Endocytic uptake of nonenzymatically glycosylated proteins is mediated by a scavenger receptor for aldehyde-modified proteins

DJ-1: Potential target for treatment of myocardial ischemia-reperfusion injury

Ischemic heart disease (IHD) is a significant global health concern, resulting in high rates of mortality and disability among patients. Although coronary blood flow reperfusion is a key treatment for IHD, it often leads to acute myocardial ischemia-reperfusion injury (IRI). Current intervention strategies have limitations in providing adequate protection for the ischemic myocardium. DJ-1, originally known as a Parkinson's disease related protein, is a highly conserved cytoprotective protein. It is involved in enhancing mitochondrial function, scavenging reactive oxygen species (ROS), regulating autophagy, inhibiting apoptosis, modulating anaerobic metabolism, and exerting anti-inflammatory effects. DJ-1 is also required for protective strategies, such as ischemic preconditioning, ischemic postconditioning, remote ischemic preconditioning and pharmacological conditioning. Therefore, DJ-1 emerges as a potential target for the treatment of myocardial IRI. Our comprehensive review delves into its protective mechanisms in myocardial IRI and the structural foundations underlying its functions.

The Role of the Endothelium in Premature Atherosclerosis: Molecular Mechanisms

Atherosclerotic disease is still one of the leading causes of mortality. Atherosclerosis is a complex progressive and systematic artery disease that involves the intima of the large and middle artery vessels. The inflammation has a key role in the pathophysiological process of the disease and the infiltration of the intima from monocytes, macrophages and T-lymphocytes combined with endothelial dysfunction and accumulated oxidized low-density lipoprotein (LDL) are the main findings of atherogenesis. The development of atherosclerosis involves multiple genetic and environmental factors. Although a large number of genes, genetic polymorphisms, and susceptible loci have been identified in chromosomal regions associated with atherosclerosis, it is the epigenetic process that regulates the chromosomal organization and genetic expression that plays a critical role in the pathogenesis of atherosclerosis. Despite the positive progress made in understanding the pathogenesis of atherosclerosis, the knowledge about the disease remains scarce.

The Effect of Advanced Glycation End-Products and Aminoguanidine on Tnfα Production by Rat Peritoneal Macrophages

Objective

To evaluate the effect of advanced glycation end-products (AGEs) and the inhibitor of their formation, aminoguanidine, on tumor necrosis factor-α (TNFα) production (as a functional marker) by rat peritoneal macrophages (PMΦ).

Design

Charles River rats underwent a daily intraperitoneal injection of peritoneal dialysis solution [(PDS), 4.25 g/dL dextrose; Dialine, Travenol, Ashdod, Israel] for a 2-month period (group E). Another group of rats was subjected to the same protocol with the addition of 25 mg/kg aminoguanidine (group A). Three control groups were utilized: ( 1 ) rats that were injected daily with aminoguanidine only (group AO), ( 2 ) rats that were injected with Dulbecco's phosphate-buffered saline (group D), and ( 3 ) rats in which no intervention was carried out (group C). After 2 months, PMΦ were isolated from rat peritoneal effluent and their TNFα production measured by ELISA in cell-free culture supernatants, in both the basal state and after 24-hour stimulation with lipopolysaccharide (LPS). The concentrations of AGEs in peritoneal effluent were assayed and correlated to TNFα levels. PMΦ obtained from normal rats were then incubated for 24 hours with ( 1 ) the peritoneal effluent of each of the above respective groups, with or without LPS; ( 2 ) increasing concentrations of AGEs (0 - 250 μg/mL); and ( 3 ) increasing concentrations of aminoguanidine (0 - 7.5 mg/mL), and TNFα secretion again determined.

Results

After 2 months of daily intraperitoneal injection of PDS, in the basal state, TNFα production was significantly higher in PMΦ isolated from the peritoneal effluent groups (groups E, A, and AO) compared to controls (group C). Following LPS stimulation, a further increase in TNFα secretion was seen, with a significantly greater response in group AO versus groups E, A, and D. Effluent AGEs were markedly elevated only in group E. No correlation was found between TNFα secretion by these PMΦ and the concentration of AGEs. On incubation with the respective peritoneal effluents (groups E, A, and AO), in both the basal and stimulated state, TNFα production by PMΦ from normal rats was significantly enhanced compared to group C. Incubation with increasing concentrations of AGEs or aminoguanidine resulted in an increase of TNFα secretion by these PMΦ.

Conclusions

Following intermittent intraperitoneal administration of glucose-based PDS, rat PMΦ are chronically activated, as evidenced by increased basal TNFα secretion. The peritoneal effluent of such treated animals is capable of stimulating TNFα production by normal rat PMΦ. These data suggest that glucose-based PDS acts as a primer of PMΦ, which retain their ability to further stimulation by LPS. Although, in vitro, AGEs promote TNFα secretion by normal rat PMΦ, in vivo, their influence is probably modulated by other factors. Aminoguanidine has a specific inducing effect on rat PMΦ, independent of glucose-based PDS.

Glycation of glucose sensitive lysine residues K36, K438 and K549 of albumin is associated with prediabetes

Prediabetes is a risk factor for the development of diabetes. Early diagnosis of prediabetes may prevent the onset and progression of diabetes and its associated complications. Therefore, this study aimed at the identification of novel markers for efficient prediction of prediabetes. In this pursuit, we have evaluated the ability of glycated peptides of albumin in predicting prediabetes. Glycated peptides of in vitro glycated albumin were characterized by data dependent acquisition and parallel reaction monitoring using LC-HRMS. Amongst 14 glycated peptides characterized in vitro, four peptides, particularly, FK(CML)DLGEENFK, K(AML)VPQVSTPTLVEVSR, K(CML)VPQVSTPTLVEVSR, and K(AML)QTALVELVK, corresponding to 3 glucose sensitive lysine residues K36, K438, and K549, respectively showed significantly higher abundance in prediabetes than control. Additionally, the abundance of three of these peptides, namely K(AML)QTALVELVK, K(CML)VPQVSTPTLVEVSR and FK(CML)DLGEENFK was >1.8-fold in prediabetes, which was significantly higher than the differences observed for FBG, PPG, and HbA1c. Further, the four glycated peptides showed a significant correlation with FBG, PPG, HbA1c, triglycerides, VLDL, and HDL. This study supports that glycated peptides of glucose sensitive lysine residues K36, K438 and K549 of albumin could be potentially useful markers for prediction of prediabetes. SIGNIFICANCE: Undiagnosed prediabetes may lead to diabetes and associated complications. This study reports targeted quantification of four glycated peptides particulary FK(CML)DLGEENFK, K(AML)VPQVSTPTLVEVSR, K(CML)VPQVSTPTLVEVSR, and K(AML)QTALVELVK, corresponding to 3 glucose sensitive lysine residues K36, K438 and K549 respectively by parallel reaction monitoring in healthy and prediabetic subjects. These peptides showed significantly higher abundance in prediabetes than healthy subjects, and showed significant correlation with various clinical parameters including FBG, PPG, HbA1c, and altered lipid profile. Therefore, together these four peptides constitute a panel of markers that can be useful for prediction of prediabetes.

In vivo imaging of advanced glycation end products (AGEs) of albumin: first observations of significantly reduced clearance and liver deposition properties in mice

Advanced glycation end products (AGEs) are associated with various diseases, especially during aging and the development of diabetes and uremia. To better understand these biological processes, investigation of the in vivo kinetics of AGEs, i.e., analysis of trafficking and clearance properties, was carried out by molecular imaging. Following the preparation of Cy7.5-labeled AGE-albumin and intravenous injection in BALB/cA-nu/nu mice, noninvasive fluorescence kinetics analysis was performed. In vivo imaging and fluorescence microscopy analysis revealed that non-enzymatic AGEs were smoothly captured by scavenger cells in the liver, i.e., Kupffer and other sinusoidal cells, but were unable to be properly cleared from the body. Overall, these results highlight an important link between AGEs and various disorders associated with them, which may serve as a platform for future research to better understand the processes and mechanisms of these disorders.

