Ascorbic acid enhances arsenic trioxide-induced cytotoxicity in multiple myeloma cells

Multiple myeloma (MM) is a clonal B-cell malignancy characterized by slow-growing plasma cells in the bone marrow (BM). Patients with MM typically respond to initial chemotherapies; however, essentially all progress to a chemoresistant state. Factors that contribute to the chemorefractory phenotype include modulation of free radical scavenging, increased expression of drug efflux pumps, and changes in gene expression that allow escape from apoptotic signaling. Recent data indicate that arsenic trioxide (As ₂O ₃) induces remission of refractory acute promyelocytic leukemia and apoptosis of cell lines overexpressing Bcl-2 family members; therefore, it was hypothesized that chemorefractory MM cells would be sensitive to AS ₂O ₃. AS ₂O ₃ induced apoptosis in 4 human MM cell lines: 8226/S, 8226/Dox40, U266, and U266/Bcl-x _L. The addition of interleukin-6 had no effect on cell death. Glutathione (GSH) has been implicated as an inhibitor of As ₂O ₃-induced cell death either through conjugating As ₂O ₃ or by sequestering reactive oxygen induced by As ₂O ₃. Consistent with this possibility, increasing GSH levels with N-acetylcysteine attenuated As ₂O ₃ cytotoxicity. Decreases in GSH have been associated with ascorbic acid (AA) metabolism. Clinically relevant doses of AA decreased GSH levels and potentiated As ₂O ₃-mediated cell death of all 4 MM cell lines. Similar results were obtained in freshly isolated human MM cells. In contrast, normal BM cells displayed little sensitivity to As ₂O ₃ alone or in combination with AA. Together, these data suggest that As ₂O ₃ and AA may be effective antineoplastic agents in refractory MM and that AA might be a useful adjuvant in GSH-sensitive therapies.