Clinical, pathophysiological and structure/function consequences of modification of albumin by Amadori-glucose adducts.

BACKGROUND: The nonenzymatic condensation of glucose with albumin results in the formation of albumin modified by Amadori glucose adducts, the principal form in which glycated albumin exists in vivo. SCOPE OF REVIEW: This review focuses on (a) the utility of measurement of Amadori-modified glycated albumin (AGA) as a biomarker in diabetes, where elevated levels attend the hyperglycemic state; (b) the role of AGA as a causal factor in the pathogenesis of complications of diabetes; (c) effects on transport properties; and (d) structural and functional consequences of the modification of albumin by Amadori glucose adducts. It does not discuss counterparts with respect to Advanced Glycation Endproducts (AGE), which may be found in other publications. MAJOR CONCLUSIONS: Nonenzymatic glycation of albumin, which is increased in diabetes, has clinical relevance and pathophysiologic importance, with ramifications for the management of this disease, the development of its complications, and the transport of endogenous and exogenous ligands. GENERAL SIGNIFICANCE: Appreciation of the manifold consequences of AGA has afforded new avenues for assessing clinical management of diabetes, awareness of the impact of nonenzymatic glycation on albumin biology, insights into the pathogenesis of vascular complications of diabetes, and avenues of investigation of and intervention strategies for these complications. This article is part of a Special Issue on albumin. This article is part of a Special Issue entitled Serum Albumin.

Anti-glycation and anti-albuminuric effects of GLY-230 in human diabetes.

BACKGROUND/AIMS: Inhibiting nonenzymatic glycation with GLY-230 lowers glycated albumin without affecting hyperglycemia and ameliorates renal dysfunction in the db/db mouse, but the effects of this compound in man have not been assessed. We report results from the first clinical trial in patients with diabetes of this new glycation inhibitor. METHODS: 21 diabetic men were randomly assigned to receive a total dose of 250, 500 or 750 mg of GLY-230 or placebo (1:1:1:1.2 ratio) daily for 14 days to evaluate safety and the effect of drug on plasma concentrations of glycated albumin and on urinary albumin. RESULTS: GLY-230 dose-responsively decreased glycated albumin in all participants, in whom HbA1c did not change. Among participants exhibiting microalbuminuria at baseline, mean albumin excretion significantly decreased in patients receiving GLY-230 (microg albumin/mg creatinine = 61.4 +/- 15.8 and 29.8 +/- 10.4 at baseline and completion, respectively; p = 0.001), but not placebo. There were no serious adverse events or laboratory abnormalities, and all safety parameters remained within normal limits. CONCLUSIONS: This first-in-diabetic man study indicates that GLY-230 lowers glycated albumin and that this decrease is associated with a reduction in urine albumin excretion in patients with preexisting microalbuminuria. These data encourage further evaluation of GLY-230 in diabetic renal dysfunction.

Reduction of Abeta42 in brains of transgenic APPswe mice by 2-3-chlorophenylaminophenylacetate.

1. Imbalanced generation of the Abeta42 peptide from the amyloid beta protein precursor (APP) is implicated in the pathogenesis of Alzheimer's disease. 2. The present study is the first to evaluate the ability of 2-[3-chlorophenylamino]phenylacetic acid (GLY-230), a new drug in clinical development for the treatment of vascular complications of diabetes, to modulate Abeta42 levels in transgenic mice expressing APP. 3. Oral administration of 7.5 mg/kg GLY-230 twice a day for 14 days to APPswe transgenic mice aged 3 months significantly reduced brain Abeta42 and increased plasma Abeta42 levels by 50 and 20%, respectively. 4. GLY-230 readily entered the brain after administration of a dose (7.5 mg/kg) that decreased brain Abeta42. 5. These results are the first to demonstrate that GLY-230, which exhibits antiglycation but no cyclo-oxygenase inhibitory properties, lowers brain Abeta42 levels in this experimental model of Alzheimer's disease.

Vitreous fluid of db/db mice exhibits alterations in angiogenic and metabolic factors consistent with early diabetic retinopathy.

