Isolation and characterization of two binding proteins for advanced glycosylation end products from bovine lung which are present on the endothelial cell surface

Beta 2-microglobulin modified with advanced glycation end products modulates collagen synthesis by human fibroblasts

Beta 2-microglobulin amyloidosis (A beta 2m) is a serious complication for patients undergoing long-term dialysis. beta 2-microglobulin modified with advanced glycation end products (beta 2m-AGE) is a major component of the amyloid in A beta 2m. It is not completely understood whether beta 2m-AGE plays an active role in the pathogenesis of A beta 2m, or if its presence is a secondary event of the disease. beta 2-microglobulin amyloid is mainly located in tendon and osteo-articular structures that are rich in collagen, and local fibroblasts constitute the principal cell population in the synthesis and metabolism of collagen. Recent identification of AGE binding proteins on human fibroblasts lead to the hypothesis that the fibroblast may be a target for the biological action of beta 2m-AGE. The present study demonstrated that two human fibroblast cell lines exhibited a decrease in procollagen type I mRNA and type I collagen synthesis after exposure to beta 2m-AGE for 72 hours. Similar results were observed using AGE-modified albumin. Antibody against the RAGE, the receptor for AGE, attenuated this decrease in synthesis, indicating that the response was partially mediated by RAGE. In addition, antibody against epidermal growth factor (EGF) attenuated the decrease in type I procollagen mRNA and type I collagen induced by beta 2m-AGE, suggesting that EGF acts as an intermediate factor. These findings support the hypothesis that beta 2m-AGE actively participates in connective tissue and bone remodeling via a pathway involving fibroblast RAGE, and at least one interposed mediator, the growth factor EGF.

Ribozyme targeting of receptor for advanced glycation end products in mouse mesangial cells

Accumulation of extracellular matrix is a characteristic of diabetic nephropathy, and advanced glycation end products (AGEs) are considered to play an important role in the mechanism. To investigate the involvement of the receptor for AGE (RAGE) in upregulation of type IV collagen by AGEs, we applied the hammerhead ribozyme for targeting RAGE. We established a stable mouse mesangial cell line that produces the RAGE-specific ribozyme (Rz-RAGE). Both the RAGE mRNA and protein were decreased in the cell line. The amount of type IV collagen mRNA increased by AGEs' treatment in control cells. In contrast, the increase of type IV collagen induced by AGEs was not observed in the Rz-RAGE-producing cells. We conclude that the induction of type IV collagen by AGEs is mediated by RAGE and this mechanism could be involved in diabetic nephropathy. This study also suggested the experimental/therapeutic potential of hammerhead ribozymes.

Advanced glycation end product (AGE)-mediated induction of tissue factor in cultured endothelial cells is dependent on RAGE

BACKGROUND

Binding of advanced glycation end products (AGEs) to the cellular surface receptor (RAGE) induces translocation of the transcription factor NF-kappaB into the nucleus and NF-kappaB-mediated gene expression. This study examines the role of RAGE in the AGE albumin-mediated induction of endothelial tissue factor, known to be partly controlled by NF-kappaB.

METHODS AND RESULTS

Endothelial cells (ECs) were incubated in the presence of an 18-mer phosphorothioate oligodeoxynucleotide antisense to the 5'-coding sequence of the RAGE gene (antisense RAGE; 0.1 micromol/L). Sense oligonucleotides (sense RAGE, 0.1 micromol/L) of the same region served as control. The cellular uptake of oligonucleotides was controlled by immunofluorescence microscopy. RAGE transcription was suppressed by antisense RAGE, as demonstrated by RT-PCR reactions. AGE albumin-mediated activation of cultured ECs was studied after 48 hours of preincubation of ECs with antisense or sense RAGE. Electrophoretic mobility shift assays and Western blot analysis demonstrated that the AGE albumin-induced translocation of NF-kappaB from the cytoplasm into the nucleus was suppressed in the presence of antisense RAGE but not by sense RAGE. In parallel, AGE albumin-mediated tissue factor transcription, activity, and antigen were significantly reduced in ECs exposed to antisense RAGE, whereas sense RAGE (and nonspecific oligonucleotides) did not influence tissue factor expression.

CONCLUSIONS

Activation of ECs and induction of tissue factor by AGE albumin in ECs is dependent on RAGE.

