Dietary advanced glycation end products and their role in health and disease

Over the past 2 decades there has been increasing evidence supporting an important contribution from food-derived advanced glycation end products (AGEs) to the body pool of AGEs and therefore increased oxidative stress and inflammation, processes that play a major role in the causation of chronic diseases. A 3-d symposium (1st Latin American Symposium of AGEs) to discuss this subject took place in Guanajuato, Mexico, on 1-3 October 2014 with the participation of researchers from several countries. This review is a summary of the different presentations and subjects discussed, and it is divided into 4 sections. The first section deals with current general knowledge about AGEs. The second section dwells on mechanisms of action of AGEs, with special emphasis on the receptor for advanced glycation end products and the potential role of AGEs in neurodegenerative diseases. The third section discusses different approaches to decrease the AGE burden. The last section discusses current methodologic problems with measurement of AGEs in different samples. The subject under discussion is complex and extensive and cannot be completely covered in a short review. Therefore, some areas of interest have been left out because of space. However, we hope this review illustrates currently known facts about dietary AGEs as well as pointing out areas that require further research.

Fueling inflammation at tumor microenvironment: the role of multiligand/RAGE axis

The receptor for advanced glycation end products (RAGE), firstly described in 1992, is a single-transmembrane and multiligand member of the immunoglobulin protein family. RAGE engagement produces activation of multiple intracellular signaling mechanisms involved in several inflammation-associated clinical entities, such as diabetes, cancer, renal and heart failures, as well as neurodegenerative diseases. Although RAGE expression has been extensively reported in many cancer types, it is now emerging as a relevant element that can continuously fuel an inflammatory milieu at the tumor microenvironment, thus changing our perception of its contribution to cancer biology. In this review, we will discuss the role of multiligand/RAGE axis, particularly at the multicellular cross talk established in the inflammatory tumor microenvironment. A better understanding of its contribution may provide new targets for tumor management and risk assessment.

Advanced glycation and endothelial functions: a link towards vascular complications in diabetes

The formation of advanced glycation end-products (AGEs), also called the Maillard reaction, occurs ubiquitously and irreversibly in patients with diabetes mellitus, and its consequences are especially relevant to vascular dysfunctions. The interaction of AGEs with their receptors (RAGE) has been implicated in the development of vascular complications. This interaction elicits remarkable vascular cell changes analogous to those observed in diabetes mellitus, including angiogenic and thrombogenic responses of endothelial cells, increased oxidative stress, and functional alterations in vascular tone control. This review focuses on AGEs formation, the interaction with their specific receptors and how the triggered intracellular events determine functional alterations of vascular endothelium. Finally, some potential pharmacological approaches undertaken to circumvent the deleterious effects of AGEs are also discussed.

Regulation of endothelial nitric oxide synthase expression by albumin-derived advanced glycosylation end products

We examined whether albumin-derived advanced glycosylation end products (AGEs) downregulate the expression of endothelial nitric oxide synthase (NOS). Significant reductions in NOS activity and cGMP levels in bovine aortic endothelial cells were observed when exposed to different concentrations of albumin-derived AGEs. Western and Northern blot analyses showed significant decreases at the protein and transcript levels. Both reductions became evident after 24 hours of exposure. Nuclear run-on assays showed that AGE-BSA did not modify the transcription rate of the NOS III gene; however, AGE-BSA treatment markedly reduced the half-life of NOS III mRNA. In addition, AGE-treated endothelial cells displayed significant reduction on their antiplatelet properties. These results indicate that NOS expression is reduced by AGEs by increasing the rate of mRNA degradation and may be relevant to the impairment of some endothelial functions observed in diabetes and aging. The full text of this article is available at http://www.circresaha.org.

Chlorogenic acid inhibits human platelet activation and thrombus formation

BACKGROUND

Chlorogenic acid is a potent phenolic antioxidant. However, its effect on platelet aggregation, a critical factor in arterial thrombosis, remains unclear. Consequently, chlorogenic acid-action mechanisms in preventing platelet activation and thrombus formation were examined.

