We showed earlier that nutritional stress like starvation or high fat diet resulted in phenotypic changes in the lipidomes of hepatocyte lipid droplets (LDs), representative for the pathophysiological status of the mouse model. Here we extend our former study by adding genetic stress due to knock-out (KO) of adipocyte triglyceride lipase (ATGL), the rate limiting enzyme in LD lipolysis. An intervention trial for 6 weeks with male wild-type (WT) and ATGL-KO mice was carried out; both genotypes were fed lab chow or were exposed to short-time starvation. Isolated LDs were analyzed by LC-MS/MS. Triacylglycerol, diacylglycerol and phosphatidylcholine lipidomes, in that order, provided best phenotypic signatures characteristic for respective stresses applied to the animals. This was evidenced at lipid species level by principal component analysis, calculation of average values for chain-lengths and numbers of double bonds, and by visualization in heat maps. Structural backgrounds for analyses and metabolic relationships were elaborated at lipid molecular species level. Relating our lipidomic data to non-alcoholic fatty liver diseases of nutritional and genetic etiologies with or without accompanying insulin resistance, phenotypic distinction in hepatocyte LDs dependent on insulin status emerged. Taken together, lipidomes of hepatocyte lipid droplets are sensitive responders to nutritional and genetic stress.

This work aims to combine chromatographic retention, high mass resolution and accuracy, MS/MS spectra, and a package for automated identification and quantitation of lipid species in one platform for lipidomic analysis. The instrumental setup elaborated comprises reversed-phase HPLC coupled to a Fourier transform ion cyclotron resonance mass spectrometer (LTQ-FT), and Lipid Data Analyzer (LDA) software. Data analysis for lipid species quantification in this platform is based on retention time, mass resolution of 200,000, and mass accuracy below 2 ppm. In addition, automatically generated MS/MS spectra provide structural information at molecular level. This LC/MS technology allows analyzing complex biological samples in a quantitative manner as shown here paradigmatically for murine lipid droplets having a huge surplus of triacylglycerol species. Chromatographic pre-separation of the bulk lipid class alleviates the problem of ion suppression of lipid species from other classes. Extension of 1D to 2D chromatography is possible, yet time consuming. The platform affords on unambiguous detection of lipid species as low as 0.1‰ within major lipid classes as low as 0.1‰. Taken together, a novel lipidomic LC/MS platform based on chromatographic retention, high mass resolution and accuracy, MS/MS analysis and quantitation software enables analysis of complex samples as demonstrated for lipid droplets.

Liver steatosis can be induced by fasting or high fat diet. We investigated by lipidomic analysis whether such metabolic states are reflected in the lipidome of hepatocyte lipid droplets (LD) from mice fed normal chow diet (FED), fasted before sacrifice (FAS), or fed a high fat diet (HFD). LC-MS/MS at levels of lipid species profiles and of lipid molecular species uncovered first a FAS phenotype enriched in triacylglycerol (TG) molecular species having very long-chain polyunsaturated fatty acyl (VLC-PUFA) residues and an HFD phenotype with less unsaturated TG species in addition to characteristic marker species. Nutritional stress did not result in dramatic structural alterations in diacylglycerol (DG) and phospholipid (PL) classes. Moreover, molecular species of bulk TG and of DG indicated concomitant de novo TG synthesis and lipase-catalyzed degradation to be active in LD. DG species with VLC-PUFA residues would be preferred precursors for phosphatidylcholine (PC) species, the others for TG molecular species. In addition, molecular species of PL classes fitted the hepatocyte Kennedy and PEMT pathways. We demonstrate that lipidomic analysis of lipid droplets enables phenotyping nutritional stress. TG species are best suited for such phenotyping, whereas structural analysis of TG, DG and PL molecular species furnishes metabolic insights.

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Supplementary files for Scientific Data manuscript Apr 2014

We present here a novel approach for the quantitation of lipids in LC-MS data. The new algorithm obtains its analytical power by two major innovations: (i) a 3D algorithm that confines the peak borders in m/z and time direction and (ii) the use of the theoretical isotopic distribution of an analyte as selection/exclusion criterion. The algorithm is integrated in the Lipid Data Analyzer (LDA) application which additionally provides standardization, a statistics module for results analysis, a batch mode for unattended analysis of several runs and a 3D viewer for the manual verification. The statistics module offers sample grouping, tests between sample groups and export functionalities, where the results are visualized by heat maps and bar charts. The presented algorithm has been applied to data from a controlled experiment and to biological data, containing analytes distributed over an intensity range of 106. Our approach shows improved sensitivity and an extremely high positive predictive value compared with existing methods. Consequently, the novel algorithm, integrated in a user-friendly application, is a valuable improvement in the high-throughput analysis of the lipidome.