Experimental Myocardial Infarction Upregulates Circulating Fibroblast Growth Factor-23

Non-Classical Effects of FGF23: Molecular and Clinical Features

This article reviews the role of fibroblast growth factor 23 (FGF23) protein in phosphate metabolism, highlighting its regulation of vitamin D, parathyroid hormone, and bone metabolism. Although it was traditionally thought that phosphate-calcium homeostasis was controlled exclusively by parathyroid hormone (PTH) and calcitriol, pathophysiological studies revealed the influence of FGF23. This protein, expressed mainly in bone, inhibits the renal reabsorption of phosphate and calcitriol formation, mediated by the α-klotho co-receptor. In addition to its role in phosphate metabolism, FGF23 exhibits pleiotropic effects in non-renal systems such as the cardiovascular, immune, and metabolic systems, including the regulation of gene expression and cardiac fibrosis. Although it has been proposed as a biomarker and therapeutic target, the inhibition of FGF23 poses challenges due to its potential side effects. However, the approval of drugs such as burosumab represents a milestone in the treatment of FGF23-related diseases.

Short-term fasting of mice elevates circulating fibroblast growth factor 23 (FGF23)

AIMS

Phosphate and vitamin D homeostasis are controlled by fibroblast growth factor 23 (FGF23) from bone suppressing renal phosphate transport and enhancing 24-hydroxylase (Cyp24a1), thereby inactivating 1,25(OH)2 D3 . Serum FGF23 is correlated with outcomes in several diseases. Fasting stimulates the production of ketone bodies. We hypothesized that fasting can induce FGF23 synthesis through the production of ketone bodies.

METHODS

UMR106 cells and isolated neonatal rat ventricular myocytes (NRVM) were treated with ketone body β-hydroxybutyrate. Mice were fasted overnight, fed ad libitum, or treated with β-hydroxybutyrate. Proteins and further blood parameters were determined by enzyme-linked immunoassay (ELISA), western blotting, immunohistochemistry, fluorometric or colorimetric methods, and gene expression by quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

β-Hydroxybutyrate stimulated FGF23 production in UMR106 cells in a nuclear factor kappa-light-chain enhancer of activated B-cells (NFκB)-dependent manner, and in NRVMs. Compared to fed animals, fasted mice exhibited higher β-hydroxybutyrate and FGF23 serum levels (based on assays either detecting C-terminal or intact, biologically active FGF23 only), cardiac, pancreatic, and thymic Fgf23 and renal Cyp24a1 expression, and lower 1,25(OH)2 D3 serum concentration as well as renal Slc34a1 and αKlotho (Kl) expression. In contrast, Fgf23 expression in bone and serum phosphate, calcium, plasma parathyroid hormone (PTH) concentration, and renal Cyp27b1 expression were not significantly affected by fasting.

CONCLUSION

Short-term fasting increased FGF23 production, as did administration of β-hydroxybutyrate, effects possibly of clinical relevance in view of the increasing use of FGF23 as a surrogate parameter in clinical monitoring of diseases. The fasting state of patients might therefore affect FGF23 tests.

Baseline fibroblast growth factor 23 is associated with long-term mortality in ST-elevation myocardial infarction-results from the augsburg myocardial infarction registry

Background

The aim of this study was to investigate the association between inflammatory plasma protein concentrations and long-term mortality in patients with ST-elevation myocardial infarction (STEMI).

Methods

For 343 STEMI patients recorded between 2009 and 2013 by the population-based Myocardial Infarction Registry Augsburg, 92 inflammatory plasma proteins were measured at the index event using the OLINK inflammation panel. In multivariable-adjusted Cox regression models, the association between each plasma protein and all-cause long-term mortality was investigated. Median follow-up time was 7.6 (IQR: 2.4) years. For plasma protein that showed a strong association with long-term mortality, a 5-year survival ROC analysis was performed.

Results

One plasma protein, namely Fibroblast Growth Factor 23 (FGF-23), was particularly well associated with long-term mortality in the multivariable-adjusted Cox model with an FDR-adjusted p-value of <0.001 and a Hazard Ratio (HR) of 1.57 [95% CI: 1.29-1.91]. In the 5-years ROC analysis, an AUC of 0.6903 [95% CI: 0.594-0.781] was estimated for FGF-23. All other plasma protein didńt show strong associations, each marker with FDR-adjusted p-values >0.05 in the multivariable-adjusted Cox models.

Conclusions

FGF-23 is independently associated with long-term mortality after STEMI and might play an important role in the response to myocardial injury. The results suggest FGF-23 to be a useful marker in the long-term treatment of STEMI patients and a potential target for drug development.

Oncostatin M is a regulator of fibroblast growth factor 23 (FGF23) in UMR106 osteoblast-like cells

Renal phosphate and vitamin D metabolism is under the control of fibroblast growth factor 23 (FGF23), an endocrine and paracrine factor predominantly produced in bone. FGF23 formation is stimulated by active vitamin D, or parathyroid hormone (PTH), which are further regulators of phosphate homeostasis. In renal, inflammatory, and other diseases, plasma FGF23 reflects disease stage and correlates with outcome. Oncostatin M is part of the interleukin-6 (IL-6) family and regulates remodeling and PTH effects in bone as well as cardiac FGF23 production in heart failure via glycoprotein gp130. Here, we studied whether oncostatin M is a regulator of FGF23 in bone cells. Experiments were performed in UMR106 osteoblast-like cells, Fgf23 mRNA was determined by qRT-PCR, FGF23 protein by Western Blotting and ELISA, and oncostatin M receptor and leukemia inhibitory factor (LIF) receptor gene knockout accomplished by siRNA. As a result, oncostatin M dose-dependently up-regulated Fgf23 expression and protein secretion. The oncostatin M effect on FGF23 was mediated by oncostatin M receptor and gp130 and involved, at least in part, STAT3 and MEK1/2. Taken together, oncostatin M is a regulator of FGF23 through oncostatin M receptor, gp130, as well as STAT3 and MEK1/2 in UMR106 osteoblasts.

Roles of PTH and FGF23 in kidney failure: a focus on nonclassical effects

Parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) each play a central role in the pathogenesis of chronic kidney disease-mineral and bone disorder (CKD-MBD). Both hormones increase as kidney function declines, presumably as a response to maintain normal phosphate balance, but when patients reach kidney failure, PTH and FGF23 fail to exert their phosphaturic effects, leading to hyperphosphatemia and further elevations in PTH and FGF23. In patients with kidney failure, the major target organ for PTH is the bone, but elevated PTH is also associated with mortality presumably through skeletal and nonskeletal mechanisms. Indeed, accumulated evidence suggests improved survival with PTH-lowering therapies, and a more recent study comparing parathyroidectomy and calcimimetic treatment further suggests a notion of "the lower, the better" for PTH control. Emerging data suggest that the link between SHPT and mortality could in part be explained by the action of PTH to induce adipose tissue browning and wasting. In the absence of a functioning kidney, the classical target organ for FGF23 is the parathyroid gland, but FGF23 loses its hormonal effect to suppress PTH secretion owing to the depressed expression of parathyroid Klotho. In this setting, experimental data suggest that FGF23 exerts adverse nontarget effects, but it remains to be confirmed whether FGF23 directly contributes to multiple organ injury in patients with kidney failure and whether targeting FGF23 can improve patient outcomes. Further efforts should be made to determine whether intensive control of SHPT improves clinical outcomes and whether nephrologists should aim at controlling FGF23 levels just as with PTH levels.

Effect of Intravenous Ferric Carboxymaltose on Exercise Capacity After Kidney Transplantation (EFFECT-KTx): rationale and study protocol for a double-blind, randomised, placebo-controlled trial

INTRODUCTION

Iron deficiency (ID) is common and has been associated with an excess mortality risk in kidney transplant recipients (KTRs). In patients with chronic heart failure and ID, intravenous iron improves exercise capacity and quality of life. Whether these beneficial effects also occur in KTRs is unknown. The main objective of this trial is to address whether intravenous iron improves exercise tolerance in iron-deficient KTRs.

METHODS AND ANALYSIS

The Effect of Ferric Carboxymaltose on Exercise Capacity after Kidney Transplantation study is a multicentre, double-blind, randomised, placebo-controlled clinical trial that will include 158 iron-deficient KTRs. ID is defined as plasma ferritin <100 µg/L or plasma ferritin 100-299 µg/L with transferrin saturation <20%. Patients are randomised to receive 10 mL of ferric carboxymaltose (50 mg Fe³⁺/mL, intravenously) or placebo (0.9% sodium chloride solution) every 6 weeks, four dosages in total. The primary endpoint is change in exercise capacity, as quantified by the 6 min walk test, between the first study visit and the end of follow-up, 24 weeks later. Secondary endpoints include changes in haemoglobin levels and iron status, quality of life, systolic and diastolic heart function, skeletal muscle strength, bone and mineral parameters, neurocognitive function and safety endpoints. Tertiary (explorative) outcomes are changes in gut microbiota and lymphocyte proliferation and function.

