Interactions Between MYC and BRD4 Result in Their Reciprocal Regulation

Division Of Basic Sciences - Nci

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Abstract

BRD4, a member of the BET family of proteins, has been identified as a key factor controlling multiple cancers and is being investigated as a therapeutic target in cancer as well as many other diseases. BRD4 is a chromatin bound protein primarily with dual enzymatic activities that plays critical roles as a chromatin remodeler and transcriptional activator. The chromatin remodeling function of BRD4 depends on its intrinsic HAT activity while its transcriptional activation function depends on its kinase activity. It is well documented that BRD4 is released from the chromatin during periods of increased transcription such as immediate early transcription at the end of mitosis or during stress. The chromatin free BRD4 recruits and activates PTEFb, a transcription elongation factor, and directly phosphorylates the RNA Pol II to activate transcription. Work from our lab has shown that the enzymatic activities of BRD4, and thereby its chromatin remodeling and transcription activation functions, are tightly cross-regulated by the substrates it interacts with. These results suggest that the chromatin remodeling and transcriptional functions of BRD4 are distinctly delineated and do not overlap temporally, leading to the conclusion that the dual-function BRD4 transitions from being a chromatin remodeler to a transcriptional activator. The central goal of this project is to define the signals and mechanism that trigger the release of BRD4 from chromatin, allowing its transition from a chromatin remodeler to a transcriptional regulator. While attempting to delineate the chromatin remodeling/HAT and transcriptional activation/kinase activities of BRD4 we recently discovered that the kinase and HAT activities of BRD4 are regulated by its interacting partners. When BRD4 is bound to the chromatin and interacting with nucleosomes its kinase activity is repressed. On the other hand, when BRD4 is interacting with its transcriptional partners such as MYC or the Pol II CTD, its HAT activity is suppressed. These findings suggest that the HAT and kinase activities of BRD4 are tightly cross-regulated and dependent on its interactions and function. BRD4 transcription/kinase functions are kept suppressed when it is functioning as a chromatin remodeler/ HAT and conversely, its chromatin remodeling/ HAT functions are repressed when it is functioning as a transcription factor/ kinase. The release of BRD4 from chromatin immediately after mitosis during early G1 gene activation is well documented and consistent with our hypothesis that BRD4 transitions from a chromatin remodeler to become a transcriptional activator upon its release from chromatin. We have exploited the observation of chromatin release at the M/G1 interface to further characterize the mechanisms underlying cross-regulation of BRD4. The primary objective of our ongoing studies is to characterize the JNK-BRD4 interaction and its effects on the functions of BRD4. We hypothesized, based on earlier reports, that the Jun-C N-terminal kinase (JNK), a key member of the MAPK signaling cascade might be involved in releasing BRD4 from chromatin during mitotic stress. Our results show that JNK interacts with BRD4 directly and causes its release from the chromatin. Preliminary results also indicate that JNK phosphorylates BRD4 at multiple sites. JNK phosphorylation of BRD4 affects both its HAT and kinase activities. We are currently further characterizing the nature of the JNK-BRD4 interaction and its functional effects. In summary, our findings suggest that the cross-regulation of BRD4 enzymatic activities lead to its transition from being a chromatin remodeler to a transcriptional activator and that the JNK kinase may play a critical role in this transition.