Antibody-Mediated Activation of FGFR1 Induces FGF23 Production and Hypophosphatemia

Ai-Luen Wu,Bo Feng,Jose Zavala-Solorio,Vineela D Gandham,Junichiro Sonoda,Mark Z Chen,Richard A D Carano,Shelby K Wyatt,Ganesh Kolumam,Makoto Makishima

doi:10.1371/journal.pone.0057322

Abstract

The phosphaturic hormone Fibroblast Growth Factor 23 (FGF23) controls phosphate homeostasis by regulating renal expression of sodium-dependent phosphate co-transporters and cytochrome P450 enzymes involved in vitamin D catabolism. Multiple FGF Receptors (FGFRs) can act as receptors for FGF23 when bound by the co-receptor Klotho expressed in the renal tubular epithelium. FGFRs also regulate skeletal FGF23 secretion; ectopic FGFR activation is implicated in genetic conditions associated with FGF23 overproduction and hypophosphatemia. The identity of FGFRs that mediate the activity of FGF23 or that regulate skeletal FGF23 secretion remains ill defined. Here we report that pharmacological activation of FGFR1 with monoclonal anti-FGFR1 antibodies (R1MAb) in adult mice is sufficient to cause an elevation in serum FGF23 and mild hypophosphatemia. In cultured rat calvariae osteoblasts, R1MAb induces FGF23 mRNA expression and FGF23 protein secretion into the culture medium. In a cultured kidney epithelial cell line, R1MAb acts as a functional FGF23 mimetic and activates the FGF23 program. siRNA-mediated Fgfr1 knockdown induced the opposite effects. Taken together, our work reveals the central role of FGFR1 in the regulation of FGF23 production and signal transduction, and has implications in the pathogenesis of FGF23-related hypophosphatemic disorders.

Highlights

Inorganic phosphate plays a crucial role in many biological processes including bone mineralization, vascular function, and cellular activity; its level in the body must be tightly regulated
R1MAb activates Fibroblast Growth Factor 23 (FGF23) pathway in vivo Previously, we reported a mild reduction in serum phosphate levels in lean C57BL/6 and diabetic db/db mice treated with an agonistic anti-FGFR1 antibody, R1MAb1, at 0.5 mg/kg [22]
We used selective antibody activators of FGFR1 to study the effects of systemic FGFR1 activation on phosphate homeostasis

Summary

Introduction

Inorganic phosphate (phosphorus) plays a crucial role in many biological processes including bone mineralization, vascular function, and cellular activity; its level in the body must be tightly regulated. Fibroblast Growth Factor 23 (FGF23) is an endocrine member of the FGF superfamily produced by osteocytes in the bone [1,2,3]. It acts as an important determinant of phosphate homeostasis by controlling renal phosphate transport as well as vitamin D catabolism. Inactivating mutations in genes such as dentin matrix acidic phosphoprotein 1 (Dmp1) [8], Phosphate regulating endopeptidase homolog, X-linked (Phex) [9], or Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (Enpp1) [10,11], have been shown to increase skeletal secretion of FGF23, resulting in hypophosphatemic rickets

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