Phosphatidylinositol 3-kinase mediates neuroprotection by estrogen in cultured cortical neurons

Ovarian steroids reduce apoptosis induced by trophic insufficiency in nerve growth factor-differentiated PC12 cells and axotomized rat facial motoneurons

Previous studies have demonstrated that ovarian steroids exert neuroprotective effects in a variety of in vitro and in vivo systems. The mechanisms underlying these effects remain poorly understood. In the present study, the neuroprotective effects of estradiol (E(2)) and progesterone (P) were examined in two models of apoptosis induced by growth factor insufficiency: partially nerve growth factor (NGF)-differentiated PC12 cells, after serum and NGF withdrawal; and axotomized immature rat facial motor motoneurons. E(2) and P both increased the survival of trophically withdrawn NGF-differentiated PC12 cells, at physiologically relevant concentrations. However, neither steroid had a significant effect on the survival of PC12 cells that had not been NGF treated. Exposure to NGF had no effect on the expression of estrogen receptor (ER)beta, but markedly increased the levels of ERalpha and altered the expression of the progesterone receptor (PR) from predominantly PR-B in NGF naive cells, to predominantly PR-A after NGF. The survival promoting effects of E(2) and P were blocked by the specific steroid receptor antagonists Faslodex (ICI 182780) and onapristone (ZK98299), respectively. Inhibitors of RNA (actinomycin D) or protein (cycloheximide) synthesis also abrogated the protective effects of both steroids. In immature rats, E(2) and P both significantly increased the numbers of surviving facial motor neurons at 21 days after axotomy. These data demonstrate significant protective effects of E(2) and P in two well-characterized models of apoptosis induced by trophic withdrawal and suggest that, at least in PC12 cells, the effects of the steroids are mediated via interaction with nuclear steroid receptor systems. The lack of steroid responsiveness in NGF-naive PC12 cells despite the presence of abundant ERbeta and PR-B are consistent with the view that ERalpha and PR-A may be particularly important as mediators of the neuroprotective effects of their corresponding hormonal ligands.

Neuroprotective effects of estrogen and tamoxifen in vitro: a facilitative role for glia?

Selective estrogen receptor modulators (SERMs) are steroidal or nonsteroidal compounds that can exhibit either estrogen-like agonistic effects or estrogen-antagonistic effects depending on the target tissue. While SERM actions in the breast, bone, and uterus have been well characterized, their effects in the brain are considerably less well understood. Previous work by our laboratory has demonstrated a beneficial effect of tamoxifen in the reduction of ischemic stroke damage in ovariectomized female rats. The present study utilized neuronal cell culture models to attempt to understand the mechanisms of tamoxifen-mediated neuroprotection. Neither physiologic doses of 17beta-E2 nor clinically therapeutic doses of tamoxifen directly protected GT1-7 neurons or purified cultures of rat cerebrocortical neurons from several forms of cell death. Reverse transcriptase polymerase chain reaction and Western blot analysis revealed that GT1-7 neurons possessed both estrogen receptor-alpha (ERalpha) and ERbeta mRNA and protein, whereas purified embryonic rat cortical neurons only expressed appreciable levels of ERalpha transcript and protein, with little to no expression of ERbeta. In contrast to the lack of protection in the purified neuronal cultures, both 17beta- E2 and tamoxifen significantly protected mixed glial/ neuronal cortical cultures from cell death, suggesting that glia may facilitate 17beta-E2-and tamoxifen-mediated neuroprotection. Furthermore, astrocyte-conditioned media and exogenous transforming growth factor-beta1, a documented astrocyte-derived cytokine, were shown to rescue purified cortical neurons from cell death. Together, these findings support a role for astrocytes in neuroprotection and raise the intriguing possibility that astrocytes may help mediate the neuroprotective effect of 17beta-E2 and tamoxifen.

Estradiol rapidly activates Akt via the ErbB2 signaling pathway

Previously, we have demonstrated that the two mitogenic growth factors epidermal growth factor and IGF-I can activate Akt and estrogen receptor-alpha (ERalpha) in the hormone-dependent breast cancer cell line, MCF-7. In this report we now show that estradiol can also rapidly activate phosphatidylinositol 3-kinase (PI 3-K)/Akt and that this effect is mediated by the ErbB2 signaling pathway. Treatment of cells with estradiol resulted in phosphorylation of Akt and a 9-fold increase in Akt activity in 10 min. Akt activation was blocked by wortmannin and LY 294,002, two inhibitors of PI 3-K; by genistein, a protein tyrosine kinase inhibitor and an ER agonist; by AG825, a selective ErbB2 inhibitor; and by the antiestrogens ICI 182,780 and 4-hydroxy-tamoxifen; but not by rapamycin, an inhibitor of the ribosomal protein kinase p70S6K; nor by AG30, a selective epidermal growth factor receptor inhibitor. Akt activation by estradiol was abrogated by an arginine-to-cysteine mutation in the pleckstrin homology domain of Akt (R25C). Growth factors also activated Akt in the ER-negative variant of MCF-7, MCF-7/ADR, but estradiol did not induce Akt activity in these cells. Transient transfection of ERalpha into these cells restored Akt activation by estradiol, suggesting that estradiol activation of Akt requires the ERalpha. Estradiol did not activate Akt in MCF-7 cells stably transfected with an anti-ErbB2-targeted ribozyme, further confirming a role for ErbB2. In vitro kinase assays using immunoprecipitation and anti-Akt1, -Akt2, and -Akt3-specific antibodies demonstrated that Akt1 is activated by estradiol in MCF-7 cells whereas Akt3 is the activated isoform in ER-negative MDA-MB231 cells, implying that selective activation of Akt subtypes plays a role in the actions of estradiol. Taken together, our data suggest that estradiol, bound to membrane ERalpha, interacts with and activates an ErbB dimer containing ErbB2, inducing activation of PI 3-K/Akt.

Estrogen levels regulate the subcellular distribution of phosphorylated Akt in hippocampal CA1 dendrites

In addition to genomic pathways, estrogens may regulate gene expression by activating specific signal transduction pathways, such as that involving phosphatidylinositol 3-kinase (PI3-K) and the subsequent phosphorylation of Akt (protein kinase B). The Akt pathway regulates various cellular events, including the initiation of protein synthesis. Our previous studies showed that synaptogenesis in hippocampal CA1 pyramidal cell dendritic spines is highest when brain estrogen levels are highest. To address the role of Akt in this process, the subcellular distribution of phosphorylated Akt immunoreactivity (pAkt-I) in the hippocampus of female rats across the estrous cycle and male rats was analyzed by light microscopy (LM) and electron microscopy (EM). By LM, the density of pAkt-I in stratum radiatum of CA1 was significantly higher in proestrus rats (or in estrogen-supplemented ovariectomized females) compared with diestrus, estrus, or male rats. By EM, pAkt-I was found throughout the shafts and in select spines of stratum radiatum dendrites. Quantitative ultrastructural analysis identifying pAkt-I with immunogold particles revealed that proestrus rats compared with diestrus, estrus, and male rats contained significantly higher pAkt-I associated with (1) dendritic spines (both cytoplasm and plasmalemma), (2) spine apparati located within 0.1 microm of dendritic spine bases, (3) endoplasmic reticula and polyribosomes in the cytoplasm of dendritic shafts, and (4) the plasmalemma of dendritic shafts. These findings suggest that estrogens may regulate spine formation in CA1 pyramidal neurons via Akt-mediated signaling events.

