B. Gabbitas et E. Canalis, RETINOIC ACID STIMULATES THE TRANSCRIPTION OF INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-6 IN SKELETAL CELLS, Journal of cellular physiology, 169(1), 1996, pp. 15-22
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.