Hydrogen peroxide attenuates the prosurvival signaling of insulin-like growth factor-1 through two pathways

Although it has been well established that oxidative stress triggering a variety of signaling pathways leads to cell death, little attention has been paid to how these pathways affect prosurvival factors such as insulin-like growth factor-1 (IGF-1). In this study, we found that the prosurvival signaling of IGF-1 was attenuated by H2O2. To study the mechanism underlying this phenomenon, cells pretreated with Trolox or various glutamate receptor antagonists [i.e. N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine maleate (MK-801), non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX), metabolic glutamate receptor antagonists LY341495 and CPCCOEt] were exposed to H2O2, and then stimulated by IGF-1. The phosphorylation statuses of IGF-1 receptors, Akt and ERK, were determined by western blotting, and cell viability was analyzed by an MTT assay. IGF-1 exerted a potent neuroprotective effect against B27 deprivation, and this effect was abolished by 100 μM H2O2. Meanwhile, the phosphorylation of IGF-1 receptors, Akt and ERK, was attenuated. Moreover, the phosphorylation of Akt was more susceptible to H2O2 insult than IGF-1 receptors. MK-801 increased the phosphorylation of IGF-1 receptors and its downstream target Akt, and thereby promoted cell survival, whereas the other glutamate receptor antagonists exerted no effect. Antioxidant Trolox did not restore IGF-1 signaling, but it increased Akt phosphorylation and also increased cell viability. These results showed that H2O2 impaired IGF-1 prosurvival signaling through two pathways. One pathway disrupted the autophosphorylation of IGF-1 receptors through NMDA receptors and the other directly dephosphorylated Akt.     

Estradiol, insulin-like growth factor-I and brain aging

The decrease in some hormones with aging, such as insulin-like growth factor-I (IGF-I) and estradiol, may have a negative impact on brain function. Estradiol and IGF-I may antagonize the damaging effects of adrenal steroids and other causes of brain deterioration. The signaling of estradiol and IGF-I interact to promote neuroprotection. Estrogen receptor alpha, in an estrogen-dependent process, can physically interact with IGF-I receptor and with the downstream signaling molecules of the phosphotidylinositol 3-kinase (PI3K)/Akt/glycogen synthase kinase 3 (GSK3) pathway. Estradiol and IGF-I have a synergistic effect on the activation of Akt, which in turn decreases the activity of GSK3. This may be one of the mechanisms used by estradiol to promote neuronal survival, since the inhibition of GSK3 is associated to the activation of surviving signaling pathways in neurons. Furthermore, estradiol may control Tau phosphorylation by modulating the interactions of estrogen receptor alpha with GSK3 and beta-catenin, another molecule involved in the regulation of neuronal survival and the reorganization of the cytoskeleton. All these actions may be involved in the neuroprotective effects of the hormone. Possible aging-associated changes in the expression or activity of these signaling molecules may affect estradiol neuroprotective effects. Therefore, it is important to determine whether aging affects the signaling of estradiol and IGF-I in the brain.     

Central opiate modulation of growth hormone and insulin-like growth factor-I

The effects of central administration of morphine-sulfate (MOR:80 μg) and morphine-6-glucuronide (M6G:1 μg) on the growth hormone (GH)/insulin-like growth factor (IGF) system were assessed. MOR and M6G were injected intracerebroventricularly (ICV) in chronically catheterized 24 h fasted rats; time-matched control animals received H₂O (5 μl). MOR increased plasma GH concentrations 3-fold 2 h after ICV injection, and transiently increased the plasma concentration and liver content of IGF-I (60% and 90%, respectively) 30 min after ICV injection. M6G did not produce any significant alterations in plasma GH and IGF-I levels at the time-points measured. Both MOR and M6G increased the concentration of IGF binding protein-1 (IGFBP-1) in plasma and liver 2 h after injection. However, MOR showed 2- to 2.5-fold greater effect than M6G in stimulating plasma and liver IGFBP-1. MOR and M6G produced similar increases in plasma epinephrine (5-fold), norepinephrine (3-fold) and corticosterone (1.5-fold). Neither opiate significantly altered circulating insulin levels. These findings suggest that opiate modulation of GH and IGF may be hormone-independent and centrally modulated. We speculate that differential affinities of MOR and M6G to the different opiate receptor subtypes might be responsible for their distinct effects on GH/IGF-I system.     

