Characterization of advanced glycation end products for biochemical studies: side chain modifications and fluorescence characteristics

Monitoring nonenzymatic glycation of human immunoglobulin G by methylglyoxal and glyoxal: A spectroscopic study

The accumulation of dicarbonyl compounds, methylglyoxal (MG) and glyoxal (G), has been observed in diabetic conditions. They are formed from nonoxidative mechanisms in anaerobic glycolysis and lipid peroxidation, and they act as advanced glycation endproduct (AGE) precursors. The objective of this study was to monitor and characterize the AGE formation of human immunoglobulin G (hIgG) by MG and G using ultraviolet (UV) and fluorescence spectroscopy, circular dichroism (CD), and matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS). hIgG was incubated over time with MG and G at different concentrations. Formation of AGE was monitored by UV and fluorescence spectroscopy. The effect of AGE formation on secondary structure of hIgG was studied by CD. Comparison of AGE profile for MG and G was performed by MALDI-MS. Both MG and G formed AGE, with MG being nearly twice as reactive as G. The combination of these techniques is a convenient method for evaluating and characterizing the AGE proteins.

Water-Soluble alpha,beta-unsaturated aldehydes of cigarette smoke induce carbonylation of human serum albumin

Cigarette smoking is a major risk factor for developing pulmonary and cardiovascular diseases as well as some forms of cancer. Understanding the mechanisms by which smoking contributes to disease remains a major research focus. Increased levels of carbonylated serum proteins are present in smokers; albumin is the major carbonylated protein in the bronchoalveolar lavage fluid of older smokers. We have investigated the susceptibility of human serum albumin (HSA) to alpha,beta-unsaturated aldehyde-induced carbonylation when exposed to whole-phase cigarette smoke extract (CSE). Fluorescence studies with fluorescent probes showed depletion of HSA Cys34 free thiol and marked decrease of free Lys residues. Spectrophotometric and immunochemical carbonyl assays after carbonyl derivatization with 2,4-dinitrophenylhydrazine revealed the formation of covalent carbonyl adducts. Nanoscale capillary liquid chromatography and electrospray tandem mass spectrometry analysis detected acrolein and crotonaldehyde Michael adducts at Cys34, Lys525, Lys351, and His39 at all the CSE concentrations tested. Lys541 and Lys545 were also found to form a Schiff base with acrolein. The carbonyl scavenger drugs, hydralazine and pyridoxamine, partially prevented CSE-induced HSA carbonylation. Carbonylation of HSA associated with cigarette smoking might result in modifications of its antioxidant properties and transport functions of both endogenous and exogenous compounds.

Green tea polyphenol epigallocatechin-3-gallate inhibits advanced glycation end product-induced expression of tumor necrosis factor-alpha and matrix metalloproteinase-13 in human chondrocytes

Introduction

The major risk factor for osteoarthritis (OA) is aging, but the mechanisms underlying this risk are only partly understood. Age-related accumulation of advanced glycation end products (AGEs) can activate chondrocytes and induce the production of proinflammatory cytokines and matrix metalloproteinases (MMPs). In the present study, we examined the effect of epigallocatechin-3-gallate (EGCG) on AGE-modified-BSA (AGE-BSA)-induced activation and production of TNFα and MMP-13 in human OA chondrocytes.

Methods

Human chondrocytes were derived from OA cartilage by enzymatic digestion and stimulated with in vitro-generated AGE-BSA. Gene expression of TNFα and MMP-13 was measured by quantitative RT-PCR. TNFα protein in culture medium was determined using cytokine-specific ELISA. Western immunoblotting was used to analyze the MMP-13 production in the culture medium, phosphorylation of mitogen-activated protein kinases (MAPKs), and the activation of NF-κB. DNA binding activity of NF-κB p65 was determined using a highly sensitive and specific ELISA. IκB kinase (IKK) activity was determined using an in vitro kinase activity assay. MMP-13 activity in the culture medium was assayed by gelatin zymography.

