Fibroblast growth factor-23 is regulated by 1alpha,25-dihydroxyvitamin D

1,25-Dihydroxyvitamin D3 regulates furin-mediated FGF23 cleavage

Intact fibroblast growth factor 23 (iFGF23) is a phosphaturic hormone that is cleaved by furin into N-terminal and C-terminal fragments. Several studies have implicated vitamin D in regulating furin in infections. Thus, we investigated the effect of 1,25-dihydroxyvitamin D3 [1,25(OH)2D] and the vitamin D receptor (VDR) on furin-mediated iFGF23 cleavage. Mice lacking VDR (Vdr-/-) had a 25-fold increase in iFGF23 cleavage, with increased furin levels and activity compared with wild-type (WT) littermates. Inhibition of furin activity blocked the increase in iFGF23 cleavage in Vdr-/- animals and in a Vdr-knockdown osteocyte OCY454 cell line. Chromatin immunoprecipitation revealed VDR binding to DNA upstream of the Furin gene, with more transcription in the absence of VDR. In WT mice, furin inhibition reduced iFGF23 cleavage, increased iFGF23, and reduced serum phosphate levels. Similarly, 1,25(OH)2D reduced furin activity, decreased iFGF23 cleavage, and increased total FGF23. In a post hoc analysis of a randomized clinical trial, we found that ergocalciferol treatment, which increased serum 1,25(OH)2D, significantly decreased serum furin activity and iFGF23 cleavage, compared with placebo. Thus, 1,25(OH)2D inhibits iFGF23 cleavage via VDR-mediated suppression of Furin expression, thereby providing a mechanism by which vitamin D can augment phosphaturic iFGF23 levels.

Chenodeoxycholic acid regulates fibroblast growth factor 23 gene expression via estrogen-related receptor γ in human hepatoma Huh7 cells

Fibroblast growth factor 23 (FGF23) is a glycoprotein that belongs to the FGF19 subfamily and participates in phosphate and vitamin D homeostasis. Chenodeoxycholic acid (CDCA), one of the primary bile acids, is reported to induce the secretion of FGF19 subfamily members, FGF21 and FGF19, in hepatocytes. However, whether and how CDCA influences FGF23 gene expression are largely unknown. Thus, we performed real-time polymerase chain reaction and Western blot analyses to determine the mRNA and protein expression levels of FGF23 in Huh7 cells. CDCA upregulated estrogen-related receptor γ (ERRγ) alongside FGF23 mRNA and protein levels, while, the knockdown of ERRγ ablated the induction effect of CDCA on FGF23 expression. Promoter studies showed that CDCA-induced FGF23 promoter activity occurred partly through ERRγ binding directly to the ERR response element (ERRE) in the human FGF23 gene promoter. Finally, the inverse agonist of ERRγ, GSK5182 inhibited the induction of FGF23 by CDCA. Overall, our results revealed the mechanism of CDCA-mediated FGF23 gene upregulation in the human hepatoma cell line. Moreover, the ability of GSK5182 to reduce CDCA-induced FGF23 gene expression might represent a therapeutic strategy to control abnormal FGF23 induction in conditions that involve elevated levels of bile acids, such as nonalcoholic fatty liver disease and biliary atresia.

Forestalling Hungry Bone Syndrome after Parathyroidectomy in Patients with Primary and Renal Hyperparathyroidism

Hungry bone syndrome (HBS), severe hypocalcemia following parathyroidectomy (PTX) due to rapid drop of PTH (parathormone) after a previous long term elevated concentration in primary (PHPT) or renal hyperparathyroidism (RHPT), impairs the outcome of underlying parathyroid disease.

OBJECTIVE

overview HBS following PTx according to a dual perspective: pre- and post-operative outcome in PHPT and RHPT. This is a case- and study-based narrative review.

INCLUSION CRITERIA

key research words "hungry bone syndrome" and "parathyroidectomy"; PubMed access; in extenso articles; publication timeline from Inception to April 2023.

EXCLUSION CRITERIA

non-PTx-related HBS; hypoparathyroidism following PTx. We identified 120 original studies covering different levels of statistical evidence. We are not aware of a larger analysis on published cases concerning HBS (N = 14,349). PHPT: 14 studies (N = 1545 patients, maximum 425 participants per study), and 36 case reports (N = 37), a total of 1582 adults, aged between 20 and 72. Pediatric PHPT: 3 studies (N = 232, maximum of 182 participants per study), and 15 case reports (N = 19), a total of 251 patients, aged between 6 and 18. RHPT: 27 studies (N = 12,468 individuals, the largest cohort of 7171) and 25 case reports/series (N = 48), a total of 12,516 persons, aged between 23 and 74. HBS involves an early post-operatory (emergency) phase (EP) followed by a recovery phase (RP). EP is due to severe hypocalcemia with various clinical elements (<8.4 mg/dL) with non-low PTH (to be differentiated from hypoparathyroidism), starting with day 3 (1 to 7) with a 3-day duration (up to 30) requiring prompt intravenous calcium (Ca) intervention and vitamin D (VD) (mostly calcitriol) replacement. Hypophosphatemia and hypomagnesiemia may be found. RP: mildly/asymptomatic hypocalcemia controlled under oral Ca+VD for maximum 12 months (protracted HBS is up to 42 months). RHPT associates a higher risk of developing HBS as compared to PHPT. HBS prevalence varied from 15% to 25% up to 75-92% in RHPT, while in PHPT, mostly one out of five adults, respectively, one out of three children and teenagers might be affected (if any, depending on study). In PHPT, there were four clusters of HBS indicators. The first (mostly important) is represented by pre-operatory biochemistry and hormonal panel, especially, increased PTH and alkaline phosphatase (additional indicators were elevated blood urea nitrogen, and a high serum calcium). The second category is the clinical presentation: an older age for adults (yet, not all authors agree); particular skeleton involvement (level of case reports) such as brown tumors and osteitis fibrosa cystica; insufficient evidence for the patients with osteoporosis or those admitted for a parathyroid crisis. The third category involves parathyroid tumor features (increased weight and diameter; giant, atypical, carcinomas, some ectopic adenomas). The fourth category relates to the intra-operatory and early post-surgery management, meaning an associated thyroid surgery and, maybe, a prolonged PTx time (but this is still an open issue) increases the risk, as opposite to prompt recognition of HBS based on calcium (and PTH) assays and rapid intervention (specific interventional protocols are rather used in RHPT than in PHPT). Two important aspects are not clarified yet: the use of pre-operatory bisphosphonates and the role of 25-hydroxyitamin D assay as pointer of HBS. In RHPT, we mentioned three types of evidence. Firstly, risk factors for HBS with a solid level of statistical evidence: younger age at PTx, pre-operatory elevated bone alkaline phosphatase, and PTH, respectively, normal/low serum calcium. The second group includes active interventional (hospital-based) protocols that either reduce the rate or improve the severity of HBS, in addition to an adequate use of dialysis following PTx. The third category involves data with inconsistent evidence that might be the objective of future studies to a better understanding; for instance, longer pre-surgery dialysis duration, obesity, an elevated pre-operatory calcitonin, prior use of cinalcet, the co-presence of brown tumors, and osteitis fibrosa cystica as seen in PHPT. HBS remains a rare complication following PTx, yet extremely severe and with a certain level of predictability; thus, the importance of being adequately identified and managed. The pre-operatory spectrum of assessments is based on biochemistry and hormonal panel in addition to a specific (mostly severe) clinical presentation while the parathyroid tumor itself might provide useful insights as potential risk factors. Particularly in RHPT, prompt interventional protocols of electrolytes surveillance and replacement, despite not being yet a matter of a unified, HBS-specific guideline, prevent symptomatic hypocalcemia, reduce the hospitalization stay, and the re-admission rates.