METHODS AND RESULTS

Chlorogenic acid in a dose-dependent manner (0.1 to 1 mmol/L) inhibited platelet secretion and aggregation induced by ADP, collagen, arachidonic acid and TRAP-6, and diminished platelet firm adhesion/aggregation and platelet-leukocyte interactions under flow conditions. At these concentrations chlorogenic acid significantly decreased platelet inflammatory mediators (sP-selectin, sCD40L, CCL5 and IL-1β) and increased intraplatelet cAMP levels/PKA activation. Interestingly, SQ22536 (an adenylate cyclase inhibitor) and ZM241385 (a potent A2A receptor antagonist) attenuated the antiplatelet effect of chlorogenic acid. Chlorogenic acid is compatible to the active site of the adenosine A2A receptor as revealed through molecular modeling. In addition, chlorogenic acid had a significantly lower effect on mouse bleeding time when compared to the same dose of aspirin.

CONCLUSIONS

Antiplatelet and antithrombotic effects of chlorogenic acid are associated with the A2A receptor/adenylate cyclase/cAMP/PKA signaling pathway.

The receptor for advanced glycation end-products: a complex signaling scenario for a promiscuous receptor

Firstly described in 1992, the receptor for advanced glycation end-products has attracted increasing attention due to its diverse ligand repertoire and involvement in several pathophysiological processes associated with inflammation such as in diabetes, cancer, autoimmune diseases and neurodegenerative diseases. This receptor in addition to its binding capacity for advanced glycation end-products also recognizes some molecules classified as both, pathogen- and damage-associated molecular patterns and thus triggering the transcription of genes encoding inflammatory mediators. Some of these ligands are common for both, the receptor of advanced glycation end-products and members of the Toll-like receptor family, generating shared signaling cascades. Furthermore, these receptors may cooperate as essential partners through the recruitment and assembly of homo- and hetero-oligomers in order to strengthen the inflammatory response. The purpose of this review is to highlight the importance of some particular features of this multiligand receptor, its signaling cascade as well as the cross-talk with some members of the Toll-like receptor family.

Is ozone pre-conditioning effect linked to Nrf2/EpRE activation pathway in vivo? A preliminary result

The present preliminary study has been focused on verifying whether ozone preconditioning may be linked to Nrf2/EpRE (nuclear factor erythroid 2/electrophile-responsive element) activation pathway in vivo. Healthy volunteers received a total of three Major Auto-Hemotherapy (MAH) treatments, with treatments administered every second day. The amount of blood used for each subject was standardized to the value obtained multiplying the subject׳s body weight by 1.3 in order to ensure the same ozone concentrations for each subject. A parallel group (n=50) age and gender matched was used as reference for the experimental variables related to the oxidative stress parameters. Levels of Nrf2 and oxidative stress index were measured throughout the study. Levels of Nrf2 (P<0.01) in peripheral blood mononuclear cells (PBMC) were found to increase immediately after ozone/oxygen exposure (35µg/ml, prior to reinfusion). This effect was still detected (P<0.05) in total circulating PBMC when measured 30min following reinfusion. After a series of 3 MAH, Nrf2 returned back to the basal level. At the end of the experiment the activities of superoxide dismutase and catalase were increased (P<0.05). These data demonstrate for the first time in vivo the activation of the Nrf2 pathway by a low dose of ozone and the promotion of the feedback mechanism that induces the synthesis of proteins which collectively favors cell survival.

Polyphenols and AGEs/RAGE axis. Trends and challenges

The formation of advanced glycation end-products (AGEs) is a key pathophysiological event linked not only to the onset and progression of diabetic complications, but also to neurodegeneration, cardiovascular diseases, cancer, and others important human diseases. AGEs contributions to pathophysiology are mainly through the formation of cross-links and by engaging the receptor for advanced glycation end-products (RAGE). Polyphenols are secondary metabolites found largely in fruits, vegetables, cereals, and beverages, and during many years, important efforts have been made to elucidate their beneficial effects on human health, mainly ascribed to their antioxidant activities. In the present review, we highlighted the beneficial actions of polyphenols aimed to diminish the harmful consequences of advanced glycation, mainly by the inhibition of ROS formation during glycation, the inhibition of Schiff base, Amadori products, and subsequent dicarbonyls group formation, the activation of the glyoxalase system, as well as by blocking either AGEs-RAGE interaction or cell signaling.

