Characterization of monomeric and homodimeric forms of osteoclastogenesis inhibitory factor

Osteoporosis induced by cellular senescence: A mathematical model

Osteoporosis is a disease characterized by loss of bone mass, where bones become fragile and more likely to fracture. Bone density begins to decrease at age 50, and a state of osteoporosis is defined by loss of more than 25%. Cellular senescence is a permanent arrest of normal cell cycle, while maintaining cell viability. The number of senescent cells increase with age. Since osteoporosis is an aging disease, it is natural to consider the question to what extend senescent cells induce bone density loss and osteoporosis. In this paper we use a mathematical model to address this question. We determine the percent of bone loss for men and women during age 50 to 100 years, and the results depend on the rate η of net formation of senescent cell, with η = 1 being the average rate. In the case η = 1, the model simulations are in agreement with empirical data. We also consider senolytic drugs, like fisetin and quercetin, that selectively eliminate senescent cells, and assess their efficacy in terms of reducing bone loss. For example, at η = 1, with estrogen hormonal therapy and early treatment with fisetin, bone density loss for women by age 75 is 23.4% (below osteoporosis), while with no treatment with fisetin it is 25.8% (osteoporosis); without even a treatment with estrogen hormonal therapy, bone loss of 25.3% occurs already at age 65.

Breast Cancer Exosomal microRNAs Facilitate Pre-Metastatic Niche Formation in the Bone: A Mathematical Model

Pre-metastatic niche is a location where cancer cells, separating from a primary tumor, find "fertile soil" for growth and proliferation, ensuring successful metastasis. Exosomal miRNAs of breast cancer are known to enter the bone and degrade it, which facilitates cancer cells invasion into the bone interior and ensures its successful colonization. In this paper, we use a mathematical model to first describe, in health, the continuous remodeling of the bone by bone-forming osteoblasts, bone-resorbing osteoclasts and the RANKL-OPG-RANK signaling system, which keeps the balance between bone formation and bone resorption. We next demonstrate how breast cancer exosomal miRNAs disrupt this balance, either by increasing or by decreasing the ratio of osteoclasts/osteoblasts, which results in abnormal high bone resorption or abnormal high bone forming, respectively, and in bone weakening in both cases. Finally we consider the case of abnormally high resorption and evaluate the effect of drugs, which may increase bone density to normal level, thus protecting the bone from invasion by cancer cells.

Rank-Rankl-Opg Axis in Multiple Sclerosis: The Contribution of Placenta

Women with multiple sclerosis (MS) can safely become pregnant and give birth, with no side effects or impediments. Pregnancy is generally accepted as a period of well-being in which relapses have a softer evolution, particularly in the third trimester. Herein, we hypothesized that the placenta, via its “secretome”, could contribute to the recognized beneficial effects of pregnancy on MS activity. We focused on a well-known receptor/ligand/decoy receptor system, such as the one composed by the receptor activator of nuclear factor-kB (RANK), its ligand (RANKL), and the decoy receptor osteoprotegerin (OPG), which have never been investigated in an integrated way in MS, pregnancy, and placenta. We reported that pregnancy at the term of gestation influences the balance between circulating RANKL and its endogenous inhibitor OPG in MS women. We demonstrated that the placenta at term is an invaluable source of homodimeric OPG. By functional studies on astrocytes, we showed that placental OPG suppresses the mRNA expression of the CCL20, a chemokine responsible for Th17 cell recruitment. We propose placental OPG as a crucial molecule for the recognized beneficial effect of late pregnancy on MS and its potential utility for the development of new and more effective therapeutic approaches.

The Roadmap of RANKL/RANK Pathway in Cancer

The receptor activator of the nuclear factor-κB ligand (RANKL)/RANK signaling pathway was identified in the late 1990s and is the key mediator of bone remodeling. Targeting RANKL with the antibody denosumab is part of the standard of care for bone loss diseases, including bone metastases (BM). Over the last decade, evidence has implicated RANKL/RANK pathway in hormone and HER2-driven breast carcinogenesis and in the acquisition of molecular and phenotypic traits associated with breast cancer (BCa) aggressiveness and poor prognosis. This marked a new era in the research of the therapeutic use of RANKL inhibition in BCa. RANKL/RANK pathway is also an important immune mediator, with anti-RANKL therapy recently linked to improved response to immunotherapy in melanoma, non-small cell lung cancer (NSCLC), and renal cell carcinoma (RCC). This review summarizes and discusses the pre-clinical and clinical evidence of the relevance of the RANKL/RANK pathway in cancer biology and therapeutics, focusing on bone metastatic disease, BCa onset and progression, and immune modulation.

Murine osteoclasts secrete serine protease HtrA1 capable of degrading osteoprotegerin in the bone microenvironment

Osteoclasts are multinucleated cells responsible for bone resorption. The differentiation of osteoclasts from bone marrow macrophages (BMMs) is induced by receptor activator of NF-κB ligand (RANKL). Osteoprotegerin (OPG), a decoy receptor of RANKL, inhibits osteoclastogenesis by blocking RANKL signaling. Here we investigated the degradation of OPG in vitro. Osteoclasts, but not BMMs, secreted OPG-degrading enzymes. Using mass spectrometry and RNA-sequencing analysis, we identified high-temperature requirement A serine peptidase 1 (HtrA1) as an OPG-degrading enzyme. HtrA1 did not degrade OPG pre-reduced by dithiothreitol, suggesting that HtrA1 recognizes the three-dimensional structure of OPG. HtrA1 initially cleaved the amide bond between leucine 90 and glutamine 91 of OPG, then degraded OPG into small fragments. Inhibitory activity of OPG on RANKL-induced osteoclastogenesis was suppressed by adding HtrA1 in RAW 264.7 cell cultures. These results suggest that osteoclasts potentially prepare a microenvironment suitable for osteoclastogenesis. HtrA1 may be a novel drug target for osteoporosis.

Cumambrin A prevents OVX-induced osteoporosis via the inhibition of osteoclastogenesis, bone resorption, and RANKL signaling pathways

Being the principal cells responsible for bone resorption and pathologic bone loss, osteoclasts have become the main target for antiresorptive treatment. Cumambrin A is a natural compound isolated from Chrysanthemum indicum L. and belongs to a member of the sesquiterpene lactone family. To date, the therapeutic effect of cumambrin A on osteoporosis and its mechanisms of action are not known. In this study, we found that cumambrin A can significantly inhibit osteoclast formation and bone resorption through the suppression of receptor activator of NF-κB ligand (RANKL)-induced NF-κB and nuclear factor of activated T-cell activity and ERK phosphorylation. Furthermore, cumambrin A inhibits the expression of osteoclast marker genes including cathepsin K, calcitonin receptor, and V-ATPase d2. Using an in vivo ovariectomized mouse model, we showed that cumambrin A protects against estrogen withdrawal-induced bone loss. Collectively, our results reveal that cumambrin A can suppress osteoclast formation, bone resorption, and RANKL-induced signaling pathways, suggesting that cumambrin A is a potential therapeutic agent for the treatment of osteoporosis.-Zhou, L., Liu, Q., Hong, G., Song, F., Zhao, J., Yuan, J., Xu, J., Tan, R. X., Tickner, J., Gu, Q., Xu, J. Cumambrin A prevents OVX-induced osteoporosis via the inhibition of osteoclastogenesis, bone resorption, and RANKL signaling pathways.