Nε-(carboxymethyl) lysine-induced mitochondrial fission and mitophagy cause decreased insulin secretion from β-cells

Nε-(carboxymethyl) lysine-conjugated bovine serum albumin (CML-BSA) is a major component of advanced glycation end products (AGEs). We hypothesised that AGEs reduce insulin secretion from pancreatic β-cells by damaging mitochondrial functions and inducing mitophagy. Mitochondrial morphology and the occurrence of autophagy were examined in pancreatic islets of diabetic db/db mice and in the cultured CML-BSA-treated insulinoma cell line RIN-m5F. In addition, the effects of α-lipoic acid (ALA) on mitochondria in AGE-damaged tissues were evaluated. The diabetic db/db mouse exhibited an increase in the number of autophagosomes in damaged mitochondria and receptor for AGEs (RAGE). Treatment of db/db mice with ALA for 12 wk increased the number of mitochondria with well-organized cristae and fewer autophagosomes. Treatment of RIN-m5F cells with CML-BSA increased the level of RAGE protein and autophagosome formation, caused mitochondrial dysfunction, and decreased insulin secretion. CML-BSA also reduced mitochondrial membrane potential and ATP production, increased ROS and lipid peroxide production, and caused mitochondrial DNA deletions. Elevated fission protein dynamin-related protein 1 (Drp1) level and mitochondrial fragmentation demonstrated the unbalance of mitochondrial fusion and fission in CML-BSA-treated cells. Additionally, increased levels of Parkin and PTEN-induced putative kinase 1 protein suggest that fragmented mitochondria were associated with increased mitophagic activity, and ALA attenuated the CML-BSA-induced mitophage formation. Our study demonstrated that CML-BSA induced mitochondrial dysfunction and mitophagy in pancreatic β-cells. The findings from this study suggest that increased concentration of AGEs may damage β-cells and reduce insulin secretion.

Expression of the cellular prion protein affects posttransfusion recovery and survival of red blood cells in mice

BACKGROUND

Cellular prion protein (PrP(C) ) is expressed on various cell types including red blood cells (RBCs). The PrP(C) plays a key role in the pathogenesis of prion diseases, but its physiologic function remains unclear. PrP(C) is expressed on CD34+ hematopoietic stem cells and its expression is regulated during blood cell differentiation including the erythroid line.

STUDY DESIGN AND METHODS

We investigated the role of PrP(C) in RBC survival in circulation by transfusing a mix of biotin-labeled RBCs from wild-type (WT) and PrP knockout (KO) mice to groups of recipient mice (WT and KO). The proportion of biotinylated RBCs in peripheral blood was estimated by flow cytometry.

RESULTS

KO RBCs displayed a markedly higher first-day posttransfusion recovery but had a decreased survival in circulation when compared to WT RBCs. Similar results were obtained in all groups of transfused mice, irrespective of RBCs biotinylation level. In addition, we confirmed this finding in an analogous study using Tga20 mice overexpressing PrP(C) and KO mice of a different genetic background.

CONCLUSION

Our results demonstrate that PrP(C) expression affects RBC recovery and survival in circulation.

Structural analysis and aggregation propensity of reduced and nonreduced glycated insulin adducts

The milieu within pancreatic β cells represents a favorable environment for glycation of insulin. Therefore, in this study, insulin samples were individually subjected to glycation under reducing and nonreducing conditions. As monitored by ortho-phthalaldehyde and fluorescamine assays, the reduced glycated insulin adduct demonstrates extensively higher level of glycation than the nonreduced glycated counterpart. Also, gel electrophoresis experiments suggest a significant impact of glycation under a reducing system on the level of insulin oligomerization. Furthermore, reduced and nonreduced glycated insulin adducts respectively exhibit full and partial resistance against dithiothreitol-induced aggregation. The results of thioflavin T and Congo red assays suggest the existence of a significant quantity of amyloid-like entities in the sample of reduced glycated insulin adduct. Both fluorescence and far-ultraviolet circular dichroism studies respectively suggest that the extents of unfolding and secondary structural alteration were closely correlated to the level of insulin glycation. Moreover, the surface tension of two glycated insulin adducts was inversely correlated to their glycation extents and to the quantity of exposed hydrophobic patches. Overall, the glucose-modified insulin molecules under reducing and nonreducing systems display different structural features having significant consequences on aggregation behaviors and surface tension properties. The particular structural constraints of glycated insulin may reduce the binding interaction of this hormone to its receptor which is important for both insulin function and clearance.

Advanced Glycation End Products (AGE)-Modified Proteins and Their Potential Relevance to Atherosclerosis

Modification of proteins by long-term incubation with glucose leads, through the formation of early products such as Schiff base and Amadori rearrangement products, to the formation of advanced glycation end products (AGE). AGE-modified proteins are characterized physicochemically by fluorescence, brown coloration, and intramolecular or intermolecular cross-linking. Biologically, they are specifically recognized by the AGE receptors of the cell surface membrane. Recent studies have provided evidence for the involvement of AGE proteins in atherosclerosis. First, in vitro experiments using Chinese hamster ovary cells overexpressing the macrophage scavenger receptor (MSR) and peritoneal macrophages from MSR-knockout mice demonstrated that MSR plays a major role as the AGE receptor in the endocytotic uptake of AGE by macrophages. Second, immunohistochemical studies using anti-AGE antibody and anti-MSR antibody revealed the presence of AGE proteins in human atherosclerotic lesions in arterial walls. Because MSR is closely associated with the formation of early atherosclerotic lesions, these results suggest a potential role played by AGE proteins or their interaction with MSR in the atherosclerotic process. (Trends Cardiovasc Med 1996;6:163-168).

Identification of 4-hydroxy-2-nonenal–histidine adducts that serve as ligands for human lectin-like oxidized LDL receptor-1

LOX-1 (lectin-like oxidized low-density lipoprotein receptor-1) is an endothelial scavenger receptor that is important for the uptake of OxLDL (oxidized low-density lipoprotein) and contributes to the pathogenesis of atherosclerosis. However, the precise structural motifs of OxLDL that are recognized by LOX-1 are unknown. In the present study, we have identified products of lipid peroxidation of OxLDL that serve as ligands for LOX-1. We used CHO (Chinese-hamster ovary) cells that stably express LOX-1 to evaluate the ability of BSA modified by lipid peroxidation to compete with AcLDL (acetylated low-density lipoprotein). We found that HNE (4-hydroxy-2-nonenal)-modified proteins most potently inhibited the uptake of AcLDL. On the basis of the findings that HNE-modified BSA and oxidation of LDL resulted in the formation of HNE–histidine Michael adducts, we examined whether the HNE–histidine adducts could serve as ligands for LOX-1. The authentic HNE–histidine adduct inhibited the uptake of AcLDL in a dose-dependent manner. Furthermore, we found the interaction of LOX-1 with the HNE–histidine adduct to have a dissociation constant of 1.22×10−8 M using a surface plasmon resonance assay. Finally, we showed that the HNE–histidine adduct stimulated the formation of reactive oxygen species and activated extracellular-signal-regulated kinase 1/2 and NF-κB (nuclear factor κB) in HAECs (human aortic endothelial cells); these signals initiate endothelial dysfunction and lead to atherosclerosis. The present study provides intriguing insights into the molecular details of LOX-1 recognition of OxLDL.

