Effect of PKK knockdown on survival of SUDHL-6 cells. (A) SUDHL-6 cells stably expressing shControl or shPKK-1 were cultured in the medium containing 10% serum. The live cells were measured by the MTT assay. The results were normalized to the absorbance readings at the time the cells were plated (0 hour). Mean results and SDs from 3 experiments are shown. (B) The SUDHL-6 cells stably expressing shControl or shPKK-1 were cultured in the medium containing the indicated concentrations of serum. The relative live cells were analyzed as described in panel A at the indicated times. (C) SUDHL-6 cells stably expressing shControl or sh-PKK-1 were cultured in the medium containing 0% or 10% serum for 24 hours. The annexin V–positive cells (dying or dead) were analyzed as described in “Cell viability and apoptosis assays.” The data represent the mean results of 3 independent experiments. (D) The shRNA-expressing SUDHL-6 cells were cultured in the medium containing 0% or 10% serum for 24 hours. The cleavage of PARP and caspase 3 was analyzed on Western blots. The analysis of IKKα was used as the equal loading control.