Unique regulation of SOST, the sclerosteosis gene, by BMPs and steroid hormones in human osteoblasts

Investigation of the Underlying Mechanism of Sclerosteosis Expression in Muscle Tissue in Multiple Myeloma with Sarcopenia

Objective

To explore the role of sclerosteosis (SOST) gene expression in the occurrence and development of multiple myeloma (MM) complicated with sarcopenia.

Methods

Analysis of the SOST expression in skeletal muscle tissue of patients with MM using high-throughput sequencing combined with transcriptomics; observation of morphological changes of the mouse C2C12 myoblasts co-cultured with SP2/0 myeloma cells in Transwell; observation of the SOST expression in the C2C12 myoblasts using the immunofluorescence labeling method; and assessment of the changes in exercise capacity of mice with MM using ethology; and the measurement of the SOST expression in muscles of mice using immunohistochemistry.

Results

The transcription level of the SOST gene in the muscle tissue was significantly higher in patients with MM and sarcopenia than in patients with MM without sarcopenia and elderly patients with sarcopenia; the area of C2C12 mouse myoblasts co-cultured with SP2/0 myeloma cells was 167,904 ± 8653.7 pix; this was significantly lower than the area of 402,994 ± 13,575.0 pix in the control group (CG); the fluorescence intensity of SOST in the cells of the experimental group (EG) was 159,389 ± 10,534 AU; this was significantly higher than the intensity of 26,338 ± 6059 AU in the CG; the differences in results of the coat-hanger test, the tail suspension test, the weight-bearing forced swimming test, and the grip strength test between the tumor-bearing mice in the EG and the CG were statistically significant; and the quantitative result of SOST expression in the muscle tissue of the EG mice was 11,515 ± 1573 pix; this was significantly higher than the result of 3399 ± 798.8 pix in the CG.

Conclusion

The SOST gene expression was significantly higher in muscle of mice in EG than in CG; and increased SOST gene expression might be a pathogenesis of MM complicated with sarcopenia.

Paracrine and endocrine functions of osteocytes

Osteocytes are dendritic-shaped cells embedded in the bone matrix and are terminally differentiated from osteoblasts. Inaccessibility due to their location has hindered the understanding of the molecular functions of osteocytes. However, scientific advances in the past few decades have revealed that osteocytes play critical roles in bone and mineral metabolism through their paracrine and endocrine functions. Sclerostin produced by osteocytes regulates bone formation and resorption by inhibiting Wnt/β-catenin signaling in osteoblast-lineage cells. Receptor activator of nuclear factor κ B ligand (RANKL) derived from osteocytes is essential for osteoclastogenesis and osteoclast activation during postnatal life. Osteocytes also secrete fibroblast growth factor 23 (FGF23), an endocrine FGF that regulates phosphate metabolism mainly by increasing phosphate excretion and decreasing 1, 25-dihydroxyvitamin D production in the kidneys. The regulation of FGF23 production in osteocytes is complex and multifactorial, involving many local and systemic regulators. Antibodies against sclerostin, RANKL, and FGF23 have emerged as new strategies for the treatment of metabolic bone diseases. Improved undrstanding of the paracrine and endocrine functions of osteocytes will provide insight into future therapeutic options.

Effect of compression on mandibular fracture haematoma-derived cells

Mechanical stress induces a variety of biochemical and morphological reactions in bone cell biology. This study aimed to investigate appropriate pressures of osteogenesis on the biological responses of 3-dimensional cultured human mandibular fracture haematoma-derived cells by compressive loading. Six patients with mandibular fractures who underwent open reduction and internal fixation were included in the study. During the operation, fracture haematomas that formed fibrin clots were manually removed before irrigation. First, pressures were applied to human mandibular fracture haematoma-derived cell-seeded collagen sponges. The sponges were subjected to mechanical compression using loading equipment applied at no compression, 0.5, or 1 mm. Compressive loading was applied to the samples prior to compression for 0, 6, 12, or 24 hours. Collagen sponge samples were collected for quantification of mRNA using several parameters including alkaline phosphatase (ALP), osteopontin (OPN), osterix (OSX), runt-related gene 2 (RUNX2), protein level, and immunocytochemistry (anti-sclerostin). Among these the 0.5 mm compression group compared with the control and 1.0 mm compression groups upregulated mRNA expression of OPN and OSX after 24 hours. Additionally, compared with the control group, a significantly higher OSX gene expression was observed in both the 0.5 mm and 1.0 mm groups after 6, 12, and 24 hours of compression (p < 0.05). However, no significant differences were observed regarding ALP and RUNX2 expression. These results indicated increased stimulation of osteogenesis of the mandibular fracture-line gap in the 0.5 mm compression group compared with the control and 1.0 mm compression groups.

Controversy of physiological vs. pharmacological effects of BMP signaling: Constitutive activation of BMP type IA receptor-dependent signaling in osteoblast lineage enhances bone formation and resorption, not affecting net bone mass

Bone morphogenetic proteins (BMPs) were first described over 50 years ago as potent inducers of ectopic bone formation when administrated subcutaneously. Preclinical studies have extensively examined the osteoinductive properties of BMPs in vitro and new bone formation in vivo. BMPs (BMP-2, BMP-7) have been used in orthopedics over 15 years. While osteogenic function of BMPs has been widely accepted, our previous studies demonstrated that loss-of-function of BMP receptor type IA (BMPR1A), a potent receptor for BMP-2, increased net bone mass by significantly inhibiting bone resorption in mice, indicating a positive role of BMP signaling in bone resorption. The physiological role of BMPs (i.e. osteogenic vs. osteoclastogenic) is still largely unknown. The purpose of this study was to investigate the physiological role of BMP signaling in endogenous long bones during adult stages. For this purpose, we conditionally and constitutively activated the Smad-dependent canonical BMP signaling thorough BMPR1A in osteoblast lineage cells using the mutant mice (Col1CreER™:caBmpr1a). Because trabecular bones were largely increased in the loss-of-function mouse study for BMPR1A, we hypothesized that the augmented BMP signaling would affect endogenous trabecular bones. In the mutant bones, the Smad phosphorylation was enhanced within physiological level three-fold while the resulting gross morphology, bodyweights, bone mass/shape/length, serum calcium/phosphorus levels, collagen cross-link patterns, and healing capability were all unchanged. Interestingly, we found; 1) increased expressions of both bone formation and resorption markers in femoral bones, 2) increased osteoblast and osteoclast numbers together with dynamic bone formation parameters by trabecular bone histomorphometry, 3) modest bone architectural phenotype with reduced bone quality (i.e. reduced trabecular bone connectivity, larger diametric size but reduced cortical bone thickness, and reduced bone mechanical strength), and 4) increased expression of SOST, a downstream target of the Smad-dependent BMPR1A signaling, in the mutant bones. This study is clinically insightful because gain-of-function of BMP signaling within a physiological window does not increase bone mass while it alters molecular and cellular aspects of osteoblast and osteoclast functions as predicted. These findings help explain the high-doses of BMPs (i.e. pharmacological level) in clinical settings required to substantially induce a bone formation, concurrent with potential unexpected side effects (i.e. bone resorption, inflammation) presumably due to a broader population of cell-types exposed to the high-dose BMPs rather than osteoblastic lineage cells.

Reduction of SOST gene promotes bone formation through the Wnt/β-catenin signalling pathway and compensates particle-induced osteolysis

The increase in bone resorption and/or the inhibition of bone regeneration caused by wear particles are the main causes of periprosthetic osteolysis. The SOST gene and Sclerostin, a protein synthesized by the SOST gene, are the characteristic marker of osteocytes and regulate bone formation and resorption. We aimed to verify whether the SOST gene was involved in osteolysis induced by titanium (Ti) particles and to investigate the effects of SOST reduction on osteolysis. The results showed osteolysis on the skull surface with an increase of sclerostin levels after treated with Ti particles. Similarly, sclerostin expression in MLO-Y4 osteocytes increased when treated with Ti particles in vitro. After reduction of SOST, local bone mineral density and bone volume increased, while number of lytic pores on the skull surface decreased and the erodibility of the skull surface was compensated. Histological analyses revealed that SOST reduction increased significantly alkaline phosphatase- (ALP) and osterix-positive expression on the skull surface which promoted bone formation. ALP activity and mineralization of MC3T3-E1 cells also increased in vitro when SOST was silenced, even if treated with Ti particles. In addition, Ti particles decreased β-catenin expression with an increase in sclerostin levels, in vivo and in vitro. Inversely, reduction of SOST expression increased β-catenin expression. In summary, our results suggested that reduction of SOST gene can activate the Wnt/β-catenin signalling pathway, promoting bone formation and compensated for bone loss induced by Ti particles. Thus, this study provided new perspectives in understanding the mechanisms of periprosthetic osteolysis.

