FGF23 induces left ventricular hypertrophy

Update on fibroblast growth factor 23 in chronic kidney disease

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, 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 end-stage renal disease, 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 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 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 helps explain the biphasic cardiovascular effects of vitamin D in chronic kidney disease

Hypovitaminosis D is highly prevalent in chronic kidney disease (CKD). Recently, vitamin D has sparked widespread interest because of its potential favorable benefits on cardiovascular disease (CVD). Evidence from clinical studies and animal models supports the existence of biphasic cardiovascular effects of vitamin D, in which lower doses suppress CVD and higher doses stimulate CVD. However, the mechanism for the different effects remains unclear. Fibroblast growth factor-23 (FGF-23) is a recently identified member of the FGF family, and thought to be actively involved in renal phosphate and vitamin D homeostasis. More specifically, Vitamin D stimulates FGF-23 secretion and is inhibited by increased FGF-23. Given this background, we hypothesize that FGF-23 may provide a unique tool to explain the biphasic cardiovascular effects of vitamin D in CKD. The data presented in this review support the hypothesis that FGF-23 may be linked with the high cardiovascular risk in CKD through accelerating the onset of vascular calcification, secondary hyperparathyroidism, left ventricular hypertrophy and endothelial dysfunction. Therefore, modulation of FGF-23 may become a potential therapeutic target to lowing cardiovascular risk in CKD. Several clinical interventions, including decreased phosphate intake, phosphate binders, cinacalcet plus concurrent low-dose vitamin D, C-terminal tail of FGF-23 and renal transplantation, have been employed to manipulate FGF-23.

Fibroblast growth factor 23 and Inflammation in CKD

BACKGROUND AND OBJECTIVES

Levels of fibroblast growth factor 23 (FGF23) and inflammatory markers are commonly elevated in CKD, and each is associated with adverse clinical outcomes. This study tested the hypothesis that FGF23 is independently associated with inflammation in CKD.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The association between levels of FGF23 and the inflammatory markers IL-6, C-reactive protein (CRP), TNF-α, and fibrinogen was assessed in a cross-sectional analysis of 3879 participants enrolled in the Chronic Renal Insufficiency Cohort (CRIC) study between June 2003 and September 2008.

RESULTS

FGF23 correlated directly with IL-6 (r=0.4), CRP (r=0.2), TNF-α (r=0.4), and fibrinogen (r=0.3; P<0.001 for each). In univariate and multivariable-adjusted linear regression analyses, natural log (ln) transformed FGF23 was significantly associated with lnIL-6, lnCRP, lnTNF-α, and fibrinogen (P<0.001 for each). Each unit higher lnFGF23 was associated with severe inflammation, defined as levels of all inflammatory markers in the highest 25th percentile, in univariate (odds ratio [OR], 2.4 [95% confidence interval (CI), 2.0-2.9]) and multivariable-adjusted (OR, 2.0 [95% CI, 1.6-2.5]) logistic regression analyses. Ascending FGF23 quartiles were independently associated with severe inflammation (OR, 5.6 for the highest versus lowest FGF23 quartile [95% CI, 2.3-13.9]; P for trend < 0.001).

CONCLUSIONS

Higher FGF23 levels are independently associated with higher levels of inflammatory markers in patients with CKD and with significantly greater odds of severe inflammation. Future studies should evaluate whether inflammation modifies the association between FGF23 and adverse outcomes in CKD.

Bone biomarkers help grading severity of coronary calcifications in non dialysis chronic kidney disease patients

Background

Osteoprotegerin (OPG) and fibroblast growth factor-23 (FGF23) are recognized as strong risk factors of vascular calcifications in non dialysis chronic kidney disease (ND-CKD) patients. The aim of this study was to investigate the relationships between FGF23, OPG, and coronary artery calcifications (CAC) in this population and to attempt identification of the most powerful biomarker of CAC: FGF23? OPG?

Methodology/Principal Findings

195 ND-CKD patients (112 males/83 females, 70.8 [27.4–94.6] years) were enrolled in this cross-sectional study. All underwent chest multidetector computed tomography for CAC scoring. Vascular risk markers including FGF23 and OPG were measured. Logistic regression analyses were used to study the potential relationships between CAC and these markers.

