FGF23 associated bone diseases

Abstract

Recently, fibroblast growth factor 23 (FGF23) has sparked widespread interest because of its potential role in regulating phosphate and vitamin D metabolism. In this review, we summarized the FGF superfamily, the mechanism of FGF23 on phosphate and vitamin D metabolism, and the FGF23 related bone disease.

Keywords

fibroblast growth factor 23 / FGF receptor / phosphate metabolism / Klotho / bone disease

Introduction

Fibroblast growth factor (FGF) was previously considered to be a growth factor and gene transcriptional regulatory factor that acted predominantly through a paracrine mechanism. However, recent research has found that the FGF family contains up to 23 members, most of which are associated with human growth, gonadal function, the immune system, bone metabolism, calcification of vessels or aging. In the last decade, great breakthroughs have been made in the understanding of serum phosphate regulation and it has been demonstrated that FGF23 is a novel type of endocrine hormone. The FGF23-leading phosphate regulatory system also includes FGF2, FGF7, FGF receptor 1 (FGFR1), FGFR2, FGFR3, dentin matrix protein 1 (DMP1), the FGF23 essential cofactor Klotho, matrix extracellular phosphoglycoprotein (MEPE), frizzled related protein 4 (FRP4) and the sodium-phosphate (NaPi) cotransporter. All of these factors, along with the parathyroid hormone (PTH)-vitamin D-calcitonin axis, contribute to phosphate metabolism, both independently and in concert.

FGF23 associated bone diseases include X-linked hypophosphatemia (XLH), autosomal recessive hypophosphatemic rickets (ARHR), autosomal dominant hypophosphatemic rickets (ADHR), tumor-induced osteomalacia (TIO), chronic kidney disease with high levels of serum FGF23 and familial tumoral calcinosis (FTC). The pathogenesis of these diseases is related to dysfunction of FGF23. Renal hypophosphatemic rickets, drug-related hypophosphatemic bone disease, osteogenesis imperfecta, McCune-Albright’s syndrome, polyostotic fibrous dysplasia, chronic kidney disease with bone and mineral disorders, and chronic osteopathy of renal acidosis are also related to elevated serum FGF23 (secondary high FGF23). This review focuses on progress made in the understanding of FGF23 associated bone diseases.

FGF superfamily and FGF23

The FGFs are a superfamily of tissue growth factors. Recent studies show that, apart from the regulation of tissue reconstruction and organ function, FGFs also function as regulatory factors in organ development. FGF19, FGF21 and FGF23 are hormone-like cytokines and are associated with chronic hemolysis, fatty liver, type 2 diabetes mellitus, obesity, Cushing’s syndrome and hypophosphatemia.

FGF superfamily

The FGF superfamily can be divided into three groups according to functions: FGF13-like (the first to be discovered; most important members are FGF11, 12, 13 and 14); FGF4-like (products of FGF13 multiplex gene expression, including FGF1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 16, 18, 20 and 22); and FGF15/19-like (hormone-like FGFs produced from FGF4-like members, including FGF15, 19, 21 and 23) [1–3] (Fig. 1). It has been demonstrated that inactivating mutations of FGF23, phosphate regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and dentin matrix protein 1 (DMP1) can cause ADHR, XLH and ARHR, respectively. Inactivating mutations of FGF23/Galnt can lead to FTC (Table 1). Members of this family are encoded by 23 different genes and their structures and bioactivities share many similarities. In tissue, FGF regulates cell metabolism and activity in a paracrine and autocrine manner, and binds heparin with high affinity. It also binds related receptors to aid cell growth, induce mitosis, and promote angiogenesis and the development of hematopoietic stem cells. It also promotes wound healing, tissue repair, cell transformation and the generation of tissue. All of these functions are mediated by the FGFR and heparan sulfate-containing proteoglycans (HSPGs).

Expression of intestinal FGF15 is mediated by cholic acid, which forms a heterodimer with the farnesoid X receptor to induce the synthesis of FGF15. When FGF15 combines with Klotho-FGFR4, it suppresses the activity of Cyp7a1 and the synthesis of cholic acid. Non-esterified fatty acid (NEFA) activates peroxisome proliferator-activated receptor α (PPARα), and PPARα induces the expression of FGF21 with retinoid X receptors (RXRs) in the liver. The heterodimer of the vitamin D receptor in bone induces the expression of FGF23 in combination with RXRs, and high levels of FGF23 activate the Klotho-FGFR1c complex to induce Cyp2 and suppress Cyp27b1. This provides a negative feedback for the production of vitamin D (Fig. 2 and Fig. 3). FGF23 is also called minhibin, and plays a key role in phosphate transport and bone calcification. In hyperphosphatemia or conditions involving high serum levels of 1,25-(OH)₂D, secretion of FGF23 by osteocytes and osteoblasts is increased. In the kidney, FGF23 is combined with FGFR1 to suppress the activation of the NaPi cotransporter and 1α-hydroxylase.

FGF2

FGF2 and FGF3 have several isomers, probably because their gene expression is correlated with their levels and there are no precursors of these genes. The isomers of FGF2 differ in the length of extensions to their N-termini, with molecular weights of 18 kDa, 22 kDa, 23 kDa, 24 kDa and 34 kDa. FGF2 is a key FGF, being expressed in the pituitary gland, brain, nerves, retina, adrenal gland and placenta. Expression of FGF2 is highest in the pituitary gland (500 µg/kg); in other organs its levels are much lower, at only about 1/50 to 1/10 of that in the pituitary. Serum and body fluid levels of FGF2 are very low. When FGF2 binds its receptor, it transduces cell signals via following pathways. (1) It activates adenylate cyclase and guanylate cyclase, resulting in phosphorylation of phosphatase C and the dissociation of phosphatidylinositol-4,5-bis-phosphate into triglyceride and inositol triphosphate, which leads to the activation of protein kinase C (PKC) and causes Ca²⁺ influx. (2) It localizes in the nucleus after binding the receptor and regulates the activity of RNA polymerase I, enhancing ribosomal transcription and accelerating the transition from G₀ to G₁ and from G₁ to S. All the above functions stimulate DNA synthesis and cell proliferation.

FGF23

FGF23 is responsible for phosphate metabolism. Missense mutations can lead to ARHR. Mutant FGF23 can result in resistance to hydrolase and cause hypophosphatemia. Ectopic expression of FGF23 in tumors can also result in hypophosphatemia (tumor-induced osteomalacia, TIO).

Dysfunction of FGF19 is related to chronic hemolysis and non-alcoholic fatty liver disease, and dysfunction of FGF21 can cause obesity and type 2 diabetes mellitus. It has been found that Cushing’s syndrome and anorexia nervosa are correlated with FGF21 to some extent.

FGF23 is a newly identified factor that regulates phosphate metabolism, calcification of bone and vitamin D. Its molecular weight is 32 kDa and it contains 251 amino acids. It functions with the assistance of Klotho (see below); only when FGF23 combines with Klotho can it bind its receptor to transduce cell signals. The FGF23 complex also contains a sucrose octasulfate-binding domain; this is related to heparin, but the mechanism remains unclear.

