The phosphatonin pathway: New insights in phosphate homeostasis

A GALNT3 mutation causing Hyperphosphatemic familial Tumoral calcinosis

Aim Hyperphosphatemic Familial Tumoral Calcinosis (HFTC) is an autosomal recessive disorder. This study investigates the etiology of HFTC in offspring from consanguineous parents.

Methods

Clinical assessment, imaging, and direct sequencing were utilized to elucidate the condition. Previously reported cases were also reviewed.

Result

We identified a consanguineous Chinese family with HFTC caused by an interesting homozygous G to A substitution in GALNT3 (c.1626 + 1G > A). The parents were carriers.

Conclusion

This study represents the first report of HFTC in a consanguineous Chinese family due to an interesting GALNT3 mutation. We reviewed known GALNT3 variants and associated clinical features of calcification disorders. The phenotypic difference between homozygous and complex heterozygous mutations is not clinically significant. Gene mutations affect the function of proteins mainly by affecting their binding to polyvalent ligands.

Unusual presentation of tumor-induced osteomalacia mismanaged due to misdiagnosis: A literature review based on a case report

Key Clinical Message

Tumor-induced osteomalacia is a rare but potentially serious disease with nonspecific misguiding manifestations that can result in a wrong diagnosis and being treated for rheumatologic or other similar diseases. In patients with unexpected fractures, resistant musculoskeletal pains, and hypophosphatemia, this diagnosis should be considered by the physicians and approached through a complete history taking, physical exam laboratory, and radiologic evaluation to give the opportunity of on-time treatment to the patient.

Abstract

Tumor-induced osteomalacia (TIO) is an uncommon mesenchymal tumor that results in disproportionate phosphorus excretion, primarily leading to bone-related symptoms. Laboratory, imaging, and histopathological evaluation can confirm this pathologic condition. In this case, we present the history and subsequent clinical parts of a 50-year-old woman who presented with an unusual presentation of generalized musculoskeletal pains and a right ankle mass. Her disease was diagnosed with multidisciplinary evaluation and was approached by a surgical treatment. The patient was treated with total resection of the tumor, which led to complete resolution of musculoskeletal and metabolic abnormalities, which were resolved following total tumor resection. TIO is a paraneoplastic disease that results in abnormal secretion of phosphatonins, particularly fibroblast growth factor 23 (FGF23). This can cause hypophosphatemia, hyperparathyroidism, lower bone density, and increased risk of pathologic fractures. These tumors are mostly cured by surgical ± radiotherapy. The present study aims to provide insight into the fact that a TIO diagnosis is not always straightforward. However, in suspicious cases such as unexplained hypophosphatemia, it should be considered to prevent delayed diagnosis of the progressive pathology. The earlier treatment can prevent several complications and reduce the risk of mortality.

Preoperative evaluation and orthopedic surgical strategies for tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is very rare, with about 1000 reported cases globally. Removing most TIO culprit tumors requires the evaluation and intervention of orthopedic doctors. However, orthopedic doctors often have a poor understanding of the optical treatment of TIO due to its rarity. In addition, most TIO patients lack specific clinical manifestations. Also, the clinical localization and qualitative diagnosis of TIO are difficult and thus can easily be misdiagnosed and mistreated. Furthermore, the true incidence rate of TIO may be underestimated. Although many breakthroughs have been made in exploring the pathogenesis, clinical diagnosis, and treatment of TIO, rational and standardized orthopedic surgical treatment experience summary and sorting for TIO patients are lacking. In this article, the recent experience and progress in the field of orthopedic surgical treatment for TIO globally have been summarized, providing a theoretical basis and new clinical practice guidance for the rational treatment of TIO patients.

Tumor-Induced Osteomalacia: A Systematic Clinical Review of 895 Cases

Tumor-induced osteomalacia (TIO) is a rare and largely underdiagnosed paraneoplastic condition. Previous reviews often reported incomplete data on clinical aspects, diagnosis or prognosis. The aim of this study was to present a systematic clinical review of all published cases of TIO. A search was conducted in Pubmed, Embase, Web of Science from inception until April 23rd, 2020. We selected case reports and case series of patients diagnosed with TIO, with information on tumor localization and serum phosphate concentration. Two reviewers independently extracted data on biochemical and clinical characteristics including bone involvement, tumor localization and treatment. 468 articles with 895 unique TIO cases were included. Median age was 46 years (range 9 months-90 years) and 58.3% were males. Hypophosphatemia and inappropriately low or normal 1,25-dihydroxyvitamin D levels, characteristic for TIO, were present in 98% of cases. Median tumor size was 2.7 cm (range 0.5 to 25.0 cm). Serum fibroblast growth factor 23 was related to tumor size (r = 0.344, P < 0.001). In 32% of the cases the tumor was detected by physical examination. Data on bone phenotype confirmed skeletal involvement: 62% of cases with BMD data had a T-score of the lumbar spine ≤ - 2.5 (n = 61/99) and a fracture was reported in at least 39% of all cases (n = 346/895). Diagnostic delay was longer than 2 years in more than 80% of cases. 10% were reported to be malignant at histology. In conclusion, TIO is a debilitating disease characterized by a long diagnostic delay leading to metabolic disturbances and skeletal impairment. Increasing awareness of TIO should decrease its diagnostic delay and the clinical consequences.

Dietary supplementation of 25-hydroxycholecalciferol as an alternative to cholecalciferol in swine diets: A review

25-hydroxycholecalciferol (25-OHD₃ ) formed via hepatic hydroxylation from vitamin D, cholecalciferol, represents the precursor of the biologically active vitamin D hormone, 1,25-dihydroxyvitamin D. Due to a higher absorption rate and the omission of one hydroxylation, dietary supplementation of 25-OHD₃ instead of vitamin D₃ is considered to be more efficient as plasma concentrations of 25-OHD₃ are increased more pronounced. The present review summarises studies investigating potential beneficial effects on mineral homeostasis, bone metabolism, health status and performance in sows, piglets and fattening pigs. Results are inconsistent. While most studies could not demonstrate any or only a slight impact of partial or total replacement of vitamin D₃ by 25-OHD₃ , some experiments indicated that 25-OHD₃ might alter physiological processes when animals are challenged, for example, by a restricted mineral supply.

