G9a is transactivated by C/EBP beta to facilitate mitotic clonal expansion during 3T3-L1 preadipocyte differentiation

In 3T3-L1 preadipocyte differentiation, the CCAAT/enhancer-binding protein-beta (C/EBP beta) is an important early transcription factor that activates cell cycle genes during mitotic clonal expansion (MCE), sequentially activating peroxisome proliferator-activated receptor-gamma(PPAR gamma) and C/EBP alpha during terminal differentiation. Although C/EBP beta acquires its DNA binding activity via dual phosphorylation at about 12- 16 h postinduction, the expression of PPAR gamma and C/EBP beta is not induced until 36-72 h. The delayed expression of PPAR gamma and C/EBP alpha ensures the progression of MCE, but the mechanism responsible for the delay remains elusive. We provide evidence that G9a, a major euchromatic methyltransferase, is transactivated by C/EBP beta and represses PPAR gamma and C/EBP alpha through H3K9 dimethylation of their promoters during MCE. Inhibitor-or siRNA-mediated G9a downregulation modestly enhances PPAR gamma and C/EBP alpha expression and adipogenesis in 3T3-L1 preadipocytes. Conversely, forced expression of G9a impairs the accumulation of triglycerides. Thus, this study elucidates an epigenetic mechanism for the delayed expression of PPAR gamma and C/ EBP alpha.