FGF23 at the crossroads of phosphate, iron economy and erythropoiesis

The impact of monoallelic inactivation mutations in the ENPP1 gene on pediatric skeletal development: a case report and literature review

Background

Recently, in our clinical work, we discovered a case of abnormal bone metabolism in children resulting from an inactivated mutation of the ENPP1 gene. Through this discovery, we highlighted the impact of the ENPP1 gene on the skeletal growth and development of children, and provided new ideas for the clinical diagnosis of bone diseases in children.

Case summary

A 17-year-old boy presented with abnormal gait and hip pain. The anteroposterior (AP) pelvis X-ray revealed bilateral abnormalities in the femoral metaphysis, acetabulum, and ilium bones, as well as slippage of the left femoral head epiphysis. After genetic testing was carried out, it was found that the patient had a monoallelic inactivation mutations in the ENPP1 gene, which is the pathogenic gene of Autosomal-Recessive Hypophosphatemic Rickets 2 (ARHR2). Genetic testing identified that the patient had an inactivating mutation in the ENPP1 gene, which is associated with Autosomal-Recessive Hypophosphatemic Rickets 2 (ARHR2). Since symptoms were present at the time of diagnosis, the current treatment plan includes symptomatic treatments, such as calcium supplementation and femoral epiphyseal fixation.

Conclusion

We discovered that the inactivating mutation of the ENPP1 gene has an influence on bone metabolism, particularly calcium and phosphorus metabolism, which can lead to severe adverse effects on the growth and development of pediatric patients. Through this case and a review of the literature, we aim to enable clinical physicians to establish a holistic perspective during pediatric consultations.

iFGF23 Plasma Levels in Transfusion-Dependent β-Thalassemia: Insights into Bone and Iron Metabolism

Background: FGF23 is a phosphate homeostasis regulator; the literature suggests a link between FGF23, iron homeostasis and erythropoiesis. Little is known about the FGF23 level variations in β-thalassemia (βT), which is characterized by ineffective erythropoiesis and iron overload. Our cross-sectional study aims to evaluate the iFGF23 level variations in a large cohort of βT patients considering their bone mineral densities (BMDs) and iron loads. Methods: Clinical, biochemical and radiological data were collected from 213 transfusion-dependent βT (TDT) adults referring to the Regional HUB Centre for Thalassaemia and Haemoglobinopathies in Ferrara, Italy. The iFGF23 levels in the TDT patients were compared to the general population's reference range. The BMDs and hearth and liver iron deposits were assessed with DEXA scans and MRI, respectively. Results: The iFGF23 distribution in the TDT subjects is significantly different from that of the general population. The iFGF23 levels are positively correlated with the age at transfusion initiation and calcium and phosphate levels and are negatively correlated with the osteocalcin plasma levels. Patients treated with deferasirox had lower iFGF23 levels than those treated with other chelators. The iFGF23 levels are not correlated with the BMD or iron status. Conclusions: These findings provide insights into the relationship between the iFGF23 and bone and iron metabolism in TDT patients. Further studies are needed to explore its potential clinical relevance.

No longer to be ignored: Hypophosphatemia following intravenous iron administration

Intravenous iron supplementation is increasingly used to safely and effectively correct iron deficiency anemia, but some formulations are linked to a renal phosphate wasting syndrome which is mediated by fibroblast growth factor 23. Unawareness among prescribers and the nonspecific clinical symptoms of hypophosphatemia result in underreporting of this complication. Even though it is often an asymptomatic and self-limiting condition, accumulating evidence from case reports and dedicated randomized controlled trials show that IV iron induced hypophosphatemia may be associated with clinical symptoms. If hypophosphatemia is not recognized and treated, a metabolic bone disease phenotype may develop, pathophysiologically reminiscent of hypophosphatemic rickets as seen in X-linked hypophosphatemic rickets or oncogenic osteomalacia. This syndrome is particularly, but not uniquely, associated with formulations containing ferric carboxymaltose, probably due to specific chemical characteristics of its carbohydrate moiety. Risk factors include repeated infusion, severity of iron deficiency, as well as normal kidney function. Coexisting vitamin D deficiency or hyperparathyroidism increase the risk of metabolic bone disease. Complications can be easily prevented by an early diagnosis and switching to another IV iron formulation. In this review, we describe the epidemiology and pathophysiology of this condition, to raise awareness among prescribing clinicians.

Vitamin D assay and supplementation: still debatable issues

Over the last decades, in addition to the improvement of pathophysiological knowledge regarding the role and mechanisms of action of vitamin D, there has been a progressive advancement in analytical technologies for its measurement, as well as in methodological standardization. A significant number of scientific works, meta-analyses, and guidelines have been published on the importance of vitamin D and the need for supplementation in deficient individuals. However, it appears necessary to clarify the fundamental elements related to the measurement of vitamin D (both at the strictly analytical and post-analytical levels) and the scientific evidence related to the efficacy/safety of supplementation. In particular, there is a need to discuss current recommended levels for deficiency, insufficiency and possible toxicity in the light of evidence from standardization projects. Additionally, given the important interrelations between vitamin D, parathyroid hormone (PTH), and fibroblast growth factor-23 (FGF23), the analytical issues and clinical utility of these biomarkers will be discussed.

FGF-based drug discovery: advances and challenges

The fibroblast growth factor (FGF) family comprises 15 paracrine-acting and 3 endocrine-acting polypeptides, which govern a multitude of processes in human development, metabolism and tissue homeostasis. Therapeutic endocrine FGFs have recently advanced in clinical trials, with FGF19 and FGF21-based therapies on the cusp of approval for the treatment of primary sclerosing cholangitis and metabolic syndrome-associated steatohepatitis, respectively. By contrast, while paracrine FGFs were once thought to be promising drug candidates for wound healing, burns, tissue repair and ischaemic ailments based on their potent mitogenic and angiogenic properties, repeated failures in clinical trials have led to the widespread perception that the development of paracrine FGF-based drugs is not feasible. However, the observation that paracrine FGFs can exert FGF hormone-like metabolic activities has restored interest in these FGFs. The recent structural elucidation of the FGF cell surface signalling machinery and the formulation of a new threshold model for FGF signalling specificity have paved the way for therapeutically harnessing paracrine FGFs for the treatment of a range of metabolic diseases.

Investigating the Pulmonary Host Response of Acinetobacter baumannii Infection-Associated Pneumonia by Metagenomic Next-Generation Sequencing

Background: For investigating the host response in Acinetobacter baumannii associated pneumonia, we analyzed the host genetic sequences obtained from metagenomic next-generation sequencing (mNGS). Methods: The samples for mNGS were bronchoalveolar lavage fluid (BALF) collected from the lungs of patients infected with A. baumannii and from patients without bacterial infections. BALF samples from patients with pneumonia were collected from the lungs of patients infected with A. baumannii with New Delhi metallo-β-lactamase (NDM, before treatment), A. baumannii with NDM (post-treatment), A. baumannii without resistant genes, and those without bacterial infection. Partek was used for investigating enriched functions and pathways related to the pulmonary host response to pneumonia caused by A. baumannii with NDM infection and A. baumannii without antimicrobial-resistant genes. The STRING was employed for identifying protein interaction pathways related to the pulmonary host response to pneumonia caused by A. baumannii without antimicrobial-resistant genes. Results: In pulmonary host response to pneumonia caused by A. baumannii with NDM, five immune system-related pathways and five pathways related to signal transduction were identified. No significant differences were observed in the immune system and signal transduction pathways in the pulmonary host response to pneumonia caused by A. baumannii without antimicrobial-resistant genes. However, significant differences were noted in the phagosome, ferroptosis, and regulation of the actin cytoskeleton in cellular processes. Conclusions: mNGS provides information not only on pathogen gene expression but also on host gene expression. In this study, we found that pneumonia with A. baumannii carrying the NDM resistance gene triggers stronger immune responses in the lung, while pneumonia with A. baumannii lacking antimicrobial resistance genes is more linked to iron-related pathways.

