Depressed expression of Klotho and FGF receptor 1 in hyperplastic parathyroid glands from uremic patients. Kidney Int. 2010;77:232-238. [PMID: 19890272 DOI: 10.1038/ki.2009.414]

Cinacalcet HCl prevents development of parathyroid gland hyperplasia and reverses established parathyroid gland hyperplasia in a rodent model of CKD

BACKGROUND

Secondary hyperparathyroidism (sHPT) represents an adaptive response to progressively impaired control of calcium, phosphorus and vitamin D in chronic kidney disease (CKD). It is characterized by parathyroid hyperplasia and excessive synthesis and secretion of parathyroid hormone (PTH). Parathyroid hyperplasia in uremic rats can be prevented by calcium-sensing receptor (CaSR) activation with the calcimimetic cinacalcet (Sensipar®/Mimpara®); however, it is unknown, how long the effects of cinacalcet persist after withdrawal of treatment or if cinacalcet is efficacious in uremic rats with established sHPT.

METHODS

We sought to determine the effect of cinacalcet discontinuation in uremic rats and whether cinacalcet was capable of influencing parathyroid hyperplasia in animals with established sHPT.

RESULTS

Discontinuation of cinacalcet resulted in reversal of the beneficial effects on serum PTH and parathyroid hyperplasia. In rats with established sHPT, cinacalcet decreased serum PTH and mediated regression of parathyroid hyperplasia. The cinacalcet-mediated decrease in parathyroid gland size was accompanied by increased expression of the cyclin-dependent kinase inhibitor p21. Prevention of cellular proliferation with cinacalcet occurred despite increased serum phosphorus and decreased serum calcium.

CONCLUSIONS

The animal data provided suggest established parathyroid hyperplasia can be reversed by modulating CaSR activity with cinacalcet and that continued treatment may be necessary to maintain reductions in PTH.

Phosphate Metabolism in Cardiorenal Metabolic Disease

Hyperphosphatemia is a major risk factor for cardiovascular disease, abnormalities of mineral metabolism and bone disease, and the progression of renal insufficiency in patients with chronic renal disease. In early renal disease, serum phosphate levels are maintained within the 'normal laboratory range' by compensatory increases in phosphaturic hormones such as fibroblast growth factor-23 (FGF-23). An important co-factor for FGF-23 is Klotho; a deficiency in Klotho plays an important role in the pathogenesis of hyperphosphatemia, renal tubulointerstitial disease, and parathyroid and bone abnormalities. Clinical hyperphosphatemia occurs when these phosphaturic mechanisms cannot counterbalance nephron loss. Hyperphosphatemia is associated with calcific uremic arteriolopathy and uremic cardiomyopathy, which may explain, in part, the epidemiologic connections between phosphate excess and cardiovascular disease. However, no clinical trials have been conducted to establish a causal relationship, and large, randomized trials with hard endpoints are urgently needed to prove or disprove the benefits and risks of therapy. In summary, hyperphosphatemia accelerates renal tubulointerstitial disease, renal osteodystrophy, as well as cardiovascular disease, and it is an important mortality risk factor in patients with chronic kidney disease.

[FGF-23: just a phosphate metabolism regulator or something else?]

Endocrine communication between the bone, kidney, and the intestine is involved in maintaining appropriate serum phosphate concentrations, which is critical for the maintenance of skeletal integrity and is central to signal transduction and cell metabolism. In addition, hyperphosphatemia is statistically associated with vascular calcification, increased morbidity and mortality; because of this, phosphate regulation has become an important field of research. In this sense, fibroblast growth factor 23 (FGF-23) has been identified as a new hormone involved in phosphate regulation through feedback mechanisms involving parathyroid hormone and vitamin D. Given what the kidney is the primary site for regulation of phosphate levels and the principal target for FGF-23, its discovery has changed the understanding of disordered mineral metabolism in chronic kidney disease, especially now, since there is clinical evidence in favor of FGF-23 playing a central role for the pathogenesis of sHPT.

Paricalcitol versus cinacalcet plus low-dose vitamin D for the treatment of secondary hyperparathyroidism in patients receiving haemodialysis: study design and baseline characteristics of the IMPACT SHPT study

Background. Paricalcitol and cinacalcet are common therapies for patients on haemodialysis with secondary hyperparathyroidism (SHPT). We conducted a multi-centre study in 12 countries to compare the safety and efficacy of paricalcitol and cinacalcet for the treatment of SHPT.

Methods. Patients aged ≥18 years with Stage 5 chronic kidney disease receiving maintenance haemodialysis and with intact parathyroid hormone (iPTH) 300–800 pg/mL, calcium 8.4–10.0 mg/dL (2.09–2.49 mmol/L) and phosphorus ≤6.5 mg/dL (2.09 mmol/L) were randomized within two strata defined by the mode of paricalcitol administration to treatment with paricalcitol- (intra-venous, US and Russian sites, IV stratum; oral, non-US and non-Russian sites, oral stratum) or cinacalcet-centred therapy. The primary endpoint is the proportion of patients in each treatment group who achieve a mean iPTH value of 150–300 pg/mL during Weeks 21–28 of treatment. Assuming efficacy response rates of 36 and 66% for cinacalcet and paricalcitol, respectively, and a 20% discontinuation rate, 124 subjects in each stratum were estimated to provide 81% power to detect a 30% absolute difference in the primary endpoint.

