RETINOIC ACID STIMULATES THE TRANSCRIPTION OF INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-6 IN SKELETAL CELLS

Retinoic acid has important actions on cell differentiation and osteob lastic function, and some of these actions may be mediated by changes in the insulin-like growth factor (IGF) axis. Skeletal cells synthesiz e IGF I and II and the six known IGF binding proteins (IGFBP). IGFBP-6 binds IGF II with high affinity and prevents IGF II-mediated effects. In fibroblasts, IGFBP-6 levels are regulated by retinoic acid, and we postulated that retinoic acid may regulate IGF II in bone by altering IGFBP-6 synthesis. We examined the effect of retinoic acid on IGFBP-6 expression in cultures of osteoblast-enriched cells from 22-day fetal rat calvariae (Ob cells). Retinoic acid caused a time- and dose-depen dent increase in IGFBP-6 mRNA levels, as determined by Northern blot a nalysis. The effect was maximal after 48 h of treatment and observed w ith retinoic acid at concentrations of 10 nM to 1 mu M. Retinoic acid increased IGFBP-6 polypeptide levels in the culture medium, as determi ned by Western immunoblot analysis. Cycloheximide at 3.6 mu M slightly decreased IGFBP-6 transcripts but did not prevent the stimulatory eff ect of retinoic acid. The decay of IGFBP-6 mRNA in transcriptionally a rrested Ob cells was similar in control and retinoic acid-treated cell s, and retinoic acid increased the rates of IGFBP-6 transcription, as determined by nuclear run on assays. In conclusion, retinoic acid enha nces IGFBP-6 expression in Ob cells by transcriptional mechanisms. Sin ce IGFBP-6 prevents the effects of IGF II, increased synthesis of IGFB P-6 could mediate selected actions of retinoic acid in bone. (C) 1996 Wiley-Liss, Inc.