Super-enhancer-controlled positive feedback loop BRD4/ER alpha-RET-ER alpha promotes ER alpha-positive breast cancer

Estrogen and estrogen receptor alpha (ER alpha)-induced gene transcription is tightly associated with ER alpha-positive breast carcinogenesis. ER alpha-occupied enhancers, particularly super-enhancers, have been suggested to play a vital role in regulating such transcriptional events. However, the landscape of ER alpha-occupied super-enhancers (ERSEs) as well as key ER alpha-induced target genes associated with ERSEs remain to be fully characterized. Here, we defined the landscape of ERSEs in ER alpha-positive breast cancer cell lines, and demonstrated that bromodomain protein BRD4 is a master regulator of the transcriptional activation of ERSEs and cognate ER alpha target genes. RET, a member of the tyrosine protein kinase family of proteins, was identified to be a key ER alpha target gene of BRD4-regulated ERSEs, which, in turn, is vital for ER alpha-induced gene transcriptional activation and malignant phenotypes through activating the RAS/RAF/MEK2/ERK/p90RSK/ER alpha phosphorylation cascade. Combination therapy with BRD4 and RET inhibitors exhibited additive effects on suppressing ER alpha-positive breast cancer both in vitro and in vivo, comparable with that of standard endocrine therapy tamoxifen. Furthermore, combination therapy re-sensitized a tamoxifen-resistant ER alpha-positive breast cancer cell line to tamoxifen treatment. Taken together, our data uncovered the critical role of a super-enhancer-associated positive feedback loop constituting BRD4/ER alpha-RET-ER alpha in ER alpha-positive breast cancer, and suggested that targeting components in this loop would provide a new therapeutic avenue for treating ER alpha-positive breast cancer in the clinic.