Glycoprotein 130 regulates bone turnover and bone size by distinct downstream signaling pathways

Communication between ephrinB2 and EphB4 within the osteoblast lineage

Members of the ephrin and Eph family are local mediators of cell function through largely contact-dependent processes in development and in maturity. Production of ephrinB2 mRNA and protein are increased by PTH and PTHrP in osteoblasts. Both a synthetic peptide antagonist of ephrinB2/EphB4 receptor interaction and recombinant soluble extracellular domain of EphB4 (sEphB4), which is an antagonist of both forward and reverse EphB4 signaling, were able to inhibit mineralization and the expression of several osteoblast genes involved late in osteoblast differentiation. The findings are consistent with ephrinB2/EphB4 signaling within the osteoblast lineage having a paracrine role in osteoblast differentiation, in addition to the proposed role of osteoclast-derived ephrinB2 in coupling of bone formation to resorption. This local regulation might contribute to control of osteoblast differentiation and bone formation at remodeling sites, and perhaps also in modeling.

The dual role of IL-6-type cytokines on bone remodeling and bone tumors

Many factors such as vitamins, hormones and cytokines, control bone metabolism and remodeling. Cytokines of the interleukin-6 family, by acting on bone cells (i.e. osteoblasts and osteoclasts), have an important role in the bone tissue but they recently appeared as double-edged swords. They sustain bone formation but they can also drive bone loss in various osteolytic pathologies. Similarly, development of bone cancers can be either prevented or enhanced by these cytokines, depending on the cell type, the stage of the tumor and the bone environment. This dual effect is also apparent at the level of the signal transducer and activator of transcription and the mitogen-activated protein kinases, the two main signaling pathways that mediate opposite effects in bone cells.

Fibroblast growth factor inhibits interferon gamma-STAT1 and interleukin 6-STAT3 signaling in chondrocytes

Activation of fibroblast growth factor receptor 3 (FGFR3) leads to attenuation of cartilage growth. The members of the STAT family of transcription factors are believed to participate in FGFR3 signaling in cartilage, however the molecular mechanism of this action is poorly understood. Here, we demonstrate that a chronic FGF stimulus leads to accumulation of STAT1, 3, 5 and 6, evident in both in vitro chondrocyte model and murine limb explant cultures. Despite the accumulation, both endogenous and cytokine-induced activation of STAT1 and STAT3 is impaired by FGF, as demonstrated by imaging of active STAT nuclear translocation and analyses of STAT activatory phosphorylation and transcriptional activation. Further, we demonstrate that FGF induces expression of CIS, SOCS1 and SOCS3 inhibitors of gp130, a common receptor for the IL6-family of cytokines. Since cytokine-gp130 signaling represents an important positive regulator of cartilage, its inhibition may contribute to the growth-inhibitory effect of FGFR3 in cartilage.

Bone remodelling: its local regulation and the emergence of bone fragility

Bone modelling prevents the occurrence of damage by adapting bone structure - and hence bone strength - to its loading circumstances. Bone remodelling removes damage, when it inevitably occurs, in order to maintain bone strength. This cellular machinery is successful during growth, but fails during advancing age because of the development of a negative balance between the volumes of bone resorbed and formed during remodelling by the basic multicellular unit (BMU), high rates of remodelling during midlife in women and late in life in both sexes, and a decline in periosteal bone formation. together resulting in bone loss and structural decay each time a remodelling event occurs. The two steps in remodelling - resorption of a volume of bone by osteoclasts and formation of a comparable volume by osteoblasts - are sequential, but the regulatory events leading to these two fully differentiated functions are not. Reparative remodelling is initiated by damage producing osteocyte apoptosis, which signals the location of damage via the osteocyte canalicular system to endosteal lining cells which forms the canopy of a bone-remodelling compartment (BRC). Within the BRC, local recruitment of osteoblast precursors from the lining cells, the marrow and circulation, direct contact with osteoclast precursors, osteoclastogenesis and molecular cross-talk between precursors, mature cells, cells of the immune system, and products of the resorbed matrix, titrate the birth, work and lifespan of the cells of this multicellular remodelling machinery to either remove or form a net volume of bone appropriate to the mechanical requirements.

IL-6 Induces Osteoblastic Differentiation of Periodontal Ligament Cells

Interleukin (IL)-6 has been considered as an osteolytic factor involved in periodontal disease. However, the function of IL-6 in osteoblastic differentiation of periodontal ligament cells is not clear. We examined the effects of IL-6 and its soluble receptor (sIL-6R) on osteoblastic differentiation of periodontal ligament cells. Osteoblastic differentiation was induced by ascorbic acid. Osteoblast markers, including alkaline phosphatase activity and Runx2 gene expression, were examined. The mechanism of action of IL-6 on osteoblastic differentiation was evaluated by insulin-like growth factor (IGF)-I production and specific inhibitors for the IL-6-signaling molecule. IL-6/sIL-6R enhanced alkaline phosphatase activity and Runx2. Alkaline phosphatase activity was reduced by anti-IGF-I antibody. Mitogen-activated protein kinase and Janus protein tyrosine kinase inhibitors diminished alkaline phosphatase induced by IL-6/sIL-6R. We conclude that IL-6/sIL-6R increases ascorbic-acid-induced alkaline phosphatase activity through IGF-I production, implying that IL-6 acts not only as an osteolytic factor, but also as a mediator of osteoblastic differentiation in periodontal ligament cells.

