Structure-activity relationship study on the binding of PBDEs with thyroxine transport proteins

Update of the risk assessment of polybrominated diphenyl ethers (PBDEs) in food

The European Commission asked EFSA to update its 2011 risk assessment on polybrominated diphenyl ethers (PBDEs) in food, focusing on 10 congeners: BDE-28, -47, -49, -99, -100, -138, -153, -154, -183 and ‑209. The CONTAM Panel concluded that the neurodevelopmental effects on behaviour and reproductive/developmental effects are the critical effects in rodent studies. For four congeners (BDE-47, -99, -153, -209) the Panel derived Reference Points, i.e. benchmark doses and corresponding lower 95% confidence limits (BMDLs), for endpoint-specific benchmark responses. Since repeated exposure to PBDEs results in accumulation of these chemicals in the body, the Panel estimated the body burden at the BMDL in rodents, and the chronic intake that would lead to the same body burden in humans. For the remaining six congeners no studies were available to identify Reference Points. The Panel concluded that there is scientific basis for inclusion of all 10 congeners in a common assessment group and performed a combined risk assessment. The Panel concluded that the combined margin of exposure (MOET) approach was the most appropriate risk metric and applied a tiered approach to the risk characterisation. Over 84,000 analytical results for the 10 congeners in food were used to estimate the exposure across dietary surveys and age groups of the European population. The most important contributors to the chronic dietary Lower Bound exposure to PBDEs were meat and meat products and fish and seafood. Taking into account the uncertainties affecting the assessment, the Panel concluded that it is likely that current dietary exposure to PBDEs in the European population raises a health concern.

QSAR and molecular docking studies on designing potent inhibitors of SARS-CoVs main protease

Background: Severe acute respiratory syndrome coronavirus (SARS-CoVs) have emerged as a global health threat, which had caused a high rate of mortality. There is an urgent need to find effective drugs against these viruses. Objective: This study aims to predict the activity of unsymmetrical aromatic disulfides by constructing a QSAR model, and to design new compounds according to the structural and physicochemical attributes responsible for higher activity towards SARS-CoVs main protease. Methods: All molecules were constructed in ChemOffice software and molecular descriptors were calculated by CODESSA software. A regression-based linear heuristic method was established by changing descriptors datasets and calculating predicted IC₅₀ values of compounds. Then, some new compounds were designed according to molecular descriptors from the heuristic method model. The compounds with predicted values smaller than a set point were constantly screened out. Finally, the properties analysis and molecular docking were conducted to further understand the structure-activity relationships of these finalized compounds. Results: The heuristic method explored the various descriptors responsible for bioactivity and gained the best linear model with R² 0.87. The success of the model fully passed the testing set validation, proving that the model has both high statistical significance and excellent predictive ability. A total of 5 compounds with ideal predicted IC₅₀ were found from the 96 newly designed derivatives and their properties analyze was carried out. Molecular docking experiments were conducted for the optimal compound 31a, which has the best compound activity with good target protein binding capability. Conclusion: The heuristic method was quite reliable for predicting IC₅₀ values of unsymmetrical aromatic disulfides. The present research provides meaningful guidance for further exploration of the highly active inhibitors for SARS-CoVs.

3D- and 2D-QSAR models’ study and molecular docking of novel nitrogen-mustard compounds for osteosarcoma

Background: The dipeptide-alkylated nitrogen-mustard compound is a new kind of nitrogen-mustard derivative with a strong anti-tumor activity, which can be used as a potential anti-osteosarcoma chemotherapy drug.

Objective: 2D- and 3D-QSAR (structure–activity relationship quantification) models were established to predict the anti-tumor activity of dipeptide-alkylated nitrogen-mustard compounds.

Method: In this study, a linear model was established using a heuristic method (HM) and a non-linear model was established using the gene expression programming (GEP) algorithm, but there were more limitations in the 2D model, so a 3D-QSAR model was introduced and established through the CoMSIA method. Finally, a series of new dipeptide-alkylated nitrogen-mustard compounds were redesigned using the 3D-QSAR model; docking experiments were carried out on several compounds with the highest activity against tumors.

