Transgenic mice overexpressing human fibroblast growth factor 23 (R176Q) delineate a putative role for parathyroid hormone in renal phosphate wasting disorders

MC8 peptide-mediated Her-2 receptor targeting based on PEI-β-CyD as gene delivery vector

A novel vector with high gene delivery efficiency and special cell targeting ability was developed using a good strategy that utilized low molecular weight polyethylenimine (PEI; molecular weight, 600 KDa [PEI600]) cross-linked to β-cyclodextrin (β-CyD) via a facile synthetic route. Human epidermal growth factor receptor 2 (Her-2) are highly expressed in a variety of human cancer cells and are potential targets for cancer therapy. MC8 peptides, which have been proven to combine especially with Her-2 on cell membranes were coupled to PEI-β-CyD using N-succinimidyl-3-(2-pyridyldithio) propionate as a linker. The ratios of PEI600, β-CyD, and peptide were calculated based on proton integral values obtained from the (1)H-NMR spectra of the resulting products. Electron microscope observations showed that MC8-PEI-β-CyD can efficiently condense plasmid DNA (pDNA) into nanoparticles of about 200 nm, and MTT assays suggested the decreased toxicity of the polymer. Experiments on gene delivery efficiency in vitro showed that MC8-PEI-β-CyD/pDNA polyplexes had significantly greater transgene activities than PEI-β-CyD/pDNA in the Skov3 and A549 cells, which positively expressed Her-2, whereas, no such effect was observed in the MCF-7 cells, which negatively expressed Her-2. Our current research indicated that the synthesized nonviral vector shows improved gene delivery efficiency and targeting specificity in Her-2 positive cells.

Mineralizing enthesopathy is a common feature of renal phosphate-wasting disorders attributed to FGF23 and is exacerbated by standard therapy in hyp mice

We have previously confirmed a paradoxical mineralizing enthesopathy as a hallmark of X-linked hypophosphatemia. X-linked hypophosphatemia is the most common of the phosphate-wasting disorders mediated by elevated fibroblast growth factor 23 (FGF23) and occurs as a consequence of inactivating mutations of the PHEX gene product. Despite childhood management of the disease, these complications of tendon and ligament insertion sites account for a great deal of the disease's morbidity into adulthood. It is unclear whether the enthesopathy occurs in other forms of renal phosphate-wasting disorders attributable to high FGF23 levels. Here we describe two patients with autosomal recessive hypophosphatemic rickets due to the Met1Val mutation in dentin matrix acidic phosphoprotein 1 (DMP1). In addition to the biochemical and skeletal features of long-standing rickets with elevated FGF23 levels, these individuals exhibited severe, debilitating, generalized mineralized enthesopathy. These data suggest that enthesophytes are a feature common to FGF23-mediated phosphate-wasting disorders. To address this possibility, we examined a murine model of FGF23 overexpression using a transgene encoding the secreted form of human FGF23 (R176Q) cDNA (FGF23-TG mice). We report that FGF23-TG mice display a similar mineralizing enthesopathy of the Achilles and plantar facial insertions. In addition, we examined the impact of standard therapy for phosphate-wasting disorders on enthesophyte progression. We report that fibrochondrocyte hyperplasia persisted in Hyp mice treated with oral phosphate and calcitriol. In addition, treatment had the untoward effect of further exacerbating the mineralization of fibrochondrocytes that define the bone spur of the Achilles insertion. These studies support the need for newer interventions targeted at limiting the actions of FGF23 and minimizing both the toxicities and potential morbidities associated with standard therapy.

New insights into NPP1 function: lessons from clinical and animal studies

The recent elucidation of rare human genetic disorders resulting from mutations in ectonucleotide pyrophosphotase/phosphodiesterase (ENPP1), also known as plasma cell membrane glycoprotein 1 (PC-1), has highlighted the vital importance of this molecule in human health and disease. Generalised arterial calcification in infants (GACI), a frequently lethal disease, has been reported in recessive inactivating mutations in ENPP1. Recent findings have also linked hypophosphataemia to a lack of NPP1 function. A number of human genetic studies have indicated that NPP1 is a vital regulator that influences a wide range of tissues through various signalling pathways and when disrupted can lead to significant pathology. The function of Enpp1 has been widely studied in rodent models, where both the mutant tiptoe walking (ttw/ttw) mouse and genetically engineered Enpp1(-/-) mice show significant alterations in skeletal and soft tissue mineralisation, calcium/phosphate balance and glucose homeostasis. These models therefore provide important tools with which to study the potential mechanisms underpinning the human diseases associated with altered NPP1. This review will focus on the recent advances in our current knowledge of the actions of NPP1 in relation to bone disease, cardiovascular pathologies and diabetes. A fuller understanding of the mechanisms through which NPP1 exerts its pathological effects may stimulate the development of novel therapeutic strategies for patients at risk from the devastating clinical outcomes associated with disrupted NPP1 function.

A novel Phex mutation in a new mouse model of hypophosphatemic rickets

X-linked hypophosphatemic rickets (XLH) is a dominantly inherited disease characterized by renal phosphate wasting, aberrant vitamin D metabolism, and defective bone mineralization. It is known that XLH in humans and in certain mouse models is caused by inactivating mutations in PHEX/Phex (phosphate-regulating gene with homologies to endopeptidases on the X chromosome). By a genome-wide N-ethyl-N-nitrosourea (ENU)-induced mutagenesis screen in mice, we identified a dominant mouse mutation that exhibits the classic clinical manifestations of XLH, including growth retardation, skeletal abnormalities (rickets/osteomalacia), hypophosphatemia, and increased serum alkaline phosphatase (ALP) levels. Mapping and sequencing revealed that these mice carry a point mutation in exon 14 of the Phex gene that introduces a stop codon at amino acid 496 of the coding sequence (Phex(Jrt) also published as Phex(K496X) [Ichikawa et al., 2012]). Fgf23 mRNA expression as well as that of osteocalcin, bone sialoprotein, and matrix extracellular phosphoglycoprotein was upregulated in male mutant long bone, but that of sclerostin was unaffected. Although Phex mRNA is expressed in bone from mutant hemizygous male mice (Phex(Jrt)/Y mice), no Phex protein was detected in immunoblots of femoral bone protein. Stromal cultures from mutant bone marrow were indistinguishable from those of wild-type mice with respect to differentiation and mineralization. The ability of Phex(Jrt)/Y osteoblasts to mineralize and the altered expression levels of matrix proteins compared with the well-studied Hyp mice makes it a unique model with which to further explore the clinical manifestations of XLH and its link to FGF23 as well as to evaluate potential new therapeutic strategies.

