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.

Subcellular localization of TCDD differs between the liver, lungs, and kidneys after acute and subchronic exposure: species/dose comparisons and possible mechanism

Subcellular localization of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds has been examined only in the liver. The objective of this study was (1) to examine and compare the subcellular distribution of TCDD within hepatic and nonhepatic (lungs/kidneys) tissues of female Sprague-Dawley rats acutely exposed to TCDD, (2) to analyze species comparisons in the subcellular localization of TCDD in multiple tissues, (3) to investigate the effect of dose on subcellular distribution of TCDD, (4) to analyze the effect of subchronic exposure on the subcellular distribution of TCDD, and (5) to examine one possible mechanism for subcellular localization of TCDD. Female Sprague-Dawley rats and B6C3F1 mice received a single oral dose of 0.1, 1.0, or 10 microg [3H]TCDD/kg body weight and subcellular fractions of the liver, lungs, and kidneys were prepared by differential centrifugation 3 days after exposure. Analysis of the rat subcellular fractions revealed that TCDD was equally distributed between the hepatic P9 (mitochondrial, lysosomal, and nuclear) and S9 (cytosol and microsomal) fractions at all doses tested. In contrast, TCDD was concentrated in the P9 of rat nonhepatic tissues at all doses studied. Differential centrifugation of the hepatic S9 showed that TCDD was localized within the hepatic P100 (microsomal) fraction at all doses tested. In contrast, TCDD localized in pulmonary and renal S100 (cytosolic) fractions at all doses. The subcellular distribution of TCDD in the liver and lungs of acutely exposed B6C3F1 mice was similar to that observed in the rats. Although TCDD was concentrated within the renal P9, the remainder of TCDD in the S9 was evenly distributed between the S100 and the P100 fractions of acutely exposed B6C3F1 mice. Subchronic exposure of B6C3F1 mice to 1.5 or 150 ng [3H]TCDD/kg/day revealed that increasing dose resulted in equal distribution of TCDD between the hepatic S9 and P9 versus concentration in the renal P9. In addition, a dose-dependent increase in accumulation of TCDD in the hepatic P100 was accompanied by a dose-dependent increase in TCDD localization in the renal S100 of mice subchronically exposed to TCDD. TCDD exposure in rats resulted in a dose-dependent increase in the induction of CYP1A1 protein and associated enzyme activity in hepatic, pulmonary, and renal microsomes. TCDD-induced CYP1A2 protein levels and associated enzymatic activity were only present in hepatic microsomes. This is the first report to suggest that subcellular distribution of TCDD differs between hepatic and nonhepatic tissues and demonstrate that the liver-specific microsomal localization of TCDD in female Sprague-Dawley rats also occurs in the liver of female B6C3F1 mice acutely or subchronically exposed to TCDD. In addition, these data are consistent with the hypothesis that the hepatic sequestration of TCDD is due to an interaction with CYP1A2. Furthermore, the lack of pulmonary/renal sequestration coupled with the lack of localization of TCDD in pulmonary/renal microsomes also supports the role of CYP1A2 as a hepatic microsomal binding protein involved in TCDD sequestration..

Rapid distribution of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to embryonic tissues in C57BL/6N mice and correlation with palatal uptake in vitro

2,3,7,8-Tetracholoridbenzo-p-dioxin (TCDD) is a developmentally toxic environmental contaminant capable of inducing cleft palate and hydronephrosis in embryonic C57BL/6N mice. In this study, the disposition of TCDD was determined in pregnant C57BL/6N mice in the 24 hr immediately following oral administration on Gestation Day (GD) 12. TCDD was detected in maternal blood, liver, and fat and in the placenta, embryonic liver, and palate within 30 min after dosing on GD 12. The levels peaked in blood and placenta at 3 hr and in the other tissues at 8 hr. Levels of TCDD decreased slightly after 8 hr in embryonic liver and palate. In vitro systems were used to study the mechanisms of action of TCDD and in these models exposure is typically reported as concentration of TCDD in the culture medium. The present study is the first to allow a direct comparison of the level of TCDD in embryonic tissue after in vivo and in vitro exposures. Uptake of TCDD was determined in embryonic palatal organ culture and tissue levels were then expressed in comparable units for both in vivo and in vitro exposures. The data provide new information on distribution in the pregnant mouse and the embryo and also show that the palatal organ culture model provides a reasonable dosimetric representation of in utero exposure.

