Turning over renal osteodystrophy dogma: direct actions of FGF23 on osteoblast ß-catenin pathway.

Although recognized as a major complication of chronic kidney disease (CKD), the pathophysiology of the CKD-related mineral and bone disorder (CKD-MBD) is not completely understood. Recently, the inhibition of Wnt/ß-catenin pathway in osteocytes by sclerostin has been shown to play a role in CKD-MBD. The study by Carrilo-Lopez et al. confirms this inhibition in an experimental model of CKD. Moreover, they describe direct actions of FGF23-Klotho on osteoblasts, increasing the expression of DKK1, another Wnt/ß-catenin pathway inhibitor.

Modeling pulmonary alveolar microlithiasis by epithelial deletion of the Npt2b sodium phosphate cotransporter reveals putative biomarkers and strategies for treatment.

Pulmonary alveolar microlithiasis (PAM) is a rare, autosomal recessive lung disorder associated with progressive accumulation of calcium phosphate microliths. Inactivating mutations in SLC34A2, which encodes the NPT2b sodium-dependent phosphate cotransporter, has been proposed as a cause of PAM. We show that epithelial deletion of Npt2b in mice results in a progressive pulmonary process characterized by diffuse alveolar microlith accumulation, radiographic opacification, restrictive physiology, inflammation, fibrosis, and an unexpected alveolar phospholipidosis. Cytokine and surfactant protein elevations in the alveolar lavage and serum of PAM mice and confirmed in serum from PAM patients identify serum MCP-1 (monocyte chemotactic protein 1) and SP-D (surfactant protein D) as potential biomarkers. Microliths introduced by adoptive transfer into the lungs of wild-type mice produce marked macrophage-rich inflammation and elevation of serum MCP-1 that peaks at 1 week and resolves at 1 month, concomitant with clearance of stones. Microliths isolated by bronchoalveolar lavage readily dissolve in EDTA, and therapeutic whole-lung EDTA lavage reduces the burden of stones in the lungs. A low-phosphate diet prevents microlith formation in young animals and reduces lung injury on the basis of reduction in serum SP-D. The burden of pulmonary calcium deposits in established PAM is also diminished within 4 weeks by a low-phosphate diet challenge. These data support a causative role for Npt2b in the pathogenesis of PAM and the use of the PAM mouse model as a preclinical platform for the development of biomarkers and therapeutic strategies.

Sclerostin, Osteocytes, and Chronic Kidney Disease - Mineral Bone Disorder.

Osteocytes respond to kidney damage by increasing production of secreted factors important to bone and mineral metabolism. These circulating proteins include the antianabolic factor, sclerostin, and the phosphaturic hormone, fibroblast growth factor 23 (FGF23). Elevated sclerostin levels correlate with increased FGF23, localized reduction in Wnt/ß-catenin signaling in the skeleton and reduced osteoblast differentiation/activity. Decreased Wnt/ß-catenin signaling occurs regardless of the overall changes in bone formation rates, suggesting that a reduction in the anabolic response may be a common feature of renal bone disorders but additional mechanisms may contribute to the diversity of osteodystrophy phenotypes. Recent preclinical studies support this hypothesis, as treatment with antisclerostin antibodies improved bone quality in the context of low but not high turnover renal osteodystrophy. Sclerostin also appears in the circulation suggesting additional roles outside the skeleton in normal and disease states. In patients with chronic kidney disease (CKD), serum levels are elevated several fold relative to healthy individuals. Emerging data suggest that these changes are associated with increased fracture rates but the relationship between sclerostin and cardiovascular disease is unclear. Additional epidemiologic studies that examine stage specific and patient sub-populations are needed to assess whether sclerostin elevations influence comorbidities associated with CKD.

Sclerostin and CKD-MBD.

Declining kidney function is associated with sequential systemic changes in mineral homeostasis leading to pathologic alterations in the cardiovascular system and the skeleton. One of the earliest changes in response to renal injury is the increased osteocyte production of secreted factors including the anti-anabolic protein, sclerostin. Elevated sclerostin is associated with reduced Wnt/ß-catenin signaling in bone and decreased osteoblast differentiation/activity. Agents that directly or indirectly inhibit ß-catenin signaling have differential skeletal effects suggesting additional mechanisms contribute to the diversity of renal osteodystrophies. Similarly, Wnt/ß-catenin activation in smooth muscle cells contributes to cardiovascular calcification yet emerging data suggests that this pathway may also be protective when elevated in neighboring tissues. The ongoing epidemiology studies examining the relationship between circulating sclerostin and cardiovascular disease, particularly those that investigate stage specific and/or patient sub-populations, will be useful in understanding the overall contributions of this pathway, its antagonist sclerostin, and the progression of CKD-MBD.

