CYP1A induction by beta-naphthoflavone, Aroclor 1254, and 2,3,7,8-tetrachlorodibenzo-p-dioxin and its influence on aflatoxin B1 metabolism and DNA adduction in zebrafish

Toxic Effects of Polychlorinated Biphenyl Congeners and Aroclors on Embryonic Growth and Development

Polychlorinated biphenyls (PCBs) cause significant health and reproductive problems in many vertebrates. Exposure during embryogenesis likely leads to defects in organ development, compromising survival and growth through adulthood. The present study identifies the impact of PCBs on the embryonic development of key organs and resulting consequences on survival and growth. Zebrafish embryos were treated with individual PCB congeners (126 or 104) or one of 4 Aroclor mixtures (1016, 1242, 1254, or 1260) and analyzed for changes in gross embryonic morphology. Specific organs were assessed for defects during embryonic development, using a variety of transgenic zebrafish to improve organ visualization. Resulting larvae were grown to adulthood while survival and growth were assayed. Embryonic gross development on PCB treatment was abnormal, with defects presenting in a concentration-dependent manner in the liver, pancreas, heart, and blood vessel organization. Polychlorinated biphenyl 126 treatment resulted in the most consistently severe and fatal phenotypes, whereas treatments with PCB 104 and Aroclors resulted in a range of more subtle organ defects. Survival of fish was highly variable although the growth rates of surviving fish were relatively normal, suggesting that maturing PCB-treated fish that survive develop compensatory strategies needed to reach adulthood. Life span analyses of fish from embryogenesis through adulthood, as in the present study, are scarce but important for the field because they help identify foci for further studies. Environ Toxicol Chem 2021;40:187-201. © 2020 SETAC.

Aryl Hydrocarbon Receptor: Its Regulation and Roles in Transformation and Tumorigenesis

AhR is an environmental response gene that mediates cellular responses to a variety of xenobiotic compounds that frequently function as AhR ligands. Many AhR ligands are classified as carcinogens or pro-carcinogens. Thus, AhR itself acts as a major mediator of the carcinogenic effect of many xenobiotics in vivo. In this concise review, mechanisms by which AhR trans-activates downstream target gene expression, modulates immune responses, and mediates malignant transformation and tumor development are discussed. Moreover, activation of AhR by post-translational modifications and crosstalk with other transcription factors or signaling pathways are also summarized.

Using Zebrafish to Analyze the Genetic and Environmental Etiologies of Congenital Heart Defects

Congenital heart defects (CHDs) are among the most common human birth defects. However, the etiology of a large proportion of CHDs remains undefined. Studies identifying the molecular and cellular mechanisms that underlie cardiac development have been critical to elucidating the origin of CHDs. Building upon this knowledge to understand the pathogenesis of CHDs requires examining how genetic or environmental stress changes normal cardiac development. Due to strong molecular conservation to humans and unique technical advantages, studies using zebrafish have elucidated both fundamental principles of cardiac development and have been used to create cardiac disease models. In this chapter we examine the unique toolset available to zebrafish researchers and how those tools are used to interrogate the genetic and environmental contributions to CHDs.

Non-coplanar and coplanar polychlorinated biphenyls potentiate genotoxicity of aflatoxin B1 in a human hepatocyte line by enhancing CYP1A2 and CYP3A4 expression

