IL-6, LIF, and TNF-α regulation of GM-CSF inhibition of osteoclastogenesis in vitro

Icaritin inhibits lung cancer-induced osteoclastogenesis by suppressing the expression of IL-6 and TNF-a and through AMPK/mTOR signaling pathway

Bone metastasis is one of the common phenomena in the late stage of lung cancer. Inhibition of bone metastasis can improve the survival of lung cancer patients. However, the current drugs for the treatment of bone metastasis have shown little effect on overall survival. Therefore, there is an urgent necessity to identify novel drugs capable of preventing and treating bone metastasis of lung cancer. Our study determined that icaritin (ICT) can inhibit lung cancer-mediated osteoclastogenesis and induce the apoptosis of osteoclasts. Exposure to ICT increased the activation of adenosine 5'-monophosphate-activated protein kinase (AMPK), reduced the activation of mammalian target of rapamycin (mTOR) and decreased the expression of bcl-2. The bioactivity of ICT on osteoclastogenesis was associated with the regulation of the AMPK/mTOR signaling pathway. Blocking AMPK significantly increased osteoclast differentiation, decreased osteoclast apoptosis and canceled the effects of ICT on the phosphorylation of AMPK as well as the inhibition of mTOR and bcl-2. Furthermore, ICT decreased the levels of IL-6 and TNF-α in osteoclasts, while the AMPK inhibitor compound C significantly abolished the inhibitory effects of ICT on IL-6 and TNF-α. Thus, the present study demonstrated that ICT may be a potential natural agent for the treatment of bone metastasis in patients with lung cancer.

Oral Exposure to ZnO Nanoparticles Disrupt the Structure of Bone in Young Rats via the OPG/RANK/RANKL/IGF-1 Pathway

Purpose

To evaluate the effects of ZnO NPs on bone growth in rats and explore the possible mechanisms of action.

Materials and Methods

Three-week-old male rats received ultrapure water or 68, 203, and 610 mg/kg zinc oxide nanoparticles (ZnO NPs) for 28 days, orally.

Results

The high-dosage groups caused significant differences in weight growth rate, body length, and tibia length (P<0.05), all decreasing with increased ZnO NP dosage. There were no significant differences in body mass index (BMI) (P>0.05). The zinc concentration in liver and bone tissue increased significantly with increased ZnO NP dosage (P<0.05). Clearly increased aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels were observed in the 610 mg/kg ZnO NP group (P>0.05), whereas alkaline phosphatase (ALP) increased in the 610 mg/kg ZnO NP group (P<0.05). Significant differences in insulin-like growth factor type 1 (IGF-1) levels and a decrease in calcium (Ca) levels were observed in 203 and 610 mg/kg ZnO NP groups (P<0.05). Phosphorus (P) levels increased and the Ca/P ratio decreased in the 610 mg/kg ZnO NP group (P<0.05). Micro-computed tomography (micro-CT) of the tibia demonstrated signs of osteoporosis, such as decreased bone density, little trabecular bone structure and reduced cortical bone thickness. Micro-CT data further demonstrated significantly decreased bone mineral density (BMD), trabecular number (Tb.N), and relative bone volume (BV/TV) with increasing dosage of ZnO NPs. Osteoprotegerin (OPG) expression and the ratio of OPG to receptor activator of nuclear factor-κB ligand (RANKL) were statistically lower in the 610 mg/kg ZnO NP group (P<0.05), whereas RANKL expression did not change significantly (P>0.05).

Conclusion

We infer that ZnO NPs affect bone growth in young rats directly or indirectly by altering IGF-1 levels. Overall, the results indicate that ZnO NPs promote osteoclast activity and increase bone loss through the OPG/RANK/RANKL/IGF-1 pathway.

The Role of Mast Cells in Bone Metabolism and Bone Disorders

Mast cells (MCs) are important sensor and effector cells of the immune system that are involved in many physiological and pathological conditions. Increasing evidence suggests that they also play an important role in bone metabolism and bone disorders. MCs are located in the bone marrow and secrete a wide spectrum of mediators, which can be rapidly released upon activation of mature MCs following their differentiation in mucosal or connective tissues. Many of these mediators can exert osteocatabolic effects by promoting osteoclast formation [e.g., histamine, tumor necrosis factor (TNF), interleukin-6 (IL-6)] and/or by inhibiting osteoblast activity (e.g., IL-1, TNF). By contrast, MCs could potentially act in an osteoprotective manner by stimulating osteoblasts (e.g., transforming growth factor-β) or reducing osteoclastogenesis (e.g., IL-12, interferon-γ). Experimental studies investigating MC functions in physiological bone turnover using MC-deficient mouse lines give contradictory results, reporting delayed or increased bone turnover or no influence depending on the mouse model used. By contrast, the involvement of MCs in various pathological conditions affecting bone is evident. MCs may contribute to the pathogenesis of primary and secondary osteoporosis as well as inflammatory disorders, including rheumatoid arthritis and osteoarthritis, because increased numbers of MCs were found in patients suffering from these diseases. The clinical observations could be largely confirmed in experimental studies using MC-deficient mouse models, which also provide mechanistic insights. MCs also regulate bone healing after fracture by influencing the inflammatory response toward the fracture, vascularization, bone formation, and callus remodeling by osteoclasts. This review summarizes the current view and understanding of the role of MCs on bone in both physiological and pathological conditions.

