Partial neuroprotection by 17-β-estradiol in neonatal γ-irradiated rat cerebellum

Neuroprotective effect of Wogonin on Rat's brain exposed to gamma irradiation

Wogonin (5,7-dihydroxy-8-methoxy flavone), an active component isolated from the root of Scutellaria baicalensis Georgi. Neurotoxic effects of γ irradiation have been established in humans and animals. The current study was designed to evaluate whether wogonin could restrain γ irradiation-induced neurotoxicity in rats and to explore the underlying mechanisms. Rats were divided into five groups, 10 rats each. Group 1 was orally administered distilled water and served as control. Group 2 received an oral daily dose of wogonin (30 mg/kg). Rats in group 3 were exposed to a whole-body single dose of γ-irradiation. Animals in group 4 received an oral daily dose of wogonin (30 mg/kg) for 15 days then exposed to a whole-body single dose of γ-irradiation. In group 5, rats were exposed to a whole-body single dose of γ-irradiation then were orally administered a daily dose of wogonin (30 mg/kg) for 15 days. There were significant increases in malondialdehyde (MDA), tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and Interleukin 6 (IL-6) levels and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) mRNA and protein expression. Whereas significant decreases in reduced glutathione (GSH), superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX) level as well as nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) mRNA and protein expression in the irradiated group when compared with the relevant control. The cerebral cortex of irradiated rats showed vacuolization and nuclear pyknosis in the neuronal cells and focal gliosis. Wogonin administration pre- or post-irradiation significantly ameliorated all these previous effects. Wogonin had antioxidant and anti-inflammatory effects and ameliorated the histopathological changes in the brain.

Local Estrogen Synthesis Regulates Parallel Fiber-Purkinje Cell Neurotransmission Within the Cerebellar Cortex

Estrogens affect cerebellar activity and cerebellum-based behaviors. Within the adult rodent cerebellum, the best-characterized action of estradiol is to enhance glutamatergic signaling. However, the mechanisms by which estradiol promotes glutamatergic neurotransmission remain unknown. Within the mouse cerebellum, we found that estrogen receptor activation of metabotropic glutamate receptor type 1a strongly enhances neurotransmission at the parallel fiber-Purkinje cell synapse. The blockade of local estrogen synthesis within the cerebellum results in a diminution of glutamatergic neurotransmission. Correspondingly, decreased estrogen availability via gonadectomy or blockade of aromatase activity negatively affects locomotor performance. These data indicate that locally derived, and not just gonad-derived, estrogens affect cerebellar physiology and function. In addition, estrogens were found to facilitate parallel fiber-Purkinje cell synaptic transmission in both sexes. As such, the actions of estradiol to support cerebellar neurotransmission and cerebellum-based behaviors might be fundamental to understanding the normal processing of activity within the cerebellar cortex.

Regulation and the Mechanism of Estrogen on Cav1.2 Gene in Rat-Cultured Cortical Astrocytes

L-type calcium channel (LTCC) gene Cav1.2 is believed to play an important role in the alteration of Ca(2+) homeostasis in brain astrocytes. Increasing evidence shows that alteration of intracellular Ca(2+) concentration is related to the effect of 17β-estradiol (E2) in a variety of neurophysiological and neuropathological conditions. In this study, we measured immunoreactivity of Cav1.2 protein expression in rat primary cortical astrocytes by using Western blots. We demonstrated that E2 upregulated Cav1.2 expression in a dose- and time-dependent manner and the effect of E2 on Cav1.2 expression were blocked by an estrogen receptor (ER) antagonist, ICI-182,780. The ER subtype-selective ERα agonists propylpyrazole triole (PPT) and ERβ agonist diarylpropionitrile (DPN) both increase the expression of Cav1.2 in a dose-dependent manner. Also, the PPT most closely mimicked the upregulation of Cav1.2 protein expression by E2. Similar experiments of 10 nM E2-treated ERα- or ERβ-knockdown astrocytes have also shown that the E2 regulation of Cav1.2 protein expression is mediated through an ERα-dependent pathway. Furthermore, we established that E2 did not change the level of Cav1.2 mRNA. The induction of E2-mediated Cav1.2 expression was inhibited by cycloheximide (CHX) but not by actinomycin D (Act-D), suggesting that E2 regulation of Cav1.2 expression occurred at a posttranscriptional level. We also found that E2 may increase Cav1.2 levels by decreasing its ubiquitination and degradation rate. These findings provide new information about the effect of E2 on Cav1.2 in astrocytes, particularly necessary for the treatment of neurological disease.

