Polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans, pesticides, and diabetes in the Anniston Community Health Survey follow-up (ACHS II): single exposure and mixture analysis approaches

Dioxins and dioxin-like compounds measurements were added to polychlorinated biphenyls (PCBs) and organochlorine pesticides to expand the exposure profile in a follow-up to the Anniston Community Health Survey (ACHS II, 2014) and to study diabetes associations. Participants of ACHS I (2005-2007) still living within the study area were eligible to participate in ACHS II. Diabetes status (type-2) was determined by a doctor's diagnosis, fasting glucose ≥125 mg/dL, or being on any glycemic control medication. Incident diabetes cases were identified in ACHS II among those who did not have diabetes in ACHS I, using the same criteria. Thirty-five ortho-substituted PCBs, 6 pesticides, 7 polychlorinated dibenzo-p-dioxins (PCDD), 10 furans (PCDF), and 3 non-ortho PCBs were measured in 338 ACHS II participants. Dioxin toxic equivalents (TEQs) were calculated for all dioxin-like compounds. Main analyses used logistic regression models to calculate odds ratios (OR) and 95 % confidence intervals (CI). In models adjusted for age, race, sex, BMI, total lipids, family history of diabetes, and taking lipid lowering medication, the highest ORs for diabetes were observed for PCDD TEQ: 3.61 (95 % CI: 1.04, 12.46), dichloro-diphenyl dichloroethylene (p,p'-DDE): 2.07 (95 % CI 1.08, 3.97), and trans-Nonachlor: 2.55 (95 % CI 0.93, 7.02). The OR for sum 35 PCBs was 1.22 (95 % CI: 0.58-2.57). To complement the main analyses, we used BKMR and g-computation models to evaluate 12 mixture components including 4 TEQs, 2 PCB subsets and 6 pesticides; suggestive positive associations for the joint effect of the mixture analyses resulted in ORs of 1.40 (95% CI: -1.13, 3.93) for BKMR and 1.32 (95% CI: -1.12, 3.76) for g-computation. The mixture analyses provide further support to previously observed associations of trans-Nonachlor, p,p'- DDE, PCDD TEQ and some PCB groups with diabetes.

An assessment of dioxin exposure across gestation and lactation using a PBPK model and new data from Seveso

On July 10, 1976, an explosion at a chemical plant in Seveso, Italy, released up to 30kg of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-the most potent dioxin congener. Twenty years later, the Seveso Women's Health Study (SWHS) initiated a follow-up assessment of a cohort of female Seveso residents. Researchers collected serial blood, measured for TCDD levels, and recorded information about the women's medical history after the explosion. The study's aims were to: 1) modify the human PBPK model for TCDD (Emond et al. 2004; Emond et al. 2005; NCEA-USEPA, 2010) to include repetitive gestation and lactation; 2) simulate TCDD blood concentrations during different life stages including pregnancy and lactation, under different exposure scenarios; and 3) use this PBPK model to compare the influence of gestation and lactation on elimination of TCDD. After optimization of the model, it was assessed using data from the SWHS cohort. The 23 women in Subcohort A, were 4-39years old and in Subcohort B, the 18 women were 3-17years old when the explosion occurred. The model accurately predicted the blood concentrations during the 20years post-exposure, including periods of pregnancy and lactation. The model was also used to analyze the contribution of gestation and lactation to the mother's elimination of TCDD. The results suggest that gestation and lactation do not significantly impact TCDD blood elimination. Future efforts will focus on using additional data to evaluate the PBPK model and improving the mathematical descriptions of lactation and multiple gestations.

5th International PCB Workshop--summary and implications

A summation of new and novel findings presented at "The Fifth PCB Workshop: New Knowledge Gained from Old Pollutants" workshop is provided in this overview, along with discussion of data gaps and research needs in the future. Relative to the previous workshop, the scientific presentations had a decreased emphasis on toxicology; rather, more than half of the sessions dealt with environmental sources, fate and transport, or transformations. Approximately 100 presentations in the form of talks and posters were included in the workshop. The presentations were generally divided into: emissions and transport of PCBs in natural and urban settings; chiral aspects of PCB transport; metabolism and distribution; new aspects of environmental metabolism of PCBs--from microbes to plants to animals; reproduction, developmental and cardiovascular effects of PCBs; updates on Anniston--the most highly exposed PCB community in the U.S. to date; and new and novel approaches for evaluating PCB mixtures (e.g., PCB toxic equivalency factors, and TEFs)--and the implications of such for risk assessment. An overarching state-of-the-science view is important to the goal of preventing negative health consequences. Currently, there are still many roadblocks to evaluating risk associated with this large group of 209 congeners--all of which have different physiochemical properties, variable fate and transport mechanism in the environment, and a range of ability for persistence, bioaccumulation, and biological activity.

Relative potency based on hepatic enzyme induction predicts immunosuppressive effects of a mixture of PCDDS/PCDFS and PCBS

The toxic equivalency factor (TEF) approach was employed to compare immunotoxic potency of mixtures containing polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans and polychlorinated biphenyls relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), using the antibody response to sheep erythrocytes (SRBC). Mixture-1 (MIX-1) contained TCDD, 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-pentachlorodibenzofuran (1-PeCDF), 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF), and 1,2,3,4,6,7,8,9-octachlorodibenzofuran (OCDF). Mixture-2 (MIX-2) contained MIX-1 and the following PCBs, 3,3',4,4'-tetrachlorobiphenyl (IUPAC No. 77), 3,3',4,4',5-pentachlorobiphenyl (126), 3,3',4,4',5,5N-hexachlorobiphenyl (169), 2,3,3',4,4'-pentachlorobiphenyl (105), 2,3',4,4',5-pentachlorobiphenyl (118), and 2,3,3',4,4',5-hexachlorobiphenyl (156). The mixture compositions were based on relative chemical concentrations in food and human tissues. TCDD equivalents (TEQ) of the mixture were estimated using relative potency factors from hepatic enzyme induction in mice [DeVito, M.J., Diliberto, J.J., Ross, D.G., Menache, M.G., Birnbaum, L.S., 1997. Dose-response relationships for polyhalogenated dioxins and dibenzofurans following subchronic treatment in mice. I .CYP1A1 and CYP1A2 enzyme activity in liver, lung and skin. Toxicol. Appl. Pharmacol. 130, 197-208; DeVito, M.J., Menache, G., Diliberto, J.J., Ross, D.G., Birnbaum L.S., 2000. Dose-response relationships for induction of CYP1A1 and CYP1A2 enzyme activity in liver, lung, and skin in female mice following subchronic exposure to polychlorinated biphenyls. Toxicol. Appl. Pharmacol. 167, 157-172] Female mice received 0, 1.5, 15, 150 or 450 ng TCDD/kg/day or approximately 0, 1.5, 15, 150 or 450 ng TEQ/kg/day of MIX-1 or MIX-2 by gavage 5 days per week for 13 weeks. Mice were immunized 3 days after the last exposure and 4 days later, body, spleen, thymus, and liver weights were measured, and antibody response to SRBCs was observed. Exposure to TCDD, MIX-1, and MIX-2 suppressed the antibody response in a dose-dependent manner. Two-way ANOVA indicated no differences in the response between TCDD and the mixtures for body weight, spleen/body weight and decreased antibody responses. The results support the use of the TEF methodology and suggest that immune suppression by dioxin-like chemicals may be of concern at or near background human exposures.

Impact of Repeated Exposure on the Toxicokinetics of BDE 47 in Mice

2,2',4,4'-Tetrabromodiphenyl ether (BDE 47) is the major polybrominated diphenyl ether (PBDE) found in environmental samples and human tissue despite its small contribution to global production and usage. Currently, three toxicokinetic studies are available investigating single-dose exposures; this is the first study to investigate toxicokinetic parameters following repeated exposure to BDE 47. The disposition and excretion of BDE 47 was monitored in adult female C57BL/6 mice for 5 days following ten consecutive 1.0-mg/kg oral doses and compared with results from our previous study. Results of the present study suggest greater retention of BDE 47 and nonlinear disposition patterns following repeated exposure to this dose in mice. No target tissues of sequestration or potential toxicity were determined; however, some tissues, such as the liver, demonstrated patterns of interest following repeated exposure that were not previously observed in acute toxicokinetic studies. Repeated exposure to BDE 47 results in higher concentrations remaining in adipose tissue, which demonstrates its potential for bioaccumulation. The data also suggest that excretion of BDE 47 may be decreased following repeated exposure. These results, in combination with evidence of its persistence and toxicity, underlie the need to further understand BDE 47 toxicokinetics across species at steady-state conditions.

Inhibition of human and rat CYP1A2 by TCDD and dioxin-like chemicals

Dioxins have been shown to bind and induce rodent CYP1A2, producing a dose-dependent hepatic sequestration in vivo. The induction of CYP1A2 activity has been used as a noninvasive biomarker for human exposure to dioxins; while there is a consistent relationship between exposure and hepatic CYP1A2 induction in rodents, this relationship has only been observed in some of the highest exposed human populations. This may be explained by inhibition of CYP1A2 activity by dioxins as some rodent studies demonstrate that rodent CYP1A2 activity can in fact be inhibited by dioxins in vitro. CYP1A2 activity was examined using a series of dioxins to inhibit human and rat CYP1A2 activity in species-specific CYP1A2 SUPERSOMES using three common CYP1A2 substrates. Methoxyresorufin was a more efficient substrate than acetanalide or caffeine in this in vitro system. Rat and human CYP1A2 enzymatic activity is inhibited by TCDD, PCDD, TCDF, 4-PeCDF, and PCBs 126, 169, 105, 118, and 156 in a concentration-dependent manner. These data demonstrate that the in vitro metabolism of prototype substrates is similar between the rat and human CYP1A2 SUPERSOME preparations and that dioxins inhibit CYP1A2 activity in both species. Because of the potential for inhibition of CYP1A2 activity by TCDD and other dioxins, studies examining CYP1A2 induction in dioxin-exposed populations using these substrates should be viewed cautiously.