Sclerostin within the chronic kidney disease spectrum

Sclerostin is sometimes presented as a promising biomarker in assessing bone health both in the general population and chronic kidney disease patients. However, it is still unclear whether it has any true added value compared to existing bone biomarkers in predicting bone turnover and/or bone density in chronic kidney disease patients. A wealth of papers has been published to evaluate the association between sclerostin and vascular calcifications development or even as prognostic biomarker for mortality, but often with conflicting results. Standardization and harmonization of analytical techniques is a prerequisite to advance clinical knowledge in sclerostin.

The BMP-2 mutant L51P: a BMP receptor IA binding-deficient inhibitor of noggin

The antagonist-specific regulation in tissue engineering constitutes important attempts to achieve an improved and rapid bone regeneration by controlling the natural biological response of the natural body growth factors. L51P is molecularly engineered bone morphogentic protein-2 (BMP-2) variant with a substitution of the 51st leucine with a proline residue. L51P is deficient in BMP receptor binding, but maintains its structure and affinity for inhibitory proteins such as noggin, chordin, and gremlin. These modifications convert the BMP-2 variant L51P into a receptor-inactive inhibitor of BMP antagonists. This current approach may prevent the uncontrolled bone overgrowth using high concentration of BMPs and thus regulates the possible growth factor's high-dose side effects. Exploring of L51P biological functions is required to broad our understanding of BMP mutant biological functions and their potential clinical applications. The progress of L51P researches would hopefully lead to the development of multiple applications for using the L51P in bone and fracture healing disorders.

The Bromodomain Inhibitor N-Methyl pyrrolidone Prevents Osteoporosis and BMP-Triggered Sclerostin Expression in Osteocytes

(1) Background: In an adult skeleton, bone is constantly renewed in a cycle of bone resorption, followed by bone formation. This coupling process, called bone remodeling, adjusts the quality and quantity of bone to the local needs. It is generally accepted that osteoporosis develops when bone resorption surpasses bone formation. Osteoclasts and osteoblasts, bone resorbing and bone forming cells respectively, are the major target in osteoporosis treatment. Inside bone and forming a complex network, the third and most abundant cells, the osteocytes, have long remained a mystery. Osteocytes are responsible for mechano-sensation and -transduction. Increased expression of the osteocyte-derived bone inhibitor sclerostin has been linked to estrogen deficiency-induced osteoporosis and is therefore a promising target for osteoporosis management. (2) Methods: Recently we showed in vitro and in vivo that NMP (N-Methyl-2-pyrrolidone) is a bioactive drug enhancing the BMP-2 (Bone Morphogenetic Protein 2) induced effect on bone formation while blocking bone resorption. Here we tested the effect of NMP on the expression of osteocyte-derived sclerostin. (3) Results: We found that NMP significantly decreased sclerostin mRNA and protein levels. In an animal model of osteoporosis, NMP prevented the estrogen deficiency-induced increased expression of sclerostin. (4) Conclusions: These results support the potential of NMP as a novel therapeutic compound for osteoporosis management, since it preserves bone by a direct interference with osteoblasts and osteoclasts and an indirect one via a decrease in sclerostin expression by osteocytes.

Sclerostin neutralization unleashes the osteoanabolic effects of Dkk1 inhibition

The WNT pathway has become an attractive target for skeletal therapies. High-bone-mass phenotypes in patients with loss-of-function mutations in the LRP5/6 inhibitor Sost (sclerosteosis), or in its downstream enhancer region (van Buchem disease), highlight the utility of targeting Sost/sclerostin to improve bone properties. Sclerostin-neutralizing antibody is highly osteoanabolic in animal models and in human clinical trials, but antibody-based inhibition of another potent LRP5/6 antagonist, Dkk1, is largely inefficacious for building bone in the unperturbed adult skeleton. Here, we show that conditional deletion of Dkk1 from bone also has negligible effects on bone mass. Dkk1 inhibition increases Sost expression, suggesting a potential compensatory mechanism that might explain why Dkk1 suppression lacks anabolic action. To test this concept, we deleted Sost from osteocytes in, or administered sclerostin neutralizing antibody to, mice with a Dkk1-deficient skeleton. A robust anabolic response to Dkk1 deletion was manifest only when Sost/sclerostin was impaired. Whole-body DXA scans, μCT measurements of the femur and spine, histomorphometric measures of femoral bone formation rates, and biomechanical properties of whole bones confirmed the anabolic potential of Dkk1 inhibition in the absence of sclerostin. Further, combined administration of sclerostin and Dkk1 antibody in WT mice produced a synergistic effect on bone gain that greatly exceeded individual or additive effects of the therapies, confirming the therapeutic potential of inhibiting multiple WNT antagonists for skeletal health. In conclusion, the osteoanabolic effects of Dkk1 inhibition can be realized if sclerostin upregulation is prevented. Anabolic therapies for patients with low bone mass might benefit from a strategy that accounts for the compensatory milieu of WNT inhibitors in bone tissue.

Role of osteocytes in mediating bone mineralization during hyperhomocysteinemia

Hyperhomocysteinemia (HHCY) is a risk factor for osteoporosis but whether HHCY affects bone mineralization or not is still ambiguous. Herein we evaluated whether homocysteine affects tissue mineral density (TMD) of cortical bone and if so the role of osteocytes. CD1 mice administered with homocysteine (5 mg/100 g body weight, i.p.) for 7, 15 and 30 days showed temporal changes in TMD and osteocyte lacunar density in femoral cortices. Short-term administration of homocysteine (day 7) increased osteocyte lacunar density and reduced TMD evidenced by microCT50 while prolonged administration of homocysteine (day 30) reinstated TMD and lacunar density to baseline values. Major differences were decreased number of nucleated osteocyte lacunae, increased number of empty lacunae and cleaved caspase 3-positive osteocyte lacunae in day 30 HHCY bone evidenced by H&E staining and immunohistochemistry. Other differences were induction in mineralization genes like Dmp1, Phex and Sost in cortical bone by real-time PCR and increased number of Dmp1- and Sost-positive osteocyte lacunae in day 30 HHCY bone evidenced by immunohistochemistry. Both HHCY day 7 and day 30 samples showed reduced Young's modulus demonstrating that biomechanical property of bone was lost during early HHCY itself, which did not improve with recovery of TMD. Our results thus demonstrate occurrence of two phases in cortical bone upon HHCY: the early phase that involved loss of TMD and increase in osteocyte numbers and a late phase that involved osteocyte reprogramming, apoptosis and mineralization, which reinstated TMD but compromised biomechanical property. To conclude, osteocytes have a potential role in arbitrating bone pathogenesis during HHCY.

Transcriptional control of Sost in bone

Sclerostin is an osteocyte derived negative regulator of bone formation. A highly specific expression pattern and the exclusive bone phenotype have made Sclerostin an attractive target for therapeutic intervention in treating metabolic bone diseases such as osteoporosis and in facilitating fracture repair. Understanding the molecular mechanisms that regulate Sclerostin transcription is of great interest as it may unveil new avenues for therapeutic approaches. Such studies may also elucidate how various signaling pathways intersect to modulate bone metabolism. Here we review the current understanding of the upstream molecular mechanisms that regulate Sost/SOST transcription, in bone.

