Interactions between CCAAT enhancer binding protein δ and estrogen receptor α control insulin-like growth factor I ( igf1) and estrogen receptor-dependent gene expression in osteoblasts

Although ambient levels of estradiol and synthesis of the osteoblast growth factor IGF-I are inversely related in vivo, estradiol has little or no direct effect on igf1 gene expression in rat osteoblasts in vitro. Rather, estradiol suppresses the effect of hormones that enhance igf1 expression through protein kinase A dependent activation of CCAAT enhancer binding protein (C/EBP) transcription factors. We show here that inhibition of C/EBP activity by estradiol relates to the level of estrogen receptor α (ERα) expression, and that a complex between hormone-activated ERα and C/EBPδ inhibits transcription by each factor. Protein fragmentation, co-immunoprecipitation, and gene expression studies identified domains for physical and functional interactions between ERα and C/EBPδ. Whereas ERα and fragments comprising its various domains associated with C/EBPδ, only ERα fragment A/B alone replicated the suppressive effect of intact ERα on endogenous C/EBPδ activity. Complementary studies showed that several carboxyl regions of C/EBPδ cooperatively inhibit ERα dependent transcription. Therefore, multiple domains of C/EBPδ and ERα can physically interact to alter gene expression in osteoblasts in selective ways that depend on variations in the local hormone environment. Their combined effects on one important gene target, igf1, may help to determine the balance in the overall rates of bone formation.