Advanced glycation end-products: a review

Study of degradation pathways of Amadori compounds obtained by glycation of opioid pentapeptide and related smaller fragments: stability, reactions, and spectroscopic properties

Reactions between biological amines and reducing sugars (the Maillard reaction) are among the most important of the chemical and oxidative changes occurring in biological systems that contribute to the formation of a complex family of rearranged and dehydrated covalent adducts that have been implicated in the pathogenesis of human diseases. In this study, chemistry of the Maillard reactions was studied in four model systems containing fructosamines (Amadori compounds) obtained from the endogenous opioid pentapeptide leucine-enkephalin (Tyr-Gly-Gly-Phe-Leu), leucine-enkephalin methyl ester, structurally related tripeptide (Tyr-Gly-Gly), or from amino acid (Tyr). The degradation of model compounds as well as their ability to develop Maillard fluorescence was investigated under oxidative conditions in methanol and phosphate buffer pH 7.4 at two different temperatures (37 and 70 degrees C). At 37 degrees C, glycated leucine-enkephalin degraded slowly in methanol (t(1/2) approximately 13 days) and phosphate buffer (t(1/2) approximately 9 days), producing a parent peptide compound as a major product throughout a three-week incubation period. Whereas fluorescence slowly increased over time at 37 degrees C, incubations off all studied Amadori compounds at 70 degrees C resulted in a rapid appearance of a brown color and sharp increase in AGE (advanced glycation end products)-associated fluorescence (excitation 320 nm/emmision 420 nm) as well as in distinctly higher amounts of fragmentation products. The obtained data indicated that the shorter the peptide chain the more degradation products were formed. These studies have also helped to identify a new chemical transformation of the peptide backbone in the Maillard reaction that lead to beta-scission of N-terminal tyrosine side chain and p-hydroxybenzaldehyde formation under both aqueous and nonaqueous conditions.

Synthesis of a novel radical trapping and carbonyl group trapping anti-AGE agent: a pyridoxamine analogue for inhibiting advanced glycation (AGE) and lipoxidation (ALE) end products

[structure: see text] Pyridoxamine is known to be an effective inhibitor of both advanced glycation (AGE) and advanced lipoxidation (ALE) end products. The synthesis of a novel multifunctional AGE and ALE inhibitor, 6-dimethylaminopyridoxamine (dmaPM, 11) is described. The 6-dimethylamino substituent increases the radical trapping ability of pyridoxamine's phenolic group. Results obtained during ribose glycations show that both the new dmaPM and a known strong radical trapping agent, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), prevent intermolecular protein cross-linking more effectively than pyridoxamine (PM).

Impairment of vascular endothelial nitric oxide synthase activity by advanced glycation end products

Endothelial damage is believed to play a key role in the development of both micro- and macrovascular disease in diabetes, and advanced glycation end products (AGEs) may contribute importantly to this. To determine whether glucose-derived AGEs can cause endothelial dysfunction, we examined the effects of albumin AGE-modified by glucose (AGE-Glu) both in vivo, after injection into rabbit femoral artery, and in vitro on rabbit aortic rings and cultured human umbilical vein endothelial cells (HUVEC). Exposure of blood vessels to AGE-Glu, in vivo and in vitro, inhibited endothelium-dependent vasorelaxation, whereas unmodified albumin did not. In isolated rabbit aorta, this effect was reversible after AGE-Glu washout, and the response to the endothelium-independent vasodilator sodium nitroprusside was unaffected by AGE-Glu. In HUVEC, AGE-Glu inhibited endothelial nitric oxide synthase activity, and this was associated with a decrease in serine phosphorylation of this enzyme. Longer term (72 h) incubation decreased HUVEC viability. Use of specific antibodies demonstrated that these effects were mediated by N(epsilon)-(carboxymethyl)lysine (CML), an important AGE found in vivo, and by the AGE-R1 receptor. Furthermore, these effects all occurred at CML concentrations similar to those found in the plasma of diabetic patients. These results suggest an important role of AGE in the pathogenesis of diabetic vasculopathy.

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.

Diabetic nephropathy in type 2 diabetes mellitus: risk factors and prevention

Diabetic nephropathy (DN) is responsible for the increasing number of patients on dialysis in developing countries, and is already the most common cause of renal replacement therapy in the developed ones. In this manuscript, we review the risk factors and point out strategies to prevent this microvascular complication in type 2 diabetic patients (DM2). There are some known genetic and non-genetic risk factors related to the development and progression of DN in DM2 patients. Candidate genes have been analysed, but there are still controversy about the genetic markers of the disease. Recognized non-genetic risk factors are poor glycemic, pressoric and lipidic control. Additionally, it has been suggested that the presence of diabetic retinopathy, autonomic neuropathy, smoking habit, higher protein ingestion, and higher normal levels of albuminuria (even within the normal range) are associated with an increased risk of developing DN. Some strategies have been investigated and proved to prevent or at least to postpone DN, such as to control blood pressure, glycemic levels and dyslipidemia. Furthermore, angiotensin-converting enzyme inhibitors and angiotensin-II blockers have independent effects, not explained by blood pressure control alone. Other therapeutic items are to consume a low protein diet and to quit smoking.

Diabetic ketoacidosis and its treatment increase plasma 3-deoxyglucosone

OBJECTIVES

Highly reactive dicarbonyl compounds are known to be increased by hyperglycemia, ketone bodies and lipid peroxidation. This study was carried out to investigate the effect of diabetic ketoacidosis (DKA) and its treatment on the plasma concentration of 3 deoxyglucosone (3-DG) one of the dicarbonyl compounds.

DESIGN AND METHODS

3-DG was measured in 7 children before, during and following correction of severe DKA. 3-DG was elevated before treatment (610 nmol/L +or/- 70) in comparison to baseline (120 h) (200 nmol/L+/or- 17) (p < 0.05). At 6 to 24 h into treatment 3-DG was further elevated (1080 nmol/L +or/- 80) in comparison to both pretreatment (p < 0.05) and baseline (p < 0.05).

CONCLUSION

3-DG is significantly elevated before the treatment of DKA and increases further during the treatment of DKA. The time course of the increase of 3-DG coincides with the time of progression of subclinical brain edema, which occurs in DKA.

Role of megalin in endocytosis of advanced glycation end products: implications for a novel protein binding to both megalin and advanced glycation end products

Advanced glycation end products (AGE) are filtered by glomeruli and reabsorbed and metabolized by proximal tubule cells (PTC). In renal failure, decreased renal AGE metabolism likely accounts for the accumulation in serum that is related to uremic complications. In diabetes, AGE generation is increased, and the handling mechanisms in PTC are likely associated with the pathogenesis of tubulointerstitial injury. It is therefore important to clarify the mechanisms of the AGE metabolism to develop a strategy for removing AGE in uremia and to elucidate the pathogenesis of diabetic nephropathy. To this end, this study focused on the molecular analysis of megalin, a multi-ligand endocytic receptor, in PTC. AGE uptake analysis was performed using the rat yolk sac-derived L2 cell line system established for the analysis of megalin's endocytic functions. The cells mediated specific internalization and degradation of AGE, which were significantly blocked by anti-megalin IgG, indicating that megalin is involved in the cellular processes. However, cell surface AGE-binding assays and ligand blot analysis revealed no evidence that megalin is a direct AGE receptor. Affinity chromatography and ligand blot analysis originally revealed that 200-kD and 400-kD proteins in the cells bind to AGE and the 200-kD protein to megalin in a Ca(2+)-dependent manner. The binding of megalin with the 200-kD protein was suppressed by receptor-associated protein (RAP), a ligand for megalin. In conclusion, megalin functions for endocytosis of AGE via an indirect mechanism. L2 cells express novel AGE-binding proteins, one of which may interact with megalin.

