Tissue-, sex- and development-specific transcription profiles of eight UDP-glucuronosyltransferase genes in zebrafish (Danio rerio) and their regulation by activator of aryl hydrocarbon receptor

Organ-specific biotransformation in salmonids: Insight into intrinsic enzyme activity and biotransformation of three micropollutants

Aquatic ecosystems continue to be threatened by chemical pollution. To what extent organisms are able to cope with chemical exposure depends on their ability to display mechanisms of defense across different organs. Among these mechanisms, biotransformation processes represent key physiological responses that facilitate detoxification and reduce the bioaccumulation potential of chemicals. Biotransformation does not only depend on the ability of different organs to display biotransformation enzymes but also on the affinity of chemicals towards these enzymes. In the present study, we explored the ability of different organs and of two freshwater fish to support biotransformation processes through the determination of in vitro phase I and II biotransformation enzyme activity, and their role in supporting intrinsic clearance and the formation of biotransformation products. Three environmentally relevant pollutants were evaluated: the polycyclic aromatic hydrocarbon (PAH) pyrene (as recommended by the OECD 319b test guideline), the fungicide azoxystrobin, and the pharmaceutical propranolol. Comparative studies using S9 sub-cellular fractions derived from the liver, intestine, gills, and brain of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) revealed significant phase I and II enzyme activity in all organs. However, organ- and species-specific differences were found. In brown trout, significant extrahepatic biotransformation was observed for pyrene but not for azoxystrobin and propranolol. In rainbow trout, the brain appeared to biotransform azoxystrobin. In this same species, propranolol appeared to be biotransformed by the intestine and gills. Biotransformation products could be detected only from hepatic biotransformation, and their profiles and formation rates displayed species-specific patterns and occurred at different magnitudes. Altogether, our findings further contribute to the current understanding of organ-specific biotransformation capacity, beyond the expression and activity of enzymes, and its dependence on specific enzyme-chemical interactions to support mechanisms of defense against exposure.

Comprehensive mRNA and microRNA analysis revealed the effect and response strategy of freshwater fish, grass carp (Ctenopharyngodon idella) under geosmin exposure

Geosmin is an environmental pollutant that causes off-flavor in water and aquatic products. The high occurrence of geosmin contamination in aquatic systems and aquaculture raises public awareness, however, few studies have investigated the response pathways of geosmin stress on freshwater fish. In this research, grass carp were exposed to 50 μg/L geosmin for 96 h, liver tissue was sequenced and validated using real-time qPCR. In total of 528 up-regulated genes and 488 down-regulated genes were observed, includes cytochrome P450 and uridine diphosphate (UDP)-glucuronosyltransferase related genes. KEGG analysis showed that chemical carcinogenesis-DNA adducts, metabolism of xenobiotics by cytochrome P450, drug metabolism-cytochrome P450 pathway was enriched. Common genes from the target genes of microRNAs and differential expression genes are enriched in metabolism of xenobiotics cytochrome P450 pathway. Two miRNAs (dre-miR-146a and miR-212-3p) down regulated their target genes (LOC127510138 and adh5, respectively) which are enriched cytochrome P450 related pathway. The results present that geosmin is genetoxic to grass carp and indicate that cytochrome P450 system and UDP-glucuronosyltransferase play essential roles in biotransformation of geosmin. MicroRNAs regulate the biotransformation of geosmin by targeting specific genes, which contributes to the development of strategies to manage its negative impacts in both natural and artificial environments.

Comparative analysis of the metabolites and biotransformation pathways of fentanyl in the liver and brain of zebrafish

The rise of fentanyl has introduced significant new challenges to public health. To improve the examination and identification of biological samples in cases of fentanyl misuse and fatalities, this study utilized a zebrafish animal model to conduct a comparative investigation of the metabolites and biotransformation pathways of fentanyl in the zebrafish's liver and brain. A total of 17 fentanyl metabolites were identified in the positive ion mode using ultra-high-pressure liquid chromatography Q Exactive HF Hybrid Quadrupole-Orbitrap mass spectrometry (UHPLC-QE HF MS). Specifically, the zebrafish's liver revealed 16 fentanyl metabolites, including 6 phase I metabolites and 10 phase II metabolites. Conversely, the zebrafish's brain presented fewer metabolites, with only 8 detected, comprising 6 phase I metabolites and 2 phase II metabolites. Notably, M'4, a metabolite of dihydroxylation, was found exclusively in the brain, not in the liver. Through our research, we have identified two specific metabolites, M9-a (monohydroxylation followed by glucuronidation) and M3-c (monohydroxylation, precursor of M9-a), as potential markers of fentanyl toxicity within the liver. Furthermore, we propose that the metabolites M1 (normetabolite) and M3-b (monohydroxylation) may serve as indicators of fentanyl metabolism within the brain. These findings suggest potential strategies for extending the detection window and enhancing the efficiency of fentanyl detection, and provide valuable insights that can be referenced in metabolic studies of other new psychoactive substances.

Physiology-informed toxicokinetic model for the zebrafish embryo test developed for bisphenols

Zebrafish embryos (ZFE) is a widely used model organism, employed in various research fields including toxicology to assess e.g., developmental toxicity and endocrine disruption. Variation in effects between chemicals are difficult to compare using nominal dose as toxicokinetic properties may vary. Toxicokinetic (TK) modeling is a means to estimate internal exposure concentration or dose at target and to enable extrapolation between experimental conditions and species, thereby improving hazard assessment of potential pollutants. In this study we advance currently existing TK models for ZFE with physiological ZFE parameters and novel experimental bisphenol data, a class of chemicals with suspected endocrine activity. We developed a five-compartment model consisting of water, plastic, chorion, yolk sack and embryo in which surface area and volume changes as well as the processes of biotransformation and blood circulation influence mass fluxes. For model training and validation, we measured internal concentrations in ZFE exposed individually to BPA, bisphenol AF (BPAF) and Z (BPZ). Bayesian inference was applied for parameter calibration based on the training data set of BPZ. The calibrated TK model predicted internal ZFE concentrations of the majority of external test data within a 5-fold error and half of the data within a 2-fold error for bisphenols A, AF, F, and tetrabromo bisphenol A (TBBPA). We used the developed model to rank the hazard of seven bisphenols based on predicted internal concentrations and measured in vitro estrogenicity. This ranking indicated a higher hazard for BPAF, BPZ, bisphenol B and C (BPB, BPC) than for BPA.