Glutamate transporters and retinal excitotoxicity

Glutamate appears to play a major role in several degenerative retinal disorders. However, exogenous glutamate is only weakly toxic to the retina when glutamate transporters on Müller glial cells are operational. In an ex vivo rat retinal preparation, we previously found that exogenous glutamate causes Müller cell swelling but does not trigger excitotoxic neurodegeneration unless very high concentrations that overwhelm the capacity of glutamate transporters are administered. To determine the role of glutamate transporters in Müller cell swelling and glutamate-mediated retinal degeneration, we examined the effects of DL-threo-beta-benzyloxyaspartate (TBOA), an agent that blocks glutamate transport but that unlike most available transport inhibitors is neither a substrate for transport nor a glutamate receptor agonist. We found that TBOA triggered severe retinal neurodegeneration attenuated by ionotropic glutamate receptor antagonists. TBOA-induced neuronal damage was also diminished by riluzole, an agent that inhibits endogenous glutamate release. In the presence of riluzole, to inhibit glutamate release plus TBOA to block glutamate uptake, the addition of low concentrations of exogenous glutamate triggered severe excitotoxic neuronal damage without inducing Müller cell swelling. We conclude that TBOA-sensitive glutamate transporters play an important role in regulating the neurodegenerative effects of glutamate in the rat retina.     

Regulation of adult olfactory neurogenesis by insulin-like growth factor-I

Insulin-like growth factor-I (IGF-I) has multiple effects within the developing nervous system but its role in neurogenesis in the adult nervous system is less clear. The adult olfactory mucosa is a site of continuing neurogenesis that expresses IGF-I, its receptor and its binding proteins. The aim of the present study was to investigate the roles of IGF-I in regulating proliferation and differentiation in the olfactory mucosa. The action of IGF-I was assayed in serum-free culture combined with bromodeoxyuridine-labelling of proliferating cells and immunochemistry for specific cell types. IGF-I and its receptor were expressed by globose basal cells (the neuronal precursor) and by olfactory neurons. IGF-I reduced the numbers of proliferating neuronal precursors, induced their differentiation into neurons and promoted morphological differentiation of neurons. The evidence suggests that IGF-I is an autocrine and/or paracrine signal that induces neuronal precursors to differentiate into olfactory sensory neurons. These effects appear to be similar to the cellular effects of IGF-I in the developing nervous system.     

Serum insulin-like growth factor-I levels in amyotrophic lateral sclerosis.

The serum concentrations of the myotrophic hormone insulin-like growth factor-I (IGF-I) in 23 patients with amyotrophic lateral sclerosis were not significantly different from those found in the sera of 13 control patients. There was no difference in binding of 125I-IGF-I by serum from patients with amyotrophic lateral sclerosis in comparison with that found in the controls. These results indicate that immunoreactive IGF-I concentrations are normal in patients with amyotrophic lateral sclerosis and that such patients do not have significant antibodies binding their endogenous IGF-I.     

Different levels of neuroprotection by two insulin-like growth factor-I splice variants

We compared the neuroprotective effects of a liver-type isoform of insulin-like growth factor-I (IGF-IEa) and its splice variant, mechano-growth factor (MGF), isolated from active skeletal muscle. cDNAs of these peptides were injected into the facial muscle of adult rats prior to facial nerve avulsion. This resulted in significant neuroprotection of 88% and 37%, respectively, of motoneurons compared to control plasmid and avulsion-only groups. MGF is markedly more effective than the liver-type, systemic IGF-I for motoneuron survival, suggesting a major role for the peripheral target in adult neuronal maintenance and survival.     

Effects of insulin-like growth factor-I on cytokine-induced sickness behavior in mice

Central administration of insulin-like growth factor-I (IGF-I) attenuates sickness behavior in response to the cytokine inducer lipopolysaccharide. The present study was designed to determine the respective roles of the two main proinflammatory cytokines, tumor necrosis factor alpha (TNFalpha) and interleukin-1beta (IL-1beta), in these effects. Male CD1 mice were injected into the lateral ventricle (i.c.v.) of the brain with optimal amounts of either TNFalpha (50 ng) or IL-1beta (2 ng) that induce sickness behavior. Behavioral responses to IGF-I (0, .1, and 1 microg) also given i.c.v. were measured at various time intervals before and after treatment with the two proinflammatory cytokines. Mice treated with TNFalpha and IL-1beta lost body weight and displayed equivalent reductions in social exploration and instances of immobility. At the dose of .1 microg, IGF-I attenuated these signs of sickness in TNFalpha-but not in IL-1beta-treated mice. At the dose of 1 microg, IGF-I attenuated IL-1beta-induced immobility and the reduction in social exploration but had no effect on loss of body weight. These findings indicate that IGF-I is more potent in attenuating sickness behavior induced by TNFalpha than that caused by IL-1beta, which is consistent with the relative specificity of the TNFalpha/IGF-I interactions in the brain.     

