Estradiol protects against injury-induced cell death in cortical explant cultures: a role for estrogen receptors

Estrogen's sex-specific effects on ischemic cell death and estrogen receptor mRNA expression in rat cortical organotypic explants

Estrogens, such as the biologically active 17-β estradiol (E2), regulate not only reproductive behaviors in adults, but also influence neurodevelopment and neuroprotection in both females and males. E2, contingent upon the timing and concentration of the therapy, is neuroprotective in female and male rodent models of stroke. In Vivo studies suggest that E2 may partially mediate this neuroprotection, particularly in the cortex, via ERα. In Vitro studies, utilizing a chemically induced ischemic injury in cortical explants from both sexes, suggest that ERα or ERβ signaling is needed to mediate the E2 protection. Since we know that the timing and concentration of E2 therapy may be sex-specific, we examined if E2 (1 nM) mediates neuroprotection when female and male cortical explants are separately isolated from postnatal day (PND) 3-4 rat. Changes in basal levels ERα, ERβ, and AR mRNA expression are compared across early post-natal development in the intact cortex and the corresponding days in vitro (DIV) for cortical explants. Following ischemic injury at 7 DIV, cell death and ERα, ERβ and AR mRNA expression was compared in female and male cortical explants. We provide evidence that E2-mediated protection is maintained in isolated cortical explants from females, but not male rats. In female cortical explants, the E2-mediated protection at 24 h occurs secondarily to a blunted transient increase in ERα mRNA at 12 h. These results suggest that cortical E2-mediated protection is influenced by sex and supports data to differentially treat females and males following ischemic injury.

Understanding the effects of opioids vs non-opioids in the treatment of neonatal abstinence syndrome, an in vitro model

Neonatal abstinence syndrome (NAS) refers to cadre of withdrawal manifestations in infants born to mothers who used illicit and licit substances during pregnancy. The increasing prevalence of NAS has been largely due to the maternal use of opioids during pregnancy. NAS contributes to increased morbidity and long-term disability in surviving infants. Clinically, oral opioid therapies for opioid exposure have been a standard treatment with morphine (MO) being the most commonly used medication. Recently, a non-opioid agent, clonidine (CD) has also been used with potentially favorable short- and long-term outcomes in infants. However, data regarding the cellular and molecular effects of these treatments on the developing brain is still lacking due to a lack of a reliable animal model that targets the neonatal brain. To address this gap in knowledge we determined the effects of MO or CD on the cell death of neonatal cortical explant cultures that were exposed to oxycodone (OXY) in utero. Sprague Dawley rats were randomized and implanted with programmable infusion pumps before mating to receive either the OXY (dose increasing from 1.21-1.90 mg/kg/day to a maximum dose of 2.86-3.49 mg/kg/day) or normal saline (NS) throughout pregnancy and until one week after delivery. Male and female rat pups were sacrificed on postnatal day 4, and the prefrontal cortex (PFC) and hippocampus (HC) were dissected and treated with MO (0.10-1.00 µM) or CD (1.20-120.00 µM) in culture media. After 5 days of treatment the explants were labeled with propidium iodide to detect cell death. Dead cells were analyzed and counted under fluorescence microscopy. In explants from the PFC, cell death was greater in those prenatally exposed to OXY and postnatally treated with MO (OXY/MO) (736.8 ± 76.5) compared to OXY/CD (620.9 ± 75.0; p = 0.005). In the HC explants, mean cell death counts were not significantly different between groups regardless of prenatal exposure or postnatal treatment (p = 0.19). The PFC is vital in controlling higher-order executive functions such as behavioral flexibility, learning and working memory. Therefore, our finding is consistent with executive function problems in children with prenatal opioid exposure.

Modification of the toxic effects of methylmercury and thimerosal by testosterone and estradiol in SH-SY5Y neuroblastoma cell line

Short-chained alkyl mercury compounds accumulate in particularly in the brain. Exposure to these compounds is associated with various neurotoxic effects. Gender-based differences are observed in neurodevelopmental disorders, and testosterone and estradiol may alter the toxic effect of the compounds. The present study aimed to investigate the toxic effects of methylmercury and thimerosal on SH-SY5Y cells in high testosterone/low estradiol and high estradiol/low testosterone containing cellular environment and estimate whether male and female brains react differently to the toxic effects of methylmercury and thimerosal. Study groups (n = 3) were designed as control: growth medium, thimerosal (T): 1.15-μM thimerosal, methylmercury (M): 2.93-μM methylmercury, high testosterone/low estradiol + thimerosal (TT): 1-μM testosterone + 0.75-μM estradiol + 1.15-μM thimerosal, high estradiol/low testosterone + thimerosal (ET): 0.1-μM testosterone + 7.5-μM estradiol + 1.15-μM thimerosal, high testosterone/low estradiol + methylmercury (TM): 1-μM testosterone + 0.75-μM estradiol + 2.93-μM methylmercury and high estradiol/low testosterone + methylmercury (EM): 0.1-μM testosterone + 7.5-μM estradiol + 2.93-μM methylmercury. While a significant decrease in glutathione levels was observed in M group, it was not seen in EM group. A significant increase in the protein carbonyl levels was detected in T group. A similar increase was observed in the TM and TT groups in which testosterone was dominant. It was determined that methylmercury, but not thimerosal, caused significant DNA damage and in TT group. The results showed that both thimerosal and methylmercury are toxic on SH-SY5Y cells and toxic effects of methylmercury are more severe than thimerosal. It has been determined that testosterone and estradiol alter the toxic effects of thimerosal and methylmercury.

Cyanide Poisoning Compromises Gene Pathways Modulating Cardiac Injury in Vivo

Smoke inhalation from a structure fire is a common route of cyanide poisoning in the U.S. Cyanide inhibits cellular respiration, often leading to death. Its rapid distribution throughout the body can result in injuries to multiple organs, and cyanide victims were reported to experience myocardial infarction and other cardiac complications. However, molecular mechanisms of such complications are yet to be elucidated. While FDA-approved CN antidotes such as sodium thiosulfate and hydroxocobalamin are clinically used, they have foreseeable limitations during mass casualty situations because they require intravenous administration. To facilitate the development of better antidotes and therapeutic treatments, a global view of molecular changes induced by cyanide exposure is necessary. As an exploratory pursuit, we performed oligonucleotide microarrays to establish cardiac transcriptomes of an animal model of nose-only inhalation exposure to hydrogen cyanide (HCN), which is relevant to smoke inhalation. We also profiled cardiac transcriptomes after subcutaneous injection of potassium cyanide (KCN). Although the KCN injection model has often been used to evaluate medical countermeasures, this study demonstrated that cardiac transcriptomes are largely different from that of the HCN inhalation model at multiple time points within 24 h after exposure. Pathway analysis identified that HCN-induced transcriptomes were enriched with genes encoding mediators of pathways critical in modulation of cardiac complications and that a large number of such genes were significantly decreased in expression. We utilized the upstream regulatory analysis to propose drugs that can be potentially employed to treat cyanide-induced cardiac complications.

4-Hydroxyestrone, an Endogenous Estrogen Metabolite, Can Strongly Protect Neuronal Cells Against Oxidative Damage

Earlier studies showed that endogenous estrogens have neuroprotective effect against oxidative damage. The present study seeks to investigate the protective effect of various endogenous estrogen metabolites against oxidative neurotoxicity in vitro and in vivo. Using immortalized mouse hippocampal neuronal cells as an in vitro model, 4-hydroxyestrone, an estrone metabolite with little estrogenic activity, is found to have the strongest neuroprotective effect against oxidative neurotoxicity among 25 endogenous estrogen metabolites tested, and its protective effect is stronger than 17β-estradiol. Similarly, 4-Hydroxyestrone also exerts a stronger protective effect than 17β-estradiol against kanic acid-induced hippocampal oxidative damage in rats. Neuroprotection by 4-hydroxyestrone involves increased cytoplasmic translocation of p53 resulting from SIRT1-mediated deacetylation of p53. Analysis of brain microsomal enzymes shows that estrogen 4-hydroxylation is the main metabolic pathway in the central nervous system. Together, these results show that 4-hydroxyestrone is an endogenous neuroestrogen that can strongly protect against oxidative neuronal damage.

