Extracts from tumors causing oncogenic osteomalacia inhibit phosphate uptake in opossum kidney cells

Clonal osteoblastic cell lines with CRISPR/Cas9-mediated ablation of Pit1 or Pit2 show enhanced mineralization despite reduced osteogenic gene expression

Multiple actions of extracellular Pi on the skeletal cells are likely to be partly mediated by type III sodium/phosphate (Na⁺/Pi) cotransporters Pit1 and Pit2, although the details are not fully understood. In the current study, to determine the roles of Pit1 and Pit2 in osteoblasts, we generated Pit1-knockout (KO) and Pit2-KO osteoblastic cells by applying CRISPR/Cas9 genome editing to an osteoblastic cell line MC3T3-E1 subclone 4. The extracellular Pi level was increased in the Pit1-KO and Pit2-KO clones due to the reduced Pi uptake. Interestingly, in vitro mineralization was accelerated in the Pit1-KO and Pit2-KO clones, although the induction of the expression of osteogenic marker genes was suppressed. In the cells before mineralization, extracellular levels of pyrophosphate (PPi) and adenosine triphosphate (ATP) were increased in the Pit1-KO and Pit2-KO clones, which might be attributable to the reduced expression and activity of tissue-nonspecific alkaline phosphatase (TNSALP). A 24-h treatment with high Pi reduced the expression and activity of TNSALP, suggesting that the suppression of TNSALP in the Pit1-KO and Pit2-KO clones was caused by the increased availability of extracellular Pi. Lentiviral gene transfer of Pit1 and Pit2 restored the changes observed in Pit1-KO and Pit2-KO clones, respectively. The expressions of P2Y2 and P2X7 which encode receptors for extracellular ATP were altered in the Pit1-KO and Pit2-KO clones, suggesting an influence on purinergic signaling. In mineralized cells after long-term culture, intracellular levels of PPi and ATP were higher in the Pit1-KO and Pit2-KO clones. Taken together, ablation of Pit1 or Pit2 in this osteoblastic cell model led to accelerated mineralization, suppressed TNSALP and altered the levels of extracellular and intracellular PPi and ATP, which might be partly mediated by changes in the availability of extracellular Pi.

Fibroblast Growth Factor 23

Traditionally, control of phosphorus in the body has been considered secondary to the tighter control of calcium by parathyroid hormone and vitamin D. However, over the past decade, substantial advances have been made in understanding the control of phosphorus by the so-called phosphatonin system, the lynchpin of which is fibroblast growth factor 23 (FGF23). FGF23 binds to the klotho/FGFR1c receptor complex in renal tubular epithelial cells, leading to upregulation of Na/Pi cotransporters and subsequent excretion of phosphorus from the body. In addition, FGF23 inhibits parathyroid hormone and the renal 1α-hydroxylase enzyme, while it stimulates 24-hydroxylase, leading to decreased 1,25-dihydroxyvitamin D3. FGF23 is intimately involved in the pathogenesis of a number of diseases, particularly the hereditary hypophosphatemic rickets group and chronic kidney disease, and is a target for the development of new treatments in human medicine. Little work has been done on FGF23 or the other phosphatonins in veterinary medicine, but increases in FGF23 are seen with chronic kidney disease in cats, and increased FGF23 expression has been found in soft tissue sarcomas in dogs.

