Neuroprotective properties of selective estrogen receptor agonists in cultured neurons

Targeting Nuclear Receptors in Neurodegeneration and Neuroinflammation

Nuclear receptors, also known as ligand-activated transcription factors, regulate gene expression upon ligand signals and present as attractive therapeutic targets especially in chronic diseases. Despite the therapeutic relevance of some nuclear receptors in various pathologies, their potential in neurodegeneration and neuroinflammation is insufficiently established. This perspective gathers preclinical and clinical data for a potential role of individual nuclear receptors as future targets in Alzheimer's disease, Parkinson's disease, and multiple sclerosis, and concomitantly evaluates the level of medicinal chemistry targeting these proteins. Considerable evidence suggests the high promise of ligand-activated transcription factors to counteract neurodegenerative diseases with a particularly high potential of several orphan nuclear receptors. However, potent tools are lacking for orphan receptors, and limited central nervous system exposure or insufficient selectivity also compromises the suitability of well-studied nuclear receptor ligands for functional studies. Medicinal chemistry efforts are needed to develop dedicated high-quality tool compounds for the therapeutic validation of nuclear receptors in neurodegenerative pathologies.

Dynamic changes in binding interaction networks of sex steroids establish their non-classical effects

Non-classical signaling in the intracellular second messenger system plays a pivotal role in the cytoprotective effect of estradiol. Estrogen receptor is a common target of sex steroids and important in mediating estradiol-induced neuroprotection. Whereas the mechanism of genomic effects of sex steroids is fairly understood, their non-classical effects have not been elucidated completely. We use real time molecular dynamics calculations to uncover the interaction network of estradiol and activator estren. Besides steroid interactions, we also investigate the co-activation of the receptor. We show how steroid binding to the alternative binding site of the non-classical action is facilitated by the presence of a steroid in the classical binding site and the absence of the co-activator peptide. Uncovering such dynamic mechanisms behind steroid action will help the structure-based design of new drugs with non-classical responses and cytoprotective potential.

Nanosuspensions of a new compound, ER-β005, for enhanced oral bioavailability and improved analgesic efficacy

Estrogen receptor-β005 (ER-β005) is a novel compound developed by our group; however, its application has been greatly hindered due to its low solubility. A nanosuspension of insoluble drugs is a nanoscale colloidal dispersion that has extremely higher drug-loading compared with other nanomedicines. In this study, nanosuspensions of ER-β005 (Nano-ER-β005) stabilized by a food protein, β-casein (β-CN), were prepared via an antisolvent-precipitation method to improve oral absorption and thus promote therapeutic efficacy. Nano-ER-β005, which has a diameter of 110nm and drug-loading of 50%, was developed. Analyses of fluorescence and circular dichroism (CD) spectra demonstrated a strong interaction between β-CN and drug particles in Nano-ER-β005, indicating that β-CN is a potent nanosuspension stabilizer. The oral bioavailability of Nano-ER-β005 was 1.6-fold greater than that of raw drug particles. Additionally, ER-β005 was confirmed to have a strong therapeutic effect against pain reactions in animal models, and inhibition of this effect was significantly increased with Nano-ER-β005 treatment. In conclusion, by using β-CN as a stabilizer, nanosuspensions of ER-β005 were developed and oral absorption was enhanced. Moreover, ER-β005 is a powerful drug that inhibits pain reactions, and its therapeutic efficacy was markedly increased in the Nano-ER-β005.

Rescue from Sexually Dimorphic Neuronal Cell Death by Estradiol and PI3 Kinase Activity

Responses of primary hippocampal and cortical neurons derived from male and female rats to cellular stressors were studied. It is demonstrated that 17β-estradiol (E2), a potent neuroprotectant, protected the female neurons but had no effects on the male neurons from CoCl2- and glutamate-induced toxicity. Agonists of the estrogen receptor (ER) subtypes ERα and ERβ, DPN and PPT, respectively, had similar effects to E2. By contrast, effects of E2 were abolished by the ER antagonist ICI-182780, further corroborating the neuroprotective role of ERs. In male neurons, CoCl2 predominately activated the apoptosis-inducing factor (AIF)-dependent pathway and AIF translocation from the cytosol to the nucleus. In comparison, CoCl2 activated the caspase pathway and cytochrome c release in female neurons. The inhibitors of these pathways, namely DiQ for AIF and zVAD for caspase, specifically rescued CoCl2-induced cell death in male and female neurons, respectively. When zVAD and ICI-182780, and E2 were applied in combination, it was demonstrated E2 acted on the caspase pathway leading to female-specific neuroprotection. Furthermore, the PI3 kinase (PI3K) inhibitor blocked the rescue effects of DiQ and zVAD on the male and female neurons, respectively, suggesting that PI3K is a common upstream regulator for both pathways. The present study suggested that both sex-specific and nonspecific mechanisms played a role in neuronal responses to stressors and protective reagents.

Neuronal differentiation dictates estrogen-dependent survival and ERK1/2 kinetic by means of caveolin-1

Estrogens promote a plethora of effects in the CNS that profoundly affect both its development and mature functions and are able to influence proliferation, differentiation, survival and neurotransmission. The biological effects of estrogens are cell-context specific and also depend on differentiation and/or proliferation status in a given cell type. Furthermore, estrogens activate ERK1/2 in a variety of cellular types. Here, we investigated whether ERK1/2 activation might be influenced by estrogens stimulation according to the differentiation status and the molecular mechanisms underling this phenomenon. ERK1/2 exert an opposing role on survival and death, as well as on proliferation and differentiation depending on different kinetics of phosphorylation. Hence we report that mesencephalic primary cultures and the immortalized cell line mes-c-myc A1 express estrogen receptor α and activate ERK1/2 upon E₂ stimulation. Interestingly, following the arrest of proliferation and the onset of differentiation, we observe a change in the kinetic of ERKs phosphorylation induced by estrogens stimulation. Moreover, caveolin-1, a main constituent of caveolae, endogenously expressed and co-localized with ER-α on plasma membrane, is consistently up-regulated following differentiation and cell growth arrest. In addition, we demonstrate that siRNA-induced caveolin-1 down-regulation or disruption by means of ß-cyclodextrin treatment changes ERK1/2 phosphorylation in response to estrogens stimulation. Finally, caveolin-1 down-regulation abolishes estrogens-dependent survival of neurons. Thus, caveolin-1 appears to be an important player in mediating, at least, some of the non-genomic action of estrogens in neurons, in particular ERK1/2 kinetics of activation and survival.

MiR-7-1 potentiated estrogen receptor agonists for functional neuroprotection in VSC4.1 motoneurons