Human adipose-derived mesenchymal stem cell-conditioned media suppresses inflammatory bone loss in a lipopolysaccharide-induced murine model

Conditioned media (CM) from mesenchymal stem cells (MSCs) contains various cytokines, growth factors and microRNAs, which may serve important roles in modulating the inflammatory process. However, the effect of MSC-CM on inflammatory bone loss remains unknown. The present study investigated the effects of conditioned media from human adipose-derived mesenchymal stem cells (AMSC-CM) on the prevention of lipopolysaccharide (LPS)-mediated bone loss in mice. To investigate the underlying mechanisms of this effect, the effects of AMSC-CM on serum levels of inflammation-associated cytokines [tumor necrosis factor (TNF)-α, interleukin (IL)-1, IL-6 and IL-10] in LPS-treated mice, in addition to their mRNA expression in LPS-treated macrophages, was investigated. Micro-computed tomography and histological analysis revealed that AMSC-CM administration effectively inhibited LPS-induced bone destruction in vivo. ELISA analysis indicated that AMSC-CM significantly reduced the serum levels of proinflammatory cytokines (TNF-α, IL-1 and IL-6) in LPS-treated mice. Furthermore, AMSC-CM treatment significantly decreased the mRNA expression levels of TNF-α, IL-1 and IL-6 in macrophages treated with LPS. These findings indicate that AMSC-CM inhibits LPS-induced bone loss by decreasing the production of proinflammatory cytokines, suggesting that the use of AMSC-CM may be a potential therapeutic strategy for the treatment of inflammatory bone loss.

Bone mesenchymal stem cell secretion of sRANKL/OPG/M-CSF in response to macrophage-mediated inflammatory response influences osteogenesis on nanostructured Ti surfaces

Although it has been well established that osteogenic differentiation of bone mesenchymal stem cells (bMSCs) as well as osteoclastic differentiation of macrophages can be manipulated by the nanostructure of biomaterial surfaces, the interactions among the effects of the surface on immune cells and bMSCs remained unknown. Therefore, in this study, the osteogenic behaviors and secretion of osteoclastogenesis-related cytokines of human bMSCs on TiO₂ nanotubular (NT) surfaces in conditioned medium (CM) generated by macrophages cultured on the respective NT surfaces (NT-CM) were analyzed. Although bMSCs showed consistent osteogenic behaviors on the NT5 and NT20 surfaces in both standard culture medium and both types of NT-CM, collagen synthesis and extracellular matrix mineralization were partially impeded on the NT20 surface in NT20-CM and bMSC cytokine secretions on the NT20 surface in NT20-CM elicited remarkable multinuclear giant cell and osteoclast formation compared with that observed on the NT5 surface in NT5-CM. After implantation in vivo, mineralized bone formation was significantly delayed around the NT20 implant compared with the NT5 implant, but both surfaces contributed to good bone formation after 12 weeks. The results obtained in this study advance our understanding of the confounding influence of the implant surface nanostructure, macrophage inflammatory response, and osteogenic differentiation of bMSCs as well as the retro-regulative effects of bMSCs on the osteoclastic differentiation of macrophages, and the culture system based on different NT surfaces and CM generated on the respective surfaces may provide a systematic research model for evaluating the performance of endosseous implants as well as a prospective approach for improving implant osseointegration via immune-regulation.

Effects of the anti-receptor activator of nuclear factor kappa B ligand denusomab on beta thalassemia major-induced osteoporosis

INTRODUCTION

Osteoporosis represents the second most common cause of endocrinopathy in patients with beta thalassemia major (BTM). Some drugs proved effective to reduce vertebral and non-vertebral fracture risk. Denosumab is a fully human monoclonal antibody to the receptor activator of nuclear factor kappa B ligand (RANKL), a member of the tumor necrosis factor receptor superfamily essential for osteoclastogenesis. The efficacy and safety of denosumab in BTM-induced osteoporosis has not been tested.

OBJECTIVE

To evaluate the efficacy and safety of anti-RANKL on the biochemical and radiological parameters of bone mineralization in patients with BTM-induced osteoporosis.

DESIGN

The study population was selected using the random sampling method from the patient's database of our thalassemia clinic. Transfusion-dependent BTM patients above 18 years with no history of treatment with bisphosphonates were randomly selected. Bone mineral density (BMD) of the lumbar spine (LS) and right femoral neck (FN) were measured by dual energy X-ray absorption (DEXA) scan using a calibrated method. Independent factors likely to be associated with low bone mass were determined and included in the analysis to ascertain possible associations.

