Emerging Evidence on Membrane Estrogen Receptors as Novel Therapeutic Targets for Central Nervous System Pathologies

Nuclear- and membrane-initiated estrogen signaling cooperate to orchestrate the pleiotropic effects of estrogens. Classical estrogen receptors (ERs) act transcriptionally and govern the vast majority of hormonal effects, whereas membrane ERs (mERs) enable acute modulation of estrogenic signaling and have recently been shown to exert strong neuroprotective capacity without the negative side effects associated with nuclear ER activity. In recent years, GPER1 was the most extensively characterized mER. Despite triggering neuroprotective effects, cognitive improvements, and vascular protective effects and maintaining metabolic homeostasis, GPER1 has become the subject of controversy, particularly due to its participation in tumorigenesis. This is why interest has recently turned toward non-GPER-dependent mERs, namely, mERα and mERβ. According to available data, non-GPER-dependent mERs elicit protective effects against brain damage, synaptic plasticity impairment, memory and cognitive dysfunctions, metabolic imbalance, and vascular insufficiency. We postulate that these properties are emerging platforms for designing new therapeutics that may be used in the treatment of stroke and neurodegenerative diseases. Since mERs have the ability to interfere with noncoding RNAs and to regulate the translational status of brain tissue by affecting histones, non-GPER-dependent mERs appear to be attractive targets for modern pharmacotherapy for nervous system diseases.

Membrane-bound estrogen receptor alpha initiated signaling is dynamin dependent in breast cancer cells

Background

Although membrane-associated estrogen receptors (mERs) have been known to play important role in steroid-induced signal transmission, we still know little about their function in the estrogen-induced proliferation of breast cancer cells.

Methods

In our current work we tried to separate membrane-initiated estrogen receptor signaling from the overall estrogenic effect in MCF-7 breast carcinoma cells. Re-analyzing expression data from multiple microarray experiments, we selected a set of key regulatory genes involved in proliferation regulation and estrogen signaling to monitor estrogen-induced transcription changes. We then compared these expression changes after 17β-estradiol and a membrane receptor selective estrogen–BSA treatment using quantitative real-time PCR. In order to follow receptor trafficking we used light and electron microscopy.

Results

Our quantitative real-time PCR results confirmed that the selective membrane receptor agonist, estrogen–BSA induces similarly pronounced expression changes regarding these genes as 17β-estradiol. Morphological study revealed that the membrane-bound form of classical estrogen receptor alpha is internalized after ligand binding via dynamin-dependent, caveola-mediated endocytosis. Inhibition of this internalization with dynamin inhibitor, dynasore practically abolished the regulatory effect of E2-BSA, suggesting that interaction and internalization with the scaffold protein is necessary for effective signaling.

Conclusions

The physiological role of plasma membrane estrogen receptor alpha is intensively studied, yet there are still several aspects of it to be resolved. The dynamin-dependent, ligand-mediated internalization of mERs seems to play an important role in estrogen signaling. Our results may serve as another example of how membrane initiated estrogen signaling and nuclear receptor initiated signaling overlap and form an intertwined system. 

Electronic supplementary material

The online version of this article (10.1186/s40001-018-0328-7) contains supplementary material, which is available to authorized users.

Estrogen: a master regulator of bioenergetic systems in the brain and body

Estrogen is a fundamental regulator of the metabolic system of the female brain and body. Within the brain, estrogen regulates glucose transport, aerobic glycolysis, and mitochondrial function to generate ATP. In the body, estrogen protects against adiposity, insulin resistance, and type II diabetes, and regulates energy intake and expenditure. During menopause, decline in circulating estrogen is coincident with decline in brain bioenergetics and shift towards a metabolically compromised phenotype. Compensatory bioenergetic adaptations, or lack thereof, to estrogen loss could determine risk of late-onset Alzheimer's disease. Estrogen coordinates brain and body metabolism, such that peripheral metabolic state can indicate bioenergetic status of the brain. By generating biomarker profiles that encompass peripheral metabolic changes occurring with menopause, individual risk profiles for decreased brain bioenergetics and cognitive decline can be created. Biomarker profiles could identify women at risk while also serving as indicators of efficacy of hormone therapy or other preventative interventions.

Estrogen attenuates lipopolysaccharide-induced nitric oxide production in macrophages partially via the nongenomic pathway

Steroid hormones exert genotropic effects through members of the nuclear hormone receptor family. In the present study, we examined the effects of 17β-estradiol (E2) on nitric oxide (NO) production following lipopolysaccharide (LPS) stimulation and investigated the mechanisms in mouse bone marrow-derived macrophages (BMMs). E2 alone did not affect NO production. In contrast, E2 inhibited LPS-induced production of NO in BMMs. Using a cell-impermeable E2 conjugated to BSA (E2-BSA), which has been used to investigate the nongenomic effects of estrogen, we found that the increase in NO production induced by LPS was also attenuated. In addition, the intracellular estrogen receptor blocker, ICI 182780, only partially antagonized the total effects of E2 on LPS-stimulated NO production capacity. E2 also attenuated the LPS activation of p38 mitogen-activated protein kinase (MAPK) but not that of extracellular-regulated protein kinase 1/2 (ERK1/2) and c-Jun NH2-terminal kinase (JNK). This attenuation was not abrogated by ICI 182780. Moreover, the p38 inhibitor, SB 203580, greatly reduced the LPS-induced NO production, and the remaining NO levels were no longer regulated by E2. Additionally, E2-BSA inhibited LPS-mediated changes in p38 MAPK activation to the same extent as E2. Moreover, E2 and E2-BSA inhibited LPS-induced activation of nuclear factor-kappa B (NF-κB) and activator protein 1 (AP-1). This inhibitory effect of E2 was only partially antagonized by ICI 182780. Taken together, these results suggest that E2 has an inhibitory effect on LPS-induced NO production in BMMs through inhibition of p38 MAPK phosphorylation, and blockade of NF-κB and AP-1 activation. These effects are mediated at least in part via a nongenomic pathway.