Skeletal and extraskeletal actions of denosumab

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

Modulation of osteoclast differentiation and function by the new members of the tumor necrosis factor receptor and ligand families.

Osteoblasts/stromal cells are essentially involved in osteoclast differentiation and function through cell-to-cell contact (Fig. 8). Although many attempts have been made to elucidate the mechanism of the so-called "microenvironment provided by osteoblasts/stromal cells," (5-8) it has remained an open question until OPG and its binding molecule were cloned. The serial discovery of the new members of the TNF receptor-ligand family members has confirmed the idea that osteoclast differentiation and function are regulated by osteoblasts/stromal cells. RANKL, which has also been called ODF, TRANCE, or OPGL, is a member of the TNF ligand family. Expression of RANKL mRNA in osteoblasts/stromal cells is up-regulated by osteotropic factors such as 1 alpha, 25(OH)2D3, PTH, and IL-11. Osteoclast precursors express RANK, a TNF receptor family member, recognize RANKL through cell-to-cell interaction with osteoblasts/stromal cells, and differentiate into pOCs in the presence of M-CSF. RANKL is also involved in the survival and fusion of pOCs and activation of mature osteoclasts. OPG, which has also been called OCIF or TR1, is a soluble receptor for RANKL and acts as a decoy receptor in the RANK-RANKL signaling system (Fig. 8). In conclusion, osteoblasts/stromal cells are involved in all of the processes of osteoclast development, such as differentiation, survival, fusion, and activation of osteoclasts (Fig. 8). Osteoblasts/stromal cells can now be replaced with RANKL and M-CSF in dealing with the whole life of osteoclasts. RANKL, RANK, and OPG are three key molecules that regulate osteoclast recruitment and function. Further studies on these key molecules will elucidate the molecular mechanism of the regulation of osteoclastic bone resorption. This line of studies will establish new ways to treat several metabolic bone diseases caused by abnormal osteoclast recruitment and functions such as osteopetrosis, osteoporosis, metastatic bone disease, Paget's disease, rheumatoid arthritis, and periodontal bone disease.     

CLINICAL Review #: the role of receptor activator of nuclear factor-kappaB (RANK)/RANK ligand/osteoprotegerin: clinical implications

CONTEXT: Receptor activator of nuclear factor-kappaB ligand (RANKL), receptor activator of nuclear factor-kappaB (RANK), and osteoprotegerin (OPG) play a central role in bone remodeling and disorders of mineral metabolism. EVIDENCE ACQUISITION: A PubMed search was conducted from January 1992 until 2007 for basic, observational, and clinical studies in subjects with disorders related to imbalances in the RANK/RANKL/OPG system. EVIDENCE SYNTHESIS: RANK, RANKL, and OPG are members of the TNF receptor superfamily. The pathways involving them in conjunction with various cytokines and calciotropic hormones play a pivotal role in bone remodeling. Several studies involving mutations in the genes encoding RANK and OPG concluded in the discovery of a number of inherited skeletal disorders. In addition, basic and clinical studies established a consistent relationship between the RANK/RANKL/OPG pathway and skeletal lesions related to disorders of mineral metabolism. These studies were a stepping stone in further defining the role of the RANK/RANKL/OPG pathway in osteoporosis, rheumatoid arthritis, bone loss associated with malignancy-related skeletal diseases, and its relationship to vascular calcifications. Subsequently, the further understanding of this pathway led to the development of new therapeutic modalities including the human monoclonal antibody to RANKL and recombinant OPG as a target for treatment of postmenopausal osteoporosis and multiple myeloma. CONCLUSIONS: The RANK/RANKL/OPG system mediates the effects of calciotropic hormones and, consequently, alterations in their ratio are key in the development of several clinical conditions. New agents with the potential to block effects of RANKL have emerged for treatment of postmenopausal osteoporosis and malignancy-related skeletal disease.     

