FGF-23 is readily detectable in the plasma or serum of healthy persons and can be markedly elevated in those with oncogenic osteomalacia or X-linked hypophosphatemia, suggesting that this growth factor has a role in phosphate homeostasis. FGF-23 measurements might improve the management of phosphate-wasting disorders.

Circulating FGF-23 was significantly elevated in patients with CKD and its concentration correlated with renal creatinine clearance. In healthy volunteers, FGF-23 levels did not change after phosphate deprivation or phosphate loading.

Mutations in the fibroblast growth factor 23 gene, FGF23, cause autosomal dominant hypophosphatemic rickets (ADHR). The gene product, FGF-23, is produced by tumors from patients with oncogenic osteomalacia (OOM), circulates at increased levels in most patients with X-linked hypophosphatemia (XLH) and is phosphaturic when injected into rats or mice, suggesting involvement in the regulation of phosphate (Pi) homeostasis. To better define the precise role of FGF-23 in maintaining Pi balance and bone mineralization, we generated transgenic mice that express wild-type human FGF-23, under the control of the alpha1(I) collagen promoter, in cells of the osteoblastic lineage. At 8 wk of age, transgenic mice were smaller (body weight = 17.5 +/- 0.57 vs. 24.3 +/- 0.37 g), exhibited decreased serum Pi concentrations (1.91 +/- 0.27 vs. 2.75 +/- 0.22 mmol/liter) and increased urinary Pi excretion when compared with wild-type littermates. The serum concentrations of human FGF-23 (undetectable in wild-type mice) was markedly elevated in transgenic mice (>7800 reference units/ml). Serum PTH levels were increased in transgenic mice (231 +/- 62 vs. 139 +/- 44 pg/ml), whereas differences in calcium and 1,25-dihydroxyvitamin D were not apparent. Expression of Npt2a, the major renal Na(+)/Pi cotransporter, as well as Npt1 and Npt2c mRNAs, was significantly decreased in the kidneys of transgenic mice. Histology of tibiae displayed a disorganized and widened growth plate and peripheral quantitative computerized tomography analysis revealed reduced bone mineral density in transgenic mice. The data indicate that FGF-23 induces phenotypic changes in mice resembling those of patients with ADHR, OOM, and XLH and that FGF-23 is an important determinant of Pi homeostasis and bone mineralization.

Fibroblast growth factor-23 (FGF23) is a circulating factor that decreases serum levels of inorganic phosphate (Pi) as well as 1,25-dihydroxyvitamin D 3 . Recent studies also suggest a correlation between serum levels of FGF23 and parathyroid hormone (PTH) in patients with chronic kidney disease. It is, however, unknown whether FGF23 directly modulates PTH expression, or whether the correlation is secondary to abnormalities in Pi and vitamin D metabolism. The objective of the current study was therefore to elucidate possible direct effects of FGF23 on bovine parathyroid cells in vitro. Treatment of parathyroid cells with a stabilized form of recombinant FGF23 (FGF23(R176Q)) induced a rise in early response gene-1 mRNA transcripts, a marker of FGF23 signaling. FGF23(R176Q) potently and dose-dependently decreased the PTH mRNA level within 12 h. In agreement, FGF23(R176Q) also decreased PTH secretion into conditioned media. In contrast, FGF23(R176Q) dose-dependently increased 1a-hydroxylase expression within 3 h. FGF23 (R176Q) did not affect cell viability nor induce apoptosis, whereas a small but significant increase in cell proliferation was found. We conclude that FGF23 is a negative regulator of PTH mRNA expression and secretion in vitro. Our data suggest that FGF23 may be a physiologically relevant regulator of PTH. This defines a novel function of FGF23 in addition to the previously established roles in controlling vitamin D and Pi metabolism.

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Domestication is one of the strongest forms of short-term, directional selection. Although selection is typically only exerted on one or a few target traits, domestication can lead to numerous changes in many seemingly unrelated phenotypes. It is unknown whether such correlated responses are due to pleiotropy or linkage between separate genetic architectures. Using three separate intercrosses between wild and domestic chickens, a locus affecting comb mass (a sexual ornament in the chicken) and several fitness traits (primarily medullary bone allocation and fecundity) was identified. This locus contains two tightly-linked genes, BMP2 and HAO1, which together produce the range of pleiotropic effects seen. This study demonstrates the importance of pleiotropy (or extremely close linkage) in domestication. The nature of this pleiotropy also provides insights into how this sexual ornament could be maintained in wild populations.

Osteoprotegerin (OPG) is a recently cloned member of the tumour necrosis factor receptor family. It has been suggested that this secreted glycoprotein acts as an inhibitor of osteoclastic differentiation. Expression of OPG has previously been demonstrated in a number of tissues. However, it is still unclear whether or not OPG is expressed by human osteoblasts. We have used the RNase protection assay to demonstrate the OPG transcript in primary cultured human osteoblast-like cells, human marrow stroma cells and osteosarcoma cell lines. Furthermore, we have studied the effect of glucocorticoids on OPG mRNA levels in these cells. We demonstrate that glucocorticoids decrease the OPG transcript in a dose-and time-dependent manner. The time-course study reveals that hydrocortisone (10 6 M) decreases OPG mRNA levels within 2 h. This decrease is transient, reaching control levels again after 24 h.Our findings demonstrate that human osteoblasts express the mRNA corresponding to OPG, an inhibitor of osteoclast differentiation. The finding that OPG mRNA levels are decreased by glucocorticoids indicates that a reduced production of OPG from osteoblasts and/or marrow stroma cells could, in part, explain glucocorticoidinduced bone resorption.

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