Fibroblast growth factor 23: state of the field and future directions

The endocrine FGFs axis: A systemic anti-fibrotic response that could prevent pulmonary fibrogenesis?

Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal disease for which therapeutic options are limited, with an unmet need to identify new therapeutic targets. IPF is thought to be the consequence of repeated microlesions of the alveolar epithelium, leading to aberrant epithelial-mesenchymal communication and the accumulation of extracellular matrix proteins. The reactivation of developmental pathways, such as Fibroblast Growth Factors (FGFs), is a well-described mechanism during lung fibrogenesis. Secreted FGFs with local paracrine effects can either exert an anti-fibrotic or a pro-fibrotic action during this pathological process through their FGF receptors (FGFRs) and heparan sulfate residues as co-receptors. Among FGFs, endocrine FGFs (FGF29, FGF21, and FGF23) play a central role in the control of metabolism and tissue homeostasis. They are characterized by a low affinity for heparan sulfate, present in the cell vicinity, allowing them to have endocrine activity. Nevertheless, their interaction with FGFRs requires the presence of mandatory co-receptors, alpha and beta Klotho proteins (KLA and KLB). Endocrine FGFs are of growing interest for their anti-fibrotic action during liver, kidney, or myocardial fibrosis. Innovative therapies based on FGF19 or FGF21 analogs are currently being studied in humans during liver fibrosis. Recent data report a similar anti-fibrotic action of endocrine FGFs in the lung, suggesting a systemic regulation of the pulmonary fibrotic process. In this review, we summarize the current knowledge on the protective effect of endocrine FGFs during the fibrotic processes, with a focus on pulmonary fibrosis.

Lipocalin 2-A bone-derived anorexigenic and β-cell promoting signal: From mice to humans

The skeleton is traditionally known for its structural support, organ protection, movement, and maintenance of mineral homeostasis. Over the last 10 years, bone has emerged as an endocrine organ with diverse physiological functions. The two key molecules in this context are fibroblast growth factor 23 (FGF23), secreted by osteocytes, and osteocalcin, a hormone produced by osteoblasts. FGF23 affects mineral homeostasis through its actions on the kidneys, and osteocalcin has beneficial effects in improving glucose homeostasis, muscle function, brain development, cognition, and male fertility. In addition, another osteoblast-derived hormone, lipocalin 2 (LCN2) has emerged into the researchers' field of vision. In this review, we mainly focus on LCN2's role in appetite regulation and glucose metabolism and also briefly introduce its effects in other pathophysiological conditions, such as nonalcoholic fatty liver disease, sarcopenic obesity, and cancer-induced cachexia.

Bone-derived C-terminal FGF23 cleaved peptides increase iron availability in acute inflammation

Inflammation leads to functional iron deficiency by increasing the expression of the hepatic iron regulatory peptide hepcidin. Inflammation also stimulates fibroblast growth factor 23 (FGF23) production by increasing both Fgf23 transcription and FGF23 cleavage, which paradoxically leads to excess in C-terminal FGF23 peptides (Cter-FGF23), rather than intact FGF23 (iFGF23) hormone. We determined that the major source of Cter-FGF23 is osteocytes and investigated whether Cter-FGF23 peptides play a direct role in the regulation of hepcidin and iron metabolism in response to acute inflammation. Mice harboring an osteocyte-specific deletion of Fgf23 showed a ∼90% reduction in Cter-FGF23 levels during acute inflammation. Reduction in Cter-FGF23 led to a further decrease in circulating iron in inflamed mice owing to excessive hepcidin production. We observed similar results in mice showing impaired FGF23 cleavage owing to osteocyte-specific deletion of Furin. We next showed that Cter-FGF23 peptides bind members of the bone morphogenetic protein (BMP) family, BMP2 and BMP9, which are established inducers of hepcidin. Coadministration of Cter-FGF23 and BMP2 or BMP9 prevented the increase in Hamp messenger RNA and circulating hepcidin levels induced by BMP2/9, resulting in normal serum iron levels. Finally, injection of Cter-FGF23 in inflamed Fgf23KO mice and genetic overexpression of Cter-Fgf23 in wild type mice also resulted in lower hepcidin and higher circulating iron levels. In conclusion, during inflammation, bone is the major source of Cter-FGF23 secretion, and independently of iFGF23, Cter-FGF23 reduces BMP-induced hepcidin secretion in the liver.

Fam20c regulates the calpain proteolysis system through phosphorylating Calpasatatin to maintain cell homeostasis

BACKGROUND

The family with sequence similarity 20-member C (FAM20C) kinase, a Golgi casein kinase, which is responsible for phosphorylating the majority of the extracellular phosphoproteins within S-x-E/pS motifs, and is fundamentally associated with multiple biological processes to maintain cell proliferation, biomineralization, migration, adhesion, and phosphate homeostasis. In dissecting how FAM20C regulates downstream molecules and potential mechanisms, however, there are multiple target molecules of FAM20C, particularly many phenomena remain elusive, such as changes in cell-autonomous behaviors, incompatibility in genotypes and phenotypes, and others.

METHODS

Here, assay for transposase-accessible chromatin using sequencing (ATAC-seq), RNA sequencing (RNA-seq), proteomics, and phosphoproteomics were performed in Fam20c-dificient osteoblasts and to facilitate an integrated analysis and determine the impact of chromatin accessibility, genomic expression, protein alterations, signaling pathway, and post translational modifcations.

RESULTS

By combining ATAC-seq and RNA-seq, we identified TCF4 and Wnt signaling pathway as the key regulators in Fam20c-dificient cells. Further, we showed Calpastatin/Calpain proteolysis system as a novel target axis for FAM20C to regulate cell migration and F-actin cytoskeleton by integrated analysis of proteomics and phosphoproteomics. Furthermore, Calpastatin/Calpain proteolysis system could negatively regulate the Wnt signaling pathway.

CONCLUSION

These observations implied that Fam20c knockout osteoblasts would cause cell homeostatic imbalance, involving changes in multiple signaling pathways in the conduction system.

Chenodeoxycholic acid regulates fibroblast growth factor 23 gene expression via estrogen-related receptor γ in human hepatoma Huh7 cells

Fibroblast growth factor 23 (FGF23) is a glycoprotein that belongs to the FGF19 subfamily and participates in phosphate and vitamin D homeostasis. Chenodeoxycholic acid (CDCA), one of the primary bile acids, is reported to induce the secretion of FGF19 subfamily members, FGF21 and FGF19, in hepatocytes. However, whether and how CDCA influences FGF23 gene expression are largely unknown. Thus, we performed real-time polymerase chain reaction and Western blot analyses to determine the mRNA and protein expression levels of FGF23 in Huh7 cells. CDCA upregulated estrogen-related receptor γ (ERRγ) alongside FGF23 mRNA and protein levels, while, the knockdown of ERRγ ablated the induction effect of CDCA on FGF23 expression. Promoter studies showed that CDCA-induced FGF23 promoter activity occurred partly through ERRγ binding directly to the ERR response element (ERRE) in the human FGF23 gene promoter. Finally, the inverse agonist of ERRγ, GSK5182 inhibited the induction of FGF23 by CDCA. Overall, our results revealed the mechanism of CDCA-mediated FGF23 gene upregulation in the human hepatoma cell line. Moreover, the ability of GSK5182 to reduce CDCA-induced FGF23 gene expression might represent a therapeutic strategy to control abnormal FGF23 induction in conditions that involve elevated levels of bile acids, such as nonalcoholic fatty liver disease and biliary atresia.

FGF/FGFR system in the central nervous system demyelinating disease: Recent progress and implications for multiple sclerosis

BACKGROUND

With millions of victims worldwide, multiple sclerosis is the second most common cause of disability among young adults. Although formidable advancements have been made in understanding the disease, the neurodegeneration associated with multiple sclerosis is only partially counteracted by current treatments, and effective therapy for progressive multiple sclerosis remains an unmet need. Therefore, new approaches are required to delay demyelination and the resulting disability and to restore neural function by promoting remyelination and neuronal repair.