Estrogen Receptor-β Up-Regulates IGF1R Expression and Activity to Inhibit Apoptosis and Increase Growth of Medulloblastoma

Medulloblastoma (Med) is the most common malignant brain tumor in children. The role of ESR2 [estrogen receptor (ER)-β] in promoting Med growth was comprehensively examined in three in vivo models and human cell lines. In a novel Med ERβ-null knockout model developed by crossing Esr2(-/-) mice with cerebellar granule cell precursor specific Ptch1 conditional knockout mice, the tumor growth rate was significantly decreased in males and females. The absence of Esr2 resulted in increased apoptosis, decreased B-cell lymphoma 2 (BCL2), and IGF-1 receptor (IGF1R) expression, and decreased levels of active MAPKs (ERK1/2) and protein kinase B (AKT). Treatment of Med in Ptch1(+/-) Trp53(-/-) mice with the antiestrogen chemotherapeutic drug Faslodex significantly increased symptom-free survival, which was associated with increased apoptosis and decreased BCL2 and IGF1R expression and signaling. Similar effects were also observed in nude mice bearing D283Med xenografts. In vitro studies in human D283Med cells metabolically stressed by glutamine withdrawal found that 17β-estradiol and the ERβ selective agonist 2,3-bis(4-hydroxyphenyl)-propionitrile dose dependently protected Med cells from caspase-3-dependent cell death. Those effects were associated with increased phosphorylation of IGF1R, long-term increases in ERK1/2 and AKT signaling, and increased expression of IGF-1, IGF1R, and BCL2. Results of pharmacological experiments revealed that the cytoprotective actions of estradiol were dependent on ERβ and IGF1R receptor tyrosine kinase activity and independent of ERα and G protein-coupled estrogen receptor 1 (G protein coupled receptor 30). The presented results demonstrate that estrogen promotes Med growth through ERβ-mediated increases in IGF1R expression and activity, which induce cytoprotective mechanisms that decrease apoptosis.

Estrogen regulates the expression of Ndrg2 in astrocytes

N-myc downstream-regulated gene 2 (Ndrg2) is a newly identified molecule that is mainly expressed in astrocytes within the central nervous system (CNS) and is involved in the proliferation and activation of astrocytes. 17β-estradiol (E2) is one of the most important circulating hormones, and in the CNS, astrocytes are a target and potential mediator of the action of E2. Our most recent study found that DPN, an estrogen receptor (ER) β-specific agonist, activated the Ndrg2 promoter and elevated endogenous NDRG2 protein expression in MCF7, HSG and T-47D cells. However, whether E2 regulates Ndrg2 expression in astrocytes remains unknown. Here, we conducted both in vivo and in vitro experiments and found that ERβ co-localized with NDRG2 in astrocytes. Furthermore, in primary cultured astrocytes, we demonstrated that E2 up-regulated Ndrg2 mRNA and protein expression in a dose- and time-dependent manner and that the ERβ agonist DPN but not the ERα agonist PPT up-regulated Ndrg2 expression. In vivo, we found that in the hippocampus of adult ovariectomized (OVX) female mice, Ndrg2 mRNA and protein expression were significantly decreased compared with those in normal adult female mice. After the OVX mice received continuous subcutaneous injections of 50μg/kg E2, 100μg/kg E2 or the ERβ agonist DPN for 10 days, the Ndrg2 expression significantly increased compared with that of the OVX mice. Our results indicate that E2 may affect astrocytes by regulating Ndrg2 expression.

