Targeting lysophosphatidic acid receptor type 1 with Debio 0719 inhibits spontaneous metastasis dissemination of breast cancer cells independently of cell proliferation and angiogenesis

Metastasis is the main cause of death for cancer patients. Targeting factors that control metastasis formation is a major challenge for clinicians. Lysophosphatidic acid (LPA) is a bioactive phospholipid involved in cancer. LPA activates at least six independent G protein-coupled receptors (LPA_1–6). Tumor cells frequently co-express multiple LPA receptors, puzzling the contribution of each one to cancer progression. All three receptors, LPA₁, LPA₂ and LPA₃, act as oncogenes and prometastatic factors in the mouse mammary gland. The competitive inhibitor of LPA₁ and LPA₃ receptors, Ki16425, inhibits efficiently breast cancer bone metastases in animal models. We showed here that Debio 0719, which corresponds to the R-stereoisomer of Ki16425 exhibited highest antagonist activities at LPA₁ (IC₅₀=60 nM) and LPA₃ (IC₅₀=660 nM) than Ki16425 [IC₅₀=130 nM (LPA₁); IC₅₀=2.3 μM (LPA₃)]. In vitro, Debio 0719, inhibited LPA-dependent invasion of the 4T1 mouse mammary cancer cells. In vivo, early but not late administration of Debio 0719 (50 mg/kg p.o. twice daily) to BALB/c mice during the course of orthotopic 4T1 primary tumor growth reduced the number of spontaneously disseminated tumor cells to bone and lungs without affecting the growth of primary tumors and tumor-induced angiogenesis. We found that increased LPA₁ mRNA expression in primary tumors of breast cancer patients correlated significantly with their positive lymph node status (p<0.001). Altogether, our results suggest that LPA₁ controls early events of metastasis independently of cell proliferation and angiogenesis. Therefore, targeting this receptor with Debio 0719 has a high therapeutic potential against metastasis formation for breast cancer patients.

Abstract 2368: Autotaxin/Enpp2 controls of bone metastasis formation through osteoclast function

Autotaxin (ATX, NPP-2) is a secreted protein with both an oncogenic action on mouse breast tissue and metastatic properties on breast cancer cells. Due to its lysophospholipase D activity, ATX generates lysophosphatidic acid (LPA) from lysophosphatidylcholine and controls LPA levels in the blood. We have recently showed that platelet-derived LPA promotes the progression of osteolytic bone metastases of breast cancer cells. However, how LPA is produced at the metastatic site and the role of ATX by cancer cells in this metastasis process are still unknown. Here, we found that in vitro, forced expression of ATX in human breast cancer MDA-B02 cells (MDA-B02/ATX) increased invasiness, proliferation and enhanced their pro-osteoclastic function. In vivo, intravenous injection of MDA-B02/ATX cells to inmmunodeficiency BALB/C nude mice enhanced osteolytic bone metastasis formation, as judged by increased bone loss and tumor burden, and higher active osteoclast number at the bone metastatic site. Mouse 4T1 breast cancer cells induced the formation of osteolytic bone metastases after intracardiac injection in immunocompetent BALB/C mice. We found that 4T1 cells expressed active ATX. In vitro, silencing ATX[O1] expression inhibited invasion but not proliferation of 4T1-siATX cells. In vivo, silencing ATX expression inhibited the extent of osteolytic bone lesions and decreased the number of active osteoclasts at bone the metastatic site. Altogether, these results demonstrated that expression of ATX by cancer cells controls bone metastasis formation and that targeting ATX in patients might contribute to the development of additional therapies against bone metastases.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2368.

Cancer cell expression of autotaxin controls bone metastasis formation in mouse through lysophosphatidic acid-dependent activation of osteoclasts

Background

Bone metastases are highly frequent complications of breast cancers. Current bone metastasis treatments using powerful anti-resorbtive agents are only palliative indicating that factors independent of bone resorption control bone metastasis progression. Autotaxin (ATX/NPP2) is a secreted protein with both oncogenic and pro-metastatic properties. Through its lysosphospholipase D (lysoPLD) activity, ATX controls the level of lysophosphatidic acid (LPA) in the blood. Platelet-derived LPA promotes the progression of osteolytic bone metastases of breast cancer cells. We asked whether ATX was involved in the bone metastasis process. We characterized the role of ATX in osteolytic bone metastasis formation by using genetically modified breast cancer cells exploited on different osteolytic bone metastasis mouse models.

Methodology/Principal Findings

Intravenous injection of human breast cancer MDA-B02 cells with forced expression of ATX (MDA-B02/ATX) to inmmunodeficiency BALB/C nude mice enhanced osteolytic bone metastasis formation, as judged by increased bone loss, tumor burden, and a higher number of active osteoclasts at the metastatic site. Mouse breast cancer 4T1 cells induced the formation of osteolytic bone metastases after intracardiac injection in immunocompetent BALB/C mice. These cells expressed active ATX and silencing ATX expression inhibited the extent of osteolytic bone lesions and decreased the number of active osteoclasts at the bone metastatic site. In vitro, osteoclast differentiation was enhanced in presence of MDA-B02/ATX cell conditioned media or recombinant autotaxin that was blocked by the autotaxin inhibitor vpc8a202. In vitro, addition of LPA to active charcoal-treated serum restored the capacity of the serum to support RANK-L/MCSF-induced osteoclastogenesis.

Conclusion/Significance

Expression of autotaxin by cancer cells controls osteolytic bone metastasis formation. This work demonstrates a new role for LPA as a factor that stimulates directly cancer growth and metastasis, and osteoclast differentiation. Therefore, targeting the autotaxin/LPA track emerges as a potential new therapeutic approach to improve the outcome of patients with bone metastases.

