Quantitative determination of phosphatidylcholine hydroperoxides during copper oxidation of LDL and HDL by liquid chromatography/mass spectrometry

Binding properties of recombinant LDL receptor and LOX-1 receptor to LDL measured using bio-layer interferometry and atomic force microscopy

Oxidation of low-density lipoproteins (LDLs) triggers a recognition by scavenger receptors such as lectin-like oxidized LDL receptor-1 (LOX-1) and is related to inflammation and cardiovascular diseases. Although LDLs that are recognized by LOX-1 can be risk-related LDLs, conventional LDL detection methods using commercially available recombinant receptors remain undeveloped. Using a bio-layer interferometry (BLI), we investigated the binding of recombinant LOX-1 (reLOX-1) and LDL receptors to the oxidized LDLs. The recombinant LDL receptor preferably bound minimally modified LDLs, while the reLOX-1 recognized extensively oxidized LDLs. An inversed response of the BLI was observed during the binding in the case of reLOX-1. AFM study showed that the extensively oxidized LDLs and aggregates of LDLs were observed on the surface, supporting the results. Altogether, a combined use of these recombinant receptors and the BLI method is useful in detecting high-risk LDLs such as oxidized LDLs and modified LDLs.

Oxidized Low-Density Lipoproteins Trigger Hepatocellular Oxidative Stress with the Formation of Cholesteryl Ester Hydroperoxide-Enriched Lipid Droplets

Oxidized low-density lipoproteins (oxLDLs) induce oxidative stress in the liver tissue, leading to hepatic steatosis, inflammation, and fibrosis. Precise information on the role of oxLDL in this process is needed to establish strategies for the prevention and management of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH). Here, we report the effects of native LDL (nLDL) and oxLDL on lipid metabolism, lipid droplet formation, and gene expression in a human liver-derived C3A cell line. The results showed that nLDL induced lipid droplets enriched with cholesteryl ester (CE) and promoted triglyceride hydrolysis and inhibited oxidative degeneration of CE in association with the altered expression of LIPE, FASN, SCD1, ATGL, and CAT genes. In contrast, oxLDL showed a striking increase in lipid droplets enriched with CE hydroperoxides (CE-OOH) in association with the altered expression of SREBP1, FASN, and DGAT1. Phosphatidylcholine (PC)-OOH/PC was increased in oxLDL-supplemented cells as compared with other groups, suggesting that oxidative stress increased hepatocellular damage. Thus, intracellular lipid droplets enriched with CE-OOH appear to play a crucial role in NAFLD and NASH, triggered by oxLDL. We propose oxLDL as a novel therapeutic target and candidate biomarker for NAFLD and NASH.

Organic Peroxides in Aerosol: Key Reactive Intermediates for Multiphase Processes in the Atmosphere

Organic peroxides (POs) are organic molecules with one or more peroxide (-O-O-) functional groups. POs are commonly regarded as chemically labile termination products from gas-phase radical chemistry and therefore serve as temporary reservoirs for oxidative radicals (HOx and ROx) in the atmosphere. Owing to their ubiquity, active gas-particle partitioning behavior, and reactivity, POs are key reactive intermediates in atmospheric multiphase processes determining the life cycle (formation, growth, and aging), climate, and health impacts of aerosol. However, there remain substantial gaps in the origin, molecular diversity, and fate of POs due to their complex nature and dynamic behavior. Here, we summarize the current understanding on atmospheric POs, with a focus on their identification and quantification, state-of-the-art analytical developments, molecular-level formation mechanisms, multiphase chemical transformation pathways, as well as environmental and health impacts. We find that interactions with SO2 and transition metal ions are generally the fast PO transformation pathways in atmospheric liquid water, with lifetimes estimated to be minutes to hours, while hydrolysis is particularly important for α-substituted hydroperoxides. Meanwhile, photolysis and thermolysis are likely minor sinks for POs. These multiphase PO transformation pathways are distinctly different from their gas-phase fates, such as photolysis and reaction with OH radicals, which highlights the need to understand the multiphase partitioning of POs. By summarizing the current advances and remaining challenges for the investigation of POs, we propose future research priorities regarding their origin, fate, and impacts in the atmosphere.