Diabetes mellitus and heart failure

Type 2 diabetes mellitus substantially increases the lifetime risk of both developing and dying from heart failure. While this appears to be explained in part by the well-known association of diabetes with hypertension, dyslipidemia, and coronary atherosclerosis, additional pathophysiologic mechanisms linking type 2 diabetes and heart failure have recently been suggested. These include the potentially adverse effects of hyperglycemia on endothelial function and redox state, effects of excess circulating glucose and fatty acids on cardiomyocyte ultrastructure, intracellular signaling and gene expression, and the possibility that diabetes may impair recruitment of the myocardial insulin-responsive glucose transport system in response to ischemia. Because many of these putative pathophysiologic mechanisms should be amenable to normalization of the diabetic metabolic milieu, strategies designed to more carefully control circulating levels of glucose and fatty acids might conceivably delay or prevent the development of heart failure.

The polyamines spermine and spermidine protect proteins from structural and functional damage by AGE precursors: a new role for old molecules?

Due to the importance of glycation in the genesis of diabetic complications, an intense search for synthetic new antiglycation agents is ongoing. However, a somewhat neglected avenue is the search for endogenous compounds that may inhibit the process and be a source of protodrugs. Based on their ubiquity, their polycationic nature, their essential role in growth, their relatively high concentrations in tissues, and their high concentrations in sperm, we hypothesized that polyamines inhibit glycation and that might be one of their so far elusive functions. In this study we demonstrate a potent antiglycation effect of physiological concentrations of the polyamines spermine and spermidine. We employed two approaches: in the first, we monitored structural changes on histones and ubiquitin in which polyamines inhibit glycation-induced dimer and polymer formation. In the second we monitored functional impairment of catalytic activity of antithrombin III and plasminogen. Protection is afforded against glycation by hexoses, trioses and dicarbonyls AGE precursors and is comparable to those of aminoguanidine and carnosine.

Oxidative deamination of benzylamine by glycoxidation

In the present study, model reactions for the oxidative deamination by glycoxidation using benzylamine were undertaken to elucidate the detail of the reaction. Glucose, 3-deoxyglucosone (3-DG), and methylglyoxal (MG) oxidatively deaminated benzylamine to benzaldehyde in the presence of Cu(2+) at a physiological pH and temperature but not glyoxal. 3-DG and MG were more effective oxidants than glucose. We have determined the effects of metal ions, pH, oxygen, and radical scavengers on the oxidative deamination. The formation of benzaldehyde was greatest with Cu(2+), and was accelerated at a higher pH and in the presence of oxygen. EDTA, catalase, and dimethyl sulfoxide significantly inhibited the oxidation, suggesting the participation of reactive oxygen species. From these results, we propose a mechanism for the oxidative deamination by the Strecker-type reaction and the reactive oxygen species-mediated oxidation during glycoxidation.

Nephrin expression is reduced in human diabetic nephropathy: evidence for a distinct role for glycated albumin and angiotensin II

We studied the distribution of nephrin in renal biopsies from 17 patients with diabetes and nephrotic syndrome (7 type 1 and 10 type 2 diabetes), 6 patients with diabetes and microalbuminuria (1 type 1 and 5 type 2 diabetes), and 10 normal subjects. Nephrin expression was semiquantitatively evaluated by measuring immunofluorescence intensity by digital image analysis. We found an extensive reduction of nephrin staining in both type 1 (67 +/- 9%; P < 0.001) and type 2 (65 +/- 10%; P < 0.001) diabetic patients with diabetes and nephrotic syndrome when compared with control subjects. The pattern of staining shifted from punctate/linear distribution to granular. In patients with microalbuminuria, the staining pattern of nephrin also showed granular distribution and reduction intensity of 69% in the patient with type 1 diabetes and of 62 +/- 4% (P < 0.001) in the patients with type 2 diabetes. In vitro studies on human cultured podocytes demonstrated that glycated albumin and angiotensin II reduced nephrin expression. Glycated albumin inhibited nephrin synthesis through the engagement of receptor for advanced glycation end products, whereas angiotensin II acted on cytoskeleton redistribution, inducing the shedding of nephrin. This study indicates that the alteration in nephrin expression is an early event in proteinuric patients with diabetes and suggests that glycated albumin and angiotensin II contribute to nephrin downregulation.

Protein carbonylation in human diseases

Oxidative modifications of enzymes and structural proteins play a significant role in the aetiology and/or progression of several human diseases. Protein carbonyl content is the most general and well-used biomarker of severe oxidative protein damage. Human diseases associated with protein carbonylation include Alzheimer's disease, chronic lung disease, chronic renal failure, diabetes and sepsis. Rapid recent progress in the identification of carbonylated proteins should provide new diagnostic (possibly pre-symptomatic) biomarkers for oxidative damage, and yield basic information to aid the establishment an efficacious antioxidant therapy.

Early and intermediate Amadori glycosylation adducts, oxidative stress, and endothelial dysfunction in the streptozotocin-induced diabetic rats vasculature

AIMS/HYPOTHESIS

In a model of streptozotocin-induced Type 1 diabetes mellitus in rats of 9 weeks duration, we analysed time associations between the development of hyperglycaemia, early and intermediate glycosylation Amadori adducts, or AGE compared with enhancement of oxidative stress and endothelial dysfunction.

METHODS

Endothelial function was tested at several stages of streptozotocin-induced diabetes and after treatment with insulin, resulting in different concentrations of blood glucose, glycosylated haemoglobin (an Amadori adduct), and AGE. Other animals were studied antagonising the formation of AGE with aminoguanidine.

RESULTS

Relaxation in response to acetylcholine (1 nmol/l to 10 micro mol/l) was tested in isolated segments from aorta or mesenteric microvessels. Impairment of endothelium-dependent relaxations occurred after 2 weeks of untreated diabetes. Preincubation of vessels affected with 100 U/ml superoxide dismutase improved the relaxations to acetylcholine, along the time-course of the endothelial impairment. This indicates the participation of reactive oxygen species on diabetic endothelial dysfunction. The impairment of endothelium-dependent relaxations was recovered after 3 more weeks of insulin treatment. Aminoguanidine treatment did not modify this pattern of development. The time course of the rise and disappearance of endothelial dysfunction showed a higher correlation with glycosylated haemoglobin concentrations than with blood glucose or serum AGE.

CONCLUSION/INTERPRETATION

Enhancement of early and intermediate Amadori adducts of protein glycosylation was the factor showing a better relation with the development of endothelium impairment. These results are consistent with a role for these products in the development of diabetic vasculopathy.

Two flavone C-glycosides from the style of Zea mays with glycation inhibitory activity

A new flavone C-glycoside, chrysoeriol 6-C-beta-boivinopyranosyl-7-O-beta-glucopyranoside (1), and a known flavone C-glycoside (2) were isolated from the style of Zea mays. Each compound contains the rare sugar boivinose. These flavones exhibited glycation inhibitory activity similar to that of aminoguanidine, a known glycation inhibitor.

New insights into protein crosslinking via the Maillard reaction: structural requirements, the effect on enzyme function, and predicted efficacy of crosslinking inhibitors as anti-ageing therapeutics

Protein crosslinking via the Maillard reaction with alpha-dicarbonyl compounds has been the subject of intense literature scrutiny. We report here a systematic study of three previously-neglected aspects of the reaction. Firstly, structural requirements were probed. An arginine-free peptide that contains two lysine residues, and a lysine-free peptide that contains arginine, were reacted with glyoxal, methylglyoxal and biacetyl. Methylglyoxal was able to crosslink in the absence of arginine residues, but glyoxal and biacetyl were not. Glyoxal crosslinked the lysine-free peptide via the N-terminus, but methylglyoxal and biacetyl could not. In this study, crosslinking did not require the presence of arginine but did require a free amino group, from a lysine residue, or the N-terminus. Thus specificity in structural requirements for protein crosslinking by alpha-dicarbonyls has been demonstrated. Secondly, protein function following glycation was examined by treating ribonuclease A with the three alpha-dicarbonyls, which were shown both to crosslink the enzyme and impair enzymatic activity. Thirdly, the effects of two reported Maillard reaction inhibitors, aminoguanidine and 3,5-dimethylpyrazole-1-carboxamidine on the crosslinking reaction were assessed, with a parallel measurement of the effect on enzyme activity. The results demonstrate that preventing protein crosslinking does not necessarily preserve enzyme activity. These results cast doubt on the likely efficacy of some purported anti-ageing compounds in vivo.

