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

Co-Exposure of Phenanthrene and the cyp-Inducer 3-Methylchrysene Leads to Altered Biotransformation and Increased Toxicity in Fish Egg and Larvae

Polycyclic aromatic hydrocarbons (PAHs) have frequently been suspected of governing crude oil toxicity because of similar morphological defects in fish. However, PAH concentrations are often not high enough to explain the observed crude oil toxicity. We hypothesize that one PAH can enhance the metabolism and toxicity of another PAH when administered as a mixture. Early life stage Atlantic haddock (Melanogrammus aeglefinus) were in this study exposed to phenanthrene in the presence and absence of 3-methylchrysene that is known to induce the metabolic enzyme cytochrome P450 1A via cyp1a gene expression. Uptake, metabolism, and multiple toxicity endpoints were then measured in a time-course study up to 3 days post-hatching. Passive dosing provided aqueous concentrations ≈180 μg/L for phenanthrene and ≈0.6 μg/L for 3-methylchrysene, which resulted in tissue concentrations ≈60 μg/g ww for phenanthrene and ≈0.15 μg/g ww for 3-methylchrysene. The low concentration of 3-methylchrysene led to the elevated expression of cyp1a but no toxicity. Levels of phenanthrene metabolites were 5-fold higher, and morphological defects and cardiotoxicity were consistently greater when co-exposed to both compounds relative to phenanthrene alone. This work highlights the metabolic activation of PAH toxicity by a co-occurring PAH, which can lead to excess toxicity, synergistic effects, and the overproportional contribution of PAHs to crude oil toxicity.

Alkyl-phenanthrenes in early life stage fish: differential toxicity in Atlantic haddock (Melanogrammus aeglefinus) embryos

Tricyclic polycyclic aromatic hydrocarbons (PAHs) are believed to be the primary toxic components of crude oil. Such compounds including phenanthrene are known to have direct effects on cardiac tissue, which lead to malformations during organogenesis in early life stage fish. We tested a suite of 13 alkyl-phenanthrenes to compare uptake and developmental toxicity in early life stage haddock (Melanogrammus aeglefinus) embryos during gastrulation/organogenesis beginning at 2 days post fertilization via passive dosing. The alkyl-phenanthrenes were tested at their solubility limits, and three of them also at lower concentrations. Measured body burdens were linearly related to measured water concentrations. All compounds elicited one or more significant morphological defects or functional impairment, such as decreased length, smaller eye area, shorter jaw length, and increased incidence of body axis deformities and eye deformities. The profile of developmental toxicities appeared unrelated to the position of alkyl substitution, and gene expression of cytochrome 1 a (cyp1a) was low regardless of alkylation. Mortality and sublethal effects were observed below the expected range for baseline toxicity, thus indicating excess toxicity. Additionally, PAH concentrations that resulted in toxic effects here were far greater than when measured in whole crude oil exposures that cause toxicity. This work demonstrates that, while these phenanthrenes are toxic to early life stage fish, they cannot individually account for most of the developmental toxicity of crude oil, and that other compounds and/or mixture effects should be given more consideration.

Regiospecific Photochemical Synthesis of Methylchrysenes

Methylated polycyclic aromatic hydrocarbons (PAHs) are suspected to be some of the toxic compounds in crude oil towards marine life and are needed as single compounds for environmental studies. 1-, 3- and 6-methylchrysene (3a,b,c) were prepared as single isomers by photochemical cyclization of the corresponding stilbenoids in the Mallory reaction using stoichiometric amounts of iodine in 82-88% yield. 2-methylchrysene (3d) was prepared by photochemical cyclization where the regioselectivity was controlled by elimination of an ortho-methoxy group under acidic oxygen free conditions in 72% yield. These conditions failed to form 4-methylchrysene from the corresponding stilbenoid. All stilbenoids were made from a common naphthyl Wittig salt and suitably substituted benzaldehydes. We have also demonstrated that methylchrysenes can be oxidized to the corresponding chrysenecarboxylic acids by KMnO₄ in modest yields.

Endogenous AhR agonist FICZ accumulates in rainbow trout (Oncorhynchus mykiss) alevins exposed to a mixture of two PAHs, retene and fluoranthene

Multiple studies have reported synergized toxicity of PAH mixtures in developing fish larvae relative to the additive effect of the components. From a toxicological perspective, multiple mechanisms are known to contribute to synergism, such as altered toxicodynamics and kinetics, as well as increased oxidative stress. An understudied contributor to synergism is the accumulation of endogenous metabolites, for example: the aryl hydrocarbon receptor 2 (AhR2) agonist and tryptophan metabolite 6-Formylindolo(3,2-b)carbazole (FICZ). Fish larvae exposed to FICZ, alongside knock-down of cytochrome p450 (cyp1a), has been reported to induced symptoms of toxicity similar to those observed following exposure to PAHs or the dioxin 2,3,7,8-tetrachlorodibenzo-p-dioxin. Here, we explored if FICZ accumulates in newly hatched rainbow trout alevins (Oncorhynchus mykiss) exposed to two PAHs with different properties: retene (potent AhR2 agonist) and fluoranthene (weak AhR2 agonist and Cyp1a inhibitor), either alone or as a binary mixture for 3 and 7 days. We found that exposure to the mixture resulted in accumulation of endogenous FICZ, synergized the blue sac disease index (BSD), and altered the body burden profiles of the PAHs, when compared to the alevins exposed to the individual components. It is thus very plausible that accumulation of endogenously derived FICZ contributed to the synergized BSD index and toxicity in exposed alevins. Accumulation of endogenously derived FICZ is a novel finding that extends our general understanding on PAHs toxicity in developing fish larvae, while at the same time highlighting why environmental risk assessment of PAHs should not be based solely results from the assessment of individual compounds.

Development and Applications of a Zebrafish (Danio rerio) CYP1A-Targeted Monoclonal Antibody (CRC4) with Reactivity across Vertebrate Taxa: Evidence for a Conserved CYP1A Epitope

CYP1A is a heme-thiolate enzyme associated with the cytochrome P4501A1 monooxygenase system and is inducible by a wide variety of xenobiotics and endogenous ligands that bind and activate the aryl hydrocarbon receptor (AHR). The AHR-CYP1A axis is important for detoxification of certain xenobiotics and for homeostatic balance of endogenous sex hormones, amine hormones, vitamins, fatty acids, and phospholipids. Herein, we generated and described applications of a zebrafish CYP1A-targeted monoclonal antibody (mAb CRC4) that fortuitously recognizes induced CYP1A across vertebrate taxa, including fish, chicken, mouse, rat, and human. We then demonstrated that mAb CRC4 targets a highly conserved epitope signature of vertebrate CYP1A. The unique complimentary determining region (CDR) sequences of heavy and light chains were determined, and these Ig sequences will allow for the expression of recombinant mAb CRC4, thus superseding the need for long-term hybridoma maintenance. This antibody works well for immunohistochemistry (IHC), as well as whole-mounted IHC in zebrafish embryos. Monoclonal antibody CRC4 may be particularly useful for studying the AHR-CYP1A axis in multiple vertebrate species and within the context of Oceans and Human Health research. By using archived samples, when possible, we actively promoted efforts to reduce, replace, and refine studies involving live animals.

Exposure to retene, fluoranthene, and their binary mixture causes distinct transcriptomic and apical outcomes in rainbow trout (Oncorhynchus mykiss) yolk sac alevins

Polycyclic aromatic hydrocarbons (PAHs) are widely spread environmental contaminants which affect developing organisms. It is known that improper activation of the aryl hydrocarbon receptor (AhR) by some PAHs contributes to toxicity, while other PAHs can disrupt cellular membrane function. The exact downstream mechanisms of AhR activation remain unresolved, especially with regard to cardiotoxicity. By exposing newly hatched rainbow trout alevins (Oncorhynchus mykiss) semi-statically to retene (32 µg l^-1; AhR agonist), fluoranthene (50 µg l^-1; weak AhR agonist and CYP1a inhibitor) and their binary mixture for 1, 3, 7 and 14 days, we aimed to uncover novel mechanisms of cardiotoxicity using a targeted microarray approach. At the end of the exposure, standard length, yolk area, blue sac disease (BSD) index and PAH body burden were measured, while the hearts were prepared for microarray analysis. Each exposure produced a unique toxicity profile. We observed that retene and the mixture, but not fluoranthene, significantly reduced growth by Day 14 compared to the control, while exposure to the mixture increased the BSD-index significantly from Day 3 onward. Body burden profiles were PAH-specific and correlated well with the exposure-specific upregulations of genes encoding for phase I and II enzymes. Exposure to the mixture over-represented pathways related to growth, amino acid and xenobiotic metabolism and oxidative stress responses. Alevins exposed to the individual PAHs displayed over-represented pathways involved in receptor signaling: retene downregulated genes with a role in G-protein signaling, while fluoranthene upregulated those involved in GABA signaling. Furthermore, exposure to retene and fluoranthene altered the expression of genes encoding for proteins involved in calcium- and potassium ion channels, which suggests affected heart structure and function. This study provides deeper understanding of the complexity of PAH toxicity and the necessity of investigating PAHs as mixtures and not as individual components.

