Circulating fibroblast growth factor-23 increases following intermittent parathyroid hormone (1-34) in postmenopausal osteoporosis: association with biomarker of bone formation

The Molecular Mechanisms Underlying the Systemic Effects Mediated by Parathormone in the Context of Chronic Kidney Disease

Chronic kidney disease (CKD) stands as a prominent non-communicable ailment, significantly impacting life expectancy. Physiopathology stands mainly upon the triangle represented by parathormone-Vitamin D-Fibroblast Growth Factor-23. Parathormone (PTH), the key hormone in mineral homeostasis, is one of the less easily modifiable parameters in CKD; however, it stands as a significant marker for assessing the risk of complications. The updated "trade-off hypothesis" reveals that levels of PTH spike out of the normal range as early as stage G2 CKD, advancing it as a possible determinant of systemic damage. The present review aims to review the effects exhibited by PTH on several organs while linking the molecular mechanisms to the observed actions in the context of CKD. From a diagnostic perspective, PTH is the most reliable and accessible biochemical marker in CKD, but its trend bears a higher significance on a patient's prognosis rather than the absolute value. Classically, PTH acts in a dichotomous manner on bone tissue, maintaining a balance between formation and resorption. Under the uremic conditions of advanced CKD, the altered intestinal microbiota majorly tips the balance towards bone lysis. Probiotic treatment has proven reliable in animal models, but in humans, data are limited. Regarding bone status, persistently high levels of PTH determine a reduction in mineral density and a concurrent increase in fracture risk. Pharmacological manipulation of serum PTH requires appropriate patient selection and monitoring since dangerously low levels of PTH may completely inhibit bone turnover. Moreover, the altered mineral balance extends to the cardiovascular system, promoting vascular calcifications. Lastly, the involvement of PTH in the Renin-Angiotensin-Aldosterone axis highlights the importance of opting for the appropriate pharmacological agent should hypertension develop.

The rationale for intermittent administration of PTH in the management of mineral and bone disorder of chronic kidney disease

A major complication of chronic kidney disease is the derangement of mineral metabolism, leading to increased risk of fractures and cardiovascular mortality. Current therapeutic regimens are focused on reducing parathyroid hormone levels caused by secondary hyperparathyroidism, and the active vitamin D metabolite l,25(OH)2D, with limited success. It may be a more effective approach, however, if we could target the delayed response of parathyroid hormone in the early retention of phosphate following loss of renal function.We propose intermittent administration (even in stage 2 chronic kidney disease) of parathyroid hormone, known for its bone anabolic effects compared to the catabolic effects of the continuously elevated parathyroid hormone associated with the hyperparathyroid state, to mitigate the retention of phosphate. This approach may prevent the compensatory responses of the other two major calcium- and phosphate-regulating hormones (FGF-23 and l,25(OH)2D) that lead to further worsening of the derangement of mineral metabolism.In addition to its strong theoretical basis, there are data supporting the need for further research focused on the use of intermittent parathyroid hormone in the management of chronic kidney disease-mineral bone disorder.

The Role of Fibroblast Growth Factor 19 Subfamily in Different Populations Suffering From Osteoporosis

Fibroblast growth factor (FGF) 19 subfamily, also known as endocrine fibroblast growth factors (FGFs), is a newly discovered metabolic regulator, including FGF19, FGF21 and FGF23. They play significant roles in maintaining systemic homeostasis, regulating the balance of bile acid and glucolipid metabolism in humans. Osteoporosis is a chronic disease, especially in the current status of aging population, osteoporosis is the most prominent chronic bone disease, leading to multiple complications and a significant economic burden that requires long-term or even lifelong management. Members of the FGF family have been shown to be associated with bone mineral density (BMD), fracture repair and cartilage regeneration. Studies of the FGF19 subfamily in different populations with osteoporosis have been increasing in recent years. This review summarizes the role of the FGF19 subfamily in bone metabolism, and provides new options for the treatment of bone diseases such as osteoporosis.

