Assessment of within-group variation in CYP1A mRNA inducibility in environmentally exposed and chemically treated Atlantic tomcod

Rapid in-plate screening of biotransformation products in single zebrafish embryos

A procedure was developed for rapid screening of xenobiotic biotransformation products (bioTPs) in single zebrafish (ZF; Danio rerio) embryos. Exposure was carried out from 0–120 hours post fertilization (hpf) to 6 different concentrations of the model compound propranolol (PPL). Following in-plate extraction and non-target instrumental analysis by high resolution mass spectrometry, suspected bioTPs were identified using custom data filtration scripts and matching to in silico structural predictions. A total of eight PPL bioTPs were identified (five at a level 1 confidence and one at a level 2–3 confidence). These findings supplement previously generated toxicometabolomic models derived from the same dataset, and were obtained without conducting additional exposure experiments. In addition to facilitating assessments of inter-individual variability in bioTP production in ZF embryos, we demonstrate that bioTPs can be elucidated using extremely small quantities of biomass (i.e. ∼200 μg). To the best of our knowledge, this is the first time bioTP elucidation has been carried out in single ZF embryos.

A procedure was developed for rapid screening of xenobiotic biotransformation products (bioTPs) in single zebrafish (ZF; Danio rerio) embryos.

Evidence of spatially extensive resistance to PCBs in an anadromous fish of the Hudson River

Populations of organisms that are chronically exposed to high levels of chemical contaminants may not suffer the same sublethal or lethal effects as naive populations, a phenomenon called resistance. Atlantic tomcod (Microgadus tomcod) from the Hudson River, New York, are exposed to high concentrations of polycyclic aromatic hydrocarbons (PAHs) and bioaccumulate polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs). They have developed resistance to PCBs and PCDDs but not to PAHs. Resistance is largely heritable and manifests at early-life-stage toxic end points and in inducibility of cytochrome P4501A (CYP1A) mRNA expression. Because CYP1A induction is activated by the aryl hydrocarbon receptor (AHR) pathway, as are most toxic responses to these compounds, we sought to determine the geographic extent of resistance to CYP1A mRNA induction by PCBs in the Hudson River tomcod population. Samples of young-of-the-year tomcod were collected from seven locales in the Hudson River, extending from the Battery at river mile 1 (RM 1) to RM 90, and from the Miramichi River, New Brunswick, Canada. Laboratory-reared offspring of tomcod adults from Newark Bay, in the western portion of the Hudson River estuary, were also used in this study. Fish were partially depurated in clean water and intraperitoneally injected with 10 ppm coplanar PCB-77, 10 ppm benzo[a]pyrene (BaP), or corn oil vehicle, and levels of CYP1A mRNA were determined. CYP1A was significantly inducible by treatment with BaP in tomcod from the Miramichi River, from laboratory-spawned offspring of Newark Bay origin, and from all Hudson River sites spanning 90 miles of river. In contrast, only tomcod from the Miramichi River displayed significantly induced CYP1A mRNA expression when treated with PCB-77. Our results suggest that the population of tomcod from throughout the Hudson River estuary has developed resistance to CYP1A inducibility and probably other toxicities mediated by the AHR pathway. Tomcod from the Hudson River may represent the most geographically expansive population of vertebrates with resistance to chemical pollutants that has been characterized.

Resistance to contaminants in North American fish populations

Fish from urban and industrialized estuaries are exposed among the highest levels of contaminants of any vertebrate populations. As a result, they serve as especially relevant models for determining the toxic effects and mechanisms through which environmental toxicants work. In controlled laboratory experiments, fish from highly contaminated locales sometimes exhibit resistance to contaminant-induced toxicity. Resistance may be due to genetic adaptation or physiological acclimations. Distinguishing between these possibilities is important in predicting the persistence of resistance and its potential costs to affected populations and communities. Along the Atlantic coast of North America, populations of two estuarine species, Atlantic killifish (mummichog) Fundulus heteroclitus and Atlantic tomcod Microgadus tomcod, exhibit phenotypes that are resistant to aromatic hydrocarbon (AH) contaminants, including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polycyclic aromatic hydrocarbons (PAHs). Populations of these species exhibit resistance to AH-induced lethality, early life-stage toxicities, and expression of cytochrome P4501A (CYP1A). However, some differences among populations in the occurrence and type (genetic or physiological) of AH-resistant phenotypes have been observed. In some instances, resistance was obviously genetic and resulted in its transmission to at least the F2 generation, in others, resistance had a physiological or yet to be identified epigenetic basis. In some cases, resistance was observed for all AH compounds tested, in others, it was seen only for halogenated AHs. As toxic responses to AHs are believed to be mediated by the aryl hydrocarbon receptor pathway (AHR), several studies compared the structure and expression of AHR pathway molecules between resistant and sensitive fish populations. However, no obvious differences in these molecular parameters were observed between resistant and sensitive populations at the transcriptional level. Further studies at the protein level are recommended to further evaluate the role of the AHR pathway in conferring resistance. Open-ended microarray and proteomic approaches may provide additional resolution in determining the molecular mechanisms of resistance. Also, studies that evaluate the prevalence and ecosystem cost of resistance are needed.

