RAGE recycles at the plasma membrane in S100B secretory vesicles and promotes Schwann cells morphological changes

RAGE is a multiligand receptor of the immunoglobulin superfamily involved in regeneration of injured peripheral nerve and cell motility. RAGE is implicated in the development of various chronic diseases, such as neurodegenerative disorders, inflammatory responses, and diabetic complications. The correlation between RAGE endocytic trafficking and RAGE function is still uninvestigated. S100B is one of the ligands of RAGE. The molecular mechanisms responsible of S100B translocation in exocytic vesicles are still poorly investigated. In the present study we elucidate the role of RAGE endocytic trafficking in promoting S100B secretion in Schwann cells. Here we show that RAGE-induced secretion of S100B requires phosphorylated caveolin1-dependent endocytosis of RAGE. Endocytosis of RAGE in response to ligand binding promotes the fusion of endosomes with S100B-positive secretory vesicles. Src promotes the fusion of endosomes with S100B-secretory vesicles. Inhibition of src induces RAGE degradation. RAGE-mediated src activation induces cav1 phosphorylation and relocalization in the perinuclear compartment. RAGE signaling and recycling are required for S100-induced Schwann cells morphological changes and are inhibited by high-glucose, suggesting a possible link between diabetes and peripheral nerve injury. Indeed, high glucose inhibits RAGE-mediated src activation. Src inhibition blocks RAGE recycling, S100B secretion, and morphological changes. In summary, we identified a novel pathway of vesicular trafficking required for the amplification of RAGE signaling and cytoskeleton dynamics that is potentially involved in the regeneration of injured peripheral nerve.     

Inhibition of the receptor for advanced glycation endproducts (RAGE) protects pancreatic β-cells

Advanced glycation endproducts (AGEs) and the receptor for AGEs (RAGE) have been linked to the pathogenesis of diabetic complications, such as retinopathy, neuropathy, and nephropathy. AGEs may induce β-cell dysfunction and apoptosis, another complication of diabetes. However, the role of AGE-RAGE interaction in AGE-induced pancreatic β-cell failure has not been fully elucidated. In this study, we investigated whether AGE–RAGE interaction could mediate β-cell failure. We explored the potential mechanisms in insulin secreting (INS-1) cells from a pancreatic β-cell line, as well as primary rat islets. We found that glycated serum (GS) induced apoptosis in pancreatic β-cells in a dose- and time-dependent manner. Treatment with GS increased RAGE protein production in cultured INS-1 cells. GS treatment also decreased bcl-2 gene expression, followed by mitochondrial swelling, increased cytochrome c release, and caspase activation. RAGE antibody and knockdown of RAGE reversed the β-cell apoptosis and bcl-2 expression. Inhibition of RAGE prevented AGE-induced pancreatic β-cell apoptosis, but could not restore the function of glucose stimulated insulin secretion (GSIS) in rat islets. In summary, the results of the present study demonstrate that AGEs are integrally involved in RAGE-mediated apoptosis and impaired GSIS dysfunction in pancreatic β-cells. Inhibition of RAGE can effectively protect β-cells against AGE-induced apoptosis, but cannot reverse islet dysfunction in GSIS.     

Disordered osteoclast formation in RAGE-deficient mouse establishes an essential role for RAGE in diabetes related bone loss

The mechanisms underlying diabetes-mediated bone loss are not well defined. It has been reported that the advanced glycation endproducts (AGEs) and receptor for AGEs (RAGEs) are involved in diabetic complications. Here, mice deficient in RAGE were used as a model for investigating the effects of RAGE on bone mass. We found that RAGE-/- mice have a significantly increased bone mass and bone biomechanical strength and a decreased number of osteoclasts compared to wild-type mice. The serum levels of IL-6 and bone breakdown marker pyridinoline were significantly decreased in RAGE-/- mice. RAGE-/- mice maintain bone mass following ovariectomy, whereas wild-type mice lose bone mass. Furthermore, osteoclast-like cells do express RAGE mRNA. Our data therefore indicate that RAGE serves as a positive factor to regulate the osteoclast formation, directly implicates a role for RAGE in diabetes-promoted bone destruction, and documents that the AGE-RAGE interaction may account for diabetes associated bone loss.     

