Caspase inhibition in camptothecin-treated U-937 cells is coupled with a shift from apoptosis to transient G(1) arrest followed by necrotic cell death

Leukemia U-937 cells rapidly undergo characteristic morphological changes, caspase activation, and DNA fragmentation typical of apoptosis on treatment with the DNA topoisomerase I inhibitor camptothecin (CPT), In a previous r eport (Sane, A, T., and Bertrand, R., Cancer Res,, 58: 3066-3072, 1998), we showed that, after CPT treatment, caspase inhibition by the tripeptide der ivative benzyloxycarbanyl-val-Ala-Asp(Ome)-fluoromethyl ketone (zVAD-fmk) b locked apoptosis and slowed passage of the cells through S-G(2) and caused a transient accumulation of these cells at the G(1) phase of the cell cycle . Accumulation of these cells at G(1) is not associated with major changes in expression level of cyclin-dependent kinase (cdk)2, cdk4, and cdk6; cycl in Hi and cyclin E:; or p16, p21, p27, and p57 after CPT treatment. Further more, cdk2, cdk4, and cdk6 kinase activities remain unaffected after CPT tr eatment. These results indicate that the G(1) arrest of these cells does no t correlate with a classical driven cell cycle checkpoint but with the know n effect of CPT in mediating inhibition of DNA replication and RNA transcri ption after stabilization of topoisomerase I-linked DNA strand breaks. Howe ver, persistent caspase inhibition after CPT treatment also results in cell s falling into necrosis after the transient G(1) arrest. These results indi cate that the enforced inhibition of caspase activities does not confer a s urvival advantage upon CPT-treated cells but is coupled with a shift from a poptosis to transient G(1) arrest followed by massive necrosis.