Understanding the Stony Bridge between Osteoporosis and Vascular Calcification: Impact of the FGF23/Klotho axis

A relationship between osteoporosis (OP) and vascular calcification (VC) is now proposed. There are common mechanisms underlying the regulation of them. Fibroblast growth factor- (FGF-) 23 and Klotho are hormones associated with the metabolic axis of osteovascular metabolism. Most recently, it was suggested that the FGF23-klotho axis is associated with increasing incidence of fractures and is potentially involved in the progression of the aortic-brachial stiffness ratio. Herein, we discussed the potential role of the FGF23/Klotho axis in the pathophysiology of OP and VC. We want to provide an update review in order to allow a better understanding of the potential role of the FGF23/Klotho axis in comorbidity of OP and VC. We believe that a better understanding of the relationship between both entities can help in proposing new therapeutic targets for reducing the increasing prevalence of OP and VC in the aging population.

Serum 25-hydroxy-vitamin D and the risk of fractures in the teriparatide versus risedronate VERO clinical trial

PURPOSE

Using data from the 2-year, randomized, double-dummy VERO trial, we examined the changes in 25-hydroxy-vitamin D (25[OH]D) concentrations over time, and whether the fracture risk reduction of teriparatide versus risedronate varies by baseline 25(OH)D sufficiency category.

METHODS

Postmenopausal women with established osteoporosis received subcutaneous daily teriparatide 20 μg or oral weekly risedronate 35 mg, with concomitant 500-1000 mg of elemental calcium and 400-800 IU/day of vitamin D supplements. Fracture endpoints were analyzed by predefined subgroups of 25(OH)D insufficient and sufficient patients. Heterogeneity of the treatment effect on fractures was investigated by logistic and Cox proportional hazards regression models.

RESULTS

At baseline, mean serum 25(OH)D was 31.9 ng/mL in the teriparatide group and 31.5 ng/mL in the risedronate group, and 16.8% and 17.9% of patients, respectively, were 25(OH)D insufficient. At month 6, the mean serum 25(OH)D concentration decreased in teriparatide-treated patients to 24.5 ng/mL (by approximately 23%) but remained relatively constant in risedronate-treated patients (32.2 ng/mL) (p < 0.001). Proportions of 25(OH)D insufficient patients at month 6 were 26.7% and 5.6%, respectively (p < 0.001). The risk reduction with teriparatide versus risedronate for any of the fracture endpoints did not significantly differ between subgroups by 25(OH)D sufficiency status at baseline, with nonsignificant (p > 0.1) treatment-by-25(OH)D interactions in all fracture analyses.

CONCLUSIONS

Serum 25(OH)D concentration decreases during teriparatide treatment. Fracture risk reduction with teriparatide versus risedronate did not significantly differ between the two groups of patients defined by baseline 25(OH)D.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT01709110 EudraCT Number: 2012-000123-41.

Skeletal response to treatment with teriparatide (TPD) after bisphosphonate in post-menopausal women with osteoporosis and a high prevalence of secondary risk factors in real-life setting of a metabolic bone clinic; effect of age and vitamin D status

PURPOSE

Teriparatide (TPD) is a skeletal anabolic agent used in patients with severe post-menopausal osteoporosis (PMO) and steroid-induced osteoporosis who are at hish risk of fracture. Predictors of therapeutic response to teriparatide in real-life setting are not well characterised. We investigated potential factors associated with teriparatide response in post-menopausal women with established osteoporosis.

METHODS

We carried out a retrospective survey of 48 women, aged 73.2 [7.5] years with severe osteoporosis and prevalent fractures treated with TPD according to the NICE criteria. BMD was measured at baseline, 6-12 and 18-24 months at the lumbar spine (LS), total hip (TH) and femoral neck (FN). Bone turnover markers, serum 25 (OH)vitamin D were determined at 3-12 and 12-24 months.

