Effects of intermittent parathyroid hormone (PTH) administration on SOST mRNA and protein in rat bone

Parathyroid hormone (1-34) retards the lumbar facet joint degeneration and activates Wnt/β-catenin signaling pathway in ovariectomized rats

PURPOSE

Facet joint degeneration (FJD) is a major cause of low back pain. Parathyroid hormone (PTH) (1-34) is commonly used to treat osteoporosis. However, little is known about its effects on FJD induced by estrogen deficiency. This study aims to investigate the effects of PTH (1-34) on FJD induced by estrogen deficiency and the underlying pathogenesis of the disease.

METHODS

Forty 3-month-old female Sprague-Dawley rats were randomly divided into four groups: 30 received bilateral ovariectomy (OVX) followed by 12 weeks of treatment with normal saline, PTH (1-34) or 17β-estradiol (E2), and 10 received sham surgery followed by administration of normal saline. Status and Wnt/β-catenin signaling activity in the cartilage and subchondral bone of the L4-L5 FJs and serum biomarkers were analyzed.

RESULTS

Administration of PTH (1-34) and E2 ameliorated cartilage lesions, and significantly decreased MMP-13 and caspase-3 levels and chondrocyte apoptosis. PTH (1-34) but not E2 significantly increased cartilage thickness, number of chondrocytes, and the expression of aggrecan. PTH (1-34) significantly improved microarchitecture parameters of subchondral bone, increased the expression of collagen I and osteocalcin, and decreased RANKL/OPG ratio. E2 treatment significantly increased the OPG level and decreased the RANKL/OPG ratio in the subchondral bone of ovariectomized rats, but it did not significantly improve the microarchitecture parameters of subchondral bone. Wnt3a and β-catenin expression was significantly reduced in the articular cartilage and subchondral bone in OVX rats, but PTH (1-34) could increase the expression of these proteins. E2 significantly increased the activity of Wnt/β-catenin pathway only in cartilage, but not in subchondral bone. The restoration of Wnt/β-catenin signaling had an obvious correlation with the improvement of some parameters associated with the FJs status.

CONCLUSION

Wnt/β-catenin signaling may be a potential therapeutic target for FJD induced by estrogen deficiency. PTH (1-34) is effective in treating this disease with better efficacy than 17β-estradiol, and the efficacy may be attributed to its restoration of Wnt/β-catenin signaling.

Only bioactive forms of PTH (n-oxPTH and Met18(ox)-PTH) inhibit synthesis of sclerostin - evidence from in vitro and human studies

Sclerostin (SOST) is produced by osteocytes and is known as a negative regulator of bone homeostasis. Parathyroid hormone (PTH) regulates calcium, phosphate as well as vitamin D metabolism, and is a strong inhibitor of SOST synthesis in vitro and in vivo. PTH has two methionine amino acids (positions 8 and 18) which can be oxidized. PTH oxidized at Met18 (Met18(ox)-PTH) continues to be bioactive, whereas PTH oxidized at Met8 (Met8(ox)-PTH) or PTH oxidized at Met8 and Met18 (Met8, Met18(di-ox)-PTH) has minor bioactivity. How non-oxidized PTH (n-oxPTH) and oxidized forms of PTH act on sclerostin synthesis is unknown. The effects of n-oxPTH and oxidized forms of PTH on SOST gene expression were evaluated in UMR106 osteoblast-like cells. Moreover, we analyzed the relationship of SOST with n-oxPTH and all forms of oxPTH in 516 stable kidney transplant recipients using an assay system that can distinguish in clinical samples between n-oxPTH and the sum of all oxidized PTH forms (Met8(ox)-PTH, Met18(ox)-PTH, and Met8, Met18(di-ox)-PTH). We found that both n-oxPTH and Met18(ox)-PTH at doses of 1, 3, 20, and 30 nmol/L significantly inhibit SOST gene expression in vitro, whereas Met8(ox)-PTH and Met8, Met18(di-ox)-PTH only have a weak inhibitory effect on SOST gene expression. In the clinical cohort, multivariate linear regression showed that only n-oxPTH, but not intact PTH (iPTH) nor oxPTH, is independently associated with circulating SOST after adjusting for known confounding factors. In conclusion, only bioactive PTH forms such as n-oxPTH and Met18(ox)-PTH, inhibit SOST synthesis.

Eldecalcitol Induces Minimodeling-Based Bone Formation and Inhibits Sclerostin Synthesis Preferentially in the Epiphyses Rather than the Metaphyses of the Long Bones in Rats

This study aimed to examine minimodeling-based bone formation between the epiphyses and metaphyses of the long bones of eldecalcitol (ELD)-administered ovariectomized rats. Sixteen-week-old female rats were divided into four groups: sham-operated rats receiving vehicle (Sham group), ovariectomized (OVX) rats receiving vehicle (Vehicle group), or ELDs (30 or 90 ng/kg BW, respectively; ELD30 and ELD90 groups). ELD administration increased bone volume and trabecular thickness, reducing the number of osteoclasts in both the epiphyses and metaphyses of OVX rats. The Sham and Vehicle groups exhibited mainly remodeling-based bone formation in both regions. The epiphyses of the ELD groups showed a significantly higher frequency of minimodeling-based bone formation than remodeling-based bone formation. In contrast, the metaphyses exhibited significantly more minimodeling-based bone formation in the ELD90 group compared with the ELD30 group. However, there was no significant difference between minimodeling-based bone formation and remodeling-based bone formation in the ELD90 group. While the minimodeling-induced new bone contained few sclerostin-immunoreactive osteocytes, the underlying pre-existing bone harbored many. The percentage of sclerostin-positive osteocytes was significantly reduced in the minimodeling-induced bone in the epiphyses but not in the metaphyses of the ELD groups. Thus, it seems likely that ELD could induce minimodeling-based bone formation in the epiphyses rather than in the metaphyses, and that ELD-driven minimodeling may be associated with the inhibition of sclerostin synthesis.

Pharmaceutical treatment of bone loss: From animal models and drug development to future treatment strategies

Animal models are fundamental to advance our knowledge of the underlying pathophysiology of bone loss and to study pharmaceutical countermeasures against it. The animal model of post-menopausal osteoporosis from ovariectomy is the most widely used preclinical approach to study skeletal deterioration. However, several other animal models exist, each with unique characteristics such as bone loss from disuse, lactation, glucocorticoid excess, or exposure to hypobaric hypoxia. The present review aimed to provide a comprehensive overview of these animal models to emphasize the importance and significance of investigating bone loss and pharmaceutical countermeasures from perspectives other than post-menopausal osteoporosis only. Hence, the pathophysiology and underlying cellular mechanisms involved in the various types of bone loss are different, and this might influence which prevention and treatment strategies are the most effective. In addition, the review sought to map the current landscape of pharmaceutical countermeasures against osteoporosis with an emphasis on how drug development has changed from being driven by clinical observations and enhancement or repurposing of existing drugs to today's use of targeted anti-bodies that are the result of advanced insights into the underlying molecular mechanisms of bone formation and resorption. Moreover, new treatment combinations or repurposing opportunities of already approved drugs with a focus on dabigatran, parathyroid hormone and abaloparatide, growth hormone, inhibitors of the activin signaling pathway, acetazolamide, zoledronate, and romosozumab are discussed. Despite the considerable progress in drug development, there is still a clear need to improve treatment strategies and develop new pharmaceuticals against various types of osteoporosis. The review also highlights that new treatment indications should be explored using multiple animal models of bone loss in order to ensure a broad representation of different types of skeletal deterioration instead of mainly focusing on primary osteoporosis from post-menopausal estrogen deficiency.

Sclerostin is a promising therapeutic target for oral inflammation and regenerative dentistry

Sclerostin is the protein product of the SOST gene and is known for its inhibitory effects on bone formation. The monoclonal antibody against sclerostin has been approved as a novel treatment method for osteoporosis. Oral health is one of the essential aspects of general human health. Hereditary bone dysplasia syndrome caused by sclerostin deficiency is often accompanied by some dental malformations, inspiring the therapeutic exploration of sclerostin in the oral and dental fields. Recent studies have found that sclerostin is expressed in several functional cell types in oral tissues, and the expression level of sclerostin is altered in pathological conditions. Sclerostin not only exerts similar negative outcomes on the formation of alveolar bone and bone-like tissues, including dentin and cementum, but also participates in the development of oral inflammatory diseases such as periodontitis, pulpitis, and peri-implantitis. This review aims to highlight related research progress of sclerostin in oral cavity, propose necessary further research in this field, and discuss its potential as a therapeutic target for dental indications and regenerative dentistry.

