Abstract
Abstract
The molecular pathogenesis of many cancer types, including multiple myeloma (MM), involves alterations in the PI3K/Akt/mTOR and cyclin/CDK/CDKI/Rb (Rb) pathways. We have shown the combination of a Class I-specific HDAC inhibitor (entinostat, MS-275) with the mTOR inhibitor sirolimus to be synergistic in a large panel of B cell tumor cell lines including multiple myeloma, mantle cell and Burkitt's lymphoma. While tumor outgrowth occurred in both single agent arms, the combination effectively controlled in vivo tumor growth in long-term preclinical studies. The combination was also highly effective in a tumor-cell specific bioluminescence assay where myeloma cells are co-cultured with bone marrow stromal cells to mimic the protective effects of the tumor microenvironment. We found the combination antagonized the oncogenic activation of the AKT pathway associated with single-agent rapamycin treatment, inhibited the ERK/MAPK pathway, decreased pro-survival signaling, and increased growth inhibitory signals to a much greater extent than either single agent alone. To further understand the molecular mechanisms contributing to the synergy of this combination, a functional genomics approach utilizing high-throughput RNAi screening and gene expression profiling was taken. Several cell cycle-specific kinases were identified that upon inhibition enhance the activity of sirolimus and are significantly down-regulated by entinostat. Thus far, we have confirmed by western blot that PLK1 and AURKB are decreased in a dose-dependent manner by entinostat. Specific inhibition of these targets synergizes with sirolimus. Additionally, we found very low (<EC20) entinostat/sirolimus combination doses to hyper-sensitize tumor cells to a number of cell cycle inhibitors. These findings are further supported by increased cell cycle arrest seen in the combination treatment compared to either drug alone, as well as marked reduction in the cell cycle progression marker phospho-Histone H3. Taken together, our findings indicate the anti-tumor synergy of combined HDAC/mTOR inhibition is the result of multiple cooperative effects.
Citation Format: Benjamin J. Gamache, John Simmons, Jyoti Patel, Lihui Ou, Aleksandra Michalowski, Patrick Sullivan, Bih-Rong Wei, R. Mark Simpson, Shuling Zhang, Ke Zhang, W. Michael Kuehl, Ola Landgren, Natasha Caplen, Beverly Mock. A functional genomics approach for identification of sirolimus sensitizer genes regulated by HDAC inhibitors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4408. doi:10.1158/1538-7445.AM2013-4408