ETHICS AND DISSEMINATION

The protocol of this study has been approved by the medical ethical committee of the University Medical Centre Groningen (METc 2018/482;) and is being conducted in accordance with the principles of the Declaration of Helsinki, the Standard Protocol Items: Recommendations for Interventional Trials checklist and the Good Clinical Practice guidelines provided by the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use. Study results will be disseminated through publications in peer-reviewed journals and conference presentations.

TRIAL REGISTRATION NUMBER

NCT03769441.

FGF23 in Chronic Kidney Disease: Bridging the Heart and Anemia

Fibroblast growth factor 23 (FGF23) is a phosphaturic hormone produced mainly in osteocytes. In chronic kidney disease (CKD) FGF23 levels increase due to higher production, but also as the result of impaired cleavage and reduced excretion from the body. FGF23 has a significant role in disturbed bone and mineral metabolism in CKD, which leads to a higher cardiovascular risk and mortality in these patients. Current research has emphasized the expression of FGF23 in cardiac myocytes, fibroblasts, and endothelial cells, and in addition to the effects on the kidney, its primary role is in cardiac remodeling in CKD patients. Recent discoveries found a significant link between increased FGF23 levels and anemia development in CKD. This review describes the FGF23 role in cardiac hypertrophy and anemia in the setting of CKD and discusses the best therapeutical approach for lowering FGF23 levels.

Direct and indirect effects of fibroblast growth factor 23 on the heart

Fibroblast growth factor (FGF)23 is a bone-derived phosphotropic hormone that regulates phosphate and mineral homeostasis. Recent studies have provided evidence that a high plasma concentration of FGF23 is associated with cardiac disease, including left ventricular hypertrophy (LVH), heart failure, atrial fibrillation, and cardiac death. Experimental studies have shown that FGF23 activates fibroblast growth factor receptor 4 (FGFR4)/phospholipase Cγ/calcineurin/nuclear factor of activated T-cells signaling in cardiomyocytes and induces cardiac hypertrophy in rodents. Activation of FGFR4 by FGF23 normally requires the co-receptor α-klotho, and klotho-independent signaling occurs only under conditions characterized by extremely high FGF23 concentrations. Recent studies have demonstrated that FGF23 activates the renin-angiotensin-aldosterone system (RAAS) and induces LVH, at least in part as a result of lower vitamin D activation. Moreover, crosstalk between FGF23 and RAAS results in the induction of cardiac hypertrophy and fibrosis. In this review, we summarize the results of studies regarding the relationships between FGF23 and cardiac events, and describe the potential direct and indirect mechanisms whereby FGF23 induces LVH.

Inflammatory plasma proteins predict short-term mortality in patients with an acute myocardial infarction

BACKGROUND

The aim of this study was to investigate the association between inflammatory markers and 28-day mortality in patients with ST-elevation myocardial infarction (STEMI).

METHODS

In 398 STEMI patients recorded between 2009 and 2013 by the population-based Myocardial Infarction Registry Augsburg, 92 protein biomarkers were measured in admission arterial blood samples using the OLINK inflammatory panel. In multivariable-adjusted logistic regression models, the association between each marker and 28-day mortality was investigated. The values of the biomarkers most significantly associated with mortality were standardized and summarized to obtain a prediction score for 28-day mortality. The predictive ability of this biomarker score was compared to the established GRACE score using ROC analysis. Finally, a combined total score was generated by adding the standardized biomarker score to the standardized GRACE score.

RESULTS

The markers IL-6, IL-8, IL-10, FGF-21, FGF-23, ST1A1, MCP-1, 4E-BP1, and CST5 were most significantly associated with 28-day mortality, each with FDR-adjusted (false discovery rate adjusted) p-values of < 0.01 in the multivariable logistic regression model. In a ROC analysis, the biomarker score and the GRACE score showed comparable predictive ability for 28-day mortality (biomarker score AUC: 0.7859 [CI: 0.6735-0.89], GRACE score AUC: 0.7961 [CI: 0.6965-0.8802]). By combining the biomarker score and the Grace score, the predictive ability improved with an AUC of 0.8305 [CI: 0.7269-0.9187]. A continuous Net Reclassification Improvement (cNRI) of 0.566 (CI: 0.192-0.94, p-value: 0.003) and an Integrated Discrimination Improvement (IDI) of 0.083 ((CI: 0.016-0.149, p-value: 0.015) confirmed the superiority of the combined score over the GARCE score.

CONCLUSIONS

Inflammatory biomarkers may play a significant role in the pathophysiology of acute myocardial infarction (AMI) and AMI-related mortality and might be a promising starting point for personalized medicine, which aims to provide each patient with tailored therapy.

Physiological Mechanisms of Hypertension and Cardiovascular Disease in End-Stage Kidney Disease

PURPOSE OF REVIEW

In this article, we summarize recent advances in understanding hypertension and cardiovascular disease in patients with end-stage kidney disease.

RECENT FINDINGS

Factors such as anemia, valvular and vascular calcification, vasoconstrictors, uremic toxins, hypoglycemia, carbamylated proteins, oxidative stress, and inflammation have all been associated with the progression of cardiovascular disease in end-stage kidney disease but the causality of these mechanisms has not been proven. The high risk of cardiovascular mortality has not improved as in the general population despite many advancements in cardiovascular care over the last two decades. Mechanisms that increase hypertension risk in these patients are centered on the control of extracellular fluid volume; however, over-correction of volume with dialysis can increase risks of intradialytic hypotension and death in these patients. This review presents both recent and classic work that increases our understanding of hypertension and cardiovascular disease in end-stage kidney disease.

Strategies to lower fibroblast growth factor 23 bioactivity

Fibroblast growth factor 23 (FGF23) is a circulating hormone derived from the bone whose release is controlled by many factors and exerts a multitude of systemic actions. There are congenital and acquired disorders of increased and decreased FGF23 levels. In chronic kidney disease (CKD), elevations of FGF23 levels can be 1000-fold above the upper physiological limit. It is still debated whether this high FGF23 in CKD is a biomarker or causally related to morbidity and mortality. Data from human association studies support pathogenicity, while experimental data are less robust. Knowledge of the biology and pathobiology of FGF23 has generated a plethora of means to reduce FGF23 bioactivity at many levels that will be useful for therapeutic translations. This article summarizes these approaches and addresses several critical questions that still need to be answered.

Fibroblast Growth Factor 23 and Risk of New Onset Heart Failure With Preserved or Reduced Ejection Fraction: The PREVEND Study

Background

The role of fibroblast growth factor 23 (FGF23) in the development of new‐onset heart failure (HF) with reduced (HFrEF) or preserved ejection fraction (HFpEF) in the general population is unknown. Therefore, we set out to investigate associations of C‐terminal FGF23 with development of new‐onset HF and, more specifically, with HFrEF or HFpEF in a large, prospective, population‐based cohort.

Methods and Results

We studied 6830 participants (aged 53.8±12.1 years; 49.7% men; estimated glomerular filtration rate, 93.1±15.7 mL/min per 1.73 m²) in the community‐based PREVEND (Prevention of Renal and Vascular End‐Stage Disease) study who were free of HF at baseline. Cross‐sectional multivariable linear regression analysis showed that ferritin (standardized β, −0.24; P<0.001) and estimated glomerular filtration rate (standardized β, −0.13; P<0.001) were the strongest independent correlates of FGF23. Multivariable Cox proportional hazard regression was used to study the association between baseline FGF23 and incident HF, HFrEF (ejection fraction ≤40%) or HFpEF (ejection fraction ≥50%). After median follow‐up of 7.4 [IQR 6.9–7.9] years, 227 individuals (3.3%) developed new‐onset HF, of whom 132 had HFrEF and 88 had HFpEF. A higher FGF23 level was associated with an increased risk of incident HF (fully adjusted hazard ratio, 1.29 [95% CI, 1.06–1.57]) and with an increased risk of incident HFrEF (fully adjusted hazard ratio, 1.31 [95% CI, 1.01–1.69]). The association between FGF23 and incident HFpEF lost statistical significance after multivariable adjustment (hazard ratio, 1.22 [95% CI, 0.87–1.71]).

Conclusions

Higher FGF23 is independently associated with new‐onset HFrEF in analyses fully adjusted for cardiovascular risk factors and other potential confounders. The association between FGF23 and incident HFpEF lost statistical significance upon multivariable adjustment.

Low-Dose Propranolol Prevents Functional Decline in Catecholamine-Induced Acute Heart Failure in Rats

Severe hyper-catecholaminergic states likely cause heart failure and cardiac fibrosis. While previous studies demonstrated the effects of beta-blockade in experimental models of single-catecholamine excess states, the detailed benefits of beta-blockade in more realistic models of hyper-adrenergic states are less clearly understood. In this study, we examined different therapeutic dosages and the effects of propranolol in rats with hyper-acute catecholamine-induced heart failure, and subsequent cardiopulmonary changes. Rats (n = 41) underwent a 6 h infusion of epinephrine and norepinephrine alone, with additional low-dose (1 mg/kg) or high-dose propranolol (10 mg/kg) at hour 1. Cardiac and pulmonary tissues were examined after 6 h. Catecholamine-only groups had the lowest survival rate. Higher doses of propranolol (15 mg/kg) caused similarly low survival rates and were not further analyzed. All low-dose propranolol rats survived, with a modest survival improvement in the high-dose propranolol groups. Left ventricular (LV) systolic pressure and LV end-diastolic pressure improved maximally with low-dose propranolol. Cardiac immunohistochemistry revealed an LV upregulation of FGF-23 in the catecholamine groups, and this improved in low-dose propranolol groups. These results suggest catecholamine-induced heart failure initiates early pre-fibrotic pathways through FGF-23 upregulation. Low-dose propranolol exerted cardio-preventative effects through FGF-23 downregulation and hemodynamic-parameter improvement in our model of hyper-acute catecholamine-induced heart failure.

