Synergistic effect of 2,2',4,4',5,5'-hexachlorobiphenyl and 2,3,7,8-tetrachlorodibenzo-p-dioxin on hepatic porphyrin levels in the rat

Windows of sensitivity to toxic chemicals in the development of the endocrine system: an analysis of ATSDR's toxicological profile database

This review utilizes the robust database of literature contained in toxicological profiles developed by the Agency for Toxic Substances and Disease Registry. The aim was to use this database to identify developmental toxicity studies reporting alterations in hormone levels in the developing fetus and offspring and identify windows of sensitivity. We identified 74 oral exposure studies in rats that provided relevant information on 30 chemicals from 21 profiles. Most studies located provided information on thyroid hormones, with fewer studies on anterior pituitary, adrenal medulla, ovaries, and testes. No studies pertaining to hormones of the posterior pituitary, pancreas, or adrenal cortex were located. The results demonstrate that development of the endocrine system may be affected by exposure to environmental contaminants at many different points, including gestational and/or lactational exposure. Moreover, this review demonstrates the need for more developmental toxicity studies focused on the endocrine system and specifically alterations in hormone levels.

Polychlorinated Biphenyls Induce Oxidative DNA Adducts in Female Sprague-Dawley Rats

Polychlorinated biphenyls (PCBs) are organic chemicals that were traditionally produced and widely used in industry as mixtures and are presently formed as byproducts of pigment and dye manufacturing. They are known to persist and bioaccumulate in the environment. Some have been shown to induce liver cancer in rodents. Although the mechanism of the toxicity of PCBs is unknown, it has been shown that they increase oxidative stress, including lipid peroxidation. We hypothesized that oxidative stress-induced DNA damage could be a contributor for PCB carcinogenesis and analyzed several DNA adducts in female Sprague-Dawley rats exposed to 3,3',4,4',5-pentachlorobiphenyl (PCB 126), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), and a binary mixture (PCB 126 + 153) for 14, 31, and 53 wks. Eight adducts were measured to profile oxidative DNA lesions, including 8-oxo-deoxyguanosine (8-oxo-dG), 1,N(6)-ethenodeoxyadenosine (1,N(6)-εdA), N(2),3-ethenoguanine (N(2),3-εG), 1,N(2)-ethenodeoxyguanosine (1,N(2)-εdG), as well as malondialdehyde (M1dG), acrolein (AcrdG), crotonaldehyde (CrdG), and 4-hydroxynonenal-derived dG adducts (HNEdG) by LC-MS/MS analysis. Statistically significant increases were observed for 8-oxo-dG and 1,N(6)-εdA concentrations in hepatic DNA of female rats exposed to the binary mixture (1000 ng/kg/day + 1000 μg/kg/day) but not in rats exposed to PCB 126 (1000 ng/kg/day) or PCB 153 (1000 μg/kg/day) for 14 and 31 wks. However, exposure to PCB 126 (1000 ng/kg/day) for 53 wks significantly increased 8-oxo-dG, 1,N(6)-εdA, AcrdG, and M1dG. Exposure to PCB 153 (1000 μg/kg/day) for 53 wks increased 8-oxo-dG, and 1,N(6)-εdA. Exposure to the binary mixture for 53 wks increased 8-oxo-dG, 1,N(6)-εdA, AcrdG, 1,N(2)-εdG, and N(2),3-εG significantly above control groups. Increased hepatic oxidative DNA adducts following exposure to PCB 126, PCB 153, or the binary mixture shows that an increase in DNA damage may play an important role in hepatic toxicity and carcinogenesis in female Sprague-Dawley rats.

Non-additive hepatic gene expression elicited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) co-treatment in C57BL/6 mice