Comparative toxicity of conventional and unconventional oils during rainbow trout (Oncorhynchus mykiss) embryonic development: From molecular to health consequences

Canadian freshwater ecosystems are vulnerable to oil spills from pipelines, which contain mostly diluted bitumen. This study aimed to compare the toxicity of a dilbit and a conventional oil on developing rainbow trout. A total of five exposure scenarios were performed, from 10 to 43 days, using water-accommodated fraction (WAF) with an initial loading of 1:9 oil to water ratio (w/v) in a range of dilutions from 0.32 to 32% WAF, respectively, with TPAH and VOC concentrations from 2.41 to 17.5 μg/L and 7.94-660.99 μg/L, and with or without a recovery period. Following the five exposures, several endpoints were examined, including survivorship, morphometrics, gene expression, and enzymatic activity. Significant mortality rates were measured for the highest WAF concentration of the dilbit in all five exposures (60-100% mortality at 32% WAF). In comparison, the highest WAF concentration of the conventional oil induced significant mortality in three out of the five exposure (from 35 to 100% mortality at 32% WAF). Hatching delays were noted in embryos exposed to both oils. Developmental delays were observed in dilbit-exposed embryos and are suspected to be an indicator of reduced survivorship after hatching. The induced expression of cyp1a remained a reliable biomarker of exposure and of fish malformations, though it did not always predict mortality. Using CYP1A activity in combination with cyp1a may bring more insights in studies of oil risk assessment. This study demonstrates that dilbits are more toxic to early life stages compared to conventional oils and highlights the need to consider the most sensitive stage of development when performing risk assessment studies on oils.

Salmo trutta is more sensitive than Oncorhynchus mykiss to early-life stage exposure to retene

Salmonids are known to be among the most sensitive fish to dioxin-like compounds (DLCs), but very little is known about the sensitivity of the brown trout (Salmo trutta), which has declined and is endangered in several countries of Europe and Western Asia. We investigated the sensitivity of brown trout larvae to a widespread dioxin-like PAH, retene (3.2 to 320 μg.L^-1), compared to the larvae of a salmonid commonly used in toxicology studies, the rainbow trout (Oncorhynchus mykiss). Mortality, growth, cyp1a induction and the occurrence of deformities were measured after 15 days of exposure. Brown trout larvae showed a significantly higher mortality at 320 μg.L^-1 compared to rainbow trout larvae. While the occurrence of deformities was only significantly increased at 320 μg.L^-1 for the rainbow trout, brown trout larvae displayed pericardial edemas and hemorrhages already at 10 or 100 μg.L^-1. cyp1a induction was increased significantly already at ≥3.2 μg.L^-1 for the brown trout, versus ≥32 μg.L^-1 for the rainbow trout. Least square regression analysis of the concentration-response relationships suggested that S. trutta larvae were at least 2 times more sensitive than O. mykiss larvae for cyp1a induction. The present study suggests that S. trutta larvae are more sensitive than O. mykiss larvae to a potent DLC, retene. As it is possible that S. trutta populations have declined partly because of pollution by DLCs, we recommend generating more data regarding the sensitivity of threatened fish populations, in order to ensure better risk assessment.

Lethal and sublethal effects of diluted bitumen and conventional oil on fathead minnow (Pimephales promelas) larvae exposed during their early development

The increasing extraction of bitumen from the oil sands region in Canada is creating a need for transport. Spills from current and projected pipelines represent a significant environmental risk, especially for freshwater ecosystems. The toxicity of diluted bitumen (dilbit) on freshwater fish is largely unknown. This study assessed the toxicity of two dilbits (Clearwater McMurray and Bluesky) and compared their toxicity to a conventional oil (Lloydminster Heavy) on fathead minnow (Pimephales promelas) larvae. Larvae were exposed to various concentrations of the water-accommodated fraction (WAF) of the oils during 7 days from hatching. In the WAF treatments, the concentrations of volatile organic compounds (VOCs), including benzene, toluene, ethylbenzene, xylene (BTEX), hydrocarbons containing 6 to 10 carbon atoms (C_6-10), and polycyclic aromatic hydrocarbons (PAHs) and their alkylated forms were measured. Both dilbits contained higher concentrations of light components, while the conventional oil contained the highest concentrations of PAHs and alkylated PAHs. The Clearwater McMurray dilbit induced a higher mortality, with a maximum of 65.3%, while the other oils induced a similar mortality up to 16.5% and 18.6% for Lloydminster and for Bluesky, respectively. All three oils induced an increase in gene expression of the phase I detoxification enzyme (cyp1a) with increasing total hydrocarbon concentrations. All three exposures induced a similar increase in glutathione S-transferase (GST) activity, but no change in gst gene expression. For the Bluesky and Lloydminster exposures, an increase in malondialdehyde concentration was also observed, suggesting a rate limiting capacity of GST and phase II enzymes to perform the biotransformation of the PAH metabolites. Overall, this study brings new insights on the toxicity of dilbits in comparison to conventional oils on early life stages of North American freshwater fish and demonstrated that dilbits can be more toxic than conventional oils, depending on their composition and diluent proportions.

Retene, pyrene and phenanthrene cause distinct molecular-level changes in the cardiac tissue of rainbow trout (Oncorhynchus mykiss) larvae, part 1 - Transcriptomics

Polycyclic aromatic hydrocarbons (PAHs) are contaminants of concern that impact every sphere of the environment. Despite several decades of research, their mechanisms of toxicity are still poorly understood. This study explores the mechanisms of cardiotoxicity of the three widespread model PAHs retene, pyrene and phenanthrene in the rainbow trout (Oncorhynchus mykiss) early life stages. Newly hatched larvae were exposed to each individual compound at sublethal doses causing no significant increase in the prevalence of deformities. Changes in the cardiac transcriptome were assessed after 1, 3, 7 and 14 days of exposure using custom Salmo salar microarrays. The highest number of differentially expressed genes was observed after 1 or 3 days of exposure, and retene was the most potent compound in that regard. Over-representation analyses suggested that genes related to cardiac ion channels, calcium homeostasis and muscle contraction (actin binding, troponin and myosin complexes) were especially targeted by retene. Pyrene was also able to alter similar myosin-related genes, but at a different timing and in an opposite direction, suggesting compound-specific mechanisms of toxicity. Pyrene and to a lesser extent phenanthrene were altering key genes linked to the respiratory electron transport chain and to oxygen and iron metabolism. Overall, phenanthrene was not very potent in inducing changes in the cardiac transcriptome despite being apparently metabolized at a slower rate than retene and pyrene. The present study shows that exposure to different PAHs during the first few days of the swim-up stage can alter the expression of key genes involved into the cardiac development and function, which could potentially affect negatively the fitness of the larvae in the long term.

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.

Polycyclic aromatic compounds (PACs) in the Canadian environment: Exposure and effects on wildlife

Polycyclic aromatic compounds (PACs) are ubiquitous in the environment. Wildlife (including fish) are chronically exposed to PACs through air, water, sediment, soil, and/or dietary routes. Exposures are highest near industrial or urban sites, such as aluminum smelters and oil sands mines, or near natural sources such as forest fires. This review assesses the exposure and toxicity of PACs to wildlife, with a focus on the Canadian environment. Most published field studies measured PAC concentrations in tissues of invertebrates, fish, and birds, with fewer studies of amphibians and mammals. In general, PAC concentrations measured in Canadian wildlife tissues were under the benzo[a]pyrene (BaP) guideline for human consumption. Health effects of PAC exposure include embryotoxicity, deformities, cardiotoxicity, DNA damage, changes to DNA methylation, oxidative stress, endocrine disruption, and impaired reproduction. Much of the toxicity of PACs can be attributed to their bioavailability, and the extent to which certain PACs are transformed into more toxic metabolites by cytochrome P450 enzymes. As most mechanistic studies are limited to individual polycyclic aromatic hydrocarbons (PAHs), particularly BaP, research on other PACs and PAC-containing complex mixtures is required to understand the environmental significance of PAC exposure and toxicity. Additional work on responses to PACs in amphibians, reptiles, and semi-aquatic mammals, and development of molecular markers for early detection of biological responses to PACs would provide a stronger biological and ecological justification for regulating PAC emissions to protect Canadian wildlife.

