The phosphatonin pathway: New insights in phosphate homeostasis

[FGF-23: just a phosphate metabolism regulator or something else?]

Endocrine communication between the bone, kidney, and the intestine is involved in maintaining appropriate serum phosphate concentrations, which is critical for the maintenance of skeletal integrity and is central to signal transduction and cell metabolism. In addition, hyperphosphatemia is statistically associated with vascular calcification, increased morbidity and mortality; because of this, phosphate regulation has become an important field of research. In this sense, fibroblast growth factor 23 (FGF-23) has been identified as a new hormone involved in phosphate regulation through feedback mechanisms involving parathyroid hormone and vitamin D. Given what the kidney is the primary site for regulation of phosphate levels and the principal target for FGF-23, its discovery has changed the understanding of disordered mineral metabolism in chronic kidney disease, especially now, since there is clinical evidence in favor of FGF-23 playing a central role for the pathogenesis of sHPT.

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.

A dairy product fermented by lactobacilli cancels the adverse effects of hypochlorhydria induced by a proton pump inhibitor on bone metabolism in growing rats

The purpose of the present study is to investigate the effects of hypochlorhydria induced by proton pump inhibitor (PPI) administration and intake of a dairy product fermented by lactobacilli (DFL) on bone metabolism in growing rats. Male rats, aged 3 weeks, were divided into two groups: a control group fed a casein-based diet and a group fed a DFL-based diet. Each group was fed its respective experimental diets for 9 d. At day 5 of the feeding period, each group was divided into two subgroups: one that received a saline injection and one that received a PPI injection. Rats were subcutaneously administered saline or PPI for 5 d. Faecal Ca excretion was determined from day 6 to day 9. At the end of the experiment, plasma and femurs were collected. Administration of PPI significantly decreased bone mineral density (shown by X-ray computerised tomography) and bone strength (shown by a three-point bending test) in the control group. Plasma osteocalcin, type I collagen C-telopeptides, 1,25-dihydroxyvitamin D and parathyroid hormone concentrations were elevated by PPI administration in the control group. Faecal Ca excretion and urinary P excretion in the control group were remarkably increased by PPI administration. On the other hand, these adverse effects of PPI were not observed in the DFL group. These results suggest that hypochlorhydria-induced bone loss may result from high bone turnover induced by secondary hyperparathyroidism due to Ca malabsorption and that DFL intake cancels these adverse effects probably via improving Ca malabsorption in growing rats.

Unique roles of phosphorus in endochondral bone formation and osteocyte maturation

The mechanisms by which inorganic phosphate (P(i)) homeostasis controls bone biology are poorly understood. Here we used Dmp1 null mice, a hypophosphatemic rickets/osteomalacia model, combined with a metatarsal organ culture and an application of neutralizing fibroblast growth factor 23 (FGF-23) antibodies to gain insight into the roles of P(i) in bone biology. We showed (1) that abnormal bone remodeling in Dmp1 null mice is due to reduced osteoclast number, which is secondary to a reduced ratio of RANKL/OPG expressed by osteoclast supporting cells and (2) that osteoblast extracellular matrix mineralization, growth plate maturation, secondary ossification center formation, and osteoblast differentiation are phosphate-dependent. Finally, a working hypothesis is proposed to explain how phosphate and DMP1 control osteocyte maturation.

Role of Klotho in aging, phosphate metabolism, and CKD

The klotho gene (KL) was identified first as a putative aging-suppressor gene that extended life span when overexpressed and accelerated aging-like phenotypes when disrupted in mice. It encodes a single-pass transmembrane protein and is expressed predominantly in kidney, where it functions as an obligate coreceptor for fibroblast growth factor 23 (FGF-23). FGF-23 is a bone-derived hormone that suppresses phosphate reabsorption and 1,25 dihydroxyvitamin D(3) (vitamin D) synthesis in the kidney. Klotho also is expressed in the parathyroid gland, where FGF-23 decreases parathyroid hormone expression and secretion, further suppressing vitamin D synthesis in kidney. Thus, FGF-23 functions as a phosphaturic hormone and a counter-regulatory hormone for vitamin D, thereby inducing negative phosphate balance. Mice lacking either FGF-23 or Klotho show hyperphosphatemia in addition to developing multiple aging-like phenotypes, which can be rescued by resolving phosphate retention. These findings have unveiled an unexpected link between aging and phosphate. In patients with chronic kidney disease (CKD), phosphate retention is seen universally and has been associated with increased mortality risk. Patients with CKD have high serum FGF-23 levels with decreased klotho expression in the kidney and parathyroid, rendering FGF-23 and Klotho as potential biomarkers and therapeutic targets for CKD. The Klotho protein not only serves as a coreceptor for FGF-23, but also functions as a humoral factor. Klotho's extracellular domain is released into blood and urine by ectodomain shedding and exerts various functions independently of FGF-23, including regulation of multiple ion channels and transporters. Decreased urinary Klotho protein level has been identified as one of the earliest biomarkers of CKD progression. This review focuses on the current understanding of Klotho protein function, with emphasis on its potential involvement in the pathophysiologic process of CKD.

Sevelamer carbonate for the treatment of hyperphosphatemia in patients with kidney failure (CKD III - V)

IMPORTANCE OF THE FIELD

Altered mineral metabolism in chronic kidney disease (CKD) is associated with increased morbidity, mortality, hospitalization, cost of care and reduced quality of life. Phosphorus control, one component of CKD metabolic derangements, is potentially related to impaired outcomes and has significant room for improvement.

AREAS COVERED IN THIS REVIEW

Historical, present and future aspects of treatment of hyperphosphatemia focusing on sevelamer hydrochloride and sevelamer carbonate.

WHAT THE READER WILL GAIN

Comprehensive insight into the background and controversies regarding phosphate binders.

TAKE HOME MESSAGE

While calcium-free phosphate binders with a sevelamer backbone may offer therapeutic advantages for CKD patients at risk, more studies comprising significant patient numbers are warranted to answer compelling clinical questions.

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.