The parathyroid glands contain abundant receptors for Klotho and FGF23. When FGF23 binds Klotho, the complex can activate the MAPK pathway. It phosphorylates ERK1/2, inhibits secretion of PTH and stimulates the expression of 1α-hydroxylase. As a result, the function of the parathyroids is suppressed. The N-terminal contains 71 amino acids and shares a homologous sequence domain with FGF. With the assistance of Klotho, FGF23 binds the FGFR to stimulate the excretion of phosphorus in the kidney and inhibit the synthesis of 1,25-(OH)₂D [4–6]. The synthesis and secretion of FGF23 are regulated at the gene expression, post-transcriptional modification, secretion and extracellular processing levels. 1,25-(OH)₂D stimulates the expression of FGF23 in bone tissue. Serum FGF23 is undetectable in vitamin D receptor knockout mice, but is increased after administration of 1,25-(OH)₂D.

Other FGFs

There are at least two isomers of FGF3, with molecular weights of 31.5 kDa and 28.5 kDa. In bone and cartilage, the function of FGF18 is similar to that of FGF2, and FGF2 can be totally replaced by FGF18. FGF10 has no effect on the regulation of osteoblast, chondrocyte and osteoclast function. FGF9, 18 and 20 participate in the development of the skull and limbs. FGF18 binds FGFR3 to stimulate the growth of osteoblasts and has an inhibitory effect on chondrogenesis.

FGFR

The FGFR contains three Ig-like domains; the second part of the third Ig-like domain can be divided into FGFR1, FGFR3IIIb and FGFR3IIIc. When FGF binds the FGFR and HSPGs, the resulting FGFR-FGF-HSPG complex causes a cascade reaction with biological effects such as receptor phosphorylation. For example, in the renal proximal tubule, FGF23 binds to the FGFR1α-Klotho complex to activate ERK1/2 kinase and cause phosphorylation of serum/glucocorticoid regulated kinase 1 (SGK1). Phosphorylated SGK1 phosphorylates Na⁺/H⁺ exchange regulatory cofactor-1 (NHERF-1) and internalizes and degrades NaPi-2a. PTH activates protein kinase A (PKA) and PKC, which also phosphorylate NHERF-1, with biological effects (Fig. 4).

Different types of FGFR and their mechanisms of action

Paracrine and autocrine regulatory pathways are mediated by FGFRs on the cell membrane. Target cells have at least three receptor types. The first receptor type is an integral membrane protein. The extracellular domain contains an Ig-like domain and the intracellular domain has endogenous tyrosine kinase activity. The tyrosine kinase domain contains hydrophilic amino acid residues and can undergo autophosphorylation. FGF binds tightly to this FGFR (K_d = 2–10 pmol/L). The second receptor type is of low affinity (K_d = 50–500 nmol/L) and is called HSPG. It does not contain an endogenous tyrosine kinase activity domain and cannot transduce FGF signals, but is a key regulatory factor for FGF bioavailability. The third receptor type is rich in cysteine and is called the cysteine-rich receptor (CFR). Its nature and effects are unclear.

High affinity FGFR

The high affinity FGFR is encoded by four genes. It has several names, including FGFR/fly, k-sam, fly-2, cek-2, cek-3, FGFR2 and FGFR3. A mutated FGFR gene can result in osteochondrodysplasia or craniosynostosis. The structures of the various FGFRs differ, but all bind FGF (though with varying affinities and biological effects). This is because the FGFR contains various ligand-binding sites. Like other soluble molecules, soluble FGFR may regulate the amount of bound ligand. When FGFR enters the cell, it localizes to the nuclear membrane, but whether FGFR can bind FGF2, which is abundant in cells, is unclear, as is its location in the nuclear membrane. High affinity FGFR transduces FGF signals and is a key regulatory factor in the growth and development of bone. Numerous types of dysostosis and deformities are related to mutated FGFR, including achondroplasia, deadly osteodyplasia (FGFR3 mutation), Crouzon syndrome, Jackson-Weiss syndrome (FGFR2 mutation) and Pferffer syndrome (FGFR1 mutation).

Low affinity FGFR and FGF binding protein (FGFBP)

Low affinity FGFR, also called HSPG, is located in the cell surface or extracellular matrix. It contains a proteoglycanolytic binding site. As discussed above, high affinity FGFR transduction requires HSPG, perlecan and syndecan-1, and the FGF-HSPG complex is a form of FGF storage. Whether and how much it can be utilized depends on the HSPGs and FGFBP. At least three FGFBPs have been identified, but it is estimated that many remain undiscovered.

CFR

The CFR is unable to transduce but can bind FGF. Another receptor-like protein, MG-160, is also able to bind FGF2. MG-160 shares 90% sequence homogeneity with CFR. Both exist in the Golgi body, but their functions are unknown. It is thought that they may degrade FGF1 and FGF2 and thus may have a regulatory effect on the bioavailability of FGF.

Function of the FGFR and its pathophysiological significance

FGFR and phosphorous metabolism

Andrukhova hypothesized the mechanism by which the FGF23 signal transduction pathway in proximal tubule cells regulates phosphorous metabolism, suggesting that FGF23 in the blood binds to the FGFR1α-Klotho complex in proximal tubule cells, activating ERK1/2 kinase and resulting in phosphorylation of NHERF-1. This leads to the phosphorylation of SGK-1, internalization of NHERF-1 and degradation of NaPi-2a. Finally, phosphorus is excreted in the urine. This is discussed further below.

FGFR and tumors

Dysfunction of the FGFR is related to numerous human diseases, particularly tumors. Mutations in the FGFR may occur in the extracellular part, the transmembrane part or the intracellular part, but the relationship between the location of mutations and tumors is unclear. Skin, lung, endometrial and brain cancer are all associated with FGFR mutation, and point mutations in the C-terminal are related to rhabdomyosarcoma. The pathogenesis of stem cell leukemia/lymphoma is complex but is closely related to an FGFR fusion protein, which is considered a key factor.

FGF23-Klotho-MEPE phosphorus metabolic system

The FGF23 mediated phosphorus metabolic system includes MEPE, FRP4, FGF2, FGF7, FGFR1, FGFR2 and FGFR3, Klotho, NaPi-2, DMP1 and certain inflammatory factors [7].

Regulation of FGF23 secretion and its physiological effects

Phosphorus in the diet regulates the secretion of FGF23

Bone is the main source of inorganic phosphorus, and serum phosphorus is regulated by both intestinal absorption and excretion in the kidney. Intestinal absorption of phosphorus is determined by the amount in the diet but also by 1,25-(OH)₂D. Elevation of blood phosphorus inhibits the secretion of PTH and then expression of NPT2a in the kidney tubules, increasing the excretion of phosphorus in the urine to maintain blood phosphorus in the normal range. Calcium and phosphorus in the diet and serum, as well as 1,25-(OH)₂D, stimulate the synthesis of FGF23, which directly suppresses the secretion of 1,25-(OH)₂D. Simultaneously, it also suppresses 1,25-(OH)₂D synthesis by the inhibition of PTH. FGF23 inhibits reabsorption of phosphorus in the kidney tubules to decrease blood phosphorus, and low levels of phosphorus stimulate the secretion of 1,25-(OH)₂D. Consequently, serum 1,25-(OH)₂D levels depend on both FGF23 and blood phosphorus. The amount of phosphorus in the diet regulates the secretion of FGF23. Animal experiments have demonstrated that a diet containing no phosphorus decreases serum FGF23 levels by seven times. Human trials have shown similar results, but the effect of a diet lacking phosphorus on FGF23 is significantly less than that of phosphorus level on FGF23 and the effect was independent of 1,25-(OH)₂D [8,9].

