Induction of endothelial apoptosis by 4-hydroxyhexenal

Involvement of Metabolic Lipid Mediators in the Regulation of Apoptosis

Apoptosis is the physiological mechanism of cell death and can be modulated by endogenous and exogenous factors, including stress and metabolic alterations. Reactive oxygen species (ROS), as well as ROS-dependent lipid peroxidation products (including isoprostanes and reactive aldehydes including 4-hydroxynonenal) are proapoptotic factors. These mediators can activate apoptosis via mitochondrial-, receptor-, or ER stress-dependent pathways. Phospholipid metabolism is also an essential regulator of apoptosis, producing the proapoptotic prostaglandins of the PGD and PGJ series, as well as the antiapoptotic prostaglandins of the PGE series, but also 12-HETE and 20-HETE. The effect of endocannabinoids and phytocannabinoids on apoptosis depends on cell type-specific differences. Cells where cannabinoid receptor type 1 (CB1) is the dominant cannabinoid receptor, as well as cells with high cyclooxygenase (COX) activity, undergo apoptosis after the administration of cannabinoids. In contrast, in cells where CB2 receptors dominate, and cells with low COX activity, cannabinoids act in a cytoprotective manner. Therefore, cell type-specific differences in the pro- and antiapoptotic effects of lipids and their (oxidative) products might reveal new options for differential bioanalysis between normal, functional, and degenerating or malignant cells, and better integrative biomedical treatments of major stress-associated diseases.

Cyanidin-3-glucoside Alleviates 4-Hydroxyhexenal-Induced NLRP3 Inflammasome Activation via JNK-c-Jun/AP-1 Pathway in Human Retinal Pigment Epithelial Cells

Recently, the NLRP3 inflammasome activation in the eyes has been known to be associated with the pathogenesis of age-related macular degeneration. The aim of this study was to investigate the protective effects of cyanidin-3-glucoside (C3G), an important anthocyanin with great potential for preventing eye diseases, against 4-hydroxyhexenal- (HHE-) induced inflammatory damages in human retinal pigment epithelial cells, ARPE-19. We noticed that C3G pretreatment to the ARPE-19 cells rescued HHE-induced antiproliferative effects. Cell apoptosis ratio induced by HHE was also decreased by C3G, measured by flow cytometry. The activation of NLRP3 inflammasome induced by HHE was found with increases of caspase-1 activity, proinflammatory cytokine releases (IL-1β and IL-18), and NLRP3 inflammasome-related gene expressions (NLRP3, IL-1β, IL-18, and caspase-1). The C3G showed potent inhibitive effects on these NLRP3 inflammasome activation hallmarks induced by HHE. Moreover, we noticed that the C3G's pretreatment leads to a delayed and a decreased JNK activation in HHE-challenged ARPE-19 cells. Finally, using a luciferase reporter gene assay system, we demonstrated that HHE-induced activation protein- (AP-) 1 transcription activity was abolished by C3G pretreatment in a dose-dependent manner. Taken together, these data showed that HHE leads to inflammatory damages to ARPE-19 cells while C3G has great protective effects, highlighting future potential applications of C3G against AMD-associated inflammation.

Modulation of oxidative stress response by flaxseed oil: Role of lipid peroxidation and underlying mechanisms

Polyunsaturated fatty acids (PUFA's) are majorly classified as ω-3 and ω-6 fatty acids. The eicosapentaenoic acid (EPA, ω-3:20-5), docosahexaenoic acid (DHA, ω-3:22-6) and alpha-linolenic acid (ALA, ω-3:18-3) are known ω-3 fatty acids, extracted from animal (e.g fish oil) and plant sources (e.g flaxseed oil). Furthermore, linoleic acid (LA, ω-6:18-2) is recognized as ω-6 fatty acid and the most prominent biological fatty acid with a pro-inflammatory response. Flaxseed oil has variety of biological roles, due to the significant amount of ω-3/ω-6 fatty acids. Numerous studies have reported that ALA (ω-3:18-3) and LA (ω-6:18-2) has diverse pharmacological activities. The ALA (ω-3:18-3) and LA (ω-6:18-2) are recognised to be the pharmacological antagonist. For example, ALA (ω-3:18-3) is recognised as anti-inflammatory, whereas LA (ω-6:18-2) is considered to be pro-inflammatory. PUFA's get oxidized in three ways; firstly, free radical-mediated pathway, secondly non-free radical non-enzymatic metabolism, and lastly enzymatic degradation. The present report is an attempt to summarize various modes of PUFA's metabolism and elaborate biological effects of the associated metabolites concerning flaxseed oil.

Pro-apoptotic effects of lipid oxidation products: HNE at the crossroads of NF-κB pathway and anti-apoptotic Bcl-2