Bone loss in inflammatory arthritis: mechanisms and therapeutic approaches with bisphosphonates

The inflammatory process in rheumatoid arthritis provokes intense bone resorption, evidenced as bone erosions, juxta-articular osteopenia and generalized osteoporosis. These types of bone loss share a common pathogenesis, and the role of osteoclasts in focal bone erosion was verified in elegant animal studies. The tumour necrosis factor (TNF) family of cytokines and receptors--specifically TNF-alpha, RANKL, RANK and OPG--are dominant regulators of osteoclastic bone resorption in rheumatoid arthritis. The confirmation of the osteoclast mechanism provides new insight into the structural joint protection afforded by disease-modifying drugs and suggests innovative approaches to limit bone destruction. Emerging treatment strategies for bone disease in rheumatoid arthritis are the use of monoclonal antibodies to neutralize RANKL, and powerful bisphosphonates that target pathogenic osteoclasts.     

Role of RANKL-RANK/osteoprotegerin molecular complex in bone remodeling and its immunopathologic implications

Bone remodeling is a cyclic and continuous physiological process, which ensures the conservation and renewal of the bone matrix. Osteosynthesis of the bone matrix is achieved by osteoblasts and coordinated within this complex machinery of bone remodeling with resorption of extracellular bone matrix performed by osteoclasts. The mismatch between the activities of osteoblasts and osteoclasts has immunopathologic implications associated with either a decrease or increase of bone mass mineral density. The balance of the trimolecular control factor complex composed of osteoprotegerin (OPG), RANKL (osteoprotegerin ligand) and RANK maintains physiologic bone remodeling. This trimolecular complex functions as receptors and ligands and belongs to the superfamily of tumor necrosis factor (TNF). This mini review highlights the complex interplay of the RANKL-RANK/OPG axis and their immunopathologic implications in clinical medicine.     

Pathophysiology of bone metabolism

Osteoporosis is a multifactorial disease entailing a high risk to sustain fragility fractures. Its susceptibility is determined by genetic and environmental factors and underlying diseases. Bone is rebuilt and regenerated by osteoclasts, osteoblasts and osteocytes. Local and systemic growth and differentiation factors such as Insulin-like growth factors, bone morphogenetic proteins and wnt-proteins confer anabolic signals, while the RANK/RANK-Ligand and Osteoprotegerin (OPG) system regulates bone resorption. The ratio of osteoclast stimulating RANKL and its soluble decoy receptor OPG is modulated by sex hormones, vitamin D, parathyroid hormone, local growth factors and mechanical loading. Osteocytes regulate bone mass via the bone formation inhibitor sclerostin. Bone is tightly interconnected with and regulated by the calcium/phosphate/vitamin D system via the parathyroid gland, the gut, liver and kidneys. Sex hormones are important for bone formation during adolescence and their loss in menopause/andropause exaggerates bone resorption. Basically over-activity of osteoclasts and/or functional deficits of osteoblasts can cause negative bone balance and favor osteoporosis.     

Pathophysiologie des Knochenstoffwechsels

Osteoporosis is a multifactorial disease entailing a high risk to sustain fragility fractures. Its susceptibility is determined by genetic and environmental factors and underlying diseases. Bone is rebuilt and regenerated by osteoclasts, osteoblasts and osteocytes. Local and systemic growth and differentiation factors such as Insulin-like growth factors, bone morphogenetic proteins and wnt-proteins confer anabolic signals, while the RANK/RANK-Ligand and Osteoprotegerin (OPG) system regulates bone resorption. The ratio of osteoclast stimulating RANKL and its soluble decoy receptor OPG is modulated by sex hormones, vitamin D, parathyroid hormone, local growth factors and mechanical loading. Osteocytes regulate bone mass via the bone formation inhibitor sclerostin. Bone is tightly interconnected with and regulated by the calcium/phosphate/vitamin D system via the parathyroid gland, the gut, liver and kidneys. Sex hormones are important for bone formation during adolescence and their loss in menopause/andropause exaggerates bone resorption. Basically over-activity of osteoclasts and/or functional deficits of osteoblasts can cause negative bone balance and favor osteoporosis.     

