ERO1α primes the ryanodine receptor to respond to arsenite with concentration dependent Ca2+ release sequentially triggering two different mechanisms of ROS formation

Abstract

A 6 h exposure of U937 cells to 2.5 μM arsenite stimulates low Ca²⁺ release from the inositol 1, 4, 5-triphosphate receptor (IP₃R), causing a cascade of causally connected events, i.e., endoplasmic reticulum oxidoreductin-1α (ERO1α) expression, activation of the ryanodine receptor (RyR), mitochondrial Ca²⁺ accumulation, mitochondrial superoxide formation and further ERO1α expression. At greater arsenite concentrations, the release of the cation from the IP₃R and the ensuing ERO1α expression remained unchanged but were nevertheless critical to sequentially promote concentration-dependent increases in Ca²⁺ release from the RyR, NADPH oxidase activation and a third mechanism of ERO1α expression which, in analogy to the one driven by mitochondrial superoxide, was also mediated by reactive oxygen species (ROS) and devoid of effects on Ca²⁺ homeostasis. Thus, concentration-independent stimulation of Ca²⁺ release from the IP₃R is of pivotal importance for the effects of arsenite on Ca²⁺ homeostasis. It stimulates the expression of a fraction of ERO1α that primes the RyR to respond to the metalloid with concentration-dependent Ca²⁺-release, triggering the formation of superoxide in the mitochondrial respiratory chain and via NADPH oxidase activation. The resulting dose-dependent ROS formation was associated with a progressive increase in ERO1α expression, which however failed to affect Ca²⁺ homeostasis, thereby suggesting that ROS, unlike IP₃R-dependent Ca²⁺ release, promote ERO1α expression in sites distal from the RyR.

Keywords: Arsenite; Ca(2+) homeostasis; ERO1α; Mitochondrial superoxide; NADPH oxidase-derived superoxide; Ryanodine receptor.