Osteoprotegerin reduces osteoclast numbers and prevents bone erosion in collagen-induced arthritis

Rheumatoid arthritis is characterized by progressive synovial inflammation and joint destruction. While matrix metalloproteinases (MMPs) are implicated in the erosion of unmineralized cartilage, bone destruction involves osteoclasts, the specialized cells that resorb calcified bone matrix. RANK ligand (RANKL) expressed by stromal cells and T cells, and its cognate receptor, RANK, were identified as a critical ligand-receptor pair for osteoclast differentiation and survival. A decoy receptor for RANKL, osteoprotegerin, (OPG) impinges on this system and regulates osteoclast numbers and activity. RANKL is also expressed in collagen-induced arthritis (CIA) in which focal collections of osteoclasts are prominent at sites of bone destruction. To determine the role of RANK signaling events in the effector phase of CIA, we investigated effects of Fc-osteoprotegerin fusion protein (Fc-OPG) in CIA. After induction of CIA in Dark Agouti rats, test animals were treated with or without Fc-OPG (3 mg/kg/day) subcutaneously for 5 days, beginning at the onset of disease. Paraffin-embedded joints were then analyzed histologically and the adjacent bone assessed by histomorphometry. Osteoclasts were identified using TRAP staining and expression of the mRNA for OPG and RANKL was identified by in situ hybridization. The results indicated that short-term Fc-OPG effectively prevented joint destruction, even though it had no impact on the inflammatory aspects of CIA. In arthritic joints, Fc-OPG depleted osteoclast numbers by over 75% and diminished bone erosion scores by over 60%. Although cartilage loss was also reduced by Fc-OPG, the effects on cartilage were less striking than those on bone. In arthritic joints OPG mRNA was highly expressed and co-localized with RANK ligand, and treatment with Fc-OPG did not affect the expression of endogenous RANKL or OPG mRNA. These data demonstrate that short term Fc-OPG treatment has powerful anti-erosive effects, principally on bone, even though synovitis is not affected. These findings indicate the potential utility of disrupting RANK signaling to preserve skeletal integrity in inflammatory arthritis.

The COOH-terminus of parathyroid hormone-related protein (PTHrP) interacts with beta-arrestin 1B

Parathyroid hormone-related protein (PTHrP) has a diverse range of proposed biological activities participating in both extracellular and intracellular signaling. In order to identify candidate protein effectors, yeast two-hybrid screens were conducted using mature human PTHrP (residues 1-141) and the COOH-terminus (residues 107-141). Both PTHrP baits interacted with a beta-arrestin 1B fragment, an important component of G-protein-coupled receptor desensitization and MAPK signaling. Co-immunoprecipitation, in vitro binding assays and colocalization experiments confirmed this interaction in human cells and this required residues 122-141 of PTHrP. These findings suggest that beta-arrestin 1 acts as an effector for a novel function of PTHrP in cytoplasm.

Parathyroid hormone-related protein (PTHrP)-(1-139) isoform is efficiently secreted in vitro and enhances breast cancer metastasis to bone in vivo

Parathyroid hormone-related peptide (PTHrP) is a mediator of local osteolysis due to breast cancer. Three isoforms of PTHrP, (1-139), (1-141), and (1-173), are products of alternative splicing in humans, but the specific contribution of each of these isoforms to osteolytic metastasis caused by breast cancer has not been evaluated. To determine the role of PTHrP isoforms in breast cancer metastasis to bone, the human breast cancer cell line MDA-MB-231 (MDA-231) was stably transfected with cDNAs for human prepro PTHrP-(1-139), -(1-141), or -(1-173). Stable MDA/PTHrP-(1-139) clones expressed more PTHrP mRNA and secreted more PTHrP protein, compared with MDA/PTHrP-(1-141), -(1-173), or parental MDA-231. Parental MDA-231 cells and clones expressing each isoform had similar growth rates in vitro. In a mouse model of bone metastases, the osteolytic lesion area of radiographs was greatest in mice bearing MDA/PTHrP-(1-139) compared with those bearing MDA/PTHrP-(1-141), -(1-173), or parental MDA-231. Ca(++) and plasma PTHrP concentrations were significantly higher in the MDA/PTHrP-(1-139) compared with the MDA/PTHrP-(1-141), -(1-173), or parental MDA-231 groups. These data demonstrate that the PTHrP-(1-139) isoform was produced to a greater extent than PTHrP-(1-141) or -(1-173), and in vivo enhanced osteolysis with increased plasma PTHrP concentrations and hypercalcemia compared with overexpression of PTHrP-(1-141) or -(1-173).

