Induction of MuRF1 Is Essential for TNF-α-Induced Loss of Muscle Function in Mice

Abstract

BackgroundHumoral circulating inflammatory cytokines such as tumor necrosis factor alpha (TNF-α) can impair skeletal muscle contractility. Furthermore, TNF-α expression correlates with elevated levels of atrogin-like muscle-specific ubiquitin E3 ligases, which are presumed to mediate muscle protein breakdown and atrophy. However, the casual relationships between MuRF1 and TNF-α and their relative contributions to muscle function impairment are not known. MethodsTNF-α or saline was injected into either C57Bl6 or MuRF1−/− mice. After 16–24 h, the expression of MuRF1 in skeletal muscle was quantified by quantitative reverse transcription–PCR and Western blot analysis. Muscle function was measured in an organ bath. To obtain a broader overview on potential alterations, two-dimensional gel electrophoresis was performed. ResultsWild-type animals injected with TNF-α had higher MuRF1 mRNA expression (saline versus TNF-α: 56.6±12.1 versus 133.6±30.3 arbitrary units; p<0.05) and protein expression (saline versus TNF-α: 0.38±0.11 versus 1.07±0.25 arbitrary units; p<0.05) as compared to saline-injected littermates. Furthermore, TNF-α reduced force development at 150 Hz by 25% in C57Bl6 animals (saline versus TNF-α: 2412±120 versus 1799±114 g/cm2; p<0.05), but not in MuRF1−/− mice (saline versus TNF-α: 2424±198 versus 2431±180 g/cm2; p=NS). Proteome analysis revealed a significant down-regulation of fast skeletal muscle troponin T in wild-type animals treated with TNF-α as compared to MuRF1−/− mice that received TNF-α. ConclusionThe results of this study demonstrate for the first time that TNF-α-induced reduction in skeletal muscle force development depends on the induction of the atrophy-related E3 ubiquitin ligase MuRF1. A link for the reduction in muscle force may be the TNF-α/MuRF1-mediated down-regulation of fast skeletal muscle troponin T.