A rare combination of tumor-induced osteomalacia caused by sinonasal glomangiopericytoma and coexisting parathyroid adenoma: case report and literature review

BACKGROUND

Tumor-induced osteomalacia (TIO) is a rare, acquired disease of renal phosphate wasting and disturbed vitamin D homeostasis as a result of the action of a phosphaturic protein - FGF-23, produced by a neoplasm. Although the clinical and biochemical profile of the syndrome is characteristic, it remains underreported and unrecognized by clinicians. Hyperparathyroidism is rarely associated with oncogenic osteomalacia, but it should be considered because of potentially life-threatening hypophosphatemia caused by both conditions.

CASE PRESENTATION

We report a case of a 42-year-old woman admitted to the Department of Otolaryngology of the Military Institute of Medicine in Warsaw for the endoscopic resection of hormonally active glomangiopericytoma extending into the anterior skull base. She presented with a 5-year history of musculoskeletal pain and progressive weakness of the extremities which finally led her to become bedridden. After the excision of the tumor her symptoms and laboratory results gradually improved except increasing PTH serum levels. Further examination revealed a parathyroid proliferative tumor, which was surgically removed. The patient walked without aids at follow-up 16 months after the surgery.

CONCLUSIONS

This case is unusual because of tumor-induced osteomalacia and parathyroid adenoma occurring concomitantly. Further investigations of FGF-23 and PTH interplay should be conducted to elucidate the pathogenesis of hyperparathyroidism and tumorigenesis in some cases of TIO. By presenting this case, we wanted to remind clinicians of a rare and misdiagnosed paraneoplastic syndrome and highlight the importance of monitoring PTH concentrations during the follow-up of patients with TIO.

Fungal commensalism modulated by a dual-action phosphate transceptor

Successful host colonization by fungi in fluctuating niches requires response and adaptation to multiple environmental stresses. However, our understanding about how fungal species thrive in the gastrointestinal (GI) ecosystem by combing multifaceted nutritional stress with respect to homeostatic host-commensal interactions is still in its infancy. Here, we discover that depletion of the phosphate transceptor Pho84 across multiple fungal species encountered a substantial cost in gastrointestinal colonization. Mechanistically, Pho84 enhances the gastrointestinal commensalism via a dual-action activity, coordinating both phosphate uptake and TOR activation by induction of the transcriptional regulator Try4 and downstream commensalism-related transcription. As such, Pho84 promotes Candida albicans commensalism, but this does not translate into enhanced pathogenicity. Thus, our study uncovers a specific nutrient-dependent dual-action regulatory pathway for Pho84 on fungal commensalism.

Novel mineral regulatory pathways in ovine pregnancy: I. phosphate, klotho signaling, and sodium-dependent phosphate transporters

Appropriate mineralization of the fetal skeleton requires an excess of phosphate in the fetus compared to the mother. However, mechanisms for placental phosphate transport are poorly understood. This study aimed to identify phosphate regulatory pathways in ovine endometria and placentae throughout gestation. Suffolk ewes were bred with fertile rams upon visual detection of estrus (Day 0). On Days 9, 12, 17, 30, 70, 90, 110, and 125 of pregnancy (n = 3-14/Day), ewes were euthanized and hysterectomized. Phosphate abundance varied across gestational days in uterine flushings, allantoic fluid, and homogenized endometria and placentae (P < 0.05). The expression of mRNAs for sodium-dependent phosphate transporters (SLC20A1 and SLC20A2) and klotho signaling mediators (FGF7, FGF21, FGF23, FGFR1-4, KL, KLB, ADAM10, and ADAM17) were quantified by qPCR. Day 17 conceptus tissue expressed SLC20A1, SLC20A2, KLB, FGF7, FGF21, FGF23, FGFR1, and FGFR2 mRNAs. Both sodium-dependent phosphate transporters and klotho signaling mediators were expressed in endometria and placentae throughout gestation. Gestational day influenced the expression of SLC20A1, ADAM10, ADAM17, FGF21, FGFR1, and FGFR3 mRNAs in both endometria and placentae (P < 0.05). Gestational day influenced endometrial expression of FGF7 (P < 0.001), and placental expression of FGF23 (P < 0.05). Immunohistochemistry confirmed that both FGF23 and KL proteins were expressed in endometria and placentae throughout gestation. The observed spatiotemporal profile of KL-FGF signaling suggests a potential role in the establishment of pregnancy and regulation of fetal growth. This study provides a platform for further mechanistic investigation into the role for KL-FGF signaling in the regulation of phosphate transport at the ovine maternal-conceptus interface.

The Causes of Hypo- and Hyperphosphatemia in Humans

Phosphate homeostasis involves several major organs that are the skeleton, the intestine, the kidney, and parathyroid glands. Major regulators of phosphate homeostasis are parathormone, fibroblast growth factor 23, 1,25-dihydroxyvitamin D, which respond to variations of serum phosphate levels and act to increase or decrease intestinal absorption and renal tubular reabsorption, through the modulation of expression of transcellular transporters at the intestinal and/or renal tubular level. Any acquired or genetic dysfunction in these major organs or regulators may induce hypo- or hyperphosphatemia. The causes of hypo- and hyperphosphatemia are numerous. This review develops the main causes of acquired and genetic hypo- and hyperphosphatemia.

An Overview of the Pathophysiology of Vascular Calcification in Chronic Kidney Disease

Abnormalities of calcium–phosphate balance, with subsequent bone metabolism disorders, are among the key and earliest features of chronic kidney disease (CKD). Recently, another consequence of these abnormalities was brought to light—namely, vascular calcification. Most studies performed in patients on dialysis suggest that their vascular calcification is more advanced than that seen in the general population. Furthermore, the progression of vessel wall mineralization is much more dynamic in patients with CKD.

Apart from the commonly assessed factors that promote vascular calcification, such as age, duration of dialysis, or poor control of calcium–phosphate status, several other factors have recently been identified. In the spectrum of substances involved in the regulation of the process of soft-tissue calcification, the most extensively studied in the nephrology literature are bone morphogenetic protein 7, osteoprotegerin, matrix Gla protein, fetuin-A, and the phosphatonins. Better understanding of the mechanisms underlying excess vascular mineralization have led to the development of promising new therapies.