Increased expansion of the lung stimulates calmodulin 2 expression in fetal sheep

Obstruction of the fetal trachea causes the lungs to expand with accumulated liquid. Although this is a potent stimulus for lung growth, the mechanisms involved are unknown. Our aim was to identify genes that are differentially expressed as a result of increased fetal lung expansion. Using differential display RT-PCR, we isolated a cDNA fragment partially encoding calmodulin 2 (CALM2) and identified the remainder of the coding region by 5'-rapid amplification of cDNA ends. Differential expression of CALM2 was confirmed by Northern blot analysis; CALM2 mRNA levels were increased to 161 +/- 5% of control at 2 days of increased lung expansion, induced by tracheal obstruction (TO), and had returned to control levels at days 4 and 10. Using in situ hybridization analysis, we found that the proportion of CALM2-labeled cells increased from 10.3 +/- 1.0% to 21.4 +/- 6.8% by 2 days of TO. This increase in CALM2 expression was reflected by a tendency for calmodulin protein levels to increase from 122.7 +/- 17.3 to 156.5 +/- 17.7 at 2 days of TO. Thus increases in fetal lung expansion result in time-dependent changes in CALM2 mRNA levels, which closely parallels the changes in lung DNA synthesis rates. As calmodulin is essential for cell proliferation, increased CALM2 mRNA levels may reflect an important role for calmodulin in expansion-induced fetal lung growth.

Involvement of receptor activator of NFkappaB ligand and tumor necrosis factor-alpha in bone destruction in rheumatoid arthritis

Bone loss represents a major unsolved problem in rheumatoid arthritis (RA). The skeletal complications of RA consist of focal bone erosions and periarticular osteoporosis at sites of active inflammation, and generalized bone loss with reduced bone mass. New evidence indicates that osteoclasts are key mediators of all forms of bone loss in RA. TNF-alpha is one of the most potent osteoclastogenic cytokines produced in inflammation and is pivotal in the pathogenesis of RA. Production of tumor necrosis factor-alpha (TNF-alpha) and other proinflammatory cytokines in RA is largely CD4(+) T-cell dependent and mostly a result of interferon-gamma (IFN-gamma) secretion. Synovial T cells contribute to synovitis by secreting IFN-gamma and interleukin (IL)-17 as well as directly interacting with macrophages and fibroblasts through cell-to-cell contact mechanisms. Activated synovial T cells express both membrane-bound and soluble forms of receptor activator of NF-kappaB ligand (RANKL). In rheumatoid synovium, fibroblasts also provide an abundant source of RANKL. Furthermore, TNF-alpha and IL-1 target stromal-osteoblastic cells to increase IL-6, IL-11, and parathyroid hormone-related protein (PTHrP) production as well as expression of RANKL. In the presence of permissive levels of RANKL, TNF-alpha acts directly to stimulate osteoclast differentiation of macrophages and myeloid progenitor cells. In addition, TNF-alpha induces IL-1 release by synovial fibroblasts and macrophages, and IL-1, together with RANKL, is a major survival and activation signal for nascent osteoclasts. Consequently, TNF-alpha and IL-1, acting in concert with RANKL, can powerfully promote osteoclast recruitment, activation, and osteolysis in RA. The most convincing support for this hypothesis has come from in vivo studies of animal models. Protection of bone in the presence of continued inflammation in arthritic rats treated with osteoprotegerin (OPG) supports the concept that osteoclasts mediate bone loss, providing further evidence that OPG protects bone integrity by downregulating osteoclastogenesis and promoting osteoclast apoptosis. Modulation of the RANKL/OPG equilibrium in arthritis may provide additional skeletal benefits, such as chondroprotection. The nexus between T-cell activation, TNF-alpha overproduction, and the RANKL/OPG/RANK ligand-receptor system points to a unifying paradigm for the entire spectrum of skeletal pathology in RA. Strategies that address osteoclastic bone resorption will represent an important new facet of therapy for RA.

