Sulfonation of environmental estrogens by zebrafish cytosolic sulfotransferases

Physiologically Based Toxicokinetic Modeling of Bisphenols in Zebrafish (Danio rerio) Accounting for Variations in Metabolic Rates, Brain Distribution, and Liver Accumulation

Bisphenol A (BPA) is an industrial chemical, which has raised human health and environmental concerns due to its endocrine-disrupting properties. BPA analogues are less well-studied despite their wide use in consumer products. These analogues have been detected in water and aquatic organisms around the world, with some analogues showing toxic effects in various species including fish. Here, we present novel organ-specific time-course distribution data of bisphenol Z (BPZ) in female zebrafish (Danio rerio), including concentrations in the ovaries, liver, and brain, a rarely sampled organ with high toxicological relevance. Furthermore, fish-specific in vitro biotransformation rates were determined for 11 selected bisphenols. A physiologically based toxicokinetic (PBTK) model was adapted for four of these bisphenols, which was able to predict levels in the gonads, liver, and brain as well as the whole body within a 2-5-fold error with respect to experimental data, covering several important target organs of toxicity. In particular, predicted liver concentrations improved compared to currently available PBTK models. Predicted data indicate that studied bisphenols mainly distribute to the carcass and gonads and less to the brain. Our model provides a tool to increase our understanding on the distribution and kinetics of a group of emerging pollutants.

The toxicokinetics of bisphenol A and its metabolites in fish elucidated by a PBTK model

Bisphenol A (BPA) is a chemical of major concern due to its endocrine disrupting function, high production volume, and persistence in the aquatic environment. Consequently, organisms such as fish are subject to chronic exposure to BPA. However, physiologically-based toxicokinetic (PBTK) models, which are valuable tools to improve the understanding of a chemical's fate in an organism, have never been specifically adapted to model BPA toxicokinetics (TK) in fish. In our work, an existing PBTK developed for four different fish species was modified to model BPA ADME processes (absorption, distribution, metabolization and excretion). The metabolization of BPA into BPA-monoglucuronide (BPA gluc) and BPA-monosulfate (BPA sulf) and their TK in various organs was taking into account in the model. Experiments were performed to generate BPA TK data in a model species commonly used in ecotoxicology, the stickleback. The model structure had to include two sites of metabolization to simulate BPA TK accurately in stickleback organs. Thus, the fish liver may not be the only site of the metabolization of BPA: plasma or gills could also play a role in BPA metabolization. The PBTK model predictive performance evaluated on literature data in zebrafish and rainbow trout concurs with this conclusion. Finally, a calibration mixing data from the three species was compared to the calibration on stickleback data only.

Updated perspectives on the cytosolic sulfotransferases (SULTs) and SULT-mediated sulfation

The cytosolic sulfotransferases (SULTs) are Phase II detoxifying enzymes that mediate the sulfate conjugation of numerous xenobiotic molecules. While the research on the SULTs has lagged behind the research on Phase I cytochrome P-450 enzymes and other Phase II conjugating enzymes, it has gained more momentum in recent years. This review aims to summarize information obtained in several fronts of the research on the SULTs, including the range of the SULTs in different life forms, concerted actions of the SULTs and other Phase II enzymes, insights into the structure-function relationships of the SULTs, regulation of SULT expression and activity, developmental expression of SULTs, as well as the use of a zebrafish model for studying the developmental pharmacology/toxicology.

Transcriptome alterations in zebrafish embryos after exposure to environmental estrogens and anti-androgens can reveal endocrine disruption

Exposure to environmental chemicals known as endocrine disruptors (EDs) is in many cases associated with an unpredictable hazard for wildlife and human health. The identification of endocrine disruptive properties of chemicals certain to enter the aquatic environment relies on toxicity tests with fish, assessing adverse effects on reproduction and sexual development. The demand for quick, reliable ED assays favored the use of fish embryos as alternative test organisms. We investigated the application of a transcriptomics-based assay for estrogenic and anti-androgenic chemicals with zebrafish embryos. Two reference compounds, 17α-ethinylestradiol and flutamide, were tested to evaluate the effects on development and the transcriptome after 48h-exposures. Comparison of the transcriptome response with other estrogenic and anti-androgenic compounds (genistein, bisphenol A, methylparaben, linuron, prochloraz, propanil) showed commonalities and differences in regulated pathways, enabling us to classify the estrogenic and anti-androgenic potencies. This demonstrates that different mechanism of ED can be assessed already in fish embryos.

