Protection against methylglyoxal-derived AGEs by regulation of glyoxalase 1 prevents retinal neuroglial and vasodegenerative pathology

Diabetic retinopathy: hyperglycaemia, oxidative stress and beyond

Diabetic retinopathy remains a relevant clinical problem. In parallel with diagnostic and therapeutic improvements, the role of glycaemia and reactive metabolites causing cell stress and biochemical abnormalities as treatment targets needs continuous re-evaluation. Furthermore, the basic mechanisms of physiological angiogenesis, remodelling and pruning give important clues about the origins of vasoregression during the very early stages of diabetic retinopathy and can be modelled in animals. This review summarises evidence supporting a role for the neurovascular unit-composed of neuronal, glial and vascular cells-as a responder to the biochemical changes imposed by reactive metabolites and high glucose. Normoglycaemic animal models developing retinal degeneration, provide valuable information about common pathways downstream of progressive neuronal damage that induce vasoregression, as in diabetic models. These models can serve to assess novel treatments addressing the entire neurovascular unit for the benefit of early diabetic retinopathy.

Pyridoxamine improves survival and limits cardiac dysfunction after MI

Advanced glycation end products (AGEs) play a key role in the progression of heart failure. Whether treatments limiting AGEs formation would prevent adverse left ventricular remodeling after myocardial infarction (MI) remain unknown. We investigated whether pyridoxamine (PM) could limit adverse cardiac outcome in MI. Rats were divided into MI, MI + PM and Sham. Echocardiography and hemodynamic parameters were used to assess cardiac function 8 weeks post-surgery. Total interstitial collagen, collagen I and collagen III were quantified using Sirius Red and polarized light microscopy. PM improved survival following LAD occlusion. Pre-treatment with PM significantly decreased the plasma AGEs levels. MI rats treated with PM displayed reduced left ventricular end-diastolic pressure and tau compared to untreated MI rats. Deformation parameters were also improved with PM. The preserved diastolic function was related to the reduced collagen content, in particular in the highly cross-linked collagen type I, mainly in the peri-infarct region, although not via TGF-β1 pathway. Our data indicate that PM treatment prevents the increase in AGEs levels and reduces collagen levels in a rat model of MI, resulting in an improved cardiac phenotype. As such, therapies targeting formation of AGEs might be beneficial in the prevention and/or treatment of maladaptive remodeling following MI.

Dicarbonyl stress and glyoxalase enzyme system regulation in human skeletal muscle

Skeletal muscle insulin resistance is a hallmark of Type 2 diabetes (T2DM) and may be exacerbated by protein modifications by methylglyoxal (MG), known as dicarbonyl stress. The glyoxalase enzyme system composed of glyoxalase 1/2 (GLO1/GLO2) is the natural defense against dicarbonyl stress, yet its protein expression, activity, and regulation remain largely unexplored in skeletal muscle. Therefore, this study investigated dicarbonyl stress and the glyoxalase enzyme system in the skeletal muscle of subjects with T2DM (age: 56 ± 5 yr.; BMI: 32 ± 2 kg/m²) compared with lean healthy control subjects (LHC; age: 27 ± 1 yr.; BMI: 22 ± 1 kg/m²). Skeletal muscle biopsies obtained from the vastus lateralis at basal and insulin-stimulated states of the hyperinsulinemic (40 mU·m^-2·min^-1)-euglycemic (5 mM) clamp were analyzed for proteins related to dicarbonyl stress and glyoxalase biology. At baseline, T2DM had increased carbonyl stress and lower GLO1 protein expression (-78.8%), which inversely correlated with BMI, percent body fat, and HOMA-IR, while positively correlating with clamp-derived glucose disposal rates. T2DM also had lower NRF2 protein expression (-31.6%), which is a positive regulator of GLO1, while Keap1 protein expression, a negative regulator of GLO1, was elevated (207%). Additionally, insulin stimulation during the clamp had a differential effect on NRF2, Keap1, and MG-modified protein expression. These data suggest that dicarbonyl stress and the glyoxalase enzyme system are dysregulated in T2DM skeletal muscle and may underlie skeletal muscle insulin resistance. Whether these phenotypic differences contribute to the development of T2DM warrants further investigation.

Higher Plasma Methylglyoxal Levels Are Associated With Incident Cardiovascular Disease in Individuals With Type 1 Diabetes: A 12-Year Follow-up Study

Methylglyoxal (MGO), a major precursor for advanced glycation end products, is increased in diabetes. In diabetic rodents, inhibition of MGO prevents cardiovascular disease (CVD). Whether plasma MGO levels are associated with incident CVD in people with type 1 diabetes is unknown. We included 159 individuals with persistent normoalbuminuria and 162 individuals with diabetic nephropathy (DN) from the outpatient clinic at Steno Diabetes Center. We measured MGO at baseline and recorded fatal and nonfatal CVD over a median follow-up of 12.3 years (interquartile range 7.6-12.5 years). Data were analyzed by Cox regression, with adjustment for sex, age, HbA_1c, DN, diabetes duration, smoking, systolic blood pressure, antihypertensive medication, and BMI. During follow-up, 73 individuals suffered at least one CVD event (36 fatal and 53 nonfatal). Higher MGO levels were associated with total, fatal, and nonfatal incident CVD (hazard ratios [HRs] 1.47 [95% CI 1.13-1.91], 1.42 [1.01-1.99], and 1.46 [1.08-1.98], respectively). We observed a similar trend for total mortality (HR 1.24 [0.99-1.56]). This study shows for the first time in our knowledge that plasma MGO levels are associated with cardiovascular events in individuals with type 1 diabetes. MGO may explain, at least in part, the increased risk for CVD in type 1 diabetes.

Methylglyoxal-induced dicarbonyl stress in aging and disease: first steps towards glyoxalase 1-based treatments

Dicarbonyl stress is the abnormal accumulation of dicarbonyl metabolites leading to increased protein and DNA modification contributing to cell and tissue dysfunction in aging and disease. It is produced by increased formation and/or decreased metabolism of dicarbonyl metabolites. MG (methylglyoxal) is a dicarbonyl metabolite of relatively high flux of formation and precursor of the most quantitatively and functionally important spontaneous modifications of protein and DNA clinically. Major MG-derived adducts are arginine-derived hydroimidazolones of protein and deoxyguanosine-derived imidazopurinones of DNA. These are formed non-oxidatively. The glyoxalase system provides an efficient and essential basal and stress-response-inducible enzymatic defence against dicarbonyl stress by the reduced glutathione-dependent metabolism of methylglyoxal by glyoxalase 1. The GLO1 gene encoding glyoxalase 1 has low prevalence duplication and high prevalence amplification in some tumours. Dicarbonyl stress contributes to aging, disease and activity of cytotoxic chemotherapeutic agents. It is found at a low, moderate and severe level in obesity, diabetes and renal failure respectively, where it contributes to the development of metabolic and vascular complications. Increased glyoxalase 1 expression confers multidrug resistance to cancer chemotherapy and has relatively high prevalence in liver, lung and breast cancers. Studies of dicarbonyl stress are providing improved understanding of aging and disease and the basis for rational design of novel pharmaceuticals: glyoxalase 1 inducers for obesity, diabetes and cardiovascular disease and glyoxalase 1 inhibitors for multidrug-resistant tumours. The first clinical trial of a glyoxalase 1 inducer in overweight and obese subjects showed improved glycaemic control, insulin resistance and vascular function.

Ager Deletion Enhances Ischemic Muscle Inflammation, Angiogenesis, and Blood Flow Recovery in Diabetic Mice

OBJECTIVE

Diabetic subjects are at higher risk of ischemic peripheral vascular disease. We tested the hypothesis that advanced glycation end products (AGEs) and their receptor (RAGE) block angiogenesis and blood flow recovery after hindlimb ischemia induced by femoral artery ligation through modulation of immune/inflammatory mechanisms.

