Antagonism of TCDD-induced ethoxyresorufin-O-deethylation activity by polybrominated diphenyl ethers (PBDEs) in primary cynomolgus monkey (Macaca fascicularis) hepatocytes

Toxicological evaluation of TBBPA by common carp (Cyprinus carpio) about the in vivo/vitro disturbance of the AHR pathway

Tetrabromobisphenol A (TBBPA) is a widely used plastic additive with high bioaccumulation potential and toxicity on both humans and wildlife. Currently, research on its ecotoxicity and the underlying mechanism is limited. Using common carp (Cyprinus carpio), we evaluated the toxicity of TBBPA, especially focusing on its alteration of a key metabolism-related pathway aryl hydrocarbon receptor (AHR), using in vivo/vitro assays and in silico simulation. The 96 h LC50 of TBBPA of common carp was 4.2 mg/L and belonged to the acute toxic level II. The bioaccumulation potential of TBBPA follows the role of liver > gill > brain and varies between 3- and 14-day exposure. On the AHR pathway respect, as expected, the metabolism-related cyp1a1 and cyp1b1 were upregulated in the liver and brain. Ahr2, the receptor, was also upregulated in the brain under TBBPA exposure. The alteration of gene expression was tissue-specific while the difference between 3- or 14-day exposure was minor. AHR inhibition assay indicated the 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD)-induced AHR transactivation can be inhibited by TBBPA suggesting it is not a potent agonist but a competitive antagonist. In silico analysis indicated TBBPA can be successfully docked into the binding cavity with similar poses but still have AHR-form-specific interactions. Molecular dynamics simulation proved TBBPA can be more flexible than the coplanar ligand TCDD, especially in ccaAHR1b with greater root-mean-square deviation (RMSD), of which TCDD-induced transactivation seemed not to be blocked by TBBPA. This research increased the understanding of TBBPA toxicity and alteration of the AHR pathway, and pointed out the need to perform additional toxicology evaluation of emerging contaminants, especially on non-model species.

The aryl hydrocarbon receptor: A predominant mediator for the toxicity of emerging dioxin-like compounds

The aryl hydrocarbon receptor (AHR) is a member of the basic helix-loop-helix/Per-ARNT-Sim (bHLH-PAS) family of transcription factors and has broad biological functions. Early after the identification of the AHR, most studies focused on its roles in regulating the expression of drug-metabolizing enzymes and mediating the toxicity of dioxins and dioxin-like compounds (DLCs). Currently, more diverse functions of AHR have been identified, indicating that AHR is not just a dioxin receptor. Dioxins and DLCs occur ubiquitously and have diverse health/ecological risks. Additional research is required to identify both shared and compound-specific mechanisms, especially for emerging DLCs such as polyhalogenated carbazoles (PHCZs), polychlorinated diphenyl sulfides (PCDPSs), and others, of which only a few investigations have been performed at present. Many of the toxic effects of emerging DLCs were observed to be predominantly mediated by the AHR because of their structural similarity as dioxins, and the in vitro TCDD-relative potencies of certain emerging DLC congeners are comparable to or even greater than the WHO-TEFs of OctaCDD, OctaCDF, and most coplanar PCBs. Due to the close relationship between AHR biology and environmental science, this review begins by providing novel insights into AHR signaling (canonical and non-canonical), AHR's biochemical properties (AHR structure, AHR-ligand interaction, AHR-DNA binding), and the variations during AHR transactivation. Then, AHR ligand classification and the corresponding mechanisms are discussed, especially the shared and compound-specific, AHR-mediated effects and mechanisms of emerging DLCs. Accordingly, a series of in vivo and in vitro toxicity evaluation methods based on the AHR signaling pathway are reviewed. In light of current advances, future research on traditional and emerging DLCs will enhance our understanding of their mechanisms, toxicity, potency, and ecological impacts.

