Role of bromodomain 4 in breast cancer progression and metastasis

2483 Bromodomain 4 (Brd4) is a mammalian bromodomain-containing protein which interacts with acetylated chromatin in a bromodomain-dependent manner. Recent work showed that Brd4 dysregulation may play an important role in breast cancer progression. Gene expression analysis of mouse tumors with a different propensity to metastasize revealed a consistent down-regulation of Brd4 in aggressive tumors. Furthermore, polymorphisms in a Brd4-binding partner, Sipa1, were shown to have a major effect on metastatic efficiency, in both mouse and human, suggesting that the Brd4-Sipa1 complex may be an important node in metastatic progression. In vitro analysis of a highly metastatic mouse mammary tumor cell line Mvt-1 ectopically expressing Brd4 demonstrates that Brd4 activation significantly reduces mammary tumor cell invasiveness and cell migration without altering intrinsic growth rate. However, subcutaneous implantation of these cells into mice showed that Brd4 activation profoundly reduces tumor growth and metastatic capacity. This implies that activation of Brd4 may somehow be manipulating response to tumor microenvironment in the in vivo setting. To further investigate the role of Brd4 in breast cancer progression, immunohistochemistry of Brd4 in tissue microarray (TMA) was performed and Brd4 is found to be expressed in most of the breast cancer tissues from different human populations and to a different extent. Moreover, Brd4, which was described as a nuclear protein, is found to be expressed both in the nucleus and the cytoplasm, consistent with the recent finding that its binding partner Sipa1 is both nuclear and cytoplasmic. In addition, real-time PCR (qPCR) experiments show that ectopic expression of Brd4 up-regulates the expression of genes involved in transcriptional control and chromatin remodeling. The expression of the immediate-early gene Fos (FBJ Osteosarcoma oncogene), for example, is up-regulated in the mammary tumor cell line Mvt-1 ectopically expressing Brd4 compared to control cells on both the RNA and the protein levels. Furthermore, P-TEFb (positive transcription elongation factor b), a general transcriptional elongation factor that stimulates the processivity of Pol II elongation, is known to bind to Brd4 to form the transcriptionally active P-TEFb. P-TEFb was also shown to bind to the hexamethylene bis-acetamide inducible 1 (HEXIM1) and 7SK-snRNA to form a separate and inactive complex. Treating the mammary tumor cell line Mvt-1 with HMBA, an inducer of HEXIM1, down-regulates the mRNA levels of the Fos gene, indicating that HEXIM1 might exert an opposing effect on the control of gene transcription exerted by Brd4. ChIP-on-chip experiments are in progress to understand the role of Brd4 in transcription and to identify genome-wide targets of Brd4 regulation that may play important roles in breast cancer progression.