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

Insulin-like growth factor-I receptor and estrogen receptor crosstalk mediates hormone-induced neurite outgrowth in PC12 cells

Estradiol (E(2)) and insulin-like growth factor-I (IGF-I) can act independently or in concert to promote neurite outgrowth in vivo and in cultured neurons. This study examined the role of crosstalk between estrogen receptor (ER)alpha and the IGF-I receptor as a critical mediator of hormone- and growth factor-dependent neurite outgrowth in a homogenous cell system. We used control PC12 cells and PC12 cells stably transfected with ER alpha, both of which express IGF-I receptor. Cells were treated for 1 week with vehicle, 1 nM E(2) or 100 ng/ml IGF-I alone or with E(2) or IGF-I in the presence of either the IGF-I receptor antagonist JB1 or the ER antagonist ICI 182,780. IGF-I significantly increased neurite outgrowth, as measured by the percentage of process-bearing cells, and absolute neurite length per cell in both control and ER alpha-transfected PC12 cells. In contrast, E(2) increased process formation and extension only in PC12 cells that were stably transfected with ER alpha. ICI 182,780 and JB1 blocked the IGF-I-induced increases in neurite length in both cell types. The efficacy of ICI 182,780 in control PC12 cells may have been due to the upregulation of ER alpha in these cells by the 7-day treatment with IGF-I. The ER and IGF-I receptor antagonists similarly blocked the E(2)-induced increase in neurite lengths in ER alpha-transfected cells. Immunofluorescent analysis of the cellular distribution of an axonal marker, phospho-neurofilament, verified that the processes extended by PC12 cells were neurites. These data suggest that receptor crosstalk between IGF-I receptors and ER alpha has an important role in neurite formation and extension even in a single-cell system.

Differential protective properties of estradiol and tamoxifen against methamphetamine-induced nigrostriatal dopaminergic toxicity in mice

Mechanisms implicated in protective potential of estrogens are poorly understood. Tamoxifen, a selective estrogen receptor modulator (SERM), presents a neuroprotective effect against methamphetamine (MA)- and methoxy-phenyltetrahydropyridine (MPTP)-induced toxicity when used alone but abolishes estrogen's positive effects when combined with this hormone. In order to understand tamoxifen's protective properties, the present study compared it to estradiol on several markers of dopaminergic neurons to achieve a relatively comprehensive comparison between these two agents. Estradiol benzoate (E) or tamoxifen were used at different concentrations (E: 1, 10 or 40 microg; tamoxifen: 12.5, 125 or 500 microg) 24 h prior to a MA injection in ovariectomized CD-1 mice. The effects of the lesion and treatments were studied on striatal dopamine (DA) concentrations, dopamine and monoamine vesicular transporters (DAT and VMAT2), and preproenkephalin (PPE) mRNA levels. Both treatments, at all concentrations, prevented the MA-induced decrease of striatal DA concentrations and VMAT2 binding. Only E was able to prevent loss of DAT binding in the lateral striatum and to attenuate the MA-induced increase in striatal PPE mRNA levels (at 1 or 40 microg). Therefore, in this paradigm, E and tamoxifen differentially modulated MA-induced neuronal damages. While both treatments prevented the DA decrease, E protected more efficiently other dopaminergic parameters suggesting that overall E is more effective than tamoxifen as a neuroprotectant of the nigrostriatal dopaminergic system.

Ovariectomy and gonadal hormone treatment: effects on insulin-like growth factor-1 receptors in the rat brain

Various studies demonstrate that estradiol regulates structure and function of adult neurons. Long-term effect of estradiol in terms of neuroprotection is less documented compared to short-term one. It is well documented that estradiol interacts with insulin-like growth factor-1 (IGF-I) in the brain. The present study examines the effect of ovariectomy and two doses of ovarian hormone treatment on IGF-I receptor density in the adult rat by receptor autoradiography using (125)I-IGF-I as a ligand. Our result showed that ovariectomy decreased IGF-I receptor density in hippocampus, hypothalamus and parietal cortex compared to that of the sham-operated group. Treatment with low or high dose estrogen restored IGF-I receptor density to the control levels in nearly all areas studied in this investigation. It seems that low dose estrogen has more pronounced effect than the high dose in restoring IGF-I receptor density. On the other hand, progesterone treatment in high but not in low dose restored IGF-I receptor density to that of the control. These results demonstrate that both estrogen and progesterone significantly affects IGF-I receptor density in different areas of the brain. These effects indicate a dose-dependent modulator effect of ovarian hormones on IGF-I activity in the brain.

