17-Alpha-estradiol ameliorating oxygen-induced retinopathy in a murine model

G-protein-coupled estrogen receptor protects retinal ganglion cells via inhibiting endoplasmic reticulum stress under hyperoxia

Abnormal development of immature retinal vascular structure in preterm infants under the condition of hyperoxia is the primary cause of retinopathy of prematurity (ROP), which has become the leading cause of blindness in children. Retinal ganglion cells (RGCs) play a critical role in the normal growth of retinal vessels. Previous studies have indicated that estrogen can alleviate retinal lesions in the ROP animal model by inhibiting reactive oxygen species, which is associated with endoplasmic reticulum (ER) stress. This study aimed to investigate the protecting effect of G-protein coupled estrogen receptor (GPER), one of the estrogen receptors distributed in ER, on RGCs in the early stage of ROP and its relationship with ER stress. We found that GPER was widely expressed in primary cultured murine RGCs. GPER activation by its agonist G-1 increased cell vitality and decreased apoptosis and autophagy of RGCs under hyperoxia. GPER activation by G-1 decreased the expressions of the ER stress proteins, including inositol-requiring kinase/endonuclease 1α, pancreatic ER stress kinase, and cleaved activating transcription factor 6 in ER of RGCs under hyperoxia. GPER activation decreased IP3R activity and increased Ca²⁺  concentration in ER of RGCs under hyperoxia. In addition, GPER antagonist (G-15) reversed all these effects of the GPER agonist mentioned above. This study suggested that GPER activation can protect the survival of RGCs in the early stage of ROP via reducing ER stress in RGCs under the condition of hyperoxia.

Inhibiting endoplasmic reticulum stress by activation of G-protein-coupled estrogen receptor to protect retinal astrocytes under hyperoxia

Retinal vascularization is arrested at the early (hyperoxia) stage in retinopathy of prematurity (ROP), a leading cause of blindness in children. Estrogen was reported to alleviate ROP by inhibiting reactive oxygen species, the upstream signaling molecules of endoplasmic reticulum stress (ERS). Astrocytes have long been proposed to guide angiogenesis, because they form a reticular network that provides a substrate for migrating endothelial cells. However, the factors that control the vascularization of the immature retina and the therapeutic mechanism of estrogen in early ROP remain poorly understood. This study aimed to investigate the role of G-protein-coupled estrogen receptor (GPER), an estrogen receptor distributed in the endoplasmic reticulum (ER), in protecting retinal astrocytes under hyperoxia and the association with ERS. The results showed that GPER was widely expressed in retinal astrocytes. GPER activation increases cell viability, decreases apoptosis, and autophagy of retinal astrocytes, decreases inositol-1,4,5-triphosphate receptor activity, and increases Ca²⁺ concentration in ER of astrocytes under hyperoxia. GPER blockade reversed all of these changes. Together, our findings indicate that GPER can protect the survival of retinal astrocytes by inhibiting ERS under hyperoxia.

G-protein coupled estrogen receptor activation protects the viability of hyperoxia-treated primary murine retinal microglia by reducing ER stress

In this study, we investigated the effects of G-protein coupled estrogen receptor (GPER) activation in the early phase of retinopathy of prematurity (ROP) and its association with endoplasmic reticulum (ER) stress using primary murine retinal microglia as an experimental model. Fluorescence microscopy results show that the CD11c-positive primary retinal microglia in vitro cultured for 14 days were GPER-positive. GPER activation using GPER-agonist G-1 reduced autophagy and increased the viability of the hyperoxia-treated primary murine retinal microglia. Furthermore, GPER activation reduced the expression of ER stress-related proteins, IRE1α, PERK and ATF6 in the hyperoxia-treated primary murine retinal microglia compared to the corresponding controls. GPER activation significantly reduced a time-dependent increase in IP3R-dependent calcium release from the ER, thereby maintaining higher calcium levels in the ER of hyperoxia-treated primary retinal microglia. However, the protective effects of G-1 on the hyperoxia-treated primary retinal microglia were eliminated by inactivation of GPER using the GPER-antagonist, G-15. In conclusion, our study demonstrates that GPER activation enhances the survival of hyperoxia-treated primary retinal microglia by reducing ER stress. Our study demonstrates the therapeutic potential of GPER agonists such as G-1 in the early phase of ROP.