Approach to determining etiology of hypophosphatemia in a patient with coexisting phosphaturic mesenchymal tumor and fibrous dysplasia

Dysregulated FGF23 production is a demonstrated cause of hypophosphatemia and osteomalacia. Diseases associated with these conditions include phosphaturic mesenchymal tumor (PMT) causing tumor induced osteomalacia, various forms of rickets, and fibrous dysplasia (FD). Coexistence of 2 conditions that can increase FGF23 concentrations is rare. We report a case of a 79-yr-old man who presented with rib and right flank pain. Imaging revealed bone lesions in the right iliac wing, left supra-acetabular area, and L4 vertebral body. Biopsies showed a right iliac PMT and left supra-acetabular FD. Cryoablation of both lesions resolved the phosphaturia with normalization of phosphorus level. Coexistence of PMT and FD in this patient with hypophosphatemia raised questions about the source of the FGF23, meaning of coexistence of PMT and FD in the same patient and, about the nature of the third lesion in the L4 vertebral body. Using FGF23 mRNA chromogenic in situ hybridization, we identified the PMT, rather than the FD, as the source of FGF23. Lack of GNAS mutation in the PMT suggested it being independent of FD. Assessment by the intact FGF23: total FGF23 ratio as well as gallium-DOTATATE scan suggested that the vertebral body lesion could represent FD. Other than understanding difference in underlying molecular processing of FGF23 in PMT and FD, testing for mutations, imaging studies as well as in situ hybridization helped solve the questions arising from this unique case of coexistence of PMT and FD.

Advances in the mechanisms of vascular calcification in chronic kidney disease

Vascular calcification (VC) is common in patients with advanced chronic kidney disease (CKD).A series of factors, such as calcium and phosphorus metabolism disorders, uremic toxin accumulation, inflammation and oxidative stress and cellular senescence, cause osteoblast-like differentiation of vascular smooth muscle cells, secretion of extracellular vesicles, and imbalance of calcium regulatory factors, which together promote the development of VC in CKD. Recent advances in epigenetics have provided better tools for the investigation of VC etiology and new approaches for finding more accurate biomarkers. These advances have not only deepened our understanding of the pathophysiological mechanisms of VC in CKD, but also provided valuable clues for the optimization of clinical predictors and the exploration of potential therapeutic targets. The aim of this article is to provide a comprehensive overview of the pathogenesis of CKD VC, especially the new advances made in recent years, including the various key factors mentioned above. Through the comprehensive analysis, we expect to provide a solid theoretical foundation and research direction for future studies targeting the specific mechanisms of CKD VC, the establishment of clinical predictive indicators and the development of potential therapeutic strategies.

Decoding FGF/FGFR Signaling: Insights into Biological Functions and Disease Relevance

Fibroblast Growth Factors (FGFs) and their cognate receptors, FGFRs, play pivotal roles in a plethora of biological processes, including cell proliferation, differentiation, tissue repair, and metabolic homeostasis. This review provides a comprehensive overview of FGF-FGFR signaling pathways while highlighting their complex regulatory mechanisms and interconnections with other signaling networks. Further, we briefly discuss the FGFs involvement in developmental, metabolic, and housekeeping functions. By complementing current knowledge and emerging research, this review aims to enhance the understanding of FGF-FGFR-mediated signaling and its implications for health and disease, which will be crucial for therapeutic development against FGF-related pathological conditions.

Why cells need iron: a compendium of iron utilisation

Iron deficiency is globally prevalent, causing an array of developmental, haematological, immunological, neurological, and cardiometabolic impairments, and is associated with symptoms ranging from chronic fatigue to hair loss. Within cells, iron is utilised in a variety of ways by hundreds of different proteins. Here, we review links between molecular activities regulated by iron and the pathophysiological effects of iron deficiency. We identify specific enzyme groups, biochemical pathways, cellular functions, and cell lineages that are particularly iron dependent. We provide examples of how iron deprivation influences multiple key systems and tissues, including immunity, hormone synthesis, and cholesterol metabolism. We propose that greater mechanistic understanding of how cellular iron influences physiological processes may lead to new therapeutic opportunities across a range of diseases.

Safety and Efficacy of Ferric Carboxymaltose for Iron Deficiency Anemia in Inflammatory Bowel Disease: A Systematic Review

Ulcerative colitis and Crohn's disease, two types of inflammatory bowel disease (IBD), often cause anemia, primarily due to iron deficiency and chronic inflammation. Anemia negatively affects patients' daily functioning and quality of life, causing symptoms including headaches, exhaustion, and dyspnea. In IBD, iron deficiency arises from reduced intake, chronic blood loss, and impaired absorption. While oral iron supplements are commonly used, their poor absorption and gastrointestinal side effects limit their effectiveness, especially in IBD patients. The European Crohn's and Colitis Organization (ECCO) recommends intravenous iron, such as ferric carboxymaltose (FCM), as iron deficiency anemia in IBD can be managed using a safe and efficient substitute. With regard to treating iron deficiency anemia in patients with IBD, the purpose of this study is to investigate the safety and effectiveness of intravenous ferric carboxymaltose. We conducted a thorough search of medical databases, such as the Cochrane library, PubMed, and ResearchGate, to gather relevant literature. Using the databases, we found a total of 297 relevant articles. The identified studies have been screened, eligibility criteria have been introduced, and 14 research studies were selected for inclusion. This review adhered to the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines, with quality assessments conducted using the Cochrane risk of bias 2 scale for randomized tests and the Newcastle-Ottawa scale for observational examination. We reviewed 14 research articles involving 2,493 patients. Among these, five were randomized controlled trials (RCTs), and the remaining nine were observational studies. The primary outcomes assessed were the therapeutic response (defined as hemoglobin ≥2 g/dL rise or normalization, improvement in iron profile parameters) and any adverse effects after FCM is administered to IBD patients. FCM was found to be highly effective in improving hemoglobin and iron profile parameters, with a generally good safety profile. Ferric carboxymaltose was the most efficient and well-tolerated intravenous (IV) iron formulation, proving safer and more effective than other iron therapies in patients suffering from IBD. However, severe hypophosphatemia can lead to serious complications, including heart failure, pulmonary failure, rhabdomyolysis, fractures, and osteomalacia, which may worsen its long-term impact. Therefore, the risk of hypophosphatemia associated with prolonged FCM use requires careful monitoring and further research to ensure its long-term safety and assess its effects on patients' quality of life.

Developmental regulation of endothelial-to-hematopoietic transition from induced pluripotent stem cells

Hematopoietic stem cells (HSCs) arise in embryogenesis from a specialized hemogenic endothelium (HE). In this process, HE cells undergo a unique fate change termed endothelial-to-hematopoietic transition, or EHT. While induced pluripotent stem cells (iPSCs) give rise to HE with robust hemogenic potential, the generation of bona fide HSCs from iPSCs remains a challenge. Here, we map single cell dynamics of EHT during embryoid body differentiation from iPSCs and integrate it with human embryo datasets to identify key transcriptional differences between in vitro and in vivo cell states. We further map ligand-receptor interactions associated with differential expression of developmental programs in the iPSC system. We found that the expression of endothelial genes was incompletely repressed during iPSC EHT. Elevated FGF signaling by FGF23, an endothelial pathway ligand, was associated with differential gene expression between in vitro and in vivo EHT. Chemical inhibition of FGF signaling during EHT increased HSPC generation in the zebrafish, while an FGF agonist had the opposite effect. Consistently, chemical inhibition of FGF signaling increased hematopoietic output from iPSCs. In summary, we map the dynamics of EHT from iPSCs at single cell resolution and identify ligand-receptor interactions that can be modulated to improve iPSC differentiation protocols. We show, as proof of principle, that chemical inhibition of FGF signaling during EHT improves hematopoietic output in zebrafish and the iPSC system.

