Proteasome inhibitor bortezomib increases radiation sensitivity in androgen independent human prostate cancer cells.

ABSTRACT

OBJECTIVES: To investigate the effects of a strong proteasome inhibitor, bortezomib alone or in combination with radiotherapy on androgen-independent DU145 human prostate cancer cells. Proteasomes play important roles in cell cycle, proliferation, apoptosis, angiogenesis, and cellular resistance to chemotherapy and radiotherapy. METHODS: Increasing concentrations of bortezomib alone or in combination with radiation were applied to DU145 cells and IC(50) 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. RESULTS: The IC(50) value of bortezomib was found to be 28 microm although 400- and 800-cGy radiation decreased the cell proliferation by 14% and 28%, respectively. In 400- and 800-cGy radiation applied DU145 cells, IC(50) value of bortezomib decreased to 23- and 12 microm, respectively. Exposure to 5 microm 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 microm 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. CONCLUSIONS: Bortezomib increases radiation sensitivity in androgen-independent human DU145 prostate cancer cells through inhibition of Bcl-2 and induction of caspase-3 genes.