AIMS

The article reviews the latest literature in this field.

MATERIALS AND METHODS

The fibroblast growth factor (FGF) signaling pathway is a promising target in progressive multiple sclerosis.

DISCUSSION

FGF signal transduction contributes to establishing the oligodendrocyte lineage, neural stem cell proliferation and differentiation, and myelination of the central nervous system. Furthermore, FGF signaling is implicated in the control of neuroinflammation. In recent years, interventions targeting FGF, and its receptor (FGFR) have been shown to ameliorate autoimmune encephalomyelitis symptoms in multiple sclerosis animal models moderately.

CONCLUSION

Here, we summarize the recent findings and investigate the role of FGF/FGFR signaling in the onset and progression, discuss the potential therapeutic advances, and offer fresh insights into managing multiple sclerosis.

Between a rock and a hard place: Regulation of mineralization in the periodontium

The periodontium supports and attaches teeth via mineralized and nonmineralized tissues. It consists of two, unique mineralized tissues, cementum and alveolar bone. In between these tissues, lies an unmineralized, fibrous periodontal ligament (PDL), which distributes occlusal forces, nourishes and invests teeth, and harbors progenitor cells for dentoalveolar repair. Many unanswered questions remain regarding periodontal biology. This review will focus on recent research providing insights into one enduring mystery: the precise regulation of the hard-soft tissue borders in the periodontium which define the interfaces of the cementum-PDL-alveolar bone structure. We will focus on advances in understanding the molecular mechanisms that maintain the unmineralized PDL "between a rock and a hard place" by regulating the mineralization of cementum and alveolar bone.

Does the RGD region of certain proteins affect metabolic activity?

A better understanding of the role of mineralized tissues and their associated factors in governing whole-body metabolism should be of value toward informing clinical strategies to treat mineralized tissue and metabolic disorders, such as diabetes and obesity. This perspective provides evidence suggesting a role for the arginine-glycine-aspartic acid (RGD) region, a sequence identified in several proteins secreted by bone cells, as well as other cells, in modulating systemic metabolic activity. We focus on (a) two of the SIBLING (small integrin-binding ligand, N-linked glycoprotein) family genes/proteins, bone sialoprotein (BSP) and osteopontin (OPN), (b) insulin-like growth factor-binding protein-1 & 2 (IGFBP-1, IGFBP-2) and (c) developmental endothelial locus 1 (DEL1) and milk fat globule–EGF factor-8 (MFG-E8). In addition, for our readers to appreciate the mounting evidence that a multitude of bone secreted factors affect the activity of other tissues, we provide a brief overview of other proteins, to include fibroblast growth factor 23 (FGF23), phosphatase orphan 1 (PHOSPHO1), osteocalcin (OCN/BGLAP), tissue non-specific alkaline phosphatase (TNAP) and acidic serine aspartic-rich MEPE-associated motif (ASARM), along with known/suggested functions of these factors in influencing energy metabolism.

Unusual Presentation and Surgical Treatment of a Phosphaturic Mesenchymal Tumor in a Knee

A 30-year-old woman presented to our hospital with an 11-year history of gradually enlarging masses around the left knee and 2-year history of progressively worsening bone pain. Tumor-induced osteomalacia (TIO), a rare paraneoplastic syndrome caused by phosphaturic mesenchymal tumors (PMTs) was suspected, but the postoperative pathology of her two operations was both reported as tenosynovial giant cell tumor (TGCT), making its diagnosis confusing. The possibility of hypophosphatemia, insufficient blood supply, innervation of the left lower limbs, as well as the unclear pathology, make it unreasonable to perform tumor-type knee prosthesis replacement directly. Finally, we placed static polymethylmethacrylate (PMMA) spacer at first, then when the concentration of blood phosphorus level rose to the normal range, the pathology was confirmed to be TIO, the blood supply and innervation was satisfying, tumor-type knee prosthesis replacement was performed. She was discharged post operative day 15 after the prothesis implantation without incident. One and a half years after her surgery, the concentration of blood phosphorus was still in the normal range, the symptom of systemic bone pain had improved significantly, the prosthesis was still in a good position and no recurrence was caught.

Iron and 25-hydroxyvitamin D in postmenopausal women with osteoporosis

Iron and vitamin D deficiencies are some of the most common health problems in the world. Iron is essential in oxygen transport and participates in many enzymatic systems in the body, with important roles in vitamin D metabolism. Osteoporosis is one of the most prevalent chronic disease of the elderly in the world as well as in the Saudi population. The relationship between iron, vitamin D deficiency and bone health comes from clinical observations in iron overload patients who suffered bone loss. The opposite scenario, whether iron and vitamin D deficiencies affect bone metabolism, has not been fully addressed. This is of great interest, as this nutrient deficiency is a worldwide public health problem and at the same time osteoporosis and bone alterations are highly prevalent. The relationship between 25(OH)D and iron deficiencies with osteoporosis is unknown up to date. This review presents the current knowledge on nutritional iron and vitamin D deficiencies in bone remodeling, and discuss the link between iron and bone metabolism among postmenopausal women. Finally, it is hypothesized that chronic iron and vitamin D deficiencies induces bone resorption and risk of osteoporosis, thus complete recovery from anemia and its prevention should be promoted in order to improve quality of life including bone health. Several mechanisms are implicated; hence, further investigation on the possible impact of iron and vitamin D deficiencies on the development of osteoporosis is needed.

A novel FGF23 mutation in hyperphosphatemic familial tumoral calcinosis and its deleterious effect on protein O-glycosylation

BACKGROUND

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a rare disease characterized by hyperphosphatemia and ectopic calcification, predominantly at periarticular locations. This study was performed to characterize the clinical profile of tumoral calcinosis and to identify gene mutations associated with HFTC and elucidated its pathogenic role.

METHODS

The three subjects (two male and one female) were aged 30, 25 and 15 years, respectively. The clinical features, histopathological findings, and outcomes of three subjects with HFTC were retrospectively reviewed. The three subjects were analyzed for FGF23, GALNT3 and KL mutations. Function of mutant gene was analyzed by western blotting and wheat germ agglutinin affinity chromatography.

RESULTS

All subjects had hyperphosphatemia and elevated calcium-phosphorus product. Calcinosis positions included the left shoulder, left index finger, and right hip. Bone and joint damage were present in two cases and multiple foci influenced body growth in one case. The histopathological features were firm, rubbery masses comprising multiple nodules of calcified material bordered by the proliferation of mononuclear or multinuclear macrophages, osteoclastic-like giant cells, fibroblasts, and chronic inflammatory cells. The novel mutation c.484A>G (p.N162D) in exon 3 of FGF23 was identified in one subject and his family members. Measurement of circulating FGF23 in the subject confirmed low intact FGF23 and increased C-terminal fragment. In vitro experiments showed that the mutant FGF23 proteins had defective O-glycosylation and impaired protein proteolysis protection.

CONCLUSION

We identified a novel FGF23 missense mutation, and confirmed its damaging role in FGF23 protein O-glycosylation. Our findings expand the current spectrum of FGF23 variations that influence phosphorus metabolism.

Hyperphosphatemic Familial Tumoral Calcinosis Hidden in Plain Sight for 73 Years: A Case Report

While dental pulp calcifications and root anomalies may be inconsequential incidental findings in dental radiographs, they can, especially in combination, represent a clue, hidden in plain sight, for the diagnosis of hyperphosphatemic familial tumoral calcinosis (HFTC). HFTC is an autosomal recessive disease of mineral metabolism characterized by sometimes massive, painful calcification around large joints, systemic inflammation, dental pulp calcification, and thistle-shaped roots. This paper describes a woman with HFTC who endured not only the symptoms of HFTC for decades, but also the frustration of not knowing the cause. The diagnosis was finally made at the age of 73 years, when the connection between a large right shoulder calcification and hyperphosphatemia was made. The dental findings were likely present on her initial radiographs taken in childhood. Increased awareness of the association between characteristic dental findings and HFTC may allow for earlier diagnosis and interventions to improve the care of patients with this rare condition.