Eph receptors and ephrin signaling pathways: a role in bone homeostasis

The maintenance of bone homeostasis is tightly controlled, and largely dependent upon cellular communication between osteoclasts and osteoblasts, and the coupling of bone resorption to bone formation. This tight coupling is essential for the correct function and maintenance of the skeletal system, repairing microscopic skeletal damage and replacing aged bone. A range of pathologic diseases, including osteoporosis and cancer-induced bone disease, disrupt this coupling and cause subsequent alterations in bone homeostasis. Eph receptors and their associated ligands, ephrins, play critical roles in a number of cellular processes including immune regulation, neuronal development and cancer metastasis. Eph receptors are also expressed by cells found within the bone marrow microenvironment, including osteoclasts and osteoblasts, and there is increasing evidence to implicate this family of receptors in the control of normal and pathological bone remodeling.

EphrinB2 regulation by PTH and PTHrP revealed by molecular profiling in differentiating osteoblasts

With the aim of identifying new pathways and genes regulated by PTH(1-34) and PTH-related protein 1-141 [PTHrP(1-141)] in osteoblasts, this study was carried out using a mouse marrow stromal cell line, Kusa 4b10, that acquires features of the osteoblastic phenotype in long-term culture conditions. After the appearance of functional PTH receptor 1 (PTHR1) in Kusa 4b10 cells, they were treated with either PTH(1-34) or PTHrP(1-141), and RNA was subjected to Affymetrix whole mouse genome array. The microarray data were validated using quantitative real-time RT-PCR on independently prepared RNA samples from differentiated Kusa 4b10, UMR106 osteosarcoma cells, and primary mouse calvarial osteoblasts, as well as in vivo using RNA from metaphyseal bone after a single PTH injection to 3-wk-old and 6-mo-old ovariectomized rats. Of the 45,101 probes used on the microarray, 4675 were differentially expressed by >or=1.5 fold, with a false discovery rate <0.1. Among the regulated genes, ephrinB2 mRNA was upregulated in response to both PTH and PTHrP. This was confirmed by quantitative real-time PCR in vitro and in vivo. Increased ephrinB2 protein was also shown in vitro by Western blotting, and immunostaining of femur sections showed ephrinB2 in both osteoclasts and osteoblasts. Production of ephrinB2, as well as other ephrins or Eph family members, did not change during differentiation of Kusa 4b10 cells. Blockade of ephrinB2/EphB4 interaction resulted in inhibition of mineralization of Kusa 4b10 cells. Together with the shown effect of ephrinB2 promoting osteoblast differentiation and bone formation through action on EphB4, the data raise the possibility that PTH or PTHrP might regulate ephrinB2 to act in a paracrine or autocrine manner on EphB4 or EphB2 in the osteoblast, contributing as a local event to the anabolic action of PTH or PTHrP.

Interleukin-6 inhibits receptor activator of nuclear factor kappaB ligand-induced osteoclastogenesis by diverting cells into the macrophage lineage: key role of Serine727 phosphorylation of signal transducer and activator of transcription 3

Osteoclasts are bone-resorptive cells that differentiate from hematopoietic precursors upon receptor activator of nuclear factor kappaB ligand (RANKL) activation. Previous studies demonstrated that IL-6 indirectly stimulates osteoclastogenesis through the production of RANKL by osteoblasts. However, few data described the direct effect of IL-6 on osteoclasts. To investigate this effect, we used several models: murine RAW264.7 cells, mouse bone marrow, and human blood monocytes. In the three models used, the addition of IL-6 inhibited RANKL-induced osteoclastogenesis. Furthermore, IL-6 decreased the expression of osteoclast markers and up-modulated macrophage markers. To elucidate this inhibition, signal transducer and activator of transcription (STAT) 3, the main signaling molecule activated by IL-6, was analyzed. Addition of two STAT3 inhibitors completely abolished RANKL-induced osteoclastogenesis, revealing a key role of STAT3. We demonstrated that a basal level of phosphorylated-STAT3 on Serine(727) associated with an absence of phosphorylation on Tyrosine(705) is essential for osteoclastogenesis. Furthermore, a decrease of Serine(727) phosphorylation led to an inhibition of osteoclast differentiation, whereas an increase of Tyrosine(705) phosphorylation upon IL-6 stimulation led to the formation of macrophages instead of osteoclasts. In conclusion, we showed for the first time that IL-6 inhibits RANKL-induced osteoclastogenesis by diverting cells into the macrophage lineage, and demonstrated the functional role of activated-STAT3 and its form of phosphorylation in the control of osteoclastogenesis.

Prediction of osteoporosis candidate genes by computational disease-gene identification strategy

Osteoporosis is a complex disease with a strong genetic component. To date, more than 20 genome-wide linkage scans across multiple populations have been launched to hunt for osteoporosis susceptibility genes. Some significant or suggestive chromosomal regions of linkage to bone mineral density have been identified and replicated in genome-wide linkage screens. However, identification of key candidate genes within these confirmed regions is challenging. We used five freely available bioinformatics tools (Prioritizer, GeneSeeker, PROSPECTR and SUSPECTS, Disease Gene Prediction, and Endeavor) to analyze the 13 well-replicated osteoporosis susceptibility loci: 1p36, 1q21-25, 2p22-24, 3p14-25, 4q25-34, 6p21, 7p14-21, 11q14-25, 12q23-24, 13q14-34, 20p12, 2q24-32, and 5q12-21. Pathways and regulatory network analyses were performed using the Ingenuity Pathways Analysis (IPA) software. We identified a subset of most likely candidate osteoporosis susceptibility genes that are largely involved in transforming growth factor (TGF)-beta signaling, granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling, axonal guidance signaling, peroxisome proliferator-activated receptor (PPAR) signaling, and Wnt/beta-catenin signaling pathway. Six nonoverlapping networks were generated by IPA 5.0 from 88 out of the 91 candidate genes. The list of most likely candidate genes and the associated pathway identified will assist researchers in prioritizing candidate disease genes for further empirical analysis and understanding the pathogenesis of osteoporosis.

