The MuRF1 Promoter is Synergistically Activated by the Glucocorticoid Receptor and the Forkhead Family Member, FoxO1

Abstract

Skeletal muscle wasting is observed in a wide range of physiological conditions. The recently discovered ubiquitin E3 ligases, MAFbx and MuRF1, have been implicated in the regulation of protein degradation during muscle atrophy. Analysis of the MAFbx promoter has led to the observation that the FoxO family of transcription factors plays a critical role in regulating expression during atrophic conditions. Interestingly, MuRF1 and MAFbx expression in vivo has been shown to increase in response to treatment with dexamethasone, however no functional glucocorticoid receptor binding element (GRE) has been found in the MAFbx proximal promoter. To understand the regulation of MuRF1 expression, a 5000 base pair fragment of the mouse MuRF1 promoter was cloned. Analysis of the sequence revealed a GRE that is conserved in the mouse, rat and human promoter, as well as, a conserved FoxO binding element (FBE) that lies immediately adjacent to the GRE. Transcriptional reporter assays performed in both HepG2 and C2C12 cells demonstrate that the MuRF1 promoter is highly responsive to dexamethasone-activated glucocorticoid receptor (GR) and FoxO1 individually, while co-overexpression of GR and FoxO1 leads to a dramatic synergistic increase in reporter activity. Furthermore, mutation of either the GRE or the FBE alone or in combination is sufficient to significantly impair GR and/or FoxO1-mediated transcriptional activation of the MuRF1 promoter. These initial findings strongly suggest that corticosteroid-activated GR and FoxO1 act synergistically to drive MuRF1 expression under atrophy-inducing conditions.