Protein glycation inhibitory and antioxidative activities of some plant extracts in vitro

The protein glycation inhibitory activity of aqueous ethanolic extracts from 25 plant tissues was evaluated in vitro using the model system of bovine serum albumin and fructose. The most bioactive plant tissue was Allium cepa (skin), followed by Illicium religiosum (bark and wood), Fagopyrum esculentum (hull), Origanum officinalis (leaf), Rosmarinus officinalis (leaf), Pyrus pyrifolia (bark),Acanthopanax senticosus (bark), Eugenia caryophllata (leaf), and Erigeron annuus (whole). The extracts with glycation inhibitory activity also showed antioxidative activity when a micellar linoleic acid peroxidation system was applied followed by 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) radical cation decolorization and 1,1-diphenyl-2-picrylhydrazyl free radical scavenging assays. The glycation inhibitory activity was significantly correlated with the antioxidative potency of the extracts. The positive glycation inhibitory and antioxidative activities of these plants might suggest a possible role in targeting aging and diabetic complications.

Acute hyperglycemia causes intracellular formation of CML and activation of ras, p42/44 MAPK, and nuclear factor kappaB in PBMCs

Twenty-three nondiabetic volunteers were divided into three groups. In group A (n = 9), the glucose infusion was adjusted to maintain blood glucose at 5 mmol/l (euglycemic clamp). In group B (n = 9), the glucose infusion was adjusted to maintain blood glucose at 10 mmol/l (hyperglycemic clamp) over 2 h. Group C consisted of five volunteers who were studied as the control group. Peripheral blood mononuclear cells (PBMCs) were isolated before and at the end of a 2-h clamp. In group C, PBMCs were isolated before and after 2 h without performing a clamp. The euglycemic clamp as well as "no clamp" had no effects on all parameters studied. In contrast, a significant increase in carboxymethyllysine (CML) content and p21(ras) and p42/44 mitogen-activated protein kinase (MAPK) phosphorylation was observed at the end of a 2-h hyperglycemic clamp. The nuclear factor (NF)-kappaB (but not Oct-1) binding activity increased significantly in the hyperglycemic clamp. Western blots confirmed NF-kappaB-p65-antigen translocation into the nucleus. IkappaBalpha did not change significantly in both groups. Hyperglycemia-mediated NF-kappaB activation and increase of CML content, p21(ras), and p42/44 MAPK phosphorylation was also seen in ex vivo-isolated PBMCs stimulated with 5 or 10 mmol/l glucose. Addition of insulin did not influence the results. Inhibition of activation of ras, MAPK, or protein kinase C blocked hyperglycemia-mediated NF-kappaB activation in ex vivo-isolated PBMCs stimulated with 10 mmol/l glucose. Similar data were obtained using an NF-kappaB-luciferase reporter plasmid. Therefore, we can conclude that an acute hyperglycemia-mediated mononuclear cell activation is dependent on activation of ras, p42/p44 MAPK phosphorylation, and subsequent NF-kappaB activation and results in transcriptional activity in PBMCs.

Abnormal angiogenesis in diabetes mellitus

The adverse long-term effects of diabetes mellitus have been well described and involve many organ systems. While diabetes management has largely focused on control of hyperglycemia, the presence of abnormalities of angiogenesis may cause or contribute to many of the clinical manifestations of diabetes. When compared with non-diabetic subjects, diabetics demonstrate vascular abnormalities of the retina, kidneys, and fetus. Diabetics have impaired wound healing, increased risk of rejection of transplanted organs, and impaired formation of coronary collaterals. In each of these conditions, and possibly in diabetic neuropathy as well, abnormalities of angiogenesis can be implicated in the pathogenesis. A perplexing feature of the aberrant angiogenesis is that excessive and insufficient angiogenesis can occur in different organs in the same individual. In this review, the clinical features, molecular mechanisms, and potential therapeutic options of abnormal angiogenesis in diabetes will be reviewed.

Improvement in quality of diabetes control and concentrations of AGE-products in patients with type 1 and insulin-treated type 2 diabetes mellitus studied over a period of 10 years (JEVIN)

Advanced glycation end (AGE)-products, a complex and heterogeneous group of compounds, have been implicated in diabetes-related long-term complications. Up to the present, only few data exist about serum levels of the AGE-proteins N- epsilon -carboxymethyllysine (CML) and pentosidine in selection-free populations of patients with type 1 and insulin-treated type 2 diabetes mellitus. In the present 10-year, population-based trial of patients with insulin-treated diabetes mellitus, serum CML and pentosidine levels were examined in correlation to the patients' quality of diabetes control and the prevalence of diabetes-related long-term complications. Jena's St. Vincent Trial (JEVIN) was started in 1989/1990. At this time, a centralised diabetes care system existed. After the baseline examination of 190 patients (83% of the target population) with insulin-treated diabetes mellitus, follow-up examinations were performed in 1994/1995 and 1999/2000. In 1994/1995, the CML concentration in patients with type 1/type 2 diabetes mellitus was 1096.47+/-405.50/1136.43+/-405.24 ng/ml. In 1999/2000, it was significantly lower (727.49+/-342.91 ng/ml, P=.033/743.76+/-312.47 ng/ml, P<.0001). The same tendency showed the AGE-protein pentosidine (type 1: 1994/1995 203.18+/-118.88 vs. 1999/2000 156.59+/-104.84 pmol/ml [P=.029], type 2: 1994/1995 189.72+/-67.66 vs. 1999/2000 151.54+/-127.73 pmol/ml [P=.020]). Parallel to the decrease in the mean concentration of the AGE-products CML and pentosidine mean HbA1c improved and the prevalence of diabetic long-term complications (retino-, neuro-, and nephropathy) remained comparable 1999/2000-1989/1990. Comparing the data of 1999/2000 with those from 1994/1995, there was not only a substantial improvement in patients' quality of diabetes control but also a decrease in the concentration of AGE-products. In patients with diabetes mellitus, the AGE-products seem to be mainly influenced by the quality of diabetes control. However, the most important parameter reflecting the risk for development and progression of diabetes-related long-term complications seems not to be the AGE-products, but patients' HbA1c.

A study of capillary pericyte viability on extracellular matrix produced by endothelial cells in high glucose

AIMS/HYPOTHESIS

Thickening of the basement membrane and selective loss of pericytes occur early in diabetic retinopathy. As we showed previously that pericyte adhesion is impaired on extracellular matrix produced by endothelial cells in high hexose concentrations, we aimed to verify if altered adhesion could influence pericyte viability and replication.

METHODS

Conditioned extracellular matrices were obtained by growing human umbilical vein endothelial cells in media containing 28 mmol/l D-glucose, with or without the inhibitors of protein glycation thiamine or aminoguanidine, and D-galactose or L-glucose up to 28 mmol/l. Having removed the endothelium, bovine retinal pericytes were grown on these matrices and, in separate experiments, on laminin, fibronectin or type IV collagen. Pericyte viability and replication were measured by cell counts and DNA synthesis after 7 days, cell cycle traversal after 2 days and apoptosis after 18 h, 2 days and 7 days.

RESULTS

Pericyte counts and DNA synthesis were reduced on matrices produced in high D-glucose and D-galactose, whilst matrix obtained in L-glucose reduced DNA synthesis but not counts. Both thiamine and aminoguanidine corrected reduced pericyte viability when added to high D-glucose. Cell cycle and apoptosis were not affected by growing pericytes on different conditioned matrices. Laminin, fibronectin and type IV collagen did not modify pericyte replication.

