Overexpression of fibroblast growth factor 23 suppresses osteoblast differentiation and matrix mineralization in vitro

A novel miR-466l-3p/FGF23 axis promotes osteogenic differentiation of human bone marrow mesenchymal stem cells

BACKGROUND

MicroRNAs (miRNAs) regulate osteogenic differentiation processes and influence the development of osteoporosis (OP). This study aimed to investigate the potential role of miR-466 l-3p in OP.

METHODS

The expression levels of miR-466 l-3p and fibroblast growth factor 23 (FGF23) were quantified in the trabeculae of the femoral neck of 40 individuals with or without OP using quantitative reverse transcription-polymerase chain reaction (qRT-PCR). The impact of miR-466 l-3p or FGF23 expression on cell proliferation and the expression levels of runt-related transcription factor 2 (RUNX2), type I collagen (Col1), osteocalcin (OCN), osterix (OSX) and dentin matrix protein 1 (DMP1) was quantified in human bone marrow mesenchymal stem cells (hBMSCs) overexpressing miR-466 l-3p. Furthermore, alkaline phosphatase (ALP) staining and alizarin red staining were performed to measure ALP activity and the levels of calcium deposition, respectively. In addition, bioinformatics analysis, luciferase reporter assays, and RNA pull-down assays were conducted to explore the molecular mechanisms underlying the effects of miR-466 l-3p and FGF23 in osteogenic differentiation of hBMSCs.

RESULTS

The expression levels of miR-466 l-3p were significantly lower in femoral neck trabeculae of patients with OP than in the control cohort, whereas FGF23 levels exhibited the opposite trend. Furthermore, miR-466 l-3p levels were upregulated and FGF23 levels were downregulated in hBMSCs during osteogenic differentiation. Moreover, the high miR-466 l-3p expression enhanced the mRNA expression of RUNX2, Col1, OCN, OSX and DMP1, as well as cell proliferation, ALP activity, and calcium deposition in hBMSCs. FGF23 was found to be a direct target of miR-466 l-3p. FGF23 overexpression downregulated the expression of osteoblast markers and inhibited the osteogenic differentiation induced by miR-466 l-3p overexpression. qRT-PCR and Western blot assays showed that miR-466 l-3p overexpression decreased the expression levels of mRNAs and proteins associated with the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) signaling pathway, whereas FGF23 upregulation exhibited the opposite trend.

CONCLUSION

In conclusion, these findings suggest that miR-466 l-3p enhances the osteogenic differentiation of hBMSCs by suppressing FGF23 expression, ultimately preventing OP.

Fibroblast growth factor 23 inhibition attenuates steroid-induced osteonecrosis of the femoral head through pyroptosis

Steroid-induced osteonecrosis of the femoral head (SONFH) is the predominant cause of non-traumatic osteonecrosis of the femoral head (ONFH). Impaired blood supply and reduced osteogenic activity of the femoral head are the key pathogenic mechanisms of SONFH. Fibroblast growth factor 23 (FGF23) levels are not only a biomarker for early vascular lesions caused by abnormal mineral metabolism, but can also act directly on the peripheral vascular system, leading to vascular pathology. The aim of this study was to observe the role of FGF23 on bone microarchitecture and vascular endothelium, and to investigate activation of pyroptosis in SONFH. Lipopolysaccharide (LPS) combined with methylprednisolone (MPS) was applied for SONFH mouse models, and adenovirus was used to increase or decrease the level of FGF23. Micro-CT and histopathological staining were used to observe the structure of the femoral head, and immunohistochemical staining was used to observe the vascular density. The cells were further cultured in vitro and placed in a hypoxic environment for 12 h to simulate the microenvironment of vascular injury during SONFH. The effect of FGF23 on osteogenic differentiation was evaluated using alkaline phosphatase staining, alizarin red S staining and expression of bone formation-related proteins. Matrigel tube formation assay in vitro and immunofluorescence were used to detect the ability of FGF23 to affect endothelial cell angiogenesis. Steroids activated the pyroptosis signaling pathway, promoted the secretion of inflammatory factors in SONFH models, led to vascular endothelial dysfunction and damaged the femoral head structure. In addition, FGF23 inhibited the HUVECs angiogenesis and BMSCs osteogenic differentiation. FGF23 silencing attenuated steroid-induced osteonecrosis of the femoral head by inhibiting the pyroptosis signaling pathway, and promoting osteogenic differentiation of BMSCs and angiogenesis of HUVECs in vitro.

Characteristics of oral health of patients with X-linked hypophosphatemia: case reports and literature review

BACKGROUND

Oral health is impaired in X-linked hypophosphatemia (XLH), resulting in delayed dental development, malocclusion, and radiographic abnormalities. This study investigates the oral manifestations in Slovenian XLH patients, focusing on enamel and dentin abnormalities and a literature review of spontaneous periapical abscesses in XLH cases.

OBJECTIVES

To report XLH patients with specific oral signs and symptoms, histological analysis of affected teeth, and review of reported cases of XLH patients with spontaneous periapical abscesses.

METHODS

Case reports: Seven XLH patients from the National Registry of Patients with Rare Diseases underwent a detailed oral examination, including X-ray reviews. The patients who were expected to have tooth exfoliation or extraction were asked to donate their teeth for histological analysis by scanning electron microscopy.

LITERATURE SEARCH

A literature search of four electronic databases and a manual bibliography search aimed to identify documented cases of XLH with periapical abscesses up to January 21, 2024. Inclusion criteria were confirmed XLH patients with periapical abscesses in English peer-reviewed publications.

RESULTS

Tooth samples from three XLH patients showed reduced dentin mineralisation, affecting one-third to one-half of the outer dentin. Inadequate mineralisation, uneven dentin tubules, and cracks and chipping in the enamel were observed, indicating mineralisation deviations. Similar cracks extended into the dentin and were also present in the root of the examined tooth. Based on the content of the 75 items identified in the search, spontaneous abscesses are not uncommon in patients with XLH.

CONCLUSIONS

XLH significantly affects patients' lives and requires lifelong treatment. Dental examinations consistently revealed oral problems, including malocclusion. Histological analysis confirmed structural changes, especially in the dentin. Despite continued treatment, XLH patients may have an increased risk of oral pathologies. Further research is needed to understand the impact of XLH and its treatment on dental health.

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.

FGF23 Expression Is a Promising Immunohistochemical Diagnostic Marker for Undifferentiated Pleomorphic Sarcoma of Bone (UPSb)

Background: Undifferentiated pleomorphic sarcoma of bone (UPSb) is a rare primary bone sarcoma that lacks a specific line of differentiation. Distinguishing between UPSb and other malignant bone sarcomas, including dedifferentiated chondrosarcoma and osteosarcoma, is challenging due to their overlapping features. We have previously identified that UPSb tumours have elevated mRNA levels of Fibroblast Growth Factor 23 (FGF23) transcripts compared to other sarcomas including osteosarcoma. In the present study, we evaluated the specificity and practicality of FGF23 immunoreactivity as a specific diagnostic tool to differentiate UPSb tumours from osteosarcomas and dedifferentiated chondrosarcomas. Methods: A total of 10 UPSb, 10 osteosarcoma, and 10 dedifferentiated chondrosarcoma cases (all high-grade), were retrieved and immunohistochemistry for FGF23 was performed. Results: FGF23 protein was expressed at high levels in 80-90% of undifferentiated pleomorphic sarcoma of the bone cases, whereas it was expressed at significantly lower levels in dedifferentiated chondrosarcoma and osteosarcoma cases. A semiquantitative analysis, considering the intensity of immunoreactivity, confirmed significantly elevated FGF23 expression levels in UPSb tissues compared to those observed in osteosarcoma and dedifferentiated chondrosarcoma tissues. Conclusions: The results we present here suggest that FGF23 immunohistochemistry may be a useful tool to aid in differentiating UPSb from morphologically similar malignant bone sarcomas, especially in situations where sampling is restricted and there is limited clinical information available.

Fibroblast growth factor 23 and its role in bone diseases

Fibroblast growth factor 23 (FGF23) has been casually linked to numerous hypophosphatemic bone diseases, however connection with bone loss or fragility fractures is still a matter of debate. The purpose of this review is to explore and summarise the known actions of FGF23 in various pathological bone conditions. Besides implication in bone mineralisation, elevated FGF23 showed a pathological effecton bone remodelling, primarily by inhibiting osteoblast function. Unlike the weak association with bone mineral density, high values of FGF23 have been connected with fragility fracture prevalence. This review shows that its effects on bone are concomitantly present on multiple levels, affecting both qualitative and quantitative part of bone strength, eventually leading to impaired bone strength and increased tendency of fractures. Recognising FGF23 as a risk factor for the development of bone diseases and correcting its levels could lead to the reduction of morbidity and mortality in specific groups of patients.

