Distinct hepatic lipid profile of hypertriglyceridemic mice determined by easy ambient sonic-spray ionization mass spectrometry

Mitochondrial Adaptive Responses to Hypertriglyceridemia and Bioactive Lipids

Significance: Altered plasma triglyceride metabolism and changes in dietary fatty acid types and levels are major contributors to the development of metabolic and cardiovascular diseases such as fatty liver disease, obesity, diabetes, and atherosclerosis. Lipid accumulation in visceral adipose tissue and ectopically in other organs, as well as lipid-induced redox imbalance, is connected to mitochondrial dysfunction in a range of oxidative stress-associated metabolic and degenerative disorders. Recent Advances: Successful mitochondrial adaptive responses in the context of hypertriglyceridemia and dietary bioactive polyunsaturated fatty acids contribute to increase body energy expenditure and reduce oxidative stress, thus allowing several cell types to cope with metabolic challenges and stresses. These responses include mitochondrial redox signaling, mild uncoupling, and changes in network dynamic behavior. Critical Issues: Mitochondrial bioenergetics and redox changes in a lipid overload context are relatively well characterized. However, the turning point between adaptive and maladaptive mitochondrial responses remains a critical issue to be elucidated. In addition, the relationship between changes in fusion/fission machinery and mitochondrial function is less well understood. Future Directions: The effective mitochondrial responses described here support the research for new drug design and diet or nutraceutical formulations targeting mitochondrial mild uncoupling and effective quality control as putative strategies for cardiometabolic diseases. Antioxid. Redox Signal. 36, 953-968.

Mapping Lipid C=C Location Isomers in Organ Tissues by Coupling Photochemical Derivatization and Rapid Extractive Mass Spectrometry

Lipid desaturation plays important roles in biological processes and the disease states. Here, we report a simple but efficient method for mapping unsaturated phospholipids including the spatial distribution of lipid C=C location isomers in animal organs by coupling the C=C specific derivatization with direct analysis mass spectrometry (MS). Lipids are sampled directly by a stainless-steel wire from rat brain or kidney, extracted, and derivatized via the Paternò-Büchi reaction in a glass emitter of the nanoelectrospray ionization (nanoESI) source. Subsequent analysis by nanoESI-tandem mass spectrometry reveals C=C locations and relative quantities of lipid C=C location isomers. Unsaturated lipids, such as phospholipids and free fatty acids, have been identified with ion intensities spanning two orders of magnitude in rat brain. Typical sample consumption is less than 10 μg/measurement and the time for each analysis is about 3 min. This method should serve as a complementary method to high spatial resolution mass spectrometry imaging techniques, because it offers a streamlined experimental workflow for rapid profiling of lipids with C=C specificity to enable such applications as point-of-care disease diagnostics.

Rapid In Situ Profiling of Lipid C═C Location Isomers in Tissue Using Ambient Mass Spectrometry with Photochemical Reactions

Rapid and in situ profiling of lipids using ambient mass spectrometry (AMS) techniques has great potential for clinical diagnosis, biological studies, and biomarker discovery. In this study, the online photochemical reaction involving carbon-carbon double bonds was coupled with a surface sampling technique to develop a direct tissue-analysis method with specificity to lipid C═C isomers. This method enabled the in situ analysis of lipids from the surface of various tissues or tissue sections, which allowed the structural characterization of lipid isomers within 2 min. Under optimized reaction conditions, we have established a method for the relative quantitation of lipid C═C location isomers by comparing the abundances of the diagnostic ions arising from each isomer, which has been proven effective through the established linear relationship ( R² = 0.999) between molar ratio and diagnostic ion ratio of the FA 18:1 C═C location isomers. This method was then used for the rapid profiling of unsaturated lipid C═C isomers in the sections of rat brain, lung, liver, spleen, and kidney, as well as in normal and diseased rat tissues. Quantitative information on FA 18:1 and PC 16:0-18:1 C═C isomers was obtained, and significant differences were observed between different samples. To the best of our knowledge, this is the first study to report the direct analysis of lipid C═C isomers in tissues using AMS. Our results demonstrated that this method can serve as a rapid analytical approach for the profiling of unsaturated lipid C═C isomers in biological tissues and should contribute to functional lipidomics and clinical diagnosis.

