Advanced glycosylation endproducts: Role in diabetic and non-diabetic vascular disease

Selectively lighting up glyoxal in living cells using an o-phenylenediamine fused hemicyanine

Glyoxal (GL) is a reactive α-dicarbonyl compound generated from glycated proteins in the Maillard reaction. It has attracted particular attention over the past few years because of its possible clinical significance in chronic and age-related diseases. In this work, a reaction-based red emission fluorescent probe GL1 has been synthesized successfully by grafting an alkyl group onto an amino group to regulate its selectivity for GL. Under physiological conditions, the fluorescence intensity of GL1 at 640 nm obviously increased with the increase of GL concentration, and it exhibited high selectivity for GL over other reactive carbonyl compounds, as well as a lower detection limit (0.021 μM) and a larger Stokes shift (112 nm). At the same time, GL1 can selectively accumulate in mitochondria and can be used to detect exogenous and endogenous GL in living cells with low cytotoxicity.

Attenuation of hyperglycemia and amadori products by aminoguanidine in alloxan-diabetic rabbits occurs via enhancement in antioxidant defenses and control of stress

The micro- and macro-complications in diabetes mellitus (DM) mainly arise from the damage induced by Amadori and advanced glycation end products, as well as the released free radicals. The primary goal of DM treatment is to reduce the risk of micro- and macro-complications. In this study, we looked at the efficacy of aminoguanidine (AG) to prevent the production of early glycation products in alloxan-diabetic rabbits. Type1 DM was induced in rabbits by a single intravenous injection of alloxan (90 mg/kg body weight). Another group of rabbits was pre-treated with AG (100 mg/kg body weight) prior to alloxan injection; this was followed by weekly treatment with 100 mg/kg of AG for eight weeks. Glucose, insulin, and early glycation products (HbA1C and fructosamine) were measured in control, diabetic and AG treated diabetic rabbits. The effects of hyperglycemia on superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (Gpx), reduced glutathione (rGSH), nitric oxide, lipid peroxides, and protein carbonyl were investigated. Alloxan-diabetic rabbits had lower levels of SOD, CAT, Gpx, and rGSH than control rabbits. Nitric oxide levels were considerably greater. AG administration restored the activities of SOD, CAT, Gpx enzymes up to 70-80% and ameliorated the nitric oxide production. HbA1c and fructosamine levels were considerably lower in AG-treated diabetic rabbits. The observed control of hyperglycemia and amadori adducts in alloxan-diabetic rabbits by AG may be attributed to decrease of stress and restoration of antioxidant defenses.

Fewer Exposed Lysine Residues May Explain Relative Resistance of Chicken Serum Albumin to In Vitro Protein Glycation in Comparison to Bovine Serum Albumin

Protein glycation and formation of advanced glycation end products is associated with several diseases resulting from high blood glucose concentrations. Plasma albumin is directly exposed to circulating glucose concentrations and is therefore at greater risk of glycation than hemoglobin. As plasma glucose concentrations in birds are 1.5-2 times higher than mammals of similar mass, avian albumin may be particularly at risk of glycation. Thus, the goal of the present study was to compare the in vitro formation of glycated albumin in chicken serum albumin (CSA) and bovine serum albumin (BSA) exposed to a range of glucose concentrations over a 16-week period. The level of glycation for CSA and BSA was quantified using boronate affinity columns to separate glycated albumin from non-glycated albumin and calculating the difference in protein concentration of each sample. The results indicate that CSA is glycated to a lesser degree than BSA when the albumins are exposed to increasing concentrations of glucose (38.8-500 mM). This was most apparent at week sixteen (500 mM glucose) where BSA expressed a higher degree of glycation (37.8 ± 0.76%) compared to CSA (19.7 ± 1.06%, P < 0.05). Additionally, percent glycation at week sixteen was significantly higher than the glucose-free solutions for both BSA and CSA, indicating that glycation is glucose-dependent. Analyses of the protein structures suggest that the relative resistance of CSA to glycation may be due to fewer lysine residues and variations in protein folding that shield more lysine residues from the plasma. Moreover, comparisons of reconstructed ancestral albumin sequences show that the ancestor of birds had 6-8 fewer lysine residues compared to that of mammals.

