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Research Articles: Therapeutics, Targets, and Development

The natural product honokiol preferentially inhibits cellular FLICE-inhibitory protein and augments death receptor–induced apoptosis

Departments of ¹ Hematology and Medical Oncology and ² Dermatology, Winship Cancer Institute, Emory University School of Medicine, Atlanta, Georgia

Requests for reprints: Shi-Yong Sun, Winship Cancer Institute, Emory University School of Medicine, 1365-C Clifton Road Northeast, C3088, Atlanta, GA 30322. Phone: 404-778-2170; Fax: 404-778-5520. E-mail: ssun{at}emory.edu

Abstract

Targeting death receptor–mediated apoptosis has emerged as an effective strategy for cancer therapy. However, certain types of cancer cells are intrinsically resistant to death receptor–mediated apoptosis. In an effort to identify agents that can sensitize cancer cells to death receptor–induced apoptosis, we have identified honokiol, a natural product with anticancer activity, as shown in various preclinical studies, as an effective sensitizer of death receptor–mediated apoptosis. Honokiol alone moderately inhibited the growth of human lung cancer cells; however, when combined with tumor necrosis factor–related apoptosis-inducing ligand (TRAIL), greater effects on decreasing cell survival and inducing apoptosis than TRAIL alone were observed, indicating that honokiol cooperates with TRAIL to enhance apoptosis. This was also true to Fas-induced apoptosis when combined with Fas ligand or an agonistic anti-Fas antibody. Among several apoptosis-associated proteins tested, cellular FLICE-inhibitory protein (c-FLIP) was the only one that was rapidly down-regulated by honokiol in all of the tested cell lines. The down-regulation of c-FLIP by honokiol could be prevented by the proteasome inhibitor MG132. Moreover, honokiol increased c-FLIP ubiquitination. These results indicate that honokiol down-regulates c-FLIP by facilitating its degradation through a ubiquitin/proteasome-mediated mechanism. Enforced expression of ectopic c-FLIP abolished the ability of honokiol to enhance TRAIL-induced apoptosis. Several honokiol derivatives, which exhibited more potent effects on down-regulation of c-FLIP than honokiol, showed better efficacy than honokiol in inhibiting the growth and enhancing TRAIL-induced apoptosis as well. Collectively, we conclude that c-FLIP down-regulation is a key event for honokiol to modulate the death receptor–induced apoptosis. [Mol Cancer Ther 2008;7(7):2212–23]

Grant support: Georgia Cancer Coalition Distinguished Cancer Scholar award (S-Y. Sun), Department of Defense grant W81XWH-04-1-0142-VITAL (S-Y. Sun for Project 4), NIH/National Cancer Institute Specialized Program of Research Excellence P50 grant CA128613-01 (S-Y. Sun for Project 2), NIH grant 5R01AR050727 (J.L. Arbiser), Jamie Rabinowitch-Davis Foundation and Minsk Foundation (J.L. Arbiser), and Veterans Affairs Merit award (J.L. Arbiser).

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.

Notes: S.M. Raja and S. Chen share equal first authorship. F.R. Khuri and S-Y. Sun are Georgia Cancer Coalition Distinguished Cancer Scholars. This work was part of S.M. Raja's honor thesis. S.M. Raja is currently a medical student at Jefferson Medical College (Philadelphia, PA). T.M. Acker is a Ph.D. graduate student in the Molecular and Systems Pharmacology Graduate Program at Emory University.

³ Supplementary material for this article is available at Molecular Cancer Therapeutics Online (http://mct.aacrjournals.org/).

Received 12/19/07; revised 3/20/08; accepted 4/15/08.