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

The role of macrophage scavenger receptor 1 (MSR1) in inflammatory disorders and cancer

Macrophage scavenger receptor 1 (MSR1), also named CD204, holds key inflammatory roles in multiple pathophysiologic processes. Present primarily on the surface of various types of macrophage, this receptor variably affects processes such as atherosclerosis, innate and adaptive immunity, lung and liver disease, and more recently, cancer. As highlighted throughout this review, the role of MSR1 is often dichotomous, being either host protective or detrimental to the pathogenesis of disease. We will discuss the role of MSR1 in health and disease with a focus on the molecular mechanisms influencing MSR1 expression, how altered expression affects disease process and macrophage function, the limited cell signalling pathways discovered thus far, the emerging role of MSR1 in tumour associated macrophages as well as the therapeutic potential of targeting MSR1.

Glycation of albumin and its implication in Diabetes: A comprehensive analysis using mass spectrometry

AIM

To understand the mechanism of glycation of albumin and effects on cysteinylation and methionine oxidation.

METHODS

The in vitro glycation of HSA and BSA was studied with varying concentrations of glucose. Clinical blood samples of diabetic subjects with varying HbA1c values, were analyzed to assess in vivo glycation. All samples and their tryptic digests were analyzed using liquid chromatography/mass spectrometry. Glycation sites were mapped on to the three-dimensional structure of the HSA and BSA.

RESULTS

A total thirty-one sites for glycation and eight sites of N^ε-carboxymethyl-lysine (CML) modification were identified on albumin. The site selectivity of glycation was correlated with the environment of the reactive residue in the three-dimensional structure.

CONCLUSIONS

The maximum percentage glycation under extreme conditions was in the range of ~55 to 88% in four weeks. Two major glycation sites K-233 and K-525 were identified, which together accounted for 40-50% of total glycation. A correlation was observed between glycation and oxidation of methionine residues in samples glycated in vitro. The role of spatially proximate residues in facilitating the glycation process is evident. The tri- and tetra-glycated isoforms of albumin can serve as biomarkers for the severe uncontrolled diabetic state.

Low-Dose Radiation Promotes the Proliferation and Migration of AGE-Treated Endothelial Progenitor Cells Derived from Bone Marrow via Activating SDF-1/CXCR4/ERK Signaling Pathway

Low-dose radiation (LDR) has been confirmed to mobilize bone marrow-derived endothelial progenitor cells (EPCs) and promote diabetic wound healing. But it is unclear whether LDR acts directly on EPCs and promotes their proliferation and migration. Given the key role of advanced glycosylation end products (AGE) in the pathogenesis of diabetes, we used AGE to induce EPC damage. We then investigated the effect of LDR on the proliferation and migration of AGE-treated EPCs and explored the underlying mechanisms. EPCs cultured in vitro were treated with different concentrations of AGE, and the cells were then exposed to different low doses and treated with a specific antagonist for CXCR4, AMD3100 (1 lmol/l). The proliferation and migration abilities of EPCs were detected using the CCK-8 and wound healing assays, respectively. The mRNA and protein expression of SDF-1 and CXCR4 in AGE-treated EPCs were measured using qPCR and Western blot analysis, respectively. The expressions of ERK and phosphorylated ERK (pERK) were detected using Western blot analysis. The results showed that 200 mg/l and 400 mg/l AGE had an inhibitory effect on the proliferation of EPCs, and this inhibitory effect was exerted in a dose- and time-dependent manner. AGE significantly reduced the migration ability of EPCs cultured in vitro. After the cells received either 50 or 75 mGy low-dose irradiation, the proliferation of EPCs and AGE-treated EPCs was clearly increased; in addition, LDR also enhanced cell migration ability, but this enhancement was counteracted by AMD3100. Results from qPCR and Western blot analysis showed that LDR increased the mRNA and protein expression of SDF-1/ CXCR4. LDR also upregulated pERK expression in EPCs and AGE-treated EPCs, but LDR-induced upregulation of pERK expression was inhibited by AMD3100. These findings indicate that LDR can directly activate the SDF-1/CXCR4 biological axis and downstream ERK signaling pathway, and promote the proliferation and migration abilities of EPCs by increasing the expression of SDF-1, CXCR4 and pERK in EPCs.

