Development of a neurotoxic equivalence scheme of relative potency for assessing the risk of PCB mixtures

Neuroendocrine impacts of endocrine-disrupting chemicals in birds: life stage and species sensitivities

Assessing potential risk associated with exposure to endocrine-disrupting chemicals (EDC) has been difficult due to species specific variation in vulnerability and to both short- and long-term effects produced by EDC. In precocial birds, embryonic exposure to EDC impacts sexual differentiation of neuroendocrine systems and behavior. Often, detectable nonlethal effects of EDC diminish as the organism matures such that the chronic impact of EDC may appear relatively innocuous by the time an individual is sexually mature. In addition, studies have not addressed lifetime effects of EDC exposure on birds. Consequently, it is difficult to assess chronic effects of nonlethal exposure on the fitness of an individual and whether there is a potential risk to a wild population. Assessing behavioral and neuroendocrine consequences of exposure is complicated by individual and species variation in sensitivity as well as exposure to complex mixtures. Our studies are designed to examine effects of individual EDC administered to the embryo as well as in a multigenerational dietary study in which birds received low doses of the pesticide methoxychlor (MXC). The influence of dietary MXC exposure was also compared between Japanese quail and northern bobwhite quail. The effects of dietary exposures to 0.5, 5, or 10 ppm that are relatively environmentally low were determined. The selection of these doses was to mimic levels that might be encountered in the field and higher doses that might potentially reveal effects of exposure at relatively low exposures. These doses were also based on the regulations by the U.S. Environmental Protection Agency that mandate a limit 0.04 ppm MXC in drinking water, with a limit of no more than 0.05 ppm in water that children drink. Further there is a limit of 1-100 ppm for crops and other food for human and livestock consumption; bottled water has a 0.1 ppm limit for MXC content. Our data are discussed in the context of applicability of toxicological yardsticks, including the toxic equivalent (TEQ) and neurotoxic equivalent (NEQ) as predictive indices for short- and long-term outcomes to nonlethal concentrations of EDC. Other approaches have been developed to address inconsistencies in effects and incorporate diverse data into potency estimates. Perhaps it is time to develop a more inclusive estimation method for endocrine and neuroendocrine effects. An endocrine disruption index (EDI) would (1) complement other indices, (2) focus on endocrine disruption, and (3) include effects beyond those mediated by the aryl hydrocarbon receptor (AhR) for a comparative assessment of nonlethal EDC effects.

A review of the tissue residue approach for organic and organometallic compounds in aquatic organisms

This paper reviews the tissue residue approach (TRA) for toxicity assessment as it applies to organic chemicals and some organometallic compounds (Sn, Hg, and Pb) in aquatic organisms. Specific emphasis was placed on evaluating key factors that influence interpretation of critical body residue (CBR) toxicity metrics including data quality issues, lipid dynamics, choice of endpoints, processes that alter toxicokinetics and toxicodynamics, phototoxicity, species- and life stage-specific sensitivities, and biotransformation. The vast majority of data available on TRA is derived from laboratory studies of acute lethal responses to organic toxicants exhibiting baseline toxicity. Application of the TRA to various baseline toxicants as well as substances with specific modes of action via receptor-mediated processes, such as chlorinated aromatic hydrocarbons, pesticides, and organometallics is discussed, as is application of TRA concepts in field assessments of tissue residues. In contrast to media-based toxicity relationships, CBR values tend to be less variable and less influenced by factors that control bioavailability and bioaccumulation, and TRA can be used to infer mechanisms of toxic action, evaluate the toxicity of mixtures, and interpret field data on bioaccumulated toxicants. If residue-effects data are not available, body residues can be estimated, as has been done using the target lipid model for baseline toxicants, to derive critical values for risk assessment. One of the primary unresolved issues complicating TRA for organic chemicals is biotransformation. Further work on the influence of biotransformation, a better understanding of contaminant lipid interactions, and an explicit understanding of the time dependency of CBRs and receptor-mediated toxicity are all required to advance this field. Additional residue-effects data on sublethal endpoints, early life stages, and a wider range of legacy and emergent contaminants will be needed to improve the ability to use TRA for organic and organometallic compounds.

