Targeting the BD2 BET bromodomain to inhibit proliferation of pancreatic ductal adenocarcinoma cells

Pancreatic ductal adenocarcinoma (PDAC), the most common type of pancreatic cancer, is a highly aggressive tumor with a 5-year survival of We and others document that inhibiting the function of BET bromodomain proteins suppresses PDAC tumor growth in preclinical models. The BET family of proteins is characterized by two N-terminal bromodomains, BD1 and BD2, which bind to acetylated lysine residues on specific histones. The binding of BET proteins to acetylated lysines promotes the formation of transcriptional complexes, and impacts gene transcription and chromatin structure. The literature suggests the potential specific roles for BD1 vs BD2 using pharmacological inhibitors and dominant negative mutational analyses. However, the relative importance of each bromodomain with respect to transcriptional regulation compared to chromatin structure is not well understood. Further, there is suggestion in the literature that each bromodomain function may differ in specific tumor cell types. Functional differences between BD1 and BD2 in PDAC cells have not been reported. The goal of the current study was to compare the efficacy of the BD2-specific BET inhibitor (BETi) SJ018 with the efficacy of the pan-BET inhibitor I-BET762 in PDAC cells. We exposed four PDAC cell lines (Panc1, BxPC3, MiaPaCa2, and CFPAC) to a range of concentrations of SJ018 or I-BET762 for 96 hours, and assessed cell viability using alamarBlue. Data, analyzed using GraphPad Prism 7, demonstrated that SJ018 exerted up to ~60-fold lower IC50 values across all cell lines tested compared to the pan-BET inhibitor I-BET762. Further, we recently made a novel observation that the pan-BETi JQ1 induced DNA damage and inhibits expression of the DNA repair protein Rad51 in PDAC cells. Immunoblot analysis for phosphorylated Ser-139 H2AX (γH2AX), a marker of DNA damage, demonstrated that SJ018 and I-BET762 also increased DNA damage, and that the BD2-specific SJ018 induces DNA damage to a greater extent and at a lower dose than the BD1/BD2 inhibitor I-BET762 in BxPC3 cells. The data also showed that both SJ018 and I-BET762 inhibited expression of the homologous recombination protein Rad51 in vitro. We conclude that SJ018 is more potent than I-BET762 in PDAC cells as a single agent, and that degree of potency is reflected by higher levels of DNA damage. Citation Format: Aubrey Lynn Miller, Peter J. Slavish, Brandon M. Young, Philip M. Potter, Anang A. Shelat, Karina J. Yoon. Targeting the BD2 BET bromodomain to inhibit proliferation of pancreatic ductal adenocarcinoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5794.