APPROACH AND RESULTS

Wild-type mice rendered diabetic with streptozotocin and subjected to unilateral femoral artery ligation displayed increased accumulation and expression of AGEs and RAGE in ischemic muscle. In diabetic wild-type mice, femoral artery ligation attenuated angiogenesis and impaired blood flow recovery, in parallel with reduced macrophage content in ischemic muscle and suppression of early inflammatory gene expression, including Ccl2 (chemokine [C-C motif] ligand-2) and Egr1 (early growth response gene-1) versus nondiabetic mice. Deletion of Ager (gene encoding RAGE) or transgenic expression of Glo1 (reduces AGEs) restored adaptive inflammation, angiogenesis, and blood flow recovery in diabetic mice. In diabetes mellitus, deletion of Ager increased circulating Ly6C^hi monocytes and augmented macrophage infiltration into ischemic muscle tissue after femoral artery ligation. In vitro, macrophages grown in high glucose display inflammation that is skewed to expression of tissue damage versus tissue repair gene expression. Further, macrophages grown in high versus low glucose demonstrate blunted macrophage-endothelial cell interactions. In both settings, these adverse effects of high glucose were reversed by Ager deletion in macrophages.

CONCLUSIONS

These findings indicate that RAGE attenuates adaptive inflammation in hindlimb ischemia; underscore microenvironment-specific functions for RAGE in inflammation in tissue repair versus damage; and illustrate that AGE/RAGE antagonism may fill a critical gap in diabetic peripheral vascular disease.

Involvement of a gut-retina axis in protection against dietary glycemia-induced age-related macular degeneration

Age-related macular degeneration (AMD) is the major cause of blindness in developed nations. AMD is characterized by retinal pigmented epithelial (RPE) cell dysfunction and loss of photoreceptor cells. Epidemiologic studies indicate important contributions of dietary patterns to the risk for AMD, but the mechanisms relating diet to disease remain unclear. Here we investigate the effect on AMD of isocaloric diets that differ only in the type of dietary carbohydrate in a wild-type aged-mouse model. The consumption of a high-glycemia (HG) diet resulted in many AMD features (AMDf), including RPE hypopigmentation and atrophy, lipofuscin accumulation, and photoreceptor degeneration, whereas consumption of the lower-glycemia (LG) diet did not. Critically, switching from the HG to the LG diet late in life arrested or reversed AMDf. LG diets limited the accumulation of advanced glycation end products, long-chain polyunsaturated lipids, and their peroxidation end-products and increased C3-carnitine in retina, plasma, or urine. Untargeted metabolomics revealed microbial cometabolites, particularly serotonin, as protective against AMDf. Gut microbiota were responsive to diet, and we identified microbiota in the Clostridiales order as being associated with AMDf and the HG diet, whereas protection from AMDf was associated with the Bacteroidales order and the LG diet. Network analysis revealed a nexus of metabolites and microbiota that appear to act within a gut-retina axis to protect against diet- and age-induced AMDf. The findings indicate a functional interaction between dietary carbohydrates, the metabolome, including microbial cometabolites, and AMDf. Our studies suggest a simple dietary intervention that may be useful in patients to arrest AMD.

The pathology associated with diabetic retinopathy

This review summarizes the pathological features of diabetic retinopathy. The lesions occurring in the diabetic retina have been described over many decades using descriptive and experimental approaches based on clinical studies on patients, human post-mortem material, animal models and various in vitro systems. We have also accumulated a wealth of knowledge about basic molecular mechanisms and key pathogenic processes that drive these abnormalities in diabetic retina. Despite these advances, there are still limited therapeutic options for diabetic retinopathy with those currently available only addressing late-stage disease. With a particular focus on the earlier stages of diabetes, there is growing appreciation the complex neuronal, glial and microvascular abnormalities which progressively disrupt retinal function. This is especially true from the perspective of the neurovascular unit during health and disease. Based on a strong appreciation of cellular and molecular pathology that underpins diabetic retinopathy, further advances are anticipated as we drive towards development of efficacious therapeutic options that can address all stages of disease.

The Glyoxalase System and Methylglyoxal-Derived Carbonyl Stress in Sepsis: Glycotoxic Aspects of Sepsis Pathophysiology

Sepsis remains one of the leading causes of death in intensive care units. Although sepsis is caused by a viral, fungal or bacterial infection, it is the dysregulated generalized host response that ultimately leads to severe dysfunction of multiple organs and death. The concomitant profound metabolic changes are characterized by hyperglycemia, insulin resistance, and profound transformations of the intracellular energy supply in both peripheral and immune cells. A further hallmark of the early phases of sepsis is a massive formation of reactive oxygen (ROS; e.g., superoxide) as well as nitrogen (RNS; e.g., nitric oxide) species. Reactive carbonyl species (RCS) form a third crucial group of highly reactive metabolites, which until today have been not the focus of interest in sepsis. However, we previously showed in a prospective observational clinical trial that patients suffering from septic shock are characterized by significant methylglyoxal (MG)-derived carbonyl stress, with the glyoxalase system being downregulated in peripheral blood mononuclear cells. In this review, we give a detailed insight into the current state of research regarding the metabolic changes that entail an increased MG-production in septicemia. Thus, we point out the special role of the glyoxalase system in the context of sepsis.

Methylglyoxal in Metabolic Disorders: Facts, Myths, and Promises

Glucose and fructose metabolism originates the highly reactive byproduct methylglyoxal (MG), which is a strong precursor of advanced glycation end products (AGE). The MG has been implicated in classical diabetic complications such as retinopathy, nephropathy, and neuropathy, but has also been recently associated with cardiovascular diseases and central nervous system disorders such as cerebrovascular diseases and dementia. Recent studies even suggested its involvement in insulin resistance and beta-cell dysfunction, contributing to the early development of type 2 diabetes and creating a vicious circle between glycation and hyperglycemia. Despite several drugs and natural compounds have been identified in the last years in order to scavenge MG and inhibit AGE formation, we are still far from having an effective strategy to prevent MG-induced mechanisms. This review summarizes the endogenous and exogenous sources of MG, also addressing the current controversy about the importance of exogenous MG sources. The mechanisms by which MG changes cell behavior and its involvement in type 2 diabetes development and complications and the pathophysiological implication are also summarized. Particular emphasis will be given to pathophysiological relevance of studies using higher MG doses, which may have produced biased results. Finally, we also overview the current knowledge about detoxification strategies, including modulation of endogenous enzymatic systems and exogenous compounds able to inhibit MG effects on biological systems.

Skin autofluorescence, renal insufficiency and retinopathy in patients with type 2 diabetes

OBJECTIVES

Advanced glycation end-products (AGEs) are involved in diabetic retinopathy (DR). Their accumulation in tissues can be analyzed by measuring the skin autofluorescence (sAF). We hypothesized that renal insufficiency, another cause of high sAF, may disturb the relation between sAF and DR.

RESEARCH DESIGN AND METHODS

We measured sAF with an AGE-Reader in 444 patients with type 2 diabetes (T2D), and we analyzed their retinal status. The associations of sAF with DR, and interaction with renal insufficiency were estimated by multivariate logistic regression analysis.

RESULTS

Mean age was 62years (standard deviation (SD) 10years), diabetes duration 13 (9) years and mean HbA1C 8.9% (1.8). The prevalence of DR was 21.4% and increased with age, diabetes duration, arterial hypertension, renal parameters (serum creatinine and albumin excretion rates), and sAF. The prevalence of macular edema (ME) was 8.6% and increased with the duration of diabetes, but not with sAF (p=0.11). There was a significant interaction between renal insufficiency and sAF for the relation with DR or ME (p=0.02). For the 83% patients without renal insufficiency (estimated GFR>60mL/min/1.73m2), sAF was related to DR or ME after multivariate adjustment: OR 1.87 (1.09-3.19). The 17% patients with renal insufficiency had the highest rates of DR or ME (38.6%) and the highest sAF, unrelated to each other.

CONCLUSIONS

In T2D patients with renal insufficiency, the high sAF does not relate to retinopathy, which should be systematically searched due to its high frequency. For other patients, a high sAF argues for DR screening.