Effect of chemical chaperones on glucose-induced lysozyme modifications

Nonenzymatic glycation of biomacromolecules occurs due to the diabetes mellitus and ageing. A number of small molecules, known as chemical chaperones, stabilize protein conformation against thermal and chemically induced denaturation. These compounds are including: polyamines (e.g. spermine and spermidine), amino acids (e.g. lysine) and polyols (e.g. glycerol). In this study the effect of spermidine (Spd), spermine (Spm), and glycerol on glycation, structure and function of lysozyme (LZ), as an extra-cellular protein, by different techniques is investigated. LZ is incubated with or without glucose (50 or 100 mM) in the absence or presence of Spd/Spm/glycerol at 37 °C up to 16 weeks. All the observed changes of glycated-LZ in comparison with the native protein, including: increased fluorescence emission, alteration in the secondary and tertiary structure, and reduced electrophoretic mobility- indicate its structural changes that are accompanied with its reduced activity. Glucose in the presence or absence of Spd induces the protein dimerization, but glucose plus Spm induces its trimmerization. In contrast, glycerol inhibits the LZ glycation and prevents the large changes on its structure and function. Glucose binds lysine residues, decreases the protein positive charges and induces some alterations in its structure and activity. Polyamines also directly bind to LZ, increase its positive charges and hence induce more glycation; more conformational changes, oligomerization and its inactivation in the presence of glucose, but glycerol affect the protein environment and preserve protein from these harmful effects.

Calycosin protects HUVECs from advanced glycation end products-induced macrophage infiltration

ETHNOPHARMACOLOGICAL RELEVANCE

Astragali radix is a traditional Chinese medicine that has long been used for treatment of diabetes and diabetes-associated disease, but its active component and mechanism on the disease is not well defined.

AIM OF THE STUDY

Infiltration of leukocytes within the glomeruli and vasculature is one of the early and characteristic features of diabetic nephropathy. Advanced glycation end products (AGEs) play pivotal role in the progression of diabetic-associated diseases. The present study was designed to explore the therapeutic effect of calycosin, an active component from A. radix, on AGEs-induced macrophages infiltration in HUVECs.

MATERIALS AND METHODS

Transwell HUVEC-macrophage co-culture system was established to evaluate macrophage migration and adhesion. Immunocytochemistry was applied to examine TGF-beta1, ICAM-1 and RAGE protein expressions; real-time PCR was carried out to determine mRNA expression of TGF-beta1, ICAM-1 and RAGE. Immunofluorescence was carried out to observe estrogen receptor-alpha, ICAM-1, RAGE expression and the phosphorylation status of ERK1/2 and NF-κB.

RESULTS

Calycosin significantly reduced AGEs-induced macrophage migration and adhesion to HUVEC. Pre-treatment with calycosin strikingly down-regulated HUVEC TGF-beta1, ICAM-1 and RAGE expressions in both protein and mRNA levels. Furthermore, calycosin incubation significantly increased estrogen receptor expression and reversed AGEs-induced ERK1/2 and NF-κB phosphorylation and nuclear translocation in HUVEC, and this effect of calycosin could be inhibited by estrogen receptor inhibitor, ICI182780.

CONCLUSIONS

These findings suggest that calycosin can reduce AGEs-induced macrophage migration and adhesion to endothelial cells and relieve the local inflammation; furthermore, this effect was via estrogen receptor-ERK1/2-NF-κB pathway.

Oxidative stress-associated rise of hepatic protein glycation increases inflammatory liver injury in uncoupling protein-2 deficient mice

Mitochondrial dysfunction seems to be intrinsically involved in the pathogenesis of multiple organ failure because of enhanced production of reactive oxygen species and induction of oxidative damage. Chronic oxidative stress in turn causes an accumulation of advanced glycation end products (AGEs). To investigate whether mitochondrial dysfunction-associated oxidative stress leads to increased formation and accumulation of AGE, we studied hepatic glycation in uncoupling protein-2 (UCP2-/-) knockout mice. Using the galactosamine/lipopolysaccharide (G/L)-induced liver injury model, we further tested the hypothesis that a mitochondrial dysfunction-associated increase of hepatic glycation is causative for increased liver injury. Under baseline conditions, UCP2-/- mice showed higher malondialdehyde levels and reduced glutathione/glutathione disulfide ratios as well as significantly higher hepatic levels of AGE and hepatic expression of receptor for AGE (RAGE) when compared with UCP2+/+ mice, indicative for increased oxidative stress and hepatic glycation. Further, livers of G/L-challenged UCP2-/- mice revealed significantly more pronounced tissue injury and were found to express higher levels of AGE and RAGE compared with wild-type mice. Functional blockade of RAGE by application of recombinant RAGE significantly diminished liver damage particularly in UCP2-/- mice. This in turn increased survival from 30% in UCP2+/+ mice to 50% in UCP2-/- mice. In summary, we show for the first time that mitochondrial dysfunction-associated oxidative stress enhances hepatic protein glycation, which aggravates inflammation-induced liver injury. Targeting the AGE/RAGE interaction by the blockade of RAGE might be of therapeutic value for the oxidative stress-exposed liver.

Lipid peroxidation generates body odor component trans-2-nonenal covalently bound to protein in vivo

trans-2-Nonenal is an unsaturated aldehyde with an unpleasant greasy and grassy odor endogenously generated during the peroxidation of polyunsaturated fatty acids. 2-Nonenal covalently modified human serum albumin through a reaction in which the aldehyde preferentially reacted with the lysine residues. Modified proteins were immunogenic, and a specific monoclonal antibody (mAb) 27Q4 that cross-reacted with the protein covalently modified with 2-nonenal was raised from mouse. To verify the presence of the protein-bound 2-nonenal in vivo, the mAb 27Q4 against the 2-nonenal-modified keyhole limpet hemocyanin was raised. It was found that a novel 2-nonenal-lysine adduct, cis- and trans-N(epsilon)-3-[(hept-1-enyl)-4-hexylpyridinium]lysine (HHP-lysine), constitutes an epitope of the antibody. The immunoreactive materials with mAb 27Q4 were detected in the kidney of rats exposed to ferric nitrilotriacetate, an iron chelate that induces free radical-mediated oxidative tissue damage. Using high performance liquid chromatography with on-line electrospray ionization tandem mass spectrometry, we also established a highly sensitive method for detection of the cis- and trans-HHP-lysine and confirmed that the 2-nonenal-lysine adducts were indeed formed during the lipid peroxidation-mediated modification of protein in vitro and in vivo. Furthermore, we examined the involvement of the scavenger receptor lectin-like oxidized low density lipoprotein receptor-1 in the recognition of 2-nonenal-modified proteins and established that the receptor recognized the HHP-lysine adducts as a ligand.