Suppression subtractive hybridization and microarray identification of estrogen-regulated hypothalamic genes

The gonadal steroid estrogen is a pleiotropic hormone that has multiple effects on numerous cellular functions. One of estrogen's major targets is the brain, where the steroid not only affects growth, differentiation, and survival of neurons, but also regulates cell excitability. Because estrogen modulates multiple, overlapping signaling pathways, it has been difficult to scrutinize the transcriptional activity of the steroid. Therefore, we still lack a global picture of how different genes interact and are regulated by estrogen. Herein we report the use of suppression subtractive hybridization followed by custom microarray analysis of thousands of genes that are differentially expressed during the negative feedback phase of the female reproductive cycle. We have found a number of key transcripts that are regulated by estrogen and contribute to the alteration in synaptic transmission and hence excitability of hypothalamic neurons (e.g., GABA neurons). These include gec-1, GABA(B)R2, PI3 kinase subunit p55gamma, and a number of proteins containing pleckstrin homology domains that are critical for plasma membrane targeting. Studies are underway to refine our analysis to individual nuclei and individual cells. However, what has emerged from this highly sensitive microarray analysis is that estrogen affects neuronal plasticity in hypothalamic neurons not only by transcription of new membrane proteins (e.g., receptors and channels), but also by altering expression of downstream signaling molecules and proteins involved in neurosecretory pathways.

Fibroblast growth factor-2 over-rides insulin-like growth factor-I induced proliferation and cell survival in human neuroblastoma cells

The insulin-like growth factor (IGF) system is a key regulator of cell growth, survival and differentiation, and these functions are co-modulated by other growth factors including fibroblast growth factor-2 (FGF-2). To investigate IGF/FGF interactions in neuronal cells, we employed neuroblastoma cells (SK-N-MC). In serum free conditions proliferation of the SK-N-MC cells was promoted by IGF-I (25 ng/ml), but blunted by FGF-2 (50 ng/ml). IGF-I-induced proliferation was abolished in the presence of FGF-2 even when IGF-I was used at 100 ng/ml. In addition to our previously described FGF-2 induced proteolytic cleavage of IGFBP-2, we found that FGF-2 increased IGFBP-6 levels in conditioned medium (CM) without affecting IGFBP-6 mRNA abundance. Modulation of IGFBP-2 and -6 levels were not significant mechanisms involved in the blockade of IGF-I action since the potent IGF-I analogues [QAYL]IGF-I and des(1-3)IGF-I (minimal IGFBP affinity) were unable to overcome FGF-2 inhibition of cell proliferation. FGF-2 treated cells showed morphological differentiation expressing the TUJ1 neuronal marker while cells treated with IGF-I alone showed no morphological change. When IGF-I was combined with FGF-2, however, cell morphology was indistinguishable from that seen with FGF-2 alone. FGF-2 inhibited proliferation and enhanced differentiation was also associated with a 70% increase in cell death. Although IGF-I alone was potently anti-apoptotic (60% decreased), IGF-I was unable to prevent apoptosis when administrated in combination with FGF-2. Gene-array analysis confirmed FGF-2 activation of the intrinsic and extrinsic apoptotic pathways and blockade of IGF anti-apoptotic signaling. FGF-2, directly and indirectly, overcomes the proliferative and anti-apoptotic activity of IGF-I by complex mechanisms, including enhancement of differentiation and apoptotic pathways, and inhibition of IGF-I induced anti-apoptotic signalling. Modulation of IGF binding protein abundance by FGF-2 does not play a significant role in inhibition of IGF-I induced mitogenesis.