The type 1 lysophosphatidic acid receptor is a target for therapy in bone metastases

Platelet-derived lysophosphatidic acid (LPA) supports the progression of breast and ovarian cancer metastasis to bone. The mechanisms through which LPA promotes bone metastasis formation are, however, unknown. Here we report that silencing of the type 1 LPA receptor (LPA(1)) in cancer cells blocks the production of tumor-derived cytokines that are potent activators of osteoclast-mediated bone destruction and significantly reduces the progression of osteolytic bone metastases. Moreover, functional blockade of LPA action on its cognate receptor LPA(1) using a pharmacological antagonist mimics the effects of silencing LPA(1) in tumor cells in vitro and substantially reduces bone metastasis progression in animals. Overall, these results suggest that inhibition of platelet-derived LPA action on LPA(1) expressed by tumor cells may be a promising therapeutic target for patients with bone metastases.

Platelet-derived lysophosphatidic acid supports the progression of osteolytic bone metastases in breast cancer

The role of lysophosphatidic acid (LPA) in cancer is poorly understood. Here we provide evidence for a role of LPA in the progression of breast cancer bone metastases. LPA receptors LPA(1), LPA(2), and LPA(3) were expressed in human primary breast tumors and a series of human breast cancer cell lines. The inducible overexpression of LPA(1) in MDA-BO2 breast cancer cells specifically sensitized these cells to the mitogenic action of LPA in vitro. In vivo, LPA(1) overexpression in MDA-BO2 cells enhanced the growth of subcutaneous tumor xenografts and promoted bone metastasis formation in mice by increasing both skeletal tumor growth and bone destruction. This suggested that endogenous LPA was produced in the tumor microenvironment. However, MDA-BO2 cells or transfectants did not produce LPA. Instead, they induced the release of LPA from activated platelets which, in turn, promoted tumor cell proliferation and the LPA(1)-dependent secretion of IL-6 and IL-8, 2 potent bone resorption stimulators. Moreover, platelet-derived LPA deprivation in mice, achieved by treatment with the platelet antagonist Integrilin, inhibited the progression of bone metastases caused by parental and LPA(1)-overexpressing MDA-BO2 cells and reduced the progression of osteolytic lesions in mice bearing CHO-beta3wt ovarian cancer cells. Overall, our data suggest that, at the bone metastatic site, tumor cells stimulate the production of LPA from activated platelets, which enhances both tumor growth and cytokine-mediated bone destruction.

[Does tumor angiogenesis play a role in bone metastatic process?]

Angiogenesis plays an important role in the regulation of tumor growth and metastasis of numerous cancers. However, the role of angiogenesis in the bone metastasis process is totally unknown. Using an experimental model of bone metastasis caused by MDA-MB-231/B02 breast cancer cells in nude mice, we show that beside its well-know anti-angiogenic activity, angiostatin inhibits bone metastasis formation through a direct antiosteoclastic activity.

Angiostatin inhibits bone metastasis formation in nude mice through a direct anti-osteoclastic activity

Bone is a very common metastatic site for breast cancer. In bone metastasis, there is a vicious circle wherein bone-residing metastatic cells stimulate osteoclast-mediated bone resorption, and bone-derived growth factors released from resorbed bone promote tumor growth. The contribution of tumor angiogenesis in the growth of bone metastases is, however, unknown. By using an experimental model of bone metastasis caused by MDA-MB-231/B02 breast cancer cells that quite closely mimics the conditions likely to occur in naturally arising metastatic human breast cancers, we demonstrate here that when MDA-MB-231/B02 cells were engineered to produce at the bone metastatic site an angiogenesis inhibitor, angiostatin, there was a marked inhibition in the extent of skeletal lesions. Inhibition of skeletal lesions came with a pronounced reduction in tumor burden in bone. However, although angiostatin produced by MDA-MB-231/B02 cells was effective at inhibiting in vitro endothelial cell proliferation and in vivo angiogenesis in a Matrigel implant model, we have shown that it inhibited cancer-induced bone destruction through a direct inhibition of osteoclast activity and generation. Overall, these results indicate that, besides its well known anti-angiogenic activity, angiostatin must also be considered as a very effective inhibitor of bone resorption, broadening its potential clinical use in cancer therapy.

Integrin alpha(v)beta3 expression confers on tumor cells a greater propensity to metastasize to bone

The reasons why tumor cells metastasize to bone remain obscure. There is some evidence to support the theory that integrins (acting as cell surface adhesion receptors) play a role in mediating metastasis in certain organs. Here, we report that overexpression of a functionally active integrin alpha(v)b3 in Chinese hamster ovary (CHO) tumor cells drastically increased the incidence, number, and area of bone metastases in nude mice compared with those observed in mock-transfected CHO cells (CHO dhfr+) or in CHO cells expressing a functionally inactive integrin alpha(v)b3 (CHO beta3Delta744). Moreover, a breast cancer cell line (B02) established from bone metastases caused by MDA-MB-231 cells constitutively overexpressed integrin alpha(v)b3, whereas the cell surface expression level of other integrins remained unchanged. In vivo, the extent of bone metastases in B02-bearing mice was significantly increased compared with that of MDA-MB-231-bearing mice. In vitro, B02 cells and CHO cells expressing a functionally active integrin alpha(v)b3 exhibited substantially increased invasion of and adhesion to mineralized bone, bone sialoprotein, and collagen compared with those found with MDA-MB-231, CHO dhfr+, and CHO beta3Delta744 cells, respectively. Overall, our findings suggest that integrin alpha(v)b3 expression in tumor cells accelerates the development of osteolytic lesions, presumably through increased invasion of and adhesion to bone.

