Volume 52, Issue 1 - Special issue: Frontiers in research on the Maillard reaction in aging and chronic disease / Guest Editors: John Baynes, Columbia, SC, USA and Philipp Gillery, Reims, France

Increase in life expectancy concerns most populations but more importantly developed countries. This increase is accompanied by the shift of chronic diseases to the senior population, especially cardiovascular diseases and diabetes type II. Aging mechanisms, mostly post-genetic, comprise among others the Maillard reaction which strongly contributes by several harmful processes to the age-dependent decline of tissue structure and function. Several of these mechanisms were studied in our laboratory at the cellular-molecular level and will be described in this review with respect to their role in aging and age-related pathologies, especially cardiovascular diseases.

Chronic kidney disease (CKD), especially in its end stage, is marked by extremely high cardiovascular rates of morbidity and mortality; hemodialysis patients have a five-fold shorter life expectancy than healthy subjects of the same age. In CKD the metabolic products that accumulate in the body are so-called uremic toxins. These include advanced glycation end-products (AGE). AGE levels are markedly increased in CKD patients not only because of impaired excretion but also because of increased production. AGE formation has initially been described as a non-enzymatic reaction between proteins and glucose in the so-called Maillard reaction, but they are also more rapidly formed during oxidative stress and subsequent formation of reactive carbonyl compounds like (methyl)glyoxal. AGE accumulate in tissue where they cross-link with proteins, e.g., collagen, inducing tissue stiffening of blood vessels and skin. They may also interact with receptor of AGE (RAGE) and other receptors, which lead to activation of intracellular transduction mechanisms resulting in cytokine release and further tissue damage in CKD. The accumulation of AGE in the skin can be measured non-invasively using autofluorescence. The skin autofluorescence is a strong marker of cardiovascular mortality in CKD. The focus of this review is on the role of tissue and plasma AGE, and of skin autofluorescence as a proxy of tissue AGE accumulation, in the increase in cardiovascular disease in end stage renal disease (ESRD). This review will also present the possibility of reducing the AGE accumulation in ESRD patients using the following five methods: 1) use of low AGE peritoneal dialysis solutions; 2) use of advanced hemodialysis techniques; 3) use of AGE reducing drugs; 4) optimizing the nutrition of hemodialysis patients; and 5) renal transplantation.

Advanced glycation end products (AGEs) represent a family of protein, peptide, amino acid, nucleic acid and lipid adducts formed by the reaction of carbonyl compounds derived directly or indirectly from glucose, ascorbic acid and other metabolites such as methylglyoxal. AGE formation in diabetes is of growing importance for their role as markers and potential culprits of diabetic complications, in particular retinopathy, nephropathy and neuropathy. Development of sensitive and specific assays utilizing liquid chromatography mass spectrometry with isotope dilution method has made it possible to detect and quantitate non-UV active AGEs such as carboxymethyl-lysine and glucosepane, the most prevalent AGE and protein crosslink of the extracellular matrix. Below we review studies on AGE formation in two skin biopsies obtained near the closeout of the Diabetes Control and Complications Trial (DCCT), one of which was processed in 2011 for assay of novel AGEs. The results of these analyses show that while several AGEs are associated and predict complication progression, the glucose/fructose-lysine/glucosepane AGE axis is one of the most robust markers for microvascular disease, especially retinopathy, in spite of adjustment for past or future average glycemia. Yet overall little biological and clinical information is available on glucosepane, making this review a call for data in a field of growing importance for diabetes and chronic metabolic diseases of aging.

