Fibroblast growth factor 23 (FGF23) is responsible for
phosphate wasting and the phenotypic changes observed in human diseases such as
X-linked hypophosphatemia (XLH). Targeted overexpression of nuclear high-molecular weight
fibroblast growth factor 2 isoforms (HMW
isoforms) in osteoblasts resulted in a transgenic mouse with phenotypic changes similar to XLH, including increased FGF23,
hypophosphatemia, and
rickets/
osteomalacia. The goal of this study was to assess whether HMW
isoforms also reduced mineralized bone formation via
phosphate-independent effects in bone marrow stromal cells (BMSCs) by modulating FGF23/
FGF receptor (FGFR)/
extracellular signal-regulated kinase (ERK) signaling. To determine if decreased bone formation in BMSC cultures from HMW transgenic mice could be rescued by blocking this pathway, an FGF23
neutralizing antibody, the FGFR
tyrosine kinase inhibitor SU5402 and the
mitogen-activated protein kinase (MAPK) inhibitor
PD98059 were used. FGF23 levels in the
conditioned medium of HMW BMSC cultures were dramatically increased compared to BMSC from control (Vector) mice. Mineralized nodule formation was significantly decreased in HMW BMSC cultures compared with control cultures. The decreased nodule formation in HMW cultures was partially rescued by the FGF23
neutralizing antibody,
SU5402 and
PD98059.
mRNA levels for the osteoblast-related genes,
osteocalcin, Runt-related
transcription factor 2 (Runx2), and osterix, and the osteocyte-related gene dentin matrix acidic
phosphoprotein 1 (Dmp1) were significantly decreased in HMW cultures compared with control cultures, and the decreases were partially rescued by
SU5402 or
PD98059 treatment.
Matrix-gla-protein (Mgp)
mRNA was significantly higher in HMW cultures compared with control cultures, reduced by
SU5402, but further increased by
PD98059. Our results suggest that
phosphate-independent effects of HMW
isoforms in vitro may be directly mediated in part via FGF23 and that HMW
isoforms signal via FGF23/FGFR/MAPK to inhibit bone formation in vitro.