PATIENTS AND METHODS

We studied 30 patients with BTM-induced osteoporosis as per World Health Organization criteria (T Score of less than - 1.0 being defined as osteopenic and a T Score of less than - 2.5 being referred as osteoporotic). 19 males and 11 females aged between 18 and 32 years, with full pubertal development (Tanner's stage 5) at the time of the study. Their mean serum ferritin concentration was 3557 ng ± 1488 ng/ml. Every patient underwent DEXA scan as a baseline and after 12 months of denosumab therapy. Biochemical evaluation including serum concentrations of creatinine, Na, K, calcium, phosphorus, parathormone, bone specific alkaline phosphatase and type 1 collagen carboxy telopetide (ICCT) using enzyme-linked immunosorbent assay (Nordic Bioscience Diagnostics A/S) was done at baseline, after a month and then every 3 months for 12 months after starting denosumab. 60 mg of denosumab was administered subcutaneously twice yearly for a year. The mean BMD T Scores at baseline were -2.7 at the LS and -2.1 at the FN.

RESULTS

Denosumab therapy for a year was associated with a significant increase in BMD of 9.2% (95% confidence interval [CI], 8.2-10.1) at the LS and 6.0% (95% CI, 5.2-6.7) at the FN. Denosumab treatment decreased serum ICCT levels by 56% at 1 month and normalized them in all patients at 1 year. Significant correlations were found between BMD T Score before and 1 year after denosumab in LS (r = 0.752, P < 0.001) and FN (r = 0.758 P < 0.001), respectively. The most common side effects were pain in the back and extremities (12%) and nausea (10%). Asymptomatic hypocalcaemia occurred in two patients.

CONCLUSION

Denosumab therapy for a year significantly increased BMD density at LS and FN of patients with BTM and was associated with a rapid and sustained reduction in ICCT levels. Further studies are required to confirm long-term effects of this therapy.

Osteoimmunology and the influence of pro-inflammatory cytokines on osteoclasts

Bone and immune system are functionally interconnected. Immune and bone cells derive from same progenitors in the bone marrow, they share a common microenvironment and are being influenced by similar mediators. The evidence on increased bone resorption associated with inappropriate activation of T cells such as during inflammation, is well established. However, the molecular mechanisms beyond this clinical observation have begun to be intensively studied with the advancement of osteoimmunology. Now days, we have firm evidence on the influence of numerous proinflammatory cytokines on bone cells, with the majority of data focused on osteoclasts, the bone resorbing cells. It has been shown that some proinflammatory cytokines could possess osteoclastogenic and/or anti-osteoclastogenic properties and can target osteoclasts directly or via receptor activator of nuclear factor κB (RANK)/RANK ligand(RANKL)/osteoprotegerin (OPG) system. Several studies have reported opposing data regarding (anti)osteoclastogenic properties of these cytokines.

Therefore, the first part of this review is summarizing current evidence on the influence of pro-inflammatory cytokines on osteoclasts and thus on bone resorption. In the second part, the evidence on the role of pro-inflammatory cytokines in osteoporosis and osteoarthritis is reviewed to show that unravelling the mechanisms beyond such complex bone diseases, is almost impossible without considering skeletal and immune systems as an indivisible integrated system.

Infection, inflammation, and bone regeneration: a paradoxical relationship

Various strategies have been developed to promote bone regeneration in the craniofacial region. Most of these interventions utilize implantable materials or devices. Infections resulting from colonization of these implants may result in local tissue destruction in a manner analogous to periodontitis. This destruction is mediated via the expression of various inflammatory mediators and tissue-destructive enzymes. Given the well-documented association among microbial biofilms, inflammatory mediators, and tissue destruction, it seems reasonable to assume that inflammation may interfere with bone healing and regeneration. Paradoxically, recent evidence also suggests that the presence of certain pro-inflammatory mediators is actually required for bone healing. Bone injury (e.g., subsequent to a fracture or surgical intervention) is followed by a choreographed cascade of events, some of which are dependent upon the presence of pro-inflammatory mediators. If inflammation resolves promptly, then proper bone healing may occur. However, if inflammation persists (which might occur in the presence of an infected implant or graft material), then the continued inflammatory response may result in suboptimal bone formation. Thus, the effect of a given mediator is dependent upon the temporal context in which it is expressed. Better understanding of this temporal sequence may be used to optimize regenerative outcomes.

