Estrogen receptor protein interaction with phosphatidylinositol 3-kinase leads to activation of phosphorylated Akt and extracellular signal-regulated kinase 1/2 in the same population of cortical neurons: A unified mechanism of estrogen action

17 beta-Estradiol (E-2)-induced neuroprotection is dependent on mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-kinase (PI3K) signaling cascades. We sought to determine whether E-2 neuroprotective mechanisms are mediated by a unified signaling cascade activated by estrogen receptor (ER)-PI3K interaction within the same population of neurons or whether E-2 activation of extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt are independent signaling events in different neuronal populations. Immunoprecipitation of E-2-treated cortical neurons was conducted to determine a protein-protein interaction between ER and the PI3K regulatory subunit p85. Subsequently, cortical neurons were treated with E-2 alone or in presence of MAPK inhibitors or PI3K inhibitors. Results of these analyses indicated a protein-protein interaction between ER and p85 that was time-dependent and consistent with the temporal profile for generation of Akt (pAkt) and ERK1/2 phosphorylation (pERK1/2). E-2-induced phosphorylation of Akt, was first apparent at 10 min and maximal at 30 min. Simultaneously, E-2-induced pERK1/2 was first apparent at 5-10 min and maximal at 30 min. Inhibition of PI3K completely blocked E-2 activation of pAkt at 10 and 30 min and blocked E-2 activation of ERK1/2 at 10 min, which revealed a PI3K-independent activation of ERK at 30 min. Double immunocytochemical labeling for pERK1/2 and pAkt demonstrated that E-2 induced both signaling pathways in the same neurons. These results indicate a unified signaling mechanism for rapid E-2 action that leads to the coordinated activation of both pERK1/2 and pAkt in the same population of neurons. Implications of these results for understanding estrogen mechanism of action in neurons and therapeutic development are considered.