Superoxide-dependent and -independent mitochondrial signaling during apoptosis in multiple myeloma cells

Superoxide (O ₂ ^-) radicals have been linked to apoptosis. Here, we show that 2-methoxyestradiol (2ME2)-induced apoptosis in multiple myeloma (MM) cells is associated with O ₂ ^- generation, whereas dexamethasone (Dex)-induced apoptosis occurs without concurrent increase in O ₂ ^-. In contrast, both these agents decrease mitochondrial transmembrane potential (Δψ _m). Treatment of MM cells with an antioxidant N-acetyl-L-cysteine blocks 2ME2, but not Dex-induced apoptosis as well as release of mitochondrial proteins cytochrome c (cyto c) and Smac. Taken together, these results demonstrate that there are at least two distinct apoptotic pathways: one dependent on O ₂ ^-, which is induced by 2ME2 and is associated with release of cyto c and Smac; and the other an independent of O ₂ ^-, which is triggered by Dex and associated with Smac release.