SMAD3 negatively regulates serum irisin and skeletal muscle FNDC5 and PGC-1α during exercise

Abstract Beige adipose cells are a distinct and inducible type of thermogenic fat cell that express the mitochondrial uncoupling protein-1 and thus represent a powerful target for treating obesity. Mice lacking the transforming growth factor-beta (TGF-β) effector protein SMAD3 are protected against diet-induced obesity due to browning of their white adipose tissue (WAT) leading to increased whole body energy expenditure. However, the role SMAD3 plays in WAT browning is not clearly understood. Irisin is an exercise-induced skeletal muscle hormone that induces WAT browning similar to that observed in SMAD3 deficient mice. Together these observations suggested that SMAD3 may negatively regulate irisin production and/or secretion from skeletal muscle. To address this question we used wild type and SMAD3 knockout (Smad3-/-) mice subjected to an exercise regime and C2C12 myotubes treated with TGF-β, a TGF-β receptor-1 pharmacological inhibitor, adenovirus expressing constitutively active SMAD3 or siRNA against SMAD3. We find that in Smad3-/- mice exercise increases serum irisin and skeletal muscle FNDC5 (irisin precursor) and its upstream activator peroxisome proliferator-activated receptor gamma coactivator 1-α (PGC-1α) to a greater extent than in wild type mice. In C2C12 myotubes TGF-β suppresses FNDC5 and PGC-1α mRNA and protein levels via SMAD3 and promotes SMAD3 binding to the FNDC5 and PGC-1α promoters. These data establish that SMAD3 suppresses FNDC5 and PGC-1α in skeletal muscle cells. These findings shed light on the poorly understood regulation of irisin/FNDC5 by demonstrating a novel association between irisin and SMAD3 signaling in skeletal muscle.