Osteoblast–osteoclast interactions

Bone homeostasis depends on the resorption of bones by osteoclasts and formation of bones by the osteoblasts. Imbalance of this tightly coupled process can cause diseases such as osteoporosis. Thus, the mechanisms that regulate communication between osteoclasts and osteoblasts are critical to bone cell biology. It has been shown that osteoblasts and osteoclasts can communicate with each other through direct cell–cell contact, cytokines, and extracellular matrix interaction. Osteoblasts can affect osteoclast formation, differentiation, or apoptosis through several pathways, such as OPG/RANKL/RANK, RANKL/LGR4/RANK, Ephrin2/ephB4, and Fas/FasL pathways. Conversely, osteoclasts also influence formation of bones by osteoblasts via the d2 isoform of the vacuolar (H+) ATPase (v-ATPase) V0 domain (Atp6v0d2), complement component 3a, semaphorin 4D or microRNAs. In addition, cytokines released from the resorbed bone matrix, such as TGF-β and IGF-1, also affect the activity of osteoblasts. Drugs could be developed by enhancing or restricting some of these interactions. Several reviews have been performed on the osteoblast–osteoclast communication. However, few reviews have shown the research advances in the recent years. In this review, we summarized the current knowledge on osteoblast–osteoclast communication.     

Human bone marrow adipocytes support dexamethasone-induced osteoclast differentiation and function through RANKL expression

The TNF-family molecule, Receptor Activator of Nuclear factor κ B Ligand (RANKL) is known as a key regulator for bone remodeling, and is essential for the development and activation of osteoclasts. In this study, we examined the regulation of RANKL in primary human bone marrow adipocytes and the relationship between bone marrow adipocytes and bone metabolism. RANKL expression and the RANKL/osteoprotegerin (OPG) mRNA ratio in marrow adipocytes increased following dexamethasone treatment. In co-cultures of human osteoclast precursors and bone marrow adipocytes with dexamethasone, osteoclast precursors differentiated to TRAP-positive multinuclear cells. Moreover, the ability of bone resorption was confirmed in co-culture in flasks coated with calcium phosphate film. Osteoclast precursor differentiation and bone resorption were blocked by RANKL antibody pretreatment. TRAP-positive multinuclear cells did not form in coculture without cell-to-cell contact conditions. We conclude that primary human bone marrow adipocytes have the ability to promote osteoclast differentiation and activities, similar to osteoblasts and other RANKL-expressing cells.     

OPG/RANKL/RANK信号通路在骨巨细胞瘤发病机制中的作用

文题释义：OPG/RANKL/RANK 信号通路：是骨代谢中至关重要的一条信号通路,它是成骨细胞与破骨细胞之间相互作用的信号通道,同时也是骨巨细胞瘤影响骨代谢的主要途径。 骨巨细胞瘤(Giant cell tumor of bone,GCTB)：是常见的原发性骨肿瘤之一,其发病率占所有原发性骨肿瘤的4%-10%,多发生于20-40岁的青壮年患者,好发于股骨远端、胫骨近端或桡骨远端。骨巨细胞瘤组织学来源尚不清楚,一般认为起始于骨髓内间叶组织。该肿瘤具有较强的侵袭性,对骨质有较大的破坏和侵蚀作用,但很少有患者出现反应性新骨生成或是自愈倾向。 背景：研究表明,骨保护素/核因子κB受体活化因子配体/核因子κB受体活化因子(OPG/RANKL/RANK)信号通路与骨巨细胞瘤发病机制之间有一定的相关性,通过控制OPG/RANKL/RANK信号通路影响成骨细胞与破骨细胞之间相互作用,对该病起到一定的治疗作用。 目的：介绍 OPG/RANKL/RANK信号通路与骨巨细胞瘤发病机制的关系,总结并讨论OPG/RANKL/RANK信号通路在骨巨细胞瘤发病机制中的最新研究进展。 方法：检索 PubMed 数据库、Web of science数据库及万方数据库中2001至2019年相关文献,检索词分别为“OPG/RANKL/RANK,giant cell tumor of bone,pathogenesis,signal pathway, bone metabolism,OPG/RANK/RANKL,骨巨细胞瘤,发病机制,信号通路,骨代谢”。排除较陈旧及重复的文献,通过整理,共纳入53篇文献进行分析探讨。 结果与结论：①骨保护素抑制破骨细胞增殖及分化,降低成熟破骨细胞活性,阻断核因子κB受体活化因子配体与核因子κB受体活化因子结合,减缓破骨;②核因子κB受体活化因子配体与破骨细胞前体细胞表面的核因子κB受体活化因子结合,促进破骨细胞前体细胞分化,增殖,进而加速破骨;③核因子κB受体活化因子配体与其受体结合后,激活核因子κB等信号因子促进破骨细胞的增殖、分化并激活破骨细胞,同时调节相关基因的转录及表达;④OPG/RANK/RANKL与骨巨细胞瘤发病机制相关。 ORCID: 0000-0002-2756-4848(梁晨亮) 中国组织工程研究杂志出版内容重点：组织构建;骨细胞;软骨细胞;细胞培养;成纤维细胞;血管内皮细胞;骨质疏松;组织工程     