The axis between lipid oxidation products and cell death is explicitly linked. 4-Hydroxynonenal (HNE), as well as other lipid oxidation products was also established to induce apoptosis in various experimental settings. Yet, the decision leading to apoptotic execution not only includes upregulation of pro-apoptotic signals but also involves a downregulation of anti-apoptotic signals. Within the frames of this paradigm, HNE acts significantly different from other lipid oxidation products in the regulation of two widely known anti-apoptotic elements, Nuclear Factor-κB (NF-κB) transcription factors and its target anti-apoptotic B-Cell Lymphoma-2 (Bcl-2) protein. Even so, a review inclusively linking these anti-apoptotic factors and their crosstalk upon HNE exposure is still at demand. In order to elucidate presence of such crosstalk, reports on the link between HNE and NF-κB pathway, on the link between HNE and anti-apoptotic Bcl-2 and on the crossroad of these links during HNE exposure were summarized and discussed. IKK, the upstream kinase of NF-κB, has been shown to regulate HNE mediated phosphorylation and inactivation of Bcl-2 by our group. Based on this observation and other studies reporting on HNE-NF-κB pathway interaction, IKK was proposed to mediate the crosstalk of NF-κB pathway and anti-apoptotic Bcl-2 protein, when HNE is present. These reports further suggested that HNE based inhibition of NF-κB pathway is highly likely. Besides, evidence on the HNE-anti-apoptotic Bcl-2 axis supported the deduction of HNE mediated NF-κB pathway inhibition and IKK mediated Bcl-2 inactivation. In conclusion, through combining all evidences, three possible scenarios intervening the HNE mediated crosstalk between NF-κB pathway and anti-apoptotic Bcl-2 protein, was extrapolated.

4-Hydroxyhexenal and 4-hydroxynonenal are mediators of the anti-cachectic effect of n-3 and n-6 polyunsaturated fatty acids on human lung cancer cells

Cachexia, the most severe paraneoplastic syndrome, occurs in about 80% of patients with advanced cancer; it cannot be reverted by conventional, enteral, or parenteral nutrition. For this reason, nutritional interventions must be based on the use of substances possessing, alongside nutritional and energetic properties, the ability to modulate production of the pro-inflammatory factors responsible for the metabolic changes characterising cancer cachexia. In light of their nutritional and anti-inflammatory properties, polyunsaturated fatty acids (PUFAs), and in particular n-3, have been investigated for treating cachexia; however, the results have been contradictory. Since both n-3 and n-6 PUFAs can affect cell functions in several ways, this research investigated the possibility that the effects of both n-3 and n-6 PUFAs could be mediated by their major aldehydic products of lipid peroxidation, 4-hydroxyhexenal (HHE) and 4-hydroxynonenal (HNE), and by their anti-inflammatory properties. An "in vitro" cancer cachexia model, consisting of human lung cancer cells (A427) and murine myoblasts (C2C12), was used. The results showed that: 1) both n-3 and n-6 PUFAs reduced the growth of lung cancer cells without causing cell death, increased lipid peroxidation and Peroxisome Proliferator-Activated Receptor (PPAR)α, and decreased TNFα; 2) culture medium conditioned by A427 cells grown in the absence of PUFAs blocked myosin production and the differentiation of C2C12 muscle cells; conversely, muscle cells grown in culture medium conditioned by the same cells in the presence of PUFAs showed myosin expression and formed myotubes; 3) adding HHE or HNE directly to C2C12 cells maintained in culture medium conditioned by A427 cells in the absence of PUFAs stimulated myosin production and myotube formation; 4) putative consensus sequences for (PPARs) have been found in genes encoding fast isoforms of myosin heavy chain, by a bioinformatics approach. The overall results show, first, the ability of both n-3 and n-6 PUFAs and their lipid peroxidation products to prevent the blocking of myosin expression and myotube formation caused in C2C12 cells by medium conditioned by human lung tumour cells. The C2C12 cell differentiation can be due to direct effect of lipid peroxidation products, as evidenced by treating C2C12 cells with HHE and HNE, and to the decrease of pro-inflammatory TNFα in A427 cell culture medium. The presence of consensus sequences for PPARs in genes encoding the fast isoforms of myosin heavy chain suggests that the effects of PUFAs, HHE, and HNE are PPAR-mediated.

Duality of n-3 Polyunsaturated Fatty Acids on Mcp-1 Expression in Vascular Smooth Muscle: A Potential Role of 4-Hydroxy Hexenal

N-3 polyunsaturated fatty acids such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) have protective effects against atherosclerosis. Monocyte chemotactic protein (MCP)-1 is a major inflammatory mediator in the progression of atherosclerosis. However, little is known about the regulation of MCP-1 by DHA and EPA in vessels and vascular smooth muscle cells (VSMCs). In this study, we compared the effect of DHA and EPA on the expression of Mcp-1 in rat arterial strips and rat VSMCs. DHA, but not EPA, suppressed Mcp-1 expression in arterial strips. Furthermore, DHA generated 4-hydroxy hexenal (4-HHE), an end product of n-3 polyunsaturated fatty acids (PUFAs), in arterial strips as measured by liquid chromatography-tandem mass spectrometry. In addition, 4-HHE treatment suppressed Mcp-1 expression in arterial strips, suggesting 4-HHE derived from DHA may be involved in the mechanism of this phenomenon. In contrast, Mcp-1 expression was stimulated by DHA, EPA and 4-HHE through p38 kinase and the Keap1-Nuclear factor erythroid-derived 2-like 2 (Nrf2) pathway in VSMCs. In conclusion, there is a dual effect of n-3 PUFAs on the regulation of Mcp-1 expression. Further study is necessary to elucidate the pathological role of this phenomenon.

