorigenic effects of I3C in MCF-7 human breast cancer cells may arise from its ability to reduce ER-alpha expression through the binding of its metabolite, DIM, to the nuclear AhR.

Metabolism and metabolic inhibition of gambogic acid in rat liver microsomes

AIM

To study the metabolism of gambogic acid (GA) and the effects of selective cytochrome P-450 (CYP450) inhibitors on the metabolism of GA in rat liver microsomes in vitro.

METHODS

Rat liver microsomes were used to perform metabolism studies. Various selective CYP450 inhibitors were used to investigate their effects on the metabolism of GA and the principal CYP450 isoform involved in the formation of major metabolite M(1) in rat liver microsomes. Types of inhibition in an enzyme kinetics model were used to model the interaction.

RESULTS

GA was rapidly metabolized to two phase I metabolites, M(1) and M(2), in rat liver microsomes. M(1) and M(2) were tentatively presumed to be the hydration metabolite and epoxide metabolite of GA, respectively. alpha-Naphthoflavone uncompetitively inhibited the formation of M(1) while ketoconazole, sulfaphenazole, diethyl dithiocarbamate and quinidine had little or no inhibitory effects on the formation of M(1).

CONCLUSION

GA is rapidly metabolized in rat liver microsomes and M(1) is crucial for the elimination of GA. Cytochrome P-450 1A2 is the major rat CYP involved in the metabolism of GA.

Elucidation of the Effects of the CYP1A2 Deficiency Polymorphism in the Metabolism of 4-Cyclohexyl-1-ethyl-7-methylpyrido[2,3-d]pyrimidine-2-(1H)-one (YM-64227), a Phosphodiesterase Type 4 Inhibitor, and Its Metabolites in Dogs

The canine CYP1A2 1117 C>T single nucleotide polymorphism is responsible for a substantial portion of the interindividual variability seen in the pharmacokinetics of 4-cyclohexyl-1-ethyl-7-methylpyrido[2,3-d]pyrimidine-2-(1H)-one (YM-64227). The purpose of this study is to investigate the contribution of CYP1A2 to the metabolism of YM-64227 and its five metabolites (MM-1 to MM-5), as well as to determine the interindividual variability between the pharmacokinetic profiles of the compounds with respect to the CYP1A2 deficiency polymorphism. alpha-Naphthoflavone and anti-CYP1A1/2 antibody inhibited the metabolic activities at which MM-2 and MM-3 were formed from YM-64227 in C/C- and C/T-type microsomes. In T/T type, the rate of MM-2 and MM-3 formation was lower, and alpha-naphthoflavone and anti-CYP1A1/2 antibody were shown to have no effect. A positive correlation between the overall metabolism of YM-64227 and phenacetin O-deethylation, a CYP1A2 activity marker, was observed in all the genotypes. The in vitro metabolic clearances in the T/T type of MM-1, MM-3, MM-4, and MM-5 were less than 50% lower than those in the C/C type. The anti-CYP1A1/2 antibody inhibited the metabolism of MM-1, MM-3, MM-4, and MM-5 in the C/C and C/T types. These results suggest that the formation of MM-2 and MM-3 from YM-64227 is catalyzed by CYP1A2, and that CYP1A2 contributes mainly to the subsequent metabolism of the primary metabolites of YM-64227, with the exception of MM-2. It is possible that the interindividual variability of YM-64227 with respect to the CYP1A2 deficiency polymorphism is caused by a decrease in the metabolic activities of both the unchanged drug and its metabolites.

Induction of ROS formation, poly(ADP-ribose) polymerase-1 activation, and cell death by PCB126 and PCB153 in human T47D and MDA-MB-231 breast cancer cells

