The aim of the present study was to investigate the radiosensitizing effect of genistein, and the corresponding mechanisms of action on breast cancer cells with different estrogen receptor (ER) status. Human breast cancer cell lines such as MCF-7 (ER-positive, harboring wild-type p53) and MDA-MB-231 (ER-negative, harboring mutant p53) were irradiated with X-rays in the presence or absence of genistein. Cell survival, DNA damage and repair, cell cycle distribution, cell apoptosis, expression of proteins related to G₂/M cell cycle checkpoint and apoptosis were measured with colony formation assays, immunohistochemistry, flow cytometry and western blot analysis, respectively. Genistein showed relatively weak toxicity to both cell lines at concentrations in the range of 5–20 μM. Using the dosage of 10 μM genistein, the sensitizer enhancement ratios after exposure to X-rays at a 10% cell survival (IC₁₀) were 1.43 for MCF-7 and 1.36 for MDA-MB-231 cells, respectively. Significantly increased DNA damages, arrested cells at G₂/M phase, decreased homologous recombination repair protein Rad51 foci formation and enhanced apoptotic rates were observed in both cell lines treated by genistein combined with X-rays compared with the irradiation alone. The combined treatment obviously up-regulated the phosphorylation of ATM, Chk2, Cdc25c and Cdc2, leading to permanent G₂/M phase arrest, and up-regulated Bax and p73, down-regulated Bcl-2, finally induced mitochondria-mediated apoptosis in both cell lines. These results suggest that genistein induces G₂/M arrest by the activation of the ATM/Chk2/Cdc25C/Cdc2 checkpoint pathway and ultimately enhances the radiosensitivity of both ER+ and ER- breast cancer cells through a mitochondria-mediated apoptosis pathway.

Radioresistance remains a critical obstacle in the clinical management of glioblastoma (GBM) by radiotherapy. Therefore, it is necessary to explore the molecular mechanisms underlying radioresistance to improve patient response to radiotherapy and increase the treatment efficacy. The present study aimed to elucidate the role of specificity protein 1 (Sp1) in the radioresistance of GBM cells. Different human GBM cell lines and tumor-bearing mice were exposed to ionizing radiation (IR). Cell survival was determined by the colony formation assay. The expression of genes and proteins in the cells and tissues was analyzed by RT-PCR and western blotting, respectively. The γ-H2AX, p-Sp1 and dependent protein kinase catalytic subunit (DNA-PKcs phospho S2056) foci were analyzed by immunofluorescence. Apoptotic rates were measured by flow cytometry. Sp1 was upregulated after IR in vitro and in vivo and knocking down Sp1-sensitized GBM cells to IR. Sp1 activated the DNA-PKcs promoter and increased its expression and activity. Furthermore, the loss of Sp1 delayed double-strand breaks (DSB) repair and increased IR-induced apoptosis of GBM cells. Taken together, IR upregulates Sp1 expression in GBM cells, enhancing the activity of DNA-PKcs and promoting IR-induced DSB repair, thereby leading to increased radioresistance.

More than 70% of cancer patients receive radiotherapy during their treatment, with consequent various side effects on normal cells due to high ionizing radiation doses despite tumor shrinkage. To date, many radioprotectors and radiosensitizers have been investigated in preclinical studies, but their use has been hampered by the high toxicity to normal cells or poor tumor radiosensitization effects. Genistein is a naturally occurring isoflavone found in soy products. It selectively sensitizes tumor cells to radiation while protecting normal cells from radiation-induced damage, thus improving the efficacy of radiotherapy and consequent therapeutic outcomes while reducing adverse effects. Genistein protects normal cells by its potent antioxidant effect that reduces oxidative stress and mitigates radiation-induced apoptosis and inflammation. Conversely, genistein increases the radiosensitivity of tumor cells through specific mechanisms such as the inhibition of DNA repair, the arrest of the cell cycle in the G₂/M phase, the generation of reactive oxygen species (ROS), and the modulation of apoptosis. These effects increase the cytotoxicity of radiation. Preclinical studies demonstrated genistein efficacy in various cancer models, such as breast, prostate, and lung cancer. Despite limited clinical studies, the existing evidence supports the potential of genistein in improving the therapeutic effect of radiotherapy. Future research should focus on dosage optimization and administration, the exploration of combination therapies, and long-term clinical trials to establish genistein benefits in clinical settings. Hence, the unique ability of genistein to improve the radiosensitivity of tumor cells while protecting normal cells could be a promising strategy to improve the efficacy and safety of radiotherapy.