PHEX gene and hypophosphatemia

Targeted ablation of Fgf23 demonstrates an essential physiological role of FGF23 in phosphate and vitamin D metabolism

Inorganic phosphate is essential for ECM mineralization and also as a constituent of important molecules in cellular metabolism. Investigations of several hypophosphatemic diseases indicated that a hormone-like molecule probably regulates serum phosphate concentration. FGF23 has recently been recognized as playing important pathophysiological roles in several hypophosphatemic diseases. We present here the evidence that FGF23 is a physiological regulator of serum phosphate and 1,25-dihydroxyvitamin D (1,25[OH]2D) by generating FGF23-null mice. Disruption of the Fgf23 gene did not result in embryonic lethality, although homozygous mice showed severe growth retardation with abnormal bone phenotype and markedly short life span. The Fgf23(-/-) mice displayed significantly high serum phosphate with increased renal phosphate reabsorption. They also showed an elevation in serum 1,25(OH)2D that was due to the enhanced expression of renal 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-OHase) from 10 days of age. These phenotypes could not be explained by currently known regulators of mineral homeostasis, indicating that FGF23 is essential for normal phosphate and vitamin D metabolism.

FGF-23 transgenic mice demonstrate hypophosphatemic rickets with reduced expression of sodium phosphate cotransporter type IIa

Fibroblast growth factor (FGF)-23 was identified as a causative factor of tumor-induced osteomalacia and also as a responsible gene for autosomal dominant hypophosphatemic rickets. To clarify the pathophysiological roles of FGF-23 in these diseases, we generated its transgenic mice. The transgenic mice expressing human FGF-23 reproduced the common clinical features of these diseases such as hypophosphatemia probably due to increased renal phosphate wasting, inappropriately low serum 1,25-dihydroxyvitamin D level, and rachitic bone. The renal phosphate wasting in the transgenic mice was accompanied by the reduced expression of sodium phosphate cotransporter type IIa in renal proximal tubules. These results reinforce the notion that the excessive action of FGF-23 plays a causative role in the development of several hypophosphatemic rickets/osteomalacia.

MEPE has the properties of an osteoblastic phosphatonin and minhibin

Matrix extracellular phosphoglycoprotein (MEPE) is expressed exclusively in osteoblasts, osteocytes and odontoblasts with markedly elevated expression found in X-linked hypophosphatemic rickets (Hyp) osteoblasts and in oncogenic hypophosphatemic osteomalacia (OHO) tumors. Because these syndromes are associated with abnormalities in mineralization and renal phosphate excretion, we examined the effects of insect-expressed full-length human-MEPE (Hu-MEPE) on serum and urinary phosphate in vivo, (33)PO(4) uptake in renal proximal tubule cultures and mineralization of osteoblast cultures. Dose-dependent hypophosphatemia and hyperphosphaturia occurred in mice following intraperitoneal (IP) administration of Hu-MEPE (up to 400 microg kg(-1) 31 h(-1)), similar to mice given the phosphaturic hormone PTH (80 microg kg(-1) 31 h(-1)). Also the fractional excretion of phosphate (FEP) was stimulated by MEPE [65.0% (P < 0.001)] and PTH groups [53.3% (P < 0.001)] relative to the vehicle group [28.7% (SEM 3.97)]. In addition, Hu-MEPE significantly inhibited (33)PO(4) uptake in primary human proximal tubule renal cells (RPTEC) and a human renal cell line (Hu-CL8) in vitro (V(max) 53.4% inhibition; K(m) 27.4 ng/ml, and V(max) 9.1% inhibition; K(m) 23.8 ng/ml, respectively). Moreover, Hu-MEPE dose dependently (50-800 ng/ml) inhibited BMP2-mediated mineralization of a murine osteoblast cell line (2T3) in vitro. Inhibition of mineralization was localized to a small (2 kDa) cathepsin B released carboxy-terminal MEPE peptide (protease-resistant) containing the acidic serine-aspartate-rich motif (ASARM peptide). We conclude that MEPE promotes renal phosphate excretion and modulates mineralization.

The phosphatonin pathway: New insights in phosphate homeostasis

Serum phosphate concentrations are maintained within a defined range by processes that regulate the intestinal absorption and renal excretion of inorganic phosphate. The hormones currently believed to influence these processes are parathyroid hormone (PTH) and the active metabolite of vitamin D, 1alpha,25-dihydroxyvitamin D (1alpha,25(OH)2D). A new class of phosphate-regulating factors, collectively known as the phosphatonins, have been shown to be associated with the hypophosphatemic diseases, tumor-induced osteomalacia (TIO), X-linked hypophosphatemic rickets (XLH), and autosomal-dominant hypophosphatemic rickets (ADHR). These factors, which include fibroblast growth factor 23 (FGF23) and secreted frizzled-related protein 4 (FRP4), decrease extracellular fluid phosphate concentrations by directly reducing renal phosphate reabsorption and by suppressing 1alpha,25(OH)2D formation through the inhibition of 25-hydroxyvitamin D 1alpha-hydroxylase. The role of these substances under normal or pathologic conditions is not yet clear. For example, it is unknown whether any of the phosphatonins are directly responsible for the decreased concentrations of 1alpha,25(OH)2D observed in chronic and end-stage kidney disease or whether they are induced in an attempt to correct the hyperphosphatemia seen in late stages of chronic renal failure. Future experiments should clarify their physiologic and pathologic roles in phosphate metabolism.