Role of NPT2b in health and chronic kidney disease.

PURPOSE OF REVIEW: Maintaining phosphate homeostasis is essential and any deviation can lead to several acute and chronic disease states. To maintain normal physiological levels, phosphate needs to be tightly regulated. This is achieved through a complex relationship of organ cross-talk via hormonal regulation of the type II sodium-dependent phosphate co-transporters. This editorial provides evidence of the importance of intestinal NPT2b in health and chronic kidney disease (CKD). RECENT FINDINGS: The advent of the different Npt2b knockout mice has increased our understanding of how the intestinal phosphate co-transporter contributes to the regulation of systemic phosphate. In addition, these studies have suggested that Npt2b may participate in the phosphate-sensing machinery important for organ cross-talk. Studies using Drosophila have expanded our knowledge of phosphate sensing mechanisms and may provide a foundation for delineating these pathways in humans. Several preclinical studies using different agents to modulate Npt2b, and clinical studies using nicotinamide, have provided evidence that Npt2b is a viable therapeutic target for the management of hyperphosphatemia. SUMMARY: Over the last couple of years, new experimental approaches have increased our understanding of the important role of Npt2b in maintaining phosphate homeostasis. In addition, several clinical studies have associated the detrimental effects of elevated phosphate with cardiovascular events, and decreased lifespan. Although several key questions about intestinal phosphate transport remain to be answered, it is clear that the intestine is an important player, with current evidence suggesting that it is a prime target for regulating phosphate uptake and improving health outcomes in CKD.

Role of TGF-ß in a mouse model of high turnover renal osteodystrophy.

Altered bone turnover is a key pathologic feature of chronic kidney disease-mineral and bone disorder (CKD-MBD). Expression of TGF-ß1, a known regulator of bone turnover, is increased in bone biopsies from individuals with CKD. Similarly, TGF-ß1 mRNA and downstream signaling is increased in bones from jck mice, a model of high-turnover renal osteodystrophy. A neutralizing anti-TGF-ß antibody (1D11) was used to explore TGF-ß's role in renal osteodystrophy. 1D11 administration to jck significantly attenuated elevated serum osteocalcin and type I collagen C-telopeptides. Histomorphometric analysis indicated that 1D11 administration increased bone volume and suppressed the elevated bone turnover in a dose-dependent manner. These effects were associated with reductions in osteoblast and osteoclast surface areas. Micro-computed tomography (µCT) confirmed the observed increase in trabecular bone volume and demonstrated improvements in trabecular architecture and increased cortical thickness. 1D11 administration was associated with significant reductions in expression of osteoblast marker genes (Runx2, alkaline phosphatase, osteocalcin) and the osteoclast marker gene, Trap5. Importantly, in this model, 1D11 did not improve kidney function or reduce serum parathyroid hormone (PTH) levels, indicating that 1D11 effects on bone are independent of changes in renal or parathyroid function. 1D11 also significantly attenuated high-turnover bone disease in the adenine-induced uremic rat model. Antibody administration was associated with a reduction in pSMAD2/SMAD2 in bone but not bone marrow as assessed by quantitative immunoblot analysis. Immunostaining revealed pSMAD staining in osteoblasts and osteocytes but not osteoclasts, suggesting 1D11 effects on osteoclasts may be indirect. Immunoblot and whole genome mRNA expression analysis confirmed our previous observation that repression of Wnt/ß-catenin expression in bone is correlated with increased osteoclast activity in jck mice and bone biopsies from CKD patients. Furthermore, our data suggest that elevated TGF-ß may contribute to the pathogenesis of high-turnover disease partially through inhibition of ß-catenin signaling.

Npt2b deletion attenuates hyperphosphatemia associated with CKD.

