Update on fibroblast growth factor 23 in chronic kidney disease. Kidney Int. 2012;82:737-747. [PMID: 22622492 PMCID: PMC3434320 DOI: 10.1038/ki.2012.176]

Functional diversity of fibroblast growth factors in bone formation

The functional significance of fibroblast growth factor (FGF) signaling in bone formation has been demonstrated through genetic loss-of-function and gain-of-function approaches. FGFs, comprising 22 family members, are classified into three subfamilies: canonical, hormone-like, and intracellular. The former two subfamilies activate their signaling pathways through FGF receptors (FGFRs). Currently, intracellular FGFs appear to be primarily involved in the nervous system. Canonical FGFs such as FGF2 play significant roles in bone formation, and precise spatiotemporal control of FGFs and FGFRs at the transcriptional and posttranscriptional levels may allow for the functional diversity of FGFs during bone formation. Recently, several research groups, including ours, have shown that FGF23, a member of the hormone-like FGF subfamily, is primarily expressed in osteocytes/osteoblasts. This polypeptide decreases serum phosphate levels by inhibiting renal phosphate reabsorption and vitamin D3 activation, resulting in mineralization defects in the bone. Thus, FGFs are involved in the positive and negative regulation of bone formation. In this review, we focus on the reciprocal roles of FGFs in bone formation in relation to their local versus systemic effects.

A longitudinal study of inflammation, CKD-mineral bone disorder, and carotid atherosclerosis after renal transplantation

BACKGROUND AND OBJECTIVES

The role of reversibility of nontraditional risk factors, like inflammation and CKD-mineral bone disorder, in the reduction of cardiovascular risk after renal transplantation is still scarcely defined.

DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS

The longitudinal relationship between C-reactive protein, CKD-mineral bone disorder biomarkers, and intima media thickness was investigated in a series of 178 patients (age=32±10 years) with stage 5 CKD maintained on chronic dialysis who underwent echo-color Doppler studies of the carotid arteries before and after renal transplantation. Smokers and patients with diabetes were excluded from the study. In all patients, immunosuppression was performed by a standard regimen on the basis of calcineurin inhibitors. Healthy controls were specifically selected to match the age and sex distribution of the patients. Biochemical and intima media thickness assessments were repeated 6 months after transplantation.

RESULTS

Before transplantation, intima media thickness in patients with stage 5 CKD on dialysis (average=0.9±0.2 mm) was higher (P<0.001) than in well matched healthy controls (0.6±0.1 mm) and reduced substantially (-22%; 95% confidence interval, -24% to -20%) after transplantation (P=0.001). GFR (multivariable-adjusted β=0.23; P<0.001), C-reactive protein (β=0.15; P<0.001), and fibroblast growth factor 23 (β=0.28; P<0.001) were the strongest independent correlates of intima media thickness before transplantation. Similarly, longitudinal changes in the same biomarkers were the sole independent correlates of simultaneous changes in intima media thickness (C-reactive protein: β=0.25; fibroblast growth factor 23: β=0.26; P<0.001 for both) after renal transplantation. The evolution of intima media thickness after transplantation was largely independent of classic risk factors, including BP, LDL cholesterol, and insulin resistance, as measured by homeostatic model assessment.

CONCLUSIONS

Intima media thickness improves after renal transplantation. Such an improvement associates with parallel changes in serum C-reactive protein and fibroblast growth factor 23. These observations are in keeping with the hypothesis that the decline in cardiovascular risk after transplantation, in part, depends on partial resolution of nontraditional cardiovascular risk factors, like inflammation and CKD-mineral bone disorder.

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

BACKGROUND

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

[FGF23 and the heart]

The prevalence of chronic kidney disease (CKD) has now reached epidemic proportions and it is very likely that it will continue to rise with the increasing prevalence of juvenile diabetes mellitus, hypertension and aging population. CKD is a risk factor for cardiovascular disease (CVD) and cardiovascular disease can lead to CKD. It is also well known that patients with CKD have a higher risk of death from CVD than of progressing to end-stage renal disease that requires renal replacement therapy. In patients with CKD, there is a higher mortality from sudden cardiac death and congestive heart failure than coronary artery disease, which is not the case in the general population. The high prevalence of congestive heart failure in CKD is due to cardiac remodeling which progresses from concentric remodeling to concentric and eccentric hypertrophy, leading to left ventricular hypertrophy with both systolic and diastolic dysfunction. Recent studies have suggested that, in patients with chronic kidney disease, common traditional risk factors for cardiovascular disease such as hypertension, hyperlipidemia and obesity may not be the main determinants of cardiovascular disease. Among the various non-traditional cardiovascular risk factors present in patients with chronic kidney disease, abnormalities of CKD related mineral and bone disorder, which includes elevated fibroblast growth factor 23 (FGF23) have been one of the most extensively studied. However, after many years of research, the debate over the exact pathways by which FGF23 may lead to increased CVD still continues. FGF23 may have both direct and indirect effects on the cardiovascular system. Better understanding of the most relevant pathophysiologic pathways for FGF23 may lead to therapeutic interventions against cardiovascular disease in patients with CKD.

Elevated cardiac markers in chronic kidney disease as a consequence of hyperphosphatemia-induced cardiac myocyte injury

Background

Elevated cardiac markers (CMs) and hyperphosphatemia are commonly encountered in patients with chronic kidney diseases (CKD), but the causal relationship between them has not been established.

Material/Methods

We enrolled 151 patients with different kidney functions in a cross-sectional study to explore the relationship of serum phosphorus with CMs, including cardiac troponin T (cTnT), myoglobin (MYO), creatine kinase-MB (CK-MB), and brain natriuretic peptide (BNP). Then, the effect of reducing phosphorus levels on CMs by taking phosphate binder for 3 months was prospectively observed in 64 hemodialysis patients. Finally, human cardiomyocytes were exposed to different concentrations of inorganic phosphorus to examine its underlying mechanism.

Results

1) Serum phosphorus and CMs gradually increased as the glomerular filtration rate declined in CKD patients (p<0.01). 2) Elevation of CMs was much greater and cardiac structure and function were worse in CKD patients who had higher serum phosphorus concentrations (p<0.05). 3) Serum phosphorus level positively correlated with cTnT, MYO, and BNP in CKD patients (p<0.001). 4) In hemodialysis patients, the reduction of cTnT, MYO, and CK-MB was synchronous with the pharmacologically-induced decline of serum phosphorus level. However, levels of serum Fibroblast growth factor 23 (FGF23) had no statistical decrease. 5) Simulated hyperphosphatemia inhibited proliferation of human cardiomyocytes in a time- and concentration-dependent manner.

Conclusions

Hyperphosphatemia may induce myocardial damage in CKD patients, possibly through triggering apoptosis of human cardiomyocytes, and this could account for the elevated cardiac markers in CKD patients.

Fibroblast growth factor-23 in obese, normotensive adolescents is associated with adverse cardiac structure

OBJECTIVES

Fibroblast growth factor-23 (FGF23) is a biomarker for cardiovascular disease. Obesity may promote FGF23 production in the absence of chronic kidney disease. We sought to determine among normotensive African American adolescents whether FGF23 levels are greater in obese compared with normal-weight adolescents and to determine the relationship of FGF23 with markers of cardiac structure and insulin resistance.

STUDY DESIGN

Cross-sectional data were obtained from a cohort of 130 normotensive, African American adolescents ages 13-18 years without chronic kidney disease; 74 were obese; 56 were normal weight. Plasma C-terminal FGF23, fasting glucose and insulin, and high-sensitivity C-reactive protein were measured; participants underwent M-mode echocardiography.

RESULTS

FGF23 was skewed and approximately normally distributed after natural log transformation (logFGF23). FGF23 levels were greater in obese vs normal-weight participants (geometric mean 43 vs 23 RU/mL, P < .01). FGF23 values were significantly greater in participants with eccentric or concentric cardiac hypertrophy compared with those without hypertrophy P < .01). LogFGF23 directly correlated with body mass index, body mass index z-score, waist circumference, fasting insulin levels, and homeostasis model assessment scores. Regression models adjusted for age, sex, and high-sensitivity C-reactive protein suggest that each 10% increase in FGF23 is associated with a 1.31 unit increase in left ventricular mass (P < .01), a 0.29-unit increase in left ventricular mass index (P < .01), and a 0.01-unit increase in left atrial dimension indexed to height (P = .02).