Estrogen receptor alpha forms estrogen-dependent multimolecular complexes with insulin-like growth factor receptor and phosphatidylinositol 3-kinase in the adult rat brain

Estradiol and insulin-like growth factor-I (IGF-I) have numerous functional interactions in the brain, including the regulation of neuroendocrine events, the control of reproductive behavior and the promotion of synaptic plasticity and neuronal survival. To explore the mechanisms involved in these interdependent actions of estradiol and IGF-I in the adult brain, the potential interactions of estrogen receptors with components of the IGF-I signaling system were assessed in this study. Systemic estradiol administration resulted in a transient immunocoprecipitation of the IGF-I receptor with the estrogen receptor alpha and in a transient increase in tyrosine phosphorylation of the IGF-I receptor in the hypothalamus of adult ovariectomized Wistar rats. Both effects were coincident in time, with a peak between 1 and 3 h after systemic estradiol administration. Three hours after estradiol treatment, there was an enhanced immunocoprecipitation of estrogen receptor alpha with p85 subunit of phosphatidylinositol 3-kinase, as well as an enhanced immunocoprecipitation of p85 with insulin receptor substrate-1. The interaction with the IGF-I receptor was specific for the alpha form of the estrogen receptor and was also induced by intracerebroventricular injection of IGF-I. These hormonal actions may be part of the mechanism by which estradiol activates IGF-I receptor signaling pathways in the brain and may explain the interdependence of estrogen receptors and the IGF-I receptor in synaptic plasticity, neuroprotection and other neural events.

Heregulin-beta1 regulates the estrogen receptor-alpha gene expression and activity via the ErbB2/PI 3-K/Akt pathway

This study examines whether the serine/threonine protein kinase, Akt, is involved in the crosstalk between the ErbB2 and estrogen receptor-alpha (ER-alpha) pathways. Treatment of MCF-7 cells with 10(-9) M heregulin-beta1 (HRG-beta1) resulted in a rapid phosphorylation of Akt and a 15-fold increase in Akt activity. Akt phosphorylation was blocked by inhibitors of phosphatidylinositol 3-kinase (PI 3-K), by antiestrogens, the protein tyrosine kinase inhibitor, genistein, and by AG825, a selective ErbB2 inhibitor; but not by AG30, a selective EGFR inhibitor. Akt phosphorylation by HRG-beta1 was abrogated by an arginine to cysteine mutation (R25C) in the pleckstrin homology (PH) domain of Akt, and HRG-beta1 did not induce Akt phosphorylation in the ER-negative variant of MCF-7, MCF-7/ADR. Transient transfection of ER-alpha into these cells restored Akt phosphorylation by HRG-beta1, suggesting the requirement of ER-alpha. HRG-beta1 did not activate Akt in MCF-7 cells stably transfected with an anti-ErbB2-targeted ribozyme, further confirming a role for ErbB2. Stable transfection of the cells with a dominant negative Akt or with the R25C-Akt mutant, as well as PI 3-K inhibitors, blocked the effect of HRG-beta1 on ER-alpha expression and activity and on the growth of MCF-7 cells. Stable transfection of MCF-7 cells with a constitutively active Akt mimicked the effect of HRG-beta1. Experiments employing selective ErbB inhibitors demonstrate that the effect of HRG-beta1 on ER-alpha expression and activity is also mediated by ErbB2 and not by EGFR, demonstrating that ErbB2 is the primary mediator of the effects of HRG-beta1 on ER-alpha regulation. Taken together, our data suggest that HRG-beta1, bound to the ErbB2 ErbB3 heterodimer, in the presence of membrane ER-alpha, interacts with and activates PI 3-K/Akt. Akt leads to nuclear ER-alpha phosphorylation, thereby altering its expression and transcriptional activity.

Estrogen activates protein kinase C in neurons: role in neuroprotection

It has been previously demonstrated that estrogen can protect neurons from a variety of insults, including beta-amyloid (Abeta). Recent studies have shown that estrogen can rapidly modulate intracellular signaling pathways involved in cell survival. In particular, estrogen activates protein kinase C (PKC) in a variety of cell types. This enzyme plays a key role in many cellular events, including regulation of apoptosis. In this study, we show that 17beta-estradiol (E2) rapidly increases PKC activity in primary cultures of rat cerebrocortical neurons. A 1 h pre-treatment with E2 or phorbol-12-myristate-13-acetate (PMA), a potent activator of PKC, protects neurons against Abeta toxicity. Protection afforded by both PMA and E2 is blocked by pharmacological inhibitors of PKC. Further, depletion of PKC levels resulting from prolonged PMA exposure prevents subsequent E2 or PMA protection. Our results indicate that E2 activates PKC in neurons, and that PKC activation is an important step in estrogen protection against Abeta. These data provide new understanding into the mechanism(s) underlying estrogen neuroprotection, an action with therapeutic relevance to Alzheimer's disease and other age-related neurodegenerative disorders.

The phosphatidylinositol 3-kinase inhibitor LY294002 binds the estrogen receptor and inhibits 17beta-estradiol-induced transcriptional activity of an estrogen sensitive reporter gene

Estrogen receptors (ERs) are members of the superfamily of ligand-activated transcription factors. In addition to the classical, hormone-mediated activation, ERs may alternatively be activated in a ligand-independent manner by a variety of agents including growth factors, neurotransmitters and cAMP. It has been demonstrated that the phosphatidylinositol 3 (PI3)-dependent kinase/Akt pathway may activate the ER alpha by increasing the activity of both estrogen independent activation function-1 and estrogen-dependent activation function-2 domains. The Akt phosphorylation site in the ER is Ser167. Phosphorylation of this residue is inhibited by LY294002, which blocks the PI3-kinase/Akt pathway. In the course of studies examining the effects of LY294002 on ligand-independent activation of ERs in L cells, we found that LY294002 exhibits antiestrogenic effects in a dose-dependent manner. By competition binding assays, we found that LY294002 specifically displaced radiolabelled estradiol from ERs with an IC(50) of 11+/-0.06 nM, being an estradiol competitor as effective as the antiestrogens ICI182,780 (IC(50), 21+/-0.13) and 4-OH-tamoxifen (IC(50), 15+/-0.09). Further, LY294002 irreversibly blocked estrogen-induced transactivation of an estradiol-sensitive reporter gene. These findings are of particular importance in the interpretation of studies demonstrating ERs inactivation by the PI3-kinase inhibitor. Our studies show that an apparent block of ER activation cannot be dissociated from inhibition of ligand-mediated events. Thus, this effect can be the result of the ability of LY294002 to bind the ERs and inhibit transactivation of estrogen-regulated genes.

