Klotho Lacks an FGF23‐Independent Role in Mineral Homeostasis

Fibroblast growth factor‐23 (FGF23) is a bone‐derived hormone regulating vitamin D hormone production and renal handling of minerals by signaling through an FGF receptor/αKlotho (Klotho) receptor complex. Whether Klotho has FGF23‐independent effects on mineral homeostasis is a controversial issue. Here, we aimed to shed more light on this controversy by comparing male and female triple knockout mice with simultaneous deficiency in Fgf23 and Klotho and a nonfunctioning vitamin D receptor (VDR) (Fgf23/Klotho/VDR) with double (Fgf23/VDR, Klotho/VDR, and Fgf23/Klotho) and single Fgf23, Klotho, and VDR mutants. As expected, 4‐week‐old Fgf23, Klotho, and Fgf23/Klotho knockout mice were hypercalcemic and hyperphosphatemic, whereas VDR, Fgf23/VDR, and Klotho/VDR mice on rescue diet were normocalcemic and normophosphatemic. Serum levels of calcium, phosphate, and sodium did not differ between 4‐week‐old triple Fgf23/Klotho/VDR and double Fgf23/VDR or Klotho/VDR knockout mice. Notably, 3‐month‐old Fgf23/Klotho/VDR triple knockout mice were indistinguishable from double Fgf23/VDR and Klotho/VDR compound mutants in terms of serum calcium, serum phosphate, serum sodium, and serum PTH, as well as urinary calcium and sodium excretion. Protein expression analysis revealed increased membrane abundance of sodium‐phosphate co‐transporter 2a (NaPi‐2a), and decreased expression of sodium‐chloride co‐transporter (NCC) and transient receptor potential cation channel subfamily V member 5 (TRPV5) in Fgf23/Klotho/VDR, Fgf23/VDR, and Klotho/VDR mice, relative to wild‐type and VDR mice, but no differences between triple and double knockouts. Further, ex vivo treatment of live kidney slices isolated from wild‐type and Klotho/VDR mice with soluble Klotho did not induce changes in intracellular phosphate, calcium or sodium accumulation assessed by two‐photon microscopy. In conclusion, our data suggest that the main physiological function of Klotho for mineral homeostasis in vivo is its role as co‐receptor mediating Fgf23 action. © 2017 American Society for Bone and Mineral Research.     

Pharmacological inhibition of fibroblast growth factor (FGF) receptor signaling ameliorates FGF23‐mediated hypophosphatemic rickets

Fibroblast growth factor 23 (FGF23) is a circulating factor secreted by osteocytes that is essential for phosphate homeostasis. In kidney proximal tubular cells FGF23 inhibits phosphate reabsorption and leads to decreased synthesis and enhanced catabolism of 1,25‐dihydroxyvitamin D₃ (1,25[OH]₂D₃). Excess levels of FGF23 cause renal phosphate wasting and suppression of circulating 1,25(OH)₂D₃ levels and are associated with several hereditary hypophosphatemic disorders with skeletal abnormalities, including X‐linked hypophosphatemic rickets (XLH) and autosomal recessive hypophosphatemic rickets (ARHR). Currently, therapeutic approaches to these diseases are limited to treatment with activated vitamin D analogues and phosphate supplementation, often merely resulting in partial correction of the skeletal aberrations. In this study, we evaluate the use of FGFR inhibitors for the treatment of FGF23‐mediated hypophosphatemic disorders using NVP‐BGJ398, a novel selective, pan‐specific FGFR inhibitor currently in Phase I clinical trials for cancer therapy. In two different hypophosphatemic mouse models, Hyp and Dmp1‐null mice, resembling the human diseases XLH and ARHR, we find that pharmacological inhibition of FGFRs efficiently abrogates aberrant FGF23 signaling and normalizes the hypophosphatemic and hypocalcemic conditions of these mice. Correspondingly, long‐term FGFR inhibition in Hyp mice leads to enhanced bone growth, increased mineralization, and reorganization of the disturbed growth plate structure. We therefore propose NVP‐BGJ398 treatment as a novel approach for the therapy of FGF23‐mediated hypophosphatemic diseases. © 2013 American Society for Bone and Mineral Research.     

