Increasing concentrations of bortezomib alone or in combination with radiation were applied to DU145 cells and IC50 values that inhibited cell growth by 50 % were determined by 3-(4,5-dimethylthiazol-2-yl)-2,5 diphenyltetrazolium-bromide assay. Apoptosis was determined using annexin V staining by flow cytometry. mRNA levels of proapoptotic caspase-3 and antiapoptotic Bcl-2 genes were examined by reverse transcriptase polymerase chain reaction.
The IC50 value of bortezomib was found to be 28 μm although 400- and 800-cGy radiation decreased the cell proliferation by 14 % and 28 % , respectively. In 400- and 800-cGy radiation applied DU145 cells, IC50 value of bortezomib decreased to 23- and 12 μm, respectively. Exposure to 5 μm bortezomib for 48 hours caused apoptosis in 35 % of the population whereas 800-cGy radiation resulted apoptosis in 14 % of cells. However, 42 % of DU145 cells that were exposed to 800 cGy and 5 μm bortezomib underwent apoptosis. Reverse transcriptase polymerase chain reaction results showed a significant decrease in mRNA levels of antiapoptotic Bcl-2 gene and an increase in proapoptotic caspase-3 gene expression in the combination group compared to control group.
Bortezomib increases radiation sensitivity in androgen-independent human DU145 prostate cancer cells through inhibition of Bcl-2 and induction of caspase-3 genes.