Phosphate and FGF-23

Fibroblast growth factor (FGF)-23 is probably the most important regulator of serum phosphate and calcitriol (1,25(OH)2D3) levels. It is secreted by osteocytes and osteoblasts in response to oral phosphate loading or increased serum 1,25(OH)2D3 levels. In human chronic kidney disease (CKD), plasma FGF-23 appears to be a sensitive biomarker of abnormal renal phosphate handling, as FGF-23 levels increase during early stages of kidney malfunction. In humans and animals with CKD, elevated FGF-23 levels increase fractional phosphate excretion, reduce serum phosphate levels, and reduce 1α-hydroxylase activity, which reduces 1,25(OH)2D3 formation thereby increasing parathyroid hormone (PTH) secretion. FGF-23 thus has a key adaptive role in maintaining normophosphatemia. Plasma FGF-23 continues to increase as CKD progresses, increasing by orders of magnitude in end-stage renal disease. At the same time, responsiveness to FGF-23 declines as the number of intact nephrons declines, which is associated with reduced expression of Klotho, the co-receptor required for FGF-23 signaling. In late CKD, FGF-23 cannot reduce serum phosphate levels, and abnormally high plasma FGF-23 concentrations appear to exert unwarranted off-target effects, including left ventricular hypertrophy, faster CKD progression, and premature mortality. Lowering serum phosphate levels through the use of oral phosphate binders and/or long-acting PTH agents may reduce FGF-23 levels in early CKD stages, thereby limiting off-target effects, which may improve patient outcomes. Fibroblast growth factor (FGF)-23 is probably the most important regulator of serum phosphate and calcitriol (1,25(OH)2D3) levels. It is secreted by osteocytes and osteoblasts in response to oral phosphate loading or increased serum 1,25(OH)2D3 levels. In human chronic kidney disease (CKD), plasma FGF-23 appears to be a sensitive biomarker of abnormal renal phosphate handling, as FGF-23 levels increase during early stages of kidney malfunction. In humans and animals with CKD, elevated FGF-23 levels increase fractional phosphate excretion, reduce serum phosphate levels, and reduce 1α-hydroxylase activity, which reduces 1,25(OH)2D3 formation thereby increasing parathyroid hormone (PTH) secretion. FGF-23 thus has a key adaptive role in maintaining normophosphatemia. Plasma FGF-23 continues to increase as CKD progresses, increasing by orders of magnitude in end-stage renal disease. At the same time, responsiveness to FGF-23 declines as the number of intact nephrons declines, which is associated with reduced expression of Klotho, the co-receptor required for FGF-23 signaling. In late CKD, FGF-23 cannot reduce serum phosphate levels, and abnormally high plasma FGF-23 concentrations appear to exert unwarranted off-target effects, including left ventricular hypertrophy, faster CKD progression, and premature mortality. Lowering serum phosphate levels through the use of oral phosphate binders and/or long-acting PTH agents may reduce FGF-23 levels in early CKD stages, thereby limiting off-target effects, which may improve patient outcomes. Phosphate is essential for many cellular functions. It is a constituent of DNA, membrane lipids, high-energy phosphates, and second messengers, that is, inositol trisphosphate, cyclic adenosine monophosphate (cAMP), and cyclic guanosine monophosphate, and protein phosphorylation is an essential process, which helps regulate enzyme and receptor activities. Because phosphate is scarce in nature, vertebrate physiology has evolved to conserve phosphate through the efficient use of three sodium-dependent phosphate co-transporters (NaPi-IIa, NaPi-IIb, and NaPi-IIc, also referred to as NPT2a, NPT2b, and NPT2c) that are highly conserved throughout vertebrate taxa, from fish1.Coloso R.M. King K. Fletcher J.W. et al.Dietary P regulates phosphate transporter expression, phosphatase activity, and effluent P partitioning in trout culture.J Comp Physiol B. 2003; 173: 519-530Crossref PubMed Scopus (30) Google Scholar to humans.2.Dominguez J.H. Gray R.W. Lemann Jr, J. Dietary phosphate deprivation in women and men: effects on mineral and acid balances, parathyroid hormone and the metabolism of 25-OH-vitamin D.J Clin Endocrinol Metab. 