Effects of erythropoietin on fibroblast growth factor 23 in mice and humans

Nephrectomy Does not Exacerbate Cancellous Bone loss in Thalassemic Mice

Patients with β-thalassemia have an increased risk of developing chronic kidney disease which is associated with osteoporosis and periodontitis. The purpose of this study was to evaluate mandibular and femoral bone change in heterozygous β-globin knockout (BKO) mice following 5/6 nephrectomy (Nx). Female and male BKO mouse blood smears demonstrated microcytic hypochromic anemia. Serum urea nitrogen, creatinine, calcium, and phosphorus levels were not changed in BKO mice. Nx increased the serum levels of urea nitrogen in both wild type (WT) and BKO mice and the level was much higher in BKO males. Serum level of creatinine was increased in Nx WT but not BKO mice. However, serum calcium and phosphorus levels were not altered. Nx induced comparable renal fibrosis in BKO mice and WT controls. Bone loss was observed in mandibular cancellous bone but not cortical bone of both male and female BKO mice. Nx decreased cancellous bone volume and cortical thickness in WT. Interestingly, BKO mice were resistant to Nx-induced cancellous bone loss. However, cortical thickness and cortical bone mineral density were reduced in Nx male BKO mice. Nx increased mRNA levels of type I collagen, Osx and Trap in WT but not BKO mice. Similarly, Nx reduced cancellous bone volume in femurs and increased osteoblast number and osteoclast number in WT not BKO mice. Serum FGF23 and erythropoietin levels were markedly increased in BKO mice. Nx decreased serum erythropoietin but not FGF23 levels. Since WT treated with erythropoietin exhibited a significant reduction in cancellous bone volume, it was possible that lower level of erythropoietin in Nx BKO mice prevented the Nx-induced cancellous bone loss.

Crosstalk between fibroblast growth factor 23, iron, erythropoietin, and inflammation in kidney disease

PURPOSE OF REVIEW

Recent research has revealed that regulation of the bone-secreted hormone fibroblast growth factor 23 (FGF23) is not limited to classical mineral factors. Specifically, bidirectional relationships have been described between FGF23 production and anemia, iron status, and inflammation. Here, we will review the latest published articles on the crosstalk between FGF23 and the aforementioned nonclassical factors.

RECENT FINDINGS

It has been recently reported that erythropoietin, iron deficiency, and inflammation increase FGF23 production and metabolism. Moreover, FGF23 promotes anemia and regulates inflammatory responses. These findings are particularly important in the setting of chronic kidney disease which is characterized by elevated FGF23 levels and several associated comorbidities.

SUMMARY

Regulation of FGF23 is complex and involves many bone and renal factors. More recently, erythropoietin, iron deficiency, and inflammation have been also shown to affect FGF23 transcription and cleavage. Importantly, FGF23 has emerged as a regulator of erythropoiesis, iron metabolism, and inflammation. These findings provide novel and important insights into the pathophysiologic mechanisms of chronic kidney disease and may present new opportunities for therapeutic clinical interventions.

High Plasma Erythropoietin Predicts Incident Fractures in Elderly Men with Normal Renal Function: The MrOS Sweden Cohort