Early detection of bone metastases in a murine model using fluorescent human breast cancer cells: application to the use of the bisphosphonate zoledronic acid in the treatment of osteolytic lesions

A very common metastatic site for human breast cancer is bone. The traditional bone metastasis model requires human MDA-MB-231 breast carcinoma cell inoculation into the left heart ventricle of nude mice. MDA-MB-231 cells usually develop osteolytic lesions 3-4 weeks after intracardiac inoculation in these animals. Here, we report a new approach to study the formation of bone metastasis in animals using breast carcinoma cells expressing the bioluminescent jellyfish protein (green fluorescent protein [GFP]). We first established a subclone of MDA-MB-231 cells by repeated in vivo passages in bone using the heart injection model. On stable transfection of this subclone with an expression vector for GFP and subsequent inoculation of GFP-expressing tumor cells (B02/GFP.2) in the mouse tail vein, B02/GFP.2 cells displayed a unique predilection for dissemination to bone. Externally fluorescence imaging of live animals allowed the detection of fluorescent bone metastases approximately 1 week before the occurrence of radiologically distinctive osteolytic lesions. The number, size, and intensity of fluorescent bone metastases increased progressively with time and was indicative of breast cancer cell progression within bone. Histological examination of fluorescent long bones from B02/GFP.2-bearing mice revealed the occurrence of profound bone destruction. Treatment of B02/GFP.2-bearing mice with the bisphosphonate zoledronic acid markedly inhibited the progression of established osteolytic lesions and the expansion of breast cancer cells within bone. Overall, this new bone metastasis model of breast cancer combining both fluorescence imaging and radiography should provide an invaluable tool to study the effectiveness of pharmaceutical agents that could suppress cancer colonization in bone.

Influence of mature adipocytes on osteoblast proliferation in human primary cocultures

It has been shown that the bone loss occurring with aging in spongy bone is associated with a reduced osteoblastic bone formation and an increased volume of marrow adipose tissue. This observation suggests a relationship between cells from the osteoblastic and adipogenic lineages. The purpose of the present study was to evaluate the influence of mature adipocytes on osteoblastic proliferation and activity in a model of coculture. Human primary osteoblastic (hBOB) cells were derived from femoral bone explants collected in patients undergoing orthopedic surgery. Human stromal osteoblastic (hMSOB) cells were obtained from bone marrow samples collected by aspiration during orthopedic surgery. Extramedullary and medullary mature adipocytes (hAd) showing similar functions, except for their response to insulin, hAd were isolated from mammary adipose tissue collected in women undergoing tumorectomy. Cells were cocultured, with hAd being separated from osteoblastic cells (hBOB or hMSOB) by a porous membrane (0.4 microm). When hBOB cells were seeded on the upper side of the insert and hAd were floating on the lower side, cell contacts between the two cell types were possible through the pores of the membrane. At the end of the experiment, proliferation of the osteoblastic cells was evaluated by [(3)H]-thymidine incorporation and alkaline phosphatase (AP) activity was measured. After 20 h of coculture, proliferation of the hBOB cells was significantly decreased when compared with control hBOB (-40 +/- 6%, p < 0.05). To establish whether or not the influence of hAd on hBOB proliferation required intercellular communications, hAd and hBOB cells were cocultured far from the porous membrane. Six other independent experiments confirmed an inhibition of hBOB proliferation under both experimental conditions (p < 0.05): -35 +/- 7% with possible intercellular contacts, and -30 +/- 7% without any contact. In contrast, the proliferation of hMSOB cells was not significantly modified after coculture with hAd. In addition, the presence of hAd did not significantly modify the AP activity of hBOB (0.163 +/- 0.143 and 0.181 +/- 0.114 nmol/min per microgram of protein in controls and after coculture, respectively). No reproducible effect of hAd-conditioned medium was noted on hBOB- and hMSOB-cell proliferation or hBOB-cell activity. In conclusion, mature adipocytes induced an inhibition of hBOB-cells proliferation, probably mediated by a factor secreted by hAd. This effect may contribute to the age-related reduction of bone formation and bone loss.

Platelet-osteosarcoma cell interaction is mediated through a specific fibrinogen-binding sequence located within the N-terminal domain of thrombospondin 1

Approximately 20% of patients with osteosarcoma have metastatic disease in lungs or bones at diagnosis. The requirement of platelets in hematogenous dissemination of metastatic cells is now well established. Tumor cells interact with platelets and induce platelet aggregation. In this respect, metastatic potential of tumor cells correlates with their capacity to aggregate platelets in vitro. We have previously shown that thrombospondin 1 (TSP-1) is synthesized and expressed on the surface of MG-63 osteosarcoma cells and mediates platelet-osteosarcoma cell interaction. However, active sites mimicking the function of TSP-1 during platelet-osteosarcoma cell interaction are not known. In this study, a panel of antibodies directed against the N-terminal and C-terminal domains and type 1, type 2, and type 3 repeats of TSP-1 were first used to delineate the structural requirement for the binding of osteosarcoma cell surface-associated TSP-1 to platelets. A drastic inhibition of the platelet-aggregating activity of MG-63 cells was obtained in the presence of a monoclonal antibody directed against the N-terminal domain of TSP-1. Among a series of 16 synthetic peptides spanning the whole N-terminal domain of TSP-1, only synthetic peptide N12/I encompassing amino acid residues 151-164 of the N-terminal domain of TSP-1 inhibited the platelet-aggregating activity of MG-63 cells. Electron microscopy studies showed that peptide N12/I strongly inhibited platelet-osteosarcoma cell interaction. A polyclonal antibody directed against peptide N12/I specifically bound to the surface of MG-63 cells, recognized TSP-1 and drastically inhibited the platelet-aggregating activity of MG-63 cells. In addition, peptide N12/I specifically bound to fibrinogen and inhibited TSP-1/fibrinogen interaction. Overall, our results provide evidence that a fibrinogen-binding sequence located within the N-terminal domain of TSP-1 mediates the binding of osteosarcoma cell surface-associated TSP-1 to platelet-bound fibrinogen.

