Abstract 1597: C646, a selective small molecule inhibitor of p300, radiosensitizes cancer cells through enhancement of mitotic catastrophe.

Background: The histone acetyltransferases (HATs) including p300, CBP and TIP60 are transcriptional co-activators implicated in many gene regulatory pathways and protein acetylation events. We have previously shown that siRNA-mediated ablation of p300 and CBP sensitized cancer cells to ionizing radiation (IR) (Ogiwara, Oncogene 2011), and that garcinol, curcumin and anacardic acid, natural compounds with multi-HATs inhibitory activity, had radiosensitizing effect in cancer cells (Oike, Int J Radiat Oncol Biol Phys 2012). However, radiosensitization by selective small molecule inhibitors targeting activity of a specific HAT, that can have much potential for clinical use, have not been well investigated. C646 is a selective small molecule inhibitor of p300 identified by structure-based and active site-directed in silico screening (Bowers, Chem Biol 2010). In this study, we investigated the in vitro effect of C646 on radiosensitization and cell death in human cancer cells. Materials and Methods: The radiosensitizing effect in A549 (lung adenocarcinoma), H460 (lung squamous cell carcinoma) and H157 (lung large cell carcinoma) cells were assessed by colony formation assay, while that in MRC-5 (fibroblast) cells lacking in colony-forming ability was examined by ATP assay. Cell cycle distribution in A549 cells was analyzed using flow cytometry. The effect on mitotic catastrophe, apoptosis and senescence in A549 cells was evaluated by DAPI, annexin V and senescence-associated β-galactosidase staining, respectively. C646 was used at concentrations less than IC50. Results: C646 radiosensitized A549, H460 and H157 cancer cells with dose enhancement ratio at 10% surviving fraction of 1.4, 1.2 and 1.2, respectively. Meanwhile, C646 unaffected the radiosensitivity of MRC-5 cells. Cell cycle analysis showed that the hyperploid cell population (> 4N DNA cells) increased in C646-treated A549 cells after IR. Analyses of mode of cell death revealed that C646 enhanced mitotic catastrophe, but not apoptosis nor senescence, induced by IR in A549 cells. Conclusion: These data indicate that C646 is a radiosensitizer which enhances mitotic catastrophe induced by IR. Citation Format: Takahiro Oike, Mayumi Komachi, Hideaki Ogiwara, Jun Yokota, Takashi Nakano, Takashi Kohno. C646, a selective small molecule inhibitor of p300, radiosensitizes cancer cells through enhancement of mitotic catastrophe. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1597. doi:10.1158/1538-7445.AM2013-1597