Glucose metabolism, advanced glycation endproducts, and cognition

The accumulation of advanced glycation endproducts (AGEs) has been implicated in the pathogenesis of diabetic complications and aging. Diabetes mellitus is associated with moderate cognitive deficit. The AGEs may exert several actions on central nervous system. To explore a possible role of AGEs for cognitive impairment, we have examined the association between serum AGEs and cognitive function in diabetic patients.
We studied 198 diabetic patients with a mean age of 74 years. Serum AGE (N-epsilon- carboxymethyllysine, CML) was measured with the enzyme immunoassay. The several domains of cognitive function were assessed using the WAIS-R, Stroop test, and Benton test. Diabetic patients had more impairment of complex pychomotor skill and visual memory as assessed with the digit symbol substitution test and Benton test than non-diabetic subjects. Among the diabetic patients, a marker of hyperglycemia, HbA1c level was associated with the cognitive impairment, while short-term glucose control partially improved the cognitive impairment. Serum AGE levels correlated with age, duration of diabetes, and HbA1c. Serum AGE levels were significantly associated with the impairment of complex psychomotor skills independent of HbA1c. In conclusion, hyperglycemia and increased AGE accumulation were associated with the cognitive impairment in patients with diabetes mellitus as a model of accelerated aging.     

Advanced glycation end-products (AGEs) and heart failure: pathophysiology and clinical implications

Advanced glycation end-products (AGEs) are molecules formed during a non-enzymatic reaction between proteins and sugar residues, called the Maillard reaction. AGEs accumulate in the human body with age, and accumulation is accelerated in the presence of diabetes mellitus. In patients with diabetes, AGE accumulation is associated with the development of cardiac dysfunction. Enhanced AGE accumulation is not restricted to patients with diabetes, but can also occur in renal failure, enhanced states of oxidative stress, and by an increased intake of AGEs. Several lines of evidence suggest that AGEs are related to the development and progression of heart failure in non-diabetic patients as well. Preliminary small intervention studies with AGE cross-link breakers in heart failure patients have shown promising results. In this review, the role of AGEs in the development of heart failure and the role of AGE intervention as a possible treatment for heart failure are discussed.     

Protein glycation: a firm link to endothelial cell dysfunction

The advanced glycation end products (AGEs) are a heterogeneous class of molecules, including the following main subgroups: bis(lysyl)imidazolium cross-links, hydroimidazolones, 3-deoxyglucosone derivatives, and monolysyl adducts. AGEs are increased in diabetes, renal failure, and aging. Microvascular lesions correlate with the accumulation of AGEs, as demonstrated in diabetic retinopathy or renal glomerulosclerosis. On endothelial cells, ligation of receptor for AGE (RAGE) by AGEs induces the expression of cell adhesion molecules, tissue factor, cytokines such as interleukin-6, and monocyte chemoattractant protein-1. A chief means by which AGEs via RAGE exert their effects is by generation of reactive oxygen species, at least in part via stimulation of NADPH oxidase. Diabetes-associated vascular dysfunction in vivo can be prevented by blockade of RAGE. Thus, agents that limit AGE formation, increase the catabolism of these species, or antagonize their binding to RAGE may provide new targets for vascular protection in diabetes.     

Advanced glycation end products induce glomerular sclerosis and albuminuria in normal rats.

High levels of tissue advanced glycation end products (AGEs) that result from the spontaneous modification of proteins by glucose occur in diabetes and aging. To address the potential pathogenic role of AGEs in the glomerulosclerosis of diabetes or nephrosclerosis of aging, doses of AGE-modified rat albumin (25 mg per kg per day, i.v.) sufficient to elevate circulating AGE levels to the range of diabetic serum were administered daily to healthy rats alone or in combination with the AGE inhibitor aminoguanidine. After 5 months, the AGE content of renal tissues in AGE-treated rats rose to 50% above controls (P < 0.025), whereas serum contained 2.8-fold greater AGE levels (P < 0.025). Light and electron microscopy of kidneys from AGE-treated rats revealed a more than 50% increase in glomerular volume compared to controls (P < 0.001), significant periodic acid/Schiff reagent-positive deposits, basement membrane widening, and mesangial extracellular matrix increase and indicated significant glomerulosclerosis compared to untreated (P < 0.002) or albumin-treated controls (P < 0.002). These changes were associated with significant loss of protein (P < 0.005) and albumin (P < 0.002) in the urine of AGE-treated rats compared to controls. Cotreatment with aminoguanidine markedly limited both the structural and functional defects. These in vivo data demonstrate that AGEs influence glomerular structure and function in a manner leading to glomerulosclerosis. The effects are AGE-specific, as they are ameliorated by a pharmacological AGE inhibitor, aminoguanidine.     

