Sensitization of Radio-Resistant Lung Cancer Cells with a B Subunit of Bacterial Cytolethal Distending Toxin from Aggregatibacter actinomycetemcomitans

Background: Combination cancer therapy is a promising strategy which employs multiple therapeutic agents with different mechanisms of action along with minimal intolerable side effects. For example, a combination of radiotherapy with gene therapy can overcome the development of resistance to therapeutic doses of irradiation (IR) and normal tissue damages caused by high-dose radiation. Recent studies have revealed radio-resistance in non-small cell lung cancer (NSCLC) cells. In this study, for the first time, subunit B of cytolethal distending toxin (cdtB)-expressing plasmid was introduced as a sensitizer of the cells to IR with a high efficacy. Methods: A vector expressing cdtB suicide gene of human periodontal bacterium Aggregatibacter actinomycetemcomitans was constructed and then transfected into A549 cell line. In the next step, cells transfected with pcDNA3.1/cdtB were irradiated and its growth inhibitory effect was evaluated in NSCLC cancer in vitro by MTT (3-(4, 5-methylthiazol-2-yl) -2, 5-diphenyl-tetrazolium bromide) assay. Terminal deoxyribonucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay were carried out in order to examine the apoptosis induction by a combination of IR with cdtB. Results: Our data indicated significant cell death in NSCLC cells in comparison with controls with an increase from 5% in response to IR up to 73.27% for combination of IR with cdtB. Moreover, the result of TUNEL assay showed significant differences in the number of apoptotic cells among the different affected groups. Conclusions: Our results confirmed that cdtB-expressing plasmid sensitizes NSCLC cells to IR and significantly increases the efficacy of radiotherapy and therefore, combining toxin with IR has a synergistic effect on NSCLC.