Shedding Light on the Complex Regulation of FGF23

Early research has suggested a rather straightforward relation between phosphate exposure, increased serum FGF23 (Fibroblast Growth Factor 23) concentrations and clinical endpoints. Unsurprisingly, however, subsequent studies have revealed a much more complex interplay between autocrine and paracrine factors locally in bone like PHEX and DMP1, concentrations of minerals in particular calcium and phosphate, calciprotein particles, and endocrine systems like parathyroid hormone PTH and the vitamin D system. In addition to these physiological regulators, an expanding list of disease states are shown to influence FGF23 levels, usually increasing it, and as such increase the burden of disease. While some of these physiological or pathological factors, like inflammatory cytokines, may partially confound the association of FGF23 and clinical endpoints, others are in the same causal path, are targetable and hence hold the promise of future treatment options to alleviate FGF23-driven toxicity, for instance in chronic kidney disease, the FGF23-associated disease with the highest prevalence by far. These factors will be reviewed here and their relative importance described, thereby possibly opening potential means for future therapeutic strategies.

Oral Acid Load Down-Regulates Fibroblast Growth Factor 23

Increased dietary acid load has a negative impact on health, particularly when renal function is compromised. Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that is elevated during renal failure. The relationship between metabolic acidosis and FGF23 remains unclear. To investigate the effect of dietary acid load on circulating levels of FGF23, rats with normal renal function and with a graded reduction in renal mass (1/2 Nx and 5/6 Nx) received oral NH4Cl for 1 month. Acid intake resulted in a consistent decrease of plasma FGF23 concentrations in all study groups when compared with their non-acidotic control: 239.3 ± 13.5 vs. 295.0 ± 15.8 pg/mL (intact), 346.4 ± 19.7 vs. 522.6 ± 29.3 pg/mL (1/2 Nx) and 988.0 ± 125.5 vs. 2549.4 ± 469.7 pg/mL (5/6 Nx). Acidosis also decreased plasma PTH in all groups, 96.5 ± 22.3 vs. 107.3 ± 19.1 pg/mL, 113.1 ± 17.3 vs. 185.8 ± 22.2 pg/mL and 504.9 ± 75.7 vs. 1255.4 ± 181.1 pg/mL. FGF23 showed a strong positive correlation with PTH (r = 0.877, p < 0.0001) and further studies demonstrated that acidosis did not influence plasma FGF23 concentrations in parathyroidectomized rats, 190.0 ± 31.6 vs. 215 ± 25.6 pg/mL. In conclusion, plasma concentrations of FGF23 are consistently decreased in rats with metabolic acidosis secondary to increased acid intake, both in animals with intact renal function and with decreased renal function. The in vivo effect of metabolic acidosis on FGF23 appears to be related to the simultaneous decrease in PTH.

PTH and FGF23 Exert Interdependent Effects on Renal Phosphate Handling: Evidence From Patients With Hypoparathyroidism and Hyperphosphatemic Familial Tumoral Calcinosis Treated With Synthetic Human PTH 1-34

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) both influence blood phosphate levels by regulating urinary phosphate reabsorption. Clinical data suggest that adequate renal phosphate handling requires the presence of both FGF23 and PTH, but robust evidence is lacking. To investigate whether the phosphaturic effects of PTH and FGF23 are interdependent, 11 patients with hypoparathyroidism, which features high blood phosphate in spite of concomitant FGF23 elevation, and 1 patient with hyperphosphatemic familial tumoral calcinosis (HFTC), characterized by deficient intact FGF23 action and resulting hyperphosphatemia, were treated with synthetic human PTH 1-34 (hPTH 1-34). Biochemical parameters, including blood phosphate, calcium, intact FGF23 (iFGF23), nephrogenic cAMP, 1,25(OH)₂ vitamin D (1,25D), and tubular reabsorption of phosphate (TRP), were measured at baseline and after hPTH 1-34 treatment. In patients with hypoparathyroidism, administration of hPTH 1-34 increased nephrogenic cAMP, which resulted in serum phosphate normalization followed by a significant decrease in iFGF23. TRP initially decreased and returned to baseline. In the patient with HFTC, hPTH 1-34 administration also increased nephrogenic cAMP, but this did not produce changes in phosphate or TRP. No changes in calcium were observed in any of the studied patients, although prolonged hPTH 1-34 treatment did induce supraphysiologic 1,25D levels in the patient with HFTC. Our results indicate that PTH and FGF23 effects on phosphate regulation are interdependent and both are required to adequately regulate renal phosphate handling. Published 2021. This article is a U.S. Government work and is in the public domain in the USA.

The effect of vitamin D supplementation on serum levels of fibroblast growth factor- 23: A systematic review and meta-analysis of randomized controlled trials

Previous studies of the effect of vtamin D on serum levels of fibroblast growth factor- 23 (FGF-23) have yeilded an inconsistent findings. This systematic review and meta-analysis of randomized controlled trials (RCTs) sought to investigate the effect of vitamin D supplementation on serum levels of FGF-23. PubMed, Scopus, ISI Web of Science, and the Cochrane Library were searched, from database inception to November 2020, for RCTs that evaluated the effects of native or active vitamin D supplementation on serum levels of FGF-23 in adults. Weighted mean difference (WMD) were calculated and random effects meta-analysis was used to estimate the overall effects. Twenty-seven trials were included in the meta-analysis. Supplementation with native vitamin D (23 studies, n = 2247 participants; weighted mean difference [WMD] = 0.5 pg/mL, 95 % CI: -0.52 to 1.51, P = 0.33; I² = 29.9 %), and active vitamin D (5 studies, n = 342 participants, WMD = 29.45 pg/mL, 95 % CI: -3.9 to 62.81, P = 0.08; I² = 99.3%) had no significant effects on serum FGF-23 concentration. In subgroup analyses, supplementation with ergocalciferol (3 studies, n = 205 participants; WMD = 18.27 pg/mL, 95 % CI: 5.36-31.17, P = 0.006), and daily dosing regimens (9 studies, n = 1374 participants; WMD = 0.41 pg/mL, 95 % CI: 0.22 to 0.59, P < 0.001) increased serum FGF-23 levels compared to control. Overall, our findings revealed no significan effect of vitamin D supplementation on serum FGF-23 concentration. However, further high quality, large-scale studies are needed to better elucidate this relationship.

C-Terminal, but Not Intact, FGF23 and EPO Are Strongly Correlatively Elevated in Patients With Gain-of-Function Mutations in HIF2A: Clinical Evidence for EPO Regulating FGF23

Fibroblast growth factor 23 (FGF23) is a key phosphate- and vitamin D-regulating hormone. FGF23 circulates as an intact 251 amino acid protein or N- and C-terminal degradation products. Hormone activity resides in the intact molecule, but it has been suggested that high levels of the C-terminal protein can interfere with intact FGF23 (iFGF23) activity. New evidence points to involvement of the hypoxia-inducible factor (HIF)/erythropoietin (EPO)/iron pathway as important in FGF23 physiology. Exactly how this pathway regulates FGF23 is not clear. Various in vitro, in vivo, and clinical studies involving perturbations in this pathway at various points have yielded conflicting results. Many of these studies are complicated by the confounding, independent effect of renal insufficiency on FGF23. To gain insight into FGF23 physiology, we studied 8 patients with a rare paraganglioma/somatostatinoma syndrome who had elevated blood EPO levels as a result of somatic gain-of-function mutations in HIF2A (EPAS1) that stimulate tumoral EPO production. All patients had normal renal function. EPO levels varied; most were very elevated and highly correlated with C-terminal FGF23 (cFGF23) levels that were also markedly elevated. Blood phosphate and intact FGF23 levels were normal. These data from patients with normal renal function in whom HIF activation was the inciting event suggest a direct role of the HIF/EPO pathway in FGF23 transcription and translation. They also demonstrate that posttranslational regulation was finely tuned to maintain normal blood phosphate levels. Additionally, normal phosphate and intact FGF23 levels in the setting of markedly increased C-terminal FGF23 levels suggest intact FGF23 action is not attenuated by C-terminal FGF23. Published 2020. This article is a U.S. Government work and is in the public domain in the USA. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

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.