Integrated in silico and in vivo approaches to investigate effects of BDE-99 mediated by the nuclear receptors on developing zebrafish

One of the most abundant polybrominated diphenyl ethers (PBDEs) is 2,2',4,4',5-pentabromodiphenyl ether (BDE-99), which persists and potentially bioaccumulates in aquatic wildlife. Previous studies in mammals have shown that BDE-99 affects development and disrupts certain endocrine functions through signaling pathways mediated by nuclear receptors. However, fewer studies have investigated the potential of BDE-99 to interact with nuclear receptors in aquatic vertebrates such as fish. In the present study, interactions between BDE-99 and nuclear receptors were investigated by in silico and in vivo approaches. This PBDE was able to dock into the ligand-binding domain of zebrafish aryl hydrocarbon receptor 2 (AhR2) and pregnane X receptor (PXR). It had a significant effect on the transcriptional profiles of genes associated with AhR or PXR. Based on the developed cytoscape of all zebrafish genes, it was also inferred that AhR and PXR could interact via cross-talk. In addition, both the in silico and in vivo approaches found that BDE-99 affected peroxisome proliferator-activated receptor alpha (PPARα), glucocorticoid receptor, and thyroid receptor. Collectively, our results demonstrate for the first time detailed in silico evidence that BDE-99 can bind to and interact with zebrafish AhR and PXR. These findings can be used to elaborate the molecular mechanism of BDE-99 and guide more objective environmental risk assessments. Environ Toxicol Chem 2018;37:780-787. © 2017 SETAC.

Identification of the metabolites of polybrominated diphenyl ether 99 and its related cytochrome P450s

OBJECTIVE

To investigate the metabolites of polybrominated diphenyl ether 99 (BDE-99) and its related cytochrome P450s in an in vitro system.

METHODS

Rat primary hepatocytes were isolated and treated with BDE-99 for 24-72 h. Metabolites were then extracted from the hepatocytes and media, and detected by GC/MS. Several mRNAs of metabolic enzymes were also extracted from the same cells and the gene expression levels were determined using quantitative real-time PCR. In addition, selected recombinant cytochrome P450s (CYPs) were expressed in a bacurovirus/sf9 system, and these were further used to explore the metabolism of BDE-99 in vitro. The parent depletion approach was used for screening the ability of CYPs to eliminate BDE-99.

RESULTS

A reductively debrominated metabolite, BDE-47, and three oxidative metabolites, 2, 4, 5-tribromophenol, 5-OH-BDE-47, and 5'-OH-BDE-99, were identified from the BDE-99-treated rat hepatocytes, whereas no MeO metabolite was detected in the system. RT-PCR analysis showed that CYP 3A23/3A1, 1A2, and 2B1/2 were induced by BDE-99. Furthermore, using the heterological expressed CYP proteins in in vitro BDE-99 metabolism experiments we found that CYP1A2 and CYP3A4 showed the highest metabolic efficiency for BDE-99, with the metabolic clearance rates of CYP1A2 and CYP3A4 being 30.3% and 27.7%, respectively. CYP1A1 and CYP2A6 displayed relatively low clearance rates, while CYP2E1 seemed not to be associated with the BDE-99 metabolism.

CONCLUSIONS

In our in vitro rat primary hepatocyte metabolism system, four metabolites of BDE-99 were identified, and CYP3A4 and CYP1A2 were demonstrated to be involved in the BDE-99 metabolism.

Molecular toxicology of polybrominated diphenyl ethers: nuclear hormone receptor mediated pathways

Polybrominated diphenyl ethers (PBDEs) are used in large quantities as flame retardant additives in commercial products. Bio-monitoring data show that PBDE concentrations have increased rapidly in the bodies of wildlife and human over the last few decades. Based on the studies on experimental animals, the toxicological endpoints of exposure to PBDEs are likely to be thyroid homeostasis disruption, neuro-developmental deficits, reproductive ineffectiveness and even cancer. Unfortunately, the available molecular toxicological evidence for these endpoints is still very limited. This review focuses on the recent studies on the molecular mechanisms of PBDE toxicities carried out through the hormone receptor pathways, including thyroid hormone receptor, estrogen receptor, androgen receptor, progesterone receptor and aryl hydrocarbon receptor pathways. The general approach in the mechanistic investigation is to examine the in vitro direct binding of a PBDE with a receptor, the in vitro recruitment of a co-activator or co-repressor by the ligand-bound receptor, and the participation of the ligand in the receptor-mediated transcription pathways in cells. It is hoped that further studies in this area would provide more insights into the potential risks of PBDEs to human health.