CONCLUSIONS/INTERPRETATIONS

Reduced pericyte counts could depend on impaired initial adhesion to the extracellular matrix produced by endothelium in high hexose concentrations, rather than impaired replication or viability. Altered cell-matrix interactions might facilitate pericyte dropout in diabetic retinopathy, independently of the effects of high glucose on pericyte replication.

A practical method to study functional impairment of proteins by glycation and effects of inhibitors using current coagulation/fibrinolysis reagent kits

Objectives We undertook the present work to device a simple method to study the effects of inhibitors on functional impairment of proteins by the action of glycating agents. Design and methods For that purpose, we first tested the feasibility and optimized the conditions to employ glycation of human plasma coupled with AT III and plasminogen activity measurement, using coagulation test kits available in most clinical laboratories. Results Using D-BUT-CHT-lys-pNA as a plasmin-specific substrate, we show that incubation of plasma with fructose, glyceraldehyde or MG but not glucose decreases plasminogen activity reaching more than 40% in 16 h. A parallel dose-dependent decrease in heparin activation of AT III by up to a 50% was demonstrated using SAR-PRO-ARG-pNA as a specific thrombin substrate. We studied the effects of aminoguanidine, carnosine, quercetin aglycone, alpha tocopherol and ascorbic acid. Conclusion The methods afforded good discrimination between the known different reactivities of glycating sugars as well as the action of known antiglycation agents. They provide a practical system for monitoring the action of putative antiglycation agents.

New therapies for advanced glycation end product nephrotoxicity: current challenges

BACKGROUND

The role of advanced glycation end products (AGEs) in diabetic nephropathy has been developed during several years of research and increasingly complex AGE biochemistry. However, the structural diversity of AGE chemistry has created new challenges in the search for AGE-based inhibition therapies.

RESULTS

The challenges include the need to standardize measurements of serum and tissue AGE levels, identifying nephrotoxic AGE compounds, understanding the cell biological state of AGEs in the diabetic kidney, determining the mechanism of action of selective inhibition of the glycation cascade, and forming complementary therapies.

CONCLUSION

Current challenges in the development of new therapies for AGE nephrotoxicity are reviewed.

Pentosidine and N(epsilon)-(carboxymethyl)-lysine in Alzheimer's disease and vascular dementia

Increasing evidence suggests an interaction of oxidative stress and the formation of advanced glycation end products (AGE) in the onset and progression of Alzheimer's disease. We studied levels of pentosidine and N(epsilon)-(carboxymethyl)-lysine (CML) in serum and cerebrospinal fluid (CSF) of 15 patients with probable Alzheimer's disease (AD), 20 patients with vascular dementia (VD), and 31 control subjects (14 matched for age, and 17 younger patients). AGE protein concentrations in CSF did not differ within controls when divided into two subgroups by age. We found significantly elevated levels of CML in CSF of AD patients and of pentosidine in CSF of patients suffering from vascular dementia when compared to controls. The concentrations of pentosidine and CML in serum apparently did not relate directly to CSF values, suggesting influence of extra-cerebral factors in serum samples. It is concluded that AGE proteins are differentially affected in these types of dementia, depending on the specific neuropathology. Furthermore, measurements of AGE products in vivo should rely on CSF rather than blood samples.

Methylglyoxal-induced glycation affects protein topography

Methylglyoxal is a metabolic byproduct that is elevated in diabetic tissue. We examined the effects of methylglyoxal on cytosolic aspartate aminotransferase (cAAT), which is an enzyme previously shown to be modified by glyceraldehyde, acrolein, and ribose 5-phosphate. In the present study we observed that methylglyoxal caused real-time changes in tryptophan (intrinsic) fluorescence. Millimolar concentrations of methylglyoxal predominately decreased the fluorescence emission at 388 nm. While micromolar concentrations also decreased emission at 388 nm, low levels of methylglyoxal caused a prominent redshift in the wavelength of maximal emission. The changes in intrinsic fluorescence reflect definable changes in protein topography. These observations are consistent with a change in conformation that is more compact than that of native cAAT, suggesting that intramolecular cross-links (i.e., lysine-lysine) or hydrophobic pockets (i.e., carboxyethyl-lysines) were formed. Methylglyoxal also inhibited activity, and the inhibition correlated with the methylglyoxal-induced change in protein conformation.

Increased serum levels of the specific AGE-compound methylglyoxal-derived hydroimidazolone in patients with type 2 diabetes

A time-delayed fluorescence immunoassay was developed for the determination of serum levels of methylglyoxal (MG)-derived hydroimidazolone using a monoclonal antiserum raised against Nalpha-acetyl-Ndelta-(5-hydro-5-methyl)-4-imidazolone, Europium-labeled anti-mouse IgG antiserum as indicator, and MG modified bovine serum albumin (BSA) as standard. Serum levels of hydroimidazolone were measured in 45 patients with type 2 diabetes aged 59.4 +/- 6.1 (mean +/- SD) years and with duration of diabetes of 7.3 +/- 3.1 years, and in 19 nondiabetic controls aged 56.3 +/- 4.3 years. The serum levels of hydroimidazolone were significantly higher in patients compared to controls: median, 3.0 (5-95 percentile, 1.6 to 5.4) U/mg protein versus 1.9 (1.2 to 2.8) U/mg protein (P =.0005). Significant positive correlations were observed between the serum levels of hydroimidazolone and serum levels of advanced glycation end products (AGEs), measured with a polyclonal anti-AGE antibody: r = 0.59 for patients (P <.0001), and r = 0.65 for controls (P =.002). Similarly, significant correlations were also found between serum levels of hydroimidazolone and N(epsilon)-(carboxymethyl)-lysine (CML): r = 0.36 in patients and r = 0.55 for controls (both P =.02). Serum hydroimidazolone levels did not correlate with fasting plasma glucose or hemoglobin A(1c) (HbA(1c)) levels. The observed differences between patients with diabetes and nondiabetic controls seem to be comparable to differences measured for other AGE compounds.

Increased urinary levels of pentosidine, pyrraline and acrolein adduct in type 2 diabetes

This study investigates whether urinary levels of pentosidine, pyrraline and acrolein adduct are increased in type 2 diabetes (DM), and whether these levels are correlated with glycemic control and clinical traits. Urinary levels of pentosidine, pyrraline and acrolein adduct in DM patients (n = 100) recruited from the outpatient clinic of our university hospital were compared with those of age- and sex-matched non-diabetic subjects (n = 50). The correlation of these urinary levels with the glycemic control and the clinical traits were examined. Furthermore, the influence of smoking habit on the levels of acrolein adduct was examined. Urinary levels of pentosidine, pyrraline and acrolein adduct were all significantly (p<0.001) higher in the DM group than in the non-DM group (pentosidine (log(pmol/mgCr)), 1.579 +/- 0.147 vs 1.427 +/- 0.142; pyrraline (log(nmol/mgCr)), 0.888 +/- 0.402 vs 0.581 +/- 0.336; acrolein adduct (log(nmol/mgCr)), 2.316 +/- 0.221 vs 2.051 +/- 0.201). Glycemic control parameters, such as fasting plasma glucose (FPG) and HbA1c, were significantly correlated with these urinary levels. Age was correlated with the urinary levels of pentosidine but not with those of pyrraline and acrolein adduct. The urinary albumin excretion rate did not correlate with any of these urinary levels. The levels of acrolein adduct were higher in the subjects with smoking habit than in those without the habit in the DM group as well as in the non-DM group (DM, 2.391 +/- 0.230 and 2.212 +/- 0.190, p=0.0004; Non-DM, 2.120 +/- 0.171 and 1.993 +/- 0.206, p=0.0503). The urinary levels of pentosidine, pyrraline and acrolein adduct were increased in DM and were significantly correlated with glycemic control levels. In addition, smoking habit seems to increase the urinary levels of acrolein adduct.

