Biochemical and pharmacological criteria define two shedding activities for TRANCE/OPGL that are distinct from the tumor necrosis factor alpha convertase

The hidden secrets of soluble RANKL in bone biology

The discovery of cytokine tumor necrosis factor (TNF) in the 20th century revealed numerous secrets about organ development. In particular, the functions identified for the receptor activator of nuclear factor kappa-β (NF-κβ) ligand (also known as the RANKL/osteoprotegerin ligand (OPGL) or RANK ligand/TNFSF11) in the homeostasis of skeletal structure, function and regulation were not anticipated. Empirical evidence established the receptor-ligand interaction of RANKL with RANK in osteoclast formation. Reverse signaling of RANKL triggers NF-κβ for the degradation of β-catenin to inhibit bone formation. There is also evidence that RANKL modifies the behavior of other cells in the bone microenvironment, including osteoblasts, chondrocytes, endothelial cells and lymphocytes during normal (homeostatic) and diseased (osteoimmune) states. Two forms of RANKL, i.e., soluble and membrane-bound RANKL, are produced by bone cells. Even though soluble RANKL (sRANKL) and membrane-bound RANKL (mRANKL) both stimulate osteoclast formation in vitro, their biological roles are different. mRANKL triggers osteoclastogenesis by binding to RANK through cell-cell interaction; however, sRANKL released from osteogenic cells binds to RANK without cell-cell interaction. This review attempts to hypothesize how sRANKL functions biologically in bone and explore how this hypothesis might influence future research.

Matrix Metalloproteases as Influencers of the Cells' Social Media

Matrix metalloproteinases (MMPs) have been studied in the context of cancer due to their ability to increase cell invasion, and were initially thought to facilitate metastasis solely through the degradation of the extracellular matrix (ECM). MMPs have also been investigated in the context of their ECM remodeling activity in several acute and chronic inflammatory diseases. However, after several MMP inhibitors failed in phase III clinical trials, a global reassessment of their biological functions was undertaken, which has revealed multiple unanticipated functions including the processing of chemokines, cytokines, and cell surface receptors. Despite what their name suggests, the matrix aspect of MMPs could contribute to a lesser part of their physiological functions in inflammatory diseases, as originally anticipated. Here, we present examples of MMP substrates implicated in cell signaling, independent of their ECM functions, and discuss the impact for the use of MMP inhibitors.

IL-6 exhibits both cis- and trans-signaling in osteocytes and osteoblasts, but only trans-signaling promotes bone formation and osteoclastogenesis

Interleukin 6 (IL-6) supports development of bone-resorbing osteoclasts by acting early in the osteoblast lineage via membrane-bound (cis) or soluble (trans) receptors. Here, we investigated how IL-6 signals and modifies gene expression in differentiated osteoblasts and osteocytes and determined whether these activities can promote bone formation or support osteoclastogenesis. Moreover, we used a genetically altered mouse with circulating levels of the pharmacological IL-6 trans-signaling inhibitor sgp130-Fc to determine whether IL-6 trans-signaling is required for normal bone growth and remodeling. We found that IL-6 increases suppressor of cytokine signaling 3 (Socs3) and CCAAT enhancer-binding protein δ (Cebpd) mRNA levels and promotes signal transducer and activator of transcription 3 (STAT3) phosphorylation by both cis- and trans-signaling in cultured osteocytes. In contrast, RANKL (Tnfsf11) mRNA levels were elevated only by trans-signaling. Furthermore, we observed soluble IL-6 receptor release and ADAM metallopeptidase domain 17 (ADAM17) sheddase expression by osteocytes. Despite the observation that IL-6 cis-signaling occurs, IL-6 stimulated bone formation in vivo only via trans-signaling. Although IL-6 stimulated RANKL (Tnfsf11) mRNA in osteocytes, these cells did not support osteoclast formation in response to IL-6 alone; binucleated TRAP+ cells formed, and only in response to trans-signaling. Finally, pharmacological, sgp130-Fc-mediated inhibition of IL-6 trans-signaling did not impair bone growth or remodeling unless mice had circulating sgp130-Fc levels > 10 μg/ml. At those levels, osteopenia and impaired bone growth occurred, reducing bone strength. We conclude that high sgp130-Fc levels may have detrimental off-target effects on the skeleton.

Soluble RANKL contributes to osteoclast formation in adult mice but not ovariectomy-induced bone loss

Receptor activator of NFkB ligand (RANKL) is a TNF-family cytokine required for osteoclast formation, as well as immune cell and mammary gland development. It is produced as a membrane-bound protein that can be shed to form a soluble protein. We created mice harboring a sheddase-resistant form of RANKL, in which soluble RANKL is undetectable in the circulation. Lack of soluble RANKL does not affect bone mass or structure in growing mice but reduces osteoclast number and increases cancellous bone mass in adult mice. Nonetheless, the bone loss caused by estrogen deficiency is unaffected by the lack of soluble RANKL. Lymphocyte number, lymph node development, and mammary gland development are also unaffected by the absence of soluble RANKL. These results demonstrate that the membrane-bound form of RANKL is sufficient for most functions of this protein but that the soluble form does contribute to physiological bone remodeling in adult mice.

RANKL is a cytokine produced as a membrane-bound and a secreted protein. Here, using mice lacking soluble RANKL, the authors show that the secreted protein is important for osteoclast function, but not for mammary gland and lymphocyte development.

IL-3 Differentially Regulates Membrane and Soluble RANKL in Osteoblasts through Metalloproteases and the JAK2/STAT5 Pathway and Improves the RANKL/OPG Ratio in Adult Mice

Bone remodeling comprises balanced activities between osteoclasts and osteoblasts, which is regulated by various factors, including hormones and cytokines. We previously reported that IL-3 inhibits osteoclast differentiation and pathological bone loss. IL-3 also enhances osteoblast differentiation and bone formation from mesenchymal stem cells. However, the role of IL-3 in regulation of osteoblast–osteoclast interactions and underlying mechanisms is not yet delineated. In this study, we investigated the role of IL-3 on the regulation of osteoblast-specific molecules, receptor activator of NF-κB ligand (RANKL), and osteoprotegerin (OPG) that modulate bone homeostasis. We found that IL-3 increases RANKL expression at both the transcriptional and translational levels, and it showed no effect on OPG expression in calvarial osteoblasts. The increased RANKL expression by IL-3 induces mononuclear osteoclasts; however, it does not induce multinuclear osteoclasts. Interestingly, IL-3 decreases soluble RANKL by reducing ectodomain shedding of membrane RANKL through downregulation of metalloproteases mainly a disintegrin and metalloproteinase (ADAM)10, ADAM17, ADAM19, and MMP3. Moreover, IL-3 increases membrane RANKL by activating the JAK2/STAT5 pathway. Furthermore, IL-3 enhances RANKL expression in mesenchymal stem cells of wild-type mice but not in STAT5a knockout mice. Interestingly, IL-3 restores RANKL expression in adult mice by enhancing bone-specific RANKL and decreasing serum RANKL. Furthermore, IL-3 increases the serum OPG level in adult mice. Thus, our results reveal, to our knowledge for the first time, that IL-3 differentially regulates two functional forms of RANKL through metalloproteases and the JAK2/STAT5 pathway, and it helps in restoring the decreased RANKL/OPG ratio in adult mice. Notably, our studies indicate the novel role of IL-3 in regulating bone homeostasis in important skeletal disorders.