Protein modifications by electrophilic lipoxidation products: adduct formation, chemical strategies and tandem mass spectrometry for their detection and identification

The post-translational modification of proteins by electrophilic oxylipids is emerging as an important mechanism that contributes to the complexity of proteomes. Enzymatic and non-enzymatic oxidation of biological lipids results in the formation of chemically diverse electrophilic carbonyl compounds, such as 2-alkenals and 4-hydroxy alkenals, epoxides, and eicosanoids with reactive cyclopentenone structures. These lipoxidation products are capable of modifying proteins. Originally considered solely as markers of oxidative insult, more recently the modifications of proteins by lipid peroxidation products are being recognized as a new mechanism of cell signaling with relevance to redox homeostasis, adaptive response and inflammatory resolution. The growing interest in protein modifications by reactive oxylipid species necessitates the availability of methods that are capable of detecting, identifying and characterizing these protein adducts in biological samples with high complexity. However, the efficient analysis of these chemically diverse protein adducts presents a considerable analytical challenge. We first provide an introduction into the chemistry and biological relevance of protein adductions by electrophilic lipoxidation products. We then provide an overview of tandem mass spectrometry approaches that have been developed in recent years for the interrogation of protein modifications by electrophilic oxylipid species.

Immunochemical detection of lipid hydroperoxide- and aldehyde-modified proteins in diseases

Polyunsaturated fatty acid (PUFA) is easily peroxidized by free radicals and enzymes. When this occurs, it results in the compromised integrity of cellular membranes and leads to lipid hydroperoxide as a major reaction product, which is decomposed into aldehyde. Lipid hydroperoxide-modified lysine is known to be an early product of the lipid peroxidation process, suggesting that it might be a PUFA-oxidative stress marker during the initial stage of oxidative stress. Lipid hydroperoxides cause or enhance ROS-mediated DNA fragmentation. The α,β-unsaturated aldehydes are end products of PUFA peroxidation. They are highly reactive and readily attack and modify the protein amino acid residues into aldehyde-modified proteins. Lipid peroxidation-derived α,β-unsaturated aldehydes are capable of inducing cellular stress-responsive processes such as cell signaling and apoptosis. The lipid hydroperoxide- and aldehyde-modified proteins have been immunohistochemically detected in diverse pathological situations such as atherosclerosis, Alzheimer's disease, Parkinson's disease, and chemical material-induced liver injury and renal tubular injury in humans and experimental animals. These findings suggest that the expression of the lipid hydroperoxide- and aldehyde-modified proteins is closely associated with the pathogenesis of these diseases in humans and experimental animals.

4-Hydroxy hexenal derived from dietary n-3 polyunsaturated fatty acids induces anti-oxidative enzyme heme oxygenase-1 in multiple organs

It has recently been reported that expression of heme oxygenase-1 (HO-1) plays a protective role against many diseases. Furthermore, n-3 polyunsaturated fatty acids (PUFAs) were shown to induce HO-1 expression in several cells in vitro, and in a few cases also in vivo. However, very few reports have demonstrated that n-3 PUFAs induce HO-1 in vivo. In this study, we examined the effect of fish-oil dietary supplementation on the distribution of fatty acids and their peroxidative metabolites and on the expression of HO-1 in multiple tissues (liver, kidney, heart, lung, spleen, intestine, skeletal muscle, white adipose, brown adipose, brain, aorta, and plasma) of C57BL/6 mice. Mice were divided into 4 groups, and fed a control, safflower-oil, and fish-oil diet for 3 weeks. One group was fed a fish-oil diet for just 1 week. The concentration of fatty acids, 4-hydroxy hexenal (4-HHE), and 4-hydroxy nonenal (4-HNE), and the expression of HO-1 mRNA were measured in the same tissues. We found that the concentration of 4-HHE (a product of n-3 PUFAs peroxidation) and expression of HO-1 mRNA were significantly increased after fish-oil treatment in most tissues. In addition, these increases were paralleled by an increase in the level of docosahexaenoic acid (DHA) but not eicosapentaenoic acid (EPA) in each tissue. These results are consistent with our previous results showing that DHA induces HO-1 expression through 4-HHE in vascular endothelial cells. In conclusion, we hypothesize that the HO-1-mediated protective effect of the fish oil diet may be through production of 4-HHE from DHA but not EPA in various tissues.

Paricalcitol attenuates 4-hydroxy-2-hexenal-induced inflammation and epithelial-mesenchymal transition in human renal proximal tubular epithelial cells

4-Hydroxy-2-hexenal (HHE), the aldehyde product of lipid peroxidation, may be responsible for the pathogenesis of progressive renal disease. Recently, paricalcitol (19-nor-1,25-dihydroxyvitamin D2) was shown to be renoprotective through its anti-inflammatory and antifibrotic effects in various experimental nephropathy models. In this study, we investigated the effects of paricalcitol on inflammation and epithelial-mesenchymal transition (EMT) after HHE-induced renal tubular epithelial cell injury. To investigate the molecular mechanisms underlying HHE-induced renal tubular cell injury, the human proximal tubular epithelial (HK-2) cells cultured with 10 µM HHE in the presence or absence of paricalcitol. In HK-2 cells, paricalcitol attenuated the HHE-induced expression of extracellular signal-regulated kinase, c-Jun N-terminal kinase, and p38 mitogen-activated protein kinase, and prevented nuclear factor-κB (NF-κB) activation. The expression of the inflammatory proteins inducible nitric oxide synthase and cyclooxygenase-2 was attenuated by paricalcitol pretreatment. In addition, HHE increased the expression of the transforming growth factor (TGF)-β/Smad signaling proteins and fibrotic proteins, such as α-smooth muscle actin and connective tissue growth factor; this inducible expression was suppressed by pretreatment with paricalcitol. Treatment with HHE resulted in the activation of the β-catenin signaling pathway, and paricalcitol pretreatment reduced the expression of β-catenin in HHE-treated HK-2 cells. Coimmunoprecipitation shows that paricalcitol induced vitamin D receptor (VDR)/β-catenin complex formation in HK-2 cells. Also immunofluorescence staining revealed that co-localization of VDR and β-catenin in the nuclei. ICG-001, an inhibitor of β-catenin, decreased the expression of TGF-β1 and attenuated HHE-induced tubular EMT. These results show that paricalcitol attenuated HHE-induced renal tubular cell injury by suppressing inflammation and EMT process through inhibition of the NF-κB, TGF-β/Smad, and β-catenin signaling pathways.