Substituted pyrrol-1-ylacetic acids that combine aldose reductase enzyme inhibitory activity and ability to prevent the nonenzymatic irreversible modification of proteins from monosaccharides

Starting from the known inhibitory activity of (3-benzoylpyrrol-1-yl)acetic acid (I) and (2-benzoylpyrrol-1-yl)acetic acid (II), a series of 3-aroyl and 2,4-bis-aroyl derivatives (54-75) were synthesized and tested for inhibition of aldose reductase, an enzyme involved in the appearance of diabetic complications. It was found that a number of the tested compounds exhibited considerable activity in the micromolar range. Important structural features for the potent compounds is the presence of substituents with relatively low Hammett sigma values and/or moieties which increase their overall aromatic area. The most active derivative was the [2,4-bis(4-methoxybenzoyl)pyrrol-1-yl]acetic acid (75), with potency favorably compared to known ARIs such as tolrestat, epalrestat, zopolrestat, and fidarestat. Four selected derivatives were also evaluated for their ability to interfere with the oxidative modification of serum albumin in an in vitro experimental glycation model of diabetes mellitus. All of them showed considerable activity, comparable to the known inhibitor trolox. Our results, taken together, indicate that compound 75 combines favorably two biological activities directly connected to a number of pathological conditions related to the chronic diabetes mellitus.

Pycnogenol for diabetic retinopathy. A review

Diabetic retinopathy represents a serious health threat to a rapidly growing number of patients with diabetes mellitus. The retinal microangiopathy is characterised by vascular lesions with exudate deposits and haemorrhages causing vision loss. Pycnogenol, a standardised extract of the bark of the French maritime pine (Pinus pinaster), is known to increase capillary resistance. Pycnogenol has been tested for treatment and prevention of retinopathy in five clinical trials with a total number of 1289 patients since the late 1960's. All but one of these studies have been reported in French and German and, today, are of limited accessibility, giving the impetus for reviewing them in detail in this article. There were two open case studies and two double blind studies (one controlled against calcium dobesilate and another against placebo) and, finally, one multi-center field study with 1169 diabetics. All of these studies unequivocally showed that Pycnogenol retains progression of retinopathy and partly recovers visual acuity. Treatment efficacy of Pycnogenol was at least as good as that of calcium dobesilate. Pycnogenol was shown to improve capillary resistance and reduce leakages into the retina. Tolerance was generally very good and side effects were rare, mostly referring to gastric discomfort. In conclusion, treatment with Pycnogenol had a favourable outcome in the majority of the patients with diabetic retinopathy.

Advanced glycation end-product (AGE)-damaged IgG and IgM autoantibodies to IgG-AGE in patients with early synovitis

Advanced glycation end-product (AGE)-damaged IgG occurs as a result of hyperglycemia and/or oxidative stress. Autoantibodies to IgG-AGE were previously demonstrated in patients with severe, longstanding rheumatoid arthritis (RA). We investigated whether IgG-AGE and anti-IgG-AGE antibodies were present early in the course of RA and other inflammatory arthropathies. We prospectively followed a cohort of 238 patients with inflammatory arthritis of duration less than 1 year. Patients were evaluated clinically and serologically, and radiographs were obtained at initial and 1-year visits. Sera were assayed for IgG-AGE and anti-IgG-AGE antibodies by enzyme-linked immunosorbent assay (ELISA). Rheumatoid factor (RF) was determined by nephelometry and ELISA. Of all patients, 29% had RF-positive RA, 15% had RF-negative RA, 18% had spondyloarthropathy, and 38% had undifferentiated arthritis. IgG-AGE was present in 19% of patients, and was similar in amount and frequency in all groups. Patients with elevated IgG-AGE levels had significantly higher levels of the inflammatory markers C-reactive protein and erythrocyte sedimentation rate, but there was no correlation with blood glucose levels. Overall, 27% of the patients had IgM anti-IgG-AGE antibodies. These antibodies were highly significantly associated with RFs (P < 0.0001) and with swollen joint count (P < 0.01). In early onset arthritis, IgG damaged by AGE was detected in all patient groups. The ability to make IgM anti-IgG-AGE antibodies, however, was restricted to a subset of RF-positive RA patients with more active disease. The persistence of the anti-IgG-AGE response was more specific to RA, and was transient in the patients with spondyloarthropathy and with undifferentiated arthritis who were initially found to be positive for anti-IgG-AGE antibodies.

Effects of diabetes, insulin and antioxidants on NO synthase abundance and NO interaction with reactive oxygen species

BACKGROUND

Earlier studies have provided evidence for increased production of reactive oxygen species (ROS) and altered nitric oxide (NO) metabolism in diabetes. This study was intended to explore the effect of type I diabetes and its treatment with insulin alone or insulin plus antioxidant-fortified diet on expression of NOS isoforms and ROS interactions with lipids, glucose and NO.

METHODS

Rats with streptozotocin-induced diabetes were divided into once-daily insulin (ultralente)-treated, insulin plus antioxidant (vitamin E and vitamin C)-treated and untreated groups. After four weeks, plasma malondialdehyde (MDA) and tissue endothelial (eNOS), neuronal (nNOS) NO synthases, carboxymethyllysine (CML) and nitrotyrosine were determined.

RESULTS

The untreated diabetic animals exhibited severe hyperglycemia, elevated blood pressure, increased plasma MDA, high tissue CML and reduced tissue nitrotyrosine denoting enhanced lipid, glucose and protein oxidation but reduced NO oxidation by ROS. This was coupled with significant reduction of eNOS and nNOS expression in renal cortex and eNOS in the left ventricle. Insulin therapy partially lowered blood pressure, tissue CML, plasma glucose and MDA, but significantly raised eNOS expression and nitrotyrosine abundance to supranormal levels. Combined insulin and antioxidant therapies resulted in normalization of blood pressure, plasma MDA, tissue CML and nitrotyrosine without affecting glucose level or NOS expression.

CONCLUSION

Oxidative stress in untreated diabetes is associated with down-regulation of NOS isoforms and increased ROS-mediated oxidation of lipid and glucose, but not NO. Amelioration of hyperglycemia with once-daily insulin administration alone results in up-regulation of NOS isoforms, reduction of lipid and glucose oxidation and increased NO oxidation. However, insulin plus antioxidant supplementation can normalize all three parameters.

A New Flavone C-Glycoside from the Style of Zea mays L. with Glycation Inhibitory Activity

A new flavone C-glycoside, chrysoeriol 6-C-beta-fucopyranoside (1), and a known flavone C-glycoside (2) were isolated from the style of Zea mays L. These structures were elucidated on the basis of spectroscopic analysis. Compound 1 exhibited glycation inhibitory activity.

The botanical extracts of Achyrocline satureoides and Ilex paraguariensis prevent methylglyoxal-induced inhibition of plasminogen and antithrombin III

Endogenously produced dicarbonyls, such as methylglyoxal (MG), are involved in advanced glycation end-product formation and thus linked to the pathophysiology of diabetic chronic complications. While the search for synthetic new antiglycation agents continues, little attention has been paid to putative antiglycation agents in natural compounds. Given the link between glycation and oxidation, in this work, we study the effects of methylglyoxal on two model systems; plasminogen and antithrombin III (AT III), then we set out to unravel a possible antiglycation effect for extracts of the flavonoid-rich common herbal species Achyrocline satureoides (AS) and Ilex paraguariensis (IP). Using SAR-PRO-ARG-pNA as a specific thrombin substrate, we show that incubation of plasma with MG decreases heparin activation of AT III by up to a 70%, in a dose-dependent manner. A parallel dose-dependent decrease in plasminogen activity reaching more than 50% was shown using D-BUT-CHT-lys-pNA as a plasmin-specific substrate. Extracts of AS and IP display a dose dependent inhibition of the action of the dicarbonyl, already significant at a 1/100 dilution of the herbal infusions. The inhibition was comparable to that obtained by using millimolar concentrations of known AGE inhibitors such as aminoguanidine and carnosine as well as micromolar concentrations of the antioxidant ascorbic acid. We believe our system of whole plasma glycation over 16 h with micromolar concentrations of MG, coupled with the measurement of activities of plasminogen and AT III by specific substrates provides a straightforward, practical method for monitoring the action of putative antiglycation agents. If predictably milder glycated forms of AT III and plasminogen were to be secreted in vivo, the loss of activities shown here could act synergistically to generate hyperthrombicity.