Gowda SG, Gowda D, Sakurai T, Sazaki I, Chiba H, Hui SP. Simple and Sensitive Method for the Quantitative Determination of Lipid Hydroperoxides by Liquid Chromatography/Mass Spectrometry

Lipid hydroperoxides (LOOH) are the initial products of the peroxidation of unsaturated lipids and play a crucial role in lipid oxidation due to their ability to decompose into free radicals and cause adverse effects on human health. Thus, LOOHs are commonly considered biomarkers of oxidative stress-associated pathological conditions. Despite their importance, the sensitive and selective analytical method for determination is limited, due to their low abundance, poor stability, and low ionizing efficiency. To overcome these limitations, in this study, we chemically synthesized eight fatty acid hydroperoxides (FAOOH), including FA 18:1-OOH, FA 18:2-OOH, FA 18:3-OOH, FA 20:4-OOH, FA 20:5-OOH, FA 22:1-OOH, FA 22:6-OOH as analytes, and FA 19:1-OOH as internal standard. Then, they were chemically labeled with 2-methoxypropene (2-MxP) to obtain FAOOMxP by one-step derivatization (for 10 min). A selected reaction monitoring assisted targeted analytical method was developed using liquid chromatography/tandem mass spectrometry (LC-MS/MS). The MxP-labelling improved the stability and enhanced the ionization efficiency in positive mode. Application of reverse-phase chromatography allowed coelution of analytes and internal standards with a short analysis time of 6 min. The limit of detection and quantification for FAOOH ranged from 0.1–1 pmol/µL and 1–2.5 pmol/µL, respectively. The method was applied to profile total FAOOHs in chemically oxidized human serum samples (n = 5) and their fractions of low and high-density lipoproteins (n = 4). The linoleic acid hydroperoxide (FA 18:2-OOH) and oleic acid hydroperoxide (FA 18:1-OOH) were the most abundant FAOOHs in human serum and lipoproteins. Overall, our validated LC-MS/MS methodology features enhanced detection and rapid separation that enables facile quantitation of multiple FAOOHs, therefore providing a valuable tool for determining the level of lipid peroxidation with potential diagnostic applications.

Identification of cadmium-produced lipid hydroperoxides, transcriptomic changes in antioxidant enzymes, xenobiotic transporters, and pro-inflammatory markers in human breast cancer cells (MCF7) and protection with fat-soluble vitamins

Cadmium (Cd) is a toxic metal that is regarded as a metallohormone with estrogen-like properties. The present study aimed at identification of lipid hydroperoxides produced in human breast cancer (MCF7) exposed to cadmium (Cd) at environmentally relevant levels. Cd induced cytotoxicity and oxidative stress and produced a series of 26 lipid hydroperoxide species including 14 phosphatidylcholine hydroperoxides (PC-OOH), 9 triacylglycerol hydroperoxides (TG-OOH), and 3 cholesteryl ester hydroperoxides (CE-OOH). Among these hydroperoxides, PC34:2-OOH, PC34:3-OOH, TG60:14-OOH, TG48:5-OOH, TG60:15-OOH, and CE20:4-OOH were produced in a dose-dependent manner, suggesting these as possible biomarkers for Cd exposure in MCF7 cells. In addition, Cd led to significant decreases in the gene expressions of antioxidants, detoxification enzymes, and xenobiotic transporters. In a protection trial, co-exposure of MCF7 cells to fat-soluble vitamins including vitamin A, D, and E reduced Cd-induced cytotoxicity, lipid peroxidation, oxidative stress, and inflammatory responses. Fat-soluble vitamins upregulated antioxidant and detoxification enzymes, and xenobiotic transporters. Therefore, dietary supplementation of such micronutrients is recommended for people at risk for exposure to Cd.