BACKGROUND: This study evaluated the postulate that the vitreous of diabetic db/db mice, a genetic model of type 2 diabetes that manifests hyperglycemia and insulin resistance, exhibits alterations in angiogenic and metabolic factors that reflect abnormalities in the retinal microvasculature participatory in the pathogenesis of diabetic retinopathy. METHODS: Vitreous obtained from db/db and age-matched nondiabetic db/m mice was analyzed by Western blot for pigment epithelium-derived factor (PEDF) and vascular endothelial growth factor (VEGF), by immunoassay for type IV collagen, and by measurement of TBARs for lipid peroxide products. RESULTS: Compared to nondiabetic db/m controls, vitreous from db/db mice contained decreased PEDF and increased VEGF (VEGF:PEDF relative ratio 2.2 +/- 0.3 and 1.0 +/- 0.1 in db/db vs. db/m, respectively; p < 0.05), and elevated concentrations of lipid peroxide products (187 +/- 43 and 84 +/- 15 ng/ml in db/db vs. db/m, respectively; p < 0.05) and type IV collagen (5.2 +/- 0.7 and 3.1 +/- 0.4 nmol/ml in db/db vs. db/m, respectively; p < 0.05). These changes were observed at age 18-20 weeks, consistent with an early stage in the development of retinal microvascular pathology. CONCLUSIONS: The findings support the potential usefulness of vitreous from the db/db mouse as a model tissue for investigation of pathogenetic factors and assessment of therapeutic interventions in early diabetic retinopathy.

Normalizing glycated albumin reduces increased urinary collagen IV and prevents renal insufficiency in diabetic db/db mice.

Increased excretion of type IV collagen accompanies the accumulation of mesangial matrix, which leads to compromise in the glomerular filtration surface area, during the development of diabetic nephropathy. We postulated that the response of urinary collagen IV would be useful in evaluating possible treatment strategies to arrest the nephropathic process while still at a reversible stage. To test this hypothesis, we examined the effect of a small molecule (22CPPA) that inhibits the formation of glycated albumin, which is causally linked to the pathogenesis of diabetic nephropathy, on collagen IV excretion, albuminuria, and renal function in db/db mice. Compared to nondiabetic db/m mice, db/db animals showed markedly increased urinary collagen IV and albumin, significantly elevated serum glycated albumin and creatinine concentrations, and a significantly reduced creatinine clearance. Treatment of db/db mice with test compound, which normalized glycated albumin concentrations, significantly lowered collagen IV and albumin excretion and ameliorated the fall in creatinine clearance and the rise in serum creatinine despite persistent hyperglycemia. The findings indicate that reduction of elevated collagen IV excretion in diabetes reflects a salutary influence on developing glomerulosclerosis, and that glycated albumin has an important nephropathogenic role that can be therapeutically addressed independent of glycemic status.

Increased plasma glycated low-density lipoprotein concentrations in diabetes: a marker of atherogenic risk.

Nonenzymatic glycation of apolipoprotein B in the low-density lipoprotein (LDL) complex has been considered a proatherogenic modification contributory to the increased susceptibility of patients with diabetes to atherosclerosis. We postulated that glycated LDL concentrations might be associated with other markers of cardiovascular disease. To explore this hypothesis, we measured glycated LDL concentrations by a monospecific immunoassay in 50 patients with type 1 and 100 patients with type 2 diabetes and examined relationships with the amount of albumin excretion and the serum cholesterol and triglyercide concentrations. Plasma glycated LDL showed a significant positive correlation (r = 0.325; P < 0.001) with urinary albumin excretion that was higher in type 1 (r = 0.463) than in type 2 (r = 0.245) patients. The mean glycated LDL concentration progressively increased with increasing albumin excretion when patients were subcategorized into groups of normoalbuminuria, low (

Inhibiting Amadori-modified albumin formation improves biomarkers of podocyte damage in diabetic rats.