Preclinical studies on the role of erythropoietin (EPO) in bone metabolism are contradictory. Regeneration models indicate an anabolic effect on bone healing, whereas models on physiologic bone remodeling indicate a catabolic effect on bone mass. No human studies on EPO and fracture risk are available. It is known that fibroblast growth factor 23 (FGF23) affects bone mineralization and that serum concentration of FGF23 is higher in men with decreased estimated glomerular filtration rate (eGFR). Recently, a direct association between EPO and FGF23 has been shown. We have explored the potential association between EPO and bone mineral density (BMD), fracture risk, and FGF23 in humans. Plasma levels of EPO were analyzed in 999 men (aged 69 to 81 years), participating in the Gothenburg part of the population-based Osteoporotic Fractures in Men (MrOS) study, MrOS Sweden. The mean ± SD EPO was 11.5 ± 9.0 IU/L. Results were stratified by eGFR 60 mL/min. For men with eGFR ≥60 mL/min (n = 728), EPO was associated with age (r = 0.13, p < 0.001), total hip BMD (r = 0.14, p < 0.001), intact (i)FGF23 (r = 0.11, p = 0.004), and osteocalcin (r = -0.09, p = 0.022). The association between total hip BMD and EPO was independent of age, body mass index (BMI), iFGF23, and hemoglobin (beta = 0.019, p < 0.001). During the 10-year follow-up, 164 men had an X-ray-verified fracture, including 117 major osteoporotic fractures (MOF), 39 hip fractures, and 64 vertebral fractures. High EPO was associated with higher risk for incident fractures (hazard ratio [HR] = 1.43 per tertile EPO, 95% confidence interval [CI] 1.35-1.63), MOF (HR = 1.40 per tertile EPO, 95% CI 1.08-1.82), and vertebral fractures (HR = 1.42 per tertile EPO, 95% CI 1.00-2.01) in a fully adjusted Cox regression model. In men with eGFR<60 mL/min, no association was found between EPO and BMD or fracture risk. We here demonstrate that high levels of EPO are associated with increased fracture risk and increased BMD in elderly men with normal renal function. © 2019 American Society for Bone and Mineral Research.

Changes in Fibroblast Growth Factor 23 and Soluble Klotho Levels After Hemodialysis Initiation

Rationale & Objective

Patients with chronic kidney failure have markedly elevated fibroblast growth factor 23 (FGF-23) levels and decreased soluble Klotho levels. However, no studies have examined the effects of hemodialysis initiation on the levels of these hormones and other parameters of mineral metabolism.

Study Design

Prospective single-arm study.

Setting & Participants

20 individuals with incident kidney failure initiating hemodialysis.

Exposure

Initiation of hemodialysis. Dose adjustments of phosphate binders and vitamin D receptor activators and use of calcimimetics, erythropoiesis-stimulating agents, and intravenous iron were prohibited.

Outcomes

Changes in serum levels of FGF-23, soluble Klotho, and other biochemical parameters of mineral metabolism, measured before and after each hemodialysis session, for a total of 4 sessions over 5 days.

Analytical Approach

Repeated-measures analysis of variance.

Results

At baseline, participants had 18-fold higher median FGF-23 levels and 1.6-fold lower mean soluble Klotho levels compared with age- and sex-matched healthy individuals. Initiation of hemodialysis led to progressive reductions in serum phosphorus, intact parathyroid hormone, and FGF-23 levels, with dialysis-related fluctuations. No reductions were observed in levels of α₁-microglobulin, which has molecular weight comparable to FGF-23. The magnitude of the FGF-23 level reductions was strongly associated with concomitant changes in serum phosphorus levels but not with the changes in intact parathyroid hormone levels. Soluble Klotho levels did not change after the initiation of hemodialysis.

Limitations

Single-arm design, small sample size, short follow-up period.

Conclusions

Initiation of hemodialysis in patients with chronic kidney failure led to progressive reductions in FGF-23 levels in association with reductions in serum phosphorus levels. These results suggest that phosphorus is a strong inducer of FGF-23 production and that regulation of FGF-23 production is a rapid process.

Does a rise in plasma erythropoietin after high-altitude exposure affect FGF23 in healthy volunteers on a normal or low-phosphorus diet?

BACKGROUND AND AIMS

Data of experimental rodent models suggest that hypoxia with subsequent increase in erythropoietin stimulates the expression of the phosphaturic hormone fibroblast growth factor 23 (FGF23).

METHODS AND RESULTS

To translate the findings of animal studies into human physiology, herein we exposed eight healthy volunteers to high altitude (2656 m above sea level) for four days. The volunteers were randomized on a low-phosphorous diet (n = 4) or a normal phosphorus diet (n = 4). Although high-altitude exposure caused a significant increase in plasma erythropoietin (EPO) (before high-altitude exposure: low phosphorus: median EPO 6.6 mIU/ml [interquartile range (IQR) 6.0; 8.2], normal phosphorus: median EPO 9.0 mIU/ml [IQR 7.9; 11.5]; at day 2: low phosphorus: median EPO 21.3 mIU/ml [IQR 19.5; 23.8], normal phosphorus: median EPO 19.4 mIU/ml [IQR 18.0; 20.8]), there was no consistent increase in plasma c-terminal FGF23 or plasma intact FGF23. We observed only a single, intermittent peak in c-terminal FGF23 levels after 5 h of maximal aerobic exercise.

