Estradiol enhances Akt activation in cortical explant cultures following neuronal injury

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

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

The stress of losing sleep: Sex-specific neurobiological outcomes

Sleep is a vital and evolutionarily conserved process, critical to daily functioning and homeostatic balance. Losing sleep is inherently stressful and leads to numerous detrimental physiological outcomes. Despite sleep disturbances affecting everyone, women and female rodents are often excluded or underrepresented in clinical and pre-clinical studies. Advancing our understanding of the role of biological sex in the responses to sleep loss stands to greatly improve our ability to understand and treat health consequences of insufficient sleep. As such, this review discusses sex differences in response to sleep deprivation, with a focus on the sympathetic nervous system stress response and activation of the hypothalamic-pituitary-adrenal (HPA) axis. We review sex differences in several stress-related consequences of sleep loss, including inflammation, learning and memory deficits, and mood related changes. Focusing on women's health, we discuss the effects of sleep deprivation during the peripartum period. In closing, we present neurobiological mechanisms, including the contribution of sex hormones, orexins, circadian timing systems, and astrocytic neuromodulation, that may underlie potential sex differences in sleep deprivation responses.

Considerations for Studying Sex as a Biological Variable in Spinal Cord Injury

In response to NIH initiatives to investigate sex as a biological variable in preclinical animal studies, researchers have increased their focus on male and female differences in neurotrauma. Inclusion of both sexes when modeling neurotrauma is leading to the identification of novel areas for therapeutic and scientific exploitation. Here, we review the organizational and activational effects of sex hormones on recovery from injury and how these changes impact the long-term health of spinal cord injury (SCI) patients. When determining how sex affects SCI it remains imperative to expand outcomes beyond locomotor recovery and consider other complications plaguing the quality of life of patients with SCI. Interestingly, the SCI field predominately utilizes female rodents for basic science research which contrasts most other male-biased research fields. We discuss the unique caveats this creates to the translatability of preclinical research in the SCI field. We also review current clinical and preclinical data examining sex as biological variable in SCI. Further, we report how technical considerations such as housing, size, care management, and age, confound the interpretation of sex-specific effects in animal studies of SCI. We have uncovered novel findings regarding how age differentially affects mortality and injury-induced anemia in males and females after SCI, and further identified estrus cycle dysfunction in mice after injury. Emerging concepts underlying sexually dimorphic responses to therapy are also discussed. Through a combination of literature review and primary research observations we present a practical guide for considering and incorporating sex as biological variable in preclinical neurotrauma studies.

Neuroactive gonadal drugs for neuroprotection in male and female models of Parkinson's disease

The existence of sex differences in Parkinson's disease (PD) incidence is well documented with greater prevalence and earlier age at onset in men than in women. These reported sex differences could be related to estrogen exposure. In PD animal models, estrogen is well documented to be neuroprotective against dopaminergic neuron loss induced by neurotoxins. Using the 1-methyl 4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) mouse model, we showed that several compounds are neuroprotective on dopaminergic neurons including estrogen, the selective estrogen receptor modulator raloxifene, progesterone, dehydroepiandrosterone, the estrogen receptor alpha (ERα) agonist PPT as well as the G protein-coupled membrane estrogen receptor (GPER1) specific agonist G1. Accumulating evidence suggests that GPER1 could be implicated in the neuroprotective effects of estrogen, raloxifene and G1 in collaboration with ERα. We recently reported that the 5α-reductase inhibitor Dutasteride is also neuroprotective and could bring an alternative to estrogens for therapy in male. Additional studies are needed to optimize therapies with these gonadal drugs into safe personalized treatments according to sex for treatment of PD.

The role of hypothalamic estrogen receptors in metabolic regulation

Estrogens regulate key features of metabolism, including food intake, body weight, energy expenditure, insulin sensitivity, leptin sensitivity, and body fat distribution. There are two 'classical' estrogen receptors (ERs): estrogen receptor alpha (ERS1) and estrogen receptor beta (ERS2). Human and murine data indicate ERS1 contributes to metabolic regulation more so than ESR2. For example, there are human inactivating mutations of ERS1 which recapitulate aspects of the metabolic syndrome in both men and women. Much of our understanding of the metabolic roles of ERS1 was initially uncovered in estrogen receptor α-null mice (ERS1(-/-)); these mice display aspects of the metabolic syndrome, including increased body weight, increased visceral fat deposition and dysregulated glucose intolerance. Recent data further implicate ERS1 in specific tissues and neuronal populations as being critical for regulating food intake, energy expenditure, body fat distribution and adipose tissue function. This review will focus predominantly on the role of hypothalamic ERs and their critical role in regulating all aspects of energy homeostasis and metabolism.

Signaling mechanisms involved in the acute effects of estradiol on 5-HT clearance

Estradiol was found previously to have an antidepressant-like effect and to block the ability of selective serotonin reuptake inhibitors (SSRIs) to have an antidepressant-like effect. The antidepressant-like effect of estradiol was due to estrogen receptor β (ERβ) and/or GPR30 activation, whereas estradiol's blockade of the effect of an SSRI was mediated by ERα. This study focuses on investigating signaling pathways as well as interacting receptors associated with these two effects of estradiol. In vivo chronoamperometry was used to measure serotonin transporter (SERT) function. The effect of local application of estradiol or selective agonists for ERα (PPT) or ERβ (DPN) into the CA3 region of the hippocampus of ovariectomized (OVX) rats on 5-hydroxytryptamine (5-HT) clearance as well as on the ability of fluvoxamine to slow 5-HT clearance was examined after selective blockade of signaling pathways or that of interacting receptors. Estradiol- or DPN-induced slowing of 5-HT clearance mediated by ERβ was blocked after inhibition of MAPK/ERK1/2 but not of PI3K/Akt signaling pathways. This effect also involved interactions with TrkB, and IGF-1 receptors. Estradiol's or PPT's inhibition of the fluvoxamine-induced slowing of 5-HT clearance mediated by ERα, was blocked after inhibition of either MAPK/ERK1/2 or PI3K/Akt signaling pathways. This effect involved interactions with the IGF-1 receptor and with the metabotropic glutamate receptor 1, but not with TrkB. This study illustrates some of the signaling pathways required for the effects of estradiol on SERT function, and particularly shows that ER subtypes elicit different as well as common signaling pathways for their actions.

