Studies of lipid turnover in cells with stable isotope and gas chromatograph-mass spectrometry

Relationship between hepatic and mitochondrial ceramides: A novel in vivo method to track ceramide synthesis

Ceramides (CERs) are key intermediate sphingolipids implicated in contributing to mitochondrial dysfunction and the development of multiple metabolic conditions. Despite the growing evidence of CERs role in disease risk, kinetic methods to measure CER turnover are lacking, particularly using in vivo models. The utility of orally-administered ¹³C₃, ¹⁵N L-serine, dissolved in drinking water, was tested to quantify CER 18:1/16:0 synthesis in 10 week-old male and female C57Bl/6 mice. To generate isotopic labeling curves, animals consumed either a control (CD) or high fat diet (HFD; n=24/diet) for two weeks and varied in the duration of the consumption of serine-labeled water (0, 1, 2, 4, 7, or 12 days; n=four animals/day/diet). Unlabeled and labeled hepatic and mitochondrial CERs were quantified using liquid chromatography tandem mass spectrometry. Total hepatic CER content did not differ between the two diet groups while total mitochondrial CERs increased with HFD feeding (60%, P<0.001). Within hepatic and mitochondrial pools, HFD induced greater saturated CER concentrations (P<0.05) and significantly elevated absolute turnover of 16:0 mitochondrial CER (mitochondria: 59%, P<0.001 versus liver: 15%, P=0.256). The data suggest cellular redistribution of CERs due to the HFD. These data demonstrate that a two-week HFD alters the turnover and content of mitochondrial CERs. Given the growing data on CERs contributing to hepatic mitochondrial dysfunction and the progression of multiple metabolic diseases, this method may now be used to investigate how CER turnover is altered in these conditions.

Quantifying ceramide kinetics in vivo using stable isotope tracers and LC-MS/MS

Numerous studies have implicated dyslipidemia as a key factor in mediating insulin resistance. Ceramides have received special attention since their levels are inversely associated with normal insulin signaling and positively associated with factors that are involved in cardiometabolic disease. Despite the growing literature surrounding ceramide biology, there are limited data regarding the activity of ceramide synthesis and turnover in vivo. Herein, we demonstrate the ability to measure ceramide kinetics by coupling the administration of [²H]water with LC-MS/MS analyses. As a "proof-of-concept" we determined the effect of a diet-induced alteration on ceramide flux; studies also examined the effect of myriocin (a known inhibitor of serine palmitoyltransferase, the first step in sphingosine biosynthesis). Our data suggest that one can estimate ceramide synthesis and draw conclusions regarding the source of fatty acids; we discuss caveats in regards to method development in this area.

Application of stable isotopes to investigate the metabolism of fatty acids, glycerophospholipid and sphingolipid species

Nature provides an enormous diversity of lipid molecules that originate from various pathways. To gain insight into the metabolism and dynamics of lipid species, the application of stable isotope-labeled tracers combined with mass spectrometric analysis represents a perfect tool. This review provides an overview of strategies to track fatty acid, glycerophospholipid, and sphingolipid metabolism. In particular, the selection of stable isotope-labeled precursors and their mass spectrometric analysis is discussed. Furthermore, examples of metabolic studies that were performed in cell culture, animal and clinical experiments are presented.

Factors to consider in using [U-C]palmitate for analysis of sphingolipid biosynthesis by tandem mass spectrometry

This study describes the use of a stable-isotope labeled precursor ([U-¹³C]palmitate) to analyze de novo sphingolipid biosynthesis by tandem mass spectrometry. It also describes factors to consider in interpreting the data, including the isotope's location (¹³C appears in three isotopomers and isotopologues: [M + 16] for the sphingoid base or N-acyl fatty acid, and [M + 32] for both); the isotopic enrichment of palmitoyl-CoA; and its elongation, desaturation, and incorporation into N-acyl-sphingolipids. For HEK293 cells incubated with 0.1 mM [U-¹³C]palmitic acid, ∼60% of the total palmitoyl-CoA was ¹³C-labeled by 3 h (which was near isotopic equilibrium); with this correction, the rates of de novo biosynthesis of C16:0-ceramide, C16:0-monohexosylceramide, and C16:0-sphingomyelins were 62 ± 3, 13 ± 2, and 60 ± 11 pmol/h per mg protein, respectively, which are consistent with an estimated rate of appearance of C16:0-ceramide using exponential growth modeling (119 ± 11 pmol/h per mg protein). Including estimates for the very long-chain fatty acyl-CoAs, the overall rate of sphingolipid biosynthesis can be estimated to be at least ∼1.6-fold higher. Thus, consideration of these factors gives a more accurate picture of de novo sphingolipid biosynthesis than has been possible to-date, while acknowledging that there are inherent limitations to such approximations.

