Serum fibroblast growth factor 23, serum iron and bone mineral density in premenopausal women

Fibroblast growth factor 23 (FGF23) circulates as active protein and inactive fragments. Low iron status increases FGF23 gene expression, and iron deficiency is common. We hypothesized that in healthy premenopausal women, serum iron influences C-terminal and intact FGF23 concentrations, and that iron and FGF23 associate with bone mineral density (BMD). Serum iron, iron binding capacity, percent iron saturation, phosphorus, and other biochemistries were measured in stored fasting samples from healthy premenopausal white (n=1898) and black women (n=994), age 20-55years. Serum C-terminal and intact FGF23 were measured in a subset (1631 white and 296 black women). BMD was measured at the lumbar spine and femur neck. Serum phosphorus, calcium, alkaline phosphatase and creatinine were lower in white women than black women (p<0.001). Serum iron (p<0.0001) and intact FGF23 (p<0.01) were higher in white women. C-terminal FGF23 did not differ between races. Phosphorus correlated with intact FGF23 (white women, r=0.120, p<0.0001; black women r=0.163, p<0.01). However, phosphorus correlated with C-terminal FGF23 only in black women (r=0.157, p<0.01). Intact FGF23 did not correlate with iron. C-terminal FGF23 correlated inversely with iron (white women r=-0.134, p<0.0001; black women r=-0.188, p<0.01), having a steeper slope at iron <50mcg/dl than ≥50mcg/dl. Longitudinal changes in iron predicted changes in C-terminal FGF23. Spine BMD correlated with iron negatively (r=-0.076, p<0.01) in white women; femur neck BMD correlated with iron negatively (r=-0.119, p<0.0001) in black women. Both relationships were eliminated in weight-adjusted models. BMD did not correlate with FGF23. Serum iron did not relate to intact FGF23, but was inversely related to C-terminal FGF23. Intact FGF23 correlated with serum phosphorus. In weight-adjusted models, BMD was not related to intact FGF23, C-terminal FGF23 or iron. The influence of iron on FGF23 gene expression is not important in determining bone density in healthy premenopausal women

Bone Mass and Strength are Significantly Improved in Mice Overexpressing Human WNT16 in Osteocytes

Recently, we demonstrated that osteoblast-specific overexpression of human WNT16 increased both cortical and trabecular bone mass and structure in mice. To further identify the cell-specific role of Wnt16 in bone homeostasis, we created transgenic (TG) mice overexpressing human WNT16 in osteocytes using Dmp1 promoter (Dmp1-hWNT16 TG) on C57BL/6 (B6) background. We analyzed bone phenotypes and serum bone biomarkers, performed gene expression analysis and measured dynamic bone histomorphometry in Dmp1-hWNT16 TG and wild-type (WT) mice. Compared to WT mice, Dmp1-hWNT16 TG mice exhibited significantly higher whole-body, spine and femoral aBMD, BMC and trabecular (BV/TV, Tb.N, and Tb.Th) and cortical (bone area and thickness) parameters in both male and female at 12 weeks of age. Femur stiffness and ultimate force were also significantly improved in the Dmp1-hWNT16 TG female mice, compared to sex-matched WT littermates. In addition, female Dmp1-hWNT16 TG mice displayed significantly higher MS/BS, MAR and BFR/BS compared to the WT mice. Gene expression analysis demonstrated significantly higher mRNA level of Alp in both male and female Dmp1-hWNT16 TG mice and significantly higher levels of Osteocalcin, Opg and Rankl in the male Dmp1-hWNT16 TG mice in bone tissue compared to sex-matched WT mice. These results indicate that WNT16 plays a critical role for acquisition of both cortical and trabecular bone mass and strength. Strategies designed to use WNT16 as a target for therapeutic interventions will be valuable to treat osteoporosis and other low bone mass conditions