CLIC4 regulates radioresistance of nasopharyngeal carcinoma by iNOS after γ-rays but not carbon ions irradiation.

Nasopharyngeal carcinoma (NPC) is a major health problem in the East and Southeast Asia, and the intensity modulated radiotherapy (IMRT) is the current preferred treatment method of NPC, but radioresistance-induced residual and recurrent tumors are the main cause of treatment failure. Till now, the mechanism of radioresistance and prognostic biomarkers of NPC are still unrevealed. In this study, we collected clinical NPC samples and established radioresistant NPC-R cell lines by irradiating NPC cells with fractionation doses of γ-rays. Using genechip assay between radioresistance and radiosensitive clinical samples and TMT assay between NPC and NPC-R cells, differential expressed genes were examined and the potential biomarker of radioresistance was screened. Immunohistochemical assay of NPC clinical specimens showed that CLIC4 was significantly up-regulated in radioresistance tumor tissues. In vitro studies confirmed that up-regulation of CLIC4 gene enhanced radioresistance in comparison with the alterations of intracellular oxidative metabolism of reactive oxygen species (ROS) and nitric oxide (NO) in an opposite way. Correspondingly, inhibition of CLIC4 sensitized NPC cells to irradiation and decreased nuclear translocation of iNOS and intracellular level of NO in NPC cells. Interestingly, the capacity for DNA repair had no difference between NPC and NPC-R cells. Moreover, because of great interests in using carbon ion irradiation to treat NPC effectively, we demonstrated that, after carbon ion irradiation, NPC-R and NPC cells had similar survival even under the status of up- or down-regulation of CLIC4. Conclusively, CLIC4 contributes to radioresistance of NPC to γ-rays but not carbon ions by regulating intracellular oxidative metabolism of nuclear translocation of iNOS.