Nitidine chloride prevents OVX-induced bone loss via suppressing NFATc1-mediated osteoclast differentiation

Nitidine chloride (NC), a bioactive alkaloid isolated from Zanthoxylum nitidum, has been used as a herbal ingredient in toothpaste that prevents cavities for decades. It also displays potential antitumor and anti-inflammation properties. However, its anticatabolic effect on bone is not known. We investigated the effect of NC on osteoclastogenesis, bone resorption and RANKL-induced NF-κB and NFATc1 signalling. In mouse-derived bone marrow monocytes (BMMs), NC suppressed RANKL-induced multinucleated tartrate-resistant acid phosphatase (TRAP)-positive osteoclast formation and bone resorption in a dose dependent manner. NC attenuated the expression of osteoclast marker genes including cathepsin K, D2, calcitonin receptor, NFATc1, and TRAP. Further, NC inhibited RANKL-activated NF-κB and NFATc1 signalling pathways. In vivo study revealed that NC abrogated oestrogen deficiency-induced bone loss in ovariectomized mice. Histological analysis showed that the number of osteoclasts was significantly lower in NC-treated groups. Collectively, our data demonstrate that NC suppressed osteoclastogenesis and prevented OVX-induced bone loss by inhibiting RANKL-induced NF-κB and NFATc1 signalling pathways. NC may be a natural and novel treatment for osteoclast-related bone lytic diseases.

Bench to bedside: elucidation of the OPG-RANK-RANKL pathway and the development of denosumab

Bone is a complex tissue that provides mechanical support for muscles and joints, protection for vital organs, a mineral reservoir that is essential for calcium homeostasis, and the environment and niches required for haematopoiesis. The regulation of bone mass in mammals is governed by a complex interplay between bone-forming cells termed osteoblasts and bone-resorbing cells termed osteoclasts, and is guided physiologically by a diverse set of hormones, cytokines and growth factors. The balance between these processes changes over time, causing an elevated risk of fractures with age. Osteoclasts may also be activated in the cancer setting, leading to bone pain, fracture, spinal cord compression and other significant morbidities. This Review chronicles the events that led to an increased understanding of bone resorption, the elucidation of the signalling pathway mediated by osteoprotegerin, receptor activator of NF-κB (RANK) and RANK ligand (RANKL) and its role in osteoclast biology, as well as the evolution of recombinant RANKL antagonists, which culminated in the development of the therapeutic RANKL-targeted antibody denosumab.

The role of osteoprotegerin (OPG) receptor activator for nuclear factor kappaB ligand (RANKL) in cardiovascular pathology - a review

Initially described as key regulators in metabolic bone disease osteoprotegerin (OPG), receptor activator of nuclear factor kappa B (RANK) and RANK ligand (RANKL) have also been discriminated as regulators in immunologic function. Cardiovascular diseases (CVD) develop over many years in life and are often triggered by inflammatory processes within the vessel wall that lead to vascular remodeling. Recently some study groups have described OPG as a prognostic parameter for mortality and morbidity in cardiovascular patients.

PEGylation of osteoprotegerin/osteoclastogenesis inhibitory factor (OPG/OCIF) results in decreased uptake into rats and human liver

OBJECTIVES

Our aim was to investigate the effect of PEGylation on the uptake of osteoprotegerin/osteoclastogenesis inhibitory factor (OPG/OCIF) into rat liver, kidney and spleen, and human liver.

METHODS

Copolymer of polyethyleneglycol allylmethylether and maleamic acid sodium salt with OCIF (poly(PEG)-OCIF) (0.5 mg/kg) was administered to rats and the concentrations of poly(PEG)-OCIF in the liver, kidney and spleen at 15 min after administration were measured by ELISA. For human liver uptake, the liver perfusion of OCIF and (3)H-labelled poly(PEG)-OCIF was conducted using fresh human liver block.

KEY FINDINGS

The tissue uptake of poly(PEG)-OCIF in rats was significantly lower compared with that of OCIF. In fresh human liver perfusion, (3)H-poly(PEG)-OCIF was rarely taken up into the liver. On the other hand, more than 50% of the perfused OCIF was taken up.

CONCLUSIONS

PEGylation of OCIF using poly(PEG) dramatically suppressed the uptake of OCIF into human liver as well as into rat liver and could be a promising approach for improving the pharmacokinetic and pharmacological effects of OCIF in the clinical setting.

Structural and functional insights of RANKL-RANK interaction and signaling

Bone remodeling involves bone resorption by osteoclasts and synthesis by osteoblasts and is tightly regulated by the receptor activator of the NF-kappaB ligand (RANKL)/receptor activator of the NF-kappaB (RANK)/osteoprotegerin molecular triad. RANKL, a member of the TNF superfamily, induces osteoclast differentiation, activation and survival upon interaction with its receptor RANK. The decoy receptor osteoprotegerin inhibits osteoclast formation by binding to RANKL. Imbalance in this molecular triad can result in diseases, including osteoporosis and rheumatoid arthritis. In this study, we report the crystal structures of unliganded RANK and its complex with RANKL and elucidation of critical residues for the function of the receptor pair. RANK represents the longest TNFR with four full cysteine-rich domains (CRDs) in which the CRD4 is stabilized by a sodium ion and a rigid linkage with CRD3. On association, RANK moves via a hinge region between the CRD2 and CRD3 to make close contact with RANKL; a significant structural change previously unseen in the engagement of TNFR superfamily 1A with its ligand. The high-affinity interaction between RANK and RANKL, maintained by continuous contact between the pair rather than the patched interaction commonly observed, is necessary for the function because a slightly reduced affinity induced by mutation produces significant disruption of osteoclast formation. The structures of RANK and RANKL-RANK complex and the biological data presented in the paper are essential for not only our understanding of the specific nature of the signaling mechanism and of disease-related mutations found in patients but also structure based drug design.

Plasma osteoprotegerin levels predict cardiovascular and all-cause mortality and deterioration of kidney function in type 1 diabetic patients with nephropathy

AIMS/HYPOTHESIS

The bone-related peptide osteoprotegerin is produced by vascular cells and is involved in the process of vascular calcification. The aim of this study was to investigate the predictive value of plasma levels of osteoprotegerin in relation to mortality, cardiovascular events and deterioration in kidney function in patients with type 1 diabetes.

METHODS

This prospective observational follow-up study included 397 type 1 diabetic patients with overt diabetic nephropathy (243 men; age [mean+/-SD] 42.1 +/- 10.6 years, duration of diabetes 28.3 +/- 9.9 years, GFR 67 +/- 28 ml min(-1) 1.73 m(2)) and a group of 176 patients with longstanding type 1 diabetes and persistent normoalbuminuria (105 men; age 42.6 +/- 9.7 years, duration of diabetes 27.6 +/- 8.3 years).

