FGF23 affects the lineage fate determination of mesenchymal stem cells

Distinct role of Klotho in long bone and craniofacial bone: skeletal development, repair and regeneration

Bone defects are highly prevalent diseases caused by trauma, tumors, inflammation, congenital malformations and endocrine abnormalities. Ideally effective and side effect free approach to dealing with bone defects remains a clinical conundrum. Klotho is an important protein, which plays an essential role in regulating aging and mineral ion homeostasis. More recently, research revealed the function of Klotho in regulating skeleton development and regeneration. Klotho has been identified in mesenchymal stem cells, osteoblasts, osteocytes and osteoclasts in different skeleton regions. The specific function and regulatory mechanisms of Klotho in long bone and craniofacial bone vary due to their different embryonic development, ossification and cell types, which remain unclear and without conclusion. Moreover, studies have confirmed that Klotho is a multifunctional protein that can inhibit inflammation, resist cancer and regulate the endocrine system, which may further accentuate the potential of Klotho to be the ideal molecule in inducing bone restoration clinically. Besides, as an endogenous protein, Klotho has a promising potential for clinical therapy without side effects. In the current review, we summarized the specific function of Klotho in long bone and craniofacial skeleton from phenotype to cellular alternation and signaling pathway. Moreover, we illustrated the possible future clinical application for Klotho. Further research on Klotho might help to solve the existing clinical difficulties in bone healing and increase the life quality of patients with bone injury and the elderly.

Exploring the Role of Hormones and Cytokines in Osteoporosis Development

The disease of osteoporosis is characterized by impaired bone structure and an increased risk of fractures. There is a significant impact of cytokines and hormones on bone homeostasis and the diagnosis of osteoporosis. As defined by the World Health Organization (WHO), osteoporosis is defined as having a bone mineral density (BMD) that is 2.5 standard deviations (SD) or more below the average for young and healthy women (T score < -2.5 SD). Cytokines and hormones, particularly in the remodeling of bone between osteoclasts and osteoblasts, control the differentiation and activation of bone cells through cytokine networks and signaling pathways like the nuclear factor kappa-B ligand (RANKL)/the receptor of RANKL (RANK)/osteoprotegerin (OPG) axis, while estrogen, parathyroid hormones, testosterone, and calcitonin influence bone density and play significant roles in the treatment of osteoporosis. This review aims to examine the roles of cytokines and hormones in the pathophysiology of osteoporosis, evaluating current diagnostic methods, and highlighting new technologies that could help for early detection and treatment of osteoporosis.

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.

Relationship between bone marrow adipose tissue and kidney function in postmenopausal women

Introduction

Bone marrow adipose tissue (BMAT) is associated with aging, osteoporosis, and chronic kidney disease (CKD). To date, the association between BMAT and kidney function in postmenopausal women has not been thoroughly investigated. The main purpose of this study was to determine whether a relationship exists between proton density fat fraction (PDFF) and kidney function in postmenopausal women.

Methods

We investigated the cross-sectional association between estimated glomerular filtration rate (eGFR) - calculated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation - and PDFF - measured at the lumbar spine and proximal femur using Water Fat Imaging (WFI) MRI - in 199 postmenopausal women from the ADIMOS cohort study. We also performed DXA scans and laboratory measurements of sclerostin and c-terminal Fibroblast Growth Factor 23 (cFGF23).

