The dual role of mitochondrial superoxide in arsenite toxicity: Signaling at the boundary between apoptotic commitment and cytoprotection

Arsenite toxicity is in numerous cellular systems dependent on the formation of reactive oxygen and or nitrogen species. This is also true in U937 cells in which the metalloid selectively promotes the formation of mitochondrial superoxide (mitoO2-) rapidly converted to diffusible H2O2. We tested the hypothesis that, under the same conditions, mitoO2-also mediates the triggering of a parallel survival signaling. We found that a low concentration of the metalloid causes an early activation of nuclear factor erythroid 2 p45-related factor 2 (Nrf2), and a downstream signaling leading to enhanced GSH biosynthesis, via a mechanism sensitive to various treatments/strategies selectively preventing mitoO2-formation. Under the same conditions, the toxic effects mediated by arsenite, leading to delayed mitochondrial permeability transition (MPT)-dependent apoptosis, were also prevented. Additional studies revealed remarkable similarities in the kinetics of mitoO2-formation, MPT induction, Nrf2 activation and GSH biosynthesis, prior to the onset of apoptosis in a small portion of the cells. Importantly, mitoO2-formation, as well as the ensuing toxic events, were significantly potentiated and anticipated under conditions associated with inhibition of de novo GSH biosynthesis triggered by the metalloid through Nrf2 activation. We conclude that, in the arsenite toxicity paradigm under investigation, mitoO2-represents the only trigger of two opposite pathways leading to activation of the Nrf2 signaling and/or to a MPT-dependent apoptotic death. The first pathway, through enhanced GSH biosynthesis, mitigates the extent of further mitoO2-formation, thereby limiting and delaying an otherwise rapid and massive apoptotic death.