Osteocyte-Specific Deletion of Fgfr1 Suppresses FGF23

PLOS ONE, Dec 2019

Increases in fibroblastic growth factor 23 (FGF23 or Fgf23) production by osteocytes result in hypophosphatemia and rickets in the Hyp mouse homologue of X-linked hypophosphatemia (XLH). Fibroblastic growth factor (FGF) signaling has been implicated in the pathogenesis of Hyp. Here, we conditionally deleted FGF receptor 1 (FGFR1 or Fgfr1) in osteocytes of Hyp mice to investigate the role of autocrine/paracrine FGFR signaling in regulating FGF23 production by osteocytes. Crossing dentin matrix protein 1 (Dmp1)-Cre;Fgfr1null/+ mice with female Hyp;Fgfr1flox/flox mice created Hyp and Fgfr1 (Fgfr1Dmp1-cKO)-null mice (Hyp;Fgfr1Dmp1-cKO) with a 70% decrease in bone Fgfr1 transcripts. Fgfr1Dmp1-cKO-null mice exhibited a 50% reduction in FGF23 expression in bone and 3-fold reduction in serum FGF23 concentrations, as well as reductions in sclerostin (Sost), phosphate regulating endopeptidase on X chromosome (PHEX or Phex), matrix extracellular phosphoglycoprotein (Mepe), and Dmp1 transcripts, but had no demonstrable alterations in phosphate or vitamin D homeostasis or skeletal morphology. Hyp mice had hypophosphatemia, reductions in 1,25(OH)2D levels, rickets/osteomalacia and elevated FGF2 expression in bone. Compared to Hyp mice, compound Hyp;Fgfr1Dmp1-cKO-null mice had significant improvement in rickets and osteomalacia in association with a decrease in serum FGF23 (3607 to 1099 pg/ml), an increase in serum phosphate (6.0 mg/dl to 9.3 mg/dl) and 1,25(OH)2D (121±23 to 192±34 pg/ml) levels, but only a 30% reduction in bone FGF23 mRNA expression. FGF23 promoter activity in osteoblasts was stimulated by FGFR1 activation and inhibited by overexpression of a dominant negative FGFR1(TK−), PLCγ and MAPK inhibitors. FGF2 also stimulated the translation of an FGF23 cDNA transfected into osteoblasts via a FGFR1 and PI3K/Akt-dependent mechanism. Thus, activation of autocrine/paracrine FGF pathways is involved in the pathogenesis of Hyp through FGFR1-dependent regulation of FGF23 by both transcriptional and post-transcriptional mechanisms. This may serve to link local bone metabolism with systemic phosphate and vitamin D homeostasis.

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Osteocyte-Specific Deletion of Fgfr1 Suppresses FGF23