Protection of motoneurons is an important goal in the treatment of spinal cord injury (SCI). We tested whether neuroprotective microRNAs (miRs) like miR-206, miR-17, miR-21, miR-7-1, and miR-106a could enhance efficacy of estrogen receptor (ER) agonists such as 1,3,5-tris (4-hydroxyphenyl)-4-propyl-1H-pyrazole (PPT, ERα agonist), Way200070 (WAY, ERβ agonist), and estrogen (EST, ERα and ERβ agonist) in preventing apoptosis in the calcium ionophore (CI)-insulted ventral spinal cord 4.1 (VSC4.1) motoneurons. We determined that 200 nM CI induced 70% cell death. Treatment with 50 nM PPT, 100 nM WAY, and 150 nM EST induced overexpression of ERα, ERβ, and both receptors, respectively, at mRNA and protein levels. Treatment with ER agonists significantly upregulated miR-206, miR-17, and miR-7-1 in the CI-insulted VSC4.1 motoneurons. Transfection with miR-206, miR-17, or miR-7-1 mimic potentiated WAY or EST to inhibit apoptosis in the CI-insulted VSC4.1 motoneurons. Overexpression of miR-7-1 maximally increased efficacy of WAY and EST for down regulation of pro-apoptotic Bax and upregulation of anti-apoptotic Bcl-2. A search using microRNA database (miRDB) indicated that miR-7-1 could inhibit the expression of L-type Ca(2+) channel protein alpha 1C (CPα1C). miR-7-1 overexpression and WAY or EST treatment down regulated CPα1C but upregulated p-Akt to trigger cell survival signaling. The same therapeutic strategy increased expression of the Ca(2+)/calmodulin-dependent protein kinase II beta (CaMKIIβ) and the phosphorylated cAMP response element binding protein (p-CREB) so as to promote Bcl-2 transcription. Whole cell membrane potential and mitochondrial membrane potential studies indicated that miR-7-1 highly potentiated EST to preserve functionality in the CI-insulted VSC4.1 motoneurons. In conclusion, our data indicated that miR-7-1 most significantly potentiated efficacy of EST for functional neuroprotection and this therapeutic strategy could be used in the future to attenuate apoptosis of motoneurons in SCI.

Oestrogens as modulators of neuronal signalosomes and brain lipid homeostasis related to protection against neurodegeneration

Oestrogens trigger several pathways at the plasma membrane that exert beneficial actions against neurodegenerative diseases, such as Alzheimer's disease and Parkinson's disease. Part of these actions takes place in lipid rafts, which are membrane domains with a singular protein and lipid composition. These microdomains also represent a preferential site for signalling protein complexes, or signalosomes. A plausible hypothesis is that the dynamic interaction of signalosomes with different extracellular ligands may be at the basis of neuronal maintenance against different neuropathologies. Oestrogen receptors are localised in neuronal lipid rafts, taking part of macromolecular complexes together with a voltage-dependent anion channel (VDAC), and other molecules. Oestradiol binding to its receptor at this level enhances neuroprotection against amyloid-β degeneration through the activation of different signal transduction pathways, including VDAC gating modulation. Moreover, part of the stability and functionality of signalling platforms lays on the distribution of lipid hallmarks in these microstructures, which modulate membrane physicochemical properties, thus favouring molecular interactions. Interestingly, recent findings indicate a potential role of oestrogens in the preservation of neuronal membrane physiology related to lipid homeostasis. Thus, oestrogens and docosahexaenoic acid may act synergistically to stabilise brain lipid structure by regulating neuronal lipid biosynthetic pathways, suggesting that part of the neuroprotective effects elicited by oestrogens occur through mechanisms aimed at preserving lipid homeostasis. Overall, oestrogen mechanisms of neuroprotection may occur not only by its interaction with neuronal protein targets through nongenomic and genomic mechanisms, but also through its participation in membrane architecture stabilisation via 'lipostatic' mechanisms.

Transcriptome meta-analysis reveals a central role for sex steroids in the degeneration of hippocampal neurons in Alzheimer's disease

Background

Alzheimer’s disease is the most prevalent form of dementia. While a number of transcriptomic studies have been performed on the brains of Alzheimer’s specimens, no clear picture has emerged on the basis of neuronal transcriptional alterations linked to the disease. Therefore we performed a meta-analysis of studies comparing hippocampal neurons in Alzheimer’s disease to controls.

Results

Homeostatic processes, encompassing control of gene expression, apoptosis, and protein synthesis, were identified as disrupted during Alzheimer’s disease. Focusing on the genes carrying out these functions, a protein-protein interaction network was produced for graph theory and cluster exploration. This approach identified the androgen and estrogen receptors as key components and regulators of the disrupted homeostatic processes.

Conclusions

Our systems biology approach was able to identify the importance of the androgen and estrogen receptors in not only homeostatic cellular processes but also the role of other highly central genes in Alzheimer’s neuronal dysfunction. This is important due to the controversies and current work concerning hormone replacement therapy in postmenopausal women, and possibly men, as preventative approaches to ward off this neurodegenerative disorder.

Progesterone-estrogen interactions in synaptic plasticity and neuroprotection

17ß-Estradiol and progesterone exert a number of physiological effects throughout the brain due to interactions with several types of receptors belonging to the traditional family of intracellular hormonal receptors as well as to membrane-bound receptors. In particular, both hormones elicit rapid modifications of neuronal excitability that have been postulated to underlie their effects on synaptic plasticity and learning and memory. Likewise, both hormones have been shown to be neuroprotective under certain conditions, possibly due to the activation of pro-survival pathways and the inhibition of pro-apoptotic cascades. Because of the similarities in their cellular effects, there have been a number of questions raised by numerous observations that progesterone inhibits the effects of estrogen. In this manuscript, we first review the interactions between 17ß-estradiol (E2) and progesterone (P4) in synaptic plasticity, and conclude that, while E2 exerts a clear and important role in long-term potentiation of synaptic transmission in hippocampal neurons, the role of P4 is much less clear, and could be accounted by the direct or indirect regulation of GABAA receptors. We then discuss the neuroprotective roles of both hormones, in particular against excitotoxicity. In this case, the neuroprotective effects of these hormones are very similar to those of the neurotrophic factor BDNF. Interestingly, P4 antagonizes the effects of E2, possibly through the regulation of estrogen receptors or of proteins associated with the receptors or interactions with signaling pathways activated by E2. Overall, this review emphasizes the existence of common molecules and pathways that participate in the regulation of both synaptic plasticity and neurodegeneration.

Oestrogen receptor α agonist improved long-term ovariectomy-induced spatial cognition deficit in young rats

Ovariectomy is known as 'surgical menopause' with decreased levels of oestrogen in female rodents and its reported risks and adverse effects include cognitive impairment. In the brain, oestrogen exerts effects through its receptors, oestrogen receptor α (ERα) and β (ERβ). However, the role of ERα or ERβ in ovariectomy-induced cognitive impairment needs further investigation. Here, we observed that bilaterally ovariectomized 3-month-old rats showed obvious spatial learning and memory deficits in the Morris water maze with significant loss of neurons and synapses in the hippocampus. In addition to the rapid decline in serum oestradiol levels, the expression of ERα, but not ERβ, was decreased in the hippocampus starting 1 wk after ovariectomy. Prompt 4,4',4″-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol (PPT) treatment (1 mg/kg.d), an agonist of ERα, improved the spatial learning and memory ability of ovariectomized rats and rescued ovariectomy-induced neuron loss by up-regulating the level of BCLxl, an important anti-apoptosis protein. Furthermore, PPT treatment also improved ovariectomy-induced hippocampal synapse loss and up-regulated the levels of synaptic proteins (synapsin I, NR2A and GluR1) and the activates of CaMK Πα, ERK and Akt. Thus, these results demonstrated that ERα plays an important role in neuroprotection and that prompt ERα rescue is effective to improve hippocampal-dependent cognition deficit after long-term ovariectomy.

