OR04-05 Dose dependent Effects of Estrogen on Genomic Regulation in Breast Cancer Cells

Abstract Disclosure: H. Kim: None. T.S. Nandu: None. W.L. Kraus: None. Estrogens are steroid hormones that play a key role in a wide range of physiological and pathological processes. Notably, estrogens act as the primary mitogen in estrogen receptor positive breast cancers (ER+ BC). The primary mode of action of these hormones is through estrogen receptor-alpha (ERα), which functions primarily as a nuclear transcription factor (TF) through ligand-dependent activation by 17β-estradiol (E2), the major naturally-occurring estrogen. The physiological levels of E2 are regulated by production, sequestration, and clearance, which can limit the amount of bioavailable E2. However, many studies examining the genomic effects of E2-regulated signaling in cell-based assays used supraphysiological levels of hormone. While this has allowed us to understand the extremes of E2 signaling, it has limited our ability to examine cellular functions which operate at lower hormone concentrations. In particular, the genomic actions of ligand-bound ERα through transcriptional enhancer formation and function are not understood at physiological E2 doses. Here, we present studies examining two largely overlooked aspects of E2 signaling: (1) genomic actions at physiological concentrations of E2 and (2) cellular accumulation and retention of E2. Using an integrated genomic approach with RNA-seq, PRO-seq, ChIP-seq, and ATAC-seq, we identified gene sets that express preferentially at either picomolar or nanomolar concentrations of E2. Expression of these genes is regulated mainly at the transcriptional level and is associated with distinct enhancer features, including chromatin accessibility and H3K27ac enrichment, that may play a role in determining hormone responsiveness. These have implications of functional differences between response at low and high doses of E2. Furthermore, we found that ERα positive breast cancer (ER+ BC) cells accumulate E2 from the culture medium and retain the hormone for an extended duration. Accumulated intracellular E2 supports sustained ERα chromatin binding and E2-dependent gene expression, as well as cell proliferation. We are exploring the link between intracellular steroid-binding proteins and E2 cellular retention. Taken together, our results are expanding the role of E2 dose in hormone signaling in ER+ BC. Presentation: Saturday, July 12, 2025