Results. Of 746 patients screened, 272 (mean age, 63 years; mean iPTH, 509 pg/mL) were randomized. Mean duration of haemodialysis at baseline was 3.7 years. Comorbidities included hypertension (90.4%), Type 2 diabetes (40.4%), congestive heart failure (17.3%), coronary artery disease (34.6%) and gastrointestinal disorders (75%).

Conclusions. The study participants are representative of a multinational cohort of patients on haemodialysis with elevated iPTH. The study results will provide valuable information on the best available treatment of SHPT in patients on haemodialysis.

Downregulation of Klotho expression by dehydration

Klotho, a transmembrane protein, protease, and hormone mainly expressed in renal tissue counteracts aging. Overexpression of Klotho substantially prolongs the life span. Klotho deficiency leads to excessive formation of 1,25(OH)(2)D(3), growth deficit, accelerated aging, and early death. Aging is frequently paralleled by dehydration, which is considered to accelerate the development of age-related disorders. The present study explored the possibility that dehydration influences Klotho expression. Klotho transcript levels were determined by RT-PCR, and Klotho protein abundance was detected by Western blotting in renal tissue from hydrated and 36-h-dehydrated mice as well as in human embryonic kidney (HEK293) cells. Dehydration was followed by a significant decline of renal Klotho transcript levels and protein abundance, accompanied by an increase in plasma osmolarity as well as plasma ADH, aldosterone, and 1,25(OH)(2)D(3) levels. Antidiuretic hormone (ADH; 50 nM) and aldosterone (1 μM) significantly decreased Klotho transcription and protein expression in HEK293 cells. In conclusion, the present observations disclose a powerful effect of dehydration on Klotho expression, an effect at least partially mediated by enhanced release of ADH and aldosterone.

Cinacalcet treatment and serum FGF23 levels in haemodialysis patients with secondary hyperparathyroidism

BACKGROUND

Elevated fibroblast growth factor 23 (FGF23) is associated with adverse clinical outcomes and development of secondary hyperparathyroidism (SHPT) refractory to active vitamin D. Cinacalcet hydrochloride is effective in treating SHPT, but little is known as to whether treatment with cinacalcet alters these levels and whether pretreatment FGF23 levels predict response to this therapy.

METHODS

We measured serum full-length FGF23 levels in 55 haemodialysis patients, who participated and completed the 52-week, multicentre, open-label single-arm trial that examined the effectiveness of cinacalcet for treating SHPT. In the study, alteration of vitamin D dosage was not permitted except for the case in which serum calcium could not be managed by calcium carbonate adjustment alone.

RESULTS

After 12 weeks of cinacalcet treatment, FGF23 levels decreased significantly concomitantly with a significant reduction in intact parathyroid hormone (PTH) levels. These responses were sustained >52 weeks. In multivariate regression analyses, changes from baseline in serum FGF23 were associated with changes in serum calcium and phosphorus but not with intact PTH at each time point of measurements (Week-12, Week-24 and Week-52). Baseline FGF23 was not associated with the likelihood of achieving an intact PTH <180 pg/mL at the study end.

CONCLUSIONS

Cinacalcet lowers serum FGF23 in haemodialysis patients with SHPT independently of the effects of active vitamin D. Pretreatment FGF23 cannot predict treatment response to cinacalcet in this setting. The precise mechanism of FGF23 reduction by cinacalcet and its clinical impact on outcomes in patients remain to be investigated.

Fibroblast growth factor 23 and risks of mortality and end-stage renal disease in patients with chronic kidney disease

CONTEXT

A high level of the phosphate-regulating hormone fibroblast growth factor 23 (FGF-23) is associated with mortality in patients with end-stage renal disease, but little is known about its relationship with adverse outcomes in the much larger population of patients with earlier stages of chronic kidney disease.

OBJECTIVE

To evaluate FGF-23 as a risk factor for adverse outcomes in patients with chronic kidney disease.

DESIGN, SETTING, AND PARTICIPANTS

A prospective study of 3879 participants with chronic kidney disease stages 2 through 4 who enrolled in the Chronic Renal Insufficiency Cohort between June 2003 and September 2008.

MAIN OUTCOME MEASURES

All-cause mortality and end-stage renal disease.

RESULTS

At study enrollment, the mean (SD) estimated glomerular filtration rate (GFR) was 42.8 (13.5) mL/min/1.73 m(2), and the median FGF-23 level was 145.5 RU/mL (interquartile range [IQR], 96-239 reference unit [RU]/mL). During a median follow-up of 3.5 years (IQR, 2.5-4.4 years), 266 participants died (20.3/1000 person-years) and 410 reached end-stage renal disease (33.0/1000 person-years). In adjusted analyses, higher levels of FGF-23 were independently associated with a greater risk of death (hazard ratio [HR], per SD of natural log-transformed FGF-23, 1.5; 95% confidence interval [CI], 1.3-1.7). Mortality risk increased by quartile of FGF-23: the HR was 1.3 (95% CI, 0.8-2.2) for the second quartile, 2.0 (95% CI, 1.2-3.3) for the third quartile, and 3.0 (95% CI, 1.8-5.1) for the fourth quartile. Elevated fibroblast growth factor 23 was independently associated with significantly higher risk of end-stage renal disease among participants with an estimated GFR between 30 and 44 mL/min/1.73 m(2) (HR, 1.3 per SD of FGF-23 natural log-transformed FGF-23; 95% CI, 1.04-1.6) and 45 mL/min/1.73 m(2) or higher (HR, 1.7; 95% CI, 1.1-2.4), but not less than 30 mL/min/1.73 m(2).

