Elena Tsourdi

Calcitonin controls bone formation by inhibiting the release of sphingosine 1-phosphate from osteoclasts

The hormone calcitonin (CT) is primarily known for its pharmacologic action as an inhibitor of bone resorption, yet CT-deficient mice display increased bone formation. These findings raised the question about the underlying cellular and molecular mechanism of CT action. Here we show that either ubiquitous or osteoclast-specific inactivation of the murine CT receptor (CTR) causes increased bone formation. CT negatively regulates the osteoclast expression of Spns2 gene, which encodes a transporter for the signalling lipid sphingosine 1-phosphate (S1P). CTR-deficient mice show increased S1P levels, and their skeletal phenotype is normalized by deletion of the S1P receptor S1P3. Finally, pharmacologic treatment with the nonselective S1P receptor agonist FTY720 causes increased bone formation in wild-type, but not in S1P3-deficient mice. This study redefines the role of CT in skeletal biology, confirms that S1P acts as an osteoanabolic molecule in vivo and provides evidence for a pharmacologically exploitable crosstalk between osteoclasts and osteoblasts.

Selective Glucocorticoid Receptor Modulation Maintains Bone Mineral Density in Mice

Glucocorticoids (GCs) are potent anti‐inflammatory drugs, but their use is limited by their adverse effects on the skeleton. Compound A (CpdA) is a novel GC receptor modulator with the potential for an improved risk/benefit profile. We tested the effects of CpdA on bone in a mouse model of GC‐induced bone loss. Bone loss was induced in FVB/N mice by implanting slow‐release pellets containing either vehicle, prednisolone (PRED) (3.5 mg), or CpdA (3.5 mg). After 4 weeks, mice were killed to examine the effects on the skeleton using quantitative computed tomography, bone histomorphometry, serum markers of bone turnover, and gene expression analysis. To assess the underlying mechanisms, in vitro studies were performed with human bone marrow stromal cells (BMSCs) and murine osteocyte‐like cells (MLO‐Y4 cells). PRED reduced the total and trabecular bone density in the femur by 9% and 24% and in the spine by 11% and 20%, respectively, whereas CpdA did not influence these parameters. Histomorphometry confirmed these results and further showed that the mineral apposition rate was decreased by PRED whereas the number of osteoclasts was increased. Decreased bone formation was paralleled by a decline in serum procollagen type 1 N‐terminal peptide (P1NP), reduced skeletal expression of osteoblast markers, and increased serum levels of the osteoblast inhibitor dickkopf‐1 (DKK‐1). In addition, serum CTX‐1 and the skeletal receptor activator of NF‐κB ligand (RANKL)/osteoprotegerin (OPG) ratio were increased by PRED. None of these effects were observed with CpdA. Consistent with the in vivo data, CpdA did not increase the RANKL/OPG ratio in MLO‐Y4 cells or the expression of DKK‐1 in bone tissue, BMSCs, and osteocytes. Finally, CpdA also failed to transactivate DKK‐1 expression in bone tissue, BMSCs, and osteocytes. This study underlines the bone‐sparing potential of CpdA and suggests that by preventing increases in the RANKL/OPG ratio or DKK‐1 in osteoblast lineage cells, GC‐induced bone loss may be ameliorated.

Skeletal and extraskeletal actions of denosumab

Osteoclasts and osteoblasts define skeletal mass, structure and strength through their respective actions in resorbing and forming bone. This remodeling process is orchestrated by the actions of hormones and growth factors, which regulate a cytokine system comprising the receptor activator of nuclear factor κB ligand (RANKL), its receptor RANK and the soluble decoy receptor osteoprotegerin (OPG). Bone resorption depends on RANKL, which determines osteoclast formation, activity and survival. Importantly, cells of the osteoblastic lineage mainly provide RANKL and therefore, are central in the regulation of osteoclast functions. Catabolic effects of RANKL are inhibited by OPG, a TNF receptor family member that binds RANKL, thereby preventing the activation of its receptor RANK, which is expressed by osteoclast precursors. Because this cytokine network is pivotal for the regulation of bone mass in health and diseases, including osteoporosis, rheumatoid arthritis and malignant bone conditions, it has been successfully used for the generation of a targeted therapy to block osteoclast actions. The clinical approval of denosumab, a fully monoclonal antibody against RANKL, provides a novel option to treat bone diseases with a potent, targeted and reversible inhibitor of bone resorption. Although RANKL is also expressed by endothelial cells, T lymphocytes, synovial fibroblasts and various tumor cells, no meaningful clinical extraskeletal effects have been reported after administration of denosumab. This article summarizes the molecular and cellular basis of the RANKL/RANK/OPG system and presents preclinical and clinical studies on the skeletal actions of denosumab.