TAGE (toxic AGEs) theory in diabetic complications

Diabetic complication is a leading cause of acquired blindness, end-stage renal failure, a variety of neuropathies and accelerated atherosclerosis. Chronic hyperglycemia is initially involved in the pathogenesis of diabetic micro- and macro-vascular complications via various metabolic derangements. High glucose increased production of various types of advanced glycation end-products (AGEs). Recently, we found that glyceraldehyde-derived AGEs (AGE-2) play an important role in the pathogenesis of angiopathy in diabetic patients. There is considerable interest in receptor for AGEs (RAGE) found on many cell types, particularly those affected in diabetes. Recent studies suggest that interaction of AGE-2 (predominantly structure of toxic AGEs; TAGE) with RAGE alters intracellular signaling, gene expression, release of pro-inflamatory molecules and production of reactive oxygen species (ROS) that contribute towards the pathology of diabetic complications. We propose three pathways for the in vivo formation of AGE-2 precursor, glyceraldehyde, such as i) glycolytic pathway, ii) polyol pathway, and iii) fructose metabolic pathway. Glyceraldehyde can be transported or can leak passively across the plasma membrane. It can react non-enzymatically with proteins to lead to accelerated formation of TAGE at both intracellularly and extracellularly. In this review, we discuss the molecular mechanisms of diabetic complications, especially focusing on toxic AGEs (TAGE) and their receptor (RAGE) system.     

Advanced glycation endproducts: from precursors to RAGE: round and round we go

The formation of advanced glycation endproducts (AGEs) occurs in diverse settings such as diabetes, aging, renal failure, inflammation and hypoxia. The chief cellular receptor for AGEs, RAGE, transduces the effects of AGEs via signal transduction, at least in part via processes requiring the RAGE cytoplasmic domain binding partner, diaphanous-1 or mDia1. Data suggest that RAGE perpetuates the inflammatory signals initiated by AGEs via multiple mechanisms. AGE–RAGE interaction stimulates generation of reactive oxygen species and inflammation—mechanisms which enhance AGE formation. Further, recent data in type 1 diabetic kidney reveal that deletion of RAGE prevents methylglyoxal accumulation, at least in part via RAGE-dependent regulation of glyoxalase-1, a major enzyme involved in methylglyoxal detoxification. Taken together, these considerations place RAGE in the center of biochemical and molecular stresses that characterize the complications of diabetes and chronic disease. Stopping RAGE-dependent signaling may hold the key to interrupting cycles of cellular perturbation and tissue damage in these disorders.     

Advanced glycation endproducts and their pathogenic roles in neurological disorders

Glycation is implicated in neurological disorders. In some cases it plays a key role in the pathogenesis, in others it plays a co-adjuvant role or it appears as a consequence of degenerative changes and protein accumulation stemming from other pathways. In this work, we attempt to provide a concise, updated review of the major recent findings concerning glycation in neurological diseases. After a short introduction covering advanced glycation endproducts (AGEs) and the receptor for AGEs (RAGE), we will discuss the impact of glycation in central nervous system disorders including Alzheimer’s, Parkinson’s and Creutzfeldt–Jakob disease, as well as peripheral diabetic polyneuropathies. Therapies directed at lowering the concentrations of RAGE ligands including AGEs, blocking RAGE signaling, preventing oxidative stress or lowering methylglyoxal (MGO) levels may significantly decrease the development of AGE-related pathologies in patients with neurological disorders. Many drugs are on the pipeline and the future clinical trials will reveal if the promising results translate into clinical application.     

RAGE and soluble RAGE: potential therapeutic targets for cardiovascular diseases

Receptor for advanced glycation end-products (RAGE) is known to be involved in microvascular complications in diabetes. RAGE is also profoundly associated with macrovascular complications in diabetes through regulation of atherogenesis, angiogenic response, vascular injury, and inflammatory response. The potential significance of RAGE in the pathogenesis of cardiovascular disease appears not to be confined solely to nondiabetic rather than diabetic conditions. Numerous truncated forms of RAGE have recently been described, and the C-terminally truncated soluble form of RAGE has received much attention. Soluble RAGE consists of several forms, including endogenous secretory RAGE (esRAGE), which is a spliced variant of RAGE, and a shedded form derived from cell-surface RAGE. These heterogeneous forms of soluble RAGE, which carry all of the extracellular domains but are devoid of the transmembrane and intracytoplasmic domains, bind ligands including AGEs and can antagonize RAGE signaling in vitro and in vivo. ELISA systems have been developed to measure plasma esRAGE and total soluble RAGE, and the pathophysiological roles of soluble RAGE have begun to be unveiled clinically. In this review, we summarize recent findings regarding pathophysiological roles in cardiovascular disease of RAGE and soluble RAGE and discuss their potential usefulness as therapeutic targets and biomarkers for the disease.     

