Human Fibroblast Growth Factor-23 Mutants Suppress Na+-dependent Phosphate Co-transport Activity and 1α,25-Dihydroxyvitamin D3 Production

Abstract

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...

Calcineurin Aβ Is Central to the Expression of the Renal Type II Na/Pi Co-transporter Gene and to the Regulation of Renal Phosphate Transport

“…The type II Na/Pi co-transporter is regulated by changes in dietary P but also by factors such as PTH and FGF23 (7). The phosphaturic response to PTH has been well studied.…”

Hyperphosphatemic familial tumoral calcinosis caused by a mutation in GALNT3 in a European kindred

“…FGF23 decreases circulating phosphate levels by downregulation of NaPiIIa (Shimada et al 2005), the major phosphate transporter in the renal proximal tubule, by inhibition of NaPiIIb, responsible for trans-intestinal phosphate transport , and by down-regulation of 1-alpha-hydroxylation of 25-hydroxycholecalciferol (Inoue et al 2005). Proteolytic degradation of FGF23 is believed to play a major role in regulating these activities (Saito et al 2003). However, recent data suggest that FGF23 is also subject to O-glycosylation (Fukumoto 2005), and it is therefore tempting to speculate that ppGalNacT3-mediated O-glycosylation may be involved in the regulation of FGF23 activity.…”

MEPE has the properties of an osteoblastic phosphatonin and minhibin

“…The specific ADHR mutations increase the stability of mutated FGF23 and the uncleaved mutated molecule is phosphaturic and elicits osteomalacia/rickets [4,77,81,98]. Wild-type FGF23 is also expressed in OHO tumors and the tumor over-expression of the wild-type FGF23 molecule is thought to swamp proteolytic mechanisms that normally inactivate full-length FGF23 [11,15,81].…”