Journal Article

Silibinin inhibits constitutive activation of Stat3, and causes caspase activation and apoptotic death of human prostate carcinoma DU145 cells

Chapla Agarwal, Alpna Tyagi, Manjinder Kaur and Rajesh Agarwal

in Carcinogenesis

Volume 28, issue 7, pages 1463-1470
Published in print July 2007 | ISSN: 0143-3334
Published online March 2007 | e-ISSN: 1460-2180 | DOI: http://dx.doi.org/10.1093/carcin/bgm042
Silibinin inhibits constitutive activation of Stat3, and causes caspase activation and apoptotic death of human prostate carcinoma DU145 cells

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Transcription factor signal transducer and activator of transcription (Stat)-3 is activated constitutively in prostate cancer (PCA) 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 anticancer 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 poly(ADP-ribose) polymerase (PARP) cleavage was observed by silibinin only under serum-starved conditions suggesting that additional survival pathways are active under serum conditions. In other studies, cells were treated with various specific pharmacological inhibitors where phosphorylation of Stat3 was not reduced by epidermal growth factor receptor and Mitogen activated protein/extracellular signal regulate kinase kinase (MEK1/2) inhibitors, suggesting lack of significant roles of these in Stat3 activation in DU145 cells. Janus kinase (JAK)-1 and JAK2 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 PCA, 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.

Journal Article.  6872 words.  Illustrated.

Subjects: Clinical Cytogenetics and Molecular Genetics

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