Elevated extracellular calcium can prevent apoptosis via the calcium-sensing receptor

Abstract

The calcium-sensing receptor (CaR) is a membrane-bound, G-protein-coupled receptor present on parathyroid cells which monitors the level of extracellular calcium (Ca(o)2+) and transduces signals involved in serum calcium regulation. Expression of CaR protein in tissues with functions unrelated to systemic calcium homeostasis, including the brain, suggests that extracellular calcium (Ca(o)2+) may act as a first messenger to regulate diverse cellular functions. To test this hypothesis, we examined the effect of increasing Ca(o)2+ on apoptosis induced by Sindbis Virus in AT-3 prostate carcinoma cells. We found a steep increase in cell survival with between 5 and 7 mM added Ca(o)2+ (EC50 = 6.1 mM). Magnesium, a less potent agonist of the calcium sensing receptor, was also protective (EC50 = 23.4 mM). Northern and immunocytochemical analyses confirmed the presence of the CaR message and protein in AT-3 prostate carcinoma cells. Enforced expression of CaR protein by stable transfection in human embryonic kidney (HEK)-293 cells, which normally don't express the receptor, resulted in resistance to SV-induced apoptosis in the presence of elevated Ca(o)2+. In addition to preventing SV-induced death, elevated Ca(o)2+ also abrogated apoptosis induced by c-Myc overexpression/serum deprivation in rat 1A fibroblasts, and these fibroblasts were shown to express CaR message and protein. Altogether, these observations suggest that Ca(o)2+ can act with the CaR to prevent apoptosis and define a novel mechanism by which calcium ions can regulate cell survival.