CONCLUSIONS

In this sample of obese African American adolescents, FGF23 blood levels were associated with abnormal cardiac structure. We postulate that FGF23 may be an early marker of cardiac injury in obese but otherwise-healthy African American adolescents.

Association of serum levels of FGF23 and α-Klotho with glomerular filtration rate and proteinuria among cardiac patients

Background

Expression and/or excretion of fibroblast growth factor-23 (FGF23) and its co-receptor Klotho are altered in patients with end-stage renal disease. The possibility that the FGF23/α-Klotho system mediates the aggravated cardiovascular outcome among patients with chronic kidney disease (CKD) has been suggested. We determined whether FGF23 and α-Klotho concentrations are altered among patients with reduced renal function and proteinuria.

Methods

Serum FGF23 and α-Klotho were measured in cardiology patients who were not undergoing chronic hemodialysis. Estimated glomerular filtration rate (eGFR) was correlated negatively with FGF23 and positively with α-Klotho.

Results

The correlation between FGF23 and the renal tubular maximum reabsorption rate of phosphate to the GFR (TmP/GFR) was not significant, but that between FGF23 and serum calcium or inorganic phosphate was significant among patients with an estimated GFR of less than 60 mL/min/m². By stepwise multivariate regression analysis, eGFR was selected as significant predictor for FGF23 or α-Klotho among patients with an estimated GFR of less than 60 mL/min/m²; however, urine albumin/creatinine ratio was not selected as a predictor for FGF23 or α-Klotho irrespective of the eGFR levels. In patients with eGFR of <60 mL/min/1.73 m², UACR was significantly associated with log(FGF23); but, this association did not remain statistically significant in a multivariate model.

Conclusions

Among cardiology patients with various stages of CKD, serum concentrations of FGF23 and α-Klotho were associated with renal function, but not with the extent of proteinuria.

Fibroblast growth factor 23 (FGF23) and mortality: the Ludwigshafen Risk and Cardiovascular Health Study

BACKGROUND

Fibroblast growth factor 23 (FGF23) is an important regulatory hormone in phosphate and vitamin D metabolism. Here, we investigated the associations of FGF23 with traditional cardiovascular risk factors and with bone metabolism parameters as well as the impact of FGF23 upon long-term mortality in a large cohort of patients referred for coronary angiography.

METHODS

We examined whether c-term FGF23 concentrations at baseline were associated with cardiovascular and total mortality in 2974 patients from the Ludwigshafen Risk and Cardiovascular Health Study (LURIC). We investigated if these associations were independent from established cardiovascular risk factors as well as from other mineral regulating factors and bone biomarkers such as calcium, parathyroid hormone (PTH), alkaline phosphatase (AP), vitamin D, and serum phosphate.

RESULTS

Mean age of participants was 63 ± 10 years; median c-term FGF23 serum levels were 54 (40-78) RU/ml. During a median follow-up of 9.9 years, 884 deaths (30%) occurred, 545 (18%) of which were cardiovascular. FGF23 significantly and inversely correlated with eGFR. AP, phosphate, and PTH increased in parallel with quartiles of FGF23. Age- and sex-adjusted hazard ratios (HRs) in the fourth quartile compared to the first quartile of FGF23 were 2.54 (95%CI, 2.09-3.09; p < 0.001) for all cause and 2.56 (95% CI, 1.99-3.28; p < 0.001) for cardiovascular mortality. These associations remained significant after additional adjustments for cardiovascular risk factors and bone biomarkers (calcium, PTH, AP, vitamin D, and phosphate): Adjusted HRs were 1.38 (95%CI, 1.26-1.52; p < 0.001) for all-cause and 1.35 (95%CI, 1.20-1.52; p < 0.001) for cardiovascular mortality for each increase by one standard deviation of c-term FGF23.

CONCLUSIONS

In patients undergoing coronary angiography baseline c-term FGF23 levels predict the risk for all-cause and cardiovascular mortality over 9.9 years of follow-up. These associations were independent of established cardiovascular risk factors and serum phosphate.

Competitive interaction between fibroblast growth factor 23 and asymmetric dimethylarginine in patients with CKD

Both fibroblast growth factor 23 (FGF-23) and asymmetric dimethylarginine (ADMA) are associated with progression of CKD. We tested the hypothesis that ADMA and FGF23 are interactive factors for CKD progression in a cohort of 758 patients with CKD in Southern Europe (mean eGFR±SD, 36±13 ml/min per 1.73 m(2)) and in a central European cohort of 173 patients with CKD (MMKD study, mean eGFR, 64±39 ml/min per 1.73 m(2)). In the first cohort, 214 patients had renal events (decrease in eGFR of >30%, dialysis, or kidney transplantation) during a 3-year follow-up. Both intact FGF-23 and ADMA predicted the incidence rate of renal events in unadjusted and adjusted analyses (P<0.001). There was a strong competitive interaction between FGF-23 and ADMA in the risk of renal events (P<0.01 in adjusted analyses); the risk associated with raised ADMA levels was highest in patients with low FGF-23 levels. These results were confirmed in the MMKD cohort, in which FGF-23 level was again an effect modifier of the relationship between plasma ADMA level and renal events (doubling of baseline serum creatinine, dialysis, or kidney transplantation) in the adjusted analyses (P<0.01). Furthermore, in the MMKD cohort there was a parallel, independent competitive interaction between symmetric dimethylarginine level and c-terminal FGF-23 level for the risk for renal events (P=0.001). These findings indicate that the association of ADMA level with the risk of CKD progression is modified by FGF-23 level and provide further evidence that dysregulation of the nitric oxide system is involved in CKD progression.

Serum fibroblast growth factor-23 is associated with incident kidney disease

Fibroblast growth factor-23 is a bone-derived hormone that increases urinary phosphate excretion and inhibits hydroxylation of 25-hydroxyvitamin D. Recent studies suggest that fibroblast growth factor-23 may be an early biomarker of CKD progression. However, its role in kidney function decline in the general population is unknown. We assessed the relationship between baseline (1990-1992) serum levels of intact fibroblast growth factor-23 and incident ESRD in 13,448 Atherosclerosis Risk in Communities study participants (56.1% women, 74.7% white) followed until December 31, 2010. At baseline, the mean age of participants was 56.9 years and the mean eGFR was 97 ml/min per 1.73 m(2). During a median follow-up of 19 years, 267 participants (2.0%) developed ESRD. After adjustment for demographic characteristics, baseline eGFR, traditional CKD risk factors, and markers of mineral metabolism, the highest fibroblast growth factor-23 quintile (>54.6 pg/ml) compared with the lowest quintile (<32.0 pg/ml) was associated with risk of developing ESRD (hazard ratio, 2.10; 95% confidence interval, 1.31 to 3.36; trend P<0.001). In a large, community-based study comprising a broad range of kidney function, higher baseline fibroblast growth factor-23 levels were associated with increased risk of incident ESRD independent of the baseline level of kidney function and a number of other risk factors.

Serum phosphorus and mortality in the Third National Health and Nutrition Examination Survey (NHANES III): effect modification by fasting

BACKGROUND

Serum phosphorus levels have been associated with mortality in some but not all studies. Because dietary intake prior to measurement can affect serum phosphorus levels, we hypothesized that the association between serum phosphorus level and mortality is strongest in those who have fasted longer.

STUDY DESIGN

Prospective cohort study.

SETTING & PARTICIPANTS

Nationally representative sample of 12,984 participants 20 years or older in the Third National Health and Nutrition Examination Survey (1988-1994).

FACTORS

Serum phosphorus level, fasting duration (dichotomized as ≥ 12 or < 12 hours).

OUTCOMES

All-cause and cardiovascular mortality determined by death certificate data from the National Death Index.

MEASUREMENTS

Serum phosphorus measured in a central laboratory and fasting duration recorded as time since food or drink other than water was consumed.

RESULTS

Individuals fasting 12 or more hours had lower serum phosphorus levels than those fasting less than 12 hours (3.34 vs 3.55 mg/dL; P < 0.001) and higher correlation with repeat measurement (0.66 vs 0.53; P = 0.002). In multivariable-adjusted Cox regression models, the highest quartile of serum phosphorus was associated with increased mortality in participants fasting 12 or more hours (adjusted HR, 1.74; 95% CI, 1.38-2.20; reference, lowest quartile) but not in participants fasting less than 12 hours (adjusted HR, 1.08; 95% CI, 0.89-1.32; P for interaction = 0.002). Relationships were consistent using 8 hours as the fasting cutoff point or cardiovascular mortality as the outcome.

