Prostate Cancer (PCA) is the most common invasive malignancy and leading cause of deaths in males. Several studies have demonstrated the potential of flavonoids for the prevention and/or intervention of various cancers including PCA. One such naturally occurring flavonoid/flavonolignan is silibinin, and in several recent studies, we have shown its efficacy against PCA in both in vivo and in vitro model systems. In the present work, for the first time, we assessed the anticancer efficacy and associated molecular mechanisms of silibinin in advanced and androgen-independent human prostate carcinoma PC3 cells in comparison with silymarin, which along with silibinin includes small amounts of several other stereoisomers including isosilybins, dihydrosilybin, silydianin and silychristin. Treatment of cells with (50-100 μg/ml) of silibinin or silymarin inhibited cell proliferation, induced cell death and caused G1 or G2-M cell cycle arrest in a dose- and a time-dependent manner. Molecular studies showed that G1 arrest was associated with a decrease in cyclin D1, cyclin D3, CDK4 and CDK2 protein levels and CDK2 and CDK4 kinase activity; however, both silibinin as well as silymarin treatments resulted in upregulation of CDK inhibitors Kip1/p27 and Cip1/p21. The G2-M arrest by silibinin and silymarin; however, was associated with a decrease in the levels of cyclin B1, cyclin A, pCdc2 (Tyr15) and Cdc2, and an inhibition of Cdc2 kinase activity. Silibinin and silymarin also resulted in the decreased level of Cdc25B and Cdc25C phosphatases associated with G2-M arrest, and resulted in an increased phosphorylation of Cdc25C at Ser216 and its translocation from nucleus to the cytoplasm. Furthermore, both of these agents increased the phosphorylation at Thr68 and Ser19 sites of Chk2, an upstream kinase that might be responsible for the increased phosphorylation of Cdc25C at Ser216 site. Transient transfection of PC3 cells with Chk2-specific small interfering RNA duplexes significantly attenuated silibinin- and silymarin-induced G2-M arrest suggesting an important role of Chk2 kinase in G2-M arrest. Taken together, these findings indicate that silibinin- and silymarin-induced G1 arrest is mediated through decrease in the levels of cyclins-CDKs and upregulation of CDKIs, while G2-M arrest is through Chk2-Cdc25C-Cdc2/cyclin B1 pathway. Furthermore, since we observed either comparable or slightly better effects with silymarin compared to silibinin when used in equal doses by weight, these findings suggest that several stereoisomers of silibinin present in silymarin possibly contribute to the additional efficacy of silymarin. More studies, therefore, are needed in future to isolate silibinin isomers from silymarin and assess their anti-cancer efficacy.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]