Nongenomic activity and subsequent c-fos induction by estrogen receptor ligands are not sufficient to promote deoxyribonucleic acid synthesis in human endometrial adenocarcinoma cells

Estrogen 17beta-estradiol (E2) rapidly modulates several signaling pathways related to cell growth, preservation, and differentiation. The physiological role of these nongenomic effects with regard to downstream outcomes, and the relationship with transcriptional estrogen activity are unclear. Furthermore, the ability of selective estrogen receptor modulators (SERMs) to trigger nongenomic actions is largely unknown. To determine whether estrogen receptor (ER) ligands exert nongenomic activity in endometrial adenocarcinoma cells, and whether this activity affects transcription and DNA synthesis, we challenged human Ishikawa cells with E2 or partial ER agonists 4-hydroxytamoxifen (OHT) and raloxifene (ral). Serum-starved Ishikawa cells exposed for 5 min to 0.1 nM E2 showed induced phosphorylation of MAPK (ERK1/2). Ral and 4-OHT each at 1 nM also stimulated ERK in a rapid transient manner. E2 and 4-OHT induced proto-oncogene c-fos mRNA expression in Ishikawa cells within 30 min, but ral had no effect. In contrast to nongenomic action, only E2 stimulated expression of an estrogen response element (ERE)-driven luciferase (LUC) reporter gene. To examine DNA synthesis, [(3)H]-thymidine incorporation was measured in serum-starved cultures exposed to E2 or partial agonists for 2 d. E2 at 1 nM stimulated thymidine uptake in an ERK-dependent manner, but 1 nM 4-OHT, 1 nM ral, and 0.1-nM concentrations of E2 had no significant effects. Taken together, these data indicate that both nongenomic and direct transcriptional ER effects are likely required to promote DNA synthesis.

Nonnuclear actions of estrogen

Estrogen has long been observed to endow cardiovascular protective effects, as evidenced by sex-specific differences in the incidence of hypertensive and coronary artery disease, the development of atherosclerosis, and myocardial remodeling after infarction. To exert its tissue-specific effects, the classic estrogen receptor (ER) functions as a ligand-dependent transcription factor. However, there is growing evidence that in response to 17beta-estradiol and heterologous signals, the ER can also mediate signaling cascades at the membrane and in the cytoplasm via various second messengers, such as receptor-mediated protein kinases. This review summarizes the current understanding of nonnuclear ER signaling and discusses the relevance to eliciting the beneficial cardiovascular effects of estrogen. These include vasodilation, inhibition of response to vessel injury, limiting myocardial injury after infarction, and attenuating cardiac hypertrophy. Defining the full repertoire of ER function promises to expose novel, highly specific targets for pharmacological interventions and may ultimately lead to the primary and secondary prevention of cardiovascular diseases.

17beta-Estradiol and the phytoestrogen genistein attenuate neuronal apoptosis induced by the endoplasmic reticulum calcium-ATPase inhibitor thapsigargin

Estrogenic compounds have been shown to protect neurons from a variety of toxic stimuli in vitro and in vivo and depletion of estrogen at menopause has been associated with increased risk of neurodegenerative diseases. Genistein is an isoflavone soy derivative that binds to estrogen receptors with selective estrogen receptor modulator (SERM) properties. Recent FDA recommendations of soy intake for cholesterol reduction have prompted investigation into the potentially estrogenic role of dietary soy phytochemicals in the brain. In this study, we have shown that 50nM genistein significantly reduces neuronal apoptosis in an estrogen receptor-dependent manner. The importance of apoptosis in the brain has been recognized with regard to organization of the developing brain as well as degeneration in response to disease or stroke; however, the effects of estrogenic compounds on neuronal apoptosis have not been thoroughly examined. We developed a model of apoptotic toxicity in primary cortical neurons by using the endoplasmic reticulum (ER) calcium-ATPase inhibitor, thapsigargin, to test potential anti-apoptotic effects of 17beta-estradiol and genistein. Estrogen receptor beta, but not estrogen receptor alpha, was detected in our primary neuron cultures. Thapsigargin-induced apoptosis was confirmed by loss of mitochondrial function, DNA laddering, nuclear condensation and fragmentation, and caspase activation. Both 17beta-estradiol and genistein reduced the number of apoptotic neurons and reduced the number of neurons containing active caspase-3. This effect was blocked by co-addition of ICI 182780. Our results demonstrate that genistein and 17beta-estradiol have comparable anti-apoptotic properties in primary cortical neurons and that these properties are mediated through estrogen receptors.

Interactions of estrogen and insulin-like growth factor-I in the brain: molecular mechanisms and functional implications

In the brain, as in other tissues, estradiol interacts with growth factors. One of the growth factors that is involved in the neural actions of estradiol is insulin-like growth factor-I (IGF-I). Estradiol and IGF-I cooperate in the central nervous system to regulate neuronal development, neural plasticity, neuroendocrine events and the response of neural tissue to injury. The precise molecular mechanisms involved in these interactions are still not well understood. In the central nervous system there is abundant co-expression of estrogen receptors (ERs) and IGF-I receptors (IGF-IRs) in the same cells. Furthermore, the expression of estrogen receptors and IGF-I receptors in the brain is cross-regulated. In addition, using specific antibodies for the phosphorylated forms of extracellular-signal regulated kinase (ERK) 1 and ERK2 and Akt/protein kinase B (Akt/PKB) it has been shown that estradiol affects IGF-I signaling pathways in the brain. Estradiol treatment results in a dose-dependent increase in the phosphorylation of ERK and Akt/PKB in the brain of adult ovariectomized rats. In addition, estradiol and IGF-I have a synergistic effects on the activation of Akt/PKB in the adult rat brain. These findings suggest that estrogen effects in the brain may be mediated in part by the activation of the signaling pathways of the IGF-I receptor.

Protective effects of estradiol against amyloid beta protein-induced inhibition of neuronal Cl(-)-ATPase activity

Low concentrations of amyloid beta proteins (Abetas, 1-10 nM) were recently demonstrated to reduce Cl(-)-ATPase activity in parallel with an increase in the intracellular Cl(-) concentration ([Cl(-)]i) and decreases in plasma membrane phosphorylated phosphatidylinositol (PIP and PIP2) levels in cultured rat hippocampal neurons. In this study, 17 beta-estradiol (estradiol) at a therapeutic concentration (1.8 nM) for Alzheimer's disease was found to block these Abeta (Abeta25-35)-induced changes. This protective effect of estradiol on Cl(-)-ATPase activity was antagonized by a pure estrogen receptor antagonist, ICI182780 and inhibitors for cyclic GMP-dependent protein kinase (PKG) (KT5823), Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) (KN62) and phosphatidylinositol (PI) 4-kinase (wortmannin and quercetin). Estradiol recovered Abeta-induced decreases in plasma membrane phosphoinositide (PIP and PIP2) levels, this effect being inhibited by KT5823 and KN62. Glutamate toxicity was augmented in neurons with elevated [Cl(-)]i either by Abeta-treatment or carbachol+KCl+LiCl-treatment. The increased glutamate toxicity in the Abeta-treated neurons was attenuated by estradiol. Thus, a therapeutic concentration of estradiol protected Abeta-treated neurons against inhibition of Cl(-)-ATPase activity and an increase in [Cl(-)]i through its receptor, probably via PKG- and CaMKII(-)mediated recovery of PI4P formation. Elevated [Cl(-)]i may be related to enhancement of glutamate toxicity.