Activation of Nrf2 attenuates carbonyl stress induced by methylglyoxal in human neuroblastoma cells: Increase in GSH levels is a critical event for the detoxification mechanism

The present study focused on the methylglyoxal (MG) detoxification mechanism in neuroblastoma cells. The involvement of nuclear factor erythroid 2-related factor 2 (Nrf2)/Kelch-like ECH-associated protein 1 (Keap1) pathway as a defense response against the formation of MG-modified proteins, which is well-known evidence of carbonyl stress, was also examined. We found that MG treatment resulted in accumulation of modified proteins bearing the structure of advanced glycation end products (AGEs) derived from MG in SH-SY5Y cells. This accumulation was suppressed by activation of the Nrf2 pathway prior to MG exposure via pre-treatment with an Nrf2 activator, carnosic acid and CDDO-Im, confirming the involvement of the Nrf2 pathway in MG detoxification. Although pre-treatment with the Nrf2 activator did not affect mRNA levels of GLO1, AKR1B1, and AKR7A2, the expressions of GCL and xCT mRNA, involved in GSH synthesis, were induced prior to increase in GSH levels. Furthermore, we demonstrated that a GSH synthesis inhibitor eliminated the MG detoxification effect derived from pretreatment with the Nrf2 activator. These results indicated that increase in GSH levels, induced by pre-treatment with carnosic acid, promoted the formation of the GLO1 substrate, hemithioacetal, thereby accelerating MG metabolism via the glyoxalase system and suppressing its toxicity. It was, therefore, determined that promotion of GSH synthesis via the Nrf2/Keap1pathway is important in the MG detoxification mechanism against neuronal MG-induced carbonyl stress, and Nrf2 activators contribute to reduction in the accumulation and toxic expression of carbonyl proteins.

Effects of the neuroprotective drugs somatostatin and brimonidine on retinal cell models of diabetic retinopathy

AIMS

Diabetic retinopathy is considered a microvascular disease, but recent evidence has underlined early involvement of the neuroretina with interactions between microvascular and neural alterations. Topical administration of somatostatin (SST), a neuroprotective molecule with antiangiogenic properties, prevents diabetes-induced retinal neurodegeneration in animals. The α₂-adrenergic receptor agonist brimonidine (BRM) decreases vitreoretinal vascular endothelial growth factor and inhibits blood-retinal barrier breakdown in diabetic rats. However, SST and BRM effects on microvascular cells have not yet been studied. We investigated the behaviour of these drugs on the crosstalk between microvasculature and neuroretina.

METHODS

Expression of SST receptors 1-5 in human retinal pericytes (HRP) was checked. We subsequently evaluated the effects of diabetic-like conditions (high glucose and/or hypoxia) with/without SST/BRM on HRP survival. Endothelial cells (EC) and photoreceptors were maintained in the above conditions and their conditioned media (CM) used to culture HRP. Vice versa, HRP-CM was used on EC and photoreceptors. Survival parameters were assessed.

RESULTS

HRP express the SST receptor 1 (SSTR1). Glucose fluctuations mimicking those occurring in diabetic subjects are more damaging for pericytes and photoreceptors than stable high glucose and hypoxic conditions. SST/BRM added to HRP in diabetic-like conditions decrease EC apoptosis. However, neither SST nor BRM changed the response of pericytes and neuroretina-vascular crosstalk under diabetic-like conditions.

CONCLUSIONS

Retinal pericytes express SSTR1, indicating that they can be a target for SST. Exposure to SST/BRM had no adverse effects, direct or mediated by the neuroretina, suggesting that these molecules could be safely evaluated for the treatment of ocular diseases.

The relationship between advanced glycation end products and ocular circulation in type 2 diabetes

AIMS

To determine whether skin autofluorescence (SAF) and serum pentosidine, biomarkers of advanced glycation end products (AGEs), were associated with ocular microcirculation in type 2 diabetes patients with early diabetic retinopathy (DR).

METHODS

This study included 46 eyes of 46 type 2 diabetes patients with no DR or non-proliferative DR. SAF was measured with an autofluorescence reader. Optic nerve head (ONH) microcirculation, represented by mean blur rate (MBR), was measured with laser speckle flowgraphy. Overall MBR, vascular MBR, and tissue MBR were calculated in software. MBR, SAF, pentosidine levels, and clinical findings, including central macular thickness (CMT), were then compared.

RESULTS

SAF in the diabetes patients was correlated with age (P=0.018). Serum pentosidine was correlated with age, vascular MBR and tissue MBR (P=0.046, P=0.035, and P=0.01, respectively). CMT was correlated with tissue MBR (P=0.016), but not with vascular MBR or overall MBR. Separate multiple regression analyses of independent contributing factors revealed that age, SAF, serum pentosidine, duration of diabetes, and pulse rate contributed to tissue MBR (P=0.041, P=0.046, P=0.022, P=0.011 and P=0.036, respectively), while SAF, HbA1c, pulse rate, tissue MBR, diastolic blood pressure, and creatinine contributed to CMT (P=0.005, P=0.039, P<0.001, P<0.001, P=0.022 and P=0.001, respectively).

CONCLUSIONS

Tissue MBR may be closely related to AGE levels and CMT in type 2 diabetes patients with early DR, suggesting that ocular circulation might be potential early biomarkers of DR.

Dicarbonyls and glyoxalase in disease mechanisms and clinical therapeutics

The reactive dicarbonyl metabolite methylglyoxal (MG) is the precursor of the major quantitative advanced glycation endproducts (AGEs) in physiological systems - arginine-derived hydroimidazolones and deoxyguanosine-derived imidazopurinones. The glyoxalase system in the cytoplasm of cells provides the primary defence against dicarbonyl glycation by catalysing the metabolism of MG and related reactive dicarbonyls. Dicarbonyl stress is the abnormal accumulation of dicarbonyl metabolites leading to increased protein and DNA modification contributing to cell and tissue dysfunction in ageing and disease. It is produced endogenously by increased formation and/or decreased metabolism of dicarbonyl metabolites. Dicarbonyl stress contributes to ageing, disease and activity of cytotoxic chemotherapeutic agents. It contributes to ageing through age-related decline in glyoxalase 1 (Glo-1) activity. Glo-1 has a dual role in cancer as a tumour suppressor protein prior to tumour development and mediator of multi-drug resistance in cancer treatment, implicating dicarbonyl glycation of DNA in carcinogenesis and dicarbonyl-driven cytotoxicity in mechanism of action of anticancer drugs. Glo-1 is a driver of cardiovascular disease, likely through dicarbonyl stress-driven dyslipidemia and vascular cell dysfunction. Dicarbonyl stress is also a contributing mediator of obesity and vascular complications of diabetes. There are also emerging roles in neurological disorders. Glo-1 responds to dicarbonyl stress to enhance cytoprotection at the transcriptional level through stress-responsive increase of Glo-1 expression. Small molecule Glo-1 inducers are in clinical development for improved metabolic, vascular and renal health and Glo-1 inhibitors in preclinical development for multidrug resistant cancer chemotherapy.

The protection conferred against ischemia-reperfusion injury in the diabetic brain by N-acetylcysteine is associated with decreased dicarbonyl stress

Diabetes, a risk factor for stroke, leads to elevated blood methylglyoxal (MG) levels. This is due to increased MG generation from the high glucose levels, and because diabetes impairs the glutathione (GSH)-glyoxalase system for MG elimination. MG glycates proteins and causes dicarbonyl stress. We investigated the contribution of MG and GSH to stroke outcome. Cerebral ischemia/reperfusion was performed in chemical-induced (streptozotocin) and genetic Akita mouse models of Type 1 diabetes. Brain infarction and functions of the GSH-dependent MG elimination pathway were determined. Diabetes increased post-ischemia-reperfusion cerebral infarct area in association with elevated MG and diminished GSH levels. Infarct size correlated with brain MG-to-GSH ratio. Expression of glutamate-cysteine ligase catalytic subunit (GCLc) was increased in diabetic brain. GCL activity was unchanged. MG-adducts were elevated in the diabetic brain and, using immunoprecipitation, we identified one of the bands as glycated occludin. This was accompanied by increased blood-brain barrier permeability. Total protein carbonyls were elevated, indicative of oxidative/carbonyl stress. N-acetylcysteine (NAC) corrected MG-to-GSH ratio, and reduced diabetic brain infarct area, occludin glycation and permeability. In addition, protein carbonyls were decreased by NAC. We showed that the diabetic brain exhibited a lower GSH-dependent potential for MG elimination, which contributed to increased protein glycation, and oxidative/carbonyl stress. The consequence of these changes was aggravated post-stroke brain injury. NAC administration protected against the exacerbated brain damage via restored GSH generation and normalization of the MG-to-GSH ratio and possibly by attenuating oxidative/carbonyl stress. This treatment could contribute to the successful management of stroke risk/outcome in diabetes.

