Abstract
We recently demonstrated that mRNA levels of three members of the CCAAT/enhancer binding factor (C/EBP) family of transcription factors are increased in skeletal muscle following 12 days of spaceflight. In the present study, we further explored the expression of C/EBP-δ in atrophying fast skeletal muscle by examining its expression in muscle from food-deprived (FD) mice, and investigated its role in regulating the expression of the secreted antigrowth factor myostatin. C/EBP-δ mRNA and protein levels were significantly increased by 2 days of food deprivation in the tibialis anterior (TA) muscle, and expression of both myostatin and C/EBP-δ mRNA during food deprivation was attenuated by injection with the glucocorticoid inhibitor RU486. The increase in myostatin mRNA levels with food deprivation appears to be at least partially transcriptionally driven, since levels of myostatin pre-mRNA were significantly increased in the TA muscle. C/EBP-δ mRNA levels and promoter activity were significantly increased by transfection of C(2)C(12) myotubes with a glucocorticoid receptor construct and 24 h of treatment with the synthetic glucocorticoid dexamethasone. Furthermore, activity of the C/EBP-δ promoter was significantly increased with as little as 1 h of dexamethasone treatment, while activity of the mouse myostatin promoter was only significantly increased with longer treatment periods of 24 h or more. Activity of the myostatin promoter-reporter construct was significantly increased in C(2)C(12) myotubes by cotransfection with expression constructs for C/EBP-α, -β, and -δ, with C/EBP-δ having the greatest effect. The myostatin promoter contains two potential C/EBP binding sequences, a CCAAT box, and a C/EBP binding element (CBE). Mutation of the CCAAT box attenuated basal myostatin promoter activity but potentiated C/EBP-δ-activated myostatin promoter activity in C(2)C(12) myotubes in vitro, while mutation of the CBE abolished glucocorticoid receptor and C/EBP-δ responsiveness. The present results support a model in which glucocorticoid-induced increases in C/EBP-δ expression may contribute to myostatin transcription during atrophic states.