The primary purpose of this research is to investigate whether exposure to polychlorinated biphenyls (PCBs), i.e. PCB153 and PCB126, is associated with induction of reactive oxygen species (ROS), poly(ADP-ribose) polymerase-1 (PARP-1) activation, and cell death in human T47D and MDA-MB-231 breast cancer cells. Results indicated that PCB153 and PCB126 induced concentration- and time-dependent increases in cytotoxic response and ROS formation in both T47D and MDA-MB-231 cells. At non-cytotoxic concentrations both PCB153 and PCB126 induced decreases in intracellular NAD(P)H and NAD+ in T47D and MDA-MB-231 cells where T47D cells were more resistant to PCB-induced reduction in intracellular NAD(P)H than MDA-MB-231 cells. Further investigation indicated that three specific PARP inhibitors completely blocked PCB-induced decreases in intracellular NAD(P)H in both T47D and MDA-MB-231 cells. These results imply that decreases in intracellular NAD(P)H in PCB-treated cells may be, in part, due to depletion of intracellular NAD+ pool mediated by PARP-1 activation through formation of DNA strand breaks. Overall, the extent of cytotoxic response, ROS formation, and PARP-1 activation generated in T47D and MDA-MB-231 cells was greater for PCB153 than for PCB126. In addition, the cytotoxicity induced by PCB153 and PCB126 in both T47D and MDA-MB-231 cells was completely blocked by co-treatment of catalase, dimethylsulfoxide, cupper (I)-/iron (II)-specific chelators, and CYP1A/2B inhibitors. This evidence suggests the involvement of ROS, Cu(I), Fe(II), and CYP1A/2B enzymes in mediating the induction of cell death by PCB153 and PCB126. Further, antagonism was observed between PCB126 and PCB153 for effects on cytotoxic response and ROS formation in T47D and MDA-MB-231 cells. Antagonism was also observed between PCB153 and PCB126 in the induction of NAD(P)H depletion at lower concentration (<10 microM) in T47D cells, but not in MDA-MB-231 cells. In conclusions, results from our investigation suggest that ROS formation induced by PCBs is a significant determinant factor in mediating the DNA damage and cell death in human breast cancer cells. The data also suggests that the status of estrogen receptor alpha may play a role in modulating the PCB-induced oxidative DNA damage and cell death in human breast cancer cells.

Heterotropic and homotropic cooperativity by a drug-metabolising mutant of cytochrome P450 BM3

Recently, we described a triple mutant of the bacterial cytochrome P450 BM3 as the first mutant with affinity for drug-like compounds. In this paper, we show that this mutant, but not wild-type BM3, is able to metabolise testosterone and several drug-like molecules such as amodiaquine, dextromethorphan, acetaminophen, and 3,4-methylenedioxymethylamphetamine that are known substrates of human P450s. Interestingly, the metabolism of 3,4-methylenedioxymethylamphetamine and acetaminophen could be stimulated up to 70-fold by the addition of caffeine, a known activator of rat P450 3A2. With testosterone metabolism, homotropic cooperativity was observed. This shows that heterotropic and homotropic cooperativity, known to occur in the P450 3A family, can also take place in BM3. BM3 therefore can be used as a model system to study atypical kinetics in mammalian P450s. Second, this study shows that BM3 can be engineered to a drug-metabolising enzyme, making it a promising candidate to use as biocatalyst in drug discovery and synthesis.

Translocation of PKC-betaII is mediated via RACK-1 in the neuronal cells following dioxin exposure

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to induce neurotoxic effects. However, the mechanism of TCDD-mediated signaling pathways and its possible molecular targets in neurons remains unknown. In this study, we analyzed effects of TCDD on neurofilament subunits, receptor for activated C kinase-1 (RACK-1), and PKC-betaII activity in developing neuronal cells. TCDD induced a significant increase of RACK-1, an adaptor protein for protein kinase C (PKC), in cerebellar granule cells in both dose- and time-dependent manner, indicating that RACK-1 is a sensitive molecular target in neuronal cells for TCDD exposure. TCDD induced a dose-dependent translocation of PKC-betaII from cytosol to membrane fractions. However, when RACK-1 induction was blocked by antisense oligonucleotide or alpha-naphthoflavone, Ah receptor (AhR) inhibitor, the translocation of PKC-betaII was inhibited. Our data suggests that TCDD activates PKC-betaII via RACK-1 in an AhR-dependent manner. This is the first report identifying RACK-1 as a target molecule involved in TCDD-mediated signaling pathways. TCDD exposure also increased the level of neurofilament-H mRNA. These results suggest that identification of target molecules may contribute to improve our understanding of TCDD-mediated signaling pathway and the risk assessment of TCDD-induced neurotoxicities.

