Fibroblast growth factor-23 and Klotho in chronic kidney disease

      The well-established increased cardiovascular risk that is a hallmark of chronic kidney disease (CKD) has directed research to metabolic changes that are typical of CKD. Epidemiological data point to derangements of mineral metabolism to be involved in this risk profile. Subsequently, newly discovered humoral factors—such as fibroblast growth factor (FGF)-23—that are involved in mineral and vitamin D homeostasis turned out to be associated with clinical outcome, independently of the minerals they regulate. Additional proteins involved in FGF-23 signaling, such as Klotho, subsequently appeared to have FGF-23-independent effects as well. In this review, the discovery, mode of action, and clinical implications of these new factors are outlined.

      Keywords

      Identification of fibroblast growth factor (FGF)-23 has in many ways revolutionized our current understanding of mineral metabolism. It was initially discovered through attempts to identify the predicted existence of ‘phosphatonins’, that is, phosphate-regulating hormones. Summarizing the accomplishments made by many different researchers, FGF-23 was found to be the primary cause of autosomal dominant hypophosphatemic rickets,
      • ADHR Consortium
      Autosomal dominant hypophosphataemic rickets is associated with mutations in FGF-23.
      as well as an ectopically overproduced phosphaturic factor in patients with tumor-induced osteomalacia.
      • Shimada T.
      • Mizutani S.
      • Muto T.
      • et al.
      Cloning and characterization of FGF-23 as a causative factor of tumor-induced osteomalacia.
      Structurally, FGF-23 was the twenty-third member of the FGF family to be discovered with approximately 25–30% homology to other FGFs. The first 24 amino acids of the amino terminus function as a signal peptide for its transport from the Golgi network to the extracellular space, and it is consequently a circulating factor. The carboxy terminus is distinct from other FGFs, providing unique characteristics in terms of glycosylation and receptor activation. Two arginines—located at residues 176 and 179, respectively—provide a consensus site for proteolytic cleavage by furin-like enzymes that inactivate and degrade the active FGF-23 protein
      • Yamashita T.
      Structural and biochemical properties of fibroblast growth factor 23.
      ,
      • White K.E.
      • Carn G.
      • Lorenz-Depiereux B.
      • et al.
      Autosomal-dominant hypophosphatemic rickets (ADHR) mutations stabilize FGF-23.
      (Figure 1).
      Figure thumbnail gr1
      Figure 1The structure of fibroblast growth factor (FGF)-23. The diagram shows its FGF-like domain, the cleavage site, and the signal peptide. Reprinted with permission from Yamashita
      • Yamashita T.
      Structural and biochemical properties of fibroblast growth factor 23.
      .

      FGF-23 MECHANISMS AND REGULATION

      There is striking concordance between the phenotypic changes found in patients with primary or secondary FGF-23 excess and/or deficiency and data from the extensive animal and in vitro studies. Collectively, FGF-23 is a potent negative regulator of circulating phosphate and 1,25-dihydroxyvitamin D (calcitriol; 1,25D) levels.
      • Shimada T.
      • Mizutani S.
      • Muto T.
      • et al.
      Cloning and characterization of FGF-23 as a causative factor of tumor-induced osteomalacia.
      • Shimada T.
      • Kakitani M.
      • Yamazaki Y.
      • et al.
      Targeted ablation of FGF-23 demonstrates an essential physiological role of FGF-23 in phosphate and vitamin D metabolism.
      FGF-23 induces phosphaturia and lowers serum phosphate level through reduction and internalization of the sodium–phosphate cotransporters Npt2a and Npt2c in the kidney proximal tubules.
      • Shimada T.
      • Mizutani S.
      • Muto T.
      • et al.
      Cloning and characterization of FGF-23 as a causative factor of tumor-induced osteomalacia.
      • Bai X.
      • Miao D.
      • Li J.
      • et al.
      Transgenic mice overexpressing human fibroblast growth factor 23 (R176Q) delineate a putative role for parathyroid hormone in renal phosphate wasting disorders.
      • Larsson T.
      • Marsell R.
      • Schipani E.
      • et al.