Beta amyloid toxicity does not require RAGE protein

It has been suggested that a receptor for advanced glycation end products (RAGE) is the nerve cell receptor for amyloid beta protein (A beta). To determine if this is indeed the case, two neural cell lines as well as rat cortical neurons were examined for the presence of the mRNA for RAGE by PCR and northern blot analysis. Although lung was strongly positive, in no case was RAGE mRNA detected in the cultured neural cells. Glycated-albumin is a major ligand for RAGE and the cell surface RAGE protein is trypsin sensitive. In agreement with the mRNA data, trypsin treatment did not alter A beta toxicity, nor did glycated albumin modify the A beta response. It follows that RAGE is not the neural receptor for A beta.

Activation of the receptor for advanced glycation end products triggers a p21(ras)-dependent mitogen-activated protein kinase pathway regulated by oxidant stress

Advanced glycation end products (AGEs) exert their cellular effects on cells by interacting with specific cellular receptors, the best characterized of which is the receptor for AGE (RAGE). The transductional processes by which RAGE ligation transmits signals to the nuclei of cells is unknown and was investigated. AGE-albumin, a prototypic ligand, activated p21(ras) in rat pulmonary artery smooth muscle cells that express RAGE, whereas nonglycated albumin was without effect. MAP kinase activity was enhanced at concentrations of AGE-albumin, which activated p21(ras) and NF-kappaB. Depletion of intracellular glutathione rendered cells more sensitive to AGE-mediated activation of this signaling pathway. In contrast, signaling was blocked by preventing p21(ras) from associating with the plasma membrane or mutating Cys118 on p21(ras) to Ser. Signaling was receptor-dependent, because it was prevented by blocking access to RAGE with either anti-RAGE IgG or by excess soluble RAGE. These data suggest that RAGE-mediated induction of cellular oxidant stress triggers a cascade of intracellular signals involving p21(ras) and MAP kinase, culminating in transcription factor activation. The molecular mechanism that triggers this pathway likely involves oxidant modification and activation of p21(ras).

Advanced glycation endproducts stimulate the MAP-kinase pathway in tubulus cell line LLC-PK1

Advanced glycation endproducts (AGEs) are suggested to play an important role in diabetic nephropathy. They induce specific cellular responses such as the release of cytokines in different cell lines. The effect of AGEs on signal transduction pathways was investigated in the renal tubulus cell line LLC-PK1. Using a serine-phosphate-specific antibody AGE-induced cellular responses associated with phosphorylation/dephosphorylation events were demonstrated. In particular, the p42MAP kinase and its downstream target, the AP-1 complex, are shown to be activated by AGE-BSA but not by BSA. In contrast, only partial phosphorylation is observed for the p70S6-kinase. Thus, AGEs appear to induce specific signal transduction pathways.

Characterization and functional analysis of the promoter of RAGE, the receptor for advanced glycation end products

The receptor for advanced glycation end products, RAGE, is a member of the immunoglobulin superfamily of cell surface molecules differentially expressed on a range of cell types. Ligation of RAGE perturbs homeostatic mechanisms and, potentially, provides a basis for cellular dysfunction in pathologic situations in which its ligands accumulate. To understand factors underlying RAGE expression, we cloned the 5'-flanking region of the RAGE gene and characterized putative regulatory motifs. Analysis of the putative promoter region revealed the presence of three potential NF-kappaB-like and two SP1 binding sites. Transient transfection of vascular endothelial and smooth muscle cells using chimeric 5'-deletion constructs linked to luciferase reporter revealed that the region -1543/-587 contributed importantly to both basal and stimulated expression of the RAGE gene. This region of the RAGE gene contained three putative NF-kappaB-like binding sites and was responsible for increased luciferase activity observed when endothelial or smooth muscle cells were stimulated with lipopolysaccharide. DNase I footprinting assays and electrophoretic mobility shift assay revealed that two of the three NF-kappaB-like binding sites (1 and 2) were likely functional and responsive to stimuli. Upon simultaneous mutation of NF-kappaB-like sites 1 and 2, both basal promoter expression and response to stimulation with LPS, as measured by relative luciferase activity, were significantly diminished. These results point to NF-kappaB-dependent mechanisms regulating cellular expression of RAGE and suggest a means of linking RAGE to the inflammatory response.

