Prostate cancer (PCA) is the most common malignancy in elderly men in the United States, and is the second leading cause of cancer related-deaths in men following lung cancer. Signal transducer and activator of transcription (Stat) 3 is a latent transcription factor required in cell proliferation, survival and tumorigenesis, and is activated constitutively in both prostate cancer tissue and cell lines such as DU145. Also noteworthy here is that activated Stat3 controls the expression of genes that are involved in virtually all aspects of malignancy, suggesting that inhibition of Stat3 activation should be an effective approach for PCA intervention. Silibinin, a flavanone isolated from milk thistle (Silybum marianum L.), is used as a dietary supplement for the betterment of the liver functions and clinically as an anti-hepatotoxic agent. More than a decade of research findings have also well-established the efficacy of silibinin as an effective anti-cancer and chemopreventive agent in various epithelial cancer models, and currently this phytochemical is in phase I/II clinical trial in PCA patients. Here we assessed whether silibinin inhibits Stat3 activation in DU145 cells, and if it does, what is the biological fate of the cells? At 50 μM or higher concentrations for 24 or 48 h, silibinin dose-dependently reduced constitutive Stat3 phosphorylation at Tyr705 and Ser727 sites under both serum and serum-starved conditions. Constitutively active Stat3-DNA binding was also inhibited dose-dependently by silibinin; however, apoptotic death together with caspase and PARP cleavage was observed by silibinin only in serum-starved cells suggesting that additional survival pathway/s are active under serum conditions. In other studies, cells were treated with different specific inhibitors where phosphorylation of Stat3 was not reduced by EGFR and MEK1/2 inhibitors suggesting lack of their roles in Stat3 activation in DU145 cells. JAK1 and JAK 2 inhibitors strongly reduced Stat3 phosphorylation, but did not result in apoptotic cell death. Interestingly, JAK1 inhibitor only in combination with silibinin resulted in a complete reduction in Stat3 phosphorylation at Tyr705, activated caspase-9 and caspase-3, and caused strong PARP cleavage and apoptotic death of DU145 cells. Given a critical role of Stat3 activation in prostate cancer, our results showing that silibinin inhibits constitutively active Stat3 and induces apoptosis in DU145 cells might have potential significance in therapeutic intervention of this deadly malignancy.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA