Hyperinsulinemia and skeletal muscle fatty acid trafficking

Endothelin-1 in the pathophysiology of obesity and insulin resistance

The association between plasma endothelin-1 (ET-1) and obesity has been documented for decades, yet the contribution of ET-1 to risk factors associated with obesity is not fully understood. In 1994, one of first papers to document this association also noted a positive correlation between plasma insulin and ET-1, suggesting a potential contribution of ET-1 to the development of insulin resistance. Both endogenous receptors for ET-1, ETA and ETB are present in all insulin-sensitive tissues including adipose, liver and muscle, and ET-1 actions within these tissues suggest that ET-1 may be playing a role in the pathogenesis of insulin resistance. Further, antagonists for ET-1 receptors are clinically approved making these sites attractive therapeutic targets. This review focuses on known mechanisms through which ET-1 affects plasma lipid profiles and insulin signalling in these metabolically important tissues and also identifies gaps in our understanding of ET-1 in obesity-related pathophysiology.

Effects of meal ingestion on intramyocellular ceramide concentrations and fractional de novo synthesis in humans

We investigated the effects of meal ingestion on intramyofibrillar (IMF) and subsarcolemmal (SS) ceramide metabolism in volunteers ranging from lean to obese. Thirty-eight women and men underwent a steady-state meal ingestion protocol that included a 6.5-h infusion of [U-¹³C]palmitate and muscle biopsies 1.5 and 6.5 h after starting the tracer infusion. We measured IMF and SS sphingolipid concentrations and the contribution of plasma palmitate to intramyocellular C16:0 ceramide by use of LC-MS-MS. In response to meal ingestion SS C24 ceramide concentrations, but not C14-C20 concentrations, increased significantly. IMF ceramide concentrations did not change. The increases in SS C24 ceramides were negatively related to parameters of insulin resistance. The fractional contribution of plasma palmitate to intramyocellular C16:0 ceramides in both IMF and SS fractions was inversely related to overweight status (β = -0.432, P = 0.0095 and β = -0.443, P = 0.0058, respectively). These data indicate that meal ingestion has differing effects on SS ceramide subspecies and suggest that the fractional de novo synthesis of intramyocellular ceramide from plasma palmitate in the postprandial condition is reduced in those who are overweight.

Diacylglycerol acyltransferase 2 links glucose utilization to fatty acid oxidation in the brown adipocytes

Brown adipose tissue uptake of glucose and fatty acids is very high during nonshivering thermogenesis. Adrenergic stimulation markedly increases glucose uptake, de novo lipogenesis, and FA oxidation simultaneously. The mechanism that enables this concerted response has hitherto been unknown. Here, we find that in primary brown adipocytes and brown adipocyte-derived cell line (IMBAT-1), acute inhibition and longer-term knockdown of DGAT2 links the increased de novo synthesis of fatty acids from glucose to a pool of TAG that is simultaneously hydrolyzed, providing FA for mitochondrial oxidation. DGAT1 does not contribute to this pathway, but uses exogenous FA and glycerol to synthesize a functionally distinct pool of TAG to which DGAT2 also contributes. The DGAT2-dependent channelling of 14C from glucose into TAG and CO2 was reproduced in β3-agonist-stimulated primary brown adipocytes. Knockdown of DGAT2 in IMBAT-1 affected the mRNA levels of UCP1 and genes important in FA activation and esterification. Therefore, in β3-agonist activated brown adipocytes, DGAT2 specifically enables channelling of de novo synthesized FA into a rapidly mobilized pool of TAG, which is simultaneously hydrolyzed to provide substrates for mitochondrial fatty acid oxidation.

Palmitoyl-carnitine production by blood cells associates with the concentration of circulating acyl-carnitines in healthy overweight women

BACKGROUND

Circulating acyl-carnitines (acyl-CNTs) are associated with insulin resistance (IR) and type 2 diabetes (T2D) in both rodents and humans. However, the mechanisms whereby circulating acyl-CNTs are increased in these conditions and their role in whole-body metabolism remains unknown. The purpose of this study was to determine if, in humans, blood cells contribute in production of circulating acyl-CNTs and associate with whole-body fat metabolism.