GP130 cytokines and bone remodelling in health and disease

Cytokines that bind to and signal through the gp130 co-receptor subunit include interleukin (IL)-6, IL-11, oncostatin M (OSM), leukemia inhibitory factor (LIF), cardiotrophin-1 (CT-1), and ciliary neutrophic factor (CNTF). Apart from contributing to inflammation, gp130 signalling cytokines also function in the maintenance of bone homeostasis. Expression of each of these cytokines and their ligand-specific receptors is observed in bone and joint cells, and bone-active hormones and inflammatory cytokines regulate their expression. gp130 signalling cytokines have been shown to regulate the differentiation and activity of osteoblasts, osteoclasts and chondrocytes. Furthermore, cytokine and receptor specific gene-knockout mouse models have identified distinct roles for each of these cytokines in regulating bone resorption, bone formation and bone growth. This review will discuss the current models of paracrine and endocrine actions of gp130-signalling cytokines in bone remodelling and growth, as well as their impact in pathologic bone remodelling evident in periodontal disease, rheumatoid arthritis, spondylarthropathies and osteoarthritis.

Reversal of chemotherapy-induced leukopenia using granulocyte macrophage colony-stimulating factor promotes bone metastasis that can be blocked with osteoclast inhibitors

Hematopoietic growth factors are used to reverse chemotherapy-induced leukopenia. However, some factors such as granulocyte macrophage colony-stimulating factor (GM-CSF) induce osteoclast-mediated bone resorption that can promote cancer growth in the bone. Accordingly, we evaluated the ability of GM-CSF to promote bone metastases of breast cancer or prostate cancer in a mouse model of chemotherapy-induced leukopenia. In this model, GM-CSF reversed cyclophosphamide-induced leukopenia but also promoted breast cancer and prostate cancer growth in the bone but not in soft tissue sites. Bone growth was associated with the induction of osteoclastogenesis, yet in the absence of tumor GM-CSF, it did not affect osteoclastogenesis. Two osteoclast inhibitors, the bisphosphonate zoledronic acid and the RANKL inhibitor osteoprotegerin, each blocked GM-CSF-induced tumor growth in the bone but did not reverse the ability of GM-CSF to reverse chemotherapy-induced leukopenia. Our findings indicate that it is possible to dissociate the bone-resorptive effects of GM-CSF, to reduce metastatic risk, from the benefits of this growth factor in reversing leukopenia caused by treatment with chemotherapy.

WNT/beta-catenin signaling is involved in regulation of osteoclast differentiation by human immunodeficiency virus protease inhibitor ritonavir: relationship to human immunodeficiency virus-linked bone mineral loss

Untreated human immunodeficiency virus (HIV) infection is accompanied by reduced bone mineral density, which appears to be exacerbated by certain HIV protease inhibitors (PIs). The mechanisms leading to this apparent paradox, however, remain unclear. We have previously shown that, the HIV envelope glycoprotein gp120 used at levels similar those in plasmas of untreated HIV(+) patients, induced expression of the osteoclast (OC) differentiation factor RANKL in CD4+ T cells. In addition, the HIV PI ritonavir abrogated the interferon-gamma-mediated degradation of the RANKL nuclear adapter protein TRAF6, a physiological block to RANKL activity. Here, using oligonucleotide microarrays and quantitative polymerase chain reaction, we explored potential upstream mechanisms for these effects. Ritonavir, but not the HIV PIs indinavir or nelfinavir, up-regulated the production of transcripts for OC growth factors and the non-canonical Wnt Proteins 5B and 7B as well as activated promoters of nuclear factor-kappaB signaling, but suppressed genes involved in canonical Wnt signaling. Similarly, ritonavir blocked the cytoplasmic to nuclear translocation of beta-catenin, the molecular node of the Wnt signaling pathway, in association with enhanced beta-catenin ubiquitination. Exposure of OC precursors to LiCl, an inhibitor of the canonical Wnt antagonist GSK-3beta, suppressed OC differentiation, as did adenovirus-mediated overexpression of beta-catenin. These data identify, for the first time, a biologically relevant role for Wnt signaling via beta-catenin in isolated OC precursors and the modulation of Wnt signaling by ritonavir. The reversal of these ritonavir-mediated changes by interferon-gamma provides a model for possible intervention in this metabolic complication of HIV therapy.

Inhibiting interleukin-6 in rheumatoid arthritis

Interleukin (IL)-6 is the most abundant proinflammatory cytokine in the circulation and synovial joints of patients with active rheumatoid arthritis. It has pivotal roles in the immune response and inflammation. In rheumatoid arthritis, it causes synovitis, joint destruction, and many systemic manifestations. Clinical trials of tocilizumab, a humanized anti-IL-6 receptor monoclonal antibody that blocks IL-6 signaling, have demonstrated therapeutic benefit. It heralds a new era of anticytokine therapy in rheumatoid arthritis.

