The glucagon like peptide 1 analogue, exendin-4, attenuates oxidative stress-induced retinal cell death in early diabetic rats through promoting Sirt1 and Sirt3 expression.Exp Eye Res. 2016;151:203-211. [PMID: 27212443 DOI: 10.1016/j.exer.2016.05.002]

Glucagon-Like Peptide 1 Receptor Agonist Stimulation Inhibits Laser-Induced Choroidal Neovascularization by Suppressing Intraocular Inflammation

Purpose

The glucagon-like peptide-1 receptor (GLP-1R), a diabetes therapy target, is expressed in multiple organs and is associated with neuroprotective, anti-inflammatory, and antitumor effects, particularly in cardiac and cerebral tissues. Although GLP-1's role in diabetic and ischemic retinopathies is well-studied, its influence on choroidal neovascularization (CNV) in exudative age-related macular degeneration (AMD) remains unclear. This study explored the effects of GLP-1 on CNV using a laser-induced mouse model.

Methods

The anti-angiogenic effects of GLP-1 were tested using ex vivo sprouting assays in 3-week-old C57BL/6J mice. In 6-week-old mice, GLP-1R localization in laser-induced CNV lesions was analyzed via immunohistochemistry. Liraglutide, a GLP-1R agonist, was administered subcutaneously for 7 days or by single intravitreal injection post-laser. Eyeballs collected on days 1 to 7 post-laser were analyzed using RT-qPCR for GLP-1R expression and inflammatory cytokines.

Results

GLP-1R-positive cells were detected in CNV lesions and were expressed in Iba-1-positive activated microglia or macrophages. They also expressed in abnormal retinal pigment epithelial cells and surrounding normal endothelial cells. NOD-like receptor protein 3 (NLRP3) inflammasome signaling was observed near CNV. Liraglutide inhibited angiogenesis in ex vivo assays and significantly reduced CNV formation with both subcutaneous and intravitreal administration. Additionally, Liraglutide inhibited expression of NLRP3, IL-1β, IL-6, and TNF expression compared with healthy controls. Intravitreal GLP-1R antagonist reduced subcutaneous effects.

Conclusions

Liraglutide suppresses CNV formation, likely via NLRP3 inflammasome inhibition. Intraocular GLP-1R appears to mediate anti-CNV effects, supporting GLP-1R agonists as potential adjunctive therapy for exudative AMD and warranting further investigation into its safety and clinical feasibility.

SIRT3 mitigates high glucose-induced damage in retinal microvascular endothelial cells via OPA1-mediated mitochondrial dynamics

Oxidative stress in endothelial cells is pivotal in diabetic retinopathy (DR), with mitochondrial homeostasis being crucial to mitigate this stress. This study explored the roles of mitochondrial sirtuins (SIRTs) in high glucose (HG)-induced oxidative stress, related endothelial impairment, and mitochondrial homeostasis damage in rat retinal microvascular endothelial cells (RMECs). RMECs were cultured under HG or equivalent osmotic conditions. Cell viability was assessed using the Cell Counting Kit-8 assay, whereas cell death and survival were determined via calcein-AM/propidium iodide double staining. Reactive oxygen species (ROS) levels were measured using 2',7'-dichlorofluorescein fluorescence. Expression of mitochondrial SIRTs3-5 and key mitochondrial homeostasis molecules was quantified by the quantitative real-time polymerase chain reaction and confirmed by western blotting. Mitochondrial morphology was evaluated using electron microscopy and the MitoTracker fluorescent probe. A SIRT3-overexpressing RMEC line was constructed to assess the role of SIRT3 in oxidative stress and mitochondrial dynamics. After 48 h of HG exposure, cell viability was significantly reduced, with a concomitant increase in cell death and ROS levels, alongside a marked decrease in SIRT3 expression and molecules associated with mitochondrial dynamics. SIRT3 overexpression reversed these effects, particularly increasing the mitochondrial fusion-related molecule, optic atrophy 1 (OPA1). However, the OPA1 inhibitor, MYLS22, blocked the protective effect of SIRT3, leading to more dead cells, a higher ROS level, and intensified mitochondrial fragmentation. These results suggest that SIRT3 is involved in HG-induced imbalanced mitochondrial dynamics of endothelial cells in DR, potentially through the OPA1 pathway.

Sirtuin insights: bridging the gap between cellular processes and therapeutic applications

The greatest challenges that organisms face today are effective responses or detection of life-threatening environmental changes due to an obvious semblance of stress and metabolic fluctuations. These are associated with different pathological conditions among which cancer is most important. Sirtuins (SIRTs; NAD+-dependent enzymes) are versatile enzymes with diverse substrate preferences, cellular locations, crucial for cellular processes and pathological conditions. This article describes in detail the distinct roles of SIRT isoforms, unveiling their potential as either cancer promoters or suppressors and also explores how both natural and synthetic compounds influence the SIRT function, indicating promise for therapeutic applications. We also discussed the inhibitors/activators tailored to specific SIRTs, holding potential for diseases lacking effective treatments. It may uncover the lesser-studied SIRT isoforms (e.g., SIRT6, SIRT7) and their unique functions. This article also offers a comprehensive overview of SIRTs, linking them to a spectrum of diseases and highlighting their potential for targeted therapies, combination approaches, disease management, and personalized medicine. We aim to contribute to a transformative era in healthcare and innovative treatments by unraveling the intricate functions of SIRTs.

Unlocking longevity with GLP-1: A key to turn back the clock?

Traditionally known for managing blood sugar, GLP-1, a gut hormone, is emerging as a potential key to both lengthening lifespan and combating age-related ailments. While widely recognized for its role in blood sugar control, GLP-1 is increasingly recognized for its diverse effects on various biological pathways beyond glucose metabolism. Research across organisms and humans suggests that activating GLP-1 receptors significantly impacts cellular processes linked to aging. Its ability to boost mitochondrial function, enhance cellular stress resistance, and quell inflammation hints at its wider influence on aging mechanisms. This intricate interplay between GLP-1 and longevity appears to act through multiple pathways. One key effect is its ability to modulate insulin sensitivity, potentially curbing age-related metabolic issues like type 2 diabetes. Its neuroprotective properties also make it a promising candidate for addressing age-related cognitive decline and neurodegenerative diseases. Furthermore, preclinical studies using GLP-1 analogs or agonists have shown promising results in extending lifespan and improving healthspan in various model organisms. These findings provide a compelling rationale for exploring GLP-1-based interventions in humans to extend healthy aging. However, despite the exciting therapeutic prospects of GLP-1 in promoting longevity, challenges remain. Determining optimal dosages, establishing long-term safety profiles, and investigating potential adverse effects require comprehensive clinical investigations before we can confidently translate these findings to humans. This article emphasises the wide applicability of GLP-1.

Effect of SIRT1 gene single-nucleotide polymorphisms on susceptibility to type 1 diabetes in a Han Chinese population

AIMS

SIRT1 deficiency has been associated with diabetes, and a variant of the SIRT1 gene has been found to be involved in human autoimmune diabetes; however, it is unclear whether this genetic variation exists in Han Chinese with type 1 diabetes (T1D) and whether it contributes to development of T1D. Therefore, we aimed to explore the association of the SIRT1 gene single-nucleotide polymorphisms (SNPs) rs10997866 and rs3818292 in a Han Chinese population with T1D.

METHODS

This study recruited 2653 unrelated Han Chinese individuals, of whom 1289 had T1D and 1364 were healthy controls. Allelic and genotypic distributions of SIRT1 polymorphisms (rs10997866 and rs3818292) were determined by MassARRAY. Basic characteristics, genotype and allele frequencies of selected SNPs were compared between the T1D patients and healthy controls. Further genotype-phenotype association analysis of the SNPs was performed on the T1D patients divided into three groups according to genotype. Statistical analyses included the chi-square test, Mann‒Whitney U test, Kruskal‒Wallis H test and logistic regression.