1976; 43: 1056-1068Crossref PubMed Scopus (114) Google Scholar Additional molecules contributing to the regulation of phosphate homeostasis include Pit2 and the 1α-hydroxylase, which allows the formation of calcitriol (1,25(OH)2D3). Expression of these proteins in the proximal renal tubules is regulated by serum phosphate concentration and different hormonal systems. For example, low serum phosphate levels induce NPT2b expression in the intestinal tract, thus enhancing absorption of this mineral from the diet. In addition, low serum phosphate levels induce NPT2a and NPT2c expression in the proximal tubules of the kidney, thus maximizing the reabsorption of phosphate and minimizing urinary losses of this mineral. The efficiency of this latter process was strikingly demonstrated in a 1976 study, in which healthy human volunteers on a low-phosphate diet (90 mg/day) reduced their phosphate excretion considerably, thereby avoiding profound hypophosphatemia.2.Dominguez J.H. Gray R.W. Lemann Jr, J. Dietary phosphate deprivation in women and men: effects on mineral and acid balances, parathyroid hormone and the metabolism of 25-OH-vitamin D.J Clin Endocrinol Metab. 1976; 43: 1056-1068Crossref PubMed Scopus (114) Google Scholar In contrast to the efficient adaptation to hypophosphatemia, renal excretion of excess phosphate is a more difficult problem for human physiology to solve. The principal hormones that regulate renal phosphate handling are parathyroid hormone (PTH), which is produced by the parathyroid gland, and fibroblast growth factor (FGF)-23, which is produced by osteocytes and osteoblasts in bone. In healthy individuals, increasing serum phosphate concentration induces secretion of PTH and FGF-23. These two phosphaturic hormones reduce expression of NPT2a and NPT2c in the proximal renal tubules, thereby diminishing phosphate reabsorption and increasing urinary phosphate excretion.3.Gattineni J. Bates C. Twombley K. et al.FGF23 decreases renal NaPi-2a and NaPi-2c expression and induces hypophosphatemia in vivo predominantly via FGF receptor 1.Am J Physiol Renal Physiol. 2009; 297: F282-F291Crossref PubMed Scopus (324) Google Scholar, 4.Bergwitz C. Jüppner H. Regulation of phosphate homeostasis by PTH, vitamin D, and FGF23.Annu Rev Med. 2010; 61: 91-104Crossref PubMed Scopus (486) Google Scholar PTH also increases the 1α-hydroxylase in the kidney, thereby increasing 1,25(OH)2D3 production, thus enhancing intestinal calcium and phosphate absorption. Some evidence suggests that PTH induces expression and secretion of FGF-23; FGF-23 in turn decreases 1,25(OH)2D3 production, which is an inhibitor of PTH production. In chronic kidney disease (CKD), increased FGF-23 production enhances the excretion of phosphate per nephron, thereby restoring normophosphatemia. However, it also reduces 1,25(OH)2D3 levels,5.Gutierrez O. Isakova T. Rhee E. et al.Fibroblast growth factor-23 mitigates hyperphosphatemia but accentuates calcitriol deficiency in chronic kidney disease.J Am Soc Nephrol. 2005; 16: 2205-2215Crossref PubMed Scopus (746) Google Scholar contributing to an increase in PTH secretion, which appears to occur after FGF-23 levels increase. This process disrupts the bone–kidney–parathyroid endocrine axis and eventually fails to prevent the development of hyperphosphatemia as CKD progresses. The resulting changes, constituting CKD-related mineral and bone disorder (CKD-MBD), are a reflection of the trade-offs postulated by Bricker's6.Bricker N.S. On the pathogenesis of the uremic state. An exposition of the ‘trade-off hypothesis’.N Engl J Med. 1972; 286: 1093-1099Crossref PubMed Scopus (238) Google Scholar intact nephron hypothesis: ‘If solute intake does not diminish as the number of excretory units diminishes, the adaptive increase in excretion rate per nephron may be accomplished only at the expense of one or more abnormalities of the uremic state.’ This review will explore preclinical and clinical studies of the role of FGF-23 in phosphate metabolism in CKD and the contribution of FGF-23 to CKD-MBD. NPT2a and NPT2c facilitate the efficient reabsorption of phosphate in the proximal tubule; expression of both transporters is regulated by PTH and FGF-23. The acute PTH-dependent regulation of NPT2a and NPT2c expression is mediated predominantly through the cAMP and protein kinase A signaling pathway.4.Bergwitz C. Jüppner H. Regulation of phosphate homeostasis by PTH, vitamin D, and FGF23.Annu Rev Med. 2010; 61: 91-104Crossref PubMed Scopus (486) Google Scholar The PTH-induced increase in urinary phosphate is coupled to cAMP excretion from proximal tubules. Cyclic AMP production requires Gsα, which appears to be derived in this portion of the kidney predominantly, if not exclusively, from the maternal allele, thereby potentially limiting the maximal amount of Gsα protein that can be generated in this tissue. As a consequence of this parent-specific expression of Gsα in the proximal renal tubules, maternally inherited, heterozygous GNAS mutations cause PTH resistance, that is, pseudohyperparathyroidism.7.Jüppner H. Bastepe M. Different mutations within or upstream of the GNAS locus cause distinct forms of pseudohypoparathyroidism.J Pediatr Endocrinol Metab. 2006; 19: 641-646PubMed Google Scholar Inactivating GNAS mutations in those exons that encode Gsα cause pseudohypoparathyroidism type Ia, whereas microdeletions within or upstream of GNAS cause pseudohypoparathyroidism type Ib and are associated with GNAS methylation changes. The mechanism by which cAMP is excreted into the urine remains unknown, but is expected to involve a specific transporter that is expressed in the proximal tubule. In the distal tubule, PTH diminishes calcium excretion through cAMP/protein kinase A- and possibly also through inositol trisphosphate/protein kinase C-dependent actions on the function of calbindin-D and TRPV5. In contrast to the findings in proximal tubular cells, Gsα is expressed in the distal tubular cells from both parental alleles, and maternally inherited GNAS mutations consequently leave the PTH-induced regulation of calcium excretion intact. Similar to the actions of PTH, FGF-23 controls NPT2a and NPT2c expression. FGF-23 is secreted by bone cells in response to 1,25(OH)2D3, which increases the mRNA levels encoding this phosphaturic hormone.8.Kolek O.I. Hines E.R. Jones M.D. et al.1α,25-Dihydroxyvitamin D3 upregulates FGF23 gene expression in bone: the final link in a renal-gastrointestinal-skeletal axis that controls phosphate transport.Am J Physiol Gastrointest Liver Physiol. 2005; 289: G1036-G1042Crossref PubMed Scopus (336) Google Scholar FGF-23 mediates its action in the kidney through an FGF receptor (FGFR)/Klotho complex to downregulate NPT2a and NPT2c expression in the proximal tubules. However, it is uncertain whether FGF-23 signals initially through receptors in the distal convoluted tubule cells in which ERK phosphorylation occurs in response to FGF-23.9.Farrow E.G. Davis S.I. Summers L.J. et al.Initial FGF23-mediated signaling occurs in the distal convoluted tubule.J Am Soc Nephrol. 