In this era of genomics, transcriptomics, and proteomics, metabolomics is emerging as an important component of the omics evolution ( 1 ). Of the four kinds of biological molecules that comprise the human body, i.e., nucleic acids, amino acids (proteins), carbohydrates (sugars), and lipids (fats), lipids stand out among the various cellular metabolites in the sheer number of distinct molecular species. Using state-of-the-art lipidomics approaches made possible by newly developed instrumentation, protocols, and bioinformatics tools ( 2 ), the LIPID MAPS Consortium Abstract The focus of the present study was to defi ne the human plasma lipidome and to establish novel analytical methodologies to quantify the large spectrum of plasma lipids. Partial lipid analysis is now a regular part of every patient's blood test and physicians readily and regularly prescribe drugs that alter the levels of major plasma lipids such as cholesterol and triglycerides. Plasma contains many thousands of distinct lipid molecular species that fall into six main categories including fatty acyls, glycerolipids, glycerophospholipids, sphingolipids, sterols, and prenols. The physiological contributions of these diverse lipids and how their levels change in response to therapy remain largely unknown. As a fi rst step toward answering these questions, we provide herein an in-depth lipidomics analysis of a pooled human plasma obtained from healthy individuals after overnight fasting and with a gender balance and an ethnic distribution that is representative of the US population. In total, we quantitatively assessed the levels of over 500 distinct molecular species distributed among the main lipid categories. As more information is obtained regarding the roles of individual lipids in health and disease, it seems likely that future blood tests will include an ever increasing number of these lipid molecules. -Quehenberger, O., A.

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Increased intramyocellular lipid concentrations are thought to play a role in insulin resistance, but the precise nature of the lipid species that produce insulin resistance in human muscle are unknown. Ceramides, either generated via activation of sphingomyelinase or produced by de novo synthesis, induce insulin resistance in cultured cells by inhibitory effects on insulin signaling. The present study was undertaken to determine whether ceramides or other sphingolipids are increased in muscle from obese insulin-resistant subjects and to assess whether ceramide plays a role in the insulin resistance of Akt in human muscle. Lean insulinsensitive and obese insulin-resistant subjects (n ‫؍‬ 10 each) received euglycemic-hyperinsulinemic clamps with muscle biopsies basally and after 30, 45, or 60 min of insulin infusion. The rate of glucose infusion required to maintain euglycemia (reflecting glucose uptake) was reduced by >50%, as expected, in the obese subjects at each time point (P < 0.01). Under basal conditions, total muscle ceramide content was increased nearly twofold in the obese subjects (46 ؎ 9 vs. 25 ؎ 2 pmol/2 mg muscle, P < 0.05). All species of ceramides were increased similarly in the obese subjects; in contrast, no other sphingolipid was increased. Stimulation of Akt phosphorylation by insulin in the obese subjects was significantly reduced after 30 min (0.96 ؎ 0.11 vs. 1.84 ؎ 0.38 arbitrary units) or 45-60 min (0.68 ؎ 0.17 vs. 1.52 ؎ 0.26) of insulin infusion (P < 0.05 for both). Muscle ceramide content was significantly correlated with the plasma free fatty acid concentration (r ‫؍‬ 0.51, P < 0.05). We conclude that obesity is associated with increased intramyocellular ceramide content. This twofold increase in ceramide may be involved in the decrease in Akt phosphorylation observed after insulin infusion and could theoretically play a role in the reduced ability of insulin to stimulate glucose uptake in skeletal muscle from obese subjects.

Sphingolipids are comprised of a backbone sphingoid base that may be phosphorylated, acylated, glycosylated, bridged to various headgroups through phosphodiester linkages, or otherwise modified. Organisms usually contain large numbers of sphingolipid subspecies and knowledge about the types and amounts is imperative because they influence membrane structure, interactions with the extracellular matrix and neighboring cells, vesicular traffic and the formation of specialized structures such as phagosomes and autophagosomes, as well as participate in intracellular and extracellular signaling. Fortunately, "sphingolipidomic" analysis is becoming feasible (at least for important subsets such as all of the backbone "signaling" subspecies: ceramides, ceramide 1-phosphates, sphingoid bases, sphingoid base 1-phosphates, inter alia) using mass spectrometry, and these profiles are revealing many surprises, such as that under certain conditions cells contain significant amounts of "unusual" species: N-mono-, di-, and tri-methyl-sphingoid bases (including N,N-dimethylsphingosine); 3-ketodihydroceramides; N-acetyl-sphingoid bases (C2-ceramides); and dihydroceramides, in the latter case, in very high proportions when cells are treated with the anticancer drug fenretinide (4-hydroxyphenylretinamide). The elevation of DHceramides by fenretinide is befuddling because the 4,5-trans-double bond of ceramide has been thought to be required for biological activity; however, DHceramides induce autophagy and may be important in the regulation of this important cellular process. The complexity of the sphingolipidome is hard to imagine, but one hopes that, when partnered with other systems biology approaches, the causes and consequences of the complexity will explain how these intriguing compounds are involved in almost every aspect of cell behavior and the malfunctions of many diseases.

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