Potential Role of Endoplasmic Reticulum Stress in Pathogenesis of Diabetic Retinopathy

Streptozotocin-Induced Diabetic Rats Showed a Differential Glycine Receptor Expression in the Spinal Cord: A GlyR Role in Diabetic Neuropathy

In the spinal cord, attenuation of the inhibitory action of glycine is related to an increase in both inflammatory and diabetic neuropathic pain; however, the glycine receptor involvement in diabetic neuropathy has not been reported. We determined the expression of the glycine receptor subunits (α1-α3 and β) in streptozotocin-induced diabetic Long-Evans rats by qPCR and Western blot. The total mRNA and protein expression (whole spinal cord homogenate) of the α1, α3, and β subunits did not change during diabetes; however, the α2 subunit mRNA, but not the protein, was overexpressed 45 days after diabetes induction. By contrast, the synaptic expression of the α1 and α2 subunits decreased in all the studied stages of diabetes, but that of the α3 subunit increased on day 45 after diabetes induction. Intradermal capsaicin produced higher paw-licking behavior in the streptozotocin-induced diabetic rats than in the control animals. In addition, the nocifensive response was higher at 45 days than at 20 days. During diabetes, the expression of the glycine receptor was altered in the spinal cord, which strongly suggests its involvement in diabetic neuropathy.

Neurovascular Cell Death and Therapeutic Strategies for Diabetic Retinopathy

Diabetic retinopathy (DR) is a major complication of diabetes and a leading cause of blindness worldwide. DR was recently defined as a neurovascular disease associated with tissue-specific neurovascular impairment of the retina in patients with diabetes. Neurovascular cell death is the main cause of neurovascular impairment in DR. Thus, neurovascular cell protection is a potential therapy for preventing the progression of DR. Growing evidence indicates that a variety of cell death pathways, such as apoptosis, necroptosis, ferroptosis, and pyroptosis, are associated with neurovascular cell death in DR. These forms of regulated cell death may serve as therapeutic targets for ameliorating the pathogenesis of DR. This review focuses on these cell death mechanisms and describes potential therapies for the treatment of DR that protect against neurovascular cell death.

Thioredoxin 1 overexpression attenuated diabetes-induced endoplasmic reticulum stress in Müller cells via apoptosis signal-regulating kinase 1

As one of the common and serious chronic complications of diabetes mellitus (DM), the related mechanism of diabetic retinopathy (DR) has not been fully understood. Müller cell reactive gliosis is one of the early pathophysiological features of DR. Therefore, exploring the manner to reduce diabetes-induced Müller cell damage is essential to delay DR. Thioredoxin 1 (Trx1), one of the ubiquitous redox enzymes, plays a vital role in redox homeostasis via protein-protein interactions, including apoptosis signal-regulating kinase 1 (ASK1). Previous studies have shown that upregulation of Trx by some drugs can attenuate endoplasmic reticulum stress (ERS) in DR, but the related mechanism was unclear. In this study, we used DM mouse and high glucose (HG)-cultured human Müller cells as models to clarify the effect of Trx1 on ERS and the underlying mechanism. The data showed that the diabetes-induced Müller cell damage was increased significantly. Moreover, the expression of ERS and reactive gliosis was also upregulated in diabetes in vivo and in vitro. However, it was reversed after Trx1 overexpression. Besides, ERS-related protein expression, reactive gliosis, and apoptosis were decreased after transfection with ASK1 small-interfering RNA in stable Trx1 overexpression Müller cells after HG treatment. Taken together, Trx1 could protect Müller cells from diabetes-induced damage, and the underlying mechanism was related to inhibited ERS via ASK1.

Lactucaxanthin Regulates the Cascade of Retinal Oxidative Stress, Endoplasmic Reticulum Stress and Inflammatory Signaling in Diabetic Rats

PURPOSE

The purpose of this study is to investigate the protective mechanism of lactucaxanthin against retinal angiogenesis in diabetic retinopathy.