Estrogen receptors and insulin-like growth factor-I receptors mediate estrogen-dependent synaptic plasticity

Previous studies have shown that estradiol induces a transient disconnection of axo-somatic inhibitory synapses in the hypothalamic arcuate nucleus of adult ovariectomized rats. The synaptic disconnection is accompanied by an increase in the levels of insulin-like growth factor-I (IGF-I) in the arcuate nucleus, suggesting that IGF-I signaling may be involved in the estrogen-induced synaptic plasticity. The role of estrogen receptors and IGF-I receptors in the synaptic changes has been studied by assessing the number of axo-somatic synapses in ovariectomized rats treated with intracerebroventricular administration of the estrogen receptor antagonist ICI 182,780 and the IGF-I receptor antagonist JBI to ovariectomized rats. Estradiol administration resulted in a significant decrease in the number of axo-somatic synapses on arcuate neurons in control ovariectomized rats. Both the estrogen receptor antagonist and the IGF-I receptor antagonist blocked the estrogen-induced synaptic decrease. This finding suggest that estrogen-induced synaptic plasticity in the arcuate nucleus is dependent on the activation of both estrogen receptors and IGF-I receptors.     

Effects of early undernutrition on the brain insulin-like growth factor-I system

Undernutrition reduces circulating concentrations of insulin-like growth factor (IGF)-I, but how it affects the brain IGF system, especially during development, is largely unknown. We have studied IGF-I, IGF-II, IGF receptor and IGF binding protein (BP)-2 mRNA expression in the hypothalamus, cerebellum and cerebral cortex of neonatal rats that were food restricted beginning on gestational day 16. One group was refed starting on postnatal day 14. Rats were killed on postnatal day 8 or 22. Undernutrition did not produce an overall reduction in brain weight at either age but, at 22 days, both the cerebellum and hypothalamus weighed significantly less. At 8 days, no change was detected in the central IGF axis in response to undernutrition. However, in 22-day-old undernourished rats, IGF-I and IGF receptor mRNA expression were increased in both the hypothalamus and cerebellum, while IGFBP-2 was decreased, but only in the hypothalamus. Refeeding had no effect on any of these parameters. These results suggest that the hypothalamus and cerebellum respond to malnutrition and the decrease in circulating IGF-I, a peptide fundamental for growth and development, by increasing the local production of both the growth factor and its receptor in attempt to maintain normal development.     

A non-transformed oligodendrocyte precursor cell line, OL-1, facilitates studies of insulin-like growth factor-I signaling during oligodendrocyte development

The process by which oligodendrocyte progenitors differentiate into mature oligodendrocytes is complex and incompletely understood in part because of the paucity of oligodendrocyte precursors cell lines that can be studied in culture. We have developed a non-immortalized rat oligodendrocyte precursor line, called OL-1, which behaves in a fashion consistent with developing oligodendrocytes in vivo. This OL-1 line provides a model for the study of oligodendrocyte development and offers an alternative to the CG-4 cell line. When OL-1 cells are propagated in conditioned growth media, they have morphology consistent with immature oligodendrocytes and exhibit A2B5 antigen positive and myelin basic protein-negative immunoreactivity. Withdrawal of conditioned growth media and culture in serum-free medium results in OL-1 cell maturation, manifested by a shift to myelin basic protein-positive immunoreactivity, A2B5 antigen-negative immunoreactivity, decreased NG2 mRNA expression, increased expression of proteolipid protein mRNA, and increased expression of CNP protein. In addition, the expression of proteolipid protein and its splicing variant DM-20 exhibit a pattern that is similar to brain proteolipid protein expression during development. When OL-1 cells are exposed to Insulin-like growth factor-I, there are significant increases in proteolipid protein mRNA expression (p<0.05), the number of cell processes (p<0.05), and cell number (p<0.05). Treatment with the caspase inhibitors Z-DEVD-FMK and Z-VAD-FMK (inhibitors of caspases 3, 6, 7, 8, 10 and 1, 3, 4, respectively), Insulin-like growth factor-I, or both, results in a similar increase in cell number. Because Insulin-like growth factor-I does not substantially increase the BrdU labeling of OL-1 cells, these data collectively indicate that Insulin-like growth factor-I increases OL-1 cell number predominately by promoting survival, rather than stimulating proliferation. This non-immortalized oligodendrocyte precursor cell line, therefore, exhibits behavior consistent with the in vivo development of oligodendrocytes and provides an excellent model for the study of developing oligodendrocytes.     

