Cellular stress triggers the human topoisomerase I damage response independently of DNA damage in a p53 controlled manner

Topoisomerase I requirement for death receptor-induced apoptotic nuclear fission

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.

DNA cleavage assay for the identification of topoisomerase I inhibitors

The inhibition of DNA topoisomerase I (Top1) has proven to be a successful approach in the design of anticancer agents. However, despite the clinical successes of the camptothecin derivatives, a significant need for less toxic and more chemically stable Top1 inhibitors still persists. Here, we describe one of the most frequently used protocols to identify novel Top1 inhibitors. These methods use uniquely 3'-radiolabeled DNA substrates and denaturing polyacrylamide gel electrophoresis to provide evidence for the Top1-mediated DNA cleaving activity of potential Top1 inhibitors. These assays allow comparison of the effectiveness of different drugs in stabilizing the Top1-DNA intermediate or cleavage (cleavable) complex. A variation on these assays is also presented, which provides a suitable system for determining whether the inhibitor blocks the forward cleavage or religation reactions by measuring the reversibility of the drug-induced Top1-DNA cleavage complexes. This entire protocol can be completed in approximately 2 d.

Topoisomerase II and tubulin inhibitors both induce the formation of apoptotic topoisomerase I cleavage complexes

Topoisomerase I (Top1) is a ubiquitous enzyme that removes DNA supercoiling generated during transcription and replication. Top1 can be trapped on DNA as cleavage complexes by the anticancer drugs referred to as Top1 inhibitors as well as by alterations of the DNA structure. We reported recently that Top1 cleavage complexes (Top1cc) are trapped during apoptosis induced by arsenic trioxide and staurosporine. In the present study, we generalize the occurrence of apoptotic Top1cc in response to anticancer drugs, which by themselves do not directly interact with Top1: the topoisomerase II inhibitors etoposide, doxorubicin, and amsacrine, and the tubulin inhibitors vinblastine and Taxol. In all cases, the Top1cc form in the early phase of apoptosis and persist throughout the apoptotic process. Their formation is prevented by the caspase inhibitor benzyloxycarbonyl-Val-Ala-DL-Asp(OMe)-fluoromethylketone and the antioxidant N-acetyl-L-cysteine. We propose that the trapping of Top1cc is a general process of programmed cell death, which is caused by alterations of the DNA structure (oxidized bases and strand breaks) induced by caspases and reactive oxygen species.

Human topoisomerase I forms double cleavage complexes on natural DNA

DNA topoisomerase I releases torsional stress generated in chromatin during transcription and replication. Usually topoisomerase I is recognized to work as a monomer, but previously we have shown that two molecules can form a dimer-like protein-protein complex on a 'suicide' DNA substrate resulting in a topoisomerase I double cleavage complex. Here we show that during the normal relaxation reaction a considerable fraction of human topoisomerase I formed transient dimers on plasmid DNA too. Recombinant as well as topoisomerase I purified from human cells formed double cleavage complexes within a distance of 12 or 14 nucleotides. When topoisomerase I was isolated from camptothecin-treated HeLa cells, a considerable fraction migrated to the same position as topoisomerase I bearing a covalently bound 12-to-14-mer oligonucleotide. Taken together our data suggest that human topoisomerase I double cleavage complexes are part of the normal catalytic cycle of this enzyme that occur in vitro and possibly also in vivo.