Polychlorinated biphenyls (PCBs) are persistent organic pollutants and hazardous to human health. Aflatoxin B1 (AFB1) is a strong carcinogen dependent on activation by cytochrome P450 (CYP) 1A2 and 3A4. Humans in some regions may be exposed to both PCBs and AFB1. Since PCBs are CYP inducers, we were interested in their combined genotoxicity. In this study, the effects of non-coplanar 2,3,3'-tri- (PCB 20), 2,2'5,5'-tetra- (PCB 52), 2,3,3',4'-tetrachlorobiphenyl (PCB 56), and coplanar 3,3',4,4',5-pentachlorobiphenyl (PCB 126) on protein levels of CYP1A1, 1A2, and 3A4, and nuclear receptors AhR, CAR and PXR in a human hepatocyte (L-02) line were investigated. Moreover, the combined effects of each PCB and AFB1 for induction of micronuclei and double-strand DNA breaks (indicated by an elevation of γ-H2AX) were analyzed. The results indicated that PCBs 20, 52 and 56 reduced the expression of AhR, while elevated that of CAR and PXR, with thresholds at low micromolar concentrations. However, they were less potent than PCB 126, which was active at sub-nanomolar levels. Overexpression of human splice variant CAR 3 in the cells increased CYP1A2 and 3A4 levels, which were further enhanced by each non-coplanar PCB, suggesting a role of CAR in modulating CYPs. Pretreatment of cells with each test PCB potentiated both micronuclei formation and DNA damage induced by AFB1. This study suggests that both non-coplanar and coplanar PCBs may enhance the genotoxicity of AFB1, through acting on various nuclear receptors; the potentiation of AFB1 genotoxicity by PCBs and the potential health implications may deserve concerns and further investigation.

Presence of unreported carcinogens, Aflatoxins and their hydroxylated metabolites, in industrialized Oaxaca cheese from Mexico City

Aflatoxins (AFs) are toxic secondary metabolites of the fungi Aspergillus flavus, A. parasiticus and A. nomius. The fungi produce these AFs in cereals, oilseeds and spices. AFs have damaging effects on all organisms, including humans, and their symptoms can be classified as acute (vomiting, hemorrhage and death) or chronic (immunodepression, Reye syndrome, Kwashiorkor, teratogenesis, hepatitis, cirrhosis, and various cancers). Basic AFs (AFB₁, AFB₂, AFG₁, and AFG₂) are metabolized in the liver or by microbes that produce hydroxylated metabolites (AFM₁, AFM₂, and AFP₁) and aflatoxicol (AFL), soluble in water and easy to dispose. Thus, AFs can be excreted in fluids, such as milk. AFs are not destroyed in the process of making cheese. The purpose of this study was to identify and quantify the AFs present in 30 samples of industrialized Oaxaca-type cheese sold in Mexico City. The average concentrations of AFs detected in the 30 samples of industrialized cheese were as follows: AFB₁ (0.1 μg kg^-1) in 20% (6/30); a trace amount of AFB₂ (0.01 < LOD) in only 3% (1/30); AFG₁ (0.14 μg kg^-1) in 10% (3/30); AFG₂ (0.6 μg kg^-1) in 30% (9/30); AFM₁ (1.7 μg kg^-1) in 57% (17/30); AFP₁ (0.03% μg kg^-1) in 3% (1/30); and AFL (13.1 μg kg^-1) in 97% (29/30). AFB₁ and AFL were the most abundant aflatoxins in Oaxaca-type cheese. However, eight aflatoxins were present, contributing an average of 15.7 μg kg^-1 AFs distributed among the 30 samples. The risk assessment analysis showed that there was no substantial risk for cancer due to AFs in industrialized Oaxaca cheese from Mexico City.

Comparative live-imaging of in vivo EROD (ethoxyresorufin-O-deethylase) induction in zebrafish (Danio rerio) and fathead minnow (Pimephales promelas) embryos after exposure to PAHs and river sediment extracts

The measurement of EROD (ethoxyresorufin-O-deethylase) activity to determine the induction of CYP1A after exposure to dioxin-like substances is a well-established biomarker in fish. For reasons of animal welfare and implementations of new chemicals regulations (REACh), in vivo methods using zebrafish (Danio rerio) and medaka (Oryzias latipes) embryos have recently been developed to quantify CYP1A induction, which is visualized as mean intensity of the autofluorescent resorufin formed in living anaesthetized embryos. In the present study, concentration ranges of three PAHs (benzo[a]pyrene, β-naphthoflavone, benzo[k]fluoranthene) as examples of known CYP1A inducers as well as extracts of two well-characterized sediment samples of the lower Neckar river (Southern Germany) were used to determine the suitability of the fathead minnow (Pimephales promelas) embryo for the in vivo EROD assay. Data for zebrafish embryos were generated for comparison. Fathead minnow embryos were principally suitable to show in vivo EROD induction via live-imaging. Since in fathead minnow embryos both signal area and fluorescence intensities are lower than in zebrafish embryos, the induction potentials of the three model PAHs and the environmental samples proved to be species-dependent. Among the three PAHs tested, benzo[k]fluoranthene lead to the strongest EROD signal followed by β-naphthoflavone and benzo[a]pyrene in comparison to the positive control. Whereas benzo[k]fluoranthene and β-naphthoflavone showed a dose-response relationship for the EROD induction, benzo[a]pyrene failed to induce a significant signal in fathead minnow embryos. If compared to the model PAHs, the extracts of both sediments from the lower Neckar River induced stronger EROD signals in both fathead minnow and zebrafish embryos. Observations thus documented fathead minnow embryos to be as suitable for biomonitoring purposes as are zebrafish embryos.