Temperature determines the rate at which retene affects trout embryos, not the concentration that is toxic

The toxicity of waterborne retene (7-isopropyl-1-methyl phenanthrene) to post-hatch embryos of rainbow trout (Oncorhynchus mykiss) was assessed at 5 and 11 °C. Survival times of retene-exposed embryos were 70 % longer at 5 °C than at 11 °C, but survival times and LC50 s did not vary when time was expressed as degree-days (thermal units), i.e., at a common stage of development. The size of survivors decreased with increasing retene concentrations, but not with temperature. Retene did not bioconcentrate to any extent (bioconcentration factors < 2) at either temperature, indicating effective biotransformation by embryos. However, concentrations of retene metabolites were slightly higher at 5 °C, suggesting slower excretion rates than at 11 °C. The relative expression of cytochrome P450 proteins (CYP1A) did not vary with temperature but increased with retene concentration, as indicated by cyp1a mRNA concentrations. The induction of CYP1A protein by retene exposure was evident in the vasculature of eye, brain, heart, kidney, liver, gill, mouth, intestine, muscle, and yolk-sac. However, immunohistochemical staining was greater at 5 than at 11 °C for all tissues except liver and muscle. Overall, temperature effects on retene toxicity disappeared when the duration of embryo development and retene exposure were expressed as thermal units (degree-days). Temperature controlled the rate of embryo development and the rate of toxicity (time to a toxic endpoint), but not the concentrations that were toxic.

Developmental and latent effects of diluted bitumen exposure on early life stages of sockeye salmon (Oncorhynchus nerka)

The early life stages of Pacific salmon are at risk of environmental exposure to diluted bitumen (dilbit) as Canada's oil sands industry continues to expand. The toxicity and latent effects of dilbit exposure were assessed in sockeye salmon (Oncorhynchus nerka) exposed to water-soluble fractions of dilbit (WSFd) from fertilization to the swim-up stage, and then reared in clean water for 8 months. Mortality was significantly higher in WSFd-exposed embryos, with cumulative mortality up to 4.6-fold higher in exposed relative to unexposed embryos. The sublethal effects of WSFd exposure included transcriptional up-regulation of cyp1a, a concentration-dependent delay in the onset and progression of hatching, as well as increased prevalence of developmental deformities at total polycyclic aromatic hydrocarbon (TPAH) concentrations ≥35 μg L^-1. Growth and body composition were negatively affected by WSFd exposure, including a concentration-specific decrease in soluble protein concentration and increases in total body lipid and triglyceride concentrations. Mortality continued during the first 2 months after transferring fish to clean water, reaching 53% in fish exposed to 100 μg L^-1 TPAH; but there was no latent impact on swimming performance, heart mass, or heart morphology in surviving fish after 8 months. A latent effect of WSFd exposure on brain morphology was observed, with fish exposed to 4 μg L^-1 TPAH having significantly larger brains compared to other treatment groups after 8 months in clean water. This study provides comprehensive data on the acute, sub-chronic, and latent impacts of dilbit exposure in early life stage sockeye, information that is critical for a proper risk analysis of the impact of a dilbit spill on this socioeconomically important fish species.

The Toxicity to Fish Embryos of PAH in Crude and Refined Oils

Oil spills are a potential threat to the recruitment and production of fish. Polycyclic aromatic hydrocarbons (PAH), particularly 3-5-ringed alkyl PAH, are components of oil that cause chronic embryotoxicity. Toxicity is related to molecular size and octanol-water partition coefficients (Kow), indicating that water-lipid partitioning controls exposure and tissue dose. Nevertheless, more than 25% of the variation in toxicity among congeners is unexplained. Congeners with the same number of rings, alkyl carbon atoms, and Kow, but different molecular shapes, have markedly different toxicities, likely due to differences in interactions with cellular receptors. The potentiation and antagonism of metabolism and toxicity in PAH mixtures suggest that measured effect concentrations for individual PAH are conservative. Because mixture interactions are not well understood, total PAH concentrations >0.1 µg/L following oil spills should be considered hazardous.

Effects of diluted bitumen exposure on juvenile sockeye salmon: From cells to performance

Diluted bitumen (dilbit; the product of oil sands extraction) is transported through freshwater ecosystems critical to Pacific salmon. This is concerning, because crude oil disrupts cardiac development, morphology, and function in embryonic fish, and cardiac impairment in salmon can have major consequences on migratory success and fitness. The sensitivity of early life-stage salmon to dilbit and its specific cardiotoxic effects are unknown. Sockeye salmon parr were exposed to environmentally relevant concentrations of the water-soluble fraction (WSF) of dilbit for 1 wk and 4 wk, followed by an examination of molecular, morphological, and organismal endpoints related to cardiotoxicity. We show that parr are sensitive to WSF of dilbit, with total polycyclic aromatic hydrocarbon (PAH) concentrations of 3.5 µg/L sufficient to induce a liver biomarker of PAH exposure, and total PAH of 16.4 µg/L and 66.7 µg/L inducing PAH biomarkers in the heart. Furthermore, WSF of dilbit induces concentration-dependent cardiac remodeling coincident with performance effects: fish exposed to 66.7 µg/L total PAH have relatively fewer myocytes and more collagen in the compact myocardium and impaired swimming performance at 4 wk, whereas the opposite changes occur in fish exposed to 3.5 µg/L total PAH. The results demonstrate cardiac sensitivity to dilbit exposure that could directly impact sockeye migratory success. Environ Toxicol Chem 2017;36:354-360. © 2016 SETAC.

Inducible headkidney cytochrome P450 contributes to endosulfan immunotoxicity in walking catfish Clarias gariepinus

The effect of endosulfan metabolites on fish immune system is not well known. It is also not clear whether endosulfan accumulates in fish immune organs and undergoes metabolic biotransformation in situ. In the present study we investigated the role of headkidney (HK), an important fish immune organ on endosulfan metabolism and the long term effects of endosulfan metabolites on the fish immune system. C. gariepinus (walking catfish) were exposed to 2.884ppb of endosulfan (1/10th LC50) for 30d followed by their maintenance in endosulfan-free water for 30d for recovery. Endosulfan induced time-dependent reduction in the HK somatic index and histo-pathological changes in renal and hemopoietic components of the organ. At cellular level, exposure to endosulfan led to death of HK leucocytes. Gas-liquid-chromatography documented the presence of both α- and β-isomers of endosulfan along with the toxic metabolite endosulfan sulfate (ESS) in the HK of exposed fishes. We report that β-endosulfan accumulates more readily in the HK. Depuration studies suggested the persistence of ESS in the HK. Enzyme-immunoassay and qPCR results demonstrated direct relationship between cytochrome P450 1A (CYP1A) expression and ESS levels in the HK. Pre-treatment of HKL with CYP1A specific inhibitor α-Naphthoflavone (ANF) led to reduction in CYP1A mRNA, protein levels, and inhibited ESS formation together implicating the role of CYP1A on endosulfan metabolism. When the exposed fish were transferred to endosulfan-free water ('recovered fish') it was observed that after 30d of recovery period the concentration of endosulfan and its metabolite in the HK were significantly reduced, compared to 30-d exposed fish. We also observed improvement in HK histo-architecture but no significant recovery in HKL number and viability. Collectively, our findings suggest that HK plays an important role in endosulfan metabolism. We propose that endosulfan induces the activation of CYP1A in HK which led to the generation of persistent metabolite, ESS, resulting in immunotoxicity.

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.

