Thyroid hormone disruption by Bisphenol A structural analogues and associated mechanisms

Abstract

Bisphenol A (BPA) is used in various commercial products such as food containers, toys and thermographic and pressure-sensitive receipt papers. Due to growing concerns about adverse health effects associated with BPA, several bisphenols, such as bisphenol F (BPF) and bisphenol S (BPS), etc., have been used in increasing amount as alternatives for BPA. Accordingly, these alternative bisphenols have recently been detected in human biological samples and in various environmental media. Endocrine disruption potentials of these bisphenols are of growing concern, but there are only limited information available on their toxicities. Thyroid disruption effects of these chemicals have rarely been investigated. Thyroid hormones (THs) play a crucial role in many physiological processes, including development, growth, metabolism, and energy balance. THs are primarily regulated by negative feedback of circulating THs via the hypothalamus-pituitary-thyroid (H-P-T) axis. THs go through several steps, i.e., TH synthesis, transport, binding, action and metabolism. Due to their importance, normal regulation of THs is important and alteration of TH regulation has significant toxicological implication. Therefore, it is important to evaluate the TH disrupting potential of BPA and its structural analogues and to understand their underlying mechanisms. For this purpose, a series of studies was carried out. In the first study, aiming to screen the TH disrupting potential of BPA and its commercially available structural analogues (BPAF, BPAP, BPB, BPC, BPF, BPM, BPP, BPS and BPZ) and to explore their possible underlying mechanisms, rat pituitary (GH3) cell and rat thyroid follicular (FRTL-5) cell-direct effects were evaluated. Significant changes in gene transcription (tshβ, trα, trβ, dio1 and dio2) in GH3 cells were observed after exposure to most of the tested bisphenols. In the FRTL-5 cells, both BPA and BPS significantly elevated transcription of nis, pax8, nkx2.1, tg and tpo. Significant gene transcription changes indicate that tested bisphenols have the potency to disrupt TH signaling or homeostasis by acting directly on the cells. The patterns of gene transcription in GH3 cells were generally similar among bisphenols, which implies common mode of action among them in the pituitary cells. The transcription of TH synthesis genes were induced by direct exposure to BPA and BPS in FRTL-5 cells. Our findings indicate that bisphenols could disrupt TH regulation by direct effects on pituitary and thyroid cells. Further investigation for TH disruption using in vivo models is needed. In the second study, the effects of BPA and three major structural analogues (BPF, BPS, and BPZ) on thyroid endocrine system in embryo stage were evaluated using a zebrafish embryo/larvae model. Zebrafish embryos (<4 hr after fertilization) were exposed to BPA, BPF, BPS, or BPZ until 120 h post fertilization (hpf). Then, the whole-body levels of THs (T3, T4) and the transcription of genes related to TH endocrine system (crh, tshβ, tshr, nkx2.1, pax8, tpo, tg, hhex, trα, trβ, ttr, dio1, dio2, and ugt1ab) were analyzed. In addition, effects on embryo/larval survival, hatching, and development were observed. Significant increases in T3 and/or T4 were observed after exposure to BPA, BPF and BPS. However, BPZ did not cause statistical significant changes in hormonal level of T3 and T4. In addition, transcription of genes related to thyroid synthesis (hhex and tg), TH transport (ttr) and TH metabolism (ugt1ab) were elevated by the exposure to most tested bisphenols. Hatching delay was observed from all the tested bisphenols. Taken together, BPA and its structural analogues generally stimulate TH synthesis, which may be associated with observed developmental delay. Our findings demonstrate that BPA and its structural analogues might increase TH synthesis and pose a threat to thyroid endocrine homeostasis in the early life stage in zebrafish. In the third study, TH disrupting potencies of BPA and BPS, a primary alternative, at adult stage were evaluated using zebrafish model. Male and female adult zebrafish were exposed to BPA and BPS for 14 d. Then, the transcription of genes related to TH system in head and liver tissues and plasma levels of THs were analyzed. Both BPA and BPS generally increased the transcription of genes related to TH synthesis (crh, tshb, tshr, or nkx2.1), transport (ttr) and metabolism (dio1, dio2, or ugt1ab) in the head and the liver though only some of changes were statistically significant. On the other hand, T3 and T4 levels were not altered by exposure to BPA, however T3 levels were sex-dependently affected by exposure to BPS. The transcription changes toward activating TH synthesis were also observed in adult stage zebrafish which indicating TH disrupting effects of BPA and BPS. However, the mechanism for this discrepant response between BPA and BPS on T3 levels was not clearly elucidated. Taken all the studies together, BPA and its structural analogues might have the potential to cause disruption of TH homeostasis. The possible mechanism might be the TH synthesis activation which could be explained by the influence of bisphenols on the binding between TH receptor (TR) and its co-regulators and by direct effects of bisphenols on thyroid cells. Overall, the results of this series of studies demonstrate the TH disrupting effects of BPA and its structural analogues and their possible mechanisms. Hence, this study contributes to the current understanding of potential health implications of BPA alternatives and can be employed to screen safer alternatives. However, this studies did not consider environmental concentration and low-dose effect. Therefore, TH disrupting effects at environmental relevant concentrations and low-dose TH disrupting effect deserve further investigations.