RESULTS

BMD increased at 6-12 months (% change mean [SEM] 6.5 [1.1] p = 0.004) and 18-24 months (8.45 % [1.2] p<0.001) at the LS. A significant increase in BMD was observed at FN (3.1 [1.3] % p = 0.02). Changes in BMD at the TH was higher in patients younger than 73 years compared to older women (% change in BMD 4.13 [1.64] % v/s -1.7 [1.1] p = 0.007). Baseline 25 (OH) vitamin D correlated with change in P1NP at 3-12 months (r = 0.45 p = 0.049).

CONCLUSIONS

TPD-induced changes in BMD at the TH appears may be dependent on age. Vitamin D status may influence the early anabolic effect to TPD. Our data suggest that these factors may be important considerations when initiating and optimising treatment with TPD, although further larger studies are needed to confirm these findings.

Intermittent PTH 1-34 administration improves the marrow microenvironment and endothelium-dependent vasodilation in bone arteries of aged rats

Inflammation coincides with diminished marrow function, vasodilation of blood vessels, and bone mass. Intermittent parathyroid hormone (PTH) administration independently improves marrow and vascular function, potentially impacting bone accrual. Currently, the influence of marrow and intermittent PTH administration on aged bone blood vessels has not been examined. Vasodilation of the femoral principal nutrient artery (PNA) was assessed in the presence and absence of marrow. Furthermore, we determined the influence of PTH 1-34 on 1) endothelium-dependent vasodilation and signaling pathways [i.e., nitric oxide (NO) and prostacyclin (PGI₂)], 2) endothelium-independent vasodilation, 3) cytokine production by marrow cells, and 4) bone microarchitecture and bone static and dynamic properties. Young (4-6 mo) and old (22-24 mo) male Fischer-344 rats were treated with PTH 1-34 or a vehicle for 2 wk. In the absence and presence of marrow, femoral PNAs were given cumulative doses of acetylcholine, with and without the NO and PGI₂ blockers, and diethylamine NONOate. Marrow-derived cytokines and bone parameters in the distal femur were assessed. Exposure to marrow diminished endothelium-dependent vasodilation in young rats. Reduced bone volume and NO-mediated vasodilation occurred with old age and were partially reversed with PTH. Additionally, PTH treatment in old rats restored endothelium-dependent vasodilation in the presence of marrow and augmented IL-10, an anti-inflammatory cytokine. Endothelium-independent vasodilation was unaltered, and PTH treatment reduced osteoid surfaces in old rats. In conclusion, the marrow microenvironment reduced vascular function in young rats, and PTH treatment improved the marrow microenvironment and vasodilation with age. NEW & NOTEWORTHY This study investigated the influence of the marrow microenvironment on bone vascular function in young and old rats. An inflamed marrow microenvironment may reduce vasodilator capacity of bone blood vessels, diminishing delivery of blood flow to the skeleton. In young rats, the presence of the marrow reduced vasodilation in the femoral principal nutrient artery (PNA). However, intermittent parathyroid hormone administration (i.e., a treatment for osteoporosis) improved the marrow microenvironment and vasodilator capacity in old PNAs.

Optimal dosing and delivery of parathyroid hormone and its analogues for osteoporosis and hypoparathyroidism - translating the pharmacology