Dietary phosphorus restriction affects bone metabolism, vitamin D metabolism and rumen fermentation traits in sheep

Homeostasis of calcium (Ca) and phosphate (P_i ) is maintained by a concerted interplay of absorption and reabsorption via the gastrointestinal tract and the kidney and by storage and mobilization from the bone regulated mainly by parathyroid hormone (PTH), 1,25-dihydroxycholecalciferol and calcitonin. The present study aimed at characterizing the effects of dietary P restriction on bone, vitamin D metabolism and rumen fermentation traits reflecting the endogenous P cycle maintaining the ruminal P supply for microbial metabolism. The experiments were done in eleven female, non-pregnant, non-lactating four- to nine-year-old Black Headed Mutton sheep allotted to two feeding groups: "P-restricted" (0.11% P/kg DM and 0.88% Ca/kg DM) and "Control" (0.38% P/kg DM and 0.88% Ca/kg DM). Dietary P restriction did not lead to hypophosphataemia, probably due to a compensation by bone mobilization, demonstrated by increased serum concentrations of a resorption marker and altered gene expression in bone tissue. In addition, the RNA expression of fibroblast growth factor 23, a bone-derived factor involved in the regulation of vitamin D metabolism, was significantly reduced with dietary P restriction. Furthermore, several genes related to vitamin D metabolism and plasma concentrations of 1,25-(OH)₂ D were associated with serum concentrations of phosphate (P_i ). In the parotid gland, the expression of the P_i transporter NaPi2b was negatively associated with serum P_i and positively with parathyroid PTH expression. Although P_i concentrations in saliva and the gastrointestinal tract were significantly reduced, we found no adverse effects of the P-restricted ration on the production of short chain fatty acids, but slight differences in the production of butyrate as well as its relationship to rumen P_i and ammonia concentrations that might indicate an impact on ruminal fermentation.

Hypophosphataemic Rickets: Diagnosis Algorithm-How Not to Make a Mistake

Hypophosphataemic rickets is a heterogeneous group of entities characterized by rickets or osteomalacia due to a phosphate deficit caused mainly by decreased renal reabsorption. They are also characterized by defective intestinal absorption of calcium and rickets or osteomalacia unresponsive to cholecalciferol. These metabolic alterations lead to growth retardation, bone pain and deformities, and short stature. For a correct diagnosis and treatment of all forms of rickets, the basic aspects of pathophysiology of the calcium-phosphorus metabolism and the relevance of the bone-kidney axis modulated by the presence of phosphaturic agents need to be known. Diagnosis of these diseases includes clinical assessment, blood and urine analytical tests, and bone x-ray. The aim of this article is to briefly describe the pathophysiology, signs, symptoms, and clinical forms of hypophosphataemic rickets, proposing a diagnosis algorithm that can help in the clinical practice.

Review: Regulation of gastrointestinal and renal transport of calcium and phosphorus in ruminants

In comparison to monogastric animals, ruminants show some peculiarities in respect to the regulation of mineral homeostasis, which can be regarded as a concerted interplay between gastrointestinal absorption, renal excretion and bone mobilisation to maintain physiological Ca and phosphate (Pi) concentrations in serum. Intestinal absorption of Ca or Pi is mediated by two general mechanisms: paracellular, passive transport dominates when luminal Ca or Pi concentrations are high and transcellular. The contribution of active transport becomes more important when dietary Ca or Pi supply is restricted or the demand increased. Both pathways are modulated directly by dietary interventions, influenced by age and regulated by endocrine factors such as 1,25-dihydroxyvitamin D3. Similar transport processes are observed in the kidney. After filtration, Ca and Pi are resorbed along the nephron. However, as urinary Ca and Pi excretion is very low in ruminants, the regulation of these renal pathways differs from that described for monogastric species, too. Furthermore, salivary secretion, as part of endogenous Pi recycling, and bone mobilisation participate in the maintenance of Ca and Pi homeostasis in ruminants. Saliva contains large amounts of Pi for buffering rumen pH and to ensure optimal conditions for the rumen microbiome. The skeleton is a major reservoir of Ca and Pi to compensate for discrepancies between demand and uptake. But alterations of the regulation of mineral homeostasis induced by other dietary factors such as a low protein diet were observed in growing ruminants. In addition, metabolic changes, for example, at the onset of lactation have pronounced effects on gastrointestinal mineral transport processes in some ruminant species. As disturbances of mineral homeostasis do not only increase the risk of the animals to develop other diseases, but are also associated with protein and energy metabolism, further research is needed to improve our knowledge of its complex regulation.

Phosphate in Virulence of Candida albicans and Candida glabrata

Candida species are the most commonly isolated invasive human fungal pathogens. A role for phosphate acquisition in their growth, resistance against host immune cells, and tolerance of important antifungal medications is becoming apparent. Phosphorus is an essential element in vital components of the cell, including chromosomes and ribosomes. Producing the energy currency of the cell, ATP, requires abundant inorganic phosphate. A comparison of the network of regulators and effectors that controls phosphate acquisition and intracellular distribution, the PHO regulon, between the model yeast Saccharomyces cerevisiae, a plant saprobe, its evolutionarily close relative C. glabrata, and the more distantly related C. albicans, highlights the need to coordinate phosphate homeostasis with adenylate biosynthesis for ATP production. It also suggests that fungi that cope with phosphate starvation as they invade host tissues, may link phosphate acquisition to stress responses as an efficient mechanism of anticipatory regulation. Recent work indicates that connections among the PHO regulon, Target of Rapamycin Complex 1 signaling, oxidative stress management, and cell wall construction are based both in direct signaling links, and in the provision of phosphate for sufficient metabolic intermediates that are substrates in these processes. Fundamental differences in fungal and human phosphate homeostasis may offer novel drug targets.

