Low Glutathione Level Favors Formation of DNA Adducts to 4-Hydroxy-2(E)-nonenal, a Major Lipid Peroxidation Product

Reconsidering the Chemical Nature of Strand Breaks Derived from Abasic Sites in Cellular DNA: Evidence for 3'-Glutathionylation

The hydrolytic loss of coding bases from cellular DNA is a common and unavoidable reaction. The resulting abasic sites can undergo β-elimination of the 3'-phosphoryl group to generate a strand break with an electrophilic α,β-unsaturated aldehyde residue on the 3'-terminus. The work reported here provides evidence that the thiol residue of the cellular tripeptide glutathione rapidly adds to the alkenal group on the 3'-terminus of an AP-derived strand break. The resulting glutathionylated adduct is the only major cleavage product observed when β-elimination occurs at an AP site in the presence of glutathione. Formation of the glutathionylated cleavage product is reversible, but in the presence of physiological concentrations of glutathione, the adduct persists for days. Biochemical experiments provided evidence that the 3'-phosphodiesterase activity of the enzyme apurinic/apyrimidinic endonuclease (APE1) can remove the glutathionylated sugar remnant from an AP-derived strand break to generate the 3'OH residue required for repair via base excision or single-strand break repair pathways. The results suggest that a previously unrecognized 3'glutathionylated sugar remnant─and not the canonical α,β-unsaturated aldehyde end group─may be the true strand cleavage product arising from β-elimination at an abasic site in cellular DNA. This work introduces the 3'glutathionylated cleavage product as the major blocking group that must be trimmed to enable repair of abasic site-derived strand breaks by the base excision repair or single-strand break repair pathways.

Electrophile Modulation of Inflammation: A Two-Hit Approach

Electrophilic small molecules have gained significant attention over the last decade in the field of covalent drug discovery. Long recognized as mediators of the inflammatory process, recent evidence suggests that electrophiles may modulate the immune response through the regulation of metabolic networks. These molecules function as pleiotropic signaling mediators capable of reversibly reacting with nucleophilic biomolecules, most notably at reactive cysteines. More specifically, electrophiles target critical cysteines in redox regulatory proteins to activate protective pathways such as the nuclear factor erythroid 2-related factor 2-Kelch-like ECH-associated protein 1 (Nrf2-Keap1) antioxidant signaling pathway while also inhibiting Nuclear Factor κB (NF-κB). During inflammatory states, reactive species broadly alter cell signaling through the oxidation of lipids, amino acids, and nucleic acids, effectively propagating the inflammatory sequence. Subsequent changes in metabolic signaling inform immune cell maturation and effector function. Therapeutic strategies targeting inflammatory pathologies leverage electrophilic drug compounds, in part, because of their documented effect on the redox balance of the cell. With mounting evidence demonstrating the link between redox signaling and metabolism, electrophiles represent ideal therapeutic candidates for the treatment of inflammatory conditions. Through their pleiotropic signaling activity, electrophiles may be used strategically to both directly and indirectly target immune cell metabolism.

Inhibition of lipid peroxidation during the reproductive period extends the lifespan of Caenorhabditis elegans

Glutathione peroxidase 4 (GPx4) is a unique antioxidant enzyme that directly reduces the phospholipid hydroperoxides (PLOOH) generated in biomembranes using glutathione as the reductant. We have previously reported that the Caenorhabditis elegans gpx-quad mutant, which lacks all homologous genes of GPx4 has a reduced lifespan compared with the wild-type. However, the mechanisms underlying the lifespan reduction remain unclear. By monitoring the change in PLOOH production with age, we found that PLOOH was elevated in the gpx-quad mutants compared with the wild-type during the reproductive period. Administration of vitamin E not only reduced the PLOOH content but also prolonged the lifespan of the gpx-quad mutants. In contrast, vitamin C did not extend the lifespan of the gpx-quad mutants. Interestingly, we found that the inhibition of lipid peroxidation by vitamin E during 5 to 10 days after hatching is important to extend the lifespan of C. elegans. These results suggest that production of PLOOH during the reproductive period strongly influences the lifespan of C. elegans.

