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

Abstract

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 (As₂O₃) 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 As₂O₃. As₂O₃ 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 As₂O₃-induced cell death either through conjugating As₂O₃ or by sequestering reactive oxygen induced by As₂O₃. Consistent with this possibility, increasing GSH levels with N-acetylcysteine attenuated As₂O₃ cytotoxicity. Decreases in GSH have been associated with ascorbic acid (AA) metabolism. Clinically relevant doses of AA decreased GSH levels and potentiated As₂O₃-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 As₂O₃ alone or in combination with AA. Together, these data suggest that As₂O₃ and AA may be effective antineoplastic agents in refractory MM and that AA might be a useful adjuvant in GSH-sensitive therapies.