Hypophosphatemia-mediated hypotension in transgenic mice overexpressing human FGF-23

Fibroblast growth factor 23-Klotho and hypertension: experimental and clinical mechanisms

Hypertension (HTN) and chronic kidney disease (CKD) are increasingly recognized in pediatric patients and represent risk factors for cardiovascular morbidity and mortality later in life. In CKD, enhanced tubular sodium reabsorption is a leading cause of HTN due to augmented extracellular fluid volume expansion. The renin-angiotensin-aldosterone system (RAAS) upregulates various tubular sodium cotransporters that are also targets of the hormone fibroblast growth factor 23 (FGF23) and its co-receptor Klotho. FGF23 inhibits the activation of 1,25-dihydroxyvitamin D that is a potent suppressor of renin biosynthesis. Here we review the complex interactions and disturbances of the FGF23-Klotho axis, vitamin D, and the RAAS relevant to blood pressure regulation and discuss the therapeutic strategies aimed at mitigating their pathophysiologic contributions to HTN.

Inflammation: a putative link between phosphate metabolism and cardiovascular disease

Dietary habits in the western world lead to increasing phosphate intake. Under physiological conditions, extraosseous precipitation of phosphate with calcium is prevented by a mineral buffering system composed of calcification inhibitors and tight control of serum phosphate levels. The coordinated hormonal regulation of serum phosphate involves fibroblast growth factor 23 (FGF23), αKlotho, parathyroid hormone (PTH) and calcitriol. A severe derangement of phosphate homeostasis is observed in patients with chronic kidney disease (CKD), a patient collective with extremely high risk of cardiovascular morbidity and mortality. Higher phosphate levels in serum have been associated with increased risk for cardiovascular disease (CVD) in CKD patients, but also in the general population. The causal connections between phosphate and CVD are currently incompletely understood. An assumed link between phosphate and cardiovascular risk is the development of medial vascular calcification, a process actively promoted and regulated by a complex mechanistic interplay involving activation of pro-inflammatory signalling. Emerging evidence indicates a link between disturbances in phosphate homeostasis and inflammation. The present review focuses on critical interactions of phosphate homeostasis, inflammation, vascular calcification and CVD. Especially, pro-inflammatory responses mediating hyperphosphatemia-related development of vascular calcification as well as FGF23 as a critical factor in the interplay between inflammation and cardiovascular alterations, beyond its phosphaturic effects, are addressed.

A Land of Controversy: Fibroblast Growth Factor-23 and Uremic Cardiac Hypertrophy

Cardiac hypertrophy is a common feature in patients with CKD. Recent studies revealed that two phosphate regulators, fibroblast growth factor-23 and α-Klotho, are highly involved in the pathophysiologic process of CKD-induced cardiac hypertrophy. With decreasing renal function, elevated fibroblast growth factor-23 and decreased α-Klotho may contribute to cardiac hypertrophy by targeting the heart directly or by inducing systemic changes, such as vascular injury, hemodynamic disorders, and inflammation. However, several studies have demonstrated that disturbances in the fibroblast growth factor-23/α-Klotho axis do not lead to cardiac hypertrophy. In this review, we describe the cardiac effects of the fibroblast growth factor-23/α-Klotho axis and summarize recent progress in this field. In addition, we present not only the main controversies in this field but also provide possible directions to resolve these disputes.

The potential of metabolomic analysis techniques for the characterisation of α1-adrenergic receptors in cultured N1E-115 mouse neuroblastoma cells

Several studies of neuropathic pain have linked abnormal adrenergic signalling to the development and maintenance of pain, although the mechanisms underlying this are not yet fully understood. Metabolomic analysis is a technique that can be used to give a snapshot of biochemical status, and can aid in the identification of the mechanisms behind pathological changes identified in cells, tissues and biological fluids. This study aimed to use gas chromatography-mass spectrometry-based metabolomic profiling in combination with reverse transcriptase-polymerase chain reaction and immunocytochemistry to identify functional α1-adrenergic receptors on cultured N1E-115 mouse neuroblastoma cells. The study was able to confirm the presence of mRNA for the α1D subtype, as well as protein expression of the α1-adrenergic receptor. Furthermore, metabolomic data revealed changes to the metabolite profile of cells when exposed to adrenergic pharmacological intervention. Agonist treatment with phenylephrine hydrochloride (10 µM) resulted in altered levels of several metabolites including myo-inositol, glucose, fructose, alanine, leucine, phenylalanine, valine, and n-acetylglutamic acid. Many of the changes observed in N1E-115 cells by agonist treatment were modulated by additional antagonist treatment (prazosin hydrochloride, 100 µM). A number of these changes reflected what is known about the biochemistry of α1-adrenergic receptor activation. This preliminary study therefore demonstrates the potential of metabolomic profiling to confirm the presence of functional receptors on cultured cells.