Lack of PTEN in osteocytes increases circulating phosphate concentrations by decreasing intact fibroblast growth factor 23 levels

Fibroblast growth factor 23 (FGF23) has been centric to the regulation of phosphate (Pi) metabolism; however, the regulatory network of FGF23 in osteocytes has not yet been defined in detail. We herein investigated the role of PTEN (phosphatase and tensin homolog deleted from chromosome 10) in this regulation. We created mice lacking PTEN expression mainly in osteocytes by crossing Pten-flox mice with Dmp1-Cre mice. The lack of PTEN in the osteocytes of these mice was associated with decreased skeletal and serum intact FGF23 levels, which, in turn, resulted in reductions of urinary Pi excretion and elevations of serum Pi levels. Mechanistically, the knockdown of PTEN expression in osteoblastic UMR106 cells activated the AKT/mTORC1 (mechanistic target of rapamycin complex 1) pathway and this was associated with reductions in Fgf23 expression. Furthermore, the suppression of Fgf23 expression by PTEN knockdown or insulin simulation in UMR106 cells was partially restored by the treatment with the mTORC1 inhibitor, rapamycin. These results suggest that FGF23 expression in osteoblastic cells is in part regulated through the AKT/mTORC1 pathway and provide new insights into our understanding of the regulatory network of Pi metabolism.

1,25(OH)2 vitamin D3 stimulates active phosphate transport but not paracellular phosphate absorption in mouse intestine

KEY POINTS

Intestinal absorption of phosphate proceeds via an active/transcellular route mostly mediated by NaPi-IIb/Slc34a2 and a poorly characterized passive/paracellular pathway. Intestinal phosphate absorption and expression of NaPi-IIb are stimulated by 1,25(OH)₂ vitamin D₃ but whether NaPi-IIb is the only target under hormonal control remains unknown. We report that administration of 1,25(OH)₂ vitamin D₃ to wild-type mice resulted in the expected increase in active transport of phosphate in jejunum, without changing paracellular fluxes. Instead, the same treatment failed to alter phosphate transport in intestinal-depleted Slc34a2-deficient mice. In both genotypes, 1,25(OH)₂ vitamin D₃ induced similar hyperphosphaturic responses and changes in the plasma levels of FGF23 and PTH. While urinary phosphate loss induced by administration of 1,25(OH)₂ vitamin D₃ did not alter plasma phosphate, further studies should investigate whether chronic administration would lead to phosphate imbalance in mice with reduced active intestinal absorption.

ABSTRACT

Intestinal absorption of phosphate is stimulated by 1,25(OH)₂ vitamin D_3. At least two distinct mechanisms underlie phosphate absorption in the gut, an active transcellular transport requiring the Na⁺ /phosphate cotransporter NaPi-IIb/Slc34a2, and a poorly characterized paracellular passive pathway. 1,25(OH)₂ vitamin D₃ stimulates NaPi-IIb expression and function, and loss of NaPi-IIb reduces intestinal phosphate absorption. However, it is remains unknown whether NaPi-IIb is the only target for hormonal regulation by 1,25(OH)₂ vitamin D₃ . Here we compared the effects of intraperitoneal administration of 1,25(OH)₂ vitamin D₃ (2 days, once per day) in wild-type and intestinal-specific Slc34a2-deficient mice, and analysed trans- vs. paracellular routes of phosphate absorption. We found that treatment stimulated active transport of phosphate only in jejunum of wild-type mice, though NaPi-IIb protein expression was upregulated in jejunum and ileum. In contrast, 1,25(OH)₂ vitamin D₃ administration had no effect in Slc34a2-deficient mice, suggesting that the hormone specifically regulates NaPi-IIb expression. In both groups, 1,25(OH)₂ vitamin D₃ elicited the expected increase of plasma fibroblast growth factor 23 (FGF23) and reduction of parathyroid hormone (PTH). Treatment resulted in hyperphosphaturia (and hypercalciuria) in both genotypes, though mice remained normophosphataemic. While increased intestinal absorption and higher FGF23 can trigger the hyperphosphaturic response in wild types, only higher FGF23 can explain the renal response in Slc34a2-deficient mice. Thus, 1,25(OH)₂ vitamin D₃ stimulates intestinal phosphate absorption by acting on the active transcellular pathway mostly mediated by NaPi-IIb while the paracellular pathway appears not to be affected.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

FGF23 and the PTH response to paricalcitol in chronic kidney disease

BACKGROUND

The parathyroid glands are endowed both with receptors responsive to FGF23 and to 1,25 vitamin D. Vitamin D receptor (VDR) activation, besides lowering PTH, also raises serum FGF23. FGF23 has been implicated in parathyroid resistance to VDR activation but the issue has never been investigated in predialysis CKD patients.

METHODS

In the Paricalcitol and Endothelial Functio in Chronic Kidney Disease (PENNY) study (NCT01680198), a 12-week randomized trial in stage G3-4 CKD patients (placebo n = 44 and paricalcitol n = 44), we measured PTH and the active form of FGF23 with no missing value across the trial.

RESULTS

At baseline, serum FGF23 and PTH were inter-related (r = .54, P < .01). Paricalcitol reduced serum PTH (-75.1 pg/mL, 95% CI: -90.4 to -59.8; P < .001) and increased FGF23 (+107 pg/mL, 95% CI: 44-170 pg/mL, P = .001). Changes in the Ca × P product in response to paricalcitol were closely related to simultaneous FGF23 changes in an analysis adjusted for changes in serum calcium and phosphate (P < .001). Of note, baseline FGF23, appropriately adjusted for baseline PTH, was unrelated with the PTH response to paricalcitol (r = -.06, P = .72). Placebo did not change neither PTH nor FGF23.

CONCLUSION

Serum FGF23 and PTH are inter-related and changes in the Ca × P product induced by paricalcitol per se correlate with the FGF23 response to this drug. Independently of serum FGF23, the parathyroid glands of patients with moderate to severe CKD maintain an intact ability to respond to VDR activation.

Regulation and Effects of FGF23 in Chronic Kidney Disease

Chronic kidney disease (CKD) is a global health epidemic that accelerates cardiovascular disease, increases risk of infection, and causes anemia and bone disease, among other complications that collectively increase risk of premature death. Alterations in calcium and phosphate homeostasis have long been considered nontraditional risk factors for many of the most morbid outcomes of CKD. The discovery of fibroblast growth factor 23 (FGF23), which revolutionized the diagnosis and treatment of rare hereditary disorders of FGF23 excess that cause hypophosphatemic rickets, has also driven major paradigm shifts in our understanding of the pathophysiology and downstream end-organ complications of disordered mineral metabolism in CKD. As research of FGF23 in CKD has rapidly advanced, major new questions about its regulation and effects continuously emerge. These are promoting exciting innovations in laboratory, patient-oriented, and epidemiological research and stimulating clinical trials of new therapies and repurposing of existing ones to target FGF23.

