Distinct roles for BET family members in estrogen receptor a enhancer function and gene regulation in breast cancer cells

The bromodomain family member proteins (BRD; BET proteins) are key coregulators for estrogen receptor alpha (ERa)-mediated transcriptional enhancers. The use of BRDselective inhibitors has gained much attention as a potential treatment for various solid tumors, including ER-positive breast cancers. However, the roles of individual BET family members have largely remained unexplored. Here, wedescribe the role of BRDs in estrogen (E2)-dependent gene expression in ERa-positive breast cancer cells. We observed that chemical inhibition of BET family proteins with JQ1 impairs E2- regulated gene expression and growth in breast cancer cells. In addition, RNAi-mediated depletion of each BET family member (BRDs 2, 3, and 4) revealed partially redundant roles at ERa enhancers and for target gene transcription. Furthermore, we found a unique role of BRD3 as a molecular sensor of total BET family protein levels and activity through compensatory control of its own protein levels. Finally, we observed that BRD3 is recruited to a subset of ERa-binding sites (ERBS) that are enriched for active enhancer features, located in clusters of ERBSs likely functioning as "super enhancers," and associated with highly E2-responsive genes. Collectively, our results illustrate a critical and specific role for BET family members in ERα-dependent gene transcription.