Uterine Carcinomas in Tetrabromobisphenol A-exposed Wistar Han Rats Harbor Increased Tp53 Mutations and Mimic High-grade Type I Endometrial Carcinomas in Women

Update of the scientific opinion on tetrabromobisphenol A (TBBPA) and its derivatives in food

The European Commission asked EFSA to update its 2011 risk assessment on tetrabromobisphenol A (TBBPA) and five derivatives in food. Neurotoxicity and carcinogenicity were considered as the critical effects of TBBPA in rodent studies. The available evidence indicates that the carcinogenicity of TBBPA occurs via non-genotoxic mechanisms. Taking into account the new data, the CONTAM Panel considered it appropriate to set a tolerable daily intake (TDI). Based on decreased interest in social interaction in male mice, a lowest observed adverse effect level (LOAEL) of 0.2 mg/kg body weight (bw) per day was identified and selected as the reference point for the risk characterisation. Applying the default uncertainty factor of 100 for inter- and intraspecies variability, and a factor of 3 to extrapolate from the LOAEL to NOAEL, a TDI for TBBPA of 0.7 μg/kg bw per day was established. Around 2100 analytical results for TBBPA in food were used to estimate dietary exposure for the European population. The most important contributors to the chronic dietary LB exposure to TBBPA were fish and seafood, meat and meat products and milk and dairy products. The exposure estimates to TBBPA were all below the TDI, including those estimated for breastfed and formula-fed infants. Accounting for the uncertainties affecting the assessment, the CONTAM Panel concluded with 90%-95% certainty that the current dietary exposure to TBBPA does not raise a health concern for any of the population groups considered. There were insufficient data on the toxicity of any of the TBBPA derivatives to derive reference points, or to allow a comparison with TBBPA that would support assignment to an assessment group for the purposes of combined risk assessment.

TBBPA rather than its main derivatives enhanced growth of endometrial cancer via p53 ubiquitination

Tetrabromobisphenol A (TBBPA) and its derivatives widely exist in various environments and biota. Although the available data indicate that TBBPA exposure is highly associated with the increased incidence of endometrial cancer (EC), the effects of TBBPA and its main derivatives on EC proliferation and the involved crucial mechanism remain unclear. The present study aimed to investigate the effects of TBBPA and its derivatives under environmental concentrations on the proliferation of EC, and the crucial mechanism on the progression of EC caused by bromine flame retardants exposure. In this research, TBBPA and two of the most common TBBPA derivatives including TBBPA bis (2-hydroxyethyl ether) (TBBPA-BHEE) and TBBPA bis (dibromopropyl ether) (TBBPA-BDBPE) were screened for their capacities in induced EC proliferation and explored the related mechanism by in vitro cell culture model and in vivo mice model. Under environmental concentrations, TBBPA promoted the proliferation of EC, the main derivatives of TBBPA (TBBPA-BHEE and TBBPA-BDBPE) did not present the similar facilitation effects. The ubiquitination degradation of p53 was crucial in TBBPA induced EC proliferation, which resulted in the increase of downstream cell cycle and decrease of apoptosis. The further molecular docking result suggested the high affinity between TBBPA and ubiquitinated proteasome. This finding revealed the effects of TBBPA and its derivatives on EC proliferation, thus providing novel insights into the underlying mechanisms of TBBPA-caused EC.

Invisible Hand behind Female Reproductive Disorders: Bisphenols, Recent Evidence and Future Perspectives

Reproductive disorders are considered a global health problem influenced by physiological, genetic, environmental, and lifestyle factors. The increased exposure to bisphenols, a chemical used in large quantities for the production of polycarbonate plastics, has raised concerns regarding health risks in humans, particularly their endocrine-disrupting effects on female reproductive health. To provide a basis for future research on environmental interference and reproductive health, we reviewed relevant studies on the exposure patterns and levels of bisphenols in environmental matrices and humans (including susceptible populations such as pregnant women and children). In addition, we focused on in vivo, in vitro, and epidemiological studies evaluating the effects of bisphenols on the female reproductive system (the uterus, ovaries, fallopian tubes, and vagina). The results indicate that bisphenols cause structural and functional damage to the female reproductive system by interfering with hormones; activating receptors; inducing oxidative stress, DNA damage, and carcinogenesis; and triggering epigenetic changes, with the damaging effects being intergenerational. Epidemiological studies support the association between bisphenols and diseases such as cancer of the female reproductive system, reproductive dysfunction, and miscarriage, which may negatively affect the establishment and maintenance of pregnancy. Altogether, this review provides a reference for assessing the adverse effects of bisphenols on female reproductive health.