Modification and quantification of in vivo EROD live-imaging with zebrafish (Danio rerio) embryos to detect both induction and inhibition of CYP1A

The visualization of specific activation of the aryl hydrocarbon receptor (AhR) directly in the zebrafish embryo (Danio rerio) via live-imaging is a reliable tool to investigate the presence of dioxin-like substances in environmental samples. The co-existence of inducers and inhibitors of cytochrome P450-dependent monooxygenases (CYP1A) is typical of complex environmental mixtures and requires modifications of the in vivo EROD assay: For this end, zebrafish embryos were used to evaluate the EROD-modifying potentials of common single-compound exposures as well as binary mixtures with the PAH-type Ah-receptor agonist β-naphthoflavone. For chemical testing, chlorpyrifos and Aroclor 1254 were selected; β-naphthoflavone served as maximum EROD induction control. Chlorpyrifos (≤EC₁₀) could be documented to be a strong CYP1A inhibitor causing characteristic edema-related toxicity. Aroclor 1254 resulted in inhibition of CYP1A catalytic activity in a concentration- and specific time-dependent manner. Next to a fast CYP1A induction, CYP1A inhibition could also be detected after 3h short-term exposure of zebrafish embryos to chlorpyrifos. This communication also describes techniques for the quantification of fluorescence signals via densitometry as a basis for subsequent statistical assessment. The co-exposure approach with zebrafish embryos accounts for the nature of potential interaction between CYP1A inducers and inhibitors and thus pays tribute to the complexity of environmental mixtures. The co-exposure EROD live-imaging assay thus facilitates a better understanding of mixture effects and allows a better assessment and interpretation of (embryo) toxic potentials.

The feasibility of using mosquitofish (Gambusia affinis) for detecting endocrine-disrupting chemicals in the freshwater environment

We evaluated the utility of gene-transcriptional responses in the liver of mosquitofish (Gambusia affinis), a species introduced to many countries and therefore widely available, for detecting endocrine-disrupting activity in water. Exposure to β-naphthoflavone, an aryl hydrocarbon receptor (AhR) agonist, significantly increased the transcript of the cytochrome P4501A gene (cyp1a), peaking at 24 h, in both sexes at concentrations of 10 µg/L or more. 17β-Estradiol (E(2) ) at 500 ng/L increased the number of males showing gene transcription of precursors of yolk protein, vitellogenin (Vtga, Vtgb, and Vtgc), at 24, 48, and 72 h. Exposure for 48 h to bisphenol A (BPA), an estrogen mimic, also increased vtg-positive males at 1 mg/L or more. Leachate from a Japanese stable-type landfill significantly increased vtg-positive males after 48 h exposure, and the in vitro activity of the leachate against the estrogen receptor (ER) was estimated as an E(2) equivalent of 240 ng/L by yeast transfected with the ER. Chemical analysis showed that major contributors to the ER activation were BPA and 4-tert-octylphenol. This leachate and drainage water from a control-type landfill had AhR activities, estimated by yeast with the AhR, but had no significant effect on cyp1a transcription. These results showed that mosquitofish are suitable for detecting in vivo AhR and ER effects, but are insensitive to E(2).

