PT - JOURNAL ARTICLE AU - Masayo Hirao-Suzuki AU - Shuso Takeda AU - Katsuhiro Okuda AU - Masufumi Takiguchi AU - Shin’ichi Yoshihara TI - Repeated Exposure to 4-Methyl-2,4-bis(4-hydroxyphenyl)pent-1-ene (MBP), an Active Metabolite of Bisphenol A, Aggressively Stimulates Breast Cancer Cell Growth in an Estrogen Receptor <em>β</em> (ER<em>β</em>)–Dependent Manner AID - 10.1124/mol.118.114124 DP - 2019 Mar 01 TA - Molecular Pharmacology PG - 260--268 VI - 95 IP - 3 4099 - http://molpharm.aspetjournals.org/content/95/3/260.short 4100 - http://molpharm.aspetjournals.org/content/95/3/260.full SO - Mol Pharmacol2019 Mar 01; 95 AB - Bisphenol A (BPA), recognized as an endocrine disruptor, is thought to exert its activity through a mechanism involving the activation of estrogen receptors (ERs) α/β. However, a major problem is that very high concentrations of BPA are required (i.e., those in excess of environmental levels) for effective activation of ERα/β-mediated transcriptional activities in vitro, despite the BPA-induced estrogenic effects observed in vivo. To elucidate the causal reasons, we successfully identified a BPA metabolite, 4-methyl-2,4-bis(4-hydroxyphenyl)pent-1-ene (MBP), which exhibits highly potent estrogenic activity both in vivo and in vitro. We have focused on the biologic relationship between breast tumor promotion and MBP/BPA, because BPA is considered to be a human carcinogen owing to its breast tumor–promoting properties. In general, humans are exposed to many endocrine disruptors, including BPA. In the present study, we used the ERα/β-positive human breast cancer cell line MCF-7 as an experimental model to investigate the effects of repeated exposure to BPA/MBP at concentrations found in the environment on the expression of ERα/β and to determine the particular ER subtype involved. We demonstrated that repeated exposure to MBP, but not to BPA, significantly downregulated ERα protein expression and stimulated the proliferation of MCF-7 cells through the activation of ERβ-mediated signaling.