Defective renal maintenance of the vitamin D endocrine system impairs vitamin D renoprotection: a downward spiral in kidney disease

In kidney disease, the progressive loss of renal capacity to produce calcitriol, the vitamin D hormone, is a key contributor to elevations in parathyroid hormone (PTH) and mineral and skeletal disorders predisposing to renal and cardiovascular damage, ectopic calcifications, and high mortality rates. Thus, the safe correction of calcitriol deficiency to suppress PTH has been the treatment of choice for decades. However, recent epidemiological and experimental data suggest that calcitriol replacement may improve outcomes through renal and cardioprotective actions unrelated to PTH suppression. Furthermore, a striking incidence of vitamin D deficiency occurs in kidney disease and associates more strongly than calcitriol deficiency with a higher risk for kidney disease progression and death. Despite the translational relevance of these findings, no prospective trials are currently available in support of the efficacy of vitamin D supplementation and/or calcitriol replacement to safely halt/moderate renal disease progression. This review updates the pathophysiology behind the vicious cycle by which kidney injury impairs the maintenance of normal vitamin D and calcitriol levels, which in turn impedes vitamin D/calcitriol renoprotective actions, a requirement for the design of prospective trials to improve current recommendations for vitamin D interventions at all stages of kidney disease.

Serum phosphate level in burn patients

INTRODUCTION

Despite plasma phosphate imbalance being rare, it is a relatively common finding in certain subsets of burn patients. It may occur due to the burn itself or as a result of the treatment. Severe hypophosphataemia (<1.0 mg dl(-1)) is associated with a significant morbidity and a fourfold increase in mortality. In this study, the relation between serum phosphate level and the total body surface area (TBSA) of the burn was compared.

METHODS

According to the percentage of TBSA of the burn, the patients (n=155) were divided into three groups: group A with 20-29% TBSA burns, group B with 30-39% and group C with more than 40% TBSA burns (62, 48 and 45 patients, respectively). Analysis of variance (ANOVA)-repeated measure was used to detect any statistically significant difference in the three post-burn time-points of 3rd, 6th and 9th days and the mean score of the serum phosphate level between the three groups.

RESULTS

The incidence of hypophosphataemia at 9th post-burn day in the three groups was 6.1%, 32.4% and 73.5%, respectively. There were significant differences (p<0.05) between mean serum phosphate levels of groups A and C, B and C and A and B as well. We found significant differences between the three post-burn follow-up time stages.

DISCUSSION

We have shown that hypophosphataemia, defined as mean serum phosphate levels below 3.0 mg dl(-1), was very common following burn, based on 75.6% of patients with more than 40% burn at the 3rd post-burn day. As the percentage of TBSA of burn increases, the incidence of hypophosphataemia significantly increases. We suggest that phosphate level be routinely measured after a major burn, especially in patients with a complicated course, so that appropriate replacement therapy may be started in a timely manner.

A potential link between phosphate and aging--lessons from Klotho-deficient mice

Phosphate homeostasis is maintained primarily by a bone-kidney endocrine axis. When phosphate is in excess, fibroblast growth factor-23 (FGF23) is secreted from bone and acts on kidney to promote phosphate excretion into urine. FGF23 also reduces serum vitamin D levels to suppress phosphate absorption from intestine. Thus, FGF23 functions as a hormone that induces negative phosphate balance. One critical feature of FGF23 is that it requires Klotho, a single-pass transmembrane protein expressed in renal tubules, as an obligate co-receptor to bind and activate cognate FGF receptors. Importantly, defects in either FGF23 or Klotho not only cause phosphate retention but also a premature-aging syndrome in mice, which can be rescued by resolving hyperphosphatemia. In addition, changes in extracellular and intracellular phosphate concentration affect glucose metabolism, insulin sensitivity, and oxidative stress in vivo and in vitro, which potentially affect aging processes. These findings suggest an unexpected link between inorganic phosphate and aging in mammals.

Cardiovascular risk in chronic kidney disease (CKD): the CKD-mineral bone disorder (CKD-MBD)

Recent advances in our understanding of the excess mortality of chronic kidney disease (CKD) due to cardiovascular complications, obtained through observational studies, demonstrate that vascular calcification and hyperphosphatemia are major cardiovascular risk factors. Mechanistic studies demonstrate that these two risk factors are related and that hyperphosphatemia directly stimulates vascular calcification. The role of hyperphosphatemia in stimulating vascular calcification in CKD is associated with a block to the skeletal reservoir function in phosphate balance due to excess bone resorption. This has led to the realization that renal osteodystrophy is linked to vascular calcification by disordered mineral homeostasis (phosphate) and that a multiorgan system fails in CKD, leading to cardiovascular mortality. In children with renal disease, the multiorgan system fails, just as in adults, but the outcomes have been less well studied, and perceptions of differences from adults are possibly incorrect. Vascular calcification and cardiovascular mortality are less prevalent among pediatric patients, but they are present. However, CKD-induced vascular disease causes stiffness of the arterial tree causing, in turn, systolic hypertension and left ventricular hypertrophy as early manifestations of the same pathology in the adult. Because of the role of the skeleton in these outcomes, renal osteodystrophy has been renamed as the CKD mineral bone disorder (CKD-MBD). This review, which focuses on the pediatric patient population, describes our current state of knowledge with regards to the pathophysiology of the CKD-MBD, including the new discoveries related to early stages of CKD. As a new necessity, cardiovascular function issues are incorporated into the CKD-MBD, and new advances in our knowledge of this critical component of the disorder will lead to improved outcomes in CKD.

Serological cardiovascular and mortality risk predictors in dialysis patients receiving sevelamer: a prospective study

BACKGROUND

Cardiovascular morbidity and mortality are massively increased in patients with chronic kidney disease (CKD). Sevelamer hydrochloride has been shown to attenuate cardiovascular calcifications in CKD and end-stage renal disease (ESRD) patients. We assessed how sevelamer hydrochloride influences the evolution of serum fetuin-A and other serological factors predicting cardiovascular outcome and survival in haemodialysis patients.

METHODS

Fifty-seven prevalent haemodialysis patients were included in a three-phase prospective interventional trial (A-B-A design; 8 weeks per phase). Sevelamer was only administered in the middle phase of the study. Within the other two phases, >or=90% of the patients received calcium acetate for phosphate binding. Detailed time courses of serum biochemistries were analysed in order to obtain detailed insight into the influence of sevelamer upon CKD-mineral and bone disorder (MBD) parameters as well as serum fetuin-A, fibroblast growth factor 23 (FGF23) and uraemic toxin levels [uric acid, indoxyl sulphate, hippuric acid, indole acetic acid, p-cresol and 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF)].

RESULTS

Forty-one patients finished the three prospective study phases (intention-to-treat analysis). After treatment with sevelamer, serum fetuin-A significantly increased (+21%), showing a delayed increase outlasting the third (non-sevelamer) study period. Total and low-density lipoprotein (LDL) cholesterol levels, as well as serum calcium, decreased significantly. The opposite occurred with albumin, C-reactive protein and intact parathyroid hormone (iPTH). FGF23, uric acid, indoxyl sulphate, hippuric acid, indole acetic acid, CMPF and serum phosphate did not change significantly during sevelamer treatment. In contrast, in parallel to sevelamer treatment, there was a significant rise in serum P-cresol.