Targeted deletion of Klotho in kidney distal tubule disrupts mineral metabolism

Renal Klotho controls mineral metabolism by directly modulating tubular reabsorption of phosphate and calcium and by acting as a co-receptor for the phosphaturic and vitamin D-regulating hormone fibroblast growth factor-23 (FGF23). Klotho null mice have a markedly abnormal phenotype. We sought to determine effects of renal-specific and partial deletion of Klotho to facilitate investigation of its roles in health and disease. We generated a mouse model with partial deletion of Klotho in distal tubular segments (Ksp-KL(-/-)). In contrast to Klotho null mice, Ksp-KL(-/-) mice were fertile, had a normal gross phenotype, and did not have vascular or tubular calcification on renal histology. However, Ksp-KL(-/-) mice were hyperphosphatemic with elevated FGF23 levels and abundant expression of the sodium-phosphate cotransporter Npt2a at the brush border membrane. Serum calcium and 1,25-dihydroxyvitamin D(3) levels were normal but parathyroid hormone levels were decreased. TRPV5 protein was reduced with a parallel mild increase in urinary calcium excretion. Renal expression of vitamin D regulatory enzymes and vitamin D receptor was higher in Ksp-KL(-/-) mice than controls, suggesting increased turnover of vitamin D metabolites and a functional increase in vitamin D signaling. There was a threshold effect of residual renal Klotho expression on FGF23: deletion of >70% of Klotho resulted in FGF23 levels 30-250 times higher than in wild-type mice. A subgroup of Ksp-KL(-/-) mice with normal phosphate levels had elevated FGF23, suggesting a Klotho-derived renal-bone feedback loop. Taken together, renal FGF23-Klotho signaling, which is disrupted in CKD, is essential for homeostatic control of mineral metabolism.

Relevance of vitamin D in reproduction

The steroid hormone vitamin D is historically recognized for its relevance to bone health and calcium homeostasis. Recent years have witnessed a shift in focus to non-skeletal benefits of vitamin D; in this latter context, an accruing body of literature attests to a relevance of vitamin D to reproductive physiology. This article reviews the existing data about the diverse and previously underappreciated roles for vitamin D in reproductive health. A large body of available literature suggests that vitamin D deficiency may be detrimental to reproductive biology. However, given that our appreciation of vitamin D's role in reproductive physiology is almost entirely shaped by 'associative' studies and that data based on prospective interventional trials are limited, these concepts remain predominantly conjectural. Exact mechanisms whereby vitamin D may participate in the regulation of reproductive physiology remain far from clear. This review underscores a need for appropriately designed intervention trials to address the existing knowledge gaps and to delineate the specific roles of vitamin D signaling in reproductive biology.

FGF23 neutralization improves chronic kidney disease-associated hyperparathyroidism yet increases mortality

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is associated with secondary hyperparathyroidism (HPT) and serum elevations in the phosphaturic hormone FGF23, which may be maladaptive and lead to increased morbidity and mortality. To determine the role of FGF23 in the pathogenesis of CKD-MBD and development of secondary HPT, we developed a monoclonal FGF23 antibody to evaluate the impact of chronic FGF23 neutralization on CKD-MBD, secondary HPT, and associated comorbidities in a rat model of CKD-MBD. CKD-MBD rats fed a high-phosphate diet were treated with low or high doses of FGF23-Ab or an isotype control antibody. Neutralization of FGF23 led to sustained reductions in secondary HPT, including decreased parathyroid hormone, increased vitamin D, increased serum calcium, and normalization of bone markers such as cancellous bone volume, trabecular number, osteoblast surface, osteoid surface, and bone-formation rate. In addition, we observed dose-dependent increases in serum phosphate and aortic calcification associated with increased risk of mortality in CKD-MBD rats treated with FGF23-Ab. Thus, mineral disturbances caused by neutralization of FGF23 limited the efficacy of FGF23-Ab and likely contributed to the increased mortality observed in this CKD-MBD rat model.

Vitamin D: metabolism

The biologically active metabolite of vitamin D, 1,25(OH)(2)D(3), affects mineral homeostasis and has numerous other diverse physiologic functions including effects on growth of cancer cells and protection against certain immune disorders. This article reviews the role of vitamin D hydroxylases in providing a tightly regulated supply of 1,25(OH)(2)D(3). The role of extrarenal 1α(OH)ase in placenta and macrophages is also discussed, as well as regulation of vitamin D hydroxylases in aging and chronic kidney disease. Understanding specific factors involved in regulating the hydroxylases may lead to the design of drugs that can selectively modulate the hydroxylases. The ability to alter levels of these enzymes would have therapeutic potential for the treatment of various diseases, including bone loss disorders and certain immune diseases.

Role of FGF23 in vitamin D and phosphate metabolism: implications in chronic kidney disease

FGF23 is a bone-derived hormone that regulates systemic phosphate homeostasis, vitamin D metabolism and α-Klotho expression through a novel bone-kidney axis. FGF23 inhibits renal tubular reabsorption of phosphate through mechanisms independent of PTH as well as reduces circulating 1, 25(OH)(2)D through its dual effects to suppress Cyp27b1 production and to stimulate Cyp24 catabolism of 1,25(OH)(2)D. 1,25(OH)(2)D and other factors regulating bone remodeling/mineralization are the major physiological regulators of FGF23 expression. FGF23 also suppresses the gene transcription of α-klotho by the kidney, which exists as a membrane and soluble protein. Membrane Klotho acts as a coreceptor for and dictates organ specificity of FGF23, whereas soluble Klotho act as an endocrine factor that regulates activity of cell surface glycoproteins and receptors in multiple tissues. Elevated FGF23 levels are responsible for several hereditary and acquired hypophosphatemic rickets disorders. FGF23 and Klotho deficiency have similar phenotypes characterized by hyperphosphatemia, elevated 1,25(OH)(2)D and tumoral calcinosis. FGF23 levels progressively increase during chronic kidney disease (CKD). FGF23 has been proposed to be the initial adaptive response leading to reductions in 1,25(OH)(2)D and secondary hyperparathyroidism (HPT) in CKD. The overall biological effect of this initial step may be to orchestrate a coordinated adaptation to protect the organism from the adverse effects of excess phosphate retention. The second step involves the effects of PTH on bone remodeling that further stimulates FGF23 production through both direct and indirect mechanisms related to alterations in extracellular matrix factors. PTH further amplifies FGF23 expression in later stages of CKD to compensate for the increased phosphate efflux from bone caused by excessive bone turnover. While many aspects of the regulation and functions of FGF23 remain to be established, the idea that FGF23 hormone is the initial adaptive hormonal response in CKD that suppresses 1,25(OH)(2)D, reduces gastrointestinal calcium and phosphate absorption and leads to a secondary HPT represents a paradigm shift in the conceptualization of the pathogenesis of secondary hyperparathyroidism. In addition, the prevalent thought that CKD is a functional "vitamin D deficient state" requiring therapy with 1,25(OH)(2)D analogs is challenged by effects of FGF23 to potentially lower both 25(OH)D and 1,25(OH)D by induction of Cyp24-mediated degradation. Finally, increments in FGF23 are associated with increased cardiovascular mortality in CKD. Whether these effects represent direct effects of FGF23 or represent a marker of other abnormalities in CKD remains to be determined.

