Advanced glycation end products of DNA: quantification of N2-(1-Carboxyethyl)-2'-deoxyguanosine in biological samples by liquid chromatography electrospray ionization tandem mass spectrometry

Dietary glycation compounds - implications for human health

The term "glycation compounds" comprises a wide range of structurally diverse compounds that are formed endogenously and in food via the Maillard reaction, a chemical reaction between reducing sugars and amino acids. Glycation compounds produced endogenously are considered to contribute to a range of diseases. This has led to the hypothesis that glycation compounds present in food may also cause adverse effects and thus pose a nutritional risk to human health. In this work, the Senate Commission on Food Safety (SKLM) of the German Research Foundation (DFG) summarized data on formation, occurrence, exposure and toxicity of glycation compounds (Part A) and systematically assessed potential associations between dietary intake of defined glycation compounds and disease, including allergy, diabetes, cardiovascular and renal disease, gut/gastrotoxicity, brain/cognitive impairment and cancer (Part B). A systematic search in Pubmed (Medline), Scopus and Web of Science using a combination of keywords defining individual glycation compounds and relevant disease patterns linked to the subject area of food, nutrition and diet retrieved 253 original publications relevant to the research question. Of these, only 192 were found to comply with previously defined quality criteria and were thus considered suitable to assess potential health risks of dietary glycation compounds. For each adverse health effect considered in this assessment, however, only limited numbers of human, animal and in vitro studies were identified. While studies in humans were often limited due to small cohort size, short study duration, and confounders, experimental studies in animals that allow for controlled exposure to individual glycation compounds provided some evidence for impaired glucose tolerance, insulin resistance, cardiovascular effects and renal injury in response to oral exposure to dicarbonyl compounds, albeit at dose levels by far exceeding estimated human exposures. The overall database was generally inconsistent or inconclusive. Based on this systematic review, the SKLM concludes that there is at present no convincing evidence for a causal association between dietary intake of glycation compounds and adverse health effects.

DNA modifications: Biomarkers for the exposome?

The concept of the exposome is the encompassing of all the environmental exposures, both exogenous and endogenous, across the life course. Many, if not all, of these exposures can result in the generation of reactive species, and/or the modulation of cellular processes, that can lead to a breadth of modifications of DNA, the nature of which may be used to infer their origin. Because of their role in cell function, such modifications have been associated with various major human diseases, including cancer, and so their assessment is crucial. Historically, most methods have been able to only measure one or a few DNA modifications at a time, limiting the information available. With the development of DNA adductomics, which aims to determine the totality of DNA modifications, a far more comprehensive picture of the DNA adduct burden can be gained. Importantly, DNA adductomics can facilitate a "top-down" investigative approach whereby patterns of adducts may be used to trace and identify the originating exposure source. This, together with other 'omic approaches, represents a major tool for unraveling the complexities of the exposome and hence allow a better a understanding of the environmental origins of disease.

Methylglyoxal in Cardiometabolic Disorders: Routes Leading to Pathology Counterbalanced by Treatment Strategies

Methylglyoxal (MGO) is the major compound belonging to reactive carbonyl species (RCS) responsible for the generation of advanced glycation end products (AGEs). Its upregulation, followed by deleterious effects at the cellular and systemic levels, is associated with metabolic disturbances (hyperglycemia/hyperinsulinemia/insulin resistance/hyperlipidemia/inflammatory processes/carbonyl stress/oxidative stress/hypoxia). Therefore, it is implicated in a variety of disorders, including metabolic syndrome, diabetes mellitus, and cardiovascular diseases. In this review, an interplay between pathways leading to MGO generation and scavenging is addressed in regard to this system's impairment in pathology. The issues associated with mechanistic MGO involvement in pathological processes, as well as the discussion on its possible causative role in cardiometabolic diseases, are enclosed. Finally, the main strategies aimed at MGO and its AGEs downregulation with respect to cardiometabolic disorders treatment are addressed. Potential glycation inhibitors and MGO scavengers are discussed, as well as the mechanisms of their action.

LC-MS/MS Analysis of Reaction Products of Arginine/Methylarginines with Methylglyoxal/Glyoxal

Methylglyoxal (MGO) and glyoxal (GO) are toxic α-dicarbonyl compounds that undergo reactions with amine containing molecules such as proteins and amino acids and result in the formation of advanced glycation end products (AGEs). This study aimed at investigating the reactivity of arginine (Arg) or dimethylarginine (SDMA or ADMA) with MGO or GO. The solutions of arginine and MGO or GO were prepared in PBS buffer (pH 7.4) and incubated at 37 °C. Direct electrospray ionization-high-resolution mass spectrometry (ESI-HRMS) analysis of the reaction mixture of Arg and MGO revealed the formation of Arg-MGO (1:1) and Arg-2MGO (1:2) products and their corresponding dehydrated products. Further liquid chromatography (LC)-MS analyses revealed the presence of isomeric products in each 1:1 and 1:2 product. The [M + H]+ of each isomeric product was subjected to MS/MS experiments for structural elucidation. The MS/MS spectra of some of the products showed a distinct structure indicative fragment ions, while others showed similar data. The types of products formed by the arginines with GO were also found to be similar to that of MGO. The importance of the guanidine group in the formation of the AGEs was reflected in similar incubation experiments with ADMA and SDMA. The structures of the products were proposed based on the comparison of the retention times and HRMS and MS/MS data interpretation, and some of them were confirmed by drawing analogy to the data reported in the literature.

Data-Independent Acquisition for the Detection of Mononucleoside RNA Modifications by Mass Spectrometry

RNA is dynamically modified in cells by a plethora of chemical moieties to modulate molecular functions and processes. Over 140 modifications have been identified across species and RNA types, with the highest density and diversity of modifications found in tRNA (tRNA). The methods used to identify and quantify these modifications have developed over recent years and continue to advance, primarily in the fields of next-generation sequencing (NGS) and mass spectrometry (MS). Most current NGS methods are limited to antibody-recognized or chemically derivatized modifications and have limitations in identifying multiple modifications simultaneously. Mass spectrometry can overcome both of these issues, accurately identifying a large number of modifications in a single run. Here, we present advances in MS data acquisition for the purpose of RNA modification identification and quantitation. Using this approach, we identified multiple tRNA wobble position modifications in Arabidopsis thaliana that are upregulated in salt-stressed growth conditions and may stabilize translation of salt stress induced proteins. This work presents improvements in methods for studying RNA modifications and introduces a possible regulatory role of wobble position modifications in A. thaliana translation.

The role of endogenous versus exogenous sources in the exposome of putative genotoxins and consequences for risk assessment

The "totality" of the human exposure is conceived to encompass life-associated endogenous and exogenous aggregate exposures. Process-related contaminants (PRCs) are not only formed in foods by heat processing, but also occur endogenously in the organism as physiological components of energy metabolism, potentially also generated by the human microbiome. To arrive at a comprehensive risk assessment, it is necessary to understand the contribution of in vivo background occurrence as compared to the ingestion from exogenous sources. Hence, this review provides an overview of the knowledge on the contribution of endogenous exposure to the overall exposure to putative genotoxic food contaminants, namely ethanol, acetaldehyde, formaldehyde, acrylamide, acrolein, α,β-unsaturated alkenals, glycation compounds, N-nitroso compounds, ethylene oxide, furans, 2- and 3-MCPD, and glycidyl esters. The evidence discussed herein allows to conclude that endogenous formation of some contaminants appears to contribute substantially to the exposome. This is of critical importance for risk assessment in the cases where endogenous exposure is suspected to outweigh the exogenous one (e.g. formaldehyde and acrolein).