      Transgenic mice expressing fibroblast growth factor 23 under the control of the alpha1(I) collagen promoter exhibit growth retardation, osteomalacia, and disturbed phosphate homeostasis.
      Further, FGF-23 directly suppresses renal 1α-hydroxylase, leading to decreased conversion of 25-hydroxyvitamin D to its active metabolite 1,25D.
      • Shimada T.
      • Mizutani S.
      • Muto T.
      • et al.
      Cloning and characterization of FGF-23 as a causative factor of tumor-induced osteomalacia.
      • Shimada T.
      • Hasegawa H.
      • Yamazaki Y.
      • et al.
      FGF-23 is a potent regulator of vitamin D metabolism and phosphate homeostasis.
      Another role of FGF-23 in vitamin D metabolism is to enhance the degradation pathway of vitamin D through stimulation of the 24-hydroxylase. In the context of physiology, more recent studies have also convincingly shown that FGF-23, at least in the short term, directly decreases the transcript level and secretion of parathyroid hormone (PTH).
      • Krajisnik T.
      • Björklund P.
      • Marsell R.
      • et al.
      Fibroblast growth factor-23 regulates parathyroid hormone and 1alpha-hydroxylase expression in cultured bovine parathyroid cells.
      • Ben-Dov I.Z.
      • Galitzer H.
      • Lavi-Moshayoff V.
      • et al.
      The parathyroid is a target organ for FGF-23 in rats.
      The role of FGF-23 in regulation of PTH and secondary hyperparathyroidism in chronic kidney disease (CKD) is currently under intense investigation (see below).
      Whereas the main target of FGF-23 is the kidney, the tissue source of FGF-23 is primarily bone, more specifically osteocytes and osteoblasts.
      • Liu S.
      • Guo R.
      • Simpson L.G.
      • et al.
      Regulation of fibroblastic growth factor 23 expression but not degradation by PHEX.
      • Riminucci M.
      • Collins M.T.
      • Fedarko N.S.
      • et al.
      FGF-23 in fibrous dysplasia of bone and its relationship to renal phosphate wasting.
      This further underscores the fact that bone, beyond its capacity to store minerals and provide mechanical support, is a highly active endocrine organ. Further, there is robust evidence for the presence of a previously unidentified bone–kidney axis. The interplay between bone and kidney is not farfetched given that the kidney is the main determinant of circulating phosphate levels and actively participates in maintaining calcium homeostasis, providing the skeleton with sufficient minerals to form hydroxyapatite crystals at the mineralization front.
      Because FGF-23 holds promise as a biomarker for patient outcome (see below), especially in patients with CKD, it is important to understand its mode of regulation. The most rapid stimuli for FGF-23 expression both in vitro and in vivo is 1,25D, evoking a response in serum FGF-23 level within 3–4 h after intravenous administration.
      • Liu S.
      • Tang W.
      • Zhou J.
      • et al.
      Fibroblast growth factor 23 is a counter-regulatory phosphaturic hormone for vitamin D.
      This completes a feedback loop between vitamin D and FGF-23, and FGF-23 can in that sense be viewed as a counter-regulatory hormone for vitamin D. As a result, the decline in vitamin D level that occurs already in the initial phase of CKD can likely be attributed to a rise in FGF-23 rather than a reduced renal mass per se.
      FGF-23 production is also promoted by high dietary phosphate intake,
      • Vervloet M.G.
      • van Ittersum F.J.
      • Büttler R.M.
      • et al.
      Effects of dietary phosphate and calcium intake on fibroblast growth factor-23.
      • Ferrari S.L.
      • Bonjour J.P.
      • Rizzoli R.
      Fibroblast growth factor-23 relationship to dietary phosphate and renal phosphate handling in healthy young men.
      • Nishida Y.
      • Taketani Y.
      • Yamanuka-Okumura H.
      • et al.
      Acute effect of oral phosphate loading on serum fibroblast growth factor 23 levels in healthy men.
      • Isakova T.
      • Gutierrez O.M.
      • Smith K.
      • et al.
      Pilot study of dietary phosphorus restriction and phosphorus binders to target fibroblast growth factor 23 in patients with chronic kidney disease.