Phosphaturic Mesenchymal Tumor of Soft Tissue of the Foot: Report of a Case With Review of the Literature

Phosphaturic mesenchymal tumor (PMT) is a rare neoplasm that ectopically secretes fibroblast growth factor 23, a bone cell-derived protein that regulates phosphate homeostasis. The overproduction of fibroblast growth factor 23 causes a paraneoplastic syndrome characterized by hyperphosphaturia, hypophosphatemia, hypovitaminosis D, and vitamin D refractory rickets/osteomalacia, effects that disappear with tumor removal. The PMT may occur in several anatomic regions, mainly in the limbs, usually involving both soft tissue and bone. Acral locations occur in 10% to 15% of the cases, mostly in the feet, with 95 cases reported in this anatomic region to date. We report a case of a PMT in a young adult male who presented in 2007 with the classic constellation of signs and symptoms. A small soft-tissue tumor was detected in his right heel, 3 years after exhaustively seeking for it by various imaging techniques performed at different institutions. Before the tumor was detected, attempts to manage this patient's osteomalacia with phosphate and vitamin D (both calcitriol and ergocalciferol) supplementation were unsuccessful. Following surgical resection, the patient experienced prompt correction of the phosphaturia and gradual reconstitution of his bone mineralization. The pathologic diagnosis was (benign) PMT, mixed connective tissue type. In 2019, 12 years after resection, the patient is asymptomatic, and his bone mineral homeostasis has been restored.

Role of fibroblast growth factor 23 and klotho cross talk in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive fibrosing interstitial pneumonia that mainly affects the elderly. Several reports have demonstrated that aging is involved in the underlying pathogenic mechanisms of IPF. α-Klotho (KL) has been well characterized as an "age-suppressing" hormone and can provide protection against cellular senescence and oxidative stress. In this study, KL levels were assessed in human plasma and primary lung fibroblasts from patients with idiopathic pulmonary fibrosis (IPF-FB) and in lung tissue from mice exposed to bleomycin, which showed significant downregulation when compared with controls. Conversely, transgenic mice overexpressing KL were protected against bleomycin-induced lung fibrosis. Treatment of human lung fibroblasts with recombinant KL alone was not sufficient to inhibit transforming growth factor-β (TGF-β)-induced collagen deposition and inflammatory marker expression. Interestingly, fibroblast growth factor 23 (FGF23), a proinflammatory circulating protein for which KL is a coreceptor, was upregulated in IPF and bleomycin lungs. To our surprise, FGF23 and KL coadministration led to a significant reduction in fibrosis and inflammation in IPF-FB; FGF23 administration alone or in combination with KL stimulated KL upregulation. We conclude that in IPF downregulation of KL may contribute to fibrosis and inflammation and FGF23 may act as a compensatory antifibrotic and anti-inflammatory mediator via inhibition of TGF-β signaling. Upon restoration of KL levels, the combination of FGF23 and KL leads to resolution of inflammation and fibrosis. Altogether, these data provide novel insight into the FGF23/KL axis and its antifibrotic/anti-inflammatory properties, which opens new avenues for potential therapies in aging-related diseases like IPF.

Hyperphosphatemic Familial Tumoral Calcinosis With Galnt3 Mutation: Transient Response to Anti-Interleukin-1 Treatments

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a rare autosomal recessive disease caused by mutations in genes involved in phosphate homeostasis and characterized by high serum phosphate concentration and occurrence of ectopic calcifications. Management of the disease includes lowering of phosphate concentration and, when clinically necessary, debulking surgery of calcifications. In addition, high inflammatory disease flares can occur. Our case is about a patient with GALNT3 mutation and several localizations of refractory calcinosis. Assuming HFTC acts like an auto-inflammatory syndrome, we report the effect of anti-interleukine-1 therapies on the evolution of the disease. Anakinra (100 mg, then 200 mg subcutaneous daily) and canakinumab (300 mg every 4 weeks) were sequentially given to the patient. Anti-IL-1 therapy was effective in controlling inflammatory flares; however, it did not prevent extension of calcinosis. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare paraneoplastic syndrome characterized by hypophosphatemia resulting from decreased tubular phosphate reabsorption, with a low or inappropriately normal level of active vitamin D. The culprit tumors of TIO could produce fibroblast growth factor 23 which plays a role in regulating renal Pi handling and 25-hydroxyvitamin D 1α-hydroxylase activity. Chronic hypophosphatemia could eventually lead to inadequate bone mineralization, presenting as osteomalacia. The diagnosis should be considered when patients manifest as hypophosphatemia and osteomalacia, or rickets and needs to be differentiated from other disorders of phosphate metabolism, such as the inhereditary diseases like X-linked hypophosphataemic rickets, autosomal dominant hypophosphataemic rickets, autosomal recessive hypophosphataemic rickets and acquired diseases like vitamin D deficiency. Localization of responsible tumors could be rather difficult since the vast majority are very small and could be everywhere in the body. A combination of thorough physical examination, laboratory tests and imaging techniques should be applied and sometimes a venous sampling may come into handy. The technology of somatostatin-receptor functional scintigraphy markedly facilitates the localization of TIO tumor. Patients undergoing complete removal of the causative neoplasm generally have favorable prognoses while a few have been reported to suffer from recurrence and metastasis. For those undetectable or unresectable cases, phosphate supplements and active vitamin D should be administrated and curative intended radiotherapy or ablation is optional.

Fibroblast growth factor 23 and Klotho contribute to airway inflammation

Circulating levels of fibroblast growth factor (FGF)23 are associated with systemic inflammation and increased mortality in chronic kidney disease. α-Klotho, a co-receptor for FGF23, is downregulated in chronic obstructive pulmonary disease (COPD). However, whether FGF23 and Klotho-mediated FGF receptor (FGFR) activation delineates a pathophysiological mechanism in COPD remains unclear. We hypothesised that FGF23 can potentiate airway inflammation via Klotho-independent FGFR4 activation.FGF23 and its effect were studied using plasma and transbronchial biopsies from COPD and control patients, and primary human bronchial epithelial cells isolated from COPD patients as well as a murine COPD model.Plasma FGF23 levels were significantly elevated in COPD patients. Exposure of airway epithelial cells to cigarette smoke and FGF23 led to a significant increase in interleukin-1β release via Klotho-independent FGFR4-mediated activation of phospholipase Cγ/nuclear factor of activated T-cells signalling. In addition, Klotho knockout mice developed COPD and showed airway inflammation and elevated FGFR4 expression in their lungs, whereas overexpression of Klotho led to an attenuation of airway inflammation.Cigarette smoke induces airway inflammation by downregulation of Klotho and activation of FGFR4 in the airway epithelium in COPD. Inhibition of FGF23 or FGFR4 might serve as a novel anti-inflammatory strategy in COPD.

