PCB153 and p,p'-DDE disorder thyroid hormones via thyroglobulin, deiodinase 2, transthyretin, hepatic enzymes and receptors

Differential gene expression and developmental pathologies associated with persistent organic pollutants in sentinel fish in Troutman Lake, Sivuqaq, Alaska

Persistent organic pollutants (POPs) are lipophilic compounds that bioaccumulate in animals and biomagnify within food webs. Many POPs are endocrine disrupting compounds that impact vertebrate development. POPs accumulate in the Arctic via global distillation and thereby impact high trophic level vertebrates as well as people who live a subsistence lifestyle. The Arctic also contains thousands of point sources of pollution, such as formerly used defense (FUD) sites. Sivuqaq (St. Lawrence Island), Alaska was used by the U.S. military during the Cold War and FUD sites on the island remain point sources of POP contamination. We examined the effects of POP exposure on ninespine stickleback (Pungitius pungitius) collected from Troutman Lake in the village of Gambell as a model for human exposure and disease. During the Cold War, Troutman Lake was used as a dump site by the U.S. military. We found that PCB concentrations in stickleback exceeded the U.S. Environmental Protection Agency's guideline for unlimited consumption despite these fish being low trophic level organisms. We examined effects at three levels of biological organization: gene expression, endocrinology, and histomorphology. We found that ninespine stickleback from Troutman Lake exhibited suppressed gonadal development compared to threespine stickleback (Gasterosteus aculeatus) studied elsewhere. Troutman Lake stickleback also displayed two distinct hepatic phenotypes, one with lipid accumulation and one with glycogen-type vacuolation. We compared the transcriptomic profiles of these liver phenotypes using RNA sequencing and found significant upregulation of genes involved in ribosomal and metabolic pathways in the lipid accumulation group. Additionally, stickleback displaying liver lipid accumulation had significantly fewer thyroid follicles than the vacuolated phenotype. Our study and previous work highlight health concerns for people and wildlife due to pollution hotspots in the Arctic, and the need for health-protective remediation.

Association between polychlorinated biphenyl exposure and thyroid hormones: a systematic review and meta-analysis

Purpose: To conduct a comprehensive meta-analysis investigating the association between polychlorinated biphenyl (PCB) exposure and serum thyroid hormone levels among adults. Methods: Eleven studies met inclusion criteria for analysis following systematic search of PubMed, Embase, and Scopus databases. Of these, 7 studies measured exposure by the total sum of PCB congeners (∑PCB), 1 study measured individual PCB congener levels, and 3 studies measured both ∑PCB levels and PCB congener levels. Correlation coefficients (r) were extracted from each study. Summary estimates were calculated for ∑PCB levels and PCB congeners reported by 2 or more studies: PCB 28, 52, 101, 105, 118, 138, 153, and 180, using random effects model. Results: Significant negative correlation was found between ∑PCBs and T3 (r: -0.09; 95% CI: -0.17, -0.02) and FT3 (r: -0.24; 95% CI: -0.36, -0.12). Congener-specific analysis found T3 to be negatively correlated with PCB-153 (r: -0.19; 95% CI: -0.34, -0.03) and PCB-180 (r: -0.14; 95% CI: -0.26, -0.01), whereas TSH was positively correlated with PCB-105 (r: 0.15; 95% CI: 0.02, 0.28). Conclusions: The present study is the first meta-analysis to investigate the association between PCB exposure and thyroid hormone dysfunction among adults. Results suggest a significant association between PCB exposure and thyroid hormone dysregulation.

Deiodinases control local cellular and systemic thyroid hormone availability

Iodothyronine deiodinases (DIO) are a family of selenoproteins controlling systemic and local availability of the major thyroid hormone l-thyroxine (T4), a prohormone secreted by the thyroid gland. T4 is activated to the active 3,3'-5-triiodothyronine (T3) by two 5'-deiodinases, DIO1 and DIO2. DIO3, a 5-deiodinase selenoenzyme inactivates both the prohormone T4 and its active form T3. DIOs show species-specific different patterns of temporo-spatial expression, regulation and function and exhibit different mechanisms of reaction and inhibitor sensitivities. The main regulators of DIO expression and function are the thyroid hormone status, several growth factors, cytokines and altered pathophysiological conditions. Selenium (Se) status has a modest impact on DIO expression and translation. DIOs rank high in the priority of selenium supply to various selenoproteins; thus, their function is impaired only during severe selenium deficiency. DIO variants, polymorphisms, SNPs and rare mutations have been identified. Development of DIO isozyme selective drugs is ongoing. A first X-ray structure has been reported for DIO3. This review focusses on the biochemical characteristics and reaction mechanisms, the relationships between DIO selenoproteins and their importance for local and systemic provision of the active hormone T3. Nutritional, pharmacological, and environmental factors and inhibitors, such as endocrine disruptors, impact DIO functions.