Evidence for a dense and intimate innervation of the bone tissue, including glutamate-containing fibers

The recent demonstration in bone cells of receptors for glutamate (Glu), a major neuromediator, suggests that Glu may also act as a signaling molecule in bone and regulate bone cell metabolism. Although bone is known to be innervated, the distribution and characteristics of nerve fibers in this tissue have not been well documented. We have studied the anatomical distribution of nerve fibers and the presence of glutamate-immunoreactive ones in sections of long bones from neonatal, 15-, and 25-day-old rats, using immunocytochemistry with antibodies directed against several neuronal markers and Glu. We showed by electron microscopy that bone is rich in nerve-like processes running along vessels adjacent to bone trabeculae, in the vicinity of hematopoietic cells and bone cells. Immunocytochemical studies at the tissue and cellular level confirmed the presence of a dense network of thin nerve processes immunolabeled for neurofilament 200, tyrosine hydroxylase, and microtubule associated protein-2, three markers of nerve fibers. Some of these nerve processes showed local dilatations in contact with medullary cells and bone cells that were immunolabeled for synaptophysin, a nerve terminal marker. Glu was largely expressed in these thin nerve processes in proximity to bone cells. These findings show evidence for a dense and intimate network of nerve processes in bone, some of which were containing Glu, suggesting glutamatergic innervation in bone.

[Thrombospondins, tumor angiogenesis and breast cancer]

Thrombospondin-1 and -2 are extracellular matrix proteins that are overexpressed in breast cancer tissue. Their role in breast cancer remains unknown. This article reviews the potential effects of thrombospondin-1 and -2 in breast cancer tumori genesis and metastatic dissemination.

Influence of magnesium substitution on a collagen-apatite biomaterial on the production of a calcifying matrix by human osteoblasts

The induction of a calcifying matrix is of great interest in the restoration of bone defects. In a previous in vitro study we demonstrated that a collagen sponge constituted of type I collagen fibrils, chondroitin sulfates, and hydroxyapatite crystals induces an earlier and a more abundant synthesis of a new extracellular calcifying matrix than do other biomaterials such as collagen or hydroxyapatite alone. Bone mineral contains various amounts of magnesium ions, either adsorbed at the surface of apatite crystals or incorporated inside the crystal structure. Magnesium is known to reduce the degradation rate of tricalcium phosphate ceramics and to influence the crystallization of mineral substance. Thus we evaluated two sponges modified with different substituted apatites. The substituted low magnesium-containing apatite sample decreased the osteoinductive properties of the sponge whereas the substituted high magnesium-containing apatite sample had a toxic effect on bone cells and prevented the formation of any extracellular matrix. Such a toxic effect can be explained by the presence of large numbers of magnesium ions released into the culture medium even though at physiological level magnesium is able to promote bone mineralization and to control the growth of hydroxyapatite crystals. Thus collagen sponges containing hydroxyapatite remain one of the most appropriately evaluated biomaterials used for the restoration of periodontal pockets and bone defects.

Glutamate receptors are expressed by bone cells and are involved in bone resorption

The excitatory amino acid glutamate (Glu) is a potent neurotransmitter in the central nervous system and exerts its action via a variety of glutamate receptors (GluRs). Because we had previously shown that a poly-glutamate (poly-Glu) peptide stimulates bone resorption in vitro, an effect specific to Glu (Raynal, C., Delmas, P. D., and Chenu, C. Bone sialoprotein stimulates in vitro bone resorption. Endocrinology 137:2347-2354; 1996), we investigated the possibility that bone cells express GluRs, and whether they may be important for osteoclast activity. Using immunocytochemistry on rat bone sections, we have shown that all mature bone cells (osteoblasts and osteoclasts) express GluRs, and that the ionotropic N-methyl-D-aspartate (NMDA) receptor 1 subunit (NMDAR1) appears most highly expressed. Osteoclasts isolated from rabbit long bones also possessed NMDAR1 GluRs. Bone resorption in vitro by isolated osteoclasts was inhibited by a monoclonal antibody directed against NMDAR1, as well as by two pharmacological antagonists of this receptor (D-AP5 and MK 801), which also antagonized poly-Glu stimulated bone resorption. These results suggest a possible new mechanism for regulating osteoclast activity and indicate that excitatory amino acids such as glutamate may be important local regulators of bone cell functions.