Methylglyoxal can mediate behavioral and neurochemical alterations in rat brain

Diabetes is associated with loss of cognitive function and increased risk for Alzheimer's disease (AD). Advanced glycation end products (AGEs) are elevated in diabetes and AD and have been suggested to act as mediators of the cognitive decline observed in these pathologies. Methylglyoxal (MG) is an extremely reactive carbonyl compound that propagates glycation reactions and is, therefore, able to generate AGEs. Herein, we evaluated persistent behavioral and biochemical parameters to explore the hypothesis that elevated exogenous MG concentrations, induced by intracerebroventricular (ICV) infusion, lead to cognitive decline in Wistar rats. A high and sustained administration of MG (3μmol/μL; subdivided into 6days) was found to decrease the recognition index of rats, as evaluated by the object-recognition test. However, MG was unable to impair learning-memory processes, as shown by the habituation in the open field (OF) and Y-maze tasks. Moreover, a single high dose of MG induced persistent alterations in anxiety-related behavior, diminishing the anxiety-like parameters evaluated in the OF test. Importantly, MG did not alter locomotion behavior in the different tasks performed. Our biochemical findings support the hypothesis that MG induces persistent alterations in the hippocampus, but not in the cortex, related to glyoxalase 1 activity, AGEs content and glutamate uptake. Glial fibrillary acidic protein and S100B content, as well as S100B secretion (astroglial-related parameters of brain injury), were not altered by ICV MG administration. Taken together, our data suggest that MG interferes directly in brain function and that the time and the levels of exogenous MG determine the different features that can be seen in diabetic patients.

A Glyoxalase-1 Knockdown Does Not Have Major Short Term Effects on Energy Expenditure and Atherosclerosis in Mice

Objective. Glyoxalase-1 is an enzyme detoxifying methylglyoxal (MG). MG is a potent precursor of advanced glycation endproducts which are regarded to be a key player in micro- and macrovascular damage. Yet, the role of Glo1 in atherosclerosis remains unclear. In this study, the effect of Glo1 on mouse metabolism and atherosclerosis is evaluated. Methods. Glo1 knockdown mice were fed a high fat or a standard diet for 10 weeks. Body weight and composition were investigated by Echo MRI. The PhenoMaster system was used to measure the energy expenditure. To evaluate the impact of Glo1 on atherosclerosis, Glo1(KD) mice were crossed with ApoE-knockout mice and fed a high fat diet for 14 weeks. Results. Glo1 activity was significantly reduced in heart, liver, and kidney lysates derived from Glo1(KD) mice. Yet, there was no increase in methylglyoxal-derived AGEs in all organs analyzed. The Glo1 knockdown did not affect body weight or body composition. Metabolic studies via indirect calorimetry did not show significant effects on energy expenditure. Glo1(KD) mice crossed to ApoE(-/-) mice did not show enhanced formation of atherosclerosis. Conclusion. A Glo1 knockdown does not have major short term effects on the energy expenditure or the formation of atherosclerotic plaques.

The progress in understanding and treatment of diabetic retinopathy

Diabetic retinopathy is the most frequently occurring complication of diabetes mellitus and remains a leading cause of vision loss globally. Its aetiology and pathology have been extensively studied for half a century, yet there are disappointingly few therapeutic options. Although some new treatments have been introduced for diabetic macular oedema (DMO) (e.g. intravitreal vascular endothelial growth factor inhibitors ('anti-VEGFs') and new steroids), up to 50% of patients fail to respond. Furthermore, for people with proliferative diabetic retinopathy (PDR), laser photocoagulation remains a mainstay therapy, even though it is an inherently destructive procedure. This review summarises the clinical features of diabetic retinopathy and its risk factors. It describes details of retinal pathology and how advances in our understanding of pathogenesis have led to identification of new therapeutic targets. We emphasise that although there have been significant advances, there is still a pressing need for a better understanding basic mechanisms enable development of reliable and robust means to identify patients at highest risk, and to intervene effectively before vision loss occurs.

The role of CTGF in diabetic retinopathy

Connective tissue growth factor (CTGF, CCN2) contributes to fibrotic responses in diabetic retinopathy, both before clinical manifestations occur in the pre-clinical stage of diabetic retinopathy (PCDR) and in proliferative diabetic retinopathy (PDR), the late clinical stage of the disease. CTGF is a secreted protein that modulates the actions of many growth factors and extracellular matrix (ECM) proteins, leading to tissue reorganization, such as ECM formation and remodeling, basal lamina (BL) thickening, pericyte apoptosis, angiogenesis, wound healing and fibrosis. In PCDR, CTGF contributes to thickening of the retinal capillary BL and is involved in loss of pericytes. In this stage, CTGF expression is induced by advanced glycation end products, and by growth factors such as vascular endothelial growth factor (VEGF) and transforming growth factor (TGF)-β. In PDR, the switch from neovascularization to a fibrotic phase - the angio-fibrotic switch - in PDR is driven by CTGF, in a critical balance with vascular endothelial growth factor (VEGF). We discuss here the roles of CTGF in the pathogenesis of DR in relation to ECM remodeling and wound healing mechanisms, and explore whether CTGF may be a potential novel therapeutic target in the clinical management of early as well as late stages of DR.

The role of methylglyoxal and the glyoxalase system in diabetes and other age-related diseases

The formation and accumulation of advanced glycation endproducts (AGEs) are related to diabetes and other age-related diseases. Methylglyoxal (MGO), a highly reactive dicarbonyl compound, is the major precursor in the formation of AGEs. MGO is mainly formed as a byproduct of glycolysis. Under physiological circumstances, MGO is detoxified by the glyoxalase system into D-lactate, with glyoxalase I (GLO1) as the key enzyme in the anti-glycation defence. New insights indicate that increased levels of MGO and the major MGO-derived AGE, methylglyoxal-derived hydroimidazolone 1 (MG-H1), and dysfunctioning of the glyoxalase system are linked to several age-related health problems, such as diabetes, cardiovascular disease, cancer and disorders of the central nervous system. The present review summarizes the mechanisms through which MGO is formed, its detoxification by the glyoxalase system and its effect on biochemical pathways in relation to the development of age-related diseases. Although several scavengers of MGO have been developed over the years, therapies to treat MGO-associated complications are not yet available for application in clinical practice. Small bioactive inducers of GLO1 can potentially form the basis for new treatment strategies for age-related disorders in which MGO plays a pivotal role.

Post-Glucose Load Plasma α-Dicarbonyl Concentrations Are Increased in Individuals With Impaired Glucose Metabolism and Type 2 Diabetes: The CODAM Study

OBJECTIVE

There is increasing evidence that postprandial glucose excursions play an important role in the development of vascular complications. The underlying mechanism is unknown, but glucose-derived formation of reactive α-dicarbonyl compounds may explain why acute hyperglycemia leads to increased risk for diabetes complications. In the current study, we investigated whether α-dicarbonyls are increased after a glucose load in individuals without or with impaired glucose metabolism (IGM) and type 2 diabetes.

RESEARCH DESIGN AND METHODS

Cross-sectional, linear analyses were performed in the Cohort on Diabetes and Atherosclerosis Maastricht (CODAM [n = 574, 61% men, 60 years old]) study. Individuals with normal glucose metabolism (n = 279), IGM (n = 120), and type 2 diabetes (n = 92) who had complete data on an oral glucose tolerance test (OGTT) and were not on insulin treatment were included in the study population. Plasma α-dicarbonyl (methylglyoxal [MGO], glyoxal [GO], and 3-deoxyglucosone [3-DG]) levels were measured in the fasting state and in samples of the OGTT by ultra-performance liquid chromatography-tandem mass spectrometry.

RESULTS

The presence of both IGM and type 2 diabetes was significantly associated with higher α-dicarbonyl incremental areas under the curve (iAUCs), as calculated from the OGTT (for IGM, MGO β = 0.190 [95% CI 0.106-0.274], GO β = 0.287 [95% CI 0.172-0.401], and 3-DG β = 0.285 [95% CI 0.221-0.349]; for type 2 diabetes, MGO β = 0.293 [95% CI 0.180-0.405], GO β = 0.536 [95% CI 0.382-0.689], and 3-DG β = 0.542 [95% CI 0.456-0.628]). Adjustment for glucose iAUC attenuated these associations. iAUCs of the α-dicarbonyls correlated highly with glucose iAUC but not with fasting glucose levels or HbA1c.

