Advanced Glycation End Products and Receptor–Oxidative Stress System in Diabetic Vascular Complications

Reducing sugars can react non‐enzymatically with amino groups of protein to form Amadori products. These early glycation products undergo further complex reactions, such as rearrangement, dehydration, and condensation, to become irreversibly cross‐linked, heterogeneous fluorescent derivatives, termed advanced glycation end products (AGEs). The formation and accumulation of AGEs have been known to progress at an accelerated rate in patients with diabetes mellitus, thus being involved in the development and progression of diabetic micro‐ and macroangiopathy. Indeed, there is accumulating evidence that an interaction between an AGE and its receptor (RAGE) generates oxidative stress and subsequently evokes vascular inflammation and thrombosis, thereby playing a central role in diabetic vascular complications. In this paper, we review the pathophysiological role of AGE‐RAGE–oxidative stress system and its therapeutic interventions in diabetic micro‐ and macroangiopathy.     

Age-related changes in cells and tissues due to advanced glycation end products (AGEs)

Advanced glycation end products (AGEs) formed by nonenzymatic glycation and oxidation of proteins accumulate during normal aging and at accelerated rate during the course of diabetes. They play a role in the pathogenesis of several other chronic diseases such as Alzheimer's disease, arthritis, atherosclerosis, pulmonary fibrosis and renal failure. AGE-formation changes the chemical and biological properties of proteins inside and outside of the cell. Binding to specific cell surface receptors induces activation of cellular signaling pathways leading to cellular dysfunction and cell death. AGEs are inducible by oxidative stress and induce oxidative stress. Subject of current studies of cell biologists is the intracellular processing of AGEs, which is accompanied by changes of the endolysosomal compartment.     

Inhibitors of advanced glycation end products (AGEs): potential utility for the treatment of cardiovascular disease

Accelerated atherosclerosis and microvascular complications are the leading causes of coronary heart disease, stroke, blindness, and end-stage renal failure, which could account for disabilities and high mortality rates in patients with diabetes. Recent clinical studies have substantiated the concept of "hyperglycemic memory" in the pathogenesis of cardiovascular disease (CVD) in diabetes. Indeed, the Diabetes Control and Complications Trial-Epidemiology of Diabetes Interventions and Complications (DCCT-EDIC) Research, has revealed that intensive therapy during the DCCT reduces the risk of cardiovascular events by about 50% in type 1 diabetic patients 11 years after the end of the trial. Among various biochemical pathways activated under diabetic conditions, the process of formation and accumulation of advanced glycation end products (AGEs) and their mode of action are most compatible with the theory "hyperglycemic memory." Further, there is a growing body of evidence that AGEs play an important role in CVD in diabetes. These observations suggest that the inhibition of AGEs formation may be a promising target for therapeutic intervention in diabetic vascular complications. Therefore, in this article, we review several agents with inhibitory effects on AGEs formation and their therapeutic implications in CVD in diabetes.     

Role of profilin-1 in vasculopathy induced by advanced glycation end products (AGEs)

AimsTo construct a simple and feasible rat model to mimic diabetic vasculopathy by chronic injection of advanced glycation end products (AGEs) and further determine the role of profilin-1 in vasculopathy in AGE-injection rats.MethodsSprague-Dawley rats were injected with AGEs-BSA (25 mg/kg/day) for 0, 20, 30, 40, and 60 days by caudal vein. Then, the morphological changes in the aorta, heart, and kidney and the expression of profilin-1 were assessed. In cultured endothelial cells, shRNA profilin-1 was used to clarify the role of profilin-1 in AGEs-induced vascular endothelial lesions and inflammatory reactions.ResultsThe aorta, heart, and kidney of the AGE-injection rats had obvious morphological changes. Also, the indicators of vascular remodeling in the aorta significantly increased, accompanied by the increased expression of profilin-1 in the aorta, heart, and kidney and polysaccharide content on the kidney basement membrane. In addition, the protein level of profilin-1 was markedly upregulated in the aorta of AGEs-injected rats and endothelial cells incubated with AGEs. shRNA profilin-1 markedly attenuated the upregulated expression of profilin-1, receptor for AGEs (RAGE), and NF-κB in endothelial cells incubated with AGEs, as well as reduced the high levels of ICAM-1, IL-8, TNF-α, ROS, and apoptosis induced by AGEs.ConclusionsExogenous AGEs can mimic diabetic vasculopathy in vivo to some extent and increase profilin-1 expression in the target organs of diabetic complications. Blockade of profilin-1 attenuates vascular lesions and inflammatory reactions, suggesting its critical role in the metabolic memory mediated by AGEs.     

