FGF23 suppresses chondrocyte proliferation in the presence of soluble α-Klotho both in vitro and in vivo

FGF23 facilitates IL-1β synthesis in rheumatoid arthritis through activating PI3K, Akt, and NF-κB pathways

Rheumatoid arthritis (RA) is a well-known autoimmune disorder related with joint pain, joint swelling, cartilage and bone degradation as well as deformity. Fibroblast growth factor 23 (FGF23) is an endocrine factor of the FGF family primarily produced by osteocytes and osteoblasts, involves an essential effect in pathogenesis of RA. IL-1β is a vital proinflammatory factor in the development of RA. However, the role of FGF23 on IL-1β synthesis in RA has not been fully explored. Our analysis of database revealed higher levels of FGF23 and IL-1β in RA samples compared with healthy controls. High-throughput screening demonstrated that IL-1β is a potential candidate factor after FGF23 treatment in RA synovial fibroblasts (RASFs). FGF23 concentration dependently promotes IL-1β synthesis in RASFs. FGF23 enhances IL-1β expression by activating the PI3K, Akt, and NF-κB pathways. Our findings support the notion that FGF23 is a promising target in the remedy of RA.

Fibroblast growth factor 23, klotho and heparin

PURPOSE OF REVIEW

Fibroblast growth factor (FGF) 23 is a bone-derived hormone that regulates phosphate and vitamin D metabolism by targeting the kidney. When highly elevated, such as in chronic kidney disease (CKD), FGF23 can also target the heart and induce pathologic remodeling. Here we discuss the mechanisms that underlie the physiologic and pathologic actions of FGF23, with focus on its FGF receptors (FGFR) and co-receptors.

RECENT FINDINGS

Klotho is a transmembrane protein that acts as an FGFR co-receptor for FGF23 on physiologic target cells. Klotho also exists as a circulating variant, and recent studies suggested that soluble klotho (sKL) can mediate FGF23 effects in cells that do not express klotho. Furthermore, it has been assumed that the actions of FGF23 do not require heparan sulfate (HS), a proteoglycan that acts as a co-receptor for other FGF isoforms. However, recent studies revealed that HS can be part of the FGF23:FGFR signaling complex and modulate FGF23-induced effects.

SUMMARY

sKL and HS have appeared as circulating FGFR co-receptors that modulate the actions of FGF23. Experimental studies suggest that sKL protects from and HS accelerates CKD-associated heart injury. However, the in vivo relevance of these findings is still speculative.

Progress of Wnt Signaling Pathway in Osteoporosis

Osteoporosis, one of the serious health diseases, involves bone mass loss, bone density diminishing, and degeneration of bone microstructure, which is accompanied by a tendency toward bone fragility and a predisposition to fracture. More than 200 million people worldwide suffer from osteoporosis, and the cost of treating osteoporotic fractures is expected to reach at least $25 billion by 2025. The generation and development of osteoporosis are regulated by genetic factors and regulatory factors such as TGF-β, BMP, and FGF through multiple pathways, including the Wnt signaling pathway, the Notch signaling pathway, and the MAPK signaling pathway. Among them, the Wnt signaling pathway is one of the most important pathways. It is not only involved in bone development and metabolism but also in the differentiation and proliferation of chondrocytes, mesenchymal stem cells, osteoclasts, and osteoblasts. Dkk-1 and SOST are Wnt inhibitory proteins that can inhibit the activation of the canonical Wnt signaling pathway and block the proliferation and differentiation of osteoblasts. Therefore, they may serve as potential targets for the treatment of osteoporosis. In this review, we analyzed the mechanisms of Wnt proteins, β-catenin, and signaling molecules in the process of signal transduction and summarized the relationship between the Wnt signaling pathway and bone-related cells. We hope to attract attention to the role of the Wnt signaling pathway in osteoporosis and offer new perspectives and approaches to making a diagnosis and giving treatment for osteoporosis.

Growth hormone treatment improves final height in children with X-linked hypophosphatemia

BACKGROUND/AIM

Despite optimal conventional treatment (oral phosphate supplements and active vitamin D analogs), about 40-50% of children with well-controlled X-linked hypophosphatemia (XLH) show linear growth failure, making them less likely to achieve an acceptable final height. Here, we studied the hypothesis that rhGH treatment improves final height in children with XLH and growth failure.

METHODS

Two cohorts of children with XLH were included in this retrospective longitudinal analysis: (1) a cohort treated with rhGH for short stature (n = 34) and (2) a cohort not treated with rhGH (n = 29). The mean duration of rhGH treatment was 4.4 ± 2.9 years. We collected the auxological parameters at various time points during follow-up until final height.

RESULTS

In rhGH-treated children, 2 years of rhGH therapy was associated with a significant increase in height from - 2.4 ± 0.9 to - 1.5 ± 0.7 SDS (p < 0.001). Their mean height at rhGH discontinuation was - 1.2 ± 0.9 SDS and at final height was - 1.3 ± 0.9 SDS corresponding to 165.5 ± 6.4 cm in boys and 155.5 ± 6.3 cm in girls. Notably, the two groups had similar final heights; i.e., the final height in children not treated with rhGH being - 1.2 ± 1.1 SDS (165.4 ± 6.8 cm in boys and 153.7 ± 7.8 cm in girls), p = 0.7.

CONCLUSION

Treatment with rhGH permits to improve final height in children with XLH and growth failure, despite optimal conventional treatment. We propose therefore that rhGH therapy could be considered as an option for short stature in the context of XLH.

Meclozine ameliorates bone mineralization and growth plate structure in a mouse model of X‑linked hypophosphatemia

X-linked hypophosphatemic rickets (XLH) is characterized by hypo-mineralization of the bone due to hypophosphatemia. XLH is caused by abnormally high levels of fibroblast growth factor 23, which trigger renal phosphate wasting. Activated fibroblast growth factor receptor 3 (FGFR3) signaling is considered to be involved in XLH pathology. Our previous study revealed that meclozine attenuated FGFR3 signaling and promoted longitudinal bone growth in an achondroplasia mouse model. The present study aimed to examine whether meclozine affected the bone phenotype in a mouse model of XLH [X-linked hypophosphatemic (Hyp) mice]. Meclozine was administered orally to 7-day-old Hyp mice for 10 days, after which the mice were subjected to blood sampling and histological analyses of the first coccygeal vertebra, femur and tibia. Villanueva Goldner staining was used to assess bone mineralization, hematoxylin and eosin staining was used to determine the growth plate structure and tartrate-resistant acid phosphatase staining was used to measure osteoclast activity. The osteoid volume/bone volume of cortical bone was lower in meclozine-treated Hyp mice compared with untreated Hyp mice. Meclozine treatment improved the abnormally thick hypertrophic zone of the growth plate and ameliorated the downregulation of osteoclast surface/bone surface in Hyp mice. However, meclozine had only a marginal effect on mineralization in the trabecular bone and on calcium and phosphate plasma levels. A 10-day-tratment with meclozine partially ameliorated bone mineralization in Hyp mice; hence, meclozine could alleviate XLH symptoms.

