New insights in myeloma-induced osteolysis

Multiple myeloma (MM) is a plasma cell malignancy localized in the bone marrow (BM) and characterized by a high capacity for bone destruction. Almost all patients with MM have early osteolytic lesions, which result mainly from increased bone resorption related to stimulation of osteoclast recruitment and activity in the immediate vicinity of myeloma cells. The recent discovery of Osteoprotegerin (OPG) and the subsequent identification of its ligand RANKL have provided new insights in the regulation of osteoclastogenesis. The ratio OPG/RANKL is critical for the regulation of bone remodeling maintaining the balance between osteoblastic and osteoclastic activity. This review summarizes the new concept that myeloma cells induce in bone environment an imbalance in the OPG/RANKL system responsible for osteolysis observed in patients. Indeed, myeloma cells increase in bone environment the expression of the potent osteoclastogenic factor RANKL and decrease the osteoprotective factor OPG production. Biological mechanisms involved in these processes are discussed. Furthermore, the chemokines MIP-1alpha and MIP-1beta belonging to the RANTES family are potent osteoclastogenic factors produced by myeloma cells and participate in myeloma-associated bone disease. These data open new avenues for the treatment of bone disease in MM and highlight the promising therapeutical interest of RANKL inhibitors (OPG and RANK-Fc) and MIP-1 inhibitors in the management of myeloma-associated osteolysis, besides bisphosphonates.     

Myeloma bone disease and treatment options

Multiple myeloma (MM) is a B-cell malignancy characterized by enhanced bone loss commonly associated with a diffuse osteopenia, focal lytic lesions, pathologic fractures, hypercalcemia, and bony pain. Bone destruction in MM results from asynchronous bone turnover wherein increased osteoclastic bone resorption is not accompanied by a comparable increase in bone formation. Recent characterization of osteoclast-activating factors (OAFs), receptor activator of nuclear factor-kappaB (RANK) ligand (RANKL)-osteoprotegerin-RANK system, and inhibitors of Wnt signaling have provided a better understanding of myeloma bone disease in molecular level. The development of minimally invasive surgical procedures such as kyphoplasty and vertebroplasty allows myeloma patients with vertebral compression fractures to have immediate improvement in quality of life and shorter hospital stays. Monthly intravenous infusion of either pamidronate or zoledronic acid have reduced the skeletal complications among MM patients and are now a mainstay of myeloma therapy. Orally administered bisphosphonates, in contrast, have shown little ability to slow the development of skeletal complications in these patients. Although pre-clinical studies suggest nitrogen-containing bisphosphonates have potent anti-tumor effects, clinical trials will be necessary, probably at higher doses given more slowly, to establish their possible anti-tumor effects clinically. As our understanding of the pathophysiology of myeloma bone disease continues to increase, new target therapies will continue to emerge offering new and more advanced options for the management of myeloma bone disease.     

Aetiology of bone disease and the role of bisphosphonates in multiple myeloma

Osteolytic bone disease is a major cause of morbidity in patients with multiple myeloma. Our understanding of the pathophysiology of multiple myeloma has increased substantially during the past decade. However the underlying mechanisms of bone destruction and the treatments available have, until recently, received relatively little specific attention. In this review, we provide an overview of the RANK/RANKL/osteoprotegerin system; we describe its interaction with other cellular mechanisms, through which malignant plasma cells drive osteolysis, and explain how bisphosphonates can be used to block this action. We also review the supporting evidence for bisphosphonates as the treatment of choice for patients with bone complications related to multiple myeloma, and discuss possible developments for targeted therapy in the future.     