Development of a synthetic PCB mixture resembling the average polychlorinated biphenyl profile in Chicago air

Studies of environmental and toxic effects of polychlorinated biphenyls (PCBs) are ideally performed with PCB mixtures reflecting the composition of environmental PCB profiles to mimic actual effects and to account for complex interactions among individual PCB congeners. Unfortunately, only a few laboratory studies employing synthetic PCB mixtures have been reported, in part because of the challenges associated with the preparation of complex PCB mixtures containing many individual PCB congeners. The objective of this study was to develop a PCB mixture that resembles the average PCB profile recorded from 1996 to 2002 at a satellite station of the Integrated Atmospheric Deposition Network located at the Illinois Institute of Technology (IIT) in Chicago, Illinois, using commercial PCB mixtures. Initial simulations, using published Aroclor profiles, showed that a mixture containing 65% Aroclor 1242 and 35% Aroclor 1254 was a good approximation of the target profile. A synthetic Chicago air mixture (CAM) was prepared by mixing the respective Aroclors in this ratio, followed by GC/MS/MS analysis. Comparison of the PCB profile of the synthetic mixture with the target profile suggests that the synthetic PCB mixture is a good approximation of the average IIT Chicago air profiles (similarity coefficient cos θ = 0.82; average relative percent difference = 84%). The synthetic CAM was also a reasonable approximation of the average of 184 PCB profiles analyzed in 2007 at 37 sites throughout Chicago as part of the University of Iowa Superfund Basic Research Program (isbrp), with a cos θ of 0.70 and an average relative percent difference of 118%. While the CAM and the two Chicago air profiles contained primarily di- to pentachlorobiphenyls, higher chlorinated congeners, including congeners with seven or eight chlorine atoms, were underrepresented in the synthetic CAM. The calculated TCDD toxic equivalency quotients of the synthetic CAM (2.7 ng/mg PCB) and the IIT Chicago air profile (1.6 ng/mg PCB) were comparable, but lower by two orders of magnitude than the isbrp Chicago air profile (865 ng/mg PCB) due to surprisingly high PCB 126 levels in Chicago air. In contrast, the calculated neurotoxic equivalency quotients of the CAM (0.33 mg/mg PCB) and the two Chicago air profiles (0.44 and 0.30 mg/mg PCB, respectively) were similar. This study demonstrates the challenges and methods of creating and characterizing synthetic, environmental mixtures of PCBs.

In utero and lactational exposure to a complex mixture of polychlorinated biphenyls: toxicity in pups dependent on the Cyp1a2 and Ahr genotypes

Polychlorinated biphenyls (PCBs) are persistent toxic pollutants occurring as complex mixtures in the environment. Humans are known genetically to have > 60-fold differences in hepatic cytochrome P450 1A2 (CYP1A2) levels and > 12-fold differences in aryl hydrocarbon receptor (AHR) affinity, both of which could affect PCB pharmacokinetics. Thus, we compared Ahr(b1)_Cyp1a2(+/+) high-affinity AHR wild-type, Ahr(d)_Cyp1a2(+/+) poor affinity AHR wild-type, Ahr(b1)_Cyp1a2(-/-) knockout, and Ahr(d)_Cyp1a2(-/-) knockout mouse lines. We chose a mixture of three coplanar and five noncoplanar PCBs to reproduce that seen in human tissues, breast milk, and the food supply. The mixture was given by gavage to the mother on gestational day 10.5 (GD10.5) and postnatal day 5 (PND5); tissues were collected from pups and mothers at GD11.5, GD18.5, PND6, PND13, and PND28. Ahr(b1)_Cyp1a2(-/-) pups showed lower weight at birth and slower rate of growth postnatally. Absence of CYP1A2 resulted in significant splenic atrophy at PND13 and PND28. Presence of high-affinity AHR enhanced thymic atrophy and liver hypertrophy in the pups. Concentrations of each congener were analyzed at all time points: maximal noncoplanar congener levels in maternal tissues were observed from GD18 until PND6, whereas the highest levels in pups were found between PND6 and PND28. Coplanar PCB concentrations were generally higher in Ahr(d)-containing pup tissues; these findings are consistent with earlier studies demonstrating the crucial importance of AHR-mediated inducible CYP1 in the gastrointestinal tract as a means of detoxication of oral planar polycyclic aromatic hydrocarbons.