RESULTS

The allelic (G vs. A) and genotypic (GA vs. AA) distributions of SIRT1 rs10997866 were significantly different in T1D patients and healthy controls (P = 0.039, P = 0.027), and rs10997866 was associated with T1D susceptibility under dominant, overdominant and additive models (P = 0.026, P = 0.030 and P = 0.027, respectively). Moreover, genotype-phenotype association analysis showed the GG genotype of rs10997866 and the GG genotype of rs3818292 to be associated with higher titers of IA-2A (P = 0.013 and P = 0.038, respectively).

CONCLUSION

SIRT1 rs10997866 is significantly associated with T1D susceptibility, with the minor allele G conferring a higher risk of T1D. Moreover, SIRT1 gene rs10997866 and rs3818292 correlate with the titer of IA-2A in Han Chinese individuals with T1D.

Glucose transport, transporters and metabolism in diabetic retinopathy

Diabetic retinopathy (DR) is the most common reason for blindness in working-age individuals globally. Prolonged high blood glucose is a main causative factor for DR development, and glucose transport is prerequisite for the disturbances in DR caused by hyperglycemia. Glucose transport is mediated by its transporters, including the facilitated transporters (glucose transporter, GLUTs), the "active" glucose transporters (sodium-dependent glucose transporters, SGLTs), and the SLC50 family of uniporters (sugars will eventually be exported transporters, SWEETs). Glucose transport across the blood-retinal barrier (BRB) is crucial for nourishing the neuronal retina in the context of retinal physiology. This physiological process primarily relies on GLUTs and SGLTs, which mediate the glucose transportation across both the cell membrane of retinal capillary endothelial cells and the retinal pigment epithelium (RPE). Under diabetic conditions, increased accumulation of extracellular glucose enhances the retinal cellular glucose uptake and metabolism via both glycolysis and glycolytic side branches, which activates several biochemical pathways, including the protein kinase C (PKC), advanced glycation end-products (AGEs), polyol pathway and hexosamine biosynthetic pathway (HBP). These activated biochemical pathways further increase the production of reactive oxygen species (ROS), leading to oxidative stress and activation of Poly (ADP-ribose) polymerase (PARP). The activated PARP further affects all the cellular components in the retina, and finally resulting in microangiopathy, neurodegeneration and low-to-moderate grade inflammation in DR. This review aims to discuss the changes of glucose transport, glucose transporters, as well as its metabolism in DR, which influences the retinal neurovascular unit (NVU) and implies the possible therapeutic strategies for treating DR.

SIRT1: a promising therapeutic target in type 2 diabetes mellitus

A significant increase in the worldwide incidence and prevalence of type 2 diabetic mellitus (T2DM) has elevated the need for studies on novel and effective therapeutic strategies. Sirtuin 1 (SIRT1) is an NAD + dependent protein deacetylase with a critical function in the regulation of glucose/lipid metabolism, insulin resistance, inflammation, oxidative stress, and mitochondrial function. SIRT1 is also involved in the regulation of insulin secretion from pancreatic β-cells and protecting these cells from inflammation and oxidative stress-mediated tissue damages. In this regard, major SIRT1 activators have been demonstrated to exert a beneficial impact in reversing T2DM-related complications including cardiomyopathy, nephropathy, retinopathy, and neuropathy, hence treating T2DM. Therefore, an accumulating number of recent studies have investigated the efficacy of targeting SIRT1 as a therapeutic strategy in T2DM. In this review we aimed to discuss the current understanding of the physiological and biological roles of SIRT1, then its implication in the pathogenesis of T2DM, and the therapeutic potential of SIRT1 in combating T2DM.

Novel antioxidant protein target therapy to counter the prevalence and severity of SARS-CoV-2

Background

This review analyzed the magnitude of the COVID-19 pandemic globally and in India and the measures to counter its effect using natural and innate immune booster molecules. The study focuses on two phases: the first focuses on the magnitude, and the second on the effect of antioxidants (natural compounds) on SARS-CoV-2.

Methods

The magnitude of the prevalence, mortality, and comorbidities was acquired from the World Health Organization (WHO) report, media, a report from the Ministry of Health and Family Welfare (MoHFW), newspapers, and the National Centre of Disease Control (NCDC). Research articles from PubMed as well as other sites/journals and databases were accessed to gather literature on the effect of antioxidants.

Results

In the elderly and any chronic diseases, the declined level of antioxidant molecules enhanced the reactive oxygen species, which in turn deprived the immune system.

Conclusion

Innate antioxidant proteins like sirtuin and sestrin play a vital role in enhancing immunity. Herbal products and holistic approaches can also be alternative solutions for everyday life to boost the immune system by improving the redox balance in COVID-19 attack. This review analyzed the counteractive effect of alternative therapy to boost the immune system against the magnitude of the COVID-19 pandemic.

SIRT1 regulates endoplasmic reticulum stress-related organ damage

Endoplasmic reticulum (ER) stress plays a key role in the pathogenesis of several organ damages. Studies show that excessive ER stress (ERS) can destroy cellular homeostasis, causing cell damage and physiological dysfunction in various organs. In recent years, Sirtuin1 (SIRT1) has become a research hotspot on ERS. Increasing evidence suggests that SIRT1 plays a positive role in various ERS-induced organ damage via multiple mechanisms, including inhibiting cellular apoptosis and promoting autophagy. SIRT1 can also alleviate liver, heart, lung, kidney, and intestinal damage by inhibiting ERS. We discuss the possible mechanism of SIRT1, explore potential therapeutic targets of diseases, and provide a theoretical basis for treating ERS-related diseases.

Epigenetic modifications and metabolic memory in diabetic retinopathy: beyond the surface

Epigenetics focuses on DNA methylation, histone modification, chromatin remodeling, noncoding RNAs, and other gene regulation mechanisms beyond the DNA sequence. In the past decade, epigenetic modifications have drawn more attention as they participate in the development and progression of diabetic retinopathy despite tight control of glucose levels. The underlying mechanisms of epigenetic modifications in diabetic retinopathy still urgently need to be elucidated. The diabetic condition facilitates epigenetic changes and influences target gene expression. In this review, we summarize the involvement of epigenetic modifications and metabolic memory in the development and progression of diabetic retinopathy and propose novel insights into the treatment of diabetic retinopathy.

Diabetes mellitus associated neurovascular lesions in the retina and brain: A review

Diabetes mellitus (DM) is now recognized as a system-wide, autoimmune, inflammatory, microvascular disorder, which, in the retina and brain results in severe multifocal injury now recognized as a leading cause, world-wide, of progressive vision loss and dementia. To address this problem, resulting primarily from variations in glycemia in the prediabetic and overt diabetic states, it must be realized that, although some of the injury processes associated with diabetes may be system wide, there are varying responses, effector, and repair mechanisms that differ from organ to organ or within varying cell structures. Specifically, within the retina, and similarly within the brain cortex, lesions occur of the "neurovascular unit", comprised of focal microvascular occlusions, inflammatory endothelial and pericyte injury, with small vessel leakage resulting in injury to astrocytes, Müller cells, and microglia, all of which occur with progressive neuronal apoptosis. Such lesions are now recognized to occur before the first microaneurysms are visible to imaging by fundus cameras or before they result in detectable symptoms or signs recognizable to the patient or clinician. Treatments, therefore, which currently are not initiated within the retina until edema develops or there is progression of vascular lesions that define the current staging of retinopathy, and in the brain only after severe signs of cognitive failure. Treatments, therefore are applied relatively late with some reduction in progressive cellular injury but with resultant minimal vision or cognitive improvement. This review article will summarize the multiple inflammatory and remediation processes currently understood to occur in patients with diabetes as well as pre-diabetes and summarize as well the current limitations of methods for assessing the structural and functional alterations within the retina and brain. The goal is to attempt to define future screening, monitoring, and treatment directions that hopefully will prevent progressive injury as well as enable improved repair and attendant function.

