Biological effects of bisphosphonates in patients with rheumatoid arthritis

Loss of bone mineral density(BMD) has frequently been observed in patients with rheumatoid arthritis (RA) and main causes of osteoporosis were reported to be steroid osteoporosis, postmenoposal osteoporosis, and disuse bone atrophy associated with polyarticular impairment. It is becoming clear that the increase in bone resorption such as these osteoporosis and RA is underling the molecular mechanism; the facilitation of osteoclast differentiation and activation by the inflammatory cytokines TNFalpha and IL-1. Bisphosphonates, which are taken up by osteoclasts and macrophages to inhibit the activity of these cytokines, are expected to function as an inhibitor of inflammation induced by these cells. Bisphosphonates reduce also osteoclast numbers and activity by induction of osteoclast apoptosis, and could be a therapeutic goal for new anti-osteoclast drugs. As for the periarticular osteoporosis, bisphosphonate has also anti inflammatory effects and inhibition of bone destruction in RA.     

Osteoblast as a target of anti-osteoporotic treatment

Osteoblasts are mesenchymal cells that play a key role in maintaining bone homeostasis; they are responsible for the production of extracellular matrix proteins, regulation of matrix mineralization, control of bone remodeling and regulate osteoclast differentiation. Osteoblasts have an essential role in the pathogenesis of many bone diseases, particularly osteoporosis. For many decades, the main current available treatments for osteoporosis have been represented by anti-resorptive drugs, such as bisphosphonates, which act mainly by inhibiting osteoclasts maturation, proliferation and activity; nevertheless, in recent years much attention has been paid on anabolic aspects of osteoporosis treatment. Many experimental evidences support the hypothesis of direct effects of the classical anti-resorptive drugs also on osteoblasts, and recent progress in understanding bone physiology have led to the development of new pharmacological agents such as anti-sclerostin antibodies and teriparatide which directly target osteoblasts, inducing anabolic effects and promoting bone formation.     

Promotion of osteoclast survival and antagonism of bisphosphonate-induced osteoclast apoptosis by glucocorticoids

Glucocorticoids depress bone formation by inhibiting osteoblastogenesis and increasing osteoblast apoptosis. However, the role of bone resorption in the initial rapid phase of bone loss characteristic of glucocorticoid-induced osteoporosis is unexplained, and the reason for the efficacy of bisphosphonates in this condition remains unknown. We report that in murine osteoclast cultures, glucocorticoids prolonged the baseline survival of osteoclasts and antagonized bisphosphonate-induced caspase activation and apoptosis by a glucocorticoid receptor-mediated action. Consistent with the in vitro evidence, in a murine model of glucocorticoid-induced osteoporosis, the number of cancellous osteoclasts increased, even though osteoclast progenitor number was reduced. Moreover, in mice receiving both glucocorticoids and bisphosphonates, the expected proapoptotic effect of bisphosphonates on osteoclasts was abrogated, as evidenced by maintenance of osteoclast numbers and, additionally, loss of bone density. In contrast, bisphosphonate administration prevented glucocorticoid-induced osteoblast apoptosis. These results indicate that the early loss of bone with glucocorticoid excess is caused by extension of the life span of pre-existing osteoclasts, an effect not preventable by bisphosphonates. Therefore, the early beneficial effects of these agents must be due, in part, to prolonging the life span of osteoblasts.     

Osteoporosis associated with rheumatoid arthritis

Patients with rheumatoid arthritis(RA) develop both generalized and periarticular osteoporosis. Both of them are believed to be associated with increased production of inflammatory cytokines(TNF alpha, IL-1 beta, IL-6) and increased formation and activation of osteoclasts. Whether glucocorticoids work positively or negatively on generalized/periarticular osteoporosis is still controversial. RANKL has been shown to be expressed on T cells and fibroblast-like cells in the synovium, thus 'RANKL-RANK' pathway is likely to play an important role in periarticular osteoporosis and bone erosion as well as generalized osteoporosis. Among various therapies for generalized/periarticular osteoporosis in RA, anti-cytokine antibodies/antagonists and osteoclast inhibitors including bisphosphonates are promising.     