Decreased beta-isomerization of the C-terminal telopeptide of type I collagen alpha 1 chain in Paget's disease of bone

In Paget's disease of bone, the normal lamellar bone is replaced by a woven structure with an irregular arrangement of collagen fibers. In this study, we investigated whether the degree of beta-isomerization within C-telopeptide of alpha 1 chain of type I collagen was altered in Paget's disease compared with other bone diseases with no alteration of bone structure. In Paget's disease (n = 26), but not in patients with primary hyperparathyroidism (n = 6) or hyperthyroidism (n = 17), the urinary excretion of nonisomerized (alpha) fragments derived from degradation of type I collagen C-telopeptide (CTX) was markedly increased compared with beta-isomerized CTX (+ 13-fold vs. + 3.5-fold over controls) resulting in an urinary alpha CTX/beta CTX ratio 3-fold higher than in controls (2.6 +/- 1.0 vs. 0.8 +/- 0.3, p < 0.001). In five pagetic patients in complete remission, as demonstrated by normal total alkaline phosphatase activity, the alpha CTX/beta CTX ratio was normal. The immunohistochemistry of normal and pagetic human bone sections showed a preferential distribution of alpha CTX within woven structure, while lamellar bone was intensely stained with an anti-beta CTX antibody, suggesting a lower degree of beta-isomerization of type I collagen in the woven pagetic bone. In collagenase digest of human bone specimens, we found a lower proportion of beta-isomerized type I collagen molecules in pagetic bone (40% of beta CTX) than in normal bone taken from trabecular (68%) and cortical compartments (71%). In conclusion, we found that in Paget's disease the alpha CTX/beta CTX ratio in bone and in urine is markedly increased. This altered beta isomerization can be accurately detected in vivo by measuring urinary degradation products arising from bone resorption.

Distribution of thrombospondin and integrin alpha V in DCIS, invasive ductal and lobular human breast carcinomas. Analysis by electron microscopy

The ultrastructural distribution of thrombospondin (TSP) and its cell surface receptor, integrin alpha V, was studied in two cases of human breast carcinoma: one of ductal carcinoma in situ (DCIS) with an invasive component, and one of invasive lobular carcinoma. In DCIS, moderate immunolabelling for TSP and integrin alpha V was observed in the rough endoplasmic reticulum and at the plasma membrane of intraductal carcinoma cells. TSP was also associated with extracellular matrix collagen fibrils surrounding in situ carcinoma cells. In the invasive part of this ductal carcinoma, most of the malignant cells were negative for TSP, while integrin alpha V was moderately expressed in these cells. In sharp contrast, typical strands of invasive lobular carcinoma cells in "Indian file" showed moderate TSP immunostaining in the rough endoplasmic reticulum and strong immunoreactivity for TSP at the plasma membrane and in the extracellular matrix. Moderate to strong immunoreactivity for integrin alpha V was also observed in invasive lobular carcinoma cells. Because of the role of TSP during cancer cell invasion, the different expression patterns of TSP in invasive ductal versus lobular carcinoma may well reflect biological differences between these two types of breast carcinoma and could account for the peculiar diffuse invasive behaviour of breast lobular carcinoma cells.

Degradation of subcutaneous implants of bone particles from normal and warfarin-treated rats

Osteoclasts are multinucleated cells specific to bone tissue and of hemopoietic origin. They are formed by fusion of mononucleated cells in a manner related to the formation of macrophage polykarions. Subcutaneous implantation of mineralized bone particles induces multinucleated giant cell recruitment. There is controversy, however, about the nature of these cells. Although subcutaneous implantation of bone particles derived from warfarin-treated animals has been applied as an in vivo model to study the role of osteocalcin in bone resorption, the exact nature of multinucleated cells elicited in this model is still unclear. In this paper, subcutaneous implants of bone particles from normal and warfarin-treated rats were implanted in Sprague-Dawley rats. Resorption was assessed in 12 and 16 day implants by chemical analysis (calcium content) and by histomorphometric measurement of the bone particle area and the number of multinucleated and tartrate-resistant acid phosphatase-positive cells. No significant difference in calcium content and bone area were observed, after 12 or after 16 days of implantation, between implants from normal and warfarin-treated rats. The number of tartrate-resistant acid phosphatase-positive cells elicited by bone particles represented less than 25% of the number of multinucleated cells and did not differ between bone particles from normal and warfarin-treated rats. By electron microscopy, a majority of multinucleated cells did not show a ruffled border in contact with bone particles, and their morphological features were suggestive of a foreign body giant cell reaction. In our experience this model appears to elicit only a few osteoclasts among multinucleated macrophagic cells and may not be the most appropriate one for the study of resorption of normal or osteocalcin-depleted bone.

A number of osteocalcin antisera recognize epitopes on proteins other than osteocalcin in cultured skin fibroblasts: implications for the identification of cells of the osteoblastic lineage in vitro

Rabbit antisera to bovine osteocalcin were produced independently in two laboratories and their specificities established by western blot analysis. By immunohistochemistry each of the five polyclonal antisera produced an intense cytoplasmic staining in human bone-derived cells. Staining intensity was strongly attenuated by preabsorption of the antisera with osteocalcin. No staining was observed using nonimmune rabbit serum. However, the choice of skin cells as negative controls for osteocalcin synthesis yielded an unexpected positive staining pattern similar to that seen with the bone-derived cells over a range of antiserum dilutions. This was not caused by the uptake of exogenous osteocalcin from the culture medium because a similar pattern of staining was observed when medium was supplemented with osteocalcin-depleted fetal calf serum. Treatment with 1,25-dihydroxyvitamin D3 induced osteocalcin mRNA expression and osteocalcin secretion in cultures of bone-derived cells but not in skin fibroblasts. The results demonstrate that these polyclonal antisera also recognize epitopes shared with other proteins synthesized in culture by skin fibroblasts. Furthermore, three mouse monoclonal antibodies to distinct regions of the osteocalcin molecule show differential staining of human bone-derived cells, skin cells, and osteosarcoma cells (MG63). These observations indicate that the shared epitope residues in the central region of osteocalcin and are consistent with the specific synthesis of osteocalcin by bone cells alone. The observed nonspecificity of many osteocalcin antisera may compromise immunocytochemical studies of the osteoblast phenotype in studies in vitro when based solely on reactivity with inadequately characterized osteocalcin antisera.