Neuroprotective Role of Steroidal Sex Hormones: An Overview

Progesterone, estrogens, and testosterone are the well-known steroidal sex hormones, which have been reported to have "nonreproductive "effects in the brain, specifically in the neuroprotection and neurotrophy. In the last one decade, there has been a surge in the research on the role of these hormones in neuroprotection and their positive impact on different brain injuries. The said interest has been sparked by a desire to understand the action and mechanisms of these steroidal sex hormones throughout the body. The aim of this article was to highlight the potential outcome of the steroidal hormones, viz. progesterone, estrogens, and testosterone in terms of their role in neuroprotection and other brain injuries. Their possible mechanism of action at both genomic and nongenomic level will be also discussed. As far as our knowledge goes, we are for the first time reporting neuroprotective effect and possible mechanism of action of these hormones in a single article.

Estrogen receptor variant ER-α36 is involved in estrogen neuroprotection against oxidative toxicity

It is well known that estrogen exerts neuroprotective effect against various neuronal damages. However, the estrogen receptor (ER) that mediates estrogen neuroprotection has not been well established. In this study, we investigated the potential receptor that mediates estrogen neuroprotection and the underlying molecular mechanisms. Hydrogen peroxide (H2O2) was chosen as an agent in our study to mimic free radicals that are often involved in the pathogenesis of many degenerative diseases. We found that in human SY5Y and IMR-32 cells, the estrogen neuroprotection against H2O2 toxicity was abrogated by knockdown of a variant of estrogen receptor-α, ER-α36. We also studied the rapid estrogen signaling mediated by ER-α36 in neuroprotective effect and found the PI3K/AKT and MAPK/ERK1/2 signaling mediated by ER-α36 is involved in estrogen neuroprotection. We also found that GPER, an orphan G protein-coupled receptor, is not involved in ER-α36-mediated rapid estrogen response. Our study thus demonstrates that ER-α36-mediated rapid estrogen signaling is involved in the neuroprotection activity of estrogen against oxidative toxicity.

Stroke: understanding the differences between males and females

Stroke is a significant cause of death and long-term disability in the USA. The incidence, mortality, and outcomes of stroke are significantly different between men and women. As with many diseases that affect men and women differently, an understanding on the reasons underlying those differences is critical to effective diagnosis and treatment. This review will examine the sex differences in stroke in both humans and animal models of stroke and review what is known about potential mechanisms underlying these differences. It is clear that there is a complex interaction between hormonal, genetic, and unknown factors at play in generating the sex differences in stroke.

Quantitative structure-activity relationships predicting the antioxidant potency of 17β-estradiol-related polycyclic phenols to inhibit lipid peroxidation

The antioxidant potency of 17β-estradiol and related polycyclic phenols has been well established. This property is an important component of the complex events by which these types of agents are capable to protect neurons against the detrimental consequences of oxidative stress. In order to relate their molecular structure and properties with their capacity to inhibit lipid peroxidation, a marker of oxidative stress, quantitative structure-activity relationship (QSAR) studies were conducted. The inhibition of Fe³⁺-induced lipid peroxidation in rat brain homogenate, measured through an assay detecting thiobarbituric acid reactive substances for about seventy compounds were correlated with various molecular descriptors. We found that lipophilicity (modeled by the logarithm of the n-octanol/water partition coefficient, logP) was the property that influenced most profoundly the potency of these compounds to inhibit lipid peroxidation in the biological medium studied. Additionally, the important contribution of the bond dissociation enthalpy of the phenolic O–H group, a shape index, the solvent-accessible surface area and the energy required to remove an electron from the highest occupied molecular orbital were also confirmed. Several QSAR equations were validated as potentially useful exploratory tools for identifying or designing novel phenolic antioxidants incorporating the structural backbone of 17β-estradiol to assist therapy development against oxidative stress-associated neurodegeneration.

Estrogen and neuroprotection: from clinical observations to molecular mechanisms

We now appreciate that estrogen is a pleiotropic gonadal steroid that exerts profound effects on the plasticity and cell survival of the adult brain. Over the past century, the life span of women has increased, but the age of the menopause remains constant. This means that women may now live over one third of their lives in a hypoestrogenic, postmenopausal state. The impact of prolonged hypoestrogenicity on the brain is now a critical health concern as we realize that these women may suffer an increased risk of cognitive dysfunction and neurodegeneration due to a variety of diseases. Accumulating evidence from both clinical and basic science studies indicates that estrogen exerts critical protective actions against neurodegenerative conditions such as Alzheimer's disease and stroke. Here, we review the discoveries that comprise our current understanding of estrogen action against neurodegeneration. These findings carry far-reaching possibilities for improving the quality of life in our aging population.

Neuroprotective effects of nonfeminizing estrogens in retinal photoreceptor neurons

PURPOSE

Retinal diseases such as macular degeneration and glaucoma are disorders that target specific retinal neurons that can ultimately lead to vision loss. Under these conditions and pathologies, retinal neurons can die via apoptosis that may be due to increased oxidative stress. The neuroprotective effects of 17β-estradiol (E2) and three synthetic nonfeminizing estrogen analogs (ZYC-26, ZYC-23, and ZYC-3) were investigated to examine their abilities to protect retinal neurons against glutamate toxicity.

METHODS

Using an in vitro model of glutamate-induced cell death in 661W cells, a mouse cone photoreceptor cell line, shown to express both estrogen receptors (ERs) via immunoblotting, was pretreated with E2 and its analogs and cell viability were assessed.

RESULTS

It was observed that E2 and estrogen analogs, ZYC-26 and ZYC-3, were protective against a 5 mM glutamate insult in 661W cells. The neuroprotective abilities of ZYC-26 and ZYC-3 were autonomous of estrogen receptor-α (ERα) and ERβ demonstrated by their ability to protect in the presence of ICI 182780, a pan-ER antagonist with a high affinity for the estrogen receptor. Treatment with PPT and DPN, ERα- and ERβ-specific agonists, respectively, did not protect the 661W cells from the glutamate insult. Studying the membrane ER (mER) or GPR30 did show that activation of the receptor by G1 protected the retinal neuron from insult, whereas G15, an antagonist of the mER was not able to antagonize the protection previously seen.

CONCLUSIONS

These data demonstrate that nonfeminizing estrogens may emerge as useful compounds for neuroprotection of retinal cells.

Neuroglobin, a novel target for endogenous neuroprotection against stroke and neurodegenerative disorders

Brain neurons and tissues respond to sublethal injury by activating endogenous protective pathways. Recently, following the failure of a large number of clinical trials for protective strategies against stroke that aim to inhibit a specific ischemia response pathway, endogenous neuroprotection has emerged as a more promising and hopeful strategy for development of therapeutics against stroke and neurodegenerative disorders. Neuroglobin (Ngb) is an oxygen-binding globin protein that is highly and specifically expressed in brain neurons. Accumulating evidence have clearly demonstrated that Ngb is an endogenous neuroprotective molecule against hypoxic/ischemic and oxidative stress-related insults in cultured neurons and animals, as well as neurodegenerative disorders such as Alzheimer’s disease, thus any pharmacological strategy that can up-regulate endogenous Ngb expression may lead to novel therapeutics against these brain disorders. In this review, we summarize recent studies about the biological function, regulation of gene expression, and neuroprotective mechanisms of Ngb. Furthermore, strategies for identification of chemical compounds that can up-regulate endogenous Ngb expression for neuroprotection against stroke and neurodegenerative disorders are discussed.