Correction of proximal tubule phosphate transport defect in Hyp mice in vivo and in vitro with indomethacin

X-linked hypophosphatemia is the most prevalent inherited form of rickets. In this disorder, rickets results from hyperphosphaturia and inappropriately normal levels of 1,25(OH)2-vitamin D. Current therapy with oral phosphate and vitamin D improves the rickets, but has significant morbidity and does not significantly affect the short stature and hypophosphatemia. In the present study, we demonstrate that Hyp mice, which have a mutation homologous to that in patients with X-linked hypophosphatemia, have a 2-fold greater urinary prostaglandin E2 (PGE2) excretion than C57/B6 mice. To determine whether PGs were involved in the pathogenesis of this disorder, Hyp and C57/B6 mice received i.p. injections with vehicle or indomethacin (1 mg/kg of body weight twice daily for 4 days) and were studied approximately 12 h after the last dose of indomethacin. In the Hyp mice, indomethacin treatment decreased the fractional excretion of phosphate from 13.0 +/- 3.2% to 2.2 +/- 1.1% (P < 0.05), and increased serum phosphate from 2.9 +/- 0.2 mg/dl to 4.1 +/- 0.2 mg/dl (P < 0.05). There was no effect of indomethacin in C57/B6 mice. Indomethacin did not affect serum creatinine or inulin clearance, demonstrating that the normalization of urinary phosphate excretion was not caused by changes in glomerular filtration rate. Indomethacin treatment increased renal brush border membrane vesicle NaPi-2 protein abundance in Hyp mice to levels comparable to that of C57/B6 mice, but had no effect in C57/B6 mice. In vitro isolated perfused proximal tubule studies demonstrate directly that 10-6 M bath indomethacin normalized the phosphate transport defect in Hyp mice but had no effect on C57/B6 mice. In conclusion, there is dysregulation of renal PG metabolism in Hyp mice, and indomethacin treatment normalizes the urinary excretion of phosphate by a direct tubular effect.

Secreted frizzled-related protein 4 is a potent tumor-derived phosphaturic agent

Tumors associated with osteomalacia elaborate the novel factor(s), phosphatonin(s), which causes phosphaturia and hypophosphatemia by cAMP-independent pathways. We show that secreted frizzled-related protein-4 (sFRP-4), a protein highly expressed in such tumors, is a circulating phosphaturic factor that antagonizes renal Wnt-signaling. In cultured opossum renal epithelial cells, sFRP-4 specifically inhibited sodium-dependent phosphate transport. Infusions of sFRP-4 in normal rats over 2 hours specifically increased renal fractional excretion of inorganic phosphate (FEPi) from 14% +/- 2% to 34% +/- 5% (mean +/- SEM, P < 0.01). Urinary cAMP and calcium excretion were unchanged. In thyro-parathyroidectomized rats, sFRP-4 increased FEPi from 0.7% +/- 0.2% to 3.8% +/- 1.2% (P < 0.05), demonstrating that sFRP-4 inhibits renal inorganic phosphate reabsorption by PTH-independent mechanisms. Administration of sFRP-4 to intact rats over 8 hours increased FEPi, decreased serum phosphate (1.95 +/- 0.1 to 1.53 +/- 0.09 mmol/l, P < 0.05) but did not alter serum 1alpha, 25-dihydroxyvitamin D, renal 25-hydroxyvitamin D 1alpha-hydroxylase cytochrome P450, and sodium-phosphate cotransporter mRNA concentrations. Infusion of sFRP-4 antagonizes Wnt action as demonstrated by reduced renal beta-catenin and increased phosphorylated beta-catenin concentrations. The sFRP-4 is detectable in normal human serum and in the serum of a patient with tumor-induced osteomalacia. Thus, sFRP-4 displays phosphatonin-like properties, because it is a circulating protein that promotes phosphaturia and hypophosphatemia and blunts compensatory increases in 1alpha, 25-dihydroxyvitamin D.

Osteomalacic and hyperparathyroid changes in fibrous dysplasia of bone: core biopsy studies and clinical correlations

Deposition, mineralization, and resorption of FD bone compared with unaffected bone from FD patients was investigated in iliac crest biopsy specimens from 13 patients. Compared with unaffected bone, lesional FD bone seemed to be very sensitive to the effects of PTH and renal phosphate wasting, which respectively bring about hyperparathyroid or osteomalacic changes in the lesional bone.

