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

Mixture effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin and polychlorinated biphenyl congeners in rats

Concern of the toxic effects and bioaccumulation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and polychlorinated biphenyls in the environment continues to be a focus of research in persistent organochlorine contaminants. Groups of five adult female S.D. rats were administered by gavage 0, 2.5, 25, 250 or 1000 ng TCDD/kg body weight/day or TCDD in combination with a mixture of PCB congeners (PCBs) at 2 or 20 microg/kg b.w./day for a period of 28 days. Growth suppression, increased absolute and relative liver weights, and decreased thymic weight were observed in either the 1000 ng TCDD group alone, or the groups receiving a mixture of 1000 ng TCDD + 2 microg PCBs. The TCDD induced increases in liver and thymic weights were not altered by co-administration with PCBs, however, growth suppression appeared to be more pronounced in the group receiving 1000 ng TCDD + 2 microg PCBs than with TCDD alone. Treatment with TCDD at 250 ng and 1000 ng/kg resulted in a significant increase in hepatic microsomal methoxy resorufin-O-demethylase and ethoxy resorufin-O-deethylase activities which were antagonized by co-administration with PCBs. Similarly, effects of 250 ng TCDD on serum cholesterol and liver UDP glucuronosyl transferase activity and ascorbic acid were significantly reduced by co-administration with 20 microg PCBs. Other biochemical effects elicited by treatment with 1000 ng TCDD, but not affected by co-administration with PCBs include the following: increased serum albumin, decreased liver vitamin A, and increased kidney vitamin A and liver microsomal glutathione-S-transferase activity. While decreased hemoglobin, platelet, packed cell volume and red cell indices were observed in TCDD treated rats, no interactive effects were seen. The above results indicate that the mixture effects of PCBs and TCDD may be additive or antagonistic depending on the dose level and endpoints measured. For the purpose of predicting mixture effects, knowledge of mechanisms of action and toxicokinetics is required.

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.

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.