FGF23 directly inhibits osteoprogenitor differentiation in Dmp1-knockout mice

Fibroblast growth factor 23 (FGF23) is a phosphate-regulating (Pi-regulating) hormone produced by bone. Hereditary hypophosphatemic disorders are associated with FGF23 excess, impaired skeletal growth, and osteomalacia. Blocking FGF23 became an effective therapeutic strategy in X-linked hypophosphatemia, but testing remains limited in autosomal recessive hypophosphatemic rickets (ARHR). This study investigates the effects of Pi repletion and bone-specific deletion of Fgf23 on bone and mineral metabolism in the dentin matrix protein 1-knockout (Dmp1KO) mouse model of ARHR. At 12 weeks, Dmp1KO mice showed increased serum FGF23 and parathyroid hormone levels, hypophosphatemia, impaired growth, rickets, and osteomalacia. Six weeks of dietary Pi supplementation exacerbated FGF23 production, hyperparathyroidism, renal Pi excretion, and osteomalacia. In contrast, osteocyte-specific deletion of Fgf23 resulted in a partial correction of FGF23 excess, which was sufficient to fully restore serum Pi levels but only partially corrected the bone phenotype. In vitro, we show that FGF23 directly impaired osteoprogenitors' differentiation and that DMP1 deficiency contributed to impaired mineralization independent of FGF23 or Pi levels. In conclusion, FGF23-induced hypophosphatemia is only partially responsible for the bone defects observed in Dmp1KO mice. Our data suggest that combined DMP1 repletion and FGF23 blockade could effectively correct ARHR-associated mineral and bone disorders.

Adipose Tissue Denervation Blunted the Decrease in Bone Formation Promoted by Obesity in Rats

The impact of obesity upon bone metabolism is controversial since both beneficial or harmful effects have been reported. Bone remodeling is modulated by the central nervous system through cytokines, hormones and neuromodulators. The present study aimed to evaluate the effects evoked by bilateral retroperitoneal white adipose tissue (rWAT) denervation (Dnx) upon bone mineral metabolism and remodeling in an experimental model of obesity in rats. Male Wistar rats were fed during 18 weeks with high-fat diet (HFD) or standard diet (SD) as controls, and rWAT Dnx or Sham surgery was performed at the 14th week. Biochemical and hormonal parameters, bone histomorphometry, rWAT and hypothalamus protein and gene expression were analyzed. The HFD group presented decreased bone formation parameters, increased serum and bone leptin and FGF23, increased serum and hypothalamic neuropeptide Y (NPY) and decreased serum 1,25-dihydroxyvitamin D3 and PTH. After rWAT Dnx, bone markers and histomorphometry showed restoration of bone formation, and serum and hypothalamic NPY decreased, without alteration in leptin levels. The present study shows that the denervation of rWAT improved bone formation in obese rats mediated by a preferential reduction in neurohormonal actions of NPY, emphasizing the relevance of the adipose tissue-brain-bone axis in the control of bone metabolism in obesity.

Advanced Glycation End Products and Bone Metabolism in Patients with Chronic Kidney Disease

Advanced glycation end products (AGEs) accumulation may be involved in the progression of CKD-bone disorders. We sought to determine the relationship between AGEs measured in the blood, skin, and bone with histomorphometry parameters, bone protein, gene expression, and serum biomarkers of bone metabolism in patients with CKD stages 3 to 5D patients. Serum levels of AGEs were estimated by pentosidine, glycated hemoglobin (A1c), and N-carboxymethyl lysine (CML). The accumulation of AGEs in the skin was estimated from skin autofluorescence (SAF). Bone AGEs accumulation and multiligand receptor for AGEs (RAGEs) expression were evaluated by immunohistochemistry; bone samples were used to evaluate protein and gene expression and histomorphometric analysis. Data are from 86 patients (age: 51 ± 13 years; 60 [70%] on dialysis). Median serum levels of pentosidine, CML, A1c, and SAF were 71.6 pmol/mL, 15.2 ng/mL, 5.4%, and 3.05 arbitrary units, respectively. AGEs covered 3.92% of trabecular bone and 5.42% of the cortical bone surface, whereas RAGEs were expressed in 0.7% and 0.83% of trabecular and cortical bone surfaces, respectively. AGEs accumulation in bone was inversely related to serum receptor activator of NF-κB ligand/parathyroid hormone (PTH) ratio (R = -0.25; p = 0.03), and RAGE expression was negatively related to serum tartrate-resistant acid phosphatase-5b/PTH (R = -0.31; p = 0.01). Patients with higher AGEs accumulation presented decreased bone protein expression (sclerostin [1.96 (0.11-40.3) vs. 89.3 (2.88-401) ng/mg; p = 0.004]; Dickkopf-related protein 1 [0.064 (0.03-0.46) vs. 1.36 (0.39-5.87) ng/mg; p = 0.0001]; FGF-23 [1.07 (0.4-32.6) vs. 44.1 (6-162) ng/mg; p = 0.01]; and osteoprotegerin [0.16 (0.08-2.4) vs. 6.5 (1.1-23.7) ng/mg; p = 0.001]), upregulation of the p53 gene, and downregulation of Dickkopf-1 gene expression. Patients with high serum A1c levels presented greater cortical porosity and Mlt and reduced osteoblast surface/bone surface, eroded surface/bone surface, osteoclast surface/bone surface, mineral apposition rate, and adjusted area. Cortical thickness was negatively correlated with serum A1c (R = -0.28; p = 0.02) and pentosidine levels (R = -0.27; p = 0.02). AGEs accumulation in the bone of CKD patients was related to decreased bone protein expression, gene expression changes, and increased skeletal resistance to PTH; A1c and pentosidine levels were related to decreased cortical thickness; and A1c levels were related to increased cortical porosity and Mlt. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

FGF23 functions and disease

The main function of fibroblast growth factor 23 (FGF23) is the regulation of phosphate metabolism through its action on the sodium-dependent phosphate cotransporters in the proximal renal tubules. Additionally, FGF23 interacts with vitamin D and parathyroid hormone in a complex metabolic pathway whose detailed mechanisms are still not clear in human physiology and disease. More recently, research has also focused on the understanding of mechanisms of FGF23 action on organs and system other than the kidneys and bone, as well as on its interaction with other metabolic pathways. Collectively, the new evidence are successfully used for the clinical evaluation and management of FGF23-related disorders, for which new therapies with many potential applications are now available.

Skeletal and extraskeletal disorders of biomineralization

The physiological process of biomineralization is complex and deviation from it leads to a variety of diseases. Progress in the past 10 years has enhanced understanding of the genetic, molecular and cellular pathophysiology underlying these disorders; sometimes, this knowledge has both facilitated restoration of health and clarified the very nature of biomineralization as it occurs in humans. In this Review, we consider the principal regulators of mineralization and crystallization, and how dysregulation of these processes can lead to human disease. The knowledge acquired to date and gaps still to be filled are highlighted. The disorders of mineralization discussed comprise a broad spectrum of conditions that encompass bone disorders associated with alterations of mineral quantity and quality, as well as disorders of extraskeletal mineralization (hyperphosphataemic familial tumoural calcinosis). Included are disorders of alkaline phosphatase (hypophosphatasia) and phosphate homeostasis (X-linked hypophosphataemic rickets, fluorosis, rickets and osteomalacia). Furthermore, crystallopathies are covered as well as arterial and renal calcification. This Review discusses the current knowledge of biomineralization derived from basic and clinical research and points to future studies that will lead to new therapeutic approaches for biomineralization disorders.

PHEXL222P Mutation Increases Phex Expression in a New ENU Mouse Model for XLH Disease

PhexL222P mouse is a new ENU mouse model for XLH disease due to Leu to Pro amino acid modification at position 222. PhexL222P mouse is characterized by growth retardation, hypophosphatemia, hypocalcemia, reduced body bone length, and increased epiphyseal growth plate thickness and femur diameter despite the increase in PHEXL222P expression. Actually, PhexL222P mice show an increase in Fgf23, Dmp1, and Mepe and Slc34a1 (Na-Pi IIa cotransporter) mRNA expression similar to those observed in Hyp mice. Femoral osteocalcin and sclerostin and Slc34a1 do not show any significant variation in PhexL222P mice. Molecular dynamics simulations support the experimental data. P222 might locally break the E217-Q224 β-sheet, which in turn might disrupt inter-β-sheet interactions. We can thus expect local protein misfolding, which might be responsible for the experimentally observed PHEXL222P loss of function. This model could be a valuable addition to the existing XLH model for further comprehension of the disease occurrence and testing of new therapies.

FGF23-Klotho Axis and Fractures in Patients Without and With Early CKD: A Case-Cohort Analysis of CARTaGENE

CONTEXT

Whether fibroblast growth factor-23 (FGF23) and α-Klotho are associated with fractures, especially in chronic kidney disease (CKD), remains controversial.

OBJECTIVE

We evaluated how FGF23, α-Klotho, and traditional mineral parameters predict fractures in individuals with and without early CKD.

METHODS

We conducted a stratified case-cohort analysis using CARTaGENE, a population-based survey from Quebec, Canada. Individuals aged 40 to 69 years were selected according to outcome and CKD status (non-CKD: eGFR > 60 mL/min/1.73 m2; CKD stage 3: eGFR 30-60 mL/min/1.73 m2]). Baseline levels of c-terminal FGF23 (cFGF23), α-Klotho, parathyroid hormone (PTH), phosphate, and calcium were analyzed for associations with osteoporotic fracture incidence from recruitment (2009-2010) through March 2016. Adjusted Cox models were used, and predictors were treated linearly or flexibly using splines.