Renal Function Is Associated with Changes in Bone Mineral Density in Postmenopausal Osteoporotic Women Treated with Denosumab: Data From a Retrospective Cohort Study

Background/Objectives: Renal function influences bone metabolism, as kidney failure can increase the risk of fractures. Denosumab is an approved osteoporosis treatment, but its efficacy in relation to renal function has not yet been studied in real-life scenarios. This study aimed to investigate the denosumab-induced change in bone mineral density (BMD) according to kidney function. Methods: A retrospective analysis was conducted at the outpatient clinic in postmenopausal women receiving denosumab (60 mg subcutaneously administered every 6 months). The glomerular filtration rate (eGFR) was measured by the 2021 CKD-EPI equation and patients were stratified for eGFR categories. BMD was measured by dual-energy X-ray absorptiometry. Results: 128 women (mean age 70.3 ± 9.4 years) were recruited. The mean denosumab treatment duration was 3.9 ± 1.4 years and all the participants had improved BMD values. In stepwise multiple regression analysis-after controlling for age, BMI, and treatment duration-the eGFR value (ß = -0.11, SE 0.04, p = 0.01) was independently associated with the lumbar spine BMD change. The same association remained when the eGFR categories were considered (ß = 3.564, SE 1.29, p = 0.007). In addition, after controlling for BMI and the duration of denosumab treatment, age (ß = -0.7915, SE 0.37, p = 0.03) and eGFR (ß = -0.3257, SE 0.1567, p = 0.04) were found to be associated with femoral neck BMD change. The association remained when considering eGFR categories (ß = 8.7339, SE 4.29, p = 0.04). Conclusions: This retrospective study suggests that eGFR is associated with denosumab efficacy in postmenopausal women treated for osteoporosis.

Endocrine fibroblast growth factors in domestic animals

Fibroblast growth factors (FGFs) are a group of structurally homologous yet functionally pleiotropic proteins. Canonical and intracellular FGFs have primarily autocrine or paracrine effects. However, the FGF19 subfamily, composed of FGF15/19, FGF21, and FGF23, act as endocrine hormones that regulate bile acid, metabolic, and phosphorus homeostasis, respectively. Current research in human and rodent models demonstrates the potential of these endocrine FGFs to target various diseases, including disorders of inherited hypophosphatemia, chronic liver disease, obesity, and insulin resistance. Many diseases targeted for therapeutic use in humans have pathophysiological overlaps in domestic animals. Despite the potential clinical and economic impact, little is known about endocrine FGFs and their signaling pathways in major domestic animal species compared with humans and laboratory animals. This review aims to describe the physiology of these endocrine FGFs, discuss their current therapeutic use, and summarize the contemporary literature regarding endocrine FGFs in domestic animals, focusing on potential future directions.

Impact of Serum Phosphate on Hemoglobin Level: A Longitudinal Analysis on a Large Cohort of Dialysis Patients

Background/Objectives: Phosphate is a macro-element involved in all cellular energetic processes. As about 90% of the phosphate filtered by the glomerulus is excreted by kidneys, the impairment of renal function and the consequent over-secretion of parathyroid hormone and fibroblast growth factor 23 results in the increase in the serum phosphate levels. The association between phosphate and hemoglobin is controversial, as both direct and indirect relationships have been reported. The present study aims to investigate the relationship between phosphate and hemoglobin in a large prospective, longitudinal cohort including dialysis patients from the Sicilian Registry of Nephrology, Dialysis, and Transplantation. Methods: In this prospective cohort study, we included 6263 hemodialysis patients to achieve a total of 120,462 repeated measurements of serum phosphate and hemoglobin over time. The longitudinal association between phosphate and hemoglobin was analyzed by univariate and multivariate Linear Mixed Models. Results: The mean age was 66 ± 16 years and the median dialysis vintage was 5 months [IQR: 2-16]. Mean and median values of hemoglobin and phosphate were 10.7 g/dL (SD 1.3 g/dL) and 4.6 mg/dL [IQR 3.9-5.5 mg/dL], respectively. The multivariate model, adjusted for potential confounders, confirmed the positive association between serum phosphate and hemoglobin [adjβ = 0.13, 95%CI 0.03-0.23, p = 0.01)]. These results were confirmed in analyses stratified for the use of phosphate binders. Conclusions: In our large cohort of dialysis patients, we found a linear, direct relationship between phosphate and hemoglobin levels. As a reduction in phosphate is associated with a parallel reduction in hemoglobin levels, hypophosphatemia can accentuate anemia in dialysis patients. Our results generate the hypothesis that monitoring serum phosphate in clinical practice might provide a better management of anemia.

Young hearts, early risks: novel cardiovascular biomarkers in former very preterm infants at kindergarten age

BACKGROUND

Preterm birth is associated with long-term cardiovascular morbidity and mortality. In adults, fibroblast growth factor-23 (FGF-23), α-Klotho, and secretoneurin have all garnered attention as cardiovascular biomarkers, but their utility in pediatric populations has not yet been ascertained. The aim of this pilot study was to evaluate these novel cardiovascular biomarkers and their association with indicators of cardiovascular impairment in the highly vulnerable population of former very preterm infants.

METHODS

Five- to seven-year-old children born at < 32 weeks' gestation were eligible for the study. Healthy same-aged children born at term served as controls. Biomarkers were quantified in fasting blood samples, and echocardiographic measurements including assessment of aortic elastic properties were obtained.

RESULTS

We included 26 former very preterm infants and 21 term-born children in the study. At kindergarten age, former very preterm infants exhibited significantly higher plasma concentrations of biologically active intact FGF-23 (iFGF-23; mean 43.2 pg/mL vs. 29.1 pg/mL, p = 0.003) and secretoneurin (median 93.8 pmol/L vs. 70.5 pmol/L, p = 0.046). iFGF-23 inversely correlated with distensibility of the descending aorta.

CONCLUSION

In preterm-born children, iFGF-23 and secretoneurin both offer prospects as valuable cardiovascular biomarkers, potentially allowing for risk stratification and timely implementation of preventive measures.

IMPACT

Former very preterm infants have increased plasma concentrations of the novel cardiovascular biomarkers intact fibroblast growth factor-23 (iFGF-23) and secretoneurin at kindergarten age. Increases in iFGF-23 concentrations are associated with decreased distensibility of the descending aorta even at this early age. Monitoring of cardiovascular risk factors is essential in individuals with a history of preterm birth. Both iFGF-23 and secretoneurin hold promise as clinically valuable biomarkers for risk stratification, enabling the implementation of early preventive measures.

High-Throughput Preosteoblastic Spheroids Elevate Fibroblast Growth Factor 23 via Parathyroid Hormone Signaling Pathway

Fibroblast growth factor 23 (FGF23) plays a crucial role in managing renal phosphate and the synthesis of 1,25(OH)2-vitamin D3, which is essential for bone homeostasis. Developing robust in vitro systems to study FGF23-regulating mechanisms is crucial for advancing our knowledge and identifying potential therapeutic targets. The traditional in vitro 2D culture system results in relatively low expression of FGF23, complicating further exploration of its regulatory mechanisms and potential therapeutic targets. Herein, we reported a high-throughput approach to generate preosteoblastic cell spheroids with enhanced FGF23 production. For this purpose, murine preosteoblast cell line (MC3T3-E1) was cultured in our previously reported nonadherent microwells (200 µm in diameter, 148 µm in depth, and 100 µm space in between) and self-assembled into spheroids with a diameter of 92.3 ± 15.0 µm after 24 h. Compared with monolayer culture, the MC3T3-E1 spheroids showed a significant upregulation of FGF23 in both gene and protein levels after 24 h of serum-free induction. RNA sequencing and western blotting analysis further suggested that the enhanced FGF23 production in MC3T3-E1 spheroids was attributed to the activation of the parathyroid hormone (PTH)/PTH1R signaling pathway. Impressively, inhibition of PTH signaling through small molecular inhibitors or short hairpin RNA targeting PTH1R effectively reduced FGF23 production. In summary, the current study revealed the efficacy of the high-throughput formation of preosteoblast cell spheroid in stimulating FGF23 expression for mechanistic studies. Importantly, our findings highlight the potential of the current 3D spheroid system for target identification and drug discovery.

Exploring endocrine FGFs - structures, functions and biomedical applications

The family of fibroblast growth factors (FGFs) consists of 22 members with diverse biological functions in cells, from cellular development to metabolism. The family can be further categorized into three subgroups based on their three modes of action. FGF19, FGF21, and FGF23 are endocrine FGFs that act in a hormone-like/endocrine manner to regulate various metabolic activities. However, all three members of the endocrine family require both FGF receptors (FGFRs) and klotho co-receptors to elicit their functions. α-klotho and β-klotho act as scaffolds to bring endocrine FGFs closer to their receptors (FGFRs) to form active complexes. Numerous novel studies about metabolic FGFs' structures, mechanisms, and physiological insights have been published to further understand the complex molecular interactions and physiological activities of endocrine FGFs. Herein, we aim to review the structures, physiological functions, binding mechanisms to cognate receptors, and novel biomedical applications of endocrine FGFs in recent years.

