Biology of RANK, RANKL, and osteoprotegerin

CC-chemokine ligand 20/macrophage inflammatory protein-3α and CC-chemokine receptor 6 are overexpressed in myeloma microenvironment related to osteolytic bone lesions

The expression of the chemokine CC-chemokine ligand 20 (CCL20)/macrophage inflammatory protein (MIP)-3alpha and its receptor CC-chemokine receptor 6 (CCR6) by multiple myeloma (MM) and microenvironment cells and their potential relationship with osteoclast (OC) formation and osteolytic bone lesions in MM patients was investigated in this study. First, we found that MM cells rarely produce CCL20/MIP-3alpha but up-regulate its production by bone marrow (BM) osteoprogenitor cells and osteoblasts in coculture with the involvement of soluble factors as interleukin-1beta and tumor necrosis factor alpha. MM cells also stimulate both CCL20/MIP-3alpha and CCR6 expression by OCs in coculture. Thereafter, we showed that CCL20/MIP-3alpha significantly increases both the number of multinucleated tartrate-resistant acid phosphatase-positive OCs and receptor activator of nuclear factor-kappaB-positive OC progenitor cells similar to CCL3/MIP-1alpha. Finally, we found that blocking anti-CCL20/MIP-3alpha and anti-CCR6 antibodies significantly inhibits MM-induced OC formation. In vitro data were further expanded in vivo analyzing a total number of 64 MM patients. Significantly higher CCL20/MIP-3alpha levels were detected in MM patients versus monoclonal gammopathy of uncertain significance (MGUS) subjects and in MM osteolytic patients versus nonosteolytic ones. Moreover, a significant increase of CCL20/MIP-3alpha-positive osteoblasts in osteolytic MM patients compared with nonosteolytic ones was observed. Interestingly, no significant difference in BM CCL20/MIP-3alpha expression and level was observed between MGUS and nonosteolytic MM patients. Our data indicate that CCL20/MIP-3alpha and its receptor CCR6 are up-regulated in the bone microenvironment by MM cells and contribute to OC formation and osteolytic bone lesions in MM patients.

Insights in the application of research-grade diagnostic kits for biomarker assessments in support of clinical drug development: bioanalysis of circulating concentrations of soluble receptor activator of nuclear factor kappaB ligand

Application of research-grade diagnostic kits in clinical drug development has grown commensurate with the increased interest in utilization of biomarkers as drug development tools. Since novel biomarkers are frequently macromolecular, immunoassay methodology comprises the 'technology-of-choice' for biomarker quantification. In particular, commercial research-grade immunoassay kits are appealing for use in biomarker quantification during clinical phase drug development because of their ready availability, ease of operation and perceived convenience. However, bioanalytical validation issues arise often during the application of commercial kits, as GLP regulatory-compliant application places greater demands on kit design and performance. In this review, we have used the receptor activator of nuclear factor kappaB ligand (RANKL) as a model system to offer some insights into the challenges that can be encountered in the application of 'research-grade' diagnostic kits in support of clinical drug development. Currently only a few assays are available commercially for the determination of circulating concentrations of sRANKL. Of these, two immunoassay designs have been most often. The first design employs human osteoprotegerin to capture unbound sRANKL from serum and, thereby, provides a measure of circulating free concentrations. In contrast, the other common assay design first involves preincubation of serum samples with human osteoprotegerin to convert the free fraction of sRANKL to the osteoprotegerin-bound complex. The bound fraction is subsequently captured by an anti-osteoprotegerin antibody. In both immunoassay designs, detection is accomplished with an anti-sRANKL enzyme conjugation system. In this report we review these sRANKL immunoassay designs critically from the perspective of their potential suitability as drug development biomarker tools. In addition, analytical challenges relevant to the application of these 'research-grade' diagnostic kits for regulatory-compliant determination of sRANKL concentrations are discussed.

Clastic cells: mineralized tissue resorption in health and disease

Clastic cells are responsible for mineralized tissue resorption. Bone resorbing cells are called osteoclasts; however, they are able to resorb mineralized dental tissues or calcified cartilage and then they are called odontoclasts and chondroclasts, respectively. They derive from mononuclear precursors of the monocyte-macrophage lineage from hemopoietic tissue, reach target mineralized tissues and degrade them under many different physiologic or pathologic stimuli. Clastic cells play a key role in calcium homeostasis, and participate in skeletal growth, tooth movement, and other physiological and pathological events. They interact tightly with forming cells in bone and dental hard tissues; their unbalance may result in disturbed resorptive activity thus, causing local or systemic diseases.