Klotho Ameliorates Vascular Calcification via Promoting Autophagy

Vascular calcification (VC) is regarded as a common feature of vascular aging. Klotho deficiency reportedly contributes to VC, which can be ameliorated by restoration of Klotho expression. However, the specific mechanisms involved remain unclear. Here, we investigated the role of autophagy in the process of Klotho-inhibiting VC. The clinical study results indicated that, based on Agatston score, serum Klotho level was negatively associated with aortic calcification. Then, Klotho-deficient mice exhibited aortic VC, which could be alleviated with the supplementation of Klotho protein. Moreover, autophagy increased in the aorta of Klotho-deficient mice and protected against VC. Finally, we found that Klotho ameliorated calcification by promoting autophagy both in the aorta of Klotho-deficient mice and in mouse vascular smooth muscle cells (MOVAS) under calcifying conditions. These findings indicate that Klotho deficiency induces increased autophagy to protect against VC and that Klotho expression further enhances autophagy to ameliorate calcification. This study is beneficial to exploring the underlying mechanisms of Klotho regulating VC, which has important guiding significance for future clinical studies in the treatment of VC.

FGF23 and SOX9 expression in hemophilic cartilage: In vitro studies of the effects of iron

INTRODUCTION

Clarifying the links between iron and FGF23, SOX9 expression in chondrocytes would be helpful for comprehending articular cartilage degradation pathogenesis in blood-induced arthritis and exploring new protective methods.

AIM

The purpose of this study was to determine iron regulation of fibroblast growth factor 23 (FGF23) and SRY-box 9 (SOX9) in human chondrocytes, an area which is unexplored in blood-induced arthritis cartilage degradation pathogenesis.

METHODS

Expression of FGF23, SOX9, MMP13 and collagen Ⅱ in articular cartilage of patients with osteoarthritis (OA) or haemophilic arthritis (HA) was determined by western blot (WB). Iron induced FGF23 and SOX9 mRNA and protein expression in primary human normal chondrocyte cells (HUM-iCell-s018) was quantifified by qRT-PCR and WB, respectively.

RESULTS

We found that compared with OA patients, the expression of FGF23, MMP13 in articular cartilage of patients with HA was up-regulated, while the expression of SOX9, collagen Ⅱ was down-regulated. Iron induced FGF23 and suppressed SOX9 expression in chondrocytes in a dose-dependent manner.

CONCLUSIONS

These findings demonstrated that iron were involved in hemophilic cartilage lesion directly via changing cartilage phenotype through regulation of FGF23 and SOX9 expression in chondrocytes.

Pathogenesis of FGF23-Related Hypophosphatemic Diseases Including X-linked Hypophosphatemia

Since phosphate is indispensable for skeletal mineralization, chronic hypophosphatemia causes rickets and osteomalacia. Fibroblast growth factor 23 (FGF23), which is mainly produced by osteocytes in bone, functions as the central regulator of phosphate metabolism by increasing the renal excretion of phosphate and suppressing the production of 1,25-dihydroxyvitamin D. The excessive action of FGF23 results in hypophosphatemic diseases, which include a number of genetic disorders such as X-linked hypophosphatemic rickets (XLH) and tumor-induced osteomalacia (TIO). Phosphate-regulating gene homologous to endopeptidase on the X chromosome (PHEX), dentin matrix protein 1 (DMP1), ectonucleotide pyrophosphatase phosphodiesterase-1, and family with sequence similarity 20c, the inactivating variants of which are responsible for FGF23-related hereditary rickets/osteomalacia, are highly expressed in osteocytes, similar to FGF23, suggesting that they are local negative regulators of FGF23. Autosomal dominant hypophosphatemic rickets (ADHR) is caused by cleavage-resistant variants of FGF23, and iron deficiency increases serum levels of FGF23 and the manifestation of symptoms in ADHR. Enhanced FGF receptor (FGFR) signaling in osteocytes is suggested to be involved in the overproduction of FGF23 in XLH and autosomal recessive hypophosphatemic rickets type 1, which are caused by the inactivation of PHEX and DMP1, respectively. TIO is caused by the overproduction of FGF23 by phosphaturic tumors, which are often positive for FGFR. FGF23-related hypophosphatemia may also be associated with McCune-Albright syndrome, linear sebaceous nevus syndrome, and the intravenous administration of iron. This review summarizes current knowledge on the pathogenesis of FGF23-related hypophosphatemic diseases.

Description of a novel SLC34A3.c.671delT mutation causing hereditary hypophosphatemic rickets with hypercalciuria in two adolescent boys and response to recombinant human growth hormone

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is an autosomal recessive disorder characterized by hypophosphatemia, rickets, hyperphosphaturia, elevated 1,25(OH)₂D, and hypercalciuria. Mutations in SLC34A3, the gene encoding the sodium-dependent cotransporter NPT2c, have previously been described as a cause of HHRH. Here, we describe two male siblings with rickets and hypercalciuric nephrolithiasis born to unrelated parents, and their response to oral phosphate supplementation and growth hormone therapy. Whole exome sequencing of the oldest brother, and polymerase chain reaction and Sanger sequence analysis of the identified SLC34A3 mutations, was performed for confirmation and to evaluate his siblings and parents. Serum and urine biochemical parameters of mineral homeostasis before and after therapy were evaluated. Whole exome sequencing analysis identified a previously reported heterozygous deletion SLC34A3.g.2259-2359del101bp on the maternal allele, and a novel heterozygous single nucleotide deletion SLC34A3.c.671delT on the paternal allele of the two affected brothers. The parents and the unaffected brother are heterozygous carriers. Recombinant human growth hormone (rHGH) plus oral phosphate in one affected brother improved the renal phosphate leak and resulted in accelerated linear growth superior to that seen with oral phosphate supplementation alone in the other affected brother. Our case study is the first to demonstrate that rHGH can be considered in addition to oral supplementation with phosphorus to improve linear growth in patients with this disorder, and suggests that renal phosphate reabsorption in response to rHGH is NPT2c-independent.

A Mutation in VWA1, Encoding von Willebrand Factor A Domain-Containing Protein 1, Is Associated With Hemifacial Microsomia

Background

Hemifacial microsomia (HFM) is a type of rare congenital syndrome caused by developmental disorders of the first and second pharyngeal arches that occurs in one out of 5,600 live births. There are significant gaps in our knowledge of the pathogenic genes underlying this syndrome.

Methods

Whole exome sequencing (WES) was performed on five patients, one asymptomatic carrier, and two marry-in members of a five-generation pedigree. Structure of WARP (product of VWA1) was predicted using the Phyre2 web portal. In situ hybridization and vwa1-knockdown/knockout studies in zebrafish using morpholino and CRISPR/Cas9 techniques were performed. Cartilage staining and immunofluorescence were carried out.