Results

Participants' mean age was 67.5 (standard deviation, SD 10.0) years. Their median eGFR was 85.0 (interquartile range, IQR 72.2-95.0) ml/min/1.73 cm2, and their mean lumbar spine PDFF was 57.9 % (SD 9.6). When classified by eGFR-based CKD stages, 41.7 % of the cohort had an eGFR ≥ 90 (n = 83), 47.2 % had an eGFR of 60-89.9 (n = 94), and 11.1 % had an eGFR of 30-59.9 (n = 22). Participants with eGFR ≥ 90 had a lower lumbar spine PDFF than those with eGFR 60-89.9 (mean 55.8 % (9.8) vs. 58.9 % (9.0), p = 0.031) and those with eGFR 30-59.9 (55.8 % (9.8) vs. 60.8 % (9.8), p = 0.043). However, the differences did not remain significant after adjusting for predetermined confounders, including age, diabetes, Charlson comorbidity index, recent history of fragility fracture, appendicular lean mass, and lumbar spine BMD. The inclusion of sclerostin and/or cFGF23 as suspected mediators did not alter the findings. When proximal hip imaging-based PDFF was considered, no significant differences were found between the eGFR categories in the unadjusted and adjusted analyses.

Conclusion

No evidence of an association between kidney function and bone marrow adiposity was found either in the lumbar spine or proximal femur in a cohort of postmenopausal women.

Klotho Supplementation Reverses Renal Dysfunction and Interstitial Fibrosis in Remnant Kidney

INTRODUCTION

While recent investigations show that klotho exerts renoprotective actions, it has not been fully addressed whether klotho protein supplementation reverses renal damage.

METHODS

The impacts of subcutaneous klotho supplementation on rats with subtotal nephrectomy were examined. Animals were divided into 3 groups: group 1 (short remnant [SR]): remnant kidney for 4 weeks, group 2 (long remnant [LR]): remnant kidney for 12 weeks, and group 3 (klotho supplementation [KL]): klotho protein (20 μg/kg/day) supplementation on the remnant kidney. Blood pressure, blood and urine compositions with conventional methods such as enzyme-linked immunosorbent assay and radioimmunoassay, kidney histology, and renal expressions of various genes were analyzed. In vitro studies were also performed to support in vivo findings.

RESULTS

Klotho protein supplementation decreased albuminuria (-43%), systolic blood pressure (-16%), fibroblast growth factor (FGF) 23 (-51%) and serum phosphate levels (-19%), renal angiotensin II concentration (-43%), fibrosis index (-70%), renal expressions of collagen I (-55%), and transforming growth factor β (-59%) (p < 0.05 for all). Klotho supplementation enhanced fractional excretion of phosphate (+45%), glomerular filtration rate (+76%), renal expressions of klotho (+148%), superoxide dismutase (+124%), and bone morphogenetic protein (BMP) 7 (+174%) (p < 0.05 for all).

CONCLUSION

Our data indicated that klotho protein supplementation inactivated renal renin-angiotensin system, reducing blood pressure and albuminuria in remnant kidney. Furthermore, exogenous klotho protein supplementation elevated endogenous klotho expression to increase phosphate excretion with resultant reductions in FGF23 and serum phosphate. Finally, klotho supplementation reversed renal dysfunction and fibrosis in association with improved BMP7 in remnant kidney.

To Investigate Growth Factor Receptor Targets and Generate Cancer Targeting Inhibitors

Receptor tyrosine kinase (RTK) regulates multiple pathways, including Mitogenactivated protein kinases (MAPKs), PI3/AKT, JAK/STAT pathway, etc. which has a significant role in the progression and metastasis of tumor. As RTK activation regulates numerous essential bodily processes, including cell proliferation and division, RTK dysregulation has been identified in many types of cancers. Targeting RTK is a significant challenge in cancer due to the abnormal upregulation and downregulation of RTK receptors subfamily EGFR, FGFR, PDGFR, VEGFR, and HGFR in the progression of cancer, which is governed by multiple RTK receptor signalling pathways and impacts treatment response and disease progression. In this review, an extensive focus has been carried out on the normal and abnormal signalling pathways of EGFR, FGFR, PDGFR, VEGFR, and HGFR and their association with cancer initiation and progression. These are explored as potential therapeutic cancer targets and therefore, the inhibitors were evaluated alone and merged with additional therapies in clinical trials aimed at combating global cancer.