Molecular imaging of alkylguanine-DNA alkyltransferase: further evaluation of radioiodinated derivatives of O6-benzylguanine

PURPOSE

An inverse correlation has been established between tumor levels of the DNA repair protein alkylguanine-DNA alkyltransferase (AGT) and a positive outcome after alkylator chemotherapy. Quantitative imaging of AGT could provide important information for patient-specific cancer treatment. Several radiolabeled analogues of O6-benzylguanine (BG), a potent AGT inactivator, have been developed and shown to be capable of labeling pure AGT protein. Herein, two of these analogues--O6-3-[*I]iodobenzylguanine ([*I]IBG) and O6-3-[*I]iodobenzyl-2'-deoxyguanosine ([*I]IBdG)--were further evaluated in two murine xenograft models. (AcO)2-[131I]IBdG, a peracetylated derivative of IBdG, also was investigated as an alternative agent.

METHODS

Several biodistribution studies of radioiodinated IBG and IBdG were performed in TE-671 human rhabdomyosarcoma and DAOY human medulloblastoma murine xenograft models. Mice were treated with BG or its nucleoside analogue dBG to deplete the tumor AGT content. The effect of unlabeled IBG and that of 7,8-benzoflavone (BF), an inhibitor of the cytochrome P-450 isozyme CYP1A2, on the tumor uptake of the tracers was determined. The uptake of (AcO)2-[131I]IBdG along with that of [125I]IBdG in DAOY cells in vitro was determined in the presence and absence of a nucleoside transporter inhibitor, dipyridamole.

RESULTS

Pretreatment of mice either with BG or dBG failed to reduce tumor levels of [*I]IBG or [*I]IBdG even though such treatments completely depleted tumor AGT content. Treatment of mice with BF increased tumor uptake of [125I]IBG by 56%; however, differentiation of tumors with and without AGT still was not possible. (AcO)2-[131I]IBdG, a peracetylated derivative of IBdG, had a higher uptake in vitro in DAOY tumor cells. However, its uptake, like that of [125I]IBdG, was blocked by dipyridamole.

CONCLUSIONS

Taken together, these results suggest that labeled agents that are more specific for cellular AGT and that are more metabolically stable are needed.

Human cytochrome p450 enzymes of importance for the bioactivation of methyleugenol to the proximate carcinogen 1'-hydroxymethyleugenol

In vitro studies were performed to elucidate the human cytochrome P450 enzymes involved in the bioactivation of methyleugenol to its proximate carcinogen 1'-hydroxymethyleugenol. Incubations with Supersomes, expressing individual P450 enzymes to a high level, revealed that P450 1A2, 2A6, 2C9, 2C19, and 2D6 are intrinsically able to 1'-hydroxylate methyleugenol. An additional experiment with Gentest microsomes, expressing the same individual enzymes to roughly average liver levels, indicated that P450 1A2, 2C9, 2C19, and 2D6 contribute to methyleugenol 1'-hydroxylation in the human liver. A study, in which correlations between methyleugenol 1'-hydroxylation in human liver microsomes from 15 individuals and the conversion of enzyme specific substrates by the same microsomes were investigated, showed that P450 1A2 and P450 2C9 are important enzymes in the bioactivation of methyleugenol. This was confirmed in an inhibition study in which pooled human liver microsomes were incubated with methyleugenol in the presence and absence of enzyme specific inhibitors. Kinetic studies revealed that at physiologically relevant concentrations of methyleugenol P450 1A2 is the most important enzyme for bioactivation of methyleugenol in the human liver showing an enzyme efficiency (kcat/Km) that is approximately 30, 50, and > 50 times higher than the enzyme efficiencies of, respectively, P450 2C9, 2C19, and 2D6. Only when relatively higher methyleugenol concentrations are present P450 2C9 and P450 2C19 might contribute as well to the bioactivation of methyleugenol in the human liver. A 5-fold difference in activities was found between the 15 human liver microsomes of the correlation study (range, 0.89-4.30 nmol min(-1) nmol P450(-1)). Therefore, interindividual differences might cause variation in sensitivity toward methyleugenol. Especially lifestyle factors such as smoking (induces P450 1A) and the use of barbiturates (induces P450 2C) can increase the susceptibility for adverse effects of methyleugenol.

