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 17β-estradiol (E₂) treatment of hippocampal neurons attenuated the excitotoxic glutamate-induced rise in bulk-free [Ca²⁺]_i despite potentiating the influx of Ca²⁺ induced by glutamate. E₂-induced attenuation of bulk-free [Ca²⁺]₁ 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 E2-treated cells, indicating an E₂-induced increase in the mitochondrial calcium ([Ca²⁺]_m) load. The increased [Ca²⁺]_m load was accompanied by increased expression of Bcl-2, which can promote mitochondrial Ca²⁺ load tolerance. These findings provide a mechanism of E₂-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.