Fibroblast growth factor 23 inhibits osteogenic differentiation and mineralization of chicken bone marrow mesenchymal stem cells

Fibroblast growth factor 23 (FGF23), a bone-derived hormone, is involved in the reabsorption of phosphate (P) and the production of vitamin D hormones in the kidney. However, whether and how FGF23 regulates chicken bone metabolism remains largely unknown. In the present study, we investigated the effect of FGF23 on osteogenic differentiation and mineralization of chicken bone marrow mesenchymal stem cells (BMSCs). First, we found that the transcription of FGF23 was inhibited by β-glycerophosphate sodium (GPS, 5 mM, 10 mM, 20 mM) and 10^-9 M 1, 25-dihydroxyvitamin D3 (1, 25(OH)₂D₃), but was stimulated by 10^-7 M 1, 25(OH)₂D₃ and parathyroid hormone (PTH, 10^-9 M, 10^-8 M, 10^-7 M). Second, overexpression of FGF23 by the FGF23 adenovirus (Adv-FGF23) suppressed the formation of mineralized nodules (P < 0.001) and alkaline phosphatase (ALP) activity (P < 0.05) in both differentiated and mineralized osteoblasts. Administration of FGF receptor 3 (FGFR3) inhibitor (50 nM) was sufficient to restore the FGF23-decreased ALP activity (P < 0.05), but not for the formation of mineralized nodules. In addition, the phosphorylation of ERK increased considerably with Adv-FGF23 overexpression (P < 0.05). Administration of an ERK-specific inhibitor (10 μM) could down-regulate the phosphorylation of ERK (P-ERK) (P < 0.05) and slightly restored the Adv-FGF23-reduction of ALP activity (P = 0.08). In summary, our data suggest that GPS, 1, 25(OH)₂D₃, and PTH could regulate FGF23 mRNA expression in vitro. FGF23 is a negative regulator of bone remodeling. FGF23 not only inhibits BMSCs osteogenesis through the FGFR3-ERK signaling pathway but also suppresses the mineralization of mature osteoblasts.

Bone Marrow Adiposity, Bone Mineral Density and Wnt/β-catenin Pathway Inhibitors Levels in Hemodialysis Patients

Background

Marrow adipose tissue (MAT) is known to accumulate in patients with chronic kidney disease. This pilot study aimed to evaluate bone mineral density (BMD), MAT, visceral adipose tissue (VAT), and subcutaneous adipose tissue (SAT) using computed tomography (CT) scans and to explore correlations between bone parameters, circulating Wnt/β-catenin pathway inhibitor levels, and adipose tissue parameters.

Methods

Single-center cross-sectional pilot study conducted in hemodialysis patients at the Centre Universitaire de Québec, Hôtel-Dieu de Québec hospital, Canada. CT-scan slices were acquired at the levels of the hip, L3 vertebra, and tibia. Volumetric and areal BMD, tibia cortical thickness, VAT and SAT area, and fat marrow index (FMI) were analyzed using the Mindways QCT Pro software. Blood levels of sclerostin, dickkopf-related protein 1 (DKK1), fibroblast growth factor 23, and α-Klotho were assessed. Spearman’s rho test was used to evaluate correlations.

Results

Fifteen hemodialysis patients (median age, 75 [66–82] years; 80% male; dialysis vintage, 39.3 [27.4–71.0] months) were included. While inverse correlations were obtained between L3 FMI and BMD, positive correlations were found between proximal tibial FMI and vertebral and tibial BMD, as well as with tibial (proximal and distal) cortical thickness. VAT had a positive correlation with α-Klotho levels, whereas L3 FMI had a negative correlation with DKK1 levels.

Conclusions

CT-scan allows simultaneous evaluation of bone and marrow adiposity in dialysis patients. Correlations between MAT and BMD vary depending on the bone site evaluated. DKK1 and α-Klotho levels correlate with adipose tissue accumulation in dialysis patients.