Fibroblast Growth Factor 23 (FGF23) and the kidney

Phosphate homeostasis in humans is a complex phenomenon involving the interplay of several different organs and circulating hormones. Among the latter, parathyroid hormone (PTH), and vitamin D3 (Vit D3) were thought to be the main regulators of serum phosphate concentration since they mediated the intestinal, renal and bone responses that follow fluctuations in serum phosphate levels. The study of three rare disorders – tumor-induced osteomalacia (TIO), autosomal dominant hypophosphatemic rickets (ADHR) and X-linked hypophosphatemic rickets (XLH) – has offered a completely new insight into phosphate metabolism by unraveling the role of a group of peptides that can directly affect serum phosphate concentration by increasing urinary phosphate excretion. Fibroblast growth factor-23 (FGF-23) is the most extensively studied “phosphatonin”. The production, mechanism of action, effects in various target tissues, and its role in common clinical disorders are the focus of this review.

The age-related changes of dietary phosphate responsiveness in plasma 1,25-dihydroxyvitamin D levels and renal Cyp27b1 and Cyp24a1 gene expression is associated with renal α-Klotho gene expression in mice

In this study, we investigated the relationship between age-related changes in renal α-Klotho gene expression, vitamin D metabolism and the responsiveness of dietary phosphate in 1, 2 and 13 month-old mice fed a high phosphate (phosphate 1.2%) diet or low phosphate (phosphate 0.02%) diet for 5 days. We found that 1,25-dihydroxyvitamin D levels in plasma were significantly lower in the high phosphate group than the low phosphate group for 1 and 2 month-old mice, but not 13 month-old mice. In addition, in the high phosphate group plasma 1,25-dihydroxyvitamin D levels were decreased in 2 month-old mice relative to 1 month-old mice, but 13 month-old mice had higher levels than 2 month-old mice. In fact, plasma 1,25-dihydroxyvitamin D levels showed a significant correlation with vitamin D metabolism gene Cyp27b1 and Cyp24a1 mRNA expression in the high phosphate group. Interestingly, renal α-Klotho mRNA and protein levels were significant change with age. Furthermore, α-Klotho mRNA expression showed a significant negative correlation with plasma 1,25-dihydroxyvitamin D levels in the high phosphate group. Our results suggest that age-related alterations in renal α-Klotho expression could affect the responsiveness of dietary phosphate to vitamin D metabolism.

Localization of Oncogenic Osteomalacia by Systemic Venous Sampling of Fibroblast Growth Factor 23

PURPOSE

Tumor-induced osteomalacia (TIO) is characterized by hypophosphatemia caused by a phosphaturic mesenchymal tumor. While surgical resection of the tumor leads to a cure, identification of the responsible tumor is challenging. Recently, several studies showed that systemic sampling of fibroblast growth factor 23 (FGF23) is helpful for localization of tumors. The present study aimed to evaluate the clinical utility of this method in Korean patients.

MATERIALS AND METHODS

Six patients compatible with TIO who were admitted to our hospital between 2006 and 2015 were analyzed. Systemic venous sampling of FGF23 was performed to detect blind lesions or to confirm a suspicious lesion identified in previous imaging studies.

RESULTS

Venous sampling helped confirming the tumor in five of the six patients. Three patients underwent surgery after sampling, and in two patients, the lesions were detected after 3 years by means of ⁶⁸Ga-DOTATOC positron emission tomography with computed tomography. In one patient, there was a local elevation of serum FGF23 without any related lesion on additional imaging.

CONCLUSION

Our data strengthened the value of venous sampling of FGF23 in predicting the location of tumors and suggested that it can be more effective in the presence of the relevant lesion in subsequent imaging analyses.

FGF23-αKlotho as a paradigm for a kidney-bone network

The vertebrate endoskeleton is not a mere frame for muscle attachment to facilitate locomotion, but is a massive organ integrated with many physiologic functions including mineral and energy metabolism. Mineral balance is maintained by tightly controlled ion fluxes that are external (intestine and kidney) and internal (between bone and other organs), and are regulated and coordinated by many endocrine signals between these organs. The endocrine fibroblast growth factors (FGFs) and Klotho gene families are complex systems that co-evolved with the endoskeleton. In particular, FGF23 and αKlotho which are primarily derived from bone and kidney respectively, are critical in maintaining mineral metabolism where each of these proteins serving highly diverse roles; abound with many unanswered questions regarding their upstream regulation and downstream functions. Genetic lesions of components of this network produce discreet disturbances in many facets of mineral metabolism. One acquired condition with colossal elevations of FGF23 and suppression of αKlotho is chronic kidney disease where multiple organ dysfunction contributes to the morbidity and mortality. However, the single most important group of derangements that encompasses the largest breadth of complications is mineral metabolism disorders. Mineral metabolic disorders in CKD impact negatively and significantly on the progression of renal disease as well as extra-renal complications. Knowledge of the origin, nature, and impact of phosphate, FGF23, and αKlotho derangements is pivotal to understanding the pathophysiology and treatment of CKD.

De Novo Synthesis and Functional Analysis of Polyphosphate-Loaded Poly(Ethylene) Glycol Hydrogel Nanoparticles Targeting Pyocyanin and Pyoverdin Production in Pseudomonas aeruginosa as a Model Intestinal Pathogen

