Scribble scrambles parathyroid hormone receptor interactions to regulate phosphate and vitamin D homeostasis.

G protein-coupled receptors, including PTHR, are pivotal for controlling metabolic processes ranging from serum phosphate and vitamin D levels to glucose uptake, and cytoplasmic interactors may modulate their signaling, trafficking, and function. We now show that direct interaction with Scribble, a cell polarity-regulating adaptor protein, modulates PTHR activity. Scribble is a crucial regulator for establishing and developing tissue architecture, and its dysregulation is involved in various disease conditions, including tumor expansion and viral infections. Scribble co-localizes with PTHR at basal and lateral surfaces in polarized cells. Using X-ray crystallography, we show that colocalization is mediated by engaging a short sequence motif at the PTHR C-terminus using Scribble PDZ1 and PDZ3 domain, with binding affinities of 31.7 and 13.4 µM, respectively. Since PTHR controls metabolic functions by actions on renal proximal tubules, we engineered mice to selectively knockout Scribble in proximal tubules. The loss of Scribble impacted serum phosphate and vitamin D levels and caused significant plasma phosphate elevation and increased aggregate vitamin D3 levels, whereas blood glucose levels remained unchanged. Collectively these results identify Scribble as a vital regulator of PTHR-mediated signaling and function. Our findings reveal an unexpected link between renal metabolism and cell polarity signaling.

A new neuropeptide insect parathyroid hormone iPTH in the red flour beetle Tribolium castaneum.

In the postgenomics era, comparative genomics have advanced the understanding of evolutionary processes of neuropeptidergic signaling systems. The evolutionary origin of many neuropeptidergic signaling systems can be traced date back to early metazoan evolution based on the conserved sequences. Insect parathyroid hormone receptor (iPTHR) was previously described as an ortholog of vertebrate PTHR that has a well-known function in controlling bone remodeling. However, there was no sequence homologous to PTH sequence in insect genomes, leaving the iPTHR as an orphan receptor. Here, we identified the authentic ligand insect PTH (iPTH) for the iPTHR. The taxonomic distribution of iPTHR, which is lacking in Diptera and Lepidoptera, provided a lead for identifying the authentic ligand. We found that a previously described orphan ligand known as PXXXamide (where X is any amino acid) described in the cuttlefish Sepia officinalis has a similar taxonomic distribution pattern as iPTHR. Tests of this peptide, iPTH, in functional reporter assays confirmed the interaction of the ligand-receptor pair. Study of a model beetle, Tribolium castaneum, was used to investigate the function of the iPTH signaling system by RNA interference followed by RNA sequencing and phenotyping. The results suggested that the iPTH system is likely involved in the regulation of cuticle formation that culminates with a phenotype of defects in wing exoskeleton maturation at the time of adult eclosion. Moreover, RNAi of iPTHRs also led to significant reductions in egg numbers and hatching rates after parental RNAi.

Serum PTH, PTH1R/ATF4 pathway, and the sRANKL/OPG system in bone as a new link between bone growth, cross-sectional geometry, and strength in young rats with experimental chronic kidney disease.

The progression of chronic kidney disease (CKD) in children is associated with deregulated parathyroid hormone (PTH), growth retardation, and low bone accrual. PTH can cause both catabolic and anabolic impact on bone, and the activating transcription factor 4 (ATF4), a downstream target gene of PTH, is related to its anabolic effect. Osteoprotegerin (OPG) and receptor activator of NF-κB ligand (RANKL) are PTH-dependent cytokines, which may play an important role in the regulation of bone remodeling. This study aimed to evaluate the impact of endogenous PTH and the bone RANKL/OPG system on bone growth, cross-sectional geometry and strength utilizing young, nephrectomized rats. The parameters of cross-sectional geometry were significantly elevated in rats with CKD during the three-month experimental period compared with the controls, and they were strongly associated with serum PTH levels and the expression of parathyroid hormone 1 receptor (PTH1R)/ATF4 genes in bone. Low bone soluble RANKL (sRANKL) levels and sRANKL/OPG ratios were also positively correlated with cross-sectional bone geometry and femoral length. Moreover, the analyzed geometric parameters were strongly related to the biomechanical properties of femoral diaphysis. In summary, the mild increase in endogenous PTH, its anabolic PTH1R/ATF4 axis and PTH-dependent alterations in the bone RANKL/OPG system may be one of the possible mechanisms responsible for the favorable impact on bone growth, cross-sectional geometry and strength in young rats with experimental CKD.