Neurodevelopmental toxicity of prenatal polychlorinated biphenyls (PCBs) by chemical structure and activity: a birth cohort study

BACKGROUND

Polychlorinated biphenyls (PCBs) are ubiquitous environmental toxins. Although there is growing evidence to support an association between PCBs and deficits of neurodevelopment, the specific mechanisms are not well understood. The potentially different roles of specific PCB groups defined by chemical structures or hormonal activities e.g., dioxin-like, non-dioxin like, or anti-estrogenic PCBs, remain unclear. Our objective was to examine the association between prenatal exposure to defined subsets of PCBs and neurodevelopment in a cohort of infants in eastern Slovakia enrolled at birth in 2002-2004.

METHODS

Maternal and cord serum samples were collected at delivery, and analyzed for PCBs using high-resolution gas chromatography. The Bayley Scales of Infant Development -II (BSID) were administered at 16 months of age to over 750 children who also had prenatal PCB measurements.

RESULTS

Based on final multivariate-adjusted linear regression model, maternal mono-ortho-substituted PCBs were significantly associated with lower scores on both the psychomotor (PDI) and mental development indices (MDI). Also a significant association between cord mono-ortho-substituted PCBs and reduced PDI was observed, but the association with MDI was marginal (p = 0.05). Anti-estrogenic and di-ortho-substituted PCBs did not show any statistically significant association with cognitive scores, but a suggestive association between di-ortho-substituted PCBs measured in cord serum and poorer PDI was observed.

CONCLUSION

Children with higher prenatal mono-ortho-substituted PCB exposures performed more poorly on the Bayley Scales. Evidence from this and other studies suggests that prenatal dioxin-like PCB exposure, including mono-ortho congeners, may interfere with brain development in utero. Non-dioxin-like di-ortho-substituted PCBs require further investigation.

PCBs in school-persistent chemicals, persistent problems

The issue of polychlorinated biphenyls (PCB) exposures resulting from occupancy of PCB-contaminated buildings is not new, but the contribution of building materials to that contamination is largely unrecognized. A rapidly emerging base of evidence shows that PCBs can be widely found in caulking and paint in masonry buildings constructed or renovated from about 1950 to the late 1970s. These materials can cause extensive PCB contamination of the building interiors and surrounding soil, and people who teach, live, or attend school in these buildings can have elevated serum PCB levels. The potential risk associated with this source of PCB exposure is not known; however, it is worth noting that the specific PCB congeners found at high levels in the building environments, and in biological samples from the occupants, include some that are suspected of being potent neurotoxins. The U. S. Environmental Protection Agency (EPA) is moving to address this issue in schools; however, the costs of remediating contaminated buildings will pose a formidable obstacle to most school districts.

Tissue distribution of polychlorinated biphenyls and organochlorine pesticides and potential toxicity to Alaskan northern fur seals assessed using PCBs congener specific mode of action schemes