Results

Through WES and a set of filtration, we identified a c.G905A:p.R302Q point mutation in a novel candidate pathogenic gene, VWA1. The Phyre2 web portal predicted alterations in secondary and tertiary structures of WARP, indicating changes in its function as well. Predictions of protein-to-protein interactions in five pathways related to craniofacial development revealed possible interactions with four proteins in the FGF pathway. Knockdown/knockout studies of the zebrafish revealed deformities of pharyngeal cartilage. A decrease of the proliferation of cranial neural crest cells (CNCCs) and alteration of the structure of pharyngeal chondrocytes were observed in the morphants as well.

Conclusion

Our data suggest that a mutation in VWA1 is functionally linked to HFM through suppression of CNCC proliferation and disruption of the organization of pharyngeal chondrocytes.

FGF/FGFR signaling in health and disease

Growing evidences suggest that the fibroblast growth factor/FGF receptor (FGF/FGFR) signaling has crucial roles in a multitude of processes during embryonic development and adult homeostasis by regulating cellular lineage commitment, differentiation, proliferation, and apoptosis of various types of cells. In this review, we provide a comprehensive overview of the current understanding of FGF signaling and its roles in organ development, injury repair, and the pathophysiology of spectrum of diseases, which is a consequence of FGF signaling dysregulation, including cancers and chronic kidney disease (CKD). In this context, the agonists and antagonists for FGF-FGFRs might have therapeutic benefits in multiple systems.

X-Linked Hypophosphataemic Rickets and Growth

X-linked hypophosphataemia (XLH) is the most prevalent form of hereditary rickets characterized by an alteration of phosphate metabolism which frequently leads to the appearance of fractures, bone deformities and growth delay. Although the mechanism of growth impairment in patients with XLH still needs to be clarified, it is known that this alteration is not due to genetic or endocrine factors. A potential explanation for the impairment of growth in this disease is the alteration of the growth plate, a structure responsible for longitudinal growth of bones. Some of the findings in the growth plate of patients with XLH include atypical organization of chondrocytes due to low rates of proliferation and apoptosis and disturbance of chondrocyte hypertrophy, overactivation of the mitogen-activated protein kinase (MAPK) signalling pathway and upregulation of phosphorylated extracellular signal-regulated kinase (pERK). Conventional treatment of XLH (consisting of oral phosphate supplements and active vitamin D analogues) is often insufficient for the longitudinal growth of bone, but other strategies based on recombinant growth hormone or therapies targeting fibroblast growth factor 23 (FGF23) or its receptor, such as burosumab, have shown promising results. This article briefly describes the relationship between XLH and growth retardation, and how to address this alteration in patients with XLH.

FGF23: Is It Another Biomarker for Phosphate-Calcium Metabolism?

Fibroblast growth factor 23 (FGF23) is a protein produced by mature osteoblasts involved in mineral homeostasis by binding to its receptor complex FGFR/Klotho located mainly in the kidneys. Although this protein participates in numerous biological processes, increase in the levels of FGF23 is responsible for many pathologies, such as X-linked hypophosphataemia (XLH), chronic kidney disease, cardiovascular disease or even mortality. For this reason, both FGF23 and its receptors have become elements of interest for the development of treatments. However, FGF23 can be altered for many other reasons, such as inflammatory processes, iron, hypoxia, heart failure or erythropoietin, that negatively affect mortality. This article will review the role of FGF23 in phosphate homeostasis, its relationship to mortality, fractures and chronic renal failure, and how the levels of this factor can be reduced.

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.

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.

CREB activation in hypertrophic chondrocytes is involved in the skeletal overgrowth in epiphyseal chondrodysplasia Miura type caused by activating mutations of natriuretic peptide receptor B

Natriuretic peptide receptor B (NPRB) produces cyclic guanosine monophosphate (cGMP) when bound by C-type natriuretic peptide (CNP). Activating mutations in NPRB cause a skeletal overgrowth disorder, which has been named epiphyseal chondrodysplasia, Miura type (ECDM; OMIM #615923). Here we explored the cellular and molecular mechanisms for the skeletal overgrowth in ECDM using a mouse model in which an activating mutant NPRB is specifically expressed in chondrocytes. The mutant mice (NPRB[p.V883M]-Tg) exhibited postnatal skeletal overgrowth and increased cGMP in cartilage. Both endogenous and transgene-derived NPRB proteins were localized at the plasma membrane of hypertrophic chondrocytes. The hypertrophic zone of growth plate was thickened in NPRB[p.V883M]-Tg. An in vivo BrdU-labeling assay suggested that some of the hypertrophic chondrocytes in NPRB[p.V883M]-Tg mice continued to proliferate, although wild-type (WT) chondrocytes stopped proliferating after they became hypertrophic. In vitro cell studies revealed that NPRB activation increased the phosphorylation of cyclic AMP-responsive element binding protein (CREB) and expression of cyclin D1 in matured chondrocytes. Treatment with cell-permeable cGMP also enhanced the CREB phosphorylation. Inhibition of cyclic adenosine monophosphate (cAMP)/protein kinase A pathway had no effects on the CREB phosphorylation induced by NPRB activation. In immunostaining of the growth plates for the proliferation marker Ki67, phosphorylated CREB and cyclin D1, most signals were similarly observed in the proliferating zone in both genotypes, but some cells in the hypertrophic zone of NPRB[p.V883M]-Tg were also positively stained. These results suggest that NPRB activation evokes its signal in hypertrophic chondrocytes to induce CREB phosphorylation and make them continue to proliferate, leading to the skeletal overgrowth in ECDM.

FGF23 and its role in X-linked hypophosphatemia-related morbidity

Background

X-linked hypophosphatemia (XLH) is an inherited disease of phosphate metabolism in which inactivating mutations of the Phosphate Regulating Endopeptidase Homolog, X-Linked (PHEX) gene lead to local and systemic effects including impaired growth, rickets, osteomalacia, bone abnormalities, bone pain, spontaneous dental abscesses, hearing difficulties, enthesopathy, osteoarthritis, and muscular dysfunction. Patients with XLH present with elevated levels of fibroblast growth factor 23 (FGF23), which is thought to mediate many of the aforementioned manifestations of the disease. Elevated FGF23 has also been observed in many other diseases of hypophosphatemia, and a range of animal models have been developed to study these diseases, yet the role of FGF23 in the pathophysiology of XLH is incompletely understood.

Methods

The role of FGF23 in the pathophysiology of XLH is here reviewed by describing what is known about phenotypes associated with various PHEX mutations, animal models of XLH, and non-nutritional diseases of hypophosphatemia, and by presenting molecular pathways that have been proposed to contribute to manifestations of XLH.