Antiglycating potential of gum arabic capped-silver nanoparticles

Advanced glycation end products are major contributors to the pathology of diabetes, Alzheimer's disease, and atherosclerosis; accordingly, identification of antiglycation compounds is attracting considerable interest. In the present study, the inhibitory effect of gum arabic capped-silver nanoparticles on advanced glycation end products formation was monitored by several biophysical techniques. Silver nanoparticles were characterized by ultraviolet-visible, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Bovine serum albumin and methylglyoxal mixtures incubated with increasing concentrations of silver nanoparticles showed significant reductions in advanced glycation end product formation that were confirmed by ultraviolet-visible, fluorescence spectrometry, and high-performance liquid chromatography techniques. High-performance liquid chromatography showed decreased adduct formation of glycated protein in the presence of silver nanoparticles. The structural changes induced by silver nanoparticles were further confirmed by circular dichroism and Fourier transform infrared spectroscopy. Strong inhibition of advanced glycation end product formation was observed in the presence of elevated silver nanoparticles. The results of this study suggest that silver nanoparticles are a potent antiglycating agent.

Glycation of glutamate cysteine ligase by 2-deoxy-d-ribose and its potential impact on chemoresistance in glioblastoma

The antioxidant glutathione (GSH) plays a critical role in maintaining intracellular redox homeostasis but in tumors the GSH biosynthetic pathway is often dysregulated, contributing to tumor resistance to radiation and chemotherapy. Glutamate-cysteine ligase (GCL) catalyzes the first and rate-limiting reaction in GSH synthesis, and enzyme function is controlled by GSH feedback inhibition or by transcriptional upregulation of the catalytic (GCLC) and modifier (GCLM) subunits. However, it has recently been reported that the activity of GCLC and the formation of GCL can be modified by reactive aldehyde products derived from lipid peroxidation. Due to the susceptibility of GCLC to posttranslational modifications by reactive aldehydes, we examined the potential for 2-deoxy-D-ribose (2dDR) to glycate GCLC and regulate enzyme activity and GCL formation. 2dDR was found to directly modify both GCLC and GCLM in vitro, resulting in a significant inhibition of GCLC and GCL enzyme activity without altering substrate affinity or feedback inhibition. 2dDR-mediated glycation also inhibited GCL subunit heterodimerization and formation of the GCL holoenzyme complex while not causing dissociation of pre-formed holoenzyme. This PTM could be of particular importance in glioblastoma (GBM) where intratumoral necrosis provides an abundance of thymidine, which can be metabolized by thymidine phosphorylase (TP) to form 2dDR. TP is expressed at high levels in human GBM tumors and shRNA knockdown of TP in U87 GBM cells results in a significant increase in cellular GCL enzymatic activity.

The in vitro inhibition effect of 2 nm gold nanoparticles on non-enzymatic glycation of human serum albumin

The reaction of amino groups of protein and the carbonyl groups of reducing sugar molecules, non-enzymatically induce a series of chemical reactions that form a heterogeneous group of compounds known as advanced glycation end products (AGEs). The accumulation of AGEs is associated with various disease conditions that include complications in diabetes, Alzheimer's disease and aging. The current study monitored the extent of non-enzymatic glycation of human serum albumin (HSA) in order to estimate the formation of HSA related AGEs in the presence of 2 nm gold nanoparticles. The rate of glycation was evaluated using several analytical methods. Physiological concentrations of HSA and glyceraldehyde mixtures, incubated with various concentrations of negatively charged 2 nm gold nanoparticles, resulted in a lower reaction rate than mixtures without 2GNP. Moreover, increasing concentrations of gold nanoparticles exhibited a pronounced reduction in AGE formation. High performance liquid chromatography, UV-visible spectroscopy and circular dichroism analytical methods provide reliable techniques for evaluating AGE formation of HSA adducts.

