Structural and Biochemical Properties of Fibroblast Growth Factor 23

Endocrine manifestations of chronic kidney disease and their evolving management: A systematic review

BACKGROUND

Chronic Kidney Disease (CKD) shows a wide range of renal abnormalities including the excretory, metabolic, endocrine, and homeostatic function of the kidney. The prognostic impact of the 'endocrine manifestations' which are often overlooked by clinicians cannot be overstated.

METHODS AND OBJECTIVES

A systematic review was attempted to provide a comprehensive overview of all endocrine abnormalities of CKD and their evolving principles of management, searching databases of PubMed, Embase, and Scopus and covering the literature between 2002 and 2022.

RESULTS

The endocrine derangements in CKD can be attributed to a myriad of pathologic processes, in particular decreased clearance, impaired endogenous hormone production, uremia-induced cellular dysfunction, and activation of systemic inflammatory pathways. The major disorders include anemia, hyperprolactinemia, insulin resistance, reproductive hormone deficiency, thyroid hormone deficiency, and serum FGF (Fibroblast Growth Factor) alteration. Long-term effects of CKD also include malnutrition and increased cardiovascular risk. The recent times have unveiled their detailed pathogenesis and have seen an evolution in the principles of management which necessitates a revision of current guidelines.

CONCLUSION

Increased advertence regarding the pathology, impact, and management of these endocrine derangements can help in reducing morbidity as well as mortality in the CKD patients by allowing prompt individualized treatment. Moreover, with timely and appropriate intervention, a long-term reduction in complications, as well as an enhanced quality of life, can be achieved in patients with CKD.

Upstream Regulators of Fibroblast Growth Factor 23

Fibroblast growth factor 23 (FGF23) has been described as an important regulator of mineral homeostasis, but has lately also been linked to iron deficiency, inflammation, and erythropoiesis. FGF23 is essential for the maintenance of phosphate homeostasis in the body and activating mutations in the gene itself or inactivating mutations in its upstream regulators can result in severe chronic hypophosphatemia, where an unbalanced mineral homeostasis often leads to rickets in children and osteomalacia in adults. FGF23 can be regulated by changes in transcriptional activity or by changes at the post-translational level. The balance between O-glycosylation and phosphorylation is an important determinant of how much active intact or inactive cleaved FGF23 will be released in the circulation. In the past years, it has become evident that iron deficiency and inflammation regulate FGF23 in a way that is not associated with its classical role in mineral metabolism. These conditions will not only result in an upregulation of FGF23 transcription, but also in increased cleavage, leaving the levels of active intact FGF23 unchanged. The exact mechanisms behind and function of this process are still unclear. However, a deeper understanding of FGF23 regulation in both the classical and non-classical way is important to develop better treatment options for diseases associated with disturbed FGF23 biology. In this review, we describe how the currently known upstream regulators of FGF23 change FGF23 transcription and affect its post-translational modifications at the molecular level.

Physiology of FGF23 and overview of genetic diseases associated with renal phosphate wasting

Phosphate is a cornerstone of several physiological pathways including skeletal development, bone mineralization, membrane composition, nucleotide structure, maintenance of plasma pH, and cellular signaling. The kidneys have a key role in phosphate homeostasis with three hormones having important functions in renal phosphate handling or intestinal absorption: parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and 1-25-dihydroxyvitamin D (1,25(OH)2D). FGF23 is mainly synthesized by osteocytes; it is a direct phosphaturic factor that also inhibits 1,25(OH)2D and PTH. In addition to crucial effects on phosphate and calcium metabolism, FGF23 also has 'off-target' effects notably on the cardiovascular, immune and central nervous systems. Genetic diseases may affect the FGF23 pathway, resulting in either increased FGF23 levels leading to hypophosphatemia (such as in X-linked hypophosphatemia) or defective secretion/action of intact FGF23 inducing hyperphosphatemia (such as in familial tumoral calcinosis). The aim of this review is to provide an overview of FGF23 physiology and pathophysiology in X-linked hypophosphatemia, with a focus on FGF23-associated genetic diseases.

Is fibroblast growth factor 23 a new cardiovascular risk marker in gestational diabetes?

OBJECTIVE

This study was designed to compare the serum levels of fibroblast growth factor 23 (FGF23) among patients with gestational diabetes mellitus (GDM) and healthy pregnant women, and to evaluate the association between hormonal and metabolic parameters.

SUBJECTS AND METHODS

A total of 82 pregnant women were consecutively enrolled in the study. Of these, 46 were diagnosed as having GDM; the remaining 36 healthy pregnant women served as controls in a cross-sectional study design. The womens' ages ranged from 22 to 38 years and gestational ages, from 24 to 28 weeks. Serum samples were analyzed for FGF23 levels using an enzyme-linked immunosorbent assay.

RESULTS

Serum FGF23 levels were increased in patients with GDM compared with controls (median, 65.3 for patients with GDM vs. 36.6 ng/mL for healthy controls; p = 0.019). Mean fasting glucose (105.6 ± 7.4 vs. 70.2 ± 7.2 mg/dL, p < 0.001), HbA1c (5.6 ± 0.5 vs. 4.9 ± 0.5%, p < 0.001), insulin (median, 11.1 vs. 8.7 µIU/mL, p = 0.006) and HOMA-IR (3.0 (1.8) vs 1.4 (0.6), p < 0.001) levels were significantly higher in patients with GDM than in controls. Serum FGF23 level was positively correlated with body mass index (r2 = 0.346, p < 0.05), FPG (r2 = 0.264, p < 0.05), insulin (r2 = 0.388, p < 0.05), HOMA-IR (r2 = 0.384, p < 0.05).