Advanced glycation end-products and the kidney

BACKGROUND

Advanced glycation end-products (AGEs) are increased in situations with hyperglycemia and oxidative stress such as diabetes mellitus. They are products of nonenzymatic glycation and oxidation of proteins and lipids. The kidney plays an important role in clearance and metabolism of AGEs.

METHODS

Medline and other relevant databases were searched. In addition, key review articles were scanned for relevant original publication. Finally, original data from our research group were also included.

RESULTS

Kidney podocytes and endothelial cells express specific receptors for AGEs. Their activation leads to multiple pathophysiological effects including hypertrophy with cell cycle arrest and apoptosis, altered migration, and generation of proinflammatory cytokines. AGEs have been primarily implicated in the pathophysiology of diabetic nephropathy and diabetic microvascular complications. AGEs are also involved in other primary renal diseases as well as in the development and progression of atherosclerosis. However, serum or plasma concentrations of AGEs do not correlate well with cardiovascular events in patients with chronic kidney disease (CKD). This is likely due to the fact that serum concentrations failed to correlate with AGEs deposited in target tissues. Several inhibitors of the AGE-RAGE axis are currently tested for various indications.

CONCLUSION

AGEs and their receptors are involved in the pathogenesis of vascular and kidney disease. The role of circulating AGEs as biomarkers for cardiovascular risk estimation is questionable. Whether putative inhibitors of AGEs will get the maturity for its therapeutic use in the future remains open.

Effect of Umbelliferone on Tail Tendon Collagen and Haemostatic Function in Streptozotocin-Diabetic Rats

Diabetes mellitus is known to affect collagen in various tissues. Umbelliferone (7-hydroxycoumarin), a natural antioxidant and benzopyrone, is found in golden apple (Aegle marmelos Correa) and bitter orange (Citrus aurantium). Plant-derived phenolic coumarins have been shown to act as dietary antioxidants. In this study, we have investigated the influence of umbelliferone on collagen content and its effects on the tail tendon in streptozotocin-diabetic rats. Male albino Wistar rats (180-200 g) were made diabetic by intraperitoneal administration of streptozotocin (40 mg/kg). Normal and diabetic rats were treated with umbelliferone for 45 days. Diabetic rats had increased glucose and decreased insulin levels. Tail tendons of diabetic rats had increased total collagen, glycation and fluorescence, and decreased levels of neutral, acid and pepsin-soluble collagens. We have studied the effect of umbelliferone on haemostatic function because umbelliferone is also a coumarin derivative like the anticoagulant, warfarin. Diabetic rats had a significant decrease in prothrombin, clotting and bleeding time, and treatment with umbelliferone made these parameters almost normal. Our results show that umbelliferone controls glycaemia and has a beneficial effect on collagen content and its properties, i.e. collagen related parameters, in the tail tendon, which indicates recovery from the risk (recovery of animals from the risk of complications) of collagen-mediated diabetic polyneuropathy and diabetic nephropathy.

Protection against glycation and similar post-translational modifications of proteins

Glycation and other non-enzymic post-translational modifications of proteins have been implicated in the complications of diabetes and other conditions. In recent years there has been extensive progress in the search for ways to prevent the modifications and prevent the consequences of the modifications. These areas are covered in this review together with newer ideas on possibilities of reversing the chemical modifications.