      • Antoniucci D.M.
      • Yamashita T.
      • Portale A.A.
      Dietary phosphorus regulates serum fibroblast growth factor-23 concentrations in healthy men.
      as well as chronic hyperphosphatemia, although rapid changes in serum phosphate concentrations may not invoke acute increments in FGF-23. One hypothesis is that FGF-23 responds to the net phosphate balance rather than the serum phosphate level, but experimental data supporting this hypothesis is weak. Further, the complete chain of events from high dietary phosphate intake and hyperphosphatemia to increased FGF-23 synthesis in bone is currently unknown.
      It also stands clear that vitamin D and phosphate regulate FGF-23 through independent pathways, because mice lacking the vitamin D receptor are still highly responsive to high dietary phosphate intake.
      • Yu X.
      • Sabbagh Y.
      • Davis S.I.
      • et al.
      Genetic dissection of phosphate- and vitamin D-mediated regulation of circulating FGF-23 concentrations.
      As a final remark, the response in FGF-23 elicited by dietary phosphate intake in humans is much weaker than that in rodents.
      Despite the fact that FGF-23 belongs to the FGF family, in which all members signal through one or several of the known FGF receptors, it has been difficult to unveil the ‘true’ FGF-23 receptor both in vivo and in vitro. A major breakthrough came from studies by Urakawa et al.,
      • Urakawa I.
      • Yamazaki Y.
      • Shimada T.
      • et al.
      Klotho converts canonical FGF receptor into a specific receptor for FGF-23.
      who demonstrated that type I membrane-bound α-Klotho (Klotho) directly binds to FGF receptor 1c, converting it into a specific FGF-23 receptor (Figure 2).
      Figure thumbnail gr2
      Figure 2The fibroblast growth factor (FGF)-23 receptor. In light blue, FGF receptor type-1 (FGFR-1). In dark blue, left-sided membrane-bound Klotho; right-sided soluble, shed Klotho. The red hexagon depicts FGF-23. FGF-23 signal transduction is established when Klotho and FGFR-1 colocalize (left side). It is unknown whether signal transduction can occur by soluble Klotho (right side). In the kidney, signaling leads to downregulation of CYP27B1 and retrieval of phosphate transporters in the proximal tubule (figure provided by J Hoenderop, Department of Physiology, University Medical Center Nijmegen, The Netherlands).
      Accordingly, FGF-23 is dependent on Klotho to induce FGF-receptor signaling, at least in the kidneys and parathyroid glands. The importance of Klotho in FGF-23 signaling is evidenced by Klotho-null mice, which harbor nearly an identical biochemical phenotype compared with FGF-23-knockout mice, despite exceptionally high circulatory FGF-23 levels.
      • Kuro-o M.
      • Matsumura Y.
      • Yamazaki Y.
      • et al.
      Mutation of the mouse klotho gene leads to a syndrome resembling ageing.
      • Shimada T.
      • Urakawa I.
      • Yamazaki Y.
      • et al.
      FGF-23 transgenic mice demonstrate hypophosphatemic rickets with reduced expression of sodium phosphate cotransporter type IIa.
      There are, however, still controversies and unresolved issues around FGF-23 receptor signaling. First, Klotho expression in the kidney is largely confined to the distal tubules, whereas renal phosphate reabsorption occurs in the proximal tubules. It is currently unclear how FGF-23 signaling in distal tubules modifies phosphate reabsorption in proximal tubules. Second, it is possible that high levels of FGF-23, as present in many patients with advanced CKD, could induce unspecific ‘off-target’ (that is, Klotho-independent) FGF-receptor signaling.
      FGF-23 and Klotho are likely to have important roles in the pathophysiology of secondary hyperparathyroidism. Although FGF-23 in the short term suppresses PTH secretion, chronically high exposure of FGF-23 may override this effect by lowering the systemic levels of 1,25D and attenuate parathyroid vitamin D receptor signaling. Equally important, it was recently demonstrated that FGF-23 reduces the expression level of Klotho and that parathyroid Klotho expression in surgically removed human parathyroid adenomas declines in parallel with loss of renal function.
