Cytoplasmic HMGB1 and HMGB1-Beclin1 complex are increased in radioresistant oral squamous cell carcinoma

Cytoplasmic high mobility group box 1 (HMGB1) is an autophagy regulator, and autophagy is important in the radioresistance of various solid cancers. We evaluated the degree of autophagy and cytoplasmic HMGB1 in radioresistant oral squamous cell carcinoma (SCC) by culturing the SCC15 and quasiliquid layer 1 (QLL1) SCC cell lines that originate from cancer of the oral tongue and a metastatic lymph node, respectively, and then delivered radiation to induce radioresistance to cells. We then compared the degree of autophagy between non-irradiated control and radioresistant cancer cells using a western blot assay. We also compared the total and cytoplasmic concentrations of HMGB1 between the non-irradiated control and radioresistant cancer cells by western blot assay, and extracellular concentrations of HMGB1 with an enzyme-linked immunosorbent assay (ELISA). Formation of an HMGB1-Beclin1 complex was evaluated by immunofluorescence and co-immunoprecipitation assays. Autophagy increased in the radioresistant SCC15 cells (compared with non-irradiated control SCC15 cells) but not in the radioresistant QLL1 cells. The total amount of HMGB1 expression within cells did not differ; however, the degree of cytoplasmic HMGB1 expression was higher in radioresistant SCC15 cells than in non-irradiated control SCC15 cells. The HMGB1-Beclin1 complex, which is a main regulator of autophagy, was also increased in radioresistant SCC15 cells compared with non-irradiated control SCC15 cells. Autophagy flux and cytoplasmic HMGB1-Beclin1 increased after the acquisition of radioresistance in oral SCC.