Glucocorticoid suppression of IGF I transcription in osteoblasts.
Glucocorticoids have profound effects on bone formation, decreasing IGF I transcription in osteoblasts, but the mechanisms involved are poorly understood. We previously showed that the bp +34 to +192 region of the rat IGF I exon 1 promoter was responsible for repression of IGF I transcription by cortisol in cultures of osteoblasts from fetal rat calvariae (Ob cells). Here, site-directed mutagenesis was used to show that a binding site for members of the CAAT/enhancer binding protein family of transcription factors, within the +132 to +158 region of the promoter, mediates this glucocorticoid effect. EMSAs demonstrated that cortisol increased binding of osteoblast nuclear proteins to the +132 to +158 region of the IGF I promoter. Supershift assays showed that CAAT/enhancer binding protein alpha, beta, and delta interact with this sequence, and binding of CAAT/enhancer binding protein delta, in particular, was increased in the presence of cortisol. Northern blot analysis showed that CAAT/enhancer binding protein delta and beta transcripts were increased by cortisol in Ob cells. Further, cortisol increased the transcription of these genes and increased the stability of CAAT/enhancer binding protein delta mRNA. In conclusion, cortisol represses IGF I transcription in osteoblasts, and CAAT/enhancer binding proteins appear to play a role in this effect.
Delany
A M
AM
Department of Research, Saint Francis Hospital and Medical Center, Hartford, Connecticut 06105, USA.
Durant
D
D
Canalis
E
E
D000818:Animals; D015152:Blotting, Northern; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000235:genetics / Q000378:metabolism; D022782:CCAAT-Enhancer-Binding Protein-beta / Q000235:genetics / Q000378:metabolism; D051743:CCAAT-Enhancer-Binding Protein-delta; D022762:CCAAT-Enhancer-Binding Proteins / Q000235:genetics / Q000378:metabolism; D002478:Cells, Cultured; D004622:Embryo, Mammalian; D005091:Exons; D005938:Glucocorticoids / Q000494:pharmacology*; D006854:Hydrocortisone / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000235:genetics*; D016297:Mutagenesis, Site-Directed; D009687:Nuclear Proteins / Q000378:metabolism; D010006:Osteoblasts / Q000187:drug effects / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D014157:Transcription Factors; D014158:Transcription, Genetic / Q000187:drug effects*; D014162:Transfection
D022763:CCAAT-Enhancer-Binding Protein-alpha; D022782:CCAAT-Enhancer-Binding Protein-beta; D022762:CCAAT-Enhancer-Binding Proteins; C497254:Cebpd protein, rat; D005938:Glucocorticoids; D009687:Nuclear Proteins; D012333:RNA, Messenger; D014157:Transcription Factors; D051743:CCAAT-Enhancer-Binding Protein-delta; D007334:Insulin-Like Growth Factor I; D006854:Hydrocortisone