Result: The 2D- and 3D-QSAR models obtained in this experiment were satisfactory. A linear model with six descriptors was obtained in this experiment using the HM through CODESSA software, where the descriptor “Min electroph react index for a C atom” has the greatest effect on the compound activity; a reliable non-linear model was obtained using the GEP algorithm model (the best model was generated in the 89th generation cycle, with a correlation coefficient of 0.95 and 0.87 for the training and test set, respectively, and a mean error of 0.02 and 0.06, respectively). Finally, 200 new compounds were designed by combining the contour plots of the CoMSIA model with each other, together with the descriptors in the 2D-QSAR, among which compound I1.10 had a high anti-tumor and docking ability.

Conclusion: Through the model established in this study, the factors influencing the anti-tumor activity of dipeptide-alkylated nitrogen-thaliana compounds were revealed, providing direction and guidance for the further design of efficient chemotherapy drugs against osteosarcoma.

3D,2D-QSAR study and docking of novel quinazolines as potential target drugs for osteosarcoma

Background: Quinazolines are an important class of benzopyrimidine heterocyclic compounds with a promising antitumor activity that can be used for the design and development of osteosarcoma target compounds. Objective: To predict the compound activity of quinazoline compounds by constructing 2D- and 3D-QSAR models, and to design new compounds according to the main influencing factors of compound activity in the two models. Methods: First, heuristic method and GEP (gene expression programming) algorithm were used to construct linear and non-linear 2D-QSAR models. Then a 3D-QSAR model was constructed using CoMSIA method in SYBYL software package. Finally, new compounds were designed according to molecular descriptors of 2D-QSAR model and contour maps of 3D-QSAR model. Several compounds with optimal activity were used for docking experiments with osteosarcoma related targets (FGFR4). Results: The non-linear model constructed by GEP algorithm was more stable and predictive than the linear model constructed by heuristic method. A 3D-QSAR model with high Q² (0.63) and R ² (0.987) values and low error values (0.05) was obtained in this study. The success of the model fully passed the external validation formula, proving that the model is very stable and has strong predictive power. 200 quinazoline derivatives were designed according to molecular descriptors and contour maps, and docking experiments were carried out for the most active compounds. Compound 19g.10 has the best compound activity with good target binding capability. Conclusion: To sum up, the two novel QSAR models constructed were very reliable. The combination of descriptors in 2D-QSAR with COMSIA contour maps provides new design ideas for future compound design in osteosarcoma.

Structural studies on the endocrine-disrupting role of polybrominated diphenyl ethers (PBDEs) in thyroid diseases

Polybrominated diphenyl ethers (PBDEs) are synthetic brominated flame retardants with extensive applications in daily-life consumer products. However, PBDEs have become ubiquitous environmental contaminants due to their leach-out capability. The hazardous human health effects and endocrine-disrupting activity of PBDEs have led many governmental organizations to impose ban on their manufacture, causing their gradual phase out from commercial products. However, PBDEs and their metabolites are still being detected from biological and environmental samples owing to their persistence and bioaccumulation. The PDBE metabolites in these samples are present in concentrations often higher and even with higher toxic potential than parent PBDEs. The two commonly detected environmental PBDE congeners, 2,2',4,4'-tetra-bromodiphenyl ether (BDE-47) and 2,2',4,4',5-penta-bromodiphenyl ether (BDE-99), and their HO- and MeO- metabolites were considered in this study for their potential disrupting activity on thyroid hormone transport. Specifically, the study involved structural binding characterization of BDE-47 and BDE-99 including their two HO- and two MeO- metabolites with thyroxine-binding globulin (TBG), which is the main thyroid hormone transport protein in blood. The results showed that the binding pattern and molecular interactions of above two PBDEs and their metabolites exhibited overall similarity to native ligand, thyroxine in dock score, binding energy, and amino acid interactions with TBG. The BDE-99 and its metabolites were predicted to have stronger binding to TBG than BDE-47 with the metabolite 5-MeO-BDE-99 showing equal binding affinity to that of thyroxine. It is concluded that BDE-47 and BDE-99 and their metabolites have the potential to disrupt thyroid hormone transport and interfere in thyroid function.

