β-Adrenergic receptor agonist, compound 49b, inhibits TLR4 signaling pathway in diabetic retina

Discovery of astragaloside IV against high glucose-induced apoptosis in retinal ganglion cells: Bioinformatics and in vitro studies

OBJECTIVE

To examine the therapeutic mechanism of astragaloside IV (AS-IV) in the management of retinal ganglion cell (RGC) injury induced by high glucose (HG), a comprehensive approach involving the integration of network pharmacology and conducting in vitro and in vivo experiments was utilized.

METHODS

A rat model of diabetic retinopathy (DR) injury was created by administering streptozotocin through intraperitoneal injection. Additionally, a model of RGC injury induced by HG was established using a glucose concentration of 0.3 mmol/mL. Optical coherence tomography (OCT) images were captured 8 weeks after the injection of AS-IV. AS-IV and FBS were added to the culture medium and incubated for 48 h. The viability of cells was assessed using a CCK-8 assay, while the content of reactive oxygen species (ROS) was measured using DCFH-DA. Apoptosis was evaluated using Annexin V-PI. To identify the targets of AS-IV, hyperglycemia, and RGC, publicly available databases were utilized. The Metascape platform was employed for conducting GO and KEGG enrichment analyses. The STRING database in conjunction with Cytoscape 3.7.2 was used to determine common targets of protein-protein interactions (PPIs) and to identify the top 10 core target proteins in the RGC based on the MCC algorithm. qRT-PCR was used to measure the mRNA expression levels of the top10 core target proteins in RGCs.

RESULTS

OCT detection indicated that the thickness of the outer nucleus, and inner and outer accessory layers of the retina increased in the AS-IV treated retina compared to that in the DM group but decreased compared to that in the CON group. Coculturing RGC cells with AS-IV after HG induction resulted in a significant increase in cell viability and a decrease in ROS and apoptosis, suggesting that AS-IV can reduce damage to RGC cells caused by high glucose levels by inhibiting oxidative stress. There were 14 potential targets of AS-IV in the treatment of RGC damage induced by high glucose levels. The top 10 core target proteins identified by the MCC algorithm were HIF1α, AKT1, CTNNB1, SMAD2, IL6, SMAD3, IL1β, PPARG, TGFβ1, and NOTCH3. qRT-PCR analysis showed that AS-IV could upregulate the mRNA expression levels of SMAD3, TGF-β1, and NOTCH3, and downregulate the mRNA expression levels of HIF1α, AKT1, CTNNB1, SMAD2, SMAD3, and IL-1β in high glucose-induced RGC cells.

CONCLUSION

The findings of this study validate the efficacy of astragaloside IV in the treatment of DR and shed light on the molecular network involved. Specifically, HIF1α, AKT1, CTNNB1, SMAD2, SMAD3, and IL-1β were identified as the crucial candidate molecules responsible for the protective effects of astragaloside IV on RGCs.

Platycodin D inhibits diabetic retinopathy via suppressing TLR4/MyD88/NF-κB signaling pathway and activating Nrf2/HO-1 signaling pathway

Diabetic retinopathy (DR) is one of the most frequently occurring diabetic complications associated with inflammation and oxidative stress. Platycodin D (PLD) is a bio-active saponin that has been reported to exhibit anti-inflammation, anti-oxidative, and antidiabetic activities. Therefore, we speculated the protective effects of PLD on DR in the present study. Our results demonstrated that PLD attenuated high glucose (HG)-induced inflammation, as evidenced by decreased production of TNF-α, IL-1β, IL-6. The HG-induced oxidative stress was prevented by PLD with decreased ROS production and malondialdehyde (MDA) level, as well as increased activities of superoxide dismutase and glutathione (GSH). In addition, treatment of PLD significantly decreased the apoptotic rate in HG-induced ARPE-19 cells. The HG-caused increases in expression of bax and cleaved capsase-3, as well a decrease in bcl-2 expression were attenuated by PLD. Furthermore, PLD suppressed the activation of TLR4/MyD88/NF-κB and enhanced the activation of Nrf2/HO-1 pathway in HG-induced ARPE-19 cells. Additionally, overexpression of TLR4 attenuated the anti-inflammatory, while knockdown of Nrf2 reversed the anti-oxidative and anti-apoptotic activities of PLD in HG-stimulated ARPE-19 cells. Furthermore, PLD attenuates retinal damage in DR rats. Finally, we demonstrated that PLD weakened the TLR4/MyD88/NF-κB p65 pathway and promoted the Nrf2/HO-1 pathway in vivo. Taken together, these findings indicated that PLD exerted protective effects against DR, which were attributed to the regulation of TLR4/MyD88/NF-κB and Nrf2/HO-1 signaling pathways.

Targeting Novel Regulated Cell Death: Pyroptosis, Necroptosis, and Ferroptosis in Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the primary causes of visual impairment in the working-age population. Retinal cell death is recognized as a prominent feature in the pathological changes of DR. Several types of cell death occurrence have been confirmed in DR, which might be the underlying mechanisms of retinal cell loss. Regulated cell death (RCD) originates from too intense or prolonged perturbations of the intracellular or extracellular microenvironment for adaptative responses to cope with stress and restore cellular homeostasis. Pyroptosis, necroptosis, and ferroptosis represent the novel discovered RCD forms, which contribute to retinal cell death in the pathogenesis of DR. This evidence provides new therapeutic targets for DR. In this review, we summarize the mechanisms of three types of RCD and analyse recent advances on the association between novel RCD and DR, aiming to provide new insights into the underlying pathogenic mechanisms and propose a potential new strategy for DR therapy.