The fully adjusted-univariate analysis clearly showed high OPG (≥10.71 pmol/L) as the only variable significantly associated with moderate CAC ([100–400[) (OR = 2.73 [1.03;7.26]; p = 0.04). Such association failed to persist for CAC scoring higher than 400. Indeed, severe CAC was only associated with high phosphate fractional excretion (FEPO₄) (≥38.71%) (OR = 5.47 [1.76;17.0]; p = 0.003) and high FGF23 (≥173.30 RU/mL) (OR = 5.40 [1.91;15.3]; p = 0.002). In addition, the risk to present severe CAC when FGF23 level was high was not significantly different when OPG was normal or high. Conversely, the risk to present moderate CAC when OPG level was high was not significantly different when FGF23 was normal or high.

Conclusions

Our results strongly suggest that OPG is associated to moderate CAC while FGF23 rather represents a biomarker of severe CAC in ND-CKD patients.

Fibroblast growth factor 23 is associated with proteinuria and smoking in chronic kidney disease: an analysis of the MASTERPLAN cohort

BACKGROUND

Fibroblast growth factor 23 (FGF23) has emerged as a risk factor for cardiovascular disease and mortality throughout all stages of chronic kidney disease (CKD), independent from established risk factors and markers of mineral homeostasis. The relation of FGF23 with other renal and non-renal cardiovascular risk factors is not well established.

METHODS

Using stored samples, plasma FGF23 was determined in 604 patients with moderate to severe kidney disease that participated in the MASTERPLAN study (ISRCTN73187232). The association of FGF23 with demographic and clinical parameters was evaluated using multivariable regression models.

RESULTS

Mean age in the study population was 60 years and eGFR was 37 (± 14) ml/min/1.73 m(2). Median proteinuria was 0.3 g/24 hours [IQR 0.1-0.9]. FGF23 level was 116 RU/ml [67-203] median and IQR. Using multivariable analysis the natural logarithm of FGF23 was positively associated with history of cardiovascular disease (B = 0.224 RU/ml; p = 0.002), presence of diabetes (B = 0.159 RU/ml; p = 0.035), smoking (B = 0.313 RU/ml; p < 0.001), phosphate level (B = 0.297 per mmol/l; p = 0.0024), lnPTH (B = 0.244 per pmol/l; p < 0.001) and proteinuria (B = 0.064 per gram/24 hrs; p = 0.002) and negatively associated with eGFR (B = -0.022 per ml/min/1.73 m(2); p < 0.001).

CONCLUSIONS

Our study demonstrates that in patients with CKD, FGF23 is related to proteinuria and smoking. We confirm the relation between FGF23 and other cardiovascular risk factors.

Long-term management of CKD-mineral and bone disorder

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is the term used to describe the abnormalities of bone and mineral metabolism that occur in the setting of kidney disease. The spectrum of these abnormalities is wide, ranging from severe high-turnover bone disease on one end to marked low bone turnover bone disease on the other. Similarly, some patients have severe vascular calcifications while others do not, and the values for biochemistry determinations, including calcium, phosphorus, and parathyroid hormone, also may vary widely among patients. This variability may be influenced by such things as the chronicity of the particular kidney disease, effects of therapies such as corticosteroids on modifying the course of kidney disease, and comorbid conditions, such as diabetes, heart disease, age, and osteoporosis. The heterogeneity of CKD-MBD makes strict protocol-driven therapeutic approaches difficult; accordingly, considerable individualized therapy is required. Using a case history, we explore several of the variables and difficulties involved in patient management.

Fibroblast growth factor-23 abolishes 1,25-dihydroxyvitamin D₃-enhanced duodenal calcium transport in male mice

Despite being widely recognized as the important bone-derived phosphaturic hormone, whether fibroblast growth factor (FGF)-23 modulated intestinal calcium absorption remained elusive. Since FGF-23 could reduce the circulating level of 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃], FGF-23 probably compromised the 1,25(OH)₂D₃-induced intestinal calcium absorption. FGF-23 may also exert an inhibitory action directly through FGF receptors (FGFR) in the intestinal cells. Herein, we demonstrated by Ussing chamber technique that male mice administered 1 μg/kg 1,25(OH)₂D₃ sc daily for 3 days exhibited increased duodenal calcium absorption, which was abolished by concurrent intravenous injection of recombinant mouse FGF-23. This FGF-23 administration had no effect on the background epithelial electrical properties, i.e., short-circuit current, transepithelial potential difference, and resistance. Immunohistochemical evidence of protein expressions of FGFR isoforms 1-4 in mouse duodenal epithelial cells suggested a possible direct effect of FGF-23 on the intestine. This was supported by the findings that FGF-23 directly added to the serosal compartment of the Ussing chamber and completely abolished the 1,25(OH)₂D₃-induced calcium absorption in the duodenal tissues taken from the 1,25(OH)₂D₃-treated mice. However, direct FGF-23 exposure did not decrease the duodenal calcium absorption without 1,25(OH)₂D₃ preinjection. The observed FGF-23 action was mediated by MAPK/ERK, p38 MAPK, and PKC. Quantitative real-time PCR further showed that FGF-23 diminished the 1,25(OH)₂D₃-induced upregulation of TRPV5, TRPV6, and calbindin-D(9k), but not PMCA(1b) expression in the duodenal epithelial cells. In conclusion, besides being a phosphatonin, FGF-23 was shown to be a novel calcium-regulating hormone that acted directly on the mouse intestine, thereby compromising the 1,25(OH)₂D₃-induced calcium absorption.