The fur seal (Callorhinus ursinus) population has decreased in their primary breeding grounds in the Bering Sea; contamination is among suspected causes. Our goal was to better understand the extent of contamination of seal tissues with certain organochlorine compounds by measuring the concentrations of polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) in fur seal tissues from St. Paul Island, to gain a better perspective of tissue congener distribution and to evaluate the observed PCB levels against toxicologically significant levels for modes of action. Concentrations of 145 PCB congeners (Sigma(145)PCBs) and 12 OCPs were measured with gas chromatography-ion trap mass spectrometry in 8 different tissues of 10 male northern fur seals. The mean concentrations of SigmaOCPs [in ng/g lipid weight (lw)] were 1180 in blubber, 985 in the heart, 1007 in the liver, 817 in the kidney, 941 in muscle, 660 in reproductive tissues, 204 in the brain, and 322 in the lung. The mean concentrations of Sigma(145)PCBs (in ng/g lw) were 823 in blubber, 777 in the liver, 732 in the heart, 646 in reproductive tissues, 638 in muscle, 587 in the kidney, 128 in the lung, and 74.3 in brain tissues. Concentrations of PCBs affecting the aryl hydrocarbon receptor expressed as total PCB toxic equivalents (SigmaPCB-TEQs) ranged from 0.3 to 545 pg/g lw for the various tissues. The major contributors to SigmaPCB-TEQs are CB-118 in muscle, brain, lung, kidney, and liver, CB-126 in blubber, and CB-118 and CB-126 equally in the heart and reproductive tissues. Concentrations of PCBs affecting Ca(2+) homeostatsis expressed as the neurotoxic equivalent (NEQ) showed SigmaPCB-NEQs ranged from 17.7 to 215 ng/g lw in all tissues. Although no composite measure of perturbation of thyroid function is available, sufficient amounts of congeners with high binding to the thyroxine transport system were present to warrant consideration of this mode of action in future studies. Analyses of 145 PCBs and mode of action evaluation suggest that PCB contamination could potentially exert an effect on the Alaskan northern fur seal population although the PCB concentrations have been decreasing in the fur seals over the last decade.

Quantitative structure-activity relationship (QSAR) studies for predicting activation of the ryanodine receptor type 1 channel complex (RyR1) by polychlorinated biphenyl (PCB) congeners

A quantitative structure-activity relationship (QSAR) was developed to predict the congener specific ryanodine receptor type RyR1 activity of all 209 polychlorinated biphenyl (PCB) congeners. A three-variable QSAR equation was obtained via stepwise forward linear regression on an unsupervised forward selection reduced data set from an initial database. Application of the QSAR towards predicting EC(2x) values for all 209 PCB congeners indicated good agreement in substitution pattern trends between the experimental and estimated data sets. The QSAR model predicts a less than two-fold increase in maximal potency among all congeners outside the experimental database, and it appears that no high-potency PCB congeners with EC(2x) values much less than 0.2 microM exist. Increasing RyR1-neuro toxicity equivalents with increasing homologue number and Aroclor chlorination likely reflect indirect molecular controls on toxicity, since congeners with multiple ortho substituents-the primary structural feature controlling a lack of coplanarity and resulting neurotoxicity-are more likely to be found in higher homologues.

Aroclor 1254 induced cytotoxicity and mitochondrial dysfunction in isolated rat hepatocytes

Polychlorinated biphenyls (PCBs) are widespread persistent environmental contaminants that display a complex spectrum of toxicological properties, including hepatotoxicity. Although Aroclor 1254 is ubiquitous in the environment, its potential cytotoxic effect on rat hepatocytes and the mechanism underlines its cytotoxicity are not fully investigated. Therefore, the present study was conducted to investigate: (1) the potential cytotoxicity of Aroclor 1254 in rat hepatocytes, and (2) characterization of the molecular mechanisms involved in the Aroclor 1254-induced hepatotoxicity, particularly the role of mitochondria, possibly a primary target in such event, could greatly explain the cytotoxic effect of Aroclor 1254 in rat hepatocytes. Hepatocytes were isolated from adult male albino rats and incubated for 24h in a fresh media containing 0, 20, 30, 40, 50 or 60muM of Aroclor 1254. At the end of incubation, hepatocytes and hepatocyte mitochondria were used for the assay. Our results showed cytotoxicity of Aroclor 1254 in rat hepatocytes starting at a concentration of 30muM as manifested by increased lactate dehydrogenase (LDH) leakage, decreased cell viability (MTT assay) and increased lipid peroxidation. As mitochondria are known to be one possible site of the cell damage, the effects of Aroclor 1254 on hepatocyte mitochondria was investigated. Aroclor 1254 induced reactive oxygen species (ROS) generation in hepatocyte mitochondria, inhibited mitochondrial respiratory chain complexes I and III and beta-oxidation of free fatty acids, depletion of mitochondrial antioxidant enzymes GPx and GR and the non-enzymatic antioxidant reduced glutathione, inhibited mitochondrial membrane potential (Deltapsi(m)), decreased mitochondrial aconitase and cardiolipin content, and elevated levels of cytochrome P450 subfamily, CYP1A and CYP2B activities as indicated by ethoxyresorufin O-deethylase (EROD) and pentoxyresorufin O-deethylase (PROD). Therefore, we can conclude that Aroclor 1254 induced rat hepatocyte toxicity and our findings provide evidence to propose that mitochondria are one of the most important and earliest cell targets in Aroclor 1254-mediated toxicity and delineate several mitochondrial processes at least, in part, by induction of oxidative stress. These findings can be useful in future cytoprotective therapy approaches. Since mitochondrial events appear to be targeted in hepatocellular damage induced by Aroclor 1254, an antioxidant therapy targeted to mitochondria may constitute an interesting strategy to ameliorate its toxicity.