CONCLUSIONS

In haemodialysis patients, treatment with sevelamer over 8 weeks was associated with a delayed yet long-lasting increase in serum fetuin-A levels. Increasing the serum level of fetuin-A, a negative acute-phase protein and systemic calcification inhibitor, might be one of the potential anti-calcification mechanisms of sevelamer. Since we failed to detect a decrease in systemic inflammation and uraemic toxins, the exact mechanisms by which sevelamer treatment affects serum fetuin-A remain to be determined.

Klotho and aging

The klotho gene encodes a single-pass transmembrane protein that forms a complex with multiple fibroblast growth factor (FGF) receptors and functions as an obligatory co-receptor for FGF23, a bone-derived hormone that induces negative phosphate balance. Defects in either Klotho or Fgf23 gene expression cause not only phosphate retention but also a premature-aging syndrome in mice, unveiling a potential link between phosphate metabolism and aging. In addition, the extracellular domain of Klotho protein is clipped on the cell surface and secreted into blood stream, potentially functioning as an endocrine factor. The secreted Klotho protein has a putative sialidase activity that modifies glycans on the cell surface, which may explain the ability of secreted Klotho protein to regulate activity of multiple ion channels and growth factors including insulin, IGF-1, and Wnt. Secreted Klotho protein also protects cells and tissues from oxidative stress through a mechanism yet to be identified. Thus, the transmembrane and secreted forms of Klotho protein have distinct functions, which may collectively affect aging processes in mammals.

Hypophosphatemia-mediated hypotension in transgenic mice overexpressing human FGF-23

Fibroblast growth factor-23 (FGF-23) is a potent circulating phosphaturic factor associated with renal phosphate wasting. The effects of FGF-23 on skeletal and phosphate homeostasis have been investigated widely; however, the effect of FGF-23 on the cardiovascular system (CVS) is unknown. To assess whether FGF-23 influences the function and structure of the CVS and whether the effect of FGF-23 on the CVS is mediated by FGF receptors directly or indirectly by hypophosphatemia, FGF-23 transgenic mice and their wild-type littermates were fed a normal diet or a high-phosphate diet comprising a normal diet plus 1.25% phosphate in drinking water from weaning for 5 wk, and the phenotypes of the CVS 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 developed hypotension. The left ventricle was appropriately hypertrophic, and plasma catecholamine and renin-angiotensin system components were upregulated, indicating compensatory mechanisms in response to the hypotension. Transgenic mice also exhibited an impaired vascular reactivity and a downregulation of vasoconstrictor receptor gene expression, possibly as pathogenetic factors contributing to the hypotension. The high-phosphate diet improved the hypophosphatemia, resulting in a rescue of the cardiovascular phenotype. This study demonstrates that FGF-23 overexpression can result in abnormalities in the CVS and that the effect of FGF-23 overexpression on the CVS is mediated by the secondary severe hypophosphatemia.

RT-PCR analysis for FGF23 using paraffin sections in the diagnosis of phosphaturic mesenchymal tumors with and without known tumor induced osteomalacia

Phosphaturic mesenchymal tumors of the mixed connective tissue type (PMTMCT) are extremely rare, histologically distinctive neoplasms, which cause tumor-induced osteomalacia (TIO) in most cases through the elaboration of a phosphaturic hormone, fibroblast growth factor-23 (FGF23). Rarely, identical tumors without known TIO may be observed. We studied a large group of PMTMCT for expression of FGF23, using a novel reverse transcription polymerase chain reaction (RT-PCR) assay for FGF23 in formalin-fixed, paraffin-embedded tissues. Twenty-nine PMTMCT (17 with and 12 without TIO) and 23 non-PMTMCT (16 various mesenchymal tumors, including 5 chondromyxoid fibroma, 8 chondroblastoma, 1 hemangiopericytoma, 1 aneurysmal bone cyst, and 1 high grade sarcoma; 5 carcinomas; and 2 non-neoplastic tissues) were retrieved. Total RNA was extracted from formalin-fixed, paraffin-embedded sections for RT-PCR analysis. FGF23 was amplified using 3 sets of primers that spanned the intron/exon boundaries to amplify the 3 exons of FGF23 gene (140, 125, and 175 bp). The housekeeping gene phosphoglycerokinase (189 bp) was coamplified to check the RNA quality. Sixteen of 17 (94%) PMTMCT with TIO were FGF23-positive. Nine of 12 (75%) PMTMCT without TIO were FGF23-positive. Two chondromyxoid fibroma and 1 aneurysmal bone cyst were positive; all other non-PMTMCT were negative. We conclude that RT-PCR for FGF23 is a sensitive and specific means of confirming the diagnosis of PMTMCT both in patients with and without TIO. FGF23 gene expression was present in more than 90% of PMTMCT with known TIO, confirming the role of FGF23 in this syndrome. Rare FGF23-negative PMTMCT with known TIO likely express other phosphaturic hormones (eg, frizzled-related protein 4). Our finding of expression of FGF23 in 75% of histologically identical tumors without known TIO confirms the reproducibility of the diagnosis of PMTMCT, even in the absence of known phosphaturia.

A familial disorder with low bone density and renal phosphate wasting

Hereditary forms of renal phosphate wasting have been studied thoroughly in the past years. X-linked Hypophosphatemic rickets (XLH), autosomal dominant hypophosphatemic rickets/osteomalacia (ADHR) and autosomal recessive hypophosphatemic rickets (ARHR) are known genetic disorders in which a disturbance of phosphatonins is a causative factor in the pathogenesis. We describe a comparable but yet undescribed disorder in a family in which a 53 year old man presented with a spontaneous fracture after suffering for years with severe fatigue and musculoskeletal pains. A low serum phosphate was discovered. The two subsequent generations of this family developed the same symptoms but at an earlier age. Almost all family members have been investigated and the majority appears to have low bone density and/or renal phosphate wasting and/or low serum phosphate. Remarkably no rickets was found. No elevation of FGF23 or mutations in the gene encoding FGF23 were found. We believe this is a new familial disorder of bone metabolism and phosphate homeostasis in which a disturbance of bone modulators may play a central role.