Interactive effects between 2,3,7,8-tetrachlorodibenzo-p-dioxin and 2,2',4,4',5,5'-hexachlorobiphenyl in female B6C3F1 mice: tissue distribution and tissue-specific enzyme induction

The distribution of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) was studied in female B6C3F1 mice. Single doses of TCDD alone (0, 0.1, 1, or 10 micrograms [3H]TCDD/kg), PCB 153 alone (0, 3.58, 35.8, or 358 mg [14C]PCB 153/kg), and all possible combinations of these doses were administered in corn oil, po. At 7 days after dosing, 11 different tissues were analyzed for radioactivity. When TCDD was administered alone, TCDD-derived radioactivity distributed to all tissues in a dose-dependent manner, increasing with dose in the liver, while decreasing (as a percentage of the administered dose) in all other tissues. When PCB 153 was administered alone, the PCB 153 concentration was dose-dependently (percentage of dose) decreased in liver, skin, lung, adrenals, kidney, and blood; no dosimetric effects were observed in the other organs. Coadministration of low doses of both TCDD and PCB 153 resulted in little or no effect on the distribution of either compound. Interactive effects occurred in the pharmacokinetic behavior of both compounds only at higher doses. For example, the amount of TCDD in the liver was increased by 358 mg PCB 153/kg. In most other organs administration of PCB 153 resulted in a dose-dependent decrease in the TCDD content. Coadministration of PCB 153 with 10 micrograms TCDD/kg increased PCB 153 in the liver, but not in other tissues. These results clearly demonstrate that interactive effects on pharmacokinetic behavior occur only at high doses.

Relative potencies of polychlorinated dibenzo-p-dioxins, dibenzofurans, and biphenyls derived from hepatic porphyrin accumulation in mice

Hepatic porphyrin accumulation was studied after subchronic dosing of female B6C3F1 mice by gavage with single congeners of polychlorinated or polybrominated dibenzo-p-dioxins (PCDDs, PBDDs), polychlorinated dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs). Quantitative hepatic porphyrin profile analyses in selected samples showed uroporphyrin and heptacarboxylporphyrin as the main porphyrins detected. Dose-dependent increases in total hepatic porphyrins were found for all congeners tested. At lower dose levels, relative potencies, based on administered dose as well as target tissue dose, of PCDDs, PCDFs, and coplanar PCBs, using 2,3,7,8-tetrachlorodibenzo-p-dioxin as a reference compound, were in the same range as those previously derived from the induction of hepatic CYP1A1 and CYP1A2 enzyme activities. CYP1A2 has been reported to be involved in the oxidation of uroporphyringen III to uroporphyrin III. All these facts suggest the involvement of an aryl hydrocarbon receptor-medicated mechanism in hepatic porphyrin accumulation, possibly via CYP1A2. However, the relative potencies of the mono-ortho-substituted PCBs were higher for hepatic porphyrin accumulation than for hepatic CYP1A1 and CYP1A2 induction. In addition, hepatic porphyrin accumulation was the highest after exposure to mono-ortho-PCBs. Since mono-ortho- substituted PCBs induce the phenobarbital-inducible CYP2B isoforms of cytochrome P450, an additional induction of delta-aminolevulinic acid synthetase may also contribute to hepatic porphyrin accumulation following subchronic exposure to these particular congeners. Relative potencies derived from hepatic porphyrin accumulation after PCDD, PCDF, or coplanar PCB administration are a useful tool in risk assessment. However, the higher potencies of the mono-ortho-substituted PCBs have important implications for risk assessment of these compounds.

Comparison of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) disposition following pulmonary, oral, dermal, and parenteral exposures to rats