Evaluation and mechanistic study of chlordecone-induced thyroid disruption: Based on in vivo, in vitro and in silico assays

The present study aimed to evaluate the thyroid-disrupting potency of chlordecone, and reveal the underlying mechanism. In the in vivo assays, rare minnow embryos were exposed to 0, 0.01, 0.1, 1 and 10 μg·L^-1 chlordecone until sexually mature. The results showed decreased T4 but increased T3 concentrations in plasma, upregulated mRNA levels of thyrotropin-releasing hormone receptor (trhr) and sodium-iodide symporter (nis) in the brain, and transthyretin (ttr), thyroid hormone receptor α (trα) and deiodinase enzymes (dio1 and dio2) in the liver of adult fish. In the in vitro assays, single chlordecone treatments promoted growth hormone (GH) and prolactin (PRL) secretion in GH3 cells. Transcription of thyroid receptor (trβ) was inhibited, but this is not likely responsible for chlordecone-induced GH secretion and altered transcription. When co-treated with T3, chlordecone acted independently of the effect of T3 on GH secretion; chlordecone-induced GH/PRL secretion and mRNA expression were further promoted when co-treated with E2, but inhibited when co-treated with ICI, indicating an important role for estrogen receptors (ERs) in chlordecone-induced changes in GH3 cells. Furthermore, in silico prediction suggested no stable interactions between chlordecone and thyroid hormone-related proteins, as well as a regulatory role for ERs in thyroid systems. Overall, our results indicated that chlordecone may have adverse effects on thyroid systems upon long-term exposure. However, rather of TRs, ERs may be responsible for thyroid disruption following chlordecone exposure.

Rno-miR-224-5p contributes to 2,2',4,4'-tetrabromodiphenyl ether-induced low triiodothyronine in rats by targeting deiodinases

Hypothyroidism is commonly associated with substantial adverse impacts on human health, and polybrominated diphenyl ether (PBDE), a kind of classic thyroid hormone disruptor, was speculated to be a potential environmental factor, but its effect on thyroxine metabolism has received little attention. In the present study, we investigated the role and mechanism of rno-miR-224-5p in deiodinase-mediated thyroxine metabolism in rats treated with 2,2',4,4'-tetrabromodiphenyl ether (BDE47), a predominant PBDE congener in humans. BDE47 decreased plasma triiodothyronine (T3) and thyroxine (T4) and increased reverse T3 (rT3) in the rats, and the expression of type 1 deiodinase (DIO1) and type 3 deiodinase (DIO3) increased in both the rats and H4-II-E cells. Rno-miR-224-5p was predicted to target dio1 instead of dio3, according to the TargetScan, miRmap.org and microRNA.org databases. Experiments showed that the rno-miR-224-5p level was decreased by BDE47 in a dose-dependent manner and confirmed that rno-miR-224-5p downregulated both DIO1 and DIO3 in the H4-II-E cells and in the rats, as determined using mimics and an inhibitor of rno-miR-224-5p. Furthermore, DIO1 was observed to be a direct functional target of rno-miR-224-5p, whereas DIO3 was indirectly regulated by rno-miR-224-5p via the phosphorylation of the MAPK/ERK (but not p38 or JNK) pathway. Reportedly, DIO1 and DIO3 act principally as inner-ring deiodinases and are responsible for the conversion of T4 to rT3, but not to T3, and the final clearance of thyroxine (mainly in the form of T2). Our results demonstrated that BDE47 induced low levels of T3 conversion through DIO1 and DIO3, which were regulated by rno-miR-224-5p. The findings suggest a novel additional mechanism of PBDE-induced thyroxine metabolism disorder that differs from that of PBDEs as environmental thyroid disruptors.

Sperm quality biomarkers complement reproductive and endocrine parameters in investigating environmental contaminants in common carp (Cyprinus carpio) from the Lake Mead National Recreation Area

Lake Mead National Recreational Area (LMNRA) serves as critical habitat for several federally listed species and supplies water for municipal, domestic, and agricultural use in the Southwestern U.S. Contaminant sources and concentrations vary among the sub-basins within LMNRA. To investigate whether exposure to environmental contaminants is associated with alterations in male common carp (Cyprinus carpio) gamete quality and endocrine- and reproductive parameters, data were collected among sub-basins over 7 years (1999-2006). Endpoints included sperm quality parameters of motility, viability, mitochondrial membrane potential, count, morphology, and DNA fragmentation; plasma components were vitellogenin (VTG), 17ß-estradiol, 11-keto-testosterone, triiodothyronine, and thyroxine. Fish condition factor, gonadosomatic index, and gonadal histology parameters were also measured. Diminished biomarker effects were noted in 2006, and sub-basin differences were indicated by the irregular occurrences of contaminants and by several associations between chemicals (e.g., polychlorinated biphenyls, hexachlorobenzene, galaxolide, and methyl triclosan) and biomarkers (e.g., plasma thyroxine, sperm motility and DNA fragmentation). By 2006, sex steroid hormone and VTG levels decreased with subsequent reduced endocrine disrupting effects. The sperm quality bioassays developed and applied with carp complemented endocrine and reproductive data, and can be adapted for use with other species.