Oxidative Stress in Neutrophils: Implications for Diabetic Cardiovascular Complications

Significance: Neutrophil behavior and function are altered by hyperglycemia associated with diabetes. Aberrant activation by hyperglycemia causes neutrophils to respond with increased production of reactive oxidative species (ROS). Excess ROS, a signature of primed neutrophils, can intracellularly induce neutrophils to undergo NETosis, flooding surrounding tissues with ROS and damage-associated molecular patterns such as S100 calcium binding proteins (S100A8/A9). The cargo associated with NETosis also attracts more immune cells to the site and signals for increased immune cell production. This inflammatory response to diabetes can accelerate other associated conditions such as atherosclerosis and thrombosis, increasing the risk of cardiovascular disease. Recent Advances: As the prevalence of diabetes continues to grow, more attention has been focused on developing effective treatment options. Currently, glucose-lowering medications and insulin injections are the most widely utilized treatments. As the disease progresses, medications are usually stacked to maintain glucose at desired target levels, but this approach often fails and does not effectively reduce cardiovascular risk, even with the latest drugs. Critical Issues: Despite advances in treatment options, diabetes remains a progressive disease as glucose lowering alone has failed to abolish the associated cardiovascular complications. Future Directions: Significant interest is being generated in developing treatments that do not solely focus on glucose control but rather mitigate glucotoxicity. Several therapies have been proposed that target cellular dysfunction downstream of hyperglycemia, such as using antioxidants to scavenge ROS, inhibiting ROS production from NOX, and suppressing neutrophil release of S100A8/A9 proteins. Antioxid. Redox Signal. 36, 652-666.

Diet and Obesity-Induced Methylglyoxal Production and Links to Metabolic Disease

The obesity rate in the United States is 42.4% and has become a national epidemic. Obesity is a complex condition that is influenced by socioeconomic status, ethnicity, genetics, age, and diet. Increased consumption of a Western diet, one that is high in processed foods, red meat, and sugar content, is associated with elevated obesity rates. Factors that increase obesity risk, such as socioeconomic status, also increase consumption of a Western diet because of a limited access to healthier options and greater affordability of processed foods. Obesity is a public health threat because it increases the risk of several pathologies, including atherosclerosis, diabetes, and cancer. The molecular mechanisms linking obesity to disease onset and progression are not well understood, but a proposed mechanism is physiological changes caused by altered lipid peroxidation, glycolysis, and protein metabolism. These metabolic pathways give rise to reactive molecules such as the abundant electrophile methylglyoxal (MG), which covalently modifies nucleic acids and proteins. MG-adducts are associated with obesity-linked pathologies and may have potential for biomonitoring to determine the risk of disease onset and progression. MG-adducts may also play a role in disease progression because they are mutagenic and directly impact protein stability and function. In this review, we discuss how obesity drives metabolic alterations, how these alterations lead to MG production, the association of MG-adducts with disease, and the potential impact of MG-adducts on cellular function.

A Review of Stable Isotope Labeling and Mass Spectrometry Methods to Distinguish Exogenous from Endogenous DNA Adducts and Improve Dose-Response Assessments

Cancer remains the second most frequent cause of death in human populations worldwide, which has been reflected in the emphasis placed on management of risk from environmental chemicals considered to be potential human carcinogens. The formation of DNA adducts has been considered as one of the key events of cancer, and persistence and/or failure of repair of these adducts may lead to mutation, thus initiating cancer. Some chemical carcinogens can produce DNA adducts, and DNA adducts have been used as biomarkers of exposure. However, DNA adducts of various types are also produced endogenously in the course of normal metabolism. Since both endogenous physiological processes and exogenous exposure to xenobiotics can cause DNA adducts, the differentiation of the sources of DNA adducts can be highly informative for cancer risk assessment. This review summarizes a highly applicable methodology, termed stable isotope labeling and mass spectrometry (SILMS), that is superior to previous methods, as it not only provides absolute quantitation of DNA adducts but also differentiates the exogenous and endogenous origins of DNA adducts. SILMS uses stable isotope-labeled substances for exposure, followed by DNA adduct measurement with highly sensitive mass spectrometry. Herein, the utilities and advantage of SILMS have been demonstrated by the rich data sets generated over the last two decades in improving the risk assessment of chemicals with DNA adducts being induced by both endogenous and exogenous sources, such as formaldehyde, vinyl acetate, vinyl chloride, and ethylene oxide.

Elevated glucose increases genomic instability by inhibiting nucleotide excision repair

Exposure to chronic, elevated glucose inhibits nucleotide excision repair, which leads to accumulation of DNA glycation adducts, increased DNA strand breaks, and activation of the DNA damage response.

We investigated potential mechanisms by which elevated glucose may promote genomic instability. Gene expression studies, protein measurements, mass spectroscopic analyses, and functional assays revealed that elevated glucose inhibited the nucleotide excision repair (NER) pathway, promoted DNA strand breaks, and increased levels of the DNA glycation adduct N²-(1-carboxyethyl)-2ʹ-deoxyguanosine (CEdG). Glycation stress in NER-competent cells yielded single-strand breaks accompanied by ATR activation, γH2AX induction, and enhanced non-homologous end-joining and homology-directed repair. In NER-deficient cells, glycation stress activated ATM/ATR/H2AX, consistent with double-strand break formation. Elevated glucose inhibited DNA repair by attenuating hypoxia-inducible factor-1α–mediated transcription of NER genes via enhanced 2-ketoglutarate–dependent prolyl hydroxylase (PHD) activity. PHD inhibition enhanced transcription of NER genes and facilitated CEdG repair. These results are consistent with a role for hyperglycemia in promoting genomic instability as a potential mechanism for increasing cancer risk in metabolic disease. Because of the pleiotropic functions of many NER genes beyond DNA repair, these results may have broader implications for cellular pathophysiology.

Methylglyoxal induces chromosomal instability and mitotic dysfunction in lymphocytes

Type 2 diabetes is associated with elevated levels of DNA damage, in particular micronuclei (MNi) which are formed by acentric chromosome fragments caused by double-stranded DNA breaks (DSBs), or whole chromosomes which fail to segregate during mitosis. We investigated if methylglyoxal (MGO), a reactive dicarbonyl known to be elevated in type 2 diabetes is capable of increasing chromosomal instability and DNA damage as measured by the cytokinesis block micronucleus cytome (CBMNcyt) assay in B-lymphoblastoid WIL2-NS cells and primary peripheral blood lymphocytes (PBL). We also investigated the level of various dicarbonyl stress biomarkers, including extracellular and intracellular MGO, protein and MGO modifications of DNA. WIL2-NS cells exposed to either MGO or a glyoxalase 1 inhibitor showed increases in MNi and nuclear buds, which were associated with an increase in intracellular MGO. DNA damage in the form of MNi and nucleoplasmic bridges were observed in primary PBL exposed to 10 µM MGO, suggesting low concentrations of MGO may be genotoxic. Furthermore, we showed, using fluorescent in situ hybridization, that the majority of MNi caused by MGO in WIL2-NS cells were caused by whole chromosome loss events, rather than DSBs. Our data suggest that MGO, a reactive metabolite elevated in type 2 diabetes and other pathologies, can affect genomic integrity by impairing chromosome segregation during mitosis.