CONCLUSIONS

The increased levels of α-dicarbonyls during an OGTT in individuals with IGM and type 2 diabetes underline the potential importance of α-dicarbonyl stress as a candidate to explain the increased risk of diabetes complications in individuals with postprandial hyperglycemia.

Role of the receptor for advanced glycation endproducts (RAGE) in retinal vasodegenerative pathology during diabetes in mice

AIMS/HYPOTHESIS

The receptor for AGEs (RAGE) is linked to proinflammatory pathology in a range of tissues. The objective of this study was to assess the potential modulatory role of RAGE in diabetic retinopathy.

METHODS

Diabetes was induced in wild-type (WT) and Rage (-/-) mice (also known as Ager (-/-) mice) using streptozotocin while non-diabetic control mice received saline. For all groups, blood glucose, HbA1c and retinal levels of methylglyoxal (MG) were evaluated up to 24 weeks post diabetes induction. After mice were killed, retinal glia and microglial activation, vasopermeability, leucostasis and degenerative microvasculature changes were determined.

RESULTS

Retinal expression of RAGE in WT diabetic mice was increased after 12 weeks (p < 0.01) but not after 24 weeks. Rage (-/-) mice showed comparable diabetes but accumulated less MG and this corresponded to enhanced activity of the MG-detoxifying enzyme glyoxalase I in their retina when compared with WT mice. Diabetic Rage (-/-) mice showed significantly less vasopermeability, leucostasis and microglial activation (p < 0.05-0.001). Rage (-/-) mice were also protected against diabetes-related retinal acellular capillary formation (p < 0.001) but not against pericyte loss.

CONCLUSIONS/INTERPRETATION

Rage (-/-) in diabetic mice is protective against many retinopathic lesions, especially those related to innate immune responses. Inhibition of RAGE could be a therapeutic option to prevent diabetic retinopathy.

Prevention of protein glycation by natural compounds

Non-enzymatic protein glycosylation (glycation) contributes to many diseases and aging of organisms. It can be expected that inhibition of glycation may prolong the lifespan. The search for inhibitors of glycation, mainly using in vitro models, has identified natural compounds able to prevent glycation, especially polyphenols and other natural antioxidants. Extrapolation of results of in vitro studies on the in vivo situation is not straightforward due to differences in the conditions and mechanism of glycation, and bioavailability problems. Nevertheless, available data allow to postulate that enrichment of diet in natural anti-glycating agents may attenuate glycation and, in consequence, ageing.

Dicarbonyl stress in cell and tissue dysfunction contributing to ageing and disease

Dicarbonyl stress is the abnormal accumulation of dicarbonyl metabolites leading to increased protein and DNA modification contributing to cell and tissue dysfunction in ageing and disease. Enzymes metabolising dicarbonyls, glyoxalase 1 and aldoketo reductases, provide an efficient and stress-response enzyme defence against dicarbonyl stress. Dicarbonyl stress is produced by increased formation and/or decreased metabolism of dicarbonyl metabolites, and by exposure to exogenous dicarbonyls. It contributes to ageing, disease and activity of cytototoxic chemotherapeutic agents.

Primary retinal cultures as a tool for modeling diabetic retinopathy: an overview

Experimental models of diabetic retinopathy (DR) have had a crucial role in the comprehension of the pathophysiology of the disease and the identification of new therapeutic strategies. Most of these studies have been conducted in vivo, in animal models. However, a significant contribution has also been provided by studies on retinal cultures, especially regarding the effects of the potentially toxic components of the diabetic milieu on retinal cell homeostasis, the characterization of the mechanisms on the basis of retinal damage, and the identification of potentially protective molecules. In this review, we highlight the contribution given by primary retinal cultures to the study of DR, focusing on early neuroglial impairment. We also speculate on possible themes into which studies based on retinal cell cultures could provide deeper insight.

Plasma advanced glycation end products are associated with incident cardiovascular events in individuals with type 2 diabetes: a case-cohort study with a median follow-up of 10 years (EPIC-NL)

Experimental data suggest a role for advanced glycation end products (AGEs) in cardiovascular disease (CVD), particularly in type 2 diabetes (T2DM). However, epidemiological evidence of an association between high plasma AGEs and increased cardiovascular risk remains inconclusive. Therefore, in a case-cohort study comprising 134 cardiovascular case subjects and a random subcohort of 218 individuals (including 65 cardiovascular case subjects), all with T2DM and nested in the European Prospective Investigation into Cancer and Nutrition in the Netherlands (EPIC-NL) study, plasma levels of protein-bound Nε-(carboxymethyl)lysine, Nε-(carboxyethyl)lysine, and pentosidine were measured with liquid chromatography. AGEs were loge-transformed, combined in a z-score, and the association with incident cardiovascular events was analyzed with Cox proportional hazard regression, adapted for case-cohort design (Prentice method). After multivariable adjustment (sex, age, cohort status, diabetes duration, total cholesterol to HDL-cholesterol ratio, smoking, systolic blood pressure, BMI, blood pressure-, cholesterol- and glucose-lowering treatment, prior cardiovascular events, and triglycerides), higher plasma AGE z-scores were associated with higher risk of incident cardiovascular events in individuals without prior cardiovascular events (hazard ratio 1.31 [95% CI: 1.06-1.61]). A similar trend was observed in individuals with prior cardiovascular events (1.37 [0.63-2.98]). In conclusion, high plasma AGEs were associated with incident cardiovascular events in individuals with T2DM. These results underline the potential importance of AGEs in development of CVD.

Methylglyoxal as a new biomarker in patients with septic shock: an observational clinical study

Introduction

The role of reactive carbonyl species, such as methylglyoxal (MG), has been overlooked within the context of the sepsis syndrome. The aims of this study were to assess the impact of MG formation in different inflammatory settings and to evaluate its use for early diagnosis as well as prognosis of the sepsis syndrome.

Methods

In total, 120 patients in three groups were enrolled in this observational clinical pilot study. The three groups included patients with septic shock (n = 60), postoperative controls (n = 30), and healthy volunteers (n = 30). Plasma samples from patients with septic shock were collected at sepsis onset and after 24 hours and 4, 7, 14, and 28 days. Plasma samples from postoperative controls were collected prior to surgery, immediately following the end of the surgical procedure as well as 24 hours later and from healthy volunteers once. Plasma levels of MG were determined by high-performance liquid chromatography. Additionally, plasma levels of procalcitonin, C-reactive protein, soluble CD14 subtype, and interleukin-6 were determined.

Results

Patients with septic shock showed significantly higher plasma levels of MG at all measured times, compared with postoperative controls. MG was found to identify patients with septic shock more effectively—area under the curve (AUC): 0.993—than procalcitonin (AUC: 0.844), C-reactive protein (AUC: 0.791), soluble CD14 subtype (AUC: 0.832), and interleukin-6 (AUC: 0.898) as assessed by receiver operating characteristic (ROC) analysis. Moreover, plasma levels of MG in non-survivors were significantly higher than in survivors (sepsis onset: *P = 0.018 for 90-day survival; **P = 0.008 for 28-day survival). Plasma levels of MG proved to be an early predictor for survival in patients with septic shock (sepsis onset: ROC-AUC 0.710 for 28-day survival; ROC-AUC 0.686 for 90-day survival).

Conclusions

MG was identified as a marker for monitoring the onset, development, and remission of sepsis and was found to be more useful than routine diagnostic markers. Further studies are required to determine the extent of MG modification in sepsis and whether targeting this pathway could be therapeutically beneficial to the patient.

Trial registration

German Clinical Trials Register DRKS00000505. Registered 8 November 2010.

Electronic supplementary material

The online version of this article (doi:10.1186/s13054-014-0683-x) contains supplementary material, which is available to authorized users.

Glyoxalase 1 overexpression does not affect atherosclerotic lesion size and severity in ApoE-/- mice with or without diabetes

AIMS

Advanced glycation end-products (AGEs) and their precursors have been associated with the development of atherosclerosis. We recently discovered that glyoxalase 1 (GLO1), the major detoxifying enzyme for AGE precursors, is decreased in ruptured human plaques, and that levels of AGEs are higher in rupture-prone plaques. We here investigated whether overexpression of human GLO1 in ApoE(-/-) mice could reduce the development of atherosclerosis.

