Measurement of caspase-3 activity in DN-treated A549 cells. (A) A549 cells were treated with 30 μM DN for 24, 48 and 72 h. (B) A549 cells were treated with 30 μM DN for 24 h or pretreated for 1 h with 25 μM z-VAD-fmk inhibitor prior to the addition of 30 μM DN and further incubation for 24 h. After these treatments, cell lysates were prepared, and enzymatic activities of caspase-3 were measured by the CaspACE™ colorimetric assay. Each bar graph represents the mean ± SD of at least three independent experiments. The caspase-3 activities in untreated cells are taken as 1-fold, and the figures on the chart display the relative changes of these activities in the treated cells. *Statistical significance (p <0.05) versus control at equal incubation periods.