Results

The pathophysiology of XLH is complex, involving a range of molecular pathways that variously contribute to different manifestations of the disease. Hypophosphatemia due to elevated FGF23 is the most obvious contributor, however localised fluctuations in tissue non-specific alkaline phosphatase (TNAP), pyrophosphate, calcitriol and direct effects of FGF23 have been observed to be associated with certain manifestations.

Conclusions

By describing what is known about these pathways, this review highlights key areas for future research that would contribute to the understanding and clinical treatment of non-nutritional diseases of hypophosphatemia, particularly XLH.

Mutant hFGF23(A12D) stimulates osteoblast differentiation through FGFR3

Fibroblast growth factor (FGF) 23 is a member of the FGF family involved in bone development by interacting with FGFRs. In a previous study, we discovered a mutant human FGF (hFGF) 23 (A12D) in the mandibular prognathism (MP) pedigree. However, the exact role of hFGF23(A12D) during bone formation remains unclear. The aim of this study was to identify the function of hFGF23(A12D) in bone formation. We infected isolated rat calvaria (RC) cells with the recombinant lentivirus containing mutant hFGF23(A12D) and WT hFGF23 respectively. Real‐time PCR, western blot and enzyme‐linked immunosorbent assay confirmed that hFGF23(A12D) failed to be secreted. We measured cell growth via the CCK‐8 assay based on Zsgreen expression, detected cell differentiation ability via alkaline phosphatase staining, performed RT‐PCR and found that hFGF23(A12D) inhibited proliferation of RC cells and stimulated the differentiation of RC cells to osteoblasts. Through RNA sequencing, RT‐PCR and western blot, we found increased expression of FGFR3. Through co‐immunoprecipitation assays and immunofluorescence staining, we revealed that hFGF23(A12D) activated the mitogen‐activated protein kinase signalling pathway through interactions with the intracellular domain of FGFR3. In summary, we determined the mechanisms of hFGF23(A12D) involved in osteoblast generation and formation which is specifically due to its interaction with FGFR3.

The effects of elevated fibroblast growth factor 23 on mandibular growth in rats

OBJECTIVE

The aim of this study is to elucidate the local effects of fibroblast growth factor 23 (FGF23) in on mandibular condylar growth in growing rats.

DESIGN

Growing Sprague-Dawley rats received intra-temporomandibular joint injections of phosphate buffer solution (PBS), adenovirus-mediated green fluorescent protein (Ad-GFP) or adenovirus-mediated fibroblast growth factor 23 (Ad-FGF23), which were marked as groups A, B, and C, respectively. In vitro, we treated rat mandibular cartilage chondrocytes with PBS, Ad-GFP, and Ad-FGF23.

RESULTS

The mandibular condyles in group C grew smaller sizes than those in the other control groups due to significant differences among the experimental and control groups with the value of C-D, Q-R (P ≤ 0.05), accompanied by diminished bone mass of sub-cartilage condyles via micro CT analysis. Histologically, the length of the hypertrophic zone was diminished and was associated with decreasing chondrocyte proliferation in group C. Quantitative real-time PCR indicated significant decreases in the expression of chondrogenesis marker genes, including Type X collagen (Col X) and SRY-type box 9 (Sox 9). Moreover, elevated Ad-FGF23 suppressed chondrocyte proliferation and the expression of the chondrogenic differentiation markers Col X and Sox 9 of in vitro.

CONCLUSIONS

Local injection of FGF23 suppressed the development and decreased the bone mass of condyles through the decreasing the formation of condylar cartilage, specifically by regulating condylar cartilage cell viability and chondrogenesis expression.

Secreted α-Klotho maintains cartilage tissue homeostasis by repressing NOS2 and ZIP8-MMP13 catabolic axis

Progressive loss of tissue homeostasis is a hallmark of numerous age-related pathologies, including osteoarthritis (OA). Accumulation of senescent chondrocytes in joints contributes to the age-dependent cartilage loss of functions through the production of hypertrophy-associated catabolic matrix-remodeling enzymes and pro-inflammatory cytokines. Here, we evaluated the effects of the secreted variant of the anti-aging hormone α-Klotho on cartilage homeostasis during both cartilage formation and OA development. First, we found that α-Klotho expression was detected during mouse limb development, and transiently expressed during in vitro chondrogenic differentiation of bone marrow-derived mesenchymal stem cells. Genome-wide gene array analysis of chondrocytes from OA patients revealed that incubation with recombinant secreted α-Klotho repressed expression of the NOS2 and ZIP8/MMP13 catabolic remodeling axis. Accordingly, α-Klotho expression was reduced in chronically IL1β-treated chondrocytes and in cartilage of an OA mouse model. Finally, in vivo intra-articular secreted α-Kotho gene transfer delays cartilage degradation in the OA mouse model. Altogether, our results reveal a new tissue homeostatic function for this anti-aging hormone in protecting against OA onset and progression.

Soluble Klotho causes hypomineralization in Klotho-deficient mice

The type I transmembrane protein αKlotho (Klotho) serves as a coreceptor for the phosphaturic hormone fibroblast growth factor 23 (FGF23) in kidney, while a truncated form of Klotho (soluble Klotho, sKL) is thought to exhibit multiple activities, including acting as a hormone, but whose mode(s) of action in different organ systems remains to be fully elucidated. FGF23 is expressed primarily in osteoblasts/osteocytes and aberrantly high levels in the circulation acting via signaling through an FGF receptor (FGFR)-Klotho coreceptor complex cause renal phosphate wasting and osteomalacia. We assessed the effects of exogenously added sKL on osteoblasts and bone using Klotho-deficient (kl/kl) mice and cell and organ cultures. sKL induced FGF23 signaling in bone and exacerbated the hypomineralization without exacerbating the hyperphosphatemia, hypercalcemia and hypervitaminosis D in kl/kl mice. The same effects were seen in rodent bone models in vitro, in which we also detected formation of a sKL complex with FGF23-FGFR and decreased Phex (gene responsible for X-linked hypophosphatemic rickets (XLH)/osteomalacia) expression. Further, sKL-FGF23-dependent hypomineralization in vitro was rescued by soluble PHEX. These data suggest that exogenously added sKL directly participates in FGF23 signaling in bone and that PHEX is a downstream effector of the sKL-FGF23-FGFR axis in bone.

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.

FTO demethylase activity is essential for normal bone growth and bone mineralization in mice

The Fto gene locus has been linked to increased body weight and obesity in human population studies, but the role of the actual FTO protein in adiposity has remained controversial. Complete loss of FTO protein in mouse and of FTO function in human patients has multiple and variable effects. To determine which effects are due to the ability of FTO to demethylate mRNA, we genetically engineered a mouse with a catalytically inactive form of FTO. Our results demonstrate that FTO catalytic activity is not required for normal body composition although it is required for normal body size and viability. Strikingly, it is also essential for normal bone growth and mineralization, a previously unreported FTO function.

