BET Proteins Exhibit Transcriptional and Functional Opposition in the Epithelial-to-mesenchymal Transition

Abstract: Transcriptional programs in embryogenesis and cancer, such as the epithelial-to-mesenchymal transition (EMT), ensure cellular plasticity, an essential feature of carcinoma progression. As effectors of signal transduction, the bromodomain and extraterminal (BET) proteins are well suited to support plasticity because they function as co-activators or co-repressors of mammalian transcriptomes. Here, using both hormone-sensitive and triple-negative breast cancer (TNBC) model systems, we systematically altered EMT transcriptional profiles by manipulating individual BET proteins and found that BRD2 positively regulates EMT, whereas BRD3 and BRD4 repress this program. Knockdown of individual BET proteins revealed independent transcriptional networks that differed from each other and from the small-molecule pan-BET inhibitor JQ1, which previously had been misleadingly asserted to be BRD4-selective. Available small-molecule pan-BET inhibitors, proposed as anti-proliferative agents in cancer clinical trials, obscure these biological differences. Transcriptional profiling reveals that individual BET proteins, inhibited separately, engage in and control EMT through unique processes. Implications: The distinct and opposing functions of BET proteins in the EMT process suggests the need for more member-selective epigenetic targeting agents.