Advanced glycation end products in serum predict changes in the kidney morphology of patients with insulin-dependent diabetes mellitus

The biochemical mechanisms that cause the development and progression of diabetic nephropathy are unknown. Advanced glycation end products (AGEs) might play a role, as shown by increased levels of tissue-bound and circulating AGEs that correlate with the severity of diabetic nephropathy. The aim of the present study was to investigate if circulating AGEs predict the progression of morphological pathology in patients with diabetic nephropathy. We have developed an immunoassay to determine serum levels of AGEs. In a prospective clinical trial of young insulin-dependent diabetes mellitus (IDDM) patients with microalbuminuria, kidney biopsies were taken at baseline and after 24 to 36 months. The biopsies were analyzed for structural changes in the glomeruli by quantitative morphometry (electron microscopy). We have retrospectively analyzed serum AGEs. The mean serum level of AGEs at the start of the study was 18.7 U/mL (95% confidence interval [CI], 16.9 to 20.5). A positive correlation between serum AGE levels at the start of study and changes from baseline to follow-up study in basement membrane thickness (r = .56, P < .02) and matrix/glomerular volume fraction (r = .57, P < .02) was demonstrated. In a stepwise regression analysis with changes in the matrix/glomerular volume fraction as the dependent variable, serum AGE levels at the start of the study proved to be a significant independent variable (P < .02), whereas the mean hemoglobin A1c (HbA1c) or HbA1c at the start was not. This study shows that serum AGEs predict the progression of early morphological kidney damage during 2.5 years in patients with IDDM.

A role for macrophage scavenger receptors in atherosclerosis and susceptibility to infection

Macrophage type-I and type-II class-A scavenger receptors (MSR-A) are implicated in the pathological deposition of cholesterol during atherogenesis as a result of receptor-mediated uptake of modified low-density lipoproteins (mLDL). MSR-A can bind an extraordinarily wide range of ligands, including bacterial pathogens, and also mediates cation-independent macrophage adhesion in vitro. Here we show that targeted disruption of the MSR-A gene in mice results in a reduction in the size of atherosclerotic lesions in an animal deficient in apolipoprotein E. Macrophages from MSR-A-deficient mice show a marked decrease in mLDL uptake in vitro, whereas mLDL clearance from plasma occurs at a normal rate, indicating that there may be alternative mechanisms for removing mLDL from the circulation. In addition, MSR-A-knockout mice show an increased susceptibility to infection with Listeria monocytogenes or herpes simplex virus type-1, indicating that MSR-A may play a part in host defence against pathogens.

Accumulation of pyrraline-modified albumin in phagocytes due to reduced degradation by lysosomal enzymes

Previous studies suggested that the interaction between proteins modified by advanced glycation end products (AGEs) and cells, such as macrophages, may be involved in diabetic angiopathy. Pyrraline is one of the AGEs and known to be elevated in plasma of diabetic rats and humans, and is present in vascular lesions of diabetic and elderly subjects. We examined whether modification of albumin by pyrraline influences its degradation by macrophage-like cell line, P388D1 cells. Degradation of pyrraline-modified albumin by these cells was diminished, causing accumulation of the albumin in these cells. The susceptibility of pyrraline-modified albumin to lysosomal proteolytic enzymes was reduced by approximately 40% in vitro, while lysosomal activity in the cells per se was not affected. This phenomenon was also observed when human monocytes were used instead of P388D1 cells. Our results suggest that accumulation of pyrraline-modified albumin in P388D1 cells is due to the reduced susceptibility of the protein to lysosomal enzymatic degradation. Such alterations in the interaction between AGEs-modified protein and phagocytes may contribute to angiopathy in elderly subjects and patients with diabetes.

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

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

Glycation of collagen: the basis of its central role in the late complications of ageing and diabetes

The most serious late complication of ageing and diabetes mellitus follow similar patterns in the dysfunction of retinal capillaries, renal tissue, and the cardiovascular system. The changes are accelerated in diabetic patients owing to hyerglycaemia and are the major cause of premature morbidity and mortality. These tissues and their optimal functioning are dependent on the integrity of their supporting framework of collagen. It is the modification of the properties by glycation that results in many of the damaging late complications. Initially glycation affects the interactions of collagen with cells and other matrix components, but the most damaging effects are caused by the formation of glucose-mediated intermolecular cross-links. These cross-links decrease the critical flexibility and permeability of the tissues and reduce turnover. In contrast to the renal and retinal tissue, the cardiovascular system also contains a significant proportion of other fibrous connective tissue protein elastin, and its properties are similarly modified by glycation. The nature of these glycation cross-links is now being unravelled and this knowledge is crucial in any attempt to inhibit these deleterious glycation reactions.