Relationship between advanced glycation end products and increased lipid peroxidation in semen of diabetic men

Aims

Majority of diabetic male patients have disturbances in their reproductive systems. However, the mechanisms underlying these disturbances are largely unknown. Since advanced glycation end products (AGE) have a key role in oxidative stress and cell damage in diabetic complications, we hypothesize that AGEs may be involved sperm lipid peroxidation.

Methods

Total AGEs in seminal plasma of 32 diabetic and 35 non-diabetic men was determined by spectrofluorimetric method and carboxy methyl lysine (CML) level was assayed using ELISA. Contents of lipid peroxidation in sperm and seminal plasma were determined by thiobarbituric acid reaction. Total antioxidant capacity (TAC) was measured by a colorimetric assay.

Results

Total AGEs were found significantly higher in seminal plasma of diabetic men than non-diabetic group (p < 0.001) whereas no significant differences in seminal plasma CML values between two groups was observed. Moreover, sperm and seminal plasma lipid peroxidation were significantly higher in diabetic subjects than non-diabetic men and a significantly lower TAC was detected in diabetic group compare to non-diabetics.

Conclusions

These results showed an increment in AGEs in seminal plasma of diabetic subjects and may suggest a key role for glycation process and increased oxidative stress in reproductive system dysfunction.     

The AGE‐receptor in the pathogenesis of diabetic complications

Native glucose‐derived glycation derivatives (advanced glycation end products, AGE) in vascular, renal and neuronal tissues contribute to organ damage. Glycation derivatives include a number of chemically and cell‐reactive substances, also termed glycoxidation products or glycotoxins (GT). Cell‐associated AGE‐specific receptors (AGE‐Rs), AGE‐R1‐3, RAGE, as well as the scavenger receptors ScR‐II and CD‐36 that are present on vascular, renal, hemopoietic, and neuronal/glial cells, serve in the regulation of AGE uptake and removal. AGE‐Rs also modulate cell activation, growth‐related mediators, and cell proliferation, consequently influencing organ structure/function. This occurs via oxidant stress triggered via receptor‐dependent or ‐independent pathways, and leads to signal activation pathways, resulting in pro‐inflammatory responses. In susceptible individuals, the AGE‐R expression/function may be subject to environmental or gene‐related modulation, which in turn may influence tissue‐specific gene functions. In this context, altered expression and activity of AGE‐R components has recently been found in both mouse diabetes models and humans with diabetic complications. Although several gene polymorphisms are detected in most AGE‐R components, no significant correlation to diabetic complications has as yet been found. Further investigation is underway to define whether primary or secondary genetic links of pathogenic significance exist in this system. Various AGE‐binding peptides or soluble receptors have emerged as potential sequestering agents for toxic AGEs as potential therapies for diabetic complications. Copyright © 2001 John Wiley & Sons, Ltd.     