10β,17α-Dihydroxyestra-1,4-dien-3-one: A Bioprecursor Prodrug Preferentially Producing 17α-Estradiol in the Brain for Targeted Neurotherapy

Uterotrophic effect of 17α-estradiol, the C17 epimer of the main human estrogen 17β-estradiol, was shown to manifest in animal models at doses lower than those necessary for central outcome raising concerns about its potential to treat maladies of the central nervous system. We introduce here 10β,17α-dihydroxyestra-1,4-dien-3-one (α-DHED) that acts as a bioprecursor prodrug producing 17α-estradiol with remarkable selectivity to the brain and, therefore, without appreciable exposure of the periphery to the parent steroid. This distinguishing feature of α-DHED is shown by using an estrogen-responsive mouse model with complementary LC-MS/MS measurement of drug contents in target tissues. Our data warrant further research to fully establish the potential of α-DHED for a safe and efficacious 17α-estradiol-based neurotherapy.

Evaluating the neuroprotective effect of 17β-estradiol in rodent models of oxidative retinopathies

PURPOSE

To determine the neuroprotective effect of estrogen on the structure and function of the retina exposed to an oxidative stress.

METHODS

Male Sprague-Dawley rat pups were exposed to either hyperoxia (O₂E: from P8 to P14) or bright light (LE: from P14 to P28) with or without 17 β-estradiol (βE₂) treatment. Retinal structure (histology) and function (ERG) were assessed at selected time points.

RESULTS

In the O₂E model, βE₂ injections caused a significant reduction of the ERG and a significantly thinner OPL compared to untreated oxygen-exposed group (O₂-exposed) rats. In contrast, in the LE model βE₂, treatment was beneficial to the retinal structure (thicker ONL) and function (better preserved ERG amplitudes) compared to untreated light-exposed group (light-exposed rats).

CONCLUSION

Our results show that in conditions where the primary target of the oxidative stress is the outer retina (i.e., the photoreceptors) estrogen can protect the retina, while in situations where the inner retina (or retinal vasculature) is the main site of oxidative damage, estrogen may potentiate the detrimental effect of oxidative stress on the retina.

The progress of prophylactic treatment in retinopathy of prematurity

Retinopathy of prematurity (ROP) is a retinal vascular disorder frequently found in premature infants. Different therapeutic strategies have been developed to treat ROP. However, there are still many children with ROP suffering by severe limitations in vision or even blindness. Recently, ROP has been suggested to be caused by abnormal development of the retinal vasculature, but not simply resulted by retinal neovascularization which takes about 4 to 6wk after birth in premature infants. Thus, instead of focusing on how to reduce retinal neovascularization, understanding the pathological changes and mechanisms that occur prior to retinal neovascularization is meaningful, which may lead to identify novel target(s) for the development of novel strategy to promote the healthy growth of retinal blood vessels rather than passively waiting for the appearance of retinal neovascularization and removing it by force. In this review, we discussed recent studies about, 1) the pathogenesis prior to retinal neovascularization in oxygen-induced retinopathy (OIR; a ROP in animal model) and in premature infants with ROP; 2) the preclinical and clinical research on preventive treatment of early OIR and ROP. We will not only highlight the importance of the mechanisms and signalling pathways in regulating early stage of ROP but also will provide guidance for actively exploring novel mechanisms and discovering novel treatments for early phase OIR and ROP prior to retinal neovascularization in the future.

