Gefitinib radiosensitizes non-small cell lung cancer cells by suppressing cellular DNA repair capacity

Purpose: Overexpression of the epidermal growth factor receptor (EGFR) promotes unregulated growth, inhibits apoptosis, and likely contributes to clinical radiation resistance of non - small cell lung cancer (NSCLC). Molecular blockade of EGFR signaling is an attractive therapeutic strategy for enhancing the cytotoxic effects of radiotherapy that is currently under investigation in preclinical and clinical studies. In the present study, we have investigated the mechanism by which gefitinib, a selective EGFR tyrosine kinase inhibitor, restores the radiosensitivity of NSCLC cells. Experimental Design: Two NSCLC cell lines, A549 and H1299, were treated with 1 mu mol/L gefitinib for 24 h before irradiation and then tested for clonogenic survival and capacity for repairing DNA double strand breaks (DSB). Four different repair assays were used: host cell reactivation, detection of gamma-H2AX and pNBS1 repair foci using immunofluorescence microscopy, the neutral comet assay, and pulsed-field gel electrophoresis. Results: In clonogenic survival experiments, gefitinib had significant radiosensitizing effects on both cell lines. Results from all four DNA damage repair analyses in cultured A549 and H1299 cells showed that gefitinib had a strong inhibitory effect on the repair of DSBs after ionizing radiation. The presence of DSBs was especially prolonged during the first 2 h of repair compared with controls. Immunoblot analysis of selected repair proteins indicated that pNBS1 activation was prolonged by gefitinib correlating with its effect on pNBS1-labeled repair foci. Conclusions: Overall, we conclude that gefitinib enhances the radioresponse of NSCLC cells by suppressing cellular DNA repair capacity, thereby prolonging the presence of radiation-induced DSBs.