Relevance of Peptide Homeostasis in Metabolic Retinal Degenerative Disorders: Curative Potential in Genetically Modified Mice

The mammalian retina contains approximately 30 neuropeptides that are synthetized by different neuronal cell populations, glia, and the pigmented epithelium. The presence of these neuropeptides leaves a mark on normal retinal molecular processes and physiology, and they are also crucial in fighting various pathologies (e.g., diabetic retinopathy, ischemia, age-related pathologies, glaucoma) because of their protective abilities. Retinal pathologies of different origin (metabolic, genetic) are extensively investigated by genetically manipulated in vivo mouse models that help us gain a better understanding of the molecular background of these pathomechanisms. These models offer opportunities to manipulate gene expression in different cell types to help reveal their roles in the preservation of retinal health or identify malfunction during diseases. In order to assess the current status of transgenic technologies available, we have conducted a literature survey focused on retinal disorders of metabolic origin, zooming in on the role of retinal neuropeptides in diabetic retinopathy and ischemia. First, we identified those neuropeptides that are most relevant to retinal pathologies in humans and the two clinically most relevant models, mice and rats. Then we continued our analysis with metabolic disorders, examining neuropeptide-related pathways leading to systemic or cellular damage and rescue. Last but not least, we reviewed the available literature on genetically modified mouse strains to understand how the manipulation of a single element of any given pathway (e.g., signal molecules, receptors, intracellular signaling pathways) could lead either to the worsening of disease conditions or, more frequently, to substantial improvements in retinal health. Most attention was given to studies which reported successful intervention against specific disorders. For these experiments, a detailed evaluation will be given and the possible role of converging intracellular pathways will be discussed. Using these converging intracellular pathways, curative effects of peptides could potentially be utilized in fighting metabolic retinal disorders.

Liraglutide Treatment Does Not Induce Changes in the Peripapillary Retinal Nerve Fiber Layer Thickness in Patients with Diabetic Retinopathy

Purpose: Liraglutide treatment has shown promising anti-inflammatory and nerve regenerative results in preclinical and clinical trials. We sought to assess if liraglutide treatment would induce nerve regeneration through its anti-inflammatory and neurotrophic mechanisms by increasing peripapillary retinal nerve fiber layer (RNFL) thickness in individuals with long-term type 1 diabetes. Methods: Secondary analyses were performed on a prospective, double-blinded, randomized, placebo-controlled trial on adults with type 1 diabetes, distal symmetric polyneuropathy (DSPN), and confirmed diabetic retinopathy, who were randomized 1:1 to either 26 weeks placebo or liraglutide treatment. The primary endpoint was a change in peripapillary RNFL thickness between treatments, assessed by optical coherence tomography. Results: Thirty-seven participants were included in the secondary analysis. No differences in mean peripapillary RNFL thickness (overall ΔMean RNFL thickness; liraglutide -1 (±8) μm (-1%) vs. placebo -1 (±5) μm (-1%), P = 0.78, n = 37) or any of the quadrants. Peripapillary RNFL thicknesses were shown between treatments in either nonproliferative (ΔMean RNFL thickness; liraglutide -1 (±5) μm (-1%) vs. placebo 0 (±4) μm (0%), P = 0.80, N = 26) or proliferative diabetic retinopathy subgroup (ΔMean RNFL thickness; liraglutide -2 (±14) μm (-3%) vs. placebo -1 (±6) μm (-2%), P = 0.88, N = 11). Conclusions: In this study, 26 weeks of liraglutide treatment did not induce measurable changes in the assessed optic nerve thickness. Thus, this methodology does not support the induction of substantial nerve regeneration in this cohort with established retinopathy and DSPN. The trial was approved by the Danish Health and Medicines Authority. Informed consent was obtained from all participants. TODINELI study: EUDRA CT: 2013-004375-12, Ethics Ref: N-20130077 Clinical trial registration number: clinicaltrials.gov NCT02138045.

The Protective Effects of α-Mangostin Attenuate Sodium Iodate-Induced Cytotoxicity and Oxidative Injury via Mediating SIRT-3 Inactivation via the PI3K/AKT/PGC-1α Pathway

It is well known that age-related macular degeneration (AMD) is an irreversible neurodegenerative disease that can cause blindness in the elderly. Oxidative stress-induced retinal pigment epithelial (RPE) cell damage is a part of the pathogenesis of AMD. In this study, we evaluated the protective effect and mechanisms of alpha-mangostin (α-mangostin, α-MG) against NaIO3-induced reactive oxygen species (ROS)-dependent toxicity, which activates apoptosis in vivo and in vitro. MTT assay and flow cytometry demonstrated that the pretreatment of ARPE-19 cells with α-MG (0, 3.75, 7.5, and 15 μM) significantly increased cell viability and reduced apoptosis from NaIO3-induced oxidative stress in a concentration-dependent manner, which was achieved by the inhibition of Bax, cleaved PARP-1, cleaved caspase-3 protein expression, and enhancement of Bcl-2 protein. Furthermore, pre-incubation of ARPE-19 cells with α-MG markedly inhibited the intracellular ROS and extracellular H2O2 generation via blocking of the abnormal enzyme activities of superoxide dismutase (SOD), the downregulated levels of catalase (CAT), and the endogenous antioxidant, glutathione (GSH), which were regulated by decreasing PI3K-AKT-PGC-1α-STRT-3 signaling in ARPE-19 cells. In addition, our in vivo results indicated that α-MG improved retinal deformation and increased the thickness of both the outer nuclear layer and inner nuclear layer by inhibiting the expression of cleaved caspase-3 protein. Taken together, our results suggest that α-MG effectively protects human ARPE-19 cells from NaIO3-induced oxidative damage via antiapoptotic and antioxidant effects.

Cellular targets in diabetic retinopathy therapy

Despite the existence of treatment for diabetes, inadequate metabolic control triggers the appearance of chronic complications such as diabetic retinopathy. Diabetic retinopathy is considered a multifactorial disease of complex etiology in which oxidative stress and low chronic inflammation play essential roles. Chronic exposure to hyperglycemia triggers a loss of redox balance that is critical for the appearance of neuronal and vascular damage during the development and progression of the disease. Current therapies for the treatment of diabetic retinopathy are used in advanced stages of the disease and are unable to reverse the retinal damage induced by hyperglycemia. The lack of effective therapies without side effects means there is an urgent need to identify an early action capable of preventing the development of the disease and its pathophysiological consequences in order to avoid loss of vision associated with diabetic retinopathy. Therefore, in this review we propose different therapeutic targets related to the modulation of the redox and inflammatory status that, potentially, can prevent the development and progression of the disease.

Neurovascular Unit: A New Target for Treating Early Stages of Diabetic Retinopathy

The concept of diabetic retinopathy as a microvascular disease has evolved and is now considered a more complex diabetic complication in which neurovascular unit impairment plays an essential role and, therefore, can be considered as a main therapeutic target in the early stages of the disease. However, neurodegeneration is not always the apparent primary event in the natural story of diabetic retinopathy, and a phenotyping characterization is recommendable to identify those patients in whom neuroprotective treatment might be of benefit. In recent years, a myriad of treatments based on neuroprotection have been tested in experimental models, but more interestingly, there are drugs with a dual activity (neuroprotective and vasculotropic). In this review, the recent evidence concerning the therapeutic approaches targeting neurovascular unit impairment will be presented, along with a critical review of the scientific gaps and problems which remain to be overcome before our knowledge can be transferred to clinical practice.

