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

17beta-estradiol protects male mice from cuprizone-induced demyelination and oligodendrocyte loss

In addition to regulating reproductive functions in the brain and periphery, estrogen has tropic and neuroprotective functions in the central nervous system (CNS). Estrogen administration has been demonstrated to provide protection in several animal models of CNS disorders, including stroke, brain injury, epilepsy, Parkinson's disease, Alzheimer's disease, age-related cognitive decline and multiple sclerosis. Here, we use a model of toxin-induced oligodendrocyte death which results in demyelination, reactive gliosis, recruitment of oligodendrocyte precursor cells and subsequent remyelination to study the potential benefit of 17beta-estradiol (E2) administration in male mice. The results indicate that E2 partially ameliorates loss of oligodendrocytes and demyelination in the corpus callosum. This protection is accompanied by a delay in microglia accumulation as well as reduced mRNA expression of the pro-inflammatory cytokine, tumor necrosis factor alpha (TNFalpha), and insulin-like growth factor-1 (IGF-1). E2 did not significantly alter the accumulation of astrocytes or oligodendrocyte precursor cells, or remyelination. These data obtained from a toxin-induced, T cell-independent model using male mice provide an expanded view of the beneficial effects of estrogen on oligodendrocyte and myelin preservation.

Role of sex hormones in the modulation of cholangiocyte function

Over the last years, cholangiocytes, the cells that line the biliary tree, have been considered an important object of study for their biological properties which involves bile formation, proliferation, injury repair, fibrosis and angiogenesis. Cholangiocyte proliferation occurs in all pathologic conditions of liver injury where it is associated with inflammation and regeneration. During these processes, biliary cells start to secrete different cytokines, growth factors, neuropeptides and hormones which represent potential mechanisms for cross talk with other liver cells. Several studies suggest that hormones, and in particular, sex hormones, play a fundamental role in the modulation of the growth of this compartment in the injured liver which functionally conditions the progression of liver disease. Understanding the mechanisms of action and the intracellular pathways of these compounds on cholangiocyte pathophysiology will provide new potential strategies for the management of chronic liver diseases. The purpose of this review is to summarize the recent findings on the role of sex hormones in cholangiocyte proliferation and biology.

The selective beta1-adrenoceptor antagonist nebivolol is a potential oestrogen receptor agonist with neuroprotective abilities

BACKGROUND AND PURPOSE

Nebivolol, a selective beta(1)-adrenoceptor antagonist mediating rapid vasodilating effects, is used clinically to treat hypertension. Recently, it was reported that nebivolol also acts as an oestrogen receptor (ER) agonist. To investigate the neuroprotective potential of oestrogens, we assessed the oestrogenic effects of nebivolol in several in vitro neuronal models.

EXPERIMENTAL APPROACH

Human neuroepithelioma SK-N-MC cells stably transfected with human ER alpha and beta, and mouse N2A neuroblastoma cells expressing human APP695(SWE)[N2Aswe, stably transfected with the Swedish mutation form of the Alzheimer-associated amyloid precursor protein (APPswe, K670M/N671L)] were incubated with different concentrations of nebivolol and 17beta-oestradiol (E2) for 24-48 h. ER activation was detected in a specific reporter assay, and ER-dependent gene expression was measured by quantitative real-time PCR (qRT PCR). Furthermore, cell survival rates were determined, and oxidative stress was induced by hydrogen peroxide and paraquat. Amyloid beta protein precursor (APP) processing was investigated, and the cleavage fragments sAPPalpha and Abeta were quantified via alpha-, beta- and gamma-secretase activity assays. Alterations of secretase expression levels were determined by qRT PCR.

KEY RESULTS

Nebivolol induces oestrogen-dependent gene transcription, and protects neuronal cells against oxidative stress even at low and physiological concentrations (10(-8) M). Moreover, nebivolol modulates processing of APP in mouse neuronal N2Aswe cells by increasing alpha-secretase activity, ultimately leading to enhanced release of soluble non-amyloidogenic sAPPalpha.

CONCLUSIONS AND IMPLICATIONS

We showed that nebivolol acts as ER agonist in neuronal cell lines, and suggest oestrogen-like neuroprotective effects mediated by nebivolol.

Interactions of estradiol and insulin-like growth factor-I signalling in the nervous system: new advances

Estradiol and insulin-like growth factor-I (IGF-I) interact in the brain to regulate a variety of developmental and neuroplastic events. Some of these interactions are involved in the control of hormonal homeostasis and reproduction. However, the interactions may also potentially impact on affection and cognition by the regulation of adult neurogenesis in the hippocampus and by promoting neuroprotection under neurodegenerative conditions. Recent studies suggest that the interaction of estradiol and IGF-I is also relevant for the control of cholesterol homeostasis in neural cells. The molecular mechanisms involved in the interaction of estradiol and IGF-I include the cross-regulation of the expression of estrogen and IGF-I receptors, the regulation of estrogen receptor-mediated transcription by IGF-I and the regulation of IGF-I receptor signalling by estradiol. Current investigations are evidencing the role exerted by key signalling molecules, such as glycogen synthase kinase 3 and beta-catenin, in the cross-talk of estrogen receptors and IGF-I receptors in neural cells.

Neuroprotective actions of selective estrogen receptor modulators

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

Oestradiol and insulin-like growth factor-1 reduce cell loss after global ischaemia in middle-aged female rats

Whereas the ability of oestradiol and insulin-like growth factor (IGF)-1 to afford neuroprotection against ischaemia-induced neuronal death in young female and male rodents is well established, the impact of IGF-1 in middle-aged animals is largely unknown. The present study assessed the efficacy of oestradiol and IGF-1 with respect to reducing neuronal death after transient global ischaemia in middle-aged female rats after 8 weeks of hormone withdrawal. Rats were ovariohysterectomised and implanted 8 weeks later with an osmotic mini-pump delivering IGF-1 or saline into the lateral ventricle. Some rats also received physiological levels of oestradiol by subcutaneous pellet. Two weeks later, rats were subjected to global ischaemia or sham operation. Surviving hippocampal CA1 neurones were quantified. Ischaemia produced massive CA1 cell death compared to sham-operated animals, which was evident at 14 days. Significantly more neurones survived in animals treated with either oestradiol or IGF-1, but simultaneous treatment produced no additive effect. IGF-1, an endogenous growth factor, may be a clinically useful therapy in preventing human brain injury, with neuroprotective equivalence to oestradiol but without the harmful side-effects.

