Regulated Nuclear-Cytoplasmic Localization of CCAAT/Enhancer-binding Protein δ in Osteoblasts

Insulin-like growth factor I (IGF-I) plays a central role in skeletal growth by promoting bone cell replication and differentiation. Prostaglandin E₂ (PGE₂) and parathyroid hormone enhance cAMP production in cultured rat osteoblasts and stimulate IGF-I expression through a transcriptional mechanism mediated by cAMP-dependent protein kinase (PKA). We previously showed that PGE₂ activated the transcription factor CCAAT/enhancer-binding protein δ (C/EBPδ) in osteoblasts and induced its binding to a DNA element within the IGF-I promoter. We report here that a PKA-dependent pathway stimulates nuclear translocation of C/EBPδ. Under basal conditions, C/EBPδ was cytoplasmic but rapidly accumulated in the nucleus after PGE₂ treatment (t_1/2 < 30 min). Nuclear translocation occurred without concurrent protein synthesis and was maintained in the presence of hormone. Nuclear localization required PKA and was blocked by a dominant-interfering regulatory subunit of the enzyme, even though C/EBPδ was not a PKA substrate. Upon removal of hormonal stimulus, C/EBPδ quickly exited the nucleus (t_1/2 < 12 min) through a pathway blocked by leptomycin B. Mutagenesis studies indicated that the basic domain of C/EBPδ was necessary for nuclear localization and that the leucine zipper region permitted full nuclear accumulation. We thus define a pathway for PKA-mediated activation of C/EBPδ through its regulated nuclear import.