Interactions between environmental contaminants can lead to non-additive effects that may affect the toxicity and risk assessment of a mixture. Comprehensive time course and dose-response studies with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), non-dioxin-like 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) and their mixture were performed in immature, ovariectomized C57BL/6 mice. Mice were gavaged once with 30 μg/kg TCDD, 300 mg/kg PCB153, a mixture of 30 μg/kg TCDD with 300 mg/kg PCB153 (MIX) or sesame oil vehicle for 4,12, 24,72 or 168 h. In the 24h dose-response study, animals were gavaged with TCDD (0.3,1, 3, 6, 10, 15, 30, 45 μg/kg), PCB153 (3,10, 30, 60, 100, 150, 300, 450 mg/kg), MIX (0.3+3, 1+10, 3+30, 6+60, 10+100, 15+150, 30+300, 45 μg/kg TCDD+450 mg/kg PCB153, respectively) or vehicle. All three treatments significantly increased relative liver weights (RLW), with MIX eliciting significantly greater increases compared to TCDD and PCB153 alone. Histologically, MIX induced hepatocellular hypertrophy, vacuolization, inflammation, hyperplasia and necrosis, a combination of TCDD and PCB153 responses. Complementary lipid analyses identified significant increases in hepatic triglycerides in MIX and TCDD samples, while PCB153 had no effect on lipids. Hepatic PCB153 levels were also significantly increased with TCDD co-treatment. Microarray analysis identified 167 TCDD, 185 PCB153 and 388 MIX unique differentially expressed genes. Statistical modeling of quantitative real-time PCR analysis of Pla2g12a, Serpinb6a, Nqo1, Srxn1, and Dysf verified non-additive expression following MIX treatment compared to TCDD and PCB153 alone. In summary, TCDD and PCB153 co-treatment elicited specific non-additive gene expression effects that are consistent with RLW increases, histopathology, and hepatic lipid accumulation.

Effect of PCBs on the lactational transfer of methyl mercury in mice: PBPK modeling

MeHg and PCB exposure to lactating mice were analyzed and a physiologically-based pharmacokinetic (PBPK) model was developed to describe the lactational transfer of MeHg in mice. The influence of albumin on the lactational transfer of MeHg was incorporated into the PBPK model. Experimental results with lactating mice and their pups showed that co-exposure with PCB congeners increased the lactational transfer of MeHg to the pups, which was associated with the rise of albumin levels in maternal blood. Observed results were matched with PBPK model simulations conducted under the assumptions that (1) MeHg bound to plasma albumin is transferred to maternal milk, and (2) PCB congeners may increase the lactational transfer of MeHg by escalating albumin levels in maternal blood. Further refinement of PBPK model quantitatively described the pharmacokinetic changes of MeHg by co-exposure with PCBs in pup's tissues.

Accumulation of M1dG DNA adducts after chronic exposure to PCBs, but not from acute exposure to polychlorinated aromatic hydrocarbons

Oxidative DNA damage is one of the key events thought to be involved in mutation and cancer. The present study examined the accumulation of M1dG, 3-(2'-deoxy-beta-D-erythro-pentofuranosyl)-pyrimido[1,2-a]-purin-10(3H)-one, DNA adducts after single dose or 1-year exposure to polyhalogenated aromatic hydrocarbons (PHAH) in order to evaluate the potential role of oxidative DNA damage in PHAH toxicity and carcinogenicity. The effect of PHAH exposure on the number of M1dG adducts was explored initially in female mice exposed to a single dose of either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or a PHAH mixture. This study demonstrated that a single exposure to PHAH had no significant effect on the number of M1dG adducts compared to the corn oil control group. The role of M1dG adducts in polychlorinated biphenyl (PCB)-induced toxicity and carcinogenicity was further investigated in rats exposed for a year to PCB 153, PCB 126, or a mixture of the two. PCB 153, at doses up to 3000 microg/kg/day, had no significant effect on the number of M1dG adducts in liver and brain tissues from the exposed rats compared to controls. However, 1000 ng/kg/day of PCB 126 resulted in M1dG adduct accumulation in the liver. More importantly, coadministration of equal proportions of PCB 153 and PCB 126 resulted in dose-dependent increases in M1dG adduct accumulation in the liver from 300 to 1000 ng/kg/day of PCB 126 with 300-1000 microg/kg/day of PCB 153. Interestingly, the coadministration of different amounts of PCB 153 with fixed amounts of PCB 126 demonstrated more M1dG adduct accumulation with higher doses of PCB 153. These results are consistent with the results from cancer bioassays that demonstrated a synergistic effect between PCB 126 and PCB 153 on toxicity and tumor development. In summary, the results from the present study support the hypothesis that oxidative DNA damage plays a key role in toxicity and carcinogenicity following long-term PCB exposure.