Extracellular matrix glycation and receptor for advanced glycation end-products activation: a missing piece in the puzzle of the association between diabetes and cancer

A growing body of epidemiologic evidence suggests that people with diabetes are at a significantly higher risk of many forms of cancer. However, the molecular mechanisms underlying this association are not fully understood. Cancer cells are surrounded by a complex milieu, also known as tumor microenvironment, which contributes to the development and metastasis of tumors. Of note, one of the major components of this niche is the extracellular matrix (ECM), which becomes highly disorganized during neoplastic progression, thereby stimulating cancer cell transformation, growth and spread. One of the consequences of chronic hyperglycemia, the most frequently observed sign of diabetes and the etiological source of diabetes complications, is the irreversible glycation and oxidation of proteins and lipids leading to the formation of the advanced glycation end-products (AGEs). These compounds may covalently crosslink and biochemically modify structure and functions of many proteins, and AGEs accumulation is particularly high in long-living proteins with low biological turnover, features that are shared by most, if not all, ECM proteins. AGEs-modified proteins are recognized by AGE-binding proteins, and thus glycated ECM components have the potential to trigger Receptor for advanced glycation end-products-dependent mechanisms. The biological consequence of receptor for advanced glycation end-products activation mechanisms seems to be connected, in different ways, to drive some hallmarks of cancer onset and tumor growth. The present review intends to highlight the potential impact of ECM glycation on tumor progression by triggering receptor for advanced glycation end-products-mediated mechanisms.

Dietary AGEs as Exogenous Boosters of Inflammation

Most chronic modern non-transmissible diseases seem to begin as the result of low-grade inflammation extending over prolonged periods of time. The importance of diet as a source of many pro-inflammatory compounds that could create and sustain such a low-grade inflammatory state cannot be ignored, particularly since we are constantly exposed to them during the day. The focus of this review is on specific components of the diet associated with inflammation, specifically advanced glycation end products (AGEs) that form during thermal processing of food. AGEs are also generated in the body in normal physiology and are widely recognized as increased in diabetes, but many people are unaware of the potential importance of exogenous AGEs ingested in food. We review experimental models, epidemiologic data, and small clinical trials that suggest an important association between dietary intake of these compounds and development of an inflammatory and pro-oxidative state that is conducive to chronic diseases. We compare dietary intake of AGEs with other widely known dietary patterns, such as the Mediterranean and the Dietary Approaches to Stop Hypertension (DASH) diets, as well as the Dietary Inflammation Index (DII). Finally, we delineate in detail the pathophysiological mechanisms induced by dietary AGEs, both direct (i.e., non-receptor-mediated) and indirect (receptor-mediated).

HMGB1 enhances the protumoral activities of M2 macrophages by a RAGE-dependent mechanism

The monocyte-macrophage lineage shows a high degree of diversity and plasticity. Once they infiltrate tissues, they may acquire two main functional phenotypes, being known as the classically activated type 1 macrophages (M1) and the alternative activated type 2 macrophages (M2). The M1 phenotype can be induced by bacterial products and interferon-γ and exerts a cytotoxic effect on cancer cells. Conversely, the alternatively activated M2 phenotype is induced by Il-4/IL13 and promotes tumor cell growth and vascularization. Although receptor for advanced glycation end-products (RAGE) engagement in M1 macrophages has been reported by several groups to promote inflammation, nothing is known about the functionality of RAGE in M2 macrophages. In the current study, we demonstrate that RAGE is equally expressed in both macrophage phenotypes and that RAGE activation by high-mobility group protein box1 (HMGB1) promotes protumoral activities of M2 macrophages. MKN45 cells co-cultured with M2 macrophages treated with HMGB1 at different times displayed higher invasive abilities. Additionally, conditioned medium from HMGB1-treated M2 macrophages promotes angiogenesis in vitro. RAGE-targeting knockdown abrogates these activities. Overall, the present findings suggest that HMGB1 may contribute, by a RAGE-dependent mechanism, to the protumoral activities of the M2 phenotype.

