Biological activity of FGF-23 fragments

Exploring endocrine FGFs - structures, functions and biomedical applications

The family of fibroblast growth factors (FGFs) consists of 22 members with diverse biological functions in cells, from cellular development to metabolism. The family can be further categorized into three subgroups based on their three modes of action. FGF19, FGF21, and FGF23 are endocrine FGFs that act in a hormone-like/endocrine manner to regulate various metabolic activities. However, all three members of the endocrine family require both FGF receptors (FGFRs) and klotho co-receptors to elicit their functions. α-klotho and β-klotho act as scaffolds to bring endocrine FGFs closer to their receptors (FGFRs) to form active complexes. Numerous novel studies about metabolic FGFs' structures, mechanisms, and physiological insights have been published to further understand the complex molecular interactions and physiological activities of endocrine FGFs. Herein, we aim to review the structures, physiological functions, binding mechanisms to cognate receptors, and novel biomedical applications of endocrine FGFs in recent years.

Vitamin D Supplementation for 12 Months in Older Adults Alters Regulators of Bone Metabolism but Does Not Change Wnt Signaling Pathway Markers

Vitamin D status and supplementation regulates bone metabolism and may modulate Wnt signaling. We studied the response of hormonal regulators of bone metabolism, markers of Wnt signaling and bone turnover and bone mineral density (BMD) and bone mineral content (BMC) in a randomized vitamin D intervention trial (12,000 IU, 24,000 IU, 48,000 IU/mo for 1 year; men and women aged >70 years; n = 379; ISRCTN35648481). Associations with total and free 25(OH)D concentrations were analyzed by linear regression. Baseline vitamin D status was (mean ± SD) 25(OH)D: 40.0 ± 20.1 nmol/L. Supplementation dose-dependently increased total and free 25(OH)D concentrations and decreased plasma phosphate and parathyroid hormone (PTH) (all p < 0.05). The procollagen 1 intact N-terminal (PINP)/C-terminal telopeptide (CTX) ratio, C-terminal fibroblast growth factor-23 (cFGF23), and intact FGF23 (iFGF23) significantly increased with no between-group differences, whereas Klotho was unchanged. 1,25(OH)₂D and PINP significantly increased in the 24 IU and 48,000 IU groups. Sclerostin (SOST), osteoprotegerin (OPG), receptor activator of NF-κB ligand (RANKL), BMD, BMC, and CTX remained unchanged. Subgroup analyses with baseline 25(OH)D <25 nmol/L (n = 94) provided similar results. Baseline total and free 25(OH)D concentrations were positively associated with 1,25(OH)₂D, 24,25(OH)₂D (p < 0.001), vitamin D binding protein (DBP) (p < 0.05), BMD, and BMC (p < 0.05). Associations with PTH (p <0.001), cFGF23 (p < 0.01), and BAP (p < 0.05) were negative. After supplementation, total and free 25(OH)D concentrations remained positively associated only with 24,25(OH)₂D (p < 0.001) and DBP (p < 0.001) and negatively with estimated glomerular filtration rate (eGFR) (p < 0.01). PTH and SOST were significantly associated only with free 25(OH)D. There were no significant relationships with BMD and BMC after supplementation. The decrease in PTH and increase in PINP/CTX ratio suggest a protective effect of supplementation on bone metabolism, although no significant effect on BMD or pronounced changes in regulators of Wnt signaling were found. The increase in FGF23 warrants caution because of its negative association with skeletal and cardiovascular health. Associations of total and free 25(OH)D with biomarkers were similar and known positive associations between vitamin D status and BMD were confirmed. The change in associations after supplementation might suggest a threshold effect. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The Saga of Endocrine FGFs

Fibroblast growth factors (FGFs) are cell-signaling proteins with diverse functions in cell development, repair, and metabolism. The human FGF family consists of 22 structurally related members, which can be classified into three separate groups based on their action of mechanisms, namely: intracrine, paracrine/autocrine, and endocrine FGF subfamilies. FGF19, FGF21, and FGF23 belong to the hormone-like/endocrine FGF subfamily. These endocrine FGFs are mainly associated with the regulation of cell metabolic activities such as homeostasis of lipids, glucose, energy, bile acids, and minerals (phosphate/active vitamin D). Endocrine FGFs function through a unique protein family called klotho. Two members of this family, α-klotho, or β-klotho, act as main cofactors which can scaffold to tether FGF19/21/23 to their receptor(s) (FGFRs) to form an active complex. There are ongoing studies pertaining to the structure and mechanism of these individual ternary complexes. These studies aim to provide potential insights into the physiological and pathophysiological roles and therapeutic strategies for metabolic diseases. Herein, we provide a comprehensive review of the history, structure–function relationship(s), downstream signaling, physiological roles, and future perspectives on endocrine FGFs.

Antenatal iron supplementation, FGF23, and bone metabolism in Kenyan women and their offspring: secondary analysis of a randomized controlled trial

BACKGROUND

Fibroblast growth factor-23 (FGF23) regulates body phosphate homeostasis primarily by increasing phosphaturia. It also acts as a vitamin D-regulating hormone. Maternal iron deficiency is associated with perturbed expression and/or regulation of FGF23 and hence might be implicated in the pathogenesis of hypophosphatemia-driven rickets in their offspring.

OBJECTIVES

We aimed to determine the effect of antenatal oral iron supplementation on FGF23 concentration and maternal and infant markers of bone-mineral regulation.

METHODS

We performed a secondary analysis of a trial in which 470 rural Kenyan women with singleton pregnancies and hemoglobin concentrations ≥ 90 g/L were randomly allocated to daily, supervised supplementation with 60 mg elemental iron as ferrous fumarate or placebo from 13-23 weeks of gestation until 1 mo postpartum. As previously reported, iron supplementation improved iron status in mothers and neonates. For the present study, we reanalyzed all available plasma samples collected in mothers and neonates at birth, with primary outcomes being concentrations of FGF23, measured by 2 assays: 1 that detects intact hormone and C-terminal cleavage products (total-FGF23) and another that detects the intact hormone only (intact-FGF23).

RESULTS

Analysis was performed on 433 women (n = 216, iron group; n = 217, placebo group) and 414 neonates (n = 207, iron group; n = 207, placebo group). Antenatal iron supplementation reduced geometric mean total-FGF23 concentrations in mothers and neonates by 62.6% (95% CI: 53.0%, 70.3%) and 15.2% (95% CI: -0.3%, 28.4%, P = 0.06), respectively. In addition, it increased geometric mean neonatal intact-FGF23 concentrations by 21.6% (95% CI: 1.2%, 46.1%), increased geometric mean maternal hepcidin concentrations by 136.4% (95% CI: 86.1%, 200.3%), and decreased mean maternal 25-hydroxyvitamin D concentrations by 6.1 nmol/L (95% CI: -11.0, -1.2 nmol/L).

