Involvement of ferroptosis‐defensive xCT/GPX4 axis in radioresistance and its impacts on prognosis in oral squamous cell carcinoma

Abstract Aim Radiotherapy, the mainstay treatment for patients with advanced oral squamous cell carcinoma (OSCC), has low clinical efficacy, resulting in poor prognosis. Radiation induces ferroptosis, a form of cell death driven by iron‐dependent lipid peroxidation. Since the xCT/glutathione peroxidase 4 (GPX4) axis is predominantly involved in the ferroptosis defense system, targeting this axis may potentiate cancer cell vulnerability to ferroptosis. Methods We examined the expression of xCT and GPX4 in the tumor cells of biopsy specimens using immunohistochemistry and evaluated the radiation‐mediated antitumor effects of the ferroptosis inducers, erastin and RSL3, that act by inhibiting xCT and GPX4, respectively. Results Univariate analysis revealed that high immunohistochemical expression of xCT was correlated with shorter survival, and high GPX4 expression was an independent poor prognostic factor, indicating that the xCT/GPX4 axis influences the clinical outcome of OSCC. Analyses using two types of OSCC cell lines revealed that the clonogenic survival of irradiated cells was increased by the ferroptosis inhibitor ferrostatin‐1 and decreased by erastin and RSL3. The enhanced antitumor effects of erastin and RSL3 were accompanied by increased lipid peroxidation, which was suppressed by the iron chelator deferoxamine. Erastin and RSL3 enhance the effects of radiation on OSCC cells in a mouse xenograft model; these effects were associated with increased expression of the lipid peroxidation marker 4‐hydroxynonenal in tumor cells. Conclusions xCT/GPX4 axis is involved in the tumor resistance against radiation through the inhibition of ferroptosis in OSCC. Furthermore, radiotherapy combined with ferroptosis induction by targeting the xCT/GPX4 axis may improve patient prognosis in advanced OSCC.