In living organisms, proteins are progressively modified by spontaneous non-enzymatic reactions generating many post-translational modification derived products (PTMDPs) which exert deleterious effects and may be considered endogenous toxins in diabetes mellitus and chronic renal failure. Non-enzymatic glycation, which refers to the spontaneous binding of reducing sugars to free amino groups, is increased in diabetes mellitus because of hyperglycemia and is amplified by oxidative processes (‘glycoxidation’). Glycoxidation leads to the formation of ‘advanced glycation end products’ (AGEs), together with products of other oxidative pathways. AGEs alter tissue organization and cell-protein interactions, mainly in the case of long-lived extracellular matrix proteins, and interact with membrane receptors, among which RAGE (receptor of AGEs), a multiligand receptor which triggers intracellular signaling pathways stimulating inflammatory functions. Another major protein modification, carbamylation, is increased in chronic renal failure, which may occur during the course of diabetes mellitus. Carbamylation is due to the binding of isocyanic acid on the α-NH 2 extremity of proteins or amino acids, or on ε-NH 2 lysine groups, generating homocitrulline, a potential biomarker in atherosclerosis. Isocyanic acid is formed in vivo either by spontaneous dissociation of urea or by myeloperoxidase action on thiocyanate. Carbamylated proteins exhibit altered properties. For example, carbamylated collagen is unable to stimulate oxidative functions of polymorphonuclear neutrophils but increases matrix metalloproteinase-9 production by monocytes. Lipoprotein functions are altered by carbamylation and may contribute to atherogenesis. Thus, the numerous PTMDPs may be considered both hallmarks of protein damage in chronic diseases and endogenous toxins acting at the molecular and cellular levels.

Non-enzymatic modification of proteins in hyperglycemia is a major proposed mechanism of diabetic complications. Specifically, advanced glycation end products (AGEs) derived from hyperglycemia-induced reactive carbonyl species (RCS) can have pathogenic consequences when they target functionally critical protein residues. Modification of a small number of these critical residues, often undetectable by the methodologies relying on measurements of total AGE levels, can cause significant functional damage. Therefore, detection of specific sites of protein damage in diabetes is central to understanding the molecular basis of diabetic complications and for identification of biomarkers which are mechanistically linked to the disease. The current paradigm of RCS-derived protein damage places a major focus on methylglyoxal (MGO), an intermediate of cellular glycolysis. We propose that glyoxal (GO) is a major contributor to extracellular matrix (ECM) damage in diabetes. Here, we review the current knowledge and provide new data about GO-derived site-specific ECM modification in experimental diabetes.

Lipid oxidation plays a role in the pathophysiology of several diseases, including diabetes. Patients with type 2 diabetes show abnormally high plasma levels of phosphatidylcholine hydroperoxide (PCOOH). However, little is known about the biochemical processes that increase plasma PCOOH in diabetes. We hypothesized that “glycated lipid moieties” may form in diabetic plasma and cause oxidative stress resulting in PCOOH formation. To evaluate this hypothesis, liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods were developed to analyze Amadori-glycated phosphatidylethanolamine (Amadori-PE, an early stage Maillard product), as well as the advanced glycation end products (AGE) carboxymethyl-PE (CM-PE) and carboxyethyl-PE (CE-PE). The product ion scan, neutral loss scanning, and multiple reaction monitoring provide useful structural and quantitative information about Amadori-PE, CM-PE, and CE-PE in diabetic plasma and erythrocytes. We found that plasma and erythrocyte Amadori-PE concentrations were significantly higher in diabetic patients (757±377 nM plasma, 2793±989 nM packed cells) than in normal subjects (165±66 nM plasma, 712±52 nM packed cells), and that Amadori-PE concentrations were positively correlated with PCOOH. By contrast, no significant differences were observed in blood AGE-PE concentrations between diabetic patients (CM-PE: 7.7±3.5 nM plasma, 528±83 nM packed cells; CE-PE: 2.5±1.1 nM plasma, 82±24 nM packed cells) and normal subjects (CM-PE: 6.6±3.1 nM plasma, 705±533 nM packed cells; CE-PE: 4.2±1.5 nM plasma, 68±16 nM packed cells). These results suggest that Amadori-PE is more prone to accumulation in the blood with diabetes than CM-PE or CE-PE. This review describes the involvement of blood lipid glycation and lipid oxidation in the development of diabetes.