Identification of novel bioactive aldehyde-modified phosphatidylethanolamines formed by lipid peroxidation

Lipid aldehydes generated by lipid peroxidation induce cell damage and inflammation. Recent evidence indicates that γ-ketoaldehydes (isolevuglandins, IsoLGs) form inflammatory mediators by modifying the ethanolamine headgroup of phosphatidylethanolamines (PEs). To determine if other species of aldehyde-modified PEs (al-PEs) with inflammatory bioactivity were generated by lipid peroxidation, we oxidized liposomes containing arachidonic acid and characterized the resulting products. We detected PE modified by IsoLGs, malondialdehyde (MDA), and 4-hydroxynonenal (HNE), as well as a novel series of N-acyl-PEs and N-carboxyacyl-PEs in these oxidized liposomes. These al-PEs were also detected in high-density lipoproteins exposed to myeloperoxidase. When we tested the ability of al-PEs to induce THP-1 monocyte adhesion to cultured endothelial cells, we found that PEs modified by MDA, HNE, and 4-oxononenal induced adhesion with potencies similar to those of PEs modified by IsoLGs (∼2μM). A commercially available medium-chain N-carboxyacyl-PE (C11:0CAPE) also stimulated adhesion, whereas C4:0CAPE and N-acyl-PEs did not. PEs modified by acrolein or by glucose were only partial agonists for adhesion. These studies indicate that lipid peroxidation generates a large family of al-PEs, many of which have the potential to drive inflammation.

A comparative 'bottom up' proteomics strategy for the site-specific identification and quantification of protein modifications by electrophilic lipids

We report a mass spectrometry-based comparative "bottom up" proteomics approach that combines d(0)/d(4)-succinic anhydride labeling with commercially available hydrazine (Hz)-functionalized beads (Affi-gel Hz beads) for detection, identification and relative quantification of site-specific oxylipid modifications in biological matrices. We evaluated and applied this robust and simple method for the quantitative analysis of oxylipid protein conjugates in cardiac mitochondrial proteome samples isolated from 3- and 24-month-old rat hearts. The use of d(0)/d(4)-succinic anhydride labeling, Hz-bead based affinity enrichment, nanoLC fractionation and MALDI-ToF/ToF tandem mass spectrometry yielded relative quantification of oxylipid conjugates with residue-specific modification information. Conjugation of acrolein (ACR), 4-hydroxy-2-hexenal (HHE), 4-hydroxy-2-nonenal (HNE) and 4-oxo-2-noneal (ONE) to cysteine, histidine and lysine residues were identified. HHE conjugates were the predominant subset of Michael-type adducts detected in this study. The HHE conjugates showed higher levels in mitochondrial preparations from young heart congruent with previous findings by others that the n-3/n-6 PUFA ratio is higher in young heart mitochondrial membranes. Although this study focuses on protein adducts of reactive oxylipids, the method might be equally applicable to protein carbonyl modifications caused by metal catalyzed oxidation reactions.

Bioactive aldehyde-modified phosphatidylethanolamines

Lipid peroxidation generates a variety of lipid aldehydes, which have been recognized to modify protein and DNA, causing inflammation and cancer. However, recent studies demonstrate that phosphatidylethanolamine (PE) is a major target for these aldehydes, forming aldehyde-modified PEs (al-PEs) as a novel family of mediators for inflammation. This review summarizes our current understanding of these al-PEs, including formation, detection, structural characterization, physiological relevance and mechanism of action.

Inflammation-related gene expression by lipid oxidation-derived products in the progression of atherosclerosis

Vascular areas of atherosclerotic development persist in a state of inflammation, and any further inflammatory stimulus in the subintimal area elicits a proatherogenic response; this alters the behavior of the artery wall cells and recruits further inflammatory cells. In association with the inflammatory response, oxidative events are also involved in the development of atherosclerotic plaques. It is now unanimously recognized that lipid oxidation-derived products are key players in the initiation and progression of atherosclerotic lesions. Oxidized lipids, derived from oxidatively modified low-density lipoproteins (LDLs), which accumulate in the intima, strongly modulate inflammation-related gene expression, through involvement of various signaling pathways. In addition, considerable evidence supports a proatherogenic role of a large group of potent bioactive lipids called eicosanoids, which derive from oxidation of arachidonic acid, a component of membrane phospholipids. Of note, LDL lipid oxidation products might regulate eicosanoid production, modulating the enzymatic degradation of arachidonic acid by cyclooxygenases and lipoxygenases; these enzymes might also directly contribute to LDL oxidation. This review provides a comprehensive overview of current knowledge on signal transduction pathways and inflammatory gene expression, modulated by lipid oxidation-derived products, in the progression of atherosclerosis.