INTRODUCTION

Fibrous dysplasia is a genetic noninherited disease caused by activating mutations of the GNAS1 gene, resulting in the deposition of qualitatively abnormal bone and marrow. This study was designed to learn more about the local processes of bone deposition, mineralization, and resorption within lesional fibrous dysplasia (FD) bone compared with unaffected bone of FD patients.

METHODS

Histology, histomorphometry, and quantitative back-scattered electron imaging (qBSE) analysis was conducted on affected and unaffected biopsy specimens from 13 patients and correlated to markers of bone metabolism.

RESULTS AND CONCLUSIONS

There was a marked excess of unmineralized osteoid with a nonlamellar structure and a reduced mineral content in mineralized bone within FD lesions (p < 0.001). A negative correlation (p = 0.05) between osteoid thickness (O.Th) and renal tubular phosphate reabsorption (measured as TmP/GFR) was observed for lesional bone, but not for unaffected bone, in which no histological or histomorphometric evidence of osteomalacia was observed in patients with renal phosphate wasting. Histological and histomorphometric evidence of increased bone resorption was variable in lesional bone and correlated with serum levels of parathyroid hormone (PTH). Hyperparathyroidism-related histological changes were observed in fibrous dysplastic bone, but not in the unaffected bone, of patients with elevated serum PTH secondary to vitamin D deficiency. Our data indicate that, compared with unaffected bone, lesional FD bone is very sensitive to the effects of PTH and renal phosphate wasting, which, respectively, bring about hyperparathyroid or osteomalacic changes in the lesional bone. Osteomalacic and hyperparathyroid changes, which emanate from distinct metabolic derangements (which superimpose on the local effects of GNAS1 mutations in bone), influence, in turn, the severity and type of skeletal morbidity in FD.

The neuropeptide-degrading enzyme NL1 is expressed in specific neurons of mouse brain

Metalloendopeptidases of the M13 family were shown to play critical roles in normal physiological processes such as pain control, hypertension and phosphate metabolism, and in pathological states such as Alzheimer's disease. Recently, NL1, a novel member of the family, has been identified and shown to be expressed in several tissues both as a membrane-bound and a secreted protein. As a further step to understand the physiological role(s) of NL1 in mouse, we mapped NL1 mRNA expression pattern in embryos and in young animals at postnatal days p1 and p3, and in adult nervous tissue, using in situ hybridization at the cellular level. No expression could be detected in embryos and young animals. In contrast, NL1 expression was evident in adult brain, pituitary gland and spinal cord. In the central nervous system (CNS), NL1 mRNA was predominantly found in the ventro-posterior regions, which are mostly associated with vegetative functions. At the cellular level, NL1 mRNA was non-uniformly distributed within subpopulations of neurons. In the spinal cord, specific signal was observed in the gray matter. Then, in order to identify putative relevant substrates for NL1, we studied its enzymatic activity towards peptides known to be co-expressed in the NL1-positive domains. Our study showed that NL1 degrades several of these peptides in vitro, the most readily degraded peptides being Bradykinin and Substance P. These results suggest that NL1 is likely to play a critical role in the central nervous system.

Osteoblastoma as a cause of osteomalacia assessed by bone scan

A 27-year-old female patient was admitted to our hospital with a history of leg pain and mass. She had a benign osteoblastoma in right tibia. Resection of the tumor without treatment by vitamin D antagonist resulted in rapid cure of the osteomalacia. Bone scintigraphy with Tc-99m MDP revealed multiple hot uptakes in initial scan, and follow up scan showed a clear resolution of the lesions.

Oral and dental manifestations of vitamin D-dependent rickets type I: report of a pediatric case

Vitamin D-dependent rickets type I (VDDRI) represents an autosomal recessive hereditary defect in vitamin D metabolism. Patients with VDDRI have mutations of chromosome 12 that affect the gene for the enzyme 1-alpha-hydroxylase, resulting in decreased levels of 1,25(OH)(2) vitamin D. Clinical features include growth failure, hypotonia, weakness, rachitic rosary, convulsions, tetany, open fontanels, and pathologic fractures. The oral and dental manifestations of VDDRI have not been described. Here we present the case of a 10-year-old girl affected by VDDRI, as established by the combination of clinical and radiographic findings, family history, and laboratory values. Dental examination revealed markedly hypoplastic, yellowish-to-brownish enamel in all permanent teeth, malocclusion, and chronic periodontal disease. Large quadrangular pulp chambers and short roots were evident in dental radiographs. Light microscopic and ultrastructural examination showed abnormalities of dental hard tissues, affecting both enamel and dentin. The differential diagnosis and treatment of VDDRI are discussed.