For several decades, researchers have been investigating the relations between the consumption of Maillard reaction products (MRPs) and biological activities, whether harmful or beneficial. Interest in this area is growing because the intake of these compounds has increased in parallel with the consumption of thermally processed foods. MRPs are bioavailable to some degree and, although there is little consensus about possible harmful effects or pathways of action, there are pathological implications regarding the progress and development of various degenerative disorders. Besides these considerations, it has been reported that some MRPs, particularly melanoidins, have beneficial effects as antioxidant, anti-inflammatory or anti-aging factors, among others. Some interesting aspects of this dual nature of MRPs are addressed in the present review. In this context, it is difficult to draw firm conclusions about the physiological consequences resulting from MRPs intake, due to the diversity of compounds formed in the different food matrixes. Maintaining a balanced and varied diet, not only in the food consumed but also in how it is processed, is increasingly viewed as the best strategy to limit the negative effects and to preserve the positive actions of dietary MRPs. Much remains to be done to elucidate the chemical structures of these compounds and to relate it to specific physiologic actions. For that purpose, setting the chromatographic determinations as the most reliable analytical tools to measure them in foods and biological tissues/fluids seems essential. Moreover, designing more realistic and better controlled clinical studies, based on the real diet of the population, will be the only method to perform an effective health-risk assessment of MRPs consumption.

Food allergies are abnormal responses to a food triggered by the immune system. The majority of allergenic foods are often subjected to thermal processing before consumption. The Maillard reaction is a non-enzymatic reaction between reducing sugars and compounds with free amino groups such as amino acids and proteins, and takes place during thermal processing and storage of foods. Among many other effects the reaction leads to modification of proteins with various types of glycation structures such as Nε(carboxymethyl)lysine (CML), pentosidine, pyrraline and methylglyoxal-H1, which are collectively called advanced glycation end-products (AGEs). Notably, evidence has accumulated that some glycation structures of AGEs function as immune epitopes. Here we discuss the possible involvement of food allergen AGEs in the pathogenesis of food allergies.

In a recent article, we presented the hypothesis that decompartmentalized metal ions are a major contributor to the development of diabetic complications and supported the use of chelation therapy for the treatment of diabetic complications [Nagai R, Murray DB, Metz TO, Baynes JW. Chelation: a fundamental mechanism of action of AGE inhibitors, AGE breakers, and other inhibitors of diabetes complications. Diabetes 2012;61:549–59]. Evidence in support of this hypothesis included the observation that many drugs used in the treatment of diabetes are chelators, that advanced glycation end product (AGE) inhibitors and AGE breakers lack carbonyl-trapping or AGE-breaker activity but are potent chelators, and that simple copper chelators inhibit vascular pathology in diabetes and aging. In the present article, we extend this hypothesis, proposing the interplay between copper and iron in the development of pathology in diabetes and other chronic age-related diseases, including atherosclerosis and neurodegenerative diseases. We also discuss the need and provide a framework for the development of a clinical laboratory test to assess plasma autoxidative catalytic activity and transition metal homeostasis in vivo.

Background: We hypothesized that genetic variability in genes encoding enzymes metabolizing glycolytic intermediates produced in excess under hyperglycemic conditions [i.e., transketolase ( TKT ), transaldolase, TKT-like protein 1, fructosamine 3-kinase ( FN3K ), glyoxalase 1 and glucose-6-phosphate dehydrogenase] could influence progression of diabetic nephropathy (DN) and diabetes-related morbidity and mortality. Methods: A total of 19 single nucleotide polymorphisms (SNPs) in six candidate genes were studied in 314 type 2 diabetic subjects with variable stage of kidney disease (normo- and microalbuminuria, proteinuria, end-stage renal disease). SNP selection criteria were based on known functional effect and gene coverage. SNPs were detected using polymerase chain reaction based methods. Subjects were followed up for median of 38 months. Time-to-event analysis considered three end-points: 1) DN progression by at least one stage; 2) major cardiovascular event; and 3) all-cause mortality. Results: We found combined effect of TKT SNP rs11130362 and FN3K SNP rs1056534 on DN progression (p<0.01). Additionally, TKT rs3736156 alone and also in combination with the previous two SNPs exhibited significant effect on incidence of major cardiovascular events (p<0.01 and p=0.01, respectively). Conclusions: Genetic variability in rate-limiting enzymes of pathways proposed to confer hypothetical protection against hyperglycemia might act as an important determinant of hyperglycemia toxicity in long-standing diabetes.