Associations of Intact and C-Terminal FGF23 with Inflammatory Markers in Older Patients Affected by Advanced Chronic Kidney Disease

Background: In patients with chronic kidney disease (CKD), Fibroblast Growth Factor 23 (FGF23) is markedly increased and has been proposed to interact with systemic inflammation. Methods: In this cross-sectional study, we evaluated the correlations of intact FGF23, c-terminal FGF23, and the FGF23 ratio (c-terminal to intact) with some inflammatory cytokines in 111 elderly patients with advanced CKD not yet in dialysis. Results: Estimated glomerular filtration rate (eGFR) was inversely correlated with intact FGF23 and c-terminal FGF23, as well as with interleukin 6 (IL-6), tumor necrosis factor alpha (TNFα), and monocyte chemoattractant protein-1 (MCP-1). Intact FGF23 levels were directly correlated with IL-6 (r = 0.403; p < 0.001) and TNFα (r = 0.401; p < 0.001) while c-terminal FGF23 was directly correlated with MCP-1 (r = 0.264; p = 0.005). The FGF23 ratio was, instead, inversely correlated with IL-6 (r = -0.326; p < 0.001). Multivariate analysis revealed that intact FGF23 was directly associated with TNFα [B = 0.012 (95% CI 0.006, 0.019); p = 0.003] and c-terminal FGF23 was directly associated with MCP-1 [B = 0.001 (95% CI 0.000, 0.002); p = 0.038], while the FGF23 ratio was inversely correlated with IL-6 [B = -0.028 (95% CI -0.047, -0.010); p = 0.002]. Conclusions: Our data demonstrate that, in CKD patients, intact FGF23 and the metabolites deriving from its proteolytic cleavage are differently associated with some inflammatory pathways. In particular, intact FGF23 is mainly associated with IL-6 and TNFα, c-terminal FGF23 with MCP-1, and the FGF23 ratio with IL6.

[Electrolyte disorders in oncological patients]

Electrolyte disorders (ED) are common in patients with cancer and in most cases, the etiologies do not differ from the general population. They may also be induced by the cancer, its therapy or paraneoplastic syndromes. ED are associated with poor outcomes, increased morbidity and mortality in this population. Hyponatremia is the most common disorder, often multifactorial, iatrogenic or secondary to the syndrome of inappropriate antidiuretic hormone secretion, usually due to small cell lung cancer. More rarely, hyponatremia may reveal adrenal insufficiency. Hypokalemia is generally multifactorial and associated with other ED. Cisplatin and ifosfamide induce proximal tubulopathies with hypokalemia and/or hypophosphatemia. Hypomagnesemia is often iatrogenic, related to cisplatin or cetuximab, but can be prevented by supplementation. Hypercalcemia can impair life quality and be life-threatening in the most severe cases. Hypocalcemia is less common and often of iatrogenic origin. Finally, the tumor lysis syndrome is a diagnostic and therapeutic emergency that affects the prognosis of patients. Its incidence tends to increase in solid oncology, related to the improvement of therapies. Prevention and early diagnosis of ED are essential to optimize the overall management of patients with underlying cancer and cancer therapy. The aim of this review is to synthesize most frequent ED and their management.

Effect of ferric citrate hydrate on fibroblast growth factor 23 and platelets in non-dialysis-dependent chronic kidney disease and non-chronic kidney disease patients with iron deficiency anemia

BACKGROUND

Iron deficiency anemia (IDA) increases levels of C-terminal fibroblast growth factor 23 (cFGF23) and platelet count (PLT), each of which is associated with cardiovascular events. Therefore, we hypothesized that iron replacement with ferric citrate hydrate (FC) would decrease cFGF23 levels and PLT in patients with IDA.

METHODS

In a randomized, open-label, multicenter, 24-week clinical trial, patients with non-dialysis-dependent chronic kidney disease (CKD) and non-CKD complicated by IDA (8.0 ≤ hemoglobin < 11.0 g/dL; and serum ferritin < 50 ng/mL [CKD]; < 12 ng/mL [non-CKD]) were randomized 1:1 to FC-low (500 mg: approximately 120 mg elemental iron/day) or FC-high (1000 mg: approximately 240 mg elemental iron/day). If sufficient iron replacement had been achieved after week 8, further treatment was discontinued.

RESULTS

Seventy-three patients were allocated to FC-low (CKD n = 21, non-CKD n = 15) and FC-high (CKD n = 21, non-CKD n = 16). Regardless of CKD status, FC increased serum ferritin and transferrin saturation, did not change intact FGF23 or serum phosphorus, but decreased cFGF23. In FC-low group, median changes in cFGF23 from baseline to week 8 were -58.00 RU/mL in CKD and -725.00 RU/mL in non-CKD; in FC-high group, the median changes were -66.00 RU/mL in CKD and -649.50 RU/mL in non-CKD. By week 8, FC treatment normalized PLT in all patients with high PLT at baseline (>35.2 × 104/µL; FC-low: 1 CKD, 8 non-CKD; FC-high: 3 CKD, 8 non-CKD).

CONCLUSION

Regardless of CKD status, iron replacement with FC decreased elevated cFGF23 levels and normalized elevated PLT in patients with IDA.

CLINICAL TRIAL REGISTRATION NUMBER

jRCT2080223943.

Glycosylation of FGF/FGFR: An underrated sweet code regulating cellular signaling programs

Fibroblast growth factors (FGFs) and their receptors (FGFRs) constitute plasma-membrane localized signaling hubs that transmit signals from the extracellular environment to the cell interior, governing pivotal cellular processes like motility, metabolism, differentiation, division and death. FGF/FGFR signaling is critical for human body development and homeostasis; dysregulation of FGF/FGFR units is observed in numerous developmental diseases and in about 10% of human cancers. Glycosylation is a highly abundant posttranslational modification that is critical for physiological and pathological functions of the cell. Glycosylation is also very common within FGF/FGFR signaling hubs. Vast majority of FGFs (15 out of 22 members) are N-glycosylated and few FGFs are O-glycosylated. Glycosylation is even more abundant within FGFRs; all FGFRs are heavily N-glycosylated in numerous positions within their extracellular domains. A growing number of studies points on the multiple roles of glycosylation in fine-tuning FGF/FGFR signaling. Glycosylation modifies secretion of FGFs, determines their stability and affects interaction with FGFRs and co-receptors. Glycosylation of FGFRs determines their intracellular sorting, constitutes autoinhibitory mechanism within FGFRs and adjusts FGF and co-receptor recognition. Sugar chains attached to FGFs and FGFRs constitute also a form of code that is differentially decrypted by extracellular lectins, galectins, which transform FGF/FGFR signaling at multiple levels. This review focuses on the identified functions of glycosylation within FGFs and FGFRs and discusses their relevance for the cell physiology in health and disease.

A case of hypophosphatemia and elevated intact fibroblast growth factor 23 levels after short-term saccharated ferric oxide administration in a young woman and database analysis of adverse drug reactions in Japan

Intravenous iron replacement therapy is a common treatment for iron deficiency. Commonly used agents in this treatment include ferric carboxymaltose, ferric derisomaltose, and saccharated ferric oxide (SFO). These drugs are known to elevate fibroblast growth factor 23 levels, resulting in hypophosphatemia, but in past reports, hypophosphatemia attributable to SFO treatment has been associated mainly with prolonged administration over several weeks. The present study details our experience of a case of moderate hypophosphatemia (<2 mg/dL) in a 22-year-old woman who had no specific history of hypophosphatemia during the first 5 days of SFO treatment, and showed an increase in intact fibroblast growth factor 23 levels within the first week of treatment. Cases of hypophosphatemia have been reported as occurring as early as 1 week after the start of SFO administration in the Japanese Adverse Drug Event Report database. These cases, along with our case, underline the need for awareness of the possibility of hypophosphatemia from the early stage of SFO administration, regardless of the patient's age or dosage, as well as the need to monitor patients to prevent complications.

