Double-ratiometric fluorescence imaging of H 2 Se and O 2 ˙ - under hypoxia for exploring Na 2 SeO 3 -induced HepG2 cells' apoptosis.

Sodium selenite (Na 2 SeO 3 ), as an anti-tumor drug for inducing tumor cells' apoptosis, has been widely studied under normoxic conditions and has been shown to exhibit oxidative stress process-induced apoptosis. However, since the real tumor environment is hypoxic, the actual mechanism is still unclear. Hence, considering the main metabolite of Na 2 SeO 3 in the metabolic process to be hydrogen selenide (H 2 Se) and also that it can be converted to superoxide anion (O 2 ˙ - ) instantaneously in the presence of oxygen, a dual-ratiometric fluorescence imaging system for simultaneous monitoring of the changes of H 2 Se and O 2 ˙ - induced by Na 2 SeO 3 -guided tumor cell apoptosis under hypoxic conditions was constructed. Two molecular probes NIR-H 2 Se and dihydroethidium were used to detect H 2 Se and O 2 ˙ - , respectively, whereas Rhodamine 110 was used as the reference fluorophore. Notably, H 2 Se levels significantly increased under hypoxic conditions, but there was no change in the level of O 2 ˙ - , which is inconsistent with the results of the previous researches. Therefore, we hypothesize that the mechanism of Na 2 SeO 3 -induced apoptosis for tumor cells is caused by reductive stress; also, this method can be applied for the future study of other anti-cancer selenium compounds.