METHODS AND RESULTS

Eight non-diabetic healthy women (age: 47 ± 19 y; BMI: 26 ± 1 kg·m^-2) underwent stable isotope tracer infusion and hyperinsulinemic-euglycemic clamp study to determine in vivo whole-body fatty acid flux and insulin sensitivity. Blood samples collected at baseline (0 min) and after 3 h of clamp were used to determine the synthesis rate of palmitoyl-carnitine (palmitoyl-CNT) in vitro. The fractional synthesis rate of palmitoyl-CNT was significantly higher during hyperinsulinemia (0.788 ± 0.084 vs. 0.318 ± 0.012%·hr^-1, p = 0.001); however, the absolute synthesis rate (ASR) did not differ between the periods (p = 0.809) due to ∼30% decrease in blood palmitoyl-CNT concentration (p = 0.189) during hyperinsulinemia. The ASR of palmitoyl-CNT significantly correlated with the concentration of acyl-CNTs in basal (r = 0.992, p < 0.001) and insulin (r = 0.919, p = 0.001) periods; and the basal ASR significantly correlated with plasma palmitate oxidation (r = 0.764, p = 0.027).

CONCLUSION

In women, blood cells contribute to plasma acyl-CNT levels and the acyl-CNT production is linked to plasma palmitate oxidation, a marker of whole-body fat metabolism. Future studies are needed to confirm the role of blood cells in acyl-CNT and lipid metabolism under different physiological (i.e., in response to meal) and pathological (i.e., hyperlipidemia, IR and T2D) conditions.

PALMITIC AND OLEIC ACIDS AND THEIR ROLE IN PATHOGENESIS OF ATHEROSCLEROSIS

On the basis of phylogenetic theory of general pathology, the cause of a noninfectious disease whose occurrence in a population is more than 5–7% is an impaired biological function or reaction to the environment. From the general biology viewpoint, high mortality rate related to cardio-vascular diseases and atherosclerosis (intercellular deficiency of polyenic fatty acids (PFA)) is just extinction of the Homo sapiens population upon adaptation to new environmental factors. The biological function of throphology (feeding) and biological reaction of exotrophy (external feeding) are impaired in several aspects, the major of which is nonphysiologically high dietary content of saturated fatty acids, primarily, of palmitic fatty acid (FA). The lipoprotein system formed at early stages of phylogenesis cannot transport and provide physiological deposition of great amounts of palmitic FA, which leads to the development of an adaption (compensatory) and accumulation disease. This results in hypermipidemia, impaired bioavailability of PFA to cells, compesatory production of humoral mediators from ω-9 eicosatrienoic mead FA, disorders in physiological parameters of cell plasma membrane and integral proteins, nonphysiological conformation of apoВ-100 in lipoproteins, formation of ligandless lipoproteins (biological litter) and impairments in the biological function of endoecology, utilization of ligandless lipoproteins in arterial intima by phylogenetically early macrophages that do not hydrolyze polyenic cholesterol esters, increase in the intensity of the biological reaction of inflammation, and destructive and inflammatory lesions in arterial intima of an atheromatosis or atherothrombosis type. Atheromatous masses are catabolites of PFA which were not internalized by phylogenetically late cells via receptor-mediated pathway.

