AMPK siRNA (100 μg/mouse) or PPARδ siRNA (80 μg/mouse) was injected twice (at 48-h intervals) for 2 weeks into the HFD-fed and METRNL-treated AMPK or PPARδ-knockdown groups through the tail vein using in vivo-jetPEI for siRNA delivery into skeletal muscle. Scrambled siRNA was injected twice into each mouse in the ND group. Upon completion of the study period, all experimental mice were sacrificed under anesthesia after fasting overnight for 12 h.

Physical activity has many beneficial effects on metabolic disorders, such as obesity, insulin resistance, and diabetes. Meteorin-like protein (METRNL), a novel secreted protein homologous to the neurotrophin Metrn, is induced after exercise in the skeletal muscle. Herein, we investigated the effects of METRNL on lipid-mediated inflammation and insulin resistance in skeletal muscle via AMP-activated protein kinase (AMPK) or peroxisome proliferator-activated receptor delta (PPARdelta). Treatment with METRNL suppressed inflammatory markers, such as nuclear factor kappaB (NFkappaB) nuclear translocation, inhibitory kappaBalpha (IkappaBalpha) phosphorylation, interleukin-6 (IL-6) expression, and pro-inflammatory cytokines (such as TNFalpha and MCP-1). METRNL treatment also attenuated the impaired insulin response both in palmitate-treated differentiated C2C12 cells and the skeletal muscle of high-fat diet (HFD)-fed mice. Furthermore, METRNL administration rescued glucose intolerance and reduced HFD-induced body weight gain in mice; however, METRNL did not affect calorie intake. METRNL treatment increased AMPK phosphorylation and PPARdelta expression both in differentiated C2C12 cells and mouse skeletal muscle. siRNA-mediated suppression of AMPK and PPARdelta abrogated the suppressive effects of METRNL on palmitate-induced inflammation and insulin resistance. Moreover, METRNL augmented the mRNA expression of fatty acid oxidation-associated genes, such as carnitine palmitoyltransferase 1 (CPT1), acyl-CoA oxidase (ACO), and fatty acid binding protein 3 (FABP3). siRNAs for AMPK and PPARdelta reversed these changes. In the current study, we report for the first time that METRNL alleviates inflammation and insulin resistance and induces fatty acid oxidation through AMPK or PPARdelta-dependent signaling in skeletal muscle.