17β-estradiol ameliorates oxygen-induced retinopathy in the early hyperoxic phase

Retinopathy of prematurity (ROP) is a major and leading cause of blindness in premature infants. It has been realized that early treatment for ROP is important. However, all the early treatments of ROP are focusing on peripheral retinal ablation which does not surmount the limit of extinguishing retinal neovascularization and protecting the retinas of children with ROP from the injury of ablation. In this study, we investigated the morphological changes of retina and oxidative stress alterations in the early phase of oxygen-induced retinopathy (OIR) and tested the effects of 17β-estradiol (17β-E2), a nonselective estrogen receptor (ER) agonist, on early phase OIR development. We found that large central capillary-free areas were induced in the retinas of pups exposed to hyperoxia on postnatal day 9 (P9), whereas vascularization was almost complete in the retinas of pups exposed to normoxia at the same age. The concentrations of malondiadehyde (MDA), an end-product of oxidative stress, and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, a major enzyme producing free radicals, as well as the activity of NADPH oxidase were significantly elevated in the retinas of pups exposed to hyperoxia on P9 and postnatal day 13 (P13) compared to those in age matched pups exposed to normoxia. Treatment with 17β-E2 decreased not only the percentage of the central capillary-free area to total retina area but also the concentrations of MDA and NADPH oxidase as well as the activity of NADPH oxidase in a dose-dependent manner in pups exposed to hyperoxia on p9 and P13. The concentration of VEGF was significantly decreased on P9 but increased on P14 in the retinas of pups exposed to hyperoxia, whereas it was significantly elevated on P9 but decreased on P14 in the retinas of pups treated with 17β-E2. The effect of 17β-E2 could be reversed by the co-treatment with ICI182780, a high affinity estrogen receptor antagonist, which suggested that 17β-E2 might exert its effect on early hyperoxic phase of OIR through estrogen receptor. Our results suggest that treatment with antioxidant drugs at early hyperoxic phase of ROP even before the appearance of retinal neovascularization may be more effective than their application to ROP at late phase, which may abolish the deleterious factors that contribute to retinal neovascularization and promote retinal blood vessels to develop healthily.

17α-Oestradiol-induced neuroprotection in the brain of spontaneously hypertensive rats

17β-oestradiol is a powerful neuroprotective factor for the brain abnormalities of spontaneously hypertensive rats (SHR). 17α-Oestradiol, a nonfeminising isomer showing low affinity for oestrogen receptors, is also endowed with neuroprotective effects in vivo and in vitro. We therefore investigated whether treatment with 17α-oestradiol prevented pathological changes of the hippocampus and hypothalamus of SHR. We used 20-week-old male SHR with a blood pressure of approximately 170 mmHg receiving s.c. a single 800 μg pellet of 17α-oestradiol dissolved in cholesterol or vehicle only for 2 weeks Normotensive Wistar-Kyoto (WKY) rats were used as controls. 17α-Oestradiol did not modify blood pressure, serum prolactin, 17β-oestradiol levels or the weight of the testis and pituitary of SHR. In the brain, we analysed steroid effects on hippocampus Ki67+ proliferating cells, doublecortin (DCX) positive neuroblasts, glial fibrillary acidic protein (GFAP)+ astrocyte density, aromatase immunostaining and brain-derived neurotrophic factor (BDNF) mRNA. In the hypothalamus, we determined arginine vasopressin (AVP) mRNA. Treatment of SHR with 17α-oestradiol enhanced the number of Ki67+ in the subgranular zone and DCX+ cells in the inner granule cell layer of the dentate gyrus, increased BDNF mRNA in the CA1 region and gyrus dentatus, decreased GFAP+ astrogliosis in the CA1 subfield, and decreased hypothalamic AVP mRNA. Aromatase expression was unmodified. By contrast to SHR, normotensive WKY rats were unresponsive to 17α-oestradiol. These data indicate a role for 17α-oestradiol as a protective factor for the treatment of hypertensive encephalopathy. Furthermore, 17α-oestradiol is weakly oestrogenic in the periphery and can be used in males.