Toll-Like Receptor Signalling Pathways and the Pathogenesis of Retinal Diseases

There is growing evidence that the pathogenesis of retinal diseases such as diabetic retinopathy (DR) and age-related macular degeneration (AMD) have a significant chronic inflammatory component. A vital part of the inflammatory cascade is through the activation of pattern recognition receptors (PRR) such as toll-like receptors (TLR). Here, we reviewed the past and current literature to ascertain the cumulative knowledge regarding the effect of TLRs on the development and progression of retinal diseases. There is burgeoning research demonstrating the relationship between TLRs and risk of developing retinal diseases, utilising a range of relevant disease models and a few large clinical investigations. The literature confirms that TLRs are involved in the development and progression of retinal diseases such as DR, AMD, and ischaemic retinopathy. Genetic polymorphisms in TLRs appear to contribute to the risk of developing AMD and DR. However, there are some inconsistencies in the published reports which require further elucidation. The evidence regarding TLR associations in retinal dystrophies including retinitis pigmentosa is limited. Based on the current evidence relating to the role of TLRs, combining anti-VEGF therapies with TLR inhibition may provide a longer-lasting treatment in some retinal vascular diseases.

Role of Toll-Like Receptor 4 in Diabetic Retinopathy

Diabetic retinopathy (DR) is the most frequent microvascular complication of diabetes mellitus (DM) and a leading cause of blindness worldwide. Evidence has shown that DR is an inflammatory disease with hyperglycemia playing a causative role in the development of its main features, including inflammation, cellular apoptosis, neurodegeneration, oxidative stress, and neovascularization. Toll-like receptors (TLRs) are a well-known family of pattern recognition receptors (PRRs) responsible for the initiation of inflammatory and immune responses. TLR4 identifies both endogenous and exogenous ligands and is associated with various physiological and pathological pathways in the body. While the detailed pathophysiology of DR is still unclear, increasing data suggests a crucial role for TLR4 in the development of DR. Due to hyperglycemia, TLR4 expression increases in diabetic retina, which activates various pathways leading to DR. Considering the role of TLR4 in DR, several studies have focused on the association of TLR4 polymorphisms and risk of DR development. Moreover, evidence concerning the effect of microRNAs in the pathogenesis of DR, through their interaction with TLR4, indicates the determinant role of TLR4 in this disease. Of note, several agents have proven as effective in alleviating DR through the inhibition of the TLR4 pathway, suggesting new avenues in DR treatment. In this review, we provided a brief overview of the TLR4 structure and biological function and a more comprehensive discussion about the mechanisms of TLR4 activation in DR. Furthermore, we summarized the relationship between TLR4 polymorphisms and risk of DR and the relationship between microRNAs and TLR4 in DR. Finally, we discussed the current progress in designing TLR4 inhibitors, which could be helpful in DR clinical management.

Forskolin regulates retinal endothelial cell permeability through TLR4 actions in vitro

To investigate whether forskolin, a protein kinase A agonist, regulates toll-like receptor 4 actions on retinal endothelial cell permeability in vitro. We also evaluated whether PKA could regulate TLR4 signaling independent of exchange protein activated by cAMP in REC in culture. REC were grown in normal (5 mM) or high (25 mM) glucose. Cells were treated with forskolin to increase PKA levels, siRNA against TLR4, siRNA against myeloid differentiation primary response 88, siRNA against translocating chain associated membrane protein 1, siRNA against epac1, or scrambled siRNA, or a combination of these treatments. Western blotting was done for zonula occludens 1 and occludin protein levels, as well as TLR4 signaling cascade proteins. Permeability measurements were done for REC in culture following inhibition of TLR4 or its signaling cascades. Forskolin restored high glucose-associated decreases in ZO-1 and occludin, which was associated with improved in vitro permeability levels. Both forskolin and TLR4 inhibition reduced high glucose-induced increases in REC permeability, but the actions were not cooperative. Forskolin regulated both MyD88-dependent and -independent signaling pathways, independent of Epac1. Finally, blockade of MyD88 or TRAM1 reduced permeability in REC grown in high glucose. A PKA agonist regulated TLR4 signaling independent of Epac1. PKA agonism or TLR4 inhibition is effective at reducing high glucose-induced permeability in REC in vitro. These studies offer new avenues for therapeutic development.

Bone marrow mesenchymal stem cells-induced exosomal microRNA-486-3p protects against diabetic retinopathy through TLR4/NF-κB axis repression

AIM

Diabetic retinopathy (DR) is a chronic disease causing health and economic burdens on individuals and society. Thus, this study is conducted to figure out the mechanisms of bone marrow mesenchymal stem cells (BMSCs)-induced exosomal microRNA-486-3p (miR-486-3p) in DR.