Neuroprotective actions of brain aromatase

The steroidal regulation of vertebrate neuroanatomy and neurophysiology includes a seemingly unending list of brain areas, cellular structures and behaviors modulated by these hormones. Estrogens, in particular have emerged as potent neuromodulators, exerting a range of effects including neuroprotection and perhaps neural repair. In songbirds and mammals, the brain itself appears to be the site of injury-induced estrogen synthesis via the rapid transcription and translation of aromatase (estrogen synthase) in astroglia. This induction seems to occur regardless of the nature and location of primary brain damage. The induced expression of aromatase apparently elevates local estrogen levels enough to interfere with apoptotic pathways, thereby decreasing secondary degeneration and ultimately lessening the extent of damage. There is even evidence suggesting that aromatization may affect injury-induced cytogenesis. Thus, aromatization in the brain appears to confer neuroprotection by an array of mechanisms that involve the deceleration and acceleration of degeneration and repair, respectively. We are only beginning to understand the factors responsible for the injury-induced transcription of aromatase in astroglia. In contrast, much of the manner in which local and circulating estrogens may achieve their neuroprotective effects has been elucidated. However, gaps in our knowledge include issues about the cell-specific regulation of aromatase expression, steroidal influences of aromatization distinct from estrogen formation, and questions about the role of constitutive aromatase in neuroprotection. Here we describe the considerable consensus and some interesting differences in knowledge gained from studies conducted on diverse animal models, experimental paradigms and preparations towards understanding the neuroprotective actions of brain aromatase.

Insulin-like growth factor I: a potential neuroprotective compound for the treatment of acute ischemic stroke?

BACKGROUND AND PURPOSE

Insulin-like growth factor I (IGF-I) exerts neuroprotective effects in both white and gray matter under different detrimental conditions. The purpose of this review is to collect the evidence whether IGF-I is a candidate neuroprotective drug in patients with acute ischemic stroke.

RESULTS

IGF-I was found to be neuroprotective in animal models of focal brain ischemia when given >or=2 hours after the insult. Different routes of administration (eg, cerebroventricular, intravenous, and intranasal) were found to be effective. In addition to inhibition of apoptosis and reduction of the infarct volume, IGF-I also improved neurological outcome. Furthermore, there are strong indications that IGF-I can also stimulate the regeneration of neural tissue.

CONCLUSIONS

Additional studies are required to reveal the neuroprotective mechanisms of IGF-I in detail and to elucidate the role of IGF-binding proteins. Preclinical studies in relevant animal models for studying stroke (ie, hypertensive, diabetic, or aged animals) should be done testing different doses and routes of IGF-I administration and different combinations of IGF-I and IGF-binding proteins.

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.

Estrogen and SERM neuroprotection in animal models of Parkinson's disease

A higher prevalence and incidence of Parkinson disease (PD) is observed in men and beneficial motor effects of estrogens are observed in parkinsonian women. Lesion of the dopamine (DA) nigrostriatal pathway in animals with 1-methyl 4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) provides a model of PD and this is based on its use in humans as side-product of a drug abuse. Presently treatment of PD is mainly symptomatic. The MPTP mouse is used to study the neuroprotective roles of estrogenic drugs on the DA system. Estrogens, but not androgens, are active neuroprotectants as well as progesterone and dehydroepiandrosterone. An estrogen receptor agonist PPT and the selective estrogen receptor modulator raloxifene are also neuroprotective. Striatal DA neurons of estrogen receptor alpha knockout mice are more susceptible to MPTP toxicity than wild-type mice and neuroprotection by estradiol is associated with the activation of the PI3-K pathway involving Akt, GSK3beta, Bcl2 and BAD.

Sex differences in a transgenic rat model of Huntington's disease: decreased 17beta-estradiol levels correlate with reduced numbers of DARPP32+ neurons in males

Recent clinical studies have highlighted that female sex hormones represent potential neuroprotective mediators against damage caused by acute and chronic brain diseases. This evidence has been confirmed by experimental studies documenting the protective role of female sex hormones both in vitro and in vivo, although these studies did not specifically focus on Huntington's disease (HD). We therefore investigated the onset and course of HD in female and male transgenic (tg) HD (CAG(n51)) and control rats across age and focused on three aspects: (i) behavioral and physiological alterations (energy expenditure, home-cage activity, emotional disturbance and motor dysfunction), (ii) morphological markers (numbers and characteristics of striatal DARPP32(+) medium-sized spiny neurons (MSNs) and dopamine receptor autoradiography) and (iii) peripheral sex hormone levels as well as striatal estrogen receptor expression. Independent of their sex, tgHD rats exhibited increased levels of food intake, elevated home-cage activity scores and anxiolytic-like behavior, whereas only males showed an impairment of motor function. In line with the latter finding, loss and atrophy of DARPP32(+) MSNs were apparent only in male tgHD rats. This result was associated with a decreased striatal dopamine D1 receptor density and lower plasma levels of 17beta-estradiol at the age of 14 months. As DARPP32(+) MSNs expressed both alpha- and beta-estrogen receptors and showed a correlation between cell numbers and 17beta-estradiol levels, our findings suggest sex-related differences in the HD phenotype pointing to a substantial neuroprotective effect of sex hormones and opening new perspectives on the therapy of HD.

Gender-specific expression of multiple estrogen receptors, growth hormone receptors, insulin-like growth factors and vitellogenins, and effects of 17 beta-estradiol in the male tilapia (Oreochromis mossambicus)

Gender-specific expression of estrogen receptors (ER alpha and ER beta), growth hormone receptors (GHR1 and GHR2), insulin-like growth factors (IGF-I and IGF-II) and three vitellogenins (Vgs A-C) was examined in the liver, gonad, pituitary, and brain of sexually mature male, female, and 17 beta-estradiol (E2)-treated male tilapia (Oreochromis mossambicus). Reflecting greater growth rate in male tilapia, hepatic expression of GHR1, GHR2, IGF-I and IGF-II as well as plasma IGF-I levels were higher in males than in females, whereas the expression of Vgs A-C and ER alpha was higher in females. On the other hand, expression of all genes measured was higher in the ovary than in testis. Forty eight hours after E2 injection (5 microg/g) into male fish, hepatic expression of most transcripts measured were altered to levels that were similar to those seen in females. The changes included decreased expression of GHR1, GHR2, IGF-I, and IGF-II, and increased expression of ER alpha and Vgs A-C. E2 treatment also increased Vg and decreased IGF-I in the plasma. Brain expression of ER alpha, ER beta, GHR1, and IGF-I was higher in females than in males, whereas pituitary expression of GHR2 and IGF-I was lower in females; only brain expression of GHR1 was increased by E2 treatment. These findings suggest that E2 stimulates Vg production primarily through activation of ER alpha and down-regulation of the GH/IGF-I axis, thus shifting energy from somatic growth towards vitellogenesis at the level of the liver.

