Evaluation of tetrabromobisphenol A effects on human glucocorticoid and androgen receptors: A comparison of results from human- with yeast-based in vitro assays

Tetrabromobisphenol A reduces male rats reproductive organ coefficients and disrupting sexual hormone by causing oxidative stress

Tetrabromobisphenol A (TBBPA) has become a topic of public attention due to its pervasive detection in the environment and organisms in recent decades. However, limited information is available regarding the toxicity of TBBPA on reproductive ability of male mammals. Herein, the reproductive toxicity of TBBPA was investigated in male rats to fill the knowledge gap. In this study, male rats were exposed to TBBPA (0, 10, 100, and 1000 mg/kg) for 6 weeks. Subsequently, body and organ indexes, histopathological evaluation of testis and epididymis, ultrastructural observation of sperm, testosterone and progesterone levels, and oxidative stress indicators were conducted to reveal corresponding mechanisms. Results obtained showed that compare to the control group, the body weight, testes weight, epididymis weight, seminal vesicle and coagulation glands weight of rats in the 1000 mg/kg group lost 8.30%, 16.84%, 20.16%, 19.72% and 26.42%, respectively. Intriguingly, exposure to TBBPA (10, 100, 100 mg/kg) resulted in substantial pathological damage in testis, epididymis and sperm. TBBPA exposure also increased malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents, as well as superoxide dismutase (T-SOD) and catalase (CAT) activities in testicular tissue. What's more, the testosterone and progesterone levels in male rat serum were significantly decreased after exposure to TBBPA for 6 weeks. Meanwhile, results of molecular docking showed that TBBPA has a strong affinity with estrogen receptors (ERs). These findings demonstrated that TBBPA exposure negatively impacts the reproductive ability of male rats, thus providing new insights for risk assessment for reproductive health under TBBPA exposure.

Cellular and physiological mechanisms of halogenated and organophosphorus flame retardant toxicity

Flame retardants (FRs) are chemical substances used to inhibit the spread of fire in numerous industrial applications, and their abundance in modern manufactured products in the indoor and outdoor environment leads to extensive direct and food chain exposure of humans. Although once considered relatively non-toxic, FRs are demonstrated by recent literature to have disruptive effects on many biological processes, including signaling pathways, genome stability, reproduction, and immune system function. This review provides a summary of research investigating the impact of major groups of FRs, including halogenated and organophosphorus FRs, on animals and humans in vitro and/or in vivo. We put in focus those studies that explained or referenced the modes of FR action at the level of cells, tissues and organs. Since FRs are highly hydrophobic chemicals, their biophysical and biochemical modes of action usually involve lipophilic interactions, e.g. with biological membranes or elements of signaling pathways. We present selected toxicological information about these molecular actions to show how they can lead to damaging membrane integrity, damaging DNA and compromising its repair, changing gene expression, and cell cycle as well as accelerating cell death. Moreover, we indicate how this translates to deleterious bioactivity of FRs at the physiological level, with disruption of hormonal action, dysregulation of metabolism, adverse effects on male and female reproduction as well as alteration of normal pattern of immunity. Concentrating on these subjects, we make clear both the advances in knowledge in recent years and the remaining gaps in our understanding, especially at the mechanistic level.

Tetrabromobisphenol A and Diclazuril Evoke Tissue-Specific Changes of Thyroid Hormone Signaling in Male Thyroid Hormone Action Indicator Mice