Triacsin C reduces lipid droplet formation and induces mitochondrial biogenesis in primary rat hepatocytes

Intracellular long-chain acyl-CoA synthetases (ACSL) activate fatty acids to produce acyl-CoA, which undergoes β-oxidation and participates in the synthesis of esterified lipids such as triacylglycerol (TAG). Imbalances in these metabolic routes are closely associated with the pathogenesis of non-alcoholic fatty liver disease (NAFLD). Triacsin C is one of the few compounds that inhibit TAG accumulation into lipid droplets (LD) by suppressing ACSL activity. Here we report that treatment of primary rat hepatocytes with triacsin C at concentrations lower than the IC₅₀ (4.1 μM) for LD formation: (i) diminished LD number in a concentration-dependent manner; (ii) increased mitochondrial amount; (iii) markedly improved mitochondrial metabolism by enhancing the β-oxidation efficiency, electron transport chain capacity, and degree of coupling - treatment of isolated rat liver mitochondria with the same triacsin C concentrations did not affect the last two parameters; (iv) decreased the GSH/GSSG ratio and elevated the protein carbonyl level, which suggested an increased reactive oxygen species production, as observed in isolated mitochondria. The hepatocyte mitochondrial improvements were not related to either the transcriptional levels of PGC-1α or the content of mTOR and phosphorylated AMPK. Triacsin C at 10 μM induced hepatocyte death by necrosis and/or apoptosis through mechanisms associated with mitochondrial permeability transition pore opening, as demonstrated by experiments using isolated mitochondria. Therefore, triacsin C at sub-IC₅₀ concentrations modulates the lipid imbalance by shifting hepatocytes to a more oxidative state and enhancing the fatty acid consumption, which can in turn accelerate lipid oxidation and reverse NAFLD in long-term therapies.

Apolipoprotein CIII Overexpression-Induced Hypertriglyceridemia Increases Nonalcoholic Fatty Liver Disease in Association with Inflammation and Cell Death

Nonalcoholic fatty liver disease (NAFLD) is the principal manifestation of liver disease in obesity and metabolic syndrome. By comparing hypertriglyceridemic transgenic mice expressing apolipoprotein (apo) CIII with control nontransgenic (NTg) littermates, we demonstrated that overexpression of apoCIII, independent of a high-fat diet (HFD), produces NAFLD-like features, including increased liver lipid content; decreased antioxidant power; increased expression of TNFα, TNFα receptor, cleaved caspase-1, and interleukin-1β; decreased expression of adiponectin receptor-2; and increased cell death. This phenotype is aggravated and additional NAFLD features are differentially induced in apoCIII mice fed a HFD. HFD induced glucose intolerance together with increased gluconeogenesis, indicating hepatic insulin resistance. Additionally, the HFD led to marked increases in plasma TNFα (8-fold) and IL-6 (60%) in apoCIII mice. Cell death signaling (Bax/Bcl2), effector (caspase-3), and apoptosis were augmented in apoCIII mice regardless of whether a HFD or a low-fat diet was provided. Fenofibrate treatment reversed several of the effects associated with diet and apoCIII expression but did not normalize inflammatory traits even when liver lipid content was fully corrected. These results indicate that apoCIII and/or hypertriglyceridemia plays a major role in liver inflammation and cell death, which in turn increases susceptibility to and the severity of diet-induced NAFLD.

Long-chain acyl-CoA synthetase 6 regulates lipid synthesis and mitochondrial oxidative capacity in human and rat skeletal muscle

KEY POINTS

Long-chain acyl-CoA synthetase 6 (ACSL6) mRNA is present in human and rat skeletal muscle, and is modulated by nutritional status: exercise and fasting decrease ACSL6 mRNA, whereas acute lipid ingestion increase its expression. ACSL6 genic inhibition in rat primary myotubes decreased lipid accumulation, as well as activated the higher mitochondrial oxidative capacity programme and fatty acid oxidation through the AMPK/PGC1-α pathway. ACSL6 overexpression in human primary myotubes increased phospholipid species and decreased oxidative metabolism.

ABSTRACT

Long-chain acyl-CoA synthetases (ACSL 1 to 6) are key enzymes regulating the partitioning of acyl-CoA species toward different metabolic fates such as lipid synthesis or β-oxidation. Despite our understanding of ecotopic lipid accumulation in skeletal muscle being associated with metabolic diseases such as obesity and type II diabetes, the role of specific ACSL isoforms in lipid synthesis remains unclear. In the present study, we describe for the first time the presence of ACSL6 mRNA in human skeletal muscle and the role that ACSL6 plays in lipid synthesis in both rodent and human skeletal muscle. ACSL6 mRNA was observed to be up-regulated by acute high-fat meal ingestion in both rodents and humans. In rats, we also demonstrated that fasting and chronic aerobic training negatively modulated the ACSL6 mRNA and other genes of lipid synthesis. Similar results were obtained following ACSL6 knockdown in rat myotubes, which was associated with a decreased accumulation of TAGs and lipid droplets. Under the same knockdown condition, we further demonstrate an increase in fatty acid content, p-AMPK, mitochondrial content, mitochondrial respiratory rates and palmitate oxidation. These results were associated with increased PGC-1α, UCP2 and UCP3 mRNA and decreased reactive oxygen species production. In human myotubes, ACSL6 overexpression reduced palmitate oxidation and PGC-1α mRNA. In conclusion, ACSL6 drives acyl-CoA toward lipid synthesis and its downregulation improves mitochondrial biogenesis, respiratory capacity and lipid oxidation. These outcomes are associated with the activation of the AMPK/PGC1-α pathway.

