Saikosaponin-D enhances radiosensitivity of hepatoma cells under hypoxic conditions by inhibiting hypoxia-inducible factor-1

Background: Our previous study revealed that the combination of Saikosaponin-d (SSd) and radiation is more effective in the treatment of liver cancer than the application of either of these monotherapeutic methods. However, the molecular mechanisms of the radiosensitizing effect of SSd on liver cancer remained ill defined. Methods: Cells were treated with different interventions; afterward, cell viability, apoptosis, and cell survival of SMMC-7721 and HepG2 hepatoma cells were examined. Xenograft tumor models were established by subcutaneously injecting SMMC-7721 cells. The molecular mechanism was assessed by western blot. Results: SSd dose-dependently increased radiosensitivity of hepatoma cells under hypoxic condition. The growth inhibitory effect of the combined treatment was correlated with cell apoptosis. Further mechanistic analysis indicated that SSd induced the upregulation of p53 and Bax as well as the downregulation of Bcl-2 by attenuating HIF-1 expression under hypoxic condition. These effects were enhanced when the HIF-1 inhibitor PX-478 was introduced. In vivo data also presented a more significant suppression of tumor xenograft growth from the combined therapy than from either of the monotherapeutic methods. Conclusions: Our study provides evidence for a radiosensitizing effect of SSd on hepatoma cells under hypoxic conditions by inhibiting HIF-1 expression. Thus, SSd can be used as a potential sensitizer in hepatoma radiotherapy.