Steroid catabolism in marine and freshwater fish

Steroids play important roles in regulating many physiological functions in marine and freshwater fish. Levels of active steroid in blood and tissues are determined by the balance between synthetic and catabolic processes. This review examines what is known about pathways of catabolism of steroids, primarily sex steroids, in marine and freshwater fish. Cytochrome P450 (P450) isoforms present in hepatic microsomes catalyze steroid hydroxylation to metabolites with lower or no activity at estrogen or androgen receptors. Important pathways of steroid catabolism to readily excreted metabolites are glucuronidation and sulfonation of hydroxyl groups. Estradiol, testosterone, DHEA and hydroxylated metabolites of these and other steroids readily form glucuronide and sulfate conjugates in those fish species where these pathways have been examined. Little is known, however, of the structure and function of the UDP-glucuronosyltransferase (UGT) and sulfotransferase (SULT) enzymes involved in steroid conjugation in fish. Glucuronide and sulfate conjugates of steroids may be transported into and out of cells by organic anion transporter proteins and multi-drug resistance proteins, and there is growing evidence that these proteins play important roles in steroid conjugate transport and elimination. Induction or inhibition of any of these pathways by environmental chemicals can result in alteration of the natural balance of steroid hormones and could lead to disruption of the endocrine system. Recent studies in this area are presented, with particular focus on phase II (conjugative) pathways.

Kinetics of hepatic phase I and II biotransformation reactions in eight finfish species

Hepatic microsomes and cytosols of channel catfish (Ictalurus punctatus), rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar), red tilapia (Oreochromis sp.), largemouth bass (Micropterussalmoides), striped bass (Morone saxatilis), hybrid striped bass (M. saxatilis x M. crysops), and bluegill (Lepomis macrochuris) (n=8) were used to study the kinetics of phase I (ECOD, EROD, PROD, BROD) and phase II (UDP-glucuronosyltransferase (UDPGT)-, sulfotransferase (ST)- and glutathione-s-transferase (GST)-mediated) reactions. The best catalytic efficiency for ECOD and GST activities was performed by channel catfish, Atlantic salmon, rainbow trout and tilapia. The highest EROD catalytic efficiency was for Atlantic salmon. None of the species had either PROD or BROD activities. Rainbow trout had very similar UDPGT catalytic efficiency to tilapia, channel catfish, Atlantic salmon, largemouth bass and bluegill. Sulfotransferase conjugation had no significant differences among the species. In summary, tilapia, channel catfish, Atlantic salmon and rainbow trout had the best biotransforming capabilities; striped bass, hybrid striped bass and bluegill were low metabolizers and largemouth bass shared some capabilities with both groups.

Low salinity acclimation and thyroid hormone metabolizing enzymes in gilthead seabream (Sparus auratus)

We investigated the effect of acclimation to low salinity water of gilthead seabream (Sparus auratus), a euryhaline seawater teleost, on the activities of thyroid hormone-metabolizing enzymes in gills, kidney, and liver. Following acclimation to low salinity water, the plasma free thyroxine (T(4)) concentration increases 2.5-fold, and outer ring deiodination activities towards T(4), 3,5,3'-triiodothyronine (T(3)) and 3,3',5'-triiodothyronine (reverse T(3), rT(3)) in the gills are reduced by 20-32%. Conjugation (catalyzed by sulfotransferase and UDP-glucuronyltransferase) and deconjugation pathways (arylsulfatase, beta-glucuronidase) play a role in the biological activity of native and conjugated thyroid hormones. Branchial, renal, and hepatic activities of the enzymes involved in these metabolic pathways respond differentially to low salinity conditions. The results substantiate that thyroid hormones are involved in S. auratus osmoregulation, and that the gills are well equipped to play an important role in the modulation of plasma hormone titers.

A compilation of in vitro rate and affinity values for xenobiotic biotransformation in fish, measured under physiological conditions

Scientific literature from the past 25 years was searched to obtain in vitro biotransformation rate and affinity data for fish. To maximize the environmental relevance of this dataset, we focused on studies conducted at multiple substrate concentrations, and established acceptance criteria with respect to assay temperature and pH. Altogether, enzyme rate and affinity parameters are provided for 43 species and 77 compounds. In all but three instances, the reported reactions exhibited saturation at high substrate concentrations and could be used to calculate Michaelis-Menten rate (Vmax) and affinity (Km) constants. Most of this information was obtained using in vitro systems derived from liver tissue. Information from non-hepatic tissues was included, however, to provide a basis for comparisons among tissues. Where possible, in vitro enzyme parameters were examined to compare: (1) hepatic metabolism of a common substrate within a species, (2) hepatic metabolism of common substrates by different species, and (3) metabolism of a common substrate by different tissues of one species. Comparisons within species highlight a number of factors that may substantially influence xenobiotic metabolism in fish including gender, life stage, and acclimation temperature. Limited data suggest that Vmax and Km for the same reaction may vary by up to three orders of magnitude among species.