Optimization of differential display polymerase chain reaction as a bioindicator for the cladoceran Daphnia magna

Research on toxicant-responsive genes is providing new and important bioindicators for environmental biologists. Identifying genes whose expression is modulated by toxicant exposure provides important clues into the mechanisms underlying toxicity. In addition, toxicant-responsive genes can be developed as molecular end points that are likely to be sensitive tools for environmental assessment. Differential display polymerase chain reaction (ddPCR) is a useful approach for screening and analyzing the expression of genes. A ddPCR protocol was optimized to investigate gene expression in the cladoceran Daphnia magna. The modified protocol requires submicrogram quantities of total RNA (from <10 animals) and utilizes a sensitive fluorescent tagging system. By reverse-transcribing total RNA with arbitrary 18-nucleotide primers and PCR-amplifying the cDNA using the same arbitrary primers under low-stringency conditions, reproducible and consistent ddPCR profiles were generated. Minimal variability was introduced by reaction differences or biological variability. A trial stress (starvation) was found to generate modest differences in the ddPCR profiles. This technique promises to significantly advance knowledge regarding gene expression during toxicant insult. Furthermore, this represents the first step in the development of a novel gene fingerprinting technique that can be applied to any compound and organism of interest.

Statistical analysis of cyprinid ethoxyresorufin-O-deethylase data in a large French watershed

A comparison of ethoxyresorufin-O-deethylase (EROD) data collected in 1995 in various sites in the Rhône watershed (France) was carried out to quantify the influence of factors such as contamination and biological parameters on EROD levels and within-group variabilities. Three species of cyprinids were collected and fish chemical contamination was measured. A log transformation of EROD data provided both normalization and homogeneity of variances. The influence of female sexual maturation on the variability and EROD dimorphism was quantified. A relationship with contaminant bioaccumulation was observed. A comparison with EROD data collected during previous studies by the same laboratory was made to validate the results.

Xenobiotics and xenoestrogens in fish: modulation of cytochrome P450 and carcinogenesis

As is the case with mammals, an ever-increasing number of cytochromes P450 (CYPs) are being characterized from fish. The focus of work on fish CYPs has been primarily on environmental induction of CYP1A by pollutants such as the polycyclic aromatic hydrocarbons, polychlorinated biphenyls, dioxins and dibenzofurans. This response has been the basis for a sensitive biomonitoring tool of ecosystem health for a number of years. Studies have documented a correlation between CYP1A induction, pollutant levels and tumor incidence, especially in bottom-dwelling species. The rainbow trout has been utilized as a tumor model to document the role of CYP1A modulation in the inhibition or promotion of cancer. Fish are also very responsive to the class of chemicals known as xenoestrogens. Recent evidence is presented documenting the modulation of CYPs by xenoestrogens and their potential role as modulators of the tumor response. In this paper, we summarize the current knowledge concerning the occurrence of CYPs in fish and focus on the role of CYP1A induction in environmental monitoring of various genotoxic carcinogens and in the modulation of cancer in the trout model. Finally, the important class of aquatic pollutants known as xenoestrogens have now been shown to modulate CYP levels perhaps leading to alterations in tumor response or other adverse effects.

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

This study investigated the inductive response of cytochrome P4501A (CYP1A) in the zebrafish (Danio rerio) following exposure to Aroclor 1254, beta-naphthoflavone (betaNF), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and then investigated TCDD modulation of aflatoxin B1 (AFB1) metabolism and hepatic AFB1-DNA adduction. Aroclor 1254 fed at 500 ppm for 1 to 9 days or intraperitoneal (ip) injection of 75-200 mg Aroclor 1254/kg body weight failed to induce CYP1A protein or associated 7-ethoxyresorufin-O-deethylase (EROD) activity. By contrast, dietary betaNF at 500 ppm for 3 or 7 days induced CYP1A protein and EROD activity approximately threefold above controls. A single ip injection of 150 mg/kg betaNF showed maximal induction of CYP1A protein and EROD activity near 24 hr, both of which decreased to control levels during the next 6 days. Single ip administration of 25, 50, 100, or 150 mg betaNF/kg body weight provided dose-responsive increases in CYP1A and EROD activity. Dietary exposure to 0.75 ppm TCDD for 3 days also significantly induced CYP1A and EROD. The effect of TCDD on the metabolism of [3H]AFB1 in zebrafish was then investigated. The major [3H]AFB1 metabolites excreted in water over 24 hr in the control group were aflatoxicol, aflatoxicol-glucuronide, and parent AFB1. By contrast, the predominant metabolites in the TCDD-pretreated group were aflatoxicol-M1-glucuronide, aflatoxicol, aflatoxin M1 plus aflatoxicol-M1 (unresolved), aflatoxicol-glucuronide, and parent AFB1. Surprisingly, hepatic AFB1-DNA adduction was approximately fourfold higher in the TCDD treated group than in controls. This significant difference could not be explained by increased capacity for bioactivation of AFB1 as measured by an in vitro AFB1-exo-8, 9-epoxide trapping assay. However, it was demonstrated that both control and induced zebrafish have high capacity to bioactivate aflatoxin M1 to a reactive intermediate, such that secondary bioactivation of this genotoxic intermediate may be responsible for the increased DNA binding.

Genetic and molecular ecotoxicology: a research framework

Participants at the Napa Conference on Genetic and Molecular Ecotoxicology assessed the status of this field in light of heightened concerns about the genetic effects of exposure to hazardous substances and recent advancements in our capabilities to measure those effects. We present here a synthesis of the ideas discussed throughout the conference, including definitions of important concepts in the field and critical research needs and opportunities. While there were many opinions expressed on these topics, there was general agreement that there are substantive new opportunities to improve the impact of genetic and molecular ecotoxicology on prediction of sublethal effects of exposure to hazardous substances. Future studies should emphasize integration of genetic ecotoxicology, ecological genetics, and molecular biology and should be directed toward improving our understanding of the ecological implications of genotoxic responses. Ecological implications may be assessed at either the population or ecosystem level; however, a population-level focus may be most pragmatic. Recent technical advancements in measuring genetic and molecular responses to toxicant exposure will spur rapid progress. These new techniques have considerable promise for increasing our understanding of both mechanisms of toxicity on genes or gene products and the relevance of detrimental effects to individual fitness.