Discontinuation of Denosumab therapy for osteoporosis: A systematic review and position statement by ECTS

INTRODUCTION The optimal duration of osteoporosis treatment is controversial. As opposed to bisphosphonates, denosumab does not incorporate into bone matrix and bone turnover is not suppressed after its cessation. Recent reports imply that denosumab discontinuation may lead to an increased risk of multiple vertebral fractures. METHODS The European Calcified Tissue Society (ECTS) formed a working group to perform a systematic review of existing literature on the effects of stopping denosumab and provide advice on management. RESULTS Data from phase 2 and 3 clinical trials underscore a rapid decrease of bone mineral density (BMD) and a steep increase in bone turnover markers (BTMs) after discontinuation of denosumab. Clinical case series report multiple vertebral fractures after discontinuation of denosumab and a renewed analysis of FREEDOM and FREEDOM Extension Trial suggests, albeit does not prove, that the risk of multiple vertebral fractures may be increased when denosumab is stopped due to a rebound increase in bone resorption. CONCLUSION There appears to be an increased risk of multiple vertebral fractures after discontinuation of denosumab although strong evidence for such an effect and for measures to prevent the occurring bone loss is lacking. Clinicians and patients should be aware of this potential risk. Based on available data, a re-evaluation should be performed after 5years of denosumab treatment. Patients considered at high fracture risk should either continue denosumab therapy for up to 10years or be switched to an alternative treatment. For patients at low risk, a decision to discontinue denosumab could be made after 5years, but bisphosphonate therapy should be considered to reduce or prevent the rebound increase in bone turnover. However, since the optimal bisphosphonate regimen post-denosumab is currently unknown continuation of denosumab can also be considered until results from ongoing trials become available. Based on current data, denosumab should not be stopped without considering alternative treatment in order to prevent rapid BMD loss and a potential rebound in vertebral fracture risk.

Seizures associated with zoledronic acid for osteoporosis.

CONTEXT Bisphosphonates represent potent antiresorptive drugs that are established for therapy of patients with benign and malignant bone diseases. Zoledronic acid is an iv aminobisphosphonate that is administered annually against osteoporosis. Because of its potency and the parenteral route of administration, zoledronic acid is an alternative to oral bisphosphonates, in particular in elderly patients with multiple comorbidities. The most common side effects include an acute-phase reaction and mild and transient hypocalcemia. OBJECTIVE Here, we report three cases of seizures that developed after the administration of zoledronic acid. METHODS We review case histories and laboratory results of three patients with seizures associated with the administration of zoledronic acid. We discuss their course and comorbidities in the context of the published literature. RESULTS All three patients were elderly persons with multiple comorbidities, including neurological diseases, that required parenteral bisphosphonates for severe osteoporosis with concurrent contraindications for oral bisphosphonates. CONCLUSION We analyze potential mechanisms underlying these seizures in association with zoledronic acid exposure and discuss potential strategies to minimize this risk.

Hyperthyroidism and Hypothyroidism in Male Mice and Their Effects on Bone Mass, Bone Turnover, and the Wnt Inhibitors Sclerostin and Dickkopf-1