Effects of xenobiotics and steroids on renal and hepatic estrogen metabolism in lake trout

Experiments were conducted to (1) elucidate the biochemical pathways of E2 metabolism in the lake trout (Salvelinus namaycush) kidney and liver, and (2) test the hypothesis that specific xenobiotics and endogenous steroids inhibit E2 metabolism by these tissues. Kidney and liver tissue fragments from immature lake trout were incubated in vitro in the presence of radiolabelled E2 plus various xenobiotics or steroids. E2 metabolites were identified by liquid chromatography/mass spectroscopy, and quantified by liquid scintillation spectroscopy. A major metabolite produced by both tissues was an unidentified hydroxylated estrogen metabolite (E2-OH) with a molecular mass of 288 that was not estriol (16-OH-E2), but possibly 7alpha-OH-E2 or 2-OH-E2 (catecholestrogen). Both tissues also produced estradiol-17-glucuronide (E2-17-G), estradiol-17-sulfate (E2-17-S), and estradiol-3-glucuronide (E2-3-G). Compared to the kidney, the liver produced half the amount of conjugated metabolites, but twofold more E2-OH. The following xenobiotics (at a concentration of 100 microM) inhibited the production of water-soluble (i.e., conjugated) E2 metabolites by both the kidney and liver: 4,4'-(OH)2-3,3',5,5'- tetrachlorobiphenyl (4,4'-OH-TCB), bisphenol A (BPA), tetrabromobisphenol A (TB-BPA), tetrachlorobisphenol A (TC-BPA), tribromophenol (TBP), trichlorophenol (TCP), and pentachlorophenol (PCP). The alkylphenols, 4-n-nonylphenol (NP), and 4-octylphenol (OP), and 2,2',4,4'-tetrabromodiphenyl ether (TBDE) had no significant effect on E2 metabolism by either tissue. Testosterone and 17alpha,20beta-dihydroxy-4-pregnen-3-one inhibited the production of conjugated E2 metabolites by both the kidney and liver. Cortisol and 11-ketotestosterone inhibited E2 metabolism by the liver only. The median inhibitory concentrations (IC50) for 4,4'-OH-TCB ranged from 7-32 microM in the kidney and 0.6-1.6 microM in the liver. For BPA, IC50's ranged from 40-108 microM in the kidney and 11-18 microM in the liver. Low doses (0.1 microM) of 4,4'-OH-TCB and BPA significantly increased estrogen metabolism in the kidney. The results suggest that certain estrogenic xenobiotics and endogenous steroids may inhibit the phase II conjugation of E2 by the kidney and liver of lake trout, and some of the known biological effects of these compounds are likely mediated, at least partially, by this mechanism of action.

Steroid levels and steroid metabolism in the mussel Mytilus edulis: the modulating effect of dispersed crude oil and alkylphenols

Significant amounts of oil and alkylphenols are released into the sea by petroleum installations as a result of discharges of produced water. Some of these pollutants elicit estrogenic responses in fish, but their effects on the endocrine system of molluscs are largely unknown. In this study, mussels Mytilus edulis were exposed to North Sea oil (O) and the mixture of North Sea oil+alkylphenols (OAP), and the effects on tissue steroid levels and steroid metabolism (P450-aromatase and estradiol-sulfotransferase) were monitored. Levels of free testosterone and free estradiol were much higher in gonad tissue than in peripheral tissue, whereas esterified steroids (released after saponification) were of the same order of magnitude in both tissues. Levels of free steroids determined in gonads were not affected by exposure, but esterified steroids significantly increased in OAP exposed mussels (up to 2.4-fold). The sulfation of estradiol was investigated as a conjugation pathway, and increased activities were observed in digestive gland cytosol of both O and OAP exposure groups (up to 2.8-fold). Additionally, increased P450-aromatase activity was determined in OAP exposed mussels (up to three-fold, both in gonad and digestive gland), but not in the O group. Altogether, the results indicate that North Sea oil leads to increased sulfation of estradiol, and that in combination with alkylphenols, additional alterations are observed: increased P450-aromatase, and increased levels of esterified-steroids in gonads. Nonetheless, mussels are able to maintain gonad concentrations of free steroids unaltered, possibly via homeostatic mechanisms such as the conjugation with fatty acid or the formation of sulphate conjugates.

Endocrine alteration in juvenile cod and turbot exposed to dispersed crude oil and alkylphenols

Juvenile Atlantic cod (Gadus morhua) and turbot (Scophthalmus maximus) were exposed for 3 weeks in a continuous water flow to 0.5 ppm of dispersed North Sea crude oil, 0.5 ppm of dispersed North Sea crude oil spiked with 0.1 ppm of a mixture of alkylphenols (offshore oil production), and 30 ppb of nonylphenol (NP). As potential markers of endocrine alteration, key enzymatic activities involved in both synthesis (17beta-hydroxysteroid dehydrogenases and P450 aromatase) and metabolism (liver UDP-glucuronosyltransferases (UGT) and sulfotransferases) of steroids were assessed together with circulating levels of testosterone and estradiol in plasma. NP-exposed turbot had lower ovarian P450 aromatase, lower levels of testosterone and estradiol in plasma, and lower glucuronidation rates of sex steroids than those from the control group. In contrast, higher liver UGT-testosterone, and a trend towards higher P450 aromatase was detected in oil-exposed specimens. Those exposed to the combination oil+alkylphenols had lower levels of estradiol in plasma than controls, and no significant effects on any the enzymatic activities tested was observed. All these alterations were more evident in turbot than in cod. In fact, apart from a higher glucuronidation rate of estradiol detected in the liver of NP-exposed cod, no significant differences were observed between control and exposed cod.