No bones about fluoride.

We now have a number of effective drugs for osteoporosis. However, close inspection of clinical trials results suggests we should aim for even better ones.

Estrogen promotes apoptosis of murine osteoclasts mediated by TGF-beta.

Postmenopausal osteoporosis, the most common bone disease in the developed world, is associated with estrogen deficiency. This deficiency induces increased generation and activity of osteoclasts, which perforate bone trabeculae, thus reducing their strength and increasing fracture risk. Estrogen replacement prevents these effects, indicating that estrogen negatively regulates osteoclast formation and function, but how it does this is unclear. Because functional osteoclast life span and thus the amount of bone that osteoclasts resorb could also be enhanced following estrogen deficiency, and since sex steroids regulate apoptosis in other target tissues, we investigated whether estrogen may affect osteoclast function by promoting apoptosis. 17 beta-Estradiol promoted apoptosis of murine osteoclasts in vitro and in vivo by two- to threefold. Tamoxifen, which has estrogenic effects on bone resorption, and transforming growth factor-beta 1 (TGF-beta), whose production by osteoblasts is increased by estrogen, had similar effects in vitro. Anti-TGF-beta antibody inhibited TGF-beta-, estrogen- and tamoxifen-induced osteoclast apoptosis, indicating that TGF-beta might mediate this effect. These findings suggest that estrogen may prevent excessive bone loss before and after the menopause by limiting osteoclast life span through promotion of apoptosis. The development of analogues to promote this mechanism specifically could be a useful and novel therapeutic approach to prevent postmenopausal osteoporosis.

Prostaglandins are necessary for osteoclast-activating factor production by activated peripheral blood leukocytes.

The production of osteoclast-activating factor (OAF) by normal human peripheral blood leukocytes stimulated by phytohemagglutinin was inhibited by a series of structurally unrelated inhibitors of prostaglandin synthetase. Inhibition of OAF production by these agents was reversed by adding prostaglandins of the E series back to the leukocyte suspension. These results indicate that prostaglandin synthesis is necessary for OAF production.

Monocytes regulate osteoclast-activating factor production by releasing prostaglandins.

Osteoclast-activating factor (OAF), a powerful stimulator of osteoclastic bone resorption, is released by peripheral blood mononuclear cells on exposure to phytohemagglutinin (PHA) or a specific antigen to which the leukocytes have been previously exposed. Both lymphocytes and monocytes are required in the leukocyte population for OAF release to occur. In this study we examined the relationship between the lymphocyte and monocyte in OAF production. Biological activity, as a result of OAF, was assessed by a bioassay based on the release of previously incorporated 45Ca from fetal rodent long bones in organ culture. We found that an enriched lymphocyte population depleted of monocytes by serial adherence does not release OAF after stimulation with PHA, although the cells are activated as assessed by [3H]thymidine and 3H-amino acid incorporation. When conditioned media harvested from adherent cells which did not contain OAF was added to the enriched lymphocytes, OAF release occurred. Media harvested from adherent cells which were cultured with indomethacin (10 microM), an inhibitor of prostaglandin synthesis, did not permit OAF release by activated lymphocytes. When PGE1 and PGE2 (0.1 microM) were added exogenously to the enriched lymphocyte population, OAF release occurred after stimulation with PHA. These results indicate that, (a) the activated lymphocyte is the cell or origin of OAF, (b) prostaglandins produced by monocytes are necessary for OAF production by activated lymphocytes, and (c) monocyte prostaglandins can influence bone resorption indirectly by regulating OAF production as well as directly by osteoclast activation. The interactions of OAF and prostaglandins at bone resorbing sites may be important in inflammatory and neoplastic diseases associated with bone destruction.

Role of the tubulin-microtubule system in lymphocyte activation.

