Small interfering RNA targeting HIF-1α reduces hypoxia-dependent transcription and radiosensitizes hypoxic HT 1080 human fibrosarcoma cells in vitro

Hypoxia inducible factor-1 has been identified as a potential target to overcome hypoxia-induced radioresistance The aim of the present study was to investigate whether selective HIF-1 inhibition via small interfering RNA (siRNA) targeting hypoxia-inducible factor 1 alpha (HIF-1 alpha) affects hypoxia-induced radioresistance in HT 1080 human fibrosarcoma cells. HIF-1 alpha expression in HT 1080 human fibrosarcoma cells in vitro was silenced using HIF-1 alpha siRNA sequence primers. Quantitative real-time polymerase chain reaction assay was performed to quantify the mRNA expression of HIF-1 alpha. HIF-1 alpha protein levels were studied by Western blotting at 20% (air) or after 12 hours at 0.1% O2 (hypoxia). Cells were assayed for clonogenic survival after irradiation with 2, 5, or 10 Gy, under normoxic or hypoxic conditions in the presence of HIF-1 alpha-targeted or control siRNA sequences. A modified oxygen enhancement ratio (OERA ') was calculated as the ratio of the doses to achieve the same survival at 0.1% O-2 as at ambient oxygen tensions. OERA ' was obtained at cell survival levels of 50%, 37%, and 10%. HIF-1 alpha-targeted siRNA enhanced radiation treatment efficacy under severely hypoxic conditions compared to tumor cells treated with scrambled control siRNA. OER was reduced on all survival levels after treatment with HIF-1 alpha-targeted siRNA, suggesting that inhibition of HIF-1 activation by using HIF-1 alpha-targeted siRNA increases radiosensitivity of hypoxic tumor cells in vitro. Inhibition of HIF-1 activation by using HIF-1 alpha-targeted siRNA clearly acts synergistically with radiotherapy and increase radiosensitivity of hypoxic cells in vitro.