RAGE and its ligands in retinal disease

RAGE, the receptor for advanced glycation endproducts (AGEs), is a multiligand signal transduction receptor of the immunoglobulin superfamily of cell surface molecules that has been implicated in the pathogenesis of diabetic complications, neurodegenerative diseases, inflammatory disorders, and cancer. These diverse biologic disorders reflect the multiplicity of ligands capable of cellular interaction via RAGE that include, in addition to AGEs, amyloid-beta (Abeta) peptide, the S100/calgranulin family of proinflammatory cytokines, and amphoterin, a member of the High Mobility Group Box (HMGB) DNA-binding proteins. In the retina, RAGE expression is present in neural cells, the vasculature, and RPE cells, and it has also been detected in pathologic cellular retinal responses including epiretinal and neovascular membrane formation. Ligands for RAGE, in particular AGEs, have emerged as relevant to the pathogenesis of diabetic retinopathy and age-related macular disease. While the understanding of RAGE and its role in retinal dysfunction with aging, diabetes mellitus, and/or activation of pro-inflammatory pathways is less complete compared to other organ systems, increasing evidence indicates that RAGE can initiate and sustain significant cellular perturbations in the inner and outer retina. For these reasons, antagonism of RAGE interactions with its ligands may be a worthwhile therapeutic target in such seemingly disparate, visually threatening retinal diseases as diabetic retinopathy, age-related macular degeneration, and proliferative vitreoretinopathy.     

N(epsilon)-(carboxymethyl)lysine adducts of proteins are ligands for receptor for advanced glycation end products that activate cell signaling pathways and modulate gene expression.

Recent studies suggested that interruption of the interaction of advanced glycation end products (AGEs), with the signal-transducing receptor receptor for AGE (RAGE), by administration of the soluble, extracellular ligand-binding domain of RAGE, reversed vascular hyperpermeability and suppressed accelerated atherosclerosis in diabetic rodents. Since the precise molecular target of soluble RAGE in those settings was not elucidated, we tested the hypothesis that predominant specific AGEs within the tissues in disorders such as diabetes and renal failure, N(epsilon)-(carboxymethyl)lysine (CML) adducts, are ligands of RAGE. We demonstrate here that physiologically relevant CML modifications of proteins engage cellular RAGE, thereby activating key cell signaling pathways such as NF-kappaB and modulating gene expression. Thus, CML-RAGE interaction triggers processes intimately linked to accelerated vascular and inflammatory complications that typify disorders in which inflammation is an established component.     

Peripheral Levels of AGEs and Astrocyte Alterations in the Hippocampus of STZ-Diabetic Rats

Diabetic patients and streptozotocin (STZ)-induced diabetes mellitus (DM) models exhibit signals of brain dysfunction, evidenced by neuronal damage and memory impairment. Astrocytes surrounding capillaries and synapses modulate many brain activities that are connected to neuronal function, such as nutrient flux and glutamatergic neurotransmission. As such, cognitive changes observed in diabetic patients and experimental models could be related to astroglial alterations. Herein, we investigate specific astrocyte changes in the rat hippocampus in a model of DM induced by STZ, particularly looking at glial fibrillary acidic protein (GFAP), S100B protein and glutamate uptake, as well as the content of advanced glycated end products (AGEs) in serum and cerebrospinal fluid (CSF), as a consequence of elevated hyperglycemia and the content of receptor for AGEs in the hippocampus. We found clear peripheral alterations, including hyperglycemia, low levels of proinsulin C-peptide, elevated levels of AGEs in serum and CSF, as well as an increase in RAGE in hippocampal tissue. We found specific astroglial abnormalities in this brain region, such as reduced S100B content, reduced glutamate uptake and increased S100B secretion, which were not accompanied by changes in GFAP. We also observed an increase in the glucose transporter, GLUT-1. All these changes may result from RAGE-induced inflammation; these astroglial alterations together with the reduced content of GluN1, a subunit of the NMDA receptor, in the hippocampus may be associated with the impairment of glutamatergic communication in diabetic rats. These findings contribute to understanding the cognitive deficits in diabetic patients and experimental models.     

