Radio-sensitization effect of an mTOR inhibitor, temsirolimus, on lung adenocarcinoma A549 cells under normoxic and hypoxic conditions

The mammalian target of rapamycin (mTOR) correlates with cell survival under hypoxia and regulates hypoxia-inducible factor-1α (HIF-1α), a key protein in hypoxia-related events. However, the role of mTOR in radio-resistance has not been fully investigated. Therefore, the effect of mTOR on the radio-resistance of cancer cells under hypoxia was evaluated using the mTOR inhibitor temsirolimus. Clonogenic survival was examined in the A549 human lung adenocarcinoma cell line under normoxia or hypoxia, with or without temsirolimus. An oxygen enhancement ratio (OER) was calculated using the D10 values, the doses giving 10% survival. Western blotting was performed to investigate the effect of temsirolimus on mTOR and the HIF-1α pathway under normoxia and hypoxia. A549 cells showed a radio-resistance of 5.1 and 14.2 Gy, as indicated by D10 values under normoxia and hypoxia, respectively; the OER was 2.8. The cell survival rates under hypoxia and with temsirolimus remarkably decreased compared with those under normoxia. The D10 values of the cells under normoxia and hypoxia were 4.8 and 5.4 Gy, respectively (OER = 1.1). mTOR expression was suppressed by temsirolimus under both normoxia and hypoxia. HIF-1α expression decreased under hypoxia in the presence of temsirolimus. These results suggest that temsirolimus can overcome the radio-resistance induced by hypoxia. When the fact that mTOR acts upstream of HIF-1α is considered, our data suggest that the restoration of radiation sensitivity by temsirolimus under hypoxia may be associated with the suppression of the HIF-1α pathway. Temsirolimus could therefore be used as a hypoxic cell radio-sensitizer.