Molecular multi-effect screening of environmental pollutants using the MolDarT

Molecular effect detection is a useful approach for ecotoxicological screening of chemicals. We show here the application of the molecular DarT (MolDarT), where the expression of selected target genes is detected in short-term (120 h) exposed developing zebrafish (Danio rerio), thus allowing subacute multi-effect compound screening. The genes metallothionein 2 (mt2), cytochrome P450 1A1 (cyp1a1), and recombination activation gene 1 (rag1) are used as endpoints that describe detoxification/metal toxicity (mt2), detoxification/PAH toxicity (cyp1a1), and acquired immune system disruption (rag1). Each gene's developmental expression was studied in unexposed zebrafish during 4 to 120 h past fertilization (hpf), and all three genes were found to be expressed at 120 hpf. Furthermore, mt2 transcripts were present at high levels at 4 hpf, indicating a maternal transfer. For positive toxicity controls, freshly fertilized zebrafish eggs were exposed for 120 hpf to ZnSO(4), 1,5-dimethylnaphthalene (DMN) and CdCl(2). Exposure to 100 and 200 microM ZnSO(4) significantly induced mt2; 10 microM DMN and 20 microM DMN resulted in significantly increased cyp1a1 abundance; and 5 and 10 microM CdCl(2) significantly reduced rag1 expression levels. Furthermore, we analysed these target genes for their expression in zebrafish eggs from a previous exposure study. The eggs were exposed for 120 hpf to the environmental pollutants estradiol (E2), ethinylestradiol (EE2), nonylphenol (NP), atrazine, cyproconazol, and bisphenol A (BPA) and found differential expression of the three genes. Exposure to the (xeno-)estrogenic compound NP (0.75 microM) significantly lowered mt2 expression. This study shows the potential of short-term in vivo multi-effect screenings within one single subacute exposure using the MolDarT.

Aflatoxin B1-induced DNA damage in Labeo rohita: protective effect of an antioxidant supplement, Amrita Bindu

The present study was undertaken to investigate the effect of potent hepatocarcinogen aflatoxin B1 in adduct formation and DNA damage in Labeo rohita. Also, the salubrious efficacy of an antioxidant supplement Amrita Bindu (based on Indian system of Medicine) was investigated. Fish weighing 175-250 g were administered intraperitoneally a single dose of 100 microg aflatoxin B1/100 g body wt. and another group was given 20% solution of Amrita Bindu along with aflatoxin B1 at 100 microg/100 g body wt. On the 3rd and 6th day, the liver tissue was analyzed for aflatoxin concentration, aflatoxin-DNA adduct formation and DNA damage measured in terms of single strand breaks. The fishes administered with aflatoxin B1 showed elevated concentration of aflatoxin along with a parallel increase in the DNA adduct when compared with the controls. While the fish co-administered with Amrita Bindu showed 34% and 24% reduction in aflatoxin deposition (accumulation) and aflatoxin-DNA adduct formation respectively on the 3rd day, a further reduction by around 41% and 33% in aflatoxin deposition and DNA adduct formation respectively was observed on the 6th day. Furthermore, the increased single strand breaks (measured by alkaline single cell gel assay) and fragmentation observed in agarose gel electrophoresis in aflatoxin B1 administered fish were significantly reduced by Amrita Bindu co-administration. In conclusion, this is the first report to show aflatoxin B1-induced DNA adduct formation and DNA damage in one of the major Indian culturable fish, Labeo rohita. Also, our observations show that the antioxidant supplement, Amrita Bindu, has a potential role in ameliorating the aflatoxin B1-induced DNA damage thus suggesting its applicability in preventing the vital macromolecule DNA.

The dioxin TCDD protects against aflatoxin-induced mutation in female rats, but not in male rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant and a potent carcinogen in laboratory rodents. When combined with other environmental toxins, it has been shown to increase the (geno)toxicity of some compounds. In this study, the effect of TCDD on the mutagenicity of aflatoxin-B1 (AFB1) was examined in the rat liver using a lacI transgenic rodent mutation assay. AFB1 induces GC-->TA transversions. Since TCDD is known to have a differential effect in male and female rodents, both sexes were studied. The data showed that a 6-week pre-exposure to TCDD had no significant effect on the frequency of aflatoxin-induced mutation in the liver of male rats. However, the TCDD treatment completely prevented the aflatoxin-induced transversion mutations in female animals.

