Embryolethality and induction of 7-ethoxyresorufin O-deethylase in chick embryos by polychlorinated biphenyls and polycyclic aromatic hydrocarbons having Ah receptor affinity

Polycyclic aromatic compounds (PACs) in the Canadian environment: The challenges of ecological risk assessments

Ecological risk assessments (ERAs) of polycyclic aromatic compounds (PACs), as single congeners or in mixtures, present technical challenges that raise concerns about their accuracy and validity for Canadian environments. Of more than 100,000 possible PAC structures, the toxicity of fewer than 1% have been tested as individual compounds, limiting the assessment of complex mixtures. Because of the diversity in modes of PAC action, the additivity of mixtures cannot be assumed, and mixture compositions change rapidly with weathering. In vertebrates, PACs are rapidly oxygenated by cytochrome P450 enzymes, often to metabolites that are more toxic than the parent compound. The ability to predict the ecological fate, distribution and effects of PACs is limited by toxicity data derived from tests of a few responses with a limited array of test species, under optimal laboratory conditions. Although several models are available to predict PAC toxicity and rank species sensitivity, they were developed with data biased by test methods, and the reported toxicities of many PACs exceed their solubility limits. As a result, Canadian Environmental Quality Guidelines for a few individual PACs provide little support for ERAs of complex mixtures in emissions and at contaminated sites. These issues are illustrated by reviews of three case studies of PAC-contaminated sites relevant to Canadian ecosystems. Interactions among ecosystem characteristics, the behaviour, fate and distribution of PACs, and non-chemical stresses on PAC-exposed species prevented clear associations between cause and effect. The uncertainties of ERAs can only be reduced by estimating the toxicity of a wider array of PACs to species typical of Canada's diverse geography and environmental conditions. Improvements are needed to models that predict toxicity, and more field studies of contaminated sites in Canada are needed to understand the ecological effects of PAC mixtures.

In ovo uptake, metabolism, and tissue-specific distribution of chiral PCBs and PBDEs in developing chicken embryos

Fertilized chicken eggs were injected with environmental doses of 4 chiral polychlorinated biphenyls (PCBs) and 8 polybrominated biphenyl ethers (PBDEs) to investigate their uptake, metabolism in the embryo, and distribution in the neonate chicken. PCB95 uptake was the most efficient (80%) whereas BDE209 was the least (56%). Embryos metabolized approximately 52% of the PCBs absorbed. Though some degree of metabolism in the first 18 days, most of the PCBs and PBDEs was metabolized in the last three days, when BDE85, 99, 153, and 209 decrease by 11–37%. Enantioselective metabolism of the (+) enantiomers of PCB95, 149, and 132 and the (−) enantiomer of PCB91 was observed. The enantioselective reactivity was higher with the two penta-PCBs than the two tetra-PCBs. Liver, exhibited high affinity for high lipophilic chemicals, enrich all chemicals that was deflected in other tissues except for some special chemicals in a given tissues. Lipid composition, time of organ formation, and metabolism contribute to the distribution of chemicals in the neonate chicken. The result of this study will improve our understanding on the fate and potential adverse effects of PCBs and PBDEs in the neonate chicken.

Toxicity and cytochrome P450 1A mRNA induction by 6-formylindolo[3,2-b]carbazole (FICZ) in chicken and Japanese quail embryos

The tryptophan derivative formylindolo[3,2-b]carbazole (FICZ) binds with high ligand affinity to the aryl hydrocarbon receptor (AHR) and is readily degraded by AHR-regulated cytochrome P450 family 1 (CYP1) enzymes. Whether in vivo exposure to FICZ can result in toxic effects has not been examined and the main objective of this study was to determine if FICZ is embryotoxic in birds. We examined toxicity and CYP1 mRNA induction of FICZ in embryos from chicken (Gallus domesticus) and Japanese quail (Coturnix japonica) exposed to FICZ (2-200μgkg(-1)) by yolk and air sac injections. FICZ caused liver toxicity, embryo mortality, and CYP1A4 and CYP1A5 induction in both species with similar potency. This is in stark contrast to the very large difference in sensitivity of these species to halogenated AHR agonists. We also exposed chicken embryos to a low dose of FICZ (4μgkg(-1)) in combination with a CYP inhibitor, ketoconazole (KCZ). The mixture of FICZ and KCZ was lethal while FICZ alone had no effect at 4μgkg(-1). Furthermore, mixed exposure to FICZ and KCZ caused stronger and more long-lasting hepatic CYP1A4 induction than exposure to each compound alone. These findings indicate reduced biotransformation of FICZ by co-treatment with KCZ as a cause for the enhanced effects although additive AHR activation is also possible. To conclude, FICZ is toxic to bird embryos and it seems reasonable that the toxicity by FICZ involves AHR activation. However, the molecular targets and biological events leading to hepatic damage and mortality are unknown.