Multi-level assessment of the origin, feeding area and organohalogen contamination on salmon from the Baltic Sea

The Atlantic salmon (Salmo salar) population in the Baltic Sea consists of wild and hatchery-reared fish that have been released into the sea to support salmon stocks. During feeding migration, salmon migrate to different parts of the Baltic Sea and are exposed to various biotic and abiotic stressors, such as organohalogen compounds (OHCs). The effects of salmon origin (wild or hatchery-reared), feeding area (Baltic Main Basin, Bothnian Sea, and Gulf of Finland), and OHC concentration on the differences in hepatic proteome of salmon were investigated. Multi-level analysis of the OHC concentration, transcriptome, proteome, and oxidative stress biomarkers measured from the same salmon individuals were performed to find the key variables (origin, feeding area, OHC concentrations, and oxidative stress) that best account for the differences in the transcriptome and proteome between the salmon groups. When comparing wild and hatchery-reared salmon, differences were found in xenobiotic and amino acid metabolism-related pathways. When comparing salmon from different feeding areas, the amino acid and carbohydrate metabolic pathways were notably different. Several proteins found in these pathways are correlated with the concentrations of polychlorinated biphenyls (PCBs). The multi-level analysis also revealed amino acid metabolic pathways in connection with PCBs and oxidative stress variables related to glutathione metabolism. Other pathways found in the multi-level analysis included genetic information processes related to ribosomes, signaling and cellular processes related to the cytoskeleton, and the immune system, which were connected mainly to the concentrations of Polychlorinated biphenyls and Dichlorodiphenyltrichloroethane and their metabolites. These results suggest that the hepatic proteome of salmon in the Baltic Sea, together with the transcriptome, is more affected by the OHC concentrations and oxidative stress of the feeding area than the origin of the salmon.

Transcriptomic impacts and potential routes of detoxification in a lampricide-tolerant teleost exposed to TFM and niclosamide

Sea lamprey (Petromyzon marinus) control in the Laurentian Great Lakes of North America often relies on the application of 3-trifluoromethyl-4-nitrophenol (TFM) and niclosamide mixtures to kill larval sea lamprey. Selectivity of TFM against lampreys appears to be due to differential detoxification ability in these jawless fishes compared to bony fishes, particularly teleosts. However, the proximate mechanisms of tolerance to the TFM and niclosamide mixture and the mechanisms of niclosamide toxicity on its own are poorly understood, especially among non-target fishes. Here, we used RNA sequencing to identify specific mRNA transcripts and functional processes that responded to niclosamide or a TFM:niclosamide mixture in bluegill (Lepomis macrochirus). Bluegill were exposed to niclosamide or TFM:niclosamide mixture, along with a time-matched control group, and gill and liver tissues were sampled at 6, 12, and 24 h. We summarized the whole-transcriptome patterns through gene ontology (GO) term enrichment and through differential expression of detoxification genes. The niclosamide treatment resulted in an upregulation of several transcripts associated with detoxification (cyp, ugt, sult, gst), which may help explain the relatively high detoxification capacity in bluegill. Conversely, the TFM:niclosamide mixture resulted in an enrichment of processes related to arrested cell cycle and growth, and cell death alongside a diverse detoxification gene response. Detoxification of both lampricides likely involves the use of phase I and II biotransformation genes. Our findings strongly suggest that the unusually high tolerance of bluegill to lampricides is due to these animals having an inherently high capacity and flexible detoxification response to such compounds.

DNA repair genes play a variety of roles in the development of fish embryos

Embryogenesis is one of the most important life stages because it determines an organism's healthy growth. However, embryos of externally fertilizing species, such as most fish, are directly exposed to the environment during development and may be threatened by DNA damaging factors (pollutants, UV, reactive oxygen species). To counteract the negative effects of DNA fragmentation, fish embryos evolved complex damage response pathways. DNA repair pathways have been extensively studied in some fish species, such as zebrafish (Danio rerio). Our literature review, on the other hand, revealed a paucity of knowledge about DNA damage response and repair in non-model aquaculture fish species. Further, several pieces of evidence underlie the additional role of DNA repair genes and proteins in organogenesis, spatiotemporal localization in different tissue, and its indispensability for normal embryo development. In this review, we will summarize features of different DNA repair pathways in course of fish embryo development. We describe how the expression of DNA repair genes and proteins is regulated during development, their organogenetic roles, and how the expression of DNA repair genes changes in response to genotoxic stress. This will aid in addressing the link between genotoxic stress and embryo phenotype. Furthermore, available data indicate that embryos can repair damaged DNA, but the effects of early-life stress may manifest later in life as behavioral changes, neoplasia, or neurodegeneration. Overall, we conclude that more research on DNA repair in fish embryos is needed.

Antagonistic effects and mechanisms of carbendazim and chlorpyrifos on the neurobehavior of larval zebrafish

Residues from multiple pesticides are frequently detected on vegetables, which may produce combined toxicity not predicted by individual toxicity data. As these combined effects present additional dangers to food safety, we have compared individual to combined effects for a variety of pesticides. Carbendazim and chlorpyrifos are the two most commonly detected pesticides in vegetables, and previous studies reported that combined exposure results in synergistic developmental toxicity to zebrafish embryos. In this study, individual and combined effects on zebrafish motor activity were examined following individual and combined exposure to assess nervous system toxicity. Further, transcriptomics methods were used to identify potential molecular mechanisms for individual and combined toxicity. Carbendazim alone induced a disorganized swim pattern characterized by increased angular velocity, turn angle, meander, and acceleration during light-dark transition, while chlorpyrifos alone reduced average swim speed and light-dark acceleration. Combined treatment significantly reduced average swim velocity and total distance traveled. Combination indices indicated strong antagonism between compounds for average speed and light-dark acceleration. Transcriptomics (RNA-seq) showed that carbendazim significantly altered the expression of genes involved in antigen processing and presentation, apoptosis, autophagy, and metabolism, including ctslb, cyp7a1, hsp70l, and ugt1a1. Alternatively, chlorpyrifos significantly altered genes involved in various nervous system-related pathways, including glutamatergic, GABAergic, dopaminergic, and calcium signaling. Protein-protein interaction (PPI) network analysis suggested that chlorpyrifos significantly downregulated genes related to light transduction, resulting in decreased sensitivity to light-dark transitions, while antagonism mainly reflected divergent effects on phototransduction and retinol metabolism. Carbendazim had no significant effects on vision-related genes such as gnat1 and gngt1, while chlorpyrifos downregulated expression, an effect reversed by the combination. Comprehensive toxicity analyses must include joint effects of co-applied pesticides for enhanced food safety.