RESULTS

The median (range) follow-up period was 11.3 (0.0-12.9) years. Among patients with diabetic nephropathy, individuals with high osteoprotegerin levels (fourth quartile) had significantly higher all-cause mortality than patients with low levels (first quartile) (covariate-adjusted hazard ratio [HR] 3.00 [1.24-7.27]). High osteoprotegerin levels also predicted cardiovascular mortality (covariate-adjusted HR 4.88 [1.57-15.14]). Furthermore, patients with high osteoprotegerin levels had significantly higher risk of progression to end-stage renal disease than patients with low levels (covariate-adjusted HR 4.32 [1.45-12.87]). In addition, patients with high levels of plasma osteoprotegerin had an elevated rate of decline in GFR.

CONCLUSIONS/INTERPRETATION

High levels of osteoprotegerin predict all-cause and cardiovascular mortality in patients with diabetic nephropathy. Furthermore, high levels of osteoprotegerin predict deterioration of kidney function towards end-stage renal disease.

[Osteoprotegerin: regulator, protector and marker]

Experimental and clinical trials in the field of bone biology helped to clarify the role of receptors, which belong to the tumor necrosis factor family, such as osteoprotegerin and receptor activator of nuclear factor kappaB (RANK), in the regulation of bone remodeling. The ligand of the receptor activator of nuclear factor kappaB (RANKL) is a stimulator of bone resorption, while osteoprotegerin is the soluble "decoy" receptor to RANKL, protecting thereby bone from resorption. Pathological states of bone remodeling (like osteoporosis) are associated with imbalance in the activity of osteoprotegerin and the receptor activator of nuclear factor kappaB. Recent studies, however, also indicate that the osteoprotegerin/RANKL/RANK system has important roles in the regulation of the immune and vascular system as well. In this review we summarize the function and regulation of osteoprotegerin, its role in pathological states--primarily in cardiovascular diseases--and its relevance as a marker of cardiovascular risk. Finally, we present our prospective trial performed among the chronic dialyzed patients, where we examined the association between the cardiovascular mortality, osteoprotegerin levels and the arterial stiffness.

Receptor activator of nuclear factor kappaB ligand and osteoprotegerin regulation of bone remodeling in health and disease

Osteoclasts and osteoblasts dictate skeletal mass, structure, and strength via their respective roles in resorbing and forming bone. Bone remodeling is a spatially coordinated lifelong process whereby old bone is removed by osteoclasts and replaced by bone-forming osteoblasts. The refilling of resorption cavities is incomplete in many pathological states, which leads to a net loss of bone mass with each remodeling cycle. Postmenopausal osteoporosis and other conditions are associated with an increased rate of bone remodeling, which leads to accelerated bone loss and increased risk of fracture. Bone resorption is dependent on a cytokine known as RANKL (receptor activator of nuclear factor kappaB ligand), a TNF family member that is essential for osteoclast formation, activity, and survival in normal and pathological states of bone remodeling. The catabolic effects of RANKL are prevented by osteoprotegerin (OPG), a TNF receptor family member that binds RANKL and thereby prevents activation of its single cognate receptor called RANK. Osteoclast activity is likely to depend, at least in part, on the relative balance of RANKL and OPG. Studies in numerous animal models of bone disease show that RANKL inhibition leads to marked suppression of bone resorption and increases in cortical and cancellous bone volume, density, and strength. RANKL inhibitors also prevent focal bone loss that occurs in animal models of rheumatoid arthritis and bone metastasis. Clinical trials are exploring the effects of denosumab, a fully human anti-RANKL antibody, on bone loss in patients with osteoporosis, bone metastasis, myeloma, and rheumatoid arthritis.

Variations along the 24-hour cycle of circulating osteoprotegerin and soluble RANKL: a rhythmometric analysis

The variability of serum osteoprotegerin (OPG) and soluble RANKL (sRANKL) along the 24-h cycle was assessed in 20 healthy women. No rhythmic variations of serum OPG, sRANKL or sRANKL/OPG ratio were detected as a group phenomenon. Timing of sampling is unlikely to influence the results of measurements of circulating OPG and sRANKL.

INTRODUCTION

Physiological bone turnover shows diurnal variations. The aim of the study was to assess variability of OPG and sRANKL serum levels along the 24-h cycle.

METHODS

Blood was collected from 20 healthy women (median age 31 years, range 25-65 years) at 4-h intervals between 08:00 and 24:00 and at 2-h intervals between 24:00 and 08:00. Serum albumin, cortisol, osteocalcin (OC), C-terminal telopeptide of type I collagen (CTX), OPG and total sRANKL were measured. Temporal variations were assessed by the COSINOR model.

RESULTS

Circadian rhythms of cortisol and albumin documented a normal synchronization within the circadian structure. Serum OC and CTX showed rhythmic variations, peaking at night-time. Rhythmic variations of serum OPG, sRANKL and sRANKL/OPG ratio were not detected as a group phenomenon. On an individual basis, rhythmic changes were detected in ten patients for OPG and eight patients for sRANKL, with very small amplitudes and heterogeneous acrophases.

CONCLUSIONS

The absence of consistent rhythmic variations of circulating OPG and sRANKL levels may reflect the absence of rhythmic variations of their expression in the bone microenvironment. Were this the case, the nocturnal rise of bone resorption should be accounted for by different, not RANKL/OPG-mediated factors. Since circulating OPG and sRANKL may derive from sources other than bone, rhythmicity could be masked by non-rhythmic or non-synchronized rhythmic expression in these sources. Timing of sampling is unlikely to influence the results of measurements of circulating OPG and sRANKL.

Investigating the interaction between osteoprotegerin and receptor activator of NF-kappaB or tumor necrosis factor-related apoptosis-inducing ligand: evidence for a pivotal role for osteoprotegerin in regulating two distinct pathways

Osteoprotegerin (OPG) binds the ligand for receptor activator of nuclear factor kappaB (RANKL) to prevent association with its receptor RANK and inhibit osteoclast-mediated bone resorption. OPG has been reported, recently, to inhibit tumor necrosis factor-related apoptosis-induced ligand (TRAIL)-induced tumor cell apoptosis. This raises the possibility that OPG may play a unique role in regulating these two signaling pathways. However, there are little data on the interactions between OPG, RANKL, and TRAIL, and the relative affinity of OPG for these two ligands is unknown. In the present study we examined the ability of OPG to bind native human TRAIL and RANKL under physiological conditions. Native TRAIL was expressed in Escherichia coli, purified to homogeneity, and shown to induce human myeloma cell apoptosis. OPG inhibited native TRAIL from binding the TRAILR1 at 37 degrees C in vitro. Similarly, OPG prevented RANKL from binding to RANK. TRAIL also prevented OPG-mediated inhibition of RANKL from binding RANK. The affinity of OPG for native TRAIL and RANKL at 37 degrees C was determined by plasmon surface resonance analysis. OPG had a binding affinity for TRAIL of 45 nM, whereas the affinity of OPG for RANKL was 23 nM. These data suggest that OPG can bind both RANKL and TRAIL and that the affinity of OPG for these two ligands is of a similar order of magnitude. Furthermore, OPG prevented TRAIL-mediated reductions in cell viability, whereas TRAIL inhibited OPG-mediated inhibition of osteoclastogenesis in vitro. This highlights the pivotal role of OPG in regulating the biology of both RANKL and TRAIL.