Renal connective tissue growth factor induction in experimental diabetes is prevented by aminoguanidine

The aim of this study was to determine whether aminoguanidine (AG), an inhibitor of advanced glycation, prevents expression of the profibrotic cytokine, connective tissue growth factor (CTGF), as well as accumulation of the previously reported CTGF-dependent matrix protein, fibronectin, in a model of experimental diabetic nephropathy. Diabetic animals were randomly allocated into groups receiving 32 wk of AG or vehicle. Diabetic rats showed increases in CTGF mRNA and protein expression as well as in advanced glycation end-product (AGE) and fibronectin immunostaining, compared with nondiabetic rats. In the diabetic kidney, the increase in CTGF gene and protein expression as well as expression of the extracellular matrix protein fibronectin were prevented by AG. To further explore the relationship between AGEs and mesangial CTGF and fibronectin production, cultured human mesangial cells were exposed in vitro to soluble AGE-BSA and carboxymethyl lysine-BSA, and this led to induction of both CTGF and fibronectin. On the basis of our in vitro findings in mesangial cells linking AGEs to CTGF expression, the known prosclerotic effects of CTGF, and the ability of AG to attenuate mesangial expansion, it is postulated that the antifibrotic effects of AG in this animal model may be partially mediated by CTGF.

Hypertension and related factors in the etiology of Alzheimer's disease

The prevalence of hypertension is estimated to approach 50% in individuals above age 70. The consequences of hypertension include cerebrovascular disease, coronary heart disease, and general atherosclerosis. Several recent studies suggest that there may be an association also between hypertension and Alzheimer's disease (AD). This review will examine the evidence for this association and possible pathways between hypertension, Alzheimer encephalopathy, and clinical dementia.

Oxidative deamination of lysine residue in plasma protein of diabetic rats. Novel mechanism via the Maillard reaction

The levels of alpha-aminoadipic-delta-semialdehyde residue, the oxidative deamination product of lysine residue, in plasma protein from streptozotocin-induced diabetic rats were evaluated. alpha-Aminoadipic-delta-semialdehyde was converted to a bisphenol derivative by acid hydrolysis in the presence of phenol, and determined by high performance liquid chromatography. Analysis of plasma proteins revealed three times higher levels of alpha-aminoadipic-delta-semialdehyde in diabetic subjects compared with normal controls. Furthermore, we explored the oxidative deamination via the Maillard reaction and demonstrated that the lysine residue of bovine serum albumin is oxidatively deaminated during the incubation with various carbohydrates in the presence of Cu2+ at a physiological pH and temperature. This experiment showed that 3-deoxyglucosone and methylglyoxal are the most efficient oxidants of the lysine residue. When the reaction was initiated from glucose, a significant amount of alpha-aminoadipic-delta-semialdehyde was also formed in the presence of Cu2+. The reaction was significantly inhibited by deoxygenation, catalase, and a hydroxyl radical scavenger. The mechanism we propose for the oxidative deamination is the Strecker-type reaction and the reactive oxygen species-mediated oxidation. Based on these findings, we propose a novel mechanism for the oxidative modification of proteins in diabetes, namely the oxidative deamination of the lysine residue via the Maillard reaction.

Association of Gly82Ser polymorphism in the RAGE gene with diabetic retinopathy in type II diabetic Asian Indian patients

AIM/HYPOTHESIS

The binding of advanced glycation end products (AGE) to the receptor induces cellular oxidative stress and vascular dysfunction and this is implicated in the pathogenesis of diabetic retinopathy (DR). This study aims to investigate the frequency of Gly82Ser polymorphism in exon 3 of the receptor for AGE (RAGE) gene and its association with DR in Asian Indian patients who have type II diabetes.

METHODS

200 Asian Indian patients with at least 15-year duration of type II diabetes were identified. This group included (1) 100 patients with retinopathy (DR) and (2) 100 patients without retinopathy (DNR). Fifty unrelated healthy controls (CT) were also included in the study. Genotype frequencies of Gly82Ser polymorphism were studied by polymerase chain reaction (PCR) amplification and restriction fragment length polymorphism analysis using AluI enzyme. Later, the nucleotide change was confirmed by DNA sequencing.

RESULTS

The frequency of the Ser82 allele was significantly higher, 18% in the DNR group compared to 7% in the DR group (P=.03). The same genotype was 2% in the CT group.

CONCLUSION/INTERPRETATION

Our result suggests that Ser82 allele in the receptor for AGE gene is a low-risk allele for developing DR in Asian Indian patients who have type II diabetes.

Prolonged longevity in naked mole-rats: age-related changes in metabolism, body composition and gastrointestinal function

Aging is characterized by declines in all physiological processes and concomitant changes in body composition. Age-related changes in metabolism, body composition and gastrointestinal function were investigated in naked mole-rats (Heterocephalus glaber), rodents that exhibit extended longevity. Maximum lifespan of these 40 g rodents (>27 year) is approximately 9 times greater than predicted allometrically. We investigated changes in basal metabolic rate (BMR), body composition and intestinal glucose transport in 1, 5, 10 and 20-year-old male individuals. Body composition was measured using dual X-ray absorptiometry and activity of sodium glucose co-transporters (SGLT1) determined using everted gut sleeves. One-year-olds had lower body mass than other age cohorts, as they had not attained full adult form. Among the 5, 10, and 20-year-olds, no age-related changes in body mass, BMR, percentage body fat, fat-free mass or bone mineral density were found. SGLT1 activity declined moderately (<20%) from 5 to 20 years and was similar at 10-20 years, whereas age-related declines are 40-60% in mice. Although mole-rats have low metabolic rates, their prolonged longevity results in a lifetime energy expenditure more than 4 times that of mice. Since lifetime energy expenditure is an important index of potential exposure to oxidative damage, naked mole-rats may be valuable for studying mechanisms of aging.

The hexosamine pathway regulates the plasminogen activator inhibitor-1 gene promoter and Sp1 transcriptional activation through protein kinase C-beta I and -delta

Increased flux through the hexosamine biosynthesis pathway (HBP) has been shown to stimulate the expression of a number of genes. We previously demonstrated in glomerular mesangial and endothelial cells that both high glucose concentrations and glucosamine activated the plasminogen activator inhibitor-1 (PAI-1) gene promoter through the transcription factor, Sp1; and that the glutamine:fructose-6-phosphate amidotransferase inhibitor, 6-diazo-5-oxonorleucine, inhibited the effect of high glucose, but not that of glucosamine. Here, we examined the role of protein kinase C (PKC) isoforms in the regulation of the PAI-1 promoter and Sp1 transcriptional activity by the HBP. In transient transfections, exposure to 2 mm glucosamine or 20 mm glucose for 4 days increased the activities of a PAI-1 promoter-luciferase reporter gene as well as the Sp1 transcriptional activation domain fused to the GAL4 DNA-binding domain cotransfected with a GAL4 promoter-luciferase reporter. Cotransfected dominant negative PKC-betaI and -delta completely blocked the induction of PAI-1 promoter transcription by both sugars, whereas only dominant negative PKC-betaI interfered with Sp1-GAL4 activation. Both glucosamine and high glucose stimulated the in vitro kinase activity of immunoprecipitated PKC-betaI and -delta. Furthermore, 6-diazo-5-oxonorleucine suppressed high glucose-induced PKC kinase activity and Sp1-GAL4 transcriptional activation. These findings demonstrate a requirement for the PKC-betaI and -delta signal transduction pathways in HBP-induced transcription.