Secretion of FGF23 in chronic nephropathy

In patients with chronic nephropathy, serum FGF23 and PTH are elevated. In these patients, serum phosphorus is closely related to FGF23, but there is currently no evidence to prove that this disease directly stimulates the secretion of FGF23.

Chalybeate stimulates secretion of FGF23

Intravenous chalybeate markedly stimulates the secretion of FGF23, causing secondary high serum FGF23. Chronic nephropathy is usually treated with chalybeate to increase FGF23 secretion.

FGF23 and bone mineralization

FGF23 is an endocrine/paracrine hormone that regulates phosphorus metabolism. In an FGF23 knockout mouse model, hyperphosphatemia was found (as in tumoral calcinosis). Compared with wild type mice and mice with XLH, the expression of small integrin-binding ligand N-linked glycoproteins (SIBLINGs) was significantly altered, which indicates that FGF23 can stimulate bone metabolism under physiological conditions and is a key factor in bone matrix mineralization. When FGF23 is elevated or absent, mineralization of bone and teeth is abnormal. Absence of FGF23 can lead to over-mineralization and vice versa (Table 2).

FGF23 inhibits synthesis of PTH and 1,25-(OH)₂D

Patients with XLH have hypophosphatemia, but serum 1,25-(OH)₂D is also at a low level, so the inhibitory effect of FGF23 on 1,25-(OH)₂D is obvious. FGF23 has the same effect on PTH. Consequently, blood levels of PTH are markedly altered in patients with XLH. However, FGF23 produces paracrine factors when bound to kidney tubule receptors and these factors could inactivate the NaPi symporter and 1α-hydroxylase. The PHEX secreted by osteoblasts transforms Fi to Fa, and Fa inactivates FGF23. Due to mutation of PHEX, Fa is decreased, resulting in elevation of FGF23, consumption of phosphorus in the kidney and an inappropriate 1,25-(OH)₂D decrease. The cause of the increased FGF23 is overproduction of FGF23 in osteocytes [10,11].

Physiological effect of Klotho

The product of the Klotho gene is a single-strand transmembrane protein. The extracellular domain is cleaved by the proteases ADAM10 and ADAM17 to form Klotho. Klotho occurs in two types: transmembrane and secretory. Secretory Klotho is found in serum, urine and cerebrospinal fluid. So transmembrane Klotho binds the FGFR to form complex X, which functions as coreceptor of FGF23 in mediating the metabolism of phosphorus and vitamin D. NEFA activates PPARα, which binds RXRs to induce the expression of FGF21 in the liver. Heterodimers of the vitamin D receptor combine with RXRs to induce the expression of FGF23, and FGF23 activates the Klotho-FGFR1c complex to inhibit Cyp27b1. This comprises the negative feedback regulatory system for Klotho and FGF23. Klotho and FGF23 are necessary factors for the metabolism of vitamin D and their absence leads to hypervitaminosis D.

Interaction between Klotho and FGF23

In osteocytes, the heterotetramer produced by the combination of 1,25-(OH)₂D with the vitamin D receptor binds the promoter of the FGF23 gene and thereby promotes the expression of FGF23. When FGF23 is released into blood, it binds the Klotho-FGFR complex in the kidney (the bone-kidney axis) and the parathyroids (the bone-parathyroid axis). FGF23 also suppresses the expression of 1α-hydroxylase (CYP27B1) to block the negative feedback regulatory system for vitamin D. In the parathyroids, FGF23 inhibits the expression of PTH, which markedly induces the expression of CYP27B1, blocking another pathway in the vitamin D negative feedback regulatory system [12,13]. The extracellular part of the Klotho protein is degraded by ADAM10 and ADAM17 to form secretory Klotho. Because secretory Klotho contains sialidase activity, it can regulate the polysaccharide part of the transient receptor potential cation channel, subfamily V (TRPV5) calcium channel in the cell surface. Consequently, secretory Klotho can inhibit insulin, insulin-like growth factor 1 and Wnt, and has antioxidative stress and antioncogenesis activities [14].

Phosphate diuresis effect of Klotho

Hyperphosphatemia is the predominant observation in Klotho knockout mice. The elevated activity of the NaPi symporter in bone disease [15] indicates that Klotho is a key factor in calcium and phosphorus metabolism in the kidney [16–18]. Klotho in the proximal tubule inhibits the NaPi symporter [19]. The extracellular part of Klotho contains two tandem repeats with 20%–40% similarity with β-glucosidase, so their effects are similar [20]. NaPi-2a is a glycoprotein [21] that directly inactivates the NaPi symporter. This is probably because Klotho converts NaPi-2a to low molecular weight peptides and removes the glycosyl group. Klotho is a cell protective protein [22–24], and in patients with chronic nephropathy who lack Klotho, serum phosphorus is constantly elevated.

Klotho absence

In patients with chronic nephropathy, the risk of cardiovascular diseases is increased [25–27], and decreasing serum phosphorus has protective and curative effects. Research shows that Klotho has a protective effect on the cardiovascular system [28–30] (Fig. 5).

Physiological effect of PHEX-FRP4-DMP1

The following observations are evidence that PHEX and DMP1 regulate the synthesis and secretion of FGF23 [31–34]. (1) FGF23 is highly expressed in PHEX, FMP1 and Hyp knockout mice. (2) Hypophosphatemia is resolved and serum 1,25-(OH)₂D is “inappropriately normal” after a single injection of anti-FGF23 antibody. (3) When anti-FGF23 antibody is injected repeatedly, the bone disease is attenuated. There is currently no evidence that PHEX and DMP1 directly regulate FGF23. However, when osteoblasts transform to osteocytes, their gene expression profile changes markedly and DMP1 is one of the key factors that regulate this transformation. When expression of DMP1 increases in osteoblasts, all of the other osteoblast-specific genes are suppressed. Osteoblasts transform to osteocytes in this microenvironment. When DMP1 is inactivated, this process cannot be inhibited and the matrix cannot be calcified. High expression of the FGF23 and collagen genes causes high FGF23 syndrome and associated bone diseases.

MEPE is synthesized by osteoblasts, odontoblasts and osteocytes. It is cleaved by cathepsin B and releases acidic serine-aspartate-rich MEPE associated protein (ASARM) to inhibit calcification, reabsorb phosphorus and upregulate the production of 1,25-(OH)₂D. The combination of PHEX and MEPE blocks the effect of cathepsin B. If the PHEX gene is mutated, PHEX cannot bind to MEPE, which results in a marked increase in the activity of cathepsin B and elevation of ASARM [35]. If the activity of PHEX is insufficient, hypophosphatemia results due to accumulation of MEPE. FGF2 and 1,25-(OH)₂D inhibit the expression of MEPE. MEPE is an inhibitory factor in the calcification of bone (minhibin) that regulates osteogenesis and bone absorption; it has a similar effect to FGF23 and can inhibit the reabsorption of phosphorus in the kidney tubules to induce hypophosphatemia. Patients with XLH have elevated MEPE, which is another factor contributing to phosphorus consumption in the kidney. FRP4 is the member of frizzled protein family and is located on 7p14.1. It contains six exons and is 10.8 kb in length. The FRP4 protein contains 346 amino acids and has a molecular weight of 40 kDa. The first 21 amino acids constitute the signaling part of the protein. Blood FRP4 is elongated (48 kDa) due to the addition of saccharide molecules. FRP4 has a cysteine-rich ligand-binding domain and a hydrophilic C-terminal. With the FRP receptor, it binds to Wnt to suppress its signal.