Leukemia inhibitory factor protein and receptors are expressed in the bovine adrenal cortex and increase cortisol and decrease adrenal androgen release

The release of adrenal steroids during acute stress is primarily regulated by adrenocorticotropic hormone (ACTH). In contrast, during chronic inflammatory stress additional factors are involved in regulating adrenal function. Leukemia inhibitory factor (LIF) is a pleiotropic cytokine that increases ACTH release from the pituitary. In addition, LIF and LIF receptors (LIFR) are expressed in the human adrenal cortex and the human adrenocortical tumor cell line H295R. Furthermore, LIF increases basal and ACTH-stimulated cortisol release from H295R cells. However, the expression of LIF and LIFR in non-human adrenal glands and the effects of LIF on the release of cortisol from adrenal cells of non-human species have not been determined. Furthermore, the effects of LIF on adrenal androgen release from all species are unknown. In this study, immunohistochemistry, Western blots, RT-PCR, and nucleotide sequencing was utilized to demonstrate that LIF and its receptor are expressed throughout the bovine adrenal cortex. Although LIF did not modify basal cortisol release from dispersed cells isolated from the bovine adrenal zona fasciculate, this cytokine increased ACTH-stimulated release of cortisol from these cells in a manner dependent on the LIF concentration and exposure interval. In contrast, LIF in a concentration-dependent and time-dependent manner decreased basal and ACTH-stimulated adrenal androgen release from dispersed cells isolated from the bovine adrenal zona reticularis. Because LIF release increases during inflammatory stress and this cytokine stimulates adrenal cortisol release and inhibits adrenal androgen release, this cytokine may play an important role in regulating the release of adrenal steroids during inflammatory stress.

p38 mitogen-activated protein kinase inhibitor LY2228820 enhances bortezomib-induced cytotoxicity and inhibits osteoclastogenesis in multiple myeloma; therapeutic implications

The interaction between multiple myeloma (MM) cells and the bone marrow (BM) microenvironment induces proliferation and survival of MM cells, as well as osteoclastogenesis. This study investigated the therapeutic potential of novel p38 mitogen-activated protein kinase (p38MAPK) inhibitor LY2228820 (LY) in MM. Although cytotoxicity against MM cell lines was modest, LY significantly enhanced the toxicity of bortezomib by down-regulating bortezomib-induced heat shock protein 27 phosphorylation. LY inhibited interleukin-6 secretion from long term cultured-BM stromal cells and BM mononuclear cells (BMMNCs) derived from MM patients in remission. LY also inhibited macrophage inflammatory protein-1alpha secretion from patient MM cells and BMMNCs as well as normal CD14 positive osteoclast precursor cells. Moreover, LY significantly inhibited in vitro osteoclastogenesis from CD14 positive cells induced by macrophage-colony stimulating factor and soluble receptor activator of nuclear factor-kappaB ligand. Finally, LY also inhibited in vivo osteoclatogenesis in a severe combined immunodeficiency mouse model of human MM. These results suggest that LY represents a promising novel targeted approach to improve MM patient outcome both by enhancing the effect of bortezomib and by reducing osteoskeletal events.

Relationships between insulin-like growth factor-I (IGF-I) and OPG, RANKL, bone mineral density in healthy Chinese women

Serum IGF-I level was negatively correlated with OPG and OPG/RANKL ratio, but positively correlated with RANKL. Serum OPG level in the highest quintile of IGF-I was significantly lower than that in the lowest. We conclude that the effect of IGF-I on bone remodeling may be mediated by the OPG/RANKL system.

INTRODUCTION

Insulin-like growth factor I (IGF-I) is an important factor in coupling bone remodeling, activating both formation and resorption. Compared with the many studies on the role of IGF-I in bone formation, the information regarding its effects on bone resorption is limited and conflicting. The balance of the two peptides produced by osteoblasts, osteoprotegerin (OPG) and receptor activator of nuclear factor-kappaB ligand (RANKL), is critical for the bone resorption process. Our study was designed to analyze the relationships of serum concentrations of IGF-I with OPG, RANKL, OPG/RANKL ratio as well as BMDs in healthy Chinese women.

METHODS

BMDs at lumbar spine and proximal femur in 504 pre- and postmenopausal women were measured by DXA. Serum levels of IGF-I, OPG and RANKL were also measured. Pearson's correlation and partial correlation analysis, ANOVA, covariance analysis and stepwise multiple regression analysis were used as appropriate.

RESULTS

Age was negatively correlated with serum levels of IGF-I (r = -0.702, p < 0.001). IGF-I was negatively correlated with OPG and OPG/RANKL ratio, but positively correlated with RANKL. The relationship between IGF-I and BMDs disappeared after adjustment for age. In postmenopausal women, IGF-I was lower in women with osteoporosis than in those with normal BMD (p = 0.056), but no differences were found among OPG, RANKL and OPG/RANKL ratio. Serum levels of OPG in the highest quintile of IGF-I were significantly lower than those in the lowest quintile of IGF-I, while no difference was found in RANKL. In the multiple regression analysis model, serum levels of IGF-I were the main determinants of the bone mass in Chinese women.