CONCLUSIONS

Analysis of this randomized trial confirms that iron supplementation can reverse elevated FGF23 production caused by iron deficiency in iron-deficient mothers and their neonates. Further investigations are warranted to assess to what extent iron supplementation can prevent FGF23-mediated hypophosphatemic rickets or osteomalacia.

Hyperphosphatemia with low FGF7 and normal FGF23 and sFRP4 levels in the circulation characterizes pediatric hypophosphatasia

Hypophosphatasia (HPP) is the inborn-error-of-metabolism caused by loss-of-function mutation(s) of the ALPL gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). TNSALP in healthy individuals is on cell surfaces richly in bone, liver, and kidney. Thus, TNSALP natural substrates accumulate extracellularly in HPP, including inorganic pyrophosphate (PPi), a potent inhibitor of hydroxyapatite crystal formation and growth. Superabundance of extracellular PPi (ePPi) in HPP impairs mineralization of bones and teeth, often leading to rickets during childhood and osteomalacia in adult life and to tooth loss at any age. HPP's remarkably broad-ranging severity is largely explained by nearly four hundred typically missense mutations throughout the ALPL gene that are transmitted as an autosomal dominant or autosomal recessive trait. In the clinical laboratory, the biochemical hallmark of HPP is low serum ALP activity (hypophosphatasemia). However, our experience indicates that hyperphosphatemia from increased renal reclamation of filtered inorganic phosphate (Pi) is also common. Herein, from our prospective single-center study, we document throughout the clinical spectrum of non-lethal pediatric HPP that hyperphosphatemia reflects increased renal tubular threshold maximum for phosphorus adjusted for the glomerular filtration rate (TmP/GFR). To explore its pathogenesis, we studied mineral metabolism and quantitated circulating levels of three phosphatonins [fibroblast growth factor 23 (FGF23), secreted frizzled-related protein 4 (sFRP4), and fibroblast growth factor 7 (FGF7)] in 41 pediatric patients with HPP, 73 with X-linked hypophosphatemia (XLH), and 15 healthy pediatric control (CTR) subjects. The HPP and XLH cohorts had normal serum total and ionized calcium and parathyroid hormone levels (Ps > 0.10) and uncompromised glomerular filtration. In XLH, serum FGF23 was characteristically elevated (P < 0.0001) and despite hypophosphatemia sFRP4 was normal (P > 0.4) while FGF7 was low (P < 0.0001). In HPP, despite hyperphosphatemia serum FGF23 and sFRP4 were normal (Ps > 0.8) while FGF7 was low (P < 0.0001). Subsequently, in rats, we confirmed that FGF7 is phosphaturic. Thus, hyperphosphatemia in non-lethal pediatric HPP is associated with phosphatonin insufficiency together with, as we discuss, ePPi excess and diminished renal TNSALP activity.

Effect of etelcalcetide on cardiac hypertrophy in hemodialysis patients: a randomized controlled trial (ETECAR-HD)

Background

Fibroblast growth factor 23 (FGF23) is associated with left ventricular hypertrophy (LVH) in patients with chronic kidney disease, and calcimimetic therapy reduces plasma concentrations of FGF23. It remains unknown whether treatment with the calcimimetic etelcalcetide (ETL) reduces LVH in patients on hemodialysis.

Methods/design

This single-blinded randomized trial of 12 months duration will test the effects of ETL compared with alfacalcidol on LVH and cardiac fibrosis in maintenance hemodialysis patients with secondary hyperparathyroidism. Both treatment regimens will be titrated to equally suppress secondary hyperparathyroidism while alfacalcidol treatment causes an increase and ETL a decrease in FGF23, respectively.

Patients treated thrice weekly with hemodialysis for ≥ 3 months and ≤ 3 years with parathyroid hormone levels ≥ 300 pg/ml and LVH will be enrolled in the study.

The primary study endpoint is change from baseline to 12 months in left ventricular mass index (LVMI; g/m²) measured by cardiac magnetic resonance imaging. Sample size calculations showed that 62 randomized patients will be necessary to detect a difference in LVMI of at least 20 g/m² between the two groups at 12 months. Due to the strong association of volume overload and LVH, randomization will be stratified by residual kidney function, and regular body composition monitoring will be performed to control the volume status of patients.

Study medication will be administered intravenously by the dialysis nurses after every hemodialysis session, thus omitting adherence issues.

Secondary study endpoints are cardiac parameters measured by echocardiography, biomarker concentrations of bone metabolism (FGF23, vitamin D, parathyroid hormone, calcium, phosphate, s-Klotho), cardiac markers (pro-brain natriuretic peptide, pre- and postdialysis troponin T) and metabolites of the renin–angiotensin–aldosterone cascade (angiotensin I (Ang I), Ang II, Ang-(1–7), Ang-(1–5), Ang-(1–9), and aldosterone).

Discussion

The causal inference and pathophysiology of LVH regression by FGF23 reduction using calcimimetic treatment has not yet been shown. This intervention study has the potential to discover a new strategy for the treatment of cardiac hypertrophy and fibrosis in patients on maintenance hemodialysis. It might be speculated that successful treatment of cardiac morphology will also reduce the risk of cardiac death in this population.

Trial registration

European Clinical Trials Database, EudraCT number 2017-000222-35; ClinicalTrials.gov, NCT03182699. Registered on

Fibroblast growth factor 23 expression in human calcified vascular tissues

Vascular calcification is a major risk for cardiovascular disease and implies the transformation of smooth muscle cells to an osteoblastic phenotype as a consequence of dysregulation of calcium and phosphate metabolism. Fibroblast growth factor (FGF) 23 is the most potent phosphate regulator. Observational studies suggest that high levels of FGF23 are related to cardiovascular morbidity and mortality. In this work, we determined the levels of both the intact and the carboxi-terminal fragments of circulating FGF23 in 133 patients with established cardiovascular disease, the expression of FGF23, its receptors 1 and 3, and its co-receptor Klotho in vascular fragments of aorta, carotid and femoral in 43 out of this group of patients, and in a control group of 20 organ donors. Patients with atherosclerosis and vascular calcification presented increased levels of FGF23 respect to the control group. Vascular immunoreactivity for FGF23 was also significantly increased in patients with vascular calcification as compared to patients without calcification and to controls. Finally, gene expression of FGF23 and RUNX2 were also higher and directly related in vascular samples with calcification. Conversely, expression of Klotho was reduced in patients with cardiovascular disease when comparing to controls. In conclusion, our findings link the calcification of the vascular tissue with the expression of FGF23 in the vessels and with the elevation of circulating levels this hormone.