Thyroid hormone (TH) signaling is a prerequisite of normal tissue function. Environmental pollutants with the potential to disrupt endocrine functions represent an emerging threat to human health and agricultural production. We used our Thyroid Hormone Action Indicator (THAI) mouse model to study the effects of tetrabromobisphenol A (TBBPA; 150 mg/bwkg/day orally for 6 days) and diclazuril (10.0 mg/bwkg/day orally for 5 days), a known and a potential hormone disruptor, respectively, on local TH economy. Tissue-specific changes of TH action were assessed in 90-day-old THAI mice by measuring the expression of a TH-responsive luciferase reporter in tissue samples and by in vivo imaging (14-day-long treatment accompanied with imaging on day 7, 14 and 21 from the first day of treatment) in live THAI mice. This was followed by promoter assays to elucidate the mechanism of the observed effects. TBBPA and diclazuril impacted TH action differently and tissue-specifically. TBBPA disrupted TH signaling in the bone and small intestine and impaired the global TH economy by decreasing the circulating free T4 levels. In the promoter assays, TBBPA showed a direct stimulatory effect on the hdio3 promoter, indicating a potential mechanism for silencing TH action. In contrast, diclazuril acted as a stimulator of TH action in the liver, skeletal muscle and brown adipose tissue without affecting the Hypothalamo-Pituitary-Thyroid axis. Our data demonstrate distinct and tissue-specific effects of TBBPA and diclazuril on local TH action and prove that the THAI mouse is a novel mammalian model to identify TH disruptors and their tissue-specific effects.

Review of the environmental occurrence, analytical techniques, degradation and toxicity of TBBPA and its derivatives

BFRs (brominated flame retardants) are a class of compounds that are added to or applied to polymeric materials to avoid or reduce the spread of fire. Tetrabromobisphenol A (TBBPA) is one of the known BFR used many in industries today. Due to its wide application as an additive flame retardant in commodities, TBBPA has become a common indoor contaminant. Recent researches have raised concerns about the possible hazardous effect of exposure to TBBPA and its derivatives in humans and wildlife. This review gives a thorough assessment of the literature on TBBPA and its derivatives, as well as environmental levels and human exposure. Several analytical techniques/methods have been developed for sensitive and accurate analysis of TBBPA and its derivatives in different compartments. These chemicals have been detected in practically every environmental compartment globally, making them a ubiquitous pollutant. TBBPA may be subject to adsorption, biological degradation or photolysis, photolysis after being released into the environment. Treatment of TBBPA-containing waste, as well as manufacturing and usage regulations, can limit the release of these chemicals to the environment and the health hazards associated with its exposure. Several methods have been successfully employed for the treatment of TBBPA including but not limited to adsorption, ozonation, oxidation and anaerobic degradation. Previous studies have shown that TBBPA and its derivative cause a lot of toxic effects. Diet and dust ingestion and have been identified as the main routes of TBBPA exposure in the general population, according to human exposure studies. Toddlers are more vulnerable than adults to be exposed to indoor dust through inadvertent ingestion. Furthermore, TBBP-A exposure can occur during pregnancy and through breast milk. This review will go a long way in closing up the knowledge gap on the silent and over ignored deadly effects of TBBPA and its derivatives and their attendant consequences.

Endocrine disrupting potential of replacement flame retardants - Review of current knowledge for nuclear receptors associated with reproductive outcomes

BACKGROUND AND AIM

Endocrine disrupting chemicals (EDCs) constitute a major public health concern because they can induce a large spectrum of adverse effects by interfering with the hormonal system. Rapid identification of potential EDCs using in vitro screenings is therefore critical, particularly for chemicals of emerging concerns such as replacement flame retardants (FRs). The review aimed at identifying (1) data gaps and research needs regarding endocrine disrupting (ED) properties of replacement FRs and (2) potential EDCs among these emerging chemicals.

METHODS

A systematic search was performed from open literature and ToxCast/Tox21 programs, and results from in vitro tests on the activities of 52 replacement FRs towards five hormone nuclear receptors (NRs) associated with reproductive outcomes (estrogen, androgen, glucocorticoid, progesterone, and aryl hydrocarbon receptors) were compiled and organized into tables. Findings were complemented with information from structure-based in silico model predictions and in vivo information when relevant.

RESULTS

For the majority of the 52 replacement FRs, experimental in vitro data on activities towards these five NRs were either incomplete (15 FRs) or not found (24 FRs). Within the replacement FRs for which effect data were found, some appeared as candidate EDCs, such as triphenyl phosphate (TPhP) and tris(1,3-dichloropropyl)phosphate (TDCIPP). The search also revealed shared ED profiles. For example, anti-androgenic activity was reported for 19 FRs and predicted for another 21 FRs.