Familial tumoral calcinosis and the role of O-glycosylation in the maintenance of phosphate homeostasis

Familial tumoral calcinosis refers to a group of disorders inherited in an autosomal recessive fashion. Hyperphosphatemic tumoral calcinosis is characterized by increased re-absorption of phosphate through the renal proximal tubule, resulting in elevated phosphate concentration and deposition of calcified deposits in cutaneous and subcutaneous tissues, as well as, occasionally, in visceral organs. The disease was found to result from mutations in at least 3 genes: GALNT3, encoding a glycosyltransferase termed ppGalNacT3, FGF23 encoding a potent phosphaturic protein, and KL encoding Klotho. Recent data showed that ppGalNacT3 mediates O-glycosylation of FGF23, thereby allowing for its secretion and possibly protecting it from proteolysis-mediated inactivation. Klotho was found to serve as a co-receptor for FGF23, thereby integrating the genetic data into a single physiological system. The elucidation of the molecular basis of HFTC shed new light upon the mechanisms regulating phosphate homeostasis, suggesting innovative therapeutic strategies for the management of hyperphosphatemia in common acquired conditions such as chronic renal failure.

FGF23 decreases renal NaPi-2a and NaPi-2c expression and induces hypophosphatemia in vivo predominantly via FGF receptor 1

Fibroblast growth factor-23 (FGF23) is a phosphaturic hormone that contributes to several hypophosphatemic disorders by reducing the expression of the type II sodium-phosphate cotransporters (NaPi-2a and NaPi-2c) in the kidney proximal tubule and by reducing serum 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] levels. The FGF receptor(s) mediating the hypophosphatemic action of FGF23 in vivo have remained elusive. In this study, we show that proximal tubules express FGFR1, -3, and -4 but not FGFR2 mRNA. To determine which of these three FGFRs mediates FGF23's hypophosphatemic actions, we characterized phosphate homeostasis in FGFR3(-/-) and FGFR4(-/-) null mice, and in conditional FGFR1(-/-) mice, with targeted deletion of FGFR1 expression in the metanephric mesenchyme. Basal serum phosphorus levels and renal cortical brush-border membrane (BBM) NaPi-2a and NaPi-2c expression were comparable between FGFR1(-/-), FGFR3(-/-), and FGFR4(-/-) mice and their wild-type counterparts. Administration of FGF23 to FGFR3(-/-) mice induced hypophosphatemia in these mice (8.0 +/- 0.4 vs. 5.4 +/- 0.3 mg/dl; p < or = 0.001) and a decrease in renal BBM NaPi-2a and NaPi-2c protein expression. Similarly, in FGFR4(-/-) mice, administration of FGF23 caused a small but significant decrease in serum phosphorus levels (8.7 +/- 0.3 vs. 7.6 +/- 0.4 mg/dl; p < or = 0.001) and in renal BBM NaPi-2a and NaPi-2c protein abundance. In contrast, injection of FGF23 into FGFR1(-/-) mice had no effects on serum phosphorus levels (5.6 +/- 0.3 vs. 5.2 +/- 0.5 mg/dl) or BBM NaPi-2a and NaPi-2c expression. These data show that FGFR1 is the predominant receptor for the hypophosphatemic action of FGF23 in vivo, with FGFR4 likely playing a minor role.

A new role for vitamin D receptor activation in chronic kidney disease

Vitamin D has proven to be much more than a simple "calcium hormone." The fact that the vitamin D receptor has been found in cells not related to mineral metabolism supports that statement. The interest of nephrologists in vitamin D and its effects beyond mineral metabolism has increased over the last few years, evidencing the importance of this so-called "sunshine hormone." In the present review, we highlight the most recent developments in the traditional use of vitamin D in chronic kidney disease (CKD) patients, namely, the control of secondary hyperparathyroidism (sHPT). Furthermore, we also explore the data available regarding the new possible therapeutic uses of vitamin D for the treatment of other complications present in CKD patients, such as vascular calcification, left ventricular hypertrophy, or proteinuria. Finally, some still scarce but very promising data regarding a possible role of vitamin D in kidney transplant patients also are reviewed. The available data point to a potential beneficial effect of vitamin D in CKD patients beyond the control of mineral metabolism.

Mechanisms of renal phosphate loss in liver resection-associated hypophosphatemia

OBJECTIVE

To determine precisely the role of parathyroid hormone (PTH) and of phosphatonins in the genesis of posthepatectomy hypophosphatemia.

BACKGROUND

Posthepatectomy hypophosphatemia has recently been related to increased renal fractional excretion of phosphate (FE P). To address the cause of hypophosphatemia, we measured serum concentrations of PTH, various phosphatonins, and the number of removed hepatic segment in patients with this disorder.

METHODS

Serum phosphate (PO4), ionized calcium (Ca++), HCO3-, pH and FE P, intact PTH (I-PTH), carboxyl-terminal fibroblast growth factor 23 (C-FGF-23) and intact fibroblast growth factor 23 (I-FGF-23), FGF-7, and secreted frizzled related-protein-4 (sFRP-4) were measured before and on postoperative (po) days 1, 2, 3, 5, and 7, in 18 patients undergoing liver resection. The number of removed hepatic segments was also assessed.

RESULTS

Serum PO4 concentrations decreased within 24 hours, were lowest (0.66 +/- 0.03 mmol/L; P < 0.001) at 48 hours, and returned to normal within 5 days of the procedure. FE P peaked at 25.07% +/- 2.26% on po day 1 (P < 0.05). Decreased ionized calcium concentrations (1.10 +/- 0.01 mmol/L; P < 0.01) were observed on po day 1 and were negatively correlated with increased I-PTH concentrations (8.8 +/- 0.9 pmol/L; P < 0.01; correlation: r = -0.062, P = 0.016). FE P was positively related to I-PTH levels on po day 1 (r = 0.52, P = 0.047) and negatively related to PO4 concentrations (r = -0.56, P = 0.024). Severe hypophosphatemia and increased urinary phosphate excretion persisted for 72 hours even when I-PTH concentrations had returned to normal. I-FGF-23 decreased to its nadir of 7.8 +/- 6.9 pg/mL (P < 0.001) on po day 3 and was correlated with PO4 levels on po days 0, 3, 5, and 7 (P < 0.001). C-FGF-23, FGF-7 and sFRP-4 levels could not be related to either PO4 concentrations or FE P.

CONCLUSION

Posthepatectomy hypophosphatemia is associated with increased FE P unrelated to I-FGF-23 or C-FGF-23, FGF-7, or sFRP-4. I-PTH contributes to excessive FE P partially on po day 1 but not thereafter. Other yet defined factors should explain post hepatectomy hypophosphatemia.

