Effects of luteolin on the apoptosis of THP-1 cells with differential PTTG1 expression. (a) The undifferentiated or PMA-differentiated THP-1 cells were treated with vehicle or luteolin (25–100 μM) for 24 h, and then, the cell population in sub-G1 phase was determined by flow cytometric analysis. The experiments were triplicated, and the data are presented as the means ± SD from three independent experiments. ** p < 0.01 represents a significant difference compared to the vehicle group (veh). ## p < 0.01 represents a significant difference compared to the undifferentiated cells (Ud group). (b) Apoptotic marker proteins, including pro-caspase 3, cleaved caspase 3, PARP1 and cleaved PARP1, were analyzed by Western blot analysis. The experiments were triplicated, and a representative blot is shown. (c) The intensity of cleaved caspase 3 versus β-actin proteins is normalized and presented as the mean ± SD of triplicated experiments. (d) The intensity of cleaved PARP1 versus total PARP1 proteins is normalized and presented as the mean ± SD of triplicated experiments. ## p < 0.01 represents a significant difference compared to the undifferentiated THP-1 cells (THP-1 group).