Shedding Light on the Complex Regulation of FGF23

Early research has suggested a rather straightforward relation between phosphate exposure, increased serum FGF23 (Fibroblast Growth Factor 23) concentrations and clinical endpoints. Unsurprisingly, however, subsequent studies have revealed a much more complex interplay between autocrine and paracrine factors locally in bone like PHEX and DMP1, concentrations of minerals in particular calcium and phosphate, calciprotein particles, and endocrine systems like parathyroid hormone PTH and the vitamin D system. In addition to these physiological regulators, an expanding list of disease states are shown to influence FGF23 levels, usually increasing it, and as such increase the burden of disease. While some of these physiological or pathological factors, like inflammatory cytokines, may partially confound the association of FGF23 and clinical endpoints, others are in the same causal path, are targetable and hence hold the promise of future treatment options to alleviate FGF23-driven toxicity, for instance in chronic kidney disease, the FGF23-associated disease with the highest prevalence by far. These factors will be reviewed here and their relative importance described, thereby possibly opening potential means for future therapeutic strategies.

Chronic Kidney Disease-Mineral Bone Disease biomarkers in kidney transplant patients

Background:

Chronic Kidney Disease associated with Mineral Bone Disease (CKD-MBD) is frequent in kidney transplant patients. Post-transplantation bone disease is complex, especially in patients with pre-existing metabolic bone disorders that are further affected by immunosuppressive medications and changes in renal allograft function. Main biochemical abnormalities of mineral metabolism in kidney transplantation (KTx) include hypophosphatemia, hyperparathyroidism (HPTH), insufficiency or deficiency of vitamin D, and hypercalcemia.

Objective:

This review aimed to summarize the pathophysiology and main biomarkers of CKD-MBD in KTx.

Methods:

A comprehensive and non-systematic search in PubMed was independently made with an emphasis on biomarkers in mineral bone disease in KTx.

Results:

CKD-MBD can be associated with numerous factors including secondary HPTH, metabolic dysregulations before KTx, and glucocorticoids therapy in post-transplant subjects. Fibroblast growth factor 23 (FGF23) reaches normal levels after KTx with good allograft function, while calcium, vitamin D and phosphorus, ultimately, result in hypercalcemia, persistent vitamin D insufficiency, and hypophosphatemia respectively. As for PTH levels, there is an initial tendency of a significant decrease, followed by a raise due to secondary or tertiary HPTH. In regard to sclerostin levels, there is no consensus in the literature.

Conclusion:

KTx patients should be continuously evaluated for mineral homeostasis and bone status, both cases with successful kidney transplantation and those with reduced functionality. Additional research on CKD-MBD pathophysiology, diagnosis, and management is essential to guarantee long-term graft function, better prognosis, good quality of life, and reduced mortality for KTx patients.

Fibroblast Growth Factor 23 and Outcome Prediction in Patients with Acute Myocardial Infarction

(1) Background: Fibroblast growth factor 23 (FGF23) is associated with mortality in patients with heart failure (HF); however, less is known about mortality associations in patients with myocardial infarction (MI). (2) Methods: FGF23 was assessed in 180 patients with acute MI, 99 of whom presented with concomitant acute HF. Patients were followed up for one year, and outcome estimates by FGF23 were compared to GRACE score estimates. (3) Results: Log-transformed serum levels of intact FGF23 (logFGF23) did not differ between MI patients with and without HF, and no difference in logFGF23 was observed between 14 MI patients who died and those who survived. However, when only MI patients with concomitant HF were considered, logFGF23 was significantly higher among non-survivors compared to that in survivors. While logFGF23 was not associated with the outcome in the entire cohort, logFGF23 was fairly predictive for one-year mortality in patients with concomitant HF (AUC 0.78; 95%CI 0.61–0.95), where it outperformed GRACE score estimates (AUC 0.70; 95%CI 0.46–0.94). (4) Conclusions: FGF23 was associated with one-year mortality only in MI patients who concomitantly presented with HF, surpassing the predictive ability of GRACE score estimates. No associations were observed in patients without HF despite similar FGF23 levels at admission. Further studies are warranted to investigate whether FGF23 is causal for dismal outcome of HF.

Comprehensive Expression Analysis of Cardiac Fibroblast Growth Factor 23 in Health and Pressure-induced Cardiac Hypertrophy

Enhanced fibroblast growth factor 23 (FGF23) is associated with left ventricular hypertrophy (LVH) in patients with chronic kidney and heart disease. Experimentally, FGF23 directly induces cardiac hypertrophy and vice versa cardiac hypertrophy stimulates FGF23. Besides the bone, FGF23 is expressed by cardiac myocytes, whereas its synthesis in other cardiac cell types and its paracrine role in the heart in health and disease is unknown. By co-immunofluorescence staining of heart tissue of wild-type mice, we show that Fgf23 is expressed by cardiac myocytes, fibroblasts and endothelial cells. Cardiac Fgf23 mRNA and protein level increases from neonatal to six months of age, whereas no age-related changes in bone Fgf23 mRNA expression were noted. Cardiac myocyte-specific disruption of Fgf23 using Cre-LoxP system (Fgf23fl/fl/cre+) caused enhanced mortality, but no differences in cardiac function or structure. Although pressure overload-induced cardiac hypertrophy induced by transverse aortic constriction (TAC) resulted in a slightly worse phenotype with a more severe reduced ejection fraction, higher end-systolic volume and more enlarged systolic LV diameter in Fgf23fl/fl/cre+ mice compared to controls, this was not translated to any worse cellular hypertrophy, fibrosis or chamber remodeling. TAC induced Fgf23 mRNA expression in whole cardiac tissue in both genotypes. Interestingly, co-immunofluorescence staining revealed enhanced Fgf23 synthesis in cardiac fibroblasts and endothelial cells but not in cardiac myocytes. RNA sequencing of isolated adult cardiac myocytes, cardiac fibroblasts and endothelial cells confirmed significantly higher Fgf23 transcription in cardiac fibroblasts and endothelial cells after TAC. Our data indicate that Fgf23 is physiologically expressed in various cardiac cell types and that cardiac fibroblasts and endothelial cells might be an important source of FGF23 in pathological conditions. In addition, investigations in Fgf23fl/fl/cre+ mice suggest that cardiac myocyte-derived FGF23 is needed to maintain cardiac function during pressure overload.

Induction of FGF23-related hypophosphatemic osteomalacia by alcohol consumption

Context

Fibroblast growth factor (FGF) 23 is a hormone that regulates serum phosphate levels, the excess action of which causes chronic hypophosphatemic rickets/osteomalacia. To date, there are only two identified causes of acquired FGF23-related hypophosphatemic osteomalacia: tumor-induced osteomalacia (TIO) and osteomalacia induced by the intravenous infusion of some forms of iron preparations. In the current study, two cases of FGF23-related hypophosphatemia probably induced by chronic alcohol consumption were first introduced.

Case description

Case 1 and case 2 had been drinking high amounts of alcohol for more than twenty years until they were admitted to the hospital. Case 1 was a 43-year-old man with progressive worsening multiple pains and muscle weakness who exhibited chronic hypophosphatemia with increased intact FGF23 levels. A week after admission, the serum phosphate level recovered to the reference range, and the intact FGF23 level declined. Case 1 resumed drinking after discharge, and hypophosphatemia concomitant with high intact FGF23 levels recurred. The alleviation of FGF23-related hypophosphatemia was observed each time he temporarily abstained from drinking for a short period. Case 2 was a 60-year-old man with recurrent fractures and exacerbation of pain in multiple joints who also exhibited hypophosphatemia with increased intact FGF23 levels. After admission, the serum phosphate level gradually increased to the lower limit of the normal range. The intact FGF23 level decreased, but it was still higher than 30 pg/ml, and causative FGF23-producing tumors were not identified even with thorough examinations, including somatostatin receptor scintigraphy, fluorine-18-fluorodeoxyglucose-positron emission tomography/computed tomography (¹⁸F-FDG-PET/CT) and systemic venous FGF23 sampling. He completely abstained from alcohol after discharge. Along with the serum phosphate level, intact FGF23 was subsequently decreased and had been normalized for 5 months. Both patients had no genetic mutation related to hereditary FGF23-related hypophosphatemic rickets/osteomalacia, including autosomal dominant hypophosphatemic rickets/osteomalacia (ADHR).

