Carcinogenesis Advance Access published online on March 6, 2007
Carcinogenesis, doi:10.1093/carcin/bgm042
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Silibinin inhibits constitutive activation of Stat3, and causes caspase activation and apoptotic death of human prostate carcinoma DU145 cells
1 Department of Pharmaceutical Sciences, School of Pharmacy
2 University of Colorado Cancer Center, University of Colorado Health Sciences Center, Denver, CO 80262, USA
* To whom correspondence should be addressed: R. Agarwal, Department of Pharmaceutical Sciences, School of Pharmacy, University of Colorado Health Sciences Center, 4200 East Ninth Avenue, Box C238, Denver, CO 80262, USA; Phone: 303-315-1381, Fax: 303-315-6281, E-mail: Rajesh.Agarwal{at}uchsc.edu
Transcription factor Stat3 is activated constitutively in prostate cancer suggesting that its disruption could be an effective approach to control this malignancy. Here we assessed whether silibinin, a flavanone from Silybum marianum with proven anti-cancer efficacy in various cancer models, 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 concentration-dependently reduced constitutive Stat3 phosphorylation at Tyr705 and Ser727 residues under both serum and serum-starved conditions. Constitutively active Stat3-DNA binding was also inhibited concentration-dependently by silibinin; however, apoptotic death together with caspase and PARP cleavage was observed by silibinin only under serum-starved conditions suggesting that additional survival pathway(s) are active under serum conditions. In other studies, cells were treated with various specific pharmacological inhibitors where phosphorylation of Stat3 was not reduced by EGFR and MEK1/2 inhibitors suggesting lack of significant role(s) of these 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 showed that silibinin inhibits constitutively active Stat3 and induces apoptosis in DU145 cells, and thus might have potential significance in therapeutic intervention of this deadly malignancy.
Key Words: Chemoprevention • prostate cancer • silibinin • Stat3 • apoptosis
Received October 27, 2006; revised February 14, 2007; accepted February 16, 2007.