BONE MORPHOGENETIC PROTEIN-2 INCREASES INSULIN-LIKE GROWTH FACTOR-I AND FACTOR-II TRANSCRIPTS AND POLYPEPTIDE LEVELS IN BONE CELL-CULTURES

Insulin-like growth factors (IGF) I and II are among the most prevalen t growth factors secreted by bone cells and are presumed to act as aut ocrine regulators of bone formation. Certain growth factors, synthesiz ed by skeletal cells and known to stimulate the replication but not th e differentiated function of cells of the osteoblastic lineage, have b een shown to inhibit skeletal IGF-I and II synthesis. We postulated th at growth factors with limited mitogenic activity and with differentia tion-inducing properties, such as bone morphogenetic protein (BMP) 2, have the opposite effect and enhance IGF-I and II synthesis. We tested the effects of BMP-2 on IGF-I and II mRNA expression and polypeptide concentrations in cultures of osteoblast-enriched (OB) cells from 22 d ay fetal rat calvariae. Steady-state IGF-I and II mRNA levels were det ermined by northern blot analysis, and IGF-I and II concentrations wer e determined in acidified and fractionated culture medium by a specifi c radioimmunoassay. After 24-48 h of treatment, BMP-2 at 3.3 nM increa sed IGF-I and II transcripts by up to twofold and polypeptide levels b y up to fourfold. BMP-2 was a more potent stimulator of IGF-II synthes is, and it was active at doses as low as 0.03 nM for IGF-II mRNA and 0 .3 nM for IGF-II protein, whereas a dose of 3.3 nM was required to obs erve the effect on IGF-I synthesis. The effects of BMP-2 on IGF-I and II transcripts and polypeptide levels were dependent on protein synthe sis and decreased in the presence of cycloheximide at 3.6 mu M. In con clusion, BMP-2 increases skeletal IGF-I and II synthesis by increasing IGF-I and II transcript levels, and this effect may contribute to its actions on selected aspects of OB cell differentiated function.