CONCLUSION

Serum FGF23 levels were higher in women with GDM compared with controls. The present findings suggest that FGF23 could be a useful marker of cardiovascular disease in GDM.

Occurrence and severity of non-occlusive mesenteric ischemia (NOMI) after cardiovascular surgery correlate with preoperatively assessed FGF-23 levels

Purpose

To evaluate the value of preoperatively assessed fibroblast growth factor 23 (FGF-23) levels and to correlate FGF-23 with angiographic findings in non-occlusive mesenteric (NOMI) ischemia using a standardized scoring system.

Materials and methods

Between 2/2011 and 3/2012 a total of 865 patients (median age: 67 years) underwent cardiovascular surgery during this ethics committee approved, prospective study. 65 of these patients had clinical suspicion of NOMI and consequently underwent catheter angiography of the superior mesenteric artery. Images were assessed using a standardized reporting system (Homburg-NOMI-Score). These data were correlated to following preoperative parameters of kidney function: cystatin C, creatinine, FGF-23 and estimated glomerular filtration rate (eGFR), and outcome data (death, acute renal failure) using linear and logistic regressions, as well as nonparametric tests.

Results

Significant correlations were found between FGF-23 and the angiographic appearance of NOMI (p = 0.03). Linear regression analysis showed no significant correlation to the severity of NOMI with creatinine (p = 0.273), cystatin C (p = 0.484), cystatin C eGFR (p = 0.914) and creatinine eGFR (p = 0.380). Logistic regression revealed a significant correlation between death and the Homburg-NOMI-Score (p<0.001), but not between development of NOMI and acute renal failure (p = 0.122). The ROC Analysis yielded an area under the curve of 0.695 (95% CI: 0.627–0.763) with a sensitivity of 0.672 and specificity of 0.658.

Conclusions

FGF-23 significantly correlates with the severity of NOMI, which is in contrast to other renal function parameters. The applied scoring system allows to predict mortality in NOMI patients.

Klotho and fibroblast growth factor 23 in cerebrospinal fluid in children

The fibroblast growth factor (FGF) 23/Klotho axis is a principal regulator of phosphate hemostasis and vitamin D metabolism, but limited data is available on its role in the central nervous system. Here, we investigate soluble α-Klotho (sKlotho) and C-terminal as well as intact FGF23 in cerebrospinal fluid (CSF) and plasma and their relationship to mineral metabolism parameters in humans. In 39 children aged 0.3-16.8 years undergoing lumbar puncture for the exclusion of inflammatory neurological disease, sKlotho and FGF23 were investigated by Western blot analysis, followed by ELISA quantification in CSF and plasma. The percentage of intrathecal synthesis of both proteins was calculated by measuring both the expected and observed CSF/plasma ratios of sKlotho and FGF23. The secreted (KL1) and cleaved (KL1+KL2) isoforms of sKlotho, and FGF23 were clearly detected in CSF in all subjects, although protein levels were lower compared to those of plasma samples (each p < 0.01). The intrathecal percentage of CSF sKlotho and FGF23 synthesis amounted to 98 and 99 %, respectively. CSF sKlotho levels were higher in boys than in girls (p < 0.01), and correlated positively with plasma C-terminal FGF23 concentrations (p < 0.05) and standardized height (p < 0.01). Importantly, there were no significant correlations between plasma and CSF levels of sKlotho or FGF23. Plasma sKlotho as well as C-terminal and intact FGF23, respectively, were associated with parameters of mineral metabolism These results provide evidence that cleaved and secreted sKlotho and FGF23 are present in CSF, mainly derived from brain and affected by sex, height, and mineral metabolism parameters in children. Nevertheless, the absence of significant associations between plasma and CSF levels of Klotho and FGF23, respectively, suggest that the regulation of Klotho and FGF23 may be different between organs secreting these hormones into blood and CSF.

Cord Blood Ferritin and Fibroblast Growth Factor-23 Levels in Neonates

CONTEXT

Elevated levels of the phosphate-regulating hormone, fibroblast growth factor-23 (FGF-23) are associated with skeletal and cardiovascular disease. Levels of FGF-23 are elevated in neonates, but the mechanisms are poorly understood. Iron deficiency is a recently described stimulus for FGF-23 production.

OBJECTIVE

To test the hypothesis that lower fetal iron status, as measured by lower cord blood ferritin, is independently associated with elevated FGF-23 levels in neonates.

DESIGN AND PARTICIPANTS

This is a cross-sectional study of 64 full-term, healthy neonates.

SETTING

This study took place in a university-based, tertiary care center.

MAIN OUTCOME MEASURES

Plasma levels of second generation C-terminal FGF-23 (cFGF-23) and intact FGF-23 (iFGF-23).

RESULTS

Levels of cFGF-23 ranged from 108 to 7508 reference units (RU)/ml (median, 824 RU/ml), and iFGF-23 from undetectable (<8.5) to 135.4 pg/ml (median, <8.5 pg/mL). Ferritin ranged from 58 to 719 ng/ml (mean, 203 ng/ml). Lower cord blood ferritin levels were associated with higher cFGF-23 (r = −0.320; P = .014), but not iFGF-23 levels (r = −0.222; P = .082). In multivariate analyses adjusted for glycemic indices, maternal race, and parity, lower ferritin levels remained independently associated with higher cFGF-23 levels (B = −0.261, P = .01). In the full models, higher cord blood glucose and C-peptide levels were also independently associated with higher cFGF-23 levels.