Advanced glycation end products impair function of late endothelial progenitor cells through effects on protein kinase Akt and cyclooxygenase-2

Endothelial progenitor cells (EPCs) exhibit impaired function in the context of diabetes, and advanced glycation end products (AGEs), which accumulate in diabetes, may contribute to this. In the present study, we investigated the mechanism by which AGEs impair late EPC function. EPCs from human umbilical cord blood were isolated, and incubated with AGE-modified albumin (AGE-albumin) at different concentrations found physiologically in plasma. Apoptosis, migration, and tube formation assays were used to evaluate EPC function including capacity for vasculogenesis, and expression of the receptor for AGEs (RAGE), Akt, endothelial nitric oxide synthase (eNOS), and cycloxygenase-2 (COX-2) were determined. Anti-RAGE antibody was used to block RAGE function. AGE-albumin concentration-dependently enhanced apoptosis and depressed migration and tube formation, but did not affect proliferation, of late EPCs. High AGE-albumin increased RAGE mRNA and protein expression, and decreased Akt and COX-2 protein expression, whilst having no effect on eNOS mRNA or protein in these cells. These effects were inhibited by co-incubation with anti-RAGE antibody. These results suggest that RAGE mediates the AGE-induced impairment of late EPC function, through down-regulation of Akt and COX-2 in these cells.     

Advanced glycation and endothelial functions: a link towards vascular complications in diabetes

The formation of advanced glycation end-products (AGEs), also called the Maillard reaction, occurs ubiquitously and irreversibly in patients with diabetes mellitus, and its consequences are especially relevant to vascular dysfunctions. The interaction of AGEs with their receptors (RAGE) has been implicated in the development of vascular complications. This interaction elicits remarkable vascular cell changes analogous to those observed in diabetes mellitus, including angiogenic and thrombogenic responses of endothelial cells, increased oxidative stress, and functional alterations in vascular tone control. This review focuses on AGEs formation, the interaction with their specific receptors and how the triggered intracellular events determine functional alterations of vascular endothelium. Finally, some potential pharmacological approaches undertaken to circumvent the deleterious effects of AGEs are also discussed.     

Receptor for advanced glycation end products is subjected to protein ectodomain shedding by metalloproteinases

The receptor for advanced glycation end products (RAGE) is a 55-kDa type I membrane glycoprotein of the immunoglobulin superfamily. Ligand-induced up-regulation of RAGE is involved in various pathophysiological processes, including late diabetic complications and Alzheimer disease. Application of recombinant soluble RAGE has been shown to block RAGE-mediated pathophysiological conditions. After expression of full-length RAGE in HEK cells we identified a 48-kDa soluble RAGE form (sRAGE) in the culture medium. This variant of RAGE is smaller than a 51-kDa soluble version derived from alternative splicing. The release of sRAGE can be induced by the phorbol ester PMA and the calcium ionophore calcimycin via calcium-dependent protein kinase C subtypes. Hydroxamic acid-based metalloproteinase inhibitors block the release of sRAGE, and by RNA interference experiments we identified ADAM10 and MMP9 to be involved in RAGE shedding. In protein biotinylation experiments we show that membrane-anchored full-length RAGE is the precursor of sRAGE and that sRAGE is efficiently released from the cell surface. We identified cleavage of RAGE to occur close to the cell membrane. Ectodomain shedding of RAGE simultaneously generates sRAGE and a membrane-anchored C-terminal RAGE fragment (RAGE-CTF). The amount of RAGE-CTF increases when RAGE-expressing cells are treated with a gamma-secretase inhibitor, suggesting that RAGE-CTF is normally further processed by gamma-secretase. Identification of these novel mechanisms involved in regulating the availability of cell surface-located RAGE and its soluble ectodomain may influence further research in RAGE-mediated processes in cell biology and pathophysiology.     