Fibroblast growth factor-23 helps explain the biphasic cardiovascular effects of vitamin D in chronic kidney disease

Hypovitaminosis D is highly prevalent in chronic kidney disease (CKD). Recently, vitamin D has sparked widespread interest because of its potential favorable benefits on cardiovascular disease (CVD). Evidence from clinical studies and animal models supports the existence of biphasic cardiovascular effects of vitamin D, in which lower doses suppress CVD and higher doses stimulate CVD. However, the mechanism for the different effects remains unclear. Fibroblast growth factor-23 (FGF-23) is a recently identified member of the FGF family, and thought to be actively involved in renal phosphate and vitamin D homeostasis. More specifically, Vitamin D stimulates FGF-23 secretion and is inhibited by increased FGF-23. Given this background, we hypothesize that FGF-23 may provide a unique tool to explain the biphasic cardiovascular effects of vitamin D in CKD. The data presented in this review support the hypothesis that FGF-23 may be linked with the high cardiovascular risk in CKD through accelerating the onset of vascular calcification, secondary hyperparathyroidism, left ventricular hypertrophy and endothelial dysfunction. Therefore, modulation of FGF-23 may become a potential therapeutic target to lowing cardiovascular risk in CKD. Several clinical interventions, including decreased phosphate intake, phosphate binders, cinacalcet plus concurrent low-dose vitamin D, C-terminal tail of FGF-23 and renal transplantation, have been employed to manipulate FGF-23.

FGF receptor-mediated gene delivery using ligands coupled to PEI-β-CyD

A novel vector with high gene delivery efficiency and special cell-targeting ability was developed using a good strategy that utilized low-molecular-weight polyethylenimine (PEI; molecular weight: 600 KDa [PEI600]) crosslinked to β-cyclodextrin (β-CyD) via a facile synthetic route. Fibroblast growth factor receptors (FGFRs) are highly expressed in a variety of human cancer cells and are potential targets for cancer therapy. In this paper, CY11 peptides, which have been proven to combine especially with FGFRs on cell membranes were coupled to PEI-β-CyD using N-succinimidyl-3-(2-pyridyldithio) propionate as a linker. The ratios of PEI600, β-CyD, and peptide were calculated based on proton integral values obtained from the (1)H-NMR spectra of the resulting products. Electron microscope observations showed that CY11-PEI-β-CyD can efficiently condense plasmid DNA (pDNA) into nanoparticles of about 200 nm, and MTT assays suggested the decreased toxicity of the polymer. Experiments on gene delivery efficiency in vitro showed that CY11-PEI-β-CyD/pDNA polyplexes had significantly greater transgene activities than PEI-β-CyD/pDNA in the COS-7 and HepG2 cells, which positively expressed FGFR, whereas no such effect was observed in the PC-3 cells, which negatively expressed FGFR. Our current research indicated that the synthesized nonviral vector shows improved gene delivery efficiency and targeting specificity in FGFR-positive cells.

Skeletal secretion of FGF-23 regulates phosphate and vitamin D metabolism

The discovery of fibroblast growth factor 23 (FGF-23) has expanded our understanding of phosphate and vitamin D homeostasis and provided new insights into the pathogenesis of hereditary hypophosphatemic and hyperphosphatemic disorders, as well as acquired disorders of phosphate metabolism, such as chronic kidney disease. FGF-23 is secreted by osteoblasts and osteocytes in bone and principally targets the kidney to regulate the reabsorption of phosphate, the production and catabolism of 1,25-dihydroxyvitamin D and the expression of α-Klotho, an anti-ageing hormone. Secreted FGF-23 plays a central role in complex endocrine networks involving local bone-derived factors that regulate mineralization of extracellular matrix and systemic hormones involved in mineral metabolism. Inactivating mutations of PHEX, DMP1 and ENPP1, which cause hereditary hypophosphatemic disorders and primary defects in bone mineralization, stimulate FGF23 gene transcription in osteoblasts and osteocytes, at least in part, through canonical and intracrine FGF receptor pathways. These FGF-23 regulatory pathways may enable systemic phosphate and vitamin D homeostasis to be coordinated with bone mineralization. FGF-23 also functions as a counter-regulatory hormone for 1,25-dihydroxyvitamin D in a bone-kidney endocrine loop. FGF-23, through regulation of additional genes in the kidney and extrarenal tissues, probably has broader physiological functions beyond regulation of mineral metabolism that account for the association between FGF-23 and increased mortality and morbidity in chronic kidney disease.

Can features of phosphate toxicity appear in normophosphatemia?

Phosphate is an indispensable nutrient for the formation of nucleic acids and the cell membrane. Adequate phosphate balance is a prerequisite for basic cellular functions ranging from energy metabolism to cell signaling. More than 85% of body phosphate is present in the bones and teeth. The remaining phosphate is distributed in various soft tissues, including skeletal muscle. A tiny amount, around 1% of total body phosphate, is distributed both in the extracellular fluids and within the cells. Impaired phosphate balance can affect the functionality of almost all human systems, including muscular, skeletal, and vascular systems, leading to an increase in morbidity and mortality of the involved patients. Currently, measuring serum phosphate level is the gold standard to estimate the overall phosphate status of the body. Despite the biological and clinical significance of maintaining delicate phosphate balance, serum levels do not always reflect the amount of phosphate uptake and its distribution. This article briefly discusses the potential that some of the early consequences of phosphate toxicity might not be evident from serum phosphate levels.

Evidence for FGF23 involvement in a bone-kidney axis regulating bone mineralization and systemic phosphate and vitamin D homeostasis

Bone is involved in the maintenance of phosphate and vitamin D homeostasis via its production and secretion of FGF23 and serves as a reservoir for the storage and release of calcium and phosphate into the circulation. Alterations in mineralization of extracellular matrix and the remodeling activities of the skeleton are coupled to the kidney conservation of phosphate and production of 1,25(OH)2D via the regulation of FGF23 production by osteocytes through yet-to-be defined locally derived factors. In addition, FGF23 production is regulated by 1,25(OH)2D in a feedback loop where FGF23 stimulate Cyp24 mediated degradation of 1,25(OH)2D that serves to protect the organism from the toxic effects of vitamin D excess. In this chapter, we will review the regulation and function of FGF23.

FGF23 and the parathyroid

Klotho and fibroblast growth factor 1 (FGFR1) are expressed not only in FGF23's classical target organ, the kidney, but also in other organs such as the parathyroid. FGF23 acts on the parathyroid to decrease PTH mRNA and serum PTH levels. It does this by activating the MAPK pathway. In chronic kidney disease there are very high levels of serum FGF23 together with increased serum PTH levels, implying resistance of the parathyroid to the action of FGF23. This has been shown in parathyroid tissue surgically removed from dialysis patients as well as in experimental models of uremia to be due to down-regulation of klotho-FGFR1 expression in the parathyroid. Moreover, the parathyroids of rats with advanced uremia do not respond to administered FGF23 by activation of the MAPK pathway or inhibition of PTH secretion. Therefore, there is down-regulation of parathyroid klotho-FGFR1 in CKD which correlates with the resistance of the parathyroid to FGF23. A further subject of great interest in this field is the effect of PTH to directly increase FGF23 expression by osteoblast like cells in culture and the observations that parathyroidectomy prevents and corrects the increased serum FGF23 level of experimental CKD as well as decreases FGF23 in patients with CKD. There is therefore a negative feedback loop between bone and the parathyroid.