Molecular insight into the hepatopancreas of oriental river prawn (Macrobrachium nipponense) in response to residual chlorine stimulus

Chlorine or chloric disinfectants are cost-effective disinfectants, which are widely used to disinfect domestic and industrial water. The residual chlorine levels in some of these waters have been proven toxic to several aquatic organisms; however, the molecular mechanisms of toxicity of residual chlorine to aquatic crustaceans, including Macrobrachium nipponense, an economically important freshwater prawn native to Asian countries, have not been investigated to date. Here, M. nipponense was exposed to 0.53 mg/L of residual chlorine, and comparative transcriptomics analyses were performed to determine their response mechanisms at the molecular level. Residual chlorine caused lethal effects on prawns. Furthermore, a total of 940 differentially expressed genes (DEGs), including 501 up-regulated and 439 down-regulated genes, were identified after 48 h of residual chlorine exposure compared to the control group. After enrichment analysis of GO (Gene Ontology) functions and KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways, identified DEGs were demonstrated to be associated with a variety of functions including exerting "oxidoreductase activity", and participating in "oxidation-reduction process". In addition, cytochrome P450 family 1 subfamily A1 (CYP1A1), glutathione S-transferase (GST), and glucuronosyltransferase (UGT) were enriched in the pathway of metabolism of xenobiotics by cytochrome P450. Furthermore, protein-protein interaction (PPI) network analysis revealed interactions among actin beta/gamma 1 (ACTB_G1) gene encoding protein and a series of multiple functional DEGs (e.g., hexokinase (HK), fructose 1,6-biphosphate-aldolase A (ALDOA), cytochrome c (CYC), and elongation factor 1-alpha (EEF1A)) encoding proteins. This study laid a theoretical foundation for safety evaluation of chlorinated aquatic water and further investigation of the toxicity of chlorination to M. nipponense.

Transcriptional inhibition of steroidogenic factor 1 in vivo in Oreochromis niloticus increased weight and suppressed gonad development

Steroidogenic factor 1 (sf1) (officially designated as nuclear receptor subfamily 5 group A member 1 [NR5A1]) is an important regulator of gonad development. Previous studies on sf1 in fish have been limited to cloning and in vitro expression experiments. In this study, we used antisense RNA to down-regulate sf1 transcription and sf1 protein expression. Down-regulation of sf1 resulted in an increase in body weight and inhibition of gonadal development in both males and females with the consequent lower gonadosomatic index compared to fish in the control group. Hematoxylin-eosin staining of the gonads of fish with down-regulated sf1 revealed fewer seminiferous tubules and sperm in the testis of males. In addition, the oocytes were mainly stage II and many of them were atretic follicle. We conducted comparative transcriptome and proteome analyses between the sf1-down-regulated group and the control group. These analyses revealed multiple gene-protein pairs and pathways involved in regulating the observed changes, including 44 and 74 differently expressed genes and proteins in males and females, respectively. The results indicated that dysfunctional retinal metabolism and fatty acid metabolism could be causes of the observed weight gain and gonad abnormalities in sf1-down-regulated fish. These findings demonstrate the feasibility of using antisense RNA for gene editing in fish. This methodology allows the study gene function in species less amenable to gene editing as for example aquaculture species with long life cycles.

Global expression response of genes in sex-undifferentiated Nile tilapia gonads after exposure to trace letrozole

The aromatase inhibitor letrozole can be found in rivers, effluents, and even drinking water. Studies have demonstrated that letrozole affects various metabolic pathways and may cause reproductive toxicity, especially in fish exposed during development. However, studies on the effect of a low concentration of letrozole at the whole-gonad transcriptomic level in the early stage of fish sexual development have not been investigated. The aim of our study was to explore the potential effects of a low concentration of letrozole on the gonad transcriptome of Nile tilapia at an early stage of sexual development. In this study, 9 dpf (days postfertilization) Nile tilapia were exposed to trace letrozole for 12 days. Letrozole exposure from 9 dpf to 21 dpf persistently altered phenotypic sex development and induced the male-biased sex ratio. The transcriptome results showed that 1173 differentially expressed genes (DEGs) were present in the female control vs 1.5 μg/L letrozole-treated female comparison group and that 1576 DEGs were present in the 1.5 μg/L letrozole-treated female vs male control comparison group. Differentially expressed gene enrichment analysis revealed several crucial pathways, including the drug metabolism-cytochrome P450 pathway, the ErbB-PI3K/Akt/mTOR pathway, and the calcium signalling pathway. Further analysis of these identified DEGs indicated that some key genes correlated with metabolism and epigenetic regulation were significantly affected by letrozole, such as UDP-glucuronosyltransferase (Ugt), glutathione S-transferase omega-1 (Gsto1), lysine-specific demethylase 6bb (Kdm6bb, original name is Kdm6a), jumonji and AT-rich interaction domain containing 2 (Jarid2b, original name is Jarid2), growth arrest and DNA damage inducible gamma (Gadd45g), and chromobox protein 7 (Cbx7). The qRT-PCR validation results for twelve DEGs showed that the Pearson's correlation of the log₁₀^{fold change} values between the qPCR and RNA-Seq results was 0.90, indicating the accuracy and reliability of the RNA-Seq results. Our study is the first to report the effect of letrozole on the transcriptome of gonads from fish during early-stage sexual development. These findings will be useful for understanding the toxic effects and molecular mechanisms of letrozole exposure at the early stage of gonad development on the sexual development of aquatic organisms.

Mass spectrometry imaging in zebrafish larvae for assessing drug safety and metabolism

Drug safety assessment in the early phases of drug discovery is critical to facilitate the rapid development of novel therapeutics. Recently, teleost zebrafish (Danio rerio) has emerged as a promising vertebrate model for the assessment of drug safety. Zebrafish is a convenient model because of its small size, high fecundity, embryo transparency, and ex utero development. In this study, we developed a matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) method applied to zebrafish larvae to investigate safety and metabolism of sahaquine (Sq), an anticancer agent inhibiting histone deacetylase 6. This technique improves on prior studies using liquid chromatography-mass spectrometry (LC-MS) by adding analysis of the drug spatial distribution. Using this method, it was determined that Sq dissolved in fish water (1-2000 μM) did not reach the larval body and was mainly distributed throughout the yolk. High Sq concentration (800 μM) administered intravenously allowed the compound to reach the larval body but did not induce phenotypic abnormalities. Sq was metabolized into its glucuronidated form within 24 h and was excreted within 72 h. MALDI MSI was instrumental in showing that Sq-glucuronide was mainly formed in the gut and slightly in yolk syncytial layer, and provided valuable insights into xenobiotics elimination in zebrafish larvae. This study indicates that Sq has a good safety profile and merits further investigations in other disease models. In addition, the optimized MALDI MSI protocol provided here can be widely applied to study distribution and metabolic fate of other structurally related molecules.