Altered electromechanical coupling in the renal microvasculature during the early stage of diabetes mellitus

1. The early stage of type 1 diabetes mellitus (DM) is characterized by renal hyperfiltration, which promotes the eventual development of diabetic nephropathy. The hyperfiltration state is associated with afferent arteriolar dilation and diminished responsiveness of this vascular segment to a variety of vasoconstrictor stimuli, whereas efferent arteriolar diameter and vasoconstrictor responsiveness are typically unaltered. 2. The contractile status of preglomerular vascular smooth muscle appears to be tightly coupled to membrane potential (E(m)) and its influence on Ca(2+) influx through voltage-gated channels. Efferent arteriolar tone is largely independent of electromechanical events. Hence, defective electromechanical mechanisms in vascular smooth muscle should engender selective changes in preglomerular microvascular function, such as those evident during the early stage of DM. 3. Afferent arteriolar contractile responses to K(+)-induced depolarization and BAYK8644 are diminished 2 weeks after onset of DM in the rat. Similarly, depolarization-induced Ca(2+) influx and the resulting increase in intracellular [Ca(2+)] are abated in the preglomerular microvasculature of diabetic rats. The intracellular [Ca(2+)] response to depolarization is rapidly restored by normalization of extracellular glucose levels. These observations suggest that hyperglycaemia in DM impairs regulation of afferent arteriolar voltage-gated Ca(2+) channels. 4. Dysregulation of E(m) may also contribute to afferent arteriolar dilation in DM. Vasodilator responses to pharmacological opening of ATP-sensitive K(+) channels are exaggerated in afferent arterioles from diabetic rats. Moreover, blockade of these channels normalizes afferent arteriolar diameter in kidneys from diabetic rats. These observations suggest that increased functional availability and basal activation of ATP-sensitive K(+) channels promote afferent arteriolar dilation in DM. 5. We propose that dysregulation of E(m) (involving ATP- sensitive K(+) channels) and a diminished Ca(2+) influx response to depolarization (involving voltage-gated Ca(2+) channels) may act synergistically to promote preglomerular vasodilation during the early stage of DM.

Advanced glycosylation end products stimulate collagen mRNA synthesis in mesangial cells mediated by protein kinase C and transforming growth factor-beta

Advanced glycosylation end products (AGE) seem to be implicated in the pathogenesis of diabetic nephropathy. The present study has examined the effects of AGE on protein kinase C (PKC) activity and transforming growth factor-beta1 (TGF-beta1) in relation to collagen gene regulation in cultured human mesangial cells (HMCs). Quiescent HMCs were exposed to serum-free media containing bovine serum albumin (BSA), AGE-modified BSA (AGE-BSA), or glycated BSA in which AGE formation was prevented by the use of aminoguanidine (BSA-AM). AGE-BSA (200 microg/mL) induced a peak membrane-associated PKC activity, particularly PKC-a, at 4 hours. AGE-BSA stimulated alpha1(I) and alpha1(IV) collagen mRNA expression after 24-hour incubation with HMCs, which remained elevated until hour 60. HMCs incubated with AGE-BSA induced a significant inhibition of cell proliferation compared with cells incubated with BSA. AGE-BSA stimulated TGF-beta mRNA and protein expression in HMCs. The TGF-beta secreted by HMCs was shown by CCL-64 mink lung cell assay to be bioactive. In contrast, BSA-AM did not affect either collagen or TGF-beta mRNA or protein expression in HMCs. The stimulatory effects of AGE-BSA on collagen gene regulation in HMCs could be negated by the pretreatment of HMCs with GF 109203X for 30 minutes or with phorbol myristate acetate for 24 hours before AGE-BSA administration. Neutralizing antibody to TGF-beta inhibited increased collagen mRNA expression by HMCs exposed to AGE-BSA. These results suggest that AGE-BSA stimulates collagen mRNA expression by activating PKC and the transcriptional upregulation of TGF-beta1 in HMCs. Thus, PKC and TGF-beta may function as key signaling intermediaries in the AGE-up-regulated collagen gene expression pathway in HMCs.

