INTERLEUKIN-6 AND ITS SOLUBLE RECEPTOR REGULATE THE EXPRESSION OF INSULIN-LIKE-GROWTH-FACTOR BINDING PROTEIN-5 IN OSTEOBLAST CULTURES

Interleukin-6(IL-G), a cytokine produced by bone cells, is known to in fluence bone resorption by stimulating the development of osteoclasts from precursor cells and to have mitogenic actions on osteoblastic cel ls. Insulin-like growth factors (IGFs) are important local regulators of bone formation, and IGF binding protein (IGFBP)-5 stimulates bone c ell growth and enhances the effects of IGF-I. We tested the effects of IL-6 in the presence and absence of its soluble receptor (sIL-6R) on IGFBP-5 expression in cultures of osteoblast-enriched cells from 22-da y-old fetal rat calvariae (Ob cells). When tested individually, IL-G a nd sIL-6R had a modest stimulatory effect on IGFBP-5 messenger RNA (mR NA) levels. In contrast, when IL-6 and sIL-GR were tested in combinati on. they caused a considerable increase in IGFBP-5 mRNA levels: and IL -6 at 100 ng/ml and sIL-6R at 125 ng/ml increased IGFBP-5 transcripts by 5- to 7-fold after 24 h. The effect of IL-6 and sIL-6R on IGFBP-5 t ranscripts was not blocked by indomethacin, but cycloheximide markedly inhibited IGFBP-5 mRNA levels in control and treated cultures. IL-6 a nd sIL-6R did not modify the decay of IGFBP-5 mRNA in transcriptionall y arrested Ob cells. and stimulated the rate of IGFBP-5 transcription as demonstrated by a nuclear run-on assay, IL-6 and sIL-6R did not inc rease intact IGFBP-5 levels in the extracellular matrix and increased IGFBP-5 fragments in the culture medium. Conditioned medium from Ob ce lls induced the proteolytic fragmentation of an IGFBP-5 standard, an e ffect that was accelerated and enhanced by conditioned medium from IL- 6/sIL-6R-treated cultures and prevented by metalloprotease inhibitors. In conclusion, IL-6, in the presence of sIL-6R. stimulates IGFBP-5 mR NA expression in Ob cells by transcriptional mechanisms, and accelerat es the fragmentation of the protein.