Advanced glycation end products: role in pathology of diabetic cardiomyopathy

Increasing evidence demonstrates that advanced glycation end products (AGEs) play a pivotal role in the development and progression of diabetic heart failure, although there are numerous other factors that mediate the disease response. AGEs are generated intra- and extracellularly as a result of chronic hyperglycemia. Then, following the interaction with receptors for advanced glycation end products (RAGEs), a series of events leading to vascular and myocardial damage are elicited and sustained, which include oxidative stress, increased inflammation, and enhanced extracellular matrix accumulation resulting in diastolic and systolic dysfunction. Whereas targeting glycemic control and treating additional risk factors, such as obesity, dyslipidemia, and hypertension, are mandatory to reduce chronic complications and prolong life expectancy in diabetic patients, drug therapy tailored to reducing the deleterious effects of the AGE–RAGE interactions is being actively investigated and showing signs of promise in treating diabetic cardiomyopathy and associated heart failure. This review shall discuss the formation of AGEs in diabetic heart tissue, potential targets of glycation in the myocardium, and underlying mechanisms that lead to diabetic cardiomyopathy and heart failure along with the use of AGE inhibitors and breakers in mitigating myocardial injury.     

The role of AGEs in aging: causation or correlation

Over a dozen advanced glycation end-products (AGEs) have been identified in tissue proteins by chemical or immunological methods. Of these, about half are known to accumulate with age in collagen at a rate that correlates with the half-life of the collagen. AGEs may be formed by oxidative and non-oxidative reactions and are in some cases identical to advanced lipoxidation end-products (ALEs) formed in protein during lipid peroxidation reactions. AGEs affect the biochemical and physical properties of proteins and the extracellular matrix (ECM), including the charge, hydrophobicity, turnover and elasticity of collagen, and the cell adhesion, permeability and pro-inflammatory properties of the ECM. A number of scavenger and AGE-specific receptors have been identified that may mediate the turnover of AGE-proteins, catalyze the local production of reactive oxygen species and attract and activate tissue macrophages. Although AGEs in proteins are probably correlative, rather than causative, with respect to aging, they accumulate to high levels in tissues in age-related chronic diseases, such as atherosclerosis, diabetes, arthritis and neurodegenerative disease. Inhibition of AGE formation in these diseases may limit oxidative and inflammatory damage in tissues, retarding the progression of pathophysiology and improve the quality of life during aging.     

Advanced glycation end products (AGEs) and diabetic vascular complications

Diabetic vascular complication is a leading cause of acquired blindness, end-stage renal failure, a variety of neuropathies and accelerated atherosclerosis, which could account for disabilities and high mortality rates in patients with diabetes. Chronic hyperglycemia is essentially involved in the development and progression of diabetic micro- and macroangiopathy. Among various metabolic derangements implicated in the pathogenesis of diabetic vascular complication, advanced glycation end product (AGE) hypothesis is most compatible with the theory of 'hyperglycemic memory'. In this review, we discuss the molecular mechanisms of diabetic vascular complication, specially focusing on AGEs and their receptor (RAGE) system. Several types of AGE inhibitors and their therapeutic implications in this devastating disorder are also discussed here.     

Role of advanced glycation end product (AGE)-induced receptor (RAGE) expression in diabetic vascular complications

