Hydrogen sulfide contributes to hypoxia-induced radioresistance on hepatoma cells

Growing evidence has demonstrated that, as an endogenous signaling gasotransmitter, hydrogen sulfide (H2S) plays an important role in regulating numerous biological functions. The role of H2S in hypoxia-induced radioresistance on hepatoma cells was investigated in the present work. Results showed that, when HepG2 cells were maintained in hypoxia circumstances for 4 h, the cellular radioresistance was extensively increased so that the oxygen enhancement ratio of the survival fraction approached 2.68. Under this hypoxic condition, when the cells were treated with DL-propargylglycine (PPG) and aminooxyacetic acid (AOAA), a specific inhibitor of H2S synthase of cystathionine-γ-lyase (CSE) and cystathionine-β-synthase (CBS) respectively, radiation responses including cell killing, micronuclei (MN) formation, and caspase-3 activity were significantly enhanced. However, treatment of cells with low concentrations of NaHS (≤100 μM) protected cells from these radiation damages. Western bolting assay showed that CSE and CBS were over-expressed in the irradiated hypoxic cells in a dose dependent manner. Moreover, when the hypoxic HepG2 cells were treated with NaHS together with glibenclamide, a specific inhibitor of K+ATP channels, the role of exogenous H2S in radioprotection was partly eliminated. This study demonstrated that H2S contributed to hypoxia-induced radioresistance probably via the opening of K+ATP channels, which suggests that the endogenous H2S synthase could be a potential radiotherapeutic target for a hypoxic tumor. (author)