Background: The reactive α-oxoaldehydes glyoxal (GO), methylglyoxal (MGO) and 3-deoxyglucosone (3-DG) have been linked to diabetic complications and other age-related diseases. Numerous techniques have been described for the quantification of α-oxoaldehydes in blood or plasma, although with several shortcomings such as the need of large sample volume, elaborate extraction steps or long run-times during analysis. Therefore, we developed and evaluated an improved method including sample preparation, for the quantification of these α-oxoaldehydes in blood and plasma with ultra performance liquid chromatography tandem mass spectrometry (UPLC MS/MS). Methods: EDTA plasma and whole blood samples were deproteinized using perchloric acid (PCA) and subsequently derivatized with o-phenylenediamine (oPD). GO, MGO and 3-DG concentrations were determined using stable isotope dilution UPLC MS/MS with a run-to-run time of 8 min. Stability of α-oxoaldehyde concentrations in plasma and whole blood during storage was tested. The concentration of GO, MGO and 3-DG was measured in EDTA plasma of non-diabetic controls and patients with type 2 diabetes (T2DM). Results: Calibration curves of GO, MGO and 3-DG were linear throughout selected ranges. Recoveries of these α-oxoaldehydes were between 95% and 104%. Intra- and inter-assay CVs were between 2% and 14%. Conclusions: To obtain stable and reliable α-oxoaldehyde concentrations, immediate centrifugation of blood after blood sampling is essential and the use of EDTA as anticoagulant is preferable. Moreover, immediate precipitation of plasma protein with PCA stabilized α-oxoaldehyde concentrations for at least 120 min. With the use of the developed method, we found increased plasma concentrations of GO, MGO and 3-DG in T2DM as compared with non-diabetic controls.

Background: An impact on glycation, and possibly on diabetic complications, is attributed to fructosamine-3-kinase (FN3K) and its related protein (FN3K-RP) because they degrade Amadori compounds in vivo. Little is known about individual differences in FN3K-RP activity, which might contribute to an individual risk for diabetic complications. Methods: An HPLC-based activity assay for FN3K-RP in erythrocytes with the substrate N -α-hippuryl- N -ε-psicosyllysine was developed. The activities of FN3K and FN3K-RP were also analysed in erythrocytes of 103 consecutive participants of a health-care survey amongst a high-risk group for diabetes. The potential associations of these activities with the subjects’ health background (anthropometric data, glucose tolerance and HbA 1c , blood lipids, history of metabolic diseases in the subjects and their families, and medication) were examined. Results: The interindividual variability of FN3K-RP is less pronounced than that of FN3K [60–135 vs. 2.8–12.5 mU/g haemoglobin (Hb)]. No correlations with age, sex, body weight, blood cholesterol, or plasma glucose in an oral glucose tolerance test were observed. Subjects with kidney disease had higher activity of mainly FN3K-RP [111±15 vs. 98±18 mU/g Hb, mean±standard deviations (SDs), n=16 vs. 87, p=0.009], whereas subjects whose parents or siblings had a stroke showed lower FN3K activity (6.2±1.6 vs. 7.1±1.8 mU/g Hb, mean±SD, n=24 vs. 66, p=0.040). Conclusions: There is a likely impact of FN3K and FN3K-RP on the glycation cascade in vivo with potential positive and negative effects. The new screening method enables further studies to elucidate the function and importance of FN3K-RP.