Estrogens and androgens inhibit association of RANKL with the pre-osteoblast membrane through post-translational mechanisms

We have recently demonstrated that RUNX2 promoted, and 17β-Estradiol (E2) diminished, association of RANKL with the cell membrane in pre-osteoblast cultures. Here we show that, similar to E2, dihydrotestosterone (DHT) diminishes association of RANKL, and transiently transfected GFP-RANKL with the pre-osteoblast membrane without decreasing total RANKL mRNA or protein levels. Diminution of membrane-associated RANKL was accompanied with marked suppression of osteoclast differentiation from co-cultured pre-osteoclasts, even though DHT increased, not decreased, RANKL concentrations in pre-osteoblast conditioned media. A marked decrease in membrane-associated RANKL was observed after 30 min of either E2 or DHT treatment, and near-complete inhibition was observed by 1 hr, suggesting that the diminution of RANKL membrane association was mediated through non-genomic mechanisms. Further indicating dispensability of nuclear action of estrogen receptor, E2-mediated inhibition of RANKL membrane association was mimicked by an estrogen dendrimer conjugate (EDC) that cannot enter the cell nucleus. Finally, the inhibitory effect of E2 and DHT on RANKL membrane association was counteracted by the MMP inhibitor NNGH, and the effect of E2 (and not DHT) was antagonized by the Src inhibitor SU6656. Taken together, these results suggest that estrogens and androgens inhibit osteoblast-driven osteoclastogenesis through non-genomic mechanism(s) that entail, MMP-mediated RANKL dissociation from the cell membrane.

Bone-Seeking Matrix Metalloproteinase-2 Inhibitors Prevent Bone Metastatic Breast Cancer Growth

Bone metastasis is common during breast cancer progression. Matrix metalloproteinase-2 (MMP-2) is significantly associated with aggressive breast cancer and poorer overall survival. In bone, tumor- or host-derived MMP-2 contributes to breast cancer growth and does so by processing substrates, including type I collagen and TGFβ latency proteins. These data provide strong rationale for the application of MMP-2 inhibitors to treat the disease. However, in vivo, MMP-2 is systemically expressed. Therefore, to overcome potential toxicities noted with previous broad-spectrum MMP inhibitors (MMPIs), we used highly selective bisphosphonic-based MMP-2 inhibitors (BMMPIs) that allowed for specific bone targeting. In vitro, BMMPIs affected the viability of breast cancer cell lines and osteoclast precursors, but not osteoblasts. In vivo, we demonstrated using two bone metastatic models (PyMT-R221A and 4T1) that BMMPI treatment significantly reduced tumor growth and tumor-associated bone destruction. In addition, BMMPIs are superior in promoting tumor apoptosis compared with the standard-of-care bisphosphonate, zoledronate. We demonstrated MMP-2-selective inhibition in the bone microenvironment using specific and broad-spectrum MMP probes. Furthermore, compared with zoledronate, BMMPI-treated mice had significantly lower levels of TGFβ signaling and MMP-generated type I collagen carboxy-terminal fragments. Taken together, our data show the feasibility of selective inhibition of MMPs in the bone metastatic breast cancer microenvironment. We posit that BMMPIs could be easily translated to the clinical setting for the treatment of bone metastases given the well-tolerated nature of bisphosphonates. Mol Cancer Ther; 16(3); 494-505. ©2017 AACR.

Parathyroid hormone induces expression and proteolytic processing of Rankl in primary murine osteoblasts

Rankl, the major pro-osteoclastogenic cytokine, is synthesized as a transmembrane protein that can be cleaved by specific endopeptidases to release a soluble form (sRankl). We have previously reported that interleukin-33 (IL-33) induces expression of Tnfsf11, the Rankl-encoding gene, in primary osteoblasts, but we failed to detect sRankl in the medium. Since we also found that PTH treatment caused sRankl release in a similar experimental setting, we directly compared the influence of the two molecules. Here we show that treatment of primary murine osteoblasts with PTH causes sRankl release into the medium, whereas IL-33 only induces Tnfsf11 expression. This difference was not explainable by alternative splicing or by PTH-specific induction of endopeptidases previously shown to facilitate Rankl processing. Since sRankl release after PTH administration was blocked in the presence a broad-spectrum matrix metalloprotease inhibitor, we applied genome-wide expression analyses to identify transcriptional targets of PTH in osteoblasts. We thereby confirmed some of the effects of PTH established in other systems, but additionally identified few PTH-induced genes encoding metalloproteases. By comparing expression of these genes following administration of IL-33, PTH and various other Tnfsf11-inducing molecules, we observed that PTH was the only molecule simultaneously inducing sRankl release and Adamts1 expression. The functional relevance of the putative influence of PTH on Rankl processing was further confirmed in vivo, as we found that daily injection of PTH into wildtype mice did not only increase bone formation, but also osteoclastogenesis and sRankl concentrations in the serum. Taken together, our findings demonstrate that transcriptional effects on Tnfsf11 expression do not generally trigger sRankl release and that PTH has a unique activity to promote the proteolytic processing of Rankl.

A review of denosumab for the treatment of osteoporosis

Osteoporosis is an age-related systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility. Bone remodeling involves two types of cells: osteoblasts and osteoclasts. Receptor activator of nuclear factor-κB ligand (RANKL) is a key regulator of the formation and function of bone-resorbing osteoclasts, and its cell surface receptor, receptor activator of nuclear factor-κB (RANK), is expressed by both osteoclast precursors and mature osteoclasts. Denosumab is a fully human monoclonal anti-RANKL antibody that inhibits the binding of RANKL to RANK, thereby decreasing osteoclastogenesis and bone-resorbing activity of mature osteoclasts. Although there are many medications available for the treatment of osteoporosis, inhibition of RANKL by denosumab has been shown to significantly affect bone metabolism. Denosumab appears to be a promising, highly effective, and safe parenteral therapy with good adherence for osteoporosis. Moreover, denosumab may be cost-effective therapy compared with existing alternatives. Therefore, in this review, we focus on studies of denosumab and the risks and benefits identified for this type of treatment for osteoporosis.

The role of TNF-α and TNF superfamily members in the pathogenesis of calcific aortic valvular disease

Calcific aortic valve disease (CAVD) represents a slowly progressive pathologic process associated with major morbidity and mortality. The process is characterized by multiple steps: inflammation, fibrosis, and calcification. Numerous studies focalized on its physiopathology highlighting different "actors" for the multiple "acts." This paper focuses on the role of the tumor necrosis factor superfamily (TNFSF) members in the pathogenesis of CAVD. In particular, we discuss the clinical and experimental studies providing evidence of the involvement of tumor necrosis factor-alpha (TNF-α), receptor activator of nuclear factor-kappa B (NF-κB) ligand (RANKL), its membrane receptor RANK and its decoy receptor osteoprotegerin (OPG), and TNF-related apoptosis-inducing ligand (TRAIL) in valvular calcification.