Effects of local single dose administration of parathormone on the early stages of osseointegration: A pre-clinical study

The present study aimed to evaluate the effect of parathormone (PTH) administered directly to the implant's surface prior to insertion, using a large translational animal model. Sixty titanium implants were divided into four groups: (i) Collagen, control group, where implants were coated with Type-I Bovine-collagen, and three experimental groups, where implants received varying doses of PTH: (ii) 12.5, (iii) 25, and (iv) 50 μg, prior to placement. Fifteen female sheep (~2 years old, weighing ~65 kg) received four implants in an interpolated fashion in C3, C4 or C5 vertebral bodies. After 3-, 6- and 12-weeks, samples were harvested, histologically processed, qualitatively and quantitatively assessed for bone-to-implant contact (BIC) and bone area fraction occupancy (BAFO). BIC yielded lower values at 6-weeks for 50 μg relative to the control group, with no significant differences, when compared to the 12.5- and 25-μg. No significant differences were detected at 6-weeks between collagen, 12.5- and 25-μg groups. At 3- and 12-weeks, no differences were detected for BIC among PTH groups. With respect to BAFO, no significant differences were observed between the control and experimental groups independent of PTH concentration and time in vivo. Qualitative observations at 3-weeks indicated the presence of a more mature bone near the implant's surface with the application of PTH, however, no significant differences in new bone formation or healing patterns were observed at 6- and 12-weeks. Single local application of different concentrations of PTH on titanium implant's surface did not influence the osseointegration at any time-point evaluation in low-density bone.

Parathyroid Hormone Regulates Circulating Levels of Sclerostin and FGF23 in a Primary Hyperparathyroidism Model

Parathyroid hormone (PTH) increases fibroblast growth factor 23 (FGF23), mediated both by protein kinase A (PKA) and Wnt signaling, and decreases expression of sclerostin, a Wnt antagonist derived from osteocytes. Patients with primary hyperparathyroidism (PHPT) have lower serum sclerostin levels than healthy controls, consistent with the idea of SOST downregulation by PTH. Nevertheless, the relationship between FGF23 and sclerostin in PHPT is still unclear. We examined this issue in a mouse model of PHPT. PHPT mice had increased FGF23 and decreased sclerostin expression in calvaria and in their serum concentrations compared with wild-type (WT) mice. In UMR106 osteoblasts, PTH increased Fgf23 expression and decreased Sost expression, as well as forskolin, a PKA agonist, whereas inhibition of PKA reversed the changes in Fgf23 and Sost expression, stimulated by PTH. Sclerostin treatment had no effect on Fgf23 expression, but when it was added together with PTH, it statistically significantly abrogated the increase in Fgf23 expression. By contrast, there was no statistically significant correlation between serum FGF23 and sclerostin, whereas PTH was positively and negatively correlated with serum FGF23 and sclerostin, respectively. These results indicate that the high level of PTH in PHPT mice leads to increased FGF23 and decreased sclerostin expression in serum and calvaria. A decrease of sclerostin may further augment FGF23 in vitro; however, there was no statistically significant association between circulating FGF23 and sclerostin. It is suggested that the pathogenesis of increased FGF23 expression in PHPT mice may be modified by not only sclerostin, but also other regulatory factors modulated by PTH.

Relationship Between Changes in Serum Levels of Intact Parathyroid Hormone and Sclerostin After a Single Dose of Zoledronic Acid: Results of a Phase 1 Pharmacokinetic Study

Although changes in serum sclerostin levels at 12 months after infusion of zoledronic acid have been reported, the changes in sclerostin levels at earlier time points are poorly understood. We reanalyzed the study data of a previous phase 1 pharmacokinetic study and investigated the correlation between changes in sclerostin levels and relevant factors in calcium metabolism. A total of 24 Japanese female subjects with primary postmenopausal osteoporosis were administered a single 4- or 5-mg dose of zoledronic acid. Serum and urine samples were collected on days 15, 29, 90, 180, and 365 after administration. Serum levels of calcium, phosphate, intact parathyroid hormone (iPTH), and sclerostin were measured. Levels of serum sclerostin were unchanged from baseline on days 15 and 29, but increased significantly on day 90, subsequently decreased significantly on day 180, and returned to baseline levels on day 365. A significant negative correlation was observed between changes in iPTH levels at early time points and sclerostin levels at later time points. This suggests that sclerostin was negatively regulated by iPTH, and the decrease in sclerostin may indicate the start of bone formation during later time points after zoledronic acid injection.

Antitumor Effect of Sclerostin against Osteosarcoma

Simple Summary

Osteosarcoma is highly variable and heterogeneous, which is one of the reasons for its resistance to treatment. Because osteosarcoma is defined by abnormal bone formation, we hypothesize its suppression could lead to effective treatment for all types of osteosarcomas. Sclerostin is secreted by osteocytes and inhibits the canonical pathway by binding to LRP5/6, thereby suppressing bone formation. The resulting suppression of bone formation leads to bone loss and osteoporosis. Here, we investigated the antitumor effect of sclerostin against osteosarcoma and found that sclerostin suppressed the proliferative capacity and migratory ability of osteosarcoma cells.

Abstract

Various risk factors and causative genes of osteosarcoma have been reported in the literature; however, its etiology remains largely unknown. Bone formation is a shared phenomenon in all types of osteosarcomas, and sclerostin is an extracellular soluble factor secreted by osteocytes that prevents bone formation by inhibiting the Wnt signaling pathway. We aimed to investigate the antitumor effect of sclerostin against osteosarcoma. Osteosarcoma model mice were prepared by transplantation into the dorsal region of C3H/He and BALB/c-nu/nu mice using osteosarcoma cell lines LM8 (murine) and 143B (human), respectively. Cell proliferations were evaluated by using alamarBlue and scratch assays. The migratory ability of the cells was evaluated using a migration assay. Sclerostin was injected intraperitoneally for 7 days to examine the suppression of tumor size and extension of survival. The administration of sclerostin to osteosarcoma cells significantly inhibited the growth and migratory ability of osteosarcoma cells. Kaplan–Meier curves and survival data demonstrated that sclerostin significantly inhibited tumor growth and improved survival. Sclerostin suppressed the proliferative capacity and migratory ability of osteosarcoma cells. Osteosarcoma model mice inhibited tumor growth and prolonged survival periods by the administration of sclerostin. The effect of existing anticancer drugs such as doxorubicin should be investigated for future clinical applications.

Role of Gut Microbiota in the Skeletal Response to PTH

Exposed surfaces of mammals are colonized with 100 trillion indigenous bacteria, fungi, and viruses, creating a diverse ecosystem known as the human microbiome. The gut microbiome is the richest microbiome and is now known to regulate postnatal skeletal development and the activity of the major endocrine regulators of bone. Parathyroid hormone (PTH) is one of the bone-regulating hormone that requires elements of the gut microbiome to exert both its bone catabolic and its bone anabolic effects. How the gut microbiome regulates the skeletal response to PTH is object of intense research. Involved mechanisms include absorption and diffusion of bacterial metabolites, such as short-chain fatty acids, and trafficking of immune cells from the gut to the bone marrow. This review will focus on how the gut microbiome communicates and regulates bone marrow cells in order to modulate the skeletal effects of PTH.

Parathyroid hormone-dependent bone formation requires butyrate production by intestinal microbiota

Parathyroid hormone (PTH) is a critical regulator of skeletal development that promotes both bone formation and bone resorption. Using microbiota depletion by wide-spectrum antibiotics and germ-free (GF) female mice, we showed that the microbiota was required for PTH to stimulate bone formation and increase bone mass. Microbiota depletion lowered butyrate levels, a metabolite responsible for gut-bone communication, while reestablishment of physiologic levels of butyrate restored PTH-induced anabolism. The permissive activity of butyrate was mediated by GPR43 signaling in dendritic cells and by GPR43-independent signaling in T cells. Butyrate was required for PTH to increase the number of bone marrow (BM) regulatory T cells (Tregs). Tregs stimulated production of the osteogenic Wnt ligand Wnt10b by BM CD8+ T cells, which activated Wnt-dependent bone formation. Together, these data highlight the role that butyrate produced by gut luminal microbiota plays in triggering regulatory pathways, which are critical for the anabolic action of PTH in bone.

Intravital multiphoton microscopy as a novel tool in the field of immunopharmacology

Intravital microscopy with multiphoton excitation is a recently developed optical imaging technique for deep tissue imaging without fixation or sectioning, which permits examination of fundamental concepts regarding the dynamic nature of cells under physiological and pathological conditions in living animals. This novel technique also offers exciting opportunities for pharmacological research by providing new platforms for the study of cellular dynamics in response to drugs in vivo. Moreover, fluorescent chemical probes for functional or molecular analysis in single cells in vivo play important roles in pharmacology. For example, we have recently revealed the pharmacodynamic actions of different biological agents for the treatment of rheumatoid arthritis (RA) in vivo by directly visualizing drug-induced cellular behaviors and functions of osteoclasts on bone surfaces. This review focuses on the principles and advantages of intravital imaging for the dissection of pharmacological mechanisms, and discusses how such imaging can contribute to the drug development process, introducing recent trials that evaluated the in vivo pharmacological effects of various agents.

Relationship Between Osteoporosis and Serum Sclerostin Levels in Kidney Transplant Recipients

OBJECTIVES

Sclerostin, a peptide secreted primarily by osteocytes, suppresses osteoblast maturation, thus reducing bone formation. Here, we evaluated the relationship between sclerostin levels and osteoporosis in kidney transplant recipients.

