Bisphenol A stabilizes Nrf2 via Ca influx by direct activation of the IP receptor.

Bisphenol A (BPA) is an endocrine-disrupting chemical used in polycarbonate and epoxy resins. Previously, we found that BPA stabilized the protein levels of nuclear factor erythroid 2-related factor 2 (Nrf2) by inducing Ca efflux into the cytosol, followed by nitric oxide synthase activation, resulting in the enhanced nitrosylation of Keap1, which is a negative regulator of Nrf2. However, the mechanisms behind the stimulation of Ca efflux by BPA remain unknown. In the present study, we found that BPA stimulated Ca efflux into the cytosol from the ER, but not from outside of cells through the plasma membrane in Hep3B cells. Ca efflux and Nrf2 stabilization by BPA were inhibited by an inhibitor of the inositol 1,4,5-trisphosphate (IP) receptor, 2-aminoethoxydiphenylborane, in the endoplasmic reticulum. IP is produced by activation of phospholipase C (PLC) from a membrane lipid, phosphatidylinositol 4,5-bisphosphate (PIP). The induction of Nrf2 by BPA was not inhibited by a PLC inhibitor, U-73122, suggesting that BPA does not induce the production of IP via PLC activation. We found that BPA bound directly to the IP binding core domain of the IP receptor, and BPA competed with IP on this site. In addition, overexpression of this domain of the IP receptor in Hep3B cells inhibited the stabilization of Nrf2 by BPA. These results clarified that the IP receptor is a new target of BPA, and that BPA induces Ca efflux from the endoplasmic reticulum via activation of the IP receptor.