Potentiation of cytotoxicity of topoisomerase I poison by concurrent and sequential treatment with the checkpoint inhibitor UCN-01 involves disparate mechanisms resulting in either p53-independent clonogenic suppression or p53-dependent mitotic catastrophe

UCN-01 is a potent inhibitor of the S- and G2-M-phase cell cycle checkpoints by targeting chk1 and possibly chk2 kinases. It has been shown in some, but not all, instances that UCN-01 potentiates the cytotoxicity of DNA-damaging agents selectively in p53-defective cells. We have investigated this concept in HCT116 colon cancer cells treated with the topoisomerase I poison SN-38. SN-38 alone induced a senescence-like sustained G2 arrest without apoptosis. Sequential treatment with SN-38 followed by UCN-01 resulted in enhancement of cytotoxicity by apoptosis assay, whereas the reverse sequence or concurrent treatment did not potentiate apoptosis. Real-time visualization of HCT116 cells labeled with green fluorescent protein-histone 2B or green fluorescent protein-alpha-tubulin revealed that sequential treatment resulted in G2 checkpoint abrogation, and cells entered an aberrant mitosis despite normal assembly of bipolar spindles, resulting in either apoptosis or formation of micronucleated cells. Although p53-null cells were clearly more sensitive than parental HCT116 to undergoing checkpoint abrogation and mitotic death after sequential treatment, this was not accompanied by an increased inhibition of clonogenicity over that induced by SN-38 alone. Conversely, concurrent treatment with SN-38 and UCN-01 resulted in S-phase checkpoint override, an amplified DNA damage response including increased phosphorylation of the DNA double-strand breakage marker H2AX and augmentation of clonogenic inhibition, which was independent of p53. Thus, reported discrepancies in the pharmacology of UCN-01 and the influence of p53 status on treatment outcome appears to stem, in part, from the different schedules used, the specific checkpoints examined, and the assays used to assess cytotoxicity. Moreover, checkpoint abrogation and subsequent apoptosis induced by UCN-01 do not necessarily correlate with reproductive cell death.