      • Krajisnik T.
      • Olauson H.
      • Mirza M.A.
      • et al.
      Parathyroid Klotho and FGF-receptor 1 expression decline with renal function in hyperparathyroid patients with chronic kidney disease and kidney transplant recipients.
      This is a plausible explanation for the parathyroid FGF-23 ‘resistance’ observed both in CKD patients and in rodent models of experimentally induced renal failure.
      • Galitzer H.
      • Ben-Dov I.Z.
      • Silver J.
      • et al.
      Parathyroid cell resistance to fibroblast growth factor 23 in secondary hyperparathyroidism of chronic kidney disease.
      • Canalejo R.
      • Canalejo A.
      • Martinez-Moreno J.M.
      • et al.
      FGF23 fails to inhibit uremic parathyroid glands.
      In summary, the discovery of FGF-23 and Klotho has led to significant advances in our understanding of mineral metabolism. This knowledge is now gradually being translated into the clinic with many potential implications, including the endorsement of FGF-23 as a predictive biomarker and the possibility of FGF-23/Klotho as a novel therapeutic target.

      KLOTHO

      Klotho was discovered in 1997 in a mouse strain with a phenotype consistent with premature aging as the principle hallmark.
      • Kuro-o M.
      • Matsumura Y.
      • Yamazaki Y.
      • et al.
      Mutation of the mouse klotho gene leads to a syndrome resembling ageing.
      Klotho has a short transmembrane domain and two large extracellular domains,
      • Shiraki-Iida T.
      • Aizawa H.
      • Matsumura Y.
      • et al.
      Structure of the mouse klotho gene and its two transcripts encoding membrane and secreted protein.
      as shown in Figure 2. Klotho exists as a membrane-bound form and two circulating forms—one being the shed product of the membrane form and the other a truncated form derived from the same gene by alternative splicing.
      Besides its above-described role as a cofactor in FGF-23 signaling, as shown in Figure 2, Klotho harbors at least two important additional functions.
      • Torres P.U.
      • Irié D.
      • Molina-Blétry V.
      • et al.
      Klotho: an antiaging protein involved in mineral and vitamin D metabolism.
      First, it turns out that Klotho has enzymatic activity, which resides on the extracellular domain.
      • Tohyama O.
      • Imura A.
      • Iwano A.
      • et al.
      Klotho is a novel beta-glucuronidase capable of hydrolyzing steroid beta-glucuronides.
      The latter is of importance because Klotho retains this enzymatic activity even when it is shed from the plasma membrane. This is in contrast to the role of Klotho as a cofactor in FGF-23, where the membrane-bound form appears to be necessary.
      • Yamashita T.
      Structural and biochemical properties of fibroblast growth factor 23.
      The best-described role of this enzymatic activity of Klotho is its influence on the transient receptor potential V5 at the luminal side of the renal tubules.
      • Chang Q.
      • Hoefs S.
      • van der Kemp A.W.
      • et al.
      The beta-glucuronidase klotho hydrolyzes and activates the TRPV5 channel.
      The glucuronidase-like activity of Klotho modulates the sugar moieties, leading to sustained retention of these highly important calcium transporters in the kidney. In this way, Klotho promotes reabsorption of ultrafiltrated calcium and prevents calciuria.
      • Chang Q.
      • Hoefs S.
      • van der Kemp A.W.
      • et al.
      The beta-glucuronidase klotho hydrolyzes and activates the TRPV5 channel.
      This calcium-retaining effect of Klotho in the healthy kidney is yet another mechanism used to prevent overactivation of native vitamin D, besides its suppressive role on vitamin D metabolism mediated by FGF-23 signaling. Active vitamin D in turn upregulates Klotho expression, closing a feedback loop between vitamin D and Klotho.
      • Tsujikawa H.
      • Kurotaki Y.
      • Fujimori T.
      • et al.
      Klotho, a gene related to a syndrome resembling human premature aging, functions in a negative regulatory circuit of vitamin D endocrine system.
