A. NCI-H929 cells were treated with piperlongumine (0.5, 1, 2, 4, or 6 μM) for 24 h, and a BrdU incorporation assay was then performed. Data are representative of three independent experiments. *P < 0.05, **P < 0.01, compared to control. B-C. NCI-H929 cells were treated with 4 μM piperlongumine for 12, 24, or 48 h; cyclins and CDK2 levels were then measured, and caspase activity was measured by colorimetric assay. D. NCI-H929 cells were treated with piperlongumine for 24 h, and cleaved caspase-3, caspase-9, and caspase-8 levels were measured. E. NCI-H929 cells were treated with 4 μM piperlongumine for 12 or 24 h and Bcl-2 and Bax levels were measured. Quantitative analysis was performed using Image J software, with normalization to GAPDH expression. F. NCI-H929 cells were treated with 1 or 2.5 μM piperlongumine for 12 h; CCCP was used as the positive control. Fluorescence was then measured by flow cytometry.