Gene silencing of the BDNF/TrkB axis in multiple myeloma blocks bone destruction and tumor burden in vitro and in vivo

Osteolytic bone diseases are a prominent feature of multiple myeloma (MM), resulting from aberrant osteoclastic bone resorption that is uncoupled from osteoblastic bone formation. Myeloma stimulates osteoclastogenesis, which is largely dependent on an increase in receptor activator of NF‐κB ligand (RANKL) and a decrease in osteoprotegerin (OPG) within the bone marrow milieu. Recently, brain‐derived neurotrophic factor (BDNF) was identified as a MM‐derived factor that correlates with increased RANKL levels and contributes to osteolytic bone destruction in myeloma patients. Because tyrosine receptor kinase B (TrkB), the receptor of BDNF, is abundantly expressed in osteoblasts, we sought to evaluate the role of BDNF/TrkB in myeloma–osteoblast interactions and the effect of this pathway on the RANKL/OPG ratio and osteoclastogenesis. Coculture systems constructed with noncontact transwells revealed that, in vitro, MM‐derived BDNF increased RANKL and decreased OPG production in osteoblasts in a time‐ and dose‐dependent manner. These effects were completely abolished by a specific small interfering RNA for TrkB. BDNF regulates RANKL/OPG expression in osteoblasts through the TrkB/ERK pathway. To investigate the biological effects of BDNF on myeloma in vivo, a SCID‐RPMI8226 mice model was constructed using lentiviral short hairpin RNA‐transfected RPMI8226 cells. In this system, stable knockdown of BDNF in MM cells significantly restored the RANKL/OPG homostasis, inhibited osteolytic bone destruction and reduced angiogenesis and tumor burden. Our studies provide further support for the potential osteoclastogenic effects of BDNF, which mediates stroma–myeloma interactions to disrupt the balance of RANKL/OPG expression, ultimately increasing osteoclastogenesis in MM.     

Autologous stem cell transplantation normalizes abnormal bone remodeling and sRANKL/osteoprotegerin ratio in patients with multiple myeloma.

The osteoprotegerin (OPG)/receptor activator of NF-kappa B ligand (RANKL) system has a major role in the pathogenesis of bone disease in myeloma (MM). The effect of autologous stem cell transplantation (ASCT) on bone turnover in MM was evaluated in 51 patients (35M/16F). Markers of bone resorption (NTX, TRACP-5b), bone formation (bone-alkaline phosphatase (bALP), osteocalcin), OPG and sRANKL were measured pre- and every month post-ASCT. The median follow-up period was 12 months. Four patients were transplanted in CR, 44 were transplanted in PR and three patients had progressive/resistant disease. All patients received bisphosphonates both pre- and post-ASCT. At baseline the majority of patients had increased NTX, TRACP-5b levels, and sRANKL/OPG ratio, while markers of bone formation were strongly suppressed. ASCT produced a significant reduction of sRANKL/OPG ratio, with a concomitant decrease of NTX, and TRACP-5b levels, starting the second month post-ASCT. Bone formation markers, osteocalcin and bALP, started to increase after the 9th and 11th month post-ASCT, respectively, while the increase of OPG preceded this. These results provide biochemical evidence that ASCT normalizes the abnormal bone resorption in MM patients possibly through the decrease of RANKL/OPG ratio, while bone formation requires a longer period to return to normal.     

The role of RANK/RANKL/osteoprotegerin (OPG) triad in cancer-induced bone diseases: physiopathology and clinical implications

Bone homeostasis is maintained by the remodelling of bone which depends on a balance between osteoclast-mediated bone resorption and osteoblast-mediated bone formation. Malignant bone lesions are very common in patients with cancer; whether they result from a tumor in bone (giant cell tumour of bone, osteosarcoma, multiple myeloma...) or they are bony metastases from advanced cancers of which the most osteotropic are breast and prostate cancer. Malignant cells within the bone disrupt the normal bone remodelling process, leading to increased bone destruction and occurence of pathological fractures. Receptor activator of NF-kB (RANK) and its ligand (RANKL) play a pivotal role in the regulation of bone remodelling; by binding to RANK, RANKL stimulates osteoclastogenesis and bone resorption, whereas its cognate decoy receptor osteoprotegerin (OPG) blocks this process by interacting with RANKL. Tumour cells produce different factors that manipulate the RANK/RANKL/OPG pathway in order to stimulate bone destruction. Furthermore, pending on the tumour type, RANKL plays a role in the migration, invasion and proliferation of malignant cells within the bone, while OPG increases survival of tumour cells. Inhibition of RANK/RANKL system may therefore offer new therapeutic perspectives for the treatment of primitive and secondary bone cancers.     

