The Role of Klotho Protein in Chronic Kidney Disease

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The Role of Klotho Protein in Chronic Kidney Disease: Studies in Animals and Humans Edyta Gołembiewska1,*, Joanna Stępniewska1, Joanna Kabat-Koperska1, Karolina Kędzierska1, Maciej Domański1 and Kazimierz Ciechanowski1 Department of Nephrology, Transplantology and Internal Medicine, Pomeranian Medical University, Szczecin, Poland Please provide Abstract: The identification of Klotho gene was a major discovery as the gene encodes a protein regucorresponding author(s) photograph lating multiple functions. A defect in Klotho gene expression in mice results in a phenotype of premasize should be 4" x 4" inches ture aging including shortened life span, growth retardation, hypogonadism, skin and muscle atrophies, vascular calcification, cognition impairment, motor neuron degeneration and others. This phenotype is associated with phosphate balance disorders and underlines the major function of Klotho in mineral metabolism. As another 2 related paralogs were discovered (beta-Klotho, which is involved in bile acid and energy metabolism, and gamma-Klotho, with a yet to be defined function), this led to the revised naming of Klotho as alpha-Klotho. Two forms of alpha-Klotho protein have been reported: a membrane-bound and a soluble one. Membrane Klotho forms a complex with fibroblast growth factor (FGF) receptors and functions as an obligate co-receptor for the FGF-23 phosphatonin in distal tubules. The soluble form of Klotho seems to function as a humoral factor and regulates glycoproteins on the cell surface including ion channels and growth factors. There is data suggesting that soluble Klotho exerts phosphaturic effects independently of FGF-23. Circulating soluble Klotho is produced either by proteolytic cleavage of the extracellular domain of the transmembrane form by two membrane-anchored proteases (ADAM10 and ADAM17) or by alternative mRNA splicing. In animal models Klotho has been shown to exert pleiotropic actions, including cytoprotection, anti-oxidation, anti-apoptosis, protection of vasculature, promotion of angiogenesis and vascularization, inhibition of fibrogenesis and preservation of stem cells. The exact diagnostic and therapeutic role of Klotho in humans is not fully known yet. The article presents the role of Klotho in physiology and different stages of chronic kidney disease (CKD).

Keywords: Cardiovascular disease, chronic kidney disease, dialysis, FGF-23, Klotho protein, kidney transplantation. Received: ????????????????

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INTRODUCTION The identification of Klotho gene was a major discovery as the gene encodes a protein regulating multiple functions [1]. A defect in Klotho gene expression in mice results in a phenotype of premature aging including shortened life span, growth retardation, hypogonadism, skin and muscle atrophies, vascular calcification, cognition impairment, motor neuron degeneration and others. This phenotype is associated with phosphate balance disorders and underlines the major function of Klotho in mineral metabolism. The discovery of β-Klotho, involved in bile acid and energy metabolism [2], and γ-Klotho, have led to the revision naming of Klotho as α-Klotho. For the purpose of this review, the old nomenclature of Klotho will be used. The highest expression of Klotho is in kidney, predominantly in renal tubular cells, but it is also expressed in brain, parathyroid gland and heart [3, 4]. *Address correspondence to this author at the Dept. of Nephrology, Transplantology and Internal Medicine, Pomeranian Medical University, Szczecin, Poland, Powstancow Wielkopolskich 72, 70-111 Szczecin, Poland; Tel:/Fax : +48914661196; E-mail: [email protected] 1389-2037/16 $58.00+.00

Accepted: ????????????????

Klotho protein in mammals is present in 2 different isoforms: a membrane-bound protein and as a soluble form [4]. Full-length Klotho is a single-pass transmembrane protein that functions as an obligatory co-receptor for fibroblast growth factor-23 (FGF-23) which promotes phosphate excretion and inhibition of calcitriol synthesis. Extracellular domain is cleaved by protease ADAM10/17 (an acronym for A Disintegrin And Metalloproteinase) [5] and released into blood, urine and cerebrospinal fluid. Another way of generating soluble Klotho is an alternative mRNA splicing. More and more is known about this process, and recently, Chen et al. made another step towards identification of cleavage sites leading to the shed form of Klotho [6]. The soluble ectodomain of Klotho possesses glycosidase activity that cuts sugar chains on ion channels and transporters on the cell surface, regulating their activity. KLOTHO IN THE CHRONIC KIDNEY DISEASEMINERAL BONE DISORDER (CKD-MBD) Traditionally, the disorders in bone and mineral homeostasis, as a consequence of declining renal function, had been described as renal osteodystrophy. This definition referred mainly to altered bone morphology and remodeling and was © 2016 Bentham Science Publishers

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based on the results of bone biopsies in patients with chronic kidney disease (CKD). However, it did not capture the full spectrum of changes and in 2006, the Kidney Disease: Improving Global Outcomes group (KDIGO) established the name CKD-MBD to comprise the syndrome manifestating with abnormalities in circulating biomarkers, bone disease and extraskeletal calcification [7]. Apart from standard measurements, such as phosphatemia, parathyroid hormone (PTH) or vitamin D levels, new markers appeared on the scene, like fibroblast growth factor-23 (FGF-23), sclerostin, osteocalcin and activin-A [8, 9]. Klotho is also one of the novel components in this complex systems biology between the kidney, skeleton and cardiovascular system. In chronic kidney disease, the expression of membrane –bound Klotho, an obligatory co-receptor for FGF-23 signaling, is reduced and this limits FGF-23-stimulated signal transduction. This disrupts the negative feedback to FGF-23 secretion and the production of FGF-23 in osteocytes continues. It has been demonstrated that FGF-23 levels rise before measurable changes in calcium, phosphorus, or PTH levels and may serve as an early biomarker of CKD-MBD [8, 10]. With Klotho deficiency hyperphosphatemia remains the principal regulator of FGF-23 secretion. In the late stage of CKD abundant FGF-23 concentrations lead to activation of FGF receptors in a Klotho independent way and promote such pathologies as cardiac myocyte hypertrophy [11]. On the other hand, studies in mice showed that soluble Klotho protected against uremic cardiomyopathy, independently of FGF-23 and phosphate [12]. The association between chronic kidney disease and cardiovascular risk in this complex system has been emphasized. Patients with CKD and End Stage Renal Disease (ESRD) suffer from cardiovascular events which are a major cause of their death. This higher risk of cardiovascular episodes is associated with calcium phosphate balance disorders, as well as with vascular stiffness and calcification. In recent years, there have been more and more data that FGF23/Klotho axis is an important player in the field and constitutes another link between CKD and cardiovascular disease (CVD) [13-16]. PLEIOTROPIC ACTIONS OF SOLUBLE KLOTHO Soluble Klotho has been shown to exert pleiotropic actions, independent of FGF-23 [4, 17]. First, soluble Klotho modulates calcium channels (TRPV5, TRPV6), Na-phosphate co-transporters (NaPi- 2a, Pit-1, Pit-2) and renal outer medullary potassium channel ROMK1. The transient receptor potential vanilloid 5 (TRPV5) ion channel is a calcium channel expressed in the epithelial cells of distal convoluted tubules. It is a gate for transcellular calcium reabsorption. Klotho up-regulates TRPV5 from both the inside and outside of cells. The intracellular action of Klotho is likely due to enhanced forward trafficking of channel proteins, whereas the extracellular action is due to inhibition of endocytosis. Both effects involve putative Klotho sialidase activity and may play important roles regarding calcium reabsorption in the kidney [18, 19]. The direct suppression of NaPi-2a by soluble Klotho protein in a FGF-23-independent fashion is mediated by direct

Gołembiewska et al.

inhibition of NaPi cotransport activity without change in protein abundance on cell surface. The suggested model of direct effect of Klotho on NaPi-2a protein is as follows: glucuronate on a yet unknown substrate is removed by Klotho leading to suppression of NaPi cotransport activity which subsequently renders NaPi-2a protein more susceptible to proteases. Protease inhibitors do not reverse the Klothoinduced inhibition of transport, supporting that Klothoinduced deglycosylation is sufficient to suppress NaPi cotransport activity [20, 21]. ROMK1 is one of the major mediators of urinary potassium reabsorption and the proposed mechanism of Klotho action is similar to that of TRPV5. Klotho removes terminal sialic acids from N-glycan of ROMK1, and exposes underlying disaccharide galactose-N-acetylglucosamine, a ligand for a ubiquitous galectin-1. Binding to galectin-1 at cell surface prevents clathrin-mediated endocytosis and causes accumulation of functional ROMK1 on the plasma membrane [22]. Second, soluble Klotho is involved in regulation of the signaling of the cytokines and growth factors. Klotho protein can inhibit insulin/insulin-like growth factor (IGF1) signaling and in this way promote resistance to oxidative stress. Mammalian forkhead box O (FOXO) transcription factors, FOXO1, FOXO3a, and FOXO4, are negatively regulated by insulin/IGF-1 signaling. Activation of insulin/IGF-1 signaling leads to phosphorylation and activation of a serine-threonine kinase Akt, which in turn phosphorylates FOXOs which are inactivated. If this process is interrupted, the nuclear FOXOs then directly bind to the promoters of antioxidant enzymes including catalase and mitochondrial manganese-superoxide dismutase (SOD2), and up-regulate their expression, thereby facilitating removal of reactive oxygen species (ROS) and conferring resistance to oxidative stress [23]. Soluble Klotho protein directly binds to the type-II transforming growth factor-beta (TGF-β) receptor and inhibits TGF-β1 binding to cell surface receptors, that inhibits TGFβ1 signaling. In cultured cells, Klotho suppresses TGF-β1induced epithelial-to-mesenchymal transition (EMT) responses, including decreased epithelial marker expression, increased mesenchymal marker expression, and/or increased cell migration. In addition to TGF-β1 signaling, soluble Klotho has been shown to inhibit Wnt signal pathway that can promote EMT. Klotho blocked Wnt-triggered activation and nuclear translocation of β-catenin, as well as the expression of its target genes in tubular epithelial cells [24, 25]. Apart from obvious influence of calcium-phosphate metabolism, many of these proteins and signaling pathways play important roles in vascular function. In this context, it would be essential to know how the declining kidney function corresponds to lower Klotho expression and patients’ outcome. KLOTHO IN CHRONIC KIDNEY DISEASE: FROM CONSERVATIVE TREATMENT TO RENAL REPLACEMENT THERAPY. Chronic Kidney Disease In an animal model of chronic kidney disease, in wildtype mice, Hu et al. [26] showed very low renal, plasma and urinary levels of Klotho. Also, severe calcification in the

