Induction Of Murf-1 Is Essential For Tnf-alpha Induced Loss Of Muscle Function In Mice.

Abstract

PURPOSE: Inflammatory cytokines like tumor necrosis factor alpha (TNF-α) have the potency to reduce skeletal muscle function. Furthermore, TNF-α also induces the expression of Murf-1, a skeletal muscle specific ubiquitin E3-ligases, which plays an important role in the pathophysiology of muscle atrophy. So far it is unclear if the induction of Murf-1 by TNF-α is an important molecular step for the TNF-α induced reduction in muscle function. METHODS: TNF-α (100 ng/g) or saline was injected intraperitoneal (ip) either into C57Bl6 (n=10) or Murf-1 knockout mice (n=10). After 16-24h the animals were killed and the expression of Murf-1 in the skeletal muscle (quadriceps muscle) was quantified either by qRT-PCR or western blot. Muscle function (force frequency relation) was determined in an organ bath setting in the m. soleus. RESULTS: The ip injection of TNF-α let to a significant increase of Murf-1 mRNA (saline: 56.6±12.1 vs. TNF-α: 133.6±30.3 arb. Units; p<0.05) as well as protein expression (saline: 0.38±0.11 vs. TNF-α: 1.07±0.25 arb. Units; p<0.05). As expected no induction of Murf-1 was observed in the Murf-1 knock-out animals. Analyzing the force frequency relation in the m. soleus from the animals a 25% reduction in force development was detected at 150Hz in the C57Bl6 animals injected with TNF-a compared to the saline injected animals (saline: 2412±120 vs. TNF-α: 1799±114 g/cm2; p<0.05). This reduction in force development by TNF-α was not seen in the Murf-1 knock-out animals (saline: 2424±198 vs. TNF-α: 2431±180 g/cm2; p=NS). CONCLUSION: The results of this study demonstrate for the first time, that the TNF-α induced reduction in skeletal muscle force development depends on the induction of the atrophy related E3-ubiquitin ligase Murf-1. To explore the exact molecular mechanism, by which Murf-1 mediates the TNF-α induced force reduction, further experiments are necessary.