Relevant dose of the environmental contaminant, tributyltin, promotes histomorphological changes in the thyroid gland of male rats

Endocrine Disruptors Chemicals: Impacts of Bisphenol A, Tributyltin and Lead on Thyroid Function

The large-scale industrial production characteristic of the last century led to an increase in man-made compounds and mobilization of natural compounds, many of which can accumulate in the environment and organisms due to their bioaccumulation and biomagnification properties. The endocrine system is especially vulnerable to these compounds that are known as endocrine disruptor chemicals (EDCs). Thyroid hormones (THs) are essential for normal development and growth, besides being the main regulators of basal metabolic rate. Thus, compounds able to affect THs synthesis, transport, and action could produce important deleterious effects, impacting the development of metabolic and endocrine diseases. Herein, we will review the main effects of EDCs on the thyroid axis, with special emphasis on the widely used substances bisphenol A (BPA), employed in the synthesis of polycarbonate plastics and epoxy resins; tributyltin (TBT), an organotin chemical substance widely used in several agro-industrial applications; and lead (Pb), a ubiquitous environmental and occupational polluting heavy metal. Exposure to these EDCs occurs mainly from the ingestion of contaminated food and beverages. Furthermore, there are few epidemiological studies evaluating human risk, and experimental studies employ different exposure models, making it difficult to integrate results. However, even low doses of these EDCs warn of thyrotoxicity. Since THs homeostasis is essential for health and humans are increasingly being exposed to EDCs, it is important to clarify which substances might act as thyroid hormone system disrupting chemicals and how they act in order to try to overcome their deleterious effects and limit the exposure to these compounds.

Endocrine-disrupting chemical, methylparaben, in environmentally relevant exposure promotes hazardous effects on the hypothalamus-pituitary-thyroid axis

Methylparaben (MP) belongs to the paraben class and is widely used as a preservative in personal care products, medicines, and some foods. MP acts as an endocrine disrupting chemical (EDC) on the hypothalamic-pituitary-thyroid (HPT) axis. However, the effects of MP have not yet been completely elucidated, as published results are scarce and controversial. The objective of this work was to evaluate the effects of subacute exposure to MP on the HPT axis of male rats. To achieve this, in this study the animals were divided into four experimental groups: control, MP3, MP30 and MP300 (3, 30 and 300 μg/kg/day, respectively). The rats were gavage for 14 days and sacrificed at the end of MP treatment. Our findings demonstrated that MP can promote important changes in thyroid morphology, including a decrease in follicular area, colloid area, epithelial area, and epithelial height, affecting the homeostasis of the HPT axis, and affecting the expression of genes related to hormonal biosynthesis. Furthermore, changes in interstitial collagen deposition were also demonstrated. Finally, we conclude that exposure to MP can be harmful to health, as it is involved in the dysregulation of the thyroid gland, affecting its morphophysiology, suggesting that even doses considered safe by current legislation can be dangerous and should be reconsidered.

The hypothalamus-pituitary-thyroid axis is disrupted by exposure to a mix of tributyltin and bisphenol S

Tributyltin is a biocide and bisphenol S is a plasticizer. The effects of the TBT + BPS mix on thyroid axis function are unknown. This study evaluated the effects of subacute exposure to TBT and BPS, both in mix and alone, in female young Wistar rats. Thyroid morphophysiology, gene expression, oxidative stress and collagen deposition were evaluated. TBT and BPS exposure resulted in a decrease in thyroid hormone levels, whereas TBT alone resulted in a decrease in TSH levels. The TBT + BPS group exhibited an increase in T4 levels, a decrease in T3 levels, a decrease in TPO activity, and an increase in TSHr mRNA expression. Deiodinase 1 (D1) and 2 (D2) were increased in the hypothalamus-pituitary-thyroid (HPT) axis (except for D2 in the pituitary gland) and in the liver of the TBT + BPS group, beside increases in the pituitary TRHr and thyroid ER mRNAs. The thyroid morphology of the TBT + BPS group revealed significant expansion of both the thyroid follicle and its surrounding tissue. In contrast, the TBT and BPS groups displayed numerous thyroid follicles undergoing fusion, a decrease in epithelial height and the epithelial/colloid ratio. The BPS group was the only group that exhibited increased collagen deposition. The TBT + BPS group demonstrated significant increases in the transcript levels of nrf2 and keap1. In all groups, the number of thiol groups decreased. There was an increase in SOD activity in the TBT + BPS group. Overall, subacute exposure to a mix deregulates the HPT axis, which correspondingly affects gene expression and causes enzymatic and morphological changes in the thyroid gland.

