Bone cells synthesize and respond to fibroblast growth factor: Regulation of proliferation and expression of the osteoblast phenotype

The observation that fibroblast growth factor (FGF) is present in bone matrix, led to our proposal that FGF acts as a paracrine or autocrine growth factor for osteoblasts. To obtain evidence in support of this proposal we grew bovine bone cells in explant culture and tested the subcultured cells for their ability to produce and respond to FGF. The cells expressed phenotypic traits of osteoblasts, including high alkaline phosphatase activity, parathyroid hormone-responsive adenylate cyclase activity, and synthesis of the skeletal extracellular matrix protein, osteocalcin (bone gla protein).

The bone cells expressed transcripts that hybridized with basic FGF (bFGF) gene probes. Extracts of bone cells contained proteins with bFGF-like and acidic FGF (aFGF)-like immunoreactivity. The bone cell-derived bFGF was identified on the basis of its affinity for heparin-Sepharose, its bioactivity, and immunoreactivity. Basic FGF altered both the rate of proliferation and the expression of the osteoblast phenotype by bone cells. Bone cells proliferated in response to both bFGF (EC$\sb{50}$ = 60pg/ml) and aFGF (EC$\sb{50}$ = 2ng/ml.) Transforming growth factor-$\beta$, another growth factor present in bone matrix in vivo, potentiated these effects. bFGF stimulated the production of osteocalcin and these effects were synergistic with those of 1,25 dihydroxy Vitamin D$\sb3$ (1,25(OH)$\sb2$D$\sb3$) and ascorbic acid. In contrast, bFGF reduced alkaline phosphatase and parathyroid hormone-stimulated adenylated cyclase activity of bone cells. Our results support the proposal that cells of the osteoblast lineage are both target cells for FGF and produce FGF.

The bone cells produced an extracellular matrix (BnC-ECM) that possessed both mitogenic and morphogenic properties. The mitogenic activity of the BnC-ECM for vascular endothelial cells was eliminated by pre-incubation of the BnC-ECM with anti-bFGF antibodies, showing that bone cell derived-bFGF was sequestered in the extracellular matrix. The BnC-ECM also increased the concentration of osteocalcin in the media of cultures exposed to bFGF, ascorbic acid, or 1,25(OH)$\sb2$D$\sb3$. These results are consistent with the proposal that bFGF may function as an autocrine or paracrine growth factor for osteoblasts via its deposition into the extracellular matrix of bone.