Quantitation by liquid chromatography-nanoelectrospray ionization-high resolution tandem mass spectrometry of DNA adducts derived from methyl glyoxal and carboxyethylating agents in leukocytes of smokers and non-smokers

A liquid chromatograpy-nanoelectrospray ionization-high resolution tandem mass spectrometry (LC-NSI-HRMS/MS) method was developed for quantitation of the DNA adducts 7-(2'-carboxyethyl)guanine (7-2'-CEG) and N2-(1'-carboxyethyl)guanine (N2-1'-CEG), as their methyl esters, in human leukocyte DNA from smokers and non-smokers. 7-2'-CEG has been previously identified in all human liver samples analyzed and is formed from an unknown carboxyethylating agent while N2-1'-CEG is formed from the advanced glycation endproduct methyl glyoxal. The method was applied for the analysis of these two DNA adducts in leukocyte DNA from 20 smokers and 20 non-smokers, in part to test the hypothesis that 7-2'-CEG could be formed by endogenous nitrosation, as previously observed in rats treated with nitrosodihydrouracil and nitrite. Levels of 7-2'-CEG (mean ± S.D.) were 0.6 ± 0.2 pmol/μmol dG in smokers and 0.5 ± 0.2 pmol/μmol dG in non-smokers, while those of N2-1'-CEG were 4.5 ± 1.9 pmol/μmol dG in smokers and 4.6 ± 2 pmol/μmol dG in non-smokers. These results did not support our hypothesis that endogenous nitrosation of dihydrouracil in smokers leads to higher levels of 7-2'-CEG in leukocyte DNA than in non-smokers. However the study provides the first data on levels of these DNA adducts in human leukocyte DNA, and the LC-NSI-HRMS/MS method developed for their quantitation could be important for future studies of DNA damage by methyl glyoxal.

Non-enzymatic covalent modifications: a new link between metabolism and epigenetics

Epigenetic modifications, including those on DNA and histones, have been shown to regulate cellular metabolism by controlling expression of enzymes involved in the corresponding metabolic pathways. In turn, metabolic flux influences epigenetic regulation by affecting the biosynthetic balance of enzyme cofactors or donors for certain chromatin modifications. Recently, non-enzymatic covalent modifications (NECMs) by chemically reactive metabolites have been reported to manipulate chromatin architecture and gene transcription through multiple mechanisms. Here, we summarize these recent advances in the identification and characterization of NECMs on nucleic acids, histones, and transcription factors, providing an additional mechanistic link between metabolism and epigenetics.

DNA Adducts as Biomarkers To Predict, Prevent, and Diagnose Disease-Application of Analytical Chemistry to Clinical Investigations

Characterization of the chemistry, structure, formation, and metabolism of DNA adducts has been one of the most significant contributions to the field of chemical toxicology. This work provides the foundation to develop analytical methods to measure DNA adducts, define their relationship to disease, and establish clinical tests. Monitoring exposure to environmental and endogenous toxicants can predict, diagnose, and track disease as well as guide therapeutic treatment. DNA adducts are one of the most promising biomarkers of toxicant exposure owing to their stability, appearance in numerous biological matrices, and characteristic analytical properties. In addition, DNA adducts can induce mutations to drive disease onset and progression and can serve as surrogate markers of chemical exposure. In this perspective, we highlight significant advances made within the past decade regarding DNA adduct quantitation using mass spectrometry. We hope to expose a broader audience to this field and encourage analytical chemistry laboratories to explore how specific adducts may be related to various pathologies. One of the limiting factors in developing clinical tests to measure DNA adducts is cohort size; ideally, the cohort would allow for model development and then testing of the model to the remaining cohort. The goals of this perspective article are to (1) provide a summary of analyte levels measured using state-of-the-art analytical methods, (2) foster collaboration, and (3) highlight areas in need of further investigation.

Methylglyoxal, a Highly Reactive Dicarbonyl Compound, in Diabetes, Its Vascular Complications, and Other Age-Related Diseases

The formation and accumulation of methylglyoxal (MGO), a highly reactive dicarbonyl compound, has been implicated in the pathogenesis of type 2 diabetes, vascular complications of diabetes, and several other age-related chronic inflammatory diseases such as cardiovascular disease, cancer, and disorders of the central nervous system. MGO is mainly formed as a byproduct of glycolysis and, under physiological circumstances, detoxified by the glyoxalase system. MGO is the major precursor of nonenzymatic glycation of proteins and DNA, subsequently leading to the formation of advanced glycation end products (AGEs). MGO and MGO-derived AGEs can impact on organs and tissues affecting their functions and structure. In this review we summarize the formation of MGO, the detoxification of MGO by the glyoxalase system, and the biochemical pathways through which MGO is linked to the development of diabetes, vascular complications of diabetes, and other age-related diseases. Although interventions to treat MGO-associated complications are not yet available in the clinical setting, several strategies to lower MGO have been developed over the years. We will summarize several new directions to target MGO stress including glyoxalase inducers and MGO scavengers. Targeting MGO burden may provide new therapeutic applications to mitigate diseases in which MGO plays a crucial role.

A validated UHPLC-MS/MS method for simultaneous quantification of 9 exocyclic DNA adducts induced by 8 aldehydes

Human exposure to aldehydes is implicated in several diseases including cancer. These strong electrophilic compounds can react with nucleophilic sites in DNA to form reversible and irreversible modifications. These modifications, if not repaired, can contribute to pathogenesis. The aim of our study was to provide a mass spectrometry (MS)-based profiling method for identifying potential biomarkers of aldehydes exposure. We have developed and validated a highly sensitive method using ultra high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (UHPLC-ESI-MS/MS) for the simultaneous quantitation of 9 exocyclic DNA adducts derived from 8 main exogenous and endogenous aldehydes, namely formaldehyde, acetaldehyde, acrolein, crotonaldehyde, malondialdehyde, 4-hydroxy-2-nonenal, glyoxal and methylglyoxal. Finally, we applied the established method to quantify adducts in genomic DNA isolated from the blood of a smoker and a non-smoker blood samples in order to demonstrate its applicability.

The Impact of Minor-Groove N2-Alkyl-2'-deoxyguanosine Lesions on DNA Replication in Human Cells

Endogenous metabolites and exogenous chemicals can induce covalent modifications on DNA, producing DNA lesions. The N² of guanine was shown to be a common alkylation site in DNA; however, not much is known about the influence of the size of the alkyl group in N²-alkyldG lesions on cellular DNA replication or how translesion synthesis (TLS) polymerases modulate DNA replication past these lesions in human cells. To answer these questions, we employ a robust shuttle vector method to investigate the impact of four N²-alkyldG lesions (i.e., with the alkyl group being a methyl, ethyl, n-propyl, or n-butyl group) on DNA replication in human cells. We find that replication through the N²-alkyldG lesions was highly efficient and accurate in HEK293T cells or isogenic CRISPR-engineered cells with deficiency in polymerase (Pol) ζ or Pol η. Genetic ablation of Pol ι, Pol κ, or Rev1, however, results in decreased bypass efficiencies and elicits substantial frequencies of G → A transition and G → T transversion mutations for these lesions. Moreover, further depletion of Pol ζ in Pol κ- or Pol ι-deficient cells gives rise to elevated rates of G → A and G → T mutations and substantially decreased bypass efficiencies. Cumulatively, we demonstrate that the error-free replication past the N²-alkyldG lesions is facilitated by a specific subset of TLS polymerases, and we find that longer alkyl chains in these lesions induce diminished bypass efficiency and fidelity in DNA replication.