Role of insulin metabolism disturbances in the development of Alzheimer disease: mini review

Alzheimer disease (AD) is the most common form of dementia. Different pathogenic processes have been studied that underlie characteristic changes of AD, including A beta protein aggregation, tau phosphorylation, neurovascular dysfunction, and inflammatory processes. Insulin exerts pleiotropic effects in neurons, such as the regulation of neural proliferation, apoptosis, and synaptic transmission. In this setting, any disturbance in the metabolism of insulin in the central nervous system (CNS) may put unfavorable effects on CNS function. It seems that disturbances in insulin metabolism, especially insulin resistance, play a role in most pathogenic processes that promote the development of AD. In this article, the relationships of disturbances in the metabolism of insulin in CNS with A beta peptides aggregation, tau protein phosphorylation, inflammatory markers, neuron apoptosis, neurovascular dysfunction, and neurotransmitter modulation are discussed, and future research directions are provided.

PI3 kinase/Akt activation mediates estrogen and IGF-1 nigral DA neuronal neuroprotection against a unilateral rat model of Parkinson's disease

Recently, using the medial forebrain bundle (MFB) 6-hydroxydopmaine (6-OHDA) lesion rat model of Parkinson's disease (PD), we have demonstrated that blockade of central IGF-1 receptors (IGF-1R) attenuated estrogen neuroprotection of substantia nigra pars compacta (SNpc) DA neurons, but exacerbated 6-OHDA lesions in IGF-1 only treated rats (Quesada and Micevych [2004]: J Neurosci Res 75:107-116). This suggested that the IGF-1 system is a central mechanism through which estrogen acts to protect the nigrostriatal DA system. Moreover, these results also suggest that IGF-1R-induced intracellular signaling pathways are involved in the estrogen mechanism that promotes neuronal survival. In vitro, two convergent intracellular signaling pathways used by estrogen and IGF-1, the mitogen-activated protein kinase (MAPK/ERK), and phosphatidyl-inositol-3-kinase/Akt (PI3K/Akt), have been demonstrated to be neuroprotective. Continuous central infusions of MAPK/ERK and PI3K/Akt inhibitors were used to test the hypothesis that one or both of these signal transduction pathways mediates estrogen and/or IGF-1 neuroprotection of SNpc DA neurons after a unilateral administration of 6-OHDA into the MFB of rats. Motor behavior tests and tyrosine hydroxylase immunoreactivity revealed that the inhibitor of the PI3K/Akt pathway (LY294002) blocked the survival effects of both estrogen and IGF-1, while an inhibitor of the MAPK/ERK signaling (PD98059) was ineffective. Western blot analyses showed that estrogen and IGF-1 treatments increased PI3K/Akt activation in the SN; however, MAPK/ERK activation was decreased in the SN. Indeed, continuous infusions of inhibitors blocked phosphorylation of PI3K/Akt and MAPK/ERK. These findings indicate that estrogen and IGF-1-mediated SNpc DA neuronal protection is dependent on PI3K/Akt signaling, but not on the MAPK/ERK pathway.

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

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

Pubertal ovarian hormone exposure reduces the number of myelinated axons in the splenium of the rat corpus callosum

The size of the female rat corpus callosum decreases in response to pubertal ovarian hormone exposure, but the underlying changes in axonal composition have not been examined. In the current study, animals underwent ovariectomy or sham surgery at day 20, and the number of myelinated and unmyelinated axons were examined in young adulthood (2 months) using electron microscopy. Ovariectomized animals had a greater number of myelinated axons compared to intact animals, while total axon number was not affected. Ovarian hormone exposure seems to limit the number of axons that become myelinated in the splenium, while not affecting the number of axons crossing through the region.

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

OBJECTIVE

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

DESIGN

Laboratory study using human tissue.

SETTING

University-based laboratory.

PATIENT(S)

Women with and without endometriosis undergoing hysterectomy.

INTERVENTION(S)

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

MAIN OUTCOME MEASURE(S)

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

RESULT(S)

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

CONCLUSION(S)

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

Estrogen receptor alpha (ERalpha) and insulin-like growth factor I receptor (IGF-IR) cross-talk in the gonadotropic alphaT3-1 cell line

In reproductive tissues such as the breast and the uterus, cell proliferation and differentiation is strongly regulated by complex interactions between estrogen receptor alpha (ERalpha) and growth factor receptors. In the present study, we investigated the potential occurrence of such cross-talk in the murine, gonadotropic alphaT3-1 cell line, which expresses ERalpha and the IGF-I receptor (IGF-IR). Under estrogen-free conditions, basal cell proliferation and ER-mediated gene transcription was strongly inhibited by the selective estrogen receptor modulator (SERM) 4-hydroxy-tamoxifen (4-OH-Tam) and by the pure anti-estrogen ICI 182,780 (ICI). These effects can be reversed by either 17-beta-estradiol (E(2)) or insulin-like growth factor I (IGF-I), both exerting modest mitogenic effects in the alphaT3-1 cell line. Furthermore, IGF-I enhanced both basal and E(2)-induced ER-driven gene transcription. This may be explained, at least in part, by enhanced phosphorylation of ERalpha at serine 118, a prerequisite for the transactivation capacity of the receptor. Finally, the IGF-I-induced response on cell growth and ER-mediated transactivation can be inhibited with either ICI or 4-OH-Tam. In conclusion, our data indicate IGF-IR and ER interactions in the alphaT3-1 cell line, an in vitro model for the pituitary gonadotrophs, hereby suggesting a role of IGF-I in the regulation of gonadotropin synthesis and secretion.