Expression of receptor activator of nuclear factor kappaB ligand on bone marrow plasma cells correlates with osteolytic bone disease in patients with multiple myeloma.

PURPOSE: Increased bone resorption is a hallmark of multiple myeloma and is attributable to osteoclast activation. Recent studies showed that the receptor activator of nuclear factor kappaB ligand (RANKL) is the key mediator of osteoclastogenesis and plays a crucial role in bone destruction in malignant bone disease. We found that human myeloma cells express RANKL and analyzed the association of the RANKL expression with the presence of osteolytic bone disease in patients with multiple myeloma. EXPERIMENTAL DESIGN: Flow cytometry was performed on bone marrow samples derived from controls and multiple myeloma patients with or without osteolytic bone lesions on conventional radiography. Plasma cells were identified as CD38++/CD138+ cells. The level of RANKL expression on the surface of bone marrow plasma cells was correlated with the bone status of the patients. RESULTS: The bone marrow plasma cells from controls showed no or only a weak surface expression of RANKL, and the median mean fluorescence index (MFI) was 6. In contrast, expression of RANKL could be detected on bone marrow plasma cells from all of the patients with multiple myeloma, and median MFI was 47. The difference in MFI for RANKL expression of bone marrow plasma cells from controls and myeloma patients was highly significant (P < 0.0005). Myeloma patients with osteolytic bone lesions showed a significantly higher expression of RANKL (median MFI = 60; range, 16-2494) compared with patients without osteolysis (median MFI = 16; range, 6-229; P < 0.0005). CONCLUSIONS: These results show for the first time that the level of RANKL expression by myeloma cells correlates significantly with osteolytic bone disease.     

Mechanism of bone metastasis: The role of osteoprotegerin and of the host-tissue microenvironment-related survival factors

Osteoprotegerin (OPG), member of tumor necrosis factor (TNF) receptor superfamily, has various biological functions including bone remodeling. OPG binds to receptor activator of nuclear factor-kB ligand (RANKL) and prevents osteoclastic bone resorption. Recently, OPG has gained more clinical interest as its role in cancer-mediated bone destruction and the potential of RANKL inhibition could act as a novel treatment in tumor-induced bone disease. OPG protects prostate cancer cells from apoptotic effects of TRAIL and therefore provides tumor cells producing OPG with survival advantages. Additionally, the increased RANKL/OPG ratio in metastatic breast cancer results in severe osteolysis. Thus, bone formation and resorption are the crux of cancer metastasis, resulting in bone pain and pathological fractures. This review provides an overview of the role of OPG in cancer-induced bone disease.     

Biochemical markers of bone metabolism in multiple myeloma

Osteolytic bone disease is a major clinical feature of multiple myeloma (MM). Mechanisms of bone destruction are related to increased osteoclastic activity, which is not accompanied by a comparable increase in bone formation, as osteoblasts are functionally exhausted. Thus the lesions rarely heal and bone scans are often negative in myeloma patients with extensive lytic lesions, offering very little in the follow-up of bone disease. Biochemical markers of bone resorption, such as N- and C-terminal cross-linking telopeptide of type I collagen (NTX, CTX/ICTP, respectively), tartrate resistant acid phosphatase isoform-5b, bone formation (bone-specific alkaline phosphatase [BAP]), and osteocalcin provide useful information on bone dynamics. Several studies have shown that NTX, CTX, and ICTP are elevated in myeloma patients, reflect the extent of bone disease, and correlate with survival. Furthermore, they are useful in monitoring bone destruction during antimyeloma or bisphosphonate treatment. Markers of bone formation have produced conflicting results in trials. However, BAP correlates with bone pain, lytic lesions, and fractures in quite a few studies of MM. Novel markers, such as bone sialoprotein, receptor activator of nuclear factor-kappa B ligand (RANKL), osteoprotegerin, osteopontin, dickkopf-1, and soluble Frizzle-related protein-2 have been found of value in assessing bone lytic disease in MM, but their promising results must be confirmed in large trials. In conclusion, although no marker provides optimal analysis of MM or of MM treatments, combinations of markers have at times helped in assessing MM stages and lytic bone disease and in monitoring specific treatment modalities. The need for further research in this field is clear.     