Dual effect of 17β-estradiol on NMDA-induced neuronal death: involvement of metabotropic glutamate receptor 1

Pretreatment with 10 nm 17β-estradiol (17βE2) or 100 μm of the metabotropic glutamate 1 receptor (mGlu1R) agonist, dihydroxyphenylglycine (DHPG), protected neurons against N-methyl-d-aspartate (NMDA) toxicity. This effect was sensitive to blockade of both estrogen receptors and mGlu1R by their respective antagonists. In contrast, 17βE2 and/or DHPG, added after a low-concentration NMDA pulse (45 μm), produced an opposite effect, i.e. an exacerbation of NMDA toxicity. Again this effect was prevented by both receptor antagonists. In support of an interaction of estrogen receptors and mGlu1R in mediating a neurotoxic response, exacerbation of NMDA toxicity by 17βE2 disappeared when cultures were treated with DHPG prior to NMDA challenge, and conversely, potentiation of NMDA-induced cell death by DHPG was prevented by pretreatment with 17βE2. Addition of calpain III inhibitor (10 μm), 2 h before NMDA, prevented the increased damage induced by the two agonists, an affect that can be secondary to cleavage of mGlu1R by calpain. Accordingly, NMDA stimulation reduced expression of the full-length (140 kDa) mGluR1, an effect partially reversed by calpain inhibitor. Finally, in the presence of NMDA, the ability of 17βE2 to stimulate phosphorylation of AKT and ERK was impaired. Pretreatment with calpain inhibitor prevented the reduction of phosphorylated ERK but had no significant effect on phosphorylated AKT. Accordingly, the inhibition of ERK signaling by U0126 (1 μm) counteracted the effect of calpain inhibition on 17βE2-induced exacerbation of NMDA toxicity. The present data confirm the dual role of estrogens in neurotoxicity/neuroprotection and highlight the role of the timing of exposure to estrogens.

Neuroprotection and estrogen receptors

This review is intended to assess the state of current knowledge on the role of estrogen receptors (ERs) in the neuroprotective effects of estrogens in models for acute neuronal injury and death. We evaluate the overall evidence that estrogens are neuroprotective in acute injury and critically assess the role of ERα, ERβ, GPR 30, and nonreceptor-mediated mechanisms in these robust neuroprotective effects of this ovarian steroid hormone. We conclude that all three receptors, as well as nonreceptor-mediated mechanisms can be involved in neuroprotection, depending on the model used, the level of estrogen administrated, and the mode of administration of the steroid. Also, the signaling pathways used by both ER-dependent and ER-independent mechanisms to exert neuroprotection are considered. Finally, further studies that are needed to parse out the relative contribution of receptor versus nonreceptor-mediated signaling are discussed.

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.

Estrogen receptors and type 1 metabotropic glutamate receptors are interdependent in protecting cortical neurons against β-amyloid toxicity

We examined the interaction between estrogen receptors (ERs) and type 1 metabotropic glutamate receptors (mGlu1 receptors) in mechanisms of neurodegeneration/neuroprotection using mixed cultures of cortical cells challenged with β-amyloid peptide. Both receptors were present in neurons, whereas only ERα but not mGlu1 receptors were found in astrocytes. Addition of 17β-estradiol (17βE2) protected cultured neurons against amyloid toxicity, and its action was mimicked by the selective ERα agonist, 1,3,5-tris(4-hydroxyphenyl)-4-propyl-1H-pyrazole (PPT) as well as by a cell-impermeable bovine serum albumin conjugate of 17βE2. The selective ERβ agonist, diarylpropionitrile (DPN), was only slightly neuroprotective. The mGlu1/5 receptor agonist, 3,5-dihydroxyphenylglycine (DHPG), was also neuroprotective against amyloid toxicity, and its action was abolished by the mGlu1 receptor antagonist, (3,4-dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl)-methanone (JNJ 16259685). Neuroprotection by 17βΕ2 or PPT (but not DPN) and DHPG was less than additive, suggesting that ERα and mGlu1 receptors activate the same pathway of cell survival. More important, neuroprotection by 17βΕ2 was abolished not only by the ER antagonist fulvestrant (ICI 182,780) but also by JNJ 16259685, and neuroprotection by DHPG was abolished by ICI 182,780. ERα and mGlu1 receptors were also interdependent in activating the phosphatidylinositol-3-kinase pathway, and pharmacological blockade of this pathway abolished neuroprotection by 17βE2, DHPG, or their combination. These data provide the first evidence that ERα and mGlu1 receptors critically interact in promoting neuroprotection, information that should be taken into account when the impact of estrogen on neurodegeneration associated with central nervous system disorders is examined.

Sex hormones, aging, and Alzheimer's disease

A promising strategy to delay and perhaps prevent Alzheimer's disease (AD) is to identify the age-related changes that put the brain at risk for the disease. A significant normal age change known to result in tissue-specific dysfunction is the depletion of sex hormones. In women, menopause results in a relatively rapid loss of estradiol and progesterone. In men, aging is associated with a comparatively gradual yet significant decrease in testosterone. We review a broad literature that indicates age-related losses of estrogens in women and testosterone in men are risk factors for AD. Both estrogens and androgens exert a wide range of protective actions that improve multiple aspects of neural health, suggesting that hormone therapies have the potential to combat AD pathogenesis. However, translation of experimental findings into effective therapies has proven challenging. One emerging treatment option is the development of novel hormone mimetics termed selective estrogen and androgen receptor modulators. Continued research of sex hormones and their roles in the aging brain is expected to yield valuable approaches to reducing the risk of AD.

Oestrogen receptors and signalling pathways: implications for neuroprotective effects of sex steroids in Parkinson's disease

Parkinson's disease (PD) is an age-related neurodegenerative disorder with a higher incidence in the male population. In the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD, 17β-oestradiol but not androgens were shown to protect dopamine (DA) neurones. We report that oestrogen receptors (ER)α and β distinctly contribute to neuroprotection against MPTP toxicity, as revealed by examining the membrane DA transporter (DAT), the vesicular monoamine transporter 2 (VMAT2) and tyrosine hyroxylase in ER wild-type (WT) and knockout (ERKO) C57Bl/6 male mice. Intact ERKOβ mice had lower levels of striatal DAT and VMAT2, whereas ERKOα mice were the most sensitive to MPTP toxicity compared to WT and ERKOβ mice and had the highest levels of plasma androgens. In both ERKO mice groups, treatment with 17β-oestradiol did not provide neuroprotection against MPTP, despite elevated plasma 17β-oestradiol levels. Next, the recently described membrane G protein-coupled oestrogen receptor (GPER1) was examined in female Macaca fascicularis monkeys and mice. GPER1 levels were increased in the caudate nucleus and the putamen of MPTP-monkeys and in the male mouse striatum lesioned with methamphetamine or MPTP. Moreover, neuroprotective mechanisms in response to oestrogens transmit via Akt/glycogen synthase kinase-3 (GSK3) signalling. The intact and lesioned striata of 17β-oestradiol treated monkeys, similar to that of mice, had increased levels of pAkt (Ser 473)/βIII-tubulin, pGSK3 (Ser 9)/βIII-tubulin and Akt/βIII-tubulin. Hence, ERα, ERβ and GPER1 activation by oestrogens is imperative in the modulation of ER signalling and serves as a basis for evaluating nigrostriatal neuroprotection.

Membrane-initiated signaling of estrogen related to neuroprotection. "Social networks" are required

Numerous studies indicate that estrogens are crucial in normal brain functioning and preservation against different injuries. At the neuronal membrane, estrogens, binding to estrogen receptors (ERs) or other surface targets, exert rapid actions involving a plethora of signaling pathways that may converge in neuronal survival. Emerging work reveals that at least part of these actions may require the compartmentalization of ERs in signaling platforms, composed of macromolecular signaling proteins and particular lipid composition integrated in lipid rafts. These particular microstructures may provide the optimal microenvironment to trigger multiple ER interactions that may be crucial for neuroprotection against different brain impairments, such as Alzheimer's disease (AD). In this order of ideas, recent evidence has demonstrated that a membrane ER (mER) physically interacts with a voltage-dependent anion channel (VDAC) in lipid rafts from septal, hippocampal and cortical neurons, and these interactions may have important consequences in the alternative mechanisms developed by estrogens to achieve neuroprotection against amyloid beta (Aβ)-induced toxicity. This review includes a survey of some of the rapid mechanisms developed by estrogen to prevent neuronal death, and the ER interactions that are involved in the structural maintenance and signal transduction mechanisms important for neuronal survival against AD neuro-pathology. A special emphasis is put on the biological relevance of neuronal membrane VDAC in Aβ-related neurotoxicity, and the potential modulation of this channel as a part of a signaling complex with mER, which may be modified in AD brains.