Exendin-4 Protects Against Myocardial Ischemia-Reperfusion Injury by Upregulation of SIRT1 and SIRT3 and Activation of AMPK

This study evaluated if the cardioprotective effect of Exendin-4 against ischemia/reperfusion (I/R) injury in male rats involves modulation of AMPK and sirtuins. Adult male rats were divided into sham, sham + Exendin-4, I/R, I/R + Exendin-4, and I/R + Exendin-4 + EX-527, a sirt1 inhibitor. Exendin-4 reduced infarct size and preserved the function and structure of the left ventricles (LV) of I/R rats. It also inhibited oxidative stress and apoptosis and upregulated MnSOD and Bcl-2 in their infarcted myocardium. With no effect on SIRTs 2/6/7, Exendin-4 activated and upregulated mRNA and protein levels of SIRT1, increased levels of SIRT3 protein, activated AMPK, and reduced the acetylation of p53 and PGC-1α as well as the phosphorylation of FOXO-1. EX-527 completely abolished all beneficial effects of Exendin-4 in I/R-induced rats. In conclusion, Exendin-4 cardioprotective effect against I/R involves activation of SIRT1 and SIRT3. Graphical Abstract Exendin-4 could scavenge free radical directly, upregulate p53, and through upregulation of SIRT1 and stimulating SIRT1 nuclear accumulation. In addition, Exendin-4 also upregulates SIRT3 which plays an essential role in the upregulation of antioxidants, inhibition of reactive oxygen species (ROS) generation, and prevention of mitochondria damage. Accordingly, SIRT1 induces the deacetylation of PGC-1α and p53 and is able to bind p-FOXO-1. This results in inhibition of cardiomyocyte apoptosis through increasing Bcl-2 levels, activity, and levels of MnSOD; decreasing expression of Bax; decreasing cytochrome C release; and improving mitochondria biogenesis through upregulation of Mfn-2.

Exendin-4 inhibits high glucose-induced oxidative stress in retinal pigment epithelial cells by modulating the expression and activation of p66Shc

PURPOSE

Activation of p⁶⁶Sch, an adaptor protein, is associated with oxidative stress and apoptosis and has been implicated in the pathogenesis of diabetes-induced retinal pigment epithelial cell damage and diabetic retinopathy. Exendin-4 is a glucagon-like protein that protects against diabetic retinopathy, but the mechanism of action is not well understood. This study aimed to investigate whether Exendin-4 could protect against high glucose-induced oxidative stress and apoptosis in the adult human retinal pigment epithelial-19 cell line by modulating levels and activation of p⁶⁶Shc and to study the underlying mechanisms.

MATERIALS AND METHODS

Adult human retinal pigment epithelial-19 cells were cultured under low (5 µM) or high glucose (100 µM) conditions in the presence or absence of Exendin-4 and with or without pre-incubation with Exendin-9-39, a glucagon-like peptide-1 receptor antagonist.

RESULTS

In a dose-dependent manner, Exendin-4 inhibited high glucose-induced cell death and decreased levels of reactive oxygen species, lactate dehydrogenase release, and single single-stranded DNA. At the most effective concentration (100 µM), Exendin-4 reduced mitochondrial levels of phospho-p⁶⁶Shc (Ser³⁶), cytoplasmic levels of cleaved caspase-3 and cytochrome-c, and NADPH oxidase levels in high glucose-treated cells. It also increased levels of glutathione and magnesium superoxide dismutase and protein levels of magnesium superoxide dismutase but downregulated total protein levels of protein kinase-β and p⁶⁶Shc and inhibited c-Jun N-terminal kinase phosphorylation in both low- and high glucose-treated cells. All these Exendin-4 effects, however, were inhibited by Exendin-9-39.

CONCLUSIONS

Exendin-4 protects against high glucose-induced adult human retinal pigment epithelial-19 cell damage by increasing antioxidants, downregulating NADPH, and inhibiting mitochondria-mediated apoptosis, effects that are associated with the inhibition of c-Jun N-terminal kinase and downregulation of protein kinase-β and p⁶⁶Shc.

Neurovascular unit in diabetic retinopathy: pathophysiological roles and potential therapeutical targets

Diabetic retinopathy (DR), one of the common complications of diabetes, is the leading cause of visual loss in working-age individuals in many industrialized countries. It has been traditionally regarded as a purely microvascular disease in the retina. However, an increasing number of studies have shown that DR is a complex neurovascular disorder that affects not only vascular structure but also neural tissue of the retina. Deterioration of neural retina could precede microvascular abnormalities in the DR, leading to microvascular changes. Furthermore, disruption of interactions among neurons, vascular cells, glia and local immune cells, which collectively form the neurovascular unit, is considered to be associated with the progression of DR early on in the disease. Therefore, it makes sense to develop new therapeutic strategies to prevent or reverse retinal neurodegeneration, neuroinflammation and impaired cell-cell interactions of the neurovascular unit in early stage DR. Here, we present current perspectives on the pathophysiology of DR as a neurovascular disease, especially at the early stage. Potential novel treatments for preventing or reversing neurovascular injuries in DR are discussed as well.

Antioxidative Potentials of Incretin-Based Medications: A Review of Molecular Mechanisms

Glucagon-like peptide 1 receptor agonists and dipeptidyl-peptidase 4 inhibitors are medications used for managing diabetes, mimicking the metabolic effects of incretin hormones. Recent evidence suggests that these medications have antioxidative potentials in the diabetic milieu. The pathophysiology of most diabetic complications involves oxidative stress. Therefore, if incretin-based antidiabetic medications can alleviate the free radicals involved in oxidative stress, they can potentially provide further therapeutic effects against diabetic complications. However, the molecular mechanisms by which these medications protect against oxidative stress are not fully understood. In the current review, we discuss the potential molecular mechanisms behind these pharmacologic agents' antioxidative properties.

Diabetic retinopathy, oxidative stress, and sirtuins: an in depth look in enzymatic patterns and new therapeutic horizons

Diabetic retinopathy (DR) is one of the leading causes of blindness in the world. DR represents the most common microvascular complication of diabetes, and its incidence is constantly rising. The complex interactions between inflammation, oxidative stress, and the production of free oxygen radicals caused by prolonged exposure to hyperglycemia determine the development of DR. Sirtuins (SIRTs) are a recently discovered class of 7 histone deacetylases involved in cellular senescence, regulation of cell cycle, metabolic pathways, and DNA repair. SIRTs participate in the progress of several pathologies such as cancer, neurodegeneration, and metabolic diseases. In DR sirtuins 1,3,5, and 6 play an important role as they regulate the activation of the inflammatory response, insulin sensibility, and both glycolysis and gluconeogenesis. A wide spectrum of direct and indirect activators of SIRTs pathways (e.g., antagomiR, resveratrol, or glycyrrhizin) is currently being developed to treat the inflammatory cascade occurring in DR. We focus on the main metabolic and inflammatory pathways involving SIRTs and DR, as well as recent evidence on SIRTs activators that may be employed as novel therapeutic approaches to DR.

Oxidative stress and diabetic retinopathy: Molecular mechanisms, pathogenetic role and therapeutic implications

Oxidative stress, a cytopathic outcome of excessive generation of ROS and the repression of antioxidant defense system for ROS elimination, is involved in the pathogenesis of multiple diseases, including diabetes and its complications. Retinopathy, a microvascular complication of diabetes, is the primary cause of acquired blindness in diabetic patients. Oxidative stress has been verified as one critical contributor to the pathogenesis of diabetic retinopathy. Oxidative stress can both contribute to and result from the metabolic abnormalities induced by hyperglycemia, mainly including the increased flux of the polyol pathway and hexosamine pathway, the hyper-activation of protein kinase C (PKC) isoforms, and the accumulation of advanced glycation end products (AGEs). Moreover, the repression of the antioxidant defense system by hyperglycemia-mediated epigenetic modification also leads to the imbalance between the scavenging and production of ROS. Excessive accumulation of ROS induces mitochondrial damage, cellular apoptosis, inflammation, lipid peroxidation, and structural and functional alterations in retina. Therefore, it is important to understand and elucidate the oxidative stress-related mechanisms underlying the progress of diabetic retinopathy. In addition, the abnormalities correlated with oxidative stress provide multiple potential therapeutic targets to develop safe and effective treatments for diabetic retinopathy. Here, we also summarized the main antioxidant therapeutic strategies to control this disease.

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Oxidative stress can both contribute to and result from hyperglycemia-induced metabolic abnormalities in retina.

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Genes important in regulation of ROS are epigenetically modified, increasing ROS accumulation in retina.

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Oxidative stress is closely associated with the pathological changes in the progress of diabetic retinopathy.

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Antioxidants ameliorate retinopathy through targeting multiple steps of oxidative stress.