LIMITATIONS

Observational study, lack of fibroblast growth factor 23 or intact parathyroid hormone measurements.

CONCLUSIONS

Fasting but not nonfasting serum phosphorus levels were associated with increased mortality. Risk prognostication based on serum phosphorus may be improved using fasting levels.

Fibroblast growth factor-23 and incident coronary heart disease, heart failure, and cardiovascular mortality: the Atherosclerosis Risk in Communities study

BACKGROUND

Fibroblast growth factor-23 (FGF-23) is a hormone involved in phosphorous regulation and vitamin D metabolism that may be associated with cardiovascular risk, and it is a potential target for intervention. We tested whether elevated FGF-23 is associated with incident coronary heart disease, heart failure, and cardiovascular mortality, even at normal kidney function.

METHODS AND RESULTS

A total of 11 638 Atherosclerosis Risk In Communities study participants, median age 57 at baseline (1990-1992), were followed through 2010. Cox regression was used to evaluate the independent association of baseline serum active FGF-23 with incident outcomes. Models were adjusted for traditional cardiovascular risk factors and estimated glomerular filtration rate. During a median follow-up of 18.6 years, 1125 participants developed coronary heart disease, 1515 developed heart failure, and 802 died of cardiovascular causes. For all 3 outcomes, there was a threshold, whereby FGF-23 was not associated with risk at <40 pg/mL but was positively associated with risk at >40 pg/mL. Compared with those with FGF-23 <40 pg/mL, those in the highest FGF-23 category (≥ 58.8 pg/mL) had a higher risk of incident coronary heart disease (adjusted hazard ratio, 95% CIs: 1.65, 1.40 to 1.94), heart failure (1.75, 1.52 to 2.01), and cardiovascular mortality (1.65, 1.36 to 2.01). Associations were modestly attenuated but remained statistically significant after further adjustment for estimated glomerular filtration rate. In stratified analyses, similar results were observed in African Americans and among persons with normal kidney function.

CONCLUSIONS

High levels of serum FGF-23 were associated with increased risk of coronary heart disease, heart failure, and cardiovascular mortality in this large, biracial, population-based cohort. This association was independent of traditional cardiovascular risk factors and kidney function.

Associations of FGF-23 and sKlotho with cardiovascular outcomes among patients with CKD stages 2-4

BACKGROUND AND OBJECTIVES

CKD-mineral and bone disorders (CKD-MBD) measures contribute to cardiovascular morbidity in patients with CKD. Among these, fibroblast growth factor (FGF)-23 and its coreceptor Klotho may exert direct effects on vascular and myocardial tissues. Klotho exists in a membrane-bound and a soluble form (sKlotho). Recent experimental evidence suggests sKlotho has vasculoprotective functions.

DESIGN, SETTINGS, PARTICIPANTS, & MEASUREMENTS

Traditional and novel CKD-MBD variables were measured among 444 patients with CKD stages 2-4 recruited between September 2008 and November 2012 into the ongoing CARE FOR HOMe study. Across tertiles of baseline sKlotho and FGF-23, the incidence of two distinct combined end points was analyzed: (1) the first occurrence of an atherosclerotic event or death from any cause and (2) the time until hospital admission for decompensated heart failure or death from any cause.

RESULTS

Patients were followed for 2.6 (interquartile range, 1.4-3.6) years. sKlotho tertiles predicted neither atherosclerotic events/death (fully adjusted Cox regression analysis: hazard ratio [HR] for third versus first sKlotho tertile, 0.75 [95% confidence interval (95% CI), 0.43-1.30]; P=0.30) nor the occurrence of decompensated heart failure/death (HR for third versus first sKlotho tertile, 0.81 [95% CI, 0.39-1.66]; P=0.56). In contrast, patients in the highest FGF-23 tertile had higher risk for both end points in univariate analysis. Adjustment for kidney function attenuated the association between FGF-23 and atherosclerotic events/death (HR for third versus first FGF-23 tertile, 1.23 [95% CI, 0.58-2.61]; P=0.59), whereas the association between FGF-23 and decompensated heart failure/death remained significant after adjustment for confounders (HR for third versus first FGF-23 tertile, 4.51 [95% CI, 1.33-15.21]; P=0.02).

CONCLUSIONS

In this prospective observational study of limited sample size, sKlotho was not significantly related to cardiovascular outcomes. FGF-23 was significantly associated with future decompensated heart failure but not incident atherosclerotic events.

Treatment of established left ventricular hypertrophy with fibroblast growth factor receptor blockade in an animal model of CKD

BACKGROUND

Activation of fibroblast growth factor receptor (FGFR)-dependent signalling by FGF23 may contribute to the complex pathogenesis of left ventricular hypertrophy (LVH) in chronic kidney disease (CKD). Pan FGFR blockade by PD173074 prevented development of LVH in the 5/6 nephrectomy rat model of CKD, but its ability to treat and reverse established LVH is unknown.

METHODS

CKD was induced in rats by 5/6 nephrectomy. Two weeks later, rats began treatment with vehicle (0.9% NaCl) or PD173074, 1 mg/kg once-daily for 3 weeks. Renal function was determined by urine and blood analyses. Left ventricular (LV) structure and function were determined by echocardiography, histopathology, staining for myocardial fibrosis (Sirius-Red) and investigating cardiac gene expression profiles by real-time PCR.

RESULTS

Two weeks after inducing CKD by 5/6 nephrectomy, rats manifested higher (mean ± SEM) systolic blood pressure (208 ± 4 versus 139 ± 3 mmHg; P < 0.01), serum FGF23 levels (1023 ± 225 versus 199 ± 9 pg/mL; P < 0.01) and LV mass (292 ± 9 versus 220 ± 3 mg; P < 0.01) when compared with sham-operated animals. Thereafter, 3 weeks of treatment with PD173074 compared with vehicle did not significantly change blood pressure, kidney function or metabolic parameters, but significantly reduced LV mass (230 ± 14 versus 341 ± 33 mg; P < 0.01), myocardial fibrosis (2.5 ± 0.7 versus 5.4 ± 0.95% staining/field; P < 0.01) and cardiac expression of genes associated with pathological LVH, while significantly increasing ejection fraction (18 versus 2.5% post-treatment increase; P < 0.05).

CONCLUSIONS

FGFR blockade improved cardiac structure and function in 5/6 nephrectomy rats with previously established LVH. These data support FGFR activation as a potentially modifiable, blood pressure-independent molecular mechanism of LVH in CKD.

Effect of niacin on FGF23 concentration in chronic kidney disease

BACKGROUND

Elevated serum phosphorus and FGF23 are independent cardiovascular risk factors in patients with chronic kidney disease. In a randomized controlled trial of patients with dyslipidemia assigned to either extended release niacin (ERN) alone, ERN combined with the selective prostaglandin D2 receptor subtype 1 inhibitor laropiprant (ERN-L) or placebo, niacin lowered serum phosphorus; however, it is not known if it lowers FGF23 concentrations.

METHODS

This is an ancillary study to a multicenter, randomized, double-blind, placebo-controlled trial among patients with dyslipidemia and an estimated glomerular filtration rate (eGFR) of 30-74 ml/min/1.73 m(2). Participants were randomized to ERN-L (n = 162), ERN (n = 97), or placebo (n = 68) in a 3:2:1 ratio for 24 weeks. The primary outcome was a change in serum FGF23 concentrations, and secondary outcomes were changes in other mineral metabolism parameters.

RESULTS

Both the ERN and ERN-L groups showed significant declines in serum phosphorus, calcium and calcium·phosphorus product at 24 weeks compared to placebo. A significant decline from baseline (10.9%, p < 0.01) in the serum FGF23 concentration was observed in the ERN group compared to placebo, but not in the ERN-L group compared to placebo (p = 0.36 and 0.97 for ERN-L and placebo, respectively), despite equivalent declines in serum phosphorus. Similarly, the most marked declines in PTH occurred in the ERN-only group versus placebo; no change in PTH was observed in the ERN-L group.

CONCLUSIONS

In this ancillary study of hyperlipidemic patients with an eGFR of 30-74 ml/min/1.73 m(2), ERN alone but not in combination with laropiprant lowered FGF23 and PTH concentrations. If confirmed, niacin may provide a novel strategy to decrease phosphorus, FGF23, and PTH concentrations in patients with chronic kidney disease.