Sex steroid hormones act as growth factors

We observed that sex steroid hormones, like growth factors, stimulate the Src/Ras/erk pathway of cell lines derived from human mammary or prostate cancers. In addition, hormone-dependent pathway activation can be induced in Cos cells, upon transfection of classic steroid receptors. Cross-talks between sex steroid receptors regulate their association with Src and consequent pathway activation. Oestradiol treatment of MCF-7 cells triggers simultaneous association of ER with Src and p85, the regulatory subunit of phosphatidylinositol-3-kinase (PI3-kinase) and activation of Src- and PI3-K-dependent pathways. Activation of the latter pathway triggers cyclin D1 transcription, that is unaffected by Mek-1 activation. This suggests that simultaneous activation of different signalling effectors is required to target different cell cycle components. Thus, a novel reciprocal cross-talk between the two pathways appears to be mediated by the ER. In all tested cells, activation of the signalling pathways has a proliferative role. Transcriptionally inactive ER expressed in NIH 3T3 cells responds to hormone causing Src/Ras/Erk pathway activation and DNA synthesis. This suggests that in these cells genomic activity is required for later events of cell growth.

Enhancing effects of estrogen on inhibitory avoidance performance may be in part independent of intracellular estrogen receptors in the hippocampus

Estradiol (E(2)) can have classical actions via intracellular estrogen receptors (ERs) in the dorsal hippocampus, as well as effects independent of ERs ('non-genomic' mechanisms). These experiments investigated whether E(2)'s cognitive enhancing effects in the inhibitory avoidance task require actions at ERs in the dorsal hippocampus. Ovariectomized (ovx) rats were administered E(2) (s.c. or to the dorsal hippocampus), an E(2) conjugate (E(2):BSA), or vehicle and/or an ER antagonist, tamoxifen (10 mg/kg s.c.) or ICI 182,780 (10 microg intrahippocampally), or vehicle for 2 days prior to training (Day 3) and testing (Day 4) in the inhibitory avoidance task. Exp 1: crossover latencies in the inhibitory avoidance task were significantly increased in ovx rats with s.c. E(2) silastic capsules or s.c. injections of 1000 or 10 microg E(2) compared to vehicle-administered rats. Exp 2: bilateral inserts of E(2) to the dorsal hippocampus significantly increased crossover latencies compared to vehicle. Exp 3: s.c. tamoxifen, the ER antagonist, did not block the increased crossover latencies produced by 10 microg E(2) s.c. (compared to vehicle). Exp 4: s.c. tamoxifen did not block the increased crossover latencies produced by intrahippocampal E(2) (compared to vehicle). Exp 5: ICI 182,780 was unable to attenuate the increased crossover latencies produced by intrahippocampal E(2). Exp 6: E(2):BSA administered to the dorsal hippocampus significantly enhanced performance on the inhibitory avoidance task compared to control implants to the hippocampus. The ability of systemic and intrahippocampal E(2) to similarly enhance inhibitory avoidance performance suggests that actions of E(2) in the dorsal hippocampus are sufficient to enhance cognitive performance. Further, that neither tamoxifen nor ICI 182,780 blocked E(2)'s enhancing effects on inhibitory avoidance and that E(2):BSA was able to enhance performance suggest that non-genomic mechanisms may in part mediate E(2)'s cognitive enhancing performance in this task.

Differential Akt phosphorylation at Ser473 and Thr308 in cultured neurons after exposure to glutamate in rats

Akt kinase is involved in growth factor-mediated neuronal protection. In the present study, we found in cultured neurons exposed to glutamate, that phosphorylation at Ser473 was transiently induced, but the level of phosphorylation at Thr308 and Akt activity were unchanged. Inhibition of phosphoinositide 3-kinase with LY294002 decreased phosphorylation and Akt activity, however, pretreatment with LY294002 did not affect glutamate toxicity. Our findings suggested that the endogenous Akt pathway does not play a crucial role in cell survival after exposure to glutamate.

Protective effects of estrogen and selective estrogen receptor modulators in the brain

Within the last few years, there has been a growing interest in the neuroprotective effects of estrogen and the possible beneficial effects of estrogen in neurodegenerative diseases such as stroke, Alzheimer disease, and Parkinson disease. Here, we review the progress in this field, with a particular focus upon estrogen-induced protection from stroke-induced ischemic damage. The important issue of whether clinically relevant selective estrogen receptor modulators (SERMs) such as tamoxifen and raloxifene and estrogen replacement therapy can exert neuroprotection is also addressed. Although the mechanism of estrogen and SERM neuroprotection is not clearly resolved, we summarize the leading possibilities, including 1) a genomic estrogen receptor-mediated pathway that involves gene transcription, 2) a nongenomic signaling pathway involving activation of cell signalers such as mitogen-activated protein kinases and/or phosphatidylinositol-3-kinase /protein kinase B, and 3) a nonreceptor antioxidant free-radical scavenging pathway that is primarily observed with pharmacological doses of estrogen. The role of other potential mediatory factors such as growth factors and the possibility of an astrocyte role in neuroprotection is also discussed.

Synergistic interaction of estradiol and insulin-like growth factor-I in the activation of PI3K/Akt signaling in the adult rat hypothalamus

Estradiol and insulin-like growth factor-I (IGF-I) interact in the hypothalamus to regulate neuronal function, synaptic plasticity and neuroendocrine events. However, the molecular mechanisms involved in these interactions are still unknown. In the present study, the effect of estradiol on the signaling pathways of IGF-I receptor has been assessed in the hypothalamus of young adult ovariectomized rats, using specific antibodies for the phosphorylated forms of extracellular-signal regulated kinase (ERK) 1 and ERK2 and Akt/protein kinase B (Akt/PKB). Estradiol treatment resulted, between 6 and 24 h after systemic administration, in dose-dependent effects on the phosphorylation of ERK and Akt/PKB. Estradiol did not modify the level of ERK phosphorylation induced by intracerebroventricular administration of IGF-I. However, both hormones had a synergistic effect on the phosphorylation of Akt/PKB. These findings suggest that estrogen effects in the hypothalamus may be mediated in part by the activation of the signaling pathways of the IGF-I receptor.

Neuroprotective effects of a novel non-receptor-binding estrogen analogue: in vitro and in vivo analysis

BACKGROUND AND PURPOSE

Although estrogens are neuroprotective, hormonal effects limit their clinical application. Estrogen analogues with neuroprotective function but lacking hormonal properties would be more attractive. The present study was undertaken to determine the neuroprotective effects of a novel 2-adamantyl estrogen analogue, ZYC3.

METHODS

Cytotoxicity was induced in HT-22 cells by 10 mmol/L glutamate. 17beta-Estradiol (E2) or ZYC3 was added immediately before the exposure to glutamate. Cell viability was determined by calcein assay. The binding of E2 and ZYC3 to human alpha (ERalpha) and beta (ERbeta) estrogen receptors was determined by ligand competition binding assay. Ischemia/reperfusion injury was induced by temporary middle cerebral artery occlusion (MCAO). E2 or ZYC3 (100 microg/kg) was administered 2 hours or immediately before MCAO, respectively. Infarct volume was determined by 2,3,5-triphenyltetrazolium chloride staining. Cerebral blood flow was recorded during and within 30 minutes after MCAO by a hydrogen clearance method.