The Role of Fibroblast Growth Factor 23 in Inflammation and Anemia

In patients with chronic kidney disease (CKD), adverse outcomes such as systemic inflammation and anemia are contributing pathologies which increase the risks for cardiovascular mortality. Amongst these complications, abnormalities in mineral metabolism and the metabolic milieu are associated with chronic inflammation and iron dysregulation, and fibroblast growth factor 23 (FGF23) is a risk factor in this context. FGF23 is a bone-derived hormone that is essential for regulating vitamin D and phosphate homeostasis. In the early stages of CKD, serum FGF23 levels rise 1000-fold above normal values in an attempt to maintain normal phosphate levels. Despite this compensatory action, clinical CKD studies have demonstrated powerful and dose-dependent associations between FGF23 levels and higher risks for mortality. A prospective pathomechanism coupling elevated serum FGF23 levels with CKD-associated anemia and cardiovascular injury is its strong association with chronic inflammation. In this review, we will examine the current experimental and clinical evidence regarding the role of FGF23 in renal physiology as well as in the pathophysiology of CKD with an emphasis on chronic inflammation and anemia.

FGF23, Biomarker or Target?

Fibroblast growth factor 23 (FGF23) plays a key role in the complex network between the bones and other organs. Initially, it was thought that FGF23 exclusively regulated phosphate and vitamin D metabolism; however, recent research has demonstrated that an excess of FGF23 has other effects that may be detrimental in some cases. The understanding of the signaling pathways through which FGF23 acts in different organs is crucial to develop strategies aiming to prevent the negative effects associated with high FGF23 levels. FGF23 has been described to have effects on the heart, promoting left ventricular hypertrophy (LVH); the liver, leading to production of inflammatory cytokines; the bones, inhibiting mineralization; and the bone marrow, by reducing the production of erythropoietin (EPO). The identification of FGF23 receptors will play a remarkable role in future research since its selective blockade might reduce the adverse effects of FGF23. Patients with chronic kidney disease (CKD) have very high levels of FGF23 and may be the population suffering from the most adverse FGF23-related effects. The general population, as well as kidney transplant recipients, may also be affected by high FGF23. Whether the association between FGF23 and clinical events is causal or casual remains controversial. The hypothesis that FGF23 could be considered a therapeutic target is gaining relevance and may become a promising field of investigation in the future.

Tumor-induced osteomalacia: experience from three tertiary care centers in India

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by recalcitrant hypophosphatemia. Reports from the Indian subcontinent are scarce, with most being single center experiences involving few patients. Herein, we conducted a retrospective analysis of 30 patients of TIO diagnosed at three tertiary care hospitals in India. Patients with persistent hypophosphatemia (despite correction of hypovitaminosis D), normocalcemia, elevated alkaline phosphatase, low TmP/GFR and elevated or 'inappropriately normal' FGF23 levels were labeled as having TIO. They were sequentially subjected to functional followed by anatomical imaging. Patients with a well-localized tumor underwent excision; others were put on phosphorous and calcitriol supplementation. The mean age at presentation was 39.6 years with female:male ratio of 3:2. Bone pain (83.3%) and proximal myopathy (70%) were the chief complaints; 40% of cases had fractures. The mean delay in diagnosis was 3.8 years. Tumors were clinically detectable in four patients (13.3%). The mean serum phosphate was 0.50 mmol/L with a median serum FGF23 level of 518 RU/mL. Somatostatin receptor-based scintigraphy was found to be superior to FDG-PET in tumor localization. Lower extremities were the most common site of the tumor (72%). Tumor size was positively correlated with serum FGF23 levels. Twenty-two patients underwent tumor resection and 16 of them had phosphaturic mesenchymal tumors. Surgical excision led to cure in 72.7% of patients whereas disease persistence and disease recurrence were seen in 18.2% and 9.1% of cases, respectively. At the last follow-up, serum phosphate in the surgically treated group was significantly higher than in the medically managed group.

Plasma Fibroblast Growth Factor 23 Is Elevated in Pediatric Primary Hypertension

Fibroblast growth factor 23 (FGF 23), an endocrine hormone regulating the homeostasis of phosphate and vitamin D, has been shown to play a role in cardiovascular disease. Increased blood FGF 23 is found to be associated with elevated blood pressure in adults. However, measurement of FGF 23 in hypertensive children has not been documented. In this study, a total of 98 children with primary hypertension and 37 controls were recruited, and blood FGF 23 was comparatively investigated. Additionally, FGF 23 levels were compared between the subgroups of patients after hypertensive children were sub-grouped according to their cardiac geometry, hypertension stages, insulin levels, and weight. The case group had a FGF 23 level of 48.99 (16.42), expressed as the median (the interquartile range), significantly higher than the 41.72 (7.05) from the control group (p = 0.0002). While no remarkable differences were observed in FGF 23 levels between non-obese and obese hypertensive children, between patients with stage 1 and stage 2 hypertension, or between patients with normal and high insulin levels; hypertensive children with abnormal cardiac geometry had significantly higher levels of FGF 23 than patients with normal cardiac geometry (p = 0.0085). Our data revealed for the first time that hypertensive children have higher levels of FGF 23. Further studies are needed to examine if lowering FGF 23 improves the cardiac geometry in hypertensive children with higher FGF 23.

The stability and variability of serum and plasma fibroblast growth factor-23 levels in a haemodialysis cohort

Background

Serum fibroblast growth factor 23 (FGF-23) levels are markedly elevated in haemodialysis patients and have been linked to mortality outcomes. Small studies in health and chronic kidney disease, have demonstrated marked intra- and inter-individual variability in measured FGF-23 levels, and variable degradation in serum as compared to plasma samples. In end-stage kidney disease (ESKD), the intra- and inter-individual variability of FGF-23 levels, and the optimal collection methods remain poorly characterized. In this study we assessed the variability of FGF-23 levels in a cohort of stable haemodialysis patients. Secondly, in a subset of patients, we assessed the effects of different collection methods on measured FGF-23 levels.

Methods

To assess the variability of FGF-23, pre-dialysis blood samples were collected over 3 consecutive weeks from 75 haemodialysis patients. The effects of different specimen collection methods were examined in a subset of patients (n = 23), with pre-dialysis blood collected into different tubes: plain (serum), EDTA (plasma) and EDTA with the addition of a protease inhibitor (EDTA-PI). All analyses were performed in the main cohort and repeated in each subgroup. Variability over a 3-week period was assessed using repeated measures ANOVA and random effects linear regression models. Intra-class correlation coefficients were calculated to assess agreement, and coefficients of variation were calculated to assess intra- and inter-individual variability.

Results

Over the 3-week study period the mean FGF-23 levels were not significantly different in the serum (p = 0.26), EDTA (p = 0.62) and EDTA-PI (p = 0.55) groups. FGF-23 levels demonstrated marked intra- and inter-individual variability with a CV of 36 and 203.2%, respectively. In the subgroup analysis, the mean serum FGF-23 levels were significantly lower than the EDTA (p < 0.001) or EDTA-PI (p < 0.001) groups, however there was no difference in mean FGF-23 levels between EDTA and EDTA-PI (p = 0.54).