2009; 20: 955-960Crossref PubMed Scopus (191) Google Scholar These cells are adjacent to NPT2a-expressing proximal tubular cells and it has therefore been speculated that a paracrine signal from the distal to the proximal tubules is required for decreasing NPT2a expression. Even small changes in plasma FGF-23 levels are associated with significant changes in urinary phosphate excretion, as shown by the small increase in FGF-23 levels after unilateral nephrectomy in healthy kidney donors, which is associated with increased urinary phosphate excretion.10.Westerberg P.A. Ljunggren O. Larsson T.E. et al.Fibroblast growth factor-23 and mineral metabolism after unilateral nephrectomy.Nephrol Dial Transplant. 2010; 25: 4068-4071Crossref PubMed Scopus (18) Google Scholar Besides its effect on tubular phosphate handling, FGF-23 reduces PTH secretion, and it inhibits 1α-hydroxylase leading to a decrease in 1,25(OH)2D3 production, which contributes to the development of hypocalcemia and leads to an increase in PTH production.11.Hasegawa H. Nagano N. Urakawa I. et al.Direct evidence for a causative role of FGF23 in the abnormal renal phosphate handling and vitamin D metabolism in rats with early-stage chronic kidney disease.Kidney Int. 2010; 78: 975-980Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar The 1,25(OH)2D3 itself increases FGF-23 production. FGF-23 appears to be an important biomarker for an abnormal regulation of phosphate homeostasis in CKD and is likely to be involved in CKD-MBD pathophysiology. As CKD progresses, plasma FGF-23 levels increase. This increase occurs earlier and to a greater extent than observed for serum phosphate; in late CKD, plasma FGF-23 levels can be elevated by several orders of magnitude.5.Gutierrez O. Isakova T. Rhee E. et al.Fibroblast growth factor-23 mitigates hyperphosphatemia but accentuates calcitriol deficiency in chronic kidney disease.J Am Soc Nephrol. 2005; 16: 2205-2215Crossref PubMed Scopus (746) Google Scholar, 12.van Husen M. Fischer A.K. Lehnhardt A. et al.Fibroblast growth factor 23 and bone metabolism in children with chronic kidney disease.Kidney Int. 2010; 78: 200-206Abstract Full Text Full Text PDF PubMed Scopus (69) Google Scholar Findings in experimental renal disease suggest that the FGF-23 increase also precedes the increase in PTH levels.11.Hasegawa H. Nagano N. Urakawa I. et al.Direct evidence for a causative role of FGF23 in the abnormal renal phosphate handling and vitamin D metabolism in rats with early-stage chronic kidney disease.Kidney Int. 2010; 78: 975-980Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar In fact, bone biopsies of patients with CKD have shown increased expression of FGF-23 already by CKD stage 2, along with a marked increase in DMP1 protein (a negative regulator of FGF-23), which may be improperly processed and could thus be inactive.13.Pereira R.C. Jüppner H. Azucena-Serrano C.E. et al.Patterns of FGF-23, DMP1, and MEPE expression in patients with chronic kidney disease.Bone. 2009; 45: 1161-1168Abstract Full Text Full Text PDF PubMed Scopus (209) Google Scholar Furthermore, in a prospective study of patients with mild-to-moderate CKD,14.Fliser D. Kollerits B. Neyer U. et al.Fibroblast growth factor 23 (FGF23) predicts progression of chronic kidney disease: the Mild to Moderate Kidney Disease (MMKD) Study.J Am Soc Nephrol. 2007; 18: 2600-2608Crossref PubMed Scopus (605) Google Scholar higher plasma FGF-23 levels were shown to predict a more rapid progression toward end-stage renal disease (ESRD); similarly elevated FGF-23 levels predict CKD progression in patients with diabetic nephropathy.15.Titan S.M. Zatz R. Jorgetti V. et al.FGF-23 as a predictor of renal outcome in diabetic nephropathy. (Renal week 2009 abstract F-PO1872).J Am Soc Nephrol. 2009; 20: 540AGoogle Scholar In addition, elevated FGF-23 levels are independently associated with left ventricular hypertrophy in patients with CKD,16.Gutiérrez O.M. Januzzi J.L. Isakova T. et al.Fibroblast growth factor 23 and left ventricular hypertrophy in chronic kidney disease.Circulation. 