METHODS

Streptozotocin-induced diabetic rats were orally gavaged with either lactucaxanthin or lutein (n=12/group) for 8 weeks. Serum and retina collected from euthanized rats were subjected to assess oxidative stress, ER stress and inflammatory response.

RESULTS

Lactucaxanthin administration was found to lower oxidative stress markers (protein carbonylation and lipid peroxidation) by augmenting antioxidant activity expression and ameliorated VEGF-A levels in diabetic group. Likewise, it suppressed the expression of ER stress (ATF4, ATF6, and XBP1), and inflammatory (TNF-α, IL-6, NF-κB, and ICAM-1) markers in diabetic retina. In addition, lactucaxanthin improved glucose tolerance and lipid profile under diabetic condition and suppressed the crosstalk between OS, ER stress, and inflammation.

CONCLUSION

Lactucaxanthin could be used as a promising therapeutic bioactive for treating DR condition, and retinal angiogenesis.

EXPERT OPINION

Limitation of the study includes the sample size and the duration of treatment. Despite these limitations, this study has revealed the potential of lactucaxanthin in treating eye related diabetic complications. To validate the results obtained from this study, clinical study must be performed to understand the relative benefit of lactucaxanthin in DR treatment.

Retinal Nrf2 expression in normal and early streptozotocin-diabetic rats

Diabetic retinopathy is the most common cause of vision loss among diabetic patients. Although hyperglycemia produces retinal oxidative stress in long-standing diabetes, the pathogenesis mechanism is unknown. The Nuclear factor erythroid 2-related factor 2 (Nrf2) plays a central role in cell responses against oxidative damage. We used adult Long Evans rats where diabetes was induced by streptozotocin. Normal and treated rats were sacrificed at 7, 20, and 45 days after streptozotocin injection. We analyzed Nrf2 and Keap1 expression in retinal homogenates, cytoplasmic, and nuclear retinal fractions. Normal retina showed Nrf2 expression in all retina nuclear layers. We found a transitory decrease of Nrf2 mRNA and protein expression at 7 and 20 days after the streptozotocin injection that recovered later on: moreover, the protein level increased after 45 days. Keap1 immunoprecipitation revealed similar levels as Nrf2 in normal and diabetic rat retinas, indicating that the diabetic condition did not lead to dissociation of the Keap1-Nrf2 complex. Indeed, glutathione levels and superoxide dismutase activity were not altered in the treated rat retinas. These results do not support oxidative stress in the retina shortly after diabetes induction.

In Vivo Anti-Inflammation Potential of Aster koraiensis Extract for Dry Eye Syndrome by the Protection of Ocular Surface

Dry eye syndrome (DES) is a corneal disease often characterized by an irritating, itching feeling in the eyes and light sensitivity. Inflammation and endoplasmic reticulum (ER) stress may play a crucial role in the pathogenesis of DES, although the underlying mechanism remains elusive. Aster koraiensis has been used traditionally as an edible herb in Korea. It has been reported to have wound-healing and inhibitory effects against insulin resistance and inflammation. Here, we examined the inhibitory effects of inflammation and ER stress by A. koraiensis extract (AKE) in animal model and human retinal pigmented epithelial (ARPE-19) cells. Oral administration of AKE mitigated DE symptoms, including reduced corneal epithelial thickness, increased the gap between lacrimal gland tissues in experimental animals and decreased tear production. It also inhibited inflammatory responses in the corneal epithelium and lacrimal gland. Consequently, the activation of NF-κB was attenuated by the suppression of cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2). Moreover, AKE treatment ameliorated TNF-α-inducible ocular inflammation and thapsigargin (Tg)-inducible ER stress in animal model and human retinal pigmented epithelial (ARPE-19) cells. These results prove that AKE prevents detrimental functional and histological remodeling on the ocular surface and in the lacrimal gland through inhibition of inflammation and ER stress, suggesting its potential as functional food material for improvement of DES.

