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Molecular and Cellular Biology, January 2007, p. 689-698, Vol. 27, No. 2
0270-7306/07/$08.00+0     doi:10.1128/MCB.01505-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

An Inhibitor of the Kinesin Spindle Protein Activates the Intrinsic Apoptotic Pathway Independently of p53 and De Novo Protein Synthesis ^,

Weikang Tao,^1* Victoria J. South,^1, Ronald E. Diehl,^1, Joseph P. Davide,¹ Laura Sepp-Lorenzino,¹ Mark E. Fraley,² Kenneth L. Arrington,² and Robert B. Lobell¹

Department of Cancer Research,¹ Department of Medicinal Chemistry, Merck Research Laboratories, West Point, Pennsylvania 19486²

Received 12 August 2006/ Returned for modification 8 September 2006/ Accepted 27 October 2006

The kinesin spindle protein (KSP), a microtubule motor protein, is essential for the formation of bipolar spindles during mitosis. Inhibition of KSP activates the spindle checkpoint and causes apoptosis. It was shown that prolonged inhibition of KSP activates Bax and caspase-3, which requires a competent spindle checkpoint and couples with mitotic slippage. Here we investigated how Bax is activated by KSP inhibition and the roles of Bax and p53 in KSP inhibitor-induced apoptosis. We demonstrate that small interfering RNA-mediated knockdown of Bax greatly attenuates KSP inhibitor-induced apoptosis and that Bax activation is upstream of caspase activation. This indicates that Bax mediates the lethality of KSP inhibitors and that KSP inhibition provokes apoptosis via the intrinsic apoptotic pathway where Bax activation is prior to caspase activation. Although the BH3-only protein Puma is induced after mitotic slippage, suppression of de novo protein synthesis that abrogates Puma induction does not block activation of Bax or caspase-3, indicating that Bax activation is triggered by a posttranslational event. Comparison of KSP inhibitor-induced apoptosis between matched cell lines containing either functional or deficient p53 reveals that inhibition of KSP induces apoptosis independently of p53 and that p53 is dispensable for spindle checkpoint function. Thus, KSP inhibitors should be active in p53-deficient tumors.

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* Corresponding author. Mailing address: Department of Cancer Research, Merck Research Laboratories, 770 Sumneytown Pike, WP26-462, West Point, PA 19486. Phone: (215) 652-1006. Fax: (215) 993-3398. E-mail: weikang_tao{at}merck.com.

Published ahead of print on 13 November 2006.

Supplemental material for this article may be found at http://mcb.asm.org/.

These authors contributed equally to this work.

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Molecular and Cellular Biology, January 2007, p. 689-698, Vol. 27, No. 2
0270-7306/07/$08.00+0     doi:10.1128/MCB.01505-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

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