Dual targeting of PI3K and MEK enhances the radiation response of K-RAS mutated non-small cell lung cancer

// Mahmoud Toulany 1 , Mari Iida 2 , Simone Keinath 1 , Firdevs F. Iyi 1 , Katharina Mueck 1 , Birgit Fehrenbacher 3 , Wael Y. Mansour 4, 5 , Martin Schaller 3 , Deric L. Wheeler 2, * , H. Peter Rodemann 1, * 1 Division of Radiobiology and Molecular Environmental Research, Department of Radiation Oncology, University of Tuebingen, Tuebingen, Germany 2 Department of Human Oncology, University of Wisconsin School of Medicine and Public Health, Wisconsin Institute for Medical Research, Madison, WI, USA 3 Department of Dermatology, University of Tuebingen, Tuebingen, Germany 4 Tumor Biology Department, National Cancer Institute, Cairo University, Cairo, Egypt 5 Laboratory of Radiobiology and Experimental Radiooncology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany * Shared last authorship Correspondence to: Mahmoud Toulany, email: mahmoud.toulany@uni-tuebingen.de Keywords: NSCLC, PI3K/Akt, MAPK/ERK, radiotherapy, double-strand breaks Received: January 1, 2016     Accepted: May 12, 2016     Published: May 27, 2016 ABSTRACT Despite the significant contribution of radiotherapy to non-small lung cancer (NSCLC), radioresistance still occurs. One of the major radioresistance mechanisms is the hyperactivation of the PI3K/Akt pathway in which Akt facilitates the repair of DNA double-strand breaks (DSBs) through the stimulation of DNA-PKcs. We investigated if targeting PI3K would be a potential approach for enhancing the radiosensitivity of K-RAS mutated (K-RASmut) NSCLC cell lines A549 and H460. Short-term (1-2 h) pre-treatment of 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 after 24 h of PI-103 pre-treatment was due to reactivation of K-Ras/MEK/ERK-dependent Akt. However, long-term treatment with the combination of PI-103 and MEK inhibitor PD98059 completely blocked reactivation of Akt and impaired DSBs repair through non-homologous end joining (NHEJ) leading to radiosensitization. The effect of PI3K inhibition on Akt signaling was also tested in A549 mouse xenografts. P-Akt and P-DNA-PKcs were inhibited 30 min post-irradiation in xenografts, which were pretreated by PI-103 30 min before irradiation. However, Akt was reactivated 30 min post-irradiation in tumors, which were pre-treated for 3 h with PI-103 before irradiation. After a 24 h pretreatment with PI-103, a significant reactivation of Akt was achieved 24 h after irradiation. Thus, due to MEK/ERK-dependent reactivation of Akt, targeting PI3K alone is not a suitable approach for radiosensitizing K-RASmut NSCLC cells, indicating that dual targeting of PI3K and MEK is an efficient approach to improve radiotherapy outcome.