Abstract A51: FOXM1 inhibition by thiostrepton synergizes with olaparib by attenuating adaptive response in ovarian cancer cells

Abstract Background: High-grade serous ovarian carcinoma (HGS-OvCa) is the most common subtype of epithelial ovarian cancer and accounts for around 70% of all ovarian cancers in the US. Cancer cells that show “BRCAness” are highly sensitive to poly (ADP-ribose) polymerase inhibitors (PARPi), including those with deleterious mutations in BRCA1/2 or defects in other components that are crucial for homologous recombination (HR) repair pathway. In HGS-OvCa, genes involved in HR are altered in about 50% of cases, making these cancers sensitive to PARPi. However, FOXM1 transcription factor network is activated in more than 84% of cases in HGS-OvCa, and activation of the FOXM1 pathway has been shown to upregulate genes involved in HR, which can lead to PARPi resistance. Therefore, FOXM1 becomes an attractive therapeutic target for exploring new combination therapies to overcome PARPi resistance and enhancing its therapeutic effects. Hypothesis: We hypothesize that FOXM1 inhibition induces “BRCAness” and sensitizes ovarian cancer cells to PARP inhibitors. Results: Olaparib increases the expression and nuclear localization of FOXM1 levels and FOXM1 target genes, such as BRCA1 and RAD51, suggesting that FOXM1 may play an important role in adaptive response after olaparib treatment. Furthermore, FOXM1 expression levels inversely correlates with olaparib sensitivity in multiple ovarian cancer cells. Downregulation of FOXM1 by siRNA sensitizes ovarian cancer cells to olaparib, while pharmacologic inhibition of FOXM1 by thiostrepton synergizes with olaparib as shown by colony formation assays. Thiostrepton downregulates DNA repair and anti-apoptotic genes, such as BRCC3, BRCA1/2, FANCF, and BCL2, and upregulates pro-apoptotic genes DDIT3, DDIT4, and PD-L1 in multiple ovarian cancer cell lines. Consistently, thiostrepton activates caspase 3 activity, induces apoptosis, and enhances DNA damage with combination of thiostrepton and olaparib in comet assay. Thiostrepton also increases PARP1 trapping onto chromatin in OVCA420 cells treated with olaparib. Finally, thiostrepton resensitizes rucaparib-resistant cells derived from MDA-MB-436 to rucaparib in cell viability assay and colony formation assay. Thiostrepton downregulates the stabilized mutant BRCA1, which has been shown to cause the resistance to PARPi in these cells, suggesting that FOXM1 may play a key role in acquired PARPi resistance, which can be overcome by inhibition of FOXM1. Conclusions: These data demonstrate that FOXM1 plays an important role in adaptive response induced by olaparib in ovarian cancer cells. Inhibition of FOXM1 by thiostrepton increases apoptosis and sensitizes ovarian cancer cells to olaparib by inducing “BRCAness” through downregulating HR genes. Interestingly, we also find that thiostrepton could increase PARP trapping onto chromatin and further enhance DNA damage. Moreover, FOXM1 inhibition by thiostrepton could resensitize acquired PARPi-resistant cells, probably through downregulating stabilized mutant BRCA1. Together, all these data suggest that FOXM1 inhibition represents an effective strategy to overcome PARPi resistance, and inhibition of adaptive response by FOXM1 may produce better therapeutic effects for PARP inhibitors. Citation Format: Pingping Fang, Jill Madden, Lisa Neums, Jeremy Chien. FOXM1 inhibition by thiostrepton synergizes with olaparib by attenuating adaptive response in ovarian cancer cells. [abstract]. In: Proceedings of the AACR Conference: Addressing Critical Questions in Ovarian Cancer Research and Treatment; Oct 1-4, 2017; Pittsburgh, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(15_Suppl):Abstract nr A51.