Skeletal Muscle Catabolism in TNBS-Induced Murine Colitis

The present study determined whether the muscle atrophy produced by colitis is associated with altered rates of muscle protein synthesis or degradation, as well as the potential role of the local (e.g., muscle) insulin-like growth factor (IGF) system and muscle-specific ubiquitin E3 ligases atrogin-1 and MuRF1 in mediating altered muscle protein balance. Colitis was induced in C57BL/6 mice by intra-rectal administration of trinitrobenzne sulfonic acid (TNBS), and blood and tissues collected on day 10. Mice with inflammatory bowel disease (IBD) demonstrated reduced skeletal muscle mass and protein content, whereas colonic segment weight and gross damage score were both increased in mice with colitis, compared to time-matched control values. There was no change in muscle protein synthesis in mice with IBD, but there was an increased protein breakdown (45%), proteasome activity (85%), and mRNA expression for atrogin-1 and MuRF1 (200–300%) in muscle. These changes were associated with a reduction in liver (but not muscle) IGF-I mRNA as well as a reduction in both total and free IGF-I in the blood. Colitis decreased the hepatic content of IGF binding protein (IGFBP)-3 mRNA by 40% and increased IGFBP-1 mRNA by 100%. In contrast, colitis did alter IGFBP mRNAs in muscle. The TNFα, IL-6 and NOS2 mRNA content of both liver and skeletal muscle was increased in TNBS-treated mice, and plasma TNFα and IL-6 concentrations were also elevated. These data suggest TNBS-induced colitis is independent of a change in muscle protein synthesis but dependent on stimulation of protein degradation via increased expression of muscle-specific atrogenes, which may be mediated in part by the reduction in circulating concentration of IGF-I and the concomitant increase in inflammatory mediators observed in the blood and muscle per se.