Polybrominated diphenyl ether (DE-71) interferes with thyroid hormone action independent of effects on circulating levels of thyroid hormone in male rats

Abstract

Polybrominated diphenyl ethers (PBDEs) are routinely found in human tissues including cord blood and breast milk. PBDEs may interfere with thyroid hormone (TH) during development, which could produce neurobehavioral deficits. An assumption in experimental and epidemiological studies is that PBDE effects on serum TH levels will reflect PBDE effects on TH action in tissues. To test whether this assumption is correct, we performed the following experiments. First, five concentrations of diphenyl ether (0-30 mg/kg) were fed daily to pregnant rats to postnatal day 21. PBDEs were measured in dam liver and heart to estimate internal dose. The results were compared with a separate study in which four concentrations of propylthiouracil (PTU; 0, 1, 2, and 3 ppm) was provided to pregnant rats in drinking water for the same duration as for diphenyl ether. PBDE exposure reduced serum T4 similar in magnitude to PTU, but serum TSH was not elevated by PBDE. PBDE treatment did not affect the expression of TH response genes in the liver or heart as did PTU treatment. PTU treatment reduced T4 in liver and heart, but PBDE treatment reduced T4 only in the heart. Tissue PBDEs were in the micrograms per gram lipid range, only slightly higher than observed in human fetal tissues. Thus, PBDE exposure reduces serum T4 but does not produce effects on tissues typical of low TH produced by PTU, demonstrating that the effects of chemical exposure on serum T4 levels may not always be a faithful proxy measure of chemical effects on the ability of thyroid hormone to regulate development and adult physiology.

Figure 1.

Effects of PTU or PBDE treatment on maternal hormone concentrations. A, Serum total T₄ levels in dams at the time the animals were killed on P21, after PTU (solid bars) or PBDE (open bars) treatment. A one-way ANOVA was performed separately for each of the two experiments. For PTU, F_3,28 = 27.12; P < .0001; for PBDE, F_4,31 = 14.02; P < .001. B, Serum free T₄ levels in dams after PTU (solid bars) or PBDE (open bars) treatment. For PTU, F_{3, 27} = 20.90; P < .0001; for PBDE, F_{4, 31} = 11.71; P < .0001. C, Serum total T₃ in dams treated with PTU (solid bars) or PBDE (open bars) treatment. One-way ANOVA revealed no significant differences among treatment groups. D, Serum TSH in dams treated with PTU (solid bars) or PBDE (open bars). For PTU, a one-way ANOVA demonstrated that TSH levels were significantly increased (F_{3, 51} = 14.83; P < .0001). This was not the case for PBDE, in which F_4,29 = 1.37; P = .27. Bars represent mean ± SEM (n = 5–10 per group). *, P < .05; ***, P < .0001.

Figure 2.

Effect of PTU or PBDE treatment on serum hormones in 21-day pups. A, Serum total T₄ levels in pups after PTU (solid bars) or PBDE (open bars) treatment. For PTU, F_{3, 27} = 12.83; P < .0001; for PBDE, F_{4, 27} = 65.68; P < .0001. B, Serum free T₄ levels in pups after PTU (solid bars) or PBDE (open bars) treatment. For PTU, F_{3, 27} = 3.45; P = .0304; for PBDE, F_{4, 27} = 27.84; P < .0001. C, Serum total T₃ levels were not significantly affected by PTU (solid bars) or PBDE (open bars) treatment. D, Serum TSH in P21 pups after PTU (solid bars) or PBDE (open bars) treatment. For PTU, F_3,11 = 4.9; P < .02; for PBDE treatment, F_4,24 = 1.13; P = NS. *, P < .05; **, P < .01; ***, P < .0001.

Figure 3.

Effect of PTU or PBDE treatment on thyroid hormone action in liver on P21. A, ME mRNA levels were affected by treatment with either PTU (F_3,27 = 6.96; P < .001, n = 4–10/group) or PBDE (F_4,22 = 25.9; P < .0001, n = 4–7/group). However, PTU treatment significantly reduced whereas PBDE treatment significantly elevated ME mRNA levels. B, S14 mRNA levels were significantly affected by PTU (F_3,27 = 9.6; P < .0002, n = 4–10/group) but not by PBDE treatment (F_4,23 = 2.5; P = NS, n = 5–7/group). C, Liver MDR1a mRNA levels were significantly affected by PTU treatment (F_3,27 = 4.2; P < .01, n = 4–10/group) but not by PBDE treatment (F_4,25 = 1.6; P = NS, n = 5–8/group). D, IGFBP mRNA levels in the liver were not affected by either PTU (F_3,23 = 0.32; P = NS, n = 4–10/group) or PBDE (F_4,27 = 1.85; P = NS, n = 5–8/group) treatment. *, P < .05; **, P < .01; ***, P < .0001.

Figure 4.

Effect of PTU or PBDE treatment on thyroid hormone action in heart on P21. A, myHC6 mRNA levels were significantly affected by PTU treatment (F_3,28 = 10.4; P < .0001, n = 5–10/group). Post hoc analysis revealed that myHC6 mRNA levels were significantly reduced in animals treated with 3 ppm PTU compared with controls. In contrast, PBDE treatment did not affect myHC6 mRNA levels (F_4,27 = 0.83; P = NS, n = 5–8/group). B, myHC7 mRNA levels were significantly affected by PTU treatment (F_3,26 = 4.34; P < .01, n = 5–10/group). Post hoc analysis revealed that myHC7 mRNA levels were significantly higher in animals treated with 3 ppm PTU compared with controls. In contrast, myHC7 mRNA levels were not affected by PBDE treatment (F_4,27 = 0.51; P = NS, n = 5–8/group). C, MDR1a mRNA levels in heart were significantly reduced by PTU treatment (F_3,27 = 3.8; P < .02, 5–10/group). Post hoc analysis showed that all MDR1a mRNA levels were reduced in all PTU-treated animals compared with controls. In contrast, PBDE treatment did not affect MDR1a mRNA levels in heart (F_4,25 = 0.8; P = NS, n = 5–8/group).

Figure 5.

Effects of PTU or PBDE treatment on tissue T₄ and deiodinases expression. A, Liver T₄ levels were significantly reduced by PTU treatment (F_3,26 = 5.09; P < .006, n = 5–9/group) but not by PBDE treatment (F_4,27 = 0.18; P = NS, n = 5–8/group). B, PTU exerted a significant effect on T₄ levels of the heart (F_3,26 = 4.58; P < .01, n = 4–10/group). However, the only significant difference among mean T₄ levels was between animals treated with 1 and 3 ppm PTU. In contrast, PBDE treatment very significantly reduced T₄ levels in the heart (F_4,26 = 26.56; P < .0001, n = 5–8/group). Post hoc analysis revealed that T₄ levels in the heart of animals treated with either 10 or 30 mg/kg PBDE were significantly lower than that of control animals. C, Type 1 deiodinase mRNA in the liver was marginally reduced by PTU treatment (F_3,24 = 3.38; P < .03, n = 4–9/group) and was limited to animals treated with 3 ppm PTU compared with controls., More robust reductions in D1 mRNA levels were induced by PBDE treatment (F_4,27 = 11.01; P < .0001, n = 5–8/group). Post hoc analysis revealed that D1 mRNA levels were significantly lower than control levels in all PBDE-treated animals with the exception of those treated with 1 mg/kg. D, Type 2 deiodinase mRNA was not affected by PTU treatment (F_3,25 = 2.69; P = NS, n = 5–9/group) or PBDE treatment (F _4,25 = 1.02; P = NS, n = 4–8/group).