Inhibition of BET proteins impairs estrogen-mediated growth and transcription in breast cancers by pausing RNA polymerase advancement

Estrogen (E₂)-induced transcription requires coordinated recruitment of estrogen receptor α (ER) and multiple factors at the promoter of activated genes. However, the precise mechanism by which this complex stimulates the RNA polymerase II activity required to execute transcription is largely unresolved. We investigated the role of bromodomain (BRD) containing bromodomain and extra-terminal (BET) proteins, in E₂-induced growth and gene activation. JQ1, a specific BET protein inhibitor, was used to block BET protein function in two different ER-positive breast cancer cell lines (MCF7 and T47D). Real-time PCR and ChIP assays were used to measure RNA expression and to detect recruitment of various factors on the genes, respectively. Protein levels were measured by Western blotting. JQ1 suppressed E₂-induced growth and transcription in both MCF7 and T47D cells. The combination of E₂ and JQ1 down-regulated the levels of ER protein in MCF7 cells but the loss of ER was not responsible for JQ1-mediated inhibition of E₂ signaling. JQ1 did not disrupt E₂-induced recruitment of ER and co-activator (SRC3) at the E₂-responsive DNA elements. The E₂-induced increase in histone acetylation was also not altered by JQ1. However, JQ1 blocked the E₂-induced transition of RNA polymerase II from initiation to elongation by stalling it at the promoter region of the responsive genes upstream of the transcription start site. This study establishes BET proteins as the key mediators of E₂-induced transcriptional activation. This adds another layer of complexity to the regulation of estrogen-induced gene activation that can potentially be targeted for therapeutic intervention.