Abstract 1418: Mapping the FOXA1 interactome in ER+ breast cancer cells using proximity labeling reveals a novel interaction with NR2C2

Abstract Approximately 70% of breast cancers are driven by the estrogen receptor alpha (ER), and despite the advent of endocrine therapy to block ER signaling pathways, a significant proportion of women develop resistance to these drugs. The pioneer factor FOXA1 has been shown to facilitate nearly all DNA-binding events of ER in response to estrogen in ER-positive breast cancer (ER+BC). Notably, up-regulation of FOXA1 is a hallmark of endocrine-resistant tumors. Furthermore, FOXA1 has been shown to alter the transcriptome in endocrine therapy resistance. FOXA1 interacting partners, such as ER, androgen receptor (AR), MLL3, YAP1 and GATA-3 mediate transcription activation of genes important for cancer progression. There have been previous studies showing interactors of FOXA1, but none have addressed a comprehensive list of interactors of FOXA1, especially in breast cancer. To address this challenge, we generated MCF-7 cell lines stably expressing the biotin ligase miniTurbo fused to the N- or C-terminus of FOXA1 to classify interactors of FOXA1 in ER+BC. Using proximity labeling (PL) coupled with LC-MS/MS, we aimed to comprehensively catalog interactors of FOXA1. We were able to identify expected proteins such as ER, AR, MLL3, YAP1, and GATA-3. Moreover, we have discovered 157 previously unpublished interactors of FOXA1. Importantly, poorer prognosis for relapse-free survival among ER+BC patients is associated with several of these novel interactors, including NR2C2, an orphan nuclear receptor previously shown to regulate various nuclear receptor signaling but whose roles in breast cancer remain unclear. NR2C2 ChIP-seq in MCF-7 cells under FOXA1-depleted conditions revealed NR2C2 binding loci dependent on FOXA1 levels and binding. By integrating proteomic and genomic approaches, we highlight new mechanisms of FOXA1-mediated transcriptional regulation, with promising potential for endocrine therapy resistance. Citation Format: Carla Sofia Rodriguez-Tirado, Rosemary Plagens, Shen Li, Christine Mills, Laure Herring, Hector Franco. Mapping the FOXA1 interactome in ER+ breast cancer cells using proximity labeling reveals a novel interaction with NR2C2. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1418.