Estrogen protection against mitochondrial toxin-induced cell death in hippocampal neurons: antagonism by progesterone

Previously we demonstrated that mitochondrial dysfunction plays a critical role in the pathogenesis of Alzheimer's disease. Further, we have shown that the neuroprotective effects of 17β-estradiol (E2) are dependent upon mitochondrial function. In the current study, we sought to identify mitochondrial sites of E2 action that mediate neuroprotection by assessing the efficacy of E2 to protect neurons against inhibitors of mitochondrial respiration which target specific complexes within the respiratory chain. Subsequently, the impact of progesterone (P4) on E2-induced prevention against mitochondrial toxins was investigated. Mitochondrial inhibitors, rotenone, 3-NPA, antimycin, KCN, and oligomycin, exhibited concentration dependent toxicity in primary hippocampal neurons. The concentration inducing 30% cell death (LD30) was selected for analyses assessing the neuroprotective efficacy of ovarian hormones (E2 and P4). Pretreatment of hippocampal neurons with E2 significantly protected against 3-NPA (7.5mM) and antimycin (125 μM) induced cell death and was moderately neuroprotective against rotenone (3 μM). E2 was ineffective against KCN and oligomycin-induced cell death. Pretreatment with P4 was without effect against these mitochondrial inhibitors. Co-administration of P4 with E2 abolished E2 induced neuroprotection against 3-NPA and antimycin. Additional metabolic analyses indicated that E2 and P4 separately increased mitochondrial respiratory capacity whereas the co-administration of E2 and P4 resulted in diminished mitochondrial respiration. These findings indicate that E2 protects against mitochondrial toxins that target complexes I, II and III whereas P4 was without effect. The data also predict that continuous combined co-administration of estrogen and progesterone common to many hormone therapy regimens is unlikely to prevent the deficits in mitochondrial function.

Estrogen receptor agonists and estrogen attenuate TNF-α-induced apoptosis in VSC4.1 motoneurons

Tumor necrosis factor-alpha (TNF-α) may cause apoptosis and inflammation in amyotrophic lateral sclerosis (ALS) and spinal cord injury (SCI). Recent studies suggest that estrogen (EST) provides neuroprotection against SCI. We tested whether 1,3,5-tris (4-hydroxyphenyl)-4-propyl-1H-pyrazole (PPT) (EST receptor alpha (ERα) agonist), 2,3-bis (4-hydroxyphenyl) propionitrile (DPN) (EST receptor beta (ERβ) agonist), or EST itself would prevent apoptosis in VSC4.1 motoneurons following exposure to TNF-α. Cells were exposed to TNF-α and 15 min later treated with PPT, DPN, or EST. Posttreatment with 50 nM PPT, 50 nM DPN, or 150 nM EST prevented cell death in VSC4.1 motoneurons. Treatment of VSC4.1 motoneurons with PPT, DPN, or EST induced overexpression of ERα, ERβ, or both, which contributed to neuroprotection by upregulating expression of anti-apoptotic proteins (p-AKT, p-CREB, Bcl-2, and p-Src). Our analyses also revealed that EST agonists and EST increased phosphorylation of extracellular signal-regulated kinase (ERK). The L-type Ca(2+) channel inhibitor, nifedipine (10 μM), partially inhibited EST agonist and EST-induced increase in phosphorylated ERK expression. The mitogen-activated protein kinase inhibitor, PD98059 (5 μM), partially prevented ER agonists and EST from providing neuroprotection to TNF-α toxicity. Presence of the nuclear ER antagonist, ICI 182 780 (10 μM), blocked the neuroprotection provided by all three ER agonists tested. Taken together, our data indicate that both ERα and ERβ contribute to PPT, DPN, or EST-mediated neuroprotection with similar signaling profiles. Our data strongly imply that PPT, DPN, or EST can be used as effective neuroprotective agents to attenuate motoneuron death in ALS and SCI.

Roles of estrogen receptors alpha and beta in sexually dimorphic neuroprotection against glutamate toxicity

Although most agree that 17β-estradiol is neuroprotective via a variety of mechanisms, less is known about the role that biological sex plays in receptor-mediated estradiol neuroprotection. To address this issue we isolated primary cortical neurons from rat pups sorted by sex and assessed the ability of estradiol to protect the neurons from death induced by glutamate. Five-minute pretreatment with 10-50 nM 17β-estradiol protected female but not male neurons from glutamate toxicity 24 h later. Both estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) are expressed in these cultures. Experiments using an ERα selective agonist or antagonist indicate that this receptor is important for neuroprotection in female cortical neurons. The ERβ selective agonist conveys a small degree of neuroprotection to both male and female cortical neurons. Interestingly, we found that 17α estradiol and the novel membrane estrogen receptor (mER) agonist STX, but not bovine serum albumin conjugated estradiol or the GPR30 agonist G1 were neuroprotective in both male and female neurons. Taken together these data highlight a role for ERα in sexually dimorphic neuroprotection.

Bcl-2 and Caspase 3 mRNA levels in the testes of gudgeon, Gobio gobio, exposed to ethinylestradiol (EE2)

Apoptosis inhibition has been reported in the male reproductive tract of teleost fish exposed to 17beta-estrogen or estrogen-like compounds. In order to understand the molecular mechanisms of cell death inhibition, this study examined 2 genes involved in the apoptotic pathway, Bcl-2 and Caspase 3, an anti-apoptotic and a pro-apoptotic genes, respectively. Partial cDNA sequences of Bcl-2 and Caspase 3 were cloned from gudgeon (Gobio gobio), a common European cyprinid fish. To follow mRNA levels of Bcl-2 and Caspase 3 under xenoestrogen exposure, we first performed an in vitro experiment on fish testis exposed to the most potent xenoestrogen found in the environment, ethinylestradiol (EE2). We further studied mRNA expression of both genes in the testis of fish exposed to xenoestrogens in situ. In the in vitro experiment, fragments of gudgeon testis were exposed for 21 days to 10(-3), 10(-2), 10(-1), 1 and 10 microg/L of EE2, as well as to positive (10(-1) microg/L of E2) and ethanol control medium. Results showed a significant induction of Bcl-2 mRNA at 10(-1) microg/L (p<0.05). Surprisingly, Caspase 3, a cell death effector, displayed the same profile as observed for the anti-apoptotic gene Bcl-2. In the experiment on wild gudgeon exposed from birth to an estrogenic sewage treatment plant effluent, the mRNA expression of Bcl-2 and Caspase 3 in feminized fish (ovotestis) was not significantly different due to high variability of expression between individuals. At the current state of knowledge on spermatogenesis disruption in teleost fish, in vitro studies seem better adapted than in situ investigations to enlighten the molecular pathway of apoptosis inhibition in testis exposed to xenoestrogens.

