Rhabdomyosarcoma (RMS) is the most common pediatric soft tissue sarcoma. Intensive multi-modal therapeutic regimens have increased overall survival in the last 25 years, but 25% of patients still succumb to disease, thus necessitating the identification of novel targets for therapy. In this study, we identified that Bub1b is necessary for the growth and survival of both ARMS and ERMS cells using a loss of function high-throughput shRNA screen. Using many additional Bub1b shRNA sequences, we were able to confirm that human RMS cells were more sensitive to Bub1b knockdown compared to human fibroblasts or human ovarian cancer cells. Knockdown of Bub1b also resulted in significant suppression of RMS xenografts growth in vivo. Mechanistically, flow cytometry analysis of knockdown Bub1b cells had an increase in >4N DNA content. Live-cell time-lapse microscopy studies provided direct evidence that knockdown of Bub1b promotes endoreduplication, eventually causing mitotic catastrophe. Further, ChIP assay demonstrated that Forkhead Box M1 (FoxM1), an essential transcriptional factor for G1/S transition and mitotic progression, directly binds to the Bub1b promoter. Luciferase reporter assay showed that FoxM1-binding region is critical for the activation of the Bub1b promoter. Suppression of FoxM1, either by shRNA or the inhibitor siomycin A, led to reduction of Bub1b expression and inhibition of cell growth and survival in vitro. These findings indicate that the FoxM1-Bub1b axis is important for the growth and survival of RMS cells and may be a target for RMS therapy in the future.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A203.