Cathepsin K inhibitors: a novel target for osteoporosis therapy

Osteoporosis is characterized by low bone mass with skeletal fragility and an increased risk of fracture. This bone loss is brought about by an imbalance between bone resorption and formation. Cathepsin K is the most abundant cysteine protease expressed in the osteoclast and is believed to be instrumental in bone matrix degradation necessary for bone resorption. Cathepsin K inhibitors represent a novel target for developing agents to treat osteoporosis and other disorders characterized by increased bone resorption.     

Novel therapies in benign and malignant bone diseases

With an ageing population and improving cancer therapies, the two most common benign and malignant bone diseases, osteoporosis and bone metastases, will continue to affect an increasing number of patients. Our expanding knowledge of the molecular processes underlying these conditions has resulted in novel bone targets that are currently being explored in clinical trials. Clearly, the approval of denosumab, a monoclonal antibody directed against RANKL, has just marked the beginning of a new era for bone therapy with several additional new therapies lining up for clinical approval in the coming years. Potential agents targeting the osteoclast include cathepsin K, currently in phase 3 trials, and src inhibitors. Amongst anabolic agents, inhibitors of the Wnt-inhibitor sclerostin and dickkopf-1 are promising in clinical trials. Here, we will provide a comprehensive overview of the most promising agents currently explored for the treatment of bone diseases.     

Pathogenesis of osteoporosis--overview

Pathogenesis of osteoporosis was overviewed from standpoints of individual level, organ level and cellular level, respectively. As for the individual level, it is important to discriminate the pathogenetic factors related to low BMD or fractures. Independent from BMD, age, previous fractures, bone geometry, microstructure of the bone are related to fractures by influencing the bone quality. As for the organ level, pathogenesis of osteoporosis should be discussed from the view point of bone remodeling and its controlling factors. However, details of this process are still unclear. As for the cellular level, clarification of the roles of osteoblast, osteoclast and their interactions are essential in understanding the pathogenesis of osteoporosis. Recent discovery of Cbfa1, RANK/RANKL signaling and osteoprotegerin may open the new insight in the pathogenesis of osteoporosis.     

OPG, anti-rANKL antibody

Identification of RANKL/ODF (receptor activator of NF-kappaB ligand/osteoclast differentiation factor), RANK(receptor activator of NF-kappaB) and OPG/OCIF(osteoprotegerin/ osteoclastogenesis inhibitory factor) revealed the mechanisms regulating osteoclast differentiation and function. RANKL-RANK signaling is essential for the physiological osteoclast development and plays a major role in the pathological bone destruction. OPG and anti-RANKL antibody act as a specific inhibitor of RANKL and are useful and applicable to osteoporosis and rheumatoid arthritis.     

Oncostatin M promotes bone formation independently of resorption when signaling through leukemia inhibitory factor receptor in mice

Effective osteoporosis therapy requires agents that increase the amount and/or quality of bone. Any modification of osteoclast-mediated bone resorption by disease or drug treatment, however, elicits a parallel change in osteoblast-mediated bone formation because the processes are tightly coupled. Anabolic approaches now focus on uncoupling osteoblast action from osteoclast formation, for example, by inhibiting sclerostin, an inhibitor of bone formation that does not influence osteoclast differentiation. Here, we report that oncostatin M (OSM) is produced by osteoblasts and osteocytes in mouse bone and that it has distinct effects when acting through 2 different receptors, OSM receptor (OSMR) and leukemia inhibitory factor receptor (LIFR). Specifically, mouse OSM (mOSM) inhibited sclerostin production in a stromal cell line and in primary murine osteoblast cultures by acting through LIFR. In contrast, when acting through OSMR, mOSM stimulated RANKL production and osteoclast formation. A key role for OSMR in bone turnover was confirmed by the osteopetrotic phenotype of mice lacking OSMR. Furthermore, in contrast to the accepted model, in which mOSM acts only through OSMR, mOSM inhibited sclerostin expression in Osmr^–/– osteoblasts and enhanced bone formation in vivo. These data reveal what we believe to be a novel pathway by which bone formation can be stimulated independently of bone resorption and provide new insights into OSMR and LIFR signaling that are relevant to other medical conditions, including cardiovascular and neurodegenerative diseases and cancer.     

