Ceramide Content Is Increased in Skeletal Muscle From Obese Insulin-Resistant Humans

Abstract

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.

Human skeletal muscle ceramide content is not a major factor in muscle insulin sensitivity

“…This observation is consistent with our results in a previous study on young male untrained and endurance-trained participants, presumably having different insulin sensitivities, in which muscle ceramide content was similar in the resting overnight fasted condition [14]. As mentioned above, our finding is in contrast to two studies that found higher muscle ceramide content in young obese insulinresistant participants compared with lean controls [11,12]. This discrepancy could be explained by the presence of an increased muscle ceramide concentration in obese individuals.…”

“…Effect of insulin on muscle ceramide content In the present study, muscle ceramide content was not influenced by stimulation with low or high insulin levels during the clamp, which is consistent with prior observations showing no effect of insulin on muscle ceramide content in human skeletal muscle [11,15,16]. Muscle ceramides are generated primarily through de novo synthesis from palmitate and serine, or through the breakdown of sphingomyelin in the membranes [22,28,29].…”

“…It is not clear whether the generation of ceramide from sphingomyelin degradation in human skeletal muscle is influenced by insulin infusion and/or the ceramide degradation is influenced by insulin. However, the presence of unchanged muscle ceramide content in this and prior studies [11,15] and the unchanged muscle ceramide fatty acid composition in the present study suggest that this is not the case.…”

“…Two studies have demonstrated higher muscle ceramide content in obese insulin-resistant individuals compared with lean insulin-sensitive individuals [11,12], and a recent study found that 8weeks of training reduced muscle ceramide content in young obese insulin-resistant individuals and improved glucose tolerance [13]. However, in contrast, we observed similar muscle ceramide content in endurancetrained compared with untrained normal young men [14].…”

Inhibition of adipose tissue lipolysis increases intramuscular lipid use in type 2 diabetic patients

“…More recent insights from various lipid infusion studies have led to an alternative mechanism [9][10][11][12][13]. The latter suggests that elevated NEFA delivery and/or impaired FA oxidation result in intramyocellular accumulation of triacylglycerol (TG) and FA metabolites (such as fatty acyl-CoA, ceramides and diacylglycerol), which induce defects in the insulin signalling cascade, causing skeletal muscle insulin resistance [9,10,[13][14][15][16][17]. Insulin resistance can subsequently lead to development of the hyperglycaemic and/or hyperinsulinaemic state that is associated with type 2 diabetes.…”