RelB-dependent differential radiosensitization effect of STI571 on prostate cancer cells

Radiation therapy is an effective treatment for localized prostate cancer. However, when high-risk factors are present, such as increased prostate-specific antigen, elevated Gleason scores and advanced T stage, undetected spreading of the cancer, and development of radiation-resistant cancer cells are concerns. Thus, additional therapeutic agents that can selectively sensitize advanced prostate cancer to radiation therapy are needed. Imatinib mesylate (Gleevec, STI571), a tyrosine kinase inhibitor, was evaluated for its potential to enhance the efficacy of ionizing radiation (IR) against aggressive prostate cancer cells. STI571 significantly enhances the IR-induced cytotoxicity of androgen-independent prostate cancer cells but not of androgen-responsive prostate cancer cells. The differential cytotoxic effects due to STI571 are associated with the nuclear level of RelB in prostate cancer cells. STI571 inhibits IR-induced RelB nuclear translocation, leading to increased radiosensitivity in aggressive androgen-independent PC-3 and DU-145 cells. In contrast, STI571 enhances RelB nuclear translocation in androgen-responsive LNCaP cells. The different effects of STI571 on RelB nuclear translocation are consistent with RelB DNA binding activity and related target gene expression. STI571 inhibits the phosphoinositide 3-kinase-AKT-IkappaB kinase-alpha pathway in PC-3 cells by decreasing the phosphorylation levels of phosphoinositide 3-kinase (Tyr458) and AKT (Ser473), whereas STI571 increases NF-kappaB inducible kinase (Thr559) phosphorylation, leading to activation of IkappaB kinase-alpha in LNCaP cells. These results reveal that STI571 exhibits differential effects on the upstream kinases leading to different downstream effects on the NF-kappaB alternative pathway in prostate cancer cells and suggest that STI571 is effective for the treatment of androgen-independent prostate cancer in the context of high constitutive levels of RelB. Mol Cancer Ther; 9(4); 803-12. (c)2010 AACR.