Bone: a new endocrine organ at the heart of chronic kidney disease and mineral and bone disorders

Recent reports of several bone-derived substances, some of which have hormonal properties, have shed new light on the bone-cardiovascular axis. Deranged concentrations of humoral factors are not only epidemiologically connected to cardiovascular morbidity and mortality, but can also be causally implicated, especially in chronic kidney disease. FGF23 rises exponentially with advancing chronic kidney disease, seems to reach maladaptive concentrations, and then induces left ventricular hypertrophy, and is possibly implicated in the process of vessel calcification. Sclerostin and DKK1, both secreted mainly by osteocytes, are important Wnt inhibitors and as such can interfere with systems for biological signalling that operate in the vessel wall. Osteocalcin, produced by osteoblasts or released from mineralised bone, interferes with insulin concentrations and sensitivity, and its metabolism is disturbed in kidney disease. These bone-derived humoral factors might place the bone at the centre of cardiovascular disease associated with chronic kidney disease. Most importantly, factors that dictate the regulation of these substances in bone and subsequent secretion into the circulation have not been researched, and could provide entirely new avenues for therapeutic intervention.

Fibroblast growth factor 23 and left ventricular mass index in maintenance hemodialysis patients: standard versus long nocturnal hemodialysis

Elevated levels of fibroblast growth factor 23 (FGF23) and phosphorus (P) have been linked to greater risks of left ventricular hypertrophy (LVH) in patients with end stage renal disease (ESRD). The aim of this study was to test if differences exist in a long nocturnal HD group in comparison with a group treated with standard daily thrice weekly dialysis. The attempt was to evaluate if elevated FGF-23 levels, intact parathyroid hormone and P might be associated with left ventricular mass index (LVMI). Quantitative echocardiographic analyses were performed at baseline in 50 maintenance HD patients (17 women and 33 men, mean age: 56.4 ± 15.35 years, mean HD vintage: 9.06 ± 8.86 years, all patients are on HD thrice a week-median duration 15 h/week, 10 of them on long nocturnal HD, median duration 24 h/week). LVMIs were calculated. FGF23 was measured in duplicate using a second generation C-terminal enzyme-linked immunosorbent assay and log of FGF-23 values were computed. Mean LVMI was 136.44 ± 44.44 g/m(2) . Serum FGF-23 levels were elevated when compared to population data with preserved kidney function (median 1388.5 RU/mL, range 252 to 24 336 RU/mL). There were no correlations recorded between log FGF-23 levels and LVMI (r = 0.2, P = 0.66). LVMI was significantly lower in HD patients on long nocturnal dialysis procedure (r = -0.31, P = 0.05). Patients treated with long nocturnal HD showed lower LV mass, lower P-values and higher 25-OH-D3 supply. Plasma FGF-23 concentration was comparable between the groups and was not associated with LVMI in our maintenance HD patients.

Coronary artery calcification in chronic kidney disease: An update

Arterial calcification is a well-recognized complication of advanced atherosclerosis. Chronic kidney disease (CKD) is characterized by significantly more pronounced, disseminated and fast-progressing calcification of the vascular system, including the coronary arteries. New computed tomography-based imaging techniques allow for the noninvasive assessment and monitoring of calcification in different vascular sites. Coronary artery calcification (CAC) develops early in the course of CKD and is tightly associated with mineral and bone disorders, which include but are not limited to secondary hyperparathyroidism. In this review, recent data on the pathogenesis of CAC development and progression are discussed, with a special emphasis on fibroblast growth factor 23 and its co-receptor, klotho. The prevalence, progression and prognostic significance of CAC are reviewed separately for patients with end-stage renal disease treated with dialysis, kidney transplant recipients and patients with earlier stages of CKD. In the last section, therapeutic considerations are discussed, with special attention paid to the importance of treatment that addresses mineral and bone disorders of CKD.

Renal osteodystrophy in children: pathogenesis, diagnosis and treatment

PURPOSE OF REVIEW

Disturbances in calcium-phosphate homeostasis play an important role in children with chronic kidney disease, and not only cause renal osteodystrophy but also result in increased cardiovascular morbidity and mortality. This review outlines the current aspects in the pathogenesis, diagnostic approach and treatment of renal osteodystrophy.

RECENT FINDINGS

The pathogenesis of renal osteodystrophy is under strong influence of the fibroblast growth factor 23/Klotho system, which is able to enhance phosphate excretion and reduce calcitriol synthesis in the kidney. Fibroblast growth factor 23 increases tissue calcinosis and is cardiotoxic, and is independently associated with mortality. Despite improvement in diagnostic imaging (bone density measurements), determination of biomarkers, mainly parathyroid hormone, still plays a central role. New treatment options resulted in improved bone health and also a reduction in mortality was achieved in adults with calcium-free phosphate binders. Substitution of active and inactive vitamin D is important and also has a beneficial effect on proteinuria.

SUMMARY

Knowledge about the biochemical and molecular mechanisms of renal osteodystrophy is increasing dramatically and has an impact not only to bone health but also overall morbidity and mortality. This will ultimately translate into further improved diagnostic approaches and novel treatment options.

The extracellular matrix in the kidney: a source of novel non-invasive biomarkers of kidney fibrosis?

Interstitial fibrosis is the common endpoint of end-stage chronic kidney disease (CKD) leading to kidney failure. The clinical course of many renal diseases, and thereby of CKD, is highly variable. One of the major challenges in deciding which treatment approach is best suited for a patient but also in the development of new treatments is the lack of markers able to identify and stratify patients with stable versus progressive disease. At the moment renal biopsy is the only means of diagnosing renal interstitial fibrosis. Novel biomarkers should improve diagnosis of a disease, estimate its prognosis and assess the response to treatment, all in a non-invasive manner. Existing markers of CKD do not fully and specifically address these requirements and in particular do not specifically reflect renal fibrosis. The aim of this review is to give an insight of the involvement of the extracellular matrix (ECM) proteins in kidney diseases and as a source of potential novel biomarkers of renal fibrosis. In particular the use of the protein fingerprint technology, that identifies neo-epitopes of ECM proteins generated by proteolytic cleavage by proteases or other post-translational modifications, might identify such novel biomarkers of renal fibrosis.

Aging and chronic kidney disease

A recent report has dealt with geriatric nephrology, including epidemiology and pathophysiology of chronic kidney disease (CKD), attempting to get nephrologists to pay more attention to elderly CKD patients. The aims of this article are to summarize the morphological and functional properties of the aging kidney, and to better understand nephrology care for elderly CKD patients. The kidneys are affected by the aging process, which results in numerous effects on the renal system. In addition, the elderly population is hetereogenous - some have a decline in GFR explained by diseases that complicate aging such as arteriosclerosis with hypertension, whereas in the most of healthy adults the decline in GFR is much more modest and not inevitable. The values for normal estimated glomerular filtration rate (eGFR) in aging population have important implications for the diagnosis of CKD in the elderly. However, the MDRD equation underestimates mean eGFR by 25% and the CKD-EPI equation underestimates mean GFR by 16%. This bias may lead to misclassifying healthy older persons as having CKD. It is also still unknown whether and how age influences the predictive role of other risk factors for end-stage renal disease (ESRD) and death in referred as well as unreferred patients. The risk of ESRD was reported to be higher than the risk of death without ESRD for ages <60 years, and independent of eGFR. Proteinuria significantly increased the risk of ESRD with advancing age. In older patients on nephrology care, the risk of ESRD prevailed over mortality even when eGFR was not severely impaired. Proteinuria increases the risk of ESRD, while the predictive role of other modifiable risk factors was unchanged compared with younger patients. The decision to initiate renal replacement therapy in the elderly is complicated by more challenges than in younger patients. Calorie restriction and Klotho deficiency may be a candidate therapeutic target for attenuating kidney aging. © 2014 S. Karger AG, Basel.

Laboratory abnormalities in CKD-MBD: markers, predictors, or mediators of disease?

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is characterized by bone abnormalities, vascular calcification, and an array of laboratory abnormalities. The latter classically include disturbances in the parathyroid hormone/vitamin D axis. More recently, fibroblast growth factor 23 (FGF23) and klotho also have been identified as important regulators of mineral metabolism. Klotho deficiency and high circulating FGF23 levels precede secondary hyperparathyroidism in CKD patients. Levels of FGF23 and parathyroid hormone increase along the progression of CKD to maintain mineral homeostasis and to overcome end-organ resistance. It is hard to define when the increase of both hormones becomes maladaptive. CKD-MBD is associated with adverse outcomes including cardiovascular disease and mortality. This review summarizes the complex pathophysiology of CKD-MBD and outlines which laboratory abnormalities represent biomarkers of disease severity, which laboratory abnormalities are predictors of cardiovascular disease, and which laboratory abnormalities should be considered (direct) uremic toxins exerting organ damage. This information may help to streamline current and future therapeutic efforts.

