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J. Biol. Chem., Vol. 283, Issue 34, 23200-23208, August 22, 2008

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Topoisomerase I Requirement for Death Receptor-induced Apoptotic Nuclear Fission^*

From the Laboratory of Molecular Pharmacology, Center for Cancer Research, NCI, National Institutes of Health, Bethesda, Maryland 20892-4255

Topoisomerase I (Top1) is known to relax DNA supercoiling generated by transcription, replication, and chromatin remodeling. However, it can be trapped on DNA as cleavage complexes (Top1cc) by oxidative and carcinogenic DNA lesions, base damage, and camptothecin treatment. We show here that Top1 is also functionally involved in death receptor-induced programmed cell death. In cells exposed to TRAIL or Fas ligand, Top1cc form at the onset of apoptosis. Those apoptotic Top1cc are prevented by caspase inhibition and Bax inactivation, indicating that both caspases and the mitochondrial death pathway are required for their formation. Accordingly, direct activation of the mitochondrial pathway by BH3 mimetic molecules induces apoptotic Top1cc. We also show that TRAIL-induced apoptotic Top1cc are preferentially formed by caspase-3-cleaved Top1 at sites of oxidative DNA lesions with an average of one apoptotic Top1cc/100 kbp. Examination of Top1 knock-down cells treated with TRAIL revealed similar DNA fragmentation but a marked decrease in apoptotic nuclear fission with reduced formation of nuclear bodies. Thus, we propose that Top1 contributes to the full apoptotic responses induced by TRAIL.

Received for publication, February 12, 2008 , and in revised form, June 12, 2008.

^* This work was supported, in whole or in part, by a National Institutes of Health grant from the National Cancer Institute, Center for Cancer Research Intramural Research Program. 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.

¹ To whom correspondence should be addressed: NIH, Bldg. 37, Rm. 5068, Bethesda, MD 20892-4255. Tel.: 301-496-5944; Fax: 301-402-0752; E-mail: pommier{at}nih.gov.

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