Carboxymethyllysine Modification Research Articles

Role of Glycated High Mobility Group Box-1 in Gastric Cancer.

Advanced glycation end products (AGEs) are produced in response to a high-glucose environment and oxidative stress and exacerbate various diseases. Nε-(Carboxymethyl)lysine (CML) is an AGE that is produced by the glycation of lysine residues of proteins. There are a few reports on alterations in protein function due to CML modification; however, its association with cancer is not clear. We investigated the significance of CML modification in high mobility group box protein-1 (HMGB1), a cytokine that is significantly associated with cancer progression. Treatment of the gastric cancer cell lines TMK1 and MKN74 with glyoxal or glucose resulted in increased CML modification compared to untreated cells. CML-HMGB1 was modified via oxidation and more pronouncedly activated the receptor for AGE and downstream AKT and NF-κB compared to naïve HMGB1 and oxidized HMGB1. CML-HMGB1 bound with reduced affinity to DNA and histone H3, resulting in enhanced extranuclear translocation and extracellular secretion. Treatment of gastric cancer cells with CML-HMGB1 enhanced cell proliferation and invasion, sphere formation, and protection from thapsigargin-induced apoptosis, and decreased 5-FU sensitivity in comparison to HMGB1. Further, CML-HMGB1 was detected at various levels in all the 10 gastric cancer tumor specimens. HMGB1 levels correlated with primary tumor progression and distant metastasis, whereas CML-HMGB1 levels were associated with primary tumor progression, lymph node metastasis, distant metastasis, and stage. In addition, CML-HMGB1 levels correlated with oxidative stress in cancer tissues and resistance to neoadjuvant therapy. Therefore, CML modification of HMGB1 enhanced the cancer-promoting effect of HMGB1. In this study, CML-HMGB1 has been highlighted as a new therapeutic target, and analysis of the molecular structure of CML-HMGB1 is desired in the future.

Comprehensive Quantification of Carboxymethyllysine-Modified Peptides in Human Plasma.

A prolonged hyperglycemic condition in diabetes mellitus results in glycation of plasma proteins. N(ε)-Carboxymethyllysine (CML) is a well-known protein advanced glycation end product, and one of its mechanisms of formation is through further oxidation of Amadori compound modified lysine (AML). Unlike enrichment of AML peptides using boronate affinity, biochemical enrichment methods are scarce for comprehensive profiling of CML-modified peptides. To address this problem, we used AML peptide sequence and site of modification as template library to identify and quantify CML peptides. In this study, a parallel reaction monitoring workflow was developed to comprehensively quantify CML modified peptides in Type 1 diabetic subjects' plasma with good and poor glycemic control (n = 20 each). A total of 58 CML modified peptides were quantified, which represented 57 CML modification sites in 19 different proteins. Out of the 58 peptides, five were significantly higher in poor glycemic control samples with the area under the receiver operating characteristic curve ≥0.83. These peptides could serve as promising indicators of glycemic control in Type 1 diabetes management.

Single Posttranslational Modifications in the Central Repeat Domains of Tau4 Impact its Aggregation and Tubulin Binding.

A variety of methods have been employed to study the impact of posttranslational modifications on Tau protein function. Here, a semisynthesis strategy is described that enables selective modification within the central repeat domain of Tau4 (residues 291‐321), comprising a major interaction motive with tubulin as well as one of the key hexapeptides involved in Tau aggregation. This strategy has led to the preparation of four semisynthetic Tau variants with phosphoserine residues in different positions and one with a so far largely ignored carboxymethyllysine modification that results from a non‐enzymatic posttranslational modification (nPTM). The latter modification inhibits tubulin polymerization but exhibits an aggregation behavior very similar to unmodified Tau. In contrast, phosphorylated Tau variants exhibit similar binding to tubulin as unmodified Tau4 but show lower tendencies to aggregate.

Single Posttranslational Modifications in the Central Repeat Domains of Tau4 Impact its Aggregation and Tubulin Binding

AbstractA variety of methods have been employed to study the impact of posttranslational modifications on Tau protein function. Here, a semisynthesis strategy is described that enables selective modification within the central repeat domain of Tau4 (residues 291‐321), comprising a major interaction motive with tubulin as well as one of the key hexapeptides involved in Tau aggregation. This strategy has led to the preparation of four semisynthetic Tau variants with phosphoserine residues in different positions and one with a so far largely ignored carboxymethyllysine modification that results from a non‐enzymatic posttranslational modification (nPTM). The latter modification inhibits tubulin polymerization but exhibits an aggregation behavior very similar to unmodified Tau. In contrast, phosphorylated Tau variants exhibit similar binding to tubulin as unmodified Tau4 but show lower tendencies to aggregate.