MATERIALS AND METHODS

This cross-sectional study included 78 kidney transplantrecipients > 18 years old and at least 6 months posttransplant. In our center, unrelated living-donor kidney transplants are not performed. Patients with parathyroid adenoma or parathyroidectomy history were excluded. Lumbar and femoral neck bone mineral densities andT and Z scores were obtained by dual-energy X-ray absorptiometry; results were used to divide patients into osteoporotic and nonosteoporotic groups. Serum sclerostin was measured by enzyme-linked immunosorbent assay.

RESULTS

: Of total patients, 43% had osteoporosis, mean age was 40.8 years, and 70% were male. Groups had similar ages, male-female distribution, time posttransplant, cumulative corticosteroid dose, estimated glomerular filtration rates, and 25-hydroxyvitamin D2 levels (P > .05). The osteoporotic group had lower sclerostin (405.9 ± 234.9 vs 521.7 ± 233.5 ng/dL; P = .035) and higherintact parathyroid hormone levels (110.9 ± 68.0 vs 84.8 ± 41.4 pg/mL; P = .04) than the nonosteoporotic group. Sclerostin levels were not correlated with cumulative corticosteroid dose, intact parathyroid hormone, bone mineral density, and T scores at any site but were weakly negatively correlated with age (P = .04, r = -0.25). In multiple regression analyses, only intact parathyroid hormone had negative effects on lumbar bone mineral density (P = .02) andT scores (P = .036). Serum sclerostin levels, age, and cumulative corticosteroid dose did not affect lumbar or hip bone mineral density and T scores (P > .05).

CONCLUSIONS

Sclerostin levels were low in our osteoporotic patients;therefore, sclerostin may not be a contributing factor to osteoporosis development. Because sclerostin is an osteocyte-derived peptide, its serum levels only reflect total osteocyte number and bone mass.

Effects of sequential treatment with intermittent parathyroid hormone and zoledronic acid on particle-induced implant loosening: Evidence from a rat model

Particle-induced implant loosening is a major challenge to long-term survival of joint prostheses. Administration of intermittent parathyroid hormone (PTH) has shown potential in the treatment of cases of early-stage periprosthetic osteolysis, while sequential administration of intermittent PTH (iPTH) and bisphosphonates (Bps) has achieved significant effects on treatment of postmenopausal osteoporosis. The objective of this study was to determine whether sequential treatment could preserve bone mass and implant fixation during a pathological course of peri-implant osteolysis in a rat model. Ninety male Sprague Dawley rats were randomly divided into nine groups, four of which were used for confirmation of establishment of the peri-implant osteolysis model at two time points, while the other five were used to determine the efficiency of the sequential treatment on peri-implant osteolysis. Implant fixation and peri-implant bone mass were evaluated using biomechanical testing, micro-CT analysis, and histology at 6 and 12 weeks postoperative. The biomechanical test demonstrated that the maximum loading force during a push-out test was significantly elevated in the sequential treatment group compared to the osteolysis group and iPTH withdrawal group at 12 weeks. Peri-implant bone morphology also indicated a robust increase in bone volume in the sequential treatment group. Sequential administration of iPTH and Bps was effective in preventing experimental peri-implant osteolysis, resulting in improved implant fixation and increased peri-implant bone volume. Clinical significance: The innovative application of sequential treatment in peri-implant osteolysis could be used clinically to improve the prognosis of patients with early-stage periprosthetic osteolysis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1489-1497, 2019.

FGF-2 targets sclerostin in bone and myostatin in skeletal muscle to mitigate the deleterious effects of glucocorticoid on musculoskeletal degradation

AIM

Myokines are associated with regulation of bone and muscle mass. However, limited information is available regarding the impact of myokines on glucocorticoid (GC) mediated adverse effects on the musculoskeletal system. This study investigates the role of myokine fibroblast growth factor-2 (FGF-2) in regulating GC-induced deleterious effects on bone and skeletal muscle.

METHODS

Primary osteoblast cells and C2C12 myoblast cell line were treated with FGF-2 and then exposed to dexamethasone (GC). FGF-2 mediated attenuation of the inhibitory effect of GC on osteoblast and myoblast differentiation and muscle atrophy was assessed through quantitative PCR and western blot analysis. Further, FGF-2 was administered subcutaneously to dexamethasone treated mice to collect bone and skeletal muscle tissue for in vivo analysis of bone microarchitecture, mechanical strength, histomorphometry and for histological alterations in treated tissue samples.

KEY FINDINGS

FGF-2 abrogated the dexamethasone induced inhibitory effect on osteoblast differentiation by modulating BMP-2 pathway and inhibiting Wnt antagonist sclerostin. Further, dexamethasone induced atrophy in C2C12 cells was mitigated by FGF-2 as evident from down regulation of atrogenes expression. FGF-2 prevented GC-induced impairment of mineral density, biomechanical strength, trabecular bone volume, cortical thickness and bone formation rate in mice. Additionally, skeletal muscle tissue from GC treated mice displayed weak myostatin immunostaining and reduced expression of atrogenes following FGF-2 treatment.

SIGNIFICANCE

FGF-2 mitigated GC induced effects through inhibition of sclerostin and myostatin expression in bone and muscle respectively. Taken together, this study exhibited the role of exogenous FGF-2 in sustaining osteoblastogenesis and inhibiting muscle atrophy in presence of glucocorticoid.

Sclerostin and parathyroid hormone responses to acute whole-body vibration and resistance exercise in young women

Whole-body vibration (WBV) has been shown to improve bone mineral density, and muscle strength and power. No studies to date have examined sclerostin and parathyroid hormone (PTH) responses to WBV combined with resistance exercise (RE). This randomized crossover study compared acute serum sclerostin and PTH responses to RE and WBV + RE in young women (n = 9) taking oral contraceptives. Participants were exposed to 5 1-min bouts of vibration (20 Hz, 3.38 peak-peak displacement, separated by 1 min of rest) before high intensity resistance exercise. Fasting blood samples were obtained before (PRE), immediately after WBV (POSTWBV), immediately post RE (IP) and 30 min post RE (30P). Pre-exercise sclerostin and PTH levels were not significantly different between conditions. Sclerostin levels significantly (p < 0.05) increased from PRE to IP for the WBV + RE condition, then decreased back to the pre-exercise level. PTH significantly decreased from PRE to 30P (p < 0.05) and IP to 30P (p < 0.01) for both conditions. Correcting for hemoconcentration eliminated the significant sclerostin responses, but the significant decrease in PTH remained (p < 0.05). There were no significant relationships found between sclerostin and PTH. In conclusion, sclerostin concentrations increased in response to the WBV + RE condition, which may have been mediated by plasma volume shifts. There was no transient PTH increase, but it showed a large decrease at 30P for both conditions. Based on these findings, the addition of WBV exposures prior to high intensity RE did not alter sclerostin and PTH responses to RE in young women.

Loss of Gsα in osteocytes leads to osteopenia due to sclerostin induced suppression of osteoblast activity

The stimulatory subunit of G-protein, Gsα, acts as a secondary messenger of G-protein coupled receptors (GPCRs) that primarily activates cAMP-induced signaling. GPCRs, such as the parathyroid hormone receptor (PTHR), are critical regulators of bone formation as shown by number of genetic manipulation studies targeting early osteoblast lineage cells. In this study, we have examined the role of Gsα in osteocytes, the terminally differentiated and most abundant cells of the osteoblast lineage. Mice lacking the stimulatory subunit of G-proteins (Gsα) in osteocytes (DMP1-GsαKO) have significant decrease of both trabecular and cortical bone, as assessed by μCT. Histomorphometric analysis showed that the osteopenia was mostly driven by more than 90% decrease in osteoblast numbers and activity whereas osteoclasts were only slightly decreased. The decrease in osteoblast number was associated with a striking lack of endocortical osteoblasts. We have previously shown that loss of the stimulatory subunit of G-proteins (Gsα) in osteocytes in vitro or in vivo induces high expression of sclerostin. To determine if the increased sclerostin levels contributed to the decreased endosteal bone lining cells and osteopenia, we treated wild-type mice with recombinant sclerostin and the DMP1-GsαKO mice with anti-sclerostin antibody. Treatment of wild-type mice with 100 μg/kg sclerostin for 3-weeks significantly reduced the numbers of bone lining cells and led to osteopenia. Next, the DMP1-GsαKO and control littermates were treated with the anti-sclerostin antibody (25 mg/kg, 2 times per week) for 4-weeks. Upon the antibody treatment, the endocortical osteoblasts reappeared in the DMP1-GsαKO mice to a comparable level to that of the vehicle treated control littermates. In control mice, E11/gp38 positive osteocytes were observed in parallel with the endocortical osteoblasts with higher dendrite density towards the endocortical osteoblasts. In DMP1-GsαKO mice, E11/gp38 positive osteocytes were lacking dendrites and were randomly scattered throughout the bone matrix. After treatment with anti-sclerostin antibody, DMP1-GsαKO mice showed increased E11/gp38 positive osteocytes near the endosteal bone surface and endosteal osteoblasts. The anti-sclerostin antibody treatment proportionally increased the bone volume but it could not completely rescue the osteopenia in the DMP1-GsαKO mice. Taken together, this data suggests that Gsα signaling in osteocytes leads to osteopenia driven, at least in part, by increased secretion of sclerostin.