The ecotoxicological contaminant tris(4-chlorophenyl)methanol (TCPMOH) impacts embryonic development in zebrafish (Danio rerio)

Tris(4-chlorophenyl)methanol (TCPMOH) is a water contaminant with unknown etiology, but is believed to be a byproduct of DDT manufacturing. It is highly persistent in the environment, and bioaccumulates in marine species. TCPMOH has also been measured in human breast milk, which poses a risk for developing infants. However, almost no toxicity data is currently available. In this study, we investigate the hazard posed by developmental TCPMOH exposures using the zebrafish model (Danio rerio). Zebrafish (Danio rerio) embryos were exposed to 0, 0.1, 0.5, 1, or 5 µM TCPMOH beginning at 24 h post fertilization (hpf). Embryonic mortality and incidence of morphological deformities increased in a concentration-dependent manner with TCPMOH exposure. RNA sequencing assessed changes in gene expression associated with acute (4 hour) exposures to 50 nM TCPMOH. Developmental exposure to TCPMOH decreased expression of ahr2, as well as metabolic enzymes cyp1a1, cyp1b1, cyp1c1, cyp1c2, and cyp2y3 (p<0.05). These findings were concordant with decreased Cyp1a1 induction measured by the ethoxyresorufin-O-deethylase (EROD) assay (p<0.05). Pathways associated with xenobiotic metabolism, lipid metabolism, and transcriptional and translational regulation were decreased. Pathways involved in DNA replication and repair, carbohydrate metabolism, and endocrine function were upregulated. Overall, this study demonstrates that TCPMOH is acutely toxic to zebrafish embryos at elevated concentrations.

Embryo/larval toxicity and transcriptional effects in zebrafish (Danio rerio) exposed to endocrine active riverbed sediments

Sediment toxicity plays a fundamental role in the health of inland fish communities; however, the assessment of the hazard potential of contaminated sediments is not a common objective in environmental diagnostics or remediation. This study examined the potential of transcriptional endpoints investigated in zebrafish (Danio rerio) exposed to riverbed sediments in ecotoxicity testing. Embryo-larval 10-day tests were conducted on sediment samples collected from five sites (one upstream and four downstream of the city of Milan) along a polluted tributary of the Po River, the Lambro River. Sediment chemistry showed a progressive downstream deterioration in river quality, so that the final sampling site showed up to eight times higher concentrations of, for example, triclosan, galaxolide, PAH, PCB, BPA, Ni, and Pb, compared with the uppermost site. The embryo/larval tests showed widespread toxicity although the middle river sections evidenced worse effects, as evidenced by delayed embryo development, hatching rate, larval survival, and growth. At the mRNA transcript level, the genes encoding biotransformation enzymes (cyp1a, gst, ugt) showed increasing upregulations after exposure to sediment from further downstream sites. The genes involved in antioxidant responses (sod, gpx) suggested that more critical conditions may be present at downstream sites, but even upstream of Milan there seemed to be some level of oxidative stress. Indirect evidences of potential apoptotic activity (bcl2/bax < 1) in turn suggested the possibility of genotoxic effects. The genes encoding for estrogen receptors (erα, erβ1, erβ2) showed exposure to (xeno)estrogens with a progressive increase after exposure to sediments from downstream sites, paralleled by a corresponding downregulation of the ar gene, likely related to antiandrogenic compounds. Multiple levels of thyroid disruption were also evident particularly in downstream zebrafish, as for thyroid growth (nkx2.1), hormone synthesis and transport (tg, ttr, d2), and signal transduction (trα, trβ). The inhibition of the igf2 gene reasonably reflected larval growth inhibitions. Although none of the sediment chemicals could singly explain fish responses, principal component analysis suggested a good correlation between gene transcripts and the overall trend of contamination. Thus, the combined impacts from known and unknown covarying chemicals were proposed as the most probable explanation of fish responses. In summary, transcriptional endpoints applied to zebrafish embryo/larval test can provide sensitive, comprehensive, and timeliness information which may greatly enable the assessment of the hazard potential of sediments to fish, complementing morphological endpoints and being potentially predictive of longer studies.

Development of the brain vasculature and the blood-brain barrier in zebrafish

To ensure tissue homeostasis the brain needs to be protected from blood-derived fluctuations or pathogens that could affect its function. Therefore, the brain capillaries develop tissue-specific properties to form a selective blood-brain barrier (BBB), allowing the passage of essential molecules to the brain and blocking the penetration of potentially harmful compounds or cells. Previous studies reported the presence of this barrier in zebrafish. The intrinsic features of the zebrafish embryos and larvae in combination with optical techniques, make them suitable for the study of barrier establishment and maturation. In this review, we discuss the most recent contributions to the development and formation of a functional zebrafish BBB. Moreover, we compare the molecular and cellular characteristic of the zebrafish and the mammalian BBB.

Glutathione S-Transferase Protein Expression in Different Life Stages of Zebrafish (Danio rerio)

Zebrafish is a widely used animal model in biomedical sciences and toxicology. Although evidence for the presence of phases I and II xenobiotic defense mechanisms in zebrafish exists on the transcriptional and enzyme activity level, little is known about the protein expression of xenobiotic metabolizing enzymes. Given the important role of glutathione S-transferases (GSTs) in phase II biotransformation, we analyzed cytosolic GST proteins in zebrafish early life stages and different organs of adult male and female fish, using a targeted proteomics approach. The established multiple reaction monitoring-based assays enable the measurement of the relative abundance of specific GST isoenzymes and GST classes in zebrafish through a combination of proteotypic peptides and peptides shared within the same class. GSTs of the classes alpha, mu, pi and rho are expressed in zebrafish embryo as early as 4 h postfertilization (hpf). The majority of GST enzymes are present at 72 hpf followed by a continuous increase in expression thereafter. In adult zebrafish, GST expression is organ dependent, with most of the GST classes showing the highest expression in the liver. The expression of a wide range of cytosolic GST isoenzymes and classes in zebrafish early life stages and adulthood supports the use of zebrafish as a model organism in chemical-related investigations.

From mRNA Expression of Drug Disposition Genes to In Vivo Assessment of CYP-Mediated Biotransformation during Zebrafish Embryonic and Larval Development

The zebrafish (Danio rerio) embryo is currently explored as an alternative for developmental toxicity testing. As maternal metabolism is lacking in this model, knowledge of the disposition of xenobiotics during zebrafish organogenesis is pivotal in order to correctly interpret the outcome of teratogenicity assays. Therefore, the aim of this study was to assess cytochrome P450 (CYP) activity in zebrafish embryos and larvae until 14 d post-fertilization (dpf) by using a non-specific CYP substrate, i.e., benzyloxy-methyl-resorufin (BOMR) and a CYP1-specific substrate, i.e., 7-ethoxyresorufin (ER). Moreover, the constitutive mRNA expression of CYP1A, CYP1B1, CYP1C1, CYP1C2, CYP2K6, CYP3A65, CYP3C1, phase II enzymes uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1) and sulfotransferase 1st1 (SULT1ST1), and an ATP-binding cassette (ABC) drug transporter, i.e., abcb4, was assessed during zebrafish development until 32 dpf by means of quantitative PCR (qPCR). The present study showed that trancripts and/or the activity of these proteins involved in disposition of xenobiotics are generally low to undetectable before 72 h post-fertilization (hpf), which has to be taken into account in teratogenicity testing. Full capacity appears to be reached by the end of organogenesis (i.e., 120 hpf), although CYP1-except CYP1A-and SULT1ST1 were shown to be already mature in early embryonic development.

Zebrafish (Danio rerio): A valuable tool for predicting the metabolism of xenobiotics in humans?