Nutritional Strategies for Managing Iron Deficiency in Adolescents: Approaches to a Challenging but Common Problem

Iron deficiency (ID) is a common and challenging problem in adolescence. In order to prevent, recognize, and treat ID in this age range, it is critical to understand the recommended daily intake of iron in relation to an adolescent's activity, dietary habits, and basal iron losses. Adolescents following vegetarian or vegan diets exclusively rely on plant-based, nonheme iron, which has decreased bioavailability compared with heme iron and requires increased total iron intake. Individuals with disordered eating habits, excessive menstrual blood loss, and certain chronic health conditions (including inflammatory bowel disease and heart failure) are at high risk of ID and the development of symptomatic iron deficiency anemia (IDA). Adolescent athletes and those with sleep and movement disorders may also be more sensitive to changes in iron status. Iron deficiency is typically treated with oral iron supplementation. To maximize iron absorption, oral iron should be administered no more than once daily, ideally in the morning, while avoiding foods and drinks that inhibit iron absorption. Oral iron therapy should be provided for ≥3 mo in the setting of ID to reach a ferritin of 20 ng/mL before discontinuation. Intravenous iron is being increasingly used in this population and has demonstrated efficacy and safety in adolescents. It should be considered in those with persistent ID despite a course of oral iron, severe and/or symptomatic IDA, and chronic inflammatory conditions characterized by decreased gastrointestinal iron absorption.

Energy homeostasis in the bone

The bone serves as an energy reservoir and actively engages in whole-body energy metabolism. Numerous studies have determined fuel requirements and bioenergetic properties of bone under physiological conditions as well as the dysregulation of energy metabolism associated with bone metabolic diseases. Here, we review the main sources of energy in bone cells and their regulation, as well as the endocrine role of the bone in systemic energy homeostasis. Moreover, we discuss metabolic changes that occur as a result of osteoporosis. Exploration in this area will contribute to an enhanced comprehension of bone energy metabolism, presenting novel possibilities to address metabolic diseases.

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.

The Intricacies of Renal Phosphate Reabsorption-An Overview

To maintain an optimal body content of phosphorus throughout postnatal life, variable phosphate absorption from food must be finely matched with urinary excretion. This amazing feat is accomplished through synchronised phosphate transport by myriads of ciliated cells lining the renal proximal tubules. These respond in real time to changes in phosphate and composition of the renal filtrate and to hormonal instructions. How they do this has stimulated decades of research. New analytical techniques, coupled with incredible advances in computer technology, have opened new avenues for investigation at a sub-cellular level. There has been a surge of research into different aspects of the process. These have verified long-held beliefs and are also dramatically extending our vision of the intense, integrated, intracellular activity which mediates phosphate absorption. Already, some have indicated new approaches for pharmacological intervention to regulate phosphate in common conditions, including chronic renal failure and osteoporosis, as well as rare inherited biochemical disorders. It is a rapidly evolving field. The aim here is to provide an overview of our current knowledge, to show where it is leading, and where there are uncertainties. Hopefully, this will raise questions and stimulate new ideas for further research.

The differential effect of modern intravenous iron on fibroblast growth factor 23 and phosphate in non-dialysis dependent CKD - the exploratory randomized controlled double-blind ExplorIRON-CKD study

BACKGROUND

Intravenous iron is commonly used in patients with non-dialysis-dependent chronic kidney disease (CKD). Modern intravenous iron compounds (e.g. ferric derisomaltose (FDI), ferric carboxymaltose (FCM)) are increasingly utilized with similar efficacy. A differential effect in terms of hypophosphatemia has been noted following administration of FCM, which may be related to fibroblast growth factor 23 (FGF23). This study was designed to examine the comparative effects of FDI and FCM on FGF23, phosphate and other markers of bone turnover.

METHODS

The single-center double-blind randomized controlled trial "Iron and Phosphaturia - ExplorIRON-CKD" primarily assessed the effects of FCM and FDI on intact FGF23 and phosphate, whilst also studying the impact on vitamin D, parathyroid hormone and phosphaturia. Bone markers including alkaline phosphatase, bone-specific alkaline phosphatase, procollagen type 1 N-terminal propeptide and carboxy-terminal collagen cross-linked telopeptide were monitored. Non-dialysis-dependent CKD patients (stage 3a-5) with iron deficiency with/without anemia (serum ferritin < 200 µg/L or transferrin saturation = 20% and serum ferritin 200-299 µg/L) were randomized to receive FDI or FCM in a 1:1 ratio. At baseline 1000 mg of intravenous iron was administered followed by 500-1000 mg at 1 month to achieve replenishment. Measurements were performed at baseline, 1-2 days following iron administration, 2 weeks, 1 month (second iron administration), 1-2 days following second administration, 2 months and 3 months following initial infusion.

RESULTS

Twenty-six patients participated in the trial; 14 randomized to FDI and 12 to FCM. Intact FGF23 increased following administration of iron, and the increase was significantly higher with FCM compared to FDI (Baseline to 1-2 days following 1st administration: FDI: 3.0 (IQR: - 15.1 - 13.8) % vs. FCM: 146.1 (IQR: 108.1-203.1) %; p < 0.001 and Baseline to 1-2 days following 2nd administration: FDI: 3.2 (IQR: - 3.5 - 25.4) % vs. FCM: 235.1 (138.5-434.6) %; p = 0.001). Phosphate levels decreased in the FCM group, causing a significant difference versus FDI 2 weeks following administration of the first dose. A significantly greater decrease in 1,25 (OH)2 Vitamin D was noted with FCM. Several markers of bone turnover significantly changed following administration of FCM but not FDI.

CONCLUSIONS

The study suggests a differential effect on FGF23 following administration of FCM compared to FDI in non-dialysis-dependent CKD patients, similar to other patient groups. This may lead to changes consistent with hypovitaminosis D and alterations in bone turnover with potential clinical consequences. Further definitive studies are required to understand these differences of intravenous iron compounds.

TRIAL REGISTRATION

European Union Drug Regulating Authorities Clinical Trials Database (EudraCT) number: 2019-004370-26 ( https://www.clinicaltrialsregister.eu/ctr-search/trial/2019-004370-26/GB ) (First date of trial registration: 03/12/2019).

The basics of phosphate metabolism

Phosphorus is an essential mineral that is, in the form of inorganic phosphate (Pi), required for building cell membranes, DNA and RNA molecules, energy metabolism, signal transduction and pH buffering. In bone, Pi is essential for bone stability in the form of apatite. Intestinal absorption of dietary Pi depends on its bioavailability and has two distinct modes of active transcellular and passive paracellular absorption. Active transport is transporter mediated and partly regulated, while passive absorption depends mostly on bioavailability. Renal excretion controls systemic Pi levels, depends on transporters in the proximal tubule and is highly regulated. Deposition and release of Pi into and from soft tissues and bone has to be tightly controlled. The endocrine network coordinating intestinal absorption, renal excretion and bone turnover integrates dietary intake and metabolic requirements with renal excretion and is critical for bone stability and cardiovascular health during states of hypophosphataemia or hyperphosphataemia as evident from inborn or acquired diseases. This review provides an integrated overview of the biology of phosphate and Pi in mammals.

Crosstalk Between the Neuroendocrine System and Bone Homeostasis

The homeostasis of bone microenvironment is the foundation of bone health and comprises 2 concerted events: bone formation by osteoblasts and bone resorption by osteoclasts. In the early 21st century, leptin, an adipocytes-derived hormone, was found to affect bone homeostasis through hypothalamic relay and the sympathetic nervous system, involving neurotransmitters like serotonin and norepinephrine. This discovery has provided a new perspective regarding the synergistic effects of endocrine and nervous systems on skeletal homeostasis. Since then, more studies have been conducted, gradually uncovering the complex neuroendocrine regulation underlying bone homeostasis. Intriguingly, bone is also considered as an endocrine organ that can produce regulatory factors that in turn exert effects on neuroendocrine activities. After decades of exploration into bone regulation mechanisms, separate bioactive factors have been extensively investigated, whereas few studies have systematically shown a global view of bone homeostasis regulation. Therefore, we summarized the previously studied regulatory patterns from the nervous system and endocrine system to bone. This review will provide readers with a panoramic view of the intimate relationship between the neuroendocrine system and bone, compensating for the current understanding of the regulation patterns of bone homeostasis, and probably developing new therapeutic strategies for its related disorders.