Renal accumulation and clearance of advanced glycation end-products in type 2 diabetic nephropathy: effect of angiotensin-converting enzyme and vasopeptidase inhibition

Aims/hypothesis Renal accumulation of AGEs may contribute to the progression of diabetic nephropathy. We evaluated the effect of ramipril (a pure ACE inhibitor) and AVE7688 (a dual inhibitor of ACE and neutral endopeptidase) on renal accumulation of the advanced glycation end-product (AGE) 3-deoxyglucosone-imidazolone, carboxymethyllysine (CML) and pentosidine, and on clearance of CML in type 2 diabetes.Methods Male Zucker diabetic fatty rats (ZDF, Gmi-fa/fa) rats were treated from age 10 to 37 weeks with ramipril (1 mg·kg^–1·day^–1), AVE7688 (45 mg·kg^–1·day^–1) or without drug. Ramipril and AVE7688 reduced albuminuria by 30 and 90%, respectively.Results ZDF rats showed increased renal accumulation of the AGE subtypes 3-deoxyglucosone-imidazolone, pentosidine and CML by about 40, 55 and 55%, respectively compared with heterozygous, non-diabetic control animals at the age of 37 weeks. AVE7688 but not ramipril attenuated the renal accumulation of 3-deoxyglucosone-imidazolone, pentosidine and CML and improved CML clearance in ZDF rats. During glycation reactions in vitro, AVE7688 also demonstrated potent chelating activity and inhibited metal-catalysed formation of pentosidine and CML.Conclusions/interpretation Improved AGE clearance and direct inhibition of AGE formation by chelation may contribute to reduced accumulation of renal AGEs and to the nephroprotective effects of vasopeptidase inhibition in type 2 diabetes.     

Serum levels of low molecular weight advanced glycation end products in diabetic subjects

AIMS: One of the principal theories of the development of diabetic complications proposes that increased levels of advanced glycation end products (AGE) are formed in diabetes by prolonged exposure of proteins, lipids and nucleotides to glucose. Such AGEs may contribute to the development of diabetic complications by a number of mechanisms. Circulating AGEs can be detected in serum, and in the present study, we analysed the clinical correlates of circulating serum low molecular weight AGE (LMW-AGE). METHODS: Serum LMW-AGE was measured in 106 non-diabetic and 499 diabetic subjects using fluorescence spectroscopy. Results were calibrated against an in-house AGE albumin preparation, and expressed as absolute fluorescence units (AFU). RESULTS: Serum LMW-AGE values were significantly higher in diabetic than non-diabetic subjects [median 7.5 (range 0-595.5) vs. 5.3 (1.0-15.5) AFU, P<0.01]. In the normal subjects, there were significant correlations between serum LMW-AGE and age (r=0.42, P<0.01) and serum creatinine (r=0.39, P<0.01). In the diabetic patients, serum LMW-AGE correlated significantly with age (r=0.315, P<0.01), systolic blood pressure (r=0.141, P=0.002), serum creatinine (r=0.449, P<0.01) and urinary albumin/creatinine ratio (ACR) (r=0.265, P<0.01). There was no correlation between serum LMW-AGE and HbA1c. On regression analysis, with serum LMW-AGE as the dependent variable, serum creatinine emerged as the most significant factor (t=8.1, P<0.01), followed by age (t=4.0, P<0.01) and ACR (t=2.9, P=0.004). There was no significant difference in serum LMW-AGE between those with and without retinopathy or in those with vascular disease. CONCLUSIONS: We conclude that circulating LMW-AGEs are increased in diabetic subjects. The major determinant appears to be renal dysfunction in the form of raised albumin/creatinine ratio or creatinine. There was no association with other markers of vascular disease or presence of diabetic complications.     

Advanced glycation end products in senile diabetic and nondiabetic patients with cataract

BackgroundAdvanced glycation end products (AGE) have been reported to contribute to aging and cataract formation in the lens. In the present study, AGE immunoreactivity in human serum samples of normal senile subjects (n=31), senile diabetic patients without cataract (n=33), senile diabetic patients with cataract (n=30), senile nondiabetic with cataract (n=30), and normal young subjects (n=31) was investigated.MethodsA noncompetitive ELISA with polyclonal anti-AGE antibody was performed. The patients were selected on clinical grounds from Eye Ward, Jinnah Postgraduate Medical Centre, Karachi, Pakistan.ResultsFasting blood glucose, glycosylated hemoglobin, and serum fructosamine were estimated. Fasting blood glucose, HbA_1C, and serum fructosamine levels were significantly (P<.001) increased in senile diabetic patients with and without cataract as compared to nondiabetic senile patients with cataract and senile control subjects. However, the serum AGEs were found to be significantly (P<.001) increased in senile diabetic patients with cataract and senile nondiabetic patients with cataract followed by the diabetic patients without cataract as compared to senile control and young control subjects. In contrast to all four senile groups, the serum AGEs were significantly (P<.001) lower in young control subjects.ConclusionsThe AGE distribution in the senile groups corroborates the hypothesis that the advanced glycation process might have a role in cataract formation, which in diabetic patients occurs vigorously as compared with nondiabetic cataract patients.     

