OSTEOGENIC PROTEIN-1-MEDIATED INSULIN-LIKE GROWTH-FACTOR GENE-EXPRESSION IN PRIMARY CULTURES OF RAT OSTEOBLASTIC CELLS

Osteogenic protein-1 (OP-1) is a member of the bone morphogenetic prot ein family and has been shown to induce new bone formation in vivo. In the present study, we determined whether the expression of the IGF sy stem, a significant growth factor system in bone, was altered by OP-1 in primary cultures of fetal rat calvarial cells. Levels of messenger RNA (mRNA) encoding insulin-like growth factor I (IGF-I), IGF-II, IGF- I receptor, and IGF-binding proteins (IGFBF-1 to -6) were determined a fter OP-1 treatment. The level of total IGF-I mRNA was elevated in an OP-1 concentration (0-1000 ng/ml)-dependent manner, with maximal stimu lation of IGF-I mRNA of 2- to 3-fold apparent 24 h after treatment. Th e increase in the IGF-I mRNA level involved a preferential stimulation of transcripts initiated at start site 2 in the exon 1 promoter. The level of IGF-II mRNA also increased by approximately 2-fold in OP-1-tr eated cells in a concentration-dependent manner. The level of IGF-I re ceptor mRNA was not altered by treatment. Whereas IGFBP-1 mRNA was not detected in these cells, IGFBP-2 mRNA was expressed, but the expressi on was not changed after treatment for 48 h in the concentration range (0-1000 ng/ml) tested. The IGFBP-5 mRNA level was increased slightly 48 h after OP-1 treatment in a concentration-dependent manner. The IGF BP-4, -5, and -6 mRNA levels decreased dramatically in an OP-1 concent ration-dependent manner. In addition, coincubation of antisense oligon ucleotides corresponding to IGF-I or -II mRNA sequence with OP-1 reduc ed the OP-1-induced elevation in alkaline phosphatase activity. The pr esent results suggest that the differentiation of rat osteoblastic cel ls in response to OP-1 is mediated in part by increased IGF-I and -II gene expression and alterations in the gene expression of different IG FBPs.