Mechanisms of Action Point Towards Combined PBDE/NDL-PCB Risk Assessment

At present, human risk assessment of the structurally similar non-dioxin-like (NDL) PCBs and polybrominated diphenylethers (PBDEs) is done independently for both groups of compounds. There are however obvious similarities between NDL-PCBs and PBDEs with regard to modulation of the intracellular calcium homeostasis (basal calcium levels, voltage-gated calcium channels, calcium uptake, ryanodine receptor) and thyroid hormone (TH) homeostasis (TH levels and transport). which are mechanisms of action related to neurobehavioral effects (spontaneous activity, habituation and learning ability). There also similarities in agonistic interactions with the hepatic nuclear receptors PXR and CAR. Several effects on developmental (reproductive) processes have also been observed, but results were more dispersed and insufficient to compare both groups of compounds. The available mechanistic information is sufficient to warrant a dose addition model for NDL-PCBs and PBDEs, including their hydroxylated metabolites.Although many of the observed effects are similar from a qualitative point of view for both groups, congener or tissue specific differences have also been found. As this is a source of uncertainty in the combined hazard and risk assessment of these compounds, molecular entities involved in the observed mechanisms and adverse outcomes associated with these compounds need to be identified. The systematical generation of (quantitative) structure-activity information for NDL-PCBs and PBDEs on these targets (including potential non-additive effects) will allow a more realistic risk estimation associated with combined exposure to both groups of compounds during early life. Additional validation studies are needed to quantify these uncertainties for risk assessment of NDL-PCBs and PBDEs.

Relationship between the binding free energy and PCBs' migration, persistence, toxicity and bioaccumulation using a combination of the molecular docking method and 3D-QSAR

The molecular docking method was used to calculate the binding free energies between biphenyl dioxygenase and 209 polychlorinated biphenyl (PCB) congeners. The relationships between the calculated binding free energies and migration (octanol-air partition coefficients, KOA), persistence (half-life, t1/2), toxicity (half maximal inhibitory concentration, IC50), and bioaccumulation (bioconcentration factor, BCF) values for the PCBs were used to gain insight into the degradation of PCBs in the presence of biphenyl dioxygenase. The relationships between the calculated binding free energies and the molecular weights, KOA, BCF, and t1/2 values for the PCBs were statistically significant (P < 0.01), whereas the relationship between the calculated binding free energies and the IC50 for the PCBs was not statistically significant (P > 0.05). The electrostatic field, derived from three-dimensional quantitative structure-activity relationship studies, was a primary factor governing the binding free energy, which agreed with literature findings for KOA, t1/2, and BCF. Comparative molecular field analysis and comparative molecular similarity indices analysis contour maps showed that the binding free energies, KOA, t1/2, and BCF values for the PCBs decreased simultaneously when substituents with electropositive groups at the 3-position or electronegative groups at the 3'-position were introduced. This indicated the binding free energy was correlated with the persistent organic pollutant characteristics of PCBs. Furthermore, low binding free energies improved the degradation of the PCBs and simultaneously decreased the KOA, t1/2, and BCF values, thereby reducing the persistent organic pollutant characteristics of PCBs in the environment. These results are expected to be beneficial in providing a theoretical foundation for further elucidation of the degradation and molecular modification of PCBs.

Structural aspects of thyroid hormone binding to proteins and competitive interactions with natural and synthetic compounds

Thyroid hormones and their metabolites constitute a vast class of related iodothyronine compounds that contribute to the regulation of metabolic activity and cell differentiation. They are in turn transported, transformed and recognized as signaling molecules through binding to a variety of proteins from a wide range of evolutionary unrelated protein families, which renders these proteins and their iodothyronine binding sites an example for extensive convergent evolution. In this review, we will briefly summarize what is known about iodothyronine binding sites in proteins, the modes of protein/iodothyronine interaction, and the ligand conformations. We will then discuss physiological and synthetic compounds, including popular drugs and food components, that can interfere with iodothyronine binding and recognition by these proteins. The discussion also includes compounds persisting in the environment and acting as endocrine disrupting chemicals.