CONCLUSIONS

In conclusion, the relationship between decreasing IGF-I and BMDs in healthy Chinese women influenced by age, whereas the effect of IGF-I on bone remodeling (bone resorption) may be mediated by the OPG/RANKL system.

Callus mineralization and maturation are delayed during fracture healing in interleukin-6 knockout mice

IL-6 is a pleiotropic cytokine involved in cell signaling in the musculoskeletal system, but its role in bone healing remains uncertain. The purpose of this study was to examine the role of IL-6 in fracture healing. Eight-week-old male C57BL/6 and IL-6 -/- mice were subjected to transverse, mid-diaphyseal osteotomies on the right femora. Sacrifice time points were 1, 2, 4, or 6 weeks post-fracture (N=14 per group). Callus tissue properties was analyzed by microcomputed tomography (micro-CT) and Fourier transform infrared imaging spectroscopy (FT-IRIS). Cartilage and collagen content, and osteoclast density were measured histologically. In intact unfractured bone, IL-6 -/- mice had reduced crystallinity, mineral/matrix ratio, tissue mineral density (TMD), and bone volume fraction (BVF) compared to wildtype mice. This suggests that there was an underlying deficit in baseline bone quality in IL-6 -/- mice. At 2 weeks post-fracture, the callus of IL-6 -/- mice had reduced crystallinity and mineral/matrix ratio. These changes were less evident at 4 weeks. At 2 weeks, the callus of the IL-6 -/- mice had an increased tissue mineral density (TMD), an increased cartilage and collagen content, and reduced osteoclast density compared to these parameters in wildtype mice. By 4 and 6 weeks, these parameters were no longer different between the two strains of mice. In conclusion, IL-6 -/- mice had delayed callus maturity, mineralization, and remodeling compared with the callus of the wildtype mice. These effects were transient indicating that the role of IL-6 appears to be most important in the early stages of fracture healing.

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.

Macrophage lineage phenotypes and osteoclastogenesis--complexity in the control by GM-CSF and TGF-beta

Bone-resorbing osteoclasts (OCs) derive from macrophage lineage precursors under the potential control of many factors. Addition of macrophage-colony stimulating factor (M-CSF or CSF-1) to murine bone marrow cells gives rise to so-called bone marrow-derived macrophages (BMM); this adherent population can then be quantitatively converted into OC lineage cells when receptor activator of NFkappaB ligand (RANKL) is included. The effect of another CSF, granulocyte macrophage-CSF (GM-CSF), on OC differentiation in vitro is quite complex with both enhancing and suppressive actions being described. We report here that GM-CSF can generate a population of adherent macrophage lineage cells from murine bone marrow precursors (GM-BMM) which is also capable of giving rise to OC lineage cells in the presence of M-CSF and RANKL as effectively as BMM. The degree of this differentiation was surprising considering that GM-BMM are often referred to as immature dendritic cells and that, for both BMM and the GM-BMM, GM-CSF suppressed subsequent OC differentiation governed by M-CSF and RANKL. Unlike for BMM, this GM-CSF-mediated suppression for GM-BMM appeared to be independent of c-fos expression. The effects on bone of another cytokine, transforming growth factor-beta (TGF-beta), are also quite complex although usually found to be stimulatory for OC differentiation. Unexpectedly, we observed that TGF-beta1 also potently suppressed M-CSF+RANKL-driven OC differentiation from both BMM and GM-BMM. Using cells from gene-deficient mice, this inhibition of OC differentiation by both GM-CSF and TGF-beta1 appeared to be independent of endogenous interferon alpha/beta production. It appears therefore that the influence of GM-CSF and TGF-beta on osteoclastogenesis depends on the presence or otherwise of other stimuli such as RANKL and possibly upon the maturation state of the OC precursors. It is proposed that the findings have particular relevance for the control of bone resorption in pathology, for example, in inflammatory lesions.

Activated T lymphocytes suppress osteoclastogenesis by diverting early monocyte/macrophage progenitor lineage commitment towards dendritic cell differentiation through down-regulation of receptor activator of nuclear factor-kappaB and c-Fos