Food quality and authenticity screening via easy ambient sonic-spray ionization mass spectrometry

This review is the first to summarize a decade of studies testing the use of easy ambient sonic-spray ionization mass spectrometry (EASI-MS) and its several sister techniques, Venturi (V-EASI), thermal imprinting (TI-EASI) and Spartan (S-EASI) mass spectrometry in food quality control and authentication. Since minimal or no sample preparation is required, such ambient desorption/ionization techniques have been shown to provide direct, fast and selective fingerprinting characterization at the molecular level based on the pools of the most typical components. They have also been found to be applicable on intact, undisturbed samples or on simple solvent extracts. Fundamentals of EASI-MS and its sister techniques, including mechanisms, devices, parameters and strategies, as well as the many applications reported for food analysis, are summarized and discussed.

Flowing atmospheric pressure afterglow combined with laser ablation for direct analysis of compounds separated by thin-layer chromatography

A thin-layer chromatography-mass spectrometry (TLC-MS) setup for characterization of low molecular weight compounds separated on standard TLC plates has been constructed. This new approach successfully combines TLC separation, laser ablation, and ionization using flowing atmospheric pressure afterglow (FAPA) source. For the laser ablation, a low-priced 445-nm continuous-wave diode laser pointer, with a power of 1 W, was used. The combination of the simple, low-budget laser pointer and the FAPA ion source has made this experimental arrangement broadly available, also for small laboratories. The approach was successfully applied for the characterization of low molecular weight compounds separated on TLC plates, such as a mixture of pyrazole derivatives, alkaloids (nicotine and sparteine), and an extract from a drug tablet consisting of paracetamol, propyphenazone, and caffeine. The laser pointer used was capable of ablating organic compounds without the need of application of any additional substances (matrices, staining, etc.) on the TLC spots. The detection limit of the proposed method was estimated to be 35 ng/cm² of a pyrazole derivative.

Graphical abstract

Desorption/ionization efficiencies of triacylglycerols and phospholipids via EASI-MS

Knowledge of the major effects governing desorption/ionization efficiency is required for the development and application of ambient mass spectrometry. Although all triacylglycerols (TAG) have the same favorable protonation and cationization sites, their desorption/ionization efficiencies can vary dramatically during easy ambient sonic-spray ionization because of structural differences in the carbon chain. To quantify this somewhat surprising and drastic effect, we have performed a systematic investigation of desorption/ionization efficiencies as a function of unsaturation and length for TAG as well as for diacylglycerols, monoacylglycerols and several phospholipids (PL). Affinities for Na(+) as a function of unsaturation level have also been assayed via comprehensive metadynamics calculations to understand the influence of this phenomenon on the ionization efficiency. The results suggest that dipole-dipole interactions within a carbon chain tuned by unsaturation sites govern ionization efficiency of TAG and PL.

Surface analysis of lipids by mass spectrometry: more than just imaging

Mass spectrometry is now an indispensable tool for lipid analysis and is arguably the driving force in the renaissance of lipid research. In its various forms, mass spectrometry is uniquely capable of resolving the extensive compositional and structural diversity of lipids in biological systems. Furthermore, it provides the ability to accurately quantify molecular-level changes in lipid populations associated with changes in metabolism and environment; bringing lipid science to the "omics" age. The recent explosion of mass spectrometry-based surface analysis techniques is fuelling further expansion of the lipidomics field. This is evidenced by the numerous papers published on the subject of mass spectrometric imaging of lipids in recent years. While imaging mass spectrometry provides new and exciting possibilities, it is but one of the many opportunities direct surface analysis offers the lipid researcher. In this review we describe the current state-of-the-art in the direct surface analysis of lipids with a focus on tissue sections, intact cells and thin-layer chromatography substrates. The suitability of these different approaches towards analysis of the major lipid classes along with their current and potential applications in the field of lipid analysis are evaluated.