Results

EGCG significantly decreased AGE-stimulated gene expression and production of TNFα and MMP-13 in human chondrocytes. The inhibitory effect of EGCG on the AGE-BSA-induced expression of TNFα and MMP-13 was mediated at least in part via suppression of p38-MAPK and JNK activation. In addition, EGCG inhibited the phosphorylating activity of IKKβ kinase in an in vitro activity assay and EGCG inhibited the AGE-mediated activation and DNA binding activity of NF-κB by suppressing the degradation of its inhibitory protein IκBα in the cytoplasm.

Conclusions

These novel pharmacological actions of EGCG on AGE-BSA-stimulated human OA chondrocytes provide new suggestions that EGCG or EGCG-derived compounds may inhibit cartilage degradation by suppressing AGE-mediated activation and the catabolic response in human chondrocytes.

Cytotoxicity of advanced glycation endproducts in human micro- and astroglial cell lines depends on the degree of protein glycation

Advanced glycation endproducts (AGEs) arise from the reaction of sugars with side chains and the N-terminus of proteins and are thought to be involved in the pathogenesis of several diseases by inducing oxidative stress, inflammation and cell death presumably mediated through activation of the receptor of AGE (RAGE). To address the question whether the cell damaging effect of AGE depends on the degree of its protein glycation, differential modified AGEs derived from incubating human serum albumin with increasing concentrations of methyl glyoxal were tested on cell viability, reactive oxygen species (ROS) formation, intracellular ATP levels, and activation of caspases 3/7 in two human glial cell lines, which were used as a model for human glia cells. All AGEs tested, regardless of their degree of modification, were found to induce ROS formation in both microglial (CHME-5) and astroglial cells (U373 MG), while only highly modified AGEs were able to decrease the cell viability and to induce apoptosis. This indicates that apoptotic events may be involved in the change of physiological parameters.

Structural basis for pattern recognition by the receptor for advanced glycation end products (RAGE)

The receptor for advanced glycated end products (RAGE) is a multiligand receptor that is implicated in the pathogenesis of various diseases, including diabetic complications, neurodegenerative disorders, and inflammatory responses. The ability of RAGE to recognize advanced glycated end products (AGEs) formed by nonenzymatic glycoxidation of cellular proteins places RAGE in the category of pattern recognition receptors. The structural mechanism of AGE recognition was an enigma due to the diversity of chemical structures found in AGE-modified proteins. Here, using NMR spectroscopy we showed that the immunoglobulin V-type domain of RAGE is responsible for recognizing various classes of AGEs. Three distinct surfaces of the V domain were identified to mediate AGE-V domain interactions. They are located in the positively charged areas of the V domain. The first interaction surface consists of strand C and loop CC ', the second interaction surface consists of strand C ', strand F, and loop FG, and the third interaction surface consists of strand A ' and loop EF. The secondary structure elements of the interaction surfaces exhibit significant flexibility on the ms-micros time scale. Despite highly specific AGE-V domain interactions, the binding affinity of AGEs for an isolated V domain is low, approximately 10 microm. Using in-cell fluorescence resonance energy transfer we show that RAGE is a constitutive oligomer on the plasma membrane. We propose that constitutive oligomerization of RAGE is responsible for recognizing patterns of AGE-modified proteins with affinities less than 100 nm.

Interaction of human serum albumin and its clinically relevant modification with oligoribonucleotides

Human serum albumin (HSA) was shown to mediate oligoribonucleotide cleavage. Nonenzymatic glycation of HSA decreased the ribonuclease-like activity of the protein. According to (31)P NMR data, both native and glycated albumins induced hydrolysis of RNA molecule through 2',3'-cyclophosphate intermediates. A feasible mechanism of RNA hydrolysis by native albumin and its clinically relevant modification was discussed.