Localization of thrombospondin, CD36 and CD51 during prenatal development of the human mammary gland

Thrombospondin (TSP) is a 450 kDa extracellular matrix glycoprotein expressed in normal, hyperplastic, and neoplastic human breast. In this study, the patterns of expression of TSP were determined during development of the human fetal mammary gland between the 15th and the 39th week of gestation. Using immunohistochemistry, TSP is found in the dense mesenchyme immediately adjacent to the mammary bud, and at the membrane of budding epithelial cells invading the surrounding mesenchyme. As formation of the ductal tree system occurs, TSP is deposited at the myoepithelial-stromal junction of mammary ducts. Such an immunolocalization of TSP in buds and ducts of the fetal mammary gland has been confirmed at the mRNA level using in situ hybridization. Presence of TSP transcripts in nascent breast tissue has been also demonstrated by polymerase chain reaction assay. Comparison of TSP immunolocalization with that of two known TSP cell surface receptors, CD36 and CD51, reveals no codistribution of TSP with these receptors during mammary gland development. As opposed to TSP, CD36 is strongly expressed at the membrane of preadipocytes present in the fat pad tissue, but absent from budding epithelial cells. CD51 is only weakly expressed by malpighian epithelial cells and does not colocalize with TSP. In lactating ducts of a newborn, TSP disappears from the myoepithelial-stromal junction of ducts and is synthesized at the apices of secretory epithelial cells of lactating ducts together with CD36. In conclusion, our findings support the existence of an important role for TSP during development of the human fetal mammary gland.(ABSTRACT TRUNCATED AT 250 WORDS)

Osseointegration of titanium implants in the tibia. Electron microscopy of biopsies from 4 patients

We studied the ultrastructure of bone tissue around implants of pure titanium inserted into the tibia in 4 patients with arthrosis or rheumatoid arthritis. Three main appearances of the interface were noted. First, a close contact between titanium and calcified bone with living osteocytes inside the newly-formed bone was observed in all samples. Secondly, a close contact was also seen between the implant and osteoid, the newly formed collagenous matrix being either uncalcified or calcifying. Thirdly, a loose extracellular matrix with fibrillar and nonfibrillar materials was sometimes observed between bone mineral and implant. There was no inflammatory reaction at the interface. We concluded that the titanium implants were osseointegrated, but the calcification of the bone tissue was not complete even after 20 months. However, mineralization of osteoid and living bone cells revealed the presence of an active tissue.

Role of platelet membrane glycoproteins Ib/IX and IIb/IIIa, and of platelet alpha-granule proteins in platelet aggregation induced by human osteosarcoma cells

We have previously shown that the platelet-aggregating activity of human MG-63 and HOS osteosarcoma cells depends at least in part upon tumor cell surface-associated thrombospondin, and suggested that platelet-osteosarcoma cell interactions could occur through interactions with specific platelet membrane receptors. In this study, the platelet-aggregating activity of MG-63 and HOS cells was studied by using a variety of platelet disorders. Both osteosarcoma cell lines induced a biphasic platelet aggregation response when added to normal platelet-rich plasma, while the second phase of aggregation was absent when added to gray platelets (deficiency in alpha-granule proteins) and to aspirin-treated platelets. Platelets from two unrelated patients with type I Glanzmann's thrombasthenia (deficiency in glycoprotein (GP) GPIIb/IIIa) did not aggregate at all with osteosarcoma cells. Using giant platelets from three patients with Bernard-Soulier syndrome (deficiency in GPIb/IX), the aggregation response induced by MG-63 and HOS cells was monophasic and reversible when compared to normal-sized platelets and to giant platelets from a patient with May-Hegglin anomaly (no membrane GP defect). Because GPIb serves as a receptor for von Willebrand factor during hemostasis, aggregation experiments were also conducted with the platelet-rich plasma of two patients with a low plasma von Willebrand factor concentration (type I von Willebrand's disease) before and after the infusion of deamino-D-arginine vasopressin. MG-63 and HOS cells induced biphasic platelet aggregation both before and after deamino-D-arginine vasopressin treatment, while the ristocetin-dependent binding of von Willebrand factor to platelets only occurred after deamino-D-arginine vasopressin treatment. Preincubation of normal platelet-rich plasma with monoclonal antibody SZ-2 directed against the von Willebrand binding domain of GPIb did not inhibit the platelet-aggregation activity of osteosarcoma cells, whereas anti-GPIb antibody SZ-2 did inhibit ristocetin-induced platelet agglutination. In addition, anti-GPIX antibodies did not affect platelet-osteosarcoma cell interactions. In conclusion, our data demonstrate that the first phase of the platelet-aggregating activity of human osteosarcoma cells is initiated by the interaction of these tumor cells with platelet membrane GPIIb/IIIa, whereas the second phase, even if plasma von Willebrand factor is deficient, involves platelet membrane GPIb and the participation of platelet alpha-granule proteins in membrane-mediated events, making aggregation irreversible.