CONCLUSIONS

cFGF-23, but not iFGF-23 levels, are elevated in cord blood of healthy term neonates and independently associated with lower serum ferritin and higher glycemic indices.

Klotho Prevents Translocation of NFκB

Klotho protein is a β-glucuronidase capable of hydrolyzing steroid β-glucuronides. Two molecules are produced by the Klotho gene, a membrane bound form and a circulating form. This protein is recognized as an antiaging gene with pleiotropic functions. The activation of cellular systems is associated with the pathogenesis of several chronic and degenerative diseases associated with an inflammatory state. Inflammation is characterized by an activation of NFκB. Klotho suppresses nuclear factor NFκB activation and the subsequent transcription of proinflammatory genes. This review focuses on the current understanding of Klotho protein function and its relationship with NFκB regulation, emphasizing its potential involvement in the pathophysiologic process.

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.

Key role of the kidney in the regulation of fibroblast growth factor 23

High circulating levels of fibroblast growth factor 23 (FGF23) have been demonstrated in kidney failure, but mechanisms of this are not well understood. Here we examined the impact of the kidney on the early regulation of intact FGF23 in acute uremia as induced by bilateral or unilateral nephrectomy (BNX and UNX, respectively) in the rat. BNX induced a significant increase in plasma intact FGF23 levels from 112 to 267 pg/ml within 15 min, which remained stable thereafter. UNX generated intact FGF23 levels between that seen in BNX and sham-operated rats. The intact to C-terminal FGF23 ratio was significantly increased in BNX rats. The rapid rise in FGF23 after BNX was independent of parathyroid hormone or FGF receptor signaling. No evidence of early stimulation of FGF23 gene expression in the bone was found. Furthermore, acute severe hyperphosphatemia or hypercalcemia had no impact on intact FGF23 levels in normal and BNX rats. The half-life of exogenous recombinant human FGF23 was significantly prolonged from 4.4 to 11.8 min in BNX rats. Measurements of plasma FGF23 in the renal artery and renal vein demonstrated a significant renal extraction. Thus the kidney is important in FGF23 homeostasis by regulation of its plasma level and metabolism.

Fibroblast growth factor 23/klotho axis in chronic kidney disease

Fibroblast growth factor-23 (FGF23) is a bone-derived hormone that regulates phosphate and 1,25-hydroxyvitamin D [1,25(OH)2D] metabolism. FGF23 binds to FGF receptor 1 with its coreceptor Klotho and maintains serum phosphate levels within the normal range by increasing renal phosphate excretion. In addition, FGF23 reduces the synthesis and accelerates the degradation of 1,25(OH)2D to reduce intestinal phosphate absorption. Moreover, FGF23 acts at the parathyroid gland to decrease parathyroid hormone synthesis and secretion. In chronic kidney disease (CKD), serum FGF23 levels rise exponentially as renal function declines long before a significant increase in serum phosphate concentration occurs. Although there is room for argument, FGF23 and Klotho are recently reported contributors to vascular calcification. Finally, prospective observational studies have shown that serum FGF23 concentrations predict mortality not only among dialysis patients but among predialysis CKD patients. In addition to being a coreceptor for FGF23, Klotho circulates as an endocrine substance and exerts a multitude of effects. This review describes recent advances in research on the FGF23-Klotho axis in CKD. © 2014 S. Karger AG, Basel.

Method-specific differences in plasma fibroblast growth factor 23 measurement using four commercial ELISAs

BACKGROUND

There is growing interest in measuring plasma fibroblast growth factor 23 (FGF23) concentration in a number of clinical settings. However, data comparing current commercial intact and C-terminal FGF23 assays is lacking.

METHODS

We used plasma samples collected from a cohort of healthy adults and patients undergoing chronic haemodialysis therapy (n=67) to compare the precision, recovery, linearity and pre-analytical stability characteristics of four commercial FGF23 assays from Kainos, Millipore and Immutopics Inc. Method agreement was evaluated using Passing-Bablok regression and difference plot analysis.

RESULTS

Both Millipore and Immutopics intact FGF23 kits demonstrated marked negative proportional bias relative to Kainos assay readout, particularly in the haemodialysis group, and poor recovery of purified FGF23 standard at high spiking concentrations. Dilution of high-reading samples with saline as recommended by the Immutopics kit resulted in significant deviation from linearity. Immutopics C-terminal FGF23 concentrations displayed a strong association with intact FGF23 concentrations determined with all three intact assays in the haemodialysis group, but showed no significant correlation within the physiological range. Only intact FGF23 measurements made with the Immutopics assay demonstrated evidence of significant instability 8 h after venepuncture.

CONCLUSIONS

Current ELISA kits for plasma intact FGF23 measurement show poor analytical agreement, and cannot be used interchangeably. This is mainly due to differences in calibration. Harmonisation of available assays using a common international standard would facilitate more meaningful interpretation of data from studies using different kits. Discordance between intact and C-terminal FGF23 assay measurements is more marked at physiological concentrations than in patients undergoing haemodialysis.