PCB body burdens in US women of childbearing age 2001-2002: An evaluation of alternate summary metrics of NHANES data

An extensive body of epidemiologic data associates prenatal exposure to polychlorinated biphenyls (PCBs) with neurodevelopmental deficits and other childhood health effects. Neurological effects and other adverse health effects may also result from exposure during infancy, childhood, and adulthood. Although manufacture and use of PCBs were banned in the US in 1977, exposure to PCBs is a continuing concern due to the widespread distribution of these compounds in the environment and their persistence. The National Health and Nutrition Examination Survey provides PCB body burden measurements representative of the US population for the years 1999-2002. Interpretation of these data is challenging due to the large number of PCB congeners reported. We examined 6 PCB body burden metrics to identify an approach for summarizing the NHANES data and for characterizing changes over time in potential risks to children's health. We focused on women of childbearing age, defined here as 16-39 years, because in utero exposures have been associated with neurodevelopmental effects, and used only the 2001-2002 data because of higher detection rates. The 6 metrics, each consisting of different combinations of the 9 most frequently detected congeners, were as follows: total PCBs (all 9 congeners); highly chlorinated PCBs (2 congeners); dioxin-like PCBs (3 congeners, weighted by toxic equivalency factors); non-dioxin-like PCBs (6 congeners); a 4-congener metric (PCBs 118, 138, 153, and 180); and PCB-153 alone. The PCB metrics were generally highly correlated with each other. There was a strong association of PCB body burdens with age for all metrics. Median body burdens of Mexican American women were lower than those of non-Hispanic White and non-Hispanic Black women for 5 of the 6 metrics, and there were no significant differences in body burdens between the latter two groups. Body burdens of women with incomes above poverty level were greater than those for lower-income women at the median and 95th percentiles, but the differences were not statistically significant for any metric. We conclude that the 4-congener and total PCBs metrics are the most promising approaches for tracking changes in body burdens over time and for comparing body burdens of different subgroups in NHANES.

Identification of the functional domain of thyroid hormone receptor responsible for polychlorinated biphenyl-mediated suppression of its action in vitro

BACKGROUND

Polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins, and poly-chlorinated dibenzofurans adversely affect the health of humans and various animals. Such effects might be partially exerted through the thyroid hormone (TH) system. We previously reported that one of the hydroxylated PCB congeners suppresses TH receptor (TR)-mediated transcription by dissociating TR from the TH response element (TRE). However, the binding site of PCB within TR has not yet been identified.

OBJECTIVES

We aimed to identify the functional TR domain responsible for the PCB-mediated suppression of TR action by comparing the magnitude of suppression using several representative PCB/dioxin congeners.

MATERIALS AND METHODS

We generated chimeric receptors by combining TR and glucocorticoid receptor (GR) and determined receptor-mediated transcription using transient transfection-based reporter gene assays, and TR-TRE binding using electrophoretic mobility shift assays.