CONCLUSION

Elevated FGF-23 is an independent risk factor for end-stage renal disease in patients with relatively preserved kidney function and for mortality across the spectrum of chronic kidney disease.

The potential roles of FGF23 and Klotho in the prognosis of renal and cardiovascular diseases

Fibroblast growth factor (FGF) 23 and Klotho are two factors associated with several metabolic disorders. Similar to humans, accelerated aging processes characterized by chronic vascular disease, bone demineralization, skin atrophy and emphysema have been recognized in FGF23-null mice and Klotho-deficient mice. The role of these factors in the control of mineral metabolism homeostasis have been shown recently, particularly at the level of parathyroid cells and also in modulating active vitamin D production, two phenomena which are relevant in the presence of chronic kidney disease. In addition, the hormonal affect of circulating FGF23 and Klotho proteins on vascular reactivity, either directly on endothelial cell functions or indirectly by modulating the brain endothelin-1-dependent sympathetic nervous system activity, has contributed to understanding their role in the pathophysiology of hypertension and atherosclerotic vasculopathies. Consequently, very recent clinical investigations seem to confirm the involvement of Klotho in modulating the severity and prognosis of human cardiovascular (CV) disorders and longevity. The present review reports data related to the possible interactive effects of Klotho and FGF23 on the prognosis of renal and CV diseases.

Effect of intravenous saccharated ferric oxide on serum FGF23 and mineral metabolism in hemodialysis patients

BACKGROUND/AIMS

Fibroblast growth factor-23 (FGF23) plays a central role in the development of hypophosphatemia and inappropriately low 1,25-dihydroxyvitamin D induced by iron therapy for iron-deficiency anemia. The aim of this study was to examine the effect of intravenous saccharated ferric oxide on serum FGF23 levels and mineral metabolism in hemodialysis patients.

METHODS

This prospective study enrolled 27 hemodialysis patients who had iron-deficiency anemia defined by a hemoglobin concentration < 10.5 g/dl and serum ferritin < 100 ng/ml. Intravenous saccharated ferric oxide at a dose of 40 mg was administered three times weekly over 3 weeks. The dose of active vitamin D and phosphate binders was kept unchanged. Serum FGF23, intact parathyroid hormone (PTH) and other parameters were prospectively monitored for 5 weeks.

RESULTS

Serum FGF23 levels were markedly elevated [3,453 (338-6,383) pg/ml] at baseline. After 3 weeks of intravenous saccharated ferric oxide treatment, serum FGF23 further increased to 4,701 (1,251-14,396) pg/ml, and returned to the baseline values after 2 weeks of observation. There was also a significant decrease in intact PTH but no changes in serum calcium and phosphorus.

CONCLUSIONS

Intravenous saccharated ferric oxide induces further increase in elevated FGF23 levels in hemodialysis patients. This increase does not induce hypophosphatemia and inappropriately low 1,25-dihydroxyvitamin D in the absence of functioning kidney, but may result in transient PTH suppression - possibly by directly acting on the parathyroid.

Role of Klotho in aging, phosphate metabolism, and CKD

The klotho gene (KL) was identified first as a putative aging-suppressor gene that extended life span when overexpressed and accelerated aging-like phenotypes when disrupted in mice. It encodes a single-pass transmembrane protein and is expressed predominantly in kidney, where it functions as an obligate coreceptor for fibroblast growth factor 23 (FGF-23). FGF-23 is a bone-derived hormone that suppresses phosphate reabsorption and 1,25 dihydroxyvitamin D(3) (vitamin D) synthesis in the kidney. Klotho also is expressed in the parathyroid gland, where FGF-23 decreases parathyroid hormone expression and secretion, further suppressing vitamin D synthesis in kidney. Thus, FGF-23 functions as a phosphaturic hormone and a counter-regulatory hormone for vitamin D, thereby inducing negative phosphate balance. Mice lacking either FGF-23 or Klotho show hyperphosphatemia in addition to developing multiple aging-like phenotypes, which can be rescued by resolving phosphate retention. These findings have unveiled an unexpected link between aging and phosphate. In patients with chronic kidney disease (CKD), phosphate retention is seen universally and has been associated with increased mortality risk. Patients with CKD have high serum FGF-23 levels with decreased klotho expression in the kidney and parathyroid, rendering FGF-23 and Klotho as potential biomarkers and therapeutic targets for CKD. The Klotho protein not only serves as a coreceptor for FGF-23, but also functions as a humoral factor. Klotho's extracellular domain is released into blood and urine by ectodomain shedding and exerts various functions independently of FGF-23, including regulation of multiple ion channels and transporters. Decreased urinary Klotho protein level has been identified as one of the earliest biomarkers of CKD progression. This review focuses on the current understanding of Klotho protein function, with emphasis on its potential involvement in the pathophysiologic process of CKD.

