Glucocorticoid suppression of IGF I transcription in osteoblasts

Glucocorticoids have profound effects on bone formation, decreasing IGF I t ranscription in osteoblasts, but the mechanisms involved are poorly underst ood. We previously showed that the bp +34 to +192 region of the rat IGF I e xon I promoter was responsible for repression of IGF I transcription by cor tisol 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, with in the +132 to +158 region of the promoter, mediates this glucocorticoid ef fect. EMSAs demonstrated that cortisol increased binding of osteoblast nucl ear proteins to the +132 to +158 region of the IGF I promoter. Supershift a ssays showed that CAAT/enhancer binding protein alpha, beta, and delta inte ract with this sequence, and binding of CAAT/enhancer binding protein delta , in particular, was increased in the presence of cortisol. Northern blot a nalysis showed that CAAT/enhancer binding protein delta and beta transcript s were increased by cortisol in Ob cells. Further, cortisol increased the t ranscription of these genes and increased the stability of CAAT/enhancer bi nding protein delta mRNA. In conclusion, cortisol represses IGF I transcrip tion in osteoblasts, and CAAT/enhancer binding proteins appear to play a ro le in this effect.