Liver sinusoidal endothelial cells are the principal site for elimination of unfractionated heparin from the circulation

The mechanism of elimination of blood borne heparin was studied. To this end unfractionated heparin (UFH) was tagged with FITC, which served as both a visual marker and a site of labeling with (125)I-iodine. UFH labeled in this manner did not alter the anticoagulant activity or binding specificity of the glycosaminoglycan. Labeled heparin administered intravenously to rats (0.1 IU/kg) had a circulatory t(1/2) of 1.7 min, which was increased to 16 min upon coinjection with unlabeled UFH (100 IU/kg). At 15 min after injection, 71% of recovered radioactivity was found in liver. Liver cell separation revealed the following relative uptake capacity, expressed per cell: liver sinusoidal endothelial cell (LSEC)-parenchymal cell-Kupffer cell = 15:3.6:1. Fluorescence microscopy on liver sections showed accumulation of FITC-UFH only in cells lining the liver sinusoids. No fluorescence was detected in parenchymal cells or endothelial cells lining the central vein. Fluorescence microscopy of cultured LSECs following binding of FITC-UFH at 4 degrees C and chasing at 37 degrees C, showed accumulation of the probe in vesicles located at the periphery of the cells after 10 min, with transfer to large, evenly stained vesicles in the perinuclear region after 2 h. Immunogold electron microscopy of LSECs to probe the presence of FITC following injection of FITC-UFH along with BSA-gold to mark lysosomes demonstrated colocalization of the probe with the gold particles in the lysosomal compartment. Receptor-ligand competition experiments in primary cultures of LSECs indicated the presence of a specific heparin receptor, functionally distinct from the hyaluronan/scavenger receptor (Stabilin2). The results suggest a major role for LSECs in heparin elimination.

Beneficial effect of Corni Fructus, a constituent of Hachimi-jio-gan, on advanced glycation end-product-mediated renal injury in Streptozotocin-treated diabetic rats

Previous investigations have demonstrated that Hachimi-jio-gan, a Chinese prescription consisting of eight crude drugs, has a therapeutic potential in diabetes and diabetic nephropathy, using these model rats. To add to these findings, we performed this study to assess whether one of the crude drugs, Corni Fructus (Cornus officinalis SIEB. et ZUCC.), had an effect on streptozotocin-induced diabetic rats as a major active constituent, compared with an inhibitor of advanced glycation end-product (AGE) formation, aminoguanidine. Diabetic rats were orally administrated Corni Fructus extract (50, 100, 200 mg/kg body weight/d) or aminoguanidine (100 mg/kg body weight/d). Treatment with Corni Fructus for 10 d suppressed hyperglycemia, proteinuria, renal AGE formation, and related protein expressions, i.e., receptor for AGEs, nuclear factor-kappaB, transforming growth factor-beta1, and Nepsilon-(carboxymethyl)lysine, in the same way as with aminoguanidine. However, improvement of renal function, shown via serum creatinine (Cr) and Cr clearance, was superior to aminoguanidine treatment. In conclusion, the present study supported the hypothesis that Corni Fructus plays an important role against diabetic pathogenesis, i.e., reducing glucose toxicities, up-regulating renal function, and consequently ameliorating glycation-associated renal damage; thus, this study may provide a new recognition of crude drugs to clarify the mechanisms of Chinese prescriptions.

Soluble RAGE blocks scavenger receptor CD36-mediated uptake of hypochlorite-modified low-density lipoprotein

Engagement of the receptor for advanced glycation end products (RAGE) by its signal transduction ligands evokes inflammatory cell infiltration and activation of the vessel wall. However, soluble RAGE (sRAGE), the truncated form spanning the extracellular binding domain of RAGE, has potent anti-inflammatory properties by acting as a decoy for RAGE ligands. We now show that sRAGE binds with high affinity to atherogenic low-density lipoprotein (LDL) modified by hypochlorous acid (HOCl), the major oxidant generated by the myeloperoxidase-H2O2-chloride system of phagocytes activated during inflammation. We further demonstrate that sRAGE can be coprecipitated with HOCl-LDL from spiked serum. To determine the functional significance of sRAGE binding to HOCl-LDL, cell association studies with macrophages were performed. sRAGE effectively inhibited cellular uptake of HOCl-LDL and subsequent lipid accumulation. Using Chinese hamster ovary cells overexpressing class B scavenger receptor CD36 or SR-BI, two preferential scavenger receptors for HOCl-LDL, we demonstrate that sRAGE only interferes with CD36-mediated uptake of HOCl-LDL. The present findings indicate that sRAGE acts as a sink for HOCl-LDL, which is abundantly present in human atherosclerotic lesions. We propose that sRAGE represents a physiological antagonist that interferes with scavenger receptor-mediated cholesterol accumulation and foam cell formation of macrophages.

Hypochlorite-modified albumin colocalizes with RAGE in the artery wall and promotes MCP-1 expression via the RAGE-Erk1/2 MAP-kinase pathway

Signal transduction via the endothelial receptor for advanced glycation end products (RAGE) plays a key role in vascular inflammation. Recent observations have shown that the myeloperoxidase-H2O2-chloride system of activated phagocytes is highly up-regulated under inflammatory conditions where hypochlorous acid (HOCl) is formed as the major oxidant. Albumin, an in vivo carrier for myeloperoxidase is highly vulnerable to oxidation and a major representative of circulating advanced oxidized proteins during inflammatory diseases. Immunohistochemical studies performed in the present study revealed marked colocalization of HOCl-modified epitopes with RAGE and albumin in sections of human atheroma, mainly at the endothelial lining. We show that albumin modified with physiologically relevant concentrations of HOCl, added as reagent or generated by the myeloperoxidase-H2O2-chloride system, is a high affinity ligand for RAGE. Albumin, modified by HOCl in the absence of free amino acids/carbohydrates/lipids to exclude formation of AGE-like structures, induced a rapid, RAGE-dependent activation of extracellular signal-regulated kinase 1/2 and up-regulation of the proinflammatory mediator monocyte chemoattractant protein-1. Cellular activation could be blocked either by a specific polyclonal anti-RAGE IgG and/or a specific mitogen-activated protein-kinase kinase inhibitor. The present study demonstrates that HOCl-modified albumin acts as a ligand for RAGE and promotes RAGE-mediated inflammatory complications.

Toxicity of the AGEs generated from the Maillard reaction: On the relationship of food-AGEs and biological-AGEs

Advanced glycation end products (AGEs) are generated in the late stages of Maillard reaction in foods and biological systems. These products are mostly formed by the reactions of reducing sugar or degradation products of carbohydrates, lipids, and ascorbic acid. AGEs exist in high concentration in foods, but in relatively low concentrations in most of the biological systems. Recently, some AGEs have been reported to be toxic, and were proposed to be causative factors for various kinds of diseases, especially diabetes and kidney disorder, through the association with receptor of AGE (RAGE). It has also been reported that food-derived AGEs (food-AGEs) may not be a causative factor for pro-oxidation. However, the relationship of food-AGEs and biological-derived AGEs (biological-AGEs) is not clear. In this review, the following issues are discussed: the formation of AGEs in foods and biological systems; identification of the main AGEs in foods and biological systems; absorption of food-AGEs; the effects of AGEs in vivo; relationship between food-AGEs and biological-AGEs; possible defense mechanism against AGEs in vivo and finally, the problems to be solved concerning the toxicity of AGEs.

Association of Advanced Glycation End Products with A549 Cells, a Human Pulmonary Epithelial Cell Line, Is Mediated by a Receptor Distinct from the Scavenger Receptor Family and RAGE

Cellular interactions with advanced glycation end products (AGE)-modified proteins are known to induce several biological responses, not only endocytic uptake and degradation, but also the induction of cytokines and growth factors, combined responses that may be linked to the development of diabetic vascular complications. In this study we demonstrate that A549 cells, a human pulmonary epithelial cell line, possess a specific binding site for AGE-modified bovine serum albumin (AGE-BSA) (K(d) = 27.8 nM), and additionally for EN-RAGE (extracellular newly identified RAGE binding protein) (K(d) = 118 nM). Western blot and RT-PCR analysis showed that RAGE (receptor for AGE) is highly expressed on A549 cells, while the expression of other known AGE-receptors such as galectin-3 and SR-A (class A scavenger receptor), are below the level of detection. The binding of (125)I-AGE-BSA to these cells is inhibited by unlabeled AGE-BSA, but not by EN-RAGE. In contrast, the binding of (125)I-EN-RAGE is significantly inhibited by unlabeled EN-RAGE and soluble RAGE, but not by AGE-BSA. Our results indicate that A549 cells possess at least two binding sites, one specific for EN-RAGE and the other specific for AGE-BSA. The latter receptor on A549 cells is distinct from the scavenger receptor family and RAGE.