Recent studies have shown that urinary excretion of podocyte proteins is an indicator of podocyte injury, and that podocyte abnormalities and elevated concentrations of Amadori-modified glycated albumin (AGA) are linked to the development of diabetic nephropathy and to each other. We evaluated relationships between urinary markers of podocyte damage, increased AGA and filtration function in rats made diabetic by streptozotocin injection and treated for 8 weeks with a compound that inhibits the formation of AGA, with age-matched nondiabetic and diabetic rats serving as controls. Blood and urine were collected for measurement of glycated albumin, creatinine, albumin, nephrin, podocalyxin, and ßig-h3 protein. The elevated circulating concentrations of glycated albumin and higher urinary levels of these podocyte markers as well as of albumin that were observed in diabetic rats compared with nondiabetic controls were significantly reduced in animals receiving test compound, and decrease in urinary biomarkers correlated with reduction in AGA. The results provide evidence that lowering the concentration of AGA, independent of filtration status and hyperglycemia, reduces urinary nephrin, podocalyxin, and ßig-h3 protein, linking the increased glycated albumin associated with diabetes to podocyte abnormalities and shedding of podocyte proteins into the urine.

Effects of nonenzymatic glycation and fatty acids on functional properties of human albumin.

Human serum albumin nonenzymatically condenses with glucose to form stable Amadori adducts that are increased with the hyperglycemia of diabetes. The present study evaluated the influence of fatty acids, which are major endogenous ligands, on albumin glycation and of glycation on albumin conformation and exogenous ligand binding. Physiologic concentrations of palmitate, oleate, and linoleate reduced the ability of albumin to form glucose adducts, whereas glycation decreased intrinsic fluorescence, lowered the affinity for dansylsarcosine, and diminished the fatty acid-induced increase in limiting fluorescence of protein-bound warfarin that was observed with nonglycated albumin. The findings indicate that fatty acids impede the ability of albumin to undergo Amadori glucose modification and induce conformational changes affecting exogenous ligand binding, and that nonenzymatic glycation of albumin induces alterations in structural and functional properties that may have import in lipid transport and atherogenesis.

Inhibiting low-density lipoprotein glycation ameliorates increased cholesteryl ester synthesis in macrophages and hypercholesterolemia and aortic lipid peroxidation in streptozotocin diabetic rats.

Increased nonenzymatic glycation of apolipoprotein (apo) B-containing lipoproteins impairs uptake and metabolism by the high-affinity low-density lipoprotein receptor and is one of the postsecretory modifications contributory to accelerated atherosclerosis in diabetes. The present study evaluated in vitro and in vivo effects of 2,2-chlorophenylaminophenylacetate to probe the influence of glycated lipoprotein on cholesterol homeostasis. This compound prevented the increased formation of glycated products in low-density lipoprotein incubated with 200 mmol/L glucose and the increased cholesteryl ester synthesis in THP-1 macrophages induced by apo B-containing lipoproteins preincubated with high glucose concentration. The elevated circulating concentrations of glycated lipoprotein and cholesterol and higher vascular levels of lipid peroxidation products observed in streptozotocin diabetic rats compared with nondiabetic controls were significantly reduced in diabetic animals treated for 6 months with test compound. These results are the first to demonstrate that inhibiting nonenzymatic glycation of apo B-containing lipoproteins ameliorates abnormalities contributory to hypercholesterolemia and atherogenic risk in diabetes.

Amelioration of diabetes-associated abnormalities in the vitreous fluid by an inhibitor of albumin glycation.

PURPOSE: Albumin modified by Amadori glucose adducts is a plasma-borne factor that activates cell signaling pathways, modulates the expression of growth factors and cytokines, and participates in the pathogenesis of microvascular complications of diabetes. In the present study, streptozotocin diabetic rats were treated with an orally administered compound that inhibits the nonenzymatic glycation of albumin to evaluate whether increased glycated albumin contributes to diabetes-associated abnormalities in the vitreous fluid. METHODS: Vitreous obtained from age-matched nondiabetic and streptozotocin-diabetic rats, half of which received the test compound 2-(3-chlorophenylamino) phenylacetic acid (23CPPA) by oral gavage for 26 weeks, was analyzed by immunoassay for pigment epithelium-derived factor (PEDF), vascular endothelial growth factor (VEGF) and glycated albumin content, by measurement of thiobarbituric acid reactive substances (TBARs) for lipid peroxide products and by colorimetric assay for hyaluronan content. RESULTS: Compared with that of nondiabetic controls, vitreous of diabetic rats contained decreased PEDF, increased VEGF, higher VEGF/ PEDF ratio, and elevated levels of TBARs, glycated albumin, and hyaluronan. These changes were significantly attenuated in rats treated with test compound despite the presence of marked hyperglycemia. CONCLUSIONS: Results indicate that inhibiting the formation of glycated albumin, which is increased in diabetes, ameliorates vitreous changes in angiogenic and metabolic factors associated with the development of diabetic retinopathy. The observed improvement in vitreous alterations associated with reductions in glycated albumin suggests that elevated levels of glycated albumin play a retinopathogenic role in diabetes that is operative and that can be therapeutically addressed independently of glycemic status.