DISCUSSION

This comprehensive review points to critical gaps in knowledge on ED potential for many replacement FRs, including chemicals to which the general population is likely exposed. Although this review does not cover all possible characteristics of ED, it allowed the identification of potential EDCs associated with reproductive outcomes, calling for deeper evaluation and possibly future regulation of these chemicals. By identifying shared ED profiles, this work also raises concerns for mixture effects since the population is co-exposed to several FRs and other chemicals.

Mechanistic in silico modeling of bisphenols to predict estrogen and glucocorticoid disrupting potentials

Endocrine disrupting chemicals (EDCs) can act as agonists, antagonists or mixed agonists/antagonists toward estrogen receptor α (ERα) and glucocorticoid receptor (GR) in a tissue- and cell-specific manner. However, the activation/inhibition mechanism by which structurally different chemicals induce various types of disruption remain ambiguous. This unrevealed theory limited the in silico modeling of EDCs and the prioritization of potential EDCs for experimental testing. As a kind of chemical widely used in manufacture, bisphenols (BPs) have attracted great attentions on their potential endocrine disrupting effects. BPs used in this study exhibited pure agonistic, pure antagonistic or mixed agonistic/antagonistic activities toward ERα and/or GR. According to the mechanistic modeling, the pure agonistic and pure antagonistic activities were attributed to a single type of protein conformation induced by BPs-ERα and/or BPs-GR interactions, whereas the mixed agonistic/antagonistic activities were attributed to multiple conformations that concomitantly exist. After interacting with BPs, the active conformation recruits coactivator to induce agonistic activity and the blocked conformation inhibits coactivator to induce antagonistic activity, whereas the concomitantly-existing multiple conformations (active, blocked and competing conformations) recruit coactivator, recruit corepressor and/or inhibit coactivator to dually induce the agonistic and antagonistic activities. Therefore, the in silico modeling in this study can not only predict ERα and GR disrupting activities but also, especially, identify the potential mechanisms. This mechanistic study breaks the current bottleneck of computational toxicology and can be widely used to prioritize potential estrogen/glucocorticoid disruptor for experimental testing in both pre-clinic and clinic studies.

Glucocorticoid and mineralocorticoid receptors and corticosteroid homeostasis are potential targets for endocrine-disrupting chemicals

Endocrine-disrupting chemicals (EDCs) have received significant concern, since they ubiquitously exist in the environment and are able to induce adverse health effects on human and wildlife. Increasing evidence shows that the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR), members of the steroid receptor subfamily, are potential targets for EDCs. GR and MR mediate the actions of glucocorticoids and mineralocorticoids, respectively, which are two main classes of corticosteroids involved in many physiological processes. The effects of EDCs on the homeostasis of these two classes of corticosteroids have also gained more attention recently. This review summarized the effects of environmental GR/MR ligands on receptor activity, and disruption of corticosteroid homeostasis. More than 130 chemicals classified into 7 main categories were reviewed, including metals, metalloids, pesticides, bisphenol analogues, flame retardants, other industrial chemicals and pharmaceuticals. The mechanisms by which EDCs interfere with GR/MR activity are primarily involved in ligand-receptor binding, nuclear translocation of the receptor complex, DNA-receptor binding, and changes in the expression of endogenous GR/MR genes. Besides directly interfering with receptors, enzyme-catalyzed synthesis and prereceptor regulation pathways of corticosteroids are also important targets for EDCs. The collected evidence suggests that corticosteroids and their receptors should be considered as potential targets for safety assessment of EDCs. The recognition of relevant xenobiotics and their underlying mechanisms of action is still a challenge in this emerging field of research.

Differences in reproductive toxicity of TBBPA and TCBPA exposure in male Rana nigromaculata

