Mechanisms of fibroblast growth factor-2 modulation of vascular endothelial growth factor expression by osteoblastic cells

Abstract

Normal bone growth and repair is dependent on angiogenesis. Fibroblast growth factor-2 (FGF-2), vascular endothelial growth factor (VEGF), and transforming growth factor-β (TGFβ) have all been implicated in the related processes of angiogenesis, growth, development, and repair. The purpose of this study was to investigate the relationships between FGF-2 and both VEGF and TGFβ in nonimmortalized and clonal osteoblastic cells. Northern blot analysis revealed 6-fold peak increases in VEGF mRNA at 6 h in fetal rat calvarial cells and MC3T3-E1 osteoblastic cells after stimulation with FGF-2. Actinomycin D inhibited these increases in VEGF mRNA, whereas cycloheximide did not. The stability of VEGF mRNA was not increased after FGF-2 treatment. Furthermore, FGF-2 induced dose-dependent increases in VEGF protein levels (P < 0.01). Although in MC3T3-E1 cells, TGFβ1 stimulates a 6-fold peak increase in VEGF mRNA after 3 h of stimulation, we found that both TGFβ2 and TGFβ3 yielded 2- to 3-fold peak increases in VEGF mRNA levels noted after 6 h of stimulation. Similarly, both TGFβ2 and TGFβ3 dose dependently increased VEGF protein production. To determine whether FGF-2-induced increases in VEGF mRNA may have occurred independently of TGFβ, we disrupted TGFβ signal transduction (using adenovirus encoding a truncated form of TGFβ receptor II), which attenuated TGFβ1 induction of VEGF mRNA, but did not impede FGF-2 induction of VEGF mRNA. In summary, FGF-2-induced VEGF expression by osteoblastic cells is a dose-dependent event that may be independent of concomitant FGF-2-induced modulation of TGFβ activity.