B. Gabbitas et E. Canalis, BONE MORPHOGENETIC PROTEIN-2 INHIBITS THE SYNTHESIS OF INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-5 IN BONE CELL-CULTURES, Endocrinology, 136(6), 1995, pp. 2397-2403
Previous work from our laboratory indicated that bone morphogenetic pr
otein-2 (BMP-3) enhances the synthesis of insulin-like growth factor-I
(IGF-I) and IGF-II by skeletal cells. The activity of EGF-I and -II i
s regulated by six known IGF-binding proteins (IGFBPs). Although most
IGFBPs inhibit the actions of IGF on bone growth, IGFBP-8 is stimulato
ry, and its synthesis correlates with changes in osteoblast cell growt
h. We tested the effects of BMP-2 on IGFBP-B expression in cultures of
osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cel
ls). Treatment of Ob cells with BMP-2 caused a time- and dose-dependen
t decrease in IGFBP-8 messenger RNA (mRNA) levels, as determined by No
rthern blot analysis. The effect was maximal after 24 h of treatment a
nd occurred at BMP-2 concentrations of 0.03-3.3 nM. Treatment with BMP
-2 for 24 h also decreased IGFBP-5 polypeptide levels in the extracell
ular matrix, as determined by Western blot analysis. The effects of BM
P-2 on IGFBP-5 transcripts were independent of cell division, as they
were observed in the presence and absence of hydroxyurea (1 mM). IGFBP
-5 transcripts were barely detectable in the presence of the protein s
ynthesis inhibitor cyclohexidmide at 3.6 mu M, and further suppressive
effects of BMP-2 on IGFBP-5 mRNA could not be determined. BMP-2 did n
ot modify the decay of IGFBP-5 mRNA in transcriptionally arrested Ob c
ells. In addition, BMP-2 inhibited IGFBP-5 heterogeneous nuclear RNA,
determined by reverse transcription polymerase chain reaction, after 2
-6 h of treatment, suggesting an inhibition of IGFBF-5 transcription o
r processing. In conclusion, BMP-2 inhibits IGFBP-5 expression in Ob c
ells through pathways that are independent of its mitogenic activity a
nd through mechanisms that may involve decreased transcription or alte
red RNA processing.