Phosphate: a stealthier killer than previously thought?

The kidneys excrete excess dietary phosphate, and patients with chronic kidney disease may suffer from phosphate overload and hyperphosphatemia. In chronic kidney disease, hyperphosphatemia has emerged as a risk factor for vascular calcification, cardiovascular mortality, left ventricular hypertrophy, and progression of chronic kidney disease. Serum phosphate at the upper limits of normal has also been associated with adverse outcomes in patients with relatively preserved kidney function. Of note, hyperphosphatemia is not a sensitive indicator of phosphate overload. In this regard, increased circulating fibroblast growth factor-23, a phosphatonin that is released in response to phosphate overload, is independently associated with adverse outcomes in patients with and without chronic kidney disease. Direct effects of extracellular phosphate on vascular calcification or cardiovascular cell biology; adverse consequences of adaptive mechanisms activated to limit phosphate overload, such as left ventricular hypertrophy induced by fibroblast growth factor-23; or epidemiological associations of additional cardiovascular risk factors with chronic kidney disease may underlie these observations. We now review the pathophysiology of phosphate, its relationship with cardiovascular outcomes, the potential consequences for patient care related to dietary phosphate and phosphate binders, and the clinical relevance for patients without overt chronic kidney disease.

FGF23, albuminuria, and disease progression in patients with chronic IgA nephropathy

BACKGROUND AND OBJECTIVES

Fibroblast growth factor-23 (FGF23) regulates mineral metabolism. Circulatory FGF23 levels are increased and predict outcomes in CKD. However, the relation of FGF23 to albuminuria and disease progression in patients with CKD and one underlying diagnosis is unknown.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Prospective, observational study in 180 patients with IgA nephropathy (IgAN), CKD stage 1-4, and median 55-month follow-up (range, 12-177 months). Primary outcomes were (1) time-averaged albuminuria, (2A) progression to CKD stage 5 or ≥50% loss of estimated GFR, (2B) progression to CKD stage 5 or ≥25% loss of estimated GFR within 10 years, and (3) annual loss of estimated GFR.

RESULTS

FGF23 was independently associated with baseline and time-averaged albuminuria (change in 1 g/24 hour albuminuria per increase in log FGF23: β = 0.26; P=0.02). Log FGF23 predicted CKD progression in crude models and after adjustment for mineral metabolites (endpoints 2A and 2B). It remained significant after adjustments for age, sex, serum albumin, calcium, phosphate, parathyroid hormone, 25-hydroxyvitamin D, baseline albuminuria, baseline estimated GFR, mean arterial BP, body mass index, and angiotensin-converting enzyme inhibitors/angiotensin-receptor blocker use in endpoint 2B (hazard ratio, 2.53; P=0.02) but not endpoint 2A (hazard ratio, 2.01; P=0.43). Log FGF23 predicted annual loss of estimated GFR in the same model (change in ml/min per 1.73 m(2) per increase in log FGF23, 1.50; P=0.008).

CONCLUSIONS

In patients with CKD and IgAN, FGF23 was associated with albuminuria and CKD progression, a finding that suggests its role as a potential biomarker in IgAN.

Daily variability in mineral metabolites in CKD and effects of dietary calcium and calcitriol

BACKGROUND AND OBJECTIVES

Primary prevention of disordered mineral metabolism in CKD necessitates knowledge of its early pathophysiology. This study evaluated daily fluctuations in mineral metabolites in patients with CKD stages 3 and 4 before and after short-term calcitriol treatment and tested the effects of dietary calcium and calcitriol supplementation on these parameters in the dynamic postprandial setting.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Twelve CKD patients received calcitriol (0.25 μg daily for 1 week) with hourly assessments of mineral metabolites made throughout the day and in the context of standardized meals before and after treatment. Calcium content (250 versus 500 mg) in the breakfasts constituted the dietary calcium intervention. Twelve healthy volunteers were used as controls.