The Effects of Long-term Administration of rhPTH(1-84) in Hypoparathyroidism by Bone Histomorphometry

Hypoparathyroidism is a rare disorder that is associated with abnormal bone properties. Recombinant human parathyroid hormone (1-84) [rhPTH(1-84)] in short-term studies has beneficial skeletal effects. Although rhPTH(1-84) will likely be used indefinitely, long-term effects on skeletal microstructure are unknown. We therefore studied histomorphometric changes with transiliac crest bone biopsies before and after 8.3 ± 1 years of rhPTH(1-84) in 13 hypoparathyroid subjects compared with 45 controls. Before institution of rhPTH(1-84), skeletal remodeling indices were markedly suppressed. With long-term treatment, indices of bone remodeling increased. Mineralizing surface increased by 26-fold (0.3 ± 1 to 7.9 ± 7%, p = 0.003), bone formation rate increased by 15-fold (0.003 ± 0.01 to 0.047 ± 0.05 μm² /μm/day, p = 0.007), osteoid width doubled (1.9 ± 1 to 4.3 ± 1 lamellae, p = 0.017), and osteoid surface tripled (3.3 ± 3 to 10.8 ± 6%, p = 0.011). Bone resorption as measured by eroded surface increased (4.6 ± 2 to 7.5 ± 3%, p = 0.021). Structural changes demonstrated intratrabecular tunneling, with increases in cancellous bone volume (19.6 ± 5 to 29.1 ± 11%, p = 0.017) and trabecular number (1.8 ± 1 to 2.5 ± 1 #/mm, p = 0.025). Cortical porosity tended to increase (6.3 ± 5 to 9.5 ± 3%, p = 0.07). Mineralizing surface, osteoid surface, and eroded surface surpassed control levels, as did cancellous bone volume, trabecular number, and cortical porosity. These data, the first to reflect such long exposure of any PTH for any disease, illustrate that PTH establishes and maintains a new skeletal state for at least 8 years in hypoparathyroidism. © 2018 American Society for Bone and Mineral Research.

PTH (1-34) and growth hormone in prevention of disuse osteopenia and sarcopenia in rats

Osteopenia and sarcopenia develops rapidly during disuse. The study investigated whether intermittent parathyroid hormone (1-34) (PTH) and growth hormone (GH) administered alone or in combination could prevent or mitigate disuse osteopenia and sarcopenia in rats. Disuse was achieved by injecting 4IU botulinum toxin A (BTX) into the right hindlimb musculature of 12-14-week-old female Wistar rats. Seventy-two rats were divided into six groups: 1. Baseline; 2. Ctrl; 3. BTX; 4. BTX+GH; 5. BTX+PTH; 6. BTX+PTH+GH. PTH (1-34) (60μg/kg/day) and GH (5mg/kg/day). The animals were sacrificed after 6weeks of treatment. Sarcopenia was established by histomorphometry, while the skeletal properties were determined using DXA, μCT, mechanical testing, and dynamic bone histomorphometry. Disuse resulted in lower muscle mass (-63%, p<0.05), trabecular BV/TV (-28%, p<0.05), Tb.Th (-11%, p<0.05), lower diaphyseal cortical thickness (-10%, p<0.001), and lower bone strength at the distal femoral metaphysis (-27%, p<0.001) compared to Ctrl animals. PTH fully counteracted the immobilization-induced lower BV/TV, Tb.Th, and distal femoral metaphyseal strength. GH increased muscle mass (+17%, p<0.05) compared to BTX, but did not prevent the immobilization-induced loss of bone strength, BV/TV, and cortical trabecular thickness. Combination of PTH and GH increased distal femoral metaphyseal bone strength (+45%, p<0.001), BV/TV (+50%, p<0.05), Tb.Th (+40%, p<0.05), and whole femoral aBMD (+15%, p<0.001) compared to BTX and muscle mass (+21%, p<0.05) compared to BTX+PTH. In conclusion, PTH and GH in combination is more efficient at preventing the disuse-related deterioration of bone strength, density, and micro-architecture than either PTH or GH given as monotherapy. Furthermore, GH, either alone or in combination with PTH, attenuated disuse-induced loss of muscle mass. The combination of PTH and GH resulted in a more effective treatment than PTH and GH as monotherapy.

Effect of Teriparatide Treatment on Circulating Periostin and Its Relationship to Regulators of Bone Formation and BMD in Postmenopausal Women With Osteoporosis

CONTEXT

Treatment of postmenopausal osteoporosis with teriparatide parathyroid hormone amino terminal 1-34 increases bone formation and improves bone microarchitecture. A possible modulator of action is periostin. In vitro experiments have shown that periostin might regulate osteoblast differentiation and bone formation through Wnt signaling. The effect of teriparatide on periostin is not currently known.

OBJECTIVES

To determine the effect of teriparatide treatment on circulating levels of periostin and other regulators of bone formation and investigate how changes in periostin relate to changes in bone turnover markers, regulators of bone formation, and bone mineral density (BMD).

PARTICIPANTS AND DESIGN

Twenty women with osteoporosis; a 2-year open-label single-arm study.

INTERVENTION

Teriparatide 20 µg was administered by subcutaneous injection daily for 104 weeks. Periostin, sclerostin, and Dickkopf-related protein 1, procollagen type I N-terminal propeptide (PINP), and C-telopeptide of type I collagen were measured in fasting serum collected at baseline (two visits) and then at weeks 1, 2, 4, 12, 26, 52, 78, and 104. BMD was measured at the lumbar spine, total hip, and femoral neck using dual energy x-ray absorptiometry.

RESULTS

Periostin levels increased by 6.6% [95% confidence interval (CI), -0.4 to 13.5] after 26 weeks of teriparatide treatment and significantly by 12.5% (95% CI, 3.3 to 21.0; P < 0.01) after 52 weeks. The change in periostin correlated positively with the change in the lumbar spine BMD at week 52 (r = 0.567; 95% CI, 0.137 to 0.817; P < 0.05) and femoral neck BMD at week 104 (r = 0.682; 95% CI, 0.261 to 0.885; P < 0.01).

CONCLUSIONS

Teriparatide therapy increases periostin secretion; it is unclear whether this increase mediates the effect of the drug on bone.

Regulatory T cells are expanded by Teriparatide treatment in humans and mediate intermittent PTH-induced bone anabolism in mice

Teriparatide is a bone anabolic treatment for osteoporosis, modeled in animals by intermittent PTH (iPTH) administration, but the cellular and molecular mechanisms of action of iPTH are largely unknown. Here, we show that Teriparatide and iPTH cause a ~two-threefold increase in the number of regulatory T cells (Tregs) in humans and mice. Attesting in vivo relevance, blockade of the Treg increase in mice prevents the increase in bone formation and trabecular bone volume and structure induced by iPTH Therefore, increasing the number of Tregs is a pivotal mechanism by which iPTH exerts its bone anabolic activity. Increasing Tregs pharmacologically may represent a novel bone anabolic therapy, while iPTH-induced Treg increase may find applications in inflammatory conditions and transplant medicine.

Chronological immunolocalization of sclerostin and FGF23 in the mouse metaphyseal trabecular and cortical bone

To assess the chronological participation of sclerostin and FGF23 in bone metabolism, this study tracked the immunolocalization of sclerostin and FGF23 in the metaphyses of murine long bones from embryonic day 18 (E18) through 1 day after birth, 1 week, 2 weeks, 4 weeks, 8 weeks, and 20 weeks of age. We have selected two regions in the metaphyseal trabeculae for assessing sclerostin and FGF23 localization: close to the chondro-osseous junction, i.e., bone modeling site even in the adult animals, and the trabecular region distant from the growth plate, where bone remodeling takes place. As a consequence, sclerostin-immunopositive osteocytes could not be observed in both close and distant trabecular regions early at the embryonic and young adult stages. However, osteocytes gradually started to express sclerostin in the distant region earlier than in the close region of the trabeculae. Immunoreactivity for FGF23 was observed mainly in osteoblasts in the early stages, but detectable in osteocytes in the later stages of growth in trabecular and cortical bones. Fgf23 was weakly expressed in the embryonic and neonatal stages, while the receptors, Fgfr1c and αKlotho were strongly expressed in femora. At the adult stages, Fgf23 expression became more intense while Fgfr1c and aKlotho were weakly expressed. These findings suggest that sclerostin is secreted by osteocytes in mature bone undergoing remodeling while FGF23 is synthesized by osteoblasts and osteocytes depending on the developmental/growth stage. In addition, it appears that FGF23 acts in an autocrine and paracrine fashion in fetal and neonatal bones.