Background: Circulating levels of pro-inflammatory advanced glycation end products (AGEs) are increased in diabetes and other conditions characterized by chronically elevated oxidant stress (OS). OS also increases after acute trauma and is implicated in the development of complications such as multiple organ failure. Herein, we assess the effect of acute OS on circulating levels of AGEs in a cohort of acute trauma victims. Methods: An observational study was performed at a large Level 1 Trauma Center. Blood samples for measurement of two AGEs, carboxymethyllysine (CML) and methylglyoxal (MG), were obtained at admission, and serially afterwards in patients admitted to the ICU. Demographics, dietary history, markers of injury severity and ICU morbidity and mortality data were collected. Results: One hundred and fifty-six trauma patients (TP) (age: 39±17 years, 83% males, injury severity score: 18±14) were included in the study. TP had significantly higher serum AGE levels than normal healthy controls (CML, TP 12.4±8.2 U/mL vs. controls 8.9±5.3 U/mL, p<0.001; MG, TP 2.1±1.4 nmol/mL vs. controls 0.79±0.3 nmol/mL, p<0.001). Admission serum AGE levels in 49 severe TP admitted to the ICU were lower than those who were not. However, among the ICU patients, serum AGEs increased further for about 7 days in patients with an uncomplicated course, and remained markedly elevated in those with a complicated course. Conclusions: Circulating AGEs are transiently increased after acute trauma and persistently elevated AGE levels are associated with greater severity of injury.

Background: The receptor for AGE (RAGE) is a key mediator in cerebral ischemia. Based on the evidence from animal studies and the presence of increased high mobility group box 1 protein (HMGB1, a RAGE ligand) in the serum of stroke patients, we hypothesized that soluble RAGE (sRAGE) increase in serum after ischemic and hemorrhagic stroke and that the levels decrease with patient improvement. Methods: We performed a longitudinal study of the acute changes of sRAGE levels in a series of 15 ischemic and hemorrhagic stroke patients at admission and over a period averaging 1 week and extending for up to more than a month in some of the cases. Serum sRAGE was measured by an enzyme-linked immunosorbent assay (R&D Systems Inc., Minneapolis, MN, USA). Results: Serum sRAGE at admission were not significantly different between patients and healthy controls, p=0.17. Over the following days after the event, stroke patients displayed an increase of the serum levels of sRAGE, which at peak ranged between 26% and 296%, p>0.001. Similar changes are seen for both types of events, hemorrhagic and ischemic. sRAGE changes paralleled recovery and recurrence or aggravation of the episodes. Biological variability of sRAGE as measured daily in healthy subjects over 21 days showed a CV of only 8.9%. Conclusions: Our results provide for the first time a proof of principle that circulating sRAGE increase after ischemic and hemorrhagic stroke and may become candidate biomarkers to consider in larger studies exploring prognostic or follow-up value.

Background: The histopathological hallmarks in Alzheimer’s disease (AD) include neuronal cell death, formation of amyloid plaques and neurofibrillary tangles. Glycoxidation plays a crucial role in AD pathogenesis, as pentosidine and Nε- carboxymethyl-lysine (CML), were detected in AD hallmarks, and in vivo cerebrospinal fluid (CSF). However, the definitive role of AGEs in the neuropathology of AD is inconclusive. The aim of this preliminary study was to assess the level of pentosidine in CSF of patients affected by neurological disorders, including probable AD, in order to assess the feasibility of AGEs detection in CSF and to explore pentosidine as a potential biomarker in AD. Methods: Twenty-five patients diagnosed with AD (NINCDS ADRDA criteria) and different neurological disorders were enrolled. Diabetic patients were excluded. Pentosidine, CML, amyloid β1–42 were assessed by high performance liquid chromatography (HPLC) by Odetti modified method,and by sandwich ELISA respectively. Results: Our data showed the presence of pentosidine in all CSF samples, a significant increase in CSF pentosidine levels with age (p<0.05) and a significant decreased concentration of pentosidine in four AD subjects (p<0.01), after normalization to CSF protein concentration. Conclusions: The study showed that AGEs concentration in CSF might benefit from age correction, at least for pentosidine, originally addressing a potential systemic age-dependent AGEs accumulation. The significant decrease of CSF pentosidine in AD, even in 4 patients, might conceive that different AGEs inform specific types of neurodegeneration, depending on oxidative stress levels, blood – brain barrier permeability, brain localization and systemic risk factors.