Advanced glycation end products: a Nephrologist's perspective

Advanced glycation end products (AGEs) are a heterogeneous group of molecules that accumulate in plasma and tissues with advancing age, diabetes, and renal failure. There is emerging evidence that AGEs are potential uremic toxins and may have a role in the pathogenesis of vascular and renal complications associated with diabetes and aging. AGEs are formed when a carbonyl of a reducing sugar condenses with a reactive amino group in target protein. These toxic molecules interact with specific receptors and elicit pleiotropic responses. AGEs accelerate atherosclerosis through cross-linking of proteins, modification of matrix components, platelet aggregation, defective vascular relaxation, and abnormal lipoprotein metabolism. In vivo and in vitro studies indicate that AGEs have a vital role in the pathogenesis of diabetic nephropathy and the progression of renal failure. The complications of normal aging, such as loss of renal function, Alzheimer's disease, skin changes, and cataracts, may also be mediated by progressive glycation of long-lived proteins. AGEs accumulate in renal failure as a result of decreased excretion and increased generation resulting from oxidative and carbonyl stress of uremia. AGE-modified beta(2)-microglobulin is the principal pathogenic component of dialysis-related amyloidosis in patients undergoing dialysis. Available dialytic modalities are not capable of normalizing AGE levels in patients with end-stage renal disease. A number of reports indicated that restoration of euglycemia with islet-cell transplantation normalized and prevented further glycosylation of proteins. Aminoguanidine (AGN), a nucleophilic compound, not only decreases the formation of AGEs but also inhibits their action. A number of studies have shown that treatment with AGN improves neuropathy and delays the onset of retinopathy and nephropathy. N-Phenacylthiazolium bromide is a prototype AGE cross-link breaker that reacts with and can cleave covalent AGE-derived protein cross-links. Thus, there is an exciting possibility that the complications of diabetes, uremia, and aging may be prevented with these novel agents.

Age-dependent accumulation of advanced glycation endproducts is accelerated in combined hyperlipidemia and hyperglycemia, a process attenuated by L-arginine

In this study we have investigated the occurrence of "classical" Amadori rearrangement products of AGE-proteins in the vascular mesenteric bed and in the lens of Golden Syrian hamsters (12 weeks old) rendered simultaneous hyperlipidemics-diabetics (HD), or hyperlipidemics (H) for 24 weeks. For the next 4 weeks the hamsters in HD and H groups received by gavage a solution of 3 mM L-arginine, with the intent to look for the potential effects of L-arginine on the fluorescence of tissular AGE-proteins. Age-matched normal hamsters were used as controls (C). The AGE-products of proteins, and the AGE-collagen isolated from the mesenteric bed were quantitated by fluorescence spectroscopy at ex: 370 nm/em: 440 nm. The results showed that: (i) compared to the fluorescence levels of AGE-proteins detected at C goup, in HD group the fluorescence of AGE-proteins was found 2.78 and 7.41 fold increased in the vascular mesenteric bed and lens, respectively; (ii) in H group the fluorescence of AGE-proteins was 2.36 fold augumented in the vascular mesenteric bed, and 5.43 fold in the lens (versus the C goup); (iii) the aging occurring during the 24 weeks of the experiment induced a small increase in AGE-proteins fluorescence in both mesentery (1.76 fold) and lens (3.83 fold), compared to the levels measured in C group at the inception of the study (12 weeks old hamsters); (iv) the fluorescence of AGE-proteins in the vascular mesenteric bed and in the lens of hamsters in HD and H groups correlated with the increase in circulating plasma glucose and cholesterol concentrations throughout the experiment; (v) L-arginine dietary supplementation in HD and H groups, diminished the AGE-collagen fluorescence in the mesentery to ∼ 35% and ∼ 17%, respectively; in the lens the fluorescence of AGE-proteins was reduced to 65-70% of the levels found in HD and H groups (at 24 weeks). This study showed for the first time that simultaneous hyperlipidemia-hyperglycemia induced an enhanced accumulation of fluorescent AGE-proteins in the mesentery and lens (comparatively to the effect of hyperlipidemia and of chronological aging monitored during the experiment), and that in vivo L-arginine administration decreased the fluorescence of tissular AGE-proteins (AGE-collagen included). The latter observation may bring another area of potential intervention in the adjunct efforts to find out inhibitors of AGE formation, and thus to reduce the increased levels of AGE-proteins accumulated in tissues when diabetes is additionally complicated with atherosclerosis.