Paracrine inhibition of osteoblast differentiation induced by neuroblastoma cells

The aim of our study was to investigate whether the defective function of osteogenic cells induced by neuroblastoma might play a role in the development of skeletal metastases. This mechanism has been extensively demonstrated for multiple myeloma, in which the blockage of osteoblast differentiation has been ascribed to the inhibitors of canonical Wingless pathway (Wnt), namely Dickkopf 1 (Dkk1). Our purpose was to verify if neuroblastoma cells derived from bone marrow metastases (SH-SY5Y, LAN1) or primaries (NB100, CHP212) hamper the differentiation of mesenchymal stem cells (hMSCs) into osteoblasts in a paracrine manner, and to test whether this ability depends on Dkk1 activity. We found that all neuroblastoma cells increased the proliferation of hMSCs collected from pediatric-aged donors, with a corresponding decrease in osteoblast differentiation markers, including alkaline phosphatase (ALP), analyzed as gene expression, enzymatic activity and number of ALP-positive colony forming units, osteoprotegerin (OPG) release, OPG and osteocalcin gene-expression. Dkk1 mRNA and protein were detectable in all cell lines, and the use of neutralizing anti-Dkk1 antibody reversed the effects induced by SH-SY5Y cells. Taken together, our results confirm that neuroblastoma hinders osteoblastogenesis, and that Dkk1 release seems to play a crucial role in blocking the differentiation of osteoprogenitor cells, though the ability to promote osteoclast activation remains an essential requirement for the development of skeletal metastases. Finally, our findings suggest that strategies regulating Wnt signaling and Dkk1 activity could be considered for adjuvant therapies in neuroblastoma metastasizing to the skeleton.

Biological response of human bone cells to zinc-modified Ca-Si-based ceramics

Calcium silicate (CaSiO(3)) ceramics have received considerable attention in recent years due to their excellent bioactivity and degradability. However, their poor chemical stability limits their biological applications. Hardystonite (Ca(2)ZnSi(2)O(7)) ceramics are Ca-Si-based materials developed by incorporating zinc into the Ca-Si system to improve their chemical stability. However, the biological responses of Ca(2)ZnSi(2)O(7) to bone cells are unknown. The objective of this study is to investigate and compare the in vitro responses of human osteoblast-like cells (HOBs) and osteoclasts when cultured on Ca(2)ZnSi(2)O(7) and CaSiO(3) ceramic disks. The ability of Ca(2)ZnSi(2)O(7) ceramics to support HOB attachment, cytoskeleton organization, proliferation and differentiation was assessed by scanning electron microscopy, confocal microscopy, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, alkaline phosphatase activity and quantitative real-time polymerase chain reaction. Our results show that Ca(2)ZnSi(2)O(7) supported HOB attachment with a well-organized cytoskeleton structure, and significantly increased cellular proliferation and differentiation compared to CaSiO(3). In addition, Ca(2)ZnSi(2)O(7) showed increased expression levels of osteoblast-related mRNAs (alkaline phosphatase, collagen type I, osteocalcin, receptor activator of NF(kappa)B ligand and osteoprotegerin) compared to CaSiO(3). Ca(2)ZnSi(2)O(7) ceramic supported the formation of mature and functional osteoclasts and formed resorption imprints. On CaSiO(3) ceramics, the cells failed to differentiate from the monocytes into osteoclasts. Taken together, these results indicate that Hardystonite ceramics are conducive to both types of bone cells, osteoblast-like cells and osteoclasts, suggesting their potential use for skeletal tissue regeneration and as coatings onto currently available orthopedic and dental implants.

The roles of the classical and alternative nuclear factor-kappaB pathways: potential implications for autoimmunity and rheumatoid arthritis

Nuclear factor-κB (NF-κB) is an inducible transcription factor controlled by two principal signaling cascades, each activated by a set of signal ligands: the classical/canonical NF-κB activation pathway and the alternative/noncanonical pathway. The former pathway proceeds via phosphorylation and degradation of inhibitor of NF-κB (IκB) and leads most commonly to activation of the heterodimer RelA/NF-κB1(p50). The latter pathway proceeds via phosphorylation and proteolytic processing of NF-κB2 (p100) and leads to activation, most commonly, of the heterodimer RelB/NF-κB2 (p52). Both pathways play critical roles at multiple levels of the immune system in both health and disease, including the autoimmune inflammatory response. These roles include cell cycle progression, cell survival, adhesion, and inhibition of apoptosis. NF-κB is constitutively activated in many autoimmune diseases, including diabetes type 1, systemic lupus erythematosus, and rheumatoid arthritis (RA). In this review we survey recent developments in the involvement of the classical and alternative pathways of NF-κB activation in autoimmunity, focusing particularly on RA. We discuss the involvement of NF-κB in self-reactive T and B lymphocyte development, survival and proliferation, and the maintenance of chronic inflammation due to cytokines such as tumor necrosis factor-α, IL-1, IL-6, and IL-8. We discuss the roles played by IL-17 and T-helper-17 cells in the inflammatory process; in the activation, maturation, and proliferation of RA fibroblast-like synovial cells; and differentiation and activation of osteoclast bone-resorbing activity. The prospects of therapeutic intervention to block activation of the NF-κB signaling pathways in RA are also discussed.