Hypophosphatemia in juvenile patients with systemic lupus erythematosus

BACKGROUND

Hypophosphatemia is observed in both genetic diseases and acquired disorders, although it has never been described in systemic lupus erythematosus (SLE). The present study reports hypophosphatemia observed in patients with SLE, and investigates whether serum phosphorus (P) concentration is associated with disease activity and some cytokines.

METHODS

Six girls with SLE (age 10 to 16 years) and seven age-matched controls were studied. Relationships between serum P and parameters that represented disease activity were evaluated. Interleukin (IL) 6 and tumor necrosis factor alpha (TNF-alpha) levels were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Two of the patients with SLE developed marked hypophosphatemia during active disease. Their serum P significantly correlated positively with serum complements, red blood cell (RBC) and platelet counts, and negatively with anti-double-stranded DNA antibody (dsDNAab). Three of the remaining four patients showed the same trend, in which their serum P significantly correlated with RBC, platelets, dsDNAab and/or serum complement concentrations. Serum and urinary IL-6 and serum TNF-alpha were higher in active SLE patients than those with inactive disease or in the control group. In addition, the concentrations of these cytokines significantly correlated inversely with serum P. Renal tubular reabsorption of P was significantly lower in patients with active disease both with and without lupus nephritis.

CONCLUSIONS

Hypophosphatemia may be another sign of juvenile SLE, and serum P may represent disease activity. Both TNF-alpha and IL-6 may be related to hypophosphatemia in patients with SLE. Waste of P from renal tubules may be a possible mechanism of hypophosphatemia in SLE.

Human fibroblast growth factor-23 mutants suppress Na+-dependent phosphate co-transport activity and 1alpha,25-dihydroxyvitamin D3 production

The human fibroblast growth factor 23 (hFGF23) and its autosomal dominant hypophosphatemic rickets (ADHR) mutant genes were incorporated into animals by naked DNA injection to investigate the action on phosphate homeostasis in vivo. The hFGF23 mutants (R176Q, R179Q, and R179W) markedly reduced serum phosphorus (6.2-6.9 mg/dl) compared with the plasmid MOCK (8.5 mg/dl). However, native hFGF23 did not affect serum phosphorus (8.6 mg/dl). Both hFGF23 and hFGF23R179Q mRNAs were expressed more than 100-fold in the liver 4 days after injection, however, the C-terminal portion of hFGF23 was detected only in the serum from hFGF23R179Q-injected animals (1109 pg/ml). hFGF23R179Q mutant was secreted as a 32-kDa protein, whereas, native hFGF23 was detected as a 20-kDa protein in the cell-conditioned media. These results suggest the hFGF23R179Q protein is resistant to intracellular proteolytic processing. The hFGF23R179Q suppressed Na/P(i) co-transport activities both in kidney and in small intestine by 45 and 30%, respectively, as well as serum 1alpha,25-dihydroxyvitamin D(3) to less than 15 pg/ml. However, it had little effect on serum parathyroid hormone (PTH). Infusion of hFGF23R179Q protein normalized serum phosphorus in thyroparathyroidectomized rats without affecting serum calcium. Taken together, the FGF23 mutants reduce both phosphate uptake in intestine and phosphate reabsorption in kidney, independent of PTH action.

Age-dependent phosphate homeostasis is regulated by a circulating factor

To evaluate the age-dependent phosphate homeostasis, we studied the serum inorganic phosphate (P(i)) concentration in 78 recipients, aged 5-25 years, a year after renal transplantation (RT). The significant age-dependent decline of the serum P(i) concentration was observed in recipients (p < 0.0001) as well as in normal children. Our study revealed that a circulating factor may play a central role in the age-dependent change of phosphate regulation in human.

Acute and chronic effect of dietary phosphorus restriction on protein expression in young rat renal proximal tubules

Renal proximal tubules play a vital role in phosphorus (P) homeostasis. It is well known that dietary P restriction up-regulates the activities of 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1-OHase), an enzyme that is involved in activation of vitamin D and thereby maintaining P balance. However, the mechanism involved in such regulation is not known. In the present study, we aim to identify proteins that might be involved in the renal adaptation to dietary P restriction using a proteomic approach. Renal proximal tubules were harvested from young rats fed either normal P diet or low P diet (LPD) for 1 to 7 days. Western blotting analysis of 1-OHase and signaling proteins in insulin-like growth factor I axis indicated an increase in expression of these proteins upon dietary P restriction. Using two-dimensional electrophoresis, we found that LPD reduced the total number of protein species expressed in renal proximal tubules. Differentially expressed proteins were analyzed and located using the software Melanie III, and their identities were found using matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Our results showed that beta-actin, gamma-actin, major urinary protein, phosphatidylinositol transfer protein beta isoform, and G1/S-specific cyclin D3 are up-regulated and nonspecific lipid transfer protein is down-regulated by LPD.