Association Between FGF-23 Levels and Risk of Fracture in Women With Systemic Sclerosis

INTRODUCTION/BACKGROUND

The purpose of this study was to evaluate the association between Fracture Risk Assessment Tool (FRAX) and serum fibroblast grow factor-23 (FGF-23) levels in SSc women patients compared with healthy controls.

METHODOLOGY

This cross-sectional study was performed in San Cecilio Hospital, Granada (Spain) from November 2017 to May 2019. Sixty-two women with SSc and 62 age and sex matched healthy controls were included in this study. FGF-23 serum concentration was evaluated by indirect enzyme-linked immunosorbent assay. The FRAX scoring tool was applied using the on-line calculator (www.shef.ac.uk/FRAX).

RESULTS

Even though there was no significant difference in FGF-23 levels between SSc women patients and healthy controls (78.2 ± 60.5 vs 80.3 ± 56.3 pg/mL, p = 0.662). FGF-23 levels were positively associated with FRAX index within the study group.

CONCLUSIONS

This study shows that FGF-23 status is associated with FRAX index in women with SSc. FGF-23 could be a promising biomarker for detecting risk fracture in SSc women patients.

FGF gene family characterization provides insights into its adaptive evolution in Carnivora

Fibroblast growth factors (FGFs) encoded by the FGF gene family can regulate development and physiology in animals. However, their evolutionary characteristics in Carnivora are largely unknown. In this study, we identified 660 sequences of three types of FGF genes from 30 unannotated genomes of Carnivora animals (before 7th May 2020), and the FGF genes from 52 Carnivora species were analyzed through the method of comparative genomics. Phylogenetic and selective pressure analyses were carried out based on the FGF genes of these 52 Carnivora species. The phylogenetic analysis results demonstrated that the FGF gene family was divided into 10 subfamilies and that FGF5 formed one clade rather than belonging to the subfamilies of FGF4 and FGF6. The evolutionary analysis results showed that the FGF genes were prominently subjected to purifying selection and were highly conserved in the process of Carnivora evolution. We also carried out phylogenetic comparative analyses, which indicated that the habitat was one of the factors that shaped the evolution of Carnivora FGF genes. The FGF1 and FGF6 genes were positively selected in the Carnivora animals, and positive selection signals were detected for the FGF19 gene in semiaquatic Carnivora animals. In summary, we clarified the phylogenetic and evolutionary characteristics of Carnivora FGF genes and provided valuable data for future studies on evolutionary characterization of Carnivora animals.

Extended Whole-body Ga-68 DOTATATE PET-CT in evaluating Tumour-Induced Osteomalacia: Case report and review of literature

Tumour-induced osteomalacia is a rare paraneoplastic syndrome that manifests as chronic hypophosphataemia, non-specific bone pain and muscle weakness. It is generally caused by phosphaturic mesenchymal tumour (PMT), which is uncommonly associated with synchronous tumours. However, diagnosis is often delayed for several years due to the rarity, indolent growing nature and non-specific symptoms of the disease, often resulting in an overlook by clinicians during assessments. The patient initially presented with hypophosphataemia and generalised skeletal pain with multiple atraumatic fractures. Blood tests revealed serum calcium levels at the upper limit and extremely low inorganic phosphate levels. Herein, we report a case where two synchronous PMTs from two different sites were detected by 'extended' whole-body Ga-68 DOTATATE PET-CT, leading to remission of the disease after complete surgical removal. Early detection and diagnosis of PMT neoplasm is crucial, as complete surgical resection of this tumour is the only definitive treatment currently known. Upon excision, this curable disease will result in complete resolution of symptoms and blood parameters, leading to remission of the disease which significantly improves the patient's quality of life. PMT often over-expresses somatostatin receptors (SSTR), predominantly subtype 2A, and Ga-68 DOTATATE PET-CT is a selective SSTR imaging that targets this characteristic over-expression in these tumours. The high diagnostic accuracy of Ga-68 DOTATATE PET-CT should be the primary imaging modality for full evaluation of this disease.

Skeletal endocrinology: where evolutionary advantage meets disease

The regulation of whole-body homeostasis by the skeleton is mediated by its capacity to secrete endocrine signaling molecules. Although bone-derived hormones confer several adaptive benefits, their physiological functions also involve trade-offs, thus eventually contributing to disease. In this manuscript, we discuss the origins and functions of two of the best-studied skeletal mediators, fibroblast growth factor 23 and osteocalcin, in an evolutionary context. Moreover, we provide a theoretical framework seeking to explain the broad involvement of these two hormones in amniote physiology as well as their potential to fuel the development and progression of diseases. Vice versa, we outline which perturbations might be amenable to manipulation of these systems and discuss limitations and ongoing challenges in skeletal endocrine research. Finally, we summarize unresolved questions and potential future studies in this thriving field.

A Cross-Sectional Cohort Study of the Effects of FGF23 Deficiency and Hyperphosphatemia on Dental Structures in Hyperphosphatemic Familial Tumoral Calcinosis

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a rare autosomal recessive disorder caused by mutations in FGF23, GALNT3, KLOTHO, or FGF23 autoantibodies. Prominent features include high blood phosphate and calcific masses, usually adjacent to large joints. Dental defects have been reported, but not systematically described. Seventeen patients with HFTC followed at the National Institutes of Health underwent detailed clinical, biochemical, molecular, and dental analyses. Studies of teeth included intraoral photos and radiographs, high-resolution μCT, histology, and scanning electron microscopy (SEM). A scoring system was developed to assess the severity of tooth phenotype. Pulp calcification was found in 13 of 14 evaluable patients. Short roots and midroot bulges with apical thinning were present in 12 of 13 patients. Premolars were most severely affected. μCT analyses of five HFTC teeth revealed that pulp density increased sevenfold, whereas the pulp volume decreased sevenfold in permanent HFTC teeth compared with age- and tooth-matched control teeth. Histology revealed loss of the polarized odontoblast cell layer and an obliterated pulp cavity that was filled with calcified material. The SEM showed altered pulp and cementum structures, without differences in enamel or dentin structures, when compared with control teeth. This study defines the spectrum and confirms the high penetrance of dental features in HFTC. The phenotypes appear to be independent of genetic/molecular etiology, suggesting hyperphosphatemia or FGF23 deficiency may be the pathomechanistic driver, with prominent effects on root and pulp structures, consistent with a role of phosphate and/or FGF23 in tooth development. Given the early appearance and high penetrance, cognizance of HFTC-related features may allow for earlier diagnosis and treatment. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

The ABCs of the atypical Fam20 secretory pathway kinases

The study of extracellular phosphorylation was initiated in late 19th century when the secreted milk protein, casein, and egg-yolk protein, phosvitin, were shown to be phosphorylated. However, it took more than a century to identify Fam20C, which phosphorylates both casein and phosvitin under physiological conditions. This kinase, along with its family members Fam20A and Fam20B, defined a new family with altered amino acid sequences highly atypical from the canonical 540 kinases comprising the kinome. Fam20B is a glycan kinase that phosphorylates xylose residues and triggers peptidoglycan biosynthesis, a role conserved from sponges to human. The protein kinase, Fam20C, conserved from nematodes to humans, phosphorylates well over 100 substrates in the secretory pathway with overall functions postulated to encompass endoplasmic reticulum homeostasis, nutrition, cardiac function, coagulation, and biomineralization. The preferred phosphorylation motif of Fam20C is SxE/pS, and structural studies revealed that related member Fam20A allosterically activates Fam20C by forming a heterodimeric/tetrameric complex. Fam20A, a pseudokinase, is observed only in vertebrates. Loss-of-function genetic alterations in the Fam20 family lead to human diseases such as amelogenesis imperfecta, nephrocalcinosis, lethal and nonlethal forms of Raine syndrome with major skeletal defects, and altered phosphate homeostasis. Together, these three members of the Fam20 family modulate a diverse network of secretory pathway components playing crucial roles in health and disease. The overarching theme of this review is to highlight the progress that has been made in the emerging field of extracellular phosphorylation and the key roles secretory pathway kinases play in an ever-expanding number of cellular processes.