Tobacco smoke is a source of toxic reactive glycation products

Smokers have a significantly higher risk for developing coronary and cerebrovascular disease than nonsmokers. Advanced glycation end products (AGEs) are reactive, cross-linking moieties that form from the reaction of reducing sugars and the amino groups of proteins, lipids, and nucleic acids. AGEs circulate in high concentrations in the plasma of patients with diabetes or renal insufficiency and have been linked to the accelerated vasculopathy seen in patients with these diseases. Because the curing of tobacco takes place under conditions that could lead to the formation of glycation products, we examined whether tobacco and tobacco smoke could generate these reactive species that would increase AGE formation in vivo. Our findings show that reactive glycation products are present in aqueous extracts of tobacco and in tobacco smoke in a form that can rapidly react with proteins to form AGEs. This reaction can be inhibited by aminoguanidine, a known inhibitor of AGE formation. We have named these glycation products “glycotoxins.” Like other known reducing sugars and reactive glycation products, glycotoxins form smoke, react with protein, exhibit a specific fluorescence when cross-linked to proteins, and are mutagenic. Glycotoxins are transferred to the serum proteins of human smokers. AGE-apolipoprotein B and serum AGE levels in cigarette smokers were significantly higher than those in nonsmokers. These results suggest that increased glycotoxin exposure may contribute to the increased incidence of atherosclerosis and high prevalence of cancer in smokers.     

Aging: role and control of glycation

PURPOSE: Advanced glycation end-products (AGEs) accumulate in aging tissues and organs during rheumatoid arthritis and Alzheimer disease. These aging toxins are especially involved in cell alteration during diabetes mellitus (glycotoxin) and renal failure (uremic toxin). AGEs participate to the endothelial dysfunction leading to diabetic macro but also micro-angiopathy. AGEs binding to cell receptors are critical steps in the deleterious consequences of AGE excess. AGE-receptor activation altered cell and organ functions by a pro-inflammatory, pro-coagulant and pro-fibrosis factors cell response. CURRENT KNOWLEDGE AND KEY POINTS: Non-enzymatic glycation and glycoxidation with glucose auto-oxidation represent the two main pathways resulting in AGE formation. No exclusive AGE classification is actually available. Pathophysiological mechanisms are described to explain AGE toxicity. AGEs bind to cell receptors inducing deleterious consequences such as endothelial dysfunction after endothelial RAGE activation. AGEs can also have deleterious effects through glycated protein accumulation or in situ protein glycation. FUTURE PROSPECTS AND PROJECTS: Many in vitro or animal studies demonstrated that AGE deleterious effects can be prevented by glycation inhibitors, AGE cross-link breakers or AGE-RAGE interaction inhibition. New molecules are actually studied as new strategy to prevent or treat the deleterious effects of these aging toxins.     

Serum concentrations of advanced glycation endproducts are associated with the development of atherosclerosis as well as diabetic microangiopathy in patients with type 2 diabetes

