title: Hexa-D-arginine treatment increases 7B2•PC2 activity in hyp-mouse osteoblasts and rescues the HYP phenotype.
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.
authors.affiliation: Department of Medicine, University of Wisconsin-Madison and Geriatric Research and Education Center, William S. Middleton Memorial Veterans Hospital, Madison, WI, USA.
authors.forename: Baozhi
authors.initials: B
authors.lastname: Yuan
authors.forename: Jian Q
authors.initials: JQ
authors.lastname: Feng
authors.forename: Stephen
authors.initials: S
authors.lastname: Bowman
authors.forename: Ying
authors.initials: Y
authors.lastname: Liu
authors.forename: Robert D
authors.initials: RD
authors.lastname: Blank
authors.forename: Iris
authors.initials: I
authors.lastname: Lindberg
authors.forename: Marc K
authors.initials: MK
authors.lastname: Drezner
mesh_terms: D000818:Animals; D001842:Bone and Bones / Q000000981:diagnostic imaging / Q000187:drug effects / Q000378:metabolism / Q000473:pathology; D016326:Extracellular Matrix Proteins / Q000378:metabolism; D053098:Familial Hypophosphatemic Rickets / Q000000981:diagnostic imaging / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology*; D005260:Female; D000089703:Fibroblast Growth Factor-23; D005346:Fibroblast Growth Factors / Q000378:metabolism; D005786:Gene Expression Regulation / Q000187:drug effects; D040181:Genetic Diseases, X-Linked; D006801:Humans; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008817:Mice, Mutant Strains; D051859:Neuroendocrine Secretory Protein 7B2 / Q000235:genetics / Q000378:metabolism*; D009842:Oligopeptides / Q000494:pharmacology / Q000627:therapeutic use*; D010006:Osteoblasts / Q000187:drug effects / Q000201:enzymology / Q000473:pathology*; D010641:Phenotype; D045706:Proprotein Convertase 2 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011859:Radiography
chemical_list: C513308:Dmp1 protein, mouse; D016326:Extracellular Matrix Proteins; C000717427:FGF23 protein, human; C000717428:Fgf23 protein, mouse; D051859:Neuroendocrine Secretory Protein 7B2; D009842:Oligopeptides; D012333:RNA, Messenger; C497679:Sgne1 protein, mouse; C492739:hexaarginine amide; D005346:Fibroblast Growth Factors; D000089703:Fibroblast Growth Factor-23; C473668:Pcsk2 protein, mouse; D045706:Proprotein Convertase 2