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Abstract
Bone sialoprotein (BSP), an early marker of osteoblast differentiation, has been implicated in the nucleation of hydroxyapatite during de novo bone formation. Basic fibroblast growth factor (FGF2) is recognized as a potent mitogen for a variety of mesenchymal cells. In skeletal tissues, FGF2 produced by osteoblasts accumulates in the bone matrix and acts as an autocrine/paracrine regulator of bone cells. To determine the molecular mechanism of FGF2 regulation of osteogenesis, we have analyzed the effects of FGF2 on the expression of BSP in the rat osteosarcoma cell line ROS 17/2.8. FGF2 at 10 ng/ml, increased BSP mRNA levels ~4-fold; the stimulation was first evident at 3 hr, reached maximal levels at 6 hr. The stability of the BSP mRNA was not significantly affected by FGF2, suggesting that the increased mRNA was due to increased transcription. From transient transfection analyses using various BSP promoter-luciferase constructs, a FGF2 response element (FRE) (nts m 92 to m 85, "GGTGAGAA") was identified as a target of transcriptional activation by FGF2. Ligation of two copies of the FRE 5' to an SV40 promoter was sufficient to confer FGF responsive transcription. A sequence-specific protein-DNA complex, formed with a double-stranded oligonucleotide encompassing the FRE and nuclear extracts from ROS 17/2.8 cells, but not from fibroblasts, was increased following FGF2 stimulation. Several point mutations within the critical FRE sequence abrogated the formation of this complex and suppressed both basal and FGF2-mediated promoter activity. Thus, we have identified a novel FRE within the proximal promoter of the BSP gene that mediates both constitutive and FGF2-induced BSP transcription.