Epigenetic mechanisms in cerebral ischemia

Treatment efficacy for ischemic stroke represents a major challenge. Despite fundamental advances in the understanding of stroke etiology, therapeutic options to improve functional recovery remain limited. However, growing knowledge in the field of epigenetics has dramatically changed our understanding of gene regulation in the last few decades. According to the knowledge gained from animal models, the manipulation of epigenetic players emerges as a highly promising possibility to target diverse neurologic pathologies, including ischemia. By altering transcriptional regulation, epigenetic modifiers can exert influence on all known pathways involved in the complex course of ischemic disease development. Beneficial transcriptional effects range from attenuation of cell death, suppression of inflammatory processes, and enhanced blood flow, to the stimulation of repair mechanisms and increased plasticity. Most striking are the results obtained from pharmacological inhibition of histone deacetylation in animal models of stroke. Multiple studies suggest high remedial qualities even upon late administration of histone deacetylase inhibitors (HDACi). In this review, the role of epigenetic mechanisms, including histone modifications as well as DNA methylation, is discussed in the context of known ischemic pathways of damage, protection, and regeneration.

Neuroprotective effects of estradiol on motoneurons in a model of rat spinal cord embryonic explants

Amyotrophic lateral sclerosis (ALS) is an adult-onset degenerative disorder characterized by motoneuron death. Clinical and experimental studies in animal models of ALS have found gender differences in the incidence and onset of disease, suggesting that female hormones may play a beneficial role. Cumulative evidence indicates that 17β-estradiol (17βE2) has a neuroprotective role in the central nervous system. We have previously developed a new culture system by using rat spinal cord embryonic explants in which motoneurons have the singularity of migrating outside the spinal cord, growing as a monolayer in the presence of glial cells. In this study, we have validated this new culture system as a useful model for studying neuroprotection by estrogens on spinal cord motoneurons. We show for the first time that spinal cord motoneurons express classical estrogen receptors and that 17βE2 activates, specifically in these cells, the Akt anti-apoptotic signaling pathway and two of their downstream effectors: GSK-3β and Bcl-2. To further validate our system, we demonstrated neuroprotective effects of 17βE2 on spinal cord motoneurons when exposed to the proinflammatory cytokines TNF-α and IFN-γ. These effects of 17βE2 were fully reverted in the presence of the estrogen receptor antagonist ICI 182,780. Our new culture model and the results presented here may provide the basis for further studies on the effects of estrogens, and selective estrogen receptor modulators, on spinal cord motoneurons in the context of ALS or other motoneuron diseases.

G protein-coupled estrogen receptor in energy homeostasis and obesity pathogenesis

Obesity and its related metabolic diseases have reached a pandemic level worldwide. There are sex differences in the prevalence of obesity and its related metabolic diseases, with men being more vulnerable than women; however, the prevalence of these disorders increases dramatically in women after menopause, suggesting that sex steroid hormone estrogens play key protective roles against development of obesity and metabolic diseases. Estrogens are important regulators of several aspects of metabolism, including body weight and body fat, caloric intake and energy expenditure, and glucose and lipid metabolism in both males and females. Estrogens act in complex ways on their nuclear estrogen receptors (ERs) ERα and ERβ and transmembrane ERs such as G protein-coupled estrogen receptor. Genetic tools, such as different lines of knockout mouse models, and pharmacological agents, such as selective agonists and antagonists, are available to study function and signaling mechanisms of ERs. We provide an overview of the evidence for the physiological and cellular actions of ERs in estrogen-dependent processes in the context of energy homeostasis and body fat regulation and discuss its pathology that leads to obesity and related metabolic states.

Estradiol promotes neural stem cell differentiation into endothelial lineage and angiogenesis in injured peripheral nerve

Neural stem cells (NSCs) differentiate into endothelial cells (ECs) and neuronal cells. Estradiol (E2) is known to exhibit proangiogenic effects on ischemic tissues via EC activation. Therefore, we hypothesized that E2 can promote the therapeutic potential of NSC transplantation for injured nerve repair via the differentiation of NSCs into ECs during neovascularization. NSCs isolated from newborn mouse brains were transplanted into injured sciatic nerves with (NSC/E2 group) or without E2-conjugated gelatin hydrogel (E2 group). The NSC/E2 group exhibited the greatest recovery in motor nerve conduction velocity, voltage amplitude, and exercise tolerance. Histological analyses revealed increased intraneural vascularity and blood perfusion as well as striking NSC recruitment to the neovasculature in the injured nerves in the NSC/E2 group. In vitro, E2 enhanced the NSC migration and proliferation inhibiting apoptosis. Fluorescence-activated cell sorting analysis also revealed that E2 significantly increased the percentage of CD31 in NSCs, and the effect of E2 was completely neutralized by the estrogen receptor antagonist ICI. The combination of E2 administration and NSC transplantation cooperatively improved the functional recovery of injured peripheral nerves, at least in part, via E2-associated NSC differentiation into ECs. These findings provide a novel mechanistic insight into both NSC biology and the biological effects of endogenous E2.

