IL-6 controls resistance to radiation by suppressing oxidative stress via the Nrf2-antioxidant pathway in oral squamous cell carcinoma

BACKGROUND: In promoting tumour malignancy IL-6 signalling is considered to have an important role. However, the biological roles of IL-6 on radiosensitivity in oral squamous cell carcinoma (OSCC) remain largely unclear. The objective of this study is to determine the effects and molecular mechanisms of IL-6 on radiosensitivity in OSCC. METHODS: Two OSCC cell lines, and OSCC tissue samples with radioresistant cells were used. We examined the effects of IL-6, or tocilizumab, a humanised anti-human IL-6 receptor antibody, or both on radiosensitivity and DNA damage after X-ray irradiation in vitro. In addition, we investigated the involvement of the Nrf2-antioxidant pathway in IL-6-mediated radioresistant mechanisms using OSCC cell lines and tissues. RESULTS: Increased levels of IL-6 suppressed radiation-induced cell death, and the blockade of IL-6 signalling by tocilizumab sensitised tumour cells to radiation. The radioresistant effect of IL-6 was associated with decreased DNA damage after radiation. We also found that IL-6 promotes the activation of not only the downstream molecule STAT3 but also the Nrf2-antioxidant pathway, leading to a significant decrease in oxidative stress by upregulating Mn-SOD. CONCLUSIONS: These results indicate that the blockade of IL-6 signalling combined with conventional radiotherapy could augment the treatment response and survival rate in patients with radioresistant OSCC.