Thyroid hormones are key regulators of bone homeostasis, and Wnt signaling has been implicated in thyroid hormone-associated bone loss. Here we tested whether hyperthyroidism and hypothyroidism interfere with dickkopf-1 (DKK1) and sclerostin, two inhibitors of Wnt signaling. Twelve-week-old male C57BL/6 mice were rendered either hyperthyroid or hypothyroid. Hyperthyroid mice displayed decreased trabecular (-54%, P < .001) and cortical bone density (-5%, P < .05) and reduced cortical thickness (-15%, P < .001), whereas hypothyroid mice showed a higher trabecular bone density (+26%, P < .001) with unchanged cortical bone parameters. Histomorphometry and biochemical markers of bone remodeling indicated high bone turnover in hyperthyroid mice and low bone turnover in hypothyroid mice. In vivo, serum DKK1 concentrations were decreased in hyperthyroid mice (-24%, P < .001) and increased in hypothyroid mice (+18%, P < .01). The increase of the number of DKK1-positive cells in hypothyroid mice was confirmed at the tissue level. Interestingly, sclerostin was increased in both disease models, although to a higher extent in hyperthyroid mice (+50%, P < .001, and +24%, P < .05). Serum sclerostin concentrations adjusted for bone mass were increased by 3.3-fold in hyperthyroid (P < .001) but not in hypothyroid mice. Consistently, sclerostin mRNA expression and the number of sclerostin-positive cells were increased in hyperthyroid but not in hypothyroid mice. Our data show that thyroid hormone-induced changes in bone remodeling are associated with a divergent regulation of DKK1 and sclerostin. Thus, the modulation of Wnt signaling by thyroid hormones may contribute to thyroid hormone-associated bone disease and altered expression of Wnt inhibitors may emerge as potential therapeutic targets.

Sclerostin Blockade and Zoledronic Acid Improve Bone Mass and Strength in Male Mice With Exogenous Hyperthyroidism

Hyperthyroidism in mice is associated with low bone mass, high bone turnover, and high concentrations of sclerostin, a potent Wnt inhibitor. Here, we explored the effects of either increasing bone formation with sclerostin antibodies (Scl-Ab) or reducing bone turnover with bisphosphonates on bone mass and strength in hyperthyroid mice. Twelve-week-old C57BL/6 male mice were rendered hyperthyroid using l-thyroxine (T4; 1.2 µg/mL added to the drinking water) and treated with 20 mg/kg Scl-Ab twice weekly or 100 µg/kg zoledronic acid (ZOL) once weekly or phosphate-buffered saline for 4 weeks. Hyperthyroid mice displayed a lower trabecular bone volume at the spine (-42%, P < 0.05) and the distal femur (-55%, P < 0.05) compared with euthyroid controls. Scl-Ab and ZOL treatment of hyperthyroid mice increased trabecular bone volume at the spine by threefold and twofold, respectively. Serum bone formation and resorption markers were increased in hyperthyroid mice and suppressed by treatment with ZOL but not Scl-Ab. Trabecular bone stiffness at the lumbar vertebra was 63% lower in hyperthyroid mice (P < 0.05) and was increased fourfold by Sci-Ab (P < 0.001) and threefold by ZOL treatment (P < 0.01). Bone strength based on ultimate load, which was 10% lower in hyperthyroidism, was increased by Scl-Ab by 71% and ZOL by 22% (both P < 0.001). Increased proportion of low mineralized bone seen in hyperthyroid mice was restored by treatment with Scl-Ab and ZOL. Thus, bone-forming and antiresorptive drugs prevent bone loss in hyperthyroid mice via different mechanisms.

Glycosaminoglycans and their sulfate derivatives differentially regulate the viability and gene expression of osteocyte-like cell lines

Collagen and glycosaminoglycans, such as hyaluronan and chondroitin sulfate, are the major components of bone extracellular matrix, and extracellular matrix composites are being evaluated for a wide range of clinical applications. The molecular and cellular effects of native and sulfate-modified glycosaminoglycans on osteocytes were investigated as critical regulators of bone remodeling. The effects of glycosaminoglycans on viability, necrosis, apoptosis, and regulation of gene expression were tested in two osteocyte-like cell lines, the murine MLO-Y4 and the rat UMR 106-01 cells. Glycosaminoglycans were non-toxic and incorporated by osteocytic cells. In MLO-Y4 cells, sulfation of glycosaminoglycans led to a significant inhibition of osteocyte apoptosis, 42% inhibition for highly sulfated chondroitin sulfate and 58% for highly sulfated hyaluronan, respectively. Cell proliferation was not affected. While treatment with highly sulfated chondroitin sulfate increased cell viability by 20% compared to the native chondroitin sulfate. In UMR 106-01 cells, treatment with highly sulfated hyaluronan reduced the receptor activator of nuclear factor-κB ligand/osteoprotegerin ratio by 58% compared to the non-sulfated form, whereas highly sulfated chondroitin sulfate led to 60% reduction in the receptor activator of nuclear factor-κB ligand/osteoprotegerin ratio in comparison to the native chondroitin sulfate. The expression of SOST, the gene encoding sclerostin, was reduced by 50% and 45% by highly sulfated hyaluronan and chondroitin sulfate, respectively, compared to their native forms. The expression of BMP-2, a marker of osteoblast differentiation, was doubled after treatment with the highly sulfated hyaluronan in comparison to its native form. In conclusion, highly sulfated glycosaminoglycans inhibit osteocyte apoptosis in vitro and promote an osteoblast-supporting gene expression profile.