Sinonasal phosphaturic mesenchymal tumor: Case report and systematic review

We report a case of sinonasal phosphaturic mesenchymal tumor (PMT) and conduct a systematic review of the literature to highlight a unique paraneoplastic syndrome associated with PMT. We used English language publications from Medline and Cochrane databases (1970-2013) as data sources. A systematic review of the literature was conducted. All reported cases of head and neck PMTs were included. The presence or absence of the associated paraneoplastic syndrome was noted. We found 33 cases of PMT in the head and neck reported in the literature, 17 of which occurred in the sinonasal area. Approximately 5% of all PMTs are located in the head and neck. Just greater than half are concentrated in the sinonasal area, and the remaining involve various bony and soft tissue structures of the head and neck. PMT is sometimes associated with a paraneoplastic syndrome of tumor-induced (oncogenic) osteomalacia (TIO) causing bone pain, muscle weakness, and pathologic fractures. We present the 18th reported case of sinonasal PMT. A smooth mucosa-covered midline intraseptal mass filling the posterior nasal cavity with destruction and erosion of the skull base was found in an adult male. The patient underwent successful endoscopic resection with wide negative margins and is without recurrence at 24-month follow-up. PMT is a benign, locally aggressive tumor with rare malignant transformation. Knowledge of the bony invasion and destruction caused by this tumor is essential in planning surgical resection with wide negative margins. Familiarity with the associated TIO is essential to investigate for and manage any associated bony morbidity.

Recurrent phosphaturic mesenchymal tumour of the temporal bone causing deafness and facial nerve palsy

OBJECTIVE

We describe the first reported case of a phosphaturic mesenchymal tumour, mixed connective tissue variant, invading the temporal bone.

CASE REPORT

A female patient presented with increasing deafness. On examination there appeared to be a mass behind an intact tympanic membrane. Further radiological investigation showed a vascular mass occupying the middle ear, mastoid and internal auditory meatus. This was surgically resected and revealed to be a benign phosphaturic mesenchymal tumour, mixed connective tissue variant. The tumour recurred a year later, presenting as facial nerve palsy. A revision procedure was carried out; the tumour was excised with the sacrifice of a segment of the facial nerve, and a facial-hypoglossal nerve anastomosis was performed.

CONCLUSION

This case report highlights the occurrence of this benign but sometimes aggressive tumour, of which both clinicians and pathologists should be aware. Early recognition of the condition remains of utmost importance to minimise the debilitating consequences of long-term osteomalacia in affected patients, and to prevent extracranial and intracranial complications caused by the tumour.

Both FGF23 and extracellular phosphate activate Raf/MEK/ERK pathway via FGF receptors in HEK293 cells

Fibroblast growth factor 23 (FGF23) is a phosphaturic hormone produced by bone and exerts its function in the target organs by binding the FGF receptor (FGFR) and Klotho. Since recent studies suggested that extracellular inorganic phosphate (Pi) itself triggers signal transduction and regulates gene expression in some cell types, we tested the notion that extracellular Pi induces signal transduction in the target cells of FGF23 also and influences its signaling, utilizing a human embryonic kidney cell line HEK293. HEK293 cells expressed low levels of klotho, and treatment with a recombinant FGF23[R179Q], a proteolysis-resistant mutant of FGF23, resulted in phosphorylation of ERK1/2 and induction of early growth response-1 (EGR1) expression. Interestingly, increased extracellular Pi resulted in activation of the Raf/MEK/ERK pathway and expression of EGR1, which involved type III sodium/phosphate (Na(+)/Pi) cotransporter PiT-1. Since the effects of an inhibitor of Na(+)/Pi cotransporter on FGF23 signaling suggested that the signaling triggered by increased extracellular Pi shares the same downstream cascade as FGF23 signaling, we further investigated their convergence point. Increasing the extracellular Pi concentration resulted in the phosphorylation of FGF receptor substrate 2α (FRS2α), as did treatment with FGF23. Knockdown of FGFR1 expression diminished the phosphorylation of both FRS2α and ERK1/2 induced by the Pi. Moreover, overexpression of FGFR1 rescued the decrease in Pi-induced phosphorylation of ERK1/2 in the cells where the expression of PiT-1 was knocked down. These results suggest that increased extracellular Pi triggers signal transduction via PiT-1 and FGFR and influences FGF23 signaling in HEK293 cells.

