Reduced insulin-mediated citrate synthase activity in cultured skeletal muscle cells from patients with type 2 diabetes: evidence for an intrinsic oxidative enzyme defect.
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.
Ortenblad
Niels
N
Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark.
Mogensen
Martin
M
Petersen
Ingrid
I
Højlund
Kurt
K
Levin
Klaus
K
Sahlin
Kent
K
Beck-Nielsen
Henning
H
Gaster
Michael
M
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
D007328:Insulin; D019308:Palmitic Acid; D015094:3-Hydroxyacyl CoA Dehydrogenases; D002950:Citrate (si)-Synthase; D005947:Glucose