Conclusion

Two cases of FGF23-related hypophosphatemia probably induced by alcohol were first introduced in this study. Identifying this reversible condition among acquired FGF23-related hypophosphatemic osteomalacia is critical to obtain better patient outcomes and save medical resources. This condition is similar to iron infusion-induced FGF23-related hypophosphatemia in terms of the dysregulation of FGF23 due to exogenous factors. Future research to elucidate the precise mechanism of these conditions is warranted.

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Two adults with acquired FGF23-related hypophosphatemic osteomalacia were studied.

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Tumor-induced osteomalacia was suspected initially, but no tumor was identified.

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Cessation of alcohol led to recovery from FGF23-related hypophosphatemia.

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Alcohol is a well-recognized cause of hypophosphatemia, but osteomalacia is uncommon.

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Alcohol-induced FGF23-related osteomalacia is a totally new and distinct phenomenon.

Fibroblast Growth Factor-23-Klotho Axis in Cardiorenal Syndrome: Mediators and Potential Therapeutic Targets

Cardiorenal syndrome (CRS) is a complex disorder that refers to the category of acute or chronic kidney diseases that induce cardiovascular disease, and inversely, acute or chronic heart diseases that provoke kidney dysfunction. There is a close relationship between renal and cardiovascular disease, possibly due to the presence of common risk factors for both diseases. Thus, it is well known that renal diseases are associated with increased risk of developing cardiovascular disease, suffering cardiac events and even mortality, which is aggravated in those patients with end-stage renal disease or who are undergoing dialysis. Recent works have proposed mineral bone disorders (MBD) as the possible link between kidney dysfunction and the development of cardiovascular outcomes. Traditionally, increased serum phosphate levels have been proposed as one of the main factors responsible for cardiovascular damage in kidney patients. However, recent studies have focused on other MBD components such as the elevation of fibroblast growth factor (FGF)-23, a phosphaturic bone-derived hormone, and the decreased expression of the anti-aging factor Klotho in renal patients. It has been shown that increased FGF-23 levels induce cardiac hypertrophy and dysfunction and are associated with increased cardiovascular mortality in renal patients. Decreased Klotho expression occurs as renal function declines. Despite its expression being absent in myocardial tissue, several studies have demonstrated that this antiaging factor plays a cardioprotective role, especially under elevated FGF-23 levels. The present review aims to collect the recent knowledge about the FGF-23-Klotho axis in the connection between kidney and heart, focusing on their specific role as new therapeutic targets in CRS.

Cardiac Fibroblast Growth Factor 23 Excess Does Not Induce Left Ventricular Hypertrophy in Healthy Mice

Fibroblast growth factor (FGF) 23 is elevated in chronic kidney disease (CKD) to maintain phosphate homeostasis. FGF23 is associated with left ventricular hypertrophy (LVH) in CKD and induces LVH via klotho-independent FGFR4-mediated activation of calcineurin/nuclear factor of activated T cells (NFAT) signaling in animal models, displaying systemic alterations possibly contributing to heart injury. Whether elevated FGF23 per se causes LVH in healthy animals is unknown. By generating a mouse model with high intra-cardiac Fgf23 synthesis using an adeno-associated virus (AAV) expressing murine Fgf23 (AAV-Fgf23) under the control of the cardiac troponin T promoter, we investigated how cardiac Fgf23 affects cardiac remodeling and function in C57BL/6 wild-type mice. We report that AAV-Fgf23 mice showed increased cardiac-specific Fgf23 mRNA expression and synthesis of full-length intact Fgf23 (iFgf23) protein. Circulating total and iFgf23 levels were significantly elevated in AAV-Fgf23 mice compared to controls with no difference in bone Fgf23 expression, suggesting a cardiac origin. Serum of AAV-Fgf23 mice stimulated hypertrophic growth of neonatal rat ventricular myocytes (NRVM) and induced pro-hypertrophic NFAT target genes in klotho-free culture conditions in vitro. Further analysis revealed that renal Fgfr1/klotho/extracellular signal-regulated kinases 1/2 signaling was activated in AAV-Fgf23 mice, resulting in downregulation of sodium-phosphate cotransporter NaPi2a and NaPi2c and suppression of Cyp27b1, further supporting the bioactivity of cardiac-derived iFgf23. Of interest, no LVH, LV fibrosis, or impaired cardiac function was observed in klotho sufficient AAV-Fgf23 mice. Verified in NRVM, we show that co-stimulation with soluble klotho prevented Fgf23-induced cellular hypertrophy, supporting the hypothesis that high cardiac Fgf23 does not act cardiotoxic in the presence of its physiological cofactor klotho. In conclusion, chronic exposure to elevated cardiac iFgf23 does not induce LVH in healthy mice, suggesting that Fgf23 excess per se does not tackle the heart.

Acute cardiac overload does not induce cardiac or skeletal expression of fibroblast growth factor 23 in rats

Elevated fibroblast growth factor 23 (FGF23) is associated with cardiovascular events, particularly heart failure. Although FGF23 has been reported to induce cardiac hypertrophy, recent studies demonstrated that cardiac hypertrophy and myocardial infarction induce FGF23 production by cardiomyocytes. We aimed to explore whether acute cardiac overload increases cardiac and skeletal FGF23 expression and circulating FGF23 levels.

Communication Between Cardiomyocytes and Fibroblasts During Cardiac Ischemia/Reperfusion and Remodeling: Roles of TGF-β, CTGF, the Renin Angiotensin Axis, and Non-coding RNA Molecules

Communication between cells is a foundational concept for understanding the physiology and pathology of biological systems. Paracrine/autocrine signaling, direct cell-to-cell interplay, and extracellular matrix interactions are three types of cell communication that regulate responses to different stimuli. In the heart, cardiomyocytes, fibroblasts, and endothelial cells interact to form the cardiac tissue. Under pathological conditions, such as myocardial infarction, humoral factors released by these cells may induce tissue damage or protection, depending on the type and concentration of molecules secreted. Cardiac remodeling is also mediated by the factors secreted by cardiomyocytes and fibroblasts that are involved in the extensive reciprocal interactions between these cells. Identifying the molecules and cellular signal pathways implicated in these processes will be crucial for creating effective tissue-preserving treatments during or after reperfusion. Numerous therapies to protect cardiac tissue from reperfusion-induced injury have been explored, and ample pre-clinical research has attempted to identify drugs or techniques to mitigate cardiac damage. However, despite great success in animal models, it has not been possible to completely translate these cardioprotective effects to human applications. This review provides a current summary of the principal molecules, pathways, and mechanisms underlying cardiomyocyte and cardiac fibroblast crosstalk during ischemia/reperfusion injury. We also discuss pre-clinical molecules proposed as treatments for myocardial infarction and provide a clinical perspective on these potential therapeutic agents.

Myocardial Hypertrophy and Fibrosis Are Associated with Cardiomyocyte Beta-Catenin and TRPC6/Calcineurin/NFAT Signaling in Spontaneously Hypertensive Rats with 5/6 Nephrectomy

BACKGROUND

Arterial hypertension (AH) is associated with heart and chronic kidney disease (CKD). However, the precise mechanisms of myocardial remodeling (MR) in the settings of CKD remain elusive. We hypothesized that TRPC6, calcineurin/NFAT, and Wnt/β-catenin signaling pathways are involved in the development of MR in the background of CKD and AH.

METHODS

Early CKD was induced by performing a 5/6 nephrectomy (5/6NE) in spontaneously hypertensive rats (SHR-NE). Sham-operated (SO) SHR (SHR-SO) and Wistar Kyoto (WKY-SO) rats served as controls. Systolic blood pressure (SBP), heart rate, myocardial mass index (MMI), serum creatinine, cardiomyocyte diameter (dCM), myocardial fibrosis (MF), serum and kidney α-Klotho levels, myocardial expression of calcineurin (CaN), TRPC6, and β-catenin were measured two months after 5/6NE or SO.

RESULTS

NE-induced kidney dysfunction corresponded to mild-to-moderate human CKD and was associated with an increase in FGF23 and a decrease in renal α-Klotho. The levels of SBP, MMI, dCM, and MF were higher in SHRs compared to WKY-SO as well as in SHR-NE vs. SHR-SO. The MR was associated with increased cardiomyocyte expression of CaN/NFAT and β-catenin along with its intracellular re-distribution. TRPC6 protein levels were substantially elevated in both SHR groups with higher Trpc6 mRNA expression in SHR-NE.

CONCLUSIONS

The Wnt/β-catenin and TRPC6/CaN/NFAT hypertrophic signaling pathways seem to be involved in myocardial remodeling in the settings of AH and CKD and might be mediated by FGF23 and α-Klotho axis.