New insights into phosphate homeostasis: fibroblast growth factor 23 and frizzled-related protein-4 are phosphaturic factors derived from tumors associated with osteomalacia

PURPOSE OF REVIEW

Studies of patients with tumors associated with osteomalacia (tumor-induced osteomalacia), X-linked hypophosphatemia (XLH) and autosomal-dominant hypophosphatemic rickets have provided important new insights into the identity and mechanisms of action of factors that play a role in controlling renal phosphate excretion and serum phosphate concentrations. In the present review I discuss how these disorders may be mechanistically related to one another.

RECENT FINDINGS

Patients (or mice) with these disorders manifest rickets as a result of excessive urinary phosphate losses. Tumors associated with osteomalacia elaborate factors ('phosphatonins') that increase renal phosphate excretion and reduce serum phosphate concentrations. These factors include fibroblast growth factor (FGF) 23 and frizzled-related protein-4. Mice with XLH (Hyp) elaborate a circulating factor that induces changes in mineral metabolism similar to those in patients with tumor-induced osteomalacia. In mice and humans with XLH, a mutant enzyme, phex/PHEX, cannot degrade the phosphaturic factor. Patients with autosomal-dominant hypophosphatemic rickets produce a mutant FGF 23 that is resistant to proteolytic degradation. Excessive FGF 23 activity is associated with increased renal phosphate excretion and hypophosphatemia.

SUMMARY

In tumor-induced osteomalacia, excessive production of factors such as FGF 23 and frizzled-related protein-4 is associated with inability of endogenous proteolytic enzymes to degrade these individual substances, with resultant hyperphosphaturia, hypophosphatemia, and rickets. In XLH, mutant PHEX/phex (phosphate-regulating gene with homology to endopeptidases located on the X-chromosome) activity prevents degradation of a phosphaturic factor. In autosomal-dominant hypophosphatemic rickets, a mutant form of FGF 23 that is resistant to proteolytic degradation causes increased renal phosphate losses and hypophosphatemia.

Phosphatonins: a new class of phosphate-regulating proteins

PURPOSE OF REVIEW

There is an intimate relationship between phosphate and calcium homeostasis throughout the animal kingdom. One traditional assumption is that all phosphate-regulating hormones are primarily calcium-regulating hormones. Although the notion of a circulating substance dedicated to phosphate homeostasis has existed for more than a decade, it is not until recently that these hormones have been identified. The molecular characterization of these substances will prove to be critical for understanding phosphate physiology and clinical disorders of phosphate metabolism.

RECENT FINDINGS

This review will focus primarily on the first two proteins recently shown to have phosphatonin properties. Using three human diseases as models and a combination of positional cloning and differential gene expression, fibroblast growth factor 23 and frizzled-related protein 4 were shown to be associated with one or more of these diseases. Although both of these substances have phosphaturic action, their biological effects are likely to extend beyond epithelial phosphate transport.

SUMMARY

The phosphatonins are a growing family of substances that may act on multiple organs in autocrine, paracrine, and endocrine modes to regulate phosphate metabolism. As this list expands, the need for a more rigid definition of the term phosphatonin becomes evident. The identification and characterization of these phosphate-regulatory compounds will provide a clearer understanding of how individual phosphatonins regulate phosphate in normal and disease physiology.

Effects of PHEX antisense in human osteoblast cells

X-linked hypophosphatemia (XLH) is an X-linked dominant disorder that is characterized by rachitic bone disease and hypophosphatemia due to renal phosphate transport defect. The candidate gene for XLH, PHEX, has recently been identified and found to share high homology with endopeptidases. PHEX is expressed in various tissues, including bones, and the available evidence today indicates that bones can release abnormal humoral factors that affect bone mineralization and proximal tubule phosphate transport in XLH. It was, therefore, hypothesized that the inactivating mutations of PHEX in bone may lead to the release of humoral factors and contribute to the phenotypic expression of the disease. To test this possibility, clones of MG-63 cells, a human osteoblast cell line, were produced and stably transfected with PHEX-antisense vectors, resulting in a decrease in PHEX expression at mRNA and protein levels. It was found that these antisense-transfected cells had impaired mineralization, with a decrease in 45Ca incorporation and calcification nodule formation. It was also found that the conditioned culture media collected from these antisense-transfected cells exhibited inhibitory activities on 45Ca incorporation by the nontransfected MG-63 cells and 32P uptake by the opossum kidney proximal tubular cells. The results of the study, therefore, provide strong evidence that supports the link between PHEX mutations and the pathogenesis of XLH.

Evidence for a PTH-independent humoral mechanism in post-transplant hypophosphatemia and phosphaturia

BACKGROUND

Patients undergoing successful kidney transplantation often manifest overt hypophosphatemia associated with exaggerated phosphaturia during the early post-transplant period (2 weeks to 3 months). The mechanism for this phenomenon has not been fully elucidated. We tested the hypothesis that a circulating serum factor [non-parathyroid hormone (non-PTH)], which operates during chronic renal failure (CRF) to maintain phosphate (Pi) homeostasis, can increase fractional excretion of Pi (FE(PO4)) in normal functioning kidney grafts during the early post-transplant period, thereby causing phosphaturia and hypophosphatemia.