In vitro glycation of human serum albumin by dihydroxyacetone and dihydroxyacetone phosphate

Amino groups in proteins can non-enzymatically react with reducing sugars to generate a structurally diverse group of compounds referred to as advanced glycation end products (AGEs). The in vivo formation of AGEs contributes to some of the complications of diabetes including atherosclerosis, cataract formation, and renal failure. The formation of AGEs is dependent on both sugar and protein concentrations. Increases in temperature, pH, and exposure time of sugars to the proteins also play a significant role in the rate of AGE formation. This study focuses on the use of a combination of analytical techniques to study the in vitro AGE formation of HSA with dihydroxyacetone phosphate (DHAP), a ketose generated during glycolysis, and its dephosphorylated analog, dihydroxy acetone (DHA), commonly used as a browning reagent in skin tanning preparations. The extent of AGE formation was affected by DHAP and DHA concentrations and by the duration of HSA exposure to these glycating agents. Increases in temperature and pH sped the glycation process and enhanced the formation of the AGEs of HSA. MALDI-TOF mass spectroscopic data provided a reliable result to evaluate the extent of the AGE formation.

Quantitative analysis of glycation patterns in human serum albumin using 16O/18O-labeling and MALDI-TOF MS

BACKGROUND

The glycation of human serum albumin (HSA) during diabetes can affect the ability of this protein to bind drugs and small solutes in blood. This study describes the use of (16)O/(18)O-labeling and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to compare the levels of modification that occur throughout HSA under various glycation conditions in vitro. These quantitative studies build on a recent report that has identified the early and advanced glycation products that are formed on such samples of HSA.

METHODS

Glycated HSA samples were prepared by incubating 42 g/l HSA with 0 to 15 mmol/l glucose at pH 7.4 and 37°C for up to 5 weeks. A control HSA sample was digested in (16)O-enriched water and glycated HSA samples were digested in the presence of (18)O-enriched water. These 2 types of samples were then mixed and the amounts of (16)O- vs. (18)O-labeled peptides were measured to determine the levels of modification that were occurring throughout HSA.

RESULTS

The largest levels of modification occurred in residues 101-119, 1-10 or 42-51, 87-100, 360-372, 521-531, and 275-286 of HSA after 2 weeks of glycation, and in residues 21-41, 1-10 or 42-51, 521-531, 82-93, and 146-160 after 5 weeks of glycation. Some of these regions contained the N-terminus, K199, K439, and K525, which have been previously identified as major glycation sites on HSA. The glycation pattern of HSA was dominated by early glycation products (e.g., fructosyl-lysine) after a reaction period of 2 weeks for mildly glycated HSA, while advanced glycation end products became more prominent at longer reaction times.

CONCLUSIONS

The time course of the observed modifications indicated that the pattern of glycation products changed as HSA was incubated over longer periods of time with glucose. Several regions found to have significant levels of modification were at or near the major drug binding regions on HSA. These results explain why the interaction of some drugs with HSA has been observed to vary with the level of glycation for this protein.

Endothelial dysfunction: from molecular mechanisms to measurement, clinical implications, and therapeutic opportunities

Endothelial dysfunction has been implicated as a key factor in the development of a wide range of cardiovascular diseases, but its definition and mechanisms vary greatly between different disease processes. This review combines evidence from cell-culture experiments, in vitro and in vivo animal models, and clinical studies to identify the variety of mechanisms involved in endothelial dysfunction in its broadest sense. Several prominent disease states, including hypertension, heart failure, and atherosclerosis, are used to illustrate the different manifestations of endothelial dysfunction and to establish its clinical implications in the context of the range of mechanisms involved in its development. The size of the literature relating to this subject precludes a comprehensive survey; this review aims to cover the key elements of endothelial dysfunction in cardiovascular disease and to highlight the importance of the process across many different conditions.