METHODS AND RESULTS

We crossed C57BL/6 ApoE(-/-) mice with C57BL/6 GLO1 overexpressing mice (huGLO1(+/-)) to generate ApoE(-/-) (n = 16) and ApoE(-/-) huGLO1(+/-) (n = 20) mice. To induce diabetes, we injected a subset with streptozotocin (STZ) to generate diabetic ApoE(-/-) (n = 8) and ApoE(-/-) huGLO1(+/-) (n = 13) mice. All mice were fed chow and sacrificed at 25 weeks of age. The GLO1 activity was three-fold increased in huGLO1(+/-) aorta, but aortic root lesion size and phenotype did not differ between mice with and without huGLO1(+/-) overexpression. We detected no differences in gene expression in aortic arches, in AGE levels and cytokines, in circulating cells, and endothelial function between ApoE(-/-) mice with and without huGLO1(+/-) overexpression. Although diabetic mice showed decreased GLO1 expression (P < 0.05) and increased lesion size (P < 0.05) in comparison with non-diabetic mice, GLO1 overexpression also did not affect the aortic root lesion size or inflammation in diabetic mice.

CONCLUSION

In ApoE(-/-) mice with or without diabetes, GLO1 overexpression did not lead to decreased atherosclerotic lesion size or systemic inflammation. Increasing GLO1 levels does not seem to be an effective strategy to reduce glycation in atherosclerotic lesions, likely due to increased AGE formation through GLO1-independent mechanisms.

Differential effects of glyoxalase 1 overexpression on diabetic atherosclerosis and renal dysfunction in streptozotocin-treated, apolipoprotein E-deficient mice

The reactive dicarbonyls, glyoxal and methylglyoxal (MG), increase in diabetes and may participate in the development of diabetic complications. Glyoxal and MG are detoxified by the sequential activities of glyoxalase 1 (GLO1) and glyoxalase 2. To determine the contribution of these dicarbonyls to the etiology of complications, we have genetically manipulated GLO1 levels in apolipoprotein E‐null (Apoe^−/−) mice. Male Apoe^−/− mice, hemizygous for a human GLO1 transgene (GLO1TGApoe^−/− mice) or male nontransgenic Apoe^−/− litter mates were injected with streptozotocin or vehicle and 6 or 20 weeks later, aortic atherosclerosis was quantified. The GLO1 transgene lessened streptozotocin (STZ)‐induced increases in immunoreactive hydroimidazolone (MG‐H1). Compared to nondiabetic mice, STZ‐treated GLO1TGApoe^−/− and Apoe^−/− mice had increased serum cholesterol and triglycerides and increased atherosclerosis at both times after diabetes induction. While the increased GLO1 activity in the GLO1TGApoe^−/− mice failed to protect against diabetic atherosclerosis, it lessened glomerular mesangial expansion, prevented albuminuria and lowered renal levels of dicarbonyls and protein glycation adducts. Aortic atherosclerosis was also quantified in 22‐week‐old, male normoglycemic Glo1 knockdown mice on an Apoe^−/− background (Glo1KDApoe^−/− mice), an age at which Glo1KD mice exhibit albuminuria and renal pathology similar to that of diabetic mice. In spite of ~75% decrease in GLO1 activity and increased aortic MG‐H1, the Glo1KDApoe^−/− mice did not show increased atherosclerosis compared to age‐matched Apoe^−/− mice. Thus, manipulation of GLO1 activity does not affect the development of early aortic atherosclerosis in Apoe^−/− mice but can dictate the onset of kidney disease independently of blood glucose levels.

Increased levels of methylglyoxal and methylglyoxal‐derived advanced glycation end products may contribute to the development of diabetic complications. We show that overexpression of an enzyme that participates in the pathway of methylglyoxal detoxification, glyoxalase 1, protects streptozotocin‐treated, apolipoprotein E‐deficient mice from diabetic kidney disease but not from diabetes‐induced accelerated aortic atherosclerosis.

Emerging role of advanced glycation-end products (AGEs) in the pathobiology of eye diseases

Advanced glycation end products (AGEs) have been implicated in vision loss associated with macula degeneration, cataract formation, diabetic retinopathy and glaucoma. This pathogenic potential is mainly attributed to their accumulation in ocular tissues where they mediate aberrant crosslinking of extracellular matrix proteins and disruption of endothelial junctional complexes that affects cell permeability, mediates angiogenesis and breakdown of the inner blood-retinal barrier. Furthermore, AGEs severely affect cellular metabolism by disrupting ATP production, enhancing oxidative stress and modulating gene expression of anti-angiogenic and anti-inflammatory genes. Elucidation of AGE-induced mechanisms of action in different eye compartments will help in the understanding of the complex cellular and molecular processes associated with eye diseases. Several pharmaceutical agents with anti-glycating and anti-oxidant properties as well as AGE crosslink 'breakers' have been currently applied to eye diseases. The role of diet and the beneficial effects of certain nutriceuticals provide an alternative way to manage chronic visual disorders that affect the quality of life of millions of people.

The pathobiology of diabetic vascular complications--cardiovascular and kidney disease

With the increasing incidence of obesity and type 2 diabetes, it is predicted that more than half of Americans will have diabetes or pre-diabetes by 2020. Diabetic patients develop vascular complications at a much faster rate in comparison to non-diabetic individuals, and cardiovascular risk is increased up to tenfold. With the increasing incidence of diabetes across the world, the development of vascular complications will become an increasing medical burden. Diabetic vascular complications affect the micro- and macro-vasculature leading to kidney disease often requiring dialysis and transplantation or cardiovascular disease increasing the risk for myocardial infarction, stroke and amputations as well as leading to premature mortality. It has been suggested that many complex pathways contribute to the pathobiology of diabetic complications including hyperglycaemia itself, the production of advanced glycation end products (AGEs) and interaction with the receptors for AGEs such as the receptor for advanced glycation end products (RAGE), as well as the activation of vasoactive systems such as the renin-angiotensin aldosterone system (RAAS) and the endothelin system. More recently, it has been hypothesised that reactive oxygen species derived from NAD(P)H oxidases (Nox) may represent a common downstream mediator of vascular injury in diabetes. Current standard treatment of care includes the optimization of blood glucose and blood pressure usually including inhibitors of the renin-angiotensin system. Although these interventions are able to delay progression, they fail to prevent the development of complications. Thus, there is an urgent medical need to identify novel targets in diabetic vascular complications which may include the blockade of Nox-derived ROS formation, as well as blockade of AGE formation and inhibitors of RAGE activation. These strategies may provide superior protection against the deleterious effects of diabetes on the vasculature.

Reducing methylglyoxal as a therapeutic target for diabetic heart disease

Diabetes is a well-known risk factor for the development of cardiovascular diseases. Diabetes affects cardiac tissue through several different, yet interconnected, pathways. Damage to endothelial cells from direct exposure to high blood glucose is a primary cause of deregulated heart function. Toxic by-products of non-enzymatic glycolysis, mainly methylglyoxal, have been shown to contribute to the endothelial cell damage. Methylglyoxal is a precursor for advanced glycation end-products, and, although it is detoxified by the glyoxalase system, this protection mechanism fails in diabetes. Recent work has identified methylglyoxal as a therapeutic target for the prevention of cardiovascular complications in diabetes. A better understanding of the glyoxalase system and the effects of methylglyoxal may lead to more advanced strategies for treating cardiovascular complications associated with diabetes.

Glyoxalase I (Glo1) and its metabolites in vascular disease

Glo1 (glyxoalase I) is a cytosolic protein expressed in all mammalian cells. Its physiological function is the detoxification of MG (methylglyoxal), which is a potent precursor of AGEs (advanced glycation end-products). Although the impact of AGEs on different forms of vascular diseases has been intensively investigated, the evidence for the involvement of Glo1 and MG is still scarce. Recently, several studies have provided significant evidence for Glo1 having a protective effect on microvascular complications in diabetic patients, such as retinopathy and nephropathy. Regarding macrovascular complications, especially atherosclerotic lesions, the impact of Glo1 is even less clear. In the present article, we review the latest findings regarding the role of Glo1 and MG in vascular biology and the pathophysiology of micro- and macro-vascular disease.