Involvement of endogenous nitric oxide in myeloperoxidase mediated benzo(a)pyrene induced polymorphonuclear leukocytes injury

The present study was undertaken to investigate the involvement of nitric oxide in the augmentation of benzo(a)pyrene induced cellular injury in polymorphonuclear leukocytes (PMNs). Polymorphs were isolated from the blood collected from Wistar rats treated with and without benzo(a)pyrene (50mg/kg, i.p.) through cardiac puncture. Catalase, superoxide dismutase (SOD), glutathione-s-transferase (GST), myeloperoxidase (MPO) and nitrite content were estimated in PMNs using standard procedures. Inducible nitric oxide synthase (iNOS) and cytochrome P-4501A1 (CYP1A1) expression in PMNs were also analyzed in presence or absence of nitric oxide synthase (NOS) inhibitors, aminoguanidine (AG, 5mM) and L-NG nitro L-arginine methyl ester (L-NAME, 1mM). A significant augmentation was observed in the nitrite content, activities of superoxide dismutase, MPO and GST and the expressions of iNOS and CYP1A1, however, catalase activity was attenuated in PMNs of benzo(a)pyrene treated rats as compared with their respective controls. AG and L-NAME resulted in a significant attenuation in nitrite content, MPO activity and iNOS expression; however, no significant alteration was observed in CYP1A1 expression. CYP1A1 inhibitor alpha-naphthoflavone inhibited the expression of iNOS in PMNs of benzo(a)pyrene treated animals significantly. The results obtained thus suggest that CYP1A1 induces iNOS expression leading to the generation of endogenous nitric oxide (NO) that could be responsible for the augmentation of myeloperoxidase-mediated benzo(a)pyrene-induced injury in PMNs.

Characterization of anti-apoptotic action of TCDD as a defensive cellular stress response reaction against the cell damaging action of ultra-violet irradiation in an immortalized normal human mammary epithelial cell line, MCF10A

It was originally shown by Woerner and Schrenk [Woerner, W., Schrenk, D., 1998. 2,3,7,8-Tetrachlorodibenzo-p-dioxin suppresses apoptosis and leads to hyperphosphorylation of p53 in rat hepatocytes. Environ. Toxicol. Pharmacol. 6, 239-247] that TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) acts as an antagonist against the action of UV-irradiation to induce apoptosis in rat primary hepatocytes. Since prevention of apoptosis has been shown to promote carcinogenesis, we have decided to investigate this phenomenon in a human mammary gland epithelial cell line, MCF10A. We found that, in this cell line, TCDD can antagonize apoptosis that was induced by a variety of treatments, such as UV- and gamma-irradiation, growth factor starvation and trypsinization, or by the addition of H(2)O(2), TGFbeta, and staurosporine. Furthermore, other agents that are known to elicit defensive cellular responses, such as LPS, Fe(3+), nitric oxide and hypoxia could also antagonize UV induced apoptosis just as in the case of TCDD. In addition, we found that, in this cell line, such anti-apoptotic action of TCDD resembles that of exogenously added EGF or TGF alpha. To study the basic mechanism of such an action of TCDD, we tested a variety of diagnostic agents to reverse the effect of TCDD. Antagonists of TCDD which were found to be effective in this way were (a) inhibitors of c-Src kinase, such as PP-2 and CGP77675, (b) those known to block the action of TGF alpha, such as anti-TGF alpha antibody, and alpha(1)-antitrypsin, (c) PD98059, a specific inhibitor of ERK activation, but not SB202190 (an inhibitor of p38 MAPK activation) or SP600125 (a JNK inhibitor) and (d) Ah receptor antagonists, alpha-naphthoflavone and 1, 10-phenanthroline. These results support the notion that TCDD acts as an anti-apoptotic agent by mimicking the action of EGF through activation of the c-Src/ERK signaling pathway.

An aryl hydrocarbon receptor agonist amplifies the mitogenic actions of estradiol in granulosa cells: evidence of involvement of the cognate receptors

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that, besides mediating toxic responses, may have a central role in ovarian physiology. Studying the actions of AHR ligands on granulosa cells function, we have found that beta-naphthoflavone amplifies the comitogenic actions of FSH and 17beta-estradiol in a dose-dependent manner. This amplification was even greater in cells that overexpress the AHR and was reversed by cotreatment with the AHR antagonist alpha-naphthoflavone, suggesting that this effect is mediated by the AHR. The estrogen receptor is likewise implicated in this phenomenon, because a pure antiestrogen abolished the described synergism. However, the more traditional inhibitory AHR-estrogen receptor interaction was observed on the estrogen response element-driven transcriptional activity. On the other hand, alpha-naphthoflavone inhibited dose-dependently the mitogenic actions of FSH and 17beta-estradiol. Beta-naphthoflavone induced the expression of Cyp1a1 and Cyp1b1 transcripts, two well-characterized AHR-inducible genes that code for hydroxylases that metabolize estradiol to catecholestrogens. Nevertheless, the positive effect of beta-naphthoflavone on proliferation was not caused by increased metabolism of estradiol to catecholestrogens, because these compounds inhibited the hormonally stimulated DNA synthesis. This latter inhibition exerted by catecholestrogens suggests that these hydroxylases would play a regulatory point in granulosa cell proliferation. Our study indicates that AHR ligands modulate the proliferation of rat granulosa cells, and demonstrates for the first time that an agonist of this receptor is able to amplify the comitogenic action of classical hormones through a mechanism that might implicate a positive cross-talk between the AHR and the estrogen receptor pathways.

