AUTOCRINE REGULATION AND CELL-GROWTH AND SECRETION OF INSULIN-LIKE GROWTH-FACTOR-I (IGF-1) IN OSTEOBLASTIC CELL-LINE MC3T3-E1

Bone cells maintained in culture produce different growth factors whic h modulate cell growth via a mechanism of auto/paracrine regulation. I GF-1 is abundantly produced by murine bone cells where it acts as a mi togenic agent. The aim of this work was to study the effect of IGF-II, TGFbeta1, basic FGF (FGFb) and PDGF on cell growth and production of IGF-1 in the murine osteoblastic clonal cell line MC3T3-E1. IGF-1 was assayed by RIA after elimination of the IGF binding proteins. After 24 th of treatment in culture conditions without serum, incorporation of [H-3] methylthymidine increased significantly in MC3T3-E1 treated with IGF-II, FGFb and PDGF. The effect was dose-dependent. At low cell den sity (2.5 X 10(4) cemm/cm2) and after 24 h treatment, IGF-II at 10 ng/ ml led to a 220% increase in IGF-I production in MC3T3-E1 cells (9.5 /- 1.5 vs 4.2 +/- 0.44 ng/mug protein, p < 0.001) while TGFbeta1, FGFb and PDGF at 1 ng/ml led to a significant decrease (65, 95 and 85% res pectively) in IGF-I (TGFbeta1: 1.5 +/- 0.3 ng/ mug; FGBb: 0.21 +/- 0.0 4 ng/mug; PDGF: 0.66 +/- 0.1 ng/mug; p < 0.001). Production of IGF-I w as controlled by a dose-dependent relationship and varied as a functio n of incubation time and cell density. IGF-II led to an increase in mR NA coding for IGF-1 as early as the first hour after IGF-II addition w ith a maximal effect at 6 hours. The effects of IGF-II and TGFbeta1 on cell growth and on the regulation of IGF-I production were coherent w hile FGFb and PDGF acted as both powerful mitogenic agents and inhibit eurs of MC3T3-E1 cell line secretion of IGF-I. These results demonstra te the complexity of local regulation mechanisms and the important rol e of auto/paracrine mechanisms in controlling bone remodelling.