4-Hydroxynonenal induces mitochondrial-mediated apoptosis and oxidative stress in SH-SY5Y human neuronal cells

Excessive and sustained increases in oxidative stress and apoptosis have been implicated in the pathogenesis of many diseases. In this study, we demonstrated that 4-hydroxynonenal (4-HNE), a product of lipid peroxidation in a range of concentration (0.1-50 μM) showed cytotoxic effects on SH-SY5Y cell culture at a concentration >5 μM at 4 hr of exposure. 4-HNE dose dependently decreased cell viability and significantly promoted reactive oxygen species formation and enhanced oxidative stress as reflected in the increased level of lipid peroxidation and catalase activity and decreased glutathione peroxidase activity as well as glutathione levels. 4-HNE-induced oxidative stress was associated with increased transcriptional and translational expressions of Bax and p53 in SH-SY5Y cells. Mitochondrial-mediated apoptosis was confirmed by increased expression and activity of caspase-3. Our data demonstrate that 4-HNE induces neuronal cell death through abnormal expression of apoptotic markers (p53, Bax and caspase-3). Oxidative stress may be involved in the initial priming of SH-SY5Y cells to 4-HNE-induced cytotoxicity in vitro.

Phosphatidylethanolamines modified by γ-ketoaldehyde (γKA) induce endoplasmic reticulum stress and endothelial activation

Peroxidation of plasma lipoproteins has been implicated in the endothelial cell activation and monocyte adhesion that initiate atherosclerosis, but the exact mechanisms underlying this activation remain unclear. Lipid peroxidation generates lipid aldehydes, including the γ-ketoaldehydes (γKA), also termed isoketals or isolevuglandins, that readily modify the amine headgroup of phosphatidylethanolamine (PE). We hypothesized that aldehyde modification of PE could mediate some of the proinflammatory effects of lipid peroxidation. We found that PE modified by γKA (γKA-PE) induced THP-1 monocyte adhesion to human umbilical cord endothelial cells. γKA-PE also induced expression of adhesion molecules and increased MCP-1 and IL-8 mRNA in human umbilical cord endothelial cells. To determine the structural requirements for γKA-PE activity, we tested several related compounds. PE modified by 4-oxo-pentanal induced THP-1 adhesion, but N-glutaroyl-PE and C(18:0)N-acyl-PE did not, suggesting that an N-pyrrole moiety was essential for cellular activity. As the N-pyrrole headgroup might distort the membrane, we tested the effect of the pyrrole-PEs on membrane parameters. γKA-PE and 4-oxo-pentanal significantly reduced the temperature for the liquid crystalline to hexagonal phase transition in artificial bilayers, suggesting that these pyrrole-PE markedly altered membrane curvature. Additionally, fluorescently labeled γKA-PE rapidly internalized to the endoplasmic reticulum (ER); γKA-PE induced C/EBP homologous protein CHOP and BiP expression and p38 MAPK activity, and inhibitors of ER stress reduced γKA-PE-induced C/EBP homologous protein CHOP and BiP expression as well as EC activation, consistent with γKA-PE inducing ER stress responses that have been previously linked to inflammatory chemokine expression. Thus, γKA-PE is a potential mediator of the inflammation induced by lipid peroxidation.

Aldose reductase inhibition suppresses oxidative stress-induced inflammatory disorders

Oxidative stress-induced inflammation is a major contributor to several disease conditions including sepsis, carcinogenesis and metastasis, diabetic complications, allergic asthma, uveitis and after cataract surgery posterior capsular opacification. Since reactive oxygen species (ROS)-mediated activation of redox-sensitive transcription factors and subsequent expression of inflammatory cytokines, chemokines and growth factors are characteristics of inflammatory disorders, we envisioned that by blocking the molecular signals of ROS that activate redox-sensitive transcription factors, various inflammatory diseases could be ameliorated. We have indeed demonstrated that ROS-induced lipid peroxidation-derived lipid aldehydes such as 4-hydroxy-trans-2-nonenal (HNE) and their glutathione-conjugates (e.g. GS-HNE) are efficiently reduced by aldose reductase to corresponding alcohols which mediate the inflammatory signals. Our results showed that inhibition of aldose reductase (AKR1B1) significantly prevented the inflammatory signals induced by cytokines, growth factors, endotoxins, high glucose, allergens and auto-immune reactions in cellular as well as animal models. We have demonstrated that AKR1B1 inhibitor, fidarestat, significantly prevents tumor necrosis factor-alpha (TNF-α)-, growth factors-, lipopolysachharide (LPS)-, and environmental allergens-induced inflammatory signals that cause various inflammatory diseases. In animal models of inflammatory diseases such as diabetes, cardiovascular, uveitis, asthma, and cancer (colon, breast, prostate and lung) and metastasis, inhibition of AKR1B1 significantly ameliorated the disease. Our results from various cellular and animal models representing a number of inflammatory conditions suggest that ROS-induced inflammatory response could be reduced by inhibition of AKR1B1, thereby decreasing the progression of the disease and if the therapy is initiated early, the disease could be eliminated. Since fidarestat has already undergone phase III clinical trial for diabetic neuropathy and found to be safe, though clinically not very effective, our results indicate that it can be developed for the therapy of a number of inflammation-related diseases. Our results thus offer a novel therapeutic approach to treat a wide array of inflammatory diseases.

