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 IG F-II is present at high concentrations in bone, we postulated that glu cocorticoids may regulate IGF-II by altering IGFBP-6 synthesis. We tes ted 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 m essenger RNA levels, as determined by Northern blot analysis. The effe ct was maximal after 48 h of treatment and observed with cortisol conc entrations of 10 nM to 1 mu M. Treatment with cortisol also increased IGFBP-6 polypeptide levels in the medium, as determined by Western imm unoblot analysis. Cycloheximide at 3.6 mu M decreased IGFBP-6 transcri pts and prevented the stimulatory effect of cortisol. Cortisol did not modify the decay of IGFBP-6 messenger RNA in transcriptionally arrest ed Ob cells. In addition, cortisol increased the rate of IGFBP-6 trans cription, as determined by nuclear run-on assays. In conclusion, corti sol stimulates IGFBP-6 expression in Ob cells by transcriptional mecha nisms. As IGFBP-6 binds to and prevents the effect of IGF-II, its incr eased synthesis could be relevant to the inhibitory actions of cortiso l in bone.