Advanced glycation end products modulate transcriptional regulation in mesangial cells

Advanced glycation end products (AGEs) stimulate synthesis of extracellular matrix (ECM) in a receptor-mediated manner on mesangial cells. In the present study, we examined the transcriptional regulation of the gene for type IV collagen [(IV)collagen], which is one of the major components of mesangial sclerosis, after stimulation of AGEs on mesangial cells. The methylation pattern of the promoter/enhancer region of (IV)collagen gene was similar in AGE-treated and control cells. AGEs significantly increased the transcriptional activity of the (IV)collagen gene, as measured by transient transfection assays using the reporter gene construct containing (IV)collagen promoter/enhancer and the chloramphenicol acetyltransferase gene. AGEs also increased smooth muscle alpha-actin mRNA levels as well as its transcriptional activity. Nuclear factor binding of the promoter of (IV)collagen gene was stimulated by AGEs. Furthermore, AGEs dramatically decreased the mRNA levels of (IV)collagen promoter binding protein (MSW), a larger subunit of DNA replication complex, AP1. These results suggest that AGEs increase expression of (IV)collagen gene by modulating the levels of promoter binding proteins. These transcriptional events may play a critical role in ECM accumulation in response to AGEs.

Advanced glycation endproducts (AGEs) induce oxidant stress in the gingiva: a potential mechanism underlying accelerated periodontal disease associated with diabetes

We hypothesized that one mechanism underlying advanced periodontal disease in diabetes may involve oxidant stress in the gingiva, induced by the effects of Advanced Glycation Endproducts (AGEs), the irreversible products of non-enzymatic glycation and oxidation of proteins and lipids which accumulate in diabetic plasma and tissue. Infusion of AGE albumin, a prototypic ligand, into mice resulted in increased generation of thiobarbituric acid reactive substances (TBARS) compared with infusion of non-glycated albumin in the gingiva, as well as in the lung, kidney and brain. Pretreatment of the animals with the antioxidants probucol or N-acetylcysteine (NAC) prevented the generation of TBARS in the gingiva. Affinity-purified antibody to AGEs demonstrated increased immunoreactivity for AGEs in the vasculature and connective tissues of the gingiva in streptozotocin-induced diabetic mice compared to non-diabetic controls. Increased immunoreactivity for AGEs was also demonstrated in the gingiva of diabetic humans compared with non-diabetic individuals via immunohistochemistry and ELISA. Consistent with these data, immunohistochemistry for heme oxygenase-1, a marker of enhanced oxidant stress, was increased in the gingival vasculature of diabetic mice and humans compared with non-diabetic controls. These data suggest that AGEs present in diabetic gingiva may be associated with a state of enhanced oxidant stress, a potential mechanism for accelerated tissue injury.

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.

RAGE and amyloid-beta peptide neurotoxicity in Alzheimer's disease

Amyloid-beta peptide is central to the pathology of Alzheimer's disease, because it is neurotoxic--directly by inducing oxidant stress, and indirectly by activating microglia. A specific cell-surface acceptor site that could focus its effects on target cells has been postulated but not identified. Here we present evidence that the 'receptor for advanced glycation end products' (RAGE) is such a receptor, and that it mediates effects of the peptide on neurons and microglia. Increased expressing of RAGE in Alzheimer's disease brain indicates that it is relevant to the pathogenesis of neuronal dysfunction and death.

Advanced glycation endproducts-associated parameters in the peripheral blood of patients with Alzheimer's disease

Advanced glycation endproducts (AGEs), structural components of beta-amyloid plaques and neurofibrillary tangels, have been implicated in the pathogenesis of Alzheimer's disease. AGE levels, measured by fluorescence, and their precursor molecules such as glucose and its Amadori product, fructosylamine, were measured to examine the question whether the reported increased level of AGEs in the brain is reflected in an increase in AGE-associated parameters in peripheral blood. Lactoferrin, proposed to play an important role in the interaction of AGEs with their receptors, was determined by ELISA. All AGE-associated parameters showed trends to lower values in patients with Alzheimer's disease compared with non-demented controls. Albumin and total iron were not significantly different between the groups. In contrast to diabetes and renal failure, where high levels of AGEs and their precursors are present in tissue as well as in peripheral blood, elevated CNS AGE levels in patients with Alzheimer's disease are manifested without detectable peripheral changes.