Signaling of extracellular inorganic phosphate up-regulates cyclin D1 expression in proliferating chondrocytes via the Na+/Pi cotransporter Pit-1 and Raf/MEK/ERK pathway

As chondrocytes mature, the concentration of inorganic phosphate (Pi) increases in the extracellular milieu. It was demonstrated that the progressive accumulation of Pi started from the proliferative zone and peaked in the hypertrophic zone of growth plate. Although extracellular Pi is reported to be involved in the apoptosis and mineralization of mature chondrocytes, its role in proliferating chondrocytes remains unclear. Here we investigated this role utilizing ATDC5, an established cell model of chondrocytic differentiation. In proliferating ATDC5 cells, we found that the expression of cyclin D1 was up-regulated, and that of alkaline phosphatase (ALP) was down-regulated in response to an increase in extracellular Pi within 24h. Moreover, an increase in extracellular Pi-induced activation of the Raf/MEK/ERK pathway, and treatment with a MEK inhibitor PD98059 abolished the effects on the expression of cyclin D1 and ALP, indicating that extracellular Pi regulates the expression of these genes through the Raf/MEK/ERK pathway. Consistent with its up-regulation of cyclin D1 expression, the extracellular Pi facilitated the proliferation of ATDC5 cells. Treatment with phosphonoformic acid (PFA), an inhibitor of sodium/phosphate (Na(+)/Pi) cotransporters, abrogated the activation of the Raf/MEK/ERK pathway and gene expression induced by the increase in extracellular Pi. Knocking down of the type III Na(+)/Pi cotransporter Pit-1 diminished the responsiveness of ATDC5 cells to the increase in extracellular Pi. Interestingly, the increased extracellular Pi induced the phosphorylation of fibroblast growth factor receptor substrate 2α (FRS2α), which was also cancelled by knocking down of the expression of Pit-1. In primary chondrocytes isolated from mouse rib cages as well, increased extracellular Pi induced the phosphorylation of ERK1/2 and alterations in the expression of cyclin D1 and ALP, both of which were abolished by treatment with PFA. These results suggest that signaling by extracellular Pi is mediated by Pit-1 and FRS2α, and leads to activation of the Raf/MEK/ERK pathway and increased expression of cyclin D1, which facilitates the proliferation of immature chondrocytes.

Phosphaturic mesenchymal tumor, mixed connective tissue variant, of the mandible: report of a case and review of the literature

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome that results in renal phosphate wasting with hypophosphatemia. In most cases, the underlying cause of TIO is a small mesenchymal neoplasm that is often difficult to detect, resulting in delayed diagnosis. One such neoplasm is the phosphaturic mesenchymal tumor, mixed connective tissue variant (PMTMCT), an unusual entity with unique morphologic and biochemical features. Most of these tumors are found at appendicular sites with only rare cases reported in the jaws. We describe a PMTMCT involving the mandible in a patient with a protracted history of osteomalacia. A review of the current literature is provided with emphasis on the clinical and histologic features, etiopathogenesis, and management of PMTMCT in the setting of TIO.

Oncogenic hypophosphatemic osteomalacia associated with a nasal hemangiopericytoma

We report a patient with a nasal hemangiopericytoma associated with an oncogenic hypophosphatemic osteomalacia (OHO). This syndrome results from tumor products that decrease renal tubular phosphate resorption, leading to the osteomalacia. This patient presented with classic bone manifestations of osteomalacia and a nasal tumor. Laboratory studies performed before the first resection of the tumor included normal serum calcium, hypophosphatemia due to decreased tubular reabsorption of phosphate, and an undetectable serum 1,25 dihydroxy vitamin D level. Serum parathormone level was normal. Anterior iliac crest bone biopsy showed characteristic signs of osteomalacia that included increased osteoid and delayed mineralization. A partial resection of the nasal tumor was performed. After the first surgery the patient showed detectable serum level of 1,25 dihydroxy vitamin D, and transient normalization of the tubular reabsorption of phosphate. The patient was also treated with phosphate supplements and vitamin D with transient control of her clinical manifestations and improvement of the radiographic signs of osteomalacia. Three months after surgery, the serum level of 1,25 dihydroxy vitamin D level again became undetectable. After selective embolization of the tumor, followed by an apparent complete tumor resection and postoperative radiation therapy, her hypophosphatemia and decreased phosphate tubular reabsorption persisted. Therefore, biochemical changes associated with hemangiopericytoma induced OHO may persist even after apparent total tumor resection. Clinicians should be aware of the oncogenic basis for some osteomalacia, as seen in this patient.