Mechanisms of bone loss in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune inflammatory disease in which destruction of bone in the joints causes major morbidity. Recent research has shed light on the cell and molecular mechanisms that lead to this osteolysis, all due directly or indirectly to the chronic inflammation. The aspects of this research covered in this review include the alteration of cell proliferation and survival that results in growth of the RA synovium. This process depends upon an increase in angiogenesis and local blood flow, which is also a feature of increased bone turnover. In addition, the inflammatory environment increases expression of chemokines, which are involved in the recruitment of monocytic osteoclast precursors. Chronic inflammation also promotes an overall catabolic state, with increased osteoclast differentiation and resorptive activity, driven by disregulation of receptor activator of NF-kappaB ligand (RANKL) and the synergistic activity of inflammatory cytokines such as tumor necrosis factor-alpha and interleukin-1. Osteoclast survival is increased in this environment, but osteoblast differentiation and survival are decreased, with a consequent reduction in bone formation and a net loss of bone. Recognition of these processes and the factors involved will enable more effective and targeted treatments for RA.

Juvenile Paget's disease: the second reported, oldest patient is homozygous for the TNFRSF11B "Balkan" mutation (966_969delTGACinsCTT), which elevates circulating immunoreactive osteoprotegerin levels

The oldest person (60 yr) with juvenile Paget's disease is homozygous for the TNFRSF11B mutation 966_969delTGACinsCTT. Elevated circulating levels of immunoreactive OPG and soluble RANKL accompany this genetic defect that truncates the OPG monomer, preventing formation of OPG homodimers.

INTRODUCTION

Juvenile Paget's disease (JPD), a rare autosomal recessive disorder, features skeletal pain, fracture, and deformity from extremely rapid bone turnover. Deafness and sometimes retinopathy also occur. Most patients have diminished osteoprotegerin (OPG) inhibition of osteoclastogenesis caused by homozygous loss-of-function defects in TNFRSF11B, the gene that encodes OPG. Circulating immunoreactive OPG (iOPG) is undetectable with complete deletion of TNFRSF11B but normal with a 3-bp in-frame deletion.

MATERIALS AND METHODS

We summarize the clinical course of a 60-yr-old Greek man who is the second reported, oldest JPD patient, including his response to two decades of bisphosphonate therapy. Mutation analysis involved sequencing all exons and adjacent mRNA splice sites of TNFRSF11B. Over the past 4 yr, we used ELISAs to quantitate his serum iOPG and soluble RANKL (sRANKL) levels.

RESULTS

Our patient suffered progressive deafness and became legally blind, although elevated markers of bone turnover have been normal for 6 yr. He carries the same homozygous mutation in TNFRSF11B (966_969delTGACinsCTT) reported in a seemingly unrelated Greek boy and Croatian man who also have relatively mild JPD. This frame-shift deletes 79 carboxyterminal amino acids from the OPG monomer, including a cysteine residue necessary for homodimerization. Nevertheless, serum iOPG and sRANKL levels are persistently elevated.

CONCLUSIONS

Homozygosity for the TNFRSF11B "Balkan" mutation (966_969delTGACinsCTT) causes JPD in the second reported, oldest patient. Elevated circulating iOPG and sRANKL levels complement evidence that this deletion/insertion omits a cysteine residue at the carboxyterminus needed for OPG homodimerization.

RANKL/OPG/TRAIL plasma levels and bone mass loss evaluation in antiretroviral naive HIV-1-positive men

Osteopenia and osteoporosis are common in HIV-1-infected individuals and represent a challenge in clinical and therapeutic management. This report investigated osteopenia/osteoporosis in a group of 31 antiretroviral naive HIV-1-positive men and the role of specific molecules belonging to TNF and the TNF-receptor family in HIV-1-related bone mass loss. Osteoprotegerin (OPG), the receptor activator of NF-kappab-ligand (RANKL), and the TNF-related apoptosis-inducing ligand (TRAIL) were significantly increased in the plasma of antiretroviral naive HIV-1-positive patients compared to a control group of healthy blood donors. In addition, TRAIL and RANKL plasma concentrations were positively correlated to HIV-1-RNA viral load. Measurement of bone mineral density in 20 out of 31 HIV-1-positive subjects disclosed osteopenia/osteoporosis in 40% of these patients. The antiretroviral naive HIV-1-positive subjects with low bone mineral density had a decreased plasma OPG/RANKL ratio and a plasma RANKL concentration >500 pg/ml. Together, these data indicate that plasma concentrations of specific factors involved in bone homeostasis were increased during HIV-1 infection and that RANKL and OPG/RANKL ratio deregulation may be involved in osteopenia/osteoporosis occurring in antiretroviral naive HIV-1 individuals.

Functional Dissection of Osteoprotegerin and Its Interaction with Receptor Activator of NF-κB Ligand

The receptor activator of NF-kappaB (RANK) belongs to the neuregulin/tumor necrosis factor (TNF) receptor superfamily and is activated by RANK ligand (RANK-L), a homotrimeric, TNF-like cytokine. RANK is present on the surface of osteoclast cell precursors, where its interaction with RANK-L induces their terminal differentiation into osteoclasts, thus increasing bone breakdown. The secreted, soluble receptor osteoprotegerin (OPG) interrupts this activation by binding directly to RANK-L. Therefore, osteoclast maturation (and bone homeostasis) is regulated in vivo by OPG levels of expression. We have studied the assembly state and affinity of OPG for RANK-L by sedimentation analyses and surface plasmon resonance (Biacore). Full-length, homodimeric OPG binds to RANK-L with a KD of 10 nM. OPG is also a member of the TNF receptor superfamily and contains four disulfide-rich ligand-binding domains, yet lacks a transmembrane region separating the ligand-binding region from the two death domains, as observed for other receptor family members. We showed that dimerization of OPG results from noncovalent interactions mediated by the death domains and to a lesser extent by a C-terminal heparin-binding region. In contrast, a C-terminal intermolecular disulfide bond does not contribute to the formation or stability of OPG dimers. A truncate of osteoprotegerin, containing the ligand-binding domains but lacking the dimerization domains, bound RANK-L with a KD of approximately 3 microM, indicating that monomer oligomerization for the OPG provides an increase of 3 orders of magnitude in the affinity for RANK-L. Therefore, OPG dimer formation is required for the mechanism of inhibition of the RANK-L/RANK receptor interaction.

An intermediate form of juvenile Paget's disease caused by a truncating TNFRSF11B mutation

Juvenile Paget's disease (JPD) is a rare condition with an autosomal recessive mode of inheritance. Typically presenting in infancy or early childhood, the disorder is characterized by a generalized widening of the long bones and thickening of the skull combined with sustained elevation of serum alkaline phosphatase levels. The extremely rapid bone turnover results in osteopenia, fractures, and progressive skeletal deformity. In 2002, mutations in TNFRSF11B, the gene encoding osteoprotegerin, were described as underlying JPD. We evaluated a patient with JPD at the clinical, biochemical, radiological, and molecular level. Mutation analysis of TNFRSF11B revealed a homozygous insertion/deletion in exon 5, predicted to result in truncation of the protein at amino acid 325. The residual activity of the mutated protein product was investigated by Western blotting and ELISA upon transient overexpression. Absence of the C-terminal domain abolished homodimerization and was shown to lead to a decreased capacity of the mutant protein to bind its ligand RANKL. We conclude that truncation of the C-terminal part of osteoprotegerin negatively affects functional activity. As a consequence, osteoclast formation and function are up-regulated, causing the increased bone turnover seen in this patient.