Pathophysiological implications of fibroblast growth factor-23 and Klotho and their potential role as clinical biomarkers

BACKGROUND

Fibroblast growth factor-23 (FGF-23) and Klotho constitute the main regulatory system of phosphorus homeostasis. Beyond this physiological role, there is growing evidence suggesting that this system has relevant pathophysiological implications in different clinical processes.

CONTENT

In this review we discuss the pathophysiological implications of the FGF-23/Klotho system and the potential utility that measurements of its components may have as clinical biomarkers in different clinical settings, such as progression of chronic kidney disease, acute renal failure, and secondary hyperparathyroidism, as well as vascular dysfunction, atherosclerosis, and cardiovascular morbidity and mortality. We outline and discuss the current commercially available assays for determination of FGF-23 and Klotho and the assay limitations that must be overcome to translate these biomarkers into reliable indicators in clinical practice.

SUMMARY

In addition to its physiological role, the FGF-23/Klotho system appears to provide important information regarding the pathophysiology of several clinical conditions. Although there has been increasing study of the components of this new biological system and their potential use as clinical biomarkers, the ultimate value of this system in clinical practice will not be known until remaining assay limitations can be overcome and adequately designed studies have been conducted to demonstrate its clinical utility.

Preoperative plasma FGF23 levels predict acute kidney injury in children: results of a pilot study

BACKGROUND

Acute kidney injury (AKI) carries a large burden of morbidity and mortality. Early diagnosis may lead to better strategies of clinical care. Cardiac surgery involving cardiopulmonary bypass is associated with a significant incidence of AKI. The study objective was to determine whether or not preoperative fibroblast growth factor-23 (FGF23) levels differed among pediatric patients who did or did not develop AKI following cardiac surgery.

METHODS

A nested case-control study was performed. FGF23 levels were measured pre- and post-operatively in 19 children without chronic kidney disease (CKD) who underwent cardiopulmonary bypass. Five patients developed AKI and 14 patients served as controls.

RESULTS

FGF23 levels in patients who developed AKI following cardiac surgery were elevated above normal levels, both pre-operatively and post-operatively compared with those patients who did not develop AKI. Relative risk of developing AKI when the pre-operative FGF23 level was >86 RU/mL was 2.0 (p = 0.033). Preoperative FGF23 levels correlated with post-operative fluid gain (correlation coefficient 0.607, p = 0.0059).

CONCLUSIONS

FGF23 may serve as a pre-operative prognostic indicator of the development of AKI following cardiopulmonary bypass surgery in pediatric patients without CKD. Identifying patients more likely to have AKI following surgery provides a means of achieving closer clinical management of AKI and fluid balance.

Fibroblast growth factor 23: associations with cardiovascular disease and mortality in chronic kidney disease

Fibroblast growth factor-23 (FGF-23) has emerged as an important hormone involved in phosphorus and vitamin D homeostasis. Chronic kidney disease (CKD) is the most common clinical condition in which FGF-23 levels are persistently and markedly elevated. Abnormal phosphate homeostasis and high circulating levels of FGF-23 are early complications of CKD. Although increases in FGF-23 levels may help maintain serum phosphate levels in the normal range in CKD, the long-term effects of its sustained elevated levels are unclear. Patients with CKD have high risks of developing end-stage renal disease (ESRD), cardiovascular disease, and premature death. Recent prospective studies in populations with predialysis CKD, ESRD on hemodialysis, and kidney transplant recipients demonstrate that elevated FGF-23 levels are independently associated with cardiovascular events and mortality. It was originally thought that FGF-23 was only a biomarker of disturbed phosphate balance; however, recent studies have shown that FGF-23 can have a direct effect on the heart, inducing left ventricular hypertrophy. This suggests that elevated FGF-23 levels may be a novel mechanism that explains the poor cardiovascular outcomes in CKD patients. Interventional studies are required in order to clarify the relation of causality between FGF-23 and cardiovascular mortality in this population.

Reliability of plasma fibroblast growth factor 23 as risk biomarker in epidemiological studies measured over a four-month period

Background

Identified as a biomarker of altered calcium–phosphorus metabolism in chronic kidney disease, fibroblast growth factor 23 (FGF-23) can also be used as a biomarker of risk for cardiovascular disease in the general population. However, it is crucial to first evaluate the reproducibility (reliability) of plasma FGF-23 concentrations.

Methods

We assessed the reliability of plasma FGF-23 concentrations using replicate blood samples taken four months apart of 207 participants from the European Prospective Investigation into Cancer and Nutrition-Potsdam Study.