Nuclear transport of parathyroid hormone (PTH)-related protein is dependent on microtubules

PTH-related protein (PTHrP) was first discovered as a circulating factor secreted by certain cancers and is responsible for the syndrome of humoral hypercalcemia of malignancy induced by various tumors. The similarity of its N terminus to that of PTH enables PTHrP to share the signaling properties of PTH, but the rest of the molecule possesses distinct functions, including a role in the nucleus/nucleolus in reducing apoptosis and enhancing cell proliferation. PTHrP nuclear import is mediated by importin beta1. In this study we use the technique of fluorescence recovery after photobleaching to demonstrate the ability of PTHrP to shuttle between cytoplasm and nucleus and to visualize directly the transport of PTHrP into the nucleus in living cells. Endogenous and transfected PTHrP was demonstrated to colocalize with microtubule structures in situ using various high-resolution microscopic approaches, as well as in in vitro binding studies, where importin beta1, but not importin alpha, enhanced the microtubular association of PTHrP with microtubules. Significantly, the dependence of PTHrP nuclear import on microtubules was shown by the inhibitory effect of pretreatment with the microtubule-disrupting agent nocodazole on nuclear-cytoplasmic flux. These results indicate that PTHrP nuclear/nucleolar import is dependent on microtubule integrity and are consistent with a direct role for the cytoskeleton in protein transport to the nucleus.

The generation of highly enriched osteoclast-lineage cell populations

Osteoclasts form when hematopoietic cells are stimulated by macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-kappaB ligand (RANKL) or tumor necrosis factor-alpha (TNFalpha). Osteoclast precursors derive from M-CSF-dependent proliferating hematopoietic cells but cannot yet be purified from mixed populations. M-CSF stimulation of bone marrow cells results in large numbers of nonadherent, proliferating macrophage precursors. These rapidly form adherent bone marrow macrophages (BMM). BMM and their precursors can be isolated free from mesenchymal and lymphocytic cells. BMM precursors derived from CBA-strain mouse bone marrow, when cocultured with ST2 cells (which express RANKL and M-CSF), formed numerous mononuclear osteoclasts, which resorbed bone and expressed tartrate-resistant acid phosphatase (TRAP) and calcitonin receptors (CTR). Addition of approximately 10 BMM precursors to ST2 cultures resulted in over 80% of these cocultures forming functional osteoclasts, suggesting that they are a highly enriched source of osteoclast progenitors. Supporting this, recombinant RANKL/M-CSF-stimulated BMM precursors formed populations in which all cells expressed TRAP. While only a small proportion of these cells (8.6%) expressed CTR, with transforming growth factor-beta (TGFbeta) present RANKL/M-CSF-stimulated BMM precursors formed almost pure (98.4%) CTR-positive osteoclasts after 7 days. This suggests that TGFbeta stimulated the maturation rate of these cells. Passaged or viably frozen BMM precursors gave rise to BMM that also all formed osteoclasts lineage cells after RANKL/M-CSF stimulation. These data suggest that BMM precursors derived from CBA mice are an expanded pool of osteoclast progenitors. These can be employed to generate osteoclast populations of high purity and in large numbers when stimulated by TGFbeta, which greatly augments the osteoclastogenic effects of RANKL.

Dynamics of leptomycin B-sensitive nucleocytoplasmic flux of parathyroid hormone-related protein

Parathyroid hormone-related protein is responsible for hypercalcemia induced by various tumors. The similarity of its N-terminus to that of parathyroid hormone enables parathyroid hormone-related protein to share parathyroid hormone's signaling properties, but the rest of the molecule possesses distinct functions including a role in the nucleus/nucleolus in reducing apoptosis and enhancing cell proliferation. We have previously shown that parathyroid hormone-related protein nuclear import is mediated by importin beta1. Here we use fluorescence recovery after photobleaching for the first time to show that, in living cells, parathyroid hormone-related protein is exported from the nucleus in a leptomycin B-sensitive manner, implicating CRM1 as the parathyroid hormone-related protein nuclear export receptor. Leptomycin B treatment significantly reduced the rate of nuclear export 4 -10-fold, thereby increasing parathyroid hormone-related protein concentration in the nucleus/nucleolus about 2-fold. Intriguingly, this also led to a 2-fold reduced nuclear import rate. Inhibiting the nuclear export of a protein able to shuttle between nucleus and cytoplasm through distinct receptors thus can also affect nuclear import, indicating that the subcellular localization of a protein containing distinct nuclear import and export signals is the product of an integrated system. Although there have been several recent studies examining the dynamics of intranuclear transport using fluorescence recovery after photobleaching, this represents, to our knowledge, the first use of the technique to examine the kinetics of nucleocytoplasmic flux in living cells.