Systemic hypoxia led to little retinal neuronal loss and dramatic optic nerve glial response

Vision loss is a devastating consequence of systemic hypoxia, but the cellular mechanisms are unclear. We investigated the impact of acute hypoxia in the retina and optic nerve. We induced systemic hypoxia (10% O₂) in 6-8w mice for 48 h and performed in vivo imaging using optical coherence tomography (OCT) at baseline and after 48 h to analyze structural changes in the retina and optic nerve. We analyzed glial cellular and molecular changes by histology and immunofluorescence and the impact of pretreatment with 4-phenylbutyric acid (4-PBA) in oligodendroglia survival. After 48 h hypoxia, we found no change in ganglion cell complex thickness and no loss of retinal ganglion cells. Despite this, there was significantly increased expression of CCAAT-enhancer-binding protein homologous protein (CHOP), a marker of endoplasmic reticulum stress, in the retina and optic nerve. In addition, hypoxia induced obvious increase of GFAP expression in the anterior optic nerve, where it co-localized with CHOP, and significant loss of Olig2⁺ oligodendrocytes. Pretreatment with 4-PBA, which has been shown to reduce endoplasmic reticulum stress, rescued total Olig2⁺ oligodendrocytes and increased the pool of mature (CC-1⁺) but not of immature (PDGFRa+) oligodendrocytes. Consistent with a selective vulnerability of the retina and optic nerve in hypoxia, the most striking changes in the 48 h murine model of hypoxia were in glial cells in the optic nerve, including increased CHOP expression in the astrocytes and loss of oligodendrocytes. Our data support a model where glial dysfunction is among the earliest events in systemic hypoxia - suggesting that glia may be a novel target in treatment of hypoxia.

Metabolomic profile of diabetic retinopathy: a GC-TOFMS-based approach using vitreous and aqueous humor

AIM

To identify the potential metabolite markers in diabetic retinopathy (DR) by using gas chromatography coupled with time-of-flight mass spectrometry (GC-TOFMS).

METHODS

GC-TOFMS spectra were acquired from vitreous and aqueous humor (AH) samples of patients with DR and non-diabetic participants. Comparative analysis was used to elucidate the distinct metabolites of DR. Metabolic pathway was employed to explicate the metabolic reprogramming pathways involved in DR. Logistic regression and receiver-operating characteristic analyses were carried out to select and validate the biomarker metabolites and establish a therapeutic model.

RESULTS

Comparative analysis showed a clear separation between disease and control groups. Eight differentiating metabolites from AH and 15 differentiating metabolites from vitreous were highlighted. Out of these 23 metabolites, 11 novel metabolites have not been detected previously. Pathway analysis identified nine pathways (three in AH and six in vitreous) as the major disturbed pathways associated with DR. The abnormal of gluconeogenesis, ascorbate-aldarate metabolism, valine-leucine-isoleucine biosynthesis, and arginine-proline metabolism might weigh the most in the development of DR. The AUC of the logistic regression model established by D-2,3-Dihydroxypropanoic acid, isocitric acid, fructose 6-phosphate, and L-Lactic acid in AH was 0.965. The AUC established by pyroglutamic acid and pyruvic acid in vitreous was 0.951.

CONCLUSIONS

These findings have expanded our understanding of identified metabolites and revealed for the first time some novel metabolites in DR. These results may provide useful information to explore the mechanism and may eventually allow the development of metabolic biomarkers for prognosis and novel therapeutic strategies for the management of DR.

Water-soluble C60 fullerene ameliorates astroglial reactivity and TNFa production in retina of diabetic rats

