Hexa-D-arginine Treatment Increases 7B2•PC2 Activity in Hyp-Mouse Osteoblasts and Rescues the HYP Phenotype

J Bone Miner Res. 2013 Jan;28(1):56-72. doi: 10.1002/jbmr.1738.

Abstract

Inactivating mutations of the "phosphate regulating gene with homologies to endopeptidases on the X chromosome" (PHEX/Phex) underlie disease in patients with X-linked hypophosphatemia (XLH) and the hyp-mouse, a murine homologue of the human disorder. Although increased serum fibroblast growth factor 23 (FGF-23) underlies the HYP phenotype, the mechanism(s) by which PHEX mutations inhibit FGF-23 degradation and/or enhance production remains unknown. Here we show that treatment of wild-type mice with the proprotein convertase (PC) inhibitor, decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone (Dec), increases serum FGF-23 and produces the HYP phenotype. Because PC2 is uniquely colocalized with PHEX in osteoblasts/bone, we examined if PC2 regulates PHEX-dependent FGF-23 cleavage and production. Transfection of murine osteoblasts with PC2 and its chaperone protein 7B2 cleaved FGF-23, whereas Signe1 (7B2) RNA interference (RNAi) transfection, which limited 7B2 protein production, decreased FGF-23 degradation and increased Fgf-23 mRNA and protein. The mechanism by which decreased 7B2•PC2 activity influences Fgf-23 mRNA was linked to reduced conversion of the precursor to bone morphogenetic protein 1 (proBMP1) to active BMP1, which resulted in limited cleavage of dentin matrix acidic phosphoprotein 1 (DMP1), and consequent increased Fgf-23 mRNA. The significance of decreased 7B2•PC2 activity in XLH was confirmed by studies of hyp-mouse bone, which revealed significantly decreased Sgne1 (7B2) mRNA and 7B2 protein, and limited cleavage of proPC2 to active PC2. The expected downstream effects of these changes included decreased FGF-23 cleavage and increased FGF-23 synthesis, secondary to decreased BMP1-mediated degradation of DMP1. Subsequent Hexa-D-Arginine treatment of hyp-mice enhanced bone 7B2•PC2 activity, normalized FGF-23 degradation and production, and rescued the HYP phenotype. These data suggest that decreased PHEX-dependent 7B2•PC2 activity is central to the pathogenesis of XLH.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Bone and Bones / diagnostic imaging
  • Bone and Bones / drug effects
  • Bone and Bones / metabolism
  • Bone and Bones / pathology
  • Extracellular Matrix Proteins / metabolism
  • Familial Hypophosphatemic Rickets / diagnostic imaging
  • Familial Hypophosphatemic Rickets / drug therapy*
  • Familial Hypophosphatemic Rickets / metabolism
  • Familial Hypophosphatemic Rickets / pathology*
  • Female
  • Fibroblast Growth Factors / metabolism
  • Gene Expression Regulation / drug effects
  • Genetic Diseases, X-Linked*
  • Humans
  • Male
  • Mice
  • Mice, Inbred C57BL
  • Mice, Mutant Strains
  • Neuroendocrine Secretory Protein 7B2 / genetics
  • Neuroendocrine Secretory Protein 7B2 / metabolism*
  • Oligopeptides / pharmacology
  • Oligopeptides / therapeutic use*
  • Osteoblasts / drug effects
  • Osteoblasts / enzymology
  • Osteoblasts / pathology*
  • Phenotype
  • Proprotein Convertase 2 / antagonists & inhibitors
  • Proprotein Convertase 2 / metabolism*
  • RNA, Messenger / genetics
  • RNA, Messenger / metabolism
  • Radiography

Substances

  • Dmp1 protein, mouse
  • Extracellular Matrix Proteins
  • Neuroendocrine Secretory Protein 7B2
  • Oligopeptides
  • RNA, Messenger
  • Sgne1 protein, mouse
  • hexaarginine amide
  • Fibroblast Growth Factors
  • fibroblast growth factor 23
  • Pcsk2 protein, mouse
  • Proprotein Convertase 2