The incidence of cardiovascular events and mortality strongly correlates with serum phosphate in individuals with CKD. The Npt2b transporter contributes to maintaining phosphate homeostasis in the setting of normal renal function, but its role in CKD-associated hyperphosphatemia is not well understood. Here, we used adenine to induce uremia in both Npt2b-deficient and wild-type mice. Compared with wild-type uremic mice, Npt2b-deficient uremic mice had significantly lower levels of serum phosphate and attenuation of FGF23. Treating Npt2b-deficient mice with the phosphate binder sevelamer carbonate further reduced serum phosphate levels. Uremic mice exhibited high turnover renal osteodystrophy; treatment with sevelamer significantly decreased the number of osteoclasts and the rate of mineral apposition in Npt2b-deficient mice, but sevelamer did not affect bone formation and rate of mineral apposition in wild-type mice. Taken together, these data suggest that targeting Npt2b in addition to using dietary phosphorus binders may be a therapeutic approach to modulate serum phosphate in CKD.

Repression of osteocyte Wnt/ß-catenin signaling is an early event in the progression of renal osteodystrophy.

Chronic kidney disease-mineral bone disorder (CKD-MBD) is defined by abnormalities in mineral and hormone metabolism, bone histomorphometric changes, and/or the presence of soft-tissue calcification. Emerging evidence suggests that features of CKD-MBD may occur early in disease progression and are associated with changes in osteocyte function. To identify early changes in bone, we utilized the jck mouse, a genetic model of polycystic kidney disease that exhibits progressive renal disease. At 6 weeks of age, jck mice have normal renal function and no evidence of bone disease but exhibit continual decline in renal function and death by 20 weeks of age, when approximately 40% to 60% of them have vascular calcification. Temporal changes in serum parameters were identified in jck relative to wild-type mice from 6 through 18 weeks of age and were subsequently shown to largely mirror serum changes commonly associated with clinical CKD-MBD. Bone histomorphometry revealed progressive changes associated with increased osteoclast activity and elevated bone formation relative to wild-type mice. To capture the early molecular and cellular events in the progression of CKD-MBD we examined cell-specific pathways associated with bone remodeling at the protein and/or gene expression level. Importantly, a steady increase in the number of cells expressing phosphor-Ser33/37-ß-catenin was observed both in mouse and human bones. Overall repression of Wnt/ß-catenin signaling within osteocytes occurred in conjunction with increased expression of Wnt antagonists (SOST and sFRP4) and genes associated with osteoclast activity, including receptor activator of NF-κB ligand (RANKL). The resulting increase in the RANKL/osteoprotegerin (OPG) ratio correlated with increased osteoclast activity. In late-stage disease, an apparent repression of genes associated with osteoblast function was observed. These data confirm that jck mice develop progressive biochemical changes in CKD-MBD and suggest that repression of the Wnt/ß-catenin pathway is involved in the pathogenesis of renal osteodystrophy.

Intestinal phosphate transport.

Phosphate is absorbed in the small intestine by a minimum of 2 distinct mechanisms: paracellular phosphate transport which is dependent on passive diffusion, and active transport which occurs through the sodium-dependent phosphate cotransporters. Despite evidence emerging for other ions, regulation of the phosphate-specific paracellular pathways remains largely unexplored. In contrast, there is a growing body of evidence that active transport through the sodium-dependent phosphate cotransporter, Npt2b, is highly regulated by a diverse set of hormones and dietary conditions. Furthermore, conditional knockout of Npt2b suggests that it plays an important role in maintenance of phosphate homeostasis by coordinating intestinal phosphate absorption with renal phosphate reabsorption. The knockout mouse also suggests that Npt2b is responsible for the majority of sodium-dependent phosphate uptake. The type-III sodium-dependent phosphate transporters, Pit1 and Pit2, contribute to a minor role in total phosphate uptake. Despite coexpression along the apical membrane, differential responses of Pit1 and Npt2b regulation to chronic versus dietary changes illustrates another layer of phosphate transport control. Finally, a major problem in patients with CKD is management of hyperphosphatemia. The present evidence suggests that targeting key regulatory pathways of intestinal phosphate transport may provide novel therapeutic approaches for patients with CKD.

Altered renal FGF23-mediated activity involving MAPK and Wnt: effects of the Hyp mutation.

