Received for publication September 28, 2007.
Revised December 17, 2007.
Accepted for publication December 17, 2007.

Estrogen Receptor Independent Neuroprotection Via Protein Phosphatase Preservation and Attenuation of Persistent ERK1/2 Activation

Abstract

The mechanism of estrogen-mediated neuroprotection is not yet clear. Estrogens have a variety of modes of action including transducing signaling events such as activation and/or suppression of the MAPK pathway. We have previously shown protein phosphatases to be involved in 17-estradiol mediated neuroprotection. In the present study, we assessed the role of estrogen-receptors (ER) in estrogen-mediated neuroprotection from oxidative/excitotoxic stress and the consequential effects on MAPK signaling. Okadaic acid and calyculin A, non-specific serine/threonine phosphatase inhibitors, were exposed to cells at various concentrations in the presence or absence of 17-estradiol, the enantiomer of 17-estradiol, 2-(1-adamantyl)-3-hydroxyestra-1,3,5(10)-trien-17-one (ZYC3, non-ER binding estrogen analog) and/or glutamate. All three compounds, which we have shown have little or no binding to ER and ER, were protective against glutamate toxicity, but not against okadaic acid and calyculin A toxicity. Additionally, in the presence of effective concentrations of these inhibitors, the protective effects of these estrogen analogues were lost. Glutamate treatment caused a 50% decrease in levels of PP1, PP2A, and PP2B protein. Co-administration of ZYC3 with glutamate prevented the decreases in PP1, PP2A, and PP2B levels. Furthermore, glutamate treatment caused a persistent increase in phosphorylation of ERK1/2 that corresponds with the decrease protein levels of serine/threonine phosphatases. ZYC3 blocked this persistent increase in ERK phosphorylation. These results suggest that estrogens protect cells against glutamate-induced oxidative stress through an ER-independent mediated mechanism that serves to preserve phosphatase activity in the face of oxidative insults resulting in attenuation of the persistent phosphorylation of ERK associated with neuronal death.

Key words: ERK, estrogen receptor, estrogens, neuroprotection, neurotoxicity, serine/threonine phosphatase