Dioxin-like potency of HO- and MeO- analogues of PBDEs' the potential risk through consumption of fish from eastern China

Polybrominated diphenyl ethers (PBDEs) and their analogues, such as hydroxylated PBDE (HO-PBDEs) and methoxylated PBDE (MeO-PBDEs) are of interest due to their wide distribution, bioaccumulation and potential toxicity to humans and wildlife. While information on the toxicity/biological potencies of PBDEs was available, information on analogues of PBDEs was limited. Dioxin-like toxicity of 34 PBDEs analogues was evaluated by use of the H4IIE-luc, rat hepatoma transactivation bioassay in 384-well plate format at concentrations ranging from 0 to 10 000 ng/mL. Among the 34 target analogues of PBDEs studied here, 19 activated the aryl hydrocarbon receptor (AhR) and induced significant dioxin-like responses in H4IIE-luc cells. Efficacies of the analogues of PBDEs ranged from 5.0% to 101.8% of the maximum response caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD-max) and their respective 2,3,7,8-TCDD potency factors (ReP(H4IIE-luc)) ranged from 7.35 × 10(-12) to 4.00 × 10(-4), some of which were equal to or more potent than some mono-ortho-substituted PCBs (TEF-(WHO) = 3 × 10(-5)). HO-PBDEs exhibited greater dioxin-like activity than did the corresponding MeO-PBDEs. Analogues of PBDEs were detected mostly in marine organisms. Of these 11 detected analogues of PBDEs, 6 were found to have measurable dioxin-like potency. Though some analogues of PBDEs exhibited significant dioxin-like potency as measured by responses of the H4IIE-luc transactivation assay, concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) equivalents ((PBDEs analogues)TEQ(H4IIE-luc)), calculated as the sum of the product of concentrations of individual PBDE and their ReP(H4IIE-luc), were less than the tolerance limit proposed by European Union and the oral reference dose (RfD) derived by U.S. Environmental Protection Agency, respectively. (Hazard Quotients (HQ) < 0.005) Additional investigations should be conducted to evaluate the toxic potencies of these chemicals, especially for 2'-MeO-BDE-28, 4-HO-BDE-90, 6-HO-BDE-47, and 6-MeO-BDE-47, which had been detected in other environmental media, including human blood.

In vitro steroidogenic effects of mixtures of persistent organic pollutants (POPs) extracted from burbot (Lota lota) caught in two Norwegian lakes

This study investigated the effects of two mixtures of persistent organic pollutants (POPs) on steroidogenesis in the H295R cell line. The two mixtures were obtained from the livers of burbot (Lota lota) caught in two Norwegian lakes (Mjøsa and Losna) with different contaminant profiles. Steroid hormone levels in the cell culture medium and mRNA levels of 16 genes involved in steroidogenesis were investigated. The crude Lake Mjøsa extract had to be diluted ten times more than the Lake Losna extract in order to prevent cytotoxicity. The ten times diluted Lake Mjøsa mixture had higher levels of DDT and derivates (∑DDTs, 1.7 times) and brominated flame retardants (∑BDEs and HBCD, 15-25 times) than the Lake Losna mixture, which, on the other hand, had higher concentrations of ∑PCBs (1.5 times higher) and also of HCB, ∑HCH isomers and ∑chlordane isomers (5-20 times higher). In the cell culture media, only cortisol levels were increased at the highest exposure concentration to the Lake Mjøsa mixture, while both cortisol and estradiol levels were increased following exposure to the two highest Lake Losna mixture exposure concentrations. Testosterone levels decreased only at the highest exposure concentration of the Lake Losna mixture. Multivariate models suggested that ∑PCBs, and to a lesser extent ∑DDTs, were responsible for the cortisol responses, while estradiol and testosterone alterations were best explained by HCB and ∑PCBs, respectively. Exposure to the mixtures generally increased mRNA levels, with smaller effects exerted by the Lake Mjøsa mixture than the Lake Losna mixture. It was concluded that both mixtures affected steroidogenesis in the H295R cells. Small differences in mixture composition, rather than the high content of brominated flame retardants in the Lake Mjøsa mixture, were suggested to be the most probable reason for the apparent differences in potencies of the two mixtures.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced cytotoxicity accompanied by oxidative stress in rat Sertoli cells: Possible role of mitochondrial fractions of Sertoli cells