Tetrabromobisphenol A (TBBPA) and tetrachlorobisphenol A (TCBPA) are persistent toxic environmental pollutants that cause severe reproductive toxicity in animals. The goal of this study was to compare the reproductive toxic effects of TBBPA and TCBPA on male Rana nigromaculata and to expound on the mechanisms leading to these effects. Healthy adult frogs were exposed to 0, 0.001, 0.01, 0.1, and 1 mg/L of TBBPA and TCBPA for 14 days. Sperm numbers were counted by erythrometry. Sperm mobility and deformities were observed under a light microscope (400 × ). We used commercial ELISA kits to determine the serum content of testosterone (T), estradiol (E2), luteinizing hormone (LH) and follicle stimulating hormone (FSH). Expression of androgen receptor (AR) mRNA was detected using real-time qPCR. Sperm numbers and sperm mobility were significantly decreased and sperm deformity was significantly increased in a concentration dependent manner following exposure to TBBPA and TCBPA. Sperm deformity was significantly greater in the 1 mg/L TCBPA (0.549) treatment group than in the 1 mg/L TBBPA (0.397) treatment group. Serum T content was significantly greater in the 0.01, 0.1 and 1 mg/L TBBPA and TCBPA experimental groups compared with controls, while E2 content was significantly greater in only the 1 mg/L TBBPA and TCBPA experimental groups. Expression levels of LH and FSH significantly decreased in the 1 mg/L TBBPA and TCBPA treatment groups. AR mRNA expression decreased markedly in all the treated groups. Our results indicated that TBBPA and TCBPA induced reproductive toxicity in a dose-dependent manner, with TCBPA having greater toxicity than TBBPA. Furthermore, changes in T, E2, LH, and FSH levels induced by TBBPA and TCBPA exposure, which led to endocrine disorders, also caused disturbance of spermatogenesis through abnormal gene expressions of AR in the testes.

An ex vivo assay for screening glucocorticoid signaling disruption based on glucocorticoid-response gene transcription in Xenopus tails

There is a pressing need for developing in vivo or ex vivo assays to screen the glucocorticoid (GC) signaling disruption of chemicals. Thus, we aimed to establish an ex vivo assay for screening GC signaling disruption based on the GC-response gene transcription in Xenopus laevis tails cultured ex vivo. Firstly, we investigated effects of corticosterone (CORT, a main GC in frogs) on GC-response gene expression, and determined the six genes as molecular endpoints for assaying the GC signaling disruption. CORT in the range of 1.56-400nmol/L was found to up-regulate transcription of the six GC-response genes, exhibiting comparable or higher sensitivity than previously reported assays. To validate this ex vivo assay, then, we examined effects of dexamethasone (a known GC signaling agonist) on GC-response gene expression. Dexamethasone displayed an agonistic action in a concentration-dependent manner, further demonstrating the efficiency of the established assay. Finally, we applied the ex vivo assay to evaluate the GC signaling disruption of bisphenol A (BPA). In accordance with previous reports, we found a concentration-dependent agonistic activity of BPA, showing that the established assay is effective for detecting the GC signaling disrupting activity of environmental chemicals. Correspondingly, the GC signaling agonistic actions of CORT and BPA in ex vivo tails accorded with the observations in vivo, indicating that the ex vivo assay is able to detect the actions of chemicals in vivo. Overall, we established an ex vivo assay that can effectively screen GC signaling disruption of environmental chemicals.

Co-exposure of Freshwater Microalgae to Tetrabromobisphenol A and Sulfadiazine: Oxidative Stress Biomarker Responses and Joint Toxicity Prediction

Combined toxicity and oxidative stress biomarker responses were determined for tetrabromobisphenol A (TBBPA) and sulfadiazine (SDZ) to the unicellular green alga Scenedesmus obliquus. Concentration-response analyses were performed for single toxicants and for mixtures containing TBBPA and SDZ with two different mixture ratios. The effect concentrations and the observed effects of the mixtures were compared to the predictions of the joint toxicity by the concentration addition (CA) model and independent action (IA) model. Results showed that the observed joint toxicity was within the scope of the highest (TBBPA) and lowest (SDZ) toxicity observed for the individual components. Furthermore, co-exposure of S. obliquus to TBBPA and SDZ provided preliminary evidence that the mixtures induced oxidative stress leading to cell damage. The CA and IA models proved to be valid for the prediction of the joint toxicity of TBBPA and SDZ. This study highlights a combined environmental risk assessment for two emerging pollutants.