32 wk old C3H/HeJ mice actively respond to mechanical loading

Numerous studies indicate that C3H/HeJ (C3H) mice are mildly responsive to mechanical loading compared to C57BL/6J (C57) mice. Guided by data indicating high baseline periosteal osteoblast activity in 16 wk C3H mice, we speculated that simply allowing the C3H mice to age until basal periosteal bone formation was equivalent to that of 16 wk C57 mice would restore mechanoresponsiveness in C3H mice. We tested this hypothesis by subjecting the right tibiae of 32 wk old C3H mice and 16 wk old C57 mice to low magnitude rest-inserted loading (peak strain: 1235 mu epsilon) and then exposing the right tibiae of 32 wk C3H mice to low (1085 mu epsilon) or moderate (1875 mu epsilon) magnitude cyclic loading. The osteoblastic response to loading on the endocortical and periosteal surfaces was evaluated via dynamic histomorphometry. At 32 wk of age, C3H mice responded to low magnitude rest-inserted loading with significantly elevated periosteal mineralizing surface, mineral apposition rate and bone formation compared to unloaded contralateral bones. Surprisingly, the periosteal bone formation induced by low magnitude rest-inserted loading in C3H mice exceeded that induced in 16 wk C57 mice. At 32 wk of age, C3H mice also demonstrated an elevated response to increased magnitudes of cyclic loading. We conclude that a high level of basal osteoblast function in 16 wk C3H mice appears to overwhelm the ability of the tissue to respond to an otherwise anabolic mechanical loading stimulus. However, when basal surface osteoblast activity is equivalent to that of 16 wk C57 mice, C3H mice demonstrate a clear ability to respond to either rest-inserted or cyclic loading.

Expression and synthesis of bone morphogenetic proteins by osteoclasts: a possible path to anabolic bone remodeling

Skeletal remodeling is a finely orchestrated process coupling bone formation to bone resorption. The dynamics of coupling is regulated by the microenvironment at the bone remodeling site, which in turn is influenced by the intercellular communication between cells like osteoclasts and osteoblasts. Understanding the dynamics of coupling is important in devising new therapeutic approaches to the treatment of skeletal diseases characterized by disturbances in the bone remodeling process. In this study, we report the localization of bone morphogenetic proteins (BMPs) in osteoclasts generated from primary cocultures of bone marrow cells from mouse femur and tibia with mouse calvarial osteoblasts, using immunocytochemistry and in situ hybridization. Positive staining was seen in osteoclasts for BMP-2, -4, -6, and -7. Real-time PCR was used to quantitatively confirm the expression of transcripts for BMP-2, BMP-4, and BMP-6 mRNA in murine osteoclasts. Finally, the presence of BMP-2, -4, -6, and-7 proteins was confirmed in osteoclast lysates by Western blotting. Overall, our data suggest a possible direct role for osteoclasts in promoting bone formation via expression and synthesis of BMPs, which then would play an important role in promoting the recruitment, proliferation, and differentiation of osteoblasts at bone resorption sites.

STAT1 and STAT3 do not participate in FGF-mediated growth arrest in chondrocytes

Activating mutations in fibroblast growth factor receptor 3 (FGFR3) cause several human skeletal dysplasias as a result of attenuation of cartilage growth. It is believed that FGFR3 inhibits chondrocyte proliferation via activation of signal transducers and activators of transcription (STAT) proteins, although the exact mechanism of both STAT activation and STAT-mediated inhibition of chondrocyte growth is unclear. We show that FGFR3 interacts with STAT1 in cells and is capable of activating phosphorylation of STAT1 in a kinase assay, thus potentially serving as a STAT1 kinase in chondrocytes. However, as demonstrated by western blotting with phosphorylation-specific antibodies, imaging of STAT nuclear translocation, STAT transcription factor assays and STAT luciferase reporter assays, FGF does not activate STAT1 or STAT3 in RCS chondrocytes, which nevertheless respond to a FGF stimulus with potent growth arrest. Moreover, addition of active STAT1 and STAT3 to the FGF signal, by means of cytokine treatment, SRC-mediated STAT activation or expression of constitutively active STAT mutants does not sensitize RCS chondrocytes to FGF-mediated growth arrest. Since FGF-mediated growth arrest is rescued by siRNA-mediated downregulation of the MAP kinase ERK1/2 but not STAT1 or STAT3, our data support a model whereby the ERK arm but not STAT arm of FGF signaling in chondrocytes accounts for the growth arrest phenotype.

Osteoclast receptors and signaling

Osteoclasts are bone-resorbing cells derived from hematopoietic precursors of the monocyte-macrophage lineage. Besides the well known Receptor Activator of Nuclear factor-kappaB (RANK), RANK ligand and osteoprotegerin axis, a variety of factors tightly regulate osteoclast formation, adhesion, polarization, motility, resorbing activity and life span, maintaining bone resorption within physiological ranges. Receptor-mediated osteoclast regulation is rather complex. Nuclear receptors, cell surface receptors, integrin receptors and cell death receptors work together to control osteoclast activity and prevent both reduced or increased bone resorption. Here we will discuss the signal transduction pathways activated by the main osteoclast receptors, integrating their function and mechanisms of action.

Correlation of obesity and osteoporosis: effect of fat mass on the determination of osteoporosis

It was previously believed that obesity and osteoporosis were two unrelated diseases, but recent studies have shown that both diseases share several common genetic and environmental factors. Body fat mass, a component of body weight, is one of the most important indices of obesity, and a substantial body of evidence indicates that fat mass may have beneficial effects on bone. Contrasting studies, however, suggest that excessive fat mass may not protect against osteoporosis or osteoporotic fracture. Differences in experimental design, sample structure, and even the selection of covariates may account for some of these inconsistent or contradictory results. Despite the lack of a clear consensus regarding the impact of effects of fat on bone, a number of mechanistic explanations have been proposed to support the observed epidemiologic and physiologic associations between fat and bone. The common precursor stem cell that leads to the differentiation of both adipocytes and osteoblasts, as well the secretion of adipocyte-derived hormones that affect bone development, may partially explain these associations. Based on our current state of knowledge, it is unclear whether fat has beneficial effects on bone. We anticipate that this will be an active and fruitful focus of research in the coming years.