Maternal exposure to tributyltin alters the breast milk, hormonal profile, and thyroid morphology of dams and induces sex-specific changes in neonate rat offspring

Tributyltin (TBT) is the chemical substance commonly used worldwide to prevent biofouling of vessels. Due to its ability to bioaccumulate and biomagnify, even after being banned, significant concentrations of TBT can be detected in sediment, affecting marine and human life. Although studies have shown that direct exposure to TBT alters physiological parameters in mammals, the relationship between exposure to TBT during pregnancy and lactation, considered critical windows for metabolic programming, has not been fully elucidated. Our hypothesis is that offspring whose mothers were exposed to TBT during critical stages of development may exhibit dysfunctions in endocrine-metabolic parameters. We used pregnant Wistar rats that were divided into groups and received the following treatments from gestational day 7 until the end of lactation by intragastric gavage: vehicle (ethanol 0.01%; Control), low TBT dose (100 ng/kg of body weight (bw)/day; TBT100ng) and high TBT dose (1000 ng/kg bw/day; TBT1000ng). Dams and offspring at birth and weaning (21 days old) were studied. Maternal exposure to TBT promoted dose-dependent changes in dams. The findings for adiposity, milk composition and lipid profile were more pronounced in TBT100 ng dam; however, thyroid morphology was altered in TBT1000 ng dam. Female offspring were differentially affected by the dose of exposure. At birth, females in the TBT100ng group had low body weight, lower naso-anal length (NAL), and higher plasma T4, and at weaning, females in the TBT100ng group had lower insulin and leptin levels. Females in the TBT1000ng group had lower NAL at birth and lower leptinemia and weight of white adipose tissue at weaning. Male offspring from TBT groups showed high T3 at birth, without biometric alterations at birth or weaning. Despite these findings, both sexes exhibited dose-dependent morphological changes in the thyroid gland. Thus, maternal exposure to TBT constitutes an important route of contamination for both dams and offspring.

Assessment of five typical environmental endocrine disruptors and thyroid cancer risk: a meta-analysis

Introduction

There are conflicting reports on the association between environmental endocrine disruptors (EEDs) and thyroid cancer. This meta-analysis aimed to elucidate the relationship between EEDs and thyroid cancer.

Methods

We searched for epidemiological studies on EEDs and thyroid cancer published in PubMed and Web of Science up to December 2022. We then screened the articles that could extract data on EEDs concentration levels in both thyroid cancer patients and healthy controls. We excluded articles that could not calculate effect sizes, focused on other thyroid diseases, or lacked controls. Standardized mean difference (SMD) was calculated to analyze the association between EEDs and thyroid cancer. We measured the heterogeneity among the included studies using I2, assessed publication bias by Egger's and Begg's test, and evaluated article quality using the Newcastle-Ottawa Quality Score (NOS). In the end, fifteen eligible case-control studies were included.

Results

Our comprehensive analysis revealed that polychlorinated biphenyls (PCBs) were negatively associated with thyroid cancer{ SMD = -0.03, 95% confidence interval (CI) = (-0.05, -0.00), P = 0.03}, while polybrominated diphenyl ethers (PBDEs), phthalates (PAEs), and heavy metals were positively associated with thyroid cancer{PBDEs: SMD = 0.14, 95%CI = (0.04, 0.23), P = 0.007; PAEs: SMD = 0.30, 95%CI = (0.02, 0.58), P = 0.04; heavy metals: SMD = 0.21, 95%CI = (0.11, 0.32), P < 0.001}. We did not find a statistically significant relationship between bisphenol A (BPA) and thyroid cancer. Most of the included studies did not show publication bias, except for those on PCBs.