The 2005 World Health Organization reevaluation of human and Mammalian toxic equivalency factors for dioxins and dioxin-like compounds

In June 2005, a World Health Organization (WHO)-International Programme on Chemical Safety expert meeting was held in Geneva during which the toxic equivalency factors (TEFs) for dioxin-like compounds, including some polychlorinated biphenyls (PCBs), were reevaluated. For this reevaluation process, the refined TEF database recently published by Haws et al. (2006, Toxicol. Sci. 89, 4-30) was used as a starting point. Decisions about a TEF value were made based on a combination of unweighted relative effect potency (REP) distributions from this database, expert judgment, and point estimates. Previous TEFs were assigned in increments of 0.01, 0.05, 0.1, etc., but for this reevaluation, it was decided to use half order of magnitude increments on a logarithmic scale of 0.03, 0.1, 0.3, etc. Changes were decided by the expert panel for 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.3), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.03), octachlorodibenzo-p-dioxin and octachlorodibenzofuran (TEFs = 0.0003), 3,4,4',5-tetrachlorbiphenyl (PCB 81) (TEF = 0.0003), 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) (TEF = 0.03), and a single TEF value (0.00003) for all relevant mono-ortho-substituted PCBs. Additivity, an important prerequisite of the TEF concept was again confirmed by results from recent in vivo mixture studies. Some experimental evidence shows that non-dioxin-like aryl hydrocarbon receptor agonists/antagonists are able to impact the overall toxic potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds, and this needs to be investigated further. Certain individual and groups of compounds were identified for possible future inclusion in the TEF concept, including 3,4,4'-TCB (PCB 37), polybrominated dibenzo-p-dioxins and dibenzofurans, mixed polyhalogenated dibenzo-p-dioxins and dibenzofurans, polyhalogenated naphthalenes, and polybrominated biphenyls. Concern was expressed about direct application of the TEF/total toxic equivalency (TEQ) approach to abiotic matrices, such as soil, sediment, etc., for direct application in human risk assessment. This is problematic as the present TEF scheme and TEQ methodology are primarily intended for estimating exposure and risks via oral ingestion (e.g., by dietary intake). A number of future approaches to determine alternative or additional TEFs were also identified. These included the use of a probabilistic methodology to determine TEFs that better describe the associated levels of uncertainty and "systemic" TEFs for blood and adipose tissue and TEQ for body burden.

Differential effects of two lots of aroclor 1254 on enzyme induction, thyroid hormones, and oxidative stress

Aroclor 1254 is a commercial mixture of polychlorinated biphenyls (PCBs), which is defined as being 54% chlorine by weight. However, the congener composition varies from lot to lot. Two lots which have been used in toxicity studies, 124-191 and 6024 (AccuStandard), were analyzed for their congener composition. Lot 6024 has approximately 10 times the dioxin toxic equivalents (TEQ) of lot 124-191. The purpose of this study was to determine if the difference in the TEQ of the two lots explains the different in vivo responses seen on a weight basis. Male Long-Evans rats (70 days old) were treated orally with a single dose of 0-1,000 mg/kg of each lot. Hepatic ethoxy-, methoxy-, and pentoxyresorufin O-deethylase (EROD, MROD, and PROD, respectively) activities as well as serum thyroxine (T(4)) concentrations and measures of oxidative stress were determined 4 days after treatment. Results, on a weight basis, indicate that lot 6024 led to a greater induction of EROD, MROD, and PROD but not total T(4) reduction. The differences in TEQ between the lots explained the differential induction of EROD and MROD but did not account for the induction of PROD nor decreases in T(4). PROD induction is not due to dioxin-like congeners, whereas the decrease in serum T(4) levels may involve multiple mechanisms. Effects on the antioxidants ascorbic acid and uric acid were seen only at the highest mass dose for both lots and were not explained by the difference in TEQ. These results illustrate that the differences in the TEQ explain the differences in the strict dioxin-like effects (EROD, MROD induction), but the non-dioxin-like congeners cause other effects that are not associated with the aryl hydrocarbon receptor (e.g., PROD). In addition, supra-additive effects also occur in the mixture (T(4), oxidative stress). Thus, current results demonstrate that overall toxicity cannot be predicted on the basis of the TEQ values. It is also critical that the lot number is reported in studies conducted with Aroclor 1254 because the congener composition and therefore the effects observed can be very different.