RESULTS

ZYC3 significantly decreased toxicity of glutamate with a potency 10-fold that of E2. ZYC3 did not bind to either ERalpha or ERbeta. Infarct volume was significantly reduced to 122.4+/-17.6 and 83.1+/-19.3 mm(3) in E2 and ZYC3 groups, respectively, compared with 252.6+/-15.6 mm(3) in the ovariectomized group. During MCAO, both E2 and ZYC3 significantly increased cerebral blood flow in the nonischemic side, while no significant differences were found in the ischemic side. However, E2 and ZYC3 significantly increased cerebral blood flow in both sides within 30 minutes after reperfusion.

CONCLUSIONS

Our study shows that ZYC3, a non-receptor-binding estrogen analogue, possesses both neuroprotective and vasoactive effects, which offers the possibility of clinical application for stroke without the side effects of estrogens. It also suggests that both the neuroprotective and vasoactive effects of estrogen are receptor independent.

Adenosine-mediated activation of Akt/protein kinase B in the rat hippocampus in vitro and in vivo

Adenosine is considered an endogenous neuroprotective metabolite that through activation of the A(1) receptor results in reduction of neuronal damage following cerebral ischemia. Protein kinase B, also known as Akt/PKB, is part of an endogenous pathway that exerts effective neuroprotection from both necrotic and apoptotic cell death. Using a rat model of unilateral common carotid artery occlusion coupled with hypoxia, and using in vitro rat hippocampal slices, we examined the ability of adenosine to directly activate Akt/PKB. Western blot analysis revealed that levels of phosphorylated Akt/PKB were elevated in vivo under ischemic conditions in an adenosine A(1)-dependent manner and elevated in hippocampal slices treated with an adenosine A(1) agonist. We conclude from these studies that the activation of an adenosine A(1) receptor-mediated signal transduction pathway, either by endogenous adenosine (in vivo) or by an adenosine A(1) agonist (in vitro), results in the activation of the neurotrophic kinase Akt/PKB.

Cell type-specificity of nonclassical estrogen signaling in the developing midbrain

Estrogens have widespread biological functions in the CNS involving the coordination of developmental processes, the regulation of cell physiology, and the control of neuroendocrine systems. In the midbrain, estrogens promote the survival, maturation, and function of neurons and, in particular, of dopamine cells. Aside from classical signaling through nuclear estrogen receptors, we have provided evidence that cellular transmission of estrogen effects in the midbrain comprises a complex intracellular signaling scenario. The major conclusion drawn from our studies is that estrogens interact with yet unidentified membrane receptor complexes which stimulate the phospholipase C and induce the formation of inosite-tri-phosphate (IP(3)). This causes a rapid and transitory rise in intracellular free calcium. The modulation of calcium homeostasis is the primary nonclassical physiological response to estrogens in all cell types. Surprisingly, a different secondary downstream signaling cascade seems to be activated in each estrogen-responsive cell population, i.e. phosphatidylinositol-3 kinase (PI3-kinase) in GABAergic and cAMP/ protein kinase A (PKA) in dopaminergic neurons, mitogen-activated protein kinase (MAP-kinase) in astrocytes. The precise biological role of estrogens for the different cell types is still fragmentary. We assume that estrogens positively influence intracellular signaling mechanisms which are important for cell differentiation and survival. It remains to be elucidated what determines the cell type-specificity of these estrogen responses.

Rapid actions of steroid receptors in cellular signaling pathways

Steroid hormones regulate cellular processes by binding to intracellular receptors that, in turn, interact with discrete nucleotide sequences to alter gene expression. Because most steroid receptors in target cells are located in the cytoplasm, they need to get into the nucleus to alter gene expression. This process typically takes at least 30 to 60 minutes. In contrast, other regulatory actions of steroid hormones are manifested within seconds to a few minutes. These time periods are far too rapid to be due to changes at the genomic level and are therefore termed nongenomic or rapid actions, to distinguish them from the classical steroid hormone action of regulation of gene expression. The rapid effects of steroid hormones are manifold, ranging from activation of mitogen-activated protein kinases (MAPKs), adenylyl cyclase (AC), protein kinase C (PKC), and heterotrimeric guanosine triphosphate-binding proteins (G proteins). In some cases, these rapid actions of steroids are mediated through the classical steroid receptor that can also function as a ligand-activated transcription factor, whereas in other instances the evidence suggests that these rapid actions do not involve the classical steroid receptors. One candidate target for the nonclassical receptor-mediated effects are G protein-coupled receptors (GPCRs), which activate several signal transduction pathways. One characteristic of responses that are not mediated by the classical steroid receptors is insensitivity to steroid antagonists, which has contributed to the notion that a new class of steroid receptors may be responsible for part of the rapid action of steroids. Evidence suggests that the classical steroid receptors can be localized at the plasma membrane, where they may trigger a chain of reactions previously attributed only to growth factors. Identification of interaction domains on the classical steroid receptors involved in the rapid effects, and separation of this function from the genomic action of these receptors, should pave the way to a better understanding of the rapid action of steroid hormones.

Estrogen-astrocyte interactions: implications for neuroprotection

BACKGROUND

Recent work has suggested that the ovarian steroid 17beta-estradiol, 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 estrogen have consistently been shown to be protective in vivo, direct protection upon purified neurons is controversial, with many investigators unable to show a direct protection in highly purified primary neuronal cultures. These findings suggest that while direct protection may occur in some instances, an alternative or parallel pathway for protection may exist which could involve another cell type in the brain.

PRESENTATION OF THE HYPOTHESIS

A hypothetical indirect protective mechanism is proposed whereby physiological levels of estrogen stimulate the release of astrocyte-derived neuroprotective factors, which aid in the protection of neurons from cell death. This hypothesis is attractive as it provides a potential mechanism for protection of estrogen receptor (ER)-negative neurons through an astrocyte intermediate. It is envisioned that the indirect pathway could act in concert with the direct pathway to achieve a more widespread global protection of both ER+ and ER- neurons.

TESTING THE HYPOTHESIS

We hypothesize that targeted deletion of estrogen receptors in astrocytes will significantly attenuate the neuroprotective effects of estrogen.

IMPLICATIONS OF THE HYPOTHESIS

If true, the hypothesis would significantly advance our understanding of endocrine-glia-neuron interactions. It may also help explain, at least in part, the reported beneficial effects of estrogen in neurodegenerative disorders. Finally, it also sets the stage for potential extension of the hypothetical mechanism to other important estrogen actions in the brain such as neurotropism, neurosecretion, and synaptic plasticity.

