Role of Serine/Threon protein Phosphateases in Estrogen Mediated Neuroprotection
The Signaling pathways that mediate neurodegeneration are complex and involve a balance between phosphorylation and dephosphorylation of Signaling and Structural proteins. Estrogens have a variety of mode of action including transducing signaling events including the activation and/or supression of survival pathways. The purpose of this study was to delineate the role of protein phosphatases (PP) in estrogen neuroprotection. We assessed the role of PP in neuroprotection mediated by estrogen and its analogues. We also determined the role of estrogen receptors (ER) and MAPK signaling. Okadaic Acid (OA) and calyculin A (CA), non-specific serine/threonine PP inhibitors, were exposed to cells at various concentrations in the presence or absence of 17beta-estradiol, 17alpha-estradiol, the enantiomer of 17beta-estradiol (ENT E2), 2-(l-adamantyl)-3-hydroxyestra-1,3,5(10)-trien-17-one (ZYC3, non-ER binding estrogen analog) and/or glutamate. OA and CA caused a dose-dependent decrease in cell viability. None of the estrogen and its analogues showed protection against neurotoxic concentrations of either OA or CA, while all estrogens attenuated glutamate toxicity. However in the presence of these PP inhibitors, estrogen mediated protection against glutamate toxicity was lost. Glutamate treatment caused a 50% decrease in levels of PP1, PP2A and PP2B protein; while, co-administration of estrogen or its analogues with glutamate prevented the decrease in PP1, PP2A, and PP2B levels. In addition, PP2A and calcineurin activities were significantly suppressed with treatment of glutamate and/or OA; while the presence of these estrogens attenuated the decreases in PP activity. Moreover, an increase in reactive oxygen species, protein cabonylation, lipid peroxidation, caspase-3 activity, and mitochondrial dysfunction were evidence in both glutamate and OA mediated cell death. Estrogens attenuate these increases in glutamate-mediated cell death, but were ineffective in OA-induced neuronal death. Furthermore, gluatmate treatment caused a persistent increase in phosphorylation of ERK1/2 that corresponds with the decrease protein levels of PPs. Treatment of estrogens protect cells against glutamate-induced oxidative stress and excitotoxicity through an ER-independent mediated mechanism that serves to preserve phosphatase activity in the face of oxidative/excitotoxic insults resulting in attenuation of the persistent phosphorylation of ERK1/2 associated with neuronal death.