Momordica charantia extracts protect against inhibition of endothelial angiogenesis by advanced glycation endproducts in vitro

Diabetes mellitus characterized by hyperglycemia favors formation of advanced glycation endproducts (AGEs) capable of triggering vascular complications by interfering with imbalanced inflammation and angiogenesis to eventually impede wound-healing. Momordica charantia (MC, bitter melon) has been shown to prevent AGE formation and to promote angiogenesis in diabetic wounds in animal models. However, the mechanism underlying its effects on angiogenesis is unclear. We investigated the effects of methanolic extracts of MC pulp (MCP), flesh (MCF) and charantin (active component of MC) using an in vitro model of angiogenesis. MC extracts or low concentrations of bovine serum albumin-derived AGEs (BSA-AGEs) stimulated proliferation, migration (using wound-healing assay) and tube formation (using Matrigel™-embedded 3D culture) of bovine aortic endothelial cells (BAEC) together with increases in the phosphorylation of extracellular signal-regulated kinase (ERK)1/2, the key angiogenic signaling cytoplasmic protein. Blocking the receptor for AGEs (RAGE) inhibited low BSA-AGE- and MC extract-induced ERK1/2 phosphorylation and tube formation, indicating the crucial role of RAGE in the pro-angiogenic effects of MC extracts. Moreover, inhibitory effects of high BSA-AGE concentration on cell proliferation and migration were reduced by the addition of MC extracts, which reversed the BSA-AGE anti-angiogenic effect on tube formation. Thus, MC extracts exert direct pro-angiogenic signaling mediated via RAGE to overcome the anti-angiogenic effects of high BSA-AGEs, highlighting the biphasic RAGE-dependent mechanisms involved. This study enhances our understanding of the mechanisms underlying the pro-angiogenic effects of MC extracts in improvement of diabetes-impaired wound-healing.

Identification and characterization of reaction products of 5-hydroxytryptamine with methylglyoxal and glyoxal by liquid chromatography/tandem mass spectrometry

RATIONALE

Methylglyoxal (MGO) and glyoxal (GO) are known to be at high levels in humans with diabetes. They react with amine-containing proteins and amino acids to form advanced glycation end products, however, their reactivity with other amine-containing metabolites, such as neurotransmitters, has not been explored. In this study, we aimed at studying the reactivity of 5-hydroxytryptamine (5-HT) with MGO or GO, which may alter the metabolic function of 5-HT.

METHODS

Stock solutions of 5-HT, MGO and GO were made in PBS buffer at pH 7.4 and 5-HT was incubated with MGO or GO at different concentrations. The reactions were also performed at physiological concentrations. The reaction mixtures collected at different incubation times were analyzed by direct ESI-HRMS, LC/MS and LC/MS/MS to detect/characterize the products. Agilent 6545 Q-TOF and Agilent 6420 triple quadrupole mass spectrometers were used for the study, and LC separations were performed on a C18 column.

RESULTS

The direct ESI-HRMS data of the reaction mixtures showed formation of three and four reaction products when 5-HT was reacted with MGO and GO, respectively. All the products showed dominant [M + H]⁺ ions. The products were characterized by HRMS, LC/MS/MS and literature reports on similar compounds. The products can easily be identified by LC/MS based on the accurate mass values together with retention time information. The MS/MS of the reaction products showed structure-indicative fragment ions.

CONCLUSIONS

5-HT reacts with one or two MGO/GO to form a set of reaction products. The reaction between 5-HT and MGO or GO was faster at higher concentrations of MGO/GO (<10 min), and the same products were found even at physiological concentrations (<48 h). The LC/MS/MS (SRM) method can be used to screen the reaction products when present at low levels.

Inhibition of GLO1 in Glioblastoma Multiforme Increases DNA-AGEs, Stimulates RAGE Expression, and Inhibits Brain Tumor Growth in Orthotopic Mouse Models

Cancers that exhibit the Warburg effect may elevate expression of glyoxylase 1 (GLO1) to detoxify the toxic glycolytic byproduct methylglyoxal (MG) and inhibit the formation of pro-apoptotic advanced glycation endproducts (AGEs). Inhibition of GLO1 in cancers that up-regulate glycolysis has been proposed as a therapeutic targeting strategy, but this approach has not been evaluated for glioblastoma multiforme (GBM), the most aggressive and difficult to treat malignancy of the brain. Elevated GLO1 expression in GBM was established in patient tumors and cell lines using bioinformatics tools and biochemical approaches. GLO1 inhibition in GBM cell lines and in an orthotopic xenograft GBM mouse model was examined using both small molecule and short hairpin RNA (shRNA) approaches. Inhibition of GLO1 with S-(p-bromobenzyl) glutathione dicyclopentyl ester (p-BrBzGSH(Cp)₂) increased levels of the DNA-AGE N²-1-(carboxyethyl)-2′-deoxyguanosine (CEdG), a surrogate biomarker for nuclear MG exposure; substantially elevated expression of the immunoglobulin-like receptor for AGEs (RAGE); and induced apoptosis in GBM cell lines. Targeting GLO1 with shRNA similarly increased CEdG levels and RAGE expression, and was cytotoxic to glioma cells. Mice bearing orthotopic GBM xenografts treated systemically with p-BrBzGSH(Cp)₂ exhibited tumor regression without significant off-target effects suggesting that GLO1 inhibition may have value in the therapeutic management of these drug-resistant tumors.

Dicarbonyls and Advanced Glycation End-Products in the Development of Diabetic Complications and Targets for Intervention

Advanced glycation end-products (AGEs) are non-enzymatic protein and amino acid adducts as well as DNA adducts which form from dicarbonyls and glucose. AGE formation is enhanced in diabetes and is associated with the development of diabetic complications. In the current review, we discuss mechanisms that lead to enhanced AGE levels in the context of diabetes and diabetic complications. The methylglyoxal-detoxifying glyoxalase system as well as alternative pathways of AGE detoxification are summarized. Therapeutic approaches to interfere with different pathways of AGE formation are presented.

DNA Advanced Glycation End Products (DNA-AGEs) Are Elevated in Urine and Tissue in an Animal Model of Type 2 Diabetes

More precise identification and treatment monitoring of prediabetic/diabetic individuals will require additional biomarkers to complement existing diagnostic tests. Candidates include hyperglycemia-induced adducts such as advanced glycation end products (AGEs) of proteins, lipids, and DNA. The potential for DNA-AGEs as diabetic biomarkers was examined in a longitudinal study using the Lepr^db/db animal model of metabolic syndrome. The DNA-AGE, N²-(1-carboxyethyl)-2'-deoxyguanosine (CEdG) was quantified by mass spectrometry using isotope dilution from the urine and tissue of hyperglycemic and normoglycemic mice. Hyperglycemic mice (fasting plasma glucose, FPG, ≥ 200 mg/dL) displayed a higher median urinary CEdG value (238.4 ± 112.8 pmol/24 h) than normoglycemic mice (16.1 ± 11.8 pmol/24 h). Logistic regression analysis revealed urinary CEdG to be an independent predictor of hyperglycemia. Urinary CEdG was positively correlated with FPG in hyperglycemic animals and with HbA1c for all mice. Average tissue-derived CEdG was also higher in hyperglycemic mice (18.4 CEdG/10⁶ dG) than normoglycemic mice (4.4 CEdG/10⁶ dG). Urinary CEdG was significantly elevated in Lepr^db/db mice relative to Lepr^wt/wt, and tissue CEdG values increased in the order Lepr^wt/wt < Lepr^wt/db < Lepr^db/db. These data suggest that urinary CEdG measurement may provide a noninvasive quantitative index of glycemic status and augment existing biomarkers for the diagnosis and monitoring of diabetes.