FGF23 and Associated Disorders of Phosphate Wasting

Fibroblast growth factor 23 (FGF23), one of the endocrine fibroblast growth factors, is a principal regulator in the maintenance of serum phosphorus concentration. Binding to its cofactor αKlotho and a fibroblast growth factor receptor is essential for its activity. Its regulation and interaction with other factors in the bone-parathyroid-kidney axis is complex. FGF23 reduces serum phosphorus concentration through decreased reabsorption of phosphorus in the kidney and by decreasing 1,25 dihydroxyvitamin D (1,25(OH)2D) concentrations. Various FGF23-mediated disorders of renal phosphate wasting share similar clinical and biochemical features. The most common of these is X-linked hypophosphatemia (XLH). Additional disorders of FGF23 excess include autosomal dominant hypophosphatemic rickets, autosomal recessive hypophosphatemic rickets, fibrous dysplasia, and tumor-induced osteomalacia. Treatment is challenging, requiring careful monitoring and titration of dosages to optimize effectiveness and to balance side effects. Conventional therapy for XLH and other disorders of FGF23-mediated hypophosphatemia involves multiple daily doses of oral phosphate salts and active vitamin D analogs, such as calcitriol or alfacalcidol. Additional treatments may be used to help address side effects of conventional therapy such as thiazides to address hypercalciuria or nephrocalcinosis, and calcimimetics to manage hyperparathyroidism. The recent development and approval of an anti-FGF23 antibody, burosumab, for use in XLH provides a novel treatment option.

Regulation of fibroblast growth factor 23 (FGF23) in health and disease

Fibroblast growth factor 23 (FGF23) is mainly produced in the bone and, upon secretion, forms a complex with a FGF receptor and coreceptor αKlotho. FGF23 can exert several endocrine functions, such as inhibiting renal phosphate reabsorption and 1,25-dihydroxyvitamin D3 production. Moreover, it has paracrine activities on several cell types, including neutrophils and hepatocytes. Klotho and Fgf23 deficiencies result in pathologies otherwise encountered in age-associated diseases, mainly as a result of hyperphosphataemia-dependent calcification. FGF23 levels are also perturbed in the plasma of patients with several disorders, including kidney or cardiovascular diseases. Here, we review mechanisms controlling FGF23 production and discuss how FGF23 regulation is perturbed in disease.

The dynamic relationships between the active and catabolic vitamin D metabolites, their ratios, and associations with PTH

Vitamin D status, assessed by serum concentration of 25(OH)D, is the prime candidate marker for many disease-association studies, but the interplay between the subsequent 1,25-dihydroxyvitamin D (1,25(OH)₂D) and 24,25-dihydroxyvitamin D (24,25(OH)₂D) metabolites is unclear. In this study, we conducted an analysis from a large cohort of healthy, physically fit, young army recruits (n = 940). We found a significant, inverse relationship between serum 25(OH)D and 1,25(OH)₂D:24,25(OH)₂D vitamin D metabolite ratio (VMR) (r²Exp = 0.582, p < 0.0001), and demonstrated a significant association with increasing PTH concentration (p < 0.001). Circannual rhythms were evident for all vitamin D metabolites and VMRs except for 1,25(OH)₂D when fitted to Cosinor curves. We estimated 1,25(OH)₂D:24,25(OH)₂D VMR of ≥35 to be the threshold value for vitamin D insufficiency, and ≥51 to be predictive of vitamin D deficiency. Our three-dimensional model provides mechanistic insight into the vitamin D-PTH endocrine system, and further substantiates the role of 24,25(OH)₂D in human physiology. The model sets a new paradigm for vitamin D treatment strategy, and may help the establishment of vitamin D-adjusted PTH reference intervals. The study was approved by the UK Ministry of Defence research ethics committee (MODREC 165/Gen/10 and 692/MoDREC/15). ClinicalTrials.gov Identifier NCT02416895.

Questioning the Safety of Calcidiol in Hemodialysis Patients

Background. Epidemiological studies have suggested a survival benefit for hemodialysis patients on paricalcitol or calcitriol, but nutritional vitamin D supplementation of patients already on vitamin D receptor (VDR) activators is controversial. Methods. This observational retrospective cohort study was conducted with prospectively collected data from all consecutive patients with chronic kidney disease (CKD) who underwent hemodialysis under routine clinical practice conditions for two years. Results. Of the 129 patients, 89 were treated with calcidiol, paricalcitol, and/or calcitriol. The patients with any vitamin D formulation had higher serum concentrations of 25-hydroxy vitamin D and fibroblast growth factor-23 and tended to have higher mortality rates (42% vs. 25%, p = 0.07). On subgroup analysis, any calcidiol treatment or calcidiol combined with paricalcitol associated with significantly higher mortality rates than no treatment (47% and 62.5%, p = 0.043 and 0.008, respectively). The association between calcidiol/paricalcitol treatment and elevated mortality remained significant after adjusting for age, sex, diabetes, C-reactive protein, and hemodialysis vintage. Any calcidiol and calcidiol/paricalcitol treatment exhibited a dose-response relationship with mortality (p for trend: 0.002 and 0.005, respectively). Conclusions. These data draw attention to the hitherto unexplored safety of calcidiol supplementation in patients on hemodialysis, especially in those already on vitamin D. Until clinical trials demonstrate the safety and efficacy of this approach, caution should be exercised when prescribing these patients ≥0.5 calcidiol mg/month.

Hypophosphatemic Rickets

Hypophosphatemic rickets, mostly of the X-linked dominant form caused by pathogenic variants of the PHEX gene, poses therapeutic challenges with consequences for growth and bone development and portends a high risk of fractions and poor bone healing, dental problems and nephrolithiasis/nephrocalcinosis. Conventional treatment consists of PO4 supplements and calcitriol requiring monitoring for treatment-emergent adverse effects. FGF23 measurement, where available, has implications for the differential diagnosis of hypophosphatemia syndromes and, potentially, treatment monitoring. Newer therapeutic modalities include calcium sensing receptor modulation (cinacalcet) and biological molecules targeting FGF23 or its receptors. Their long-term effects must be compared with those of conventional treatments.

Phosphate wasting disorders in adults

A cause of hypophosphatemia is phosphate wasting disorders. Knowledge concerning mechanisms involved in phosphate wasting disorders has greatly increased in the last decade by the identification of phosphatonins, among them FGF-23. FGF-23 is a primarily bone derived factor decreasing renal tubular reabsorption of phosphate and the synthesis of calcitriol. Currently, pharmacological treatment of these disorders offers limited efficacy and is potentially associated to gastrointestinal, renal, and parathyroid complications; therefore, efforts have been directed toward newer pharmacological strategies that target the FGF-23 pathway. This review focuses on phosphate metabolism, its main regulators, and phosphate wasting disorders in adults, highlighting the main issues related to diagnosis and current and new potential treatments.

Heterotrimeric G proteins in the control of parathyroid hormone actions

Parathyroid hormone (PTH) is a key regulator of skeletal physiology and calcium and phosphate homeostasis. It acts on bone and kidney to stimulate bone turnover, increase the circulating levels of 1,25 dihydroxyvitamin D and calcium and inhibit the reabsorption of phosphate from the glomerular filtrate. Dysregulated PTH actions contribute to or are the cause of several endocrine disorders. This calciotropic hormone exerts its actions via binding to the PTH/PTH-related peptide receptor (PTH1R), which couples to multiple heterotrimeric G proteins, including G_s and G_q/11 Genetic mutations affecting the activity or expression of the alpha-subunit of G_s, encoded by the GNAS complex locus, are responsible for several human diseases for which the clinical findings result, at least partly, from aberrant PTH signaling. Here, we review the bone and renal actions of PTH with respect to the different signaling pathways downstream of these G proteins, as well as the disorders caused by GNAS mutations.

Effect of Calcitriol on FGF23 Level in Healthy Adults and its Dependence on Phosphate Level

AIM

To evaluate the short-term effects of calcitriol and sevelamer hydrochloride on fibroblast growth factor-23 (FGF23) in humans and to determine whether the effect is direct or indirect through calcitriol-induced increased absorption of phosphorus from the intestine.

PATIENTS AND METHODS

A total of 15 healthy individuals were tested at three time points and stages, for 24 h and at 1-week intervals. During each stage, blood samples were taken at three time points (0, 8 and 24 h); baseline stage: under no intervention; second stage, while receiving 0.5 μg calcitriol orally twice daily; and at the third stage, while receiving 0.5 μg calcitriol orally twice daily and sevelamer hydrochloride during meals. The changes in FGF23, parathyroid hormone, calcitriol, Ca, and phosphorus were determined.

