Differential growth factor control of bone formation through osteoprogenitor differentiation

The osteogenic factors bone morphogenetic protein (BMP-7), platelet-derived growth factor (PDGF)-BB, and fibroblast growth factor (FGF-2) regulate the recruitment of osteoprogenitor cells and their proliferation and differentiation into mature osteoblasts. However, their mechanisms of action on osteoprogenitor cell growth, differentiation, and bone mineralization remain unclear. Here, we tested the hypothesis that these osteogenic agents were capable of regulating osteoblast differentiation and bone formation in vitro. Normal human bone marrow stromal (HBMS) cells were treated with BMP-7 (40 ng ml−1), PDGF-BB (20 ng ml−1), FGF-2 (20 ng ml−1), or FGF-2 plus BMP-7 for 28 days in a serum-containing medium with 10 mM β-glycerophosphate and 50 μg ml−1 ascorbic acid. BMP-7 stimulated a morphological change to cuboidal-shaped cells, increased alkaline phosphatase (ALKP) activity, bone sialoprotein (BSP) gene expression, and alizarin red S positive nodule formation. Hydroxyapatite (HA) crystal deposition in the nodules was demonstrated by Fourier transform infrared (FTIR) spectroscopy only in BMP-7- and dexamethasone (DEX)-treated cells. DEX-treated cells appeared elongated and fibroblast-like compared to BMP-7-treated cells. FGF-2 did not stimulate ALKP, and cell morphology was dystrophic. PDGF-BB had little or no effect on ALKP activity and biomineralization. Alizarin Red S staining of cells and calcium assay indicated that BMP-7, DEX, and FGF-2 enhanced calcium mineral deposition, but FTIR spectroscopic analysis demonstrated no formation of HA similar to human bone in control, PDGF-BB-, and FGF-2-treated samples. Thus, FGF-2 stimulated amorphous octacalcium phosphate mineral deposition that failed to mature into HA. Interestingly, FGF-2 abrogated BMP-7-induced ALKP activity and HA formation. Results demonstrate that BMP-7 was competent as a sole factor in the differentiation of human bone marrow stromal cells to bone-forming osteoblasts confirmed by FTIR examination of mineralized matrix. Other growth factors, PDGF, and FGF-2 were incompetent as sole factors, and FGF-2 inhibited BMP-7-stimulated osteoblast differentiation.