RANKL subcellular trafficking and regulatory mechanisms in osteocytes

The receptor activator of the NF-κB ligand (RANKL) is the central player in the regulation of osteoclastogenesis, and the quantity of RANKL presented to osteoclast precursors is an important factor determining the magnitude of osteoclast formation. Because osteoblastic cells are thought to be a major source of RANKL, the regulatory mechanisms of RANKL subcellular trafficking have been studied in osteoblastic cells. However, recent reports showed that osteocytes are a major source of RANKL presentation to osteoclast precursors, prompting a need to reinvestigate RANKL subcellular trafficking in osteocytes. Investigation of molecular mechanisms in detail needs well-designed in vitro experimental systems. Thus, we developed a novel co-culture system of osteoclast precursors and osteocytes embedded in collagen gel. Experiments using this model revealed that osteocytic RANKL is provided as a membrane-bound form to osteoclast precursors through osteocyte dendritic processes and that the contribution of soluble RANKL to the osteoclastogenesis supported by osteocytes is minor. Moreover, the regulation of RANKL subcellular trafficking, such as OPG-mediated transport of newly synthesized RANKL molecules to lysosomal storage compartments, and the release of RANKL to the cell surface upon stimulation with RANK are confirmed to be functional in osteocytes. These results provide a novel understanding of the regulation of osteoclastogenesis.

RANKL employs distinct binding modes to engage RANK and the osteoprotegerin decoy receptor

Osteoprotegerin (OPG) and receptor activator of nuclear factor κB (RANK) are members of the tumor necrosis factor receptor (TNFR) superfamily that regulate osteoclast formation and function by competing for RANK ligand (RANKL). RANKL promotes osteoclast development through RANK activation, while OPG inhibits this process by sequestering RANKL. For comparison, we solved crystal structures of RANKL with RANK and RANKL with OPG. Complementary biochemical and functional studies reveal that the monomeric cytokine-binding region of OPG binds RANKL with ∼500-fold higher affinity than RANK and inhibits RANKL-stimulated osteoclastogenesis ∼150 times more effectively, in part because the binding cleft of RANKL makes unique contacts with OPG. Several side chains as well as the C-D and D-E loops of RANKL occupy different orientations when bound to OPG versus RANK. High affinity OPG binding requires a 90s loop Phe residue that is mutated in juvenile Paget's disease. These results suggest cytokine plasticity may help to fine-tune specific tumor necrosis factor (TNF)-family cytokine/receptor pair selectivity.

Soluble rank ligand produced by myeloma cells causes generalised bone loss in multiple myeloma

Patients with multiple myeloma commonly develop focal osteolytic bone disease, as well as generalised osteoporosis. The mechanisms underlying the development of osteoporosis in patients with myeloma are poorly understood. Although disruption of the RANKL/OPG pathway has been shown to underlie formation of focal osteolytic lesions, its role in the development of osteoporosis in myeloma remains unclear. Increased soluble RANKL in serum from patients with myeloma raises the possibility that this molecule plays a key role. The aim of the present study was to establish whether sRANKL produced by myeloma cells contributes directly to osteoporosis. C57BL/KaLwRij mice were injected with either 5T2MM or 5T33MM murine myeloma cells. 5T2MM-bearing mice developed osteolytic bone lesions (p<0.05) with increased osteoclast surface (p<0.01) and reduced trabecular bone volume (p<0.05). Bone volume was also reduced at sites where 5T2MM cells were not present (p<0.05). In 5T2MM-bearing mice soluble mRANKL was increased (p<0.05), whereas OPG was not altered. In contrast, 5T33MM-bearing mice had no changes in osteoclast surface or trabecular bone volume and did not develop osteolytic lesions. Soluble mRANKL was undetectable in serum from 5T33MM-bearing mice. In separate experiments, RPMI-8226 human myeloma cells were transduced with an human RANKL/eGFP construct, or eGFP alone. RPMI-8226/hRANKL/eGFP cells, but not RPMI-8226/eGFP cells, stimulated osteoclastic bone resorption (p<0.05) in vitro. Sub-cutaneous injection of NOD/SCID mice with RPMI-8226/hRANKL/eGFP or RPMI-8226/eGFP cells resulted in tumour development in all mice. RPMI-8226/hRANKL/eGFP-bearing mice exhibited increased serum soluble hRANKL (p<0.05) and a three-fold increase in osteoclast number (p<0.05) compared to RPMI-8226/eGFP-bearing mice. This was associated with reduced trabecular bone volume (27%, p<0.05), decreased trabecular number (29%, p<0.05) and increased trabecular thickness (8%, p<0.05). Our findings demonstrate that soluble RANKL produced by myeloma cells causes generalised bone loss, suggesting that targeting RANKL may prevent osteoporosis in patients with myeloma.

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.

New regulation mechanisms of osteoclast differentiation

Osteoclasts play a crucial role in both physiological and pathological bone resorption. It is, thus, of compelling importance to understand the molecular mechanisms of osteoclast regulation. Because receptor activator of nuclear factor-κB ligand (RANKL) is the key cytokine that induces osteoclast differentiation, we have focused on the investigation of RANKL signaling and RANKL-expressing cells. Here, we summarize the recent advances in the understanding of osteoclastogenic signaling and the cells that express RANKL in the context of osteoimmunology. The scope of osteoimmunology has been extended to now encompass a wide range of molecular and cellular interactions, and its framework provides a scientific basis for future therapeutic approaches to diseases related to the bone and/or immune systems.

Interleukin-1 stimulates ADAM17 through a mechanism independent of its cytoplasmic domain or phosphorylation at threonine 735

ADAM17 (a disintegrin and metalloproteinase) is a membrane-anchored metalloproteinase that regulates the release of EGFR-ligands, TNFα and other membrane proteins from cells. ADAM17 can be rapidly activated by a variety of signaling pathways, yet little is known about the underlying mechanism. Several studies have demonstrated that the cytoplasmic domain of ADAM17 is not required for its rapid activation by a variety of stimuli, including phorbol esters, tyrosine kinases and some G-protein coupled receptors. However, phosphorylation of cytoplasmic residue T735 was recently reported as a crucial step for activation of ADAM17 by IL-1β and by the p38 MAP-kinase pathway. One possible mechanism to reconcile these results would be that T735 has an inhibitory role and that it must be phosphorylated as a pre-requisite for the activation of ADAM17, which would then proceed via a mechanism that is independent of its cytoplasmic domain. To test this hypothesis, we performed rescue experiments of Adam17−/− cells with wild type and mutant forms of ADAM17. However, these experiments showed that an inactivating mutation (T735A) or an activating mutation (T735D) of cytoplasmic residue T735 or the removal of the cytoplasmic domain of ADAM17 did not significantly affect the stimulation of ADAM17 by IL-1β or by activation of MAP-kinase with anisomycin. Moreover, we found that the MAP-kinase inhibitor SB203580 blocked activation of cytoplasmic tail-deficient ADAM17 and of the T735A mutant by IL-1β or by anisomycin, providing further support for a model in which the activation mechanism of ADAM17 does not rely on its cytoplasmic domain or phosphorylation of T735.

Strontium ranelate inhibits key factors affecting bone remodeling in human osteoarthritic subchondral bone osteoblasts

INTRODUCTION

In osteoarthritis (OA) the progression of cartilage degeneration has been associated with remodeling of the subchondral bone. Human OA subchondral bone osteoblasts were shown to have an abnormal phenotype and altered metabolism leading to an abnormal resorptive process. Bone resorption is suggested to occur, at least in part, through the increased levels of two proteolytic enzymes, MMP-2 and MMP-9, and RANKL, which are mainly produced by osteoblasts. In this study, we investigated in human OA subchondral bone osteoblasts the modulatory effect of strontium ranelate on the above key factors.