METHODS

The putative miR-486-3p binding sites to 3'untranslated region of Toll-like receptor 4 (TLR4) was verified by luciferase reporter assay. High glucose (HG)-treated Muller cells were transfected with miR-486-3p or TLR4-related oligonucleotides and plasmids to explore theirs functions in DR. Additionally, HG-treated Muller cells were co-cultured with BMSC-derived exosomes, exosomes collected from BMSCs that had been transfected with miR-486-3p or TLR4-related oligonucleotides and plasmids to explore their functions in DR. MiR-486-3p, TLR4 and nuclear factor-kappaB (NF-κB) expression, angiogenesis-related factors, oxidative stress factors, viability and apoptosis in HG-treated Muller cells were detected by RT-qPCR, western blot analysis, ELISA, MTT assay and flow cytometry, respectively.

RESULTS

MiR-486-3p was poorly expressed while TLR4 and NF-κB were highly expressed in HG-treated Muller cells. TLR4 was a target of miR-486-3p. Upregulating miR-486-3p or down-regulating TLR4 inhibited oxidative stress, inflammation and apoptosis, and promoted proliferation of HG-treated Muller cells. Meanwhile, BMSC-derived exosomes inhibited oxidative stress, inflammation and apoptosis, and promoted proliferation of HG-treated Muller cells. Restoring miR-486-3p further enhanced, while up-regulating TLR4 reversed, the improvement of exosomes treatment.

CONCLUSION

Our study highlights that up-regulation of miR-486-3p induced by BMSC-derived exosomes played a protective role in DR mice via TLR4/NF-κB axis repression.

Epac1 regulates TLR4 signaling in the diabetic retinal vasculature

Toll-like receptor 4 (TLR4) polymorphisms occur in diabetic patients. Previous work showed that TLR4 is in the retina of diabetic mice, as well as in retinal endothelial cells (REC) and Müller cells. Since we have shown that exchange protein activated by cAMP 1 (Epac1) can reduce inflammatory mediators, we hypothesized that Epac1 would inhibit TLR4 signaling. We also hypothesized that direct TLR4 inhibition would protect the diabetic retina. Human REC in normal and high glucose were treated with an Epac1 agonist to explore the actions of Epac1 on TLR4 signaling in vitro. Subsequently, 2-month diabetic endothelial cell specific knockout mice for Epac1 (Cdh5Cre-Epac1) and Epac1 floxed mice retinas were used for Western blotting for TLR4 signaling pathways. We also used direct inhibition of TLR4 via Tak242 to investigate diabetes-induced changes in retinal permeability and neuronal loss in the mice. The Epac1 agonist reduced TLR4 signaling in REC grown in high glucose. TLR4 levels and both MyD88-dependent and -independent signaling pathways are increased in Cdh5Cre-Epac1 mice compared to Epac1 floxed mice. Tak242 reduced TLR4 signaling in diabetic mice and reduced diabetes-induced increases in permeability and cell loss in the ganglion cell layer in the Epac1 floxed and Cdh5Cre-Epac1 mice. In conclusion, Epac1 reduced TLR4 signaling in the retina and in REC. Direct inhibition of TLR4 was able to protect the retina against diabetes-induced changes in permeability and cell numbers in the ganglion cell layer.

MD2 blockade prevents modified LDL-induced retinal injury in diabetes by suppressing NADPH oxidase-4 interaction with Toll-like receptor-4

Modified LDL-induced inflammation and oxidative stress are involved in the pathogenesis of diabetic retinopathy. Recent studies have also shown that modified LDL activates Toll-like receptor 4 (TLR4) to mediate retinal injury. However, the mechanism by which modified LDL activates TLR4 and the potential role of the TLR4 coreceptor myeloid differentiation protein 2 (MD2) are not known. In this study, we inhibited MD2 with the chalcone derivatives L2H17 and L6H21 and showed that MD2 blockade protected retinal Müller cells against highly oxidized glycated-LDL (HOG-LDL)-induced oxidative stress, inflammation, and apoptosis. MD2 inhibition reduced oxidative stress by suppressing NADPH oxidase-4 (NOX4). Importantly, HOG-LDL activated TLR4 and increased the interaction between NOX4 and TLR4. MD2 was required for the activation of these pathways, as inhibiting MD2 prevented the association of NOX4 with TLR4 and reduced NOX4-mediated reactive oxygen species production and TLR4-mediated inflammatory factor production. Furthermore, treatment of diabetic mice with L2H17 significantly reduced LDL extravasation in the retina and prevented retinal dysfunction and apoptosis by suppressing the TLR4/MD2 pathway. Our findings provide evidence that MD2 plays a critical role in mediating modified LDL-induced cell injury in the retina and suggest that targeting MD2 may be a potential therapeutic strategy.

Blocking the activity of a protein involved in triggering inflammation and oxidative stress in the retina may reduce diabetes-induced visual impairment and blindness. Besides hyperglycemia, plasma lipids and lipoproteins have also been proposed as potential risk factors for diabetic retinopathy. However, the precise mechanisms controlling low-density lipoprotein-induced retinal damage are unclear. In experiments on mice, Yi Wang at Wenzhou Medical University in Wenzhou, China, and co-workers found that retinal injury caused by highly oxidized glycated-LDL is mediated by the myeloid differentiation protein 2 (MD2). Blocking MD2 with an inhibitor suppressed two key signaling pathways, reducing the production of reactive oxygen species and inflammatory signaling proteins in the retina. Further investigations are needed, but the team believe MD2 could be a vital therapeutic target for diabetic retinopathy.