Interaction studies of polybrominated diphenyl ethers (PBDEs) with human serum albumin (HSA): Molecular docking investigations

This work has evaluated the interactions of HSA and typical PBDEs (BDE47, BDE99, BDE100, BDE153 and BDE209) at molecular level by modelling. Apart from the BDE209, PBDEs with higher molecular weight show higher binding energy with the residues of HSA. The BDE209 without H atoms has the lowest binding energy (-ΔG_binding: 4.30calmol^-1) than other PBDEs (-ΔG_binding: 7.93-8.42calmol^-1). The BDE99 shows a higher binding energy than its isomer (BDE100). On the other hand, the lgK_ow-depth plotting figure shows that a higher K_ow value (hydrophobicity) of PBDEs is accompanied by a deeper binding site within the central channel of HSA. This work may provide a theoretical method to assess the transport and distribution of PBDEs in human body.

Different binding mechanisms of neutral and anionic poly-/perfluorinated chemicals to human transthyretin revealed by In silico models

Chemical forms-dependent binding interactions between phenolic compounds and human transthyretin (hTTR) have been elaborated previously. However, it is not known whether the binding interactions between ionizable halogenated alphatic compounds and hTTR also have the same manner. In this study, poly-/perfluorinated chemicals (PFCs) were selected as model compounds and molecular dynamic simulation was performed to investigate the binding mechanisms between PFCs and hTTR. Results show the binding interactions between the halogenated aliphatic compounds and hTTR are related to the chemical forms. The ionized groups of PFCs can form electrostatic interactions with the -NH⁺₃ groups of Lys 15 residues in hTTR and form hydrogen bonds with the residues of hTTR. By analyzing the molecular orbital energies of PFCs, we also found that the anionic groups (nucleophile) in PFCs could form electron donor - acceptor interactions with the -NH⁺₃ groups (electrophile) in Lys 15. The aforementioned orientational interactions make the ionized groups of the PFCs point toward the entry port of the binding site. The roles of fluorine atoms in the binding interactions were also explored. The fluorine atoms can influence the binding interactions via inductive effects. Appropriate molecular descriptors were selected to characterize these interactions, and two quantitative structure-activity relationship models were developed.

Understanding the microscopic binding mechanism of hydroxylated and sulfated polybrominated diphenyl ethers with transthyretin by molecular docking, molecular dynamics simulations and binding free energy calculations

Polybrominated diphenyl ethers (PBDEs), one typical type of persistent environmental contaminant, have toxicological effects such as disrupting thyroid homeostasis in the human body. The high binding affinities of hydroxylated metabolites of PBDEs (OH-PBDEs) with transthyretin (TTR) were considered to be one major reason for their extraordinary capacity of passing through the blood-brain barrier via competitive thyroid hormone (T4) transport protein binding. Recent findings showed that sulfated PBDEs can be formed in human liver cytosol as phase-II metabolites. However, experimentally determined data for the TTR binding potential of the sulfated PBDEs are still not available. Therefore, molecular docking and molecular dynamics (MD) simulations were employed in the present study to probe the molecular basis of TTR interacting with hydroxylated and sulfated PBDEs at the atomic level. The docking scores of LeDock were used to construct the structure-based predictive model. The calculated results showed that the sulfated PBDEs have stronger affinity for TTR than the corresponding OH-PBDEs. Further analysis of structural characteristics based on MD simulations indicated that upon the binding of PBDE metabolites, the stability of TTR was enhanced and the dissociation rate of the tetrameric protein structure was potentially decreased. Subsequent binding free energy calculations implied that van der Waals interactions are the dominant forces for the binding of these metabolites of PBDEs at the T4 site of TTR. The residues Ser117/Ser117' and Lys15/Lys15' were identified, by both residue energy decomposition and computational alanine-scanning mutagenesis methods, as key residues which play an important role in determining the binding orientations of the -OSO₃^- group of sulfated PBDEs by formation of either hydrogen bonds or electrostatic interactions, respectively. In general, the combination of docking calculations with MD simulations provided a theoretically toxicological assessment for the metabolites of PBDEs, deep insight into the recognition mechanism of TTR for these compounds, and thus more comprehensive understanding of the thyroid-related toxic effects of PBDEs as well.