The role of polybrominated diphenyl ethers in the induction of cancer: a systematic review of insight into their mechanisms

Environmental pollution caused by persistent organic pollutants (POPs) has increased the challenge for the scientific communities. Polybrominated diphenyl ethers (PBDEs), classified as POPs, are widely applied in various materials as brominated flame retardants (BFRs). Because of the nature of these chemical compounds including toxicity, stability, and capability to bioaccumulate and biomagnify, PBDEs have posed a great challenge and risk to human health and wildlife. Therefore, the side effects of exposure to PBDEs as ubiquitous pollutants in the environment on cancer progression were investigated using a systematic review (SR) survey. To achieve this goal, forty studies were considered after defining the search terms and inclusion criteria, and/or exclusion criteria; the eligible records were collected from the international bibliographic databases. Based on the findings of the reviewed records, environmental exposure to the BFRs including PBDEs has a positive association with different mechanisms that induce cancer progression. However, the findings of the reviewed studies were not totally consistent with the mode of action and side effects are yet to be fully elucidated. Several articles have reported that BFRs can be carcinogenic and induce epithelial to mesenchymal transition via different mechanisms. The main mode of action involved in the environmental exposure to BFRs and the risk of cancer progression is endoplasmic reticulum and oxidative stress (OS). Generally, the imbalance of antioxidant mechanisms, reactive nitrogen species (RNSs) and reactive oxygen species (ROSs), during damage in cells, and stress caused OS, which increases tumorigenesis via multiple mechanisms, such as DNA damage, inflammation, and angiogenesis.

Tetrabromobisphenol A induces THR β-mediated inflammation and uterine injury in mice at environmentally relevant exposure concentrations

Tetrabromobisphenol A (TBBPA) is a widely used flame retardant, but the adverse outcomes induced by TBBPA has not been fully elucidated. In this study, TBBPA was detected in 54.9% of 102 female Chinese volunteers with an average serum concentration of 0.34 ng/mL. To investigate whether TBBPA induces adverse outcomes at environmentally relevant exposure doses, the mice were exposed to TBBPA for 14 and 28 days. The internal doses of TBBPA in mice serum were nearly the internal doses in volunteers. TBBPA significantly increased the secretion of some pro-inflammatory cytokines and suppressed immune responses in mice under such serum concentrations after 14- and 28-days exposure. Interestingly, uterine edema was observed in TBBPA-treated mice. In primary uterine cells model, the results showed TBBPA exposure suppressed THRβ expression, leading to the activation of the inflammatory PI3K/NF-κB signaling pathway. Our findings indicated that the uterus is the susceptible target organ of TBBPA and TBBPA exposure might increase risk of uterine cancer through deregulating inflammation pathways.

Differential Receptor Tyrosine Kinase Phosphorylation in the Uterus of Rats Following Developmental Exposure to Tetrabromobisphenol A

Tetrabromobisphenol A (TBBPA) is a brominated flame retardant that induces endometrial adenocarcinoma and other uterine tumors in Wistar Han rats; however, early molecular events or biomarkers of TBBPA exposure remain unknown. We investigated the effects of TBBPA on growth factor receptor activation (phospho-RTK) in uteri of rats following early-life exposures. Pregnant Wistar Han rats were exposed to TBBPA (0, 0.1, 25, 250 mg/kg/day) via oral gavage on gestation day 6 through weaning of pups (PND 21). Pups were exposed in utero, through lactation, and by daily gavage from PND 22 to PND 90. Uterine horns were collected (at PND 21, PND 33, PND 90) and formalin-fixed or frozen for histologic, immunohistochemical, phospho-RTK arrays, or western blot analysis. At PND 21, the phosphor-RTKs, FGFR2, FGFR3, TRKC and EPHA1 were significantly increased at different treatment concentrations. Several phospho-RTKs were also significantly overexpressed at PND 33 which included epithelial growth factor receptor (EGFR), Fibroblast Growth Factor Receptor 3-4 (FGFR2, FGFR3, FGFR4), insulin-like growth factor receptor 1 (IGF1R), INSR, AXL, MERTK, PDGFRa and b, RET, Tyrosine Kinase with Immunoglobulin Like and EGF Like Domains 1 and 2 (TIE1; TIE2), TRKA, VEGFR2 and 3, and EPHA1 at different dose treatments. EGFR, an RTK overexpressed in endometrial cancer in women, remained significantly increased for all treatment groups at PND 90. Erb-B2 Receptor Tyrosine Kinase 2 (ERBB2) and IGF1R were overexpressed at PND 33 and remained increased through PND 90, although ERBB2 was statistically significant at PND 90. The phospho-RTKs, FGFR3, AXL, DTK, HGFR, TRKC, VEGFR1 and EPHB2 and 4 were also statistically significant at PND 90 at different dose treatments. The downstream effector, phospho-MAPK44/42 was also increased in uteri of treated rats. Our findings show RTKs are dysregulated following early life TBBPA exposures and their sustained activation may contribute to TBBPA-induced uterine tumors observed in rats later in life.