Parathyroid resistance to FGF23 in kidney transplant recipients: back to the past or ahead to the future?

Fibroblast growth factor 23 (FGF23) modulates the metabolism of minerals and vitamin D. In chronic kidney disease (CKD), this process is disturbed owing to decreased parathyroid expression of FGF23's receptor complex Klotho-FGF receptor 1. In this issue, Krajisnik and colleagues demonstrate that similar alterations occur in parathyroid glands from kidney transplant recipients in association with a decline in allograft function. Is it possible that these data can be extrapolated to general early-stage CKD patients?

Secondary hyperparathyroidism: pathogenesis, disease progression, and therapeutic options

Secondary hyperparathyroidism (SHPT) is a challenge frequently encountered in the management of patients with chronic kidney disease (CKD). Downregulation of the parathyroid vitamin D and calcium-sensing receptors represent critical steps that lead to abnormalities in mineral metabolism: high phosphate, low calcium, and vitamin D deficiency. These imbalances result in parathyroid hyperplasia and contribute to vascular calcification. New studies have established a central role for fibroblast growth factor 23 (FGF-23) in the regulation of phosphate-vitamin D homeostasis. FGF-23 concentration increases in CKD and contributes to SHPT. Achieving current targets for the key mineral parameters in the management of SHPT set by the Kidney Disease Improving Global Outcomes (KDIGO) guidelines can be challenging. This review summarizes the current understanding and evidence supporting strategies for SHPT treatment in CKD patients. Treatment should include a combination of dietary phosphorus restriction, phosphate binders, vitamin D sterols, and calcimimetics. Parathyroidectomy is effective in suitable candidates refractory to medical therapy and the standard against which new approaches should be measured. Future strategies may focus on the stimulation of apoptotic activity of hyperplastic parathyroid cells.

Fibroblast growth factor 23 is elevated before parathyroid hormone and phosphate in chronic kidney disease

Fibroblast growth factor 23 (FGF23) regulates phosphorus metabolism and is a strong predictor of mortality in dialysis patients. FGF23 is thought to be an early biomarker of disordered phosphorus metabolism in the initial stages of chronic kidney disease (CKD). We measured FGF23 in baseline samples from 3879 patients in the Chronic Renal Insufficiency Cohort study, which is a diverse cohort of patients with CKD stage 2-4. Mean serum phosphate and median parathyroid hormone (PTH) levels were in the normal range, but median FGF23 was markedly greater than in healthy populations, and increased significantly with decreasing estimated glomerular filtration rate (eGFR). High levels of FGF23, defined as being above 100 RU/ml, were more common than secondary hyperparathyroidism and hyperphosphatemia in all strata of eGFR. The threshold of eGFR at which the slope of FGF23 increased was significantly higher than the corresponding threshold for PTH based on non-overlapping 95% confidence intervals. Thus, increased FGF23 is a common manifestation of CKD that develops earlier than increased phosphate or PTH. Hence, FGF23 measurements may be a sensitive early biomarker of disordered phosphorus metabolism in patients with CKD and normal serum phosphate levels.

Molecular regulation of phosphate metabolism by fibroblast growth factor-23-klotho system

Phosphorus is an essential nutrient and is routinely assimilated through consumption of food. The body's need of phosphate is usually fulfilled by intestinal absorption of this element from the consumed food, whereas its serum level is tightly regulated by renal excretion or reabsorption. Sodium-dependent phosphate transporters, located in the luminal side of the proximal tubular epithelial cells, have a molecular control on renal phosphate excretion and reabsorption. The systemic regulation of phosphate metabolism is a complex multiorgan process, and the identification of fibroblast growth factor-23 (FGF23)-Klotho system as a potent phosphatonin has provided new mechanistic insights into the homeostatic control of phosphate. Hypophosphatemia as a result of an increase in urinary phosphate wasting after activation of the FGF23-Klotho system is a common phenomenon, observed in both animal and human studies, whereas suppression of the FGF23-Klotho system leads to the development of hyperphosphatemia. This article will briefly summarize how delicate interactions of the FGF23-klotho system can regulate systemic phosphate homeostasis.

The regulation of renal phosphate transport

Renal phosphate transport is mediated by the abundance and activity of the sodium-dependent phosphate transporters, Npt2a, Npt2c, and PiT-2, present within the apical brush border membrane of the proximal tubule. Recent studies have demonstrated differential expression and activity of these sodium-dependent phosphate transporters within the proximal tubule. In general, phosphate transport is regulated by a variety of physiological stimuli, including parathyroid hormone, glucocorticoids, vitamin D3, estrogen, and thyroid hormone. Phosphatonins are now recognized as major regulators of phosphate transport activity. Other factors that affect phosphate transport include dopamine, dietary phosphate, acid-base status, lipid composition, potassium deficiency, circadian rhythm, and hypertension. Studies have shown that the PDZ-containing sodium/hydrogen exchanger regulatory factor (NHERF) proteins, specifically NHERF-1 and NHERF-3, play a critical role in the physiological regulation of phosphate transport, particularly in response to dietary phosphate. In addition, recent studies have found that NHERF-1 is also important in both the parathyroid hormone- and dopamine-mediated inhibition of phosphate transport. This review will detail the various hormones and agents involved in the regulation of phosphate transport as well as provide a brief summary of the signaling pathways and cytoskeletal proteins active in the transport of phosphate in the renal proximal tubule.