Local synthesis and dual actions of progesterone in the nervous system: neuroprotection and myelination

Progesterone (PROG) is synthesized in the brain, spinal cord and peripheral nerves. Its direct precursor pregnenolone is either derived from the circulation or from local de novo synthesis as cytochrome P450scc, which converts cholesterol to pregnenolone, is expressed in the nervous system. Pregnenolone is converted to PROG by 3beta-hydroxysteroid dehydrogenase (3beta-HSD). In situ hybridization studies have shown that this enzyme is expressed throughout the rat brain, spinal cord and dorsal root ganglia (DRG) mainly by neurons. Macroglial cells, including astrocytes, oligodendroglial cells and Schwann cells, also have the capacity to synthesize PROG, but expression and activity of 3beta-HSD in these cells are regulated by cellular interactions. Thus, Schwann cells convert pregnenolone to PROG in response to a neuronal signal. There is now strong evidence that P450scc and 3beta-HSD are expressed in the human nervous system, where PROG synthesis also takes place. Although there are only a few studies addressing the biological significance of PROG synthesis in the brain, the autocrine/paracrine actions of locally synthesized PROG are likely to play an important role in the viability of neurons and in the formation of myelin sheaths. The neuroprotective effects of PROG have recently been documented in a murine model of spinal cord motoneuron degeneration, the Wobbler mouse. The treatment of symptomatic Wobbler mice with PROG for 15 days attenuated the neuropathological changes in spinal motoneurons and had beneficial effects on muscle strength and the survival rate of the animals. PROG may exert its neuroprotective effects by regulating expression of specific genes in neurons and glial cells, which may become hormone-sensitive after injury. The promyelinating effects of PROG were first documented in the mouse sciatic nerve and in co-cultures of sensory neurons and Schwann cells. PROG also promotes myelination in the brain, as shown in vitro in explant cultures of cerebellar slices and in vivo in the cerebellar peduncle of aged rats after toxin-induced demyelination. Local synthesis of PROG in the brain and the neuroprotective and promyelinating effects of this neurosteroid offer interesting therapeutic possibilities for the prevention and treatment of neurodegenerative diseases, for accelerating regenerative processes and for preserving cognitive functions during aging.

Estradiol inhibits GSK3 and regulates interaction of estrogen receptors, GSK3, and beta-catenin in the hippocampus

Estrogens regulate a wide set of neuronal functions such as gene expression, survival and differentiation in a manner not very different from that exerted by neurotrophins or by growth factors. The best-studied hormonal action is the transcriptional activation mediated by estrogen receptors. However, the direct effects of estrogen on growth factor signaling have not been well clarified. The present data show that estradiol, in vivo, induces a transient activation of GSK3 in the adult female rat hippocampus, followed by a more sustained inhibition, as inferred from phosphorylation levels of Tau. Similar data was obtained from cultured hippocampal neurons when treated with the hormone. The transient activation was confirmed by direct measure of GSK3 kinase activity. In addition, our results show a novel complex of estrogen receptor alpha, GSK3, and beta-catenin. The presence of the hormone removes beta-catenin from this complex. There is a second complex, also affected by estradiol, in which Tau is associated with GSK3, beta-catenin, and elements of the PI3 kinase complex. Considering the role of GSK3 in neurodegeneration, our data suggest that part of the neuroprotective effects of estrogen may be due to the control of GSK3.

Sex steroids and growth factors in the regulation of mammary gland proliferation, differentiation, and involution

The mammary gland is subjected to major morphological and biochemical changes during the lactation cycle. It is therefore not surprising that this dynamic process is strictly controlled. The importance of the sex steroid hormones 17beta-estradiol and progesterone for normal development of the mammary gland was recognized several decades ago and has been unequivocally confirmed since. Furthermore, it is now also established that the influence of sex steroids is not restricted to mammogenesis, but that these hormones also control involution. Another important regulatory role is played by growth factors that have been shown to modulate survival (epidermal growth factor, amphiregulin, transforming growth factor alpha, insulin like growth factor, and tumor necrosis factor alpha) or apoptosis (tumor necrosis factor alpha, transforming growth factor beta) of mammary cells. However, the molecular mechanism underlying the influence of sex steroid hormones and/or growth factors on the development and function of the mammary gland remains largely unknown to date. Also scarce is information on the interaction between both groups of modulators. Nevertheless, based on the current indications compiled in this review, an important functional role for sex steroid hormones in the lactation cycle in co-operation with growth factors can be suggested.