Conclusions

The measured FGF-23 levels were significantly lower in serum as compared to plasma, and the addition of a protease inhibitor did not confer an additional benefit. Importantly in this cohort of ESKD patients, FGF-23 levels showed marked intra- and inter-individual variability. The routine measurement of FGF-23 in ESKD remains challenging, however this study suggests the plasma is the optimal collection method for FGF-23 analysis.

Association between resistin and fibroblast growth factor 23 in patients with type 2 diabetes mellitus

Fibroblast growth factor 23 (FGF23) is associated with cardiovascular disease and all-cause mortality in patients with diabetes mellitus. Insulin resistance has recently been reported to increase FGF23 levels, and resistin is a peptide that mainly regulates insulin resistance. However, few studies have investigated the association between FGF23 and resistin. A total of 422 patients with diabetes mellitus were recruited for this cross-sectional study to examine the association between resistin and intact FGF23. The mean ( ± standard deviation) age was 63.1 ± 11.9 years, and the median HbA1c was 6.7% (range, 6.1-7.1%). The mean estimated glomerular filtration rate (eGFR) was 66.2 ± 23.1 mL/min/m2. Multiple regression analysis for resistin showed that logFGF23 (coefficient (Coef): 1.551; standard error (SE): 0.739; P = 0.036), C-peptide (Coef: 0.798; SE: 0.229; P = 0.001), ghrelin (Coef: 1.061; SE: 0.332; P = 0.001), intact parathyroid hormone (Coef: 0.022; SE: 0.099; P = 0.030), and eGFR (Coef: -0.091; SE: 0.017; P < 0.001) were all significantly associated with the resistin level. These associations were modified in patients with higher age, lower body mass index, and higher vitamin D levels. These results suggest that resistin is positively associated with serum FGF23 levels.

Fibroblast Growth Factor 23 Associates with Death in Critically Ill Patients

BACKGROUND AND OBJECTIVES

Dysregulated mineral metabolism is a common and potentially maladaptive feature of critical illness, especially in patients with AKI, but its association with death has not been comprehensively investigated. We sought to determine whether elevated plasma levels of the osteocyte-derived, vitamin D-regulating hormone, fibroblast growth factor 23 (FGF23), are prospectively associated with death in critically ill patients with AKI requiring RRT, and in a general cohort of critically ill patients with and without AKI.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We measured plasma FGF23 and other mineral metabolite levels in two cohorts of critically ill patients (n=1527). We included 817 patients with AKI requiring RRT who enrolled in the ARF Trial Network (ATN) study, and 710 patients with and without AKI who enrolled in the Validating Acute Lung Injury biomarkers for Diagnosis (VALID) study. We hypothesized that higher FGF23 levels at enrollment are independently associated with higher 60-day mortality.

RESULTS

In the ATN study, patients in the highest compared with lowest quartiles of C-terminal (cFGF23) and intact FGF23 (iFGF23) had 3.84 (95% confidence interval, 2.31 to 6.41) and 2.08 (95% confidence interval, 1.03 to 4.21) fold higher odds of death, respectively, after adjustment for demographics, comorbidities, and severity of illness. In contrast, plasma/serum levels of parathyroid hormone, vitamin D metabolites, calcium, and phosphate were not associated with 60-day mortality. In the VALID study, patients in the highest compared with lowest quartiles of cFGF23 and iFGF23 had 3.52 (95% confidence interval, 1.96 to 6.33) and 1.93 (95% confidence interval, 1.12 to 3.33) fold higher adjusted odds of death.

CONCLUSIONS

Higher FGF23 levels are independently associated with greater mortality in critically ill patients.

Is there a role for newer biomarkers in chronic kidney disease-mineral and bone disorder management?

The current management of Chronic Kidney Disease-Mineral Bone Disorder (CKD-MBD) relies largely on clinical judgement and assessment of biochemical parameters including serum calcium, phosphate and intact parathyroid hormone concentrations. In the past two decades, there has been a leap in the understanding of the pathophysiology of CKD-MBD, leading to the discovery of novel biomarkers. The potential utility of these markers in this clinical setting is an area of intense investigation. In the absence of any guidelines aiding the clinician's understanding and application of these markers, we summarise the current available literature surrounding fibroblast growth factor-23, α-Klotho, sclerostin and serum calcification propensity testing and their respective assays in the context of CKD-MBD management.

FGF23 Actions on Target Tissues-With and Without Klotho

Fibroblast growth factor (FGF) 23 is a phosphaturic hormone whose physiologic actions on target tissues are mediated by FGF receptors (FGFR) and klotho, which functions as a co-receptor that increases the binding affinity of FGF23 for FGFRs. By stimulating FGFR/klotho complexes in the kidney and parathyroid gland, FGF23 reduces renal phosphate uptake and secretion of parathyroid hormone, respectively, thereby acting as a key regulator of phosphate metabolism. Recently, it has been shown that FGF23 can also target cell types that lack klotho. This unconventional signaling event occurs in an FGFR-dependent manner, but involves other downstream signaling pathways than in "classic" klotho-expressing target organs. It appears that klotho-independent signaling mechanisms are only activated in the presence of high FGF23 concentrations and result in pathologic cellular changes. Therefore, it has been postulated that massive elevations in circulating levels of FGF23, as found in patients with chronic kidney disease, contribute to associated pathologies by targeting cells and tissues that lack klotho. This includes the induction of cardiac hypertrophy and fibrosis, the elevation of inflammatory cytokine expression in the liver, and the inhibition of neutrophil recruitment. Here, we describe the signaling and cellular events that are caused by FGF23 in tissues lacking klotho, and we discuss FGF23's potential role as a hormone with widespread pathologic actions. Since the soluble form of klotho can function as a circulating co-receptor for FGF23, we also discuss the potential inhibitory effects of soluble klotho on FGF23-mediated signaling which might-at least partially-underlie the pleiotropic tissue-protective functions of klotho.

FGF23 is synthesised locally by renal tubules and activates injury-primed fibroblasts

In kidney disease, higher circulating levels of the mineral-regulating hormone fibroblast growth factor (FGF)-23 are predictive of disease progression but direct pathogenic effects on the kidney are unknown. We sought evidence of local renal synthesis in response to unilateral ureteric obstruction in the mouse, and pro-fibrotic actions of FGF23 on the fibroblast in vitro. Acute tubulointerstitial injury due to unilateral ureteric obstruction stimulated renal FGF23 synthesis by tubules, and downregulated inactivating proprotein convertases, without effects on systemic mineral metabolism. In vitro, FGF23 had divergent effects on fibroblast activation in cells derived from normal and obstructed kidneys. While FGF23 failed to stimulate fibrogenesis in normal fibroblasts, in those primed by injury, FGF23 induced pro-fibrotic signalling cascades via activation of TGF-β pathways. Effects were independent of α-klotho. Tubule-derived FGF23 may amplify myofibroblast activation in acute renal injury, and might provide a novel therapeutic target in renal fibrosis.