In primary hyperparathyroidism (PHPT), bone loss results from the resorptive effects of excess parathyroid hormone (PTH). Under physiological conditions, PTH has actions that are more targeted to homeostasis and to bone accrual. The predominant action of PTH, either catabolic, anabolic or homeostatic, can be understood in molecular and pharmacokinetic terms. When administered intermittently, PTH increases bone mass, but when present continuously and in excess (e.g. PHPT), bone loss ensues. This dual effect of PTH depends not only on the dosing regimen, continuous or intermittent, but also on how the PTH molecule interacts with various states of its receptor (PTH/PTHrP receptor) influencing downstream signalling pathways differentially. Altering the amino-terminal end of PTH or PTHrP could emphasize the state of the receptor that is linked to an osteoanabolic outcome. This concept led to the development of a PTHrP analogue that interacts preferentially with the transiently linked state of the receptor, emphasizing an osteoanabolic effect. However, designing PTH or PTHrP analogues with prolonged state of binding to the receptor would be expected to be linked to a homeostatic action associated with the tonic secretory state of the parathyroid glands that is advantageous in treating hypoparathyroidism. Ideally, further development of a drug delivery system that mimics the physiological tonic, circadian, and pulsatile profile of PTH would be optimal. This review discusses basic, translational and clinical studies that may well lead to newer approaches to the treatment of osteoporosis as well as to different PTH molecules that could become more advantageous in treating hypoparathyroidism.

Efficacy and safety of osteoporosis medications in a rat model of late-stage chronic kidney disease accompanied by secondary hyperparathyroidism and hyperphosphatemia

This study showed that bisphosphonate was safe and effective for the treatment of bone disorders in stage 4 chronic kidney disease (CKD) rats. Intermittent teriparatide therapy showed an anabolic action on bone even under secondary hyperparathyroidism conditions without having an adverse effect on mineral metabolism in late-stage CKD.

INTRODUCTION

Patients with late-stage CKD are at high risk for fragility fractures. However, there are no consensus on the efficacy and safety of osteoporosis medications for patients with late-stage CKD. In the present study, we aimed to examine the efficacy and safety of alendronate (ALN) and teriparatide (TPD) for treating bone disorder in late-stage CKD with pre-existing secondary hyperparathyroidism using a rat model of CKD.

METHODS

Male 10-week-old Sprague-Dawley rats were subjected to a 5/6 nephrectomy or sham surgery and randomized into the following four groups: sham, vehicle (saline subcutaneous (sc) daily), ALN (50 μg/kg sc daily), and TPD (40 μg/kg sc daily). Medications commenced at 24 weeks of age and continued for 4 weeks. Micro-computed tomography, histological analysis, infrared spectroscopic imaging, and serum assays were performed.

RESULTS

Nephrectomized rats developed hyperphosphatemia, secondary hyperparathyroidism (SHPT), and high creatinine, equivalent to CKD stage 4 in humans. ALN suppressed the bone turnover and increased the degree of mineralization in cortical bone, resulting in an improvement in the mechanical properties. TPD further increased the bone turnover and significantly increased the degree of mineralization, micro-geometry, and bone volume, resulting in a significant improvement in the mechanical properties. Both ALN and TPD had no adverse effect on renal function and mineral metabolism.

CONCLUSIONS

BP is safe and effective for the treatment of bone disorders in stage 4 CKD rats. Intermittent TPD therapy showed an anabolic action on bone even under SHPT conditions without having an adverse effect on mineral metabolism in late-stage CKD.

Relationship between bone turnover and density with teriparatide, denosumab or both in women in the DATA study

BACKGROUND

While changes in biochemical markers of bone turnover (BTM) have been reported to predict changes in bone mineral density (BMD), the relationship between changes in BMD and BTMs with combined antiresorptive/anabolic therapy is unknown.

METHODS

In the DATA study, 94 postmenopausal osteoporotic women (ages 51-91) received either teriparatide 20-mcg SC daily, denosumab 60-mg SC every 6months, or both for 2years. Pearson's correlation coefficients (R) were calculated to determine the relationship between baseline and early changes in BTMs (as well as serum sclerostin) and 2-year changes in BMD.

RESULTS

In women receiving teriparatide, baseline BTMs did not correlate with 2-year BMD changes though 12-month increases in osteocalcin and P1NP were associated with 2-year increases in spine BMD. In women receiving denosumab, spine and hip BMD gains correlated with both baseline and changes in P1NP and C-telopeptide. In women receiving combined teriparatide/denosumab, while both baseline and decreases in P1NP were associated with spine BMD gains, distal radius increases were associated with less CTX suppression. Neither baseline nor changes in serum sclerostin correlated with BMD in any treatment group.