Klotho, FGF23, and FGF receptors in chronic kidney disease: a yin-yang situation?

Secondary hyperparathyroidism in chronic kidney disease (CKD) develops in response to disturbances in calcium and phosphate metabolism associated with CKD, including FGF23 and klotho. FGF23 activates its receptor FGFR1, splice variant IIIC, in the parathyroid gland via a klotho-dependent mechanism and suppresses parathyroid hormone (PTH) secretion. Klotho also may regulate PTH secretion in an FGF23-independent mode, by modulating parathyroid Na+/K+-ATPase activity. The persistence of hyperparathyroidism with progressing CKD despite high serum FGF23 is indicative of FGF23 resistance.

Defective renal maintenance of the vitamin D endocrine system impairs vitamin D renoprotection: a downward spiral in kidney disease

In kidney disease, the progressive loss of renal capacity to produce calcitriol, the vitamin D hormone, is a key contributor to elevations in parathyroid hormone (PTH) and mineral and skeletal disorders predisposing to renal and cardiovascular damage, ectopic calcifications, and high mortality rates. Thus, the safe correction of calcitriol deficiency to suppress PTH has been the treatment of choice for decades. However, recent epidemiological and experimental data suggest that calcitriol replacement may improve outcomes through renal and cardioprotective actions unrelated to PTH suppression. Furthermore, a striking incidence of vitamin D deficiency occurs in kidney disease and associates more strongly than calcitriol deficiency with a higher risk for kidney disease progression and death. Despite the translational relevance of these findings, no prospective trials are currently available in support of the efficacy of vitamin D supplementation and/or calcitriol replacement to safely halt/moderate renal disease progression. This review updates the pathophysiology behind the vicious cycle by which kidney injury impairs the maintenance of normal vitamin D and calcitriol levels, which in turn impedes vitamin D/calcitriol renoprotective actions, a requirement for the design of prospective trials to improve current recommendations for vitamin D interventions at all stages of kidney disease.

Effective use of cinacalcet for the treatment of secondary hyperparathyroidism in Austrian dialysis patients--results of the Austrian cohort of the ECHO study

BACKGROUND

Despite extensive use of standard therapy for secondary hyperparathyroidism (sHPT) in dialysis patients, still most patients do not achieve the recommended treatment targets. In a pan-European observational study (ECHO), the effectiveness of the calcimimetic cinacalcet for the treatment of sHPT was evaluated in real-world clinical practice. A sub-analysis of the entire Austrian study cohort is presented.

METHODS

Adult dialysis patients who had initiated cinacalcet therapy were included. Data on biochemical parameters of bone and mineral metabolism (intact parathyroid hormone [iPTH], calcium [Ca] and phosphorus [P]) and concurrent medication were collected 6 months prior to the initiation of cinacalcet, at initiation (baseline) and after up to 12 months of active treatment.

RESULTS

A total of 320 patients (mean age (±SD): 56 (±14) years) from 34 Austrian dialysis centres were enrolled. At baseline, patients presented with elevated serum iPTH (median 605 pg/ml) and hyperphosphataemia (median 2.1 mmol/l). After 12 months of cinacalcet treatment, serum iPTH (median percentage change -48%), calcium (-2%) and phosphorus (-6%) decreased. The greatest iPTH reduction (-66%) was found in patients with most severe sHPT (>800 pg/ml at baseline). The proportion of patients achieving the recommended NKF/K-DOQI(™) treatment targets increased from baseline to month 12 for iPTH (3-36%) and phosphorus (24 to 39%) and remained stable for calcium (51 to 50%), respectively. No patient had all 3 parameters simultaneously within NKF/K-DOQI(™) treatment targets at baseline, while 7% of patients achieved this treatment goal after 12 months. During the study the use of the phosphate binder sevelamer remained fairly stable, while the relative percentage use of calcium-based phosphate binders increased and the usage of aluminium-containing binders decreased; vitamin D analogue use remained stable.

CONCLUSION

Additional use of cinacalcet improved biochemical parameters of bone and mineral metabolism and enabled more patients to achieve and maintain the KDOQI(™) treatment targets for serum iPTH, calcium and phosphorus.

Minireview: fibroblast growth factor 23 in phosphate homeostasis and bone metabolism

Fibroblast growth factor 23 (FGF23) was identified in 2000. Since then, FGF23 has been found to physiologically regulate phosphate metabolism and aberrant actions of FGF23 results in several disorders of phosphate and bone metabolism. In addition, FGF23 plays an important role in the development of chronic kidney disease-mineral and bone disorder. However, further investigations are necessary, especially with regard to the regulation of FGF23 expression. In this minireview, we focus on the physiological and pathophysiological significance of FGF23 in phosphate and bone metabolism.