Functional grouping of osteoclast genes revealed through microarray analysis

We describe for the first time functional clusters of genes that are modulated during the differentiation of osteoclasts. Pathway analysis was applied to gene array data generated from affymetrix chips hybridized to RNA isolated from RAW264.7 cells exposed to RANK-ligand (RANK-L) for 5 days. This analysis revealed major functional gene clusters that were either up- or down-regulated during osteoclastogenesis. Some of the genes within the clusters have known functions, while others do not. We discuss herein the relevance of these functional gene clusters and their modulation to biological processes underlying the formation, function, and fate of osteoclasts.

Rutile and titanium particles differentially affect the production of osteoblastic local factors

Titanium and its alloys are widely used as implant materials for dental and orthopaedic applications. To improve their wear and corrosion resistance, several surface modifications that give rise to an outer ceramic layer of rutile have been developed. It is expected that after a long period of functional loading, rutile debris will arise from these modified surfaces. We have compared the in vitro biocompatibility of subcytotoxic doses of rutile and titanium particles of phagocytosable size in primary cultures of human osteoblasts. Particles were visualized using a spectral confocal microscope by reflection. Both types of particles aggregated in the culture media and were efficiently internalized by osteoblasts as agglomerates. Treatment of isolated cultures of osteoblasts with rutile particles stimulated the release of IL-6, PGE2, and GM-CSF to a lesser extent than titanium. The influence of macrophages on the particle-induced stimulation of those local factors was analyzed by coculturing TPA-differentiated THP-1 cells with osteoblasts. Under these conditions, levels of IL-6 and PGE2 after treatment of cocultured osteoblasts with rutile particles were lower than after exposure to titanium. These results indicate that rutile debris shows a lower bioreactivity than titanium when tested in cultures of human osteoblasts and support the improved biocompatibility of titanium-based implants modified to create an outer layer of rutile on their surfaces.

Cartilage-selective genes identified in genome-scale analysis of non-cartilage and cartilage gene expression

Background

Cartilage plays a fundamental role in the development of the human skeleton. Early in embryogenesis, mesenchymal cells condense and differentiate into chondrocytes to shape the early skeleton. Subsequently, the cartilage anlagen differentiate to form the growth plates, which are responsible for linear bone growth, and the articular chondrocytes, which facilitate joint function. However, despite the multiplicity of roles of cartilage during human fetal life, surprisingly little is known about its transcriptome. To address this, a whole genome microarray expression profile was generated using RNA isolated from 18–22 week human distal femur fetal cartilage and compared with a database of control normal human tissues aggregated at UCLA, termed Celsius.

Results

161 cartilage-selective genes were identified, defined as genes significantly expressed in cartilage with low expression and little variation across a panel of 34 non-cartilage tissues. Among these 161 genes were cartilage-specific genes such as cartilage collagen genes and 25 genes which have been associated with skeletal phenotypes in humans and/or mice. Many of the other cartilage-selective genes do not have established roles in cartilage or are novel, unannotated genes. Quantitative RT-PCR confirmed the unique pattern of gene expression observed by microarray analysis.

Conclusion

Defining the gene expression pattern for cartilage has identified new genes that may contribute to human skeletogenesis as well as provided further candidate genes for skeletal dysplasias. The data suggest that fetal cartilage is a complex and transcriptionally active tissue and demonstrate that the set of genes selectively expressed in the tissue has been greatly underestimated.

Distinct functional motifs within the IL-17 receptor regulate signal transduction and target gene expression

IL-17 is the founding member of a novel family of proinflammatory cytokines that defines a new class of CD4+ effector T cells, termed "Th17." Mounting evidence suggests that IL-17 and Th17 cells cause pathology in autoimmunity, but little is known about mechanisms of IL-17RA signaling. IL-17 through its receptor (IL-17RA) activates genes typical of innate immune cytokines, such as TNFalpha and IL-1beta, despite minimal sequence similarity in their respective receptors. A previous bioinformatics study predicted a subdomain in IL-17-family receptors with homology to a Toll/IL-1R (TIR) domain, termed the "SEFIR domain." However, the SEFIR domain lacks motifs critical for bona fide TIR domains, and its functionality was never verified. Here, we used a reconstitution system in IL-17RA-null fibroblasts to map functional domains within IL-17RA. We demonstrate that the SEFIR domain mediates IL-17RA signaling independently of classic TIR adaptors, such as MyD88 and TRIF. Moreover, we identified a previously undescribed"TIR-like loop" (TILL) required for activation of NF-kappaB, MAPK, and up-regulation of C/EBPbeta and C/EBPdelta. Mutagenesis of the TILL domain revealed a site analogous to the LPS(d) mutation in TLR4, which renders mice insensitive to LPS. However, a putative salt bridge typically found in TIR domains appears to be dispensable. We further identified a C-terminal domain required for activation of C/EBPbeta and induction of a subset IL-17 target genes. This structure-function analysis of a IL-17 superfamily receptor reveals important differences in IL-17RA compared with IL-1/TLR receptors.

Are nonresorbing osteoclasts sources of bone anabolic activity?