Effects of global ischemia and estradiol pretreatment on phosphorylation of Akt, CREB and STAT3 in hippocampal CA1 of young and middle-aged female rats

Transient global ischemia induces selective, delayed neuronal death of pyramidal neurons in the hippocampal CA1. Whereas long term treatment of middle-aged female rats with estradiol at physiological doses ameliorates neuronal death, the signaling pathways that mediate the neuroprotection are, as yet, unknown. Protein kinase B (Akt) and downstream transcription factors, the cAMP response element binding protein (CREB) and signal transducer and activator of transcription (STAT3) are critical players in cellular survival following injury. The present study was undertaken to determine whether long term estradiol alters the phosphorylation status and activity of Akt, STAT3 and CREB in ovariohysterectomized, middle-aged and young female rats subjected to global ischemia. Irrespective of either hormone or ischemic condition, middle-aged females exhibited lower levels of p-CREB and higher levels of Akt and STAT3 in CA1 than young females, as assessed by Western blot. In middle-aged animals, ischemia increased the phosphorylation status/activity of Akt and STAT3, and decreased the phosphorylation status/activity of CREB in the hippocampal CA1. Whereas estradiol did not detectably alter the phosphorylation status/activity of Akt or STAT3, it prevented the ischemia-induced decrease in nuclear p-CREB. Similar results were observed for the young females. Collectively, these data demonstrate that CREB, STAT3, and Akt are involved in the molecular response to global ischemia and that age influences the status of CREB, STAT3 and Akt activity in CA1 under physiological as well as pathological conditions, further emphasizing the importance of including older rodents in neuroprotection studies.

Estradiol triggers sonic-hedgehog-induced angiogenesis during peripheral nerve regeneration by downregulating hedgehog-interacting protein

Both estradiol (E2) and Sonic Hedgehog (Shh) contribute to angiogenesis and nerve regeneration. Here, we investigated whether E2 improves the recovery of injured nerves by downregulating the Shh inhibitor hedgehog-interacting protein (HIP) and increasing Shh-induced angiogenesis. Mice were treated with local injections of E2 or placebo one week before nerve-crush injury; 28 days after injury, nerve conduction velocity, exercise duration, and vascularity were significantly greater in E2-treated mice than in placebo-treated mice. E2 treatment was also associated with higher mRNA levels of Shh, the Shh receptor Patched-1, and the Shh transcriptional target Gli1, but with lower levels of HIP. The E2-induced enhancement of nerve vascularity was abolished by the Shh inhibitor cyclopamine, and the effect of E2 treatment on Shh, Gli1, and HIP mRNA expression was abolished by the E2 inhibitor ICI. Gli-luciferase activity in human umbilical-vein endothelial cells (HUVECs) increased more after treatment with E2 and Shh than after treatment with E2 alone, and E2 treatment reduced HIP expression in HUVECs and Schwann cells without altering Shh expression. Collectively, these findings suggest that E2 improves nerve recovery, at least in part, by reducing HIP expression, which subsequently leads to an increase in Shh signaling and Shh-induced angiogenesis.

Effect of a chronic treatment with 17β-estradiol on striatal dopamine neurotransmission and the Akt/GSK3 signaling pathway in the brain of ovariectomized monkeys

The present experiments sought the effect of chronic treatment with 17β-estradiol on striatal dopaminergic activity and the Akt/GSK3 signaling pathway in the brain of monkeys. Eight female monkeys (Macacca fascicularis) were ovariectomized (OVX) and a month later, half received a month treatment with 17β-estradiol and the other with vehicle. The DA transporter (DAT) was measured by autoradiography with [(125)I]RTI-121 and the vesicular DA transporter (VMAT(2)) with [(3)H]TBZ-OH at three rostro-caudal levels (anterior, middle and posterior) of the caudate nucleus and putamen subdivided in their lateral/medial, ventral/dorsal sub-regions. Specific binding to DAT was increased in all sub-regions of the caudate nucleus and the putamen of 17β-estradiol-treated compared to vehicle-treated monkeys whereas specific binding to VMAT(2) remained unchanged. We measured by Western blot the phosphorylated forms of Akt at serine 473 and threonine 308, GSK3β at serine 9 and tyrosine 216 and GSK3α at serine 21 in anterior, middle and posterior caudate nucleus and putamen. 17β-Estradiol treatment increased in all the caudate nucleus and putamen pAkt (Ser473)/βIII-tubulin, pGSK3β (Ser9)/βIII-tubulin and in putamen Akt/βIII-tubulin compared to vehicle-treated monkeys. In anterior and middle putamen, pAkt (Thr308)/βIII-tubulin was also increased in monkeys treated with 17β-estradiol. pGSK3β (Tyr216)/βIII-tubulin and pGSK3α (Ser21)/βIII-tubulin remained unchanged by the 17β-estradiol treatment. These results suggest that 17β-estradiol activates striatal DA neurotransmission in primates as reflected with increased DAT specific binding and downstream activation of Akt/GSK3 signaling. This supports a beneficial role of a chronic treatment with 17β-estradiol by increasing the activity of signaling pathways implicated in cell survival.

Estrogen neuroprotection and the critical period hypothesis

17β-Estradiol (estradiol or E2) is implicated as a neuroprotective factor in a variety of neurodegenerative disorders. This review focuses on the mechanisms underlying E2 neuroprotection in cerebral ischemia, as well as emerging evidence from basic science and clinical studies, which suggests that there is a "critical period" for estradiol's beneficial effect in the brain. Potential mechanisms underlying the critical period are discussed, as are the neurological consequences of long-term E2 deprivation (LTED) in animals and in humans after natural menopause or surgical menopause. We also summarize the major clinical trials concerning postmenopausal hormone therapy (HT), comparing their outcomes with respect to cardiovascular and neurological disease and discussing their relevance to the critical period hypothesis. Finally, potential caveats, controversies and future directions for the field are highlighted and discussed throughout the review.