FGF-23: the rise of a novel cardiovascular risk marker in CKD

Elevated plasma levels of the phosphaturic hormone fibroblast growth factor 23 (FGF-23) are a hallmark of chronic kidney disease (CKD)-mineral and bone disorder. FGF-23 allows serum phosphate levels within physiological limits to be maintained in progressive CKD until end-stage renal disease is reached. Despite its seemingly beneficial role in phosphate homeostasis, several prospective studies in dialysis patients and in patients with less advanced CKD associated elevated FGF-23 with poor cardiovascular and renal outcome. Moreover, very recent evidence suggests an adverse prognostic impact of elevated FGF-23 even in subjects without manifest CKD. These epidemiological data are supplemented by laboratory findings that reveal a pathophysiological role of FGF-23 in the pathogenesis of myocardial injury. In aggregate, these clinical and experimental data identify FGF-23 as a promising target of novel therapeutic interventions in CKD and beyond, which should be tested in future clinical trials.

Fibroblast Growth Factor-23 and Hypophosphatemia in Chronic Obstructive Pulmonary Disease Patients

Impaired serum phosphate levels may contribute to respiratory muscle weakness that further negatively impacts Chronic Obstructive Pulmonary Disease (COPD) patients. Recently, Fibroblast Growth Factor 23 (FGF-23) has been shown to play an important role in the regulation of body phosphate. The current study includes 2 groups: 70 COPD patients and 34 control subjects. Blood samples were taken for a panel of routine lab tests. FGF-23 was measured using a commercially available ELISA kit. Plasma FGF-23 levels were significantly higher in the patient group compared to the control group (P=0.000). Tubular maximum absorption of phosphate was significantly reduced in COPD patients compared to the control group (P=0.04). Plasma FGF-23 negatively correlated with FEV1 and serum albumin. Elevated plasma FGF-23 levels found in COPD patients correlated with disease severity and may represent an additional factor causing low serum phosphate.

FGF23 analysis of a Chinese family with autosomal dominant hypophosphatemic rickets

Autosomal dominant hypophosphatemic rickets (ADHR; MIM 193100) is a hereditary disorder characterized by isolated renal phosphate wasting, hypophosphatemia, and inappropriately normal 1,25-dihydroxyvitamin D(3) levels. Recent studies have shown that the fibroblast growth factor 23 (FGF23) gene is responsible for this disease. FGF23 protein is a phosphaturic factor that is elevated in several diseases associated with hypophosphatemia and rickets but varies with disease status in ADHR. In the present study we observed a Chinese family of Han ethnic origin diagnosed with ADHR. The proband is a 30-year-old woman with no history of rickets but with multiple tooth abscesses as a young adult. She presented with progressive painful swelling of the left ankle after a blunt trauma at 26 years of age. She developed back pain, generalized weakness, and fatigue, and she could barely walk at age 27. She was found to have severe hypophosphatemia, low ratio of phosphorus tubule maximum (TmP) to glomerular filtration rate (GFR) (TmP/GFR), and elevated alkaline phosphatase at age 28. Her brother, 26 years old, presented with fatigue at 24 years of age and is normophosphatemic. The parents of this family had no history of rickets or hypophosphatemia. Direct sequence analysis of genomic DNA demonstrated a single heterozygous c.527G>A (p.R176Q) mutation in the FGF23 gene in three family members, including the proband, her brother, and their mother. Intact FGF23 assay of seven time points during the oral phosphate loading test showed no significant relationship between intact FGF23 and serum phosphorus levels of the subject with ADHR and a control. It is probably the first report of a Chinese family with ADHR.

Can features of phosphate toxicity appear in normophosphatemia?

Phosphate is an indispensable nutrient for the formation of nucleic acids and the cell membrane. Adequate phosphate balance is a prerequisite for basic cellular functions ranging from energy metabolism to cell signaling. More than 85% of body phosphate is present in the bones and teeth. The remaining phosphate is distributed in various soft tissues, including skeletal muscle. A tiny amount, around 1% of total body phosphate, is distributed both in the extracellular fluids and within the cells. Impaired phosphate balance can affect the functionality of almost all human systems, including muscular, skeletal, and vascular systems, leading to an increase in morbidity and mortality of the involved patients. Currently, measuring serum phosphate level is the gold standard to estimate the overall phosphate status of the body. Despite the biological and clinical significance of maintaining delicate phosphate balance, serum levels do not always reflect the amount of phosphate uptake and its distribution. This article briefly discusses the potential that some of the early consequences of phosphate toxicity might not be evident from serum phosphate levels.

Fibroblast growth factor 23 in chronic kidney disease: New insights and clinical implications

Fibroblast growth factor 23 (FGF-23) is a recently discovered regulator of phosphate and mineral metabolism. Its main physiological function is the enhancement of renal phosphate excretion. FGF-23 levels are inversely related to renal function and in patients with chronic kidney disease (CKD) elevation in FGF-23 precedes the rise of serum phosphate. Studies have demonstrated an important role for FGF-23 in the development of secondary hyperparathyroidism through an effect on parathyroid hormone and calcitriol. In cross-sectional studies FGF-23 has been associated with surrogate markers of cardiovascular disease such as endothelial dysfunction and arterial stiffness. FGF-23 has also been associated with both progression of CKD and mortality in dialysis patients. The discovery of FGF-23 has provided a profound new insight into bone and mineral metabolism, and it may become an important biomarker and therapeutic target in CKD.