Klotho Protein in CKD

mice’s vessels was observed. On the other hand, in transgenic mice that overexpressed Klotho, despite induction of CKD, levels of Klotho were preserved with better renal function and there was much less calcification compared to wildtype mice with CKD. The beneficial effect of Klotho was not only a result of better kidney function, but the authors suggested a direct effect of Klotho on vasculature. Klotho suppressed sodium-dependent uptake of phosphate and preserved differentiation in vascular smooth muscle cells. The authors also proved that in vitro treatment with recombinant soluble Klotho protein directly inhibited high phosphate induced calcification. The levels of Klotho expression and its relation to serum levels of FGF-23 and soluble Klotho were examined in the population of CKD stage 1-5 (with the subgroup of CKD stage 5 subjects on regular hemodialysis therapy) patients who underwent renal biopsy [27]. Both renal Klotho and Klotho mRNA levels assessed using PCR, declined significantly with the progression of renal dysfunction in CKD. Renal Klotho levels were significantly and positively correlated only with estimated glomerular filtration rate (eGFR). They were also correlated with parameters of mineral metabolism (serum calcium, phosphate, 1,25 – dihydroxyvitamin D3 and intact parathyroid hormone). Serum FGF-23 levels were not correlated with renal Klotho levels at stages 1-3, but there was a significant inverse correlation at stages 4 and 5. However, in multiple regression FGF-23 levels correlated significantly only with eGFR and serum phosphate, but not with renal Klotho. Thus, the authors suggested that loss of renal Klotho is not a primary factor enhancing FGF-23 secretion, especially in early CKD stages. The authors also found that the declining kidney function and levels of renal Klotho were associated with the decrease in soluble Klotho levels. Serum soluble Klotho levels significantly correlated with renal Klotho. Also, patients who underwent regular maintenance hemodialysis expressed very little renal Klotho and their serum soluble Klotho levels were significantly lower than in patients with CKD stage 5 but still on conservative treatment (pre-dialysis patients). Obtained results suggested that soluble Klotho level could be a useful biomarker of renal Klotho expression. In Shimamura’s study [28] with 292 patients with CKD enrolled, the authors found that serum soluble Klotho level significantly decreased in CKD stage 2 patients compared to the CKD 1 group. This way Klotho can serve as a sensitive biomarker of early CKD diagnosis, reflecting decreased expression of membrane Klotho. Levels of serum soluble Klotho and 24-hour urinary excretion Klotho were measured in 131 ambulatory CKD patients in Akimoto’s study [29]. Significant correlation was found between eGFR and the amount of 24-hour urinary excreted Klotho as well as between eGFR and the serum Klotho levels. However, the results showed that eGFR was independently associated only with the log-transformed value of the amount of 24-hour urinary excreted Klotho what led the authors to the conclusion that the amount of urinary Klotho, rather than the serum Klotho levels, should be linked to the magnitude of the functioning nephrons in CKD patients.