Involvement of phosphatidylinositol 3-kinase and insulin-like growth factor-I in YXLST-mediated neuroprotection

In the present study, we examine the neuroprotective role of the external Qi of YXLST in cultured retinal neurons. Primary retinal neuronal cultures were grown from retinas of 0-2-day-old Sprague-Dawley rats. Cultures were treated directly with external Qi of YXLST 30 min prior to H(2)O(2) exposure in most experiments. Cell viability was measured by 3,(4,5-dimethylthiazol-2-yl)2,5-diphenyl-tetrazolium bromide (MTT) assay. Apoptotic cell death was evaluated by the TdT-mediated digoxigenin-dUTP nick-end labeling TUNEL assay, and by DNA laddering analysis. Northern blot analysis was performed to examine the level of insulin-like growth factor-I (IGF-I) gene expression. Phosphatidylinositol 3-kinase (PI3K) assay was performed to study the PI3K activity. The results showed that treatment of external Qi of YXLST significantly attenuated neuronal death that was induced by 24-h exposure to hydrogen peroxide, and greatly inhibited hydrogen peroxide-induced apoptosis. External Qi of YXLST also upregulated IGF-I gene expression and increased PI3K activity. These observations indicate that external Qi-mediated IGF-I expression and PI3K signaling could be one of the mechanisms in neuroprotection by YXLST.     

Differential patterns of insulin-like growth factor-I and -II mRNA expression in medulloblastoma

Insulin-like growth factors (IGFs) play an important role in tumour growth and development. We hypothesized that this is also the case for medulloblastomas, which are highly malignant cerebellar brain tumours usually occurring in children. In these tumours the expression patterns of IGF-I and -II mRNA were studied. Tumour specimens obtained from 12 children and two adults at diagnosis were hybridized in situ with digoxigenin-labelled cRNA probes for hIGF-I and hIGF-II mRNAs. In all cases, tumour cells showed abundant expression of IGF-I mRNA. Nine of the 14 tumours showed variable but significant IGF-II expression. In these tumours, the hybridization signal almost exclusively colocalized with a subpopulation of Ki-M1P positive cells that were identified as ramified microglia (RM) cells. In the five tumours without IGF-II expression, microglia/brain macrophages with a more rounded amoeboid-like morphology predominated. RM cells in normal cerebellar tissues, residing abundantly in areas of the white and, to a less extent, in the grey matter, were IGF-II mRNA-negative. These RM cells showed a thinner and more extensively branched appearance and were more evenly distributed than those encountered in medulloblastoma. Probably, during the transformation from the resting ramified towards the amoeboid morphology (or vice versa) IGF-II mRNA expression is only temporarily induced. The physiological meaning of the induction of IGF-II mRNA expression by these cells in medulloblastoma remains unclear but any IGF-II peptide synthesized could exert unfavourable mitogenic and antiapoptotic effects on adjacent tumour cells. However, in this relatively small number of cases we could not find any indications for a relationship between clinical characteristics of the various cases and the extent of IGF-II mRNA expression.     

Low levels of insulin-like growth factor-I in cerebrospinal fluid in children with autism

Autism is a behaviourally defined syndrome characterized by disturbances of social interaction and communication and restrictions of behaviour patterns and imagination. The pathogenesis of autism is unknown but it is suspected that a number of genetic factors may be involved. Neurotrophic factors such as insulin-like growth factor-I (IGF-I) play a role in early brain development. The aim of this study was to determine whether IGF-I levels might be associated with the development of autism. IGF-I levels were measured in the CSF of 11 children with autism (4 females, 7 males; mean age 3.8 years, SD 1.1) using a sensitive radioimmunoassay method and compared with levels in 11 control participants (6 females, 5 males; mean age 3.8 years). Levels of IGF-I in the CSF were statistically significantly lower in the children with autism than in the control children (p=0.03). IGF-I may play a role in pathogenetic mechanisms of autism and the role of neurotrophic factors in autism and other neurodevelopmental diseases should be studied further.     