The promotion of tetrabromobisphenol A exposure on Ishikawa cells proliferation and pivotal role of ubiquitin-mediated IκB' degradation

Tetrabromobisphenol A (TBBPA), one of the highly common industrial brominated flame retardants (BFRs), has been recently reported to influence the progression of endometrial carcinoma. However, the underlying mechanism between them has not been fully illuminated. Our findings demonstrated that treatment with low concentrations of TBBPA significantly induced the proliferation of Ishikawa cells in a concentration- and time-dependent manner. Mechanically, TBBPA stimulation led to the elevation of NF-κB expression, accompanied by the occurrence of ubiquitin-mediated IκB' degradation. Additionally, the upregulation of pro-inflammatory cytokines upon TBBPA exposure was observed in both mRNA and protein levels. Interestingly, the above toxic effects of TBBPA on Ishikawa cells were markedly attenuated by the addition of MG-132, a proteasome inhibitor, suggesting the crucial role of ubiquitin-mediated IκB' degradation in the TBBPA-stimulated proliferation of Ishikawa cells. Confirmation using in vivo model was also presented in this work. Accordingly, our data indicated that ubiquitin-mediated IκB' degradation and inflammatory response could serve as critical and sensitive biomarkers for the TBBPA-induced endometrial carcinoma, which would be helpful for the future carcinogenic risk assessments of TBBPA exposure on uterus.

TBBPA stimulated cell migration of endometrial cancer via the contribution of NOX-generated ROS in lieu of energy metabolism

Due to the extensive applications and deleterious effects of Tetrabromobisphenol A (TBBPA), the health risk and possible mechanisms have been a topic of concern. However, the knowledge on carcinogenic risk of TBBPA and corresponding mechanisms remains scarce. In this study, endometrial cancer cells were exposed to low doses of TBBPA and its main derivatives including TBBPA bis (2,3-dibromopropyl ether) (TBBPA-BDBPE) and TBBPA bis (2-hydroxyethyl ether) (TBBPA-BHEE). The data from wound healing and transwell assays demonstrated that TBBPA treatment exhibited the strongest enhanced effect on cell migration among other tested treatments. Of note, the process of invasion rather than epithelial-mesenchymal transition (EMT) was accompanied by the occurrence of migration elevated by TBBPA. Furthermore, the levels of several metabolite indicators were measured to assess the underlying mechanisms involved in TBBPA-induced cell migration. The findings suggested that NADPH oxidase (NOX)-driven ROS instead of energy metabolism was sensitive to TBBPA stimulation. In addition, molecular docking supported a link between TBBPA ligand and NOX receptor. Accordingly, this study has provided new insights for TBBPA-induced carcinogenic effects and may arise peoples' vigilance to environmental pollution of brominated flame retardant.

Derivation of a no-significant-risk-level for tetrabromobisphenol A based on a threshold non-mutagenic cancer mode of action

A no‐significant‐risk‐level of 20 mg day^–1 was derived for tetrabromobisphenol A (TBBPA). Uterine tumors (adenomas, adenocarcinomas, and malignant mixed Müllerian) observed in female Wistar Han rats from a National Toxicology Program 2‐year cancer bioassay were identified as the critical effect. Studies suggest that TBBPA is acting through a non‐mutagenic mode of action. Thus, the most appropriate approach to derivation of a cancer risk value based on US Environmental Protection Agency guidelines is a threshold approach, akin to a cancer safe dose (RfD_cancer). Using the National Toxicology Program data, we utilized Benchmark dose software to derive a benchmark dose lower limit (BMDL₁₀) as the point of departure (POD) of 103 mg kg^–1 day^–1. The POD was adjusted to a human equivalent dose of 25.6 mg kg^–1 day^–1 using allometric scaling. We applied a composite adjustment factor of 100 to the POD to derive an RfD_cancer of 0.26 mg kg^–1 day^–1. Based on a human body weight of 70 kg, the RfD_cancer was adjusted to a no‐significant‐risk‐level of 20 mg day^–1. This was compared to other available non‐cancer and cancer risk values, and aligns well with our understanding of the underlying biology based on the toxicology data. Overall, the weight of evidence from animal studies indicates that TBBPA has low toxicity and suggests that high doses over long exposure durations are needed to induce uterine tumor formation. Future research needs include a thorough and detailed vetting of the proposed adverse outcome pathway, including further support for key events leading to uterine tumor formation and a quantitative weight of evidence analysis.