Low serum iron is associated with high serum intact FGF23 in elderly men: The Swedish MrOS study

BACKGROUND

Fibroblast growth factor (FGF23) is a protein that is produced by osteoblasts and osteocytes. Increased serum levels of FGF23 have been associated with increased risks of osteoporotic fractures and cardiovascular disease, particularly in participants with poor renal function. Serum iron (Fe) has been suggested as a regulator of FGF23 homeostasis.

OBJECTIVE

To determine whether Fe and iron status are determinants of the levels of intact FGF23 (iFGF23) in elderly men.

METHODS

The MrOS study is a population-based study of elderly men (N=1010; mean age, 75.3years; range, 69-81years). The levels of Fe, transferrin saturation (TS), and ferritin were evaluated in relation to the serum concentrations of iFGF23 before and after adjustments for confounders.

RESULTS

TS <15% was found in 3.5% (34/977) of the participants, who had a higher median level iFGF23 compared with the remaining subjects (47.4μmol/L vs. 41.9μmol/L, p=0.008). The levels of iFGF23 correlated negatively (un-adjusted) with the levels of Fe (r=-0.17, p<0.001), TS (r=-0.16, p<0.001) and serum ferritin (r=-0.07, p=0.022). In addition, in participants with estimated glomerular filtration rate eGFRCystatin C>60mL/min, the levels of iFGF23 correlated (age-adjusted) negatively with the levels of Fe (r=-0.15, p<0.001) and TS (r=-0.17, p<0.001). The level of iFGF23 correlated positively (un-adjusted) with lumbar spine bone mineral density (BMD) (r=0.14, p<0.001), total body BMD (r=0.11, p=0.001), and total hip BMD (r=0.09, p=0.004). The corresponding correlations, when adjusted for age, weight, and height were: r=0.08, p=0.018; r=0.05, p=0.120; and r=0.02, p=0.624, respectively. No associations were found between BMD and the levels of Fe or TS. Multiple step-wise linear regression analyses [adjusting for age, body mass index (BMI), comorbidity index, cystatin C, C-reactive protein (hs-CRP), serum vitamin D 25-OH (25OHD), phosphate, calcium, parathyroid hormone (PTH), erythropoietin, hemoglobin, lumbar spine BMD, apolipoprotein B/A1 ratio] were performed in three separate models with Fe, TS or ferritin as potential explanatory variables. Fe and TS, but not ferritin, were independent predictors of iFGF23 level (standardized β-values: -0.10, p<0.001; -0.10, p<0.001; and -0.05, p=0.062, respectively).

CONCLUSION

Low levels of Fe in elderly men are associated with high levels of iFGF23, independently of markers of inflammation and renal function, suggesting an iron-related pathway for FGF23 regulation.

Abnormalities in biomarkers of mineral and bone metabolism in kidney donors

Previous studies have suggested that kidney donors may have abnormalities of mineral and bone metabolism typically seen in chronic kidney disease. This may have important implications for the skeletal health of living kidney donors and for our understanding of the pathogenesis of long-term mineral and bone disorders in chronic kidney disease. In this prospective study, 182 of 203 kidney donors and 173 of 201 paired normal controls had markers of mineral and bone metabolism measured before and at 6 and 36 months after donation (ALTOLD Study). Donors had significantly higher serum concentrations of intact parathyroid hormone (24.6% and 19.5%) and fibroblast growth factor-23 (9.5% and 8.4%) at 6 and 36 months, respectively, as compared to healthy controls, and significantly reduced tubular phosphate reabsorption (-7.0% and -5.0%) and serum phosphate concentrations (-6.4% and -2.3%). Serum 1,25-dihydroxyvitamin D3 concentrations were significantly lower (-17.1% and -12.6%), while 25-hydroxyvitamin D (21.4% and 19.4%) concentrations were significantly higher in donors compared to controls. Moreover, significantly higher concentrations of the bone resorption markers, carboxyterminal cross-linking telopeptide of bone collagen (30.1% and 13.8%) and aminoterminal cross-linking telopeptide of bone collagen (14.2% and 13.0%), and the bone formation markers, osteocalcin (26.3% and 2.7%) and procollagen type I N-terminal propeptide (24.3% and 8.9%), were observed in donors. Thus, kidney donation alters serum markers of bone metabolism that could reflect impaired bone health. Additional long-term studies that include assessment of skeletal architecture and integrity are warranted in kidney donors.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

The effects of maternal iron deficiency on infant fibroblast growth factor-23 and mineral metabolism

Fibroblast growth factor-23 (FGF23), a phosphate(Phos)-regulating hormone, is abnormally elevated in hypophosphataemic syndromes and an elevated FGF23 is a predictor of mortality in kidney disease. Recent findings suggest iron deficiency as a potential mediator of FGF23 expression and murine studies have shown in utero effects of maternal iron deficiency on offspring FGF23 and phosphate metabolism. Our aim was to investigate the impact of maternal iron status on infant FGF23 and mineral metabolites over the first 2years of life. Infants born to mothers with normal (NIn=25,) and low (LIn=25) iron status during pregnancy, from a mother-infant trial (ISRCTN49285450) in rural Gambia, West Africa, had blood and plasma samples analysed at 12, 24, 52, 78 and 104weeks (wk) of age. Circulating intact-FGF23 (I-FGF23), Phos, total alkaline phosphatase (TALP) and haemoglobin (Hb) decreased and estimated glomerular filtration rate increased over time [all P≤0.0001)]. C-terminal-FGF23 (C-FGF23) and TALP were significantly higher in LI compared with NI, from 52wk for C-FGF23 [Beta coefficient (SE) 18.1 (0.04) %, P=0.04] and from 24wk for TALP [44.7 (29.6) U/L, P=0.04]. Infant Hb was the strongest negative predictor of C-FGF23 concentration [-21% (4%) RU/mL, P≤0.0001], Phos was the strongest positive predictor of I-FGF23 [32.0(3.9) pg/mL, P≤0.0001] and I-FGF23 did not predict C-FGF23 over time [-0.5% (0.5%), P=0.3]. In conclusion, this study suggests that poor maternal iron status is associated with a higher infant C-FGF23 and TALP but similar I-FGF23 concentrations in infants and young children. These findings further highlight the likely public health importance of preventing iron deficiency during pregnancy. Whether or not children who are born to iron deficient mothers have persistently high concentrations of these metabolites and are more likely to be at risk of impaired bone development and pre-disposed to rickets requires further research.

