Endogenous CCAAT/Enhancer Binding Protein β and p300 Are Both Regulated by Growth Hormone to Mediate Transcriptional Activation

The regulation of c-fos transcription by GH involves multiple factors, including CCAAT/enhancer binding protein (C/EBP) beta. Knockdown of C/EBPbeta by RNA interference prevents stimulation of endogenous c-fos mRNA by GH, indicating a key role for C/EBPbeta in GH-stimulated c-fos transcription. GH rapidly increases the occupancy of both endogenous C/EBPbeta and p300 on the c-fos promoter in 3T3-F442A preadipocytes as indicated by chromatin immunoprecipitation. The transient occupancy of p300 on c-fos and the presence of p300 in the anti-C/EBPbeta immunoprecipitate coincide with the transient increase in c-fos transcription with GH, suggesting that a nuclear complex containing both p300 and C/EBPbeta occupies the c-fos promoter in response to GH. Expression of p300 with C/EBPbeta markedly increases c-fos promoter activity when neither alone is effective, indicating that p300 coactivates C/EBPbeta-mediated c-fos promoter activation. Such coactivation can determine a baseline for c-fos activation by GH. Furthermore, the occupancy of phosphorylated murine C/EBPbeta (T188) on c-fos upon GH treatment is simultaneous with increased occupancy by p300, suggesting that phospho-C/EBPbeta recruits p300 in response to GH. Thus, endogenous C/EBPbeta and p300 on c-fos are dynamically regulated by GH to determine transcriptional activation. Phosphorylated C/EBPbeta and p300 appear to function as part of a regulated complex that mediates GH-stimulated transcription.