SUMMARY AND CONCLUSIONS

In women treated with teriparatide or denosumab, early BTM changes (increases and decreases, respectively) predict 2-year BMD gains, especially at the spine. In women treated with combined teriparatide/denosumab therapy, BMD increases at the distal radius were associated with less suppression of bone turnover. These results suggest that efficacy of combination therapy at cortical sites such as the radius may depend on residual bone remodeling despite RANKL inhibition.

One Year of Abaloparatide, a Selective Activator of the PTH1 Receptor, Increased Bone Formation and Bone Mass in Osteopenic Ovariectomized Rats Without Increasing Bone Resorption

Abaloparatide is a novel 34-amino acid peptide selected to be a potent and selective activator of the parathyroid hormone receptor (PTH1R) signaling pathway with 41% homology to PTH(1-34) and 76% homology to PTHrP(1-34). A 12-month treatment study was conducted in osteopenic ovariectomized (OVX) rats to characterize the mechanisms by which abaloparatide increases bone mass. Sprague-Dawley (SD) rats were subjected to OVX or sham surgery at age 6 months and left untreated for 3 months to allow OVX-induced bone loss. Ten OVX rats were euthanized after this bone depletion period, and the remaining OVX rats received daily subcutaneous injections of vehicle (n = 18) or abaloparatide at 1, 5, or 25 μg/kg/d (n = 18/dose level) for 12 months. Sham controls (n = 18) received vehicle daily. Bone densitometry and biochemical markers of bone formation and resorption were assessed longitudinally, and L₃ vertebra and tibia were collected at necropsy for histomorphometry. Abaloparatide increased biochemical bone formation markers without increasing bone resorption markers or causing hypercalcemia. Abaloparatide increased histomorphometric indices of bone formation on trabecular, endocortical, and periosteal surfaces without increasing osteoclasts or eroded surfaces. Abaloparatide induced substantial increases in trabecular bone volume and density and improvements in trabecular microarchitecture. Abaloparatide stimulated periosteal expansion and endocortical bone apposition at the tibial diaphysis, leading to marked increases in cortical bone volume and density. Whole-body bone mineral density (BMD) remained stable in OVX-Vehicle controls while increasing 25% after 12 months of abaloparatide (25 μg/kg). Histomorphometry and biomarker data suggest that gains in cortical and trabecular bone mass were attributable to selective anabolic effects of abaloparatide, without evidence for stimulated bone resorption. © 2016 American Society for Bone and Mineral Research.

Frequency of Teriparatide Administration Affects the Histological Pattern of Bone Formation in Young Adult Male Mice

Evidence supports that daily and once-weekly administration of teriparatide, human (h)PTH(1-34), enhance bone mass in osteoporotic patients. However, it is uncertain whether different frequencies of hPTH(1-34) administration would induce bone formation similarly in terms of quantity and quality. To investigate that issue, mice were subjected to different frequencies of PTH administration, and their bones were histologically examined. Frequencies of administration were 1 time/2 days, 1 time a day, and 2 and 4 times a day. Mice were allocated to either to control or to 3 different dosing regimens: 80 μg/kg of hPTH(1-34) per injection (80 μg/kg per dose), 80 μg/kg of hPTH(1-34) per day (80 μg/kg · d), or 20 μg/kg of hPTH(1-34) per day (20 μg/kg · d). With the regimens of 80 μg/kg per dose and 80 μg/kg · d, high-frequency hPTH(1-34) administration increased metaphyseal trabecular number. However, 4 doses per day induced the formation of thin trabeculae, whereas the daily PTH regimen resulted in thicker trabeculae. A similar pattern was observed with the lower daily hPTH(1-34) dose (20 μg/kg · d): more frequent PTH administration led to the formation of thin trabeculae, showing a thick preosteoblastic cell layer, several osteoclasts, and scalloped cement lines that indicated accelerated bone remodeling. On the other hand, low-frequency PTH administration induced new bone with mature osteoblasts lying on mildly convex surfaces representative of arrest lines, which suggests minimodeling-based bone formation. Thus, high-frequency PTH administration seems to increase bone mass rapidly by forming thin trabeculae through accelerated bone remodeling. Alternatively, low-frequency PTH administration leads to the formation of thicker trabeculae through bone remodeling and minimodeling.

