PARP1 cleavage correlates with apoptosis. (A) Western blot analysis showing cleavage of procaspases 3, 7, and 9 after 5 Gy IR to indicate apoptosis. Cleavage of full-length procaspases 7 and 9 and appearance of the largest cleavage product of caspase 3 are shown. Actin shows equal loading. r indicates relapse; *, delayed PARP1 cleavage. (i) Apoptosis-proficient ALL tumors showed complete cleavage of caspases 3, 7, and 9 after IR. (ii) Apoptosis-resistant ALL tumors showed absence of cleavage of caspases 3, 7, and 9 up to 24 hours after IR. (B) FACS analysis of Annexin V apoptosis assay indicating loss of plasma membrane and showing the percentage of cells undergoing IR-induced apoptosis at 0 and 8 hours after 5 Gy IR. Background apoptosis has been excluded. Data represent the average and SE from 3 apoptosis-proficient ALL tumors (ALL 92, ALL 90, and ALL 88; solid line) and 3 apoptosis-resistant ALL tumors (ALL 20, ALL 82, and ALL 85; broken line). (C) Annexin V apoptosis assay and FACS analysis showing the percentage of cells undergoing spontaneous apoptosis at 0, 8, and 24 hours. Data represent the average and SE from 3 apoptosis-proficient ALL tumors (ALL 92, ALL 90, and ALL 88; solid line) and 3 apoptosis-resistant ALL tumors (ALL 20, ALL 82, and ALL 85; broken line).