The human gastrointestinal tract is the primary site of colonization of multidrug resistant pathogens and the major source of life-threatening complications in critically ill and immunocompromised patients. Eradication measures using antibiotics carry further risk of antibiotic resistance. Furthermore, antibiotic treatment can adversely shift the intestinal microbiome toward domination by resistant pathogens. Therefore, approaches directed to prevent replacement of health promoting microbiota with resistant pathogens should be developed. The use of non-microbicidal drugs to create microenvironmental conditions that suppress virulence of pathogens is an attractive strategy to minimize the negative consequences of intestinal microbiome disruption. We have previously shown that phosphate is depleted in the intestinal tract following surgical injury, that this depletion is a major "cue" that triggers bacterial virulence, and that the maintenance of phosphate abundance prevents virulence expression. However, the use of inorganic phosphate may not be a suitable agent to deliver to the site of the host-pathogen interaction since it is readily adsorbed in small intestine. Here we propose a novel drug delivery approach that exploits the use of nanoparticles that allow for prolonged release of phosphates. We have synthesized phosphate (Pi) and polyphosphate (PPi) crosslinked poly (ethylene) glycol (PEG) hydrogel nanoparticles (NP-Pi and NP-PPi, respectively) that result in sustained delivery of Pi and PPi. NP-PPi demonstrated more prolonged release of PPi as compared to the release of Pi from NP-Pi. In vitro studies indicate that free PPi as well NP-PPi are effective compounds for suppressing pyoverdin and pyocyanin production, two global virulence systems of virulence of P. aeruginosa. These studies suggest that sustained release of polyphosphate from NP-PPi can be exploited as a target for virulence suppression of lethal pathogenic phenotypes in the gastrointestinal tract.

Phosphate Toxicity in CKD: The Killer among Us

Maintenance of a normal serum phosphate level depends on absorption in the gut, reabsorption and excretion by the kidney, and the flux between the extracellular and skeletal pools. Phosphate homeostasis is a coordinated, complex system of crosstalk between the bone, intestine, kidney, and parathyroid gland. Dysfunction of this system has serious clinical consequences in healthy individuals and those with conditions, such as CKD, in which hyperphosphatemia is associated with increased risks of cardiovascular morbidity and mortality. The last half-century of renal research has helped define the contribution of the parathyroid hormone, calcitriol, fibroblast growth factor 23, and Klotho in the regulation of phosphate. However, despite new discoveries and insights gained during this time, what remains unchanged is the recognition that phosphate retention is the initiating factor for the development of many of the complications observed in CKD, namely secondary hyperparathyroidism and bone and cardiovascular diseases. Controlling phosphate load remains the primary goal in the treatment of CKD. This review discusses the clinical effects of dysregulated phosphate metabolism, particularly in CKD, and its association with cardiovascular disease. The importance of early control of phosphate load in the treatment of CKD is emphasized, and the latest research in the treatment of phosphate retention is discussed.

Bone mineral disorder in chronic kidney disease: Klotho and FGF23; cardiovascular implications

Cardiovascular factors are one of the main causes of morbidity and mortality in patients with chronic kidney disease. Bone mineral metabolism disorders and inflammation are pathological conditions that involve increased cardiovascular risk in chronic kidney disease. The cardiovascular risk involvement of bone mineral metabolism classical biochemical parameters such as phosphorus, calcium, vitamin D and PTH is well known. The newest markers, FGF23 and klotho, could also be implicated in cardiovascular disease.

The Role and Mechanism of α-Klotho in the Calcification of Rat Aortic Vascular Smooth Muscle Cells

OBJECTIVE

To investigate the role and possible mechanism of α-Klotho in the calcification and the osteogenic transition of cultured VSMCs.

METHODS

VSMCs were cultured in vitro and divided into 5 groups, each using a different medium: (1) control; (2) β-GP; (3) β-GP + Klotho; (4) β-GP + LiCl; (5) β-GP + Klotho + LiCl. Calcium deposits were visualized using Alizarin Red S staining. The calcium concentrations were determined by the o-cresolphthalein complexone method. BMP2, Runx2 and β-catenin levels were estimated by western blotting, and the level of α-SMA was determined by using immunofluorescence at day 12.

RESULTS

β-GP induced an increase in the expression of BMP2, Runx2, and β-catenin. The calcium content increased, and the expression of α-SMA decreased. Alizarin Red S staining was positive under the high phosphorus conditions. BMP2, Runx2, and β-catenin levels and the calcium content decreased when the cells were cultured with rmKlotho; however, the levels of each were upregulated after treatment with the LiCl.

CONCLUSIONS

Klotho can ameliorate the calcification and osteogenic transition of VSMCs induced by β-GP. The mechanism of Klotho in preventing calcification in VSMCs may be partially mediated by the inhibition of the Wnt/β-catenin signaling pathway.

Estrogen directly and specifically downregulates NaPi-IIa through the activation of both estrogen receptor isoforms (ERα and ERβ) in rat kidney proximal tubule

We have previously demonstrated that estrogen (E2) downregulates phosphate transporter NaPi-IIa and causes phosphaturia and hypophosphatemia in ovariectomized rats. In the present study, we examined whether E2 directly targets NaPi-IIa in the proximal tubule (PT) and studied the respective roles of estrogen receptor isoforms (ERα and ERβ) in the downregulation of NaPi-IIa using both in vivo and an in vitro expression systems. We found that estrogen specifically downregulates NaPi-IIa but not NaPi-IIc or Pit2 in the kidney cortex. Proximal tubules incubated in a "shake" suspension with E2 for 24 h exhibited a dose-dependent decrease in NaPi-IIa protein abundance. Results from OVX rats treated with specific agonists for either ERα [4,4',4″;-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol, PPT] or ERβ [4,4',4″-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol, DPN] or both (PPT + DPN), indicated that only the latter caused a sharp downregulation of NaPi-IIa, along with significant phosphaturia and hypophosphatemia. Lastly, heterologous expression studies demonstrated that estrogen downregulated NaPi-IIa only in U20S cells expressing both ERα and ERβ, but not in cells expressing either receptor alone. In conclusion, these studies demonstrate that rat PT cells express both ERα and ERβ and that E2 induces phosphaturia by directly and specifically targeting NaPi-IIa in the PT cells. This effect is mediated via a mechanism involving coactivation of both ERα and ERβ, which likely form a functional heterodimer complex in the rat kidney proximal tubule.

