title: Reduced insulin-mediated citrate synthase activity in cultured skeletal muscle cells from patients with type 2 diabetes: evidence for an intrinsic oxidative enzyme defect.
abstract: In myotubes established from patients with type 2 diabetes (T2D), lipid oxidation and insulin-mediated glucose oxidation are reduced, whereas in myotubes from obese non-diabetic subjects, exposure to palmitate impairs insulin-mediated glucose oxidation. To determine the underlying mechanisms of these metabolic malfunctions, we studied mitochondrial respiration, uncoupled respiration and oxidative enzyme activities (citrate synthase (CS), 3-hydroxy-acyl-CoA-dehydrogenase activity (HAD)) before and after acute exposure to insulin and/or palmitate in myotubes established from healthy lean and obese subjects and T2D patients. Basal CS activity was lower (14%) in diabetic myotubes compared with myotubes from lean controls (P=0.03). Incubation with insulin (1 microM) for 4 h increased the CS activity (26-33%) in myotubes from both lean (P=0.02) and obese controls (P<0.001), but not from diabetic subjects. Co-incubation with palmitate (0.6 mM) for 4 h abolished the stimulatory effect of insulin on CS activity in non-diabetic myotubes. No differences were detected in mitochondrial respiration and HAD activity between myotubes from non-diabetic subjects and T2D patients, and none of these measures responded to high levels of insulin and/or palmitate. These results provide evidence for an intrinsic defect in CS activity, which may play a role in the pathogenesis of T2D. Moreover, the data suggest that insulin resistance at the CS level can be induced by exposure to high free fatty acid levels.
authors.affiliation: Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark.
authors.forename: Niels
authors.initials: N
authors.lastname: Ortenblad
authors.forename: Martin
authors.initials: M
authors.lastname: Mogensen
authors.forename: Ingrid
authors.initials: I
authors.lastname: Petersen
authors.forename: Kurt
authors.initials: K
authors.lastname: Højlund
authors.forename: Klaus
authors.initials: K
authors.lastname: Levin
authors.forename: Kent
authors.initials: K
authors.lastname: Sahlin
authors.forename: Henning
authors.initials: H
authors.lastname: Beck-Nielsen
authors.forename: Michael
authors.initials: M
authors.lastname: Gaster
mesh_terms: D015094:3-Hydroxyacyl CoA Dehydrogenases / Q000378:metabolism; D002478:Cells, Cultured; D002950:Citrate (si)-Synthase / Q000378:metabolism*; D003924:Diabetes Mellitus, Type 2 / Q000201:enzymology* / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D007333:Insulin Resistance; D008875:Middle Aged; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism; D018482:Muscle, Skeletal / Q000187:drug effects* / Q000378:metabolism; D009765:Obesity / Q000201:enzymology / Q000378:metabolism; D010084:Oxidation-Reduction; D010101:Oxygen Consumption; D019308:Palmitic Acid / Q000378:metabolism
chemical_list: D007328:Insulin; D019308:Palmitic Acid; D015094:3-Hydroxyacyl CoA Dehydrogenases; D002950:Citrate (si)-Synthase; D005947:Glucose