Disentangling the relationship between bone turnover and glucose homeostasis: A prospective, population-based twin study

Background

Biochemical markers of bone turnover are lower in patients with type 2 diabetes, which may be explained by genetic variants being associated with type 2 diabetes and bone turnover as well as environmental factors. We hypothesized that bone turnover markers associate with and predict changes in glucose homeostasis after control for genetics and shared environment.

Methods

1071 healthy, non-diabetic (at baseline, 1997-2000) adult mono- and dizygotic twins participating in the prospective study GEMINAKAR were reassessed between 2010 and 2012 with clinical evaluation, biochemical tests and oral glucose tolerance test. Fasting bone turnover markers (CTX, P1NP and osteocalcin) were measured. The association between bone turnover, glucose homeostasis and the ability of bone turnover markers to predict changes in glucose homeostasis were assessed in cross-sectional and longitudinal analyses. Analyses were performed both at an individual level and adjusted for shared environmental and genetic factors.

Results

Glucose levels increased with age, and 33 (3%) participants had developed type 2 diabetes at follow-up. In women, bone turnover markers increased with age, whereas for men only osteocalcin increased with age. Bone turnover markers were not associated with fasting glucose, insulin, or HOMA-IR at baseline or follow-up before or after adjustment for age, sex, BMI, smoking, and use of medication at baseline. Variation in bone turnover markers was mainly explained by unique environmental factors, 70%, 70% and 55% for CTX, P1NP and osteocalcin, respectively, whereas additive genetic factors explained 7%, 13% and 45% of the variation in CTX, P1NP and osteocalcin.

Conclusions

Bone turnover markers were not associated with baseline plasma glucose levels and did not predict changes in glucose homeostasis. Variation in bone turnover markers is mainly explained by environmental factors, however, compared to CTX and P1NP, genetic factors have a larger impact on osteocalcin levels.

C-Terminal, but Not Intact, FGF23 and EPO Are Strongly Correlatively Elevated in Patients With Gain-of-Function Mutations in HIF2A: Clinical Evidence for EPO Regulating FGF23

Fibroblast growth factor 23 (FGF23) is a key phosphate- and vitamin D-regulating hormone. FGF23 circulates as an intact 251 amino acid protein or N- and C-terminal degradation products. Hormone activity resides in the intact molecule, but it has been suggested that high levels of the C-terminal protein can interfere with intact FGF23 (iFGF23) activity. New evidence points to involvement of the hypoxia-inducible factor (HIF)/erythropoietin (EPO)/iron pathway as important in FGF23 physiology. Exactly how this pathway regulates FGF23 is not clear. Various in vitro, in vivo, and clinical studies involving perturbations in this pathway at various points have yielded conflicting results. Many of these studies are complicated by the confounding, independent effect of renal insufficiency on FGF23. To gain insight into FGF23 physiology, we studied 8 patients with a rare paraganglioma/somatostatinoma syndrome who had elevated blood EPO levels as a result of somatic gain-of-function mutations in HIF2A (EPAS1) that stimulate tumoral EPO production. All patients had normal renal function. EPO levels varied; most were very elevated and highly correlated with C-terminal FGF23 (cFGF23) levels that were also markedly elevated. Blood phosphate and intact FGF23 levels were normal. These data from patients with normal renal function in whom HIF activation was the inciting event suggest a direct role of the HIF/EPO pathway in FGF23 transcription and translation. They also demonstrate that posttranslational regulation was finely tuned to maintain normal blood phosphate levels. Additionally, normal phosphate and intact FGF23 levels in the setting of markedly increased C-terminal FGF23 levels suggest intact FGF23 action is not attenuated by C-terminal FGF23. Published 2020. This article is a U.S. Government work and is in the public domain in the USA. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Tumor-Induced Osteomalacia

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by tumoral production of fibroblast growth factor 23 (FGF23). The hallmark biochemical features include hypophosphatemia due to renal phosphate wasting, inappropriately normal or frankly low 1,25-dihydroxy-vitamin D, and inappropriately normal or elevated FGF23. TIO is caused by typically small, slow growing, benign phosphaturic mesenchymal tumors (PMTs) that are located almost anywhere in the body from the skull to the feet, in soft tissue or bone. The recent identification of fusion genes in a significant subset of PMTs has provided important insights into PMT tumorigenesis. Although management of this disease may seem straightforward, considering that complete resection of the tumor leads to its cure, locating these often-tiny tumors is frequently a challenge. For this purpose, a stepwise, systematic approach is required. It starts with thorough medical history and physical examination, followed by functional imaging, and confirmation of identified lesions by anatomical imaging. If the tumor resection is not possible, medical therapy with phosphate and active vitamin D is indicated. Novel therapeutic approaches include image-guided tumor ablation and medical treatment with the anti-FGF23 antibody burosumab or the pan-FGFR tyrosine kinase inhibitor, BGJ398/infigratinib. Great progress has been made in the diagnosis and treatment of TIO, and more is likely to come, turning this challenging, debilitating disease into a gratifying cure for patients and their providers.

New Therapies for Hypophosphatemia-Related to FGF23 Excess

FGF23 is a hormone produced by osteocytes in response to an elevation in the concentration of extracellular phosphate. Excess production of FGF23 by bone cells, or rarely by tumors, is the hormonal basis for several musculoskeletal syndromes characterized by hypophosphatemia due to renal phosphate wasting. FGF23-dependent chronic hypophosphatemia causes rickets and osteomalacia, as well as other skeletal complications. Genetic disorders of FGF23-mediated hypophosphatemia include X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets (ADHR), autosomal recessive hypophosphatemic rickets (ARHR), fibrous dysplasia of bone, McCune-Albright syndrome, and epidermal nevus syndrome (ENS), also known as cutaneous skeletal hypophosphatemia syndrome (CSHS). The principle acquired form of FGF23-mediated hypophosphatemia is tumor-induced osteomalacia (TIO). This review summarizes current knowledge about the pathophysiology and clinical presentation of the most common FGF23-mediated conditions, with a focus on new treatment modalities. For many decades, calcitriol and phosphate supplements were the mainstay of therapy. Recently, burosumab, a monoclonal blocking antibody to FGF23, has been approved for treatment of XLH in children and adults, and an active comparator trial in children has shown good efficacy and safety for this drug. The remainder of FGF23-mediated hypophosphatemic disorders continue to be treated with phosphate and calcitriol, although ongoing trials with burosumab for treatment of tumor-induced osteomalacia show early promise. Burosumab may be an effective treatment for the remainder of FGF23-mediated disorders, but clinical trials to support that possibility are at present not available.

Erythropoiesis stimulating agents are associated with serum fibroblast growth factor 23 metabolism in patients on hemodialysis

Background

This study aimed to determine associations among short- and long-acting erythropoiesis stimulating agents (ESAs), changes in serum fibroblast growth factor 23 (FGF23) and biomarkers of iron metabolism.

Methods

Among 108 patients on hemodialysis (HD), 44 received every 2 weeks or monthly doses of continuous erythropoiesis receptor activator (CERA), 31 received weekly doses of darbepoetin-α, 24 received three doses per week of epoetin-β and 9 were not treated with an ESA. Intact and C-terminal FGF23 and transferrin saturation (TSAT), ferritin, erythroferrone and hepcidin 25 were measured in blood samples collected before the HD session at the end of the dialysis week (baseline, Day 0) and on Days 3, 5, 7 and 14 thereafter.

