NF-kB but not FoxO sites in the MuRF1 promoter are required for transcriptional activation in disuse muscle atrophy

The muscle-specific ring finger protein 1 (MuRF1) gene is required for most types of skeletal muscle atrophy yet we have little understanding of its transcriptional regulation. The purpose of this study is to identify whether NF-kB and/or FoxO response elements in the MuRF1 promoter are required for MuRF1 gene activation during skeletal muscle atrophy due to the removal of hindlimb weight bearing (unloading). Both NF-kB -dependent and FoxO-dependent luciferase reporter activities were significantly increased at 5 days of unloading. Using a 4.4-kb MuRF1 promoter reporter construct, a fourfold increase in reporter (i. e., luciferase) activity was found in rat soleus muscles after 5 days of hindlimb unloading. This activation was abolished by mutagenesis of either of the two distal putative NF-kB sites or all three putative NF-kB sites but not by mutagenesis of all four putative FoxO sites. This work provides the first direct evidence that NF-kB sites, but not FoxO sites, are required for MuRF1 promoter activation in muscle disuse atrophy in vivo.