The parathyroid hormone-vitamin D 3 endocrine system, as well as dietary phosphorus, plays an important role in regulating renal and gastrointestinal absorption of phosphate. Recently, emerging evidence suggests that other systemic and/or paracrine/autocrine factors are present in bones for maintaining phosphate homeostasis, such as fibroblast growth factor-23 (FGF-23), 1 frizzled-related protein-4 (FRP-4), and matrix extracellular phosphoglycoprotein (MEPE) (1-11). These three factors were highly expressed in tumors isolated from oncogenic osteomalacia patients and reduced phosphate transport in kidney. Among these factors, FGF-23 strongly suppressed 1␣,25(OH) 2 D 3 production and elicited hypophosphatemia. Administration of the recombinant FGF-23 protein reduced serum phosphorus without affecting serum calcium, as well as increasing renal phosphorus excretion in mice (12). Mice bearing FGF-23-expressing Chinese hamster ovary cells showed suppressed 25-hydroxyvitamin D 3 1␣-hydroxylase mRNA expression in the kidney (3). FGF-23 mRNA is expressed in a variety of tissues such as thymus, brain, bone, thyroid/parathyroid gland, and heart (2, 3, 13). Recent studies (13, 14) indicated FGF-23 mRNA as well as FGF-23 protein was elevated in bones from patients with McCune-Albright syndrome and also in bones from HYP mouse, mouse homologue to X-linked hypophosphatemic (XLH) rickets. However, the level of serum FGF-23 in hypophosphatemic patients with XLH is still controversial (15-17). Hyperphosphatemic patients with chronic kidney disease showed significant elevation in circulating FGF-23, which correlated with serum phosphorus and creatinine (16, 18 -20), suggesting (a) serum phosphorus was a possible regulator of FGF-23 production or (b) circulating FGF-23 accumulated in chronic renal failure.The purpose of this study was to evaluate the effects of dietary phosphorus and 1␣,25(OH) 2 D 3 on FGF-23 production. Administration of FGF-23 protein or overexpression of Fgf23 gene in rodent suppressed 1␣,25(OH) 2 D 3 production by reducing 25-hydroxyvitamin D 3 1␣-hydroxylase in the proximal tubules (12, 21-23). On the contrary, Fgf23-null mice reported increased circulating 1␣,25(OH) 2 D 3 despite hyperphosphatemia, hypercalcemia, and low PTH levels (24). Administration of 1␣,25(OH) 2 D 3 increased serum FGF-23 in normal mice (25). These observations suggested mutual regulation between FGF-23 and 1␣,25(OH) 2 D 3 ; however, 1␣,25(OH) 2 D 3 administration also increases intestinal phosphate uptake and suppresses PTH. Thus, we used thyroparathyroidectomized rats as well as 5/6 nephrectomized rats fed a diet with various kinds of phosphorus content to examine the direct effect of 1␣,25(OH) 2 D 3 administration on serum FGF-23.* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.§ To whom correspondence should be addressed. Tel.: 81-550-87-6735; Fax: 81-550-8...

The human fibroblast growth factor 23 (hFGF23) and its autosomal dominant hypophosphatemic rickets (ADHR) mutant genes were incorporated into animals by naked DNA injection to investigate the action on phosphate homeostasis in vivo. The hFGF23 mutants (R176Q, R179Q, and R179W) markedly reduced serum phosphorus (6.2-6.9 mg/dl) compared with the plasmid MOCK (8.5 mg/dl). However, native hFGF23 did not affect serum phosphorus (8.6 mg/dl). Both hFGF23 and hFGF23R179Q mRNAs were expressed more than 100-fold in the liver 4 days after injection, however, the C-terminal portion of hFGF23 was detected only in the serum from hFGF23R179Q-injected animals (1109 pg/ ml). hFGF23R179Q mutant was secreted as a 32-kDa protein, whereas, native hFGF23 was detected as a 20-kDa protein in the cell-conditioned media. These results suggest the hFGF23R179Q protein is resistant to intracellular proteolytic processing. The hFGF23R179Q suppressed Na/P i co-transport activities both in kidney and in small intestine by 45 and 30%, respectively, as well as serum 1␣,25-dihydroxyvitamin D 3 to less than 15 pg/ml. However, it had little effect on serum parathyroid hormone (PTH). Infusion of hFGF23R179Q protein normalized serum phosphorus in thyroparathyroidectomized rats without affecting serum calcium. Taken together, the FGF23 mutants reduce both phosphate uptake in intestine and phosphate reabsorption in kidney, independent of PTH action.The FGF23 protein is a novel, secreted protein that consists of 251 amino acids, including a putative N-terminal signal peptide (residues 1-24) (1). Patients with autosomal dominant hypophosphatemic rickets (ADHR), 1 a rare renal phosphatewasting disorder, have been found to have three missense mutations in two arginine residues, R176Q, R179Q, and R179W in the FGF23 gene (2, 3). Also, the FGF23 was identified as a causative factor of oncogenic osteomalacia (OHO) (4). It was highly expressed in the tumor isolated from OHO patients (5), whereas, it was expressed at a very low level in normal tissues. Patients with OHO share similar manifestations with ADHR such as hypophosphatemia, decreased or inappropriately normal 1␣,25-dihydroxyvitamin D concentrations, and osteomalacia. Recent studies indicated administration of the recombinant FGF23 protein reduced serum phosphate without affecting serum calcium, as well as increasing renal phosphate excretion in mice. Mice bearing FGF23-expressing CHO cells showed a suppressed 25-hydroxyvitamin D 1␣-hydroxylase mRNA expression in the kidney (4). These results suggested FGF23 might inhibit renal sodium-dependent phosphate (Na/Pi) co-transporter activity. However, according to the results from the in vitro studies using opossum kidney cells, the FGF23 effects on Na/P i co-transporter are still controversial (4, 6). Yamashita et al. reported that heparin was required to activate FGF23⅐FGFR3c complex and phosphate reabsorption in opossum kidney cells. Also, the mitogen-activating protein kinase signal pathway was a major pathway for the FGF23 signaling (7). Shimada et al. a...