METHODS

Human subchondral bone osteoblasts were cultured in a medium containing 0.1, 1 and 2 mM of strontium ranelate for 18 h for mRNA and 72 h for protein determination. The effect of strontium ranelate was evaluated on the expression (qPCR) of MMP-2, MMP-9, OPG, RANKL (total), RANKL-1, and RANKL-3, on the production of OPG (ELISA), membranous RANKL (flow cytometry), and MT1-MMP, ADAM17, and ADAM19 (Western blot). After incubation of osteoblasts with pre-osteoclasts (i.e., differentiated human peripheral blood mononuclear cells), the resorbed surface was measured using a sub-micron synthetic calcium phosphate thin film.

RESULTS

Firstly, the expression levels of MMP-2, MMP-9, OPG, and RANKL were determined in normal and OA subchondral bone osteoblasts. As expected, the gene expression of MMP-9 and RANKL were not detectable in normal cells, whereas MMP-2 was very low but detectable and OPG demonstrated high gene expression. Further experiments looking at the effect of strontium ranelate on expression levels, except for OPG, were performed only on the OA subchondral bone osteoblasts. In OA cells, the expression levels of MMP-2 and MMP-9 were significantly decreased by strontium ranelate at 1mM (p≤0.005, p≤0.02, respectively) and 2 mM (p≤0.003, p≤0.007), and for MMP-9 only at 0.1 mM (p≤0.05). In normal cells, the expression of OPG was increased with strontium ranelate at 2 mM, and in OA both the expression (p≤0.02) and synthesis (p≤0.002) of OPG were significantly increased with strontium ranelate at 1 and 2 mM. RANKL (total) as well as the isoforms RANKL-1 and RANKL-3 were significantly increased by strontium ranelate at 1 and 2 mM. Of note, it is known that the different RANKL isoforms differentially regulate RANKL membranous localization: RANKL-3, in contrast to RANKL-1, prevents such membranous localization. This is reflected by the significant (p≤0.02) reduction in the level of membranous RANKL by strontium ranelate at 2 mM. This latter finding was not likely to be related to a proteolytic cleavage of membranous RANKL, as the enzymes known to cleave it, MT1-MMP, ADAM17 and ADAM19, were unaffected by strontium ranelate. In addition, OA osteoblasts treated with strontium ranelate induced a significant (p≤0.002) decrease in resorbed surface at the three tested concentrations.

CONCLUSION

This study provides new insights into the mode of action of strontium ranelate on the metabolism of human OA subchondral bone osteoblasts. These data suggest that strontium ranelate may exert a positive effect on OA pathophysiology by inhibiting, in these cells, the synthesis of key factors leading to bone resorption, a feature associated with the OA process.

A comprehensive manually curated reaction map of RANKL/RANK-signaling pathway

Receptor activator of nuclear factor-kappa B ligand (RANKL) is a member of tumor necrosis factor (TNF) superfamily that plays a key role in the regulation of differentiation, activation and survival of osteoclasts and also in tumor cell migration and bone metastasis. Osteoclast activation induced by RANKL regulates hematopoietic stem cell mobilization as part of homeostasis and host defense mechanisms thereby linking regulation of hematopoiesis with bone remodeling. Binding of RANKL to its receptor, Receptor activator of nuclear factor-kappa B (RANK) activates molecules such as NF-kappa B, mitogen activated protein kinase (MAPK), nuclear factor of activated T cells (NFAT) and phosphatidyl 3-kinase (PI3K). Although the molecular and cellular roles of these molecules have been reported previously, a systematic cataloging of the molecular events induced by RANKL/RANK interaction has not been attempted. Here, we present a comprehensive reaction map of the RANKL/RANK-signaling pathway based on an extensive manual curation of the published literature. We hope that the curated RANKL/RANK-signaling pathway model would enable new biomedical discoveries, which can provide novel insights into disease processes and development of novel therapeutic interventions.

Database URL:http://www.netpath.org/pathways?path_id=NetPath_21

Matrix metalloproteinases as master regulators of the vicious cycle of bone metastasis

Bone remodeling is a delicate balancing act between the bone matrix synthesizing osteoblasts and bone resorbing osteoclasts. Active bone metastases typically subvert this process to generate lesions that are comprised of extensive areas of pathological osteogenesis and osteolysis. The resultant increase in bone matrix remodeling enhances cytokine/growth factor bioavailability thus creating a vicious cycle that stimulates tumor progression. Given the extent of matrix remodeling occurring in the tumor-bone microenvironment, the expression of matrix metalloproteinases (MMPs) would be expected, since collectively they have the ability to degrade all components of the extracellular matrix (ECM). However, in addition to being "matrix bulldozers", MMPs control the bioavailability and bioactivity of factors such as RANKL and TGFβ that have been described as crucial for tumor-bone interaction, thus implicating MMPs as key regulators of the vicious cycle of bone metastases.

Molecular and cellular mechanisms of ectodomain shedding

The extracellular domain of several membrane-anchored proteins is released from the cell surface as soluble proteins through a regulated proteolytic mechanism called ectodomain shedding. Cells use ectodomain shedding to actively regulate the expression and function of surface molecules, and modulate a wide variety of cellular and physiological processes. Ectodomain shedding rapidly converts membrane-associated proteins into soluble effectors and, at the same time, rapidly reduces the level of cell surface expression. For some proteins, ectodomain shedding is also a prerequisite for intramembrane proteolysis, which liberates the cytoplasmic domain of the affected molecule and associated signaling factors to regulate transcription. Ectodomain shedding is a process that is highly regulated by specific agonists, antagonists, and intracellular signaling pathways. Moreover, only about 2% of cell surface proteins are released from the surface by ectodomain shedding, indicating that cells selectively shed their protein ectodomains. This review will describe the molecular and cellular mechanisms of ectodomain shedding, and discuss its major functions in lung development and disease.

Shedding of RANKL by tumor-associated MT1-MMP activates Src-dependent prostate cancer cell migration

Membrane type 1 matrix metalloproteinase (MT1-MMP) plays an essential role in protease-mediated extracellular matrix (ECM) degradation, but it also functions as a sheddase releasing non-ECM substrates such as receptor activator of NF-kappaB ligand (RANKL), an osteoclastogenic factor typically confined to the surface of osteoblasts. We previously found high expression of MT1-MMP in skeletal metastasis of prostate cancer patients, in a pattern similar to RANKL expression. We also showed that overexpression of MT1-MMP in prostate cancer cells increases tumor growth and osteolysis in an intratibial mouse model of bone metastasis, and that soluble factor(s) shed by tumor-derived MT1-MMP enhance osteoclast differentiation in a RANKL-dependent manner. Recent evidence indicates that the cognate receptor for RANKL, RANK, is expressed in prostate cancer cells, suggesting the presence of an autocrine pathway. In this study, we show that MT1-MMP-expressing LNCaP prostate cancer cells display enhanced migration. Moreover, conditioned medium from LNCaP cells expressing both RANKL and MT1-MMP stimulates the migration of MT1-MMP-deficient C42b prostate cancer cells. This enhanced chemotaxis can be abrogated by osteoprotegerin (soluble decoy receptor of RANKL), MIK-G2 (a selective inhibitor for MT1-MMP), and PP2 (a Src inhibitor). These findings indicate that tumor-derived MT1-MMP enhances tumor cell migration through initiation of an autocrine loop requiring ectodomain shedding of membrane-bound RANKL in prostate cancer cells, and that Src is a key downstream mediator of RANKL-induced migration of prostate cancer cells.

