Role of Protein Phosphatases in Estrogen-Mediated Neuroprotection

Abstract

The signaling pathways that mediate neurodegeneration are complex and involve a balance between phosphorylation and dephosphorylation of signaling and structural proteins. We have shown previously that 17␤-estradiol and its analogs are potent neuroprotectants. The purpose of this study was to delineate the role of protein phosphatases (PPs) in estrogen neuroprotection against oxidative stress and excitotoxicity. HT-22 cells, C6-glioma cells, and primary rat cortical neurons were exposed to the nonspecific serine/threonine protein phosphatase inhibitors okadaic acid and calyculin A at various concentrations in the presence or absence of 17␤-estradiol and/or glutamate. Okadaic acid and calyculin A caused a dose-dependent decrease in cell viability in HT-22, C6-glioma, and primary rat cortical neurons. 17␤-Estradiol did not show protection against neurotoxic concentrations of either okadaic acid or calyculin A in these cells. In the absence of these serine/threonine protein phosphatase inhibitors, 17␤-estradiol attenuated glutamate toxicity. However, in the presence of effective concentrations of these protein phosphatase inhibitors, 17␤-estradiol protection against glutamate toxicity was lost. Furthermore, glutamate treatment in HT-22 cells and primary rat cortical neurons caused a 50% decrease in levels of PP1, PP2A, and PP2B protein, whereas coadministration of 17␤-estradiol with glutamate prevented the decrease in PP1, PP2A, and PP2B levels. These results suggest that 17␤-estradiol may protect cells against glutamate-induced oxidative stress and excitotoxicity by activating a combination of protein phosphatases.

Estrogen Receptor-Independent Neuroprotection via Protein Phosphatase Preservation and Attenuation of Persistent Extracellular Signal-Regulated Kinase 1/2 Activation

“…5C). The time course and magnitude of the response to ZYC3 was similar to that which we have reported for 17␤-estradiol (Yi et al, 2005). Similar trends were seen with PP2A and PP2B expression (data not shown).…”

“…Although PP2A has been shown to regulate ER␣ by mRNA stabilization (Keen et al, 2005) as well as direct interaction with ER␣ in the absence of estrogen (Lu et al, 2003), our data indicate that effects of estrogens on protein phosphatases occur without ER interactions. An interesting but yet unexplained observation is that although 17␤-estradiol (Yi et al, 2005) or ZYC3 (present study) alone have little effect on protein phosphatase levels, they cause a prompt increase in protein phosphatase concentrations and a persistent resistance to glutamate-induced decline in serine/threonine phosphatase levels. The absence of effects of estrogens alone and the rapidity of the response to estrogens in the face of insult suggest that estrogens either reduce or prevent the clearance of these serine/threonine phosphatases that are activated by oxida- In view of the relative lack of binding of these analogs to ER␣ or ER␤ (Perez et al, 2005), intracellular signaling pathways may be involved in mediating their neuroprotective effects.…”

“…Estrogens seem to protect neurons through the prevention of insult-induced decrease in phosphatase activity and the resulting neurotoxic, persistent hyperphosphorylation of proteins in multiple signaling pathways that are detrimental to cell survival. We have demonstrated that the neuroprotective effects of estrogen analogs against glutamate toxicity are opposed by phosphatase inhibition and that these analogs prevent insult-induced reduction in protein phosphatase levels as seen with 17␤-estradiol (Yi et al, 2005). These data demonstrate that the protective effects of estrogens against neurotoxicity induced by oxidative stress and excitotoxicity are receptor-independent and involve protein phosphatase activation through a signal transduction pathway mediated by MAPK (ERK1/2).…”

“…We have previously shown that serine/threonine phosphatases are involved in neuroprotective mechanism of 17␤-estradiol (Yi et al, 2005). We show that inhibition of serine/threonine protein phosphatases attenuates the neuroprotective effects of estrogen analogs that have little or no ER␣-or ER␤-binding properties and involves the MAPK signaling pathway.…”

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

“…In conclusion, we sought to determine whether a single mechanism could serve as a unifying mechanism for E 2 activation of Akt and MAPK, two signaling cascades demonstrated to be required for estrogen-inducible neuroprotection against degenerative insults (McEwen, 1992;Toran-Allerand et al, 1999;Brinton, 2001;Yi et al, 2005). Results of this investigation demonstrate that a protein-protein interaction between estrogen receptor and the regulatory subunit p85 of PI3K leads to activation of both the Akt and MAPK signaling pathways in cortical neurons.…”