The role of the 18-kDa isoform of fibroblast growth factor-2 (FGF2) in the maintenance of bone mass was examined in Col3.6 -18-kDa FGF2-IRES-GFPsaph transgenic (18-kDa TgFGF2) mice in which a 3.6-kb fragment of the type I collagen 5-regulatory region (Col3.6) drives the expression of only the 18-kDa isoform of FGF2 with green fluorescent protein-sapphire (GFPsaph). Vector only transgenic mice (Col3.6-IRESGFPsaph, VTg) were also developed as a control, and mice specifically deficient in 18-kDa FGF2 (FGF2 lmw؊/؊ ) were also examined. Bone mineral density, femoral bone volume, trabecular thickness, and cortical bone area and thickness were significantly increased in 18-kDa TgFGF2 mice compared with VTg. Bone marrow cultures (BMSC) from 18-kDa TgFGF2 mice produced more mineralized nodules than VTg. Increased bone formation was associated with reduced expression of the Wnt antagonist secreted frizzled receptor 1 (sFRP-1). In contrast to 18-kDa TgFGF2 mice, FGF2 lmw؊/؊ mice have significantly reduced bone mineral density and fewer mineralized nodules, coincident with increased expression of sFRP-1 in bones and BMSC. Moreover, silencing of sFRP-1 in BMSC from FGF2 lmw؊/؊ mice reversed the decrease in -catenin and Runx2 mRNA. Assay of Wnt/-catenin-mediated transcription showed increased and decreased TCF-luciferase activity in BMSC from 18-kDa TgFGF2 and FGF2 lmw؊/؊ mice, respectively. Collectively, these results demonstrate that the 18-kDa FGF2 isoform is a critical determinant of bone mass in mice by modulation of the Wnt signaling pathway.
A variety of tissues, including bone, produce FGF22 where osteoblasts deposit it in newly forming bone matrix (1). A single fgf2 gene encodes multiple protein isoforms from alternative translation start sites (2, 3). Humans express four FGF2 isoforms, including three high molecular mass 22-, 23-, and 24-kDa proteins that have nuclear localization sequences and a low molecular mass 18-kDa FGF2 protein that is exported from cells. In rodents, there are two high molecular mass isoforms of 22 and 21 kDa and a low molecular mass 18-kDa FGF2 protein that is exported from cells. Translation of each of the three high molecular mass human FGF2 isoforms (22, 23, and 24 kDa) is initiated from an unconventional CUG translation initiation codon. In contrast, translation of the 18-kDa FGF2 isoform is initiated from a classical AUG codon located downstream of the CUG codons. Thus, multiple isoforms of FGF2 protein can be expressed from a single mRNA as a result of translation at either AUG (18-kDa protein) or CUG (22-, 23-, and 24-kDa proteins) start sites (2-4).Previous studies showed that constitutive overexpression of all the human FGF2 protein isoforms in transgenic (TgFGF2) mice resulted in chondrodysplasia (5), decreased bone mineral density (BMD), and decreased bone mass (6). We have also reported that overexpression of the 18-kDa FGF2 isoform increased osteoblastic ROS17/2.8 cell proliferation (7). Counterintuitively, targeted deletion or "knock-out" (KO) of all FGF2 protein isoforms in...