Inorganic phosphate homeostasis and the role of dietary phosphorus

Inorganic phosphate (Pi) is required for cellular function and skeletal mineralization. Serum Pi level is maintained within a narrow range through a complex interplay between intestinal absorption, exchange with intracellular and bone storage pools, and renal tubular reabsorption. The crucial regulated step in Pi homeostasis is the transport of Pi across the renal proximal tubule. Type II sodium-dependent phosphate (Na/Pi) cotransporter (NPT2) is the major molecule in the renal proximal tubule and is regulated by Pi, parathyroid hormone and by 1,25-dihydroxyvitamin D. Recent studies of inherited and acquired hypophosphatemia [X-linked hypophosphatemic rickets/osteomalacia (XLH), autosomal dominant hypophosphatemic rickets/osteomalacia (ADHR) and tumor-induced rickets/osteomalacia (TIO)], which exhibit similar biochemical and clinical features, have led to the identification of novel genes, PHEX and FGF23, that play a role in the regulation of Pi homeostasis. The PHEX gene, which is mutated in XLH, encodes an endopeptidase, predominantly expressed in bone and teeth, but not in kidney. FGF-23 may be a substrate of this endopeptidase and may therefore accumulate in patients with XLH. In the case of ADHR mutations in the furin cleavage site, which prevent the processing of FGF-23 into fragments, lead to the accumulation of a "stable" circulating form of the peptide which also inhibits renal Pi reabsorption. In the case of TIO, ectopic overproduction of FGF-23 overwhelms its processing and degradation by PHEX, leading to the accumulation of FGF-23 in the circulation and inhibition of renal Pi reabsorption. Mice homozygous for severely hypomorphic alleles of the Klotho gene exhibit a syndrome resembling human aging, including atherosclerosis, osteoporosis, emphysema, and infertility. The KLOTHO locus is associated with human survival, defined as postnatal life expectancy, and longevity, defined as life expectancy after 75. In considering the relationship of klotho expression to the dietary Pi level, the klotho protein seemed to be negatively controlled by dietary Pi.

Surface plasmon resonance (SPR) confirms that MEPE binds to PHEX via the MEPE-ASARM motif: a model for impaired mineralization in X-linked rickets (HYP)

Matrix Extracellular Phospho-glycoprotEin (MEPE) and proteases are elevated and PHEX is defective in HYP. PHEX prevents proteolysis of MEPE and release of a protease-resistant MEPE-ASARM peptide, an inhibitor of mineralization (minhibin). Thus, in HYP, mutated PHEX may contribute to increased ASARM peptide release. Moreover, binding of MEPE by PHEX may regulate this process in normal subjects. The nature of the PHEX-MEPE nonproteolytic interaction(s) (direct or indirect) is/are unknown. Our aims were to determine (1) whether PHEX binds specifically to MEPE, (2) whether the binding involves the ASARM motif region, and (3) whether free ASARM peptide affects mineralization in vivo in mice. Protein interactions between MEPE and recombinant soluble PHEX (secPHEX) were measured using surface plasmon resonance (SPR). Briefly, secPHEX, MEPE, and control protein (IgG) were immobilized on a Biacore CM5 sensor chip, and SPR experiments were performed on a Biacore 3000 high-performance research system. Pure secPHEX was then injected at different concentrations, and interactions with immobilized proteins were measured. To determine MEPE sequences interacting with secPHEX, the inhibitory effects of MEPE-ASARM peptides (phosphorylated and nonphosphorylated), control peptides, and MEPE midregion RGD peptides on secPHEX binding to chip-immobilized MEPE were measured. ASARM peptide and etidronate-mediated mineralization inhibition in vivo and in vitro were determined by quenched calcein fluorescence in hind limbs and calvariae in mice and by histological Sanderson stain. A specific, dose-dependent and Zn-dependent protein interaction between secPHEX and immobilized MEPE occurs (EC50 of 553 nM). Synthetic MEPE PO4-ASARM peptide inhibits the PHEX-MEPE interaction (K(D(app)) = 15 uM and B(max/inhib) = 68%). In contrast, control and MEPE-RGD peptides had no effect. Subcutaneous administration of ASARM peptide resulted in marked quenching of fluorescence in calvariae and hind limbs relative to vehicle controls indicating impaired mineralization. Similar results were obtained with etidronate. Sanderson-stained calvariae also indicated a marked increase in unmineralized osteoid with ASARM peptide and etidronate groups. We conclude that PHEX and MEPE form a nonproteolytic protein interaction via the MEPE carboxy-terminal ASARM motif, and the ASARM peptide inhibits mineralization in vivo. The binding of MEPE and ASARM peptide by PHEX may explain why loss of functional osteoblast-expressed PHEX results in defective mineralization in HYP.