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Dialysis Similarly to the study performed in CKD patients on conservative treatment [29], Akimoto examined soluble Klotho levels in the serum, urine and peritoneal dialysate obtained from peritoneal dialysis (PD) patients [30]. Like in CKD patients, urinary Klotho excretion was significantly correlated with residual renal function. However, there was no apparent correlation between the serum soluble Klotho and eGFR. As to the peritoneal dialysate, the amount of soluble Klotho was correlated with the amount of albumin in dialysate collection. The amount of urinary excreted Klotho was significantly associated with the 24-hour urine volume. The impact of 24-hour urine volume in the context of Klotho metabolism has been confirmed in Golembiewska’s et al. study which included 35 incident PD patients [31]. The results showed that serum soluble alpha-Klotho was strongly negatively correlated with 24-hour diuresis. Soluble Klotho level was also negatively correlated with renal phosphate clearance, but not with eGFR. As soluble Klotho is derived by shedding of the extracellular portion of Klotho from distal tubules, its lower serum level might be associated with a higher cleavage of Klotho into urine in patients with higher residual diuresis. Hu et al. [20] described a direct, FGF-23 independent, phosphaturic effect of Klotho in mice. In this mode of soluble Klotho action, Klotho protein has to gain access to the lumen of the proximal tubule and then regulates phosphate excretion via the sodium-phosphate transporter NaPi-2a. It seems that Klotho is too large to get to the lumen of the proximal tubule through glomerular filtration. The appearance of Klotho in the proximal lumen could be due to the cleavage and then transcytosis of plasma- or distal tubulederived Klotho. The phosphaturic action of soluble Klotho in the proximal tubule could be the reason of the correlation between renal phosphate clearance and serum Klotho level observed in the study. Results in Buiten’s study [32] performed in 127 patients undergoing regular maintenance hemodialysis or peritoneal dialysis, showed that there was a significant correlation between the mode of dialysis and soluble Klotho levels. Patients performing peritoneal dialysis had higher levels of soluble Klotho. This finding was quite unexpected, since soluble Klotho is excreted in the peritoneal dialysate and, on the other hand, its molecular weight is too high (approximately 130 kDa) to allow clearance by hemodialysis. It has to be underlined, however, that PD patients had higher residual renal function what might reflect healthier kidney. As Klotho constitutes the link between CKD and cardiovascular disease, in this study, plasma levels of soluble Klotho were related to such parameters as left ventricular mass index (LVMI), left ventricular ejection fraction (LVEF) and coronary artery calcium score assessed using computed tomography angiography. After dividing the patients into 2 groups with high (> 460 pg/mL) and low serum Klotho level, patients with high soluble Klotho level had significantly less coronary artery disease (CAD) diagnosis and less LV dysfunction. However, there was no independent association between soluble Klotho and CVD. One of possible explanations was the fact that patients with ESRD had been exposed


to many factors predisposing to vascular calcifications for many years. In that study there was one measurement of soluble Klotho at the time of enrollment and this might therefore not reflect the total ‘burden’ of Klotho deficiency. Nevertheless, it is possible that soluble Klotho has no independent contribution to CVD in dialysis patients. Other studies [33, 34] showed that in patients with CKD stages 2-4, soluble Klotho was not associated with abdominal aortic calcification score (AAC score) nor was it predictive for cardiovascular outcomes. In Kitagawa’s study [35], decreases in the serum soluble Klotho levels were independently associated with signs of vascular dysfunction such as arterial stiffness, but not endothelial dysfunction, atherosclerosis or vascular calcification. In prospective study performed in a large group of hemodialysed patients, Nowak et al. [36] found that FGF-23 but not Klotho levels were associated with mortality. However, both low Klotho and high FGF-23 levels were associated with atrial fibrillation (AF). One of the possible explanations was that AF was associated with atherosclerotic changes due to Klotho deficiency. Still, another possible hypothesis is that Klotho might be essential for the function of ion channels which are responsible for the peacemaking activity in the sinoatrial node, and thus, Klotho may be protective against AF. Kidney Transplantation There is paucity of data on the role of Klotho protein in kidney transplant recipients, and still the results of studies are conflicting. In the study of Wan et al. [37] they examined the levels of soluble Klotho, FGF-23 and other parameters of calcium-phosphate metabolism in the group of 44 children who received a kidney transplantation. Plasma Klotho levels in this group were higher than in CKD group (CKD1-5 and dialysis group). There was no correlation between FGF-23 and soluble Klotho level. Also, no association was found between soluble Klotho and eGFR, even in the subgroup of transplanted patients with eGFR below 60 mL/min. Another study in children by Sawires et al. [38] examined relationships between FGF-23, serum Klotho and other variables regulating phosphate metabolism in the group of CKD (stages 2-5) patients on conservative treatment, CKD patients undergoing regular hemodialysis and kidney transplant recipients. In children with functioning graft, the mean duration after transplantation was 2.28 ± 0.37 years. The levels of Klotho in transplanted subjects were higher than the levels in hemodialysed patients, but still lower than in healthy volunteers. Similarly to Wan et al. study [37], in patients with kidney allograft, no correlation was found between soluble Klotho and FGF-23 levels. On the other hand, in these patients the authors found an inverse significant correlation between serum calcium and Klotho level. The study showed that age, calcium, phosphate, FGF-23 and 1,25dihydroxyvitamin D3 were independent predictors of Klotho. An interesting study, though on the limited number of subjects, was conducted by Akimoto et al. [39]. The levels of soluble Klotho were measured both in 10 living donors and their renal transplant recipients before and several days after retroperitoneoscopic nephrectomy. The levels of soluble