A novel recessive mutation of fibroblast growth factor-23 in tumoral calcinosis

BACKGROUND

Tumoral calcinosis is a rare disease characterized by hyperphosphatemia due to hypophosphaturia and by ectopic calcifications. Phosphatonins are important hormones that regulate phosphorus homeostasis. Tumoral calcinosis is a rare congenital disorder in which the differential diagnosis from other syndromes associated with extraskeletal calcifications may be difficult. Mutations in the UDP-N-acetyl-alpha-D-galactosamine: polypeptide N-acetylgalactosaminyltransferase-3 (GALNT3) and fibroblast growth factor-23 (FGF23) genes have been described. Mutational analysis is important for the early recognition of the disorder, for prevention of its complications, and for family screening strategies. We examined two unrelated white patients affected by tumoral calcinosis.

METHODS

The first patient was a woman with a history of an ectopic calcification in the left shoulder. The second patient was a man with a history of an ectopic calcification in the right buttock. Routine biochemistry and FGF-23 assays were performed on serum samples. Genomic DNA was extracted from peripheral blood. The FGF23 and GALNT3 genes were analyzed by direct sequencing.

RESULTS

A new homozygous H41Q codon 41, C-->A transversion at position 123 (c.123C>A) in exon 1 of the FGF23 gene was evidenced in both patients. No mutation of the GALNT3 gene was detected in these patients. As determined by an ELISA assay, intact FGF-23 circulating protein was low in both patients.

CONCLUSIONS

This is the fourth mutation of the FGF23 gene described in subjects with tumoral calcinosis.

Novel mechanisms in the regulation of phosphorus homeostasis

Phosphorus plays a critical role in diverse biological processes, and, therefore, the regulation of phosphorus balance and homeostasis are critical to the well being of the organism. Changes in environmental, dietary, and serum concentrations of inorganic phosphorus are detected by sensors that elicit changes in cellular function and alter the efficiency by which phosphorus is conserved. Short-term, post-cibal responses that occur independently of hormones previously thought to be important in phosphorus homeostasis may play a larger role than previously appreciated in the regulation of phosphorus homeostasis. Several hormones and regulatory factors such as the vitamin D endocrine system, parathyroid hormone, and the phosphatonins (FGF-23, sFRP-4, MEPE) among others, may play a role only in the long-term regulation of phosphorus homeostasis. In this review, we discuss how organisms sense changes in phosphate concentrations and how changes in hormonal factors result in the conservation or excretion of phosphorus.

Fibroblast growth factor 23: a possible cause of left ventricular hypertrophy in hemodialysis patients

BACKGROUND

Left ventricular hypertrophy (LVH) is a common cardiovascular disorder and an independent risk factor for cardiovascular death in dialysis patients. Hyperphosphatemia is associated with LVH. Previous studies have shown that fibroblast growth factor 23 (FGF23), which has an important role in phosphate metabolism, is elevated in chronic hemodialysis patients.

OBJECTIVES

The aim of this study is to determine the association of FGF23 and LVH and the prognostic value of FGF23 in chronic hemodialysis patients.

MATERIALS AND METHODS

One hundred twenty-four end-stage renal disease patients were evaluated for LVH by echocardiography. Serum FGF23 levels were measured using a commercial enzyme-linked immunosorbent assay kit.

RESULTS

Patients with LVH were more likely to have poor urea clearance (Kt/V), higher systolic blood pressure, and comorbidity of diabetes mellitus and coronary artery disease. LVH was also associated with higher levels of FGF23. Multivariate analysis indicated that FGF23 level, systolic blood pressure, and comorbidity of diabetes mellitus and coronary artery disease remained correlated with LVH. This suggested that serum FGF23 level is independently associated with LVH in our hemodialysis patients. Cox analysis indicated no significant difference in risk of death for patients with elevated levels of FGF23.

CONCLUSION

LVH has a high prevalence in hemodialysis patients, and FGF23 is independently associated with LVH but is not a predictor for short-term prognosis (2-year follow-up).

Regulation of vitamin D homeostasis: implications for the immune system

Vitamin D homeostasis in the immune system is the focus of this review. The production of both the activating (25- and 1alpha-hydroxylase) and the metabolizing (24-hydroxylase) enzymes by cells of the immune system itself, indicates that 1,25(OH)(2)D(3) can be produced locally in immune reaction sites. Moreover, the strict regulation of these enzymes by immune signals is highly suggestive for an autocrine/paracrine role in the immune system, and opens new treatment possibilities.

The Klotho gene family as a regulator of endocrine fibroblast growth factors

The Klotho gene encodes a single-pass transmembrane protein and functions as an aging-suppressor gene, which extends lifespan when overexpressed and accelerates the development of aging-like phenotypes when disrupted in mice. Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that regulates phosphate and vitamin D homeostasis. It has been shown that Klotho-deficient mice and Fgf23 knockout mice exhibit identical phenotypes. This observation led to the identification of Klotho as a cofactor essential for interactions between FGF23 and FGF receptors. In addition to the Klotho-FGF23 axis, recent studies has shown that betaKlotho, a Klotho family protein, also functions as a cofactor required for FGF19 and FGF21 signaling and determines the tissue-specific metabolic activities of FGF19 and FGF21. This review summarizes recent progress in understanding of Klotho and betaKlotho function in the regulation of tissue-specific metabolic activity of the endocrine fibroblast growth factors (FGF19, FGF21, and FGF23).

Kidney and phosphate metabolism

The serum phosphorus level is maintained through a complex interplay between intestinal absorption, exchange intracellular and bone storage pools, and renal tubular reabsorption. The kidney plays a major role in regulation of phosphorus homeostasis by renal tubular reabsorption. Type IIa and type IIc Na⁺/P_i transporters are important renal Na⁺-dependent inorganic phosphate (P_i) transporters, which are expressed in the brush border membrane of proximal tubular cells. Both are regulated by dietary P_i intake, vitamin D, fibroblast growth factor 23 (FGF23) and parathyroid hormone. The expression of type IIa Na⁺/P_i transporter result from hypophosphatemia quickly. However, type IIc appears to act more slowly. Physiological and pathophysiological alteration in renal P_i reabsorption are related to altered brush border membrane expression/content of the type II Na⁺/P_i cotransporter. Many studies of genetic and acquired renal phosphate wasting disorders have led to the identification of novel genes. Two novel P_i regulating genes, PHEX and FGF23, play a role in the pathophysiology of genetic and acquired renal phosphate wasting disorders and studies are underway to define their mechanism on renal P_i regulation. In recent studies, sodium-hydrogen exchanger regulatory factor 1 (NHERF1) is reported as another new regulator for P_i reabsorption mechanism.