RESULTS

During calcitriol administration, the FGF23 level changed significantly (p=0.008), with the level at 24 h levels being significantly higher than at 8 h (8.8 pg/ml vs. 13.0 pg/ml, p=0.036). There was a statistically significant difference in the percentage change, among the three stages, at time 8 to 24 h and 0 to 24 h for FGF23 (p=0.014 and p=0.015, respectively), with significant differences between baseline vs. calcitriol for 8 to 24 h FGF23 change (-9.23% vs. 26.98%, p=0.003) and a trend between baseline vs. calcitriol (p=0.061) and calcitriol plus sevelamer (p=0.069) for 0 to 24 h FGF23 change.

CONCLUSION

Administration of calcitriol to healthy individuals increases the circulating level of FGF23 within 24 h. Combined calcitriol and sevelamer administration restrains the increase of FGF23, suggesting that calcitriol-induced increased absorption of phosphate from the intestine might also be involved in the increase of FGF23.

FGF23 Is Not Associated With Age-Related Changes in Phosphate, but Enhances Renal Calcium Reabsorption in Girls

CONTEXT

Fibroblast growth factor (FGF)23 is a critical determinant of phosphate homeostasis. The role of FGF23, however, in regulating physiologic changes in serum phosphate and renal phosphate handling across childhood is not well described. In addition, animal models have suggested a role for FGF23 in regulating renal calcium excretion.

OBJECTIVE

To assess changes in FGF23 concentrations across childhood in relation to changes in mineral ions and hormones of mineral ion homeostasis.

DESIGN

This was a cross-sectional study.

SETTING

The study was conducted at a Clinical Research Center at a tertiary care hospital.

PATIENTS OR OTHER PARTICIPANTS

Ninety healthy girls ages 9 to 18 years were recruited from the surrounding community.

MAIN OUTCOME MEASURES

The associations of intact and C-terminal FGF23 concentrations with measures of mineral ion homeostasis were determined by univariable and multivariable linear regression.

RESULTS

Serum phosphate and renal phosphate excretion varied with age, as expected (R = -0.49, P < 0.001 and R = -0.48, P < 0.001, respectively). Neither intact nor C-terminal FGF23 varied with age, and FGF23 was not correlated with serum or urinary phosphate. Intact FGF23 was positively correlated with serum calcium (R = 0.39, P < 0.001) and negatively correlated with urinary calcium/creatinine ratio (R = -0.27, P = 0.011).

CONCLUSIONS

The changes in serum and urinary phosphate handling across childhood do not appear to be determined by alterations in FGF23 concentrations. These data may point to a role for FGF23 in calcium regulation in human physiology.

Fibroblast Growth Factor 23-Mediated Bone Disease

Fibroblast growth factor 23 (FGF23) is an important regulator of phosphate and vitamin D metabolism and its excessive or insufficient production leads to a wide variety of skeletal disorders. This article reviews the FGF23-α-Klotho signaling pathway, highlighting the latest developments in FGF23 regulation and action, and describes the disorders associated with FGF23 excess or deficiency.

Fibroblast Growth Factor-23-A Potential Uremic Toxin

Fibroblast growth factor-23 (FGF23) is a circulating member of the FGF family produced mainly by the osteocytes and osteoblasts that can act as a hormone. The main action of FGF23 is to lower phosphatemia via the reduction of urinary phosphate reabsorption and the decrease of 1,25(OH)₂-D generation in the kidney. In the course of chronic kidney disease (CKD), plasma FGF23 concentration rises early, most probably to compensate the inability of the deteriorating kidneys to excrete an adequate amount of phosphate. However, this comes at the cost of FGF23-related target organ toxicity. Results of clinical studies suggest that elevated plasma FGF23 concentration is independently associated with the increased risk of CKD progression, occurrence of cardio-vascular complications, and mortality in different stages of CKD. FGF23 also contributes to cardiomyocyte hypertrophy, vascular calcification, and endothelial dysfunction. The impact of FGF23 on heart muscle is not dependent on Klotho, but rather on the PLCγ–calcineurin–NFAT (nuclear factor of activated T-cells) pathway. Among the factors increasing plasma FGF23 concentration, active vitamin D analogues play a significant role. Additionally, inflammation and iron deficiency can contribute to the increase of plasma FGF23. Among the factors decreasing plasma FGF23, dietary phosphate restriction, some intestinal phosphate binders, cinacalcet (and other calcimimetics), and nicotinamide can be enumerated. Anti-FGF23 antibodies have also recently been developed to inhibit the action of FGF23 in target organs. Still, the best way to normalize plasma FGF23 in maintenance hemodialysis patients is restoring kidney function by successful kidney transplantation.

The FGF23/KLOTHO Regulatory Network and Its Roles in Human Disorders

The functions of Klotho (KL) are multifaceted and include the regulation of aging and mineral metabolism. It was originally identified as the gene responsible for premature aging-like symptoms in mice and was subsequently shown to function as a coreceptor in the fibroblast growth factor (FGF) 23 signaling pathway. The discovery of KL as a partner for FGF23 led to significant advances in understanding of the molecular mechanisms underlying phosphate and vitamin D metabolism, and simultaneously clarified the pathogenic roles of the FGF23 signaling pathway in human diseases. These novel insights led to the development of new strategies to combat disorders associated with the dysregulated metabolism of phosphate and vitamin D, and clinical trials on the blockade of FGF23 signaling in X-linked hypophosphatemic rickets are ongoing. Molecular and functional insights on KL and FGF23 have been discussed in this review and were extended to how dysregulation of the FGF23/KL axis causes human disorders associated with abnormal mineral metabolism.

Ablation of the Stimulatory G Protein α-Subunit in Renal Proximal Tubules Leads to Parathyroid Hormone-Resistance With Increased Renal Cyp24a1 mRNA Abundance and Reduced Serum 1,25-Dihydroxyvitamin D

PTH regulates serum calcium, phosphate, and 1,25-dihydroxyvitamin D (1,25(OH)2D) levels by acting on bone and kidney. In renal proximal tubules (PTs), PTH inhibits reabsorption of phosphate and stimulates the synthesis of 1,25(OH)2D. The PTH receptor couples to multiple G proteins. We here ablated the α-subunit of the stimulatory G protein (Gsα) in mouse PTs by using Cre recombinase driven by the promoter of type-2 sodium-glucose cotransporter (Gsα(Sglt2KO) mice). Gsα(Sglt2KO) mice were normophosphatemic but displayed, relative to controls, hypocalcemia (1.19 ±0.01 vs 1.23 ±0.01 mmol/L; P < .05), reduced serum 1,25(OH)2D (59.3 ±7.0 vs 102.5 ±12.2 pmol/L; P < .05), and elevated serum PTH (834 ±133 vs 438 ±59 pg/mL; P < .05). PTH-induced elevation in urinary cAMP excretion was blunted in Gsα(Sglt2KO) mice (2- vs 4-fold over baseline in controls; P < .05). Relative to baseline in controls, PTH-induced reduction in serum phosphate tended to be blunted in Gsα(Sglt2KO) mice (-0.39 ±0.33 vs -1.34 ±0.36 mg/dL; P = .07). Gsα(Sglt2KO) mice showed elevated renal vitamin D 24-hydroxylase and bone fibroblast growth factor-23 (FGF23) mRNA abundance (∼3.4- and ∼11-fold over controls, respectively; P < .05) and tended to have elevated serum FGF23 (829 ±76 vs 632 ±60 pg/mL in controls; P = .07). Heterozygous mice having constitutive ablation of the maternal Gsα allele (E1(m-/+)) (model of pseudohypoparathyroidism type-Ia), in which Gsα levels in PT are reduced, also exhibited elevated serum FGF23 (474 ±20 vs 374 ±27 pg/mL in controls; P < .05). Our findings indicate that Gsα is required in PTs for suppressing renal vitamin D 24-hydroxylase mRNA levels and for maintaining normal serum 1,25(OH)2D.