Toxicokinetics of BDE 47 in female mice: effect of dose, route of exposure, and time

2,2',4,4'-Tetrabromodiphenyl ether (BDE 47) is present in commercial mixtures of polybrominated diphenyl ethers (PBDEs), which are used as flame retardants in a wide variety of consumer products. Despite its small contribution to PBDE global production and usage, BDE 47 is the major congener found in environmental samples and human tissue. No human data are currently available regarding the toxicokinetics of BDE 47 either as an individual congener or in the commercial mixture. Because previous studies have suggested potential toxicokinetic differences between rodent species, this study was conducted in an effort to fully characterize absorption, distribution, and excretion parameters following a single dose with respect to dose, time, and route of exposure in female C57BL/6 mice. Over 80% of the administered dose was absorbed after oral or intratracheal administration, whereas approximately 62% was absorbed when the dose was applied dermally. Disposition was dictated by lipophilicity as adipose and skin were major depot tissues. BDE 47 was rapidly excreted in the urine and feces. Of particular interest was the amount of parent compound found in the urine, which was a major factor in determining an initial whole-body half life of 1.5 days after a single oral exposure. Elimination, both whole-body and from individual tissues, was biphasic. Initial half-lives were 1-3 days, whereas terminal half-lives were much longer, suggesting the potential for bioaccumulation. This toxicokinetic behavior has important implications for extrapolation of toxicological studies to the assessment of health risk in humans.

CYP1A2 is not required for 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced immunosuppression

One of the most sensitive and reproducible immunotoxic endpoints of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure is suppression of the antibody response to sheep red blood cells (SRBCs) in mice. Immunosuppression occurs in concert with hepatomegaly and associated induction of several hepatic cytochrome P450 enzymes, including CYP1A2 which is responsible for the hepatic sequestration of TCDD. In this study, TCDD-induced immunosuppression was evaluated in C57BL/6N CYP1A2 (+/+) wild-type and compared with that of age-matched CYP1A2 (-/-) knockout and CYP1A2 (+/-) heterozygous female mice. Groups of mice were given a single gavage dose of 0, 0.03, 0.1, 0.3, 1.0, 3.0 or 10.0microg TCDD/kg, followed 7 days later by immunization with SRBCs. Serum was obtained 5 days after immunization and body, spleen, thymus and liver weights were measured. sheep red blood cell (SRBC) antibody titers were determined by an enzyme-linked immunosorbent assay (ELISA). Anti-SRBC titers were suppressed at 1.0, 1.0 and 0.3microg TCDD/kg for CYP1A2 (+/+), CYP1A2 (+/-), and CYP1A2 (-/-) mice, respectively, which indicated a three-fold increase in TCDD-induced immunosuppression for the CYP1A2 (-/-) mice. This increase in TCDD-induced immunosuppression may be due to the inability of CYP1A2 (-/-) mice to sequester TCDD in the liver leading to a higher dose to the immune system. In CYP1A2 (+/+) mice, a dose of 3.0microg TCDD/kg was sufficient to increase the liver weight, while in CYP1A2 (-/-) mice no increase in liver weight was observed. Application of analysis of variance and dose-response modeling approaches indicate that there is little evidence that the immunosuppression dose-response curves, for the three strains, differ in the lower part of the dose-response range. Thus, CYP1A2 is not required for TCDD-induced immunosuppression in the mouse.

A mixture of dioxins, furans, and non-ortho PCBs based upon consensus toxic equivalency factors produces dioxin-like reproductive effects

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD; dioxin) and related polyhalogenated aromatic hydrocarbons (PHAHs) alter the reproductive development of laboratory animals. Therefore, we exposed animals to a mixture of dioxins, furans, and polychlorinated biphenyls (PCBs) that included TCDD, 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-pentachlorodibenzofuran (1-PeCDF), 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF), octachlorodibenzofuran (OCDF), 3,3',4,4'-tetrachlorobiphenyl (PCB77), 3,3',4,4',5-pentachlorobiphenyl (PCB126), and 3,3',4,4',5,5'-hexachlorobiphenyl (PCB169). The mixture composition approximated the relative abundance of these compounds in foodstuff (L. S. Birnbaum and M. J. DeVito, 1995, Toxicology Vol. 105, pp. 391-401). Following the work of Gray et al. with TCDD (1997, Toxicology and Applied Pharmacology Vol. 146, pp. 11-20), we exposed time-pregnant dams on gestation day (GD) 15 at doses up to 1.0 microgram TCDD toxic equivalency (TEQ)/kg and the development of offspring was monitored. This mixture significantly increased the time to puberty in both male and female offspring. At postnatal day (PND) 32 seminal vesicle weights were decreased; however, only ventral prostate weight was affected at PND 49 and no effects were seen at PND 63. In female offspring, the mixture caused dose-dependent increases in the incidence of vaginal thread. Ethoxyresorufin-O-deethylase (EROD) activity was higher than with TCDD the comparable TEQ exposure. Based on the slightly lowered responsiveness to the mixture, we used 2.0 microgram TEQ/kg to examine reproductive effects. This dose elicited the responses observed with 1.0 microgram TCDD/kg. Results indicate that the mixture causes a similar spectrum of effects seen with TCDD and the slightly lowered degree of response based on administered dose appears to be due to decreased transfer of mixture components to the offspring. Thus, the use of the WHO consensus TEFs (M. Van den Berg et al., 1998, Environ. Health Perspec. 106, 775-792) reasonably predicts the developmental toxicity of this mixture of dioxin-like PHAHs.

TCDD-mediated oxidative stress in male rat pups following perinatal exposure

2,3,7,8-tetrachlorododibenzo-p-dioxin (TCDD) is a highly persistent trace environmental contaminant and is one of the most potent toxicants known. Exposure to TCDD has been shown to cause oxidative stress in a variety of animal models. In this study, pregnant Long Evans rats were dosed with 1 microg TCDD/kg on gestational day (GD) 15 so as to investigate oxidative stress in the liver of male pups following gestational exposure to TCDD. Lipid peroxidation (TBARS), production of reactive oxygen species (ROS), and total glutathione (GSH) were assayed to identify changes in oxidative stress parameters in the pup liver at GD 21 and postnatal days (PND) 4, 25, 32, 49, and 63. Mean ROS levels in pups were elevated at all time points tested with a significant elevation at PND 4 and PND 25. However, pup hepatic lipid peroxidation was unchanged throughout the time course. In addition, hepatic total GSH levels were not significantly changed although the means for the TCDD-treated groups were less than those of the controls at all time points except PND 49. The results indicate that although the levels of ROS are increased following gestational/lactational exposure, this increase does not translate to direct oxidative damage or significant changes to endogenous antioxidant defense mechanisms. Further investigation into the effect of gestational/lactational exposure in pups should include additional endpoints for further characterization of the time course of the response, the effect upon extrahepatic tissues, and investigation of differences between male and female offspring.

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.

Differential effects of two lots of aroclor 1254: congener-specific analysis and neurochemical end points

Aroclor 1254 is a widely studied commercial polychlorinated biphenyl (PCB) mixture which, by definition, contains 54% chlorine by weight. Recent reports indicate substantial differences in the congener composition among Aroclor lots and hence their biologic effects. We designed the current study to compare the effects of two lots of Aroclor 1254 (lots 6024 and 124-191). We analyzed these two lots for PCB congeners, polychlorinated dibenzofurans (PCDFs), polychlorinated naphthalenes (PCNs), and polychlorinated dibenzodioxins (PCDDs). We used previously established techniques for analyzing intracellular Ca(2+) buffering and protein kinase C (PKC) translocation to test their biologic activity in neuronal preparations. PCB congener-specific analysis indicated that ortho and non-ortho congeners in these two lots varied in their percent contribution. Among all congeners, the percentages of non-ortho congeners (PCBs 77, 81, 126, and 169) were higher in lot 6024 (2.9% of total) than in lot 124-191 (0.02% of total). We detected no dioxins in these two lots (< 2 ppb). Although there are some differences in the congener composition, total PCNs were similar in both lots: 171 ppm in lot 6024 and 155 ppm in lot 124-191. However, total PCDFs were higher in lot 6024 (38.7 ppm) than in lot 124-191 (11.3 ppm). When we tested these two Aroclors on Ca(2+) buffering and PKC translocation in brain preparations, the effects were significantly different. Although lot 124-191 was more potent on PKC translocation than lot 6024, lot 6024 was slightly more active on Ca(2+) buffering than lot 124-191. These effects could not be attributed to the differences in the percentage of non-ortho congeners or PCDFs because they were inactive on these two parameters. The effects could not be attributed to PCNs because the levels were almost similar. The effects seen with two lots of Aroclor 1254 in neuronal cells were also not predicted based on the TCDD toxic equivalents (TEQs), although TEQs predicted the effects on ethoxyresorufin-O-deethylase (EROD) or methoxyresorufin-O-deethylase (MROD) activities. It is possible that the differential effects seen in neuronal cells could be caused by differences in the composition of ortho-congeners in these two mixtures, because PCBs with ortho-lateral substitutions can exhibit different activities on the selected neurochemical end points. Because of these differential effects with different lot numbers, the composition of Aroclor mixtures used in investigations should be disclosed.