International Union of Basic and Clinical Pharmacology. XCIII. The parathyroid hormone receptors--family B G protein-coupled receptors.

The type-1 parathyroid hormone receptor (PTHR1) is a family B G protein-coupled receptor (GPCR) that mediates the actions of two polypeptide ligands; parathyroid hormone (PTH), an endocrine hormone that regulates the levels of calcium and inorganic phosphate in the blood by acting on bone and kidney, and PTH-related protein (PTHrP), a paracrine-factor that regulates cell differentiation and proliferation programs in developing bone and other tissues. The type-2 parathyroid hormone receptor (PTHR2) binds a peptide ligand, called tuberoinfundibular peptide-39 (TIP39), and while the biologic role of the PTHR2/TIP39 system is not as defined as that of the PTHR1, it likely plays a role in the central nervous system as well as in spermatogenesis. Mechanisms of action at these receptors have been explored through a variety of pharmacological and biochemical approaches, and the data obtained support a basic "two-site" mode of ligand binding now thought to be used by each of the family B peptide hormone GPCRs. Recent crystallographic studies on the family B GPCRs are providing new insights that help to further refine the specifics of the overall receptor architecture and modes of ligand docking. One intriguing pharmacological finding for the PTHR1 is that it can form surprisingly stable complexes with certain PTH/PTHrP ligand analogs and thereby mediate markedly prolonged cell signaling responses that persist even when the bulk of the complexes are found in internalized vesicles. The PTHR1 thus appears to be able to activate the Gα(s)/cAMP pathway not only from the plasma membrane but also from the endosomal domain. The cumulative findings could have an impact on efforts to develop new drug therapies for the PTH receptors.

Oxidation inhibits PTH receptor signaling and trafficking.

Reactive Oxygen Species (ROS) increase during aging, potentially affecting many tissues including brain, heart, and bone. ROS alter signaling pathways and constitute potential therapeutic targets to limit oxidative damaging effects in aging-associated diseases. Parathyroid hormone receptors (PTHR) are widely expressed and PTH is the only anabolic therapy for osteoporosis. The effects of oxidative stress on PTHR signaling and trafficking have not been elucidated. Here, we used Fluorescence Resonance Energy Transfer (FRET)-based cAMP, ERK, and calcium fluorescent biosensors to analyze the effects of ROS on PTHR signaling and trafficking by live-cell imaging. PTHR internalization and recycling were measured in HEK-293 cells stably transfected with HA-PTHR. PTH increased cAMP production, ERK phosphorylation, and elevated intracellular calcium. Pre-incubation with H2O2 reduced all PTH-dependent signaling pathways. These inhibitory effects were not a result of PTH oxidation since PTH incubated with H2O2 triggered similar responses. PTH promoted internalization and recycling of the PTHR. Both events were significantly reduced by H2O2 pre-incubation. These findings highlight the role of oxidation on PTHR signaling and trafficking, and suggest the relevance of ROS as a putative target in diseases associated with oxidative stress such as age-related osteoporosis.

Synergistic effects of high dietary calcium and exogenous parathyroid hormone in promoting osteoblastic bone formation in mice.

In the present study, we investigated whether high dietary Ca and exogenous parathyroid hormone 1-34 fragments (PTH 1-34) have synergistic effects on bone formation in adult mice, and explored the related mechanisms. Adult male mice were fed a normal diet, a high-Ca diet, a PTH-treated diet, or a high-Ca diet combined with subcutaneously injected PTH 1-34 (80 µg/kg per d) for 4 weeks. Bone mineral density, trabecular bone volume, osteoblast number, alkaline phosphatase (ALP)- and type I collagen-positive areas, and the expression levels of osteoblastic bone formation-related genes and proteins were increased significantly in mice fed the high-Ca diet, the PTH-treated diet, and, even more dramatically, the high-Ca diet combined with PTH. Osteoclast number and surface and the ratio of receptor activator for nuclear factor-κB ligand (RANKL):osteoprotegerin (OPG) were decreased in the high-Ca diet treatment group, increased in the PTH treatment group, but not in the combined treatment group. Furthermore, third-passage osteoblasts were treated with high Ca (5 mM), PTH 1-34 (10⁻8 M) or high Ca combined with PTH 1-34. Osteoblast viability and ALP activity were increased in either the high Ca-treated or PTH-treated cultures and, even more dramatically, in the cultures treated with high Ca plus PTH, with consistent up-regulation of the expression levels of osteoblast proliferation and differentiation-related genes and proteins. These results indicate that dietary Ca and PTH play synergistic roles in promoting osteoblastic bone formation by stimulating osteoblast proliferation and differentiation.

