MIBE acts as antagonist ligand of both estrogen receptor alpha and GPER in breast cancer cells

Introduction: The multiple biological responses to estrogens are mainly mediated by the classical estrogen receptors ER alpha and ER beta, which act as ligand-activated transcription factors. ERa exerts a main role in the development of breast cancer; therefore, the ER antagonist tamoxifen has been widely used although its effectiveness is limited by de novo and acquired resistance. Recently, GPR30/GPER, a member of the seven-transmembrane G protein-coupled receptor family, has been implicated in mediating the effects of estrogens in various normal and cancer cells. In particular, GPER triggered gene expression and proliferative responses induced by estrogens and even ER antagonists in hormone-sensitive tumor cells. Likewise, additional ER ligands showed the ability to bind to GPER eliciting promiscuous and, in some cases, opposite actions through the two receptors. We synthesized a novel compound (ethyl 3-[5-(2-ethoxycarbonyl-1-methylvinyloxy)-1-methyl-1H-indol-3-yl] but-2-enoate), referred to as MIBE, and investigated its properties elicited through ER alpha and GPER in breast cancer cells. Methods: Molecular modeling, binding experiments and functional assays were performed in order to evaluate the biological action exerted by MIBE through ER alpha and GPER in MCF7 and SkBr3 breast cancer cells. Results: MIBE displayed the ability to act as an antagonist ligand for ER alpha and GPER as it elicited inhibitory effects on gene transcription and growth effects by binding to both receptors in breast cancer cells. Moreover, GPER was required for epidermal growth factor receptor (EGFR) and ERK activation by EGF as ascertained by using MIBE and performing gene silencing experiments. Conclusions: Our findings provide novel insights on the functional cross-talk between GPER and EGFR signaling. Furthermore, the exclusive antagonistic activity exerted by MIBE on ER alpha and GPER could represent an innovative pharmacological approach targeting breast carcinomas which express one or both receptors at the beginning and/or during tumor progression. Hence, the simultaneous inhibition of both ER alpha and GPER may guarantee major therapeutic benefits in respect to the use of a selective estrogen receptor antagonist.