The complications of both first and second types of diabetes mellitus patients are important cause of decline in quality of life and mortality worldwide. Diabetic retinopathy (DR) is a widespread complication that affects almost 60% of patients with prolonged (at least 10–15 years) diabetes. The critical role of glial cells has been shown in retinopathy initiation in the last decades. Furthermore, glial reactivity and inflammation could be key players in early pathogenesis of DR. Despite the large amount of research data, the approaches of effective DR therapy remain unclear. The progress of DR is accompanied by pro-inflammatory and pro-oxidative changes in retinal cells including astrocytes and Muller cells. Glial reactivity is a key pathogenetic factor of various disorders in neural tissue. Fullerene C60 nanoparticles were confirmed for both antioxidant and anti-inflammatory capability. In the presented study glioprotective efficacy of water-soluble hydrated fullerene C60 (C60HyFn) was tested in a STZ-diabetes model during 12 weeks. Exposure of the STZ-diabetic rat group to C60HyFn ameliorated the astrocyte reactivity which was determined via S100β and PARP1 overexpression. Moreover, C60HyFn induced the decrease of TNFα production in the retina of STZ-diabetic rats. By contrast, the treatment with C60HyFn of the normal control rat group didn’t change the content of all abovementioned markers of astrogliosis and inflammation. Thus, diabetes-induced abnormalities in the retina were suppressed via the anti-oxidant, anti-inflammatory and glioprotective effects of C60HyFn at low doses. The presented results demonstrate that C60HyFn can ensure viability of retinal cells viability through glioprotective effect and could be a new therapeutic nano-strategy of DR treatment.

Heme, Heme Oxygenase, and Endoplasmic Reticulum Stress-A New Insight into the Pathophysiology of Vascular Diseases

The prevalence of vascular disorders continues to rise worldwide. Parallel with that, new pathophysiological pathways have been discovered, providing possible remedies for prevention and therapy in vascular diseases. Growing evidence suggests that endoplasmic reticulum (ER) stress is involved in a number of vasculopathies, including atherosclerosis, vascular brain events, and diabetes. Heme, which is released from hemoglobin or other heme proteins, triggers various pathophysiological consequence, including heme stress as well as ER stress. The potentially toxic free heme is converted by heme oxygenases (HOs) into carbon monoxide (CO), iron, and biliverdin (BV), the latter of which is reduced to bilirubin (BR). Redox-active iron is oxidized and stored by ferritin, an iron sequestering protein which exhibits ferroxidase activity. In recent years, CO, BV, and BR have been shown to control cellular processes such as inflammation, apoptosis, and antioxidant defense. This review covers our current knowledge about how heme induced endoplasmic reticulum stress (HIERS) participates in the pathogenesis of vascular disorders and highlights recent discoveries in the molecular mechanisms of HO-mediated cytoprotection in heme stress and ER stress, as well as crosstalk between ER stress and HO-1. Furthermore, we focus on the translational potential of HIERS and heme oxygenase-1 (HO-1) in atherosclerosis, diabetes mellitus, and brain hemorrhage.

Associations between Fatty Acid-Binding Protein 4⁻A Proinflammatory Adipokine and Insulin Resistance, Gestational and Type 2 Diabetes Mellitus

There is ample scientific evidence to suggest a link between the fatty acid-binding protein 4 (FABP4) and insulin resistance, gestational (GDM), and type 2 (T2DM) diabetes mellitus. This novel proinflammatory adipokine is engaged in the regulation of lipid metabolism at the cellular level. The molecule takes part in lipid oxidation, the regulation of transcription as well as the synthesis of membranes. An involvement of FABP4 in the pathogenesis of obesity and insulin resistance seems to be mediated via FABP4-dependent peroxisome proliferator-activated receptor γ (PPARγ) inhibition. A considerable number of studies have shown that plasma concentrations of FABP4 is increased in obesity and T2DM, and that circulating FABP4 levels are correlated with certain clinical parameters, such as body mass index, insulin resistance, and dyslipidemia. Since plasma-circulating FABP4 has the potential to modulate the function of several types of cells, it appears to be of extreme interest to try to develop potential therapeutic strategies targeting the pathogenesis of metabolic diseases in this respect. In this manuscript, representing a detailed review of the literature on FABP4 and the abovementioned metabolic disorders, various mechanisms of the interaction of FABP4 with insulin signaling pathways are thoroughly discussed. Clinical aspects of insulin resistance in diabetic patients, including women diagnosed with GDM, are analyzed as well.

