CCAAT/enhancer-binding protein β is required for mitotic clonal expansion during adipogenesis

Hormonal induction of growth-arrested 3T3-L1 preadipocytes triggers a signaling cascade that culminates in adipogenesis. CCAAT/enhancer-binding protein (C/EBP)beta is expressed immediately but gains DNA-binding activity only after a long lag as the cells synchronously begin mitotic clonal expansion (MCE). After MCE, a process required for adipogenesis, C/EBPbeta activates expression of C/EBPalpha and peroxisome proliferator-activated receptor gamma, which then transcriptionally activate genes that produce the adipocyte phenotype. When mouse embryo fibroblasts (MEFs) are subjected to the same differentiation protocol, a subset of the MEFs undergoes a similar program of events. Similar to 3T3-L1 preadipocytes, the MEFs reenter the cell cycle (as indicated by the synchronous expression of cyclin A) and undergo MCE as evidenced by the incorporation of BrdUrd into DNA and the formation of mitotic foci of cells that undergo adipogenesis. C/EBPbeta is expressed immediately after induction but exhibits delayed acquisition of DNA-binding activity followed by expression of adipocyte markers and the accumulation of cytoplasmic triglyceride. MEFs from C/EBPbeta(-/-) mice, however, neither undergo MCE nor differentiate into adipocytes. Forced expression of C/EBPbeta (LAP) but not dominant-negative C/EBPbeta (LIP) in C/EBPbeta(-/-) MEFs restores MCE, expression of adipocyte markers, and the capacity to form mitotic foci of cells that undergo adipogenesis. These findings demonstrate that expression of C/EBPbeta is a prerequisite for MCE in the adipocyte-differentiation program.