Hormonal and systemic regulation of sclerostin

The Wnt/β-catenin signaling pathway plays an essential role in osteoblast biology. Sclerostin is a soluble antagonist of Wnt/β-catenin signaling secreted primarily by osteocytes. Current evidence indicates that sclerostin likely functions as a local/paracrine regulator of bone metabolism rather than as an endocrine hormone. Nonetheless, circulating sclerostin levels in humans often reflect changes in the bone microenvironment, although there may be exceptions to this observation. Using existing assays, circulating sclerostin levels have been shown to be altered in response to both hormonal stimuli and across a variety of normal physiological and pathophysiological conditions. In both rodents and humans, parathyroid hormone provided either intermittently or continuously suppresses sclerostin levels. Likewise, most evidence from both human and animal studies supports a suppressive effect of estrogen on sclerostin levels. Efforts to examine non-hormonal/systemic regulation of sclerostin have in general shown less consistent findings or have provided associations rather than direct interventional information, with the exception of mechanosensory studies which have consistently demonstrated increased sclerostin levels with skeletal unloading, and conversely decreases in sclerostin with enhanced skeletal loading. Herein, we will review the existent literature on both hormonal and non-hormonal/systemic factors which have been studied for their impact on sclerostin regulation.

Sclerostin measurement in human disease: Validity and current limitations

Sclerostin a potent regulator of bone formation, is an antagonist of the Wnt-signaling pathway. The advent of assays to measure circulating sclerostin has enabled research to be performed with the aim to understand the potential role of circulating sclerostin as a pathophysiological marker in a variety of clinical settings. At this time, however, assays to measure circulating sclerostin are still relatively new and have not demonstrated consistent internal agreement in addition to which there are differences between serum and plasma levels. Nevertheless, measurement of sclerostin in the circulation has the potential to reflect the dynamics of bone formation with particular reference to situations in which osteocytes, the major source of circulating sclerostin, may be perturbed. Because of technical uncertainties regarding sclerostin assays that are currently available, circulating sclerostin measurements should be interpreted cautiously with attention to reference ranges for each assay and whether or not the measurement is made in serum or plasma.

PTH-Induced Osteoblast Proliferation Requires Upregulation of the Ubiquitin-Specific Peptidase 2 (Usp2) Expression

Osteoporosis is a common disease that increases individual's fragility fracture risk. PTH is the only therapeutic agent for severe osteoporosis that requires anabolic action of bone formation. Although a part of the PTH actions is explained by increased proliferation of osteoblastic precursor cells, the mechanisms involved in the proliferation of osteoblastic cells by PTH have not been clarified yet. Therefore, in this study, we investigated the effects of PTH on gene expression in the cultured osteoblastic MC3T3-E1 cells, and found that the ubiquitin-specific peptidase 2 (Usp2) may be one of the direct target genes of PTHR signaling. Usp2 is a deubiquitination enzyme targeting various factors including CyclinD1 in cancer cells and PTH receptor 1 in osteoblasts. We confirmed that consistent induction of Usp2 expression peaked at 1 h by PTH1-34 (teriparatide) in MC3T3-E1 cells and primary calvarial osteoblasts. Among the three known splicing variants of the Usp2, we found the isoforms 1 and 2 are predominantly expressed in osteoblasts. Live-imaging analysis of the Fucci-transgenic mouse-derived primary osteoblasts indeed demonstrated that Usp2 is required for the PTH1-34-induced osteoblast proliferation. Western blotting analysis of the CyclinD1 indicated that Usp2 knock-down influences the paradoxical changes of CyclinD1 protein levels in this condition. Our data indicate that Usp2 is required for the PTH1-34-induced proliferation of osteoblasts.

Short-term intermittent administration of parathyroid hormone facilitates osteogenesis by different mechanisms in cancellous and cortical bone

Intermittent administration of human parathyroid hormone (1–34)[hPTH(1–34)] induces anabolic action on the bones. To understand the mechanism underlying the early phase of hPTH(1–34)-induced anabolic action, we investigated the expression profiles of osterix and sclerostin after short-term intermittent administration of hPTH(1–34) using immunohistochemistry in adult rats. In the cancellous bone, hPTH(1–34) administration greatly increased the number of osterix-positive cells in the bone marrow on day 1, but the cells gradually decreased on days 3 and 5. Injections of hPTH(1–34) induced no significant changes in the number of sclerostin-positive osteocytes in the cancellous bone. In the cortical bone, intermittent administration of hPTH(1–34) significantly reduced the number of sclerostin-positive osteocytes. The serum sclerostin level was downregulated and the osteocalcin level was upregulated on day 5 after intermittent administration of hPTH(1–34). Intermittent hPTH(1–34) injections increased osteoblast surface, osteoid thickness, and osteoid surface in cancellous bone, but not in cortical bone. This study suggested that the increase in osterix-positive osteoprogenitors in cancellous bone and the decrease in sclerostin-positive osteocytes in cortical bone play important roles in anabolic action on osteogenesis induced by short-term administration of hPTH(1–34).

•

We analyzed the effects of hPTH(1–34) injection into rats at early phase.

•

hPTH(1–34) injection increased the osterix-positive cells in bone marrow.

•

hPTH(1–34) injection decreased sclerostin-positive cells in cortical bone.

•

hPTH(1–34) exerts different effects in cancellous and cortical bone.

T cells, osteoblasts, and osteocytes: interacting lineages key for the bone anabolic and catabolic activities of parathyroid hormone

Osteoimmunology is a field of research dedicated to the study of the interactions between the immune system and bone. Among the cells of the immune system that regulate bone turnover and the responsiveness of bone cells to calciothropic hormones are bone marrow T lymphocytes. T cells secrete osteoclastogenic cytokines such as RANKL and TNF-α, as well as factors that stimulate bone formation, one of which is Wnt10b. In addition, T cells regulate the differentiation and life span of stromal cells (SCs) and their responsiveness to parathyroid hormone (PTH) via costimulatory molecules expressed on their surface. The conditioning effect of T cells on SCs is inherited by the osteoblastic and osteocytic progeny of SCs. As a result, osteoblastic cells of T cell-deficient mice have functional characteristics different from corresponding cells of T cell-replete mice. These differences include the ratio of RANKL/OPG produced in response to continuous PTH treatment, and the osteoblastogenic response to intermittent PTH treatment. This article reviews the evidence indicating that the effects of PTH are mediated not only by osteoblasts and osteocytes but also by T cells.

DMP-1-mediated Ghr gene recombination compromises skeletal development and impairs skeletal response to intermittent PTH

Bone minerals are acquired during growth and are key determinants of adult skeletal health. During puberty, the serum levels of growth hormone (GH) and its downstream effector IGF-1 increase and play critical roles in bone acquisition. The goal of the current study was to determine how bone cells integrate signals from the GH/IGF-1 to enhance skeletal mineralization and strength during pubertal growth. Osteocytes, the most abundant bone cells, were shown to orchestrate bone modeling during growth. We used dentin matrix protein (Dmp)-1-mediated Ghr knockout (DMP-GHRKO) mice to address the role of the GH/IGF axis in osteocytes. We found that DMP-GHRKO did not affect linear growth but compromised overall bone accrual. DMP-GHRKO mice exhibited reduced serum inorganic phosphate and parathyroid hormone (PTH) levels and decreased bone formation indices and were associated with an impaired response to intermittent PTH treatment. Using an osteocyte-like cell line along with in vivo studies, we found that PTH sensitized the response of bone to GH by increasing Janus kinase-2 and IGF-1R protein levels. We concluded that endogenously secreted PTH and GHR signaling in bone are necessary to establish radial bone growth and optimize mineral acquisition during growth.

1α,25-dihydroxyvitamin D3 stimulates human SOST gene expression and sclerostin secretion

Sclerostin, the SOST gene product, is a negative regulator of bone formation and a positive regulator of bone resorption. In this study, treatment of human primary osteoblasts, including cells differentiated to an osteocyte-like stage, with 1α,25-dihydroxyvitaminD3 (1,25D) resulted in the dose-dependent increased expression of SOST mRNA. A similar effect was observed in human trabecular bone samples cultured ex vivo, and in osteocyte-like cultures of differentiated SAOS2 cells. Treatment of SAOS2 cells with 1,25D resulted in the production and secretion of sclerostin protein. In silico analysis of the human SOST gene revealed a single putative DR3-type vitamin D response element (VDRE) at position -6216 bp upstream of the transcription start site (TSS). This sequence was confirmed to have strong VDRE activity by luciferase reporter assays and electrophoretic mobility shift analysis (EMSA). Sequence substitution in the VDR/RXR half-sites abolished VDRE reporter activity and binding of nuclear proteins. A 6.3 kb fragment of the human proximal SOST promoter demonstrated responsiveness to 1,25D. The addition of the evolutionary conserved region 5 (ECR5), a known bone specific enhancer region, ahead of the 6.3 kb fragment increased basal promoter activity but did not increase 1,25D responsiveness. Site-specific mutagenesis abolished the responsiveness of the 6.3 kb promoter to 1,25D. We conclude that 1,25D is a direct regulator of human SOST gene and sclerostin protein expression, extending the pathways of control of sclerostin expression. At least some of this responsiveness is mediated by the identified classical VDRE however the nature of the transcriptional regulation by 1,25D warrants further investigation.