Background: Skin autofluorescence (SAF), as a proxy of AGE accumulation, is predictive of cardiovascular (CVD) complications in i.a. type 2 diabetes mellitus and renal failure, independently of most conventional CVD risk factors. The present exploratory substudy of the Groningen Overweight and Lifestyle (GOAL)-project addresses whether SAF is related to Systematic COronary Risk Evaluation (SCORE) risk estimation (% 10-year CVD-mortality risk) in overweight/obese persons in primary care, without diabetes/renal disease, and if after 3-year treatment of risk factors (change in, Δ) SAF is related to ΔSCORE. Methods: In a sample of 65 participants from the GOAL study, with a body mass index (BMI) >25–40 kg/m 2 , hypertension and/or dyslipidemia, but without diabetes/renal disease, SAF and CVD risk factors were measured at baseline, and after 3 years of lifestyle and pharmaceutical treatment. Results: At baseline, the mean SCORE risk estimation was 3.1±2.6%, mean SAF 2.04±0.5AU. In multivariate analysis SAF was strongly related to age, but not to other risk factors/SCORE. After 3 years ΔSAF was 0.34±0.45 AU (p<0.001). ΔSAF was negatively related to Δbodyweight but not to ΔSCORE%, or its components. At follow-up, SAF was higher in 11 patients with a history of CVD compared to 54 persons without CVD (p=0.002). Conclusions: Baseline and 3-year-Δ SAF are not related to (Δ)SCORE, or its components, except age, in the studied population. ΔSAF was negatively related to Δweight. As 3-year SAF was higher in persons with CVD, these results support a larger study on SAF to assess its contribution to conventional risk factors/SCORE in predicting CVD in overweight persons with low-intermediate cardiovascular risk.

Background: High levels of circulating advanced glycation end products (AGEs) can initiate chronic low-grade activation of the immune system (CLAIS) with each of these factors independently associated with cardiovascular (CV) morbidity and mortality. Therefore, our objective was to characterize the relationship between serum AGEs, CLAIS and other risk factors for CV disease in normotensive non-diabetic individuals. Methods: We measured body mass index (BMI), waist-to-hip ratio (WHR), blood pressure, lipid and glucose profile in 44 non-diabetic volunteers (17 female, 27 males). Carboxymethyl-lysine (CML) was measured by ELISA as a marker for circulating AGEs and NF-κB p65 activity as an inflammatory marker by DNA-binding in peripheral blood mononuclear cells lysates (PBMC). Results: Plasma CML concentrations were related to diastolic blood pressure (r=−0.51, p<0.01) independently of age, sex, BMI and WHR (p<0.05). Diastolic blood pressure was also related to NF-κB activity in PBMC (r=0.47, p<0.01) before and after adjustment for age, sex, BMI and WHR (p<0.05). Plasma CML concentrations were related to the pulse pressure before (r=0.42; p<0.05) and after adjustment for age, sex, BMI and waist (p<0.05). Neither CML nor NF-κB activity were related to systolic blood pressure (both p=ns). Plasma CML concentrations were not associated with plasma lipid or glucose concentrations (all p=ns). Conclusions: Plasma AGE levels and NF-κB activity in PBMC were independent determinants of diastolic and pulse pressure in healthy normotensive individuals. This association suggests a role for AGEs in the etiology of hypertension, possibly via the initiation of CLAIS and aortic stiffening.