Estrogen and brain vulnerability

Accumulated clinical and basic evidence suggests that gonadal steroids affect the onset and progression of several neurodegenerative diseases and schizophrenia, and the recovery from traumatic neurological injury such as stroke. Thus, our view on gonadal hormones in neural function must be broadened to include not only their function in neuroendocrine regulation and reproductive behaviors, but also to include a direct participation in response to degenerative disease or injury. Recent findings indicate that the brain up-regulates both estrogen synthesis and estrogen receptor expression at sites of injury. Genetic or pharmacological inactivation of aromatase, the enzyme involved in estrogen synthesis, indicates that the induction of this enzyme in the brain after injury has a neuroprotective role. Some of the mechanisms underlying the neuroprotective effects of estrogen may be independent of the classically defined nuclear estrogen receptors (ERs). Other neuroprotective effects of estrogen do depend on the classical nuclear ERs, through which estrogen alters expression of estrogen responsive genes that play a role in apoptosis, axonal regeneration, or general trophic support. Yet another possibility is that non-classical ERs in the membrane or cytoplasm alter phosphorylation cascades, such as those involved in the signaling of insulin-like growth factor-1 (IGF-1). Indeed, ERs and IGF-1 receptor interact in the activation of PI3K and MAPK signaling cascades and in the promotion of neuroprotection. The decrease in estrogen and IGF-1 levels with aging may thus result in an increased risk for neuronal pathological alterations after different forms of brain injury.

Steroid hormone mimics: molecular mechanisms of cell growth and apoptosis in normal and malignant mammary epithelial cells

Anti-estrogen (anti-E2) therapy with E2 receptor antagonists has a significant benefit in women with breast cancer, but it may also increase the risk for developing hormone-independent breast cancer for which there is no therapy similar to that used in hormone-dependent breast cancer. Therefore, there is a significant interest in the development of compounds that may provide therapeutic benefit for hormone-independent breast cancer without untoward risks and adverse effects. The estrogen receptor (ER) modulators with both agonistic as well as antagonistic properties may, thus, be exploited for the development of the next generation of compounds for the prevention and/or treatment of breast cancer. In this article, we have discussed the clinical indications, risks, benefits and mechanisms of action of ER modulators and related compounds, particularly indole-3-carbinol (I3C), which may open new avenues for the prevention and/or treatment of breast cancer.

Rapid vascular cell responses to estrogen and membrane receptors

There is a growing interest in the effects of estrogen on the vascular wall, due to the marked gender difference in the incidence of clinically apparent coronary heart disease, when comparing premenopausal women with age-matched males. Estrogen has numerous effects on vascular endothelial and smooth muscle cells, both of which express estrogen receptors (ERs). Although ERs are classically defined as ligand-activated transcription factors, it has become increasingly clear that estrogen-stimulated, ER-dependent cellular responses can be rapid consequences of signal transduction cascades. The cellular localization and molecular form of the ER(s) which mediates rapid signaling are poorly defined. In this review, we describe the mounting evidence for membrane-localized ERs that vary in structure from classical forms. We also discuss ER-catalyzed molecular complex formations and a variety of estrogen-triggered signal transduction cascades, including those involving phosphatidylinositol 3-kinase/Akt, MAP kinase and G-protein-coupled receptors, all of which may induce "protective" profiles in vascular cells.

Rapid stimulation of the PI3-kinase/Akt signalling pathway in developing midbrain neurones by oestrogen

Oestrogen promotes the differentiation of neurones in the central nervous system. In the rodent midbrain, the maturation of dopaminergic neurones appears to be under oestrogen control. This is supported by the fact that dopaminergic cells contain nuclear oestrogen receptors-alpha/beta (ER). Second, aromatase is transiently expressed in the developing midbrain. In previous studies, we have shown that oestrogen increases dopamine synthesis and plasticity of dopamine cells. These effects are transmitted through classical nuclear ER but require also the stimulation of nonclassical signalling pathways involving the activation of membrane receptors. This study attempted to identify nonclassical oestrogen-dependent signalling cascades which might be stimulated downstream of membrane ERs. Using cultured mouse midbrain cells, we could demonstrate by Western blotting, that oestrogen rapidly phosphorylates Akt, a kinase which is implicated in the phosphatidylinositol 3 (PI3)-kinase pathway. This effect was only seen in midbrain neurones but not astrocytes. Oestrogen-induced Akt phosphorylation was time- and dose-dependent, showing highest responses after 30 min and at a steroid concentration of 10(-8) and 10(-6) M. Immunocytochemistry for phosphorylated Akt (pAkt) demonstrated that pAkt is predominantly found in a nuclear/perinuclear position and that oestrogen exposure increased the number of pAkt-positive cells. To investigate the mechanisms which are involved in transmitting oestrogen effects on the cellular level, cells were treated with antagonists for distinct signalling pathways. The application of the nuclear ER antagonist ICI 182 780 did not abolish the oestrogen-induced Akt phosphorylation. In contrast, interrupting intracellular calcium signalling with BAPTA completely prevented this effect. The PI3-kinase inhibitor LY294002 also inhibited the activation of Akt by oestrogen. Our study clearly indicates that oestrogen can rapidly stimulate the PI3-kinase/Akt signalling cascade in differentiating midbrain neurones. This effect requires the intermediate activation of calcium-dependent signalling pathways. In conclusion, oestrogen effects in the developing midbrain appear to be connected with the PI3-kinase/Akt signalling mechanism.

Oestrogen-mediated suppression of tumour necrosis factor alpha-induced apoptosis in MCF-7 cells: subversion of Bcl-2 by anti-oestrogens

In oestrogen receptor (ER)-positive breast carcinoma cells, 17beta-oestradiol suppresses a dose-dependent induction of cell death by tumour necrosis factor alpha (TNF). The ability of oestrogens to promote cell survival in ER-positive breast carcinoma cells is linked to a coordinate increase in Bcl-2 expression, an effect that is blocked with the pure anti-oestrogen ICI 182,780. The role of Bcl-2 in MCF-7 cell survival was confirmed by stable overexpression of Bcl-2 which resulted in suppression of apoptosis induced by doxorubicin (DOX), paclitaxel (TAX) and TNF as compared to vector-control cells. The pure anti-oestrogen ICI 182,780 in combination with TNF, DOX or TAX potentiated apoptosis in vector-transfected cells. Interestingly, pre-treatment with ICI 182,780 markedly enhanced chemotherapeutic drug- or TNF-induced apoptosis in Bcl-2 expressing cells, an effect that was correlated with ICI 182,780 induced activation of c-Jun N-terminal kinase. Our results suggest that the effects of oestrogens/anti-oestrogens on the regulation of apoptosis may involve coordinate activation of signalling events and Bcl-2 expression.