The cellular and molecular mechanisms of diabetic complications

In this article, the cellular and molecular mechanisms of diabetic complications have been reviewed. Hyperglycemia-induced mechanisms that may induce vascular dysfunction in specific sites of diabetic microvascular damage include increased polyol pathway flux, altered cellular redox state, increased formation of diacylglycerol and the subsequent activation of specific PKC isoforms, and accelerated nonenzymatic formation of advanced glycation endproducts. Several of these mechanisms may be responsible for the potentially damaging overproduction of reactive oxygen species observed with hyperglycemia. Each of these mechanisms may contribute to the known pathophysiologic features of diabetic complications by a number of mechanisms, including the upregulation of cytokines and growth factors. Diabetic macrovascular disease may arise more from insulin resistance than from hyperglycemia, and the authors speculate that this may reflect a selective loss of insulin-dependent vascular homeostasis.

Induction of synthesis and secretion of interleukin 1 beta in the human monocytic THP-1 cells by human serum albumins modified with methylglyoxal and advanced glycation endproducts

Human serum albumin modified with 1-2 methylglyoxal residues per molecule of protein (MGmin-HSA) stimulated the synthesis and secretion of interleukin 1 beta (IL-1 beta) from human monocytic THP-1 cells in vitro. It was a more potent inducer of IL-1 beta synthesis than human serum albumin highly-modified with glucose-derived advanced glycation endproducts (AGE-HSA). With 20 microM ligand. IL-1 beta synthesis was (pg/10(6) cells): MGmin-HSA 484.5 +/- 50.3; AGE-HSA 30.6 +/- 2.0 (n = 3). IL-1 beta synthesis increased markedly with MGmin-HSA concentrations > 5 microM. IL-1 beta synthesis and secretion from monocytes in response to methylglyoxal-modified proteins in vivo may contribute to the development of macro- and micro-angiopathy, particularly in diabetes mellitus.

Renal fate of circulating advanced glycated end products (AGE): evidence for reabsorption and catabolism of AGE-peptides by renal proximal tubular cells

The presence of excessive amounts of advanced glycation end products (AGE) in tissues or in the circulation may critically affect the progression of diabetic nephropathy. Circulating AGE levels, mainly in the form of small peptides, increase in diabetic patients or in patients with end-stage renal disease. This rise correlates with the severity of the nephropathy. However, so far little is known about the fate of AGE-proteins and AGE-peptides in renal tissue, and in order to elucidate this issue we undertook the present study. AGE-bovine serum albumin (AGE-BSA) and AGE-peptides were prepared, characterized by spectrophotometry, spectrofluorometry, chromatography and SDS-PAGE. AGE-peptides reacted in vitro with LDL producing biochemical and ultrastructural modifications. Using colloidal gold post-embedding immunoelectron microscopy with an anti-AGE antibody generated in our laboratory, we followed, in a short-term kinetic study, the cellular and sub-cellular localisation of circulating AGE-products throughout the nephron. AGE-peptides or AGE-BSA were injected into otherwise normal rats and detected by protein A-gold immuno-cytochemistry after 15, 30 or 45 min of circulation. Most of the AGE-BSA was found in the lumen of capillary vessels and distributed along the endothelial side of the glomerular basement membrane. Presence on mesangial matrix was also apparent. AGE-peptides were easily filtered and actively reabsorbed by the proximal convoluted tubule. At 15 min, little labelling was found in the glomerular wall. Instead, the labelling was present in the urinary space and microvilli of epithelial cells. Early endosomes displayed intense labelling as well. At 45 min, late endosomes and lysosomes added to the pattern of labelling. The distal tubule epithelial cells were devoid of labelling for any of the intervals studied. AGE-peptides but not AGE-BSA could be detected in the urine of injected rats. These observations point to participation of the endo-lysosomal apparatus of the proximal convoluted tubule to the disposal of AGE-peptides, while giving an ultrastructural support for a key role of the kidney in AGE catabolism.