4-Hydroxynonenal metabolites and adducts in pre-carcinogenic conditions and cancer

4-hydroxy-2-nonenal (HNE) is an amazing reactive compound, originating from lipid peroxidation within cells but also in food and considered as a "second messenger" of oxidative stress. Due to its chemical features, HNE is able to make covalent links with DNA, proteins and lipids. The aim of this review is to give a comprehensive summary of the chemical properties of HNE and of the consequences of its reactivity in relation to cancer development. The formation of exocyclic etheno-and propano-adducts and genotoxic effects are addressed. The adduction to cellular proteins and the repercussions on the regulation of cell signaling pathways involved in cancer development are reviewed, notably on the Nrf2/Keap1/ARE pathway. The metabolic pathways leading to the inactivation/elimination or, on the contrary, to the bioactivation of HNE are considered. A special focus is given on the link between HNE and colorectal cancer development, due to its occurrence in foodstuffs and in the digestive lumen, during digestion.

Deletion of the four phospholipid hydroperoxide glutathione peroxidase genes accelerates aging in Caenorhabditis elegans

The glutathione peroxidase (GPx) family is a major antioxidant enzyme family that catalyzes the reduction of a variety of hydroperoxides. GPxs are divided into selenium- and nonselenium-containing GPxs. Because of their efficient antioxidant activity, which depends on the presence of the amino acid residue selenocysteine, selenium-containing GPxs have been the subject of many studies. However, the physiological roles of the nonselenium GPxs remain unclear. Here, we report that the deletion of phospholipid hydroperoxide glutathione peroxidase (PHGPx) homologues causes accelerated aging that leads to a shortened lifespan in Caenorhabditis elegans. PHGPx is an antioxidant enzyme that directly reduces the phospholipid hydroperoxides generated in biomembranes. The quadruple phgpx mutant gpx-1; gpx-2; gpx-6; gpx-7 developed normally, reached adulthood and reproduced as well as the wild type. However, a lifespan analysis showed that the quadruple phgpx mutant had a short maximum lifespan, with an age-related increase in its mortality rate. The intestine is the primary tissue expressing gpx-1, gpx-2, gpx-6 and gpx-7 in C. elegans, and the expression of gpx-6 is greatly enhanced under starvation conditions. These results suggest that the C. elegans PHGPx homologues have important functions in the regulation of aging, probably by reducing oxidative damage in the intestine.

Recent developments in DNA adduct analysis by mass spectrometry: a tool for exposure biomonitoring and identification of hazard for environmental pollutants

DNA adducts represent an important category of biomarkers for detection and exposure surveillance of potential carcinogenic and genotoxic chemicals in the environment. Sensitive and specific analytical methods are required to detect and differentiate low levels of adducts from native DNA from in vivo exposure. In addition to biomonitoring of environmental pollutants, analytical methods have been developed for structural identification of adducts which provides fundamental information for determining the toxic pathway of hazardous chemicals. In order to achieve the required sensitivity, mass spectrometry has been increasingly utilized to quantify adducts at low levels as well as to obtain structural information. Furthermore, separation techniques such as chromatography and capillary electrophoresis can be coupled to mass spectrometry to increase the selectivity. This review will provide an overview of advances in detection of adducted and modified DNA by mass spectrometry with a focus on the analysis of nucleosides since 2007. Instrument advances, sample and instrument considerations, and recent applications will be summarized in the context of hazard assessment. Finally, advances in biomonitoring applying mass spectrometry will be highlighted. Most importantly, the usefulness of DNA adducts measurement and detection will be comprehensively discussed as a tool for assessment of in vitro and in vivo exposure to environmental pollutants.