Transforming growth factor beta affects osteoclast differentiation via direct and indirect actions

Transforming growth factor beta (TGF-beta) is abundant in bone and has complex effects on osteolysis, with both positive and negative effects on osteoclast differentiation, suggesting that it acts via more than one mechanism. Osteoclastogenesis is determined primarily by osteoblast (OB) expression of the tumor necrosis factor (TNF)-related molecule receptor activator of NF-kappaB ligand (RANKL) and its decoy receptor osteoprotegerin (OPG), which are increased and decreased, respectively, by osteolytic factors. A RANKL-independent osteoclastogenic mechanism mediated by TNF-alpha has also been shown. Therefore, we investigated TGF-beta effects on osteoclast formation in culture systems in which osteoclastogenic stimulus is dependent on OBs and culture systems where it was provided by exogenously added RANKL or TNF-alpha. Both OPG and TGF-beta inhibited osteoclast formation in hemopoietic cell/OB cocultures, but the kinetics of their action differed. TGF-beta also inhibited osteoclastogenesis in cocultures of cells derived from OPG null (opg-/-) mice. TGF-beta strongly decreased RANKL messenger RNA (mRNA) expression in cultured osteoblasts, and addition of exogenous RANKL to TGFbeta-inhibited cocultures of opg-/- cells partially restored osteoclastogenesis. Combined, these data indicate that the inhibitory actions of TGF-beta were mediated mainly by decreased OB production of RANKL. In contrast, in the absence of OBs, TGF-beta greatly increased osteoclast formation in recombinant RANKL- or TNF-alpha-stimulated cultures of hemopoietic cells or RAW 264.7 macrophage-like cells to levels several-fold greater than attainable by maximal stimulation by RANKL or TNF-alpha. These data suggest that TGF-beta may increase osteoclast formation via action on osteoclast precursors. Therefore, although RANKL (or TNF-alpha) is essential for osteoclast formation, factors such as TGF-beta may powerfully modify these osteoclastogenic stimuli. Such actions may be critical to the control of physiological and pathophysiological osteolysis.

Bone morphogenetic protein 2 stimulates osteoclast differentiation and survival supported by receptor activator of nuclear factor-kappaB ligand

Bone is a major storage site for TGFbeta superfamily members, including TGFbeta and bone morphogenetic proteins. It is believed that these cytokines are released from bone during bone resorption. Recent studies have shown that both RANKL and macrophage colony-stimulating factor are two essential factors produced by osteoblasts for inducing osteoclast differentiation. In the present study we examined the effects of bone morphogenetic protein-2 on osteoclast differentiation and survival supported by RANKL and/or macrophage colony-stimulating factor. Mouse bone marrow-derived macrophages differentiated into osteoclasts in the presence of RANKL and macrophage colony-stimulating factor. TGFbeta superfamily members such as bone morphogenetic protein-2, TGFbeta, and activin A markedly enhanced osteoclast differentiation induced by RANKL and macrophage colony-stimulating factor, although each cytokine alone failed to induce osteoclast differentiation in the absence of RANKL. Addition of a soluble form of bone morphogenetic protein receptor type IA to the culture markedly inhibited not only osteoclast formation induced by RANKL and bone morphogenetic protein-2, but also the basal osteoclast formation supported by RANKL alone. Either RANKL or macrophage colony-stimulating factor stimulated the survival of purified osteoclasts. Bone morphogenetic protein-2 enhanced the survival of purified osteoclasts supported by RANKL, but not by macrophage colony-stimulating factor. Both bone marrow macrophages and mature osteoclasts expressed bone morphogenetic protein-2 and bone morphogenetic protein receptor type IA mRNAs. An EMSA revealed that RANKL activated nuclear factor-kappaB in purified osteoclasts. Bone morphogenetic protein-2 alone did not activate nuclear factor-kappaB, but rather inhibited the activation of nuclear factor-kappaB induced by RANKL in purified osteoclasts. These findings suggest that bone morphogenetic protein-mediated signals cross-communicate with RANKL-mediated ones in inducing osteoclast differentiation and survival. The enhancement of RANKL-induced survival of osteoclasts by bone morphogenetic protein-2 appears unrelated to nuclear factor-kappaB activation.