Fibroblast Growth Factor 23 and Osteoporosis: Evidence from Bench to Bedside

Osteoporosis is a chronic debilitating disease caused by imbalanced bone remodeling processes that impair the structural integrity of bone. Over the last ten years, the association between fibroblast growth factor 23 (FGF23) and osteoporosis has been studied in both pre-clinical and clinical investigations. FGF23 is a bone-derived endocrine factor that regulates mineral homeostasis via the fibroblast growth factor receptors (FGFRs)/αKlotho complex. These receptors are expressed in kidney and the parathyroid gland. Preclinical studies have supported the link between the local actions of FGF23 on the bone remodeling processes. In addition, clinical evidence regarding the effects of FGF23 on bone mass and fragility fractures suggest potential diagnostic and prognostic applications of FGF23 in clinical contexts, particularly in elderly and patients with chronic kidney disease. However, inconsistent findings exist and there are areas of uncertainty requiring exploration. This review comprehensively summarizes and discusses preclinical and clinical reports on the roles of FGF23 on osteoporosis, with an emphasis on the local action, as opposed to the systemic action, of FGF23 on the bone. Current gaps in knowledge and future research directions are also suggested to encourage further rigorous research in this important field.

microRNA-199a counteracts glucocorticoid inhibition of bone marrow mesenchymal stem cell osteogenic differentiation through regulation of Klotho expression in vitro

Osteogenic differentiation (OD) of bone marrow mesenchymal stem cells (BMSCs) is critically important for mitigation of osteoporosis. Glucocorticoids (GCs) are extensively used for treating chronic inflammation, although long-term exposure to GCs is capable of triggering osteoporosis. microRNAs (miRNAs) have been reported to play a critical role in bone diseases. In the present study, we treated BMSCs with dexamethasone (DEX) during OD to stimulate GC-mediated osteoporosis. Microarray and quantitative polymerase chain reaction (Q-PCR) assays demonstrated that miR-199a was upregulated during OD of BMSCs, while DEX treatment caused a significant reduction in miR-199a. Alkaline phosphatase (ALP) activity, Alizarin red (AR) staining, and Q-PCR were applied to assess the role of miRNA-199a overexpression in DEX-triggered OD inhibition. miR-199a was able to rescue OD and ALP activity, which were inhibited by DEX. Additionally, we observed that ALP, BMP2, COL1A1, and Runx2 were increased after transfection of miRNA-199a mimics. Furthermore, we confirmed that miRNA-199a facilitates OD of BMSCs through direct inhibition of Klotho protein and messenger RNA expression affecting the downstream fibroblast growth factor receptor 1/extracellular-signal-regulated kinase and Janus kinase 1/signal transducer and activator of transcription 1 pathways. This study indicates that miR-199a plays a critical role in preventing GC-mediated osteoblast differentiation and may function as a promising miRNA biomarker for osteoporosis.

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.

FGF23 and Fetuin-A Interaction and Mesenchymal Osteogenic Transformation

Recently, we found a strict bone association between Fibroblast growth factor 23 (FGF23) and Fetuin-A, both involved in cardiovascular and mineral bone disorders. In this study, an uninvestigated bone marrow positivity for both was found. Though the role of exogenous FGF23 on mesenchymal cells (MSCs) was reported, no information is as yet available on the possible production of this hormone by MSCs. To further analyze these uninvestigated aspects, we studied human primary cells and mouse and human cell lines by means of immunostaining, qRT-PCR, enzyme linked immunosorbent assays, chromatin immunoprecipitation, transfection, and a streamlined approach for the FGF23⁻Fetuin-A interaction called Duolink proximity ligation assay. Mesenchymal cells produce but do not secrete FGF23 and its expression increases during osteo-differentiation. Fibroblast growth factor 23 is also involved in the regulation of Fetuin-A by binding directly to the Fetuin-A promoter and then activating its transcription. Both FGF23 overexpression and addition induced an upregulation of Fetuin-A in the absence of osteo-inducer factors. Fibroblast growth factor 23 and Fetuin-A promoter were increased by osteo-inducer factors with this effect being abolished after FGF23 silencing. In conclusion, both FGF23 and Fetuin-A are present and strictly linked to each other in MSCs with FGF23 driving Fetuin-A production. This mechanism suggests a role for these two proteins in the osteoblast differentiation.