Ovarian Steroid and Growth Factor Regulation of Female Reproductive Function Involves Modification of Hypothalamic α1-Adrenoceptor Signaling

The ovarian steroids estradiol (E(2)) and progesterone (P) act on target neurons in the hypothalamus and preoptic area to coordinate the expression of female reproductive behaviors with the timing of the preovulatory luteinizing hormone (LH) surge. This chapter will summarize evidence that E(2) and P facilitation of the receptive component of female reproductive behavior, lordosis, involves changes in both the expression of and intracellular signal transduction pathways engaged by alpha(1)-adrenergic receptors in the hypothalamus and preoptic area. The alpha(1)-adrenoceptors are thought to mediate the facilitatory effects of the catecholamine neurotransmitter norepinephrine on both lordosis behavior and LH release. E(2) first induces the expression of the alpha(1B)-adrenergic receptor subtype in the hypothalamus and preoptic area. P then acts in an E(2)-dependent manner to promote linkage of hypothalamic alpha(1)-adrenoceptors to an intracellular signaling pathway involving nitric oxide and cyclic GMP. This chapter will also describe recent findings that implicate brain insulin-like growth factor-I (IGF-I) receptors as obligatory co-mediators of hormonal regulation of hypothalamic alpha(1)-adrenoceptors and female neuroendocrine function. Additional studies suggest that E(2) and IGF-I facilitate lordosis behavior by activating kinases traditionally associated with growth factor signal transduction (mitogen-activated protein kinases and phosphatidlyinositol-3-kinases). These molecular events are proposed to help coordinate the timing of ovulation with the expression of sexual receptivity, thereby maximizing reproductive success.

Growth hormone prevents human immunodeficiency virus-induced neuronal p53 expression

Growth hormone (GH) is neuroprotective, presumably through its actions on GH receptor-mediated pathways. Here, we examined the effects of GH using in vitro and in vivo assays of human immunodeficiency virus (HIV)-induced neuronal injury. Neuronal cultures were in assays of neurotoxicity induced by supernatants from HIV-1 tat-transfected monocytoid cells (Tat supernatant). GH treatment reduced neuronal death compared with untreated cultures (p < 0.001), which was blocked by a GH receptor antagonist, B2036. Tat supernatant-induced p53 expression in neurons was also reduced by GH treatment. Expression of both p53 and GH receptor were increased in brain tissue from HIV-infected persons compared with controls (p < 0.05). Mice receiving intrastriatal implants of Tat supernatant and treated with GH showed less neurobehavioral abnormalities together with reduced neuroinflammation and neuronal injury compared with untreated animals (p < 0.01). Three acquired immunodeficiency syndrome-defined patients with neurocognitive impairment were serially evaluated during daily GH treatment showing a sustained improvement in neuropsychological performance (p < 0.01). GH prevents neuronal death through its actions on neurons involving a p53-mediated pathway and also improved in vivo neurological function, indicating that GH may have a role in the treatment of HIV-induced neurodegeneration.

Steroid hormones and neurosteroids in normal and pathological aging of the nervous system

Without medical progress, dementing diseases such as Alzheimer's disease will become one of the main causes of disability. Preventing or delaying them has thus become a real challenge for biomedical research. Steroids offer interesting therapeutical opportunities for promoting successful aging because of their pleiotropic effects in the nervous system: they regulate main neurotransmitter systems, promote the viability of neurons, play an important role in myelination and influence cognitive processes, in particular learning and memory. Preclinical research has provided evidence that the normally aging nervous system maintains some capacity for regeneration and that age-dependent changes in the nervous system and cognitive dysfunctions can be reversed to some extent by the administration of steroids. The aging nervous system also remains sensitive to the neuroprotective effects of steroids. In contrast to the large number of studies documenting beneficial effects of steroids on the nervous system in young and aged animals, the results from hormone replacement studies in the elderly are so far not conclusive. There is also little information concerning changes of steroid levels in the aging human brain. As steroids present in nervous tissues originate from the endocrine glands (steroid hormones) and from local synthesis (neurosteroids), changes in blood levels of steroids with age do not necessarily reflect changes in their brain levels. There is indeed strong evidence that neurosteroids are also synthesized in human brain and peripheral nerves. The development of a very sensitive and precise method for the analysis of steroids by gas chromatography/mass spectrometry (GC/MS) offers new possibilities for the study of neurosteroids. The concentrations of a range of neurosteroids have recently been measured in various brain regions of aged Alzheimer's disease patients and aged non-demented controls by GC/MS, providing reference values. In Alzheimer's patients, there was a general trend toward lower levels of neurosteroids in different brain regions, and neurosteroid levels were negatively correlated with two biochemical markers of Alzheimer's disease, the phosphorylated tau protein and the beta-amyloid peptides. The metabolism of dehydroepiandrosterone has also been analyzed for the first time in the aging brain from Alzheimer patients and non-demented controls. The conversion of dehydroepiandrosterone to Delta5-androstene-3beta,17beta-diol and to 7alpha-OH-dehydroepiandrosterone occurred in frontal cortex, hippocampus, amygdala, cerebellum and striatum of both Alzheimer's patients and controls. The formation of these metabolites within distinct brain regions negatively correlated with the density of beta-amyloid deposits.