Detergency effects of nanofibrillar amyloid formation on glycation of human serum albumin

The prolonged glycation of human serum albumin (HSA) results in significant changes in its structure. The identity of these structural changes and the influence of carbohydrates on these changes require further study. Here, we evaluated structural changes and amyloid formation of HSA upon incubation with Glc, Fru, or Rib. Fluorescence spectrophotometry, surface tension analysis, and transmission electron microscopy (TEM) were utilized to evaluate the structures of glycated HSA. The physicochemical properties including excess free energy, protein adsorption at the air-water interface, critical aggregation concentration (CAC), and surface activity indicated an increase in hydrophobicity and partial unfolding of HSA structure upon glycation. Thus, it appears that AGE products can act as detergents. Incubation of HSA with these sugars after 20 wks induced significant amyloid nanofibril formation. Together these results indicate that prolonged glycation of HSA is associated with a transition from helical structure to beta-sheet (amyloid formation).

The multiple Maillard reactions of ribose and deoxyribose sugars and sugar phosphates

Ribose 5-phosphate (R5P) undergoes the Maillard reaction with amines at significantly higher rates than most other sugars and sugar phosphates. The presence of an intramolecular phosphate group, which catalyzes the early stages of the Maillard reaction, provides the opportunity for the R5P molecule to undergo novel reaction paths creating unique Maillard products. The initial set of reactions leading to an Amadori product (phosphorylated) and to an alpha-dicarbonyl phosphate compound follows a typical Maillard reaction sequence, but an observed phosphate hydrolysis accompanying the reaction adds to the complexity of the products formed. The reaction rate for the loss of R5P is partially dependent on the pK(a) of the amine but also is correlated to the protonation of an early intermediate of the reaction sequence. In the presence of oxygen, a carboxymethyl group conjugated to the amine is a major product of the reaction of R5P with N-acetyllysine while little of this product is generated in the absence of oxygen. Despite lacking a critical hydroxyl group necessary for the Maillard reaction, 2-deoxyribose 5-phosphate (dR5P) still generates an Amadori-like product (with a carbonyl on the C-3 carbon) and undergoes phosphate cleavage. Two highly UV-absorbing products of dR5P were amine derivatives of 5-methylene-2-pyrrolone and 2-formylpyrrole. The reaction of dR5P with certain amines generates a set of products that exhibit an interesting absorbance at 340nm and a high fluorescence.

Modification of aluminum chips for LDI mass spectrometry of proteins

Matrix-assisted laser desorption ionization time-of-flight mass spectroscopy (MALDI TOFMS) combined with affinity chromatography on immobilized phenylboronic acid agarose gels was used for selective enrichment and detection of specifically modified proteins such as glycated proteins in complex biological samples. Physicochemical grafting of hydrophilic polymers on aluminum surface was developed to reduce nonspecific protein sorption and to create a proper support layer for a three-dimensional affinity hydrogel. Grafted agarose allowed the fixation of three-dimensional agarose hydrogel on the chip surface. Both pinched polymers and hydrogels were effectively derivatized. 3-Aminophenylboronic acid (mPBA) was covalently immobilized as an affinity ligand to achieve specific binding of glycated plasma proteins. Alternatively, the affinity sorbent was immersed into the hydrogel to increase binding capacity. MALDI TOFMS was used to evaluate binding efficiency and molecular mass changes of human serum albumin due to glycation. Glycated proteins were captured directly on the chip with high selectivity and efficacy, and low nonspecific binding. Thus they could easily be characterized by MALDI TOFMS.

Fibril formation of lysozyme upon interaction with sodium dodecyl sulfate at pH 9.2

Fibril formation seems to be a general property of all proteins. Its occurrence in hen or human lysozyme depends on certain conditions, namely acidic pHs or the presence of some additives. This paper studies the interaction of lysozyme with sodium dodecyl sulfate (SDS) at pH 9.2, using UV-visible spectrophotometry, circular dichroism (CD) spectropolarimetry, electron microscopy (EM) and chemometry. Based on observations such as the strange increase in absorbance at 650nm (pH 9.2) and the presence of intermediates, it is assumed that lysozyme fibrils have been formed at pH 9.2 in the presence of SDS as an anionic surfactant. Thioflavin T emission fluorescence and an EM image confirmed this assumption. beta-cyclodextrin was then used as a turbidity inhibitor to establish its effect on the distribution of intermediates that participate in fibril formation.