Distribution and localization patterns of estrogen receptor-beta and insulin-like growth factor-1 receptors in neurons and glial cells of the female rat substantia nigra: localization of ERbeta and IGF-1R in substantia nigra

Although several studies have focused on the neuroprotective effects of estrogen (E2) on stroke, there have been tantalizing reports on the potential neuroprotective role of E2 in degenerative neuronal diseases such as Alzheimer's and Parkinson's (PD). In animal models of PD, E2 protects the nigrostriatal dopaminergic (DA) system against neurotoxins. However, little is known about the cellular and molecular mechanism(s) involved by which E2 elicits its neuroprotective effects on the nigrostriatal DA system. A preferred mechanism for neuroprotection is the interaction of E2 with specific neuroprotective growth factors and receptors. One such neuroprotective factor/receptor system is insulin-like growth factor-1 (IGF-1). E2 neuroprotective effects in the substantia nigra (SN) DA system have been shown to be dependent on IGF-1. To determine whether E2 also interacts with the IGF-1 receptor (IGF-1R) and to determine the cellular localization of estrogen receptor (ER) and IGF-1R, we compared the distribution of ER and IGF-1R in the SN. Stereological measurements revealed that 40% of the subpopulation of tyrosine hydroxylase-immunoreactive (TH-ir) SN pars compacta (SNpc) DA neurons are immunoreactive for estrogen receptor-beta (ERbeta). No immunolabeling for ERalpha was observed. In situ hybridization and immunocytochemistry studies confirmed the expression of IGF-1R mRNA and revealed that almost all TH-ir SNpc DA neurons were immunoreactive for IGF-1R, respectively. Moreover, one-third of glial fibrillary acidic protein (GFAP-ir) cells in the SN were ERbeta-ir, and 67% of GFAP-ir cells expressed IGF-1R-ir. Therefore, the localization of ERbeta and IGF-1R on SNpc DA neurons and astrocytes suggests a modulatory role of E2 on IGF-1R, and this modulation may affect neuroprotection.

A high soy diet enhances neurotropin receptor and Bcl-XL gene expression in the brains of ovariectomized female rats

Estrogen is a powerful neuroprotective agent with the ability to induce trophic and antiapoptotic genes. However, concerns about negative overall health consequences of estrogen replacement after menopause have led to the adoption of other strategies to obtain estrogen's benefits in the brain, including the use of selective estrogen receptor modulators, high soy diets, or isoflavone supplements. This study sought to determine the ability of a high soy diet to induce neuroprotective gene expression in the female rat brain and compare the actions of soy with estrogen. Adult ovariectomized female rats were treated with 3 days of high dose estrogen or 2 weeks of a soy-free diet, a high soy diet, or chronic low dose estrogen. Different brain regions were microdissected and subjected to real time RT-PCR for neuroprotective genes previously shown to be estrogen-regulated. The principle findings are that a high soy diet led to the widespread increase in the mRNA for neurotropin receptors TrkA and p75-NTR, and the antiapoptotic Bcl-2 family member Bcl-X(L). Immunohistochemistry confirmed increases in both TrkA and Bcl-X(L). Chronic low dose estrogen mimicked some of these effects, but acute high dose estrogen did not. The effects of a high soy diet were particularly evident in the parietal cortex and hippocampus, two regions protected by estrogen in animal models of neurological disease and injury. These results suggest that a high soy diet may provide beneficial effects to the brain similar to low dose chronic estrogen treatment such as that used for postmenopausal hormone replacement.

Neuroprotective effects of estradiol in hippocampal neurons and glia of middle age mice

During aging the hippocampus experiences structural, molecular, and functional alterations. Protection from age-related disorders is provided by several factors, including estrogens. Since aging defects start at middle age, we studied if 17 beta-estradiol (E(2)) protected the hippocampus at this age period. Middle age (10-12 month old) male C57Bl/6 mice were implanted sc with E(2) (15 microg) or cholesterol pellets. Ten days afterwards they received bromodeoxyuridine (BrdU) 4 and 2h before killing to study cell proliferation in the dentate gyrus (DG). A pronounced depletion of BrdU+cells in the DG was found in cholesterol-treated middle age mice, accompanied by astrocytosis, and by neuronal loss in the hilus. Middle age mice receiving E(2) showed increased number of BrdU+cells while the other parameters were remarkably attenuated. When steroid treatment was prolonged for 2 months to study migration of cells in the granular layer of the DG, cell migration was unaffected by E(2). However, E(2)-treated middle age mice presented higher cell density and increased staining for doublecortin, a marker for differentiating neurons. Thus, from the three basic steps of adult neurogenesis (proliferation, migration, and differentiation), E(2) stimulated progenitor proliferation - even after long exposure to E(2) studied by Ki67 immunocytochemistry - and differentiation towards a neuronal lineage. This result, in conjunction with recovery from other aging indicators as increased deposits of the aging pigment lipofuscin in DG cells, loss of hilar neurons and astrocytosis supports a wide range protection of hippocampal function of middle age mice by estrogenic hormones.

Beta-amyloid toxicity in embryonic rat astrocytes

The senile plaques of Alzheimer's disease contain a high concentration of beta-amyloid (betaA) protein, which may affect the glial population in the septal nucleus, an area of increased risk in AD. BetaA toxicity was measured in septal glia, via a dose-response experiment, by quantifying the effects of three different doses (0.1, 1, and 10 microM) of betaA on cell survival. Astrocytes from embryonic day-16 rats were grown in serum-free media in a single layer culture. Cells were treated on day in vitro (DIV)1 and survival was determined on DIV3 to ascertain which concentration was most toxic. In a separate set of experiments, an attempt was made to protect glial cells from the degenerative effects of betaA, with treatments of growth factors and estrogen. BetaA (10 microM) treatment was administered on DIV1, on DIV2 the cells were treated with estrogen (EST, 10 nM), insulin-like growth factors (IGF1 and IGF2, each 10 ng/ml), basic fibroblast growth factor (bFGF, 5 ng/ml) or nerve growth factor (NGF, 100 ng/ml), and on DIV3 the cells were visualized and quantified by fluorescence microscopy with DAPI (4,6-diamidino-2-phenylindole). In addition to dose-response and glial protection, experiments were also conducted to determine whether toxic effects were due to apoptosis. Our results suggest that the survival of glial populations is significantly affected in all three concentrations (0.1, 1.0, and 10 microM) of betaA. Glial protection was evident in the presence of NGF, for it showed the significantly highest survival rate relative to the betaA treatment alone. Furthermore, toxic effects of betaA appear to be due primarily to apoptosis. Significant reversal of betaA-induced apoptosis was seen with bFGF and IGF1.