Receptor activator of nuclear factor-kappaB ligand expression by human myeloma cells mediates osteoclast formation in vitro and correlates with bone destruction in vivo

Multiple myeloma (MM) is an incurable B-cell malignancy able to mediate massive destruction of the axial skeleton. The aim of this study was to examine the involvement of the tumor necrosis factor-ligand family member, receptor activator of nuclear factor-kappaB ligand (RANKL), and its naturally occurring antagonist, osteoprotegerin (OPG), in MM biology. Using flow cytometry and two independent anti-RANKL antibodies, we demonstrate RANKL expression in CD38(+++)CD45(+) and CD38(+++)CD45(-) myeloma plasma cell (MPC) subpopulations derived from patients with osteolytic MM. In addition, highly purified subpopulations of MPC express mRNA for both transmembrane and soluble RANKL isoforms but lack expression of OPG mRNA and protein. We also show that RANKL expressed by MPC is functional as in vitro coculture of CD38(+++)CD45(+) and CD38(+++)CD45(-) MPC subpopulations with peripheral blood mononuclear cells resulted in the formation of multinucleate, tartrate-resistant acid phosphatase-positive osteoclasts-like cells capable of forming typical resorption pits. Furthermore, high expression of membrane-associated RANKL by CD38(+++) MPC correlated with the presence of multiple radiological bone lesions in individuals with MM. Together, our data strongly suggest that RANKL expression by MPC confers on them the ability to participate directly in the formation of osteoclast in vivo and extends our knowledge of the involvement of RANKL and OPG in the osteolysis characteristic of this disease.     

The high rate of bone resorption in multiple myeloma is due to RANK (receptor activator of nuclear factor-kappaB) and RANK Ligand expression

The excessive bone resorption observed in multiple myeloma may be due to the production of several osteoclast-activating factors either by the myeloma cells themselves or by the bone marrow microenvironment. These factors could act primarily via a common final pathway involving the recently-described members of the TNF receptor-ligand family: RANKL (Receptor Activator of NK-κB Ligand) and its corresponding RANK receptor that play a crucial role in osteoclast differentiation and activation, and osteoprotegerin (OPG), the physiological inhibitor of RANKL. RANKL expression by stromal cells is increased in myeloma and is associated with a concomitant decrease in OPG expression. This increase in RANKL-OPG ratio correlates with the extent of the myeloma bone disease. The RANKL-OPG imbalance could play a decisive role in the lytic bone lesions in myeloma, and this possibility is reinforced by several in-vivo studies that have assessed the effects of administering RANKL inhibitors in murine myeloma models. Treatment with either OPG : Fc or RANK : Fc decreased myeloma osteolysis in these models. RANKL blockade is also currently being evaluated in malignant osteolysis in humans. A therapeutic approach targeting the RANKL-RANK signaling pathway could be of great value, as RANKL inhibitors are potent anti-resorptive agents, affecting both myeloma-induced bone resorption and the tumor burden.     