Advanced glycosylation end products might promote atherosclerosis through inducing the immune maturation of dendritic cells

OBJECTIVE

Both advanced glycosylation end products (AGEs) and dendritic cells (DCs) have been shown to play a causative role in atherosclerosis. However, whether they function interactively in the process remains uncertain. We therefore studied the effects of AGE-bovine serum albumin (AGE-BSA) on the maturation of DCs and the expressions of scavenger receptor-A (SR-A) and receptor for AGEs (RAGE) on DCs.

METHODS AND RESULTS

AGE-BSA induced DCs maturation accompanied with increased expressions of CD1a, CD40, CD80, CD83, CD86, and MHC class II. The capacity of DCs to stimulate T-cell proliferation and secretion of cytokines (interferon [IFN], IFN-gamma, interleukin [IL]-10 and IL-12) was also enhanced by AGE-BSA. AGE-BSA significantly upregulated SR-A and RAGE expression on DCs and the upregulation was abolished by inhibition of mitogen-activated protein (MAP) kinase Jnk, but not by that of Erk and p38 MAP kinase. AGE-BSA-induced expression of CD83 and secretion of IL-12 were partly inhibited by either an anti-RAGE neutralizing antibody or a Jnk inhibitor.

CONCLUSIONS

AGE-BSA induces maturation of DCs and augmented their capacity to stimulate T-cell proliferation and cytokine secretions possibly through upregulation of RAGE and SR-A, which at least in part through Jnk. These findings might explain in part the interactive roles of AGEs and DCs in the processes of atherosclerosis.

Scavenger receptors on sinusoidal liver endothelial cells are involved in the uptake of aldehyde-modified proteins

Scavenger receptors on sinusoidal liver endothelial cells (SECs) eliminate potentially harmful modified proteins circulating through the liver. It was shown recently that aldehyde-modified proteins bind to scavenger receptors and are associated with the development/progression of alcoholic liver diseases. For these studies, rat livers were perfused in situ with 125I-formaldehyde-bovine serum albumin (f-Alb) or 125I-malondialdehyde-acetaldehyde-bovine serum albumin (MAA-Alb) in the presence of known scavenger receptor ligands as inhibitors. Reverse transcription-polymerase chain reaction (RT-PCR) analysis and scavenger receptor Type A (SRA) knock-out mice were used to assess the role of these receptors in mediating immune responses. The degradation of 125I-f-Alb or 125I-MAA-Alb in whole livers and isolated SECs can be inhibited by known scavenger receptor ligands, including f-Alb, maleylated bovine albumin, and fucoidan. 125I-f-Alb could not be completely inhibited by MAA-Alb. In contrast, 125I-MAA-Alb was only partially inhibited with advanced glycosylated endproduct albumin. RT-PCR data show the presence of a number of scavenger receptors on SECs that may be responsible for the binding of MAA-modified proteins. SRA seems to be one of these receptors involved in the effects mediated by MAA-modified proteins. In a study using SRA knockout mice, it was shown that a decreased antibody response to MAA-Alb resulted. By RT-PCR, CD36, LOX-1, and SR-AI are the scavenger receptors most likely involved in the degradation of MAA-Alb.

Advanced glycosylation end products induce inducible nitric oxide synthase (iNOS) expression via a p38 MAPK-dependent pathway

BACKGROUND

Advanced glycosylation end products (AGEs) accumulation in tissue has been implicated in diabetic related complications, including diabetic nephropathy. Activation of peroxisome proliferator activated receptor-gamma (PPAR-gamma) ameliorates diabetic nephropathy.

METHODS

In the present study, we investigated the effects of AGEs on inducible nitric oxide synthase (iNOS) expression and nitric oxide production, and the effects of rosiglitazone, an activator of PPAR-gamma, on AGE-induced iNOS expression and nitrite release in glomerular mesangial cells.

RESULTS

AGEs caused a dose- and time-dependent increase of iNOS induction and nitrite accumulation in mesangial cells. A protein tyrosine kinase inhibitor (genistein), or a p38 mitogen-activated protein kinase (MAPK) inhibitor (SB203580) suppressed AGE-induced iNOS expression and nitrite release from mesangial cells. Addition of bovine serum albumin (BSA)-AGEs to mesangial cells increased p38 MAPK activities. Activation of PPAR-gamma by rosiglitazone inhibited AGE-induced iNOS expression, nitrite release, and p38 MAPK activation in mesangial cells. AGE-stimulated nitrite release was attenuated by pretreatment with anti-tumor necrosis factor-alpha (TNF-alpha) and anti-transforming growth factor-beta (TGF-beta) antibodies. AGE-induced iNOS expression was inhibited by treatment with a nuclear factor-kappaB (NF-kappaB) inhibitor, pyrrolidone dithiocarbamate. Addition of BSA-AGEs to mesangial cells stimulated p65 NF-kappaB translocation from the cytosol to the nucleus.

CONCLUSION

These data suggest that cytokine release, NF-kappaB and p38 MAPK-dependent pathways may play a role in AGE-induced iNOS expression and subsequent nitric oxide production in mesangial cells. Rosiglitazone may prevent AGE-induced iNOS expression by interfering with p38 MAPK activity.

Human serum opsonization of orthopedic biomaterial particles: protein-binding and monocyte/macrophage activation in vitro

Wear particles generated after total joint arthroplasty activate monocyte/macrophages and incite formation of a granulomatous periprosthetic tissue associated with bone loss and implant loosening. This study tested the hypothesis that selective opsonization of orthopedic implant biomaterial wear particles by human serum proteins influences monocyte/macrophage activation. Serum protein binding to metallic, polymeric, and ceramic particles was determined by one-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Individual proteins bound to particles were subsequently identified using two-dimensional SDS-PAGE, microsequencing techniques, and SWISS-PROT analysis. Effects of selective protein opsonization on particle-induced monocyte/macrophage activation were assessed by quantification of interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha release. Results from one-dimensional gel analyses revealed distinct serum protein-binding patterns specific for each material tested. Two-dimensional gel analysis together with amino acid sequencing of the prominent protein species confirmed the presence of albumin and alpha-1-antitrypsin bound to all particles tested. In contrast to the metallic particles, apolipoprotein was a major species associated with polymeric particles. Opsonization of PMMA particles with purified preparations of each of the identified proteins showed that albumin significantly enhanced particle-induced monocyte/macrophage activation. These data confirm orthopedic biomaterial specific binding of human serum proteins and demonstrate that albumin exacerbates particle-induced monocyte/macrophage activation. Alterations in the chemical and surface properties of orthopedic biomaterials to modulate protein interactions may improve implant longevity.