Regulation and function of the FGF23/klotho endocrine pathways

Calcium (Ca(2+)) and phosphate (PO(4)(3-)) homeostasis are coordinated by systemic and local factors that regulate intestinal absorption, influx and efflux from bone, and kidney excretion and reabsorption of these ions through a complex hormonal network. Traditionally, the parathyroid hormone (PTH)/vitamin D axis provided the conceptual framework to understand mineral metabolism. PTH secreted by the parathyroid gland in response to hypocalcemia functions to maintain serum Ca(2+) levels by increasing Ca(2+) reabsorption and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] production by the kidney, enhancing Ca(2+) and PO(4)(3-) intestinal absorption and increasing Ca(2+) and PO(4)(3-) efflux from bone, while maintaining neutral phosphate balance through phosphaturic effects. FGF23 is a recently discovered hormone, predominately produced by osteoblasts/osteocytes, whose major functions are to inhibit renal tubular phosphate reabsorption and suppress circulating 1,25(OH)(2)D levels by decreasing Cyp27b1-mediated formation and stimulating Cyp24-mediated catabolism of 1,25(OH)(2)D. FGF23 participates in a new bone/kidney axis that protects the organism from excess vitamin D and coordinates renal PO(4)(3-) handling with bone mineralization/turnover. Abnormalities of FGF23 production underlie many inherited and acquired disorders of phosphate homeostasis. This review discusses the known and emerging functions of FGF23, its regulation in response to systemic and local signals, as well as the implications of FGF23 in different pathological and physiological contexts.

Rabbit model of primary hyperparathyroidism induced by high-phosphate diet

The objective of this study is to establish a new rabbit model of primary hyperparathyroidism (PHPT) induced by high-phosphate diet. One hundred twenty rabbits were divided into two groups of 60 each. The treatment group was fed a high-phosphate diet (Ca:P = 1:7) and the control group was given a normal animal diet (Ca:P = 1:0.7) for 1 to 6 mo. Serologic examinations, including parathyroid hormone (PTH), calcium and phosphorus levels, blood urea nitrogen, creatinine, and uric acid, and the histologic examination, including parathyroid, kidney, and bones, were performed at the end of each month for 6 mo. Compared with the control, serum PTH levels in the treatment groups were elevated at all six time points, whereas serum calcium levels were reduced, and serum phosphorus levels remain unchanged over the course of the first 3 mo. Serum calcium levels were increased, whereas serum phosphorus levels were reduced at 4, 5, and 6 mo. Parathyroid histopathological examination showed no change during the first month, whereas 60% of the animals exhibited mild hyperplasia starting at 2 mo, and 90% of the animals in the treatment group exhibited mild-to-moderate hyperplasia with gland enlargement starting from 3 mo through the end of the study. Histopathological examination of the kidneys showed no change at 1 mo, but focal parenchymal inflammation with calcium deposition was observed in the treatment groups at 2 to 6 mo. Fibrous tissue of the bone extended toward the cortex, and fibrosis was evident at the third month. The fibrous cells were found to be concentrated mainly on the inner and outer membranes of the bone cortex, and the amount of fibrous tissue increased as the disease progressed. We conclude that a new rabbit animal model of PHPT can be successfully created by the administration of a high-phosphate diet. This animal model can be used in various future studies related to PHPT.

Vitamin D metabolism in the kidney: regulation by phosphorus and fibroblast growth factor 23

1,25-Dihydroxyvitamin D (1,25(OH)(2)D) plays a critical role in calcium and phosphorus (Pi) metabolism, bone growth, and tissue differentiation. The synthesis of 1,25(OH)(2)D in the proximal renal tubule is the primary determinant of its circulating concentration and is mediated by the mitochondrial enzyme, 25-hydroxyvitamin D-1α-hydroxylase, CYP27B1). Enzyme activity in the kidney is tightly regulated by several factors, of which Pi and fibroblast growth factor 23 (FGF-23) are important determinants. In healthy human subjects and experimental animals, dietary Pi restriction and resultant hypophosphatemia stimulate renal 1,25(OH)(2)D production by transcriptional up regulation of the 1α-hydroxylase gene, and this effect is independent of serum concentrations of PTH. Dietary Pi intake and serum Pi concentration also are important determinants of the circulating concentration of FGF-23, itself a potent regulator of Pi and vitamin D metabolism. In several inherited human hypophosphatemic diseases, including X-linked hypophosphatemia, serum FGF-23 concentrations are increased, resulting in renal Pi wasting, hypophosphatemia, inappropriately low serum concentrations of 1,25(OH)(2)D, and growth retardation and rickets in children. Experimental studies demonstrate that direct administration of recombinant FGF-23 or its over-expression in mice induces a dose-dependent decrease in renal CYP27B1 mRNA expression, an increase in renal 24-hydroxylase mRNA expression, and a consequent decrease in serum 1,25(OH)(2)D concentrations. Studies in vitro and in vivo demonstrate that activation of MEK/ERK1/2 signaling in the kidney is necessary for the suppression of CYP27B1 gene expression by FGF-23. Thus, phosphorus and FGF-23 are important physiologic determinants of the renal metabolism of 1,25(OH)(2)D.

Dietary phosphate restriction normalizes biochemical and skeletal abnormalities in a murine model of tumoral calcinosis

Mutations in the GALNT3 gene cause tumoral calcinosis characterized by ectopic calcifications due to persistent hyperphosphatemia. We recently developed Galnt3 knockout mice in a mixed background, which had hyperphosphatemia with increased bone mineral density (BMD) and infertility in males. To test the effect of dietary phosphate intake on their phenotype, Galnt3 knockout mice were generated in the C57BL/6J strain and fed various phosphate diets: 0.1% (low), 0.3% (low normal), 0.6% (normal), and 1.65% (high). Sera were analyzed for calcium, phosphorus, alkaline phosphatase, creatinine, blood urine nitrogen, 1,25-dihydroxyvitamin D, osteocalcin, tartrate-resistant acid phosphatase 5b, and fibroblast growth factor 23 (Fgf23). Femurs were evaluated by dual-energy x-ray absorptiometry, dynamic histomorphometry, and/or microcomputed tomography. Galnt3 knockout mice in C57BL/6J had the same biochemical phenotype observed in our previous study: hyperphosphatemia, inappropriately normal 1,25-dihydroxyvitamin D level, decreased alkaline phosphatase activity, and low intact Fgf23 concentration but high Fgf23 fragments. Skeletal analyses of their femurs revealed significantly high BMD with increased cortical bone area and trabecular bone volume. On all four phosphate diets, Galnt3 knockout mice had consistently higher phosphorus levels and lower alkaline phosphatase and intact Fgf23 concentrations than littermate controls. The low-phosphate diet normalized serum phosphorus, alkaline phosphatase, and areal BMD but failed to correct male infertility in Galnt3 knockout mice. The high-phosphate diet did not increase serum phosphorus concentration in either mutant or control mice due to a compensatory increase in circulating intact Fgf23 levels. In conclusion, dietary phosphate restriction normalizes biochemical and skeletal phenotypes of Galnt3 knockout mice and, thus, can be an effective therapy for tumoral calcinosis.

Fibroblast growth factor 23 and the bone-vascular axis: lessons learned from animal studies

Calcification of arteries and cardiac valves is observed commonly in dialysis patients and represents a major determinant of the heightened cardiovascular risk observed during chronic kidney disease (CKD) progression. Recent advances from clinical and basic science studies suggest that vascular calcification should be considered a systemic disease in which pathologic processes occurring in the bone and kidney contribute to calcium deposition in the vasculature. Among the factors potentially involved in the vascular-bone axis dysregulation associated with CKD, there now is increasing interest in the role of the phosphaturic hormone fibroblast growth factor 23 (FGF-23). Increased FGF-23 plasma levels are observed with a decrease in kidney function and predict the risk of future cardiovascular mortality. However, clinical data are still unclear about whether a direct pathogenetic effect of FGF-23 on vascular/kidney/bone health exists. In the last few years, a series of basic science studies, performed using engineered mice, have contributed important pathophysiologic information about FGF-23 activities. This review summarizes findings from these studies and discusses the potential role of FGF-23 during the pathologic interplay between kidney, vessels, and bone in CKD.