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

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

Prognostic Factors in Multiple Sclerosis

Prognostic factors to determine the patient's likelihood of developing MS are important for several reasons. Prognostic factors are important to the patient who wants to be informed about his/her prospects, to the clinician who needs to individuate the patients who deserve immune treatments, and to the researcher who needs to improve the design and the analysis of the therapeutic trials. In addition, with the development of new immune therapies, whose early use is strongly encouraged, it is crucial to dispose of reliable clinical predictors to identify the patients who are candidates for early or aggressive therapies. Several studies have indicated that a poor prognosis is related to male gender; a late age at onset; motor, cerebellar, and sphincter involvement at onset; a progressive course at onset; a short inter-attack interval; a high number of early attacks; and a relevant early residual disability. Paraclinical support for MS prognosis is given by imaging techniques, cerebrospinal fluid analysis, and evoked potential examinations. The most sensitive paraclinical test to predict conversion from suspected demyelinating disease to definite MS is MRI.

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.

The role of insulin-like growth factor-I and growth factor-associated signal transduction pathways in estradiol and progesterone facilitation of female reproductive behaviors

We are examining the role of insulin-like growth factor-I (IGF-I) and downstream signal transduction pathways associated with growth factors (e.g., mitogen-activated protein kinase, MAPK) in estradiol and progesterone facilitation of female reproductive behavior in rats. Brain IGF-I receptor activity is required for the long-term, priming actions of estradiol on the female reproductive axis. Infusions of an IGF-I receptor antagonist during estradiol priming blocks induction of hypothalamic alpha(1B)-adrenergic receptors and luteinizing hormone surges, and attenuates lordosis behavior. Infusion of MAPK and phosphatidylinositol-3-kinase inhibitors inhibitors during estradiol priming completely blocks hormone-facilitated lordosis. Because progestin receptors (PRs) can be phosphorylated and activated by MAPKs, growth factor signaling pathways may also participate in progesterone facilitation of reproductive behaviors. Infusion of a MAPK inhibitor in estradiol-primed rats blocks progestin facilitation and sequential inhibition of lordosis and proceptive behaviors. Interference with MAPK signaling also inhibits behavioral responses to cGMP and a delta-opioid agonist, both of which can activate MAPK in some cells. Thus MAPK is involved in the facilitation of lordosis and proceptive behaviors, perhaps by phosphorylation of hypothalamic PRs.

Estrogen, cognition and female ageing

Starting from fetal life, estrogens are crucial in determining central gender dimorphism, and an estrogen-induced synaptic plasticity is well evident during puberty and seasonal changes as well as during the ovarian cycle. Estrogens act on the central nervous system (CNS) both through genomic mechanisms, modulating synthesis, release and metabolism of neurotransmitters, neuropeptides and neurosteroids, and through non-genomic mechanisms, influencing electrical excitability, synaptic function and morphological features. Therefore, estrogen's neuroactive effects are multifaceted and encompass a system that ranges from the chemical to the biochemical to the genomic mechanisms, protecting against a wide range of neurotoxic insults. Clinical evidences show that, during the climacteric period, estrogen withdrawal in the limbic system gives rise to modifications in mood, behaviour and cognition and that estrogen administration is able to improve mood and cognitive efficiency in post-menopause. Many biological mechanisms support the hypothesis that estrogens might protect against Alzheimer's disease (AD) by influencing neurotransmission, increasing cerebral blood flow, modulating growth proteins associated with axonal elongation and blunting the neurotoxic effects of beta-amyloid. On the contrary, clinical studies of estrogen replacement therapy (ERT) and cognitive function have reported controversial results, indicating a lack of efficacy of estrogens on cognition in post-menopausal women aged >or=65 years. These findings suggest the presence of a critical period for HRT-related neuroprotection and underlie the potential importance of early initiation of therapy for cognitive benefit. In this review, we shall first describe the multiple effects of steroids in the nervous system, which may be significant in the ageing process. A critical update of HRT use in women and a discussion of possible prospectives for steroid use are subsequently proposed.

Estrogens and insulin-like growth factor 1 modulate neoplastic cell growth in human cholangiocarcinoma

We investigated the expression of estrogen receptors (ERs), insulin-like growth factor 1 (IGF-1), and IGF-1R (receptor) in human cholangiocarcinoma and cholangiocarcinoma cell lines (HuH-28, TFK-1, Mz-ChA-1), evaluating the role of estrogens and IGF-1 in the modulation of neoplastic cell growth. ER-alpha, ER-beta, IGF-1, and IGF-1R were expressed (immunohistochemistry) in all biopsies (18 of 18) of intrahepatic cholangiocarcinoma. ER-alpha was expressed (Western blot) only by the HuH-28 cell line (intrahepatic cholangiocarcinoma), whereas ER-beta, IGF-1, and IGF-1R were expressed in the three cell lines examined. In serum-deprived HuH-28 cells, serum readmission induced stimulation of cell proliferation that was inhibited by ER and IGF-1R antagonists. 17beta-Estradiol and IGF-1 stimulated proliferation of HuH-28 cells to a similar extent to that of MCF7 (breast cancer) but greater than that of TFK-1 and Mz-ChA-1, inhibiting apoptosis and exerting additive effects. These effects of 17beta-estradiol and IGF-1 were associated with enhanced protein expression of ER-alpha, phosphorylated (p)-ERK1/2 and pAKT but with decreased expression of ER-beta. Finally, transfection of IGF-1R anti-sense oligonucleotides in HuH-28 cells markedly decreased cell proliferation. In conclusion, human intrahepatic cholangiocarcinomas express receptors for estrogens and IGF-1, which cooperate in the modulation of cell growth and apoptosis. Modulation of ER and IGF-1R could represent a strategy for the management of cholangiocarcinoma.

