We demonstrate here for the first time novel positive and negative effects of the FLICE-like inhibitory protein (FLIP) on human prostate cancer cell survival. A proteaosome inhibitor, MG132, mediated cell cycle arrest at G2/M and apoptosis through p38 activation. Interestingly, FLIP was stabilized by MG132 and interacted with Raf-1, resulting in enhancement of p38 signals and cytotoxicity. In contrast, overexpression of FLIP inhibited ubiquitylation and proteasomal degradation of beta-catenin, resulting in increase of the target gene cyclin D1, colony formation and invasive activity. Immunohistochemical analysis and in vitro experiments in primary culture showed FLIP to be overexpressed, statistically associated with expression of beta-catenin/cyclin D1 in metastatic cells, the FLIP/beta-catenin/cyclin D1 signals contributing to colony formation and invasion, which were canceled by FLIP knock down. In contrast, MG132-induced cytotoxicity including apoptosis was strongly inhibited by reduction of FLIP. Taken together, the results indicate that FLIP plays an important role in development of metastatic prostate cancer by inhibiting proteasomal degradation of beta-catenin, whereas it is mainly involved in proteasome inhibitior-mediated cell cycle arrest and apoptosis through activating the Raf-1/p38 pathway. Furthermore, proteasome inhibitors may be effective drugs for advanced prostate cancers overexpressing FLIP.