Participation of growth factor signal transduction pathways in estradiol facilitation of female reproductive behavior

Estradiol (E(2)) regulates female reproductive behavior (lordosis) by acting on estrogen-sensitive neurons. We recently showed that E(2) facilitation of lordosis behavior requires concurrent activation of brain IGF-I receptors. The present study confirmed this finding and sought to identify the downstream signaling pathways involved in estrogen/IGF-I priming of lordosis. Intracerebroventricular infusions of a selective IGF-I receptor antagonist were administered to ovariectomized rats every 12 h beginning 1 h before the first of two daily E(2) injections. IGF-I receptor blockade partially inhibits lordosis if the antagonist is infused throughout the 2-d estrogen treatment period but not if it is administered only during the first or last 12 h of estrogen treatment. Because E(2) and IGF-I can activate phosphatidylinositol-3-kinase (PI3K) and MAPK, we infused agents that block PI3K and/or MAPK activity as described above. Both PI3K inhibitors (wortmannin and LY294002) and MAPK inhibitors (PD98059 and U0126) partially attenuate lordosis when administered during estrogen priming. None of these drugs modifies lordosis if they are infused only once, during the last 12 h of estrogen treatment. When both wortmannin and PD98059 are infused during E(2) priming, lordosis behavior is completely abolished. These data suggest that activation of both PI3K and MAPK by E(2) and IGF-I mediates hormonal facilitation of lordosis behavior.

Raloxifene analog LY117018 enhances the regeneration of sciatic nerve in ovariectomized female mice

The aim of this study was to examine the effects of LY117018, a selective estrogen receptor modulator, on peripheral nerve regeneration, using a model of sciatic nerve crush injury in mice. Sciatic functional index, an index of functional recovery, was significantly higher in LY117018 treated mice throughout regeneration. Analysis of semi-thin sections revealed a significant increase in both the total number of regenerating nerve fibers at day 7, and the mean axonal area of myelinated fibers at 7, 14, and 21 days after injury, in LY117018 treated mice. Analysis of axonal transport through retrograde labeling of motor neurons showed that LY117018 increased transport, and ICI 182,780 blocked the effects of LY117018, delineating estrogen receptors as its target. Our study suggests that LY117018 may markedly accelerate peripheral nerve regeneration and functional recovery through activation of estrogen receptors.

An antagonist of estrogen receptors blocks the induction of adult neurogenesis by insulin-like growth factor-I in the dentate gyrus of adult female rat

