Bisphenol A, and three related alkylphenols exert rapid estrogenic actions on zebrafish (Danio rerio) oocytes to maintain meiotic arrest
Bisphenol A (BPA) is an alkylphenol compound used in plastic manufacturing, which enters the aquatic environment through wastewater treatment plants and landfill leachates. Exposure of fish to BPA results in developmental defects, decreased hatching, and increased occurrence of the egg yolk precursor protein vitellogenin in male fish. BPA is an estrogenic like compound (xenoestrogen) that can bind and activate the nuclear estrogen receptors, ERα and ERβ, causing changes in gene transcription (genomic mechanism). However, the ability of BPA to activate rapid estrogen signaling and to disrupt nongenomic physiological responses to estrogens is not known. One important nongenomic estrogen action in fish is to maintain meiotic arrest of oocytes and to prevent precocious oocyte maturation through activation of the transmembrane G-protein coupled estrogen receptor (Gper). Binding of estrogens to Gper on zebrafish oocytes results in rapid activation of epidermal growth factor receptor (Egfr) signaling and subsequent Mapk3/1 activation. We show here that BPA and three structurally-related chemicals, tetrachlorobisphenol A, tetrabromobisphenol A and nonylphenol, can mimic estrogen by activating this mechanism of meiotic arrest through Gper in zebrafish (Danio rerio). BPA bound to zebrafish Gper and inhibited spontaneous oocyte maturation (OM) of denuded oocytes. Treatment of oocytes with Actinomycin D did not block the effects of BPA, suggesting that this inhibition of OM is through a nongenomic mechanism. Incubation of oocytes with a selective GPER antibody and the specific GPER antagonist G-15 blocked the effects of BPA on OM, further suggesting that BPA inhibition of OM is through its interaction with the receptor. Various inhibitors of the EGFR pathway were utilized to determine if the inhibition of OM by BPA is mediated through this mechanism. BPA activation of the Egfr pathway resulted in Mapk3/1 (also known as Erk 1/2) phosphorylation. The results show that BPA disrupts oocyte maturation through a novel mechanism involving activation of a Gper/Egfr/Mapk3/1 pathway with potential adverse impacts on reproductive success.