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A mouse model for a human lethal FTO catalytic null mutation was established.

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Lean/fat body composition and energy metabolism parameters were unaffected.

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FTO catalytic activity was required for normal body size and viability.

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Lack of FTO enzymatic activity caused substantial bone demineralization.

Inhibition of FGFR Signaling Partially Rescues Hypophosphatemic Rickets in HMWFGF2 Tg Male Mice

Transgenic mice harboring high molecular weight fibroblast growth factor (FGF)2 isoforms (HMWTg) in osteoblast lineage cells phenocopy human X-linked hypophosphatemic rickets (XLH) and Hyp murine model of XLH demonstrating increased FGF23/FGF receptor signaling and hypophosphatemic rickets. Because HMWFGF2 was upregulated in bones of Hyp mice and abnormal FGF receptor (FGFR) signaling is important in XLH, HMWTg mice were used to examine the effect of the FGFR inhibitor NVP-BGJ398, now in clinical trials for cancer therapy, on hypophosphatemic rickets. Short-term treatment with NVP-BGJ398 rescued abnormal FGFR signaling and hypophosphatemia in HMWTg. Long-term treatment with NVP-BGJ398 normalized tail, tibia, and femur length. Four weeks NVP-BGJ398 treatment significantly increased total body bone mineral density (BMD) and bone mineral content (BMC) in HMWTg mice; however, at 8 weeks, total body BMD and BMC was indistinguishable among groups. Micro-computed tomography revealed decreased vertebral bone volume, trabecular number, and increased trabecular spacing, whereas femur trabecular tissue density was increased; however, NVP-BGJ398 rescued defective cortical bone mineralization, increased thickness, reduced porosity, and increased endosteal perimeter and cortical tissue density in HMWTg. NVP-BGJ398 improved femur cancellous bone, cortical bone structure, growth plate, and double labeling in cortical bone and also increased femur trabeculae double labeled surface, mineral apposition rate, bone formation rate, and osteoclast number and surface in HMWTg. The decreased NPT2a protein that is important for renal phosphate excretion was rescued by NVP-BGJ398 treatment. We conclude that NVP-BGJ398 partially rescued hypophosphatemic rickets in HMWTg. However, long-term treatment with NVP-BGJ398 further increased serum FGF23 that could exacerbate the mineralization defect.

Tissue expression and source of circulating αKlotho

αKlotho (Klotho), a type I transmembrane protein and a coreceptor for Fibroblast Growth Factor-23, was initially thought to be expressed only in a limited number of tissues, most importantly the kidney, parathyroid gland and choroid plexus. Emerging data may suggest a more ubiquitous Klotho expression pattern which has prompted reevaluation of the restricted Klotho paradigm. Herein we systematically review the evidence for Klotho expression in various tissues and cell types in humans and other mammals, and discuss potential reasons behind existing conflicting data. Based on current literature and tissue expression atlases, we propose a classification of tissues into high, intermediate and low/absent Klotho expression. The functional relevance of Klotho in organs with low expression levels remain uncertain and there is currently limited data on a role for membrane-bound Klotho outside the kidney. Finally, we review the evidence for the tissue source of soluble Klotho, and conclude that the kidney is likely to be the principal source of circulating Klotho in physiology.

Fibroblast-growth factor 23 promotes terminal differentiation of ATDC5 cells

Objectives

Fibroblast Growth Factor 23 (FGF23) is well documented as a crucial player in the systemic regulation of phosphate homeostasis. Moreover, loss-of-function experiments have revealed that FGF23 also has a phosphate-independent and local impact on skeletogenesis. Here, we used ATDC5 cell line to investigate the expression of FGF23 and the role it may play locally during the differentiation of these cells.

Methods

ATDC5 cells were differentiated in the presence of insulin, and treated with recombinant FGF23 (rFGF23), inorganic phosphate (Pi) and/or PD173074, an inhibitor of FGF receptors (FGFRs). The mRNA expressions of FGF23, FGFRs and markers of hypertophy, Col X and MMP13, were determined by qPCR analysis and FGF23 production was assessed by ELISA. FGFR activation was determined by immunoprecipitation and immunoblotting.

Results

FGF23 mRNA expression and production were increased during ATDC5 differentiation. At D28 in particular, rFGF23 stimulation increased hypertrophic markers expression, as Col X and MMP13, and mineralization. A synergic effect of Pi and rFGF23 stimulation was observed on these markers and on the mineralization process. The use of PD173074, a pan-FGFR inhibitor, decreased terminal differentiation of ATDC5 by preventing rFGF23 pro-hypertrophic effects.

Conclusions

Altogether, our results provide evidence that FGF23 plays an important role during differentiation of ATDC5 cell line, by promoting both hypertrophy and mineralization.

A rare case of multiple phosphaturic mesenchymal tumors along a tendon sheath inducing osteomalacia

Background

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by renal phosphate wasting, hypophosphatemia, reduction of 1,25-dihydroxyl vitamin D, and bone calcification disorders. Tumors associated with TIO are typically phosphaturic mesenchymal tumors that are bone and soft tissue origin and often present as a solitary tumor. The high production of fibroblast growth factor 23 (FGF23) by the tumor is believed to be the causative factor responsible for the impaired renal tubular phosphate reabsorption, hypophosphatemia and osteomalacia. Complete removal of the tumors by surgery is the most effective procedure for treatment. Identification of the tumors by advanced imaging techniques is difficult because TIO is small and exist within bone and soft tissue. However, systemic venous sampling has been frequently reported to be useful for diagnosing TIO patients.

Case presentation

We experienced a case of 39-year-old male with diffuse bone pain and multiple fragility fractures caused by multiple FGF23-secreting tumors found in the hallux. Laboratory testing showed hypophosphatemia due to renal phosphate wasting and high levels of serum FGF23. Contrast-enhanced MRI showed three soft tissue tumors and an intraosseous tumor located in the right hallux. Systemic venous sampling of FGF23 revealed an elevation in the right common iliac vein and external iliac vein, which suggested that the tumors in the right hallux were responsible for overproduction of FGF23. Thereafter, these tumors were surgically removed and subjected to histopathological examinations. The three soft tissue tumors were diagnosed as phosphaturic mesenchymal tumors, which are known to be responsible for TIO. The fourth tumor had no tumor structure and was consisting of hyaline cartilage and bone tissue. Immediately after surgery, we noted a sharply decrease in serum level of FGF23, associated with an improved hypophosphatemia and a gradual relief of systematic pain that disappeared within two months of surgery.

Conclusion

The authors reported an unusual case of osteomalacia induced by multiple phosphaturic mesenchymal tumors located in the hallux. Definition of tumors localization by systemic venous sampling led to successful treatment and cure this patient. The presence of osteochondral tissues in the intraosseous tumor might be developed from undifferentiated mesenchymal cells due to high level of FGF23 produced by phosphaturic mesenchymal tumors.