METHODS

Five groups of patients were studied: control subjects (group 1, N = 16), "early" (2 weeks to 1 month) post-transplant patients (group 2, N = 22), "late" (9 to 12 months) post-transplant patients (group 3, N = 14), patients with advanced CRF (glomerular filtration rate = 30 to 40 mL/min; group 4, N = 8), and patients who suffered from end-stage renal failure and were treated by chronic hemodialysis (group 5, N = 14). Group 2 manifested significant hypophosphatemia and phosphaturia when compared with groups 1 and 3 (Pi = 0.9 +/- 0.003 mg/dL, FE(PO4) = 68+/- 5%, P < 0.0005 vs. groups 1 and 3). Sera were taken from each of the five subject groups and applied to the proximal tubular opossum kidney (OK) cells. The activity of Na/Pi-type 4 (that is, OK-specific type II transporter) was evaluated by measuring Na(+)-dependent (32)Pi flux. The expression of Na/Pi type II mRNA and the abundance of Na/Pi protein were determined by Northern and Western blot assays, respectively.

RESULTS

When compared with sera from groups 1 and 3, 10% sera taken from groups 2, 4, and 5 (incubated overnight with OK cells) inhibited (32)Pi flux by 25 to 30% (P < 0.0003). Both Na/Pi mRNA and the expression of Na/Pi protein were markedly augmented under the same conditions (P < 0.05 groups 2, 4, and 5 vs. groups 1 and 3). Time-course analysis revealed that the up-regulation of Na/Pi protein by sera from groups 2, 4, and 5 was observed as early as four hours of incubation, whereas augmented abundance of Na/Pi mRNA was only seen after eight hours of incubation. The addition of PTH (1-34) to sera from groups 2, 4, and 5 abolished the augmented expression of NaPi protein. We labeled OK cell surface membrane proteins with N-hydroxysuccinimide bound to biotin (NHS-SS-biotin). Biotinylated transporters incubated with the different sera were precipitated by strepavidin and identified by Western blot analysis. Cells incubated in sera from group 2 showed increased membrane bound transporter when compared with control sera, whereas the intracellular pool of the transporter was comparable between the two groups.

CONCLUSION

A non-PTH circulating serum factor (possibly phosphatonin) that increases FE(PO4) during CRF is also responsible for phosphaturia and hypophosphatemia in the early period following successful kidney transplantation. The putative factor inactivates Na/Pi activity along with inhibition of the transporter trafficking from the cell membrane into the cytosol.

Renal phosphate wasting in fibrous dysplasia of bone is part of a generalized renal tubular dysfunction similar to that seen in tumor-induced osteomalacia

Fibrous dysplasia (FD) of bone is characterized by focal replacement of normal bone and marrow with abnormal bone and fibrous tissue. It arises from postzygotic activating mutations of the GNAS1 gene. Hypophosphatemia due to renal phosphate wasting has been reported in association with FD as a part of the McCune-Albright Syndrome (MAS), which is characterized by FD, skin hyperpigmentation, and precocious puberty. To date, the prevalence and mechanism of phosphate wasting has not been well studied. We evaluated 42 patients with FD/MAS. Serum and urine samples were tested for indices of mineral metabolism, amino acid handling, and markers of bone metabolism. Twenty (48%) patients had some degree of renal phosphate wasting. Nephrogenous cyclic adenosine monophosphate (cAMP) was normal in FD patients, suggesting that the underlying cause of phosphate wasting is not the presence of activating GNAS1 mutations in the kidney. In addition, there was evidence of a more generalized renal tubulopathy as represented by the presence of abnormal vitamin D metabolism, proteinuria in 36 (86%) patients, and aminoaciduria in 39 (94%) patients. Renal phosphate wasting significantly correlated with the degree of bone involvement, as assessed by serum and urine markers of bone metabolism, suggesting that a circulating factor produced by FD bone and impacting on the kidney may be the mechanism. These data show that phosphaturia as part of a generalized renal tubulopathy represents the most common extraskeletal manifestation of FD and that the observed tubulopathy is similar to that seen in tumor-induced osteomalacia (TIO).