Perinatal effects of persistent organic pollutants on thyroid hormone concentration in placenta and breastmilk

Thyroid hormones (TH) are known to play a critical role in regulating many biological processes including growth and development, energy homeostasis, thermogenesis, lipolysis and metabolism of cholesterol. Severe TH deficiency especially during fetal development results in cretinism, but can also lead to an imbalance in metabolism with, among others, an alteration in body weight composition. Over the past two decades, increasing evidence has shown that certain persistent organic pollutants (POP) can interfere with the endocrine system. These POP referred to as "endocrine disrupting chemicals" are widely present in the environment and populations are exposed globally. Moreover, epidemiological studies have shown that a particularly sensitive period is the pre- and postnatal time. Indeed, perinatal exposure to such chemicals could lead to the onset diseases in later life. It is known, that, maternal thyroid hormones are transported by the placenta to the fetus from 6 weeks of gestation and it seems that during the first trimester, and part of the second, the fetus is entirely dependent on maternal TH supply for its development. Interferences in the TH-network as a consequence of the exposure to such pollutants could cause variations in TH concentration. Only small changes in maternal thyroid hormone levels in early stages of pregnancy can influence fetal neurological and cardiovascular development, as well as according to recent studies, have effect on childhood body composition. With this review, we will report the most recent and important studies concerning the association between thyroid hormone concentration and POP levels measured during the perinatal period. We will mostly focus on the data recently reported on placenta and breastmilk as main sources for understanding the potential consequences of exposure. The possible link between exposure to pollutants, TH dysregulation and possible adverse outcome will also be briefly discussed. From our literature search, several studies support the hypothesis that pre- and postnatal exposure to different pollutants might play a role in causing variation in thyroid hormone concentration. However, few research papers have so far studied the relationship linking exposure to pollutants, TH concentration and possible health consequences. Therefore, this review highlights the need for further research in this direction.

Di-(2-ethylhexyl) phthalate inhibits expression and internalization of transthyretin in human placental trophoblastic cells

During pregnancy, fetal thyroid hormones (THs) are dependent on maternal placental transport and their physiological level is crucial for normal fetal neurodevelopment. Earlier research has shown that Di-(2-ethylhexyl) phthalate (DEHP) disrupts thyroid function and THs homeostasis in pregnant women and fetuses, and affects placental THs transport. However, the underlying mechanisms are poorly understood. The present study, therefore, aimed to systematically investigate the potential mechanisms of DEHP-induced disruption in the placental THs transport using two human placental trophoblastic cells, HTR-8/SVneo cells and JEG-3 cells. While the exposure of DEHP at the doses of 0-400 μM for 24 h did not affect cell viability, we found reduced consumption of T3 and T4 in the culture medium of HTR-8/Svneo cells treated with DEHP at 400 μM. DEHP treatment did not affect T3 uptake and the expression of monocarboxylate transporters 8 (MCT8) and organic anion transporters 1C1 (OATP1C1). However, DEHP significantly inhibited transthyretin (TTR) internalization, down-regulated TTR, deiodinase 2 (DIO2), and thyroid hormone receptors mRNA expression and protein levels, and up-regulated deiodinase 3 (DIO3) protein levels in a dose-dependent manner. These results indicate that DEHP acts on placental trophoblast cells, inhibits its TTR internalization, down-regulates TTR expression and affects the expression of DIO2, DIO3, and thyroid hormone receptor. These may be the mechanisms by which PAEs affects THs transport through placental.