Advanced glycation end product (AGE): characterization of the products from the reaction between D-glucose and serum albumin

We incubated bovine serum albumin (BSA) with glucose in an attempt to study how the advanced glycation end products (AGEs) are formed and what methods can be used for their identification and isolation. The reaction was monitored by boronated affinity gel, size exclusion and ion exchange chromatography, and chromatofocusing. Reaction products were also characterized by fluorescence measurement, fructosamine assay, and polyacrylamide gel electrophoresis (PAGE). Based on the measurement of AGE-associated fluorescence (excitation, 370 nm; emission, 440 nm) we found that the AGEs could be detected as early as after 3 days incubation. The fluorescence was always associated with the larger molecules of cross-linking product resulting from the reaction between BSA and glucose. The overall fluorescence intensity increased with incubation time and fluorescence of the highest intensity was found with the AGE product largest in size. As with the Amadori product, AGEs also bind to the boronated gel column but with an even higher affinity. Compared to the original albumin monomer AGE molecules are not only larger in size but also have lower isoelectric points and carry more negative charges. Both the size and the negative charges of AGEs continue to increase over time during incubation. This results in a group of cross-linking molecules heterogeneous in size and charge. These results will aid in both the isolation and selection of appropriate AGE molecules for the preparation of anti-AGE antibodies, calibrator, and control in the development of an AGE immunoassay.

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.

The receptor for advanced glycation end products (RAGE) is a cellular binding site for amphoterin. Mediation of neurite outgrowth and co-expression of rage and amphoterin in the developing nervous system

The receptor for advanced glycation end products (RAGE), a newly-identified member of the immunoglobulin superfamily, mediates interactions of advanced glycation end product (AGE)-modified proteins with endothelium and other cell types. Survey of normal tissues demonstrated RAGE expression in situations in which accumulation of AGEs would be unexpected, leading to the hypothesis that under physiologic circumstances, RAGE might mediate interaction with ligands distinct from AGEs. Sequential chromatography of bovine lung extract identified polypeptides with M(r) values of approximately 12,000 (p12) and approximately 23,000 (p23) which bound RAGE. NH2-terminal and internal protein sequence data for p23 matched that reported previously for amphoterin. Amphoterin purified from rat brain or recombinant rat amphoterin bound to purified sRAGE in a saturable and dose-dependent manner, blocked by anti-RAGE IgG or a soluble form of RAGE (sRAGE). Cultured embryonic rat neurons, which express RAGE, displayed dose-dependent binding of 125I-amphoterin which was prevented by blockade of RAGE using antibody to the receptor or excess soluble receptor (sRAGE). A functional correlate of RAGE-amphoterin interaction was inhibition by anti-RAGE F(ab')2 and sRAGE of neurite formation by cortical neurons specifically on amphoterin-coated substrates. Consistent with a potential role for RAGE-amphoterin interaction in development, amphoterin and RAGE mRNA/antigen were co-localized in developing rat brain. These data indicate that RAGE has physiologically relevant ligands distinct from AGEs which are likely, via their interaction with the receptor, to participate in physiologic processes outside of the context of diabetes and accumulation 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.

Macrophage scavenger receptor mediates the endocytic uptake and degradation of advanced glycation end products of the Maillard reaction

Modification of proteins by long-term incubation with glucose leads to the formation of advanced glycation end products (AGE). Recent immunological demonstration of the presence of AGE proteins in several human tissues suggests that they may be involved in aging, diabetic complications and atherosclerosis. AGE proteins are taken up by macrophages via the AGE receptor, which is similar to the macrophage scavenger receptor (MSR). In the present study, we examined whether MSR could mediate the endocytic uptake of AGE proteins by using Chinese hamster ovary (CHO) cells overexpressing bovine type II MSR (CHO-SRII). 125I-labelled AGE bovine serum albumin (125I-AGE-BSA) as well as 125I-acetylated low-density lipoprotein (125I-acetyl-LDL) underwent endocytic degradation by CHO-SRII cells, but not by control CHO cells. Endocytic degradation of 125I-acetyl-LDL and 125I-AGE-BSA by CHO-SRII cells was significantly inhibited by unlabeled AGE-BSA, as well as by acetyl-LDL. Immunoelectron microscopic studies using both AGE-BSA conjugated with gold particles and anti-(bovine MSR) antibody (D2) revealed co-localization of gold particles and the reactive sites for the antibody at coated pits of plasma membranes as well as in endosomes. These results clearly show that MSR mediates the endocytic uptake and degradation of AGE proteins, suggesting a new role of MSR in biological recognition of AGE in vivo.

