Managing chronic inflammation in the aging diabetic patient with CKD by diet or sevelamer carbonate: a modern paradigm shift

The maintenance of normal metabolism and body defenses depends on the balance between cellular antioxidant and anti-inflammatory factors. This balance can be disrupted by agents/mechanisms in the extracellular milieu that induce excess reactive oxygen species (ROS) and inflammation. Cytopathic advanced glycation endproducts, present in ever increasing amounts in the modern diet, are one of the major environmental factors that cause excess ROS and/or inflammation at all ages and induce complications in aging, such as chronic kidney disease (CKD) and type 2 diabetes. Increased ROS and/or inflammation are present in both aging and CKD, and are associated with reduced cellular defenses against ROS and/or inflammation. Affected individuals have reduced defenses against further stress and are predisposed to organ failure, now a well-known phenomenon in aging. Thus, new methods are urgently needed to safely reduce ROS and/or inflammation in the aging type 2 diabetes patient with CKD. Studies of both normal aging and diabetic patients with kidney disease underline the fact that increased ROS and/or inflammation can be managed in these conditions by economical, safe, and effective interventions that reduce the uptake of advanced glycation endproducts by either modifying preparation of food or an oral drug. This communication reviews these data and adds new information on the efficacy of a drug, sevelamer carbonate, required to reduce ROS and/or inflammation in the aging type 2 diabetes patient complicated by CKD. If larger and longer studies confirm the hypothesis that one or both of these interventions reduce progression of CKD, it could represent a new paradigm in the management of complications in the type 2 diabetes patient with CKD.

Differential expression of receptors for advanced glycation end-products in peritoneal mesothelial cells exposed to glucose degradation products

Autoclaving peritoneal dialysate fluid (PDF) degrades glucose into glucose degradation products (GDPs) that impair peritoneal mesothelial cell functions. While glycation processes leading to formation of advanced glycation end-products (AGE) were viewed commonly as being mediated by glucose present in the PDF, recent evidence indicates that certain GDPs are even more powerful inducers of AGE formation than glucose per se. In the present study, we examined the expression and modulation of AGE receptors on human peritoneal mesothelial cells (HPMC) cultured with GDPs, conventional PDF or PDF with low GDP content. HPMC cultured with GDPs differentially modulated AGE receptors (including RAGE, AGE-R1, AGE-R2 and AGE-R3) expression in a dose-dependent manner. At subtoxic concentrations, GDPs increased RAGE mRNA expression in HPMC. 2-furaldehyde (FurA), methylglyoxal (M-Glx) and 3,4-dideoxy-glucosone-3-Ene (3,4-DGE) increased the expression of AGE-R1 and RAGE, the receptors that are associated with toxic effects. These three GDPs up-regulated the AGE synthesis by cultured HPMC. In parallel, these GDPs also increased the expression of vascular endothelial growth factor (VEGF) in HPMC. PDF with lower GDP content exerted less cytotoxic effect than traditional heat-sterilized PDF. Both PDF preparations up-regulated the protein expression of RAGE and VEGF. However, the up-regulation of VEGF in HPMC following 24-h culture with conventional PDF was higher than values from HPMC cultured with PDF containing low GDP. We have demonstrated, for the first time, that in addition to RAGE, other AGE receptors including AGE-R1, AGE-R2 and AGE-R3 are expressed on HPMC. Different GDPs exert differential regulation on the expression of these receptors on HPMC. The interactions between GDPs and AGE receptors may bear biological relevance to the intraperitoneal homeostasis and membrane integrity.

Diabetes and advanced glycation endproducts

Bio-reactive advanced glycation endproducts (AGE) alter the structure and function of molecules in biological systems and increase oxidative stress. These adverse effects of both exogenous and endogenously derived AGE have been implicated in the pathogenesis of diabetic complications and changes associated with ageing including atherosclerosis, renal, eye and neurological disease. Specific AGE receptors and nonreceptor mechanisms contribute to these processes but also assist in the removal and degradation of AGE. The final disposal of AGE depends on renal clearance. Promising pharmacologic strategies to prevent AGE formation, reduce AGE toxicity, and/or inactivate AGE are under investigation.

The AGE-receptor in the pathogenesis of diabetic complications

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

Accelerated diabetic glomerulopathy in galectin-3/AGE receptor 3 knockout mice

Several molecules were shown to bind advanced glycation end products (AGEs) in vitro, but it is not known whether they all serve as AGE receptors and which functional role they play in vivo. We investigated the role of galectin-3, a multifunctional lectin with (anti)adhesive and growth-regulating properties, as an AGE receptor and its contribution to the development of diabetic glomerular disease, using a knockout mouse model. Galectin-3 knockout mice obtained by gene ablation and the corresponding wild-type mice were rendered diabetic with streptozotocin and killed 4 months later, together with age-matched nondiabetic controls. Despite a comparable degree of metabolic derangement, galectin-3-deficient mice developed accelerated glomerulopathy vs. the wild-type animals, as evidenced by the more pronounced increase in proteinuria, extracellular matrix gene expression, and mesangial expansion. This was associated with a more marked renal/glomerular AGE accumulation, indicating it was attributable to the lack of galectin-3 AGE receptor function. The galectin-3-deficient genotype was associated with reduced expression of receptors implicated in AGE removal (macrophage scavenger receptor A and AGE-R1) and increased expression of those mediating cell activation (RAGE and AGE-R2). These results show that the galectin-3-regulated AGE receptor pathway is operating in vivo and protects toward AGE-induced tissue injury in contrast to that through RAGE.

Lysozyme enhances renal excretion of advanced glycation endproducts in vivo and suppresses adverse age-mediated cellular effects in vitro: a potential AGE sequestration therapy for diabetic nephropathy?

BACKGROUND

Lysozyme (LZ), a host-defense protein, contains an 18 amino-acid domain with high affinity binding for sugar-derived proteins or lipids, called advanced glycation endproducts (AGE), that are implicated in diabetes- and age-dependent complications (DC).

MATERIALS AND METHODS

A) The effects of LZ on AGE- removal were tested in vivo. LZ was injected (200 ug/day, i.p., X2 weeks) in non-obese diabetic (NOD), db/db (+/+) mice, and non-diabetic, AGE-infused Sprague-Dawley rats. B) LZ: AGE interactions with macrophage-like T1B-183 cells (Mf) and mesangial cells (MC) were tested in vitro.

RESULTS

A) In NOD mice, LZ reduced the elevated basal serum AGE (sAGE) (p < 0.05), enhanced urinary AGE (uAGE) excretion by approximately 2-fold (p < 0.01), while it reduced albuminuria (UA), p < 0.005. In db/db mice, LZ infusion also reduced the elevated sAGE (p < 0.05), doubled uAGE excretion (p < 0.05), and decreased UA (p < 0.01). In addition, LZ maintained normal sAGE in normal rats infused with AGE-BSA, as it doubled the urinary AGE (uAGE) clearance (p < 0.01). B) LZ stimulated the uptake and degradation of (125) I-labeled AGE-BSA and (25) I-human serum AGE by Mf, while suppressing AGE-induced TNFalpha and IGF-I production. In MC, LZ suppressed the AGE-promoted PDGF-B, alpha1 type IV collagen, and tenascin mRNA levels, and restored the AGE-suppressed expression and activity of MMP-9, but not MMP-2.