Osteoprotegerin production by human intestinal epithelial cells: a potential regulator of mucosal immune responses

Receptor activator of NF-κB (RANK) and its ligand (RANKL) are important members of the TNF receptor (TNFR) and TNF superfamilies, respectively. RANK is expressed on osteoclasts, T-lymphocytes, and dendritic cells, and its ligation with RANKL leads to cellular activation. However, another member of the TNFR family, osteoprotegerin (OPG), acts as a decoy receptor, binding to RANKL and preventing its interaction with RANK. Furthermore, OPG also binds TNF-related apoptosis-inducing ligand (TRAIL), an important regulator of cell survival. OPG is therefore an important regulator of bone metabolism and immune responses. Although intestinal epithelial cells (IEC) express some members of the TNF/TNFR superfamilies, the roles of OPG and RANKL in the intestinal mucosa has not been investigated. Here, we report that various human IEC lines constitutively express OPG mRNA and protein as well as mRNA for RANKL. Furthermore, human colonic epithelium constitutively expressed OPG, and this expression was increased in inflamed tissue. All of the IEC lines tested released OPG into the culture supernatant under standard culture conditions. Whereas TNF-α increased OPG protein secretion by HT29 cells, the cytokines IL-1β and IFN-γ had little, if any, effect. Furthermore, the culture supernatant from untreated HT29 cells abrogated TRAIL-induced inhibition of Jurkat T-cell proliferation and inhibited osteoclast activity in an in vitro model of bone resorption. Taken together, our data indicate that OPG is constitutively produced by IEC, could be upregulated by TNF-α, and is biologically active. Thus IEC-derived OPG may represent an important mucosal immunoregulatory factor and may be involved in bone physiology.

Osteoprotegerin (OPG) acts as an endogenous decoy receptor in tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis of fibroblast-like synovial cells

We examined the role of osteoprotegerin (OPG) on tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in rheumatoid fibroblast-like synovial cells (FLS). OPG protein concentrations in synovial fluid from patients with rheumatoid arthritis (RA) correlated with those of interleukin (IL)-1beta or IL-6. A similar correlation was present between IL-1beta and IL-6 concentrations. Rheumatoid FLS in vitro expressed both death domain-containing receptors [death receptor 4 (DR4) and DR5] and decoy receptors [decoy receptor 1 (DcR1) and DcR2]. DR4 expression on FLS was weak compared with the expression of DR5, DcR1 and DcR2. Recombinant TRAIL (rTRAIL) rapidly induced apoptosis of FLS. DR5 as well as DR4 were functional with regard to TRAIL-mediated apoptosis induction in FLS; however, DR5 appeared be more efficient than DR4. In addition to soluble DR5 (sDR5) and sDR4, OPG administration significantly inhibited TRAIL-induced apoptogenic activity. OPG was identified in the culture supernatants of FLS, and its concentration increased significantly by the addition of IL-1beta in a time-dependent manner. Neither IL-6 nor tumour necrosis factor (TNF)-alpha increased the production of OPG from FLS. TRAIL-induced apoptogenic activity towards FLS was reduced when rTRAIL was added without exchanging the culture media, and this was particularly noticeable in the IL-1beta-stimulated FLS culture; however, the sensitivity of FLS to TRAIL-induced apoptosis itself was not changed by IL-1beta. Interestingly, neutralization of endogenous OPG by adding anti-OPG monoclonal antibody (MoAb) to FLS culture restored TRAIL-mediated apoptosis. Our data demonstrate that OPG is an endogenous decoy receptor for TRAIL-induced apoptosis of FLS. In addition, IL-1beta seems to promote the growth of rheumatoid synovial tissues through stimulation of OPG production, which interferes with TRAIL death signals in a competitive manner.

Osteoprotegerin and bone mineral metabolism in renal failure

PURPOSE OF REVIEW

This review focuses on the impact of a recent breakthrough in bone cell biology for understanding the pathophysiology of bone/vascular abnormalities associated with uraemia.

RECENT FINDINGS

Osteoprotegerin is a humoral osteoclastogenesis/osteoclast activation inhibitory factor, which belongs to the TNF-alpha receptor superfamily. Serum osteoprotegerin levels are elevated along with the deterioration of the glomerular filtration rate. The circulating osteoprotegerin molecules were likely to preserve the activity of osteoclastogenesis inhibition, and the levels in many dialysis patients were high enough to inhibit osteoclastogenesis. The higher serum osteoprotegerin levels were related to the development of vascular calcification.

SUMMARY

Osteoprotegerin is a potential uraemic toxin that increases the skeletal resistance to PTH. However, the roles of increased circulating osteoprotegerin on bone and/or vascular abnormalities in uraemia remain to be clarified.

Osteoprotegerin in human milk: a potential role in the regulation of bone metabolism and immune development

Osteoprotegerin (OPG) is a member of the tumor necrosis factor superfamily. It is a soluble "decoy" receptor for tumor necrosis factor-related apoptosis-inducing ligand and ligand of the receptor activator of NF-kappaB. As such, OPG inhibits osteoclast activity and regulates the immune system. Human milk is a complex biologic fluid that supplies nutritional and protective factors to the breast-fed infant. In the present study, human milk samples at various times postpartum were assessed for the presence of OPG. Using biochemical as well as immunologic and biologic techniques we showed that human milk contains OPG at a level that is 1000-fold higher than that found in normal human serum. We observed that human breast milk cells and the human mammary epithelial cell line MCF-7 express OPG, indicating that both cell types are possible sources of milk OPG in vivo. In vitro studies demonstrated that milk OPG is biologically active and suggested that it may contribute to the antiresorptive activity of milk on bone, as well as tumor necrosis factor-related apoptosis-inducing ligand-induced inhibition of T cell proliferation. OPG-like activity was also observed in bovine colostrum and milk. Furthermore, we were able to detect human OPG in the sera of rats gavaged with human milk. We discuss the relevance of our findings for the breast-fed infant and for the prevention of immune and bone disorders.

Native osteoprotegerin gene transfer inhibits the development of murine osteolytic bone disease induced by tumor xenografts

OBJECTIVE

Multiple myeloma is a plasma cell malignancy characterized by the development of osteolytic lesions leading to bone pain, pathologic fractures, and hypercalcemia. Osteoprotegerin (OPG) is a potent inhibitor of osteoclast differentiation and activation, but is limited as a therapeutic agent due to its short circulating half-life. In order to overcome these limitations, the therapeutic effects of native OPG gene transfer are examined.

MATERIALS AND METHODS

We used replication-incompetent lentiviral vectors to transfer the unmodified, native human OPG gene ex vivo into human ARH-77 cells injected into severe combined immunodeficient (SCID) mice, to determine gene transfer efficiency as well as the impact on disease progression in this in vivo model.

RESULTS

We can efficiently transfer and express either the LacZ marker gene or the native human OPG gene into human ARH-77 cells. Moreover, transfer of the OPG gene into ARH-77 cells reduces the development of osteolytic bony lesions when these cells are injected into SCID mice, compared to mice injected with either unmodified ARH-77 cells or ARH-77 cells transduced with the OPG gene in the antisense orientation. This therapeutic effect was manifested as a reduction in vertebral compression deformities and in the number and size of long-bone osteolytic lesions on skeletal radiographs, as well as a decrease in osteoclast surface on histologic analysis.