Sost, independent of the non-coding enhancer ECR5, is required for bone mechanoadaptation

Sclerostin (Sost) is a negative regulator of bone formation that acts upon the Wnt signaling pathway. Sost is mechanically regulated at both mRNA and protein level such that loading represses and unloading enhances Sost expression, in osteocytes and in circulation. The non-coding evolutionarily conserved enhancer ECR5 has been previously reported as a transcriptional regulatory element required for modulating Sost expression in osteocytes. Here we explored the mechanisms by which ECR5, or several other putative transcriptional enhancers regulate Sost expression, in response to mechanical stimulation. We found that in vivo ulna loading is equally osteoanabolic in wildtype and Sost^-/- mice, although Sost is required for proper distribution of load-induced bone formation to regions of high strain. Using Luciferase reporters carrying the ECR5 non-coding enhancer and heterologous or homologous hSOST promoters, we found that ECR5 is mechanosensitive in vitro and that ECR5-driven Luciferase activity decreases in osteoblasts exposed to oscillatory fluid flow. Yet, ECR5^-/- mice showed similar magnitude of load-induced bone formation and similar periosteal distribution of bone formation to high-strain regions compared to wildtype mice. Further, we found that in contrast to Sost^-/- mice, which are resistant to disuse-induced bone loss, ECR5^-/- mice lose bone upon unloading to a degree similar to wildtype control mice. ECR5 deletion did not abrogate positive effects of unloading on Sost, suggesting that additional transcriptional regulators and regulatory elements contribute to load-induced regulation of Sost.

Analysis of SOST expression using large minigenes reveals the MEF2C binding site in the evolutionarily conserved region (ECR5) enhancer mediates forskolin, but not 1,25-dihydroxyvitamin D3 or TGFβ1 responsiveness

Transcribed from the SOST gene, sclerostin is an osteocyte-derived negative regulator of bone formation that inhibits osteoblastogenesis via antagonism of the Wnt pathway. Sclerostin is a promising therapeutic target for low bone mass diseases and neutralizing antibody therapies that target sclerostin are in development. Diverse stimuli regulate SOST including the vitamin D hormone, forskolin (Fsk), bone morphogenic protein 2 (BMP-2), oncostatin M (OSM), dexamethasone (Dex), and transforming growth factor (TGFβ₁). To explore the mechanisms by which these compounds regulate SOST expression, we examined their ability to regulate a SOST reporter minigene containing the entire SOST locus including the downstream regionor mutant minigenes containing a deletion of the -1kb to -2kb promoter proximal region (-1kb), ECR2, ECR5, or two point mutations in the MEF2 binding site of ECR5 (ECR5/MEF2). Previous reports suggest that both the PTH and TGFβ₁ effects on SOST are mediated through ECR5 and that the action of PTH is mediated specifically via the MEF2 binding site at ECR5. Consistent with these reports, the suppressive effects of Fsk were abrogated following both ECR5 deletion and ECR5/MEF2 mutation. In contrast, we found that TGFβ₁ negatively regulated SOST and that neither ECR5 nor ECR5/MEF2 was involved. Surprisingly, none of these four deletions/mutations abrogated the suppressive effects of the vitamin D hormone, OSM, Dex, or TGFβ₁, or the positive effects of BMP-2. These data suggest that we need to move beyond ECR5 to understand SOST regulation.

Differential osteogenicity of multiple donor-derived human mesenchymal stem cells and osteoblasts in monolayer, scaffold-based 3D culture and in vivo

We set out to compare the osteogenicity of human mesenchymal stem (hMSCs) and osteoblasts (hOBs). Upon osteogenic induction in monolayer, hMSCs showed superior matrix mineralization expressing characteristic bone-related genes. For scaffold cultures, both cell types presented spindle-shaped, osteoblast-like morphologies forming a dense, interconnected network of high viability. On the scaffolds, hOBs proliferated faster. A general upregulation of parathyroid hormone-related protein (PTHrP), osteoprotegrin (OPG), receptor activator of NF-κB ligand (RANKL), sclerostin (SOST), and dentin matrix protein 1 (DMP1) was observed for both cell types. Simultaneously, PTHrP, RANKL and DMP-1 expression decreased under osteogenic stimulation, while OPG and SOST increased significantly. Following transplantation into NOD/SCID mice, μCT and histology showed increased bone deposition with hOBs. The bone was vascularized, and amounts further increased for both cell types after recombinant human bone morphogenic protein 7 (rhBMP-7) addition also stimulating osteoclastogenesis. Complete bone organogenesis was evidenced by the presence of osteocytes and hematopoietic precursors. Our study results support the asking to develop 3D cellular models closely mimicking the functions of living tissues suitable for in vivo translation.

Rat Osteosarcoma Cells as a Therapeutic Target Model for Osteoregeneration via Sclerostin Knockdown

There are various conceptually different strategies to improve bone regeneration and to treat osteoporosis, each with distinct inherent advantages and disadvantages. The use of RNA interference strategies to suppress the biological action of catabolic factors or antagonists of osteogenic proteins is promising, and such strategies can be applied locally. They are comparably inexpensive and do not suffer from stability problems as protein-based approaches. In this study, we focus on sclerostin, encoded by the SOST gene, a key regulator of bone formation and remodeling. Sclerostin is expressed by mature osteocytes but also by late osteogenically differentiated cells. Thus, it is difficult and requires long-term cultures to investigate the effects of SOST silencing on the expression of osteogenic markers using primary cells. We, therefore, selected a rat osteosarcoma cell line, UMR-106, that has been shown to express SOST and secrete sclerostin in a comparable fashion as late osteoblasts and osteocytes. We investigated the effects of differentiating supplements on SOST expression and sclerostin secretion in UMR-106 cells and found that addition of 100 ng/ml of bone morphogenetic protein (BMP)-2 strongly induced sclerostin secretion, whereas dexamethasone inhibited secretion. Effects of silencing SOST in UMR-106 cells cultured in various differentiation media including BMP-2 and/or dexamethasone were determined next with the aim to find promising test conditions for a readout system for the evaluation of future small interfering RNA release formulations for local induction of bone formation. We found a direct correlation between attenuated SOST expression and an increase in the osteogenic potential of UMR-106 cells. The combination of SOST silencing and BMP-2 could synergistically improve osteogenic factors. A lowered proliferation rate in silenced groups may indicate a faster switch to differentiation.

Sclerostin expression in bone tumours and tumour-like lesions

AIMS

To assess the immunophenotypic and mRNA expression of sclerostin in human skeletal tissues and in a wide range of benign and malignant bone tumours and tumour-like lesions.

METHODS AND RESULTS

Sclerostin expression was evaluated by immunohistochemistry and quantitative polymerase chain reaction (PCR). In lamellar and woven bone, there was strong sclerostin expression by osteocytes. Osteoblasts and other cell types in bone were negative. Hypertrophic chondrocytes in the growth plate and mineralized cartilage cells in zone 4 of hyaline articular cartilage strongly expressed sclerostin, but most chondrocytes in hyaline cartilage were negative. In primary bone-forming tumours, including osteosarcomas, there was patchy expression of sclerostin in mineralized osteoid and bone. Sclerostin staining was seen in woven bone in fibrous dysplasia, in osteofibrous dysplasia, and in reactive bone formed in fracture callus, in myositis ossificans, and in the wall of solitary bone cysts and aneurysmal bone cysts. Sclerostin was expressed by hypertrophic chondrocytes in osteochondroma and chondroblasts in chondroblastoma, but not by tumour cells in other bone tumours, including myeloma and metastatic carcinoma. mRNA expression of sclerostin was identified by quantitative PCR in osteosarcoma specimens and cell lines.

CONCLUSIONS

Sclerostin is an osteocyte marker that is strongly expressed in human woven and lamellar bone and mineralizing chondrocytes. This makes it a useful marker with which to identify benign and malignant osteogenic tumours and mineralizing cartilage tumours, such as chondroblastomas and other lesions in which there is bone formation.