Retene causes multifunctional transcriptomic changes in the heart of rainbow trout (Oncorhynchus mykiss) embryos

Fish are particularly sensitive to aryl hydrocarbon receptor (AhR)-mediated developmental toxicity. The molecular mechanisms behind these adverse effects have remained largely unresolved in salmonids, and for AhR-agonistic polycyclic aromatic hydrocarbons (PAHs). This study explored the cardiac transcriptome of rainbow trout (Oncorhynchus mykiss) eleuteroembryos exposed to retene, an AhR-agonistic PAH. The embryos were exposed to retene (nominal concentration 32 μg/L) and control, their hearts were collected before, at and after the onset of the visible signs of developmental toxicity, and transcriptomic changes were studied by microarray analysis. Retene up- or down-regulated 122 genes. The largest Gene Ontology groups were signal transduction, transcription, apoptosis, cell growth, cytoskeleton, cell adhesion/mobility, cardiovascular development, xenobiotic metabolism, protein metabolism, lipid metabolism and transport, and amino acid metabolism. Together these findings suggest that retene affects multiple signaling cascades in the heart of rainbow trout embryos, and potentially disturbs processes related to cardiovascular development and function.

Stage-dependent ethoxyresorufin-O-deethylase (EROD) in vivo activity in medaka (Oryzias latipes) embryos

Using medaka (Oryzias latipes) embryos, this study aimed to quantitatively characterize the stage-dependent in vivo ethoxyresorufin-O-deethylase (EROD) as indicator of cytochrome P4501A (CYP1A) activity. Embryos were challenged for 24-h to an agonist (β-naphthoflavone [BNF], 2.5, 5, 10, and 20 μg L(-1)) or to its combination (2.5 μg L(-1)) with an antagonist (α-naphthoflavone [ANF], 25, 50, 100, and 200 μg L(-1)), initiated at four different developmental time points (1, 3, 6, and 9 d post-fertilization [dpf]). Respective induction and competitive inhibition were evaluated over fluorescent images of whole embryo (nonorgan-specific [NOS] EROD activity) and gallbladder (organ-specific [OS] EROD activity). Both flavonoids showed signs of stability in solution. Generally speaking, the mean fluorescence intensity (MFI) values for NOS EROD increased with BNF concentration and exposure challenge. BNF co-exposure with ⩾50 μg ANF L(-1) during the 1-2 and 3-4 dpf challenges lowered NOS EROD to undetectably induced levels. Significant increments in MFIs for OS-EROD were seen from exposures to ⩾2.5 μg BNF L(-1), peaking during the 6-7 dpf challenge regardless of BNF concentration. The simultaneous BNF/ANF incubation showed competitive inhibition for OS EROD activity, although levels were generally detectably induced during all challenges and at all ANF concentrations. The morphometric in vivo gallbladder analysis indicated significant dilation in the 10 dpf-old embryos co-exposed to BNF and 200 μg ANF L(-1). This quantitative approach can be used successfully at 4 dpf at the NOS-EROD or OS-EROD levels, although the NOS-EROD response was sensitive enough to induction or inhibition, even at 2 dpf.

Quantitative structure-activity relationships for chronic toxicity of alkyl-chrysenes and alkyl-benz[a]anthracenes to Japanese medaka embryos (Oryzias latipes)

Alkylated polycyclic aromatic hydrocarbons (alkyl-PAHs) are a class of compounds found at significant concentrations in crude oils, and likely the main constituents responsible for the chronic toxicity of oil to fish. Alkyl substituents at different locations on the aromatic rings change the size and shape of PAH molecules, which results in different interactions with tissue receptors and different severities of toxicity. The present study is the first to report the toxicity of several alkylated derivatives of chrysene and benz[a]anthracene to the embryos of Japanese medaka (Oryzias latipes) using the partition controlled delivery (PCD) method of exposure. The PCD method maintained the desired exposure concentrations by equilibrium partitioning of hydrophobic test compounds from polydimethylsiloxane (PDMS) films. Test concentrations declined by only 13% over a period of 17 days. Based on the prevalence of signs of blue sac disease (BSD), as expressed by median effective concentrations (EC50s), benz[a]anthracene (B[a]A) was more toxic than chrysene. Alkylation generally increased toxicity, except at position 2 of B[a]A. Alkyl-PAHs substituted in the middle region had a lower EC50 than those substituted at the distal region. Except for B[a]A and 7-methylbenz[a]anthracene (7-MB), estimated EC50 values were higher than their solubility limits, which resulted in limited toxicity within the range of test concentrations. The regression between log EC50s and logKow values provided a rough estimation of structure-activity relationships for alkyl-PAHs, but Kow alone did not provide a complete explanation of the chronic toxicity of alkyl PAHs.

Chronic dietary exposure to pyrolytic and petrogenic mixtures of PAHs causes physiological disruption in zebrafish--part I: Survival and growth

The release of polycyclic aromatic hydrocarbons (PAHs) into the environment has increased very substantially over the last decades leading to high concentrations in sediments of contaminated areas. To evaluate the consequences of long-term chronic exposure to PAHs, zebrafish were exposed, from their first meal at 5 days post fertilisation until they became reproducing adults, to diets spiked with three PAH fractions at three environmentally relevant concentrations with the medium concentration being in the range of 4.6-6.7 μg g(-1) for total quantified PAHs including the 16 US-EPA indicator PAHs and alkylated derivatives. The fractions used were representative of PAHs of pyrolytic (PY) origin or of two different oils of differing compositions, a heavy fuel (HO) and a light crude oil (LO). Fish growth was inhibited by all PAH fractions and the effects were sex specific: as determined with 9-month-old adults, exposure to the highest PY inhibited growth of females; exposure to the highest HO and LO inhibited growth of males; also, the highest HO dramatically reduced survival. Morphological analysis indicated a disruption of jaw growth in larvae and malformations in adults. Intestinal and pancreatic enzyme activities were abnormal in 2-month-old exposed fish. These effects may contribute to poor growth. Finally, our results indicate that PAH mixtures of different compositions, representative of situations encountered in the wild, can promote lethal and sublethal effects which are likely to be detrimental for fish recruitment.

Developmental toxicity of PAH mixtures in fish early life stages. Part I: adverse effects in rainbow trout

A new gravel-contact assay using rainbow trout, Oncorhynchus mykiss, embryos was developed to assess the toxicity of polycyclic aromatic hydrocarbons (PAHs) and other hydrophobic compounds. Environmentally realistic exposure conditions were mimicked with a direct exposure of eyed rainbow trout embryos incubated onto chemical-spiked gravels until hatching at 10 °C. Several endpoints were recorded including survival, hatching delay, hatching success, biometry, developmental abnormalities, and DNA damage (comet and micronucleus assays). This bioassay was firstly tested with two model PAHs, fluoranthene and benzo[a]pyrene. Then, the method was applied to compare the toxicity of three PAH complex mixtures characterized by different PAH compositions: a pyrolytic extract from a PAH-contaminated sediment (Seine estuary, France) and two petrogenic extracts from Arabian Light and Erika oils, at two environmental concentrations, 3 and 10 μg g(-1) sum of PAHs. The degree and spectrum of toxicity were different according to the extract considered. Acute effects including embryo mortality and decreased hatching success were observed only for Erika oil extract. Arabian Light and pyrolytic extracts induced mainly sublethal effects including reduced larvae size and hemorrhages. Arabian Light and Erika extracts both induced repairable DNA damage as revealed by the comet assay versus the micronucleus assay. The concentration and proportion of methylphenanthrenes and methylanthracenes appeared to drive the toxicity of the three PAH fractions tested, featuring a toxic gradient as follows: pyrolytic < Arabian Light < Erika. The minimal concentration causing developmental defects was as low as 0.7 μg g(-1) sum of PAHs, indicating the high sensitivity of the assay and validating its use for toxicity assessment of particle-bound pollutants.

Effects of water accommodated fractions of crude oils and diesel on a suite of biomarkers in Atlantic cod (Gadus morhua)

The aim of this study was to characterize concentration- and time-dependent responses in juvenile Atlantic cod (Gadus morhua) following exposure for one and three weeks to the water-soluble fraction (WAF) of three weathered oils: Arabian Light crude oil (ALC), North Sea crude oil (NSC) and ship-diesel. The sum of polycyclic aromatic hydrocarbons (PAH) in water was highest after one week of exposure and within environmentally relevant concentrations. PAH metabolites in bile confirmed exposure to and uptake of PAHs. Hepatic cytochrome P450 1A (CYP1A) gene expression (mRNA quantification) increased dramatically following exposure to all three oil types (fold-change up to 165) and there was a time lag between gene and protein expression. Hepatic CYP1A protein concentration and ethoxyresorufin-O-deethylase (EROD) activity were more variable among individuals and treatments than gene expression. EROD activity in liver and gills increased in fish exposed to WAF from the two crude oils, but not in fish exposed to WAF from diesel. Exposure to diesel appeared to induce oxidative stress to a greater extent than exposure to crude oils. Other biomarkers (glutathione S-transferases, acetylcholine esterase, vitellogenin) did not appear to respond to the exposure and hence did not discriminate among oils. Biomarker responses in cod after exposure to weathered crude oils and diesel suggested that the CYP1A system and oxidative stress markers have the highest potential for discriminating among different oil types and to monitor the environmental consequences of spills.