The spectrum of enzymes involved in activation of 2-aminoanthracene varies with the metabolic system applied

The aim of this study was to estimate the involvement of cytochrome P450s (CYPs) in the metabolic activation of 2-aminoanthracene (2AA) by use of metabolic systems such as liver S9 or hepatocytes from untreated and beta-naphthoflavone (BNF)- or phenobarbital (PB)-treated rats. Metabolic activation was determined in the Salmonella reverse mutation assay (Ames test). Unexpectedly, both enzyme inducers, BNF and PB, significantly decreased the mutagenicity of 2AA activated by S9 fractions. 2AA mutagenicity was detected in the presence of cytochrome P450 inhibitors such as alpha-naphthoflavone (ANF), clotrimazole and N-benzylimidazole to study the contribution of CYP isoenzymes to the activation process. ANF significantly decreased the activation of 2AA by S9 from untreated rats. In contrast, ANF significantly increased the metabolic activation of 2AA by S9 from BNF- and PB-treated rats. The enhanced mutagenicity was not altered by co-incubation with clotrimazole and ANF. Pre-incubation of 2AA in the presence of N-benzylimidazole significantly increased the activation of 2AA by S9 from BNF- and PB-treated rats, which suggests that CYPs play minor role in 2AA metabolic activation by rat liver S9 fractions. In contrast with the results described above, BNF treatment of rats significantly enhanced the activation of 2AA by hepatocytes. ANF attenuated the extent of this activation suggesting that different enzymes play a major role in the activation processes in these metabolic systems. Our results indicate that identification of mutagenic hazard by use of the Ames test may depend on the metabolic system applied.

Lack of CYP1A1 expression is involved in unresponsiveness of the human hepatoma cell line SK-HEP-1 to dioxin

The aryl hydrocarbon receptor (AhR) mediates a wide variety of toxic effects due to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The human hepatoma cell line SK-HEP-1 expresses AhR and ARNT. However, TCDD failed to induce CYP1A1 and XRE-dependent reporter genes in these cells. Although CYP1A1 was not induced by TCDD exposure, both CYP1B1 and AhR repressor (AhRR) were constitutively expressed. The AhR antagonist alpha-naphthoflavone altered the basal level of XRE-dependent reporter gene expression dose-dependently. As our results suggested the activation of AhR signals by putative endogenous ligands, we established SK-HEP-1-derived cell lines that stably expressed CYP1A1. The inducibility of XRE-dependent reporter genes and CYP1B1 by TCDD was restored in these cells. Our findings demonstrated the presence of endogenous ligands in SK-HEP-1 cells due to the absence of the metabolizing enzyme CYP1A1, but not CYP1B1, which allowed the constitutive expression of AhR target genes.

alpha-Naphthoflavone, a potent antiplatelet flavonoid, is mediated through inhibition of phospholipase C activity and stimulation of cyclic GMP formation

The aim of this study was to systematically examine the inhibitory mechanisms of the flavonoid alpha-naphthoflavone (alpha-NF) in platelet activation. In this study, alpha-NF concentration dependently (5-20 microM) inhibited platelet aggregation stimulated by agonists. alpha-NF (5 and 10 microM) inhibited intracellular Ca(2+) mobilization, phosphoinositide breakdown, and thromboxane A(2) formation stimulated by collagen (1 microg/mL) in human platelets. In addition, alpha-NF (5 and 10 microM) markedly increased levels of cyclic GMP and cyclic GMP-induced vasodilator-stimulated phosphoprotein (VASP) Ser(157) phosphorylation. Rapid phosphorylation of a platelet protein of Mr 47,000 (P47), a marker of protein kinase C activation, was triggered by phorbol-12,13-dibutyrate (60 nM). This phosphorylation was markedly inhibited by alpha-NF (5 and 10 microM). However, alpha-NF (5 and 10 microM) did not reduce the electron spin resonance (ESR) signal intensity of hydroxyl radicals in collagen (1 microg/mL)-activated platelets. These results indicate that the antiplatelet activity of alpha-NF may be involved in the following pathways. (1) alpha-NF may inhibit the activation of phospholipase C, followed by inhibition of phosphoinositide breakdown, protein kinase C activation, and thromboxane A(2) formation, thereby leading to inhibition of intracellular Ca(2+) mobilization. (2) alpha-NF also activated the formation of cyclic GMP, resulting in inhibition of platelet aggregation. These results strongly indicate that alpha-NF appears to represent a novel and potent antiplatelet agent for treatment of arterial thromboembolism.