Signaling and cytotoxic functions of 4-hydroxyalkenals

The peroxidation of n-3 and n-6 polyunsaturated fatty acids (PUFAs) and of their hydroperoxy metabolites is a complex process. It is initiated by free oxygen radical-induced abstraction of a hydrogen atom from the lipid molecule followed by a series of nonenzymatic reactions that ultimately generate the reactive aldehyde species 4-hydroxyalkenals. The molecule 4-hydroxy-2E-hexenal (4-HHE) is generated by peroxidation of n-3 PUFAs, such as linolenic acid, eicosapentaenoic acid, and docosahexaenoic acid. The aldehyde product 4-hydroxy-2E-nonenal (4-HNE) is the peroxidation product of n-6 PUFAs, such as arachidonic and linoleic acids and their 15-lipoxygenase metabolites, namely 15-hydroperoxyeicosatetraenoic acid (15-HpETE) and 13-hydroperoxyoctadecadienoic acid (13-HpODE). Another reactive peroxidation product is 4-hydroxy-2E,6Z-dodecadienal (4-HDDE), which is derived from 12-hydroperoxyeicosatetraenoic acid (12-HpETE), the 12-lipoxygenase metabolite of arachidonic acid. Hydroxyalkenals, notably 4-HNE, have been implicated in various pathophysiological interactions due to their chemical reactivity and the formation of covalent adducts with macromolecules. The progressive accumulation of these adducts alters normal cell functions that can lead to cell death. The lipophilicity of these aldehydes positively correlates to their chemical reactivity. Nonetheless, at low and noncytotoxic concentrations, these molecules may function as signaling molecules in cells. This has been shown mostly for 4-HNE and to some extent for 4-HHE. The capacity of 4-HDDE to generate such "mixed signals" in cells has received less attention. This review addresses the origin and cellular functions of 4-hydroxyalkernals.

Elevated 4-hydroxyhexenal in Alzheimer's disease (AD) progression

Multiple studies have demonstrated elevations of α, β-unsaturated aldehydes including 4-hydroxynonenal (HNE) and acrolein, in vulnerable regions of mild cognitive impairment (MCI), preclinical Alzheimer's disease (PCAD), and late stage Alzheimer's disease (LAD) brain. However, there has been limited study of a third member, 4-hydroxyhexenal (HHE), a diffusible lipid peroxidation product of the ω-3 polyunstataturated fatty acids (PUFAs). In the present study levels of extractable and protein-bound HHE were quantified in the hippocampus/parahippocampal gyrus (HPG), superior and middle temporal gyri (SMTG), and cerebellum (CER) of MCI, PCAD, LAD, and normal control (NC) subjects. Levels of extractable and protein-bound HHE were increased in multiple regions in the progression of Alzheimer's disease (AD). Extractable HHE was significantly elevated in the hippocampus/parahippocampal gyrus (HPG) of PCAD and LAD subjects and protein-bound HHE was significantly higher in MCI, PCAD, and LAD HPG. A time- and concentration-dependent decrease in survival and a concentration-dependent decrease in glucose uptake were observed in primary cortical cultures treated with HHE. Together these data support a role for lipid peroxidation in the progression of Alzheimer's disease.

Trans-4-hydroxy-2-hexenal, a product of n-3 fatty acid peroxidation: make some room HNE

Lipid peroxidation yields multiple aldehyde species. Of these, trans-4-hydroxy-2-nonenal (HNE), derived from n-6 poly-unsaturated fatty acids (PUFA) is one of the most studied products of lipid peroxidation. On the other hand, oxidative damage to n-3 PUFA, e.g. docosahexaenoic acid (DHA; 22:6, n-3) and eicosapentaenoic acid, is now recognized as an important effector of oxidative stress and is of particular interest in n-3 rich tissues such as brain and retina. Trans-4-hydroxy-2-hexenal (HHE) is a major alpha,beta-unsaturated aldehyde product of n-3 PUFA oxidation and, like HNE, is an active biochemical mediator resulting from lipid peroxidation. HHE adducts are elevated in disease states, in some cases, at higher levels than the corresponding HNE adduct. HHE has properties in common with HNE, but there are important differences particularly with respect to adduction targets and detoxification pathways. In this review, the biochemistry and cell biology of HHE will be discussed. From this review, it is clear that further study is needed to determine the biochemical and physiological roles of HHE and its related aldehyde, trans-4-oxo-2-hexenal.

Transgenic expression of proximal tubule peroxisome proliferator-activated receptor-alpha in mice confers protection during acute kidney injury

Our previous studies suggest that peroxisome proliferator-activated receptor-alpha (PPARalpha) plays a critical role in regulating fatty acid beta-oxidation in kidney tissue and this directly correlated with preservation of kidney morphology and function during acute kidney injury. To further study this, we generated transgenic mice expressing PPARalpha in the proximal tubule under the control of the promoter of KAP2 (kidney androgen-regulated protein 2). Segment-specific upregulation of PPARalpha expression by testosterone treatment of female transgenic mice improved kidney function during cisplatin or ischemia-reperfusion-induced acute kidney injury. Ischemia-reperfusion injury or treatment with cisplatin in wild-type mice caused inhibition of fatty-acid oxidation, reduction of mitochondrial genes of oxidative phosphorylation, mitochondrial DNA, fatty-acid metabolism, and the tricarboxylic acid cycle. Similar injury in testosterone-treated transgenic mice resulted in amelioration of these effects. Similarly, there were increases in the levels of 4-hydroxy-2-hexenal-derived lipid peroxidation products in wild-type mice, which were also reduced in the transgenic mice. Similarly, necrosis of the S3 segment was reduced in the two injury models in transgenic mice compared to wild type. Our results suggest proximal tubule PPARalpha activity serves as a metabolic sensor. Its increased expression without the use of an exogenous PPARalpha ligand in the transgenic mice is sufficient to protect kidney function and morphology, and to prevent abnormalities in lipid metabolism associated with acute kidney injury.