Calreticulin, an antithrombotic agent which binds to vitamin K-dependent coagulation factors, stimulates endothelial nitric oxide production, and limits thrombosis in canine coronary arteries

Coagulation Factor IX/IXa has been shown to bind to cellular surfaces, and Factor IXa expresses its procoagulant activity by assembling into the intrinsic Factor X activating complex (Factors IXa/VIIIa/X), which also forms on membrane surfaces. This led us to identify cellular proteins which bind Factor IX/IXa; an approximately 55-kDa polypeptide was purified to homogeneity from bovine lung extracts based on its capacity to bind 125I-Factor IX in a dose-dependent and saturable manner. From protein sequence data of the amino terminus and internal peptides, the approximately 55-kDa polypeptide was identified as calreticulin, a previously identified intracellular calcium-binding protein. Recombinant calreticulin bound vitamin K-dependent coagulation factors, 125I-Factor IX, 125I-Factor X, and 125I-prothrombin (Kd values of approximately 2.7, 3.2, and 8.3 nM, respectively), via interaction with its C-domain, although it did not affect the coagulant properties of these proteins. 125I-Calreticulin also bound to endothelial cells in vitro (Kd approximately 7.4 nM), and mouse infusion studies showed an initial rapid phase of clearance in which calreticulin could be localized on the vascular endothelium. Exposure of endothelial cells to calreticulin led to dose-dependent, immediate, and sustained increase in the production of nitric oxide, as measured using a porphyrinic microsensor. In a canine electrically induced thrombosis model, intracoronary infusion of calreticulin (n = 7) prevented occlusion of the left circumflex coronary artery in a dose-dependent manner compared with vehicle-treated controls (n = 5). These results indicate that calreticulin interacts with the endothelium to stimulate release of nitric oxide and inhibit clot formation.

Diabetic late complications: will aldose reductase inhibitors or inhibitors of advanced glycosylation endproduct formation hold promise?

Patients suffering from the severe complications associated with both insulin- (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM): nephropathy, retinopathy, neuropathy, and atherosclerosis are still largely left without a prospect of an efficient treatment. This is the case even if it has been assumed for decades and now finally proved by the results from the Diabetes Control and Complications Trial (DCCT) that hyperglycemia is the single main cause of these complications. Improved glycemic control as a result of intensive insulin treatment has the potential to reduce the incidence and progression of complications, but implementation and monitoring of improved glycemic control in all groups of IDDM and NIDDM patients in different communities will be difficult and expensive. Results from the recently terminated DCCT have shown that even with intensive insulin treatment, there will be a significant burden of complications on the diabetic population. It will, therefore, still be of immense importance for the long-term quality of life for the diabetic patient that additional possibilities are developed for prevention and intervention against diabetic complications. Almost two decades of research, animal model testing, and clinical trials have been conducted on various efficient aldose reductase inhibitors. Now the concept of inhibition of formation of advanced glycosylation endproducts on proteins and lipids resulting from extra- and intracellular hyperglycemia is entering the scene as an alternative or perhaps supplementary approach to reduce the occurrence of diabetic complications. An overview of the results from these two fields of research and associated drug-development programs will be presented along with thoughts on possible future developments.

Glycated albumin modified by Amadori adducts modulates aortic endothelial cell biology

Increased protein glycation has been mechanistically linked to accelerated vascular pathobiology in diabetes. To test the influence of protein modified by Amadori glucose adducts on vascular cell biology, we examined the effect of glycated albumin on replicative capacity and basement membrane collagen production by aortic endothelial cells in culture. Relative to carbohydrate-free albumin, which supported cell proliferation and Type IV collagen synthesis, glycated albumin significantly inhibited 3H-thymidine incorporation and Type IV collagen production. The glycated albumin-induced effects were prevented by monoclonal antibodies (A717) that specifically react with Amadori-modified albumin, but not by IgG that was unreactive with glycated albumin. A717 had no effect on thymidine incorporation or collagen synthesis by cells cultured in the presence of nonglycated albumin. The findings indicate that the interaction of glycated albumin with endothelial cells, which have been shown to display dose-responsive, saturable receptors, limits cell replication and triggers maladaptive biosynthetic programs, which may contribute to degenerative macrovascular disease in diabetes.

Advanced protein glycosylation in diabetes and aging

Products of advanced protein glycosylation (advanced glycation end products, or AGEs) accumulate in tissues as a function of time and sugar concentration. AGEs induce permanent abnormalities in extracellular matrix component function, stimulate cytokine and reactive oxygen species production through AGE-specific receptors, and modify intracellular proteins. Pharmacologic inhibition of AGE formation in long-term diabetic animals prevents diabetic retinopathy, nephropathy, neuropathy, and arterial abnormalities in animal models. Clinical trials in humans are currently in progress.