Complex dynamics of osteoclast formation and death in long-term cultures

BACKGROUND

Osteoclasts, cells responsible for bone resorption, contribute to the development of degenerative, metabolic and neoplastic bone diseases, which are often characterized by persistent changes in bone microenvironment. We aimed to investigate the dynamics of osteoclast formation and death in cultures that considerably exceeded the length of standard protocol and to design a mathematical model describing osteoclastogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

RAW 264.7 monocytic cells fuse to form multinucleated osteoclasts upon treatment with pro-resorptive cytokine RANKL. We have found that in long-term experiments (15-26 days), the dynamics of changes in osteoclast numbers was remarkably complex and qualitatively variable in different experiments. Whereas 19 of 46 experiments exhibited single peak of osteoclast formation, in 27 experiments we observed development of successive waves of osteoclast formation and death. Periodic changes in osteoclast numbers were confirmed in long-term cultures of mouse bone marrow cells treated with M-CSF and RANKL. Because the dynamics of changes in osteoclast numbers was found to be largely independent of monocytes, a two-species model of ordinary differential equations describing the changes in osteoclasts and monocytes was ineffective in recapitulating the oscillations in osteoclast numbers. Following experimental observation that medium collected from mature osteoclasts inhibited osteoclastogenesis in fresh cultures, we introduced a third variable, factor f, to describe osteoclast-derived inhibitor. This model allowed us to simulate the oscillatory changes in osteoclasts, which were coupled to oscillatory changes in the factor f, whereas monocytes changed exponentially. Importantly, to achieve the experimentally observed oscillations with increasing amplitude, we also had to assume that osteoclast presence stimulates osteoclast formation.

CONCLUSIONS/SIGNIFICANCE

This study identifies the critical role for osteoclast autocrine regulation in controlling long-term dynamic of osteoclast formation and death and describes the complementary roles for negative and positive feedback mediators in determining the sharp dynamics of activation and inactivation of osteoclasts.

Transcriptional regulation of bone marrow thrombopoietin by platelet proteins

Platelet production is regulated primarily by the cytokine thrombopoietin (TPO). Although TPO is expressed in several different tissues, only in the bone marrow has the level of expression been reported to increase in response to reduced numbers of platelets. In these studies, we demonstrate that platelet granule proteins are able to transcriptionally repress TPO mRNA expression in a marrow stromal cell line as well as in primary bone marrow stromal cell cultures. Like TPO mRNA, secretion of TPO protein was also suppressed by serum treatment. Reporter gene constructs indicate that DNA elements located in an approximately 1.9-kb region between 250-bp upstream of the transcriptional initiation site and the middle of the second intron are able to mediate the transcriptional repression.

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.

Functions of RANKL/RANK/OPG in bone modeling and remodeling

The discovery of the RANKL/RANK/OPG system in the mid 1990s for the regulation of bone resorption has led to major advances in our understanding of how bone modeling and remodeling are regulated. It had been known for many years before this discovery that osteoblastic stromal cells regulated osteoclast formation, but it had not been anticipated that they would do this through expression of members of the TNF superfamily: receptor activator of NF-kappaB ligand (RANKL) and osteoprotegerin (OPG), or that these cytokines and signaling through receptor activator of NF-kappaB (RANK) would have extensive functions beyond regulation of bone remodeling. RANKL/RANK signaling regulates osteoclast formation, activation and survival in normal bone modeling and remodeling and in a variety of pathologic conditions characterized by increased bone turnover. OPG protects bone from excessive resorption by binding to RANKL and preventing it from binding to RANK. Thus, the relative concentration of RANKL and OPG in bone is a major determinant of bone mass and strength. Here, we review our current understanding of the role of the RANKL/RANK/OPG system in bone modeling and remodeling.

Vascular compliance and arterial calcification: impact on blood pressure reduction

PURPOSE OF REVIEW

The aim of this article is to review the relationship between vascular calcification and difficult to control hypertension. This does not address antihypertensive treatment of drug resistant hypertension per se.

RECENT FINDINGS

Vascular calcification occurs in a variety of common hypertension scenarios. Basic mechanisms of how and why vessels calcify are reviewed including new genetic insights. The potential for contributing to or improving calcification through drug therapies for nonhypertensive disorders is reviewed.

SUMMARY

Vascular calcification is common and easily recognized. Studies that target its clinical consequences (arterial stiffness) as primary treatment goals are needed.