High dietary phosphorus intake is associated with all-cause mortality: results from NHANES III

BACKGROUND

Elevated serum phosphorus is associated with all-cause mortality, but little is known about risk associated with dietary phosphorus intake.

OBJECTIVE

We investigated the association between phosphorus intake and mortality in a prospective cohort of healthy US adults (NHANES III; 1998-1994).

DESIGN

Study participants were 9686 nonpregnant adults aged 20-80 y without diabetes, cancer, or kidney or cardiovascular disease. Exposure to dietary phosphorus, which was assessed by using a 24-h dietary recall, was expressed as the absolute intake and phosphorus density (phosphorus intake divided by energy intake). All-cause and cardiovascular mortality was assessed through 31 December 2006.

RESULTS

Median phosphorus intake was 1166 mg/d (IQR: 823-1610 mg/d); median phosphorus density was 0.58 mg/kcal (0.48-0.70 mg/kcal). Individuals who consumed more phosphorus-dense diets were older, were less often African American, and led healthier lifestyles (smoking, physical activity, and Healthy Eating Index). In analyses adjusted for demographics, cardiovascular risk factors, kidney function, and energy intake, higher phosphorus intake was associated with higher all-cause mortality in individuals who consumed >1400 mg/d [adjusted HR (95% CI): 2.23 (1.09, 4.5) per 1-unit increase in ln(phosphorus intake); P = 0.03]. At <1400 mg/d, there was no association. A similar association was seen between higher phosphorus density and all-cause mortality at a phosphorus density amount >0.35 mg/kcal [adjusted HR (95% CI): 2.27 (1.19, 4.33) per 0.1-mg/kcal increase in phosphorus density; P = 0.01]. At <0.35 mg/kcal (approximately the fifth percentile), lower phosphorus density was associated with increased mortality risk. Phosphorus density was associated with cardiovascular mortality [adjusted HR (95% CI): 3.39 (1.43, 8.02) per 0.1 mg/kcal at >0.35 mg/kcal; P = 0.01], whereas no association was shown in analyses with phosphorus intake. Results were similar by subgroups of diet quality and in analyses adjusted for sodium and saturated fat intakes.

CONCLUSIONS

High phosphorus intake is associated with increased mortality in a healthy US population. Because of current patterns in phosphorus consumption in US adults, these findings may have important public health implications.

The impact of vitamin D status on the relative increase in fibroblast growth factor 23 and parathyroid hormone in chronic kidney disease

Fibroblast growth factor (FGF) 23 is an important regulator of phosphaturia. Its serum level was found to increase before that of parathyroid hormone (PTH) in early chronic kidney disease (CKD) in some but not all previous studies. As vitamin D insufficiency is associated with elevated PTH, we determined the effect of vitamin D status on FGF23 and PTH levels in relation to glomerular filtration rate (GFR) in people with CKD stage 3. Serum intact FGF23, PTH, and 25(OH)vitamin D3 were measured in 1664 patients who were prospectively recruited from primary care. Mean or median values for key variables were an age of 73 years, estimated GFR (eGFR) of 53 ml/min per 1.73 m(2), PTH 46 pg/ml, FGF23 42 pg/ml, and 25(OH)vitamin D3 53 nmol/l. FGF23 and PTH concentrations were elevated in similar proportions of people with lower eGFR in the whole cohort. However, when 752 people with vitamin D insufficiency or deficiency were excluded, FGF23 was elevated in a greater proportion than PTH at all levels of eGFR. Conversely, among people with vitamin D insufficiency, PTH was elevated in a greater proportion than FGF23 at all GFR levels of ⩾40 ml/min per 1.73 m(2). In this cohort, we were able to triangulate the relationship between 25(OH)vitamin D3, PTH, and FGF23, showing that vitamin D status critically determines whether FGF23 or PTH becomes elevated first in the context of lower GFR. Future studies of FGF23 in people with CKD should routinely determine their vitamin D status.

The role of the gastrointestinal tract in phosphate homeostasis in health and chronic kidney disease

Purpose of review

For a number of years, there has been increasing interest in the concept of directly targeting intestinal phosphate transport to control hyperphosphatemia in chronic kidney disease. However, progress has been slow due to the paucity of information on the mechanisms involved in intestinal phosphate absorption. This editorial highlights the most recent developments in our understanding of this process and the role of the intestine in the maintenance of phosphate balance.

Recent findings

Recent studies in NaPi-IIb knockout mice have confirmed that this transport protein plays a significant role in intestinal phosphate absorption and is critical in the proposed feed-forward mechanism between the small intestine and kidney, which helps to maintain normal phosphate balance and steady-state plasma phosphate concentrations. In addition, renal failure-induced hyperphosphatemia is attenuated in NaPi-IIb knockout mice, confirming that NaPi-IIb is a suitable target in the prevention and treatment of hyperphosphatemia.

Summary

Recent findings suggest that consumption of processed foods containing phosphate preservatives may lead to excessive phosphate exposure (if not overload), toxicity, and cardiovascular disease in the general population, as well as in patients with declining renal function. Therefore, establishing more effective ways of targeting the intestine to limit dietary phosphate absorption could have wide-reaching health benefits.

Ferric citrate hydrate for the treatment of hyperphosphatemia in nondialysis-dependent CKD

BACKGROUND AND OBJECTIVES

Ferric citrate hydrate is a novel iron-based phosphate binder being developed for hyperphosphatemia in patients with CKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A phase 3, multicenter, randomized, double blind, placebo-controlled study investigated the efficacy and safety of ferric citrate hydrate in nondialysis-dependent patients with CKD. Starting in April of 2011, 90 CKD patients (eGFR=9.21±5.72 ml/min per 1.73 m(2)) with a serum phosphate≥5.0 mg/dl were randomized 2:1 to ferric citrate hydrate or placebo for 12 weeks. The primary end point was change in serum phosphate from baseline to the end of treatment. Secondary end points included the percentage of patients achieving target serum phosphate levels (2.5-4.5 mg/dl) and change in fibroblast growth factor-23 at the end of treatment.

RESULTS

The mean change in serum phosphate was -1.29 mg/dl (95% confidence interval, -1.63 to -0.96 mg/dl) in the ferric citrate hydrate group and 0.06 mg/dl (95% confidence interval, -0.20 to 0.31 mg/dl) in the placebo group (P<0.001 for difference between groups). The percentage of patients achieving target serum phosphate levels was 64.9% in the ferric citrate hydrate group and 6.9% in the placebo group (P<0.001). Fibroblast growth factor-23 concentrations were significantly lower in patients treated with ferric citrate hydrate versus placebo (change from baseline [median], -142.0 versus 67.0 pg/ml; P<0.001). Ferric citrate hydrate significantly increased serum iron, ferritin, and transferrin saturation compared with placebo (P=0.001 or P<0.001). Five patients discontinued active treatment because of treatment-emergent adverse events with ferric citrate hydrate treatment versus one patient with placebo. Overall, adverse drug reactions were similar in patients receiving ferric citrate hydrate or placebo, with gastrointestinal disorders occurring in 30.0% of ferric citrate hydrate patients and 26.7% of patients receiving placebo.

CONCLUSION

In patients with nondialysis-dependent CKD, 12-week treatment with ferric citrate hydrate resulted in significant reductions in serum phosphate and fibroblast growth factor-23 while simultaneously increasing serum iron parameters.

Ramipril lowers plasma FGF-23 in patients with diabetic nephropathy

BACKGROUND/AIMS

Ramipril attenuates renal Fibroblast growth factor-23 (FGF-23) expression, ameliorates proteinuria and normalizes serum phosphate in the diabetic Zucker rat with progressive renal disease suggesting that the renoprotective effect by this drug may be in part due to a FGF-23-lowering effect of angiotensin-converting enzyme (ACE) inhibition.

METHODS

In this nonrandomized study, we tested whether ACE-inhibition reduces circulating FGF-23 in type-2 diabetics with stage-1 chronic kidney disease (CKD) and proteinuria. Intact FGF-23, the eGFR, proteinuria and the endothelium-dependent flow-mediated (FMD) response to ischemia and other parameters were measured at baseline and after 12-weeks of treatment with ramipril (n = 68) or amlodipine (n = 32).