Subchronic Exposure of [3H]-2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) in Female B6C3F1 Mice: Relationship of Steady-State Levels to Disposition and Metabolism

The present study of subchronic low exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) at or near steady-state levels tries to emulate the most probable mode for human exposure, dietary consumption. This study is the first and most intensive pharmacokinetic study to be reported with repeated dosing, multiple times, and multiple doses examining disposition of TCDD-derived radioactivity and CYP1A activities in mice. For time-course relationships, animals were dosed (daily, Monday-Friday) with 0, 1.5, or 150 ng [3H]TCDD/kg for 4, 8, 13, or 17 weeks and also for 13 weeks followed by 4 weeks with no dosing. For dose-response relationships, animals were dosed for 13 weeks (daily, Monday-Friday) with 0, 0.15, 0.45, 1.5, 4.5, 15, 45, 150, or 450 ng [3H]TCDD/kg. Additional animals dosed for 13 weeks (daily, Monday-Friday) with 1.5 or 150 ng [(3)H]TCDD/kg were housed in metabolism cages. Time- and dose-dependencies of TCDD were confirmed in all measured tissues. Liver/fat (L/F) concentration ratios ranged from 0.2-3.4 (low to high dose). Hepatic CYP1A1 enzymatic activity increased (p < 0.05) starting at 0.15 ng/kg/day with L/F of 0.2 and body burden of 2.8 ng TCDD/kg body weight. By examining TCDD exposures at or near steady state, this study reports for the first time and provides direct evidence of low-dose effects on a measured reversible response at body burdens that are within background levels of the general human population. In addition, this study emphasizes cumulative effects of daily dosing and suggests the importance of tissue dosimetry or body burden for a persistent chemical such as TCDD.

Disposition of polychlorinated dibenzo-p-dioxins, dibenzofurans, and non-ortho polychlorinated biphenyls in pregnant long evans rats and the transfer to offspring

Pharmacokinetic properties of polychlorinated dibenzo-p-dioxins, dibenzofurans (PCDFs), and non-ortho biphenyls (PCBs) play a critical role in their relative toxicity. The present study examined the transfer of these chemicals to offspring and placenta. Pregnant Long Evans rats received 0.0 (control), 0.05, 0.2, 0.8, and 1.0 microg/kg of dioxin toxic equivalence (TEQ) by oral gavage on the 15th gestational day (GD 15), using a dosing mixture that contained 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 1,2,3,7,8-pentachlorodibenzofuran (1-PeCDF), 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF), octachlorodibenzofuran (OCDF), 3,3',4,4'-tetrachlorobiphenyl (PCB 77), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), and 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) in ratios approximating that in food. Rats were euthanized on GD 16, GD 21, and postnatal day 4 (PND 4). The chemical concentrations in fetus, pup, placenta, and maternal liver, serum, and adipose tissue were determined using gas chromatography/high-resolution mass spectrometry. A dose-dependent increase in hepatic sequestration was seen with TCDD, PeCDD, 4-PeCDF, OCDF, PCB 126, and PCB 169, and the transfer to offspring was reduced at higher doses. 4-PeCDF, PeCDD and PCB 126 showed higher liver affinity than TCDD. TCDF, 1-PeCDF, and PCB 77 were metabolized rapidly. On GD 16, TCDD and the three PCBs reached equilibration between the fetus and placenta, but this did not occur with PeCDD and 4-PeCDF until GD 21, according to the lipid-based concentrations. Offspring compartments received more of the dosed compounds lactationally than transplacentally (7-28% versus 0.5-3%). The behavior of each congener was dose-dependent; therefore, extrapolation of high-dose experimental data should be used with caution.

Tissue disposition of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in maternal and developing long-evans rats following subchronic exposure

Prenatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces alterations in the reproductive system of the developing pups. The objective of this study was to determine the disposition of TCDD in maternal and fetal Long-Evans (LE) rats following subchronic exposure, since the adverse reproductive and developmental effects have been extensively characterized in this strain of rat. LE rats were dosed by gavage with 1, 10, or 30 ng [(3)H]TCDD/kg in corn oil, 5 days/week for 13 weeks. At the end of 13 weeks, females were mated and dosing continued every day throughout gestation. Dams were sacrificed on gestation day (GD) 9, GD16, GD21, and post-natal day 4 and analyzed for [(3)H]TCDD-derived activity in maternal and fetal tissues. Maternal body burdens were equivalent at different time points, indicating that the dams were at steady state. Maternal body burdens were approximately 19, 120, and 300 ng TCDD/kg following doses of 1, 10, and 30 ng TCDD/kg, respectively. Individual embryo concentrations on GD9 were 1.6, 7, and 16 pg TCDD/g after maternal exposure of 1, 10, and 30 ng/kg/d, respectively. On GD 16, fetal liver, urogenital tract, head, and body concentrations were similar and averaged 1.4, 7.8, and 16.4 pg TCDD/g after administration of 1, 10, or 30 ng TCDD/kg/d, respectively, indicating no preferential sequestration within the different fetal tissues. These concentrations of TCDD within fetal tissues after subchronic exposure are comparable to those seen after a single dose of 50, 200, or 1000 ng TCDD/kg administered on GD15, a critical period of gestation.

Dose-response relationships for induction of CYP1A1 and CYP1A2 enzyme activity in liver, lung, and skin in female mice following subchronic exposure to polychlorinated biphenyls

The Toxic Equivalency Factor (TEF) method is used to estimate potential health risks associated with exposure to dioxin-like chemicals. The TEF method is a relative potency (REP) scheme that assumes dose additivity, that the chemicals produce the same response through the same mechanism, and that the REP of a chemical is equivalent for all responses. The present study estimates the REP for five PCBs with dioxin-like activity. Mice were exposed to either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,3',4, 4'-pentachlorobiphenyl (105), 2,3',4,4',5-pentachlorobiphenyl (118), 3,3',4,4',5-pentachlorobiphenyl (126), or 2,3,3',4,4'-, 5-hexachlorobiphenyl (156), five days/week for 13 weeks by oral gavage in a corn oil vehicle. Three days after the last dose, animals were euthanized and the ethoxyresorufin-O-deethylase activity was determined in liver, lung, and skin. Acetanilide-4-hydroxylase activity was determined in liver. In addition, liver and skin disposition of the chemicals were determined. REPs were estimated using a statistical method previously described (DeVito et al., Toxicol. Appl. Pharmacol.147, 267-280, 1997). For any given compound, the REP generally varied by less than a factor of four across endpoints when calculated based on an administered dose. However, typically there was one response for every chemical in which the REP was different by an order of magnitude or more from the other responses. There was some evidence that the REPs may be dose-dependent. While, in general, these data support the use of a single point estimate of the REP, the issue of dose-dependency requires targeted investigation.

Extrapolation of a PBPK model for dioxins across dosage regimen, gender, strain, and species

A physiologically based pharmacodynamic (PBPK) model for 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) was developed based on pharmacokinetic data from acute oral exposures of TCDD to female Sprague-Dawley rats (Wang et al., 1997, Toxicol Appl. Pharmacol 147, 151-168). In the present study, the utility of this model to predict the disposition of TCDD in male and female Sprague-Dawley and female Wistar rats exposed to TCDD through different dosage regimens was examined. The ability of the model to predict the disposition of 2-iodo-3,7,8-trichlorodibenzo-p-dioxin (ITrCDD) in mice (Leung, et al., 1990, Toxicol. Appl. Pharmacol. 103, 399-410) was also examined. The ability of the model to predict across routes of exposure was assessed with intravenous injection data (5.6 microg/kg bw) (Li et al., 1995, Fundam. Appl. Toxicol. 27, 70-76) in female rats. Analysis across gender extrapolations used data for male Sprague-Dawley rats exposed intravenously to 9.25 microg TCDD/kg bw (Weber et al., 1993, Fundam. Appl. Toxicol. 21, 523-534). The analysis of across-dosage regimen and stains of rats extrapolations were assessed using data from rats exposed to TCDD through a loading/maintenance dosage regimen (Krowke et al., 1989, Arch. Toxicol. 63, 356-360). The physiological differences between gender, strain, and species were taken into account when fitting the PBPK model to these data sets. The results demonstrate that the PBPK model for TCDD developed for female Sprague-Dawley rats exposed by acute oral dosing accurately predicts the disposition of TCDD, for different gender and strain of rats across varying dosage regimens, as well as in a strain of mice. Minimal changes in fitted parameters were required to provide accurate predictions of these data sets. This study provides further confirmation of the potential use of physiological modeling in understanding pharmacokinetics and pharmacodynamics.