Comparative embryotoxicity of phenanthrene and alkyl-phenanthrene to marine medaka (Oryzias melastigma)

Alkylated polycyclic aromatic hydrocarbons (alkyl-PAHs) are the predominant form of PAHs in oil, comprising 85-95% of total PAHs. However, little attention has been paid to these chemicals in ecological risk assessment of marine oil spill. A comparative study of the toxic effects of phenanthrene and retene (7-isopropyl-1-methylphenanthrene, an alkyl-phenanthrene) on the early life stage of marine medaka (Oryzias melastigma) was conducted. Results showed that retene was significantly more toxic than phenanthrene, and marine medaka could be more sensitive to retene than some freshwater fishes. Retene had a higher excretion rate than phenanthrene during the larvae stage. Both of compounds resulted in developmental malformation of marine medaka embryos, with phenanthrene affecting on peripheral vascular system and yolk sac, while retene affecting on cardiac tissues. The toxicity of phenanthrene might be mainly related to its anesthetic effects, and that of retene might be related to the CYP1A-mediated toxicity of its metabolites.

Acute embryonic or juvenile exposure to Deepwater Horizon crude oil impairs the swimming performance of mahi-mahi (Coryphaena hippurus)

The Deepwater Horizon incident likely resulted in exposure of commercially and ecologically important fish species to crude oil during the sensitive early life stages. We show that brief exposure of a water-accommodated fraction of oil from the spill to mahi-mahi as juveniles, or as embryos/larvae that were then raised for ∼25 days to juveniles, reduces their swimming performance. These physiological deficits, likely attributable to polycyclic aromatic hydrocarbons (PAHs), occurred at environmentally realistic exposure concentrations. Specifically, a 48 h exposure of 1.2 ± 0.6 μg L(-1) ΣPAHs (geometric mean ± SEM) to embryos/larvae that were then raised to juvenile stage or a 24 h exposure of 30 ± 7 μg L(-1) ΣPAHs (geometric mean ± SEM) directly to juveniles resulted in 37% and 22% decreases in critical swimming velocities (Ucrit), respectively. Oil-exposed larvae from the 48 h exposure showed a 4.5-fold increase in the incidence of pericardial and yolk sac edema relative to controls. However, this larval cardiotoxicity did not manifest in a reduced aerobic scope in the surviving juveniles. Instead, respirometric analyses point to a reduction in swimming efficiency as a potential alternative or contributing mechanism for the observed decreases in Ucrit.

Are we in the dark ages of environmental toxicology?

Environmental toxicity is judged to be in a "dark ages" period due to longstanding limitations in the implementation of the simple conceptual model that is the basis of current aquatic toxicity testing protocols. Fortunately, the environmental regulatory revolution of the last half-century is not substantially compromised as development of past regulatory guidance was designed to deal with limited amounts of relatively poor quality toxicity data. However, as regulatory objectives have substantially increased in breadth and depth, aquatic toxicity data derived with old testing methods are no longer adequate. In the near-term explicit model description and routine assumption validation should be mandatory. Updated testing methods could provide some improvements in toxicological data quality. A thorough reevaluation of toxicity testing objectives and methods resulting in substantially revised standard testing methods, plus a comprehensive scheme for classification of modes/mechanisms of toxic action, should be the long-term objective.

CYP1A expression in liver and gills of roach (Rutilus rutilus) after waterborne exposure to two phenanthrene derivatives, 1-methylphenanthrene and 4-methylphenanthrene

Phenanthrenes (Phs) substituted with alkyl groups are a class of compound present in the environment, and they appear to be toxic to developing fish. The present study aimed to investigate the effect of waterborne exposure to two monomethyl derivatives of phenanthrene, 1-methylphenanthrene (1M-Ph) and 4-methylphenanthrene (4M-Ph), on cytochrome P450 1A (CYP1A) gene expression in fish gills and liver. Juvenile common roaches (Rutilus rutilus) were exposed to water with dimethyl sulfoxide (DMSO) solutions of 1M-Ph, 4M-Ph, benzo[a]pyrene (BaP; positive control), each at a dose of 100 µg/L, or to water with DMSO alone (negative control group) for 2 d and 7 d. Significant CYP1A responses with regard to treatment and exposure duration were noted (2-way analysis of variance [ANOVA]) in gills (p = 0.013 and p = 0.003, respectively) and liver (p < 0.001). The 2 monomethyl Phs did not induce consistent gene expression changes, except for 4-MPh, which elevated the CYP1A messenger ribonucleic acid (mRNA) level in the liver at the end of the treatment (almost 4-fold; p < 0.05; 7 d). As was expected, exposure to BaP resulted in elevation of CYP1A mRNA expression in treated fish compared with the control group. Expressions after 2 d and 7 d were approximately 220- and 180-fold higher in liver and 8- and 6-fold higher in gills respectively. The CYP1A protein levels remained stable in both tissues, with one notable exception in roach liver treated for 2 d with BaP (∼ 6-fold increase; p < 0.05). The different effects of the 1- and 4-methylphenanthrenes on CYP1A gene expression in roach liver suggest a relationship between chemical or 3-D structure of the differentially substituted monomethyl Phs and their biological activity.

Selective induction of tumor cell apoptosis by a novel P450-mediated reactive oxygen species (ROS) inducer methyl 3-(4-nitrophenyl) propiolate

Induction of tumor cell apoptosis has been recognized as a valid anticancer strategy. However, therapeutic selectivity between tumor and normal cells has always been a challenge. Here, we report a novel anti-cancer compound methyl 3-(4-nitrophenyl) propiolate (NPP) preferentially induces apoptosis in tumor cells through P450-catalyzed reactive oxygen species (ROS) production. A compound sensitivity study on multiple cell lines shows that tumor cells with high basal ROS levels, low antioxidant capacities, and p53 mutations are especially sensitive to NPP. Knockdown of p53 sensitized non-transformed cells to NPP-induced cell death. Additionally, by comparing NPP with other ROS inducers, we show that the susceptibility of tumor cells to the ROS-induced cell death is influenced by the mode, amount, duration, and perhaps location of ROS production. Our studies not only discovered a unique anticancer drug candidate but also shed new light on the understanding of ROS generation and function and the potential application of a ROS-promoting strategy in cancer treatment.

EROD activity and genotoxicity in the seabob shrimp Xiphopenaeus kroyeri exposed to benzo[a]pyrene (BaP) concentrations

Seabob shrimp Xiphopenaeus kroyeri is a marine species that lives in shallow waters of coastal environments, often impacted by polycyclic aromatic hydrocarbons (PAH) pollution. In the present study, seabob shrimp were exposed for 96 h to benzo[a]pyrene (BaP) at the nominal concentrations of 100, 200, 400 and 800 microg-L(-1). Animals of the control groups were exposed either to clean water or to the BaP-carrier (DMSO). At the end of the exposures, muscle tissues were sampled for BaP uptake assessment and hepatopancreas and hemolymph for EROD enzyme activity and hemocytes DNA damage, respectively. EROD activity and DNA damage increased significantly as a function of BaP exposure concentrations. Significant correlations between BaP uptake and both EROD activity and DNA damage suggest that they can be used as suitable tools for integrated levels of study on the biomarkers of PAH exposure.

Detection of mutagens and BaPMO inducers in river water using the Blue Cotton adsorption technique

Induction of Mixed Function Oxidase (MFO) activity and bioactivation potential were measured in experimental carp and in native fish species from two rivers with different pollution level (Sava and Mrežnica). The experimental carp were intraperitoneally exposed to various water volume equivalents of Blue Cotton extracts from both rivers. The induction of MFO measured as a Benzo(a)pyrene monooxygenase (BaPMO) activity was increased up to 9.3-fold in experimental carp and up to 11.3-fold in native fish from Sava River, whereas the values from Mrežnica River showed only a slight increase when compared with the control (highest increase of 1.8-fold in nose carp). Accordingly, bioactivation potential using modified Ames test was higher in both experiments with Sava River. Both measured parameters in experimental carp increased in a dose-dependent manner in accordance to river volume equivalents. Different induction potential of native fish species comparable between two rivers confirmed the known possibility of their usage in biomonitoring studies. These results give qualitatively a new support to the idea of using Blue Cotton extraction technique combined with induction of MFO activity and Ames test in fish as a good biomarker for assessing risk of exposure to mutagens/carcinogens such as Polycyclic Aromatic Hydrocarbons (PAHs), especially due to the simplicity and cost-effectiveness of these methods.