Inhibiting albumin glycation attenuates dysregulation of VEGFR-1 and collagen IV subchain production and the development of renal insufficiency.

Glomerular cells in culture respond to albumin containing Amadori glucose adducts (the principal serum glycated protein), with activation of protein kinase C-beta(1), increased expression of transforming growth factor (TGF)-beta1, the TGF-beta type II signaling receptor, and the extracellular matrix proteins alpha(1)(IV) collagen and fibronectin and with decreased production of the podocyte protein nephrin. Decreasing the burden of glycated albumin in diabetic db/db mice significantly reduces glomerular overexpression of TGF-beta1 mRNA, restores glomerular nephrin immunofluorescence, and lessens proteinuria, mesangial expansion, renal extracellular matrix protein production, and increased glomerular vascular endothelial growth factor (VEGF) immunostaining. In the present study, db/db mice were treated with a small molecule, designated 23CPPA, that inhibits the nonenzymatic condensation of glucose with the albumin protein to evaluate whether increased glycated albumin influences the production of VEGF receptors (VEGFRs) and type IV collagen subchains and ameliorates the development of renal insufficiency. Renal levels of VEGF and VEGFR-1 proteins and serum creatinine concentrations were significantly higher and renal levels of alpha(3)(IV) collagen and nephrin proteins and endogenous creatinine clearance values were significantly lower in control diabetic than in age-matched nondiabetic (db/m) mice. These changes were significantly attenuated in db/db littermate mice treated from 9 to 18 wk of age with 23CPPA. The findings indicate that inhibiting excess nonenzymatic glycation of serum albumin improves renal molecular biology abnormalities and protects against the development of renal insufficiency in the db/db mouse.

Evidence linking glycated albumin to altered glomerular nephrin and VEGF expression, proteinuria, and diabetic nephropathy.

BACKGROUND: Albumin modified by Amadori-glucose adducts has been linked to the development of diabetic nephropathy through its ability, independent of hyperglycemia, to activate protein kinase C-beta (PKC-beta), up-regulate the transforming growth factor-beta (TGF-beta) system, and stimulate expression of extracellular matrix proteins in glomerular cells, and by the demonstration that reducing the burden of glycated albumin ameliorates renal structural and functional abnormalities in the db/db mouse. METHODS: To probe whether the salutary effects consequent to lowering glycated albumin, which include reduction of albuminuria, relate to an influence of the Amadori-modified protein on nephrin, the podocyte protein critical to regulation of protein excretion, and on the angiogenic vascular endothelial growth factor (VEGF), which induces microvascular permeability, diabetic db/db mice were treated with a small molecule that inhibits the nonenzymatic glycation of albumin. RESULTS: Compared to nondiabetic db/m mice, diabetic controls exhibited increased urinary excretion of albumin and type IV collagen, elevated renal TGF-beta1 protein levels, reduced glomerular nephrin immunofluorescence and nephrin protein by immunoblotting, and increased glomerular VEGF immunostaining and renal VEGF protein content. Diabetic animals receiving test compound showed significant lowering of proteinuria, normalization of renal TGF-beta1 protein, and significant restoration of altered glomerular nephrin and VEGF expression. CONCLUSION: The findings causally implicate the increased glycated albumin associated with the diabetic state in the abnormal renal nephrin and VEGF expression found in diabetes, thereby promoting proteinuria and glomerulosclerosis.

Intervention strategies to prevent pathogenetic effects of glycated albumin.

