Skeletal muscle catabolism in trinitrobenzene sulfonic acid-induced murine colitis

Abstract

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 (eg, 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 intrarectal administration of trinitrobenzene sulfonic acid (TNBS), and blood and tissues were collected on day 10. Mice with inflammatory bowel disease demonstrated reduced skeletal muscle mass and protein content, whereas colonic segment weight and gross damage score were both increased in mice with colitis, compared with time-matched control values. There was no change in muscle protein synthesis in mice with inflammatory bowel disease; but there was an increased protein breakdown (45%), proteasome activity (85%), and messenger RNA (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 tumor necrosis factor-α, interleukin-6, and nitric oxide synthase 2 mRNA content of both liver and skeletal muscle was increased in TNBS-treated mice; and plasma tumor necrosis factor-α and interleukin-6 concentrations were also elevated. These data suggest that 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.

Figure 1

Effect of TNBS-induced colitis on in vitro-determined rates of muscle protein breakdown and proteasome activity. Panel A: Protein degradation in plantaris determined by tyrosine (Try) release into medium. Panels B and C: Activity of the 26S and 20S proteasome, respectively. Values are means ± SEM; n = 8–10 per group for protein degradation and 6 per group for proteasome activity. *P < 0.05 compared to time-matched control values.

Figure 2

Effect of TNBS-induced colitis on MuRF1, atrogin-1 and E3αI mRNA content in skeletal muscle. Panel A: representative autoradiographs of E3 ligases and actin (loading control). Panels B, C and D: bar graphs represent means ± SEM of n = 9–11; data were normalized to actin and expressed as arbitrary densitometry units (AU)/actin. Data from control mice were arbitrarily set at 1.0 AU. *P < 0.05 compared to time-matched control values.

Figure 3

Effect of TNBS-induced colitis on the IGF-I mRNA content of liver and skeletal muscle. Panels A and C: representative autoradiographs of IGF-I mRNA in liver and muscle (gastrocnemius/plantaris), respectively. Panels B and D: bar graphs represent means ± SEM of n = 9–11, data were normalized to actin and data from control mice were arbitrarily set at 1.0 AU. *P < 0.05 compared to time-matched control values.

Figure 4

Effect of TNBS-induced colitis on the plasma concentration of total and free IGF-I, and insulin (panels A–C, respectively). Bar graphs represent means ± SEM of n = 9–11. *P < 0.05 compared to time-matched control values.

Figure 5

Effect of TNBS-induced colitis on IGFBP-1 and IGFBP-3 mRNA content in liver. Panel A: representative autoradiographs of IGFBP-1 and IGFBP-3 mRNA in liver. Panels B and C: bar graphs represent means ± SEM of n = 9–11, where data were normalized to mouse ribosomal protein L32 mRNA signal and expressed as AU/L32. Data from control mice were arbitrarily set at 1.0 AU. *P < 0.05 compared to time-matched control values.

Figure 6

Effect of TNBS-induced colitis on muscle and liver mRNA content for TNFα, IL-6, NOS2, and IL-1α (panels A–D, respectively). Bar graphs represent means ± SEM of n = 9–11, where data were normalized to mouse ribosomal protein L32 mRNA signal and data from control mice were arbitrarily set at 1.0 AU. *P < 0.05 compared to time-matched control values for the same tissue.

Figure 7

Effect of TNBS-induced colitis on the plasma concentration of TNFα and IL-6. Bar graphs in panel A (TNFα) and panel B (IL-6) represent means ± SEM of n = 9–11. *P < 0.05 compared to time-matched control values.