Miscellaneous non-inflammatory musculoskeletal conditions. Hyperphosphatemic familial tumoral calcinosis (FGF23, GALNT3 and αKlotho)

Familial tumoral calcinosis (TC) is a rare disorder distinguished by the development of ectopic and vascular calcified masses that occur in settings of hyperphosphatemia (hFTC) and normophosphatemia (nFTC). Serum phosphorus concentrations are relatively tightly controlled by interconnected endocrine activity at the level of the intestine, kidney, and skeleton. Discovering the molecular causes for heritable forms of hFTC has shed new light on the regulation of serum phosphate balance. This review will focus upon the genetic basis and clinical approaches for hFTC, due to genes that are related to the phosphaturic hormone fibroblast growth factor-23 (FGF23). These include FGF23 itself, an FGF23-glycosylating enzyme (GALNT3), and the FGF23 co-receptor α-Klotho (αKL). Our understanding of the molecular basis of hFTC will, in the short term, aid in understanding normal phosphate balance, and in the future, provide potential insight into the design of novel therapeutic strategies for both rare and common disorders of phosphate metabolism.

FGF receptors control vitamin D and phosphate homeostasis by mediating renal FGF-23 signaling and regulating FGF-23 expression in bone

The functional interaction between fibroblast growth factor 23 (FGF-23) and Klotho in the control of vitamin D and phosphate homeostasis is manifested by the largely overlapping phenotypes of Fgf23- and Klotho-deficient mouse models. However, to date, targeted inactivation of FGF receptors (FGFRs) has not provided clear evidence for an analogous function of FGFRs in this process. Here, by means of pharmacologic inhibition of FGFRs, we demonstrate their involvement in renal FGF-23/Klotho signaling and elicit their role in the control of phosphate and vitamin D homeostasis. Specifically, FGFR loss of function counteracts renal FGF-23/Klotho signaling, leading to deregulation of Cyp27b1 and Cyp24a1 and the induction of hypervitaminosis D and hyperphosphatemia. In turn, this initiates a feedback response leading to high serum levels of FGF-23. Further, we show that FGFR inhibition blocks Fgf23 transcription in bone and that this is dominant over vitamin D-induced Fgf23 expression, ultimately impinging on systemic FGF-23 protein levels. Additionally, we identify Fgf23 as a specific target gene of FGF signaling in vitro. Thus, in line with Fgf23- and Klotho-deficient mouse models, our study illustrates the essential function of FGFRs in the regulation of vitamin D and phosphate levels. Further, we reveal FGFR signaling as a novel in vivo control mechanism for Fgf23 expression in bone, suggesting a dual function of FGFRs in the FGF-23/Klotho pathway leading to vitamin D and phosphate homeostasis.

Fibroblast growth factor 23 (FGF23) and alpha-klotho stimulate osteoblastic MC3T3.E1 cell proliferation and inhibit mineralization

Elevated serum levels of the phosphate-regulating hormone fibroblast growth factor 23 (FGF23) are found in patients with phosphate wasting diseases and chronic kidney disease-mineral and bone disorder (CKD-MBD). These diseases are associated with rickets and renal osteodystrophy, respectively. FGF23 is secreted from osteoblastic cells and signals through FGFRs, membrane coreceptor alpha-Klotho (Klotho), and, possibly, a circulating form of Klotho. Despite the absence of detectable Klotho on osteoblastic cells, studies have suggested that forced FGF23 expression in osteoblasts inhibited mineralization. Thus, we examined the effects of exogenously applied FGF23 on osteoblastic MC3T3.E1 cell proliferation and differentiation, with and without soluble Klotho. MC3T3.E1 cells were cultured in osteoblast differentiation medium, supplemented with FGF23 (0.1-1,000 ng/mL), Klotho (50 ng/mL), the combination FGF23 + Klotho, and FGF2 (100 ng/mL) as a control. Neither FGF23 nor Klotho exposure affected proliferation of day 4 growth phase cells or mineralization of day 14 cultures. In contrast, FGF23 + Klotho resulted in inhibition of mineralization and osteoblast activity markers at day 14, and a slight, reproducible induction of proliferation. Inhibition of FGFR1, but not FGFR2 or FGFR3, completely restored FGF23 + Klotho-induced inhibition of alkaline phosphatase (ALP) activity at day 7. ALP activity was partially restored by the MAPK inhibitor U0126 but not inhibitors p38 and P13K. Thus, soluble Klotho enables FGF23 signaling in MC3T3.E1 cells, likely through FGFR 1(IIIc). Elevated FGF23 actions, in part, appear to parallel FGF2 with lower potency. In addition to affecting bone via indirect phosphate wasting pathways, supraphysiological FGF23 and soluble Klotho may directly impact bone in diseases with elevated FGF23 levels.

Regulation of vitamin D metabolism

Fundamental to understanding the way in which perturbations in the vitamin D endocrine system can affect human health is an appreciation of the steps involved in the production of the well-recognized active hormonal form, 1,25-dihydroxyvitamin D(3). Thus this paper focuses first on the nature and regulation of the two enzymes responsible for the production of 1,25-dihydroxyvitamin D(3), the 25-hydroxylase in the liver and the 1α-hydroxylase in the kidney. The most important regulators of the 1α-hydroxylase in the kidney are 1,25-dihydroxyvitamin D(3) itself, parathyroid hormone and FGF23. The extent and importance of extra-renal, 1,25-dihydroxyvitamin D(3) synthesis is then considered. Finally the features of the 24R-hydroxylase, which produces 24R,25-dihydroxyvitamin D(3) in the kidney and is induced by and inactivated, 1,25-dihydroxyvitamin D(3)in target cells are described.

Bone proteins PHEX and DMP1 regulate fibroblastic growth factor Fgf23 expression in osteocytes through a common pathway involving FGF receptor (FGFR) signaling

Fibroblastic growth factor 23 (FGF23) is a circulating phosphaturic hormone. Inactivating mutations of the endopeptidase PHEX or the SIBLING protein DMP1 result in equivalent intrinsic bone mineralization defects and increased Fgf23 expression in osteocytes. The mechanisms whereby PHEX and DMP1 regulate Fgf23 expression are unknown. We examined the possibility that PHEX and DMP1 regulate Fgf23 through a common pathway by analyzing the phenotype of compound Phex and Dmp1 mutant mice (Hyp/Dmp1(-/-)). Compared to single-mutant littermates, compound-mutant Hyp/Dmp1(-/-) mice displayed nonadditive elevations of serum FGF23 (1912 ± 183, 1715 ± 178, and 1799 ± 181 pg/ml), hypophosphatemia (P(i): 6.0 ± 0.3, 5.8 ± 0.2, and 5.4 ± 0.1 mg/dl), and severity of rickets/osteomalacia (bone mineral density: -36, -36, and -30%). Microarray analysis of long bones identified gene expression profiles implicating common activation of the FGFR pathway in all the mutant groups. Furthermore, inhibiting FGFR signaling using SU5402 in Hyp- and Dmp1(-/-)-derived bone marrow stromal cells prevented the increase in Fgf23 mRNA expression (129- and 124-fold increase in Hyp and Dmp1(-/-) vs. 1.3-fold in Hyp+SU5402 and 2.5-fold in Dmp1(-/-)+SU5402, P<0.05). For all analyses, samples collected from nonmutant wild-type littermates served as controls. These findings indicate that PHEX and DMP1 control a common pathway regulating bone mineralization and FGF23 production, the latter involving activation of the FGFR signaling in osteocytes.