Glucose and reactive oxygen species

PURPOSE OF REVIEW

This review aims at presenting new concepts of glucose-induced damage in diabetes via an increased production of oxygen free radicals.

RECENT FINDINGS

Reactive oxygen species modulate various biological functions by stimulating transduction signals, some of which are involved in diabetes pathogenesis and complications.

SUMMARY

Diabetes is characterized by high glucose concentrations that lead, via several mechanisms (glucose autoxidation, stimulation of the polyol pathway, activation of the reduced form of nicotinamide adenine dinucleotide phosphate oxidase, and production of advanced glycation endproducts), to an increased production of reactive oxygen species. The resulting oxidative stress (the imbalance between reactive oxygen species production and the antioxidant defences) can play a key role in diabetes pathogenesis. Superoxide radicals generated by the reduced form of nicotinamide adenine dinucleotide phosphate oxidase may thus contribute to impaired endothelium-dependent vascular relaxation by the inactivation of nitric oxide, and more generally to vascular dysfunction, thereby contributing to accelerated atherosclerosis in diabetic patients. The increased production of reactive oxygen species induced by hyperglycaemia has also been suggested to be involved in platelet dysfunction, in tissue remodelling (via metalloproteinases), and in redox regulation of glucose transport in skeletal muscle. Beyond the classic treatments for diabetes, new therapeutic strategies involving antioxidants or anti-advanced glycation endproduct molecules are proposed. Future methods could take into account the signalling pathways and genes that are regulated by reactive oxygen species.

Nepsilon-(Carboxymethyl)lysine induces gamma-glutamylcysteine synthetase in RAW264.7 cells

Advanced glycation end products (AGEs) play an important role in the development of angiopathy in diabetes mellitus and atherosclerosis. Here, we show that adducts of N(epsilon)-(carboxymethyl)lysine (CML), a major AGE, and bovine serum albumin (CML-BSA) stimulated gamma-glutamylcysteine synthetase (gamma-GCS), which is a key enzyme of glutathione (GSH) synthesis, in RAW264.7 mouse macrophage-like cells. CML-BSA stimulated the expression of gamma-GCS heavy subunit (h) time- and dose-dependently and concomitantly increased GSH levels. CML-BSA also stimulated DNA-binding activity of activator protein-1 (AP-1) within 3h, but the stimulatory effect decreased in 5h, and nuclear factor-kappaB (NF-kappaB) with a peak activity at 1h and the stimulatory effect diminished in 3h. Studies of luciferase activity of the gamma-GCSh promoter showed that deletion and mutagenesis of the AP-1-site abolished CML-BSA-induced up-regulation, while that of NF-kappaB-site did not affect CML-BSA-induced activity. CML-BSA also stimulated the activity of protein kinase C, Ras/Raf-1, and MEK/ERK1/2. Inhibition of ERK1/2 abolished CML-BSA-stimulated AP-1 DNA-binding activity and gamma-GCSh mRNA expression. Our results suggest that induction of gamma-GCS by CML adducts seems to increase the defense potential of cells against oxidative stress produced during glycation processes.

Reactive carbonyls from tobacco smoke increase arterial endothelial layer injury

We hypothesized that reactive carbonyls generated from smoke exposure cause increased arterial low-density lipoprotein (LDL) accumulation and endothelial layer permeability. In addition, we hypothesized that estrogen supplementation was protective against chronic environmental tobacco smoke (ETS) exposure to the artery wall. Quantitative fluorescence microscopy was used to determine artery injury after exposure. For our chronic studies, ovariectomized rats treated with subcutaneous placebo or 17beta-estradiol pellets were exposed to ETS or filtered air for 6 wk. ETS exposure increased carotid artery LDL accumulation more than fourfold compared with filtered air exposure, an effect largely mediated by increased permeability. No protective effect of estradiol was observed. Acute ETS exposure of a buffer solution containing LDL resulted in a more than sixfold increase in the highly reactive carbonyl glyoxal. Perfusion of this solution through carotid arteries resulted in a 105% increase in permeability. Moreover, perfusion of glyoxal alone caused a 50% increase in carotid artery permeability. This endothelial damage and changes in lipid accumulation may serve as an initiating event in atheroma formation in individuals exposed to ETS.

Sugar-induced modification of fibroblast growth factor 2 reduces its angiogenic activity in vivo

Both clinical and animal studies have shown that angiogenesis is impaired in diabetes mellitus; however, the mechanisms responsible for this effect are poorly characterized. The major aims of the present study were to evaluate the effect of hyperglycemia on fibroblast growth factor 2 (FGF2)-induced angiogenesis in vivo and to determine whether FGF2 non-enzymatic glycation occurs in hyperglycemic mice. New blood vessel formation was examined in reconstituted basement membrane protein (Matrigel) plugs containing FGF2 in control normoglycemic CD1 and in hyperglycemic nonobese diabetic (NOD) mice. FGF2-induced angiogenesis in NOD mice was inhibited by 75% versus control mice (P < 0.001). When recombinant FGF2 was mixed with Matrigel and injected in mice, it was found that recombinant FGF2 glycation was significantly enhanced in plugs from NOD versus control mice (P < 0.01). In the Boyden chamber assay, the chemotactic effect of glycated FGF2 toward endothelial cells was lower than that of unmodified FGF2 (P < 0.01). Further, FGF2 glycated in vitro and co-injected with Matrigel in CD1 mice was a weaker angiogenic stimulus than unglycated FGF2 (P < 0.005). These results indicate that FGF2-induced angiogenesis is inhibited in diabetic mice, FGF2 glycation is enhanced in hyperglycemic mice, and glycation markedly reduces FGF2 chemotactic effect in vitro and its angiogenic properties in vivo. Thus, FGF2 glycation may represent a mechanism responsible for the impairment of angiogenesis in diabetes mellitus.

Neuronal Ca2+ disregulation in diabetes mellitus

The Ca(2+) hypothesis of brain ageing and dementia may account for part of the available data on the pathogenesis of dementia and certain neurodegenerative disorders. The hypothesis proposes that disturbances in the homeostasis of neuronal cytosolic free Ca(2+) are part of a final common pathway, ultimately leading to neuronal dysfunction and cell death. The hypothesis also proposes that a small change in cytosolic free Ca(2+) sustained over a long period of time will result in similar damage as a large change over a short period. Diabetes mellitus is associated with neurological complications in the peripheral and central nervous system, as reflected in peripheral neuropathy, modest cognitive impairments and an increased risk of dementia. In animal models of diabetes, learning impairments are associated with alterations in Ca(2+) -dependent forms of hippocampal synaptic plasticity. Disturbances in the homeostasis of cytosolic free Ca(2+) may present a final common pathway in the multifactorial pathogenesis of neurological complications of diabetes, which involves vascular changes, oxidative stress, and non-enzymatic protein glycation. In line with the Ca(2+) hypothesis of neurodegenerative disorders, a prolonged, small increase in basal cytosolic Ca(2+) levels indeed exists in sensory neurones of diabetic animals. In addition, Ca(2+) dynamics are affected. Ca(2+) channel blockers, such as nimodipine, have been shown to improve experimental peripheral neuropathy, through a vascular mechanism, possibly in combination with direct neuronal effects. Preliminary studies indicate that nimodipine may also improve Ca(2+)-dependent forms of synaptic plasticity in the hippocampus of diabetic rats.