Role of megalin in endocytosis of advanced glycation end products: implications for a novel protein binding to both megalin and advanced glycation end products

Advanced glycation end products (AGE) are filtered by glomeruli and reabsorbed and metabolized by proximal tubule cells (PTC). In renal failure, decreased renal AGE metabolism likely accounts for the accumulation in serum that is related to uremic complications. In diabetes, AGE generation is increased, and the handling mechanisms in PTC are likely associated with the pathogenesis of tubulointerstitial injury. It is therefore important to clarify the mechanisms of the AGE metabolism to develop a strategy for removing AGE in uremia and to elucidate the pathogenesis of diabetic nephropathy. To this end, this study focused on the molecular analysis of megalin, a multi-ligand endocytic receptor, in PTC. AGE uptake analysis was performed using the rat yolk sac-derived L2 cell line system established for the analysis of megalin's endocytic functions. The cells mediated specific internalization and degradation of AGE, which were significantly blocked by anti-megalin IgG, indicating that megalin is involved in the cellular processes. However, cell surface AGE-binding assays and ligand blot analysis revealed no evidence that megalin is a direct AGE receptor. Affinity chromatography and ligand blot analysis originally revealed that 200-kD and 400-kD proteins in the cells bind to AGE and the 200-kD protein to megalin in a Ca(2+)-dependent manner. The binding of megalin with the 200-kD protein was suppressed by receptor-associated protein (RAP), a ligand for megalin. In conclusion, megalin functions for endocytosis of AGE via an indirect mechanism. L2 cells express novel AGE-binding proteins, one of which may interact with megalin.

CD36-mediated endocytic uptake of advanced glycation end products (AGE) in mouse 3T3-L1 and human subcutaneous adipocytes

Interaction of advanced glycation end products (AGE) with AGE receptors induces several cellular phenomena potentially relating to diabetic complications. We here show that AGE-modified bovine serum albumin (BSA) is endocytosed by adipocytes via CD36. Upon differentiation, 3T3-L1 and human subcutaneous adipose cells showed marked increases in endocytic uptake and subsequent degradation of [(125)I]AGE-BSA, which were inhibited effectively by the anti-CD36 antibody. Ligand specificity of CD36 for modified BSAs was compared with that of LOX-1 and scavenger receptor class A. Effect of fucoidan on [(125)I]AGE-BSA binding showed a sharp contrast to that on [(125)I]-oxidized low density lipoprotein. These results implicate that CD36-mediated interaction of AGE-modified proteins with adipocytes might play a pathological role in obesity or insulin-resistance.

Carbonyl Stress and Diabetic Complications

Advanced glycation irreversibly and progressively modifies proteins over time and yields the advanced glycation end-products (AGE). AGEs are thought to contribute to the development of atherosclerosis and of diabetic and uremic complications. Their inhibition has thus become a therapeutic goal. In this article, we discuss the role of various reactive carbonyl compound (RCOs) in the genesis of AGEs, postulate the existence of "carbonyl stress" in complicated diabetes and, finally, discuss therapeutic perspectives.

Relative quantification of N(epsilon)-(Carboxymethyl)lysine, imidazolone A, and the Amadori product in glycated lysozyme by MALDI-TOF mass spectrometry

The nonenzymatic glycation of proteins by reducing sugars, also known as the Maillard reaction, has received increasing recognition from nutritional science and medical research. In this study, we applied matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) to perform relative and simultaneous quantification of the Amadori product, which is an early glycation product, and of N(epsilon)-(carboxymethyl)lysine and imidazolone A, two important advanced glycation end products. Therefore, native lysozyme was incubated with d-glucose for increasing periods of time (1, 4, 8, and 16 weeks) in phosphate-buffered saline pH 7.8 at 50 degrees C. After enzymatic digestion with endoproteinase Glu-C, the N-terminal peptide fragment (m/z 838; amino acid sequence KVFGRCE) and the C-terminal peptide fragment (m/z 1202; amino acid sequence VQAWIRGCRL) were used for relative quantification of the three Maillard products. Amadori product, N(epsilon)-(carboxymethyl)lysine, and imidazolone A were the main glycation products formed under these conditions. Their formation was dependent on glucose concentration and reaction time. The kinetics were similar to those obtained by competitive ELISA, an established method for quantification of N(epsilon)-(carboxymethyl)lysine and imidazolone A. Inhibition experiments showed that coincubation with N(alpha)-acetylargine suppressed formation of imidazolone A but not of the Amadori product or N(epsilon)-(carboxymethyl)lysine. The presence of N(alpha)-acetyllysine resulted in the inhibition of lysine modifications but in higher concentrations of imidazolone A. o-Phenylenediamine decreased the yield of the Amadori product and completely inhibited the formation of N(epsilon)-(carboxymethyl)lysine and imidazolone A. MALDI-TOF-MS proved to be a new analytical tool for the simultaneous, relative quantification of specific products of the Maillard reaction. For the first time, kinetic data of defined products on specific sites of glycated protein could be measured. This characterizes MALDI-TOF-MS as a valuable method for monitoring the Maillard reaction in the course of food processing.

Glycation by ascorbic acid causes loss of activity of ribulose-1,5-bisphosphate carboxylase/oxygenase and its increased susceptibility to proteases

Glycation is a process whereby sugar molecules form a covalent adduct with protein amino groups. In this study, we used ascorbic acid (AsA) as a glycating agent and purified cucumber (Cucumis sativus L.) ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) as a model protein in chloroplast tissues, and examined effects of glycation on the activity and susceptibility of Rubisco to proteases. Glycation proceeded via two phases during incubation with AsA and Rubisco in vitro at physiological conditions (10 mM AsA, pH 7.5, 25 degrees C in the presence of atmospheric oxygen). At the early stage of glycation (phase 1), the amount of AsA attaching to Rubisco increased at an almost linear rate (0.5-0.7 mol AsA incorporated (mol Rubisco)(-1) d(-1)). By Western blotting using monoclonal antibodies recognizing glycation adducts, a major glycation adduct, N( epsilon )-(carboxymethyl)lysine was detected. At the late stage of glycation (phase 2), incorporation of AsA reached saturation, and a glycation adduct, pentosidine mediating intramolecular cross-linking, was detected corresponding to formation of high molecular weight aggregates cross-linked between subunits. Glycation led to a decrease in Rubisco activity (half-life about 7-8 d). Furthermore, glycated Rubisco of phase 2 drastically increased protease susceptibility in contrast to unchanged susceptibility of glycated Rubisco of phase 1 compared to that of native Rubisco. Results obtained here suggest that AsA is possibly an important factor in the loss of activity and turnover of Rubisco.

Nepsilon-(Carboxymethyl)lysine induces gamma-glutamylcysteine synthetase in RAW264.7 cells

Advanced glycation end products (AGEs) play an important role in the development of angiopathy in diabetes mellitus and atherosclerosis. Here, we show that adducts of N(epsilon)-(carboxymethyl)lysine (CML), a major AGE, and bovine serum albumin (CML-BSA) stimulated gamma-glutamylcysteine synthetase (gamma-GCS), which is a key enzyme of glutathione (GSH) synthesis, in RAW264.7 mouse macrophage-like cells. CML-BSA stimulated the expression of gamma-GCS heavy subunit (h) time- and dose-dependently and concomitantly increased GSH levels. CML-BSA also stimulated DNA-binding activity of activator protein-1 (AP-1) within 3h, but the stimulatory effect decreased in 5h, and nuclear factor-kappaB (NF-kappaB) with a peak activity at 1h and the stimulatory effect diminished in 3h. Studies of luciferase activity of the gamma-GCSh promoter showed that deletion and mutagenesis of the AP-1-site abolished CML-BSA-induced up-regulation, while that of NF-kappaB-site did not affect CML-BSA-induced activity. CML-BSA also stimulated the activity of protein kinase C, Ras/Raf-1, and MEK/ERK1/2. Inhibition of ERK1/2 abolished CML-BSA-stimulated AP-1 DNA-binding activity and gamma-GCSh mRNA expression. Our results suggest that induction of gamma-GCS by CML adducts seems to increase the defense potential of cells against oxidative stress produced during glycation processes.

