Background and objective: Combined hypoxic cytotoxic drugs and chemoradiotherapy is an important mean of oncotherapy, and Tirapazamine (TPZ) is one of the most remarkable drugs. It has been shown that TPZ has a synergistic effect with radiotherapy on tumor cells, but whether TPZ would down-regulate the expression of the hypoxia-induced genes has not been reported. This study was to investigate the hypoxia-induced mRNA expressions of hypoxia inducible factor-1alpha (HIF-1alpha) and osteopontin (OPN) in human nasopharyngeal carcinoma HNE-1 and CNE-1 cells and the radiosensitization of TPZ, a hypoxia-specific drug, on HNE-1 and CNE-1 cells in vitro.
Methods: The IC50 values of TPZ for HNE-1 and CNE-1 cells were measured using MTT assay, and the mRNA expressions of HIF-1alpha and OPN in HNE-1 and CNE-1 cells was determined using RT-PCR under aerobic and hypoxic conditions, respectively. The survival rates of HNE-1 and CNE-1 cells treated with or without TPZ at IC10 in the presence or absence of oxygen for 6 h were determined using colony formation assay following exposure to 1-6 Gy of 60Co radiation. The dose-survival curves were plotted and the values of D0, Dq and SER were calculated as a single-hit multitarget model.
Results: The IC50 values of TPZ were 34.81 μmol/L and 35.02 μmol/L in HNE-1 and CNE-1 cells under aerobic condition, and 30.20 μmol/L and 28.48 μmol/L under hypoxic condition, respectively. The expressions of HIF-1alpha and OPN mRNA were reduced by TPZ in HNE-1 cells, but not in CNE-1 cells under hypoxic condition. For the HNE-1 cells, the respective values of D0 and Dq were 0.89 Gy and 0.28 Gy following normoxic irradiation versus 1.47 Gy and 0.44 Gy following hypoxic irradiation. For the CNE-1 cells, the respective values of D0 and Dq were 0.72 Gy and 0.68 Gy following normoxic irradiation versus 0.95 Gy and 0.56 Gy following hypoxic irradiation. The values of D0 and Dq for HNE-1 and CNE-1 cells treated with TPZ under hypoxic condition following irradiation were 0.66 Gy, 0.21 Gy and 0.85 Gy, 0.79 Gy, respectively.
Conclusion: TPZ can down-regulate hypoxia-induced expression of HIF-1alpha and OPN mRNA of HNE-1 cells and radiosensitize the HNE-1 cells but not CNE-1 cells, and act as a hypoxia modifier.