Results

Levels of ferritin, hepcidin 25 and erythroferrone as well as TSAT were significantly decreased or elevated in patients treated with CERA compared with other types of ESAs. Levels of C-terminal FGF23 increased in all groups during the observation period. Levels of intact FGF23 and ratios of intact FGF23 to C-terminal FGF23 gradually decreased between Days 3 and 7 in the CERA but not in the other groups. Multivariate models associated changes in hepcidin 25 and phosphate with those of intact FGF23.

Conclusion

The long-acting ESA CERA might influence levels of intact FGF23 by increasing FGF23 cleavage in patients on HD in association with prolonged hepcidin 25 suppression.

Lessons learnt from delayed diagnosis of FGF-23-producing tumour-induced osteomalacia and post-operative hungry bone syndrome

Tumour-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by a fibroblast growth-factor-23 (FGF-23)-secreting phosphaturic mesenchymal tumour (PMT) and is characterised by hypophosphataemic osteomalacia. We present a 36-year-old man initially presenting with diffuse bone and joint pain who was inappropriately treated for presumed ankylosing spondylitis for 2 years. Whole-body bone scan suggested metabolic bone disease, prompting referral to our endocrine institution. He was subsequently diagnosed with persistent hypophosphataemia, inappropriately high renal tubular phosphate excretion, 1,25-dihydroxyvitamin D₃ suppression, severe osteoporosis and severe osteomalacia. FGF-23 concentrations (140 ng/L) were raised 3-fold above the upper limit of normal. Initial Gallium-68 (⁶⁸Ga) DOTATATE positron emission tomography (PET)/CT scan missed an active lesion in the left fibular head as the field only included the mid-brain to the proximal femora. Histopathology results from tumour resection confirmed a PMT over-expressing FGF-23. Serum phosphate and FGF-23 normalised immediately post-operatively. He developed severe hypocalcaemia 3-weeks post-operatively (1.77 mmol/L) which normalised after 1 month of high-dose caltrate and calcitriol therapy. Osteomalacia, osteoporosis and associated symptoms resolved during medium-term follow-up with >100% improvement in his bone mineral density. This case report and discussion highlights the pitfalls contributing to delayed diagnosis of TIO and alerts clinicians to the potential complication of hungry bone syndrome post-tumour resection.

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.

Glucocorticoids Decrease Longitudinal Bone Growth in Pediatric Kidney Transplant Recipients by Stimulating the FGF23/FGFR3 Signaling Pathway

Renal transplantation (RTx) is an effective therapy to improve clinical outcomes in pediatric patients with terminal chronic kidney disease. However, chronic immunosuppression with glucocorticoids (GCs) reduces bone growth and BMD. The mechanisms causing GC-induced growth impairment have not been fully clarified. Fibroblast growth factor 23 (FGF23) is a peptide hormone that regulates phosphate homeostasis and bone growth. In pathological conditions, FGF23 excess or abnormal FGF receptors (FGFR) activity leads to bone growth impairment. Experimental data indicate that FGF23 expression is induced by chronic GC exposure. Therefore, we hypothesize that GCs impair bone growth by increasing FGF23 expression, which has direct effects on bone growth plate. In a post hoc analysis of a multicentric randomized clinical trial of prepubertal RTx children treated with early GC withdrawal or chronic GC treatment, we observed that GC withdrawal was associated with improvement in longitudinal growth and BMD, and lower plasma FGF23 levels as compared with a chronic GC group. In prepubertal rats, GC-induced bone growth retardation correlated with increased plasma FGF23 and bone FGF23 expression. Additionally, GC treatment decreased FGFR1 expression whereas it increased FGFR3 expression in mouse tibia explants. The GC-induced bone growth impairment in tibiae explants was prevented by blockade of FGF23 receptors using either a pan-FGFR antagonist (PD173074), a C-terminal FGF23 peptide (FGF23180-205) which blocks the binding of FGF23 to the FGFR-Klotho complex or a specific FGFR3 antagonist (P3). Finally, local administration of PD173074 into the tibia growth plate ameliorated cartilage growth impairment in GC-treated rats. These results show that GC treatment partially reduces longitudinal bone growth via upregulation of FGF23 and FGFR3 expression, thus suggesting that the FGF23/Klotho/FGFR3 axis at the growth plate could be a potential therapeutic target for the management of GC-induced growth impairment in children.

Unlocking bone for proteomic analysis and FISH

Bone tissue is critically lagging behind soft tissues and biofluids in our effort to advance precision medicine. The main challenges have been accessibility and the requirement for deleterious decalcification processes that impact the fidelity of diagnostic histomorphology and hinder downstream analyses such as fluorescence in-situ hybridization (FISH). We have developed an alternative fixation chemistry that simultaneously fixes and decalcifies bone tissue. We compared tissue morphology, immunohistochemistry (IHC), cell signal phosphoprotein analysis, and FISH in 50 patient matched primary bone cancer cases that were either formalin fixed and decalcified, or theralin fixed with and without decalcification. Use of theralin improved tissue histomorphology, whereas overall IHC was comparable to formalin fixed, decalcified samples. Theralin-fixed samples showed a significant increase in protein and DNA extractability, supporting technologies such as laser-capture microdissection and reverse phase protein microarrays. Formalin-fixed bone samples suffered from a fixation artifact where protein quantification of β-actin directly correlated with fixation time. Theralin-fixed samples were not affected by this artifact. Moreover, theralin fixation enabled standard FISH staining in bone cancer samples, whereas no FISH staining was observed in formalin-fixed samples. We conclude that the use of theralin fixation unlocks the molecular archive within bone tissue allowing bone to enter the standard tissue analysis pipeline. This will have significant implications for bone cancer patients, in whom personalized medicine has yet to be implemented.

Hyperphosphatemic Familial Tumoral Calcinosis With Galnt3 Mutation: Transient Response to Anti-Interleukin-1 Treatments

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a rare autosomal recessive disease caused by mutations in genes involved in phosphate homeostasis and characterized by high serum phosphate concentration and occurrence of ectopic calcifications. Management of the disease includes lowering of phosphate concentration and, when clinically necessary, debulking surgery of calcifications. In addition, high inflammatory disease flares can occur. Our case is about a patient with GALNT3 mutation and several localizations of refractory calcinosis. Assuming HFTC acts like an auto-inflammatory syndrome, we report the effect of anti-interleukine-1 therapies on the evolution of the disease. Anakinra (100 mg, then 200 mg subcutaneous daily) and canakinumab (300 mg every 4 weeks) were sequentially given to the patient. Anti-IL-1 therapy was effective in controlling inflammatory flares; however, it did not prevent extension of calcinosis. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Role of phosphate sensing in bone and mineral metabolism

Inorganic phosphate (P_i) is essential for signal transduction and cell metabolism, and is also an essential structural component of the extracellular matrix of the skeleton. P_i is sensed in bacteria and yeast at the plasma membrane, which activates intracellular signal transduction to control the expression of P_i transporters and other genes that control intracellular P_i levels. In multicellular organisms, P_i homeostasis must be maintained in the organism and at the cellular level, requiring an endocrine and metabolic P_i-sensing mechanism, about which little is currently known. This Review will discuss the metabolic effects of P_i, which are mediated by P_i transporters, inositol pyrophosphates and SYG1-Pho81-XPR1 (SPX)-domain proteins to maintain cellular phosphate homeostasis in the musculoskeletal system. In addition, we will discuss how P_i is sensed by the human body to regulate the production of fibroblast growth factor 23 (FGF23), parathyroid hormone and calcitriol to maintain serum levels of P_i in a narrow range. New findings on the crosstalk between iron and P_i homeostasis in the regulation of FGF23 expression will also be outlined. Mutations in components of these metabolic and endocrine phosphate sensors result in genetic disorders of phosphate homeostasis, cardiomyopathy and familial basal ganglial calcifications, highlighting the importance of this newly emerging area of research.