The FGF23/Klotho axis in the regulation of mineral and metabolic homeostasis

The function of fibroblast growth factor (FGF) 23 has been suggested to be multifaceted beyond its canonical function as a regulator of mineral metabolism. FGF23 was originally shown to play a central role in phosphate (Pi) and vitamin D metabolism, and a number of diseases associated with dysregulated Pi metabolism have been attributed to abnormal FGF23 signaling activities. The discovery of Klotho as a co-receptor for FGF23 signaling has also accelerated understanding on the molecular mechanisms underlying Pi and vitamin D metabolism. In addition to these canonical functions, FGF23 has recently been implicated in a number of metabolic diseases including chronic kidney disease-associated complications, cardiovascular diseases, and obesity-related disorders; however, the physiological significance and molecular mechanisms of these emerging roles of FGF23 remain largely unknown. Molecular and functional insights into the FGF23 pathway will be discussed in the present review, with an emphasis on its role in human disorders related to dysregulated Pi metabolism as well as metabolic disorders.

Tumor-induced rickets in a child with a central giant cell granuloma: a case report

Tumor-induced osteomalacia/rickets is a rare paraneoplastic disorder associated with a tumor-producing fibroblast growth factor 23 (FGF23). We present a child with symptoms of rickets as the first clinical sign of a central giant cell granuloma (CGCG) with high serum levels of FGF23, a hormone associated with decreased phosphate resorption. A 3-year-old boy presented with a limp and 6 months later with painless growth of the jaw. On examination gingival hypertrophy and genu varum were observed. Investigations revealed hypophosphatemia, normal 1,25 and 25 (OH) vitamin D, and high alkaline phosphatase. An MRI showed an osteolytic lesion of the maxilla. Radiographs revealed typical rachitic findings. Incisional biopsy of the tumor revealed a CGCG with mesenchymal matrix. The CGCG was initially treated with calcitonin, but the lesions continued to grow, making it necessary to perform tracheostomy and gastrostomy. One year after onset the hyperphosphaturia worsened, necessitating increasing oral phosphate supplements up to 100 mg/kg per day of elemental phosphorus. FGF23 levels were extremely high. Total removal of the tumor was impossible, and partial reduction was achieved after percutaneous computed tomography-guided radiofrequency, local instillation of triamcinolone, and oral propranolol. Compassionate use of cinacalcet was unsuccessful in preventing phosphaturia. The tumor slowly regressed after the third year of disease; phosphaturia improved, allowing the tapering of phosphate supplements, and FGF23 levels normalized. Tumor-induced osteomalacia/rickets is uncommon in children and is challenging for physicians to diagnose. It should be suspected in patients with intractable osteomalacia or rickets. A tumor should be ruled out if FGF23 levels are high.

The PTH-Vitamin D-FGF23 axis

Fibroblast growth factor 23 (FGF23) has emerged as an important regulator of phosphate and vitamin D homeostasis. It is important to understand how FGF23 interacts with vitamin D and parathyroid hormone (PTH) in a FGF23-Vitamin D-PTH axis to regulate mineral homeostasis. In this review, we discuss the genomic structure, and transcriptional, translational, and posttranslational regulation of FGF23. We describe its interaction with PTH and vitamin D, disorders of altered FGF23 states, and emerging therapies for diseases of FGF23 based upon these findings. This discussion helps redefine the role of PTH and vitamin D in relation to a complex bone-kidney-parathyroid loop, and points to areas within this complicated field in need of further clarification and research.

Fibroblast Growth Factor 23

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

Pathophysiology of parathyroid hyperplasia in chronic kidney disease: preclinical and clinical basis for parathyroid intervention

Secondary hyperparathyroidism is characterised by excessive secretion of parathyroid hormone and parathyroid hyperplasia, resulting in both skeletal and extraskeletal consequences. Recent basic and clinical studies have brought considerable advances in our understanding of the pathophysiology of parathyroid hyperplasia and have also provided practical therapeutic approaches, especially with regard to indications for parathyroid intervention. In this context, it is quite important to recognize the development of nodular hyperplasia, because the cells in nodular hyperplasia are usually resistant to calcitriol treatment. Patients with nodular hyperplasia should undergo parathyroid intervention including percutaneous ethanol injection therapy (PEIT). Selective PEIT of the parathyroid gland is an effective approach in which the enlarged parathyroid gland with nodular hyperplasia is 'selectively' destroyed by ethanol injection, and other glands with diffuse hyperplasia are then managed by medical therapy. With a more focused attention to applying parathyroid intervention, we can expect significant improvement in the management of secondary hyperparathyroidism in dialysis patients.

FGF23 is associated with disease severity and prognosis in chronic heart failure

BACKGROUND

Elevated levels of fibroblast growth factor 23 (FGF23) are associated with incident heart failure in individuals with or without chronic kidney disease. We aimed to investigate the association between serum FGF23 concentrations and disease severity and long-term outcome in patients with stable heart failure.

MATERIALS AND METHODS

Serum levels of C-term FGF23 (Ct-FGF23) concentrations, inorganic phosphate (Pi ), parathormone (PTH) and 25-hydroxyvitamin D (25(OH)D) were measured in 208 patients with nonischaemic heart failure (age 48 ± 15 years; 70% male; NYHA Class I 27·8%, NYHA Class II 43·4%, NYHA Class III/IV 28·8%; LV-EF 34 ± 15%; eGFR ≥60 mL/min/1·73 m(2) in 86%).

RESULTS

Median Ct-FGF23 levels were 18·2 RU/mL (7·5-40·8RU/mL). A dose-response relationship was found between median Ct-FGF23 levels and increasing NYHA class (I: 11·9 RU/mL, II: 15·8 RU/mL, III/IV: 38·8 RU/mL; P < 0·001). Ct-FGF23 correlated with NTproBNP (r = 0·307, P < 0·001), central venous pressure, mean pulmonary arterial pressure, pulmonary capillary wedge pressure and inversely correlated with cardiac output after adjustment for renal function (eGFR) and Pi . LnCt-FGF23 was related with the combined endpoint of death or heart transplantation (hazard ratio 1·452 [1·029-2·048]; P = 0·034) independent of Pi , PTH, 25(OH)D, age and sex.

CONCLUSION

The phosphatonin FGF23 is strongly associated with disease severity and long-term outcome in patients with nonischaemic heart failure and preserved renal function. Further studies are needed to evaluate the pathophysiologic role of FGF23 and its potential as a biomarker in heart failure.

The human response to acute enteral and parenteral phosphate loads

The human response to acute phosphate (PO4) loading is poorly characterized, and it is unknown whether an intestinal phosphate sensor mechanism exists. Here, we characterized the human mineral and endocrine response to parenteral and duodenal acute phosphate loads. Healthy human participants underwent 36 hours of intravenous (IV; 1.15 [low dose] and 2.30 [high dose] mmol of PO4/kg per 24 hours) or duodenal (1.53 mmol of PO4/kg per 24 hours) neutral sodium PO4 loading. Control experiments used equimolar NaCl loads. Maximum PO4 urinary excretory responses occurred between 12 and 24 hours and were similar for low-dose IV and duodenal infusion. Hyperphosphatemic responses were also temporally and quantitatively similar for low-dose IV and duodenal PO4 infusion. Fractional renal PO4 clearance increased approximately 6-fold (high-dose IV group) and 4-fold (low-dose IV and duodenal groups), and significant reductions in plasma PO4 concentrations relative to peak values occurred by 36 hours, despite persistent PO4 loading. After cessation of loading, frank hypophosphatemia occurred. The earliest phosphaturic response occurred after plasma PO4 and parathyroid hormone concentrations increased. Plasma fibroblast growth factor-23 concentration increased after the onset of phosphaturia, followed by a decrease in plasma 1,25(OH)2D levels; α-Klotho levels did not change. Contrary to results in rodents, we found no evidence for intestinal-specific phosphaturic control mechanisms in humans. Complete urinary phosphate recovery in the IV loading groups provides evidence against any important extrarenal response to acute PO4 loads.