The use of fibroblast growth factor 23 testing in patients with kidney disease

The emergence of fibroblast growth factor 23 as a potentially modifiable risk factor in CKD has led to growing interest in its measurement as a tool to assess patient risk and target therapy. This review discusses the analytical and clinical challenges faced in translating fibroblast growth factor 23 testing into routine practice. As for other bone mineral markers, agreement between commercial fibroblast growth factor 23 assays is poor, mainly because of differences in calibration, but also, these differences reflect the variable detection of hormone fragments. Direct comparison of readout from different assays is consequently limited and likely hampers setting uniform fibroblast growth factor 23-directed targets. Efforts are needed to standardize assay output to enhance clinical use. Fibroblast growth factor 23 is robustly associated with cardiovascular and renal outcomes in patients with CKD and adds value to risk assessments based on conventional risk factors. Compared with most other mineral markers, fibroblast growth factor 23 shows better intraindividual temporal stability, with minimal diurnal and week-to-week variability, but substantial interindividual variation, maximizing discriminative power for risk stratification. Conventional therapeutic interventions for the CKD-mineral bone disorder, such as dietary phosphate restriction and use of oral phosphate binders or calcimimetics, are associated with variable efficacy at modulating circulating fibroblast growth factor 23 concentrations, like they are for other mineral metabolites. Dual therapy with dietary phosphate restriction and noncalcium-based binder use achieves the most consistent fibroblast growth factor 23-lowering effect and seems best monitored using an intact assay. Additional studies are needed to evaluate whether strategies aimed at reducing levels or antagonizing its action have beneficial effects on clinical outcomes in CKD patients. Moreover, a better understanding of the mechanisms driving fibroblast growth factor 23 elevations in CKD is needed to inform the use of therapeutic interventions targeting fibroblast growth factor 23 excess. This evidence must be forthcoming to support the use of fibroblast growth factor 23 measurement and fibroblast growth factor 23-directed therapy in the clinic.

Predictors of intact and C-terminal fibroblast growth factor 23 in Gambian children

Elevated C-terminal fibroblast growth factor 23 (C-FGF23) concentrations have been reported in Gambian children with and without putative Ca-deficiency rickets. The aims of this study were to investigate whether i) elevated C-FGF23 concentrations in Gambian children persist long term; ii) they are associated with higher intact FGF23 concentrations (I-FGF23), poor iron status and shorter 25-hydroxyvitamin D half-life (25OHD-t1/2); and iii) the persistence and predictors of elevated FGF23 concentrations differ between children with and without a history of rickets. Children (8-16 years, n=64) with a history of rickets and a C-FGF23 concentration >125 RU/ml (bone deformity (BD), n=20) and local community children with a previously measured elevated C-FGF23 concentration (LC+, n=20) or a previously measured C-FGF23 concentration within the normal range (LC-, n=24) participated. BD children had no remaining signs of bone deformities. C-FGF23 concentration had normalised in BD children, but remained elevated in LC+ children. All the children had I-FGF23 concentration within the normal range, but I-FGF23 concentration was higher and iron status poorer in LC+ children. 1,25-dihydroxyvitamin D was the strongest negative predictor of I-FGF23 concentration (R(2)=18%; P=0.0006) and soluble transferrin receptor was the strongest positive predictor of C-FGF23 concentration (R(2)=33%; P≤0.0001). C-FGF23 and I-FGF23 concentrations were poorly correlated with each other (R(2)=5.3%; P=0.07). 25OHD-t1/2 was shorter in BD children than in LC- children (mean (s.d.): 24.5 (6.1) and 31.5 (11.5) days respectively; P=0.05). This study demonstrated that elevated C-FGF23 concentrations normalised over time in Gambian children with a history of rickets but not in local children, suggesting a different aetiology; that children with resolved rickets had a shorter 25OHD-t1/2, suggesting a long-standing increased expenditure of 25OHD, and that iron deficiency is a predictor of elevated C-FGF23 concentrations in both groups of Gambian children.

Fibroblast growth factor 23

There is growing interest in the role of fibroblast growth factor 23 (FGF23) in various diseases of disordered mineral metabolism. In chronic kidney disease (CKD), where biochemical evidence of mineral disturbances is especially common, FGF23 measurement has been advocated as an early and sensitive marker for CKD-related bone disease. In this setting, FGF23 analysis may also improve the discrimination of risk of adverse renal and cardiovascular outcomes and aid targeting of those patients that are likely to benefit from interventions. Nonetheless, while the physiological relevance of FGF23 in the control of mineral metabolism is now firmly established, relatively little attention has been paid to important preanalytical and analytical aspects of FGF23 measurement that may impact on its clinical utility. Here we review these issues and discuss the suitability of FGF23 testing strategies for routine clinical practice. The current ‘state-of-the-art’ enzyme-linked immunosorbent assay methods for FGF23 measurement show poor agreement due to differences in FGF23 fragment detection, antibody specificity and calibration. Such analytical variability does not permit direct comparison of FGF23 measurements made with different assays and is likely to at least in part account for some of the inconsistencies noted between observational studies. From a clinical perspective, the lack of concordance has implications for the development of standardized reference intervals and clinical decision limits. Finally, the inherent assay-dependent biological variability of plasma FGF23 concentration can further complicate the interpretation of results and the design of FGF23-based testing protocols. Currently, it would be premature to consider incorporating FGF23 measurements into standard testing repertoires.

The changing face of hypophosphatemic disorders in the FGF-23 era

In the past decade, research in genetic disorders of hypophosphatemia has significantly expanded our understanding of phosphate metabolism. X-linked hypophosphatemia (XLH) is the most common inherited form of rickets due to renal phosphate wasting. Recent understanding of the mechanisms of disease and role of fibroblast growth factor 23 (FGF-23) in XLH and other hypophosphatemic disorders have opened new potential therapeutic avenues. We will discuss the current standard of treatment for XLH as well as promising future directions under study.

Phosphatonins: new hormones involved in numerous inherited bone disorders

Phosphate (Pi) homeostasis is under control of several endocrine factors that play effects on bone, kidney and intestine. The control of Pi homeostasis has a significant biological importance, as it relates to numerous cellular mechanisms involved in energy metabolism, cell signaling, nucleic acid synthesis, membrane function, as well as skeletal health and integrity. Pi is essential for diverse biological processes, and negative Pi balance resulting from improperly regulated intestinal absorption, systemic utilization, and renal excretion. As results of these functions, chronic Pi deprivation causes several biological alterations, such as bone demineralization with unmineralized osateoid typical of osteomalacia in adults and rickets in developing animals and humans (1). Phosphatonins are new hormones playing an important role in the control of Pi homeostasis together with parathyroid hormone (PTH) and 1,25-dihydroxy vitamin D(3). Most insight into the underlying mechanisms was established by defining the molecular basis of different inherited disorders that are characterized by an abnormal regulation of Pi homeostasis.

Diagnosis of phosphaturic mesenchymal tumor (mixed connective tissue type) by cytopathology

Oncogenic osteomalacia (OO) is a rare paraneoplastic condition in which a bone or soft tissue tumor induces biochemical and clinical signs and symptoms of osteomalacia (or rickets) most often by the production of the phosphaturic protein, fibroblast growth factor-23. Phosphaturic mesenchymal tumor, mixed connective tissue type (PMTMCT) is a rare, histologically distinct tumor that represents the most common cause of OO. As the clinical diagnosis of OO is typically suspected on the basis of clinical and biochemical features and the presence of a bone or soft tissue tumor, cytologic examination might potentially provide the necessary pathologic confirmation of OO. In this case of a 46-year-old female with clinical stigmata of OO and a right distal humeral mass, we report that the fine-needle aspiration findings of short, cytologically bland spindled cells embedded in a fine, fibrillary stromal-rich matrix and the presence of osteoclast-type giant cells associated with the stromal matrix provide strong pathological evidence for PMTMCT and assist in pathologically confirming the clinical impression of OO, thus alleviating the need for a more invasive diagnostic surgical procedure.