Kinetics and utilization of lipid sources during acute exercise and acipimox

Overweight is associated with abnormalities of lipid metabolism, many of which are reversed by exercise. We investigated the impact of experimental antilipolysis and acute exercise on lipid kinetics and oxidation from VLDL-TG, plasma FFA, and "residual lipids" in overweight men (n = 8) using VLDL-TG and palmitate tracers in combination with muscle biopsies in a randomized, placebo-controlled design. Participants received placebo or acipimox on each study day (4 h of rest, 90 min of exercise at 50% V(O(2 max))). Exercise suppressed VLDL-TG secretion significantly during placebo but not acipimox (placebo-rest: 64.2 ± 9.4; placebo-exercise: 48.3 ± 8.0; acipimox-rest: 55.2 ± 13.4; acipimox-exercise: 52.0 ± 10.9). Resting oxidation of VLDL-TG FA and FFA was significantly reduced during acipimox compared with placebo, whereas "residual lipid oxidation" increased significantly [VLDL-TG oxidation (placebo: 18 ± 3 kcal/h; acipimox: 11 ± 2 kcal/h), FFA oxidation (placebo: 14 ± 2 kcal/h; acipimox: 4 ± 0.5 kcal/h), and residual lipid oxidation (placebo: 3 ± 5 kcal/h; acipimox: 14 ± 5 kcal/h)]. Additionally, during exercise on both placebo and acipimox, oxidation of VLDL-TG and FFA increased, but the relative contribution to total lipid oxidation diminished, except for FFA, which remained unchanged during acipimox. Residual lipid oxidation increased significantly during exercise in both absolute and relative terms. Changes in selected cellular enzymes and proteins provided no explanations for kinetic changes. In conclusion, suppressed FFA availability blunts the effect of exercise on VLDL-TG secretion and modifies the contribution of lipid sources for oxidation.

Training status diverges muscle diacylglycerol accumulation during free fatty acid elevation

How endurance training alters muscle lipid metabolism while preserving insulin sensitivity remains unclear. Because acute free fatty acid (FFA) elevation by lipid infusion reduces insulin sensitivity, we hypothesized that training status would alter accumulation of muscle triacylglycerol (TAG), diacylglycerol (DAG), ceramide, and acylcarnitine during acute FFA elevation. Trained (n = 15) and sedentary (n = 13) participants matched for age, sex, and BMI received either a 6-h infusion of lipid (20% Intralipid at 90 ml/h) or glycerol (2.25 g/100 ml at 90 ml/h) during a hyperinsulinemic euglycemic clamp. Muscle biopsies were taken at 0, 120, and 360 min after infusion initiation to measure intramyocellular concentrations of TAG, DAG, ceramides, and acylcarnitines by liquid chromatography-tandem mass spectrometry. Trained participants had a higher Vo2 max and insulin sensitivity than sedentary participants. The lipid infusion produced a comparable elevation of FFA (594 ± 90 μmol/l in trained, 721 ± 30 μmol/l in sedentary, P = 0.4) and a decline in insulin sensitivity (-44.7% trained vs. -47.2% sedentary, P = 0.89). In both groups, lipid infusion increased the linoleic and linolenic acid content of TAG without changing total TAG. In the sedentary group, lipid infusion increased total, oleic, and linoleic acid and linolenic acid content of DAG. Regardless of training status, lipid infusion did not alter total ceramide, saturated ceramide, palmitoyl-carnitine, or oleoyl-carnitine. We conclude that during acute FFA elevation, trained adults have a similar decline in insulin sensitivity with less accumulation of muscle DAG than sedentary adults, suggesting that lipid-induced insulin resistance can occur without elevation of total muscle DAG.

Fatty acid sources and their fluxes as they contribute to plasma triglyceride concentrations and fatty liver in humans

PURPOSE OF REVIEW

Different sources of fatty acids (FA) used for VLDL-triglyceride synthesis include dietary FA that clear to the liver via chylomicron uptake, FA synthesized de novo in the liver from carbohydrates, nonesterified fatty acids derived from adipose tissue, nonesterified fatty acids derived from the spillover of chylomicron-triglyceride in the fasted and fed states, and FA stored in liver lipid droplets.

RECENT FINDINGS

Data have amassed on the contributions of each of these sources to liver-triglyceride accrual, VLDL-triglyceride synthesis, and hypertriglyceridemia. Discussed here is the timing of use of FA from each of these sources for synthesis of VLDL-triglyceride. Secondly, as all of these FA sources have been shown to contribute significantly to nonalcoholic fatty liver disease (NAFLD), data are presented demonstrating how poor handling of FA and glucose in the periphery can contribute to NAFLD. Lastly, we highlight how the stress of excess FA availability on the liver can be corrected by reduction of dietary intake of sugars and fats, weight loss, and increased physical activity.

SUMMARY

A better understanding of how lifestyle factors improve FA flux will aid in the development of improved treatments for the devastating condition of NAFLD.