Spatial and temporal patterns of polycyclic aromatic hydrocarbons (PAHs) in eggs of a coastal bird from northwestern Iberia after a major oil spill

This study investigated the spatial and temporal patterns of polycyclic aromatic hydrocarbons (PAHs) in kentish plover (Charadrius alexandrinus) eggs after a major oil spill (Prestige, November 2002) in northwest Iberia. We analysed a total of 77 eggs from 10 breeding localities of the Iberian Atlantic coast, 9 located along the Galician coast (NW Spain) and one in the Ria de Aveiro (Portugal). General linear mixed models did not show a significant effect of the area on the total PAH levels and on each compound, probably due to the spread of pollution caused by the Prestige ship and the industrial and harbour pollutions of the Ria de Aveiro (Portugal). In contrast, the PAH levels were significantly affected by year. The PAH levels decreased from 2004 to 2006 but strongly increase and showed a different pattern of PAH accumulation in 2007. These results may be due to tetra- and pentacyclic compounds from forest fires that occurred during summer 2006.

3,3',4,4',5-Pentachlorobiphenyl (PCB 126) impacts hepatic lipid peroxidation, membrane fluidity and beta-adrenoceptor kinetics in chick embryos

Polychlorinated biphenyls (PCB) and other aryl hydrocarbon receptor (AHR) agonists induce oxidative stress and alter membrane lipid peroxidation and fluidity. This study tested the hypothesis that PCB-induced changes in membrane properties impact membrane beta-adrenoceptor (beta-AR) affinity and capacity in chick embryo hepatocytes. Embryos were injected into the air cell with 1.6 microg 3,3',4,4',5-pentachlorobiphenyl (PCB 126)/kg egg at day 0, and incubated to day 19 when livers were removed. This dose resulted in hepatic PCB 126 levels of 0.67 ng/g liver or 10.2 ng/g liver lipid; levels in untreated embryos were non-detectable. Hepatic microsomal EROD activity was elevated by approximately 12-fold and embryo mortality was significantly increased compared with the untreated group. Hepatic lipid peroxidation increased and membrane order (steady-state fluorescence anisotropy values) decreased with in ovo PCB 126 exposure. Consistent with changes in membrane structure, hepatic beta-AR affinity for CGP 12177 significantly decreased (Kd increased) without changes in receptor numbers. This study demonstrates that in ovo exposure to PCB 126 in chick eggs significantly impacted embryo survival, and this was correlated with altered hepatic membrane structure and ultimately membrane function.

Uptake of injected PCBs from the yolk by the developing chicken embryo

In this study, PCB uptake by the developing chicken embryo was measured after injection of two different doses of Aroclor 1254 before incubation. It was shown that 2% of the injected PCBs was absorbed on day 13, and this increased exponentially to 18% at day 19. This exponential increase could be described by a similar model for both low and high injection doses. Differences in injection dose resulted in corresponding differences in concentration in the embryos. Lipid corrected concentrations in the embryo were stable through development from day 13 up to day 19 and could be predicted from injection doses by using a conversion factor of 0.15 g(-1).

Molecular characterization and developmental expression of the aryl hydrocarbon receptor from the chick embryo

The aryl hydrocarbon receptor (AhR) was cloned from the chick embryo and its function and developmental expression characterized. Chicken AhR cDNA coded for 858 amino acid protein and 396 bp of 3' UTR. The basic helix loop helix domain exhibited 87-100% amino acid identity to avian, mammalian, and amphibian AhR, and 69-74% to piscine AhR. The PAS (Per-ARNT-Sim) region was slightly less well conserved with (a) 97% identity to other avian sequences, (b) 81-86% to amphibian and mammalian AhR, and (c) 64-69% with piscine AhR. The carboxy terminus diverged the most among species with less than 53% amino acid identity between chicken and any available mammalian and piscine AhR sequences. The chicken AhR RNA and protein were 6.1 kb and 103 kDa, respectively. Chicken AhR dimerized with human AhR nuclear translocator and bound the mammalian dioxin-response element in a ligand-dependent manner. AhR protein was detected in neural ganglia; smooth, cardiac, and skeletal muscle; and epithelium involved in epithelial-to-mesenchymal transformations, such as pituitary, gastrointestinal tract, limb apical-ectodermal ridge, and kidney collecting ducts. AhR mRNA was detected in all tissues expressing protein, except myocardium. Cytochrome P4501A4 mRNA was highly induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a subset of tissues expressing AhR, including small intestine, liver, kidney, blood vessels, and outflow tract myocardium. In conclusion, the AhR sequence and function is highly conserved between birds and mammals, and although many tissues express AhR during chick embryo development, only a subset are responsive to TCDD induction of CYP1A4.