17β-Estradiol prevents cell death and mitochondrial dysfunction by an estrogen receptor-dependent mechanism in astrocytes after oxygen-glucose deprivation/reperfusion

17β-Estradiol (E2) has been shown to protect against ischemic brain injury, yet its targets and the mechanisms are unclear. E2 may exert multiple regulatory actions on astrocytes that may greatly contribute to its ability to protect the brain. Mitochondria are recognized as playing central roles in the development of injury during ischemia. Increasing evidence indicates that mitochondrial mechanisms are critically involved in E2-mediated protection. In this study, the effects of E2 and the role of mitochondria were evaluated in primary cultures of astrocytes subjected to an ischemia-like condition of oxygen-glucose deprivation (OGD)/reperfusion. We showed that E2 treatment significantly protects against OGD/reperfusion-induced cell death as determined by cell viability, apoptosis, and lactate dehydrogenase leakage. The protective effects of E2 on astrocytic survival were blocked by an estrogen receptor (ER) antagonist (ICI-182,780) and were mimicked by an ER agonist selective for ERα (PPT), but not by an ER agonist selective for ERβ (DPN). OGD/reperfusion provoked mitochondrial dysfunction as manifested by an increase in cellular reactive oxygen species production, loss of mitochondrial membrane potential, and depletion of ATP. E2 pretreatment significantly inhibited OGD/reperfusion-induced mitochondrial dysfunction, and this effect was also blocked by ICI-182,780. Therefore, we conclude that E2 provides direct protection to astrocytes from ischemic injury by an ER-dependent mechanism, highlighting an important role for ERα. Estrogen protects against mitochondrial dysfunction at the early phase of ischemic injury. However, overall implications for protection against brain ischemia and its complex sequelae await further exploration.

Neuroprotective role of estradiol against neuronal death induced by glucose deprivation in cultured rat hippocampal neurons

Studies have reported the protective effect of estradiol (E(2)) against neuronal death induced by several insults including oxygen deprivation, mitochondrial toxins and activation of glutamate receptors. Glucose deprivation (GD) is associated with ischemia and hypoglycemia, and to date there is no effective therapeutic agent able to prevent neuronal damage induced by these conditions. In this study, we have investigated the effects of 17β-E(2) and the selective agonists of the alpha (ERα) and beta (ERβ) estrogen receptors, propyl pyrazole triol (PPT) and diarylpropionitrile (DPN), respectively, on neuronal death induced by GD in cultured rat hippocampal neurons. We have also analyzed the expression of both ER isoforms after GD. Results show that GD for 2 and 4 h reduces cell survival by 42 and 55%, respectively. Treatment with 17β-E(2) (10 nM to 10 µM) induces a dose-dependent protective effect that is blocked by ICI 182,780, an ER antagonist, and by 1,3-bis(4-hydroxyphenyl)-4-methyl-5-[4-(-piperidinylethoxy)phenol]-1H'pyrazole dihydrochloride (MPP) and 4-[2-phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-3-yl]phenol (PHTPP), selective ERα and ERβ antagonists, respectively. The ERα and ERβ agonists PPT and DPN show a similar neuroprotective effect to that of 17β-E(2), but DPN is more efficient. In addition, hippocampal neurons under normal conditions show a higher expression of the ERβ isoform. When exposed to GD during 4 h, the expression of both ER isoforms is increased, while only that of the ERβ isoform significantly increases after 2 h of GD. Results demonstrate that E(2) prevents neuronal death induced by GD through its interaction with ER, although the ERβ isoform might have a predominant role. Results also suggest that GD differentially alters the expression of ERα and ERβ in hippocampal neurons.

Effects of neuroactive steroids on cochlear hair cell death induced by gentamicin

As neuroactive steroids, sex steroid hormones have non-reproductive effects. We previously reported that 17β-estradiol (βE2) had protective effects against gentamicin (GM) ototoxicity in the cochlea. In the present study, we examined whether the protective action of βE2 on GM ototoxicity is mediated by the estrogen receptor (ER) and whether other estrogens (17α-estradiol (αE2), estrone (E1), and estriol (E3)) and other neuroactive steroids, dehydroepiandrosterone (DHEA) and progesterone (P), have similar protective effects. The basal turn of the organ of Corti was dissected from Sprague-Dawley rats and cultured in a medium containing 100 μM GM for 48h. The effects of βE2 and ICI 182,780, a selective ER antagonist, were examined. In addition, the effects of other estrogens, DHEA and P were tested using this culture system. Loss of outer hair cells induced by GM exposure was compared among groups. βE2 exhibited a protective effect against GM ototoxicity, but its protective effect was antagonized by ICI 182,780. αE2, E1, and E3 also protected hair cells against gentamicin ototoxicity. DHEA showed a protective effect; however, the addition of ICI 182,780 did not affect hair cell loss. P did not have any effect on GM-induced outer hair cell death. The present findings suggest that estrogens and DHEA are protective agents against GM ototoxicity. The results of the ER antagonist study also suggest that the protective action of βE2 is mediated via ER but that of DHEA is not related to its conversion to estrogen and binding to ER. Further studies on neuroactive steroids may lead to new insights regarding cochlear protection.

Sexual dimorphism in ischemic stroke: lessons from the laboratory

Ischemic stroke is emerging as a major health problem for elderly women. Women have lower stroke incidence than men until an advanced age, when the epidemiology of ischemic stroke shifts and incidence rises dramatically in women. Experimental models of rodent stroke have replicated this clinical epidemiology, with exacerbated injury in older compared with young female rodents. Many of the detrimental effects of aging on ischemic stroke outcome in females can be replicated by ovariectomy, suggesting that hormones such as estrogen play a neuroprotective role. However, emerging data suggest that the molecular mechanisms leading to ischemic cell death differ in the two sexes, and these effects may be independent of circulating hormone levels. This article highlights recent clinical and experimental literature on sex differences in stroke outcomes and mechanisms.

17β-Estradiol-mediated increase in Cu/Zn superoxide dismutase expression in the brain: a mechanism to protect neurons from ischemia

A number of studies have demonstrated that 17β-estradiol (E(2)) protects the brain from ischemia and yet the mechanism by which this hormone brings about its protective effect is unclear. Interestingly, like E(2), overexpression of the oxidative stress response protein Cu/Zn superoxide dismutase (SOD1), which plays a critical role in regulating reactive oxygen species, also protects the brain from ischemia. Because we previously showed that E(2) treatment of cultured mammary cells increases SOD1 expression, we hypothesized that E(2) might increase SOD1 expression in the brain and that this E(2)-mediated increase in SOD1 expression might help to protect the brain from ischemia. We now show that SOD1 is expressed in cortical neurons, that SOD1 expression is increased by exposure of brain slice cultures to E(2), and that the E(2)-mediated increase in SOD1 expression is further augmented by exposure of brain slice cultures to increased superoxide levels or oxygen and glucose deprivation. Importantly, when cortical neurons are exposed to increased superoxide levels and markers of protein and DNA damage, nitrotyrosine and 8-oxoguanine, respectively, are measured, both protein and DNA damage are reduced. In fact, E(2) reduces nitrotyrosine and 8-oxoguanine levels in brain slice cultures regardless of whether they have or have not been exposed to increased superoxide levels. Likewise, when brain slice cultures are treated with E(2) and deprived of oxygen and glucose, 8-oxoguanine levels are reduced. Taken together, these studies provide a critical link between E(2) treatment, SOD1 expression, and neuroprotection and help to define a mechanism through which E(2)-mediated neuroprotection may be conferred.

