Induction of mitochondrial manganese superoxide dismutase confers resistance to apoptosis in acute myeloblastic leukaemia cells exposed to etoposide

We investigated the possible roles of mitochondrial manganese superoxide di smutase (MnSOD) and bcl-2 in etoposide-induced cell death in acute myelobla stic leukaemia (AML) using two subclones of the OCI/AML-2 cell line, the et oposide-sensitive (ES) and the etoposide-resistant (ER), as models. Cell de ath after 24 h exposure to 10 mu mol/l etoposide was about 60% and 70% in t he ES subclone and about 20% and 25% in the ER subclone, when analysed by t rypan blue and annexin V respectively. Cytochrome c efflux from mitochondri a to cytosol was observed after 4 h of exposure in both subclones, whereas the activation of caspase-3 was not detectable until after 12 h of exposure in the ES subclone and 24 h of exposure in the ER subclone, using Western blotting. The decrease in mitochondrial membrane potential, when analysed b y the JC-1 probe fluorocytometrically, also appeared to take place later in the ER than in the ES subclone. Both subclones showed evident basal expres sion of MnSOD and bcl-2 by Western blotting. Etoposide caused a potent indu ction of MnSOD, more than 400% at 12 h, in the ER but not in the ES subclon e. No significant change in bcl-2 expression could be observed in either of the subclones during exposure to etoposide when analysed by Western blotti ng or flow cytometry. In conclusion, we suggest that MnSOD might have a spe cial role in the protection of AML cells against etoposide-induced cell dea th. Although unable to influence the cytochrome c efflux to cytosol, MnSOD might prevent the disruption of mitochondrial membrane potential, which evi dently leads to cell death by releasing various activators of apoptosis.