Activated T lymphocytes either stimulate or inhibit osteoclastogenesis from haematopoietic progenitors in different experimental models. To address this controversy, we used several modes of T lymphocyte activation in osteoclast differentiation--mitogen-pulse, anti-CD3/CD28 stimulation and in vivo and in vitro alloactivation. Osteoclast-like cells were generated from non-adherent immature haematopoietic monocyte/macrophage progenitors in murine bone-marrow in the presence of receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) and monocyte-macrophage colony-stimulating factor (M-CSF). All modes of in vivo and in vitro T lymphocyte activation and both CD4(+) and CD8(+) subpopulations produced similar inhibitory effects on osteoclastogenesis paralleled by enhanced dendritic cell (DC) differentiation. Osteoclast-inhibitory effect was associated with T lymphocyte activation and not proliferation, and could be replaced by their culture supernatants. The stage of osteoclast differentiation was crucial for the inhibitory action of activated T lymphocytes on osteoclastogenesis, because the suppressive effect was visible only on early osteoclast progenitors but not on committed osteoclasts. Inhibition was associated specifically with increased granulocyte-macrophage colony-stimulating factor (GM-CSF) expression by the mechanism of progenitor commitment toward lineages other than osteoclast because activated T lymphocytes down-regulated RANK, CD115, c-Fos and calcitonin receptor expression, and increased differentiation towards CD11c-positive DC. An activated T lymphocyte inhibitory role in osteoclastogenesis, confirmed in vitro and in vivo, mediated through GM-CSF release, may be used to counteract activated bone resorption mediated by T lymphocyte-derived cytokines in inflammatory and immune disorders. We also demonstrated the importance of alloactivation in osteoclast differentiation and the ability of cyclosporin A to abrogate T lymphocyte inhibition of osteoclastogenesis, thereby confirming the functional link between alloreaction and bone metabolism.

B and T lymphocytes are the primary sources of RANKL in the bone resorptive lesion of periodontal disease

Receptor activator of nuclear factor-kappaB (RANKL)-mediated osteoclastogenesis plays a pivotal role in inflammatory bone resorption. The aim of this study was to identify the cellular source of RANKL in the bone resorptive lesions of periodontal disease. The concentrations of soluble RANKL, but not its decoy receptor osteoprotegerin, measured in diseased tissue homogenates were significantly higher in diseased gingival tissues than in healthy tissues. Double-color confocal microscopic analyses demonstrated less than 20% of both B cells and T cells expressing RANKL in healthy gingival tissues. By contrast, in the abundant mononuclear cells composed of 45% T cells, 50% B cells, and 5% monocytes in diseased gingival tissues, more than 50 and 90% of T cells and B cells, respectively, expressed RANKL. RANKL production by nonlymphoid cells was not distinctly identified. Lymphocytes isolated from gingival tissues of patients induced differentiation of mature osteoclast cells in a RANKL-dependent manner in vitro. However, similarly isolated peripheral blood B and T cells did not induce osteoclast differentiation, unless they were activated in vitro to express RANKL; emphasizing the osteoclastogenic potential of activated RANKL-expressing lymphocytes in periodontal disease tissue. These results suggest that activated T and B cells can be the cellular source of RANKL for bone resorption in periodontal diseased gingival tissue.

The role of megakaryocytes in skeletal homeostasis and rheumatoid arthritis

PURPOSE OF REVIEW

This review provides an update on the role of megakaryocytes in skeletal homeostasis, and discusses these findings in the context of rheumatoid arthritis.

RECENT FINDINGS

Thrombocytosis is a common complication of rheumatoid arthritis, and is presumably caused by an up-regulation in megakaryocytopoiesis. In general, patients with rheumatoid arthritis exhibit localized joint bone erosion with systemic bone loss, and rheumatoid arthritis patients with thrombocytosis tend to have more severe disease. Interestingly, in addition to their role in rheumatoid arthritis with thrombocytosis, it has been demonstrated recently that megakaryocytes play a dual role in regulating skeletal mass by inhibiting bone resorption while simultaneously stimulating bone formation. This seeming contradiction in the putative role of megakaryocytes in skeletal regulation and rheumatoid arthritis is the focus of this review.

SUMMARY

In rheumatoid arthritis there are substantial increases in the levels of several pro-inflammatory pleiotropic cytokines. As would be expected, in addition to their role in inflammation, these cytokines play a critical role in the megakaryocytopoiesis seen in patients who develop reactive thrombocytosis, and these cytokines also are known to regulate osteoclastogenesis. Thus, it appears that in rheumatoid arthritis with reactive thrombocytosis, the ability of the cytokines to enhance osteoclastogenesis outweighs the ability of megakaryocytes to inhibit osteoclastogenesis.

Gene expression profiles give insight into the molecular pathology of bone in primary hyperparathyroidism

Global gene expression profiling has been used to study the molecular mechanisms of increased bone remodeling caused by PHPT. This disease is a model for chronic over-stimulation of target organs by PTH due to an inappropriate overproduction of the hormone. Hyperactivity of osteoblasts and osteoclasts lead to increased calcium and phosphate mobilization from the skeleton and hypercalcaemia. The ensemble of genes that alter expression and thus is responsible for the effects of chronic PTH stimulation is today largely unknown. The differentiated gene expression profiles revealed characteristic molecular disease modalities which define the bone remodeling abnormalities occurring in PTH dependent osteodystrophy. We analyzed mRNAs in transiliacal bone biopsies from 7 patients with PHPT using Affymetrix HG-U133A Gene Chips containing more than 22000 different probe sets. Similar analyses of the global transcriptional activity were repeated in a second bone biopsy from the same patient taken one year after surgery and reversal of disease parameters. Real time PCR was carried out on many genes for corroboration of the results. Out of more than 14500 different genes examined, 99 which were related to bone and extra-cellular matrix, showed altered expression. Of these were 85 up- and 14 down-regulated before operation. The majority of regulated genes represented structural and adhesion proteins, but included also proteases and protease regulators which promote resorption. Increased expressions of collagen type 1 and osteocalcin mRNAs in disease reflecting the PTH anabolic action were paralleled by increased concentrations of these proteins in serum. In addition, genes encoding transcriptional factors and their regulators as well as cellular signal molecules were up-regulated during disease. The identified genetic signature represents the first extensive description of the ensemble of bone and matrix related mRNAs, which are regulated by chronic PTH action. These results identify the molecular basis for this skeletal disease, and provide new insight into this clinical condition with potential bearing on future treatment.