Full-length, membrane-anchored TWEAK can function as a juxtacrine signaling molecule and activate the NF-kappaB pathway

Tumor necrosis factor (TNF) family members are initially synthesized as type II transmembrane proteins, but some of these proteins are substrates for proteolytic enzymes that generate soluble cytokines with biological activity. TWEAK (TNF-like weak inducer of apoptosis), a member of the TNF family, is a multifunctional cytokine that acts via binding to a cell surface receptor named Fn14 (fibroblast growth factor-inducible 14). Studies conducted to date indicate that TWEAK-producing cells can co-express both membrane-anchored and soluble TWEAK isoforms, but there is little information on TWEAK proteolytic processing. Also, it is presently unclear whether membrane-anchored TWEAK, like soluble TWEAK, is biologically active. Here we show that full-length human TWEAK is processed intracellularly by the serine protease furin and identify TWEAK amino acid residues 90-93 as the predominant furin recognition site. In addition, we report that full-length, membrane-anchored TWEAK can bind the Fn14 receptor on neighboring cells and activate the NF-kappaB signaling pathway. Thus, TWEAK can act in a juxtacrine manner to initiate cellular responses, and this property may be important for TWEAK function during physiological wound repair and disease pathogenesis.

Proteases and antiproteases in chronic neutrophilic lung disease - relevance to drug discovery

Chronic inflammatory lung diseases such as cystic fibrosis and emphysema are characterized by higher-than-normal levels of pulmonary proteases. While these enzymes play important roles such as bacterial killing, their dysregulated expression or activity can adversely impact on the inflammatory process. The existence of efficient endogenous control mechanisms that can dampen or halt this overexuberant protease activity in vivo is essential for the effective resolution of inflammatory lung disease. The function of pulmonary antiproteases is to fulfil this role. Interestingly, in addition to their antiprotease activity, protease inhibitors in the lung also often possess other intrinsic properties that contribute to microbial killing or termination of the inflammatory process. This review will outline important features of chronic inflammation that are regulated by pulmonary proteases and will describe the various mechanisms by which antiproteases attempt to counterbalance exaggerated protease-mediated inflammatory events. These proteases, antiproteases and their modifiers represent interesting targets for therapeutic intervention.

Involvement of a disintegrin and metalloproteinase 10 and 17 in shedding of tumor necrosis factor-alpha

Tumor necrosis factor-alpha (TNF-alpha) is initially synthesized as a membrane-bound protein and converted into a soluble form by proteolytic cleavage. Although a disintegrin and metalloproteinase 17 (ADAM17) is considered to be the primary sheddase for TNF-alpha, it is not known whether ADAM17 is solely responsible for that process in any type of cells. To identify the TNF-alpha sheddase(s) in varieties of cells, we performed experiments using a unique screening system and observed that ADAM9, ADAM10, ADAM17, and ADAM19 were capable of cleaving TNF-alpha. We then performed RNA interference experiments and confirmed that ADAM10 and ADAM17 were in fact involved in TNF-alpha shedding in 293A cells. In mouse macrophages, ADAM17 was confirmed to be the primary sheddase, but the involvement of ADAM10 was also demonstrated. In NIH3T3 cells, ADAM10 could be more important in the shedding than ADAM17. In mouse vascular endothelial cell line UVfemale2, ADAM10 and ADAM17 were equally involved in TNF-alpha shedding, whereas ADAM17 was a major sheddase in human osteoarthritic chondrocytes. From these observations and others, we concluded that both ADAM10 and ADAM17 can be a TNF-alpha sheddase and that their significance could be determined by their expression levels and the abundance of tissue inhibitor of metalloproteinases.

Matrix metalloproteinases: what do they not do? New substrates and biological roles identified by murine models and proteomics

The biological roles of the matrix metalloproteinases (MMPs) have been traditionally associated with the degradation and turnover of most of the components of the extracellular matrix (ECM). This functional misconception has been used for years to explain the involvement of the MMP family in developmental processes, cell homeostasis and disease, and led to clinical trials of MMP inhibitors for the treatment of cancer that failed to meet their endpoints and cast a shadow on MMPs as druggable targets. Accumulated evidence from a great variety of post-trial MMP degradomics studies, ranging from transgenic models to recent state-of-the-art proteomics screens, is changing the dogma about MMP functions. MMPs regulate cell behavior through finely tuned and tightly controlled proteolytic processing of a large variety of signaling molecules that can also have beneficial effects in disease resolution. Moreover, net proteolytic activity relies upon direct interactions between the different protease and protease inhibitor families, interconnected in a complex protease web, with MMPs acting as key nodal components. Such complexity renders simple interpretation of Mmp knockout mice very difficult. Indeed, the phenotype of these models reveals the response of a complex system to the loss of one protease rather than necessarily a direct effect of the lack of functional activity of a protease. Such a shift in the MMP functional paradigm, together with the difficulties associated with current methods of studying proteases this highlights the need for new high content degradomics approaches to uncover and annotate MMP activities in vivo and identify novel interactions within the protease web. Integration of these techniques with specifically designed animal models for final validation should lay the foundations for the development of new inhibitors that specifically target disease-related MMPs and/or their upstream effectors that cause deleterious effects in disease, while sparing MMP functions that are protective.

Osteoclast-derived matrix metalloproteinase-7, but not matrix metalloproteinase-9, contributes to tumor-induced osteolysis

The matrix metalloproteinases MMP-2, MMP-3, MMP-7, MMP-9, and MMP-13 are highly expressed in the tumor-bone microenvironment, and, of these, MMP-7 and MMP-9 were found to be localized to bone-resorbing osteoclasts in human breast-to-bone metastases. In a bid to define the roles of host-derived MMP-7 and MMP-9 in the tumor-bone microenvironment, the tibias of MMP-7 and MMP-9 null mice were injected with osteolytic luciferase-tagged mammary tumor cell lines. Our data show that osteoclast-derived MMP-7 significantly contributes to tumor growth and tumor-induced osteolysis whereas osteoclast-derived MMP-9 had no effect on these processes. MMP-7 is capable of processing a number of nonmatrix molecules to soluble active forms that have profound effects on cell-cell communication, such as RANKL, a crucial mediator of osteoclast precursor recruitment and maturation. Therefore, the ability of osteoclast-derived MMP-7 to promote RANKL solubilization in the tumor-bone microenvironment was explored. Results revealed that levels of soluble RANKL were significantly lower in the MMP-7 null mice compared with wild-type (WT) controls. In keeping with this observation, MMP-7 null mice had significantly fewer osteoclast numbers at the tumor-bone interface compared with the WT controls. In summary, we propose that the solubilization of RANKL by MMP-7 is a potential mechanism through which MMP-7 mediates mammary tumor-induced osteolysis. Our studies indicate that the selective inhibition of MMP-7 in the tumor-bone microenvironment may be of benefit for the treatment of lytic breast-to-bone metastases.

New perspective in osteoarthritis: the OPG and RANKL system as a potential therapeutic target?