Advanced glycation end products (AGE) inhibit scavenger receptor class B type I-mediated reverse cholesterol transport: a new crossroad of AGE to cholesterol metabolism

Advanced glycation end products (AGE) -modified proteins behave as active ligands for several receptors belonging to the scavenger receptor family. Scavenger receptor class B type I (SR-BI) was identified as the first high density lipoprotein (HDL) receptor that mediates selective uptake of HDL-cholesteryl esters (HDL-CE). This study investigated whether AGE proteins serve as ligands for SR-BI and affect SR-BI-mediated cholesterol transport using Chinese hamster ovary (CHO) cells overexpressing hamster SR-BI (CHO-SR-BI cells). [125I] AGE-bovine serum albumin (AGE-BSA) underwent active endocytosis and subsequent lysosomal degradation by CHO-SR-BI cells, indicating that SR-BI serves as an AGE receptor. SR-BI-mediated selective uptake of HDL-CE by CHO-SR-BI cells was efficiently inhibited by AGE-BSA although AGE-BSA had no effect on HDL binding to CHO-SR-BI cells. In addition, AGE-BSA significantly inhibited the efflux of [3H] cholesterol from CHO-SR-BI cells to HDL. These findings suggest the possibility that AGE proteins in the circulation interfere with the functions of SR-BI in reverse cholesterol transport by inhibiting the selective uptake of HDL-CE, as well as cholesterol efflux from peripheral cells to HDL, thereby accelerating diabetes-induced atherosclerosis.

Iterated reaction graphs: simulating complex Maillard reaction pathways

This study investigates a new method of simulating a complex chemical system including feedback loops and parallel reactions. The practical purpose of this approach is to model the actual reactions that take place in the Maillard process, a set of food browning reactions, in sufficient detail to be able to predict the volatile composition of the Maillard products. The developed framework, called iterated reaction graphs, consists of two main elements: a soup of molecules and a reaction base of Maillard reactions. An iterative process loops through the reaction base, taking reactants from and feeding products back to the soup. This produces a reaction graph, with molecules as nodes and reactions as arcs. The iterated reaction graph is updated and validated by comparing output with the main products found by classical gas-chromatographic/mass spectrometric analysis. To ensure a realistic output and convergence to desired volatiles only, the approach contains a number of novel elements: rate kinetics are treated as reaction probabilities; only a subset of the true chemistry is modeled; and the reactions are blocked into groups.

Gliclazide reduces the induction of human monocyte adhesion to endothelial cells by glycated albumin

OBJECTIVE

To examine the kinetic of human monocyte adhesion to endothelial cells stimulated by glycated albumin, the contributive role of cell adhesion molecules to this effect, and the effect of gliclazide--an hypoglycemic drug with antioxidant properties--on these parameters.

METHODS

In-vitro experiments performed in the presence and absence of gliclazide consisted of: (1) time-dependent determination of human monocyte adhesion to human endothelial cells (ECs) pre-exposed to glycated albumin; (2) evaluation of adhesion after incubation of ECs with antibodies against cell surface adhesion molecules; and (3) determination of EC surface adhesion molecules and of the activity of the transcription factor NF-kappaB.

RESULTS

Exposition of human ECs for 1-48 h to 100 microg/ml glycated albumin led to a time-dependent increase in human monocyte adhesion to endothelium. Pretreatment of ECs with 10 microg/ml gliclazide significantly decreased the glycated albumin-stimulated monocyte adhesion to these cells. Anti-antibodies against E-selectin (ELAM-1), vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) also reduced the stimulatory effect of glycated albumin on monocyte adhesion. In glycated albumin-treated ECs, an induction of both soluble and cell associated expression of ELAM-1, VCAM-1 and ICAM-1, an augmentation in the levels of these molecule transcripts and an increase in the DNA binding activity for NF-kappaB in the promoters of these antigens were observed. Gliclazide markedly inhibited the induction of all these parameters.

CONCLUSIONS

Glycated albumin stimulates human monocyte adhesion to ECs by inducing cell associated ELAM-1, ICAM-1 and VCAM-1. Gliclazide effectively inhibits monocyte adhesion to ECs by reducing glycated albumin induction of EC adhesion molecules and NF-kappaB activation. These results suggest that gliclazide may be beneficial in the prevention of endothelial disturbances associated with hyperglycemia in diabetic patients.

Immunohistochemical localisation of advanced glycation end products in pulmonary fibrosis

AIM

To investigate the presence and distribution of advanced glycation end products (AGE) in pulmonary fibrosis.