Loss of TLR4 in endothelial cells but not Müller cells protects the diabetic retina

Others have previously reported that global loss of toll-like receptor 4 (TLR4) reduced retinal inflammation. To determine cell specific actions of TLR4 in the retina, we generated diabetic endothelial cell specific and Müller cell specific TLR4 knockout mice. Diabetic Cdh5-Cre TLR4 mice, PDGFRα-Cre TLR4 mice, and TLR4 floxed mice were evaluated for retinal permeability, neuronal damage, and numbers of degenerate capillaries, all changes commonly observed in the diabetic retina. We also measured protein levels of key inflammatory mediators. We found that diabetes increased permeability, neuronal, and vascular damage in all mice. Loss of TLR4 in the retinal endothelial cells protected against these changes when compared to diabetic TLR4 floxed mice. In contrast, loss of TLR4 in Müller cells did not reduce diabetes-induced increases in permeability or neuronal and vascular damage. Elimination of TLR4 in either mouse model reduced inflammatory mediators, as well as VEGF levels. Taken together, our findings suggest that loss of TLR4 in endothelial cells is protective against diabetic-induced damage, while Müller cell TLR4 is not involved in the damage.

Bone Morphogenetic Protein-2 Induces Non-Canonical Inflammatory and Oxidative Pathways in Human Retinal Endothelial Cells

The mechanisms of diabetic retinopathy (DR), are not yet fully understood. We previously demonstrated an upregulation of retinal bone morphogenetic protein-2 (BMP2) in experimental diabetes and in retinas of diabetic human subjects. The purpose of current study was to investigate the role of non-canonical inflammatory pathway in BMP2-induced retinal endothelial cell (REC) barrier dysfunction. For this purpose, we used RT-PCR and western blotting to evaluate the levels of BMP2 signaling components (BMP2, BMP4, BMP receptors), VEGF, phosphorylated p38 MAPK and NFκB, and oxidative stress markers in cultured human retinal endothelial cells (HRECs) subjected to BMP2 (50ng/ml) for up to 24 h. Also, effect of high glucose (HG, 30mM D-glucose) on the expression of BMP2 and its downstream genes was examined in HRECs. H2-DCF is a fluorogenic dye that measures the levels of cellular reactive oxygen species (ROS) was used to measure the pro-oxidative effect of BMP2. Moreover, we evaluated the effect of inhibiting p38 and VEGF signaling on BMP2-induced HRECs barrier dysfunction by measuring the trans-endothelial cell electrical resistance (TER) using electric cell-substrate impedance sensing (ECIS). We also tested the effect of HG on the integrity of HRECs barrier in the presence or absence of inhibitors of BMP2 signaling. Our data reveals that BMP2 and high glucose upregulates BMP components of the BMP signaling pathway (SMAD effectors, BMP receptors, and TGFβ ligand itself) and induces phosphorylation of p38 MAPK and NFκB with nuclear translocation of NFκB. Inhibition of p38 or NFκB attenuated BMP2-induced VEGF expression and barrier dysfunction in HRECs. Also, inhibition of VEGFR2 attenuated BMP2-induced barrier dysfunction. Moreover, BMP2 induces generation of ROS and endothelial nitric oxide synthase (eNOS) expression and activity in HRECs. Finally, HG upregulated BMP2 and its downstream genes (SMAD, BMP4, ALKs, and TGF-β) in HRECs and BMP2 inhibitors attenuated HG-induced HRECs barrier dysfunction. Our results suggest that in addition to the regular canonical SMAD signaling BMP2 induces non-canonical inflammatory pathway in HRECs via activation of p38/NFκB pathway that causes the upregulation of VEGF and the disruption of HRECs. Inhibition of BMP2 signaling is a potential therapeutic intervention to preserve endothelial cell barrier function in DR.

Dysregulation of the NLRP3 Inflammasome in Diabetic Retinopathy and Potential Therapeutic Targets

Diabetic Retinopathy (DR) is an insidious neurovascular disorder secondary to chronic glycemic dysregulation in elderly diabetic patients. In the later stages of DR, the disease manifests as fluid infiltrating the macula, culminating in the leading cause of irreversible visual impairment in working age adults. With the current mainstay treatments preoccupied with slowing down the progression of DR, this presents an unsustainable solution from both an economic and quality of life perspective. Although the exact mechanisms by which hyperglycemia leads to retinal tissue insult are unknown, the evidence suggests that chronic low-grade inflammation in diabetic eye is in part driving the constellation of symptoms present in DR. Of the innate immune system within the eye, the NLR Family Pyrin Domain Containing 3 Inflammasome (NLRP3) has been identified in retinal cells as a causal factor in the pathogenesis of DR. Multiple pathways appear to be present in the diabetic eye that instigate prolonged activation of the NLRP3 which subsequently exerts its deleterious effects by upregulating the release of Interleukin-1Beta and Interleukin-18. In this review, we highlight the current understanding of the pathophysiology of DR, the dysregulation of the NLRP3 secondary to hyperglycemic stress in retinal cells, and novel therapeutic targets to alleviate overactivation of the inflammasome.

