Analysis of triacylglycerol hydroperoxides in human lipoproteins by Orbitrap mass spectrometer

Dietary triacylglycerol hydroperoxide is not absorbed, yet it induces the formation of other triacylglycerol hydroperoxides in the gastrointestinal tract

The in vivo presence of triacylglycerol hydroperoxide (TGOOH), a primary oxidation product of triacylglycerol (TG), has been speculated to be involved in various diseases. Thus, considerable attention has been paid to whether dietary TGOOH is absorbed from the intestine. In this study, we performed the lymph duct-cannulation study in rats and analyzed the level of TGOOH in lymph following administration of a TG emulsion containing TGOOH. As we successfully detected TGOOH from the lymph, we hypothesized that this might be originated from the intestinal absorption of dietary TGOOH [hypothesis I] and/or the in situ formation of TGOOH [hypothesis II]. To determine the validity of these hypotheses, we then performed another cannulation study using a TG emulsion containing a deuterium-labeled TGOOH (D2-TGOOH) that is traceable in vivo. After administration of this emulsion to rats, we clearly detected unlabeled TGOOH instead of D2-TGOOH from the lymph, indicating that TGOOH is not absorbed from the intestine but is more likely to be produced in situ. By discriminating the isomeric structures of TGOOH present in lymph, we predicted the mechanism by which the intake of dietary TGOOH triggers oxidative stress (e.g., via generation of singlet oxygen) and induces in situ formation of TGOOH. The results of this study hereby provide a foothold to better understand the physiological significance of TGOOH on human health.

Elucidation of Olive Oil Oxidation Mechanisms by Analysis of Triacylglycerol Hydroperoxide Isomers Using LC-MS/MS

Despite the importance of the insight about the oxidation mechanisms (i.e., radical and singlet oxygen (¹O₂) oxidation) in extra virgin olive oil (EVOO), the elucidation has been difficult due to its various triacylglycerol molecular species and complex matrix. This study tried to evaluate the mechanisms responsible for EVOO oxidation in our daily use by quantitative determination of triacylglycerol hydroperoxide (TGOOH) isomers using LC-MS/MS. The standards of dioleoyl-(hydroperoxy octadecadienoyl)-triacylglycerol and dioleoyl-(hydroperoxy octadecamonoenoyl)-triacylglycerol, which are the predominant TGOOHs contained in EVOO, were prepared. Subsequently, fresh, thermal-, and photo-oxidized EVOO were analyzed. The obtained results mostly agreed with the previously reported characteristics of the radical and ¹O₂ oxidation of linoleic acid and oleic acid. This suggests that the methods described in this paper should be valuable in understanding how different factors that determine the quality of EVOO (e.g., olive species, cultivation area, cultivation timing, and extraction methods) contribute to its oxidative stability.

Deoxynivalenol-induced alterations in the redox status of HepG2 cells: identification of lipid hydroperoxides, the role of Nrf2-Keap1 signaling, and protective effects of zinc

Deoxynivalenol (DON) is a type B trichothecenes that is widely contaminating human and animal foods, leading to several toxicological implications if ingested. Induction of oxidative stress and production of lipid peroxides were suggested to be the reasons for DON-induced cytotoxicity. However, detailed and comprehensive profiling of DON-related lipid hydroperoxides was not identified. Furthermore, the mechanisms behind DON-induced cytotoxicity and oxidative stress have received less attention. Zinc (Zn) is an essential element that has antioxidant activities; however, the protective effects of Zn against DON-induced adverse effects were not examined. Therefore, this study was undertaken to investigate DON-induced cytotoxicity and oxidative damage to human HepG2 cell lines. Furthermore, a quantitative estimation for the formed lipid hydroperoxides was conducted using LC-MS/MS. In addition, DON-induced transcriptomic changes on the inflammatory markers and antioxidant enzymes were quantitatively examined using qPCR. The protective effects of Zn against DON-induced cytotoxicity and oxidative stress, the formation of lipid hydroperoxides (LPOOH), and antioxidant status in HepG2 cells were investigated. Finally, the effects of DON and Zn on the Nrf2-Keap1 pathway were further explored. The achieved results indicated that DON caused significant cytotoxicity in HepG2 cells accompanied by significant oxidative damage and induction of the inflammatory markers. Identification of DON-related LPOOH revealed the formation of 22 LPOOH species including 14 phosphatidylcholine hydroperoxides, 5 triacylglycerol hydroperoxides, and 3 cholesteryl ester hydroperoxides. DON caused significant downregulation of Nrf2-regulated antioxidant enzymes. Zn administration led to significant protection of HepG2 cells against DON-induced adverse effects, probably via activation of the Nrf2-Keap1 pathway.