Brazil nut oil: quality control via triacylglycerol profiles provided by easy ambient sonic-spray ionization mass spectrometry

Brazil nut oil is one of the important Amazonian natural products in the global market. Despite its health benefits and applications in food and cosmetic industries, authentication and quality control of the oil are far from satisfactory. Several samples of Amazonian Brazil nut oil (authentic oils of different geographic origins, commercial oils, and oils adulterated with soybean oil) were evaluated by easy ambient sonic-spray ionization mass spectrometry (EASI-MS). The samples were characterized on the basis of triacylglycerol profiles, and their major ions were subjected to chemometric treatment (principal component analysis). The ambient mass spectrometry analysis and the set of major ions allowed the characterization of authentic oils, commercial oils, and adulterated oils (5% soybean oil) in a few minutes and without sample preparation. Therefore, it is shown that quality control of Amazonian vegetable oils can be readily accomplished by EASI-MS.

Mapping the regioisomeric distribution of fatty acids in triacylglycerols by hybrid mass spectrometry

This study describes the use of hybrid mass spectrometry for the mapping, identification, and semi-quantitation of triacylglycerol regioisomers in fats and oils. The identification was performed based on the accurate mass and fragmentation pattern obtained by data-dependent fragmentation. Quantitation was based on the high-resolution ion chromatograms, and relative proportion of sn-1(3)/sn-2 regioisomers was calculated based on generalized fragmentation models and the relative intensities observed in the product ion spectra. The key performance features of the developed method are inter-batch mass accuracy < 1 ppm (n = 10); lower limit of detection (triggering threshold) 0.1 μg/ml (equivalent to 0.2 weight % in oil); lower limit of quantitation 0.2 μg/ml (equivalent to 0.4 weight % in oil); peak area precision 6.5% at 2 μg/ml concentration and 15% at 0.2 μM concentration; inter-batch precision of fragment intensities < 1% (n = 10) independent of the investigated concentration; and averaged accuracy using the generic calibration 3.8% in the 1-10 μg/ml range and varies between 1-23% depending on analytes. Inter-esterified fat, beef tallow, pork lard, and butter fat samples were used to show how well regioisomeric distribution of palmitic acid can be captured by this method.

Ambient mass spectrometry imaging: a comparison of desorption ionization by sonic spray and electrospray

Easy ambient sonic spray ionization (EASI) and desorption electrospray ionization (DESI) were used for imaging of a number of samples, including sections of rat brain and imprints of plant material on porous Teflon. A novel approach termed Displaced Dual-mode Imaging was utilized for the direct comparison of the two methods: Images were recorded with the individual rows alternating between EASI and DESI, yielding a separate image for each technique recorded under perfectly similar conditions on the same sample. EASI works reliably for imaging of all samples, but the choice of spray solvent and flow rate is more critical in tissue imaging with EASI than with DESI. The overall sensitivity of EASI is, in general, slightly lower than that of DESI, and the representation of the dynamic range is different in images of the two techniques for some samples. However, for abundant compounds, EASI works well, resulting in images of similar quality as DESI. EASI can thus be used in imaging experiments where the application of high voltage is impractical or undesirable. The present study is in its nature also a comparison of the characteristics of the two techniques, showing results also applicable for non-imaging work, with regards to sensitivity and experimental conditions.

Administration of a murine diet supplemented with conjugated linoleic acid increases the expression and activity of hepatic uncoupling proteins

Daily intake of conjugated linoleic acid (CLA) has been shown to reduce body fat accumulation and to increase body metabolism; this latter effect has been often associated with the up-regulation of uncoupling proteins (UCPs). Here we addressed the effects of a CLA-supplemented murine diet (~2 % CLA mixture, cis-9, trans-10 and trans-10, cis-12 isomers; 45 % of each isomer on alternating days) on mitochondrial energetics, UCP2 expression/activity in the liver and other associated morphological and functional parameters, in C57BL/6 mice. Diet supplementation with CLA reduced both lipid accumulation in adipose tissues and triacylglycerol plasma levels, but did not augment hepatic lipid storage. Livers of mice fed a diet supplemented with CLA showed high UCP2 mRNA levels and the isolated hepatic mitochondria showed indications of UCP activity: in the presence of guanosine diphosphate, the higher stimulation of respiration promoted by linoleic acid in mitochondria from the CLA mice was almost completely reduced to the level of the stimulation from the control mice. Despite the increased generation of reactive oxygen species through oxi-reduction reactions involving NAD(+)/NADH in the Krebs cycle, no oxidative stress was observed in the liver. In addition, in the absence of free fatty acids, basal respiration rates and the phosphorylating efficiency of mitochondria were preserved. These results indicate a beneficial and secure dose of CLA for diet supplementation in mice, which induces UCP2 overexpression and UCP activity in mitochondria while preserving the lipid composition and redox state of the liver.