Associations of FGF23 With Change in Bone Mineral Density and Fracture Risk in Older Individuals

Elevated levels of the phosphate‐regulating hormone fibroblast growth factor 23 (FGF23) have been linked to greater risk of fractures in some studies, especially among individuals with chronic kidney disease (CKD). We evaluated FGF23 as a risk factor for bone loss and fractures in the Health, Aging, and Body Composition (Health ABC) study, which is a prospective biracial cohort of well‐functioning adults aged 70 to 79 years recruited at two clinical centers in the United States. The sample for the bone mineral density (BMD) analyses consisted of 2234 participants who had at least two serial total hip areal BMD measures. The fracture analyses included 2786 participants, 567 of whom sustained a fracture during a median follow up of 4.95 years. Linear mixed‐effects models were used for longitudinal measurements of total hip areal BMD and the proportional subdistribution hazard regression model subject to competing risks of death was used for risk of fracture. The median FGF23 was 46.7 (interquartile range [IQR] 36.7 to 60.2) pg/mL. The mean annualized percent change in total hip areal BMD did not vary significantly according to FGF23 quartile in all participants (p for trend = 0.70), but the effect was modified by CKD status (adjusted p for interaction <0.001). Among participants with CKD, the unadjusted mean annualized percent change in total hip areal BMD was greater with higher levels of FGF23 (unadjusted p for trend = 0.02), but the trend was attenuated with adjustment for estimated glomerular filtration rate and parathyroid hormone (adjusted p for trend = 0.30). FGF23 was not significantly associated with fracture risk in crude (hazard ratio [HR] per doubling of FGF23, 0.97; 95% CI, 0.85 to 1.12) or adjusted models (HR per doubling of FGF23, 1.02; 95% CI, 0.86 to 1.22), and these findings were not modified by gender or CKD status. FGF23 levels are not associated with bone loss or fracture risk in older adults with low prevalence of CKD. © 2015 American Society for Bone and Mineral Research.     

Therapeutic Effects of FGF23 c‐tail Fc in a Murine Preclinical Model of X‐Linked Hypophosphatemia Via the Selective Modulation of Phosphate Reabsorption

Fibroblast growth factor 23 (FGF23) is the causative factor of X‐linked hypophosphatemia (XLH), a genetic disorder effecting 1:20,000 that is characterized by excessive phosphate excretion, elevated FGF23 levels and a rickets/osteomalacia phenotype. FGF23 inhibits phosphate reabsorption and suppresses 1α,25‐dihydroxyvitamin D (1,25D) biosynthesis, analytes that differentially contribute to bone integrity and deleterious soft‐tissue mineralization. As inhibition of ligand broadly modulates downstream targets, balancing efficacy and unwanted toxicity is difficult when targeting the FGF23 pathway. We demonstrate that a FGF23 c‐tail‐Fc fusion molecule selectively modulates the phosphate pathway in vivo by competitive antagonism of FGF23 binding to the FGFR/α klotho receptor complex. Repeated injection of FGF23 c‐tail Fc in Hyp mice, a preclinical model of XLH, increases cell surface abundance of kidney NaPi transporters, normalizes phosphate excretion, and significantly improves bone architecture in the absence of soft‐tissue mineralization. Repeated injection does not modulate either 1,25D or calcium in a physiologically relevant manner in either a wild‐type or disease setting. These data suggest that bone integrity can be improved in models of XLH via the exclusive modulation of phosphate. We posit that the selective modulation of the phosphate pathway will increase the window between efficacy and safety risks, allowing increased efficacy to be achieved in the treatment of this chronic disease. © 2017 American Society for Bone and Mineral Research.     

