Direct Activation of Glucose Transport in Primary Human Myotubes After Activation of Peroxisome Proliferator–Activated Receptor δ

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Abstract
Activators of peroxisome proliferator–activated receptor (PPAR)γ have been studied intensively for their insulin-sensitizing properties and antidiabetic effects. Recently, a specific PPARδ activator (GW501516) was reported to attenuate plasma glucose and insulin levels when administered to genetically obese ob/ob mice. This study was performed to determine whether specific activation of PPARδ has direct effects on insulin action in skeletal muscle. Specific activation of PPARδ using two pharmacological agonists (GW501516 and GW0742) increased glucose uptake independently of insulin in differentiated C2C12 myotubes. In cultured primary human skeletal myotubes, GW501516 increased glucose uptake independently of insulin and enhanced subsequent insulin stimulation. PPARδ agonists increased the respective phosphorylation and expression of AMP-activated protein kinase 1.9-fold (P < 0.05) and 1.8-fold (P < 0.05), of extracellular signal–regulated kinase 1/2 mitogen-activated protein kinase (MAPK) 2.2-fold (P < 0.05) and 1.7-fold (P < 0.05), and of p38 MAPK 1.2-fold (P < 0.05) and 1.4-fold (P < 0.05). Basal and insulin-stimulated protein kinase B/Akt was unaltered in cells preexposed to PPARδ agonists. Preincubation of myotubes with the p38 MAPK inhibitor SB203580 reduced insulin- and PPARδ-mediated increase in glucose uptake, whereas the mitogen-activated protein kinase kinase inhibitor PD98059 was without effect. PPARδ agonists reduced mRNA expression of PPARδ, sterol regulatory element binding protein (SREBP)-1a, and SREBP-1c (P < 0.05). In contrast, mRNA expression of PPARγ, PPARγ coactivator 1, GLUT1, and GLUT4 was unaltered. Our results provide evidence to suggest that PPARδ agonists increase glucose metabolism and promote gene regulatory responses in cultured human skeletal muscle. Moreover, we provide biological validation of PPARδ as a potential target for antidiabetic therapy.

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