Mass Spectrometry-Based Quantitative Strategies for Assessing the Biological Consequences and Repair of DNA Adducts

The genetic integrity of living organisms is constantly threatened by environmental and endogenous sources of DNA damaging agents that can induce a plethora of chemically modified DNA lesions. Unrepaired DNA lesions may elicit cytotoxic and mutagenic effects and contribute to the development of human diseases including cancer and neurodegeneration. Understanding the deleterious outcomes of DNA damage necessitates the investigation about the effects of DNA adducts on the efficiency and fidelity of DNA replication and transcription. Conventional methods for measuring lesion-induced replicative or transcriptional alterations often require time-consuming colony screening and DNA sequencing procedures. Recently, a series of mass spectrometry (MS)-based strategies have been developed in our laboratory as an efficient platform for qualitative and quantitative analyses of the changes in genetic information induced by DNA adducts during DNA replication and transcription. During the past few years, we have successfully used these MS-based methods for assessing the replicative or transcriptional blocking and miscoding properties of more than 30 distinct DNA adducts. When combined with genetic manipulation, these methods have also been successfully employed for revealing the roles of various DNA repair proteins or translesion synthesis DNA polymerases (Pols) in modulating the adverse effects of DNA lesions on transcription or replication in mammalian and bacterial cells. For instance, we found that Escherichia coli Pol IV and its mammalian ortholog (i.e., Pol κ) are required for error-free bypass of N(2)-(1-carboxyethyl)-2'-deoxyguanosine (N(2)-CEdG) in cells. We also found that the N(2)-CEdG lesions strongly inhibit DNA transcription and they are repaired by transcription-coupled nucleotide excision repair in mammalian cells. In this Account, we focus on the development of MS-based approaches for determining the effects of DNA adducts on DNA replication and transcription, where liquid chromatography-tandem mass spectrometry is employed for the identification, and sometimes quantification, of the progeny products arising from the replication or transcription of lesion-bearing DNA substrates in vitro and in mammalian cells. We also highlight their applications to lesion bypass, mutagenesis, and repair studies of three representative types of DNA lesions, that is, the methylglyoxal-induced N(2)-CEdG, oxidatively induced 8,5'-cyclopurine-2'-deoxynucleosides, and regioisomeric alkylated thymidine lesions. Specially, we discuss the similar and distinct effects of the minor-groove DNA lesions including N(2)-CEdG and O(2)-alkylated thymidine lesions, as well as the major-groove O(4)-alkylated thymidine lesions on DNA replication and transcription machinery. For example, we found that the addition of an alkyl group to the O(4) position of thymine may facilitate its preferential pairing with guanine and thus induce exclusively the misincorporation of guanine nucleotide opposite the lesion, whereas alkylation of thymine at the O(2) position may render the nucleobase unfavorable in pairing with any of the canonical nucleobases and thus exhibit promiscuous miscoding properties during DNA replication and transcription. The MS-based strategies described herein should be generally applicable for quantitative measurement of the biological consequences and repair of other DNA lesions in vitro and in cells.

5'-O-Alkylpyridoxamines: Lipophilic Analogues of Pyridoxamine Are Potent Scavengers of 1,2-Dicarbonyls

Pyridoxamine (PM) is a prospective drug for the treatment of diabetic complications. In order to make zwitterionic PM more lipophilic and improve its tissue distribution, PM derivatives containing medium length alkyl groups on the hydroxymethyl side chain were prepared. The synthesis of these alkylpyridoxamines (alkyl-PMs) starting from pyridoxine offers high yields and is amenable to bulk preparations. Interestingly, alkyl-PMs were found to react with methylglyoxal (MGO), a major toxic product of glucose metabolism and autoxidation, several orders of magnitude faster than PM. This suggests the formation of nonionic pyrido-1,3-oxazine as the key step in the reaction of PM with MGO. Since the primary target of MGO in proteins is the guanidine side chain of arginine, alkyl-PMs were shown to be more effective than PM in reducing the modification of N-α-benzoylarginine by MGO. Alkyl-PMs in the presence of MGO also protected the enzymatic activity of lysozyme that contains several arginine residues next to its active site. Alkyl-PMs can be expected to trap MGO and other toxic 1,2-carbonyl compounds more effectively than PM, especially in lipophilic tissue environments, thus protecting macromolecules from functional damage. This suggests potential therapeutic uses for alkyl-PMs in diabetes and other diseases characterized by the elevated levels of toxic dicarbonyl compounds.

The role of methylglyoxal and the glyoxalase system in diabetes and other age-related diseases

The formation and accumulation of advanced glycation endproducts (AGEs) are related to diabetes and other age-related diseases. Methylglyoxal (MGO), a highly reactive dicarbonyl compound, is the major precursor in the formation of AGEs. MGO is mainly formed as a byproduct of glycolysis. Under physiological circumstances, MGO is detoxified by the glyoxalase system into D-lactate, with glyoxalase I (GLO1) as the key enzyme in the anti-glycation defence. New insights indicate that increased levels of MGO and the major MGO-derived AGE, methylglyoxal-derived hydroimidazolone 1 (MG-H1), and dysfunctioning of the glyoxalase system are linked to several age-related health problems, such as diabetes, cardiovascular disease, cancer and disorders of the central nervous system. The present review summarizes the mechanisms through which MGO is formed, its detoxification by the glyoxalase system and its effect on biochemical pathways in relation to the development of age-related diseases. Although several scavengers of MGO have been developed over the years, therapies to treat MGO-associated complications are not yet available for application in clinical practice. Small bioactive inducers of GLO1 can potentially form the basis for new treatment strategies for age-related disorders in which MGO plays a pivotal role.

Chemical structure and properties of interstrand cross-links formed by reaction of guanine residues with abasic sites in duplex DNA

A new type of interstrand cross-link resulting from the reaction of a DNA abasic site with a guanine residue on the opposing strand of the double helix was recently identified, but the chemical connectivity of the cross-link was not rigorously established. The work described here was designed to characterize the chemical structure and properties of dG-AP cross-links generated in duplex DNA. The approach involved characterization of the nucleoside cross-link "remnant" released by enzymatic digestion of DNA duplexes containing the dG-AP cross-link. We first carried out a chemical synthesis and complete spectroscopic structure determination of the putative cross-link remnant 9b composed of a 2-deoxyribose adduct attached to the exocyclic N(2)-amino group of dG. A reduced analogue of the cross-link remnant was also prepared (11b). Liquid chromatography-tandem mass spectrometric (LC-MS/MS) analysis revealed that the retention times and mass spectral properties of synthetic standards 9b and 11b matched those of the authentic cross-link remnants released by enzymatic digestion of duplexes containing the native and reduced dG-AP cross-link, respectively. These results establish the chemical connectivity of the dG-AP cross-link released from duplex DNA and provide a foundation for detection of this lesion in biological samples. The dG-AP cross-link in duplex DNA was remarkably stable, decomposing with a half-life of 22 days at pH 7 and 23 °C. The intrinsic chemical stability of the dG-AP cross-link suggests that this lesion in duplex DNA may have the power to block DNA-processing enzymes involved in transcription and replication.

Increased DNA dicarbonyl glycation and oxidation markers in patients with type 2 diabetes and link to diabetic nephropathy

AIM

The aim of this study was to assess the changes of markers of DNA damage by glycation and oxidation in patients with type 2 diabetes and the association with diabetic nephropathy.

METHODOLOGY

DNA oxidation and glycation adducts were analysed in plasma and urine by stable isotopic dilution analysis liquid chromatography-tandem mass spectrometry. DNA markers analysed were as follows: the oxidation adduct 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-OxodG) and glycation adducts of glyoxal and methylglyoxal--imidazopurinones GdG, MGdG, and N2-(1,R/S-carboxyethyl)deoxyguanosine (CEdG).

RESULTS

Plasma 8-OxodG and GdG were increased 2-fold and 6-fold, respectively, in patients with type 2 diabetes, with respect to healthy volunteers. Median urinary excretion rates of 8-OxodG, GdG, MGdG, and CEdG were increased 28-fold, 10-fold, 2-fold, and 2-fold, respectively, in patients with type 2 diabetes with respect to healthy controls. In patients with type 2 diabetes, nephropathy was associated with increased plasma 8-OxodG and increased urinary GdG and CEdG. In a multiple logistic regression model for diabetic nephropathy, diabetic nephropathy was linked to systolic blood pressure and urinary CEdG.

CONCLUSION

DNA oxidative and glycation damage-derived nucleoside adducts are increased in plasma and urine of patients with type 2 diabetes and further increased in patients with diabetic nephropathy.

