SUN-028 Role of Estrogen Receptor Alpha (ERa) Acetylation in Estrogen-Dependent Gene Regulation in Breast Cancers

Estrogens, such as 17β- estradiol (E2), act through estrogen receptor alpha (ERα), a ligand-regulated transcription factor that binds across the genome to promote enhancer formation and regulate gene expression. ERα is expressed in approximately 70% of breast cancers, where it regulates the transcription of genes involved in mitogenic and inflammatory pathways. We are exploring the acetylation of ERα on lysines (K) 266 and 268, modifications that we previously showed enhance the DNA binding and transcriptional activities of ERα in biochemical and cell-based assays. ERα acetylation is catalyzed by the lysine acetyltransferases p300 and CBP in an E2- and steroid receptor coregulator (SRC)-dependent manner. Acetylation-dependent activation of ERα has potential implications in breast cancers associated with enhanced coregulator interactions, such as SRC-3/Amplified in Breast (AIB1) gene amplifications and gain-of-function ERα mutations in endocrine resistant metastatic tumors, such as Y537S and D538G. Notably, the constitutively-activated Y537S mutant exhibits E2-independent K266/268 acetylation and further enhanced acetylation with E2 stimulation when expressed in breast cancer cells. Enhanced ERα acetylation may promote the gain-of-function ERα activity in this context. Our hypothesis is that acetylation of ERα alters its function by increasing E2-responsive gene transcription and signaling in breast cancers. We are using the ER-positive MCF-7 breast cancer cell line with a knockdown/re-expression strategy with biochemical mimics of acetylated (K266/268Q) or unacetylated (K266/268R) ERα. Genomic ERα binding profiles using chromatin immunoprecipitation-sequencing (ChIP-seq) has defined both overlapping and unique sets of transcriptional targets and recruitment kinetics for the ERα mutants. Collectively, our results suggest that the transcriptional activities of the ERα acetylation mutants share a common altered mechanism of recruitment to chromatin with potential implications on enhancer activation. Our current efforts are focused on investigating the role of ERα acetylation on chromatin accessibility, enhancer activity, and target gene transcription.