A proteomic approach to study parathyroid glands

Parathyroid tumours are heterogeneous and in some cases the diagnosis may be difficult using histological features. In this study we used a two-dimensional electrophoresis (2D)/mass spectrometry (MS)-based approach to examine the global changes of parathyroid adenoma tissues protein profile compared to the parathyroid normal tissues. Validation of protein expression was performed by immunoblotting using specific antibodies. Ingenuity software was used to identify the biological processes to which these proteins belong and to construct a potential network. A total of 30 proteins were found to be differentially expressed, of which 22 resulted in being over-expressed. Proteins identified by 2D/MS/MS proteomics were classified into functional categories and a major change (≥ 2-fold) in terms of expression was found in proteins involved in response to biotic stimuli, cell organization and signal transduction. After Ingenuity analysis, 14-3-3 ζ/δ appears to be a key protein in the network of parathyroid adenoma, where it is linked to other proteins such as annexin A2, B box and SPRY domain-containing protein (BSPRY), p53 and epidermal growth factor receptor (EGFR). Our results suggest that the proteomic approach was able to differentiate the protein profiles of normal parathyroid and parathyroid adenoma and identify a panel of proteins which are differentially expressed. The functional role of these proteins in the network of intracellular pathways is discussed.

Disorders of phosphorus homeostasis

PURPOSE OF REVIEW

The study of phosphorus physiology and investigations into clinical disorders of phosphorus metabolism has blossomed over the past decade. Recent work has confirmed and further extended our knowledge of basic mechanisms of phosphorus metabolism.

RECENT FINDINGS

This review will focus on FGF-23 and Klotho, and on the recent further dissection of their roles in phosphorus and skeletal metabolism. Additionally, this review will detail recent studies that implicate a role for these phosphaturic and vitamin D regulating factors in extraskeletal calcification, including that occurring in soft tissue and vascular beds.

SUMMARY

These findings in total provide fertile ground for investigations into the cause and treatment of abnormal skeletal and extraskeletal calcification in patients with inherited hypophosphatemic disorders. More importantly, and certainly with wider potential clinical application, these studies likewise imply a role for these factors in the pathogenesis of accelerated cardiovascular disease that occurs in patients with the most common hyperphosphatemic disorder, chronic kidney disease. Future studies are needed to confirm a harmful or possibly even beneficial role for FGF-23 and other factors in these disease states, and to determine whether therapeutic manipulation of these factors does truly affect clinical outcomes in patients with hypophosphatemia and hyperphosphatemia.

FGF23 and the parathyroid glands

Fibroblast growth factor 23 (FGF23) is a phosphatonin that is secreted by osteocytes and osteoblasts in response to hyperphosphatemia and 1,25-dihydroxyvitamin D (1,25D). It acts on its receptor complex, Klotho-FGFR1c (fibroblast growth factor receptor 1 c-splicing form), in the distal convoluted tubule to repress renal phosphorus reabsorption in the proximal tubule and suppress the renal synthesis of 1,25D. Klotho-FGFR1c is also expressed in the parathyroid glands. FGF23 acts on the receptor complex in the parathyroid glands to decrease parathyroid hormone (PTH) gene expression and PTH secretion through activation of the MAPK pathway. In chronic kidney disease (CKD), both FGF23 and PTH are increased, implying resistance of the parathyroid glands to FGF23. There is a decrease in the Klotho-FGFR1c complex in the parathyroid glands in both experimental CKD and in patients with end-stage renal disease. In addition, in advanced experimental CKD, FGF23 has a decreased ability to inhibit PTH expression.

Effectiveness of cinacalcet in patients with recurrent/persistent secondary hyperparathyroidism following parathyroidectomy: results of the ECHO study

BACKGROUND

Progressive secondary hyperparathyroidism (sHPT) is characterized by parathyroid gland hyperplasia which may ultimately require parathyroidectomy (PTX). Although PTX is generally a successful treatment for those patients subjected to surgery, a significant proportion develops recurrent sHPT following PTX. ECHO was a pan-European observational study which evaluated the achievement of KDOQI(TM) treatment targets with cinacalcet use in patients on dialysis. Previously published results showed that cinacalcet plus flexible vitamin D therapy lowered serum PTH, phosphorus and calcium in the clinical practice with similar efficacy as seen in phase III trials.

METHODS

This subgroup analysis of ECHO describes the real-world cinacalcet treatment effect in patients with recurrent or persistent sHPT after PTX (n = 153) compared to sHPT patients without prior history of PTX (n = 1696).

RESULTS

Both groups of patients had substantially elevated serum PTH with comparable sHPT severity at baseline. After 12 months of cinacalcet treatment, 20.3% (26/128) of patients with prior PTX and 18.2% (253/1388) of patients without prior PTX achieved serum PTH and Ca × P values within the recommended KDOQI(TM) target ranges.

CONCLUSIONS

Our data support the successful use of cinacalcet in patients with recurrent/persistent sHPT after PTX.

FGF-23: More than a regulator of renal phosphate handling?