Estrogen induces neurite outgrowth via Rho family GTPases in neuroblastoma cells

Estrogen (E2) has direct in vivo and in vitro effects, such as inducing neurite outgrowth, on neurons. We investigated the morphological changes and intracellular signaling pathway induced by E2 in neuroblastoma (SH-SY5Y) cells. The effect of medroxyprogesterone acetate (MPA) or progesterone (P4) on the E2-induced neurite outgrowth was also examined using SH-SY5Y cells. Neurite outgrowth was induced by E2 in association with the phosphorylation of Akt, and these effects of E2 were abolished by MPA but not by P4. Progesterone receptor antagonist RU486 blocked the inhibitory effects of MPA. Estrogen receptor antagonist ICI 182,780 and phosphatidylinositol 3-kinase inhibitor LY294002 inhibited the E2-induced neurite outgrowth. Because the Rho family of small GTPases has been shown to be involved in the regulation of neurite outgrowth, we examined the cross-talk among Rac1, Cdc42 and RhoA in the E2-induced neurite outgrowth. E2 immediately increased the Rac1 and Cdc42 activity and decreased the RhoA activity. E2-induced neurite outgrowth was attenuated in cells expressing dominant-negative mutants for Rac1 or Cdc42. These results suggest that regulation of Rho family GTPase activity by E2 is important for the neurite outgrowth in neuroblastoma cells, and that MPA may have an antagonistic effect against E2.

Adult rat hippocampal slices as in vitro models for neurodegeneration: Studies on cell viability and apoptotic processes

Adult hippocampal slice cultures were used in the modeling of apoptotic aspects of neurodegeneration. Slice viability was determined by the use of trypan blue (TB) staining, and apoptosis was assessed by caspase-3 immunohistochemistry. A large number of pyramidal cells showed signs of degeneration 30 min after sectioning (58.4% of the total number of pyramidal cells), as they exhibited TB uptake, and about 71.6% of these neurons became stained by the third day in culture, when patches in the stratum oriens also demonstrated distinct TB staining. By the sixth day of culturing, almost all cells in the pyramidal cell layer became TB positive (88.4%). The caspase-3 immunoreactivity displayed a different pattern, as the most intense immunoreactivity, detected mainly in the pyramidal cells, peaked 6 h after culturing, and then decreased steadily. The present data show that in adult hippocampal slices a large number of pyramidal cells initiate apoptotic processes as a result of irreparable damage sustained during slice preparation and culture maintenance, and support the notion that apoptosis is an integral part of the neurodegenerative processes not only in vivo but also in vitro. Elucidation of mechanisms for the apoptotic processes in adult hippocampal slice cultures could lead to the development of new therapeutic strategies; moreover, the utilization of adult hippocampal slice cultures could be a viable alternative technique to in vivo experiments in studying the mechanisms responsible for neurodegeneration.

Metabolic impact of sex hormones on obesity

Obesity and its associated health disorders and costs are increasing. Men and post-menopausal women have greater risk of developing complications of obesity than younger women. Within the brain, the hypothalamus is an important regulator of energy homeostasis. Two of its sub-areas, the ventrolateral portion of the ventral medial nucleus (VL VMN) and the arcuate (ARC) respond to hormones and other signals to control energy intake and expenditure. When large lesions are made in the hypothalamus which includes both the VL VMN and the ARC, animals eat more, have reduced energy expenditure, and become obese. The ARC and the VL VMN, in addition to other regions in the hypothalamus, have been demonstrated to contain estrogen receptors. There are two estrogen receptors, estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta). We and others have previously demonstrated that activation of ERalpha by estrogens reduces food intake and increases body weight. This review focuses on the relative contribution of activation of ERalpha by estrogens in the ARC and the VL VMN in the regulation of food intake and body weight. Additionally, estrogen receptors have been found in many peripheral tissues including adipose tissue. Estrogens are thought to have direct effects on adipose tissue and estrogens may provide anti-inflammatory properties both in the periphery and the in the central nervous system (CNS) which may protect women from diseases associated with inflammation. Understanding the mechanisms by which estrogens regulate body weight and inflammation will assist in determining potential therapeutic agents for menopausal women to decrease the propensity of diseases associated with obesity.

Estradiol-induced object memory consolidation in middle-aged female mice requires dorsal hippocampal extracellular signal-regulated kinase and phosphatidylinositol 3-kinase activation

We previously demonstrated that dorsal hippocampal extracellular signal-regulated kinase (ERK) activation is necessary for 17beta-estradiol (E(2)) to enhance novel object recognition in young ovariectomized mice (Fernandez et al., 2008). Here, we asked whether E(2) has similar memory-enhancing effects in middle-aged and aged ovariectomized mice, and whether these effects depend on ERK and phosphatidylinositol 3-kinase (PI3K)/Akt activation. We first demonstrated that intracerebroventricular or intrahippocampal E(2) infusion immediately after object recognition training enhanced memory consolidation in middle-aged, but not aged, females. The E(2)-induced enhancement in middle-aged females was blocked by intrahippocampal inhibition of ERK or PI3K activation. Intrahippocampal or intracerebroventricular E(2) infusion in middle-aged females increased phosphorylation of p42 ERK in the dorsal hippocampus 15 min, but not 5 min, after infusion, an effect that was blocked by intrahippocampal inhibition of ERK or PI3K activation. Dorsal hippocampal PI3K and Akt phosphorylation was increased 5 min after intrahippocampal or intracerebroventricular E(2) infusion in middle-aged, but not aged, females. Intracerebroventricular E(2) infusion also increased PI3K phosphorylation after 15 min, and this effect was blocked by intrahippocampal PI3K, but not ERK, inhibition. These data demonstrate for the first time that activation of dorsal hippocampal PI3K/Akt and ERK signaling pathways is necessary for E(2) to enhance object recognition memory in middle-aged females. They also reveal that similar dorsal hippocampal signaling pathways mediate E(2)-induced object recognition memory enhancement in young and middle-aged females and that the inability of E(2) to activate these pathways may underlie its failure to enhance object recognition in aged females.