The metabolic bone disease associated with the Hyp mutation is independent of osteoblastic HIF1α expression

Fibroblast growth factor-23 (FGF23) controls key responses to systemic phosphate increases through its phosphaturic actions on the kidney. In addition to stimulation by phosphate, FGF23 positively responds to iron deficiency anemia and hypoxia in rodent models and in humans. The disorder X-linked hypophosphatemia (XLH) is characterized by elevated FGF23 in concert with an intrinsic bone mineralization defect. Indeed, the Hyp mouse XLH model has disturbed osteoblast to osteocyte differentiation with altered expression of a wide variety of genes, including FGF23. The transcription factor Hypoxia inducible factor-1α (HIF1α) has been implicated in regulating FGF23 production and plays a key role in proper bone cell differentiation. Thus the goals of this study were to determine whether HIF1α activation could influence FGF23, and to test osteoblastic HIF1α production on the Hyp endocrine and skeletal phenotypes in vivo. Treatment of primary cultures of osteoblasts/osteocytes and UMR-106 cells with the HIF activator AG490 resulted in rapid HIF1α stabilization and increased Fgf23 mRNA (50–100 fold; p < 0.01–0.001) in a time- and dose-dependent manner. Next, the Phex gene deletion in the Hyp mouse was bred onto mice with a HIF1α/Osteocalcin (OCN)-Cre background. Although HIF1α effects on bone could be detected, FGF23-related phenotypes due to the Hyp mutation were independent of HIF1α in vivo. In summary, FGF23 can be driven by ectopic HIF1α activation under normal iron conditions in vitro, but factors independent of HIF1α activity after mature osteoblast formation are responsible for the disease phenotypes in Hyp mice in vivo.

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In vitro, a HIF activator stabilized HIF1α and increased Fgf23 mRNA expression.

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A novel mouse model was generated by breeding the Hyp mouse onto the HIF1α/Osteocalcin (OCN)-Cre background.

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Factors independent of HIF1α activity are responsible for the disease phenotypes in Hyp mice.

The FGF23/KLOTHO Regulatory Network and Its Roles in Human Disorders

The functions of Klotho (KL) are multifaceted and include the regulation of aging and mineral metabolism. It was originally identified as the gene responsible for premature aging-like symptoms in mice and was subsequently shown to function as a coreceptor in the fibroblast growth factor (FGF) 23 signaling pathway. The discovery of KL as a partner for FGF23 led to significant advances in understanding of the molecular mechanisms underlying phosphate and vitamin D metabolism, and simultaneously clarified the pathogenic roles of the FGF23 signaling pathway in human diseases. These novel insights led to the development of new strategies to combat disorders associated with the dysregulated metabolism of phosphate and vitamin D, and clinical trials on the blockade of FGF23 signaling in X-linked hypophosphatemic rickets are ongoing. Molecular and functional insights on KL and FGF23 have been discussed in this review and were extended to how dysregulation of the FGF23/KL axis causes human disorders associated with abnormal mineral metabolism.

Expression of osteoclastogenic factor transcripts in osteoblast-like UMR-106 cells after exposure to FGF-23 or FGF-23 combined with parathyroid hormone

As a bone-derived hormone, fibroblast growth factor-23 (FGF-23) negatively regulates phosphate and calcium metabolism, while retaining growth-promoting action for mesenchymal cell differentiation. Elevated FGF-23 levels, together with hyperparathyroidism, are often observed in chronic kidney disease, which is associated with impaired bone mineralization and enhanced bone resorption. Although overexpression of osteoblast-derived osteoclastogenic cytokines might contribute to this metabolic bone disease, whether FGF-23 alone and FGF-23 plus parathyroid hormone (PTH) directly modulated the expression of osteoblast-derived osteoclastogenic genes remained elusive. Herein, we demonstrated the direct effects of FGF-23 on proliferation and mRNA expression of osteoblast-specific differentiation and osteoclastogenic markers in rat osteoblast-like UMR-106 cells in the presence or absence of PTH. FGF-23 was found to suppress UMR-106 cell proliferation, while increasing FGF-23 expression, the latter of which suggested the presence of positive feedback regulation of FGF-23 expression in osteoblasts. FGF-23 also upregulated the mRNA expression of osteoblast differentiation markers (e.g., Runx2, osterix, AJ18, Dlx5, alkaline phosphatase, and osteopontin), osteoclastogenic factors (e.g., MCSF, MCP-1, IL-6, and TNF-α), and bone resorption regulators (RANKL and osteoprotegerin). However, combined PTH and FGF-23 exposure did not alter the levels of FGF-23-induced transcripts, suggesting that both hormones had no additive effect. In conclusion, FGF-23 directly suppressed osteoblast proliferation, while inducing osteoclastogenic gene expression in UMR-106 cells, and the FGF-23-induced transcripts were not altered by long-standing PTH exposure.

The FGF23/Klotho axis in the regulation of mineral and metabolic homeostasis

The function of fibroblast growth factor (FGF) 23 has been suggested to be multifaceted beyond its canonical function as a regulator of mineral metabolism. FGF23 was originally shown to play a central role in phosphate (Pi) and vitamin D metabolism, and a number of diseases associated with dysregulated Pi metabolism have been attributed to abnormal FGF23 signaling activities. The discovery of Klotho as a co-receptor for FGF23 signaling has also accelerated understanding on the molecular mechanisms underlying Pi and vitamin D metabolism. In addition to these canonical functions, FGF23 has recently been implicated in a number of metabolic diseases including chronic kidney disease-associated complications, cardiovascular diseases, and obesity-related disorders; however, the physiological significance and molecular mechanisms of these emerging roles of FGF23 remain largely unknown. Molecular and functional insights into the FGF23 pathway will be discussed in the present review, with an emphasis on its role in human disorders related to dysregulated Pi metabolism as well as metabolic disorders.

FGF23 Regulates Bone Mineralization in a 1,25(OH)2 D3 and Klotho-Independent Manner

Fibroblast growth factor-23 (Fgf23) is a bone-derived hormone, suppressing phosphate reabsorption and vitamin D hormone (1,25(OH)2 D3 ) production in the kidney. It has long been an enigma why lack of Fgf23 or of Klotho, the coreceptor for Fgf23, leads to severe impairment in bone mineralization despite the presence of hypercalcemia and hyperphosphatemia. Using Fgf23(-/-) or Klotho(-/-) mice together with compound mutant mice lacking both Fgf23 or Klotho and a functioning vitamin D receptor, we show that in Klotho(-/-) mice the mineralization defect is solely driven by 1,25(OH)2 D3 -induced upregulation of the mineralization-inhibiting molecules osteopontin and pyrophosphate in bone. In Fgf23(-/-) mice, the mineralization defect has two components, a 1,25(OH)2 D3 -driven component similar to Klotho(-/-) mice and a component driven by lack of Fgf23, causing additional accumulation of osteopontin. We found that FGF23 regulates osteopontin secretion indirectly by suppressing alkaline phosphatase transcription and phosphate production in osteoblastic cells, acting through FGF receptor-3 in a Klotho-independent manner. Hence, FGF23 secreted from osteocytes may form an autocrine/paracrine feedback loop for the local fine-tuning of bone mineralization.