Fluoride increases rat osteoblast function and population after in vivo administration but not after in vitro exposure

The effects of fluoride on bone tissue are now well documented by in vivo histological studies performed on both human and animal bone biopsies and demonstrating an increase in osteoblast (OB) population. In order to elucidate whether the mechanism of action of fluoride on osteoblasts was direct or indirect, 14 three-week-old Sprague-Dawley rats were selected. Seven animals received 100 ppm fluoride as sodium fluoride (NaF) in drinking water for one month. The other animals, which did not receive fluoride, were considered as controls. At the end of the experiment, femurs and vertebrae were excised and osteoblastic cells were obtained after collagenase digestion separately from each animal. The osteoblastic cells derived from control and NaF-treated rats were exposed in vitro to 10(-5) M NaF. Alkaline phosphatase (AP) activity was measured, and the cellular proliferation was assessed by 3H-thymidine incorporation. Thymidine incorporation and AP activity were significantly higher in osteoblastic cells derived from NaF-treated rats than in cells obtained from control rats (p = 0.05 and p < 0.01, respectively). In contrast, the osteoblast proliferation and activity were not modified after in vitro exposure to NaF in cells derived from control and NaF-treated rats. In conclusion, the function of osteoblasts was not modified after in vitro exposure to fluoride. In contrast, given in vivo to rats for one month, fluoride has a mitogenic effect on osteoblasts and stimulates their activity. These data emphasize the hypothesis that fluoride may act either on osteoprogenitor cells or through an indirect mechanism mediated by a cofactor.

Thrombospondin (TSP1) mediates in vitro proliferation of human MG-63 osteoblastic cells induced by alpha-thrombin

Thrombospondin (TSP) is a 450-kDa glycoprotein synthesized and secreted by human MG-63 osteoblastic cells. In this study, we have first studied the effect of alpha-thrombin on TSP expression by human MG-63 cells. In situ hybridization indicated that TSP mRNA level in thrombin-treated MG-63 cells was increased when compared to unstimulated cells. As judged by immunofluorescence, thrombin-treatment of MG-63 cells resulted in increased cell surface expression of TSP when compared to quiescent cells. Because thrombin stimulates proliferation of osteoblastic cells, the involvement of TSP in proliferation of thrombin-stimulated osteoblastic cells was then investigated using a serum-free mitogenesis assay. Both alpha-thrombin (0.01 to 0.15 U/ml) and TSP (5 to 600 ng/ml) caused a dose-dependent increase in [3H]thymidine incorporation by MG-63 cells. Proliferation of osteoblastic cells induced by alpha-thrombin or TSP was specifically and totally inhibited by anti-TSP monoclonal antibodies (3-10 micrograms/ml) or by indomethacin (1 microM), an inhibitor of prostaglandin synthesis. Anti-TSP antibodies which inhibited cell proliferation also inhibit TSP expression to the surface of these cells. Our experiments support the existence of a mechanism whereby TSP bound to the cell surface of thrombin-treated MG-63 cells stimulates secretion of prostaglandins which, in turn, allow cell proliferation to proceed.

In vitro evaluation of dose-effects of ethanol on human osteoblastic cells

Chronic alcoholism represents a high risk for fractures and osteopenia. Previous histomorphometric studies reported a decreased bone formation, but it has never been established whether ethanol has a direct toxic effect on osteoblasts. This present in vitro study was performed on human osteoblast cells derived from bone explants after collagenase digestion. The direct effect of ethanol was determined after 4 days of exposure to various doses, ranging from 0.01 to 5 g/l on the alkaline phosphatase (AP) activity, osteocalcin secretion and [3H]thymidine incorporation. The influence of the duration of exposure to 0.8 g/l ethanol was also determined. A significant and dose-dependent decrease in the cell proliferation was observed. AP activity was significantly decreased by high doses of ethanol (2-5 g/l). A biphasic effect of ethanol was noted on osteocalcin secretion according to the dose: it decreased at doses lower than 0.8 g/l and increased at the highest concentrations. At the dose of 0.8 g/l, whatever the duration of exposure, the decrease of the proliferation was of the same magnitude and no significant change in AP activity was observed. Significant ethanol-induced effects on osteocalcin secretion were observed only after 4 and 8 days of exposure. These data demonstrate that ethanol may have a direct toxic effect on osteoblast activity and proliferation. This could be one of the mechanisms of alcohol-induced osteopenia which has a multifactorial pathophysiology.

Thrombomodulin is synthesized by osteoblasts, stimulated by 1,25-(OH)2D3 and activates protein C at their cell membrane

We have previously shown that protein S, a vitamin K-dependent protein, is a bone matrix component synthesized and secreted by osteoblasts. Because protein S is a cofactor of protein C in inhibiting factor Va and VIIIa, we have looked for the presence of the proteins related to the anticoagulant protein C system in human MG 63 osteosarcoma cells and in human adult osteoblast-like cells. Using immunoblotting, we have shown that protein C, factor V, and C4b binding protein are not secreted by these cells. We have shown by enzyme-linked immunoassay, immunocytochemistry, and immunoprecipitation of labeled proteins that thrombomodulin, a transmembrane glycoprotein involved with thrombin in the activation of protein C, is present at the cell surface of osteoblasts. Moreover, using a protein C activation system where thrombin and protein C are added to the cells, we have shown that protein C could be activated at the osteoblast cell surface. This activation of exogenous protein C, reflecting the activity of thrombomodulin, as well as the expression of the thrombomodulin antigen, is regulated by some bone resorption-enhancing factors. 1,25-dihydroxyvitamin D3 and retinoic acid increase thrombomodulin expression and activity in a dose-dependent manner whereas tumor necrosis factor alpha and interleukin 1 decrease these parameters. Because thrombomodulin is known to inhibit single-chain urokinase-type plasminogen activator, a molecule present in the osteoblast microenvironment, these findings suggest that thrombomodulin could play a role in the regulation of bone resorption by modulating the plasmin system.