CONCLUSION

These data do not support a substantial effect of moderate hypoxia alone on the expression of FGF23, but they suggest that combined exercise and high-altitude exposure may temporarily induce FGF23 expression.

Elevated FGF23 and disordered renal mineral handling with reduced bone mineralization in chronically erythropoietin over-expressing transgenic mice

Fibroblast Growth Factor 23 (FGF23) is a phosphaturic factor causing increased renal phosphate excretion as well as suppression of 1,25 (OH)₂-vitamin D_3. Highly elevated FGF23 can promote development of rickets and osteomalacia. We and others previously reported that acute application of erythropoietin (EPO) stimulates FGF23 production. Considering that EPO is clinically used as chronic treatment against anemia, we used here the Tg6 mouse model that constitutively overexpresses human EPO in an oxygen-independent manner, to examine the consequences of long-term EPO therapy on mineral and bone metabolism. Six to eight weeks old female Tg6 mice showed elevated intact and C-terminal fragment of FGF23 but normal plasma levels of PTH, calcitriol, calcium and phosphate. Renal function showed moderate alterations with higher urea and creatinine clearance and mild albuminuria. Renal phosphate excretion was normal whereas mild hypercalciuria was found. Renal expression of the key proteins TRPV5 and calbindin D28k involved in active calcium reabsorption was reduced in Tg6 mice. Plasma levels of the bone turnover marker osteocalcin were comparable between groups. However, urinary excretion of deoxypyridinoline (DPD) was lower in Tg6 mice. MicroCT analysis showed reduced total, cortical, and trabecular bone mineral density in femora from Tg6 mice. Our data reveal that chronic elevation of EPO is associated with high FGF23 levels and disturbed mineral homeostasis resulting in reduced bone mineral density. These observations imply the need to study the impact of therapeutically applied EPO on bone mineralization in patients, especially those suffering from chronic kidney disease.

Anemia of Inflammation with An Emphasis on Chronic Kidney Disease

Iron is vital for a vast variety of cellular processes and its homeostasis is strictly controlled and regulated. Nevertheless, disorders of iron metabolism are diverse and can be caused by insufficiency, overload or iron mal-distribution in tissues. Iron deficiency (ID) progresses to iron-deficiency anemia (IDA) after iron stores are depleted. Inflammation is of diverse etiology in anemia of chronic disease (ACD). It results in serum hypoferremia and tissue hyperferritinemia, which are caused by elevated serum hepcidin levels, and this underlies the onset of functional iron-deficiency anemia. Inflammation is also inhibitory to erythropoietin function and may directly increase hepcidin level, which influences iron metabolism. Consequently, immune responses orchestrate iron metabolism, aggravate iron sequestration and, ultimately, impair the processes of erythropoiesis. Hence, functional iron-deficiency anemia is a risk factor for several ailments, disorders and diseases. Therefore, therapeutic strategies depend on the symptoms, severity, comorbidities and the associated risk factors of anemia. Oral iron supplements can be employed to treat ID and mild anemia particularly, when gastrointestinal intolerance is minimal. Intravenous (IV) iron is the option in moderate and severe anemic conditions, for patients with compromised intestinal integrity, or when oral iron is refractory. Erythropoietin (EPO) is used to treat functional iron deficiency, and blood transfusion is restricted to refractory patients or in life-threatening emergency situations. Despite these interventions, many patients remain anemic and do not respond to conventional treatment approaches. However, various novel therapies are being developed to treat persistent anemia in patients.