[FGF23 and Klotho: the new cornerstones of phosphate/calcium metabolism]

Since its first description as a phosphaturic agent in the early 2000s, fibroblast growth factor 23 (FGF23) has rapidly become the third key player of phosphate/calcium metabolism after PTH and vitamin D. FGF23 is a protein synthesized by osteocytes that acts mainly as a phosphaturic factor and a suppressor of 1α hydroxylase activity in the kidney. It inhibits the expression of type IIa and IIc sodium-phosphate cotransporters on the apical membrane of proximal tubular cells, thus leading to inhibition of phosphate reabsorption. Moreover, it also inhibits 1α hydroxylase activity. These two renal pathways account together for the hypophosphatemic effect of FGF23, but FGF23 has also been recently described as an inhibiting factor for PTH synthesis. Its exact role in bone remains to be defined. A transmembrane protein, Klotho, is an essential cofactor for FGF23 biological activity, but it can also act by itself for calcium and PTH regulation. This paper gives an overview of these recent data of phosphate/calcium physiology, as well as a description of clinical conditions associated with FGF23 deregulation (genetic diseases and chronic kidney disease). As a conclusion, future therapeutic consequences of the FGF23/Klotho axis are discussed.

Clinical Significance of FGF-23 in Patients with CKD

FGF23 is a bone-derived hormone that plays an important role in the regulation of phosphate and 1,25-dihydroxy vitamin D metabolism. FGF23 principally acts in the kidney to induce urinary phosphate excretion and suppress 1,25-dihydroxyvitamin D synthesis in the presence of FGF receptor 1 (FGFR1) and its coreceptor Klotho. In patients with chronic kidney disease (CKD), circulating FGF23 levels are progressively increased to compensate for persistent phosphate retention, but this results in reduced renal production of 1,25-dihydroxyvitamin D and leads to hypersecretion of parathyroid hormone. Furthermore, FGF23 is associated with vascular dysfunction, atherosclerosis, and left ventricular hypertrophy. This paper summarizes the role of FGF23 in the pathogenesis of mineral, bone, and cadiovascular disorders in CKD.

Osteo-renal regulation of systemic phosphate metabolism

Impaired kidney function and subsequent skeletal responses play a critical role in disrupting phosphate balance in chronic kidney disease (CKD) patients with mineral and bone disorder (CKD-MBD). In patients with CKD-MBD, the inability of the kidney to maintain normal mineral ion balance affects bone remodeling to induce skeletal fracture and extraskeletal vascular calcification. In physiological conditions, bone-derived fibroblast growth factor 23 (FGF23) acts on the kidney to reduce serum phosphate and 1,25-dihydroxyvitamin D levels. In humans, increased bioactivity of FGF23 leads to increased urinary phosphate excretion, which induces hypophosphatemic diseases (e.g., rickets/osteomalacia). However, reduced FGF23 activity is associated with hyperphosphatemic diseases (e.g., tumoral calcinosis). In patients with CKD, high serum levels of FGF23 fail to reduce serum phosphate levels and lead to numerous complications, including vascular calcification, one of the important determinants of mortality of CKD-MBD patients. Of particular significance, molecular, biochemical and morphological changes in patients with CKD-MBD are mostly due to osteo-renal dysregulation of mineral ion metabolism. Furthermore, hyperphosphatemia can partly contribute to the development of secondary hyperparathyroidism in patients with CKD-MBD. Relatively new pharmacological agents including sevelamer hydrochloride, calcitriol analogs and cinacalcet hydrochloride are used either alone, or in combination, to minimize hyperphosphatemia and hyperparathyroidism associated complications to improve morbidity and mortality of CKD-MBD patients. This article will briefly summarize how osteo-renal miscommunication can induce phosphate toxicity, resulting in extensive tissue injuries.

Phosphate toxicity: new insights into an old problem

Phosphorus is an essential nutrient required for critical biological reactions that maintain the normal homoeostatic control of the cell. This element is an important component of different cellular structures, including nucleic acids and cell membranes. Adequate phosphorus balance is vital for maintaining basic cellular functions, ranging from energy metabolism to cell signalling. In addition, many intracellular pathways utilize phosphate ions for important cellular reactions; therefore, homoeostatic control of phosphate is one of the most delicate biological regulations. Impaired phosphorus balance can affect the functionality of almost every human system, including musculoskeletal and cardiovascular systems, ultimately leading to an increase in morbidity and mortality of the affected patients. Human and experimental studies have found that delicate balance among circulating factors, like vitamin D, PTH (parathyroid hormone) and FGF23 (fibroblast growth factor 23), are essential for regulation of physiological phosphate balance. Dysregulation of these factors, either alone or in combination, can induce phosphorus imbalance. Recent studies have shown that suppression of the FGF23-klotho system can lead to hyperphosphataemia with extensive tissue damage caused by phosphate toxicity. The cause and consequences of phosphate toxicity will be briefly summarized in the present review.

Relationship between Fibroblast Growth Factor-23 and Mineral Metabolism in Chronic Kidney Disease

Fibroblast growth factor- (FGF-)23 is a recently discovered regulator of calcium-phosphate metabolism. FGF-23 appears to decrease in synthesis and accelerated degradation of 1,25(OH)₂D. Together with its cofactor Klotho, FGF-23 maintains serum phosphate levels within the normal range by increasing renal phosphate excretion. In chronic kidney disease (CKD), FGF-23 levels rise in parallel with the decline in renal function long before a significant increase in serum phosphate concentration occurs. Both Klotho and FGF-23, linked by a receptor mechanism, affect vitamin D synthesis and parathyroid hormone (PTH) secretion. Previous studies have shown a close association between reduced FGF-23 or Klotho activities and vascular calcification. The possible association of FGF-23 and left ventricular hypertrophy or vascular dysfunction has been proposed. Finally, prospective studies have shown that high serum FGF-23 concentrations predict more rapid disease progression in CKD patients who were not on dialysis and increased mortality in patients on maintenance hemodialysis. FGF-23 may therefore prove to be an important therapeutic target for the management of CKD.