Amygdala kindling decreases insulin-like growth factor-I receptor binding sites in the rat hippocampus

The neural excitability characteristic of kindling has been linked to structural alterations such as mossy fiber sprouting and synaptic reorganization within the hippocampus. Recent evidence suggests that growth factors may play a key role in kindling-related synaptic plasticity. Insulin-like growth factors-I and -II (IGF-I/-II) and insulin are structurally-related pleiotropic growth factors known to be involved in neural growth and differentiation. In the present study, we investigated the effect of kindling on [125I]IGF-I, [125I]IGF-II and [125I]insulin receptor binding in the hippocampus of adult rats. Our results indicate a progressive decrease in [125I]IGF-I (but not [125I]IGF-II or [125I]insulin) binding sites in the CA1, hilus and the granule cell layer of the kindled rats compared to sham-stimulated rats. These results, in keeping with the established neurotrophic effects of IGF-I, suggest a potential role for this growth factor in mediating the structural alterations associated with kindling.     

Target-derived cardiotrophin-1 and insulin-like growth factor-I promote neurite growth and survival of developing oculomotor neurons

Several trophic factors support the survival of developing motoneurons, but it is not known whether these factors act in a retrograde fashion from the motoneuron target muscle or are derived from other sources. Cardiotrophin-1 (CT-1) and the insulin-like growth factors (IGFs) are candidate target-derived motoneuron survival factors as both are expressed in muscle during naturally occurring motoneuron death and, applied systemically, support the survival of developing motoneurons. By using the embryonic chick oculomotor system, we show that CT-1 and IGF-I promote neurite outgrowth from E13-derived oculomotor explants and are retrogradely transported from muscle to nerve cell body in vivo, and injection of CT-1 or IGF-I into eye muscles increases motoneuron survival by 20 and 30%, respectively, as evidenced by calibrated stereological counting techniques. Pharmacological depletion of endogenous target-derived IGF-I in vivo reduces oculomotor neuron survival by up to 30% in a dose-dependent manner. These results significantly extend previous studies using systemic administration of trophic factors and are the first to demonstrate a target-derived retrograde mechanism of developing motoneuron survival factors.     

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.     

Role of insulin-like growth factor-I in the treatment of painful small fiber predominant neuropathy

Idiopathic, painful, small fiber predominant peripheral neuropathy is resistant to symptomatic treatment. Previous treatments have not been directed toward repairing the underlying deficit. Growth factors hold promise as agents to encourage axonal regrowth. In vitro, insulin-like growth factor-I (IGF-I) has been shown to prevent neuronal apoptosis, to increase axonal growth, and to support myelination. Using a double-blind, placebo-controlled design, 40 patients were randomized to treatment with recombinant human IGF-I (0.05 mg/kg twice daily by subcutaneous injection) or placebo for 6 months. There were no significant adverse events and minor adverse events occurred equally in both groups. The primary outcome measure was change in score on an analog pain scale. Secondary endpoints included quantitative sensory testing, quantitative autonomic testing, neuropathy impairment score, nerve conduction studies, and neuropathy symptom and change score. There was no significant difference in the primary endpoint between the two groups. Analysis of secondary endpoints and a global impression of improvement by patients and physicians did not show consistent differences between the groups. IGF-I was safe, but did not improve symptoms in this 6-month trial.     

A role for insulin-like growth factor-I in the regulation of Schwann cell survival.

During postnatal development in the peripheral nerve, differentiating Schwann cells are susceptible to apoptotic death. Schwann cell apoptosis is regulated by axons and serves as one mechanism through which axon and Schwann cell numbers are correctly matched. This regulation is mediated in part by the provision of limiting axon-derived trophic molecules, although neuregulin-1 (NRG-1) is the only trophic factor shown to date to support Schwann cell survival. In this report, we identify insulin-like growth factor-I (IGF-I) as an additional trophin that can promote Schwann cell survival in vitro. We find that IGF-I, like NRG-1, can prevent the apoptotic death of postnatal rat Schwann cells cultured under conditions of serum withdrawal. Moreover, we show that differentiating Schwann cells in the rat sciatic nerve express both the IGF-I receptor (IGF-I R) and IGF-I throughout postnatal development. These results indicate that IGF-I is likely to control Schwann cell viability in the developing peripheral nerve and, together with other findings, raise the interesting possibility that such survival regulation may switch during postnatal development from an axon-dependent mechanism to an autocrine and/or paracrine one.