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 an environmentally relevant PCB-mixture on immune function, clinical chemistry, and thyroid hormone levels in adult female B6C3F1 mice

Polychlorinated biphenyls (PCBs) have been assessed for immunotoxicity; however, humans and wildlife are exposed to multiple PCBs environmentally. Therefore, the aim of this study was to examine the effects of a complex 37 PCB congener mixture identified in blubber specific to dolphins residing in the estuarine waters of Charleston, South Carolina. Immunotoxicity was determined in adult female B₆C₃F₁ mice by assessing lymphocyte proliferation, splenic and thymic immunophenotypes, and IgM production. Mice were exposed via oral gavage to the PCB-mixture (0, 1.8, 3.6, 7.1, or 14.3 mg/kg/day) for 28 days to yield a targeted total administered dose (TAD) 0, 50, 100, 200, or 400 mg/kg. Significant increased liver weight occurred at the highest treatment. IgM production was suppressed compared to control for all treatments. Numbers of thymic CD4+/CD8+, CD4-/CD8-, and CD4+/CD8- cells were not altered, but numbers of thymic CD4-/CD8+ cells were significantly increased in the highest treatment. Lymphocyte proliferation was not markedly affected by any treatment. The numbers of splenic CD4/CD8 T-cells or MHCII+ cells were not significantly changed. Humoral immunity using the plaque-forming cell assay for determining the specific IgM antibody-forming cell response appeared to be the most sensitive endpoint affected. As the lowest concentration tested resulted in decreased IgM production and total and free thyroxine (T₄) serum levels a NOAEL was not identified. The calculated ED₅₀ for suppression of IgM production was 2.4 mg/kg/day.

Weight of Evidence and Human Relevance Evaluation of the Benfluralin Mode of Action in Rats (Part II): Thyroid carcinogenesis

Benfluralin is an herbicide of the dinitroaniline class used to control grasses and weeds. In a 2 year dietary study in rats, benfluralin increased incidences of thyroid follicular adenoma and carcinoma at high dietary concentrations (≥2500 ppm). The benfluralin toxicology database suggests the mode of action (MOA) is initiated by induction of liver metabolizing enzymes, particularly thyroid hormone specific UGTs, a major pathway for T4 clearance in rats. As reported with phenobarbital, this effect triggers negative feedback regulation, increasing thyroid stimulating hormone (TSH) release into circulating blood. When sustained over time, this leads to thyroid changes such as follicular hypertrophy, hyperplasia and thyroid follicular tumors with chronic exposures. The described MOA was previously established in rat studies with various chemical activators of xenobiotic receptors in the liver. It is generally considered as non-relevant in humans, due to differences between humans and rats in T4 turnover and susceptibility to this carcinogenic MOA. A structured methodology based on the IPCS/MOA/Human Relevance framework was used in the evaluation of available benfluralin data, and the conclusion was determined that the carcinogenic potential of benfluralin in the thyroid is not relevant 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.

Contaminants in Atlantic walruses in Svalbard Part 2: Relationships with endocrine and immune systems

Marine mammals in the Barents Sea region have among the highest levels of contaminants recorded in the Arctic and the Atlantic walrus (Odobenus rosmarus rosmarus) is one of the most contaminated species within this region. We therefore investigated the relationships bewteen blubber concentrations of lipophilic persistent organic pollutants (POPs) and plasma concentrations of perfluoroalkyl substances (PFASs) and markers of endocrine and immune functions in adult male Atlantic walruses (n = 38) from Svalbard, Norway. To do so, we assessed plasma concentrations of five forms of thyroid hormones and transcript levels of genes related to the endocrine and immune systems as endpoints; transcript levels of seven genes in blubber and 23 genes in blood cells were studied. Results indicated that plasma total thyroxine (TT4) concentrations and ratio of TT4 and reverse triiodothyronine decreased with increasing blubber concentrations of lipophilic POPs. Blood cell transcript levels of genes involved in the function of T and B cells (FC like receptors 2 and 5, cytotoxic T-lymphocyte associated protein 4 and protein tyrosine phosphatase non-receptor type 22) were increased with plasma PFAS concentrations. These results suggest that changes in thyroid and immune systems in adult male walruses are linked to current levels of contaminant exposure.