Identification of Thyroid Hormone Disruptors among HO-PBDEs: In Vitro Investigations and Coregulator Involved Simulations

Some hydroxylated polybrominated diphenyl ethers (HO-PBDEs), that have been widely detected in the environment and tissues of humans and wildlife, bind to thyroid hormone (TH) receptor (TR) and can disrupt functioning of systems modulated by the TR. However, mechanisms of TH disrupting effects are still equivocal. Here, disruption of functions of TH modulated pathways by HO-PBDEs was evaluated by assays of competitive binding, coactivator recruitment, and proliferation of GH3 cells. In silico simulations considering effects of coregulators were carried out to investigate molecular mechanisms and to predict potencies for disrupting functions of the TH. Some HO-PBDEs were able to bind to TR with moderate affinities but were not agonists. In GH3 proliferation assays, 13 out of 16 HO-PBDEs were antagonists for the TH. In silico simulations of molecular dynamics revealed that coregulators were essential for identification of TH disruptors. Among HO-PBDEs, binding of passive antagonists induced repositioning of H12, blocking AF-2 (transactivation function 2) and preventing recruitment of the coactivator. Binding of active antagonists exposed the coregulator binding site, which tended to bind to the corepressor rather than the coactivator. By considering both passive and active antagonisms, anti-TH potencies of HO-PBDEs could be predicted from free energy of binding.

Polybrominated diphenyl ethers (PBDEs) in human samples of mother-newborn pairs in South China and their placental transfer characteristics

There are limited data concerning the placenta transfer characteristics and accumulation of polybrominated diphenyl ethers (PBDEs) in infants. However, PBDEs received increasing health concerns due to their endocrine disrupt and neurodevelopment toxicity effects. The present study assessed the accumulation of PBDEs in 30 paired placenta, breast milk, fetal cord blood, and neonatal urine samples collected from five major cities of the South China. The age of mothers ranged from 21 to 39 (mean 27.6±4.56). The ∑PBDE concentrations were 15.8±9.88 ng g(-1) lipid in placenta, 13.2±7.64 ng g(-1) lipid in breast milk, 16.5±19.5 ng g(-1) lipid in fetal cord blood, and 1.80±1.99 ng ml(-1) in neonatal urine. BDE-47 was the predominant congener in all types of human sample. Octa-BDEs such as BDE-196/-197 were detected highly in placenta and cord blood while moderately in breast milk and neonatal urine. Significant (p<0.01) correlations were observed for both total and most individual PBDEs in cord blood-maternal placenta and breast milk-urine paired individual samples. The extent of placental transfer of higher brominated BDEs such as BDE-196/-197 was greater than that of BDE-47. The estimated daily intake (EDI) analysis for breast-fed infants revealed that newborns in these areas were exposed to relatively high levels of PBDEs via breast milk. Our study not only provided systematic fundamental data for PBDE distribution but also revealed the placenta transfer characteristics of PBDE congeners in South China.

Assessing reproductive and endocrine parameters in male largescale suckers (Catostomus macrocheilus) along a contaminant gradient in the lower Columbia River, USA

Persistent organochlorine pollutants such as polychlorinated biphenyls (PCBs), dichlorodiphenyldichloroethylene (p,p'-DDE), and polybrominated diphenyl ethers (PBDEs) are stable, bioaccumulative, and widely found in the environment, wildlife, and the human population. To explore the hypothesis that reproduction in male fish is associated with environmental exposures in the lower Columbia River (LCR), reproductive and endocrine parameters were studied in male resident, non-anadromous largescale sucker (Catostomus macrocheilus) (LSS) in the same habitats as anadromous salmonids having conservation status. Testes, thyroid tissue and plasma collected in 2010 from Longview (LV), Columbia City (CC), and Skamania (SK; reference) were studied. Sperm morphologies and thyrocyte heights were measured by light microscopy, sperm motilities by computer-assisted sperm motion analysis, sperm adenosine triphosphate (ATP) with luciferase, and plasma vitellogenin (VTG), thyroxine (T4), and triiodothyronine (T3) by immunoassay. Sperm apoptosis, viability, mitochondrial membrane potential, nuclear DNA fragmentation, and reproductive stage were measured by flow cytometry. Sperm quality parameters (except counts) and VTG were significantly different among sites, with correlations between VTG and 7 sperm parameters. Thyrocyte heights, T4, T3, gonadosomatic index and Fulton's condition factor differed among sites, but not significantly. Sperm quality was significantly lower and VTG higher where liver contaminants and water estrogen equivalents were highest (LV site). Total PCBs (specifically PCB-138, -146, -151, -170, -174, -177, -180, -183, -187, -194, and -206) and total PBDEs (specifically BDE-47, -100, -153, and -154) were negatively correlated with sperm motility. PCB-206 and BDE-154 were positively correlated with DNA fragmentation, and pentachloroanisole and VTG were positively correlated with sperm apoptosis and negatively correlated with ATP. BDE-99 was positively correlated with sperm counts and motility; T4 was negatively correlated with counts and positively correlated with motility, thus indicating possible androgenic mechanisms and thyroid endocrine disruption. Male LSS proved to be an informative model for studying reproductive and endocrine biomarkers in the LCR.