Alginate as an antiglycating agent for human serum albumin

Hyperglycemia and the accumulation of advanced glycation endproducts (AGEs) in tissues and serum have important roles in diabetic complications. Therefore, the identification of anti-glycation compounds is attracting considerable interest. In this study, the interaction of human serum albumin (HSA) with fructose, in the absence and presence of alginate, was studied by circular dichroism, absorbance and fluorescence techniques. The characterization study of AGEs was performed using autofluorescence, fibrillar formation, the increase in absorbance and the quantification of free lysine side chains. The results indicate that alginate inhibits the fructation of HSA as observed by a reduction in the formation of fluorescent AGEs and fibrils. Furthermore, alginate reduces the amount of modified lysine side chains, signified by the lack of increase in absorbance, and increases the helicity of this protein.

The binding of advanced glycation end products to cell surfaces can be measured using bead-reconstituted cellular membrane proteins

Advanced glycation end products (AGEs) that arise from the reaction of sugars with protein side chains are supposed to be involved in the pathogenesis of several diseases and therefore the effects of AGEs on cells are the objective of numerous investigations. Although different cellular responses to AGEs can be measured in cell culture studies, knowledge about the nature of AGE-binding and the involved cell surface receptors is poor. The measurement of AGE-binding to cell surfaces bears the potential to gain a deeper understanding about the nature of AGE-binding to cell surface proteins and could be applied as a preliminary test before performing cell culture studies on AGE effects. Herein, a new material and method for the detection of AGE-binding to cell surfaces is introduced, which has the potential to facilitate the detection of binding. In the present paper, the detection of AGE-binding to cell surface proteins using an artificial system of cellular membrane proteins reconstituted on beads (TRANSIL CaCo-2) is described. The binding of a BSA-AGE derived from a 37 degrees C incubation with 500 mM Glc (BSA-Glc 500) and the corresponding control to this artificial system was compared with the binding to intact cells and was found to be in good agreement. Additionally, the K(d) for the binding of the BSA-Glc 500 used in the study to CaCo-2 surfaces was determined using FITC-labelled samples in a flow cytometric approach. Competitive binding studies were performed using a set of non-labelled BSA-AGEs to compete with FITC-labelled BSA-Glc 500 for the cell surface binding sites. The binding was found to be inhibited to different extends, virtually depending on the degree of arginine modifications within the modified protein used for competition. Additionally, the effects of all AGEs used in the study on CaCo-2 cells was measured using the detection of reactive oxygen species (ROS), which are known to be induced as a primary result of AGE-receptor binding. The induction of ROS was found to linearly correlate to the capacity of the individual AGE to displace FITC-labelled BSA-Glc 500 in competitive binding studies. Therefore, the data indicate, that at least in case of CaCo-2 cells the detection of cell surface binding can serve as a reliable preliminary test for a potential cell-damaging effect of AGEs.

Comparison of results of the CellTiter Blue, the tetrazolium (3-[4,5-dimethylthioazol-2-yl]-2,5-diphenyl tetrazolium bromide), and the lactate dehydrogenase assay applied in brain cells after exposure to advanced glycation endproducts