CONCLUSION

LZ may act to: a) accelerate renal in-vivo AGE clearance, b) suppress macrophage and mesangial cell- specific gene activation in vitro, and c) improve albuminuria due to diabetes. These data suggest that LZ by sequestering AGEs may protect against diabetic renal damage.

Polymorphism screening of four genes encoding advanced glycation end-product putative receptors. Association study with nephropathy in type 1 diabetic patients

Advanced glycation end-products (AGEs) may play an important role in the pathogenesis and progression of cardiovascular and renal complications of diabetes. Four putative AGE receptors (RAGEs), AGE-R1, AGE-R2, and AGE-R3 have been described. In this study, we scanned the sequence of the genes encoding these AGE receptors in 48 patients with type 1 diabetes and investigated the identified polymorphisms (n = 19) in 199 type 1 diabetic patients with nephropathy and 193 type 1 diabetic patients without nephropathy. Overall, none of the polymorphisms was strongly associated with nephropathy. The minor allele of a polymorphism located in the promoter region of the RAGE gene (C-1152A) conferred a weak protective effect (P < 0.05) and was associated with a longer duration of nephropathy-free diabetes (P = 0.08).

Presence of diabetic complications in type 1 diabetic patients correlates with low expression of mononuclear cell AGE-receptor-1 and elevated serum AGE

BACKGROUND

Receptors for advanced glycation endproducts (AGE-R) mediate AGE turnover, but can also trigger inflammatory genes that promote diabetic tissue injury and diabetic complications (DC). High AGE levels and reduced AGE-R sites in kidneys of NOD mice prone to type 1 diabetes (T1D) and to renal disease (RD) suggested that impaired AGE-R function may contribute to RD in these mice.

MATERIALS AND METHODS

In this study, after confirming reduced AGE-R1 expression in NOD mouse peritoneal macrophages, we tested for differences in AGE-R1, -R2, and -R3 gene expression in 54 human subjects by RT-PCR and Western analysis. Fresh peripheral blood mononuclear cells (PBMN) were isolated from 36 persons: 18 T1D patients with severe RD (DC); 11 age-and DM-duration matched patients without DC (n-DC); and 7 normal volunteers (NL). EBV-transformed lymphoblasts were obtained from an additional 18 subjects (12 T1D patients, 6 with and 6 without DC, and 6 nondiabetics).

RESULTS

AGE-R1 mRNA and protein of PBMN from n-DC patients were enhanced (p < .05 versus NL) in proportion to serum AGE levels (sAGE) (p < .005 versus NL). In contrast, PBMN from DC patients exhibited no up-regulation of AGE-R1 mRNA or protein, despite higher sAGE levels (p < .005 versus NL). A similar unresponsiveness in AGE-R1 gene expression was observed in EBV-transformed lymphoblasts from DC patients versus NL (p < .01), but not in n-DC (p = NS). AGE-R2 and -R3 mRNA and protein levels were enhanced in both T1D groups (DC > n-DC) (n-DC AGE-R3, p < .05, DC AGE-R3, p < .05) compared to NL. AGE-R2 mRNA levels correlated with sAGE levels (r = .61, p < .05), and with creatinine clearance (r = -.63, p < .05). No differences were noted in AGE-R2 and -R3 mRNA expression in cultured cells.

CONCLUSIONS

The consistent pattern of elevated serum AGE and low expression of AGE-R1 gene in macrophages from T1D mice (NOD), fresh PBMN and EBV-transformed cells from T1D patients with advanced DC suggests ineffective regulation of R1-mediated AGE turnover, possibly of genetic basis.

Advanced glycation end-product receptor interactions on microvascular cells occur within caveolin-rich membrane domains

Advanced glycation end products (AGEs) have an important role in diabetic complications, with many responses mediated through AGE-receptors. The current study has investigated the binding and uptake of AGEs by retinal microvascular endothelium in an attempt to understand the nature of AGE-interaction with receptors on the cell surface. There has been special emphasis placed on the R1, R2, and R3 components of AGE-receptor complex (AGE-RC) and their localization to caveolin-rich membrane domains. Retinal microvascular endothelial cells (RMECs) were exposed to either AGE-modified BSA (AGE-BSA) or native BSA conjugated to colloidal gold (gAGE, gBSA) for various time periods, fixed, and processed for transmission electron microscopy (TEM). Localization of AGE-RC components in caveolae was investigated using confocal microscopy and ultrastructural immunogold labeling. Caveolae were extracted from RMECs using differential Triton X-100 solubility, and Western analysis was conducted to test for caveolae enrichment and the presence of AGE-RC complex components. Ligand blots determined 125I-AGE-BSA binding to caveolae-enriched extracts. Colloidal gold conjugates of AGE-BSA bound to caveolae and were internalized to be trafficked to lysosomal-like compartments. AGE-receptor complex components were significantly enriched within caveolae. The data suggest that AGEs interact with their receptors within caveolae. It is significant that the AGE-R complex localizes to these organelles, because this may have implications for AGE binding, internalization, signal transduction, and the modulation of AGE-receptor-mediated vascular cell dysfunction.

Intervening in atherogenesis: lessons from diabetes

Age-related atherogenesis resembles diabetes, a model in which glucose looms as an important culprit. Within the body, the sugar becomes a glue able to harden and obstruct blood vessels while also activating such cells as macrophages and T lymphocytes. A new discovery is that large quantities of glycation endproducts may arrive preformed in foods prepared by routine cooking methods.

Differential expression of renal AGE-receptor genes in NOD mice: possible role in nonobese diabetic renal disease

BACKGROUND

Nonobese diabetic mice (NOD) are prone to glomerular pathology, which is accelerated with the onset of diabetes. Advanced glycation end product (AGE) interactions with AGE-receptors (AGE-Rs) in kidneys can contribute to glomerular injury and diabetic nephropathy (DN). The significant elevation in kidney AGE deposits noted in prediabetic NOD mice suggested that delayed AGE turnover in this model may contribute to its propensity toward DN.

METHODS

To explore whether excess tissue AGE was linked to altered AGE-R status in the kidney, mRNA/protein expression, and of several AGE-Rs [AGE-R1, AGE-R2, AGE-R3, scavenger receptor II (ScR-II), and receptor for AGE (RAGE)], was determined in renal cortex and in mesangial cells (MCs) isolated from ND-, D-NOD, and ILE mice (N = 20 per group). Ligand binding, receptor site number, and affinity were determined in MCs from the same mouse groups.