CONCLUSIONS

A lentiviral vector can efficiently transfer the native human OPG gene to myeloma cells ex vivo and inhibit myeloma-induced bone destruction, thereby suggesting a therapeutic potential for unmodified, native OPG gene transfer for osteoclast-dependent skeletal disorders.

Circulating osteoprotegerin in women during GnRH-agonist treatment and their relationships with mineral components and biomarkers of bone turnover

A novel cytokine termed osteoprotegerin (OPG) that is also called osteoclastogenesis-inhibitory factor, which inhibits osteoclast maturation and activity, was recently isolated. In order to determine the influence of estrogen deficiency on the levels of circulating OPG in women, we studied the changes in the levels of circulating OPG in 10 Japanese women ages 25-49 (mean +/- SD, 34.0 +/- 6.9) years with endometriosis receiving gonadotropin-releasing hormone agonists (GnRH-a) therapy. We further analyzed whether the levels of circulating OPG have relations with the levels of the biomarkers of bone turnover or those of circulating mineral components in these patients during GnRH-a treatment. The patients were treated with a monthly injection of 3.75 mg leuprolide acetate depot for 6 months. In all patients, the concentrations of serum estradiol decreased after 6 months of GnRH-a treatment. The bone mineral density of the lumber spines in these patients significantly (P < 0.01) decreased (percentage change: mean +/- SD, -5.4 +/- 2.1%), while circulating OPG levels significantly (P < 0.01) increased after 6 months of treatment. The values of circulating OPG had significant correlations with those of urinary pyridinoline (r = 0.59, P < 0.01), urinary deoxypylridinoline (Dpd) (r = 0.46, P < 0.05), and serum alkaline phosphatase (r = 0.66, P < 0.01) but not with those of serum carboxy-terminal propeptide of type I procollagen during GnRH-a treatment. The values of circulating OPG also correlated significantly with those of serum calcium (Ca) and phosphorus (P) (r = 0.65 and 0.72, P < 0.01). Further analyses revealed that the percentage change in the value of circulating OPG had a significant correlation with that of urinary Dpd (r = 0.84, P < 0.01). These results suggest that circulating OPG levels rise against the increase in osteoblastic bone resorption and circulating Ca levels in the case of estrogen deficiency, possibly as a compensatory mechanism serving to limit circulating Ca levels and bone density.

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.

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.

High levels of soluble syndecan-1 in myeloma-derived bone marrow: modulation of hepatocyte growth factor activity

Syndecan-1 is a heparan sulfate proteoglycan expressed on the surface of, and actively shed by, myeloma cells. Hepatocyte growth factor (HGF) is a cytokine produced by myeloma cells. Previous studies have demonstrated elevated levels of syndecan-1 and HGF in the serum of patients with myeloma, both of negative prognostic value for the disease. Here we show that the median concentrations of syndecan-1 (900 ng/mL) and HGF (6 ng/mL) in the marrow compartment of patients with myeloma are highly elevated compared with healthy controls and controls with other diseases. We show that syndecan-1 isolated from the marrow of patients with myeloma seems to exist in an intact form, with glucosaminoglycan chains. Because HGF is a heparan-sulfate binding cytokine, we examined whether it interacted with soluble syndecan-1. In supernatants from myeloma cells in culture as well as in pleural effusions from patients with myeloma, HGF existed in a complex with soluble syndecan-1. Washing myeloma cells with purified soluble syndecan-1 could effectively displace HGF from the cell surface, suggesting that soluble syndecan-1 can act as a carrier for HGF in vivo. Finally, using a sensitive HGF bioassay (interleukin-11 production from the osteosarcoma cell line Saos-2) and intact syndecan-1 isolated from the U-266 myeloma cell line, we found that the presence of high concentrations of syndecan-1 (more than 3 μg/mL) inhibited the HGF effect, whereas lower concentrations potentiated it. HGF is only one of several heparin-binding cytokines associated with myeloma. These data indicate that soluble syndecan-1 may participate in the pathology of myeloma by modulating cytokine activity within the bone marrow.

High levels of soluble syndecan-1 in myeloma-derived bone marrow: modulation of hepatocyte growth factor activity

Syndecan-1 is a heparan sulfate proteoglycan expressed on the surface of, and actively shed by, myeloma cells. Hepatocyte growth factor (HGF) is a cytokine produced by myeloma cells. Previous studies have demonstrated elevated levels of syndecan-1 and HGF in the serum of patients with myeloma, both of negative prognostic value for the disease. Here we show that the median concentrations of syndecan-1 (900 ng/mL) and HGF (6 ng/mL) in the marrow compartment of patients with myeloma are highly elevated compared with healthy controls and controls with other diseases. We show that syndecan-1 isolated from the marrow of patients with myeloma seems to exist in an intact form, with glucosaminoglycan chains. Because HGF is a heparan-sulfate binding cytokine, we examined whether it interacted with soluble syndecan-1. In supernatants from myeloma cells in culture as well as in pleural effusions from patients with myeloma, HGF existed in a complex with soluble syndecan-1. Washing myeloma cells with purified soluble syndecan-1 could effectively displace HGF from the cell surface, suggesting that soluble syndecan-1 can act as a carrier for HGF in vivo. Finally, using a sensitive HGF bioassay (interleukin-11 production from the osteosarcoma cell line Saos-2) and intact syndecan-1 isolated from the U-266 myeloma cell line, we found that the presence of high concentrations of syndecan-1 (more than 3 μg/mL) inhibited the HGF effect, whereas lower concentrations potentiated it. HGF is only one of several heparin-binding cytokines associated with myeloma. These data indicate that soluble syndecan-1 may participate in the pathology of myeloma by modulating cytokine activity within the bone marrow.

Osteoprotegerin produced by osteoblasts is an important regulator in osteoclast development and function

Osteoprotegerin (OPG), a soluble decoy receptor for receptor activator of nuclear factor-kappaB ligand (RANKL)/osteoclast differentiation factor, inhibits both differentiation and function of osteoclasts. We previously reported that OPG-deficient mice exhibited severe osteoporosis caused by enhanced osteoclastic bone resorption. In the present study, potential roles of OPG in osteoclast differentiation were examined using a mouse coculture system of calvarial osteoblasts and bone marrow cells prepared from OPG-deficient mice. In the absence of bone-resorbing factors, no osteoclasts were formed in cocultures of wild-type (+/+) or heterozygous (+/-) mouse-derived osteoblasts with bone marrow cells prepared from homozygous (-/-) mice. In contrast, homozygous (-/-) mouse-derived osteoblasts strongly supported osteoclast formation in the cocultures with homozygous (-/-) bone marrow cells, even in the absence of bone-resorbing factors. Addition of OPG to the cocultures with osteoblasts and bone marrow cells derived from homozygous (-/-) mice completely inhibited spontaneously occurring osteoclast formation. Adding 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] to these cocultures significantly enhanced osteoclast differentiation. In addition, bone-resorbing activity in organ cultures of fetal long bones derived from homozygous (-/-) mice was markedly increased, irrespective of the presence and absence of bone-resorbing factors, in comparison with that from wild-type (+/+) mice. Osteoblasts prepared from homozygous (-/-), heterozygous (+/-), and wild-type (+/+) mice constitutively expressed similar levels of RANKL messenger RNA, which were equally increased by the treatment with 1alpha,25(OH)2D3. When homozygous (-/-) mouse-derived osteoblasts and hemopoietic cells were cocultured, but direct contact between them was prevented, no osteoclasts were formed, even in the presence of 1alpha,25(OH)2D3 and macrophage colony-stimulating factor. These findings suggest that OPG produced by osteoblasts/stromal cells is a physiologically important regulator in osteoclast differentiation and function and that RANKL expressed by osteoblasts functions as a membrane-associated form.

