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Background: Loss of muscle mass is commonly seen in patients with sepsis, severe injury, and cancer. Recent studies suggest that increased levels and activity of the transcription factor C/EBP? is involved in muscle wasting. Previous studies suggest that TNF? may potentiate the effects of glucocorticoids on muscle protein breakdown. We tested the hypothesis that TNF-? potentiates the effects of dexamethasone on C/EBP? expression and activity in cultured muscle cells and that this effect of TNF-? is regulated by an increased expression of the glucocorticoid receptor (GR).
Methods: Experiments were performed in cultured L6 myoblasts, a rat skeletal muscle cell line. Cells were treated for 24 h with dexamethasone (1 uM), TNF? (10 ng/ml), or both. C/EBP? and GR mRNA levels were determined by real-time PCR, protein levels by Western blotting, and activity by using a DNA-binding ELISA kit.
Results: Treatment of the cultured L6 muscle cells with dexamethasone resulted in a 4-fold increase in C/EBP? mRNA levels. TNF? potentiated the effect of dexamethasone on C/EBP? mRNA. There were similar increases in C/EBP? protein levels and DNA-binding activity. The affects of dexamethasone and TNF?-induced increase in C/EBP? mRNA and protein levels were abolished by the glucocorticoid receptor antagonist, RU38486, suggesting that the regulation of C/EBP? under the present experimental conditions was GR-dependent.
Conclusion: TNF? may participate in the regulation of muscle mass by potentiating the effects of glucocortiocids on C/EBP? expression and activity. Understanding mechanisms of corticosteroid-regulated loss of muscle mass has important clinical implications.