Estrogen Receptor-Independent Neuroprotection via Protein Phosphatase Preservation and Attenuation of Persistent Extracellular Signal-Regulated Kinase 1/2 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 mitogen-activated protein kinase (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 (ERs) in estrogen-mediated neuroprotection from oxidative/excitotoxic stress and the consequential effects on MAPK signaling. Okadaic acid and calyculin A, nonspecific 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 to have little or no binding to ER␣ and ER␤, were protective against glutamate toxicity but not against okadaic acid and calyculin A toxicity. In addition, in the presence of effective concentrations of these inhibitors, the protective effects of these estrogen analogs were lost. Glutamate treatment caused a 50% decrease in levels of protein phosphatase 1 (PP1), protein phosphatase 2A (PP2A), and protein phosphatase 2B (calcineurin) (PP2B). Coadministration of ZYC3 with glutamate prevented the decreases in PP1, PP2A, and PP2B levels. Furthermore, glutamate treatment caused a persistent increase in phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 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.

Structure–Nongenomic Neuroprotection Relationship of Estrogens and Estrogen‐Derived Compounds

“…Changes in the activity of these enzymes can regulate the phosphorylation of numerous cellular substrates such as intermediary signaling proteins Rsk, p38, JNK, and the nuclear transcriptional factors CREB and cfos/c-Jun, which may ultimately mediate cell survival changes (for a review, see [55]). However, ERK1/2 is rapidly and persistently phosphorylated in response to oxidative and excitotoxic stresses caused by glutamate and the presence of estrogens prevents this persistent phosphorylation (Figure 2.4); [56]. These data suggest that prolonged phosphorylation of ERK1/2 is detrimental to cell survival; this is supported by the observations by Smith and colleagues who noted aberrant neuronal expression of phosphorylated ERK1/2 and other MAPKs in AD brains in association with markers of oxidative stress [57].…”

“…Indeed, it has been shown that estrogens alone cause a rapid but transient increase in ERK1/2 phosphorylation [52,61]. In addition, it has also demonstrated that estrogens attenuate the persistent ERK1/2 phosphorylation caused by cytotoxic insults [56][57][58][59][60]. Glutamate-induced toxicity causes an increase in phosphorylation of ERK1/2 ([56]; Figure 2.4).…”

“…However, estrogens had no effect on okadaic acid-induced ERK1/2 phosphorylation, and the presence of this broad-spectrum protein phosphatase inhibitor abolished the estrogen-mediated reduction in phosphorylation ERK1/2 caused by glutamate (Figure 2.4). Phosphorylation of ERK1/2 in response to glutamate toxicity corresponds to decreases in serine/threonine phosphatase expression, and inhibitors of ERK1/2 phosphorylation attenuate the glutamate-induced cytotoxicity [56,62]; Figure 2.4). Therefore, one of the mechanisms of estrogen-mediated neuroprotection is by preservation of serine/threonine phosphatase protein expression and activity via an ER-independent pathway that ultimately prevents persistent ERK1/2 hyperphosphorylation as evidenced by the similar effects of the estrogen analogs as compared to βE2 [56,[62][63][64].…”

Characterization of Enantiomeric Bile Acid-induced Apoptosis in Colon Cancer Cell Lines

“…Other ent-steroids have been synthesized (58) and evaluated as probes of natural steroid interactions including ent-cholesterol (59,60), ent-progesterone (61), ent-estrogen (62), and enantiomeric neuroactive steroids (63,64). These reports illustrate that natural and enantiomeric steroids have distinctly different interactions with chiral environments, such as receptor-binding pockets.…”

Sex-Specific Role of Thioredoxin in Neuroprotection Against Iron-Induced Brain Injury Conferred by Estradiol

“…17 Estrogen can also provide neuroprotection through an ER-independent pathway. 20,21 However, the exact neuroprotective mechanism of E 2 under neuronal iron overload is still unclear. Surprisingly, the results from our present study showed that Trx induction by FC in the CN was significantly greater in male than female rats.…”