Non-carcinogenic effects of TCDD in animals

Exposure to TCDD and related chemicals leads to a plethora of effects in multiple species, tissues, and stages of development. Responses range from relatively simple biochemical alterations through overtly toxic responses, including lethality. The spectrum of effects shows some species variability, but many effects are seen in multiple wildlife, domestic, and laboratory species, ranging from fish through birds and mammals. The same responses can be generated regardless of the route of exposure, although the administered dose may vary. The body burden appears to be the most appropriate dosimetric. Many of the effects often attributed to TCDD are associated with relatively high doses: lethality, wasting, lymphoid and gonadal atrophy, chloracne, hepatotoxicity, adult neurotoxicity, and cardiotoxicity. Changes in multiple endocrine and growth factor systems have been reported in a manner which is tissue, sex, and age-dependent. The most sensitive adverse effects observed in multiple species appear to be developmental, including effects on the developing immune, nervous, and reproductive systems. Such effects have been observed at maternal body burdens in the range of 30-80 ng/kg in both non-human primates and rodents. Biochemical effects on cytokine expression and metabolizing enzymes occur at body burdens which are within a factor of ten of the clearly adverse developmental responses. Thus, effects on the immune system, learning, and the developing reproductive system of multiple animals occur at body burdens which are close to those present in the background human population.

Oxidative stress in female B6C3F1 mice following acute and subchronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a highly persistent trace environmental contaminant and is one of the most potent toxicants known to man. Hassoun et al. (1998, Toxicol. Sci. 42, 23-27) reported an increase in the production of reactive oxygen species (ROS) in the brain of female B6C3F1 mice following subchronic exposure to TCDD at doses as low as 0.45 ng/kg/day. In the present study, oxidative stress was characterized in liver, spleen, lung, and kidney following subchronic (0.15-150 ng/kg; 5 days/week for 13 weeks, po) or acute exposure (0.001-100 microg/kg, po) to TCDD in order to investigate the interaction between tissue concentration and time for production of ROS. Seven days following acute administration of TCDD, mice were sacrificed; they demonstrated increases in liver superoxide anion production (SOAP) and thiobarbituric acid reactive substances (TBARS) at doses of 10 and 100 microg/kg, associated with hepatic TCDD concentrations of 55 and 321 ng/g, respectively. Liver obtained from mice following subchronic TCDD exposure demonstrated an increase in SOAP and TBARS above controls at doses of 150 ng/kg/day with liver TCDD concentration of only 12 ng/g. Interestingly, glutathione (GSH) levels in lung and kidney following sub-chronic TCDD exposure were decreased at the low dose of 0.15 ng/kg/day. This effect disappeared at higher TCDD doses. The data suggest that higher tissue TCDD concentrations are required to elicit oxidative stress following acute dosing than with subchronic TCDD exposure. Therefore, the mechanism of ROS production following TCDD exposure does not appear to be solely dependent upon the concentration of TCDD within the tissue. In addition, very low doses of TCDD that result in tissue concentrations similar to the background levels found in the human population produced an effect on an oxidative stress endogenous defense system. The role of this effect in TCDD-mediated toxicity is not known and warrants further investigation.

In utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin alters postnatal development of seminal vesicle epithelium

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been shown to alter male reproductive development of laboratory animals through in utero and lactational exposure. As a result of exposure, the accessory glands of the male reproductive tract, including the seminal vesicle, are decreased in size as determined by total weight of the tissue. Analysis of seminal vesicle weights over time suggests that the changes may be transient. Administration of 1.0 microg/kg TCDD during gestation caused a significant decrease in seminal vesicle weights of offspring 8-11 months of age. We examined the effects of TCDD on seminal vesicles from rats exposed in utero and lactationally. Pregnant Long Evans rats were gavaged on gestation day 15 with 1.0 microg/kg TCDD in corn oil. Male pups were euthanized and necropsied on postnatal days (PND) 15, 25, 32, 49, 63, and 120. Seminal vesicles were weighed and then fixed in 10% neutral buffered formalin and processed for microscopic examination. Seminal vesicle weights were not significantly decreased until PND 32. Androgen receptor mRNA expression in PND 25 seminal vesicles was not different from control. In the present study, TCDD exposure decreased seminal vesicle epithelial branching and differentiation. Control epithelial cells had tall columnar morphology with relatively abundant cytoplasm, whereas TCDD-treated cells had rounded nuclei and less cytoplasm. In addition, immunolocalization of proliferating nuclear antigen was confined to undifferentiated basal epithelial cells of controls but was found in both basal and luminal cells of the treated seminal vesicle. Results indicate that the TCDD-induced impaired growth of the rat seminal vesicles is associated with a dramatic decrease in the development of the epithelium.

Acute administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in pregnant Long Evans rats: association of measured tissue concentrations with developmental effects

Prenatal exposure to TCDD interferes with fetal development at doses lower than those causing overt toxicity in adult animals. Exposure to TCDD during development produces alterations in the reproductive system of the developing pups- delayed puberty and reduced sperm counts in males and malformations in the external genitalia of females. The objectives of this study were to determine maternal and fetal tissue concentrations of TCDD after acute exposure and whether these tissue concentrations can be used to estimate the intensity of the developmental abnormalities reported by other laboratories. Pregnant Long Evans rats received a single, oral dose of 0.05, 0.20, 0.80, or 1.0 microg [3H]-TCDD/kg on gestation day (GD) 15, and maternal and fetal tissue concentrations of TCDD were measured on GD16 and GD21. On GD16, maternal liver contained the greatest amount of TCDD (30-47% administered dose). One day after administration of 0.20 microg TCDD/kg on GD15, there were 13.2 pg TCDD/g present in an individual fetus. This concentration is associated with delayed puberty and decreased epididymal sperm counts in male pups as well as malformations in the external genitalia of females. For the responses studied, tissue concentration measured during a critical period of gestation adequately predicts the intensity of the response. In addition, there was a strong correlation between fetal body burden and maternal body burden on GD16. A dose of 0.05 microg TCDD/kg resulted in maternal body burdens of 30.6+/-3.1 and 26.6+/-3.1 ng TCDD/kg on GD16 and GD21, respectively. In conclusion, low-level TCDD exposure during the perinatal stage of life can produce adverse effects within the developing pups and that tissue concentration measured during a critical period is the appropriate dose metric to predict adverse reproductive and developmental effects.

Effect of prenatal exposure to TCDD on the promotion of endometriotic lesion growth by TCDD in adult female rats and mice

Several lines of research led to our hypothesis that perinatal exposure to TCDD may alter the sensitivity of adult rodents to the promotional effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on endometriosis. Pregnant rats and mice were treated on gestation day (GD) 8 with either 1 (rats) or 3 (mice) microg TCDD/kg or vehicle. Female offspring were reared to adulthood, and endometriosis was induced surgically. All animals received 0, 3, or 10 microg TCDD/kg 3 weeks prior to surgery, at the time of surgery, and 3, 6, and 9 weeks after surgery. Necropsies were performed 12 weeks after surgery. Measurements at necropsy included the diameter of endometriotic lesions and body, uterine, ovarian and liver weights. While no effect of treatment on lesion diameter was found in rats, analyses revealed that perinatal plus adult exposure to TCDD can increase the size of endometriotic lesions surgically induced in mice. These and additional data on body and organ weights are consistent with previous work. These data confirm the sensitivity of mice to the promotion of endometriotic lesion growth by TCDD and indicate a perinatal effect of TCDD on this parameter when perinatal exposure on GD8 is supplemented with adult exposure to TCDD of female mice.

Dose-dependent localization of TCDD in isolated centrilobular and periportal hepatocytes

Dose-response relationships for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) suggest a differential sensitivity of liver cell types to the induction of cytochrome P450 gene expression, and that the induction of hepatic protein CYP1A2 causes sequestration of TCDD. In addition, immunolocalization of hepatic CYP1A1/1B1/1A2 proteins is not uniform after exposure to TCDD. The mechanism for the regio-specific induction of hepatic P450s by TCDD is unknown, but may involve the differential distribution of participants in the AhR-mediated pathway and/or regional P450 isozymes, as well as, non-uniform distribution/sequestration of TCDD. Therefore, this study examined the effects of TCDD in unfractionated, centrilobular and periportal hepatocytes isolated from female Sprague-Dawley rats acutely exposed (3 days) to a single oral dose of 0.01-10.0 microg [3H]TCDD/kg. A dose-dependent increase in concentration of TCDD was accompanied by a dose-dependent increase in CYP1A1, CYP1A2, and CYP1B1 mRNA expression and associated enzymes in all liver-cell populations. Centrilobular hepatocytes showed a 2.7- to 4.5-fold higher concentration of TCDD as compared to the periportal hepatocytes at doses up to 0.3 microg TCDD/kg. Centrilobular hepatocytes also exhibited an elevated MROD activity as compared to the periportal hepatocytes at doses up to 0.3 microg TCDD/kg. Furthermore, centrilobular hepatocytes showed an elevated concentration of induced CYP1A2 and CYP1B1 mRNA as compared to periportal hepatocytes within the 0.01- and 0.3-microg TCDD/kg-treatment groups. This is the first study to demonstrate that a dose-dependent difference in distribution of TCDD exists between centrilobular and periportal cells that might be related to regional differences in P450 induction.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-mediated oxidative stress in CYP1A2 knockout (CYP1A2-/-) mice

The objective of the study was to compare alterations in indicators of oxidative stress following 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in cytochrome P4501A2 (CYP1A2) knockout mice and their parental lineage strains (C57BL/6N and 129/Sv). This study will aid in determining the role, if any, of CYP1A2 in TCDD-mediated oxidative stress. Formation of thiobarbituric acid-reactive substances (TBARS) as a measurement of lipid peroxidation, production of reactive oxygen species (ROS) via the in vitro reduction of cytochrome c in tissue homogenate, and changes in the biochemical antioxidant glutathione were monitored to determine oxidative stress 7 days following a single oral dose of 25 microg TCDD/kg. TBARS, reduction of cytochrome c, and changes in glutathione demonstrated a similar response in CYP1A2 knockout and parental strains. These data suggest that CYP1A2 does not play a critical role in the acute oxidative stress response following TCDD exposure.