Interleukin-1 (IL-1) greatly induces osteoclast formation and stimulates bone resorption of mouse calvaria in culture. In the presence of soluble IL-6 receptor (sIL-6R), IL-6 similarly induces osteoclast formation, but the potency of IL-6 in inducing bone resorption in organ culture is weaker than that of IL-1. To study the differences in bone-resorbing activity between IL-1 and IL-6, we examined the effects of the two cytokines on the induction of matrix metalloproteinases (MMPs). In mouse calvarial cultures, IL-1 markedly enhanced the messenger RNA (mRNA) expression of MMP-13 (collagenase 3), MMP-2 (gelatinase A), MMP-9 (gelatinase B), and MMP-3 (stromelysin 1), which associated with increases in bone matrix degradation. A hydroxamate inhibitor of MMPs significantly suppressed bone-resorbing activity induced by IL-1. Gelatin zymography showed that both pro- and active-forms of MMP-2 and MMP-9 were detected in the conditioned medium collected from calvarial cultures, and IL-1 markedly stimulated both pro- and active-forms of the two gelatinases. IL-6 with sIL-6R also stimulated mRNA expression and biological activities of these MMPs, but the potency was much weaker than that of IL-1. Conditioned medium collected from IL-1-treated calvariae degraded native type I collagen, but 3/4- and 1/4-length collagen fragments were not detected, suggesting that both collagenases and gelatinases synergistically degraded type I collagen into smaller fragments. In mouse osteoblastic cells, the expression ofMMP-2, MMP-3, and MMP-13 mRNAs could be detected, and they were markedly enhanced by IL-1alpha on days 2 and 5. IL-6 with sIL-6R also induced expression of MMP-13 and MMP-2 mRNAs on day 2, but the expression was rather transient. These results demonstrate that the potency of induction of MMPs by IL-1 and IL-6 is closely linked to the respective bone-resorbing activity, suggesting that MMP-dependent degradation of bone matrix plays a key role in bone resorption induced by these cytokines.

These results suggest that sevelamer treatment might contribute to the suppression of ectopic calcification and ROD.

We have found that an estrogen deficiency causes a marked increase in bone marrow cells. To examine the effect of estrogen on hemopoiesis, we characterized the increased population of bone marrow cells after ovariectomy (OVX). In OVX mice, the percentage of myeloid cells and granulocytes was decreased, whereas that of B220-positive B lymphocytes was selectively increased 2-4 wk after surgery. The total number of myeloid cells and granulocytes did not change appreciably, but that of B220-positive cells was greatly increased by OVX. When OVX mice were treated with estrogen, the increased B lymphopoiesis returned to normal. B220-positive cells were classified into two subpopulations, B220I0W and B220i9'h. The majority of the B220'W cells were negative for the IgM ,u chain, whereas most of the B220"'4" cells were ,i-positive. OVX selectively increased the precursors of B lymphocytes identified by B2201w. ,unegative phenotype, suggesting that an estrogen deficiency stimulates accumulation of B lymphocyte precursors. When bone marrow-derived stromal cells (ST2) were pretreated with estrogen then co-cultured with bone marrow cells in the presence of estrogen, the stromal cell-dependent B lymphopoiesis was greatly inhibited. The present study suggests that estrogen plays an important role in the regulation of B lymphocyte development in mouse bone marrow. (J. Clin.