We measured serum concentrations of advanced glycation endproducts (AGEs) in patients with type 2 diabetes, to elucidate the mechanisms underlying the elevated serum concentrations of AGEs and to clarify the relationship between serum AGE concentrations and the development of microangiography and macroangiopathy. Serum AGEs were significantly higher in diabetic patients than in age-matched control subjects (p < 0.0001). In diabetic patients, serum AGEs were positively correlated with HbA1c (r = 0.47, p < 0.0001), urinary albumin excretion (UAE) (r = 0.42, p < 0.0001), diabetes duration (r = 0.31, p = 0.0030), and fasting plasma glucose (r = 0.34, p = 0.0010). Multiple regression analysis disclosed that only the HbA1c and UAE levels independently correlated with serum AGE levels. Serum AGEs in diabetic patients with progressive retinopathy and overt nephropathy were significantly higher than in those with less severe retinopathy and nephropathy. Serum AGEs were significantly higher in the diabetic patients with coronary heart disease (CHD) than in those without CHD. These results suggest that the HbA1c and UAE levels are independent risk factors for increased serum AGE concentrations in type 2 diabetic patients, and that higher serum AGE concentrations are associated with increased severity of diabetic retinopathy and nephropathy. Serum AGE concentrations may be a useful marker not only for the severity of diabetic microangiopathy but also for the development of CHD in patients with type 2 diabetes mellitus. Received: 8 May 2000 / Accepted in revised form: 5 September 2000     

Advanced glycation end products in diabetic and non-diabetic human subjects suffering from cataract

Advanced glycation end products (AGEs) play a pivotal role in loss of lens transparency, i.e., cataract. AGEs formation occurs as a result of sequential glycation and oxidation reaction between reducing sugars and protein. AGEs production takes place throughout the normal aging process but its accumulation is found to be more rapid in diabetic patients. In this study, we quantified AGEs and N-(carboxyethyl) lysine (CEL) in human cataractous lenses from non-diabetic (n = 50) and diabetic patients (n = 50) using ELISA. We observed significantly higher (p < 0.001) levels of lens AGEs and CEL in diabetic patients with cataract as compared with their respective controls. The presence of AGEs and CEL was also determined by western blotting and immuno-histochemical analysis. Furthermore, isolated β-crystallin from cataractous lenses of non-diabetic and diabetic patients was incubated with different sugars to evaluate the extent of glycation in a time dependent manner. Our data indicated more pronounced glycation in patients suffering from diabetes as compared to non-diabetics subjects demonstrating the need to focus on developing normoglycemic approaches. Such studies may provide an insight in developing therapeutic strategies and may have clinical implications.     

Increased advanced glycation end products in atherosclerotic lesions of patients with end-stage renal disease

Although advanced glycation end products (AGEs) are increased in the serum and tissues of patients with end-stage renal disease, little is known about the role of AGEs in atherogenesis. We therefore carried out an immunohistochemical study on the accumulation of AGEs and apolipoprotein B in the human aortas of diabetic and nondiabetic subjects with end-stage renal disease. The atherosclerotic lesions included diffuse intimal thickening, fatty streaks and atherosclerotic plaque. We used antibodies against two different epitopes of AGE structures, i.e. an N^ε-(carboxymethyl)lysine-protein adduct (CML) and a structure(s) other than CML (nonCML). The area that was positive for an antigen as a percentage of the total area (%Ar) was determined morphometrically, using an NIH-image program. In diffuse intimal thickening, atherosclerotic plaque and tunica media, the %Ar of CML and nonCML was significantly greater in diabetic or nondiabetic subjects with end-stage renal disease than in control subjects without end-stage renal disease. In fatty streaks, the %Ar of nonCML was significantly greater in nondiabetic subjects with end-stage renal disease than in control subjects, while no difference in the %Ar of CML was found between the subjects with or without end-stage renal disease. Nondiabetic subjects with end-stage renal disease showed a significantly increased %Ar of apolipoprotein B in fatty streaks and atherosclerotic plaque than the control subjects. The %Ar of CML and nonCML significantly correlated with the duration of hemodialysis in diffuse intimal thickening and atherosclerotic plaque of subjects with end-stage renal disease, but not in fatty streaks. On the other hand, the %Ar was not related to the duration of diabetes in any of the lesions in the diabetic subjects with end-stage renal disease. In diffuse intimal thickening and atherosclerotic plaque, subjects with end-stage renal disease showed a significant correlation between the %Ar of apolipoprotein B and AGEs (CML and nonCML), as well as their immunohistochemical colocalization. These results suggest that impaired AGE clearance may cause the increased accumulation of AGEs in the aortic wall of subjects with end-stage renal disease, thus resulting in the rapid progression of atherosclerosis. The accumulation of AGEs may be related to an enhanced LDL deposition in atherosclerotic lesions of subjects with end-stage renal disease.     