Discussion

Our results indicate that exposure to certain EEDs, such as PBDEs, PAEs, and heavy metals, increases the risk of thyroid cancer. However, further large-scale epidemiological studies and mechanism studies are needed to verify these potential relationships and understand the underlying biological mechanisms.

The potential role of environmental factors in modulating mitochondrial DNA epigenetic marks

Many studies implicate mitochondrial dysfunction in the development and progression of numerous chronic diseases. Mitochondria are responsible for most cellular energy production, and unlike other cytoplasmic organelles, mitochondria contain their own genome. Most research to date, through investigating mitochondrial DNA copy number, has focused on larger structural changes or alterations to the entire mitochondrial genome and their role in human disease. Using these methods, mitochondrial dysfunction has been linked to cancers, cardiovascular disease, and metabolic health. However, like the nuclear genome, the mitochondrial genome may experience epigenetic alterations, including DNA methylation that may partially explain some of the health effects of various exposures. Recently, there has been a movement to understand human health and disease within the context of the exposome, which aims to describe and quantify the entirety of all exposures people encounter throughout their lives. These include, among others, environmental pollutants, occupational exposures, heavy metals, and lifestyle and behavioral factors. In this chapter, we summarize the current research on mitochondria and human health, provide an overview of the current knowledge on mitochondrial epigenetics, and describe the experimental and epidemiologic studies that have investigated particular exposures and their relationships with mitochondrial epigenetic modifications. We conclude the chapter with suggestions for future directions in epidemiologic and experimental research that is needed to advance the growing field of mitochondrial epigenetics.

Environmentally relevant dose of the endocrine disruptor tributyltin disturbs redox balance in female thyroid gland

Tributyltin (TBT) is an endocrine disruptor used as a biocide in nautical paints. Even though many TBT effects in marine species are known, data in mammals are scarce, especially regarding the thyroid gland. The present study aimed to evaluate the effect of a subchronic exposure to TBT on thyroid oxidative stress of female Wistar rats. Rats received vehicle (control group), 200 or 1000 ng TBT/kg body weight/day for 40 days. After euthanasia, one part of the thyroids were collected in order to assess iodide uptake; activity and/or mRNA expression of thyroperoxidase (TPO) and dual oxidases (DUOXs); activity and/or mRNA expression of catalase, glutathione peroxidase, superoxide dismutase and NADPH oxidase 4 (CAT, GPx, SOD and NOX4); 4-hydroxynonenal (4-HNE) expression and total thiol groups levels; and mRNA expression of estrogen receptors alpha and beta (ERα and ERβ). The remaining part of the thyroid was processed for morphological analysis of estrogen receptor alpha (ERα) and for collagen deposition. Iodide uptake was not changed with treatments. TPO activity and expression were increased in the TBT1000 group (259.81% and 95.17%). The activity, but not mRNA, of CAT (17.36% TBT200; 27.10% TBT1000) and GPx (29.24% TBT200; 28.97% TBT1000) were decreased by TBT. SOD and NADPH oxidase activity, as well as thiol group and 4-HNE levels remained unchanged. Interstitial collagen deposition increased in the TBT200 group (39.54%). The mRNA expression of ERα increased in TBT-treated rats (44.87% TBT200; 36.43% TBT1000), while protein expression was increased but not reaching significance (TBT1000, p = 0.056) by TBT. Therefore, our results show that TBT increases TPO expression and reduces antioxidant enzyme activities in the thyroid gland leading to oxidative stress. Some of these effects could be mediated by the ERα pathway.