Effects of dietary phosphate and calcium intake on fibroblast growth factor-23

BACKGROUND AND OBJECTIVES

Little is known about the influence of dietary phosphate intake on fibroblast growth factor-23 (FGF23) and its subsequent effects on vitamin D levels. This study addresses changes in intact FGF23 (iFGF23) and C-terminal FGF23 (cFGF23), phosphaturia, and levels of vitamin D on high and low phosphate and calcium intake.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Ten healthy subjects adhered to a diet low or high in phosphate and calcium content for 36 hours each with a 1-week interval during which subjects adhered to their usual diet. Serum phosphate, calcium, vitamin D metabolites, parathyroid hormone (PTH), and FGF23 levels (cFGF23 and iFGF23) were measured several times a day. Phosphate, calcium, and creatinine excretion was measured in 24-hour urine on all study days.

RESULTS

Serum phosphate levels and urinary phosphate increased during high dietary phosphate intake (from 1.11 to 1.32 mmol/L, P<0.0001 and 21.6 to 28.8 mmol/d, P=0.0005, respectively). FGF23 serum levels increased during high dietary phosphate/calcium intake (cFGF23 from 60 to 72 RU/ml, P<0.001; iFGF23 from 33 to 37 ng/L, P=0.003), whereas PTH declined. 1,25-dihydroxyvitamin D (1,25D) showed an inverse relation with FGF23.

CONCLUSIONS

Variation in dietary phosphate and calcium intake induces changes in FGF23 (on top of a circadian rhythm) and 1,25D blood levels as well as in urinary phosphate excretion. These changes are detectable the day after the change in the phosphate content of meals. Higher FGF23 levels are associated with phosphaturia and a decline in 1,25D levels.

The osteocyte--a novel endocrine regulator of body phosphate homeostasis

Although osteocytes are the most abundant cell type in bone, much of their biology remains enigmatic. They are known to transduce mechanical stress into signals that initiate local bone remodeling, and are targets for systemic and local endocrine signals that affect bone architecture and mineral homeostasis. However, recent data reveal that osteocytes themselves act as endocrine cells that synthesize fibroblast growth factor 23 (FGF23) and several other phosphatonins, shown to underpin the systemic regulation of phosphate homeostasis. This review will synthesize the emerging discoveries concerning the osteocytic endocrine role in phosphate homeostasis through the biology and pathophysiology of these phosphatonins. We also suggest future research paths that might resolve existing uncertainties, and look ahead at how greater understanding might improve the management of clinical disorders of phosphate homeostasis.

Phosphate homeostasis and the renal-gastrointestinal axis

Transport of phosphate across intestinal and renal epithelia is essential for normal phosphate balance, yet we know less about the mechanisms and regulation of intestinal phosphate absorption than we do about phosphate handling by the kidney. Recent studies have provided strong evidence that the sodium-phosphate cotransporter NaPi-IIb is responsible for sodium-dependent phosphate absorption by the small intestine, and it might be that this protein can link changes in dietary phosphate to altered renal phosphate excretion to maintain phosphate balance. Evidence is also emerging that specific regions of the small intestine adapt differently to acute or chronic changes in dietary phosphate load and that phosphatonins inhibit both renal and intestinal phosphate transport. This review summarizes our current understanding of the mechanisms and control of intestinal phosphate absorption and how it may be related to renal phosphate reabsorption; it also considers the ways in which the gut could be targeted to prevent, or limit, hyperphosphatemia in chronic and end-stage renal failure.

Production and characterization of monoclonal antibodies to human sclerostin

We developed and characterized monoclonal antibodies directed against the amino-terminal and carboxy-terminal regions of human and mouse sclerostin (scl). Amino-terminal and carboxy-terminal scl peptides with limited homology to scl domain-containing protein-1 were synthesized using f-moc chemistry. The peptides were conjugated to keyhole limpet hemocyanin and the conjugates were used for immunization of mice. Monoclonal antibodies were obtained and characterized using bacterially expressed and insect cell-expressed recombinant scl. The amino-terminal (IgG 2aK) and carboxy-terminal (IgG 2bK) antibodies bound bioactive sclerostin that was expressed in an insect-cell expression system with dissociation constants in the nanomolar range. The antibodies are potentially useful agents that can be used for modulating sclerostin bioactivity.

Circulating fibroblast growth factor 23 in patients with end-stage renal disease treated by peritoneal dialysis is intact and biologically active

CONTEXT

Fibroblast growth factor 23 (FGF23) regulates phosphorus homeostasis and vitamin D metabolism. Circulating FGF23 levels are elevated in inherited and acquired hypophosphatemic disorders that can cause rickets or osteomalacia. Particularly increased concentrations of FGF23 are observed in patients with chronic kidney disease (CKD), in which increased FGF23 is associated with more rapid disease progression, improved bone mineralization, the development of left ventricular hypertrophy, and increased mortality.

OBJECTIVE

Our objective was to determine whether the markedly elevated levels of immunoreactive FGF23 in CKD represent accumulation of intact, biologically active hormone, C-terminal cleavage fragments, or both.

DESIGN

Biologically active FGF23 in plasma from CKD patients treated by peritoneal dialysis was quantified using a cell-based Egr-1 reporter assay; bioactive FGF23 levels were compared with those measured with immunometric FGF23 assays detecting either intact hormone alone or intact hormone and C-terminal fragments.

SETTING AND PATIENTS

Adult and pediatric patients with end-stage renal disease treated with peritoneal dialysis participated in the study at a tertiary referral center.

RESULTS

Serially diluted patient samples revealed levels of bioactive FGF23 that ran in parallel to CHO cell-derived recombinant human FGF23. FGF23 bioactivity was inhibited by an anti-FGF23 antibody. Levels of bioactive and immunoreactive FGF23 were tightly correlated, and Western blot analysis of FGF23 immunoprecipitated with anti-FGF23 antibodies from plasma of dialysis patients revealed only a single prominent protein band, which was indistinguishable from recombinant intact FGF23, without clear evidence for FGF23 fragments.

CONCLUSIONS

Our results provide strong evidence for the conclusion that virtually all circulating FGF23 in dialysis patients is intact and biologically active.

