Abstract 1582: Inhibition of protein phosphatase 2A radiosensitizes pancreatic cancer cells by modulation of CDC25C.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC The combination of radiation with chemotherapy improves survival in patients with locally advanced pancreatic cancer. However, the median survival remains at approximately one year, and strategies to improve upon chemoradiation therapy for pancreatic cancer are warranted. We screened an 8,800 gene RNAi library in order to identify novel targets for sensitization to chemoradiation. In MiaPaCa-2 pancreatic cancer cells, we found PPP2R1A, a scaffolding subunit of protein phosphatase 2A (PP2A), among the top 5 identified targets. Thus, we went on to test the ability of LB100 (Lixte Biotechnology, East Setauket, NY), a small molecule PP2A inhibitor, to sensitize pancreatic cancer cells and tumors to radiation. In both MiaPaCa-2 and Panc-1 cells, treatment with LB100 (3uM; 2 hours pre- and 48 hours post- radiation) caused significant (P<0.05) radiosensitization (similar to that observed with PPP2R1A siRNA) with radiation enhancement ratios (ER) of 1.3 ± 0.03 and 1.4 ± 0.04, respectively. Importantly, LB100 did not cause radiosensitization in normal small intestine cells. Because PP2A regulates intermediates in the DNA damage response, we investigated several of these known PP2A substrates. We found, that treatment with LB100 in combination with radiation caused an increase in pCDC25C (T130, an activating phosphorylation), which was associated with decreased pCDK1 (Y15, an inhibitory phosphorylation). To begin to determine whether CDC25C modulation was responsible for the observed radiosensitization by LB100, we depleted CDC25C from MiaPaCa-2 cells and assessed radiosensitization in response to LB100. We found that CDC25C siRNA partially rescued LB100-mediated radiosensitization in MiaPaCa-2 cells (ER: control siRNA+LB100 1.32 ± 0.03; CDC25C siRNA + LB100 1.16 ± 0.03, P<0.05) and rescued pCDK1 (Y15) levels in response to LB100+radiation treatment. To test the ability of LB100 to sensitize in vivo, mice bearing MiaPaCa-2 xenografts were treated with LB100 (1.5mg/kg) followed 2 hours later by radiation (1.2Gy/fraction) Mon-Fri for 2 cycles. Treatment with LB100 caused minimal weight loss (<10%) and a significant delay in the time required for tumor volume doubling relative to radiation alone (LB100+radiation 34 days; radiation 24 days; P<0.05) and was associated with accumulation of pCDC25C (T130) and a corresponding decrease in pCDK1 (Y15) in tumor cells. Collectively, these data demonstrate that inhibition of PP2A induces radiosensitization in pancreatic cancer cells and tumors in part via modulation of CDC25C. Future studies will address the functional consequences of modulation of CDC25C and CDK1 by LB100 which lead to radiosensitization. Citation Format: Dongping Wei, Leslie A. Parsels, Mary A. Davis, Lili Zhao, Jonathan Maybaum, Theodore S. Lawrence, Yi Sun, Meredith A. Morgan. Inhibition of protein phosphatase 2A radiosensitizes pancreatic cancer cells by modulation of CDC25C. [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 1582. doi:10.1158/1538-7445.AM2013-1582