serum Klotho significantly decreased at 2 days and 5 days after nephrectomy. On the other hand, serum Klotho levels in renal transplant recipients did not significantly change. However, in these subjects, higher Klotho levels were associated with lower serum creatinine levels. It could have been supposed that serum Klotho levels should increase after renal transplantation. However, this short-term observation included the time of acute kidney injury related to transplantation, which has been regarded as a state of acute reversible Klotho deficiency due to reduced tubular Klotho expression as a result of hemodynamic alterations. Another important issue refers to the use of immunosuppressive agents in different regimens that might modulate the expression of Klotho within graft and, in consequence, its release into the circulation. Recently, the study was performed in stable kidney recipients on triple immunosuppressive therapy, in which the authors assessed the levels of FGF-23 and Klotho in relation to comorbidities and markers of endothelial dysfunction [40]. In this study, Klotho levels were significantly higher in patients with eGFR > 60 mL/min compared to subjects with eGFR below 60 mL/min. Klotho significantly correlated with the N-terminal of the prohormone brain natriuretic peptide (NT-proBNP), von Willebrandt factor, calcium and age. The relation of FGF-23-Klotho system to markers of endothelial injury may play an important role in the pathogenesis of cardiovascular complications in these patients. On the other hand, interesting is the data from the study by Przybylowski et al. [41]. Authors evaluated FGF-23 and Klotho concentrations in 84 stable heart transplant recipients in relation to markers of renal function and markers of heart failure. Kidneys in these patients are especially exposed to the development of CKD due to side effects of the immunosuppressive regimen applied to this group. Special attention should be paid to the use of calcineurin inhibitors (cyclosporine, tacrolimus) as these drugs contribute to chronic renal dysfunction. Chronic cyclosporine nephropathy involves enhanced angiotensin II, transforming growth factorbeta, and vascular endothelial growth factor expression together with downregulation of nitric oxide synthesis. In Przybylowski’s et al. [41] study serum FGF-23 concentration correlated with kidney function parameters (eGFR, cystatin C, neutrophil gelatinase-associated lipocalin - NGAL) and increased immensely as kidney function declined. Klotho protein correlated with eGFR and was significantly lower in the group of patients with eGFR below 60 mL/min. In the study tacrolimus or cyclosporine was used in 99% of patients. FGF-23 correlated significantly with markers of heart failure, such as N-terminal pro-B-type natriuretic peptide or intraventricular septum thickness. The authors conclude that FGF-23-Klotho system disorders in heart transplant recipients are related to cardiovascular system function and chronic kidney disease. This can contribute to increased risk of cardiovascular diseases. Some studies evaluated the expression of Klotho and FGF receptor 1 in parathyroid glands in kidney transplant recipients [42, 43]. FGF-23 acts directly to decrease PTH mRNA and its release. In patients with ESRD, parathyroid resistance to FGF-23 might be caused by decreased expression of Klotho-FGF receptor 1 complex in parathyroid gland.

Klotho Protein in CKD

Krajisnik et al. [42] found that in hyperplastic parathyroid glands from kidney transplant recipients, the expression of this complex was significantly reduced. Lower expression of this complex correlated significantly with the decline in eGFR. Similar results were obtained by Hong et al. [43]. They also found that the expression of Klotho and FGFR1 in ESRD patients and in transplanted patients was significantly reduced compared to healthy subjects. Interestingly, there was no significant difference in Klotho expression between the group of transplanted patients and ESRD group. The authors postulated that incomplete recovery of Klotho levels might contribute to tertiary hyperparathyroidism after kidney transplantation.

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[12]

[13] [14]

CONCLUSION Since the introduction of Klotho protein much data has already been known. Experimental studies showed that Klotho might alleviate kidney injury, slow down the progression of chronic kidney disease and ameliorate vascular calcification. Clinical data are still scarce and further studies are needed to confirm the significance of Klotho in clinical nephrology. CONFLICT OF INTEREST The author(s) confirm that this article content has no conflict of interest. ACKNOWLEDGEMENTS

[15]

[16]

[17] [18]

[19]

Declared none. REFERENCES [1]

[2] [3] [4] [5] [6]

[7]

[8] [9] [10]

