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

Abstract

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

FIG. 1. Effect of PI3-K/Akt on E2-induced TGF-β release in rat cortical astrocytes

A, Rat cortical astrocytes were cotreated with E2 in the presence or absence or the PI3K inhibitors, LY294002 (20 μM) and wortmannin (200 nM), or Akt inhibitor prevented the induction of TGF-β1 release by E2 after an 18-h treatment. B, Cotreatment of rat cortical astrocytes with E2 in the presence of MAPK kinase inhibitors, PD98059 (30 μM) and U0126 (10 μM), was without effect on E2-induced TGF-β1 release. C, Overexpression of a dnAkt construct prevented E2-induced release of TGF-β1 in rat cortical astrocytes. Conversely, myrAkt significantly increased TGF-β1 release compared with that by empty vector-transfected astrocytes. For all studies, there were six wells per group, and experiments were performed in three independent cultures. Different superscripts denote significant differences between groups (P < 0.05, by one-way ANOVA, Student-Newman-Keuls post hoc test). Reproduced with permission from: Dhandapani, K. M. et al. Endocrinology 2005;146:2749-2759.

FIG. 2. Effect of astrocyte-derived TGF-β on E2- and TMX-mediated neuroprotection from camptothecin (CPT) in mixed cortical cultures

A, Effects of E2, TMX, and E2-BSA on cell death induced by CPT in mixed glial-neuronal cultures. Mixed cultures were pretreated for 24 h with 10 nM E2, 1 μM TMX, or 100 nM E2-BSA before 10 μM CPT treatment for 24 h before determination of cell viability. B, Treatment of glial-neuronal mixed cultures with E2, TMX, or E2-BSA at the time of CPT treatment does not affect cell death 24 h later. C, Pretreatment of purified cortical neurons with E2, TMX, or E2-BSA does not reverse the cell death induced by 24 h 10 μM CPT. D, Mixed cultures were pretreated with E2, TMX, or E2-BSA in the presence or absence of a pan-specific TGF-β isoform-neutralizing antibody (α-pan-TGFβ) or a TGF-β1-specific neutralizing antibody (α-TGF-β1), followed by a 24-h CPT exposure. Cell viability was determined 24 h after treatment with CPT. E, Effects of the ER antagonist, ICI182,780, and the PI3-K inhibitor, LY294002, on E2- and TMX-mediated rescue. Mixed cultures were pretreated with E2, TMX, or E2-BSA in the presence or absence of 1 μM ICI182,780 or 20 μM LY294002 for 24 h. Cultures were then exposed to CPT for another 24 h, followed by determination of cell viability. For all studies, cellular viability was determined using the MTT assay. Vehicle-treated cultures were considered to be 100% viable, and all treatment groups were compared with these control cultures. Viability was also confirmed using lactate dehydrogenase release assays (data not shown). In all panels, data are expressed as the mean ± SEM, and groups with different superscripts are significantly different from each other (P < 0.05, by one-way ANOVA, Student-Newman-Keuls post hoc test). For all studies, there were six wells per treatment group and experiments were performed in three independent sets of cultures for verification of results. Reproduced with permission from: Dhandapani, K. M. et al. Endocrinology 2005;146:2749-2759.

FIG. 3. Effect of 17β-Estradiol (E2) on superoxide anion O₂^- production in the ipsilateral cortex in ovariectomized animals 1h following permanent middle cerebral artery occlusion (pMCAO)

Panels A-C: Representative photomicrographs showing O₂^- production 1h after pMCAO in contralateral (non-injured) (Panel A) and ipsilateral (injured) cortex (Panel B) of placebo-treated ovariectomized adult rat, and in ipsilateral cortex of E2-treated ovariectomized adult rat (Panel C). O₂^- production was shown by oxidized HEt signals (bright red). Panels D-F: Corresponding 2-dimensional histograms of the fluorescence intensity for the images shown in panels A-C. Panel G: Graph showing difference in number of oxidized HEt-positive cells in the ipsilateral cortex of Placebo- vs. E2-treated animals at 1h following pMCAO. The results show that E2 markedly decreases O₂^- production as measured by oxidized HEt intensity and number of oxidized HEt positive cells in the ipsilateral cortex at 1h after pMCAO. N = 3 animals per group, 5-6 sections counted per group, and the experiment was repeated twice with similar results. P < 0.05 vs. Placebo group.

FIG. 4. Proposed multicellular mechanism of estrogen neuroprotection

Estrogen neuroprotection is proposed to be mediated by genomic, nongenomic and anti-inflammatory mechanisms. Both direct effects on neurons and indirect effects mediated via astrocytes, endothelial cells and microglia are suggested to contribute to the overall protective actions of estrogen in the brain. E2 = 17β-Estradiol, ER = estrogen receptor, ERK = extracellular signal regulated kinase, JNK = Jun NH(2)-terminal kinase, (?) = unknown mediators. See text for further detailed description.