Lipid-Induced Insulin Resistance in Human Muscle Is Associated With Changes in Diacylglycerol, Protein Kinase C, and IκB-α

Abstract

The possibility that lipid-induced insulin resistance in human muscle is related to alterations in diacylglycerol (DAG)/protein kinase C (PKC) signaling was investigated in normal volunteers during euglycemic-hyperinsulinemic clamping in which plasma free fatty acid (FFA) levels were increased by a lipid/heparin infusion. In keeping with previous reports, rates of insulin-stimulated glucose disappearance (G Rd ) were normal after 2 h but were reduced by 43% (from 52.7 ؎ 8.2 to 30.0 ؎ 5.3 mol ⅐ kg -1 ⅐ min -1 , P < 0.05) after 6 h of lipid infusion. No changes in PKC activity or DAG mass were seen in muscle biopsy samples after 2 h of lipid infusion; however, at ϳ6 h, PKC activity and DAG mass were increased approximately fourfold, as were the abundance of membrane-associated PKC-␤II and -␦. A threefold increase in membrane-associated PKC-␤II was also observed at ϳ2 h but was not statistically significant (P ‫؍‬ 0.058). Ceramide mass was not changed at either time point. To evaluate whether the fatty acidinduced insulin activation of PKC was associated with a change in the IkB kinase (IKK)/nuclear factor (NF)-B pathway, we determined the abundance in muscle of IB-␣, an inhibitor of NF-B that is degraded after its phosphorylation by IKK. In parallel with the changes in DAG/PKC, no change in IB-␣ mass was observed after 2 h of lipid infusion, but at ϳ6 h, IB-␣ was diminished by 70%. In summary, the results indicated that the insulin resistance observed in human muscle when plasma FFA levels were elevated during euglycemic-hyperinsulinemic clamping was associated with increases in DAG mass and membrane-associated PKC-␤II and -␦ and a decrease in IB-␣. Whether acute FFA-induced insulin resistance in human skeletal muscle is caused by the activation of these specific PKC isoforms and the IKK-␤/IB/NFB pathway remains to be established. Diabetes

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

“…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].…”

“…However, in contrast, we observed similar muscle ceramide content in endurancetrained compared with untrained normal young men [14]. Studies using lipid infusion to decrease insulin sensitivity under a euglycaemic-hyperinsulinaemic clamp have also produced conflicting results: one group observed no change in muscle ceramide levels after 6 h [15], whereas another group found that ceramide levels were increased after 4h [16]. Overall, it is not clear from the literature whether muscle ceramide content plays a major role in skeletal muscle insulin sensitivity.…”

“…This implies that, at least in rodents, the type of fatty acid available, which is dependent upon both dietary consumption and stored fatty acids, may play a very important role in the metabolic events that lead to insulin resistance. Similar data are not available for humans, but there is evidence that lipid infusion using primarily unsaturated fatty acids leads to insulin resistance without a change in muscle ceramide [15,27]. In the present study, we cannot exclude the possibility that the composition of the available fatty acids may have influenced the muscle ceramide concentration and that this may be an important factor for the mechanisms that determine insulin sensitivity.…”

“…During insulin stimulation, plasma fatty acids are markedly decreased through insulin-induced inhibition of adipose tissue lipolysis, and de novo synthesis is therefore very limited as a result of reduced palmitate availability. When plasma NEFA levels were increased by lipid infusion during an euglycaemic-hyperinsulinaemic clamp, muscle ceramide content remained unchanged [15] or was increased [16] in healthy men. This inconsistency between studies may have been due to the somewhat higher plasma NEFA concentrations observed after lipid heparin infusion in the study by Straczkowski [16].…”

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.…”