Estrogen receptor α genetic variants and the risk of stroke in a South Indian population from Andhra Pradesh

BACKGROUND

Stroke is a complex disease caused by combination of multiple risk factors. Recent findings have suggested that stroke has a strong genetic component. Evidence suggests that variations in the estrogen receptor α (ESR1) gene may influence stroke risk.

AIMS

The present study was carried out to investigate the role of ESR1 gene polymorphisms [PvuII (rs 2234693) and XbaI (rs 9340799)] with stroke in a South Indian population from Andhra Pradesh. The relationship between ESR1 genotypes with estradiol levels was also investigated in pre- and postmenopausal women.

METHODS

Four hundred patients with ischemic stroke and three hundred and eighty subjects were enrolled in this case-control study. Ischemic stroke subtypes were classified according to TOAST (Trial of Org 10172 in Acute Stroke Treatment) classification. The ESR1 PvuII and XbaI genotypes were determined by PCR-RFLP method. Serum estradiol was measured by ELISA.

RESULTS

In case of PvuII polymorphism statistically significant difference was observed in the genotypic and allelic frequencies between patients and controls (joint analysis of men and women) (p=0.003 and 0.004 respectively). However, the XbaI genotypes and alleles did not show an association with stroke in the study population. When the analysis was carried out separately for men and women, the PvuII polymorphism did not show significant association with stroke in men; women showed a significant association. Further when women were grouped in to premenopausal and postmenopausal, the premenopausal group did not show a significant association with the polymorphism but significant association with stroke was found in postmenopausal women. A stepwise multiple logistic regression analysis confirmed these findings. Women with pp genotype had low estradiol levels in comparison with PP genotypic individuals (p<0.05). Further evaluating the association of this polymorphism with stroke subtypes, we found significant association of PvuII polymorphism with extracranial atherosclerosis, lacunar and cardioembolic stroke.

CONCLUSION

In conclusion our results suggest the PvuII gene polymorphism is significantly associated with stroke in postmenopausal women in a South Indian population from Andhra Pradesh. The pp genotypes have average 17β estradiol levels which are significantly low in comparison with PP genotypes. Therefore postmenopausal women with a high frequency of pp genotype are more predisposed to ischemic stroke. However, this is a preliminary study and the results need to be confirmed in a larger cohort.

Neuroprotective mechanism conferred by 17beta-estradiol on the biochemical basis of Alzheimer's disease

Estrogen (17beta-estradiol) plays key regulatory roles in a variety of physiological and biological processes. Several lines of evidence also support its role as a protective factor in Alzheimer's disease; however, the basis of this effect is unclear. Here we show that an early-onset Alzheimer's disease transgenic mouse model expressing the double-mutant form of human amyloid precursor protein (APP); Swedish (K670N/M671L) and Indiana (V717F) undergoing treatment with 17beta-estradiol show significantly lower levels of APP processing through beta-secretase and enhanced alpha-secretase processing resulting in marked reductions of APP-CTFbeta, Abeta42 and plaque burden, along with increased levels of the non-amyloidogenic sAPPalpha. Moreover, 17beta-estradiol resulted in elevated brain levels of transthyretin, which inhibits aggregation of Abeta into plaques; though the insulin-degrading enzyme, which breaks down Abeta, was significantly reduced. These results illustrate a multifaceted effect of 17beta-estradiol on the biochemical basis of Alzheimer's disease, through effects on APP processing, Abeta levels and factors that affect its clearance and aggregation. Overall, these results support the need for further long-term longitudinal studies to elucidate consequences of menopause as well as hormone therapy on Alzheimer's disease, and explore its potential as a therapeutic avenue for the disease.

Comparable attenuation of Abeta(25-35)-induced neurotoxicity by quercitrin and 17beta-estradiol in cultured rat hippocampal neurons

In the present work, potential protective effects of quercitrin (a phytoestrogen) on Abeta-induced neurotoxicity in cultured rat hippocampal neurons were investigated in comparison with 17beta-estradiol. Cell viability, oxidative status, and antioxidative potentials were used as comparative parameters. Co-exposure of cultured neurons to Abeta(25-35) with either quercitrin or 17beta-estradiol (50-100 microM) for 72 h attenuated Abeta(25-35)-induced neurotoxicity and lipid peroxidation, but not Abeta(25-35)-induced ROS accumulation. However, only 17beta-estradiol counteracted a reduction in glutathione content and only quercitrin counteracted a reduction in glutathione peroxidase activity. Both compounds displayed no effects on superoxide dismutase activity. A specific estrogen receptor antagonist, ICI 182780, did not abolish neuroprotective effects of quercitrin and 17beta-estradiol. These findings suggested that quercitrin and 17beta-estradiol attenuated Abeta(25-35)-induced neurotoxicity in a comparable manner. Underlying neuroprotective mechanisms of both compounds were probably not related to estrogen receptor-mediated genomic mechanisms but might involve with their antioxidant and free radical scavenging properties.

Estrogen-receptor-mediated protection of cerebral endothelial cell viability and mitochondrial function after ischemic insult in vitro

Protective effects of estrogen against experimental stroke and neuronal ischemic insult are well-documented, but it is not known whether estrogen prevents ischemic injury to brain endothelium, a key component of the neurovascular unit. Increasing evidence indicates that estrogen exerts protective effects through mitochondrial mechanisms. We previously found 17beta-estradiol (E2) to improve mitochondrial efficiency and reduce mitochondrial superoxide in brain blood vessels and endothelial cells. Thus we hypothesized E2 will preserve mitochondrial function and protect brain endothelial cells against ischemic damage. To test this, an in vitro ischemic model, oxygen-glucose deprivation (OGD)/reperfusion, was applied to immortalized mouse brain endothelial cells (bEnd.3). OGD/reperfusion-induced cell death was prevented by long-term (24, 48 h), but not short-term (0.5, 12 h), pretreatment with 10 nmol/L E2. Protective effects of E2 on endothelial cell viability were mimicked by an estrogen-receptor (ER) agonist selective for ERalpha (PPT), but not by one selective for ERbeta (DPN). In addition, E2 significantly decreased mitochondrial superoxide and preserved mitochondrial membrane potential and ATP levels in early stages of OGD/reperfusion. All of the E2 effects were blocked by the ER antagonist, ICI-182,780. These findings indicate that E2 can preserve endothelial mitochondrial function and provide protection against ischemic injury through ER-mediated mechanisms.

Estrogen suppresses MLK3-mediated apoptosis sensitivity in ER+ breast cancer cells

Little knowledge exists about the mechanisms by which estrogen can impede chemotherapy-induced cell death of breast cancer cells. 17beta-Estradiol (E(2)) hinders cytotoxic drug-induced cell death in estrogen receptor-positive (ER(+)) breast cancer cells. We noted that the activity of the proapoptotic mixed lineage kinase 3 (MLK3) kinase was relatively higher in estrogen receptor-negative (ER(-)) breast tumors, suggesting that E(2) might inhibit MLK3 activity. The kinase activities of MLK3 and its downstream target, c-Jun NH(2)-terminal kinase, were rapidly inhibited by E(2) in ER(+) but not in ER(-) cells. Specific knockdown of AKT1/2 prevented MLK3 inhibition by E(2), indicating that AKT mediated this event. Furthermore, MLK3 inhibition by E(2) involved phosphorylation of MLK3 Ser(674) by AKT, attenuating the proapoptotic function of MLK3. We found that a pan-MLK inhibitor (CEP-11004) limited Taxol-induced cell death and that E(2) accentuated this limitation. Taken together, our findings indicate that E(2) inhibits the proapoptotic function of MLK3 as a mechanism to limit cytotoxic drug-induced death of ER(+) breast cancer cells.