The Complex Relationship between Diabetic Retinopathy and High-Mobility Group Box: A Review of Molecular Pathways and Therapeutic Strategies

High-mobility group box 1 (HMGB1) is a protein that is part of a larger family of non-histone nuclear proteins. HMGB1 is a ubiquitary protein with different isoforms, linked to numerous physiological and pathological pathways. HMGB1 is involved in cytokine and chemokine release, leukocyte activation and migration, tumorigenesis, neoangiogenesis, and the activation of several inflammatory pathways. HMGB1 is, in fact, responsible for the trigger, among others, of nuclear factor-κB (NF-κB), tumor necrosis factor-α (TNF-α), toll-like receptor-4 (TLR-4), and vascular endothelial growth factor (VEGF) pathways. Diabetic retinopathy (DR) is a common complication of diabetes mellitus (DM) that is rapidly growing in number. DR is an inflammatory disease caused by hyperglycemia, which determines the accumulation of oxidative stress and cell damage, which ultimately leads to hypoxia and neovascularization. Recent evidence has shown that hyperglycemia is responsible for the hyperexpression of HMGB1. This protein activates numerous pathways that cause the development of DR, and HMGB1 levels are constantly increased in diabetic retinas in both proliferative and non-proliferative stages of the disease. Several molecules, such as glycyrrhizin (GA), have proven effective in reducing diabetic damage to the retina through the inhibition of HMGB1. The main focus of this review is the growing amount of evidence linking HMGB1 and DR as well as the new therapeutic strategies involving this protein.

Role of HMGB1 signaling in the inflammatory process in diabetic retinopathy

High mobility group box 1 (HMGB1) is a key player in retinal inflammation. HMGB1 is a danger associated protein pattern receptor which can sense high glucose as a stressor. Increased HMGB1 levels have been found in patients with late stage diabetic retinopathy. HMGB1 can bind toll-like receptor 4 (TLR4) and the receptor for advanced glycation end-products (RAGE), leading to increased inflammation commonly through nuclear factor kappa beta (NFkB). Because diabetic patients have been found to have increased HMGB1 and RAGE levels, as well as polymorphisms of TLR4, a number of investigations have focused on inhibition of these pathways in the diabetic retina. Work in diabetic animal models and cell culture have demonstrated a number of factors that can inhibit HMGB1/TLR4/RAGE signaling. This regulation offers potential new avenues for therapeutic development. This review is focused on HMGB1 signaling and downstream pathways leading to inflammation in the diabetic retina.

Glycyrrhizin protects IGFBP-3 knockout mice from retinal damage

We previously reported that insulin-like growth factor binding protein 3 (IGFBP-3) knockout (KO) mice have neuronal and vascular damage to the retina. We also reported that glycyrrhizin, a high mobility growth factor binding protein 1 (HMGB1) inhibitor, is protective to the diabetic retina. In this study, we investigated whether glycyrrhizin could reduce neuronal and vascular damage in the IGFBP-3 KO mouse retina. We used measurements of retinal thickness, cell number in the ganglion cell layer, degenerate capillaries, reactive oxygen species (ROS) and protein levels of HMGB1, tumor necrosis factor alpha (TNFα), interleukin-1-beta (IL-1β) and sirtuin 1 (SIRT1) to determine whether glycyrrhizin could protect the retina. Data show that glycyrrhizin in the drinking water was effective in reducing neuronal damage at 2 months and vascular damage at 6 months. Glycyrrhizin reduced ROS levels at 6 months, and reduced levels of HMGB1, TNFα, and IL-1β at both 2 and 6 months. Taken together, the data suggest that glycyrrhizin is protective to the retina of IGFBP-3 KO mice through anti-inflammatory mechanisms.

Glycyrrhizin Protects the Diabetic Retina against Permeability, Neuronal, and Vascular Damage through Anti-Inflammatory Mechanisms

Damage associated molecular pattern (DAMPs), such as high mobility group box 1 (HMGB1), may be involved in retinal inflammation in response to high glucose. To test whether HMGB1 inhibition could protect the diabetic retina, C57BL/6J mice were made diabetic and treated with glycyrrhizin, a HMGB1 inhibitor, for up to six months. Measurements of permeability, neuronal, and vascular changes were done, as well as assessments of HMGB1, tumor necrosis factor alpha (TNFα), and interleukin-1-beta (IL1β) levels. Retinal endothelial cells (REC) treated with glycyrrhizin had reduced IL1β and cleaved caspase 3 levels. Data also demonstrate that glycyrrhizin effectively reduced HMGB1 levels throughout the retina, as well as maintained normal retinal permeability and retinal capillary coverage. Glycyrrhizin maintained normal cell numbers in the ganglion cell layer and prevented thinning of the retina at two months. These histological changes were associated with reduced reactive oxygen species, as well as reduced HMGB1, TNFα, and IL1β levels. The data strongly imply that HMGB1 inhibition prevented diabetic retinal changes through anti-inflammatory pathways.

ATF4 Involvement in TLR4 and LOX-1-Induced Host Inflammatory Response to Aspergillus fumigatus Keratitis

Purpose

Activating transcription factor 4 (ATF4) is induced by various stressors. Here, we investigated the expression of ATF4 in the host inflammatory response to Aspergillus fumigatus (A. fumigatus) keratitis.