Carboxymethyl-lysine-modified plasma proteins in age-related macular degeneration

To identify increased or decreased levels of carboxymethyl lysine (CML)-modified proteins in the plasma of age-related macular degeneration (AMD) patients and age-matched controls for the identification of possible plasma biomarkers of AMD and its progression. Plasma samples from patients with wet AMD with choroidal neovascularization (wAMD group) (n = 10), intermediate dry AMD (dAMD group) (n = 10), and age-matched controls (control group) (n = 10) were immunoprecipitated to select peptides with CML modification. LC-MS/MS was used to identify proteins in each group with CML modification. Among the identified CML enriched proteins, 6 commonly abundant proteins were identified in the plasma. There was no significant difference in amount of these proteins among the three groups, except CML-modified albumin, which was significantly decreased with progressed AMD (P = 0.0015). Five proteins were identified in the control group only, and 10 other proteins were identified in the dAMD group, and 4 other proteins in the wAMD group. Proteins involved in inflammatory responses and immunologic response were found to contain CML in the less advanced dAMD group. In the more advanced wAMD group, intracellular proteins associated with transcription or molecular activation associated with angiogenesis or blood vessel formation were found to be altered with CML. Proteins involved in inflammatory and immunologic response were CML-modified in earlier dry AMD while proteins associated with angiogenesis and blood vessel formation were CML-modified in advanced wet AMD, suggesting the possible role of oxidative change of proteins with different functions as the pathophysiology of the development and progression of AMD.

Nε-(Carboxymethyl)lysine Modification of Elastin Alters Its Biological Properties: Implications for the Accumulation of Abnormal Elastic Fibers in Actinic Elastosis

Accumulation of degenerated elastic fibers in the sun-exposed skin designated as actinic elastosis is a histological hallmark of photodamaged skin. Previous studies have indicated that the elastic fibers of actinic elastosis interact with lysozyme and are modified by N(ɛ)-(carboxymethyl)lysine (CML), one of the major advanced glycation end products (AGEs). We studied here how CML modification of elastin is involved in the pathogenesis of actinic elastosis. The CML-modified insoluble elastin became resistant to neutrophil elastase digestion, which was reversed by treatment with aminoguanidine, a potent inhibitor of AGE formation. In a temperature-dependent aggregation assay, CML-modified elastin rapidly formed self-aggregates, the size of which was larger than unmodified elastin. The elastic fiber sheets prepared from CML-modified α-elastin showed 3D wider diameter, tortuous appearance, and decreased elasticity on tensile tests. The CML-modified α-elastin, but not unmodified α-elastin, was found to bind to lysozyme in vitro, supporting the immunohistochemical findings that the antibodies for lysozyme and CML reacted simultaneously with the elastic fibers of actinic elastosis and UV-irradiated skin. The glycated elastin is likely to cause the accumulation of abnormally aggregated elastic fibers and unusual interaction with lysozyme in actinic elastosis.

The presence of Nε-(Carboxymethyl) lysine in the human epidermis

It is well known that advanced glycation end products (AGEs) are formed in long-lived dermal proteins such as collagen, and that their formation is related to skin aging. To examine the distribution of AGEs in skin tissue, we performed immunofluorescence studies on the human skin using an anti-AGEs antibody. Interestingly, AGEs signals were observed not only in the dermis but also in the epidermis. The objectives of this study were to confirm the presence of N ε-(Carboxymethyl) lysine (CML), an AGE structure, in the epidermis and to characterize the CML-modified proteins. The presence of CML in the stratum corneum (SC) was examined using liquid chromatography–electrospray ionization time-of-flight mass spectrometry. Concordance between the retention times of a compound in the SC hydrolysate and authentic CML, as well as with the specific mass transition of CML, was detected. This result showed that CML is present in the epidermis. In order to characterize the CML-modified proteins in the epidermis, protein samples extracted from the SC were analyzed using two-dimensional electrophoresis followed by an amino acid sequence analysis. The clarified peptide sequences covered approximately 27% of the amino acid sequences of cytokeratin 10 (K10). In the immunoblotting experiment following the two-dimensional electrophoresis, where protein samples extracted from whole epidermis were used, the position of the major CML-positive spots corresponded to those of K10. Taken together these results showed that CML is present in the human epidermis, and suggest that K10 is one of the target molecules for CML modification in the epidermis.

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.

