Opposing Functions of BRD4 Isoforms in Breast Cancer

Shwu-Yuan Wu, Chien-Fei Lee, Hsien-Tsung Lai, Cheng-Tai Yu, Ji-Eun Lee, Hao Zuo, Sophia Y. Tsai, Ming-Jer Tsai, Kai Ge, Yihong Wan, Cheng-Ming Chiang
Molecular Cell2020
Bromodomain-containing protein 4 (BRD4) is a cancer therapeutic target in ongoing clinical trials disrupting primarily BRD4-regulated transcription programs. The role of BRD4 in cancer has been attributed mainly to the abundant long isoform (BRD4-L). Here we show, by isoform-specific knockdown and endogenous pro- tein detection, along with transgene expression, the less abundant BRD4 short isoform (BRD4-S) is onco- genic while BRD4-L is tumor-suppressive in breast cancer cell proliferation and migration, as well as mam- mary tumor formation and metastasis. Through integrated RNA-seq, genome-wide ChIP-seq, and CUT&RUN association profiling, we identify the Engrailed-1 (EN1) homeobox transcription factor as a key BRD4-S cor- egulator, particularly in triple-negative breast cancer. BRD4-S and EN1 comodulate the extracellular matrix (ECM)-associated matrisome network, including type II cystatin gene cluster, mucin 5, and cathepsin loci, via enhancer regulation of cancer-associated genes and pathways. Our work highlights the importance of tar- geted therapies for the oncogenic, but not tumor-suppressive, activity of BRD4.
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