[Alzheimer disease, memory and estrogen]

Epidemiological studies of Alzheimer disease have shown a higher prevalence of women. Some data argue for a link between Alzheimer disease and the decrease of estrogen in post-menopausal women. Animal studies have shown a beneficial effect of estrogen on memory with a decrease of amyloid deposition in models of AD, whereas estrogen has a positive effect on BDNF. Six studies have shown a positive effect of estrogen therapy on memory and studies on structural and functional imaging have shown a beneficial effect of estrogens but the largest study on prevention of dementia with estrogens (WHI) showed a deleterious effect. To better understand this paradoxical situation, we reviewed the literature on estrogens, memory and Alzheimer disease. We first discuss the promnesic effect of estrogen on mice and rats, second the neuroprotector effect of estrogen on animal models of Alzheimer disease, and third the available human studies. We hypothesize a link with the time of instauration of the estrogen treatment. Nevertheless this hypothesis remains to be demonstrated.

17alpha-ethinylestradiol induces an imbalance between apoptosis and cell proliferation to sex steroid disruption in a testis culture of gudgeon, Gobio gobio

The aim of this study was to investigate the effect of the most potent xenoestrogen currently found in the environment, ethinylestradiol (EE2), on some physiological events occurring during early spermatogenesis of gudgeon (Gobio gobio), a common European fish species. Physiological pathways studies were apoptosis, cell proliferation, and steroidogenesis on sex steroids (testosterone [T], 11-ketotestosterone [11-KT], and 17beta-estradiol [E2]). Testis pieces were cultured in vitro during 21 d at 10(-4), 10(-3), 10(-2), 10(-1), 1 and 10 microg/L of EE2 as well as in positive (10(-1) microg/L of E2) and ethanol control medium. Apoptosis and cell proliferation displayed opposite responses related to the EE2 concentration. When apoptosis inhibition was observed, cell proliferation was induced at 10(-2) and 10(-1) microg/L of EE2 as well as in the positive control. In contrast, a massive cell death was detected for high EE2 concentrations (1 and 10 microg/L). Steroidogenesis was also disrupted in a dose-related manner. 11-Ketotestosterone was depressed at 10(-2) and 10(-1) microg/L of EE2 whereas E2 was detectable in the medium only at 10(-3), 10(-2), and 10(-1) microg/L of EE2. High concentrations of T were detected in the medium at 10(-3), 10(-2), and 10(-1) microg/L of EE2 but depressed at 1 and 10 microg/L of EE2. In conclusion, intermediate EE2 concentrations (10(-2) and 10(-1) microg/L) used in this experimental design have obviously disrupted early spermatogenesis, leading to an imbalance between cell death and cell proliferation in a sex steroid environment toward E2. The results of the present study could be the basis conditions for oocyte development within the testis of a common teleost fish under xenoestrogen exposure.

Neuroprotective actions of selective estrogen receptor modulators

Decreasing levels of sex hormones with aging may have a negative impact on brain function, since this decrease is associated with the progression of neurodegenerative disorders, increased depressive symptoms and other psychological disturbances. Extensive evidence from animal studies indicates that sex steroids, in particular estradiol, are neuroprotective. However, the potential benefits of estradiol therapy for the brain are counterbalanced by negative, life-threatening risks in the periphery. A potential therapeutic alternative to promote neuroprotection is the use of selective estrogen receptor modulators (SERMs), which may be designed to act with tissue selectivity as estrogen receptor agonists in the brain and not in other organs. Currently available SERMs act not only with tissue selectivity, but also with cellular selectivity within the brain and differentially modulate the activation of microglia, astroglia and neurons. Finally, SERMs may promote the interaction of estrogen receptors with the neuroprotective signaling of growth factors, such as the phosphatidylinositol 3-kinase/glycogen synthase kinase 3 pathway.

Protective actions of sex steroid hormones in Alzheimer's disease

Risk for Alzheimer's disease (AD) is associated with age-related loss of sex steroid hormones in both women and men. In post-menopausal women, the precipitous depletion of estrogens and progestogens is hypothesized to increase susceptibility to AD pathogenesis, a concept largely supported by epidemiological evidence but refuted by some clinical findings. Experimental evidence suggests that estrogens have numerous neuroprotective actions relevant to prevention of AD, in particular promotion of neuron viability and reduction of beta-amyloid accumulation, a critical factor in the initiation and progression of AD. Recent findings suggest neural responsiveness to estrogen can diminish with age, reducing neuroprotective actions of estrogen and, consequently, potentially limiting the utility of hormone therapies in aged women. In addition, estrogen neuroprotective actions are also modulated by progestogens. Specifically, continuous progestogen exposure is associated with inhibition of estrogen actions whereas cyclic delivery of progestogens may enhance neural benefits of estrogen. In recent years, emerging literature has begun to elucidate a parallel relationship of sex steroid hormones and AD risk in men. Normal age-related testosterone loss in men is associated with increased risk to several diseases including AD. Like estrogen, testosterone has been established as an endogenous neuroprotective factor that not only increases neuronal resilience against AD-related insults, but also reduces beta-amyloid accumulation. Androgen neuroprotective effects are mediated both directly by activation of androgen pathways and indirectly by aromatization to estradiol and initiation of protective estrogen signaling mechanisms. The successful use of hormone therapies in aging men and women to delay, prevent, and or treat AD will require additional research to optimize key parameters of hormone therapy and may benefit from the continuing development of selective estrogen and androgen receptor modulators.

17beta-estradiol-mediated neuroprotection and ERK activation require a pertussis toxin-sensitive mechanism involving GRK2 and beta-arrestin-1

17-beta-Estradiol (E2) is a steroid hormone involved in numerous bodily functions, including several brain functions. In particular, E2 is neuroprotective against excitotoxicity and other forms of brain injuries, a property that requires the extracellular signal-regulated kinase (ERK) pathway and possibly that of other signaling molecules. The mechanism and identity of the receptor(s) involved remain unclear, although it has been suggested that E2 receptor alpha (ERalpha) and G proteins are involved. We, therefore, investigated whether E2-mediated neuroprotection and ERK activation were linked to pertussis toxin (PTX)-sensitive G-protein-coupled effector systems. Biochemical and image analysis of organotypic hippocampal slices and cortical neuronal cultures showed that E2-mediated neuroprotection as well as E2-induced ERK activation were sensitive to PTX. The sensitivity to PTX suggested a possible role of G-protein- and beta-arrestin-mediated mechanisms. Western immunoblots from E2-treated cortical neuronal cultures revealed an increase in phosphorylation of both G-protein-coupled receptor-kinase 2 and beta-arrestin-1, a G-protein-coupled receptor adaptor protein. Transfection of neurons with beta-arrestin-1 small interfering RNA prevented E2-induced ERK activation. Coimmunoprecipitation experiments indicated that E2 increased the recruitment of beta-arrestin-1 and c-Src to ERalpha. These findings suggested that ERalpha is regulated by a mechanism associated with receptor desensitization and downregulation. In support of this idea, we found that E2 treatment of cortical synaptoneurosomes resulted in internalization of ERalpha, whereas treatment of cortical neurons with the ER agonists E-6-BSA-FITC [beta-estradiol-6-(O-carboxymethyl)oxime-bovine serum albumin conjugated with fluorescein isothiocyanate] and E-6-biotin [1,3,5(10)-estratrien-3,17beta-diol-6-one-6-carboxymethloxime-NH-propyl-biotin] resulted in agonist internalization. These results demonstrate that E2-mediated neuroprotection and ERK activation involve ERalpha activation of G-protein- and beta-arrestin-mediated mechanisms.