Estradiol: a hormone with diverse and contradictory neuroprotective actions

The concept that estrogens exert important neuroprotective actions has gained considerable attention during the past decade. Numerous studies have provided a deep understanding of the seemingly contradictory actions of estrogens. We realize more than ever that the effects of estrogens (with and without simultaneous or sequential progestins) are diverse and sometimes opposite, depending on the use of different estrogenic and progestinic compounds, on different delivery routes, on different concentrations, on treatment sequence, and on the age and health status of the women who receive hormone therapy. During the past few years, we have gained an increasing appreciation of the impact of estrogens on the immune system and on inflammation. In addition, we have learned that estrogens cannot only protect against cell death, but can also stimulate the birth of new neurons. Here we posit the concept that estrogen's modulation of the immune status may be the basic mechanism that underlies its ability to protect against neurodegeneration and its powerful neuroregenerative actions. We hope that this update will encourage even richer dialogues between basic and clinical scientists to ensure that future clinical studies fully consider the information that can be derived from basic science studies. Only then will we have a better understanding of the impact of hormones on the menopausal and postmenopausal period in a woman's life.

Estrogen partially down-regulates PTEN to prevent apoptosis in VSC4.1 motoneurons following exposure to IFN-gamma

PTEN is a tumor suppressor gene that is either mutated or deleted in a number of human cancers. PTEN acts as a negative regulator of the PI3K/Akt survival pathway and thus plays an important role in cell fate, proliferation, growth, and migration. Recent evidence suggests that PTEN may also be involved in the pathophysiology of neurodegenerative disorders such as spinal cord injury (SCI). Overexpression of PTEN appears to cause inactivation/dephosphorylation of Akt in neurons, resulting in increased cell death. Given this newly discovered role for PTEN, it has been identified as a potential molecular target for the development of novel therapeutic strategies against neurodegeneration. Motoneuron degeneration following SCI may occur due to up-regulation of pro-inflammatory and cytotoxic cytokines including IFN-gamma. Exposure of VSC4.1 motoneurons to IFN-gamma (10 ng/ml) for 24 h resulted in significant overexpression of PTEN and decreased levels of activated Akt. Up-regulation of PTEN following IFN-gamma exposure was associated with decreased overall cell viability due to increased apoptosis, as assessed by Wright staining and analysis of cell death markers including Bax, Bcl-2, calpain activity, and caspase-3 activity, indicating a prominent role for PTEN in suppression of the PI3K/Akt survival pathway to promote motoneuron death. Addition of estrogen (100 nM) to VSC4.1 cells for 1 h prior to IFN-gamma exposure partially decreased PTEN expression, allowing adequate activation or phosphorylation of Akt (p-Akt) to prevent apoptotic cell death. Thus, it appears that estrogen may mediate neuroprotection through decrease in PTEN expression. In conclusion, our studies suggest that PTEN inactivation may be used as an important parameter for evaluation of the efficacy of estrogen in prevention of neuronal loss in neurodegenerative disorders.

Neuroprotective effects of estrogens following ischemic stroke

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

Estrogen and tamoxifen protect against Mn-induced toxicity in rat cortical primary cultures of neurons and astrocytes

Chronic exposure to manganese (Mn) leads to a neurological disorder, manganism, which shares multiple common features with idiopathic Parkinson disease (IPD). 17beta-Estradiol (E2) and some selective estrogen receptor modulators, including tamoxifen (TX), afford neuroprotection in various experimental models of neurodegeneration. However, the neuroprotective effects and mechanisms of E2/TX in Mn-induced toxicity have yet to be documented. Herein, we studied the ability of E2/TX to protect rat cortical primary neuronal and astroglial cultures from Mn-induced toxicity. Cell viability, Western blot, and reactive oxygen species (ROS) generation were assessed. Results established that both E2 (10nM) and TX (1 microM) attenuated Mn-induced toxicity. The protective effects of E2/TX were more pronounced in astrocytes versus neurons. The E2-mediated attenuation of Mn-induced ROS generation in astrocytes at 6-h treatment (where no cell death was detected) was mediated by a classical estrogen receptor (ER) pathway and the TX-mediated effect on Mn-induced ROS generation was not mediated via classical ER-dependent mechanisms and likely by its antioxidant properties. The phosphatidylinositol-3 kinase (PI3K)/Akt signaling pathway was involved in both E2- and TX-induced attenuation of Mn-induced ROS formation (6 h) in astrocytes. Treatments with Mn for a longer duration (24 h) led to significant cell death, and the protective effects of E2 and TX were (1) not mediated by a classical ER pathway and (2) associated with activation of both mitogen-activated protein kinase/extracellular signal-regulated kinase and PI3K/Akt signaling pathways. Taken together, the results suggest that both E2 and TX offer effective therapeutic means for neuroprotection against Mn-induced toxicity.

Role of estrogen receptor alpha in membrane-initiated signaling in neural cells: interaction with IGF-1 receptor

The mechanisms of action of estradiol in the nervous system involve nuclear-initiated steroid signaling and membrane-initiated steroid signaling. Estrogen receptors (ERs) are involved in both mechanisms. ERalpha interacts with the signaling of IGF-1 receptor in neural cells: ERalpha transcriptional activity is regulated by IGF-1 receptor signaling and estradiol regulates IGF-1 receptor signaling. The interaction between ERalpha and the IGF-1 receptor in the brain may occur at the plasma membrane of neurons and glial cells. Caveolin-1 may provide the scaffolding for the interaction of different membrane-associated molecules, including voltage-dependent anion channel, ERalpha and IGF-I receptor.