      Very recently, it was shown that Klotho, besides its involvement in calcium handling in the kidney, also has direct effects on NaPt2a, the principle phosphate transporter in the proximal tubule.
      • Hu M.C.
      • Shi M.
      • Zhang J.
      • et al.
      Klotho: a novel phosphaturic substance acting as an autocrine enzyme in the renal proximal tubule.
      Remarkable about the latter finding is the fact that this effect is located on the proximal tubule, although most investigators found the distal tubule to be the most prominent site of Klotho expression. The other remarkable fact is that the Klotho-induced phosphaturic effects were also observed in FGF-23-knockout mice, indicating a direct effect on NaPt2a.
      A third recognized mode of action of Klotho is its protective effect against oxidative stress. This was demonstrated by Kuro-o et al.
      • Kuro-o M.
      Klotho as a regulator of oxidative stress and senescence.
      • Yamamoto M.
      • Clark J.D.
      • Pastor J.V.
      • et al.
      Regulation of oxidative stress by the anti-aging hormone klotho.
      (who discovered Klotho in 1997). They showed that the shed and circulating form of Klotho activates FoxO, leading to enhanced expression of superoxide dismutase.
      Future research will likely unveil additional intriguing effects of Klotho. Most recently, Klotho appeared to be involved in endothelial integrity
      • Kusaba T.
      • Okigawa M.
      • Matui A.
      • et al.
      Klotho is associated with VEGF receptor-2 and the transient receptor potential canonical-1 Ca2+ channel to maintain endothelial integrity.
      and endothelial-dependent vasodilatation.
      • Nagai R.
      • Saito Y.
      • Ohyama Y.
      • et al.
      Endothelial dysfunction in the klotho mouse and downregulation of klotho gene expression in various animal models of vascular and metabolic diseases.
      • Nakamura T.
      • Saito Y.
      • Ohyama Y.
      • et al.
      Production of nitric oxide, but not prostacyclin, is reduced in klotho mice.
      Also, Klotho was shown to bind to type II TGF-β receptor, thereby inhibiting TGF-β signaling and suppression of renal interstitial fibrosis in an animal model.
      • Doi S.
      • Zou Y.
      • Togao O.
      • et al.
      Klotho inhibits transforming growth factor-beta1 (TGF-beta1) signaling and suppresses renal fibrosis and cancer metastasis in mice.
      Finally, Klotho is expressed in the sinoatrial node and its decreased expression leads to sinoatrial node malfunction and premature death.
      • Takeshita K.
      • Fujimori T.
      • Kurotaki Y.
      • et al.
      Sinoatrial node dysfunction and early unexpected death of mice with a defect of klotho gene expression.
      All these findings are likely to be most relevant to CKD patients, as both renal expression and circulating levels of Klotho are reduced in this vulnerable population.
      • Kuro-o M.
      Phosphate and Klotho.

      CLINICAL ASPECTS OF FGF-23

      Given its central role in regulating mineral metabolism, the obvious question arises: How important is FGF-23 clinically? In early CKD, FGF-23 appears to be beneficial, compensating for reduced phosphate excretory capacity by increasing fractional excretion of phosphate. Although phosphate retention is also a stimulus for PTH secretion, in early-stage CKD the rise in FGF-23 is more pronounced than that of PTH, possibly because of the inhibitory effects of FGF-23 on PTH.
      • Krajisnik T.
      • Björklund P.
      • Marsell R.
      • et al.
      Fibroblast growth factor-23 regulates parathyroid hormone and 1alpha-hydroxylase expression in cultured bovine parathyroid cells.
      • Ben-Dov I.Z.
      • Galitzer H.
      • Lavi-Moshayoff V.
      • et al.
      The parathyroid is a target organ for FGF-23 in rats.
      A trade-off of the increase of FGF-23 may be a reduction in levels of 1,25D by the mechanism described above. As CKD progresses, the efficacy of FGF-23 to induce phosphaturia declines, due to at least two mechanisms. First, the loss of functioning nephrons reduces the amount of phosphate being ultrafiltrated; second, lowered renal Klotho expression dismantles the FGF-23 receptor, leading to higher phosphate reabsorption per nephron. For these reasons it is expected that in advanced CKD, FGF-23 could be an indicator for dismal outcome. Indeed, this has been shown for more advanced CKD, where FGF-23 independently predicted progression of disease.
