K562 cells treated with 17 μmol/L etoposide for 1 hour undergo delayed apoptosis. (A and B) Electron micrographs of K562 cells before (A) and 48 hours after (B) a 1-hour treatment with 17 μmol/L etoposide. Bar, 2 μm. (C and D) Samples of HL-60 cells (lanes 1, 2) and K562 cells (lanes 3 through 8) were treated with 17 μmol/L etoposide for 0 hours (lanes 1 and 3) or 1 hour (lanes 2 and 4 through 8) and incubated in drug-free medium thereafter. At the indicated times, samples were obtained and subjected to SDS-PAGE followed by immunoblotting with monoclonal anti-PARP (C) or agarose gel electrophoresis (D). (E) Total cell number of K562 cells following a 1-hour exposure to 17 μmol/L etoposide and cell-cycle distribution of the resulting nonapoptotic cells. (Inset) DNA histograms obtained before and 1 day after a 1-hour exposure of K562 cells to 17 μmol/L etoposide. (F) Percentage of K562 cells that exhibit apoptotic morphology and trypan blue uptake after a 1-hour exposure to 17 μmol/L etoposide. (Inset) Photograph of K562 cells stained with 1 μg/mL Hoechst 33342 at the indicated time after a 1-hour etoposide treatment. Arrows, cells with chromatin condensation and nuclear fragmentation indicative of apoptosis. Each panel is representative of at least three experiments.