Teriparatide Therapy Reduces Serum Phosphate and Intima-Media Thickness at the Carotid Wall Artery in Patients with Osteoporosis

Although cross-sectional and longitudinal studies report a relationship between osteoporosis and cardiovascular disorders (known as the bone-cardiovascular axis), the benefits of osteoporosis treatment on atherosclerosis are largely unclear. Teriparatide is a bone-forming agent that increases urinary phosphate excretion. Because elevated serum phosphate is associated with the development of atherosclerosis, the purpose of our study was to examine the relationship among lumbar spine bone mineral density (LS-BMD), intima-media thickness at the carotid artery (CA-IMT), and phosphate metabolism in response to daily teriparatide therapy. Osteoporotic patients (n = 28) with low LS-BMD (T-score < -2.5) and/or at least one vertebral fracture were treated with teriparatide (20 μg/day) for 12 months. Metabolic bone markers, LS-BMD, and CA-IMT were measured over the course of treatment. The LS-BMD significantly increased by 0.046 ± 0.038 g/cm(2) over the 12-month period (P < 0.001). CA-IMT decreased from 0.701 mm (interquartile range: 0.655-0.774 mm) at baseline to 0.525 mm (0.477-0.670 mm) at 12 months (P < 0.05); however, CA-IMT change was not significantly associated with LS-BMD change. Serum phosphate decreased after 1 month of teriparatide administration, and the change in serum phosphate at 1 months was associated with the change in CA-IMT at 12 months (ρ = 0.431, P = 0.025). Teriparatide improved LS-BMD and CA-IMT, suggesting the existence of the bone-cardiovascular axis. The association between serum phosphate and CA-IMT suggests that the teriparatide decreased CA-IMT in part by reducing serum phosphate, a well-known vascular toxin, in addition to the improvement of bone-cardiovascular axis.

Effects of Different 1-34 Parathyroid Hormone Dosages on Fibroblast Growth Factor-23 Secretion in Human Bone Marrow Cells following Osteogenic Differentiation

The importance of fibroblast growth factor (FGF)-23 as part of a hormonal bone-kidney-axis has been well established. Lately, FGF-23 has been suggested as an independent risk factor of death in patients on chronic hemodialysis. Hyperparathyroidism is a common feature of advanced kidney failure or end-stage renal disease. The independent effect of elevated parathyroid hormone (PTH) levels on FGF-23 secretion is still a matter of debate and has not yet been studied in an in vitro model of human bone marrow cells (BMC) during osteogenic differentiation. BMC from three different donors were cultivated for 4 weeks in cell cultures devoid of vitamin D either without 1-34 PTH or with PTH concentrations of 10 or 100 pmol/L, respectively. After 28 days, protein expression of the cells was determined by immunocytochemical staining, whereas real time-polymerase chain reaction served to analyze gene expression of several osteoblastic (osteocalcin, RANKL, Runx-2 and ostase) and osteoclastic markers (RANK, TRAP-5b). The concentrations of FGF-23, ostase and TRAP-5b were determined by ELISA at weeks 2, 3 and 4. We found a basal expression of FGF-23 with no increase in FGF-23 secretion after stimulation with 10 pmol/L 1-34 PTH. Stimulation with 100 pmol/L PTH resulted in an increase in FGF-23 expression (14.1±3.6 pg/mL with no PTH, 13.7±4.0 pg/mL with 10 pmol/L, P=0.84 and 17.6±3.4 pg/mL with 100 pmol/L, P=0.047). These results suggest a vitamin D and PTH-independent FGF-23 expression in human BMC after osteogenic stimulation. As only higher PTH levels stimulated FGF-23 expression, a threshold level might be hypothesized.