Ascorbic acid promotes detoxification and elimination of 4-hydroxy-2(E)-nonenal in human monocytic THP-1 cells

4-Hydroxy-2(E)-nonenal (HNE), a reactive aldehyde derived from oxidized lipids, has been implicated in the pathogenesis of cardiovascular and neurological diseases, in part by its ability to induce oxidative stress and by protein carbonylation in target cells. The effects of intracellular ascorbic acid (vitamin C) on HNE-induced cytotoxicity and protein carbonylation were investigated in human THP-1 monocytic leukemia cells. HNE treatment of these cells resulted in apoptosis, necrosis, and protein carbonylation. Ascorbic acid accumulated in the cells at concentrations of 6.4 or 8.9 mM after treatment with 0.1 or 1 mM ascorbate in the medium for 18 h. Pretreatment of cells with 1.0 mM ascorbate decreased HNE-induced formation of reactive oxygen species and formation of protein carbonyls. The protective effects of ascorbate were associated with an increase in the formation of GSH-HNE conjugate and its phase 1 metabolites, measured by LC-MS/MS, and with increased transport of GSH conjugates from the cells into the medium. Ascorbate pretreatment enhanced the efflux of the multidrug resistant protein (MRP) substrate, carboxy-2',7'-dichlorofluorescein (CDF), and it prevented the HNE-induced inhibition of CDF export from THP-1 cells, suggesting that the protective effect of ascorbate against HNE cytotoxicity is through modulation of MRP-mediated transport of GSH-HNE conjugate metabolites. The formation of ascorbate adducts of HNE was observed in the cell exposure experiments, but it represented a minor pathway contributing to the elimination of HNE and to the protective effects of ascorbate.

Phototoxicity in human retinal pigment epithelial cells promoted by hypericin, a component of St. John's wort

St. John's wort (SJW), an over-the-counter antidepressant, contains hypericin, which absorbs light in the UV and visible ranges. In vivo studies have determined that hypericin is phototoxic to skin and our previous in vitro studies with lens tissues have determined that it is potentially phototoxic to the human lens. To determine if hypericin might also be phototoxic to the human retina, we exposed human retinal pigment epithelial (hRPE) cells to 10(-7) to 10(-5) M hypericin. Fluorescence emission detected from the cells (lambda(ex) = 488 nm; lambda(em) = 505 nm) confirmed hypericin uptake by human RPE. Neither hypericin exposure alone nor visible light exposure alone reduced cell viability. However when irradiated with 0.7 J cm(-2) of visible light (lambda > 400 nm) there was loss of cell viability as measured by MTS and lactate dehydrogenase assays. The presence of hypericin in irradiated hRPE cells significantly changed the redox equilibrium of glutathione and a decrease in the activity of glutathione reductase. Increased lipid peroxidation as measured by the thiobarbituric acid reactive substances assay correlated to hypericin concentration in hRPE cells and visible light radiation. Thus, ingested SJW is potentially phototoxic to the retina and could contribute to retinal or early macular degeneration.

Advanced lipid peroxidation end products in oxidative damage to proteins. Potential role in diseases and therapeutic prospects for the inhibitors

Reactive carbonyl compounds (RCCs) formed during lipid peroxidation and sugar glycoxidation, namely Advanced lipid peroxidation end products (ALEs) and Advanced Glycation end products (AGEs), accumulate with ageing and oxidative stress-related diseases, such as atherosclerosis, diabetes or neurodegenerative diseases. RCCs induce the 'carbonyl stress' characterized by the formation of adducts and cross-links on proteins, which progressively leads to impaired protein function and damages in all tissues, and pathological consequences including cell dysfunction, inflammatory response and apoptosis. The prevention of carbonyl stress involves the use of free radical scavengers and antioxidants that prevent the generation of lipid peroxidation products, but are inefficient on pre-formed RCCs. Conversely, carbonyl scavengers prevent carbonyl stress by inhibiting the formation of protein cross-links. While a large variety of AGE inhibitors has been developed, only few carbonyl scavengers have been tested on ALE-mediated effects. This review summarizes the signalling properties of ALEs and ALE-precursors, their role in the pathogenesis of oxidative stress-associated diseases, and the different agents efficient in neutralizing ALEs effects in vitro and in vivo. The generation of drugs sharing both antioxidant and carbonyl scavenger properties represents a new therapeutic challenge in the treatment of carbonyl stress-associated diseases.

4-hydroxynonenal induces mitochondrial oxidative stress, apoptosis and expression of glutathione S-transferase A4-4 and cytochrome P450 2E1 in PC12 cells

An excessive and sustained increase in reactive oxygen species (ROS) production and oxidative stress have been implicated in the pathogenesis of many diseases. In the present study, we have demonstrated that 4-hydroxynonenal (4-HNE), a product of lipid peroxidation, alters glutathione (GSH) pools and induces oxidative stress in PC12 cells in culture. This increase was accompanied by alterations in subcellular ROS and glutathione (GSH) metabolisms. The GSH homeostasis was affected as both mitochondrial and extramitochondrial GSH levels, GSH peroxidase and glutathione reductase activities were inhibited and glutathione S-transferase (GST) activity was increased after 4-HNE treatment. A concentration- and time-dependent increase in cytochrome P450 2E1 (CYP 2E1) activity in the mitochondria and postmitochondrial supernatant was also observed. 4-HNE-induced oxidative stress also caused an increase in the expression of GSTA4-4, CYP2E1 and Hsp70 proteins in the mitochondria. Increased oxidative stress in PC12 cells initiated apoptosis as indicated by the release of mitochondrial cytochrome c, activation of poly-(ADP-ribose) polymerase (PARP), DNA fragmentation and decreased expression of antiapoptotic Bcl-2 proteins. Mitochondrial respiratory and redox functions also appeared to be affected markedly by 4-HNE treatment. These results suggest that HNE-induced oxidative stress and apoptosis might be associated with altered mitochondrial functions and a compromised GSH metabolism and ROS clearance.

