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Topoisomerase I (Top1), an essential enzyme expressed throughout the cell cycle, relaxes DNA supercoiling by forming transient covalent DNA cleavage complexes. Top1 can be trapped by anticancer drugs that are referred to as “Top1 inhibitors” (camptothecins, indenoisoquinolines, indolocarbazoles) as well as by endogenous and exogenous DNA lesions (http://discover.nci.nih.gov/pommier/pommier.htm). We recently reported that Top1 cleavage complexes form during apoptosis induced by arsenic trioxide and staurosporine, and we referred to such complexes as “apoptotic Top1-DNA complexes” (Sordet et al. J Biol Chem 279: 33968-75, 2004a; Sordet et al. J Biol Chem 279: 50499-504, 2004b; Sordet et al. Cell Cycle 3: 1095-7, 2004c). In the present study, we generalize the occurrence of apoptotic Top1-DNA complexes to agents that induce apoptosis by different pathways, and, which by themselves to not act as Top1 inhibitors: the death receptor ligands Fas and TRAIL, the Top2 inhibitor etoposide, the tubulin poison vinblastine, and the Bcl-2 homology domain-3 (BH3) mimetics: antimycin and BH3I-2’. In all cases, the apoptotic Top1-DNA complexes occurred in the early phase of programmed cell death, and they persist throughout the apoptotic process. Their formation is prevented by the peptide caspase inhibitor benzyloxycarbonyl-VAD, indicating a requirement of caspase activation. The formation of TRAIL-induced Top1-DNA cleavage complexes is also inhibited by N-acetyl-L-cystein (NAC), which indicates that oxidative DNA lesions are likely involved in the formation of Top1-DNA complexes. Consistently, immunofluorescence microscopy shows that TRAIL induces the formation of 8-oxoguanine in nuclear DNA. Thus, formation of Top1-DNA complexe is an early and general process during apoptosis. Experiments using stable Top1siRNA cell lines (see abstract by Miao et al) and cell-free systems are ongoing to elucidate the functional role of the apoptotic Top1-DNA complexes.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]