Serum MEPE-ASARM-peptides are elevated in X-linked rickets (HYP): implications for phosphaturia and rickets

MEPE (Matrix Extracellular PhosphoglycoprotEin) expression is markedly elevated in X-linked-hypophosphatemic-rickets (HYP) and tumor-induced osteomalacia (TIO). In normal individuals, circulating serum-levels of MEPE are tightly correlated with serum-phosphorus, parathyroid hormone (PTH) and bone mineral density (BMD). Also, MEPE derived, C-terminal ASARM-peptides are candidate minhibins and/or phosphatonins. Our aims were to determine: 1. whether MEPE-ASARM-peptide(s) are abnormally elevated in HYP/hyp serum, and, 2. whether the ASARM-peptide(s) accumulate in hyp mice kidney renal-tubules. Using a specific competitive ELISA we measured a five fold increase (P=0.007) of serum ASARM-peptide(s) in human HYP patients (normal subjects 3.25 microM n=9; S.E.M.=0.51 and HYP-patients 15.74 microM, n=9; S.E.M.=3.32). A 6.23 fold increase (P=0.008) was measured in hyp male mice compared with their normal male siblings (normal-siblings, 3.73 muM, S.E.M.=0.57, n=3; and hyp-mice 23.4 microM, n=3, S.E.M.=4.01). Renal immuno-histological screening also revealed a dramatic increase of ASARM-peptides in regions anatomically consistent with the proximal convoluted tubules. This study demonstrates for the first time that markedly elevated serum levels of protease-resistant ASARM-peptide(s) occur in HYP/hyp and they accumulate in murine hyp kidneys. These peptides are thus likely responsible for the phosphaturia and defective mineralization in HYP/hyp and TIO.

The wrickkened pathways of FGF23, MEPE and PHEX

The last 350 years since the publication of the first medical monograph on rickets (old English term wrickken) (Glisson et al., 1651) have seen spectacular advances in our understanding of mineral-homeostasis. Seminal and exciting discoveries have revealed the roles of PTH, vitamin D, and calcitonin in regulating calcium and phosphate, and maintaining healthy teeth and skeleton. However, it is clear that the PTH/Vitamin D axis does not account for the entire picture, and a new bone-renal metabolic milieu has emerged, implicating a novel set of matrix proteins, hormones, and Zn-metallopeptidases. The primary defects in X-linked hypophosphatemic rickets (HYP) and autosomal-dominant hypophosphatemic rickets (ADHR) are now identified as inactivating mutations in a Zn-metalloendopeptidase (PHEX) and activating mutations in fibroblast-growth-factor-23 (FGF23), respectively. In oncogenic hypophosphatemic osteomalacia (OHO), several tumor-expressed proteins (MEPE, FGF23, and FRP-4) have emerged as candidate mediators of the bone-renal pathophysiology. This has stimulated the proposal of a global model that takes into account the remarkable similarities between the inherited diseases (HYP and ADHR) and the tumor-acquired disease OHO. In HYP, loss of PHEX function is proposed to result in an increase in uncleaved full-length FGF23 and/or inappropriate processing of MEPE. In ADHR, a mutation in FGF23 results in resistance to proteolysis by PHEX or other proteases and an increase in half-life of full-length phosphaturic FGF23. In OHO, over-expression of FGF23 and/or MEPE is proposed to result in abnormal renal-phosphate handling and mineralization. Although this model is attractive, many questions remain unanswered, suggesting a more complex picture. The following review will present a global hypothesis that attempts to explain the experimental and clinical observations in HYP, ADHR, and OHO, plus diverse mouse models that include the MEPE null mutant, HYP-PHEX transgenic mouse, and MEPE-PHEX double-null-mutant.