RESULTS

Prediabetic NOD kidney AGE-R1 mRNA and protein level were threefold lower than that of ILE mice (P < 0.01), while AGE-R3 mRNA was enhanced by twofold (P < 0.05) and AGE-R2, RAGE, and ScR-II mRNA remained close to normal (ILE). The onset of diabetes in NOD mice, while enhancing AGE-R1 mRNA expression by approximately twofold, failed to raise it above the normal (ILE) level, despite increases in tissue, and serum AGE. The latter was associated with higher elevation in AGE-R3 (sixfold, P < 0.05), RAGE (twofold, P = NS), and ScR-II mRNA (2. 8-fold, P = NS) above control. MCs from prediabetic NOD mice showed a threefold lower level of AGE-R1 mRNA (P < 0.02 vs. ILE) and AGE-R1-protein, and AGE-binding activity (<40% of control ILE). In contrast, AGE-R3 mRNA was enhanced (twofold), while AGE-R2 showed no change. Cultured ND-NOD MCs displayed only one fourth of the AGE-binding sites/cell present on ILE MCs (1.6 x 10(6) vs. 6.6 x 10(6), P < 0.05), which after the onset of diabetes rose to the normal range (7.0 x 10(6)/cell), but failed to exceed it.

CONCLUSIONS

Reduced AGE-R1 gene expression in this strain may contribute to delayed AGE removal from and early AGE deposition in kidney tissues. This may act as a trigger for those AGE-R genes involved in growth-promoting changes, leading to DN in this strain.

A receptor for advanced glycosylation endproducts (AGEs) is colocalized with neurofilament-bound AGEs and SOD1 in motoneurons of ALS: immunohistochemical study

Neurofilament (NF)-bound AGEs colocalize immunochemically with SOD1 in the motoneurons of patients with ALS. Among three types of AGE receptors reported in the human brain, AGE-R1 (oligosaccharyltransferase family) and AGE-R2 (substrate of protein kinase C) have been found in neurons, while AGE-R3 is restricted to glia. The present study investigates which of these receptors may be responsible for binding AGEs in the NF conglomerates of motoneurons. Immunostaining of paraffin sections from eight ALS patients (five sporadic and three familial) and three control cases was performed with antibodies directed against R1 and R2, in parallel with those against AGEs and SOD1. The sites of AGE-R1 immunoreactivity (IR) in motoneurons were in conformity to those of NF-associated AGE and SOD1 IRs. By contrast, the IR of R2 was negative in NF conglomerates. Negative R2 IR for NF conglomerates was outlined by surrounding coarse R2 immunopositive granules in the perikaryon. No IR for R1 or R2 was found in hyaline or Bunina inclusions. There was no extraneuronal expression of IR for AGE-R1 or AGEs in microglia or astroglia around the NF accumulation. The colocalization of AGE, AGE-R1, and SOD1 at NF conglomerates in motoneurons supports the notion that AGE-mediated oxidative stress and protein aggregation may be implicated in NF conglomeration and ALS pathogenesis.

Dietary glycotoxins: inhibition of reactive products by aminoguanidine facilitates renal clearance and reduces tissue sequestration

Evidence indicates that the metabolic turnover of food-derived reactive orally absorbed advanced glycation end products (AGEs) or glycotoxins (GTs) is delayed, possibly contributing to the tissue damage induced by endogenous AGEs, especially in patients with diabetes and kidney disease. The aim of this study was to explore whether pharmacologic inhibition of dietary AGE bioreactivity by aminoguanidine (AG) can improve turnover and renal excretion of these substances. Normal Sprague-Dawley rats were fed single-labeled [14C]AGE-ovalbumin, double-labeled [14C-125I]AGE-ovalbumin, or control 125I-labeled ovalbumin diet plus free [14C]glucose, with or without AG (0.2% in water). [14C]AGE- and 125I-labeled peptide-associated radioactivity (RA) were compared with AGE immunoreactivity (by enzyme-linked immunosorbent assay) in tissues, serum, and 72-h urine samples. The effect of AG on dietary AGE bioreactivity was assessed by monitoring the inhibition of covalent complex formation between fibronectin (FN) peptide fragments and serum components, after a meal of labeled dietary AGE with or without AG. The radiolabeled AGE diet produced serum absorption and urinary excretion peaks kinetically distinct from those of free [14C]glucose or [125I]ovalbumin. Some 26% of the orally absorbed AGE-ovalbumin was excreted in the urine, whereas after AG treatment, urinary excre-tion of dietary AGEs increased markedly (to >50% of absorbed). More than 60% of tissue-bound RA was found covalently deposited in kidneys and liver, whereas after treatment with AG, tissue AGE deposits were reduced to <15% of the amount found in untreated AGE-fed controls. Sera enriched for dietary GTs formed covalently linked complexes with FN, a process completely inhibitable by AG cotreatment. Amelioration of dietary GT bioreactivity by AG improves renal elimination and prevents tissue deposition of food GTs. This may afford a novel and potentially protective use of AG against excessive tissue AGE toxicity in diabetic patients with renal disease.

Characterization of the advanced glycation end-product receptor complex in human vascular endothelial cells

Advanced glycation end products (AGEs) have been implicated as causal factors in the vascular complications of diabetes and it is known that these products interact with cells through specific receptors. The AGE-receptor complex, originally described as p60 and p90, has been characterised in hemopoietic cells and the component proteins identified and designated AGE-R1, -R2 and -R3. In the current study we have characterised this receptor in human umbilical vein endothelial cells (HUVECs) and elucidated several important biological properties which may impact on AGE mediated vascular disease. 125I-AGE-BSA binding to HUVEC monolayers was determined with and without various cold competitors. The synthetic AGE, 2-(2-furoyl)-4(5)-furanyl-1H-imidazole (FFI)-BSA, failed to compete with AGE-BSA binding unlike observations already reported in hemopoietic cells. The ability of 125I-AGE-BSA to bind to separated HUVEC plasma membrane (PM) proteins was also examined and the binding at specific bands inhibited by antibodies to each component of the AGE-receptor complex. Western blotting of whole cell and PM fractions, before and after exposure to AGE-BSA, revealed that AGE-R1, -R2 and -R3 are subject to upregulation upon exposure to their ligand, a phenomenon which was also demonstrated by immunofluorescence of non-permeabilised cells. mRNA expression of each AGE-receptor component was apparent in HUVECs, with the AGE-R2 and -R3 gene expression being upregulated upon exposure to AGEs in a time-dependent manner. A phosporylation assay in combination with AGE-R2 immunoprecipitation demonstrated that this component of the receptor complex is phosphorylated by acute exposure to AGE-BSA. These results indicate the presence of a conserved AGE-receptor complex in vascular endothelium which demonstrates subtle differences to other cell-types. In response to AGE-modified molecules, this complex is subject to upregulation, while the AGE-R2 component also displays increased phosphorylation possibly leading to enhanced signal transduction.