Sphingolipidomics: a valuable tool for understanding the roles of sphingolipids in biology and disease

The sphingolipidome is the portion of the lipidome that encompasses all sphingoid bases and their derivatives. Whereas the most studied sphingoid base is sphingosine [(2S,3R,4E)-2-aminooctadecene-1,3-diol], mammals have dozens of structural variants, and hundreds of additional types have been found in other eukaryotic organisms and some bacteria and viruses. Multiplying these figures by the N-acyl-derivatives ("ceramides") and the more than 500 phospho- and glyco- headgroups places the number of discrete molecular species in the tens of thousands or higher. Structure-specific, quantitative information about a growing fraction of the sphingolipidome can now be obtained using various types of chromatography coupled with tandem mass spectrometry, and application of these methods is producing many surprises regarding sphingolipid structure, metabolism, and function. Such large data sets can be difficult to interpret, but the development of tools that display results from genomic and lipidomic studies in a pathway relational, nodal, context can make it easier for investigators to deal with this complexity.

(Glyco)sphingolipidology: an amazing challenge and opportunity for systems biology

Sphingolipids are found in essentially all eukaryotes and in some prokaryotes and viruses, where they influence cell structure, signaling and interactions with the extracellular environment. Because of the combinatorial nature of their biosynthesis, the sphingolipidome comprises untold thousands of species that encompass bioactive backbones and complex phospho- and glycolipids. Mass spectrometry is able to analyze a growing fraction of the sphingolipidome and is beginning to provide information about localization. Use of these structure specific, quantitative methods is producing insights, and surprises, regarding sphingolipid structure, metabolism, function and disease. Dealing with such large data sets poses special challenges for systems biology, but the intrinsic and elegant interrelationships among these compounds might provide a key to dealing with the complexity of the sphingolipidome.

Mass spectrometry determination of endonuclear phospholipid composition and dynamics

Mammalian cell lipid analyses using tandem electrospray ionization mass spectrometry, in conjunction with stable isotope labeling, permit unparalleled access to membrane phospholipid molecular species compositions and turnover. Lipidomic data from isolable compartments of lipid second messenger generation, such as membrane-free nuclei, can provide dynamic insights into the topology of phospholipid turnover. For example, ESI-MS/MS precursor scans of characteristic phosphocholine m/z 184(+) fragments reveal a highly saturated endonuclear phosphatidylcholine pool with homeostatic maintenance properties. A spatially distinct CDPcholine pathway yields, within minutes of choline-d(9) labeling, unsaturated endonuclear phosphatidylcholines progressively remodeled to more saturated species evidenced by tracking the deuteriated headgroup through precursor scans of phosphocholine-d(9) (m/z 193(+) fragment). Among the other endonuclear phospholipids, diacyl phosphatidylethanolamines (neutral loss of m/z 141(+)) are also highly saturated compared with those of whole cell whereas, phophatidylinositols (precursor scans of m/z 241(-) fragment) are essentially identical in nuclei and whole cells. Moreover, the pattern of myo-inositol-d(6) acquisition into endonuclear phosphatidylinositol (precursor scans of m/z 247(-) fragment) is inconsistent with compartment-specific synthesis. Endonuclear sphingomyelins (seen in precursor scans of m/z 184(+) and confirmed from precursor scans of m/z 168(-) fragments) are enriched but similar in composition to whole cell species whereas endonuclear phosphatidylserines (neutral loss of m/z 87(-)) are more saturated than their whole cell counterparts. The focus of described methodologies emphasize their value in probing the compositions and dynamics of endonuclear phospholipids, but in principle may be extended to exploration of other isolable compartments including ER or plasma membranes.

Ceramide metabolite, not intact ceramide molecule, may be responsible for cellular toxicity

Ceramides, which are produced from the hydrolysis of sphingomyelin or synthesized from serine and palmitate in a de novo pathway, are regarded as important cellular signals for inducing apoptosis. However, controversy over this proposed role of ceramides exists. Using stable isotope labelling coupled with GC (gas chromatography)-MS and mass isotopomer distribution analysis, we have studied the metabolism of exogenous long-chain ceramides in HL60 cells. Our results do not support the concept of enhanced ceramide transport into cells induced by solvent mixtures of ethanol and hydrocarbons. In addition, cell toxicity does not correlate with the amount of intact ceramide in the cells. Our results are more consistent with a disturbance of sphingomyelin metabolism induced by the solvent mixture. The characteristics of this disturbed sphingolipid disposition are the inhibition of dihydroceramide desaturation and an enhanced degradation of sphingomyelin. As a consequence, dihydroceramides accumulate and the cellular sphingomyelin content decreases. Inhibition of these pathways is most likely to be induced by the increased production of novel ceramide metabolites instead of by intact ceramides. Octadecane-1,2-diol is identified as a possible mediator. Treatments that divert ceramide degradation to the novel pathway are potential strategies in cancer therapy for inducing cell toxicity.

Fatty acyl CoA-mediated inhibition of endoplasmic reticulum assembly

The protein machinery that mediates homotypic fusion of mammalian endoplasmic reticulum (ER) membranes is becoming increasing well defined. However, little is known of how acylation of constituent membrane components might impact upon this event. This is particularly important as acylation has been shown to promote both fusion and fission of heterotypic membranes. Using a previously characterised cell-free ER fusion assay, I show here that incubation of membranes in the presence of either palmitoyl CoA or myristoyl CoA potently inhibits assembly. Furthermore, inhibition does not occur when membranes are incubated in the constituent palmitate or CoA moieties alone. These findings suggest that not only do palmitoyl CoA and myristoyl CoA inhibit ER assembly, but that they might instead be functioning to actively facilitate ER membrane fission.