An Overview of FGF-23 as a Novel Candidate Biomarker of Cardiovascular Risk

Fibroblast growth factor-23 (FGF)-23 is a phosphaturic hormone involved in mineral bone metabolism that helps control phosphate homeostasis and reduces 1,25-dihydroxyvitamin D synthesis. Recent data have highlighted the relevant direct FGF-23 effects on the myocardium, and high plasma levels of FGF-23 have been associated with adverse cardiovascular outcomes in humans, such as heart failure and arrhythmias. Therefore, FGF-23 has emerged as a novel biomarker of cardiovascular risk in the last decade. Indeed, experimental data suggest FGF-23 as a direct mediator of cardiac hypertrophy development, cardiac fibrosis and cardiac dysfunction via specific myocardial FGF receptor (FGFR) activation. Therefore, the FGF-23/FGFR pathway might be a suitable therapeutic target for reducing the deleterious effects of FGF-23 on the cardiovascular system. More research is needed to fully understand the intracellular FGF-23-dependent mechanisms, clarify the downstream pathways and identify which could be the most appropriate targets for better therapeutic intervention. This review updates the current knowledge on both clinical and experimental studies and highlights the evidence linking FGF-23 to cardiovascular events. The aim of this review is to establish the specific role of FGF-23 in the heart, its detrimental effects on cardiac tissue and the possible new therapeutic opportunities to block these effects.

Excessive fibroblast growth factor 23 promotes renal fibrosis in mice with type 2 cardiorenal syndrome

Cardiorenal syndrome (CRS) has a high mortality, but its pathogenesis remains elusive. Fibroblast growth factor 23 (FGF23) is increased in both renal dysfunction and cardiac dysfunction, and FGF receptor 4 (FGFR4) has been identified as a receptor for FGF23. Deficiency of FGF23 causes growth retardation and shortens the lifespan, but it is unclear whether excess FGF23 is detrimental in CRS. This study sought to investigate whether FGF23 plays an important role in CRS-induced renal fibrosis. A mouse model of CRS was created by surgical myocardial infarction for 12 weeks. CRS mice showed a significant increase of circulatory and renal FGF23 protein levels, as well as an upregulation of p-GSK, active-β-catenin, TGF-β, collagen I and vimentin, a downregulation of renal Klotho expression and induction of cardiorenal dysfunction and cardiorenal fibrosis. These changes were enhanced by cardiac overexpression of FGF23 and attenuated by FGF receptor blocker PD173074 or β-catenin blocker IGC001. In fibroblasts (NRK-49F), expression of FGFR4 rather than Klotho was detected. Recombinant FGF23 upregulated the expression of p-GSK, active-β-catenin, TGF-β, collagen I and vimentin proteins. These changes were attenuated by FGFR4 blockade with BLU9931 or β-catenin blockade with IGC001. We concluded that FGF23 promotes CRS-induced renal fibrosis mediated by partly activating FGFR4/β-catenin signaling pathway.

Fibroblast Growth Factor 23 and Left Ventricular Hypertrophy in Chronic Kidney Disease-A Pediatric Perspective

Cardiovascular diseases (CVD) are a hallmark in pediatric patients with chronic kidney disease (CKD) contributing to an enhanced risk of all-cause and CV morbidity and mortality in these patients. The bone-derived phosphaturic hormone fibroblast growth factor (FGF) 23 progressively rises with declining kidney function to maintain phosphate homeostasis, with up to 1,000-fold increase in patients with kidney failure requiring dialysis. FGF23 is associated with the development of left ventricular hypertrophy (LVH) and thereby accounts to be a CVD risk factor in CKD. Experimentally, FGF23 directly induces hypertrophic growth of cardiac myocytes in vitro and LVH in vivo. Further, clinical studies in adult CKD have observed cardiotoxicity associated with FGF23. Data regarding prevalence and determinants of FGF23 excess in children with CKD are limited. This review summarizes current data and discusses whether FGF23 may be a key driver of LVH in pediatric CKD.

New Aspects of the Kidney in the Regulation of Fibroblast Growth Factor 23 (FGF23) and Mineral Homeostasis

The bone-derived hormone fibroblast growth factor 23 (FGF23) acts in concert with parathyroid hormone (PTH) and the active vitamin D metabolite calcitriol in the regulation of calcium (Ca) and phosphate (P) homeostasis. More factors are being identified to regulate FGF23 levels and the endocrine loops between the three hormones. The present review summarizes the complex regulation of FGF23 and the disturbed FGF23/Klotho system in chronic kidney disease (CKD). In addition to the reduced ability of the injured kidney to regulate plasma levels of FGF23, several CKD-related factors have been shown to stimulate FGF23 production. The high circulating FGF23 levels have detrimental effects on erythropoiesis, the cardio-vascular system and the immune system, all contributing to the disturbed system biology in CKD. Moreover, new factors secreted by the injured kidney and the uremic calcified vasculature play a role in the mineral and bone disorder in CKD and create a vicious pathological crosstalk.

Fibroblast growth factor 23 (FGF23) level is associated with ultrafiltration rate in patients on hemodialysis

Fibroblast growth factor 23 (FGF23) is a bone-derived hormone that regulates renal phosphate reabsorption and vitamin D synthesis in renal proximal tubules. High circulating FGF23 levels are associated with increased mortality in patients with chronic kidney disease and those on dialysis. Current data also suggest higher circulating levels of FGF23 are associated with cardiovascular mortality, vascular calcification, and left ventricular hypertrophy; however, evidence on the role of FGF23 in patients on dialysis is incomplete, and some of the data, especially those on cardiovascular disease (CVD), are controversial. This study aimed to evaluate factors associated with FGF23 in hemodialysis patients with or without CVD. Randomly selected 76 patients on maintenance hemodialysis at a single hemodialysis center were enrolled. After the exclusion of eight patients with extremely outlying FGF23 levels, 68 patients, including 48 males and 46 patients with a CVD history, were included in the study. The mean age was 64.4 ± 12.1 years, and the mean dialysis duration was 12.7 ± 7.1 years. Dialysis duration, time-averaged concentration of urea (TAC-urea), ultrafiltration rate (UFR), blood pressure during hemodialysis session, laboratory data, and echocardiographic parameters including interventricular septum thickness (IVST), left ventricular mass indices (LVMI), and ejection fraction were included in univariate and multivariate analyses. The median lgFGF23 levels in the overall cohort and in those with and without CVD were 2.14 (interquartile range, IQR − 0.43 to − 4.23), 2.01 (− 0.52 to 4.12), and 2.59 (0.07 to 4.32), respectively, and there was no difference between the patients with and without CVD (p = 0.14). The univariate analysis revealed that FGF23 was significantly associated with age (r =  − 0.12, p < 0.01), duration of hemodialysis (r =  − 0.11, p < 0.01), TAC-urea (r = 0.29, p = 0.01), UFR (r = 0.26, p = 0.04), alkaline phosphatase (ALP; r =  − 0.27, p = 0.03), corrected serum calcium (cCa; r = 0.32, p < 0.01), serum phosphate (iP, r = 0.57, p < 0.01), intact parathyroid hormone (iPTH; r = 0.38, p < 0.01), IVST (r = 0.30, p = 0.01), and LVMI (r = 0.26, p = 0.04). In multivariate regression analysis, FGF23 was significantly associated with cCa (F = 25.6, p < 0.01), iP (F = 22.5, p < 0.01), iPTH (F = 19.2, p < 0.01), ALP (F = 5.34, p = 0.03), and UFR (F = 3.94, p = 0.05). In addition, the univariate analysis after the categorization of patients according to CVD indicated that FGF23 was significantly associated with cCa (r = 0.34, p = 0.02), iP (r = 0.41, p < 0.01), iPTH (r = 0.39, p = 0.01), and TAC-urea (r = 0.45, p < 0.01) in patients with CVD, whereas only IVST (r = 0.53, p = 0.04) was associated with FGF23 in those without CVD. FGF23 levels in hemodialysis patients were extremely high and associated not only with mineral bone disease-related factors but also with UFR. Additionally, dialysis efficacy might be associated with lower FGF23 levels in patients with CVD.

Endocrine fibroblast growth factors as potential biomarkers for chronic kidney disease

INTRODUCTION

Among the family of fibroblast growth factors (FGFs), FGF19, FGF21, and FGF23 act as circulating hormones and are called endocrine FGFs. FGF19 and FGF21 regulate bile acid and energy homeostasis, respectively, whereas FGF23 regulates vitamin D and phosphate homeostasis. Accumulating evidence suggests that FGF23 plays a critical role in disturbed mineral metabolisms, left ventricular hypertrophy, immunosuppression, inflammation, among others in patients with chronic kidney disease (CKD), highlighting the potential both as a biomarker and a therapeutic target. Several studies have also examined the potential role of FGF19 and FGF21 in CKD patients.

AREAS COVERED

In this review, we present a brief overview of the biology of FGF19, FGF21, and FGF23, and summarize recent clinical and experimental studies on the pathophysiological roles of endocrine FGFs, mainly FGF23, in CKD patients.

EXPERT OPINION

Among the endocrine FGFs, FGF23 represents the most promising biomarker in CKD patients. If future studies confirm that FGF23 is directly toxic in CKD patients, FGF23 could be regarded as a therapeutic target and its measurement would be valuable if applied in clinical practice. Despite their potentially important roles, the clinical relevance of FGF19 and FGF21 in CKD patients is unclear, and much more studies are required.

Fibroblast growth factor 23: are we ready to use it in clinical practice?