      • Fliser D.
      • Kollerits B.
      • Never U.
      • et al.
      Fibroblast growth factor 23 (FGF-23) predicts progression of chronic kidney disease: the Mild to Moderate Kidney Disease (MMKD) Study.
      • Titan S.M.
      • Zatz R.
      • Graciolli F.G.
      • et al.
      FGF-23 as a predictor of renal outcome in diabetic nephropathy.
      In the study by Fliser et al., the predictive value of FGF-23 for disease progression was surpassed only by proteinuria.
      • Fliser D.
      • Kollerits B.
      • Never U.
      • et al.
      Fibroblast growth factor 23 (FGF-23) predicts progression of chronic kidney disease: the Mild to Moderate Kidney Disease (MMKD) Study.
      Despite their independency in predicting disease progression, FGF-23 levels and proteinuria are positively associated across several ranges of CKD, as recently demonstrated by Vervloet et al.
      • Vervloet M.
      • van Zuilen A.D.
      • Blankenstijn P.J.
      • et al.
      Fibroblast growth factor 23 is associated with proteinuria.
      The most convincing argument for the clinical meaning of FGF-23 comes from the analysis of a large cohort of dialysis patients, in which an independent association between FGF-23 and mortality was demonstrated, as shown in Figure 3.
      • Gutierrez O.M.
      • Mannstadt M.
      • Isakova T.
      • et al.
      Fibroblast growth factor 23 and mortality among patients undergoing hemodialysis.
      Even after correcting for established predictors of mortality, the hazard ratios for the higher ranges of FGF-23 outranked the others.
      Figure thumbnail gr3
      Figure 3Odds ratios for death in hemodialysis patients in relation to quartiles of fibroblast growth factor (FGF)-23 levels. Both unadjusted and fully adjusted models demonstrate a strong and graded association. *P<0.05. Reprinted with permission from Gutierrez et al.
      • Gutierrez O.M.
      • Mannstadt M.
      • Isakova T.
      • et al.
      Fibroblast growth factor 23 and mortality among patients undergoing hemodialysis.
      R, reference.
      The fact that correcting for phosphate level, PTH, and vitamin D use did not mitigate predictive value of FGF-23 for dismal outcome is remarkable, because these parameters were thought to be in the same causal pathway as FGF-23. This could mean that FGF-23 is a sensitive marker for phosphate burden, or induces harm itself. In support for the first hypothesis, FGF-23 has, in large observational studies of elderly subjects with normal or only mildly impaired renal function, been associated with vascular dysfunction,
      • Mirza M.A.
      • Larsson A.
      • Lind L.
      • et al.
      Circulating fibroblast growth factor-23 is associated with vascular dysfunction in the community.
      total atherosclerotic burden,
      • Mirza M.A.
      • Hansen T.
      • Johansson L.
      • et al.
      Relationship between circulating FGF-23 and total body atherosclerosis in the community.
      and left ventricular hypertrophy.
      • Mirza M.A.
      • Larsson A.
      • Melhus H.
      • et al.
      Serum intact FGF-23 associate with left ventricular mass, hypertrophy and geometry in an elderly population.
      Further, FGF-23 is also a predictor of fracture risk in this population, another central feature of CKD–mineral and bone disorder (CKD–MBD).
      • Mirza M.A.
      • Karlsson M.K.
      • Mellström D.
      • et al.
      Serum fibroblast growth factor-23 (FGF-23) and fracture risk in elderly men.
      Arguments for the latter come from a recent study indicating that FGF-23 influences flow-mediated vasodilation in CKD stage 3 and 4 patients.
      • Yilmaz M.I.
      • Sonmez A.
      • Saglam M.
      • et al.
      FGF-23 and vascular dysfunction in patients with stage 3 and 4 chronic kidney disease.
      Additional arguments, albeit indirect, come from recent research indicating FGF-23 as a factor associated with left ventricular mass index independent from brain natriuretic peptide,
      • Negishi K.
