Cortisol enhances the transcription of insulin-like growth factor-binding protein-6 in cultured osteoblasts

Previous work indicated that glucocorticoids inhibit the synthesis of insulin-like growth factor I (IGF-I) and IGF-binding protein-3 (IGFBP-3), -4, and -5, but not IGFBP-6, in osteoblast cultures. IGFBP-6 binds IGF-II with high affinity and prevents IGF-II-mediated effects. As IGF-II is present at high concentrations in bone, we postulated that glucocorticoids may regulate IGF-II by altering IGFBP-6 synthesis. We tested the expression of IGFBP-6 in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells). Treatment of Ob cells with cortisol caused a time- and dose-dependent increase in IGFBP-6 messenger RNA levels, as determined by Northern blot analysis. The effect was maximal after 48 h of treatment and observed with cortisol concentrations of 10 nM to 1 mu M. Treatment with cortisol also increased IGFBP-6 polypeptide levels in the medium, as determined by Western immunoblot analysis. Cycloheximide at 3.6 mu M decreased IGFBP-6 transcripts and prevented the stimulatory effect of cortisol. Cortisol did not modify the decay of IGFBP-6 messenger RNA in transcriptionally arrested Ob cells. In addition, cortisol increased the rate of IGFBP-6 transcription, as determined by nuclear run-on assays. In conclusion, cortisol stimulates IGFBP-6 expression in Ob cells by transcriptional mechanisms. As IGFBP-6 binds to and prevents the effect of IGF-II, its increased synthesis could be relevant to the inhibitory actions of cortisol in bone.