Toxicokinetic mixture interactions between chlorinated aromatic hydrocarbons in the liver of the C57BL/6J mouse: 2. Polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and biphenyls (PCBs)

Physiologically based modeling of the accumulation in plasma and tissue lipids of a mixture of PCB congeners in female Sprague-Dawley rats

This study aimed to develop a physiologically based model for simulating the concentrations of polychlorinated biphenyls (PCBs) in tissue and plasma lipids of rats exposed to PCB mixtures. The model was based on the assumption that the neutral lipid fraction is the only critical determinant of the tissue distribution of PCBs, and that the solubility/retention in other tissue components is negligible. The volumes of the model compartments reflected the volumes of neutral lipids, whereas the flow rates corresponded to those of the neutral lipids in blood. Since the equilibrium ratio of PCB concentrations in neutral lipids of tissues and plasma equals 1, the present modeling approach does not require the use of tissue:blood partition coefficients. Metabolism rates were derived from the best visual fit of the model to the PCB concentrations in hepatic lipids determined on d 41 and 90 in rats exposed to a mixture containing 5, 50, or 500 microg PCBs (118, 138, 153, 170, 180 and 187) per kilogram body weight according to various protocols: (a) every-day dosing, (b) once-a-week dosing, (c) consecutive dosing for 13 d with no further treatment, and (d) 13 irregularly spaced doses. The resulting model consistently simulated the concentrations of PCBs in adipose tissue and plasma lipids of rats exposed according to the four described protocols. The physiologically based pharmacokinetic (PBPK) model developed in this study should be useful as a basis for interpretating blood or plasma lipid concentration data on PCBs collected during biomonitoring studies.

Induction of oxidative stress in the tissues of rats after chronic exposure to TCDD, 2,3,4,7,8-pentachlorodibenzofuran, and 3,3',4,4',5-pentachlorobiphenyl

The abilities of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB126), and mixtures of these xenobiotics (toxic equivalents, TEQs) to induce oxidative stress in hepatic and brain tissues of rats have been investigated after chronic (30 wk) exposure to these congeners. TCDD, PeCDF, PCB126, and TEQs were administered daily to groups of rats at doses that corresponded to their toxic equivalency factors (TEFs), and the biomarkers of oxidative stress, including the production of superoxide anion, lipid peroxidation, and DNA single-strand breaks (SSBs), were determined in hepatic and brain tissues at the end of the exposure period. The three chemicals caused similar dose-dependent increases in the production of superoxide anion, lipid peroxidation, and DNA SSBs, which plateaued at certain dose ranges, followed by secondary increases at the higher dose levels. Similar effects were also produced by the TEQs; however, the dose-dependent increases in the biomarkers of oxidative stress were continuous and never achieved plateau levels. Except for PCB126, where statistical analyses revealed greater productions of superoxide anion and lipid peroxidation in brain tissues as compared with hepatic tissues, no significant differences were revealed between the two tissues in response to the other xenobiotics or the TEQs. Nonsignificant differences were also revealed when comparing the effects induced by the TEQs with those induced by the individual chemicals.

Production of superoxide anion, lipid peroxidation and DNA damage in the hepatic and brain tissues of rats after subchronic exposure to mixtures of TCDD and its congeners

In this study the induction of oxidative stress in the hepatic and brain tissues of rats after subchronic exposure to various mixtures of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and two of its congeners, namely 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) and 3,3',4,4',5-pentachlorobiphenyl (PCB 126) was investigated. Four mixtures of TCDD and its congeners, corresponding to 10, 22, 46 and 100 ng of toxic equivalence (TEQ) kg(-1) day(-1), were administered to groups of rats for 13 weeks. The animals were sacrificed at the end of the exposure period and the biomarkers of oxidative stress, including the production of superoxide anion, lipid peroxidation and DNA single-strand breaks (SSBs), were determined in the hepatic and brain tissues. All mixtures caused dose-dependent increases in the production of superoxide anion, lipid peroxidation and DNA SSBs in both tissues, with significantly higher damage in the hepatic compared with the brain tissues. The 22 ng TEQ dose level (TEQ = 22) contains TCDD, PeCDF and PCB 126 at levels that correspond to 7.3, 14.5 and 73.3 ng kg(-1) day(-1), respectively, and it produced effects that correspond to ca. 50% of the maximal production of superoxide anion, lipid peroxidation and DNA SSBs in the hepatic and brain tissues of those animals. Relative to the doses that are required to produce 50% of the maximal production of the biomarkers of oxidative stress by the individual congeners in hepatic and brain tissues of rats, the concentrations of the congeners in TEQ = 22 did result in significant interactivity, probably in the form of additive effects in the hepatic but not in brain tissues.

