Chronic kidney disease (CKD) is a public health epidemic that affects millions of people worldwide. Presence of CKD predisposes individuals to high risks of end-stage renal disease (ESRD), ...cardiovascular disease, and premature death. Disordered phosphate homeostasis with elevated circulating levels of fibroblast growth factor 23 (FGF23) is an early and pervasive complication of CKD. CKD is likely the most common cause of chronically elevated FGF23 levels, and the clinical condition in which levels are most markedly elevated. Although increases in FGF23 levels help maintain serum phosphate in the normal range in CKD, prospective studies in populations of pre-dialysis CKD, incident and prevalent ESRD, and kidney transplant recipients demonstrate that elevated FGF23 levels are independently associated with progression of CKD and development of cardiovascular events and mortality. It was originally thought that these observations were driven by elevated FGF23 levels acting as a highly sensitive biomarker of toxicity due to phosphate. However, FGF23 itself has now been shown to mediate ‘off-target,’ direct, end-organ toxicity in the heart, which suggests that elevated FGF23 levels may be a novel mechanism of adverse outcomes in CKD. This report reviews recent advances in FGF23 biology relevant to CKD, the classical effects of FGF23 on mineral homeostasis, and the studies that established FGF23 excess as a biomarker and novel mechanism of cardiovascular disease. The report concludes with a critical review of the effects of different therapeutic strategies targeting FGF23 reduction and how these might be leveraged in a future randomized trial aimed at improving outcomes in CKD.
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.
Chronic kidney disease (CKD) is a global health epidemic that accelerates cardiovascular disease, increases risk of infection, and causes anemia and bone disease, among other complications that ...collectively increase risk of premature death. Alterations in calcium and phosphate homeostasis have long been considered nontraditional risk factors for many of the most morbid outcomes of CKD. The discovery of fibroblast growth factor 23 (FGF23), which revolutionized the diagnosis and treatment of rare hereditary disorders of FGF23 excess that cause hypophosphatemic rickets, has also driven major paradigm shifts in our understanding of the pathophysiology and downstream end-organ complications of disordered mineral metabolism in CKD. As research of FGF23 in CKD has rapidly advanced, major new questions about its regulation and effects continuously emerge. These are promoting exciting innovations in laboratory, patient-oriented, and epidemiological research and stimulating clinical trials of new therapies and repurposing of existing ones to target FGF23.
Disturbances in phosphate homeostasis are common in patients with chronic kidney disease. As kidney function declines, circulating concentrations of phosphate and the phosphate-regulatory hormone, ...fibroblast growth factor (FGF)-23, rise progressively. Higher serum levels of phosphate and FGF-23 are associated with an increased risk of adverse outcomes, including all-cause mortality and cardiovascular events. The associations between higher FGF-23 levels and adverse cardiovascular outcomes are generally independent of serum phosphate levels, and might be strongest for congestive heart failure. Higher serum phosphate levels are also modestly associated with an increased risk of cardiovascular events even after accounting for FGF-23 levels. This observation suggests that FGF-23 and phosphate might promote distinct mechanisms of cardiovascular toxicity. Indeed, animal models implicate high serum phosphate as a mechanism of vascular calcification and endothelial dysfunction, whereas high levels of FGF-23 are implicated in left ventricular hypertrophy. These seemingly distinct, but perhaps additive, adverse effects of phosphate on the vasculature and FGF-23 on the heart suggest that future population-level and individual-level interventions will need to simultaneously target these molecules to reduce the risk of associated cardiovascular events.
Contemporary intravenous iron formulations allow administration of high doses of elemental iron and enable correction of total iron deficit in one or two infusions. An important but underappreciated ...complication of certain formulations is hypophosphatemia caused by increased secretion of the phosphaturic hormone, fibroblast growth factor 23 (FGF23). The pathophysiology of FGF23-induced hypophosphatemia due to certain intravenous iron formulations has been recently investigated in prospective clinical trials. To reach the correct diagnosis, clinicians must recognize the typical clinical manifestations of intravenous iron-induced hypophosphatemia and identify a specific pattern of biochemical changes (hyperphosphaturic hypophosphatemia triggered by high FGF23 that causes low 1,25 (OH)2 vitamin D, hypocalcemia and secondary hyperparathyroidism). Physicians and patients should be aware of hypophosphatemia as a common complication of intravenous iron therapy and monitor serum phosphate concentrations in patients receiving repeated doses of specific intravenous iron formulations. Symptoms of hypophosphatemia are associated with severity and duration. Persistent hypophosphatemia can occur with iron therapy and can cause debilitating diseases including myopathy, osteomalacia and fractures. This review summarizes the current understanding of the iron-phosphate axis as well as complications of intravenous iron-induced hypophosphatemia.
•Certain intravenous iron formulations frequently cause hypophosphatemia.•This is mediated by increased FGF23 which can cause secondary hyperparathyroidism.•Hypophosphatemia can be complicated by osteomalacia.•Recognition of this side-effect is needed to prevent and treat complications.
