Calcium signals between the ryanodine receptor- and mitochondria critically regulate the effects of arsenite on mitochondrial superoxide formation and on the ensuing survival vs apoptotic signaling

A low concentration of arsenite (6 h), selectively stimulating the intraluminal crosstalk between the inositol-1, 4, 5-triphosphate receptor and the ryanodine receptor (RyR), increased the mitochondrial transport of RyR-derived Ca(2+) through the mitochondrial Ca(2+) uniporter. This event was characterized in intact and permeabilized cells, and was shown to be critical for mitochondrial superoxide (mitoO(2)(.-)) formation. Inhibition of mitochondrial Ca(2+) accumulation therefore prevented the effects of arsenite, in both the mitochondrial (e.g., cardiolipin oxidation) and extramitochondrial (e.g., DNA single- strand breakage) compartments, and suppressed the Nrf2/GSH survival signaling. The effects of arsenite on Ca(2+) homeostasis and mitoO(2)(.-) formation were reversible, as determined after an additional 10 h incubation in fresh culture medium and by measuring long-term viability. A 16 h continuous exposure to arsenite instead produced a sustained increase in the cytosolic and mitochondrial Ca(2+) concentrations, a further increased mitoO(2)(.-) formation and mitochondrial permeability transition. These events, followed by delayed apoptosis (48 h), were sensitive to treatments/manipulations preventing mitochondrial Ca(2+) accumulation. Interestingly, cells remained viable under conditions in which the deregulated Ca(2+) homeostasis was not accompanied by mitoO(2)(.-)formation. In conclusion, we report that the fraction of Ca(2+) taken up by the mitochondria in response to arsenite derives from the RyR. Mitochondrial Ca(2+) appears critical for mitoO(2)(.-) formation and for the triggering of both the cytoprotective and apoptotic signaling. The effects of arsenite were reversible, whereas its prolonged exposure caused a sustained increase in mitochondrial Ca(2+) and mitoO(2)(.-) formation, and the prevalence of the apoptotic vs survival signaling.