Zebrafish has become a popular model organism in several lines of biological research sharing physiological, morphological and histological similarities with mammals. In fact, many human cytochrome P450 (CYP) enzymes have direct orthologs in zebrafish, suggesting that zebrafish xenobiotic metabolic profiles may be similar to those in mammals. The focus of the review is to analyse the studies that have evaluated the metabolite production in zebrafish over the years, either of the drugs themselves or xenobiotics in general (environmental pollutants, natural products, etc.), bringing a vision of how these works were performed and comparing, where possible, with human metabolism. Early studies that observed metabolic production by zebrafish focused on environmental toxicology, and in recent years the main focus has been on toxicity screening of pharmaceuticals and drug candidates. Nevertheless, there is still a lack of standardization of the model and the knowledge of the extent of similarity with human metabolism. Zebrafish screenings are performed at different life stages, typically being carried out in adult fish through in vivo assays, followed by early larval stages and embryos. Studies comparing metabolism at the different zebrafish life stages are also common. As with any non-human model, the zebrafish presents similarities and differences in relation to the profile of generated metabolites compared to that observed in humans. Although more studies are still needed to assess the degree to which zebrafish metabolism can be compared to human metabolism, the facts presented indicate that the zebrafish is an excellent potential model for assessing xenobiotic metabolism.

New toxicogenetic insights and ranking of the selected pharmaceuticals belong to the three different classes: A toxicity estimation to confirmation approach

Tetracycline hydrochloride (TH), indomethacin (IM), and bezafibrate (BF) belong to the three different important classes of pharmaceuticals, which are well known for their toxicity and environmental concerns. However, studies are still elusive to highlight the mechanistic toxicity of these pharmaceuticals and rank them using both, the toxicity prediction and confirmation approaches. Therefore, we employed the next generation toxicity testing in 21st century (TOX21) tools and estimated the in vitro/vivo toxic endpoints of mentioned pharmaceuticals, and then confirmed them using in vitro/vivo assays. We found significant resemblance in the results obtained via both approaches, especially in terms of in vivo LC50 s and developmental toxicity that ranked IM as most toxic among the studied pharmaceuticals. However, TH appeared most toxic with the lowest estimated AC50s, the highest experimental IC50s, and DNA damages in vitro. Contrarily, IM was found as congener with priority concern to activate the Pi3k-Akt-mTOR pathway in vitro at concentrations substantially lower than that of TH and BF. Further, IM exposure at lower doses (2.79-13.97 μM) depressed the pharmaceuticals detoxification phase I (CYP450 s), phase II (UGTs, SULTs), and phase III (TPs) pathways in zebrafish, whereas, at relatively higher doses, TH (2.08-33.27 μM) and BF (55.28-884.41 μM) partially activated these pathways, which ultimately caused the developmental toxicity in the following order: IM > TH > BF. In addition, we also ranked these pharmaceuticals in terms of their particular toxicity to myogenesis, hematopoiesis, and hepatogenesis in zebrafish embryos. Our results revealed that IM significantly affected myogenesis, hematopoiesis, and hepatogenesis, while TH and BF induced prominent effects on hematopoiesis via significant downregulation of associated genetic markers, such as drl, mpx, and gata2a. Overall, our findings confirmed that IM has higher toxicity than that of TH and BF, therefore, the consumption of these pharmaceuticals should be regulated in the same manner to ensure human and environmental safety.

The combined use of chemical and biochemical markers in Rutilus rutilus to assess the effect of dredging in the lower course of the Ebro River

The lower course of the Ebro River is polluted with high concentrations of organochlorine compounds dumped by a chloro-alkali plant during the last century. A remediation plan, including building of a protective wall, removal and disposal of polluted sediments started in 2012. With the aim of assessing the effects of dredging of contaminated sediments and potential alterations of water quality, areas located upstream (RR) and downstream (BE, A) the chemical plant (FL) were monitored prior (October 2012) and during dredging (June 2013) using roach (Rutilus rutilus) as sentinel organisms. Concentrations of organochlorine compounds (OCs) in fish muscle and biliary levels of polycyclic aromatic hydrocarbons (PAHs), galaxolide (HHCB) and alkyphenols (APEs) were determined together with selected enzymatic activities (7-ethoxyresorufin-O-deethylase (EROD), 7-benzyloxy-4-trifluoromethyl-coumarin O-debenzyloxylase (BFCOD) and UDP-glucuronyltransferase (UGT)) in the liver. The obtained results proved the effectiveness of the wall retaining suspended particles and avoiding further contamination of downstream sites as fish sampled at downstream sites showed up to 9-fold higher concentrations of OCs in muscle during wall construction than during dredging. EROD and UGT activities were induced in fish from downstream sites; however, no clear response to the observed pollution gradient was detected.

Response of PXR signaling pathway to simvastatin exposure in mosquitofish (Gambusia affinis) and its histological changes

As a widely used lipid lowering agent, simvastatin recently has been frequently detected in aquatic environment and the potential adverse effects from simvastatin exposure to non-target organisms such as fish is worthy of more attention. The aim of this study was to reveal the responses of detoxification system in fish to simvastatin exposure. In this investigation a ubiquitous small freshwater fish, mosquito fish (Gambusia affinis), was employed as test organism, and the transcriptional expression of nucleus transcriptional factor pregnane X receptor (PXR) and its downstream genes, including P-glycoprotein (P-gp), cytochrome 3A (CYP3A), multidrug resistance protein 2 (MRP2), UDP-glucuronosyl transferase (UGT) in mosquito fish were investigated by qRT-PCR methods under the exposure of concentrations of simvastatin (0.5 μg L-1, 5 μg L-1, 50 μgL-1, 500 μg L-1) for 24 h, 72 h and 168 h. The related enzyme activity (Erythromycin-N-Demethylase, ERND), the protein expression of PXR and the histological changes of liver tissues in fish were also determined via west blotting and transmission electron microscope approaches in the same conditions. Results showed that the mRNA expression of PXR, CYP3A and P-gp showed significantly changes under simvastatin exposure, exhibiting an obvious time/dose-effect relationship with the prolong of exposure time. ERND activity also showed time-effect at 24 h, and western blotting showed PXR protein displaying a dose-effect relationship to some extent. Hepatocyte cellular of mosquito fish exposed to simvastatin (5 μg L-1, 168 h) exhibited obvious histological changes in form of swelling, incomplete fragmentary structure etc. Overall, simvastatin altered the expression of PXR signaling pathway and subsequently bring about changes in high-levels of mosquito fish.