Ethoxyresorufin-O-deethylase (EROD) activity in fish as a biomarker of chemical exposure

This review compiles and evaluates existing scientific information on the use, limitations, and procedural considerations for EROD activity (a catalytic measurement of cytochrome P4501A induction) as a biomarker in fish. A multitude of chemicals induce EROD activity in a variety of fish species, the most potent inducers being structural analogs of 2,3,7,8-tetracholordibenzo-p-dioxin. Although certain chemicals may inhibit EROD induction/activity, this interference is generally not a drawback to the use of EROD induction as a biomarker. The various methods of EROD analysis currently in use yield comparable results, particularly when data are expressed as relative rates of EROD activity. EROD induction in fish is well characterized, the most important modifying factors being fish species, reproductive status and age, all of which can be controlled through proper study design. Good candidate species for biomonitoring should have a wide range between basal and induced EROD activity (e.g., common carp, channel catfish, and mummichog). EROD activity has proven value as a biomarker in a number of field investigations of bleached kraft mill and industrial effluents, contaminated sediments, and chemical spills. Research on mechanisms of CYP1A-induced toxicity suggests that EROD activity may not only indicate chemical exposure, but also may also precede effects at various levels of biological organization. A current research need is the development of chemical exposure-response relationships for EROD activity in fish. In addition, routine reporting in the literature of EROD activity in standard positive and negative control material will enhance confidence in comparing results from different studies using this biomarker.

Multi-bioassay approach for assessing the potency of complex mixtures of polycyclic aromatic hydrocarbons

The chick embryotoxicity screening test (CHEST) and the Salmonella/microsome bioassay were used to evaluate embryotoxic and mutagenic endpoints from crude coal tar (CT) and its fractionated polycyclic aromatic hydrocarbon (PAH) mixtures (designated as A, B, C, D and E). In the CHEST assay, CT and PAH mixtures were injected into the egg yolk. A dose-dependent increase in embryo mortality was observed for all fractions. The E fraction resulted in 47% embryo mortality at a dose of 0.125 mg/kg and was more toxic than CT. At a dose of 1 mg/kg, 85-100% embryonic deaths occurred in fractions C and D and these two fractions were more potent than fractions A and B. The main visual toxic manifestations were liver lesions, discoloration of the liver, and edema. Both CT and fractionated PAH mixtures were also tested in the Salmonella/microsome plate incorporation assay with Salmonella typhimurium strain TA98 and were evaluated with and without metabolic activation at five dose levels. In the presence of S9, the CT and fractions C, D and E induced a dose-dependent positive response. Results from the Salmonella/microsome assay were in good agreement with findings from the CHEST assay suggesting that these two bioassays in combination may facilitate the rapid detection and ranking of complex PAH mixtures.

EROD induction by environmental contaminants in avian embryo livers

The CYP1A (EROD)-inducing potencies of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3',4,4',5-pentachlorobiphenyl (PCB126) and benzo(k)fluoranthene (B(k)F) were studied in avian embryo livers. TCDD and PCB126 proved to be much more potent as inducers in the chicken than in the other species examined. This finding is consistent with a considerably higher sensitivity of the chicken compared with a number of other avian species to the embryotoxic effects of these compounds. Furthermore, the relative potencies of the tested Ah receptor agonists as CYP1A inducers differed substantially between species. B(k)F and PCB126 showed similar induction potencies in domestic duck embryos, whereas PCB126 is much more potent than B(k)F in the chicken. Also, the potency of PCB126, relative to that of TCDD, was much lower in quail embryo liver in vitro than in chicken embryo liver. Thus, there are large interspecific differences in birds in the sensitivity to CYP1A inducers and furthermore, the relative potencies of these compounds may differ substantially between species.

