BET Bromodomain Inhibitors Block Growth of Pancreatic Cancer Cells in Three-Dimensional Collagen

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is associated with pronounced fibrosis that contributes to chemoresistance, in part, through increased histone acetylation. Since bromodomain (BRD) and extra terminal domain (BET) proteins are ‘readers’ of histone acetylation marks, we targeted BET proteins in PDAC cells grown in three-dimensional collagen. We show that treatment with BET inhibitors decreases growth of PDAC cells (AsPC1, CD18 and Panc1) in collagen. Transfection with siRNA against BRD4, which is increased in human PDAC tumors, also decreases growth of PDAC cells. BET inhibitors additionally decrease growth in collagen of PDAC cells that have undergone epithelial-to-mesenchymal transition or have become resistant to chemotherapy. Although BET inhibitors and BRD4 siRNA repress c-MYC only in AsPC1 and CD18 cells, downregulating c-MYC decreases growth of all three PDAC cell lines in collagen. FOSL1, which is also targeted by BET inhibitors and BRD4 siRNA in AsPC1, CD18 and Panc1 cells, additionally regulates growth of all three PDAC cell lines in collagen. BET inhibitors and BRD4 siRNA repress HMGA2, an architectural protein that modulates chromatin state and also contributes to chemoresistance, in PDAC cells grown in collagen. Importantly, we show that there is a statistically significant correlation between BRD4 and HMGA2 in human PDAC tumors. Significantly, overexpression of HMGA2 partially mitigates the effect of BET inhibitors on growth and c-MYC and/or FOSL1 expression in collagen. Overall, these results demonstrate that BET inhibitors block growth of PDAC cells in collagen and that BET proteins may be potential targets for the treatment of pancreatic cancer.

Brg1 promotes both tumor-suppressive and oncogenic activities at distinct stages of pancreatic cancer formation

“…One of the up-regulated genes was Hmga2, a transcription factor known to induce EMT and metastasis in gastric, mammary, and pancreatic cancer (Thuault et al 2008;Watanabe et al 2009;Zha et al 2013). In a recent study, Munshi and colleagues (Sahai et al 2014) showed that the BET bromodomain inhibitor JQ1 suppresses Hmga2 expression in pancreatic cancer cell lines, and we observed the same response in Kras G12D ; Brg1 f/f PDC lines. Notably, reactivation of Brg1 up-regulates Hmga2 expression, raising the question of whether combining redifferentiation toward a mature duct state with inhibition of the mesenchymal phenotype would be efficient in reducing tumor progression.…”

“…In other words, our aim was to explore whether combined down-regulation of Pdx1 and Hnf4a coupled with Hmga2 repression could be used to reduce tumor progression. Fortunately, JQ1, a drug in clinical trial for hematological malignancies (Filippakopoulos et al 2010;Delmore et al 2011), has been shown recently to decrease tumorigenicity of pancreatic cancer cells through Hmga2 repression (Sahai et al 2014). Furthermore, JQ1 targets the BET family of transcription factors and disrupts the interaction between them and acetylated H3K27 (H3K27ac) to favor a more repressive gene expression profile (De Raedt et al 2014).…”

Functions of BET proteins in erythroid gene expression

“…Many reports attribute the effects of BET inhibitors to BRD4 based on experiments in which individual BETs are depleted. 32,58,66,67 It is important to consider that because of the functional overlap of BRD2 and BRD3, it is possible that the combined contribution of these molecules has been underestimated. Future dissection of the mechanisms through which BETs act distinctly or compensate for each other will be critical when considering the development of BET inhibitors directed against specific members of this family.…”

Concepts to Target MYC in Pancreatic Cancer

“…In PDAC in vitro models, BET-I reduced the growth of cells cultured in collagen (53). However, JQ1 controls MYC not in all of the investigated PDAC cell lines, and here, FOSL1 and HMGA2 (high-mobility group AT-hook 2) were shown to be additional oncogenic drivers downregulated upon BET inhibition (53).…”

“…However, JQ1 controls MYC not in all of the investigated PDAC cell lines, and here, FOSL1 and HMGA2 (high-mobility group AT-hook 2) were shown to be additional oncogenic drivers downregulated upon BET inhibition (53). In human PDAC patient-derived xenotransplants (PdX), JQ1 showed activity (54).…”