A novel osteoblast-derived C-type lectin that inhibits osteoclast formation

We have cloned and expressed murine osteoclast inhibitory lectin (mOCIL), a 207-amino acid type II transmembrane C-type lectin. In osteoclast formation assays of primary murine calvarial osteoblasts with bone marrow cells, antisense oligonucleotides for mOCIL increased tartrate-resistant acid phosphatase-positive mononucleate cell formation by 3-5-fold, whereas control oligonucleotides had no effect. The extracellular domain of mOCIL, expressed as a recombinant protein in Escherichia coli, dose-dependently inhibited multinucleate osteoclast formation in murine osteoblast and spleen cell co-cultures as well as in spleen cell cultures treated with RANKL and macrophage colony-stimulating factor. Furthermore, mOCIL acted directly on macrophage/monocyte cells as evidenced by its inhibitory action on adherent spleen cell cultures, which were depleted of stromal and lymphocytic cells. mOCIL completely inhibited osteoclast formation during the proliferative phase of osteoclast formation and resulted in 70% inhibition during the differentiation phase. Osteoblast OCIL mRNA expression was enhanced by parathyroid hormone, calcitriol, interleukin-1alpha and -11, and retinoic acid. In rodent tissues, Northern blotting, in situ hybridization, and immunohistochemistry demonstrated OCIL expression in osteoblasts and chondrocytes as well as in a variety of extraskeletal tissues. The overlapping tissue distribution of OCIL mRNA and protein with that of RANKL strongly suggests an interaction between these molecules in the skeleton and in extraskeletal tissues.

IL-12 alone and in synergy with IL-18 inhibits osteoclast formation in vitro

IL-12, like IL-18, was shown to potently inhibit osteoclast formation in cultures of cocultures of murine osteoblast and spleen cells, as well as in adult spleen cells treated with M-CSF and receptor activator of NF-kappaB ligand (RANKL). Neither IL-12 nor IL-18 was able to inhibit RANKL-induced osteoclast formation in cultured RAW264.7 cells, demonstrating that IL-12, like IL-18, was unable to act directly on osteoclastic precursors. IL-12, like IL-18, was found to act by T cells, since depletion of T cells from the adult spleen cell cultures ablated the inhibitory action of IL-12 and addition of either CD4 or CD8 T cells from C57BL/6 mice to RANKL-stimulated RAW264.7 cultures permitted IL-12 or IL-18 to be inhibitory. Additionally, IL-12 was still able to inhibit osteoclast formation in cocultures with osteoblasts and spleen cells from either GM-CSF R(-/-) mice or IFN-gamma R(-/-) mice, indicating that neither GM-CSF nor IFN-gamma was mediating osteoclast inhibition in these cultures. Combined, IL-18 and IL-12 synergistically inhibited osteoclast formation at concentrations 20- to 1000-fold less, respectively, than when added individually. A candidate inhibitor could not be demonstrated using neutralizing Abs to IL-4, IL-10, or IL-13 or from mRNA expression profiles among known cytokine inhibitors of osteoclastogenesis in response to IL-12 and IL-18 treatment, although the unknown inhibitory molecule was determined to be secreted from T cells.

Molecular dissection of the importin beta1-recognized nuclear targeting signal of parathyroid hormone-related protein

Produced by various types of solid tumors, parathyroid hormone-related protein (PTHrP) is the causative agent of humoral hypercalcemia of malignancy. The similarity of PTHrP's amino-terminus to that of parathyroid hormone enables it to share some of the latter's signalling properties, but its carboxy-terminus confers distinct functions including a role in the nucleus/nucleolus in reducing apoptosis and enhancing cell proliferation. PTHrP nuclear import occurs via a novel importin beta1-mediated pathway. The present study uses several different direct binding assays to map the interaction of PTHrP with importin beta using a series of alanine mutated PTHrP peptides and truncated human importin beta1 derivatives. Our results indicate that PTHrP amino acids 83-93 (KTPGKKKKGK) are absolutely essential for importin beta1 recognition with residues 71-82 (TNKVETYKEQPL) additionally required for high affinity binding; residues 380-643 of importin beta1 are required for the interaction. Binding of importin beta1 to PTHrP is reduced in the presence of the GTP-bound but not GDP-bound form of the guanine nucleotide binding protein Ran, consistent with the idea that RanGTP binding to importin beta is involved in the release of PTHrP into the nucleus following translocation across the nuclear envelope. This study represents the first detailed examination of a modular, non-arginine-rich importin beta1-recognized nuclear targeting signal.