Lower fibroblast growth factor 23 levels in young adults with Crohn disease as a possible secondary compensatory effect on the disturbance of bone and mineral metabolism

Fibroblast growth factor 23 (FGF-23) is a bone-derived circulating phosphaturic factor that decreases serum concentration of phosphate and vitamin D, suggested to actively participate in a complex renal-gastrointestinal-skeletal axis. Serum FGF-23 concentrations, as well as various other laboratory parameters involved in bone homeostasis, were measured and analyzed with regard to various diseases and patients' characteristics in 44 patients with Crohn disease (CD) and 20 healthy controls (HCs) included in this cross-sectional study. Serum FGF-23 levels were significantly lower in patients with CD (900.42 ± 815.85pg/mL) compared with HC (1410.94 ± 1000.53pg/mL), p = 0.037. Further analyses suggested FGF-23 as a factor independent from various parameters including age (r = -0.218), body mass index (r = -0.115), 25-hydroxy vitamin D (r = 0.126), parathyroid hormone (r = 0.084), and bone mineral density (BMD) of hip and lumbar (r = 0.205 and r = 0.149, respectively). This observation remained even after multivariate analyses, exhibiting that BMD was not affected by FGF-23, although parameters such as age (p = 0.026), cumulative prednisolone dose (p < 0.0001), and smoking status (p = 0.024) were strong determinants of BMD regarding hip. Lower FGF-23 levels in patients with bowel inflammation are accompanied but not directly correlated with lower vitamin D levels, showing no impact on BMD determination of young adults with CD. The downregulation of serum FGF-23 levels in CD appears as a secondary compensatory effect on the bone and mineral metabolism induced by chronic intestinal inflammation.

Plasma FGF23 levels increase rapidly after acute kidney injury

Emerging evidence suggests that fibroblast growth factor 23 (FGF23) levels are elevated in patients with acute kidney injury (AKI). In order to determine how early this increase occurs we used a murine folic acid nephropathy model and found that plasma FGF23 levels increased significantly from baseline already after 1 hour of AKI, with an 18-fold increase at 24 hours. Similar elevations of FGF23 levels were found when AKI was induced in mice with osteocyte-specific parathyroid hormone receptor ablation or the global deletion of parathyroid hormone or vitamin D receptor, indicating that the increase in FGF23 was independent of parathyroid hormone and vitamin D signaling. Furthermore, FGF23 levels increased to a similar extent in wild-type mice maintained on normal or phosphate-depleted diets prior to induction of AKI, indicating that the marked FGF23 elevation is at least partially independent of dietary phosphate. Bone production of FGF23 was significantly increased in AKI. The half-life of intravenously administered recombinant FGF23 was only modestly increased. Consistent with the mouse data, plasma FGF23 levels rose 15.9-fold by 24 hours following cardiac surgery in patients who developed AKI. The levels were significantly higher than in those without postoperative AKI. Thus, circulating FGF23 levels rise rapidly during AKI in rodents and humans. In mice this increase is independent of established modulators of FGF23 secretion.

Bone turnover markers: use in osteoporosis

Biochemical markers of bone turnover (bone turnover markers, BTMs) can be used to study changes in bone remodelling in osteoporosis. Investigators and clinicians should be aware of the appropriate sample collection and storage conditions for optimum measurements of these markers. Improvements in the variability of BTM measurements have resulted from the development of assays for automated analysers, and from international consensus regarding their use. Appropriate reference intervals should be used for the optimum interpretation of results. BTMs can provide information that is useful for the management of patients with osteoporosis, for both the initial clinical assessment and for guiding and monitoring of treatment. BTMs are clinically useful to determine possible causes of secondary osteoporosis by identifying patients with high bone turnover and rapid bone loss. In the follow-up of treatment response, BTM levels respond rapidly to both anabolic and antiresorptive treatments. BTM changes can also be used for understanding the mechanism of action of drugs in development and identifying the correct dose; they are also potentially useful as surrogate biomarkers for fracture.