Critical role of RAGE in lung physiology and tumorigenesis: a potential target of therapeutic intervention?

Lung cancer is one of the most common malignancies in the world and one of the leading causes of death from cancer. In the search for molecules that may be involved in lung tumor induction and progression, the receptor of advanced glycation end products (RAGE) comes across as a critical regulator of lung physiology. RAGE is a multiligand receptor that presents a differential expression pattern in lung epithelial cells compared to other cell types being gradually increased from fetal to birth and adult life. Under stress conditions, RAGE expression and activation are rapidly elevated resulting in chronic inflammation, which, in turn, in many instances, promotes epithelial cell malignant transformation. RAGE overexpression in normal lung alveolar type I epithelial cells is followed by rapid downregulation upon malignant transformation, being associated with increased aggressiveness. This is a striking paradox, since in every other cell type the pattern of RAGE expression follows the opposite direction, suggesting the involvement of RAGE in the well-functioning of lung cells. Additionally, RAGE has been attributed with the role of adhesion molecule, since it can stabilize mature alveolar epithelial cells to their substrate (basal lamina) by interacting electrostatically with other molecules. However, the reduction of RAGE observed in lung tumorigenesis interrupts cell-to-cell and cell-to-substrate communication, which is a critical step for cancer cell induction, progression and migration. This review addresses the differential properties of RAGE in lung physiology and carcinogenesis, providing evidence of therapeutic possibilities.

Inhibitory effect of metformin and pyridoxamine in the formation of early, intermediate and advanced glycation end-products

Background

Non-enzymatic glycation is the addition of free carbonyl group of reducing sugar to the free amino groups of proteins, resulting in the formation of a Schiff base and an Amadori product. Dihydroxyacetone (DHA) is one of the carbonyl species which reacts rapidly with the free amino groups of proteins to form advanced glycation end products (AGEs). The highly reactive dihydroxyacetone phosphate is a derivative of dihydroxyacetone (DHA), and a product of glycolysis, having potential glycating effects to form AGEs. The formation of AGEs results in the generation of free radicals which play an important role in the pathophysiology of aging and diabetic complications. While the formation of DHA-AGEs has been demonstrated previously, no extensive studies have been performed to assess the inhibition of AGE inhibitors at all the three stages of glycation (early, intermediate and late) using metformin (MF) and pyridoxamine (PM) as a novel inhibitor.

Methodology/Principal Findings

In this study we report glycation of human serum albumin (HSA) & its characterization by various spectroscopic techniques. Furthermore, inhibition of glycation products at all the stages of glycation was also studied. Spectroscopic analysis suggests structural perturbations in the HSA as a result of modification which might be due to generation of free radicals and formation of AGEs.

Conclusion

The inhibition in the formation of glycation reaction reveals that Pyridoxamine is a better antiglycating agent than Metformin at all stages of the glycation (early, intermediate and late stages).

A quantitative assay for assessing the effects of DNA lesions on transcription

Most mammalian cells in nature are quiescent but actively transcribing mRNA for normal physiological processes; thus, it is important to investigate how endogenous and exogenous DNA damage compromises transcription in cells. Here we describe a new competitive transcription and adduct bypass (CTAB) assay to determine the effects of DNA lesions on the fidelity and efficiency of transcription. Using this strategy, we demonstrate that the oxidatively induced lesions 8,5'-cyclo-2'-deoxyadenosine (cdA) and 8,5'-cyclo-2'-deoxyguanosine (cdG) and the methylglyoxal-induced lesion N(2)-(1-carboxyethyl)-2'-deoxyguanosine (N(2)-CEdG) strongly inhibited transcription in vitro and in mammalian cells. In addition, cdA and cdG, but not N(2)-CEdG, induced transcriptional mutagenesis in vitro and in vivo. Furthermore, when located on the template DNA strand, all examined lesions were primarily repaired by transcription-coupled nucleotide excision repair in mammalian cells. This newly developed CTAB assay should be generally applicable for quantitatively assessing how other DNA lesions affect DNA transcription in vitro and in cells.

Cockayne syndrome b maintains neural precursor function

Neurodevelopmental defects are observed in the hereditary disorder Cockayne syndrome (CS). The gene most frequently mutated in CS, Cockayne Syndrome B (CSB), is required for the repair of bulky DNA adducts in transcribed genes during transcription-coupled nucleotide excision repair. CSB also plays a role in chromatin remodeling and mitochondrial function. The role of CSB in neural development is poorly understood. Here we report that the abundance of neural progenitors is normal in Csb(-/-) mice and the frequency of apoptotic cells in the neurogenic niche of the adult subependymal zone is similar in Csb(-/-) and wild type mice. Both embryonic and adult Csb(-/-) neural precursors exhibited defective self-renewal in the neurosphere assay. In Csb(-/-) neural precursors, self-renewal progressively decreased in serially passaged neurospheres. The data also indicate that Csb and the nucleotide excision repair protein Xpa preserve embryonic neural stem cell self-renewal after UV DNA damage. Although Csb(-/-) neural precursors do not exhibit altered neuronal lineage commitment after low-dose UV (1J/m(2)) in vitro, neurons differentiated in vitro from Csb(-/-) neural precursors that had been irradiated with 1J/m(2) UV exhibited defective neurite outgrowth. These findings identify a function for Csb in neural precursors.

Quantitation of DNA adducts by stable isotope dilution mass spectrometry

Exposure to endogenous and exogenous chemicals can lead to the formation of structurally modified DNA bases (DNA adducts). If not repaired, these nucleobase lesions can cause polymerase errors during DNA replication, leading to heritable mutations and potentially contributing to the development of cancer. Because of their critical role in cancer initiation, DNA adducts represent mechanism-based biomarkers of carcinogen exposure, and their quantitation is particularly useful for cancer risk assessment. DNA adducts are also valuable in mechanistic studies linking tumorigenic effects of environmental and industrial carcinogens to specific electrophilic species generated from their metabolism. While multiple experimental methodologies have been developed for DNA adduct analysis in biological samples, including immunoassay, HPLC, and ³²P-postlabeling, isotope dilution high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (HPLC-ESI-MS/MS) generally has superior selectivity, sensitivity, accuracy, and reproducibility. As typical DNA adduct concentrations in biological samples are between 0.01-10 adducts per 10⁸ normal nucleotides, ultrasensitive HPLC-ESI-MS/MS methodologies are required for their analysis. Recent developments in analytical separations and biological mass spectrometry, especially nanoflow HPLC, nanospray ionization MS, chip-MS, and high resolution MS, have pushed the limits of analytical HPLC-ESI-MS/MS methodologies for DNA adducts, allowing researchers to accurately measure their concentrations in biological samples from patients treated with DNA alkylating drugs and in populations exposed to carcinogens from urban air, drinking water, cooked food, alcohol, and cigarette smoke.

Conservation of thiol-oxidative stress responses regulated by SigR orthologues in actinomycetes

Numerous thiol-reactive compounds cause oxidative stress where cells counteract by activation of survival strategies regulated by thiol-based sensors. In Streptomyces coelicolor, a model actinomycete, a sigma/antisigma pair SigR/RsrA controls the response to thiol-oxidative stress. To unravel its full physiological functions, chromatin immuno-precipitation combined with sequence and transcript analyses were employed to identify 108 SigR target genes in S. coelicolor and to predict orthologous regulons across actinomycetes. In addition to reported genes for thiol homeostasis, protein degradation and ribosome modulation, 64 additional operons were identified suggesting new functions of this global regulator. We demonstrate that SigR maintains the level and activity of the housekeeping sigma factor HrdB during thiol-oxidative stress, a novel strategy for stress responses. We also found that SigR defends cells against UV and thiol-reactive damages, in which repair UvrA takes a part. Using a refined SigR-binding sequence model, SigR orthologues and their targets were predicted in 42 actinomycetes. This revealed a conserved core set of SigR targets to function for thiol homeostasis, protein quality control, possible modulation of transcription and translation, flavin-mediated redox reactions, and Fe-S delivery. The composition of the SigR regulon reveals a robust conserved physiological mechanism to deal with thiol-oxidative stress from bacteria to human.