In this manner, FGF23, MEPE and FRP4 constitute another phosphorus regulatory system. Many other factors are involved in this system, including FGF2 and FGF7, FGFR1, FGFR2 and FGFR3, Klotho, NaPi-2, DMP1 and certain inflammatory factors.

Physiological effect of the NaPi symporter

Abnormal reabsorption of phosphorus in the kidney tubules is another key factor in the development of hypophosphatemia [36], and the reabsorption of phosphorus in the kidney primarily depends on the regulation of the NaPi symporter. The type IIa and IIc symporters are located in the apical membrane of renal proximal tubule epithelial cells. Their expression levels affect the reabsorption of phosphorus and blood phosphorus levels, and are regulated by PTH, phosphorus in the diet and FGF23. For example, mutation of the type IIc symporter leads to hereditary hypophosphatemic rickets with hypercalcemia (HHRH). Lack of inorganic phosphorus is a common cause of rickets; thus, according to changes of serum PTH and FGF23 (elevated PTH and FGF23, diuresis of phosphorus in the kidney), chronic hypophosphatemia associated bone diseases are divided into three groups: hypophosphatemia-hyper-PTH rickets/osteomalacia, hypophosphatemia-hyper-FGF23 rickets/osteomalacia, and phosphorus deficiency rickets/osteomalacia [37–40].

Type 1 NaPi symporter (NPT1)

NPT1 is mainly expressed at the apical membrane of renal proximal tubule cells and in the liver. It is a nonspecific anion carrier; its function as a phosphorus transporter [41–43] and its mechanism of action are unclear.

Type 2 NaPi cotransporters (NPT2s)

NPT2s are divided into three subgroups: NPT2a, NPT2b and NPT2c. NPT2b is expressed in the kidney, lung and intestine. Vitamin D upregulates the expression of NPT2b in the intestine. The location of NPT2b in the kidney and the mechanism of its regulation of the reabsorption of phosphorus are unclear. NPT2a and NPT3b are mainly expressed at the apical membrane of renal proximal tubule epithelial cells and function to reabsorb phosphorus. NPT2a can carry three Na⁺ and one phosphate, whereas NPT2c can carry only two Na⁺. With advancing age, expression of NPT2c decreases, and its reaction with PTH differs from that of NPT2b.

Type 3 phosphate transporters

Type 3 phosphate transporters are divided into two types, PiT1 and PiT2, both of which have high affinity to inorganic phosphorus. They are located in all tissue, are involved in energy metabolism and provide phosphonate substrate for cells. In vitro experiments show that PiT1 is highly expressed in high-phosphorus culture media, which suggests that it plays a role in the calcification of vessels.

PTH and phosphorus transporters

PTH decreases the reabsorption of phosphorus in the kidney. With the assistance of NHERF-1, the type 1 PTH receptor induces expression of NPT2a, but how PTH regulates NPT2c is unclear [44].

FGF23 and phosphorus transporters

FGF23 downregulates the expression of NPT2a and NPT2c, upregulates NPT2b in the intestine and downregulates 1α-hydroxylase in the kidney through 1,25-(OH)₂D. It also upregulates 24-hydroxylase, resulting in a decrease in the synthesis of 1,25-(OH)₂D.

Relationship between FGF23-Klotho-MEPE axis and Ca²⁺-PTH-vitamin D axis

1,25-(OH)₂D

Calcium, phosphorus and 1,25-(OH)₂D in the diet and serum stimulate the synthesis of FGF23, while FGF23 directly inhibits the secretion of 1,25-(OH)₂D and blocks the production of 1,25-(OH)₂D through suppressing PTH. FGF23 also suppresses the reabsorption of phosphorus to decrease blood phosphorus, and the latter stimulates secretion of 1,25-(OH)₂D. Thus, 1,25-(OH)₂D levels are dependent on both FGF23 and blood phosphorus.

PTH

PTH directly regulates the secretion of FGF23. In patients with chronic nephropathy with hyperphosphatemia, serum FGF23 is significantly elevated, but becomes normal after parathyroidectomy. Patients with Jansen’s metaphyseal chondrodysplasia due to an activating mutation of the type 1 PTH receptor have significantly elevated serum FGF23 [45–48], even with a low phosphorus diet and normal levels of 1,25-(OH)₂D. However, some researchers do not support this position. For example, extraneous 1,25-(OH)₂D can still increase serum FGF23 when PTH is deficient, but whether this is related to PTH secreted from the thymus needs further investigation.

Metabolism of phosphorus and regulation of serum phosphorus

The metabolic regulation of phosphorus varies between different organs and tissue. Besides PTH and vitamin D, several cytokines and metabolic enzymes, such as FGF23, secretory FRP4 and MEPE, are involved. Phosphorus metabolism is regulated by the Ca²⁺-PTH-1,25-(OH)₂D system (Fig. 6) and the Pi-FGF23-PTH-1,25-(OH)₂D system (Fig. 7), which interact with each other to control concentrations of Ca²⁺ and phosphorus within the strict physiological range through bone, kidney and intestine.

Any factor that increases the secretion of PTH leads to decreased reabsorption of phosphorus in the kidney tubules, resulting in hypophosphatemia. Hypophosphatemia caused by PTH-related protein is also associated with elevated clearance of phosphorus. If calcium is absent from the diet, the absorption of calcium in the bowel is 10%–15%, while that of phosphorus reaches 50%–60%. This ultimately leads to secondary hyperparathyroidism. Elevated PTH increases the expression of RANKL in osteoblasts and induces formation of osteoclasts, increasing the transport of calcium and phosphorus from bone to blood. At the same time, PTH decreases the reabsorption of phosphorus in the kidney tubules, leading to hypophosphatemia and rickets. These patients have normal blood calcium, reduced blood phosphorus and normal/elevated 1,25-(OH)₂D. Other factors (e.g., dysfunction of vitamin D metabolism) that lead to vitamin D deficiency, or mutated CYP2R1, also lower serum 25-(OH)D.

Regulation of blood calcium

Hypercalcemia causes diuresis. However, in patients with severe hypercalcemia, hydration and decreased glomerular filtration rate (GFR) can conceal the presence of hypophosphatemia. In contrast, patients suffer from severe hypophosphatemia if hypomagnesemia is corrected in the presence of hypocalcemia and hypomagnesemia. This is related to massive secretion of PTH stimulated by constant hypocalcemia [49–51].

Regulation of phosphorylation and dephosphorylation

Phosphorylation/dephosphorylation is a basic metabolic pathway in cells. Enzymes, signal molecules, receptors and energy are all associated with phosphorylation and more than 2000 chemical reactions involve phosphonates. Phosphorylation is catalyzed by protein kinases and dephosphorylation by phosphatases. Phosphorus transport intracellularly and in extracellular fluid and blood is the basis of phosphorus metabolism and all biological reactions. Phosphorus balance is primarily maintained by the FGF23-Klotho-MEPE system and the PTH-vitamin D-calcitonin system through its transport in the bowel-kidney-bone-thyroid system.