The inhibitory effect of diarylpropionitrile, a selective agonist of estrogen receptor beta, on the growth of MC38 colon cancer line

The protective role of estrogens in the colon carcinogenesis has been suggested for many years and attributed mainly to estrogen receptor beta (ERbeta). However, the direct effect of estrogens and their action through ERbeta on the growth of colon cancer have been rarely studied. The aim of this study was to examine the effect of various concentrations (10(-4)-10(-12)M) of diarylpropionitrile (DPN)--a selective agonist of ERbeta--on the growth of murine MC38 colon cancer line. Moreover, the aim of this paper was the immunohistochemical assessment of estrogen and progesterone receptor expression in human colon tissues and in MC38 cells (only ERbeta). We found that DPN induced a growth inhibition of MC38 cancer (50-94% of control group) at the highest (10(-4)M) and two lowest concentrations (10(-11) and 10(-12)M). Furthermore, we detected a nuclear-cytoplasmic expression of ERbeta in human normal and neoplastic colon tissues and in the studied MC38 cancer cells. The inhibitory effect of DPN on the growth of MC38 colon cancer line suggests a possibility of using a selective estrogen receptor agonist in the treatment of colon cancer.

Gender and the injured brain

Anesthesiologists are frequently confronted with patients who are at risk for neurological complications due to perioperative stroke or prior traumatic brain injury. In this review, we address the growing and fascinating body of data that suggests gender and sex steroids influence the pathophysiology of injury and outcome for these patients. Cerebral ischemia, traumatic brain injury, and epilepsy are reviewed in the context of potential sex differences in mechanisms and outcomes of brain injury and the role of estrogen, progesterone, and androgens in shaping these processes. Lastly, implications for current and future perioperative and intensive care are identified.

Selective estrogen receptor modulators differentially regulate Alzheimer-like changes in female 3xTg-AD mice

Estrogen-based hormone therapy (HT) in postmenopausal women may reduce the risk of Alzheimer's disease (AD), although HT remains controversial. One key concern with HT is the potential of adverse outcomes such as breast and uterine cancer. A promising strategy to maximize HT benefits and minimize HT risks is the use of selective estrogen receptor modulators (SERMs) that exert tissue-specific estrogenic effects. To begin investigating the SERM approach in reducing the risk of AD, we investigated whether AD-like neuropathology in the 3xTg-AD mouse model of AD is regulated by the SERMs propylpyrazole triol (PPT) and diarylpropionitrile (DPN) that exhibit relative specificity for estrogen receptor-alpha and -beta, respectively. Consistent with our previous observations, we found that ovariectomy-induced hormone depletion in adult female 3xTg-AD mice significantly increased accumulation of beta-amyloid protein (Abeta) and decreased hippocampal-dependent behavioral performance. Treatment with 17beta-estradiol (E2) prevented the ovariectomized-induced worsening of both pathologies. PPT treatment was similar to E2 in terms of reducing Abeta accumulation in hippocampus, subiculum, and amygdala but comparatively less effective in frontal cortex. In contrast, DPN did not significantly reduce Abeta accumulation in hippocampus and subiculum, was partially effective in frontal cortex, and nearly as effective as E2 in amygdala. Furthermore, PPT but not DPN mimicked the ability of E2 to improve behavioral performance. These findings provide initial evidence of beneficial actions of SERMs in a mouse model of AD and support continued investigation of SERMs as an alternative to estrogen-based HT in reducing the risk of AD in postmenopausal women.

Axogenic effect of estrogen in male rat hypothalamic neurons involves Ca(2+), protein kinase C, and extracellular signal-regulated kinase signaling

17-beta-Estradiol (E2) stimulates the growth of axons in male-derived hypothalamic neurons in vitro. This effect is not exerted through the classical intracellular estrogen receptor (ER) but depends on a membrane mechanism involving TrkB. In the present study, we investigate the intracellular signaling cascade that mediates the axogenic effect of E2. Treatment with an intracellular Ca(2+) chelator, a Ca(2+)-dependent protein kinase C (PKC) inhibitor, or two specific inhibitors of extracellular signal-regulated kinases (ERK) mitogen-activated protein kinases (MAPK) completely inhibited the E2-induced axogenesis. E2 and the membrane-impermeant construct E2BSA rapidly induced phosphorylation of ERK, which was blocked by the specific inhibitor of the ERK pathway UO126 but not by the ER antagonist ICI 182,780. Decrease of intracellular free Ca(2+) or disruption of PKC activation by Ro 32-0432 attenuated ERK activation, indicating the confluence of signals in the MAPK pathway. Subcellular analysis of ERK demonstrated that the phospho-ERK signal is augmented in the nucleus after 15 min of E2 stimulation. We have also shown that E2 increased phosphorylation of CREB via ERK signaling. In summary, this study demonstrates that E2, probably via a membrane-associated receptor, induces axonal growth by activating CREB phosphorylation through ERK signaling by a mechanism involving Ca(2+) and PKC activation.

17alpha-Estradiol is neuroprotective in male and female rats in a model of early brain injury

One of the most critical times in the human lifespan is the late embryonic/early postnatal period, due to the careful orchestration of numerous events leading to normal brain development. This period is also characterized by a heightened incidence of harmful events that act via the GABAergic system, including hypoxia-ischemia, seizures and drug exposure from maternal circulation (e.g., alcohol, barbiturates). Unfortunately, there are few effective means of attenuating damage in the immature brain. In the current investigation, we documented the effect of 17alpha-estradiol, a natural epimer of 17beta-estradiol that has potent estrogen receptor-independent actions, on excessive GABA(A) receptor-induced damage to the neonatal brain. We observed that treatment with 17alpha-estradiol significantly attenuates the GABA(A) receptor-induced reduction in hippocampal volume and impaired hippocampal-dependent performance on the Morris water maze and radial arm maze. 17alpha-Estradiol-mediated neuroprotection is hypothesized to be achieved by attenuating GABA(A) receptor-induced cell loss, assessed in primary hippocampal cultures using both the lactate dehydrogenase assay and TUNEL, with equivalent prevention of cell loss in the presence or absence of the estrogen receptor antagonist, ICI-182,780. These data highlight one of the initial investigations of the neuroprotective potential of 17alpha-estradiol in an in vivo model of injury to the immature brain.

Contribution of estrogen receptors alpha and beta to the effects of estradiol in the brain

Clinical and experimental studies show a modulatory role of estrogens in the brain and suggest their beneficial action in mental and neurodegenerative diseases. The estrogen receptors ERalpha and ERbeta are present in the brain and their targeting could bring selectivity and reduced risk of cancer. Implication of ERs in the effect of estradiol on dopamine, opiate and glutamate neurotransmission is reviewed. The ERalpha agonist, PPT, is shown as estradiol to modulate hippocampal NMDA receptors and AMPA receptors in cortex and striatum of ovariectomized rats whereas the ERbeta agonist DPN is inactive. Striatal DPN activity suggests implication of ERbeta in estradiol modulation of D2 receptors and transporters in ovariectomized rats and is supported by the lack of effect of estradiol in ERbeta knockout (ERKObeta) mice. Both ERalpha and ERbeta agonists modulate striatal preproenkephalin (PPE) gene expression in ovariectomized rats. In male mice PPT protects against MPTP toxicity to striatal dopamine; this implicates Akt/GSK3beta signaling and the apoptotic regulators Bcl2 and Bad. This suggests a role for ERalpha in striatal dopamine neuroprotection. ERKOalpha mice are more susceptible to MPTP toxicity and not protected by estradiol; differences in ERKObeta mice are subtler. These results suggest therapeutic potential for the brain of ER specific agonists.