Successful induction of sclerostin in human-derived fibroblasts by 4 transcription factors and its regulation by parathyroid hormone, hypoxia, and prostaglandin E2

Sclerostin, coded by SOST, is a secretory protein that is specifically expressed in osteocytes and suppresses osteogenesis by inhibiting WNT signaling. The regulatory mechanism underlying SOST expression remains unclear mainly due to the absence of an adequate human cell model. Thus, we herein attempted to establish a cell model of human dermal fibroblasts in order to investigate the functions of sclerostin. We selected 20 candidate transcription factors (TFs) that induce SOST expression by analyzing gene expression patterns in the human sarcoma cell line, SaOS-2, between differentiation and maintenance cultures using microarrays. An effective set of TFs to induce SOST expression was sought by their viral transduction into fibroblasts, and a combination of four TFs: ATF3, KLF4, PAX4, and SP7, was identified as the most effective inducer of SOST expression. Quantitative PCR demonstrated that the expression levels of SOST in fibroblasts treated with the 4 TFs were 199- and 1439-fold higher than those of the control after 1-week and 4-week cultures, respectively. The level of sclerostin in the conditioned medium, as determined by ELISA, was 21.2pmol/l 4weeks after the transduction of the 4 TFs. Interestingly, the production of Dickkopf1 (DKK1), another secreted inhibitor of WNT signaling, was also increased by transduction of these 4 TFs. Parathyroid hormone (PTH) significantly suppressed the induced SOST by 38% and sclerostin by 82% that of the vehicle. Hypoxia increased the induced SOST by 62% that of normoxia. Furthermore, prostaglandin E2 (PGE2) increased SOST expression levels to 16-fold those of the vehicle. In conclusion, the efficient induction of SOST expression and sclerostin production was achieved in human dermal fibroblasts by the transduction of ATF3, KLF4, PAX4, and SP7, and the induced SOST and sclerostin were regulated by PTH, hypoxia, and PGE2. This model may contribute to elucidating the regulatory mechanisms underlying SOST expression and advancing drug development for metabolic bone diseases.

Curcumin alleviates glucocorticoid-induced osteoporosis through the regulation of the Wnt signaling pathway

It is known that prolonged glucocorticoid (GC) treatment results in osteoporosis. This study aimed to evaluate the protective effects of curcumin on the bones of rats with dexamethasone (DXM)-induced osteoporosis. In the present study, rats were administered DXM for 60 days to induce osteoporosis, and they were then treated with curcumin (100 mg/kg/day) for a further 60 days. H&E staining was used to observe the pathological changes in the femurs. Serum osteocalcin levels and collagen-type I fragments (CTX) were examined as bone metabolism markers. The results revealed that treatment with curcumin attenuated DXM-induced bone injury in femurs, increased the serum levels of osteocalcin and decreased the levels of CTX. In addition, in in vitro experiments, primary rat osteoblasts treated with curcumin at 0.5, 1 and 2 µM were exposed to 100 nM DXM. An MTT assay was used to determine the proliferative ability of the cells. Alkaline phosphatase activity, and the mRNA expression levels of runt-related transcription factor 2 (Runx2), osterix, osteocalcin, collagen, type 1, alpha 1 (Col1A1) and osteonectin were detected to assess transcription factor-associated osteogenic differentiation. The mRNA and protein expression levels of osteoprotegerin (OPG) and receptor activator for nuclear factor-kappa B ligand (RANKL) were detected to assess cytokine-associated osteoclastogenesis. The results demonstrated that curcumin prevented the DXM-induced inhibition of the proliferative ability of the osteoblasts in a dose-dependent manner. In addition, curcumin upregulated the mRNA expression levels of transcription factors that favor osteoblast differentiation and increased the ratio of OPG to RANKL. Moreover, the effects of curcumin on the Wnt signaling pathway were also investigated. RT-qPCR and western blot analysis demonstrated that the Wnt signaling pathway, which was inhibited by DXM, was re-activated upon treatment with curcumin. Immunofluorescence staining revealed that curcumin restored the intranuclear staining of β-catenin in the DXM-stimulated osteoblasts. Collectively, our data demonstrate that curcumin may be a potential therapeutic agent for the treatment of GC-induced osteoporosis.

Romosozumab/CDP7851 for the treatment of osteoporosis

Osteoporosis is an important worldwide health problem, conferring significant costs on healthcare. Current osteoporosis therapies are anti-resorptive and have proven anti-fracture efficacy, while there is a paucity of osteoanabolic therapies. Romosozumab is a humanized monoclonal antibody against sclerostin, an inhibitor of osteoblastic activity. Two-year follow-up data from initial clinical studies show rapid and robust increases in bone mineral density at all sites, except the wrist. Significant increases in bone formation markers have also been observed after administration of romosozumab. Notably, and unprecedented among any currently available therapy, this increase in bone formation is accompanied with control of bone resorption, allowing an enhanced anabolic potential compared with the only other currently available anabolic therapy, teriparatide. Romosozumab has been well tolerated in initial studies and its effects on BMD are augmented by follow-on anti-resorptive therapy. Ongoing Phase III studies will provide data regarding anti-fracture efficacy and comparisons with alendronate, as well as longer-term safety.

Estrogen Modulates Bone Morphogenetic Protein-Induced Sclerostin Expression Through the Wnt Signaling Pathway

Clinical data show that estrogen levels are inversely associated with the production of sclerostin, a Wnt antagonist that recently attracted great attention over the use of its antibody in the anabolic treatment of osteoporotic conditions. However, the molecular link between sclerostin expression and estrogen signaling is not yet known. We investigated the mechanisms by which estrogen modulates sclerostin (SOST) gene expression at the cellular level in human osteoblast cells in association with bone morphogenetic protein (BMP)2 signaling given that BMP2 is a potential inducer of SOST in human mesenchymal stromal cells (hMSCs). 17β-Estradiol (E2) alone had no effect on SOST expression, which was significantly induced by treatment with BMP2 in hMSCs and osteoblasts derived from the mandibles of female donors. However, E2 suppressed the induction of SOST and other BMP2 target genes by BMP2 in hMSCs. E2 signaling was independent of the Smad pathway, which plays a critical role in SOST induction mediated by BMP2. Instead, E2 increased the transcriptional expression of β-catenin and levels of its activated form. Silencing of the gene encoding estrogen receptor (ER)α decreased E2 activity in β-catenin activation and the suppression of SOST induction by BMP2, but had no influence on BMP2-mediated SOST induction in the same conditions. Similar results were obtained after treatment with ERα antagonist as a Wnt inhibitor. In human osteoblasts, the effect of E2 on SOST expression was either suppressive or absent, depending on the cell donor. Interestingly, the SOST expression pattern after treatment with BMP2 or BMP2/E2 in human osteoblasts showing a pattern of E2 suppression on SOST induction by BMP2 correlated with the ratio of receptor activator of nuclear factor kappa-B ligand (RANKL) to osteoprotegerin (OPG) expression. These results demonstrate that estrogen signaling in osteoblasts negatively regulates SOST expression in an indirect manner through interaction with BMP2 signaling and that this regulation involves the Wnt/ERα and β-catenin pathways. This study highlights several interactions between estrogen and BMP cascades in osteoblasts that may provide a basis for therapeutic intervention for the modification of bone mass density.

Heavy Metal Ion Regulation of Gene Expression: MECHANISMS BY WHICH LEAD INHIBITS OSTEOBLASTIC BONE-FORMING ACTIVITY THROUGH MODULATION OF THE Wnt/β-CATENIN SIGNALING PATHWAY

Exposure to lead (Pb) from environmental sources remains an overlooked and serious public health risk. Starting in childhood, Pb in the skeleton can disrupt epiphyseal plate function, constrain the growth of long bones, and prevent attainment of a high peak bone mass, all of which will increase susceptibility to osteoporosis later in life. We hypothesize that the effects of Pb on bone mass, in part, come from depression of Wnt/β-catenin signaling, a critical anabolic pathway for osteoblastic bone formation. In this study, we show that depression of Wnt signaling by Pb is due to increased sclerostin levels in vitro and in vivo. Downstream activation of the β-catenin pathway using a pharmacological inhibitor of GSK-3β ameliorates the Pb inhibition of Wnt signaling activity in the TOPGAL reporter mouse. The effect of Pb was determined to be dependent on sclerostin expression through use of the SOST gene knock-out mice, which are resistant to Pb-induced trabecular bone loss and maintain their mechanical bone strength. Moreover, isolated bone marrow cells from the sclerostin null mice show improved bone formation potential even after exposure to Pb. Also, our data suggest that the TGFβ canonical signaling pathway is the mechanism by which Pb controls sclerostin production. Taken together these results support our hypothesis that the osteoporotic-like phenotype observed after Pb exposure is, in part, regulated through modulation of the Wnt/β-catenin pathway.

