Despite the fact that radiotherapy is a main therapeutic modality in cancer treatment, recent evidence suggests that fractionated radiotherapy (FR) might confer radioresistance through epithelial‐mesenchymal transition (EMT). Nevertheless, the effects of FR on EMT phenotype and the potential link between EMT induction and radioresistance development yet to be clarified. The aim of this study was to assess whether FR could promote EMT, and to elucidate if induction of EMT contributes to the acquisition of radioresistance. To this end, two human cancer cell lines (A549 and HT‐29) were irradiated (2 Gy/day) and analyzed using wound healing, transwell migration and invasion assays, real‐time polymerase chain reaction (for E‐cadherin, N‐cadherin, Vimentin, CD44, CD133, Snail, and Twist), clonogenic assay, Annexin V/PI, and 3‐[4,5‐dimethylthiazol‐2‐yl]‐2,5 diphenyl tetrazolium bromide (MTT) assay. Irradiation of A549 (for 5 or 10 consecutive days) resulted in morphological changes including elongation of cytoplasm and nuclei and pleomorphic nuclei. Also, irradiation‐enhanced migratory and invasive potential of A549. These phenotypic changes were in agreement with decreased expression of the epithelial marker (E‐cadherin), enhanced expression of mesenchymal markers (N‐cadherin, Vimentin, Snail, and Twist) and increased stemness factors (CD44 and CD133). Moreover, induction of EMT phenotype was accompanied with enhanced radioresistance and proliferation of irradiated A549. However, FR (for 5 consecutive days) did not increase HT‐29 motility. Furthermore, molecular alterations did not resemble EMT phenotype (downregulation of E‐cadherin, Vimentin, ALDH, CD44, CD133, and Snail). Eventually, FR led to enhanced radiosensitivity and decreased proliferation of HT‐29. Altogether, our findings suggest that FR might induce EMT and confer radioresistance in a cell context‐dependent manner.