Identification of lead-produced lipid hydroperoxides in human HepG2 cells and protection using rosmarinic and ascorbic acids with a reference to their regulatory roles on Nrf2-Keap1 antioxidant pathway

Lead (Pb) is one of the toxic heavy metals that have several toxicological implications including cytotoxicities and oxidative stress. The release of reactive oxygen species (ROS) usually initiates lipid peroxidation and resulting in inflammation and tissue injury. However, the detailed identification of the Pb-produced lipid hydroperoxides has received little attention. Furthermore, the mechanisms behind such effects are less informed. Therefore, this study firstly investigated Pb-produced lipid hydroperoxides in human HepG2 cells using LC/MS. The effects of Pb on the antioxidant enzymes were additionally examined using qPCR and their dependent activities. As a protection trial, the ameliorative effects of rosmarinic (RMA) and ascorbic (ASA) acids on Pb-induced cytotoxicity and oxidative stress and their regulatory effects on Nrf2/Keap1 pathway were investigated. The achieved results confirmed cytotoxicity and oxidative damage of Pb on HepG2 cells. In addition, 20 lipid hydroperoxides (LOOH) were identified including 11 phosphatidylcholine hydroperoxides (PCOOH), 5 triacylglycerol hydroperoxides (TGOOH) and 4 cholesteryl ester hydroperoxides (CEOOH). The most dominant LOOH species were PCOOH 34:2, PCOOH 34:3, PCOOH 38:7, TGOOH 60:14, TGOOH 60:15, CEOOH 18:3 and CEOOH 20:4. Pb significantly downregulated Nrf2-regulated antioxidant enzymes at both the pretranscriptional and functional levels. Co-exposure of HepG2 cells to RMA and ASA significantly reduced Pb-produced adverse outcomes. This protection occurred via activation Nrf2-Keap1 antioxidant pathway.

Identification of molecular species of phosphatidylcholine hydroperoxides in native and copper-oxidized triglyceride-rich lipoproteins in humans

Background

Triglyceride-rich lipoproteins are considered to be independent predictors of atherosclerotic cardiovascular disease. The molecular basis of its atherogenicity is uncertain. Here, we aim to identify molecular species of phosphatidylcholine hydroperoxides (PCOOH) in triglyceride-rich lipoproteins. For comparison, copper-oxidized triglyceride-rich lipoproteins were investigated as well.

Methods

A fasting EDTA blood sample was collected from six healthy human volunteers to isolate two major triglyceride-rich lipoproteins fractions – very low-density lipoproteins (VLDL) and intermediate-density lipoproteins (IDL) using sequential ultracentrifugation. Triglyceride-rich lipoproteins and plasma samples were studied for PCOOH by liquid chromatography (LC) coupled with Orbitrap mass spectrometry.

Results

Twelve molecular species of PCOOH in triglyceride-rich lipoproteins and/or plasma were identified using the following criteria: (1) high-resolution mass spectrometry (MS) with mass accuracy within 5 ppm, (2) retention time in LC and (3) fragmentation pattern in MS2 and MS3. PC36:4-OOH was most often detected in VLDL, IDL and plasma. The ratio of total PCOOH to phosphatidylcholine progressively increased with the duration of oxidation in both VLDL and IDL.

Conclusion

This study demonstrated the presence of 12 molecular species of PCOOH in native triglyceride-rich lipoproteins. The frequent detection of PCOOH in triglyceride-rich lipoproteins provides a molecular basis of the atherogenicity of triglyceride-rich lipoproteins. PCOOH in triglyceride-rich lipoproteins might serve as a molecular basis of the atherogenicity of triglyceride-rich lipoproteins.

Choline and Ethanolamine Plasmalogens Prevent Lead-Induced Cytotoxicity and Lipid Oxidation in HepG2 Cells

Plasmalogens derived from dietary phospholipids are considered to be potential protectors against oxidation-related disorders, while lead (Pb) is an environmental contaminant worldwide and is known to induce oxidative stress. However, the protective and antilipid oxidative effects of individual plasmalogen species against Pb damage have received little attention. In this study, six plasmalogen species (with either choline or ethanolamine as the headgroup and p16:0/18:1, p16:0/18:2, or p16:0/20:5 as the side chains) were evaluated in human hepatoma cells. Plasmalogen species showed a remarkable recovery in cell viability as well as elimination of reactive oxygen species and suppressed the accumulation of phosphatidylcholine hydroperoxides (from 63.6 ± 1.8% to 80.3 ± 2.9%) and phosphatidylethanolamine hydroperoxides (from 25.7 ± 9.3% to 76.1 ± 3.7%). Moreover, plasmalogens significantly upregulated the gene expression levels of a series of antioxidant enzymes that are regulated via the Nrf-2-dependent pathway. This study suggested that choline and ethanolamine plasmalogens could prevent Pb-induced cytotoxicity and lipid oxidation in HepG2 cells.