Formation of Nε-(carboxymethyl)lysine in inflammatory and noninflammatory conditions of nerve and muscle and in inflammatory cells in vitro

We studied the occurrence of Nε-(carboxymethyl)lysine (CML), the major in vivo Maillard product in nerve and muscle in various inflammatory and noninflammatory diseases using a previously characterised specific CML antiserum. We found an increased CML staining in the perineurium of biopsies from B12-deficient, diabetic and vasculitic neuropathy and to a lesser extent, in alcoholic neuropathy and in mononuclear inflammatory cells. In diabetic neuropathy CML, the receptor for advanced glycation endproducts (RAGE) and nuclear factor kappa B (activated NFκB) were colocalised indicating possible interaction. In myositis, intracellular CML depositions were observed in infiltrating monocytes, myoblasts and necrotic fibers and Western blot analysis showed a marked increase of CML-modified proteins. To evaluate the impact of inflammation-associated cells on CML formation, we designed in vitro experiments using granulocytes and the monocytic cell line Mono-Mac6 (MM6). Oxidative burst of zymosan-stimulated granulocytes led to CML modification of extracellular proteins within 60–120 min. Predifferentiated MM6 cells, exerting NADPH oxidase activity but lacking myeloperoxidase activity, showed CML modification of distinct intracellular proteins within 1–3 days upon stimulation. Our findings suggest that CML modification occurs in vivo and in vitro when oxidative stress from e.g. inflammatory cells is present, indicating that CML may represent a marker for oxidative stress.

Study of autoantibodies against advanced glycation endproducts of the Maillard reaction

Long-term incubation of proteins with reducing sugar proceeds to advanced glycation end products (AGEs). We previously demonstrated the presence of AGE structures in human and animal tissues, and suggested the implication of AGEs in aging and diabetic complications. In this study, we tested whether AGEs present in vivo, such as N ε-(carboxymethyl) lysine (CML) adduct, could serve as immunogens for generating autoantibodies. Plasma samples from STZ-induced diabetic rats reacted positively with AGE-proteins as well as CML-BSA, suggesting the presence of autoantibodies against AGE-structures, particularly CML in diabetic rats; the activity of the autoantibody increased with the duration of diabetic states, reflecting the accumulation of AGE proteins in these diabetic rats. An autoantibody fraction purified from the plasma of diabetic patients showed a positive reaction to AGE-proteins as well as to CML proteins, and also to human lens proteins which are known to undergo CML modification in vivo. Patients with renal failure caused by diabetes or nondiabetic pathologies had a higher autoantibody activity against AGE structures than that in normal subjects or diabetic patients without renal failure. These results suggest that AGEs accumulated in vivo serve as immunological epitopes for developing autoantibodies against AGEs, particularly against a CML structure, which might be used as an indicator of diabetic nephropathy or chronic renal failure.

Autoantibody against N(epsilon)-(carboxymethyl)lysine: an advanced glycation end product of the Maillard reaction.

Prolonged incubation of proteins with reducing sugar produces advanced glycation end products (AGEs), which are implicated as factors for aging and diabetic complications. We previously demonstrated the presence of N(epsilon)-(carboxymethyl)lysine (CML), one of the main AGE structures, in human and animal tissues using a monoclonal anti-CML antibody (6D12). These findings suggest that CML structures present in vivo could serve as immunogens to generate autoantibodies. This suggestion was tested in the present study. First, plasma samples from diabetic rats reacted positively with AGE bovine serum albumin (BSA). These reactivities increased with the duration of diabetic states and were inhibited specifically by CML-BSA. Second, a fraction purified from plasma of diabetic patients, which bound to AGE-BSA, showed a positive reaction to CML-BSA and furthermore also to human lens proteins, which are known to undergo CML modification in vivo. Finally, patients with renal failure caused by diabetes or nondiabetic pathologies had a higher autoantibody activity against CML structure than that in normal subjects or diabetic patients without renal failure. These results indicate that CML accumulated in vivo serves as an immunological epitope to generate an autoantibody specific for CML that might be used as a potential marker for diabetic nephropathy or chronic renal failure.

Increased accumulation of the glycoxidation product N(epsilon)-(carboxymethyl)lysine in human tissues in diabetes and aging.

N(epsilon)-(Carboxymethyl)lysine (CML), a major product of oxidative modification of glycated proteins, has been suggested to represent a general marker of oxidative stress and long-term damage to proteins in aging, atherosclerosis, and diabetes. To investigate the occurrence and distribution of CML in humans an antiserum specifically recognizing protein-bound CML was generated. The oxidative formation of CML from glycated proteins was reduced by lipoic acid, aminoguanidine, superoxide dismutase, catalase, and particularly vitamin E and desferrioxamine. Immunolocalization of CML in skin, lung, heart, kidney, intestine, intervertebral discs, and particularly in arteries provided evidence for an age-dependent increase in CML accumulation in distinct locations, and acceleration of this process in diabetes. Intense staining of the arterial wall and particularly the elastic membrane was found. High levels of CML modification were observed within atherosclerotic plaques and in foam cells. The preferential location of CML immunoreactivity in lesions may indicate the contribution of glycoxidation to the processes occurring in diabetes and aging. Additionally, we found increased CML content in serum proteins in diabetic patients. The strong dependence of CML formation on oxidative conditions together with the increased occurrence of CML in diabetic serum and tissue proteins suggest a role for CML as endogenous biomarker for oxidative damage.