Timo Hamers

Competitive Binding of Poly- and Perfluorinated Compounds to the Thyroid Hormone Transport Protein Transthyretin

Due to their unique surfactant properties, poly- and perfluorinated compounds (PFCs) have been extensively used and can be found all over the environment. Concern about their environmental fate and toxicological properties has initiated several research projects. In the present study, we investigated if PFCs can compete with thyroxine (T(4), i.e., the transport form of thyroid hormone) for binding to the human thyroid hormone transport protein transthyretin (TTR). Such competitive capacity may lead to decreased thyroid hormone levels as previously reported for animals exposed to PFCs. Twenty-four PFCs, together with 6 structurally similar natural fatty acids, were tested for binding capacity in a radioligand-binding assay. The binding potency decreased in the order: perfluorohexane sulfonate > perfluorooctane sulfonate/perfluorooctanoic acid > perfluoroheptanoic acid > sodium perfluoro-1-octanesulfinate > perfluorononanoic acid, with TTR binding potencies 12.5-50 times lower than the natural ligand T(4). Some lower molecular weight compounds with structural similarity to these PFCs were > 100 times less potent than T(4). Simple descriptors based on the two-dimensional molecular structures of the compounds were used to visualize the chemical variation and to model the structure-activity relationship for the competitive potencies of the TTR-binding compounds. The models indicated the dependence on molecular size and functional groups but demanded a more detailed description of the chemical properties and data for validation and further quantitative structure-activity relationship (QSAR) development. Competitive binding of PFCs to TTR, as observed for human TTR in the present study, may explain altered thyroid hormone levels described for PFC-exposed rats and monkeys. Median human blood levels of the most potent TTR-binding PFCs are one to two orders of magnitude lower than concentration at 50% inhibition (IC(50)) values determined in the present study. In addition, this study contributes to the understanding of the bioaccumulation of PFCs in man and possibly in other wildlife species.

Quantitative structure‐activity relationship modeling on in vitro endocrine effects and metabolic stability involving 26 selected brominated flame retardants

In this work, quantitative structure-activity relationships (QSARs) were developed to aid human and environmental risk assessment processes for brominated flame retardants (BFRs). Brominated flame retardants, such as the high-production-volume chemicals polybrominated diphenyl ethers (PBDEs), tetrabromobisphenol A, and hexabromocyclododecane, have been identified as potential endocrine disruptors. Quantitative structure-activity relationship models were built based on the in vitro potencies of 26 selected BFRs. The in vitro assays included interactions with, for example, androgen, progesterone, estrogen, and dioxin (aryl hydrocarbon) receptor, plus competition with thyroxine for its plasma carrier protein (transthyretin), inhibition of estradiol sulfation via sulfotransferase, and finally, rate of metabolization. The QSAR modeling, a number of physicochemical parameters were calculated describing the electronic, lipophilic, and structural characteristics of the molecules. These include frontier molecular orbitals, molecular charges, polarities, log octanol/water partitioning coefficient, and two- and three-dimensional molecularproperties. Experimental properties were included and measured for PBDEs, such as their individual ultraviolet spectra (200-320 nm) and retention times on three different high-performance liquid chromatography columns and one nonpolar gas chromatography column. Quantitative structure-activity relationship models based on androgen antagonism and metabolic degradation rates generally gave similar results, suggesting that lower-brominated PBDEs with bromine substitutions in ortho positions and bromine-free meta- and para positions had the highest potencies and metabolic degradation rates. Predictions made for the constituents of the technical flame retardant Bromkal 70-5DE found BDE 17 to be a potent androgen antagonist and BDE 66, which is a relevant PBDE in environmental samples, to be only a weak antagonist.