Relationship between the PTH-1,25-(OH)₂D axis and FGF23-Klotho axis

The metabolism of phosphorus is primarily regulated by the PTH-1,25-(OH)₂D axis and the FGF23-Klotho axis. The function of the PTH-1,25-(OH)₂D axis is to regulate calcium metabolism and the blood calcium balance. When blood calcium decreases, it stimulates PTH secretion from the parathyroids; PTH then decreases calcium excretion in the urine and increases the activity of 1α-hydroxylase and the excretion of phosphate. PTH releases calcium to the blood from bone, whereas 1,25-(OH)₂D stimulates the absorption of calcium and phosphorus in the intestine and inhibits the secretion of PTH.

FGF23-Klotho primarily functions to regulate phosphorus metabolism and maintain blood phosphorus levels. FGF23 produced by osteocytes increases the excretion of phosphorus from the kidney and lowers blood phosphorus. 1,25-(OH)₂D decreases the excretion of phosphate and the activity of 1α-hydroxylase. FGFR23 in the kidney occurs in the Klotho-FGFR1 complex, which is located in the distal convoluted tubule. There is a possible reflex mechanism between the proximal and distal convoluted tubules that regulates the activity of FGF23 in the proximal convoluted tubule. FGF23 and TRPV5 interact with each other, decreasing the expression of Klotho in the kidney and reducing the reabsorption of calcium in the kidney tubules. FGF23 also targets the parathyroids to inhibit the secretion of PTH, and 1,25-(OH)₂D can increase FGF23 through decreasing the secretion of PTH. In this way, the two pathways interact in the metabolism of phosphorus by 1,25-(OH)₂D in the kidney tubules, to strictly regulate serum calcium and phosphorus in the physiological range.

FGF23 associated bone diseases

Classification of FGF23 associated bone diseases

FGF23 associated bone diseases are a group of conditions in which the amount or function of FGF23, non-FGF23 related phosphorus consumption or dysfunction of vitamin D causes secondary low serum levels of FGF23. Chronic nephropathy and Klotho deficiency can cause secondary high FGF23 syndrome [52]. Tumoral calcinosis is caused by a defect of saccharification of post-transcriptional FGF23. The biochemical characteristics of these diseases are increased serum levels of inactivated C-terminal FGF23 fragments accompanied with significantly decreased bioactivated FGF23. Klotho deficiency is characterized by elevated 1,25-(OH)₂D due to loss of inhibition of 1α-hydroxylase. The Klotho deficiency and tumoral calcinosis share similar clinical manifestations. Patients with 1α-hydroxylase deficiency also have low serum 1,25-(OH)₂D (Table 3).

Osteoglophonic dysplasia (OGD) is a developmental disorder caused by mutation of FGFR1, FGFR2 and FGFR3, which regulate bone development. Mutation of FGFR1 or FGFR2 causes craniosynostosis. Most dwarfism syndromes are associated with FGFR3 mutation. OGD is a crossover bone dysplasia with clinical features including prominent supraorbital ridges, depressed nasal bridge, rhizomelic dwarfism and non-ossifying damage. Non-ossifying damage occurs mainly in the long bones and can be multiple or single. Pathological sites exhibit lack of calcification and low mineral densities of various size and shape.

FGF23 is the pathophysiological basis of TIO, XLH and ADHR, all of which are associated with hypophosphatemia and characterized by: (1) phosphorus diuresis and hyperphosphatemia; (2) 1,25-(OH)₂D dyssynthesis; (3) hypocalcemia and low calcium in the urine; (4) rickets and osteomalacia; and (5) high serum FGF23 with slightly elevated blood PTH (some patients have normal serum FGF23); some patients have normal 1,25-(OH)₂D, and are said to have an “inappropriate decrease” [53,54].

There are two ways to determine serum FGF23. C-terminal testing technology cross-reacts with the complete FGF23, whereas the complete FGF23 testing method does not have such a problem. Unlike PTH, FGF23 is rarely degraded in blood, which suggests that complete FGF23 levels should be measured.

Primary high FGF23 syndrome and osteomalacia

FGF23 and Klotho are newly identified factors that regulate phosphorus metabolism, bone calcification and vitamin D. FGF23 is a protein of molecular weight 32 kDa. Its N-terminal contains domain homologous in sequence to the N-terminal of FGF containing 71 amino acids. With the assistance of Klotho, FGF23 binds the FGFR. The primary function of FGF23 is to stimulate the excretion of phosphorus in the kidney and inhibit the synthesis of 1,25-(OH)₂D. Serum FGF23 can be measured with an immunometric assay or by determining the amount of intact FGF23 or C-terminal fragments of FGF23. The sensitivity of these tests has recently been improved. FGF23 can be used to reflect the progression of illness in dialysis patients. It is reported that the normal level of intact FGF23 is 44±37 pg/ml, but this is influenced by age, gender, body weight and GFR. Patients with chronic nephropathy have elevated serum FGF23, which is also a predictor for cardiovascular diseases.

Patients with ADHR and TIO caused by mutation of FGF23 have associated hypophosphatemia. In patients with XLH, autosomal recessive hypophosphatemia, ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) and DMP1, FGF23 is increased or “inappropriately normal.” Other diseases, such as Jansen’s metaphyseal chondrodysplasia, OGD (FGFR1 mutation) and linear nevus sebaceous syndrome, are associated with elevated FGF23.

Research on tumorous hypophosphatemia shows that FRP4, MEPE and FGF23 are all key factors involved in phosphorus metabolism. In vivo and in vitro experiments have found that Klotho is an antioxidative hormone. Klotho gene knockout mice exhibited decreased body weight, dysgenopathy, shortened lifetime, decreased secretion of insulin and increased insulin sensitivity, hyperphosphatemia and ectopic calcification. These pathophysiological changes are associated with human hypogonadism, sterility, disturbance of the immune function of the thymus, low bone mass, vessel calcification, ataxia, deafness and some aging-related diseases [55]. Blood α-Klotho in normal humans is 239–1266 pg/ml; in Klotho deficient patients, it is generally believed to be<200 pg/ml. However, the significance of this in aging or other diseases needs further investigation.

TIO/rickets is often caused by a mesenchymal neoplasm, such as a sclerosing hemangioma, intravascular tumor, hemangiopericytoma, giant cell tumor, giant cell reparative granuloma, angiofibroma, neuroma, non-ossifying fibroma, prostate cancer or epidermal nevus syndrome. Nearly one-half of cases have hemangiomas, especially hemangiopericytomas. Most are benign; malignant tumors are associated with epidermal carcinoma, endoderm carcinoma, fibrous dysplasia or neurofibroma. TIO is a paraneoplastic syndrome that can be cured if the neoplasm is resected. There are three diseases related to the cause of TIO: (1) benign tumors originating from bone or soft tissue, such as hemangioma, histiocytoma, hemangiopericytoma, bone giant cell tumor, soft tissue giant cell tumor, non-ossifying fibroma, ossifying fibroma, chondromyxoid fibroma, fibroangioma or anomalous mixed connective tissue disease; (2) malignant tumors— some malignant tumors can cause TIO, including prostate cancer, breast cancer, small cell lung cancer, multiple myeloma, osteosarcoma, sarcoma, angiosarcoma, malignant fibrous histiocytoma, chondrosarcoma, malignant neurocytoma and malignant Schwann’s cytoma; and (3) non-tumorous diseases — the non-tumorous diseases that can cause TIO are epidermal nevus, neurofibromatosis, McCune-Albright’s syndrome, Paget’s bone disease and fibrous osteohyperplasia.