Advanced glycation end products in vitreous: Structural and functional implications for diabetic vitreopathy

PURPOSE

Advanced glycation end products (AGEs) form irreversible cross-links with many macromolecules and have been shown to accumulate in tissues at an accelerated rate in diabetes. In the present study, AGE formation in vitreous was examined in patients of various ages and in patients with diabetes. Ex vivo investigations were performed on bovine vitreous incubated in glucose to determine AGE formation and cross-linking of vitreous collagen.

METHODS

By means of an AGE-specific enzyme-linked immunosorbent assay (ELISA), AGE formation was investigated in vitreous samples obtained after pars plana vitrectomy in patients with and without diabetes. In addition, vitreous AGEs were investigated in bovine vitreous collagen after incubation in high glucose, high glucose with aminoguanidine, or normal saline for as long as 8 weeks. AGEs and AGE cross-linking was subsequently determined by quantitative and qualitative assays.

RESULTS

There was a significant correlation between AGEs and increasing age in patients without diabetes (r = 0.74). Furthermore, a comparison between age-matched diabetic and nondiabetic vitreous showed a significantly higher level of AGEs in the patients with diabetes (P < 0.005). Collagen purified from bovine vitreous incubated in 0.5 M glucose showed an increase in AGE formation when observed in dot blot analysis, immunogold labeling, and AGE ELISA. Furthermore, there was increased cross-linking of collagen in the glucose-incubated vitreous, when observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and protein separation. This cross-linking was effectively inhibited by coincubation with 10 mM aminoguanidine.

CONCLUSIONS

This study suggests that AGEs may form in vitreous with increasing age. This process seems to be accelerated in the presence of diabetes and as a consequence of exposure to high glucose. Advanced glycation and AGE cross-linking of the vitreous collagen network may help to explain the vitreous abnormalities characteristic of diabetes.

Alzheimer's disease--synergistic effects of glucose deficit, oxidative stress and advanced glycation endproducts

Many approaches have been undertaken to understand Alzheimer's disease (AD) but the heterogeneity of the etiologic factors makes it difficult to define the clinically most important factor determining the onset and progression of the disease. However, there is increasing evidence that the previously so-called "secondary factors" such as a disturbed glucose metabolism, oxidative stress and formation of "advanced glycation endproducts" (AGEs) and their interaction in a vicious cycle are also important for the onset and progression of AD. AGEs are protein modifications that contribute to the formation of the histopathological and biochemical hallmarks of AD: amyloid plaques, neurofibrillary tangles and activated microglia. Oxidative modifications are formed by a complex cascade of dehydration, oxidation and cyclisation reactions, subsequent to a non-enzymatic reaction of sugars with amino groups of proteins. Accumulation of AGE-crosslinked proteins throughout life is a general phenomenon of ageing. However, AGEs are more than just markers of ageing since they can also exert adverse biologic effects on tissues and cells, including the activation of intracellular signal transduction pathways, leading to the upregulation of cytokine and free radical production (oxidative stress). Oxidative stress is involved in various divergent events leading to cell damage, including an increase in membrane rigidity, DNA strand breaks and an impairment in glucose uptake. In addition, other age-related metabolic changes such as depletion of antioxidants or decreased energy production by a disturbed glucose metabolism diminish the ability of the cell to cope with the effects of radical-induced membrane, protein and DNA damage. With our improving understanding of the molecular basis for the clinical symptoms of dementia, it is hoped that the elucidation of the etiologic causes, particularly the positive feedback loops involving radical damage and a reduced glucose metabolism, will help to develop novel "neuroprotective" treatment strategies able to interrupt this vicious cycle of oxidative stress and energy shortage in AD.

Receptors for advanced glycosylation endproducts in human brain: role in brain homeostasis

BACKGROUND

Advanced glycation end products (AGEs) are the reactive derivatives of nonenzymatic glucose-macromolecule condensation products. Aging human tissues accumulate AGEs in an age-dependent manner and contribute to age-related functional changes in vital organs. We have shown previously that AGE scavenger receptors are present on monocyte/macrophages, lymphocytes, and other cells. However, it remains unclear whether the human brain can efficiently eliminate AGE-modified proteins and whether excessive AGEs can contribute to inflammatory changes leading to brain injury in aging.

MATERIALS AND METHODS

To explore the expression and characteristics of AGE-binding proteins on CNS glia components and their putative function, such as degradation of AGE-modified proteins, primary human astrocytes and human monocytes (as a microglial cell surrogate) and murine microglia (N9) cells and cell membrane extracts were used. Immunohistochemistry was used to examine the distribution of AGE-binding proteins in the human hippocampus; RT-PCR techniques were used to examine the biologic effects of AGEs and a model AGE compound, FFI, on AGE-binding protein modulation and cytokine responses of human astrocytes and monocytes.

RESULTS

Our results showed that AGE-binding proteins AGE-R1, -R2, and -R3 are present in glial cells. Western blot analyses and radiolabeled ligand binding studies show that AGE-R1 and -R3 from human astrocytes bind AGE-modified proteins; binding could be blocked by anti-AGE-R1 and anti-AGE-R3 antibodies, respectively. Immunohistochemistry showed that AGE-R1 and -R2 are expressed mainly in neurons; only some glial cells express these AGE-binding proteins. In contrast, AGE-R3 was found only on those astrocytes whose positively stained foot processes extend and surround the sheath of microcapillaries. RT-PCR results showed that mRNAs of the three AGE-binding proteins are expressed constitutively in human astrocytes and monocytes, and receptor transcripts are not regulated by exogenous AGEs, the model AGE compound FFI, or phorbol ester. At the concentrations used, GM-CSF appears to be the only cytokine whose transcript and protein levels are regulated in human astrocytes by exogenous AGEs.

CONCLUSIONS

The selective presence of AGE-binding proteins in pyramidal neurons and glial cells and their roles in degrading AGE-modified protein in glial cells suggest that the human brain has a mechanism(s) to clear AGE-modified proteins. Without this capacity, accumulation of AGEs extracellularly could stimulate glial cells to produce the major inflammatory cytokine GM-CSF, which has been shown to be capable of up-regulating AGE-R3. It remains to be determined whether AGE-binding proteins could be aberrant or down-regulated under certain pathological conditions, resulting in an insidious inflammatory state of the CNS in some aging humans.