RESULTS

A total of 312 patients (159 non-CKD; 153 CKD) were included; 98 had ≥ 1 fracture at any site during a median follow up of 70 months. Compared with non-CKD, CKD patients had increased levels of cFGF23 but similar levels of α-Klotho. cFGF23 was linearly associated with increased fracture incidence (adjusted HR = 1.81 [1.71, 1.93] per doubling for all participants). The association of α-Klotho with fracture followed a U-curve (overall P = 0.019) but was attenuated by adjustment for potential mediators (bone mineral density, phosphate, PTH). PTH and phosphate also had U-shaped associations with fracture. Associations were mostly similar between non-CKD and CKD. Adjustment for cFGF23 strongly attenuated the association between CKD status and fractures.

CONCLUSION

cFGF23 is associated linearly with fracture incidence while α-Klotho, PTH, and phosphate levels have a U-shaped association.

New concepts in phosphorus homeostasis and its impact on renal health with particular reference to the cat

New discoveries relating to phosphorus homeostasis include the hormones fibroblast growth factor-23 and klotho produced by bone and kidney. These hormones, together with novel understanding of how calcium and phosphate ions are carried in colloidal form as calciprotein particles, have changed our view of how phosphorus is regulated. Recognition that high dietary intake of inorganic forms of phosphorus in humans is a risk factor for both cardiovascular and renal diseases have led to re-examination of the impact of inorganic sources of phosphorus in prepared cat foods on renal health. Data suggest that when homeostatic mechanisms lead to proximal tubular (S3 segment) phosphate concentrations exceeding 3.25mmol/L for a significant part of the day, tubular stress and structural kidney damage ensues. Recent experimental rodent studies support the concept that calciprotein particles form in the proximal tubule at these prevailing phosphate concentrations and trigger proximal tubular damage. Long-term feeding studies in cats suggest that carefully formulated prepared diets containing 1g/Mcal of inorganic phosphorus (in the form of sodium tripolyphosphate or potassium monophosphate and pyrophosphate), resulting in estimated tubular phosphate concentrations <2.5mmol/L can be fed to healthy adult cats without detectable adverse effects on renal health.

The Role of Alterations in Alpha-Klotho and FGF-23 in Kidney Transplantation and Kidney Donation

Cardiovascular disease and mineral bone disorders are major contributors to morbidity and mortality among patients with chronic kidney disease and often persist after renal transplantation. Ongoing hormonal imbalances after kidney transplant (KT) are associated with loss of graft function and poor outcomes. Fibroblast growth factor 23 (FGF-23) and its co-receptor, α-Klotho, are key factors in the underlying mechanisms that integrate accelerated atherosclerosis, vascular calcification, mineral disorders, and osteodystrophy. On the other hand, kidney donation is also associated with endocrine and metabolic adaptations that include transient increases in circulating FGF-23 and decreases in α-Klotho levels. However, the long-term impact of these alterations and their clinical relevance have not yet been determined. This manuscript aims to review and summarize current data on the role of FGF-23 and α-Klotho in the endocrine response to KT and living kidney donation, and importantly, underscore specific areas of research that may enhance diagnostics and therapeutics in the growing population of KT recipients and kidney donors.

Interconnections of fibroblast growth factor 23 and klotho with erythropoietin and hypoxia-inducible factor

Bone marrow (BM) hematopoiesis is tightly regulated process and bone components such as osteoblasts, extracellular matrix, and minerals influence hematopoiesis via regulation of hematopoietic stem cell function. Erythropoietin (EPO) secreted mostly by renal EPO producing (REP) cells which employ the hypoxia-inducible factor (HIF) pathway. When tissue hypoxia occurs, HIFs bind to hypoxia response element in the EPO promoter and induce EPO production. EPO binds to the EPO receptor on red cell progenitors in the BM and triggers expansion of red cell mass. Fibroblast growth factor-23 (FGF23) which is secreted mostly by osteoblasts and less by BM impacts hematopoiesis by influencing EPO production. Reciprocally, increases of EPO (acute or chronic) influence both FG23 production and cleavage resulting in variation of c fragment FGF23 (cFGF23) and intact FGF23 (iFGF23) ratios. As HIFs stimulate EPO production, they indirectly affect FGF23. Direct stimulation of FGF23 synthesis by binding of HIF on FGF23 promoter is also suggested. FGF23 cleavage by furin is another potential mechanism affecting FGF23 levels. Klotho is present in membrane-bound (transmembrane) and free (circulating) forms. Transmembrane klotho is the co-receptor of FGF23 and forms complexes with FGF23 receptors in the membrane surface and required for FGF23 actions. Recent evidence showed that klotho is also associated with EPO and HIF production suggesting a complex relationship between FGF23, klotho, EPO, and HIF. In this review, we have summarized the connections between FGF23, klotho, HIF, and EPO and their reflections to hematopoiesis.

Molecular Mechanisms Involved in Hypoxia-Induced Alterations in Bone Remodeling

Bone is crucial for the support of muscles and the protection of vital organs, and as a reservoir of calcium and phosphorus. Bone is one of the most metabolically active tissues and is continuously renewed to adapt to the changes required for healthy functioning. To maintain normal cellular and physiological bone functions sufficient oxygen is required, as evidence has shown that hypoxia may influence bone health. In this scenario, this review aimed to analyze the molecular mechanisms involved in hypoxia-induced bone remodeling alterations and their possible clinical consequences. Hypoxia has been associated with reduced bone formation and reduced osteoblast matrix mineralization due to the hypoxia environment inhibiting osteoblast differentiation. A hypoxic environment is involved with increased osteoclastogenesis and increased bone resorptive capacity of the osteoclasts. Clinical studies, although with contradictory results, have shown that hypoxia can modify bone remodeling.

[Tumor localization and treatment of tumor-induced osteomalacia]

Tumor-induced osteomalacia (TIO) or oncogenic osteomalacia (OOM) is a rare paraneoplastic renal phosphate wasting syndrome. The disease is mostly triggered by small, benign mesenchymal tumors that express somatostatin receptors (SSTR) and produce excessive levels of fibroblast growth factor 23 (FGF 23) or other phosphatonins. These reduce the phosphate back resorption in the proximal tubules of the kidneys, thereby causing hypophosphatemia and lead to an absolute or relatively low calcitriol serum concentration. The main symptoms include muscle weakness, bone pain and recurrent insufficiency fractures secondary to sometimes pronounced osteomalacia. The suspected diagnosis can only be confirmed by determination of the phosphate level. It can often take years before the tumor is successfully localized. The necessary tumor localization is often the most difficult step in the treatment before the OOM can be curatively treated by open surgical resection of the tumor. In recent years new approaches for faster tumor localization and treatment of the tumor have been developed. Positron emission tomography (PET) in co-registration with computed tomography (⁶⁸Ga-DOTA-TATE PET/CT) is currently the most sensitive imaging methodology for tumor detection. The application of the monoclonal FGF 23 antibody burosumab represents a promising new option in the treatment of inoperable adult OOM.

Bone Volumetric Density, Microarchitecture, and Estimated Bone Strength in Tumor-Induced Rickets/Osteomalacia Versus X-linked Hypophosphatemia in Chinese Adolescents

Tumor-induced rickets/osteomalacia (TIR/O) severely impairs bone microarchitecture and bone strength. However, no study has described the microarchitectural quality of bone in adolescent patients with TIR/O. TIR/O affects bone quality more severely than the inherited causes of hypophosphatemia, the most common form of which is X-linked hypophosphatemia (XLH). Nevertheless, differences of the microarchitectural quality of the bone between TIR/O and XLH have never been clarified. Therefore, in this study, we used high-resolution peripheral quantitative computed tomography to assess bone microarchitecture in five Chinese adolescent TIR/O patients, and these were compared with 15 age- and gender-matched XLH patients as well as 15 age- and gender-matched healthy controls. Compared with the healthy controls, the TIR/O patients presented with significantly lower volumetric bone mineral densities (vBMDs), severely affected bone microarchitecture, and profoundly weaker bone strength. The distal tibia was more severely affected than the distal radius. Compared with the XLH patients, the TIR/O patients showed deteriorated bone quality notably at the distal tibia and in the cancellous compartment, reflected by 45.9% lower trabecular vBMD (p = 0.029), 40.2% lower trabecular fraction (p = 0.020), 40.6% weaker stiffness (p = 0.058), and 42.7% weaker failure load (p = 0.039) at the distal tibia. The correlation analysis showed that a higher level of serum FGF23 and a lower level of serum phosphate were associated with a poorer bone microarchitecture and a weaker estimated bone strength in the hypophosphatemic patients of our study. In conclusion, our study demonstrated significantly lower vBMDs, severely impaired bone microarchitecture, and profoundly weaker bone strength in Chinese adolescent patients with TIR/O, notably at the distal tibia, compared with the same parameters in age- and sex-matched healthy controls and XLH patients, which was possibly caused by excessive FGF23 production and secretion, chronically severe hypophosphatemia, and weak mechanical stimulus at the lower extremities. These findings further our understanding of the impact of different kinds of hypophosphatemic rickets/osteomalacia on bone quality.