Soy phytoestrogens are neuroprotective against stroke-like injury in vitro

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

Pubertal maturation modifies the regulation of insulin-like growth factor-I receptor signaling by estradiol in the rat prefrontal cortex

The transition from adolescence to adulthood is accompanied by substantial plastic modifications in the cerebral cortex, including changes in the growth and retraction of neuronal processes and in the rate of synaptic formation and neuronal loss. Some of these plastic changes are prevented in female rats by prepubertal ovariectomy. The ovarian hormone estradiol modulates neuronal differentiation and survival and these effects are in part mediated by the interaction with insulin-like growth factor-I (IGF-I). In this study, we have explored whether the activation by estradiol of some components of IGF-I receptor signaling is altered in the prefrontal cortex during puberty. Estradiol administration to rats ovariectomized after puberty resulted, 24 h after the hormonal administration, in a sustained phosphorylation of Akt and glycogen synthase kinase 3 beta in the prefrontal cortex. However, this hormonal effect was not observed in animals ovariectomized before puberty. These findings suggest that during pubertal maturation there is a programming by ovarian hormones of the future regulatory actions of estradiol on IGF-I receptor signaling in the prefrontal cortex. The modification in the regulation of IGF-I receptor signaling by estradiol during pubertal maturation may have implications for the developmental changes occurring in the prefrontal cortex in the transition from adolescence to adulthood.

Gender differences and estrogen effects in parkin null mice

Estrogens are considered neurotrophic for dopamine neurons. Parkinson's disease is more frequent in males than in females, and more prevalent in females with short reproductive life. Estrogens are neuroprotective against neurotoxic agents for dopamine neurons in vivo and in vitro. Here, we have investigated the role of estrogens in wild-type (WT) and parkin null mice (PK-/-). WT mice present sexual dimorphisms in neuroprotective mechanisms (Bcl-2/Bax, chaperones, and GSH), but some of these inter-sex differences disappear in PK-/-. Tyrosine hydroxylase (TH) protein and TH+ cells decreased earlier and more severely in female than in male PK-/- mice. Neuronal cultures from midbrain of WT and PK-/- mice were treated with estradiol from 10 min to 48 h. Short-term treatments activated the mitogen-activated protein kinase pathway of WT and PK-/- neurons and the phosphatidylinositol 3'-kinase/AKT/glycogen synthase kinase-3 pathway of WT but not of PK-/- cultures. Long-term treatments with estradiol increased the number of TH+ neurons, the TH expression, and the extension of neurites, and decreased the level of apoptosis, the expression of glial fibrillary acidic protein, and the number of microglial cells in WT but not in PK-/- cultures. The levels of estrogen receptor-alpha were elevated in midbrain cultures and in the striatum of adult PK-/- male mice, suggesting that suppression of parkin changes the estrogen receptor-alpha turnover. From our data, it appears that parkin participates in the cellular estrogen response which could be of interest in the management of parkin-related Parkinson's disease patients.

The role of steroid hormones in the NF1 phenotype: focus on pregnancy

The Neurofibromatosis Type 1 (NF1) gene functions as a tumor suppressor gene. Loss of its protein, neurofibromin, in the autosomal dominant disorder NF1 is associated with peripheral nervous system tumors, particularly neurofibromas, benign lesions in which the major cell type is the Schwann Cell (SC). Benign and malignant human tumors found in NF1 patients are heterogeneous with respect to their cellular composition. The number and size of neurofibromas in NF1 patients has been shown to increase during pregnancy, with, in some cases, post-partum regression, which suggests hormonal involvement in this increase. However, in this review, we consider evidence from the literature that both direct hormonal influence on tumor growth and on angiogenesis may contribute to these effects.

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

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

Non-genomic actions of estrogens and their interaction with genomic actions in the brain

Ligands for the nuclear receptor superfamily have at least two mechanisms of action: (a) classical transcriptional regulation of target genes (genomic mechanisms); and (b) non-genomic actions, which are initiated at the cell membrane, which could also impact transcription. Though transcriptional mechanisms are increasingly well understood, membrane-initiated actions of these ligands are incompletely understood. This has led to considerable debate over the physiological relevance of membrane-initiated actions of hormones versus genomic actions of hormones, with genomic actions predominating in the endocrine field. There is good evidence that the membrane-limited actions of hormones, particularly estrogens, involve the rapid activation of kinases and the release of calcium and that these are linked to physiologically relevant scenarios in the brain. We show evidence in this review, that membrane actions of estrogens, which activate these rapid signaling cascades, can also potentiate nuclear transcription in both the central nervous system and in non-neuronal cell lines. We present a theoretical scenario which can be used to understand this phenomenon. These signaling cascades may occur in parallel or in series but subsequently, converge at the modification of transcriptionally relevant molecules such as nuclear receptors and/or coactivators. In addition, other non-cognate hormones or neurotransmitters may also activate cascades to crosstalk with estrogen receptor-mediated transcription, though the relevance of this is less clear. The idea that coupling between membrane-initiated and genomic actions of hormones is a novel idea in neuroendocrinology and provides us with a unified view of hormone action in the central nervous system.

