Role of cdk2 in the sequential phosphorylation/activation of C/EBPβ during adipocyte differentiation

Upon induction of differentiation, growth-arrested (G₁ phase) 3T3-L1 preadipocytes express CCAAT/enhancer binding protein-β (C/EBPβ), initiating a transcriptional cascade. C/EBPβ immediately undergoes a priming phosphorylation (on Thr₁₈₈) by MAPK/ERK. However, the acquisition of DNA binding and transactivation capacity of C/EBPβ is delayed until further phosphorylation (on Ser₁₈₄ or Thr ₁₇₉) by GSK3β occurs. Phosphorylation by glycogen synthase kinase-3β (GSK3β) induces S phase entry and thereby mitotic clonal expansion (MCE), a requirement for terminal differentiation. Because MAPK activity is down-regulated before S phase is completed, we sought to identify the kinase that maintains C/EBPβ in the primed phosphorylated state throughout S phase and MCE. We show here that cdk2/cyclinA, whose expression is activated at the onset of S phase, functions in this capacity. Ex vivo and in vitro experiments show that cdk2/cyclinA catalyzes this delayed priming phosphorylation. Mass spectrometric analysis revealed that cdk2/cyclinA phosphorylates C/EBPβ on Thr₁₈₈ and is required for phosphorylation (on Ser₁₈₄ or Thr₁₇₉) of C/EBPβ by GSK3β and maintenance of DNA binding activity. Suppression of cdk2 activity by RNA interference or pharmacologic inhibitor disrupts subsequent events in the differentiation program. Thus, MAPK and cdk2/cyclinA act sequentially to maintain Thr₁₈₈ of C/EBPβ in the primed phosphorylated state during MCE and thereby progression of terminal differentiation.