Biochemical and X-ray micro-computed tomographic analyses of critical size bone defects grafted with autogenous bone and mercerized bacterial cellulose membranes salified with alendronate

OBJECTIVES

This study aimed to evaluate the repair of critical-sized bone defects grafted with autogenous bone and mercerized bacterial cellulose membranes (BCm) salified with alendronate (ALN).

METHODS

Forty-eight male Wistar rats underwent surgery to create a 5 mm-diameter bone defect in the calvarium. The removed bone was particularized, regrafted into the defect, and covered by a BCm according to the group: control group (CG), simply mercerized BCm; group 1 (G1), negatively charged BCm (BCm-CM^-) salified with ALN; and group 2 (G2), positively charged BCm (BCm-DEAE⁺) salified with ALN. Serum samples were collected preoperatively and before euthanasia to analyze osteoprotegerin (OPG), parathyroid hormone (PTH), sclerostin (SOST), and fibroblast growth factor 23 (FGF23) levels. The animals were euthanized after 15 or 60 d. Calvaria were analyzed using quantitative microtomography (μCT).

RESULTS

There was an increased level of PTH in the CG compared to the G2 group, at day 60 (p = 0.019). When analyzing the same group over time, G1 presented an increased FGF23 level on days 15 and 60 (p < 0.05). CG presented an increase in PTH (p = 0.037) at day 60. The μCT analysis detected increased trabecular separation on day 15 in G2 compared to G1 (p = 0.040).

CONCLUSIONS

Salification of ionized BCm with ALN had no direct effect on bone repair; however, BCm-CM^- increased the levels of FGF23 over time. BCm-DEAE⁺ decreased PTH levels compared to mercerized BCm. BCm-CM-salified with ALN-induced superior bone quality, with respect to trabecular separation, compared to BCm-DEAE⁺.

Phosphate Balance and CKD-Mineral Bone Disease

Chronic kidney disease-mineral bone disorder (CKD-MBD) is a common comorbidity in patients with CKD. Characterized by laboratory abnormalities, bone abnormality, and vascular calcification, CKD-MBD encompasses a group of mineral and hormone disturbances that are strongly associated with increased cardiovascular (CV) morbidity and mortality. Abnormal serum phosphate concentrations are an independent risk factor for CV morbidity and mortality, and overall mortality. Phosphate retention plays a central role in initiating and driving many other disturbances in CKD-MBD (e.g., increased parathyroid hormone and fibroblast growth factor 23 concentrations, hypocalcemia, low vitamin D) that are also linked to increased CV risk. Thus, effective phosphate control is a logical therapeutic target for CKD-MBD treatment. Current phosphate management strategies (dietary restrictions, dialysis, phosphate binders) are insufficient to consistently achieve and maintain target phosphate concentrations in patients on dialysis. Phosphate binders reduce available phosphate for intestinal absorption but do not impair the dominant phosphate absorption pathway. Novel therapies that consider new mechanistic understandings of intestinal phosphate absorption are needed. One such therapy is tenapanor, a targeted sodium-hydrogen exchanger isoform 3 inhibitor that has been shown to reduce serum phosphate concentrations in multiple clinical trials. Tenapanor has a novel mechanism of action that reduces intestinal phosphate absorption in the primary paracellular phosphate absorption pathway.

Potential Role of Perilacunar Remodeling in the Progression of Osteoporosis and Implications on Age-Related Decline in Fracture Resistance of Bone

PURPOSE OF REVIEW

We took an interdisciplinary view to examine the potential contribution of perilacunar/canalicular remodeling to declines in bone fracture resistance related to age or progression of osteoporosis.

RECENT FINDINGS

Perilacunar remodeling is most prominent as a result of lactation; recent advances further elucidate the molecular players involved and their effect on bone material properties. Of these, vitamin D and calcitonin could be active during aging or osteoporosis. Menopause-related hormonal changes or osteoporosis therapies affect bone material properties and mechanical behavior. However, investigations of lacunar size or osteocyte TRAP activity with age or osteoporosis do not provide clear evidence for or against perilacunar remodeling. While the occurrence and potential role of perilacunar remodeling in aging and osteoporosis progression are largely under-investigated, widespread changes in bone matrix composition in OVX models and following osteoporosis therapies imply osteocytic maintenance of bone matrix. Perilacunar remodeling-induced changes in bone porosity, bone matrix composition, and bone adaptation could have significant implications for bone fracture resistance.

Impaired bone microarchitecture and strength in patients with tumor-induced osteomalacia

Some studies based on bone biopsy have demonstrated that in patients with tumor-induced osteomalacia (TIO) the mineralization process of the bone matrix is profoundly disturbed. However, the interrelationship between clinical and biochemical features and bone microarchitecture in this disease needs further analysis. With this purpose in mind, we set out three objectives: (i) to determine bone microarchitecture and estimated bone strength in a group of patients with tumor-induced osteomalacia using high-resolution peripheral quantitative computed tomography (HR-pQCT) and finite element analysis (FEA), (ii) to investigate correlations between duration of disease, biochemical features, bone density, HR-pQCT and FEA parameters, and (iii) to compare HR-pQCT and FEA parameters with a healthy control group. Ten patients with TIO were included. All patients had non-resolved disease. At the distal radius, all bone microarchitecture parameters were significantly affected in patients with TIO in comparison with healthy controls. At the distal tibia, all parameters were significantly impaired, except for trabecular thickness. All the parameters were more affected in the distal tibia than in the distal radius. Women with TIO (n = 7) had significantly lower bone strength parameters than healthy controls. In men (n = 3), bone strength parameters were significantly lower than in the control group at the distal tibia. Alkaline phosphatase levels exhibited a negative correlation with microarchitecture parameters, failure load, and stiffness. Higher levels of parathyroid hormone correlated with poorer microarchitecture parameters. We believe that in TIO, hormonal disturbances and the lack of mechanical stimulus specially converge to generate an extremely harmful combination for bone health. © 2021 American Society for Bone and Mineral Research (ASBMR).

A feline-focused review of chronic kidney disease-mineral and bone disorders - Part 1: Physiology of calcium handling

Mineral derangements are a common consequence of chronic kidney disease (CKD). Despite the well-established role of phosphorus in the pathophysiology of CKD, the implications of calcium disturbances associated with CKD remain equivocal. Calcium plays an essential role in numerous physiological functions in the body and is a fundamental structural component of bone. An understanding of calcium metabolism is required to understand the potential adverse clinical implications and outcomes secondary to the (mal)adaptation of calcium-regulating hormones in CKD. The first part of this two-part review covers the physiology of calcium homeostasis (kidneys, intestines and bones) and details the intimate relationships between calcium-regulating hormones (parathyroid hormone, calcitriol, fibroblast growth factor 23, α-Klotho and calcitonin) and the role of the calcium-sensing receptor.

Advances in the occurrence and biotherapy of osteoporosis

Osteoporosis (OP) is a bone metabolic disease, is characterized by degeneration of bone structure and decreased bone mass. It happens in more than 1/3 women and 1/5 men of over than 50 years old, which affects the health and lives of people. The main mechanism of OP is mainly that the dynamic balance between the bone formation and resorption is broken, so that bone resorption is more than bone formation. It is prone to result in bone metabolism disorder. There are many precipitating factor such as elder age, low hormone level, genetic factors and bad hobbies. At the same time, the occurrence of the OP and its complications has different degrees of impact on people's quality of life. Based on the current understanding of the OP, we summarized the etiology, current clinical drugs and potential targeting therapy for OP. Although the research have made many progress in explore what is the novel mechanism and how to improve the effect, there are still many problems in the treatment method that limit its application prospects and need to be solved. In this review, we mainly focus on the mechanism of OP and related research on the targeted treatment of OP. Hopefully, our summary will provide a reference to develop some novel strategies for the target therapy of OP.

Fibroblast growth factor 21 (FGF21) is a sensitive marker of osteoporosis in haemodialysis patients: a cross-sectional observational study

Introduction

Osteoporosis is one of the important bone abnormalities in chronic kidney disease-mineral and bone disorder (CKD-MBD) and still lacks a sensitive biomarker to diagnose. Fibroblast growth factor 21 (FGF21) can stimulate bone loss in patients with diabetes and increase in CKD patients. In this study, we investigated whether FGF21 could serve as a biomarker to predict osteoporosis in a haemodialysis cohort.

Methods

We recorded demographic information, biochemical data, and serum FGF21 and FGF23 levels and measured the CT attenuation values of 339 haemodialysis patients from two large medical centres. We assessed the correlation of CT attenuation values with serum FGF21 and FGF23 levels and tested whether they were independent factors for osteoporosis. ROC curves were constructed to compare the prognostic value of FGF21 and FGF23 for osteoporosis.

Results

Based on the CT attenuation value, serum FGF21 levels were higher in our osteoporosis group (median 640.86 pg/ml vs. 245.46 pg/ml, P ˂ 0.01). Meanwhile, FGF21 (r = -0.136, P < 0.05) and FGF23 (r = -0.151, P < 0.05) were both negatively associated with osteoporosis. Moreover, FGF21 (β = -0.067, P < 0.05) was an independent factor for osteoporosis. Furthermore, FGF21 combined with age yielded a marked specificity (90.5 %) and sensitivity (61.8 %) in predicting osteoporosis of haemodialysis patients with less residual renal function.