Bone remodelling is tightly regulated by a molecular triad composed of OPG/RANK/RANKL. The receptor activator of NF-kappaB ligand (RANKL) (localized on osteoblasts) enhances osteoclastogenesis via interaction with its receptor RANK (localized on osteoclasts), whereas osteoprotegerin (OPG) (produced by osteoblasts) inhibits this osteoclastogenesis by binding to RANKL. The equilibrium between OPG and RANKL plays a crucial role in the pathophysiology of bone. Although some studies have shown the efficacy of OPG as a therapeutic agent against bone resorption, its bioavailability and mechanism of action after binding to RANKL have only recently been studied. A mechanistic investigation based on what becomes of OPG after binding to cells expressing membranous RANKL demonstrated an internalization process of OPG through the clathrin pathway prior to proteasomal and/or lysosomal degradation. Interestingly, the OPG internalization process reduced the half-life of RANKL. Recent evidence has shown that subchondral bone alterations in osteoarthritis (OA) are intimately involved in cartilage degradation, and that OPG/RANKL may be implicated. Data show that human OA subchondral bone osteoblasts have abnormal OPG and RANKL levels and consequently an altered OPG and RANKL ratio. Further data also reveal the involvement of some osteotropic factors in these altered levels and that some of these factors generally target RANKL with a differential modulation of the RANKL isoforms. Altogether, data suggest that this system could be targeted as a new strategy for the treatment of OA.

Unraveling metalloproteinase function in skeletal biology and disease using genetically altered mice

The metalloproteinase family includes MMP, ADAM and ADAMTS proteases. Mice deficient in individual or pairs of metalloproteinases have been generated, and a number of these genetic models spontaneously develop skeletal abnormalities. Here we review metalloproteinase function in endochondral and intramembranous ossification, as well as in postnatal bone remodeling. We highlight how metalloproteinases enable interactions between distinct bone cell types and how this communication contributes to the skeletal phenotypes observed in knockout mice. In addition to the physiological actions of metalloproteinases in the skeletal system, the experimental manipulation of metalloproteinase-deficient mice has revealed substantial roles for these enzymes in osteoarthritis and rheumatoid arthritis. MMP, ADAM and ADAMTS proteases thus emerge as key players in the development and homeostasis of the skeletal system.

Enhanced expression and shedding of receptor activator of NF-kappaB ligand during tumor-bone interaction potentiates mammary tumor-induced osteolysis

The bone microenvironment plays a critical role in tumor-induced osteolysis and osteolytic metastasis through tumor-bone (TB)-interaction. Receptor activator of nuclear factor-kappaB (RANK) ligand (RANKL) is one of the critical signaling molecules involved in osteolysis and bone metastasis. However, the regulation and functional significance of RANKL at the TB-interface in tumor-induced osteolysis remains unclear. In this report, we examined the role of tumor-stromal interaction in the regulation of RANKL expression and its functional significance in tumor-induced osteolysis. Using a novel mammary tumor model, we identified that RANKL expression was upregulated at the TB-interface as compared to the tumor alone area. We demonstrate increased generation of sRANKL at the TB-interface, which is associated with tumor-induced osteolysis. The ratio of RANKL to osteoprotegrin (OPG), a decoy receptor for RANKL, at the TB-interface was also increased. Targeting RANKL expression with antisense oligonucleotides (RANKL-ASO), significantly abrogated tumor-induced osteolysis, decreased RANKL expression and the RANKL:OPG ratio at the TB-interface. Together, these results demonstrate that upregulation of RANKL expression and sRANKL generation at the TB-interface potentiates tumor-induced osteolysis.

Osteoimmunology: crosstalk between the immune and bone systems

INTRODUCTION

The interaction between the immune and skeletal systems has long been acknowledged, but investigation into rheumatoid arthritis (RA) as well as the various bone phenotypes found in immunocompromised gene-deficient mice has highlighted the importance of the dynamic interplay between the two systems. This has led to the recent emergence and subsequent rapid evolution of the field of osteoimmunology. BONE DESTRUCTION WITH ARTHRITIS AS A RANKL DISEASE: In the bone destruction associated with RA, IL-17-producing helper T cells (T(H)17) play a major role by inducing receptor activator of nuclear factor-kappaB ligand (RANKL). RANKL stimulates osteoclastogenesis through nuclear factor of activated T cells cytoplasmic 1 (NFATc1), which is well known as a crucial regulator of immunity.

NEW PLAYERS IN OSTEOIMMUNOLOGY

In addition to cellular interactions via cytokines, the immune and skeletal systems share various molecules, including transcription factors, signaling molecules, and membrane receptors.

CONCLUSION

The scope of osteoimmunology has grown to encompass a wide range of molecular and cellular interactions, the elucidation of which will provide a scientific basis for future therapeutic approaches to diseases of both the immune and skeletal systems.

RANK/RANKL: regulators of immune responses and bone physiology

Bone-related diseases, such as osteoporosis and rheumatoid arthritis, affect hundreds of millions of people worldwide and pose a tremendous burden to health care. By deepening our understanding of the molecular mechanisms of bone metabolism and bone turnover, it became possible over the past years to devise new and promising strategies for treating such diseases. In particular, three tumor necrosis factor (TNF) family molecules, the receptor activator of NF-kappaB (RANK), its ligand RANKL, and the decoy receptor of RANKL, osteoprotegerin (OPG), have attracted the attention of scientists and pharmaceutical companies alike. Genetic experiments revolving around these molecules established their pivotal role as central regulators of osteoclast development and osteoclast function. RANK-RANKL signaling not only activates a variety of downstream signaling pathways required for osteoclast development, but crosstalk with other signaling pathways also fine-tunes bone homeostasis both in normal physiology and disease. In addition, RANKL and RANK have essential roles in lymph node formation, establishment of the thymic microenvironment, and development of a lactating mammary gland during pregnancy. Consequently, novel drugs specifically targeting RANK, RANKL, and their signaling pathways in osteoclasts are expected to revolutionize the treatment of various ailments associated with bone loss, such as arthritis, periodontal disease, cancer metastases, and osteoporosis.

RANKL/RANK as key factors for osteoclast development and bone loss in arthropathies

Osteoporosis or rheumatoid arthritis are bone diseases affecting hundreds of millions of people worldwide and thus pose a tremendous burden to health care. Ground-breaking discoveries made in basic science over the last decade shed light on the molecular mechanisms of bone metabolism and bone turnover. Thereby, it became possible over the past years to devise new and promising strategies for treating such diseases. In particular, three molecules, the receptor activator of NF-kappaB (RANK), its ligand RANKL and the decoy receptor of RANKL, osteoprotegerin (OPG), have been a major focus of scientists and pharmaceutical companies alike, since experiments using mice in which these genes have been inactivated unanimously established their pivotal role as central regulators ofosteoclast function. RANK(L) signaling not only activates a variety of downstream signaling pathways required for osteoclast development, but crosstalk with other signaling pathways also fine-tunes bone homeostasis both in normal physiology and disease. Consequently, novel drugs specifically targeting RANK-RANKL and their signaling pathways in osteoclasts are expected to revolutionize the treatment ofvarious bone diseases, such as cancer metastases, osteoporosis, or arthropathies.

Osteoclasts and the immune system

Investigation into arthritis as well as the numerous bone phenotypes found in mice lacking immune-related genes has highlighted the importance of the dynamic interplay between the bone and immune systems. It has recently led to both the emergence and subsequent rapid evolution of the field of osteoimmunology. Receptor activator of nuclear factor-kappaB ligand (RANKL) stimulates osteoclastogenesis through the nuclear factor of activated T cells, cytoplasmic 1 (NFATc1), which is well known as a crucial regulator of immunity. Studies on RANKL signaling revealed various immune-related genes which are involved in the regulation of osteoclastogenesis. Bone destruction in rheumatoid arthritis is caused by the enhanced activity of osteoclasts resulting from the activation of T cells. Here we describe our efforts to address the challenging question as to how abnormal T-cell activation mechanistically induces bone destruction. The scope of osteoimmunology has been extended to encompass a wide range of molecular and cellular interactions, the elucidation of which will provide a scientific basis for future therapeutic approaches to diseases related to both the bone and immune systems.