Bone-remodeling transcript levels are independent of perching in end-of-lay white leghorn chickens.

Osteoporosis is a bone disease that commonly results in a 30% incidence of fracture in hens used to produce eggs for human consumption. One of the causes of osteoporosis is the lack of mechanical strain placed on weight-bearing bones. In conventionally-caged hens, there is inadequate space for chickens to exercise and induce mechanical strain on their bones. One approach is to encourage mechanical stress on bones by the addition of perches to conventional cages. Our study focuses on the molecular mechanism of bone remodeling in end-of-lay hens (71 weeks) with access to perches. We examined bone-specific transcripts that are actively involved during development and remodeling. Using real-time quantitative PCR, we examined seven transcripts (COL2A1 (collagen, type II, alpha 1), RANKL (receptor activator of nuclear factor kappa-B ligand), OPG (osteoprotegerin), PTHLH (PTH-like hormone), PTH1R (PTH/PTHLH type-1 receptor), PTH3R (PTH/PTHLH type-3 receptor), and SOX9 (Sry-related high mobility group box)) in phalange, tibia and femur. Our results indicate that the only significant effect was a difference among bones for COL2A1 (femur > phalange). Therefore, we conclude that access to a perch did not alter transcript expression. Furthermore, because hens have been used as a model for human bone metabolism and osteoporosis, the results indicate that bone remodeling due to mechanical loading in chickens may be a product of different pathways than those involved in the mammalian model.

[Rare abnormalities of parathyroid gland function and parathyroid hormone receptor action]. / Rzadkie zaburzenia funkcji przytarczyc i dzialania receptorowego parathormonu.

The parathyroid glands, located near or within the posterior surface of the thyroid gland and secreting parathyroid hormone, are essential organs for the regulation of calcium and phosphate metabolism. As they are necessary to sustain life and maintain homeostasis, undetected or misdiagnosed parathyroid disorders may pose a significant threat to health outcomes, as their presence may increase morbidity and mortality in affected individuals. The clinical picture of some disorders associated with abnormal parathyroid hormone secretion and receptor action is sometimes complicated by coexisting abnormalities, and in these cases establishing the correct diagnosis is challenging. The remarkable progress of recent years in the area of hormonal assessment, imaging procedures and molecular biology, has resulted in a great improvement in the identification, differentiation and treatment of various parathyroid disorders and has made it possible to identify several new clinical entities. In this paper, we discuss the present state-of-art on the etiopathogenesis, clinical manifestations, diagnosis and treatment of chosen rare abnormalities of parathyroid gland function and parathyroid hormone receptor action.

A mutant PTH/PTHrP type I receptor in enchondromatosis.

Enchondromas are common benign cartilage tumors of bone. They can occur as solitary lesions or as multiple lesions in enchondromatosis (Ollier and Maffucci diseases). Clinical problems caused by enchondromas include skeletal deformity and the potential for malignant change to chondrosarcoma. The extent of skeletal involvement is variable in enchondromatosis and may include dysplasia that is not directly attributable to enchondromas. Enchondromatosis is rare, obvious inheritance of the condition is unusual and no candidate loci have been identified. Enchondromas are usually in close proximity to, or in continuity with, growth-plate cartilage. Consequently, they may result from abnormal regulation of proliferation and terminal differentiation of chondrocytes in the adjoining growth plate. In normal growth plates, differentiation of proliferative chondrocytes to post-mitotic hypertrophic chondrocytes is regulated in part by a tightly coupled signaling relay involving parathyroid hormone related protein (PTHrP) and Indian hedgehog (IHH). PTHrP delays the hypertrophic differentiation of proliferating chondrocytes, whereas IHH promotes chondrocyte proliferation. We identified a mutant PTH/PTHrP type I receptor (PTHR1) in human enchondromatosis that signals abnormally in vitro and causes enchondroma-like lesions in transgenic mice. The mutant receptor constitutively activates Hedgehog signaling, and excessive Hedgehog signaling is sufficient to cause formation of enchondroma-like lesions.