Effect of different calcium phosphate scaffold ratios on odontogenic differentiation of human dental pulp cells

Calcium phosphate (CaP) scaffolds have been widely and successfully used with osteoblast cells for bone tissue regeneration. However, it is necessary to investigate the effects of these scaffolds on odontoblast cells' proliferation and differentiation for dentin tissue regeneration. In this study, three different hydroxyapatite (HA) to beta tricalcium phosphate (β-TCP) ratios of biphasic calcium phosphate (BCP) scaffolds, BCP20, BCP50, and BCP80, with a mean pore size of 300μm and 65% porosity were prepared from phosphoric acid (H2PO4) and calcium carbonate (CaCO3) sintered at 1000°C for 2h. The extracts of these scaffolds were assessed with regard to cell viability and differentiation of odontoblasts. The high alkalinity, more calcium, and phosphate ions released that were exhibited by BCP20 decreased the viability of human dental pulp cells (HDPCs) as compared to BCP50 and BCP80. However, the cells cultured with BCP20 extract expressed high alkaline phosphatase activity and high expression level of bone sialoprotein (BSP), dental matrix protein-1 (DMP-1), and dentin sialophosphoprotein (DSPP) genes as compared to that cultured with BCP50 and BCP80 extracts. The results highlighted the effect of different scaffold ratios on the cell microenvironment and demonstrated that BCP20 scaffold can support HDPC differentiation for dentin tissue regeneration.

Osteocyte control of bone remodeling: is sclerostin a key molecular coordinator of the balanced bone resorption-formation cycles?

Osteocytes, entrapped within a newly mineralized bone matrix, possess a unique cellular identity due to a specialized morphology and a molecular signature. These features endow them to serve as a bone response mechanism for mechanical stress in their microenvironment. Sclerostin, a primarily osteocyte product, is widely considered as a mechanotranduction key molecule whose expression is suppressed by mechanical loading, or it is induced by unloading. This review presents a model suggesting that sclerostin is major mediator for integrating mechanical, local, and hormonal signals, sensed by the osteocytes, in controlling the remodeling apparatus. This central role is achieved through interplay between two opposing mechanisms: (1) unloading-induced high sclerostin levels, which antagonize Wnt-canonical-β-catenin signaling in osteocytes and osteoblasts, permitting simultaneously Wnt-noncanonical and/or other pathways in osteocytes and osteoclasts, directed at bone resorption; (2) mechanical loading results in low sclerostin levels, activation of Wnt-canonical signaling, and bone formation. Therefore, adaptive bone remodeling occurring at a distinct bone compartment is orchestrated by altered sclerostin levels, which regulate the expression of the other osteocyte-specific proteins, such as RANKL, OPG, and proteins encoded by "mineralization-related genes" (DMP1, PHEX, and probably FGF23). For example, under specific terms, sclerostin regulates differential RANKL and OPG production, and creates a dynamic RANKL/OPG ratio, leading either to bone formation or resorption. It also controls the expression of PHEX, DMP1, and most likely FGF23, leading to either bone matrix mineralization or its inhibition. Such opposing up- or down-regulation of remodeling phases allows osteocytes to function as an "external unit", ensuring transition from bone resorption to bone formation.Mini Abstract: The osteocyte network plays a central role in directing bone response either to mechanical loading, or to unloading, leading correspondingly to bone formation or resorption. This review shows a key role of the osteocyte-produced sclerostin as a major mediator of the molecular mechanisms involved in the process of adaptive bone remodeling.

Expression of fibroblast growth factor 23, vitamin D receptor, and sclerostin in bone tissue from hypercalciuric stone formers

BACKGROUND AND OBJECTIVES

Increased bone resorption, low bone formation, and abnormal mineralization have been described in stone formers with idiopathic hypercalciuria. It has been previously shown that the receptor activator of NF-κB ligand mediates bone resorption in idiopathic hypercalciuria (IH). The present study aimed to determine the expression of fibroblast growth factor 23 (FGF-23), vitamin D receptor (VDR), and sclerostin in bone tissue from IH stone formers.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Immunohistochemical analysis was performed in undecalcified bone samples previously obtained for histomorphometry from 30 transiliac bone biopsies of idiopathic hypercalciuria stone-forming patients between 1992 and 2002 and 33 healthy individuals (controls). Serum parameters were obtained from their medical records.

RESULTS

Histomorphometry disclosed 21 IH patients with high and 9 IH patients with normal bone resorption. Importantly, eroded surfaces (ES/BS) from IH patients but not controls were significantly correlated with VDR immunostaining in osteoblasts (r=0.51; P=0.004), sclerostin immunostaining in osteocytes (r=0.41; P=0.02), and serum 1,25-dihydroxyvitamin D (r=0.55; P<0.01). Of note, both VDR and sclerostin immunostaining were significantly correlated with serum 1,25-dihydroxyvitamin D in IH patients (r=0.52; P=0.01 and r=0.53; P=0.02, respectively), although VDR and sclerostin expression did not differ between IH and controls. IH patients with high bone resorption exhibited a significantly stronger sclerostin immunostaining than IH patients with normal bone resorption. FGF-23 expression in osteocytes from IH patients did not differ from controls and was not correlated with any histomorphometric parameter.

CONCLUSIONS

These findings suggest the contribution of VDR and sclerostin, as well as 1,25-dihydroxyvitamin D, to increase bone resorption in idiopathic hypercalciuria but do not implicate FGF-23 in the bone alterations seen in these patients.

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.

Vitamin D endocrine system and osteocytes

The physiological role of the osteocyte, the most numerous of the three bone cell types, was significantly underestimated until recently. It is now known that they not only coordinate bone remodeling but also have an endocrine function as part of the regulatory network for calcium and phosphate homeostasis. Vitamin D and osteocytes interact in numerous ways to accomplish these activities. The major source of active vitamin D (1,25(OH)2D3) is the kidney but there is evidence that osteocytes can produce it as well. Renal 1,25(OH)2D3 regulates osteocyte production of fibroblast growth factor 23 (FGF23), a powerful phosphaturic factor with far-reaching physiological effects. The function of 1,25(OH)2D3 produced by osteocytes themselves is poorly understood and is an area of active research. Osteocytes affect local bone remodeling by producing regulatory factors for osteoblasts and osteoclasts in response to mechanical loading and to endocrine signals such as serum 1,25(OH)2D3. In addition, 1,25(OH)2D3 may inhibit mineralization in osteocyte lacunae. Whether 1,25(OH)2D3 has a role in osteocytic perilacunar remodeling is currently unknown. This short review presents the current state of our knowledge about the physiologically and clinically significant roles of vitamin D signaling in osteocytes.

Sclerostin serum levels in prostate cancer patients and their relationship with sex steroids

The role of sclerostin on bone metabolism and its relation to sex steroids in patients with prostate cancer (PC) is not well known. We found that sclerostin levels are significantly increased in PC patients, particularly in those with androgen deprivation therapy (ADT), and there is an inverse relationship between sclerostin levels and testosterone.

INTRODUCTION

Recent studies have evaluated sclerostin levels in bone diseases as osteoporosis. However, there are few data in PC patients, particularly in patients with hypogonadism related to ADT. The aim of the present study was to compare serum sclerostin levels in ADT/non-ADT-treated PC patients and healthy controls and to evaluate their relationship with sex steroids and bone metabolism.

METHODS

We performed a cross-sectional study involving 81 subjects: 25 ADT-treated PC patients, 34 PC patients without ADT treatment, and 22 healthy controls. We measured serum sclerostin levels, bone turnover markers, bone mineral density (BMD) in all individuals, and sex steroids levels in PC patients.

RESULTS

Serum sclerostin levels were significantly higher in PC patients compared to those in control subjects. ADT-treated patients had significantly higher sclerostin levels than PC patients without ADT treatment: ADT 64.52 ± 27.21 pmol/L, non-ADT 48.24 ± 15.93 pmol/L, healthy controls 38.48 ± 9.19 pmol/L, p < 0.05. In PC patients, we found a negative relationship between serum sclerostin levels and androgens after age adjustment (total testosterone: r = -0.309, p = 0.029; bioavailable testosterone: r = -0.280, p = 0.049; free testosterone: r = -0.299, p = 0.035). We did not observe any relationship between sclerostin levels and bone turnover markers or BMD in any group.