Effects of estrogen on AF64A-induced apoptosis in NG108-15 cells

In this study, we show that pretreatment with physiological concentrations (1-100 nM) of 17beta-estradiol decreased apoptosis induced by ethylcholine aziridinium (AF64A), a choline toxin, in the cholinergic neuronal cell line NG108-15. These protective effects were observed after short-term (30 min) pretreatment, and were blocked by treatment with an estrogen receptor antagonist and inhibitors of phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase kinase (MEK). The protective effects were, however, not reversed by a protein synthesis inhibitor. Furthermore, we examined the effects of 17beta-estradiol on choline uptake in NG108-15 cells. Although choline uptake was inhibited by a selective inhibitor of choline uptake, hemicholinium-3, it was not altered by treatment with 17beta-estradiol. These results indicated that the protective effect of 17beta-estradiol on AF64A-induced apoptosis could be nongenomic, and that this effect may be due to the activation of PI3K/Akt and/or MEK/extracellular signal-regulated kinase (ERK) pathways.

Estrogen receptor alpha inhibits the estrogen-mediated suppression of HIV transcription in astrocytes: implications for estrogen neuroprotection in HIV dementia

Many human immunodeficiency virus (HIV) proteins including Tat are produced by HIV-infected astrocytes and secreted into the brain resulting in extensive neuronal damage that contributes to the pathogenesis of HIV dementia. The neuroprotective hormone 17beta-estradiol (E2) is known to negatively regulate the HIV transcriptional promoter in human fetal astrocytes (SVGA cell line) in a Tat-dependent manner. In the present study we extended our investigation in HIV-infected SVGA cells and found a reduction in HIV p24 levels following E2 treatment in comparison to control. Although many E2-mediated events occur through estrogen receptor alpha (ERalpha), we found low levels of ERalpha mRNA and failed to detect ERalpha protein in SVGA cells. Paradoxically, when ERalpha was overexpressed the E2-mediated decrease in Tat transactivation of the promotor was prevented. To determine whether ERalpha expression is altered in the human brain following HIV infection, postmortum hippocampal tissue was obtained from cognitively normal HIV- and HIV+ patients, patients diagnosed with either mild cognitive/motor disorder (MCMD) or HIV-associated dementia (HAD). Immunohistochemistry and quantitative real-time PCR (qRT-PCR) for ERalpha and glial fibrillary acidic protein (GFAP) showed that ERalpha mRNA levels were not significantly different between groups, while GFAP increased in the hippocampus in the HIV+ compared to the HIV- group and was decreased in the MCMD and HAD subgroups compared to HIV+ controls. Notably the ratio of ERalpha-positive reactive astrocytes to total reactive astrocytes increased and significantly correlated with the severity of cognitive impairment following HIV infection. The data suggest that E2 would have the most dramatic effect in reducing HIV transcription early in the disease process when the subpopulation of astrocytes expressing ERalpha is low.

Neuroprotective effects of estrogens following ischemic stroke

Our laboratory has investigated whether and how 17beta-estradiol (E(2)) protects the brain against neurodegeneration associated with cerebrovascular stroke. We have discovered that low, physiological concentrations of E(2), which are strikingly similar to low-basal circulating levels found in cycling mice, dramatically protect the brain against stroke injury, and consequently revealed multiple signaling pathways and key genes that mediate protective action of E(2). Here we will review the discoveries comprising our current understanding of neuroprotective actions of estrogens against ischemic stroke. These findings may carry far reaching implications for improving the quality of life in aging populations.

Estradiol rescues neurons from global ischemia-induced cell death: multiple cellular pathways of neuroprotection

The potential neuroprotective role of sex hormones in chronic neurodegenerative disorders and acute brain ischemia following cardiac arrest and stroke is of a great therapeutic interest. Long-term pretreatment with estradiol and other estrogens affords robust neuroprotection in male and female rodents subjected to focal and global ischemia. However, the receptors (e.g., cell surface or nuclear), intracellular signaling pathways and networks of estrogen-regulated genes that intervene in neuronal apoptosis are as yet unclear. We have shown that estradiol administered at physiological levels for two weeks before ischemia rescues neurons destined to die in the hippocampal CA1 and ameliorates ischemia-induced cognitive deficits in ovariectomized female rats. This regimen of estradiol treatment involves classical intracellular estrogen receptors, transactivation of IGF-1 receptors and stimulation of the ERK/MAPK signaling pathway, which in turn maintains CREB activity in the ischemic CA1. We also find that a single, acute injection of estradiol administrated into the brain ventricle immediately after an ischemic event reduces both neuronal death and cognitive deficits. Because these findings suggest that hormones could be used to treat patients when given after brain ischemia, it is critical to determine whether the same or different pathways mediate this form of neuroprotection. We find that an agonist of the membrane estrogen receptor GPR30 mimics short latency estradiol facilitation of synaptic transmission in the hippocampus. Therefore, we are testing the hypothesis that GPR30 may act together with intracellular estrogen receptors to activate cell signaling pathways to promote neuron survival after global ischemia.

Soy phytoestrogens are neuroprotective against stroke-like injury in vitro

Diets high in soy are neuroprotective in experimental stroke. This protective effect is hypothesized to be mediated by phytoestrogens contained in soy, because some of these compounds have neuroprotective effects in in vitro models of cell death. We tested the ability of the soy phytoestrogens genistein, daidzein, and the daidzein metabolite equol to protect embryonic rat primary cortical neurons from ischemic-like injury in vitro at doses typical of circulating concentrations in human populations (0.1-1 microM). All three phytoestrogens inhibited lactate dehydrogenase (LDH) release from cells exposed to glutamate toxicity or the calcium-ATPase inhibitor, thapsigargin. In cells exposed to hypoxia or oxygen-glucose deprivation (OGD), pretreatment with the phytoestrogens inhibited cell death in an estrogen receptor (ER) dependent manner. Although OGD results in multiple modes of cell death, examination of alpha-spectrin cleavage and caspase-3 activation revealed that the phytoestrogens were able to inhibit apoptotic cell death in this model. In addition, blockade of phosphoinositide 3-kinase prevented the protective effects of genistein and daidzein, and blockade of mitogen-activated protein kinase prevented genistein-dependent neuroprotection. These results suggest that pretreatment with dietary levels of soy phytoestrogens can mimic neuroprotective effects observed with estrogen and appear to use the same ER-kinase pathways to inhibit apoptotic cell death.

Estrogen actions on mitochondria--physiological and pathological implications

Estrogens are potent neuroprotective hormones and mitochondria are the site of cellular life-death decisions. As such, it is not surprising that we and others have shown that estrogens have remarkable effects on mitochondrial function. Herein we provide evidence for a primary effect of estrogens on mitochondrial function, achieved in part by the import of estrogen receptor beta (ERbeta) into the mitochondria where it mediates a number of estrogen actions on this vital organelle. ERbeta is imported into the mitochondria, through tethering to cytosolic chaperone protein and/or through direct interaction with mitochondrial import proteins. In the mitochondria, ERbeta can affect transcription of critical mitochondrial genes through the interaction with estrogen response elements (ERE) or through protein-protein interactions with mitochondrially imported transcription factors. The potent effects of estrogens on mitochondrial function, particularly during mitochondrial stress, argues for a role of estrogens in the treatment of mitochondrial defects in chronic neurodegenerative diseases like Alzheimer's disease (AD) and Parkinson's disease (PD) and more acute conditions of mitochondrial compromise, like cerebral ischemia and traumatic brain injury.

Dynamic regulation of estrogen receptor-alpha gene expression in the brain: a role for promoter methylation?