Avian forms of cytochrome P450

Despite the importance of avian P450 forms in modulation of the toxicity of pesticides and other environmental chemicals, relatively little work has been done upon them, and very few forms have been fully characterised. An avian form that appears to belong to family 1A is readily inducible by planar molecules (e.g. coplanar PCB's, PCDD and certain PAHs) and has been the basis of a biomarker assay used in field studies. Although it is recognised by antibodies for mammalian P450 1A1, it evidently differs from the mammalian forms of the enzyme in catalytic properties. Phenobarbitone induces two forms of P450 in the domestic fowl (2H1 and 2H2) which have been purified, and these resemble P450 2B1 and P450 2B2 of the rat respectively. Two further phenobarbitone inducible forms, PB-A and PB-B have been partially purified. Also there is an acetone inducible form that resembles rodent P450 2E. In field studies evidence has been produced for the induction of P450s recognised by antibodies to mammalian forms of P450 1A1 and P450 2B in avian liver (adults and embryos), in response to environmental levels of PCBs. Fungicides which act as ergosterol biosynthesis inhibitors (EBI fungicides) such as prochloraz and propiconazole potentiate the toxicity of certain phosphorothionates to birds.

Formation of micronuclei in incubated hen's eggs as a measure of genotoxicity

The formation of micronuclei (MN) is a widely used and accepted endpoint of genotoxicity testing. The micronucleus assay provides a simple and rapid indirect measure of the induction of structural or numerical chromosome aberrations. In this work we describe hen's eggs, incubated for 11 days, as ex vivo assay system for the detection of micronucleus formation in young erythrocytes (Hen's Egg Test for Micronucleus Induction, HET-MN). At this stage of development the chick embryo presents a high metabolic competency which allows an adequate activation of several types of promutagens, as previously reported by several authors. As all stages of maturing erythrocytes are present in the bloodstream of the chick embryo, we could conveniently use samples of peripheral blood for scoring micronuclei as well as for determining the ratio between mature and immature erythrocytes as a measure of an undisturbed erythropoiesis. The obtained blood smears were stained by a modified May-Gruenwald-Giemsa procedure and scored microscopically. The examinations were facilitated by using a semiautomatic image analysis system. We could demonstrate a strong increase of the micronucleus frequency after the administration of the promutagens diethylnitrosamine (DENA), 7,12-dimethyl-benz[a]anthracene (DMBA), cyclophosphamide (CP), ifosphamide (IF), mitomycin C (MMC), and the direct-acting mutagen methanesulfonic acid methyl ester (MMS) compared to the concomitant negative controls. CP was used to demonstrate a dose-response relation and the effect of using two different routes of application (air cell and albumen). Nuclear aberrations, other than MN, were demonstrated after application of high doses of CP or IF. Expanded exposure times revealed a similar effect. The HET-MN, as an ex vivo assay, is a simple, inexpensive, and rapid assay system for genotoxicity testing, positioned between pure in vitro and in vivo assays, strictly in line with animal protection regulations and ethical aspects.

P450 induction by Aroclor 1254 and 3,3',4,4'-tetrachlorobiphenyl in cultured hepatocytes from rat, quail and man: interspecies comparison

Polychlorobiphenyls are potent inducers of hepatic cytochrome P450 in various species. Until now, no model based on cultured cells can be considered as a universal surrogate for in vivo metabolism. In this respect, cultured rat hepatocytes, quail hepatocytes, and human hepatoma (HepG2) cells were used to study the effects of 3,3',4,4'-tetrachlorobiphenyl (3,3',4,4'-TCB) and Aroclor 1254 on drug-metabolizing enzymes. The presence of dexamethasone in the culture medium allows the expression and the induction of several cytochrome P450 isoenzymes found in adult cells. Induction of ethoxycoumarin-(ECOD) and ethoxyresorufin-O-deethylase (EROD), activities were measured. Induced P450s were identified by immunoblotting and Northern blotting. Aroclor 1254 induced ECOD activity in all three cell types, but the effect was much stronger in fetal rat hepatocytes than in human or quail cells. Aroclor failed to induce EROD activity in quail cells, had a slight inducer effect in HepG2 cells, and a marked effect in rat hepatocytes. 3,3',4,4'-TCB had no effect in HepG2 cells but significantly increased EROD and ECOD activities, especially the latter, in rat and quail cells. On the immunoblots, specific antibodies revealed essentially CYP1A1 in fetal rat hepatocytes, CYP2B1/2 in quail hepatocytes and CYP3A1 in HepG2 cells. Analysis of Northern blots showed an hybridization with CYP1A1, 2B1 and 3A1 mRNA in fetal rat hepatocytes, CYP3A and 1A mRNA in HepG2 cells, and a form of CYP2 mRNA in fetal quail hepatocytes closely related to homolog rat CYP2E or CYP2C. In quail hepatocytes, induction did not increase proportionally with the concentration of inducer in the culture medium. Instead, the dose-response curves (for EROD activity especially) peaked sharply at 1 muM Aroclor 1254, an effect attributed to changes in membrane fluidity or lipid content. Our results highlight the advantage of using several types of cultured hepatocytes to investigate fundamental aspects of drug-metabolism-linked toxicity, the balance between xenobiotic bioactivation and detoxication being differently affected by PCBs in different animal species.