Methods

A. fumigatus keratitis mouse models developed by intrastromal injection as well as corneal epithelium scratching were examined daily with a slit lamp microscope for corneal opacification and ulceration. Subsequent in vitro experimentation was carried out in human corneal epithelial cells (HCECs) as well as THP-1 macrophages infected with A. fumigatus. Inhibitors, including CLI-095, Poly (I), SCH772984, and SP600125, were used to assess the role of proteins like toll-like receptor 4 (TLR4), lectin-type oxidized LDL receptor 1 (LOX-1), extracellular signal-regulated kinases (ERK1/2), and c-Jun N-terminal kinase (JNK) in ATF4 expression as a response to A. fumigatus infection. This assessment was made in both mouse models and HCECs using western blot.

Results

Compared to the controls, ATF4 was increased in corneas from two kinds of A. fumigatus keratitis models at 3 days after infection. ATF4 expression was upregulated with A. fumigatus conidia both in HCECs and THP-1 macrophages 16 hours after stimulation. Furthermore, ATF4 expression in response to A. fumigatus infection was shown to be dependent on TLR4 and LOX-1 expression, and ERK1/2 and JNK contributed to the expression of ATF4 in response to A. fumigatus.

Conclusion

Our results clearly indicate that ATF4 was involved in the host antifungal immune response to A. fumigatus keratitis; expression was found to be dependent on TLR4, LOX-1 expression, and MAPKs pathway.

Association between miRNAs expression and signaling pathways of oxidative stress in diabetic retinopathy

Diabetic retinopathy (DR) is a major cause of vision reduction in diabetic patients. Hyperglycemia is a known instigator for the development of DR, even though the role of oxidative stress pathways in the pathogenesis of DR is established. The studies indicate that microRNAs (miRNAs) are significant to the etiology of DR; changes in miRNAs expression levels may be associated with onset and progression of DR. In addition, miRNAs have emerged as a useful disease marker due to their availability and stability in detecting the severity of DR. The relationship between miRNAs expression levels and oxidative stress pathways has been investigated in several studies. The aim of this study is the examination of function and expression levels of target miRNAs in oxidative stress pathway and pathogenesis of diabetic retinopathy.

TRAF6 mediates high glucose-induced endothelial dysfunction

To investigate the role of tumor necrosis factor-associated factor 6 (TRAF6) in high glucose-induced endothelial cell dysfunction. Human aortic endothelial cells (HAECs) were cultured in high glucose medium, and TRAF6 expression was assayed by quantitative real-time Polymerase Chain Reaction (PCR) and western blotting. The effect of TRAF6 on in vitro endothelial cell viability, apoptosis, migration, and endothelial-monocyte adhesion was investigated by gene knockdown. The expression of TRAF6 and related adhesion molecules was assayed in a mouse streptozotocin-induced type I diabetes model. The signaling pathways associated with TRAF6 effects on endothelial cells were investigated in high glucose HAEC cultures. Culture of HAECs in high glucose medium significantly increased TRAF6 mRNA and protein expression in a time dependent manner. High glucose markedly reduced HAEC viability, apoptosis, and migration, and these effects was significantly reversed by TRAF6 knockdown. High glucose significantly increased intercellular adhesion of THP-1 monocytic cells and HAECs via upregulation of ICAM-1 and VCAM-1 expression, and TRAF6 knockdown attenuated the effect on THP-1 cell adhesion. TRAF6, ICAM-1, and VCAM-1 expression were increased in aorta tissue of mice with streptozotocin-induced diabetes. The free radical scavenger N-acetyl-L-cysteine attenuated TRAF6 expression in HAECs cultured in high glucose medium, and TRAF6 knockdown inhibited high glucose-induced IκB-α degradation and JNK phosphorylation. TRAF6 mediated high glucose-induced endothelial dysfunction via NF-κB- and AP-1-dependent signaling. Targeting TRAF6 may delay progression of vascular diseases during diabetes mellitus and atherosclerosis.

MicroRNA-27a protects retinal pigment epithelial cells under high glucose conditions by targeting TLR4

The present study aimed to investigate whether microRNA-27a (miRNA27a) is associated with the pathogenesis of diabetic retinopathy, and to elucidate the underlying molecular mechanism of any potential association. In retinal pigment epithelial (RPE) cells treated with high glucose, miRNA27a expression, determined by reverse transcription-quantitative polymerase chain reaction analysis, was decreased. Caspase-3/9 activity and B-cell lymphoma 2-associated X (Bax) protein expression was increased in RPE cells subjected to high glucose. Inhibition of miRNA27a suppressed the viability, and increased the caspase-3/9 activity and Bax protein expression of RPE cells treated with high glucose. Inhibition of miRNA27a expression also increased the expression of interleukin (IL)-6, IL-1β, tumor necrosis factor (TNF)-α and Toll-like receptor 4 (TLR4) in RPE cells treated with high glucose. Treatment with immunostimulatory (is)RNA directed against TLR4 was observed to inhibit caspase-3/9 activity, decrease the expression of TLR4, Bax, IL-6, IL-1β and TNF-α, and increase the viability of RPE cells subjected to high glucose following the inhibition miRNA27a. In conclusion, the results of the present study suggest that miRNA27a protects RPE cells subjected to high glucose via inhibiting inflammation and apoptosis through targeting TLR4.

