Anti-estrogenic activity of tris(2,3-dibromopropyl) isocyanurate through disruption of co-activator recruitment: experimental and computational studies

As a potential endocrine disruptor, tris(2,3-dibromopropyl) isocyanurate (TBC) has previously been demonstrated to reduce expression of estrogen-dependent vitellogenin (vtg) mRNA in adult zebrafish. However, the underlying toxicity pathways and molecular mechanisms involved in TBC-induced endocrine disruption remain elusive. In the current study, E-Screen and MVLN assays were employed to explore the potential anti-estrogenic effects of TBC via the estrogen receptor alpha (ER alpha)-mediated signaling pathway. Within a dose range between 1 x 10(-9) and 1 x 10(-7) M, TBC significantly inhibited 17 beta-estradiol (E-2)-induced cell proliferation in a breast cancer cell line. The luciferase activity induced by E-2 was also significantly inhibited by TBC in a dose-dependent manner. Moreover, neither TBC nor E-2 affected proliferation of the ER alpha-negative breast cancer cell line MDA-MB-231. These experimental results confirmed that TBC has anti-estrogenic effects by affecting the ER alpha-mediated signaling pathway. By comparing TBC with known antagonists of ER alpha, we found that TBC has similar molecular structure as certain co-activator binding inhibitors. Therefore, using molecular docking and molecular dynamics simulations, TBC was further predicted to competitively occupy the surface site of ER alpha rather than the canonical E-2-binding pocket of ER alpha, thus disrupt subsequent co-activator recruitment and transcription activation. Our findings elucidate the anti-estrogenic mechanism of TBC at the atomic level and highlight the biological importance of surface sites of nuclear receptors for a risk assessment of potential environmental pollutants.