脉冲电磁场对成骨和破骨代谢的影响

背景:脉冲电磁场对成骨细胞、破骨细胞、骨基质合成均有明显作用。目的:旨在探究脉冲电磁场在成骨和破骨代谢中的作用以及治疗骨质疏松的机制,以促进其临床应用。方法:由第一作者应用计算机检索PubMed、中国期刊全文数据库(CNKI)、维普数据库和万方数据库1997-05/2011-08相关文献。在标题、摘要、关键词中以"pulsed electromagnetic field(PEMFs),bone metabolism,osteoporosis,osteoblast,osteoclast,bone marrow mesenchymal stem cells(BMMSCs)"或"脉冲电磁场,骨代谢,骨质疏松,成骨细胞,破骨细胞,骨髓间充质干细胞"为检索词进行检索。选择文章内容与脉冲电磁场有关者,同一领域文献则选择近期发表在权威杂志文章进行分析。结果与结论:初检得到389篇文献,根据纳入标准选择关于假体无菌性松动分析及处理的46篇文献进行综述。脉冲电磁场可促进成骨代谢,抑制破骨代谢,参与调节转化生长因子β,白细胞介素6等骨代谢相关的细胞因子,促进骨基质形成,改善骨生长微环境,对骨髓间充质干细胞的增殖分化亦有促进作用。推测脉冲电磁场治疗骨质疏松及其并发症有重要的临床价值。     

Inflammatory cytokines for osteoclastogenesis

Bone homeostasis is maintained by a balance between bone resorption by osteoclasts and bone formation by osteoblasts. Osteoclast maturation requires stimulation by RANKL on osteoblasts and various stimuli. Pro-inflammatory cytokines such as IL-1 and TNF-alpha cause an imbalance in bone metabolism by favouring bone resorption via the induction of RANKL on osteoblasts and induction of osteoclast maturation. These inflammatory signals originate from the immune system, the largest source of cell-derived regulatory signals and such immunological signals to the bone induce osteoclast maturation, resulting in secondary osteoporosis. Actually, such phenomena mainly occur at the interface between proliferating synovium and bone tissue in rheumatoid arthritis (RA). Thus, therapeutic strategies for these conditions, an anti-TNF-alpha antibody, effective for treating RA disease activity, also reduce secondary osteoporosis and joint destruction.     

脉冲电磁场对成骨和破骨代谢的影响

背景：脉冲电磁场对成骨细胞、破骨细胞、骨基质合成均有明显作用。目的：旨在探究脉冲电磁场在成骨和破骨代谢中的作用以及治疗骨质疏松的机制,以促进其临床应用。方法：由第一作者应用计算机检索PubMed、中国期刊全文数据库（CNKI）、维普数据库和万方数据库1997-05/2011-08相关文献。在标题、摘要、关键词中以＂pulsed electromagnetic field（PEMFs）,bone metabolism,osteoporosis,osteoblast,osteoclast,bone marrow mesenchymal stem cells（BMMSCs）＂或＂脉冲电磁场,骨代谢,骨质疏松,成骨细胞,破骨细胞,骨髓间充质干细胞＂为检索词进行检索。选择文章内容与脉冲电磁场有关者,同一领域文献则选择近期发表在权威杂志文章进行分析。结果与结论：初检得到389篇文献,根据纳入标准选择关于假体无菌性松动分析及处理的46篇文献进行综述。脉冲电磁场可促进成骨代谢,抑制破骨代谢,参与调节转化生长因子β,白细胞介素6等骨代谢相关的细胞因子,促进骨基质形成,改善骨生长微环境,对骨髓间充质干细胞的增殖分化亦有促进作用。推测脉冲电磁场治疗骨质疏松及其并发症有重要的临床价值。     