Lipoprotein receptor-related protein 6 is required for parathyroid hormone-induced Sost suppression

Parathyroid hormone (PTH) suppresses the expression of the bone formation inhibitor sclerostin (Sost) in osteocytes by inducing nuclear accumulation of histone deacetylases (HDACs) to inhibit the myocyte enhancer factor 2 (MEF2)-dependent Sost bone enhancer. Previous studies revealed that lipoprotein receptor-related protein 6 (LRP6) mediates the intracellular signaling activation and the anabolic bone effect of PTH. Here, we investigated whether LRP6 mediates the inhibitory effect of PTH on Sost using an osteoblast-specific Lrp6-knockout (LRP6-KO) mouse model. An increased level of Sost mRNA expression was detected in femur tissue from LRP6-KO mice, compared to wild-type littermates. The number of osteocytes expressing sclerostin protein was also increased in bone tissue of LRP6-KO littermates, indicating a negative regulatory role of LRP6 on Sost/sclerostin. In wild-type littermates, intermittent PTH treatment significantly suppressed Sost mRNA expression in bone and the number of sclerostin(+) osteocytes, while the effect of PTH was much less significant in LRP6-KO mice. Additionally, PTH-induced downregulation of MEF2C and 2D, as well as HDAC changes in osteocytes, were abrogated in LRP6-KO mice. These data indicate that LRP6 is required for PTH suppression of Sost expression.

T cell-expressed CD40L potentiates the bone anabolic activity of intermittent PTH treatment

T cells are known to potentiate the bone anabolic activity of intermittent parathyroid hormone (iPTH) treatment. One of the involved mechanisms is increased T cell secretion of Wnt10b, a potent osteogenic Wnt ligand that activates Wnt signaling in stromal cells (SCs). However, additional mechanisms might play a role, including direct interactions between surface receptors expressed by T cells and SCs. Here we show that iPTH failed to promote SC proliferation and differentiation into osteoblasts (OBs) and activate Wnt signaling in SCs of mice with a global or T cell-specific deletion of the T cell costimulatory molecule CD40 ligand (CD40L). Attesting to the relevance of T cell-expressed CD40L, iPTH induced a blunted increase in bone formation and failed to increase trabecular bone volume in CD40L(-/-) mice and mice with a T cell-specific deletion of CD40L. CD40L null mice exhibited a blunted increase in T cell production of Wnt10b and abrogated CD40 signaling in SCs in response to iPTH treatment. Therefore, expression of the T cell surface receptor CD40L enables iPTH to exert its bone anabolic activity by activating CD40 signaling in SCs and maximally stimulating T cell production of Wnt10b.

Estrogen alone or in combination with parathyroid hormone can decrease vertebral MEF2 and sclerostin expression and increase vertebral bone mass in ovariectomized rats

The study is about the regulatory effects of estrogen and parathyroid hormone (PTH) on sclerostin, a protein that inhibits the Wnt/β-catenin pathway. The results indicate that estrogen may down-regulate sclerostin expression and that estrogen displays synergistic action with PTH. These results provide a new perspective on the relationship between estrogen and bone.

PURPOSE

To investigate whether estrogen can down-regulate SOST and MEF2 (myocyte enhancer factor 2) expression and whether co-treatment with estrogen and PTH has a stronger effect on suppressing SOST than PTH applied alone in ovariectomized rats.

METHODS

Forty-three-month-old virgin female Sprague-Dawley (SD) rats were ovariectomized and divided into four groups (n = 10). Another ten age-matched rats received sham operations as controls. After allowing 8 weeks for the development of vertebral osteopenia, the rats were administered the drug intervention. For this intervention, the estrogen group was subcutaneously injected with 17β-estradiol at 25 μg/kg body weight, the PTH group was injected with 80 μg/kg synthetic human PTH (1-34), and the co-treatment group was concurrently treated with PTH and estrogen at the above dosage. The OVX group and sham group were treated with vehicle. The drug treatment was conducted for 12 weeks. After the lumbar spine bone mineral density (BMD) was measured, the rats were sacrificed, and the lumbar spine and blood were collected for qPCR, Western blot, immunohistochemistry and other tests.

RESULTS

Estrogen can down-regulate MEF2 and sclerostin expression, and co-treatment with estrogen and PTH has a stronger effect on suppressing MEF2 and SOST mRNA than PTH alone. The co-treatment group displayed slightly higher bone mass and biomechanical properties than the PTH group, but the differences were not significant.

CONCLUSIONS

Estrogen appears to be a regulator of sclerostin, and the effect may involve suppressing MEF2s. Combined treatment with PTH and estrogen is not more beneficial for vertebral bone mass and strength than treatment with PTH alone in ovariectomized rats.

Additive effect of PTH (1-34) and zoledronate in the prevention of disuse osteopenia in rats

Immobilization is known to cause a rapid bone loss due to increased osteoclastic bone resorption and decreased osteoblastic bone formation. Zoledronate (Zln) is a potent anti-resorptive pharmaceutical, while intermittent PTH is a potent bone anabolic agent. The aim of the present study was to investigate whether PTH or Zln alone or in combination could prevent immobilization-induced osteopenia. Immobilization was achieved by injecting 4IU Botox (BTX) into the right hind limb musculature. Seventy-two 16-week-old female Wistar rats were randomized into 6 groups; baseline (Base), control (Ctrl), BTX, BTX+PTH, BTX+Zln, and BTX+PTH+Zln. PTH (1-34) (80μg/kg) was given 5days/week and Zln (100μg/kg) was given once at study start. The animals were killed after 4weeks of treatment. The bone properties were evaluated using DEXA, μCT, dynamic bone histomorphometry, and mechanical testing. BTX resulted in lower femoral trabecular bone volume fraction (BV/TV) (-25%, p<0.05), lower tibial trabecular bone formation rate (BFR/BS) (-29%, p<0.05), and lower bone strength (Fmax) at the distal femur (-19%, p<0.001) compared with Ctrl. BTX+PTH resulted in higher femoral BV/TV (+31%, p<0.05), higher tibial trabecular BFR/BS (+297%, p<0.05), and higher Fmax at the distal femur (+11%, p<0.05) compared with BTX. BTX+Zln resulted in higher femoral BV/TV (+36%, p<0.05), lower tibial trabecular BFR/BS (-93%, p<0.05), and higher Fmax at the distal femur (+10%, p<0.05) compared with BTX. BTX+PTH+Zln resulted in higher femoral BV/TV (+70%, p<0.001), higher tibial trabecular BFR/BS (+59%, p<0.05), and higher Fmax at the distal femur (+32%, p<0.001) compared with BTX. In conclusion, BTX-induced immobilization led to lower BV/TV, BFR/BS, and Fmax. In general, PTH or Zln alone prevented the BTX-induced osteopenia, whereas PTH and Zln given in combination not only prevented, but also increased BV/TV and BFR/BS, and maintained Fmax at the distal femoral metaphysis compared with Ctrl.

Enhanced immunoprotective effects by anti-IL-17 antibody translates to improved skeletal parameters under estrogen deficiency compared with anti-RANKL and anti-TNF-α antibodies

Activated T cell has a key role in the interaction between bone and immune system. T cells produce proinflammatory cytokines, including receptor activator of NF-κB ligand (RANKL), tumor necrosis factor α (TNF-α), and interleukin 17 (IL-17), all of which augment osteoclastogenesis. RANKL and TNF-α are targeted by inhibitors such as denosumab, a human monoclonal RANKL antibody, and infliximab, which neutralizes TNF-α. IL-17 is also an important mediator of bone loss, and an antibody against IL-17 is undergoing phase II clinical trial for rheumatoid arthritis. Although there are a few studies showing suppression of Th17 cell differentiation and induction of regulatory T cells (Tregs) by infliximab, the effect of denosumab remains poorly understood. In this study, we investigated the effects of anti-TNF-α, anti-RANKL, or anti-IL-17 antibody administration to estrogen-deficient mice on CD4(+) T-cell proliferation, CD28 loss, Th17/Treg balance and B lymphopoesis, and finally, the translation of these immunomodulatory effects on skeletal parameters. Adult Balb/c mice were treated with anti-RANKL/-TNF-α/-IL-17 subcutaneously, twice a week, postovariectomy (Ovx) for 4 weeks. Animals were then autopsied; bone marrow cells were collected for FACS and RNA analysis and serum collected for ELISA. Bones were dissected for static and dynamic histomorphometry studies. We observed that although anti-RANKL and anti-TNF-α therapies had no effect on Ovx-induced CD4(+) T-cell proliferation and B lymphopoesis, anti-IL-17 effectively suppressed both events with concomitant reversal of CD28 loss. Anti-IL-17 antibody reduced proinflammatory cytokine production and induced Tregs. All three antibodies restored trabecular microarchitecture with comparable efficacy; however, cortical bone parameters, bone biomechanical properties, and histomorphometry were best preserved by anti-IL-17 antibody, likely attributable to its inhibitory effect on osteoblast apoptosis and increased number of bone lining cells and Wnt10b expression. Based on the superior immunoprotective effects of anti-IL-17, which appears to translate to a better skeletal preservation, we propose beginning clinical trials using a humanized antibody against IL-17 for treatment of postmenopausal osteoporosis.