The role of the tubulin-microtubule system was examined in human peripheral blood leukocytes after activation with phytohemagglutinin (PHA). Soluble tubulin and microtubules were measured with a [(3)H]colchicine-binding assay. It was found that the tubulin content of PHA-activated lymphocytes was consistently increased relative to total protein content after 36 h of culture. There was no increase in the proportion of total tubulin synthesis which was present as microtubules at 36 h. Nevertheless, as a result of increased tubulin synthesis, there was a two-to three-fold increase in total microtubular mass. Colchicine, which disrupts microtubles, was used to assess the role of microtubule assembly in the sequence of events which follow lymphocyte activation, namely lymphokine release, protein synthesis, RNA synthesis, and DNA synthesis. Colchicine consistently inhibited DNA synthesis but did not inhibit release of the lymphokine, osteoclast activating factor (OAF). Protein and RNA syntheses were inhibited much less than DNA synthesis. The fact that some effects of PHA on lymphocytes appear to require intact microtubules and at least one does not suggest that the microtubule dependent step in PHA-stimulated lymphocyte activation occurs at a stage after propagation of the signal from the membrane to the cell interior.

Dual role for the latent transforming growth factor-beta binding protein in storage of latent TGF-beta in the extracellular matrix and as a structural matrix protein.

The role of the latent TGF-beta binding protein (LTBP) is unclear. In cultures of fetal rat calvarial cells, which form mineralized bonelike nodules, both LTBP and the TGF-beta 1 precursor localized to large fibrillar structures in the extracellular matrix. The appearance of these fibrillar structures preceded the appearance of type I collagen fibers. Plasmin treatment abolished the fibrillar staining pattern for LTBP and released a complex containing both LTBP and TGF-beta. Antibodies and antisense oligonucleotides against LTBP inhibited the formation of mineralized bonelike nodules in long-term fetal rat calvarial cultures. Immunohistochemistry of fetal and adult rat bone confirmed a fibrillar staining pattern for LTBP in vivo. These findings, together with the known homology of LTBP to the fibrillin family of proteins, suggest a novel function for LTBP, in addition to its role in matrix storage of latent TGF-beta, as a structural matrix protein that may play a role in bone formation.

Identification and characterization of osteoclast-like cells and their progenitors in cultures of feline marrow mononuclear cells.

The predominant cell responsible for bone resorption, the multinucleated osteoclast, has been difficult to study because of inaccessibility. When feline marrow-derived mononuclear cells are established in long-term culture, multinucleated cells form within 48 h, reaching maximum numbers at 16 d. We have observed that these cultured cells have many of the features of osteoclasts. Morphologically, they are multinucleated, contain large numbers of branched mitochondria, have a peripheral cytoplasm lacking organelles (a clear zone), and have extensive cell-surface processes. In addition to these ultrastructural features, the cells contain a tartrate-resistant acid phosphatase, the activity of which is increased by parathyroid hormone (PTH) and inhibited by calcitonin. PTH, prostaglandin E2, and 1,25(OH)2 vitamin D3 increased multinucleated cell formation, while calcitonin inhibited the stimulatory effects of PTH. Time-lapse cinemicrographic and autoradiographic studies indicated that the multinucleated cells formed by fusion of the mononuclear progenitors. The multinucleated cells were phagocytic and stained with nonspecific esterase, consistent with their being derived from immature monocytes. Further, cell populations enriched for multinucleated cells release 45Ca from devitalized bone. Density-gradient centrifugation on Percoll was used to enrich and characterize the mononuclear progenitors of these multinucleated cells. The progenitor cells were found predominantly in Percoll density layers of 1.065 to 1.08 g/ml and were enriched up to 30-fold as compared to unfractionated cells. The bone marrow mononuclear cells that formed the multinucleated cells were initially nonadherent to plastic, stained heavily with nonspecific esterase, and appeared to be immature monocytes histologically. These data indicate that the multinucleated osteoclast-like cells in our cultures are derived from nonadherent monocytic progenitor cells that are responsive to osteotropic hormones. The ability to grow and characterize these cells in vitro should facilitate studies to elucidate the role these cells play in normal and pathologic states of bone resorption.