Early-glycation of apolipoprotein E: effect on its binding to LDL receptor, scavenger receptor A and heparan sulfates

Glycation is responsible for disruption of lipoprotein functions leading to the development of atherosclerosis in diabetes. The effects of apolipoprotein E (apoE) glycation were investigated with respect to its interaction with receptors. The interaction of apoE with the low density lipoprotein receptor (LDL-R) and scavenger receptor A (SR-A) was measured by competition experiments performed using, respectively, on a human fibroblast cell line 125I-LDL, and on a murine macrophage cell line (J774) 125I-acetylated LDL, and unlabeled apoE/phospholipid complexes. Glycated apoE binding to heparin and heparan sulfates (HS) was assessed by surface plasmon resonance (SPR) technology. Site-directed mutagenesis was then performed on Lys-75, the major glycation site of the protein. The prepared mutant protein proved to be useful as a tool to study the role of Lys-75 in apoE glycation. The findings showed that, although glycation has no effect on apoE binding either to the LDL-R or to SR-A, it impairs its binding to immobilized heparin and HS. The glycation of Lys-75 was found to be proceed rapidly and contributed significantly to total protein glycation. We propose that, in the case of diabetes, glycation may lead to the atherogenicity of apoE-containing lipoproteins disturbing their uptake via the HS proteoglycan pathway.

Photosensitization of DNA damage by glycated proteins

Photosensitized DNA damage in skin is thought to be an important mechanism of UV phototoxicity. Here we demonstrate that proteins modified by advanced glycation endproducts (AGE-proteins) are photosensitizers of DNA damage and show that multiple mechanisms are involved in AGE-sensitization. AGE-chromophores accumulate on long-lived skin proteins such as collagen and elastin as a consequence of glycation, the spontaneous amino-carbonyl reaction of protein-bound lysine and arginine residues with reactive carbonyl species. AGE-proteins accumulate in both the nucleus and the cytoplasm of mammalian cells. To test the hypothesis that protein-bound AGEs in close proximity to DNA are potent UV-photosensitizers, a simple plasmid DNA cleavage assay was established. Irradiation of supercoiled phiX 174 DNA with solar simulated light in the presence of AGE-modified bovine serum albumin or AGE-modified RNAse A induced DNA single strand breaks. The sensitization potency of the glycated protein correlated with increased AGE-modification and the unmodified protein displayed no photosensitizing activity. AGE-sensitized formation of reactive oxygen species was not fully responsible for the observed DNA damage and other mechanisms such as direct electron transfer interaction between photoexcited AGE and DNA are likely to be involved. Glycated proteins in skin may equally function as potent photosensitizers of DNA damage with implications for photoaging and photocarcinogenesis.

Reactive oxygen species accelerate production of vascular endothelial growth factor by advanced glycation end products in RAW264.7 mouse macrophages

Advanced glycation end products (AGEs) are believed to play an important role in the development of angiopathy in diabetes mellitus. Previous reports suggested a correlation between accumulation of AGEs and production of vascular endothelial growth factor (VEGF) in human diabetic retina. However, the mechanisms involved were not revealed. In this study, we investigated the transcriptional regulation of the expression of vascular endothelial growth factor (VEGF) by AGEs, and possible involvement of reactive oxygen species (ROS) in the induction. We employed an AGE of bovine serum albumin (BSA) prepared by an incubation of BSA with D-glucose for 40 weeks and N(epsilon)-(carboxymethyl)lysine (CML), a major AGE. The expression of VEGF was induced by CML-BSA in RAW264.7 mouse macrophage-like cells. CML-BSA stimulated the DNA-binding activity of activator protein-1 (AP-1). Promoter assay showed that the induction of VEGF was dependent on AP-1. The activity of Ras/Raf-1/MEK/ERK1/2 was involved in the CML-BSA-stimulated signaling pathways to activate the AP-1 transcription with a peak at 1 h. AGE-BSA also induced VEGF mediated by AP-1, however, there was a difference of effect between AGE-BSA and CML-BSA in the activation of AP-1. AGE-BSA-stimulated AP-1 activity showed a peak at 5 h, which paralleled the formation of ROS. Reduction of AGE-BSA with NaBH(4) or addition of vitamin E attenuated the AGE-BSA-stimulated signaling pathways leading to the same pattern as for CML-BSA-stimulated signals. These results suggest an important role for AGEs in stimulation of the development of angiogenesis observed in diabetic complications, and that ROS accelerates the AGE-stimulated VEGF expression.

Uptake of acetaldehyde-modified (ethylated) low-density lipoproteins by mouse peritoneal macrophages

The uptake of acetaldehyde-modified (ethylated) low-density lipoproteins (LDLs) by murine peritoneal macrophages is described and compared with the uptake of acetylated LDLs. The fluorescent marker DiI was used. No competition between ethylated and acetylated LDLs was observed. Ethylated LDL uptake was not inhibited by polyinosinic acid or fucoidin. Our conclusion is that uptake of ethylated and acetylated LDLs can be done by two different receptors.

Adduction of soluble proteins with malondialdehyde-acetaldehyde (MAA) induces antibody production and enhances T-cell proliferation

BACKGROUND

The alcohol metabolites malondialdehyde and acetaldehyde can combine to form stable adducts (MAA) which are found in the livers of humans and rats after significant alcohol ingestion. While adducted proteins induce antibody responses in the absence of adjuvants, the mechanisms by which these responses occur are unknown. Thus, it was the purpose of these studies to investigate how MAA modification stimulates antibody and T-cell responses in the absence of adjuvants.

METHODS

Hen egg lysozyme (HEL) was modified with increasing levels of MAA and was used as an immunogen, and antibody and T-cell responses were determined. The role of scavenger receptors in the immunogenicity of MAA-adducted proteins was also investigated.

RESULTS

Maximum antibody response was induced after immunization with 1.8 nM MAA/nM HEL, and was primarily an IgG1 response to HEL as determined by inhibition ELISAs. T-cell proliferative responses after immunization with HEL-MAA were solely to HEL. Immunization with a scavenger receptor ligand in conjunction with HEL-MAA increased the predominant IgG1 response and sharply decreased the IgG2a response by approximately 50%. Binding of HEL-MAA by splenocytes was determined by flow cytometry to be approximately 15% greater than HEL alone, showing a doubling of the geometric mean fluorescence. Also, most of the cells that bound HEL-MAA were class II positive, indicating that antigen-presenting cells can bind the MAA-adducted HEL, and potentially initiate immune responses.

CONCLUSIONS

MAA modification of proteins induces antibody and T-cell proliferative responses in vivo. Initial studies suggest that these responses may be mediated by scavenger receptors that recognize MAA-adducted proteins. This suggests a mechanism by which proteins modified with oxidative products associated with chronic ethanol consumption may alter immune responses that may play an active role in the development and/or progression of alcoholic liver disease.

Diabetes and advanced glycation endproducts

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

Lectin-like oxidized low density lipoprotein receptor-1 (LOX-1) serves as an endothelial receptor for advanced glycation end products (AGE)

Advanced glycation end products (AGE) are known to serve as ligands for the scavenger receptors such as SR-A, CD36 and SR-BI. In the current study, we examined whether AGE is recognized by lectin-like oxidized low density lipoprotein receptor-1 (LOX-1). Cellular binding experiments revealed that AGE-bovine serum albumin (AGE-BSA) showed the specific binding to CHO cells overexpressing bovine LOX-1 (BLOX-1), which was effectively suppressed by an anti-BLOX-1 antibody. Cultured bovine aortic endothelial cells also showed the specific binding for AGE-BSA, which was suppressed by 67% by the anti-BLOX-1 antibody. Thus, LOX-1 is identified as a novel endothelial receptor for AGE.