Kuro-o, M.; Matsumura, Y.; Aizawa, H.; Kawaguchi, H.; Suga, T.; Utsugi, T.; Ohyama, Y.; Kurabayashi, M.; Kaname, T.; Kume, E.; Iwasaki, H.; Iida, A.; Shiraki-Iida, T.; Nishikawa, S.; Nagai, R.; Nabeshima, Y.I. Mutation of the mouse klotho gene leads to a syndrome resembling ageing. Nature, 1997, 390(6655), 45-51. Moschetta, A.; Kliewer, S.A. Weaving betaKlotho into bile acid metabolism. J. Clin. Invest., 2005, 115(8), 2075-2077. Hu, M.C.; Kuro-o, M.; Moe, O.W. Renal and extra-renal actions of Klotho. Semin. Nephrol., 2013, 33(2), 118-129. Lewin, E.; Olgaard, K. The vascular secret of Klotho. Kidney Int., 2015, 87, 1089-1091. Hu, M.C.; Kuro-o, M.; Moe, O.W. The emerging role of Klotho in clinical nephrology. Nephrol. Dial. Transplant., 2012, 27(7), 26502657. Chen, C.D.; Tung, T.Y.; Liang, J.; Zeldich, E.; Tucker Zhou, T.B.; Turk, B.E.; Abraham, C.R. Identification of cleavage sites leading to the shed form of the anti-aging protein klotho. Biochemistry, 2014, 53(34), 5579-5587. Moe, S.; Drueke, T.; Cunnihgham, J.; Goodman, W.; Martin, K.; Olgaard, K.; Ott, S.; Sprague, S.; Lameire, N.; Eknoyan, G. Definition, evaluation, and classification of renal osteodystrophy: A position statement from Kidney Disease: Improving Global Outcomes (KDIGO). Kidney Int., 2006, 69, 1945-1953. Nitta, K.; Nagano, N.; Tsuchiya, K. Fibroblast Growth Factor 23/Klotho Axis In Chronic Kidney Disease. Nephron Clin. Pract., 2014, 128, 1-10. Evenepoel, P.; D’Haese, P.; Brandenburg, V. Sclerostin and DKK1: new players in renal bone and vascular disease. Kidney Int., 2015, 88, 235-240. Nakano, C.; Hamano, T.; Fujii, N.; Matsui, I.; Tomida, K.;Mikami, S.; Inoue, K.; Obi, Y.; Okada, N.; Tsubahikara, Y.; Isaka, Y.; Rakuqi, H. Combined use of vitamin D status and FGF23 for risk stratification of renal outcome. Clin. J. Am. Soc. Nephrol., 2012, 7, 810-819.

[20]

[21]

[22] [23]

[24]

[25] [26] [27]

[28]

