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

Arsenite is an established human carcinogen that induces cytotoxic and genotoxic effects through poorly defined mechanisms involving the formation of reactive oxygen species (ROS) and deregulated Ca homeostasis. We used variants of the U937 cell line to address the central issue of the mechanism whereby arsenite affects Ca homeostasis. We found that 6-hour exposure to the metalloid (2.5 M), although not associated with an immediate or delayed toxicity, causes a significant increase in the intracellular Ca concentration ([Ca]) through a mechanism characterized by the following components: 1) it was not affected by ROS produced under the same conditions; 2) a small amount of Ca was mobilized from the inositol-1,4,5-trisphosphate receptor (IPR), and this response was not augmented by greater concentrations of the metalloid; 3) large amounts of Ca were instead dose dependently mobilized from the ryanodine receptor (RyR) in response to IPR stimulation; 4) the cells maintained an intact responsiveness to agonist-stimulated Ca mobilization from both channels; 5) arsenite, even at 5-10 M, failed to directly mobilize Ca from the RyR; and 6) arsenite failed to enhance Ca release from the RyR under conditions in which the [Ca] was increased by either RyR agonists or ionophore-stimulated Ca uptake. We therefore conclude that arsenite elevates the [Ca] by directly targeting the IPR 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.