Fibroblast growth factor-23 (FGF23), a hormone central to renal phosphate handling, is elevated in multiple hypophosphatemic disorders. Initial FGF23-dependent Erk1/2 activity in the kidney localizes to the distal convoluted tubule (DCT) with the co-receptor α-Klotho (KL), distinct from Npt2a in proximal tubules (PT). The Hyp mouse model of X-linked hypophosphatemic rickets (XLH) is characterized by hypophosphatemia with increased Fgf23, and patients with XLH elevate FGF23 following combination therapy of phosphate and calcitriol. The molecular signaling underlying renal FGF23 activity, and whether these pathways are altered in hypophosphatemic disorders, is unknown. To examine Npt2a in vivo, mice were injected with FGF23. Initial p-Erk1/2 activity in the DCT occurred within 10 min; however, Npt2a protein was latently reduced in the PT at 30-60  min, and was independent of Npt2a mRNA changes. KL-null mice had no DCT p-Erk1/2 staining following FGF23 delivery. Under basal conditions in Hyp mice, c-Fos and Egr1, markers of renal Fgf23 activity, were increased; however, KL mRNA was reduced 60% (P<0.05). Despite the prevailing hypophosphatemia and elevated Fgf23, FGF23 injections into Hyp mice activated p-Erk1/2 in the DCT. FGF23 injection also resulted in phospho-ß-catenin (p-ß-cat) co-localization with KL in wild-type mice, and Hyp mice demonstrated strong p-ß-cat staining under basal conditions, indicating potential crosstalk between mitogen-activated protein kinase and Wnt signaling. Collectively, these studies refine the mechanisms for FGF23 bioactivity, and demonstrate novel suppression of Wnt signaling in a KL-dependent DCT-PT axis, which is likely altered in XLH. Finally, the current treatment of phosphate and calcitriol for hypophosphatemic disorders may increase FGF23 activity.

Intestinal npt2b plays a major role in phosphate absorption and homeostasis.

Intestinal phosphate absorption occurs through both a paracellular mechanism involving tight junctions and an active transcellular mechanism involving the type II sodium-dependent phosphate cotransporter NPT2b (SLC34a2). To define the contribution of NPT2b to total intestinal phosphate absorption, we generated an inducible conditional knockout mouse, Npt2b(-/-) (Npt2b(fl/fl):Cre(+/-)). Npt2b(-/-) animals had increased fecal phosphate excretion and hypophosphaturia, but serum phosphate remained unchanged. Decreased urinary phosphate excretion correlated with reduced serum levels of the phosphaturic hormone FGF23 and increased protein expression of the renal phosphate transporter Npt2a. These results demonstrate that the absence of Npt2b triggers compensatory renal mechanisms to maintain phosphate homeostasis. In animals fed a low phosphate diet followed by acute administration of a phosphate bolus, Npt2b(-/-) animals absorbed approximately 50% less phosphate than wild-type animals, confirming a major role of this transporter in phosphate regulation. In vitro analysis of active phosphate transport in ileum segments isolated from wild-type or Npt2b(-/-) mice demonstrated that Npt2b contributes to >90% of total active phosphate absorption. In summary, Npt2b is largely responsible for intestinal phosphate absorption and contributes to the maintenance of systemic phosphate homeostasis.

Thyroid-stimulating hormone restores bone volume, microarchitecture, and strength in aged ovariectomized rats.