Altered expression of cytochrome P450 and possible correlation with preneoplastic changes in early stage of rat hepatocarcinogenesis

AIM

Correlation of cytochrome P450 (CYPs) with preneoplastic changes in the early stage of hepatocarcinogenesis is still unclear. To detect the expression of carcinogen-metabolizing related microsomal P450 enzymes, namely the CYP1A1, CYP1A2, CYP2B1/2, CYP2E1, and CYP3A, we performed the medium-term bioassay of Ito's model in Sprague-Dawley rats.

METHODS

The amount and activity of CYP were assessed by biochemical and immunohistochemical methods in week 8. The correlation between CYP expression and microsomal oxidative stress was investigated by comparing the generation of microsomal lipid peroxidation in the presence or absence of specific CYP inhibitor.

RESULTS

In the DEN-2-AAF and 2-AAF alone groups, the expression of CYP1A1 and CYP2E1 were up-regulated and the expression of CYP2B1/2 and CYP1A2 were quite the contrary. Strong staining of CYP2E1 and CYP2B1/2 was found around the centrolobular vein and weak staining in the altered hepatic foci revealed by immunohistochemical procedure. There was no significant change in the activity of CYP3A among the 4 groups. Altered hepatic tissue bore more microsomal NADPH (nicotinamide adenine dinucleotide phosphate,reduced form)-dependent lipid peroxidation than normal tissue. And the difference among the 4 groups disappeared when CYP2E1 was inhibited. More microsomal lipid peroxidation was generated when incubated with CYP1A inhibitor a-naphthoflavone.

CONCLUSION

CYP altered their expression levels and these alterations can play important roles in the alteration of cell redox status of preneoplastic tissue in the early stage of hepatocarcinogenesis.

Effects of TCDD and estradiol-17beta on the proliferation and Na+/glucose cotransporter in renal proximal tubule cells

TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) is a highly toxic environmental toxicant that alters cell proliferation and function. Estrogens are noted for their ability to stimulate cell proliferation in various tissues. However, little is known about any interaction between TCDD and estradiol-17beta (E(2)) that affects renal proximal tubule cell proliferation and Na(+)/glucose cotransporters' activity. Thus, the effects of TCDD and E(2) on [(3)H]-thymidine incorporation and on alpha-methyl-d-glucopyranoside (alpha-MG) uptake were investigated in the primary rabbit kidney proximal tubule cells (PTCs). TCDD (>10(-10) M >1 h) inhibited [(3)H]-thymidine incorporation and c-fos transcripts in real-time RT-PCR, whereas E(2) (>10(-9) M, 24 h) stimulated them. Aryl hydrocarbon receptor (AhR) agonists, beta-naphthoflavone (beta-NF) and polychlorinated biphenyls (PCBs) (10(-6) M) synergistically increased the TCDD-induced inhibition of [(3)H]-thymidine incorporation. However, the AhR antagonist, alpha-naphthoflavone (alpha-NF) as well as E(2) blocked TCDD-induced inhibition of [(3)H]-thymidine incorporation. TCDD (10(-8) M, 48 h) specifically inhibited alpha-MG uptake and its effect was due to V(max) value but not K(m) value. Indeed, TCDD decreased Na(+)/glucose cotransporter 1, 2 (SGLT1, 2) protein level compared with control. In addition, TCDD-induced inhibition of alpha-MG uptake was blocked by alpha-NF or E(2). In conclusion, TCDD inhibited [(3)H]-thymidine incorporation and alpha-MG uptake, and E(2) blocked TCDDs effects in primary cultured renal proximal tubule cells.