The association between prenatal exposure to organochlorine compounds and neonatal thyroid hormone levels: a systematic review

In this systematic review, the association between prenatal exposure to organochlorine pesticides (OCPs) and neonatal thyroid hormone levels was studied. A systematic search of scientific literature was performed from the PubMed, SCOPUS and ISI web of science electronic bibliographic databases. The search strategy for the review was [(organochlorine OR "organochlorine pesticides" OR "organochlorine pollutants" OR "organochlorine pollutant") AND ("thyroid hormone" OR triiodothyronine OR Thyroxine OR "fetal thyroid function" OR "thyroid function" OR "Thyroid Stimulating Hormone" AND "prenatal" AND "maternal exposure")] in English sources. In this review, 305 papers (PubMed: 30; Scopus: 29; ISI: 246) were identified through an electronic database search. Twenty-seven articles were assessed for eligibility, from which 16 qualified articles were selected for the final evaluation. The most common OCP metabolites which were evaluated in order were hexachlorobenzene (HCB) (13 studies), pp-dichlorodiphenyldichloroethylene (pp-DDE) (13 studies), hexachlorocyclohexane (HCH) (10 studies) and dichlorodiphenyltrichlorethane (DDT) (eight studies). A review of the documents related to the association of prenatal exposure of OCPs with fetal or neonatal thyroid function tests provides us with heterogeneous data in this field. Factors such as differences in the studied populations and their area, ethnic and genetic background, time and rate of exposure, possible interaction of other thyroid-disrupting environmental factors and dietary intake of micronutrients such as iodine and/or selenium are considered the main limitations for making an accurate conclusion. For some OCPs including DDT, DDE, HCH and HCB, there are supporting evidences, and it is suggested that their exposure could potentially alter the fetal thyroid function and consequently impair the neurodevelopment process of the infants.

Effects of the organochlorine p,p'-DDT on MCF-7 cells: Investigating metabolic and immune modulatory transcriptomic changes

The organochlorine pesticide dichloro-diphenyl-trichloroethane (DDT) is persistent in the environment and leads to adverse human health effects. High levels in breast milk pose a threat to both breast tissue and nursing infants. The objectives of this study were to investigate DDT-induced transcriptomic alterations in enzymes and transporters involved in xenobiotic metabolism, immune responses, oxidative stress markers, and cell growth in a human breast cancer cell line. MCF-7 cells were exposed to both environmentally-relevant and previously-tested concentrations of p,p'-DDT in a short-term experiment. Significant up-regulation of metabolizing enzymes and transporters (ACHE, GSTO1, NQO1 and ABCC2) and oxidative stress markers (CXCL8, HMOX-1, NFE2L2 and TNF) was clearly observed. Conversely, UGT1A6, AHR and cell growth genes (FGF2 and VEGFA) were severely down-regulated. Identification of these genes helps to identify mechanisms of p,p'-DDT action within cells and may be considered as useful biomarkers for exposure to DDT contamination.

Disruption of thyroid hormone regulated proteins and gene expression by polychlorinated biphenyls, polybrominated diphenyl ethers and new flame retardants in residents of an e-waste region

Polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and new flame retardants (NFRs) are known thyroid hormone (TH) disruptors, but their disrupting mechanisms in humans are not completely understood. In this study, we aimed to explore the disrupting mechanisms of the aforementioned chemicals via examining TH-regulated proteins and gene expression in human serum. Adult participants from an e-waste dismantling (exposed group) and a control region (control group) in South China provided blood samples for the research. Some compounds of PCBs, PBDEs, and NFRs showed strong binding affinity to the thyroid-stimulating hormone (TSH), thyroglobulin, thyroxine-binding globulin (TBG), gene expression of TH receptor α (TRα) and β, and iodothyronine deiodinase I (ID1). The highly exposed individuals had lower levels of TBG, TSH, and expression of TRα, but higher expression of ID1 than those of the control group. The disruption of TH-regulated proteins and gene expression suggested the exertion of different and, at times, even contradictory effects on TH disruption. However, no statistically significant difference was found in the TH levels between the exposed and the control group, implying that the TH disruption induced by these chemicals depends on the combined influence of multiple mechanisms. Gene expression appears to be an effective approach for investigations of TH disruption and the potential health effects.

