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Research Articles: Therapeutics, Targets, and Development

Plumbagin induces G₂-M arrest and autophagy by inhibiting the AKT/mammalian target of rapamycin pathway in breast cancer cells

¹ Cell Biology Laboratory, Department of Biotechnology and ² Department of Pharmacy, Chia-Nan University of Pharmacy and Science, Tainan, Taiwan; and ³ Graduate Institute of Natural Products, Kaohsiung Medical University, Kaohsiung, Taiwan

Requests for reprints: Po-Lin Kuo, Cell Biology Laboratory, Department of Biotechnology, Chia-Nan University of Pharmacy and Science, Tainan 717, Taiwan. Phone: 886-6-266-4911, ext. 520; Fax: 886-6-266-2135. E-mail: kuopolin{at}seed.net.tw

Abstract

This study is the first to investigate the anticancer effect of plumbagin in human breast cancer cells. Plumbagin exhibited cell proliferation inhibition by inducing cells to undergo G₂-M arrest and autophagic cell death. Blockade of the cell cycle was associated with increased p21/WAF1 expression and Chk2 activation, and reduced amounts of cyclin B1, cyclin A, Cdc2, and Cdc25C. Plumbagin also reduced Cdc2 function by increasing the association of p21/WAF1/Cdc2 complex and the levels of inactivated phospho-Cdc2 and phospho-Cdc25C by Chk2 activation. Plumbagin triggered autophagic cell death but not predominantly apoptosis. Pretreatment of cells with autophagy inhibitor bafilomycin suppressed plumbagin-mediated cell death. We also found that plumbagin inhibited survival signaling through the phosphatidylinositol 3-kinase/AKT signaling pathway by blocking the activation of AKT and downstream targets, including the mammalian target of rapamycin, forkhead transcription factors, and glycogen synthase kinase 3ß. Phosphorylation of both of mammalian target of rapamycin downstream targets, p70 ribosomal protein S6 kinase and 4E-BP1, was also diminished. Overexpression of AKT by AKT cDNA transfection decreased plumbagin-mediated autophagic cell death, whereas reduction of AKT expression by small interfering RNA potentiated the effect of plumbagin, supporting the inhibition of AKT being beneficial to autophagy. Furthermore, suppression of AKT by plumbagin enhanced the activation of Chk2, resulting in increased inactive phosphorylation of Cdc25C and Cdc2. Further investigation revealed that plumbagin inhibition of cell growth was also evident in a nude mouse model. Taken together, these results imply a critical role for AKT inhibition in plumbagin-induced G₂-M arrest and autophagy of human breast cancer cells. [Mol Cancer Ther 2006;5(12):3209–21]

Grant support: National Science Council of Taiwan grant NSC 94-2320-B-041-007.

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 8/ 9/06; revised 9/20/06; accepted 10/20/06.

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