Patients with chronic kidney disease (CKD) have a greatly enhanced risk of cardiovascular morbidity and mortality. Over the past decade it has come clear that a disturbed calcium-phosphate metabolism, with Fibroblast Growth Factor-23 as a key hormone, is partly accountable for this enhanced risk. Numerous studies have been performed unravelling FGF23s actions and its association with clinical conditions. As FGF23 is strongly associated with adverse outcome it may be a promising biomarker for risk prediction or, even more important, targeting FGF23 may be a strategy to improve patient outcome. This review elaborates on the clinical usefulness of FGF23 measurement. Firstly it discusses the reliability of the FGF23 measurement. Secondly, it evaluates whether FGF23 measurement may lead to improved patient risk classification. Finally, and possibly most importantly, this review evaluates if lowering of FGF23 should be a target for therapy. For this, the review discusses the current evidence indicating that FGF23 may be in the causal pathway to cardiovascular pathology, provides an overview of strategies to lower FGF23 levels and discusses the current evidence concerning the benefit of lowering FGF23.

Fibrosis growth factor 23 is a promoting factor for cardiac fibrosis in the presence of transforming growth factor-β1

Myocardial fibrosis is often associated with cardiac hypertrophy; indeed, fibrosis is one of the most critical factors affecting prognosis. We aimed to identify the molecules involved in promoting fibrosis under hypertrophic stimuli. We previously established a rat model of cardiac hypertrophy by pulmonary artery banding, in which approximately half of the animals developed fibrosis in the right ventricle. Here, we first comprehensively analyzed mRNA expression in the right ventricle with or without fibrosis in pulmonary artery banding model rats by DNA microarray analysis (GSE141650 at NCBI GEO). The expression levels of 19 genes were up-regulated more than 1.5-fold in fibrotic hearts compared with non-fibrotic hearts. Among them, fibrosis growth factor (FGF) 23 showed one of the biggest increases in expression. Real-time PCR analysis also revealed that, among the FGF receptor (FGFR) family, FGFR1 was highly expressed in fibrotic hearts. We then found that FGF23 was expressed predominantly in cardiomyocytes, while FGFR1 was predominantly expressed in fibroblasts in the rat ventricle. Next, we added FGF23 and transforming growth factor (TGF)-β1 (10-50 ng/mL of each) to isolated fibroblasts from normal adult rat ventricles and cultured them for three days. While FGF23 itself did not directly affect the expression levels of any fibrosis-related mRNAs, FGF23 enhanced the effect of TGF-β1 on increasing the expression levels of α-smooth muscle actin (α-SMA) mRNA. This increase in xx-SMA mRNA levels due to the combination of TGF-β1 and FGF23 was attenuated by the inhibition of FGFR1 or the knockdown of FGFR1 in fibroblasts. Thus, FGF23 synergistically promoted the activation of fibroblasts with TGF-β1, transforming fibroblasts into myofibroblasts via FGFR1. Thus, we identified FGF23 as a paracrine factor secreted from cardiomyocytes to promote cardiac fibrosis under conditions in which TGF-β1 is activated. FGF23 could be a possible target to prevent fibrosis following myocardial hypertrophy.

FGF23 and Cardiovascular Risk

FGF23 is a protein secreted in the plasma by bone cells. In the kidney, FGF23 can activate an FGF receptor in the presence of its co-receptor αKlotho. FGF23 controls the renal phosphate reabsorption and calcitriol metabolism. When renal function declines, plasma FGF23 concentration raises and FGF23 can stimulate FGFRs in the absence of αKlotho. This induces cardiac hypertrophy, modifies cardiomyocyte contractility and increases the risk of arrhythmic events in cardiac cells. There is still no evidence that decreasing FGF23 concentration in patients on dialysis could improve their survival. In different cardiac disorders cardiomyocyte can produce FGF23, which can reveal a way of adaptation to the stress.

Renal and extrarenal effects of fibroblast growth factor 23

Fibroblast growth factor 23 (FGF23) is a hormone with a central role in the regulation of phosphate homeostasis. This regulation is accomplished by the coordinated modulation of renal phosphate handling, vitamin D metabolism and parathyroid hormone secretion. Patients with kidney disease have increased circulating levels of FGF23 and in other patient populations and in healthy individuals, FGF23 levels also rise following an increase in dietary phosphate intake. Maladaptive increases in FGF23 have a detrimental effect on several organs and tissues and, importantly, these pathological changes most likely contribute to increased morbidity and mortality. For example, in the context of heart disease, FGF23 is involved in the development of pathological hypertrophy that can lead to congestive heart failure. Increased FGF23 concentrations can also lead to microcirculatory changes, in particular reduced vasodilatory capacity, and collectively these cardiovascular changes can compromise tissue perfusion. In addition, FGF23 is associated with inflammation and an increased risk of infection; other potentially detrimental effects of FGF23 are likely to emerge in the future. Most importantly, recent insights demonstrate that FGF23 can be therapeutically targeted, which holds promise for the treatment of many patients in a variety of clinical settings.

Regulation and Effects of FGF23 in 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.

Iron Infusion and Induced Hypophosphatemia: The Role of Fibroblast Growth Factor-23

The mechanism of action of fibroblast growth factor-23 (FGF23) is becoming increasingly clearer as a result of studies that have defined its structure and pleiotropic effects. Furthermore, data are emerging on the effects exerted on this hormone by iron administration. Ten main iron formulations are recognized (with clear differences in composition and possible reactions of intolerance and anaphylaxis), which are indicated for iron deficiency anemia, including nephropathic subjects, as suggested by medical guidelines. With some types of iron formulation (especially iron carboxymaltose) a particular side effect has been observed: hypophosphatemia, mediated by FGF23. This review aims to draw attention to this correlation and the contradiction represented by the presence of both positive and negative modulation by FGF23, with the effects induced by its increase even after long-term treatment with iron formulation. However, more evidence is needed to understand the reasons for this differential stimulation.

Effect of etelcalcetide on cardiac hypertrophy in hemodialysis patients: a randomized controlled trial (ETECAR-HD)

Background

Fibroblast growth factor 23 (FGF23) is associated with left ventricular hypertrophy (LVH) in patients with chronic kidney disease, and calcimimetic therapy reduces plasma concentrations of FGF23. It remains unknown whether treatment with the calcimimetic etelcalcetide (ETL) reduces LVH in patients on hemodialysis.

Methods/design

This single-blinded randomized trial of 12 months duration will test the effects of ETL compared with alfacalcidol on LVH and cardiac fibrosis in maintenance hemodialysis patients with secondary hyperparathyroidism. Both treatment regimens will be titrated to equally suppress secondary hyperparathyroidism while alfacalcidol treatment causes an increase and ETL a decrease in FGF23, respectively.

Patients treated thrice weekly with hemodialysis for ≥ 3 months and ≤ 3 years with parathyroid hormone levels ≥ 300 pg/ml and LVH will be enrolled in the study.

The primary study endpoint is change from baseline to 12 months in left ventricular mass index (LVMI; g/m²) measured by cardiac magnetic resonance imaging. Sample size calculations showed that 62 randomized patients will be necessary to detect a difference in LVMI of at least 20 g/m² between the two groups at 12 months. Due to the strong association of volume overload and LVH, randomization will be stratified by residual kidney function, and regular body composition monitoring will be performed to control the volume status of patients.

Study medication will be administered intravenously by the dialysis nurses after every hemodialysis session, thus omitting adherence issues.

Secondary study endpoints are cardiac parameters measured by echocardiography, biomarker concentrations of bone metabolism (FGF23, vitamin D, parathyroid hormone, calcium, phosphate, s-Klotho), cardiac markers (pro-brain natriuretic peptide, pre- and postdialysis troponin T) and metabolites of the renin–angiotensin–aldosterone cascade (angiotensin I (Ang I), Ang II, Ang-(1–7), Ang-(1–5), Ang-(1–9), and aldosterone).

Discussion

The causal inference and pathophysiology of LVH regression by FGF23 reduction using calcimimetic treatment has not yet been shown. This intervention study has the potential to discover a new strategy for the treatment of cardiac hypertrophy and fibrosis in patients on maintenance hemodialysis. It might be speculated that successful treatment of cardiac morphology will also reduce the risk of cardiac death in this population.

Trial registration

European Clinical Trials Database, EudraCT number 2017-000222-35; ClinicalTrials.gov, NCT03182699. Registered on

Fibroblast growth factor 23 is induced by an activated renin-angiotensin-aldosterone system in cardiac myocytes and promotes the pro-fibrotic crosstalk between cardiac myocytes and fibroblasts

Background

Fibroblast growth factor 23 (FGF23) is discussed as a new biomarker of cardiac hypertrophy and mortality in patients with and without chronic kidney disease (CKD). We previously demonstrated that FGF23 is expressed by cardiac myocytes, enhanced in CKD and induces cardiac hypertrophy via activation of FGF receptor 4 independent of its co-receptor klotho. The impact of FGF23 on cardiac fibrosis is largely unknown.

Methods

By conducting a retrospective case-control study including myocardial autopsy samples from 24 patients with end-stage CKD and in vitro studies in cardiac fibroblasts and myocytes, we investigated the pro-fibrotic properties of FGF23.

