Quantitative dissection of radioresistance in cancer cells: Oxygen depletion-induced biological versus chemical mechanisms and baseline mechanisms governed by HIF-1β-NRF2/TXNRD1.

The oxygen environment within malignant solid tumors is highly heterogeneous, with anoxic regions arising where oxygen supply is insufficient. It is known that oxygen depletion is one of the most influential factors that confer radioresistance on cancer cells. The resistance under low oxygen conditions has been attributed to both hypoxia-inducible genes-dependent biological mechanisms and genes-independent chemical mechanisms resulting from reduced production of radiation-induced reactive oxygen species. However, the relative contributions of these mechanisms have remained unclear. Moreover, although hypoxia-inducible factors (HIFs) are suggested to play key roles in hypoxia-inducible genes-dependent biological mechanisms, it has remained uncertain whether and how HIFs affect the intrinsic radioresistance of cancer cells. Here, we developed an approach to separately analyze the mechanisms behind radioresistance under anoxia as either hypoxia-inducible genes-dependent biological or genes-independent chemical mechanisms and found that the relative contributions in HeLa cells were 64.4 ± 9.06% and 35.6 ± 4.32%, respectively. In addition, HIF-1β alone, rather than heterodimeric HIF-1, HIF-2, or HIF-1β/AhR, was found to enhance the baseline radioresistance of cancer cells, irrespective of oxygen availability. RNA-sequencing-based screening further identified NRF2 and TXNRD1 as downstream effectors of HIF-1β underlying this baseline radioresistance. Taken together, these findings delineate the quantitative contributions of distinct mechanisms and identify HIF-1β-mediated signaling as a key determinant of baseline radioresistance, providing a framework for understanding how oxygen availability shapes tumor radioresistance and, in turn, a basis for developing strategies to overcome it.