Phosphaturic mesenchymal tumors: what an endocrinologist should know

Tumor-induced osteomalacia (TIO), also known as "oncogenic osteomalacia", is a rare cause of osteomalacia. TIO often has an insidious onset characterized clinically by progressive muscle weakness and bone pain with fractures. The hallmark biochemical finding is a persistent low serum phosphorus concentration due to renal phosphate wasting. The vast majority of cases of TIO result from production of the phosphaturic hormone fibroblast growth factor 23 (FGF23) by a histologically distinctive mesenchymal tumor, termed "phosphaturic mesenchymal tumor" (PMT). Circulating FGF23 induces internalization of renal sodium/phosphate co-transporters resulting in reduced proximal tubular phosphate reabsorption. FGF23 also inhibits production of 1α,25-dihydroxyvitamin D which is inappropriately low or normal in the context of hypophosphatemia. Diagnosis is often delayed owing to the rarity of the condition and an underappreciation for the role of phosphorus as a cause for the constellation of symptoms. Primary treatment for TIO is identification of the offending tumor and surgical removal. However, these tumors are notoriously difficult to find, precluding the opportunity for a curative surgery in many. In such cases, phosphate and calcitriol therapy is used to improve symptoms and heal the osteomalacia. Recently, molecular genetic studies have shown recurrent genetic events in PMT, including the novel fusions FN1-FGFR1 and less commonly FN1-FGF1. These fusion events are hypothesized to result in autocrine/paracrine signaling loops within the tumor, spurring tumorigenesis. This review will cover the clinical features, imaging characteristics, pathologic features, molecular genetic aspects, and therapy of PMT, with a brief discussion of other neoplasms that may cause TIO.

Monitoring early developed low bone mineral density in HIV-infected patients by intact parathyroid hormone and circulating fibroblast growth factor 23

BACKGROUND/PURPOSE

HIV-infected patients have a high prevalence of low bone mineral density (BMD), but BMD changes remain unclear. This cross-sectional retrospective observational study aimed to characterize the prevalence and associated factors of low BMD in HIV patients.

METHODS

Between 1 January 2015 and 31 December 2016, all patients aged 20 years or greater who sought for HIV care were included. BMD was measured by dual-energy X-ray absorptiometry. Multivariable analyses of the association with HIV disease status, treatment and anthropometric parameters were performed. Circulating fibroblast growth factor 23 and intact parathyroid hormone were measured.

RESULTS

A total of 137 patients was included; their median age was 39 years old; 97.8% were treated with combination antiretroviral therapy (cART); Body mass index (BMI) was 21.97 kg/m². Sixty-one patients (44.5%) showed low BMD (osteopenia and osteoporosis) based on the WHO criteria. The median BMD was -0.80 g/cm² (IQR, -1.5 to -0.2). The prevalence rate of low BMD was 37% in those who were aged 20-29 years, 45.2% in those who were aged 30-39 years, 45.2% in those who were aged 40-49 years, 45.8% in those who were aged 50-59 years, and 53.8% in those who were aged ≧60 years. More than half of patients (50.4%, 69/137) were younger than 40 years. Compared with normal BMD group, the low BMD group has a higher proportion of secondary hyperparathyroidism (18.0% vs 5.3%, p: 0.026) and a lower median C-terminal FGF23 level (48.92 vs 62.61 pg/ml, p: 0.008). Univariate and multivariate analyses of the factors associated with low BMD. We found that only serum intact-parathyroid hormone (iPTH) > 69 pg/ml (OR, 3.86; 95% CI, 1.14-13.09) was statistically significant associated with low BMD in multivariate analysis.

CONCLUSIONS

This cohort-based survey showed a high prevalence of low BMD among HIV-infected adults which included young-age patient in an university hospital. Secondary hyperparathyroidism was significantly associated with low BMD. There was no association between FGF23 and low BMD.

Hyperphosphatemic familial tumoral calcinosis secondary to fibroblast growth factor 23 (FGF23) mutation: a report of two affected families and review of the literature

Hyperphosphatemic familial tumoral calcinosis (HFTC), secondary to fibroblast growth factor 23 (FGF23) gene mutation, is a rare genetic disorder characterized by recurrent calcified masses. We describe young Lebanese cousins presenting with HFTC, based on a retrospective chart review and a prospective case study. In addition, we present a comprehensive review on the topic, based on a literature search conducted in PubMed and Google Scholar, in 2014 and updated in December 2017. While the patients had the same previously reported FGF23 gene mutation (homozygous c.G367T variant in exon 3 leading to a missense mutation), they presented with variable severity and age of disease onset (at 4 years in patient 1 and at 23 years in patient 2). A review of the literature revealed several potential patho-physiologic pathways of HFTC clinical manifestations, some of which may be independent of hyperphosphatemia. Most available treatment options aim at reducing serum phosphate level, by stimulating renal excretion or by inhibiting intestinal absorption. HFTC is a challenging disease. While the available medical treatment has a limited and inconsistent effect on disease symptomatology, surgical resection of calcified masses remains the last resort. Research is needed to determine the safety and efficacy of FGF23 replacement or molecular therapy, targeting the specific genetic aberration. Hyperphosphatemic familial tumoral calcinosis is a rare genetic disorder characterized by recurrent calcified masses, in addition to other visceral, skeletal, and vascular manifestations. It remains a very challenging disease.

Sucroferric oxyhydroxide decreases serum phosphorus level and fibroblast growth factor 23 and improves renal anemia in hemodialysis patients

Objective

Sucroferric oxyhydroxide, a novel iron-based phosphate-binder, has been shown to have beneficial effects in lowering serum phosphorus levels and improving renal anemia in clinical studies. Although an effect of this agent on fibroblast growth factor 23 (FGF23) has been reported in an animal study, there is little clinical data supporting this finding. This study aimed to evaluate the effect on chronic kidney disease-mineral and bone disorder, FGF23, renal anemia, iron-related parameters, adverse events of sucroferric oxyhydroxide in hemodialysis patients.

Results

Hemodialysis patients, receiving existing hyperphosphatemia drugs with insufficient benefit, were administered sucroferric oxyhydroxide with/without calcium carbonate for 16 weeks. Serum phosphorus level declined rapidly in Week 8 (p < 0.0001) and this decrease persisted until Week 16 (p < 0.0001). FGF23 decreased (p = 0.0412, Week 16), and hemoglobin increased (p < 0.0001, Week 16). Cumulative dose of erythropoiesis-stimulating agents (p = 0.0122, Week 16), and intravenous iron (p = 0.0233, Week 12) decreased. All adverse reactions were mild, and diarrhea was the most frequently observed adverse reaction (16.7%). Therefore, hyperphosphatemia treatment with sucroferric oxyhydroxide may safely improve serum phosphorus level, renal anemia, FGF23, and other factors that affect the prognosis of hemodialysis patients.

Electronic supplementary material

The online version of this article (10.1186/s13104-018-3483-6) contains supplementary material, which is available to authorized users.

Ferric Citrate Decreases Fibroblast Growth Factor 23 and Improves Erythropoietin Responsiveness in Hemodialysis Patients

BACKGROUND

Serum phosphate and vitamin D receptor activator regulate fibroblast growth factor 23 (FGF23), and iron may modulate FGF23 metabolism. The aim of the present study was to elucidate the effects of ferric citrate hydrate and lanthanum carbohydrate on serum FGF23 levels in hemodialysis patients.

METHODS

This prospective, open-label, multicenter study enrolled 60 patients on hemodialysis treated with lanthanum carbonate. Patients were randomly assigned to 2 groups: those switching from lanthanum carbonate to ferric citrate hydrate (ferric citrate group, n = 30) or those continuing lanthanum carbonate (control group, n = 30). Patients were monitored for 24 weeks. Endpoints included changes in FGF23, phosphate, and the dose of erythropoiesis stimulating agent (ESA), erythropoietin responsiveness index (ERI), and adverse events.