Advanced Glycation and ROS: a link between diabetes and heart failure

Despite many advances achieved to date, heart failure (HF) remains a leading cause of morbidity and mortality. There is a widely-accepted consensus that HF and diabetes are strongly linked by at least 3 mechanisms: associated comorbidities, coronary atherosclerosis or a specific diabetic cardiomiopathy. For the last 2 mechanisms, advanced glycation end-products may contribute to trigger key processes relevant to HF by affecting cardiac function through cross-linking or receptor engagement. This review focuses on the main effects of advanced glycation end-products on cardiomyocytes and endothelial cell function. Some pharmacological approaches are also discussed.

The immunobiology of the receptor of advanced glycation end-products: trends and challenges

Pattern-recognition receptors have been highly conserved in evolution. They recognize danger signals including both pathogen- and damage-associated molecular patterns, also known as alarmins. Several signaling pathways leading to an inflammatory reaction as part of an effective defensive response, are thus triggered. RAGE, a receptor initially considered for advanced glycation end-products, is also known to be activated by several danger signals, thus functioning as a pattern-recognition receptor. As a new member of this family, attempts to unraveling its functioning show that RAGE activation not only results in innate immune response but also contributes to promote and shape the acquired immune reaction. As reported for other members of the family, RAGE presents many polymorphic variants and additional studies are needed to elucidate its significance in immune response and disease susceptibility. Here we describe recent advances unraveling RAGE functions, as well as its significance and challenges in immunobiology.

Gallstones, Body Mass Index, C-Reactive Protein, and Gallbladder Cancer: Mendelian Randomization Analysis of Chilean and European Genotype Data

BACKGROUND AND AIMS

Gallbladder cancer (GBC) is a neglected disease with substantial geographical variability: Chile shows the highest incidence worldwide, while GBC is relatively rare in Europe. Here, we investigate the causal effects of risk factors considered in current GBC prevention programs as well as C-reactive protein (CRP) level as a marker of chronic inflammation.

APPROACH AND RESULTS

We applied two-sample Mendelian randomization (MR) using publicly available data and our own data from a retrospective Chilean and a prospective European study. Causality was assessed by inverse variance weighted (IVW), MR-Egger regression, and weighted median estimates complemented with sensitivity analyses on potential heterogeneity and pleiotropy, two-step MR, and mediation analysis. We found evidence for a causal effect of gallstone disease on GBC risk in Chileans (P = 9 × 10-5 ) and Europeans (P = 9 × 10-5 ). A genetically elevated body mass index (BMI) increased GBC risk in Chileans (P = 0.03), while higher CRP concentrations increased GBC risk in Europeans (P = 4.1 × 10-6 ). European results suggest causal effects of BMI on gallstone disease (P = 0.008); public Chilean data were not, however, available to enable assessment of the mediation effects among causal GBC risk factors.

CONCLUSIONS

Two risk factors considered in the current Chilean program for GBC prevention are causally linked to GBC risk: gallstones and BMI. For Europeans, BMI showed a causal effect on gallstone risk, which was itself causally linked to GBC risk.

Albumin-derived advanced glycation end-products trigger the disruption of the vascular endothelial cadherin complex in cultured human and murine endothelial cells

Endothelial cell (EC) junctions regulate in large part the integrity and barrier function of the vascular endothelium. Advanced glycation end-products (AGEs), the irreversibly formed reactive derivatives of non-enzymic glucose-protein condensation reactions, are strongly implicated in endothelial dysfunction that distinguishes diabetes- and aging-associated vascular complications. The aim of the present study was to determine whether AGEs affect EC lateral junction proteins, with particular regard to the vascular endothelial cadherin (VE-cadherin) complex. Our results indicate that AGE-modified BSA (AGE-BSA), a prototype of advanced glycated proteins, disrupts the VE-cadherin complex when administered to ECs. AGE-BSA, but not unmodified BSA, was found to induce decreases in the levels of VE-cadherin, beta-catenin and gamma-catenin in the complex and in total cell extracts, as well as a marked reduction in the amount of VE-cadherin present at the cell surface. In contrast, the level of platelet endothelial cell adhesion molecule-1 (PECAM-1), which is located at lateral junctions, was not altered. Supplementation of the cellular antioxidative defences abolished these effects. Finally, the loss of components of the VE-cadherin complex was correlated with increases in vascular permeability and in EC migration. These findings suggest that some of the AGE-induced biological effects on the endothelium could be mediated, at least in part, by the weakening of intercellular contacts caused by decreases in the amount of VE-cadherin present.