CONCLUSIONS

Circulating sclerostin levels are significantly increased in patients with PC and particularly in those receiving ADT. The inverse relationship between serum sclerostin and testosterone in these patients suggests that androgens are key regulators of bone metabolism in this population.

Sclerostin regulates release of bone mineral by osteocytes by induction of carbonic anhydrase 2

The osteocyte product sclerostin is emerging as an important paracrine regulator of bone mass. It has recently been shown that osteocyte production of receptor activator of NF-κB ligand (RANKL) is important in osteoclastic bone resorption, and we reported that exogenous treatment of osteocytes with sclerostin can increase RANKL-mediated osteoclast activity. There is good evidence that osteocytes can themselves liberate mineral from bone in a process known as osteocytic osteolysis. In the current study, we investigated sclerostin-stimulated mineral dissolution by human primary osteocyte-like cells (hOCy) and mouse MLO-Y4 cells. We found that sclerostin upregulated osteocyte expression of carbonic anhydrase 2 (CA2/Car2), cathepsin K (CTSK/Ctsk), and tartrate-resistant acid phosphatase (ACP5/Acp5). Because acidification of the extracellular matrix is a critical step in the release of mineral from bone, we further examined the regulation by sclerostin of CA2. Sclerostin stimulated CA2 mRNA and protein expression in hOCy and in MLO-Y4 cells. Sclerostin induced a decrease in intracellular pH (pHi) in both cell types as well as a decrease in extracellular pH (pHo) and the release of calcium ions from mineralized substrate. These effects were reversed in the co-presence of the carbonic anhydrase inhibitor, acetozolamide. Car2-siRNA knockdown in MLO-Y4 cells significantly inhibited the ability of sclerostin to both reduce the pHo and release calcium from a mineralized substrate. Knockdown in MLO-Y4 cells of each of the putative sclerostin receptors, Lrp4, Lrp5 and Lrp6, using siRNA, inhibited the sclerostin induction of Car2, Catk and Acp5 mRNA, as well as pHo and calcium release. Consistent with this activity of sclerostin resulting in osteocytic osteolysis, human trabecular bone samples treated ex vivo with recombinant human sclerostin for 7 days exhibited an increased osteocyte lacunar area, an effect that was reversed by the co-addition of acetozolamide. These findings suggest a new role for sclerostin in the regulation of perilacunar mineral by osteocytes.

Involvement of WNT/β-catenin signaling in the treatment of osteoporosis

Osteoblast differentiation is predominantly regulated by the WNT/β-catenin signaling (canonical WNT pathway), which, together with bone morphogenetic proteins, acts as the master regulator of osteogenesis. The recent characterization of the canonical WNT pathway in the regulation of bone modeling and remodeling provided important insights for our understanding of the pathophysiology of a number of conditions and of the mechanism of action of hormones or drugs with important effect on bone metabolism. This review is mainly focused on the growing therapeutic implications of these new findings. WNT/β-catenin signaling plays a key role in bone tissue by determining the differentiation of stem cells into mature osteoblasts rather than into chondrocytes and adipocytes. Its regulation is predominantly driven by the production of two WNT signaling antagonists: sclerostin (SOST) and Dickkopf-related protein 1 (DKK1). The most proximate regulator of SOST expression by osteocytes and its serum levels is bone mechanical load. SOST expression is increased with advancing age, by glucocorticoid treatment and during treatment with antiresorptive agents such as bisphosphonates and denosumab, while it is decreased by parathyroid hormone excess or administration of estrogens. Correlation between DKK1 serum levels and bone formation in various pathological conditions or during osteoporosis treatment has been reported. Inhibitors of the negative regulators of WNT/β-catenin signaling ("inhibiting the endogenous inhibitors") are potential candidates for the prevention and treatment of bone loss. Inactivating monoclonal antibodies against SOST appears to be the most attractive strategy because SOST is the only component of the WNT pathway expressed almost exclusively by osteocytes.

Role of BMPs in the regulation of sclerostin as revealed by an epigenetic modifier of human bone cells

Sclerostin, encoded by the SOST gene, is specifically expressed by osteocytes. However osteoblasts bear a heavily methylated SOST promoter and therefore do not express SOST. Thus, studying the regulation of human SOST is challenged by the absence of human osteocytic cell lines. Herein, we explore the feasibility of using the induction of SOST expression in osteoblasts by a demethylating agent to study the mechanisms underlying SOST transcription, and specifically, the influence of bone morphogenetic proteins (BMPs). Microarray analysis and quantitative PCR showed that AzadC up-regulated the expression of several BMPs, including BMP-2, BMP-4 and BMP-6, as well as several BMP downstream targets. Recombinant BMP-2 increased the transcriptional activity of the SOST promoter cloned into a reporter vector. Likewise, exposing cells transfected with the vector to AzadC also resulted in increased transcription. On the other hand, inhibition of the canonical BMP signaling blunted the effect of AzadC on SOST. These results show that the AzadC-induced demethylation of the SOST promoter in human osteoblastic cells may be a valuable tool to study the regulation of SOST expression. As a proof of concept, it allowed us to demonstrate that BMPs stimulate SOST expression by a mechanism involving BMPR1A receptors and downstream Smad-dependent pathways.

BMP signaling in development and diseases: a pharmacological perspective

Bone morphogenetic protein (BMP) signaling has been implicated in several processes during embryonic development and in adult tissue homeostasis. Maintenance of many organs such as skin, intestinal villi, bones and bone marrow requires continuous regeneration and subsequent differentiation of stem cells in order to maintain organ shape and size necessary for proper functioning. Although BMPs were initially identified as osteogenic factors present in demineralized bone capable of inducing ectopic bone formation, it is now evident that BMPs perform several other functions during embryonic development as well as during the adult life of an organism. Many disorders have been linked to either the BMPs or the molecules functioning downstream of BMP signaling pathway. This review summarizes the existing literature describing the role of BMP signaling during embryonic development and in adult tissue homeostasis to provide a perspective on pharmacological interventions of BMP signaling pathway to mitigate several disease conditions.

[Wnt pathway and sclerostin as new targets for assessment and treatment of osteoporosis]

The increasing knowledge of bone biology has allowed the identification of new intracellular pathways involved in the regulation of remodelling and osteoblast activity. In this respect, the characterization of the Wnt pathway has been a breakthrough for its involvement and role in disorders of mineral metabolism. A better understanding of these signaling pathways may allow the development of new diagnostic markers and new drugs for metabolic bone disease, where despite extensive available therapies, unmet needs still persist. In this review, we make an approach to the discovery and functions of the Wnt pathway with a focus on bone effects. Next, we briefly review the main data about their endogenous antagonist, sclerostin, precisely where drug research is more advanced.

Successful treatment of thyrotoxicosis is accompanied by a decrease in serum sclerostin levels

Sclerostin, a product of a SOST gene, is a protein expressed by osteocytes that inhibits osteoblastic bone formation. Several hormones, including PTH and glucocorticosteroids, have been suggested to be possible regulators of sclerostin production. The influence of thyroid hormones on sclerostin synthesis has not been investigated, so far. The aim of the study was to evaluate sclerostin concentrations in patients before and after treatment of thyrotoxicosis.

PATIENTS AND METHODS

The study involved 15 patients (4 men), mean age 51.8±15.3 years, mean BMI value - 24.7±3.5, with thyrotoxicosis due to Graves' disease or toxic multinodular goitre. Serum sclerostin was measured by immunoassay at diagnosis of thyrotoxicosis and after 6-10 weeks of treatment with thiamazole. The data were analysed by means of simple descriptive statistics of location and dispersion and Mann-Whitney U test for pairs of results, before and after thiamazole therapy. Association between variables was evaluated with use of Spearman`s correlation coefficient.