What have we learnt about the regulation of phosphate metabolism?

PURPOSE OF REVIEW

The search for hormones which specifically regulate phosphate metabolism has fuelled recent tantalizing studies. These studies have been motivated by diseases involving renal phosphate wasting, including tumor-induced osteomalacia, X-linked hypophosphatemic rickets, and autosomal dominant hypophosphatemia. This review focuses on likely candidate 'phosphatonins' and their possible physiological significance.

RECENT FINDINGS

Candidate phosphatonins include fibroblast growth factor 23, matrix extracellular phosphoglycoprotein, stanniocalcin, and Frizzled-related protein 4. Fibroblast growth factor 23 has emerged as the prime candidate explaining pathophysiology of these diseases. FGF-23 is expressed in most tumors in tumor-induced osteomalacia. Serum fibroblast growth factor 23 is increased in most patients with X-linked hypophosphatemic rickets and tumor-induced osteomalacia. Injection of recombinant fibroblast growth factor 23 induces phosphaturia, hypophosphatemia, and suppression of 1,25-dihydroxyvitamin D in animals. Many unanswered questions remain, including the relationship between PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) mutations and elevated fibroblast growth factor 23. It is also not clear whether these candidate phosphatonins play a role in phosphate or vitamin D metabolism in healthy humans, or that this role is endocrine. The most compelling evidence derives from the fibroblast growth factor 23-knockout mouse which shows hyperphosphatemia and increased serum 1,25-dihydroxyvitamin D. A physiologically relevant phosphatonin should explain renal adaptation to variable dietary phosphate intake. The tissue source and determinants of serum fibroblast growth factor 23 are unknown.

SUMMARY

Pathophysiological and animal studies serve as a logical foundation on which to base further questions of human physiology. The definition of what is or is not a phosphatonin may need to be refined. There is a need to return to 'old-fashioned' human physiology studies to place recent findings in perspective.

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 regulation and function of phosphate in the human body

Inorganic phosphate (Pi) is required for cellular function and skeletal mineralization. Serum Pi level is maintained within a narrow range through a complex interplay between intestinal absorption, exchange with intracellular and bone storage pools, and renal tubular reabsorption. Pi is abundant in the diet, and intestinal absorption of Pi is efficient and minimally regulated. The kidney is a major regulator of Pi homeostasis and can increase or decrease its Pi reabsorptive capacity to accommodate Pi need. The crucial regulated step in Pi homeostasis is the transport of Pi across the renal proximal tubule. Type II sodium-dependent phosphate (Na/Pi) cotransporter (NPT2) is the major molecule in the renal proximal tubule and is regulated by hormones and nonhormonal factors. Recent studies of inherited and acquired hypophosphatemia which exhibit similar biochemical and clinical features, have led to the identification of novel genes, phosphate regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and fibroblast growth factor-23 (FGF-23), that play a role in the regulation of Pi homeostasis. The PHEX gene encodes an endopeptidase, predominantly expressed in bone and teeth but not in kidney. FGF-23 may be a substrate of this endopeptidase and inhibit renal Pi reabsorption. In a survey in the United States and in Japan, the amount of phosphorus from food is gradually increasing. It is thought that excess amounts of phosphorus intake for long periods are a strong factor in bone impairment and ageing. The restriction of phosphorus intake seems to be important under low calcium intake to keep QOL on high level.