The role of GABARAPL1/GEC1 in autophagic flux and mitochondrial quality control in MDA-MB-436 breast cancer cells

GABARAPL1/GEC1 is an early estrogen-induced gene which encodes a protein highly conserved from C. elegans to humans. Overexpressed GABARAPL1 interacts with GABAA or kappa opioid receptors, associates with autophagic vesicles, and inhibits breast cancer cell proliferation. However, the function of endogenous GABARAPL1 has not been extensively studied. We hypothesized that GABARAPL1 is required for maintaining normal autophagic flux, and plays an important role in regulating cellular bioenergetics and metabolism. To test this hypothesis, we knocked down GABARAPL1 expression in the breast cancer MDA-MB-436 cell line by shRNA. Decreased expression of GABARAPL1 activated procancer responses of the MDA-MB-436 cells including increased proliferation, colony formation, and invasion. In addition, cells with decreased expression of GABARAPL1 exhibited attenuated autophagic flux and a decreased number of lysosomes. Moreover, decreased GABARAPL1 expression led to cellular bioenergetic changes including increased basal oxygen consumption rate, increased intracellular ATP, increased total glutathione, and an accumulation of damaged mitochondria. Taken together, our results demonstrate that GABARAPL1 plays an important role in cell proliferation, invasion, and autophagic flux, as well as in mitochondrial homeostasis and cellular metabolic programs.

Protein modification by oxidized phospholipids and hydrolytically released lipid electrophiles: Investigating cellular responses

Oxygen is essential for the growth and function of mammalian cells. However, imbalances in oxygen or abnormalities in the ability of a cell to respond to oxygen levels can result in oxidative stress. Oxidative stress plays an important role in a number of diseases including atherosclerosis, rheumatoid arthritis, cancer, neurodegenerative diseases and asthma. When membrane lipids are exposed to high levels of oxygen or derived oxidants, they undergo lipid peroxidation to generate oxidized phospholipids (oxPL). Continual exposure to oxidants and decomposition of oxPL results in the formation of reactive electrophiles, such as 4-hydroxy-2-nonenal (HNE). Reactive lipid electrophiles have been shown to covalently modify DNA and proteins. Furthermore, exposure of cells to lipid electrophiles results in the activation of cytoprotective signaling pathways in order to promote cell survival and recovery from oxidant stress. However, if not properly managed by cellular detoxification mechanisms, the continual exposure of cells to electrophiles results in cytotoxicity. The following perspective will discuss the biological importance of lipid electrophile protein adducts including current strategies employed to identify and isolate protein adducts of lipid electrophiles as well as approaches to define cellular signaling mechanisms altered upon exposure to electrophiles. This article is part of a Special Issue entitled: Oxidized phospholipids-their properties and interactions with proteins.

Differences in susceptibility to inactivation of human aldehyde dehydrogenases by lipid peroxidation byproducts

Aldehyde dehydrogenases (ALDHs) are involved in the detoxification of aldehydes generated as byproducts of lipid peroxidation. In this work, it was determined that, among the three most studied human ALDH isoforms, ALDH2 showed the highest catalytic efficiency for oxidation of acrolein, 4-hydroxy-2-nonenal (4-HNE), and malondialdehyde. ALDH1A1 also exhibited significant activity with these substrates, whereas ALDH3A1 only showed activity with 4-HNE. ALDH2 was also the most sensitive isoform to irreversible inactivation by these compounds. Remarkably, ALDH3A1 was insensitive to these aldehydes even at concentrations as high as 20 mM. Formation of adducts of ALDH1A1 and ALDH2 with acrolein increased their K(d) values for NAD(+) by 2- and 3-fold, respectively. NADH exerted a higher protection than propionaldehyde to the inactivation by acrolein, and this protection was additive. These results suggested that both binding sites, those for aldehyde and NAD(+) in ALDH2, are targets for the inactivation by lipid peroxidation products. Thus, with the advantage of being relatively inactivation-insensitive, ALDH1A1 and ALDH3A1 may be actively participating in the detoxification of these aldehydes in the cells.

