PARP1 inhibition using 5 mM 3-AB restored IR-induced apoptosis in apoptosis-resistant ALL tumors with increased NF-κB signaling by reducing the transcriptional activity of NF-κB. (A) Western blot analysis showing inhibition of PARP1 activity with 3-AB after 5 Gy IR. pADPr attached to the PARP1 protein (PARP1-pADPr) indicates PARP1 activity. Cleavage of procaspases 7 and 9 and the appearance of the largest cleavage product of caspase 3 was measured before and after IR and PARP1 inhibition. Actin shows equal loading. (i) Treatment with 3-AB and IR initiated procaspase cleavage in 3 apoptosis-resistant ALL tumors with abnormal NF-κB signaling. (ii) Treatment with 3-AB and IR failed to initiate procaspase cleavage in an apoptosis-resistant ALL tumor with defective p53 activation. (B) Adding 3-AB reduced the expression of the NF-κB–responsive protein cIAP1 and of TRAF6 but not of TRAF5, and it induced caspase 3 cleavage after 5 Gy IR in an apoptosis-resistant leukemia (ALL 82). Actin serves as a loading control. (C) EMSA showing the effect of 3-AB on NF-κB activation and binding to a 29-bp HIV-κB oligonucleotide (see “Materials and methods”) after 5 Gy IR. (i) Adding 3-AB abrogated NF-κB DNA binding 8 hours after IR in the nuclear protein. (ii) Adding 3-AB had no effect on the degradation of the NF-κB inhibitory protein IκBα in the cytoplasmic protein.