Toxicity of hydroxylated alkyl-phenanthrenes to the early life stages of Japanese medaka (Oryzias latipes)

Polycyclic aromatic hydrocarbons (PAH) are hydrophobic environmental contaminants with petrogenic, biogenic, and pyrogenic sources. Alkylated PAH predominate in crude oils, are found in sediment downstream of pulp and paper mills, and can be more toxic than their non-alkylated homologues. The enzymatic metabolism of alkyl phenanthrenes generates ring and chain hydroxylated derivatives. The main objective of this research was to estimate the potential role of hydroxylation in PAH toxicity and secondly to better understand the relative risk of different PAH in complex mixtures. This project assessed the toxicity of ring and chain hydroxylated 1-methylphenanthrenes to the early life stages of Japanese medaka (Oryzias latipes). Phenols were more toxic than benzylic alcohols, and some phenols were more than four times more toxic than their non-hydroxylated counterpart. Ring hydroxylation can increase PAH toxicity, and metabolism may enhance alkyl-PAH toxicity through the generation of such metabolites. This paper is the first to describe the relative toxicity of a suite of hydroxylated alkyl-PAH to the early life stages of fish, proposing an association between the preferential formation of para-quinones and enhanced toxicity.

Effect-directed analysis of municipal landfill soil reveals novel developmental toxicants in the zebrafish Danio rerio

Effect-directed analysis (EDA) is an approach used to identify (unknown) contaminants in complex samples which cause toxicity, using a combination of biology and chemistry. The goal of this work was to apply EDA to identify developmental toxicants in soil samples collected from a former municipal landfill site. Soil samples were extracted, fractionated, and tested for developmental effects with an embryotoxicity assay in the zebrafish Danio rerio. Gas chromatograph mass selective detection (GC-MSD) chemical screening was used to reveal candidate developmental toxicants in fractions showing effects. In a parallel study, liquid chromatography-hybrid linear ion trap Orbitrap mass spectrometry was also applied to one polar subfraction (Hoogenboom et al. J. Chromatogr. A2009, 1216, 510-519). EDA resulted in the identification of a number of previously unknown developmental toxicants, which were confirmed to be present in soil by GC-MS. These included 11H-benzo[b]fluorene, 9-methylacridine, 4-azapyrene, and 2-phenylquinoline, as well as one known developmental toxicant (retene). This work revealed the presence of novel contaminants in the environment that may affect vertebrate development, which are not subject to monitoring or regulation under current soil quality assessment guidelines.

Biota: sediment partitioning of aluminium smelter related PAHs and pulp mill related diterpenes by intertidal clams at Kitimat, British Columbia

The question of polycyclic aromatic hydrocarbon (PAH) bioavailability and its relationship to specific PAH sources with different PAH binding characteristics is an important one, because bioavailability drives PAH accumulation in biota and ultimately the biochemical responses to the PAH contaminants. The industrial harbour at Kitimat (British Columbia, Canada) provides an ideal location to study the bioavailability and bioaccumulation of sediment hydrocarbons to low trophic level biota. Samples of soft shell clams (Mya arenaria) and intertidal sediment collected from multiple sites over six years at various distances from an aluminium smelter and a pulp and paper mill were analysed for 106 PAHs, plant diterpenes and other aromatic fraction hydrocarbons. Interpretation using PAH source ratios and multivariate data analysis reveals six principal hydrocarbon sources: PAHs in coke, pitch and emissions from anode combustion from the aluminium smelter, vascular plant terpenes and aromatised terpenes from the pulp and paper mill, petroleum PAHs from shipping and other anthropogenic activities and PAHs from natural plant detritus. Harbour sediments predominantly contain either pitch or pyrogenic PAHs from the smelter, while clams predominantly contain plant derived PAHs and diterpenes from the adjacent pulp mill. PAHs from the smelter have low bioavailability to clams (Biota-Sediment Accumulation Factors; BSAFs <1 for pitch and coke; <10 for anode combustion, decreasing to ∼0.1 for the mass 300 and 302 PAHs), possibly due to binding to pitch or soot carbon matrices. Decreases in PAH isomer ratios between sediments and clams likely reflect a combination of variation in uptake kinetics of petroleum PAHs and compound specific metabolism, with the importance of petroleum PAHs decreasing with increasing molecular weight. Plant derived compounds exhibit little natural bioaccumulation at reference sites, but unsaturated and aromatised diterpenes released from resins by industrial pulping processes are readily accumulated by the clams (BSAFs >500). Thus while most of the smelter associated PAHs in sediments may not be bioavailable to benthic organisms, the plant terpenes (including retene, totarol, ferruginol, manool, dehydroabietane and other plant terpenes that form the chemical defence mechanism of conifers) released by pulp mills are bioavailable and possess demonstrated toxic properties. The large scale release of plant terpenes by some of the many pulp mills located in British Columbia and elsewhere represents a largely undocumented risk to aquatic biota.

Mechanistic characterization of GS-9190 (Tegobuvir), a novel nonnucleoside inhibitor of hepatitis C virus NS5B polymerase

GS-9190 (Tegobuvir) is a novel imidazopyridine inhibitor of hepatitis C virus (HCV) RNA replication in vitro and has demonstrated potent antiviral activity in patients chronically infected with genotype 1 (GT1) HCV. GS-9190 exhibits reduced activity against GT2a (JFH1) subgenomic replicons and GT2a (J6/JFH1) infectious virus, suggesting that the compound's mechanism of action involves a genotype-specific viral component. To further investigate the GS-9190 mechanism of action, we utilized the susceptibility differences between GT1b and GT2a by constructing a series of replicon chimeras where combinations of 1b and 2a nonstructural proteins were encoded within the same replicon. The antiviral activities of GS-9190 against the chimeric replicons were reduced to levels comparable to that of the wild-type GT2a replicon in chimeras expressing GT2a NS5B. GT1b replicons in which the β-hairpin region (amino acids 435 to 455) was replaced by the corresponding sequence of GT2a were markedly less susceptible to GS-9190, indicating the importance of the thumb subdomain of the polymerase in this effect. Resistance selection in GT1b replicon cells identified several mutations in NS5B (C316Y, Y448H, Y452H, and C445F) that contributed to the drug resistance phenotype. Reintroduction of these mutations into wild-type replicons conferred resistance to GS-9190, with the number of NS5B mutations correlating with the degree of resistance. Analysis of GS-9190 cross-resistance against previously reported NS5B drug-selected mutations showed that the resistance pattern of GS-9190 is different from other nonnucleoside inhibitors. Collectively, these data demonstrate that GS-9190 represents a novel class of nonnucleoside polymerase inhibitors that interact with NS5B likely through involvement of the β-hairpin in the thumb subdomain.

AhR2-mediated, CYP1A-independent cardiovascular toxicity in zebrafish (Danio rerio) embryos exposed to retene

In the embryo-larval stages of fish, alkylphenanthrenes such as retene (7-isopropyl-1-methylphenanthrene) produce a suite of developmental abnormalities typical of exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), including pericardial and yolk sac edema, cardiovascular dysfunction, and skeletal deformities. To investigate the mechanism and target tissue of retene toxicity, we used observational, histological, and protein knockdown techniques in zebrafish (Danio rerio) embryos. The primary overt signs of toxicity are pericardial edema and reduced blood flow, first observed at 36 h post-fertilization (hpf). The most pronounced effects at this stage are a reduced layer of cardiac jelly in the atrium and reduced diastolic filling. Conversely, an increased layer of cardiac jelly is observed at 72 hpf in retene-exposed embryos. Induction of cytochrome P4501A (CYP1A) is apparent in a subset of cardiomyocytes by 48 hpf suggesting that early cardiac effects may be due to AhR activation in the myocardium. Myocardial CYP1A induction is transient, with only endocardial induction observed at 72 hpf. Knockdown of cyp1a by morpholino oligonucleotides does not affect retene toxicity; however, ahr2 knockdown prevents toxicity. Thus, the mechanism of retene cardiotoxicity is AhR2-mediated and CYP1A-independent, similar to TCDD; however, the onset and proximate signs of retene toxicity differ from those of TCDD. Retene cardiotoxicity also differs mechanistically from the cardiac effects of non-alkylated phenanthrane, illustrating that alkyl groups can alter toxic action. These findings have implications for understanding the toxicity of complex mixtures containing alkylated and non-alkylated polycyclic aromatic hydrocarbons.