Tobacco smoke is a source of toxic reactive glycation products

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

Reduction of plasma apolipoprotein-B by effective removal of circulating glycation derivatives in uremia

Patients with diabetes and renal insufficiency (Db/ESRD), a group subject to accelerated atherosclerosis exhibit marked increases in the levels of circulating, glycation-derived reactive substances, termed advanced glycation endoproducts (AGEs). These products have been previously shown to react covalently with apoliprotein B (ApoB) to form AGE-ApoB, a modification that results in delayed low density lipoprotein (LDL) clearance and possibly to dyslipidemia. Because the effect of hemodialysis on AGE removal was shown to be unsatisfactory, based on single intradialytic studies, we examined the effect of long-term hemodialysis therapy on serum AGE-ApoB levels, as well as on total serum ApoB of 25 Db/ESRD patients treated by two types of hemodialysis filters, the Fresenius Inc. F8, as the low flux (LF), or high-flux polysulfone AN69 (HF) for two months using an AGE-specific ELISA. At the end of eight weeks, circulating AGE-ApoB levels were reduced significantly (by 35%) from baseline (P = 0.039) in patients treated by HF compared to a modest 16% reduction noted in patients treated by LF (P = 0.05) N = 12, P = 0.047). Of note, total plasma ApoB was reduced by 27% from baseline (P = 0.02) in patients treated by HF compared to a 6% reduction noted in those treated with LF (P = 0.8). In vitro comparison of AGE mass balance, and mass adsorption by the different filters revealed that the higher efficiency of HF filter was due to greater adsorption. The association of reduced AGE-ApoB levels with a decrease in total circulating ApoB by HF and not by LF dialysis suggests: (1) a causal link between AGE clearance and dyslipidemia in diabetic ESRD, and, (2) that more efficient modes of renal replacement treatment and AGE removal could significantly benefit clinical outcome.

Upregulation of mesangial growth factor and extracellular matrix synthesis by advanced glycation end products via a receptor-mediated mechanism

Enhanced advanced glycosylation end product (AGE) formation has been shown to participate in the pathogenesis of diabetes-induced glomerular injury by mediating the increased extracellular matrix (ECM) deposition and altered cell growth and turnover leading to mesangial expansion. These effects could be exerted via an AGE-receptor-mediated upregulation of growth factors, such as the IGFs and transforming growth factor-beta (TGF-beta). We tested this hypothesis in human and rat mesangial cells grown on nonglycated or native bovine serum albumin (BSA), glycated BSA with AGE formation (BSA-AGE), or glycated BSA in which AGE formation was prevented by the use of aminoguanidine (BSA-AM), in the presence or absence of an antibody, alpha-p60, directed against the p60/OST protein named AGE-receptor 1 (AGE-R1), or normal control (pre-immune) serum. The mRNA and/or protein levels of IGF-I, IGF-II, IGF receptors, IGF binding proteins (IGFBPs), TGF-beta1 and the ECM components fibronectin, laminin, and collagen IV were measured, together with cell proliferation. Both human and rat mesangial cells grown on BSA-AGE showed increased IGF-I and total and bioactive TGF-beta medium levels and enhanced IGF-I, IGF-II, and TGF-beta1 gene expression, compared with cells grown on BSA, whereas total IGFBP and IGFBP-3 medium content, IGF receptor density and affinity, and IGF-I receptor transcripts were unchanged. Moreover, cells grown on BSA-AGE showed increased ECM protein and mRNA levels versus cells cultured on BSA, whereas cell proliferation was unchanged in human mesangial cells and slightly reduced in rat mesangial cells. Growing cells on BSA-AM did not affect any of the measured parameters. Co-incubation of BSA-AGE with anti-AGE-R1, but not with pre-immune serum, prevented AGE-induced increases in IGF-I, TGF-beta1, and ECM production or gene expression; anti-AGE-R1 also reduced growth factor and matrix synthesis in cells grown on BSA. These results demonstrate that mesangial IGF and TGF-beta1 synthesis is upregulated by AGE-modified proteins through an AGE-receptor-mediated mechanism. The parallelism with increased ECM production raises the speculation that the enhanced synthesis of these growth factors resulting from advanced nonenzymatic glycation participates in the pathogenesis of hyperglycemia-induced mesangial expansion.

Elevated AGE-modified ApoB in sera of euglycemic, normolipidemic patients with atherosclerosis: relationship to tissue AGEs

BACKGROUND

Advanced glycation endproducts (AGEs) are implicated in the pathogenesis of atherosclerotic vascular disease of diabetic and nondiabetic etiology. Recent research suggests that advanced glycation of ApoB contributes to the development of hyperlipidemia. AGE-specific receptors, expressed on vascular endothelium and mononuclear cells, may be involved in both the clearance of, and the inflammatory responses to AGEs. The aim of this study was to examine whether there is a relationship between serum AGE-ApoB and AGEs in arterial tissue of older normolipidemic nondiabetic patients with occlusive atherosclerotic disease, compared with age-matched and younger asymptomatic persons.

MATERIALS AND METHODS

Serum AGE-ApoB was measured by ELISA in 21 cardiac bypass patients. Furthermore, an AGE-specific monoclonal antibody, and polyclonal antibodies against anti-AGE-receptor (anti-AGE-R) 1 and 2 were used to explore the localization and distribution of AGEs and AGE-R immunoreactivity (IR) in arterial segments excised from these patients.

RESULTS

Serum AGE-ApoB levels were significantly elevated in the asymptomatic, older population, compared with those in young healthy persons (259 +/- 24 versus 180 +/- 21 AGE U/mg of ApoB, p < 0.01). Higher AGE-ApoB levels were observed in those patients with atherosclerosis (329 +/- 23 versus 259 +/- 24 AGE U/mg ApoB, p < 0.05). Comparisons of tissue AGE-collagen with serum AGE-ApoB levels showed a significant correlation (r = 0.707, p < 0.01). In early lesions, AGE-IR occurred mostly extracellularly. In fatty streaks and dense, cellular atheromatous lesions, AGE-IR was visible within lipid-containing smooth muscle cells and macrophages, while in late-stage, acellular plaques, AGE-IR occurred mostly extracellularly. AGE-R1 and -R2 were observed on vascular endothelial and smooth-muscle cells and on infiltrating mononuclear cells in the early-stage lesions, whereas in dense, late-stage plaques, they colocalized mostly with lipid-laden macrophages. On tissue sections, scoring of AGE-immunofluorescence correlated with tissue AGE and plasma AGE-ApoB.

CONCLUSIONS

(1) The correlation between arterial tissue AGEs and circulating AGE-ApoB suggests a causal link between AGE modification of lipoproteins and atherosclerosis. AGE-specific receptors may contribute to this process. (2) Serum AGE-ApoB may serve to predict atherosclerosis in asymptomatic patients.