Interdependence between estradiol and insulin-like growth factor-I has been documented for different neural events, including neuronal differentiation, synaptic plasticity, neuroendocrine regulation and neuroprotection. In the present study we have assessed whether both factors interact in the regulation of neurogenesis in the adult rat dentate gyrus. Wistar albino female rats were bilaterally ovariectomized and treated with estradiol, insulin-like growth factor-I and/or the estrogen receptor antagonist ICI 182,780. Estradiol was administered in a subcutaneous silastic capsule. Insulin-like growth factor-I and ICI 182,780 were delivered in the lateral cerebral ventricle. Animals received six daily injections of 5-bromo-2-deoxyuridine and were killed 24 h after the last injection. The total number of 5-bromo-2-deoxyuridine-positive neurons was significantly increased in animals treated with insulin-like growth factor-I, compared with rats treated with vehicles, while rats treated with both insulin-like growth factor-I and estradiol showed a higher number of 5-bromo-2-deoxyuridine-positive neurons than rats treated with insulin-like growth factor-I or estradiol alone. The antiestrogen ICI 182,780 blocked the effect of insulin-like growth factor-I on the number of 5-bromo-2-deoxyuridine neurons with independence of whether the animals were treated or not with estradiol. These findings suggest that estrogen receptors are involved in the induction of adult neurogenesis by insulin-like growth factor-I in the dentate gyrus, and that estradiol and insulin-like growth factor-I have a cooperative interaction to promote neurogenesis. The interaction between insulin-like growth factor-I and estradiol may participate in changes in the rate of neurogenesis during different endocrine and physiological conditions, and may be related to the decline in neurogenesis with ageing.

Cellular strategies of estrogen-mediated neuroprotection during brain development

The role of estrogen during brain development is well documented. Estrogen influences cell survival and differentiation and also controls the formation and maintenance of neural networks. Knowledge of trophic estrogen action in the central nervous system (CNS) was the basis for the establishment of research programs directed toward a potential function of estrogen as a neuroprotective factor in the adult brain. Considerable evidence has accumulated over the years supporting this hypothesis. Experimental and epidemiologic studies as well as clinical trials have demonstrated that estrogen is beneficial for the course of neurodegenerative disorders such as Parkinson and Alzheimer diseases but may also protect neurons from postischemic neuronal degeneration. In this article, we aim to unravel potential physiologic responses and cell survival strategies that allow a more detailed understanding of estrogen-mediated neuroprotection in the brain. In particular, we focus on the participation of estrogen in the regulation of apoptotic processes. Furthermore, we present data on reciprocal estrogen-growth factor interactions. Both of these mechanisms were found to operate during brain development and to conciliate estrogen effects on neurons. This makes them likely candidates for taking part in conveying estrogen-dependent neuroprotection in the adult CNS.

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.

Effect of 17 beta-estradiol on gene expression in lumbar spinal cord following sciatic nerve crush injury in ovariectomized mice

Previously, we observed that estrogen treatment enhances regeneration of the sciatic nerve after crush injury [Brain Res. 943 (2002) 283]. In this research, we studied expression of estrogen receptors and effects of estrogen on gene expression in the lumbar spinal cord, following sciatic nerve crush injury. Using the Atlas Mouse 1.2 Array, changes in the expression of 267 of 1176 genes were registered 4 days after nerve injury. Those genes that exhibited a change in signal intensity ratios of 2-fold or greater were selected as up-regulated (42) or down-regulated (21). In estrogen treated mice, we have observed up-regulation of the genes known to control apoptosis, cell proliferation, and growth, which might account for the positive effects of estrogen on the regeneration of motor neurons. Immunohistochemical staining revealed estrogen receptor-alpha and estrogen receptor-beta localized in the nucleus and cytoplasm of lumbar motor neurons, and in the regenerating neurites of the sciatic nerve. Expression of estrogen receptor-alpha and estrogen receptor-beta mRNA in lumbar spinal cord was shown by traditional RT-PCR. Using real-time quantitative RT-PCR, we demonstrated increased expression of estrogen receptors-alpha and -beta mRNA on the injured side of the lumbar spinal cord. Western blot analysis showed the accumulation of ERs in regenerating sciatic nerve, and revealed a 40% increase of activated ERK1/2 in estrogen treated mice, compared to placebo. Our findings indicate that: (i). axotomized motor neurons increase expression of estrogen receptors-alpha and -beta mRNA, (ii). estrogen mediates the expression of genes which accelerate the growth and maturation of axons, and (iii). estrogen receptors are transported from the perikaryon into regenerating neurites, and estrogen promotes regeneration locally through the non-genomic ERK-activated signaling pathway.

Are gonadal steroid hormones involved in disorders of brain aging?