RESULTS

Blood Pressure (BP) fell to a similar extent (p < 0.001) in the two groups. However, 24 h proteinuria and the FMD improved more (both p < 0.01) in ramipril-treated patients than in amlodipine-treated patients. Changes in proteinuria (r = 0.47) and in FMD (r = -0.49) by ramipril were closely associated (p < 0.001) with simultaneous changes in FGF-23 and this link was confirmed in multiple regression analyses. In these analyses, the relationship between FMD and proteinuria changes attained statistical significance (p < 0.01) only in a model excluding FGF-23 suggesting that endothelial dysfunction and FGF-23 share a common pathway conducive to renal damage.

CONCLUSION

Findings in this study contribute to generate the hypothesis that FGF-23 may be implicated in proteinuria and in endothelial dysfunction in diabetic nephropathy (clinicaltrials.gov (NCT01738945)).

High serum phosphorus and FGF 23 levels are associated with progression of coronary calcifications

BACKGROUND

Coronary calcifications (CC) portend increased mortality in adults receiving hemodialysis (HD), however the risk factors associated with CC progression are not well known in pediatric patients. Our previous cross-sectional studies demonstrated high CC prevalence (31 %) in pediatric patients, which were significantly associated with high serum phosphorus (P), fibroblast growth factor 23 (FGF) levels, dialysis vintage, and low cholesterol. The current study was undertaken to determine and elucidate CC progression in pediatric HD patients.

METHODS

A 1-year prospective longitudinal study of 16 pediatric patients (ten male; mean age, 16.9 ± 3 years; range, 10.1-20.4 years) receiving chronic HD was conducted.

RESULTS

CC were observed in five of 16 (31.3 %) patients on baseline computed tomogram (CT) scan; 14/16 patients underwent 1-year CT. All patients with initial CC who completed CT at 1 year (3/5) progressed; one patient had new CC and none of the patients had resolved CC. Mean Agatston score increased from 23.4 ± 18.06 HU (baseline) to 169 ± 298.9 HU. Patients with CC progression had higher mean serum P (8.6 ± 1.8 mg/dl vs. 6.3 ± 1.1 mg/dl, p = 0.015) and FGF 23 levels (3,994 ± 860.5 pg/ml vs. 2,327 ± 1,206.4 pg/ml, p = 0.028). Serum P and FGF 23 levels were positively correlated with final Agatston scores (R = 0.65, p = 0.01 for serum P and R = 0.54, p = 0.045 for FGF 23) and change in Agatston scores (R = 0.65, p = 0.01 for serum P and R = 0.52, p = 0.048 for FGF 23).

CONCLUSIONS

Our study shows that CC is progressive in pediatric patients receiving HD and that increased serum P and FGF 23 levels are associated with this progression.

High dose intravenous iron, mineral homeostasis and intact FGF23 in normal and uremic rats

BACKGROUND

High iron load might have a number of toxic effects in the organism. Recently intravenous (iv) iron has been proposed to induce elevation of fibroblast growth factor 23 (FGF23), hypophosphatemia and osteomalacia in iron deficient subjects. High levels of FGF23 are associated with increased mortality in the chronic kidney disease (CKD) population. CKD patients are often treated with iv iron therapy in order to maintain iron stores and erythropoietin responsiveness, also in the case of not being iron depleted. Therefore, the effect of a single high iv dose of two different iron preparations, iron isomaltoside 1000 (IIM) and ferric carboxymaltose (FCM), on plasma levels of FGF23 and phosphate was examined in normal and uremic iron repleted rats.

METHODS

Iron was administered iv as a single high dose of 80 mg/kg bodyweight and the effects on plasma levels of iFGF23, phosphate, Ca2+, PTH, transferrin, ferritin and iron were examined in short and long term experiments (n = 99). Blood samples were obtained at time 0, 30, 60, 180 minutes, 24 and 48 hours and in a separate study after 1 week. Uremia was induced by 5/6-nephrectomy.

RESULTS

Nephrectomized rats had significant uremia, hyperparathyroidism and elevated FGF23. Iron administration resulted in significant increases in plasma ferritin levels. No significant differences were seen in plasma levels of iFGF23, phosphate and PTH between the experimental groups at any time point within 48 hours or at 1 week after infusion of the iron compounds compared to vehicle.

CONCLUSIONS

In non-iron depleted normal and uremic rats a single high dose of either of two intravenous iron preparations, iron isomaltoside 1000, and ferric carboxymaltose, had no effect on plasma levels of iFGF23 and phosphate for up to seven days.

Phosphate wasting and fibroblast growth factor-23

PURPOSE OF REVIEW

The purpose of this study is to review the regulation of phosphate and recent progress in fibroblast growth factor-23 (FGF-23), a key phosphate regulatory hormone.

RECENT FINDINGS

Phosphate is required for mineralization of bone, muscle strength and a host of biologic functions. Phosphate is sensed by bone that responds with secretion of FGF-23. The major action of FGF-23 is to stimulate phosphaturia. Feedback loops between FGF-23, 1,25-dihydroxyvitamin D3 and parathyroid hormone maintain phosphate homeostasis. Information about FGF-23 has accumulated from studies in patients with oncogenic osteomalacia and inherited disorders of phosphate wasting rickets that explains the pathophysiology. Exciting new discoveries have highlighted FGF-23 as an independent risk factor for cardiovascular disease in patients with chronic kidney disease. The phosphate sensor triggering FGF-23 production remains to be identified.

SUMMARY

Derangements in FGF-23 production, half-life or downstream response are responsible for several disorders of phosphate wasting, rickets and oncogenic osteomalacia. Very high levels of FGF-23 in renal failure are an independent risk for cardiovascular disease.

Fibroblast growth factor 23

There is growing interest in the role of fibroblast growth factor 23 (FGF23) in various diseases of disordered mineral metabolism. In chronic kidney disease (CKD), where biochemical evidence of mineral disturbances is especially common, FGF23 measurement has been advocated as an early and sensitive marker for CKD-related bone disease. In this setting, FGF23 analysis may also improve the discrimination of risk of adverse renal and cardiovascular outcomes and aid targeting of those patients that are likely to benefit from interventions. Nonetheless, while the physiological relevance of FGF23 in the control of mineral metabolism is now firmly established, relatively little attention has been paid to important preanalytical and analytical aspects of FGF23 measurement that may impact on its clinical utility. Here we review these issues and discuss the suitability of FGF23 testing strategies for routine clinical practice. The current ‘state-of-the-art’ enzyme-linked immunosorbent assay methods for FGF23 measurement show poor agreement due to differences in FGF23 fragment detection, antibody specificity and calibration. Such analytical variability does not permit direct comparison of FGF23 measurements made with different assays and is likely to at least in part account for some of the inconsistencies noted between observational studies. From a clinical perspective, the lack of concordance has implications for the development of standardized reference intervals and clinical decision limits. Finally, the inherent assay-dependent biological variability of plasma FGF23 concentration can further complicate the interpretation of results and the design of FGF23-based testing protocols. Currently, it would be premature to consider incorporating FGF23 measurements into standard testing repertoires.

Dietary and pharmacological modification of fibroblast growth factor-23 in chronic kidney disease

Increased levels of phosphorus and fibroblast growth factor-23 (FGF-23) are strong predictors of cardiovascular morbidity and mortality. From a physiological perspective and supported by some data, phosphorus is the main driver for FGF-23 secretion. Therefore, it is conceivable that interventions aiming at restriction of phosphorus uptake from the gastrointestinal tract may lower serum FGF-23 levels and improve cardiovascular risk and subsequently survival. It is not currently known to what extend phosphorus and FGF-23 are independent risk factors, and therefore both need to be targeted. However, their respective metabolisms are tightly connected. Control of phosphorus levels in chronic kidney disease (CKD) patients is attempted mainly by restriction of dietary intake and the use of phosphorus binders. In this review, it is outlined that not just the amount of dietary phosphorus intake is important but also its type (organic vs. inorganic), its source (animal vs. plant derived), and the protein-to-phosphorus ratio in the bioavailability of phosphorus from food. This qualitative aspect of diet is likely a neglected aspect of dietary counseling in CKD. However, in more advanced stages of CKD, dietary restriction of phosphorus alone is usually not sufficient to control hyperphosphatemia, and phosphorus binders are indicated. The inexpensive, calcium-containing dietary phosphorus binders are used commonly worldwide. However, they are not suitable for every patient because of the association with elevated serum calcium, increase in vascular and valvular calcification scores, and cardiovascular and all-cause mortality. The calcium content itself in these binders has recently been implicated to upregulate FGF-23. For that reason, the noncalcium, aluminum-free agents such as sevelamer and lanthanum are being advocated. However, these drugs do not have a clearly defined effect on circulating levels of FGF-23. Although it is conceivable that targeting FGF-23 may lead to improved clinical outcomes, this remains speculative. Therefore, more studies are needed to answer the question if this can be achieved with any of the phosphorus binders, or by another (additional) pharmacological intervention.