Phosphate Homeostasis and Disorders of Phosphate Metabolism

Phosphate is indispensable for human life and evolutionary changes over several millions of years have established tightly regulated mechanisms to ensure phosphate homeostasis. In this process, calcium and phosphate metabolism have come to be intricately linked together. Three hormones (PTH, FGF23 and Calcitriol) maintain the fine balance of calcium and phosphate metabolism through their actions at three sites (the gut, the kidneys and the skeleton). Disorders that disrupt this balance can have serious clinical consequences. Acute changes in serum phosphate levels can result in life threatening complications like respiratory failure and cardiac arrythmias. Chronic hypophosphataemia predominantly affects the musculoskeletal system and presents as impaired linear growth, rickets, osteomalacia and dental problems. Hyperphosphataemia is very common in the setting of chronic kidney disease and can be difficult to manage. A thorough understanding of calcium and phosphate homeostasis is essential to diagnose and treat conditions associated with hypo and hyperphosphataemia. In this review, we will discuss the calcium and phosphate metabolism, aetiologies and management of hypo and hyperphosphataemia.

Pharmacology of Mammalian Na+-Dependent Transporters of Inorganic Phosphate

Inorganic phosphate (Pi) is an essential component of many biologically important molecules such as DNA, RNA, ATP, phospholipids, or apatite. It is required for intracellular phosphorylation signaling events and acts as pH buffer in intra- and extracellular compartments. Intestinal absorption, uptake into cells, and renal reabsorption depend on a set of different phosphate transporters from the SLC20 (PiT transporters) and SLC34 (NaPi transporters) gene families. The physiological relevance of these transporters is evident from rare monogenic disorders in humans affecting SLC20A2 (Fahr's disease, basal ganglia calcification), SLC34A1 (idiopathic infantile hypercalcemia), SLC34A2 (pulmonary alveolar microlithiasis), and SLC34A3 (hereditary hypophosphatemic rickets with hypercalciuria). SLC34 transporters are inhibited by millimolar concentrations of phosphonoformic acid or arsenate while SLC20 are relatively resistant to these compounds. More recently, a series of more specific and potent drugs have been developed to target SLC34A2 to reduce intestinal Pi absorption and to inhibit SLC34A1 and/or SLC34A3 to increase renal Pi excretion in patients with renal disease and incipient hyperphosphatemia. Also, SLC20 inhibitors have been developed with the same intention. Some of these substances are currently undergoing preclinical and clinical testing. Tenapanor, a non-absorbable Na+/H+-exchanger isoform 3 inhibitor, reduces intestinal Pi absorption likely by indirectly acting on the paracellular pathway for Pi and has been tested in several phase III trials for reducing Pi overload in patients with renal insufficiency and dialysis.

FGF23 and klotho at the intersection of kidney and cardiovascular disease

Cardiovascular disease is the leading cause of death in patients with chronic kidney disease (CKD). As CKD progresses, CKD-specific risk factors, such as disordered mineral homeostasis, amplify traditional cardiovascular risk factors. Fibroblast growth factor 23 (FGF23) regulates mineral homeostasis by activating complexes of FGF receptors and transmembrane klotho co-receptors. A soluble form of klotho also acts as a 'portable' FGF23 co-receptor in tissues that do not express klotho. In progressive CKD, rising circulating FGF23 levels in combination with decreasing kidney expression of klotho results in klotho-independent effects of FGF23 on the heart that promote left ventricular hypertrophy, heart failure, atrial fibrillation and death. Emerging data suggest that soluble klotho might mitigate some of these effects via several candidate mechanisms. More research is needed to investigate FGF23 excess and klotho deficiency in specific cardiovascular complications of CKD, but the pathophysiological primacy of FGF23 excess versus klotho deficiency might never be precisely resolved, given the entangled feedback loops that they share. Therefore, randomized trials should prioritize clinical practicality over scientific certainty by targeting disordered mineral homeostasis holistically in an effort to improve cardiovascular outcomes in patients with CKD.

Incidence of Hypophosphatemia After Intravenous Administration of Iron: A Matching-Adjusted Indirect Comparison of Data from Japanese Randomized Controlled Trials

INTRODUCTION

Intravenous (IV) administration of iron is considered a safe and efficacious treatment for iron deficiency anemia (IDA), recommended in patients requiring rapid replenishment of iron, or intolerant or unresponsive to oral administration of iron. Recent randomized controlled trials (RCTs) have shown high incidence of hypophosphatemia after administration of two IV iron preparations: saccharated ferric oxide (SFO) and ferric carboxymaltose (FCM). The present study aimed to conduct matching-adjusted indirect comparison (MAIC) of hypophosphatemia incidence with these iron formulations and ferric derisomaltose (FDI) based on data from head-to-head RCTs conducted in Japan.

METHODS

A MAIC of hypophosphatemia incidence was conducted on the basis of data from two head-to-head RCTs. The relative odds of hypophosphatemia with FDI versus SFO were obtained from patient-level data from a recent RCT and adjusted for cumulative iron dose, while parametric models of serum phosphate levels from a separate RCT were used to estimate the relative odds of hypophosphatemia with FCM with SFO. An anchored MAIC was then conducted comparing FDI with FCM.

RESULTS

The adjusted odds of experiencing hypophosphatemia were significantly lower with FDI than SFO [odds ratio (OR) of 0.02; 95% confidence interval (CI) 0.01-0.05]. The parametric models of serum phosphate from the RCT comparing FCM with SFO provided an estimated OR of 1.17 for the incidence of hypophosphatemia with FCM versus SFO. Combining the two estimates in the MAIC showed that the odds of experiencing hypophosphatemia would be 52.5 (95% CI 27.7-99.4) times higher with FCM than FDI in patients with IDA associated with heavy menstrual bleeding in Japan.

CONCLUSIONS

Direct comparison of patient-level data and a MAIC from two RCTs in Japanese patients with heavy menstrual bleeding indicated that hypophosphatemia is less frequent in patients treated with FDI than those with FCM or SFO. Results are in agreement with RCTs comparing FDI and FCM in patients with various etiologies conducted in the USA and Europe.

Case Report: Unexplained Mild Hypophosphatemia and Very High Serum FGF23 Concentrations

Fibroblast growth factor (FGF)23 is one of the major regulators of phosphate homeostasis. Hypophosphatemia can lead to muscle weakness, fatigue, and osteomalacia. In the setting of hypophosphatemia, serum FGF23 can be measured to differentiate between FGF23-mediated and non-FGF23-mediated renal phosphate wasting. C-terminal FGF23 (cFGF23) assays detect both cFGF23 and intact FGF23 (iFGF23). Circulating FGF23 is regulated by 1.25-dihydroxy-vitamin D, parathyroid hormone (PTH), serum phosphate, and serum calcium but also by, for example, iron status, inflammation, erythropoietin, and hypoxia-inducible-factor-1-α. We present the case of a 48-year-old woman with unexplained mild hypophosphatemia, very high cFGF23, and normal iFGF23. The patient proved to have an iron deficiency. Iron deficiency alters the iFGF23-to-cFGF23 ratio. After initiation of iron treatment, cFGF23 strongly decreased. This case report illustrates the limitation of cFGF23 assays and urges clinicians to be aware that cFGF23 concentrations do not necessarily reflect iFGF23 concentrations and that alternative causes for its elevation should be considered (eg, iron deficiency). © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

Current Status of Mineral and Bone Disorders in Transplant Recipients

Most patients with end-stage kidney disease undergoing kidney transplantation are affected by the chronic kidney disease-mineral and bone disorder. This entity encompasses laboratory abnormalities, calcification of soft tissues, and the bone abnormalities of renal osteodystrophy that together result in an increased risk of fracture, cardiovascular events, and mortality. Although many biochemical disturbances associated with end-stage kidney disease improve in the first year after transplantation, hyperparathyroidism commonly persists, and residual changes of renal osteodystrophy are slow to resolve. When superimposed on common, traditional risk factors, post-transplant glucocorticoid treatment, the possibility of tubular disturbances and post-transplant chronic kidney disease, rates of incident fracture remain high. This review examines hormonal and biochemical changes before and after kidney transplantation, fracture risk assessment tools and imaging modalities, a staged approach to management and concerns associated with antiresorptive and anabolic therapies. A multidisciplinary approach is proposed as the best means to improve patient-level outcomes.

