INSULIN-LIKE-GROWTH-FACTOR-II PROMOTER EXPRESSION IN CULTURED RODENT OSTEOBLASTS AND ADULT-RAT BONE

Insulin-like growth factor (IGF)-II stimulates bone formation by incre asing the replication of cells of the osteoblastic lineage and by enha ncing the differentiated function of the osteoblast. Although IGF-II i s synthesized by skeletal cells, little is known about the mechanisms involved and its regulation by growth factors. IGF-II expression is ti ssue specific and is developmentally regulated, in the present study, we examined the expression of IGF-II in fetal rat, newborn mouse and M C3T3-E1 osteoblastic (Ob) cells, and in adult rat calvariae. We also d etermined mechanisms involved in the regulation of IGF-II by platelet- derived growth factor (PDGF) BB, fibroblast growth factor-2 (FGF-2), a nd transforming growth factor (TGF)beta 1. Northern analysis revealed IGF-II transcripts of 3.6 and 1.2 kb in osteoblastic cells and adult r at calvariae. Ribonuclease (RNase) protection assay using probes speci fic to the three known IGF-II promoters, P1, P2, and P3, demonstrated messenger RNA (mRNA) expression driven by P3 in osteoblasts and adult rat calvariae, but no expression of P1 or P2 transcripts. PDGF BB, FGF -2, and TGF beta 1 inhibited the expression of IGF-II P3 mRNA by 50%. PDGF BE, FGF-2, and TGF beta 1 also decreased the rates of IGF-II tran scription in rat Ob cells as determined by nuclear run-on assays and d id not modify the decay of IGF-II in transcriptionally arrested rat Ob cells. In conclusion, the synthesis of IGF-II in osteoblastic cells a nd in adult rat calvariae is driven by IGF-II P3 and is regulated by s keletal growth factors acting at the transcriptional level using the I GF-II P3.