The Association between Fibroblast Growth Factor-23 and Vascular Calcification Is Mitigated by Inflammation Markers

BACKGROUND

Fibroblast growth factor-23 (FGF-23) has been linked to vascular calcification, ventricular hypertrophy and mortality in chronic kidney disease (CKD), although these links may not be direct and independent. Similar grave outcomes are associated with inflammation and oxidative stress in CKD. Recently, accumulating evidence has linked components of phosphate homeostasis to inflammation and oxidative stress. The interaction between the triad of inflammation, FGF-23 and cardiovascular outcomes is underinvestigated.

METHODS

We studied 65 patients with stage 5 CKD on hemodialysis. Serum levels of FGF-23, high-sensitivity C-reactive protein (hsCRP), endogenous soluble receptor of advanced glycation end products (esRAGE), advanced oxidation protein products (AOPP), parathormone, lipids, calcium and phosphorous were measured. The aortic calcification index (ACI) was determined using non-contrast CT scans of the abdominal aorta.

RESULTS

FGF-23 was elevated (mean: 4,681 pg/ml, SD: 3,906) and correlated with hsCRP, esRAGE, AOPP, dialysis vintage and phosphorus in univariate analysis. In multiple regression analysis, hsCRP, AOPP and phosphorus but not esRAGE were all significantly correlated to FGF-23 (R² = 0.7, p < 0.001). In univariate analysis, ACI correlated with hsCRP, esRAGE, FGF-23, dialysis vintage, systolic blood pressure (BP) and serum cholesterol. In multiple regression analysis not including inflammation markers, ACI was associated with FGF-23. However, inclusion of inflammation markers in another multiple regression analyses showed that ACI correlated with hsCRP, BP, dialysis vintage and esRAGE but not with FGF-23 (R² = 0.65, p < 0.001).

CONCLUSION

FGF-23 is strongly correlated to various markers of inflammation and oxidative stress in hemodialysis patients. The association between FGF-23 and vascular calcification was mitigated when corrected for inflammation markers.

From bone abnormalities to mineral metabolism dysregulation in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic cause of kidney failure. It is a systemic disorder, not only affecting the kidneys, but also associated with cyst formation in other organs such as the liver, spleen, pancreas, and seminal vesicles. Other extra-renal symptoms may consist of intracranial arterial aneurysms, cardiac valvular defects, abdominal and inguinal hernias and colonic diverticulosis. Very little is known regarding bone involvement in ADPKD; however, recent evidence has revealed the potential role of fibroblast growth factor 23 (FGF23). FGF23 is an endocrine fibroblast growth factor acting in the kidney as a phosphaturic hormone and a suppressor of active vitamin D with key effects on the bone/kidney/parathyroid axis, and has been shown to increase in patients with ADPKD, even with normal renal function. The aim of this review is to provide an overview of bone and mineral abnormalities found in experimental models and in patients with ADPKD, and to discuss the possible role of FGF23 in this disease.

Fibroblast growth factor-23 and cardiovascular events in CKD

An elevated level of fibroblast growth factor-23 (FGF-23) is the earliest abnormality of mineral metabolism in CKD. High FGF-23 levels promote left ventricular hypertrophy but not coronary artery calcification. We used survival analysis to determine whether elevated FGF-23 is associated with greater risk of adjudicated congestive heart failure (CHF) and atherosclerotic events (myocardial infarction, stroke, and peripheral vascular disease) in a prospective cohort of 3860 participants with CKD stages 2-4 (baseline estimated GFR [eGFR], 44±15 ml/min per 1.73 m(2)). During a median follow-up of 3.7 years, 360 participants were hospitalized for CHF (27 events/1000 person-years) and 287 had an atherosclerotic event (22 events/1000 person-years). After adjustment for demographic characteristics, kidney function, traditional cardiovascular risk factors, and medications, higher FGF-23 was independently associated with graded risk of CHF (hazard ratio [HR], 1.45 per doubling [95% confidence interval (CI), 1.28 to 1.65]; HR for highest versus lowest quartile, 2.98 [95% CI, 1.97 to 4.52]) and atherosclerotic events (HR per doubling, 1.24 [95% CI, 1.09 to 1.40]; HR for highest versus lowest quartile, 1.76 [95% CI, 1.20 to 2.59]). Elevated FGF-23 was associated more strongly with CHF than with atherosclerotic events (P=0.02), and uniformly was associated with greater risk of CHF events across subgroups stratified by eGFR, proteinuria, prior heart disease, diabetes, BP control, anemia, sodium intake, income, fat-free mass, left ventricular mass index, and ejection fraction. Thus, higher FGF-23 is independently associated with greater risk of cardiovascular events, particularly CHF, in patients with CKD stages 2-4.

Phosphate restriction significantly reduces mortality in uremic rats with established vascular calcification

The role of hyperphosphatemia in the pathogenesis of secondary hyperparathyroidism, cardiovascular disease, and progression of renal failure is widely known. Here we studied effects of dietary phosphate restriction on mortality and vascular calcification in uremic rats. Control and uremic rats were fed a high-phosphate diet and at 3 months a portion of rats of each group were killed. Serum phosphate and the calcium phosphate product increased in uremic rats, as did aortic calcium. Of the rats, 56% had positive aortic staining for calcium (von Kossa), RUNX2, and osteopontin. The remaining uremic rats were continued on diets containing high phosphate without and with sevelamer, or low phosphate, and after 3 more months they were killed. Serum phosphate was highest in uremic rats on high phosphate. Serum PTH and FGF-23 were markedly lower in rats on low phosphate. Mortality on high phosphate was 71.4%, with sevelamer reducing this to 37.5% and phosphate restriction to 5.9%. Positive aortic staining for von Kossa, RUNX2, and osteopontin was increased, but phosphate restriction inhibited this. Kidneys from low-phosphate and sevelamer-treated uremic rats had less interstitial fibrosis, glomerulosclerosis, and inflammation than those of uremic rats on high phosphate. Importantly, kidneys from rats on low phosphate showed improvement over kidneys from high-phosphate rats at 3 months. Left ventricles from rats on low phosphate had less perivascular fibrosis and smaller cardiomyocyte size compared to rats on high phosphate. Thus, intensive phosphate restriction significantly reduces mortality in uremic rats with severe vascular calcification.

Associations between fibroblast growth factor 23 and cardiac characteristics in pediatric heart failure

BACKGROUND

In adults with heart failure, elevated levels of fibroblast growth factor 23 (FGF23) are associated with mortality. Data on FGF23 levels in pediatric heart failure are lacking.

PATIENTS AND METHODS

We conducted a cross-sectional study of 17 healthy children (mean age 13 years) and 20 pediatric patients with heart failure (mean age 12 years) who underwent echocardiography and for whom the following measurements were taken: plasma FGF23 and parathyroid hormone (PTH) and serum phosphate, creatinine and N-terminal prohormone brain natriuretic peptide (NT-proBNP). Symptom severity was assessed with the New York Heart Association and the Ross classification systems.

RESULTS

Of the 20 patients, 11 had dilated cardiomyopathy, four had congenital heart disease, three had hypertrophic cardiomyopathy, one had a failing heart transplant and one had pulmonary hypertension. Mean phosphate levels in these patients were within the reported reference range for healthy children. Median PTH levels were in the normal range in patients and controls. The median FGF23 level was higher in patients versus controls (110.9 vs. 66.4 RU/ml; P = 0.03) and higher in patients on diuretics versus other patients (222.4 vs. 82.1 RU/ml; P = 0.01). Levels of FGF23 and NT-proBNP were directly correlated (r = 0.47, P = 0.04), and patients with greater physical functional impairment had higher FGF23 levels (142.5 in those with moderate-severe limitation vs. 92.8 RU/ml in those with no limitation; P = 0.05). Among patients with dilated cardiomyopathy, higher FGF23 levels were associated with a greater left ventricular end-diastolic diameter (r = 0.63, P = 0.04).