Extracellular Vesicles and MicroRNA: Putative Role in Diagnosis and Treatment of Diabetic Retinopathy

Diabetic retinopathy (DR) is a complex, progressive, and heterogenous retinal degenerative disease associated with diabetes duration. It is characterized by glial, neural, and microvascular dysfunction, being the blood-retinal barrier (BRB) breakdown a hallmark of the early stages. In advanced stages, there is formation of new blood vessels, which are fragile and prone to leaking. This disease, if left untreated, may result in severe vision loss and eventually legal blindness. Although there are some available treatment options for DR, most of them are targeted to the advanced stages of the disease, have some adverse effects, and many patients do not adequately respond to the treatment, which demands further research. Oxidative stress and low-grade inflammation are closely associated processes that play a critical role in the development of DR. Retinal cells communicate with each other or with another one, using cell junctions, adhesion contacts, and secreted soluble factors that can act in neighboring or long-distance cells. Another mechanism of cell communication is via secreted extracellular vesicles (EVs), through exchange of material. Here, we review the current knowledge on deregulation of cell-to-cell communication through EVs, discussing the changes in miRNA expression profiling in body fluids and their role in the development of DR. Thereafter, current and promising therapeutic agents for preventing the progression of DR will be discussed.

Rodent retinal microcirculation and visual electrophysiology following simulated microgravity

How the absence of gravity affects the physiology of human beings is generating global research interest as space exploration, including missions aboard the International Space Station, continues to push boundaries. Here, we examined changes in retinal microcirculation and visual electrophysiology in mice suspended by their tails to simulate the cephalad movement of blood that occurs under microgravity conditions. Tail suspension was performed with a head-down tilt with a recommended angle of 30°. Mice in the control groups were similarly attached to a tether but could maintain a normal position. Morphologically, the 15-day tail-suspended mice showed retinal microvascular dilation, tortuosity, and a relatively long fluorescence retention; however, the average diameter of the major retinal vessels was not notably changed. In addition, optical coherence tomography showed their optic nerve head had an increased diameter. However, the mice could adapt to the change, with microcirculation and the optic nerve head recovering following 30-day tail suspension. Expression of rhodopsin and cone-opsins was not notably changed, and no retinal apoptotic-positive cells were detected between 15- and 30-day tail suspensions. Moreover, the three experimental groups of suspended mice showed normal retinal layers and thickness. Functionally, following 15-day tail suspension, scotopic electroretinograms showed a decline in the oscillatory potentials (OPs), but not in the b wave; simultaneously, the peak time of flash visual evoked potential component N₁ was delayed compared to its baseline and the time-matched control. Following 30-day tail suspension, the OPs (O2) amplitude recovered to approximately 97% of its baseline or 86% of the time-matched control level. By simulating cephalad shifting of blood, short-term tail suspension can affect rodent retinal microcirculation, the optic nerve head, and disturb visual electrophysiology. However, the change is reversible with no permanent injury observed in the retina. The mice could adapt to the short-term change of retinal microcirculation, indicating new conditions that could be combined with, or could enhance, simulated microgravity for further studying the impact of short- or long-term outer space conditions on the retina.

Oxidative Stress and Microvascular Alterations in Diabetic Retinopathy: Future Therapies

Diabetes is a disease that can be treated with oral antidiabetic agents and/or insulin. However, patients' metabolic control is inadequate in a high percentage of them and a major cause of chronic diseases like diabetic retinopathy. Approximately 15% of patients have some degree of diabetic retinopathy when diabetes is first diagnosed, and most will have developed this microvascular complication after 20 years. Early diagnosis of the disease is the best tool to prevent or delay vision loss and reduce the involved costs. However, diabetic retinopathy is an asymptomatic disease and its development to advanced stages reduces the effectiveness of treatments. Today, the recommended treatment for severe nonproliferative and proliferative diabetic retinopathy is photocoagulation with an argon laser and intravitreal injections of anti-VEGF associated with, or not, focal laser for diabetic macular oedema. The use of these therapeutic approaches is severely limited, such as uncomfortable administration for patients, long-term side effects, the costs they incur, and the therapeutic effectiveness of the employed management protocols. Hence, diabetic retinopathy is the widespread diabetic eye disease and a leading cause of blindness in adults in developed countries. The growing interest in using polyphenols, e.g., resveratrol, in treatments related to oxidative stress diseases has spread to diabetic retinopathy. This review focuses on analysing the sources and effects of oxidative stress and inflammation on vascular alterations and diabetic retinopathy development. Furthermore, current and antioxidant therapies, together with new molecular targets, are postulated for diabetic retinopathy treatment.

Relationships Between Neurodegeneration and Vascular Damage in Diabetic Retinopathy

Diabetic retinopathy (DR) is a common complication of diabetes and constitutes a major cause of vision impairment and blindness in the world. DR has long been described exclusively as a microvascular disease of the eye. However, in recent years, a growing interest has been focused on the contribution of neuroretinal degeneration to the pathogenesis of the disease, and there are observations suggesting that neuronal death in the early phases of DR may favor the development of microvascular abnormalities, followed by the full manifestation of the disease. However, the mediators that are involved in the crosslink between neurodegeneration and vascular changes have not yet been identified. According to our hypothesis, vascular endothelial growth factor (VEGF) could probably be the most important connecting link between the death of retinal neurons and the occurrence of microvascular lesions. Indeed, VEGF is known to play important neuroprotective actions; therefore, in the early phases of DR, it may be released in response to neuronal suffering, and it would act as a double-edged weapon inducing both neuroprotective and vasoactive effects. If this hypothesis is correct, then any retinal stress causing neuronal damage should be accompanied by VEGF upregulation and by vascular changes. Similarly, any compound with neuroprotective properties should also induce VEGF downregulation and amelioration of the vascular lesions. In this review, we searched for a correlation between neurodegeneration and vasculopathy in animal models of retinal diseases, examining the effects of different neuroprotective substances, ranging from nutraceuticals to antioxidants to neuropeptides and others and showing that reducing neuronal suffering also prevents overexpression of VEGF and vascular complications. Taken together, the reviewed evidence highlights the crucial role played by mediators such as VEGF in the relationship between retinal neuronal damage and vascular alterations and suggests that the use of neuroprotective substances could be an efficient strategy to prevent the onset or to retard the development of DR.

Neuroprotective Peptides in Retinal Disease

In the pathogenesis of many disorders, neuronal death plays a key role. It is now assumed that neurodegeneration is caused by multiple and somewhat converging/overlapping death mechanisms, and that neurons are sensitive to unique death styles. In this respect, major advances in the knowledge of different types, mechanisms, and roles of neurodegeneration are crucial to restore the neuronal functions involved in neuroprotection. Several novel concepts have emerged recently, suggesting that the modulation of the neuropeptide system may provide an entirely new set of pharmacological approaches. Neuropeptides and their receptors are expressed widely in mammalian retinas, where they exert neuromodulatory functions including the processing of visual information. In multiple models of retinal diseases, different peptidergic substances play neuroprotective actions. Herein, we describe the novel advances on the protective roles of neuropeptides in the retina. In particular, we focus on the mechanisms by which peptides affect neuronal death/survival and the vascular lesions commonly associated with retinal neurodegenerative pathologies. The goal is to highlight the therapeutic potential of neuropeptide systems as neuroprotectants in retinal diseases.

