Radiotherapy remains a major approach to adjuvant therapy for patients with advanced rectal cancer. Nevertheless, the effects of radiation on malignant processes have yet to be clarified. The aim of this study was to assess the biological effects of radiation on colorectal cancer (CRC) cells with special reference to epithelial-mesenchymal transition (EMT), a key developmental program often activated during cancer invasion and metastasis. We investigated the effect of radiation on two colorectal cancer cell lines, CaR1 and DLD1, assessing cell morphology, motility, migration and invasive ability. Expression of molecules associated with EMT was determined using RT-PCR, Western blotting, and immunofluorescence staining in control and irradiated cells. We also used real-time RT-PCR to examine the expression of molecules associated with EMT before and after chemoradiotherapy. Thus, we studied 26 rectal cancer patients who received preoperative chemoradiotherapy followed by radical surgery. In addition, we examined the relationship between disease recurrence and the expression of a number of proteins. Irradiation caused CRC cells to undergo phenotypic changes characteristic of EMT: spindle-cell shape, loss of polarity, intercellular separation and pseudopodia formation. Irradiation enhanced cell migration and invasiveness. In irradiated CRC cells, molecular changes consistent with EMT were observed. In clinical samples, we observed molecular changes consistent with EMT, and those changes were significantly enhanced in patients with recurring disease. These results indicate that irradiation induces an alteration to a malignant phenotype consistent with EMT in colorectal cancer cells.

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