The roles of osteoprotegerin and osteoprotegerin ligand in the paracrine regulation of bone resorption

Although multiple hormones and cytokines regulate various aspects of osteoclast formation, the final two effectors are osteoprotegerin ligand (OPG-L)/osteoclast differentiation factor (ODF), a recently cloned member of the tumor necrosis factor superfamily, and macrophage colony-stimulating factor. OPG-L/ODF is produced by osteoblast lineage cells and exerts its biological effects through binding to its receptor, osteoclast differentiation and activation receptor (ODAR)/receptor activator of NF-kappa B (RANK), on osteoclast lineage cells, in either a soluble or a membrane-bound form, the latter of which requires cell-to-cell contact. Binding results in rapid differentiation of osteoclast precursors in bone marrow to mature osteoclasts and, at higher concentrations, in increased functional activity and reduced apoptosis of mature osteoclasts. The biological activity of OPG-L/ODF is neutralized by binding to osteoprotegerin (OPG)/osteoclastogenesis inhibitory factor (OCIF), a member of the TNF-receptor superfamily that also is secreted by osteoblast lineage cells. The biological importance of this system is underscored by the induction in mice of severe osteoporosis by targeted ablation of OPG/OCIF and by the induction of osteopetrosis by targeted ablation of OPG-L/ODF or overexpression of OPG/OCIF. Thus, osteoclast formation may be determined principally by the relative ratio of OPG-L/ODF to OPG/OCIF in the bone marrow microenvironment, and alterations in this ratio may be a major cause of bone loss in many metabolic disorders, including estrogen deficiency and glucocorticoid excess. That changes in but two downstream cytokines mediate the effects of large numbers of upstream hormones and cytokines suggests a regulatory mechanism for osteoclastogenesis of great efficiency and elegance.

Osteoblasts/stromal cells stimulate osteoclast activation through expression of osteoclast differentiation factor/RANKL but not macrophage colony-stimulating factor: receptor activator of NF-kappa B ligand

We previously reported that osteoblasts/stromal cells are essentially involved in the activation as well as differentiation of osteoclasts through a mechanism involving cell-to-cell contact between osteoblasts/stromal cells and osteoclast precursors/osteoclasts. Osteoclast differentiation factor (ODF, also called RANKL/OPGL/TRANCE) and macrophage colony-stimulating factor (M-CSF, also called CSF-1) are two essential factors produced by osteoblasts/stromal cells for osteoclastogenesis. In other words, osteoblasts/stromal cells were not necessary to generate osteoclasts from spleen cells in the presence of both ODF/RANKL and M-CSF. In the present study, we examined the precise roles of ODF/RANKL and M-CSF in the activation of osteoclasts induced by calvarial osteoblasts. Osteoclasts were formed in mouse bone marrow cultures on collagen gel-coated dishes in response to a soluble form of ODF/RANKL (sODF/sRANKL) and M-CSF, and recovered by collagenase digestion. When recovered osteoclasts were further cultured on plastic dishes, most of the osteoclasts spontaneously died within 24 h. Osteoclasts cultured for 24 h on dentine slices could not form resorption pits. Addition of sODF/sRANKL to the recovered osteoclasts markedly enhanced their survival and pit-forming activity. M-CSF similarly stimulated the survival of osteoclasts, but did not induce their pit-forming activity. When primary mouse osteoblasts were added to the recovered osteoclasts, resorption pits were formed on dentine slices. Bone-resorbing factors such as 1alpha,25-dihydroxyvitamin D3, parathyroid hormone, or prostaglandin E2 enhanced pit-forming activity of osteoclasts only in the presence of osteoblasts. M-CSF-deficient osteoblasts prepared from op/op mice similarly enhanced pit-forming activity of osteoclasts. The pit-forming activity of osteoclasts induced by sODF/sRANKL or osteoblasts was completely inhibited by simultaneous addition of osteoprotegerin/osteoclastogenesis inhibitory factor, a decoy receptor of ODF/RANKL. Primary osteoblasts constitutively expressed ODF/RANKL mRNA, and its level was upregulated by treatment with 1alpha,25-dihydroxyvitamin D3, parathyroid hormone, and prostaglandin E2. These results, obtained by using an assay system that unequivocally assesses osteoclast activation, suggest that ODF/RANKL but not M-CSF mediates osteoblast-induced pit-forming activity of osteoclasts, and that bone-resorbing factors stimulate osteoclast activation through upregulation of ODF/RANKL by osteoblasts/stromal cells.

Parathyroid hormone stimulates TRANCE and inhibits osteoprotegerin messenger ribonucleic acid expression in murine bone marrow cultures: correlation with osteoclast-like cell formation

We studied the effects of PTH on the expression of tumor necrosis factor-related activation-induced cytokine (TRANCE), osteoprotegerin (OPG), and receptor activator of NF kappaB (RANK) messenger RNA (mRNA) in cultured murine bone marrow, calvaria, and osteoblasts. TRANCE, OPG, and RANK are recently identified regulators of osteoclast formation. Bone marrow cells were cultured with or without PTH(1-34) for 6 days. TRANCE, OPG, and RANK mRNA were measured by RT-PCR. In 6-day cultures, PTH stimulated the number of OCL/well in a dose-dependent manner. A time course showed significant (P < 0.01) increases in OCL/well after 24 h of PTH (100 ng/ml). TRANCE mRNA expression, like OCL formation, increased dose dependently and was maximal, with 10-100 ng/ml PTH. In contrast, OPG mRNA expression was decreased by 0.1 ng/ml PTH (40%) and completely abolished by 1 ng/ml. TRANCE mRNA expression was rapidly stimulated by PTH (maximal response at 1 h, 8.1-fold over control). Expression declined by 40% at 24 h but was still much greater than control at 6 days (4.6-fold) in a time-course study. PTH caused a transient stimulation of OPG mRNA at 1 h (2-fold), which returned to basal levels by 2 h. After 6 h, PTH completely inhibited OPG mRNA. There were only minor effects of PTH on RANK mRNA expression. PTH had less potent effects on TRANCE and OPG mRNA expression in calvaria organ cultures and osteoblasts. In mouse calvaria cultures, TRANCE expression was detectable in controls and was increased 2.9-fold by PTH at 24 h. PTH treatment of calvaria decreased OPG expression by 30% at 6 h. MC3T3 E-1 osteoblastic cells expressed minimal levels of TRANCE mRNA either before or after PTH treatment. OPG mRNA was present in MC3T3 E-1 cells, but levels were not modulated by PTH. In primary osteoblastic cells, PTH stimulated TRANCE mRNA expression 4-fold at 2 h and inhibited OPG mRNA expression by 46%. These results demonstrate a tight correlation between the ability of PTH to stimulate OCL formation in marrow culture and expression of TRANCE (r = 0.87, P < or = 0.05) and OPG mRNA (r = -0.88, P < or = 0.05). Reciprocal regulation of TRANCE and OPG mRNA by PTH preceded its effects on OCL formation by 18-23 h. Hence, it is likely that PTH regulates bone resorption, at least in part, via its effects on TRANCE and OPG expression.

