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 (GRd) were normal after 2 h but were reduced by 43% (from 52.7 ± 8.2 to 30.0 ± 5.3 μmol · kg–1 · 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 ∼6 h, PKC activity and DAG mass were increased approximately fourfold, as were the abundance of membrane-associated PKC-βII and -δ. A threefold increase in membrane-associated PKC-βII was also observed at ∼2 h but was not statistically significant (P = 0.058). Ceramide mass was not changed at either time point. To evaluate whether the fatty acid–induced insulin activation of PKC was associated with a change in the IkB kinase (IKK)/nuclear factor (NF)-κB pathway, we determined the abundance in muscle of IκB-α, an inhibitor of NF-κB that is degraded after its phosphorylation by IKK. In parallel with the changes in DAG/PKC, no change in IκB-α mass was observed after 2 h of lipid infusion, but at ∼6 h, IκB-α 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-βII and -δ and a decrease in IκB-α. Whether acute FFA-induced insulin resistance in human skeletal muscle is caused by the activation of these specific PKC isoforms and the IKK-β/IκB/NFκB pathway remains to be established.
Address correspondence and reprint requests to Dr. Guenther Boden, Temple University Hospital, 3401 N. Broad St., Philadelphia, PA 19140. E-mail:.
Received for publication 6 March 2002 and accepted in revised form 6 May 2002. Posted on the World Wide Web at http://www.diabetes.org/diabetes/rapidpubs.shtml on 7 June 2002.
DAG, diacylglycerol; FFA, free fatty acid; GRa, total body glucose appearance; GRd, total body glucose disappearance; IKK, IκB kinase; IMCL-TG, intramyocellular triglyceride; NF, neclear factor; NIH, National Institutes of Health; PI, phosphatidylinositol; PKC, protein kinase C.