Antioxidant properties of aminoguanidine

It is well known that aminoguanidine (AG) can diminish advanced glycosylation of proteins, which might be beneficial in preventing chronic diabetic complications. Recent reports suggested an inter-relationship between glycosylation of protein and free radical damage. In the present study, we examined the free radical scavenging properties of AG. Electron paramagnetic resonance using the spin-trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was performed to determine the superoxide and hydroxyl radical scavenging abilities of AG. These experiments revealed that AG was an effective hydroxyl radical scavenger even though it expressed a direct inhibitory effect on the xanthine oxidase activity at high concentrations (AG > or = 5 mM). In the second part of the study, allophycocyanin was used as an indicator of free radical mediated protein damage. In the assay, 2,2'-azobis(2-amidinopropane) hydrochloride (AAPH) was used as a peroxyl radical generator, and the loss of allophycocyanin fluorescence was monitored. The antioxidant effect of AG was expressed in oxygen-radical absorbing capacity (ORAC), where one ORAC unit equals the net protection produced by 1 microM Trolox (a water soluble analogue of vitamin E) as a control standard. AG exhibited a significant dose-dependent effect against free radical damage. These radical scavenging properties of AG may contribute to protective effects during glycation and explain the prevention of diabetic complications.

Increased macrophage uptake of irreversibly glycated albumin modified-low density lipoproteins of normal and diabetic subjects is mediated by non-saturable mechanisms

Diabetes mellitus is known as an independent risk factor in atherosclerosis. Among the prominent biochemical changes that occur in diabetic state, are the enhanced formation of advanced glycosylation end products (AGE) (especially linked to albumin and collagen) and the impaired oxidative-antioxidative balance. Previously, we have shown that AGE-albumin (AGE-Alb) significantly alters the physico-chemical characteristics of low density lipoproteins of normal (nLDL) and diabetic (dLDL) subjects. In this study we tried to establish if incubation of nLDL or dLDL, with AGE-Alb in autoxidative conditions, modifies the rate and/or the pathway of their uptake by macrophages. To this purpose, nLDL and dLDL were exposed to AGE-Alb, and after re-isolation and radiolabeling the lipoproteins were incubated with U937 or peritoneal macrophages (for various time and concentrations), in the absence or presence of different competitors (native LDL, acetylated LDL, AGE-Alb, mannan) or cytochalasin D. As controls, nLDL and dLDL, maintained in similar conditions, but without AGE-Alb, were used. The results showed that preincubation for 24 h and 72 h with AGE-Alb augmented the macrophage uptake for both nLDL and dLDL (1.7-fold). Either pre-incubated or not with AGE-Alb, dLDL was taken up at a constantly higher rate than nLDL; the difference appeared more prominent at 72 h (1.5 vs. 4 micrograms LDL protein/mg cell protein). The increased level of glycation of native dLDL as compared to native nLDL (266 +/- 35 vs. 160 +/- 24 mmol HMF/mol apoB) as well as of the lipid peroxides (1.34 +/- 0.47 vs. 0.3 +/- 0.09 nmol MDA/mg apoB) could account for the greater uptake of dLDL at any preincubation time. Competition experiments indicated that, generally, incubation with AGE-Alb diminished the apo B100,E receptor-mediated uptake in favour of 'scavenger' receptor pathway and phagocytosis. Macrophage uptake of AGE-Alb modified dLDL was reduced approximately 30% by native nLDL, approximately 70% by acetylated LDL and approximately 38% by cytochalasin D. Together, these data suggest that the consequence of the alterations induced by AGE-Albumin on LDL is the increased macrophage uptake, via non-saturable pathways, that ultimately may lead to accelerated formation of atherosclerotic plaques in diabetics.

