Extrapolation of plasma clearance to understand species differences in toxicokinetics of bisphenol A

Re-evaluation of the risks to public health related to the presence of bisphenol A (BPA) in foodstuffs

In 2015, EFSA established a temporary tolerable daily intake (t-TDI) for BPA of 4 μg/kg body weight (bw) per day. In 2016, the European Commission mandated EFSA to re-evaluate the risks to public health from the presence of BPA in foodstuffs and to establish a tolerable daily intake (TDI). For this re-evaluation, a pre-established protocol was used that had undergone public consultation. The CEP Panel concluded that it is Unlikely to Very Unlikely that BPA presents a genotoxic hazard through a direct mechanism. Taking into consideration the evidence from animal data and support from human observational studies, the immune system was identified as most sensitive to BPA exposure. An effect on Th17 cells in mice was identified as the critical effect; these cells are pivotal in cellular immune mechanisms and involved in the development of inflammatory conditions, including autoimmunity and lung inflammation. A reference point (RP) of 8.2 ng/kg bw per day, expressed as human equivalent dose, was identified for the critical effect. Uncertainty analysis assessed a probability of 57-73% that the lowest estimated Benchmark Dose (BMD) for other health effects was below the RP based on Th17 cells. In view of this, the CEP Panel judged that an additional uncertainty factor (UF) of 2 was needed for establishing the TDI. Applying an overall UF of 50 to the RP, a TDI of 0.2 ng BPA/kg bw per day was established. Comparison of this TDI with the dietary exposure estimates from the 2015 EFSA opinion showed that both the mean and the 95th percentile dietary exposures in all age groups exceeded the TDI by two to three orders of magnitude. Even considering the uncertainty in the exposure assessment, the exceedance being so large, the CEP Panel concluded that there is a health concern from dietary BPA exposure.

Last piece in the puzzle of bisphenols BPA, BPS and BPF metabolism: Kinetics of the in vitro sulfation reaction

Bisphenols are endocrine-disrupting chemicals ubiquitously present in the environment, with the consequent exposure to humans. In humans, bisphenols are metabolized to glucuronide and sulfate conjugates. Recent studies indicate that sulfation represents an important bisphenol metabolic pathway for the most vulnerable humans, such as the growing fetus. Our aim was to evaluate sulfation kinetics of commonly detected bisphenols in biological samples: bisphenol A (BPA), bisphenol S (BPS), and bisphenol F (BPF). Furthermore, we evaluated estrogenic agonist potencies and long-term stability of these bisphenol sulfates. BPS and BPF sulfates were prepared by chemical synthesis. Sulfation kinetics of the selected bisphenols were tested in pooled human liver cytosol, as a source for soluble phase II enzymes, including liver sulfotransferases, with quantification by LC-MS/MS. A validated transactivation assay using the hERα-Hela 9903 cell line was used to determine estrogenic agonist potencies. Moreover, BPA, BPS, and BPF sulfate stabilities were examined under various conditions and during storage. In vitro sulfation of BPA and BPS followed Michaelis-Menten kinetics; BPF sulfation followed a substrate inhibition model. Sulfation rates were comparable for these bisphenols, although their K_M values indicated some large differences in affinities. BPA and BPS sulfates are not hERα agonists. The bisphenol sulfates can be considered stable for at least 2 days under these tested media conditions. These data indicate that bisphenol sulfation is an important route in their biotransformation. Compared to glucuronidation, these bisphenols show slower sulfation rates but similar K_M values. BPA and BPS metabolic biotransformation by sulfation provides their detoxification as they are without estrogenic activities.

In vivo and in vitro transcriptomics meta-analyses reveal that BPA may affect TGF-beta signaling regardless of the toxicology system employed

Bisphenol A (BPA) is a high-production-volume monomer for the manufacture of a wide variety of polycarbonate plastics and resins. Evidence suggests BPA can induce carcinogenesis, reproductive toxicity, abnormal inflammatory or immune response, and developmental disorders of the brain or nervous system. However, whether BPA affects the very same basic molecular processes in all the in vivo and in vitro systems employed to exert its molecular mechanisms of toxicity remains to be clarified. In this study, we collected multi-source global transcriptomics datasets for BPA-exposed organisms and cells, and evaluated the adverse effects of BPA by using data integration and gene functional enrichment analyses. We found that BPA may affect basic cellular processes, such as cell growth, survival, proliferation, differentiation, and apoptosis, independent of species and specific in vivo or in vitro systems. Mechanistically, BPA could regulate cell-extra cellular matrix interactions via challenging TGF-beta signaling pathways. Furthermore, we compared our in vitro BPA-dependent mouse embryoid body (EB) global differentiation transcriptomics with all the other datasets. We verified the EB-based toxicological system could recapitulate several in vivo and other in vitro findings very efficiently, and in a less time- and resource-consuming fashion. Taken together, this study emphasizes the utility of meta-analyses to understand common molecular mechanisms of toxicity of synthetic chemicals.

Pregnancy-specific physiologically-based toxicokinetic models for bisphenol A and bisphenol S

Predictions from physiologically based toxicokinetic (PBTK) models can help inform human health risk assessment for potentially toxic chemicals in the environment. Bisphenol S (BPS) is the second most abundant bisphenol detected in humans in the United States, after bisphenol A (BPA). We have recently demonstrated that BPS, much like BPA, can cross the placental barrier and disrupt placental function. Differences in physicochemical properties, toxicokinetics, and exposure outcomes between BPA and other bisphenols prevent direct extrapolation of existing BPA PBTK models to BPS. The current study aimed to develop pregnancy-specific PBTK (p-PBTK) models for BPA and BPS, using a common p-PBTK model structure. Novel paired maternal and fetal pregnancy data sets for total, unconjugated, and conjugated BPA and BPS plasma concentrations from three independent studies in pregnant sheep were used for model calibration. The nine-compartment (maternal blood, liver, kidney, fat, placenta and rest of body, and fetal liver, blood and rest of body) models simulated maternal and fetal experimental data for both BPA and BPS within one standard deviation for the majority of the experimental data points, highlighting the robustness of both models. Simulations were run to examine fetal exposure following daily maternal exposure to BPA or BPS at their tolerable daily intake dose over a two-week period. These predictive simulations show fetal accumulation of both bisphenols over time. Interestingly, the steady-state approximation following this dosing strategy achieved a fetal concentration of unconjugated BPA to levels observed in cord blood from human biomonitoring studies. These models advance our understanding of bisphenolic compound toxicokinetics during pregnancy and may be used as a quantitative comparison tool in future p-PBTK models for related chemicals.