RESULTS

There was a significant decrease in free T3 (FT3) and free T4 (FT4) concentrations (from 8.74±4.79 pg/ml to 3.54±2.40 pg/ml, and from 4.48±2.21 ng/ml to 1.02±1.07 ng/ml, respectively, p<0.001). This was accompanied by a marked decrease of serum sclerostin levels from 55.46±20.90 pmol/l to 35.73±15.70 pmol/l, p<0.0015). Interestingly, enough, sclerostin levels did not correlate with serum FT3 or FT4 concentrations.

CONCLUSIONS

Restoration of a euthyroid state in patients with thyrotoxicosis results in a significant decrease in serum sclerostin concentrations. The above mentioned phenomenon may reflect lowering of bone metabolism, but a possible direct influence of thyroid hormones on SOST gene needs to be investigated.

Sclerostin and Dickkopf-1 as therapeutic targets in bone diseases

The processes of bone growth, modeling, and remodeling determine the structure, mass, and biomechanical properties of the skeleton. Dysregulated bone resorption or bone formation may lead to metabolic bone diseases. The Wnt pathway plays an important role in bone formation and regeneration, and expression of two Wnt pathway inhibitors, sclerostin and Dickkopf-1 (DKK1), appears to be associated with changes in bone mass. Inactivation of sclerostin leads to substantially increased bone mass in humans and in genetically manipulated animals. Studies in various animal models of bone disease have shown that inhibition of sclerostin using a monoclonal antibody (Scl-Ab) increases bone formation, density, and strength. Additional studies show that Scl-Ab improves bone healing in models of bone repair. Inhibition of DKK1 by monoclonal antibody (DKK1-Ab) stimulates bone formation in younger animals and to a lesser extent in adult animals and enhances fracture healing. Thus, sclerostin and DKK1 are emerging as the leading new targets for anabolic therapies to treat bone diseases such as osteoporosis and for bone repair. Clinical trials are ongoing to evaluate the effects of Scl-Ab and DKK1-Ab in humans for the treatment of bone loss and for bone repair.

Circulating sclerostin is elevated in short-term and reduced in long-term SCI

Spinal cord injury (SCI) causes profound bone loss due to muscle paralysis resulting in the inability to walk. Sclerostin, a Wnt signaling pathway antagonist produced by osteocytes, is a potent inhibitor of bone formation. Short-term studies in rodent models have demonstrated increased sclerostin in response to mechanical unloading that is reversed with reloading. Although sclerostin inhibition has been proposed as a potential therapy for bone loss, it is not known if sclerostin levels vary with duration of SCI in humans. We analyzed circulating sclerostin in 155 men with varying degrees of SCI who were 1 year or more post-injury. We report that sclerostin levels are greatest in subjects with short-term SCI (≤5 years post-injury) and decrease significantly over the first 5 years post-injury. There was no association between sclerostin and injury duration in subjects with long-term SCI (>5 years post-injury). In subjects with long-term SCI, sclerostin levels were positively associated with lower extremity bone density and bone mineral content. These data suggest that sclerostin levels are initially increased after SCI in response to mechanical unloading. This response is time-limited and as bone loss progresses, circulating sclerostin is lowest in subjects with severe osteoporosis. These findings support a dual role for sclerostin after SCI: a therapeutic target in acute SCI, and a biomarker of osteoporosis severity in chronic SCI.

Advancing our understanding of osteocyte cell biology

Osteocytes were the forgotten bone cell until the bone community could become convinced that these cells do serve an important role in bone function and maintenance. In this review we trace the history of osteocyte characterization and present some of the major observations that are leading to the conclusion that these cells are not passive placeholders residing in the bone matrix, but are indeed, major orchestrators of bone remodeling.

Osteocyte regulation of bone mineral: a little give and take

Osteocytes actively participate in almost every phase of mineral handling by bone. They regulate the mineralisation of osteoid during bone formation, and they are also a major RANKL-producing cell. Osteocytes are thus able to liberate bone mineral by regulating osteoclast differentiation and activity in response to a range of stimuli, including bone matrix damage, bone disuse and mechanical unloading, oestrogen deficiency, high-dose glucocorticoid and chemotherapeutic agents. At least some of these activities may be regulated by the osteocyte-secreted product, sclerostin. There is also mounting evidence that in addition to regulating phosphate homeostasis systemically, osteocytes contribute directly to calcium homeostasis in the mature skeleton. Osteocyte cell death and the local loss of control of bone mineralisation may be the cause of focal hypermineralisation of bone and osteopetrosis, as seen in aging and pathology. The sheer number of osteocytes in bone means that "a little give and take" in terms of regulation of bone mineral content translates into a powerful whole organism effect.

Serum sclerostin levels in Paget's disease and prostate cancer with bone metastases with a wide range of bone turnover

Evidence has been accumulating for the role of osteocytes as key players in the regulation of bone remodeling. One of the main products of these cells, sclerostin, inhibits bone formation and may also stimulate bone resorption. Circulating sclerostin has been evaluated in humans, but data are scarce in patients with different rates of bone turnover. To address this issue we evaluated serum sclerostin levels in patients with Paget's disease of bone (PD) and in patients with prostate cancer metastatic to the skeleton (PC). Sclerostin levels were measured in 88 patients with PD, 20 patients with PC and 237 healthy individuals (113 men and 124 women, aged 20 to 77 years). Bone turnover was evaluated by measuring serum levels of procollagen type 1 amino-terminal propeptide (P1NP) in all individuals studied and β-carboxy-terminal cross-linking telopeptide of type I collagen (β-CTX) only in patients. Patients were aged between 45 and 88 years and had a wide range of bone turnover: serum P1NP 9.2 to 1872 ng/ml and β-CTX 50 to 3120 pg/ml. Patients with PD and with PC had significantly higher mean serum sclerostin levels (53.1 ± 22.7 pg/ml and 56.6 ± 25.8 pg/ml, respectively) compared to healthy controls (38.1 ± 12.1 pg/ml) (p<0.001). Serum sclerostin levels were significantly correlated with P1NP in all (n=345) studied subjects (r=0.32, p<0.001). Circulating sclerostin levels are significantly increased in patients with increased bone turnover, regardless of underlying pathology. These increased levels may be due to a compensatory response to the increased number of osteoblasts at affected skeletal sites and may contribute to the increased bone resorption in patients with PC .

Combinatorial use of bone morphogenetic protein 6, noggin and SOST significantly predicts cancer progression

Emerging evidence has indicated a role of the bone morphogenetic proteins (BMP) in the pathogenesis of certain cancers. The signaling of BMP family members is tightly regulated by their antagonists, including noggin and SOST, which are, in turn, positively regulated by BMP, thereby forming a negative feedback loop. Consequently, the expression of these antagonists should be taken into account in studies on the prognostic significance of BMP. In the present paper, we correlated protein and mRNA expression levels of BMP6, noggin and SOST, alone or in combination, with patient survival in various types of cancer. We found that BMP6 alone was not significantly correlated with esophageal squamous cell carcinoma patient survival. Instead, a high level of inhibitor of differentiation 1, a downstream factor of BMP6, was associated with shorter survival in patients whose tumors stained strongly for BMP6. Knockdown of noggin in esophageal cancer cell line EC109, which expresses BMP6 strongly and SOST weakly, enhanced the non-adherent growth of the cells. Noggin and SOST expression levels, when analyzed alone, were not significantly correlated with patient survival. However, high BMP6 activity, defined by strong BMP6 expression coupled with weak noggin or SOST expression, was significantly associated with shorter survival in esophageal squamous cell carcinoma patients. We further confirmed that BMP6 activity could be used as a prognostic indicator in prostate, bladder and colorectal cancers, using publicly available data on BMP6, noggin and SOST mRNA expression and patient survival. Our results strongly suggest that BMP6, noggin and SOST could be used in combination as a prognostic indicator in cancer progression.