TCDD, as an endocrine disruptor, is known to impair testicular functions and fertility. To elucidate the mechanism(s) underlying the testicular effects of TCDD, the potential toxicity of TCDD on Sertoli cells was investigated. Furthermore, the study aims to delineate whether mitochondrial fractions of Sertoli cells are involved in mediating the testicular effects of TCDD. Adult rat Sertoli cells were incubated with (5, 10 or 15nM) of TCDD for 6, 12 or 24h. Cell viability, lactate and LDH leakage into media along with lipid peroxidation, ROS generation, SOD, CAT, GPx, GR, γ-GT and β-glucuronidase activities, GSH content and Δψ(m) were measured. Superoxide anion production, COX and cardiolipin content were measured in mitochondrial fractions. Cell viability was significantly decreased while lactate and LDH leakage into media were increased. ROS generation along with lipid peroxidation was also increased. SOD, CAT, GPx, GR activities and GSH content were significantly decreased. γ-GT and β-glucuronidase activities were also decreased. Superoxide anion production was increased while COX activity and cardiolipin content were decreased in mitochondrial fractions. Moreover, the Δψ(m) was significantly decreased as measured in Sertoli cells. In conclusion, TCDD impairs Sertoli cell functions and this effect is, at least in part, attributed to oxidative stress. We have also found that TCDD increases mitochondrial superoxide anion production and decreases Δψ(m), COX activity and mitochondrial cardiolipin content. Our findings suggest that mitochondria may play an important role in ROS production, leading to the TCDD-induced oxidative stress response and resulting toxicological consequences in rat Sertoli cells.

Polybrominated diphenyl ethers and their hydroxylated/methoxylated analogs: environmental sources, metabolic relationships, and relative toxicities

Brominated compounds are ubiquitous in the aquatic environment. The polybrominated diphenyl ether (PBDE) flame retardants are anthropogenic compounds of concern. Studies suggest that PBDEs can be biotransformed to hydroxylated brominated diphenyl ethers (OH-BDE). However, the rate of OH-BDE formation observed has been extremely small. OH-BDEs have also been identified as natural compounds produced by some marine invertebrates. Another class of compounds, the methoxylated BDEs (MeO-BDEs), has also been identified as natural compounds in the marine environment. Both the OH-BDEs and MeO-BDEs bioaccumulate in higher marine organisms. Recent studies have demonstrated that MeO-BDEs can be biotransformed to OH-BDEs and this generates greater amounts of OH-BDEs than could be generated from PBDEs. Consequently, MeO-BDEs likely represent the primary source of metabolically derived OH-BDEs. Given that for some endpoints OH-BDEs often exhibit greater toxicity compared to PBDEs, it is prudent to consider OH-BDEs as chemicals of concern, despite their seemingly "natural" origins.

Toxic effect of PBDE-47 on thyroid development, learning, and memory, and the interaction between PBDE-47 and PCB153 that enhances toxicity in rats

Polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) are widespread environmental contaminants. There are potential interactive effects between PBDEs and PCBs, as these compounds share similar structures. The developmental neurotoxicity of 2, 2', 4, 4'-tetrabromodiphenyl ether (PBDE-47) and the interaction of PBDE-47 with 2, 2', 4, 4', 5, 5'-hexachlorobipheny (PCB153) were investigated herein, as the dominant congener forms of PBDEs and PCBs, respectively. SD rats were exposed to a single oral dose of PBDE-47 (1, 5, and 10 μg/g) and/or PCB153 (5 μg/g) on post-natal day (PND) 10. Concentrations of PBDE-47, triiodothyronine (T(3)), thyroxine (T(4)), and thyroid-stimulating hormone (TSH) in serum; organ-to-body weight ratios; as well as long-term learning and memory were measured in 2-month-old rats. The present study found that some doses of PBDE-47 decreased the organ-to-body weight ratios of the thyroid and uterus, decreased the concentration of T(4) in serum, and increased the organ-to-body weight ratio of the ovaries (p < 0.05). PCB153 could increase the action of PBDE-47 during combined exposure, but this interaction was not found between PBDE-47 and PCB153. In a Morris water maze experiment, the latency periods were significantly prolonged and time ratios were obviously depressed in all PBDE-47-treated groups compared to the control (p < 0.05); furthermore, significant interactions between PBDE-47 and PCB153 were observed (p < 0.05). In conclusion, PBDE-47 may depress thyroid development as well as the long-term learning and memory capabilities in adult rats exposed to PBDE-47 on PND 10. PCB153 can interact with PBDE-47, resulting in an increase in developmental neurotoxicity.