Estrogen has long been known to play an important role in coordinating the neuroendocrine events that control sexual development, sexual behavior and reproduction. Estrogen actions in other, non-reproductive areas of the brain have also been described. It is now known that estrogen can also influence learning, memory, and emotion and has neurotrophic and neuroprotective properties. The actions of estrogen are largely mediated through at least two intracellular estrogen receptors. Both estrogen receptor-alpha and estrogen receptor-beta are expressed in a wide variety of brain regions. Estrogen receptor-alpha (ERalpha), however, undergoes developmental and brain region-specific changes in expression. The precise molecular mechanisms that regulate its expression at the level of gene transcription are not well understood. Adding to the complexity of its regulation, the estrogen receptor gene contains multiple promoters that drive its expression. In the cortex in particular, the ERalpha mRNA expression is dynamically regulated during postnatal development and again following neuronal injury. Epigenetic modification of chromatin is increasingly being understood as a mechanism of neuronal gene regulation. This review examines the potential regulation of the ERalpha gene by such epigenetic mechanisms.

Developmental exposure to polychlorinated biphenyls influences stroke outcome in adult rats

BACKGROUND

The "developmental origins of adult disease" hypothesis was originally derived from evidence linking low birth weight to cardiovascular diseases including stroke. Subsequently, it has been expanded to include developmental exposures to environmental contaminants as risk factors for adult onset disease.

OBJECTIVE

Our goal in this study was to test the hypothesis that developmental exposure to poly-chlorinated biphenyls (PCBs) alters stroke outcome in adults.

METHODS

We exposed rats to the PCB mixture Aroclor 1254 (A1254) at 0.1 or 1 mg/kg/day in the maternal diet throughout gestation and lactation. Focal cerebral ischemia was induced at 6-8 weeks of age via middle cerebral artery occlusion, and infarct size was measured in the cerebral cortex and striatum at 22 hr of reperfusion. PCB congeners were quantified in brain tissue by gas chromatography with microelectron capture detection, and cortical and striatal expression of Bcl2 and Cyp2C11 were quantified by quantitative reverse transcriptase-polymerase chain reaction.

RESULTS

Developmental exposure to A1254 significantly decreased striatal infarct in females and males at 0.1 and 1 mg/kg/day, respectively. Predominantly ortho-substituted PCB congeners were detected above background levels in brains of adult females and males exposed to A1254 at 1 but not 0.1 mg/kg/day. Effects of developmental A1254 exposure on Bcl2 and Cyp2C11 expression did not correlate with effects on infarct volume.

CONCLUSION

Our data provide proof of principle that developmental exposures to environmental contaminants influence the response of the adult brain to ischemic injury and thus represent potentially important determinants of stroke susceptibility.

Estradiol attenuates the focal cerebral ischemic injury through mTOR/p70S6 kinase signaling pathway

We previously showed that estradiol prevents neuronal cell death through the activation of Akt and its downstream targets Bad and FKHR. This study investigated whether estradiol modulates the survival pathway through other downstream targets of Akt, including mammalian target of rapamycin (mTOR) and p70S6 kinase. It is known that mTOR is a downstream target of Akt and a central regulator of protein synthesis, cell growth, and cell cycle progression. Adult female rats were ovariectomied and treated with estradiol prior to middle cerebral artery occlusion (MCAO). Brains were collected 24h after MCAO and infarct volumes were analyzed. We confirmed that estradiol significantly reduces infarct volume and decreases the number of positive cells for TUNEL staining in the cerebral cortex. Brain injury-induced a decrease in phospho-mTOR and phospho-p70S6 kinase. Estradiol prevented the injury-induced decrease in Akt activation and phosphorylation of mTOR and p70S6 kinases, and the subsequent decrease in S6 phosphorylation. Our findings suggest that estradiol plays a potent protective role against brain injury by preventing the injury-induced decrease of mTOR and p70S6 kinase phosphorylation.

Epigenetic regulation of the estrogen receptor alpha promoter in the cerebral cortex following ischemia in male and female rats

Permanent middle cerebral artery occlusion (MCAO) causes neuronal cell death in the striatum and cortex. In rodents, estradiol treatment protects the cortex from cell death in an estrogen receptor alpha (ERalpha) dependent manner. ERalpha is only transiently expressed in the cortex during neonatal development and is very low in uninjured adult cortex. Following MCAO, ERalpha mRNA expression is upregulated in the cortex of female rats, but the mechanism of this increase is still unknown. It is also unknown whether a similar increase in ERalpha expression in seen in males. In the following studies, male and vehicle or estradiol-treated ovariectomized (OVX) female rats underwent MCAO to investigate the regulation of ERalpha expression after ischemia. Twenty-four hours after surgery, mRNA or genomic DNA was collected from 1 mm micropunches taken from 300 mum brain sections for quantitative reverse transcription-polymerase chain reaction (RT-PCR) or methylation-specific (MSP) PCR, respectively. Additionally, adjacent 20 mum sections were processed for ERalpha immunohistochemistry. In OVX females, ERalpha mRNA and protein were increased in the ischemic cortex, but unchanged in males. We hypothesized that this increase in ERalpha in females is due to a reversal of gene silencing by DNA methylation. Using MSP targeting of CpG islands within the 5' untranslated region (UTR) of the rat ERalpha gene, we found that ischemia decreased methylation in the ischemic cortex of both groups of females, but there was no change in methylation in males. Using chromatin immunoprecipitation, we found that MeCP2 associates with ERalpha 5'UTR corresponding with the methylation status of the promoter. These data are the first to demonstrate a difference in the regulation of ERalpha expression in response to MCAO between males and females and that methylation of the ERalpha gene corresponds with mRNA levels in the brain.

Androgens predispose males to GABAA-mediated excitotoxicity in the developing hippocampus

Clinical evidence and animal models indicate greater brain damage in newborn males following injury. In adults, glutamate is the primary source of excitotoxic cell death and the steroid, estradiol, is neuroprotective. In neonatal brain, membrane depolarization following activation of GABAA receptors is the major source of excitation. Consequent influx of calcium via L-type channels is normally trophic, but becomes excitotoxic during periods of excessive activation of GABAA receptors, such as hypoxia-ischemia, alcohol exposure and seizures. The use of sex-specific hippocampal cultures revealed greater cell death induced by the GABAA agonist, muscimol, in male- versus female-derived cultures. Pretreatment with the androgen, dihydrotestosterone (DHT) increased muscimol-induced death in both sexes. Exploration of calcium dynamics indicated that, counter to expectation, female neurons achieved higher [Ca2+]i than male, but the calcium transient duration was shorter due to faster rise and decay. However, a second exposure to muscimol within minutes of the first, caused significant attenuation of [Ca2+]i in female neurons. In contrast, while male neurons exposed to muscimol for the first time exhibited lower maximal [Ca2+]i, when exposed to muscimol again there was no attenuation in [Ca2+]i. The latter effect was induced in females by DHT, and inversely correlated with the amount of gamma2 subunit of the GABAA receptor. This novel effect of androgen on GABA-mediated excitotoxicty suggests a unique opportunity for a sex-specific therapeutic approach involving antagonism of the androgen receptor in neonatal males at risk for brain injury.

Different types of small nerve fibers in eutopic endometrium and myometrium in women with endometriosis

OBJECTIVE

To investigate types of nerve fibers in endometrium and myometrium in women with endometriosis.

DESIGN

Laboratory study using human tissue.

SETTING

University-based laboratory.

PATIENT(S)

Women with and without endometriosis undergoing hysterectomy.

INTERVENTION(S)

Histologic sections of contiguous endometrial and myometrial tissues were prepared from hysterectomies performed on women with and without endometriosis.

MAIN OUTCOME MEASURE(S)

Types and density of nerve fibers in endometrium and myometrium in women with and without endometriosis were determined using a series of specific markers for neuronal structure and function: PGP9.5, NF, SP, CGRP, TH, VAChT, VIP, and NPY.