Decreased c-Src expression enhances osteoblast differentiation and bone formation.

c-src deletion in mice leads to osteopetrosis as a result of reduced bone resorption due to an alteration of the osteoclast. We report that deletion/reduction of Src expression enhances osteoblast differentiation and bone formation, contributing to the increase in bone mass. Bone histomorphometry showed that bone formation was increased in Src null compared with wild-type mice. In vitro, alkaline phosphatase (ALP) activity and nodule mineralization were increased in primary calvarial cells and in SV40-immortalized osteoblasts from Src(-/-) relative to Src(+/+) mice. Src-antisense oligodeoxynucleotides (AS-src) reduced Src levels by approximately 60% and caused a similar increase in ALP activity and nodule mineralization in primary osteoblasts in vitro. Reduction in cell proliferation was observed in primary and immortalized Src(-/-) osteoblasts and in normal osteoblasts incubated with the AS-src. Semiquantitative reverse transcriptase-PCR revealed upregulation of ALP, Osf2/Cbfa1 transcription factor, PTH/PTHrP receptor, osteocalcin, and pro-alpha 2(I) collagen in Src-deficient osteoblasts. The expression of the bone matrix protein osteopontin remained unchanged. Based on these results, we conclude that the reduction of Src expression not only inhibits bone resorption, but also stimulates osteoblast differentiation and bone formation, suggesting that the osteogenic cells may contribute to the development of the osteopetrotic phenotype in Src-deficient mice.

Differential roles for bone morphogenetic protein (BMP) receptor type IB and IA in differentiation and specification of mesenchymal precursor cells to osteoblast and adipocyte lineages.

Cumulative evidence indicates that osteoblasts and adipocytes share a common mesenchymal precursor and that bone morphogenetic proteins (BMPs) can induce both osteoblast and adipocyte differentiation of this precursor. In the present study, we investigated the roles of BMP receptors in differentiation along these separate lineages using a well-characterized clonal cell line, 2T3, derived from the mouse calvariae. BMP-2 induced 2T3 cells to differentiate into mature osteoblasts or adipocytes depending upon culture conditions. To test the specific roles of the type IA and IB BMP receptor components, truncated and constitutively active type IA and IB BMP receptor cDNAs were stably expressed in these cells. Overexpression of truncated type IB BMP receptor (trBMPR-IB) in 2T3 cells completely blocked BMP-2-induced osteoblast differentiation and mineralized bone matrix formation. Expression of trBMPR-IB also blocked mRNA expression of the osteoblast specific transcription factor, Osf2/ Cbfa1, and the osteoblast differentiation-related genes, alkaline phosphatase (ALP) and osteocalcin (OC). BMP-2-induced ALP activity could be rescued by transfection of wild-type (wt) BMPR-IB into 2T3 clones containing trBMPR-IB. Expression of a constitutively active BMPR-IB (caBMPR-IB) induced formation of mineralized bone matrix by 2T3 cells without addition of BMP-2. In contrast, overexpression of trBMPR-IA blocked adipocyte differentiation and expression of caBMPR-IA induced adipocyte formation in 2T3 cells. Expression of the adipocyte differentiation-related genes, adipsin and PPARgamma, correlated with the distinct phenotypic changes found after overexpression of the appropriate mutant receptors. These results demonstrate that type IB and IA BMP receptors transmit different signals to bone-derived mesenchymal progenitors and play critical roles in both the specification and differentiation of osteoblasts and adipocytes.

Cadherin-6 mediates the heterotypic interactions between the hemopoietic osteoclast cell lineage and stromal cells in a murine model of osteoclast differentiation.