The pathomorphological alterations of endocardial endothelium in experimental diabetes and diabetes associated with hyperlipidemia

The structural alterations of endocardial endothelial cells of the heart right atrium and left ventricle were investigated in Golden Syrian hamsters subjected to streptozotocin-induced diabetes and to a combination of diabetes and diet-induced hyperlipidemia. Animals were examined at time intervals ranging from 2 weeks to 6 months. Anionic sites of the endothelial plasmalemma were visualized by in situ perfusion of cationized ferritin. The results indicated that: (a) both atrial and ventricular endocardial endothelium are affected in streptozotocin-induced diabetes: endothelium converts from continuous into a fenestrated type, (b) although the anionic charge of the plasmalemma decreased in advanced diabetes, the newly formed fenestrae highly bound cationized ferritin, (c) combined diabetes and hyperlipidemia induced more severe alterations of endocardial endothelium: new permeable endothelial structures were formed (transendothelial channels, open intercellular junctions, fused plasmalemmal vesicles), and the cells became particularly enriched in cytoskeleton (intermediate filaments and microtubules), (d) the thick subendocardial layer of connective tissue contained, in the combined experimental model, macrophage derived foam cells indicative for the occurrence of alterations of atherosclerotic type.

Long-term assessment of glucose control by haemoglobin-AGE measurement

BACKGROUND

Control of blood glucose is important in reducing both the incidence and the severity of complications in diabetes mellitus. One consequence of long- term hyperglycaemia is the formation and accumulation of advanced glycation end-products (AGEs) on tissue macromolecules. An AGE-modified form of human haemoglobin (Hb-AGE) present at high levels in the red cells of diabetic patients, differs from glucose-derived Amadori product HbA1c in being chemically irreversible and thus persisting for the circulating life of the red cell. We therefore compared Hb- AGE with HbA1c as indicators of long-term blood glucose control.

METHODS

In an open study we measured circulating HbA1c and Hb-AGE concentrations in eight patients with poorly controlled non-insulin-dependent diabetes after a switch to subcutaneous insulin therapy and careful blood glucose monitoring.

RESULTS

After 16 weeks of insulin therapy, the mean HbA1c had decreased from 13.3 (SD 1.2) to 7.3 (0.9)% and the mean Hb-AGE from 12.1 (1.5) to 7.3 (1.3) U/mg Hb. The rate of Hb-AGE decline was 23% slower than that of HbA1c (p=0.044).

INTERPRETATION

The observation that Hb-AGE declines more slowly than HbA1c is consistent with the irreversible nature of the AGE product. Because of this property, Hb-AGE may prove superior to HbA1c as a long-term index of circulating glucose concentrations.

Oxidative stress and platelet activation in diabetes mellitus

The role of oxidative stress and platelet activation in the development and evolution of diabetic vascular complications is unclear. They can both be the consequence of established vascular disease or a contributing factor to the evolution of atherosclerosis. Free radical generation may both lead to or result from platelet activation, suggesting that oxidative stress and platelet activation may be closely interrelated. Controversial results may partly reflect methodological constraints. Novel techniques for the measurement of in vivo indices of oxidant injury have been developed and will be used in conjunction with pharmacological probes to establish whether oxidative stress is enhanced in diabetes and whether this preceeds the onset of micro and macrovascular disease.

Identification of the major site of apolipoprotein B modification by advanced glycosylation end products blocking uptake by the low density lipoprotein receptor

Advanced glycosylation end products (AGEs) arise from glucose-derived Amadori products and have been implicated in the pathogenesis of diabetic vascular disease. We recently reported the presence of an AGE-modified form of low density lipoprotein (LDL) that circulates in high amounts in patients with diabetes or renal insufficiency and that exhibits impaired plasma clearance kinetics. We utilized AGE-specific antibodies to identify the major sites of AGE modification within protease-digested preparations of apolipoprotein B that impair the binding of the AGE-modified form of LDL by human fibroblast LDL receptors. The predominant site of AGE immunoreactivity was found to lie within a single, 67-amino acid region located 1791 residues NH2-terminal of the putative LDL receptor binding domain. These data point to the high reactivity and specificity of this site for AGE formation and provide further evidence for important structural interactions between the LDL receptor binding domain and remote regions of the apolipoprotein B polypeptide.