Identification of pyridinoline,a collagen crosslink,as a novel intrinsic ligand for the receptor for advanced glycation end-products (RAGE)

Advanced glycation end-products (AGEs) elicit inflammatory responses via the receptor for AGEs (RAGE) and participate in the pathogenesis of diabetic complications. An earlier study showed that 3-hydroxypyridinium (3-HP), a common moiety of toxic AGEs such as glyceraldehyde-derived pyridinium (GLAP) and GA-pyridine, is essential for the interaction with RAGE. However, the physiological significance of 3-HP recognition by RAGE remains unclear. We hypothesized that pyridinoline (Pyr), a collagen crosslink containing the 3-HP moiety, could have agonist activity with RAGE. To test this hypothesis, we purified Pyr from bovine achilles tendons and examined its cytotoxicity to rat neuronal PC12 cells. Pyr elicited toxicity to PC12 cells in a concentration-dependent manner, and this effect was attenuated in the presence of either the anti-RAGE antibody or the soluble form of RAGE. Moreover, surface plasmon resonance-based analysis showed specific binding of Pyr to RAGE. These data indicate that Pyr is an intrinsic ligand for RAGE.

Abbreviations: AGEs: advanced glycation end-products; RAGE: receptor for advanced glycation end-products; DAMPs: damage-associated molecular patterns; PRR: pattern recognition receptor; TLR: toll-like receptor; GLAP: glyceraldehyde-derived pyridinium; 3-HP: 3-hydroxypyridinium; Pyr: pyridinoline; HFBA: heptafluorobutyric acid; GST: glutathione S-transferase; SPR: surface plasmon resonance; ECM: extracellular matrix; EMT: epithelial to mesenchymal transition     

Advanced glycation end product ligands for the receptor for advanced glycation end products: biochemical characterization and formation kinetics

Advanced glycation end products (AGEs) accumulate with age and at an accelerated rate in diabetes. AGEs bind cell-surface receptors including the receptor for advanced glycation end products (RAGE). The dependence of RAGE binding on specific biochemical characteristics of AGEs is currently unknown. Using standardized procedures and a variety of AGE measures, the present study aimed to characterize the AGEs that bind to RAGE and their formation kinetics in vitro. To produce AGEs with varying RAGE binding affinity, bovine serum albumin (BSA) AGEs were prepared with 0.5M glucose, fructose, or ribose at times of incubation from 0 to 12 weeks or for up to 3 days with glycolaldehyde or glyoxylic acid. The AGE-BSAs were characterized for RAGE binding affinity, fluorescence, absorbance, carbonyl content, reactive free amine content, molecular weight, pentosidine content, and N-epsilon-carboxymethyl lysine content. Ribose-AGEs bound RAGE with high affinity within 1 week of incubation in contrast to glucose- and fructose-AGE, which required 12 and 6 weeks, respectively, to generate equivalent RAGE ligands (IC50=0.66, 0.93, and 1.7 microM, respectively). Over time, all of the measured AGE characteristics increased. However, only free amine content robustly correlated with RAGE binding affinity. In addition, detailed protocols for the generation of AGEs that reproducibly bind RAGE with high affinity were developed, which will allow for further study of the RAGE-AGE interaction.     

Advanced glycation end product recognition by the receptor for AGEs

Nonenzymatic protein glycation results in the formation of advanced glycation end products (AGEs) that are implicated in the pathology of diabetes, chronic inflammation, Alzheimer's disease, and cancer. AGEs mediate their effects primarily through a receptor-dependent pathway in which AGEs bind to a specific cell surface associated receptor, the Receptor for AGEs (RAGE). N(?)-carboxy-methyl-lysine (CML) and N(?)-carboxy-ethyl-lysine (CEL), constitute two of the major AGE structures found in tissue and blood plasma, and are physiological ligands of RAGE. The solution structure of a CEL-containing peptide-RAGE V domain complex reveals that the carboxyethyl moiety fits inside a positively charged cavity of the V domain. Peptide backbone atoms make specific contacts with the V domain. The geometry of the bound CEL peptide is compatible with many CML (CEL)-modified sites found in plasma proteins. The structure explains how such patterned ligands as CML (CEL)-proteins bind to RAGE and contribute to RAGE signaling.     