Human aging is associated with a decrease of circulating gonadal steroid hormones. Since these hormones act as trophic factors for neurones and glia, it is possible that the decrease in sex steroid levels may contribute to the increased risk of neurodegenerative disorders with advanced age. Sex steroids are neuroprotective in several animal models of central and peripheral neurodegenerative diseases, and clinical data suggest that these hormones may reduce the risk of neural pathology in aged humans. Potential therapeutic approaches for aged-associated neural disorders may emerge from studies conducted to understand the mechanisms of action of sex steroids in the nervous system of aged animals. Alterations in the endogenous capacity of the aged brain to synthesize and metabolize sex steroids, as well as possible aged-associated modifications in the signalling of sex steroid receptors in the nervous system, are important areas for future investigation.

The biological actions of estrogens on skin

There is still extensive disparity in our understanding of how estrogens exert their actions, particularly in non-reproductive tissues such as the skin. Although it has been recognized for some time that estrogens have significant effects on many aspects of skin physiology and pathophysiology, studies on estrogen action in skin have been limited. However, estrogens clearly have an important function in many components of human skin including the epidermis, dermis, vasculature, hair follicle and the sebaceous, eccrine and apocrine glands, having significant roles in skin aging, pigmentation, hair growth, sebum production and skin cancer. The recent discovery of a second intracellular estrogen receptor (ERbeta) with different cell-specific roles to the classic estrogen receptor (ERalpha), and the identification of cell surface estrogen receptors, has provided further challenges to understanding the mechanism of estrogen action. It is now time to readdress many of the outstanding questions regarding the role of estrogens in skin and improve our understanding of the physiology and interaction of steroid hormones and their receptors in human skin. Not only will this lead to a better understanding of estrogen action, but may also provide a basis for further interventions in pathological processes that involve dysregulation of estrogen action.

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.

Sexual differentiation of the brain: genes, estrogen, and neurotrophic factors

Based on evidence obtained during the past 50 years, the current hypothesis to explain the sexual dimorphism of structure and function in the brain of vertebrates maintains that these differences are produced by the epigenetic action of gonadal hormones. However, evidence has progressively accumulated suggesting that genetic mechanisms controlling sexual-specific neuronal characteristics precede, or occur in parallel with, hormonal effects. 1. In cultures of hypothalamic neurons taken from gestation day 16 (GD16) embryos, treatment of sexually segregated cultures with estradiol (E2) induces axon growth in neurons from male neurons, but not from female neurons. In these cultures treatment with E2 increased the levels of tyrosine kinase type B (TrkB) and insulin-like growth factor I (IGF-I) receptors in male but not in female neurons. This and other sex differences cannot be explained by differences in hormonal environment, because the donor embryos were obtained when gonadal secretion of steroids is just beginning, before the perinatal surge of testosterone that determines development of the male brain beginning at GD17/18. 2. The response to estrogen is contingent upon coculture with heterotopic glia (mostly astrocytes) from a target region (amygdala) harvested from same-sex fetuses at GD16, whereas in the presence of homotopic glia or in cultures without glia, E2 had no effect. It was concluded that the axogenic effect of E2 depends on interaction between neurons and glia from a target region and that neurons from fetal male donors appear to mature earlier than neurons from females, a differentiated response that takes place prior to divergent exposure to gonadal secretions. 3. The effects of target and nontarget glia-conditioned media (CM) on the E2-induced growth of neuronal processes of hypothalamic neurons obtained from sexually segregated fetal donors were also studied. Estrogen added to media conditioned by target glia modified the number of primary neurites and the growth of axons of hypothalamic neurons of males but not of females. 4. Neither the Type III steroidal receptor blocker tamoxifen nor Type I antiestrogen ICI 182,780 prevented the axogenic effects of the hormone. Estradiol made membrane-impermeable by conjugation to a protein of high molecular weight (E2-BSA) preserved its axogenic capacity, suggesting the possibility of a membrane effect responsible for the action of E2. 5. Western blot analysis of the tyrosine kinase type A (TrkA), type B (TrkB), type C (TrkC), and insulin-like growth factor (IGF-I R) receptors in extracts from homogenates of cultured hypothalamic neurons showed that in cultures of male-derived neurons grown with E2 and CM from target glia, the amounts of TrkB and IGF-I R increased notably. Densitometric quantification showed that these cultures had more TrkB than cultures with CM alone or E2 alone. On the contrary, in cultures of female-derived neurons, the presence of CM alone induced maximal levels of TrkB, which were not further increased by E2; female-derived neurons in all conditions did not contain IGF-I R. Levels of TrkC were not modified by any experimental condition in male- or female-derived cultures and Trk A was not found in the homogenates. These results are compared with similar data from other laboratories and integrated in a model for the confluent interaction of estrogen and neurotrophic factors released by glia that may contribute to the sexual differentiation of the brain.

