ΔN-p73 down-regulation potentiates ATO-induced apoptosis in K562 and NB4 cells. (A) Transfection of ΔN-p73 siRNA, but not the unrelated GFP siRNA, led to a decrease in ΔN-p73 in K562 cell counts without affecting the levels of the unrelated protein actin; K562 cells were transfected with an expression vector encoding for an HA-tagged version of ΔNp73 alone or in the presence of siRNAΔNp73 or siRNAGFP. Cells were lysed 24 hours after transfection, and ΔN-p73 expression was assessed by anti-HA immunoblot. Sequences of the siRNA ΔNp73 are siRNA ΔNp73 sense 5′-CGUCGGUGACCCCGCACGGUU-3′ and siRNA ΔNp73 antisense 5′-CCGUGCGGGGUCACCGACGUU-3′. (B) Percentages of sub-G₁ apoptotic K562 and NB4 cells treated with ATO were significantly increased in cells transfected with ΔN-p73 siRNA relative to cells transfected with control siRNA (*P < .01; Dunnett test). K562 and NB4 cells were transfected with the indicated siRNAs and subsequently treated with ATO (2 μM for K562 and 1 μM for NB4) for 72 hours before apoptosis analysis. Values are mean ± SD of 4 independent experiments.