Serum fibroblast growth factor‐23 (FGF‐23) and fracture risk in elderly men

A normal mineral metabolism is integral for skeletal development and preservation of bone integrity. Fibroblast growth factor 23 (FGF‐23) is a bone‐derived circulating factor that decreases serum concentrations of inorganic phosphorous (P_i) and 1,25‐dihydroxyvitamin D₃ [1,25(OH)₂D₃]. Increased FGF‐23 expression is a direct or indirect culprit in several skeletal disorders; however, the relation between FGF‐23 and fracture risk remains undetermined. We evaluated the prospective relation between serum intact FGF‐23 (measured by a two‐site monoclonal antibody ELISA) and fracture risk employing the Swedish part of the population‐based Osteoporotic Fractures in Men Study (MrOS; n = 2868; mean age 75.4 ± 3.2 years; median follow‐up period 3.35 years). The incidence of at least one validated fracture after baseline was 20.4 per 1000 person‐years. FGF‐23 was directly related to the overall fracture risk [age‐adjusted hazard ratio (HR) per SD increase = 1.20, 95% confidence interval (CI) 1.03–1.40] and vertebral fracture risk (HR = 1.33, 95% CI 1.02–1.75). Spline models revealed a nonlinear relation between FGF‐23 and fracture risk, with the strongest relation at FGF‐23 levels above 55.7 pg/mL. FGF‐23 levels above 55.7 pg/mL also were associated with an increased risk for hip and nonvertebral fractures (HR = 2.30, 95% CI 1.16–4.58, and HR = 1.63, 95% CI 1.01–2.63, respectively). These relations remained essentially unaltered after adjustment for bodymass index (BMI), bone mineral density (BMD), glomerular filtration rate, 25(OH)₂D₃, parathyroid hormone (PTH), and other fracture risk factors. In conclusion, FGF‐23 is a novel predictor of fracture risk in elderly men. © 2011 American Society for Bone and Mineral Research.     

A Randomized,Double‐Blind,Placebo‐Controlled,Phase 3 Trial Evaluating the Efficacy of Burosumab,an Anti‐FGF23 Antibody,in Adults With X‐Linked Hypophosphatemia: Week 24 Primary Analysis

In X‐linked hypophosphatemia (XLH), inherited loss‐of‐function mutations in the PHEX gene cause excess circulating levels of fibroblast growth factor 23 (FGF23), leading to lifelong renal phosphate wasting and hypophosphatemia. Adults with XLH present with chronic musculoskeletal pain and stiffness, short stature, lower limb deformities, fractures, and pseudofractures due to osteomalacia, accelerated osteoarthritis, dental abscesses, and enthesopathy. Burosumab, a fully human monoclonal antibody, binds and inhibits FGF23 to correct hypophosphatemia. This report summarizes results from a double‐blind, placebo‐controlled, phase 3 trial of burosumab in symptomatic adults with XLH. Participants with hypophosphatemia and pain were assigned 1:1 to burosumab 1 mg/kg (n = 68) or placebo (n = 66) subcutaneously every 4 weeks (Q4W) and were comparable at baseline. Across midpoints of dosing intervals, 94.1% of burosumab‐treated participants attained mean serum phosphate concentration above the lower limit of normal compared with 7.6% of those receiving placebo (p < 0.001). Burosumab significantly reduced the Western Ontario and the McMaster Universities Osteoarthritis Index (WOMAC) stiffness subscale compared with placebo (least squares [LS] mean ± standard error [SE] difference, –8.1 ± 3.24; p = 0.012). Reductions in WOMAC physical function subscale (–4.9 ± 2.48; p = 0.048) and Brief Pain Inventory worst pain (–0.5 ± 0.28; p = 0.092) did not achieve statistical significance after Hochberg multiplicity adjustment. At week 24, 43.1% (burosumab) and 7.7% (placebo) of baseline active fractures were fully healed; the odds of healed fracture in the burosumab group was 16.8‐fold greater than that in the placebo group (p < 0.001). Biochemical markers of bone formation and resorption increased significantly from baseline with burosumab treatment compared with placebo. The safety profile of burosumab was similar to placebo. There were no treatment‐related serious adverse events or meaningful changes from baseline in serum or urine calcium, intact parathyroid hormone, or nephrocalcinosis. These data support the conclusion that burosumab is a novel therapeutic addressing an important medical need in adults with XLH.© 2018 The Authors. Journal of Bone and Mineral Research Published by Wiley Periodicals, Inc.     