Thy-1 (CD90) promotes bone formation and protects against obesity

Thy-1 on mesenchymal stem cells promotes osteogenesis while inhibiting adipogenesis and obesity. Stem cells’ balancing act Mesenchymal stem cells (MSCs) differentiate into multiple cell types. Picke et al. investigated how MSC differentiation is regulated to maintain homeostasis between the bone and fat lineages. Genetic deletion of Thy-1, a protein expressed on multiple cell types including MSCs, prevented MSC differentiation into osteoblasts but promoted differentiation into adipocytes. Thy-1–deficient mice had increased body fat and decreased bone mass. High-fat diet induced obesity in wild-type mice and concurrent reduction in bone formation, which was associated with decreased Thy-1 expression on MSCs. Obese human subjects and subjects with osteoporosis showed reductions in serum soluble Thy-1. This study suggests that Thy-1 regulates the balance between bone and fat lineages, with possible implications for bone and metabolic disorders. Osteoporosis and obesity result from disturbed osteogenic and adipogenic differentiation and present emerging challenges for our aging society. Because of the regulatory role of Thy-1 in mesenchyme-derived fibroblasts, we investigated the impact of Thy-1 expression on mesenchymal stem cell (MSC) fate between osteogenic and adipogenic differentiation and consequences for bone formation and adipose tissue development in vivo. MSCs from Thy-1–deficient mice have decreased osteoblast differentiation and increased adipogenic differentiation compared to MSCs from wild-type mice. Consistently, Thy-1–deficient mice exhibited decreased bone volume and bone formation rate with elevated cortical porosity, resulting in lower bone strength. In parallel, body weight, subcutaneous/epigonadal fat mass, and bone fat volume were increased. Thy-1 deficiency was accompanied by reduced expression of specific Wnt ligands with simultaneous increase of the Wnt inhibitors sclerostin and dickkopf-1 and an altered responsiveness to Wnt. We demonstrated that disturbed bone remodeling in osteoporosis and dysregulated adipose tissue accumulation in patients with obesity were mirrored by reduced serum Thy-1 concentrations. Our findings provide new insights into the mutual regulation of bone formation and obesity and open new perspectives to monitor and to interfere with the dysregulated balance of adipogenesis and osteogenesis in obesity and osteoporosis.

Immunoadsorption Followed by Rituximab as a Definitive Treatment for Insulin Autoimmune Syndrome (Hirata Syndrome): A Case Report

A 59-year-old Caucasian man (170 cm, 78 kg, BMI 27 kg/m2) was referred for evaluation of recurrent episodes of severe hypoglycemia, which had commenced a week earlier. Computed tomography and MRI scans excluding abnormalities of the pancreas had already been performed. The patient had previously suffered from arterial hypertension, hyperlipoproteinemia, and an acute myocardial infarction the year before. Pharmacological therapy consisted of a β-receptor antagonist, a statin, low dose aspirin, clopidogrel, and vitamin D. Importantly, there was no history of alcohol abuse or diabetes and no previous exposure to antidiabetes medication (Fig. 1). Upon admission, the patient showed a sustained need for intravenous glucose supplementation with repetitive symptomatic declines of blood glucose to levels of relevant hypoglycemia (<2.5 mmol/L) immediately after discontinuation of parenteral glucose. Figure 1 Serum parameters in relation to the course of treatment. Results of IAB, protein, and IgG are depicted in the graph during the …