Research integrity: a government perspective

What is research integrity? At the United States Environmental Protection Agency (U.S. EPA) research integrity can be defined as conducting and fostering research to define, anticipate, and understand environmental problems; and generating sound, appropriate, credible, and effective solutions to those problems. Whether in government, academia, or industry, integrity is required at all stages of research--from data generation to data analysis. What constitutes research integrity? Simply put, Did we do the right thing? Did we do it the right way? Did we honestly document what we did? This is especially important if the research is used as a basis for public policy. The extensive and intensive use of the results of science in EPA's standard setting, regulatory, and enforcement responsibilities means that scientific misconduct can lead to costly and inappropriate actions through unnecessary expenditure or inadequate protection. The soundness, effectiveness, and credibility of EPA's regulations ultimately rest on the scientific and technical bases for these actions. Careful attention to research record keeping can help ensure data quality and integrity. The U.S. Environmental Protection Agency, its research requirements, and the work of the National Health and Environmental Effects Research Laboratory are discussed below.

Effects of CYP1A2 on disposition of 2,3,7, 8-tetrachlorodibenzo-p-dioxin, 2,3,4,7,8-pentachlorodibenzofuran, and 2,2',4,4',5,5'-hexachlorobiphenyl in CYP1A2 knockout and parental (C57BL/6N and 129/Sv) strains of mice

TCDD is the prototype and most potent member of the highly lipophilic polyhalogenated aromatic hydrocarbons (PHAHs), which are persistent and ubiquitous environmental contaminants. In both acute and subchronic animal studies, there is a specific accumulation of TCDD in liver greater than in adipose tissue. The inducible hepatic binding protein responsible for this hepatic sequestration of TCDD and its congeners has been shown by our laboratory to be CYP1A2 (J. J. Diliberto, D. Burgin, and L. S. Birnbaum, 1997, Biochem. Biophys. Res. Commun. 236, 431-433). The present study was conducted using knockout (KO) mice lacking expression of CYP1A2 (CYP1A2-/-) in order to investigate the role of CYP1A2 gene on the disposition of TCDD, 4-PeCDF (a dioxin-like PHAH), and PCB 153 (a nondioxin-like PCB) in KO (CYP1A2-/-) mice and age-matched parental mice strains (C57BL/6N: CYP1A2+/+, Ah(b/b) and 129/Sv: CYP1A2+/+, Ah(d/d)). Mice were dosed (25 microgram [(3)H]TCDD/kg, 300 microgram [(14)C]4-PeCDF/kg, or 35.8 mg [(14)C]PCB 153/kg bw in a corn oil vehicle) orally and terminated after 4 days. Residues of administered compounds in collected tissues and daily excreta were quantitated using (3)H or (14)C activity. Results demonstrated differential effects in disposition for the various treatments within the three genetically different groups of mice. In KO mice, TCDD, 4-PeCDF, and PCB 153 had very little hepatic localization of chemical, and the major depot was adipose tissue. In contrast, parental strains demonstrated hepatic sequestration of TCDD and 4-PeCDF, whereas disposition of PCB 153 in parental strains was similar to that in KO mice. Another difference between KO mice and parental strains was the enhanced urinary excretion of 4-PeCDF. This study demonstrates the importance of CYP1A2 in pharmacokinetic behavior and mechanistic issues for TCDD and related compounds.

Characterization of potential endocrine-related health effects at low-dose levels of exposure to PCBs

This article addresses issues related to the characterization of endocrine-related health effects resulting from low-level exposures to polychlorinated biphenyls (PCBs). It is not intended to be a comprehensive review of the literature but reflects workshop discussions. "The Characterizing the Effects of Endocrine Disruptors on Human Health at Environmental Exposure Levels," workshop provided a forum to discuss the methods and data needed to improve risk assessments of endocrine disruptors. This article contains an overview of endocrine-related (estrogen and thyroid system) interactions and other low-dose effects of PCBs. The data set on endocrine effects includes results obtained from mechanistic methods/ and models (receptor based, metabolism based, and transport protein based), as well as from (italic)in vivo(/italic) models, including studies with experimental animals and wildlife species. Other low-dose effects induced by PCBs, such as neurodevelopmental and reproductive effects and endocrine-sensitive tumors, have been evaluated with respect to a possible causative linkage with PCB-induced alterations in endocrine systems. In addition, studies of low-dose exposure and effects in human populations are presented and critically evaluated. A list of conclusions and recommendations is included.

Characterization of potential endocrine-related health effects at low-dose levels of exposure to PCBs

This article addresses issues related to the characterization of endocrine-related health effects resulting from low-level exposures to polychlorinated biphenyls (PCBs). It is not intended to be a comprehensive review of the literature but reflects workshop discussions. "The Characterizing the Effects of Endocrine Disruptors on Human Health at Environmental Exposure Levels," workshop provided a forum to discuss the methods and data needed to improve risk assessments of endocrine disruptors. This article contains an overview of endocrine-related (estrogen and thyroid system) interactions and other low-dose effects of PCBs. The data set on endocrine effects includes results obtained from mechanistic methods/ and models (receptor based, metabolism based, and transport protein based), as well as from (italic)in vivo(/italic) models, including studies with experimental animals and wildlife species. Other low-dose effects induced by PCBs, such as neurodevelopmental and reproductive effects and endocrine-sensitive tumors, have been evaluated with respect to a possible causative linkage with PCB-induced alterations in endocrine systems. In addition, studies of low-dose exposure and effects in human populations are presented and critically evaluated. A list of conclusions and recommendations is included.

Methoxyresorufin: an inappropriate substrate for CYP1A2 in the mouse

Hepatic microsomes derived from Cypla2(-/-) knockout (KO) and parental strains of mice, C57BL/6N and 129Sv, were used to examine the specificity of methoxyresorufin and acetanilide as substrates for CYP1A2 activity. In addition, animals from each group were exposed to CYP1-inducing compounds. As expected, microsomes from untreated 1a2 KO mice did not have immunodetectable CYP1A2 protein; however, methoxyresorufin-O-demethylase (MROD, 25.5+/-6.1 pmol/min/mg protein) and acetanilide-4-hydroxylation (ACOH, 0.64+/-0.04 nmol/min/mg protein) activities were still present. Furthermore, induction of ethoxyresorufin-O-deethylase (EROD) by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in 1a2 KO mice was accompanied by a greater than 70-fold increase in MROD activity. In contrast, ACOH was only induced 2-fold by TCDD. As with 1a2 KO mice, the parental strains exposed to TCDD or 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF) showed substantial EROD and MROD induction, whereas ACOH activity was induced to a lesser degree. PCB153 (2,2',4,4',5,5'-hexachlorobiphenyl) resulted in low levels of both EROD and MROD induction. Results indicate that both substrates are subject to metabolism by non-CYP1A2 sources, and the apparent contribution of CYP1A1 activity to methoxyresorufin metabolism makes MROD unsuitable for differentiating CYP1A1 and CYP1A2 activities in the mouse.

Dose-response relationships for disposition and hepatic sequestration of polyhalogenated dibenzo-p-dioxins, dibenzofurans, and biphenyls following subchronic treatment in mice

Humans are exposed to mixtures of polyhalogenated dibenzo-p-dioxins, dibenzofurans, and biphenyls mainly through the diet. Many of these chemicals are dioxin-like and their relative toxicity is related to their ability to bind and activate the Ah receptor. The present study examines the structure-activity relationship for disposition of these chemicals in female B6C3F1 mice following subchronic exposures. Mice were treated 5 days/week for 13 weeks by oral gavage with different doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD),2,3,7,8-tetrabromodibenzo-p-dioxin (TBDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-pentachlorodibenzofuran (1-PeCDF), 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF), octachlorodibenzofuran (OCDF), 3,3',4,4',5-pentachlorobiphenyl (126), 3,3',4,4',5,5'-hexachlorobiphenyl (169), 2,3,3',4,4'-pentachlorobiphenyl (105), 2,3',4,4',5-pentachlorobiphenyl (118), and 2,3,3',4,4',5-hexachlorobiphenyl (156). All of the chemicals examined exhibited dose-dependent increases in the liver/fat concentrations except PCBs 105, 118, and 156. While TCDD is the most potent toxicant in this class of chemicals, 4-PeCDF, PeCDD, OCDF, TCDF, and PCB126 were sequestered in hepatic tissue to a greater extent than was TCDD. The high affinity for hepatic tissue supports the presence of an inducible hepatic binding protein for some dixin-like chemicals. The differences in disposition between these chemicals suggests that pharmacokinetic differences between congeners is important in the relative potency of these chemicals.

Daily cycle of bHLH-PAS proteins, Ah receptor and Arnt, in multiple tissues of female Sprague-Dawley rats

The aryl hydrocarbon receptor (AhR) shares a common PAS domain with a number of genes that exhibit a pronounced circadian rhythm. Therefore, this study examined the daily cycle of AhR and AhR nuclear translocator (Arnt) protein expression in multiple tissues of female Sprague-Dawley rats. Rats were euthanized at 4, 7, and 11 am and 4, 7, and 11 pm after which whole tissue homogenates were made from multiple tissues. Western blot analysis showed that the daily cycle of relative AhR protein expression exhibits a similar oscillation pattern in the liver, lungs, and thymus. The daily cycle of relative Arnt protein expression exhibits a similar oscillation pattern in the liver and lungs. The apparent daily cycle of AhR and Arnt protein expression in multiple tissues was not observed within the spleen. This preliminary report is the first study to suggest that the PAS proteins, AhR and Arnt, exhibit a daily oscillation pattern within multiple target tissues which may give insight into the tissue-specific toxic and biochemical responses mediated through this dimerization pair, as well as the physiological function of these proteins.