Generation of mice encoding a conditional null allele of Gcm2

Glial cells missing homolog 2 (GCM2) is a transcription factor that is expressed predominately in the pharyngeal pouches and, at later stages, in the developing and mature parathyroid glands. In humans, loss of GCM2 function, either through recessive apomorphic mutations or dominant inhibitor mutations in the human GCM2 gene, leads to isolated hypoparathyroidism. In mice, homozygous disruption of Gcm2 by conventional gene targeting results in parathyroid aplasia and hypoparathyroidism. In this study, we report the generation and functional characterization of mice encoding a conditional null allele of Gcm2. We demonstrate the functional integrity of the conditional Gcm2 allele and report successful in vivo deletion of exon 2 using Cre recombinase. The mice with conditional deletion of Gcm2 displayed phenotypes similar to those previously described for a conventional Gcm2 knockout, including perinatal lethality, hypocalemia, low or undetectable serum levels of parathyroid hormone, and absent parathyroid glands. The production of a conditional mutant allele for Gcm2 represents a valuable resource for the study of the temporal- and spatial-specific roles for Gcm2, and for understanding the postnatal activities of GCM2 protein.

Renal-specific and inducible depletion of NaPi-IIc/Slc34a3, the cotransporter mutated in HHRH, does not affect phosphate or calcium homeostasis in mice

The proximal renal epithelia express three different Na-dependent inorganic phosphate (Pi) cotransporters: NaPi-IIa/SLC34A1, NaPi-IIc/SLC34A3, and PiT2/SLC20A2. Constitutive mouse knockout models of NaPi-IIa and NaPi-IIc suggested that NaPi-IIa mediates the bulk of renal reabsorption of Pi whereas the contribution of NaPi-IIc to this process is minor and probably restricted to young mice. However, many reports indicate that mutations of NaPi-IIc in humans lead to hereditary hypophosphatemic rickets with hypercalciuria (HHRH). Here, we report the generation of a kidney-specific and inducible NaPi-IIc-deficient mouse model based on the loxP-Cre system. We found that the specific removal of the cotransporter from the kidneys of young mice does not impair the capacity of the renal epithelia to transport Pi. Moreover, the levels of Pi in plasma and urine as well as the circulating levels of parathyroid hormone, FGF-23, and vitamin D3 remained unchanged. These findings are in agreement with the data obtained with the constitutive knockout model and suggest that, under steady-state conditions of normal dietary Pi, NaPi-IIc is not an essential Na-Pi cotransporter in murine kidneys. However, and unlike the constitutive mutants, the kidney-specific depletion of NaPi-IIc does not result in alteration of the homeostasis of calcium. This suggests that the calcium-related phenotype observed in constitutive knockout mice may not be related to inactivation of the cotransporter in kidney.

Phosphate metabolism and vitamin D

Phosphate plays many essential roles in our body. To accomplish these functions, serum phosphate needs to be maintained in a certain range. Serum phosphate level is regulated by intestinal phosphate absorption, renal phosphate handling and equilibrium of extracellular phosphate with that in bone or intracellular fluid. Several hormones such as parathyroid hormone, 1,25-dihydroxyvitamin D (1,25(OH)2D) and fibroblast growth factor 23 (FGF23) regulate serum phosphate by modulating intestinal phosphate absorption, renal phosphate reabsorption and/or bone metabolism. In addition, dietary phosphate rapidly enhances renal phosphate excretion, although detailed mechanisms of this adaptation remain to be clarified. Physiologically, extracellular concentrations of phosphate and these hormones are maintained by several negative feedback loops. For example, 1,25(OH)2D enhances FGF23 production and FGF23 reduces 1,25(OH)2D level. In addition, phosphate affects 1,25(OH)2D and FGF23 levels. Dysfunction of these negative feedback loops results in several diseases with abnormal phosphate and 1,25(OH)2D levels. Especially, excess actions of FGF23 cause several hypophosphatemic rickets/osteomalacia with relatively low level of 1,25(OH)2D that had been classified as vitamin D-resistant rickets/osteomalacia. In contrast, deficient actions of FGF23 cause hyperphosphatemic familial tumoral calcinosis. However, there still remain several unanswered questions regarding phosphate and vitamin D metabolism.

Fibrous dysplasia and fibroblast growth factor-23 regulation

Fibrous dysplasia (FD) is a skeletal disorder caused by activating mutations in Gsα that result in elevations in cAMP. A feature of FD is elevated blood levels of the bone cell-derived phosphaturic hormone, fibroblast growth factor-23 (FGF23). FGF23 regulates serum phosphorus and active vitamin D levels by action on proximal renal tubule cells. An essential step in the production of biologically active FGF23 is glycosylation by the UDP-N-acetyl-α-D-galactosamine:polypeptide N-acetylgalactosaminyl transferase (ppGalNAc-T3). In the absence of glycosylation, FGF23 is processed into inactive N- and C-terminal proteins by a subtilisin proprotein convertase, probably furin. Normally, most if not all circulating FGF23 is intact. In FD, C-terminal levels are elevated, suggesting altered FGF23 processing. Altered processing in FD is the result of a cAMP-dependent, coordinated decrease in ppGalNAc-T3 and an increase in furin enzyme activity. These findings, and emerging data from other diseases, suggest regulation of FGF23 processing may be a physiologically important process.

Ultraviolet B activated 1,25(OH)(2)D affects the level of fibroblast growth factor-23 in human

Fibroblast growth factor 23 (FGF-23) is known as a phosphaturic factor regulating phosphate homeostasis. Several studies suggest that dietary phosphate, serum phosphate and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] are candidate regulators of FGF-23. While the human studies, which modulated the dietary or serum phosphate showed in rather controversial results, manipulation of the active vitamin D definitely affected FGF-23 in animals. This study was conducted to elucidate the relationship between active vitamin D directly stimulated by ultraviolet B (UVB) exposure and FGF-23 level in human. Ten healthy young adults were recruited to get the UVB exposure thrice a week at sub-minimal erythemal dose with gradual increment by 10% only for 4 weeks. Serum calcium, phosphate, mineral-related hormones and bone turnover markers were analyzed before and after the UVB exposure every 4 week for 12 whole weeks. Twenty five-hydroxyvitamin D [25(OH)D] increased by 115% (19.8 ng/mL to 40.5 ng/mL, p < 0.001) after 4 weeks of UVB exposure. While 1,25(OH)(2)D increased by 75% (49.9 pg/mL to 64.4 pg/mL, p < 0.001) then both level decreased after 4 weeks of withdrawal. C-telopeptide peaked at 2nd week then decreased, while osteocalcin increased gradually. FGF-23 started to increase from the 4th week of UVB exposure then significantly at the 4th week after withdrawal of UVB (27.8 pg/mL to 41.4 pg/mL, p < 0.05). UVB exposure effectively increased 1,25(OH)(2)D with delayed stimulatory effect on FGF-23. This result could support the regulatory loop of 1,25(OH)(2)D and FGF-23 in human, FGF-23 regulation by 1,25(OH)(2)D.