Toxic equivalency factors (TEFs) for PCBs, PCDDs, PCDFs for humans and wildlife

An expert meeting was organized by the World Health Organization (WHO) and held in Stockholm on 15-18 June 1997. The objective of this meeting was to derive consensus toxic equivalency factors (TEFs) for polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and dioxinlike polychlorinated biphenyls (PCBs) for both human, fish, and wildlife risk assessment. Based on existing literature data, TEFs were (re)evaluated and either revised (mammals) or established (fish and birds). A few mammalian WHO-TEFs were revised, including 1,2,3,7,8-pentachlorinated DD, octachlorinated DD, octachlorinated DF, and PCB 77. These mammalian TEFs are also considered applicable for humans and wild mammalian species. Furthermore, it was concluded that there was insufficient in vivo evidence to continue the use of TEFs for some di-ortho PCBs, as suggested earlier by Ahlborg et al. [Chemosphere 28:1049-1067 (1994)]. In addition, TEFs for fish and birds were determined. The WHO working group attempted to harmonize TEFs across different taxa to the extent possible. However, total synchronization of TEFs was not feasible, as there were orders of a magnitude difference in TEFs between taxa for some compounds. In this respect, the absent or very low response of fish to mono-ortho PCBs is most noticeable compared to mammals and birds. Uncertainties that could compromise the TEF concept were also reviewed, including nonadditive interactions, differences in shape of the dose-response curve, and species responsiveness. In spite of these uncertainties, it was concluded that the TEF concept is still the most plausible and feasible approach for risk assessment of halogenated aromatic hydrocarbons with dioxinlike properties.

Toxic equivalency factors (TEFs) for PCBs, PCDDs, PCDFs for humans and wildlife

An expert meeting was organized by the World Health Organization (WHO) and held in Stockholm on 15-18 June 1997. The objective of this meeting was to derive consensus toxic equivalency factors (TEFs) for polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and dioxinlike polychlorinated biphenyls (PCBs) for both human, fish, and wildlife risk assessment. Based on existing literature data, TEFs were (re)evaluated and either revised (mammals) or established (fish and birds). A few mammalian WHO-TEFs were revised, including 1,2,3,7,8-pentachlorinated DD, octachlorinated DD, octachlorinated DF, and PCB 77. These mammalian TEFs are also considered applicable for humans and wild mammalian species. Furthermore, it was concluded that there was insufficient in vivo evidence to continue the use of TEFs for some di-ortho PCBs, as suggested earlier by Ahlborg et al. [Chemosphere 28:1049-1067 (1994)]. In addition, TEFs for fish and birds were determined. The WHO working group attempted to harmonize TEFs across different taxa to the extent possible. However, total synchronization of TEFs was not feasible, as there were orders of a magnitude difference in TEFs between taxa for some compounds. In this respect, the absent or very low response of fish to mono-ortho PCBs is most noticeable compared to mammals and birds. Uncertainties that could compromise the TEF concept were also reviewed, including nonadditive interactions, differences in shape of the dose-response curve, and species responsiveness. In spite of these uncertainties, it was concluded that the TEF concept is still the most plausible and feasible approach for risk assessment of halogenated aromatic hydrocarbons with dioxinlike properties.

Hexachlorobenzene as a possible major contributor to the dioxin activity of human milk

A dioxinlike compound is a compound that binds to the aryl hydrocarbon (Ah) receptor, results in dioxinlike effects, and bioaccumulates. These are the three factors for including dioxinlike chemicals in the toxic equivalency factor (TEF) concept. Risk assessment of dioxinlike compounds is based on using these TEFs. Hexachlorobenzene (HCB) has all three features and should therefore be included in this TEF concept. Relative potency values express the potency of a specific compound in comparison to 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD), the most potent dioxinlike compound, with a relative potency value of 1. For the estimation of the total dioxin activity in an environmental biological sample, the TEF value of a compound is multiplied by the concentration in the specific matrix. This results in a certain amount of toxic equivalents (TEQs) for this compound. The summation of all TEQs in a certain mixture gives the total dioxin activity of this mixture. HCB binds to the Ah receptor about 10,000 times less than TCDD. HCB is also about 10,000 times less potent than TCDD based on in vitro cytochrome P4501A induction and porphyrin accumulation. Using a relative potency value of 0.0001, HCB could add 10-60% to the total TEQ in human milk samples in most countries. In a few countries such as Spain, Slovakia, and the Czech Republic, HCB levels in human milk expressed as TEQ could contribute up to a factor of six to the total TEQ in comparison to the contribution of polychlorinated dioxins, dibenzofurans, and biphenyls together, i.e., up to a daily intake of about 1 ng TEQ/kg for a breast-fed infant. The HCB levels in human milk in these countries are about the same as in India. Biochemical, immunological, and neurological alterations have been observed in infants fed breast milk in countries with relatively low TEQ levels in human milk. Based on the above information, it is clear that HCB should be classified as a dioxinlike compound, that more studies are needed to reduce the uncertainty in the estimation of a relative potency value for HCB, and that epidemiological studies should be undertaken in infants fed breast milk in countries with high HCB exposure levels. Furthermore, measurements of HCB levels in human and environmental samples in conjunction with other dioxinlike compounds is a prerequisite to estimate the total dioxin activity in these samples.