In skeletal muscle advanced glycation end products (AGEs) inhibit insulin action and induce the formation of multimolecular complexes including the receptor for AGEs

Chronic hyperglycemia promotes insulin resistance at least in part by increasing the formation of advanced glycation end products (AGEs). We have previously shown that in L6 myotubes human glycated albumin (HGA) induces insulin resistance by activating protein kinase Calpha (PKCalpha). Here we show that HGA-induced PKCalpha activation is mediated by Src. Coprecipitation experiments showed that Src interacts with both the receptor for AGE (RAGE) and PKCalpha in HGA-treated L6 cells. A direct interaction of PKCalpha with Src and insulin receptor substrate-1 (IRS-1) has also been detected. In addition, silencing of IRS-1 expression abolished HGA-induced RAGE-PKCalpha co-precipitation. AGEs were able to induce insulin resistance also in vivo, as insulin tolerance tests revealed a significant impairment of insulin sensitivity in C57/BL6 mice fed a high AGEs diet (HAD). In tibialis muscle of HAD-fed mice, insulin-induced glucose uptake and protein kinase B phosphorylation were reduced. This was paralleled by a 2.5-fold increase in PKCalpha activity. Similarly to in vitro observations, Src phosphorylation was increased in tibialis muscle of HAD-fed mice, and co-precipitation experiments showed that Src interacts with both RAGE and PKCalpha. These results indicate that AGEs impairment of insulin action in the muscle might be mediated by the formation of a multimolecular complex including RAGE/IRS-1/Src and PKCalpha.     

Interaction of the RAGE cytoplasmic domain with diaphanous-1 is required for ligand-stimulated cellular migration through activation of Rac1 and Cdc42

Cellular migration is a fundamental process linked to diverse pathological states such as diabetes and its complications, atherosclerosis, inflammation, and cancer. The receptor for advanced glycation end products (RAGE) is a multiligand cell surface macromolecule which binds distinct ligands that accumulate in these settings. RAGE-ligand interaction evokes central changes in key biological properties of cells, including proliferation, generation of inflammatory mediators, and migration. Although RAGE-dependent signal transduction is critically dependent on its short cytoplasmic domain, to date the proximate mechanism by which this RAGE domain engages and stimulates cytoplasmic signaling pathways has yet to be identified. Here we show that the RAGE cytoplasmic domain interacts with Diaphanous-1 (Dia-1) both in vitro and in vivo. We employed the human RAGE cytoplasmic domain as "bait" in the yeast two-hybrid assay and identified the formin homology (FH1) domain of Dia-1 as a potential binding partner of this RAGE domain. Immunoprecipitation studies revealed that the RAGE cytoplasmic domain interacts with the FH1 domain of Dia-1. Down-regulation of Dia-1 expression by RNA interference blocks RAGE-mediated activation of Rac-1 and Cdc42 and, in parallel, RAGE ligand-stimulated cellular migration. Taken together, these findings indicate that the interaction of the RAGE cytoplasmic domain with Dia-1 is required to transduce extracellular environmental cues evoked by binding of RAGE ligands to their cell surface receptor, a chief consequence of which is Rac-1 and Cdc42 activation and cellular migration. Because RAGE and Dia-1 are implicated in the regulation of inflammatory, vascular, and transformed cell migration, these findings highlight this interaction as a novel target for therapeutic intervention in inflammation, atherosclerosis, diabetes, and cancer.     

Soluble RAGE, diabetic nephropathy and genetic variability in the AGER gene

Diabetes mellitus, especially when complicated with decline of renal function due to diabetic nephropathy (DN), is associated with accumulation of advanced glycation end products (AGEs) exerting their adverse effects via receptor of AGE (RAGE). Soluble RAGE (sRAGE) is a truncated form of RAGE functioning as an inhibitor of AGE-mediated signalling. We studied relationships between sRAGE, renal function and genetic variability in the AGER gene in diabetic subjects. Study comprised a total of 265 diabetics (type 1 or 2 or LADA) with normoalbuminuria (n = 94) or DN (n = 171). sRAGE (assessed by ELISA) was significantly higher in DN than normoalbuminuria subjects (P = 0.007) and positively correlated with age, S-urea, S-creatinine and albuminuria and AGEs (determined spectrofluorimetrically), negatively with GFR (all P < 0.05); however, multivariate regression revealed that GFR was the only independent variable associated with sRAGE (P = 0.047). sRAGE did not correspond with carrier state of risk-haplotype copies (RAGE2) (P > 0.05). In conclusion, GFR is a principal determinant of sRAGE concentration and gradual sRAGE increase in subjects with advancing impairment of renal function is paralleled by AGEs.