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 osteoblastic function, and some of these actions may be mediated by changes in the insulin-like growth factor (IGF) axis. Skeletal cells synthesize 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-dependent increase in IGFBP-6 mRNA levels, as determined by Northern blot analysis. The effect was maximal after 48 h of treatment and observed with retinoic acid at concentrations of 10 nM to 1 microM. Retinoic acid increased IGFBP-6 polypeptide levels in the culture medium, as determined by Western immunoblot analysis. Cycloheximide at 3.6 microM slightly decreased IGFBP-6 transcripts but did not prevent the stimulatory effect of retinoic acid. The decay of IGFBP-6 mRNA in transcriptionally arrested Ob cells was similar in control and retinoic acid-treated cells, and retinoic acid increased the rates of IGFBP-6 transcription, as determined by nuclear run on assays. In conclusion, retinoic acid enhances IGFBP-6 expression in Ob cells by transcriptional mechanisms. Since IGFBP-6 prevents the effects of IGF II, increased synthesis of IGFBP-6 could mediate selected actions of retinoic acid in bone.
Gabbitas
B
B
Department of Research, Saint Francis Hospital and Medical Center, Hartford, Connecticut 06105, USA.
Canalis
E
E
D000818:Animals; D015152:Blotting, Northern; D015153:Blotting, Western; D001842:Bone and Bones / Q000166:cytology / Q000502:physiology*; D002478:Cells, Cultured; D004004:Dichlororibofuranosylbenzimidazole / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D018976:Insulin-Like Growth Factor Binding Protein 6 / Q000235:genetics*; D019384:Nucleic Acid Synthesis Inhibitors / Q000494:pharmacology; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats / Q000196:embryology; D013997:Time Factors; D014158:Transcription, Genetic / Q000187:drug effects*; D014212:Tretinoin / Q000494:pharmacology*
D018976:Insulin-Like Growth Factor Binding Protein 6; D019384:Nucleic Acid Synthesis Inhibitors; D012333:RNA, Messenger; D004004:Dichlororibofuranosylbenzimidazole; D014212:Tretinoin