Estrogen Inhibits Bone Resorption by Directly Inducing Apoptosis of the Bone-resorbing Osteoclasts

Estrogen deficiency causes bone loss, which can be prevented by estrogen replacement therapy. Using a recently developed technique for isolation of highly purified mammalian osteoclasts, we showed that 17 β-estradiol (E₂) was able to directly inhibit osteoclastic bone resorption. At concentrations effective for inhibiting bone resorption, E₂ also directly induced osteoclast apoptosis in a dose- and time-dependent manner. ICI164,384 and tamoxifen, as pure and partial antagonists, respectively, completely or partially blocked the effect of E₂ on both inhibition of osteoclastic bone resorption and induction of osteoclast apoptosis. These data suggest that the protective effects of estrogen against postmenopausal osteoporosis are mediated in part by the direct induction of apoptosis of the bone-resorbing osteoclasts by an estrogen receptor– mediated mechanism.     

Synergistic effects of miR-708-5p and miR-708-3p accelerate the progression of osteoporosis

BackgroundBone homeostasis is a tightly orchestrated process maintained by osteoblasts and osteoclasts, and a disruption of their steady-state equilibrium can lead to the occurrence of osteoporosis (OP).MethodsWe investigated the differential expression of micro (mi)RNAs in the bone tissues of a postmenopausal osteoporosis rat model induced by ovariectomy (OVX). Real-time PCR was used to verify the differentially expressed miRNAs in bone samples from OP patients and controls. The specific targets of two differentially expressed miRNAs in osteogenic or osteoclast differentiation were determined by bioinformatic prediction, and mRNA and protein detection.ResultsmiR-708-5p and miR-708-3p were highly expressed in the bone tissue of OVX rats and OP patients. miR-708-5p negatively regulated osteoblast differentiation in bone marrow mesenchymal stem cells by targeting SMAD specific E3 ubiquitin protein ligase 2, while miR-708-3p positively regulated osteoclast differentiation in bone marrow monocytes by targeting cerebellar degeneration associated protein 1 antisense RNA. miR-708-5p and miR-708-3p were shown to originate from the same precursor miRNA and to have a synergistic effect on the development of osteoporosis with different temporal and spatial patterns.ConclusionOur findings provide a referential theoretical basis and targets for the prevention and treatment of osteoporosis.     

Denosumab: mechanism of action and clinical outcomes

Aims: To describe the mechanisms of action of denosumab, a novel antiresorptive agent, contrasting it with other antiresorptive and anabolic osteoporosis treatments. Methods: Published papers related to the mechanism of action of approved osteoporosis treatments were sought through MEDLINE searches. Findings: Osteoporotic fractures carry a substantial burden of morbidity and mortality, but pharmacotherapy can prevent such fractures in high‐risk individuals. Antiresorptive drugs (e.g. bisphosphonates, oestrogen, denosumab) reduce bone turnover by distinct mechanisms. Denosumab, a recently approved therapy, is a fully human monoclonal antibody that binds the cytokine RANKL (receptor activator of NFκB ligand), an essential factor initiating bone turnover. RANKL inhibition blocks osteoclast maturation, function and survival, thus reducing bone resorption. In contrast, bisphosphonates bind bone mineral, where they are absorbed by mature osteoclasts, inducing osteoclast apoptosis and suppressing resorption. These differences in mechanism influence both the onset and reversibility of treatment. Discussion: Effective pharmacotherapy is necessary for patients at high risk of fracture. Among the treatment options for postmenopausal osteoporosis, there are significant differences in mechanism and dosing. Denosumab acts by a novel mechanism and is administered twice yearly by subcutaneous injection. Identified by Osteoporosis Canada Clinical Practice Guidelines as a first‐line agent for treatment of postmenopausal osteoporosis, denosumab represents an important addition to our treatment options.     