Results

Plasma FGF-23 concentrations at baseline (geometric mean: 24.7 RU/mL; 95% confidence interval [CI] in RU/mL: 21.8–27.9) were not significantly different from those measured four months later (geometric mean: 23.7 RU/mL; 95% CI in RU/mL: 20.6–27.1; P = 0.42). The intraclass correlation coefficients were 0.69 (95% CI: 0.62–0.76) for all; 0.64 (95% CI: 0.50–0.75) for men and 0.73 (95% CI: 0.64–0.81) for women.

Conclusions

Plasma FGF-23 concentrations showed good reliability over time. Our findings suggest that in epidemiological studies, a single plasma FGF-23 measurement may be sufficient to derive the relative risk in prospective cohort studies.

Chronic kidney disease and vascular remodelling: molecular mechanisms and clinical implications

CKD (chronic kidney disease) is a severe and complex disease with a very high prevalence of CV (cardiovascular) complications. CKD patients are exposed to haemodynamic disturbances in addition to severe metabolic abnormalities that lead to a specific form of arterial remodelling, which contributes to the development of CV disease. Arterial calcification is a major event in the arterial remodelling process and is strongly linked to mineral metabolism abnormalities associated with CKD. Arterial remodelling is not limited to arterial calcification and modifications in arterial wall composition are also observed. Activation of the RAS (renin-angiotensin system), ET-1 (endothelin-1), endothelial dysfunction, oxidative stress and ADMA (asymmetric ω-NG,NG-dimethylarginine), as well as the anti-aging molecule Klotho, are implicated in this process. The present review details the mechanisms involved in arterial calcification and arterial remodelling associated with CKD, and provides the clinical consequences of large and small artery stiffness and remodelling in CKD patients.

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.

Phosphate: the new cholesterol? The role of the phosphate axis in non-uremic vascular disease

Higher serum phosphate levels within the normal range are associated with substantially increased risk of cardiovascular disease events. Whether this reflects a causative relationship is unknown. Phosphate-responsive hormones (fibroblast growth factor-23, parathyroid hormone and calcitriol) are also predictors of cardiovascular mortality in populations without kidney disease or recognised disturbances of bone mineral metabolism. The high bioavailable phosphate content of Western diets may contribute to this apparent discrepancy between 'normal' and optimal phosphate axis parameters. Although uremic hyperphosphatemia is recognised to cause vascular medial calcification, this does not readily explain the association of higher-normal phosphate with common athero-occlusive phenomena. The phosphate axis may in fact play a role in atherogenesis; observational data link higher levels of phosphate and fibroblast growth factor-23 with coronary atheroma burden, whilst dietary phosphate supplementation accelerates atherosclerosis in a mouse model. In vitro studies show adverse effects of phosphate increases on both vascular smooth muscle cells and endothelium, though these observations have not yet been extended to phosphate increments within the normal range. Receptors for phosphate-responsive hormones are present throughout the cardiovascular system and may mediate atherogenic effects. Since interventions are already available to manipulate the phosphate axis, this is an important issue. If an atherogenic role for phosphate exposure is demonstrated then phosphate binders could become the new statins.

Impairment of spatial learning and memory in transgenic mice overexpressing human fibroblast growth factor-23

Fibroblast growth factor-23 (FGF-23) is a potent circulating phosphaturic factor associated with renal phosphate wasting. Effects of FGF-23 on skeleton, phosphate homeostasis, and cardiovascular system have been investigated; however, the effect of FGF-23 on the central nervous system (CNS) is unknown. To assess whether FGF-23 influences the function and structure of the CNS and whether the effect of FGF-23 on the CNS is mediated by FGF receptors directly or by hypophosphatemia indirectly, FGF-23 transgenic mice and their wild-type littermates were fed a normal diet or a high-phosphate diet containing a normal diet plus 1.25% phosphate in drinking water from weaning for 5weeks and the phenotypes of the CNS were compared between FGF-23 transgenic mice and their wild-type littermates on the same diet. At the end of this time period, transgenic animals on the normal diet showed impaired spatial learning and memory. Furthermore, these mice exhibited the impairment of long-term potentiation in hippocampal CA1 region, and the reduction of hippocampal adenosine-triphosphate content and of choline acetyltransferase-positive neurons in basal forebrain, possibly as pathogenetic factors contributing to the cognitive deficit. The central nervous phenotypes of transgenic mice were rescued following improved hypophosphatemia by the high-phosphate diet intake. This study demonstrates that FGF-23 overexpression can result in abnormalities in the CNS mediated by the secondary severe hypophosphatemia.