FGF23 is endogenously phosphorylated in bone cells

Iris Lindberg, Hong Weng Pang, Joseph P. Stains, David Clark, Austin J. Yang, Lynda Bonewald, Kevin Z. Li

Research output: Contribution to journalArticle

20 Citations (Scopus)

Abstract

Levels of serum phosphate are controlled by the peptide hormone FGF23, secreted from bone osteocytes. Elevated levels of circulating FGF23 are a key factor in several hypophosphatemic disorders and play a role in chronic kidney disease. Posttranslational processing of FGF23 includes multi-site O-glycosylation, which reduces intracellular cleavage by proprotein convertases. The FGF23 protein also contains four serine phosphorylation consensus sequences (S-X-D/E); in this work, we asked whether FGF23 is a substrate for secretory phosphorylation. Both HEK cells as well as IDG-SW3 cells, an osteocyte model, incorporated radiolabeled orthophosphate into intact FGF23, as well as into the 14-kDa carboxy-terminal-but not the 17-kDa N-terminal-fragment. Sequential serine-to-alanine site-directed mutagenesis of four kinase consensus sites showed that labeling occurred on three serines within the carboxy-terminal fragment, Ser180 (adjacent to the cleavage site), Ser207, and Ser212. Liquid chromatography-coupled mass spectroscopy indicated the presence of phosphate at Ser212 in recombinant R&D mouse FGF23(R179Q) , confirming labeling results. A phosphopeptide-specific antibody was raised against phospho-Ser212 and exhibited immunoreactivity in osteocytes present in mouse long bone, providing further evidence that FGF23 is naturally phosphorylated in bone. Bone SIBLING proteins are serine-phosphorylated by the ubiquitous Golgi secretory kinase FAM20C. Cotransfection of HEK and MC3T3 cells with FGF23 and active, but not inactive, FAM20C kinase increased the storage and release of FGF23 in radiolabeling experiments, indicating potential effects of phosphorylation on FGF23 stability. Collectively, these data point to an important role for phosphorylation of FGF23 in bone.

Original languageEnglish (US)
Pages (from-to)449-454
Number of pages6
JournalJournal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research
Volume30
Issue number3
DOIs
StatePublished - Mar 1 2015
Externally publishedYes

Fingerprint

Osteocytes
Serine
Bone and Bones
Phosphorylation
Phosphotransferases
Phosphates
Phospho-Specific Antibodies
Proprotein Convertases
Peptide Hormones
Consensus Sequence
Site-Directed Mutagenesis
Chronic Renal Insufficiency
Glycosylation
Liquid Chromatography
Alanine
Mass Spectrometry
Proteins
Serum

Keywords

  • BONE
  • FAM20C
  • FGF23
  • KINASE
  • OSTEOCYTE
  • PHOSPHATE HOMEOSTASIS

ASJC Scopus subject areas

  • Endocrinology, Diabetes and Metabolism
  • Orthopedics and Sports Medicine

Cite this

FGF23 is endogenously phosphorylated in bone cells. / Lindberg, Iris; Pang, Hong Weng; Stains, Joseph P.; Clark, David; Yang, Austin J.; Bonewald, Lynda; Li, Kevin Z.

In: Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research, Vol. 30, No. 3, 01.03.2015, p. 449-454.

Research output: Contribution to journalArticle

Lindberg, Iris ; Pang, Hong Weng ; Stains, Joseph P. ; Clark, David ; Yang, Austin J. ; Bonewald, Lynda ; Li, Kevin Z. / FGF23 is endogenously phosphorylated in bone cells. In: Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research. 2015 ; Vol. 30, No. 3. pp. 449-454.
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AU - Stains, Joseph P.

AU - Clark, David

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AU - Bonewald, Lynda

AU - Li, Kevin Z.

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AB - Levels of serum phosphate are controlled by the peptide hormone FGF23, secreted from bone osteocytes. Elevated levels of circulating FGF23 are a key factor in several hypophosphatemic disorders and play a role in chronic kidney disease. Posttranslational processing of FGF23 includes multi-site O-glycosylation, which reduces intracellular cleavage by proprotein convertases. The FGF23 protein also contains four serine phosphorylation consensus sequences (S-X-D/E); in this work, we asked whether FGF23 is a substrate for secretory phosphorylation. Both HEK cells as well as IDG-SW3 cells, an osteocyte model, incorporated radiolabeled orthophosphate into intact FGF23, as well as into the 14-kDa carboxy-terminal-but not the 17-kDa N-terminal-fragment. Sequential serine-to-alanine site-directed mutagenesis of four kinase consensus sites showed that labeling occurred on three serines within the carboxy-terminal fragment, Ser180 (adjacent to the cleavage site), Ser207, and Ser212. Liquid chromatography-coupled mass spectroscopy indicated the presence of phosphate at Ser212 in recombinant R&D mouse FGF23(R179Q) , confirming labeling results. A phosphopeptide-specific antibody was raised against phospho-Ser212 and exhibited immunoreactivity in osteocytes present in mouse long bone, providing further evidence that FGF23 is naturally phosphorylated in bone. Bone SIBLING proteins are serine-phosphorylated by the ubiquitous Golgi secretory kinase FAM20C. Cotransfection of HEK and MC3T3 cells with FGF23 and active, but not inactive, FAM20C kinase increased the storage and release of FGF23 in radiolabeling experiments, indicating potential effects of phosphorylation on FGF23 stability. Collectively, these data point to an important role for phosphorylation of FGF23 in bone.

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