Healing of osteoporotic bone defects by micro-/nano-structured calcium phosphate bioceramics

The micro-/nano-structured calcium phosphate bioceramic exhibited a higher new bone substitution rate in an osteoporotic bone defect rat model.

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.

Chronic kidney disease and vitamin D metabolism in human bone marrow-derived MSCs

Vitamin D that is synthesized in the skin or is ingested undergoes sequential steps of metabolic activation via a cascade of cytochrome P450 enzymatic hydroxylations in the liver and kidney to produce 1α,25-dihydroxyvitamin D (1α,25(OH)2 D). There are many tissues that are able to synthesize 1α,25(OH)2 D, but the biological significance of extrarenal hydroxylases is unresolved. Human marrow-derived mesenchymal stem cells (marrow stromal cells, hMSCs) give rise to osteoblasts, and their differentiation is stimulated by 1α,25(OH)2 D. In addition to being targets of 1α,25(OH)2 D, hMSCs can synthesize it; on the basis of those observations, we further examined the local autocrine/paracrine role of vitamin D metabolism in osteoblast differentiation. Research with hMSCs from well-characterized subjects provides an innovative opportunity to evaluate the effects of clinical attributes on the regulation of hMSCs. Like the renal 1α-hydroxylase, the enzyme in hMSCs is constitutively decreased with age and chronic kidney disease (CKD); both are regulated by PTH1-34, insulin-like growth factor 1, calcium, 1α,25(OH)2 D, 25(OH)D, and fibroblast growth factor 23. CKD is associated with impaired renal biosynthesis of 1α,25(OH)2 D, low bone mass, and increased fracture risk. Studies with hMSCs from CKD patients or aged subjects indicate that circulating 25(OH)D may have an important role in osteoblast differentiation on vitamin D metabolism and action in hMSCs.

Soluble Factors on Stage to Direct Mesenchymal Stem Cells Fate

Mesenchymal stem cells (MSCs) are multipotent stromal cells that are identified by in vitro plastic adherence, colony-forming capacity, expression of a panel of surface molecules, and ability to differentiate at least toward osteogenic, adipogenic, and chondrogenic lineages. They also produce trophic factors with immunomodulatory, proangiogenic, and antiapoptotic functions influencing the behavior of neighboring cells. On the other hand, a reciprocal regulation takes place; in fact, MSCs can be isolated from several tissues, and depending on the original microenvironment and the range of stimuli received from there, they can display differences in their essential characteristics. Here, we focus mainly on the bone tissue and how soluble factors, such as growth factors, cytokines, and hormones, present in this microenvironment can orchestrate bone marrow-derived MSCs fate. We also briefly describe the alteration of MSCs behavior in pathological settings such as hematological cancer, bone metastasis, and bone marrow failure syndromes. Overall, the possibility to modulate MSCs plasticity makes them an attractive tool for diverse applications of tissue regeneration in cell therapy. Therefore, the comprehensive understanding of the microenvironment characteristics and components better suited to obtain a specific MSCs response can be extremely useful for clinical use.

Physiological Bone Remodeling: Systemic Regulation and Growth Factor Involvement

Bone remodeling is essential for adult bone homeostasis. It comprises two phases: bone formation and resorption. The balance between the two phases is crucial for sustaining bone mass and systemic mineral homeostasis. This review highlights recent work on physiological bone remodeling and discusses our knowledge of how systemic and growth factors regulate this process.