Standardizing the immunological measurement of advanced glycation endproducts using normal human serum

Advanced glycation endproducts (AGEs) have been linked to many sequelae of diabetes, renal disease and aging. To detect AGE levels in human tissues and blood samples, a competitive enzyme-linked immunosorbent assay (ELISA) has been widely used. As no consensus or standard research method for the quantitation of AGEs currently exists, nor a universally defined AGE unit available, the comparative quantitation of AGEs between research laboratories is problematic and restricts the usefulness of interlaboratory clinical data. By comparing the cross-reactivities of five different anti-AGE antisera with five different in vitro AGE-modified proteins, we found that the immunological recognition of AGEs by competitive ELISA is both AGE-carrier protein- and anti-AGE antibody-dependent. This suggests that in vitro AGE-modified proteins might not be appropriate standards for AGEs that occur naturally in vivo. Based on our observation that serum AGE levels in the normal human population are consistently within a narrow range and several folds lower than in diabetics, we propose a method to standardize AGE units against normal human serum (NHS). In this new method, one AGE unit is defined as the inhibition that results from 1:5 diluted NHS in the competitive AGE-ELISA; thus the AGE value in NHS is 5 units/ml. This NHS method requires a competitive AGE-ELISA with reasonable sensitivity such that 1:5 NHS produces a 25 to 40% inhibition of anti-AGE antibody binding to immobilized AGE-proteins. By using this standardized method we found that the AGE levels in normal human serum (5.0 +/- 2.2 units/ml; mean +/- SD, n = 34) fit a normal distribution (chi 2-test, p < 0.01), and the serum AGE levels in diabetic patients (20.3 +/- 3.8 units/ml, n = 7) are significantly higher than that of the normal population (p < 0.0001). Since AGE units can now be defined against a universally available standard, NHS, the results of quantitative AGE measurements using this method should be comparable between assays and between different laboratories. Taken together, standardizing the AGE-ELISA protocol as described here provides a simple and quantitative method that should facilitate the expanded application of clinical AGE data.

Depletion of reactive advanced glycation endproducts from diabetic uremic sera using a lysozyme-linked matrix

Diabetic uremic sera contain excessive amounts of reactive advanced glycation endproducts (AGEs), which accelerate the vasculopathy of diabetes and end-stage renal disease. To capture in vivo-derived toxic AGEs, high affinity AGE-binding protein lysozyme (LZ) was linked to a Sepharose 4B matrix. Initial studies showed that > 80% of 125I-AGE-BSA was retained by the LZ matrix, compared with < 10% retained by a control matrix. More than 60% of AGE-lysine was captured by the LZ matrix, and the LZ-bound fraction retained immunoreactivity and cross-linking activity, but had little intrinsic fluorescence (370/440 nm). After passage through the LZ matrix, AGE levels in diabetic sera (0.37+/-0.04 U/mg) were significantly reduced to a level (0.09+/-0.01 U/mg; n = 10; P < 0. 0001) comparable with the level of normal human serum, whereas total protein absorption was < 3%. The AGE-enriched serum fraction exhibited cross-linking activity, which was completely prevented by aminoguanidine. Among numerous LZ-bound proteins in diabetic uremic sera, three major proteins "susceptible" to AGE modification were identified: the immunoglobulin G light chain, apolipoprotein J (clusterin/SP-40,40), and the complement 3b beta chain. These findings indicate that the LZ-linked AGE affinity column may serve as an efficient method for the depletion of toxic AGEs from sera, including specific AGE-modified proteins that may be linked to altered immunity, lipoprotein metabolism, and accelerated vasculopathy in renal failure patients with or without diabetes.

Orally absorbed reactive glycation products (glycotoxins): an environmental risk factor in diabetic nephropathy

Endogenous advanced glycation endproducts (AGEs) include chemically crosslinking species (glycotoxins) that contribute to the vascular and renal complications of diabetes mellitus (DM). Renal excretion of the catabolic products of endogenous AGEs is impaired in patients with diabetic or nondiabetic kidney disease (KD). The aim of this study was to examine the oral absorption and renal clearance kinetics of food AGEs in DM with KD and whether circulating diet-derived AGEs contain active glycotoxins. Thirty-eight diabetics (DM) with or without KD and five healthy subjects (NL) received a single meal of egg white (56 g protein), cooked with (AGE-diet) or without fructose (100 g) (CL-diet). Serum and urine samples, collected for 48 hr, were monitored for AGE immunoreactivity by ELISA and for AGE-specific crosslinking reactivity, based on complex formation with 125I-labeled fibronectin. The AGE-diet, but not the CL-diet, produced distinct elevations in serum AGE levels in direct proportion to amount ingested (r = 0.8, P < 0.05): the area under the curve for serum ( approximately 10% of ingested AGE) correlated directly with severity of KD; renal excretion of dietary AGE, although normally incomplete (only approximately 30% of amount absorbed), in DM it correlated inversely with degree of albuminuria, and directly with creatinine clearance (r = 0.8, P < 0.05), reduced to <5% in DM with renal failure. Post-AGE-meal serum exhibited increased AGE-crosslinking activity (two times above baseline serum AGE, three times above negative control), which was inhibited by aminoguanidine. In conclusion, (i) the renal excretion of orally absorbed AGEs is markedly suppressed in diabetic nephropathy patients, (ii) daily influx of dietary AGEs includes glycotoxins that may constitute an added chronic risk for renal-vascular injury in DM, and (iii) dietary restriction of AGE food intake may greatly reduce the burden of AGEs in diabetic patients and possibly improve prognosis.

Glycation products in aged thioglycollate medium enhance the elicitation of peritoneal macrophages

Thioglycollate medium (TGM) is widely used as a stimulatory agent to induce non-infectious peritoneal inflammation for elicitation of macrophages from mice and rats. It has been known for a long time that aged, autoclaved TGM is more efficient than freshly prepared TGM, however, the mechanism responsible for this enhanced activity of aged TGM remains obscure. The aging of TGM apparently favors the non-enzymatic reactions between proteins and reducing sugars in TGM that may lead to the generation of advanced glycation endproducts (AGEs). We have found that aged TGM contains 40-fold more AGEs than fresh TGM. The formation of AGEs in TGM was completely blocked by co-incubation with an AGE inhibitor, aminoguanidine. Intraperitoneal injection of aged TGM into rats elicited approximately 2-fold more macrophages than fresh TGM. However, the addition of AGE-modified proteins to fresh TGM increased the yield of peritoneal cells to a level which was significantly higher than both fresh and aged TGM. The injection of AGE-modified proteins alone did not elicit significantly more macrophages than the level of resident peritoneal cells. These results suggest that the formation of AGEs during aging of TGM is responsible for the enhanced macrophage-eliciting activity in aged TGM. AGEs may act as an enhancing agent to augment the existing inflammatory responses. AGE-supplemented TGM may provide an efficient method for eliciting peritoneal macrophages or establishing an inflammatory animal model.