In vitro induction of a calcifying matrix by biomaterials constituted of collagen and/or hydroxyapatite: an ultrastructural comparison of three types of biomaterials

The induction of a calcifying matrix was studied in vitro and compared for three biomaterials (collagen sponge, hydroxyapatite material and a mixture of both (Biostite)) cultured with human osteoblast-like cells. The influence of biomaterials on organic matrix synthesis and the calcification process was analysed at the ultrastructural level (transmission electron microscopy and X-ray microanalysis). Biomaterials were well tolerated by bone cells. Whichever biomaterial was used, osteoblasts proliferated and synthesized a new matrix constituted of fibrillar and non-fibrillar elements. This activity appeared earlier and was more intense with Biostite than with collagen sponge alone. A deposition of a mineral substance in this newly formed matrix was observed with the collagen sponge and Biostite, but never with hydroxyapatite alone. The mineral deposits were identified as hydroxyapatite crystals, similar to those observed and analysed in bone tissue. These in vitro observations clearly demonstrated the property of Biostite to produce a calcified collagenous matrix similar to bone tissue. However, in vivo confirmation is required before extending the use of this biomaterial to periodontology.

Protein-S, a vitamin K-dependent protein, is a bone matrix component synthesized and secreted by osteoblasts

Protein-S is a vitamin K (Vit K)-dependent protein synthesized by hepatocytes, megakaryocytes, and endothelial cells and plays an important role in the regulation of hemostasis. Two cases of free protein-S congenital deficiency were recently reported to be associated with osteopenia. We hypothesized that this osteopenia could be the result of a bone deficit of protein-S synthesized by bone cells. Using enzyme-linked immunoassay, immunocytochemistry, immunoblotting, and immunoprecipitation after labeling with [35S]methionine, we have shown that this protein is secreted by three human osteosarcoma cell lines and by human adult osteoblast-like cells. In addition, protein-S was present in protein extracts of human bone matrix. Protein-S secreted by MG 63 cells increased linearly from 1-7 days of culture, was biologically active, and was regulated by warfarin, as previously described for the other cell types secreting protein-S. Vit K had no direct effect on protein-S secretion or activity, but could overcome the effects of warfarin. In conclusion, in addition to osteocalcin and matrix gamma-carboxyglutamic acid (Gla) protein, osteoblasts secrete another Vit K-dependent protein, which is a constituent of the bone matrix. Our data suggest that osteopenia occurring in patients with congenital protein-S deficiency might be related to a deficiency of protein-S secretion by the osteoblasts. This finding raises the intriguing possibility that protein-S might play a role in bone turnover and bone mass.

Thrombospondin binds to the surface of human osteosarcoma cells and mediates platelet-osteosarcoma cell interaction

We have previously shown that thrombospondin (TSP) is synthesized and secreted by human MG-63 osteosarcoma cells. In this study, the secretion and cell surface expression of TSP by two different human osteosarcoma cell lines (MG-63 and TE-85) as well as the involvement of TSP in the platelet-aggregating activity of these tumor cells were studied. Using a sandwich enzyme-linked immunosorbent assay, MG-63 cells secreted 3-fold as much TSP as TE-85 cells at 48 h (0.17 +/- 0.01 (SD) versus 0.06 +/- 0.006 micrograms/10(6) cells, P = 0.007). Binding of exogenous 125I-TSP to MG-63 and TE-85 cells in monolayer indicated that binding was time and concentration dependent, saturable, and inhibited by excess cold TSP. However, despite a similar affinity, MG-63 cells had 10-fold more TSP-binding sites than TE-85 cells (402,394 +/- 130,346 versus 36,748 +/- 7,708 TSP-binding sites/cell; P = 0.002). Similar binding differences of 125I-TSP were observed with both osteosarcoma cell lines in suspension. A fluorescence-activated cell-sorting analysis was used in conjunction with an anti-TSP polyclonal antibody, and binding of endogenous TSP to MG-63 and TE-85 cells in suspension was investigated. Addition of an anti-TSP antibody to MG-63 and TE-85 cells in suspension increased the mean fluorescence intensity 50-fold when compared to an irrelevant antibody. Moreover, the fluorescence intensity of MG-63 cells with an anti-TSP polyclonal antibody was increased by 40% when compared to TE-85 cells. Since TSP was expressed on the surface of osteosarcoma cells, the involvement of this glycoprotein in the platelet-aggregating activity of MG-63 and TE-85 cells was therefore investigated using an anti-TSP polyclonal antibody and two monoclonal antibodies (P10 and MA-II), the epitopes of which lie within the Mr 140,000 non-heparin-binding fragment and the Mr 25,000 heparin-binding fragment of TSP, respectively. Preincubation of MG-63 cells (1 x 10(6) cells/ml) with either an anti-TSP polyclonal antibody (100 micrograms/ml) or monoclonal antibody P10 (15 micrograms/ml) inhibited by 80% other platelet-aggregating activity of these tumor cells, while anti-TSP monoclonal antibody MA-II (15 micrograms/ml) had no effect. In sharp contrast, the anti-TSP polyclonal antibody (100 micrograms/ml) only exhibited a slight inhibitory effect on platelet aggregation induced by TE-85 cells when using a low concentration of tumor cells (0.6 x 10(6) cells/ml).(ABSTRACT TRUNCATED AT 400 WORDS)