Cross-Talk between Sirtuin 1 and the Proinflammatory Mediator High-Mobility Group Box-1 in the Regulation of Blood-Retinal Barrier Breakdown in Diabetic Retinopathy

Purpose: High-mobility group box-1 (HMGB1) mediates inflammation and breakdown of blood-retinal barrier (BRB) in diabetic retina. Sirtuin-1 (SIRT1) has protective effects against inflammation and oxidative stress. The aim of this study was to investigate the interaction between HMGB1 and SIRT1 in regulating BRB breakdown in diabetic retina. Methods: BRB breakdown was assessed in vivo with fluorescein isothiocyanate-conjugated dextran. Vitreous samples from 47 proliferative diabetic retinopathy (PDR) and 19 nondiabetic patients, and epiretinal membranes from 13 patients with PDR were studied by enzyme-linked immunosorbent assay and immunohistochemistry. Retinas from 4-week diabetic rats and from normal rats intravitreally injected with HMGB1 were studied by spectrophotometric assay, Western blot analysis, and RT-PCR. We also studied the effect of the HMGB1 inhibitor glycyrrhizin and the SIRT1 activator resveratrol on diabetes-induced biochemical changes in the retina. Results: HMGB1 levels in vitreous samples from PDR patients were significantly higher than in nondiabetic controls, whereas SIRT1 levels were significantly lower in vitreous samples from patients with inactive PDR than those in patients with active PDR and nondiabetic controls. In epiretinal membranes, SIRT1 was expressed in vascular endothelial cells and stromal cells. Diabetes and intravitreal injection of HMGB1 in normal rats downregulated SIRT1expression, whereas glycyrrhizin and resveratrol normalized diabetes-induced downregulation of SIRT1. Resveratrol significantly attenuated diabetes-induced downregulation of occludin and upregulation of HMGB1 and receptor for advanced glycation end products in the retina and breakdown of BRB. Conclusions: Our findings suggest that a functional link between SIRT1 and HMGB1 is involved in regulating of BRB breakdown in diabetic retina.

GLP-1's role in neuroprotection: a systematic review

Glucagon-like peptide 1 (GLP-1) is a target for treatment of diabetes; however, its function in the brain is not well studied. In this systematic review, we aimed to analyze the neuroprotective role of GLP-1 and its defined mechanisms. Methods: We searched 'Web of Science' and 'Pubmed' to identify relevant studies using GLP-1 as the keyword. Two hundred and eighty-nine clinical and preclinical studies have been included. Data have been presented by grouping neurodegenerative, neurovascular and specific cell culture models. Results: Recent literature shows that GLP-1 and its agonists, DPP-4 inhibitors and combined GLP-1/GIP molecules are effective in partially or fully reversing the effects of neurotoxic compounds, neurovascular complications of diabetes, neuropathological changes related with Alzheimer's disease, Parkinson's disease or vascular occlusion. Possible mechanisms that provide neuroprotection are enhancing the viability of the neurons and restoring neurite outgrowth by increased neurotrophic factors, increasing subventricular zone progenitor cells, decreasing apoptosis, decreasing the level of pro-inflammatory factors, and strengthening blood-brain barrier. Conclusion: Based on the preclinical studies, GLP-1 modifying agents are promising targets for neuroprotection. On the other hand, the number of clinical studies that investigate GLP-1 as a treatment is low and further clinical trials are needed for a benchside to bedside translation of recent findings.

The expression of sirtuins, superoxide dismutase, and lipid peroxidation status in peripheral blood from patients with diabetes and hypothyroidism

BACKGROUND

Sirtuin 1 (SIRT1) and sirtuin 3 (SIRT3) proteins have an important role in counteracting oxidative stress. Although diabetes and hypothyroidism (HT) are both characterized by oxidative stress, the mechanisms are not fully understood. This study investigated the effects of type 1 diabetes (T1D), type 2 diabetes (T2D), and HT on the expression levels of SIRT1, SIRT3, and manganese superoxide dismutase (SOD2).

METHODS

Gene expression of SIRT1, SIRT3, and SOD2 was measured using real-time PCR. The protein expression of SOD2 and lipid peroxidation (thiobarbituric acid reactive substances) was measured by the TBARS Assay kit and enzyme-linked immunosorbent assay (ELISA) respectively.

RESULTS

The results showed that the SIRT1 and SIRT3 levels were lower in peripheral blood samples from patients with T1D, T2D, or HT than in healthy individuals. Interestingly, the mRNA and protein expression levels of SOD2 were higher in all three patient groups. Lipid peroxidation was higher in the patients with HT than in the healthy individuals.

CONCLUSIONS

These results indicate alterations in the expression levels of sirtuins and superoxide dismutase in diabetes and HT, which may be related, at least in part, to the oxidative stress. Identifying such alterations in those patients will pave the way towards the development of drugs to enhance SIRT1 and SIRT3 expression and their activity to prevent the damaging effect of oxidative stress.

Celastrol protects human retinal pigment epithelial cells against hydrogen peroxide mediated oxidative stress, autophagy, and apoptosis through sirtuin 3 signal pathway

Age-related macular degeneration (AMD), one of the most common causes of visual impairment, often occurrs in the elderly in developed countries. Oxidative stress, autophagy, and apoptosis of retinal pigment epithelial (RPE) cells play roles in the pathogenesis of AMD. In the current study, the protective effect of celastrol against hydrogen peroxide (H₂ O₂ )-induced oxidative stress and apoptosis was investigated using a human RPE cell line (ARPE-19). H₂ O₂ inhibited ARPE-19 cells' survival and autophagy and induced their oxidative stress and apoptosis. Compared with the H₂ O₂ group, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay showed that celastrol increased ARPE-19 cells' survival in a dose- and time-dependent manner. Further, studies have suggested that celastrol has antioxidative stress and antiapoptosis effects in H₂ O₂ -treated ARPE-19 cells. Also, cell autophagy is activated by celastrol in H₂ O₂ -treated ARPE-19 cells. Reverse transcription polymerase chain reaction and Western blot showed that celastrol elevated the messenger RNA (mRNA) and protein expression of sirtuin 3 (SIRT3) in H₂ O₂ -induced ARPE-19 cells. Inhibition of the level of SIRT3 by SIRT3 small interfering RNA (siRNA) reversed the effects of celastrol on oxidative stress, autophagy, and apoptosis in H₂ O₂ -induced ARPE-19 cells. In conclusion, these observations suggest that celastrol activates the SIRT3 pathway in RPE cells and protects against H₂ O₂ -induced oxidative stress and apoptosis.

Protective Effect of Hydrogen on Sodium Iodate-Induced Age-Related Macular Degeneration in Mice

Oxidative stress is one of the main causes of AMD. Hydrogen has anti-oxidative stress and apoptotic effects on retinal injury. However, the effect of hydrogen on AMD is not clear. In this study, fundus radiography, OCT, and FFA demonstrated that HRW reduced the deposition of drusen-like structures in RPE layer, prevented retina from thinning and leakage of ocular fundus vasculature induced by NaIO₃. ERG analysis confirmed that HRW effectively reversed the decrease of a-wave and b-wave amplitude in NaIO₃-mice. Mechanistically, HRW greatly reduced the oxidative stress reaction through decreased MDA levels, increased SOD production, and decreased ROS content. The OGG1 expression was downregulated which is a marker of oxidative stress. Involvement of oxidative stress was confirmed using oxidative stress inhibitor ALCAR. Moreover, oxidative stress reaction was associated with expression of Sirt1 level and HRW significantly inhibited the downregulation of Sirt1 expression. This result was further confirmed with AICAR which restore Sirt1 expression and activity. In addition, NaIO₃-induced retinal damage was related to apoptosis via caspase 8 and caspase 9, but not the caspase 3 pathways, which led to upregulation of Bax and p53, downregulation of Bcl-2, and increase in Jc-1-positive cells in mice. However, HRW effectively reversed these effects that apoptosis induced. These results suggest that HRW protects retinal functions against oxidative stress injury through inhibiting downregulation of Sirt1 and reducing retinal apoptosis. Therefore, we speculated that hydrogen administration is a promising treatment for AMD therapy.

Reduced expression of SIRT1 and SOD-1 and the correlation between these levels in various regions of the brains of patients with Alzheimer's disease

AIMS

This study was designed to explore the expression and distribution of silent information regulator 1 (SIRT1) and superoxide dismutase 1 (SOD-1) in various regions of the brains of patients with Alzheimer's disease (AD), as well as to assess potential correlations between the levels of these proteins and also between these proteins and the Braak stage of AD.

METHODS

In the temporal and frontal cortices, hippocampus and cerebellum of 10 patients with AD and 10 age-matched control subjects, expression of SIRT1 and SOD-1, together with histopathology, were assessed by immunohistochemical and immunofluorescent stainings. Relationships between variables were examined with the Pearson correlation test.