Contribution of planar (0-1 ortho) and nonplanar (2-4 ortho) fractions of Aroclor 1260 to the induction of altered hepatic foci in female Sprague-Dawley rats

The hepatic tumor promoting activity of the planar 0-1 ortho ( approximately 9.7% w/w) and the nonplanar 2-4 ortho ( approximately 90.3% w/w) fraction of the commercial PCB mixture Aroclor 1260 was studied using a medium-term two-stage initiation/promotion bioassay in female Sprague-Dawley rats. Fractionation was carried out on an activated charcoal column. The composition of the effluent from the column was tested by GC-ECD. The absence of planar compounds in the 2-4 ortho fraction was confirmed by GC-MS analysis. The dioxin-like toxic potency of the fractions was determined with the DR-CALUX assay. The animal experiment was started with the initiation procedure (diethylnitrosamine injection, 30 mg/kg body wt ip, 24 h after (2)/(3) hepatectomy), followed 6 weeks later by the promotion treatment, which consisted of a weekly subcutaneous injection during 20 weeks. Exposure groups (n = 10) received the following treatments (dose/kg body wt/week): Aroclor 1260 (10 mg), 0-1 ortho fraction (0.97 mg), 2-4 ortho fraction (1, 3, or 9 mg), a reconstituted 0-4 ortho fraction (9.97 mg), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153; 1 or 9 mg), 2,3,7,8-TCDD (1 microg; positive control) or corn oil (1 ml; vehicle control). One group did not receive a promotion treatment. All exposure groups exhibited a significantly increased volume fraction of the liver occupied by hepatic foci positive for the placental form of glutathione-S-transferase-p compared to the corn oil control, except for the groups treated with 0-1 ortho fraction and 1 mg PCB 153/kg body wt/week. Approximately 80% of the total tumor promoting capacity of the reconstituted 0-4 ortho fraction could be explained by the 2-4 ortho PCB fraction while the 0-1 ortho fraction had only a negligible contribution. These results suggest that the majority of the tumor promotion potential of PCB mixtures resides in the non-dioxin-like fraction, which is not taken into account in the toxic equivalency factor (TEF) approach for risk assessment of PCBs. This may result in an underestimation of the tumor promotion potential of environmental PCB mixtures.

Toxicokinetics

The toxicokinetic determinants of dioxin and related chemicals depend on three major properties: lipophilicity, metabolism, and binding to CYP1A2 in the liver. The induction of CYP1A2 is partially under the control of the aryl hydrocarbon receptor (AhR). Lipophilicity increases with more chlorination and controls absorption and tissue partitioning. Metabolism is the rate-limiting step for elimination. Induction of CYP1A2 leads to hepatic sequestration of TCDD. Binding to this inducible hepatic protein results in non-linear dose-dependent tissue distribution: with increasing doses, the relative concentration in extra-hepatic tissues decreases while that in liver increases. The induction of this protein occurs in both animals and humans and results in an increase in the liver to fat ratio of these compounds. Humans have similar sensitivities to rodents for dioxin-like compounds when using tissue concentration (from in vitro studies), body burden, average lifetime serum lipid concentration, or lifetime area-under-the-curve concentration based on both low dose (biochemical) and high dose (cancer) driven endpoints. To reach the same tissue concentration in humans as rodents however, humans need a lower daily intake than rodents based on differences in pharmacokinetic behaviour. This clearly indicates that physiologically based pharmacokinetic models should be explored for the estimation of the daily intake of dioxin-like compounds in humans based on tissue dose levels or derivatives of those.

Human risk assessment and TEFs

The concept of toxic equivalency factors (TEFs) has been developed to facilitate risk assessment and regulatory control of exposure to complex PCDD, PCDF and PCB mixtures. Recently the European Centre for Environment and Health of the World Health Organization (WHO-ECEH) and the International Programme on Chemical Safety (IPCS) jointly re-evaluated the TEFs of PCDDs, PCDFs and dioxin-like PCBs for mammals and derived consensus TEFs for birds and fish (Stockholm, 1997). From a mechanistic point of view it can be concluded that, although the quantitative response will vary depending on the congener involved, the occurrence of a common mechanism (binding to the Ah receptor) legitimates the use of the TEF concept across species. But there also is criticism regarding the TEF concept. Pharmacokinetic differences between species can significantly influence the TEF value, and uncertainties due to additive or non-additive interactions, to differences in species responsiveness and to differences in the shape of the dose-response curve might hamper the derivation of consensus TEF values. In this context it should be noted, however, that using TCDD alone, as the only measure of exposure to dioxin-like PCDDs, PCDFs and PCBs, would severely underestimate the risk from exposure to these compounds. Therefore, it can be concluded that, for pragmatic reasons, the TEF concept remains the most feasible approach for risk assessment purposes, in spite of the uncertainties associated with its use.