Impairment of spatial learning and memory in transgenic mice overexpressing human fibroblast growth factor-23

Fibroblast growth factor-23 (FGF-23) is a potent circulating phosphaturic factor associated with renal phosphate wasting. Effects of FGF-23 on skeleton, phosphate homeostasis, and cardiovascular system have been investigated; however, the effect of FGF-23 on the central nervous system (CNS) is unknown. To assess whether FGF-23 influences the function and structure of the CNS and whether the effect of FGF-23 on the CNS is mediated by FGF receptors directly or by hypophosphatemia indirectly, FGF-23 transgenic mice and their wild-type littermates were fed a normal diet or a high-phosphate diet containing a normal diet plus 1.25% phosphate in drinking water from weaning for 5weeks and the phenotypes of the CNS were compared between FGF-23 transgenic mice and their wild-type littermates on the same diet. At the end of this time period, transgenic animals on the normal diet showed impaired spatial learning and memory. Furthermore, these mice exhibited the impairment of long-term potentiation in hippocampal CA1 region, and the reduction of hippocampal adenosine-triphosphate content and of choline acetyltransferase-positive neurons in basal forebrain, possibly as pathogenetic factors contributing to the cognitive deficit. The central nervous phenotypes of transgenic mice were rescued following improved hypophosphatemia by the high-phosphate diet intake. This study demonstrates that FGF-23 overexpression can result in abnormalities in the CNS mediated by the secondary severe hypophosphatemia.

FGF23 in skeletal modeling and remodeling

Fibroblast growth factor 23 (FGF23), a hormone primarily produced in bone cells, targets the kidney to accelerate phosphate excretion into the urine and suppresses vitamin D synthesis, thereby inducing a negative phosphate balance. Excessive serum FGF23 due to hereditary disorders such as hypophosphatemic rickets leads to phosphate wasting and impaired bone mineralization. In contrast, deficiencies in FGF23 are associated with hyperphosphatemia, elevated 1,25(OH)(2)D(3), ectopic ossification in soft tissues, and defects in skeletal mineralization. Recent studies of human genetic disorders and genetically engineered mice, as well as the in vitro approaches, have clarified some mysteries in FGF23 regulation and its potential roles in bone modeling and remodeling, which are summarized in this review article.

Hormone-like fibroblast growth factors and metabolic regulation

The family of fibroblast growth factors (FGFs) consisting now of 22 members is generally considered to control a wide range of biological functions such as development, differentiation and survival. However, research during the past decade provided substantial evidence that a so called "hormone-like" subgroup of FGFs, comprised of FGF19, FGF21 and FGF23, is involved in the regulation of diverse metabolic pathways to control glucose, lipid, bile acid, phosphate and vitamin D metabolism. The unique properties of these FGFs include predominant production of the factors in selective tissues, their abundance in the blood due to the lack of extracellular heparin-mediated sequestration, and highly specific tissue-targeted action via engagement of their respective co-receptors. The important metabolic context of FGF19, FGF21, and FGF23 actions has revealed important novel roles for FGFs and provided significant means to explore an opportunity for therapeutic targeting of these factors and their corresponding pathways.

FGF23 or PTH: which comes first in CKD ?

In the past 40 years, disordered mineral metabolism has been among the most intensely studied areas of nephrology. A June 2010 PubMed search for 'secondary hyperparathyroidism and kidney disease' yielded 5866 references. Among these are papers documenting the development and application of numerous therapeutic agents-including calcitriol, vitamin D analogs, phosphate binders, and cinacalcet-that remain in widespread use in the day-to-day management of dialysis patients worldwide. However, almost 6000 papers later, fundamental pathophysiological concepts remain unclear, particularly regarding the early pathogenesis of disordered mineral metabolism.

Osteo-renal regulation of systemic phosphate metabolism

Impaired kidney function and subsequent skeletal responses play a critical role in disrupting phosphate balance in chronic kidney disease (CKD) patients with mineral and bone disorder (CKD-MBD). In patients with CKD-MBD, the inability of the kidney to maintain normal mineral ion balance affects bone remodeling to induce skeletal fracture and extraskeletal vascular calcification. In physiological conditions, bone-derived fibroblast growth factor 23 (FGF23) acts on the kidney to reduce serum phosphate and 1,25-dihydroxyvitamin D levels. In humans, increased bioactivity of FGF23 leads to increased urinary phosphate excretion, which induces hypophosphatemic diseases (e.g., rickets/osteomalacia). However, reduced FGF23 activity is associated with hyperphosphatemic diseases (e.g., tumoral calcinosis). In patients with CKD, high serum levels of FGF23 fail to reduce serum phosphate levels and lead to numerous complications, including vascular calcification, one of the important determinants of mortality of CKD-MBD patients. Of particular significance, molecular, biochemical and morphological changes in patients with CKD-MBD are mostly due to osteo-renal dysregulation of mineral ion metabolism. Furthermore, hyperphosphatemia can partly contribute to the development of secondary hyperparathyroidism in patients with CKD-MBD. Relatively new pharmacological agents including sevelamer hydrochloride, calcitriol analogs and cinacalcet hydrochloride are used either alone, or in combination, to minimize hyperphosphatemia and hyperparathyroidism associated complications to improve morbidity and mortality of CKD-MBD patients. This article will briefly summarize how osteo-renal miscommunication can induce phosphate toxicity, resulting in extensive tissue injuries.

Compound deletion of Fgfr3 and Fgfr4 partially rescues the Hyp mouse phenotype

Uncertainty exists regarding the physiologically relevant fibroblast growth factor (FGF) receptor (FGFR) for FGF23 in the kidney and the precise tubular segments that are targeted by FGF23. Current data suggest that FGF23 targets the FGFR1c-Klotho complex to coordinately regulate phosphate transport and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] production in the proximal tubule. In studies using the Hyp mouse model, which displays FGF23-mediated hypophosphatemia and aberrant vitamin D, deletion of Fgfr3 or Fgfr4 alone failed to correct the Hyp phenotype. To determine whether FGFR1 is sufficient to mediate the renal effects of FGF23, we deleted Fgfr3 and Fgfr4 in Hyp mice, leaving intact the FGFR1 pathway by transferring compound Fgfr3/Fgfr4-null mice on the Hyp background to create wild-type (WT), Hyp, Fgfr3(-/-)/Fgfr4(-/-), and Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice. We found that deletion of Fgfr3 and Fgfr4 in Fgfr3(-/-)/Fgfr4(-/-) and Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice induced an increase in 1,25(OH)(2)D. In Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice, it partially corrected the hypophosphatemia (P(i) = 9.4 ± 0.9, 6.1 ± 0.2, 9.1 ± 0.4, and 8.0 ± 0.5 mg/dl in WT, Hyp, Fgfr3(-/-)/Fgfr4(-/-), and Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice, respectively), increased Na-phosphate cotransporter Napi2a and Napi2c and Klotho mRNA expression in the kidney, and markedly increased serum FGF23 levels (107 ± 20, 3,680 ± 284, 167 ± 22, and 18,492 ± 1,547 pg/ml in WT, Hyp, Fgfr3(-/-)/Fgfr4(-/-), and Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice, respectively), consistent with a compensatory response to the induction of end-organ resistance. Fgfr1 expression was unchanged in Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice and was not sufficient to transduce the full effects of FGF23 in Hyp/Fgfr3(-/-)/Fgfr4(-/-) mice. These studies suggest that FGFR1, FGFR3, and FGFR4 act in concert to mediate FGF23 effects on the kidney and that loss of FGFR function leads to feedback stimulation of Fgf23 expression in bone.

