A possible mechanism for 2,2',4,4',5,5'-hexachlorobiphenyl-mediated decrease in serum thyroxine level in mice

Polychlorinated biphenyls and thyroid function: a scoping review

OBJECTIVE

Numerous evidence indicates the association between polychlorinated biphenyls (PCBs), an endocrine disrupter, with thyroid hormone disruption, contradictory findings also exist. Herein, we tried to address this question by performing a scoping review.

CONTENT

The search was performed on PubMed, Scopus, Web of Science, and Google Scholar databases from 2010 onwards. Animal studies on PCBs' effect on thyroid function were searched. The SYRCLE's RoB scale assessed the risk of bias. I2 and Q tests are used for investigating heterogeneity. A random-effects model with the pooled standard means difference (SMD) and 95 % confidence interval (CI) was performed for the TSH, TT4, TT3, and FT4 outcomes using Comprehensive Meta-Analyses (CMA) Software version 3. Also, we conducted subgroup analyses based on the different types of PCB. The initial search identified 1,279 publications from the main databases 26 of them fulfilled our eligibility criteria for the study, and then five studies among selected studies had sufficient data for analysis. Meta-analysis of data revealed that Aroclor 1260 (SDM: -0.47, 95 % CI: -0.92, -0.01, p=0.044) and PCB 126 (SDM: 0.17, 95 % CI: -0.40, 0.75, p=0.559) significantly increased TSH concentration in the exposed groups vs. the control groups. Related to the effects of PCBs on the TT4, our findings indicated a significant reduction the TT4 concentration of animals exposed to Aroclor 1260 (SDM: -5.62, 95 % CI: -8.30, -2.94, p=0.0001), PCB 118 (SDM: -6.24, 95 % CI: -7.76, -4.72, p=0.0001), PCB 126 (SDM: -1.81, 95 % CI: -2.90, -0.71, p=0.001), and PCB 153 (SDM: -1.32, 95 % CI: -2.29, -0.35, p=0.007) vs. the controls. Our meta-analysis indicated a significant increase in TT3 concentration following exposure to PCB 118 and PCB 153 (SDM: -0.89, 95 % CI: -1.36, -0.42, p=0.0001, and SDM: -1.45, 95 % CI: -2.15, -0.75, p=0.0001, respectively). Aroclor 1254 and PCB 126 significantly decreased TT3 concentration (SDM: 1.25, 95 % CI: 0.29, 2.21, p=0.01 and SDM: 3.33, 95 % CI: 2.49, 4.18, p=0.0001, respectively). PCB 126 significantly decreased FT4 in the exposed groups vs. the control groups (SDM: -7.80, 95 % CI: -11.51, -5.35, p=0.0001).

SUMMARY

Our findings showed an association between PCBs exposure and hypothyroidism in rodents, fish, and chicken embryos.

OUTLOOK

Regarding to the most evidence of hypothyroidism effects of PCBs in animal species, it is necessary to consider large cohort studies to address the association between PCBs exposure and thyroid function impairment in humans.

Effects of PCB exposure on serum thyroid hormone levels in dogs and cats

Polychlorinated biphenyls (PCBs) and their hydroxylated metabolites (OH-PCBs) might disrupt thyroid function. However, there is no clear evidence of PCB exposure disrupting thyroid hormone (TH) homeostasis in dogs and cats. The present study conducted in vivo experiments to evaluate the effects of a mixture of 12 PCB congeners (CB18, 28, 70, 77, 99, 101, 118, 138, 153, 180, 187 and 202, each congener 0.5 mg/kg BW, i.p. administration) on serum TH levels in male dogs (Canis lupus familiaris) and male cats (Felis silvestris catus). In PCB-exposed dogs, the time courses of higher-chlorinated PCBs and L-thyroxine (T4)-like OH-PCBs (4-OH-CB107 and 4-OH-CB202) concentrations were unchanged or tended to increase, whereas those of lower-chlorinated PCBs and OH-PCBs tended to decrease after 24 h. In PCB-exposed cats, concentrations of PCBs increased until 6 h and then remained unchanged. The levels of lower-chlorinated OH-PCBs including 4'-OH-CB18 increased until 96 h and then decreased. In PCB-exposed dogs, free T4 concentrations were higher than those in the control group at 48 and 96 h after PCB administration and positively correlated with the levels of T4-like OH-PCBs, suggesting competitive binding of T4 and T4-like OH-PCBs to a TH transporter, transthyretin. Serum levels of total T4 and total 3,3',5-triiodo-L-thyronine (T3) in PCB-exposed dogs were lower than in the control group at 24 and 48 h and negatively correlated with PCB concentrations, implying that PCB exposure enhanced TH excretion by increasing TH uptake and TH conjugation enzyme activities in the dog liver. In contrast, no obvious changes in TH levels were observed in PCB-exposed cats. This could be explained by the lower levels of T4-like OH-PCBs and lower hepatic conjugation enzyme activities in cats compared with dogs. Different effects on serum TH levels in PCB-exposed dogs and cats are likely to be attributable to species-specific PCB and TH metabolism.