Interaction between LDL-mimetic liposomes and acid-treated carbon nanotube electrode during Cu2+-mediated oxidation

Oxidation of low-density lipoproteins (LDL) causes atherosclerosis. Detection of oxidation of LDL-mimetic liposomes using an electrode might serve as a convenient tool in the search of antioxidants for the prevention of atherosclerosis. This report proposes a reaction mechanism between LDL-mimetic liposomes and an acid-treated carbon nanotube (CNT) electrode. Oxidation of the liposomes, mediated by Cu²⁺, was monitored by the change in electrode potential, and the fluorescence intensity generated by diphenyl-1-pyrenylphosphine (DPPP) as control. The electrode potential and fluorescence intensity increased concomitantly during oxidation, followed by a gradual decrease. Although the electrical potential peaked faster than the fluorescence intensity, addition of CNT to the DPPP reaction accelerated the latter, suggesting the role of CNT as an accelerator of liposome oxidation. Atomic force microscopy showed increased binding of liposomes to CNT along with liposomal deformation. Further, binding of Cu²⁺ to the liposome-bound CNT surface was observed by quartz crystal microbalance. In conclusion, the interaction of liposomes with Cu²⁺ and CNT surface explains the rapid response of the electrode in liposome oxidation.

An Efficient Glycoblotting-Based Analysis of Oxidized Lipids in Liposomes and a Lipoprotein

Although widely occurring lipid oxidation, which is triggered by reactive oxygen species (ROS), produces a variety of oxidized lipids, practical methods to efficiently analyze oxidized lipids remain elusive. Herein, it is shown that the glycoblotting platform can be used to analyze oxidized lipids. Analysis is based on the principle that lipid aldehydes, one of the oxidized lipid species, can be captured selectively, enriched, and detected. Moreover, 3-methyl-1-p-tolyltriazene (MTT) methylates phosphoric and carboxylic acids, and this MTT-mediated methylation is, in combination with conventional tandem mass spectrometry (MS/MS) analysis, an effective method for the structural analysis of oxidized lipids. By using three classes of standards, liposomes, and a lipoprotein, it is demonstrated that glycoblotting represents a powerful approach for focused lipidomics, even in complex macromolecules.

Oxidative lipidomics coming of age: advances in analysis of oxidized phospholipids in physiology and pathology

SIGNIFICANCE

Oxidized phospholipids are now well recognized as markers of biological oxidative stress and bioactive molecules with both pro-inflammatory and anti-inflammatory effects. While analytical methods continue to be developed for studies of generic lipid oxidation, mass spectrometry (MS) has underpinned the advances in knowledge of specific oxidized phospholipids by allowing their identification and characterization, and it is responsible for the expansion of oxidative lipidomics.

RECENT ADVANCES

Studies of oxidized phospholipids in biological samples, from both animal models and clinical samples, have been facilitated by the recent improvements in MS, especially targeted routines that depend on the fragmentation pattern of the parent molecular ion and improved resolution and mass accuracy. MS can be used to identify selectively individual compounds or groups of compounds with common features, which greatly improves the sensitivity and specificity of detection. Application of these methods has enabled important advances in understanding the mechanisms of inflammatory diseases such as atherosclerosis, steatohepatitis, leprosy, and cystic fibrosis, and it offers potential for developing biomarkers of molecular aspects of the diseases.

CRITICAL ISSUES AND FUTURE DIRECTIONS

The future in this field will depend on development of improved MS technologies, such as ion mobility, novel enrichment methods and databases, and software for data analysis, owing to the very large amount of data generated in these experiments. Imaging of oxidized phospholipids in tissue MS is an additional exciting direction emerging that can be expected to advance understanding of physiology and disease.

Elastic modulus of low-density lipoprotein as potential indicator of its oxidation

BACKGROUND

Evaluation of low-density lipoprotein oxidation is important in the risk assessment of cardiovascular disease. Atomic force microscope is widely used to evaluate the physical properties including stiffness on a single-particle scale. In this study, the effect of low-density lipoprotein oxidation on the low-density lipoprotein stiffness was investigated using an atomic force microscope.

METHODS

Isolated low-density lipoprotein particles with or without oxidation were densely bound to an Au substrate on mica, and then pressed and deformed by the atomic force microscope tip. The stiffness of each low-density lipoprotein particle was estimated as the elastic modulus obtained by the force curve analysis. Biochemical change of low-density lipoprotein due to oxidation was studied by electrophoresis.