Quantification of Estradiol Uptake in Zebrafish Embryos and Larvae

Zebrafish are a powerful model system to assess the molecular and cellular effects of exposure to toxic chemicals during embryonic development. To study the effects of environmental endocrine disruptors, embryos and larvae are commonly exposed to supraphysiologic concentrations of these compounds in the water, but their bioavailability in zebrafish is largely unknown. One hypothesis is that supraphysiologic concentrations of estrogens in the water are required to achieve physiologic levels in vivo; however, this has not been directly tested. To test this hypothesis, we developed an assay using radiolabeled estradiol ([3H]E2) to measure uptake from water at multiple concentrations and exposure durations in developing zebrafish from 0 to 5 days postfertilization (dpf). We found that [3H]E2 uptake increased with increasing concentration, duration, and developmental stage. Percent uptake from the total volume of treatment solution increased with increasing exposure duration and developmental stage, but remained constant with increasing concentration. We also found that the chorion, an acellular envelope surrounding embryos through 3 dpf, did not substantially affect [3H]E2 uptake. Finally, we found that at 1 dpf, E2 was preferentially taken up by the yolk at multiple exposure durations, while at 2 dpf E2 was preferentially taken up into the embryonic body. Our results support the hypothesis that exposing zebrafish embryos and larvae to supraphysiologic concentrations of estrogens is required to achieve physiologically relevant doses in vivo. The isotopic assay reported here will provide a foundation for determining the uptake of other compounds for teratogenicity, toxicology and drug discovery studies.

Assaying uptake of endocrine disruptor compounds in zebrafish embryos and larvae

To study the effects of environmental endocrine disruptor compounds (EDCs) on aquatic animals, embryos and larvae are typically incubated in water containing defined concentrations of EDCs. However, the amount of EDC uptake into the animal is often difficult to determine. Using radiolabeled estradiol ([³H]E2), we previously developed a rapid, straightforward assay to measure estradiol uptake from water into zebrafish embryos and larvae. Here, we extend this approach to measure the uptake of two additional EDCs, bisphenol A (BPA) and ethinyl estradiol (EE2). As with E2, the uptake of each compound by individual larvae was low (<6%), and increased with increasing concentration, duration, and developmental stage. We found that E2 and EE2 had similar uptake under equivalent exposure conditions, while BPA had comparatively lower uptake. One application of this assay is to test factors that influence EDC uptake or efflux. It has been suggested that persistent organic pollutants (POPs) inhibit ABC transporters that may normally efflux EDCs and their metabolites, inducing toxicity in aquatic organisms. We measured [³H]E2 levels in zebrafish in the presence or absence of the POP PDBE-100, and cyclosporine A, a known inhibitor of ABC transporters. Neither chemical significantly affected [³H]E2 levels in zebrafish, suggesting that zebrafish maintain estradiol efflux in the presence of PDBE-100, independently of cyclosporine A-responsive transporters. These uptake results will be a valuable reference for EDC exposure studies in developing zebrafish, and provide a rapid assay to screen for chemicals that influence estrogen-like EDC levels in vivo.

Zebrafish: A promising in vivo model for assessing the delivery of natural products, fluorescence dyes and drugs across the blood-brain barrier

The blood brain barrier (BBB) is the network of capillaries that controls the passage of substances from the blood into the brain and other parts of the central nervous system (CNS). As this barrier is the major obstacle for drug delivery into CNS, a credible BBB model is very necessary to assess the BBB permeability of novel neuroactive compounds including thousands of bioactive compounds which have been extracted from medicinal plants and have the potential for the treatment of CNS diseases. Increasing reports indicated that zebrafish has emerged as a timely, reproducible model for BBB permeability assessment. In this review, the development and functions of the BBB in zebrafish, such as its anatomical morphology, tight junctions, drug transporters and enzyme expression, are compared with those in mammals. The studies outlined in this review describe the utilization of the zebrafish as a BBB model to investigate the permeability and distribution of fluorescent dyes and drugs. Particularly, this review focuses on the use of zebrafish to evaluate the delivery of natural products and nanosized drug delivery systems across the BBB. Due to the highly conserved nature of both the structure and function of the BBB between zebrafish and mammals, zebrafish has the potential to be developed as a model for assessing and predicting the permeability of BBB to novel compounds.

Effects of diclofenac on the expression of Nrf2 and its downstream target genes in mosquito fish (Gambusia affinis)

Diclofenac (DCF) is one of widely used non-steroidal anti-inflammatory drugs. Recently, this drug has been universally detected in aquatic environment. However, its potential adverse effects and oxidative stress toxic mechanisms on fish remain unclear. In the present study, we first cloned the crucial partial sequences of some key oxidative stress related genes, which include NF-E2-related factor 2 (Nrf2), NAD(P)H: quinoneoxidoreductase (NQO1), glutamate-cysteine ligase catalytic subunit (GCLC), Cu-Zn superoxide dismutase (SOD2), catalase (CAT), alpha-glutathione S-transferase (GSTA), and UDP-glucuronosyltransferases (UGT) in mosquito fish (Gambusia affinis). We also deduced amino acids of Nrf2 and then constructed the phylogenetic trees of Nrf2, NQO1 and GCLC, respectively. Results showed that a high identity percentage was founded between G. affinis and other bony fish species, such as Xiphophorus maculates and Poecilia reticulate. The transcriptional expression of these genes and partly related enzymes activities were then investigated under the included environmental relevant concentration DCF exposure (0μmolL^-1, 1.572×10^-3μmolL^-1, 1.572×10^-2μmolL^-1, 0.1572μmolL^-1 and 1.572μmolL^-1) for 24h and 168h. The expression of Nrf2 was inhibited at 24h but induced at 168h, exhibiting a significant time and/or dose-effect relationship under DCF exposure. Similar observation was found in its downstream target genes. However, Nrf2-mediated antioxidant enzymes activities displayed differently under the same concentration of DCF exposure for the same time. Under DCF exposure for 168h, the genes exhibited dramatic induction trend, but there were no significant changes in enzyme activities and MDA content. Overall, mRNA responses were more sensitive than enzyme changes in mosquito fish under DCF exposure.

Developmental toxicity in flounder embryos exposed to crude oils derived from different geographical regions

Crude oils from distinct geographical regions have distinct chemical compositions, and, as a result, their toxicity may be different. However, developmental toxicity of crude oils derived from different geographical regions has not been extensively characterized. In this study, flounder embryos were separately exposed to effluents contaminated by three crude oils including: Basrah Light (BLO), Pyrenees (PCO), and Sakhalin Vityaz (SVO), in addition to a processed fuel oil (MFO-380), to measure developmental toxicity and for gene expressions. Each oil possessed a distinct chemical composition. Edema defect was highest in embryos exposed to PCO and MFO-380 that both have a greater fraction of three-ring PAHs (33% and 22%, respectively) compared to BLO and SVO. Observed caudal fin defects were higher in embryos exposed to SVO and MFO-380, which are both dominated by naphthalenes (81% and 52%, respectively). CYP1A gene expressions were also highest in embryos exposed to SVO and MFO-380. Higher incidence of cardiotoxicity and lower nkx 2.5 expression were detected in embryos exposed to PCO. Unique gene expression profiles were observed in embryos exposed to crude oils with distinct compositions. This study demonstrates that crude oils of different geographical origins with different compositional characteristics induce developmental toxicity to different degrees.