The common characteristics of FGF23 associated bone diseases are elevated FGF23 in serum and hypophosphatemia. The causes of elevated FGF23, which results in loss of phosphate and hypophosphatemia, are complex; and most are non-PTH/vitamin D dependent. XLH and ADHR are accompanied by hypophosphatemia and their common characteristics are: (1) diuresis of phosphorus and high phosphorus levels in urine; (2) 1,25-(OH)₂D dyssynthesis; (3) hypocalcemia and hypocalciuria; (4) rickets or osteomalacia; and (5) high serum FGF23 with slightly elevated or normal blood PTH. Serum 1,25-(OH)₂D is normal, but laboratory tests show an “inappropriate” decrease.

The most common causes of non-genetic hypophosphatemia are congenital or acquired dysfunction of reabsorption of phosphorus in the kidney tubules and vitamin D deficiency syndrome. The most common cause of genetic hypophosphatemic rickets is XLH, with an incidence of 1/20 000; it is caused by mutation of the PHEX gene. Mutation of the ENPP1 gene leads to type 2 ARHR. All types of hypophosphatemic rickets exhibit characteristic excess or overactivity of FGF23 (Fig. 9)[56,57].

In XLH, the factors that influence serum PTH include: (1) decreased serum 1,25-(OH)₂D, which increases PTH; (2) decreased blood calcium, which slightly elevates PTH; (3) increased phosphorus, which increases PTH; (4) mutation of PHEX in the parathyroid, which elevates PTH; (5) FGF23 directly inhibiting the secretion of PTH; (6) hypophosphatemia, which decreases PTH; (7) bone resistance to PTH, resulting in elevated PTH; and (8) other hormones that consume phosphorus, such as minhibin (maybe a multiple molecule) and MEPE.

Tumor or bone fibrous dysphasia leads to over-secretion and deficiency of FGF23 degradation (Table 4). Elevated 1,25-(OH)₂D inhibits the transcriptional activity of PHEX, which is secreted by osteoblasts. Patients with XLH have elevated FGF23 and decreased 1,25-(OH)₂D and phosphorus consumption due to the inactivating mutation of PHEX. Mutation of FGF23, GALNT3 (which influences the post-translational modification of FGF23) or the gene encoding Klotho (the cofactor that transforms FGFR1 to FGFR23) leads to severe hypophosphatemia and tumoral calcinosis. 25-(OH)D is converted to 1,25-(OH)₂D by 1α-hydroxylase in osteoblasts, and 1,25-(OH)₂D targets nuclear receptors and promotes FGF23 expression. Although studies have shown that FGF23 is assembled by PHEX, recent findings have not confirmed that cleavage of FGF23 is reliant on PHEX. Consequently, mutation of the furin domain is a reasonable explanation for ADHR. Other than HHRH, which leads to decreased serum FGF23, high FGF23 syndrome is mainly due to over-secretion of FGF23, mutation of FGF23 or mutation of PHEX [58–61].

Secondary high FGF23 syndrome and osteomalacia

FGF23 regulates the metabolism of phosphorus in a negative feedback system and increases the excretion of phosphorus with PTH. However, FGF23 and PTH act differently in the synthesis of 1,25-(OH)₂D; FGF23 inhibits its synthesis, whereas PTH elevates it. FGF23 also inhibits the secretion of PTH, and PTH stimulates the secretion of FGF23. Consequently, secondary parathyroidism aggravates high FGF23 syndrome and leads to exacerbation of chronic nephropathy and cardiovascular diseases.

High FGF23 syndrome damages the kidneys and cardiovascular system; the mechanism is unclear but is known to be related to the FGF23 response gene. It is therefore important to study changes in the gene profile of the heart and kidney in high FGF23 syndrome. Dai et al. determined changes in the response gene profile in a hypophosphatemic mouse model with IV collagen α3 deficiency nephropathy and high FGF23 syndrome. Studies of the response gene can help improve understanding of the pathophysiology and pathogenic significance of high FGF23 syndrome.

Low FGF23 syndrome and FGF23 deficiency

These conditions can be divided into primary low FGF23 syndrome and secondary low FGF23 syndrome.

Primary low FGF23 syndrome

This is characterized by significantly decreased serum FGF23 level and activity, and elevated 1,25-(OH)₂D. Low FGF23 syndrome and FGF23 deficiency are commonly seen in tumoral calcinosis and FTC, which is caused by an inactivating mutation of FGF23. The common pathogenesis is mutation of FGF23, Klotho, GALNT3 or SAMD9. FTC can be divided into hyperphosphatemic FTC (HFTC) and normophosphatemic FTC. The pathogenesis of HTFC is possibly a deficiency of phosphorus diuresis and hyperphosphatemia caused by mutation of FGF23. Mutated Klotho can lead to deficiency of molecules that are associated with the FGFR23, and mutated GALNT3 leads to an O-saccharification defect of FGF23 and ultimately to hyperphosphatemia.

Secondary low FGF23 syndrome

This is characterized by a severe decrease of serum FGF23 accompanied by normal or decreased blood phosphorus and elevated 1,25-(OH)₂D. Secondary low FGF23 syndrome is commonly seen in people with a low phosphorus diet, mutated vitamin D receptor, mutated 1α-hydroxylase, mutated or deficient NaPi-2a or mutated NaPi-2c (HHRH). Vitamin D dependent rickets type I (VDRR1, OMIM264700) is caused by 1α-hydroxylase deficiency leading to dysfunction of bone mineralization. The manifestations of VDDR1 are similar to those of vitamin D deficiency rickets but start earlier (normally before 2 years of age), with more severe symptoms and faster progression. The biochemical changes are also more marked. Serum calcium and phosphorus are both decreased, serum 25-(OH)D is normal or elevated and serum 1,25-(OH)₂D is markedly decreased. Vitamin D resistant rickets type II is a disease in which the vitamin D receptor is resistant to 1,25-(OH)₂D; it is also called hereditary vitamin D resistant rickets. It is characterized by rickets or osteomalacia, with clinical manifestations after birth. Patients suffer from ostalgia, myasthenia, decreased muscle tension and tetany caused by hypocalcemia. It also manifests as cessation of the growth and development of the skeleton and teeth. The biochemical characteristics include serum 1,25-(OH)₂D increase, hypocalcemia and hypophosphatemia associated with serum alkaline phosphatase and PTH elevation. Serum 25-(OH)D remains normal, 1,25-(OH)₂D is significantly elevated and 24,25-(OH)₂D is normal or decreased.