Fibroblast growth factor 23: state of the field and future directions

Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that regulates and is regulated by blood levels of phosphate and active vitamin D. Post-translational glycosylation by the enzyme GALNT3 and subsequent processing by furin have been demonstrated to be a regulated process that plays a role in regulating FGF23 levels. In physiologic states, FGF23 signaling is mediated by an FGF receptor and the coreceptor, Klotho. Recent work identifying a role for iron/hypoxia pathways in FGF23 physiology and their implications are discussed. Beyond its importance in primary disorders of mineral metabolism, recent work implicates FGF23 in renal disease-associated morbidity, as well as possible roles in cardiovascular disease and skeletal fragility.

Vitamin D: an overview of its role in skeletal muscle physiology in children and adolescents

Many children may have insufficient serum concentrations of vitamin D, which could prevent optimal muscle development and function. Vitamin D deficiency in animal models results in negative effects on muscle fiber structure and calcium/phosphorus handling, suggesting an integral role of vitamin D in skeletal muscle function. While there is a dearth of data in humans, the available evidence demonstrates a positive association between vitamin D status and muscle function. This review focuses on the important role of vitamin D in muscle function in children and adolescents who live in North American regions where exposure to ultraviolet B radiation is limited and who are thus at increased risk for vitamin D insufficiency. The effects of vitamin D on muscle cell proliferation and differentiation, muscle fiber structure, and calcium and phosphorus handling are discussed. Moreover, the roles of vitamin D and the vitamin D receptor and their genomic and nongenomic actions in muscle function are explored in depth. Future research should aim to establish a vitamin D status consistent with optimal musculoskeletal development and function in young children.

A physiologic-based approach to the evaluation of a patient with hyperphosphatemia

Phosphate is required for skeletal mineralization, cellular energy regulation, synthesis of cell membranes and nucleic acids, and a variety of cell signaling pathways. Extracellular serum phosphate concentration is determined by the balance of gastrointestinal phosphate absorption, skeletal turnover, distribution in intracellular compartments, and renal phosphate excretion. An integrated system of hormones, receptors, and phosphate transporters regulates phosphate homeostasis, and a variety of hereditary and acquired perturbations in these regulators can result in hyperphosphatemia. Although kidney failure is the most common cause of hyperphosphatemia encountered by nephrologists, hyperphosphatemia that presents in patients with early stages of chronic kidney disease or normal kidney function should prompt a detailed evaluation that can be diagnostically challenging. In this teaching case, we describe a case of hyperphosphatemia out of proportion to the degree of decrease in glomerular filtration rate. We present a practical parathyroid hormone-based diagnostic approach that illustrates the current understanding of phosphate regulation in clinically meaningful terms for the practicing nephrologist. Finally, we illustrate how measurement of fibroblast growth factor 23 could be integrated in the future when the test becomes more widely available.

Update on fibroblast growth factor 23 in chronic kidney disease

Chronic kidney disease (CKD) is a public health epidemic that affects millions of people worldwide. Presence of CKD predisposes individuals to high risks of end-stage renal disease, cardiovascular disease and premature death. Disordered phosphate homeostasis with elevated circulating levels of fibroblast growth factor 23 (FGF23) is an early and pervasive complication of CKD. CKD is likely the most common cause of chronically elevated FGF23 levels, and the clinical condition in which levels are most markedly elevated. Although increases in FGF23 levels help maintain serum phosphate in the normal range in CKD, prospective studies in populations of pre-dialysis CKD, incident and prevalent end-stage renal disease, and kidney transplant recipients demonstrate that elevated FGF23 levels are independently associated with progression of CKD and development of cardiovascular events and mortality. It was originally thought that these observations were driven by elevated FGF23 acting as a highly sensitive biomarker of toxicity due to phosphate. However, FGF23 itself has now been shown to mediate “off-target,” direct, end-organ toxicity in the heart, which suggests that elevated FGF23 may be a novel mechanism of adverse outcomes in CKD. This report reviews recent advances in FGF23 biology relevant to CKD, the classical effects of FGF23 on mineral homeostasis, and the studies that established FGF23 excess as a biomarker and novel mechanism of cardiovascular disease. The report concludes with a critical review of the effects of different therapeutic strategies targeting FGF23 reduction and how these might be leveraged in a future randomized trial aimed at improving outcomes in CKD.

FGF23, albuminuria, and disease progression in patients with chronic IgA nephropathy

BACKGROUND AND OBJECTIVES

Fibroblast growth factor-23 (FGF23) regulates mineral metabolism. Circulatory FGF23 levels are increased and predict outcomes in CKD. However, the relation of FGF23 to albuminuria and disease progression in patients with CKD and one underlying diagnosis is unknown.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Prospective, observational study in 180 patients with IgA nephropathy (IgAN), CKD stage 1-4, and median 55-month follow-up (range, 12-177 months). Primary outcomes were (1) time-averaged albuminuria, (2A) progression to CKD stage 5 or ≥50% loss of estimated GFR, (2B) progression to CKD stage 5 or ≥25% loss of estimated GFR within 10 years, and (3) annual loss of estimated GFR.

RESULTS

FGF23 was independently associated with baseline and time-averaged albuminuria (change in 1 g/24 hour albuminuria per increase in log FGF23: β = 0.26; P=0.02). Log FGF23 predicted CKD progression in crude models and after adjustment for mineral metabolites (endpoints 2A and 2B). It remained significant after adjustments for age, sex, serum albumin, calcium, phosphate, parathyroid hormone, 25-hydroxyvitamin D, baseline albuminuria, baseline estimated GFR, mean arterial BP, body mass index, and angiotensin-converting enzyme inhibitors/angiotensin-receptor blocker use in endpoint 2B (hazard ratio, 2.53; P=0.02) but not endpoint 2A (hazard ratio, 2.01; P=0.43). Log FGF23 predicted annual loss of estimated GFR in the same model (change in ml/min per 1.73 m(2) per increase in log FGF23, 1.50; P=0.008).

CONCLUSIONS

In patients with CKD and IgAN, FGF23 was associated with albuminuria and CKD progression, a finding that suggests its role as a potential biomarker in IgAN.

Randomized trial assessing the effects of ergocalciferol administration on circulating FGF23

BACKGROUND AND OBJECTIVES

Fibroblast growth factor 23 is a phosphate- and vitamin D-regulating hormone. The objective of this study was to determine the effect of ergocalciferol administration on fibroblast growth factor 23 levels in healthy vitamin D-deficient subjects.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In this 12-week trial conducted in a clinical research center, 18- to 45-year-old subjects (n=90) with 25-hydroxyvitamin D levels ≤20 ng/ml (by chemiluminescent immunoassay) were randomized to weekly ergocalciferol treatment of 50,000 international units or placebo, while consuming a self-selected diet. Changes in fibroblast growth factor 23, 25-hydroxyvitamin D (by liquid chromatography/tandem mass spectroscopy), 1,25-dihydroxyvitamin D, parathyroid hormone, and serum phosphate were measured.

RESULTS

Mean 25-hydroxyvitamin D (P<0.0001), 1,25-dihydroxyvitamin D (P=0.01), and fibroblast growth factor 23 (P=0.003) increased in the treatment versus placebo group. In the treatment group, 25-hydroxyvitamin D increased from 18 ± 7 to 40 ± 12 ng/ml at week 4 (P<0.0001) and remained stable at 43 ± 12 ng/ml at week 12 (P<0.0001); 1,25-dihydroxyvitamin D increased from 42 ± 17 to 52 ± 18 pg/ml at week 4 (P<0.001) and then remained stable, and fibroblast growth factor 23 increased from 43 ± 17 to 60 ± 33 pg/ml at week 8 (P=0.001) and 74 ± 42 pg/ml at week 12 (P<0.0001). Urinary phosphate excretion increased within the treatment group, but parathyroid hormone and serum phosphate were unchanged.

CONCLUSIONS

Ergocalciferol administration increases circulating fibroblast growth factor 23. When measuring fibroblast growth factor 23, concurrent 25-hydroxyvitamin D measurements should be obtained, because vitamin D deficiency may lower circulating fibroblast growth factor 23 levels.