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Research Articles: Therapeutics, Targets, and Development

Silibinin inhibits cytokine-induced signaling cascades and down-regulates inducible nitric oxide synthase in human lung carcinoma A549 cells

¹ Department of Pharmaceutical Sciences, School of Pharmacy, and ² University of Colorado Cancer Center, University of Colorado Denver, Denver, Colorado; and ³ Cancer Biology Laboratory, School of Life Sciences, Jawaharlal Nehru University, New Delhi, India

Requests for reprints: Rajesh Agarwal, Department of Pharmaceutical Sciences, School of Pharmacy, University of Colorado Denver, Box C238, East 4200 9th Street, Denver, CO 80262. Phone: 303-315-1381; Fax: 303-315-6281. E-mail: Rajesh.Agarwal{at}uchsc.edu

Abstract

Recently, we reported that silibinin inhibits primary lung tumor growth and progression in mice and down-regulates inducible nitric oxide synthase (iNOS) expression in tumors; however, the mechanisms of silibinin action are largely not understood. Also, the activation of signaling pathways inducing various transcription factors are associated with lung carcinogenesis and their inhibition could be an effective strategy to prevent and/or treat lung cancer. Herein, we used human lung epithelial carcinoma A549 cells to explore the potential mechanisms and observed strong iNOS expression by cytokine mixture (containing 100 units/mL IFN- + 0.5 ng/mL interleukin-1β + 10 ng/mL tumor necrosis factor-). We also examined the cytokine mixture–activated signaling cascades, which could potentially up-regulate iNOS expression, and then examined the effect of silibinin (50-200 µmol/L) on these signaling cascades. Silibinin treatment inhibited, albeit to different extent, the cytokine mixture–induced activation of signal transducer and activator of transcription 1 (Tyr⁷⁰¹), signal transducer and activator of transcription 3 (Tyr⁷⁰⁵), activator protein-1 family of transcription factors, and nuclear factor-B. The results for activator protein-1 were correlated with the decreased nuclear levels of phosphorylated c-Jun, c-Jun, JunB, JunD, phosphorylated c-Fos, and c-Fos. Further, silibinin also strongly decreased cytokine mixture–induced phosphorylation of extracellular signal-regulated kinase 1/2 but only marginally affected JNK1/2 phosphorylation. Silibinin treatment also decreased constitutive p38 phosphorylation in the presence or absence of cytokine mixture. Downstream of these pathways, silibinin strongly decreased cytokine mixture–induced expression of hypoxia-inducible factor-1 without any considerable effect on Akt activation. Cytokine mixture–induced iNOS expression was completely inhibited by silibinin. Overall, these results suggest that silibinin could target multiple cytokine-induced signaling pathways to down-regulate iNOS expression in lung cancer cells and that could contribute to its overall cancer preventive efficacy against lung tumorigenesis. [Mol Cancer Ther 2008;7(7):1817–26]

Grant support: National Cancer Institute grant RO1 CA113876.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 3/15/08; revised 5/ 1/08; accepted 5/17/08.