Kanechlor 500-mediated changes in serum and hepatic thyroxine levels primarily occur in a transthyretin-unrelated manner

The effects of Kanechlor-500 (KC500) on the levels of serum total thyroxine (T₄ ) and hepatic T₄ in wild-type C57BL/6 (WT) and its transthyretin (TTR)-deficient (TTR-null) mice were comparatively examined. Four days after a single intraperitoneal injection with KC500 (100 mg/kg body weight), serum total T₄ levels were significantly decreased in both WT and TTR-null mice. The KC500 pretreatment also promoted serum [¹²⁵ I]T₄ clearance in both strains of mice administrated with [¹²⁵ I]T₄ , and the promotion of serum [¹²⁵ I]T₄ clearance in WT mice occurred without inhibition of the [¹²⁵ I]T₄ -TTR complex formation. Furthermore, the KC500 pretreatment led to significant increases in liver weight, steady-state distribution volume of [¹²⁵ I]T₄ , hepatic accumulation level of [¹²⁵ I]T₄ , and concentration ratio of the liver to serum in both strains of mice. The present findings indicate that the KC500-mediated decrease in serum T₄ level occurs in a TTR-unrelated manner and further suggest that KC500-promoted T₄ accumulation in the liver occurs through the development of liver hypertrophy and the promotion of T₄ transportation from serum to liver.

Mechanisms of Action Point Towards Combined PBDE/NDL-PCB Risk Assessment

At present, human risk assessment of the structurally similar non-dioxin-like (NDL) PCBs and polybrominated diphenylethers (PBDEs) is done independently for both groups of compounds. There are however obvious similarities between NDL-PCBs and PBDEs with regard to modulation of the intracellular calcium homeostasis (basal calcium levels, voltage-gated calcium channels, calcium uptake, ryanodine receptor) and thyroid hormone (TH) homeostasis (TH levels and transport). which are mechanisms of action related to neurobehavioral effects (spontaneous activity, habituation and learning ability). There also similarities in agonistic interactions with the hepatic nuclear receptors PXR and CAR. Several effects on developmental (reproductive) processes have also been observed, but results were more dispersed and insufficient to compare both groups of compounds. The available mechanistic information is sufficient to warrant a dose addition model for NDL-PCBs and PBDEs, including their hydroxylated metabolites.Although many of the observed effects are similar from a qualitative point of view for both groups, congener or tissue specific differences have also been found. As this is a source of uncertainty in the combined hazard and risk assessment of these compounds, molecular entities involved in the observed mechanisms and adverse outcomes associated with these compounds need to be identified. The systematical generation of (quantitative) structure-activity information for NDL-PCBs and PBDEs on these targets (including potential non-additive effects) will allow a more realistic risk estimation associated with combined exposure to both groups of compounds during early life. Additional validation studies are needed to quantify these uncertainties for risk assessment of NDL-PCBs and PBDEs.

Comparison of the in vitro effects of TCDD, PCB 126 and PCB 153 on thyroid-restricted gene expression and thyroid hormone secretion by the chicken thyroid gland

The aim of this study was to compare the in vitro effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 3,3',4,4',5-pentachlorobiphenyl (PCB 126; a coplanar PCB congener) and 2,2'4,4',5,5'-hexachlorobiphenyl (PCB153; non-coplanar PCB) on mRNA expression of thyroid-restricted genes, i.e. sodium iodide symporter (NIS), thyroid peroxidase (TPO) and thyroglobulin (TG), and thyroid hormone secretion from the thyroid gland of the laying chicken. Relative expression levels of NIS, TG and TPO genes and thyroxine (T4) and triiodothyronine (T3) secretion from the thyroidal explants were quantified by the real-time qPCR and RIA methods, respectively. In comparison with the control group, TCDD and PCB 126 significantly increased mRNA expression of TPO and TG genes. TCDD did not affect NIS mRNA levels, but PCB 126 decreased its expression. No effect of PCB 153 on the expression of these genes was observed. TCDD and PCB 126 significantly decreased T4 and T3 secretion. There was no significant effect of PCB 153 on these hormone secretions. In conclusion, the results obtained show that in comparison with non-coplanar PCB 153, TCDD and coplanar PCB 126 can directly affect thyroid hormone synthesis and secretion, and in consequence, they may disrupt the endocrine function of the thyroid gland of the laying chicken.

Maternal urinary bisphenol a during pregnancy and maternal and neonatal thyroid function in the CHAMACOS study

BACKGROUND

Bisphenol A (BPA) is widely used in the manufacture of polycarbonate plastic bottles, food and beverage can linings, thermal receipts, and dental sealants. Animal and human studies suggest that BPA may disrupt thyroid function. Although thyroid hormones play a determinant role in human growth and brain development, no studies have investigated relations between BPA exposure and thyroid function in pregnant women or neonates.

OBJECTIVE

Our goal was to evaluate whether exposure to BPA during pregnancy is related to thyroid hormone levels in pregnant women and neonates.

METHODS

We measured BPA concentration in urine samples collected during the first and second half of pregnancy in 476 women participating in the CHAMACOS (Center for the Health Assessment of Mothers and Children of Salinas) study. We also measured free thyroxine (T4), total T4, and thyroid-stimulating hormone (TSH) in women during pregnancy, and TSH in neonates.

RESULTS

Associations between the average of the two BPA measurements and maternal thyroid hormone levels were not statistically significant. Of the two BPA measurements, only the one taken closest in time to the TH measurement was significantly associated with a reduction in total T4 (β = -0.13 µg/dL per log2 unit; 95% CI: -0.25, 0.00). The average of the maternal BPA concentrations was associated with reduced TSH in boys (-9.9% per log2 unit; 95% CI: -15.9%, -3.5%) but not in girls. Among boys, the relation was stronger when BPA was measured in the third trimester of pregnancy and decreased with time between BPA and TH measurements.

CONCLUSION

Results suggest that exposure to BPA during pregnancy is related to reduced total T4 in pregnant women and decreased TSH in male neonates. Findings may have implications for fetal and neonatal development.