In Vitro toxicity profiling of ultrapure non-dioxin-like polychlorinated biphenyl congeners and their relative toxic contribution to PCB mixtures in humans

The toxic equivalency concept used for the risk assessment of polychlorinated biphenyls (PCBs) is based on the aryl hydrocarbon receptor (AhR)-mediated toxicity of coplanar dioxin-like (DL) PCBs. Most PCBs in the environment, however, are non-dioxin-like (NDL) PCBs that cannot adopt a coplanar structure required for AhR activation. For NDL-PCBs, no generally accepted risk concept is available because their toxicity is insufficiently characterized. Here, we systematically determined in vitro toxicity profiles for 24 PCBs regarding 10 different mechanisms of action. Prior to testing, NDL-PCB standards were purified to remove traces of DL compounds. All NDL-PCBs antagonized androgen receptor activation and inhibited gap junctional intercellular communication (GJIC). Lower chlorinated NDL-PCBs were weak estrogen receptor (ER) agonists, whereas higher chlorinated NDL-PCBs were weak ER antagonists. Several NDL-PCBs inhibited estradiol-sulfotransferase activity and bound to transthyretin (TTR) but with much weaker potencies than reported for hydroxylated PCB metabolites. AhR-mediated expression of uridine-glucuronyl transferase isozyme UGT1A6 was induced by DL-PCBs only. Hierarchical cluster analysis of the toxicity profiles yielded three separate clusters of NDL-PCBs and a fourth cluster of reference DL-PCBs. Due to small differences in relative potency among congeners, the highly abundant indicator PCBs 28, 52, 101, 118, 138, 153, and 180 also contributed most to the antiandrogenic, (anti)estrogenic, antithyroidal, tumor-promoting, and neurotoxic potencies calculated for PCB mixtures reported in human samples, whereas the most potent AhR-activating DL-PCB-126 contributed at maximum 0.2% to any of these calculated potencies. PCB-168 is recommended as an additional indicator congener, given its relatively high abundance and antiandrogenic, TTR-binding, and GJIC-inhibiting potencies.

A 28-day oral dose toxicity study enhanced to detect endocrine effects of a purified technical pentabromodiphenyl ether (pentaBDE) mixture in Wistar rats

A 28-day subacute oral toxicity study was performed in Wistar rats with a purified preparation of the commercial pentabromodiphenyl ether (pentaBDE), DE-71. The applied OECD407 protocol was enhanced for endocrine and immune parameters, and to enable benchmark dose analysis. A vehicle control group and 7 dose groups were included, which received 0.27, 0.82, 2.47, 7.4, 22.2, 66.7 or 200 mg pentaBDE/kg bw/d (mkd). The liver appeared to be a key target organ, showing a marked increase of weight and centrilobular hepatocellular hypertrophy, probably due to the observed induction of P450 enzymes, notably CYP1A and CYP2B. A marked decrease of circulating total thyroxine (TT4) and an increase of plasma cholesterol were probably secondary to the liver effects. Furthermore, dose-dependently decreased weight of epididymis, seminal vesicles, and prostate, as well as sperm head deformities in males, and induction of CYP17 activity in adrenals in females were observed, all possibly related to anti-androgenic activity. Finally, we observed a substantial increase of large unstained cells in the blood and a decrease of apolar retinoids in the liver. All these effects had benchmark doses at the lower confidence bound (BMDL) in the low- or mid-dose range, but particular sensitive, potentially adverse effects were TT4 decrease (BMDLs 1.1 in males and 1.8 mkd in females), and decrease of hepatic apolar retinoids (BMDLs 0.5 mkd in males and 2.3 mkd in females). These results contribute to refinement of the hazard identification of pentaBDE and improved risk assessment of human exposure to this industrial chemical and environmental pollutant.

Effects of metal pollution on earthworm communities in a contaminated floodplain area: Linking biomarker, community and functional responses

Effects on earthworms in the contaminated floodplain area the Biesbosch, the Netherlands, were determined at different levels of organization using a combination of field and laboratory tests. The species Lumbricus rubellus, collected from different polluted sites in the Biesbosch, showed reduced values for the biomarker neutral red retention time (NRRT), mainly explained by high metal concentrations in the soil and the resulting high internal copper concentrations in the earthworms. Organic pollutant levels in earthworms were low and did not explain reduced NRRTs. Earthworm abundance and biomass were not correlated with pollutant levels in the soil. Litterbag decomposition and bait-lamina feeding activity, measures of the functional role of earthworms, were not affected by metal pollution and did not show any correlation with metal concentrations in soil or earthworms nor with NRRT. Effects at the biochemical level therefore did not result in a reduced functioning of earthworm communities.