Anti‐FGF‐23 neutralizing antibodies ameliorate muscle weakness and decreased spontaneous movement of Hyp mice

Fibroblast growth factor 23 (FGF‐23) plays causative roles in the development of several hypophosphatemic rickets/osteomalacia such as X‐linked hypophosphatemic rickets/osteomalacia (XLH) and tumor‐induced rickets/osteomalacia. Patients with hypophosphatemic rickets/osteomalacia often complain of muscle weakness and bone pain that severely affect daily activities of these patients. The purpose of this study was to examine whether anti‐FGF‐23 antibodies, which have been shown to improve hypophosphatemia and rachitic changes of juvenile Hyp mice in a murine model of XLH, also ameliorate hypophosphatemic osteomalacia and affect muscle force and spontaneous motor activity in adult Hyp mice. Repeated injections of anti‐FGF‐23 antibodies increased serum phosphate and 1,25‐dihydroxyvitmain D levels and enhanced mineralization of osteoid in adult Hyp mice, whereas bone length did not change. We found that grip strength was weaker and that spontaneous movement was less in adult Hyp mice than in wild‐type mice. In addition, FGF‐23 antibodies increased grip strength and spontaneous movement. These results suggest that the inhibition of excess FGF‐23 action not only ameliorates hypophosphatemia and impaired mineralization of bone but also improves muscle weakness and daily activities of patients with FGF‐23‐related hypophosphatemic rickets/osteomalacia. © 2011 American Society for Bone and Mineral Research.     

1,25‐Dihydroxyvitamin D Alone Improves Skeletal Growth,Microarchitecture, and Strength in a Murine Model of XLH,Despite Enhanced FGF23 Expression

X‐linked hypophosphatemia (XLH) is characterized by impaired renal tubular reabsorption of phosphate owing to increased circulating FGF23 levels, resulting in rickets in growing children and impaired bone mineralization. Increased FGF23 decreases renal brush border membrane sodium‐dependent phosphate transporter IIa (Npt2a) causing renal phosphate wasting, impairs 1‐α hydroxylation of 25‐hydroxyvitamin D, and induces the vitamin D 24‐hydroxylase, leading to inappropriately low circulating levels of 1,25‐dihydroxyvitamin D (1,25D). The goal of therapy is prevention of rickets and improvement of growth in children by phosphate and 1,25D supplementation. However, this therapy is often complicated by hypercalcemia and nephrocalcinosis and does not always prevent hyperparathyroidism. To determine if 1,25D or blocking FGF23 action can improve the skeletal phenotype without phosphate supplementation, mice with XLH (Hyp) were treated with daily 1,25D repletion, FGF23 antibodies (FGF23Ab), or biweekly high‐dose 1,25D from d2 to d75 without supplemental phosphate. All treatments maintained normocalcemia, increased serum phosphate, and normalized parathyroid hormone levels. They also prevented the loss of Npt2a, α‐Klotho, and pERK1/2 immunoreactivity observed in the kidneys of untreated Hyp mice. Daily treatment with 1,25D decreased urine phosphate losses despite a marked increase in bone FGF23 mRNA and in circulating FGF23 levels. Daily 1,25D was more effective than other treatments in normalizing the growth plate and metaphyseal organization. In addition to being the only therapy that normalized lumbar vertebral height and body weight, daily 1,25D therapy normalized bone geometry and was more effective than FGF23Ab in improving trabecular bone structure. Daily 1,25D and FGF23Ab improved cortical microarchitecture and whole‐bone biomechanical properties more so than biweekly 1,25D. Thus, monotherapy with 1,25D improves growth, skeletal microarchitecture, and bone strength in the absence of phosphate supplementation despite enhancing FGF23 expression, demonstrating that 1,25D has direct beneficial effects on the skeleton in XLH, independent of its role in phosphate homeostasis. © 2016 American Society for Bone and Mineral Research.     