RESULTS AND CONCLUSION

The elastic modulus of low-density lipoprotein particles ranged between 0.1 and 2 MPa. The oxidation of low-density lipoprotein increased the number of low-density lipoprotein particles with smaller elastic moduli, indicating the decrease in low-density lipoprotein stiffness. The elastic modulus of low-density lipoprotein might be potentially useful to evaluate low-density lipoprotein oxidation.

Oxidized high-density lipoprotein impairs endothelial progenitor cells' function by activation of CD36-MAPK-TSP-1 pathways

AIMS

High-density lipoprotein (HDL) levels inversely correlate with cardiovascular events due to the protective effects on vascular wall and stem cells, which are susceptible to oxidative modifications and then lead to potential pro-atherosclerotic effects. We proposed that oxidized HDL (ox-HDL) might lead to endothelial progenitor cells (EPCs) dysfunction and investigated underlying mechanisms.

RESULTS

ox-HDL was shown to increase apoptosis and intracellular reactive oxygen species levels, but to reduce migration, angiogenesis, and cholesterol efflux of EPCs in a dose-dependent manner. p38 mitogen-activated protein kinase (MAPK) and NF-κB were activated after ox-HDL stimulation, which also upregulated thrombospondin-1 (TSP-1) expression without affecting vascular endothelial growth factor. Effects caused by ox-HDL could be significantly attenuated by pretreatment with short hairpin RNA-mediated CD36 knockdown or probucol. Data of in vivo experiments and the inverse correlation of ox-HDL and circulating EPC numbers among patients with coronary artery diseases (CAD) or CAD and type 2 diabetes also supported it. Meanwhile, HDL separated from such patients could significantly increase cultured EPC's caspase 3 activity, further supporting our proposal.

INNOVATION

This is the most complete study to date of how ox-HDL would impair EPCs function, which was involved with activation of CD36-p38 MAPK-TSP-1 pathways and proved by not only the inverse relationship between ox-HDL and circulating EPCs in clinic but also pro-apoptotic effects of HDL separated from patients' serum.

CONCLUSION

Activation of CD36-p38 MAPK-TSP-1 pathways contributes to the pathological effects of ox-HDL on EPCs' dysfunction, which might be one of the potential etiological factors responsible for the disturbed neovascularization in chronic ischemic disease.

Identification of molecular species of cholesteryl ester hydroperoxides in very low-density and intermediate-density lipoproteins

Background

Oxidation of lipoproteins is thought to play a crucial role in atherogenesis. Role for triglyceride-rich lipoproteins in atherogenesis is unclear. Thus, we aimed to investigate whether cholesteryl ester hydroperoxides (CEOOH) are present in very low-density lipoproteins (VLDL) and intermediate-density lipoproteins (IDL) by using highly sensitive liquid chromatography/mass spectrometry.

Methods

Total lipids were extracted from the plasma of healthy donors ( n = 6) and their fractions of VLDL and IDL. Additional three plasma samples were analysed freshly for CEOOH. Detection and identification of CEOOH was conducted by liquid chromatography/LTQ ion trap mass spectrometry/Orbitrap high mass accuracy mass spectrometry. Authentic standards of CEOOH were used for unequivocal identification on the basis of their mass spectra.

Results

We identified six molecular CEOOH species overall, namely, Ch18:1-OOH, Ch18:2-OOH, Ch18:3-OOH, Ch20:4-OOH, Ch20:5-OOH and Ch22:6-OOH. Of them, Ch18:2-OOH, Ch20:5-OOH, Ch20:4-OOH and Ch22:6-OOH were detected in all IDL samples, while only Ch20:4-OOH was detected in all VLDL samples. All of CEOOH species except for Ch18:3-OOH were detected in plasma, with constant detection of Ch20:5-OOH, and Ch22:6-OOH in all plasma samples.

Conclusion

The presence of CEOOH species in VLDL and IDL was confirmed with the analytical sensitivity of 0.1 pmol, showing the constant appearance of more CEOOH species in IDL than VLDL. This finding might add biochemical evidences of atherogenicity of these lipoproteins. Clinical utility of measuring CEOOH level in these lipoproteins need to be investigated for the risk assessment of the cardiovascular disease.