Conclusions

FGF21 has a positive relationship with the incidence of osteoporosis in patients on haemodialysis. FGF21 combined with age is a good predictive biomarker for osteoporosis in patients on haemodialysis, especially those with less residual renal function.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-021-02393-z.

Giantin is required for intracellular N-terminal processing of type I procollagen

Knockout of the golgin giantin leads to skeletal and craniofacial defects driven by poorly studied changes in glycosylation and extracellular matrix deposition. Here, we sought to determine how giantin impacts the production of healthy bone tissue by focusing on the main protein component of the osteoid, type I collagen. Giantin mutant zebrafish accumulate multiple spontaneous fractures in their caudal fin, suggesting their bones may be more brittle. Inducing new experimental fractures revealed defects in the mineralization of newly deposited collagen as well as diminished procollagen reporter expression in mutant fish. Analysis of a human giantin knockout cell line expressing a GFP-tagged procollagen showed that procollagen trafficking is independent of giantin. However, our data show that intracellular N-propeptide processing of pro-α1(I) is defective in the absence of giantin. These data demonstrate a conserved role for giantin in collagen biosynthesis and extracellular matrix assembly. Our work also provides evidence of a giantin-dependent pathway for intracellular procollagen processing.

The downstream RAF-1 signaling of fibroblast growth factor-23 participates in the osteogenetic effect caused by C-type natriuretic peptide in vitro

PURPOSE

Several studies have demonstrated that C-type natriuretic peptide (CNP) stimulates osteoblastic proliferation seemly via antagonizing the expression of fibroblast growth factor (FGF)-23 in vitro. The main aim of the present study is to probe whether the post-receptor pathways of FGF-23 participate in osteogenesis caused by CNP.

METHODS

Osteoblasts were cultured in the absence or presence of CNP: 0, 10, and 100 ​pmol/L, for 24 ​h, 48 ​h and 72 ​h, respectively.

RESULTS

The findings of the present study indicated that osteoblastic proliferation was directly promoted by exogenous CNP in a dose-dependent manner; osteoblastic FGF-23 was significantly down-regulated by CNP at 24 ​h post-treatment; RAF-1, extracellular signal-regulated kinases (ERK), and P38 were substantially suppressed by CNP in a dose- and time-dependent manner; and signal transducer and activator of transcription (STAT)-1 was not changed on the premise of the down-regulated FGF-23 in osteoblasts treated with CNP.

CONCLUSION

CNP may promote osteogenesis via inhibiting ERK and P38, rather than STAT-1, in the downstream of FGF-23/RAF-1 pathway.

Pulp chamber features, prevalence of abscesses, disease severity, and PHEX mutation in X-linked hypophosphatemic rickets

INTRODUCTION

Rickets, growth failure, and recurrent periapical abscesses with fistulae are main signs in patients with X-linked hypophosphatemic rickets (XLH). Prevalence of abscesses, pulp chamber features, biochemical findings, disease severity, and PHEX gene mutation were examined.

MATERIALS AND METHODS

Pulp chambers size, shape, and morphology were assessed by orthopantomography in XLH patients (n = 24, age 5.8 ± 1.6 years) and in sex and age-matched healthy controls (n = 23, age 6.2 ± 1.4 years). XLH patients received conventional treatment (3.5 ± 1.9 years). Pulp chamber features were assessed in teeth of primary dentition and in the permanent left mandibular first molar and compared with those of controls. Rickets severity score was assessed at wrist, knee, and ankle.

RESULTS

The mean pulp chamber area/tooth area ratio, mean pulp chamber height/pulp chamber width ratio, and prominence of pulp horns into the tooth crown in primary and secondary molars were significantly higher in patients than in controls and in patients suffered abscesses than in patients without abscesses. Sixteen patients (67%) had a history of abscesses; incisors were affected more than canines and molars. Severity of rickets and mean serum parathyroid hormone (PTH) levels were significantly higher, and mean serum 1,25-dihydroxyvitamin D [1,25(OH)₂D] levels significantly lower in patients suffered abscesses than in patients without abscesses. PHEX gene mutations were not correlated with dental phenotype and disease severity.

CONCLUSION

Enlarged pulp chambers with altered shape and morphology affected the majority of XLH patients predisposing to recurrent periapical abscesses with fistulae. Dental phenotype was associated with severity of rickets, high serum PTH, and low serum 1,25(OH)₂D levels.

Investigating the causal effect of fibroblast growth factor 23 on osteoporosis and cardiometabolic disorders: A Mendelian randomization study

Pathological excess of fibroblast growth factor 23 (FGF23) causes mineral and bone disorders. However, the causality of FGF23 in the development of osteoporosis remains unknown. Whether FGF23 has systemic effects on cardiometabolic disorders beyond regulating mineral metabolism is also controversial. In this study, we investigated the causal effect of FGF23 on osteoporosis and cardiometabolic disorders using Mendelian randomization (MR) analysis. Summary statistics for single-nucleotide polymorphisms with traits of interest were obtained from the relevant genome-wide association studies. As a result, FGF23 was found to be inversely associated with femoral neck-BMD (odds ratio [OR] 0.682, 95% confidence interval [CI] 0.546-0.853, p = 8e-04) and heel estimated BMD (eBMD) (OR 0.898, 95%CI 0.820-0.985, p = 0.022) in the inverse-variance-weighted analysis, but not lumbar spine-BMD and fractures. The results were supported by the weighted-median analysis, and there was no evidence of pleiotropy in the MR-Egger analysis. FGF23 was associated with FN-BMD and eBMD after adjustment for estimated glomerular filtration rate, height, and body mass index in multivariable MR analysis. On the other hand, there was no association between FGF23 and cardiometabolic traits including cardio artery disease, brachial-ankle pulse wave velocity, intima-media thickness of carotid arteries, systolic and diastolic blood pressure, fasting glucose, high and low-density lipoprotein cholesterol, and triglycerides. Therefore, this MR study established that FGF23 was involved in bone loss and, in contrast, was not involved in cardiometabolic disorders. Our findings provide important insights into the role of FGF23 in the pathogenesis of osteoporosis and cardiometabolic disorders.

Aberrant Bone Homeostasis in AML Is Associated with Activated Oncogenic FLT3-Dependent Cytokine Networks

Acute myeloid leukaemia (AML) is a haematopoietic malignancy caused by a combination of genetic and epigenetic lesions. Activation of the oncoprotein FLT3 ITD (Fms-like tyrosine kinase with internal tandem duplications) represents a key driver mutation in 25–30% of AML patients. FLT3 is a class III receptor tyrosine kinase, which plays a role in cell survival, proliferation, and differentiation of haematopoietic progenitors of lymphoid and myeloid lineages. Mutant FLT3 ITD results in an altered signalling quality, which causes cell transformation. Recent evidence indicates an effect of FLT3 ITD on bone homeostasis in addition to haematological aberrations. Using gene expression data repositories of FLT3 ITD-positive AML patients, we identified activated cytokine networks that affect the formation of the haematopoietic niche by controlling osteoclastogenesis and osteoblast functions. In addition, aberrant oncogenic FLT3 signalling of osteogenesis-specific cytokines affects survival of AML patients and may be used for prognosis. Thus, these data highlight the intimate crosstalk between leukaemic and osteogenic cells within the osteohaematopoietic niche.

Active sites of human MEPE-ASARM regulating bone matrix mineralization

The proteolytic fragment ASARM (acidic serine- and aspartate-rich motif) of MEPE (matrix extracellular phosphoglycoprotein) (MEPE-ASARM) may act as an endogenous anti-mineralization factor involved in X-linked hypophosphatemic rickets/osteomalacia (XLH). We synthesized MEPE-ASARM peptides and relevant peptide fragments with or without phosphorylated Ser residues (pSer) to determine the active site(s) of MEPE-ASARM in a rat calvaria cell culture model. None of the synthetic peptides elicited changes in cell death, proliferation or differentiation, but the peptide (pASARM) with three pSer residues inhibited mineralization without causing changes in gene expression of osteoblast markers tested. The anti-mineralization effect was maintained in peptides in which any one of three pSer residues was deleted. Polyclonal antibodies recognizing pASARM but not ASARM abolished the pASARM effect. Deletion of six N-terminal residues but leaving the recognition sites for PHEX (phosphate regulating endopeptidase homolog, X-linked), a membrane endopeptidase responsible for XLH, intact and two C-terminal amino acid residues did not alter the anti-mineralization activity of pASARM. Our results strengthen understanding of the active sites of MEPE-pASARM and allowed us to identify a shorter more stable sequence with fewer pSer residues still exhibiting hypomineralization activity, reducing peptide synthesis cost and increasing reliability for exploring biological and potential therapeutic effects.