Advanced glycation endproducts (AGEs) arise in vivo from the reaction of proteins with sugars or dicarbonyl compounds. They are thought to be involved in the pathogenesis of several diseases such as atherosclerosis, diabetes mellitus, renal failure, and Alzheimer's disease (AD). Several binding molecules for AGEs have been described and it is assumed that many of the effects of AGEs are mediated by receptors like the receptor for AGEs (RAGE). AGEs are known to induce the release of inflammatory cytokines from activated glia in the AD brain and thus AGEs affect the cell viability of neurons and glia. In cell culture experiments controversial effects of AGEs on cell growth and viability were reported by different research groups ranging from stimulation to inhibition of the cell viability. In the present study, the effect of in vitro prepared highly modified AGEs on the viability and the membrane integrity of cultured brain cells was investigated. Three different brain cell lines were treated with glucose human serum albumin AGEs (Glc-AGEs) and methyl glyoxal human serum albumin AGEs (MG-AGEs). To investigate the effect of these model AGEs on cell viability the CellTiter Blue (CTB) and the tetrazolium (3-[4,5-dimethylthioazol-2-yl]-2,5-diphenyl tetrazolium bromide) (MTT) were used. The membrane integrity after exposure to AGEs was assayed using the lactate dehydrogenase (LDH) assay. When using the CTB assay for evaluation all AGEs were found to reduce the viability compared with the native protein in all three cell lines. Additionally, all AGEs were found to affect the membrane integrity compared with the native protein in all cell lines. When using the MTT assay for evaluation only MG-AGEs were found to cause a decrease in the viability in all cell lines used. The results of the MTT assay in Glc-AGEs treated cells varied between the cell lines. To gain a deeper understanding of the cellular responses after exposure of cells to AGEs, the present study compares results obtained when using the CTB, the MTT or the LDH assay in identically AGE treated cells.

Role of electrostatic interactions in 2,2,2-trifluoroethanol-induced structural changes and aggregation of alpha-chymotrypsin

It has been recently demonstrated that alpha-chymotrypsin (CT) can be driven toward amyloid aggregation by addition of 2,2,2-trifluoroethanol (TFE), at intermediate concentrations. In the present article, the process of TFE-induced CT aggregation was investigated in more detailed kinetic terms where the effects of medium conditions, such as temperature, presence of kosmotropic and chaotropic salts, pH and chemical modification of lysine residues were examined. Various techniques, including light scattering, fluorescence and circular dichroism spectroscopy, were used to follow and characterize this process. The kinetics of aggregation was found to obey a second-order reaction with respect to protein concentration. The aggregation-prone A-state and aggregation-deficient TFE- or T-state of CT were found to be induced at lower TFE concentrations in the presence of salts. Use of acidic and alkaline conditions and lysine modification also promoted the formation of the T-state. Results presented suggest a role for electrostatic interactions in the aggregation process.

Non-enzymatic interactions of glyoxylate with lysine, arginine, and glucosamine: a study of advanced non-enzymatic glycation like compounds

Glyoxylate is a 2 carbon aldo acid that is formed in hepatic tissue from glycolate. Once formed, the molecule can be converted to glycine by alanine-glyoxylate aminotransferase (AGAT). In defects of AGAT, glyoxylate is transformed to oxalate, resulting in high levels of oxalate in the body. The objective of this study was 2-fold. First, it was to determine, if akin to D-glucose, D-fructose or DL-glyceraldehyde, glyoxylate was susceptible to non-enzymatic attack by amino containing molecules such as lysine, arginine or glucosamine. Second, if by virtue of its molecular structure and size, glyoxylate was as reactive a reagent in non-enzymatic reactions as DL-glyceraldehyde; i.e., a glycose that we previously demonstrated to be a more effective glycating agent than D-glucose or D-fructose. Using capillary electrophoresis (CE), high performance liquid chromatography and UV and fluorescence spectroscopy, glyoxylate was found to be a highly reactive precursor of advanced glycation like end products (AGLEs) and a more effective promoter of non-enzymatic end products than D-glucose, D-fructose or DL-glyceraldehyde.