Chemopreventive effects of in vitro digested and fermented bread in human colon cells

PURPOSE

Bread as a staple food product represents an important source for dietary fibre consumption. Effects of wheat bread, wholemeal wheat bread and wholemeal rye bread on mechanisms which could have impact on chemoprevention were analysed in colon cells after in vitro fermentation.

METHODS

Effects of fermented bread samples on gene expression, glutathione S-transferase activity and glutathione content, differentiation, growth and apoptosis were investigated using the human colon adenoma cell line LT97. Additionally, apoptosis was studied in normal and tumour colon tissue by determination of caspase activities.

RESULTS

The expression of 76 genes (biotransformation, differentiation, apoptosis) was significantly upregulated (1.5-fold) in LT97 cells. The fermented bread samples were able to significantly increase glutathione S-transferase activity (1.8-fold) and glutathione content (1.4-fold) of the cells. Alkaline phosphatase activity as a marker of differentiation was also significantly enhanced (1.7-fold). The fermented bread samples significantly inhibited LT97 cell growth and increased the level of apoptotic cells (1.8-fold). Only marginal effects on apoptosis in tumour compared to normal tissue were observed.

CONCLUSIONS

This is the first study which presents chemopreventive effects of different breads after in vitro fermentation. In spite of differences in composition, the results were comparable between the bread types. Nevertheless, they indicate a potential involvement of this staple food product regarding the prevention of colon cancer.

Metabolism of 4-hydroxy-2-nonenal in human polymorphonuclear leukocytes

Intracellular metabolism of 4-hydroxy-2-nonenal (HNE), a major product and mediator of oxidative stress and inflammation, is analyzed in resting and fMLP-stimulated human polymorphonuclear leukocytes (PMNL), where this compound is generated during activation of the respiratory burst. HNE consumption rate in PMNL is very low, if compared to other cell types (rat hepatocytes, rabbit fibroblasts), where HNE metabolism is always an important part of secondary antioxidative defense mechanisms. More than 98% of HNE metabolites are identified. The pattern of HNE intermediates is quite similar in stimulated and resting PMNL - except for higher water formation in resting PMNL - while the initial velocity of HNE degradation is somewhat higher in resting cells, 0.44 instead of 0.28 nmol/(min×10(6) cells). The main products of HNE metabolism are 4-hydroxynonenoic acid (HNA), 1,4-dihydroxynonene (DHN) and the glutathione adducts with HNE, HNA, and DHN. Protein-bound HNE and water account for about 3-4% of the total HNE derivatives in stimulated cells, while in resting cells protein-bound HNE and water are 4% and 20%, respectively. Cysteinyl-glycine-HNE adduct and mercapturic acids contribute to about 5%.

NGF induced differentiated PC12 cells as in vitro tool to study 4-hydroxynonenal induced cellular damage

Investigations were carried out to examine the suitability of PC12 cells as an in vitro tool to examine 4-hydroxynonenal (4-HNE)-induced toxicity in nervous tissue. On day 8 of differentiation, markers of neural effects and oxidative stress were measured following exposure of PC12 cells to 1-50 microM 4-HNE for 1-8h. Endpoints included dopamine DA-D(2) receptor and glutathione S-transferase (GSTP1-1) protein levels, 4-HNE-protein binding, glutathione (GSH) concentrations and intracellular calcium levels. GSH levels were maximally depleted after 4h. 4-HNE also induced depletion of GSTP1-1 and increased intracellular Ca(++), with the latter seen as early as 1h after exposure. Responses at 8h were not greater than responses at earlier times. The experiments suggest that PC12 cells could be an in vitro tool for understanding toxicant-cell interactions, especially those that result in oxidative stress.

Signaling actions of electrophiles: anti-inflammatory therapeutic candidates

Over the past several years, research on biologically relevant electrophiles has been replete with new insights, expanding our understanding of the roles electrophiles play in vivo. Importantly, many electrophiles can form reversible covalent adducts with both proteins and small-molecule thiols in cells. This post-translational protein modification has important ramifications, including changes in protein enzymatic activity, the transduction of signals within and between cells, and alterations in gene expression. Electrophiles modulate a variety of cellular signaling processes that are involved in several major diseases with inflammatory components. The electrophilic fatty-acid derivatives discussed in this work are naturally occurring products of redox reactions and enzymatic activity. Furthermore, several of these electrophilic species and their derivatives represent potential therapeutic candidates.