RANKL and RANK as novel therapeutic targets for arthritis

The TNF-family molecule receptor activator of nuclear factor kappa B (NFkappaB) ligand (RANKL) (OPGL, TRANCE, ODF) and its receptor activator of NFkappaB (RANK) are key regulators of bone remodeling and regulate T cell/dendritic cell communications, and lymph node formation. Moreover, RANKL and RANK are expressed in mammary gland epithelial cells and control the development of a lactating mammary gland during pregnancy. Genetically, RANKL and RANK are essential for the development and activation of osteoclasts and bone loss in response to virtually all triggers tested. Inhibition of RANKL function via the natural decoy receptor osteoprotegerin (OPG, TNFRSF11B) prevents bone loss in postmenopausal osteoporosis and cancer metastases. Importantly, RANKL appears to be the pathogenetic principle that causes bone and cartilage destruction in arthritis, and OPG treatment prevents bone loss at inflamed joints and has partially beneficial effects on cartilage destruction in all arthritis models studied so far. Modulation of these systems provides a unique opportunity to design novel therapeutics to inhibit bone loss and crippling in arthritis.     

Molecules which are involved in osteoclastic bone resorption: from the aspect of targets of treatment for osteoporosis

Bone in vertebrates is continuously destroyed and reformed to maintain bone volume throughout their life. In normal bone remodeling, osteoblastic bone formation follows osteoclastic bone resorption in a precise and quantitative manner. In osteoporosis, the bone resorption surpasses the bone formation. Recent studies have shown that osteoclastic bone resorption is tightly regulated by bone-forming osteoblasts. Osteoblasts express RANKL (receptor activator of nuclear factor-KappaB ligand), which plays central roles in osteoclast differentiation and function. In this review article, we explain molecules which are involved in the RANKL-RANK interaction in the regulation of osteoclastic bone resorption.     

Proteasome inhibitors and bone disease

Bone disease in patients with multiple myeloma (MM) is characterized by increase in the numbers and activity of bone-resorpting osteoclasts and decrease in the number and function of bone-formation osteoblasts. MM-triggered inhibition of bone formation may stem from suppression of Wnt/β-catenin signaling, a pivotal pathway in the differentiation of mesenchymal stem cells (MSC) into osteoblasts, and regulating production of receptor activator of nuclear factor-κB ligand (RANKL)/osteoprotegerin (OPG) axis by osteoblasts. Proteasome inhibitors (PIs), such as bortezomib (Bz), induce activation of Wnt/β-catenin pathway and MSC differentiation toward osteoblasts. PIs also suppress osteoclastogenesis, possibly through regulating multiple pathways including NF-κB, Bim, and the ratio of RANKL/OPG. The critical role of PI in increasing osteoblast function and suppression of osteoclast activity is highlighted by clinical evidence of increases in bone formation and decreases in bone resorption makers. This review will discuss the function of PIs in stimulating bone formation and suppression of bone resorption, and the mechanism underlying this process that leads to inhibition bone disease in MM patients.     

OPG/RANK/RANKL系统与骨折和类风湿性关节炎

骨保护素(OPG)、细胞核因子-κB受体活化因子(RANK)和RANK配体(RANKL)是偶联成骨细胞、基质细胞和破骨细胞分化、活化及生物活性的3种主要细胞因子,其形成的局部调节体系在骨代谢中起十分重要的作用。本文简要介绍了OPG/RANK/RANKL系统及该系统在骨质疏松性骨折发生中的作用,RANKL/OPG比值与骨折的关系,OPG和RANKL对骨折愈合的作用,血清OPG或RAN-KL水平与骨折的联系,OPG基因多态性与骨折关系的研究结果。另外还介绍其在类风湿性关节炎发病机制中的作用,OPG/RANK/RANKL与滑膜组织的联系,OPG治疗的相关实验进展。     