Parathyroid hormone and its effects on dental tissues.

In the current era, various pharmacological agents exist for osteoporosis management, and synthetic parathyroid hormone (PTH) (Teriparatide, Forteo) is one of the treatment options. Depending on the timing of administration, PTH has a unique ability to cause both bone apposition and bone resorption. This review focuses on the effects of PTH on the bone, specifically the jaw bones mandible and maxilla. The article briefly describes the fundamental mechanism of PTH action at the molecular level, as well as in experimental animals and in humans. It differentiates intermittent administration of PTH, especially at doses tolerated by humans that increase bone strength and prevent bone fractures, from continuous use that may lead to bone loss. In particular, it shows how intermittent administration of PTH can play a significant role in periodontal repair and implant success via stimulation of bone mineral content especially in the pre-alveolar region.

Actions of Parathyroid Hormone Ligand Analogues in Humanized PTH1R Knockin Mice.

Rodent models are commonly used to evaluate parathyroid hormone (PTH) and PTH-related protein (PTHrP) ligands and analogues for their pharmacologic activities and potential therapeutic utility toward diseases of bone and mineral ion metabolism. Divergence, however, in the amino acid sequences of rodent and human PTH receptors (rat and mouse PTH1Rs are 91% identical to the human PTH1R) can lead to differences in receptor-binding and signaling potencies for such ligands when assessed on rodent vs human PTH1Rs, as shown by cell-based assays in vitro. This introduces an element of uncertainty in the accuracy of rodent models for performing such preclinical evaluations. To overcome this potential uncertainty, we used a homologous recombination-based knockin (KI) approach to generate a mouse (in-host strain C57Bl/6N) in which complementary DNA encoding the human PTH1R replaces a segment (exon 4) of the murine PTH1R gene so that the human and not the mouse PTH1R protein is expressed. Expression is directed by the endogenous mouse promoter and hence occurs in all biologically relevant cells and tissues and at appropriate levels. The resulting homozygous hPTH1R-KI (humanized) mice were healthy over at least 10 generations and showed functional responses to injected PTH analog peptides that are consistent with a fully functional human PTH1R in target bone and kidney cells. The initial evaluation of these mice and their potential utility for predicting behavior of PTH analogues in humans is reported here.

Carboxyl-terminal parathyroid hormone fragments: biologic effects.

Carboxyl-terminal PTH fragments (C-PTH), are generated by both direct secretion from parathyroids in relation to serum calcium levels and catabolism of PTH operated by the Kupffer cells in the liver. These molecular fragments have been till recently regarded as inert byproducts of PTH metabolism, since they do not interact with the PTH/PTH-related peptide (rP) receptor, which mediates the classical hormone actions. Current findings instead indicate that C-PTH would interact with a putative C-PTH receptor. This way, C-PTH seem to exert specific effects on calcium homeostasis and bone metabolism, opposite to those of the synthetic agonist of PTH/PTHrP receptor (i.e. PTH 1-34). In vitro and in vivo data actually indicate that C-PTH, by interacting with specific receptors, could have an anti-calcemic action, as well as a pro-apoptotic effect on both osteocytes and osteoclasts. This in turn could result in a reduced activity of the latter cells, with a consequent inhibition of bone resorption.

Dual role of parathyroid hormone in endothelial progenitor cells and marrow stromal mesenchymal stem cells.

Hematopoietic stem cells derive regulatory information also from parathyroid hormone (PTH). To explore the possibility that PTH may have a role in regulation of other stem cells residing in bone marrow, such as mesenchymal stem cells (MSCs) and endothelial progenitor cells (EPCs) we assessed the effect of this hormone on the in vitro behavior of MSCs and EPCs. We evidenced that MSCs were much more responsive to PTH than EPCs. PTH increased the proliferation rate of MSCs with a diminution of senescence and apoptosis. Taken together, our results may suggest a protective effect of PTH on MSCs that reduces stress phenomena and preserve genome integrity. At the opposite, PTH did not modify the fate of EPCs in culture.

Effects of parathyroid hormone on immune function.