Some osteopetrotic mutations lead to low resorption, increased numbers of osteoclasts, and increased bone formation, whereas other osteopetrotic mutations lead to low resorption, low numbers of osteoclasts, and decreased bone formation. Elaborating on these findings, we discuss the possibility that osteoclasts are the source of anabolic signals for osteoblasts. In normal healthy individuals, bone formation is coupled to bone resorption in a tight equilibrium. When this delicate balance is disturbed, the net result is pathological situations, such as osteopetrosis or osteoporosis. Human osteopetrosis, caused by mutations in proteins involved in the acidification of the resorption lacuna (ClC-7 or the a3-V-ATPase), is characterized by decreased resorption in face of normal or even increased bone formation. Mouse mutations leading to ablation of osteoclasts (e.g., loss of macrophage-colony stimulating factor [M-CSF] or c-fos) lead to secondary negative effects on bone formation, in contrast to mutations where bone resorption is abrogated with sustained osteoclast numbers, such as the c-src mice. These data indicate a central role for osteoclasts, and not necessarily their resorptive activity, in the control of bone formation. In this review, we consider the balance between bone resorption and bone formation, reviewing novel data that have shown that this principle is more complex than originally thought. We highlight the distinct possibility that osteoclast function can be divided into two more or less separate functions, namely bone resorption and stimulation of bone formation. Finally, we describe the likely possibility that bone resorption can be attenuated pharmacologically without the undesirable reduction in bone formation.

New 19-nor-(20S)-1alpha,25-dihydroxyvitamin D3 analogs strongly stimulate osteoclast formation both in vivo and in vitro

2-Methylene-19-nor-(20S)-1alpha,25-dihydroxyvitamin D3 (2MD), an analog of 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], has been shown to strongly induce bone formation both in vitro and in vivo. We have synthesized four substituents at carbon 2 of 2MD (2MD analogs), four stereoisomers at carbon 20 of the respective 2MD analogs (2MD analog-C20 isomers) and four 2MD analogs with an oxygen atom at carbon 22 (2MD-22-oxa analogs) and examined their ability to stimulate osteoclastogenesis and induce hypercalcemia. 2MD analogs were 100 times as potent as 1alpha,25(OH)2D3 in stimulating the formation of osteoclasts in vitro and in inducing the expression of receptor activator of NF-kappaB ligand (RANKL) and 25-hydroxyvitamin D3-24 hydroxylase mRNAs in osteoblasts. The osteoclast-inducing activities of 2MD analog-C20 isomers and 2MD 22-oxa analogs were much weaker than those of 2MD analogs. In addition, the activity of a 2MD analog in inducing dentine resorption was much stronger than that of 1alpha,25(OH)2D3 in the pit formation assay. Affinities to the vitamin D receptor and transcriptional activities of these compounds did not always correlate with their osteoclastogenic activities. Osteoprotegerin-deficient (OPG-/-) mice provide a suitable model for investigating in vivo effects of 2MD analogs because they exhibit extremely high concentrations of serum RANKL. The same amounts of 2MD analogs and 1alpha,25(OH)2D3 were administered daily to OPG-/- mice for 2 days. The elevation in serum concentrations of RANKL and calcium was much greater in 2MD analog-treated OPG-/- mice than in 1alpha,25(OH)2D3-treated ones. A 2MD analog was much more potent than 1alpha,25(OH)2D3 in causing hypercalcemia and in increasing soluble RANKL with enhanced osteoclastogenesis even in wild-type mice. In contrast, the administration of the 2MD analog to c-fos-deficient mice failed to induce osteoclastogenesis and hypercalcemia. These results suggest that new substituents at carbon 2 of 2MD strongly stimulate osteoclast formation in vitro and in vivo, and that osteoclastic bone resorption is indispensable for their hypercalcemic action of 2MD analogs in vivo.

Impaired skeletal development in interleukin-6-transgenic mice: a model for the impact of chronic inflammation on the growing skeletal system

OBJECTIVE

To identify the mediator responsible for the impact of chronic inflammation on skeletal development in children (bone loss, defective peak bone mass accrual, stunted growth), we evaluated the effects of chronic interleukin-6 (IL-6) overexpression on the skeletons of growing prepubertal mice.

METHODS

We studied IL-6-transgenic mice that had high circulating IL-6 levels since birth. Trabecular and cortical bone structure were analyzed by microcomputed tomography. Epiphyseal ossification, growth plates, and calvariae were studied by histology/histomorphometry. Osteoclastogenesis, osteoblast function/differentiation, and the effects of IL-6 on bone cells were studied in vitro. Osteoblast gene expression was evaluated by reverse transcriptase-polymerase chain reaction. The mineral apposition rate was evaluated dynamically in cortical bone by in vivo double fluorescence labeling.

RESULTS

In prepubertal IL-6-transgenic mice, we observed osteopenia, with severe alterations in cortical and trabecular bone microarchitecture, as well as uncoupling of bone formation from resorption, with decreased osteoblast and increased osteoclast number and activity. Increased osteoclastogenesis and reduced osteoblast activity, secondary to decreased precursor proliferation and osteoblast function, were present. IL-6-transgenic mice also showed impaired development of growth plates and epiphyseal ossification centers. Intramembranous and endochondral ossification and the mineral apposition rate were markedly affected, showing the presence of defective ossification.

CONCLUSION

Chronic overexpression of IL-6 alone induces a skeletal phenotype closely resembling growth and skeletal abnormalities observed in children with chronic inflammatory diseases, pointing to IL-6 as a pivotal mediator of the impact of chronic inflammation on postnatal skeletal development. We hypothesize that IL-6-modifying drugs may reduce skeletal defects and prevent the growth retardation associated with these diseases.

