Cancer Proteasome Inhibitor Bortezomib Increases Radiation Sensitivity in Androgen Independent Human Prostate Cancer Cells Serdar Goktas, Yusuf Baran, Ali U. Ural, Sertac Yazici, Emin Aydur, Seref Basal, Ferit Avcu, Aysel Pekel, Bahar Dirican, and Murat Beyzadeoglu 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 M 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 M, respectively. Exposure to 5 M 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 M 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. UROLOGY 75: 793–798, 2010. © 2010 Elsevier Inc. P rostate adenocarcinoma is the most common non- cutaneous malignancy diagnosed in males and the second leading cause of cancer death in North America. 1 Although clinically localized, prostate cancer can be treated effectively with surgery or radiation ther- apy, approximately 15% of patients present with locally advanced or metastatic disease, and recurrent disease develops after definitive local therapy in 40% of pa- tients. 2-4 Androgen ablation remains the main initial treatment of advanced prostate cancer and provides palliation of symptoms and survival benefit. 5 However, androgen-in- dependent cells are eventually selected during androgen deprivation therapy, and progression to an androgen- independent state remains the primary cause of mortality in these patients within an average of 1.5 years. 5 Data from randomized trials of chemotherapy suggest an im- provement in overall survival, pain relief, and quality of life with this form of therapy for hormone refractory prostate cancer. 6 Moreover, it is concluded, based on these data, that docetaxel should be considered for first- line treatment of metastatic hormone-refractory prostate cancer. 6 However, patients with hormone-refractory prostate cancer have not traditionally been offered che- motherapy as a routine treatment because of treatment- related toxicity and poor responses. Thus, there is a strong push to develop new compounds that interact with novel biological targets, either for use as single agents or more commonly in combination with front-line chemo- and radiotherapy or to modulate the response of estab- lished treatment regimens (i.e., restoring sensitivity to chemotherapy or radiotherapy). 7 The proteasomes play a central role in regulation of cell cycle, proliferation, apoptosis, angiogenesis, metasta- From the Department of Urology, Gulhane Military Medical School, Ankara, Turkey; Hematology Research Center, Gulhane Military Medical School, Ankara, Turkey; Department of Molecular Biology and Genetics, Izmir Institute of Technology, Izmir, Turkey; Department of Urology, Hacettepe University School of Medicine, Ankara, Turkey; and Departments of Immunology, and Radiation Oncology, Gulhane Military Medical School, Ankara Reprint requests: Sertac Yazici, M.D., Department of Urology, Gulhane Military Medical School, Etlik, Ankara, Turkey. E-mail: syazic@hacettepe.edu.tr Submitted: March 28, 2009, accepted (with revisions) © 2010 Elsevier Inc. 0090-4295/10/$34.00 793 All Rights Reserved doi:10.1016/j.urology.2009.07.1215