Advanced glycation end products (AGEs) co-localize with AGE receptors in the retinal vasculature of diabetic and of AGE-infused rats

Advanced glycation end products (AGEs), formed from the nonenzymatic glycation of proteins and lipids with reducing sugars, have been implicated in many diabetic complications; however, their role in diabetic retinopathy remains largely unknown. Recent studies suggest that the cellular actions of AGEs may be mediated by AGE-specific receptors (AGE-R). We have examined the immunolocalization of AGEs and AGE-R components R1 and R2 in the retinal vasculature at 2, 4, and 8 months after STZ-induced diabetes as well as in nondiabetic rats infused with AGE bovine serum albumin for 2 weeks. Using polyclonal or monoclonal anti-AGE antibodies and polyclonal antibodies to recombinant AGE-R1 and AGE-R2, immunoreactivity (IR) was examined in the complete retinal vascular tree after isolation by trypsin digestion. After 2, 4, and 8 months of diabetes, there was a gradual increase in AGE IR in basement membrane. At 8 months, pericytes, smooth muscle cells, and endothelial cells of the retinal vessels showed dense intracellular AGE IR. AGE epitopes stained most intensely within pericytes and smooth muscle cells but less in basement membrane of AGE-infused rats compared with the diabetic group. Retinas from normal or bovine-serum-albumin-infused rats were largely negative for AGE IR. AGE-R1 and -R2 co-localized strongly with AGEs of vascular endothelial cells, pericytes, and smooth muscle cells of either normal, diabetic, or AGE-infused rat retinas, and this distribution did not vary with each condition. The data indicate that AGEs accumulate as a function of diabetes duration first within the basement membrane and then intracellularly, co-localizing with cellular AGE-Rs. Significant AGE deposits appear within the pericytes after long-term diabetes or acute challenge with AGE infusion conditions associated with pericyte damage. Co-localization of AGEs and AGE-Rs in retinal cells points to possible interactions of pathogenic significance.

Molecular identity and cellular distribution of advanced glycation endproduct receptors: relationship of p60 to OST-48 and p90 to 80K-H membrane proteins

Advanced glycation endproducts (AGEs) are derivatives of nonenzymatic reactions between sugars and protein or lipids, and together with AGE-specific receptors are involved in numerous pathogenic processes associated with aging and hyperglycemia. Two of the known AGE-binding proteins isolated from rat liver membranes, p60 and p90, have been partially sequenced. We now report that the N-terminal sequence of p60 exhibits 95% identity to OST-48, a 48-kDa member of the oligosaccharyltransferase complex found in microsomal membranes, while sequence analysis of p90 revealed 73% and 85% identity to the N-terminal and internal sequences, respectively, of human 80K-H, a 80- to 87-kDa protein substrate for protein kinase C. AGE-ligand and Western analyses of purified oligosaccharyltransferase complex, enriched rough endoplasmic reticulum, smooth endoplasmic reticulum, and plasma membranes from rat liver or RAW 264.7 macrophages yielded a single protein of approximately 50 kDa recognized by both anti-p60 and anti-OST-48 antibodies, and also exhibited AGE-specific binding. Immunoprecipitated OST-48 from rat rough endoplasmic reticulum fractions exhibited both AGE binding and immunoreactivity to an anti-p60 antibody. Immune IgG raised to recombinant OST-48 and 80K-H inhibited binding of AGE-bovine serum albumin to cell membranes in a dose-dependent manner. Immunostaining and flow cytometry demonstrated the surface expression of OST-48 and 80K-H on numerous cell types and tissues, including mononuclear, endothelial, renal, and brain neuronal and glial cells. We conclude that the AGE receptor components p60 and p90 are identical to OST-48, and 80K-H, respectively, and that they together contribute to the processing of AGEs from extra- and intracellular compartments and in the cellular responses associated with these pathogenic substances.

Advanced glycation end-products and atherosclerosis

The late rearrangements of the covalent nonenzymatic modification of proteins by glucose, called advanced glycation end-products (AGEs), have been shown to accumulate in diabetic and ageing tissues. AGEs elicit a wide range of cell-mediated responses leading to vascular dysfunction, matrix expansion and athero- and glomerulosclerosis. Cellular responses are thought to be largely induced through an AGE-specific cell-surface receptor complex (AGEr). Interaction of AGE-modified proteins with these cells may serve diverse purposes, including disposal of senescent AGE-modified molecules and initiation of tissue repair and protein turnover. In humans, the normal renal clearance rate for the AGE-degradation products found in serum, AGE peptides (AGEp), correlates inversely with renal creatinine clearance rate. Of note, circulating AGEp include reactive intermediates which readily attach covalently to either insoluble matrix collagen or serum proteins, e.g. low-density lipoproteins (LDL), to form AGEp collagen and AGEp-LDL. Consistent with this, diabetic and nondiabetic patients with renal failure (a group highly susceptible to accelerated atherosclerosis) exhibit markedly elevated AGE-modified serum LDL. In summary, in addition to glucose-derived AGEs, the endogenously produced degradation products, AGE peptides, can amplify tissue damage and thus account as distinct toxins. The effects may particularly accelerate glucose toxicity in certain individuals that are genetically susceptible to diabetic renal and extrarenal disease.

Glycation and microglial reaction in lesions of Alzheimer's disease

Single, double, and triple immunostaining of cryostat sections of elderly normal and Alzheimer disease (AD) brain was performed with monoclonal and polyclonal antibodies to advanced glycation end products (AGE). The sections were counterstained with thioflavin-S or with immunocytochemistry for A beta and also stained with markers for microglia. AGE-immunoreactivity was detected in senile plaques and neurofibrillary tangles (NFT). AGE immunoreactivity was most intense in dense or reticular amyloid deposits and extracellular NFT, while intracellular NFT and diffuse amyloid had less AGE immunoreactivity. This pattern of immunoreactivity was similar to that noted in previous studies with antibodies to apolipoprotein-E (apo-E). Therefore, double labeling with antibodies to apo-E and AGE was performed. AGE immunoreactivity colocalized to a very high degree with apo-E immunoreactivity, except that relatively more intense apo-E immunoreactivity was detected in amyloid deposits and more intense AGE immunoreactivity in NFT. The lesions that were immunostained with antibodies to AGE and apo-E were often, but not always, associated with a local microglial reaction. The results raise the possibility that apo-E or a fragment of apo-E may be glycated. Biochemical studies are needed to determine the extent of possible apo-E glycation in AD. The present results raise the possibility that glycation may serve as one of the signals for activation of microglia associated with amyloid deposits and extracellular NFT.

Recent progress in advanced glycation and diabetic vascular disease: role of advanced glycation end product receptors

Advanced glycosylation end products (AGEs) form principally from the rearrangement of early glycation products, i.e., Amadori products, which produce a class of stable moieties that possess distinctive chemical crosslinking and biological properties. It has been generally believed that proteins with half-lives of longer than a few weeks are most susceptible to advanced glycosylation and that the highest levels of AGEs occur on proteins that comprise the long-lived structural components of connective tissue matrix and basement membrane.