Reactive electrophilic oxylipins: pattern recognition and signalling

Oxidized lipids in plants comprise a variety of reactive electrophiles that contain an alpha,beta-unsaturated carbonyl group. While some of these compounds are formed enzymatically, many of them are formed by non-enzymatic pathways. In addition to their chemical reactivity/toxicity low levels of these compounds are also biologically active. Despite their structural diversity and biosynthetic origin, common biological activities such as induction of defense genes, activation of detoxification responses and growth inhibition have been documented. However, reactive electrophilic oxylipins are poorly defined as a class of compounds but have at least two properties in common, i.e., lipophilicity and thiol-reactivity. Thiol-reactivity is a property of reactive oxylipins (RES) shared by reactive oxygen and nitrogen species (ROS and RNS) and enables these agents to modify proteins in vivo. Thiol-modification is assumed to represent a key mechanism involved in signal transduction. A metaanalysis of proteomic studies reveals that RES oxylipins, ROS and RNS apparently chemically modify a similar set of highly sensitive proteins, virtually all of which are targets for thioredoxins. Moreover, most of these proteins are redox-regulated, i.e., posttranslational thiol-modification alters the activity or function of these proteins. On the transcriptome level, effects of RES oxylipins and ROS on gene induction substantially overlap but are clearly different. Besides electrophilicity other structural properties such as target affinity apparently determine target selectivity and biological activity. In this context, different signalling mechanisms and signal transduction components identified in plants and non-plant organisms as well as putative functions of RES oxylipins are discussed.

Mechanisms of primary cancer prevention by butyrate and other products formed during gut flora-mediated fermentation of dietary fibre

Dietary fibres are indigestible food ingredients that reach the colon and are then fermented by colonic bacteria, resulting mainly in the formation of short-chain fatty acids (SCFA) such as acetate, propionate, and butyrate. Those SCFA, especially butyrate, are recognised for their potential to act on secondary chemoprevention by slowing growth and activating apoptosis in colon cancer cells. Additionally, SCFA can also act on primary prevention by activation of different drug metabolising enzymes. This can reduce the burden of carcinogens and, therefore, decrease the number of mutations, reducing cancer risk. Activation of GSTs by butyrate has been studied on mRNA, protein, and enzyme activity level by real-time RT-PCR, cDNA microarrays, Western blotting, or photometrical approaches, respectively. Butyrate had differential effects in colon cells of different stages of cancer development. In HT29 tumour cells, e.g., mRNA GSTA4, GSTP1, GSTM2, and GSTT2 were induced. In LT97 adenoma cells, GSTM3, GSTT2, and MGST3 were induced, whereas GSTA2, GSTT2, and catalase (CAT) were elevated in primary colon cells. Colon cells of different stages of carcinogenesis differed in post-transcriptional regulatory mechanisms because butyrate increased protein levels of different GST isoforms and total GST enzyme activity in HT29 cells, whereas in LT97 cells, GST protein levels and activity were slightly reduced. Because butyrate increased histone acetylation and phosphorylation of ERK in HT29 cells, inhibition of histone deacetylases and the influence on MAPK signalling are possible mechanisms of GST activation by butyrate. Functional consequences of this activation include a reduction of DNA damage caused by carcinogens like hydrogen peroxide or 4-hydroxynonenal (HNE) in butyrate-treated colon cells. Treatment of colon cells with the supernatant from an in vitro fermentation of inulin increased GST activity and decreased HNE-induced DNA damage in HT29 cells. Additional animal and human studies are needed to define the exact role of dietary fibre and butyrate in inducing GST activity and reducing the risk of colon cancer.