The proteasome inhibitor, bortezomib, and the histone deacetylase inhibitor, depsipeptide (FK228), up-regulate tumor death receptors. Therefore, we investigated whether pretreatment of malignant cells with these agents would potentiate natural killer (NK)-mediated tumor killing. NK cells isolated from healthy donors and patients with cancer were expanded in vitro and then tested for cytotoxicity against tumor cell lines before and after exposure to bortezomib or depsipeptide. In 11 of 13 (85%) renal cell carcinoma cell lines and in 16 of 37 (43%) other cancer cell lines, exposure to these drugs significantly increased NK cell-mediated tumor lysis compared with untreated tumor controls (P < 0.001). Furthermore, NK cells expanded from patients with metastatic renal cell carcinoma were significantly more cytotoxic against autologous tumor cells when pretreated with either bortezomib or depsipeptide compared with untreated tumors. Tumors sensitized to NK cell cytotoxicity showed a significant increase in surface expression of DR5 [tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-R2; P < 0.05]; in contrast, surface expression of MHC class I, MIC-A/B, DR4 (TRAIL-R1), and Fas (CD95) did not change. The enhanced susceptibility to NK cell killing was completely abolished by blocking TRAIL on NK cells, and partially abolished by blocking DR5 on tumor cells. These findings show that drug-induced sensitization to TRAIL could be used as a novel strategy to potentiate the anticancer effects of adoptively infused NK cells in patients with cancer.