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 increasing the replication of cells of the osteoblastic lineage and by enhancing the differentiated function of the osteoblast. Although IGF-II is synthesized by skeletal cells, little is known about the mechanisms involved and its regulation by growth factors. IGF-II expression is tissue specific and is developmentally regulated. In the present study, we examined the expression of IGF-II in fetal rat, newborn mouse and MC3T3-E1 osteoblastic (Ob) cells, and in adult rat calvariae. We also determined mechanisms involved in the regulation of IGF-II by platelet-derived growth factor (PDGF) BB, fibroblast growth factor-2 (FGF-2), and transforming growth factor (TGF) beta1. Northern analysis revealed IGF-II transcripts of 3.6 and 1.2 kb in osteoblastic cells and adult rat calvariae. Ribonuclease (RNase) protection assay using probes specific 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 beta1 inhibited the expression of IGF-II P3 mRNA by 50%. PDGF BB, FGF-2, and TGF beta1 also decreased the rates of IGF-II transcription in rat Ob cells as determined by nuclear run-on assays and did not modify the decay of IGF-II in transcriptionally arrested rat Ob cells. In conclusion, the synthesis of IGF-II in osteoblastic cells and in adult rat calvariae is driven by IGF-II P3 and is regulated by skeletal growth factors acting at the transcriptional level using the IGF-II P3.