Small Molecule Therapeutics Targeted Silencing of MLL5b Inhibits Tumor Growth and Promotes Gamma-Irradiation Sensitization in HPV16/ 18-Associated Cervical Cancers

We previously identified a novel MLL5 isoform, MLL5b, which was essential for E6 and E7 transcriptional activation in HPV16/18-associated cervical cancers. In this report, we investigated the potential of RNAimediated silencing ofMLL5b through the use ofMLL5b-siRNA as a novel therapeutic strategy for HPV16/18positive cervical cancer. We observed concurrent downregulation of E6 and E7 after MLL5b silencing, leading to growth inhibition via the activation of apoptosis and senescence in the HeLa cell model. This corresponded with the enhanced antitumor effects of MLL5b-siRNA compared with E6or E7-siRNA single treatments. Significant reduction in tumor size after MLLb-siRNA treatment in the HeLa xenograft tumor model further emphasized the importance of MLL5b in HPV16/18-associated tumor growth and the potential of RNAi therapeutics that target MLL5b. We also identified MLL5b as a modulator of gamma-irradiation (IR) sensitization properties of cisplatin. We observed that while MLL5b silencing alone was enough to evoke cisplatin-like IR sensitization in tumor cells in vitro, overexpression ofMLL5b inhibited the ability of cisplatin to sensitize HeLa cells to IR-induced cytotoxicity. MLL5b-siRNA-IR cotreatment was also observed to enhance tumor growth inhibition in vivo. Taken together, our findings highlight the potential of targeted silencing of MLL5b via the use ofMLL5b-siRNAas a novel therapeutic strategy andpropose thatMLL5b-siRNA could be a viable alternative for cisplatin in the current cisplatin-based chemotherapeutics for HPV16/18-associated cervical cancers. Mol Cancer Ther; 13(11); 1–11. 2014 AACR.