Application of mass spectrometry in the analysis of polybrominated diphenyl ethers

This review summarized the applications of mass spectrometric techniques for the analysis of the important flame retardants polybrominated diphenyl ethers (PBDEs) to understand the environmental sources, fate and toxicity of PBDEs that were briefly discussed to give a general idea for the need of analytical methodologies. Specific performance of various mass spectrometers hyphenated with, for example, gas chromatograph, liquid chromatograph, and inductively coupled plasma (GC/MS, LC/MS, and ICP/MS, respectively) for the analysis of PBDEs was compared with an objective to present the information on the evolution of MS techniques for determining PBDEs in environmental and human samples. GC/electron capture negative ionization quadrupole MS (GC/NCI qMS), GC/high resolution MS (GC/HRMS) and GC ion trap MS (GC/ITMS) are most commonly used MS techniques for the determination of PBDEs. New analytical technologies such as fast tandem GC/MS and LC/MS become available to improve analyses of higher PBDEs. The development and application of the tandem MS techniques have helped to understand environmental fate and transformations of PBDEs of which abiotic and biotic degradation of decaBDE is thought to be one major source of Br(1-9)BDEs present in the environment in addition to direct loading from commercial mixtures. MS-based proteomics will offer an insight into the molecular mechanisms of toxicity and potential developmental and neurotoxicity of PBDEs.

Improved 3D-QSAR analyzes for the predictive toxicology of polybrominated diphenyl ethers with CoMFA/CoMSIA and DFT

With the popular methods of CoMFA and CoMSIA, three-dimensional quantitative structure-activity relationships (QSARs) were newly developed for the toxicity of polybrominated diphenyl ethers (PBDEs). The choice of optimized geometries by density functional theory (DFT) as molecular template and the RMSD-based molecular alignment strategy might mostly contribute to the QSAR improvement, which was highlighted specifically by the increased q2 of 0.870 for CoMFA, 0.887 for CoMSIA, respectively. QSARs analyzes indicated that the steric effects from ortho- and meta-substitution and the correlated hydrophobicities have the greatest impact on the binding affinities of aryl hydrocarbon receptor (AhR) to PBDEs. Though the effects of electrostatics were comparatively inferior in the AhR binding, the aromatic interaction and possible charge transfer proved to be indispensable for toxicity mediation. Consistent with that proposed previously for other structurally similar compounds, such as dioxins and polychlorinated biphenyls, the predictive toxicology was helpful to understand the congener-specificity of toxicity of PBDEs.

DFT study on the bromination pattern dependence of electronic properties and their validity in quantitative structure-activity relationships of polybrominated diphenyl ethers

With quantum chemical computation of density functional theory (DFT), the electronic properties including the polarisabilities, polarisability anisotropies and quadrupole moments of a total of 209 congeners of polybrominated diphenyl ethers (PBDEs) were evaluated. The electronic properties were shown to be highly dependent on the bromination pattern, i.e. their values changed sensitively with the number and sites of bromination. Being similar to the 2,3,7,8-, 1,4,6,9-chlorination of dioxins, respectively, 3,3',4,4'-, 2,2',5,5'-bromination of PBDEs can impose relatively greater effects on the electronic properties. Some of electronic properties were found to be potent in explaining the variance of toxicity, and the potency was verified by the development of quantitative structure-activity relationships (QSARs). To further improve the stability and predictability of QSARs for toxicity, two-dimensional topological indices were introduced. In QSARs, polarisability anisotropy was more significant than other polarisability tensors, indicating the implicit occurrence of dispersion interaction between the ligand and aryl hydrocarbon receptor (AhR). For PBDEs, the quadrupole moment was as significant as shown previously for dioxins. As interesting descriptors with encoded information about dispersion and electronics, the electronic properties analysed herein are helpful in obtaining a better understanding of the congener-specific toxicities of PBDEs, and are applicable and may be extended to research into the toxicology of structurally similar compounds, such as halogenated aromatics.

