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

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.
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.
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.
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.