Evaluation of a method for fibroblast growth factor-23: a novel biomarker of adverse outcomes in patients with renal disease

Abstract Background: Fibroblast growth factor 23 (FGF-23), a phosphaturic peptide hormone secreted by the osteoblasts, is an important regulator of phosphorus and vitamin D metabolism. In chronic kidney disease, FGF-23 levels rise with declining kidney function. Increasing FGF-23 levels are associated with increasing risk of mortality in dialysis patients. Two assays for FGF-23 have been reported. One assay detects only full-length/intact FGF-23. In contrast, the carboxy-terminal assay recognizes both intact and carboxy-terminal FGF-23.

AIM/METHODS

The aim of this study was to evaluate both assays for FGF-23. Test samples were analyzed with both the intact and carboxy-terminal FGF-23 enzyme-linked immunosorbent assay (ELISA) kits according to manufacturers' instructions.

RESULTS

Carboxy-terminal FGF-23 showed very good precision with coefficients of variation (CV) ranging from 4% to 10.5%, whereas the CVs for intact FGF-23 were not very good (6-37.5%). The carboxy-terminal assay was linear, stable in plasma samples, and was not affected by common interferents. Also, the carboxy-terminal FGF-23 assay appeared to correlate better with worsening of kidney function as assessed by plasma creatinine and calculated estimated glomerular filtration rate (eGFR).

CONCLUSION

Thus, the carboxy-terminal FGF-23 assay is robust and can be used in prospective trials to validate its utility as a biomarker of adverse outcomes in patients with renal disease.

Fibroblast growth factor 23 concentrations in healthy term infants during the early postpartum period

Fibroblast growth factor 23 (FGF23) is a potent regulator of Pi and 1,25-(OH)(2)D homeostasis. Early postpartum infants show intriguing changes in serum levels of Ca, Pi, PTH and 1,25-(OH)(2)D. However, the role of FGF23 in the early neonatal mineral metabolism has not been clarified. In order to evaluate the significance of FGF23 during the early postpartum period, we examined the circulating FGF23 levels using an intact FGF23 ELISA and a C-terminal FGF23 ELISA either in 22 umbilical cord blood samples (the cord blood) or in 22 term infants at 5days of life (the 5-day-old infant). We also compared these ranges with those of 11 healthy adults. Data were expressed as mean+/-SD, and analyzed by two-way ANOVA, followed by the Tukey's test. C-terminal FGF23 in the cord blood, the 5-day-old infants and the healthy adults were 73.3+/-22.4, 81.0+/-28.2 and 39.0+/-7.8 RU/ml, respectively. Intact FGF23 in the cord blood, the 5-day-old infants and the healthy adults were 3.9+/-1.6, 21.8+/-17.6, and 27.6+/-7.3 pg/ml, respectively. Immunoprecipitation assays using anti-FGF23 antibodies demonstrated that the intact 32 kDa FGF23 was low and the fragmented FGF23 of 18kDa was abundant in the cord blood compared with those in the healthy adults. In conclusion, our observations indicated that the intact FGF23/C-terminal FGF23 ratio was very low due to the fragmentation of FGF23 during the early postpartum period and might have a considerable contribution to the Pi homeostasis in the healthy term infants.

The influence of glomerular filtration rate and age on fibroblast growth factor 23 serum levels in pediatric chronic kidney disease

BACKGROUND

Fibroblast growth factor 23 (FGF23) is a phosphaturic factor and a suppressor of 1alpha-hydroxylase activity in the kidney. Although its importance in chronic kidney disease (CKD) has been demonstrated in adults, there is little information in pediatric patients.

OBJECTIVES

The aims of this study were: 1) to determine reference values for FGF23 serum levels according to glomerular filtration rate (GFR) (measured by the reference standard, inulin clearance), gender, and age; and 2) to evaluate the effects of different etiologies and treatments on FGF23 serum levels in a prospective single-center cohort of 227 CKD children (119 boys).

RESULTS

Age, body weight, height, and GFR (mean +/- sd) values were: 11.3 +/- 4.1 yr, 37 +/- 16 kg, 140 +/- 20 cm, and 98 +/- 34 ml/min per 1.73 m(2), respectively. Calcium, phosphate, PTH, 25 hydroxyvitamin D, 1,25 dihydroxyvitamin D, C-terminal FGF23, and intact FGF23 (mean +/- sd) levels were: 2.43 +/- 0.11 mmol/liter, 1.41 +/- 0.22 mmol/liter, 41 +/- 23 pg/ml, 24 +/- 10 ng/ml, 152 +/- 72 pmol/liter, 76 +/- 134 relative units/ml, and 44 +/- 37 pg/ml, respectively. There was a wide range of FGF23 serum levels, but FGF23 levels increased when GFR decreased. FGF23 serum levels were not modified by gender, but they increased with age. In univariate analysis, corticosteroid therapy seemed to be associated with increased FGF23 serum levels. A multivariate linear regression analysis found a significant impact of GFR, body mass index, and solid organ transplantation on FGF23 serum levels.

CONCLUSION

Age, GFR, body mass index, and solid organ transplantation seem to influence FGF23 serum levels in a pediatric population. The impact of corticosteroids on FGF23 metabolism should be further investigated; further longitudinal studies will also help to better define the prognostic impact of FGF23 serum levels in pediatric CKD in terms of disease progression, cardiovascular morbidities, and bone disabilities.