Changes in toxicity and Ah receptor agonist activity of suspended particulate matter during flood events at the rivers Neckar and Rhine - a mass balance approach using in vitro methods and chemical analysis

BACKGROUND, AIM, AND SCOPE

As a consequence of flood events, runoff and remobilized sediments may cause an increase of ecotoxicologically relevant effects from contaminant reservoirs. Aquatic and terrestrial organisms as well as cattle and areas of settlement are exposed to dislocated contaminants during and after flood events. In this study, the impacts of two flood events triggered by intense rain at the rivers Neckar and Rhine (Southern Germany) were studied. Effects in correlation to flood flow were assessed at the river Neckar using samples collected at frequent intervals. River Rhine suspended particulate matter (SPM) was sampled over a longer period at normal flow and during a flood event. Three cell lines (H4L1.1c4, GPC.2D.Luc, RTL-W1) were used to compare Ah receptor agonist activity in different biotest systems. Multilayer fractionation was performed to identify causative compounds, focusing on persistent organic contaminants.

MATERIALS AND METHODS

Native water and SPM of flood events were collected at the river Neckar and at the monitoring station (Rheinguetestation, Worms, Germany) of the river Rhine. Water samples were XAD-extracted. SPM were freeze-dried and Soxhlet-extracted using acetone and finally dissolved in dimethyl sulfoxide. Resulting crude extracts were analyzed for cytotoxicity with the neutral red assay. Aryl hydrocarbon receptor (AhR) agonist activity was measured in a set of biological test systems (DR-CALUX, GPC.2D, and ethoxyresorufin-O-deethylase (EROD) assay) and different cell lines. In addition, crude extracts were fractionated using a combined method of multilayer (sequence of acidified silica layers) and carbon fractionation. Fractions from the multilayer fractionation contained persistent organic compounds (polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), polychlorinated biphenyls (PCBs), and some polycyclic aromatic hydrocarbon (PAHs)); fractions from the carbon fractionation were separated into a PCDD/F and a PCB fraction. Dioxin-like activity of multilayer and carbon fractions was determined in the EROD assay and expressed as biological toxicity equivalency concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (bio-TEQs). The calculation of chemical equivalency concentrations (chem-TEQs) and comparison to bio-TEQ values allowed the determination of the contribution of the analyzed persistent compounds to the total biological effects measured.

RESULTS

Soluble compounds in native and extracted water samples resulted in no or minor activity in the toxicity tests, respectively. Filter residues of native water caused increased AhR-mediated activity at the peak of the flood. Activities of SPM of the river Neckar correlated well with the flow rate indicating a flood-dependent increase of toxicity culminating at the peak of flow. River Rhine SPM showed a decrease of activity regarding an SPM sample of the flood event compared to a long-term sample. Excellent correlations with AhR agonistic activity were determined for DR-CALUX and EROD assay, while the GPC.2D assay did not correlate with both other biotests. The activity of persistent dioxin-like acting compounds in multilayer and carbon fractionated PCDD/F and PCB fractions was low if compared to corresponding crude extracts. The congener pattern of PCDD/F revealed that the contaminations mainly originated from products and productions of the chlorine and organochlorine industries.

DISCUSSION

Native and extracted water samples could be shown to contain little or no cytotoxic or AhR agonistic compounds. In contrast, particle-bound compounds were shown to be the relevant effect-causing fraction, as indicated by the activities of filter residues of native water and SPM. Compounds other than fractionated persistent PCBs and PCDD/Fs were more relevant to explain AhR-mediated activities of crude flood SPM at both rivers assessed. Biologically detected activities could at least in part be traced back to chemically analyzed and quantified compounds.

CONCLUSIONS

The calculation of the portion of persistent PCBs and PCDD/Fs in multilayer fractions causing the high inductions in the EROD assay in combination with chemical analysis provides a suitable tool to assess dioxin-like activity of persistent compounds in SPM sampled over the course of flood events. Depending on the catchment area and annual course of flood events, end points may either indicate an increase or a decrease of activity. In order to determine the ecological hazard potential of mobilized contaminants during flood events, the focus should be set on particle-bound pollutants. Furthermore, PCDD/Fs and PCBs, commonly expected to be the most relevant pollutants in river systems, could be shown to contribute only to a minor portion of the overall AhR-mediated activity. However, they might be most relevant for human exposure when considering persistence and bioaccumulation-biomagnification in the food chain.

