Manipulating the hypoxia induced radioresistance in rectal cancer

Abstract Introduction Locally advanced rectal cancer is primarily treated with radiotherapy and chemotherapy. Tumour hypoxia induces strong radioresistance, although the reasons for this characteristic are still unclear. This research explores the mechanistic and biological factors underlying hypoxia-induced radioresistance, and development of novel therapeutic strategies to overcome this. Methods HCT116 and HT29 CRC cell lines were incubated in normoxia (21% O2) and hypoxic conditions (1% O2). Cells were irradiated at varying dosages of x-ray radiation before undergoing clonogenic survival assay, comet assay to assess DNA double strand break damage and repair, or immunobloting to investigate cell death-related factors. HIF1α and HIF1b siRNA knockdowns were used to assess radioresponse in hypoxia. Metformin was used to treat cells prior to radiation and assess cell survival. Results Hypoxia induces strong radioresistance in CRC cells compared to relative normoxia in clonogenic survival assays. Neutral comet assays showed no significant difference in double strand break (DSB) repair kinetics following irradiation in hypoxia compared to normoxia. Immunoblotting analysis demonstrates activation of hypoxia-inducible factor 1a (HIF1a) in hypoxia but not in normoxia. HIF1α and HIF1b siRNA knockdowns did not show any improvement in radioresistance. Metformin did not improve radiosensitivity of CRC cells under different oxygen conditions. Conclusion Hypoxia induces strong radioresistance in cultured CRC cells which is not explained by x-ray-induced DSB damage and the efficiency of its repair. HIF1a is activated in hypoxia alone and may contribute to this resistance. Metformin alone is unable to overcome this resistance and other therapeutic targets need to be explored.