ADAM function in embryogenesis

Cleavage of proteins inserted into the plasma membrane (shedding) is an essential process controlling many biological functions including cell signaling, cell adhesion and migration as well as proliferation and differentiation. ADAM surface metalloproteases have been shown to play an essential role in these processes. Gene inactivation during embryonic development have provided evidence of the central role of ADAM proteins in nematodes, flies, frogs, birds and mammals. The relative contribution of four subfamilies of ADAM proteins to developmental processes is the focus of this review.

Activation of the epidermal growth factor receptor (EGFR) by a novel metalloprotease pathway

Neutrophil Elastase (NE) is a pro-inflammatory protease present at higher than normal levels in the lung during inflammatory disease. NE regulates IL-8 production from airway epithelial cells and can activate both EGFR and TLR4. TACE/ADAM17 has been reported to trans-activate EGFR in response to NE. Here, using 16HBE14o-human bronchial epithelial cells we demonstrate a new mechanism by which NE regulates both of these events. A high molecular weight soluble metalloprotease activity detectable only in supernatants from NE-treated cells by gelatin and casein zymography was confirmed to be meprin alpha by Western immunoblotting. In vitro studies demonstrated the ability of NE to activate meprin alpha, which in turn could release soluble TGFalpha and induce IL-8 production from 16HBE14o- cells. These effects were abrogated by actinonin, a specific meprin inhibitor. NE-induced IL-8 expression was also inhibited by meprin alpha siRNA. Immunoprecipitation studies detected EGFR/TLR4 complexes in NE-stimulated cells overexpressing these receptors. Confocal studies confirmed colocalization of EGFR and TLR4 in 16HBE14o- cells stimulated with meprin alpha. NFkappaB was also activated via MyD88 in these cells by meprin alpha. In bronchoalveolar lavage fluid from NE knock-out mice infected intra-tracheally with Pseudomonas aeruginosa meprin alpha was significantly decreased compared with control mice, and was significantly increased and correlated with NE activity, in bronchoalveolar lavage fluid from individuals with cystic fibrosis but not healthy controls. The data describe a previously unidentified lung metalloprotease meprin alpha, and its role in NE-induced EGFR and TLR4 activation and IL-8 production.

Cathepsin G enhances mammary tumor-induced osteolysis by generating soluble receptor activator of nuclear factor-kappaB ligand

Breast cancer commonly causes osteolytic metastases in bone, a process that is dependent on tumor-stromal interaction. Proteases play an important role in modulating tumor-stromal interactions in a manner that favors tumor establishment and progression. Whereas several studies have examined the role of proteases in modulating the bone microenvironment, little is currently known about their role in tumor-bone interaction during osteolytic metastasis. In cancer-induced osteolytic lesions, cleavage of receptor activator of nuclear factor-kappaB ligand (RANKL) to a soluble version (sRANKL) is critical for widespread osteoclast activation. Using a mouse model that mimics osteolytic changes associated with breast cancer-induced bone metastases, we identified cathepsin G, cathepsin K, matrix metalloproteinase (MMP)-9, and MMP13 to be proteases that are up-regulated at the tumor-bone interface using comparative cDNA microarray analysis and quantitative reverse transcription-PCR. Moreover, we showed that cathepsin G is capable of shedding the extracellular domain of RANKL, generating active sRANKL that is capable of inducing differentiation and activation of osteoclast precursors. The major source of cathepsin G at the tumor-bone interface seems to be osteoclasts that up-regulate production of cathepsin G via interaction with tumor cells. Furthermore, we showed that in vitro osteoclastogenesis is reduced by inhibition of cathepsin G in a coculture model and that in vivo inhibition of cathepsin G reduces mammary tumor-induced osteolysis. Together, our data indicate that cathepsin G activity at the tumor-bone interface plays an important role in mammary tumor-induced osteolysis and suggest that cathepsin G is a potentially novel therapeutic target in the treatment of breast cancer bone metastasis.

Pervanadate-induced shedding of the intercellular adhesion molecule (ICAM)-1 ectodomain is mediated by membrane type-1 matrix metalloproteinase (MT1-MMP)

In several vascular diseases, the ectodomain of intercellular adhesion molecule (ICAM)-1 is shed by the proteolytic activity of a zinc-dependent endopeptidase, releasing a soluble form of the protein (sICAM-1), a common marker for inflammatory diseases. Since reactive oxygen species (ROS) generated during prolonged inflammation are known to induce shedding or cleavage of several transmembrane proteins, we sought to explore the cleavage and enzymatic effects that the pervanadate, via oxidation and subsequent inactivation of protein tyrosine phosphatase, has on ICAM-1 cleavage. In these studies, we used endothelial cells (ECs) and 293 human embryonic kidney (HEK) cells expressing high-levels of surface ICAM-1. In addition, use of specific tissue inhibitors of metalloproteinases (TIMPs), small interfering (si)RNA designed to knockdown endopeptidase activity, and an immunocolocalization assay were employed to determine the identity of a specific metalloproteinase mediating pervanadate-induced sICAM-1 shedding. Our data indicate that membrane type-1 matrix metalloproteinase (MT1-MMP) is involved in pervanadate-mediated shedding of the sICAM-1 ectodomain in both cell types. Immunostaining and confocal microscopy provide visual evidence that ICAM-1 and MT1-MMP colocalize at the cellular surface following pervanadate treatment, further implicating the involvement of MT1-MMP activity in this mode of ICAM-1 shedding.

The dynamic interplay between osteoclasts and the immune system

Investigation into arthritis, as well as numerous bone phenotypes found in mice lacking immune-related genes, has highlighted the importance of the interplay between the bone and immune systems, which has led to the emergence and evolution of the field of osteoimmunology. RANKL stimulates osteoclastogenesis through nuclear factor of activated T cells (NFAT) c1, which is also a crucial regulator of immunity. In rheumatoid arthritis, bone destruction is caused by the enhanced activity of osteoclasts, which is mainly dependent on interleukin-17-producing helper T cells (T(H)17). The scope of osteoimmunology has been extended to encompass a wide range of molecular and cellular interactions. The framework of osteoimmunology will provide a scientific basis for future therapeutic approaches to diseases related to both of these systems.

Intracellular and surface RANKL are differentially regulated in patients with ankylosing spondylitis

Ankylosing spondylitis (AS) is characterized by ankylosis of axial joints but osteoporosis is also a well-reported feature. T cells have been implicated as a source of receptor activator of NFkappaB ligand (RANKL) in inflammatory bone diseases. Hence, we assessed whether T cells in patients with AS act as a source of RANKL too. Therefore, we investigated the expression of RANKL on T cells from 21 patients with AS by flow cytometry. Bone mineral density (BMD) was evaluated by quantitative computer tomography (QCT) and dual X-ray absorptiometry (DXA) and correlated with serum levels of osteoprotegerin (OPG) and RANKL. BMD was decreased in 45% of all patients when measured with DXA (48% with QCT) and correlated negatively with OPG. Expression of intracellular RANKL was increased on CD4+ (84 vs. 70%) and CD8+ (85.2 vs. 65.3%, P < 0.05) T cells in patients with AS, whereas expression of membrane-bound RANKL was significantly lower (CD4+: 2.2 vs. 8.5% and CD8+: 0.7 vs. 3.2%, P < 0.01). Our results indicate that surface and intracellular RANKL production is differentially regulated on T cells of patients with AS.