Clinical relevance of FGF-23 in chronic kidney disease

Fibroblast growth factor (FGF)-23 is a recently discovered regulator of calcium-phosphate metabolism. Whereas other known FGFs mainly act in a paracrine manner, FGF-23 has significant systemic effects. Together with its cofactor Klotho, FGF-23 enhances renal phosphate excretion in order to maintain serum phosphate levels within the normal range. In patients with chronic kidney disease (CKD), FGF-23 levels rise in parallel with declining renal function long before a significant increase in serum phosphate concentration can be detected. However, in cross-sectional studies increased FGF-23 levels in patients with CKD were found to be associated not only with therapy-resistant secondary hyperparathyroidism but were also independently related to myocardial hypertrophy and endothelial dysfunction after adjustment for traditional markers of calcium-phosphate metabolism. Finally, in prospective studies high serum FGF-23 concentrations predicted faster disease progression in CKD patients not on dialysis, and increased mortality in patients receiving maintenance hemodialysis. FGF-23 may therefore prove to be an important therapeutic target in the management of CKD.

FGF-23-Klotho signaling stimulates proliferation and prevents vitamin D-induced apoptosis

Fibroblast growth factor 23 (FGF-23) and Klotho are secretory proteins that regulate mineral-ion metabolism. Fgf-23(-/-) or Klotho(-/-) knockout mice exhibit several pathophysiological processes consistent with premature aging including severe atrophy of tissues. We show that the signal transduction pathways initiated by FGF-23-Klotho prevent tissue atrophy by stimulating proliferation and preventing apoptosis caused by excessive systemic vitamin D. Because serum levels of active vitamin D are greatly increased upon genetic ablation of Fgf-23 or Klotho, we find that these molecules have a dual role in suppression of apoptotic actions of vitamin D through both negative regulation of 1alpha-hydroxylase expression and phosphoinositide-3 kinase-dependent inhibition of caspase activity. These data provide new insights into the physiological roles of FGF-23 and Klotho.

Genetic evidence of serum phosphate-independent functions of FGF-23 on bone

Maintenance of physiologic phosphate balance is of crucial biological importance, as it is fundamental to cellular function, energy metabolism, and skeletal mineralization. Fibroblast growth factor-23 (FGF-23) is a master regulator of phosphate homeostasis, but the molecular mechanism of such regulation is not yet completely understood. Targeted disruption of the Fgf-23 gene in mice (Fgf-23^−/−) elicits hyperphosphatemia, and an increase in renal sodium/phosphate co-transporter 2a (NaPi2a) protein abundance. To elucidate the pathophysiological role of augmented renal proximal tubular expression of NaPi2a in Fgf-23^−/− mice and to examine serum phosphate–independent functions of Fgf23 in bone, we generated a new mouse line deficient in both Fgf-23 and NaPi2a genes, and determined the effect of genomic ablation of NaPi2a from Fgf-23^−/− mice on phosphate homeostasis and skeletal mineralization. Fgf-23^−/−/NaPi2a^−/− double mutant mice are viable and exhibit normal physical activities when compared to Fgf-23^−/− animals. Biochemical analyses show that ablation of NaPi2a from Fgf-23^−/− mice reversed hyperphosphatemia to hypophosphatemia by 6 weeks of age. Surprisingly, despite the complete reversal of serum phosphate levels in Fgf-23^−/−/NaPi2a^−/−, their skeletal phenotype still resembles the one of Fgf23^−/− animals. The results of this study provide the first genetic evidence of an in vivo pathologic role of NaPi2a in regulating abnormal phosphate homeostasis in Fgf-23^−/− mice by deletion of both NaPi2a and Fgf-23 genes in the same animal. The persistence of the skeletal anomalies in double mutants suggests that Fgf-23 affects bone mineralization independently of systemic phosphate homeostasis. Finally, our data support (1) that regulation of phosphate homeostasis is a systemic effect of Fgf-23, while (2) skeletal mineralization and chondrocyte differentiation appear to be effects of Fgf-23 that are independent of phosphate homeostasis.

Regulation of phosphate homeostasis is a tightly controlled hormonal process involving the intestine, kidneys, and bone, and imbalance of this homeostasis may influence overall mineralization. Fibroblast growth factor-23 (FGF-23) is a circulating hormone produced in the bone that mainly targets the kidneys to control the activity of the sodium/phosphate co-transporters NaPi2a and NaPi2c. These transporters are responsible for actively reabsorbing phosphate ions into the body to maintain physiological serum phosphate levels. Changes in FGF-23 activity lead to human disorders associated with either phosphate wasting or retention. Genetically altered mice in which Fgf-23 activity is lost exhibit severe hyperphosphatemia accompanied by increased NaPi2a activity, and they develop abnormal bone mineralization. Here we describe a new mouse model in which we eliminated NaPi2a from Fgf-23 null mice and show reversal of hyperphosphatemia to hypophosphatemia, suggesting that NaPi2a is the major regulator of phosphate homeostasis. However, the skeletal mineralization defect observed in mice lacking Fgf-23 function remained unchanged in the absence of NaPi2a in these mice. Thus our data indicate that Fgf-23 has a role in controlling bone mineralization independent of systemic phosphate levels.

