Mechanism of estrogen-mediated neuroprotection: Regulation of mitochondrial calcium and Bcl-2 expression

Estrogens are neuroprotective against glutamate excitotoxicity caused by an excessive rise in intracellular calcium ([Ca2+](i)). In this study, we demonstrate that 17beta-estradiol (E-2) treatment of hippocampal neurons attenuated the excitotoxic glutamate-induced rise in bulk-free [Ca2+](i) despite potentiating the influx of Ca2+ induced by glutamate. E-2-induced attenuation of bulk-free [Ca2+](i) depends on mitochondrial sequestration of Ca2+, which is blocked in the presence of the combination of rotenone and oligomycin or in the presence of antimycin, which collapse the mitochondrial membrane potential, thereby preventing mitochondrial Ca2+ transport. Release of mitochondrial Ca2+ by carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) after excitotoxic glutamate treatment resulted in a greater [Ca2+](i) in E-2-treated cells, indicating an E-2-induced increase in the mitochondrial calcium ([Ca2+](i)) load. The increased [Ca2+](i) load was accompanied by increased expression of Bcl-2, which can promote mitochondrial Ca2+ load tolerance. These findings provide a mechanism of E-2-induced neuronal survival by attenuation of excitotoxic glutamate [Ca2+](i) rise via increased mitochondrial sequestration of cytosolic Ca2+ coupled with an increase in Bcl-2 expression to sustain mitochondrial Ca2+ load tolerance and function.