Results

The accumulation of fibrillar collagens I and III was increased in myocardial tissue of CKD patients and correlated with dialysis vintage, klotho deficiency and enhanced cardiac angiotensinogen (AGT) expression. Using human fibrosis RT2 Profiler PCR array analysis, transforming growth factor (TGF)-β and its related TGF-β receptor/Smad complexes, extracellular matrix remodeling enzymes and pro-fibrotic growth factors were upregulated in myocardial tissue of CKD patients. FGF23 stimulated cell proliferation, migration, pro-fibrotic TGF-β receptor/Smad complexes and collagen synthesis in cultured cardiac fibroblasts. In isolated cardiac myocytes, FGF23 enhanced collagen remodeling, expression of pro-inflammatory genes and pro-survival pathways and induced pro-hypertrophic genes. FGF23 stimulated AGT expression in cardiac myocytes and angiotensin II and aldosterone, as components of the renin-angiotensin-aldosterone system (RAAS), induced FGF23 in cardiac myocytes.

Conclusions

Our data demonstrate that activated RAAS induces FGF23 expression in cardiac myocytes and thereby stimulates a pro-fibrotic crosstalk between cardiac myocytes and fibroblasts, which may contribute to myocardial fibrosis in CKD.

FGF23-Mediated Activation of Local RAAS Promotes Cardiac Hypertrophy and Fibrosis

Patients with chronic kidney disease (CKD) are prone to developing cardiac hypertrophy and fibrosis, which is associated with increased fibroblast growth factor 23 (FGF23) serum levels. Elevated circulating FGF23 was shown to induce left ventricular hypertrophy (LVH) via the calcineurin/NFAT pathway and contributed to cardiac fibrosis by stimulation of profibrotic factors. We hypothesized that FGF23 may also stimulate the local renin–angiotensin–aldosterone system (RAAS) in the heart, thereby further promoting the progression of FGF23-mediated cardiac pathologies. We evaluated LVH and fibrosis in association with cardiac FGF23 and activation of RAAS in heart tissue of 5/6 nephrectomized (5/6Nx) rats compared to sham-operated animals followed by in vitro studies with isolated neonatal rat ventricular myocytes and fibroblast (NRVM, NRCF), respectively. Uremic rats showed enhanced cardiomyocyte size and cardiac fibrosis compared with sham. The cardiac expression of Fgf23 and RAAS genes were increased in 5/6Nx rats and correlated with the degree of cardiac fibrosis. In NRVM and NRCF, FGF23 stimulated the expression of RAAS genes and induced Ngal indicating mineralocorticoid receptor activation. The FGF23-mediated hypertrophic growth of NRVM and induction of NFAT target genes were attenuated by cyclosporine A, losartan and spironolactone. In NRCF, FGF23 induced Tgfb and Ctgf, which were suppressed by losartan and spironolactone, only. Our data suggest that FGF23-mediated activation of local RAAS in the heart promotes cardiac hypertrophy and fibrosis.

Fibroblast Growth Factor 23 and Klotho in AKI

Acute kidney injury (AKI) is associated with many of the same mineral metabolite abnormalities that are observed in chronic kidney disease. These include increased circulating levels of the osteocyte-derived, vitamin D-regulating hormone, fibroblast growth factor 23 (FGF23), and decreased renal expression of klotho, the co-receptor for FGF23. Recent data have indicated that increased FGF23 and decreased klotho levels in the blood and urine could serve as novel predictive biomarkers of incident AKI, or as novel prognostic biomarkers of adverse outcomes in patients with established AKI. In addition, because FGF23 and klotho exert numerous classic as well as off-target effects on a variety of organ systems, targeting their dysregulation in AKI may represent a unique opportunity for therapeutic intervention. We review the pathophysiology, kinetics, and regulation of FGF23 and klotho in animal and human studies of AKI, and we discuss the challenges and opportunities involved in targeting FGF23 and klotho therapeutically.

Circadian rhythm of activin A and related parameters of mineral metabolism in normal and uremic rats

Activin A is a new fascinating player in chronic kidney disease-mineral and bone disorder (CKD-MBD), which is implicated in progressive renal disease, vascular calcification, and osteodystrophy. Plasma activin A rises early in the progression of renal disease. Disruption of circadian rhythms is related to increased risk of several diseases and circadian rhythms are observed in mineral homeostasis, bone parameters, and plasma levels of phosphate and PTH. Therefore, we examined the circadian rhythm of activin A and CKD-MBD-related parameters (phosphate, PTH, FGF23, and klotho) in healthy controls and CKD rats (5/6 nephrectomy) on high-, standard- and low-dietary phosphate contents as well as during fasting conditions. Plasma activin A exhibited circadian rhythmicity in healthy control rats with fourfold higher values at acrophase compared with nadir. The rhythm was obliterated in CKD. Activin A was higher in CKD rats compared with controls when measured at daytime but not significantly when measured at evening/nighttime, stressing the importance of time-specific reference intervals when interpreting plasma values. Plasma phosphate, PTH, and FGF23 all showed circadian rhythms in control rats, which were abolished or disrupted in CKD. Plasma klotho did not show circadian rhythm. Thus, the present investigation shows, for the first time, circadian rhythm of plasma activin A. The rhythmicity is severely disturbed by CKD and is associated with disturbed rhythms of phosphate and phosphate-regulating hormones PTH and FGF23, indicating that disturbed circadian rhythmicity is an important feature of CKD-MBD.

FGF23 effects on the heart-levels, time, source, and context matter

Fibroblast growth factor 23 (FGF23) has appeared as a hormone that is massively elevated in patients with chronic kidney disease. Whether FGF23 is a risk factor that associates with cardiac pathologies and cardiovascular mortality, as suggested by a variety of clinical studies, or additionally acts as a causative factor that induces cardiac injury, as more recently indicated by cell culture and animal studies, is under debate and the center of many ongoing experimental studies.

Update on Chronic Kidney Disease Mineral and Bone Disorder in Cardiovascular Disease

Chronic kidney disease mineral and bone disorder (MBD) encompasses changes in mineral ion and vitamin D metabolism that are widespread in the setting of chronic kidney disease and end-stage renal disease. MBD components associate with cardiovascular disease in many epidemiologic studies. Through impacts on hypertension, activation of the renin-angiotensin-aldosterone system, vascular calcification, endothelial function, and cardiac remodeling and conduction, MBD may be a direct and targetable cause of cardiovascular disease. However, assessment and treatment of MBD is rife with challenges owing to biological tensions between its many components, such as calcium and phosphorus with their regulatory hormones fibroblast growth factor 23 and parathyroid hormone; fibroblast growth factor 23 with its co-receptor klotho; and vitamin D with control of calcium and phosphorus. These complex interactions between MBD components hinder the simple translation to clinical trials, which ultimately are needed to prove the benefits of treating MBD. Deeper investigation using precision medicine tools and principles, including genomics and individualized risk assessment and therapy, may help move the field closer toward clinical applications. This review provides a high-level overview of conventional and precision epidemiology in MBD, potential mechanisms of cardiovascular disease pathogenesis, and guiding therapeutic principles for established and emerging treatments.

Bone marrow and plasma FGF-23 in heart failure patients: novel insights into the heart-bone axis

Aims

Fibroblast growth factor 23 (FGF‐23) is known to be elevated in patients with congestive heart failure (CHF). As FGF‐23 is expressed in the bone but can also be expressed in the myocardium, the origin of serum FGF‐23 in CHF remains unclear. It is also unclear if FGF‐23 expressed in the bone is associated with outcome in CHF. The aim of the present study was to investigate FGF‐23 levels measured in bone marrow plasma (FGF‐23‐BM) and in peripheral blood (FGF‐23‐P) in CHF patients to gain further insights into the heart–bone axis of FGF‐23 expression. We also investigated possible associations between FGF‐23‐BM as well as FGF‐23‐P and outcome in CHF patients.

Methods and results

We determined FGF‐23‐P and FGF‐23‐BM levels in 203 CHF patients (85% male, mean age 61.3 years) with a left ventricular ejection fraction (LVEF) ≤45% and compared them with those of 48 healthy controls (48% male, mean age 39.2 years). We investigated the association between FGF‐23‐BM and FGF‐23‐P with all‐cause mortality in CHF patients, 32 events, median follow‐up 1673 days, interquartile range [923, 1828]. FGF‐23‐P (median 60.3 vs. 22.0 RU/mL, P < 0.001) and FGF‐23‐BM (median 130.7 vs. 93.1 RU/mL, P < 0.001) levels were higher in CHF patients compared with healthy controls. FGF‐23‐BM levels were significantly higher than FGF‐23‐P levels in both CHF patients and in healthy controls (P < 0.001). FGF‐23‐P and FGF‐23‐BM correlated significantly with LVEF (r = −0.37 and r = −0.33, respectively), N terminal pro brain natriuretic peptide levels (r = 0.57 and r = 0.6, respectively), New York Heart Association status (r = 0.28 and r = 0.25, respectively), and estimated glomerular filtration rate (r = −0.43 and r = −0.41, respectively) (P for all <0.001) and were independently associated with all‐cause mortality in CHF patients after adjustment for LVEF, estimated glomerular filtration rate, New York Heart Association status, and N terminal pro brain natriuretic peptide, hazard ratio 2.71 [confidence interval: 1.18–6.20], P = 0.018, and hazard ratio 2.80 [confidence interval: 1.19–6.57], P = 0.018, respectively.