Programming of metabolic effects in C57BL/6JxFVB mice by exposure to bisphenol A during gestation and lactation

The global rise in prevalence of obesity is not fully explained by genetics or life style factors. The developmental origins of health and disease paradigm suggests that environmental factors during early life could play a role. In this perspective, perinatal exposure to bisphenol A (BPA) has been indicated as a programming factor for obesity and related metabolic disorders later in life. Here we study early life programming by BPA using an experimental design that is relevant for human exposure. C57BL/6JxFVB hybrid mice were exposed during gestation and lactation via maternal feed to 8 non-toxic doses (0-3000 μg/kg body weight/day (μg/kg bw/d)) of BPA. After weaning, offspring were followed for 20 weeks without further exposure. Adult male offspring showed dose-dependent increases of body and liver weights, no effects on fat pad weights and a dose-dependent decrease in circulating glucagon. Female offspring showed a dose-dependent decrease in body weight, liver, muscle and fat pad weights, adipocyte size, serum lipids, serum leptin and adiponectin. Physical activity was decreased in exposed males and suggested to be increased in exposed females. Brown adipose tissue showed slightly increased lipid accumulation in males and lipid depletion in females, and ucp1 expression was dose-dependently increased in females. The effects in females were more reliable and robust than in males due to wide confidence intervals and potential confounding by litter size for male data. The lowest derived BMDL (lower bound of the (two-sided) 90%-confidence interval for the benchmark dose) of 233 μg/kg bw/d (for interscapular weight in females) was below the proposed BMDL of 3633 μg/kg bw/d as a basis for tolerable daily intake. Although these results suggest that BPA can program for an altered metabolic phenotype, the sexual dimorphism of effects and diversity of outcomes among studies similar in design as the present study do not mark BPA as a specific obesogen. The consistency within the complex of observed metabolic effects suggests that upstream key element(s) in energy homeostasis are modified. Sex-dependent factors contribute to the final phenotypic outcome.

Transcriptional and epigenetic mechanisms underlying enhanced in vitro adipocyte differentiation by the brominated flame retardant BDE-47

Recent studies suggest that exposure to endocrine-disrupting compounds (EDCs) may play a role in the development of obesity. EDCs such as the flame retardant 2,2′,4,4′-tetrabrominated diphenyl ether (BDE-47) have been shown to enhance adipocyte differentiation in the murine 3T3-L1 model. The mechanisms by which EDCs direct preadipocytes to form adipocytes are poorly understood. Here, we examined transcriptional and epigenetic mechanisms underlying the induction of in vitro adipocyte differentiation by BDE-47. Quantitative high content microscopy revealed concentration-dependent enhanced adipocyte differentiation following exposure to BDE-47 or the antidiabetic drug troglitazone (TROG). BDE-47 modestly activated the key adipogenic transcription factor peroxisome proliferator-activated receptor gamma (PPARγ) in COS7 cells, transiently transfected with a GAL4 reporter construct. Increased gene expression was observed for Pparγ2, leptin (Lep), and glucose-6-phophatase catalytic subunit (G6pc) in differentiated 3T3-L1 cells after BDE-47 exposure compared to TROG. Methylation-sensitive high resolution melting (MS-HRM) revealed significant demethylation of three CpG sites in the Pparγ2 promoter after exposure to both BDE-47 and TROG in differentiated 3T3-L1 cells. This study shows the potential of BDE-47 to induce adipocyte differentiation through various mechanisms that include Pparγ2 gene induction and promoter demethylation accompanied by activation of PPARγ, and possible disruption of glucose homeostasis and IGF1 signaling.

The OBELIX project: early life exposure to endocrine disruptors and obesity

The hypothesis of whether early life exposure (both pre- and early postnatal) to endocrine-disrupting chemicals (EDCs) may be a risk factor for obesity and related metabolic diseases later in life will be tested in the European research project OBELIX (OBesogenic Endocrine disrupting chemicals: LInking prenatal eXposure to the development of obesity later in life). OBELIX is a 4-y project that started in May 2009 and which has the following 5 main objectives: 1) to assess early life exposure in humans to major classes of EDCs identified as potential inducers of obesity (ie, dioxin-like compounds, non-dioxin-like polychlorinated biphenyls, organochlorine pesticides, brominated flame retardants, phthalates, and perfluorinated compounds) by using mother-child cohorts from 4 European regions with different food-contaminant exposure patterns; 2) to relate early life exposure to EDCs with clinical markers, novel biomarkers, and health-effect data related to obesity; 3) to perform hazard characterization of early life exposure to EDCs for the development of obesity later in life by using a mouse model; 4) to determine mechanisms of action of obesogenic EDCs on developmental programming with in vivo and in vitro genomics and epigenetic analyses; and 5) to perform risk assessments of prenatal exposure to obesogenic EDCs in food by integrating maternal exposure through food-contaminant exposure and health-effect data in children and hazard data in animal studies.