Elevated Fibroblast Growth Factor 23 Exerts Its Effects on Placenta and Regulates Vitamin D Metabolism in Pregnancy of Hyp Mice

Fibroblast growth factor 23 (FGF23) functions in an endocrine fashion and requires α‐Klotho to exert its effects on the target organs. We have recently demonstrated that the human placenta also expresses α‐Klotho, which led us to hypothesize that FGF23 may exert effects on the placenta. Immunohistochemical analysis demonstrated the expression of FGF receptor 1 (FGFR1) as well as that of α‐Klotho in the feto‐maternal interface of both mouse and human normal‐term placentas, which suggested that these areas might be receptive to FGF23. Therefore, we next investigated whether FGF23 has some roles in the placenta using Hyp mice with high levels of circulating FGF23. Hyp and wild‐type (WT) females were mated with WT males, and the mothers and their male fetuses were analyzed. FGF23 levels in Hyp mothers were elevated. FGF23 levels were about 20‐fold higher in Hyp fetuses than in Hyp mothers, whereas WT fetuses from Hyp mothers exhibited low levels of FGF23, as did fetuses from WT mothers. We analyzed the placental gene expression and found that the expression of Cyp24a1 encoding 25OHD‐24‐hydroxylase, a target gene for FGF23 in the kidney, was increased in the placentas of fetuses from Hyp mothers compared with fetuses from WT mothers. In an organ culture of WT placentas, treatment with plasma from Hyp mothers markedly increased the expression of Cyp24a1, which was abolished by the simultaneous addition of anti‐FGF23 neutralizing antibody. The direct injection of recombinant FGF23 into WT placentas induced the expression of Cyp24a1. The increase in the placental expression of Cyp24a1 in fetuses from Hyp mothers resulted in decreased plasma 25‐hydroxyvitamin D levels. These results suggest that increased levels of circulating FGF23 in pathological conditions such as Hyp mice exerts direct effects on the placenta and affects fetal vitamin D metabolism via the regulation of Cyp24a1 expression. © 2014 American Society for Bone and Mineral Research.     

Biomarkers in WNT1 and PLS3 Osteoporosis: Altered Concentrations of DKK1 and FGF23

Recent advancements in genetic research have uncovered new forms of monogenic osteoporosis, expanding our understanding of the molecular pathways regulating bone health. Despite active research, knowledge on the pathomechanisms, disease-specific biomarkers, and optimal treatment in these disorders is still limited. Mutations in WNT1, encoding a WNT/β-catenin pathway ligand WNT1, and PLS3, encoding X chromosomally inherited plastin 3 (PLS3), both result in early-onset osteoporosis with prevalent fractures and disrupted bone metabolism. However, despite marked skeletal pathology, conventional bone markers are usually normal in both diseases. Our study aimed to identify novel bone markers in PLS3 and WNT1 osteoporosis that could offer diagnostic potential and shed light on the mechanisms behind these skeletal pathologies. We measured several parameters of bone metabolism, including serum dickkopf-1 (DKK1), sclerostin, and intact and C-terminal fibroblast growth factor 23 (FGF23) concentrations in 17 WNT1 and 14 PLS3 mutation-positive subjects. Findings were compared with 34 healthy mutation-negative subjects from the same families. Results confirmed normal concentrations of conventional metabolic bone markers in both groups. DKK1 concentrations were significantly elevated in PLS3 mutation-positive subjects compared with WNT1 mutation-positive subjects (p < .001) or the mutation-negative subjects (p = .002). Similar differences were not seen in WNT1 subjects. Sclerostin concentrations did not differ between any groups. Both intact and C-terminal FGF23 were significantly elevated in WNT1 mutation-positive subjects (p = .039 and p = .027, respectively) and normal in PLS3 subjects. Our results indicate a link between PLS3 and DKK1 and WNT1 and FGF23 in bone metabolism. The normal sclerostin and DKK1 levels in patients with impaired WNT signaling suggest another parallel regulatory mechanism. These findings provide novel information on the molecular networks in bone. Extended studies are needed to investigate whether these biomarkers offer diagnostic value or potential as treatment targets in osteoporosis. © 2020 American Society for Bone and Mineral Research.     