UNLABELLED: We show the systemic administration of low levels of TSH increases bone volume and improves bone microarchitecture and strength in aged OVX rats. TSH's actions are mediated by its inhibitory effects on RANKL-induced osteoclast formation and bone resorption coupled with stimulatory effects on osteoblast differentiation and bone formation, suggesting TSH directly affects bone remodeling in vivo. INTRODUCTION: Thyroid-stimulating hormone (TSH) receptor haploinsufficient mice with normal circulating thyroid hormone levels have reduced bone mass, suggesting that TSH directly affects bone remodeling. We examined whether systemic TSH administration restored bone volume in aged ovariectomized (OVX) rats and influenced osteoclast formation and osteoblast differentiation in vitro. MATERIALS AND METHODS: Sprague-Dawley rats were OVX at 6 months, and TSH therapy was started immediately after surgery (prevention mode; n = 80) or 7 mo later (restoration mode; n = 152). Hind limbs and lumbar spine BMD was measured at 2- or 4-wk intervals in vivo and ex vivo on termination at 8-16 wk. Long bones were subjected to microCT, histomorphometric, and biomechanical analyses. The direct effect of TSH was examined in osteoclast and osteoblast progenitor cultures and established rat osteosarcoma-derived osteoblastic cells. Data were analyzed by ANOVA Dunnett test. RESULTS: In the prevention mode, low doses (0.1 and 0.3 microg) of native rat TSH prevented the progressive bone loss, and importantly, did not increase serum triiodothyroxine (T3) and thyroxine (T4) levels in aged OVX rats. In restoration mode, animals receiving 0.1 and 0.3 microg TSH had increased BMD (10-11%), trabecular bone volume (100-130%), trabecular number (25-40%), trabecular thickness (45-60%), cortical thickness (5-16%), mineral apposition and bone formation rate (200-300%), and enhanced mechanical strength of the femur (51-60%) compared with control OVX rats. In vitro studies suggest that TSH's action is mediated by its inhibitory effects on RANKL-induced osteoclast formation, as shown in hematopoietic stem cells cultivated from TSH-treated OVX rats. TSH also stimulates osteoblast differentiation, as shown by effects on alkaline phosphatase activity, osteocalcin expression, and mineralization rate. CONCLUSIONS: These results show for the first time that systemically administered TSH prevents bone loss and restores bone mass in aged OVX rats through both antiresorptive and anabolic effects on bone remodeling.

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.

FGF-23 and sFRP-4 in chronic kidney disease and post-renal transplantation.

BACKGROUND: The phosphatonins fibroblast growth factor-23 (FGF-23) and FRP-4 are inhibitors of tubular phosphate reabsorption that may play a role in the hyperphosphatemia associated with chronic kidney disease (CKD) or in the hypophosphatemia associated with renal transplants. METHODS: Plasma FGF-23, FRP-4, phosphorus and parathyroid hormone were measured in patients at all stages of CKD. Phosphate regulation of FGF-23 and secreted frizzled related protein-4 (sFRP-4) was examined in end-stage renal disease patients in the presence and absence of therapeutic phosphate binder usage. In renal transplant patients, plasma FGF-23, sFRP-4 and phosphorus concentrations were determined before and 4-5 days after transplantation. RESULTS: Plasma FGF-23 correlated with creatinine clearance (r2 = -0.584, p < 0.0001) and plasma phosphorus (r2 = 0.347, p < 0.001) in CKD patients and with plasma phosphorus (r2 = 0.448, p < 0.001) in end-stage renal disease patients. Phosphate binder withdrawal increased FGF-23 levels. In kidney transplant patients, dramatic decreases in FGF-23 (-88.8 +/- 5.4%) and phosphorus (-64 +/- 10.2%) were observed by 4-5 days post-transplantation. In patients with post-transplant hypophosphatemia, FGF-23 levels correlated inversely with plasma phosphorus (r2 = 0.661, p < 0.05). sFRP-4 levels did not change with creatinine clearance or hyperphosphatemia in CKD or end-stage renal disease patients, and no relation was noted between post-transplant sFRP-4 levels and hypophosphatemia. CONCLUSIONS: In CKD, FGF-23 levels rose with decreasing creatinine clearance rates and increasing plasma phosphorus levels, and rapidly decreased post-transplantation suggesting FGF-23 is cleared by the kidney. Residual FGF-23 may contribute to the hypophosphatemia in post-transplant patients.

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.

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.

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.

Tumors associated with oncogenic osteomalacia express genes important in bone and mineral metabolism.

Oncogenic osteomalacia (OOM) is associated with primitive mesenchymal tumors that secrete phosphaturic factors resulting in low serum concentrations of phosphate and calcitriol, phosphaturia, and defective bone mineralization. To identify overexpressed genes in these tumors, we compared gene expression profiles of tumors resected from patients with OOM and histologically similar control tumors using serial analysis of gene expression (SAGE). Three hundred and sixty-four genes were expressed at least twofold greater in OOM tumors compared with control tumors. A subset of 67 highly expressed genes underwent validation with an extended set of OOM and control tumors using array analysis or reverse-transcription polymerase chain reaction (RT-PCR). Ten of these validated genes were consistently overexpressed in all OOM tumors relative to control tumors. Strikingly, genes with roles in bone matrix formation, mineral ion transport, and bone mineralization were highly expressed in the OOM tumors.