Bone disease in myeloma

Opinion statement The major clinical manifestation of multiple myeloma results from osteolytic bone destruction. The only currently Food and Drug Administration-approved drug for the treatment of the bony complications of multiple myeloma is monthly intravenous pamidronate at a dose of 90 mg infused over 4 hours. Recent studies have shown the safety of 2-hour infusions. A randomized trial comparing pamidronate to placebo continued to show benefits throughout the 21-month trial. Although the duration of therapy has not been firmly determined, it is likely that discontinuation of this drug will be met by enhanced bone loss and an increased risk of bony complications for these patients. Thus, it is recommended that the drug be continued indefinitely. Support for this recommendation also comes from the reduced bone density observed in women with postmenopausal osteoporosis following the withdrawal of bisphosphonate treatment. Recent attempts to give higher doses, more frequent infusions (every 2 weeks or less), or more rapid infusions (1 hour or less) of pamidronate have occasionally been associated with albuminuria and azotemia. These modifications should therefore be avoided. Importantly, the drug can be safely administered at 90 mg monthly to patients with poor renal function. The use of pamidronate for myeloma patients without lytic bone involvement or with Durie-Salmon stages I or II disease has not been evaluated. However, it is recognized that most patients with earlier stages of disease or without lytic bone involvement also develop bony complications. There is no reason to believe that these patients would not benefit from monthly intravenous infusions of pamidronate. The potential antimyeloma effect of this agent is another reason to administer this drug in these types of patients. Thus, it is our practice to administer monthly pamidronate to myeloma patients regardless of stage or bone involvement. However, trials evaluating oral bisphosphonates have produced inconsistent clinical results, probably as a result of the erratic and scanty poor absorption as well as poor oral tolerability of these drugs. Although these oral agents may be useful in some patients, it is impossible to identify which myeloma patients will benefit from orally administered bisphosphonates. The more potent nitrogen-containing bisphosphonate zoledronic acid more effectively reverses hypercalcemia of malignancy than pamidronate, and it appears promising in reducing bone loss in cancer patients. However, its efficacy in preventing skeletal complications is still being evaluated. Many other types of new agents are in early clinical trials, but their efficacy remains unproven at the present time.     

Management of Metastatic Bone Disease and Hypercalcemia of Malignancy

Thirty years of research have established bisphosphonates as the most effective agents for the inhibition of osteoclast-mediated bone resorption, and they play an important role in the management of malignant bone disease. Bisphosphonates have been systematically improved through chemical engineering, and the newest nitrogen-containing compounds, including zoledronic acid and ibandronate, are 1000-fold more potent than first-generation compounds. Consequently, they can be administered at low molar doses via short intravenous infusions without compromising renal safety. Bisphosphonates have a variety of metabolic effects on osteoclasts. Nitrogen-containing bisphosphonates inhibit protein prenylation via the mevalonate pathway, thereby inhibiting osteoclast activation and inducing apoptosis. Preclinical studies suggest that bisphosphonates also have direct and indirect antitumor activity. In animal models, bisphosphonates reduced skeletal tumor burden and bone metastases. Currently, intravenous bisphosphonates are the standard therapy for hypercalcemia of malignancy, and they have become an integral part of the treatment of bone metastases in conjunction with standard antineoplastic agents. Intravenous bisphosphonates quickly normalize serum calcium, reduce skeletal complications, and palliate bone pain in patients with bone metastases. Intravenous pamidronate (90mg via 2-hour infusion every 3–4 weeks) has, until recently, been the international standard for the treatment of osteolytic bone lesions from breast cancer or multiple myeloma. However, 4mg zoledronic acid (via 15-minute infusion) is quickly becoming the new standard based on evidence that it is as safe and effective as 90mg pamidronate in patients with breast cancer and multiple myeloma and significantly more effective for hypercalcemia of malignancy. Consequently, the American Society of Clinical Oncology guidelines for breast cancer and multiple myeloma recommend pamidronate or zoledronic acid for patients with radiographic evidence of osteolytic bone destruction. Moreover, 4mg zoledronic acid is the only bisphosphonate that has demonstrated significant clinical benefit in patients with other solid tumors, including lung cancer, and prostate cancer patients with primarily osteoblastic bone metastases. Bisphosphonates also may have activity in the adjuvant setting to prevent or delay the development of bone metastases. Studies with oral clodronate in early breast cancer have provided clinical evidence that bone metastases can be inhibited, and the studies are ongoing with more potent bisphosphonates. Bisphosphonates have also been shown to prevent cancer treatment-induced bone loss. These and other studies continue to redefine the role of bisphosphonates in the treatment of malignant bone disease and the management of bone health in cancer patients.     