Modification of proteins by nonenzymatic glycation is one of the underlying factors contributory to the development of complications of diabetes. In general, the nature of this structural modification falls into two broad categories: nonenzymatic glycation per se, which refers to the attachment of free carbohydrate to proteins in the Amadori construct, and Advanced Glycation Endproducts (AGE), which refers to a heterogeneous group of carbohydrate-modified products generated from the Amadori adduct by oxidation, polymerization, and other spontaneous reactions. This review will focus on the role of nonenzymatically glycated proteins, and in particular glycated serum albumin, in the pathogenesis of diabetic complications, and on pharmacologic approaches to mitigate their deleterious effects. Potential intervention strategies to lessen the influence of AGE-modified proteins, as well as of other contributory abnormalities, are discussed elsewhere in this volume.

Glycated albumin increases oxidative stress, activates NF-kappa B and extracellular signal-regulated kinase (ERK), and stimulates ERK-dependent transforming growth factor-beta 1 production in macrophage RAW cells.

Albumin modified by Amadori glucose adducts has been shown to modulate signal transduction and induce alterations in renal glomerular cells that contribute to the development of diabetic nephropathy. However, the participation of this nonenzymatically glycated protein in the pathobiology of atherosclerotic cardiovascular disease in diabetes has not been established. To probe this issue, we used macrophage RAW cells to assess the effects of glycated albumin on molecular events implicated in the pathogenesis of diabetes-related vascular complications. RAW cells were cultured in medium containing 5.5 mmol/L glucose and glycated or nonglycated albumin, with and without the addition of PD98059, a specific inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK), followed by analysis of phosphorylated ERK and the nuclear translocation of nuclear factor (NF)-kappa B and measurement of cellular content of thiobarbituric acid-reactive substances and the concentration of transforming growth factor (TGF)-beta(1) in the spent medium. We demonstrate, for the first time, that glycated albumin activates RAW cell ERK and promotes ERK-dependent increases in TGF-beta(1) production, oxidative stress, and NF-kappa B activation. Preincubation with the antioxidant alpha-lipoic acid partially prevented the glycated albumin-induced increase in NF-kappa B activation. These findings indicate that Amadori-modified glycated albumin modulates macrophage cell biology independent of high glucose concentration. The effects of glycated albumin on RAW cell molecular mediators and cytokine production may have pathophysiologic significance with respect to the accelerated atherosclerosis that occurs in diabetes.

Inhibiting albumin glycation in vivo ameliorates glomerular overexpression of TGF-beta1.

BACKGROUND: Glycated albumin has been causally linked to the pathobiology of diabetic renal disease through its ability to stimulate the expression of transforming growth factor-beta1 (TGF-beta1), activate protein kinase C (PKC) and extracellular signal-regulated kinase (ERK), and promote production of extracellular matrix proteins in cultured glomerular cells. Whether glycated albumin modulates glomerular TGF-beta1 expression in vivo is not known. To address this issue, we assessed glomerular TGF-beta1 expression and pathology in response to reducing the burden of glycated albumin in vivo. METHODS: We measured serum glycated albumin, urine protein, glomerular TGF-beta1 expression and morphometry, and collagen IV and fibronectin mRNA in db/m and db/db controls and in db/db mice treated for eight weeks with a synthetic compound that inhibits the condensation of glucose with albumin. RESULTS: In situ hybridization studies showed markedly increased glomerular TGF-beta1 mRNA in control db/db mice, which was significantly reduced in db/db mice treated for eight weeks with test compound. The treatment protocol, which normalized serum glycated albumin, concomitantly reduced the elevated protein excretion and the renal overexpression of mRNAs encoding fibronectin and collagen IV, and significantly decreased the mesangial matrix expansion, observed in db/db control animals. CONCLUSIONS: These findings, to our knowledge, provide the first evidence that glomerular overexpression of TGF-beta1 in diabetes derives at least in part from elevated glycated albumin concentrations, and can be partially suppressed by inhibiting the formation of this glycated protein. The results further suggest that glycated albumin has an important nephropathogenic role in diabetes that is operative, and can be therapeutically addressed, independent of glycemic status.