RESULT(S)

Nerve fibers stained with PGP9.5 and NF in endometrium and myometrium were significantly increased in women with endometriosis compared with women without endometriosis. Nerve fibers in the functional layer of endometrium in women with endometriosis were likely to be sensory C, a mixture of sensory A delta, sensory C, and adrenergic fibers in the basal layer of the endometrium, a mixture of sensory A delta, sensory C, adrenergic and cholinergic fibers in the myometrium.

CONCLUSION(S)

Increased nerve fiber density in endometrium and myometrium, and sensory C fibers and adrenergic nerve fibers in the endometrium in women with endometriosis may play an important role in the mechanisms of pain generation in this condition.

17Beta-estradiol prevents the glutamate-induced decrease of Akt and its downstream targets in HT22 cells

Estradiol is known to exert neuroprotective effect against glutamate toxicity in hippocampal-derived cell line (HT22). This study investigated whether estradiol modulates the anti-apoptotic signal through the phosphorylation of Akt and its downstream targets, including Bad, forkhead transcription factors FKHR and FKHRL1. Pretreatment with 17beta-estradiol decreased glutamate toxicity-induced cell death in HT22 cells. Also, pretreatment with 17beta-estradiol significantly decreased the positive cells of TUNEL stain, compared to that of only glutamate-treated cells. Potential activation was measured by phosphorylation of Akt at Ser(473), Bad at Ser(136), FKHR at Ser(256), and FKHRL1 at Thr(32) using Western blot analysis. 17Beta-estradiol pretreatment prevented the glutamate-induced decrease of pAkt, pBad, pFKHR, and pFKHRL1. These findings clearly confirm that 17beta-estradiol plays a potent neuroprotective role against glutamate-induced toxicity and suggest that phosphorylation of Akt and its downstream targets by 17beta-estradiol mediated these protective effects.

Neurotrophic and neuroprotective actions of estrogen: basic mechanisms and clinical implications

Estrogen is an important hormone signal that regulates multiple tissues and functions in the body. This review focuses on the neurotrophic and neuroprotective actions of estrogen in the brain, with particular emphasis on estrogen actions in the hippocampus, cerebral cortex and striatum. Sex differences in the risk, onset and severity of neurodegenerative disease such as Alzheimer's disease, Parkinson's disease and stroke are well known, and the potential role of estrogen as a neuroprotective factor is discussed in this context. The review assimilates a complex literature that spans research in humans, non-human primates and rodent animal models and attempts to contrast and compare the findings across species where possible. Current controversies regarding the Women's Health Initiative (WHI) study, its ramifications, concerns and the new studies needed to address these concerns are also addressed. Signaling mechanisms underlying estrogen-induced neuroprotection and synaptic plasticity are reviewed, including the important concepts of genomic versus nongenomic mechanisms, types of estrogen receptor involved and their subcellular targeting, and implicated downstream signaling pathways and mediators. Finally, a multicellular mode of estrogen action in the regulation of neuronal survival and neurotrophism is discussed, as are potential future directions for the field.

Mitochondrial mechanisms of estrogen neuroprotection

Oxidative stress, bioenergetic failure and mitochondrial dysfunction are all implicated in the etiology of neurodegenerative diseases such as Alzheimer's disease (AD). The mitochondrial involvement in neurodegenerative diseases reflects the regulatory role mitochondrial failure plays in both necrotic cell death and apoptosis. The potent feminizing hormone, 17 beta-estradiol (E2), is neuroprotective in a host of cell and animal models of stroke and neurodegenerative diseases. The discovery that 17alpha-estradiol, an isomer of E2, is equally as neuroprotective as E2 yet is >200-fold less active as a hormone, has permitted development of novel, more potent analogs where neuroprotection is independent of hormonal potency. Studies of structure-activity relationships and mitochondrial function have led to a mechanistic model in which these steroidal phenols intercalate into cell membranes where they block lipid peroxidation reactions, and are in turn recycled. Indeed, the parental estrogens and novel analogs stabilize mitochondria under Ca(2+) loading otherwise sufficient to collapse membrane potential. The neuroprotective and mitoprotective potencies for a series of estrogen analogs are significantly correlated, suggesting that these compounds prevent cell death in large measure by maintaining functionally intact mitochondria. This therapeutic strategy is germane not only to sudden mitochondrial failure in acute circumstances, such as during a stroke or myocardial infarction, but also to gradual mitochondrial dysfunction associated with chronic degenerative disorders such as AD.

Timing of estrogen therapy after ovariectomy dictates the efficacy of its neuroprotective and antiinflammatory actions

Recent studies describing the seemingly contradictory actions of estrogens in ischemic stroke injury have led us to reevaluate the circumstances under which estrogen therapy (ET) provides benefits against cerebral stroke and decipher its mechanisms of action. One prominent feature that follows stroke injury is massive central and peripheral inflammatory responses. Evidence now suggests that postischemic inflammatory responses strongly contribute to the extent of brain injury, and 17beta-estradiol (E(2)) may protect the ischemic brain by exerting antiinflammatory actions. In an attempt to explain recently reported dichotomous effects of E(2) in stroke injury, we tested the hypothesis that an extended period of hypoestrogenicity both prevents E(2) from protecting the brain against ischemia and simultaneously suppresses its antiinflammatory actions. We report that E(2) exerts profound neuroprotective action when administered immediately upon ovariectomy, but not when administered after 10 weeks of hypoestrogenicity. Consistently, E(2) treatment given immediately at the time of ovariectomy attenuated central and peripheral production of proinflammatory cytokines after ischemic stroke. In contrast, E(2) did not suppress production of proinflammatory molecules when it was administered after 10 weeks postovariectomy. These results demonstrate that a prolonged period of hypoestrogenicity disrupts both neuroprotective and antiinflammatory actions of E(2). Our findings may help to explain the results of the Women's Health initiative that reported no beneficial effect of ET against stroke because the majority of the subjects initiated ET after an extended period of hypoestrogenicity.

Estrogen, beta-amyloid metabolism/trafficking, and Alzheimer's disease

Estrogen plays key regulatory roles in a variety of biological actions besides its classic function as a sex hormone. Recently, estrogen has been linked to neurodegenerative diseases including Alzheimer's disease (AD) and Parkinson's disease (PD). Several lines of evidence support the notion that brain estrogen exerts neuroprotective effects against various types of neurotoxicity in different cellular and animal models. Despite some controversies, estrogen replacement therapy (ERT) at an early stage, especially when given prior to menopause, has been shown to reduce the risk of AD in postmenopausal women. In addition, multiple lines of evidence have proven the neuroprotective effects of estrogen, such as enhancing neurotrophin signaling and synaptic activities pertinent to memory functions and protecting neurons against oxidative injuries and beta-amyloid toxicity; the latter is widely accepted as the prime culprit known to trigger the pathogenesis of AD. Here we will summarize our findings that estrogen decreased generation and secretion of beta-amyloid peptides in cultured cells and primary neurons and that administration of estrogen in estrogen-deprived mice reversed the elevated levels of brain Abeta. We will also discuss the molecular and cellular mechanisms underlying estrogen's effects on Abeta metabolism, which is highlighted by our demonstration that estrogen increases intracellular trafficking of beta-amyloid precursor protein (betaAPP) and hence reduces maximal Abeta generation within the trans-Golgi network (TGN), a subcellular compartment in which APP is known to be cleaved by the secretase enzymes to generate Abeta.

