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Abstract: TH-OR043

Calciprotein Particles Cause FGF23 Induction via TLR4 Stimulation in Osteoblasts

Session Information

Category: Bone and Mineral Metabolism

  • 401 Bone and Mineral Metabolism: Basic


  • Akiyama, Kenichi, Tokyo Women's Medical University, Tokyo, Japan
  • Shiizaki, Kazuhiro, Jichi Medical University, Shimotsuke, Japan
  • Tanaka, Nobue, Tokyo Women's Medical University, Tokyo, Japan
  • Hanafusa, Norio, Tokyo Women's Medical University, Tokyo, Japan
  • Nitta, Kosaku, Tokyo Women's Medical University, Tokyo, Japan
  • Tsuchiya, Ken, Tokyo Women's Medical University, Tokyo, Japan
  • Kuro-o, Makoto, Jichi Medical University, Shimotsuke, Japan

Calciprotein particles (CPP) are polydispersed colloidal nanoparticles composed of solid-phase calcium-phosphate (CaPi) and serum protein fetuin-A. Two types of CPP with different CaPi properties exist: Primary CPP contain amorphous CaPi, whereas secondary CPP contain crystalline CaPi. We previously reported that CPP induced FGF23 expression/secretion in cultured osteoblastic cells (UMR106). We also reported that a single dose of phosphate gavage in mice increased plasma CPP levels followed by increase in FGF23 expression in the bone and FGF23 levels in the blood. However, the mechanism by which osteoblasts sense CPP remains unknown. In this study, we tested the hypothesis that Toll-like receptor-4 (TLR4) might function as a receptor for CPP. We also determined which CPP, primary or secondary, contribute to FGF23 induction.


In vitro experiments: CPP were generated in the culture medium of UMR106 by increasing concentrations of calcium (Ca) and phosphate (P). To inhibit formation of secondary CPP, we added bisphosphonate (BP) to the medium (BP inhibits transition of CaPi from the amorphous phase to the crystalline phase) and measured physical properties of CPP in the medium by small angle X-ray scattering (SAXS). FGF23 mRNA levels were determined by quantitative RT-PCR. In vivo experiments: To inhibit formation of secondary CPP in vivo, we injected BP in wild-type (WT) mice fed high P diet for 10 days. To test if FGF23 induction might depend on TLR4, we administered a single dose of P by oral gavage in WT mice and mice lacking TLR4 (TLR4 KO). These mice were evaluated by measuring P, Ca, and FGF23 levels in the blood and FGF23 mRNA levels in the cranial bone.


Secondary CPP with a hydrostatic diameter of approximately 35 nm were generated in the medium. In the presence of BP, these CPP disappeared and primary CPP with a diameter of around 9.2 nm were generated. FGF23 mRNA levels were much higher when BP was present in the medium. Administration of BP increased FGF23 levels in the blood, but suppressed FGF23 mRNA levels in the skull. A single dose of P ingestion increased FGF23 mRNA and circulating FGF23 levels in WT mice but not in TLR4 KO.


Primary CPP were a more potent inducer of FGF23 expression than secondary CPP. We also suggest that TLR4 is necessary for appropriate regulation of FGF23 expression/secretion.