Comparative Serum Analyses Identify Cytokines and Hormones Commonly Dysregulated as Well as Implicated in Promoting Osteolysis in MMP-2-Deficient Mice and Children

Deficiency of matrix metalloproteinase 2 (MMP-2) causes a complex syndrome characterized by multicentric osteolysis, nodulosis, and arthropathy (MONA) as well as cardiac valve defects, dwarfism and hirsutism. MMP-2 deficient (Mmp2 ^-/-) mice are a model for this rare multisystem pediatric syndrome but their phenotype remains incompletely characterized. Here, we extend the phenotypic characterization of MMP-2 deficiency by comparing the levels of cytokines and chemokines, soluble cytokine receptors, angiogenesis factors, bone development factors, apolipoproteins and hormones in mice and humans. Initial screening was performed on an 8-year-old male presenting a previously unreported deletion mutation c1294delC (Arg432fs) in the MMP2 gene and diagnosed with MONA. Of eighty-one serum biomolecules analyzed, eleven were upregulated (>4-fold), two were downregulated (>4-fold) and sixty-eight remained unchanged, compared to unaffected controls. Specifically, Eotaxin, GM-CSF, M-CSF, GRO-α, MDC, IL-1β, IL-7, IL-12p40, MIP-1α, MIP-1β, and MIG were upregulated and epidermal growth factor (EGF) and ACTH were downregulated in this patient. Subsequent analysis of five additional MMP-2 deficient patients confirmed the upregulation in Eotaxin, IL-7, IL-12p40, and MIP-1α, and the downregulation in EGF. To establish whether these alterations are bona fide phenotypic traits of MMP-2 deficiency, we further studied Mmp2 ^-/- mice. Among 32 cytokines measured in plasma of Mmp2 ^-/- mice, the cytokines Eotaxin, IL-1β, MIP-1α, and MIG were commonly upregulated in mice as well as patients with MMP-2 deficiency. Moreover, bioactive cortisol (a factor that exacerbates osteoporosis) was also elevated in MMP-2 deficient mice and patients. Among the factors we have identified to be dysregulated in MMP-2 deficiency many are osteoclastogenic and could potentially contribute to bone disorder in MONA. These new molecular phenotypic traits merit being targeted in future research aimed at understanding the pathological mechanisms elicited by MMP-2 deficiency in children.

Increased serum levels of fibroblast growth factor 23 after an ultradistance run

OBJECTIVES

Healthy bones need to be loaded on a regular basis. However, overstrenuous exercise causes uncoupling of bone metabolism. Thus, it is important to be aware of exercise-induced alterations in bone metabolism. The aim of this observational study was to determine whether participation in an ultradistance run has an impact on the phosphaturic hormone fibroblast growth factor 23 (FGF23), which is produced by osteocytes and suppresses osteoblast differentiation as well as matix mineralization.

DESIGN

Observational study.

METHODS

Nine participants of the Spartathlon (246km) had venous blood samples taken before and within 15min after finishing the race as well as during recovery. Serum levels of FGF23, phosphate, and blood urea nitrogen were determined.

RESULTS

FGF23 increased 6.5-fold from pre-race to post-race (2.2pmol/L [IQR: 0.4; 3.2pmol/L] to 14.4pmol/L [IQR: 4.7; 20.0pmol/L]; p=0.001). Thereafter, serum levels of FGF23 fell to 1.4pmol/L [IQR: 0.5; 1.7pmol/L] (p<0.0001). The differences in FGF23 levels between pre-race and recovery (3 days after the start) did not achieve statistical significance (p=0.614). Serum levels of phosphate and blood urea nitrogen also did not change significantly.

CONCLUSIONS

Since FGF23 plays a central role in mineral homeostasis, the transient overexpression of FGF23 may be an important contributor to the short-term uncoupling of bone metabolism induced by overstrenuous exercise.

Higher Fibroblast Growth Factor 23 Levels Are Causally Associated With Lower Bone Mineral Density of Heel and Femoral Neck: Evidence From Two-Sample Mendelian Randomization Analysis

Background: Previous observational studies have indicated that high levels of fibroblast growth factor 23 (FGF23), a phosphoric hormone that inhibits calcitriol synthesis, in the blood is associated with the reduced bone mineral density (BMD); however, whether this association is causal remains unclear. In this study, we conducted a Mendelian Random (MR) study to investigate whether the genetic predisposition of higher FGF23 levels was causally associated with lower BMD in adults.

Methods: A two-sample MR was performed with five single nucleotide polymorphisms significantly associated with FGF23, selected as instrumental variables. Two-sample MR estimates were derived from summary-level data of large-sample genome-wide association studies for BMD and the levels of bone metabolism characteristics.

Results: The two-sample MR analysis showed that for every 1-unit increase in the log-transformed blood FGF23 level (pg/mL), the decreased levels of adult heel BMD (β = −0.201, se = 0.084, P = 0.016) and femoral neck BMD (β = −0.286, se = 0.126, P = 0.022) were noted, indicative of a causal relationship based on the inverse variance weighting method. However, FGF23 levels were not correlated with adult lumbar spine BMD (β = −0.166, se = 0.193, P = 0.389), and forearm BMD (β = −0.186, se = 0.366, P = 0.610). Moreover, the two-sample MR analysis suggested that there was no evidence for associations between FGF23 and adult calcium, phosphorus, 25(OH)D, creatinine, and magnesium levels.

Conclusions: This study suggests that there may be a causal relationship between blood FGF23 levels and BMD of the heel and femoral neck in adults; however, more investigations are necessary to determine whether FGF23 may be a potential biomarker and/or therapeutic target for diseases that affect bone mineralization.

Skeletal manifestations in end-stage renal disease patients and relation to FGF23 and Klotho

Chronic kidney disease affects patients of all ages and, as it progresses, it greatly affects their lives, especially with the complications it causes. One major complication is renal osteodystrophy (ROD) which starts to develop from the early stages of the disease, but becomes most apparent in patients in need of renal replacement therapy. Diagnosing ROD in the early stages remains a challenge, which brings up the need to find novel biomarkers. Studies are focusing on the role of fibroblast growth factor 23 and Klotho in the bone and mineral homeostasis, but the results are conflicting. ROD remains a major complication in CKD patients, therefore we need to gain a better understanding from the pathophysiological point of view, in order to be able to adjust the medical therapy.

Increased FGF-23 levels are linked to ineffective erythropoiesis and impaired bone mineralization in myelodysplastic syndromes

Myelodysplastic syndromes (MDS) are clonal malignant hematopoietic disorders in the elderly characterized by ineffective hematopoiesis. This is accompanied by an altered bone microenvironment, which contributes to MDS progression and higher bone fragility. The underlying mechanisms remain largely unexplored. Here, we show that myelodysplastic NUP98‑HOXD13 (NHD13) transgenic mice display an abnormally high number of osteoblasts, yet a higher fraction of nonmineralized bone, indicating delayed bone mineralization. This was accompanied by high fibroblast growth factor-23 (FGF-23) serum levels, a phosphaturic hormone that inhibits bone mineralization and erythropoiesis. While Fgf23 mRNA expression was low in bone, brain, and kidney of NHD13 mice, its expression was increased in erythroid precursors. Coculturing these precursors with WT osteoblasts induced osteoblast marker gene expression, which was inhibited by blocking FGF-23. Finally, antibody-based neutralization of FGF-23 in myelodysplastic NHD13 mice improved bone mineralization and bone microarchitecture, and it ameliorated anemia. Importantly, higher serum levels of FGF‑23 and an elevated amount of nonmineralized bone in patients with MDS validated the findings. C‑terminal FGF‑23 correlated negatively with hemoglobin levels and positively with the amount of nonmineralized bone. Thus, our study identifies FGF-23 as a link between altered bone structure and ineffective erythropoiesis in MDS with the prospects of a targeted therapeutic intervention.

Trabecular bone score may indicate chronic kidney disease-mineral and bone disorder (CKD-MBD) phenotypes in hemodialysis patients: a prospective observational study

BACKGROUND

In the general population, the trabecular bone score (TBS) represents the bone microarchitecture and predicts fracture risk independent of bone mineral density (BMD). A few studies reported that TBS is significantly reduced in dialysis patients. Chronic kidney disease-mineral and bone disorder (CKD-MBD) are accompanied by increased fracture risk, cardiovascular morbidity, and mortality. We investigated whether TBS is associated with comorbidity related to CKD-MBD or frailty in hemodialysis patients.

METHODS

In this prospective observational study, TBS was obtained using the TBS iNsight software program (Med-Imaps) with BMD dual energy x-ray absorptiometry (DXA) images (L1-L4) from prevalent hemodialysis patients. A Tilburg frailty indicator was used to evaluate frailty, and hand grip strength and bio-impedance (InBody) were measured. A patient-generated subjective global assessment (PG-SGA) was used for nutritional assessment. The history of cardiovascular events (CVE) and demographic, clinical, laboratory, and biomarker data were collated. We then followed up patients for the occurrence of CKD-MBD related complications.

