['PS-341 (bortezomib, Velcade), the first proteasome inhibitor approved by the Food and Drug Administration for the treatment of patients with relapsed multiple myeloma, induces apoptosis in human cancer cell lines. Vitamin C (ascorbic acid) is an essential water-soluble vitamin required for many normal physiologic functions and has to be obtained through diet or supplemental tablets in humans. Here we studied the potential effect of vitamin C on the anticancer activity of PS-341 in human cancer cell lines.', 'The effects of vitamin C on apoptosis induction by PS-341 alone and by PS-341 combined with tumor necrosis factor-related apoptosis-inducing ligand were studied. In addition, the effects of vitamin C and other antioxidants on PS-341-mediated proteasome inhibition were also examined. Finally, the direct chemical interaction between vitamin C and PS-341 was determined.', 'Vitamin C abrogated the ability of PS-341 to induce apoptosis in various human cancer cell lines, to induce G(2)-M arrest, and to augment apoptosis induced by tumor necrosis factor-related apoptosis-inducing ligand. Moreover, vitamin C suppressed PS-341-mediated inhibition of proteasome activity. PS-341 itself did not induce generation of intracellular reactive oxygen species whereas other antioxidants failed to abrogate its biological activity. Importantly, we detected a direct chemical interaction between vitamin C and PS-341.'] Does vitamin C inactivate the proteasome inhibitor PS-341 in human cancer cells?