Pharmacological Inhibition of CBP/p300 Blocks Estrogen Receptor Alpha (ERα) Function through Suppressing Enhancer H3K27 Acetylation in Luminal Breast Cancer

Estrogen receptor alpha (ER) is the oncogenic driver for ER+ breast cancer (BC). ER antagonists are the standard-of-care treatment for ER+ BC; however, primary and acquired resistance to these agents is common. CBP and p300 are critical ER co-activators and their acetyltransferase (KAT) domain and acetyl-lysine binding bromodomain (BD) represent tractable drug targets, but whether CBP/p300 inhibitors can effectively suppress ER signaling remains unclear. We report that the CBP/p300 KAT inhibitor A-485 and the BD inhibitor GNE-049 downregulate ER, attenuate estrogen-induced c-Myc and Cyclin D1 expression, and inhibit growth of ER+ BC cells through inducing senescence. Microarray and RNA-seq analysis demonstrates that A-485 or EP300 (encoding p300) knockdown globally inhibits expression of estrogen-regulated genes, confirming that ER inhibition is an on-target effect of A-485. Using ChIP-seq, we report that A-485 suppresses H3K27 acetylation in the enhancers of ER target genes (including MYC and CCND1) and this correlates with their decreased expression, providing a mechanism underlying how CBP/p300 inhibition downregulates ER gene network. Together, our results provide a preclinical proof-of-concept that CBP/p300 represent promising therapeutic targets in ER+ BC for inhibiting ER signaling.