Tumor Necrosis Factor-α-converting Enzyme (TACE) Levels in Periodontal Diseases

Tumor necrosis factor-α-converting enzyme (TACE) is a metalloprotease which can shed several cytokines from the cell membrane, including receptor activator of NF-κB ligand (RANKL). This study aimed to investigate the hypothesis that TACE would be elevated in the gingival crevicular fluid (GCF) of persons with periodontitis. Total TACE amounts in GCF were higher in persons with chronic and aggressive periodontitis than in those with gingivitis or in healthy persons. TACE concentrations in GCF were higher in persons with chronic and aggressive periodontitis than in those with gingivitis, although not significantly higher than in healthy persons. Persons with chronic periodontitis receiving immunosuppressive treatment exhibited over 10-fold lower TACE levels than the other periodontitis groups. TACE was positively correlated with probing pocket depth, clinical attachment levels, and RANKL concentrations in GCF. In conclusion, the increased GCF TACE levels in persons with periodontitis and their positive correlation with RANKL may indicate an association of this enzyme with alveolar bone loss, and may warrant special attention in future therapeutic approaches.

Shedding of the urinary biomarker kidney injury molecule-1 (KIM-1) is regulated by MAP kinases and juxtamembrane region

Kidney injury molecule-1 (KIM-1) is markedly upregulated in renal proximal tubule cells by stimuli that promote dedifferentiation, including ischemic or toxic injury, as well as in cases of tubulointerstitial disease, polycystic kidney disease, and renal cell carcinoma. Structurally, KIM-1 possesses a single transmembrane domain and undergoes membrane-proximal cleavage, which leads to the release of soluble KIM-1 ectodomain into the urine. Urinary KIM-1 ectodomain is a promising sensitive and specific biomarker for acute kidney injury in humans, and therefore it is important to determine what regulates KIM-1 shedding. We found that constitutive cleavage of KIM-1 is mediated by ERK activation, and that cleavage is accelerated by p38 MAP kinase activation. After cleavage, a 14-kD membrane-bound fragment of KIM-1, which contains two highly conserved tyrosine residues, was tyrosine-phosphorylated. Mutagenesis studies demonstrated that the juxtamembrane secondary structure, not the primary amino acid sequence, was critical to the cleavage of KIM-1.

Circulating RANKL is inversely related to RANKL mRNA levels in bone in osteoarthritic males

Introduction

The relationship of circulating levels of receptor activator of nuclear factor-κB ligand (RANKL) and osteoprotegerin (OPG) with the expression of these molecules in bone has not been established. The objective of this study was to measure, in humans, the serum levels of RANKL and OPG, and the corresponding levels in bone of mRNA encoding these proteins.

Methods

Fasting blood samples were obtained on the day of surgery from patients presenting for hip replacement surgery for primary osteoarthritis (OA). Intraoperatively, samples of intertrochanteric trabecular bone were collected for analysis of OPG and RANKL mRNA, using real time RT-PCR. Samples were obtained from 40 patients (15 men with age range 50 to 79 years, and 25 women with age range 47 to 87 years). Serum total RANKL and free OPG levels were measured using ELISA.

Results

Serum OPG levels increased over the age range of this cohort. In the men RANKL mRNA levels were positively related to age, whereas serum RANKL levels were negatively related to age. Again, in the men serum RANKL levels were inversely related (r = -0.70, P = 0.007) to RANKL mRNA levels. Also in the male group, RANKL mRNA levels were associated with a number of indices of bone structure (bone volume fraction relative to bone tissue volume, specific surface of bone relative to bone tissue volume, and trabecular thickness), bone remodelling (eroded surface and osteoid surface), and biochemical markers of bone turnover (serum alkaline phosphatase and osteocalcin, and urinary deoxypyridinoline).

Conclusion

This is the first report to show a relationship between serum RANKL and the expression of RANKL mRNA in bone.

Neurosecretases provide strategies to treat sporadic and familial Alzheimer disorders

Recent discoveries on neurosecretases and their trafficking to release fibril-forming neuropeptides or other products, are of interest to pathology, cell signaling and drug discovery. Nomenclature arose from the use of amyloid precursor protein (APP) as a prototypic type-1 substrate leading to the isolation of beta-secretase (BACE), multimeric complexes (gamma-secretase, gamma-SC) for intramembranal cleavage, and attributing a new function to well-characterized metalloproteases of the ADAM family (alpha-secretase) for normal APP turnover. While purified alpha/beta-secretases facilitate drug discovery, gamma-SC presents greater challenges for characterization and mechanisms of catalysis. The review comments on links between mutation or polymorphisms in relation to enzyme mechanisms and disease. The association between lipoprotein receptor LRP11 variants and sporadic Alzheimer's disease (SAD) offers scope to integrate components of pre- and post-Golgi membranes, or brain clathrin-coated vesicles within pathways for trafficking as targets for intervention. The presence of APP and metabolites in brain clathrin-coated vesicles as significant cargo with lipoproteins and adaptors focuses attention as targets for therapeutic intervention. This overview emphasizes the importance to develop new therapies targeting neurosecretases to treat a major neurological disorder that has vast economic and social implications.

Proteases, growth factors, chemokines, and the microenvironment in prostate cancer bone metastasis

The arrival of cancer cells in the marrow upsets the delicate homeostatic nature of the bone microenvironment. Cell surface or secreted factors brought in by cancer cells perturb the web-like communication network between different bone cell types and bone matrix. Chemokines not only attract cancer cells from the circulation into the marrow, they also stimulate a cell signaling process leading to attachment, invasion, and further stimulation of bone matrix turnover. Cancer cell surface-associated proteases have also been associated with tumor growth and bone matrix turnover. Recent data indicate that autocrine proteolytic shedding of cell surface chemokines further promotes osteoclastogenesis. Proteases also contribute to autocrine and paracrine shedding of growth factors, another mechanism of promoting growth and expansion of the metastatic deposit. Studies of the bone microenvironment have thus revealed multiple potential targets of intervention with regard to the expanding metastatic deposit.

The biochemical, biological, and pathological kaleidoscope of cell surface substrates processed by matrix metalloproteinases

Matrix metalloproteinases (MMPs) constitute a family of more than 20 endopeptidases. Identification of specific matrix and non-matrix components as MMP substrates showed that, aside from their initial role as extracellular matrix modifiers, MMPs play significant roles in highly complex processes such as the regulation of cell behavior, cell-cell communication, and tumor progression. Thanks to the comprehensive examination of the expanded MMP action radius, the initial view of proteases acting in the soluble phase has evolved into a kaleidoscope of proteolytic reactions connected to the cell surface. Important classes of cell surface molecules include adhesion molecules, mediators of apoptosis, receptors, chemokines, cytokines, growth factors, proteases, intercellular junction proteins, and structural molecules. Proteolysis of cell surface proteins by MMPs may have extremely diverse biological implications, ranging from maturation and activation, to inactivation or degradation of substrates. In this way, modification of membrane-associated proteins by MMPs is crucial for communication between cells and the extracellular milieu, and determines cell fate and the integrity of tissues. Hence, insights into the processing of cell surface proteins by MMPs and the concomitant effects on physiological processes as well as on disease onset and evolution, leads the way to innovative therapeutic approaches for cancer, as well as degenerative and inflammatory diseases.