Phosphate, the renal tubule, and the musculoskeletal system

A component of ATP, phosphate is at the hub of the energy-related mechanisms operative in muscle cells. Together with calcium, phosphate is involved in bone tissue mineralization: thus, a chronic alteration in the metabolism of phosphate can induce bone and joint disorders. Diagnosis of chronic hypophosphatemia. Serum phosphate, calcium, and creatinine should be assayed simultaneously. Serum calcium is increased in hypophosphatemia caused by hyperparathyroidism and decreased in osteomalacia. Urinary phosphate excretion should be measured in patients with a normal serum calcium level and a serum phosphate level lower than 0.80 mmol/L. A decrease in urinary phosphate excretion to less than 10 mmol/24 h strongly suggests a gastrointestinal disorder, such as malabsorption, antacid use, or chronic alcohol abuse. In patients with a urinary phosphate excretion greater than 20 mmol/24 h, the maximal rate of tubular reabsorption of phosphate (TmPO4) and the ratio of TmPO4 over glomerular filtration rate (GFR) should be determined to look for phosphate diabetes. Manifestations and causes of phosphate diabetes in adults. Moderately severe phosphate diabetes in adults manifests as chronic fatigue, depression, spinal pain, and polyarthralgia, with osteoporosis ascribable to increased bone resorption. Although many cases are idiopathic, investigations should be done to look for X-linked vitamin D-resistant rickets missed during childhood, a mesenchymatous tumor, or Fanconi's syndrome with renal wasting of phosphate, glucose, and amino acids. Management of phosphate diabetes. Phosphate supplementation and, in patients with normal urinary calcium excretion, calcitriol produce some improvement in the symptoms and increase the bone mineral density. Whether dipyramidole is clinically effective remains unclear.

Effect of polyamine depletion on caspase activation: a study with spermine synthase-deficient cells

Activation of the caspase proteases represents a central point in apoptosis. The requirement for spermine for the processes leading to caspase activation has been studied in transformed embryonic fibroblasts obtained from gyro (Gy) mutant male mice. These cells lack spermine synthase activity and thus provide a valuable model to study the role of spermine in cell processes. Gy fibroblasts do not contain spermine and have a higher spermidine content. However, when compared with fibroblasts obtained from normal male littermates (N cells), Gy fibroblasts were observed to grow normally. The lack of spermine did not affect the expression of Bcl-2, and caspases 3 and 9 were activated by etoposide in both N and Gy cells, indicating that spermine is dispensable for caspase activation. Spermine deficiency did not significantly influence caspase activity in cells treated with etoposide, cycloheximide or staurosporine, but sensitized the cells to UV irradiation, which triggered significantly higher caspase activity in Gy cells compared with N cells. alpha-Difluoromethylornithine (DFMO), an inhibitor of polyamine synthesis that is able to deplete cells of putrescine and spermidine, but usually does not influence spermine content, was able to produce a more complete polyamine depletion in Gy cells. This depletion, which included spermine deficiency, dramatically increased caspase activation and cell death in Gy fibroblasts exposed to UV irradiation. On the other hand, in either N or Gy cells, DFMO treatment did not influence caspase activity triggered by staurosporine, but inhibited it when the inducers were cycloheximide or etoposide. In Gy cells depleted of polyamines by DFMO, polyamine replenishment with either spermidine or spermine was sufficient to restore caspase activity induced by etoposide, indicating that, in this model, polyamines have an interchangeable role in supporting caspase activation. Therefore, spermine is not required for such activation, and the effect and specificity of polyamine depletion on caspase activity may be very different, depending on the role of polyamines in the specific death pathways engaged by different stimuli. Some inducers of apoptosis, for example etoposide, absolutely require polyamines for caspase activation, yet the lack of polyamines, particularly spermine, strongly increases caspase activation when induced by UV irradiation.

Hypophosphatemic rickets presenting as recurring pedal stress fractures in a middle-aged woman

Stress fractures frequently occur from overtraining. When stress fractures recur, underlying metabolic abnormalities should be ruled out. We report a middle-aged woman in whom such an evaluation demonstrated previously undiagnosed hypophosphatemic rickets after she presented with recurring stress fractures in her feet. Treatment with phosphate and calcitriol was associated with clinical improvement that would likely not have occurred without this intervention. Any patient with recurring stress fractures should be evaluated with several screening laboratory tests, metabolic bone x-rays, and a measurement of bone mineral density.

Renal expression of the sodium/phosphate cotransporter gene, Npt2, is not required for regulation of renal 1 alpha-hydroxylase by phosphate