TGF-β regulates sclerostin expression via the ECR5 enhancer

Wnt signaling is critical for skeletal development and homeostasis. Sclerostin (Sost) has emerged as a potent inhibitor of Wnt signaling and, thereby, bone formation. Thus, strategies to reduce sclerostin expression may be used to treat osteoporosis or non-union fractures. Transforming growth factor-beta (TGF-β) elicits various effects upon the skeleton both in vitro and in vivo depending on the duration and timing of administration. In vitro and in vivo studies demonstrate that TGF-β increases osteoprogenitor differentiation but decreases matrix mineralization of committed osteoblasts. Because sclerostin decreases matrix mineralization, this study aimed to examine whether TGF-β achieves such inhibitory effects via transcriptional modulation of Sost. Using the UMR106.01 mature osteoblast cell line, we demonstrated that TGF-βTGF-β(1)-β(2)-β(3) and Activin A increase Sost transcript expression. Pharmacologic inhibition of Alk4/5/7 in vitro and in vivo decreased endogenous Sost expression, and siRNA against Alk4 and Alk5 demonstrated their requirement for endogenous Sost expression. TGF-β(1) targeted the Sost bone enhancer ECR5 and did not affect the transcriptional activity of the endogenous Sost promoter. These results indicate that TGF-β(1) controls Sost transcription in mature osteoblasts, suggesting that sclerostin may mediate the inhibitory effect of TGF-β upon osteoblast differentiation.

Mef2c deletion in osteocytes results in increased bone mass

Myocyte enhancer factors 2 (MEF2) are required for expression of the osteocyte bone formation inhibitor Sost in vitro, implying these transcription factors in bone biology. Here, we analyzed the in vivo function of Mef2c in osteocytes in male and female mice during skeletal growth and aging. Dmp1-Cre-induced Mef2c deficiency led to progressive decreases in Sost expression by 40% and 70% in femoral cortical bone at 3.5 months and 5 to 6 months of age. From 2 to 3 months onward, bone mass was increased in the appendicular and axial skeleton of Mef2c mutant relative to control mice. Cortical thickness and long bone and vertebral trabecular density were elevated. To assess whether the increased bone mass was related to the decreased Sost expression, we characterized 4-month-old heterozygous Sost-deficient mice. Sost heterozygotes displayed similar increases in long bone mass and density as Mef2c mutants, but the relative increases in axial skeletal parameters were mostly smaller. At the cellular level, bone formation parameters were normal in 3.5-month-old Mef2c mutant mice, whereas bone resorption parameters were significantly decreased. Correspondingly, cortical expression of the anti-osteoclastogenic factor and Wnt/β-catenin target gene osteoprotegerin (OPG) was increased by 70% in Mef2c mutant males. Furthermore, cortical expression of the Wnt signaling modulators Sfrp2 and Sfrp3 was strongly deregulated in both sexes. In contrast, heterozygous Sost deficient males displayed mildly increased osteoblastic mineral apposition rate, but osteoclast surface and cortical expression of osteoclastogenic regulators including OPG were normal and Sfrp2 and Sfrp3 were not significantly changed. Together, our data demonstrate that Mef2c regulates cortical Sfrp2 and Sfrp3 expression and is required to maintain normal Sost expression in vivo. Yet, the increased bone mass phenotype of Mef2c mutants is not directly related to the reduced Sost expression. We identified a novel function for Mef2c in control of adult bone mass by regulation of osteoclastic bone resorption.

Sclerostin expression is induced by BMPs in human Saos-2 osteosarcoma cells but not via direct effects on the sclerostin gene promoter or ECR5 element

Sclerostin is a secreted inhibitor of Wnt signaling and plays an essential role in the regulation of bone mass. The expression of sclerostin is largely restricted to osteocytes although its mode of transcriptional regulation is not well understood. We observed regulated expression of sclerostin mRNA and protein that was directly correlated with the mineralization response in cultured human Saos-2 osteosarcoma cells and rat primary calvarial cells. Sclerostin mRNA and protein levels were increased following treatment of cells with BMP2, BMP4 and BMP7. Analysis of deletion mutants from the -7.4 kb upstream region of the human sclerostin promoter did not reveal any specific regions that were responsive to BMPs, Wnt3a, PTH, TGFβ1 or Activin A in Saos-2 cells. The downstream ECR5 element did not show enhancer activity in Saos-2 cells and also was not affected when Saos-2 cells were treated with BMPs or PTH. Genome-wide microarray analysis of Saos-2 cells treated with BMP2 showed significant changes in expression of several transcription factors with putative consensus DNA binding sites in the region of the sclerostin promoter. However, whereas most factors tested showed either a range of inhibitory activity (DLX family, MSX2, HEY1, SMAD6/7) or lack of activity on the sclerostin promoter including SMAD9, only MEF2B showed a positive effect on both the promoter and ECR5 element. These results suggest that the dramatic induction of sclerostin gene expression by BMPs in Saos-2 cells occurs indirectly and is associated with late stage differentiation of osteoblasts and the mineralization process.

Sclerostin stimulates osteocyte support of osteoclast activity by a RANKL-dependent pathway

Sclerostin is a product of mature osteocytes embedded in mineralised bone and is a negative regulator of bone mass and osteoblast differentiation. While evidence suggests that sclerostin has an anti-anabolic role, the possibility also exists that sclerostin has catabolic activity. To test this we treated human primary pre-osteocyte cultures, cells we have found are exquisitely sensitive to sclerostin, or mouse osteocyte-like MLO-Y4 cells, with recombinant human sclerostin (rhSCL) and measured effects on pro-catabolic gene expression. Sclerostin dose-dependently up-regulated the expression of receptor activator of nuclear factor kappa B (RANKL) mRNA and down-regulated that of osteoprotegerin (OPG) mRNA, causing an increase in the RANKL∶OPG mRNA ratio. To examine the effects of rhSCL on resulting osteoclastic activity, MLO-Y4 cells plated onto a bone-like substrate were primed with rhSCL for 3 days and then either mouse splenocytes or human peripheral blood mononuclear cells (PBMC) were added. This resulted in cultures with elevated osteoclastic resorption (approximately 7-fold) compared to untreated co-cultures. The increased resorption was abolished by co-addition of recombinant OPG. In co-cultures of MLO-Y4 cells with PBMC, SCL also increased the number and size of the TRAP-positive multinucleated cells formed. Importantly, rhSCL had no effect on TRAP-positive cell formation from monocultures of either splenocytes or PBMC. Further, rhSCL did not induce apoptosis of MLO-Y4 cells, as determined by caspase activity assays, demonstrating that the osteoclastic response was not driven by dying osteocytes. Together, these results suggest that sclerostin may have a catabolic action through promotion of osteoclast formation and activity by osteocytes, in a RANKL-dependent manner.

Sclerostin is a locally acting regulator of late-osteoblast/preosteocyte differentiation and regulates mineralization through a MEPE-ASARM-dependent mechanism

The identity of the cell type responsive to sclerostin, a negative regulator of bone mass, is unknown. Since sclerostin is expressed in vivo by mineral-embedded osteocytes, we tested the hypothesis that sclerostin would regulate the behavior of cells actively involved in mineralization in adult bone, the preosteocyte. Differentiating cultures of human primary osteoblasts exposed to recombinant human sclerostin (rhSCL) for 35 days displayed dose- and time-dependent inhibition of in vitro mineralization, with late cultures being most responsive in terms of mineralization and gene expression. Treatment of advanced (day 35) cultures with rhSCL markedly increased the expression of the preosteocyte marker E11 and decreased the expression of mature markers DMP1 and SOST. Concomitantly, matrix extracellular phosphoglycoprotein (MEPE) expression was increased by rhSCL at both the mRNA and protein levels, whereas PHEX was decreased, implying regulation through the MEPE-ASARM axis. We confirmed that mineralization by human osteoblasts is exquisitely sensitive to the triphosphorylated ASARM-PO4 peptide. Immunostaining revealed that rhSCL increased the endogenous levels of MEPE-ASARM. Importantly, antibody-mediated neutralization of endogenous MEPE-ASARM antagonized the effect of rhSCL on mineralization, as did the PHEX synthetic peptide SPR4. Finally, we found elevated Sost mRNA expression in the long bones of HYP mice, suggesting that sclerostin may drive the increased MEPE-ASARM levels and mineralization defect in this genotype. Our results suggest that sclerostin acts through regulation of the PHEX/MEPE axis at the preosteocyte stage and serves as a master regulator of physiologic bone mineralization, consistent with its localization in vivo and its established role in the inhibition of bone formation.