Receptor activator of nuclear factor kappa B ligand (RANKL): another link between breast and bone

Mice rendered null for the genes encoding receptor activator of nuclear factor kappa B ligand (RANKL) or its receptor, RANK, are osteopetrotic because of failure of osteoclast development. The failure of lactation owing to the lack of development of lobulo-alveolar structures during pregnancy, despite earlier stages of mammary gland development being normal, is now added to each of these phenotypes. The breast phenotype in RANKL-/- (but not in RANK-/-) mice is rescued by treatment of pregnant mice with RANKL, indicating a key role for these tumour necrosis factor (TNF) ligand and receptor family members in a crucial terminal step in breast development and lactation. Both RANKL and RANK are synthesized by mammary epithelial cells, with both prolactin and parathyroid hormone-related protein (PTHrP) able to enhance production of mRNA for RANKL. These findings reveal a paracrine-autocrine system in lactation control, with novel signalling pathways that reflect intercellular communication processes in bone.

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.

Nuclear and nucleolar localization of parathyroid hormone-related protein

Parathyroid hormone-related protein (PTHrP) was first discovered as the factor causing hypercalcaemia produced by solid tumours frequently associated with the head and neck, breast, lung and kidney. The homology of its amino-terminus to parathyroid hormone (PTH; eight of the first 13 residues are identical), enables it to share the same receptor and perform similar biological functions to PTH. The sequences of PTHrP C-terminal to its PTH-like region confer functions such as transplacental calcium transport, renal bicarbonate excretion and in vitro osteoclast inhibition. Recent findings have shown that PTHrP is a nuclear/nucleolar protein in certain tissues and that this localization is cell cycle-regulated, mediated by the middle portion of the molecule, and involves the nuclear import receptor importin beta1. The present review discusses what is known about the pathway by which PTHrP localizes to the nucleus/nucleolus and the putative roles it may have there.

Fibroblastic stromal cells express receptor activator of NF-kappa B ligand and support osteoclast differentiation

Osteoclast formation in bone is supported by osteoblasts expressing receptor activator of NF-kappa B ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) expression. Numerous osteotropic factors regulate expression levels of RANKL and the RANKL decoy receptor osteoprotegerin (OPG) in osteoblasts, thereby affecting osteoclast differentiation. However, not only in RANKL widely expressed in soft tissues, but osteoclasts have been noted in extraskeletal lesions. We found that cultured skin fibroblastic cells express RANKL, M-CSF, and OPG messenger (mRNA). Stimulation by 1 alpha,25 dihydroxyvitamin D3 [1,25(OH)2D3] plus dexamethasone (Dex) augmented RANKL and diminished OPG mRNA expression in fibroblastic cells and caused the formation of numerous osteoclasts in cocultures of skin fibroblastic cells with hemopoietic cells or monocytes. The osteoclasts thus formed expressed tartrate-resistant acid phosphatase (TRAP) and calcitonin (CT) receptors and formed resorption pits in cortical bone. Osteoclast formation also was stimulated (in the presence of Dex) by prostaglandin E2 (PGE2), interleukin-11 (IL-11), IL-1, tumor necrosis factor-alpha (TNF-alpha), and parathyroid hormone-related protein (PTHrP), factors which also stimulate osteoclast formation supported by osteoblasts. In addition, granulocyte-macrophage-CSF (GM-CSF), transforming growth factor-beta (TGF-beta), and OPG inhibited osteoclast formation in skin fibroblastic cell-hemopoietic cell cocultures; CT reduced only osteoclast nuclearity. Fibroblastic stromal cells from other tissues (lung, respiratory diaphragm, spleen, and tumor) also supported osteoclast formation. Thus, RANKL-positive fibroblastic cells in extraskeletal tissues can support osteoclastogenesis if osteolytic factors and osteoclast precursors are present. Such mesenchymally derived cells may play a role in pathological osteolysis and may be involved in osteoclast formation in extraskeletal tissues.

Expression of osteoclast differentiation factor at sites of bone erosion in collagen-induced arthritis

OBJECTIVE

To investigate the cellular mechanism of bone destruction in collagen-induced arthritis (CIA).

METHODS

After induction of CIA in DA rats, a histologic study of the advanced arthritic lesion was carried out on whole, decalcified joints from the hindpaws of affected animals. To conclusively identify osteoclasts, joint tissue sections were stained for tartrate-resistant acid phosphatase (TRAP) enzyme activity, and calcitonin receptors (CTR) were identified using a specific rabbit polyclonal antibody. The expression of messenger RNA (mRNA) for the osteoclast differentiation factor (also known as receptor activator of nuclear factor kappaB ligand [RANKL]) was investigated using in situ hybridization with a specific riboprobe.