Regulation of Long Bone Growth in Vertebrates; It Is Time to Catch Up

The regulation of organ size is essential to human health and has fascinated biologists for centuries. Key to the growth process is the ability of most organs to integrate organ-extrinsic cues (eg, nutritional status, inflammatory processes) with organ-intrinsic information (eg, genetic programs, local signals) into a growth response that adapts to changing environmental conditions and ensures that the size of an organ is coordinated with the rest of the body. Paired organs such as the vertebrate limbs and the long bones within them are excellent models for studying this type of regulation because it is possible to manipulate one member of the pair and leave the other as an internal control. During development, growth plates at the end of each long bone produce a transient cartilage model that is progressively replaced by bone. Here, we review how proliferation and differentiation of cells within each growth plate are tightly controlled mainly by growth plate-intrinsic mechanisms that are additionally modulated by extrinsic signals. We also discuss the involvement of several signaling hubs in the integration and modulation of growth-related signals and how they could confer remarkable plasticity to the growth plate. Indeed, long bones have a significant ability for "catch-up growth" to attain normal size after a transient growth delay. We propose that the characterization of catch-up growth, in light of recent advances in physiology and cell biology, will provide long sought clues into the molecular mechanisms that underlie organ growth regulation. Importantly, catch-up growth early in life is commonly associated with metabolic disorders in adulthood, and this association is not completely understood. Further elucidation of the molecules and cellular interactions that influence organ size coordination should allow development of novel therapies for human growth disorders that are noninvasive and have minimal side effects.

Molecular basis of Klotho: from gene to function in aging

The discovery of the Klotho (KL) gene, which was originally identified as a putative aging-suppressor gene, has generated tremendous interest and has advanced understanding of the aging process. In mice, the overexpression of the KL gene extends the life span, whereas mutations to the KL gene shorten the life span. The human KL gene encodes the α-Klotho protein, which is a multifunctional protein that regulates the metabolism of phosphate, calcium, and vitamin D. α-Klotho also may function as a hormone, although the α-Klotho receptor(s) has not been found. Point mutations of the KL gene in humans are associated with hypertension and kidney disease, which suggests that α-Klotho may be essential to the maintenance of normal renal function. Three α-Klotho protein types with potentially different functions have been identified: a full-length transmembrane α-Klotho, a truncated soluble α-Klotho, and a secreted α-Klotho. Recent evidence suggests that α-Klotho suppresses the insulin and Wnt signaling pathways, inhibits oxidative stress, and regulates phosphatase and calcium absorption. In this review, we provide an update on recent advances in the understanding of the molecular, genetic, biochemical, and physiological properties of the KL gene. Specifically, this review focuses on the structure of the KL gene and the factors that regulate KL gene transcription, the key sites in the regulation of α-Klotho enzyme activity, the α-Klotho signaling pathways, and the molecular mechanisms that underlie α-Klotho function. This current understanding of the molecular biology of the α-Klotho protein may offer new insights into its function and role in aging.

Expression of Fgf23 in activated dendritic cells and macrophages in response to immunological stimuli in mice

Fibroblast growth factors (Fgfs) are polypeptide growth factors with diverse biological activities. While several studies have revealed that Fgf23 plays important roles in the regulation of phosphate and vitamin D metabolism, the additional physiological roles of Fgf23 remain unclear. Although it is believed that osteoblasts/osteocytes are the main sources of Fgf23, we previously found that Fgf23 mRNA is also expressed in the mouse thymus, suggesting that it might be involved in the immune system. In this study we examined the potential roles of Fgf23 in immunological responses. Mouse serum Fgf23 levels were significantly increased following inoculation with Escherichia coli or Staphylococcus aureus or intraperitoneal injection of lipopolysaccharide. We also identified activated dendritic cells and macrophages that potentially contributed to increased serum Fgf23 levels. Nuclear factor-kappa B (NF-κB) signaling was essential for the induction of Fgf23 expression in dendritic cells in response to immunological stimuli. Moreover, we examined the effects of recombinant Fgf23 protein on immune cells in vitro. Fgfr1c, a potential receptor for Fgf23, was abundantly expressed in macrophages, suggesting that Fgf23 might be involved in signal transduction in these cells. Our data suggest that Fgf23 potentially increases the number in macrophages and induces expression of tumor necrosis factor-α (TNF-α), a proinflammatory cytokine. Collectively, these data suggest that Fgf23 might be intimately involved in inflammatory processes.

Functional diversity of fibroblast growth factors in bone formation

The functional significance of fibroblast growth factor (FGF) signaling in bone formation has been demonstrated through genetic loss-of-function and gain-of-function approaches. FGFs, comprising 22 family members, are classified into three subfamilies: canonical, hormone-like, and intracellular. The former two subfamilies activate their signaling pathways through FGF receptors (FGFRs). Currently, intracellular FGFs appear to be primarily involved in the nervous system. Canonical FGFs such as FGF2 play significant roles in bone formation, and precise spatiotemporal control of FGFs and FGFRs at the transcriptional and posttranscriptional levels may allow for the functional diversity of FGFs during bone formation. Recently, several research groups, including ours, have shown that FGF23, a member of the hormone-like FGF subfamily, is primarily expressed in osteocytes/osteoblasts. This polypeptide decreases serum phosphate levels by inhibiting renal phosphate reabsorption and vitamin D3 activation, resulting in mineralization defects in the bone. Thus, FGFs are involved in the positive and negative regulation of bone formation. In this review, we focus on the reciprocal roles of FGFs in bone formation in relation to their local versus systemic effects.

Recent research on the growth plate: Advances in fibroblast growth factor signaling in growth plate development and disorders

Skeletons are formed through two distinct developmental actions, intramembranous ossification and endochondral ossification. During embryonic development, most bone is formed by endochondral ossification. The growth plate is the developmental center for endochondral ossification. Multiple signaling pathways participate in the regulation of endochondral ossification. Fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling has been found to play a vital role in the development and maintenance of growth plates. Missense mutations in FGFs and FGFRs can cause multiple genetic skeletal diseases with disordered endochondral ossification. Clarifying the molecular mechanisms of FGFs/FGFRs signaling in skeletal development and genetic skeletal diseases will have implications for the development of therapies for FGF-signaling-related skeletal dysplasias and growth plate injuries. In this review, we summarize the recent advances in elucidating the role of FGFs/FGFRs signaling in growth plate development, genetic skeletal disorders, and the promising therapies for those genetic skeletal diseases resulting from FGFs/FGFRs dysfunction. Finally, we also examine the potential important research in this field in the future.