2009; 119: 2545-2552Crossref PubMed Scopus (676) Google Scholar an important finding because CKD-MBD, and particularly hyperphosphatemia, worsen the cardiovascular prognosis in CKD. Aortic calcification, a major reason for cardiovascular morbidity, in hemodialysis recipients is independently predicted by plasma FGF-23.17.Nasrallah M.M. El-Shehaby A.R. Salem M.M. et al.Fibroblast growth factor-23 (FGF-23) is independently correlated to aortic calcification in haemodialysis patients.Nephrol Dial Transplant. 2010; 25: 2679-2685Crossref PubMed Scopus (157) Google Scholar Phosphate is a major regulator of FGF-23 expression.18.Perwad F. Azam N. Zhang M.Y. et al.Dietary and serum phosphorus regulate fibroblast growth factor 23 expression and 1,25-dihydroxyvitamin D metabolism in mice.Endocrinology. 2005; 146: 5358-5364Crossref PubMed Scopus (342) Google Scholar Dietary phosphate loading increases FGF-23 expression, whereas phosphate depletion with binders decreases the circulating levels of this hormone.19.Burnett S.M. Gunawardene S.C. Bringhurst F.R. et al.Regulation of C-terminal and intact FGF-23 by dietary phosphate in men and women.J Bone Miner Res. 2006; 21: 1187-1196Crossref PubMed Scopus (392) Google Scholar In mild CKD, FGF-23 seems to function as a protective factor, as it triggers adaptive changes that maintain normophosphatemia. For example, in animal models, FGF-23 protects against hyperphosphatemia by increasing urinary phosphate excretion and reducing 1,25(OH)2D3 production; diminished 1,25(OH)2D3 levels lead to hypocalcemia and thus to an increase in PTH levels, which further enhances renal phosphate excretion.11.Hasegawa H. Nagano N. Urakawa I. et al.Direct evidence for a causative role of FGF23 in the abnormal renal phosphate handling and vitamin D metabolism in rats with early-stage chronic kidney disease.Kidney Int. 2010; 78: 975-980Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar The importance of FGF-23 in mediating these effects is shown by the effects of anti-FGF-23 antibodies in rats with experimental CKD.11.Hasegawa H. Nagano N. Urakawa I. et al.Direct evidence for a causative role of FGF23 in the abnormal renal phosphate handling and vitamin D metabolism in rats with early-stage chronic kidney disease.Kidney Int. 2010; 78: 975-980Abstract Full Text Full Text PDF PubMed Scopus (284) Google Scholar Animals with early CKD that were treated with anti-FGF-23 antibodies showed an increase in 1,25(OH)2D3 levels, a normalization of serum calcium levels, and a decrease in PTH levels. Furthermore, fractional phosphate excretion decreased because of inactivation of FGF-23 resulting in hyperphosphatemia. As CKD progresses, elevated FGF-23 levels are no longer able to enhance urinary phosphate excretion, thus leading to the development of hyperphosphatemia. This may be partly related to declining Klotho expression and a reduction in functional nephrons. When patients require treatment by dialysis, the levels of immunoreactive FGF-23 can be markedly elevated (Figure 1).20.Isakova T. Gutierrez O.M. Wolf M. A blueprint for randomized trials targeting phosphorus metabolism in chronic kidney disease.Kidney Int. 2009; 76: 705-716Abstract Full Text Full Text PDF PubMed Scopus (94) Google Scholar However, unlike the appearance of large amounts of C-terminal PTH fragments, most of which are biologically inactive, practically all of the circulating FGF-23 present in patients with ESRD is intact and biologically active. This conclusion is based on observations with a reporter-cell assay (human embryonic kidney cells expressing Klotho and a luciferase reporter expressed under the control of the EGR-1 promoter) designed to quantify biologically active FGF-23, which demonstrated excellent correlation with immunoreactive measurement of FGF-23. The immunometric assays used for these measurements quantify the intact FGF-23 molecule alone (Kainos assay) or the intact protein along with a C-terminal fragment (Immutopics assay).21.Shimada T. Urakawa I. Isakova T. et al.Circulating fibroblast growth factor 23 in patients with end-stage renal disease treated by peritoneal dialysis is intact and biologically active.J Clin Endocrinol Metab. 