Parathyroid hormone (PTH) function as immunologic mediator has become interesting with the recent usage of PTH analogue (teriparatide) in the management of osteoporosis. Since the early 1980s, PTH receptors were found on most immunologic cells (neutrophils, B and T cells). The in vitro evaluations for a possible role of PTH as immunomodulator have shown inconsistent results mainly due to methodological heterogeneity of these studies: it used different PTH formulations (rat, bovine, and human), at different dosages and different incubating periods. In some of these studies, the lymphocytes were collected from uremic patients or animals, which renders the interpretation of the results problematic due to the effect of uremic toxins. Parathyroidectomy has been found to reverse the immunologic defect in patients with high PTH levels. Nonetheless, the clinical significance of these findings is unclear. Further studies are needed to define if PTH does have immunomodulatory effects.

Osteoblastic cells regulate the haematopoietic stem cell niche.

Stem cell fate is influenced by specialized microenvironments that remain poorly defined in mammals. To explore the possibility that haematopoietic stem cells derive regulatory information from bone, accounting for the localization of haematopoiesis in bone marrow, we assessed mice that were genetically altered to produce osteoblast-specific, activated PTH/PTHrP receptors (PPRs). Here we show that PPR-stimulated osteoblastic cells that are increased in number produce high levels of the Notch ligand jagged 1 and support an increase in the number of haematopoietic stem cells with evidence of Notch1 activation in vivo. Furthermore, ligand-dependent activation of PPR with parathyroid hormone (PTH) increased the number of osteoblasts in stromal cultures, and augmented ex vivo primitive haematopoietic cell growth that was abrogated by gamma-secretase inhibition of Notch activation. An increase in the number of stem cells was observed in wild-type animals after PTH injection, and survival after bone marrow transplantation was markedly improved. Therefore, osteoblastic cells are a regulatory component of the haematopoietic stem cell niche in vivo that influences stem cell function through Notch activation. Niche constituent cells or signalling pathways provide pharmacological targets with therapeutic potential for stem-cell-based therapies.

Altered selectivity of parathyroid hormone (PTH) and PTH-related protein (PTHrP) for distinct conformations of the PTH/PTHrP receptor.

PTH and PTHrP use the same G protein-coupled receptor, the PTH/PTHrP receptor (PTHR), to mediate their distinct biological actions. The extent to which the mechanisms by which the two ligands bind to the PTHR differ is unclear. We examined this question using several pharmacological and biophysical approaches. Kinetic dissociation and equilibrium binding assays revealed that the binding of [(125)I]PTHrP(1-36) to the PTHR was more sensitive to GTPgammaS (added to functionally uncouple PTHR-G protein complexes) than was the binding of [(125)I]PTH(1-34) ( approximately 75% maximal inhibition vs. approximately 20%). Fluorescence resonance energy transfer-based kinetic analyses revealed that PTHrP(1-36) bound to the PTHR more slowly and dissociated from it more rapidly than did PTH(1-34). The cAMP signaling response capacity of PTHrP(1-36) in cells decayed more rapidly than did that of PTH(1-34) (t(1/2) = approximately 1 vs. approximately 2 h). Divergent residue 5 in the ligand, Ile in PTH and His in PTHrP, was identified as a key determinant of the altered receptor-interaction responses exhibited by the two peptides. We conclude that whereas PTH and PTHrP bind similarly to the G protein-coupled PTHR conformation (RG), PTH has a greater capacity to bind to the G protein-uncoupled conformation (R(0)) and, hence, can produce cumulatively greater signaling responses (via R(0)-->RG isomerization) than can PTHrP. Such conformational selectivity may relate to the distinct modes by which PTH and PTHrP act biologically, endocrine vs. paracrine, and may help explain reported differences in the effects that the ligands have on calcium and bone metabolism when administered to humans.

Ligand-receptor interactions at the parathyroid hormone receptors: subtype binding selectivity is mediated via an interaction between residue 23 on the ligand and residue 41 on the receptor.

Parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) bind and activate the PTH/PTHrP receptor (PTH-1R). However, while the related receptor PTH-2R responds potently to PTH, it is not activated by PTHrP. Two hormone sites are known to be responsible for these different potencies. First, the absence of efficacy for PTHrP at PTH-2R is due to the presence of His-5 in PTHrP (Ile-5 in PTH), which interacts with the receptor's juxtamembrane domain. Second, PTHrP has lower affinity than PTH for PTH-2R because of the presence of Phe-23 (Trp-23 in PTH), which interacts with the receptor's N-terminal extracellular domain. We used these different receptor subtype properties to demonstrate that residue 41 in PTH-1R, when either the native Leu or substituted by Ile or Met, can accommodate either Phe or Trp at position 23 of the ligand. However, when Leu-41 is substituted by a smaller side chain, either Ala or Val (its equivalent residue in PTH-2R), the receptor becomes highly selective for those peptide ligands with Trp-23. Hence, despite the conservative nature of the substitutions found in the native ligands (Phe for Trp) and receptors (Leu for Val), they nevertheless enable a significant degree of selectivity to be achieved. Analysis of this functionally important ligand-receptor contact, within the context of the recent X-ray structure of the peptide-bound PTH-1R N domain, reveals the nature of the selectivity filter and how it is by-passed in PTH-1R.

Bone growth: coordinating chondrocyte differentiation.

Two signalling molecules-Indian hedgehog and parathyroid hormone-related peptide-have been found to function in a negative feedback loop that is crucial for the coordinated regulation of the rate and extent of endochondral bone growth.

Fetal parathyroids are not required to maintain placental calcium transport.

We used Hoxa3 knockout mice and other mouse models to study the role of the fetal parathyroids in fetal calcium homeostasis. Hoxa3-null fetuses lack parathyroid glands, and absence of parathyroid hormone (PTH) was confirmed with a rodent PTH immunoradiometric assay. The ionized calcium level of Hoxa3-null fetuses was significantly lower than that of wild-type or heterozygous littermates or of the mother. Both the rate of placental calcium transfer and the plasma PTHrP level were normal in Hoxa3 mutants and their heterozygous siblings. Because we had previously observed an increase in placental calcium transfer in PTH/PTHrP receptor 1-null (Pthr1-null) fetuses, we assayed plasma PTHrP in those mice. Pthr1-null fetuses had plasma PTHrP levels 11-fold higher than those of their littermates. Northern analysis, immunohistochemical, and in situ hybridization studies of Pthr1-null fetuses indicated that liver and placenta had increased expression of PTHRP: In summary, loss of fetal parathyroids in Hoxa3-null fetuses caused marked hypocalcemia but did not alter placental calcium transfer or the circulating PTHrP level. The findings in the Pthr1-null fetuses indicate that several tissues may contribute to the circulating PTHrP level in fetal mice.

Synovium as a source of increased amino-terminal parathyroid hormone-related protein expression in rheumatoid arthritis. A possible role for locally produced parathyroid hormone-related protein in the pathogenesis of rheumatoid arthritis.

Proinflammatory cytokines, including tumor necrosis factor (TNF) and interleukin 1 (IL-1), mediate the joint destruction that characterizes rheumatoid arthritis (RA). Previous studies have shown that parathyroid hormone-related protein (PTHrP) is a member of the cascade of proinflammatory cytokines induced in parenchymal organs during lethal endotoxemia. To test the hypothesis that NH2-terminal PTHrP, a potent bone resorbing agent, could also be a member of the synovial cascade of tissue-destructive cytokines whose expression is induced in RA, PTHrP expression was examined in synovium and synoviocytes obtained from patients with RA and osteoarthritis (OA). PTHrP production, as determined by measurement of immunoreactive PTHrP(1-86) in tissue explant supernatants, was increased 10-fold in RA versus OA synovial tissue. Synovial lining cells and fibroblast-like cells within the pannus expressed both PTHrP and the PTH/PTHrP receptor, findings that were confirmed by in vitro studies of cultured synoviocytes. TNF-alpha and IL-1beta stimulated PTHrP expression in synoviocytes, while dexamethasone and interferon-gamma, agents with some therapeutic efficacy in the treatment of RA, inhibited PTHrP release. Treatment of synoviocytes with PTHrP(1-34) stimulated IL-6 secretion. These results suggest that proinflammatory cytokine-stimulated production of NH2-terminal PTHrP by synovial tissue directly invading cartilage and bone in RA may mediate joint destruction through direct effects on cartilage or bone, or, indirectly, via the induction of mediators of bone resorption in the tumor-like synovium.