Gastrodin inhibits high glucose‑induced human retinal endothelial cell apoptosis by regulating the SIRT1/TLR4/NF‑κBp65 signaling pathway

Diabetic retinopathy (DR), one of the most common complications of late‑phase diabetes, is associated with the ectopic apoptosis of microvascular cells. Gastrodin, a phenolic glucoside derived from Gastrodia elata Blume, has been reported to have antioxidant and anti‑inflammation activities. The aim of the present study was to investigate the effects of gastrodin on high glucose (HG)‑induced human retinal endothelial cell (HREC) injury and its underlying mechanism. The results demonstrated that HG induced cell apoptosis in HRECs, which was accompanied by increased levels of reactive oxygen species production. Gastrodin treatment significantly alleviated HG‑induced apoptosis and oxidative stress. Furthermore, HG stimulation decreased the levels of SIRT1, which was accompanied by an increase in Toll‑like receptor 4 (TLR4) expression and the levels of phosphorylated nuclear factor (NF)‑κBp65. However, the administration of gastrodin significantly inhibited the activation of the sirtuin 1 (SIRT1)/TLR4/NF‑κBp65 signaling pathway in HRECs exposed to HG. Collectively, the present study demonstrated that gastrodin may be effective against HG‑induced apoptosis and its action may be exerted through the regulation of the SIRT1/TLR4/NF‑κBp65 signaling pathway.

Loss of TLR4 in mouse Müller cells inhibits both MyD88-dependent and -independent signaling

Müller cells are key to metabolic and ionic regulation in the retina. They also produce a number of inflammatory mediators and are significantly affected in diabetic retinopathy. To investigate the role of toll-like receptor 4 (TLR4) in retinal Müller cells, we crossed TLR4 floxed with PDGFRα-Cre mice to eliminate TLR4 in retinal Müller cells. We performed Western blotting and ELISA analyses to determine whether loss of TLR4 affected myeloid differentiation primary response protein (MyD88)-dependent or –independent signaling, leading to reduced levels of tumor necrosis factor alpha (TNFα) and interleukin 1 beta (IL1β) in whole retinal lysates from the TLR4 floxed and TLR4-PDGFRα-Cre mice. Data show that TLR4-PDGFRα-Cre mice have reduced levels of both the MyD88-dependent and -independent signaling pathways. These studies confirm successful development of a Müller cell-specific TLR4 knockout mouse colony. These mice have reduced MyD88-dependent and –independent signaling pathway proteins, as well as reduced TNFα and IL1β levels. These mice can be used to dissect TLR4 signaling in disorders affecting retinal Müller cells.

Toll-like receptor 4 regulates insulin signal transduction in retinal Müller cells

Dysfunctional insulin signalling is a causative factor in type-2 diabetes. While insulin signal transduction has been well investigated in many tissues, less is known in retinal tissues. We have previously reported that toll-like receptor 4 (TLR4) is involved in retinal damage in diabetes. We used TLR4 retinal Müller cell-specific knockout mice and Müller cells in culture to investigate the effects of loss of TLR4 on Müller cell insulin signal transduction. Loss of TLR4 in the mouse retinal Müller cells led to increased insulin receptor and Akt phosphorylation, with reduced insulin receptor substrate 1 (IRS-1) phosphorylation on serine 307, which was associated with reduced cleavage of caspase 3. In retinal Müller cells grown in high glucose, insulin signal transduction was impaired, but these responses were reduced with cells were transfected with TLR4 siRNA. Taken together, the data suggest that TLR4 regulates insulin signal transduction in retinal Müller cells.

Toll-Like Receptor 4 Reduces Occludin and Zonula Occludens 1 to Increase Retinal Permeability Both in vitro and in vivo

We reported that β-adrenergic receptors regulate toll-like receptor 4 (TLR4) signaling in the retina of diabetic mice and in retinal endothelial cells (REC) and Müller cells. We hypothesized that TLR4 regulates retinal permeability both in vitro and in vivo in the retinal vasculature. We used REC cultured in normal and high-glucose conditions and TLR4 siRNA treatments for cell culture studies of permeability and protein analyses of tumor necrosis factor α, occludin, and zonula occludens 1 (ZO-1). We used endothelial cell (EC)-specific Cre-Lox TLR4 knockout mice to study retinal permeability and neuronal and vascular changes following exposure to ocular ischemia/reperfusion (I/R) used as a retinal stressor. We found that the loss of TLR4 in the EC led to the reduced permeability following I/R and fewer degenerate capillaries. Retinal permeability was increased in REC grown in high-glucose conditions but was inhibited by TLR4 siRNA treatment. High-glucose culture conditions significantly reduced occludin and ZO-1 levels in REC, and TLR4 siRNA treatment restored levels to baseline. In conclusion, these studies demonstrate that TLR4 in EC strongly regulates retinal permeability and neuronal and vascular changes following exposure to stressors such as I/R.

TLR4 regulates insulin-resistant proteins to increase apoptosis in the mouse retina

OBJECTIVE AND DESIGN

Work in multiple organs has suggested that toll-like receptor 4 (TLR4) may play a role in insulin resistance. Additional studies have shown a negative role for TLR4 on retinal health. We have previously reported that β-adrenergic receptors can regulate both TLR4 signal transduction, as well as insulin signaling in the retina and in retinal endothelial cells. Thus, we hypothesized that TLR4 would regulate retinal insulin signaling.

MATERIALS AND METHODS

We used endothelial cell-specific TLR4 knockout mice, as well as TLR4-overexpressing mice for these studies.