In evaluating human health risks posed by dioxins, it is necessary to accurately predict systematic dosimetry or the fate of these chemicals. Pharmacokinetic parameters pertaining to inhalation, ingestion, and dermal absorption may be estimated using animal models. The present study was designed to assess absorption, tissue distribution, and elimination of TCDD following intratracheal instillation (itr.), oral gavage (p.o.), or intravenous administration (i.v.) of 1 nmol [3H]TCDD/kg to male rats; experimental conditions were chosen to permit comparison to a previous dermal disposition study (Banks and Birnbaum, Toxicol. Appl. Pharmacol. 107, 302-310, 1991). After treatment, rats were housed in individual metabolism cages for 3 days with daily excreta collection. Following termination, radioactivity was quantified in tissues and excreta. By 3 days postexposure, fecal excretion accounted for 22 (i.v.), 26 (itr.), and 32% (p.o.) of dose, while urinary excretion was only 2.2 (i.v.), 1.3 (itr), and 1.4% (p.o.). Pulmonary absorption was calculated as 95% of administered dose, while oral absorption was 88%. Dermal absorption of an equivalent administered dose was 40% (Banks and Birnbaum, 1991). For all exposure routes by 3 days, major tissue depots for absorbed dose were liver and fat. Distribution of absorbed dose was 37% (i.v.) and 35% (itr.) to liver and 21% (i.v.) and 16% (itr.) to fat. Oral gavage-treated rats had similar dosimetry (28-30% absorbed dose) in both liver and fat. In contrast following dermal exposure, distribution to liver and fat was 52 and 22%, respectively (Banks and Birnbaum, 1991). Results suggest that inhalation can be an important route for systemic absorption of dioxins. Moreover, all environmentally relevant routes of exposure (oral, dermal, and respiration) must be uniquely considered as important routes of systemic exposure for TCDD and related compounds.

Relationship between CYP1A enzyme activities and protein levels in rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Induction of CYP1A1 is one of the best characterized responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). EROD activity has been used as an enzymatic marker for CYP1A1 following TCDD treatment. Enzymatic markers for the induction of CYP1A2 by TCDD are not as well characterized. The present study examines the relationship between CYP1A1 and CYP1A2 protein and the corresponding enzymatic markers. Induction of hepatic ethoxyresorufin O-deethylase (EROD) activity and methoxyresorufin O-demethylase (MEROD) and acetanilide 4-hydroxylase (ACOH) activity (both markers for CYP1A2) were analyzed in 8-wk-old male and female Fischer 344 rats treated orally with either 0, 0.1, 0.3, 1.0, or 3.0 micrograms TCDD/kg. There were no sex differences in basal EROD or ACOH activity. MEROD activity was significantly greater in control males than in control females. Significant induction of EROD activity in females occurred at slightly lower doses of TCDD compared to males (0.1 vs. 0.3 micrograms/kg, respectively); however, a greater absolute and a larger fold induction of EROD activity was seen in males compared to females at all doses tested except 0.1 micrograms/kg. EROD activity did not attain a maximum in either sex. Similarly, MEROD activity was induced at lower doses of TCDD in females than in males (0.1 vs. 0.3 micrograms/kg, respectively). MEROD activity was maximally induced at 0.3 micrograms/kg in males. In females, MEROD did not attain maximum induction at the doses tested. ACOH activity was induced at doses as low as 0.3 micrograms/kg in both sexes, and the dose-dependent increases in activity were equivalent in males and females. Both ACOH and MEROD activity correlated well with CYP1A2 levels as determined by Western blot analysis, although there was a greater fold induction of protein than either MEROD or ACOH. Although MEROD and ACOH are both markers for the same response, MEROD activity may be a more useful marker because it is the quicker and more sensitive of the two assays.

Effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on influenza virus host resistance in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes numerous immunotoxic effects including thymic involution and an immunosuppression of nonspecific as well as specific cell- and humoral-mediated immunity. TCDD administration to laboratory animals also results in a decreased resistance to numerous bacteria, viruses, and parasites. Effects on virus host resistance appear to be among the most sensitive effects of TCDD immunotoxicity. However, previous studies have not achieved a no effect level. The present studies utilized an influenza virus host resistance model in mice to quantify the sensitivity of this model to TCDD and to determine the NOAEL (no observed adverse effect level) of TCDD for influenza virus. Results indicated that a single dose of TCDD at 0.10, 0.05, or 0.01 microgram/kg resulted in an increased mortality to Hong Kong influenza virus when mice were challenged 7 days after TCDD administration. Increased mortality was not correlated with increased virus titers in the lungs. TCDD at 0.005 or 0.001 micrograms/kg had no effect on influenza-induced mortality. TCDD alone did not affect thymus weight at any dose administered in this study. TCDD also did not alter the virus-enhanced increase in lung weight:body weight ratio nor the virus-induced decrease in thymus weight. Thus, low levels of TCDD exposure lead to enhanced mortality to influenza virus; however, the mechanism of this effect remains to be elucidated. Nonetheless, enhanced mortality to influenza virus in mice following a single dose of 10 ng TCDD/kg represents the most sensitive adverse effect yet reported for TCDD.