Nontoxic concentration of DNA‐PK inhibitor NU7441 radio‐sensitizes lung tumor cells with little effect on double strand break repair

Abstract

High‐linear energy transfer ( LET) heavy ions have been increasingly employed as a useful alternative to conventional photon radiotherapy. As recent studies suggested that high LET radiation mainly affects the nonhomologous end‐joining ( NHEJ) pathway of DNA double strand break ( DSB) repair, we further investigated this concept by evaluating the combined effect of an NHEJ inhibitor ( NU7441) at a non‐toxic concentration and carbon ions. NU7441‐treated non‐small cell lung cancer ( NSCLC) A549 and H1299 cells were irradiated with X‐rays and carbon ions (290 MeV/n, 50 keV/μm). Cell survival was measured by clonogenic assay. DNA DSB repair, cell cycle distribution, DNA fragmentation and cellular senescence induction were studied using a flow cytometer. Senescence‐associated protein p21 was detected by western blotting. In the present study, 0.3 μM of NU7441, nontoxic to both normal and tumor cells, caused a significant radio‐sensitization in tumor cells exposed to X‐rays and carbon ions. This concentration did not seem to cause inhibition of DNA DSB repair but induced a significant G2/M arrest, which was particularly emphasized in p53‐null H1299 cells treated with NU7441 and carbon ions. In addition, the combined treatment induced more DNA fragmentation and a higher degree of senescence in H1299 cells than in A549 cells, indicating that DNA‐ PK inhibitor contributes to various modes of cell death in a p53‐dependent manner. In summary, NSCLC cells irradiated with carbon ions were radio‐sensitized by a low concentration of DNA‐ PK inhibitor NU7441 through a strong G2/M cell cycle arrest. Our findings may contribute to further effective radiotherapy using heavy ions.