FOXM1-mediated regulation of reactive oxygen species and radioresistance in oral squamous cell carcinoma cells

Radioresistance is a major obstacle to the successful treatment of oral squamous cell carcinoma (OSCC). To help overcome this issue, we have developed clinically relevant radioresistant (CRR) cell lines, generated by irradiating parental cells over time, which are useful for OSCC research. In the present study, we conducted gene expression analysis using CRR cells and their parental lines to investigate the regulation of radioresistance in OSCC cells. Based on gene expression changes over time in CRR cells and parental lines subjected to irradiation, forkhead box M1 (FOXM1) was selected for further analysis in terms of its expression OSCC cell lines, including CRR cell lines, and clinical specimens. We suppressed or upregulated the expression of FOXM1 in OSCC cell lines, including CRR cell lines, and examined radiosensitivity, DNA damage, and cell viability under various conditions. The molecular network regulating radiotolerance was also investigated, especially the redox pathway, and the radiosensitizing effect of FOXM1 inhibitors was examined as a potential therapeutic application. We found that FOXM1 was not expressed in normal human keratinocytes but was expressed in several OSCC cell types. The expression of FOXM1 was upregulated in CRR cells compared with that detected in the parental cell lines. In clinical specimens, FOXM1 expression was upregulated in cells that survived irradiation. FOXM1-specific siRNA treatment increased radiosensitivity, whereas FOXM1 overexpression decreased radiosensitivity, and DNA damage was altered significantly under both conditions as well as the levels of redox-related molecules and reactive oxygen species production. Treatment with the FOXM1 inhibitor thiostrepton had a radiosensitizing effect and overcame radiotolerance in CRR cells. According to these results, the FOXM1-mediated regulation of reactive oxygen species could be a novel therapeutic target for the treatment of radioresistant OSCC; thus, treatment strategies targeting this axis might overcome radioresistance in this disease. SAS and SAS-R cells were irradiated with X-rays at 2 Gy per day for 5 days, and total RNA was collected 1, 3, 6, and 12 h after irradiation. Total RNA was also extracted from nonirradiated SAS and SAS-R cells that were cultured for use as control samples.