RESULTS

Although several PCB congeners, including the hydroxylated forms, suppressed TR-mediated transcription to various degrees, 2,3,7,8-tetrachlorodibenzo-p-dioxin did not alter TR action, but 2,3,4,7,8-pentachlorodibenzofuran weakly suppressed it. The magnitude of suppression correlated with that of TR-TRE dissociation. The suppression by PCB congeners was evident from experiments using chimeric receptors containing a TR DNA-binding domain (DBD) but not a GR-DBD.

CONCLUSIONS

Several nondioxin-like PCB congeners and hydroxylated PCB compounds suppress TR action by dissociating TR from TRE through interaction with TR-DBD.

Extensive biodegradation of polychlorinated biphenyls in Aroclor 1242 and electrical transformer fluid (Askarel) by natural strains of microorganisms indigenous to contaminated African systems

Evidence for substantial aerobic degradation of Aroclor 1242 and Askarel fluid by newly characterized bacterial strains belonging to the Enterobacter, Ralstonia and Pseudomonas genera is presented. The organisms exhibited degradative activity in terms of total PCB/Askarel degradation, degradation of individual congeners and diversity of congeners attacked. Maximal degradation by the various isolates of Askarel ranged from 69% to 86% whereas, Aroclor 1242, with the exception of Ralstonia sp. SA-4 (9.7%), was degraded by 37% to 91%. PCB analysis showed that at least 45 of the representative congeners in Aroclor 1242 were extensively transformed by benzoate-grown cells without the need for biphenyl as an inducer of the upper degradation pathway. In incubations with Aroclor 1242, no clear correlation was observed between percentage of congener transformed and the degree of chlorination, regardless of the presence or absence of biphenyl. Recovery of significant but nonstoichiometric amounts of chloride from the culture media showed partial dechlorination of congeners and suggested production of partial degradation products. Addition of biphenyl evidently enhanced dechlorination of the mixture by some isolates. With the exception of Ralstonia sp. SA-5, chloride released ranged from 24% to 60% in the presence of biphenyl versus 0.35% to 15% without biphenyl.

Alteration of gene expression profiles in the brain of Japanese medaka (Oryzias latipes) exposed to KC-400 or PCB126

Polychlorinated biphenyls (PCBs) are known as neurotoxic chemicals and possibly alter animal behavior. We previously reported that PCB-exposure induced abnormal schooling behavior in Japanese medaka (Oryzias latipes). This abnormal behavior might be caused by the functional alteration of central or terminal nervous system. To understand the mechanism(s) of behavioral change by PCB-exposure, we analyzed the gene expression profiles in the brain of medaka exposed to 3,3',4,4',5-pentachlorobiphenyl (PCB126) or a PCB mixture (Kanechlor-400: KC-400) using a cDNA microarray that we constructed. Twelve FLF-II strain medaka (six individuals per treatment) were dietary exposed to PCB126 (0.01 microg/g b.w./day) or KC-400 (1 microg/g b.w./day) for three weeks. For the control, six fish were fed a control diet. After the exposure period, fish were dissected, and the brain samples were collected. The samples from control fish were pooled and used as a common reference in the microarray experiment. Microarray data were normalized by the LOWESS method, and we screened the genes whose expression levels were altered more than 1.5-fold. Gene expression profiling showed 97 down-regulated and 379 up-regulated genes in the brain of medaka exposed to PCB126. KC-400 exposure suppressed 15 genes and induced 266 genes in medaka brain. Among these genes, the expression levels of 7 and 188 genes were commonly down- or up-regulated, respectively in both treatment groups. On the other hand, 31 gene expressions were significantly different between PCB126 and KC-400 treatment groups, and three out of 31 genes were received opposite effects. In addition, the microarray data showed that thyroid hormone-responsive genes were up-regulated by PCB-exposure, which may imply that PCBs or their metabolites mimic thyroid hormone effects in the brain of PCB-exposed medaka.