Estradiol-17beta-induced human neural progenitor cell proliferation is mediated by an estrogen receptor beta-phosphorylated extracellularly regulated kinase pathway

Estradiol-17beta (E(2)) induces rodent hippocampal neural progenitor cell (NPC) proliferation in vitro, in vivo, and after brain injury. The purpose of the present investigation was to determine whether E(2)-induced proliferation observed in rodent model systems generalized to cells of human neural origin and the signaling pathway by which E(2) promotes mitosis of human NPCs (hNPCs). Results of these analyses indicate that E(2) induced a significant increase in hNPC proliferation in a time- and dose-dependent manner. E(2)-induced hNPC DNA replication was paralleled by elevated cell cycle protein expression and centrosome amplification, which was associated with augmentation of total cell number. To determine whether estrogen receptor (ER) and which ER subtype were required for E(2)-induced hNPC proliferation, ER expression was first determined by real-time RT-PCR, followed by Western blot analysis, and subsequently verified pharmacologically using ERalpha or beta-selective ligands. Results of these analyses indicated that ERbeta expression was predominant relative to ERalpha, which was barely detectable in hNPCs. Activation of ERbeta by the ERbeta-selective ligand, diarylpropionitrile, led to an increase in phosphorylated extracellular signal-regulated kinase, and subsequent centrosome amplification and hNPC proliferation, which were blocked by the MEKK antagonist, UO126, but not its inactive analog, UO124. These findings, for the first time, demonstrate the molecular cascade and related cell biology events involved in E(2)-induced hNPC proliferation in vitro. Therapeutic implications of these findings relevant to hormone therapy and prevention of neurodegenerative disease are discussed.

Upregulation of neurotrophic factor-related gene expression in retina with experimental autoimmune uveoretinitis by intravitreal injection of tacrolimus (FK506)

AIM

The current study was designed to determine whether intravitreal injection of tacrolimus (FK506) modulates the gene expression of neurotrophic factor-related molecules in the retina from eyes with induced experimental autoimmune uveoretinitis (EAU) in rats.

METHODS

Rats were immunised with interphotoreceptor retinoid binding protein peptide (R14) and given intravitreal injection of tacrolimus on day 12 after immunisation. As control, immunised rats received intravitreal injection of vehicle. On day 15 after immunisation, changes in the genetic programme associated with neuroprotection and inflammatory responses in the retinas from both groups were determined by DNA microarray analyses and confirmed by real-time PCR analyses.

RESULTS

The gene expression of inflammatory responses was markedly reduced in tacrolimus-treated eyes. Genes for molecules associated with neuroprotection (oestrogen receptor, erythropoietin receptor, gamma-aminobutyric acid receptor, protein kinase C, glial cell line-derived neurotrophic factor receptor, fibroblast growth factor and neuropeptide Y receptor) were upregulated in the retinas from tacrolimus-treated eyes.

CONCLUSIONS

Intravitreal injection of tacrolimus modulated the genes related to neuroprotection in the retina during the ongoing process of EAU. This treatment may be useful for the neuroprotection of retina with severe uveitis as well as for immunosuppression in the uveitic eyes.

Estrogen receptor alpha and beta differentially regulate intracellular Ca(2+) dynamics leading to ERK phosphorylation and estrogen neuroprotection in hippocampal neurons

Our previous analyses indicated that both estrogen receptor (ER) subtypes, ERalpha and ERbeta, contribute to estrogen neuroprotection [Zhao, L., Wu, T.-W., Brinton, R.D., 2004. Estrogen receptor subtypes alpha and beta contribute to neuroprotection and increased Bcl-2 expression in primary hippocampal neurons. Brain Res. 1010, 22-34]. In the present study, we sought to determine the underlying mechanisms by which ERalpha and ERbeta promote neuronal function, with a focus on neuroprotection, and whether these mechanisms are consistent with a classical nuclear or membrane ER-mediated response. Results of these analyses demonstrated that both the ERalpha-selective agonist, PPT (100 pM), and the ERbeta-selective agonist, DPN (100 pM), were effective in dynamically but differentially regulating intracellular calcium (Ca(2+)) signaling in hippocampal neurons. Consistent with the direct measurement of neuroprotective outcomes [Zhao, L., Wu, T.-W., Brinton, R.D., 2004. Estrogen receptor subtypes alpha and beta contribute to neuroprotection and increased Bcl-2 expression in primary hippocampal neurons. Brain Res. 1010, 22-34], PPT and DPN exerted comparable efficacy in attenuating excitotoxic glutamate (200 microM)-induced intracellular Ca(2+) rise. In contrast, DPN was more efficacious than PPT in potentiating a physiological concentration of glutamate (25 microM)-induced intracellular Ca(2+) rise in these neurons. Further analyses revealed that both PPT and DPN increased ERK phosphorylation, however, the temporal profile and magnitude of response were unique to each molecule. The presence of the L-type Ca(2+) channel inhibitor, nifedipine (10 microM), partially inhibited 17beta-estradiol- and PPT-induced increase in phosphorylated ERK expression, whereas it induced a complete inhibition of DPN-induced increase in ERK phosphorylation. Additional neuroprotective experiments demonstrated that the MAPK inhibitor, PD 98059 (5 microM), partially blocked 17beta-estradiol-induced promotion of neuronal survival against excitotoxic glutamate (200 microM)-induced neurotoxicity, whereas it completely blocked both PPT- and DPN-induced neuroprotection. The presence of the nuclear ER antagonist, ICI 182,780 (1 microM), not only failed to block all 3 molecule-induced neuroprotection, but coadministration of ICI 182,780 and each single molecule exerted a comparable or even greater neuroprotection. Taken together, as an expansion of our previous analyses, these data indicate that both ERalpha and ERbeta contribute to neuronal mechanisms leading to estrogen promotion of neuronal function but with unique signaling profiles. Activation of ERbeta and induction of intracellular Ca(2+) influx via the L-type channels appears to be more closely associated with estrogen promotion of memory mechanisms. However, ERalpha and ERbeta play an equivalently important role in mediating estrogen neuroprotection, and, which is dependent upon the activation of the MAPK signaling. Further, the present analyses suggest that separate from a classical nuclear ER-mediated response, estrogen promotes neuronal survival likely through a non-nuclear cytoplasm or membrane-associated ER-mediated rapid signaling cascade.

The activation by estrogen receptor agonists of the BKCa-channel in human cardiac fibroblasts

The agonists selective for estrogen receptor (ER)-alpha (4,4',4''-(4-propyl-[(1)H]-pyrazole-1,3,5-triyl) tris-phenol, PPT) and ER-beta (2,3-bis(4-hydroxyphenyl)-propionitrile, DPN) are known to stimulate ER-alpha and ER-beta receptors, respectively. It remains unknown whether these two agents regulate the activity of ion channels via a direct stimulation. In this study, we tested the hypothesis that DPN or PPT stimulates the large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels in cultured human cardiac fibroblasts (HCFs). In whole-cell configuration, depolarizing pulses evoked K(+) outward currents in an outward rectification in HCFs, the amplitude of which was increased in the presence of DPN or PPT. In inside-out patches, the activity of BK(Ca)-channel with a conductance of 167+/-8 pS was observed in these cells. PPT or DPN applied to the intracellular face of the membrane enhanced the activity of BK(Ca) channels with no change in single-channel conductance. DPN and PPT increased BK(Ca)-channel activity with an EC(50) value of 2.3 and 2.6 microM, respectively. The mean closed time of these channels during the exposure to these compounds was reduced with no change in the gating charge of the channels. Intracellular Ca(2+) was not altered by these two compounds. RT-PCR analysis revealed that no change in the transcriptional level of the BK(Ca)-channel alpha-subunit was observed in chronic treatment with these two compounds. PPT- and DPN-stimulated increase in BK(Ca) channels reveal novel pharmacological properties attributable to the activity of these channels, and their increase in BK(Ca) channels activity in HCFs may contribute to cell function.

