Silibinin Suppresses Growth of Human Prostate Carcinoma PC-3 Orthotopic Xenograft via Activation of Extracellular Signal-Regulated Kinase 1/2 and Inhibition of Signal Transducers and Activators of Transcription Signaling

Purpose: Silibinin is currently under phase II clinical trial in prostate cancer patients; however, its antitumor effects and mechanisms are not completely understood. Herein, we studied the efficacy and associated mechanisms of silibinin against orthotopically growing advanced human prostate carcinoma PC-3 tumors. Experimental Design: Athymic male mice were orthotopically implanted with PC-3 cells in prostate and 1 week later after surgical recovery were gavaged daily with silibinin (100 mg/kg body weight) for 7 weeks. Results: Silibinin treatment reduced the lower urogenital weight (including tumor, prostate, and seminal vesicle) by 40% (P < 0.05) without any toxicity in mice. Silibinin decreased proliferating cell nuclear antigen expression and proliferating cells (P < 0.001) but increased cleaved caspase-3-positive cells (P < 0.01) and apoptotic cells (P < 0.001) and suppressed tumor microvessel density (P < 0.001) and vascular endothelial growth factor expression (P = 0.02). Decreased levels of cyclin-dependent kinases 2, 4, and 6, CDC2, and cyclins D1, D3, E, and A were observed, indicating an inhibitory effect of silibinin on cell cycle progression. Silibinin showed a tremendous increase in extracellular signal-regulated kinase 1/2 phosphorylation but decreased c-Jun NH2-terminal kinase 1/2 and p38 mitogen-activated protein kinase phosphorylation. A moderate decrease in phosphorylated and total levels of Akt was also noted. A marked inhibitory effect of silibinin on signal transducers and activators of transcription (STAT) 1 (Tyr(701)), STAT1 (Ser(727)), STAT3 (Tyr(705)), STAT3 (Ser(727)), and STAT5 (Tyr(794)) phosphorylation together with a decrease in their total levels was also observed. Conclusions: These findings provide evidence for antitumor efficacy of silibinin against orthotopically growing prostate tumor in mice with multitargeted mechanistic insights and support its clinical investigation in prostate cancer,