Biological activity of FGF-23 fragments

Abstract

The phosphaturic activity of intact, full-length, fibroblast growth factor-23 (FGF-23) is well documented. FGF-23 circulates as the intact protein and as fragments generated as the result of proteolysis of the full-length protein. To assess whether short fragments of FGF-23 are phosphaturic, we compared the effect of acute, equimolar infusions of full-length FGF-23 and various FGF-23 fragments carboxyl-terminal to amino acid 176. In rats, intravenous infusions of full-length FGF-23 and FGF-23 176-251 significantly and equivalently increased fractional phosphate excretion (FE Pi) from 14 +/- 3 to 32 +/- 5% and 15 +/- 2 to 33 +/- 2% (p < 0.001), respectively. Chronic administration of FGF-23 176-251 reduced serum Pi and serum concentrations of 1alpha,25-dihydroxyvitamin D. Shorter forms of FGF-23 (FGF-23 180-251 and FGF-23 184-251) retained phosphaturic activity. Further shortening of the FGF-23 carboxyl-terminal domain, however, abolished phosphaturic activity, as infusion of FGF-23 206-251 did not increase urinary phosphate excretion. Infusion of a short fragment of the FGF-23 molecule, FGF-23 180-205, significantly increased FE Pi in rats and reduced serum Pi in hyperphosphatemic Fgf-23 ( -/- ) knockout mice. The activity of FGF-23 180-251 was confirmed in opossum kidney cells in which the peptide reduced Na(+)-dependent Pi uptake and enhanced internalization of the Na(+)-Pi IIa co-transporter. We conclude that carboxyl terminal fragments of FGF-23 are phosphaturic and that a short, 26-amino acid fragment of FGF-23 retains significant phosphaturic activity.

Figure 1

chematic representation of the structure of full length FGF-23 and FGF-23 fragments.

Figure 2

Bioactivity of various FGF-23 peptides. Effect of full length recombinant FGF-23 and FGF-23 fragments 176-251, 180-251, 184-251, 206-251 and 180-205 on the excretions of phosphate (FE Pi) and sodium (FE Na). The solute excretion in period C1 (control) was subtracted form the value in the experimental period (C2). An * indicates a significant difference, C 1 compared to C 2, paired T test, p<0.05. Data are expressed as mean±SEM.

Figure 3

Serum phosphate levels of Fgf-23^−/− mice before and after ip injection of vehicle (saline), FGF23 R176Q, and FGF23 180-205. NS = not significant, ** = P<0.01

Figure 4. Inhibition of Sodium-dependent Phosphate Transport in OK Cells by FGF-23 and FGF-23 180-251

Effect of FGF-23 or FGF-23 180-251 on sodium-dependent phosphate transport in opossum kidney cells. An ^* indicates statistically significant differences, experimental vs. control, non-paired T test, p<0.05. Data are expressed as mean±SEM.

Figure 5

Internalization by FGF-23 or FGF-23 180-251 of the sodium-dependent co-transporter IIa in OK cells.