2,3,7,8-Tetrachlorodibenzo-p-dioxin in pregnant Long Evans rats: disposition to maternal and embryo/fetal tissues

Prenatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) interferes with fetal development at doses lower than those causing overt toxicity in adult animals. In a multigeneration study (Murray et al., 1979), female rats that were administered 0.01 microgram TCDD/kg/day in their diet did not experience reduced fertility; however, reduced fertility was seen in the F1 and F2 generations. Exposure to TCDD during development produces alterations in the reproductive system of the developing pups, such as delayed puberty and reduced sperm counts in males (Mably et al., 1992a; Gray et al., 1995) and malformations in the external genitalia of females (Gray and Ostby, 1995). Therefore, the objectives of this study were to determine maternal and fetal tissue concentrations of TCDD that are associated with the adverse reproductive effects seen by Gray and co-workers. Pregnant Long Evans rats received a single oral dose of 1.15 micrograms [3H]TCDD/kg on Gestation Day (GD) 8 and maternal as well as fetal tissue concentrations of TCDD were measured on GD9, GD16, and GD21. On GD9, the highest level of TCDD localized in the maternal liver (25.1% dose). In addition, the amount reaching all the embryos on GD9 was 0.01% of the administered dose, which resulted in a concentration of 0.02% dose/g. The amount of TCDD reaching the fetal compartment (fetuses + placentas) increased to 0.12% dose/tissue on GD16 and 0.71% by GD21. The concentration of TCDD within the fetal compartment (0.01% dose/g) on GD16 was comparable to that found in the maternal blood and spleen. Concentrations of TCDD in a single embryo/fetus were 39.6, 18.1, and 22.1 pg/g on GD9, GD16, and GD21, respectively. Estimates of hepatic half-life of elimination in pregnant rats suggested that TCDD may be eliminated faster in pregnant LE rats. Therefore, measurements of biliary elimination were made in pregnant and nonpregnant LE rats to compare rates of metabolism; however, biliary elimination of TCDD is not affected by pregnancy. In conclusion, this dose administered during a critical period of organogenesis causes adverse effects on the developing reproductive system of rodents. This dose produced a body burden of 22.1 pg TCDD/g within a single fetus on GD21. This indicates that low-level TCDD exposure during the perinatal stage of life can produce adverse effects within the developing pups.

A pharmacodynamic analysis of TCDD-induced cytochrome P450 gene expression in multiple tissues: dose- and time-dependent effects

The ability of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) to alter gene expression and the demonstration that the induction of CYP1A2 is responsible for hepatic TCDD sequestration suggest that both pharmacokinetic and pharmacodynamic events must be incorporated for a quantitative description of TCDD disposition. In this paper, a biologically based pharmacodynamic (BBPD) model for TCDD-induced biochemical responses in multiple tissues was developed. The parameters responsible for tissue response were estimated simultaneously with a refined physiologically based pharmacokinetic (PBPK) model developed by Wang et al. (1997a), by using the time-dependent effects of TCDD on induced CYP1A1/CYP1A2 gene expression in multiple target tissues (liver, lungs, kidneys, and skin) of female Sprague-Dawley rats treated with 10 microgram TCDD/kg for 30 min, 1, 3, 8, or 24 h, or 7, 14, or 35 days. This refined BBPD model developed based on the time-course of TCDD-induced CYP1A1/CYP1A2 protein expression, and associated enzymatic activities well described the dose-dependent effects of TCDD on cytochrome P450 protein expression and associated enzyme activities in the multiple tissues of female Sprague-Dawley rats at 3 days following a single exposure to TCDD (0.01-30.0 micromgram TCDD/kg). This is the first BBPD model to quantitatively describe the time- and dose-dependent effects of TCDD on induced CYP1A1/CYP1A2 protein expression and associated enzyme activities in multiple target tissues for TCDD-induced biochemical responses.

Effects of age, sex, and pharmacologic agents on the biliary elimination of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in F344 rats

The extreme biological persistence of 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) is due primarily to its resistance to metabolic transformation. Previous studies in several species have found hepatic metabolism to be rate-limiting for TCDD elimination, with resulting metabolites excreted primarily in feces via the bile. Using short-term biliary excretion of [3H]TCDD metabolites as an indirect measure of metabolism, groups of F344 rats were used to evaluate separately the effects of age, sex, and acute induction or inhibition of key hepatic enzymes. Adult and juvenile male and female rats were used for sex comparisons, and senescent male rats were used to explore possible changes in TCDD metabolism with age. Various pretreatments were used: phenobarbital (PB) and dexamethasone (DEX), to induce hepatic cytochrome P450 isozymes; and suicide substrate 1-aminobenzotriazole (ABT), to produce P450 inhibition. For all animals, surgical cannulation of the common bile duct and 6-hr bile collection were performed under constant anesthesia. [3H]TCDD (1 nmol/kg) was administered via the femoral vein. Naive adult male and female rats excreted approximately 0.7% and approximately 0.4% of [3H]TCDD-derived radioactivity, respectively. Biliary excretion of radioactivity in both male and female juvenile rats was similar to that of adult males; senescent male rats excreted less. Pretreatment with PB, DEX, or ABT resulted in similar decrease in biliary excretion of TCDD-derived radioactivity as observed in senescent male rats.

Subchronic/chronic toxicity of a mixture of four chlorinated dibenzo-p-dioxins in rats. II. Biochemical effects

Groups of 20 male and 20 female rats were given five different oral doses of a mixture of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1, 2,3,7,8-pentaCDD, (PCDD) 1,2,3,4,7,8-hexaCDD (HxCDD), and 1,2,3,4,6, 7,8-heptaCDD (HpCDD) divided into four daily loading doses and six biweekly maintenance doses. PCDD and HxCDD were used as positive controls. The dosing period was 13 weeks, after which half of the rats were necropsied and the rest provided with an off-dose period of another 13 weeks. Liver ethoxyresorufin O-deethylase (EROD) activity was dose-dependently increased in rats dosed with the mixture starting at the lowest dose (13- to 16-fold increase), with the effect reaching maximum at the middle dosage (74- to 112-fold increase), as well as in the positive control groups. There was some indication of reversibility at the lower doses and in positive controls during the off-dose period. The activity of phosphoenolpyruvate carboxykinase (PEPCK) in liver was dose-dependently decreased (maximally by 51%). This effect was more distinct in males than in females. Liver tryptophan 2,3-dioxygenase (TdO) activity decreased maximally by 53% at the two highest doses. This effect was more distinct in females than in males. Serum tryptophan concentrations were increased in rats moribund due to wasting. Some reversibility was apparent by the end of the off-dose period regarding all three biochemical markers of CDD toxicity. Serum glucose concentrations were decreased at the three highest doses of the mixture and in positive controls, maximally by 30%, with some reversibility during the off-dose period. There was a dose-dependent decrease of serum thyroxine (T4) concentrations in rats given the mixture and in the PCDD and HxCDD dosage groups (maximally by 69%), with some reversibility in males during the off-dose period. Serum triiodothyronine (T3) levels were not much affected, except that they tended to be decreased in rats moribund with hemorrhage or anemia. The results demonstrate that comparable biochemical changes occur after multiple as after single dosing with CDDs and that TEFs derived from acute studies can be used to predict the toxicity of mixtures of CDDs regardless whether they are administered as single compounds or as a mixture. This study supports the validity of the toxic equivalency factor (TEF) method and the notion of additive toxicity for CDDs as currently used in the risk assessment of these compounds.

Subchronic/chronic toxicity of a mixture of four chlorinated dibenzo-p-dioxins in rats. I. Design, general observations, hematology,and liver concentrations

Groups of 20 male and 20 female rats were administered five different doses of a mixture of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7,8-pentaCDD (PCDD), 1,2,3,4,7,8-hexaCDD (HxCDD), and 1,2,3,4,6,7,8-heptaCDD (HpCDD). Doses were selected based on relative potency factors derived from acute toxicity data and a previous subchronic study with HpCDD. The mixture was constituted such that each of the congeners contributed one fourth to total toxic equivalency. Total doses were divided into four daily loading doses and six biweekly maintenance doses. The highest total dose for males was 17.5 microg/kg of TCDD, 87.5 microg/kg of PCDD, 350 microg/kg of HxCDD, and 2500 microg/kg of HpCDD. Positive controls were administered PCDD (350 microg/kg) or HxCDD (1400 microg/kg). Females were given 1.5 times lower doses than males. The dosing period was 13 weeks, after which half of the rats were necropsied and the rest provided with an off-dose period of another 13 weeks. Liver concentrations as determined by GC-MS reflected the doses administered. Body weight gain was dose-dependently reduced throughout the study. Mortality rates at the end of the off-dose period were 80 and 60% for the two highest dosages (mixture) in males and 70 and 10% for females. Clinical signs and necropsy findings suggested that the cause of death was related to wasting, hemorrhage, and anemia. Prothrombin times were prolonged and platelet counts were decreased in some rats receiving high doses. This study provides in vivo support for the validity of the assumption of additive toxicity of CDDs as currently used in the toxicity equivalency factor approach to assess the toxicity of mixtures of CDDs.