FGF23 at the crossroads of phosphate, iron economy and erythropoiesis

Fibroblast growth factor 23 (FGF23) was initially characterized as an important regulator of phosphate and calcium homeostasis. New research advances demonstrate that FGF23 is also linked to iron economy, inflammation and erythropoiesis. These advances have been fuelled, in part, by the serendipitous development of two distinct FGF23 assays that can substitute for invasive bone biopsies to infer the activity of the three main steps of FGF23 regulation in bone: transcription, post-translational modification and peptide cleavage. This 'liquid bone biopsy for FGF23 dynamics' enables large-scale longitudinal studies of FGF23 regulation that would otherwise be impossible in humans. The balance between FGF23 production, post-translational modification and cleavage is maintained or perturbed in different hereditary monogenic conditions and in acquired conditions that mimic these genetic disorders, including iron deficiency, inflammation, treatment with ferric carboxymaltose and chronic kidney disease. Looking ahead, a deeper understanding of the relationships between FGF23 regulation, iron homeostasis and erythropoiesis can be leveraged to devise novel therapeutic targets for treatment of anaemia and states of FGF23 excess, including chronic kidney disease.

Plasma total fibroblast growth factor 23 levels are associated with acute kidney injury and mortality in children with acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) has high rates of mortality and multisystem morbidity. Pre-clinical data suggest that fibroblast growth factor 23 (FGF23) may contribute to pulmonary pathology, and FGF23 is associated with mortality and morbidity, including acute kidney injury (AKI), in non-ARDS cohorts. Here, we assess whether FGF23 is associated with AKI and/or mortality in a cohort of 161 pediatric ARDS patients. Plasma total (intact + C-terminal) FGF23 and intact FGF23 concentrations were measured within 24 hours of ARDS diagnosis (Day 1), and associations with Day 3 AKI and 60-day mortality were evaluated. 35 patients (22%) developed AKI by 3 days post-ARDS diagnosis, and 25 (16%) died by 60 days post-ARDS diagnosis. In unadjusted models, higher Day 1 total FGF23 was associated with Day 3 AKI (odds ratio (OR) 2.22 [95% confidence interval (CI) 1.62, 3.03], p<0.001), but Day 1 intact FGF23 was not. In a model adjusted for demographics and disease severity, total FGF23 remained associated with AKI (OR 1.52 [95% CI 1.02, 2.26], p = 0.039). In unadjusted models, both higher Day 1 total and intact FGF23 were associated with 60-day mortality (OR 1.43 [95% CI 1.07, 1.91], p = 0.014; and OR 1.44 [95% CI 1.02, 2.05], p = 0.039, respectively). In the adjusted model, only total FGF23 remained associated with 60-day mortality (OR 1.62 [95% CI 1.07, 2.45], p = 0.023). In a subgroup analysis of patients with Day 1 plasma IL-6 concentrations available, inflammation partially mediated the association between total FGF23 and AKI. Our data suggest both inflammation-dependent and inflammation-independent associations between total FGF23 and clinical outcomes in pediatric ARDS patients.

The Role of Fibroblast Growth Factor 23 in Inflammation and Anemia

In patients with chronic kidney disease (CKD), adverse outcomes such as systemic inflammation and anemia are contributing pathologies which increase the risks for cardiovascular mortality. Amongst these complications, abnormalities in mineral metabolism and the metabolic milieu are associated with chronic inflammation and iron dysregulation, and fibroblast growth factor 23 (FGF23) is a risk factor in this context. FGF23 is a bone-derived hormone that is essential for regulating vitamin D and phosphate homeostasis. In the early stages of CKD, serum FGF23 levels rise 1000-fold above normal values in an attempt to maintain normal phosphate levels. Despite this compensatory action, clinical CKD studies have demonstrated powerful and dose-dependent associations between FGF23 levels and higher risks for mortality. A prospective pathomechanism coupling elevated serum FGF23 levels with CKD-associated anemia and cardiovascular injury is its strong association with chronic inflammation. In this review, we will examine the current experimental and clinical evidence regarding the role of FGF23 in renal physiology as well as in the pathophysiology of CKD with an emphasis on chronic inflammation and anemia.

