Rescue of Schizosaccharomyces pombe from camptothecin-mediated death by a DNA topoisomerase I inhibitor, TAN-1518 A

The topoisomerase I poison camptothecin generates a Chk1-dependent DNA damage checkpoint signal in fission yeast

The protein kinase Chk1 is essential for the DNA damage checkpoint. Cells lacking Chk1 are hypersensitive to DNA-damaging agents such as UV light and gamma-irradiation because they fail to arrest the cell cycle when DNA damage is generated. Phosphorylation of Chk1 occurs after DNA damage and is dependent on the integrity of the DNA damage checkpoint pathway. We have tested whether a topoisomerase I inhibitor, camptothecin (CPT), generates DNA damage in the fission yeast Schizosaccharomyces pombe that results in Chk1 phosphorylation. We demonstrate that Chk1 is phosphorylated in response to CPT treatment in a time- and dose-dependent manner and that phosphorylation is dependent on an intact DNA damage checkpoint pathway. Furthermore, we show that cells must be actively dividing in order for CPT to generate a Chk1-responsive DNA damage signal. This observation is consistent with a model whereby the cytotoxic event caused by CPT treatment is the production of a DNA double-strand break resulting from the collision of a DNA replication fork with a trapped CPT-topoisomerase I cleavable complex. Cells lacking Chk1 are hypersensitive to CPT treatment, suggesting that the DNA damage checkpoint pathway can be an important determinant for CPT sensitivity or resistance. Finally, as a well-characterized, soluble agent that specifically causes DNA damage, CPT will allow a biochemical analysis of the checkpoint pathway that responds to DNA damage.

Effect of inhibitors of DNA replication on early zebrafish embryos: evidence for coordinate activation of multiple intrinsic cell-cycle checkpoints at the mid-blastula transition

We address the developmental activation, in the zebrafish embryo, of intrinsic cell-cycle checkpoints which monitor the DNA replication process and progression through the cell cycle. Eukaryotic DNA replication is probably carried out by a multiprotein complex containing numerous enzymes and accessory factors that act in concert to effect processive DNA synthesis (Applegren, N. et al. (1995) J. Cell. Biochem. 59, 91-107). We have exposed early zebrafish embryos to three chemical agents which are predicted to specifically inhibit the DNA polymerase alpha, topoisomerase I and topoisomerase II components of the DNA replication complex. We present four findings: (1) Before mid-blastula transition (MBT) an inhibition of DNA synthesis does not block cells from attempting to proceed through mitosis, implying the lack of functional checkpoints. (2) After MBT, the embryo displays two distinct modes of intrinsic checkpoint operation. One mode is a rapid and complete stop of cell division, and the other is an 'adaptive' response in which the cell cycle continues to operate, perhaps in a 'repair' mode, to generate daughter nuclei with few visible defects. (3) The embryo does not display a maximal capability for the 'adaptive' response until several hours after MBT, which is consistent with a slow transcriptional control mechanism for checkpoint activation. (4) The slow activation of checkpoints at MBT provides a window of time during which inhibitors of DNA synthesis will induce cytogenetic lesions without killing the embryo. This could be useful in the design of a deletion-mutagenesis strategy.