A non‐mutagenic threshold mode of action for tetrabromobisphenol A was utilized to derive a no‐significant‐risk‐level of 20 mg day^–1 for uterine tumors (adenomas, adenocarcinomas, and malignant mixed Müllerian combined) observed in female Wistar Han rats from a National Toxicology Program 2‐year bioassay. The most recent available techniques were utilized, including literature review, benchmark dose software, mode of action analysis, and threshold extrapolation. The derived no‐significant‐risk‐level aligns well with other available non‐cancer and cancer risk values based on the biology of these tumors.

Do flame retardant chemicals increase the risk for thyroid dysregulation and cancer?

PURPOSE OF REVIEW

Flame retardant chemicals are added to consumer products to reduce fire incidence and severity; approximately 1.5 million tons of these chemicals are used annually. However, their widespread use has led to their ubiquitous presence in the environment and chronic accumulation in human tissues. We summarize current trends in human flame retardant chemical exposure, and review recent data highlighting concerns for thyroid dysregulation and cancer risk in human populations.

RECENT FINDINGS

Polybrominated diphenyl ethers were once commonly used as flame retardant chemicals, but recently were phased out. Exposure is associated with thyroid dysregulation (mainly T4 reductions) in animals, with new work focusing on specific mechanisms of action. Polybrominated diphenyl ethers also impact human thyroid regulation and are related to clinical thyroid disease, but associations appear both dose and life-stage dependent. Emerging data suggest that common alternate flame retardant chemicals may be more potent thyroid disruptors than their predecessors, which is particularly concerning given increasing levels of exposure.

SUMMARY

Potential health impacts of flame retardant chemicals are only beginning to be understood for 'legacy flame retardant chemicals' (i.e., polybrominated diphenyl ethers), and are largely unevaluated for newer-use chemicals. Cumulatively, current data suggest impact on thyroid regulation is likely, potentially implicating flame retardant chemicals in thyroid disease and cancers for which thyroid dysregulation impacts risk or prognosis.

Mechanochemical conversion of brominated POPs into useful oxybromides: a greener approach

Brominated organic pollutants are considered of great concern for their adverse effect on human health and the environment, so an increasing number of such compounds are being classified as persistent organic pollutants (POPs). Mechanochemical destruction is a promising technology for POPs safe disposal because it can achieve their complete carbonization by solvent-free high energy ball milling at room temperature. However, a large amount of co-milling reagent usually is necessary, so a considerable volume of residue is produced. In the present study a different approach to POPs mechanochemical destruction is proposed. Employing stoichiometric quantities of Bi₂O₃ or La₂O₃ as co-milling reagent, brominated POPs are selectively and completely converted into their corresponding oxybromides (i.e. BiOBr and LaOBr), which possess very peculiar properties and can be used for some actual and many more potential applications. In this way, bromine is beneficially reused in the final product, while POPs carbon skeleton is safely destroyed to amorphous carbon. Moreover, mechanochemical destruction is employed in a greener and more sustainable manner.

A high dose mode of action for tetrabromobisphenol A-induced uterine adenocarcinomas in Wistar Han rats: A critical evaluation of key events in an adverse outcome pathway framework

TBBPA is a non-genotoxic flame retardant used to improve fire safety in a wide variety of consumer products. Estimated human exposures to TBBPA are very low (<0.000084 mg/kg-day), relative to the doses (500 and 1000 mg/kg-day of TBBPA) administered in a recent bioassay that resulted in uterine tumors in Wistar Han rats following chronic exposure. As part of an effort to characterize the relevance of the uterine tumors to humans, data and biological knowledge relevant to the progression of events associated with TBBPA-induced uterine tumors in female rats were organized in an adverse outcome pathway (AOP) framework. Based on a review of possible MOAs for chemically induced uterine tumors and available TBBPA data sets, a plausible molecular initiating event (MIE) was the ability of TBBPA to bind to and inhibit estrogen sulfotransferases, the enzymes responsible for sulfation of estradiol. Subsequent key events in the AOP, including increased bioavailability of unconjugated estrogens in uterine tissue, would occur as a result of decreased sulfation, leading to a disruption in estrogen homeostasis, increased expression of estrogen responsive genes, cell proliferation, and hyperplasia. Available data support subsequent key events, including generation of reactive quinones from the metabolism of estrogens, followed by DNA damage that could contribute to the development of uterine tumors. Uncertainties associated with human relevance are highlighted by potential strain/species sensitivities to development of uterine tumors, as well as the characterization of a dose-dependent MIE. For the latter, it was determined that the TBBPA metabolic profile is altered at high doses (such as those used in the cancer bioassay), and thus an MIE that is only operative under repeated high dose, administration. The MIE and subsequent key events for the development of TBBPA-induced uterine tumors are not feasible in humans given differences in the kinetic and dynamic factors associated with high dose exposures in rats relative to human exposure levels to TBBPA.