Female Sprague-Dawley rats exposed to a single oral dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin exhibit sustained depletion of aryl hydrocarbon receptor protein in liver, spleen, thymus, and lung

There is currently little information concerning the time-dependent relationship between 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure and aryl hydrocarbon receptor (AHR) and aryl hydrocarbon receptor nuclear translocator (ARNT) protein concentration in vivo. Therefore, female Sprague-Dawley rats were given a single oral dose of TCDD (10 micrograms/kg), and the AHR and ARNT protein concentrations in liver, spleen, thymus, and lung determined by Western blotting. In liver, the concentration of AHR protein was significantly reduced 8 and 24 h postdosing as compared to time-matched controls. In spleen and lung, the concentration of AHR protein was reduced 3, 8, 24, and 168 h posttreatment compared to time-matched controls but returned to control levels by 336 h. In thymus, reductions in AHR protein concentration were observed 8, 24, 168, and 336 h postdosing as compared to time-matched controls. Significant reductions in the concentration of ARNT protein were not observed in any of the TCDD-exposed tissues. Functional studies in cell culture showed that exposure of a mouse hepatoma cell line (Hepa-1c1c7) and a rat smooth muscle cell line (A-7) to TCDD (1 nM) for 12 days resulted in a 50% reduction in TCDD-inducible reporter gene expression following subsequent challenge by an additional dose of TCDD (1 nM). Collectively, these results show that (i) TCDD-mediated depletion of AHR occurs in vivo, (ii) AHR protein does not rapidly recover to pretreatment levels even though the tissue concentration of TCDD has fallen, and (iii) reduction in AHR protein concentration correlates with reduction in TCDD-mediated reporter gene expression in mammalian culture cells.

Induction of oxidative stress in brain tissues of mice after subchronic exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The ability of single doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to induce oxidative stress in hepatic and some extrahepatic tissues of animals is well documented. However, no previous study has examined the ability of TCDD to induce oxidative stress and tissue damage in brain in vivo. In this study the ability of TCDD to induce oxidative stress in brain tissues of mice was studied after subchronic exposures. Groups of female B6C3F1 mice were treated orally with TCDD (0, 0.45, 1.5, 15, and 150 ng/kg/day) for 13 weeks, 5 days/week. The animals were euthanized 3 days after the last treatment and brain tissues were collected. Biomarkers of oxidative stress including production of superoxide anion, lipid peroxidation, and DNA-single-strand breaks (SSB) were determined. TCDD treatment resulted in significant and dose-dependent increases in the production of superoxide anion as assessed by reduction of cytochrome c. Significant increases were also observed in lipid peroxidation and DNA-SSB in those tissues, as assessed by the presence of thiobarbituric acid-reactive substances and the alkaline elution technique, respectively. These results clearly indicate that subchronic exposure to low doses of TCDD can induce oxidative tissue damage in brain tissues which may at least in part play a role in the effects of TCDD on the central nervous system.

Dose-response relationships for polyhalogenated dioxins and dibenzofurans following subchronic treatment in mice. I. CYP1A1 and CYP1A2 enzyme activity in liver, lung, and skin

The dose-response relationships for induction of liver, lung, and skin ethoxyresorufin-O-deethylase (EROD) activity and liver acetanilide-4-hydroxylase (ACOH) activity following subchronic exposure to either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1,2,3,7,8-pentachlorodibenzo-p-dioxin, 2,3,7,8-tetrabromodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,7,8-pentachlorodibenzofuran (1-PeCDF), 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF), or octachlorodibenzofuran (OCDF) were determined in female B6C3F1 mice in order to estimate the relative enzyme inducing potency of these chemicals in three different tissues. The relative potencies were calculated based on tissue concentrations as well as administered dose. A dose-dependent induction of EROD activity in liver, lung, and skin and of ACOH activity in liver was found for all seven chemicals. When based on administered dose, the relative potencies for specific congeners did not vary substantially among tissues. The relative potencies for TCDF and 1-PeCDF, congeners which have much shorter half-lives than TCDD, increased for all enzymes when estimated from tissue concentrations. The relative potency of OCDF, which is poorly absorbed, was greater when estimated from tissue concentrations than when estimated from administered dose. 4-PeCDF is highly sequestered in hepatic tissue and when the relative potency was estimated based on tissue concentration, its potency for skin enzyme induction increased. These data indicate that the relative potency of these chemicals is influenced not only by the relative binding affinity to the Ah receptor, but also by differences in pharmacokinetic properties of these chemicals. In addition, it may be useful to derive two sets of toxic equivalency factor values, one used for estimating intake equivalents and the other for estimating tissue equivalents.

Determination of parameters responsible for pharmacokinetic behavior of TCDD in female Sprague-Dawley rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most toxic member of a class of planar and halogenated chemicals. Improvements in exposure assessment of TCDD require scientific information on the distribution of TCDD in target tissues and cellular responses induced by TCDD. Since 1980, several physiologically based pharmacokinetic (PBPK) models for TCDD and related compounds have been reported. Some of these models incorporated the induction of a hepatic binding protein in response to interactions of TCDD, the Ah receptor, and DNA binding sites and described the TCDD disposition in a biological system for certain data sets. Due to the limitations of the available experimental data, different values for the same physical parameters of these models were obtained from the different studies. The inconsistencies of the parameter values limit the application of PBPK models to risk assessment. Therefore, further refinement of previous models is necessary. This paper develops an improved PBPK model to describe TCDD disposition in eight target tissues. The interaction of TCDD with the Ah receptor and with hepatic inducible CYP1A2 were also incorporated into the model. This model accurately described the time course distribution of TCDD following a single oral dose of 10 microg/kg, as well as the TCDD concentration on Day 3 after six different doses, 0.01, 0.1, 0.3, 1, 10, and 30 microg TCDD/kg, in target tissues. This study extends previous TCDD models by illustrating the validity and the limitation of the model and providing further confirmation of the potential PBPK model for us in optimal experimental design and extrapolation across doses and routes of exposure. In addition, this study demonstrated some critical issues in PBPK modeling.

Subchronic/chronic toxicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) in rats. Part I. Design, general observations, hematology, and liver concentrations

Groups of 20 male and 20 female adult Sprague-Dawley rats were given five different doses of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD). Total doses for males and females (30.9/18.5, 370/222, 2222/1333, 6667/4000, and 10000/6000 microg/kg) were divided into four daily loading doses and six biweekly maintenance doses. Positive controls were administered 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD, total dose 70/41.9 microg/kg). Liver concentrations, as determined by GC/MS, reflected quite accurately the calculated dose ratios. The dosing period was 13 weeks, after which half of the rats were necropsied and the rest provided with an off-dose period of another 13 weeks. Body weight gain was dose-dependently reduced throughout the study. Mortality occurred dose-dependently, starting on Day 22 and continuing until the end of the off-dose period. Mortality rates at the end of the off-dose period were 90 and 40% for males and 60 and 10% for females in the two highest dose groups. Clinical signs and necropsy findings suggested that the cause of death was related to wasting (early deaths), gastrointestinal and nasal hemorrhage (between Days 64 and 126), or anemia (late deaths, after Day 111). Prothrombin times were prolonged intermittently, mainly at the highest dose of HpCDD. Platelet counts were dose-dependently decreased at the two highest doses of HpCDD and in the TCDD-treated group. This study demonstrates that the relative potency derived from acute toxicity studies is the same as that observed in this subchronic/chronic toxicity study of HpCDD and TCDD, confirming the validity of 0.007 as the toxic equivalency factor (TEF) for HpCDD, which is in good agreement with the international TEF of 0.01.

Subchronic/chronic toxicity of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) in rats. Part II. Biochemical effects

Groups of 20 male and 20 female rats were given five different oral doses of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) or one dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) divided into four daily loading doses and six biweekly maintenance doses. The dosing period was 13 weeks, after which half of the rats were necropsied and the rest assigned to an off-dose period of another 13 weeks. At the end of the dosing period, liver ethoxyresorufin O-deethylase (EROD) activity was dose-dependently increased starting at the lowest dose (7- to 10-fold) with maximum induction (50- to 100-fold) at the middle or second highest dose. There was a slight reversibility of this effect in HpCDD-treated rats, particularly at lower doses, and a pronounced reversibility in TCDD-dosed rats, both in accordance with respective toxicokinetics. The activity of phosphoenolpyruvate carboxykinase in liver was dose-dependently decreased (up to 60%) at the two or three highest doses of HpCDD and also in the TCDD dosage group. Liver tryptophan 2,3-dioxygenase activity was decreased at the two highest doses of HpCDD (up to 41%), particularly in females. Serum tryptophan concentrations were elevated in rats found moribund due to wasting. There was a dose-dependent decrease in serum glucose concentrations (up to 30%) at the end of the dosing period. Serum thyroxin (T4) concentrations showed a dose-dependent decrease (78% at the highest dose) beginning in the middle dose for HpCDD and in the TCDD dosage group. Serum triiodothyronine (T3) concentrations were only slightly affected, except that they were somewhat decreased in moribund animals. The results demonstrate that similar biochemical changes occur in rats after single as after multiple dosing with HpCDD and TCDD. Based on these endpoints, the relative potency of HpCDD after subchronic exposure is in agreement with the international toxic equivalency factor (I-TEF) of 0.01 and, more specifically, with a TEF of 0.007 based on LD50 values in the same strain of rats.