(1-34) Parathyroid hormone infusion acutely lowers fibroblast growth factor 23 concentrations in adult volunteers

BACKGROUND AND OBJECTIVES

Fibroblast growth factor 23 (FGF23) regulates phosphorus and vitamin D metabolism. Parathyroid hormone (PTH) infusion for 24 hours stimulated FGF23 secretion in healthy volunteers. The extent to which this was due to a direct stimulatory effect of PTH versus an indirect effect of increasing 1,25-dihydroxyvitamin D [1,25(OH)(2)D] levels was unclear.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Changes in FGF23 in 26 adults undergoing 6-hour (1-34) PTH infusion were examined, focusing particularly on the effects of PTH on FGF23 in the early period of infusion before sustained increases in 1,25(OH)(2)D.

RESULTS

FGF23 levels declined in parallel with serum phosphate during infusion (P<0.05 for both), with both analytes decreasing within the first hour and reaching their respective nadirs at 6 hours. These changes were observed despite no change in 1,25(OH)(2)D levels during the first hour and a significant increase in 1,25(OH)(2)D from baseline after 6 hours (P<0.001). There were no differences in these responses by race. However, modest racial differences in the phosphaturic response to (1-34) PTH were observed (P=0.04 for interaction), with a higher rate of increase in fractional phosphate excretion in blacks than in whites.

CONCLUSIONS

During short-term (1-34) PTH infusion, FGF23 levels decreased in parallel with serum phosphate levels and despite significant increases in 1,25(OH)(2)D. When coupled with the results of prior longer-term infusion studies, these findings suggest that acute increases in PTH initially act to suppress FGF23 secretion, perhaps to mitigate urinary phosphate losses, before the stimulatory effect of 1,25(OH)(2)D on FGF23 eventually begins to predominate.

Increased circulating levels of FGF23: an adaptive response in primary hyperparathyroidism?

INTRODUCTION

Fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH) are major players in the bone-parathyroid-kidney axis controlling phosphate homeostasis. In patients with primary hyperparathyroidism (PHPT), data on the relationship between PTH and FGF23 are scarce and not always concordant.

OBJECTIVE

The aim of our study was to evaluate the relationship between PTH and FGF23 in patients with PHPT and in euparathyroid patients cured after successful parathyroidectomy (PTx).

PATIENTS AND METHODS

Twenty-one patients with PHPT and 24 patients in long-term cure after successful PTx (EuPTH) were studied. All patients underwent biochemical evaluation of renal function, parathyroid status, vitamin D status, bone turnover markers, and serum intact FGF23 levels.

RESULTS

Mean serum FGF23 concentration was significantly higher in PHPT than in EuPTH patients (50.8±6.1 vs 33.1±2.6 pg/ml, P=0.01). FGF23 levels significantly correlated with PTH levels (r=0.361, P=0.02), also after correction for 1,25(OH)(2)D levels (r=0.419, P=0.01). FGF23 levels showed a significant negative correlation with 1,25(OH)(2)D, which was more pronounced in PHPT than in EuPTH patients (r=-0.674, P=0.001, vs r=-0.509, P=0.01).

CONCLUSION

Our findings suggest that in PHPT, FGF23 levels are increased independent of 1,25(OH)(2)D levels. The more pronounced negative relationship between FGF23 and 1,25(OH)(2)D in the presence of high circulating PTH levels suggests that the increase in FGF23 levels may be an adaptive mechanism to counteract the PTH-induced increase in 1,25(OH)(2)D levels, although not completely overriding it.