A review of the tissue residue approach for organic and organometallic compounds in aquatic organisms

This paper reviews the tissue residue approach (TRA) for toxicity assessment as it applies to organic chemicals and some organometallic compounds (Sn, Hg, and Pb) in aquatic organisms. Specific emphasis was placed on evaluating key factors that influence interpretation of critical body residue (CBR) toxicity metrics including data quality issues, lipid dynamics, choice of endpoints, processes that alter toxicokinetics and toxicodynamics, phototoxicity, species- and life stage-specific sensitivities, and biotransformation. The vast majority of data available on TRA is derived from laboratory studies of acute lethal responses to organic toxicants exhibiting baseline toxicity. Application of the TRA to various baseline toxicants as well as substances with specific modes of action via receptor-mediated processes, such as chlorinated aromatic hydrocarbons, pesticides, and organometallics is discussed, as is application of TRA concepts in field assessments of tissue residues. In contrast to media-based toxicity relationships, CBR values tend to be less variable and less influenced by factors that control bioavailability and bioaccumulation, and TRA can be used to infer mechanisms of toxic action, evaluate the toxicity of mixtures, and interpret field data on bioaccumulated toxicants. If residue-effects data are not available, body residues can be estimated, as has been done using the target lipid model for baseline toxicants, to derive critical values for risk assessment. One of the primary unresolved issues complicating TRA for organic chemicals is biotransformation. Further work on the influence of biotransformation, a better understanding of contaminant lipid interactions, and an explicit understanding of the time dependency of CBRs and receptor-mediated toxicity are all required to advance this field. Additional residue-effects data on sublethal endpoints, early life stages, and a wider range of legacy and emergent contaminants will be needed to improve the ability to use TRA for organic and organometallic compounds.

Characterization of the recalcitrant CYP1 phenotype found in Atlantic killifish (Fundulus heteroclitus) inhabiting a Superfund site on the Elizabeth River, VA

Fundulus heteroclitus (Atlantic killifish) found at the Atlantic Wood Industries Superfund site on the Elizabeth River (ER) in Portsmouth, VA (USA), have been shown to be resistant to the teratogenic effects of creosote-contaminated sediments found at this highly contaminated site. Many of the polycyclic aromatic hydrocarbons (PAHs) found at the ER are known to activate the aryl hydrocarbon receptor (AHR), and are thought to mediate their toxic effects through this pathway. Activation of the AHR results in the induction of several Phase I and II metabolic enzymes. It has been previously shown that the AHR of killifish from the ER are refractory to induction by AHR agonists. To more fully characterize this altered AHR response, we exposed embryos from the ER and from a reference site on King's Creek, VA (KC) to two PAHs, benzo[alpha]pyrene (BaP) and benzo[k]fluoranthene (BkF), and to the dioxin-like compound (DLC), 3,3',4,4',5-pentachlorobiphenyl (PCB126). We compared their developmental and molecular responses by screening the embryos for CYP1A enzyme activity, cardiac deformities, and mRNA expression of CYP1A, CYP1B1, CYP1C1, and AHR2. Basal gene expression of both CYP1A and CYP1B1 was 40% higher in the KC control embryos compared to those from the ER, while AHR2 and CYP1C1 were not significantly different between the populations. Exposure of KC embryos to BaP, BkF, and PCB126 induced CYP1A activity and cardiac deformities. In contrast, CYP1A activity was induced in ER embryos only in response to BkF exposure, although this induction in ER embryos was significantly lower than that observed in KC fish at comparable concentrations. ER embryos did not develop cardiac deformities in response to any of the chemicals tested. CYP1A, CYP1B1 and CYP1C1 mRNA were all significantly induced in the KC embryos after exposure to BaP, BkF and PCB126. Exposure to BaP and BkF in ER embryos resulted in a significant induction of CYP1A mRNA, albeit significantly lower than observed in KC fish. Interestingly, BaP exposure resulted in induction of CYP1B1 at comparable levels in embryos from both populations. CYP1s were not induced in ER embryos in response to PCB126, nor was CYP1C1 for any treatment examined. Additionally, AHR2 was not significantly induced for any of the treatment groups. This study further characterizes the AHR response in killifish, and provides greater insight into the adapted ER phenotype. The ER adaptation involves the suppression of normal AHR-inducible gene expression for all three CYP1 genes, and therefore is likely an alteration in AHR signaling or control.

Dynamic zebrafish interactome reveals transcriptional mechanisms of dioxin toxicity

Background

In order to generate hypotheses regarding the mechanisms by which 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) causes toxicity, we analyzed global gene expression changes in developing zebrafish embryos exposed to this potent toxicant in the context of a dynamic gene network. For this purpose, we also computationally inferred a zebrafish (Danio rerio) interactome based on orthologs and interaction data from other eukaryotes.

Methodology/Principal Findings

Using novel computational tools to analyze this interactome, we distinguished between dioxin-dependent and dioxin-independent interactions between proteins, and tracked the temporal propagation of dioxin-dependent transcriptional changes from a few genes that were altered initially, to large groups of biologically coherent genes at later times. The most notable processes altered at later developmental stages were calcium and iron metabolism, embryonic morphogenesis including neuronal and retinal development, a variety of mitochondria-related functions, and generalized stress response (not including induction of antioxidant genes). Within the interactome, many of these responses were connected to cytochrome P4501A (cyp1a) as well as other genes that were dioxin-regulated one day after exposure. This suggests that cyp1a may play a key role initiating the toxic dysregulation of those processes, rather than serving simply as a passive marker of dioxin exposure, as suggested by earlier research.

Conclusions/Significance

Thus, a powerful microarray experiment coupled with a flexible interactome and multi-pronged interactome tools (which are now made publicly available for microarray analysis and related work) suggest the hypothesis that dioxin, best known in fish as a potent cardioteratogen, has many other targets. Many of these types of toxicity have been observed in mammalian species and are potentially caused by alterations to cyp1a.

Effects of fish cytochromes P450 inducers and inhibitors on difloxacin N-demethylation in kidney of Chinese idle (Ctenopharyngodon idellus)

Cytochromes P450 (CYPs) play key roles in drug metabolism which are widely distributed in kidney in aquatic organisms. CYP(s) mainly catalyzed the N-demethylation reaction of difloxacin (DIF) biotransformation to sarafloxacin (SAR). However, limited information is available about CYP investigation in fish. In order to supply useful information on CYP(s) characterization for DIF N-demethylation, the present study assessed the effects of fish potent CYP inducers and inhibitors on DIF N-demethylation and the inductive and inhibitive enzyme kinetics in kidney of Chinese idle (Ctenopharyngodon idellus) by reversed-phase high-performance liquid chromatography (RP-HPLC). Results demonstrated that the amounts of SAR formation pretreated by β-naphthoflavone (BNF) increased by 1.1-fold and α-naphthoflavone (ANF) inhibited SAR formation level by 0.6-fold at the third day. Enzymatic parameters V(max) and Cl(int) of DIF N-demethylase were increased by 0.56- and 0.38-fold due to β-naphthoflavone (BNF) pretreatment. DIF N-demethylation inhibition by varying ANF concentrations represented a mixed-type inhibition with the value of the inhibition constants (K(i)) 12.9mg/kg. BNF and ANF are the separate typical inducer and inhibitor for CYP1A in fish. Thus, we suggest that CYP1A may be responsible for DIF N-demethylation in kidney. This study provides instructive information to ensure treatment success in fisheries medication with two or more drugs.