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

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

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

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

17beta-estradiol attenuates glycogen synthase kinase-3beta activation and tau hyperphosphorylation in Akt-independent manner

Decline of estrogen is associated with high incidence of Alzheimer's disease (AD) characterized pathologically with tau hyperphosphorylation, and glycogen synthase kinase-3beta (GSK-3beta) is a major tau kinase. However, the role of estrogen on GSK3beta-induced tau hyperphosphorylation is elusive. Here, we treated N2a cells with wortmannin (Wort) and GF-109203X (GFX) or gene transfection to activate GSK-3beta and to induce tau hyperphosphorylation and then the effects of 17beta-estradiol (betaE2) on tau phosphorylation and GSK-3beta activity were studied. We found that betaE2 could attenuate tau hyperphosphorylation at multiple AD-related sites, including Ser396/404, Thr231, Thr205, and Ser199/202, induced by Wort/GFX or transient overexpression of GSK-3beta. Simultaneously, it increased the level of Ser9-phosphorylated (inactive) GSK-3beta. To study whether the protective effect of betaE2 on GSK-3beta and tau phosphorylation involves protein kinase B (Akt), an upstream effector of GSK-3, we transiently expressed the dominant negative Akt (dnAkt) in the cells. We found that betaE2 could attenuate Wort/GFX-induced GSK-3beta activation and tau hyperphosphorylation with Akt-independent manner. It suggests that betaE2 may arrest AD-like tau hyperphosphorylation by directly targeting GSK-3beta.

Extranuclear steroid receptors: nature and actions

Rapid effects of steroid hormones result from the actions of specific receptors localized most often to the plasma membrane. Fast-acting membrane-initiated steroid signaling (MISS) leads to the modification of existing proteins and cell behaviors. Rapid steroid-triggered signaling through calcium, amine release, and kinase activation also impacts the regulation of gene expression by steroids, sometimes requiring integration with nuclear steroid receptor function. In this and other ways, the integration of all steroid actions in the cell coordinates outcomes such as cell fate, proliferation, differentiation, and migration. The nature of the receptors is of intense interest, and significant data suggest that extranuclear and nuclear steroid receptor pools are the same proteins. Insights regarding the structural determinants for membrane localization and function, as well as the nature of interactions with G proteins and other signaling molecules in confined areas of the membrane, have led to a fuller understanding of how steroid receptors effect rapid actions. Increasingly, the relevance of rapid signaling for the in vivo functions of steroid hormones has been established. Examples include steroid effects on reproductive organ development and function, cardiovascular responsiveness, and cancer biology. However, although great strides have been made, much remains to be understood concerning the integration of extranuclear and nuclear receptor functions to organ biology. In this review, we highlight the significant progress that has been made in these areas.

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

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

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

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

Membrane-initiated actions of estrogens in neuroendocrinology: emerging principles

Hormonal ligands for the nuclear receptor superfamily have at least two interacting mechanisms of action: 1) classical transcriptional regulation of target genes (genomic mechanisms); and 2) nongenomic actions that are initiated at the cell membrane, which could impact transcription. Although transcriptional mechanisms are increasingly well understood, membrane-initiated actions of these ligands are incompletely understood. Historically, this has led to a considerable divergence of thought in the molecular endocrine field. We have attempted to uncover principles of hormone action that are relevant to membrane-initiated actions of estrogens. There is evidence that the membrane-limited actions of hormones, particularly estrogens, involve the rapid activation of kinases and the release of calcium. Membrane actions of estrogens, which activate these rapid signaling cascades, can also potentiate nuclear transcription. These signaling cascades may occur in parallel or in series but subsequently converge at the level of modification of transcriptionally relevant molecules such as nuclear receptors and/or coactivators. In addition, other hormones or neurotransmitters may also activate cascades to crosstalk with estrogen receptor-mediated transcription. The idea of synergistic coupling between membrane-initiated and genomic actions of hormones fundamentally revises the paradigms of cell signaling in neuroendocrinology.

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

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

Role of astrocytes in estrogen-mediated neuroprotection

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

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

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

Cross-talk between estrogen receptors and insulin-like growth factor-I receptor in the brain: cellular and molecular mechanisms

Accumulating evidence suggests that insulin-like growth factor-I (IGF-I) and estradiol interact to regulate neural function. In this review, we focus on the cellular and molecular mechanisms involved in this interaction. The expression of estrogen receptors (ERs) and IGF-I receptor is cross-regulated in the central nervous system and many neurons and astrocytes coexpress both receptors. Furthermore, estradiol activates IGF-I receptor and its intracellular signaling. This effect may involve classical ERs since recent findings suggest that ERalpha may affect IGF-I actions in the brain by a direct interaction with some of the components of IGF-I signaling. In turn, IGF-I may regulate ER transcriptional activity in neuronal cells. In conclusion, ERs appear to be part of the signaling mechanism of IGF-I, and IGF-I receptor part of the mechanism of estradiol signaling in the nervous system.

Minireview: estrogen receptor-mediated rapid signaling

In addition to nuclear-initiated (genomic) responses, estrogen receptors (ERs) have the ability to facilitate rapid, membrane-initiated, estrogen-triggered signaling cascades via a plasma membrane-associated form of the receptor. These rapid responses are dependent on assembly of membrane ER-centered multimolecular complexes, which can transduce ligand-activated signals to affect a variety of enzymatic pathways, often occurring in a cell-type-specific fashion with tissue-specific physiological outcomes. In some instances, cross-talk occurs between these membrane-initiated and nuclear responses, ultimately regulating transcriptional activation. The role of splice variants in membrane-initiated estrogen responses has been described, notably those within the vascular endothelium. In this review, we describe the evidence for membrane ERs, the molecular components of the aforementioned signaling complexes and pathways, the relevance of ER splice variants, and ER-mediated responses in specific tissues. Our growing understanding of ER-mediated actions at a molecular level will provide insight into the controversies surrounding hormone replacement therapy in postmenopausal women.