Osteoclasts are multinucleated cells of hemopoietic origin that are responsible for bone resorption during physiological bone remodeling and in a variety of bone diseases. Osteoclast development requires direct heterotypic cell-cell interactions of the hemopoietic osteoclast precursors with the neighboring osteoblast/stromal cells. However, the molecular mechanisms underlying these heterotypic interactions are poorly understood. We isolated cadherin-6 isoform, denoted cadherin-6/2 from a cDNA library of human osteoclast-like cells. The isolated cadherin-6/2 is 3,423 bp in size consisting of an open reading frame of 2,115 bp, which encodes 705 amino acids. This isoform lacks 85 amino acids between positions 333 and 418 and contains 9 different amino acids in the extracellular domain compared with the previously described cadherin-6. The human osteoclast-like cells also expressed another isoform denoted cadherin-6/1 together with the cadherin-6. Introduction of cadherin-6/2 into L-cells that showed no cell-cell contact caused evident morphological changes accompanied with tight cell-cell association, indicating the cadherin-6/2 we isolated here is functional. Moreover, expression of dominant-negative or antisense cadherin-6/2 construct in bone marrow-derived mouse stromal ST2 cells, which express only cadherin-6/2, markedly impaired their ability to support osteoclast formation in a mouse coculture model of osteoclastogenesis. Our results suggest that cadherin-6 may be a contributory molecule to the heterotypic interactions between the hemopoietic osteoclast cell lineage and osteoblast/bone marrow stromal cells required for the osteoclast differentiation. Since both osteoclasts and osteoblasts/bone marrow stromal cells are the primary cells controlling physiological bone remodeling, expression of cadherin-6 isoforms in these two cell types of different origin suggests a critical role of these molecules in the relationship of osteoclast precursors and cells of osteoblastic lineage within the bone microenvironment.

Chemotactic responses of tumor cells to products of resorbing bone.

To explore possible mechanisms for the metastasis of malignant cells to bone, a model of tumor cell migration was developed, using Walker carcinosarcoma or malignant lymphoma cells. It was found that bone contains a factor that is strongly chemotactic for tumor cells. This factoor is released by a variety of agents that induce resorption of bone.

Stimulation of bone resorption in vitro by synthetic transforming growth factor-alpha.

Experiments were conducted to test the hypothesis that tumor-derived transforming growth factor-alpha (TGF-alpha) is responsible for the increased bone resorption and hypercalcemia seen in some malignant diseases. Homogeneous synthetic TGF-alpha prepared by the solid-phase synthesis method stimulated bone resorption directly in vitro in a concentration-dependent manner. Incubation times of 72 hours or more were required to stimulate resorption, which is similar to the time course of bone resorption by epidermal growth factor.

Tumor-derived growth factor increases bone resorption in a tumor associated with humoral hypercalcemia of malignancy.

Evidence is presented that a tumor-derived transforming growth factor is responsible for stimulating bone resorption and causing hypercalcemia in an animal tumor model of the hypercalcemia of malignancy. Both conditioned medium harvested from cultured tumor cells and tumor extracts of the transplantable rat Leydig cell tumor associated with hypercalcemia contained a macromolecular bone resorbing factor with the chemical characteristics of a tumor-derived transforming growth factor.

Effects of inhibition of microtubule assembly on bone mineral release and enzyme release by human breast cancer cells.

When supernates from the established human breast cancer cell line MCF-7 were applied to fetal rat long bones that had been labeled with 45Ca and devitalized to remove endogenous bone cells, mineral was released from the bones. The release of bone mineral by MCF-7 supernates was associated with increased basal release of hydrolytic enzyme activity by the tumor cells. The basal release of lysosomal enzymes and collagenolytic activity by MCF-7 cells with approximately twice that of mouse 3T3 cells, which did not cause mineral release by the fetal rat bones. Release of hydrolytic enzymes and bone mineral-releasing activity was increased by colchicine and vinblastine, drugs that inhibit microtubule assembly, but not affected by lumicolchicine. Time-course experiments performed on MCF-7 cells with or without colchicine showed that release of cathepsin D and collagenolytic activity was associated more closely with release of bone mineral and degradation of bone matrix than was the release of N-acetylglucosaminidase. The release of previously incorporated [3H]proline from the bones exposed to MCF-7 cell cultures was more closely associated with release of collagenolytic activity by MCF-7 cells than with release of cathepsin D or N-acetylglucosaminidase. These data suggest that breast cancer-mediated bone resorption in vitro is positively correlated with release of hydrolytic enzymes by the tumor cells, and release of these enzymes is enhanced by disassembly of microtubules.

Transforming growth factor beta inhibits bone resorption in fetal rat long bone cultures.

