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

We studied the effect of polychlorinated biphenyls (PCBs) on hepatic porphyrin accumulation in female Sprague-Dawley rats by feeding them diets containing 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), 2,3,3',4,4',5-hexachlorobiphenyl (PCB 156), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), or combinations of the single PCB congeners with TCDD for 13 weeks. A dose-dependent increase in hepatic porphyrin accumulation occurred after TCDD, PCB 126, or PCB 156 administration, reaching maximal levels of about twice control values. The lowest dose levels for which a significant increase in hepatic porphyrin accumulation was found were 0.7 microgram TCDD/kg diet, 50 micrograms PCB 126/kg diet, or 6 mg PCB 156/kg diet. These doses are equivalent to 47 ng TCDD/kg/day, 3.2 micrograms PCB 126/kg/day, and 365 micrograms PCB 156/kg/day. Relative potencies for hepatic porphyrin accumulation, using TCDD as a reference, ranged from 0.015 to 0.06 for PCB 126 and from 0.0001 to 0.0003 for PCB 156. CYP1A2 activities significantly correlated with hepatic porphyrin levels, with coefficients of 0.629, 0.483, or 0.808 for TCDD, PCB 126, or PCB 156, respectively. Administration of PCB 153 alone did not result in hepatic porphyrin accumulation. Co-administration of PCB 153 and TCDD revealed a strong synergistic effect on porphyrin accumulation (about 800 times control levels). This synergistic effect was significant in rats fed diets containing any combination of PCB 153 with TCDD. Uroporphyrin III and heptacarboxylic porphyrin were accumulated in porphyrinogenic livers. These results suggest that TCDD induction of CYP1A2 may be involved, leading to oxidation of uroporphyrinogen III to uroporphyrin III, in combination with an increase in delta-aminolevulinic acid synthetase induced by PCB 153. Under porphyrinogenic conditions, an inhibitor of CYP1A2 activity may also be formed. The interactive effects on porphyrin accumulation after co-administration of dioxinlike and non-dioxinlike compounds may have significant implications for the risk assessment of these chemicals.

Subchronic effects of 2,3,7,8-TCDD or PCBs on thyroid hormone metabolism: use in risk assessment.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), or 2,3,3',4,4',5-hexachlorobiphenyl (PCB 156) on thyroid hormone metabolism were studied in 13-week feeding studies in female Sprague-Dawley rats. The diets were supplemented with the compounds tested at concentrations ranging from 0.2 to 20 micrograms/kg diet for TCDD, 7 to 180 micrograms/kg diet for PCB 126, or 1.2 to 12 mg/kg diet for PCB 156, respectively. Significant correlations were found for all three compounds between reductions in plasma total thyroxine (TT4) levels and inductions of the microsomal phase II enzyme UDP-glucuronosyltransferase by using T4 as a substrate (T4UGT). Furthermore, the coinduction of certain phase I and II isozymes, i.c., cytochrome P450 1A1 (CYP1A1) and UGT1A1, by these compounds, clearly suggests the involvement of an Ah receptor-mediated mechanism in the disturbance of thyroid hormone metabolism by these polyhalogenated aromatic compounds. These results provide a mechanistic base for the use of certain effects on thyroid hormone metabolism by polyhalogenated aromatic compounds in risk assessment. By using these effects, potencies of PCB 126 and PCB 156 relative to TCDD ranged from 0.008 to 0.1 for PCB 126, and from 0.00007 to 0.004 for PCB 156, respectively. These values correspond very well with relative potencies of PCB 126 and PCB 156 by using some other well-known Ah receptor-mediated toxic and biochemical parameters.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin or 2,2',4,4',5,5'-hexachlorobiphenyl on vitamin K-dependent blood coagulation in male and female WAG/Rij-rats.

Newborns are susceptible to hemorrhages (hemorrhagic disease of the newborn or HDN) due to vitamin K deficiency. Induction of cytochrome P450 in the fetal liver by maternal anticonvulsant therapy such as phenobarbital or phenytoin is considered to be a major cause. An observed increase in late hemorrhagic disease (LHD) in breast fed neonates gave rise to the hypothesis that PCBs and dioxins, P450-inducing contaminants present in human milk, might effect vitamin K-dependent blood coagulation. This hypothesis was studied in rats. Administration of a single oral dose of 0.003, 0.03, 0.3, 3 or 30 nmol 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) per kg bodyweight or 0.75, 4, 20, 100 or 500 micromol 2,2',4,4',5,5'-hexachlorobiphenyl/kg bw (HxCB) to female and male rats resulted in dose-related reductions of the vitamin K-dependent coagulation factor VII. The highest factor VII reduction in female rats was 44%, observed after TCDD exposure. The Lowest Observed Adverse Effect Level (LOAEL) of TCDD on female factor VII levels was 0.3 nmol/kg bw (96 ng/kg). There was a significant inverse correlation between Factor VII levels and induction of hepatic ethoxyresorufin O-deethylating (EROD) activity, reflecting CYP1A1, and total P450 content. HxCB had no effect on female coagulation factors. In contrast, in male rats only exposure to HxCB, which induces mainly CYP2B1 and 2B2, decreased both coagulation factors dramatically up to 88%. The LOAEL of HxCB on factor VII in male rats was 100 micromol/kg bw (36 mg/kg). In general, effects on coagulation factors in male rats exceeded those in females. In addition, sex-dependent differences of TCDD and HxCB were observed on the hepatic vitamin K cycle enzyme activities in female and male rats. Vitamin K-dependent (gamma-glutamyl carboxylase activity was mainly induced in female rats; 2.3-fold in the highest dose group of TCDD. In male rats only vitamin K 2,3-epoxide reductase (KO-reductase) activity was induced 1.7-fold by the highest dose of HxCB. KO-reductase activity in female rats was also increased by TCDD, however, less pronounced than the carboxylase activity. Concluding, the hepatic vitamin K cycle still functions and is not blocked by TCDD or HxCB, thus explaining the observed reduction in factor VII. Finally, the possible role of P450 in vitamin K deficiency is discussed. Based on these results it is suggested to investigate the possible role of PCBs and dioxin-like compounds in LHD in more detail.