Anti-IL-20 monoclonal antibody inhibits the differentiation of osteoclasts and protects against osteoporotic bone loss

IL-20 is a proinflammatory cytokine of the IL-10 family that is involved in psoriasis, rheumatoid arthritis, atherosclerosis, and stroke. However, little is known about the role of IL-20 in bone destruction. We explored the function of IL-20 in osteoclastogenesis and the therapeutic potential of anti-IL-20 monoclonal antibody 7E for treating osteoporosis. Higher serum IL-20 levels were detected in patients with osteopenia and osteoporosis and in ovariectomized (OVX) mice. IL-20 mediates osteoclastogenesis by up-regulating the receptor activator of NF-κB (RANK) expression in osteoclast precursor cells and RANK ligand (RANKL) in osteoblasts. 7E treatment completely inhibited osteoclast differentiation induced by macrophage colony-stimulating factor (M-CSF) and RANKL in vitro and protected mice from OVX-induced bone loss in vivo. Furthermore, IL-20R1-deficient mice had significantly higher bone mineral density (BMD) than did wild-type controls. IL-20R1 deficiency also abolished IL-20-induced osteoclastogenesis and increased BMD in OVX mice. We have identified a pivotal role of IL-20 in osteoclast differentiation, and we conclude that anti-IL-20 monoclonal antibody is a potential therapeutic for protecting against osteoporotic bone loss.     

Cloning and identification of human Sca as a novel inhibitor of osteoclast formation and bone resorption.

Increased osteoclast activity is responsible for the enhanced bone destruction in postmenopausal osteoporosis, Paget's disease, bone metastasis, and hypercalcemia of malignancy. However, the number of known inhibitory factors that block osteoclast formation and bone resorption are limited. Therefore, we used an expression-cloning approach to identify novel factors produced by osteoclasts that inhibit osteoclast activity. A candidate clone was identified and isolated from a human osteoclast-like multinucleated cell (MNC) cDNA library, named osteoclast inhibitory peptide-1 (OIP-1), and the cDNA sequence was determined. This sequence matched that of the recently identified human stem cell antigen, was structurally similar to the mouse Ly-6 gene family, and the sequence predicted it was a glycosyl phosphatidyl inositol (GPI)-anchored protein that had a cleavable COOH-terminal peptide. Western blot analysis of conditioned media from 293 cells transfected with the OIP-1 cDNA clone confirmed that OIP-1 was released into the media as a membrane-bound GPI-linked protein. Interestingly, both recombinant OIP-1 expressed in Escherichia coli (which does not have GPI linker) and OIP-1 expressed by mammalian cells significantly reduced osteoclast-like MNC formation induced by 1,25-dihydroxyvitamin D3 or PTH-related protein in mouse and human bone marrow cultures, and inhibited 45Ca release from prelabeled bone in fetal rat organ cultures. In contrast, recombinant OIP-1 did not inhibit the growth of a variety of other cell types. These data indicate that OIP-1 is a novel, specific inhibitor of osteoclast formation and bone resorption.     

Osteoporosis in men and women

Osteoporosis is a cause of significant morbidity and mortality in postmenopausal women as well as men. In both men and women, increasing age and low bone mineral density (BMD) are the 2 most important independent risk factors for an initial vertebral or nonvertebral fracture. Although the prevalence of osteoporosis is greater in women, mortality after fracture is higher among men. In both men and women, the incidence of vertebral fracture increases with age, although the increase is more marked in women than in men. The diagnostic criteria for postmenopausal osteoporosis in women are well established; however, there is ongoing debate about the appropriate T-scores and BMD thresholds to diagnose osteoporosis in men. Alendronate and risedronate are considered first-line therapy for the treatment of both postmenopausal osteoporosis and male osteoporosis. The efficacy and safety of these agents have been evaluated extensively in randomized clinical trials. Studies suggest that these agents are similarly efficacious in men and women. The anabolic agent teriparatide may also be used to treat men with osteoporosis at high risk for fracture. Studies suggest that treatment with an anabolic agent like teriparatide should be followed by an antiresorptive agent.