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Abstract: Both arsenic trioxide (ATO) and bortezomib show separate antileukemic activity. With the purpose of evaluating whether the combination of ATO and bortezomib would be an option for patients with acute leukemia, we incubated HL60 leukemic cells with ATO alone and in combination with bortezomib. ATO and bortezomib cooperated to induce cell death and to inhibit proliferation and apoptosis in a synergistic way. The combined treatment resulted in a stronger activation of caspase 8 and 9, moderate activation of caspase 3, and increased expression of Fas and tumor necrosis factor–related apoptosis-inducing ligand (TRAIL)-DR5 receptors. When bortezomib was added, some proapoptotic genes (CARD9, TRAIL) were upregulated, and some antiapoptotic genes (BCL2, BCL3, FLICE) were downregulated. When coincubated, approximately 80% of cells showed altered mitochondrial membrane permeability. Moreover, ATO alone and in combination with bortezomib abrogated DNA-binding activity of nuclear factor kappa beta (NF-κB). Gene expression assays showed that more deregulated genes were related to proliferation of leukocytes, tumorigenesis, control of cell cycle, hypoxia and oxidative stress, cytokines, PI3K-AKT, ERK-MAPK, EGF pathways, and ubiquitination. Finally, in three cases of acute myeloid leukemia, the addition of bortezomib to ATO significantly increased cytotoxicity. We conclude that the combination of bortezomib and ATO may be efficacious in the treatment of myeloid disorders. [Copyright &y& Elsevier]