Association between endoplasmic reticulum stress and risk factors of diabetic retinopathy

Diabetic retinopathy (DR) is one of the most common and challenging ocular complications of diabetes mellitus. As a chronic, progressive ocular disease that poses a serious threat to vision, DR has gradually become a leading cause of blindness worldwide. Emerging evidence points to an important role of endoplasmic reticulum (ER) stress in not only maintaining the steady-state equilibrium in the body, but also in intracellular synthesis, protein folding, and other essential functions. Recent studies have demonstrated clear associations between ER stress-related physiological functions and the pathogenesis of DR. When cells are stimulated by external stimuli, UPR pathway is activated firstly to protect it. However, long-term harmful factors can induce ER stress. which interferes with the physiological metabolism of retinal cells and participates in the occurrence of DR via the ATF6 pathway, PERK pathway and IRE1 pathway. At present, ER stress blocker is expected to become a new anti-DR therapy. Thus, understanding the relationship between ER stress and DR will help to develop new effective preventative treatments. In this review, we summarize the risk factors of DR pathogenesis induced by ER stress toward revealing potentially new therapeutic targets.

Mitochondrial activity in different regions of the brain at the onset of streptozotocin-induced diabetes in rats

Diabetes affects a variety of tissues including the central nervous system; moreover, some evidence indicates that memory and learning processes are disrupted. Also, oxidative stress triggers alterations in different tissues including the brain. Recent studies indicate mitochondria dysfunction is a pivotal factor for neuron damage. Therefore, we studied mitochondrial activity in three brain regions at early type I-diabetes induction. Isolated mitochondria from normal hippocampus, cortex and cerebellum revealed different rates of oxygen consumption, but similar respiratory controls. Oxygen consumption in basal state 4 significantly increased in the mitochondria from all three brain regions from diabetic rats. No relevant differences were observed in the activity of respiratory complexes, but hippocampal mitochondrial membrane potential was reduced. However, ATP content, mitochondrial cytochrome c, and protein levels of β-tubulin III, synaptophysin, and glutamine synthase were similar in brain regions from normal and diabetic rats. In addition, no differences in total glutathione levels were observed between normal and diabetic rat brain regions. Our results indicated that different regions of the brain have specific metabolic responses. The changes in mitochondrial activity we observed at early diabetes induction did not appear to cause metabolic alterations, but they might appear at later stages. Longer-term streptozotocin treatment studies must be done to elucidate the impact of hyperglycemia in brain metabolism and the function of specific brain regions.

Lutein and Zeaxanthin Isomers Protect against Light-Induced Retinopathy via Decreasing Oxidative and Endoplasmic Reticulum Stress in BALB/cJ Mice

Oxidative stress (OS) and endoplasmic reticulum stress (ERS) are the major factors underlying photoreceptor degeneration. Lutein, RR-zeaxanthin (3R,3’R-zeaxanthin) and RS (meso)-zeaxanthin (3R,3’S-RS- zeaxanthin) (L/Zi) could protect against cell damage by ameliorating OS in retina. In this study, we examined the effect of L/Zi supplementation in a mouse model of photoreceptor degeneration and investigated whether the treatment of L/Zi ameliorated OS and ERS. BALB/cJ mice after light exposure were used as the animal model. The protective effects of L/Zi were observed by electroretinography (ERG) and terminal deoxyuridine triphosphate nick-end labeling (TUNEL) analysis. The underlying mechanisms related to OS and ERS were explored by Western blotting. After L/Zi treatment, the ERG amplitudes were significantly higher, and the number of TUNEL-positive cells was significantly reduced compared to that of the vehicle group. Western blotting results revealed that OS was ameliorated according to the significant downregulation of phosphorylated c-Jun N-terminal kinase (p-JNK), and significant upregulation of nuclear factor erythroid 2-related factor 2 (Nrf2). In addition, ERS was reduced according to the significant downregulation of 78 kDa glucose-regulated protein (GRP78), phosphorylated protein kinase RNA-like endoplasmic reticulum kinase (p-PERK), activating transcription factor 4 (ATF4) and activating transcription factor (ATF6). Our data shows that L/Zi provided functional and morphological preservation of photoreceptors against light damage, which is probably related to its mitigation of oxidative and endoplasmic reticulum stress.