Phosphate: known and potential roles during development and regeneration of teeth and supporting structures

Inorganic phosphate (P(i)) is abundant in cells and tissues as an important component of nucleic acids and phospholipids, a source of high-energy bonds in nucleoside triphosphates, a substrate for kinases and phosphatases, and a regulator of intracellular signaling. The majority of the body's P(i) exists in the mineralized matrix of bones and teeth. Systemic P(i) metabolism is regulated by a cast of hormones, phosphatonins, and other factors via the bone-kidney-intestine axis. Mineralization in bones and teeth is in turn affected by homeostasis of P(i) and inorganic pyrophosphate (PPi), with further regulation of the P(i)/PP(i) ratio by cellular enzymes and transporters. Much has been learned by analyzing the molecular basis for changes in mineralized tissue development in mutant and knock-out mice with altered P(i) metabolism. This review focuses on factors regulating systemic and local P(i) homeostasis and their known and putative effects on the hard tissues of the oral cavity. By understanding the role of P(i) metabolism in the development and maintenance of the oral mineralized tissues, it will be possible to develop improved regenerative approaches.

Disorders involving calcium, phosphorus, and magnesium

Disorders of mineral metabolism are common in both the office and hospital setting. The diagnosis can be simplified by remembering the target organs involved--intestine, kidney, and bone--and by assessing the presence of kidney disease, levels of parathyroid hormone, and vitamin D status. Although the list of possible causes for these derangements is long, most patients who have hypercalcemia have hyperparathyroidism or malignancy; those who have hypocalcemia, hypophosphatemia, and hypomagnesemia have reduced gastrointestinal absorption, and those who have hyperphosphatemia and hypermagnesemia have increased intake in the setting of kidney disease.

Phosphatonins: new hormones that control phosphorus homeostasis

Phosphorus (Pi) plays an important role in nucleic acid synthesis, energy metabolism, bone mineralization and cell signaling, and is also present in sugars, phospholipids and phosphoproteins. Phosphate homeostasis is controlled by processes that regulate the intestinal absorption and renal excretion of Pi, and bone turnover. These processes are influenced by peptide and sterol hormones, such as parathyroid hormone and 1α,25-dihydroxyvitamin D (1α,25[OH]₂D₃). Recently, a new class of phosphate-regulating peptides has been discovered: phosphatonins. These factors, such as FGF-23, secreted frizzled-related protein-4, matrix extracellular phosphoglycoprotein and FGF-7, are circulating peptides with potent phosphaturic activity. These peptides inhibit Na/Pi transporters in renal epithelial cells and, therefore, increase renal Pi excretion. In addition, FGF-23 and secreted frizzled-related protein-4 inhibit 25-hydroxyvitamin D 1α-hydroxylase activity, reducing 1α,25(OH)2D3 synthesis and, thus, intestinal Pi absorption. Phosphatonins have been associated with hypophosphatemic diseases, such as tumor-induced osteomalacia, X-linked hypophosphatemic rickets, autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets and hyperphosphatemic disease (e.g., tumoral calcinosis). The aim of this article is to review the role of phosphatonins in Pi metabolism in normal and pathologic conditions and also to investigate the correlations among the various phosphatonins.

Estrogen downregulates the proximal tubule type IIa sodium phosphate cotransporter causing phosphate wasting and hypophosphatemia

Estrogen treatment causes significant hypophosphatemia in patients. To determine the mechanisms responsible for this effect, we injected ovariectomized rats with either 17beta-estradiol or vehicle for three days. Significant renal phosphate wasting and hypophosphatemia occurred in estrogen-treated rats despite a decrease in their food intake. The mRNA and protein levels of the renal proximal tubule sodium phosphate cotransporter (NaPi-IIa) were significantly decreased in estradiol-treated ad-libitum or pair-fed groups. Estrogen did not affect NaPi-III or NaPi-IIc expression. In ovariectomized and parathyroidectomized rats, 17beta-estradiol caused a significant decrease in NaPi-IIa mRNA and protein expression compared to vehicle. Estrogen receptor alpha isoform blocker significantly blunted the anorexic effect of 17beta-estradiol but did not affect the downregulation of NaPi-IIa. Our studies show that renal phosphate wasting and hypophosphatemia induced by estrogen are secondary to downregulation of NaPi-IIa in the proximal tubule. These effects are independent of food intake or parathyroid hormone levels and likely not mediated through the activation of estrogen receptor alpha subtype.

Serum FGF21 levels are increased in obesity and are independently associated with the metabolic syndrome in humans

OBJECTIVE

Fibroblast growth factor 21 (FGF21) is a metabolic regulator with multiple beneficial effects on glucose homeostasis and insulin sensitivity in animal models. This study aimed to investigate the relationship between its serum levels and various cardiometabolic parameters in humans.

RESEARCH DESIGN AND METHODS

A newly developed immunoassay was used to measure serum FGF21 levels in 232 Chinese subjects recruited from our previous cross-sectional studies. The mRNA expression levels of FGF21 in the liver and adipose tissues were quantified by real-time PCR.

RESULTS

Serum FGF21 levels in overweight/obese subjects were significantly higher than in lean individuals. Serum FGF21 correlated positively with adiposity, fasting insulin, and triglycerides but negatively with HDL cholesterol, after adjusting for age and BMI. Logistic regression analysis demonstrated an independent association between serum FGF21 and the metabolic syndrome. Furthermore, the increased risk of the metabolic syndrome associated with high serum FGF21 was over and above the effects of individual components of the metabolic syndrome. Our in vitro study detected a differentiation-dependent expression of FGF21 in 3T3-L1 adipocytes and human adipocytes. In db/db obese mice, FGF21 mRNA expression was markedly increased in both the liver and adipose tissue compared with that in their lean littermates. Furthermore, FGF21 expression in subcutaneous fat correlated well with its circulating concentrations in humans.

CONCLUSIONS

FGF21 is a novel adipokine associated with obesity-related metabolic complications in humans. The paradoxical increase of serum FGF21 in obese individuals, which may be explained by a compensatory response or resistance to FGF21, warrants further investigation.