Disorders of phosphate metabolism--pathomechanisms and management of hypophosphataemic disorders

Hypophosphataemia does not necessarily indicate phosphate (Pi) depletion. In acute emergencies such as septicaemia, alkalosis or re-feeding, hypophosphataemia may result from redistribution of Pi from the extracellular to the intracellular space. Hypophosphataemia from true Pi depletion gives rise to skeletal (osteomalacia) and extraskeletal (myopathy, cardiomyopathy) disorders. It is practically never the result of diminished nutritional intake. The most severe syndromes of Pi depletion result from diminished tubular Pi re-absorption and renal Pi wasting. In the differential diagnosis mainly four conditions have to be considered: (i) tumour-associated osteomalacia, (ii) X-linked hypophosphataemia (XLH), (iii) autosomal dominant hypophosphataemia, and (iv) hypercalcaemic renal phosphate wasting. Recent molecular insight has put fibroblast growth factor (FGF-23) into the centre of pathophysiological considerations because of (i) overproduction (tumour-associated osteomalacia) or (ii) hypothetically, accumulation resulting from mutations causing resistance to processing or degradation (autosomal dominant hypophosphataemia) or (iii) loss-of-function of a protease (PHEX) interfering with FGF-23 breakdown (XLH). In oncogenic osteomalacia the treatment of choice is resection of the tumour. Recently, pharmacological treatment has also become possible, i.e. administration of octreotide. XLH and autosomal dominant hypophosphataemia must be managed by oral administration of phosphate and calcitriol. In patients with gastrointestinal intolerance to phosphate or with severely symptomatic bone disease, prolonged intravenous administration of Pi is necessary.

Disordered regulation of renal 25-hydroxyvitamin D-1alpha-hydroxylase gene expression by phosphorus in X-linked hypophosphatemic (hyp) mice

X-linked hypophosphatemic (Hyp) mice exhibit hypophosphatemia, impaired renal phosphate reabsorption, defective skeletal mineralization, and disordered regulation of vitamin D metabolism: In Hyp mice, restriction of dietary phosphorus induces a decrease in serum concentration of 1,25-dihydroxyvitamin D and renal activity of 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-hydroxylase), and induces an increase in renal activity of 25-hydroxyvitamin D-24-hydroxylase (24-hydroxylase). In contrast, in wild-type mice, phosphorus restriction stimulates renal 1alpha-hydroxylase gene expression and suppresses that of 24-hydroxylase. To determine the molecular basis for the disordered regulation of vitamin D metabolism in Hyp mice, we determined renal mitochondrial 1alpha-hydroxylase activity and the renal abundance of p450c1alpha and p450c24 mRNA in wild-type and Hyp mice fed either control, low-, or high-phosphorus diets for 5 d. In wild-type mice, phosphorus restriction increased 1alpha-hydroxylase activity and p450c1alpha mRNA expression by 6-fold and 3-fold, respectively, whereas in the Hyp strain the same diet induced changes of similar magnitude but opposite in direction. Phosphorus supplementation was without effect in wild-type mice, whereas in Hyp mice the same diet induced 3-fold and 2-fold increases, respectively, in enzyme activity and p450c1alpha mRNA abundance. In wild-type mice, both renal 1alpha-hydroxylase activity and p450c1alpha mRNA abundance varied inversely and significantly with serum phosphorus concentrations, whereas in Hyp mice the relationship between both renal parameters and serum phosphorus concentration was direct. In Hyp mice, phosphorus restriction induced a significant increase in renal p450c24 mRNA abundance, in contrast to the lack of effect observed in wild-type mice. The present findings demonstrate that regulation of both the p450c1alpha and p45024 genes by phosphorus is disordered in Hyp mice at the level of renal 1alpha-hydroxylase activity and renal p450c1alpha and p450c24 mRNA expression.

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.