Impaired Activation of AMP-Kinase and Fatty Acid Oxidation by Globular Adiponectin in Cultured Human Skeletal Muscle of Obese Type 2 Diabetics
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Adiponectin is an adipocyte-derived hormone associated with antidiabetic actions. In rodent skeletal muscle, globular adiponectin (gAD) activates AMP-kinase (AMPK) and stimulates fatty acid oxidation effects mediated through the adiponectin receptors, AdipoR1 and AdipoR2. In the present study, we examined the mRNA expression of adiponectin receptors and the effects of gAD on AMPK activity and fatty acid oxidation in skeletal muscle myotubes from lean, obese, and obese type 2 diabetic subjects. Myotubes from all groups expressed approximately 4.5-fold more AdipoR1 mRNA than AdipoR2, and obese subjects tended to have higher AdipoR1 expression (P = 0.052). In lean myotubes, gAD activates AMPKalpha1 and -alpha2 by increasing Thr172 phosphorylation, an effect associated with increased acetyl-coenzyme A carboxylase (ACCbeta) Ser221 phosphorylation and enhanced rates of fatty acid oxidation, effects similar to those observed after pharmacological AMPK activation by 5-aminoimidazole-4-carboxamide riboside. In obese myotubes, the activation of AMPK signaling by gAD at low concentrations (0.1 mug/ml) was blunted, but higher concentrations (0.5 mug/ml) stimulated AMPKalpha1 and -alpha2 activities, AMPK and ACCbeta phosphorylation, and fatty acid oxidation. In obese type 2 diabetic myotubes, high concentrations of gAD stimulated AMPKalpha1 activity and AMPK phosphorylation; however, ACCbeta phosphorylation and fatty acid oxidation were unaffected. Reduced activation of AMPK signaling and fatty acid oxidation in obese and obese diabetic myotubes was not associated with reduced protein expression of AMPKalpha and ACCbeta or the expression and activity of the upstream AMPK kinase, LKB1. These data suggest that reduced activation of AMPK by gAD in obese and obese type 2 diabetic subjects is not caused by reduced adiponectin receptor expression but that aspects downstream of the receptor may inhibit AMPK signaling.
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