Faul, C.; Amaral, A.P.; Oskouei, B.; Hu, M.C.; Sloan, A.; Isakova, T.; Gutierrez, O.M.; Aguillon-Prada, R.; Lincoln, J.; Hare, J.M.; Mundel, P.; Morales, A.; Scialla, J.; Fischer, M.; Soliman, E.Z.; Chen, J.; Go, A.S.; Rosas, S.E.; Nessel, L.; Townsend, R.R.; Feldman, H.I.; St John Sutton, M.; Ojo, A.; Gadegbeku, C.; Di Marco, G.S.; Reuter, S.; Kentrup, D.; Tiemann, K.; Brand, M.; Hill, J.A.; Moe, O.W.; Kuro-O, M.; Kusek, J.W.; Keane, M.G.; Wolf, M. FGF23 induces left ventricular hypertrophy. J. Clin. Invest., 2011, 121(11), 4393-4408. Xie, J.; Yoon, J.; An, S.W.; Kuro-O, M.; Huang, C.L. Soluble Klotho Protects against Uremic Cardiomyopathy Independently of Fibroblast Growth Factor 23 and Phosphate. J. Am. Soc. Nephrol., 2015, 26(5), 1150-1160. Heine, G.H.; Seiler, S.; Fliser, D. FGF-23: the rise of a novel cardiovascular risk marker in CKD. Nephrol. Dial. Transplant., 2012, 27(8), 3072-3081. Asicioglu, E.; Kahveci, A.; Arikan, H.; Koc, M.; Tuglular, S.; Ozener, C.I. Fibroblast Growth Factor-23 Levels Are Associated with Vascular Calcifications in Peritoneal Dialysis Patients. Nephron Clin. Pract., 2013, 124, 89-93 . Sany, D.; Elsawy, A.E.; Aziz, A.; Elshahawy, Y.; Ahmed, H.; Aref, H.; El Rahman, M.A. The value of serum FGF-23 as a cardiovascular marker in HD patients. Saudi J. Kidney Dis. Transpl., 2014, 25, 44-52. Chen, Z.; Chen, X.; Xie, J.; Ma, X.; Zhong, F.; Hou, L.; Ling, H.; Li, X.; Ren, H.; Chen, N. Fibroblast Growth Factor 23 is a Predictor of Aortic Artery Calcification in Maintenance Hemodialysis Patients. Renal Failure, 2013, 35(5), 660-666. Wang, Y.; Sun, Z. Current understanding of Klotho. Ageing Res. Rev., 2009, 8(1), 43-51. Wolf, M.T.; An, S.W.; Nie, M.; Bal, M.S.; Huang, C.L. Klotho upregulates renal calcium channel transient receptor potential vanilloid 5 (TRPV5) by intra- and extracellular N-glycosylationdependent mechanisms. J. Biol. Chem., 2014, 289(52), 3584935857. Leunissen, E.H.; Nair, A.V.; Büll, C.; Lefeber, D.J.; van Delft, F.L.; Bindels, R.J.; Hoenderop, J.G. The epithelial calcium channel TRPV5 is regulated differentially by klotho and sialidase. J. Biol. Chem., 2013, 288(41), 29238-29246. Hu, M.C.; Shi, M.; Zhang, J.M.; Pastor, J.; Nakatani, T.; Lanske, B.; Razzaque, M.S.; Rosenblatt, K.P.; Baum, M.G.; Kuro-o, M.; Moe, O.W. Klotho: a novel phosphaturic substance acting as an autocrine enzyme in the renal proximal tubule. FASEB J., 2010, 24, 3438-3450. Dermaku-Sopjani, M.; Sopjani, M.; Saxena, A.; Shojaiefard, M.; Bogatikov, E.; Alesutan, I.; Eichenmüller, M.; Lang, F. Downregulation of NaPi-IIa and NaPi-IIb Na-coupled phosphate transporters by coexpression of Klotho. Cell Physiol. Biochem., 2011, 28(2), 251-258. Cha, S.K.; Hu, M.C.; Kurosu, H.; Kuro-o, M.; Moe, O.; Huang, C.L. Regulation of renal outer medullary potassium channel and renal K(+) excretion by Klotho. Mol. Pharmacol., 2009, 76, 38-46. Kops, G.J.; Dansen, T.B.; Polderman, P.E.; Saarloos, I.; Wirtz, K.W.; Coffer, P.J.; Huang, T.T.; Bos, J.L.; Medema, R.H.; Burgering, B.M. Forkhead transcription factor FOXO3a protects quiescent cells from oxidative stress. Nature, 2002, 419(6904), 316-321. Doi, S.; Zou, Y.; Togao, O.; Pastor, J.V.; John, G.B.; Wang, L.; Shiizaki, K.; Gotschall, R.; Schiavi, S.; Yorioka, N.; Takahashi, M.; Boothman, D.A.; Kuro-o, M. Klotho inhibits transforming growth factor-beta1 (TGF-beta1) signaling and suppresses renal fibrosis and cancer metastasis in mice. J. Biol. Chem., 2011, 286(10), 8655-8665. Zhou, L.; Li, Y.; Zhou, D.; Tan, R.J.; Liu, Y. Loss of Klotho Contributes to Kidney Injury by Derepression of Wnt/β-Catenin Signaling. JASN, 2013, 24(5), 771-785. Hu, M.C.; Shi, M.; Zhang, J.; Quinones, H.; Griffith, C.; Kuro-o, M.; Moe, O.W. Klotho Deficiency Causes Vascular Calcification in Chronic Kidney Disease. J. Am. Soc. Nephrol., 2011, 22, 124-136. Sakan, H.; Nakatani, K.; Asai, O.; Imura, A.; Tanaka, T.; Yoshimoto, S.; Iwamoto, N.; Kurumatani, N.; Iwano, M.; Nabeshima, Y.; Konishi, N.; Saito, Y. Reduced renal α-Klotho expression in CKD patients and its effect on renal phosphate handling and vitamin D metabolism. PLoS One, 2014, 9(1), e86301. Shimamura, Y.; Hamada, K.; Inoue, K.; Ogata, K.; Ishihara, M.; Kagawa, T.; Inoue, M.; Fujimoto, S.; Ikebe, M.; Yuasa, K.; Yamanaka, S.; Sugiura, T.; Terada, Y. Serum levels of soluble secreted


[29]

[30]

[31]

[32]

[33]

[34] [35] [36] [37] [38]

