ORF11-04 - Ligand Specific Estrogen Receptor Conformational Dynamics

Estrogen receptor alpha is overexpressed in 70% of breast cancers. Hormone binding to ERα ligand binding domain (LBD) promotes nuclear translocation, coregulator recruitment, and subsequent activation of proliferative ERα-dependent transcriptional programs. Selective Estrogen Receptor Modulators (SERMs) or Degraders (SERDs) are competitive allosteric regulators that repopulate functional ERα complexes to oppose estrogenic actions within breast cancer cells. SERDs achieve complete estrogen receptor antagonism by also inducing receptor degradation. Despite the functional differences between SERMs and SERDs, static x-ray co-crystal structures of ERα LBD complexes are nearly superimposable, failing to accurately describe the allosteric networks that drive pharmacologic responses. In this study, we Hydrogen Deuterium Exchange Mass Spectroscopy (HDX-MS) on ERα LBD in complex with structurally distinct therapeutic ligands to reveal the ligand-specific dynamic differences that differentiate SERM versus SERD function. Our studies show that different ligands, even within the same drug-class, impact unique repertoire of ERα LBD structural elements, previously undescribed by existing x-ray co-crystal structures. As transcriptional co-regulators bind across the LBD surface, these dynamic differences predict alterations to the functional complexes that would drive pharmacologic response. Unexpectedly, dramatic differences were measured at the LBD Helix 1 (H1), which is distant from the binding pocket. Mutational analyses were undertaken to understand the role of H1 dynamics in SERM and SERD actions. Molecular dynamics simulations were used to nominate mutations that would affect H1 mobility. NanoBiT assay was used to test how these mutations affected the ability of SERMs and SERDs to inhibit ERα-SRC3 complex formation in situ. Together, our studies show that SERMs and SERDs reshape the surface topology of ERα LBD to elicit anti-cancer activities.