Blood Plasma Sample Preparation Method for the Assessment of Thyroid Hormone-Disrupting Potency in Effect-Directed Analysis

A sample preparation method combining solid-phase extraction (SPE) and liquid-liquid extraction (LLE) was developed to be used in Effect-Directed Analysis (EDA) of blood plasma. Until now such a method was not available. It can be used for extraction of a broad range of thyroid hormone (TH)-disruptors from plasma with high recoveries. Validation of the method using spiked cow plasma showed good recoveries for hydroxylated polybrominated diphenyl ethers (OH-PBDEs; 93.8 ± 19.5%), hydroxylated polychlorinated biphenyls (OH-PCBs; 93.8 ± 15.5%), other halogenated phenols (OHPs; 107 ± 8.1%), and for short-chain (<8 C-atoms) perfluoroalkyl substances (PFASs; 85.2 ± 24.6%). In the same extracts, the potency of the compound classes spiked to the cow plasma to competitively bind to transthyretin (TTR) was recovered by 84.9 ± 8.8%. Furthermore, the SPE-LLE method efficiently removed endogenous THs from the extracts, thereby eliminating their possible contribution to the binding assay response. The SPE-LLE method was applied to polar bear plasma samples to investigate its applicability in future EDA studies focusing on TH-disrupting compounds in this top predator species that is exposed to relatively high levels of bioaccumulating pollutants. A first screening revealed TTR-binding potency in the polar bear plasma extracts, which could be explained for 60-85% by the presence of OH-PCBs.

Perfluoroalkyl substances in polar bear mother-cub pairs: A comparative study based on plasma levels from 1998 and 2008

Perfluoroalkyl substances (PFASs) are protein-binding blood-accumulating contaminants that may have detrimental toxicological effects on the early phases of mammalian development. To enable an evaluation of the potential health risks of PFAS exposure for polar bears (Ursus maritimus), an exposure assessment was made by examining plasma levels of PFASs in polar bear mothers in relation to their suckling cubs-of-the-year (~4 months old). Samples were collected at Svalbard in 1998 and 2008, and we investigated the between-year differences in levels of PFASs. Seven perfluorinated carboxylic acids (∑₇PFCAs: PFHpA, PFOA, PFNA, PFDA, PFUnDA, PFDoDA, and PFTrDA) and two perfluorinated sulfonic acids (∑₂PFSAs: PFHxS and PFOS) were detected in the majority of the mothers and cubs from both years. In mothers and cubs, most PFCAs were detected in higher concentrations in 2008 than in 1998. On the contrary, levels of PFOS were lower in 2008 than in 1998, while levels of PFHxS did not differ between the two sampling years. PFOS was the dominating compound in mothers and cubs both in 1998 and in 2008. Concentration of PFHpA did not differ between mothers and cubs, while concentrations of PFOA, PFNA, PFDA, PFUnDA, PFDoDA, PFTrDA, PFHxS, and PFOS were higher in mothers than in their cubs. Except from PFHpA, all compounds correlated significantly between mothers and their cubs. The mean cub to mother ratios ranged from 0.15 for PFNA to 1.69 for PFHpA. On average (mean±standard error of mean), the levels of ∑₇PFCAs and ∑₂PFSAs in cubs were 0.24±0.01 and 0.22±0.01 times the levels in their mothers, respectively. Although maternal transfer appears to be a substantial source of exposure for the cubs, the low cub to mother ratios indicate that maternal transfer of PFASs in polar bears is relatively low in comparison with hydrophobic contaminants (e.g. PCBs). Because the level of several PFASs in mothers and cubs from both sampling years exceeded the levels associated with health effects in humans, our findings raise concern on the potential health effects of PFASs in polar bears from Svalbard. Effort should be made to examine the potential health effects of PFASs in polar bears.