AimsVascular complications are the major causes of morbidity and mortality in diabetic subjects. Interaction of advanced glycation end products (AGEs) with their receptor (RAGE) induces signal transduction that culminates in vascular complications. Therefore, in the present study we investigated the dependence of RAGE expression on circulating AGEs and evaluated the outcome of AGE–RAGE interaction by the oxidative stress and nature of vascular complications in type 2 diabetes mellitus (T2DM) patients.MethodsRAGE expression was determined by quantitative real-time PCR and western blotting, serum AGEs were estimated by ELISA and spectrofluorometry and oxidative stress markers namely protein carbonyl (PCO), advanced oxidation protein products (AOPP) and lipid peroxidation (MDA) were assayed spectrophotometerically in 75 T2DM patients (DM without vascular complication n = 25; DM with microvascular complications n = 25; DM with macrovascular complications n = 25) and 25 healthy controls.ResultsSerum AGE level was significantly higher in diabetic patients having vascular complications as compared to T2DM without complications (p < 0.01). RAGE m-RNA expression level in PBMCs assayed by quantitative real time PCR was four times higher in diabetic subjects without vascular complications while DM patients having microvascular and macrovascular complications showed 12 fold and 8 fold higher RAGE m-RNA expression respectively compared to healthy controls. Circulating AGE level showed significant positive correlation with RAGE m-RNA expression and oxidative stress markers.ConclusionAGE-mediated exacerbation of RAGE expression may contribute to oxidative stress generation that plays a key role in pathogenesis of vascular complications in diabetes.     

Lipoprotein Modification by Advanced Glycosylation Endproducts (AGEs): Role in Atherosclerosis

Recent progress in our understanding of advanced glycosylation reactions in vivo has affirmed the hypothesis that these products play an important role in the evolution of both diabetic and nondiabetic vascular disease. Utilizing newly developed advanced glycosylation end-products (AGE)-specific enzyme-linked immunosorbent assay (ELISA) techniques, AGEs have been identified to be present on a variety of vascular wall, lipoprotein, and lipid constituents. Vascular wall AGEs contribute to vascular pathology by increasing vascular permeability, enhancing subintimal protein and lipoprotein deposition, and inactivating nitric oxide. Lipid-linked AGEs present in low-density lipoprotein (LDL) also have been shown to initiate oxidative modification, promoting oxidation reactions that may proceed without the involvement of free metals or other radical generating systems. AGE-specific ELISA analysis has demonstrated a significantly increased level of AGE-modified LDL in the plasma of diabetic patients when compared to normal controls. AGE-modification impairs LDL-receptor-mediated clearance mechanisms in vivo and may contribute to elevated LDL levels in patients with diabetes. This concept has been substantiated further by the recent clinical observations that administration of the advanced glycosylation inhibitor aminoguanidine to diabetic patients significantly decreases circulating LDL levels. (Trends Cardiovasc Med 1997;7:39-47). ? 1997, Elsevier Science Inc.     

Olmesartan blocks advanced glycation end products (AGEs)-induced angiogenesis in vitro by suppressing receptor for AGEs (RAGE) expression

We have previously shown that advanced glycation end products (AGEs)-their receptor (RAGE) interaction elicits angiogenesis through autocrine production of vascular endothelial growth factor (VEGF), thus suggesting the active involvement of the AGEs-RAGE system in proliferative diabetic retinopathy (PDR). Since the crosstalk between the AGEs-RAGE and the renin-angiotensin system has also been proposed in the pathogenesis of PDR, we investigated here whether olmesartan, an angiotensin II type 1 receptor blocker, inhibited the AGEs-elicited angiogenesis in vitro by suppressing the NF-kappaB-mediated RAGE expression. Olmesartan significantly inhibited the AGEs-induced NF-kappaB promoter activity and RAGE gene expression in cultured microvascular endothelial cells (ECs). Further, olmesartan was found to block the AGEs-induced up-regulation of VEGF mRNA levels and consequent increase in DNA synthesis in ECs. These results demonstrated for the first time that olmesartan inhibited the AGEs signaling to angiogenesis by suppressing RAGE expression in ECs. Our present study suggests that blockade of the renin-angiotensin system by olmesartan may play a protective role against PDR by attenuating the deleterious effects of AGEs via down-regulation of RAGE.     

Osmotic stress induced oxidative damage: possible mechanism of cataract formation in diabetes

Chronic hyperglycemia causes increased level of reactive oxygen species which is thought to be involved in the pathogenesis of diabetes associated complications including cataract. In diabetic cataractous lens, over production of free radicals and decreased capacity of antioxidant defense system are the major contributors to oxidative damage by polyol pathway and advanced glycation end products. The current study focused on analysis of factors associated with osmotic imbalance and oxidative stress in aging and diabetic human cataractous lenses. We examined activities of polyol pathway enzymes, G6PD and glutathione system in lenses from subjects suffering from cataract due to aging and diabetes. We observed elevated activities of aldose reductase and sorbitol dehydrogenase while G6PD and glutathione system enzyme activities were found to be lower in cataractous subjects suffering from diabetes. The findings from the current study support the premise that osmotic imbalance, AGEs formation and oxidative stress contribute synergistically to the development of lens opacity in hyperglycemia.     