RESULTS

TRAP-positive and CTR-positive multinucleated cells were invariably detected in arthritic lesions that were characterized by bone destruction. Osteoclasts were identified at the pannus-bone and pannus-subchondral bone junctions of arthritic joints, where they formed erosive pits in the bone. TRAP-positive multinucleated cells were detected within synovium and at the bone erosive front; however, CTR-positive multinucleated cells were present only at sites adjacent to bone. RANKL mRNA was highly expressed in the synovial cell infiltrate in arthritic joints, as well as by osteoclasts at sites of bone erosion.

CONCLUSION

Focal bone erosion in CIA is attributed to cells expressing definitive features of osteoclasts, including CTR. The expression of RANKL by cells within inflamed synovium suggests a mechanism for osteoclast differentiation and activation at sites of bone erosion. Inhibitors of RANKL may represent a novel approach to treatment of bone loss in rheumatoid arthritis.

Localization of RANKL (receptor activator of NF kappa B ligand) mRNA and protein in skeletal and extraskeletal tissues

RANKL (receptor activator of NFkappaB ligand) is a membrane-associated osteoblastic molecule, and along with macrophage-colony-stimulating factor, is crucial for osteoclast formation. RANKL is known to be strongly expressed in osteoblasts and lymphoid tissues. We have sought to determine the skeletal and extraskeletal sites of production of RANKL mRNA and protein using the techniques of in situ hybridization and immunohistochemistry. Expression of RANKL mRNA and protein were determined in the developmental progression of endochondral bone formation in mouse, intramembranous bone formation in a rabbit model (mRNA only), in human giant cell tumors of bone, and at extraskeletal sites in the mouse. RANKL mRNA was expressed in prehypertrophic and hypertrophic chondrocytes at day E15 embryonic mouse long bone, and its expression was maintained at these sites throughout development. In newborn and adult mice, high levels of RANKL mRNA were expressed in mesenchymal cells of the periosteum and in mature osteoblasts, while megakaryocytes within the marrow microenvironment expressed RANKL mRNA from 1 week of age. Immunohistochemical analysis revealed a similar localization pattern of RANKL protein at the sites described. In the intramembranous bone formation model, RANKL mRNA was expressed in mesenchymal cells and in actively synthesizing osteoblasts, but not in flattened lining osteoblasts or late osteocytes. Expression of RANKL mRNA and protein in osteoclasts was variable with those within resorption lacunae showing the strongest signal/staining. Likewise, expression varied in osteoclasts from giant cell tumor of bone with a minority of tartrate-resistant acid phosphatase-positive multinucleated cells having no detectable RANKL mRNA or protein. In extraskeletal tissues, RANKL mRNA and protein were detected in the brain, heart, kidney, skeletal muscle, and skin throughout mouse development, suggesting the possibility of several other functions of the molecule. RANKL was also developmentally regulated, as evidenced by its expression in the intestine, liver, and lung at E15 and newborn mouse but not in the adult.

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.

Activated T lymphocytes support osteoclast formation in vitro

Osteoblastic stromal cells are capable of supporting osteoclast formation from hematopoietic precursors in the presence of osteotropic factors such as 1alpha,25(OH)(2)D(3), PTH, and IL-11. Osteoblastic stromal cells produce receptor activator of NF-kappaB ligand (RANKL), a type II membrane protein of the TNF ligand family, in response to these agents. Activated T lymphocytes also produce RANKL; however, the ability of this cell type to support osteoclast formation in vitro is unknown. Human PBMC-derived T cells, extracted using alphaCD3-coated magnetic beads, were cocultured with adherent murine spleen cells in the presence of Con A and a panel of cytokines. In the presence of Con A, bona fide osteoclasts were formed in vitro with activated T cells: IL-1alpha and TGFbeta further enhanced osteoclast numbers. PBMC-derived lymphocytes showed an increase in the mRNA expression of RANKL within 24 h of treatment with the same agents that were used to induce osteoclast formation. In synovial tissue sections with lymphoid infiltrates from RA patients, the expression of RANKL was demonstrated in CD3(+) T cells. The ability of activated T lymphocytes to support osteoclast formation may provide a mechanism for the potentiation of osteoclast formation and bone resorption in disease states such as rheumatoid arthritis.