The potential role of dietary advanced glycation endproducts in the development of chronic non-infectious diseases: a narrative review

Increasing clinical and experimental evidence accumulated during the past few decades supports an important role for dietary advanced glycation endproducts (AGE) in the pathogenesis of many chronic non-infectious diseases, such as type 2 diabetes, CVD and others, that are reaching epidemic proportions in the Western world. Although AGE are compounds widely recognised as generated in excess in the body in diabetic patients, the potential importance of exogenous AGE, mostly of dietary origin, has been largely ignored in the general nutrition audience. In the present review we aim to describe dietary AGE, their mechanisms of formation and absorption into the body as well as their main mechanisms of action. We will present in detail current evidence of their potential role in the development of several chronic non-infectious clinical conditions, some general suggestions on how to restrict them in the diet and evidence regarding the potential benefits of lowering their consumption.

Role of nitric oxide pathway in the protection against lethal endotoxemia afforded by low doses of lipopolysaccharide

Survival after lipopolysaccharide challenge (LD80, 20 mg.kg-1, i.p.) was significantly enhanced by previous treatment with a microdose of LPS (50 micrograms.kg-1, i.v.). When NG-monomethyl-L-arginine, a specific inhibitor of the formation of nitric oxide from L-arginine, was given 30 minutes before microdose, survival was significantly reduced. When we monitored the serum Tumor Necrosis Factor (TNF) levels in both groups a significant reduction of TNF level after the microdose was observed in mice previously treated with L-NMMA. The ability of L-NNMA to reduce TNF release was dose dependent.

Role of multiligand/RAGE axis in platelet activation

In the context of plaque progression, platelet hyperactivity associated with hyperlipidemia contributes to the development of a pro-thrombotic state. In this context, it has been demonstrated that advanced glycation end products (AGEs) significantly increases platelet activation and receptor for AGEs (RAGE) expression at the platelet surface membrane. In addition to AGEs, other ligands (S100, HMGB1 and amyloid β, among others) of RAGE have raised particular attention in platelet activation. Therefore, in this article we describe platelet hyperactivity by AGEs via RAGE-independent and RAGE-dependent pathways.

Chlorpromazine inhibits both the constitutive nitric oxide synthase and the induction of nitric oxide synthase after LPS challenge

The effects of chlorpromazine on either the activity of mouse brain nitric oxide synthase or the induction of lung nitric oxide synthase in mice and rats were studied. Chlorpromazine inhibited the nitric oxide synthase activity in mouse brain cytosol. This effect could be reversed by adding an excess of calmodulin. In addition, chlorpromazine was able to inhibit the induction of lung nitric oxide synthase, in both species, after LPS administration. Furthermore, chlorpromazine also inhibited arginase activity in mouse lung cytosol.

Adipose tissue macrophages as a therapeutic target in obesity-associated diseases

Obesity is an increasing problem in developed and developing countries. Individuals with obesity have a higher risk of several diseases, such as cardiovascular disease, increased risk of insulin resistance, type 2 diabetes, infertility, degenerative disorders, and also certain types of cancer. Adipose tissue (AT) is considered an extremely active endocrine organ, and the expansion of AT is accompanied by the infiltration of different types of immune cells, which induces a state of low-grade, chronic inflammation and metabolic dysregulation. Even though the exact mechanism of this low-grade inflammation is not fully understood, there is clear evidence that AT-infiltrating macrophages (ATMs) play a significant role in the pro-inflammatory state and dysregulated metabolism. ATMs represent the most abundant class of leukocytes in AT, constituting 5% of the cells in AT in individuals with normal weight. However, this percentage dramatically increases up to 50% in individuals with obesity, suggesting an important role of ATMs in obesity and its associated complications. In this review, we discuss current knowledge of the function of ATMs during steady-state and obesity and analyze its contribution to different obesity-associated diseases, highlighting the potential therapeutic target of ATMs in these pathological conditions.