Neuroprotection by estrogen against MPP+-induced dopamine neuron death is mediated by ERalpha in primary cultures of mouse mesencephalon

Estrogen involvement in neuroprotection is now widely accepted, although the specific molecular and cellular mechanisms of estrogen action in neuroprotection remain unclear. This study examines estrogenic effects in a mixed population of cells in attempts to identify the contributing cells that result in estrogen-mediated neuroprotection. Utilizing primary mesencephalic neurons, we found expression of both estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta) with a predominance of ERalpha on both dopamine neurons and astrocytes. We also found that 17beta-estradiol protects dopamine neurons from injury induced by the complex I inhibitor, 1-methyl-4-phenyl pyridinium (MPP(+)) in a time- and ER-dependent manner. At least 4 h of estrogen pre-treatment was required to elicit protection, an effect that was blocked by the ER antagonist, ICI 182,780. Moreover, ERalpha mediated the protection afforded by estrogen since only the ERalpha agonist, HPTE, but not the ERbeta agonist, DPN, protected against dopamine cell loss. Since glial cells were shown to express significant levels of ERalpha, we investigated a possible indirect mechanism of estrogen-mediated neuroprotection through glial cell interaction. Removal of glial cells from the cultures by application of the mitotic inhibitor, 5-fluoro-2'-deoxyuridine, significantly reduced the neuroprotective effects of estrogen. These data indicate that neuroprotection provided by estrogen against MPP(+) toxicity is mediated by ERalpha and involves an interplay among at least two cell types.

Neuroprotective effects of icaritin against beta amyloid-induced neurotoxicity in primary cultured rat neuronal cells via estrogen-dependent pathway

Beta-amyloid protein (Abeta) is the hallmark of pathogenic neurotoxins which contribute greatly to Alzheimer's disease (AD)-associated cascade including severe neuronal loss. In present study, icaritin, an active natural ingredient from a Chinese plant, Epimedium sagittatum maxim, was investigated to assess its neuroprotective effect against the toxicity induced with Abeta(25-35) in primary cultured rat cortical neuronal cells as well as the underlying mechanisms. Abeta(25-35) induced neuronal toxicity, characterized by decreased cell viability, lactate dehydrogenase (LDH) release, and neuronal DNA condensation, which is associated with both the loss of membrane potential and the alteration of the expression of Bcl-2 family proteins. The phenotype alternation induced by Abeta(25-35) could be reversed by icaritin. Furthermore, the neuroprotective effects of icaritin mentioned above were estrogen receptor dependent due to the blocking action induced by estrogen receptor antagonist ICI 182,780 and well matched binding affinity with estrogen receptor by a receptor-ligand docking experiment. mitogen-activated protein kinase/extracellular signal-regulated kinase kinase inhibitor PD98059 weakened the protective effects, which implied mitogen-activated protein kinase/extracellular signal-regulated kinase pathway may also be involved in and partly contributed to the neuroprotective effects of icaritin.

Multiple pathways transmit neuroprotective effects of gonadal steroids

Numerous preclinical studies suggest that gonadal steroids, particularly estrogen, may be neuroprotective against insult or disease progression. This paper reviews the mechanisms contributing to estrogen-mediated neuroprotection. Rapid signaling pathways, such as MAPK, PI3K, Akt, and PKC, are required for estrogen's ability to provide neuroprotection. These rapid signaling pathways converge on genomic pathways to modulate transcription of E2-responsive genes via ERE-dependent and ERE-independent mechanisms. It is clear that both rapid signaling and transcription are important for estrogen's neuroprotective effects. A mechanistic understanding of estrogen-mediated neuroprotection is crucial for the development of therapeutic interventions that enhance quality of life without deleterious side effects.

Activation of membrane estrogen receptors induce pro-survival kinases

Experimental and epidemiological data suggest a neuroprotective role for estrogen (E(2)). We have recently shown that, in PC12 cells, non-permeable estradiol conjugated to bovine serum albumin (BSA) prevent serum-deprivation induced apoptosis through activation of specific membrane estrogen receptors (mER). In the present study, we explored in detail the early signaling events involved in this anti-apoptotic action, downstream to activation of mER. Our findings suggest that mER is associated to G-proteins, and its activation with non-permeable E(2)-BSA results in the activation of the following downstream pro-survival kinases pathways: (1) the PKB/Akt pathway, (2) the Src-->MEK-->ERK kinases and finally (3) the MAPK-->ERK kinases. Activation of these pro-survival signals leads to CREB phosphorylation and NFkappaB nuclear translocation, two transcription factors controlling the expression of anti-apoptotic Bcl-2 proteins. These data suggest that major pro-survival kinases are involved in the mER-mediated anti-apoptotic effects of estrogen. This is further supported by experiments with specific kinases inhibitors, which partially but significantly reversed the mER-mediated anti-apoptotic effect of E(2)-BSA. Our findings suggest that estrogen act via mER as potent cytoprotective factors, downstream activating pro-survival kinases, assuring thus an efficient and multipotent activation of the anti-apoptotic machinery.

Dehydroepiandrosterone sulfate is neuroprotective when administered either before or after injury in a focal cortical cold lesion model

Dehydroepiandrosterone and its sulfate (DHEAS) are sex hormone precursors that exert marked neurotrophic and/or neuroprotective activity in the central nervous system. The present study evaluated the effects of DHEAS and 17beta-estradiol (E2) in a focal cortical cold lesion model, in which DHEAS (50 mg/kg, sc) and E2 (35 mg/kg, sc) were administered either as pretreatment (two subsequent injections 1 d and 1 h before lesion induction) or posttreatment (immediately after lesion induction). The focal cortical cold lesion was induced in the primary motor cortex by means of a cooled copper cylinder placed directly onto the cortical surface. One hour later, the animals were killed, the brains cut into 0.4-mm-thick slices, and the sections stained with 1% triphenyltetrazolium chloride. The volume of the hemispheric lesion was calculated for each animal. The results demonstrated that the lesion area was significantly attenuated in both the DHEAS- and E2- pre- and posttreated groups and that in the presence of letrozole, a nonsteroidal aromatase inhibitor, no neuroprotection was observed, suggesting that the beneficial effect of DHEAS on the cold injury might depend on the conversion of DHEAS to E2 within the brain. It is concluded that even a single posttraumatic administration of DHEAS may be of substantial therapeutic benefit in the treatment of focal brain injury with vasogenic edema.

Conventional protein kinase C isoforms mediate neuroprotection induced by phorbol ester and estrogen

Rapid signal transduction pathways play a prominent role in mediating neuroprotective actions of estrogen in the CNS. We have previously shown that estrogen-induced neuroprotection of primary cerebrocortical neurons from beta-amyloid peptide (Abeta) toxicity depends on activation of protein kinase C (PKC). PKC activation with phorbol-12-myristate-13-acetate (PMA) also provides neuroprotection in this paradigm. Because the PKC family includes several isoforms that have opposing roles in regulating cell survival, we sought to identify which PKC isoforms contribute to neuroprotection induced by PMA and estrogen. We detected protein expression of multiple PKC isoforms in primary neuron cultures, including conventional (alpha, betaI, betaII), novel (delta, epsilon, theta) and atypical (zeta, iota/lambda) PKC. Using a panel of isoform-specific peptide inhibitors and activators, we find that novel and atypical PKC isoforms do not participate in the mechanism of either PMA or estrogen neuroprotection. In contrast, a selective peptide activator of conventional PKC isoforms provides dose-dependent neuroprotection against Abeta toxicity. In addition, peptide inhibitors of conventional, betaI, or betaII PKC isoforms significantly reduce protection afforded by PMA or 17beta-estradiol. Taken together, these data provide evidence that conventional PKC isoforms mediate phorbol ester and estrogen neuroprotection of cultured neurons challenged by Abeta toxicity.