Immunological detection of large oxidized lipoproteins in hypertriglyceridemic serum

BACKGROUND

Triglyceride-rich, low-density lipoproteins (TG-rich LDL) have been reported as an oxidized lipoprotein species in patients with severe liver disease. Using TG-rich LDL as an immunogen, we obtained a monoclonal antibody (G11-6) that reacted with TG-rich LDL from patients with liver disease and with metal-oxidized LDL only in the early process of the oxidation reaction. This study determined the G11-6-reactive lipoproteins in hypertriglyceridemic serum.

METHODS

Serum samples from healthy volunteers (n = 12) and hypertriglyceridemic patients (n = 9) were fractionated by gel filtration and subjected to a sandwich enzyme-linked immunosorbent assay (ELISA) using G11-6 and polyclonal anti-apolipoprotein B antibodies.

RESULTS

Small LDL and larger lipoproteins reacted with G11-6. G11-6-reactive small LDL was identified in both the healthy subjects and hypertriglyceridemic patients, whereas G11-6-reactive larger lipoproteins were found only in the hypertriglyceridemic patients.

CONCLUSIONS

G11-6 is a useful tool for detecting increased large oxidized lipoproteins in hypertriglyceridemic patients.

Analysis of triacylglycerol hydroperoxides in human lipoproteins by Orbitrap mass spectrometer

Herein, we represent a simple method for the detection and characterization of molecular species of triacylglycerol monohydroperoxides (TGOOH) in biological samples by use of reversed-phase liquid chromatography with a LTQ Orbitrap XL mass spectrometer (LC/LTQ Orbitrap) via an electrospray ionization source. Data were acquired using high-resolution, high-mass accuracy in Fourier-transform mode. Platform performance, related to the identification of TGOOH in human lipoproteins and plasma, was estimated using extracted ion chromatograms with mass tolerance windows of 5 ppm. Native low-density lipoproteins (nLDL) and native high-density lipoproteins (nHDL) from a healthy donor were oxidized by CuSO4 to generate oxidized LDL (oxLDL) and oxidized HDL (oxHDL). No TGOOH molecular species were detected in the nLDL and nHDL, whereas 11 species of TGOOH molecules were detected in the oxLDL and oxHDL. In positive-ion mode, TGOOH was found as [M + NH4](+). In negative-ion mode, TGOOH was observed as [M + CH3COO](-). TGOOH was more easily ionized in positive-ion mode than in negative-ion mode. The LC/LTQ Orbitrap method was applied to human plasma and three molecular species of TGOOH were detected. The limit of detection is 0.1 pmol (S/N = 10:1) for each synthesized TGOOH.

Detection and characterization of cholesteryl ester hydroperoxides in oxidized LDL and oxidized HDL by use of an Orbitrap mass spectrometer

Oxidation of cholesteryl esters in lipoproteins by reactive oxygen species yields cholesteryl ester hydroperoxides (CEOOH). In this study, we developed a novel method for identification and characterization of CEOOH molecules in human lipoproteins by use of reversed-phase liquid chromatography with an hybrid linear ion trap-Orbitrap mass spectrometer (LC-LTQ Orbitrap). Electrospray ionization tandem mass spectrometric analysis was performed in both positive-ion and negative-ion modes. Identification of CEOOH molecules was completed by use of high-mass-accuracy (MA) mass spectrometric data obtained by using the spectrometer in Fourier-transform (FT) mode. Native low-density lipoproteins (nLDL) and native high-density lipoproteins (nHDL) from a healthy donor were oxidized by CuSO(4), furnishing oxidized LDL (oxLDL) and oxidized HDL (oxHDL). No CEOOH molecules were detected in the nLDL and the nHDL, whereas six CEOOH molecules were detected in the oxLDL and the oxHDL. In positive-ion mode, CEOOH was detected as [M + NH(4)](+) and [M + Na](+) ions. In negative-ion mode, CEOOH was detected as [M + CH(3)COO](-) ions. CEOOH were more easily ionized in positive-ion mode than in negative-ion mode. The LC-LTQ Orbitrap method was applied to human plasma and six species of CEOOH were detected. The limit of detection was 0.1 pmol (S/N = 5:1) for synthesized CEOOH.