      • Kobayashi M.
      • Ochiai I.
      • et al.
      Association between fibroblast growth factor 23 and left ventricular hypertrophy in maintenance hemodialysis patients. Comparison with B-type natriuretic peptide and cardiac troponin T.
      myocardial performance,
      • Kirkpantur A.
      • Balci M.
      • Gurbuz C.A.
      • et al.
      Serum fibroblast growth factor-23 (FGF-23) levels are independently associated with left ventricular mass and myocardial performance index in maintenance haemodialysis patients.
      and coronary artery disease.
      • Kanbay M.
      • Nicoleta M.
      • Selcoki Y.
      • et al.
      Fibroblast growth factor 23 and fetuin A are independent predictors for the coronary artery disease extent in mild chronic kidney disease.
      If FGF-23 indeed turns out to be either a sensitive biomarker of phosphate load, or has some different pathological effect in advanced stages of CKD, the next clinical question would be: Is it modifiable? From a theoretical point of view, options to lower FGF-23 would be to reduce levels of active vitamin D, PTH, and phosphate burden. Indeed, parathyroidectomy has been shown to induce a significant decline in FGF-23,
      • Sato T.
      • Tominaga Y.
      • Ueki T.
      • et al.
      Total parathyroidectomy reduces elevated circulating fibroblast growth factor 23 in advanced secondary hyperparathyroidism.
      possibly because of the direct effect of PTH on FGF-23 production.
      • Lavi-Moshayoff V.
      • Wasserman G.
      • Meir T.
      • et al.
      PTH increases FGF-23 gene expression and mediates the high-FGF-23 levels of experimental kidney failure: a bone parathyroid feedback loop.
      Lowering dietary phosphate intake decreases FGF-23 in healthy volunteers with a lag time,
      • Vervloet M.G.
      • van Ittersum F.J.
      • Büttler R.M.
      • et al.
      Effects of dietary phosphate and calcium intake on fibroblast growth factor-23.
      but this approach is not likely to be sufficient in CKD. The use of phosphate binders has shown inconsistent results as FGF-23-lowering agents. Sevelamer declined FGF-23 in a dose-dependent manner in an experimental model of uremia,
      • Nagano N.
      • Miyata S.
      • Abe M.
      • et al.
      Effect of manipulating serum phosphorus with phosphate binder on circulating PTH and FGF-23 in renal failure rats.
      and this was confirmed clinically after 6 weeks of treatment.
      • Oliveira R.B.
      • Cancela A.L.
      • Graciolli F.G.
      • et al.
      Early control of PTH and FGF-23 in normophosphatemic CKD patients: a new target in CKD–MBD therapy?.
      Use of calcium-based binder therapy did not change FGF-23 significantly in that study. These findings were confirmed in a cohort of 72 hemodialysis patients followed up for 1 year.
      • Cancela A.L.
      • Oliveira R.B.
      • Graciolli F.G.
      • et al.
      Fibroblast growth factor 23 in hemodialysis patients: effects of phosphate binder, calcitriol and calcium concentration in the dialysate.
      Use of lanthanum carbonate, however, while significantly reducing 24-h urine phosphate excretion, indicating adequate phosphate binding, did not change FGF-23.
      • Isakova T.
      • Gutierrez O.M.
      • Smith K.
      • et al.
      Pilot study of dietary phosphorus restriction and phosphorus binders to target fibroblast growth factor 23 in patients with chronic kidney disease.
      This absence of effect of lanthanum carbonate on FGF-23 in this study, however, could have been caused by a too short study period of two weeks only, since more time may be required for FGF-23 to slow down. Indeed, in a very recent study, extended use of lanthanum carbonate in an otherwise comparable study population did significantly suppress FGF-23 by almost 25%.
      • Gonzalez-Parra E.
      • Gonzalez-Casaus M.L.
      • Galán A.
      • et al.
      Lanthanum carbonate reduces FGF23 in chronic kidney disease stage 3 patients.
      Lowering levels of 1,25D in CKD as a way to decline FGF-23 is not an attractive option for obvious reasons.