Thyroid-disrupting chemicals and brain development: an update

This review covers recent findings on the main categories of thyroid hormone-disrupting chemicals and their effects on brain development. We draw mostly on epidemiological and experimental data published in the last decade. For each chemical class considered, we deal with not only the thyroid hormone-disrupting effects but also briefly mention the main mechanisms by which the same chemicals could modify estrogen and/or androgen signalling, thereby exacerbating adverse effects on endocrine-dependent developmental programmes. Further, we emphasize recent data showing how maternal thyroid hormone signalling during early pregnancy affects not only offspring IQ, but also neurodevelopmental disease risk. These recent findings add to established knowledge on the crucial importance of iodine and thyroid hormone for optimal brain development. We propose that prenatal exposure to mixtures of thyroid hormone-disrupting chemicals provides a plausible biological mechanism contributing to current increases in the incidence of neurodevelopmental disease and IQ loss.

Effect of exposure to p,p´-DDE during the first half of pregnancy in the maternal thyroid profile of female residents in a Mexican floriculture area

BACKGROUND

Dichlorodiphenyldichloroethene (p,p´-DDE), the main metabolite of dichlorodiphenyltrichloroethane (DDT), has been associated with changes in human thyroid hormone levels. Maternal thyroid hormones are essential for adequate fetal neurodevelopment during the first half of pregnancy.

OBJECTIVE

To evaluate the association between maternal p,p´-DDE concentration and the maternal thyroid profile during the first half of pregnancy.

MATERIALS AND METHODS

We analyzed the information of 430 pregnant women from a Mexican floriculture area, with a gestational age ≤16 weeks. By questionnaire, we obtained sociodemographic, reproductive, and life-style, information. Serum concentrations of thyroid stimulating hormone (TSH), and total and free T3 and T4 were determined by means of Enzyme-Linked ImmunoSorbent Assay (ELISA). p,p´-DDE was analyzed by Gas Chromatography. The association between p,p´-DDE and thyroid profile was assessed through linear and logistic regression models.

RESULTS

Thirty eight percent of women had p,p´-DDE levels below the Limit of Detection and 12.3% below the Limit of Quantification. Within the quantifiable range, median was 53.03ng/g. TSH >2.5 mIU/L was present in 9.3% of women; 47.7% had isolated hypothyroxinemia; 3.5% had subclinical hypothyroidism, and 5.8% had overt hypothyroidism. We observed a significant positive association between quantifiable p,p´-DDE and total T3 serum levels in comparison with those with concentrations below the Limit of Detection (β=0.19; 95% CI=0.06, 0.34). There were no significant associations with other hormones of the thyroid profile or with clinical diagnosis.

CONCLUSIONS

Our findings suggest that p,p´-DDE exposure, even at low concentrations, could disrupt thyroid homeostasis during pregnancy.

Association between organochlorine pesticide exposure and thyroid hormones in floriculture workers

Several studies have suggested that exposure to DDT may be related to changes in thyroid hormone levels in animals and humans, even though results across studies are inconsistent. The aim of this study was to assess the association between exposure to p,p'-DDE (a stable metabolite of DDT) and serum levels of thyroid hormones in floriculture workers. A longitudinal study was conducted on 136 male subjects from the States of Mexico and Morelos, Mexico, who were occupationally exposed to pesticides, during agricultural periods of high (rainy season) and low (dry season) levels of pesticide application. Using a structured questionnaire, a survey was carried out on socio-demographic characteristics, anthropometry, clinical history, alcohol and tobacco consumption, residential chemical exposure, and occupational history. Blood and urine samples were collected to determine serum levels of TSH, total T3, total T4, and p,p'-DDE, and metabolites of organophosphate pesticides (OP), respectively. The analysis of the associations between p,p'-DDE levels and thyroid hormone profile adjusting by potential confounding variables including urinary OP metabolites was carried out using multivariate generalized estimating equation (GEE) models. Our results showed that the geometric means of p,p'-DDE levels were 6.17 ng/ml and 4.71 ng/ml in the rainy and dry seasons, respectively. We observed positive associations between the serum levels of p,p'-DDE and those of total T3 (β=0.01, 95% CI: -0.009, 0.03), and total T4 (β=0.08, 95% CI:0.03, 0.14) and negative but no significant changes in TSH in male floricultural workers, supporting the hypothesis that acts as thyroid disruptor in humans.