METHODS

Western blotting and ELISA analyses were done for investigations of insulin receptor, insulin receptor substrate 1 (IRS-1) serine 307, and Akt phosphorylation, as well as cleaved caspase 3 levels in the mouse retina.

RESULTS

We found that loss of TLR4 led to increased insulin receptor and Akt phosphorylation, as well as decreased IRS-1^Ser307 levels. In support of these results, TLR4 overexpression decreased insulin signaling and the cleavage of caspase 3.

CONCLUSIONS

Therefore, these results suggest that TLR4 plays a key role in insulin signaling in the retina. Reduction of TLR4 levels may be protective to the retina.

Inhibition of HMGB1 protects the retina from ischemia-reperfusion, as well as reduces insulin resistance proteins

The role of inflammation in diabetic retinal amage is well accepted. While a number of cytokines and inflammatory mediators are responsible for these changes, upstream regulators are less well studied. Additionally, the role for these upstream mediators in retinal health is unclear. In this study, we hypothesized that inhibition of high mobility group box 1 (HMGB1) could restore normal insulin signaling in retinal endothelial cells (REC) grown in high glucose, as well as protect the retina against ischemia/reperfusion (I/R)-induced retinal damage. REC were grown in normal (5mM) or high glucose (25mM) and treated with Box A or glycyrrhizin, two different HMGB1 inhibitors. Western blotting was done for HMGB1, toll-like receptor 4 (TLR4), insulin receptor, insulin receptor substrate-1 (IRS-1), and Akt. ELISA analyses were done for tumor necrosis factor alpha (TNFα) and cleaved caspase 3. In addition, C57/B6 mice were treated with glycyrrhizin, both before and after ocular I/R. Two days following I/R, retinal sections were processed for neuronal changes, while vascular damage was measured at 10 days post-I/R. Results demonstrate that both Box A and glycyrrhizin reduced HMGB1, TLR4, and TNFα levels in REC grown in high glucose. This led to reduced cleavage of caspase 3 and IRS-1^Ser307 phosphorylation, and increased insulin receptor and Akt phosphorylation. Glycyrrhizin treatment significantly reduced loss of retinal thickness and degenerate capillary numbers in mice exposed to I/R. Taken together, these results suggest that inhibition of HMGB1 can reduce retinal insulin resistance, as well as protect the retina against I/R-induced damage.

Epac1 Blocks NLRP3 Inflammasome to Reduce IL-1β in Retinal Endothelial Cells and Mouse Retinal Vasculature

Inflammation is an important component of diabetic retinal damage. We previously reported that a novel β-adrenergic receptor agonist, Compound 49b, reduced Toll-like receptor 4 (TLR4) signaling in retinal endothelial cells (REC) grown in high glucose. Others reported that TLR4 activates high-mobility group box 1 (HMGB1), which has been associated with the NOD-like receptor 3 (NLRP3) inflammasome. Thus, we hypothesized that Epac1, a downstream mediator of β-adrenergic receptors, would block TLR4/HMGB1-mediated stimulation of the NLRP3 inflammasome, leading to reduced cleavage of caspase-1 and interleukin-1 beta (IL-1β). We generated vascular specific conditional knockout mice for Epac1 and used REC grown in normal and high glucose treated with an Epac1 agonist and/or NLRP3 siRNA. Protein analyses were done for Epac1, TLR4, HMGB1, NLRP3, cleaved caspase-1, and IL-1β. Loss of Epac1 in the mouse retinal vasculature significantly increased all of the inflammatory proteins. Epac1 effectively reduced high glucose-induced increases in TLR4, HMGB1, cleaved caspase-1, and IL-1β in REC. Taken together, the data suggest that Epac1 reduces formation of the NLRP3 inflammasome to reduce inflammatory responses in the retinal vasculature.

Epac1 agonist decreased inflammatory proteins in retinal endothelial cells, and loss of Epac1 increased inflammatory proteins in the retinal vasculature of mice

PURPOSE

Increased inflammatory mediator levels are reported in diabetic retinopathy. We previously reported that β-adrenergic receptor agonists reduced inflammatory mediators in the diabetic retina; however, these agents cannot be given systemically. Here, we investigated whether Epac1 is key to the protective effects of β-adrenergic receptor agonists.

METHODS

We cultured primary human retinal endothelial cells (RECs) in normal (5 mM) or high (25 mM) glucose and treated them with an Epac1-specific agonist. Additionally, we generated Epac1 conditional vascular endothelial cell knockout mice by breeding Epac1 floxed mice with Cdh5 Cre mice to investigate the role of Epac1 in the retinal levels of tumor necrosis factor alpha (TNF-α), interleukin-1β (IL-1β), nuclear factor kappa beta (NFκB), and inhibitor of kappa beta (IκB). Confocal microscopy was performed to localize Epac1 in the mouse retina.

RESULTS

Data showed that high glucose increased the TNF-α and IL-1β levels in the RECs, which were reduced cells treated with the Epac1 agonist. The loss of Epac1 in the retinas of the conditional knockout mice resulted in statistically significantly increased levels of TNF-α and IL-1β, as well as NFκB.

CONCLUSIONS

These data indicate that Epac1 may be protective to the retina through inhibition of key inflammatory mediators.