X-linked hypophosphatemic rickets associated with respiratory failure

We report a 5-year-old girl who presented to our emergency room with respiratory arrest and limb deformities and was subsequently diagnosed with X-linked hypophosphatemic rickets. On normalization of the serum phosphorus concentration, her respiratory distress resolved, illustrating that untreated X-linked hypophosphatemic rickets can lead to life-threatening respiratory distress.

Postprandial mineral metabolism and secondary hyperparathyroidism in early CKD

Normophosphatemia and normocalcemia are maintained in chronic kidney disease (CKD) by increased levels of fibroblast growth factor-23 (FGF-23) and parathyroid hormone (PTH), but the stimuli for secretion of these hormones in early CKD are incompletely understood. Most human physiologic studies have focused on random or fasting measurements of phosphorus, calcium, FGF-23, and PTH, but in this study, the hypothesis was that measurements in the postprandial state may reveal intermittent stimuli that lead to increased FGF-23 and PTH levels. The 4-h postprandial response in 13 patients with CKD and fasting normophosphatemia and normocalcemia (mean GFR 41 +/- 8 ml/min per m(2)) was compared with 21 healthy volunteers. Compared with healthy subjects, fasting patients with CKD had significantly higher levels of FGF-23 and fractional excretion of phosphorus; lower fractional excretion of calcium; and no difference in serum calcium, phosphorus, and PTH levels. After standardized meals, urinary phosphorus excretion in both groups increased despite unchanged serum phosphorus and FGF-23 levels. Postprandial urinary calcium excretion also increased in both groups, and this was accompanied by significantly reduced serum calcium and increased PTH levels in patients with CKD only; therefore, FGF-23 does not seem to be an acute postprandial regulator of phosphaturia in CKD or in health, but inappropriate postprandial calciuria with episodic, relative hypocalcemia may represent a previously unreported mechanism of secondary hyperparathyroidism in CKD.

Tissue-specific expression of betaKlotho and fibroblast growth factor (FGF) receptor isoforms determines metabolic activity of FGF19 and FGF21

The fibroblast growth factor (FGF) 19 subfamily of ligands, FGF19, FGF21, and FGF23, function as hormones that regulate bile acid, fatty acid, glucose, and phosphate metabolism in target organs through activating FGF receptors (FGFR1-4). We demonstrated that Klotho and betaKlotho, homologous single-pass transmembrane proteins that bind to FGFRs, are required for metabolic activity of FGF23 and FGF21, respectively. Here we show that, like FGF21, FGF19 also requires betaKlotho. Both FGF19 and FGF21 can signal through FGFR1-3 bound by betaKlotho and increase glucose uptake in adipocytes expressing FGFR1. Additionally, both FGF19 and FGF21 bind to the betaKlotho-FGFR4 complex; however, only FGF19 signals efficiently through FGFR4. Accordingly, FGF19, but not FGF21, activates FGF signaling in hepatocytes that primarily express FGFR4 and reduces transcription of CYP7A1 that encodes the rate-limiting enzyme for bile acid synthesis. We conclude that the expression of betaKlotho, in combination with particular FGFR isoforms, determines the tissue-specific metabolic activities of FGF19 and FGF21.

Tumoral calcinosis: new insights for the rheumatologist into a familial crystal deposition disease

A growing body of evidence points to extraosseous calcification (calcification occurring in nonosseous tissues) as a major cause of morbidity and mortality in humans. The term familial tumoral calcinosis encompasses a number of rare recessive diseases, often associated with increased reabsorption of phosphate through the renal proximal tubule, which manifests with periarticular or acral calcium deposition. Recently, the molecular pathogenesis of this group of disorders has been elucidated, leading to the identification of several proteins playing pivotal roles in the regulation of extraosseous calcification. This report reviews these advances as well as the potential implications of these discoveries for the management of acquired conditions associated with abnormal calcification.

BetaKlotho is required for metabolic activity of fibroblast growth factor 21

Fibroblast growth factor 21 (FGF21) is a liver-derived endocrine factor that stimulates glucose uptake in adipocytes. Here, we show that FGF21 activity depends on betaKlotho, a single-pass transmembrane protein whose expression is induced during differentiation from preadipocytes to adipocytes. BetaKlotho physically interacts with FGF receptors 1c and 4, thereby increasing the ability of these FGF receptors to bind FGF21 and activate the MAP kinase cascade. Knockdown of betaKlotho expression by siRNA in adipocytes diminishes glucose uptake induced by FGF21. Importantly, administration of FGF21 into mice induces MAP kinase phosphorylation in white adipose tissue and not in tissues without betaKlotho expression. Thus, betaKlotho functions as a cofactor essential for FGF21 activity.

Growth in X-linked hypophosphatemic rickets

Growth failure appears frequently in children with X-linked hypophosphatemic rickets (XLHR) due to hypophosphatemia, disease severity, body disproportion, and primary bone abnormality. Recombinant human growth hormone (rhGH) increases phosphate tubular reabsorption and phosphate level in blood and, thus, constitutes an attractive but controversial therapy in short children with XLHR, those efficacy was demonstrated in small uncontrolled series. Our aim was to report our experience regarding growth in XLHR. Twenty-seven children with XLHR--20 girls, seven boys--diagnosed at a median (md) of 1.46 years of age, (range 0.39-8.5 years), were studied at 10.12 years of age (1.58-18.56), md (range). All received oral treatment with phosphate and calcitriol. At the first visit, grouped Z-height was -1; (-4.58; 0.54) md (range). After 5 years' follow-up (0.92-15.6), Z-height was -0.91 (- 4.56; 0.17), not different from that at baseline (P = 0.465). In 16 children entirely controlled in our program upon presentation, a "catch up" phenomenon after the rickets had healed (P = 0.823) or throughout the long-term was not observed (P = 0.995). Eight patients had a Z-height </= -2SD at the last visit, and impaired linear growth was associated with age >2 years at diagnosis, male gender and non-adherence to treatment. Four children, all boys, received rhGH, and in two cases with sufficient follow up stature normalized. No rhGH side effects were observed, and phosphate and calcitriol doses remained stable. Linear growth failure appeared in a third of XLHR children. Efforts need to be made to reduce the age of diagnosis and to improve adherence to treatment. Treatment with rhGH should be considered early, after the rickets has been controlled, in those patients with impaired growth or delayed diagnosis.