Perinatal exposure to BDE-99 causes decreased protein levels of cyclin D1 via GSK3β activation and increased ROS production in rat pup livers

We here examined the potential liver toxicity in rat pups from dams exposed during the gestational and lactation periods to 2,2',4,4',5-pentabromodiphenyl ether (BDE-99). Dams were exposed to 0, 1, and 2mg/kg/day of BDE-99 from gestation day 6 to postnatal day 21. When the pups were weaning, the liver from 1 pup of each litter was excised to evaluate oxidative stress markers and the messenger RNA (mRNA) expression of multiple cytochrome P450 (CYP) isoforms. To determine whether thyroid hormone (TH) was disrupted, the protein and mRNA expressions of several TH receptor (TR) isoforms, as well as the protein levels of cyclin D1 and the phosphorylated protein kinases Akt and glycogen synthase kinase 3 beta (GSK3β), were evaluated. Perinatal exposure to BDE-99 produced decreased levels of cyclin D1 in rat pup livers. A decrease in the active form of Akt and an increase in the active form of GSK3β were observed. The decreased Akt pathway may be due to a potential disruption of the nongenomic actions of TH by BDE-99 and its metabolites. This possible TH disruption was noted as a decrease in TR isoforms expression. By contrast, we observed an upregulation of CYP2B1 gene expression, which is correlated with an increase in reactive oxygen species production. This outcome indicates activation of the nuclear constitutive androstane receptor, which could induce the expression of other enzymes capable of metabolizing TH. The present findings support the hypothesis that perinatal exposure to PBDEs, at levels found in humans, may have serious implications for metabolic processes in rat pup livers.

In silico investigations of anti-androgen activity of polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) have attracted great concern as global environmental pollutants and representative endocrine disruptors. In this work, a molecular model study combining three-dimensional quantitative structure-activity relationship (3D-QSAR), molecular docking, and molecular dynamics (MD) simulations was performed to explore the structural requirement for the anti-androgen activities of PCBs and to reveal the binding mode between the PCBs and androgen receptor (AR). The best comparative molecular similarity indices analysis (CoMSIA) model, obtained from receptor-based alignment, shows leave-one-out cross-validated correlation coefficient (q(2)) of 0.665 and conventional correlation coefficient (R(2)) of 0.945. The developed model has a highly predictive ability in both internal and external validation. Furthermore, the interaction mechanisms of PCBs to AR were analyzed by molecular docking and MD simulation. Molecular docking indicated that all the PCBs in the data set docked in a hydrophobic pocket. The Binding free energies calculated by Molecular mechanics-Poisson Boltzmann surface area (MM-PBSA) not only exhibited a good correlation with the experimental activity, but also could explain the activity difference of the studied compounds. The binding free energy decomposition analysis indicates that the van der Waals interaction is the major driving force for the binding process.

In silico study on hydroxylated polychlorinated biphenyls as androgen receptor antagonists

Hydroxylated polychlorinated biphenyls (HO-PCBs), major metabolites of PCBs, may have the potential to disrupt androgen hormone homeostasis. However, there is a lack of systematic investigation into the intermolecular interaction mechanism between HO-PCBs and the androgen receptor (AR). In this study, the combination of three-dimensional quantitative structure-activity relationship (3D-QSAR), molecular docking, and molecular dynamics (MD) simulations was performed to elucidate structural characteristics that influence the anti-androgen activity of HO-PCBs, and to provide a better understanding of the binding modes between HO-PCBs and AR. A predictive comparative molecular field analysis (CoMFA) model was developed with good robustness and predictive ability. Graphical interpretation of the model provided some insights into the structural features that affect the anti-androgen activity of HO-PCBs. The hydrogen bond interaction with Gln711, and hydrophobic interactions with residues in the hydrophobic pocket played important roles in the binding of ligand with receptor. These results are expected to be beneficial to predict anti-androgen activities of other HO-PCBs and provided possible clues for further elucidation of the binding mechanism of HO-PCBs with AR.

