Regulated nuclear-cytoplasmic localization of CCAAT/enhancer-binding protein delta in osteoblasts

Insulin-like growth factor I (IGF-I) plays a central role in skeletal growt h by promoting bone cell replication and differentiation. Prostaglandin E-2 (PGE(2)) and parathyroid hormone enhance cAMP production in cultured rat o steoblasts and stimulate IGF-I expression through a transcriptional mechani sm mediated by cAMP dependent protein kinase (PKA), We previously showed th at PGE(2) activated the transcription factor CCAAT/enhancer-binding protein delta (C/EBP delta) in osteoblasts and induced its binding to a DNA elemen t within the IGF-I promoter. We report here that a PKA-dependent pathway st imulates nuclear translocation of C/EBP delta. Under basal conditions, C/EB P delta was cytoplasmic but rapidly accumulated in the nucleus after PGE(2) treatment (t(1/2) < 30 min). Nuclear translocation occurred without concur rent protein synthesis and was maintained in the presence of hormone. Nucle ar localization required PKA and was blocked by a dominant-interfering regu latory subunit of the enzyme, even though C/EBP<delta> was not a PKA substr ate. Upon removal of hormonal stimulus, C/EBP delta quickly exited the nucl eus (t(1/2) < 12 min) through a pathway blocked by leptomycin B. Mutagenesi s studies indicated that the basic domain of C/EBP<delta> was necessary for nuclear localization and that the leucine zipper region permitted full nuc lear accumulation. We thus define a pathway for PKA-mediated activation of C/EBP delta through its regulated nuclear import.