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.

Novel Fluorescence-Based Method To Characterize the Antioxidative Effects of Food Metabolites on Lipid Droplets in Cultured Hepatocytes

A fluorescence microscopic method for characterizing size, quantity, and oxidation of lipid droplets (LDs) in HepG2 cells was developed. LDs were induced by palmitic (PA), oleic (OA), or linoleic acids (LA) and stained with two fluorescent probes for neutral lipids and lipid peroxides. Each fatty acid increased the number of LDs and oxidized LDs (oxLDs) and the degree of LD oxidation time dependently, as well as increased intracellular triglyceride hydroperoxides. LDs induced by LA without 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH) showed the most significant oxidation degree over PA and OA, especially in large LDs (area ≥ 3 μm², oxLD/LD = 52.3 ± 21.7%). Under this condition, two food-derived antioxidants were evaluated, and both of them significantly improved the LD characteristics. Moreover, chlorogenic acid reduced the quantity of large LDs by 74.0-87.6% in a dose-dependent manner. The proposed method provides a new approach to evaluate the effect of dietary antioxidants on LD characteristics.

"Only a Life Lived for Others Is Worth Living": Redox Signaling by Oxygenated Phospholipids in Cell Fate Decisions

SIGNIFICANCE

Oxygenated polyunsaturated lipids are known to play multi-functional roles as essential signals coordinating metabolism and physiology. Among them are well-studied eicosanoids and docosanoids that are generated via phospholipase A2 hydrolysis of membrane phospholipids and subsequent oxygenation of free polyunsaturated fatty acids (PUFA) by cyclooxygenases and lipoxygenases. Recent Advances: There is an emerging understanding that oxygenated PUFA-phospholipids also represent a rich signaling language with yet-to-be-deciphered details of the execution machinery-oxygenating enzymes, regulators, and receptors. Both free and esterified oxygenated PUFA signals are generated in cells, and their cross-talk and inter-conversion through the de-acylation/re-acylation reactions is not sufficiently explored.

CRITICAL ISSUES

Here, we review recent data related to oxygenated phospholipids as important damage signals that trigger programmed cell death pathways to eliminate irreparably injured cells and preserve the health of multicellular environments. We discuss the mechanisms underlying the trans-membrane redistribution and generation of oxygenated cardiolipins in mitochondria by cytochrome c as pro-apoptotic signals. We also consider the role of oxygenated phosphatidylethanolamines as proximate pro-ferroptotic signals.

FUTURE DIRECTIONS

We highlight the importance of sequential processes of phospholipid oxygenation and signaling in disease contexts as opportunities to use their regulatory mechanisms for the identification of new therapeutic targets.

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.

Synthesis of (2β,3α,6-²H₃cholesteryl linoleate and cholesteryl oleate as internal standards for mass spectrometry

The accurate analysis of trace component in complex biological matrices requires the use of reliable standards. For liquid chromatography/mass spectrometry analysis, the stable isotope-labeled derivatives of the analyte molecules are the most appropriate internal standards. We report here the synthesis of (2β,3α,6-(2)H3)cholesteryl linoleate and oleate containing three non-exchangeable deuterium in the steroid ring. The principal reactions used were: (1) trans diaxial opening of 2α,3α-epoxy-6-oxo-5α-cholestane with LiAlD4 and subsequent oxidation of the resulting (2β,6α-(2)H2)-3α,6β-diol with Jones' reagent, followed by reduction of the resulting (2β-(2)H)-3,6-dione with NaBD4 leading to the (2β,3α,6α-(2)H3)-3β,6β-dihydroxy-5α-cholestane, (2) selective protection of the 3β-hydroxy group as the tert-butyldimethylsilyl ether, (3) dehydration of the 6β-hydroxy group with POCl3 and removal of tert-butyldimethylsilyloxy groups with 5M HCl in acetone, and (4) esterification of the resultant (2β,3α,6-(2)H3)cholesterol with linoleic and oleic acids using 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide. The isotopic purity was found to be satisfactory by mass spectrometry, and nuclear magnetic resonance properties of the new compounds were tabulated. The labeled compounds can be used as internal standards in liquid chromatography/mass spectrometry assays for clinical and biochemical studies.

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.