Effect of serum fibroblast growth factor-23, matrix Gla protein and Fetuin-A in predicting osteoporosis in maintenance hemodialysis patients

This study is aimed at exploring the role of serum fibroblast growth factor-23 (FGF-23), matrix Gla (MGP) and Fetuin-A in the calcium-phosphate metabolism and estimate the value of serum FGF-23, MGP and Fetuin-A levels in predicting osteoporosis in maintenance hemodialysis (MHD) patients. This study included 64 patients who receive hemodialysis in our hospital. The serum FGF-23, MGP and Fetuin-A were analyzed by enzyme-linked immunosorbent assay (ELlSA). Bone mineral density (BMD) at the femoral neck was measured by dual-energy X-ray absorptiometry. The 64 patients (30 males, 34 females, 60.6 ± 11.3 years of age) received an average of 6.88 ± 2.94 years of dialysis. Body mass index (BMI), Kt/V, dialysis vintage, patient age, serum levels of FGF-23, Fetuin-A, bone isoenzyme of alkaline phosphatase (ALP-B), and calcium were different in statistical significance among the three groups of patients in terms of normal bone mass (N = 10), osteopenia (N = 24), or osteoporosis (N = 30). BMI, Kt/V, ALP-B, dialysis vintage and serum Fetuin-A level were identified as independent variables of femoral neck BMD by stepwise multiple regression analysis. The area under ROC curve showed that serum Fetuin-A was useful for identifying osteoporosis in MHD patients. The cutoff value corresponding to the highest Youden's index was serum Fetuin-A ≤ 89 μg/mL, which was defined as the optimal predictor of osteoporosis. Its sensitivity/specificity was 71%/77.8%. The incidence of osteoporosis is high in MHD patients. Serum Fetuin-A level is closely correlated with osteoporosis and it may serve as a predictor of osteoporosis.

A phosphate-centric paradigm for pathophysiology and therapy of chronic kidney disease

Extracellular phosphate is toxic to the cell at high concentrations. When the phosphate level is increased in the blood by impaired urinary phosphate excretion, premature aging ensues. When the phosphate level is increased in the urine by dietary phosphate overload, this may lead to kidney damage (tubular injury and interstitial fibrosis). Extracellular phosphate exerts its cytotoxicity when it forms insoluble nanoparticles with calcium and fetuin-A, referred to as calciprotein particles (CPPs). CPPs are highly bioactive ligands that can induce various cellular responses, including osteogenic transformation of vascular smooth muscle cells and cell death in vascular endothelium and renal tubular epithelium. CPPs are detected in the blood of animal models and patients with chronic kidney disease (CKD) and associated with adaptation of the endocrine axes mediated by fibroblast growth factor-23 (FGF23) and Klotho that regulate mineral metabolism and aging. These observations have raised the possibility that CPPs may contribute to the pathophysiology of CKD. This notion, if validated, is expected to provide new diagnostic and therapeutic targets for CKD.

Hypophosphatemic vitamin D-resistant osteomalacia: A case report

This study concerns the case of a 59-year-old woman who was admitted to hospital after experiencing systemic bone pain and muscle weakness for more than 2 years. The patient was diagnosed with numerous fractures via bone imaging emission computed tomography (ECT) and hypophosphatemia. Laboratory results showed a high alkaline phosphatase concentration. Thyroid nodules were visible on the thyroid and parathyroid glands during ultrasound examinations. However, no symptomatic or biochemical improvement was observed following a thyroid nodulectomy. The patient was administered large doses of neutral phosphate preparations, vitamin D3 and calcium. A gradual improvement in the condition of the patient was observed. For bone pain associated with multiple fractures in elderly patients, calcium and phosphorus metabolism disorders, and active vitamin D deficiency should be considered and an early diagnosis should be performed.

Sclerostin alters serum vitamin D metabolite and fibroblast growth factor 23 concentrations and the urinary excretion of calcium

Inactivating mutations of the SOST (sclerostin) gene are associated with overgrowth and sclerosis of the skeleton. To determine mechanisms by which increased amounts of calcium and phosphorus are accreted to enable enhanced bone mineralization in the absence of sclerostin, we measured concentrations of calciotropic and phosphaturic hormones, and urine and serum calcium and inorganic phosphorus in mice in which the sclerostin (sost) gene was replaced by the β-D-galactosidase (lacZ) gene in the germ line. Knockout (KO) (sost(-/-)) mice had increased bone mineral density and content, increased cortical and trabecular bone thickness, and greater net bone formation as a result of increased osteoblast and decreased osteoclast surfaces compared with wild-type (WT) mice. β-Galactosidase activity was detected in osteocytes of sost KO mice but was undetectable in WT mice. Eight-week-old, male sost KO mice had increased serum 1α,25-dihydroxyvitamin D, decreased 24,25-dihydroxyvitamin D, decreased intact fibroblast growth factor 23, and elevated inorganic phosphorus concentrations compared with age-matched WT mice. 25-Hydroxyvitamin D 1α-hydroxylase cytochrome P450 (cyp27B1) mRNA was increased in kidneys of sost KO mice compared with WT mice. Treatment of cultured proximal tubule cells with mouse recombinant sclerostin decreased cyp27B1 mRNA transcripts. Urinary calcium and renal fractional excretion of calcium were decreased in sost KO mice compared with WT mice. Sost KO and WT mice had similar serum calcium and parathyroid hormone concentrations. The data show that sclerostin not only alters bone mineralization, but also influences mineral metabolism by altering concentrations of hormones that regulate mineral accretion.

Protective roles of DMP1 in high phosphate homeostasis

PURPOSE

Dmp1 (dentin matrix protein1) null mice (Dmp1(-/-)) display hypophosphatemic rickets with a sharp increase in fibroblast growth factor 23 (FGF23). Disruption of Klotho (the obligatory co-receptor of FGF23) results in hyperphosphatemia with ectopic calcifications formed in blood vessels and kidneys. To determine the role of DMP1 in both a hyperphosphatemic environment and within the ectopic calcifications, we created Dmp1/Klotho compound deficient (Dmp1(-/-)kl/kl) mice.

PROCEDURES

A combination of TUNEL, immunohistochemistry, TRAP, von Kossa, micro CT, bone histomorphometry, serum biochemistry and Scanning Electron Microscopy techniques were used to analyze the changes in blood vessels, kidney and bone for wild type control, Dmp1(-/-), Klotho deficient (kl/kl) and Dmp1(-/-)kl/kl animals.