Background: Interaction of advanced glycation end products (AGEs) with their specific cell-surface receptor for AGEs (RAGE) induces production of reactive oxygen species, pro-diabetic, pro-inflammatory, and pro-atherogenic responses. The metabolic syndrome (Metsy) imposes a high risk of development of cardiovascular disease and unequivocally predisposes the non-diabetics to type 2 diabetes mellitus. The aim of the study was to investigate the association between circulating soluble RAGE (sRAGE), N ε -(carboxymethyl)lysine (CML) or AGE-associated fluorescence of plasma (AGE-Fl) with the number of manifested Metsy risk factors in young-to-middle-aged medication-free non-diabetic subjects. Methods: Metsy was classified according to NCEP/ATP III criteria; plasma sRAGE and total CML were determined by ELISA methods and AGE-Fl fluorimetrically. Results: From among 437 participants aged 33±11 years, 58% were females. In total 174 subjects were Metsy risk factors-free, 142 presented one, 59 presented two risk factors, and 62 suffered from Metsy. Plasma sRAGE and CML/albumin levels decreased with increasing number of Metsy risk factors (p<0.01, both), while AGE-Fl/albumin levels remained similar. Multivariate analysis selected waist circumference as a main determinant of plasma sRAGE as well as CML/albumin levels. Conclusions: In young-to-middle-aged non-diabetic medication-free subjects plasma total CML/albumin and sRAGE levels decrease prior to the manifestation of Metsy. With regards to RAGE-mediated CML trapping into adipose tissue inducing dysregulation of pro-inflammatory cytokines, adipokines, and the development of obesity-related insulin resistance, and the potential involvement of sRAGE in feedback regulation of the toxic effects of AGE/RAGE-mediated signaling, this early decline might be of clinical impact in development of type 2 diabetes and its complications.

Background: Advanced glycation end products (AGEs), the final products of the Maillard reaction, have been shown to impair endothelial proliferation and function, thus contributing to endothelial cell injury present in diabetes, inflammatory and cardiovascular diseases. Endoplasmic reticulum (ER) stress triggered under hyperglycemic, hypoxic and oxidative conditions has been implicated in endothelial dysfunction through activation of the unfolded protein response (UPR). The present study investigates the role of AGEs in ER stress induction in human aortic endothelial cells exposed to variable AGE treatments. Methods: Human aortic endothelial cells (HAEC) were treated with increasing concentrations (100, 200 μg/mL) of AGE-bovine serum albumin (AGE-BSA) at different time-points (24, 48, 72 h). The induction of ER stress and the involved UPR components were investigated on mRNA and protein levels. Apoptosis was quantitatively determined by flow cytometry detecting propidium iodide expression and annexin V binding simultaneously. Results: AGEs administration significantly reduced HAEC proliferation in a time- and dose-dependent manner. An immediate induction of the ER chaperones GRP78, GRP94 and the transcriptional activator, XBP-1 was observed at 24 h and 48 h. A later induction of the phospho-elF2α and proapoptotic transcription factor CHOP was observed at 48 h and 72 h, being correlated with elevated early apoptotic cell numbers at the same time-points. Conclusions: The present study demonstrates that AGEs directly induce ER stress in human aortic endothelial cells, playing an important role in endothelial cell apoptosis. Targeting AGEs signaling pathways in order to alleviate ER stress may prove of therapeutic potential to endothelial dysfunction-related disorders.