Induction of reactive oxygen species and cell survival in the presence of advanced glycation end products and similar structures

Advanced glycation end products (AGEs) that arise from the reaction of sugars with protein side chains and the terminal amino group are supposed to be involved in the pathogenesis of several diseases and therefore the effects of AGEs on cells are the objective of numerous investigations. The effects of AGEs on cells are commonly assumed to be transduced via the receptor for AGEs (RAGE) but there are also other receptors known to interact with AGEs and they are likely to be involved in signal transduction. The primary cellular effect of AGEs on cultured cells was found to be the formation of reactive oxygen species (ROS). For the present study one murine and three human cell lines were used. The effects of a set of different highly modified AGEs and AGE-like compounds derived from the incubation of different modifiers with BSA were tested for their effects on these cells. Almost all AGEs tested induced the production of reactive oxygen species (ROS) in the different cell lines although the intensity of the detected signals varied considerably between the cell lines and are strongly dependent on the AGE used for cell activation. The most highly modified BSA-species were shown to inhibit cell growth in all cell lines, whereas a moderately modified glucose derived BSA-AGE and BSA-GA(red) did not show any inhibitory effect on cell growth even when a high ROS formation was detected.

Affinity separation of polyribonucleotide-binding human blood proteins

Using affinity columns with immobilized poly(A), poly(G), poly(U), poly(C), and poly(A).poly(U) and poly(G) x poly(C) duplexes several polyribonucleotide-binding blood plasma proteins have been captured. Albumin and keratins K1 and K2e have been detected to bind polypurine tracts. The in vitro glycated albumin binds poly(A) and poly(G) more efficiently than the unmodified protein. The major polypyrimidine-binding blood plasma protein (28 kDa) can catalyze the hydrolysis of poly(U).

Two analytical methods to study the interaction of AGEs with cell surface proteins

Advanced glycation end products (AGEs) are sugar-modified proteins that are known to appear in vivo and are suspected to be involved in the pathogenesis of several diseases. Although different cellular responses to AGEs can be measured in cell culture studies, knowledge about the nature of AGE-binding and their cell surface receptors is poor. In the present paper a method for the purification of AGE-binding proteins from membrane fractions derived from different rat organs as well as a method for assaying the binding of fluorescein labelled AGEs to the surface of cells of different cell lines are described. The presence of more than 10 proteins interacting with AGEs could be shown in membrane fractions obtained from rat organs. Additionally, binding of AGE-modified BSA to different cells could be shown using fluorescence-labelled ligands in a flow cytometric approach. The presented methods provide an option to isolate AGE-interacting proteins which is a precondition for the identification of these proteins. Furthermore, the measurement of AGE-binding to cell surfaces bears the potential to gain a deeper understanding about the nature of AGE-binding to cell surface proteins and might be applied as a preliminary test before performing cell culture studies about AGE effects.

Characterization of advanced glycation end products: Mass changes in correlation to side chain modifications

Advanced glycation end products (AGEs) that arise from the reaction of sugars with protein side chains are supposed to be involved in the pathogenesis of several diseases; therefore, the effects of AGEs on cells are the objective of numerous investigations. Because AGE modifications are an extremely heterogeneous group of side chain modifications, the exact characterization of an AGE-modified protein is impossible. To gain a deeper understanding about AGE formation kinetics and structures, AGEs can be characterized with respect to the degree of modification, specific side chain modifications, absorbance and fluorescence characteristics, and changes in the protein structure and molecular weight. For this study, human serum albumin (HSA)-AGEs derived from different concentrations of glucose, methyl glyoxal, and glyoxylic acid were used. The molecular mass of the obtained AGEs was determined using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The mass data were compared with earlier results concerning the degree of lysine and arginine side chain modifications and AGE-specific fluorescence and absorbance data. The molecular masses were found to gradually increase with increasing concentrations of the individual modifier without reaching a plateau. The mass increase correlates very well with the AGE-specific absorbance at 360 nm and with the degree of side chain modifications. The mass spectrometric data prove, for the first time, that an increasing absorbance at 360 nm is directly correlated to a mass increase during the AGE formation process.