C/EBPβ regulates dexamethasone-induced muscle cell atrophy and expression of atrogin-1 and MuRF1

Muscle wasting in catabolic patients is in part mediated by glucocorticoids and is associated with increased expression and activity of the transcription factor C/EBPβ. It is not known, however, if C/EBPβ is causally linked to glucocorticoid-induced muscle atrophy. We used dexamethasone-treated L6 myoblasts and myotubes to test the role of C/EBPβ in glucocorticoid-induced expression of the muscle-specific ubiquitin ligases atrogin-1 and MuRF1, protein degradation, and muscle atrophy by transfecting cells with C/EBPβ siRNA. In myoblasts, silencing C/EBPβ expression with siRNA inhibited dexamethasone-induced increase in protein degradation, atrogin-1 and MuRF1 expression, and muscle cell atrophy. Similar effects of C/EBPβ siRNA were seen in myotubes except that the dexamethasone-induced increase in MuRF1 expression was not affected by C/EBPβ siRNA in myotubes. In additional experiments, overexpressing C/EBPβ did not influence atrogin-1 or MuRF1 expression in myoblasts or myotubes. Taken together, our observations suggest that glucocorticoid-induced muscle wasting is at least in part regulated by C/EBPβ. Increased C/EBPβ expression alone, however, is not sufficient to upregulate atrogin-1 and MuRF1 expression.