Preservation of the antioxidant status in chemically-induced diabetes mellitus by melatonin

Oxidant stress is believed to be enhanced in patients with diabetes mellitus, which may lead to endothelial dysfunction and the development of atherosclerosis. In diabetes, hyperglycemia drives non-enzymatic glycation and oxidation of proteins and lipids which enhances the formation of advanced glycation end products (AGEs), which may be involved in the pathogenesis of diabetic vascular disease. The macrovascular complications of diabetes seem to be due to enhanced cellular oxidant stress by the interaction of AGEs with their receptor. It would be worthwhile to devise methods to reduce this oxidant stress. In alloxan-induced diabetic rats lipid peroxidation products were increased, while levels of nitric oxide glutathione peroxidase and superoxide dismutase were reduced. Melatonin restored these biochemical abnormalities to normalcy independent of hyperglycemia. This model can be used to study the role of oxidant stress in the development of macrovascular complications in diabetes mellitus.     

Serum levels of soluble form of receptor for advanced glycation end products (sRAGE) are positively associated with circulating AGEs and soluble form of VCAM-1 in patients with type 2 diabetes

We have recently found that soluble form of receptor for advanced glycation end products (sRAGE) levels are positively associated with inflammatory biomarkers and the presence of coronary artery disease (CAD) in type 2 diabetic patients. Since advanced glycation end products (AGEs) up-regulate RAGE expression and endogenous sRAGE could be generated from the cleavage of cell surface RAGE, it is conceivable that sRAGE is positively associated with circulating AGEs levels in diabetes. In this study, we examined whether sRAGE were correlated to circulating levels of AGEs and soluble forms of vascular cell adhesion molecule-1 (sVCAM-1) and intercellular adhesion molecule-1 (sICAM-1) in patients with type 2 diabetes. Eighty-two Japanese type 2 diabetic patients underwent a complete history and physical examination, determination of blood chemistries, sRAGE, AGEs, sVCAM-1 and sICAM-1. Multiple regression analysis revealed that serum levels of AGEs and sVCAM-1 were independently correlated with sRAGE. This study demonstrated that serum levels of sRAGE were positively associated with circulating AGEs and sVCAM-1 levels in type 2 diabetic patients. Our present observations suggest sRAGE level may be elevated in response to circulating AGEs, thus being a novel marker of vascular injury in patients with type 2 diabetes.     

AGEs对免疫细胞作用的研究进展

糖尿病患者长期高血糖导致晚期糖基化终末产物（AGEs）在体内过多蓄积，引起体内免疫系统一系列病理生理变化。AGEs可作用于巨噬细胞、淋巴细胞、树突状细胞等多种细胞，激活细胞内信号途径，促进免疫细胞分化，分泌多种细胞因子。同时，AGEs诱发体液免疫，产生抗AGEs自身抗体，共同参与糖尿病急慢性并发症的发生。     

Advanced glycation end products and diabetic foot disease

Diabetic foot disease is an important complication of diabetes. The development and outcome of foot ulcers are related to the interplay between numerous diabetes-related factors such as nerve dysfunction, impaired wound healing and microvascular and/or macrovascular disease.The formation of advanced glycation end products (AGEs) has been recognized as an important pathophysiological mechanism in the development of diabetic complications. Several mechanisms have been proposed by which AGEs lead to diabetic complications such as the accumulation of AGEs in the extracellular matrix causing aberrant cross-linking, the binding of circulating AGEs to the receptor of AGEs (RAGE) on different cell types and activation of key cell signalling pathways with subsequent modulation of gene expression, and intracellular AGE formation leading to quenching of nitric oxide and impaired function of growth factors. In the last decade, many experimental studies have shown that these effects of AGE formation may play a role in the pathogenesis of micro- and macrovascular complications of diabetes, diabetic neuropathy and impaired wound healing. In recent years also, several clinical studies have shown that glycation is an important pathway in the pathophysiology of those complications that predispose to the development of foot ulcers. Currently, there are a number of ways to prevent or decrease glycation and glycation-induced tissue damage. Although not in the area of neuropathy or wound healing, recent clinical studies have shown that the AGE-breakers may be able to decrease adverse vascular effects of glycation with few side effects.     