Methylglyoxal and glyoxalase I in atherosclerosis

Cardiovascular disease, caused predominantly by atherosclerotic plaque rupture, remains one of the leading causes of death. However, the mechanism of plaque rupture remains largely unknown. Recent studies have linked high metabolic activity in inflamed atherosclerotic plaques to the development of plaque rupture. AGEs (advanced glycation end-products) are known to be formed as a result of high metabolic activity and are higher in rupture-prone than stable plaques. Furthermore, AGEs seem to be more than mere markers of metabolic activity, as recent studies have elucidated that AGEs and their major precursor, MG (methylglyoxal), may have an important role in the progression of atherosclerosis and plaque rupture. MG can be detoxified by Glo1 (glyoxalase I), thereby preventing the accumulation of MG and MG-derived AGEs. In the present review, data concerning MG, Glo1 and AGEs in the context of plaque phenotype are discussed.

The proinflammatory cytokine high-mobility group box-1 mediates retinal neuropathy induced by diabetes

To test the hypothesis that increased expression of proinflammatory cytokine high-mobility group box-1 (HMGB1) in epiretinal membranes and vitreous fluid from patients with proliferative diabetic retinopathy and in retinas of diabetic rats plays a pathogenetic role in mediating diabetes-induced retinal neuropathy. Retinas of 1-month diabetic rats and HMGB1 intravitreally injected normal rats were studied using Western blot analysis, RT-PCR and glutamate assay. In addition, we studied the effect of the HMGB1 inhibitor glycyrrhizin on diabetes-induced biochemical changes in the retina. Diabetes and intravitreal injection of HMGB1 in normal rats induced significant upregulation of HMGB1 protein and mRNA, activated extracellular signal-regulated kinase 1 and 2 (ERK1/2), cleaved caspase-3 and glutamate; and significant downregulation of synaptophysin, tyrosine hydroxylase, glutamine synthetase, and glyoxalase 1. Constant glycyrrhizin intake from the onset of diabetes did not affect the metabolic status of the diabetic rats, but it significantly attenuated diabetes-induced upregulation of HMGB1 protein and mRNA, activated ERK1/2, cleaved caspase-3, and glutamate. In the glycyrrhizin-fed diabetic rats, the decrease in synaptophysin, tyrosine hydroxylase, and glyoxalase 1 caused by diabetes was significantly attenuated. These findings suggest that early retinal neuropathy of diabetes involves upregulated expression of HMGB1 and can be ameliorated by inhibition of HMGB1.

The role of O-GlcNAc signaling in the pathogenesis of diabetic retinopathy

Diabetic retinopathy is a leading cause of blindness worldwide. Despite laser and surgical treatments, antiangiogenic and other therapies, and strict metabolic control, many patients progress to visual impairment and blindness. New insights are needed into the pathophysiology of diabetic retinopathy in order to develop new methods to improve the detection and treatment of disease and the prevention of blindness. Hyperglycemia and diabetes result in increased flux through the hexosamine biosynthetic pathway, which, in turn, results in increased PTM of Ser/Thr residues of proteins by O-linked β-N-acetylglucosamine (O-GlcNAc). O-GlcNAcylation is involved in regulation of many nuclear and cytoplasmic proteins in a manner similar to protein phosphorylation. Altered O-GlcNAc signaling has been implicated in the pathogenesis of diabetes and may play an important role in the pathogenesis of diabetic retinopathy. The goal of this review is to summarize the biology of the hexosamine biosynthesis pathway and O-GlcNAc signaling, to present the current evidence for the role of O-GlcNAc signaling in diabetes and diabetic retinopathy, and to discuss future directions for research on O-GlcNAc in the pathogenesis of diabetic retinopathy.

Advanced glycation end products and diabetic complications

During long standing hyperglycaemic state in diabetes mellitus, glucose forms covalent adducts with the plasma proteins through a non-enzymatic process known as glycation. Protein glycation and formation of advanced glycation end products (AGEs) play an important role in the pathogenesis of diabetic complications like retinopathy, nephropathy, neuropathy, cardiomyopathy along with some other diseases such as rheumatoid arthritis, osteoporosis and aging. Glycation of proteins interferes with their normal functions by disrupting molecular conformation, altering enzymatic activity, and interfering with receptor functioning. AGEs form intra- and extracellular cross linking not only with proteins, but with some other endogenous key molecules including lipids and nucleic acids to contribute in the development of diabetic complications. Recent studies suggest that AGEs interact with plasma membrane localized receptors for AGEs (RAGE) to alter intracellular signaling, gene expression, release of pro-inflammatory molecules and free radicals. The present review discusses the glycation of plasma proteins such as albumin, fibrinogen, globulins and collagen to form different types of AGEs. Furthermore, the role of AGEs in the pathogenesis of diabetic complications including retinopathy, cataract, neuropathy, nephropathy and cardiomyopathy is also discussed.

Advanced glycation end products and diabetic complications

During long standing hyperglycaemic state in diabetes mellitus, glucose forms covalent adducts with the plasma proteins through a non-enzymatic process known as glycation. Protein glycation and formation of advanced glycation end products (AGEs) play an important role in the pathogenesis of diabetic complications like retinopathy, nephropathy, neuropathy, cardiomyopathy along with some other diseases such as rheumatoid arthritis, osteoporosis and aging. Glycation of proteins interferes with their normal functions by disrupting molecular conformation, altering enzymatic activity, and interfering with receptor functioning. AGEs form intra- and extracellular cross linking not only with proteins, but with some other endogenous key molecules including lipids and nucleic acids to contribute in the development of diabetic complications. Recent studies suggest that AGEs interact with plasma membrane localized receptors for AGEs (RAGE) to alter intracellular signaling, gene expression, release of pro-inflammatory molecules and free radicals. The present review discusses the glycation of plasma proteins such as albumin, fibrinogen, globulins and collagen to form different types of AGEs. Furthermore, the role of AGEs in the pathogenesis of diabetic complications including retinopathy, cataract, neuropathy, nephropathy and cardiomyopathy is also discussed.

Protective effects of the neuropeptides PACAP, substance P and the somatostatin analogue octreotide in retinal ischemia: a metabolomic analysis

Ischemia is a primary cause of neuronal death in retinal diseases and the somatostatin subtype receptor 2 agonist octreotide (OCT) is known to decrease ischemia-induced retinal cell death. Using a recently optimized ex vivo mouse model of retinal ischemia, we tested the anti-ischemic potential of two additional neuropeptides, pituitary adenylate cyclase activating peptide (PACAP) and substance P (SP), and monitored the major changes occurring at the metabolic level. Metabolomics analyses were performed via fast HPLC online using a microTOF-Q MS instrument, a workflow that is increasingly becoming the gold standard in the field of metabolomics. The metabolomic approach allowed detection of the most significant alterations induced in the retina by ischemia and of the significance of the protective effects exerted by OCT, PACAP or SP. All treatments were shown to reduce ischemia-induced cell death, vascular endothelial growth factor over-expression and glutamate release. The metabolomic analysis showed that OCT and, to a lesser extent, also PACAP or SP, were able to counteract the ischemia-induced oxidative stress and to promote, with various efficacies, (i) decreased accumulation of glutamate and normalization of glutathione homeostasis; (ii) reduced build-up of α-ketoglutarate, which might serve as a substrate for the enhanced biosynthesis of glutamate in response to ischemia; (iii) reduced accumulation of peroxidized lipids and inflammatory mediators; (iv) the normalization of glycolytic fluxes and thus preventing the over-accumulation of lactate or either promoting the down-regulation of the glyoxalate anti-oxidant system; (v) a reduced metabolic shift from glycolysis towards the PPP or either a blockade at the non-oxidative phase of the PPP; and (vi) tuning down of purine metabolism. In addition, OCT seemed to stimulate nitric oxide production. None of the treatments was able to restore ATP production, although ATP reservoirs were partly replenished by OCT, PACAP or SP. These data indicate that, in addition to that of somatostatin, peptidergic systems such as those of PACAP and SP deserve attention in view of peptide-based therapies to treat ischemic retinal disorders.