The regulation of renal phosphate transport

Renal phosphate transport is mediated by the abundance and activity of the sodium-dependent phosphate transporters, Npt2a, Npt2c, and PiT-2, present within the apical brush border membrane of the proximal tubule. Recent studies have demonstrated differential expression and activity of these sodium-dependent phosphate transporters within the proximal tubule. In general, phosphate transport is regulated by a variety of physiological stimuli, including parathyroid hormone, glucocorticoids, vitamin D3, estrogen, and thyroid hormone. Phosphatonins are now recognized as major regulators of phosphate transport activity. Other factors that affect phosphate transport include dopamine, dietary phosphate, acid-base status, lipid composition, potassium deficiency, circadian rhythm, and hypertension. Studies have shown that the PDZ-containing sodium/hydrogen exchanger regulatory factor (NHERF) proteins, specifically NHERF-1 and NHERF-3, play a critical role in the physiological regulation of phosphate transport, particularly in response to dietary phosphate. In addition, recent studies have found that NHERF-1 is also important in both the parathyroid hormone- and dopamine-mediated inhibition of phosphate transport. This review will detail the various hormones and agents involved in the regulation of phosphate transport as well as provide a brief summary of the signaling pathways and cytoskeletal proteins active in the transport of phosphate in the renal proximal tubule.

Hyperphosphatemia in Renal Failure

The recent recognition that hyperphosphatemia is a strong predictor of survival on dialysis has rekindled interest in the regulation and control of serum phosphate. In incipient renal failure hyperphosphatemia is prevented by increased fractional renal phosphate excretion mediated via an increase in parathyroid hormone and the novel phosphaturic hormone FGF-23 (and possibly others). At a glomerular filtration rate of approximately 30 ml/min this compensatory mechanism fails and hyperphosphatemia ensues. Pre-dialytic serum phosphate concentrations of >6 mg/dl increase cardiac mortality presumably to a large extent, but not exclusively, via promoting vascular calcification. It has recently been recognized that vascular calcification is not only a passive precipitation process following transgression of the critical Ca-x-P product, but is an active process accompanied by expression of osteoblastic bone markers in the vessel wall. Because of the recent recognition of the relation between vascular calcification and serum phosphate as well as serum calcium, there is a need for novel calcium-free phosphate binders. Currently sevelamer and lanthanum carbonate have been introduced and trivalent iron preparations are under development.

Relationship between Fibroblast Growth Factor-23 and Mineral Metabolism in Chronic Kidney Disease

Fibroblast growth factor- (FGF-)23 is a recently discovered regulator of calcium-phosphate metabolism. FGF-23 appears to decrease in synthesis and accelerated degradation of 1,25(OH)₂D. Together with its cofactor Klotho, FGF-23 maintains serum phosphate levels within the normal range by increasing renal phosphate excretion. In chronic kidney disease (CKD), FGF-23 levels rise in parallel with the decline in renal function long before a significant increase in serum phosphate concentration occurs. Both Klotho and FGF-23, linked by a receptor mechanism, affect vitamin D synthesis and parathyroid hormone (PTH) secretion. Previous studies have shown a close association between reduced FGF-23 or Klotho activities and vascular calcification. The possible association of FGF-23 and left ventricular hypertrophy or vascular dysfunction has been proposed. Finally, prospective studies have shown that high serum FGF-23 concentrations predict more rapid disease progression in CKD patients who were not on dialysis and increased mortality in patients on maintenance hemodialysis. FGF-23 may therefore prove to be an important therapeutic target for the management of CKD.

The osteocyte--a novel endocrine regulator of body phosphate homeostasis

Although osteocytes are the most abundant cell type in bone, much of their biology remains enigmatic. They are known to transduce mechanical stress into signals that initiate local bone remodeling, and are targets for systemic and local endocrine signals that affect bone architecture and mineral homeostasis. However, recent data reveal that osteocytes themselves act as endocrine cells that synthesize fibroblast growth factor 23 (FGF23) and several other phosphatonins, shown to underpin the systemic regulation of phosphate homeostasis. This review will synthesize the emerging discoveries concerning the osteocytic endocrine role in phosphate homeostasis through the biology and pathophysiology of these phosphatonins. We also suggest future research paths that might resolve existing uncertainties, and look ahead at how greater understanding might improve the management of clinical disorders of phosphate homeostasis.

Direct evidence for a causative role of FGF23 in the abnormal renal phosphate handling and vitamin D metabolism in rats with early-stage chronic kidney disease

Circulating levels of fibroblast growth factor 23 (FGF23) are elevated in patients with early chronic kidney disease (CKD) and are postulated to cause low blood levels of 1,25-dihydroxyvitamin D, as well as normal phosphate levels. In order to provide more direct evidence for the pathophysiological role of FGF23 in the settings of mineral ion homeostasis typically seen in early CKD, we studied rats with progressive CKD treated with anti-FGF23 neutralizing antibody. Without antibody treatment, rats with CKD exhibited high circulating levels of FGF23 and parathyroid hormone, low 1,25-dihydroxyvitamin D, and normal serum phosphate levels, accompanied by increased fractional excretion of phosphate. Antibody treatment, however, lessened fractional excretion of phosphate, thus increasing serum phosphate levels, and normalized serum 1,25-dihydroxyvitamin D by increased 1α-OHase and decreased 24-OHase expressions in the kidney. These antibody-induced changes were followed by increased serum calcium levels, leading to decreased serum parathyroid hormone. Hence, our study shows that FGF23 normalizes serum phosphate and decreases 1,25-dihydroxyvitamin D levels in early-stage CKD, and suggests a pathological sequence of events for the development of secondary hyperparathyroidism triggered by increased FGF23, followed by a reduction of 1,25-dihydroxyvitamin D and calcium levels, thereby increasing parathyroid hormone secretion.

The bone and the kidney

Renal tubular diseases may present with osteopenia, osteoporosis or osteomalacia, as a result of significant derangements in body electrolytes. In case of insufficient synthesis of calcitriol, as in renal failure, the more complex picture of renal osteodystrophy may develop. Hypothetically, also disturbed renal production of BMP-7 and Klotho could cause bone disease. However, the acknowledgment that osteocytes are capable of producing FGF23, a phosphaturic hormone at the same time modulating renal synthesis of calcitriol, indicates that it is also bone that can influence renal function. Importantly, a feed-back mechanism exists between FGF23 and calcitriol synthesis, while Klotho, produced by the kidney, determines activity and selectivity of FGF23. Identification of human diseases linked to disturbed production of FGF23 and Klotho underlines the importance of this new bone-kidney axis. Kidney and bone communicate reciprocally to regulate the sophisticated machinery responsible for divalent ions homeostasis and for osseous or extraosseous mineralisation processes.

