Mechanisms of Radiosensitizing Effect of Niclosamide in MCF-7 Mammospheres with Increased Number of Cancer Stem Cells
Α small subpopulation of cancer stem cells (CSCs) plays a fundamental role in the tumor process development and metastasis. The aim of the work was to study the molecular mechanisms of the radiosensitizing effect of niclozamide in human CSCs, obtained from breast adenocarcinoma line MCF-7. Cells cultivated as mammospheres were irradiated at a dose of 2 Gy (γ-radiation source <sup>60</sup>Co, dose rate 0.54 Gy/min) 1 hour after the addition of niclosamide (2 μmol/L). 7 days after radiation exposure, the number of mammospheres was counted using a light microscope. 24 hours after irradiation the activity of the Wnt signaling pathway was examined and the level of DNA double-strand breaks (DSB) was determined by the level of histone γH2AX. The maximum decrease in the number of mammospheres was detected after the combined action of niclozamide and γ-radiation. 24 hours after irradiation, the amount of the signal pathway Wnt proteins β-catenin and Dvl-2 increased by 8% and 38%, respectively. The combined effect of niclozamid and γ-radiation led to a decrease in the amount of β-catenin by 23%, and Dvl-2 by 30% as compared to the action of irradiation only. 24 hours after irradiation, the level of histone γH2AX was 10% higher than the control level, and at a combined action of niclozamide and γ-radiation – by 30%. Mechanisms of the radiosensitizing effect of niclozamide in MCF-7 CSCs are associated with inhibition of Wnt signaling cascade and the rate of DNA DR repair.