Implications for estrogens in Parkinson's disease: an epidemiological approach

Evidence from experimental and epidemiological studies suggests a role of sex hormones in the pathogenic process leading to neurodegenerative diseases, (i.e., Alzheimer's and Parkinson's disease). The effects of sexual steroid hormones are complex and vary with the events of women's fertile life. Estrogens are supposed to influence dopamine synthesis, metabolism, and transport; however, there is no consensus regarding the direction, locus, and mechanism of the effect of estrogens on the dopaminergic system. A neuroprotective effect of estrogens has been demonstrated in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-animal models of Parkinson's disease (PD). Epidemiological studies indicate gender differences regarding the onset and the prognosis of PD. Most of the analytical studies explored the relationship between PD and exogenous estrogens. Only three studies investigated the role of endogenous estrogens in the risk of developing PD. These studies reported an increased risk of PD in conditions causing an early reduction in endogenous estrogens (early menopause, reduced fertile life length). Longer cumulative length of pregnancies has also been associated with an increased PD risk. A lack of consensus still exists on the effect of the type of menopause (surgical vs. natural) on PD risk. Finally, the effect of postmenopausal estrogen replacement therapy is still debated. Inconsistencies across studies are in part explained by the complexity of the mechanisms of action of sexual hormones and by the paucity of analytical studies.

Changes in estrogen receptor-alpha mRNA in the mouse cortex during development

Estrogen plays a critical role in brain development and is responsible for generating sex differences in cognition and emotion. Studies in rodent models have shown high levels of estrogen binding in non-reproductive areas of the brain during development, including the cortex and hippocampus, yet binding is diminished in the same areas of the adult brain. These binding studies demonstrated that estrogen receptors decline in the cortex during development but did not identify which of the two estrogen receptors was present. In the current study, we examined the expression of estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta) in the mouse cortex during the first month of life. Messenger RNA was isolated from cortical tissue taken from C57BL/6 mice on postnatal day (PND) 1, 4, 10, 18 and 25 and expression levels were determined by real-time PCR. ERalpha mRNA expression in the mouse cortex at PND 25 was significantly reduced as compared to PND 1 (p<0.01). ERbeta mRNA expression at PND 25 was significantly increased as compared to PND 1 (p<0.05). Although the increase in ERbeta mRNA was statistically significant, the ERbeta levels were extremely low in the isocortex compared to ERalpha mRNA levels, suggesting that ERalpha may play a more critical role in the developmental decrease of estradiol binding than ERbeta. Additionally, we measured ERalpha mRNA expression in organotypic explant cultures of cortex taken from PND 3 mice. Explants were maintained in vitro for 3 weeks. mRNA was isolated at several time points and ERalpha and ERbeta mRNA was measured by real-time RT-PCR. ERalpha and ERbeta mRNA levels reflected a similar pattern in vitro and in vivo, suggesting that signals outside the cortex are not needed for this developmental change. This study lays the groundwork for an understanding of the mechanisms of the developmental regulation of ERalpha mRNA.

Estradiol prevents the injury-induced decrease of 90 ribosomal S6 kinase (p90RSK) and Bad phosphorylation

Estradiol prevents neuronal cell death through the activation of cell survival signals and the inhibition of apoptotic signals. This study investigated whether estradiol modulates the anti-apoptotic signal through the activation of Raf-MEK-ERK and its downstream targets, including 90 ribosomal S6 kinase (p90RSK) and Bad. Adult female rats were ovariectomied and treated with estradiol prior to middle cerebral artery occlusion (MCAO). Brains were collected 24h after MCAO and infarct volumes were analyzed. We confirmed that estradiol significantly reduces infarct volume and decreases the positive cells of TUNEL staining in the cerebral cortex. Estradiol prevents the injury-induced decrease of Raf-1, MEK1/2, and ERK1/2 phosphorylation. Also, it inhibits the injury-induced decrease of p90RSK and Bad phosphorylation. Further, in the presence of estradiol, the interaction of phospho-Bad and 14-3-3 increased, compared with that of oil-treated animals. Our findings suggest that estradiol prevents cell death due to brain injury and that Raf-MEK-ERK cascade activation and its downstream targets, p90RSK, Bad phosphorylation by estradiol mediated these protective effects.

Role of astrocytes in estrogen-mediated neuroprotection

Recent work has suggested that the ovarian steroid hormone, 17beta-estradiol (E2), at physiological concentrations, may exert protective effects in neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and acute ischemic stroke. While physiological concentrations of E2 have consistently been shown to be protective in vivo, direct protection of neurons remains controversial, suggesting that while direct protection of neurons may occur in some instances, an alternative or parallel pathway for protection may exist which could involve another cell type in the brain. In the present review, we summarize the data in support of a possible role for astrocytes in the mediation of neuroprotection by E2. We also summarize the data suggesting a non-classical estrogen receptor may underlie some of the protective effects of E2 by activating cellular signaling pathways, such as extracellular-regulated kinase (ERK) and phosphatidylinositol 3-kinase/Akt. A possible indirect pathway involving astrocytes may act in concert with the proposed direct pathway to achieve a widespread, global protection of both ER positive and negative neurons.

17beta-estradiol pretreatment prevents the global ischemic injury-induced decrease of Akt activation and bad phosphorylation in gerbils

Estradiol acts as a neuroprotective factor against ischemic brain injury. This study investigated whether estradiol modulates neuroprotective mechanism through the activation of Akt and its downstream target such as Bad in global ischemic injury. Adult female gerbils were ovariectomized and treated with estradiol prior to ischemic injury. Transient cerebral ischemia was accomplished by bilateral clipping of the common carotid artery for 5 min. Brains were collected on 1, 3, 5 day after injury. In hippocampal CA1 region of non-treated gerbils, most of neuronal cells exhibited pyknotic nuclei and showed the positive reaction of TUNEL staining on 5 day after injury. However, estradiol significantly reduced the neuronal cell death. Potential activation was measured by phosphorylation of Akt at Ser473 and Bad at Ser136 using western blot analysis. The levels of pAkt and pBad were significantly decreased in non-treated gerbils on 1-5 day after injury. However, estradiol prevents the global ischemic injury-induced decrease of pAkt and pBad. Our findings suggest that estradiol prevents cell death due to global ischemic injury and that Akt activation and Bad phosphorylation by estradiol mediated these protective effects.

Inhibition of MLK3-MKK4/7-JNK1/2 pathway by Akt1 in exogenous estrogen-induced neuroprotection against transient global cerebral ischemia by a non-genomic mechanism in male rats

Numerous studies have demonstrated the neuroprotective effects of estrogen in experimental cerebral ischemia. To investigate molecular mechanisms of estrogen neuroprotection in global ischemia, immunoblotting, immunohistochemistry and Nissel-staining analysis were used. Our results showed that chronic pretreatment with beta-estradiol 3-benzoate (E2) enhanced Akt1 activation and reduced the activation of mixed-lineage kinase 3 (MLK3), mitogen-activated protein kinase kinase 4/7 (MKK4/7), and c-Jun N-terminal kinase 1/2 (JNK1/2) in the hippocampal CA1 subfield during reperfusion after 15 min of global ischemia. In addition, E2 reduced downstream JNK nuclear and non-nuclear components, c-Jun and Bcl-2 phosphorylation and Fas ligand protein expression induced by ischemia/reperfusion. Administration of phosphoinositide 3-kinase (PI3K) inhibitor LY 294,002 prevented both activation of Akt1 and inhibition of MLK3, MKK4/7 and JNK1/2. The interaction between ERalpha and the p85 subunit of PI3K was also examined. E2 and antiestrogen ICI 182,780 promoted and prevented this interaction, respectively. Furthermore, ICI 182,780 blocked both the activation of Akt1 and the inhibition of MLK3, MKK4/7 and JNK1/2. Photomicrographs of cresyl violet-stained brain sections showed that E2 reduced CA1 neuron loss after 5 days of reperfusion, which was abolished by ICI 182,780 and LY 294,002. Our data indicate that in response to estrogen, ERalpha interacts with PI3K to activate Akt1, which may inhibit the MLK3-MKK4/7-JNK1/2 pathway to protect hippocampal CA1 neurons against global cerebral ischemia in male rats.