Chemopreventive agent sulforaphane enhances radiosensitivity in human tumor cells

Sulforaphane (SFN), an isothiocyanate derived from broccoli and other cruciferous vegetables, is a positive regulator of phase II detoxification enzymes and is highly effective in protection against chemically induced cancers by inducing apoptosis and cell cycle arrest. Here, we report that SFN also enhances radiosensitivity in human tumor cells. Cell survival in HeLa human cervix carcinoma cells pretreated with SFN was significantly lower than in cells treated with radiation only. Constant-field gel electrophoresis and a gamma-H2AX foci assay showed marked inhibition of DSB repair in irradiated cells treated with SFN, while little inhibition was observed in cells with DMSO (control). In addition, immunofluorescence experiments revealed a significant delay in Rad51 (a key protein for homologous recombination repair) foci formation and disappearance in irradiated cells treated with SFN when compared to the cells with X-irradiation alone. The dephosphorylation of DNA-PKcs (a critical nonhomologous end joining protein) was also markedly delayed by SFN pretreatment in irradiated cells. These DSB repair inhibition data partially support the high apoptotic frequency of irradiated cells pretreated with SFN. Furthermore, the combined treatment of X-rays and SFN (i.p. 300 micromol/kg) in the xenograft model with HeLa cells showed efficient inhibition of in vivo tumor growth. To the best of our knowledge, our study is the first report showing SFN-enhanced radiosensitivity of tumor cells in vitro and in vivo, which opens the door for a multitude of clinical applications for chemoradiotherapy using SFN.