Evidence for reduced bone formation surface relative to bone resorption surface in female femoral fragility fracture patients

Fragility fractures, including neck of femur fractures, result from reductions in the amount, quality and architecture of bone. The aim of this study was to compare the cancellous bone structure, and static indices of bone turnover, in female patients, who had sustained fragility fracture at the femoral neck, with age-matched females without fragility fracture. Bone samples were taken from the intertrochanteric region of the proximal femur of female patients undergoing hip arthroplasty surgery for a subcapital fragility fracture of the femoral neck (#NOF) or from age-matched female control individuals at routine autopsy. Contiguous bone samples were analyzed for undecalcified histomorphometry and for mRNA expression. The histomorphometric data, which were normally distributed, indicated no difference between the mean values for any of the structural parameters in control and fracture samples. In particular, the bone volume (BV/TV) values were not different and did not change significantly with age in these cohorts of individuals aged >65 years. The static indices of bone turnover, eroded surface (ES/BS) and osteoid surface (OS/BS), were positively correlated with age in the >65-year-old control group (p < 0.055 and p < 0.03, respectively). The median values for these indices were not different between the fracture and control groups. However, both the median and the range of OS/BS values were increased for >65-year-old controls compared with a group of younger females aged <65 years, suggesting an increase in bone formation surface in older females in the proximal femur after 65 years of age. When the data were further interrogated, a reduction in the percentage osteoid surface to eroded surface quotient (OS/ES) was found for the fracture group compared with the age-matched control group suggesting a reduced adaptive modeling drift capability in the fracture group. In contiguous bone samples, increased median values for receptor activator of nuclear factor kappa beta (RANK) and interleukin-6 (IL-6) mRNA expression were observed in the fracture group. Study of cultured human osteoblasts showed that recombinant human IL-6 (rhIL-6) inhibited osteoblast differentiation, as measured by an increase in the immature osteoblast marker, STRO-1 and concomitantly decreased expression of the osteoblast maturation marker, alkaline phosphatase. Importantly, cells cultured in the presence of IL-6 showed significantly less mineral deposition in vitro compared with control cultures. These data suggest that perturbations in bone formation surface, relative to resorption surface, are potentially important in producing bone in the proximal femur with increased propensity to fracture.

A critical role for interleukin-6 family-mediated Stat3 activation in osteoblast differentiation and bone formation

Signal transduction and activator of transcription (Stat) 3 is a transcription factor that is activated by a variety of cytokines and growth factors, including IL-6 family cytokines. These cytokines regulate bone homeostasis and have been reported to regulate the differentiation of osteoblasts and osteoclasts through Stat3 activation in vitro, but the in vivo physiological role of Stat3 in bone homeostasis is unknown. Here, we report that gp130 knock-in mice gp130(F759/F759), in which IL-6 family cytokine-mediated Stat3 activation is enhanced, showed an osteosclerotic phenotype. To further clarify the role of Stat3 in bone formation, we generated mice with osteoblast-specific disruption of the Stat3 gene via the Cre-LoxP recombination system using alpha1(I)-collagen promoter Cre transgenic mice. The alpha1(I)Cre;Stat3(flox/-) mice showed an osteoporotic phenotype because of a reduced bone formation rate. Thus, the Stat3 signal in osteoblasts plays a pivotal role in bone formation in vivo.

Characterization of human fetal osteoblasts by microarray analysis following stimulation with 58S bioactive gel-glass ionic dissolution products

Bioactive glasses dissolve upon immersion in culture medium, releasing their constitutive ions in solution. There is evidence suggesting that these ionic dissolution products influence osteoblast-specific processes. Here, we investigated the effect of 58S sol-gel-derived bioactive glass (60 mol % SiO2, 36 mol % CaO, 4 mol % P2O5) dissolution products on primary osteoblasts derived from human fetal long bone explant cultures (hFOBs). We used U133A human genome GeneChip oligonucleotide arrays to examine 22,283 transcripts and variants, which represent over 18,000 well-substantiated human genes. Hybridization of samples (biotinylated cRNA) derived from monolayer cultures of hFOBs on the arrays revealed that 10,571 transcripts were expressed by these cells, with high confidence. These included transcripts representing osteoblast-related genes coding for growth factors and their associated molecules or receptors, protein components of the extracellular matrix (ECM), enzymes involved in degradation of the ECM, transcription factors, and other important osteoblast-associated markers. A 24-h treatment with a single dosage of ionic products of sol-gel 58S dissolution induced the differential expression of a number of genes, including IL-6 signal transducer/gp130, ISGF-3/STAT1, HIF-1 responsive RTP801, ERK1 p44 MAPK (MAPK3), MAPKAPK2, IGF-I and IGFBP-5. The over 2-fold up-regulation of gp130 and MAPK3 and down-regulation of IGF-I were confirmed by real-time RT-PCR analysis. These data suggest that 58S ionic dissolution products possibly mediate the bioactive effect of 58S through components of the IGF system and MAPK signaling pathways.

Gene expression during fracture healing in rats comparing intramedullary fixation to plate fixation by DNA microarray

OBJECTIVE

This study was designed to compare mRNA gene expression in healing diaphyseal femoral fractures between those injuries treated with intramedullary nails and those treated with internal plate fixation.

DESIGN

RNA gene expression was measured at 1 day, 3 days, and 1, 2, 4, and 6 weeks after surgery in the fracture callus of rats randomized to femoral shaft fracture with intramedullary nail fixation, rigid plate fixation, or sham fracture.

SETTING

AAALAC-accredited vivarium of an independent academic medical center.

ANIMALS

Fifty-seven, adult, female, Sprague-Dawley rats at 16 weeks of age.

INTERVENTION

Femoral fracture with intramedullary nail fixation, femoral fracture with plate and screw fixation, or sham surgery with no fracture.

MAIN OUTCOME MEASUREMENTS

RNA expression for 8700 genes was measured with 19 Affymetrix U34A microarrays. The fracture callus was significantly larger with intramedullary nail fixation than with plate fixation. Most genes responded to fracture with a change in mRNA expression. Most of the responding genes followed the same time course for both fixation methods. This included genes related to growth factors, bone matrix, mast cells, most nerve factors, and hematopoiesis. The intramedullary nail group had significantly greater up-regulation for transcripts related to cartilage, cell division, inflammation, and the acetylcholine receptor. There was significantly greater up-regulation in the plate group for genes related to macrophage activity.

