Oxidative Stress and Lipid Dysregulation in Lipid Droplets: A Connection to Chronic Kidney Disease Revealed in Human Kidney Cells

Chronic kidney disease (CKD), which is defined as a condition causing the gradual loss of kidney function, shows renal lipid droplet (LD) accumulation that is associated with oxidative damage. There is a possibility that an LD abnormality in quality plays a role in CKD development. This study aimed to explore the chemical composition of LDs that are induced in human kidney cells during exposure to free fatty acids as an LD source and oxidized lipoproteins as oxidative stress. The LDs were aspirated directly from cells using nanotips, followed by in-tip microextraction, and the LD lipidomic profiling was conducted using nanoelectrospray mass spectrometry. As a result, the free fatty acids increased the LD lipid content and, at the same time, changed their composition significantly. The oxidized lipoproteins caused distorted proportions of intact lipids, such as triacylglycerols (TG), phosphatidylcholines (PC), phosphatidylethanolamines (PE), and cholesteryl esters (CE). Notably, the oxidized lipids, including the hydroperoxides of TG, PC, and PE, exhibited significant elevations in dose-dependent manners. Furthermore, the dysregulation of intact lipids was paralleled with the accumulation of lipid hydroperoxides. The present study has revealed that the oxidation of lipids and the dysregulation of the lipid metabolism coexisted in LDs in the kidney cells, which has provided a potential new target for diagnosis and new insights into CKD.

Looking in Depth at Oxidized Cholesteryl Esters by LC-MS/MS: Reporting Specific Fragmentation Fingerprints and Isomer Discrimination

Cholesteryl esters (CE) are prone to oxidation under increased oxidative stress conditions, but little is known about oxidized CE species (oxCE). To date, only a few oxCE have been identified, however, mainly based on the detection of molecular ions by mass spectrometry (MS) or target approaches for specific oxCE. The study of oxCE occurring from radical oxidation is still scarcely addressed. In this work, we made a comprehensive assessment of oxCE derivatives and their specific fragmentation patterns to identify detailed structural features and isomer differentiation using high-resolution C18 HPLC-MS- and MS/MS-based lipidomic approaches. The LC-MS/MS analysis allowed us to pinpoint oxCE structural isomers of long-chain and short-chain species, eluting at different retention times (t_R). Data analysis revealed that oxCE can be modified either in the fatty acyl moiety or in the cholesterol ring. The location of the hydroxy/hydroperoxy group originates characteristic fragment ions, namely the unmodified cholestenyl cation (m/z 369) for the isomer with oxidation in the fatty acyl chain or ions at m/z 367 and m/z 385 (369 + 16) when oxygenation occurs in the cholesterol ring. Additionally, we identified CE 18:2 and 20:4 aldehydic and carboxylic short-chain products that showed a clear fragmentation pattern that confirmed the modification in the fatty acyl chain. Specific fragmentation fingerprinting allowed discrimination of the isobaric short-chain species, namely carboxylic short-chain products, from hydroxy aldehyde short-chain products, with a hydroxycholesterol moiety. This new information is important to identify different oxCE in biological samples and will contribute to unraveling their role in biological conditions and diseases such as cardiovascular disease.

Deep mining and quantification of oxidized cholesteryl esters discovers potential biomarkers involved in breast cancer by liquid chromatography-mass spectrometry

Oxidized cholesteryl ester (OxCE) is produced by the oxidation of cholesteryl ester (CE) in the cores of lipoproteins. OxCE production and oxidative stress have been largely associated with breast cancer. Herein, we developed a novel reverse-phase liquid chromatography coupling quadrupole time-of-flight mass spectrometry (RPLC‒Q-TOF‒MS) method based on the iterative acquisition mode and used the MS/MS mode for deep mining and simultaneous quantification of cholesterol (Chol), CEs and OxCEs in human serum. A mathematical model was used to globally profile 57 molecular species of both CEs and OxCEs in the serum of both healthy volunteers and patients with breast cancer, and the qualitative results were verified based on the retention regularity. An abnormal elevation of OxCEs was found in serum samples of breast cancer patients, where OxCEs were produced by the oxidation of the fatty acyl chain of CE (20:4), such as CE (20:1)+3O, CE (20:2)+2O and CE (20:3)+O, which could be regarded as biomarkers. This comprehensive method for the global profiling of Chol, OxCEs and CEs sheds light on the role OxCEs and CEs play in breast cancer and has enabled the discovery of breast cancer biomarkers.

Recent development on liquid chromatography-mass spectrometry analysis of oxidized lipids

Polyunsaturated fatty acids (PUFAs) in the cellular membrane can be oxidized by various enzymes or reactive oxygen species (ROS) to form many oxidized lipids. These metabolites are highly bioactive, participating in a variety of physiological and pathophysiological processes. Mass spectrometry (MS), coupled with Liquid Chromatography, has been increasingly recognized as an indispensable tool for the analysis of oxidized lipids due to its excellent sensitivity and selectivity. We will give an update on the understanding of the molecular mechanisms related to generation of various oxidized lipids and recent progress on the development of LC-MS in the detection of these bioactive lipids derived from fatty acids, cholesterol esters, and phospholipids. The purpose of this review is to provide an overview of the formation mechanisms and technological advances in LC-MS for the study of oxidized lipids in human diseases, and to shed new light on the potential of using oxidized lipids as biomarkers and mechanistic clues of pathogenesis related to lipid metabolism. The key technical problems associated with analysis of oxidized lipids and challenges in the field will also discussed.

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.

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.

Identification of molecular species of oxidized triglyceride in plasma and its distribution in lipoproteins

The role of triglycerides carried in the triglyceride-rich lipoproteins (TRL) in the progression of atherosclerosis is uncertain. Identification of oxidized triglycerides and its possible association with atherosclerosis were largely ignored. Here we applied mass spectrometric approach to detect and identify triglyceride hydroperoxides (TGOOH) in human plasma and lipoproteins.

EDTA plasma was collected from healthy human volunteers (n=9) after 14–16 h of fasting. Very low-density lipoprotein (VLDL)

We identified 11 molecular species of TGOOH in either plasma or VLDL and IDL, of which TGOOH-18:1/18:2/16:0, TGOOH-18:1/18:1/16:0, TGOOH-16:0/18:2/16:0, TGOOH-18:1/18:1/18:1, and TGOOH-16:0/20:4/16:0 were most dominant. These TGOOH molecules are carried by TRL but not by LDL and HDL. Mean concentration of TGOOH in plasma, VLDL and IDL were, respectively, 56.1±25.6, 349.8±253.6 and 512.5±173.2 μmol/mol of triglycerides.

This is the first report to identify several molecular species of oxidized triglycerides in TRL. Presence of oxidized triglyceride may contribute to the atherogenicity of TRL. Further work is needed to elucidate the association of the oxidized triglyceride in atherosclerosis.