Circulating levels of fibroblast growth factor 23 (FGF23) are elevated in patients with chronic kidney disease (CKD), but the mechanisms are poorly understood. Here we tested whether inflammation and ...iron deficiency regulate FGF23. In wild-type mice, acute inflammation induced by single injections of heat-killed Brucella abortus or interleukin-1β (IL-1β) decreased serum iron within 6 h, and was accompanied by significant increases in osseous Fgf23 mRNA expression and serum levels of C-terminal FGF23, but no changes in intact FGF23. Chronic inflammation induced by repeated bacteria or IL-1β injections decreased serum iron, increased osseous Fgf23 mRNA, and serum C-terminal FGF23, but modestly increased biologically active, intact FGF23 serum levels. Chronic iron deficiency mimicked chronic inflammation. Increased osseous FGF23 cleavage rather than a prolonged half-life of C-terminal FGF23 fragments accounted for the elevated C-terminal FGF23 but near-normal intact FGF23 levels in inflammation. IL-1β injection increased Fgf23 mRNA and C-terminal FGF23 levels similarly in wildtype and Col4a3ko mice with CKD but markedly increased intact FGF23 levels only in the CKD mice. Inflammation increased Fgf23 transcription by activating Hif1α signaling. Thus, inflammation and iron deficiency stimulate FGF23 production. Simultaneous upregulation of FGF23 cleavage in osteocytes maintains near-normal levels of biologically active, intact circulating FGF23, whereas downregulated or impaired FGF23 cleavage may contribute to elevated intact serum FGF23 in CKD.
ABSTRACT
Fibroblast growth factor 23 (FGF23) is an osteocyte‐derived hormone that regulates phosphate and vitamin D homeostasis. Through unknown mechanisms, certain intravenous iron preparations ...induce acute, reversible increases in circulating FGF23 levels that lower serum phosphate in association with inappropriately low levels of calcitriol, similar to genetic diseases of primary FGF23 excess. In contrast, studies in wild‐type mice suggest that iron deficiency stimulates fgf23 transcription but does not result in hypophosphatemia because FGF23 is cleaved within osteocytes by an unknown catabolic system. We tested the association of iron deficiency anemia with C‐terminal FGF23 (cFGF23) and intact FGF23 (iFGF23) levels in 55 women with a history of heavy uterine bleeding, and assessed the longitudinal biochemical response over 35 days to equivalent doses of randomly‐assigned, intravenous elemental iron in the form of ferric carboxymaltose (FCM) or iron dextran. Iron deficiency was associated with markedly elevated cFGF23 (807.8 ± 123.9 relative units RU/mL) but normal iFGF23 (28.5 ± 1.1 pg/mL) levels at baseline. Within 24 hours of iron administration, cFGF23 levels fell by approximately 80% in both groups. In contrast, iFGF23 transiently increased in the FCM group alone, and was followed by a transient, asymptomatic reduction in serum phosphate <2.0 mg/dL in 10 women in the FCM group compared to none in the iron dextran group. Reduced serum phosphate was accompanied by increased urinary fractional excretion of phosphate, decreased calcitriol levels, and increased parathyroid hormone levels. These findings suggest that iron deficiency increases cFGF23 levels, and that certain iron preparations temporarily increase iFGF23 levels. We propose that intravenous iron lowers cFGF23 in humans by reducing fgf23 transcription as it does in mice, whereas carbohydrate moieties in certain iron preparations may simultaneously inhibit FGF23 degradation in osteocytes leading to transient increases in iFGF23 and reduced serum phosphate.
To present emerging data on the role of fibroblast growth factor 23 (FGF23) in mineral metabolism and adverse outcomes in chronic kidney disease (CKD).
FGF23 regulates phosphorus and vitamin D ...metabolism. Its levels increase progressively beginning in early CKD, presumably as a physiological adaptation to maintain normal serum phosphate levels or normal phosphorus balance. FGF23 promotes phosphaturia and decreases production of calcitriol. Recent studies suggest that increased FGF23 is associated with mortality, left ventricular hypertrophy, endothelial dysfunction and progression of CKD. These results were consistently independent of serum phosphate levels.
FGF23 is emerging as a novel and exciting biomarker that may help identify which CKD patients might benefit most from aggressive management of disordered phosphorus metabolism. Future studies should determine whether increased FGF23 levels exert direct end-organ toxicity, such as in the heart, vessels and kidneys.
Background Recent studies have indicated that phosphorus may play an independent pathogenic role in chronic kidney disease (CKD) progression, but some of those studies were underpowered and yielded ...inconsistent results. Study Design Systematic review and meta-analysis. Setting & Population Non–dialysis-dependent patients with CKD (transplant recipients were excluded). Selection Criteria for Studies Studies assessing the risk ratio of serum phosphorus level on kidney failure and mortality for non–dialysis-dependent patients with CKD published from January 1950 to June 2014 were included following systematic searching of MEDLINE, EMBASE, and the Cochrane Library. Predictor Serum phosphorus level. Outcome Kidney failure, defined as doubled serum creatinine level, 50% decline in estimated glomerular filtration rate, or end-stage kidney disease. Results In 12 cohort studies with 25,546 patients, 1,442 (8.8%) developed kidney failure and 3,089 (13.6%) died. Overall, every 1-mg/dL increase in serum phosphorus level was associated independently with increased risk of kidney failure (hazard ratio, 1.36; 95% CI, 1.20-1.55) and mortality (hazard ratio, 1.20; 95% CI, 1.05-1.37). Limitations Existence of potential residual confounding could not be excluded. Conclusions This meta-analysis suggests an independent association between serum phosphorus level and kidney failure and mortality among non–dialysis-dependent patients with CKD and suggests that large-scale randomized controlled trials should target disordered phosphorus homeostasis in CKD.
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