RESULTS

At baseline, compared with controls, fasting CKD subjects had higher parathyroid hormone and fibroblast growth factor 23 levels and greater fractional excretion of phosphate. After breakfast, urinary calcium excretion increased and parathyroid hormone levels dipped transiently in both groups, but they rose soon thereafter, reaching higher peaks in CKD. Calcitriol decreased fasting parathyroid hormone levels, and when combined with dietary calcium load, it normalized the postprandial parathyroid and calcemic responses. Daily variability in mineral metabolites was preserved in CKD before and after calcitriol. Fibroblast growth factor 23 levels increased after calcitriol, although the response was heterogeneous.

CONCLUSIONS

Short-term treatment with calcitriol and dietary calcium supplementation normalizes the parathyroid and calcemic postprandial responses in patients with CKD, in whom the diurnal rhythms of mineral metabolites are preserved. Future studies should investigate the variable fibroblast growth factor 23 response to calcitriol in CKD.

FGF23/Klotho axis: phosphorus, mineral metabolism and beyond

In this work we summarizes the steps that allowed the identification of the fibroblast growth factor (FGF) 23/Klotho axis as the principal regulator of phosphate homeostasis, exerting actions on intestine, bone, parathyroid glands, and kidney. We review the not fully understood mechanisms of action of this axis on the regulation of mineral homeostasis and, in addition, we discuss its potential role in the pathophysiology of chronic kidney disease and the associated complications. We also reflect the actual tendency to consider the components of this system as better predictors of the pathological conditions frequently associated to mineral disorders, and review some recent studies linking these components to cardiovascular disease even in population without mineral disorders. Finally, we consider the numerous processes in which Klotho is involved, including anti-ageing and mineral control processes, independently of its functions as obligated-coreceptor for FGF23.

What's new in FGF23 research?

FGF23 is a hormone that regulates phosphate and vitamin D metabolism by binding to Klotho-fibroblast growth factor (FGF) receptor complex. Excess actions of FGF23 cause several kinds of hypophosphatemic diseases. The mechanism of overproduction of FGF23 in some of these diseases is becoming clear, whereas it is not yet completely understood. Several specific methods to inhibit FGF23 actions have been reported as candidates for new therapies for these FGF23-related hypophosphatemic diseases. On the other hand, many epidemiological studies indicated the association between high FGF23 levels and several adverse events in cardiovascular system, kidney, bone and mortality. FGF23 was recently shown to induce ventricular hypertrophy in a Klotho-independent manner. However, it is not yet shown whether this Klotho-independent action of FGF23 can explain all the results of epidemiological studies.

Developmental and regional patterns of GAP-43 immunoreactivity in a metamorphosing brain

Growth-associated protein-43 is typically expressed at high levels in the nervous system during development. In adult animals, its expression is lower, but still observable in brain areas showing structural or functional plasticity. We examined patterns of GAP-43 immunoreactivity in the brain of the bullfrog, an animal whose nervous system undergoes considerable reorganization across metamorphic development and retains a strong capacity for plasticity in adulthood. Immunolabeling was mostly diffuse in hatchling tadpoles, but became progressively more discrete as larval development proceeded. In many brain areas, intensity of immunolabel peaked at metamorphic climax, the time of final transition from aquatic to semi-terrestrial life. Changes in intensity of GAP-43 expression in the medial vestibular nucleus, superior olivary nucleus, and torus semicircularis appeared correlated with stage-dependent functional changes in processing auditory stimuli. Immunolabeling in the Purkinje cell layer of the cerebellum and in the cerebellar nucleus was detectable at most developmental time points. Heavy immunolabel was present from early larval stages through the end of climax in the thalamus (ventromedial, anterior, posterior, central nuclei). Immunolabel in the tadpole telencephalon was observed around the lateral ventricles, and in the medial septum and ventral striatum. In postmetamorphic animals, immunoreactivity was confined mainly to the ventricular zones and immediately adjacent cell layers. GAP-43 expression was present in olfactory, auditory and optic cranial nerves throughout larval and postmetamorphic life. The continued expression of GAP-43 in brain nuclei and in cranial nerves throughout development and into adulthood reflects the high regenerative potential of the bullfrog's central nervous system.