Low Concentrations of Arsenite Target the Intraluminal Inositol 1, 4, 5-Trisphosphate Receptor/Ryanodine Receptor Crosstalk to Significantly Elevate Intracellular Ca2

Arsenite is an established human carcinogen inducing cyto- and genotoxic effects through poorly defined mechanisms involving the formation of reactive oxygen species (ROS) and deregulated Ca2+ homeostasis. We used variants of the U937 cell line to address the central issue of the mechanism whereby arsenite affects Ca2+ homeostasis. We found that a 6 h exposure to the metalloid (2.5 μM), while not associated to an immediate or delayed toxicity, causes a significant increase in the intracellular Ca2+ concentration ([Ca2+]i) through a mechanism characterized by the following components: i) it was not affected by ROS produced under the same conditions; ii) a small amount of Ca2+ was mobilized from the inositol-1,4,5-trisphosphate receptor (IP3R). This response was not further augmented by greater concentrations of the metalloid; iii) large amounts of Ca2+ were instead dose-dependently mobilized from the ryanodine receptor (RyR) in response to IP3R stimulation; iv) the cells maintained an intact responsiveness to agonist-stimulated Ca2+ mobilization from both channels; v) arsenite, even at 5-10 μM, failed to directly mobilize Ca2+ from the RyR; vi) arsenite failed to enhance Ca2+ release from the RyR under conditions in which the [Ca2+]i was increased by either RyR agonists or ionophore-stimulated Ca2+ uptake. We therefore conclude that arsenite elevates the [Ca2+]i by directly targeting the IP3R and its intraluminal crosstalk with the RyR. This mechanism likely mediates mitochondrial superoxide formation, downstream damage on various biomolecules, including genomic DNA, and mitochondrial dysfunction/apoptosis eventually occurring after longer incubation to, or exposure to greater concentrations of, arsenite.