New therapeutics for osteoporosis

Two new approaches for the treatment of osteoporosis are summarized, each having arisen out of important new discoveries in bone biology. Odanacatib (ODN) inhibits the enzyme, cathepsin K, that is essential for the resorbing activity of osteoclasts. It is effective in preventing ovariectomy-induced bone loss in preclinical studies, and a phase II clinical study has shown inhibition of resorption sustained over five years. Outcome of a phase III study is awaited. The finding from mouse and human genetics that Wnt signaling is a powerful inducer of bone formation led to developments aimed at enhancing this pathway. Of the several approaches towards this, the most advanced is with a neutralizing antibody against sclerostin, the osteocyte-derived inhibitor of Wnt signaling. Preclinical studies show a powerful bone anabolic effect, and a clinical phase II study shows dose-dependent increases in bone formation and decreases in bone resorption markers.

Update on romosozumab : a humanized monoclonal antibody to sclerostin

INTRODUCTION

Disorders with inactivating mutations of the SOST gene result in reduced or absent expression of sclerostin and are associated with high bone mass. Sclerostin is an important regulator of bone formation due to its inhibitory actions in the osteoanabolic Wnt signaling pathway. Advances in understanding the mechanisms of action of this signaling molecule have led to the development of a pharmacological inhibitor of sclerostin with potential clinical applications as an osteoanabolic drug for the treatment of osteoporosis.

AREAS COVERED

Romosozumab is the first humanized monoclonal sclerostin antibody to be tested in clinical trials. Similar to preclinical animal studies with sclerostin antibodies, initial clinical studies show that romosozumab increases bone formation and bone mineral density.

EXPERT OPINION

Blocking sclerostin action with romosozumab is a promising new therapeutic approach to osteoanabolic therapy of osteoporosis; efficacy and safety data on large controlled studies are awaited.

Bone biology and anabolic therapies for bone: current status and future prospects

Bone is continuously remodelled at many sites asynchronously throughout the skeleton, with bone formation and resorption balanced at these sites to retain bone structure. Negative balance resulting in bone loss and osteoporosis, with consequent fractures, has mainly been prevented or treated by anti-resorptive drugs that inhibit osteoclast formation and/or activity, with new prospects now of anabolic treatments that restore bone that has been lost. The anabolic effectiveness of parathyroid hormone has been established, and an exciting new prospect is presented of neutralising antibody against the osteocyte protein, sclerostin. The cellular actions of these two anabolic treatments differ, and the mechanisms will need to be kept in mind in devising their best use. On present evidence it seems likely that treatment with either of these anabolic agents will need to be followed by anti-resorptive treatment in order to maintain bone that has been restored. No matter how effective anabolic therapies for the skeleton become, it seems highly likely that there will be a continuing need for safe, effective anti-resorptive drugs.

Sclerostin is differently immunolocalized in metaphyseal trabecules and cortical bones of mouse tibiae

Sclerostin, an osteocyte-derived molecule, has been reported to serve as a negative regulator of osteoblastic activity as well as bone remodeling. However, there is no report that verified the regional difference for sclerostin synthesis, and in this study we have investigated immunolocalization of sclerostin by comparing dentin matrix protein (DMP) 1, an osteocyte-derived factor broadly expressed in tibial metaphyses and cortical bone. In metaphyseal primary trabecules, a site of bone modeling, strong DMP1-reactivity was observed in osteocytic lacunar-canalicular system (OLCS), while faint staining for sclerostin was visible only in a few osteocytes. In secondary trabecules, in which bone remodeling begins, some osteocytes showed intense sclerostin-immunopositivity, though there were many DMP1-positive osteocytes. In cortical bone, there were more osteocytes reactive for sclerostin, when compared with those in the secondary trabecules. Silver impregnation verified that immature, primary trabecules contained randomly-oriented OLCS, while mature, cortical bone showed geometrically well-arrangement of OLCS. Taken together, though DMP1 is broadly synthesized in bone, sclerostin appears to be abundantly synthesized in regular OLCS of cortical bone, but less produced in irregular OLCS as seen in primary trabecules, indicating the regional difference for sclerostin synthesis.

The sclerostin-independent bone anabolic activity of intermittent PTH treatment is mediated by T-cell-produced Wnt10b

Both blunted osteocytic production of the Wnt inhibitor sclerostin (Scl) and increased T-cell production of the Wnt ligand Wnt10b contribute to the bone anabolic activity of intermittent parathyroid hormone (iPTH) treatment. However, the relative contribution of these mechanisms is unknown. In this study, we modeled the repressive effects of iPTH on Scl production in mice by treatment with a neutralizing anti-Scl antibody (Scl-Ab) to determine the contribution of T-cell-produced Wnt10b to the Scl-independent modalities of action of iPTH. We report that combined treatment with Scl-Ab and iPTH was more potent than either iPTH or Scl-Ab alone in increasing stromal cell production of OPG, osteoblastogenesis, osteoblast life span, bone turnover, bone mineral density, and trabecular bone volume and structure in mice with T cells capable of producing Wnt10b. In T-cell-null mice and mice lacking T-cell production of Wnt10b, combined treatment increased bone turnover significantly more than iPTH or Scl-Ab alone. However, in these mice, combined treatment with Scl-Ab and iPTH was equally effective as Scl-Ab alone in increasing the osteoblastic pool, bone volume, density, and structure. These findings demonstrate that the Scl-independent activity of iPTH on osteoblasts and bone mass is mediated by T-cell-produced Wnt10b. The data provide a proof of concept of a more potent therapeutic effect of combined treatment with iPTH and Scl-Ab than either alone.

Role of TGF-β in a mouse model of high turnover renal osteodystrophy

Altered bone turnover is a key pathologic feature of chronic kidney disease-mineral and bone disorder (CKD-MBD). Expression of TGF-β1, a known regulator of bone turnover, is increased in bone biopsies from individuals with CKD. Similarly, TGF-β1 mRNA and downstream signaling is increased in bones from jck mice, a model of high-turnover renal osteodystrophy. A neutralizing anti-TGF-β antibody (1D11) was used to explore TGF-β's role in renal osteodystrophy. 1D11 administration to jck significantly attenuated elevated serum osteocalcin and type I collagen C-telopeptides. Histomorphometric analysis indicated that 1D11 administration increased bone volume and suppressed the elevated bone turnover in a dose-dependent manner. These effects were associated with reductions in osteoblast and osteoclast surface areas. Micro-computed tomography (µCT) confirmed the observed increase in trabecular bone volume and demonstrated improvements in trabecular architecture and increased cortical thickness. 1D11 administration was associated with significant reductions in expression of osteoblast marker genes (Runx2, alkaline phosphatase, osteocalcin) and the osteoclast marker gene, Trap5. Importantly, in this model, 1D11 did not improve kidney function or reduce serum parathyroid hormone (PTH) levels, indicating that 1D11 effects on bone are independent of changes in renal or parathyroid function. 1D11 also significantly attenuated high-turnover bone disease in the adenine-induced uremic rat model. Antibody administration was associated with a reduction in pSMAD2/SMAD2 in bone but not bone marrow as assessed by quantitative immunoblot analysis. Immunostaining revealed pSMAD staining in osteoblasts and osteocytes but not osteoclasts, suggesting 1D11 effects on osteoclasts may be indirect. Immunoblot and whole genome mRNA expression analysis confirmed our previous observation that repression of Wnt/β-catenin expression in bone is correlated with increased osteoclast activity in jck mice and bone biopsies from CKD patients. Furthermore, our data suggest that elevated TGF-β may contribute to the pathogenesis of high-turnover disease partially through inhibition of β-catenin signaling.

The osteocyte: an endocrine cell ... and more

Few investigators think of bone as an endocrine gland, even after the discovery that osteocytes produce circulating fibroblast growth factor 23 that targets the kidney and potentially other organs. In fact, until the last few years, osteocytes were perceived by many as passive, metabolically inactive cells. However, exciting recent discoveries have shown that osteocytes encased within mineralized bone matrix are actually multifunctional cells with many key regulatory roles in bone and mineral homeostasis. In addition to serving as endocrine cells and regulators of phosphate homeostasis, these cells control bone remodeling through regulation of both osteoclasts and osteoblasts, are mechanosensory cells that coordinate adaptive responses of the skeleton to mechanical loading, and also serve as a manager of the bone's reservoir of calcium. Osteocytes must survive for decades within the bone matrix, making them one of the longest lived cells in the body. Viability and survival are therefore extremely important to ensure optimal function of the osteocyte network. As we continue to search for new therapeutics, in addition to the osteoclast and the osteoblast, the osteocyte should be considered in new strategies to prevent and treat bone disease.