Inhibition of RANK/RANKL signal transduction pathway: a promising approach for osteoporosis treatment

Osteoporosis is a bone disease causing impaired bone strength. It is characterized by increased osteoclast formation or enhanced bone resorption, leading to an increased risk of fragility fractures. Its prevalence increases with age. The advent of an aging population suggests that progressively more individuals will develop this disease in the aging population. A number of drugs for the prevention and treatment of osteoporosis act by inhibiting bone resorption. However, the effectiveness of osteoporosis treatment in clinical practice is limited. Since the osteoclast is the only cell in the body that is capable of resorbing bone, understanding its biology will be necessary for developing a new therapeutic approach for osteoporosis. Recently, it was discovered that the receptor activator of nuclear factor kappaB (RANK)/RANK ligand (RANKL)/osteoprotegerin (OPG) system is an important signal transduction pathway that regulates osteoclast formation. The binding of OPG to RANKL inhibits the binding between RANKL and RANK; this, in turn, prevents osteoclast precursors from differentiating and fusing to form mature osteoclasts. Therefore, the inhibition of the RANK/RANKL pathway inhibits osteoclast formation, differentiation, activation, and bone resorption. A potential clinical antiresorptive therapy can be developed by using an anti-RANKL monoclonal antibody, such as denosumab, that binds to RANKL with high affinity and specificity and blocks RANKL-RANK interactions.     

Pharmacological management of osteogenesis

Osteogenesis and bone remodeling are complex biological processes that are essential for the formation of new bone tissue and its correct functioning. When the balance between bone resorption and formation is disrupted, bone diseases and disorders such as Paget''s disease, fibrous dysplasia, osteoporosis and fragility fractures may result. Recent advances in bone cell biology have revealed new specific targets for the treatment of bone loss that are based on the inhibition of bone resorption by osteoclasts or the stimulation of bone formation by osteoblasts. Bisphosphonates, antiresorptive agents that reduce bone resorption, are usually recommended as first-line therapy in women with postmenopausal osteoporosis. Numerous studies have shown that bisphosphonates are able to significantly reduce the risk of femoral and vertebral fractures. Other antiresorptive agents indicated for the treatment of osteoporosis include selective estrogen receptor modulators, such as raloxifene. Denosumab, a human monoclonal antibody, is another antiresorptive agent that has been approved in Europe and the USA. This agent blocks the RANK/RANKL/OPG system, which is responsible for osteoclastic activation, thus reducing bone resorption. Other approved agents include bone anabolic agents, such as teriparatide, a recombinant parathyroid hormone that improves bone microarchitecture and strength, and strontium ranelate, considered to be a dual-action drug that acts by both osteoclastic inhibition and osteoblastic stimulation. Currently, anti-catabolic drugs that act through the Wnt-β catenin signaling pathway, serving as Dickkopf-related protein 1 inhibitors and sclerostin antagonists, are also in development. This concise review provides an overview of the drugs most commonly used for the control of osteogenesis in bone diseases.     