2010; 95: 578-585Crossref PubMed Scopus (185) Google Scholar, 22.Heijboer A.C. Levitus M. Vervloet M.G. et al.Determination of fibroblast growth factor 23.Ann Clin Biochem. 2009; 46: 338-340Crossref PubMed Scopus (63) Google Scholar Consistent with the excellent correlation between bioactive and immunoreactive FGF23, Western blot analyses of plasma FGF-23 from dialysis patients showed that most of the circulating FGF-23 is intact.21.Shimada T. Urakawa I. Isakova T. et al.Circulating fibroblast growth factor 23 in patients with end-stage renal disease treated by peritoneal dialysis is intact and biologically active.J Clin Endocrinol Metab. 2010; 95: 578-585Crossref PubMed Scopus (185) Google Scholar Thus, with CKD progression, the excess of biologically active FGF-23, which is thought to occur in response to hyperphosphatemia, ceases to be protective and may lead to pathological off-target effects that potentially contribute to the increase in mortality as FGF-23 increases in patients with ESRD23.Gutiérrez O.M. Mannstadt M. Isakova T. et al.Fibroblast growth factor 23 and mortality among patients undergoing hemodialysis.N Engl J Med. 2008; 359: 584-592Crossref PubMed Scopus (1409) Google Scholar, 24.Jean G. Terrat J.C. Vanel T. et al.High levels of serum fibroblast growth factor (FGF)-23 are associated with increased mortality in long haemodialysis patients.Nephrol Dial Transplant. 2009; 24: 2792-2796Crossref PubMed Scopus (307) Google Scholar (Figure 2).Figure 2Mortality in patients with end-stage renal disease (ESRD) receiving hemodialysis in relationship to quartiles of serum fibroblast growth factor (FGF)-23 concentration at initiation of dialysis. R, reference.Reproduced with permission from Gutiérrez et al.23.Gutiérrez O.M. Mannstadt M. Isakova T. et al.Fibroblast growth factor 23 and mortality among patients undergoing hemodialysis.N Engl J Med. 2008; 359: 584-592Crossref PubMed Scopus (1409) Google Scholar Copyright The Massachusetts Medical that serum as oral phosphate binders to prevent intestinal absorption or possibly long-acting PTH that reduce NPT2a or NPT2c expression, may prevent CKD-related FGF-23 increases (Figure For example, two long-acting PTH and were to reduce NPT2a expression in proximal tubules, leading to hypophosphatemia in M. H. et of sodium-dependent phosphate transporter NPT2a predominantly the as by parathyroid hormone 286: PubMed Scopus Google Scholar the proximal tubules with long-acting PTH agents in early CKD may to the effects of phosphate binders in serum phosphate levels and may an increase in FGF-23, PTH has a role in the and/or secretion of FGF-23. to the development of hyperphosphatemia in CKD may thus prevent an increase in FGF-23 and the off-target effects in CKD-MBD. In of this a of a randomized clinical has shown that treatment with phosphate binders can markedly decrease FGF-23 levels in patients undergoing et al.Fibroblast growth factor 23 in hemodialysis effects of phosphate calcitriol and calcium concentration in the Clin PubMed Scopus Google Scholar FGF-23 is a major regulator of phosphate which appears to be an important biomarker of phosphate homeostasis in patients with CKD. Plasma FGF-23 concentrations to increase early in CKD, with increases by orders of magnitude by the patients FGF-23 levels are independently associated with left ventricular hypertrophy, CKD progression, and possibly through off-target Lowering serum phosphate levels through the use of phosphate binders may FGF-23 levels. is to whether FGF-23 levels in patients with CKD.