Natural products as anti-glycation agents: possible therapeutic potential for diabetic complications

Diabetes mellitus is characterised by hyperglycaemia, lipidaemia and oxidative stress and predisposes affected individuals to long-term complications afflicting the eyes, skin, kidneys, nerves and blood vessels. Increased protein glycation and the subsequent build-up of tissue advanced glycation endproducts (AGEs) contribute towards the pathogenesis of diabetic complications. Protein glycation is accompanied by generation of free radicals through autoxidation of glucose and glycated proteins and via interaction of AGEs with their cell surface receptors (referred to as RAGE). Glycationderived free radicals can damage proteins, lipids and nucleic acids and contribute towards oxidative stress in diabetes. There is interest in compounds with anti-glycation activity as they may offer therapeutic potential in delaying or preventing the onset of diabetic complications. Although many different compounds are under study, only a few have successfully entered clinical trials but none have yet been approved for clinical use. Whilst the search for new synthetic inhibitors of glycation continues, little attention has been paid to anti-glycation compounds from natural sources. In the last few decades the traditional system of medicine has become a topic of global interest. Various studies have indicated that dietary supplementation with combined anti-glycation and antioxidant nutrients may be a safe and simple complement to traditional therapies targeting diabetic complications. Data for forty two plants/constituents studied for anti-glycation activity is presented in this review and some commonly used medicinal plants that possess anti-glycation activity are discussed in detail including their active ingredients, mechanism of action and therapeutic potential.     

Receptor for advanced glycation endproducts (RAGE) and vascular inflammation: Insights into the pathogenesis of macrovascular complications in diabetes

The incidence and severity of atherosclerosis is increased in patients with diabetes. Indeed, accelerated macrovascular disease in diabetic patients has emerged as a leading cause of morbidity and mortality in the United States and worldwide. Multiple investigations have suggested that there are numerous potential contributory factors that underlie these observations. Our laboratory has focused on the contribution of receptor for advanced glycation endproducts (RAGE) and its proinflammatory ligands, advanced glycation endproducts (AGEs) and S100/calgranulins in vascular perturbation, manifested as enhanced atherogenesis or accelerated restenosis after angioplasty. In rodent models of diabetic complications, blockade of RAGE suppressed vascular hyperpermeability, accelerated atherosclerotic lesion area and complexity in diabetic apolipoprotein E-deficient mice, and prevented exaggerated neointimal formation in hyperglycemic fatty Zucker rats subjected to injury of the carotid artery. In this review, we summarize these findings and provide an overview of distinct mechanisms that contribute to the development of accelerated diabetic macrovascular disease. Insights into therapeutic strategies to prevent or interrupt these processes are presented.     

Glycotoxins in the diet promote diabetes and diabetic complications

Oxidant stress underlies diabetes and diabetic complications, including cardiovascular, renal, and retinal disease. Advanced glycation end products (AGEs), or glycotoxins, are a significant contributor to oxidant stress in diabetes. The diet is a major, unrecognized source of AGEs. Importantly, reduction of dietary AGEs decreases circulating inflammatory markers in both diabetic patients and prediabetic patients and complications in animal models. This beneficial outcome requires only a 50% decrease in dietary AGEs, making this necessary intervention practical and inexpensive.     

糖尿病血管内皮细胞代谢改变的研究进展

血管内皮细胞功能障碍是糖尿病（DM）血管并发症的始动环节。高血糖所致的氧化应激水平升高、异常糖代谢途径激活、晚期糖基化终产物（AGEs）累积和蛋白激酶C（PKC）通路活化等代谢改变均可介导内皮细胞损伤。如何抑制内皮细胞异常代谢并改善内皮细胞功能一直是DM血管并发症的研究重点。本文主要就DM患者血管内皮细胞的代谢改变作一综述。