Aflatoxin B1and its interconverting metabolite aflatoxicol in milk: the situation in Mexico

Between 1996 and 1998, 580 litres of milk in Mexico were surveyed for aflatoxin B(1) (AFB(1)) and its metabolite aflatoxicol (AFL), which are mutagenic and carcinogenic mycotoxins that interconvert AFB(1)-AFL-AFB(1). The seven most consumed brands from different regions of Mexico included pasteurized and ultrapasteurized milk with four different fat levels: whole fat (28-33 g l(-1)), half-skimmed (10-20 g l(-1)), light (1-4 g l(-1)) and with vegetable oil (33 g l(-1)). Aflatoxins in each sample were concentrated with total aflatoxin immunoaffinity columns and quantitated by high-performance liquid chromatography. A milk sample was considered contaminated if it contained >/=0.05 microg l(-1) AFL. Pasteurization and ultrapasteurization of milk did not control contamination with AFL, which was present in 13% of samples at >/=0.05 microg l(-1) and in 8% at >/=0.5 microg l(-1), with a range of AFL from 0 to 12.4 microg l(-1). AFB(1) was present mainly in traces (0-0.4 microg l(-1)). The safest milk in relation to AFL contamination was imported milk powder with vegetable oil. There was a significant correlation between contamination of milk with AFL and the autumn (p<0.0002); the fat content was not significant.

Differential expression of CYP1A1 mRNA in gill, intestine and liver of tilapia fed with PCB Aroclor-1254 and Aroclor-1260 spiked food

The differential expression of cytochrome P4501A1 gene expression (CYP1A1) in liver, intestine and gill of juvenile tilapia following oral exposure to polychlorinated biphenyl (PCB) Aroclor-1254 or Aroclor-1260 spiked food were investigated. The fish was fed with 0.1 g/g of body weight each day of the spiked food with a concentration of approximately 8 ppm. Fish fed with betaNF- or acetone-spiked food were used as positive and negative controls, respectively. On day 1, 3 and 7 of post-treatment, the fish were sampled and analyzed for the total accumulated levels of PCBs, as well as the induced levels of CYP1A1 mRNA. Our results indicated that the fish fed with PCBs-spiked food showed a time-dependent bioaccumulation of PCBs. The time course studies also revealed that in the first 3 days of feeding, significant increases in CYP1A1 were found in intestine (betaNF: 320% and Aroclor-1260: 290%), followed by liver (betaNF: 144%, Aroclor-1260: 123% and Aroclor-1254: 110%). The transcript levels, then declined slowly. There was no induction of gill CYP1A1 mRNA expression in all treatment groups. Among all the tested tissues, the highest intestinal CYP1A1 transcripts induction has highlighted its prompt response to dietary PCBs exposure. Our study supported the important role of intestine in response to dietary PCBs exposure. In addition, in the present study the relative potency of Aroclor-1260 over Aroclor-1254 in stimulating CYP1A1 expression was discussed, indicating that the highly chlorinated PCBs had a higher potency of bioaccumulation and CYP1A1 mRNA induction.