Analysis of Ah receptor binding affinities of polybrominated diphenyl ethers via in silico molecular docking and 3D-QSAR

Polybrominated diphenyl ethers (PBDEs) have become ubiquitous contaminations due to their use as flame retardants. The structural similarity of PBDE to some dioxin-like compounds suggested that they may share similar toxicological effects: they might activate the aryl hydrocarbon receptor (AhR) signal transduction pathway and thus might have adverse effects on wildlife and humans. In this study, in silico computational workflow combining molecular docking and three-dimensional quantitative structure-activity relationship (3D-QSAR) was performed to investigate the binding interactions between PBDEs and AhR and the structural features affecting the AhR binding affinity of PBDE. The molecular docking showed that hydrogen-bond and hydrophobic interactions were the major driving forces for the binding of ligands to AhR, and several key amino acid residues were also identified. The CoMSIA model was developed from the conformations obtained from molecular docking and exhibited satisfactory results as q (2) of 0.605 and r (2) of 0.996. Furthermore, the derived model had good robustness and statistical significance in both internal and external validations. The 3D contour maps generated from CoMSIA provided important structural features influence the binding affinity. The obtained results were beneficial to better understand the toxicological mechanism of PBDEs.

Molecular docking, molecular dynamics simulation, and structure-based 3D-QSAR studies on estrogenic activity of hydroxylated polychlorinated biphenyls

Hydroxylated polychlorinated biphenyls (HO-PCBs), major metabolites of PCBs, have been reported to present agonist or antagonist interactions with estrogen receptor α (ERα) and induce ER-mediated responses. In this work, a multistep framework combining molecular docking, molecular dynamics (MD) simulations, and structure-based three-dimensional quantitative structure-activity relationship (3D-QSAR) studies were performed to explore the influence of structural features on the estrogenic activities of HO-PCBs, and to investigate the molecular mechanism of ERα-ligand interactions. The CoMSIA (comparative molecular similarity indices analysis) model was developed from the conformations obtained from molecular docking. The model exhibited statistically significant results as the cross-validated correlation coefficient q² was 0.648, the non-cross-validated correlation coefficient r² was 0.968, and the external predictive correlation coefficient r(pred)² was 0.625. The key amino acid residues were identified by molecular docking, and the detailed binding modes of the compounds with different activities were determined by MD simulations. The binding free energies correlated well with the experimental activity. An energetic analysis, MM-GBSA energy decomposition, revealed that the van der Waals interaction was the major driving force for the binding of compounds to ERα. The hydrogen bond interactions between the ligands and residue His524 help to stabilize the conformation of ligands at the binding pocket. These results are expected to be beneficial to predict estrogenic activities of other HO-PCB congeners and helpful for understanding the binding mechanism of HO-PCBs and ERα.

Gestational exposure to BDE-99 produces toxicity through upregulation of CYP isoforms and ROS production in the fetal rat liver

On gestation day (GD) 6 to GD 19, pregnant Sprague Dawley rats were orally exposed to 0, 0.5, 1, and 2 mg/kg/day to one of the most prevalent polybrominated diphenyl ethers congeners found in humans, 2,2',4,4',5-pentaBDE (BDE-99). All dams were euthanized on GD 20, and live fetuses were evaluated for sex, body weight, and external, internal, and skeletal malformations and developmental variations. The liver from one fetus of each litter was excised for the evaluation of oxidative stress markers and the messenger RNA expression of multiple cytochrome P450 (CYP) isoforms. Exposure to BDE-99 during the gestational period produced delayed ossification, slight hypertrophy of the heart, and enlargement of the liver in fetuses. A transplacental effect of BDE-99, evidenced by the activation of nuclear hormones receptors that induce the upregulation of CYP1A1, CYP1A2, CYP2B1, and CYP3A2 isoforms, was also found in fetal liver. These isoforms are correlated with the activity level of the enzyme catalase and the levels of thiobarbituric acid reactive substances. However, teratogenic effects from BDE-99 exposure were not observed. Clear signs of embryo/fetal toxicity, due to a possible hormonal disruption, were evidenced by a large increase in the CYP system and the production of reactive oxygen species in fetal liver.