Peripheral effect of α-melanocyte-stimulating hormone on fatty acid oxidation in skeletal muscle

To study the peripheral effects of melanocortin on fuel homeostasis in skeletal muscle, we assessed palmitate oxidation and AMP kinase activity in α-melanocyte-stimulating hormone (α-MSH)-treated muscle cells. After α-MSH treatment, carnitine palmitoyltransferase-1 and fatty acid oxidation (FAO) increased in a dose-dependent manner. A strong melanocortin agonist, NDP-MSH, also stimulated FAO in primary culture muscle cells and C2C12 cells. However, [Glu⁶]α-MSH-ND, which has ample MC4R and MC3R agonistic activity, stimulated FAO only at high concentrations (10^-5 M). JKC-363, a selective MC4R antagonist, did not suppress α-MSH-induced FAO. Meanwhile, SHU9119, which has both antagonistic activity on MC3R and MC4R and agonistic activity on both MC1R and MC5R, increased the effect of α-MSH on FAO in both C2C12 and primary muscle cells. Small interference RNA against MC5R suppressed the α-MSH-induced FAO effectively. cAMP analogues mimicked the effect of α-MSH on FAO, and the effects of both α-MSH and cAMP analogue-mediated FAO were antagonized by a protein kinase A inhibitor (H89) and a cAMP antagonist ((R_p)-cAMP). Acetyl-CoA carboxylase activity was suppressed by α-MSH and cAMP analogues by phosphorylation through AMP-activated protein kinase activation in C2C12 cells. Taken together, these results suggest that α-MSH increases FAO in skeletal muscle, in which MC5R may play a major role. Furthermore, these results suggest that α-MSH-induced FAO involves cAMP-protein kinase A-mediated AMP-activated protein kinase activation.