Lithium Increases Radiosensitivity by Abrogating DNA Repair in Breast Cancer Spheroid Culture

Lithium chloride (LiCl), a drug for bipolar disorder, has antiproliferative and apoptotic effects on certain breast cancer cell lines. This study was conducted to determine the effect of LiCl on radiosensitivity in a human breast cancer cell line in monolayer culture and the more realistic tumor model, multicellular tumor spheroid. Monolayer and spheroid cells were treated with LiCl (20 mM) for 24 hours. The clonogenic assay was used to indicate changes in survival after x-ray radiation. The percentage of apoptotic cells was determined by acridine orange/ethidium bromide double staining. The amounts of DNA damage and repair after exposure to ionizing radiation were assessed by comet assay. Mre11 mRNA level was determined by RT-PCR. GSK-3β and β-catenin protein levels were measured by Western blotting. Treatment with LiCl decreased surviving fraction at 2, 3 and 6 Gy doses of x-ray (P < 0.01). The sensitizer enhancement ratio was higher in spheroids than monolayer culture. LiCl also decreased DNA repair (P < 0.05) and Mre11 mRNA level (P < 0.01) in T47D cells. These decreases were more prominent in spheroids than monolayer culture. Treatment of T47D cells with LiCl sensitized this breast cancer cell line to ionizing radiation in monolayer and especially in the tumor-like spheroid culture. This radiosensitization was attributed, in part, to decline in DNA repair. Decrease in Mre11 mRNA level upon LiCl treatment was suggested to be an important cause for the decreased DNA repair in T47D monolayer and spheroid cells.