Myostatin Augments Muscle-Specific Ring Finger Protein-1 Expression Through an NF-kB Independent Mechanism in SMAD3 Null Muscle

Abstract

Smad (Sma and Mad-related protein) 2/3 are downstream signaling molecules for TGF- and myostatin (Mstn). Recently, Mstn was shown to induce reactive oxygen species (ROS) in skeletal muscle via canonical Smad3, nuclear factor-B, and TNF- pathway. However, mice lacking Smad3 display skeletal muscle atrophy due to increased Mstn levels. Hence, our aims were first to investigate whether Mstn induced muscle atrophy in Smad3 / mice by increasing ROS and second to delineate Smad3-independent signaling mechanism for Mstn-induced ROS. Herein we show that Smad3 / mice have increased ROS levels in skeletal muscle, and inactivation of Mstn in these mice partially ablates the oxidative stress. Furthermore, ROS induction by Mstn in Smad3 / muscle was not via nuclear factor-B (p65) signaling but due to activated p38, ERK MAPK signaling and enhanced IL-6 levels. Consequently, TNF-, nicotinamide adenine dinucleotide phosphate oxidase, and xanthine oxidase levels were up-regulated, which led to an increase in ROS production in Smad3 / skeletal muscle. The exaggerated ROS in the Smad3 / muscle potentiated binding of C/EBP homology protein transcription factor to MuRF1 promoter, resulting in enhanced MuRF1 levels leading to muscle atrophy. (Molecular Endocrinology 28: 317-330, 2014) S mads (Sma and Mad-related proteins) are a family of intracellular proteins that transduce extracellular signals from TGF- superfamily members (1, 2). Upon TGF- ligand binding and subsequent receptor activation, Smad2 and Smad3 get phosphorylated to form a complex with the common mediator Smad4. The Smad2/3-Smad4 complex then translocates into the nucleus to activate or repress the transcription of TGF- target genes. Several reports have indicated that disruption of TGF- and/or the Smad signaling pathway is causal to the pathogenesis of several disorders (3, 4) and fibrogenesis (5).