Effects of PTH on osteocyte function

Osteocytes are ideally positioned to detect and respond to mechanical and hormonal stimuli and to coordinate the function of osteoblasts and osteoclasts. However, evidence supporting the involvement of osteocytes in specific aspects of skeletal biology has been limited mainly due to the lack of suitable experimental approaches. Few crucial advances in the field in the past several years have markedly increased our understanding of the function of osteocytes. The development of osteocytic cell lines initiated a plethora of in vitro studies that have provided insights into the unique biology of osteocytes and continue to generate novel hypotheses. Genetic approaches using promoter fragments that direct gene expression to osteocytes allowed the generation of mice with gain or loss of function of particular genes revealing their role in osteocyte function. Furthermore, evidence that Sost/sclerostin is expressed primarily in osteocytes and inhibits bone formation by osteoblasts, fueled research attempting to identify regulators of this gene as well as other osteocyte products that impact the function of osteoblasts and osteoclasts. The discovery that parathyroid hormone (PTH), a central regulator of bone homeostasis, inhibits sclerostin expression generated a cascade of studies that revealed that osteocytes are crucial target cells of the actions of PTH. This review highlights these investigations and discusses their significance for advancing our understanding of the mechanisms by which osteocytes regulate bone homeostasis and for developing therapies for bone diseases targeting osteocytes.

Paradoxical Sost gene expression response to mechanical unloading in metaphyseal bone

The Sost gene encodes Sclerostin, an inhibitor of Wnt-signaling, generally considered a main response gene to mechanical loading in bone. Several papers describe that unloading leads to upregulation of Sost, which in turn may lead to loss of bone. These studies were based on whole bone homogenates or cortical bone. By serendipity, we noted an opposite response to unloading in the proximal rat tibia. Therefore, we hypothesized that Sost-expression in response to changes in mechanical load is bone site specific. One hind limb of male, 3 month old rats was unloaded by paralyzing the extensors with Botulinium toxin A (Botox) injections. A series of experiments compared the expression of Sost mRNA in the unloaded and contralateral, loaded limbs, after 3 or 10 days, in metaphyseal cancellous bone, metaphyseal cortical bone, and diaphyseal cortical bone. We also conducted μCT to confirm changes in bone volume density related to unloading. Sost mRNA expression in the cancellous metaphyseal bone was downregulated almost 2-fold, both 3 days and 10 days after unloading (P<0.05). A similar tendency was seen in the metaphyseal cortical bone, in which Sost was 1.5-fold downregulated (P<0.05) after 10days, but not significantly changed after 3days. In contrast, diaphyseal cortical Sost expression was instead upregulated 1.4-fold (P<0.05) following 3-day unloading, while there was no significant change after 10days. Cancellous bone volume density was 58% lower (P<0.001, compared to cage controls) in the unloaded limb but not significantly affected in the loaded limb. The results suggest that Sost mRNA expression in metaphyseal bone responds to mechanical unloading in an opposite direction to that observed in diaphyseal cortical bone. This proposes a more complex expression pattern for Sost in response to unloading. Therapeutics that target Sclerostin during altered loading conditions may result in local bone mass changes that are difficult to predict.

Association of serum sclerostin with bone mineral density, bone turnover, steroid and parathyroid hormones, and fracture risk in postmenopausal women: the OFELY study

SUMMARY

Sclerostin is a key regulator of bone formation. In a population of 572 postmenopausal women (mean age, 67 years) followed prospectively for a median of 6 years, there was no significant association between baseline levels of serum sclerostin and incidence of all fractures which occurred in 64 subjects.

INTRODUCTION

Sclerostin, an osteocyte soluble factor, is a major negative regulator of osteoblastic activity. Circulating sclerostin levels were reported to increase with age and to be modestly associated with bone mineral density (BMD) and bone turnover, but there are no data on the association with fracture risk.

METHODS

We investigated 572 postmenopausal women (mean age, 67 ± 8.5 years) from the OFELY population-based cohort. The associations of serum sclerostin measured with a new two-site ELISA and spine and hip BMD by DXA, serum β-isomerized C-terminal crosslinking of type I collagen (CTX), intact N-terminal propeptide of type I collagen (PINP), intact PTH, 25-hydroxyvitamin D [25(OH)D], estradiol, testosterone, and fracture risk were analyzed. At the time of sclerostin measurements, 98 postmenopausal women had prevalent fractures. After a median of 6 years (interquartile range, 5-7 years) follow-up, 64 postmenopausal sustained an incident fracture.

RESULTS

Serum sclerostin correlated positively with spine (r = 0.35, p < 0.0001) and total hip (r = 0.25, <0.0001) BMD. Conversely, serum sclerostin was weakly negatively associated with the bone markers PINP (r = -0.10, p = 0.014) and CTX (r = -0.13, p = 0.0026) and with intact PTH (r = -0.13, p = 0.0064). There was no significant association of serum sclerostin with 25(OH)D, estradiol, free estradiol index, or testosterone. Serum sclerostin considered as a continuous variable or in quartiles was not significantly associated with the risk of prevalent or incident fracture.

CONCLUSION

Serum sclerostin is weakly correlated with BMD, bone turnover, and PTH in postmenopausal women. It was not significantly associated with the risk of all fractures, although the number of incident fractures recorded may not allow detecting a modest association.

Estrogen receptor α mediates proliferation of osteoblastic cells stimulated by estrogen and mechanical strain, but their acute down-regulation of the Wnt antagonist Sost is mediated by estrogen receptor β

Mechanical strain and estrogens both stimulate osteoblast proliferation through estrogen receptor (ER)-mediated effects, and both down-regulate the Wnt antagonist Sost/sclerostin. Here, we investigate the differential effects of ERα and -β in these processes in mouse long bone-derived osteoblastic cells and human Saos-2 cells. Recruitment to the cell cycle following strain or 17β-estradiol occurs within 30 min, as determined by Ki-67 staining, and is prevented by the ERα antagonist 1,3-bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylethoxy)phenol]-1H-pyrazole dihydrochloride. ERβ inhibition with 4-[2-phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-β]pyrimidin-3-yl] phenol (PTHPP) increases basal proliferation similarly to strain or estradiol. Both strain and estradiol down-regulate Sost expression, as does in vitro inhibition or in vivo deletion of ERα. The ERβ agonists 2,3-bis(4-hydroxyphenyl)-propionitrile and ERB041 also down-regulated Sost expression in vitro, whereas the ERα agonist 4,4′,4″-[4-propyl-(1H)-pyrazol-1,3,5-triyl]tris-phenol or the ERβ antagonist PTHPP has no effect. Tamoxifen, a nongenomic ERβ agonist, down-regulates Sost expression in vitro and in bones in vivo. Inhibition of both ERs with fulvestrant or selective antagonism of ERβ, but not ERα, prevents Sost down-regulation by strain or estradiol. Sost down-regulation by strain or ERβ activation is prevented by MEK/ERK blockade. Exogenous sclerostin has no effect on estradiol-induced proliferation but prevents that following strain. Thus, in osteoblastic cells the acute proliferative effects of both estradiol and strain are ERα-mediated. Basal Sost down-regulation follows decreased activity of ERα and increased activity of ERβ. Sost down-regulation by strain or increased estrogens is mediated by ERβ, not ERα. ER-targeting therapy may facilitate structurally appropriate bone formation by enhancing the distinct ligand-independent, strain-related contributions to proliferation of both ERα and ERβ.

Serum sclerostin levels decline in post-menopausal women with osteoporosis following treatment with intermittent parathyroid hormone

OBJECTIVE

This study was carried out in order to evaluate the effect of 18-month treatment with PTH (1-34) or PTH (1-84) on serum sclerostin levels in humans.

SUBJECTS AND METHODS

We investigated 10 women with severe osteoporosis, previously treated with alendronate and 20 untreated osteoporotic women. Subjects with severe osteoporosis were randomly divided into 2 groups of 5 patients each; the first group was treated with 20 μg of PTH (1-34) and the second one with 100 μg of PTH (1-84) according to an open-label design. Fasting blood samples were collected at baseline and at 2, 4, and 24 h after hormone administration. The same protocol was followed at month 1, 6, 12, 18. Serum sclerostin levels were measured at each time point by a sandwich-type enzyme-linked immunosorbent assay.

RESULTS

Basal serum sclerostin levels were not significantly different between patients previously treated with alendronate and those never treated. No significant acute change of serum sclerostin levels was observed after PTH administration. Fitting a mixed effect regression model, we found a significant time effect (p=0.0012) using the sclerostin level as the response variable and the month of drug administration as a single covariate. Treatment with both PTH molecules induced a monthly mean reduction of sclerostin levels of 0.1956 pmol/l.

CONCLUSIONS

Our results indicate that long-term therapy with PTH (1-34) or PTH (1-84) in women with osteoporosis previously treated with alendronate is associated with a reduction in circulating sclerostin levels. This is a putative mechanism through which PTH performs its anabolic action.