Antioxidant responses and NRF2 in synergistic developmental toxicity of PAHs in zebrafish

Early piscine life stages are sensitive to polycyclic aromatic hydrocarbon (PAH) exposure, which can cause pericardial effusion and craniofacial malformations. We previously reported that certain combinations of PAHs cause synergistic developmental toxicity, as observed with coexposure to the aryl hydrocarbon receptor agonist beta-naphthoflavone (BNF) and cytochrome P4501A inhibitor alpha-naphthoflavone (ANF). Herein, we hypothesized that oxidative stress is a component of this toxicity. We examined induction of antioxidant genes in zebrafish embryos (Danio rerio) exposed to BNF or ANF individually, a BNF + ANF combination, and a prooxidant positive control, tert-butylhydroperoxide (tBOOH). We measured total glutathione (GSH) and attempted to modulate deformities using the GSH synthesis inhibitor L-buthionine (S,R)-sulfoximine (BSO) and increase GSH pools with N-acetyl cysteine (NAC). In addition, we used a morpholino to knockdown expression of the antioxidant response element transcription factor NRF2 to determine if this would alter gene expression or increase deformity severity. BNF + ANF coexposure significantly increased expressions of superoxide dismutase 1 and 2, glutathione peroxidase 1, pi class glutathione-s-transferase, and glutamate cysteine-ligase to a greater extent than tBOOH, BNF, or ANF alone. BSO pretreatment decreased some GSH levels, but did not worsen deformities, nor did NAC diminish toxicity. Knockdown of NRF2 increased mortality following tBOOH challenge, prevented significant upregulation of antioxidant genes following both tBOOH and BNF + ANF exposures, and exacerbated BNF + ANF-related deformities. Collectively, these findings demonstrate that antioxidant responses are a component of PAH synergistic developmental toxicity and that NRF2 is protective against prooxidant and PAH challenges during development.

Fluoranthene, but not benzo[a]pyrene, interacts with hypoxia resulting in pericardial effusion and lordosis in developing zebrafish

Previous research has documented several PAHs that interact synergistically, causing severe teratogenicity in developing fish embryos. The coexposure of CYP1A inhibitors (e.g. FL or ANF) with AHR agonists (e.g. BaP or BNF) results in a synergistic increase in toxicity. As with chemical CYP1A inhibitors, it has also been shown that CYP1A morpholinos exacerbate BNF-induced embryotoxicity. We hypothesized that a hypoxia-induced reduction in CYP1A activity in BNF or BaP-exposed zebrafish embryos would similarly enhance pericardial effusion and other developmental abnormalities. BaP, BNF, ANF, and FL exposures, both individually and as BaP+FL or BNF+ANF combinations, were performed under hypoxia and normoxia. CYP1A activity in the BaP+hypoxia and BNF+hypoxia embryos was reduced by approximately 60% relative to normoxia embryos. Although CYP1A activity was significantly reduced, we did not observe any increase in pericardial effusion in either group. An unexpected yet particularly interesting result of these experiments was the observed interaction of both FL and ANF with hypoxia. Relatively high, yet environmentally relevant concentrations of FL (100-500 microg L(-1)) interact with moderate hypoxia (7.3% DO) through an unknown mechanism, resulting in pericardial effusion and severe lordosis. Additionally, ANF exposures (100 microg L(-1)) which are not normally teratogenic caused dramatic pericardial effusion, but not lordosis, when embryos were coexposed to hypoxia. These results suggest that reduced CYP1A activity may not exclusively underlie observed developmental toxicity, and that hypoxia may exacerbate the developmental toxicity of some PAH mixtures.

Evidence for multiple mechanisms of toxicity in larval rainbow trout (Oncorhynchus mykiss) co-treated with retene and alpha-naphthoflavone

Alkylated polycyclic aromatic hydrocarbons, such as retene (7-isopropyl-1-methylphenanthrene), induce cytochrome P450 1A (CYP1A) enzymes and produce dioxin-like toxicity in the embryo-larval stages of fish characterized by the signs of blue sac disease (BSD). The signs of toxicity are well characterized; however, the mechanism is not well understood. To elucidate the role of CYP1A in retene toxicity, larval rainbow trout (Oncorhynchus mykiss) were co-treated with a range of concentrations of alpha-naphthoflavone (ANF), a known CYP1A inhibitor. The co-treatment produced synergistic toxicity at 3.2-100 microg/L ANF, after which toxicity at 180 microg/L ANF dropped to levels typical of retene-only. At 320 microg/L ANF, toxicity increased with or without retene, indicating that ANF alone was capable of inducing BSD. In addition, the additive toxicity of retene-only and 320 microg/L ANF-only approximately equalled that of the co-exposed larvae (100 microg/L retene+320 microg/L ANF), indicating response addition. Thus, two mechanisms of action occurred in co-exposed larvae at different concentrations of ANF. In trout larvae, there was a correlation between toxicity and CYP1A protein concentrations, and in juvenile trout, ANF produced a concentration-dependent inhibition of ethoxyresorufin-O-deethylase (EROD) activity without a measurable drop in CYP1A protein. Taken together, the mechanism underlying the synergistic toxicity is EROD-independent and may be AhR-dependent. This study demonstrated that multiple, exposure-dependent mechanisms can occur in mixture toxicity, suggesting that current risk assessment models may drastically underestimate toxicity, particularly of mixtures containing both CYP1A inducers and inhibitors.

Synergistic induction of AHR regulated genes in developmental toxicity from co-exposure to two model PAHs in zebrafish

Polycyclic aromatic hydrocarbons (PAHs) are pollutants created by the incomplete combustion of carbon, and are increasing in the environment largely due to the burning of fossil fuels. PAHs occur as complex mixtures, and some combinations have been shown to cause synergistic developmental toxicity in fish embryos, characterized by pericardial edema and craniofacial malformations. Previous studies have indicated that in the zebrafish model, this toxicity is mediated by the aryl hydrocarbon receptor 2 (AHR2), and enhanced by inhibition of CYP1A activity. In this study, we further examined this interaction of the model PAH and AHR agonist beta-naphthoflavone (BNF) with and without the AHR partial agonist/antagonist and CYP1A inhibitor alpha-naphthoflavone (ANF) to determine (1) whether ANF was acting as an AHR antagonist, (2) what alterations BNF and ANF both alone and in combination had on mRNA expression of the AHR regulated genes cytochrome P450 (cyp) 1a, 1 b 1, and 1 c 1, and the AHR repressor (ahrr2) prior to versus during deformity onset, and (3) compare CYP1A enzyme activity with mRNA induction. Zebrafish embryos were exposed from 24-48 or 24-96 hpf to BNF, 1-100 microg/L, ANF, 1-150 microg/L, a BNF+ANF co-exposure (1 microg/L+100 microg/L), or a DMSO solvent control. RNA was extracted and examined by quantitative real-time PCR. Both BNF and ANF each individually resulted in a dose dependent increase CYP1A, CYP1B1, CYP1C1, and AHRR2 mRNA, confirming their activities as AHR agonists. In the BNF+ANF co-exposures prior to deformity onset, expression of these genes was synergistic, and expression levels of the AHR regulated genes resembled the higher doses of BNF alone. Gene induction during deformities was also significantly increased in the co-exposure, but to a lesser magnitude than prior to deformity onset. EROD measurements of CYP1A activity showed ANF inhibited activity induction by BNF in the co-exposure group; this finding is not predicted by mRNA expression, which is synergistically induced in this treatment. This suggests that inhibition of CYP1A activity may alter metabolism and/or increase the half-life of the AHR agonist(s), allowing for increased AHR activation. This study furthers a mechanistic understanding of interactions underlying PAH synergistic toxicity.

Nonadditive effects of PAHs on Early Vertebrate Development: mechanisms and implications for risk assessment

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants. Traditionally, much of the research has focused on the carcinogenic potential of specific PAHs, such as benzo(a)pyrene, but recent studies using sensitive fish models have shown that exposure to PAHs alters normal fish development. Some PAHs can induce a teratogenic phenotype similar to that caused by planar halogenated aromatic hydrocarbons, such as dioxin. Consequently, mechanism of action is often equated between the two classes of compounds. Unlike dioxins, however, the developmental toxicity of PAH mixtures is not necessarily additive. This is likely related to their multiple mechanisms of toxicity and their rapid biotransformation by CYP1 enzymes to metabolites with a wide array of structures and potential toxicities. This has important implications for risk assessment and management as the current approach for complex mixtures of PAHs usually assumes concentration addition. In this review we discuss our current knowledge of teratogenicity caused by single PAH compounds and by mixtures and the importance of these latest findings for adequately assessing risk of PAHs to humans and wildlife. Throughout, we place particular emphasis on research on the early life stages of fish, which has proven to be a sensitive and rapid developmental model to elucidate effects of hydrocarbon mixtures.