Epoetin Beta and C-Terminal Fibroblast Growth Factor 23 in Patients With Chronic Heart Failure and Chronic Kidney Disease

Background In patients with chronic heart failure and chronic kidney disease, correction of anemia with erythropoietin-stimulating agents targeting normal hemoglobin levels is associated with an increased risk of cardiovascular morbidity and mortality. Emerging data suggest a direct effect of erythropoietin on fibroblast growth factor 23 (FGF23), elevated levels of which have been associated with adverse outcomes. We investigate effects of erythropoietin-stimulating agents in patients with both chronic heart failure and chronic kidney disease focusing on FGF23. Methods and Results In the EPOCARES (Erythropoietin in CardioRenal Syndrome) study, we randomized 56 anemic patients (median age 74 [interquartile range 69-80] years, 66% male) with both chronic heart failure and chronic kidney disease into 3 groups, of which 2 received epoetin beta 50 IU/kg per week for 50 weeks, and the third group served as control. Measurements were performed at baseline and after 2, 26, and 50 weeks. Data were analyzed using linear mixed-model analysis. After 50 weeks of erythropoietin-stimulating agent treatment, hematocrit and hemoglobin levels increased. Similarly, C-terminal FGF23 levels, in contrast to intact FGF23 levels, rose significantly due to erythropoietin-stimulating agents as compared with the controls. During median follow-up for 5.7 (2.0-5.7) years, baseline C-terminal FGF23 levels were independently associated with increased risk of mortality (hazard ratio 2.20; 95% CI, 1.35-3.59; P=0.002). Conclusions Exogenous erythropoietin increases C-terminal FGF23 levels markedly over a period of 50 weeks, elevated levels of which, even at baseline, are significantly associated with an increased risk of mortality. The current results, in a randomized trial setting, underline the strong relationship between erythropoietin and FGF23 physiology in patients with chronic heart failure and chronic kidney disease. Clinical Trial Registration URL: http://www.clinicaltrials.gov. Unique identifier: NCT00356733.

Iron deficiency, elevated erythropoietin, fibroblast growth factor 23, and mortality in the general population of the Netherlands: A cohort study

BACKGROUND

Emerging data in chronic kidney disease (CKD) patients suggest that iron deficiency and higher circulating levels of erythropoietin (EPO) stimulate the expression and concomitant cleavage of the osteocyte-derived, phosphate-regulating hormone fibroblast growth factor 23 (FGF23), a risk factor for premature mortality. To date, clinical implications of iron deficiency and high EPO levels in the general population, and the potential downstream role of FGF23, are unclear. Therefore, we aimed to determine the associations between iron deficiency and higher EPO levels with mortality, and the potential mediating role of FGF23, in a cohort of community-dwelling subjects.

METHODS AND FINDINGS

We analyzed 6,544 community-dwelling subjects (age 53 ± 12 years; 50% males) who participated in the Prevention of Renal and Vascular End-Stage Disease (PREVEND) study-a prospective population-based cohort study, of which we used the second survey (2001-2003)-and follow-up was performed for a median of 8 years. We measured circulating parameters of iron status, EPO levels, and plasma total FGF23 levels. Our primary outcome was all-cause mortality. In multivariable linear regression analyses, ferritin (ß = -0.43), transferrin saturation (TSAT) (ß = -0.17), hepcidin (ß = -0.36), soluble transferrin receptor (sTfR; ß = 0.33), and EPO (ß = 0.28) were associated with FGF23 level, independent of potential confounders. During median (interquartile range [IQR]) follow-up of 8.2 (7.7-8.8) years, 379 (6%) subjects died. In multivariable Cox regression analyses, lower levels of TSAT (hazard ratio [HR] per 1 standard deviation [SD], 0.84; 95% confidence interval [CI], 0.75-0.95; P = 0.004) and higher levels of sTfR (HR, 1.15; 95% CI 1.03-1.28; P = 0.01), EPO (HR, 1.17; 95% CI 1.05-1.29; P = 0.004), and FGF23 (HR, 1.20; 95% CI 1.10-1.32; P < 0.001) were each significantly associated with an increased risk of death, independent of potential confounders. Adjustment for FGF23 levels markedly attenuated the associations of TSAT (HR, 0.89; 95% CI 0.78-1.01; P = 0.06), sTfR (HR, 1.08; 95% CI 0.96-1.20; P = 0.19), and EPO (HR, 1.10; 95% CI 0.99-1.22; P = 0.08) with mortality. FGF23 remained associated with mortality (HR, 1.15; 95% CI 1.04-1.27; P = 0.008) after adjustment for TSAT, sTfR, and EPO levels. Mediation analysis indicated that FGF23 explained 31% of the association between TSAT and mortality; similarly, FGF23 explained 32% of the association between sTfR and mortality and 48% of the association between EPO and mortality (indirect effect P < 0.05 for all analyses). The main limitations of this study were the observational study design and the absence of data on intact FGF23 (iFGF23), precluding us from discerning whether the current results are attributable to an increase in iFGF23 or in C-terminal FGF23 fragments.