Expression patterns and action analysis of genes associated with drug-induced liver diseases during rat liver regeneration

AIM: To study the action of the genes associated with drug-induced liver diseases at the gene transcriptional level during liver regeneration (LR) in rats.

METHODS: The genes associated with drug-induced liver diseases were obtained by collecting the data from databases and literature, and the gene expression changes in the regenerating liver were checked by the Rat Genome 230 2.0 array.

RESULTS: The initial and total expression numbers of genes occurring in phases of 0.5-4 h after partial hepatectomy (PH), 4-6 h after PH (G0/G1 transition), 6-66 h after PH (cell proliferation), 66-168 h after PH (cell differentiation and structure-function reconstruction) were 21, 3, 9, 2 and 21, 9, 19, 18, respectively. It is illustrated that the associated genes were mainly triggered at the initial stage of LR and worked at different phases. According to their expression similarity, these genes were classified into 5 types: only up-regulated (12 genes), predominantly up-regulated (4 genes), only down-regulated (11 genes), predominantly down-regulated (3 genes), and approximately up-/down-regulated (2 genes). The total times of their up- and down-expression were 130 and 79, respectively, demonstrating that expression of most of the genes was increased during LR, while a few decreased. The cell physiological and biochemical activities during LR were staggered according to the time relevance and were diverse and complicated in gene expression patterns.

CONCLUSION: Drug metabolic capacity in regenerating liver was enhanced. Thirty-two genes play important roles during liver regeneration in rats.

Complex actions of sex steroids in adipose tissue, the cardiovascular system, and brain: Insights from basic science and clinical studies

Recent publications describing the results of the Women's Health Initiative (WHI) and other studies reporting the impact of hormone therapy on aging women have spurred reexamination of the broad use of estrogens and progestins during the postmenopausal years. Here, we review the complex pharmacology of these hormones, the diverse and sometimes opposite effects that result from the use of different estrogenic and progestinic compounds, given via different delivery routes in different concentrations and treatment sequence, and to women of different ages and health status. We examine our new and growing appreciation of the role of estrogens in the immune system and the inflammatory response, and we pose the concept that estrogen's interface with this system may be at the core of some of the effects on multiple physiological systems, such as the adipose/metabolic system, the cardiovascular system, and the central nervous system. We compare and contrast clinical and basic science studies as we focus on the actions of estrogens in these systems because the untoward effects of hormone therapy reported in the WHI were not expected. The broad interpretation and publicity of the results of the WHI have resulted in a general condemnation of all hormone replacement in postmenopausal women. In fact, careful review of the extensive literature suggests that data resulting from the WHI and other recent studies should be interpreted within the narrow context of the study design. We argue that these results should encourage us to perform new studies that take advantage of a dialogue between basic scientists and clinician scientists to ensure appropriate design, incorporation of current knowledge, and proper interpretation of results. Only then will we have a better understanding of what hormonal compounds should be used in which populations of women and at what stages of menopausal/postmenopausal life.

Estradiol prevents the injury-induced decrease of Akt/glycogen synthase kinase 3β phosphorylation

Estradiol prevents neuronal cell death through the activation of cell survival signals and the inhibition of apoptotic signals. This study investigated whether estradiol modulates the anti-apoptotic signal through the phosphorylation of Akt and its downstream target, glycogen synthase kinase 3beta (GSK3beta). Adult female rats were ovariectomized and treated with estradiol prior to middle cerebral artery occlusion (MCAO). Brains were collected 24 h after MCAO and infarct volumes were analyzed. Estradiol administration significantly reduced infarct volume and decreased the positive cells of TUNEL staining in the cerebral cortex. Potential activation was measured by phosphorylation of Akt at Ser(473) and GSK3beta at Ser(9) using Western blot analysis and immunohistochemistry. Estradiol prevented the injury-induced decrease of pAkt and pGSK3beta. Furthermore, pretreatment with estradiol decreased glutamate toxicity-induced cell death in a hippocampal cell line (HT22). Also, estradiol prevented the glutamate toxicity-induced decrease of pAkt and pGSK3beta in HT22 cells. Our findings suggest that estradiol plays a potent protective role against brain injury and that phosphorylation of Akt and GSK3beta by estradiol mediated these protective effects.

Differential modulation of estrogen receptors (ERs) in ischemic brain injury: a role for ERalpha in estradiol-mediated protection against delayed cell death

Estradiol enhances plasticity and survival of the injured brain. Our previous work demonstrates that physiological levels of estradiol protect against cerebral ischemia in the young and aging brain through actions involving estrogen receptors (ERs) and alterations in gene expression. The major goal of this study was to establish mechanisms of neuroprotective actions induced by low levels of estradiol. We first examined effects of estradiol on the time-dependent evolution of ischemic brain injury. Because estradiol is known to influence apoptosis, we hypothesized that it acts to decrease the delayed phase of cell death observed after middle cerebral artery occlusion (MCAO). Furthermore, because ERs are pivotal to neuroprotection, we examined the temporal expression profiles of both ER subtypes, ERalpha and ERbeta, after MCAO and delineated potential roles for each receptor in estradiol-mediated neuroprotection. We quantified cell death in brains at various times after MCAO and analyzed ER expression by RT-PCR, in situ hybridization, and immunohistochemistry. We found that during the first 24 h, the mechanisms of estradiol-induced neuroprotection after MCAO are limited to attenuation of delayed cell death and do not influence immediate cell death. Furthermore, we discovered that ERs exhibit distinctly divergent profiles of expression over the evolution of injury, with ERalpha induction occurring early and ERbeta modulation occurring later. Finally, we provide evidence for a new and functional role for ERalpha in estradiol-mediated protection of the injured brain. These findings indicate that physiological levels of estradiol protect against delayed cell death after stroke-like injury through mechanisms requiring ERalpha.