Interactive effects of environmentally relevant polychlorinated biphenyls and dioxins on [3H]phorbol ester binding in rat cerebellar granule cells

Polychlorinated biphenyls (PCBs) are persistent contaminants that exist as complex mixtures in the environment. One problem faced by risk assessors is that the possible interactive effects of specific PCB congeners and related chemicals found in environmental and biological samples have not been systematically investigated. Some PCBs perturb Ca2+ homeostasis and cause protein kinase C (PKC) translocation in neuronal cell cultures and in brain homogenate preparations at concentrations where no cytotoxicity is observed, and these systems are necessary for the growth and normal functioning of neurons. The changes in second messenger systems appear to be associated with the extent of noncoplanarity of the PCB molecule. We studied the interactive effects of selected PCB congeners, a PCB metabolite, and a dioxin on PKC translocation, as determined by [3H]phorbol ester binding in cerebellar granule cells. The binary combinations included coplanar and noncoplanar PCB congeners or PCB congeners with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)/PCB metabolite. In addition, we tested the interactive effects of several PCB congeners (three or more) found in environmental samples such as human milk and blood, contaminated fish, and brain samples from PCB-treated animals. The results indicated that 1) the coplanar congener [3,3',4, 4'-tetrachlorobiphenyl (TeCB)] did not alter the in vitro activity of the noncoplanar (2,2',5,5'-TeCB) or coplanar [4, 4'-dichlorobiphenyl (DCB)] congeners; 2) binary mixtures of active PCB congeners (2,2',4,4'-TeCB and 2,2'-DCB; 2,2'-DCB and 3,5-DCB; 2,2',3,5',6-PeCB and 2,2',4,4',5-PeCB) interact in a dose-additive manner; 3) TCDD did not alter the activity of either coplanar (3,3', 4,4'-TeCB) or noncoplanar (2,2',5,5'-TeCB) congeners; 4) the interaction between the parent PCB congener and hydroxy metabolite of PCB is additive; 5) PCB congener mixtures at the ratios found in environmental samples are biologically active; and 6) there was no indication of synergism in any of the combinations studied. These results suggest that the biological effects of binary mixtures of PCB congeners fit a dose-additive model, indicating that there is a specific site of action for these PCB congeners which is independent of the aryl hydrocarbon receptor. Environmental mixtures contain mostly noncoplanar PCB congeners, and because they appear to be biologically active, the potential human health risk by this group of chemicals should be considered in the risk assessment of PCBs.

A morbidity study of former pentachlorophenol-production workers

Pentachlorophenol (PCP) is a pesticide that was once widely used for wood preservation. Commercial PCP contained impurities including higher chlorinated dibenzo-p-dioxins (CDDs) and chlorinated dibenzofurans (CDFs). We investigated the effects of occupational exposure to PCP and its CDD and CDF contaminants on the skin, liver, porphyrin metabolism, and central and peripheral nervous systems. In 1986 we conducted a medical survey of 366 workers who had been engaged in the production of PCP at a single plant between 1938 and 1978. The referent group consisted of 303 workers from the same plant who were not exposed to these or related compounds. Exposure was determined from computerized personnel records. The medical survey included an administered questionnaire, medical record review, physical examination by dermatologists, internists, and neurologists, and analysis of 24-hr urine for quantitative porphyrins among other tests. In this paper we present the results of analyses of the general health, chloracne, and porphyrin metabolism end points. The general health status of PCP workers was similar to unexposed workers, but 17.8% of PCP workers had evidence of current or past chloracne. PCP workers with chloracne had significantly higher mean urinary excretion of coproporphyrins (117. 0 vs. 90.6 microg/24 hr) than unexposed workers after controlling for potential confounders. Workers with chloracne who had worked with both PCP and polychlorinated biphenyls had significantly higher mean urinary excretions of hepta-, penta-, and coproporphyrins than unexposed workers. We conclude that occupational exposure to PCP is associated with chloracne and biochemical abnormalities which may persist years after exposure.