Obesity is a major risk factor for the development of insulin resistance and type 2 diabetes. The exact mechanism by which adipose tissue induces insulin resistance is still unclear. It has been demonstrated that obesity is associated with the adipocyte dysfunction, macrophage infiltration, and low-grade inflammation, which probably contributes to the induction of insulin resistance. Adipose tissue synthesizes and secretes numerous bioactive molecules, namely adipokines and cytokines, which affect the metabolism of both lipids and glucose. Disorders in the synthesis of adipokines and cytokines that occur in obesity lead to changes in lipid and carbohydrates metabolism and, as a consequence, may lead to insulin resistance and type 2 diabetes. Obesity is also associated with the accumulation of lipids. A special group of lipids that are able to regulate the activity of intracellular enzymes are biologically active lipids: long-chain acyl-CoAs, ceramides, and diacylglycerols. According to the latest data, the accumulation of these lipids in adipocytes is probably related to the development of insulin resistance. Recent studies indicate that the accumulation of biologically active lipids in adipose tissue may regulate the synthesis/secretion of adipokines and proinflammatory cytokines. Although studies have revealed that inflammation caused by excessive fat accumulation and abnormalities in lipid metabolism can contribute to the development of obesity-related insulin resistance, further research is needed to determine the exact mechanism by which obesity-related insulin resistance is induced.

Influence of fish oil on skeletal muscle mitochondrial energetics and lipid metabolites during high-fat diet.

Resistance to insulin is a pathophysiological state related to the decreased response of peripheral tissues to the insulin action, hyperinsulinemia and raised blood glucose levels caused by increased hepatic glucose outflow. All the above precede the onset of full-blown type 2 diabetes. According to the World Health Organization (WHO), in 2016 more than 1.9 billion people over 18 years of age were overweight and about 600 million were obese. Currently, the primary hypothesis explaining the probability of occurrence of insulin resistance assigns a fundamental role of lipids accumulation in adipocytes or nonadipose tissue (muscle, liver) and the locally developing chronic inflammation caused by adipocytes hypertrophy. However, the major molecular pathways are unknown. The sphingolipid ceramide is the main culprit that combines a plethora of nutrients (e.g., saturated fatty acids) and inflammatory cytokines (e.g., TNFα) to the progression of insulin resistance. The accumulation of sphingolipid ceramide in tissues of obese humans, rodents and Western-diet non-human primates is in line with diabetes, hypertension, cardiac failure or atherosclerosis. In hypertrophied adipose tissue, after adipocytes excel their storage capacity, neutral lipids begin to accumulate in nonadipose tissues, inducing organ dysfunction. Furthermore, obesity is closely related to the development of chronic inflammation and the release of cytokines directly from adipocytes or from macrophages that infiltrate adipose tissue. Enzymes taking part in ceramide metabolism are potential therapeutic targets to manipulate sphingolipids content in tissues, either by inhibition of their synthesis or through stimulation of ceramides degradation. In this review, we will evaluate the mechanisms responsible for the development of insulin resistance and possible therapeutic perspectives.

rates of FFA release under basal ( 5 ) and postprandial ( 6 ) conditions, are characteristic features of obesity. Plasma FFA concentrations are an imperfect indicator of FFA kinetics, as evidenced by the greater lipolysis rates in women than men at comparable FFA concentrations ( 7 ) and the divergence of FFA concentrations and fl ux during exercise ( 8, 9 ). Robust isotope dilution techniques have been developed to measure FFA kinetics, including radiotracer methods using high-performance liquid chromatography (HPLC) ( 10 ), gas chromatography-mass spectrometry (GC/MS) ( 11 ), and gas chromatography-combustionisotope ratio mass spectrometry (GC/C/IRMS) ( 12 ).Each of these methods requires an isolation and derivatization step followed by a relatively time-consuming chromatography separation, potentially limiting sample throughput. To improve effi ciency, we developed a new method for simultaneous measurement of concentration and stable isotopic enrichment of plasma FFA. Our method uses HPLC electrospray ionization (ESI) quadrupole mass spectrometry in the selected ion monitoring (SIM) mode. The method is simple and reliable for monitoring changes in plasma FFA concentration and enrichment. By using HPLC for the separation, we can avoid the derivatization step, allowing more rapid sample processing. In addition, by employing a tandem HPLC injection system, we are able to obtain sample data every 5 min. The results compare favorably with the GC/C/IRMS ( 12 ) approach for measuring palmitate fl ux using ultra-low tracer infusion rates. MATERIALS AND METHODS SuppliesPalmitic acid, sodium palmitate, sodium heptadecanoate, essentially fatty acid free albumin, potassium phosphate, potassium biphosphate, heptane, 2.0 M (trimethylsilyl)diazomethane solution, sulfuric acid, and ammonium acetate were purchased form Sigma-Aldrich Chemicals (St. Louis, MO). [ 13 C 16 ]palmitic acid was supplied by Cambridge Isotope Laboratories (Andover, MA). HPLC-grade acetonitrile, isooctane, isopropanol, methanol, and water were obtained from Burdick and Jackson Chemicals Abstract Measurements of plasma free fatty acids (FFA) concentration and isotopic enrichment are commonly used to evaluate FFA metabolism. Until now, gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/ IRMS) was the best method to measure isotopic enrichment in the methyl derivatives of 13 C-labeled fatty acids. Although IRMS is excellent for analyzing enrichment, it requires time-consuming derivatization steps and is not optimal for measuring FFA concentrations. We developed a new, rapid, and reliable method for simultaneous quantifi cation of 13 C-labeled fatty acids in plasma using high-performance liquid chromatography-mass spectrometry (HPLC/MS). This method involves a very quick Dole extraction procedure and direct injection of the samples on the HPLC system. After chromatographic separation, the samples are directed to the mass spectrometer for electrospray ionization (ESI) and analysis in the negative mode using single ion monitoring. By employi...

Background/Aims: Muscle bioactive lipids accumulation leads to several disorder states. The most common are insulin resistance (IR) and type 2 diabetes. There is an ongoing debate which of the lipid species plays the major role in induction of muscle IR. Our aim was to elucidate the role of particular lipid group in induction of muscle IR. Methods: The analyses were performed on muscle from the following groups of rats: 1. Control, fed standard diet, 2 HFD, fed high fat diet, 3. HFD/Myr, fed HFD and treated with myriocin (Myr), an inhibitor of ceramide de novo synthesis. We utilized [U¹³C] palmitate isotope tracer infusion and mass spectrometry to measure content and synthesis rate of muscle long-chain acyl-CoA (LCACoA), diacylglycerols (DAG) and ceramide (Cer). Results: HFD led to intramuscular accumulation of LCACoA, DAG and Cer and skeletal muscle IR. Myr-treatment caused decrease in Cer (most noticeable for stearoyl-Cer and oleoyl-Cer) and accumulation of DAG, possibly due to re-channeling of excess of intramuscular LCACoA towards DAG synthesis. An improvement in insulin sensitivity at both systemic and muscular level coincided with decrease in ceramide, despite elevated intramuscular DAG. Conclusion: The improved insulin sensitivity was associated with decreased muscle stearoyl- and oleoyl-ceramide content. The results indicate that accumulation of those ceramide species has the greatest impact on skeletal muscle insulin sensitivity in rats.