RESULTS

FGF-23 levels were significantly lower in the ferric citrate group compared with the levels in the control group (change from baseline -6,160 vs. -1,118 pg/mL; p = 0.026). There were no significant changes in serum calcium, phosphate, and intact parathyroid hormone levels in either group. The ferric citrate group had significantly increased serum iron, ferritin, and transferrin saturation. Hemoglobin levels were significantly elevated, and the dose of ESA was significantly decreased in the ferric citrate group but not in the control group. ERI and the dose of intravenous saccharated ferric oxide were significantly lower in the ferric citrate group compared with those of the control group (p = 0.015 and p = 0.002).

CONCLUSION

In patients on hemodialysis, 24-week treatment with ferric citrate hydrate resulted in significant reduction in FGF23 and ERI independently of serum phosphate level.

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.

Tumor-induced osteomalacia in association with PTEN-negative Cowden syndrome

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic condition in which phosphaturic mesenchymal tumors (PMTs) secrete high levels of fibroblast growth factor 23 (FGF23) into the circulation. This results in renal phosphate wasting, hypophosphatemia, muscle weakness, bone pain, and pathological fractures. Recent studies suggest that fibronectin-fibroblast growth factor receptor 1 (FN1-FGFR1) translocations may be a driver of tumorigenesis. We present a patient with TIO who also exhibited clinical findings suggestive of Cowden syndrome (CS), a rare autosomal dominant disorder characterized by numerous benign hamartomas, as well as an increased risk for multiple malignancies, such as thyroid cancer. While CS is a clinical diagnosis, most, but not all, harbor a mutation in the tumor suppressor gene PTEN. Genetic testing revealed a somatic FN1-FGFR1 translocation in the FGF23-producing tumor causing TIO; however, a germline PTEN mutation was not identified. To our knowledge, this is the first reported case of concurrent TIO and CS.

FGF23 Actions on Target Tissues-With and Without Klotho

Fibroblast growth factor (FGF) 23 is a phosphaturic hormone whose physiologic actions on target tissues are mediated by FGF receptors (FGFR) and klotho, which functions as a co-receptor that increases the binding affinity of FGF23 for FGFRs. By stimulating FGFR/klotho complexes in the kidney and parathyroid gland, FGF23 reduces renal phosphate uptake and secretion of parathyroid hormone, respectively, thereby acting as a key regulator of phosphate metabolism. Recently, it has been shown that FGF23 can also target cell types that lack klotho. This unconventional signaling event occurs in an FGFR-dependent manner, but involves other downstream signaling pathways than in "classic" klotho-expressing target organs. It appears that klotho-independent signaling mechanisms are only activated in the presence of high FGF23 concentrations and result in pathologic cellular changes. Therefore, it has been postulated that massive elevations in circulating levels of FGF23, as found in patients with chronic kidney disease, contribute to associated pathologies by targeting cells and tissues that lack klotho. This includes the induction of cardiac hypertrophy and fibrosis, the elevation of inflammatory cytokine expression in the liver, and the inhibition of neutrophil recruitment. Here, we describe the signaling and cellular events that are caused by FGF23 in tissues lacking klotho, and we discuss FGF23's potential role as a hormone with widespread pathologic actions. Since the soluble form of klotho can function as a circulating co-receptor for FGF23, we also discuss the potential inhibitory effects of soluble klotho on FGF23-mediated signaling which might-at least partially-underlie the pleiotropic tissue-protective functions of klotho.

FAM20C regulates osteoblast behaviors and intracellular signaling pathways in a cell-autonomous manner

Recent studies indicate that Family with sequence similarity 20 member C (FAM20C) catalyzes the phosphorylation of secreted proteins, and participates in a variety of biological processes, including cell proliferation, migration, mineralization, and phosphate homeostasis. To explore the local influences of FAM20C on osteoblast, Fam20c-deficient osteoblasts were generated by treating the immortalized Fam20c^f/f osteoblasts with CMV-Cre-IRES-EGFP lentivirus. Compared with the normal Fam20c^f/f osteoblasts, the expression of Bone sialoprotein (Bsp), Osteocalcin (Ocn), Fibroblast growth factor 23 (Fgf23), and transcription factors that promote osteoblast maturation were up-regulated in the Fam20c-deficient osteoblasts. In contrast, the expression of Dental matrix protein 1 (Dmp1), Dentin sialophosphoprotein (Dspp), Osteopontin (Opn), type I Collagen a 1 (Col1a1), and Alkine phosphatase (Alp) were down-regulated in the Fam20c-deficient cells. These alterations disclosed the primary regulation of Fam20c on gene expression. The Fam20c-deficient osteoblasts showed a remarkable reduction in the ability of forming mineralized nodules. However, supplements of extracellular matrix proteins extracted from the normal bone failed to rescue the reduced mineralization, suggesting that FAM20C may affect the biomineralization by the means more than local phosphorylation of extracellular matrix proteins and systemic phosphorus homeostasis. Moreover, although Fam20c deficiency had little impact on cell proliferation, it significantly reduced cell migration and lowered the levels of p-Smad1/5/8, p-Erk and p-p38, suggesting that the kinase activity of FAM20C might be essential to cell mobility and the activity of BMP ligands. In summary, these findings provide evidences that FAM20C may regulate osteoblast maturation, migration, mineralization, and BMP signaling pathways in a cell-autonomous manner.

Tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome clinically characterized by bone pain, fractures and muscle weakness. It is caused by tumoral overproduction of fibroblast growth factor 23 (FGF23) that acts primarily at the proximal renal tubule, decreasing phosphate reabsorption and 1α-hydroxylation of 25 hydroxyvitamin D, thus producing hypophosphatemia and osteomalacia. Lesions are typically small, benign mesenchymal tumors that may be found in bone or soft tissue, anywhere in the body. In up to 60% of these tumors, a fibronectin-1(FN1) and fibroblast growth factor receptor-1 (FGFR1) fusion gene has been identified that may serve as a tumoral driver. The diagnosis is established by the finding of acquired chronic hypophosphatemia due to isolated renal phosphate wasting with concomitant elevated or inappropriately normal blood levels of FGF23 and decreased or inappropriately normal 1,25-OH₂-Vitamin D (1,25(OH)₂D). Locating the tumor is critical, as complete removal is curative. For this purpose, a step-wise approach is recommended, starting with a thorough medical history and physical examination, followed by functional imaging. Suspicious lesions should be confirmed by anatomical imaging, and if needed, selective venous sampling with measurement of FGF23. If the tumor is not localized, or surgical resection is not possible, medical therapy with phosphate and active vitamin D is usually successful in healing the osteomalacia and reducing symptoms. However, compliance is often poor due to the frequent dosing regimen and side effects. Furthermore, careful monitoring is needed to avoid complications such us secondary/tertiary hyperparathyroidism, hypercalciuria, and nephrocalcinosis. Novel therapeutical approaches are being developed for TIO patients, such as image-guided tumor ablation and medical treatment with the anti-FGF23 monoclonal antibody KRN23 or anti FGFR medications. The case of a patient with TIO is presented to illustrate the importance of adequate and appropriate evaluation of patients with bone pain and hypophosphatemia, as well as an step-wise localization study of patients with suspected TIO.