FINDINGS

Interestingly, Dmp1(-/-)kl/kl mice show a dramatic improvement of rickets and an identical serum biochemical phenotype to kl/kl mice (extremely high FGF23, hyperphosphatemia and reduced parathyroid hormone (PTH) levels). Unexpectedly, Dmp1(-/-)kl/kl mice presented elevated levels of apoptosis in osteocytes, endothelial and vascular smooth muscle cells in small and large blood vessels, and within the kidney as well as dramatic increase in ectopic calcification in all these tissues, as compared to kl/kl.

CONCLUSION

These findings suggest that DMP1 has an anti-apoptotic role in hyperphosphatemia. Discovering this novel protective role of DMP1 may have clinical relevance in protecting the cells from apoptosis in high-phosphate environments as observed in chronic kidney disease (CKD).

Distribution and regulation of the 25-hydroxyvitamin D3 1α-hydroxylase in human parathyroid glands

Parathyroid glands express the 25-hydroxyvitamin D(3) 1α-hydroxylase (1αOHase). 1,25-dihydroxyvitamin D(3) (calcitriol) synthesized by extrarenal tissues generally does not enter the circulation, but plays an autocrine/paracrine role specific to the cell type, and is regulated by the needs of that particular cell. While the role of calcitriol produced in the parathyroid glands presumably is to suppress PTH and cell growth, its regulation in this cell type has not been defined. In the present study, we found that regulation of the human parathyroid 1αOHase differs from the renal enzyme in that it is induced by FGF-23 and extracellular calcium. Hyperplastic parathyroid glands from patients with chronic kidney failure normally display a heterogeneous cellularity. We found that the 1αOHase is expressed at much higher levels in oxyphil cells than in chief cells in these patients. Recent findings indicate that oxyphil cell content is increased by treatment with calcium receptor activators (calcimimetics). Here, we demonstrate that the calcimimetic cinacalcet increases the expression of 1αOHase in human parathyroid cultures. Additionally, we found that the 1αOHase in human parathyroid cultures is functionally active, as evidenced by the ability of the enzyme to 1-hydroxylate 25(OH)D(3) in parathyroid monolayers. Calcium, as well as cinacalcet, also induced expression of the degradation enzyme 24-hydroxylase, indicating the presence of a negative feedback system in the parathyroid cells. Therefore, local production of 1αOHase suggests an autocrine/paracrine role in regulating parathyroid function and may mediate, in part, the suppression of PTH by calcium and FGF-23.

Phosphate inhibits in vitro Fe3+ loading into transferrin by forming a soluble Fe(III)-phosphate complex: a potential non-transferrin bound iron species

In chronic kidney diseases, NTBI can occur even when total iron levels in serum are low and transferrin is not saturated. We postulated that elevated serum phosphate concentrations, present in CKD patients, might disrupt Fe(3+) loading into apo-transferrin by forming Fe(III)-phosphate species. We report that phosphate competes with apo-transferrin for Fe(3+) by forming a soluble Fe(III)-phosphate complex. Once formed, the Fe(III)-phosphate complex is not a substrate for donating Fe(3+) to apo-transferrin. Phosphate (1-10mM) does not chelate Fe(III) from diferric transferrin under the conditions examined. Complexed forms of Fe(3+), such as iron nitrilotriacetic acid (Fe(3+)-NTA), and Fe(III)-citrate are not susceptible to this phosphate complexation reaction and efficiently deliver Fe(3+) to apo-transferrin in the presence of phosphate. This reaction suggests that citrate might play an important role in protecting against Fe(III), phosphate interactions in vivo. In contrast to the reactions of Fe(3+) and phosphate, the addition of Fe(2+) to a solution of apo-transferrin and phosphate lead to rapid oxidation and deposition of Fe(3+) into apo-transferrin. These in vitro data suggest that, in principle, elevated phosphate concentrations can influence the ability of apo-transferrin to bind iron, depending on the oxidation state of the iron.

Host stress and virulence expression in intestinal pathogens: development of therapeutic strategies using mice and C. elegans

The intestinal tract of a host exposed to extreme physiologic stress and modern medical intervention represents a relatively unexplored yet important area of infection research, given the frequency with which this site becomes colonized by highly pathogenic microorganisms that cause subsequent sepsis. Our laboratory has focused on the host tissue derived environmental cues that are released into the intestinal tract during extreme physiologic stress that induce the expression of virulence in colonizing pathogens with the goal of developing novel gut directed therapies that maintain host pathogen neutrality through the course of host stress. Here we demonstrate that maintenance of phosphate sufficiency/ abundance within the intestinal microenvironment may be considered as a universal strategy to prevent virulence activation across a broad range of pathogens that colonize the gut and cause sepsis, given that phosphate depletion occurs following stress and is a universal cue that activates the virulence of a wide variety of organisms. Using small animal models (Caenorhabditis elegans and mice) to create local phosphate depletion at sites of colonization of Pseudomonas aeruginosa, a common cause of lethal gut-derived sepsis, we demonstrate the importance of maintaining phosphate sufficiency to suppress the expression of a lethal phenotype during extreme physiologic stress. The molecular details and potential therapeutic implications are reviewed.

Phosphate Metabolism in Cardiorenal Metabolic Disease

Hyperphosphatemia is a major risk factor for cardiovascular disease, abnormalities of mineral metabolism and bone disease, and the progression of renal insufficiency in patients with chronic renal disease. In early renal disease, serum phosphate levels are maintained within the 'normal laboratory range' by compensatory increases in phosphaturic hormones such as fibroblast growth factor-23 (FGF-23). An important co-factor for FGF-23 is Klotho; a deficiency in Klotho plays an important role in the pathogenesis of hyperphosphatemia, renal tubulointerstitial disease, and parathyroid and bone abnormalities. Clinical hyperphosphatemia occurs when these phosphaturic mechanisms cannot counterbalance nephron loss. Hyperphosphatemia is associated with calcific uremic arteriolopathy and uremic cardiomyopathy, which may explain, in part, the epidemiologic connections between phosphate excess and cardiovascular disease. However, no clinical trials have been conducted to establish a causal relationship, and large, randomized trials with hard endpoints are urgently needed to prove or disprove the benefits and risks of therapy. In summary, hyperphosphatemia accelerates renal tubulointerstitial disease, renal osteodystrophy, as well as cardiovascular disease, and it is an important mortality risk factor in patients with chronic kidney disease.