Figure 3: Shikonin induces cell cycle arrest and apoptosis in U937 cells. (a) Typical DNA content histograms of U937 cells treated with increasing concentrations of shikonin for 24 h. (b) Statistical analysis of cell cycle distribution of U937 cells after treatment with different concentrations of shikonin for 24 h. Data points are means of at least three independent experiments. (c) Enzymatic activity of caspase 3/7 and caspase 9 after 6 h of shikonin treatment in U937 cells. The caspase activity (mean ± SD of at least three experiments) is expressed as percentage relative to the untreated control (*significant difference according to Student’s -test, ).