TGF-beta 1 is a polypeptide that is abundant in bone matrix, is produced by bone cells, and modulates proliferation and differentiated functions of osteoblastic cells in vitro. TGF-beta 2 is a closely related polypeptide that was originally isolated from bone matrix. TGF-beta 1 has been shown previously to stimulate prostaglandin production in cultures of neonatal mouse calvariae, which causes these bones to resorb. We found similar effects with TGF-beta 2. In comparison, TGF-beta 1 and TGF-beta 2 failed to stimulate bone resorption in fetal rat long bone cultures during a 3-d incubation period in concentrations up to 50-100 times greater than those capable of inducing bone resorption in calvariae. Incubation with TGF-beta 1 for a further 3 d decreased bone resorption up to 30%. Moreover, bone resorption induced by the bone-resorbing agents IL 1 and 1,25-dihydroxyvitamin D3 was partially or completely inhibited by TGF-beta 1 and TGF-beta 2 during the second half of the 6-d incubation period. Inhibition of DNA synthesis with hydroxyurea inhibited bone resorption in long bones in a similar pattern to that seen with TGF-beta 1. The inhibitory effects of TGF-beta 1 and TGF-beta 2 on bone resorption in long bone cultures may therefore be due to inhibition of osteoclast precursor proliferation.

Collagen and collagen-derived fragments are chemotactic for tumor cells.

Organs that are rich in collagen such as liver, lungs, and bone are frequently sites of tumor cell metastasis. In this study, we have found that cultured tumor cells of human and rat origin migrated unidirectionally in response to collagen in vitro. Synthetic di- and tri-peptides that contained amino acid sequences found frequently in the collagen helix caused similar effects. These results are consistent with the hypothesis that collagen or collagen fragments released during connective tissue remodeling may be important in tumor cell metastasis.

The role of cadherin in the generation of multinucleated osteoclasts from mononuclear precursors in murine marrow.

A critical step in bone resorption is the fusion of mononuclear osteoclast precursors to form multinucleated osteoclasts. However, little is known of the molecular mechanisms that are responsible for this important process. Since the expression of proteins in the cadherin family of homophilic calcium-dependent cell adhesion molecules is involved in the fusion process for certain other cells, we examined their role in osteoclast formation. Immunohistochemical examination of human and mouse bone using monoclonal antibodies to human and mouse E-cadherin clearly demonstrated positive staining in osteoclasts. N- and P-cadherin were not detected. In cultures of murine marrow mononuclear cells in which osteoclasts form by cell fusion, E-cadherin expression determined by Western blotting reached the highest levels as fusion was taking place. Expression of E-cadherin gene fragment was also detected in the marrow cultures by polymerase chain reaction. To study the functional role of E-cadherin expression in osteoclastic differentiation, neutralizing monoclonal antibodies were examined for their effects on osteoclast formation. The antibodies decreased the number of tartrate-resistant acid phosphatase (a marker of murine osteoclast)-positive multinucleated cell (TRAP-positive MNC) by inhibiting the fusion of mononuclear osteoclast precursors, but not proliferation of these cells or their attachment to plastic dish surfaces. This inhibitory effect was reversible. Furthermore, synthetic peptides containing the cell adhesion recognition sequence of cadherins also decreased TRAP-positive MNC formation. The antibodies and peptides inhibited not only osteoclast formation but also bone resorption. Antibodies to other types of cadherins and control rat IgG had no effects in these culture systems. Our findings suggest that E-cadherin expression may be involved in fusion (differentiation) of hemopoietic osteoclast precursors into mature multinucleated osteoclasts.

Requirement of pp60c-src expression for osteoclasts to form ruffled borders and resorb bone in mice.

Targeted disruption of the c-src proto-oncogene in mice has shown that src expression is required for normal bone resorption, since the src-deficient mutants develop osteopetrosis. To evaluate the mechanisms by which src-deficiency affects osteoclast function, we treated src-deficient mice with the stimulants of bone resorption, IL-1, parathyroid hormone, and parathyroid hormone-related protein, and analyzed the effects by quantitative bone histomorphometry and electron microscopy. Increased numbers of multinucleated cells with the morphological characteristics of osteoclasts appeared on bone surfaces, but these cells did not form ruffled borders or normal resorption lacunae. To confirm these in vivo findings, we cultured src-mutant bone marrow cells on dentine slices in the presence of 1,25 dihydroxyvitamin D3. Increased numbers of multinucleated cells were formed, but unlike normal murine bone marrow cells, they did not form resorption pits. These results indicate that osteoclasts appear in the absence of pp60c-src, but that pp60c-src expression is required for mature osteoclasts to form ruffled borders and resorb bone.