Fish bioaccumulation and biomarkers in environmental risk assessment: a review

In this review, a wide array of bioaccumulation markers and biomarkers, used to demonstrate exposure to and effects of environmental contaminants, has been discussed in relation to their feasibility in environmental risk assessment (ERA). Fish bioaccumulation markers may be applied in order to elucidate the aquatic behavior of environmental contaminants, as bioconcentrators to identify certain substances with low water levels and to assess exposure of aquatic organisms. Since it is virtually impossible to predict the fate of xenobiotic substances with simple partitioning models, the complexity of bioaccumulation should be considered, including toxicokinetics, metabolism, biota-sediment accumulation factors (BSAFs), organ-specific bioaccumulation and bound residues. Since it remains hard to accurately predict bioaccumulation in fish, even with highly sophisticated models, analyses of tissue levels are required. The most promising fish bioaccumulation markers are body burdens of persistent organic pollutants, like PCBs and DDTs. Since PCDD and PCDF levels in fish tissues are very low as compared with the sediment levels, their value as bioaccumulation markers remains questionable. Easily biodegradable compounds, such as PAHs and chlorinated phenols, do not tend to accumulate in fish tissues in quantities that reflect the exposure. Semipermeable membrane devices (SPMDs) have been successfully used to mimic bioaccumulation of hydrophobic organic substances in aquatic organisms. In order to assess exposure to or effects of environmental pollutants on aquatic ecosystems, the following suite of fish biomarkers may be examined: biotransformation enzymes (phase I and II), oxidative stress parameters, biotransformation products, stress proteins, metallothioneins (MTs), MXR proteins, hematological parameters, immunological parameters, reproductive and endocrine parameters, genotoxic parameters, neuromuscular parameters, physiological, histological and morphological parameters. All fish biomarkers are evaluated for their potential use in ERA programs, based upon six criteria that have been proposed in the present paper. This evaluation demonstrates that phase I enzymes (e.g. hepatic EROD and CYP1A), biotransformation products (e.g. biliary PAH metabolites), reproductive parameters (e.g. plasma VTG) and genotoxic parameters (e.g. hepatic DNA adducts) are currently the most valuable fish biomarkers for ERA. The use of biomonitoring methods in the control strategies for chemical pollution has several advantages over chemical monitoring. Many of the biological measurements form the only way of integrating effects on a large number of individual and interactive processes in aquatic organisms. Moreover, biological and biochemical effects may link the bioavailability of the compounds of interest with their concentration at target organs and intrinsic toxicity. The limitations of biomonitoring, such as confounding factors that are not related to pollution, should be carefully considered when interpreting biomarker data. Based upon this overview there is little doubt that measurements of bioaccumulation and biomarker responses in fish from contaminated sites offer great promises for providing information that can contribute to environmental monitoring programs designed for various aspects of ERA.

The state of the art of the zebrafish model for toxicology and toxicologic pathology research--advantages and current limitations

The zebrafish (Danio rerio) is now the pre-eminent vertebrate model system for clarification of the roles of specific genes and signaling pathways in development. The zebrafish genome will be completely sequenced within the next 1-2 years. Together with the substantial historical database regarding basic developmental biology, toxicology, and gene transfer, the rich foundation of molecular genetic and genomic data makes zebrafish a powerful model system for clarifying mechanisms in toxicity. In contrast to the highly advanced knowledge base on molecular developmental genetics in zebrafish, our database regarding infectious and noninfectious diseases and pathologic lesions in zebrafish lags far behind the information available on most other domestic mammalian and avian species, particularly rodents. Currently, minimal data are available regarding spontaneous neoplasm rates or spontaneous aging lesions in any of the commonly used wild-type or mutant lines of zebrafish. Therefore, to fully utilize the potential of zebrafish as an animal model for understanding human development, disease, and toxicology we must greatly advance our knowledge on zebrafish diseases and pathology.

The zebrafish (Danio rerio) aryl hydrocarbon receptor type 1 is a novel vertebrate receptor

Fish are known to have two distinct classes of aryl hydrocarbon receptors, and their roles in mediating xenobiotic toxicity remain unclear. In this study, we have identified and characterized a cDNA tentatively named zebrafish AHR1 (zfAHR1). Analysis of the deduced amino acid sequence reveals that the protein is distinct from zfAHR2 and is more closely related to the mammalian aryl hydrocarbon receptor (AHR). zfAHR1 and zfAHR2 share 40% amino acid identity overall and 58% in the N-terminal half. The zfAHR1 gene maps to linkage group 16 in a region that shares conserved synteny with human chromosome 7 containing the human AHR, suggesting that the zfAHR1 is the ortholog of the human AHR. zfAHR2 maps to a separate linkage group (LG22). Both zfAHR mRNAs are expressed in early development, but they are differentially expressed in adult tissues. zfAHR2 can dimerize with zfARNT2b and binds with specificity to dioxin-responsive elements (DREs). Under identical conditions, zfAHR1/zfARNT2b/DRE complexes are formed; however, the interactions are considerably weaker. In COS-7 cells expressing zfARNT2b and zfAHR2, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure leads to a significant induction of dioxin-responsive reporter genes. In identical experiments, TCDD exposure fails to induce the reporter gene in zfAHR1-expressing cells. Ligand-binding experiments suggested that the differential zfAHR activities are attributable to differences in TCDD binding because only zfAHR2 exhibits high-affinity binding to [(3)H]TCDD or beta-naphthoflavone. Finally, using chimeric zfAHR1/zfAHR2 constructs, the lack of TCDD-mediated transcriptional activity was localized to the ligand-binding and C-terminal domains of zfAHR1.