Tumour-induced osteomalacia

Tumour-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare paraneoplastic disorder caused by tumours that secrete fibroblast growth factor 23 (FGF23). Owing to the role of FGF23 in renal phosphate handling and vitamin D synthesis, TIO is characterized by decreased renal tubular reabsorption of phosphate, by hypophosphataemia and by low levels of active vitamin D. Chronic hypophosphataemia ultimately results in osteomalacia (that is, inadequate bone mineralization). The diagnosis of TIO is usually suspected when serum phosphate levels are chronically low in the setting of bone pain, fragility fractures and muscle weakness. Locating the offending tumour can be very difficult, as the tumour is often very small and can be anywhere in the body. Surgical removal of the tumour is the only definitive treatment. When the tumour cannot be located or when complete resection is not possible, medical treatment with phosphate salts or active vitamin D is necessary. One of the most promising emerging treatments for unresectable tumours that cause TIO is the anti-FGF23 monoclonal antibody KRN23. The recent identification of a fusion of fibronectin and fibroblast growth factor receptor 1 (FGFR1) as a molecular driver in some tumours not only sheds light on the pathophysiology of TIO but also opens the door to a better understanding of the transcription, translocation, post-translational modification and secretion of FGF23, as well as suggesting approaches to targeted therapy. Further study will reveal if the FGFR1 pathway is also involved in tumours that do not harbour the translocation.

Tumor-induced osteomalacia: experience from a South American academic center

The majority of tumor-induced osteomalacia cases have been reported in the Northern Hemisphere and Asia. In this first series of South American patients, we show that the clinical presentation and sensitivity of plasmatic fibroblast growth factor 23 and somatostatin analog-based imaging are similar to those described in other populations.

INTRODUCTION

Describe the experience of clinical presentation, diagnostic study, and treatment of patients with tumor-induced osteomalacia (TIO) in a South American academic center in comparison to literature.

METHODS

Analysis of the records of patients diagnosed with TIO. The clinical presentation, diagnostic studies, and treatment were analyzed. Fibroblast growth factor 23 (FGF23) was measured by ELISA.

RESULTS

Six patients were diagnosed with TIO during the studied period. The patients' median age was 53 years (range 22-64). All patients presented with weakness and pain in the extremities. Four experienced fractures during their evolution. The median time to diagnosis was 4.5 years (1-20). Biochemical studies showed hypophosphatemia, median of 1.4 mg/dL (1.2-1.6), with low maximum rates of tubular reabsorption of phosphate adjusted for glomerular filtration rate. FGF23 was elevated in 4/6 patients and inappropriately normal in the other two. In three patients, the location of the tumor was clinically evident and confirmed with anatomical imaging. In the remaining patients, two tumors were located with ⁶⁸Ga DOTATATE-PET/CT and one with OctreoScan. The causal tumors were located in the lower extremities in five patients and invading the frontal sinus in one patient. In all patients, tumors were successfully removed. Within 14 days, there was normalization of phosphate and FGF23 levels and resolution of clinical symptoms in all patients. In all cases, the histopathology was compatible with a phosphaturic mesenchymal tumor.

CONCLUSIONS

The clinical presentation, delay time to diagnosis, FGF23 diagnostic sensitivity and histopathology in this first series of South American patients is similar to those described in other populations. The success of localization by somatostatin analog-based imaging, suggests this may the optimal imaging modality.

Low serum iron is associated with high serum intact FGF23 in elderly men: The Swedish MrOS study

BACKGROUND

Fibroblast growth factor (FGF23) is a protein that is produced by osteoblasts and osteocytes. Increased serum levels of FGF23 have been associated with increased risks of osteoporotic fractures and cardiovascular disease, particularly in participants with poor renal function. Serum iron (Fe) has been suggested as a regulator of FGF23 homeostasis.

OBJECTIVE

To determine whether Fe and iron status are determinants of the levels of intact FGF23 (iFGF23) in elderly men.

METHODS

The MrOS study is a population-based study of elderly men (N=1010; mean age, 75.3years; range, 69-81years). The levels of Fe, transferrin saturation (TS), and ferritin were evaluated in relation to the serum concentrations of iFGF23 before and after adjustments for confounders.

RESULTS

TS <15% was found in 3.5% (34/977) of the participants, who had a higher median level iFGF23 compared with the remaining subjects (47.4μmol/L vs. 41.9μmol/L, p=0.008). The levels of iFGF23 correlated negatively (un-adjusted) with the levels of Fe (r=-0.17, p<0.001), TS (r=-0.16, p<0.001) and serum ferritin (r=-0.07, p=0.022). In addition, in participants with estimated glomerular filtration rate eGFRCystatin C>60mL/min, the levels of iFGF23 correlated (age-adjusted) negatively with the levels of Fe (r=-0.15, p<0.001) and TS (r=-0.17, p<0.001). The level of iFGF23 correlated positively (un-adjusted) with lumbar spine bone mineral density (BMD) (r=0.14, p<0.001), total body BMD (r=0.11, p=0.001), and total hip BMD (r=0.09, p=0.004). The corresponding correlations, when adjusted for age, weight, and height were: r=0.08, p=0.018; r=0.05, p=0.120; and r=0.02, p=0.624, respectively. No associations were found between BMD and the levels of Fe or TS. Multiple step-wise linear regression analyses [adjusting for age, body mass index (BMI), comorbidity index, cystatin C, C-reactive protein (hs-CRP), serum vitamin D 25-OH (25OHD), phosphate, calcium, parathyroid hormone (PTH), erythropoietin, hemoglobin, lumbar spine BMD, apolipoprotein B/A1 ratio] were performed in three separate models with Fe, TS or ferritin as potential explanatory variables. Fe and TS, but not ferritin, were independent predictors of iFGF23 level (standardized β-values: -0.10, p<0.001; -0.10, p<0.001; and -0.05, p=0.062, respectively).

CONCLUSION

Low levels of Fe in elderly men are associated with high levels of iFGF23, independently of markers of inflammation and renal function, suggesting an iron-related pathway for FGF23 regulation.

MC4R-dependent suppression of appetite by bone-derived lipocalin 2

Bone has recently emerged as a pleiotropic endocrine organ that secretes at least two hormones, FGF23 and osteocalcin, which regulate kidney function and glucose homeostasis, respectively. These findings have raised the question of whether other bone-derived hormones exist and what their potential functions are. Here we identify, through molecular and genetic analyses in mice, lipocalin 2 (LCN2) as an osteoblast-enriched, secreted protein. Loss- and gain-of-function experiments in mice demonstrate that osteoblast-derived LCN2 maintains glucose homeostasis by inducing insulin secretion and improves glucose tolerance and insulin sensitivity. In addition, osteoblast-derived LCN2 inhibits food intake. LCN2 crosses the blood-brain barrier, binds to the melanocortin 4 receptor (MC4R) in the paraventricular and ventromedial neurons of the hypothalamus and activates an MC4R-dependent anorexigenic (appetite-suppressing) pathway. These results identify LCN2 as a bone-derived hormone with metabolic regulatory effects, which suppresses appetite in a MC4R-dependent manner, and show that the control of appetite is an endocrine function of bone.

Analysis of the FGF gene family provides insights into aquatic adaptation in cetaceans

Cetacean body structure and physiology exhibit dramatic adaptations to their aquatic environment. Fibroblast growth factors (FGFs) are a family of essential factors that regulate animal development and physiology; however, their role in cetacean evolution is not clearly understood. Here, we sequenced the fin whale genome and analysed FGFs from 8 cetaceans. FGF22, a hair follicle-enriched gene, exhibited pseudogenization, indicating that the function of this gene is no longer necessary in cetaceans that have lost most of their body hair. An evolutionary analysis revealed signatures of positive selection for FGF3 and FGF11, genes related to ear and tooth development and hypoxia, respectively. We found a D203G substitution in cetacean FGF9, which was predicted to affect FGF9 homodimerization, suggesting that this gene plays a role in the acquisition of rigid flippers for efficient manoeuvring. Cetaceans utilize low bone density as a buoyancy control mechanism, but the underlying genes are not known. We found that the expression of FGF23, a gene associated with reduced bone density, is greatly increased in the cetacean liver under hypoxic conditions, thus implicating FGF23 in low bone density in cetaceans. Altogether, our results provide novel insights into the roles of FGFs in cetacean adaptation to the aquatic environment.