alpha-Klotho are decreased in the early stages of chronic kidney disease, making it a probable novel biomarker for early diagnosis. Clin. Exp. Nephrol., 2012, 16, 722-729. Akimoto, T.; Yoshizawa, H.; Watanabe, Y.; Numata, A.; Yamazaki, T.; Takeshima, E.; Iwazu, K.; Komada, T.; Otani, N.;, Morishita, Y.; Ito, C.; Shiizaki, K.; Ando, Y.; Muto, S.; Kuro-o, M.; Kusano, E.: Characteristics of urinary and serum soluble Klotho protein in patients with different degrees of chronic kidney disease. BMC Nephrol., 2012, 13, 155. Akimoto, T.; Shiizaki, K.; Sugase, T.; Watanabe, Y.; Yoshizawa, H.; Otani, N.; Numata, A.; Takeshima, E.; Yamazaki, T.; Miki, T.; Ito, C.; Pastor, J.V.; Iwazu, Y.; Saito, O.; Muto, S.; Kuro-o, M.; Kusano, E. The relationship between the soluble Klotho protein and the residual renal function among peritoneal dialysis patients. Clin. Exp. Nephrol., 2012, 16, 442-447. Golembiewska, E.; Safranow, K.; Kabat-Koperska, J.; Myslak, M.; Ciechanowski, K. Serum soluble Klotho protein level is associated with residual diuresis in incident peritoneal dialysis patients. Acta Biochimica Polonica, 2013, 60, 191-194. Buiten, M.S.; de Bie, M.K.; Bouma-de Krijger, A.; van Dam, B.; Dekker, F.W.; Jukema, J.W.; Rabelink, T.J.;Rotmans, J.I. Soluble Klotho is not independently associated with cardiovascular disease in a population of dialysis patients. BMC Nephrol., 2014, 15, 197. Seiler, S.; Rogacev, K.S.; Roth, H.J.; Shafein, P.; Emrich, I.; Nauhaus, S.; Floege, J.; Fliser, D.; Heine, G.H. Associations of FGF-23 and sKlotho with cardiovascular outcomes among patients with CKD stages 2-4. Clin. J. Am. Soc. Nephrol., 2014, 9(6), 10491058. Craver, L.; Dusso, A.; Martinez-Alonso, M.; Sarro, F.; Valdivielso, J.M.; Fernandez, E. A low fractional excretion of Phosphate/Fgf23 ratio is associated with severe abdominal Aortic calcification in stage 3 and 4 kidney disease patients. BMC Nephrol., 2013, 14, 221. Kitagawa, M.; Sugiyama, H.; Morinaga, H.; Inoue, T.; Takiue, K.; Ogawa, A.; Yamanari, T.; Kikumoto, Y.; Uchida, H.A.; Kitamura,


[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

S.; Maeshima, Y.; Nakamura, K.; Ito, H.; Makino, H. A decreased level of serum soluble Klotho is an independent biomarker associated with arterial stiffness in patients with chronic kidney disease. PLoS One, 2013, 8(2), e56695. Nowak, A.; Friedrich, B.; Artunc, F.; Serra, A.L.; Breidthardt, T.; Twerenbold, R.; Peter, M.; Mueller, C. Prognostic Value and Link to Atrial Fibrillation of Soluble Klotho and FGF23 in Hemodialysis Patients. PLos One, 2014, 9(7), e100688. Wan, M.; Smith, C.; Shah, V.; Gullet, A.; Wells, D.; Rees, L.; Shroff, R. Fibroblast growth factor 23 and soluble klotho in children with chronic kidney disease. Nephrol. Dial. Transplant., 2013, 28, 153-161. Sawires, H.K.; Essam, R.M.; Morgan, M.F.; Mahmoud, R.A. Serum Klotho: Relation to Fibroblast Growth Factor-23 and Other Regulators of Phosphate Metabolism in Children with Chronic Kidney Disease. Nephron., 2015, 129, 293-299. Akimoto, T.; Kimura, T.; Watanabe, N.; Ishikawa, N.; Iwazu, Y.; Saito, O.; Muto, S.; Yagisawa, T.; Kusano, E. The impact of Nephrectomy and Renal Transplantation on Serum Levels of Soluble Klotho Protein. Transplantat. Proc., 2013, 45, 134-136. Malyszko, J.; Koc-Zorawska, E.; Matuszkiewicz-Rowinska, J.; Malyszko, J. FGF23 and Klotho in relation to markers of endothelial dysfunction in kidney transplant recipients. Transplant. Proc., 2014, 46(8), 2647-2650. Przybylowski, P.; Wasilewski, G.; Janik, L.; Kozłowska, S.; Nowak, E.; Malyszko, J. Fibroblast Growth factor 23 and Klotho as Cardiovascular Risk Factors in Heart Transplant Recipients. Transplant. Proc., 2014, 46, 2848-2851. Krajisnik, T.; Olauson, H.; Mirza, M.A.I.; Hellman, P.; Akerstrom, G.; Westin, G.; Larsson, T.E.; Bjorklund, P. Parathyroid Klotho and FGF-receptor 1 expression decline with renal function in hyperparathyroid patients with chronic kidney disease and kidney transplant recipients. Kidney Int., 2010, 78, 1024-1032. Hong, Y.A.; Choi, D.E.; Lim, S.W., Yang, C.W.; Chang, Y.K. Decreased parathyroid Klotho expression is associated with persistent hyperparathyroidism after kidney transplantation. Transplant. Proc., 2013, 45(8), 2957-2962.