Abstract PS3-05-15: Targeting Breast Cancer via Dual Antagonism of Estrogen Receptor α and GPER

Abstract Background Estrogen’s diverse biological effects are primarily driven by the classical receptors ERα and ERβ,which function as ligand-activated transcription factors. Among these, ERα plays a pivotal role inbreast cancer development, making it a key target for therapies such as tamoxifen. However,tamoxifen’s clinical success is often compromised due to both inherent and acquired resistance.Notably, tamoxifen inhibits ERα but paradoxically acts as an agonist for GPER/GPR30, estrogenreceptor that mediates non-classical estrogen signaling in normal and cancerous cells. Activationof GPER can promote gene expression and cell proliferation, contributing to tamoxifen resistancein hormone-sensitive tumors. Our study addresses this challenge by focusing on a dual-targetingstrategy that simultaneously inhibits both ERα and GPER, aiming to overcome resistance andimprove therapeutic outcomes in breast cancer treatment. Methods GPER expression was evaluated in breast cancer cell lines and tumor tissues using immunoblotting,qRT-PCR, and immunohistochemistry. Cell proliferation was assessed via MTT and colonyformation assays. Migration and invasion capabilities were measured using wound healing andtranswell invasion assays. Intracellular calcium signaling was monitored using fluorescence-basedcalcium imaging following GPER stimulation. Metagenomic analysis of Hispanic breast cancertissues were performed using 16S rRNA sequencing to identify microbial populations correlatedwith GPER expression and tumor subtypes. Statistical analyses included correlation tests andsubtype stratification. Results GPER expression was detected in both breast cancer cell lines and tumor tissues. Our resultsdemonstrate that GPER is significantly overexpressed in ERα-positive breast tumors, with aunique correlation observed in a Hispanic breast cancer patient population (P 0.001). GPERexpression was also observed in Triple-Negative subtypes. The correlation of GPER expressionwas analyzed across different breast cancer subtypes, classified as Luminal A (HR+/HER2−),Luminal B (HR+/HER2+), HER2-Enriched (HR−/HER2+), and Triple-Negative (HR−/HER2−).In vitro studies confirmed the presence of GPER protein and RNA in breast cancer cells of varioussubtypes. Furthermore, this study identifies Ormeloxifene (ORM), a selective estrogen receptormodulator (SERM), as a dual antagonist of ERα and GPER. GPER expression was associated withincreased cancer aggressiveness, migration, and invasion, all of which were effectively inhibitedby Ormeloxifene. ORM acts as an antagonist ligand for both ERα and GPER by suppressing genetranscription and inhibiting growth signaling pathways in breast cancer cells. Mechanistically,Ormeloxifene suppresses GPER-induced calcium signaling and downstream activation of EGFR,ERK, YAP, and TAZ pathways, thereby inhibiting gene transcription and tumor growth. Additionally, metagenomic analysis of Hispanic breast cancer tissues revealed specific microbialpopulations associated with GPER expression and tumor subtypes. Conclusions GPER is significantly overexpressed in ERα-positive and some Triple-Negative breast cancers,especially within the Hispanic population. The exclusive antagonistic activity exerted byOrmeloxifene on both ERα and GPER represents an innovative pharmacological approach fortargeting breast carcinomas expressing one or both receptors at diagnosis or during progression.Simultaneous inhibition of ERα and GPER may offer greater therapeutic benefits compared toselective estrogen receptor antagonists alone. Additionally, the association of GPER expressionwith specific microbial populations suggests new directions for personalized breast cancertreatment. Citation Format: S. SINGH, S. Vidaurri, M. Noorani, S. Goyal, A. Dhasmana, S. Dhasmana, M. Yallapu, S. Fofana, S. C. Chauhan, D. Nguyen, S. Khan. Targeting Breast Cancer via Dual Antagonism of Estrogen Receptor α and GPER abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-05-15.