Inhibition of RANKL blocks skeletal tumor progression and improves survival in a mouse model of breast cancer bone metastasis

Bone metastases cause severe skeletal morbidity including fractures and hypercalcemia. Tumor cells in bone induce activation of osteoclasts, which mediate bone resorption and release of growth factors from bone matrix, resulting in a “vicious cycle” of bone breakdown and tumor proliferation. Receptor activator of NF-κB ligand (RANKL) is an essential mediator of osteoclast formation, function, and survival, and is blocked by a soluble decoy receptor, osteoprotegerin (OPG). In human malignancies that metastasize to bone, dysregulation of the RANK/RANKL/OPG pathway can increase the RANKL:OPG ratio, a condition which favors excessive osteolysis. In a mouse model of bone metastasis, RANKL protein levels in MDA-MB-231 (MDA-231) tumor-bearing bones were significantly higher than tumor-free bones. The resulting tumor-induced osteoclastogenesis and osteolysis was dose-dependently inhibited by recombinant OPG-Fc treatment, supporting the essential role for RANKL in this process. Using bioluminescence imaging in a mouse model of metastasis, we monitored the anti-tumor efficacy of RANKL inhibition on MDA-231 human breast cancer cells in a temporal manner. Treatment with OPG-Fc in vivo inhibited growth of MDA-231 tumor cells in bony sites when given both as a preventative (dosed day 0) and as a therapeutic agent for established bone metastases (dosed day 7). One mechanism by which RANKL inhibition reduced tumor burden appears to be indirect through inhibition of the “vicious cycle” and involved an increase in tumor cell apoptosis, as measured by active caspase-3. Here, we demonstrate for the first time that OPG-Fc treatment of mice with established bone metastases resulted in an overall improvement in survival.     

Osteoprotegerin reduces osteoclast numbers and prevents bone erosion in collagen-induced arthritis

Rheumatoid arthritis is characterized by progressive synovial inflammation and joint destruction. While matrix metalloproteinases (MMPs) are implicated in the erosion of unmineralized cartilage, bone destruction involves osteoclasts, the specialized cells that resorb calcified bone matrix. RANK ligand (RANKL) expressed by stromal cells and T cells, and its cognate receptor, RANK, were identified as a critical ligand-receptor pair for osteoclast differentiation and survival. A decoy receptor for RANKL, osteoprotegerin, (OPG) impinges on this system and regulates osteoclast numbers and activity. RANKL is also expressed in collagen-induced arthritis (CIA) in which focal collections of osteoclasts are prominent at sites of bone destruction. To determine the role of RANK signaling events in the effector phase of CIA, we investigated effects of Fc-osteoprotegerin fusion protein (Fc-OPG) in CIA. After induction of CIA in Dark Agouti rats, test animals were treated with or without Fc-OPG (3 mg/kg/day) subcutaneously for 5 days, beginning at the onset of disease. Paraffin-embedded joints were then analyzed histologically and the adjacent bone assessed by histomorphometry. Osteoclasts were identified using TRAP staining and expression of the mRNA for OPG and RANKL was identified by in situ hybridization. The results indicated that short-term Fc-OPG effectively prevented joint destruction, even though it had no impact on the inflammatory aspects of CIA. In arthritic joints, Fc-OPG depleted osteoclast numbers by over 75% and diminished bone erosion scores by over 60%. Although cartilage loss was also reduced by Fc-OPG, the effects on cartilage were less striking than those on bone. In arthritic joints OPG mRNA was highly expressed and co-localized with RANK ligand, and treatment with Fc-OPG did not affect the expression of endogenous RANKL or OPG mRNA. These data demonstrate that short term Fc-OPG treatment has powerful anti-erosive effects, principally on bone, even though synovitis is not affected. These findings indicate the potential utility of disrupting RANK signaling to preserve skeletal integrity in inflammatory arthritis.