Role of CYP1A2 in hepatic sequestration of dioxin: studies using CYP1A2 knock-out mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, dioxin), most potent of the polyhalogenated aromatic hydrocarbons, has been studied in a variety of genetically normal species. Transgenic mice lacking a cytochrome P450 1A2 gene were used to study the influence of the CYP1A2 gene on the hepatic sequestration and distribution of TCDD, 4-PeCDF (2,3,4,7,8-pentachlorodibenzofuran; dioxin-like compound), and PCB 153 (2,2',4,4',5,5'-hexachlorobiphenyl; non-dioxin-like PCB (polychlorinated biphenyl)). The knock-out mice were compared to their age-matched lineage strains of C57BL/6N (1A2+/+; Ah(b)) and 129/Sv (1A2+/+; Ah(d)) for each compound. As demonstrated by the liver-to-adipose tissue (L/F) concentration ratios, there was no hepatic sequestering of TCDD and 4-PeCDF in the transgenic knock-out mice.

Promotion of endometriosis in mice by polychlorinated dibenzo-p-dioxins, dibenzofurans, and biphenyls

Previous studies showed exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) enhances the development of endometriotic lesions. In this study we examined the effects of other polyhalogenated aromatic hydrocarbons on endometriotic proliferation. B6C3F1 female mice were treated via oral gavage a total of five times, with 3 weeks between each dosing, with 0, 1, 3, or 10 micrograms 2,3,7,8,-TCDD/kg body weight (bw); 3 or 30 mg 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153)/kg bw; 100, 300, or 1000 micrograms 3,3',4,4',5-pentachlorobiphenyl (PCB 126)/kg bw; 10, 30, or 100 micrograms 2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF)/kg bw; or 2 or 20 mg 1,3,6,8-TCDD/kg at 10 ml/kg bw. Endometriosis was surgically induced during the week of the second dosing. Three weeks following the final dose, the mice were euthanized and endometriotic lesions, whole body, liver, ovaries, uterine horn, and thymus were weighted, and lesion diameters were measured. Lesions, uterine horns, and ovaries were fixed for histopathology and livers were processed for measurement of ethoxyresorufin O-deethylase (EROD) activity. Both 2,3,7,8-TCDD (1 and 3 micrograms/kg bw) and 4-PeCDF (100 micrograms/kg bw) significantly enhanced the growth of endometrial lesions. No statistically significant increase in endometriotic lesion size was detected in animals treated with either PCB 126 or with the highest dose of 2,3,7,8-TCDD, possibly due to the effects of histologically observed ovarian toxicity. The nondioxin-like compounds, PCB 153 and 1,3,6,8-TCDD, produced no observable effects on endometriosis. Hepatic EROD activity was significantly induced by 2,3,7,8-TCDD, 4-PeCDF, and PCB 126, but not by PCB 153 or 1,3,6,8-TCDD. The results of this study provide preliminary support for the hypothesis that halogenated aromatic hydrocarbon-promoted endometriosis may be Ah receptor mediated.

Opposite effects of 2,2',4,4',5,5'-hexachlorobiphenyl and 2,3,7,8-tetrachlorodibenzo-p-dioxin on the antibody response to sheep erythrocytes in mice

The effect that cotreatment with 2,2',4,4',5,5'-hexachlorobiphenyl (PCB153) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has on the antibody plaque-forming cell (PFC) response to sheep red blood cells (SRBCs) was determined in female B6C3F1 mice. Groups of eight mice per group were given a single oral dose of PCB153 alone (0, 3.58, 35.8, or 358 mg/kg), TCDD alone (0, 0.1, 1, or 10 micrograms/kg), and all possible combinations of these doses in corn oil 7 days prior to immunization with SRBCs. Separate groups of mice were given phenobarbital (PB) parenterally by intraperitoneal injection at a dosage of 160 mg/kg/day for 3 days. Four days after intravenous immunization, body, spleen, thymus, and liver weights and the PFC response to SRBCs were determined. Exposure to TCDD alone resulted in a dose-related suppression of the PFC response, with significant suppression at 1 and 10 micrograms/kg. In contrast, exposure to PCB153 alone resulted in the enhancement of the PFC response at 358 mg/kg. Combined exposure to 358 mg/ kg PCB153 and TCDD resulted in no change (PCB153 + 0.1 microgram/ kg TCDD) or suppression (PCB153 + 1 or 10 micrograms/kg TCDD) of the PFC response relative to PCB153 alone; however, the PFC response was enhanced (PCB153 + 0.1 microgram/kg TCDD), unaffected (PCB153 + 1 microgram/kg TCDD), or suppressed (PCB153 + 10 micrograms/kg TCDD) relative to corn oil controls. PB did not affect the PFC response to SRBCs, despite a 13-fold induction of hepatic pentoxyresorufin O-dealkylase (PROD) activity. These results suggest that PCB153 enhancement of the PFC response is not related to PROD induction and that it acts as a functional antagonist rather than an aryl hydrocarbon receptor or dispositional antagonist. By enhancing the PFC response to SRBCs, PCB153 raises the "setpoint" response level. Consequently, cotreatment with an immunosuppressive dose of TCDD fails to suppress the PFC response relative to corn oil controls, while clearly suppressing it relative to the appropriate control, PCB153 alone.

A multicompartment geometric model of the liver in relation to regional induction of cytochrome P450s

A geometric, multicompartment model of the liver was developed to examine regional protein induction and to provide model output suitable for predicting the degree of induction in both the whole liver and in specific regions. The model was based on functional hexagonal arrays within the liver. A geometric representation was used to divide these functional units into five zones: a concentric periportal zone, a fenestrated periportal region that interconnects among multiple functional units, and three concentric centrilobular areas, referred to, respectively, as zones 1 through 5. The surface areas (and volumes for hexagonal cylinders) of these live zones were, respectively, 13.5, 25.2, 33.9, 20.3, and 6.8% of the total liver. The pharmacokinetic model for induction had dissociation constants (Kd) and Hill constants (n) for interactions of transcriptional activator-ligand complexes with response elements on DNA. Estimates of regional induction were converted to color intensities to "paint" the two-dimensional liver for a visual comparison with immunohistochemical observations. To obtain sharp moving boundaries of induced areas with increasing dose (as noted in various experiments), n values in each subcompartment must be large. To create realistic total induction curves that are relatively smooth, the differences in Kd values between adjacent subcompartments must be less than fivefold. Because of the high n values, the low-dose induction characteristics predicted with the multicompartment liver model differ significantly from those predicted with a model that considers the liver as a single homogeneous compartment.

Regional hepatic CYP1A1 and CYP1A2 induction with 2,3,7,8-tetrachlorodibenzo-p-dioxin evaluated with a multicompartment geometric model of hepatic zonation

A physiologically based pharmacokinetic (PBPK) model for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was combined with a five-compartment geometric model of hepatic zonation to predict both total and regional induction of CYP450 proteins within the liver. Three literature studies on TCDD pharmacokinetics and protein induction in female rats were analyzed. In simulating low-dose behavior for mRNA in whole liver and, particularly, in representing immunohistochemical observations, the five-compartment model was more successful than conventional homogeneous one-compartment liver models. The five-compartment liver model was used with the affinity of TCDD for the Ah receptor (AhR) held constant across all the liver (Kb = 0.2 nM). The presumed affinities of the AhR-TCDD complex for TCDD responsive elements in the CYP1A1 (Kd1) and CYP1A2 (Kd2) genes varied between adjacent compartments by a factor of 3. This parameterization leads to predicted 81-fold differences in affinities between the centrilobular and the periportal regions. The affinities used for AhR-TCDD complex binding to TCDD response elements for CYP1A2 in compartment 3 (the midzonal area) ranged from 0.08 to 1.0 nM in the three studies modeled. For CYP1A1 the corresponding dissociation constant in compartment 3 varied from 0.6 to 2.0 nM. In each compartment, the Hill coefficient for induction had to be 4 or greater to match the immunohistochemical results. This multi-compartment liver model is consistent with data on protein and mRNA induction throughout the liver and on the regional distribution of these proteins. No previous model has incorporated regional variations in induction. The PBPK analysis based on the multicompartment liver model suggests that the low-dose behavior for hepatic CYP1A1/CYP1A2 induction by TCDD is highly non-linear.

Differential time-course and dose-response relationships of TCDD-induced CYP1B1, CYP1A1, and CYP1A2 proteins in rats

This study examined the relationship between dose- and time-dependent hepatic localization of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and expression of CYP1B1, CYP1A1 and CYP1A2 proteins. A dose-dependent increase in hepatic TCDD in female Sprague-Dawley rats treated with 0.01-30.0 microg TCDD/kg was observed. TCDD induced CYP1A1 protein in rats treated with 0.3 microg TCDD/kg or higher. TCDD induced CYP1A2 and CYP1B1 proteins in rats treated with 1.0 microg TCDD/kg or higher. The in vivo ED50 (microg TCDD/kg) for TCDD-induced CYP1A1, CYP1A2 and CYP1B1 proteins were 0.22, 0.40 and 5.19, respectively. Hepatic accumulation of TCDD reached a maximum at 8 hours post dosing with a t1/2 of approximately 10 days. TCDD-induced CYP1A1/CYP1A2 protein expression was increased time-dependently, reaching a maximum at 3 days after dosing and remaining elevated for 35 days. In contrast, TCDD-induced CYP1B1 protein showed significant expression at 3 days after dosing and decreased to basal concentrations by 35 days. This study demonstrates that TCDD exhibits differential dose-response and time-course relationships on hepatic localization and cytochrome P-450 protein expression.