Activation of p53 enhances XIAP inhibition-induced cell death by promoting mitochondrial release of SMAC and by inducing expression of caspase-6. (A) OCI-AML3 cells were treated for 24 hours with various concentrations of nutlin-3a and levels of cytosolic and mitochondrial SMAC were determined by Western blot. Cells were also treated with various concentrations of 1396-11 + nutlin-3a with or without 2μM ABT-10 and cell death was assessed. (B) OCI-AML3 cells were treated with nutlin-3a for 24 hours and OCI-AML3vec and OCI-AML3p53shRNA cells were treated with 5μM nutlin-3a for various times. p53 and caspase-6 protein levels were determined by Western blot. (C) OCI-AML3 cells were treated with 5μM nutlin-3a for different time periods, as indicated. Caspase-6 mRNA levels were determined by real-time RT-PCR. (D) OCI-AML3 cells were treated with 1396-11, nutlin-3a, or both for 24 hours. p53 and caspase-6 protein levels were determined by Western blot.