Mutated NaPi-2a can cause familial autosomal recessive hereditary Fanconi’s syndrome and hypophosphatemic rickets. Mutated NaPi-2c (SLC34A3) causes HHRH, and Tencza et al. have reported a family with HHRH [62]; its clinical treatment is shown in Table 5.

References

[1] Beenken A, Mohammadi M. The structural biology of the FGF19 subfamily. Adv Exp Med Biol2012; 728: 1–24

[2] Donate-Correa J, Muros-de-Fuentes M, Mora-Fernández C, Navarro-González JF. FGF23/Klotho axis: phosphorus, mineral metabolism and beyond. Cytokine Growth Factor Rev2012; 23(1–2): 37–46

[3] Rowe PS. Regulation of bone-renal mineral and energy metabolism: the PHEX, FGF23, DMP1, MEPE ASARM pathway. Crit Rev Eukaryot Gene Expr2012; 22(1): 61–86

[4] Jones SA. Physiology of FGF15/19. Adv Exp Med Biol2012; 728: 171–182

[5] Potthoff MJ, Kliewer SA, Mangelsdorf DJ. Endocrine fibroblast growth factors 15/19 and 21: from feast to famine. Genes Dev2012; 26(4): 312–324

[6] Silver J, Naveh-Many T. FGF23 and the parathyroid. Adv Exp Med Biol2012; 728: 92–99

[7] Razzaque MS. FGF23, klotho and vitamin D interactions: What have we learned from in vivo mouse genetics studies? Adv Exp Med Biol2012; 728: 84–91

[8] Perwad F, Azam N, Zhang MY, Yamashita T, Tenenhouse HS, Portale AA. Dietary and serum phosphorus regulate fibroblast growth factor 23 expression and 1,25-dihydroxyvitamin D metabolism in mice. Endocrinology2005; 146(12): 5358–5364

[9] Burnett SM, Gunawardene SC, Bringhurst FR, Jüppner H, Lee H, Finkelstein JS. Regulation of C-terminal and intact FGF-23 by dietary phosphate in men and women. J Bone Miner Res2006; 21(8): 1187–1196

[10] Rodríguez M, López I, Muñoz J, Aguilera-Tejero E, Almaden Y. FGF23 and mineral metabolism, implications in CKD-MBD. Nefrologia2012; 32(3): 275–278

[11] Martin A, David V, Quarles LD. Regulation and function of the FGF23/klotho endocrine pathways. Physiol Rev2012; 92(1): 131–155

[12] Kienitz T, Ventz M, Kaminsky E, Quinkler M. Novel PHEX nonsense mutation in a patient with X-linked hypophosphatemic rickets and review of current therapeutic regimens. Exp Clin Endocrinol Diabetes2011; 119(7): 431–435

[13] Bernheim J, Benchetrit S. The potential roles of FGF23 and Klotho in the prognosis of renal and cardiovascular diseases. Nephrol Dial Transplant2011; 26(8): 2433–2438

[14] Kuro-o M. Klotho and βKlotho. Adv Exp Med Biol2012; 728: 25–40

[15] Segawa H, Yamanaka S, Ohno Y, Onitsuka A, Shiozawa K, Aranami F, Furutani J, Tomoe Y, Ito M, Kuwahata M, Imura A, Nabeshima Y, Miyamoto K. Correlation between hyperphosphatemia and type II Na-Pi cotransporter activity in klotho mice. Am J Physiol Renal Physiol2007; 292(2): F769–F779

[16] Chang Q, Hoefs S, van der Kemp AW, Topala CN, Bindels RJ, Hoenderop JG. The beta-glucuronidase klotho hydrolyzes and activates the TRPV5 channel. Science2005; 310(5747): 490–493

[17] Cha SK, Ortega B, Kurosu H, Rosenblatt KP, Kuro-O M, Huang CL. Removal of sialic acid involving Klotho causes cell-surface retention of TRPV5 channel via binding to galectin-1. Proc Natl Acad Sci USA2008; 105(28): 9805–9810

[18] Imura A, Tsuji Y, Murata M, Maeda R, Kubota K, Iwano A, Obuse C, Togashi K, Tominaga M, Kita N, Tomiyama K, Iijima J, Nabeshima Y, Fujioka M, Asato R, Tanaka S, Kojima K, Ito J, Nozaki K, Hashimoto N, Ito T, Nishio T, Uchiyama T, Fujimori T, Nabeshima Y. alpha-Klotho as a regulator of calcium homeostasis. Science2007; 316(5831): 1615–1618

[19] Hu MC, Shi M, Zhang J, Pastor J, Nakatani T, Lanske B, Razzaque MS, Rosenblatt KP, Baum MG, Kuro-o M, Moe OW. Klotho: a novel phosphaturic substance acting as an autocrine enzyme in the renal proximal tubule. FASEB J2010; 24(9): 3438–3450

[20] Tohyama O, Imura A, Iwano A, Freund JN, Henrissat B, Fujimori T, Nabeshima Y. Klotho is a novel beta-glucuronidase capable of hydrolyzing steroid beta-glucuronides. J Biol Chem2004; 279(11): 9777–9784

[21] Hayes G, Busch A, Lötscher M, Waldegger S, Lang F, Verrey F, Biber J, Murer H. Role of N-linked glycosylation in rat renal Na/Pi-cotransport. J Biol Chem1994; 269(39): 24143–24149

[22] Rakugi H, Matsukawa N, Ishikawa K, Yang J, Imai M, Ikushima M, Maekawa Y, Kida I, Miyazaki J, Ogihara T. Anti-oxidative effect of Klotho on endothelial cells through cAMP activation. Endocrine2007; 31(1): 82–87

[23] Papaconstantinou J. Insulin/IGF-1 and ROS signaling pathway cross-talk in aging and longevity determination. Mol Cell Endocrinol2009; 299(1): 89–100

[24] Diepeveen SH, Verhoeven GH, van der Palen J, Dikkeschei BL, van Tits LJ, Kolsters G, Offerman JJ, Bilo HJ, Stalenhoef AF. Oxidative stress in patients with end-stage renal disease prior to the start of renal replacement therapy. Nephron Clin Pract2004; 98(1): c3–c7

[25] Voormolen N, Noordzij M, Grootendorst DC, Beetz I, Sijpkens YW, van Manen JG, Boeschoten EW, Huisman RM, Krediet RT, Dekker FW; PREPARE study group. High plasma phosphate as a risk factor for decline in renal function and mortality in pre-dialysis patients. Nephrol Dial Transplant2007; 22(10): 2909–2916

[26] Jean G, Terrat JC, Vanel T, Hurot JM, Lorriaux C, Mayor B, Chazot C. High levels of serum fibroblast growth factor (FGF)-23 are associated with increased mortality in long haemodialysis patients. Nephrol Dial Transplant2009; 24(9): 2792–2796

[27] Gordon PL, Frassetto LA. Management of osteoporosis in CKD Stages 3 to 5. Am J Kidney Dis2010; 55(5): 941–956

[28] Maekawa Y, Ishikawa K, Yasuda O, Oguro R, Hanasaki H, Kida I, Takemura Y, Ohishi M, Katsuya T, Rakugi H. Klotho suppresses TNF-alpha-induced expression of adhesion molecules in the endothelium and attenuates NF-kappaB activation. Endocrine2009; 35(3): 341–346