CONCLUSIONS

There were differentially expressed genes present between the 2 surgical groups that may give insight into the control of fracture repair.

Interleukin-6: An osteotropic factor influencing bone formation?

Interleukin (IL)-6 has long been considered as an osteoresorptive factor. However, recent data indicate that IL-6 could influence bone formation in conditions of increased bone turnover. In this paper, the effects of IL-6 and its soluble receptor on osteoblast proliferation, differentiation and apoptosis are readdressed. A brief summary of IL-6 signaling after binding to its receptor is provided and hypotheses concerning IL-6 and the central control of bone formation are also highlighted.

GP130/OSMR is the only LIF/IL-6 family receptor complex to promote osteoblast differentiation of calvaria progenitors

Leukemia inhibitory factor (LIF) and its receptor (LIFR) are "twins" of Oncostatin M (OSM) and OSMR, respectively, likely having arisen through gene duplications. We compared their effects in a bone nodule-forming model of in vitro osteogenesis, rat calvaria (RC) cell cultures. Using a dominant-negative LIF mutant (hLIF-05), we showed that in RC cell cultures mouse OSM (mOSM) activates exclusively glycoprotein 130 (gp130)/OSMR. In treatments starting at early nodule formation stage, LIF, mOSM, IL-11, and IL-6 + sIL-6R inhibit bone nodule formation, that is, osteoprogenitor differentiation. Treatment with mOSM, and no other cytokine of the family, in early cultures (day 1-3 or 1-4) increases bone colony numbers. hLIF-05 also dose dependently stimulates bone nodule formation, confirming the inhibitory action of gp130/LIFR on osteogenesis. In pulse treatments at successive stages of bone nodule formation and maturation, LIF blocks osteocalcin (OCN) expression by differentiated osteoblasts, but has no effect on bone sialoprotein (BSP) expression. Mouse OSM inhibits OCN and BSP expression in preconfluent cultures with no or progressively reduced effects at later stages, reflecting the disruption of early nodules, possibly due to the strong apoptotic action of mOSM in RC cell cultures. In summary, LIFR and OSMR display differential effects on differentiation and phenotypic expression of osteogenic cells, most likely through different signal transduction pathways. In particular, gp130/OSMR is the only receptor complex of the family to stimulate osteoprogenitor differentiation in the RC cell culture model.

Osteoclast-derived activity in the coupling of bone formation to resorption

The cells of bone and the immune system communicate by means of soluble and membrane-bound cytokines and growth factors. Through local signalling mechanisms, cells of the osteoblast lineage control the formation and activity of osteoclasts and, therefore, the resorption of bone. Both T and B lymphocytes produce activators and inhibitors of osteoclast formation. A local 'coupling factor' linking bone resorption to subsequent formation in the bone multicellular unit (BMU) has long been proposed as the key regulator of the bone remodelling process, but never identified. There is evidence in support of the view that the coupling mechanism is dependent on growth factors released from the bone matrix during resorption, or is generated from maturing osteoblasts. We argue that osteoclasts contribute in important ways to the transiently activated osteoclast, and stimulate osteoblast lineage cells to begin replacing the resorbed bone in each BMU.

Interleukin-11 receptor signaling is required for normal bone remodeling

IL-6 and -11 regulate bone turnover and have been implicated in estrogen deficiency-related bone loss. In this study, deletion of IL-11 signaling, but not that of IL-6, suppressed osteoclast differentiation, resulting in high trabecular bone volume and reduced bone formation. Furthermore, IL-11 signaling was not required for the effects of estradiol or estrogen deficiency on the mouse skeleton.

INTRODUCTION

Interleukin (IL)-6 and -11 stimulate osteoclastogenesis and bone formation in vitro and have been implicated in bone loss in estrogen deficiency. Because of their common use of the gp130 co-receptor signaling subunit, the roles of these two cytokines are linked, and each may compensate for the absence of the other to maintain trabecular bone volume and bone cell differentiation.

MATERIALS AND METHODS

To determine the interactions in bone between IL-11 and IL-6 in vivo and whether IL-11 is required for normal bone turnover, we examined the bone phenotype of mature male and female IL-11 receptor knockout mice (IL-11Ralpha1-/-) and compared with the bone phenotype of IL-6-/- mice and mice lacking both IL-6 and IL-11Ralpha. To determine whether IL-11 is required for the effects of estrogen on trabecular bone, mature IL-11Ralpha1-/- mice were ovariectomized and treated with estradiol.

RESULTS

In both male and female IL-11Ralpha1-/- mice, trabecular bone volume was significantly higher than that of wildtype controls. This was associated with low bone resorption and low bone formation, and the low osteoclast number generated by IL-11Ralpha1-/- precursors was reproduced in ex vivo cultures, whereas elevated osteoblast generation was not. Neither trabecular bone volume nor bone turnover was altered in IL-6-/- mice, and compound IL-6-/- :IL-11Ralpha1-/- mice showed an identical bone phenotype to IL-11Ralpha1-/- mice. The responses of IL-11Ralpha1-/- mice to ovariectomy and estradiol treatment were the same as those observed in wildtype mice.

CONCLUSIONS

IL-11 signaling is clearly required for normal bone turnover and normal trabecular bone mass, yet not for the effects of estradiol or estrogen deficiency on the skeleton. In the absence of IL-11Ralpha, increased trabecular bone mass seems to result from a cell lineage-autonomous reduction in osteoclast differentiation, suggesting a direct effect of IL-11 on osteoclast precursors. The effects of IL-11Ralpha deletion on the skeleton are not mediated or compensated for by changes in IL-6 signaling.