CONCLUSIONS AND RELEVANCE

In this study, we found that functional iron deficiency and higher EPO levels were each associated with an increased risk of death in the general population. Our findings suggest that FGF23 could be involved in the association between functional iron deficiency and increased EPO levels and death. Investigation of strategies aimed at correcting iron deficiency and reducing FGF23 levels is warranted.

The EPO-FGF23 Signaling Pathway in Erythroid Progenitor Cells: Opening a New Area of Research

We provide an overview of the evidence for an erythropoietin-fibroblast growth factor 23 (FGF23) signaling pathway directly influencing erythroid cells in the bone marrow. We outline its importance for red blood cell production, which might add, among others, to the understanding of bone marrow responses to endogenous erythropoietin in rare hereditary anemias. FGF23 is a hormone that is mainly known as the core regulator of phosphate and vitamin D metabolism and it has been recognized as an important regulator of bone mineralization. Osseous tissue has been regarded as the major source of FGF23. Interestingly, erythroid progenitor cells highly express FGF23 protein and carry the FGF receptor. This implies that erythroid progenitor cells could be a prime target in FGF23 biology. FGF23 is formed as an intact, biologically active protein (iFGF23) and proteolytic cleavage results in the formation of the presumed inactive C-terminal tail of FGF23 (cFGF23). FGF23-knockout or injection of an iFGF23 blocking peptide in mice results in increased erythropoiesis, reduced erythroid cell apoptosis and elevated renal and bone marrow erythropoietin mRNA expression with increased levels of circulating erythropoietin. By competitive inhibition, a relative increase in cFGF23 compared to iFGF23 results in reduced FGF23 receptor signaling and mimics the positive effects of FGF23-knockout or iFGF23 blocking peptide. Injection of recombinant erythropoietin increases FGF23 mRNA expression in the bone marrow with a concomitant increase in circulating FGF23 protein. However, erythropoietin also augments iFGF23 cleavage, thereby decreasing the iFGF23 to cFGF23 ratio. Therefore, the net result of erythropoietin is a reduction of iFGF23 to cFGF23 ratio, which inhibits the effects of iFGF23 on erythropoiesis and erythropoietin production. Elucidation of the EPO-FGF23 signaling pathway and its downstream signaling in hereditary anemias with chronic hemolysis or ineffective erythropoiesis adds to the understanding of the pathophysiology of these diseases and its complications; in addition, it provides promising new targets for treatment downstream of erythropoietin in the signaling cascade.

FGF23, Biomarker or Target?