Partial purification of osteoclast-activating factor from phytohemagglutinin-stimulated human leukocytes.

Osteoclast-activating factor (OAF) is a soluble mediator found in supernates of human peripheral leukocytes which have been cultured with antigens or phytomitogens. OAF is a potent stimulator of osteoclastic resorption of fetal bone in organ culture. The present studies were designed to characterize OAF chemically. Bone resorbing activity from supernates of leukocytes cultured without added plasma was not lost on dialysis using a membrane with a molecular weight cutoff of 3,500, but was lost when heated to 60 degrees C for 30 min. The activity was lost after treatment with trypsin or pronase but not after treatment with ribonuclease or neuraminidase. Papain, which inactivated parathyroid hormone at a concentration of 25 mug/ml, did not inactivate OAF at 250 mug/ml. OAF did not react with an antibody to bovine parathyroid hormone which cross-reacts with human parathyroid hormone. OAF was also distinguished from active metabolites of vitamin D and from prostaglandin by extraction procedures and immunoassay for prostaglandin E(2). When the medium from activated leukocytes cultured with autologous plasma was fractionated by gel filtration on Sephadex, bone resorbing activity eluated both with plasma proteins and in lower molecular weight fractions. However, when medium from leukocytes cultured without added plasma was chromatographed, all the OAF activity was eluted in a sharp low molecular weight peak located between chymotrypsinogen (25,000 molecular weight) and ribonuclease A (13,700 molecular weight). This peak contained about 4% of the total protein originally present in the supernate. Its activity was destroyed by overnight incubation at 37 degrees C at pH 6 or 8, but not at pH 7.2. After incubation at 4 degrees C, the activity was lost at pH 3 or 10, but not at pH 4-9. The active fraction from Sephadex G-100 was therefore chromatographed at pH 7.2 on DEAE cellulose and carboxymethyl cellulose. The active material was not adsorbed; however, about sevenfold further purification was achieved by removal of contaminants. The material obtained after sequential Sephadex, DEAE and, carboxymethyl cellulose chromatography stimulated resorption of fetal rat bone in culture at concentrations of 0.75-3 mug protein/ml, indicating that this preparation of OAF was nearly as potent as bovine parathyroid hormone in this system.

Failure of parathyroid hormone antagonists to inhibit in vitro bone resorbing activity produced by two animal models of the humoral hypercalcemia of malignancy.

The humoral hypercalcemia of malignancy (HHM) is caused by tumor cells that release a circulating factor which stimulates osteoclastic bone resorption. Recently, it has been reported that tumors associated with HHM contain factors that stimulate renal and bone cell adenylate cyclase. The activity was inhibited by parathyroid hormone (PTH) antagonists, and this led to the hypothesis that hypercalcemia is due to bone resorbing factors that engage PTH receptors in bone. Since it is not known whether the bone resorbing factors act via PTH receptors in bone, we examined the effects of PTH antagonists on PTH-stimulated bone resorption and bone resorbing activity that was produced by two tumor models of HHM which also release these adenylate cyclase stimulating factors. The PTH antagonists [8,18norleucine, 34tyrosine]bovine PTH (3-34) amide and [34tyrosine]bovine PTH (7-34) completely inhibited PTH-stimulated bone resorption. Neither antagonist inhibited bone resorption that was stimulated by the conditioned medium from cells that were derived from the Walker rat 256 tumor model of HHM. Both antagonists also failed to inhibit bone resorption that was stimulated by culture media from cells that were derived from the rat Leydig cell tumor. These data suggest that in these two models of HHM, the bone resorbing factors do not exert their effects by interacting with PTH receptors on bone.