How Fibroblast Growth Factor 23 Works
ABSTRACT There is a discontinuum of hereditary and acquired disorders of phosphate homeostasis that are caused by either high or low circulating levels of the novel phosphaturic hormone fibroblastic growth factor 23 (FGF23). Disorders that are caused by high circulating levels of FGF23 are characterized by hypophosphatemia, decreased production of 1,25-dihydroxyvitamin D, and rickets/osteomalacia. On the other end of the spectrum are disorders that are caused by low circulating levels of FGF23, which are characterized by hyperphosphatemia, elevated production of 1,25-dihydroxyvitamin D, soft tissue calcifications, and hyperostosis. Knowledge of the genetic basis of these hereditary disorders of phosphate homeostasis and studies of their mouse homologues have uncovered a bone-kidney axis and new systems biology that govern bone mineralization, vitamin D metabolism, parathyroid gland function, and renal phosphate handling. Further understanding of this primary phosphate homeostatic pathway has the potential to have a significant impact on the diagnosis and treatment of disorders of bone and mineral metabolism.
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- "It is established, for example, that both DMP1 and PHEX induce a reduction in FGF23 levels by inhibiting the promoter sequences of the FGF23 gene  . DMP1 was proposed to act as a transcription factor  and MEPE is probably involved in regulating FGF23 indirectly, by inhibiting PHEX . In addition to DMP1 function as a negative regulator of FGF23 production in osteocytes, it is suggested to have a direct role in initiating nucleation associated with mineralized tissue morphology and formation  . "
ABSTRACT: Body phosphate homeostasis is regulated by a hormonal counter-balanced intestine-bone-kidney axis. The major systemic hormones involved in this axis are parathyroid hormone (PTH), 1,25-dihydroxyvitamin-D, and fibroblast growth factor-23 (FGF23). FGF23, produced almost exclusively by the osteocytes, is a phosphaturic hormone that plays a major role in regulation of the bone remodeling process. Remodeling composite components, bone mineralization and resorption cycles create a continuous influx-efflux loop of the inorganic phosphate (Pi) through the skeleton. This “bone Pi loop,” which is formed, is controlled by local and systemic factors according to phosphate homeostasis demands. Although FGF23 systemic actions in the kidney, and for the production of PTH and 1,25-dihydroxyvitamin-D are well established, its direct involvement in bone metabolism is currently poorly understood. This review presents the latest available evidence suggesting two aspects of FGF23 bone local activity: (a) Regulation of FGF23 production by both local and systemic factors. The suggested local factors include extracellular levels of Pi and pyrophosphate (PPi), (the Pi/PPi ratio), and another osteocyte-derived protein, sclerostin. In addition, 1,25-dihydroxyvitamin-D, synthesized locally by bone cells, may contribute to regulation of FGF23 production. The systemic control is achieved via PTH and 1,25-dihydroxyvitamin-D endocrine functions. (b) FGF23 acts as a local agent, directly affecting bone mineralization. We support the assumption that under balanced physiological conditions, sclerostin, by para- autocrine signaling, upregulates FGF23 production by the osteocyte. FGF23, in turn, acts as a mineralization inhibitor, by stimulating the generation of the major mineralization antagonist—PPi. © 2014 BioFactors, 2014BioFactors 11/2014; 40(6). DOI:10.1002/biof.1186 · 3.00 Impact Factor
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- "FGF-23 potently lowers serum phosphate levels by inducing renal phosphate wasting; FGF-23 inhibits the sodium phosphate cotransporter types IIa and IIc (NPT2a/2c) in the proximal tubule. It also reduces renal expression of CYB27B1 by diminishing 1.25(OH)2D3 synthesis and FGF-23 reduces parathyroid hormone (PTH) secretion  . FGF-23 is a sensitive biomarker of abnormal renal phosphate handling and levels rise earlier, and to a greater extent, than serum phosphate in progressive CKD . "
ABSTRACT: Elevated FGF-23 is a predictor of mortality and is associated with LVH in CKD. It may be a biomarker or a direct toxin. We assessed the relationship between FGF-23 and LVH in CKD using CMRI. In vitro we studied the effect of phosphate, FGF-23, and Klotho on E-selectin and VCAM production in HUVECs. FGF-23 concentration correlates negatively with eGFR and positively with LVMI. FGF-23 was an independent predictor of LVH in CKD. E-selectin and VCAM production was elevated in HUVECs cultured in high phosphate with FGF-23 or Klotho. This effect was attenuated in cells exposed to both FGF-23 and Klotho. FGF-23 is an independent predictor of LVH as measured by CMRI. We show preliminary data which supports that FGF-23 is toxic resulting in activation of the vascular endothelium. We do not prove causality with elevated FGF-23 and LVH. Further research should ascertain if lowering levels of FGF-23 translates to improved clinical outcomes.08/2011; 2011(2090-214X):297070. DOI:10.4061/2011/297070
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- "Sclerostin, a product of SOST gene, suppresses the activity of osteoblasts and viability of osteoblasts and osteocytes; its expression is restricted to osteocytes in adult bone (van Bezooijen et al., 2004; Lin et al., 2009). Investigations into the biological functions of FGF23 have broadened our understanding of the systemic regulation of phosphate homeostasis as well as of the maintenance of proper mineralization in the bone matrix (Liu and Quarles, 2007). The histological alternation of osteocytes around implants has not been fully investigated despite several studies on orthopedic implants by DMP1 and SOST (Xie et al., 2004; Atkins et al., 2009). "
ABSTRACT: Osseointegration is the most preferable interface of dental implants and newly formed bone. However, the cavity preparation for dental implants often gives rise to empty lacunae or pyknotic osteocytes in bone surrounding the dental implant. This study aimed to examine the chronological alternation of osteocytes in the bone surrounding the titanium implants using a rat model. The distribution of the osteocytic lacunar canalicular system (OLCS) in bone around the titanium implants was examined by silver impregnation according to Bodian's staining. We also performed double staining for alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP), as well as immunohistochemistry for fibroblast growth factor (FGF) 23--a regulator for the serum concentration of phosphorus--and sclerostin, which has been shown to inhibit osteoblastic activities. Newly formed bone and the injured bone at the early stage exhibited an irregularly distributed OLCS and a few osteocytes positive for sclerostin or FGF23, therefore indicating immature bone. Osteocytes in the surrounding bone from Day 20 to Month 2 came to reveal an intense immunoreactivity for sclerostin. Later on, the physiological bone remodeling gradually replaced such immature bone into a compact profile bearing a regularly arranged OLCS. As the bone was remodeled, FGF23 immunoreactivity became more intense, but sclerostin became less so in the well-arranged OLCS. In summary, it seems likely that OLCS in the bone surrounding the dental implants is damaged by cavity formation, but later gradually recovers as bone remodeling takes place, ultimately inducing mature bone.The Anatomical Record Advances in Integrative Anatomy and Evolutionary Biology 06/2011; 294(6):1074-82. DOI:10.1002/ar.21391 · 1.53 Impact Factor