Fibroblast growth factor 23 (FGF-23) is likely to be the most important regulator of phosphate homeostasis, which mediates its functions through FGF receptors and the coreceptor Klotho. Besides reducing expression of the sodium-phosphate cotransporters NPT2a and NPT2c in the proximal tubules, FGF-23 inhibits the renal 1α-hydroxylase and stimulates the 24-hydroxylase, and it appears to reduce parathyroid hormone (PTH) secretion in short-term studies. FGF-23 synthesis and secretion by osteocytes and osteoblasts is upregulated through 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and through an increased dietary phosphate intake. FGF-23 levels are elevated or inappropriately normal in patients with tumor-induced osteomalacia and several inherited hypophosphatemic disorders, but the most significant increases are found in patients with chronic kidney disease (CKD). During the early stages of CKD, increased FGF-23 production enhances urinary phosphate excretion and thus prevents the development of hyperphosphatemia, reduces the circulating levels of 1,25(OH)(2)D(3), and therefore contributes to the development of secondary hyperparathyroidism. In patients with end-stage renal disease (ESRD), FGF-23 levels can be extremely high and were shown to be predictors of bone mineralization, left ventricular hypertrophy, vascular calcification, and mortality. It remains to be determined, however, whether FGF-23 represents simply a sensitive biomarker of an abnormal phosphate homeostasis or has, independent of serum phosphate levels, potentially negative "off-target" effects. Nonetheless, reducing the production and/or the biologic activity of FGF-23 may be an important therapeutic goal for this patient population.

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

Circulating levels of fibroblast growth factor 23 (FGF23) are elevated in patients with early chronic kidney disease (CKD) and are postulated to cause low blood levels of 1,25-dihydroxyvitamin D, as well as normal phosphate levels. In order to provide more direct evidence for the pathophysiological role of FGF23 in the settings of mineral ion homeostasis typically seen in early CKD, we studied rats with progressive CKD treated with anti-FGF23 neutralizing antibody. Without antibody treatment, rats with CKD exhibited high circulating levels of FGF23 and parathyroid hormone, low 1,25-dihydroxyvitamin D, and normal serum phosphate levels, accompanied by increased fractional excretion of phosphate. Antibody treatment, however, lessened fractional excretion of phosphate, thus increasing serum phosphate levels, and normalized serum 1,25-dihydroxyvitamin D by increased 1α-OHase and decreased 24-OHase expressions in the kidney. These antibody-induced changes were followed by increased serum calcium levels, leading to decreased serum parathyroid hormone. Hence, our study shows that FGF23 normalizes serum phosphate and decreases 1,25-dihydroxyvitamin D levels in early-stage CKD, and suggests a pathological sequence of events for the development of secondary hyperparathyroidism triggered by increased FGF23, followed by a reduction of 1,25-dihydroxyvitamin D and calcium levels, thereby increasing parathyroid hormone secretion.

Reciprocal control of 1,25-dihydroxyvitamin D and FGF23 formation involving the FGF23/Klotho system

Fibroblast growth factor 23 (FGF23) is a circulating hormone that is synthesized by osteocytes and osteoblasts. This glycosylated peptide controls phosphate balance by modulating urinary phosphate excretion and indirectly intestinal phosphate absorption by reducing expression of the renal and intestinal sodium phosphate transporters. In a feedback loop, 1,25-dihydroxyvitamin D and phosphate intake control FGF23 production. FGF23 is inactivated by cleavage by a still unidentified enzyme. FGF23 cleavage occurs within cells and probably in the circulation. Klotho, a protein expressed at the cell surface of few organs, forms complexes with FGF receptors, which increases their affinity for FGF23. Klotho is also released into the plasma and urine by an enzymatic cleavage. FGF23 plays a central role in vitamin D metabolism: It inhibits calcitriol synthesis in the kidney and stimulates the catabolism of active vitamin D sterols. In turn, calcitriol stimulates FGF23 and Klotho expression. In chronic kidney diseases, FGF23 concentration increases as GFR declines, whereas Klotho tissue expression decreases. The modifications of FGF23 and Klotho expression are probably involved in the genesis of hyperparathyroidism and the resistance to vitamin D receptor (VDR) activation in chronic kidney disease. Low vitamin D, high FGF23 concentrations, and defects in VDR activation are associated with similar risks, which evoke the possibility that potential FGF23 toxicity might be partly mediated by FGF23-induced decrease in calcitriol or 25-hydroxyvitamin D. Conversely, VDR activators could be used to modulate Klotho or FGF23 expression.

Parathyroid Klotho and FGF-receptor 1 expression decline with renal function in hyperparathyroid patients with chronic kidney disease and kidney transplant recipients

Current studies suggest that short-term exposure of parathyroid glands to fibroblast growth factor 23 (FGF23) reduces parathyroid hormone secretion. However, patients with chronic kidney disease (CKD) develop secondary hyperparathyroidism despite high levels of serum FGF23, indicating a parathyroid FGF23 'resistance'. Here we analyzed the expression of the FGF23 receptors Klotho and FGF receptor 1 (FGFR1) in 88 hyperplastic parathyroid glands from 31 patients with CKD (including 21 renal allograft recipients), and their regulation in isolated bovine and human hyperplastic parathyroid cells. Glandular expression was variable, yet the Klotho and FGFR1 mRNA levels declined in parallel with the decreasing glomerular filtration rate, significantly decreasing over CKD stages. We found no association between the expression of Klotho, FGFR1, and the proliferation marker Ki67. In vitro treatment of bovine cells with FGF23 or calcium reduced the Klotho level, whereas active vitamin D(3) compounds increased its expression. Phosphate and parathyroid hormone had no effect. Treatment had less impact on Klotho in cultured human cells than in the bovine healthy cell model, whereas FGFR1 expression was induced in the hyperplastic cells. Thus parathyroid Klotho and FGFR1 decrease with declining renal function, possibly because of alterations in mineral metabolism related to the failing kidney. This could explain the observed parathyroid resistance to FGF23 in late CKD.

