Regulation of BRD4 Function

Division Of Basic Sciences - Nci

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Abstract

The switching of BRD4 from a chromatin regulator to a transcription factor suggests the existence of a signaling event that triggers its release from chromatin. Earlier reports implicated the involvement of the c-Jun N-terminal kinase (JNK) in releasing BRD4 from chromatin through an unknown mechanism during drug induced mitotic stress or UV stress. The JNK subfamily of kinases consist of three closely related paralogs JNK1, JNK2 and JNK3, with the first two expressed ubiquitously in most tissues and JNK3 being restricted to the central nervous system. JNK1 and JNK2 are believed to have identical substrates and functions, although the strength of their enzymatic activities varies widely across cell types and context. The JNK kinases are part of the mitogen activated protein kinase (MAPK) signaling pathway and control cellular responses to a very wide range of abiotic and biotic stress. In addition, like BRD4, they also modulate critical cellular and physiological processes, such as the cell cycle, apoptosis and differentiation during immunological, inflammatory responses as well as tumorigenic and normal embryonic development. Therefore, we investigated the effect of JNK kinase on BRD4 with the hypothesis that it plays a broad role in releasing BRD4 from chromatin genome wide and triggers its switch from a chromatin to transcription regulator. Our results show that JNK directly interacts with BRD4 and phosphorylates it specifically at three independent sites. Phosphorylation of BRD4 at these sites is responsible for its release from chromatin during multiple stress responses, normal thymic stimulation, and epithelial to mesenchymal transition (EMT) during cancer progression, resulting in activated transcription. Dephosphorylation of these sites by the PP4 phosphatase allows BRD4 interaction with RNA Pol II. Further, we report that the kinase and HAT activities of BRD4 are cross-regulated by its substrates and the release of BRD4 from chromatin is key to its switching from a HAT to kinase and thus transitioning from a chromatin to transcription regulator. Importantly, we show that the release of BRD4 from chromatin by JNK phosphorylation leads to significantly increased transcription of genes involved in inflammatory and immune responses as well as those responsible for EMT, while also increasing alternative splicing genome-wide. In summary, our findings characterize a novel and specific mechanism for the release of BRD4 from chromatin, facilitating its role transition from a chromatin regulator to transcription activator during periods requiring increased transcriptional activity.