Expression and activity of C/EBPβ and δ are upregulated by dexamethasone in skeletal muscle

The influence of glucocorticoids on the expression and activity of the transcription factors CCAAT/enhancer binding protein (C/EBP)β and δ in skeletal muscle was examined by treating rats or cultured L6 myotubes with dexamethasone. Treatment of rats with 10 mg/kg of dexamethasone resulted in increased C/EBPβ and δ DNA binding activity in the extensor digitorum longus muscle as determined by electrophoretic mobility shift assay (EMSA) and supershift analysis. A similar response was noticed in dexamethasone-treated myotubes. In other experiments, myocytes were transfected with a plasmid containing a promoter construct consisting of multiple C/EBP binding elements upstream of a luciferase reporter gene. Treatment of these cells Is with dexamethasone resulted in a fourfold increase in luciferase activity, suggesting that glucocorticoids increase C/EBP-dependent gene activation in muscle cells. In addition, dexamethasone upregulated the protein and gene expression of C/EBPβ and δ in the myotubes in a time- and dose-dependent fashion as determined by Western blotting and real-time PCR, respectively. The results suggest that glucocorticoids increase C/EBPβ and δ activity and expression through a direct effect in skeletal muscle.