Over 70% of patients with advanced stage ovarian cancer relapse within 2 years, with long term overall survival being only 25%. Tumor-initiating cells (TICs) are thought to contribute to this high rate of recurrence as they can resist killing by platinum-based chemotherapy. In this study we sought to identify pathways unique to TICs and to disrupt these pathways in order to more effectively target this population of cells. We conducted an siRNA screen which identified NF-kappaB signaling as an important pathway in the maintenance of ovarian cancer cells grown in non-adherent spheroid cultures that mimic TICs. Downregulation of several known NF-kappaB target genes, including NXF1, RAC2, CDC42, BCL2L1 and SERPINB3, significantly decreased growth of TIC cultures but not the corresponding adherent cultures, thereby indicating that expression of these genes is necessary for growth in TIC cultures. These findings are consistent with previous studies which showed that higher NF-kappaB activity is linked with drug resistance in patients. In parallel, we conducted a drug screen to identify compounds that target TICs. One clear candidate was bardoxolone methyl, which has been shown to inhibit IKK, an upstream NF-kappaB signaling protein. We confirmed the results of the drug screen in a panel of ovarian cancer cell lines, showing that bardoxolone methyl decreased known TIC markers CD133 and ALDH by flow cytometry, and decreased NF-kappaB proteins by western blot. To further elucidate the mechanism of bardoxolone methyl on TICs we conducted an RNAi screen +/- bardoxolone methyl. Interestingly, the knockdown of genes including BRAF, MAP3K8, LMNA and SMAD6 acted in a synthetically lethal manner with bardoxolone methyl in the TICs. To begin translating to a clinically relevant model, we proceeded to test whether bardoxolone methyl could eliminate TICs that remained after treatment with platinum-based chemotherapy. We confirmed in vitro that bardoxolone methyl in combination with chemotherapy reduced TIC populations. Our ongoing studies will address whether this drug is effective in preventing relapse in vivo after chemotherapy treatment of ovarian cancer in mouse models.
Citation Format: Michelle K. Ozaki, Carrie D. House, Krystyna Mazan-Mamczarz, Madhu Lal-Nag, Craig Thomas, Christina M. Annunziata. Targeting the NF-kappaB pathway with bardoxolone methyl to inhibit ovarian cancer spheroid formation [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 167.