METHODS

Lung tissue samples obtained from seven necropsy cases with idiopathic pulmonary fibrosis and seven with normal pulmonary parenchyma were examined immunohistochemically with a monoclonal antibody specific for AGE: 6D12. We also tested three cases with diffuse alveolar damage.

RESULTS

All the specimens from cases with pulmonary fibrosis and diffuse alveolar damage showed strong AGE expression on macrophages. Lung specimens from normal parenchyma showed positive AGE immunoreactivity on macrophages from only two of seven cases.

CONCLUSIONS

These findings suggest that AGE modified proteins accumulate in alveolar macrophages in patients with diffuse alveolar damage and idiopathic pulmonary fibrosis.

Immunochemical approaches to AGE-structures: characterization of anti-AGE antibodies

Recent immunological approaches have greatly helped broaden our understanding of the biomedical significance of advanced glycation end products (AGEs) in aging and age-enhanced disease processes. Recently, Nepsilon-(carboxymethyl) lysine (CML), one of the glycoxidation products of AGEs, was demonstrated to be a major immunological epitope among AGEs. In the subsequent study, we characterized 13 different polyclonal anti-AGE antibodies and showed that these antibodies could be classified into three groups (Groups I, II and III). Group I was specific for CML and both Group II and Group III were specific for other epitopes (non-CML). Time-course study suggested that the epitope of Group II was formed earlier than that of Group III. In the present study, we prepared two monoclonal anti-AGE antibodies (2A2 and 3A3) whose epitope structures appeared to be closely related to Group III and Group II, respectively. The result indicates that AGE-proteins express at least two major non-CML epitopes.

Insulin enhances macrophage scavenger receptor-mediated endocytic uptake of advanced glycation end products

Hyperglycemia accelerates the formation and accumulation of advanced glycation end products (AGE) in plasma and tissue, which may cause diabetic vascular complications. We recently reported that scavenger receptors expressed by liver endothelial cells (LECs) dominantly mediate the endocytic uptake of AGE proteins from plasma, suggesting its potential role as an eliminating system for AGE proteins in vivo (Smedsrod, B., Melkko, J., Araki, N., Sano, H., and Horiuchi, S. (1997) Biochem. J. 322, 567-573). In the present study we examined the effects of insulin on macrophage scavenger receptor (MSR)-mediated endocytic uptake of AGE proteins. LECs expressing MSR showed an insulin-sensitive increase of endocytic uptake of AGE-bovine serum albumin (AGE-BSA). Next, RAW 264.7 cells expressing a high amount of MSR were overexpressed with human insulin receptor (HIR). Insulin caused a 3.7-fold increase in endocytic uptake of 125I-AGE-BSA by these cells. The effect of insulin was inhibited by wortmannin, a phosphatidylinositol-3-OH kinase (PI3 kinase) inhibitor. To examine at a molecular level the relationship between insulin signal and MSR function, Chinese hamster ovary (CHO) cells expressing a negligible level of MSR were cotransfected with both MSR and HIR. Insulin caused a 1.7-fold increase in the endocytic degradation of 125I-AGE-BSA by these cells, the effect of which was also inhibited by wortmannin and LY294002, another PI3 kinase inhibitor. Transfection of CHO cells overexpressing MSR with two HIR mutants, a kinase-deficient mutant, and another lacking the binding site for insulin receptor substrates (IRS) resulted in disappearance of the stimulatory effect of insulin on endocytic uptake of AGE proteins. The present results indicate that insulin may accelerate MSR-mediated endocytic uptake of AGE proteins through an IRS/PI3 kinase pathway.

Conversion of Amadori product of Maillard reaction to Nepsilon-(carboxymethyl)lysine in alkaline condition

Nepsilon-(carboxymethyl)lysine (CML) is known to be formed by oxidative cleavage of Amadori products between C-2 and C-3 of the carbohydrate chain. We report here that CML formation from Amadori compounds is highly accelerated under alkaline conditions. Incubation of glycated human serum albumin (HSA) in 0.1 N NaOH led to the formation of CML whereas glycated HSA reduced by NaCNBH3 or non-glycated HSA did not generate CML. Nalpha-t-butyloxycarbonyl-Nepsilon-fructoselysine (Boc-FL), a model compound of Amadori product, was converted to CML under alkaline conditions. CML level of human sera (n=224) preincubated with 0.1 N NaOH correlated well with glycated albumin value (r=0.912) and hemoglobin A1c (r=0.797).