Age-dependent accumulation of advanced glycosylation end products in human neurons

Glucose can react nonenzymatically with free amino groups on proteins and form advanced glycosylation end-products (AGEPs), that have been previously isolated and characterised in aging human connective tissues. In this study, we used immunocytochemistry to examine the distribution of AGEPs in the aging human brain. Our findings show that the pyramidal neurons selectively accumulate AGEP-containing vesicles in an age-dependent manner. In addition, our results demonstrate that AGEPs accumulate in the same type of neuron that degenerates in Alzheimer's disease.

Cellular receptors for advanced glycation end products. Implications for induction of oxidant stress and cellular dysfunction in the pathogenesis of vascular lesions

Advanced glycation end products (AGEs) form by the interaction of aldoses with proteins and the subsequent molecular rearrangements of the covalently linked sugars, eventuating in a diverse group of fluorescent compounds of yellow-brown color. This heterogeneous class of nonenzymatically glycated proteins or lipids is found in the plasma and accumulates in the vessel wall and tissues even in normal aging. As a consequence of hyperglycemia, AGE formation and deposition are much enhanced in diabetes, in which their presence has been linked to secondary complications, especially microvascular disease. This review summarizes the cellular interactions of AGEs and describes the central role of a novel receptor for AGE (RAGE). RAGE, an immunoglobulin superfamily member, mediates the binding of AGEs to endothelial cells and mononuclear phagocytes, interacts with a lactoferrin-like polypeptide that also binds AGEs, and appears to activate intracellular signal transduction mechanisms consequent to its interaction with the glycated ligand. RAGE is expressed by ECs, mononuclear phagocytes, smooth muscle cells, mesangial cells, and neurons, indicating a potential role in the regulation of their properties in homeostasis and/or their dysfunction in the development of diabetic complications. Since AGEs have been shown to generate reactive oxygen intermediates, tethering of AGEs to the cell surface by their receptors focuses oxidant stress on cellular targets, resulting in changes in gene expression and the cellular phenotype. The discovery of RAGE and development of reagents to block its interaction with AGEs should provide insights into the role of this ligand-receptor interaction in the pathogenesis of diabetic complications and, potentially, atherosclerosis.

Topology of CNS myelin proteolipid protein: evidence for the nonenzymatic glycosylation of extracytoplasmic domains in normal and diabetic animals

Myelin proteolipid protein (PLP), the main integral membrane protein in the central nervous system myelin, was labeled at the extracytoplasmic domains with the membrane impermeant reagents pyridoxal 5'-phosphate and tritiated borohydride. Lysine-217, located in the fourth hydrophilic domain of PLP, was found to be the major labeled residue, which defined this domain to be extracytoplasmic in agreement with our previously proposed topological model. The remarkably high reactivity in vitro of this residue as compared to all other lysines in PLP led us to investigate the possible modification of PLP in vivo by other carbonyl compounds. We demonstrate that PLP is the most highly nonenzymatically glycosylated membrane protein in murine and bovine brain. The degree of modification increases significantly under hyperglycemic conditions, as studied in diabetic mice. The majority of the glycosylation sites are also located at extracytoplasmic domains. The degree of nonenzymatic glycosylation of PLP may be related to late diabetic complications affecting the central nervous system.

Binding sites for short-term glycated albumin on peritoneal cells of the rat

The interaction of in vitro short-term glycated rat serum albumin with rat peritoneal cells (40% macrophages) was investigated. Using 125I-labeled albumins the following results were obtained. Glycated albumins showed a binding reaction at 4 degrees C, which appeared to reach equilibrium within 2 h. The concentration-dependent binding of glycated albumin showed saturation. Binding data evaluated for glycated albumin using the Sips equation are: average association constant Ko = 3.15 x 10(7) M-1 with a heterogeneity index of a = 0.8 and 1.12 x 10(4) binding sites per cell. Such binding sites were identified in 40% of the peritoneal cell preparations studied. Native albumins, maleylated albumin, chondroitinsulfates, polylysine, lysine, fructose, glucose and hexitol-lysine could not compete with radio-labeled glycated rat albumin for its binding site on peritoneal cells. Effective competitors were glycated human serum albumin, glycated polylysine and fructose-lysine. Although the contamination with minute amounts of advanced glycosylation end products (AGE) could not be excluded, short-term glycated albumin was found to be bound to membranes of peritoneal phagocytotic cells by fructose-lysine specific proteins, whose approximately defined molecular masses of 290 kDa are distinct from hitherto described binding proteins for AGE- and aldehyde-modified proteins or for the scavenger receptors.