Toxicokinetics of Polar Chemicals in Zebrafish Embryo (Danio rerio): Influence of Physicochemical Properties and of Biological Processes

The time-resolved uptake of 17 nonionic and ionic polar compounds (logD ≤ 2) with a diversity of functional groups into zebrafish embryos (ZFE) was studied over 96 h of exposure. Among them were pharmaceuticals, pesticides and plant active ingredients. Uptake rates for the diffusion controlled passive uptake through the ZFE membrane ranged from 0.02 to 24 h(-1) for the nonionic compounds and were slower for ionic compounds (<0.008-0.08 h(-1)). The study compounds did not enrich much in the ZFE (median bioconcentration factor of 1, max. 7). Biotransformation significantly influenced the internal concentration of some of the test compounds over time (benzocaine, phenacetin, metribuzin, phenytoin, thiacloprid, valproic acid). For benzocaine, valproic acid and phenacetin several transformation products (TPs) were observed by LC-MS already at early life-stages (before 28 hpf); for benzocaine the TPs comprised >90% of the initial amount taken up into the ZFE. For six compounds internal concentrations remained very low (rel. int. conc. < 0.2). Besides biotransformation (sulfamethoxazole), poor membrane permeability (cimetidine, colchicine) and also affinity to efflux transporters (atropine and chloramphenicol) are the likely reasons for these low internal concentrations. This study outlines that the uptake of polar compounds into ZFE is influenced by their physicochemical properties. However, biological processes, biotransformation and, likely, efflux can strongly affect the internal concentrations already in early developmental stages of the ZFE. This should be considered in future toxicokinetic modeling. The evaluation of the toxicity of chemicals by ZFE requires toxicokinetic studies of the test compounds and their TPs to increase comparability to effects in fish.

Role of SULT-mediated sulfation in the biotransformation of 2-butoxyethanol and sorbitan monolaurate: A study using zebrafish SULTs

2-Butoxyethanol and sorbitan monolaurate are major components of oil dispersants that are applied in large quantities to control oil spill in the aquatic environment. An important question is whether aquatic animals are equipped with mechanisms for the detoxification of these oil dispersant compounds. The current study aimed to examine whether zebrafish cytosolic sulfotransferases (SULTs) are capable of sulfating 2-butoxyethanol and sorbitan monolaurate. A systematic analysis of 18 zebrafish SULTs revealed that SULT3 ST1 showed the strongest sulfating activity toward 2-butoxyethanol, while SULT1 ST3 displayed the strongest sulfating activity toward sorbitan monolaurate. The pH-dependence of these two SULTs in mediating the sulfation of 2-butoxyethanol or sorbitan monolaurate was examined. Taken together, these results implied that SULT-mediated sulfation may function in the detoxification of these two oil dispersant compounds.

Metabolism of clofibric acid in zebrafish embryos (Danio rerio) as determined by liquid chromatography-high resolution-mass spectrometry

The zebrafish embryo (ZFE) is increasingly used in ecotoxicology research but detailed knowledge of its metabolic potential is still limited. This study focuses on the xenobiotic metabolism of ZFE at different life-stages using the pharmaceutical compound clofibric acid as study compound. Liquid chromatography with quadrupole-time-of-flight mass spectrometry (LC-QToF-MS) is used to detect and to identify the transformation products (TPs). In screening experiments, a total of 18 TPs was detected and structure proposals were elaborated for 17 TPs, formed by phase I and phase II metabolism. Biotransformation of clofibric acid by the ZFE involves conjugation with sulfate or glucuronic acid, and, reported here for the first time, with carnitine, taurine, and aminomethanesulfonic acid. Further yet unknown cyclization products were identified using non-target screening that may represent a new detoxification pathway. Sulfate containing TPs occurred already after 3h of exposure (7hpf), and from 48h of exposure (52hpf) onwards, all TPs were detected. The detection of these TPs indicates the activity of phase I and phase II enzymes already at early life-stages. Additionally, the excretion of one TP into the exposure medium was observed. The results of this study outline the high metabolic potential of the ZFE with respect to the transformation of xenobiotics. Similarities but also differences to other test systems were observed. Biotransformation of test chemicals in toxicity testing with ZFE may therefore need further consideration.

ABC transporters and xenobiotic defense systems in early life stages of rainbow trout (Oncorhynchus mykiss)

Embryos of oviparous fish, in contrast to (ovo) viviparous species, develop in the aquatic environment, and therefore need solute transport systems at their body surfaces for maintaining internal homeostasis and defending against potentially harmful substances. We hypothesized that solute transporters undergo changes in tissue distribution from the embryo to the larval stage. We therefore studied the mRNA profiles of eight ABC transporters (abcb1a, abcb1b, abcc1, abcc2, abcc3, abcc4, abcc5, abcg2) and three solute carriers (oatp1d, putative oatp2 putative, mate1) in different body regions (head, yolk sac epithelium, abdominal viscera, skin/muscles) of developing rainbow trout. Additionally, we investigated mRNA levels of phase I (cyp1a, cyp3a) and phase II (gstp, putative ugt1, putative ugt2) biotransformation enzymes. The study covered the developmental period from the eleuthero-embryo stage to the first-feeding larval stage (1-20days post-hatch, dph). At 1dph, transcripts of abcc2, abcc4, abcg2, cyp3a, gstp, putative mate1, and putative oatp2 occurred primarily in the yolk sac epithelium, whereas at later stages expression of these genes was predominantly observed in the abdominal viscera. The functional activity of ABC transporters in fish early life stages was assessed by rhodamine B accumulation assays. Finally, we investigated the potential impact of xenobiotics (clotrimazole, clofibric acid) on the ABC and biotransformation systems of trout early life stages. While clofibric acid had no effect, clotrimazole lead to an increased rhodamine B accumulation. The results provide evidence that the transition from the eleuthero-embryo to the larval stage is accompanied by a major alteration in tissue expression of ABC transporters.

Transcriptional profiles of glutathione-S-Transferase isoforms, Cyp, and AOE genes in atrazine-exposed zebrafish embryos

Glutathione-S-transferase (GST) superfamily consists of multiple members involved in xenobiotic metabolism. Expressional pattern of the GST isoforms in adult fish has been used as a biomarker of exposure to environmental chemicals. However, GST transcriptional responses vary across organs, thus requiring a cross-tissue examination of multiple mRNAs for GST profiling in an animal after chemical exposure. Zebrafish embryos express all GST isoforms as adult fish and could therefore represent an alternative model for identification of biomarkers of exposure. To evaluate such a possibility, we studied a set of cytosolic and microsomal GST isoform-specific expression profiles in the zebrafish embryos after exposure to atrazine, a widely used herbicide. Expression of the GST isoforms was compared with that of CYP genes involved in the phase I of xenobiotic metabolism and antioxidant enzyme (AOE) genes. Using quantitative real-time PCR, we showed dynamic changes in the expressional pattern of twenty GST isoforms, cyp1a, cyp3a65, ahr2, and four AOEs in early development of zebrafish. Acute (48 and 72 h) exposure of 24 h-old embryos to atrazine, from environmentally relevant (0.005 mg/L) to high (40 mg/L) concentrations, caused a variety of transient, albeit minor changes (<2.5-fold) in the GST isoforms, ahr2 and AOE genes response. However, expression of cyp1a and cyp3a65 mRNA was markedly and consistently induced by high doses of atrazine (5 and 40 mg/L). In summary, an analysis of the response of multiple systems in the zebrafish embryos provided a comprehensive understanding of atrazine toxicity and its potential impact on biological processes.