In vitro effect of advanced glycation end-products on human polymorphonuclear superoxide production

BACKGROUND

Advanced glycation end-products (AGEs) are elevated in the sera of diabetic patients. The latter are prone to severe bacterial infections. Advanced glycation end-products have been shown to modulate immune competent cell activities. In this study we examined the in vitro effect of advanced glycation end-products on superoxide anion generation by human polymorphonuclear leukocytes.

MATERIALS AND METHODS

Advanced glycation end-products were prepared by incubation of bovine serum albumin (BSA) with glucose for 90 days. Superoxide production was measured as the superoxide dismutase-inhibitable reduction of ferricytochrome c. The effect of advanced glycation end-products on superoxide production was evaluated in both baseline (nonstimulated) and stimulated (by either formyl-methionyl-leucyl-phenylalanine, or phorbol-myristate-acetate) polymorphonuclear leukocytes.

RESULTS

The baseline superoxide production of polymorphonuclear leukocytes was significantly increased by advanced glycation end-products in a dose-dependent manner. In contrast, in stimulated polymorphonuclear leukocytes advanced glycation end-products significantly inhibited superoxide production, again in a dose-dependent manner. This inhibitory effect of advanced glycation end-products was observed after dialyzing AGE-BSA, thereby eliminating the possible influence of reactive carbohydrates. No modification of superoxide production was seen with BSA and only a mild inhibitory effect of glucose at high concentrations.

CONCLUSIONS

Advanced glycation end-products depress superoxide production by stimulated polymorphonuclear leukocytes. As superoxide plays an essential role in bactericidal activity, this polymorphonuclear leukocyte dysfunction may be a contributory factor to the increased prevalence and severity of bacterial infection seen in diabetic patients.

The AGE-receptor in the pathogenesis of diabetic complications

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

Cd36, a member of the class b scavenger receptor family, as a receptor for advanced glycation end products

Interaction of advanced glycation end products (AGE) with AGE receptors induces several cellular phenomena potentially relating to diabetic complications. Five AGE receptors identified so far are RAGE (receptor for AGE), galectin-3, 80K-H, OST-48, and SRA (macrophage scavenger receptor class A types I and II). Since SRA is known to belong to the class A scavenger receptor family, and the scavenger receptor collectively represents a family of multiligand lipoprotein receptors, it is possible that CD36, although belonging to the class B scavenger receptor family, can recognize AGE proteins as ligands. This was tested at the cellular level in this study using Chinese hamster ovary (CHO) cells overexpressing human CD36 (CD36-CHO cells). Cellular expression of CD36 was confirmed by immunoblotting and immunofluorescent microscopy using anti-CD36 antibody. Upon incubation at 37 degrees C, (125)I-AGE-bovine serum albumin (AGE-BSA) and (125)I-oxidized low density lipoprotein (LDL), an authentic ligand for CD36, were endocytosed in a dose-dependent fashion and underwent lysosomal degradation by CD36-CHO cells, but not wild-type CHO cells. In binding experiments at 4 degrees C, (125)I-AGE-BSA exhibited specific and saturable binding to CD36-CHO cells (K(d) = 5.6 microg/ml). The endocytic uptake of (125)I-AGE-BSA by these cells was inhibited by 50% by oxidized LDL and by 60% by FA6-152, an anti-CD36 antibody inhibiting cellular binding of oxidized LDL. Our results indicate that CD36 expressed by these cells mediates the endocytic uptake and subsequent intracellular degradation of AGE proteins. Since CD36 is one of the major oxidized LDL receptors and is up-regulated in macrophage- and smooth muscle cell-derived foam cells in human atherosclerotic lesions, these results suggest that, like oxidized LDL, AGE proteins generated in situ are recognized by CD36, which might contribute to the pathogenesis of diabetic macrovascular complications.

Resveratrol inhibits AGEs-induced proliferation and collagen synthesis activity in vascular smooth muscle cells from stroke-prone spontaneously hypertensive rats

Advanced glycation end-products (AGEs) of plasma proteins and/or matrix proteins are candidate mediators for various vascular complications such as atherosclerosis. We previously reported a significantly larger accumulation of AGEs of the aorta in stroke-prone spontaneously hypertensive rats (SHRSP) than in age-matched Wistar-Kyoto rats (WKY). In this study, we examined the effects of AGEs on vascular smooth muscle cells (VSMC) from SHRSP and WKY rats. We also studied the in vitro effects of resveratrol (3, 4',5-trihydroxystilbene), a natural phytestrogen, on VSMC proliferation, DNA synthesis, and collagen synthesis activity in SHRSP-VSMC. AGEs accelerated the proliferation of SHRSP- or WKY-VSMC in a time- and dose-dependent manner. VSMC from SHRSP were more sensitive to AGEs than VSMC from normotensive WKY. AGEs also significantly increased DNA synthesis and prolyl hydroxylase activity, a marker for collagen synthesis, in SHRSP-VSMC. AGEs-induced increases in TGF-beta1 mRNA in SHRSP-VSMC were significantly greater than in WKY-VSMC. Resveratrol inhibited AGEs-stimulated proliferation, DNA synthesis, and prolyl hydroxylase activity in SHRSP-VSMC in a dose-dependent manner. ICI 182780, a specific estrogen receptor antagonist, partly blocked the inhibitory effects of resveratrol on AGEs-stimulated proliferation, DNA synthesis, and prolyl hydroxylase activity. Resveratrol significantly inhibited AGEs-induced TGF-beta1 mRNA increases in a dose-dependent manner. Thus, resveratrol may confer protective effects on the cardiovascular system by attenuating vascular remodeling and may be clinically useful as a safer substitute for feminizing estrogens in preventing cardiovascular disease.

Increased levels of advanced glycosylation end products in the kidney and liver from spontaneously diabetic Chinese hamsters determined by immunochemical assay

Increased levels of advanced glycosylation end products (AGEs) have been reported in tissues in association with diabetes mellitus. Thus, we measured tissue AGE levels and detected an accumulation of AGEs in the kidney and liver from spontaneously diabetic Chinese hamsters (CHAD) to determine the relationship between AGEs and diabetes mellitus. Diabetic CHAD aged 12 to 13 months were studied together with age-matched nondiabetic CHAD. We used an AGE-specific noncompetitive enzyme-linked immunosorbent assay (ELISA) with polyclonal anti-AGE-bovine serum albumin (BSA) antibody to measure tissue AGE levels. The samples extracted from the kidney and liver obtained from diabetic and nondiabetic CHAD reacted with anti-AGE-BSA antibody. When the absorbance of standard AGE-BSA (0.1 microg/mL) was expressed as 1 U, AGE levels in the kidney and liver from diabetic CHAD were significantly increased as compared with nondiabetic CHAD (kidney, 0.26 +/- 0.05 v 0.10 +/- 0.03 U/microg protein, P< .01; liver, 0.20 +/- 0.03 v 0.09 +/- 0.02 U/microg protein, P< .01). Positive AGE staining was observed in the renal cortex, especially in the tubules of diabetic CHAD, but little AGE staining was observed in the glomerulus by the immunohistochemical study. AGE staining was diffuse in the hepatocytes. These AGE levels were significantly correlated with fasting plasma glucose and glycated hemoglobin (P < .01, respectively). In conclusion, we have confirmed that AGE structures were expressed in the kidney and liver from CHAD, and these AGE levels were increased in diabetic CHAD. AGE staining was observed in the renal tubules and hepatocytes. Tissue AGE levels were positively correlated with glycemic control in CHAD.