RESULTS

We enrolled 57 patients in total. The mean age was 56.8 ± 15.9 years (50.9% female). Prevalence of Diabetes mellitus (DM) was 40.4% and CVE was 36.8%. Mean TBS was 1.44 ± 0.10. TBS significantly reduced in the CVE group (1.38 ± 0.08 vs. 1.48 ± 0.10, p <  0.001). Multivariable regression analysis was conducted adjusting for age, sex, dialysis vintage, DM, CVE, albumin, intact parathyroid hormone, fibroblast growth factor 23, handgrip strength, and phosphate binder dose. Age (ß = - 0.030; p = 0.001) and CVE (ß = - 0.055; p = 0.024) were significant predictors of TBS. During the follow up period after TBS measurements (about 20 months), four deaths, seven incident fractures, and six new onset CVE were recorded. Lower TBS was associated with mortality (p = 0.049) or new onset fracture (p = 0.007, by log-rank test).

CONCLUSION

Lower TBS was independently associated with increased age and CVE prevalence in hemodialysis patients. Mortality and fracture incidence were significantly higher in patients with lower TBS values. These findings suggest that TBS may indicate a phenotype of frailty and also a CKD-MBD phenotype reciprocal to CVE.

"Take My Bone Away?" Hypoxia and bone: A narrative review

To maintain normal cellular and physiological function, sufficient oxygen is required. Recently, evidence has suggested that hypoxia, either pathological or environmental, may influence bone health. It appears that bone cells are distinctly responsive to hypoxic stimuli; for better or worse, this is still yet to be elucidated. Hypoxia has been shown to offer potentially therapeutic effects for bone by inducing an osteogenic-angiogenic response, although, others have noted excessive osteoclastic bone resorption instead. Much evidence suggests that the hypoxic-inducible pathway is integral in mediating the changes in bone metabolism. Furthermore, many factors associated with hypoxia including changes in energy metabolism, acid-base balance and the increased generation of reactive oxygen species, are known to influence bone metabolism. This review aims to examine some of the putative mechanisms responsible for hypoxic-induced alterations of bone metabolism, with regard to osteoclasts and osteoblasts, both positive and negative.

C-type natriuretic peptide stimulates osteoblastic proliferation and collagen-X expression but suppresses fibroblast growth factor-23 expression in vitro

BACKGROUND

The effects of C-type natriuretic peptide (CNP) and fibroblast growth factor (FGF)-23 appear to oppose each other during the process of bone formation, whereas few studies exist on the interaction between CNP and FGF-23. The main objective of the present study is to probe whether CNP is directly responsible for the regulation of osteoblast or via antagonizing FGF-23.

METHODS

Osteoblasts were cultured in the absence or presence of CNP (0, 10, and 100 pmol/L) for 24 h, 48 h and 72 h, respectively.

RESULTS

The findings of the present study indicated that: (1) CNP significantly stimulated osteoblastic proliferation and collagen (Col)-X expression; (2) both osteoblastic (osteocalcin, procollagen type I carboxy-terminal propeptide, total alkaline phosphatase and bone-specific alkaline phosphatase) and osteolytic (tartrate-resistant acid phosphatase and cross-linked carboxyterminal telopeptide of type I collagen) bone turnover biomarkers were up-regulated by CNP in osteoblasts; (3) FGF-23 mRNA and protein were significantly down-regulated at 24 h by CNP in osteoblasts, but the expression of FGF receptor-1/Klotho had no significant change.

CONCLUSIONS

CNP stimulates osteoblastic proliferation and Col-X expression via the down-regulation of FGF-23 possibly in vitro. However, the specific mechanisms of the interaction between CNP and FGF-23 in osteoblasts are still unclear according to our findings. A further study on osteoblasts cultured with CNP and FGF-23 inhibitor will be undertaken in our laboratory.

The relationship between serum fibroblast growth factor 23, Klotho, and lumbar spine bone mineral density in northern Chinese postmenopausal women

OBJECTIVES

Changes in serum protein levels of fibroblast growth factor 23 (FGF23) and Klotho resulting from bone metabolism are still controversial. The purpose of this study was to observe the relationship between FGF23 and Klotho serum proteins and lumbar spine bone mineral density (LBMD) in northern Chinese postmenopausal women.

METHODS

This was a community-based cross-sectional study carried out in Shenyang, a northern Chinese city. The study included 355 postmenopausal women with an average age of 62.92 ± 8.78 years. FGF23 and Klotho serum proteins were measured using a sandwich enzyme immunoassay. LBMD was examined using dual-energy X-ray absorptiometry. Pearson's correlation and regression analyses were performed to investigate the associations among them.

RESULTS

The LgKlotho was positively correlated with LBMD (r = 0.105). There was a linear relationship between LgKlotho serum levels and LBMD (P = 0.007) after adjusting for BMI, and the relationship still existed after adjustments for many confounding variables (P = 0.045), including age, BMI, systolic blood pressure, diastolic blood pressure, total protein, total bilirubin, high-density lipoprotein cholesterol, fasting blood glucose, serum calcium, estimated glomerular filtration rate, serum uric acid, estradiol, cigarette smoking, alcohol consumption, milk intake, calcium and vitamin D supplements, physical exercise, and fracture history in postmenopausal women. FGF23 serum levels were, however, not significantly associated with LBMD.

CONCLUSIONS

Klotho was positively correlated with LBMD, and there was a linear relationship between Klotho serum protein levels and LBMD; however, the levels of serum Klotho were not independently associated with reduced LBMD in northern Chinese postmenopausal women. Moreover, serum FGF23 levels were not significantly related to LBMD in this sample population.

Fibroblast growth factor 23 counters vitamin D metabolism and action in human mesenchymal stem cells

Chronic kidney disease (CKD) is associated with elevated circulating fibroblast growth factor 23 (FGF23), impaired renal biosynthesis of 1α,25-dihydroxyvitamin D (1α,25(OH)₂D), low bone mass, and increased fracture risk. Our previous data with human mesenchymal stem cells (hMSCs) indicated that vitamin D metabolism in hMSCs is regulated as it is in the kidney and promotes osteoblastogenesis in an autocrine/paracrine manner. In this study, we tested the hypothesis that FGF23 inhibits vitamin D metabolism and action in hMSCs. hMSCs were isolated from discarded marrow during hip arthroplasty, including two subjects receiving hemodialysis and a series of 20 subjects (aged 49-83 years) with estimated glomerular filtration rate (eGFR) data. The direct in vitro effects of rhFGF23 on hMSCs were analyzed by RT-PCR, Western immunoblot, and biochemical assays. Ex vivo analyses showed positive correlations for both secreted and membrane-bound αKlotho gene expression in hMSCs with eGFR of the subjects from whom hMSCs were isolated. There was downregulated constitutive expression of αKlotho, but not FGFR1 in hMSCs obtained from two hemodialysis subjects. In vitro, rhFGF23 countered vitamin D-stimulated osteoblast differentiation of hMSCs by reducing the vitamin D receptor, CYP27B1/1α-hydroxylase, biosynthesis of 1α,25(OH)₂D₃, and signaling through BMP-7. These data demonstrate that dysregulated vitamin D metabolism in hMSCs may contribute to impaired osteoblastogenesis and altered bone and mineral metabolism in CKD subjects due to elevated FGF23. This supports the importance of intracellular vitamin D metabolism in autocrine/paracrine regulation of osteoblast differentiation in hMSCs.

Iron Overload Associated Endocrine Dysfunction Leading to Lower Bone Mineral Density in Thalassemia Major

CONTEXT

Patients with thalassemia major (TM) have a lower bone mineral density (BMD) and higher risk of fracture than the general population. The possible mechanisms include anemia, iron overload, malnutrition, and hormonal deficiency, but these have not been thoroughly investigated.

OBJECTIVE

To identify major mineral and hormonal factors related to BMD in adult TM patients to provide human evidence for the proposed mechanisms.

DESIGN

Retrospective study.

SETTING

Referral center.

PATIENTS

Twenty-nine patients with β-TM, aged 23 to 44 years who were followed-up during 2017 to 2018 were enrolled.

OUTCOME MEASUREMENTS

Endocrine profiles, including thyroid, parathyroid, and pituitary function, glucose, vitamin D, calcium, phosphate, and fibroblast growth factor 23 (FGF23) were obtained. The relationships among the above parameters, body height, fractures, and BMD were analyzed.

RESULTS

Abnormal BMD was observed in 42.9% of women and 23.1% of men. The mean final heights of women and men were 3.7 cm and 7.3 cm lower than the mean expected values, respectively. Fracture history was recorded in 26.7% of women and 35.7% of men. BMD was negatively correlated with parathyroid hormone, FGF23, thyrotropin, and glycated hemoglobin (HbA1c) levels, and positively correlated with testosterone, IGF-1, and corticotropin levels (all P < .05). Moreover, hypothyroidism was associated with lower BMD in both the lumbar spine (P = .024) and the femoral neck (P = .004). Patients with hypothyroidism had a higher percentage of abnormal BMD (P = .016).

CONCLUSION

Hypothyroidism, higher HbA1c, and lower adrenocorticotropin were predictors of abnormal BMD in patients with β-TM. Whether the correction of these factors improves BMD warrants further research.