[29] Nagai R, Saito Y, Ohyama Y, Aizawa H, Suga T, Nakamura T, Kurabayashi M, Kuroo M. Endothelial dysfunction in the klotho mouse and downregulation of klotho gene expression in various animal models of vascular and metabolic diseases. Cell Mol Life Sci2000; 57(5): 738–746

[30] Nakamura T, Saito Y, Ohyama Y, Masuda H, Sumino H, Kuro-o M, Nabeshima Y, Nagai R, Kurabayashi M. Production of nitric oxide, but not prostacyclin, is reduced in klotho mice. Jpn J Pharmacol2002; 89(2): 149–156

[31] Yuan B, Takaiwa M, Clemens TL, Feng JQ, Kumar R, Rowe PS, Xie Y, Drezner MK. Aberrant Phex function in osteoblasts and osteocytes alone underlies murine X-linked hypophosphatemia. J Clin Invest2008; 118(2): 722–734

[32] Aono Y, Yamazaki Y, Yasutake J, Kawata T, Hasegawa H, Urakawa I, Fujita T, Wada M, Yamashita T, Fukumoto S, Shimada T. Therapeutic effects of anti-FGF23 antibodies in hypophosphatemic rickets/osteomalacia. J Bone Miner Res 2009; 24(11): 1879–1888

[33] Zhang R, Lu Y, Ye L, Yuan B, Yu S, Qin C, Xie Y, Gao T, Drezner MK, Bonewald LF, Feng JQ. Unique roles of phosphorus in endochondral bone formation and osteocyte maturation. J Bone Miner Res2011; 26(5): 1047–1056

[34] Lu Y, Yuan B, Qin C, Cao Z, Xie Y, Dallas SL, McKee MD, Drezner MK, Bonewald LF, Feng JQ. The biological function of DMP-1 in osteocyte maturation is mediated by its 57-kDa C-terminal fragment. J Bone Miner Res2011; 26(2): 331–340

[35] Razzaque MS. Osteo-renal regulation of systemic phosphate metabolism. IUBMB Life2011; 63(4): 240–247

[36] Murer H, Biber J. Phosphate transport in the kidney. J Nephrol2010; 23(Suppl 16): S145–S151

[37] Segawa H, Aranami F, Kaneko I, Tomoe Y, Miyamoto K. The roles of Na/Pi-II transporters in phosphate metabolism. Bone2009; 45(Suppl 1): S2–S7

[38] Biber J, Hernando N, Forster I, Murer H. Regulation of phosphate transport in proximal tubules. Pflugers Arch2009; 458(1): 39–52

[39] Miyamoto K, Ito M, Tatsumi S, Kuwahata M, Segawa H. New aspect of renal phosphate reabsorption: the type IIc sodium-dependent phosphate transporter. Am J Nephrol2007; 27(5): 503–515

[40] Laroche M, Boyer JF. Phosphate diabetes, tubular phosphate reabsorption and phosphatonins. Joint Bone Spine2005; 72(5): 376–381

[41] Hruska KA, Mathew S, Lund R, Qiu P, Pratt R. Hyperphosphatemia of chronic kidney disease. Kidney Int2008; 74(2): 148–157

[42] Prié D, Ureña Torres P, Friedlander G. Latest findings in phosphate homeostasis. Kidney Int2009; 75(9): 882–889

[43] Wolf M. Fibroblast growth factor 23 and the future of phosphorus management. Curr Opin Nephrol Hypertens2009; 18(6): 463–468

[44] Wahl P, Wolf M. FGF23 in chronic kidney disease. Adv Exp Med Biol2012; 728: 107–125

[45] Silver J. Molecular mechanisms of secondary hyperparathyroidism. Nephrol Dial Transplant2000; 15(Suppl 5): 2–7

[46] Lavi-Moshayoff V, Wasserman G, Meir T, Silver J, Naveh-Many T. PTH increases FGF23 gene expression and mediates the high-FGF23 levels of experimental kidney failure: a bone parathyroid feedback loop. Am J Physiol Renal Physiol2010; 299(4): F882–F889

[47] Saji F, Shiizaki K, Shimada S, Okada T, Kunimoto K, Sakaguchi T, Hatamura I, Shigematsu T. Regulation of fibroblast growth factor 23 production in bone in uremic rats. Nephron, Physiol2009; 111(4): 59–66

[48] Günther T, Chen ZF, Kim J, Priemel M, Rueger JM, Amling M, Moseley JM, Martin TJ, Anderson DJ, Karsenty G. Genetic ablation of parathyroid glands reveals another source of parathyroid hormone. Nature2000; 406(6792): 199–203

[49] Mughal MZ. Rickets. Curr Osteoporos Rep2011; 9(4): 291–299

[50] Henry HL. Regulation of vitamin D metabolism. Best Pract Res Clin Endocrinol Metab2011; 25(4): 531–541

[51] Fukumoto S, Shimizu Y. Fibroblast growth factor 23 as a phosphotropic hormone and beyond. J Bone Miner Metab2011; 29(5): 507–514

[52] Dai B, David V, Martin A, Huang J, Li H, Jiao Y, Gu W, Quarles LD. A comparative transcriptome analysis identifying FGF23 regulated genes in the kidney of a mouse CKD model. PLoS ONE2012; 7(9): e44161

[53] Bergwitz C, Jüppner H. FGF23 and syndromes of abnormal renal phosphate handling. Adv Exp Med Biol2012; 728: 41–64

[54] Gattineni J, Baum M. Genetic disorders of phosphate regulation. Pediatr Nephrol2012; 27(9):1477–1487

[55] Kinoshita Y, Saito T, Shimizu Y, Hori M, Taguchi M, Igarashi T, Fukumoto S, Fujita T. Mutational analysis of patients with FGF23-related hypophosphatemic rickets. Eur J Endocrinol2012; 167(2): 165–172

[56] Quarles LD. Role of FGF23 in vitamin D and phosphate metabolism: implications in chronic kidney disease. Exp Cell Res2012; 318(9): 1040–1048

[57] Carpenter TO. The expanding family of hypophosphatemic syndromes. J Bone Miner Metab2012; 30(1): 1–9

[58] Chong WH, Molinolo AA, Chen CC, Collins MT. Tumor-induced osteomalacia. Endocr Relat Cancer2011; 18(3): R53–R77

[59] Owen C, Chen F, Flenniken AM, Osborne LR, Ichikawa S, Adamson SL, Rossant J, Aubin JE. A novel Phex mutation in a new mouse model of hypophosphatemic rickets. J Cell Biochem2012; 113(7): 2432–2441

[60] Khaliq W, Cheripalli P, Tangella K. Tumor-induced osteomalacia (TIO): atypical presentation. South Med J2011; 104(5): 348–350

[61] van der Rest C, Cavalier E, Kaux JF, Krzesinski JM, Hustinx R, Reginster JY, Delanaye P. Tumor-induced osteomalacia: the tumor may stay hidden! Clin Biochem2011; 44(14-15): 1264–1266

[62] Magen D, Berger L, Coady M J, Ilivitzki A, Militianu D, Tieder M, Selig S, Lapointe J Y, Zelikovic I, Skorecki K. A loss-of-function mutation in NaPi-IIa and renal Fanconi’s syndrome. N Engl J Med2010; 362(12): 1102–1109

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