Prevention of CCAAT/enhancer-binding protein beta DNA binding by hypoxia during adipogenesis.

Upon exposure to adipogenesis-inducing hormones, confluent 3T3-L1 preadipocytes express C/EBPbeta (CCAAT/enhancer binding protein beta). Early induced C/EBPbeta is inactive but, after a lag period, acquires its DNA-binding capability by sequential phosphorylation. During this period, preadipocytes pass the G(1)/S checkpoint synchronously. Thr(188) of C/EBPbeta is phosphorylated initially to prime the factor for subsequent phosphorylation at Ser(184) or Thr(179) by GSK3beta, which translocates into the nuclei during the G(1)/S transition. Many events take place during the G(1)/S transition, including reduction in p27(Kip1) protein levels, retinoblastoma (Rb) phosphorylation, GSK3beta nuclear translocation, and C/EBPbeta binding to target promoters. During hypoxia, hypoxia-inducible factor-1alpha (HIF-1alpha) is stabilized, thus maintaining expression of p27(Kip1), which inhibits Rb phosphorylation. Even under normoxic conditions, constitutive expression of p27(Kip1) blocks the nuclear translocation of GSK3beta and DNA binding capability of C/EBPbeta. Hypoxia also blocks nuclear translocation of GSK3beta and DNA binding capability of C/EBPbeta in HIF-1alpha knockdown 3T3-L1 cells that fail to induce p27(Kip1). Nonetheless, under hypoxia, these cells can block Rb phosphorylation and the G(1)/S transition. Altogether, these findings suggest that hypoxia prevents the nuclear translocation of GSK3beta and the DNA binding capability of C/EBPbeta by blocking the G(1)/S transition through HIF-1alpha-dependent induction of p27(Kip1) and an HIF-1alpha/p27-independent mechanism.