Rheumatic diseases: the effects of inflammation on bone

Rheumatoid arthritis, juvenile idiopathic arthritis, the seronegative spondyloarthropathies including psoriatic arthritis, and systemic lupus erythematosus are all examples of rheumatic diseases in which inflammation is associated with skeletal pathology. Although some of the mechanisms of skeletal remodeling are shared among these diseases, each disease has a unique impact on articular bone or on the axial or appendicular skeleton. Studies in human disease and in animal models of arthritis have identified the osteoclast as the predominant cell type mediating bone loss in arthritis. Many of the cytokines and growth factors implicated in the inflammatory processes in rheumatic diseases have also been demonstrated to impact osteoclast differentiation and function either directly, by acting on cells of the osteoclast-lineage, or indirectly, by acting on other cell types to modulate expression of the key osteoclastogenic factor receptor activator of nuclear factor (NF) kappaB ligand (RANKL) and/or its inhibitor osteoprotegerin (OPG). Further elucidation of the mechanisms responsible for inflammation-induced bone loss will potentially lead to the identification of novel therapeutic strategies for the prevention of bone loss in these diseases. In this review, we provide an overview of the cell types, inflammatory mediators, and mechanisms that are implicated in bone loss and new bone formation in inflammatory joint diseases.

Alumina particles influence the interactions of cocultured osteoblasts and macrophages

The purpose of the current study was to evaluate the effects of alumina particles on secretion of several cytokines involved in bone resorption in cocultures of macrophages and osteoblasts. To distinguish the contribution of each individual cell type, we have established a heterologous in vitro system that makes use of mouse J774 cells and primary cultured human osteoblasts. J744 cells decreased the production of TNF-alpha when they were cocultured with osteoblasts. Treatment of J744 cells with alumina particles increased TNF-alpha secretion, but the induction was lower when cells were cocultured with osteoblasts. Secretion of IL-6 by J744 cells was very low, and increased in the presence of osteoblasts. Alumina particles were only able to stimulate the release of IL-6 by J744 cells when cells were cocultured with osteoblasts. On the other hand, incubation of osteoblasts with alumina particles enhanced the release of IL-6 and GM-CSF. Coculturing osteoblasts with J744 cells induced them to release IL-6 and GM-CSF, and treatment with alumina further increased the secretion of both mediators by osteoblasts. According to these in vitro results, it seems rather plausible that alumina particles are able to initiate an inflammatory response in vivo.

GH/IGF-I and bone resorption in vivo and in vitro

IGF-I may act as one of several coupling agents by activating bone formation and bone resorption. In vivo studies in normal subjects, postmenopausal women and patients with excess or diminished GH production (acromegaly and GHD) indicate that both GH and IGF-I activate osteoclasts, but that GH has a more pronounced effect, independently of IGF-I. In vitro, GH and IGF receptors have been demonstrated on osteoclasts and both GH and IGF-I may directly modify osteoclast function and activity. In addition to direct effects on osteoclasts, GH and IGF-I may affect bone resorption indirectly by stimulating release of paracrine mediators that regulate osteoclastic resorption (cytokines). Critical for the bone resorptive process is the balance between OPG and RANKL, which is regulated by many systemic factors. In vivo and in vitro, GH/IGF-I may modulate this balance but these studies are difficult to interpret, reflecting the complexity of this system. Increased OPG expression may possibly protect against GH/IGF-I-induced bone resorption and potentially be important for the long-term beneficial effects of GH replacement. Further studies investigating the OPG/RANKL ratio and system in experimental and transgenic GH/IGF models may clarify these issues.

Modulation of osteoclast formation

Osteoclasts are derived following the fusion of precursors of hematopoietic and myelomonocytic origin after appropriate stimulus, such as that afforded by RANKL and M-CSF. Thus the osteoclast can be considered as a specialized type of macrophage, and several of the factors that affect osteoclast formation also have affects upon macrophage differentiation. Inhibitors of osteoclast formation may perturb RANKL or M-CSF signalling or affect other signalling pathways. Several of these inhibitors are discussed with the view of their capacity to influence osteoclast differentiation, but not necessarily their activity.