OPG, a possible candidate for the treatment of rheumatoid arthritis

Osteoprotegerin (OPG) is an osteoclastogenesis inhibitory factor that we have cloned, and is a decoy receptor that inhibits the binding of an osteoclast differentiation factor, RANKL and its receptor RANK. Pharmacological and developmental approaches have demonstrated that OPG inhibits osteoclastogenesis and bone resorption in vivo. OPG may be useful for and applicable to the treatment of bone destruction in rheumatoid arthritis.     

Recent progress of pathogenesis and treatments in rheumatoid arthritis

Bone homeostasis is maintained by a balance between bone resorption by osteoclasts and bone formation by osteoblasts. Osteoclast maturation requires stimulation by RANKL expressed on osteoblasts and other cells. During the processes, pro-inflammatory cytokines such as TNF-alpha cause an imbalance in bone metabolism via direct and/or indirect effects on osteoclasts. These inflammatory signals originate from the immune system, the largest source of cell-derived regulatory signals and such immunological signals to the bone results in secondary osteoporosis and bone destruction. Actually, such phenomena mainly occur at the interface between proliferating synovium and bone tissue in rheumatoid arthritis (RA). Thus, therapeutic strategies using biologics including infliximab and etanercept, effective for treating RA disease activity, also reduce joint destruction. Based on an improved understanding of immune signals, investigation of the suppression of cell functions may lead to improved understanding and better treatment of bone destruction and osteoporosis.     

Evaluation of bone mineral density, bone metabolism, osteoprotegerin and receptor activator of the NFkappaB ligand serum levels during treatment with infliximab in patients with rheumatoid arthritis

OBJECTIVES: To examine whether treatment with anti-tumour necrosis factor (TNF) alpha prevents loss of bone mineral density (BMD) at the spine and hip (generalised) and in the hands (local) of patients with rheumatoid arthritis, and to study the changes in markers of bone metabolism, including receptor activator of the NFkappaB ligand (RANKL) and osteoprotegerin (OPG), during anti-TNF treatment. PATIENTS AND METHODS: 102 patients with active rheumatoid arthritis, who were treated with infliximab during 1 year, were included in this open cohort study. The BMD of the spine and hip (dual x ray absorptiometry) and hands dual x ray radiogrammetry was measured before the start of treatment and after 1 year. Changes in osteocalcin formation, beta-isomerised carboxy terminal telopeptide of type 1 collagen (beta-CTx, resorption), RANKL and OPG were determined at 0, 14, 30 and 46 weeks. RESULTS: The BMD of the spine and hip was unchanged during treatment with infliximab, whereas BMD of the hand decreased significantly by 0.8% (p<0.01). The BMD of the hip in patients with a good European League Against Rheumatism response showed a favourable change compared with patients not achieving such a response. Serum beta-CTx and RANKL were both considerably decreased compared with baseline at all time points. The decrease in beta-CTx was associated with the decrease in Disease Activity Score of 28 joints and C reactive protein during the 0-14 weeks interval. CONCLUSION: In patients with rheumatoid arthritis treated with infliximab, spine and hip bone loss is arrested, whereas metacarpal cortical hand bone loss is not stopped. The outcome of the study also supports a relationship between clinical response, in terms of reduced inflammatory activity, and changes in bone loss of the spine, hip and hands.     