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.     

Serum levels of non-carboxymethyllysine advanced glycation endproducts are correlated to severity of microvascular complications in patients with Type 1 diabetes

We investigated whether serum levels of N-(carboxymethyl)lysine (CML), non-CML advanced glycation endproducts (AGEs), or pentosidine are associated with severity of diabetic microvascular complications in patients with Type 1 diabetes. Serum levels of CML, non-CML AGE, and pentosidine were measured by an enzyme-linked immunosorbent assay in 38 males and 47 females aged 31+/-8 years (mean+/-S.D.) with Type 1 diabetes for 18.7+/-7.0 years. There was a significant correlation between serum levels of CML or non-CML AGE and current HbA(1c) level (P<.01 and P<.05, respectively). The serum levels of non-CML AGE, but not CML or pentosidine, were significantly increased as normal renal status advanced to microalbuminuria, clinical nephropathy, and hemodialysis (P<.0001) and were positively correlated with urinary albumin excretion (UAE) in patients with Type 1 diabetes (P<.0001). A significant elevation of serum non-CML AGE was found in association with the severity of diabetic retinopathy (P<.0001). We found in the present study that CML levels were also increased in the stage of simple retinopathy, the early stage of clinically evident retinopathy (P<.05). Serum levels of non-CML AGE were significantly associated with the severity of diabetic nephropathy and retinopathy, suggesting a role of non-CML AGE in the progression of microvascular complications in patients with Type 1 diabetes. Since serum levels of CML were significantly increased in patients with simple retinopathy, CML may participate in the initiation of diabetic retinopathy.     

Advanced glycation end products and their receptors co-localise in rat organs susceptible to diabetic microvascular injury

Summary Advanced glycation end products (AGEs) are believed to play an important role in the development of diabetic complications. AGEs are increased in experimental diabetes and treatment with the inhibitor of advanced glycation end products, aminoguanidine, has been shown to attenuate the level of these products in tissues undergoing complications. Recently, an AGE-binding protein has been isolated from bovine lung endothelial cells and termed the receptor for advanced glycated end products (RAGE). The present study sought to determine the distribution of AGE and RAGE in tissues susceptible to the long-term complications of diabetes including the kidney, eye, nerve, arteries as well as in a tissue resistant to such complications, the lung. Using polyclonal antisera both AGE and RAGE were found to co-localize in the renal glomerulus. AGE staining was clearly increased with age and was further increased by diabetes. Aminoguanidine treatment reduced AGE accumulation in the kidney. Co-localisation of AGE and RAGE was demonstrated in the inner plexiform layer and the inner limiting membrane of the retina and in nerve bundles from mesenteric arteries. In the aorta, both AGE and RAGE were found in the intima, media and adventitia. Medial staining was increased in diabetes and was reduced by aminoguanidine treatment. A similar pattern was observed for RAGE in the aorta. In the lung, RAGE was found widely distributed throughout the lung whereas the distribution of AGE staining was more limited, primarily localising to macrophages. The co-localisation of AGEs and RAGE in sites of diabetic microvascular injury suggests that this ligand-receptor interaction may represent an important mechanism in the genesis of diabetic complications. [Diabetologia (1997) 40: 619–628] Received: 16 October 1996 and in final revised form: 17 February 1997     

Differential accumulation of advanced glycation end products in the course of diabetic retinopathy