      • Vervloet M.G.
      • Twisk J.W.
      Mortality reduction by vitamin D receptor activation in end-stage renal disease: a commentary on the robustness of current data.
      Currently, there is no evidence supporting the fact that FGF-23 is a modifiable risk factor that leads to improvement of clinical outcomes such as mortality. Prospective trials are underway that study changes in vascular function after FGF-23-targeted interventions. A critical question is whether FGF-23 is purely a biomarker of phosphate exposure or if it has ‘off-target’ effects directly contributing to vascular toxicity. If FGF-23 levels are reduced by means of improving phosphate control, it will be difficult to analyze whether changes in outcomes are related to a more stringent phosphate control or a consequence of a lower FGF-23 level per se. Another option that could shed light on this question would be to intervene at the level of FGF-23-producing cells, either osteocytes in bone or ectopic production from bone cells in the vessel wall in the presence of calcified arterial lesions,
      • Voigt M.
      • Fischer D.C.
      • Rimpau M.
      • et al.
      Fibroblast growth factor (FGF)-23 and fetuin-A in calcified carotid atheroma.
      by using FGF-23 blocking agents or upstream transcriptional inhibitors of FGF-23.
      From the convincing epidemiological association between FGF-23 and mortality in dialysis patients,
      • Gutierrez O.M.
      • Mannstadt M.
      • Isakova T.
      • et al.
      Fibroblast growth factor 23 and mortality among patients undergoing hemodialysis.
      an enigma emerges: How is it possible that a hormone such as FGF-23 has such an impact on outcome, while its main target organ—the kidney—is non-functioning? Of course, the parathyroid is another target for FGF-23; however, in CKD, PTH suppression by FGF-23 is abolished by parathyroid resistance.
      • Krajisnik T.
      • Olauson H.
      • Mirza M.A.
      • et al.
      Parathyroid Klotho and FGF-receptor 1 expression decline with renal function in hyperparathyroid patients with chronic kidney disease and kidney transplant recipients.
      • Canalejo R.
      • Canalejo A.
      • Martinez-Moreno J.M.
      • et al.
      FGF23 fails to inhibit uremic parathyroid glands.
      Although it is likely that all FGF-23 actions require the presence of the FGF receptor 1c and membrane Klotho,
      • Farrow E.G.
      • Davis S.I.
      • Summers L.J.
      • et al.
      Initial FGF-23-mediated signaling occurs in the distal convoluted tubule.
      theoretically, other tissues expressing Klotho and FGF receptor 1c might be unidentified targets for FGF-23. Recently, both Klotho and FGF receptor-1 were shown to be present in human aortic smooth muscle cell,
      • Lim K.
      • Lu T.S.
      • Zehnder D.
      • et al.
      Development of Klotho-FGFR1/3 dependent resistance to FGF-23 in human aortic smooth muscle cells exposed to calcifying stress.
      but at present it is unclear whether this is membrane-bound or soluble Klotho, and whether actual signal transduction by FGF-23 can occur at this site. Nevertheless, the arterial wall and cardiomyocytes as target tissues for FGF-23 action is an attractive concept, given the central role of cardiovascular disease in CKD-associated morbidity. Alternatively, indirect effects of FGF-23 on the arterial wall are also plausible. These could be mediated by phosphate itself or by a reduction in Klotho level, given that FGF-23 suppresses Klotho,
      • Krajisnik T.
      • Olauson H.
      • Mirza M.A.
      • et al.
      Parathyroid Klotho and FGF-receptor 1 expression decline with renal function in hyperparathyroid patients with chronic kidney disease and kidney transplant recipients.
      and recent data suggest that it is implicated in the process of arterial calcification.
      • Hu M.C.
      • Shi M.
      • Zhang J.
      • et al.
      Klotho deficiency causes vascular calcification in chronic kidney disease.
      In conclusion, data pointing to FGF-23 and Klotho being active factors in the burden of CKD are ever increasing. Both epidemiological data and pathophysiological mechanisms have set the stage for targeted intervention in the clinical setting. These new apparently important factors in CKD provide hope for improved future targeted therapy in CKD.

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