Breast cancer cells interact with osteoblasts to support osteoclast formation

Breast cancers commonly cause osteolytic metastases in bone, a process that is dependent upon osteoclast-mediated bone resorption. Recently the osteoclast differentiation factor (ODF), better termed RANKL (receptor activator of NF-kappaB ligand), expressed by osteoblasts has been cloned as well as its cognate signaling receptor, receptor activator of NFkappaB (RANK), and a secreted decoy receptor osteoprotegerin (OPG) that limits RANKL's biological action. We determined that the breast cancer cell lines MDA-MB-231, MCF-7, and T47D as well as primary breast cancers do not express RANKL but express OPG and RANK. MCF-7, MDA-MB-231, and T47D cells did not act as surrogate osteoblasts to support osteoclast formation in coculture experiments, a result consistent with the fact that they do not express RANKL. When MCF-7 cells overexpressing PTH-related protein (PTHrP) were added to cocultures of murine osteoblasts and hematopoietic cells, osteoclast formation resulted without the addition of any osteotropic agents; cocultures with MCF-7 or MCF-7 cells transfected with pcDNAIneo required exogenous agents for osteoclast formation. When MCF-7 cells overexpressing PTHrP were cultured with murine osteoblasts, osteoblastic RANKL messenger RNA (mRNA) levels were enhanced and osteoblastic OPG mRNA levels diminished; MCF-7 parental cells had no effect on RANKL or OPG mRNA levels when cultured with osteoblastic cells. Using a murine model of breast cancer metastasis to bone, we established that MCF-7 cells that overexpress PTHrP caused significantly more bone metastases, which were associated with increased osteoclast formation, elevated plasma PTHrP concentrations and hypercalcaemia compared with parental or empty vector controls.

Calcitonin receptor antibodies in the identification of osteoclasts

Osteoclasts are the cells responsible for bone resorption, and their number and rate of formation are critical in determining bone mass. To identify and quantify osteoclasts, as well as to study their formation in bone and in osteoclastogenic cultures, osteoclast-specific cell markers are required. Only the calcitonin receptor (CTR) expression unambiguously identifies osteoclasts and distinguishes them from macrophage polykaryons. However, present autoradiographic methods for CTR detection are cumbersome and time consuming. We have developed rabbit polyclonal antibodies specific for the C-terminal intracellular domain of the mouse and rat Cla CTR. These antibodies labeled HEK-293 cells stably transfected with CTR (but not untransfected HEK-293 cells). This labeling is abrogated by preabsorbing the antibodies with the recombinant antigen. The antibodies immunostained primary mouse and rat osteoclasts as well as osteoclasts in sections of mouse bone. Osteoclasts (both mononuclear and multinucleated) formed from mouse bone marrow or spleen cells cocultured with osteoblasts in the presence of 1,25 dihydroxyvitamin D3 and prostaglandin E2 were also specifically immunostained by the CTR antibodies. Cocultures incubated under conditions that did not allow osteoclastogenesis (i.e., omission of mediators or osteoblasts, or culture for less than 4 days) were not immunostained by CTR antibodies. Autoradiographic detection of 125I-labeled salmon calcitonin combined with CTR immunohistochemistry showed that both methods labeled the same cells. A CTR polyclonal antibody and monoclonal antibody F4/80 were used in combination to show immunofluorescence labeling of murine osteoclasts and macrophage populations, respectively, in marrow/osteoblast cocultures. These results indicate that simple and rapid CTR antibody-based methods can be used to identify osteoclasts, and can be used to characterize the antigenic profile of osteoclasts by using double immunofluorescence analysis.

Phosphorylation at the cyclin-dependent kinases site (Thr85) of parathyroid hormone-related protein negatively regulates its nuclear localization

Parathyroid hormone-related protein (PTHrP) is expressed by a wide variety of cells and is considered to act as a secreted factor; however, evidence is accumulating for it to act in an intracrine manner. We have determined that PTHrP localizes to the nucleus at the G1 phase of the cell cycle and is transported to the cytoplasm when cells divide. PTHrP contains a putative nuclear localization sequence (NLS) (residues 61-94) similar to that of SV40 T-antigen, which may be implicated in the nuclear import of the molecule. We identified that Thr85 immediately prior to the NLS of PTHrP was phosphorylated by CDC2-CDK2 and phosphorylation was cell cycle-dependent. Mutation of Thr85 to Ala85 resulted in nuclear accumulation of PTHrP, while mutation to Glu85 to mimic a phosphorylated residue resulted in localization of PTHrP to the cytoplasm. Combined, the data demonstrate that the intracellular localization of PTHrP is phosphorylation- and cell cycle-dependent, and such control further supports a potential intracellular role (10,34,35) for PTHrP.