Neurotrophin‐3 Induces BMP‐2 and VEGF Activities and Promotes the Bony Repair of Injured Growth Plate Cartilage and Bone in Rats

Injured growth plate is often repaired by bony tissue causing bone growth defects, for which the mechanisms remain unclear. Because neurotrophins have been implicated in bone fracture repair, here we investigated their potential roles in growth plate bony repair in rats. After a drill‐hole injury was made in the tibial growth plate and bone, increased injury site mRNA expression was observed for neurotrophins NGF, BDNF, NT‐3, and NT‐4 and their Trk receptors. NT‐3 and its receptor TrkC showed the highest induction. NT‐3 was localized to repairing cells, whereas TrkC was observed in stromal cells, osteoblasts, and blood vessel cells at the injury site. Moreover, systemic NT‐3 immunoneutralization reduced bone volume at injury sites and also reduced vascularization at the injured growth plate, whereas recombinant NT‐3 treatment promoted bony repair with elevated levels of mRNA for osteogenic markers and bone morphogenetic protein (BMP‐2) and increased vascularization and mRNA for vascular endothelial growth factor (VEGF) and endothelial cell marker CD31 at the injured growth plate. When examined in vitro, NT‐3 promoted osteogenesis in rat bone marrow stromal cells, induced Erk1/2 and Akt phosphorylation, and enhanced expression of BMPs (particularly BMP‐2) and VEGF in the mineralizing cells. It also induced CD31 and VEGF mRNA in rat primary endothelial cell culture. BMP activity appears critical for NT‐3 osteogenic effect in vitro because it can be almost completely abrogated by co‐addition of the BMP inhibitor noggin. Consistent with its angiogenic effect in vivo, NT‐3 promoted angiogenesis in metatarsal bone explants, an effect abolished by co‐treatment with anti‐VEGF. This study suggests that NT‐3 may be an osteogenic and angiogenic factor upstream of BMP‐2 and VEGF in bony repair, and further studies are required to investigate whether NT‐3 may be a potential target for preventing growth plate faulty bony repair or for promoting bone fracture healing. © 2016 American Society for Bone and Mineral Research.     

Associations of 25‐Hydroxyvitamin D and 1,25‐Dihydroxyvitamin D With Bone Mineral Density,Bone Mineral Density Change,and Incident Nonvertebral Fracture