Role of fibroblast growth factor 23 in phosphate homeostasis and pathogenesis of disordered mineral metabolism in chronic kidney disease

The discovery of fibroblast growth factor 23 (FGF23), a novel bone-derived hormone that inhibits phosphate reabsorption and calcitriol production by the kidney, has uncovered primary regulatory pathways and new systems biology governing bone mineralization, vitamin D metabolism, parathyroid gland function and renal phosphate handling. This phosphaturic hormone, which is made predominately by osteocytes in bone, appears to have a physiologic role as a counter-regulatory hormone for vitamin D. Evidence has also emerged to support the existence of a bone-kidney axis to coordinate the mineralization of bone with renal handling of phosphate. Pathologically, high circulating levels of FGF23 result in hypophosphatemia, decreased production of 1,25(OH)(2)D, elevated parathyroid hormone and rickets/osteomalacia in patients with functioning kidneys, whereas low levels are associated with tumoral calcinosis, hyperphosphatemia and elevated 1,25(OH)(2)D. In addition, patients with chronic kidney disease (CKD) exhibit marked elevations of circulating FGF23. While the significance of increased FGF23 levels in CKD remains to be defined, it might contribute to phosphate excretion and suppression of 1,25(OH)(2)D levels in CKD stages 3 and 4, as well as potentially contribute to secondary hyperparathyroidism through direct actions on the parathyroid gland in more advanced renal failure. As our knowledge expands regarding the regulation and functions of FGF23, the assessment of FGF23 will become an important diagnostic marker as well as a therapeutic target for management of disordered mineral metabolism in a variety of acquired and hereditary disorders.

Mineral metabolism and aging: the fibroblast growth factor 23 enigma

PURPOSE OF REVIEW

The regulation of phosphate homeostasis was thought to be passively mediated by the calciotrophic hormones parathyroid hormone and 1,25(OH)2D3. This article summarizes the emerging trends that show an active regulation of phosphate homeostasis by fibroblast growth factor 23 (FGF-23) - a process fairly independent of calcium homeostasis - and how altered mineral ion metabolism may affect the aging process.

RECENT FINDINGS

A major breakthrough in FGF-23 biology has been achieved by the demonstration of strikingly similar physical/biochemical phenotypes of Fgf-23(-/-) and klotho hypomorph mice, which eventually led to the identification of klotho as a cofactor in FGF-23 and its receptor interactions. Furthermore, FGF-23 has emerged as a counter regulator of the renal 1alpha(OH)ase and sodium-phosphate cotransporter activities to modulate phosphate homeostasis. Finally, studies point towards a role of dentine matrix protein 1 in affecting phosphate homeostasis, in coordination with FGF-23.

SUMMARY

Recent mouse genetic studies have broadened our understanding of biochemical/molecular pathways involved in phosphate homeostasis, and linked FGF-23 to such regulation. Understanding the molecular interactions of essential calcium and phosphate regulators will enhance our knowledge of the coordinated regulation of mineral ion metabolism, and will help to redefine the molecular pathology of age-associated lesions accompanied by abnormal mineral ion metabolism such as vascular calcifications and osteoporosis.

Emerging role of fibroblast growth factor 23 in a bone–kidney axis regulating systemic phosphate homeostasis and extracellular matrix mineralization

PURPOSE OF REVIEW

To describe emerging understanding of fibroblast growth factor 23 (FGF23) - a bone-derived hormone that inhibits phosphate reabsorption and calcitriol production by kidney and participates as the principle phosphaturic factor in a bone-kidney axis coordinating systemic phosphate homeostasis and bone mineralization.

RECENT FINDINGS

FGF23 (a circulating factor made by osteocytes in bone) inhibits phosphate reabsorption and 1,25(OH)2D production by kidney. Physiologically, FGF23 is a counter-regulatory phosphaturic hormone for vitamin D and coordinates systemic phosphate homeostasis with skeletal mineralization. Pathologically, high circulating FGF23 levels cause hypophosphatemia, decreased 1,25(OH)2D production, elevated parathyroid hormone and rickets/osteomalacia. FGF23 mutations impairing its degradation cause autosomal dominant hypophosphatemic rickets. Respective loss-of-function mutations of osteocyte gene products DMP1 and Phex cause autosomal recessive hypophosphatemic rickets and X-linked hypophosphatemic rickets, initiating increased FGF23 production. Low FGF23 levels lead to hyperphosphatemia, elevated 1,25(OH)2D, and soft-tissue calcifications. FGF23 is markedly increased in chronic renal disease, but its role remains undefined.

SUMMARY

FGF23 discovery has uncovered primary regulatory pathways and new systems biology governing bone mineralization, vitamin D metabolism, parathyroid gland function, and renal phosphate handling. FGF23 assessment will become important in diagnosing hypophosphatemic and hyperphosphatemic disorders, for which pharmacological regulation of FGF23 levels may provide novel treatments.

FGF23 induces expression of two isoforms of NAB2, which are corepressors of Egr-1

Fibroblast growth factor 23 (FGF23) is a key humoral factor in phosphate homeostasis and skeletogenesis, though the nature of its intracellular signaling is still unclear. Recently, Egr-1, a zinc-finger transcription factor, was identified as an immediate early response gene of FGF23 in the kidney. We report here, that FGF23 induces not only Egr-1 but also two isoforms of NAB2, which are specific co-repressors of Egr-1. Both isoforms of NAB2 induced by FGF23 were localized in the nucleus and suppressed the transcriptional activity of Egr-1. A negative feedback loop established by Egr-1 and NAB2 may thus be involved in mediating the physiological effects of FGF23.