Fibroblast growth factor 23 (FGF23) plays a key role in the complex network between the bones and other organs. Initially, it was thought that FGF23 exclusively regulated phosphate and vitamin D metabolism; however, recent research has demonstrated that an excess of FGF23 has other effects that may be detrimental in some cases. The understanding of the signaling pathways through which FGF23 acts in different organs is crucial to develop strategies aiming to prevent the negative effects associated with high FGF23 levels. FGF23 has been described to have effects on the heart, promoting left ventricular hypertrophy (LVH); the liver, leading to production of inflammatory cytokines; the bones, inhibiting mineralization; and the bone marrow, by reducing the production of erythropoietin (EPO). The identification of FGF23 receptors will play a remarkable role in future research since its selective blockade might reduce the adverse effects of FGF23. Patients with chronic kidney disease (CKD) have very high levels of FGF23 and may be the population suffering from the most adverse FGF23-related effects. The general population, as well as kidney transplant recipients, may also be affected by high FGF23. Whether the association between FGF23 and clinical events is causal or casual remains controversial. The hypothesis that FGF23 could be considered a therapeutic target is gaining relevance and may become a promising field of investigation in the future.

FGF23 Synthesis and Activity

PURPOSE OF REVIEW

The phosphaturic hormone FGF23 is produced primarily in osteoblasts/osteocytes and is known to respond to increases in serum phosphate and 1,25(OH)2 vitamin D (1,25D). Novel regulators of FGF23 were recently identified, and may help explain the pathophysiologies of several diseases. This review will focus on recent studies examining the synthesis and actions of FGF23.

RECENT FINDINGS

The synthesis of FGF23 in response to 1,25D is similar to other steroid hormone targets, but the cellular responses to phosphate remain largely unknown. The activity of intracellular processing genes control FGF23 glycosylation and phosphorylation, providing critical functions in determining the serum levels of bioactive FGF23. The actions of FGF23 largely occur through its co-receptor αKlotho (KL) under normal circumstances, but FGF23 has KL-independent activity during situations of high concentrations.

SUMMARY

Recent work regarding FGF23 synthesis and bioactivity, as well as considerations for diseases of altered phosphate balance will be reviewed.

Regulation of Fibroblast Growth Factor 23 by Iron, EPO, and HIF

Purpose of review

Fibroblast growth factor-23 (FGF23) is the key hormone produced in bone critical for phosphate homeostasis. Elevated serum phosphorus and 1,25dihydroxyvitaminD stimulates FGF23 production to promote renal phosphate excretion and decrease 1,25dihydroxyvitaminD synthesis. Thus completing the feedback loop and suppressing FGF23. Unexpectedly, studies of common and rare heritable disorders of phosphate handling identified links between iron and FGF23 demonstrating novel regulation outside the phosphate pathway.

Recent Findings

Iron deficiency combined with an FGF23 cleavage mutation was found to induce the autosomal dominant hypophosphatemic rickets phenotype. Physiological responses to iron deficiency, such as erythropoietin production as well as hypoxia inducible factor activation, have been indicated in regulating FGF23. Additionally, specific iron formulations, used to treat iron deficiency, alter post-translational processing thereby shifting FGF23 protein secretion.

Summary

Molecular and clinical studies revealed that iron deficiency, through several mechanisms, alters FGF23 at the transcriptional and post-translational level. This review will focus upon the novel discoveries elucidated between iron, its regulators, and their influence on FGF23 bioactivity.

Non-renal-Related Mechanisms of FGF23 Pathophysiology

PURPOSE OF REVIEW

We will review non-renal-related mechanisms of fibroblast growth factor 23 (FGF23) pathophysiology.

RECENT FINDINGS

FGF23 production and metabolism may be affected by many bone, mineral, and kidney factors. However, it has recently been demonstrated that other factors, such as iron status, erythropoietin, and inflammation, also affect FGF23 production and metabolism. As these non-mineral factors are especially relevant in the setting of chronic kidney disease (CKD), they may represent emerging determinants of CKD-associated elevated FGF23 levels. Moreover, FGF23 itself may promote anemia and inflammation, thus contributing to the multifactorial etiologies of these CKD-associated comorbidities. CKD-relevant, non-mineral-related, bidirectional relationships exist between FGF23 and anemia, and between FGF23 and inflammation. Iron deficiency, anemia, and inflammation affect FGF23 production and metabolism, and FGF23 itself may contribute to anemia and inflammation, highlighting complex interactions that may affect aspects of CKD pathogenesis and treatment.