Background: Nitric oxide (NO) and its metabolites can nitrosylate hemoglobin (Hb) through the heme iron. Nitrihemoglobin (nitriHb) can be formed as result of porphyrin vinyl group modification with nitrite. However, in those with diabetes the non-enzymatic glycation of Hb amino acids residues (the Maillard reaction) can take place. The objectives of this study were to investigate effects of the Maillard reaction on the interaction of methemoglobin (metHb) with S-nitrosoglutathione (GSNO) and nitrite. Methods: Nitrosylhemoglobin production was registered using increasing optical density at 572 nm and compared with 592 nm, and with EPR spectroscopy. Formation of nitriHb was determined using an absorbance band of reduced hemochromogen (582 nm) in the alkaline pyridine solution. Accumulation of fluorescent advanced glycation end-products of Hb was measured through increasing of fluorescence at 385–395 nm (excitation λ=320 nm). Results: We determined that NO metabolites such as GSNO and nitrite at physiological pH values and aerobic conditions caused modification of metHb porphyrin vinyl groups with nitriHb formation. It was ascertained that this formation was inhibited by superoxide dismutase. In microaerobic conditions metHb was nitrosylated under the action of GSNO or GSNO with methylglyoxal. Nitrite nitrosylated metHb only in the presence of methylglyoxal. It was shown that GSNO inhibited accumulation of fluorescent products which formed during Hb glycation with methylglyoxal. Conclusions: The assumption was made that intermediates of the Hb glycation reaction play an important role both in vinyl group nitration and in heme iron nitrosylation. Oxygen content in reaction medium is an important factor influencing these processes. These effects can play an important role in pathogenesis of the diseases connected with carbonyl, oxidative and nitrosative stresses.

Background: Aging is the result of several mechanisms which operate simultaneously. Among them, glycation is of particular interest because it is a reaction which affects slowly renewing tissues and macromolecules with elevated half-life, like the dermis, a skin compartment highly affected by aging. Glycation produces crosslinks between macromolecules thereby providing an explanation for the increased age-related stiffness of the skin. Glycation products, also called AGEs (advanced glycation end products), accumulate primarily in extracellular matrix molecules like collagen or elastin. Methods: In order to reproduce this phenomenon in vitro we have created a model of reconstructed skin modified by glycation of the collagen used to fabricate the dermal compartment. Results: This system allowed us to uncover biological modifications of dermal markers, and more surprisingly epidermal markers, as well as an increase of metalloproteinases responsible for degradation of the dermal matrix. Consequently, the imbalance between synthesis and degradation that results from glycation, may contribute to skin aging, as shown in this model. Moreover these modifications were shown to be prevented by the addition of aminoguanidine, a well-known inhibitor of glycation. Conclusions: Using this experimental approach our results taken together stress the importance and possibly central role of glycation in skin aging and the usefulness of the reconstructed skin as a model of physiological aging.

Background: Protein glycation refers to the spontaneous reaction of reducing sugars with proteins and the subsequent formation of stable advanced glycation end products (AGEs). Glycation is linked with oxidative stress, and this association is called “glycoxidation”. Glycoxidation alters the protein structure and function and causes tissue aging, as seen in human skin. Therefore, research on substances inhibiting glycoxidation appears to be crucial in the prevention of skin aging. With this aim, several plant extracts have been screened for antiglycation activity, and the results of the best candidates are presented in this article. Methods: Glycation was studied on human skin proteins (collagen, elastin, and albumin) and on a model of reconstructed skin. Oxidative stress has been addressed by testing the copper-induced low-density lipoprotein oxidation, ultraviolet irradiation of glycated dermis, and carbonyl activation of human dermal fibroblasts. A clinical test evaluated the extent of oxidative stress induced by ultraviolet A irradiation. Results: Among the tested products, several plant extracts have decreased the glycation effects on skin proteins collagen, elastin, and albumin. In addition, a plant extract has significantly inhibited the different forms of oxidative stress associated with protein glycation. Conclusions: We have demonstrated that plant extracts can relieve the deleterious effects of glycation on human skin. Moreover, a plant extract rich in antioxidant molecules has also significantly preserved the human skin from glycoxidation attacks.