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.

Growth factors and steroid hormones: a complex interplay in the hypothalamic control of reproductive functions

The mechanisms through which LHRH-secreting neurons are controlled still represent a crucial and debated field of research in the neuroendocrine control of reproduction. In the present review, we have specifically considered two potential signals reaching these hypothalamic neurons: steroid hormones and growth factors. Examples of the relevant physiological role of the interactions between these two families of biologically acting molecules have been provided. In many cases, these interactions occur at the level of hypothalamic astrocytes, which are presently accepted as functional partners of the LHRH-secreting neurons. On the basis of the observations here summarized, we have formulated the hypothesis that a functional co-operation of steroid hormones and growth factors occurring in the hypothalamic astrocytic compartment represents a key factor in the neuroendocrine control of reproductive functions.

Intracellular pathways mediating estrogen-induced cholangiocyte proliferation in the rat

The aim of this study was to explore the intracellular signaling pathways involved in the stimulatory effects of estrogens on cholangiocyte proliferation. We investigated the tyrosine kinase-receptor pathway by evaluating the protein expression of total and phosphorylated mitogen-activated protein kinase (MAPK) isoform p44/p42 (e.g., extracellular signal-regulated kinase [ERK]1/2), the steroid-receptor coactivator Src and Shc (Src-homology/collagen protein). The study was performed in 3-week-old bile duct-ligated (BDL) rats, BDL rats treated with the antiestrogens, tamoxifen or Ici 182,780, and normal control rats. Proliferation was also evaluated in normal purified cholangiocytes treated with 17 beta estradiol in the presence or absence of tamoxifen, Ici 182,780, ERK, or Src inhibitors. After bile duct ligation, cholangiocyte proliferation was associated with a marked immunohistochemical nuclear positivity for phosphorylated (p)-ERK1/2, which was inhibited by in vivo treatment with tamoxifen or Ici 182,780. Protein expression of total and p-ERK1/2, and Shc in cholangiocytes isolated from BDL rats was markedly increased compared with controls and was inhibited by in vivo treatment with antiestrogens. In vitro, 17 beta estradiol-induced proliferation of isolated normal cholangiocyte was associated with increased (P <.01) protein expression of p-ERK1/2, Src, and Shc. Specific inhibitors of ER (Ici 182,780), ERK (U0125), and Src (PP2) inhibited in vitro 17 beta estradiol-induced cholangiocyte proliferation. In conclusion, this study showed that estrogens induced cholangiocyte proliferation by activating the Src/Shc/ERK pathway. This might suggest that pharmacologic modulation of ER, ERK, and/or Src could be proposed for the treatment of human pathology characterized by dysregulation of cholangiocyte proliferation.

Cerebral ischemia and trauma-different etiologies yet similar mechanisms: neuroprotective opportunities

Cerebral ischemia leads to brain damage caused by pathogenetic mechanisms that are also activated by neurotrauma. These mechanisms include among others excitotoxicity, over production of free radicals, inflammation and apoptosis. Furthermore, cerebral ischemia and trauma both trigger similar auto-protective mechanisms including the production of heat shock proteins, anti-inflammatory cytokines and endogenous antioxidants. Neuroprotective therapy aims at minimizing the activation of toxic pathways and at enhancing the activity of endogenous neuroprotective mechanisms. The similarities in the damage-producing and endogenous auto-protective mechanisms may imply that neuroprotective compounds found to be active against one of these conditions may indeed be also protective in the other. This review summarizes the pathogenetic events of ischemic and traumatic brain injury and reviews the neuroprotective strategies employed thus far in each of these conditions with a special emphasize on their clinical relevance and on future directions in the field of neuronal protection.

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.