Hypophosphatemic rickets due to perturbations in renal tubular function

The common denominator for all types of rickets is hypophosphatemia, leading to inadequate supply of the mineral to the growing bone. Hypophosphatemia can result from insufficient uptake of the mineral from the gut or its disproportionate losses in the kidney, the latter being caused by either tubular abnormalities per se or the effect on the tubule of circulating factors like fibroblast growth factor-23 and parathyroid hormone (PTH). High serum levels of the latter result in most cases from abnormalities in vitamin D metabolism which lead to decreased calcium absorption in the gut and hypocalcemia, triggering PTH secretion. Rickets is a disorder of the growth plate and hence pediatric by definition. However, it is important to recognize that the effect of hypophosphatemia on other parts of the skeleton results in osteomalacia in both children and adults. This review addresses the etiology, pathophysiologic mechanisms, clinical manifestations and treatment of entities associated with hypophosphatemic rickets due to perturbations in renal tubular function.

FGF23 and Klotho in chronic kidney disease

PURPOSE OF REVIEW

The wealth of data regarding fibroblast growth factor-23 (FGF23) and Klotho in chronic kidney disease (CKD) has risen exponentially over the past decade. This review is an attempt to summarize pivotal aspects of previous research, provide an update of recent findings and define important areas for future investigation.

RECENT FINDINGS

The phosphaturic hormone FGF23 increases dramatically as renal function declines. Identification of contributing stimuli to the rise in FGF23 is fundamental and recent evidence suggest a multifactorial cause which entails perturbed osteocyte function and renal mechanisms such as Klotho deficiency and, somewhat paradoxically, systemic Klotho excess. Circulating FGF23 predicts adverse outcomes, particularly cardiovascular disease, in CKD as well as in the general population. The concept of FGF23 merely as a biomarker and regulator of mineral metabolism is currently challenged by data linking FGF23 to pathological processes such as cardiac hypertrophy. Conversely, tissue level of the FGF23 coreceptor Klotho declines in early CKD and this deficiency is linked to accelerated ageing, cellular senescence, vascular calcification, oxidative stress and renal fibrosis. At present, methodological difficulties limit the utility of soluble Klotho measurements. Animal proof-of-concept studies have demonstrated beneficial effects of Klotho delivery in CKD, whereas anti-FGF23 therapy using neutralizing antibodies improved biochemical and bone parameters at the expense of increased vascular calcification and mortality.

SUMMARY

Pathological alterations of FGF23-Klotho in CKD are implicated as clinical biomarkers and may provide novel therapeutic strategies to alleviate the cardiovascular risk and slow CKD progression.

FGF23 affects the lineage fate determination of mesenchymal stem cells

FGF23 is a bone-derived hormone that regulates mineral metabolism by inhibiting renal tubular phosphate reabsorption and suppressing circulating 1,25(OH)2D and PTH levels. These effects are mediated by FGF-receptor binding and activation in the presence of its coreceptor Klotho, which is expressed in the distal tubules of the kidney. Recently, expression of Klotho in skeletal tissues has been reported, indicating a direct, yet unclear, extrarenal effect of FGF23 on cells involved with bone development and remodeling. In the present study, we found that bone marrow stromal cells harvested from Klotho null mice developed fewer osteoblastic but more adipocytic colonies than cells from wild-type mice. The underlying mechanism was explored by experiments on mouse C3H10T1/2 cells. We found that Klotho was weakly expressed and that FGF23 dose-dependently affected the lineage fate determination. The effects of FGF23 on cell differentiation can be diminished by SU 5402, a specific tyrosine kinase inhibitor for FGF receptors. Our results indicate that FGF23 directly affects the differentiation of bone marrow stromal cells.

Fibroblast growth factor 23

There is growing interest in the role of fibroblast growth factor 23 (FGF23) in various diseases of disordered mineral metabolism. In chronic kidney disease (CKD), where biochemical evidence of mineral disturbances is especially common, FGF23 measurement has been advocated as an early and sensitive marker for CKD-related bone disease. In this setting, FGF23 analysis may also improve the discrimination of risk of adverse renal and cardiovascular outcomes and aid targeting of those patients that are likely to benefit from interventions. Nonetheless, while the physiological relevance of FGF23 in the control of mineral metabolism is now firmly established, relatively little attention has been paid to important preanalytical and analytical aspects of FGF23 measurement that may impact on its clinical utility. Here we review these issues and discuss the suitability of FGF23 testing strategies for routine clinical practice. The current ‘state-of-the-art’ enzyme-linked immunosorbent assay methods for FGF23 measurement show poor agreement due to differences in FGF23 fragment detection, antibody specificity and calibration. Such analytical variability does not permit direct comparison of FGF23 measurements made with different assays and is likely to at least in part account for some of the inconsistencies noted between observational studies. From a clinical perspective, the lack of concordance has implications for the development of standardized reference intervals and clinical decision limits. Finally, the inherent assay-dependent biological variability of plasma FGF23 concentration can further complicate the interpretation of results and the design of FGF23-based testing protocols. Currently, it would be premature to consider incorporating FGF23 measurements into standard testing repertoires.

Growth hormone and Klotho

Acromegaly is characterized by excessively high GH and IGF1 levels. Recent data suggest that soluble Klotho (sKlotho) is also elevated in patients with active acromegaly. sKlotho decreases towards normal following removal of the GH-producing pituitary adenoma. The Klotho gene was identified in mice following its accidental disruption by ectopic DNA. It is an ageing suppressor gene of restricted expression (mainly in kidneys, brain, and parathyroid and pituitary glands) encoding a transmembrane protein, mKlotho. mKlotho serves as a co-receptor in fibroblast growth factor 23 (FGF23) signalling. FGF23 promotes urinary phosphate excretion and inhibits the synthesis of calcitriol. The ectodomain of mKlotho is enzymatically released to result in a humoral factor, sKlotho, which exerts systemic effects (on ion channels and signalling pathways), possibly by working as an enzyme that modifies glycans of cell surface glycoproteins. GH enhances renal phosphate reabsorption and calcitriol production, i.e. exerts effects in the proximal tubule opposing those attributed to mKlotho, and attenuates calciuria in the distal tubule similar to sKlotho. sKlotho can be measured in extracellular fluids (serum, urine and cerebrospinal fluid (CSF)) by an ELISA. In line with predominant expression of Klotho in kidneys and choroid plexus, concentrations of sKlotho are particularly high in urine and CSF. Determination of sKlotho in serum and urine (both presumably reflecting GH action on the kidneys) could be used as a supplementary tool in the diagnosis and follow-up of patients with acromegaly. The question arises whether GH exerts selected actions via modifying activities of Klotho.