GROWTH-FACTOR REGULATION OF INSULIN-LIKE-GROWTH-FACTOR BINDING PROTEIN-6 EXPRESSION IN OSTEOBLASTS

Previously we have shown that transforming growth factor beta (TGF bet a) 1, basic fibroblast growth factor (FGF), and platelet-derived growt h factor (PDGF) BB inhibit the synthesis of insulin-like growth factor (IGF) II, but their effects on IGF binding protein (IGFBP)-6 in osteo blast cultures are not known. IGFBP-6 binds IGF II with high affinity and prevents IGF II-mediated effects, so that a possible mode of regul ating the IGF II available to bone cells would be by changing the leve ls of IGFBP-6. To enhance our understanding of the actions of growth f actors on the IGF II axis in bone, we tested the effects of TGF beta 1 , basic FGF, PDGF BB, IGF I, and IGF II on the expression of IGFBP-6 i n cultures of osteoblast-enriched cells from 22 day fetal rat calvaria e (Ob cells). Treatment of Ob cells with TGF beta 1 caused a time- and dose-dependent decrease in IGFBP-6 mRNA levels, as determined by Nort hern blot analysis. The effect was maximal after 48 h and observed wit h TGF beta 1 concentrations of 0.04 nM and higher. TGF beta 1 also dec reased IGFBP-6 polypeptide levels in the medium, as determined by West ern immunoblot analysis. Cycloheximide at 3.6 mu M decreased IGFBP-6 t ranscripts and prevented the effect of TGF beta 1. The decay of IGFBP- 6 mRNA in transcriptionally arrested Ob cells was not modified by TGF beta 1. In addition, TGF beta 1 decreased the rates of IGFBP-6 transcr iption as determined by a nuclear run-on assay. In contrast, basic FGF , PDGF BB, IGF I, and IGF II did not change IGFBP-6 mRNA levels in Ob cells. In conclusion, TGF beta 1 inhibits IGFBP-6 expression in Ob cel ls by transcriptional mechanisms. Since IGFBP-6 binds IGF II and preve nts its effects on bone cells, decreased synthesis of IGFBP-6 induced by TGF beta 1 could be a local feedback mechanism to increase the amou nt of IGF II available in the bone microenvironment. (C) 1997 Wiley-Li ss, Inc.