Prevention of cardiovascular and renal pathology of aging by the advanced glycation inhibitor aminoguanidine

Human aging is impacted severely by cardiovascular disease and significantly but less overtly by renal dysfunction. Advanced glycation endproducts (AGEs) have been linked to tissue damage in diabetes and aging, and the AGE inhibitor aminoguanidine (AG) has been shown to inhibit renal and vascular pathology in diabetic animals. In the present study, the effects of AG on aging-related renal and vascular changes and AGE accumulation were studied in nondiabetic female Sprague-Dawley (S-D) and Fischer 344 (F344) rats treated with AG (0.1% in drinking water) for 18 mo. Significant increases in the AGE content in aged cardiac (P < 0.05), aortic (P < 0.005), and renal (P < 0.05) tissues were prevented by AG treatment (P < 0.05 for each tissue). A marked age-linked vasodilatory impairment in response to acetylcholine and nitroglycerine was prevented by AG treatment (P < 0.005), as was an age-related cardiac hypertrophy evident in both strains (P < 0.05). While creatinine clearance was unaffected by aging in these studies, the AGE/ creatinine clearance ratio declined 3-fold in old rats vs. young rats (S-D, P < 0.05; F344, P < 0.01), while it declined significantly less in AG-treated old rats (P < 0.05). In S-D but not in F344 rats, a significant (P < 0.05) age-linked 24% nephron loss was completely prevented by AG treatment, and glomerular sclerosis was markedly suppressed (P < 0.01). Age-related albuminuria and proteinuria were markedly inhibited by AG in both strains (S-D, P < 0.01; F344, P < 0.01). These data suggest that early interference with AGE accumulation by AG treatment may impart significant protection against the progressive cardiovascular and renal decline afflicting the last decades of life.

The role of advanced glycosylation end-products in the pathogenesis of atherosclerosis

Coronary artery disease and cerebrovascular disease due to the rapid progression of atherosclerosis is the principal cause of death in diabetes mellitus. Modification of low-density lipoproteins (LDL) by advanced glycosylation end-products (AGE) may play a central role in the development of atherosclerosis, especially in diabetic patients. An AGE-modified form of LDL (AGE-LDL) has been found to circulate in human plasma, and AGE modifications have been identified as being present on both the apoprotein (ApoB) and the phospholipid components of LDL. By utilizing an AGE-specific ELISA, we measured the AGE attached to the ApoB and lipid components of LDL from normal controls and diabetic patients with or without end-stage renal disease (ESRD), as well as lipid oxidation. AGE-ApoB, AGE-lipid and oxidized LDL (Ox-LDL) in diabetic patients were significantly higher than those in patients without diabetes. The correlation between AGE-ApoB and AGE-lipid were highly significant. An especially marked elevation of AGE-LDL was found in diabetic patients with ESRD. The correlation between the serum total cholesterol and the AGE-LDL (AGE-ApoB and AGE-lipid) was significant. In addition, based on the known biological properties of AGE-modified peptide (AGE-peptide), we have proposed that these chemically reactive circulating AGE-peptides contribute to tissue injury by reattaching to susceptible target proteins both within and outside the vasculature, and that this process accelerates vascular pathology in diabetic patients. These data indicate that AGE-modified LDLs may represent a particularly atherogenic form of LDL, and AGE-LDLs as well as AGE-peptides are likely to contribute to the development of atherosclerosis in diabetic patients.

Advanced glycosylation end products in diabetic renal and vascular disease

An increasing body of experimental data supports the important, etiologic role of advanced glycosylation end products (AGEs) in the development of the renal and vascular complications of diabetes. Advanced glycosylation end products arise from glucose-derived Amadori products and act to increase vascular permeability, enhance protein and lipoprotein deposition, inactivate nitric oxide, and promote matrix protein synthesis and glomerular sclerosis. Loss of normal renal function increases the level of circulating plasma AGEs and contributes markedly to their ultimate tissue toxicity. Aminoguanidine, a recently developed pharmacologic inhibitor of advanced glycosylation, is presently undergoing phase II/III clinical trials in diabetic nephropathy and may offer a specific therapeutic modality for diminishing the formation and toxicity of AGEs.

Antibacterial activity of lysozyme and lactoferrin is inhibited by binding of advanced glycation-modified proteins to a conserved motif

Why diabetes is associated with abnormally high susceptibility to infection remains unknown, although two major antibacterial proteins, lysozyme and lactoferrin, have now been shown to specifically bind glucose-modified proteins bearing advanced glycation end products (AGEs). Exposure to AGE-modified proteins inhibits the enzymatic and bactericidal activity of lysozyme, and blocks the bacterial agglutination and bacterial killing activities of lactoferrin. Peptide mapping revealed a single AGE binding domain in lysozyme and two AGE binding domains in lactoferrin; each domain contains a 17- to 18- amino acid cysteine-bounded loop motif (CX15-16C) that is markedly hydrophilic. Synthetic peptides corresponding to these motifs in lysozyme and lactoferrin exhibited AGE binding activity, and similar domains are also present in other antimicrobial proteins. These results suggest that elevated levels of AGEs in tissues and serum of diabetic patients may inhibit endogenous antibacterial proteins by binding to this conserved AGE-binding cysteine-bounded domain 'ABCD' motif, thereby increasing susceptibility to bacterial infections in the diabetic population.

Identification of galectin-3 as a high-affinity binding protein for advanced glycation end products (AGE): a new member of the AGE-receptor complex

BACKGROUND

Advanced glycation end products (AGE), the reactive derivatives of nonenzymatic glucose-protein condensation reactions, are implicated in the multiorgan complications of diabetes and aging. An AGE-specific cellular receptor complex (AGE-R) mediating AGE removal as well as multiple biological responses has been identified. By screening an expression library using antibody against a previously identified component of the AGE-R complex p90, a known partial cDNA clone was isolated with homology to galectin-3, a protein of diverse identity, and member of the galectin family.

MATERIALS AND METHODS

To explore this unexpected finding, the nature of the interactions between galectin-3 and AGE was studied using intact macrophage-like RAW 264.7 cells, membrane-associated and recombinant galectin-1 through -4, and model AGE-ligands (AGE-BSA, FFI-BSA).

RESULTS

Among the members of this family (galectin-1 through 4), recombinant rat galectin-3 was found to exhibit high-affinity 125I-AGE-BSA binding with saturable kinetics (kD 3.5 x 10(7) M-1) that was fully blocked by excess unlabeled naturally formed AGE-BSA or synthetic FFI-BSA, but only weakly inhibited by several known galectin-3 ligands, such as lactose. In addition to the p90, immunoprecipitation with anti-galectin-3, followed by 125I-AGE-BSA ligand blot analysis of RAW 264.7 cell extracts, revealed galectin-3 (28 and 32 kD), as well as galectin-3-associated proteins (40 and 50 kD) with AGE-binding activity. Interaction of galectin-3 with AGE-BSA or FFI-BSA resulted in formation of SDS-, and beta-mercaptoethanol-insoluble, but hydroxylamine-sensitive high-molecular weight complexes between AGE-ligand, galectin-3, and other membrane components.

CONCLUSIONS

The findings point toward a mechanism by which galectin-3 may serve in the assembly of AGE-R components and in the efficient cell surface attachment and endocytosis by macrophages of a heterogenous pool of AGE moieties with diverse affinities, thus contributing to the elimination of these pathogenic substances.