Oxidative stress at the vascular wall. Mechanistic and pharmacological aspects

During the process of energy production in aerobic respiration, vascular cells produce reactive oxygen species (ROS). A growing body of evidence indicates that oxidative stress refers to a condition in which cells are subjected to excessive levels of ROS. Overall vascular function is dependent upon a fine balance of oxidant and antioxidant mechanisms, which determine endothelial functions. Considerable experimental and clinical data indicate that intracellular oxidant milieu is also involved in several redox-sensitive cellular signaling pathways such as ion transport systems, protein phosphorylation, and gene expression and thus also plays important roles as modulator of vascular cell functions such as cell growth, apoptosis, migration, angiogenesis and cell adhesion. Overproduction of ROS under pathophysiologic conditions is integral in the development of cardiovascular diseases. This fact has raised an intensive search of new pharmacological approaches to improve vascular hemostasis and particularly those intended to decrease oxidative stress or augment the antioxidant defense mechanisms.

Effect of advanced glycosylation end products on the induction of nitric oxide synthase in murine macrophages

We studied the role of advanced glycosylation end products on the induction of nitric oxide synthase in peritoneal mouse macrophages previously exposed to modified BSA. A dose-dependent increment in the nitric oxide production induced by LPS and IFN-gamma was observed when cell cultures were pretreated with modified BSA for 48 hours. In addition, the up regulation of nitric oxide production was also time-dependent, being maximal at 24-48 hours. Experiments carried out in the presence of neutralizing antibodies to IL-1 and TNF-alpha, suggested that up regulation was not due to the capacity of modified BSA to induce both proinflammatory signals. The up regulation of nitric oxide production was paralleled with an increase in iNOS mRNA.

Inhibition of RAGE Axis Signaling: A Pharmacological Challenge

The Receptor for Advanced Glycation End Products (RAGE) is an important cell surface receptor, which belongs to the IgG super family and is now considered as a pattern recognition receptor. Because of its relevance in many human clinical settings, it is now pursued as a very attractive therapeutic target. However, particular features of this receptor such as a wide repertoire of ligands with different binding domains, the existence of many RAGE variants as well as the presence of cytoplasmatic adaptors leading a diverse signaling, are important limitations in the search for successful pharmacological approaches to inhibit RAGE signaling. Therefore, the present review aimed to display the most promising approaches to inhibit RAGE signaling, and provide an up to date review of progress in this area.

Activation of phospholipase D by interleukin-8 in human neutrophils

Interleukin 8 (IL-8), a member of the C-X-C branch of the chemokine superfamily, stimulated the breakdown of 1-O-[3H]alkyl-2-acyl-sn-glycero-3-phosphocholine ([3H]EAPC) and the formation of 1-O-[3H]alkyl-2-acyl-phosphatidic acid ([3H]-EAPA) in human polymorphonuclear leukocytes (PMN) in the presence of cytochalasin B. In addition, the mass of diradyl-PA was increased with similar kinetics. In the presence of ethanol, 1-O-[3H]alkyl-2-acyl-phosphatidylethanol ([3H]EAPEt) was formed at the expense of [3H]EAPA formation, indicating the activation of phospholipase D by the cytokine. The effect was time- and concentration-dependent, reaching a plateau at 30 seconds with the maximally activating concentration of 120 nmol/L IL-8. Preincubation of cells with 1 microgram/mL Bordetella pertussis toxin inhibited the breakdown of [3H]EAPC and [3H]EAPA formation, indicating a role for a pertussis toxin-sensitive guanosine triphosphate-binding protein. Formation of phosphatidic acid (PA) correlated with activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, the oxidative burst enzyme, with both events occurring in the same concentration range. Inhibition of PA formation, by the presence of ethanol, also inhibited the oxidative burst stimulation by IL-8. Pretreatment of PMN with 10 nmol/L platelet-activating factor potentiated both [3H]EAPA accumulation and activation of NADPD oxidase by IL-8. Collectively, these data show that IL-8 stimulates the metabolism of choline-containing phosphoglycerides in human PMN and support a role for PA in the signaling mechanisms used by IL-8 to stimulate PMN function.