Co-ordination of TGF-β and FGF signaling pathways in bone organ cultures

Transforming growth factor-β (TGF-β) is known to regulate chondrocyte proliferation and hypertrophic differentiation in embryonic bone cultures by a perichondrium dependent mechanism. To begin to determine which factors in the perichondrium mediate the effects of TGF-β, we studied the effect of Insulin-like Growth Factor-1 (IGF-I) and Fibroblast Growth Factors-2 and -18 (FGF2, FGF18) on metatarsal organ cultures. An increase in chondrocyte proliferation and hypertrophic differentiation was observed after treatment with IGF-I. A similar effect was seen after the perichondrium was stripped from the metatarsals suggesting IGF-I acts directly on the chondrocytes. Treatment with FGF-2 or FGF-18 resulted in a decrease in bone elongation as well as hypertrophic differentiation. Treatment also resulted in a decrease in BrdU incorporation into chondrocytes and an increase in BrdU incorporation in perichondrial cells, similar to what is seen after treatment with TGF-β1. A similar effect was seen with FGF2 after the perichondrium was stripped suggesting that, unlike TGF-β, FGF2 acts directly on chondrocytes to regulate proliferation and hypertrophic differentiation. To test the hypothesis that TGF-β regulates IGF or FGF signaling, activation of the receptors was characterized after treatment with TGF-β. Activation was measured as the level of tyrosine phosphorylation on the receptor. Treatment with TGF-β for 24 h did not alter the level of IGFR-I tyrosine phosphorylation. In contrast, treatment with TGF-β resulted in and increase in tyrosine phosphorylation on FGFR3 without alterations in total FGFR3 levels. TGF-β also stimulated expression of FGF18 mRNA in the cultures and the effects of TGF-β on metatarsal development were blocked or partially blocked by pretreatment with FGF signaling inhibitors. The results suggest a model in which FGF through FGFR3 mediates some of the effects of TGF-β on embryonic bone formation.