Mutational spectra of benzo[a]pyrene and MeIQx in rpsL transgenic zebrafish embryos

To evaluate the rpsL transgenic zebrafish (Brachydanio rerio) mutation assay, we treated the embryos with benzo[a]pyrene (B[a]P) (10 microg/ml) or 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) (300 microg/ml) for 16h and determined the mutation spectra. These treatments were previously reported to induce mutant frequencies that were 4.3 and 2.4 times the control value, respectively. In the B[a]P-treated group, half of the mutations were single base substitutions, 74% of which occurred at G:C base pairs. Among G:C base pair substitutions, G:C to T:A and G: C to C:G transversions were predominant, suggesting that B[a]P induced mutations in zebrafish embryos by mechanisms previously described in mammalian tissues. In the MeIQx-treated group, about 60% of the mutations were deletions. Some specific mutations were found, but the compound primarily amplified the background mutation level; improvement in the conditions of treatment may be required for elucidating MeIQx-mutagenesis in this system. This study showed that transgenic zebrafish may be a useful tool for detecting mutagens in aquatic environments and for elucidating mutagenic mechanisms.

Mechanism of biochemical action of substituted 4-methylbenzopyran-2-ones. Part II: Mechanism-based inhibition of rat liver microsome-mediated aflatoxin B1-DNA binding by the candidate antimutagen 7,8-diacetoxy-4-methylcoumarin

7,8-Diacetoxy-4-methylcoumarin (DAMC), with no prerequisite for oxidative biotransformation has been reported to produce suicide inactivation of microsomal cytochrome P-450-catalysed formation of aflatoxin B1-8,9-oxide that binds to DNA. Parenteral administration of DAMC to rats caused significant inhibition of AFB1 binding to hepatic DNA in vivo as well as AFB1-induced micronuclei formation in bone marrow cells. These results highlight the antimutagenic potential of DAMC.

Aryl hydrocarbon receptor function in early vertebrates: inducibility of cytochrome P450 1A in agnathan and elasmobranch fish

The mammalian aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that controls the expression of cytochrome P450 1A (CYP1A) genes in response to halogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The natural ligand and normal physiologic function of this protein are as yet unknown. One approach to understanding AHR function and significance is to determine the evolutionary history of this receptor and of processes such as CYP1A induction that are controlled by the AHR in mammals. In these studies, AHR function was evaluated in representative cartilaginous fish (little skate, Raja erinacea) and jawless fish (sea lamprey, Petromyzon marinus and Atlantic hagfish, Myxine glutinosa), using CYP1A induction as a model AHR-dependent response. Treatment of skate with beta-naphthoflavone (BNF) caused an 8-fold increase in hepatic ethoxyresorufin O-deethylase (EROD) activity as well as a 37-fold increase in the content of immunodetectable CYP1A protein. Evidence of CYP1A inducibility was also obtained for another cartilaginous fish, the smooth dogfish Mustelus canis. In contrast, hepatic EROD activity was not detected in untreated lamprey nor in lamprey treated with 3,3'4,4'-tetrachlorobiphenyl (TCB), a potent AHR agonist in teleosts. A possible CYP1A homolog was detected in lamprey hepatic microsomes by one of three antibodies to teleost CYP1A, but expression of this protein was not altered by TCB treatment. CYP1A protein and catalytic activity were measurable in hagfish, but neither was induced after treatment with TCB. These results suggest that the AHR-CYP1A signal transduction pathway is highly conserved in gnathostomes, but that there may be fundamental differences in AHR signaling or AHR-CYP1A coupling in agnathan fish. Agnathan fish such as hagfish and lamprey may be interesting model species for examining possible ancestral AHR functions not related to CYP1A regulation.