Aims/hypothesis. Glycated proteins, formed by reaction of glucose and protein, react further yielding numerous, mostly undefined advanced glycation end products (AGE). The recently characterized imidazolone-type AGE (AG-1) is non-oxidatively formed involving 3-deoxyglucosone whereas some AGEs, particularly Nɛ-(carboxymethyl)lysine (CML), are formed only in the presence of oxygen. Methods. To study the possible contribution of oxidative and non-oxidative AGE formation in the development of diabetic retinopathy antibodies directed against CML-type and imidazolone-type AGEs were characterized by dot blot analysis and used to localize these well-characterized epitops in the retinas from diabetic rats (early course) and from human Type I (insulin-dependent) diabetes mellitus with laser-treated proliferative diabetic retinopathy (late course). Results. In non-diabetic rats CML was moderately positive in neuroglial and vascular structures of non-diabetic rat retinas and increased strongly in diabetic retinas. Anti-imidiazolone antibody staining was strongly positive only in diabetic capillaries. Advanced human diabetic retinopathy showed strong CML-immunolabelling of the entire retina whereas control samples showed moderate staining of neuroglial structures only with the polyclonal CML-antibody. Anti-imidiazolone antibody staining was faint in the inner retina of control sections but were strong throughout the entire diabetic retina. Immunolabelling for the AGE-receptor was congruent with a marker of Müller cells. Conclusion/interpretation. Our data indicate that the oxidatively formed CML is present in non-diabetic retinas as a regular constituent but increases in diabetes both in neuroglial and vascular components. Imidazolone-type AGE are restricted to microvessels and spread during later stages over the entire retina, co-localizing with the expression of AGE-receptor. [Diabetologia (1999) 42: 728–736] Received: 22 October 1998 and in revised form: 11 January 1999     

Modification of low density lipoprotein by advanced glycation end products contributes to the dyslipidemia of diabetes and renal insufficiency.

Atherosclerosis develops rapidly in patients with diabetes or renal insufficiency. Plasma lipoprotein profiles are frequently abnormal in these conditions and reflect an elevation in the level of the apoprotein B (ApoB)-containing components very low density lipoprotein (VLDL) and low density lipoprotein (LDL). High levels of circulating advanced glycation end products (AGEs) also occur in diabetes and end-stage renal disease (ESRD). These products arise from glucose-derived Amadori products and include AGE-modified peptides (AGE-peptides) which result from the catabolism of AGE-modified tissue proteins. AGE-peptides have been shown to crosslink protein amino groups and to accumulate in plasma as a consequence of renal insufficiency. To address potential mechanisms for the dyslipidemia of diabetes and ESRD, we investigated the possibility that circulating AGEs react directly with plasma lipoproteins to prevent their recognition by tissue LDL receptors. AGE-specific ELISA showed a significantly increased level of AGE-modified LDL in the plasma of diabetic or ESRD patients compared with normal controls. AGE-LDL formed readily in vitro when native LDL was incubated with either synthetic AGE-peptides or AGE-peptides isolated directly from patient plasma. LDL which had been modified by AGE-peptides in vitro to the same level of modification as that present in the plasma of diabetics with renal insufficiency exhibited markedly impaired clearance kinetics when injected into transgenic mice expressing the human LDL receptor. These data indicate that AGE modification significantly impairs LDL-receptor-mediated clearance mechanisms and may contribute to elevated LDL levels in patients with diabetes or renal insufficiency. This hypothesis was further supported by the observation that the administration of the advanced glycation inhibitor aminoguanidine to diabetic patients decreased circulating LDL levels by 28%.     

Advanced glycation end-products, a pathophysiological pathway in the cardiorenal syndrome

The prevalence of heart failure (HF) is increasing. A distinction is made between diastolic HF (preserved left ventricular ejection fraction (LVEF)) and systolic HF (reduced LVEF). Advanced glycation end-products (AGEs) are crystallized proteins that accumulate during ageing, but are particularly increased in patients with diabetes mellitus and in patients with renal failure. Through the formation of collagen crosslinks, and by interaction with the AGE-receptor, which impairs calcium handling and increases fibrosis, AGE-accumulation has pathophysiologically been associated with the development of diastolic and renal dysfunction. Interestingly, diastolic dysfunction is a frequent finding in elderly patients, diabetic patients and in patients with renal failure. Taken together, this suggests that AGEs are related to the development and progression of diastolic HF and renal failure. In this review, the role of AGEs as a possible pathophysiological factor that link the development and progression of heart and renal failure, is discussed. Finally, the role of AGE intervention as a possible treatment in HF patients will be discussed.