p300-Dependent Acetylation of Activating Transcription Factor 5 Enhances C/EBPβ Transactivation of C/EBPα during 3T3-L1 Differentiation

Adipogenesis is a multistep process by which 3T3-L1 preadipocytes differentiate into mature adipocytes through mitotic clonal expansion (MCE) and terminal differentiation. The CCAAT/enhancer-binding protein beta (C/EBP beta) is an important transcription factor that takes part in both of these processes. C/EBP beta not only transactivates C/EBP alpha and the peroxisome proliferator-activated receptor gamma (PPAR gamma), which cause 3T3-L1 preadipocytes to enter terminal adipocyte differentiation, but also is required to activate cell cycle genes necessary for MCE. The identification of potential cofactors of C/EBP beta will help to explain how C/EBP beta undertakes these specialized roles during the different stages of adipogenesis. In this study, we found that activating transcription factor 5 (ATF5) can bind to the promoter of C/EBP alpha via its direct interaction with C/EBP beta (which is mediated via the p300-dependent acetylation of ATF5), leading to enhanced C/EBP beta transactivation of C/EBP alpha. We also show that p300 is important for the interaction of ATF5 with C/EBP beta as well as for the binding activity of this complex on the C/EBP alpha promoter. Consistent with these findings, overexpression of ATF5 and an acetylated ATF5 mimic both promoted 3T3-L1 adipocyte differentiation, whereas short interfering RNA-mediated ATF5 downregulation inhibited this process. Furthermore, we show that the elevated expression of ATF5 is correlated with an obese phenotype in both mice and humans. In summary, we have identified ATF5 as a new cofactor of C/EBP beta and examined how C/EBP beta and ATF5 (acetylated by a p300-dependent mechanism) regulate the transcription of C/EBP alpha.