Conclusions

In CHF patients, FGF‐23 is elevated in bone marrow plasma and is independently associated with heart failure severity and all‐cause mortality. The failing heart seems to interact via FGF‐23 within a heart–bone axis.

Cardiac FGF23: new insights into the role and function of FGF23 after acute myocardial infarction

OBJECTIVE

We aimed to elucidate the local role of FGF23 after myocardial infarction in a mouse model induced by left anterior descending artery (LAD) ligation. APPROACH AND RESULTS: (C57BL/6 N) mice underwent MI via LAD ligation and were sacrificed at different time-points post MI. The expression and influence of FGF23 on fibroblast and macrophages was also analyzed using isolated murine cells. We identified enhanced cardiac FGF23 mRNA expression in a time-dependent manner with an early increase, already on the first day after MI. FGF23 protein expression was abundantly detected in the infarcted area during the inflammatory phase. While described to be primarily produced in bone or macrophages, we identified cardiac fibroblasts as the only source of local FGF23 production after MI. Inflammatory mediators, such as IL-1β, IL-6 and TNF-α, were able to induce FGF23 expression in these cardiac fibroblasts. Interestingly, we were not able to detect FGF23 at later time points after MI in mature scar tissue or remote myocardium, most likely due to TGF-β1, which we have shown to inhibit the expression of FGF23. We identified FGFR1c to be the most abundant receptor for FGF23 in infarcted myocardium and cardiac macrophages and fibroblasts. FGF23 increased migration of cardiac fibroblast, as well as expression of Collagen 1, Periostin, Fibronectin and MMP8. FGF23 also increased expression of TGF-β1 in M2 polarized macrophages.

CONCLUSION

In conclusion, cardiac fibroblasts in the infarcted myocardium produce and express FGF23 as well as its respective receptors in a time-dependent manner, thus potentially influencing resident cell migration. The transitory local expression of FGF23 after MI points towards a complex role of FGF23 in myocardial ischemia and warrants further exploration, considering its role in ventricular remodeling.

Bone-Derived Extracellular Vesicles: Novel Players of Interorgan Crosstalk

An increasing number of studies have shown that bone plays an active role in regulating glucose metabolism, affects renal, and cardiovascular diseases and even influences the development of offspring. These novel findings have indicated that bone plays a much more important role in the human body than only providing physical support. However, further investigations of the mechanisms underlying the effects of bone are needed. Recently, extracellular vesicles (EVs) have received increased attention because they can transfer functional proteins, mRNAs, and miRNAs between cells/organs. After reviewing the existing evidence, we hypothesized that bone may be involved in interorgan communication via EVs. Further research exploring bone-derived EVs may facilitate the understanding of bone as a multifunctional organ.

Cardiotoxicity of Uremic Toxins: A Driver of Cardiorenal Syndrome

Cardiovascular disease (CVD) is highly prevalent in the setting of chronic kidney disease (CKD). Such coexistence of CVD and CKD—the so-called “cardiorenal or renocardiac syndrome”—contributes to exponentially increased risk of cardiovascular (CV) mortality. Uremic cardiomyopathy is a characteristic cardiac pathology commonly found in CKD. CKD patients are also predisposed to heart rhythm disorders especially atrial fibrillation. Traditional CV risk factors as well as known CKD-associated CV risk factors such as anemia are insufficient to explain CV complications in the CKD population. Accumulation of uremic retention solutes is a hallmark of impaired renal excretory function. Many of them have been considered inert solutes until their biological toxicity is unraveled and they become accepted as “uremic toxins”. Direct cardiotoxicity of uremic toxins has been increasingly demonstrated in recent years. This review offers a mechanistic insight into the pathological cardiac remodeling and dysfunction contributed by uremic toxins with a main focus on fibroblastic growth factor-23, an emerging toxin playing a central role in the chronic kidney disease–mineral bone disorder, and the two most investigated non-dialyzable protein-bound uremic toxins, indoxyl sulfate and p-cresyl sulfate. Potential therapeutic strategies that could address these toxins and their relevant mediated pathways since pre-dialysis stages are also discussed.

Higher Serum Fibroblast Growth Factor-23 Levels and the Risk of Stroke and Its Subtypes: Evidence From a Meta-Analysis of Prospective Studies

BACKGROUND

Epidemiologic studies have indicated conflicting associations of fibroblast growth factor-23 (FGF23) with the risk of stroke. To this end, a meta-analysis of prospective studies was conducted to assess the association.

METHODS

Relevant studies were identified by searching PubMed and Embase databases to March 23, 2018. Relative risks (RRs) with 95% confidence intervals (CIs) were combined with the fixed-effects model or random-effects model according to the degree of heterogeneity. Moreover, stratified analyses and sensitivity analysis were carried out for further analysis.

RESULTS

Seven prospective studies involving 1988 stroke events among 18048 participants were eligible for our meta-analysis. The combined RRs for total stroke were 1.29 (95% CI: 1.10, 1.52) for the highest versus lowest category of FGF23, with low heterogeneity among studies (P_{heterogeneity} = 0.38, I² = 6.1%). Stratified analyses showed that the combined RRs for ischemic stroke (IS) and hemorrhagic stroke (HS) risk were 1.12 (95% CI: 0.92, 1.37) and 2.63 (95% CI: 1.61, 4.30), respectively. In the stratification by geographic areas, the association between higher FGF23 and stroke was similar with studies performed in the United States (RR = 1.24, 95%CI: 1.03, 1.49) and Europe (RR = 1.88, 95%CI: 0.77, 4.55); however, only the results in the United States were statistically significant. Sensitivity analysis indicated the combined results were robust.

CONCLUSIONS

Our meta-analysis showed that higher FGF23 levels were associated with an increased risk of stroke. The positive association consistently existed in HS rather than in IS. Further studies are required to confirm these causal associations and to investigate the mechanisms.

Vitamin D treatment attenuates cardiac FGF23/FGFR4 signaling and hypertrophy in uremic rats

Background

Vitamin D deficiency and excess of circulating fibroblast growth factor 23 (FGF23) contribute to cardiovascular mortality in patients with chronic kidney disease (CKD). FGF23 activates FGF receptor 4 and (FGFR4) calcineurin/nuclear factor of activated T cells (NFAT) signaling in cardiac myocytes, thereby causing left ventricular hypertrophy (LVH). Here, we determined if 1,25-dihydroxyvitamin D (calcitriol) inhibits FGF23-induced cardiac signaling and LVH.

Methods

5/6 nephrectomized (5/6 Nx) rats were treated with different doses of calcitriol for 4 or 10 weeks and cardiac expression of FGF23/FGFR4 and activation of calcineurin/NFAT as well as LVH were analyzed. FGFR4 activation and hypertrophic cell growth were studied in cultured cardiac myocytes that were co-treated with FGF23 and calcitriol.

Results

In 5/6Nx rats with LVH, we detected elevated FGF23 expression in bone and myocardium, increased cardiac expression of FGFR4 and elevated cardiac activation of calcineurin/NFAT signaling. Cardiac expression levels of FGF23 and FGFR4 significantly correlated with the presence of LVH in uremic rats. Treatment with calcitriol reduced LVH as well as cardiac FGFR4 expression and calcineurin/NFAT activation. Bone and cardiac FGF23 expression were further stimulated by calcitriol in a dose-dependent manner, but levels of intact cardiac FGF23 protein were suppressed by high-dose calcitriol. In cultured cardiac myocytes, co-treatment with calcitriol blocked FGF23-induced activation of FGFR4 and hypertrophic cell growth.

Conclusions

Our data suggest that in CKD, cardioprotective effects of calcitriol stem from its inhibitory actions on the cardiac FGF23/FGFR4 system, and based on their counterbalancing effects on cardiac myocytes, high FGF23 and low calcitriol synergistically contribute to cardiac hypertrophy.

Biology of Fibroblast Growth Factor 23: From Physiology to Pathology

Fibroblast growth factor (FGF)23 is a phosphaturic hormone produced by osteocytes and osteoblasts that binds to FGF receptors in the presence of the transmembrane protein αKlotho. FGF23 mainly targets the renal proximal tubule to inhibit calcitriol production and the expression of the sodium/phosphate cotransporters NaPi2a and NaPi2c, thus inhibiting renal phosphate reabsorption. FGF23 also acts on the parathyroid glands to inhibit parathyroid hormone synthesis and secretion. FGF23 regulation involves many systemic and local factors, among them calcitriol, phosphate, and parathyroid hormone. Increased FGF23 is primarily observed in rare acquired or genetic disorders, but chronic kidney disease is associated with a reactional increase in FGF23 to combat hyperphosphatemia. However, high FGF23 levels induce left ventricular hypertrophy (LVH) and are associated with an increased risk of mortality. In this review, we describe FGF23 physiology and the pathological consequences of high or low FGF23 levels.