The FGF23-Klotho axis: endocrine regulation of phosphate homeostasis

Appropriate levels of phosphate in the body are maintained by the coordinated regulation of the bone-derived growth factor FGF23 and the membrane-bound protein Klotho. The endocrine actions of FGF23, in association with parathyroid hormone and vitamin D, mobilize sodium-phosphate cotransporters that control renal phosphate transport in proximal tubular epithelial cells. The availability of an adequate amount of Klotho is essential for FGF23 to exert its phosphaturic effects in the kidney. In the presence of Klotho, FGF23 activates downstream signaling components that influence the homeostasis of phosphate, whereas in the absence of this membrane protein, it is unable to exert such regulatory effects, as demonstrated convincingly in animal models. Several factors, including phosphate and vitamin D, can regulate the production of both FGF23 and Klotho and influence their functions. In various acquired and genetic human diseases, dysregulation of FGF23 and Klotho is associated with vascular and skeletal anomalies owing to altered phosphate turnover. In this Review, I summarize how the endocrine effects of bone-derived FGF23, in coordination with Klotho, can regulate systemic phosphate homeostasis, and how an inadequate balance of these molecules can lead to complications that are caused by abnormal mineral ion metabolism.

Novel mechanisms in the regulation of phosphorus homeostasis

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

Normal FGF23 levels in adult idiopathic phosphate diabetes

Fibroblast growth factor 23 (FGF23), a recently discovered phosphaturic substance playing a key role in genetic and oncogenic phosphate diabetes, is involved in the physiological regulation of phosphate metabolism. Moderate idiopathic phosphate diabetes (IPD) leading to male osteoporosis and diffuse pain resembling fibromyalgia has been described. The aim of our study was to define the role of FGF23 in the mechanism of IPD. The study concerned 29 patients with IPD, mean age 53 +/- 11 years, of whom 72% were men. Fifteen subjects without bone disease and with normal serum phosphate and calcium levels were used as controls. Phosphate diabetes was confirmed by phosphate reabsorption level <85% and phosphate reabsorption threshold (TmPO4/GFR) <0.83. Known causes of phosphate diabetes were excluded. Fasting level of FGF23, serum phosphate, 1-25(OH)2D3, and parathyroid hormone were measured in patients and compared with FGF23 and serum phosphate in healthy controls. Spinal and hip bone mineral density (BMD) were measured by osteodensitometry. Sixteen of 29 patients had diffuse pain, 10 had osteoporosis according to the World Health Organization criteria, and 11 had osteopenia. Serum phosphate was significantly lower in patients than in controls, but FGF23 levels did not differ. Compared to patients with normal bone status, patients with osteopenia and osteoporosis had significantly decreased FGF23 levels, whereas serum phosphate was identical in the two groups. In all patients, serum phosphate and FGF23 were positively correlated and FGF23 and 1-25(OH)2D3 were negatively correlated. FGF23 seems not be a cause of IPD, and the FGF23/phosphate/1-25(OH)2D3 axis appeared to be functional.

Phosphatonins: new hormones that control phosphorus homeostasis

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

Inherited hypophosphatemic disorders in children and the evolving mechanisms of phosphate regulation

Phosphorous is essential for multiple cellular functions and constitutes an important mineral in bone. Hypophosphatemia in children leads to rickets resulting in abnormal growth and often skeletal deformities. Among various causes of low serum phosphorous are inherited disorders associated with increased urinary excretion of phosphate, including autosomal dominant hypophosphatemic rickets (ADHR), X-linked hypophosphatemia (XLH), autosomal recessive hypophosphatemia (ARHP), and hereditary hypophosphatemic rickets with hypercalciuria (HHRH). Recent genetic analyses and subsequent biochemical and animal studies have revealed several novel molecules that appear to play key roles in the regulation of renal phosphate handling. These include a protein with abundant expression in bone, fibroblast growth factor 23 (FGF23), which has proven to be a circulating hormone that inhibits tubular reabsorption of phosphate in the kidney. Two other bone-specific proteins, PHEX and dentin matrix protein 1 (DMP1), appear to be necessary for limiting the expression of fibroblast growth factor 23, thereby allowing sufficient renal conservation of phosphate. This review focuses on the clinical, biochemical, and genetic features of inherited hypophosphatemic disorders, and presents the current understanding of hormonal and molecular mechanisms that govern phosphorous homeostasis.

What does serum fibroblast growth factor 23 do in hemodialysis patients?

Fibroblast growth factor 23 (FGF-23) is a phosphate-regulating substance largely produced by osteocytes. Its major action, in normal subjects, is the inhibition of Na-coupled reabsorption of inorganic phosphate in the renal proximal tubule. FGF-23 levels increase markedly in dialysis patients. Why do the FGF-23 levels increase in these patients, and do they have any physiologic or pathophysiologic effects?

Regulation of phosphate homeostasis by the phosphatonins and other novel mediators

A variety of factors regulate the efficiency of phosphate absorption in the intestine and phosphate reabsorption in kidney. Apart from the well-known regulators of phosphate homeostasis, namely parathyroid hormone (PTH) and the vitamin D-endocrine system, a number of peptides collectively known as the "phosphatonins" have been recently identified as a result of the study of various diseases associated with hypophosphatemia. These factors, fibroblast growth factor 23 (FGF-23), secreted frizzled-related protein 4 (sFRP-4), fibroblast growth factor 7 (FGF-7) and matrix extracellular phosphoglycoprotein (MEPE), have been shown to play a role in the pathogenesis of various hypophosphatemic and hyperphosphatemic disorders, such as oncogenic osteomalacia, X-linked hypophosphatemic rickets, autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemia and tumoral calcinosis. Whether these factors are true hormones, in the sense that they are regulated by the intake of dietary phosphorus and the needs of the organism for higher or lower amounts of phosphorus, remains to be firmly established in humans. Additionally, new information demonstrates that the intestine "senses" luminal concentrations of phosphate and regulates the excretion of phosphate in the kidney by elaborating novel factors that alter renal phosphate reabsorption.

The emerging role of the fibroblast growth factor-23-klotho axis in renal regulation of phosphate homeostasis

Normal mineral ion homeostasis is tightly controlled by numerous endocrine factors that coordinately exert effects on intestine, kidney, and bone to maintain physiological balance. The importance of the fibroblast growth factor (FGF)-23-klotho axis in regulating mineral ion homeostasis has been proposed from recent research observations. Experimental studies suggest that 1) FGF23 is an important in vivo regulator of phosphate homeostasis, 2) FGF23 acts as a counter regulatory hormone to modulate the renal 1alpha-hydroxylase and sodium-phosphate cotransporter activities, 3) there is a trend of interrelationship between FGF23 and parathyroid hormone activities, 4) most of the FGF23 functions are conducted through the activation of FGF receptors, and 5) such receptor activation needs klotho, as a cofactor to generate downstream signaling events. These observations clearly suggest the emerging roles of the FGF23-klotho axis in maintaining mineral ion homeostasis. In this brief article, we will summarize how the FGF23-klotho axis might coordinately regulate normal mineral ion homeostasis, and how their abnormal regulation could severely disrupt such homeostasis to induce disease pathology.