A novel molecular mechanism modulating osteoclast differentiation and function

Osteoclasts, the multinucleated giant cells that resorb bone, develop from hematopoietic cells of the monocyte/ macrophage lineage. Osteoblasts, as well as bone marrow stromal cells, support osteoclast development through a mechanism of cell-to-cell interaction with osteoclast progenitors. We recently purified and molecularly cloned osteoclastogenesis inhibitory factor (OCIF), which was identical to osteoprotegerin (OPG). OPG/OCIF, a secreted member of the tumor necrosis factor (TNF) receptor family, inhibited differentiation and activation of osteoclasts. A single class of high-affinity binding sites for OPG/OCIF appeared on a mouse bone marrow stromal cell line, ST2, in response to 1alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3] and dexamethasone (Dex). When the binding sites were occupied by OPG/OCIF, ST2 cells failed to support the osteoclast formation from spleen cells. To identify an OPG/OCIF ligand, we screened a cDNA expression library of ST2 cells treated with 1,25(OH)2D3 and Dex using OPG/OCIF as a probe. The cloned molecule was found to be a member of the membrane-associated TNF ligand family, and it induced osteoclast formation from mouse and human osteoclast progenitors in the presence of macrophage colony-stimulating factor (M-CSF) in vitro. Expression of its gene in osteoblasts/stromal cells was up-regulated by osteotropic factors, such as 1,25(OH)2D3, prostaglandin E2 (P(GE2), parathyroid hormone (PTH), and interleukin (IL)-11. A polyclonal antibody against this protein, as well as OPG/OCIF, negated not only the osteoclastogenesis induced by the protein, but also bone resorption elicited by various osteotropic factors in a fetal mouse long bone culture system. These findings led us to conclude that the protein is osteoclast differentiation factor (ODF), a long sought-after ligand that mediates an essential signal to osteoclast progenitors for their differentiation into active osteoclasts. Recent analyses of ODF receptor demonstrated that RANK, a member of the TNF receptor family, is the signaling receptor for ODF in osteoclastogenesis, and that OPG/OCIF acts as a decoy receptor for ODF to compete against RANK. The discovery of ODF, OPG/OCIF, and RANK opens a new era in the investigation of the regulation of osteoclast differentiation and function.

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.

Immunological characterization of circulating osteoprotegerin/osteoclastogenesis inhibitory factor: increased serum concentrations in postmenopausal women with osteoporosis

Osteoprotegerin (OPG)/osteoclastogenesis inhibitory factor (OCIF) is a soluble member of the tumor necrosis factor receptor family of proteins and plays an important role in the negative regulation of osteoclastic bone resorption. Whether OPG/OCIF circulates in human blood and how its level changes under pathological conditions is not known. To address these issues, a panel of monoclonal antibodies was generated against recombinant OPG/OCIF and screened for reactivity with solid-phase monomeric and homodimeric forms of the recombinant protein. Antibodies that showed high affinity for both forms of OPG/OCIF and those that selectively recognized the homodimer were identified, enabling development of two types of sensitive enzyme-linked immunosorbent assay (ELISA): one that detects both forms of OPG/OCIF equally and one specific for the homodimer. Characterization of circulating OPG/OCIF with these ELISAs revealed that the protein exists in human serum mainly in the monomeric form. The serum concentration of OPG/OCIF increased with age in both healthy Japanese men and women, and was significantly higher in postmenopausal women with osteoporosis than in age-matched controls. Within the osteoporotic group, serum OPG/OCIF concentrations were higher in patients with low bone mass. Serum OPG/OCIF concentrations were also significantly increased in those postmenopausal women with a high rate of bone turnover, as determined by increased serum bone-specific alkaline phosphatase and urinary excretion of pyridinoline and deoxypyridinoline. The results suggested that circulating OPG/OCIF levels are regulated by an age-related factor(s) and that the increased serum concentration may reflect a compensative response to enhanced osteoclastic bone resorption and the resultant bone loss rather than a cause of osteoporosis.

A new member of tumor necrosis factor ligand family, ODF/OPGL/TRANCE/RANKL, regulates osteoclast differentiation and function

Osteoclasts, the multinucleated giant cells that resorb bone, develop from monocyte-macrophage lineage cells. Osteoblasts or bone marrow stromal cells have been suggested to be involved in osteoclastic bone resorption. The recent discovery of new members of the tumor necrosis factor (TNF) receptor-ligand family has elucidated the precise mechanism by which osteoblasts/stromal cells regulate osteoclast differentiation and function. Osteoblasts/stromal cells express a new member of the TNF-ligand family "osteoclast differentiation factor(ODF)/osteoprotegerin ligand (OPGL)/TNF-related activation-induced cytokine (TRANCE)/receptor activator of NF-kB ligand (RANKL)" as a membrane associated factor. Osteoclast precursors which possess RANK, a TNF receptor family member, recognize ODF/OPGL/TRANCE/RANKL through cell-to-cell interaction with osteoblasts/stromal cells, and differentiate into osteoclasts in the presence of macrophage colony-stimulating factor. Mature osteoclasts also express RANK, and their bone-resorbingactivity is also induced by ODF/OPGL/TRANCE/RANKL which osteoblasts/stromal cells possess. Osteoprotegerin (OPG)/osteoclastogenesis inhibitory factor (OCIF)/TNF receptor-like molecule 1 (TR1) is a soluble decoy receptor for ODF/OPGL/TRANCE/RANKL. Activation of NF-kB and c-Jun N-terminal kinase through the RANK-mediated signaling system appears to be involved in differentiation and activation of osteoclasts.

RANK is the essential signaling receptor for osteoclast differentiation factor in osteoclastogenesis

Osteoclast differentiation factor (ODF) is a ligand for osteoclastogenesis-inhibitory factor/osteoprotegerin (OCIF/OPG), and mediates an essential signal for osteoclastogenesis. Soluble-form ODF binds directly to osteoclast progenitors, suggesting the presence of a membrane-bound receptor for ODF (ODFR) on the cells. To understand the ODF-mediated signal transduction mechanism in osteoclastogenesis, we molecularly cloned ODFR from a mouse macrophage-like osteoclast progenitor cell line, C7. Nucleotide sequence analysis revealed that ODFR is identical to RANK, a recently identified member of the tumor necrosis factor receptor (TNFR) family, which is involved in the regulation of dendritic cell function. A polyclonal antibody against the extracellular domain of RANK induced osteoclastogenesis in the presence of macrophage colony-stimulating factor (M-CSF). In contrast, both a genetically engineered soluble RANK and Fab fragment of the antibody blocked the binding of ODF to RANK and ODF-mediated osteoclastogenesis. These results indicate that RANK is the signaling receptor essential for ODF-mediated osteoclastogenesis.