Hippocampal neuropathology of diabetes mellitus is relieved by estrogen treatment

1. A recently recognized complication of uncontrolled diabetes mellitus is the encephalopathy involving, among other regions, the hippocampus. Since estrogens bring neuroprotection in cases of brain injury and degenerative diseases, we have studied if estradiol (E2) administration counteracts some hippocampal abnormalities of streptozotocin (STZ)-diabetic adult mice. 2. We first report the ability of E2 to modulate neurogenesis in the dentate gyrus (DG) and subventricular zone (SVZ) of diabetic mice. Using bromodeoxyuridine (BrdU) to label newly generated cells, a strong reduction in cell proliferation was obtained in DG and SVZ of mice sacrificed 20 days after STZ administration. The reduction was completely relieved by 10 days of E2 pellet implantation, which increased 30-fold the circulating E2 levels. 3. Diabetic mice also showed abnormal expression of astrocyte markers in hippocampus. Thus, increased number of GFAP(+) cells, indicative of astrogliosis, and increased number of apolipoprotein-E (Apo-E)(+) astrocytes, a marker of ongoing neuronal dysfunction, was found in stratum radiatum below the CA1 hippocampal subfield of diabetic mice. Both parameters were reverted to normal by the E2 regime that upregulated cell proliferation. 4. The studies demonstrated that hippocampal neuropathology of uncontrolled diabetes is a reversible condition and sensitive to estrogen treatment. Studies in animal models may open up new venues for understanding the beneficial role of steroid hormones in diabetic encephalopathy.

Prognosis of multiple sclerosis: clinical factors predicting the late evolution for an early treatment decision

With the development of new immunomodulating therapies, with which early use is strongly encouraged, it is crucial to be in possession of reliable clinical predictors of multiple sclerosis evolution. Prognostic factors are important to patients wanting to be informed about their prospects; to the clinician needing to individualize patients requiring immune treatments at an early stage of the disease; and also to the researcher needing to to improve the design and analysis of the clinical therapeutic trials and observational studies. Frequentist analyses have indicated a poor prognosis for male gender, late age at onset, motor, cerebellar and sphincter involvement at onset, progressive course at onset, short inter-attack interval, high number of early attacks; and a relevant early residual disability. A recent application of a Bayesian analysis led to the construction of more detailed models of the natural history of multiple sclerosis and the estimated risk of unfavorable evolution at an individual patient level.

Increased estrogen receptor beta expression correlates with decreased spine formation in the rat hippocampus

Estrogens play an important role in the brain function acting through two receptor types, ERalpha and ERbeta, both well-recognized as transcription factors. In this study, we investigated the ERbeta mRNA and protein levels in the rat hippocampus by using two in vivo models that are known to affect synapse formation. Natural estrous-proestrous cycle was used as a model in which a marked decrease in the density of hippocampal synapses was previously observed between proestrus and estrus. We have found that ERbeta mRNA and protein were displayed in high levels in the estrus and in low levels in the proestrous phase. By applying kainic acid (KA) to adult rats, we demonstrated that up-regulation of ERbeta mRNA and protein in hippocampal CA regions was vulnerable to KA-induced excitotoxicity. Furthermore, we note a concomitant decrease of ERbeta in the excitotoxicity-resistant denate gyrus that undergoes intense plastic changes, including synaptogenesis. These data suggested that decreases in ERbeta expression correlated with increase in synapse formation. This notion has been tested in vitro in hippocampal cultures, in which overexpression of ERbeta by means of gene transfection resulted in the lowering of the dendritic spine density that was elevated by estrogen. In summary, our results suggest that ERbeta inhibits synapse formation in hippocampal neurons.

Multiple pathways transmit neuroprotective effects of gonadal steroids

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

Implication of the phosphatidylinositol-3 kinase/protein kinase B signaling pathway in the neuroprotective effect of estradiol in the striatum of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mice

The present experiments sought to determine the implication of estrogen receptors (ERalpha and ERbeta) and their interaction with insulin-like growth factor receptor (IGF-IR) signaling pathways in neuroprotection by estradiol against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) toxicity. C57BL/6 male mice were pretreated for 5 days with 17beta-estradiol, an estrogen receptor alpha (ERalpha) agonist, 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)tris-phenol (PPT), or an estrogen receptor beta (ERbeta) agonist, 5-androsten-3beta, 17beta-diol (Delta5-diol). On day 5, mice received MPTP (9 mg/kg) or saline injections, and estrogenic treatments were continued for 5 more days. MPTP decreased striatal dopamine, measured by high-performance liquid chromatography, to 59% of control values; 17beta-estradiol and PPT but not Delta5-diol protected against this depletion. MPTP increased IGF-IR measured by Western blot, which was prevented by PPT. The phosphorylation of protein kinase B (Akt) (at serine 473), an essential mediator of IGF-I neuroprotective actions, increased after 17beta-estradiol and tended to increase with PPT but not with Delta5-diol treatments in MPTP mice. Glycogen synthase kinase 3beta (GSK3beta) phosphorylation (at serine 9) was greatly reduced in MPTP mice; this was completely prevented by PPT, whereas 17beta-estradiol and Delta5-diol treatments were less effective. The ratio between the levels of striatal Bcl-2 and BAD proteins, two apoptotic regulators, decreased after MPTP treatment. This effect was effectively prevented only in the animals treated with PPT. In nonlesioned mice, 17beta-estradiol and PPT increased phosphorylation of striatal Akt and GSK3beta, whereas the other markers measured remained unchanged. Delta5-Diol increased GSK3beta phosphorylation less than the PPT treatment. These results suggest that a pretreatment with estradiol promoted dopamine neuron survival by activating ERalpha and increasing Akt and GSK3beta phosphorylation.

Insulin-like growth factor 1 reduces age-related disorders induced by prenatal stress in female rats

Stress during the prenatal period can induce permanent abnormalities in adult life such as increased anxiety-like behavior and hyperactivity of hypothalamo-pituitary-adrenal (HPA) axis system. The present study was designed to investigate whether prenatal stress could induce spatial learning impairment in aged female rats. Furthermore, since it has been recently reported that insulin-like growth factor 1 (IGF-1) attenuates spatial learning deficits in aged rats and promotes neurogenesis in the hippocampus, we assessed the impact of a chronic infusion of IGF-1 on age-related disorders. Our results show that females stressed during prenatal life exhibit learning impairments in the water maze task. Chronic IGF-1 treatment restores their spatial abilities, reduces their HPA axis dysfunction and increases plasma estradiol levels. Parallel to these effects, chronic IGF-1 up-regulates neural proliferation in the dentate gyrus of the hippocampus. These findings support the hypothesis of an early programming of the vulnerability to some neurological diseases during senescence and reinforce the potential therapeutic interest of IGF-1 during brain aging.