Optimising treatment of bone metastases by Aredia™ and Zometa™

Metastatic bone disease develops as a result of the many interactions between tumour cells and bone cells. This leads to disruption of normal bone metabolism, with the increased osteoclast activity seen in most, if not all, tumor types providing a rational target for treatment. The clinical course of metastatic bone disease in multiple myeloma, breast and prostate cancers is relatively long, with patients experiencing sequential skeletal complications over a period of several years. These include bone pain, fractures, hypercalcaemia, and spinal cord compression, all of which may profoundly impair a patient's quality of life. External beam radiotherapy and systemic endocrine and cytotoxic treatments are the mainstay of treatment in advanced cancers. However, it is now clear that the bisphosphonates provide an additional treatment strategy, which reduces both the symptoms and complications of bone involvement. Pamidronate (Aredia(TM)) is the most widely evaluated bisphosphonate and is recommended for most patients with multiple myeloma or breast cancer with bone metastases. Current research aims include the evaluation of new potent bisphosphonates such as zoledronic acid (Zometa(TM)). It is hoped that this compound is not only more convenient and easier to administer but also more effective in inhibiting skeletal morbidity. Zometa may also have some direct anticancer activity. Preclinical studies with Zometa have demonstrated its potential in malignant bone disease. Clinical studies in treatment of hypercalcemia of malignancy have been completed, as have Phase I and II trials in patients with cancer and pre-existing bone metastases. Three randomized, double-blind, controlled Phase III trials are now ongoing to establish the efficacy and safety of Zometa in treatment of bone metastases in patients with osteolytic and osteoblastic lesions. Additionally, new specific molecules such as osteoprotogerin have been developed that are based on our improved understanding of the cellular signalling mechanisms involved in cancer induced bone disease. These potent molecules are now entering clinical trials. Ongoing research is aimed at trying to define the optimum route, dose, schedule and type of bisphosphonate in metastatic bone disease and their use in the prevention and treatment of osteoporosis in cancer patients. In vitro suggestions of direct anti-cancer activity and some promising clinical data in early breast cancer have resulted in considerable interest in the possible adjuvant use of bisphosphonates to inhibit the development of bone metastases.     

TRAIL produced from multiple myeloma cells is associated with osteolytic markers

Skeletal complications represent major clinical problems in multiple myeloma (MM). MM cells are known to induce differentiation of osteoclasts and inhibit osteoblasts. Receptor activator of nuclear factor-κB ligand (RANKL) and osteoprotegerin (OPG) are key molecules for osteoclastogenesis. Although OPG interacts with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), the contribution of TRAIL to skeletal-related events (SRE) remains a matter of debate. In the present study, we examined the role of TRAIL in MM bone lesions. Myeloma cells were purified from 56 MM patients by CD138-immunomagnetic beads. TRAIL, DKK-1 and MIP1α RNA expression in purified MM cells was analyzed by real-time PCR. Immunohistochemistry of TRAIL was performed on paraffin-embedded plasmacytoma tissue sections. The concentration of TRAIL in the serum and bone marrow plasma from MM patients was analyzed by ELISA. TRAIL expression was significantly higher in MM cells than in plasma cells from patients with monoclonal gammopathy of undetermined significance (MGUS). TRAIL staining was detected in the cytoplasm of myeloma cells. TRAIL expression in MM cells correlated with bone marrow plasma TRAIL concentration. TRAIL expression had a positive correlation with osteolytic markers, such as serum calcium and urinary deoxypyridinoline. These results suggest that TRAIL, produced from myeloma cells, may play an important role in bone resorption of MM patients. Inhibition of this pathway may lead to development of a new therapeutic approach preventing bone resorption in MM.     

骨转移生化指标研究进展

肿瘤骨转移发生时,破骨细胞和成骨细胞释放至血和尿中的骨生化指标明显升高,有望用于骨转移的早期诊断,但其准确性和有效性尚待大样本的前瞻性研究评价.大多数骨生化指标高水平与骨转移患者不良预后显著相关,对疾病进展和疗效监测则具有重要临床意义.     