Subacute and low-dose tributyltin exposure disturbs the mammalian hypothalamus-pituitary-thyroid axis in a sex-dependent manner

Tributyltin (TBT) is an endocrine disruptor chemical (EDC) capable of altering the proper function of the hypothalamus-pituitary thyroid (HPT) axis. This study aimed to evaluate the subacute effects of TBT on the HPT axis of male and female rats. A dose of 100 ng/kg/day TBT was used in both sexes over a 15-day period, and the morphophysiology and gene expression of the HPT axis were assessed. TBT exposure increased the body weight in both sexes, while food efficiency increased - only in male rats. It was also possible to note alterations in the thyroid, with the presence of a stratified epithelium, cystic degeneration, and increased interstitial collagen deposition. A reduction in T3 and T4 levels was only observed in TBT male rats. A reduction in TSH levels was observed in TBT female rats. Evaluating mRNA expression, we observed a decrease in hepatic D1 and TRH mRNA levels in TBT female rats. An increase in D2 mRNA expression in the hypothalamus was observed in TBT male rats. Additionally, no significant changes in TRH or hepatic D1 mRNA expression in TBT male rats or in hypothalamic D1 and D2 mRNA expression in TBT female rats were observed. Thus, we can conclude that TBT has different toxicological effects on male and female rats by altering thyroid gland morphophysiology, leading to abnormal HPT axis function, and even at subacute and low doses, it may be involved in complex endocrine and metabolic disorders.

Mitochondrial Epigenetics and Environmental Health: Making a Case for Endocrine Disrupting Chemicals

Recent studies implicate mitochondrial dysfunction in the development and progression of numerous chronic diseases, which may be partially due to modifications in mitochondrial DNA (mtDNA). There is also mounting evidence that epigenetic modifications to mtDNA may be an additional layer of regulation that controls mitochondrial biogenesis and function. Several environmental factors (eg, smoking, air pollution) have been associated with altered mtDNA methylation in a handful of mechanistic studies and in observational human studies. However, little is understood about other environmental contaminants that induce mtDNA epigenetic changes. Numerous environmental toxicants are classified as endocrine disrupting chemicals (EDCs). Beyond their actions on hormonal pathways, EDC exposure is associated with elevated oxidative stress, which may occur through or result in mitochondrial dysfunction. Although only a few studies have assessed the impacts of EDCs on mtDNA methylation, the current review provides reasons to consider mtDNA epigenetic disruption as a mechanism of action of EDCs and reviews potential limitations related to currently available evidence. First, there is sufficient evidence that EDCs (including bisphenols and phthalates) directly target mitochondrial function, and more direct evidence is needed to connect this to mtDNA methylation. Second, these and other EDCs are potent modulators of nuclear DNA epigenetics, including DNA methylation and histone modifications. Finally, EDCs have been shown to disrupt several modulators of mtDNA methylation, including DNA methyltransferases and the mitochondrial transcription factor A/nuclear respiratory factor 1 pathway. Taken together, these studies highlight the need for future research evaluating mtDNA epigenetic disruption by EDCs and to detail specific mechanisms responsible for such disruptions.

The impact of endocrine-disrupting chemical exposure in the mammalian hypothalamic-pituitary axis

The hypothalamic-pituitary axis (HP axis) plays a critical and integrative role in the endocrine system control to maintain homeostasis. The HP axis is responsible for the hormonal events necessary to regulate the thyroid, adrenal glands, gonads, somatic growth, among other functions. Endocrine-disrupting chemicals (EDCs) are a worldwide public health concern. There is growing evidence that exposure to EDCs such as bisphenol A (BPA), some phthalates, polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and biphenyls (PBBs), dichlorodiphenyltrichloroethane (DDT), tributyltin (TBT), and atrazine (ATR), is associated with HP axis abnormalities. EDCs act on hormone receptors and their downstream signaling pathways and can interfere with hormone synthesis, metabolism, and actions. Because the HP axis function is particularly sensitive to endogenous hormonal changes, disruptions by EDCs can alter HP axis proper function, leading to important endocrine irregularities. Here, we review the evidence that EDCs could directly affect the mammalian HP axis function.