Relationships between 1,25‐dihydroxyvitamin D (1,25(OH)₂D) and skeletal outcomes are uncertain. We examined the associations of 1,25(OH)₂D with bone mineral density (BMD), BMD change, and incident non‐vertebral fractures in a cohort of older men and compared them with those of 25‐hydroxyvitamin D (25OHD). The study population included 1000 men (aged 74.6 ± 6.2 years) in the Osteoporotic Fractures in Men (MrOS) study, of which 537 men had longitudinal dual‐energy X‐ray absorptiometry (DXA) data (4.5 years of follow‐up). A case‐cohort design and Cox proportional hazards models were used to test the association between vitamin D metabolite levels and incident nonvertebral and hip fractures. Linear regression models were used to estimate the association between vitamin D measures and baseline BMD and BMD change. Interactions between 25OHD and 1,25(OH)₂D were tested for each outcome. Over an average follow‐up of 5.1 years, 432 men experienced incident nonvertebral fractures, including 81 hip fractures. Higher 25OHD was associated with higher baseline BMD, slower BMD loss, and lower hip fracture risk. Conversely, men with higher 1,25(OH)₂D had lower baseline BMD. 1,25(OH)₂D was not associated with BMD loss or nonvertebral fracture. Compared with higher levels of calcitriol, the risk of hip fracture was higher in men with the lowest 1,25(OH)₂D levels (8.70 to 51.60 pg/mL) after adjustment for baseline hip BMD (hazard ratio [HR] = 1.99, 95% confidence interval [CI] 1.19–3.33). Adjustment of 1,25(OH)₂D data for 25OHD (and vice versa) had little effect on the associations observed but did attenuate the hip fracture association of both vitamin D metabolites. In older men, higher 1,25(OH)₂D was associated with lower baseline BMD but was not related to the rate of bone loss or nonvertebral fracture risk. However, with BMD adjustment, a protective association for hip fracture was found with higher 1,25(OH)₂D. The associations of 25OHD with skeletal outcomes were generally stronger than those for 1,25(OH)₂D. These results do not support the hypothesis that measures of 1,25(OH)₂D improve the ability to predict adverse skeletal outcomes when 25OHD measures are available. © 2015 American Society for Bone and Mineral Research.     

PTH(1‐84) Administration in Hypoparathyroidism Transiently Reduces Bone Matrix Mineralization

Patients with hypoparathyroidism have low circulating parathyroid (PTH) levels and higher cancellous bone volume and trabecular thickness. Treatment with PTH(1‐84) was shown to increase abnormally low bone remodeling dynamics. In this work, we studied the effect of 1‐year or 2‐year PTH(1‐84) treatment on cancellous and cortical bone mineralization density distribution (Cn.BMDD and Ct.BMDD) based on quantitative backscattered electron imaging (qBEI) in paired transiliac bone biopsy samples. The study cohort comprised 30 adult hypoparathyroid patients (14 treated for 1 year; 16 treated for 2 years). At baseline, Cn.BMDD was shifted to higher mineralization densities in both treatment groups (average degree of mineralization Cn.Ca_Mean +3.9% and +2.7%, p < 0.001) compared to reference BMDD. After 1‐year PTH(1‐84), Cn.Ca_Mean was significantly lower than that at baseline (–6.3%, p < 0.001), whereas in the 2‐year PTH(1‐84) group Cn.Ca_Mean did not differ from baseline. Significant changes of Ct.BMDD were observed in the 1‐year treatment group only. The change in histomorphometric bone formation (mineralizing surface) was predictive for Cn.BMDD outcomes in the 1‐year PTH(1‐84) group, but not in the 2‐year PTH(1‐84) group. Our findings suggest higher baseline bone matrix mineralization consistent with the decreased bone turnover in hypoparathyroidism. PTH(1‐84) treatment caused differential effects dependent on treatment duration that were consistent with the histomorphometric bone formation outcomes. The greater increase in bone formation during the first year of treatment was associated with a decrease in bone matrix mineralization, suggesting that PTH(1‐84) exposure to the hypoparathyroid skeleton has the greatest effects on BMDD early in treatment. © 2015 American Society for Bone and Mineral Research.     

