The muscle specific ubiquitin E3 ligase MuRF1 has been implicated as a key regulator of muscle atrophy under a variety of conditions, such as during synthetic glucocorticoid treatment. FOXO class transcription factors have been proposed as important regulators of MuRF1 expression, but its regulation by glucocorticoids is not well understood. The MuRF1 promoter contains a near-perfect palindromic glucocorticoid response element (GRE) 200 base pairs upstream of the transcription start site. The GRE is highly conserved in the mouse, rat, and human genes along with a directly adjacent FOXO binding element (FBE). Transient transfection assays in HepG2 cells and C 2C12 myotubes demonstrate that the MuRF1 promoter is responsive to both the dexamethasone (DEX)-activated glucocorticoid receptor (GR) and FOXO1, whereas coexpression of GR and FOXO1 leads to a dramatic synergistic increase in reporter gene activity. Mutation of either the GRE or the FBE significantly impairs activation of the MuRF1 promoter. Consistent with these findings, DEXinduced upregulation of MuRF1 is significantly attenuated in mice expressing a homodimerization-deficient GR despite no effect on the degree of muscle loss in these mice vs. their wild-type counterparts. Finally, chromatin immunoprecipitation analysis reveals that both GR and FOXO1 bind to the endogenous MuRF1 promoter in C 2C12 myotubes, and IGF-I inhibition of DEX-induced MuRF1 expression correlates with the loss of FOXO1 binding. These findings present new insights into the role of the GR and FOXO family of transcription factors in the transcriptional regulation of the MuRF1 gene, a direct target of the GR in skeletal muscle.forkhead transcription factor class O; muscle RING finger 1; glucocorticoid receptor SKELETAL MUSCLE IS A DYNAMIC TISSUE that has the capacity to continuously regulate its size in response to a variety of external cues, including mechanical load, neural activity, hormones/growth factors, stress, and nutritional status. In addition, skeletal muscle serves as the most significant repository for protein in the body, a source that is tapped to provide a pool of amino acids for tissue repair and gluconeogenesis under conditions of starvation and other metabolic stresses. Muscle loss or "atrophy" occurs as the result of a number of disparate conditions, including aging, immobilization, metabolic diseases, cancer, and neurodegenerative diseases, and as a serious side effect of therapeutic corticosteroid hormone treatment (15,27,32). The recently identified E3 ubiquitin ligase, muscle RING finger 1 (MuRF1

Genes that are up-and down-regulated by thyroid hormone in the tail resorption program of Xenopus laevis have been isolated by a gene expression screen, sequenced, and identified in the GenBank data base. The entire program is estimated to consist of fewer than 35 up-regulated and fewer than 10 down-regulated genes; 17 and 4 of them, respectively, have been isolated and characterized. Upregulated genes whose function can be predicted on the basis of their sequence include four transcription factors (including one ofthe thyroid hormone receptors), an extracellular matrix component (fibronectin) and membrane receptor (integrin), four proteinases, a deiodinase that degrades thyroid hormone, and a protein that binds the hypothalamic corticotropinreleasing factor, which has been implicated in controlling thyroid hormone synthesis in Xenopus tadpoles. All four down-regulated genes encode extracellular proteins that are expressed in tadpole epidermis. This survey of the program provides insights into the biology of metamorphosis.

Non-technical summary Skeletal muscle has the capacity to modify its size in response to external cues such as mechanical load, neural activity, hormones, stress and nutritional status. Pathological muscle loss or 'atrophy' occurs as the result of a number of disparate conditions including ageing, immobilization, diabetes, cancer, sepsis and as a serious side effect of corticosteroid hormone treatment. Synthetic glucocorticoids are often used to treat inflammation; however, high doses and chronic use of these hormones can lead to the loss of skeletal muscle mass and weakness. We show that in mice with a deletion of the MuRF1 protein, but not the MAFbx protein, the loss of muscle mass is attenuated relative to normal mice following 14 days of glucocorticoid treatment. Knowledge of how the MuRF1 protein functions in skeletal muscle to regulate skeletal muscle mass could lead to the development of therapeutics to prevent muscle atrophy under various conditions including glucocorticoid treatment.Abstract Skeletal muscle atrophy occurs under a variety of conditions and can result from alterations in both protein synthesis and protein degradation. The muscle-specific E3 ubiquitin ligases, MuRF1 and MAFbx, are excellent markers of muscle atrophy and increase under divergent atrophy-inducing conditions such as denervation and glucocorticoid treatment. While deletion of MuRF1 or MAFbx has been reported to spare muscle mass following 14 days of denervation, their role in other atrophy-inducing conditions is unclear. The goal of this study was to determine whether deletion of MuRF1 or MAFbx attenuates muscle atrophy after 2 weeks of treatment with the synthetic glucocorticoid dexamethasone (DEX). The response of the triceps surae (TS) and tibialis anterior (TA) muscles to 14 days of DEX treatment (3 mg kg −1 day −1 ) was examined in 4 month-old male and female wild type (WT) and MuRF1 or MAFbx knock out (KO) mice. Following 14 days of DEX treatment, muscle wet weight was significantly decreased in the TS and TA of WT mice. Comparison of WT and KO mice following DEX treatment revealed significant sparing of mass in both sexes of the MuRF1 KO mice, but no muscle sparing in MAFbx KO mice. Further analysis of the MuRF1 KO mice showed significant sparing of fibre cross-sectional area and tension output in the gastrocnemius (GA) after DEX treatment. Muscle sparing in the MuRF1 KO mice was related to maintenance of protein synthesis, with no observed increases in protein degradation in either WT or MuRF1 KO mice. These results demonstrate that MuRF1 and MAFbx do not function similarly under all atrophy models, and that the primary role of MuRF1 may extend beyond controlling protein degradation via the ubiquitin proteasome system. Abbreviations 4E-BP1, eukaryotic initiation factor 4E binding protein 1; DEX, dexamethasone; FOXO, class O Forkhead transcription factor; GA, gastrocnemius muscle; MAFbx, muscle atrophy F box; MuRF1, muscle RING finger 1; mTOR, mammalian target of rapamycin; ND, nutritional deprivation; REDD1, regulated ...