Both estrogen receptor α and estrogen receptor β agonists enhance cell proliferation in the dentate gyrus of adult female rats

This study investigated the involvement of estrogen receptors alpha and beta in estradiol-induced enhancement of hippocampal neurogenesis in the adult female rat. Subtype selective estrogen receptor agonists, propyl-pyrazole triol (estrogen receptor alpha agonist) and diarylpropionitrile (estrogen receptor beta agonist) were examined for each receptor's contribution, individual and cooperative, for estradiol-enhanced hippocampal cell proliferation. Estradiol increases hippocampal cell proliferation within 4 h [Ormerod BK, Lee TT, Galea LA (2003) Estradiol initially enhances but subsequently suppresses (via adrenal steroids) granule cell proliferation in the dentate gyrus of adult female rats. J Neurobiol 55:247-260]. Therefore, animals received s.c. injections of estradiol (10 microg), propyl-pyrazole triol and diarylpropionitrile alone (1.25, 2.5, 5.0 mg/0.1 ml dimethylsulfoxide) or in combination (2.5 mg propyl-pyrazole triol+2.5 mg diarylpropionitrile/0.1 ml dimethylsulfoxide) and 4 h later received an i.p. injection of the cell synthesis marker, bromodeoxyuridine (200 mg/kg). Diarylpropionitrile enhanced cell proliferation at all three administered doses (1.25 mg, P<0.008; 2.5 mg, P<0.003; 5 mg, P<0.005), whereas propyl-pyrazole triol significantly increased cell proliferation (P<0.0002) only at the dose of 2.5 mg. Our results demonstrate both estrogen receptor alpha and estrogen receptor beta are individually involved in estradiol-enhanced cell proliferation. Furthermore both estrogen receptor alpha and estrogen receptor beta mRNA was found co-localized with Ki-67 expression in the hippocampus albeit at low levels, indicating a potential direct influence of each receptor subtype on progenitor cells and their progeny. Dual receptor activation resulted in reduced levels of cell proliferation, supporting previous studies suggesting that estrogen receptor alpha and estrogen receptor beta may modulate each other's activity. Our results also suggest that a component of estrogen receptor-regulated cell proliferation may take place through alternative ligand and/or cell-signaling mechanisms.

Neuroprotection by cAMP: Another brick in the wall

Programmed cell death occurs in the nervous system both in normal development as well as in pathologic conditions, and is a key issue related to both brain repair and neurodegenerative diseases. Modulation of cell death in the nervous system may involve neurotrophic factors and other peptides, neurotransmitters and neuromodulators, that activate various signal transduction pathways, which in turn interact with the cell death execution machinery. Here we discuss the role of the second messenger cyclic adenosine 3'5'-monophosphate (cAMP) in cell death, and summarize current evidence that cAMP is a nodal point of neuroprotective signaling pathways.

Cross-talk between IGF-I and estradiol in the brain: focus on neuroprotection

The actions of estradiol in the brain involve the interaction with growth factors, such as insulin-like growth factor-I (IGF-I). Many cells in the brain coexpress receptors for estradiol (ERs) and IGF-I (IGF-IR) and both factors interact to regulate neural function. Several studies have shown that there is an interaction of IGF-IR and ERs in neuroprotection. Neuroprotective effects of estradiol are blocked by the inhibition of IGF-IR signaling, while the neuroprotective effects of IGF-I are blocked by the inhibition of ER signaling. These findings suggest that the neuroprotective actions of estradiol and IGF-I after brain injury depend on the coactivation of both ERs and IGF-IR in neural cells. The relationship of ERalpha with IGF-IR through the phosphatidylinositol 3-kinase/Akt/glycogen synthase kinase 3beta (PI3K/Akt/GSK3) signaling pathway may represent the point of convergence used by estradiol and IGF-I to cooperatively promote neuroprotection. Administration of estradiol to ovariectomized rats results in the association of ERalpha with IGF-IR and with components of the PI3K/Akt/GSK3 signaling pathway and in the regulation of the activity of Akt and GSK3 in the brain. Conversely, IGF-I regulates ERalpha transcriptional activity in neuroblastoma cells and the PI3K/Akt/GSK3 signaling pathway is involved in this effect.

Insulin-like growth factor I interfaces with brain-derived neurotrophic factor-mediated synaptic plasticity to modulate aspects of exercise-induced cognitive function

The ability of exercise to benefit neuronal and cognitive plasticity is well recognized. This study reveals that the effects of exercise on brain neuronal and cognitive plasticity are in part modulated by a central source of insulin-like growth factor-I. Exercise selectively increased insulin-like growth factor-I expression without affecting insulin-like growth factor-II expression in the rat hippocampus. To determine the role that insulin-like growth factor-I holds in mediating exercise-induced neuronal and cognitive enhancement, a specific antibody against the insulin-like growth factor-I receptor was used to block the action of insulin-like growth factor-I in the hippocampus during a 5-day voluntary exercise period. A two-trial-per-day Morris water maze was performed for five consecutive days, succeeded by a probe trial 2 days later. Blocking hippocampal insulin-like growth factor-I receptors did not significantly attenuate the ability of exercise to enhance learning acquisition, but abolished the effect of exercise on augmenting recall. Blocking the insulin-like growth factor-I receptor significantly reversed the exercise-induced increase in the levels of brain-derived neurotrophic factor mRNA and protein and pro-brain-derived neurotrophic factor protein, suggesting that the effects of insulin-like growth factor-I may be partially accomplished by modulating the precursor to the mature brain-derived neurotrophic factor. A molecular analysis revealed that exercise significantly elevated proteins downstream to brain-derived neurotrophic factor activation important for synaptic function, i.e. synapsin I, and signal transduction cascades associated with memory processes, i.e. phosphorylated calcium/calmodulin protein kinase II and phosphorylated mitogen-activated protein kinase II. Blocking the insulin-like growth factor-I receptor abolished these exercise-induced increases. Our results illustrate a possible mechanism by which insulin-like growth factor-I interfaces with the brain-derived neurotrophic factor system to mediate exercise-induced synaptic and cognitive plasticity.

The insulin-like growth factor system and its pleiotropic functions in brain

In recent years, much interest has been devoted to defining the role of the IGF system in the nervous system. The ubiquitous IGFs, their cell membrane receptors, and their carrier binding proteins, the IGFBPs, are expressed early in the development of the nervous system and are therefore considered to play a key role in these processes. In vitro studies have demonstrated that the IGF system promotes differentiation and proliferation and sustains survival, preventing apoptosis of neuronal and brain derived cells. Furthermore, studies of transgenic mice overexpressing components of the IGF system or mice with disruptions of the same genes have clearly shown that the IGF system plays a key role in vivo.

