Abstract 1304: The effect of FoxM1 inhibition with thiostrepton on ovarian cancer immune response and sensitivity to chemotherapyin vitro

Background: An ideal anti-cancer agent will target a pathway that is altered in cancer, produce a selective cytotoxic response in cancer cells, induce immunogenic cell death, and enhance tumor immunity. FoxM1 inhibitor, thiostrepton, may fit this profile of an ideal anti-cancer agent. FoxM1 is activated in the majority of high-grade serous carcinomas, regulates DNA repair genes, and enhances cell proliferation, metastasis, and chemotherapy resistance. We have previously shown that loss-of-function and gain-of-function TP53 mutations contribute to FoxM1 overexpression, thus the p53-FoxM1 axis represents a novel therapeutic target. However, the therapeutic potential of FoxM1 inhibition by thiostrepton has not been fully investigated in ovarian cancer. Hypothesis: We hypothesize that FoxM1 inhibition by thiostrepton increases cancer cell sensitivity to chemotherapy and enhances anti-tumor immunity. Results: Thiostrepton downregulates FoxM1 expression, induces cytotoxic effects across various ovarian cancer cell lines, and shows synergistic activities with cisplatin. In congruence with the Connectivity Map data, qRT-PCR results of A2780 cells treated with thiostrepton (10 μM) show upregulation of pro-apoptotic genes DDIT3, GADD45A and DDIT4, and downregulation of DNA repair and anti-apoptotic genes FANCF, BRCA2, BRCC3, and BCL2. These results suggest that the synergism observed may be a consequence of DNA damage induction by cisplatin combined with inhibition of DNA repair by thiostrepton, making this a lethal duo to cancer cells. Comet assay results from mouse ovarian cancer cells, ID8, support this hypothesis by demonstrating an increase in DNA damage following the combined treatment of thiostrepton and carboplatin or thiostrepton and PARP inhibitor olaparib. Western blots also show increased p-H2AX levels in these combined treatments. In addition to the cytotoxic effect, thiostrepton can alter the cancer cell immune response by affecting the expression of immunomodulatory genes. This is supported by the ENCODE data, which indicates FoxM1 binds to the cis-regulatory region of PD-L1 (programmed cell death ligand 1), a target of ongoing immunotherapy clinical trials. Flow cytometry and Western blots depict increased PD-L1 expression in ID8 cells treated with thiostrepton (5 μM). In these same cells, flow cytometry showed an increase in RAE1 and 4-1BB ligands, which may lead to increased recognition by natural killer T-cells, as well as enhanced externalization of calreticulin, a hallmark of immunogenic cell death. Conclusion: These studies demonstrate the novel effect of thiostrepton on immunogenic cancer cell death and an effect on anti-tumor immunity. These results suggest thiostrepton as an ideal anti-cancer agent with the ability to target the DNA repair response and the immune response to enhance the chemotherapeutic effects of cisplatin and olaparib. Citation Format: Jill Madden, Pingping Fang, Bernard Herrman, Mary Markiewicz, Ryan Moulder, Laird Forrest, Jeremy Chien. The effect of FoxM1 inhibition with thiostrepton on ovarian cancer immune response and sensitivity to chemotherapy in vitro. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1304.