OSTEOGENIC PROTEIN-1 AND INSULIN-LIKE-GROWTH-FACTOR-I SYNERGISTICALLYSTIMULATE RAT OSTEOBLASTIC CELL-DIFFERENTIATION AND PROLIFERATION

Previous studies have shown that osteogenic protein-1 (OP-1; also know n as BMP-7) alters the steady state levels of messenger RNA (mRNA) enc oding insulin-like growth factor I (IGF-I), IGF-II, and IGF-binding pr oteins (IGFBPs) in primary cultures of fetal rat calvaria (FRC) cells. In the present study, the effects of exogenous IGF-I on bone cell dif ferentiation and mineralized bone nodule formation induced by OP-1 wer e examined. Exogenous IGF-I synergistically and dose dependently enhan ced OP-1 action in stimulating [H-3]thymidine incorporation, alkaline phosphatase activity, PTH-dependent cAMP level, and bone nodule format ion. Maximal synergism between OP-1 and IGF-I was observed when both f actors were added simultaneously. Synergism was not observed when FRC cells were pretreated with IGF-I for 24 h, followed by OP-1 treatment. These findings suggest that IGF-I acted on OP-1-sensitized FRC cells. To examine the mechanism(s) by which this sensitization may occur, le vels of mRNA encoding OP-1 receptor, IGF-I receptor, and IGFBPs were m easured. The mRNA levels of both type I and II OP-1 receptors were ele vated by OP-1, but were not changed further by combined OP-1 and IGF-I treatment. IGF-I receptor gene expression was not changed by OP-1, IG F-I, or a combination of both factors. OP-1 alone or together with IGF -I increased the steady state IGFBP-3 mRNA level and reduced the stead y state mRNA levels of IGFBP-4, -5, and -6. IGF-I alone did not change the steady state mRNA levels of IGFBP-3, -4, and -6, but elevated tha t of IGFBP-5. Des(1-3)-IGF-I, which has a lower affinity for IGFBPs, w as more effective than the full-length IGF-I in enhancing the OP-1-ind uced alkaline phosphatase activity. Exogenous IGFBP-5 inhibited the OP -1-induced alkaline phosphatase activity and reduced the synergistic s timulatory effect of IGF-I and OP-1. These findings strongly suggest t hat the OP-1-induced downregulation of IGFBPs, especially that of IGFB P-5, is an important mechanism by which OP-1 and IGF-I synergize to st imulate FRC cell differentiation.