Fibroblast growth factor 23 regulates renal 1,25‐dihydroxyvitamin D and phosphate metabolism via the MAP kinase signaling pathway in Hyp mice

In X‐linked hypophosphatemia (XLH) and in its murine homologue, the Hyp mouse, increased circulating concentrations of fibroblast growth factor 23 (FGF‐23) are critical to the pathogenesis of disordered metabolism of phosphate (P_i) and 1,25‐dihydroxyvitamin D [1,25(OH)₂D]. In this study, we hypothesized that in Hyp mice, FGF‐23‐mediated suppression of renal 1,25(OH)₂D production and P_i reabsorption depends on activation of mitogen‐activated protein kinase (MAPK) signaling. Wild‐type and Hyp mice were administered either vehicle or the MEK inhibitor PD0325901 (12.5 mg/kg) orally daily for 4 days. At baseline, the renal abundance of early growth response 1 (egr1) mRNA was approximately 2‐fold greater in Hyp mice than in wild‐type mice. Treatment with PD0325901 greatly suppressed egr1 mRNA abundance in both wild‐type and Hyp mice. In Hyp mice, PD0325901 induced an 8‐fold increase in renal 1α‐hydroxylase mRNA expression and a 4‐fold increase in serum 1,25(OH)₂D concentrations compared with vehicle‐treated Hyp mice. Serum P_i levels in Hyp mice increased significantly after treatment with PD0325901, and the increase was associated with increased renal Npt2a mRNA abundance and brush‐border membrane Npt2a protein expression. These findings provide evidence that in Hyp mice, MAPK signaling is constitutively activated in the kidney and support the hypothesis that the FGF‐23‐mediated suppression of renal 1,25(OH)₂D production and P_i reabsorption depends on activation of MAPK signaling via MEK/ERK1/2. These findings demonstrate the physiologic importance of MAPK signaling in the actions of FGF‐23 in regulating renal 1,25(OH)₂D and P_i metabolism. © 2011 American Society for Bone and Mineral Research     

Effects of hPTH(1‐34) Infusion on Circulating Serum Phosphate, 1,25‐Dihydroxyvitamin D,and FGF23 Levels in Healthy Men

Fibroblast growth factor 23 (FGF23) promotes phosphaturia and suppresses 1,25‐dihydroxyvitamin D [1,25(OH)₂D] production. PTH also promotes phosphaturia, but, in contrast, stimulates 1,25(OH)₂D production. The relationship between FGF23 and PTH is unclear, and the acute effect of pharmacologically dosed PTH on FGF23 secretion is unknown. Twenty healthy men were infused with human PTH(1‐34) [hPTH(1‐34)] at 44 ng/kg/h for 24 h. Compared with baseline, FGF23, 1,25(OH)₂D, ionized calcium (iCa), and serum N‐telopeptide (NTX) increased significantly over the 18‐h hPTH(1‐34) infusion (p < 0.0001), whereas serum phosphate (PO₄) transiently increased and then returned to baseline. FGF23 increased from 35 ± 10 pg/ml at baseline to 53 ± 20 pg/ml at 18 h (p = 0.0002); 1,25(OH)₂D increased from 36 ± 16 pg/ml at baseline to 80 ± 33 pg/ml at 18 h (p < 0.0001); iCa increased from 1.23 ± 0.03 mM at baseline to 1.46 ± 0.05 mM at hour 18 (p < 0.0001); and NTX increased from 17 ± 4 nM BCE at baseline to 28 ± 8 nM BCE at peak (p < 0.0001). PO₄ was 3.3 ± 0.6 mg/dl at baseline, transiently rose to 3.7 ± 0.4 mg/dl at hour 6 (p = 0.016), and then returned to 3.4 ± 0.5 mg/dl at hour 12 (p = 0.651). hPTH(1‐34) infusion increases endogenous 1,25(OH)₂D and FGF23 within 18 h in healthy men. Whereas it is possible that the rise in PO₄ contributed to the observed increase in FGF23, the increase in 1,25(OH)₂D was more substantial and longer sustained than the change in serum phosphate. Given prior data that suggest that neither PTH nor calcium stimulate FGF23 secretion, these data support the assertion that 1,25(OH)₂D is a potent physiologic stimulator of FGF23 secretion.