Anti-tumour activity of bisphosphonates in human myeloma cells

Multiple myeloma is a haematological malignancy characterized by an expansion of malignant plasma cells within the bone marrow and is frequently associated with bone disease involving the development of osteolytic bone lesions, pathological fractures, osteoporosis and hypercalcaemia. A class of anti-resorptive drugs known as bisphosphonates have been in use to treat osteoclast-mediated bone diseases for the past 3 decades, and are currently proving effective in the treatment of the bone disease associated with multiple myeloma. Recent studies have suggested that bisphosphonate treatment may also result in an improvement in survival in some patients with multiple myeloma. These effects on survival may reflect an indirect effect of the bisphosphonates on tumour growth, via inhibition of osteoclast activity and hence a reduction in the release of tumour growth factors. However, it is also possible that bisphosphonates may have a direct effect on myeloma cells. In support of this we have demonstrated that bisphosphonates can decrease cell proliferation and induce apoptosis in human myeloma cells in vitro, and this review discusses the possibility that bisphosphonates may have not only an anti-resorptive action, but may also have a direct anti-tumour activity.     

Receptor activator of nuclear factor-kappaB ligand and osteoprotegerin: potential implications for the pathogenesis and treatment of malignant bone diseases

BACKGROUND: The current review summarizes the roles of the ligand, receptor activator of nuclear factor-kappaB ligand (RANKL), its receptor, receptor activator of nuclear factor-kappaB (RANK), and its decoy receptor, osteoprotegerin (OPG), on osteoclast biology and bone resorption. Furthermore, it highlights the impact of these compounds on the pathogenesis of malignant bone diseases, including tumor metastasis, humoral hypercalcemia of malignancy, and multiple myeloma. Finally, the authors discuss the therapeutic potential of OPG in the management of malignancies involving the skeleton. METHODS: After its discovery and cloning, the biologic effects of RANKL, RANK, and OPG have been characterized by in vitro experiments and in vivo studies. The generation of knock-out mice and transgenic mice has produced animal models with absent or excessive production of these cytokine components that display opposite abnormal skeletal phenotypes (osteoporosis or osteopetrosis). The potential effect of RANKL and OPG has been assessed by evaluating these compounds in various animal models of metabolic and malignant bone disease and by administering OPG to humans. RESULTS: Abnormal bone resorption due to local or systemic stimulation of osteoclast differentiation and activation is a hallmark of various benign and malignant bone diseases. RANKL, RANK, and OPG form an essential cytokine system that is capable of regulating all aspects of osteoclast functions, including proliferation, differentiation, fusion, activation, and apoptosis. The balance of bone resorption depends on the local RANKL-to-OPG ratio, which is enhanced in bone metastases and humoral hypercalcemia of malignancy. The exogenous administration of OPG to tumor-bearing animals corrects the increased RANKL-to-OPG ratio, and reverses the skeletal complications of malignancies. CONCLUSIONS: Abnormalities of the RANKL/OPG system have been implicated in the pathogenesis of various primary and secondary bone malignancies. The systemic administration of OPG appears to be a potent novel therapeutic agent for treatment of these disorders.     

New molecular targets in bone metastases

Bone metastases have a major impact on morbidity and on mortality in cancer patients. Despite its clinical relevance, metastasis remains the most poorly elucidated aspect of carcinogenesis. The biological mechanisms leading to bone metastasis establishment have been referred as "vicious circle," a complex network between cancer cells and the bone microenvironment. This review is aimed to underline the new molecular targets in bone metastases management other than bisphosphonates. Different pathways or molecules such as RANK/RANKL/OPG, cathepsin K, endothelin-1, Wnt/DKK1, Src have recently emerged as potential targets and nowadays preclinical and clinical trials are underway. The results from those in the advanced clinical phases are encouraging and underlined the need to design large randomised clinical trials to validate these results in the next future. Targeting the bone by preventing skeletal related events (SREs) and bone metastases has major clinical impact in improving survival in bone metastatic patients and in preventing disease relapse in adjuvant setting.