Citation: Xiao Z, Huang J, Cao L, Liang Y, Han X, et al. ( Osteocyte-Specific Deletion of Fgfr1 Suppresses FGF23 Zhousheng Xiao 0 Jinsong Huang 0 Li Cao 0 Yingjuan Liang 0 Xiaobin Han 0 Leigh Darryl Quarles 0 Makoto Makishima, Nihon University School of Medicine, Japan 0 University of Tennessee Health Science Center , Memphis, Tennessee , United States of America Increases in fibroblastic growth factor 23 (FGF23 or Fgf23) production by osteocytes result in hypophosphatemia and rickets in the Hyp mouse homologue of X-linked hypophosphatemia (XLH). Fibroblastic growth factor (FGF) signaling has been implicated in the pathogenesis of Hyp. Here, we conditionally deleted FGF receptor 1 (FGFR1 or Fgfr1) in osteocytes of Hyp mice to investigate the role of autocrine/paracrine FGFR signaling in regulating FGF23 production by osteocytes. Crossing dentin matrix protein 1 (Dmp1)-Cre;Fgfr1null/+ mice with female Hyp;Fgfr1flox/flox mice created Hyp and Fgfr1 (Fgfr1Dmp1-cKO)null mice (Hyp;Fgfr1Dmp1-cKO) with a 70% decrease in bone Fgfr1 transcripts. Fgfr1Dmp1-cKO-null mice exhibited a 50% reduction in FGF23 expression in bone and 3-fold reduction in serum FGF23 concentrations, as well as reductions in sclerostin (Sost), phosphate regulating endopeptidase on X chromosome (PHEX or Phex), matrix extracellular phosphoglycoprotein (Mepe), and Dmp1 transcripts, but had no demonstrable alterations in phosphate or vitamin D homeostasis or skeletal morphology. Hyp mice had hypophosphatemia, reductions in 1,25(OH)2D levels, rickets/ osteomalacia and elevated FGF2 expression in bone. Compared to Hyp mice, compound Hyp;Fgfr1Dmp1-cKO-null mice had significant improvement in rickets and osteomalacia in association with a decrease in serum FGF23 (3607 to 1099 pg/ml), an increase in serum phosphate (6.0 mg/dl to 9.3 mg/dl) and 1,25(OH)2D (121623 to 192634 pg/ml) levels, but only a 30% reduction in bone FGF23 mRNA expression. FGF23 promoter activity in osteoblasts was stimulated by FGFR1 activation and inhibited by overexpression of a dominant negative FGFR1(TK2), PLCc and MAPK inhibitors. FGF2 also stimulated the translation of an FGF23 cDNA transfected into osteoblasts via a FGFR1 and PI3K/Akt-dependent mechanism. Thus, activation of autocrine/paracrine FGF pathways is involved in the pathogenesis of Hyp through FGFR1-dependent regulation of FGF23 by both transcriptional and post-transcriptional mechanisms. This may serve to link local bone metabolism with systemic phosphate and vitamin D homeostasis. - Funding: This work was supported by grant R01-AR045955 to LDQ from the National Institutes of Health. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. The FGF family consists of canonical FGFs, intracellular FGFs, and hormone-like FGF gene products (i.e., FGF19, FGF21 and FGF23). The earliest evolved family members are likely intracellular FGFs, exemplified by high molecular weight FGF2 (HMWFGF2), which interact with intranuclear FGFR1 to directly activate gene transcription (i.e., integrative nuclear FGFR1 signaling) [1]. Canonical or FGFs evolved later to serve autocrine/paracrine functions. These secreted FGFs have high heparin affinity, limited diffusion capacity and act locally on cell surface FGF receptors [2]. More recently, circulating FGFs emerged with the capability to diffuse from tissues and specifically target FGF receptor/Klotho complexes at distal sites due to their unique C-terminal domain [3]. For example, FGF23 is a ,32 kDa hormone with an N-terminal FGF-homology domain and a novel 71 amino acid C-terminus [4] that allows it to be released into the circulation and to interact with a-Klotho, a type I membrane, bglycosidase to form a trimeric complex with FGFRs in specific tissues [49]. FGF23 is produced and secreted by osteoblasts/osteocytes in bone. FGF23 activates FGFR/a-Klotho complexes in the kidney to decrease Npt2a co-transporters leading to inhibition of renal tubular phosphate reabsorption and to reduce circulating 1,25(OH)2D levels by inhibiting enzymes regulating vitamin D metabolism (i.e., inhibiting Cyp27b1 and stimulating Cyp24) [6,7,913]. The biological functions of FGF23 are essential for maintenance normal mineral metabolism. Ablation of FGF23 in mice is lethal in the early postnatal period due to hyperphosphatemia and excessive 1,25(OH)2D production [14,15]. On the other hand, excess FGF23 causes hypophosphatemia, aberrant vitamin D metabolism and rickets/osteomalacia. Increases in FGF23 underlie acquired and hereditary forms of hypophosphatemic rickets and are involved in the pathogenesis of mineral metabolism abnormalities in chronic kidney diseases [16]. Consequently, understanding the factors that regulate FGF23 is of high clinical importance. FGF23 gene transcription in bone is complex and poorly understood. FGF23 is regulated by sys (...truncated)


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Zhousheng Xiao, Jinsong Huang, Li Cao, Yingjuan Liang, Xiaobin Han, Leigh Darryl Quarles. Osteocyte-Specific Deletion of Fgfr1 Suppresses FGF23, PLOS ONE, 2014, Volume 9, Issue 8, DOI: 10.1371/journal.pone.0104154