Several reports have suggested that the regulation of renal 1,25-dihydroxyvitamin D [1,25-(OH)(2)D] synthesis by extracellular phosphate (Pi) is dependent on normal transepithelial Pi transport by the renal tubule. Mice homozygous for the disrupted Na/Pi cotransporter gene Npt2 (Npt2(-/-)) exhibit renal Pi wasting, an approximately 85% decrease in renal brush border membrane Na/Pi cotransport, hypophosphatemia, and an increase in serum 1,25-(OH)(2)D concentration. We undertook 1) to determine the mechanism for the increased circulating levels of 1,25-(OH)(2)D in Npt2(-/-) mice and 2) to establish whether renal 1alpha-hydroxylase was appropriately regulated by dietary Pi in the absence of Npt2 gene expression. On a control diet, the 2.5-fold increase in the serum 1,25-(OH)(2)D concentration in Npt2(-/-) mice, relative to that in Npt2(+/+) littermates, is associated with a corresponding increase in renal mitochondrial 25-hydroxyvitamin D-1 alpha-hydroxylase (1 alpha-hydroxylase) activity and messenger RNA (mRNA) abundance. A low Pi diet elicits an increase in serum 1,25-(OH)(2)D concentration, renal 1alpha-hydroxylase activity, and mRNA abundance in Npt2(+/+) and Npt2(-/-) mice to similar levels in both mouse strains. A high Pi diet has no effect on serum 1,25-(OH)(2)D concentration, renal 1 alpha-hydroxylase activity, or mRNA abundance in Npt2(+/+) mice, but normalizes these parameters in Npt2(-/-) mice. In addition, renal 24-hydroxylase mRNA abundance is significantly reduced in Npt2(-/-) mice compared with that in Npt2(+/+) mice under all dietary conditions. In summary, we demonstrate that 1) increased renal synthesis of 1,25-(OH)(2)D is responsible for the increased serum 1,25-(OH)(2)D concentration in Npt2(-/-) mice; and 2) renal 1alpha-hydroxylase gene expression is appropriately regulated by dietary manipulation of serum Pi in both Npt2(+/+) and Npt2(-/-) mice. Thus, intact renal Na/Pi cotransport is not required for the regulation of renal 1alpha-hydroxylase by Pi.

Phex cDNA cloning from rat bone and studies on phex mRNA expression: tissue-specificity, age-dependency, and regulation by insulin-like growth factor (IGF) I in vivo

Phosphate regulating gene with homology to endopeptidases on the X chromosome (Phex) inactivating mutations cause X-linked hypophosphatemia (XLH). The disorder is characterized by decreased renal phosphate (Pi) reabsorption in both humans and mice, in the latter shown to be due to a reduction in mRNA and protein of type II sodium-dependent phosphate cotransporter (NadPi-II). To gain insight into the physiological role of Phex, we cloned the rat cDNA and examined tissue-specific and age-dependent mRNA expression. The rat full-length cDNA (2247 nucleotides) shares 96 and 90% identity with the mouse and human cDNA, respectively. We found 6.6 kb Phex transcripts in calvarial bone and lungs, and a weaker signal in liver of newborn rats. In adult animals, Phex mRNA signals were weaker in bone and lungs and absent in liver. Phex mRNA expression in bones and NadPi-I and -II cotransporter mRNA expression in kidney were also determined in hypophysectomized rats. These rats, which lack GH and IGF I, stop growing and exhibit decreased serum Pi levels. Treatment during 6 days with IGF I stimulated growth and increased serum Pi. Phex and NadPi-II cotransporter mRNA levels were higher in IGF I than in vehicle-treated animals, while mRNA expression of NadPi-I, 1alpha-hydroxylase and 24-hydroxylase and serum levels of calcitriol remained unaffected. Age-dependency of Phex expression suggests a role for Phex in Pi retention during growth. Moreover, our findings indicate that an increase in Phex expression in bones under the influence of IGF I may contribute to increased serum Pi by enhancing renal phosphate reabsorption. Because IGF I treatment increased NadPi-II mRNA expression and serum Pi, IGF I appears to act at least partially at pretranslational levels to increase NadPi-II mediated renal Pi retention in growing rats.

Tumor-induced osteomalacia and the regulation of phosphate homeostasis

Tumor-induced osteomalacia (TIO) is a rare and unique syndrome characterized by hypophosphatemia, excessive urinary phosphate excretion, reduced 1,25-dihydroxyvitamin D concentrations, and osteomalacia. Removal of the tumor is associated with a cure of the lesion. Several laboratories have now shown that conditioned medium derived from cultures of such tumors contain a small, heat-sensitive substance ("phosphatonin") of <25,000 daltons that specifically inhibits sodium-dependent phosphate transport in cultured renal proximal tubular epithelia. This substance does not increase cyclic adenosine monophosphate (cAMP) formation in tubular epithelial cells and does not increase cAMP excretion in urine. A substance with similar properties is present in the circulation of patients on hemodialysis. A syndrome with a remarkably similar biochemical phenotype, namely, X-linked hypophosphatemic rickets (XLH), also has a circulating factor with properties similar, if not identical, to those of the tumor-derived factor, "phosphatonin." The molecular defect in XLH has been shown to be due to a mutant endopeptidase, PHEX, whose substrate might be "phosphatonin." Hypophosphatemia and other biochemical abnormalities in TIO are due to excessive production of "phosphatonin" with normal PHEX function, whereas the biochemical abnormalities in XLH are caused by a mutant PHEX enzyme that fails to process "phosphatonin."