Toxic potency of 2,3,3',4,4',5-hexachlorobiphenyl relative to and in combination with 2,3,7,8-tetrachlorodibenzo-p-dioxin in a subchronic feeding study in the rat.

Interactive effects on toxicity and biochemical parameters were studied between 2,3,3',4,4',5-hexachlorobiphenyl (PCB 156) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a 13-week feeding study of female Sprague-Dawley rats. The diets were supplemented with PCB 156 (1.2, 6, or 12 mg/kg), with TCDD (5 micrograms/kg), or with combinations of both compounds. Estimated daily intake of 365 micrograms/kg body wt/day (6 mg/kg diet group) of PCB 156 caused a decrease in body weight gain, thymic atrophy, liver enlargement, a loss in hepatic retinoids, induction of CYP2B activity, and a decrease in plasma thyroxine concentrations. At an estimated daily intake of 81 micrograms PCB 156/kg body wt/day CYP1A1 and CYP1A2 activities were induced. Compared to a simultaneous subchronic feeding study with TCDD a toxic equivalency factor (TEF) between 0.00004 and 0.001 was estimated for PCB 156 with respect to the mentioned effects. Antagonistic effects were found between TCDD and PCB 156 for CYP2B activity and hepatic retinol levels. These effects concurred with a PCB 156 dose-dependent decrease in hepatic TCDD levels. Hepatic PCB 156 levels were found to be increased at the 1.2-mg PCB 156/kg dose group in coadministration with TCDD. In conclusion, at least part of the antagonistic effects between PCB 156 and TCDD observed have a toxico-kinetic base. Furthermore, the magnitude of the antagonistic effects may be neglected in comparison with the uncertainty in the TEF value. Therefore, the interactive effects found between PCB 156 and TCDD may have no implications for the additivity of the TEF concept.

Subchronic dose-response study of 2,3,7,8-tetrachlorodibenzo-p-dioxin in female Sprague-Dawley rats.

Toxic and biochemical potencies of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were studied in a 13-week feeding study in female Sprague-Dawley rats. The diets were supplemented with 0, 0.2, 0.4, 0.7, 5, or 20 micrograms TCDD/kg diet. The estimated daily intakes were calculated to be 0, 14, 26, 47, 320, or 1024 ng TCDD/kg body wt/day. At the end of the study, TCDD concentrations were measured in liver and adipose tissue. The lowest estimated daily intake that caused an increase in liver weight was 320 ng TCDD/kg/day, while an intake of 47 ng TCDD/kg/day resulted in a decrease in plasma thyroid hormone concentrations and a decrease in body weight gain. Decreases in relative thymus weights, loss of hepatic retinoids, and induction of CYP1A1 and CYP1A2 activities were already found at 14 ng/kg/day, the lowest dose used. Therefore, 95% confidence limits for the no-effect levels (CNELs) were calculated from the corresponding dose-response relationships by using sigmoidal curve fittings (Hill, Weibull, and a Logistic model) and a probability level of p < 0.05. For increases in CYP1A1 and CYP1A2 activities, the right critical values for the CNELs ranged from 0.7 to 4 ng TCDD/kg/day (Hill and Weibull). Based on hepatic TCDD residue levels, these right critical values for the CNELs ranged from 0.06 to 0.4 ng TCDD/g liver (wet weight) (Hill and Weibull). The CNELs in this study agree very well with the no-observed-adverse-effects levels as reported before in chronic, carcinogenicity, and reproductive studies with rats and TCDD, i.e., 1 ng/kg/day.

Toxic potency of 3,3',4,4',5-pentachlorobiphenyl relative to and in combination with 2,3,7,8-tetrachlorodibenzo-p-dioxin in a subchronic feeding study in the rat.

Toxic and biochemical potencies of 3,3',4,4',5-pentachlorobiphenyl (PCB 126) were studied relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in a 13-week feeding study in female Sprague-Dawley rats. To study possible interactive effects the combinations of both compounds were administered. The diets were supplemented with PCB 126 (7, 50, or 180 micrograms/kg diet), with TCDD (0.4 or 5 micrograms/kg diet), or with combinations of both compounds. An estimated daily intake of 0.47 micrograms PCB 126/kg body weight/day caused thymic atrophy, a dramatic loss in hepatic retinoids, and a marked induction in CYP1A1 and CYP1A2 activities. At a daily intake of 3.18 micrograms PCB 126/kg body weight/day a decrease in body weight gain, liver enlargement, and plasma thyroid hormone concentrations occurred. Based on a simultaneous subchronic feeding study with TCDD, a toxic equivalency factor range between 0.01 and 0.1 was estimated for PCB 126 for the mentioned effects. Antagonism was found between TCDD and PCB 126 for hepatic retinol levels and CYP1A2 activity. At the same time, TCDD and PCB 126 liver residue levels were slightly decreased by coadministration. However, these antagonistic effects occurred at maximum induction levels of CYP1A1 and CYP1A2, which are not likely to occur at levels relevant for humans.