Studies have reported that soy isoflavones and vitamin E, respectively, inhibit growth in a number of carcinoma cell lines. The present study investigated possible radiosensitizing effects of a soy isoflavone concentrate (ISF) and vitamin E succinate (VES) on growth of the PC3 human prostate carcinoma cell line. PC-3 is androgen-insensitive with mutated p53 alleles. Methods: DNA synthesis was analyzed by 3H-thymidine incorporation. Clonogenic survival assays were conducted at 2 different cell concentrations in quadruplicate sets in cells treated with ISF and/or VES for 48 hr, and/or after irradiation. After incubation for 10 days, each flask was stained with gentian violet and colonies containing > 50 cells were counted. Survival was expressed as the plating efficiency (PE) = # of colonies/# of cells plated x 100%. Cell cycle regulatory proteins were measured by Western immunoblot, and interleukin-8 (IL-8) secretion by ELISA. Results: ISF and VES, respectively, inhibited 3H-thymidine incorporation in a dose-dependent manner with IC50 values for ISFs of 50 ug/ml and VE of 27uM. In a clonogenic survival assay, cells treated with IC50 doses of ISFs or VE for 48 hr showed a substantial decrease in survival without exposure to IR. Thus NS and VES, either alone or in combination, reduced the number of PC-3 cells able to undergo mitosis. PC-3 cells exposed to a single dose of ionizing radiation (IR) showed decreased survival from 17% for sham-irradiated cells to 2% at 4 Grey (Gy), and to less than 1% at 6Gy. Plating efficiency was further reduced by 50% from controls when cells were treated with NS or VES, either alone or in combination, for 48h prior to exposure to radiation. Analysis of cell cycle regulatory proteins further showed that IR at 2, 4 and 6 Gy upregulated cyclin dependent kinase-1 (cdk-1) phosphorylation at Tyr15, whereas total cdk1 protein levels were unchanged. IR increased total Bcl-2 by approx 30%, whereas p53 protein was not detected. Lastly, PC-3 cells secrete a high level of the pro-inflammatory cytokine and chemokine IL-8, which increased over 72 hr. ISF alone significantly decreased IL-8 secretion, whereas VES consistently increased IL-8 by 40-50% at all time points. IR did not induce secretion of IL-8, and IR in combination with ISF or VES did not alter their respective effects on IL-8 secretion. Conclusions: PC3 cells treated with IC50 doses of ISFs or VE alone showed a substantial decrease in survival, and these antioxidants further showed enhancement to radiation-induced clonogenic inhibition. (DOD Grant PC 030904)

[Proc Amer Assoc Cancer Res, Volume 46, 2005]