Abstract B44: Dual targeting of PI3K and MEK impairs DNA double-strand break repair as a relevant mechanism for radioresistance of K-RAS mutated non-small cell lung cancer

Abstract

Abstract Introduction: One of the major mechanisms of radioresistance in solid tumors is the activa-tion of the PI3K/Akt pathway in which Akt facilitates the repair of DNA double-strand breaks (DSBs) through stimulation of DNA-dependent protein kinase catalytic subunit (DNA-PKcs). As it is known that the PI3K/Akt pathway is hyperactivated in K-RAS mutated (K-RASmut), we investigated if targeting PI3K would be a potential approach for enhancing radiosensitivity of K-RASmut cells non-small lung cancer (NSCLC). Methods: The K-RASmut NSCLC cell lines A549 and H460 were utilized to examine the effect of PI3K inhibition on Akt signaling, non-homologous end joining (NHEJ) repair of DSBs, and post-irradiation cell survival. Results: Short-term (1-2 h) pre-treatment of K-RASmut cells with the PI3K inhibitor PI-103 (1 µM) inhibited Akt/DNA-PKcs activity, blocked DSBs repair and induced radiosensitivity, while long-term (24 h) pre-treatment did not. Lack of an effect in K-RASmut cells after 24 h of PI-103 pre-treatment was due to reactivation of K-Ras/MEK/ERK-dependent Akt. However, long-term treatment with PI-103 together MEK inhibitor PD98059 completely blocked reactivation of Akt and impaired DSBs repair through NHEJ leading to radiosensitization. The effect of PI3K inhibition on Akt signaling was also tested in A549 mouse xenografts in vivo. Akt and P-DNA-PKcs activity was inhibited 30 min post-irradiation in xenografts, which were pre-treated by PI-103 30 min before irradiation. However, after a 24 h pretreatment with PI-103, a significant reactivation of Akt was achieved 24 h after irradiation. Conclusion: Due to ERK-dependent reactivation of Akt in K-RAS mutated NSCLC cells, dual targeting of PI3K and MEK is an effective approach to induce radiosensitization. Acknowledgement: This work was supported by grants from the Deutsche Forschungsge-meinschaft (DFG, RO 527/7-1) awarded to HPR /MT and GRK 1302/2 (T11) awarded to MT/HPR. Citation Format: Mahmoud Toulany, Mari Ilda, Deric L. Wheeler, H. Peter Rodemann. Dual targeting of PI3K and MEK impairs DNA double-strand break repair as a relevant mechanism for radioresistance of K-RAS mutated non-small cell lung cancer [abstract]. In: Proceedings of the AACR Special Conference on DNA Repair: Tumor Development and Therapeutic Response; 2016 Nov 2-5; Montreal, QC, Canada. Philadelphia (PA): AACR; Mol Cancer Res 2017;15(4_Suppl):Abstract nr B44.