Growth factor-mediated reversal of senescent dysfunction of ischemia-induced cardioprotection

Based on the role of tumor necrosis factor-α (TNF-α) in ischemic preconditioning (IPC) and the age-associated loss of both TNF-α-induced platelet-derived growth factor-AB (PDGF-AB)-mediated cardioprotection and IPC-mediated cardioprotection, we hypothesized that targeting of PDGF-AB-based pathways would restore cardioprotection by IPC in the aging heart. To study this, IPC was induced in 4- and 24-mo-old F344 rats. Sections of young hearts isolated 1 day post-IPC revealed increased TNF-α compared with controls. In old rats, TNF-α was higher at baseline than IPC young rats and was not significantly altered after IPC. Treatment of old rats with PDGF-AB with vascular endothelial growth factor and angiopoietin-2 (a combination termed PVA), but not PDGF-AB alone, at the time of IPC decreased TNF-α. In addition, when compared with young hearts, IPC induced greater apoptosis in the old hearts, which was decreased with PVA treatment but was markedly increased with PDGF-AB. To test the significance of these findings, additional rats underwent permanent coronary ligation 1 day post-IPC. IPC was cardioprotective in young rats [14 days postmyocardial infarction (MI), fractional shortening 29 ± 6% vs. control MI 17 ± 4%, P < 0.05; Masson’s trichrome stain MI size: 13 ± 2% vs. control MI 17 ± 4% left ventricular area (LVA); P < 0.05]. In old rats, however, IPC reduced the post-MI 14-day survival (33% vs. controls 67%; P < 0.05). Treatment of IPC-aging rats with PVA, but not PDGF-AB-alone, reversed IPC-induced mortality (PVA-IPC-MI survival, 88%; PDGF-AB-IPC-MI, 14%) and reduced myocardial injury (fractional shortening: PVA-IPC, 31 ± 1% vs. control MI, 21 ± 6%, P < 0.05; MI size: PVA-IPC, 12 ± 2% vs. control MI, 18 ± 3% LVA, P < 0.05) and thus demonstrated that PDGF-AB-based pathways can reverse the senescent impairment in IPC-mediated cardioprotection.

Membrane alteration as a basis of aging and the protective effects of calorie restriction

As has been experimentally determined, oxidative modification to biological systems can be extensive, although the identification and stochiometric relation of the reactive species that cause these alterations have not been fully elucidated. In this review, arguments are presented to support the notion that the combined effects of membrane lipid peroxidation and its by-products, reactive aldehydes are likely responsible for membrane-associated functional declines during aging. As evidence for a systemic response to overall oxidative stress, the molecular inflammation hypothesis of aging is discussed by considering that the activation of inflammatory genes act as a bridge linking normal aging to pathological processes.

Induction of endothelial iNOS by 4-hydroxyhexenal through NF-kappaB activation

Lipid peroxidation and its end-product, 4-hydroxyhexenal (HHE), are known to affect redox balance during aging, which causes various degenerative processes including vascular alterations from endothelial cell deterioration. To better understand the molecular action of HHE in the development of vascular abnormalities during the aging process, we investigated whether the upregulation of inducible endothelial nitric oxide synthase (iNOS) by HHE is mediated through nuclear factor kappaB (NF-kappaB) activation. Results indicate that HHE stimulates iNOS by the transcriptional regulation of NF-kappaB activation through cytosolic kappaB degradation inhibitors (IkappaB). Pretreatment with NF-kappaB inhibitors Bay 11-7082 and N-acetyl cysteine (NAC) suppressed the upregulation of iNOS by blunting IkappaB degradation and NF-kappaB binding activity. Because inflammatory stimuli induce iNOS to generate large amounts of nitric oxide (NO), intracellular NO levels in the presence of Bay 11-7082, NAC, and caffeic acid methyl ester were estimated. These inhibitors significantly suppressed the HHE-induced NO levels to a basal level. These findings strongly suggest that in endothelial cells, HHE induces iNOS gene expression through NF-kappaB activation, which can lead to vascular dysfunction by the activation of various proinflammatory genes.

NF-kappaB activation mechanism of 4-hydroxyhexenal via NIK/IKK and p38 MAPK pathway

4-Hydroxyhexenal (HHE) is known to affect redox balance during aging, included are vascular dysfunctions. To better understand vascular abnormality through the molecular alterations resulting from HHE accumulation in aging processes, we set out to determine whether up-regulation of mitogen-activated protein kinase (MAPK) by HHE is mediated through nuclear factor kappa B (NF-kappaB) activation in endothelial cells. HHE induced NF-kappaB activation by inhibitor of kappaB (IkappaB) phosphorylation via the IkappaB kinase (IKK)/NF-kappaB inducing kinase (NIK) pathway. HHE increased the activity of p38 MAPK and extracellular signal regulated kinase (ERK), but not c-jun NH(2)-terminal kinase, indicating that p38 MAPK and ERK are closely involved in HHE-induced NF-kappaB transactivation. Pretreatment with ERK inhibitor PD98059, and p38 MAPK inhibitor SB203580, attenuated the induction of p65 translocation, IkappaB phosphorylation, and NF-kappaB luciferase activity. These findings strongly suggest that HHE induces NF-kappaB activation through IKK/NIK pathway and/or p38 MAPK and ERK activation associated with oxidative stress in endothelial cells.