RESULTS

The numbers of both SIRT1-positive and SOD-1-positive neurons and integrated optical density of immunohistochemical staining for these proteins in the temporal and frontal cortices, and hippocampus of patients with AD were significantly decreased than those in corresponding controls. In the case of the cerebellum, very weak expression of SIRT1 and obvious expression of SOD-1 were observed in granule cells, with no significant difference between AD and the control group. Interestingly, the protein levels between SIRT1 and SOD-1, as well as the level of SIRT1 or SOD-1 and Braak stage, were significantly correlated in neurons in all regions of the AD brains investigated except for the cerebellum.

CONCLUSIONS

These findings indicate that the reduced level of SIRT1 in the brains of patients with AD may be related to the decline in SOD-1 and neuropathological changes of this disorder.

Protective effect of exendin-4 treatment on erectile dysfunction induced by chronic methylglyoxal administration in rats

OBJECTIVE

The aim of this study was to investigate the effect of chronic exendin-4 (Ex-4) treatment on corpus cavernosum (CC) dysfunction in methylglyoxal (MGO) administered rats.

METHODS

Male rats were divided into four groups as control, MGO (75 mg/kg/day in drinking water for 12 weeks), MGO + low-dose Ex-4 (0.1 μg/kg twice daily subcutaneously for 12 weeks concomitant with MGO), and MGO + high-dose Ex-4 (1 μg/kg twice daily subcutaneously for 12 weeks concomitant with MGO). Nitric oxide (NO)-mediated endothelium-dependent and neurogenic CC relaxations were evaluated by acetylcholine (ACh) and electrical field stimulation (EFS), respectively. Apoptosis was determined by TUNEL. Endothelial nitric oxide synthase (eNOS), phosphorylated eNOS (p-eNOS), NADPH oxidase subunit gp91phox (NOX2), and Rho kinase (ROCK2) expressions in CC were investigated by immunohistochemistry. Levels of the malondialdehyde (MDA) and advanced oxidation protein products (AOPP) were also measured.

RESULTS

In MGO administered rats, both endothelium-dependent and neurogenic CC relaxations were significantly impaired as compared to controls. Apoptotic cell death and levels of MDA and AOPP increased significantly in MGO administered rats. eNOS and p-eNOS expressions decreased significantly in MGO group, while gp91phox expressions increased significantly. The diminished relaxation in response to ACh or EFS as well as the changes in expression of proteins in MGO groups were significantly improved by exendin-4 treatment. TUNEL-positive cells, and levels of MDA and AOPP in MGO group rats were also significantly reduced by exendin-4.

CONCLUSION

Exendin-4 treatment improves NO-mediated CC relaxations in MGO administered rats probably by inhibiting NADPH oxidase.

Epigenetic control of gene regulation during development and disease: A view from the retina

Complex biological processes, such as organogenesis and homeostasis, are stringently regulated by genetic programs that are fine-tuned by epigenetic factors to establish cell fates and/or to respond to the microenvironment. Gene regulatory networks that guide cell differentiation and function are modulated and stabilized by modifications to DNA, RNA and proteins. In this review, we focus on two key epigenetic changes - DNA methylation and histone modifications - and discuss their contribution to retinal development, aging and disease, especially in the context of age-related macular degeneration (AMD) and diabetic retinopathy. We highlight less-studied roles of DNA methylation and provide the RNA expression profiles of epigenetic enzymes in human and mouse retina in comparison to other tissues. We also review computational tools and emergent technologies to profile, analyze and integrate epigenetic information. We suggest implementation of editing tools and single-cell technologies to trace and perturb the epigenome for delineating its role in transcriptional regulation. Finally, we present our thoughts on exciting avenues for exploring epigenome in retinal metabolism, disease modeling, and regeneration.

Role of Sirtuin 1 in the pathogenesis of ocular disease (Review)

Sirtuin (SIRT)1, a member of the SIRT family, is a highly conserved NAD+‑dependent histone deacetylase, which has a regulatory role in numerous physiological and pathological processes by removing acetyl groups from various proteins. SIRT1 controls the activity of numerous transcription factors and cofactors, which impacts the downstream gene expression, and eventually alleviates oxidative stress and associated damage. Numerous studies have revealed that dysfunction of SIRT1 is linked with ocular diseases, including cataract, age‑associated macular degeneration, diabetic retinopathy and glaucoma, while ectopic upregulation of SIRT1 protects against various ocular diseases. In the present review, the significant role of SIRT1 and the potential therapeutic value of modulating SIRT1 expression in ocular development and eye diseases is summarized.

SIRT3: A New Regulator of Cardiovascular Diseases

Cardiovascular diseases (CVDs) are the leading causes of death worldwide, and defects in mitochondrial function contribute largely to the occurrence of CVDs. Recent studies suggest that sirtuin 3 (SIRT3), the mitochondrial NAD⁺-dependent deacetylase, may regulate mitochondrial function and biosynthetic pathways such as glucose and fatty acid metabolism and the tricarboxylic acid (TCA) cycle, oxidative stress, and apoptosis by reversible protein lysine deacetylation. SIRT3 regulates glucose and lipid metabolism and maintains myocardial ATP levels, which protects the heart from metabolic disturbances. SIRT3 can also protect cardiomyocytes from oxidative stress-mediated cell damage and block the development of cardiac hypertrophy. Recent reports show that SIRT3 is involved in the protection of several heart diseases. This review discusses the progress in SIRT3-related research and the role of SIRT3 in the prevention and treatment of CVDs.

Effect of glucagon-like peptide-1 analogue; Exendin-4, on cognitive functions in type 2 diabetes mellitus; possible modulation of brain derived neurotrophic factor and brain Visfatin

BACKGROUND

Brain derived neurotrophic factor (BDNF) is one of the most essential neurotrophic factors in the brain. BDNF is involved in learning, memory and locomotion suggesting it as a target in type 2 diabetes mellitus (T2DM) associated cognitive changes. Visfatin; an adipokine discovered to be expressed in the brain; was found to have multiple effects including its participation in keeping energy supply to the cell and is consequentially involved in cell survival. Its role in cognitive functions in T2DM was not studied before. Recent studies point to the possible neuro-protective mechanisms of glucagon-like peptide 1 analogue: Exendin-4 (Ex-4) in many cognitive disorders, but whether BDNF or Visfatin are involved or not in its neuro-protective mechanisms; is still unknown.

AIMS

to study the changes in cognitive functions in T2DM, either not treated or treated with Glucagon-like peptide 1 (GLP-1) analogue: Ex-4, and to identify the possible underlying mechanisms of these changes and whether BDNF and brain Visfatin are involved.

METHODS

A total of 36 adult male wistar albino rats were divided into 4 groups; Control, Exendin-4 control, Diabetic and Exendin-4 treated groups. At the end of the study, Y-maze and open field tests were done the day before scarification to assess spatial working memory and locomotion, respectively. Fasting glucose and insulin, lipid profile and tumor necrosis factor- alpha (TNF-α) were measured in the serum. Homeostasis model assessment insulin resistance was calculated. In the brain tissue, malondialdehyde (MDA) level, gene expression and protein levels of BDNF and Visfatin, area of degenerated neurons, area of glial cells and area % of synaptophysin immunoexpression were assessed.

RESULTS

Compared with the control, the untreated diabetic rats showed insulin resistance, dyslipidemia and elevation of serum TNF-α. The brain tissue showed down-regulation of BDNF gene expression and reduction of its protein level, up-regulation of Visfatin gene expression and elevation of its protein level, increase in MDA, area of degenerated neurons and area of glial cells and reduction in area % of synaptophysin immunoexpression. These changes were paralleled with significant deterioration in spatial working memory and locomotion. Treatment of diabetic rats with Ex-4 reversed all these changes.

CONCLUSION

T2DM has a negative impact on cognitive functions through different pathological and subcellular mechanisms. The current study provides evidence for involvement of BDNF and brain Visfatin in T2DM- associated cognitive dysfunction. BDNF and brain Visfatin were also found to contribute to the neuro-protective effect of Ex-4 via modulation of inflammation, oxidative stress, neuro-degeneration and synaptic function.