Cross-talk between the interleukin-6 and prostaglandin E(2) signaling systems results in enhancement of osteoclastogenesis through effects on the osteoprotegerin/receptor activator of nuclear factor-{kappa}B (RANK) ligand/RANK system

The osteoprotegerin (OPG)/receptor activator of nuclear factor-kappaB ligand (RANKL)/receptor activator of nuclear factor-kappaB (RANK) system is the dominant and final mediator of osteoclastogenesis. Abnormalities of this system have been implicated in the pathogenesis of many skeletal diseases. Cyclooxygenase (COX)-2 and prostaglandin (PG)E(2), a major eicosanoid product of the COX-2-catalyzed pathway, play key roles in normal bone tissue remodeling. PGE(2) exerts its actions by binding and activating the E series of prostaglandin (EP) receptor. Activation of EP(2) and EP(4) receptors is associated with PGE(2)-induced osteoclast differentiation. IL-6, a major proinflammatory cytokine, has also been reported to induce osteoclast differentiation. Although interactions between the COX-2/PGE(2) and IL-6 systems have been described in bone cells, the mechanisms underlying these cooperative signaling pathways and the possible involvement of the OPG/RANKL/RANK system have not been fully elucidated. We demonstrate that COX-2, PGE(2), and IL-6 stimulate osteoblast growth and osteoclast differentiation. Effects on osteoclast differentiation, particularly with IL-6, were most marked when osteoclast precursor cells were grown in coculture with osteoblasts, indicating a possible role of the RANK/RANKL/OPG system. COX-2 and PGE(2) stimulated osteoclastogenesis through inhibition of OPG secretion, stimulation of RANKL production by osteoblasts, and up-regulation of RANK expression in osteoclasts. PGE(2) stimulated IL-6 secretion by bone cells, whereas COX-2 inhibitors decreased this same parameter. IL-6, in turn, increased PGE(2) secretion, COX-2, and EP receptor subtype expression in bone cells. Finally, IL-6 was the mediator of PGE(2)-induced suppression of OPG production by osteoblasts. These findings provide evidence for cross-talk between the PGE(2) and IL-6 signaling enhance osteoclast differentiation via effects on the OPG/RANKL/RANK system in bone cells.

Cloning and expression of the first nonmammalian interleukin-11 gene in rainbow trout Oncorhynchus mykiss

Interleukin (IL)-11 is a multifunctional cytokine that stimulates hematopoietic progenitor cells and exerts a series of important immunomodulatory effects. It was believed to be restricted to mammals, but here we report the first nonmammalian IL-11 gene, in rainbow trout (Oncorhynchus mykiss). A trout IL-11 cDNA clone was isolated that contains a 5'-untranslated region (UTR) of 400 bp, an open reading frame of 612 bp and a large 3'-UTR of 1924 bp. Analysis of a genomic DNA clone from a trout lambda library revealed that the trout IL-11 gene has the same five exon/four intron gene organization, as well as the same intron phase, as mammalian IL-11 genes. The 204 amino acid trout IL-11 translation has a predicted signal peptide of 26 amino acids and mature peptide of 178 amino acids, with a calculated molecular mass of 20.5 kDa and a theoretical pI of 9.83. The mature peptide contains a cysteine residue and a potential N-linked glycosylation site that are not present in mammals. Phylogenetic analysis clearly grouped trout IL-11 with IL-11 molecules from other species and separated from other members of the IL-6 family. The IL-11 gene is highly expressed in intestine and gills in healthy fish and its expression can also be detected in spleen, head kidney, brain, skin and muscle. Bacterial infection of rainbow trout markedly up-regulates IL-11 expression in liver, head kidney and spleen. IL-11 expression is also up-regulated in RTS-11 cells (a trout macrophage cell line), which constitutively expressed the lowest level of IL-11 of the four trout cell lines examined, after stimulation with bacteria, lipopolysaccharide, poly(I:C) and recombinant trout IL-1beta. Only a single transcript of 3.2 kb could be detected in lipopolysaccharide or recombinant IL-1beta-stimulated RNA samples by northern blotting. The expression results, showing that IL-11 is widely distributed and modulated by infection and other cytokines, suggest that fish IL-11 is an active player in the cytokine network and the host immune response to infection.

Calmodulin is a critical regulator of osteoclastic differentiation, function, and survival

Increased osteoclastic resorption and subsequent bone loss are common features of many debilitating diseases including osteoporosis, bone metastases, Paget's disease, and rheumatoid arthritis. While rapid progress has been made in elucidating the signaling pathways directing osteoclast differentiation and function, a comprehensive picture is far from complete. Here, we explore the role of the Ca(2+)-activated regulator calmodulin in osteoclastic differentiation, functional bone resorption, and apoptosis. During active bone resorption, calmodulin expression is increased, and calmodulin concentrates at the ruffled border, the organelle utilized for acid transport and bone dissolution. Pharmacologic inhibitors of calmodulin, several of which are already used clinically as anti-cancer and anti-psychotic agents, inhibit osteoclastic acid transport, suggesting their potential as bone-sparing drugs. Recent studies also implicate calmodulin in osteoclast apoptosis through a mechanism involving its direct interaction with the death receptor Fas. During osteoclastogenesis, RANKL-induction stimulates a rise in intracellular Ca2+, which in turn activates calmodulin and its downstream effectors. In particular, the Ca(2+)/calmodulin-dependent phosphatase calcineurin and its targets, the NFAT family of transcription factors, have been posited as the master regulators of osteoclastogenesis. However, recent in vivo and in vitro studies demonstrate that another Ca(2+)/calmodulin-regulated effector protein, CaMKII, is also involved. CaMKII(+/-) mutant mice have reduced osteoclast numbers, and CaMKII antagonists inhibit osteoclastogenesis in vitro. Furthermore, CaMKII is known to activate AP-1 transcription factors, which are also required for RANKL-induced osteoclast gene transcription, and recent findings suggest that CaMKII can down-regulate gp130, a cytokine receptor involved in bone remodeling and implicated in numerous osteo-articular diseases.