Nicotinamide phosphoribosyltransferase (Nampt) may serve as the marker for osteoblast differentiation of bone marrow-derived mesenchymal stem cells

Decreased bone volume and strength with aging and enhanced risk of fractures are in part due to reduced number of bone-forming mesenchymal stem cells (MSCs) and cellular dysfunction. In a previous study, we found that osteogenic differentiation of the multipotent and omnipotent preosteoblasts are accompanied by the alterations of intracellular NAD metabolism in which nicotinamide phosphoribosyltransferase (Nampt) plays a regulatory role. The increased Nampt during osteoblast differentiation, the enzyme catalyzing NAD resynthesis from nicotinamide was noted. However, whether Nampt will also be able to affect osteogenic differentiation of primary bone marrow-derived mesenchymal stem cells (BM-MSCs), it is still uncertain. Here we report the role of Nampt in regulating osteoblast differentiation in primary mouse BM-MSCs. We found that Nampt expression was progressively elevated during BM-MSCs osteogenic differentiation. The Nampt inhibitor FK866 or knock-down of Nampt in BM-MSCs led to declined osteoblastogenesis, including attenuated ALP activity, diminished matrix mineralization and down-regulated osteoblast specific marker genes. In addition, declined osteoblastogenesis by Nampt deficiency or addition of FK866 was related to lower intracellular NAD concentration and decreased Sirt1 activity. The present findings demonstrate that osteogenic differentiation in MSCs can be modulated by intracellular NAD metabolism, in which Nampt may serve as an applicable marker for the osteoblast determination.

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.

New insights into the FGF23-Klotho axis

Abnormal mineral metabolism is a hallmark in patients with advanced chronic kidney disease (CKD). Hyperphosphatemia, and the homeostatic mechanisms controlling phosphate metabolism, have received particular attention over the past decade. The phosphate-regulating hormone fibroblast growth factor-23 (FGF23) was discovered through studies of rare hypophosphatemic disorders, whereas Klotho, which subsequently turned out to be a co-receptor for FGF23, was identified in a mouse model showing hyperphosphatemia and multiple aging-like traits. The FGF23-Klotho endocrine axis is a pivotal regulator of mineral metabolism. In CKD, early onset of Klotho deficiency contributes to renal FGF23 resistance and a maladaptive increase in circulating FGF23. FGF23 is an early biomarker of renal injury and increased FGF23 predicts adverse clinical outcomes, in particular cardiovascular disease. A paradigm of FGF23 excess and Klotho deficiency is proposed, in which FGF23 preferentially stimulates left ventricular hypertrophy, and loss of Klotho augments fibrosis, endothelial dysfunction, and vascular calcification. The clinical benefit of FGF23 and Klotho measurements remain uncertain, nevertheless, the FGF23-Klotho axis is a solid candidate for a novel diagnostic and therapeutic target in CKD.

Myogenic-specific ablation of Fgfr1 impairs FGF2-mediated proliferation of satellite cells at the myofiber niche but does not abolish the capacity for muscle regeneration