Effects of pharmaceuticals present in aquatic environment on Phase I metabolism in fish

The fate of pharmaceuticals in aquatic environments is an issue of concern. Current evidence indicates that the risks to fish greatly depend on the nature and concentrations of the pharmaceuticals and might be species-specific. Assessment of risks associated with the presence of pharmaceuticals in water is hindered by an incomplete understanding of the metabolism of these pharmaceuticals in aquatic species. In mammals and fish, pharmaceuticals are primarily metabolized by cytochrome P450 enzymes (CYP450). Thus, CYP450 activity is a crucial factor determining the detoxification abilities of organisms. Massive numbers of toxicological studies have investigated the interactions of human pharmaceuticals with detoxification systems in various fish species. In this paper, we review the effects of pharmaceuticals found in aquatic environments on fish hepatic CYP450. Moreover, we discuss the roles of nuclear receptors in cellular regulation and the effects of various groups of chemicals on fish, presented in the recent literature.

Structural analysis of metabolites of asiatic acid and its analogue madecassic acid in zebrafish using LC/IT-MSn

Although zebrafish has become a significant animal model for drug discovery and screening, drug metabolism in zebrafish remains largely unknown. Asiatic acid (AA) and madecassic acid (MA), two natural pentacyclic triterpenoids mainly obtained from Centella asiatica (L.) Urban, have been found to possess many pharmacological effects. This study is to probe the metabolic capability of zebrafish via investigation of the drug metabolism of AA and MA in zebrafish, using a sensitive LC/IT-MSn method. In addition, the main fragmentation pathways of AA and MA were reported for the first time. Nineteen metabolites of AA and MA were firstly identified after zebrafish was exposed to the drug, which all were the phase I metabolites and mainly formed from hydroxylation, dehydrogenation, hydroxylation and dehydrogenation, dihydroxylation and dehydrogenation, and dehydroxylation reaction. The results indicated that zebrafish possessed strong metabolic capacity, and the metabolites of AA and MA were formed via similar metabolic pathways and well matched with the known metabolic rules in vivo and in vitro, which supports the widely use of this system in drug metabolism research. This investigation would also contribute to the novel information on the structural elucidation, in vivo metabolites and metabolic mechanism of pentacyclic triterpenoids.

The UDP-glucuronosyltransferases of the blood-brain barrier: their role in drug metabolism and detoxication

UDP-glucuronosyltransferases (UGTs) form a multigenic family of membrane-bound enzymes expressed in various tissues, including brain. They catalyze the formation of β-D-glucuronides from structurally unrelated substances (drugs, other xenobiotics, as well as endogenous compounds) by the linkage of glucuronic acid from the high energy donor, UDP-α-D-glucuronic acid. In brain, UGTs actively participate to the overall protection of the tissue against the intrusion of potentially harmful lipophilic substances that are metabolized as hydrophilic glucuronides. These metabolites are generally inactive, except for important pharmacologically glucuronides such as morphine-6-glucuronide. UGTs are mainly expressed in endothelial cells and astrocytes of the blood brain barrier (BBB). They are also associated to brain interfaces devoid of BBB, such as circumventricular organ, pineal gland, pituitary gland and neuro-olfactory tissues. Beside their key-role as a detoxication barrier, UGTs play a role in the steady-state of endogenous compounds, like steroids or dopamine (DA) that participate to the function of the brain. UGT isoforms of family 1A, 2A, 2B and 3A are expressed in brain tissues to various levels and are known to present distinct but overlapping substrate specificity. The importance of these enzyme species with regard to the formation of toxic, pharmacologically or physiologically relevant glucuronides in the brain will be discussed.

Benzotriazole UV-stabilizers and benzotriazole: Antiandrogenic activity in vitro and activation of aryl hydrocarbon receptor pathway in zebrafish eleuthero-embryos

Benzotriazole UV-stabilizers (BUVs) are applied in materials for protection against UV-irradiation. They are widely used, bioaccumulate and share structural similarities to benzotriazole. Benzotriazole (1HBT) finds application as corrosion inhibitor in dishwashing detergents, antifreeze (vehicles) and aircraft de-icing agent. BUVs and 1HBT are persistent and ubiquitous in the aquatic environment, but there is little understanding of the ecotoxicological implications. Here, we comparatively analyze the hormonal activity in vitro and effects in zebrafish eleuthero-embryos in vivo. 2-(2-Hydroxy-5-methylphenyl)benzotriazole (UV-P), 2-(3-t-butyl-2-hydroxy-5-methylphenyl)-5-chlorobenzotriazole (UV-326), UV-327, UV-328, UV-329 and UV-320 showed no estrogenicity (YES assay) and androgenicity (YAS assay). However, UV-P and 1HBT showed significant antiandrogenic activity. We assessed the transcription profiles of up to 26 genes associated with different toxicological pathways in zebrafish eleuthero-embryos to elucidate potential modes of action of UV-P, UV-326 and 1HBT. Embryos were experimentally exposed for 144hpf to three measured concentrations of 15.8, 70.8, and 690μg/L UV-P, 7.5, 31.7, and 84.3μg/L UV-326 and 7.9, 97.3 and 1197.3μg/L 1HBT. Among the 26 transcripts, the induction of the aryl hydrocarbon receptor (AHR) pathway by UV-P and UV-326 was the most significant finding. UV-P led to dose-related induction of AHR1, ARNT2 and cyp1a1, as well as of phase II enzymes glutathione-S-transferase (gstp1) and ugt1a. UV-326 led to a significant induction of cyp1a1 and AHR2, but down-regulation of gstp1 at 84μg/L. Only little transcriptional alterations occurred in genes related to apoptosis, oxidative stress, hormone receptors, and steroidogenesis including aromatase. 1HBT led to only a few expressional changes at 1197μg/L. Our data lead to the conclusion that UV-P and UV-326 activate the AHR-pathway, whereas 1HBT shows only little transcriptional alterations. It should be noted, however, that effects have been observed at concentration much higher than those occurring in the environment. Forthcoming studies should show whether the observed antiandrogenic activities and transcriptional changes translate into physiological effects .