Sclerostin Antibody Treatment Increases Bone Mass and Normalizes Circulating Phosphate Levels in Growing Hyp Mice

X-linked hypophosphatemia (XLH), caused by a loss-of-function mutation in the phosphate regulating gene with homology to endopeptidase located on the X chromosome (PHEX), is the most common form of vitamin D-resistant rickets. Loss of functional PHEX results in elevated fibroblast growth factor 23 (FGF23) levels, impaired phosphate reabsorption, and inhibited skeletal mineralization. Sclerostin, a protein produced primarily in osteocytes, suppresses bone formation by antagonizing Wnt signaling and is reported to be elevated in XLH patients. This study used the Hyp mouse model to investigate sclerostin's role in the pathophysiology of XLH by evaluating the use of a monoclonal antibody to sclerostin in a mouse model of XLH, the Hyp mouse. Male and female wild-type and Hyp littermates were injected with 25 mg/kg of vehicle or sclerostin antibody (Scl-Ab) twice weekly, beginning at 4 weeks of age and euthanized at 8 weeks of age. Scl-Ab treatment increased serum phosphate levels and suppressed circulating levels of intact FGF23 in treated wild-type and Hyp mice of both sexes. Cortical area, trabecular bone volume fraction (BV/TV), metaphyseal apparent density, and the peak load increased with Scl-Ab treatment in both sexes. This short-term treatment study suggests that Scl-Ab treatment can effectively improve some of the pathologies associated with XLH, including normalization of phosphate, and that sclerostin may play a role in regulating FGF23 and phosphate metabolism in XLH. © 2019 American Society for Bone and Mineral Research.

High-Phosphate Diet Improved the Skeletal Development of Fam20c-Deficient Mice

FAM20C (family with sequence similarity 20 - member C) is a protein kinase that phosphorylates secretory proteins, including the proteins that are essential to the formation and mineralization of calcified tissues. Previously, we reported that inactivation of Fam20c in mice led to hypophosphatemic rickets/osteomalacia along with increased circulating fibroblast growth factor 23 (FGF23) levels and dental defects. In this study, we examined whether a high-phosphate (hPi) diet could rescue the skeletal defects in Fam20c-deficient mice. Fam20c conditional knockout (cKO) mice were generated by crossing female Fam20c-floxed mice (Fam20cfl/fl) with male Sox2-Cre;Fam20cfl/+ mice. The pregnant female Fam20cfi/fl mice were fed either a normal or hPi diet until the litters were weaned. The cKO and control offspring were continuously given a normal or hPi diet for 4 weeks after weaning. Plain X-ray radiography, micro-CT, histology, immunohistochemistry (FGF23, DMP1, OPN, and SOX9), and in situ hybridization (type II and type X collagen) analyses were performed to evaluate the effects of an hPi diet on the mouse skeleton. Plain X-ray radiography and micro-CT radiography analyses showed that the hPi diet improved the shape and mineral density of the Fam20c-deficient femurs/tibiae, and rescued the growth plate defects in the long bone. Histology analyses further demonstrated that an hPi diet nearly completely rescued the growth plate-widening defects in the long bone and restored the expanded hypertrophic zone to nearly normal width. These results suggested that the hPi diet significantly improved the skeletal development of the Fam20c-deficient mice, implying that hypophosphatemia partially contributed to the skeletal defects in Fam20c-deficient subjects.

Pro-inflammatory Cytokines: Cellular and Molecular Drug Targets for Glucocorticoid-induced-osteoporosis via Osteocyte

Glucocorticoids are widely used to treat varieties of allergic and autoimmune diseases, however, long-term application results in glucocorticoid-induced osteoporosis (GIOP). Inflammatory cytokines: tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) play important regulatory roles in bone metabolism, but their roles in GIOP remain largely unknown. Osteocytes can modulate the formation and function of both osteoblasts and osteoclasts, directly via gap junctions, or indirectly by transferring molecule signaling. Apoptotic osteocytes release RANKL, HMGB1 and pro-inflammatory cytokines to stimulate osteoclastogenesis. Moreover, osteocytes can secrete FGF23 to regulate bone metabolism. Exposure to high levels of GCs can drive osteocyte apoptosis and influence gap junctions, leading to bone loss. GCs treatment is regarded to produce more FGF23 to inhibit bone mineralization. GCs also disrupt the vascular to decrease osteocyte feasibility and mineral appositional rate, resulting in a decline in bone strength. Apoptotic bodies from osteocytes induced by GCs treatment can enhance production of TNF-α and IL-6. On the other hand, TNF-α and IL-6 show synergistic effects by altering osteocytes signaling towards osteoclasts and osteoblasts. In addition, TNF-α can induce osteocyte apoptosis and attribute to a worsened bone quality in GCs. IL-6 and osteocytes may interact with each other. Therefore, we hypothesize that GCs regulate osteocyteogenesis through TNF-α and IL-6, which are highly expressed around osteocyte undergoing apoptosis. In the present review, we summarized the roles of osteocytes in regulating osteoblasts and osteoclasts. Furthermore, the mechanism of GCs altered relationship between osteocytes and osteoblasts/osteoclasts. In addition, we discussed the roles of TNF-α and IL-6 in GIOP by modulating osteocytes. Lastly, we discussed the possibility of using pro-inflammatory signaling pathway as therapeutic targets to develop drugs for GIOP.

Physiology of FGF23 and overview of genetic diseases associated with renal phosphate wasting

Phosphate is a cornerstone of several physiological pathways including skeletal development, bone mineralization, membrane composition, nucleotide structure, maintenance of plasma pH, and cellular signaling. The kidneys have a key role in phosphate homeostasis with three hormones having important functions in renal phosphate handling or intestinal absorption: parathyroid hormone (PTH), fibroblast growth factor 23 (FGF23), and 1-25-dihydroxyvitamin D (1,25(OH)2D). FGF23 is mainly synthesized by osteocytes; it is a direct phosphaturic factor that also inhibits 1,25(OH)2D and PTH. In addition to crucial effects on phosphate and calcium metabolism, FGF23 also has 'off-target' effects notably on the cardiovascular, immune and central nervous systems. Genetic diseases may affect the FGF23 pathway, resulting in either increased FGF23 levels leading to hypophosphatemia (such as in X-linked hypophosphatemia) or defective secretion/action of intact FGF23 inducing hyperphosphatemia (such as in familial tumoral calcinosis). The aim of this review is to provide an overview of FGF23 physiology and pathophysiology in X-linked hypophosphatemia, with a focus on FGF23-associated genetic diseases.

Recurrent severe hypophosphatemia following intravenous iron administration

Hypophosphatemia postintravenous iron is frequent but under-recognized. If prolonged or recurrent, it can cause osteomalacia. The likely mechanisms are direct toxicity to proximal tubular cells causing phosphate wasting, elevated Fibroblast growth factor-23 (FGF-23), and reduced 1,25-dihydroxyvitamin D (1,25(OH)2D). Hypophosphatemia may be severe and persist for months, necessitating phosphate replacement until normalization of serum levels occurs.

FGF23-klotho axis as predictive factors of fractures in type 2 diabetics with early chronic kidney disease

BACKGROUND

The aim of our study was to evaluate the relevance of FGF23-klotho axis in the predisposition for bone fractures in type 2 diabetic patients with early chronic kidney disease.

METHODS

In a prospective study we included 126 type 2 diabetic patients with CKD stages 2-3 (from 2010 to 2017). We used descriptive statistics, ANOVA and chi-square test. Our population was divided into two groups according to the occurrence of a bone fracture event or not, and the groups were compared considering several biological and laboratorial parameters. We employed a multiple regression model to identify risk factors for bone fracture events and hazard ratios (HR) were calculated using a backward stepwise likelihood ratio (LR) Cox regression.

RESULTS

Patients with a fracture event displayed higher levels of FGF-23, Phosphorus, PTH, TNF-α, OxLDL, HOMA-IR, calcium × phosphorus product and ACR and lower levels of Osteocalcin, α-Klotho, 25(OH)D3 and eGFR compared with patients without a fracture event (p < 0.001). The number of patients with a fracture event was higher than expected within inclining CKD stages (χ2, p = 0.06). The occurrence of fracture and the levels of TNF- α, klotho, 25(OH)D3 and OxLDL were found to predict patient entry into RRT (p < 0.05). Age, osteocalcin, α-Klotho and FGF-23 independently influenced the occurrence of bone fracture (p < 0.05).

CONCLUSIONS

α-Klotho and FGF-23 levels may have a good clinical use as biomarkers to predict the occurrence of fracture events.

Mineral Bone Abnormalities and Vascular Calcifications

Vascular calcification (VC) is common in chronic kidney disease, increases in prevalence as patients progress to end-stage renal disease, and is significantly associated with mortality. VC is a complex and highly regulated process similar to bone formation whereby hydroxyapatite crystals deposit in the intimal or medial layer of arteries. Mineral bone abnormalities are common in chronic kidney disease; reduction in glomerular filtration rate and changes in vitamin D, parathyroid hormone, and fibroblast growth factor 23 result in the dysregulation of phosphorus and calcium metabolism. Cell culture studies, animal models, and observational and clinical studies all suggest this abnormal mineral metabolism plays a role in the initiation and progression of VC in kidney disease. This review will focus on these mineral bone abnormalities and how they may contribute to mechanisms that induce VC in kidney disease.