MON-264 - Estrogen and Androgen Receptor Crosstalk at Cancer Risk Loci Regulates Hormone-Dependent IGFBP5 Expression in ER-Positive Breast Cancer

The estrogen receptor (ER) is the main driver of ER⁺ breast cancer (BCa), yet the majority of ER⁺ tumors also express the androgen receptor (AR). Clinical evidence shows that AR expression is associated with improved recurrence-free survival in ER⁺ BCa, suggesting a tumor-suppressive role for AR signaling. ER and AR regulate transcription of hormone responsive genes by binding to estrogen response elements (ERE) and androgen response elements (ARE) located in cis-regulatory elements of estrogen and androgen responsive genes, respectively. Mechanistically, ligand-activated AR antagonizes ER-driven transcription via competitive binding at EREs, sequestration of shared coactivators, or induction of AR-regulated tumor-suppressive genes. However, how AR-mediated suppression of ER activity affects the transcriptional landscape remains poorly understood. In this study, we investigated ER-AR crosstalk by integrating publicly available RNA-seq and ChIP-seq datasets from the ER⁺/AR⁺ BCa cell line ZR-75-1 treated with estrogen (E2), dihydrotestosterone (DHT), or E2+DHT. We identified 653 intergenic regions initially bound by ER after E2 treatment, which become sites of AR binding upon co-treatment with E2 and DHT. Around 93% of these regions are located distal (>2 kb) to the transcription start site, suggesting long-range regulatory mechanisms. We then integrated the identified intergenic regions to publicly available Capture Hi-C data of ZR-75-1 that used ER+ BCa risk loci as bait. Two distal putative ER/AR-bound regulatory elements with high contact frequencies with the insulin-like growth factor binding protein 5 (IGFBP5) promoter were identified, with the expression of IGFBP5 upregulated upon E2 treatment and attenuated upon E2+DHT treatment. IGFBP5 canonically modulates insulin-like growth factor (IGF) signaling by limiting the interaction of IGFs with the IGF-1 receptor. However, in ER⁺ BCa, elevated IGFBP5 expression has been associated with poorer disease-free survival and has been shown to promote cell adhesion and cell survival in MCF7 cells through IGF-independent mechanisms. To determine the effect of DHT on IGFBP5 expression in another ER+ BCa cell model, we generated AR-expressing MCF7 cells via lentiviral transduction and compared IGFBP5 expression relative to empty-vector-transduced control upon hormone treatment. Interestingly, DHT reduced IGFBP5 transcript levels in MCF7-AR cells relative to VEH and E2, with E2+DHT partially restoring this expression. This data hints at a mechanism by which AR signaling modulates ER transcriptional output through distal regulatory elements, particularly at BCa risk loci, leading to changes in estrogen responsiveness of genes such as IGFBP5.