Technical note. The serum concentration of the advanced glycation end-product N epsilon-(carboxymethyl)lysine is increased in uremia

Advanced glycation end products (AGEs) such as pentosidine and N epsilon-(carboxymethyl)lysine (CML) have been traditionally quantified by HPLC or gas chromatography--mass spectrometry (GC/MS). Enzyme-linked immunosorbent assays (ELISA) have been introduced as a convenient alternative to simplify the detection and measurement of AGEs in proteins and tissues, but some of these studies are limited by the lack of information on the structure of the epitopes recognized by antibodies to AGE-proteins. In this work we demonstrate that an antibody used in a previous study, reporting increased levels of AGEs in patients with diabetes or on continuous ambulatory peritoneal dialysis (CAPD) and hemodialysis (HD), recognizes CML as its major epitope. We also show that there is a significant correlation between the concentration of AGEs in serum measured by ELISA and a GC/MS assay for CML in serum proteins. Both analyses yielded comparable results, with patients on CAPD and HD having about threefold higher AGE- or CML-concentrations in their serum. Our data suggest that ELISA assays for CML should be useful for the clinical measurement of AGEs in serum proteins.

Hydroxyl radical mediates N epsilon-(carboxymethyl)lysine formation from Amadori product

Recent studies demonstrated N epsilon-(carboxymethyl)lysine (CML) in several tissue proteins. Incubation of proteins with glucose leads through a Schiff base to Amadori products. Oxidative cleavage of Amadori products is considered as a major route to CML formation in vivo, whereas it is not known which reactive oxygen species (ROS) is involved. The present study is undertaken to identify such a ROS. We prepared heavily glycated human serum albumin (HSA) which contained a high level of Amadori products, but an undetectable level of CML. Incubation of glycated HSA with FeCl2, but not with H2O2, led to CML formation which was enhanced by H2O2, but inhibited by catalase or mannitol, whereas superoxide dismutase had no effect. Similar data were obtained by experiments using Boc-fructose-lysine as a model Amadori compound. These data indicate that hydroxyl radical generated by the reaction of Fe2+ with H2O2 mediates CML formation from Amadori compounds.

Photo-enhanced modification of human skin elastin in actinic elastosis by N(epsilon)-(carboxymethyl)lysine, one of the glycoxidation products of the Maillard reaction

Long-term incubation of proteins with glucose leads to the formation of advanced glycation end products (AGEs), which are characterized by fluorescence, brown color, and cross-linking. Formation of AGEs in vitro requires oxygen and is dependent on transition metal-catalyzed oxidation of glucose or Amadori products. AGEs are thought to be involved in aging and age-enhanced diseases such as diabetic complications, atherosclerosis, dialysis-related amyloidosis, and Alzheimer's disease. Chronic exposure of the skin to sunlight induces hyperplasia of the elastic tissue in the upper dermis known as actinic elastosis. Herein we used a monoclonal anti-AGE antibody (6D12) whose epitope is N(epsilon)-(carboxymethyl)lysine (CML), one of the glycoxidation products of AGEs, and demonstrated that the lesions of actinic elastosis were modified by CML. Further immunohistochemical and immunoelectron microscopic examination with 6D12 demonstrated CML accumulates predominantly in elastic fibers especially in the amorphous electron-dense materials corresponding to photo-induced degenerated area rather than the electron-lucent region. Immunochemical analyses with enzyme-linked immunosorbent assay (ELISA) of elastase-soluble fractions demonstrated that the CML levels of the sun-exposed area were significantly higher than those of the sun-unexposed area. We conclude that ultraviolet-induced oxidation may accelerate CML formation in actinic elastosis of photoaged skin.