Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) exhibits specific tumoricidal activity and is under development for cancer therapy. Mismatch-repair-deficient colonic tumors evade TRAIL-induced apoptosis through mutational inactivation of Bax, but chemotherapeutics including Camptosar (CPT-11) restore TRAIL sensitivity. However, the signaling pathways in restoring TRAIL sensitivity remain to be elucidated. Here, we imaged p53 transcriptional activity in Bax-1-carcinomas by using bioluminescence, in vivo, and find that p53 is required for sensitization to TRAIL by CPT-11. Small interfering RNAs directed at proapoptotic p53 targets reveal TRAIL receptor KILLER DR5 contributes significantly to TRAIL sensitization, whereas Bak plays a minor role. Caspase 8 inhibition protects both CPT-11 pretreated wild-type and Bax-1-HCT116 cells from TRAIL-induced apoptosis, whereas caspase 9 inhibition only rescued the wild-type HCT116 cells from death induced by TRAIL. The results suggest a conversion in the apoptoticmechanism in HCT116 colon carcinoma from a type II pathway involving Bax and the mitochondria to a type I pathway involving efficient extrinsic pathway caspase activation. In contrast to Bax-1-cells, Bak-deficient human cancers undergo apoptosis in response to TRAIL or CPT-11, implying that these proteins have nonoverlapping functions. Our studies elucidate a mechanism for restoration of TRAIL sensitivity in MMR-deficient Bax-1- human cancers through p53-dependent activation of KILLER DR5 and reconstitution of a type I death pathway. Efforts to identify agents that up-regulate DR5 may be useful in cancer therapies restoring TRAIL sensitivity.