17β-estradiol delays 6-OHDA-induced apoptosis by acting on Nur77 translocation from the nucleus to the cytoplasm

Nuclear receptors (Nurs) represent a large family of gene expression regulating proteins. Gathering evidence indicates an important role for Nurs as transcription factors in dopamine neurotransmission. Nur77, a member of the Nur superfamily, plays a role in mediating the effects of antiparkinsonian and neuroleptic drugs. Besides, Nur77 survival and apoptotic roles depend largely on its subcellular localization. Estrogens are known for their neuroprotective properties, as demonstrated in animal and clinical studies. However, their action on Nur77 translocation pertaining to neuroprotection has not been investigated yet. The aim of our study was to perform a kinetic study on the effect of neurotoxic 6-hydroxydopamine (6-OHDA) and 17β-estradiol (E2) on the subcellular localization of Nur77 with reference to the modulation of apoptosis in PC12 cells. Our results demonstrate that E2 administration alone does not affect Nur77 cytoplasmic/nuclear ratio, mRNA levels, or apoptosis in PC12 cells. The neurotoxin 6-OHDA significantly enhances cytoplasmic localization of Nur77 after merely 3 h, while precipitating apoptosis. 6-OHDA also increases Nur77 transcription, which could partly explain the rise in cytoplasmic localization of the protein. Finally, treatment with both E2 and 6-OHDA delays Nur77 accumulation in the cytoplasm and delays cell death for a few hours in our cellular paradigm. Pre-treatment with E2 does not alter the increase in levels of Nur77 mRNA produced by 6-OHDA, suggesting that a raise in nuclear translocation is likely responsible for the stabilization of the cytoplasmic/nuclear ratio until 6 h. These results suggest an intriguing cooperation between E2 and Nur77 toward cellular fate guidance.

17β-Estradiol prevents cell death and mitochondrial dysfunction by an estrogen receptor-dependent mechanism in astrocytes after oxygen-glucose deprivation/reperfusion

17β-Estradiol (E2) has been shown to protect against ischemic brain injury, yet its targets and the mechanisms are unclear. E2 may exert multiple regulatory actions on astrocytes that may greatly contribute to its ability to protect the brain. Mitochondria are recognized as playing central roles in the development of injury during ischemia. Increasing evidence indicates that mitochondrial mechanisms are critically involved in E2-mediated protection. In this study, the effects of E2 and the role of mitochondria were evaluated in primary cultures of astrocytes subjected to an ischemia-like condition of oxygen-glucose deprivation (OGD)/reperfusion. We showed that E2 treatment significantly protects against OGD/reperfusion-induced cell death as determined by cell viability, apoptosis, and lactate dehydrogenase leakage. The protective effects of E2 on astrocytic survival were blocked by an estrogen receptor (ER) antagonist (ICI-182,780) and were mimicked by an ER agonist selective for ERα (PPT), but not by an ER agonist selective for ERβ (DPN). OGD/reperfusion provoked mitochondrial dysfunction as manifested by an increase in cellular reactive oxygen species production, loss of mitochondrial membrane potential, and depletion of ATP. E2 pretreatment significantly inhibited OGD/reperfusion-induced mitochondrial dysfunction, and this effect was also blocked by ICI-182,780. Therefore, we conclude that E2 provides direct protection to astrocytes from ischemic injury by an ER-dependent mechanism, highlighting an important role for ERα. Estrogen protects against mitochondrial dysfunction at the early phase of ischemic injury. However, overall implications for protection against brain ischemia and its complex sequelae await further exploration.

An early treatment with 17-β-estradiol is neuroprotective against the long-term effects of neonatal ionizing radiation exposure

Ionizing radiations can induce oxidative stress on target tissues, acting mainly through reactive oxygen species (ROS). The aim of this work was to investigate if 17-β-estradiol (βE) was able to prevent hippocampal-related behavioral and biochemical changes induced by neonatal ionizing radiation exposure and to elucidate a potential neuroprotective mechanism. Male Wistar rats were irradiated with 5 Gy of X-rays between 24 and 48 h after birth. A subset of rats was subcutaneously administered with successive injections of βE or 17-α-estradiol (αE), prior and after irradiation. Rats were subjected to different behavioral tasks to evaluate habituation and associative memory as well as anxiety levels. Hippocampal ROS levels and protein kinase C (PKC) activity were also assessed. Results show that although βE was unable to prevent radiation-induced hippocampal PKC activity changes, most behavioral abnormalities were reversed. Moreover, hippocampal ROS levels in βE-treated irradiated rats approached control values. In addition, αE administered to irradiated animals was effective in preventing radiation-induced alterations. In conclusion, βE was able to counteract behavioral and biochemical changes induced in irradiated animals, probably acting through an antioxidant mechanism.