RANK ligand and osteoprotegerin: paracrine regulators of bone metabolism and vascular function

In 1997, investigators isolated a secreted glycoprotein that blocked osteoclast differentiation from precursor cells, prevented osteoporosis (decreased bone mass) when administered to ovariectomized rats, and resulted in osteopetrosis (increased bone mass) when overexpressed in transgenic mice. Since then, the isolation and characterization of the protein named osteoprotegerin (OPG) has stimulated much work in the fields of endocrinology, rheumatology, and immunology. OPG functions as a soluble decoy receptor for receptor activator of nuclear factor-kappaB ligand (RANKL, or OPG ligand) and shares homologies with other members of the tumor necrosis factor receptor superfamily. OPG acts by competing with the receptor activator of nuclear factor-kappaB, which is expressed on osteoclasts and dendritic cells for specifically binding to RANKL. RANKL is crucially involved in osteoclast functions and bone remodeling as well as immune cell cross-talks, dendritic cell survival, and lymph node organogenesis. More recently, emerging evidence from in vitro studies and mouse genetics attributed OPG an important role in vascular biology. In fact, OPG could represent the long sought-after molecular link between arterial calcification and bone resorption, which underlies the clinical coincidence of vascular disease and osteoporosis, which are most prevalent in postmenopausal women and elderly people.     

Osteoblasts and bone formation

Bone is constantly being remodelled in a dynamic process where osteoblasts are responsible for bone formation and osteoclasts for its resorption. Osteoblasts are specialized mesenchymal cells that undergo a process of maturation where genes like core-binding factor alpha1 (Cbfa1) and osterix (Osx) play a very important role. Moreover, it was found recently that Wnt/ beta-catenin pathway plays a part on osteoblast differentiation and proliferation. In fact, mutations on some of the proteins involved in this pathway, like the low-density lipoprotein receptor related protein 5/6 (LRP5/6) lead to bone diseases. Osteoblast have also a role in regulation of bone resorption through receptor activator of nuclear factor-kappaB (RANK) ligand (RANKL), that links to its receptor, RANK, on the surface of pre-osteoblast cells, inducing their differentiation and fusion. On the other hand, osteoblasts secrete a soluble decoy receptor (osteoprotegerin, OPG) that blocks RANK/RANKL interaction by binding to RANKL and, thus, prevents osteoclast differentiation and activation. Therefore, the balance between RANKL and OPG determines the formation and activity of osteoclasts. Another factor that influences bone mass is leptin, a hormone produced by adipocytes that have a dual effect. It can act through the central nervous system and diminish osteoblasts activity, or can have an osteogenic effect by binding directly to its receptors on the surface of osteoblast cells.     

New pathophysological relevant metabolic pathways in osteoporosis. Future innovative therapies?

Osteoporosis is characterized by low bone mass and by changes in the microarchitecture of the bone. This leads to reduced bone stability and altered suscebtibility to fractures. Bone remodelling in healthy persons is characterized by a balance between bone resorption and bone formation. At the cellular level, bone remodelling is regulated by osteoclast and osteoblast activity. During bone loss, there is an imbalance, osteoclast activity being more pronounced. Therefore, the influende of estrogens, Wnt and the RANK/ RANKL/OPG system on osteoclastogenesis and osteoclast activity has been investigated. The RANK/RANKL/OPG-System is actively involved in the differentiation and function of osteoclasts and seems to play a central part in most pathophysiological mechanisms that are active in osteoporosis.     

Neue pathophysiologische Stoffwechselwege in der Osteoporose

Osteoporosis is characterized by low bone mass and by changes in the microarchitecture of the bone. This leads to reduced bone stability and altered suscebtibility to fractures. Bone remodelling in healthy persons is characterized by a balance between bone resorption and bone formation. At the cellular level, bone remodelling is regulated by osteoclast and osteoblast activity. During bone loss, there is an imbalance, osteoclast activity being more pronounced. Therefore, the influende of estrogens, Wnt and the RANK/RANKL/OPG system on osteoclastogenesis and osteoclast activity has been investigated. The RANK/RANKL/OPG-System is actively involved in the differentiation and function of osteoclasts and seems to play a central part in most pathophysiological mechanisms that are active in osteoporosis.