Carbonyl reductase 1 catalyzes 20β-reduction of glucocorticoids, modulating receptor activation and metabolic complications of obesity

Carbonyl Reductase 1 (CBR1) is a ubiquitously expressed cytosolic enzyme important in exogenous drug metabolism but the physiological function of which is unknown. Here, we describe a role for CBR1 in metabolism of glucocorticoids. CBR1 catalyzes the NADPH- dependent production of 20β-dihydrocortisol (20β-DHF) from cortisol. CBR1 provides the major route of cortisol metabolism in horses and is up-regulated in adipose tissue in obesity in horses, humans and mice. We demonstrate that 20β-DHF is a weak endogenous agonist of the human glucocorticoid receptor (GR). Pharmacological inhibition of CBR1 in diet-induced obesity in mice results in more marked glucose intolerance with evidence for enhanced hepatic GR signaling. These findings suggest that CBR1 generating 20β-dihydrocortisol is a novel pathway modulating GR activation and providing enzymatic protection against excessive GR activation in obesity.

Influence of Tetrabromobisphenol A, with or without Concurrent Triclosan, upon Bisphenol A and Estradiol Concentrations in Mice

BACKGROUND

Humans are commonly exposed to multiple environmental chemicals, including tetrabromobisphenol A (TBBPA; a flame retardant), triclosan (an antimicrobial agent), and bisphenol A (BPA; polycarbonate plastics). These chemicals are readily absorbed and may interact with each other.

OBJECTIVES

We sought to determine whether TBBPA, given alone or in combination with triclosan, can modulate the concentrations of BPA and 17β-estradiol (E2).

METHODS

Female and male CF-1 mice were each given a subcutaneous injection of 0-27mg TBBPA, with or without concurrent 0.33mg triclosan, followed by dietary administration of 50μg/kg body weight ¹⁴C-BPA. Radioactivity was measured in blood serum and tissues through liquid scintillation counting. In subsequent experiments, female and male CF-1 mice were each given a subcutaneous injection of 0 or 1mg TBBPA and E2 was measured in urine 2-12 h after injection.

RESULTS

Doses as low as 1mg TBBPA significantly elevated ¹⁴C-BPA concentrations in the uterus and ovaries of females; in the testes, epididymides, vesicular-coagulating glands, and preputial glands of males; and in blood serum, heart, lungs, and kidneys of both sexes; urinary E2 concentrations were also elevated. Lower doses of TBBPA or triclosan that had no effects on their own elevated ¹⁴C-BPA concentrations when the two substances were given concurrently.

CONCLUSION

These data indicate that TBBPA, triclosan, and BPA interact in vivo, consistent with evidence that TBBPA and triclosan inhibit enzymes that are critical for BPA and E2 metabolism. https://doi.org/10.1289/EHP1329.

DHRS7 (SDR34C1) - A new player in the regulation of androgen receptor function by inactivation of 5α-dihydrotestosterone?

DHRS7 (SDR34C1) has been associated with potential tumor suppressor effects in prostate cancer; however, its function remains largely unknown. Recent experiments using purified recombinant human DHRS7 suggested several potential substrates, including the steroids cortisone and Δ4-androstene-3,17-dione (androstenedione). However, the substrate and cofactor concentrations used in these experiments were very high and the physiological relevance of these observations needed to be further investigated. In the present study, recombinant human DHRS7 was expressed in intact HEK-293 cells in order to investigate whether glucocorticoids and androgens serve as substrates at sub-micromolar concentrations and at physiological cofactor concentrations. Furthermore, the membrane topology of DHRS7 was revisited using redox-sensitive green-fluorescent protein fusions in living cells. The results revealed that (1) cortisone is a substrate of DHRS7; however, it is not reduced to cortisol but to 20β-dihydrocortisone, (2) androstenedione is not a relevant substrate of DHRS7, (3) DHRS7 catalyzes the oxoreduction of 5α-dihydrotestosterone (5αDHT) to 3α-androstanediol (3αAdiol), with a suppressive effect on androgen receptor (AR) transcriptional activity, and (4) DHRS7 is anchored in the endoplasmic reticulum membrane with a cytoplasmic orientation. Together, the results show that DHRS7 is a cytoplasmic oriented enzyme exhibiting 3α/20β-hydroxysteroid dehydrogenase activity, with a possible role in the modulation of AR function. Further research needs to address the physiological relevance of DHRS7 in the inactivation of 5αDHT and AR regulation.