FGF-23 is a biomarker of RV dysfunction and congestion in patients with HFrEF

There is no biomarker reflecting right ventricular dysfunction in HFrEF patients used in clinical practice. We have aimed to look for a circulating marker of RV dysfunction employing a quantitative proteomic strategy. The Olink Proteomics Multiplex panels (Cardiovascular Disease II, III, Cardiometabolic, and Inflammation Target Panels) identified FGF-23 to be the most differentially abundant (more than 2.5-fold) in blood plasma of HF patients with severe RV dysfunction (n = 30) compared to those with preserved RV function (n = 31). A subsequent ELISA-based confirmatory analysis of circulating FGF-23 in a large cohort of patients (n = 344, 72.7% NYHA III/IV, LVEF 22.5%, 54.1% with moderate/severe RV dysfunction), followed by multivariable regression analysis, revealed that the plasma FGF-23 level was most significantly associated with RV dysfunction grade (p = 0.0004) and congestion in the systemic circulation (p = 0.03), but not with LV-ejection fraction (p = 0.69) or estimated glomerular filtration rate (eGFR, p = 0.08). FGF-23 was associated with the degree of RV dysfunction in both sub-cohorts (i.e. in patients with and without congestion, p < 0.0001). The association between FGF-23 and RV-dysfunction remained significant after the adjustment for BNP (p = 0.01). In contrast, when adjusted for BNP, FGF-23 was no longer associated with LV dysfunction (p = 0.59). The Cox proportional hazard model revealed that circulating FGF-23 was significantly associated with adverse outcomes even after adjusting for BNP, LVEF, RV dysfunction grade and eGFR. Circulating FGF-23 is thus a biomarker of right ventricular dysfunction in HFrEF patients regardless of congestion status.

Bone marrow sinusoidal endothelial cells are a site of Fgf23 upregulation in a mouse model of iron deficiency anemia

Iron deficiency is a potent stimulator of fibroblast growth factor 23 (FGF23), a hormonal regulator of phosphate and vitamin D metabolism, that is classically thought to be produced by bone-embedded osteocytes. Here, we show that iron-deficient transmembrane serine protease 6 knockout (Tmprss6-/-) mice exhibit elevated circulating FGF23 and Fgf23 messenger RNA (mRNA) upregulation in the bone marrow (BM) but not the cortical bone. To clarify sites of Fgf23 promoter activity in Tmprss6-/- mice, we introduced a heterozygous enhanced green fluorescent protein (eGFP) reporter allele at the endogenous Fgf23 locus. Heterozygous Fgf23 disruption did not alter the severity of systemic iron deficiency or anemia in the Tmprss6-/- mice. Tmprss6-/-Fgf23+/eGFP mice showed green fluorescence in the vascular regions of BM sections and showed a subset of BM endothelial cells that were GFPbright by flow cytometry. Mining of transcriptomic data sets from mice with normal iron balance revealed higher Fgf23 mRNA in BM sinusoidal endothelial cells (BM-SECs) than that in other BM endothelial cell populations. Anti-GFP immunohistochemistry of fixed BM sections from Tmprss6-/-Fgf23+/eGFP mice revealed GFP expression in BM-SECs, which was more intense than in nonanemic controls. In addition, in mice with intact Tmprss6 alleles, Fgf23-eGFP reporter expression increased in BM-SECs following large-volume phlebotomy and also following erythropoietin treatment both ex vivo and in vivo. Collectively, our results identified BM-SECs as a novel site for Fgf23 upregulation in both acute and chronic anemia. Given the elevated serum erythropoietin in both anemic models, our findings raise the possibility that erythropoietin may act directly on BM-SECs to promote FGF23 production during anemia.

1,25-Dihydroxyvitamin D3 regulates furin-mediated FGF23 cleavage

Intact fibroblast growth factor 23 (iFGF23) is a phosphaturic hormone that is cleaved by furin into N-terminal and C-terminal fragments. Several studies have implicated vitamin D in regulating furin in infections. Thus, we investigated the effect of 1,25-dihydroxyvitamin D3 [1,25(OH)2D] and the vitamin D receptor (VDR) on furin-mediated iFGF23 cleavage. Mice lacking VDR (Vdr-/-) had a 25-fold increase in iFGF23 cleavage, with increased furin levels and activity compared with wild-type (WT) littermates. Inhibition of furin activity blocked the increase in iFGF23 cleavage in Vdr-/- animals and in a Vdr-knockdown osteocyte OCY454 cell line. Chromatin immunoprecipitation revealed VDR binding to DNA upstream of the Furin gene, with more transcription in the absence of VDR. In WT mice, furin inhibition reduced iFGF23 cleavage, increased iFGF23, and reduced serum phosphate levels. Similarly, 1,25(OH)2D reduced furin activity, decreased iFGF23 cleavage, and increased total FGF23. In a post hoc analysis of a randomized clinical trial, we found that ergocalciferol treatment, which increased serum 1,25(OH)2D, significantly decreased serum furin activity and iFGF23 cleavage, compared with placebo. Thus, 1,25(OH)2D inhibits iFGF23 cleavage via VDR-mediated suppression of Furin expression, thereby providing a mechanism by which vitamin D can augment phosphaturic iFGF23 levels.

An Unusually Prolonged Case of FGF23-mediated Hypophosphatemia Secondary to Ferric Carboxymaltose Use

Ferric carboxymaltose (FCM)-induced hypophosphatemia is seen in up to 75% of patients receiving this therapy for iron deficiency anemia. Hypophosphatemia has been attributed to increased circulating levels of fibroblast growth factor-23 (FGF23), the transcription of which is upregulated in an iron-deficient state. However, hypophosphatemia typically resolves within 12 weeks of FCM administration. Here, we present a case of unusually prolonged hypophosphatemia that developed after treatment with FCM in a 39-year-old female with autosomal dominant polycystic kidney disease (ADPKD) but normal renal function. Workup was significant for low tubular reabsorption of phosphate and inappropriately normal FGF23. Genetic disorders of hypophosphatemia and a FGF23-secreting tumor were ruled out. Treatment with calcitriol was required for nearly 3.5 years. The prolonged hypophosphatemia was attributed to underlying ADPKD because these patients demonstrate inappropriately elevated FGF23 levels for the degree of severity of reduced glomerular filtration rate. However, the stimulus driving FGF23 secretion in these patients is incompletely understood. Elevated FGF23 in the kidney suppresses renal tubular phosphate reabsorption and 1α-hydroxylase activity ultimately leading to hypophosphatemia. We conclude that our patient was at a high risk of developing hypophosphatemia because of underlying ADPKD, and FCM was the likely precipitant to identify this underlying process.

Associations of FGF23 with 10-Year Change in eGFR and UACR and with Incident CKD in the CARDIA Cohort

Key Points

FGF23 has a nonlinear positive association with incident CKD among healthy, middle-aged adults. The relationship between FGF23 and incident CKD was only significant among participants with cFGF23 levels in the highest quartile. FGF23 is not associated with 10-year change in eGFR or 10-year change in UACR among healthy, middle-aged adults.

Background

The relationship of fibroblast growth factor 23 (FGF23) with incident CKD has been examined in older but not younger populations.

Methods

Linear regression models were used to examine the associations of c-terminal FGF23 (cFGF23) and intact FGF23 (iFGF23) with 10-year change (1995–96 to 2005–06) in eGFR and urine albumin-to-creatinine ratio (UACR) in the Coronary Artery Risk Development in Young Adults cohort. Cox proportional hazard models were used to assess the association of cFGF23 with incident CKD, defined as eGFR <60 ml/min per 1.73 m2 or UACR ≥30 mg/g. Multivariable models were adjusted for age, sex, race, education, field center, physical activity, body mass index, diabetes, smoking, and systolic BP.

Results

Among 2511 participants, the mean age was 45±3.6 years; mean eGFR was 96.5±14.0 ml/min per 1.73 m2; and median UACR was 4.3 (interquartile range, 3.0–6.7) mg/g. Most (62.6%) participants were nonsmokers; the prevalence of diabetes was low (6.6%); and median values for 10-year changes in eGFR and UACR were modest (−5.50 ml/min per 1.73 m2 and 0.70 mg/g, respectively). No consistent associations between cFGF23 and 10-year change in eGFR and UACR were observed. During a median follow-up of 9.98 years, incident CKD developed in 258 participants. There was a nonlinear association of cFGF23 with incident CKD, and relative to the lowest quartile of cFGF23, a significant relationship was detected only among participants in the highest quartile (hazard ratio, 1.58; 95% confidence interval, 1.09 to 2.27). Similar findings were observed for iFGF23.