Circulating Serum Fatty Acid-Binding Protein 4 Levels Predict the Development of Diabetic Retinopathy in Type 2 Diabetic Patients

PURPOSE

Fatty acid-binding protein 4 (FABP4) has been implicated in the pathology of diabetes and macrovascular diseases. Serum FABP4 levels were determined in type 2 diabetic patients without diabetic retinopathy (DR) at admission in order to investigate a possible contribution of FABP4 to the increased risk of 5-year incidence of DR.

DESIGN

Cohort study.

METHODS

A total of 738 patients with type 2 diabetes without DR were consecutively enrolled and followed up prospectively. Retinopathy evaluation was annually performed by ophthalmologists in the following 5 years. Multivariate analyses were performed using logistic regression models.

RESULTS

During the follow-up period, 152 (20.60% [95% CI: 17.68%-23.51%]) patients developed DR and 60 (8.13% [95% CI: 6.16%-10.10%]) patients developed vision-threatening DR (VTDR). Nonparametric Spearman rank correlation revealed a statistically significant positive correlation between serum FABP 4 level and international Clinical Diabetic Retinopathy Severity Scales (r = 0.348; P < .001). After adjustment for other established risk factors, in multivariate models comparing the third and fourth quartiles against the first quartile of the FABP4, levels of FABP4 were associated with DR and the adjusted risk of DR increased by 124% (OR = 2.24 [95% CI 1.65-3.68], P = .006) and 227% (3.27 [2.04-5.56], P < .001), respectively. Similarly, the adjusted risk of VTDR increased by 140% (OR = 2.40 [95% CI 1.32-3.82], P = .001) and 278% (3.78 [2.17-6.59], P < .001), respectively.

CONCLUSION

FABP4 shows potential as a novel biomarker for DR prediction in Chinese patients with T2DM, and strict glycemic control and more frequent retinal examination should be highlighted for T2DM patients with the highest quartile range of FABP4.

Thioredoxin is implicated in the anti‑apoptotic effects of grape seed proanthocyanidin extract during hyperglycemia

Diabetic retinopathy has long been recognized as a microvascular disease, however, recent research has indicated that diabetic retinopathy may also be considered a neurodegenerative disease. The elucidation of the molecular mechanisms underlying the development of diabetic retinopathy is imperative for the development of preventive and treatment strategies for patients with diabetes. In the present study, grape seed proanthocyanidin extract (GSPE) was used to upregulate the expression of thioredoxin (Trx), in order to evaluate its potential as a novel agent for the prevention and treatment of neurodegenerative diseases, including diabetic retinopathy. Hematoxylin and eosin staining was performed to observe the morphology of retinal neurons, whereas flow cytometry and terminal deoxynucleotidyl transferase 2'‑deoxyuridine, 5'‑triphosphate nick‑end labeling were employed to investigate cellular apoptosis. Reverse transcription‑quantitative polymerase chain reaction and western blot analysis were performed to assess the mRNA and protein expression of target proteins in order to investigate the underlying molecular mechanisms. In vivo, it was found that the photoreceptor cell was damaged in diabetic mice but following GSPE treatment, the process could be inhibited. In vitro, the results of the current study demonstrated that, under hyperglycemic culture conditions, the expression of 78 kDa glucose‑regulated protein, which is an endoplasmic reticulum stress marker, was upregulated. In addition, the expression of Trx was downregulated and cell apoptosis was enhanced. Notably, treatment with GSPE was revealed to inhibit the neurodegenerative process induced by hyperglycemia. However, treatment with the Trx inhibitor PX12 in combination with GSPE was demonstrated to potentiate apoptosis compared with GSPE treatment alone under hyperglycemic conditions. Furthermore, the protein expression of apoptosis signal‑regulating kinase (ASK) 1 and Trx‑interacting protein (Txnip) was also upregulated by hyperglycemia, whereas GSPE was revealed to counteract this upregulation. In conclusion, the results of the present study indicate that Trx may be implicated in the mechanisms underlying the protective effects of GSPE against hyperglycemia‑induced cell degeneration and apoptosis. The molecular mechanisms may also involve inhibition of the activation of the Trx/ASK1/Txnip signaling pathway.