CONCLUSION

FGF23 levels are elevated in children with heart failure and are associated with diuretic use, severity of heart failure and left ventricular dilation.

Association between circulating fibroblast growth factor 23, α-Klotho, and the left ventricular ejection fraction and left ventricular mass in cardiology inpatients

Background

Fibroblast growth factor 23 (FGF23), with its co-receptor Klotho, plays a crucial role in phosphate metabolism. Several recent studies suggested that circulating FGF23 and α-Klotho concentrations might be related to cardiovascular abnormalities in patients with advanced renal failure.

Purpose

Using data from 100 cardiology inpatients who were not undergoing chronic hemodialysis, the association of circulating levels of FGF23, α-Klotho, and other calcium-phosphate metabolism-related parameters with the left ventricular ejection fraction (LVEF) and left ventricular mass (LVM) was analyzed.

Methods and Results

LVEF was measured using the modified Simpson method for apical 4-chamber LV images and the LVM index (LVMI) was calculated by dividing the LVM by body surface area. Univariate analysis showed that log transformed FGF23, but not that of α-Klotho, was significantly associated with LVEF and LVMI with a standardized beta of −0.35 (P<0.001) and 0.26 (P<0.05), respectively. After adjusting for age, sex, estimated glomerular filtration rate, and serum concentrations of intact parathyroid hormone, and 25-hydroxyvitamin D as covariates into the statistical model, log-transformed FGF23 was found to be a statistically positive predictor for decreased left ventricular function and left ventricular hypertrophy.

Conclusions

In cardiology department inpatients, circulating FGF23 concentrations were found to be associated with the left ventricular mass and LVEF independent of renal function and other calcium-phosphate metabolism-related parameters. Whether modulation of circulating FGF23 levels would improve cardiac outcome in such a high risk population awaits further investigation.

Fibroblast growth factor 23 and cardiovascular mortality after kidney transplantation

BACKGROUND AND OBJECTIVES

Circulating fibroblast growth factor 23 (FGF23) is associated with adverse cardiovascular outcomes in CKD. Whether FGF23 predicts cardiovascular mortality after kidney transplantation, independent of measures of mineral metabolism and cardiovascular risk factors, is unknown.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The association between plasma C-terminal FGF23 and cardiovascular mortality was analyzed in a single-center prospective cohort of 593 stable kidney transplant recipients (mean age ± SD, 52 ± 12 years; 54% male; estimated GFR, 47 ± 16 ml/min per 1.73 m(2)), at a median of 6.1 (interquartile range, 2.7-11.7) years after transplantation. Multivariate Cox regression models were built, adjusting for measures of renal function and mineral metabolism; Framingham risk factors; the left ventricular wall strain markers midregional fragment of pro-A-type natriuretic peptide (MR-proANP) and N-terminal-pro brain natriuretic peptide (NT-proBNP); and copeptin, the stable C-terminal portion of the precursor of vasopressin.

RESULTS

In multivariate linear regression analysis, MR-proANP (β=0.20, P<0.001), NT-proBNP (β=0.18, P<0.001), and copeptin (β=0.26, P<0.001) were independently associated with FGF23. During follow-up for 7.0 (interquartile range, 6.2-7.5) years, 128 patients (22%) died, of whom 66 (11%) died due to cardiovascular disease; 54 (9%) had graft failure. FGF23 was associated with an higher risk of cardiovascular mortality in a fully adjusted multivariate Cox regression model (hazard ratio [HR], 1.88 [95% confidence interval (CI), 1.11 to 3.19]; P=0.02). FGF23 was also independently associated with all-cause mortality (full model HR, 1.86 [95% CI, 1.27 to 2.73]; P=0.001). Net reclassification improved for both cardiovascular mortality (HR, 0.07 [95% CI, 0.01 to 0.14]; P<0.05) and all-cause mortality (HR, 0.11 [95% CI, 0.05 to 0.18]; P<0.001).

CONCLUSIONS

Plasma FGF23 is independently associated with cardiovascular and all-cause mortality after kidney transplantation. The association remained significant after adjustment for measures of mineral metabolism and cardiovascular risk factors.

FGF-23 and secondary hyperparathyroidism in chronic kidney disease

The metabolic changes that occur in patients with chronic kidney disease (CKD) have a profound influence on mineral and bone metabolism. CKD results in altered levels of serum phosphate, vitamin D, calcium, parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF-23); the increased levels of serum phosphate, PTH and FGF-23 contribute to the increased cardiovascular mortality in affected patients. FGF-23 is produced by osteocytes and osteoblasts and acts physiologically in the kidney to induce phosphaturia and inhibit the synthesis of 1,25-dihydroxyvitamin D3. PTH acts directly on osteocytes to increase FGF-23 expression. In addition, the high levels of PTH associated with CKD contribute to changes in bone remodelling that result in decreased levels of dentin matrix protein 1 and the release of low-molecular-weight fibroblast growth factors from the bone matrix, which stimulate FGF-23 transcription. A prolonged oral phosphorus load increases FGF-23 expression by a mechanism that includes local changes in the ratio of inorganic phosphate to pyrophosphate in bone. Other factors such as dietary vitamin D compounds, calcium, and metabolic acidosis all increase FGF-23 levels. This Review discusses the mechanisms by which secondary hyperparathyroidism associated with CKD stimulates bone cells to overexpress FGF-23 levels.

Single FGF-23 measurement and time-averaged plasma phosphate levels in hemodialysis patients

BACKGROUND AND OBJECTIVES

Plasma phosphate levels display considerable intraindividual variability. The phosphatonin fibroblast growth factor 23 is a central regulator of plasma phosphate levels, and it has been postulated to be a more stable marker than conventional CKD-mineral and bone disorder parameters. Thus, fibroblast growth factor 23 has been hypothesized to reflect time-averaged plasma phosphate levels in CKD patients.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Among 40 patients from the outpatient dialysis center, serial measurements of plasma calcium and phosphate (before every dialysis session) as well as C-terminal fibroblast growth factor 23, parathyroid hormone, and alkaline phosphatase (one time weekly) were performed over a study period of 4 weeks in November and December of 2011. Intraindividual variability of repeated plasma fibroblast growth factor 23 measurements compared with other CKD-mineral and bone disorder markers was tested, and the association of a single plasma fibroblast growth factor 23 measurement with time-averaged plasma phosphate levels was analyzed.

RESULTS

Against expectations, intraindividual variability of fibroblast growth factor 23 (median coefficient of variation=27%; interquartile range=20-35) was not lower than variability of plasma phosphate (median coefficient of variation=15%; interquartile range=10-20), parathyroid hormone (median coefficient of variation=24%; interquartile range=15-39), plasma calcium (median coefficient of variation=3%; interquartile range=2-4), or alkaline phosphatase (median coefficient of variation=5%; interquartile range=3-10). Moreover, the correlation between the last fibroblast growth factor 23 measurement after 4 weeks and time-averaged plasma phosphate did not surpass the correlation between the last fibroblast growth factor 23 measurement and a single plasma phosphate value (r=0.67, P<0.001; r=0.76, P<0.001, respectively).

CONCLUSIONS

Surprisingly, fibroblast growth factor 23 was not more closely associated to time-averaged plasma phosphate levels than a single plasma phosphate value, and it did not show a lower intraindividual variability than other tested markers of CKD-mineral and bone disorder. Thus, fibroblast growth factor 23 should not be used in clinical practice as a reflector of time-averaged plasma phosphate levels.

Blood pressure, proteinuria, and phosphate as risk factors for progressive kidney disease: a hypothesis

Chronic kidney disease (CKD) affects approximately 500 million people worldwide and is increasingly common in both industrialized and emerging countries. Although the mechanisms underlying the inexorable progression of CKD are incompletely defined, recent discoveries may pave the way to a more comprehensive understanding of the pathophysiology of CKD progression and the development of new therapeutic strategies. In particular, there is accumulating evidence indicating a key role for the complex and yet incompletely understood system of divalent cation regulation, which includes phosphate metabolism and the recently discovered fibroblast growth factor 23 (FGF-23)/klotho system, which seems inextricably associated with vitamin D deficiency. The aim of this review is to discuss the links between high blood pressure, proteinuria, phosphate levels, and CKD progression and explore new therapeutic strategies to win the fight against CKD.