Antibodies to advanced glycation end products in children with diabetes mellitus

The tissue accumulation of advanced glycation end products (AGE) alters the structure and function of long-lived proteins. A number of studies have shown that tissue accumulation of AGE correlates with the severity of diabetic complications. Proteins containing AGE are highly immunogenic and anti-AGE antibodies were found in sera of diabetic rats and human. Considering the potential use of anti-AGE antibodies as a marker of AGE deposition during diabetes, we have investigated, by competitive ELISA, the presence of anti-AGE antibodies in sera of 58 children with Type 1 diabetes mellitus. The patients were studied for the period of 5 years. Positive for anti-AGE antibodies were 19 children with diabetes. Fourteen of them showed initial data for vascular complications. Anti-AGE antibodies were related to age (r = .25, P = .024), duration of diabetes (r = .41, P = .0001), HbA1c (r = .27, P = .016), microalbuminuria (r = .41, P = .0001), retinopathy (r = .35, P = .001), triglycerides (r = .27, P = .016), and total cholesterol (r = .19, P = .05). In conclusion, our study showed that the investigation of the levels and dynamics of anti-AGE antibodies might give the possibility for early diagnosis and prognosis of the severity of diabetic late complications.

Glucose-mediated cross-linking of collagen in rat tendon and skin

BACKGROUND

Cross-linking of macromolecules like collagen plays an important role in the development of complications in diabetes and ageing. One of the underlying mechanisms of this cross-linking is the formation of advanced glycation endproducts (AGEs).

METHODS

In this study, we assessed the use of differential scanning calorimetry (DSC) for the determination of these cross-links and the effects of an AGE inhibitor and breaker.

RESULTS

Treatment with N-phenacylthiazolium bromide (ALT-711) of diabetic rats with 2 months duration of diabetes normalized large artery stiffness, assessed by characteristic input impedance and systemic arterial compliance, but with the use of DSC, no statistical difference in cross-linking between control and treated animals could be measured. In addition, we performed in vitro incubation of collagen preparations with ribose and glucose to assess the DSC method as well as the influence of AGE breakers and inhibitors. Incubation of rat tail tendon (RTT) with 100 mmol/l glucose showed an increase in collagen cross-linking expressed as an increase in shrinkage temperature (T(s)). Addition of aminoguanidine (AG), an inhibitor of AGE formation, prior to glucose incubation showed a slower increase of the amount of glucose-derived cross-linking. Replacing glucose with ribose showed a quicker increase in cross-linking and less effect on cross-linking by adding aminoguanidine, demonstrating the higher reactivity of pentoses above hexoses. Similar experiments with rat skin samples (RSS) showed that RSS (type III collagen) are less susceptible to glucose-mediated cross-linking than RTT (type I collagen). We observed no effect of addition of ALT-711, a breaker of glucose-derived cross-links, on the extent of collagen cross-linking in both RTT and RSS.

CONCLUSION

Overall, DSC is considered a useful method for assessing glucose-mediated cross-linking in vitro with nonphysiological glucose concentrations. The in vivo use in biological samples is limited due to the lack of sensitivity. However, DSC remains a quick and well-quantitated method in comparison with other methods, like enzymatic digestibility.

Improved insulin sensitivity is associated with restricted intake of dietary glycoxidation products in the db/db mouse

Advanced glycation end products (AGEs), known promoters of diabetic complications, form abundantly in heated foods and are ingested in bioreactive forms. To test whether dietary AGEs play a role in the progression of insulin resistance, C57/BL/KsJ db/db mice were randomly placed for 20 weeks on a diet with either a low AGE content (LAD) or a 3.4-fold higher content of AGE (high AGE diet [HAD]), including (epsilon)N-carboxymethyllysine (CML) and methylglyoxal (MG). LAD-fed mice showed lower fasting plasma insulin levels throughout the study (P = 0.01). Body weight was reduced by approximately 13% compared with HAD-fed mice (P = 0.04) despite equal food intake. LAD-fed mice exhibited significantly improved responses to both glucose (at 40 min, P = 0.003) and insulin (at 60 min, P = 0.007) tolerance tests, which correlated with a twofold higher glucose uptake by adipose tissue (P = 0.02). Compared with the severe hypertrophy and morphological disorganization of islets from HAD-fed mice, LAD-fed mice presented a better-preserved structure of the islets. LAD-fed mice demonstrated significantly increased plasma HDL concentrations (P < 0.0001). Consistent with these observations, LAD-fed mice exhibited twofold lower serum CML and MG concentrations compared with HAD-fed mice (P = 0.02). These results demonstrate that reduced AGE intake leads to lower levels of circulating AGE and to improved insulin sensitivity in db/db mice.

Advanced glycation end products increase, through a protein kinase C-dependent pathway, vascular endothelial growth factor expression in retinal endothelial cells. Inhibitory effect of gliclazide

Accumulating evidence points to a causal role for advanced glycation end products (AGEs) in the development of diabetic vascular complications, including retinopathy. Possible pathogenic mechanisms linking AGEs to diabetic retinopathy include protein kinase C (PKC) activation, oxidative stress, and vascular endothelial growth factor (VEGF) expression. In the present study, we investigated the effect of AGEs on VEGF expression in bovine retinal endothelial cells (BRECs) and determined the role of PKC and oxidative stress in this effect. Incubation of BRECs with AGEs led to enhanced VEGF mRNA and protein expression. This treatment also induced PKC translocation in these cells. The AGE-induced increases in VEGF expression and PKC activation were inhibited by the pan-specific PKC inhibitor, calphostin C, and by the antioxidant drug and compounds, gliclazide, N-acetylcysteine, and vitamin E. In contrast, glyburide which does not exhibit antioxidant properties, did not affect the AGE-induced VEGF expression. Exposure of BRECs to AGEs resulted in a significant increase of nuclear protein binding to the NF-kappa B consensus sequence of the VEGF promoter region. Induction of DNA binding activity for NF-kappa B by AGEs was prevented by gliclazide. Treatment of BRECs with AGEs also increased the proliferation of these cells. This effect was abrogated by incubating the cells with an anti-VEGF antibody and was inhibited in the presence of gliclazide. Overall, these data demonstrate that AGEs increase VEGF expression in retinal endothelial cells through generation of oxidative stress and downstream activation of the PKC pathway. Targeting VEGF expression with specific pharmacological agents, such as antioxidants and PKC inhibitors, may prove efficacious for the treatment of diabetic retinopathy.

Advanced glycation end products and endothelial dysfunction in type 2 diabetes

OBJECTIVE

Data from experimental studies have suggested that the increased formation of advanced glycation end products (AGEs) is one of the causes of endothelial dysfunction in diabetes. This study was performed to investigate whether changes in endothelium-dependent vasodilation, a marker of endothelial function, were related to serum AGEs concentrations in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

For this study, 170 patients with type 2 diabetes and 83 healthy nondiabetic control subjects of similar age were recruited. Serum AGEs were assayed by competitive enzyme-linked immunosorbent assay. Endothelium-dependent and -independent vasodilation of the brachial artery was measured by high-resolution vascular ultrasound.

RESULTS

Serum AGEs were increased in diabetic patients compared with control subjects (4.6 +/- 0.7 vs. 3.1 +/- 0.8 unit/ml; P < 0.01), and both endothelium-dependent (5.1 +/- 2.5 vs. 9.1 +/- 4.1%; P < 0.01) and endothelium-independent vasodilation (13.2 +/- 4.6 vs. 16.4 +/- 5.5%; P < 0.01) were impaired. On univariate analysis of all subjects, serum AGEs correlated with endothelium-dependent vasodilation (r = -0.51, P < 0.01); a weaker association was found with endothelium-independent vasodilation (r = -0.24, P < 0.01). On multiple regression analyses including age, sex, smoking status, and plasma lipids, only serum AGEs remained a significant independent determinant of endothelium-dependent vasodilation (r(2) = 0.34, P < 0.01).

CONCLUSIONS

Increased serum concentrations of AGEs in patients with type 2 diabetes is associated with endothelial dysfunction, independent of other cardiovascular risk factors. Further studies to determine whether treatment targeting AGEs will lead to an amelioration of endothelial dysfunction are warranted.