Relationships between concentrations of selected organohalogen contaminants and thyroid hormones and vitamins A, E and D in Faroese pilot whales

Pilot whales (Globicephala melas) from the Faroe Islands, North-East Atlantic, have high body concentrations of organohalogenated compounds (OHCs), such as polychlorinated biphenyls (PCBs), organochlorinated pesticides (OCPs) and brominated flame retardants (BFRs). The aim of the present study was to examine if and to what extent blood plasma and liver concentrations of several groups of these OHCs are related to concentrations of relevant nutritional and hormonal biomarkers in pilot whales. Thyroid hormones (THs: total and free thyroxine and total and free triiodothyronine) and vitamin A (retinol), D (25-hydroxyvitamin D3) and E (α-tocopherol) were analysed in plasma (n=27) and vitamin A (total vitamin A, retinol and retinyl palmitate) and E (α- and γ-tocopherol) were analysed in liver (n=37) of Faroe Island pilot whales. Correlative relationships between the biomarkers and OHC concentrations previously analysed in the same tissues in these individuals were studied. The TH concentrations in plasma were significantly higher in juveniles than in adults. Vitamin D concentrations in plasma and α- and γ-tocopherol in liver were higher in adults than in juveniles. Multivariate statistical modelling showed that the age and sex influenced the relationship between biomarkers and OHCs. Some significant positive relationships were found between OHCs and thyroid hormone concentrations in the youngest juveniles (p<0.05). In plasma of juvenile whales α-tocopherol was also positively correlated with all the OHCs (p<0.05). Only few significant correlations were found between single OHCs and retinol and vitamin D in plasma within the age groups. There were significant negative relationships between hepatic PBDE concentrations and retinol (BDE-47) and γ-tocopherol (BDE-49, -47, -100, -99, -153) in liver. The relationships between OHCs and THs or vitamins suggest that in pilot whales OHCs seem to have minor effects on TH and vitamin concentrations.

Thyroid nodules and thyroid autoimmunity in the context of environmental pollution

Evidence suggests that in most industrialized countries autoimmune disorders, including chronic lymphocytic thyroiditis, are increasing. This increase parallels the one regarding differentiated thyroid cancer, the increment of which is mainly due to the papillary histotype. A number of studies have pointed to an association between chronic lymphocytic thyroiditis and differentiated thyroid cancer. The upward trend of these two thyroid diseases is sustained by certain environmental factors, such as polluting substances acting as endocrine disrupting chemicals. Herein we will review the experimental and clinical literature that highlights the effects of environmental and occupational exposure to polluting chemicals in the development of autoimmune thyroid disease or differentiated thyroid cancer. Stakeholders, starting from policymarkers, should become more sensitive to the consequences for the thyroid resulting from exposure to EDC. Indeed, the economic burden resulting from such consequences has not been quantified thus far.

Chemical contamination and the thyroid

Industrial chemical contaminants have a variable impact on the hypothalamic-pituitary-thyroid axis, this depending both on their class and on confounding factors. Today, mounting evidence is pointing to the role of environmental factors, and specifically EDCs, in the current distressing upsurge in the incidence of thyroid disease. The unease is warranted. These substances, which are nowadays rife in our environments (including in foodstuffs), have been shown to interfere with thyroid hormone action, biosynthesis, and metabolism, resulting in disruption of tissue homeostasis and/or thyroid function. Importantly, based on the concept of the "nonmonotonic dose-response curve", the relationship between dose and effect has often been found to be nonlinear. Thus, small doses can induce unpredictable, adverse effects, one case being polychlorinated biphenyls (PCBs), of which congener(s) may centrally inhibit the hypothalamic-pituitary-thyroid axis, or dissociate thyroid receptor and selectively affect thyroid hormone signaling and action. This means that PCBs can act as agonists or antagonists at the receptor level, underlining the complexity of the interaction. This review highlights the multifold activity of chemicals demonstrated to cause thyroid disruption. It also represents a call to action among clinicians to undertake systematic monitoring of thyroid function and registering of the classes of EDs and additionally urges broader scientific collaborations to clarify these chemicals' molecular mechanisms of action, substances whose prevalence in our environments is disrupting not only the thyroid but all life on earth.