The potential benefits of glucagon-like peptide-1 receptor agonists for diabetic retinopathy

For a long time, diabetic retinopathy (DR) has been one of the most severe complications of diabetes. The early treatment of DR is not clearly recognized. The additional benefit of hypoglycemic agents for DR has become a new research field. Glucagon-like peptide-1 receptor (GLP-1R) has been shown to be widely expressed in tissues including retina. Glucagon-like peptide-1 receptor agonists (GLP-1RA) have been generally used in the treatment of diabetic patients. Studies shows that GLP-1RA could inhibit nerve damage by decrease apoptosis of nerve cells and activation of glial cells. In addition, GLP-1RA plays a protective role for tight junction (TJ) and cells of blood retinal barrier (BRB). It also protects retina from BRB damage. In this review, we discuss the potential protective mechanisms of GLP-1RA for DR beyond the hypoglycemic effects.

miR-365 promotes diabetic retinopathy through inhibiting Timp3 and increasing oxidative stress

miRs play critical roles in oxidative stress-related retinopathy pathogenesis. miR-365 was identified in a previously constructed library from glyoxal-treated rat Müller cell. This report explores epigenetic alterations in Müller cells under oxidative stress to develop a novel therapeutic strategy. To examine the miR-365 expression pattern, in situ hybridization and quantitative RT-PCR were performed. Bioinformatical analysis and dual luciferase report assay were applied to identify and confirm target genes. Streptozotocin (STZ)-treated rats were used as the diabetic retinopathy (DR) model. Lentivirus-mediated anti-miR-365 was delivered subretinally and intravitreally into the rats' eyes. The functional and structural changes were evaluated by electroretinogram (ERG), histologically, and through examination of expression levels of metallopeptidase inhibitor 3 (Timp3), glial fibrillary acidic protein (Gfap), recoverin (Rcvrn) and vascular endothelia growth factor A (Vegfa). Oxidative stress factors and pro-inflammatory cytokines were analyzed. miR-365 expression was confirmed in the glyoxal-treated rat Müller cell line (glyoxal-treated rMC-1). In the retina, miR-365 mainly localized in the inner nuclear layer (INL). The increased miR-365 participated in Müller cell gliosis through oxidative stress aggravation, as observed in glyoxal-treated rMC-1 and DR rats before 6 weeks. Timp3 was a target and negatively regulated by miR-365. When miR-365 was inhibited, Timp3 expression was upregulated, Müller cell gliosis was alleviated, and retinal oxidative stress was attenuated. Visual function was also partially rescued as detected by ERG. miR-365 was found to be highly expressed in the retina and the abnormality of miR-365/Timp3 pathway is closely related to the pathology, like Müller gliosis, and the visual injury in DR. The mechanism might be through oxidative stress, and miR-365/Timp3 could be a potential therapeutic target for treating DR.

Topical administration of DPP-IV inhibitors prevents retinal neurodegeneration in experimental diabetes

AIMS/HYPOTHESIS

The main aims of the present study were: (1) to assess the expression and content of dipeptidyl peptidase IV (DPP-IV) in human and db/db mouse retinas, and in human vitreous fluid; and (2) to determine whether the topical administration of the DPP-IV inhibitors (DPP-IVi) would prevent retinal neurodegeneration and vascular leakage in db/db mice by reducing endogenous glucagon-like peptide 1 (GLP-1) degradation.

METHODS

To assess the expression and content of DPP-IV, human samples of vitreous fluid and retinas were obtained from participants with type 2 diabetes (n = 8) and age-matched non-diabetic individuals (n = 8), as well as from db/db (n = 72) and db/+ (n = 28) mice. The interventional study, which included 72 db/db mice, consisted of the topical administration (eye drops) of saxagliptin, sitagliptin or vehicle for 14 days. DPP-IV mRNA levels were assessed by RT-PCR, and protein content was measured by ELISA or western blotting. GLP-1 was assessed by immunofluorescence, and its downstream effector exchange protein activated by cAMP-1 (EPAC-1) was used as a measure of GLP-1 receptor activation. Retinal analyses were performed in vivo by electroretinography and ex vivo by RT-PCR (Epac-1, Iba-1 [also known as Aif1]), western blotting (EPAC-1, glial fibrillar acidic protein [GFAP], glutamate-aspartate transporter [GLAST]) and immunofluorescence measurements (GLP-1, GFAP, ionised calcium binding adaptor molecule 1 [IBA-1], TUNEL, GLAST, albumin and collagen IV). Glutamate was quantified by HPLC. In addition, vascular leakage was examined by the Evans Blue method.

RESULTS

DPP-IV was present in human vitreous fluid but in a range 100-fold less than in plasma. Both mRNA levels and protein content were much lower in the retina than in the liver or bowel, but were significantly higher in retinal pigment epithelium (RPE) from diabetic donors in comparison to non-diabetic donors (p < 0.05). Topical treatment with DPP-IVi prevented glial activation, apoptosis and vascular leakage induced by diabetes in db/db mice (p < 0.05). Moreover, it also significantly prevented diabetes-induced functional abnormalities in the electroretinogram. A significant increase of both GLP-1 and EPAC-1 was found after treatment with DPP-IVi (p < 0.05). Furthermore, GLAST downregulation induced by diabetes was prevented, resulting in a significant reduction of extracellular glutamate concentrations. All these effects were observed without any changes in blood glucose levels.

CONCLUSIONS/INTERPRETATION

The topical administration of DPP-IVi is effective in preventing neurodegeneration and vascular leakage in the diabetic retina. These effects can be attributed to an enhancement of GLP-1, but other mechanisms unrelated to the prevention of GLP-1 degradation cannot be ruled out.

GLP-1R as a Target for the Treatment of Diabetic Retinopathy: Friend or Foe?

Glucagon-like peptide 1 receptor (GLP-1R) agonists are increasingly being used as treatment for type 2 diabetes. Since the U.S. Food and Drug Administration published recommendations about the cardiovascular safety of new antidiabetes therapies for treating type 2 diabetes in 2008, the results of two outstanding clinical trials using GLP-1R agonists addressing this issue (Liraglutide Effect and Action in Diabetes: Evaluation of Cardiovascular Outcome Results-A Long Term Evaluation [LEADER] and Trial to Evaluate Cardiovascular and Other Long-term Outcomes With Semaglutide in Subjects With Type 2 Diabetes [SUSTAIN-6]) have been published. Both studies found beneficial effects in terms of reducing the rates of cardiovascular death, nonfatal myocardial infarction, and nonfatal stroke. However, their results regarding the progression of diabetic retinopathy (DR) were neutral with liraglutide (LEADER) or worse when compared with placebo in the case of semaglutide (SUSTAIN-6). These results are surprising because of the beneficial effects of GLP-1R analogs reported in experimental models of DR. In this Perspective, an overview of the mechanisms by which GLP-1R activation exerts its effects in preventing or arresting experimental DR is given. In addition, we consider the possible reasons for the negative results regarding the progression of DR in the SUSTAIN-6 study, as well as the gaps that still need to be covered to further clarify this important issue in the management of type 2 diabetes.

Oxidative Stress-Related Mechanisms and Antioxidant Therapy in Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the most common microvascular complications of diabetes and is the leading cause of blindness in young adults. Oxidative stress has been implicated as a critical cause of DR. Metabolic abnormalities induced by high-glucose levels are involved in the development of DR and appear to be influenced by oxidative stress. The imbalance between reactive oxygen species (ROS) production and the antioxidant defense system activates several oxidative stress-related mechanisms that promote the pathogenesis of DR. The damage caused by oxidative stress persists for a considerable time, even after the blood glucose concentration has returned to a normal level. Animal experiments have proved that the use of antioxidants is a beneficial therapeutic strategy for the treatment of DR, but more data are required from clinical trials. The aims of this review are to highlight the improvements to our understanding of the oxidative stress-related mechanisms underlying the development of DR and provide a summary of the main antioxidant therapy strategies used to treat the disease.