Protopine, a novel microtubule stabilizing agent, caused mitotic arrest and subsequent apoptosis in hormone-refractory prostate cancer cells.

B42 Prostate cancer is the second leading cause of cancer death in men. Because microtubule-stablizing agents, such as docetaxel which has been approved by the Food and Drug Administration for advanced prostate cancer, might have the therapeutic potential, we examined the anticancer effect of protopine in metastatic hormone-refractory prostate cancer PC-3 cells. Protopine is an isoquinoline alkaloid which is common in the plants of Papaveraceae , Fumariaceae and other families. Protopine exhibited anti-proliferative effect on PC-3 cells in a concentration-dependent manner with a GI 50 of 13.02 ± 1.70 μM as evaluated by sulforhodamine B assay. FACScan flow cytometric analysis revealed that protopine caused cell cycle accumulation at G2/M phase and subsequent at subG1 phase. Meanwhile, protopine activated cyclin-dependent kinase 1 (CDK1) through down-regulation of Tyr15 phosphorylation and up-regulation of Thr161 phosphorylation as well as induction of cyclin B1. By the immunofluorescence microscopy and tubulin polymerization assay, protopine concentration-dependently promoted tubulin polymerization. Furthermore, protopine induced massive apoptotic cell death as detected by TdT-mediated dUTP Nick-End Labeling (TUNEL) assay and oligonucleosomal DNA fragmentation. Protopine increased Bcl-2 and Mcl-1 phosphorylation and eventually caspase-3 activation following PARP cleavage. Overall, this is the first report to demonstrate that protopine stabilized microtubule organization leading to mitotic arrest and subsequent apoptosis in metastatic hormone-refractory prostate cancer PC-3 cell line.