Endogenous mitochondrial oxidative stress in MnSOD-deficient mouse embryonic fibroblasts promotes mitochondrial DNA glycation

The accumulation of somatic mutations in mitochondrial DNA (mtDNA) induced by reactive oxygen species (ROS) is regarded as a major contributor to aging and age-related degenerative diseases. ROS have also been shown to facilitate the formation of certain advanced glycation end-products (AGEs) in proteins and DNA and N(2)-carboxyethyl-2'-deoxyguanosine (CEdG) has been identified as a major DNA-bound AGE. Therefore, the influence of mitochondrial ROS on the glycation of mtDNA was investigated in primary embryonic fibroblasts derived from mutant mice (Sod2(-/+)) deficient in the mitochondrial antioxidant enzyme manganese superoxide dismutase. In Sod2(-/+) fibroblasts vs wild-type fibroblasts, the CEdG content of mtDNA was increased from 1.90 ± 1.39 to 17.14 ± 6.60 pg/μg DNA (p<0.001). On the other hand, the CEdG content of nuclear DNA did not differ between Sod2(+/+) and Sod2(-/+) cells. Similarly, cytosolic proteins did not show any difference in advanced glycation end-products or protein carbonyl contents between Sod2(+/+) and Sod2(-/+). Taken together, the data suggest that mitochondrial oxidative stress specifically promotes glycation of mtDNA and does not affect nuclear DNA or cytosolic proteins. Because DNA glycation can change DNA integrity and gene functions, glycation of mtDNA may play an important role in the decline of mitochondrial functions.

Intracellular glycation of nuclear DNA, mitochondrial DNA, and cytosolic proteins during senescence-like growth arrest

To investigate the accumulation of intracellular advanced glycation end products (AGEs), a method was established for the simultaneous analysis of glycation products of cytosolic proteins, nuclear DNA, and mitochondrial DNA (mtDNA). Nuclear DNA, mtDNA, and cytosolic proteins were simultaneously isolated from one cell lysate by differential centrifugation and combined mechanical and chemical cell disruption methods. The major DNA-AGE N(2)-carboxyethyl-2'-deoxyguanosine (CEdG) was quantified in nuclear DNA and mtDNA by ELISA, whereas the protein-AGEs N(ɛ)-(carboxymethyl)lysine (CML) and N(ɛ)-(carboxyethyl)lysine (CEL) were determined by western blot. The method was used to analyze NIH3T3 fibroblasts. In untreated cells, CEdG levels of mtDNA (14.84 ± 3.07 pg CEdG/μg mtDNA) were significantly higher compared with nuclear DNA (4.40 ± 0.64 pg CEdG/μg DNA; p < 0.001). Then, fibroblasts were analyzed after 7 days of senescence-like growth arrest. In senescent fibroblasts, the CEdG content of nuclear DNA significantly increased by 25%. However, the CEdG level of mtDNA significantly decreased to 52%; in parallel, an increase in mitochondrial mass and mtDNA was observed. Senescence did not lead to general accumulation of protein-AGEs, but two protein bands at 32 and 34 kDa showed a significant increase in the CML/CEL modification rate (208%, p < 0.001; 196%, p = 0.0016) in senescent fibroblasts compared with control cells.

DNA damage induced by endogenous aldehydes: current state of knowledge

DNA damage plays a major role in various pathophysiological conditions including carcinogenesis, aging, inflammation, diabetes and neurodegenerative diseases. Oxidative stress and cell processes such as lipid peroxidation and glycation induce the formation of highly reactive endogenous aldehydes that react directly with DNA, form aldehyde-derived DNA adducts and lead to DNA damage. In occasion of persistent conditions that influence the formation and accumulation of aldehyde-derived DNA adducts the resulting unrepaired DNA damage causes deregulation of cell homeostasis and thus significantly contributes to disease phenotype. Some of the most highly reactive aldehydes produced endogenously are 4-hydroxy-2-nonenal, malondialdehyde, acrolein, crotonaldehyde and methylglyoxal. The mutagenic and carcinogenic effects associated with the elevated levels of these reactive aldehydes, especially, under conditions of stress, are attributed to their capability of causing directly modification of DNA bases or yielding promutagenic exocyclic adducts. In this review, we discuss the current knowledge on DNA damage induced by endogenously produced reactive aldehydes in relation to the pathophysiology of human diseases.

Mutagenesis and repair induced by the DNA advanced glycation end product N2-1-(carboxyethyl)-2'-deoxyguanosine in human cells

Glycation of biopolymers by glucose-derived α-oxo-aldehydes such as methylglyoxal (MG) is believed to play a major role in the complex pathologies associated with diabetes and metabolic disease. In contrast to the extensive literature detailing the formation and physiological consequences of protein glycation, there is little information about the corresponding phenomenon for DNA. To assess the potential contribution of DNA glycation to genetic instability, we prepared shuttle vectors containing defined levels of the DNA glycation adduct N(2)-(1-carboxyethyl)-2'-deoxyguanosine (CEdG) and transfected them into isogenic human fibroblasts that differed solely in the capacity to conduct nucleotide excision repair (NER). In the NER-compromised fibroblasts, the induced mutation frequencies increased up to 18-fold relative to background over a range of ∼10-1400 CEdG adducts/10(5) dG, whereas the same substrates transfected into NER-competent cells induced a response that was 5-fold over background at the highest adduct density. The positive linear correlation (R(2) = 0.998) of mutation frequency with increasing CEdG level in NER-defective cells suggested that NER was the primary if not exclusive mechanism for repair of this adduct in human fibroblasts. Consistent with predictions from biochemical studies using CEdG-substituted oligonucleotides, guanine transversions were the predominant mutation resulting from replication of MG-modified plasmids. At high CEdG levels, significant increases in the number of AT → GC transitions were observed exclusively in NER-competent cells (P < 0.0001). This suggested the involvement of an NER-dependent mutagenic process in response to critical levels of DNA damage, possibly mediated by error-prone Y-family polymerases.

Inhibition of the formation of advanced glycation end products by thymoquinone

The inhibitory activity of thymoquinone, a major quinone from black seeds (Nigella sativa) against the formation of advanced glycation end products was studied using the hemoglobin-δ-gluconolactone, human serum albumin-glucose, and the N-acetyl-glycyl-lysine methyl ester-ribose assays. A comparison was made with the inhibitory activity of aminoguanidine. The cytotoxicity of thymoquinone was studied by the release of lactate dehydrogenase from platelets and the levels of plasma thiols. At 20μM, thymoquinone inhibited 39% of hemoglobin glycation, 82% of post-Amadori glycation products, reduced methyglyoxal-mediated human serum albumin glycation by 68%, inhibited 78% of late glycation end products. Aminoguanidine at 10mM was less effective than thymoquinone. The IC50 for thymoquinone and aminoguanidine were 7.2μM and 1.25mM, respectively. Thymoquinone at 20-50μM was not toxic to platelet lactate dehydrogenase and plasma thiols. The potential of thymoquinone in food applications is discussed.