Skeletal muscle satellite cells (SCs) are Pax7⁺ myogenic stem cells that reside between the basal lamina and the plasmalemma of the myofiber. In mature muscles, SCs are typically quiescent, but can be activated in response to muscle injury. Depending on the magnitude of tissue trauma, SCs may divide minimally to repair subtle damage within individual myofibers or produce a larger progeny pool that forms new myofibers in cases of overt muscle injury. SC transition through proliferation, differentiation and renewal is governed by the molecular blueprint of the cells as well as by the extracellular milieu at the SC niche. In particular, the role of the fibroblast growth factor (FGF) family in regulating SCs during growth and aging is well recognized. Of the several FGFs shown to affect SCs, FGF1, FGF2, and FGF6 proteins have been documented in adult skeletal muscle. These prototypic paracrine FGFs transmit their mitogenic effect through the FGFRs, which are transmembrane tyrosine kinase receptors. Using the mouse model, we show here that of the four Fgfr genes, only Fgfr1 and Fgfr4 are expressed at relatively high levels in quiescent SCs and their proliferating progeny. To further investigate the role of FGFR1 in adult myogenesis, we have employed a genetic (Cre/loxP) approach for myogenic-specific (MyoD^Cre-driven) ablation of Fgfr1. Neither muscle histology nor muscle regeneration following cardiotoxin-induced injury were overtly affected in Fgfr1-ablated mice. This suggests that FGFR1 is not obligatory for SC performance in this acute muscle trauma model, where compensatory growth factor/cytokine regulatory cascades may exist. However, the SC mitogenic response to FGF2 is drastically repressed in isolated myofibers prepared from Fgfr1-ablated mice. Collectively, our study indicates that FGFR1 is important for FGF-mediated proliferation of SCs and its mitogenic role is not compensated by FGFR4 that is also highly expressed in SCs.

Secreted klotho protein attenuates osteogenic differentiation of human bone marrow mesenchymal stem cells in vitro via inactivation of the FGFR1/ERK signaling pathway

Increasing evidence indicates that the osteogenic differentiation of mesenchymal stem cells (MSCs) is related to bone formation, heterotopic ossification, and even vascular calcification. Therefore, it is essential to understand the microenvironment that regulates these processes. The Klotho gene plays an important role in tissue mineralization, and its secreted protein functions as a hormone. We investigated the effects of secreted Klotho protein on the osteogenesis of human bone marrow MSC (hBMSCs). To this end, the cells received osteogenic medium with or without Klotho protein. The results showed that osteoblast-specific gene expression and mineral deposition were decreased when MSCs were incubated with Klotho. Klotho reduced the expression of fibroblast growth factor receptor 1 (FGFR1) and phosphorylated extracellular signal-regulated kinase 1/2. However, both MEK and FGFR1 inhibitors delayed bone mineral formation more than Klotho. These data suggest that secreted Klotho protein attenuates the osteogenic differentiation of hBMSCs in vitro through FGFR1/ERK signaling.

Bone mineralization is regulated by signaling cross talk between molecular factors of local and systemic origin: the role of fibroblast growth factor 23

Body phosphate homeostasis is regulated by a hormonal counter-balanced intestine-bone-kidney axis. The major systemic hormones involved in this axis are parathyroid hormone (PTH), 1,25-dihydroxyvitamin-D, and fibroblast growth factor-23 (FGF23). FGF23, produced almost exclusively by the osteocytes, is a phosphaturic hormone that plays a major role in regulation of the bone remodeling process. Remodeling composite components, bone mineralization and resorption cycles create a continuous influx-efflux loop of the inorganic phosphate (Pi) through the skeleton. This "bone Pi loop," which is formed, is controlled by local and systemic factors according to phosphate homeostasis demands. Although FGF23 systemic actions in the kidney, and for the production of PTH and 1,25-dihydroxyvitamin-D are well established, its direct involvement in bone metabolism is currently poorly understood. This review presents the latest available evidence suggesting two aspects of FGF23 bone local activity: (a) Regulation of FGF23 production by both local and systemic factors. The suggested local factors include extracellular levels of Pi and pyrophosphate (PPi), (the Pi/PPi ratio), and another osteocyte-derived protein, sclerostin. In addition, 1,25-dihydroxyvitamin-D, synthesized locally by bone cells, may contribute to regulation of FGF23 production. The systemic control is achieved via PTH and 1,25-dihydroxyvitamin-D endocrine functions. (b) FGF23 acts as a local agent, directly affecting bone mineralization. We support the assumption that under balanced physiological conditions, sclerostin, by para- autocrine signaling, upregulates FGF23 production by the osteocyte. FGF23, in turn, acts as a mineralization inhibitor, by stimulating the generation of the major mineralization antagonist-PPi.