PTH increases FGF23 gene expression and mediates the high-FGF23 levels of experimental kidney failure: a bone parathyroid feedback loop

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) target the kidney to cause a phosphaturia. FGF23 also acts on the parathyroid to decrease PTH expression, but in chronic kidney disease (CKD) there are high-serum PTH and FGF23 levels and resistance of the parathyroid to FGF23. We now report that PTH acts on bone to increase FGF23 expression and characterize the signal transduction pathway whereby PTH increases FGF23 expression. Remarkably, we show that PTH is necessary for the high-FGF23 levels of early kidney failure due to an adenine high-phosphorus diet. Parathyroidectomy before the diet totally prevented the fivefold increase in FGF23 levels in kidney failure rats. Moreover, parathyroidectomy of early kidney failure rats corrected their high-FGF23 levels. Therefore, in early kidney failure, the high-FGF23 levels are dependent on the high-PTH levels. PTH infusion for 3 days to mice with normal renal function increased serum FGF23 and calvaria FGF23 mRNA levels. To demonstrate a direct effect of PTH on FGF23, we added PTH to rat osteoblast-like UMR106 cells. PTH increased FGF23 mRNA levels (4-fold) and this effect was mimicked by a PKA activator, forskolin. PTH also decreased SOST mRNA levels (3-fold). SOST codes for sclerostin, a Wnt pathway inhibitor, which is a PTH receptor (PTH1R) target. The effect of PTH was prevented by added sclerostin. Therefore, PTH increases FGF23 expression which involves the PKA and Wnt pathways. The effect of PTH on FGF23 completes a bone-parathyroid endocrine feedback loop. Importantly, secondary hyperparathyroidism is essential for the high-FGF23 levels in early CKD.

Increased parathyroid expression of klotho in uremic rats

Klotho is a protein of significant importance for mineral homeostasis. It helps to increase parathyroid hormone (PTH) secretion and in the trafficking of Na+/K+-ATPase to the cell membrane; however, it is also a cofactor for fibroblast growth factor (FGF)-23 to interact with its receptor, FGFR1 IIIC, resulting in decreased PTH secretion. Studies on the regulation of parathyroid klotho expression in uremia have provided varying results. To help resolve this, we measured klotho expression in the parathyroid and its response to severe uremia, hyperphosphatemia, and calcitriol treatment in the 5/6 nephrectomy rat model of secondary hyperparathyroidism. Parathyroid klotho gene expression and protein were significantly increased in severely uremic hyperphosphatemic rats, but not affected by moderate uremia and normal serum phosphorus. Calcitriol suppressed klotho gene and protein expression in severe secondary hyperparathyroidism, despite a further increase in plasma phosphate. Both FGFR1 IIIC and Na+/K+-ATPase gene expression were significantly elevated in severe secondary hyperparathyroidism. Parathyroid gland klotho expression and the plasma calcium ion concentration were inversely correlated. Thus, our study suggests that klotho may act as a positive regulator of PTH expression and secretion in secondary hyperparathyroidism.

Phosphate homeostasis and the renal-gastrointestinal axis

Transport of phosphate across intestinal and renal epithelia is essential for normal phosphate balance, yet we know less about the mechanisms and regulation of intestinal phosphate absorption than we do about phosphate handling by the kidney. Recent studies have provided strong evidence that the sodium-phosphate cotransporter NaPi-IIb is responsible for sodium-dependent phosphate absorption by the small intestine, and it might be that this protein can link changes in dietary phosphate to altered renal phosphate excretion to maintain phosphate balance. Evidence is also emerging that specific regions of the small intestine adapt differently to acute or chronic changes in dietary phosphate load and that phosphatonins inhibit both renal and intestinal phosphate transport. This review summarizes our current understanding of the mechanisms and control of intestinal phosphate absorption and how it may be related to renal phosphate reabsorption; it also considers the ways in which the gut could be targeted to prevent, or limit, hyperphosphatemia in chronic and end-stage renal failure.

Does the downregulation of the FGF23 signaling pathway in hyperplastic parathyroid glands contribute to refractory secondary hyperparathyroidism in CKD patients?

Compared to normal tissue, hyperplastic parathyroid glands of patients with chronic kidney disease on dialysis express lower levels of FGFR1 and Klotho proteins. Similar findings are reported in uremic rats with advanced chronic kidney disease. Moreover, in these animals, FGF23 administration fails to reduce PTH serum levels in vivo and to transmit downstream signals in parathyroid cells ex vivo. These findings may explain, at least partly, the concomitant elevation of both FGF23 and PTH serum levels in chronic kidney disease secondary hyperparathyroidism.