Detection of 7-(2'-carboxyethyl)guanine but not 7-carboxymethylguanine in human liver DNA

7-Carboxymethylguanine (7-CMGua) and 7-(2'-carboxyethyl)guanine (7-CEGua) are DNA adducts that potentially could be formed upon the metabolism of the carcinogenic nitrosamines N-nitrososarcosine (NSAR) and 3-(methylnitrosamino)propionic acid (MNPA), respectively, or from other sources such as nitrosation of glycine (7-CMGua) or reaction of DNA with acrylic acid (7-CEGua). Since both NSAR and MNPA have been detected in human urine and there are plausible sources of exposure to other precursors to these adducts, we analyzed human liver DNA for 7-CMGua and 7-CEGua, using liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring (LC-ESI-MS/MS-SRM). Human hepatic DNA was mixed with [15N5]7-CMGua and [15N5]7-CEGua as internal standards and enzymatically hydrolyzed. The hydrolysate was partially purified by solid-phase extraction, and the resulting fraction was treated with acetyl chloride in methanol to convert 7-CMGua and 7-CEGua to their methyl esters. After a second solid-phase extraction, LC-ESI-MS/MS-SRM analysis was carried out using the transitions m/z 224 [M + H](+) --> m/z 164 [(M + H)-HCOOCH3]+ and m/z 238 [M + H]+ --> m/z 152 [BH]+ for the methyl esters of 7-CMGua and 7-CEGua, respectively. The method was sensitive, accurate, precise, and apparently free from artifact formation. 7-CEGua, as its methyl ester, was detected in all 24 human liver samples analyzed, mean +/- SD, 373 +/- 320 fmol/mumol Gua (74.6 adducts per 10(9) nucleotides), range 17-1189 fmol/mumol Gua, but the methyl ester of 7-CMGua was not detected in any sample. These results demonstrate the ubiquitous presence of 7-CEGua in human liver DNA. Acrylic acid may be a likely endogenous precursor to 7-CEGua.

Imidazopurinones are markers of physiological genomic damage linked to DNA instability and glyoxalase 1-associated tumour multidrug resistance

Glyoxal and methylglyoxal are reactive dicarbonyl metabolites formed and metabolized in physiological systems. Increased exposure to these dicarbonyls is linked to mutagenesis and cytotoxicity and enhanced dicarbonyl metabolism by overexpression of glyoxalase 1 is linked to tumour multidrug resistance in cancer chemotherapy. We report herein that glycation of DNA by glyoxal and methylglyoxal produces a quantitatively important class of nucleotide adduct in physiological systems—imidazopurinones. The adduct derived from methylglyoxal-3-(2′-deoxyribosyl)-6,7-dihydro-6,7-dihydroxy-6/7-methylimidazo-[2,3-b]purine-9(8)one isomers—was the major quantitative adduct detected in mononuclear leukocytes in vivo and tumour cell lines in vitro. It was linked to frequency of DNA strand breaks and increased markedly during apoptosis induced by a cell permeable glyoxalase 1 inhibitor. Unexpectedly, the DNA content of methylglyoxal-derived imidazopurinone and oxidative marker 7,8-dihydro-8-oxo-2′-deoxyguanosine were increased moderately in glyoxalase 1-linked multidrug resistant tumour cell lines. Together these findings suggest that imidazopurinones are a major type of endogenous DNA damage and glyoxalase 1 overexpression in tumour cells strives to counter increased imidazopurinone formation in tumour cells likely linked to their high glycolytic activity.

Mutagenic potential of DNA glycation: miscoding by (R)- and (S)-N2-(1-carboxyethyl)-2'-deoxyguanosine

Elevated circulating glucose resulting from complications of obesity and metabolic disease can result in the accumulation of advanced glycation end products (AGEs) of proteins, lipids, and DNA. The formation of DNA-AGEs assumes particular importance as these adducts may contribute to genetic instability and elevated cancer risk associated with metabolic disease. The principal DNA-AGE, N(2)-(1-carboxyethyl)-2'-deoxyguanosine (CEdG), is formed as a mixture of R and S isomers at both the polymer and monomer levels. In order to examine the miscoding potential of this adduct, oligonucleotides substituted with (R)- and (S)-CEdG and the corresponding triphosphates (R)- and (S)-CEdGTP were synthesized, and base-pairing preferences for each stereoisomer were examined using steady-state kinetic approaches. Purine dNTPs were preferentially incorporated opposite template CEdG when either the Klenow (Kf(-)) or Thermus aquaticus (Taq) polymerases were used. The Kf(-) polymerase preferentially incorporated dGTP, whereas Taq demonstrated a bias for dATP. Kf(-) incorporated purines opposite the R isomer with greater efficiency, but Taq favored the S isomer. Incorporation of (R)- and (S)-CEdGTP only occurred opposite dC and was catalyzed by Kf(-) with equal efficiencies. Primer extension from a 3'-terminal CEdG was observed only for the R isomer. These data suggest CEdG is the likely adduct responsible for the observed pattern of G transversions induced by exposure to elevated glucose or its alpha-oxoaldehyde decomposition product methylglyoxal. The results imply that CEdG within template DNA and the corresponding triphosphate possess different syn/anti conformations during replication which influence base-pairing preferences. The implications for CEdG-induced mutagenesis in vivo are discussed.

Quantification of N2-carboxymethyl-2'-deoxyguanosine in calf thymus DNA and cultured human kidney epithelial cells by capillary high-performance liquid chromatography-tandem mass spectrometry coupled with stable isotope dilution method

Glyoxal is generated endogenously from the degradation of glucose and the oxidation of carbohydrates, lipids, and the 2-deoxyribose moieties of DNA. Glyoxal is also widely used in industry and is present in cigarette smoke and food. Glyoxal can conjugate with nucleobases and proteins to give advanced glycation end products. N(2)-Carboxymethyl-2'-deoxyguanosine (N(2)-CMdG) and the cyclic 1,N(2)-glyoxal-dG are the major glyoxal adducts formed in DNA. In this study, we first assessed the stabilities of these two adducts. It turned out that 1,N(2)-glyoxal-dG was very unstable, with more than 70% of the adduct being decomposed to dG upon a 24 h incubation at 37 degrees C in phosphate-buffered saline. However, N(2)-CMdG was very stable; less than 0.5% of the lesion was degraded to dG after a 7 day incubation under the same conditions. We further developed a sensitive capillary liquid chromatography-electrospray ionization-tandem mass spectrometry coupled with a stable isotope dilution method and quantified the formation of N(2)-CMdG in calf thymus DNA and 293T human kidney epithelial cells that were exposed to glyoxal and in calf thymus DNA treated with d-glucose. Our results showed that N(2)-CMdG was produced at 2-134 lesions per 10(6) nucleosides in calf thymus DNA when the surrounding glyoxal concentration was increased from 10 to 500 microM and approximately 3-27 lesions per 10(7) nucleosides, while the D-glucose concentration changed from 2 to 50 mM. Furthermore, N(2)-CMdG was induced endogenously in 293T human kidney epithelial cells and exposure to glyoxal further stimulated the formation of this lesion; the level of this adduct ranged from 7 to 15 lesions per 10(8) nucleosides, while the glyoxal concentration increased from 10 microM to 1.25 mM. Collectively, our results suggested that N(2)-CMdG might serve as a biomarker for glyoxal exposure.

Methylglyoxal: possible link between hyperglycaemia and immune suppression?

No matter the cause of diabetes, the result is always hyperglycaemia. This excess glucose metabolism drives several damage pathways and raises concentrations of the reactive dicarbonyl, methylglyoxal (MG). MG can modify the structure and function of target molecules by forming advanced glycation end-products (AGEs) that act through their receptor (RAGE) to perpetuate vascular and neuronal injury responsible for long-term complications of diabetes. Diabetes patients also suffer lower resistance to many common infections, although the cause(s) for this lower resistance remains elusive. Here, we review recent evidence concerning immune suppression in diabetes and discuss the effects of MG on components of the immune system. We suggest that MG could be a missing link between hyperglycaemia and immune suppression in diabetes.