RECOMMENDATIONS AND PERSPECTIVES

As a consequence of climate change, flood events will increase in frequency and intensity at least in some regions such as Central Europe. Thus, it is crucial to identify the potential hazard of (re-)mobilized contaminants from reservoirs dislocated via floods and threatening especially aquatic organisms and cattle grazing in flood plains. Since other less persistent compounds seem to be more relevant to explain AhR-mediated activities in flood SPM, nonconventional PAHs and more polar compounds also need to be considered for risk assessment. Effect-directed analysis using broad-range fractionation methods taking into account compounds from polar to nonpolar should be applied for identification of pollutants causing biological effects, thus integrating biological and chemical parameters.

Receptor interactions by polybrominated diphenyl ethers versus polychlorinated biphenyls: a theoretical Structure-activity assessment

The extensive body of literature regarding the interaction of polychlorinated biphenyls (PCBs) with transcription factors (receptors) has great value to understand similarities and distinctions and in formulating hypotheses regarding the activity of polybrominated diphenyl ethers (PBDEs) toward those same receptors. Our goal is to present the most comprehensive overview of PBDE effects on AhR, CAR, PXR, ER, AR, PR, DHT, TH, T3, T4 and IGF, as well as hypothetical biological activities of PPAR, RyR, GR and GABA. Aside the influence of the conformation of the ligand, we discuss its constitution influencing the binding affinity: size and polarizability, hydrophilicity, Gibbs free energy of solvation, inductive and mesomeric effects. We evaluate the techniques to determine the biologically relevant conformation of these halogenated hydrocarbons, including computation methods, X-ray and microwave spectroscopy. A novel fluoro-tagged ligand approach holds promise as tools for illuminating the steric and electronic effects in ligand-receptor interaction. Based on our assessment, we predict that PBDEs do not exhibit AhR activity themselves, but impurities are responsible for these effects.

Effects of fluoro substitution on 4-bromodiphenyl ether (PBDE 3)

It is our hypothesis that fluoro substitution provides a powerful tool to modulate the desired characteristics and to increase the specificity of studies of structure-activity relationships. 4-Bromodiphenyl ether (PBDE 3) and its five corresponding monofluorinated analogues (F-PBDEs 3) have been synthesized and fully characterized (using (1)H, (13)C and (19)F NMR spectroscopy, and mass spectrometry). The accurate structure from X-ray crystal analysis was compared with iterative calculations using semi-empirical self-consistent field molecular-orbital (SCF-MO) models. The compounds studied were 4-bromodiphenyl ether (PBDE 3), the (13)C(6)-isotopically labeled PBDE 3 ((13)C(6)-PBDE 3) and 2-fluoro-4-bromodiphenyl ether (3-2F), 2'-fluoro-4-bromodiphenyl ether (3-2'F), 3-fluoro-4-bromodiphenyl ether (3-3F), 3'-fluoro-4-bromodiphenyl ether (3-3'F), and 4'-fluoro-4-bromodiphenyl ether (3-4'F). Solid-state intermolecular interactions for PBDE 3 and the F-PBDEs 3 isomers are dominated by weak C-H(F,Br)...pi and C-H...F interactions. The C-F bond lengths varied between 1.347 (2) and 1.362 (2) A, and the C4-Br bond length between 1.880 (3) and 1.904 (2) A. These bond lengths are correlated with electron-density differences, as determined by (13)C shifts, but not with the strength of the C-F couplings. The interior ring angles of ipso-fluoro substitution increased (121.9-124.0 degrees ) as a result of hyperconjugation, a phenomenon also predicted by the calculation models. An attraction between the vicinal fluoro and halo substituents (observed in fluoro substituted chlorobiphenyls) was not observed for the bromo substituted F-PBDEs. The influence of a fluoro substituent on the conformation was only observable in PBDEs with di-ortho substitution. Calculated and observed torsion angles showed a positive correlation with increasing van der Waals radii and/or the degree of substitution for mono- to tetra-fluoro, chloro, bromo and methyl substitutions in the ortho positions of diphenyl ether. These findings utilizing F-tagged analogues presented here may prove fundamental to the interpretation of the biological effects and toxicities of these persistent environmental pollutants.