Vitamin D: metabolism

The biologically active metabolite of vitamin D, 1,25(OH)(2)D(3), affects mineral homeostasis and has numerous other diverse physiologic functions including effects on growth of cancer cells and protection against certain immune disorders. This article reviews the role of vitamin D hydroxylases in providing a tightly regulated supply of 1,25(OH)(2)D(3). The role of extrarenal 1alpha(OH)ase in placenta and macrophages is also discussed, as well as regulation of vitamin D hydroxylases in aging and chronic kidney disease. Understanding specific factors involved in regulating the hydroxylases may lead to the design of drugs that can selectively modulate the hydroxylases. The ability to alter levels of these enzymes would have therapeutic potential for the treatment of various diseases, including bone loss disorders and certain immune diseases.

Forging forward with 10 burning questions on FGF23 in kidney disease

The discovery of fibroblast growth factor 23 (FGF23) as the causal factor in the pathogenesis of rare forms of hypophosphatemic rickets is rapidly reshaping our understanding of disordered mineral metabolism in chronic kidney disease (CKD). Excessive production of FGF23 by osteocytes is an appropriate compensation to help maintain normal phosphorus metabolism in these patients. Beginning in early CKD, progressive increases in levels of FGF23 enhance phosphaturia on a per-nephron basis and inhibit calcitriol production, thereby contributing centrally to the predominant phosphorus phenotype of predialysis kidney disease: normal serum phosphate, increased fractional excretion of phosphate, and calcitriol deficiency. A proliferation of studies linking phosphorus and now FGF23 excess to adverse renal and cardiovascular outcomes in patients with CKD is setting the stage for novel clinical trials that could ultimately bring FGF23 testing into the clinic. Ten burning questions must be addressed to galvanize FGF23 research further in CKD.

Does the downregulation of the FGF23 signaling pathway in hyperplastic parathyroid glands contribute to refractory secondary hyperparathyroidism in CKD patients?

Compared to normal tissue, hyperplastic parathyroid glands of patients with chronic kidney disease on dialysis express lower levels of FGFR1 and Klotho proteins. Similar findings are reported in uremic rats with advanced chronic kidney disease. Moreover, in these animals, FGF23 administration fails to reduce PTH serum levels in vivo and to transmit downstream signals in parathyroid cells ex vivo. These findings may explain, at least partly, the concomitant elevation of both FGF23 and PTH serum levels in chronic kidney disease secondary hyperparathyroidism.

Phosphaturic action of fibroblast growth factor 23 in Npt2 null mice

In the present study, we evaluated the roles of type II and type III sodium-dependent P(i) cotransporters in fibroblast growth factor 23 (FGF23) activity by administering a vector encoding FGF23 with the R179Q mutation (FGF23M) to wild-type (WT) mice, Npt2a knockout (KO) mice, Npt2c KO mice, and Npt2a(-/-)Npt2c(-/-) mice (DKO mice). In Npt2a KO mice, FGF23M induced severe hypophosphatemia and markedly decreased the levels of Npt2c, type III Na-dependent P(i) transporter (PiT2) protein, and renal Na/P(i) transport activity. In contrast, in Npt2c KO mice, FGF23M decreased plasma phosphate levels comparable to those in FGF23M-injected WT mice. In DKO mice with severe hypophosphatemia, FGF23M administration did not induce an additional increase in urinary phosphate excretion. FGF23 administration significantly decreased intestinal Npt2b protein levels in WT mice but had no effect in Npt2a, Npt2c, and DKO mice, despite marked suppression of plasma 1,25(OH)(2)D(3) levels in all the mutant mice. The main findings were as follow: 1) FGF23-dependent phosphaturic activity in Npt2a KO mice is dependent on renal Npt2c and PiT-2 protein; 2) in DKO mice, renal P(i) reabsorption is not further decreased by FGF23M, but renal vitamin D synthesis is suppressed; and 3) downregulation of intestinal Npt2b may be mediated by a factor(s) other than 1,25(OH)(2)D(3). These findings suggest that Npt2a, Npt2c, and PiT-2 are necessary for the phosphaturic activity of FGF23. Thus complementary regulation of Npt2 family proteins may be involved in systemic P(i) homeostasis.

Clinical relevance of FGF-23 in chronic kidney disease

Fibroblast growth factor (FGF)-23 is a recently discovered regulator of calcium-phosphate metabolism. Whereas other known FGFs mainly act in a paracrine manner, FGF-23 has significant systemic effects. Together with its cofactor Klotho, FGF-23 enhances renal phosphate excretion in order to maintain serum phosphate levels within the normal range. In patients with chronic kidney disease (CKD), FGF-23 levels rise in parallel with declining renal function long before a significant increase in serum phosphate concentration can be detected. However, in cross-sectional studies increased FGF-23 levels in patients with CKD were found to be associated not only with therapy-resistant secondary hyperparathyroidism but were also independently related to myocardial hypertrophy and endothelial dysfunction after adjustment for traditional markers of calcium-phosphate metabolism. Finally, in prospective studies high serum FGF-23 concentrations predicted faster disease progression in CKD patients not on dialysis, and increased mortality in patients receiving maintenance hemodialysis. FGF-23 may therefore prove to be an important therapeutic target in the management of CKD.

Fibroblast growth factor 23 and its role in phosphate homeostasis

Phosphate homeostasis is complex and incompletely understood. The identification of different factors involved in the regulation of phosphate balance, also called phosphatonins, has largely changed our view on the regulation of phosphate homeostasis. The active role of bone has been demonstrated clearly. Currently, maintaining phosphate homeostasis is considered the result of a complex network of endocrine feedback loops between parathyroid gland, kidney, and bone. This review describes current knowledge on fibroblast growth factor 23, which is one of the best studied phosphatonins.

FGF23-parathyroid interaction: implications in chronic kidney disease

Over the past few years there have been considerable advances in our understanding of the physiological regulation of mineral homeostasis. One of the most important breakthroughs is the identification of fibroblastic growth factor 23 (FGF23) and its role as a key regulator of phosphate and 1,25-dihydroxyvitamin D metabolism. FGF23 exerts its biological functions by binding to its cognate receptor in the presence of Klotho as a cofactor. FGF23 principally acts on the kidney to induce urinary phosphate excretion and suppresses 1,25-dihydroxyvitamin D synthesis, thereby indirectly modulating parathyroid hormone secretion. FGF23 also acts directly on the parathyroid to decrease parathyroid hormone synthesis and secretion. In patients with chronic kidney disease, FGF23 levels increase progressively to compensate for phosphate retention, but these elevated FGF23 levels fail to suppress the secretion of parathyroid hormone, particularly in the setting of uremia. Recent data suggest that this parathyroid resistance to FGF23 may be caused by decreased expression of Klotho-FGFR1 complex in hyperplastic parathyroid glands. This review summarizes recent insights into the role of FGF23 in mineral homeostasis and discusses the involvement of its direct and indirect interaction with the parathyroid gland, particularly focusing on the pathophysiology of secondary hyperparathyroidism in chronic kidney disease.