Interactions of estrogens and insulin-like growth factor-I in the brain: implications for neuroprotection

Data from epidemiological studies suggest that the decline in estrogen following menopause could increase the risk of neurodegenerative diseases. Furthermore, experimental studies on different animal models have shown that estrogen is neuroprotective. The mechanisms involved in the neuroprotective effects of estrogen are still unclear. Anti-oxidant effects, activation of different membrane-associated intracellular signaling pathways, and activation of classical nuclear estrogen receptors (ERs) could contribute to neuroprotection. Interactions with neurotrophins and other growth factors may also be important for the neuroprotective effects of estradiol. In this review we focus on the interaction between insulin-like growth factor-I (IGF-I) and estrogen signaling in the brain and on the implications of this interaction for neuroprotection. During the development of the nervous system, IGF-I promotes the differentiation and survival of specific neuronal populations. In the adult brain, IGF-I is a neuromodulator, regulates synaptic plasticity, is involved in the response of neural tissue to injury and protects neurons against different neurodegenerative stimuli. As an endocrine signal, IGF-I represents a link between the growth and reproductive axes and the interaction between estradiol and IGF-I is of particular physiological relevance for the regulation of growth, sexual maturation and adult neuroendocrine function. There are several potential points of convergence between estradiol and IGF-I receptor (IGF-IR) signaling in the brain. Estrogen activates the mitogen-activated protein kinase (MAPK) pathway and has a synergistic effect with IGF-I on the activation of Akt, a kinase downstream of phosphoinositol-3 kinase. In addition, IGF-IR is necessary for the estradiol induced expression of the anti-apoptotic molecule Bcl-2 in hypothalamic neurons. The interaction of ERs and IGF-IR in the brain may depend on interactions between neural cells expressing ERs with neural cells expressing IGF-IR, or on direct interactions of the signaling pathways of alpha and beta ERs and IGF-IR in the same cell, since most neurons expressing IGF-IR also express at least one of the ER subtypes. In addition, studies on adult ovariectomized rats given intracerebroventricular (i.c.v.) infusions with antagonists for ERs or IGF-IR or with IGF-I have shown that there is a cross-regulation of the expression of ERs and IGF-IR in the brain. The interaction of estradiol and IGF-I and their receptors may be involved in different neural events. In the developing brain, ERs and IGF-IR are interdependent in the promotion of neuronal differentiation. In the adult, ERs and IGF-IR interact in the induction of synaptic plasticity. Furthermore, both in vitro and in vivo studies have shown that there is an interaction between ERs and IGF-IR in the promotion of neuronal survival and in the response of neural tissue to injury, suggesting that a parallel activation or co-activation of ERs and IGF-IR mediates neuroprotection.

PI3-kinase in concert with Src promotes the S-phase entry of oestradiol-stimulated MCF-7 cells

The p85-associated phosphatidylinositol (PI) 3-kinase/Akt pathway mediates the oestradiol-induced S-phase entry and cyclin D1 promoter activity in MCF-7 cells. Experiments with Src, p85alpha and Akt dominant-negative forms indicate that in oestradiol-treated cells these signalling effectors target the cyclin D1 promoter. Oestradiol acutely increases PI3-kinase and Akt activities in MCF-7 cells. In NIH 3T3 cells expressing ERalpha, a dominant-negative p85 suppresses hormone stimulation of Akt. The Src inhibitor, PP1, prevents hormone stimulation of Akt and PI3-kinase activities in MCF-7 cells. In turn, stimulation of Src activity is abolished in ERalpha-expressing NIH 3T3 fibroblasts by co-transfection of the dominant-negative p85alpha and in MCF-7 cells by the PI3-kinase inhibitor, LY294002. These findings indicate a novel reciprocal cross-talk between PI3-kinase and Src. Hormone stimulation of MCF-7 cells rapidly triggers association of ERalpha with Src and p85. In vitro these proteins are assembled in a ternary complex with a stronger association than that of the binary complexes composed by the same partners. The ternary complex probably favours hormone activation of Src- and PI3-kinase-dependent pathways, which converge on cell cycle progression.

Alpha-lipoic acid protects rat cortical neurons against cell death induced by amyloid and hydrogen peroxide through the Akt signalling pathway

Substantial evidence suggests that the accumulation of beta-amyloid (Abeta)-derived peptides contributes to the aetiology of Alzheimer's disease (AD) by stimulating formation of free radicals. Thus, the antioxidant alpha-lipoate, which is able to cross the blood-brain barrier, would seem an ideal substance in the treatment of AD. We have investigated the potential effectiveness of alpha-lipoic acid (LA) against cytotoxicity induced by Abeta peptide (31-35) (30 microM) and hydrogen peroxide (H(2)O(2)) (100 microM) with the cellular 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) reduction and fluorescence dye propidium iodide assays in primary neurons of rat cerebral cortex. We found that treatment with LA protected cortical neurons against cytotoxicity induced by Abeta or H(2)O(2). In addition, LA-induced increase in the level of Akt in the neurons was observed by Western blot. The LA-induced neuroprotection and Akt increase were attenuated by pre-treatment with the phosphatidylinositol 3-kinase inhibitor, LY294002 (50 microM). Our data suggest that the neuroprotective effects of the antioxidant LA are partly mediated through activation of the PKB/Akt signaling pathway.

Apolipoprotein E isoform-specific disruption of phosphoinositide hydrolysis: protection by estrogen and glutathione

The mechanism(s) by which the E4 isoform of apolipoprotein E (apoE4) influences Alzheimer's disease (AD) are not fully known. We report that apoE4, but not apoE3, disrupts carbachol-stimulated phosphoinositide (PI) hydrolysis in SH-SY5Y neuroblastoma cells. Carbachol responses were also disrupted by beta-amyloid (Abeta) (1-42) and apoE4/Abeta(1-42) complexes, but not by apoE3/Abeta(1-42). Glutathione and estrogen protected against apoE4 and Abeta(1-42) effects, as well as those of H(2)O(2). Estrogen protection was partially blocked by wortmannin, suggesting the involvement of phosphatidylinositol 3-kinase. An apoE4-induced disruption of acetylcholine muscarinic receptor-mediated signalling may explain the lower effectiveness of cholinergic replacement treatments in apoE4 AD patients. Also, the beneficial effect of estrogen in AD may be partially due to its ability to protect against apoE4- and Abeta(1-42)-mediated disruption of PI hydrolysis.

Neuroprotective properties of 17beta-estradiol, progesterone, and raloxifene in MPTP C57Bl/6 mice

Previous work from our laboratory showed prevention of 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) induced dopamine depletion in striatum of C57Bl/6 mice by 17beta-estradiol, progesterone, and raloxifene, whereas 17alpha-estradiol had no effect. The present study investigated the mechanism by which these compounds exert their neuroprotective activity. The hormonal effect on the dopamine transporter (DAT) was examined to probe the integrity of dopamine neurons and glutamate receptors in order to find a possible excitotoxic mechanism. Drugs were injected daily for 5 days before MPTP (four injections, 15 mg/kg ip at 2-h intervals) and drug treatment continued for 5 more days. MPTP induced a decrease of striatal DAT-specific binding (50% of control) and DAT mRNA in the substantia nigra (20% of control), suggesting that loss of neuronal nerve terminals was more extensive than cell bodies. This MPTP-induced decrease of striatal [(125)I]RTI-121 specific binding was prevented by 17beta-estradiol (2 microg/day), progesterone (2 microg/day), or raloxifene (5 mg/kg/day) but not by 17alpha-estradiol (2 microg/day) or raloxifene (1 mg/kg/day). No treatment completely reversed the decreased levels of DAT mRNA in the substantia nigra. Striatal [(125)I]RTI-121 specific binding was positively correlated with dopamine concentrations in intact, saline, or hormone-treated MPTP mice. Striatal NMDA-sensitive [(3)H]glutamate or [(3)H]AMPA specific binding remained unchanged in intact, saline, or hormone-treated MPTP mice, suggesting the unlikely implication of changes of glutamate receptors in an excitotoxic mechanism. These results show a stereospecific neuroprotection by 17beta-estradiol of MPTP neurotoxicity, which is also observed with progesterone or raloxifene treatment. The present paradigm modeled early DA nerve cell damage and was responsive to hormones.