Regulation of glutamate transporter GLAST and GLT-1 expression in astrocytes by estrogen

Estrogen influences neuronal development and a broad spectrum of neural functions. In addition, several lines of evidence suggest a role as neuroprotective factor for estrogen in the CNS. Neuroprotection can result from direct estrogen-neuron interactions or be mediated indirectly involving the regulation of physiological properties of nonneuronal cells, such as astrocytes and microglia. Increased l-glutamate levels are associated with neurotoxic and neurodegenerative processes in the brain. Thus, the removal of l-glutamate from the extracellular space by astrocytes through the astroglial glutamate transporters GLT-1 and GLAST appears essential for maintaining a homeostatic milieu for neighboring neurons. We have therefore studied the influence of 17beta-estradiol on l-glutamate metabolism in cultured astrocytes from the neonate mouse midbrain using quantitative RT-PCR and Western blotting for both transporters as well as functional l-glutamate uptake studies. The administration of estrogen significantly increased the expression of GLT-1 and GLAST on the mRNA and protein level. Likewise, specific l-glutamate uptake by astrocytes was elevated after estrogen exposure and mimicked by dbcAMP stimulation. Induction of transporter expression and l-glutamate uptake were sensitive to ICI 182,780 treatment suggesting estrogen action through nuclear estrogen receptors. These findings indicate that estrogen can prevent l-glutamate-related cell death by decreasing extracellular l-glutamate levels through an increased l-glutamate uptake capacity by astrocytes.

The intrahepatic biliary epithelium is a target of the growth hormone/insulin-like growth factor 1 axis

BACKGROUND/AIMS

We evaluated the role and mechanisms by which the GH/IGF1 axis modulates cholangiocyte proliferation.

METHODS

GH-receptors (GH-R), IGF1, IGFBP3 (binding protein 3), IGF1-R and receptor substrates (IRS) were evaluated in cholangiocytes of normal or bile duct-ligated (BDL) rat livers. The effects of GH and IGF1 on proliferation of normal quiescent cholangiocytes and the transduction pathways involved were investigated.

RESULTS

IGF1, GH-R, IGF1-R, IRS-1/2 were expressed in normal cholangiocytes and overexpressed in cholangiocytes proliferating after BDL which also secrete IGF1 in a higher amount than normal cells. IGFBP3, which may counter-regulate IGF1 effects, was decreased in BDL cholangiocytes. IGF1 promoted cholangiocyte proliferation in association with overexpression of p-IGF1R, IRS1, IRS-2, p-ERK1/2 and p-AKT. GH induced IGF1 expression and release in isolated cholangiocytes, and reproduced the effects of IGF1 but GH effects were abolished by IGF1-R blocking antibody, suggesting IGF1 as a mediator of GH. Finally, IGF1 and 17beta-estradiol reciprocally potentiated their proliferative effects on cholangiocytes, and by interacting at both receptor and post-receptor levels.

CONCLUSIONS

Cholangiocytes respond to GH with production and release of IGF1 that modulates cell proliferation by transduction pathways involving IGF1-R, IRS1/2 and both ERK and PI3-kinase pathways. The biliary epithelium is a target of GH/IGF1 liver axis.

Growth hormone-releasing peptide-6 increases insulin-like growth factor-I mRNA levels and activates Akt in RCA-6 cells as a model of neuropeptide Y neurones

Chronic systemic administration of growth hormone (GH)-releasing peptide-6 (GHRP-6), an agonist for the ghrelin receptor, to normal adult rats increases insulin-like growth factor (IGF)-I mRNA and phosphorylated Akt (pAkt) levels in various brain regions, including the hypothalamus. Because neuropeptide Y (NPY) neurones of the arcuate nucleus express receptors for ghrelin, we investigated whether these neurones increase their IGF-I and p-Akt levels in response to this agonist. In control rats, immunoreactive pAkt was practically undetectable; however, GHRP-6 increased p-Akt immunoreactivity in the arcuate nucleus, with a subset of neurones also being immunoreactive for NPY. Immunoreactivity for IGF-I was detected in NPY neurones in both experimental groups. To determine if activation of this intracellular pathway is involved in modulation of NPY synthesis RCA-6 cells, an embryonic rat hypothalamic neuronal cell line that expresses NPY was used. We found that GHRP-6 stimulates NPY and IGF-I mRNA synthesis and activates Akt in this cell line. Furthermore, inhibition of Akt activation by LY294002 treatment did not inhibit GHRP-6 induction of NPY or IGF-I synthesis. These results suggest that some of the effects of GHRP-6 may involve stimulation of local IGF-I production and Akt activation in NPY neurones in the arcuate nucleus. However, GHRP-6 stimulation of NPY production does not involve this second messenger pathway.

From clinical evidence to molecular mechanisms underlying neuroprotection afforded by estrogens

Recent studies have highlighted that female sex hormones represent potential neuroprotective agents against damage produced by acute and chronic injuries in the adult brain. Clinical reports have documented the effectiveness of estrogens to attenuate symptoms associated with Parkinson's disease, and to reduce the risk of Alzheimer's disease and cerebrovascular stroke. This evidence is corroborated by numerous experimental studies documenting the protective role of female sex hormones both in vitro and in vivo. Accordingly, estrogens have been shown to promote survival and differentiation of several neuronal populations maintained in culture, and to reduce cell death associated with excitotoxicity, oxidative stress, serum deprivation or exposure to beta-amyloid. The neuroprotective effects of estrogens have been widely documented in animal models of neurological disorders, such as Alzheimer's and Parkinson's diseases, as well as cerebral ischemia. Although estrogens are known to exert several direct effects on neurones, the cellular and molecular mechanisms implicated in their protective actions on the brain are not completely understood. Thus, on the basis of clinical and experimental evidence, in this review, we discuss recent findings concerning the neuronal effects of estrogens that may contribute to their neuroprotective actions. Both estrogen receptor-dependent and -independent mechanisms will be described. These include modulation of cell death regulators, such as Bcl-2, Akt and calpain, as well as interaction with growth factors, such as BDNF, NGF, IGF-I and their receptors. The anti-inflammatory effects of estrogens will also be described, namely their ability to reduce brain levels of inflammatory mediators, cytokines and chemokines. Finally, a brief overview about receptor-independent mechanisms of neuroprotection will aim at describing the antioxidant effects of estrogens, as well as their ability to modulate neurotransmission.