Induction of altered hepatic foci by a mixture of dioxin-like compounds with and without 2,2',4,4',5,5'-hexachlorobiphenyl in female Sprague-Dawley rats

The hepatic tumor-promoting activity of a mixture of polyhalogenated aromatic hydrocarbons (PHAHs) was studied in a medium term two-stage initiation/promotion bioassay in female Sprague-Dawley rats. The PHAH mixture contained 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1, 2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 2,3,4,7, 8-pentachlorodibenzofuran (PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), 2,3',4,4',5-pentachlorobiphenyl (PCB 118), 2,3,3',4,4', 5-hexachlorobiphenyl (PCB 156), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) and covered >90% of the total toxic equivalents (TEQ) present in Baltic herring. To determine possible interactive effects of di-ortho-substituted PCBs, the PHAH mixture was tested with (PHAH+) and without (PHAH-) PCB 153. Rats were initiated by a diethylnitrosamine injection (30 mg/kg body wt i.p.) 24 h after a partial 23 hepatectomy. Six weeks after initiation, the PHAH mixtures were administered once a week by subcutaneous injections for 20 weeks. Treatment with the PHAH mixtures caused liver enlargement and an increased activity of the hepatic cytochrome P4501A1/2 and P4502B1/2. All PHAH exposure groups exhibited an increased occurrence of hepatic foci positive for the placental form of glutathione-S-transferase. In the PHAH-group dosed 1 microgram TEQ/kg body wt/week, the volume fraction of the liver occupied by foci was significantly lower compared to the TEQ equivalent dosed TCDD group (3.8 vs 8.7%). The volume fraction was significantly increased in the groups treated with 0.5, 1, or 2 micrograms TEQ/kg body wt/week of the PHAH+ mixture (4.5, 5.2, and 6.6%, respectively) compared to the corn oil group (2.0%), but to a lower extent than expected on basis of the TEQ doses. Overall, the TEQ-based administered dose overestimated the observed tumor-promoting effects of this PHAH mixture. The applicability of the toxic equivalency factor concept, the role of differences in toxicokinetic properties and interactive effects of PCB 153 on hepatic deposition of the dioxin-like congeners are discussed.

Toxicokinetics of an environmentally relevant mixture of dioxin-like PHAHs with or without a non-dioxin-like PCB in a semi-chronic exposure study in female Sprague Dawley rats

Female Sprague Dawley rats were treated subcutaneously for 20 weeks with an environmentally relevant mixture of dioxin-like PHAHs with (PHAH+) or without (PHAH-) 2,2',4,4',5,5'-hexachlorobiphenyl. The hepatic retention (% of given dose) of the various PHAH congeners differed considerably and in the following order: 2,3,4,7,8-pentachlorodibenzofuran (30.5-43.1%), 3,3',4,4',5-pentachlorobiphenyl (12.8-17.6%), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (6.9-10.8%), 2,3,7,8-tetrachlorodibenzo-p-dioxin (3.2-4.5%), 2,3,3',4,4',5-hexachlorobiphenyl (1.0-1.7%), 2,2',4,4',5,5'-hexachlorobiphenyl (0.5-0.8%) and 2,3',4,4',5-pentachlorobiphenyl (0.2-0.4%). A decrease of the hepatic retention of 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD and 2,3,4,7,8-PeCDF was found at increasing doses of the PHAH+ mixture. 2,2',4,4',5,5'-Hexachlorobiphenyl increased the hepatic retention (1.3-2 times) of all congeners in the PHAH+ group, compared to the TEQ equivalent dosed PHAH- group. No interactions were observed on the ethoxyresorufin-O-deethylase activity.

Inhibition of 3,3',4,4',5-pentachlorobiphenyl-induced fetal cleft palate and immunotoxicity in C57BL/6 mice by 2,2',4,4',5,5'-hexachlorobiphenyl

3,3',4,4',5-Pentachlorobiphenyl (pentaCB) caused a dose-dependent induction of fetal cleft palate in offspring from pregnant C57BL/6 mice exposed to a single dose (783 or 1044 micrograms/kg) of this compound on gestation day 10. In contrast, 2,2',4,4',5,5'-hexaCB did not cause cleft palate at a dose of 271 mg/kg and, in pregnant mice cotreated with 2,2',4,4',5,5'-hexaCB (271 mg/kg) plus 783 or 1044 micrograms/kg 3,3',4,4',5-pentaCB, fetal cleft palate formation was significantly inhibited. 3,3',4,4',5-PentaCB (6 micrograms/kg) also inhibited the splenic plaque-forming cell (PFC) response and serum IgM levels in C57BL/6 mice treated with the T cell-independent antigen trinitrophenyl-lipopolysaccharide. At doses as high as 72 mg/kg, 2,2',4,4'-5,5'-hexaCB was not immunotoxic; however, in mice cotreated with a immunotoxic dose of 3,3',4,4',5-pentaCB plus different doses of 2,2',4,4',5,5'-hexaCB (18, 36 and 72 mg/kg), there was a dose-dependent inhibition of 3,3',4,4',5-pentaCB-induced immunotoxicity. These non-additive (antagonistic) interactions of prototypical polychlorinated biphenyl (PCB) congeners may be an important consideration in development of a toxic equivalency factor approach for hazard and risk assessment of PCB mixtures.