Two novel nonsense mutations in GALNT3 gene are responsible for familial tumoral calcinosis

Ectopic periarticular calcifications associated with elevated levels of serum phosphate represent the principal clinical features of hyperphosphatemic familial tumoral calcinosis (HFTC), a rare autosomal recessive metabolic disorder. The disease can be caused by recessive mutations in at least two different genes: GalNAc transferase 3 (GALNT3), encoding a glycosyltransferase that initiates mucin-type O-glycosylation, and fibroblast growth factor 23 (FGF23), which encodes a regulator of phosphate circulating levels. In the current study, we performed mutation analyses of the GALNT3 gene in a subject with HFTC and in his relatives. Sequence analyses revealed that the proband was a compound heterozygote for two novel nonsense mutations in exon 4 (Y322X) and in exon 7 (Q481X). Cosegregation of the mutations with the disease within the family was confirmed by polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP) analysis. This is the first report describing the simultaneous presence of two different stop codons in the coding sequence of the GALNT3 gene.

The phosphatonins and the regulation of phosphate transport and vitamin D metabolism

Phosphate homeostasis is preserved during variations in phosphate intake by short-term intrinsic renal and intestinal adaptations in transport processes, and by more long-term hormonal mechanisms, which regulate the efficiency of phosphate transport in the kidney and intestine. Recently, several phosphaturic peptides such as fibroblast growth factor 23 (FGF-23), secreted frizzled-related protein-4 (sFRP-4), extracellular phosphoglycoprotein (MEPE) and fibroblast growth factor 7 (FGF-7) have been shown to play a pathogenic role in several hypophosphatemic disorders such as tumor-induced osteomalacia (TIO), autosomal dominant hypophosphatemic rickets (ADHR), X-linked hypophosphatemic rickets (XLH), the McCune-Albright syndrome (MAS) and fibrous dysplasia (FD). These proteins induce phosphaturia and hypophosphatemia in vivo, and inhibit sodium-dependent renal phosphate transport in cultured renal epithelial cells. Interestingly, despite the induction of hypophosphatemia by FGF-23 and sFRP-4 in vivo, serum 1, 25-dihydroxyvitamin D (1alpha,25(OH)(2)D) concentrations are decreased or remain inappropriately normal, suggesting an inhibitory effect of these proteins on 25-hydroxyvitamin D 1alpha-hydroxylase activity. In FGF-23 knockout mice, 25-hydroxyvitamin D 1alpha-hydroxylase expression is increased and elevated serum 1alpha,25(OH)(2)D levels cause significant hypercalcemia and hyperphosphatemia. MEPE, however, increases circulating 1alpha,25(OH)(2)D. Circulating or local concentrations of these peptides/proteins may regulate 25-hydroxyvitamin D 1alpha-hydroxylase activity in renal tissues under physiologic circumstances.

Phosphatonins and the Regulation of Phosphate Homeostasis

Inorganic phosphate (P(i)) is required for energy metabolism, nucleic acid synthesis, bone mineralization, and cell signaling. The activity of cell-surface sodium-phosphate (Na(+)-P(i)) cotransporters mediates the uptake of P(i) from the extracellular environment. Na(+)-P(i) cotransporters and organ-specific P(i) absorptive processes are regulated by peptide and sterol hormones, such as parathyroid hormone (PTH) and 1alpha,25-dihydroxyvitamin D (1alpha,25(OH)(2)D(3)), which interact in a coordinated fashion to regulate P(i) homeostasis. Recently, several phosphaturic peptides such as fibroblast growth factor-23 (FGF-23), secreted frizzled related protein-4 (sFRP-4), matrix extracellular phosphoglycoprotein, and fibroblast growth factor-7 have been demonstrated to play a pathogenic role in several hypophosphatemic disorders. By inhibiting Na(+)-P(i) transporters in renal epithelial cells, these proteins increase renal P(i) excretion, resulting in hypophosphatemia. FGF-23 and sFRP-4 inhibit 25-hydroxyvitamin D 1alpha-hydroxylase activity, reducing 1alpha,25(OH)(2)D(3) synthesis and thus intestinal P(i) absorption. This review examines the role of these factors in P(i) homeostasis in health and disease.

Biological activity of FGF-23 fragments

The phosphaturic activity of intact, full-length, fibroblast growth factor-23 (FGF-23) is well documented. FGF-23 circulates as the intact protein and as fragments generated as the result of proteolysis of the full-length protein. To assess whether short fragments of FGF-23 are phosphaturic, we compared the effect of acute, equimolar infusions of full-length FGF-23 and various FGF-23 fragments carboxyl-terminal to amino acid 176. In rats, intravenous infusions of full-length FGF-23 and FGF-23 176-251 significantly and equivalently increased fractional phosphate excretion (FE Pi) from 14 +/- 3 to 32 +/- 5% and 15 +/- 2 to 33 +/- 2% (p < 0.001), respectively. Chronic administration of FGF-23 176-251 reduced serum Pi and serum concentrations of 1alpha,25-dihydroxyvitamin D. Shorter forms of FGF-23 (FGF-23 180-251 and FGF-23 184-251) retained phosphaturic activity. Further shortening of the FGF-23 carboxyl-terminal domain, however, abolished phosphaturic activity, as infusion of FGF-23 206-251 did not increase urinary phosphate excretion. Infusion of a short fragment of the FGF-23 molecule, FGF-23 180-205, significantly increased FE Pi in rats and reduced serum Pi in hyperphosphatemic Fgf-23 ( -/- ) knockout mice. The activity of FGF-23 180-251 was confirmed in opossum kidney cells in which the peptide reduced Na(+)-dependent Pi uptake and enhanced internalization of the Na(+)-Pi IIa co-transporter. We conclude that carboxyl terminal fragments of FGF-23 are phosphaturic and that a short, 26-amino acid fragment of FGF-23 retains significant phosphaturic activity.

Phex mutation causes the reduction of npt2b mRNA in teeth

Hyp mice (murine homologue of human X-linked hypophosphatemia) have a disorder in phosphate homeostasis, and display hypomineralization in bones and teeth. We investigated whether a mutation of Phex (phosphate regulating gene homologies to endopeptidase on the X chromosome) has an effect on the expression level of type II sodium-dependent phosphate co-transporter (Npt2) in the developing teeth of the Hyp mouse. Quantitative RT-PCR analyses revealed that the amount of Npt2b mRNA, an isoform of Npt2, in Hyp mouse tooth germs was significantly lower than that in wild-type mice, in both in vivo and in vitro experiments. In addition, tooth germs from wild-type mice cultured in medium supplemented with antisense oligo-deoxynucleotide for Phex also showed a reduction of Npt2b mRNA expression. These findings suggest that the loss of Phex function is related to the defect of Npt2b expression in teeth, and Npt2b reduction is an intrinsic defect of Hyp murine teeth.