A vascular piece in the puzzle of adipose tissue dysfunction: mechanisms and consequences

In the last years, several studies unravelled many aspects of adipose tissue pathophysiology in metabolic diseases. Some studies suggested hypoxia as one of such aspects, despite the exact mechanisms and pathophysiological significance is still partially unknown. Adipose tissue was shown to be hypoxic in obesity, mainly resulting from adipocyte hypertrophy, leading to increased activation of inflammatory pathways. In animal and cell models, hypoxia-induced inflammation was shown to lead to endocrine alterations and dysmetabolism. However, recent evidences suggest that instead of a simple low oxygenation theory, adipose tissue microvasculature may be regulated by a series of factors, including vasoactive factors like angiotensin II, angiogenesis and glycation, among others. This review summarizes the current knowledge about the role of these factors in the regulation of adipose tissue irrigation and the functional consequences of adipose tissue microvascular dysfunction.

Glyoxalase-1 overexpression reduces endothelial dysfunction and attenuates early renal impairment in a rat model of diabetes

AIMS/HYPOTHESIS

In diabetes, advanced glycation end-products (AGEs) and the AGE precursor methylglyoxal (MGO) are associated with endothelial dysfunction and the development of microvascular complications. In this study we used a rat model of diabetes, in which rats transgenically overexpressed the MGO-detoxifying enzyme glyoxalase-I (GLO-I), to determine the impact of intracellular glycation on vascular function and the development of early renal changes in diabetes.

METHODS

Wild-type and Glo1-overexpressing rats were rendered diabetic for a period of 24 weeks by intravenous injection of streptozotocin. Mesenteric arteries were isolated to study ex vivo vascular reactivity with a wire myograph and kidneys were processed for histological examination. Glycation was determined by mass spectrometry and immunohistochemistry. Markers for inflammation, endothelium dysfunction and renal dysfunction were measured with ELISA-based techniques.

RESULTS

Diabetes-induced formation of AGEs in mesenteric arteries and endothelial dysfunction were reduced by Glo1 overexpression. Despite the absence of advanced nephrotic lesions, early markers of renal dysfunction (i.e. increased glomerular volume, decreased podocyte number and diabetes-induced elevation of urinary markers albumin, osteopontin, kidney-inflammation-molecule-1 and nephrin) were attenuated by Glo1 overexpression. In line with this, downregulation of Glo1 in cultured endothelial cells resulted in increased expression of inflammation and endothelium dysfunction markers. In fully differentiated cultured podocytes incubation with MGO resulted in apoptosis.

CONCLUSIONS/INTERPRETATION

This study shows that effective regulation of the GLO-I enzyme is important in the prevention of vascular intracellular glycation, endothelial dysfunction and early renal impairment in experimental diabetes. Modulating the GLO-I pathway therefore may provide a novel approach to prevent vascular complications in diabetes.

Neurodegeneration in the diabetic eye: new insights and therapeutic perspectives

Diabetic retinopathy (DR), one of the leading causes of preventable blindness, has been considered a microcirculatory disease of the retina. However, there is emerging evidence to suggest that retinal neurodegeneration is an early event in the pathogenesis of DR, which participates in the development of microvascular abnormalities. Therefore, the study of the underlying mechanisms leading to neurodegeneration and the identification of the mediators in the crosstalk between neurodegeneration and microangiopathy will be essential for the development of new therapeutic strategies. In this review, an updated discussion of the mechanisms involved in neurodegeneration, as well as the link between neurodegeneration and microangiopathy, is presented. Finally, the therapeutic implications and new perspectives based on identifying those patients with retinal neurodegeneration are given.

Reactive carbonyl species and their roles in sarcoplasmic reticulum Ca2+ cycling defect in the diabetic heart

Efficient and rhythmic cardiac contractions depend critically on the adequate and synchronized release of Ca(2+) from the sarcoplasmic reticulum (SR) via ryanodine receptor Ca(2+) release channels (RyR2) and its reuptake via sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a). It is well established that this orchestrated process becomes compromised in diabetes. What remain incompletely defined are the molecular mechanisms responsible for the dysregulation of RyR2 and SERCA2a in diabetes. Earlier, we found elevated levels of carbonyl adducts on RyR2 and SERCA2a isolated from hearts of type 1 diabetic rats and showed the presence of these posttranslational modifications compromised their functions. We also showed that these mono- and di-carbonyl reactive carbonyl species (RCS) do not indiscriminately react with all basic amino acid residues on RyR2 and SERCA2a; some residues are more susceptible to carbonylation (modification by RCS) than others. A key unresolved question in the field is which of the many RCS that are upregulated in the heart in diabetes chemically react with RyR2 and SERCA2a? This brief review introduces readers to the field of RCS and their roles in perturbing SR Ca(2+) cycling in diabetes. It also provides new experimental evidence that not all RCS that are upregulated in the heart in diabetes chemically react with RyR2 and SERCA2a, methylglyoxal and glyoxal preferentially do.

Knockdown of glyoxalase 1 mimics diabetic nephropathy in nondiabetic mice

Differences in susceptibility to diabetic nephropathy (DN) between mouse strains with identical levels of hyperglycemia correlate with renal levels of oxidative stress, shown previously to play a central role in the pathogenesis of DN. Susceptibility to DN appears to be genetically determined, but the critical genes have not yet been identified. Overexpression of the enzyme glyoxalase 1 (Glo1), which prevents posttranslational modification of proteins by the glycolysis-derived α-oxoaldehyde, methylglyoxal (MG), prevents hyperglycemia-induced oxidative stress in cultured cells and model organisms. In this study, we show that in nondiabetic mice, knockdown of Glo1 increases to diabetic levels both MG modification of glomerular proteins and oxidative stress, causing alterations in kidney morphology indistinguishable from those caused by diabetes. We also show that in diabetic mice, Glo1 overexpression completely prevents diabetes-induced increases in MG modification of glomerular proteins, increased oxidative stress, and the development of diabetic kidney pathology, despite unchanged levels of diabetic hyperglycemia. Together, these data indicate that Glo1 activity regulates the sensitivity of the kidney to hyperglycemic-induced renal pathology and that alterations in the rate of MG detoxification are sufficient to determine the glycemic set point at which DN occurs.

Glyoxalase-1 overexpression in bone marrow cells reverses defective neovascularization in STZ-induced diabetic mice

AIMS

Methylglyoxal (MG) accumulates in diabetes and impairs neovascularization. This study assessed whether overexpressing the MG-metabolizing enzyme glyoxalase-1 (GLO1) in only bone marrow cells (BMCs) could restore neovascularization in ischaemic tissue of streptozotocin-induced diabetic mice.

METHODS AND RESULTS

After 24 h of hyperglycaemic and hypoxic culture, BMCs from GLO1 overexpressing and wild-type (WT) diabetic mice were compared for migratory potential, viability, and mRNA expression of anti-apoptotic genes (Bcl-2 and Bcl-XL). In vivo, BMCs from enhanced green fluorescent protein (eGFP) mice that overexpress GLO1 were used to reconstitute the BM of diabetic mice (GLO1-diabetics). Diabetic and non-diabetic recipients of WT GFP(+) BM served as controls (WT-diabetics and non-diabetics, respectively). Following hindlimb ischaemia, the mobilization of BMCs was measured by flow cytometry. In hindlimbs, the presence of BM-derived angiogenic (GFP(+)CXCR4(+)) and endothelial (GFP(+)vWF(+)) cells and also arteriole density were determined by immunohistochemistry. Hindlimb perfusion was measured using laser Doppler. GLO1-BMCs had superior migratory potential, increased viability, and greater Bcl-2 and Bcl-XL expression, compared with WT BMCs. In vivo, the mobilization of pro-angiogenic BMCs (CXCR4(+), c-kit(+), and Flk(+)) was enhanced post-ischaemia in GLO1-diabetics compared to WT-diabetics. A greater number of GFP(+)CXCR4(+) and GFP(+)vWF(+) BMCs incorporated into the hindlimb tissue of GLO1-diabetics and non-diabetics than in WT-diabetics. Arteriole and capillary density and perfusion were also greater in GLO1-diabetics and non-diabetics.

CONCLUSION

This study demonstrates that protection from MG uniquely in BM is sufficient to restore BMC function and neovascularization of ischaemic tissue in diabetes and identifies GLO1 as a potential therapeutic target.