Conclusion

Among middle-aged adults in the Coronary Artery Risk Development in Young Adults cohort, median eGFR and UACR changes were modest and cFGF23 and iFGF23 were not consistently associated with 10-year change in eGFR or UACR. A nonlinear relationship was observed between cFGF23 and incident CKD, with individuals with highest cFGF23 levels being at risk of developing CKD.

Role of Chronic Kidney Disease (CKD)-Mineral and Bone Disorder (MBD) in the Pathogenesis of Cardiovascular Disease in CKD

Cardiovascular disease (CVD) is the leading cause of death in patients with chronic kidney disease (CKD). Multiple factors account for the increased incidence of cardiovascular morbidity and mortality in patients with CKD. Traditional risk factors for atherosclerosis and arteriosclerosis, including age, hypertension, dyslipidemia, diabetes mellitus, and smoking, are also risk factors for CKD. Non-traditional risk factors specific for CKD are also involved in CVD pathogenesis in patients with CKD. Recently, CKD-mineral and bone disorder (CKD-MBD) has emerged as a key player in CVD pathogenesis in the context of CKD. CKD-MBD manifests as hypocalcemia and hyperphosphatemia in the later stages of CKD; however, it initially develops much earlier in disease course. The initial step in CKD-MBD involves decreased phosphate excretion in the urine, followed by increased circulating concentrations of fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH), which increase urinary phosphate excretion. Simultaneously, the serum calcitriol concentration decreases as a result of FGF23 elevation. Importantly, FGF23 and PTH cause left ventricular hypertrophy, arrhythmia, and cardiovascular calcification. More recently, calciprotein particles, which are nanoparticles composed of calcium, phosphate, and fetuin-A, among other components, have been reported to cause inflammation, cardiovascular calcification, and other clinically relevant outcomes. CKD-MBD has become one of the critical therapeutic targets for the prevention of cardiovascular events and is another link between cardiology and nephrology. In this review, we describe the role of CKD-MBD in the pathogenesis of cardiovascular disorders and present the current treatment strategies for CKD-MBD.

Significance of changes in FGF23 levels in childhood primary nephrotic syndrome and children who progress to end‑stage renal disease

Fibroblast growth factor 23 (FGF23) is an important phosphaturic hormone, yet few studies have focused on FGF23 in children with primary nephrotic syndrome (PNS) and children who progressed to end-stage renal disease (ESRD). This cross-sectional study investigated the significance of changes in FGF23 levels in childhood PNS and children who progressed to ESRD. Of the 41 children included in the study, 17 had PNS with proteinuria and normal renal function (PNS group), 4 had ESRD (ESRD group), and 20 were healthy (control group). Following corticosteroid treatment, patients with PNS and proteinuria entered the remission phase. Serum levels of FGF23, phosphate, parathyroid hormone (PTH), 25-hydroxyvitamin D (25-OH-D), and calcium were measured. It was found that FGF23 levels in the PNS and ESRD groups were higher than those in the control group, while serum 25-OH-D levels were lower. Serum PTH levels increased significantly in the ESRD group. In the control group, FGF23 levels were negatively correlated with serum PTH and positively correlated with serum 25-OH-D. FGF23 levels were positively correlated with serum calcium and corrected calcium levels in children with PNS during the remission phase. Increased FGF23 levels in children with PNS, particularly in children who progressed to ESRD. It was also confirmed that serum FGF23 levels begin to rise in children with PNS prior to Stage 1 chronic kidney disease. These findings indicated that increased FGF23 levels may be associated with the progression and severity of nephrosis in children, and that serum FGF23 levels were useful for early detection of abnormal mineral metabolism in children with PNS.

Inhibition of FGF23 is a therapeutic strategy to target hematopoietic stem cell niche defects in β-thalassemia

Clinical evidence highlights a relationship between the blood and the bone, but the underlying mechanism linking these two tissues is not fully elucidated. Here, we used β-thalassemia as a model of congenital anemia with bone and bone marrow (BM) niche defects. We demonstrate that fibroblast growth factor 23 (FGF23) is increased in patients and mice with β-thalassemia because erythropoietin induces FGF23 overproduction in bone and BM erythroid cells via ERK1/2 and STAT5 pathways. We show that in vivo inhibition of FGF23 signaling by carboxyl-terminal FGF23 peptide is a safe and efficacious therapeutic strategy to rescue bone mineralization and deposition in mice with β-thalassemia, normalizing the expression of niche factors and restoring hematopoietic stem cell (HSC) function. FGF23 may thus represent a molecular link connecting anemia, bone, and the HSC niche. This study provides a translational approach to targeting bone defects and rescuing HSC niche interactions, with potential clinical relevance for improving HSC transplantation and gene therapy for hematopoietic disorders.

Methodology and Baseline Data of a Comparative Exploratory Double-Blinded Randomized Study of Intravenous Iron on Fibroblast Growth Factor 23 and Phosphate in Chronic Kidney Disease

Modern intravenous iron compounds (e.g., ferric carboxymaltose [FCM] and ferric derisomaltose [FDI]) are utilized in the treatment of iron deficiency anemia in non-dialysis-dependent chronic kidney disease (ND-CKD). Product-specific alterations in the metabolism of fibroblast growth factor 23 (FGF-23) leading to hypophosphatemia have been described for certain intravenous iron compounds, such as FCM, with potential effects on bone and cardiovascular health and quality of life. No prior head-to-head comparison between FCM and FDI exists in ND-CKD. This single-center exploratory double-blind randomized controlled trial primarily aimed to investigate the differential impact of FCM and FDI on FGF-23 and phosphate in patients with iron deficiency +/- anemia and ND-CKD (stages 3a-5 - serum ferritin <200 μg/L or serum ferritin 200-299 μg/L and transferrin saturation <20%). Patients were randomized (1:1) to receive either FCM or FDI over two infusions (1 month apart). Follow-up was 3 months. Measurements of serum intact FGF-23, phosphate, vitamin D metabolites, parathyroid hormone, other bone metabolism, cardiovascular, and quality of life markers were monitored. 168 patients were prescreened. Thirty-five patients were screened; 26 patients were randomized. The mean (standard deviation) age was 67.9 (12.4) years and 17 participants were male. Most participants had stage 4 CKD (median [interquartile range] estimated glomerular filtration rate [eGFR]: 18.0 [11.3] mL/min/1.73 m2). A higher than normal median (interquartile range) level of intact FGF-23 (212.1 [116.4] pg/mL) was noted. Serum phosphate was within normal range, while parathyroid hormone was higher and 1,25 (OH)2 vitamin D lower than the normal range. The "Iron and Phosphaturia - ExplorIRON-CKD" trial will provide important information regarding the differential effect of intravenous iron products in terms of FGF-23, phosphate, and other markers of bone and cardiovascular metabolism, alongside patient-reported outcome measures in patients with ND-CKD.

Clinical and Functional Assessment of Digenicity in Renal Phosphate Wasting

Apart from increased fluid intake, patients with kidney stone disease (KSD) due to renal phosphate wasting require specific metaphylaxis. NaPi2a, NaPi2c, and NHERF1 regulate plasma phosphate concentration by reabsorbing phosphate in proximal kidney tubules and have been found altered in monogenic hypophosphatemia with a risk of KSD. In this study, we aimed at assessing the combined genetic alterations impacting NaPi2a, NaPi2c, and NHERF1. Therefore, we screened our hereditary KSD registry for cases of oligo- and digenicity, conducted reverse phenotyping, and undertook functional studies. As a result, we identified three patients from two families with digenic alterations in NaPi2a, NaPi2c, and NHERF1. In family 1, the index patient, who presented with severe renal calcifications and a bone mineralization disorder, carried digenic alterations affecting both NaPi transporter 2a and 2c. Functional analysis confirmed an additive genetic effect. In family 2, the index patient presented with kidney function decline, distinct musculature-related symptoms, and intracellular ATP depletion. Genetically, this individual was found to harbor variants in both NaPi2c and NHERF1 pointing towards genetic interaction. In summary, digenicity and gene dosage are likely to impact the severity of renal phosphate wasting and should be taken into account in terms of metaphylaxis through phosphate substitution.