Estrogen protects against beta-amyloid-induced neurotoxicity in rat hippocampal neurons by activation of Akt

The cellular mechanisms underlying the neuroprotective effects of estrogen are only beginning to be elucidated. Here we examined the role of protein kinase B (Akt) activation in 17beta-estradiol (E2) inhibition of beta-amyloid peptide (31-35) (Abeta31-35)-induced neurotoxicity in cultured rat hippocampal neurons. Abeta31-35 (25-30 betaM) significantly decreased the total number of microtubule associated protein-2 positive cells (MAP2+). This decrease was significantly reversed by pre-treatment with 100 nM E2. Further, 100 nM E2 alone significantly increased the total number of protein kinase B and microtubule associated protein-2 positive cells compared with controls. Such E2-induced increases were inhibited by LY294002 (20 microM), a specific PI3-K inhibitor, as well as by tamoxifen, an estrogen receptor antagonist/selective estrogen receptor modulator. These results indicate that the neuroprotective effects of E2 may be mediated at least in part via estrogen receptor-mediated protein kinase B activation.

Differential mechanisms of neuroprotection by 17 beta-estradiol in apoptotic versus necrotic neurodegeneration

The major goal of this study was to compare mechanisms of the neuroprotective potential of 17 beta-estradiol in two models for oxidative stress-independent apoptotic neuronal cell death with that in necrotic neuronal cell death in primary neuronal cultures derived from rat hippocampus, septum, or cortex. Neuronal apoptosis was induced either by staurosporine or ethylcholine aziridinium (AF64A), as models for necrotic cell death glutamate exposure or oxygen-glucose deprivation (OGD) were applied. Long-term (20 hr) pretreatment (0.1 microm 17 beta-estradiol) was neuroprotective in apoptotic neuronal cell death induced by AF64A (40 microm) only in hippocampal and septal neuronal cultures and not in cortical cultures. The neuroprotective effect was blocked by the estrogen antagonists ICI 182,780 and tamoxifen and the phosphatidylinositol 3-kinase (PI3-K) inhibitor LY294002. In glutamate and OGD-induced neuronal damage, long-term pretreatment was not effective. In contrast, short-term (1 hr) pretreatment with 17 beta-estradiol in the dose range of 0.5-1.0 microm significantly reduced the release of lactate dehydrogenase and improved morphology of cortical cultures exposed to glutamate or OGD but was not effective in the AF64A model. Staurosporine-induced apoptosis was not prevented by either long- or short-term pretreatment. The strong expression of the estrogen receptor-alpha and the modulation of Bcl proteins by 17 beta-estradiol in hippocampal and septal but not in cortical cultures indicates that the prevention of apoptotic, but not of necrotic, neuronal cell death by 17 beta-estradiol possibly depends on the induction of Bcl proteins and the density of estrogen receptor-alpha.

Nongenomic antiapoptotic signal transduction by estrogen in cultured cortical neurons

Estrogen replacement therapy in menopausal women has been suggested to be beneficial in preventing the progression of cognitive impairment in Alzheimer disease. We demonstrated previously that the phosphatidylinositol 3-kinase (PI3-K)/Akt signal transduction pathway plays a pivotal role on the neuroprotection provided by 17beta-estradiol against acute glutamate toxicity. In the present study, we investigated the mechanism of neuroprotection against apoptosis because acute glutamate toxicity predominantly induced necrosis. 17beta-estradiol provided neuroprotection against apoptosis induced by staurosporine. This neuroprotection was inhibited by pretreatment with a PI3-K inhibitor, LY294002. An estrogen receptor specific antagonist, ICI182780, also suppressed the neuroprotection provided by 17beta-estradiol. Western blotting analysis demonstrated that treatment with 17beta-estradiol induced the phosphorylation of Akt within 5 min, which was suppressed by pretreatment with LY294002 and ICI182780. Furthermore, 17beta-estradiol induced phosphorylation of the cAMP response element binding protein (CREB) at Ser(133) within 15 min and then upregulated Bcl-2 in a PI3-K/Akt-dependent manner. Because CREB is known to be a transcription factor for Bcl-2, these results suggest that 17beta-estradiol exerts its antiapoptotic effects by CREB phosphorylation and Bcl-2 upregulation via nongenomic activation of the PI3-K/Akt pathway in cultured cortical neurons.

Estrogen and cerebrovascular physiology and pathophysiology

Numerous studies have uncovered a wide variety of estrogen effects with apparent cardiovascular benefits, the most recognized ones being vasodilation, anti-atherogenesis, diminished post-ischemic inflammation and anti-oxidant effects. This article provides an overview of the influence of estrogen on the cerebral vasculature, under physiologic and pathophysiologic conditions, and covers both acute and chronic effects. The discussion is primarily focused on the vasodilatory and anti-inflammatory actions of estrogen, since those particular estrogen influences have received the greatest attention in studies published to date. With respect to vasodilation, although some consideration is given to the role of other vasodilating mechanisms and factors, the emphasis is mostly placed on the endothelial isoform of nitric oxide synthase, eNOS, which has emerged as a clear target of estrogen. Some consideration is given to recent findings that suggest that estrogen can stimulate eNOS activity by decreasing the expression of the eNOS inhibitor caveolin-1. With regard to the ability of estrogen to counteract inflammation, potential mechanisms by which estrogen limits the post-ischemic leukocyte adhesion, and the expression of the inducible NOS, are discussed.

Ovarian hormones elicit phosphorylation of Akt and extracellular-signal regulated kinase in explants of the cerebral cortex

Estradiol and progesterone both have been demonstrated to afford neuroprotection against various insults. In an attempt to identify potential mechanisms underlying these neuroprotective effects, two key elements within signal transduction pathways linked to neuroprotection were evaluated. In mouse cerebral cortical explants, both estradiol and progesterone elicited the phosphorylation of Akt, a downstream effector of the phosphoinositide-3 (PI-3) kinase pathway. Progesterone also elicited the phosphorylation of extracellular-signal regulated kinase (ERK), a component of the mitogen-activated protein kinase (MAPK) pathway. These effects were not inhibited by the progesterone receptor antagonist, RU486. However, inhibition of either MAPK/ERK kinase with PD98059 or PI-3 kinase with LY294002 successfully inhibited progesterone's actions on ERK and Akt, respectively. Collectively, the data offer novel mechanisms for both progesterone and estrogen action in the central nervous system, demonstrating the functional and mechanistic diversity of gonadal hormones and supporting their neuroprotective potential for such neurodegenerative disorders as Alzheimer disease.

Estrogens: trophic and protective factors in the adult brain

Our appreciation that estrogens are important neurotrophic and neuroprotective factors has grown rapidly. Although a thorough understanding of the molecular and cellular mechanisms that underlie this effect requires further investigation, significant progress has been made due to the availability of animal models in which we can test potential candidates. It appears that estradiol can act via mechanisms that require classical intracellular receptors (estrogen receptor alpha or beta) that affect transcription, via mechanisms that include cross-talk between estrogen receptors and second messenger pathways, and/or via mechanisms that may involve membrane receptors or channels. This area of research demands attention since estradiol may be an important therapeutic agent in the maintenance of normal neural function during aging and after injury.