Developmental exposure to decabromodiphenyl ether (PBDE 209): effects on thyroid hormone and hepatic enzyme activity in male mouse offspring

Decabrominated diphenyl ether (PBDE 209) is the second most used brominated flame retardant (BFRs). Many studies have shown that some of the BFRs act as endocrine disruptors via alterations in thyroid hormone homeostasis and affect development. Little is known about the effect of prenatal exposure to PBDE 209 on the development in male offspring. Using a CD-1 mouse model, we attempt to estimate the possible effect of in utero exposure to PBDE 209 on thyroid hormone and hepatic enzymes activities in male offspring. Pregnant mice were administered different doses of PBDE 209 (10, 500, and 1500 mg/kg/day) or corn oil for controls per gavage from gestational days 0-17. In adult male offspring whose mothers had been treated with 1500 mg/kg of PBD 209, hepatic enzyme activity of S9 7-ethoxyresorufin O-deethylase (EROD) was weak but significantly increased (54%). However, no significant changes were observed in S9 4-nitrophenol uridinediphosphate-glucuronosyltransferase (UDPGT) in any of the treatment groups. Serum triiodothyronine (T3) was found to have decreased significantly (ca. 21% both 10 mg/kg and 1500 mg/kg) in offspring, but not thyroxine (T4). Histopathological examination revealed that prenatal exposure of PBDE 209 might be related with cell swelling of hepatocytes in male offspring and there were mild changes in the thyroid glands in 1500 mg/kg group. These data demonstrate that PBDE 209 is likely an endocrine disrupter in male mice following exposure during development. Further studies using environmentally relevant doses are needed for hazard identification.

Interactions of polybrominated diphenyl ethers with the aryl hydrocarbon receptor pathway

Polybrominated diphenyl ethers (PBDEs) are brominated flame retardants that have been in use as additives in various consumer products. Structural similarities of PBDEs with other polyhalogenated aromatic hydrocarbons that show affinity for the aryl hydrocarbon receptor (AhR), such as some polychlorinated biphenyls, raised concerns about their possible dioxin-like properties. We studied the ability of environmentally relevant PBDEs (BDE-47, -99, -100, -153, -154, and -183) and the "planar" congener BDE-77 to bind and/or activate the AhR in stably transfected rodent hepatoma cell lines with an AhR-responsive enhanced green fluorescent protein (AhR-EGFP) reporter gene (H1G1.1c3 mouse and H4G1.1c2 rat hepatoma). 7-Ethoxyresorufin-O-deethylation (EROD) was used as a marker for CYP1A1 activity. Dose- and bromination-specific inhibition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced responses was measured by their ability to inhibit the induction of AhR-EGFP expression and EROD activity. Individual exposure to these PBDEs did not result in any increase in induction of AhR-EGFP or CYP1A1 activity. The lower brominated PBDEs showed the strongest inhibitory effect on TCDD-induced activities in both cell lines. While the highest brominated PBDE tested, BDE-183, inhibited EROD activity, it did not affect the induction of AhR-EGFP expression. Similar findings were observed after exposing stably transfected human hepatoma (xenobiotic response element [XRE]-HepG2) cells to these PBDEs, resulting in a small but statically significant agonistic effect on XRE-driven luciferase activity. Co-exposure with TCDD resulted again in antagonistic effects, confirming that the inhibitory effect of these PBDEs on TCDD-induced responses was not only due to direct interaction at receptor level but also at DNA-binding level. This antagonism was confirmed for BDE-99 in HepG2 cells transiently transfected with a Gal4-AhR construct and the corresponding Gal4-Luc reporter gene. In addition, a chromatin immunoprecipitation assay further confirmed that BDE-99 could bind to the AhR and activate the AhR nuclear translocation and dioxin responsive element (DRE) binding in the context of the CYP1A1 promoter. However, the transactivation function of the BDE-99-activated AhR seems to be very weak. These combined results suggest that PBDEs do bind but not activate the AhR-AhR nuclear translocator protein-XRE complex.