Lethality and EROD-inducing potency of chlorinated chrysene in chick embryos

Chrysene was non-specifically chlorinated and the toxic potency of the mixture formed was studied. The chlorinated mixture was considerably more potent than the parent compound in terms of embryolethality and EROD and AHH induction in a 2-week test in chick embryos. The chlorinated chrysene caused anomalies, including edema and beak defects, similar to those previously found after treatment of chick embryos with coplanar PCBs. Chlorinated chrysene was also much more potent than chrysene as an inducer of EROD in a 72-hour test in chick embryos in ovo and in chick embryo liver in vitro. It seems that the mechanism of toxicity of chlorinated chrysene in chick embryos is similar to that of the coplanar PCBs and other Ah receptor ligands. The effects of the chlorinated mixture were mainly accounted for by 6- chlorochrysene and 6,12-dichlorochrysene.

Polychlorinated biphenyls (PCBs): environmental impact, biochemical and toxic responses, and implications for risk assessment

Commercial polychlorinated biphenyls (PCBs) and environmental extracts contain complex mixtures of congeners that can be unequivocally identified and quantitated. Some PCB mixtures elicit a spectrum of biochemical and toxic responses in humans and laboratory animals and many of these effects resemble those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons, which act through the aryl hydrocarbon (Ah)-receptor signal transduction pathway. Structure-activity relationships developed for PCB congeners and metabolites have demonstrated that several structural classes of compounds exhibit diverse biochemical and toxic responses. Structure-toxicity studies suggest that the coplanar PCBs, namely, 3,3',4,4'-tetrachlorobiphenyl (tetraCB), 3,3',4,4',5-pentaCB, 3,3',4,4',5,5'-hexaCB, and their monoortho analogs are Ah-receptor agonists and contribute significantly to the toxicity of the PCB mixtures. Previous studies with TCDD and structurally related compounds have utilized a toxic equivalency factor (TEF) approach for the hazard and risk assessment of polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) congeners in which the TCDD or toxic TEQ = sigma([PCDFi x TEFi]n)+sigma([PCDDi x TEFi]n) equivalent (TEQ) of a mixture is related to the TEFs and concentrations of the individual (i) congeners as indicated in the equation (note: n = the number of congeners). Based on the results of quantitative structure-activity studies, the following TEF values have been estimated by making use of the data available for the coplanar and monoortho coplanar PCBs: 3,3',4,4',5-pentaCB, 0.1; 3,3',4,4',5,5'-hexaCB, 0.05; 3,3',4,4'-tetraCB, 0.01; 2,3,3',4,4'-pentaCB, 0.001; 2,3',4,4',5-pentaCB, 0.0001; 2,3,3',4,4',5-hexaCB, 0.0003; 2,3,3',4,4',5'-hexaCB, 0.0003; 2',3,4,4',5-pentaCB, 0.00005; and 2,3,4,4',5-pentaCB, 0.0002. Application of the TEF approach for the risk assessment of PCBs must be used with considerable caution. Analysis of the results of laboratory animal and wildlife studies suggests that the predictive value of TEQs for PCBs may be both species- and response-dependent because both additive and nonadditive (antagonistic) interactions have been observed with PCB mixtures. In the latter case, the TEF approach would significantly overestimate the toxicity of a PCB mixture. Analysis of the rodent carcinogenicity data for Aroclor 1260 using the TEF approach suggests that this response is primarily Ah-receptor-independent. Thus, risk assessment of PCB mixtures that uses cancer as the endpoint cannot solely utilize a TEF approach and requires more quantitative information on the individual congeners contributing to the tumor-promoter activity of PCB mixtures.