Expression of the cell-adhesion molecule VCAM-1 by stromal cells is necessary for osteoclastogenesis

Thy-1 knockdown promotes the osteogenic differentiation of GMSCs via the Wnt/β-catenin pathway

Gingival mesenchymal stem cells (GMSCs) are newly developed seed cells for tissue engineering owing to their easy isolation, abundance and high growth rates. Thy-1 is an important regulatory molecule in the differentiation of mesenchymal stem cells (MSCs). In this study, we investigated the function of Thy-1 in the osteogenic differentiation of GMSCs by reducing the expression of Thy-1 using a lentivirus. The results demonstrated that Thy-1 knockdown promoted the osteogenic differentiation of GMSCs in vitro. Validation by RNA-seq revealed an obvious decrease in Vcam1 and Sox9 gene expression with Thy-1 knockdown. Kyoto Encyclopedia of Genes and Genomes pathway analysis suggested that the differentially expressed genes were enriched in the Wnt signalling pathway. We further demonstrated that Thy-1 knockdown promoted osteogenic differentiation of GMSCs by activating the Wnt/β-catenin signalling pathway. Therefore, Thy-1 has a key regulatory role in the differentiation of GMSCs and maybe a core molecule connecting transcription factors related to the differentiation of MSCs. Our study also highlighted the potential of Thy-1 to modify MSCs, which may help improve their use in tissue engineering.

Intravenous Transplantation of Human Hair Follicle-Derived Mesenchymal Stem Cells Ameliorates Trabecular Bone Loss in Osteoporotic Mice

Background: Hair follicles harbor a rich autologous stem cell pool and human hair follicle-derived mesenchymal stem cells (hHF-MSCs) have multi-lineage differentiation potential. Many sources of MSCs include hHF-MSCs have been attractive candidates for cell therapy, regenerative medicine and tissue engineering. The present study is to explore the effect of intravenous transplantation of hHF-MSCs on bone mass in osteoporotic mice and its mechanism, and provides prospects for clinical applications for the treatment of osteoporosis with hHF-MSCs.

Methods: Physically pull out about 20 hairs with intact hair follicles from the occipital area of the scalp of healthy volunteers, and extract hair follicle-derived fibroblast-like cells. These cells were cultured and characterized in vitro. Intravenous injection of hHF-MSCs was performed on ovariectomy-induced and age-related osteoporotic SCID mice for osteoporosis treatment. The mice were sacrificed 7 weeks after the second injection and samples were collected. The long bones and L1 vertebrae were collected for micro-CT scan, histomorphometry and immunohistochemical analysis. Peripheral serum were collected for ELISA analysis and antibody array.

Results: Hair follicle-derived fibroblast-like cells were defined as hHF-MSCs. Intravenous transplantation of hHF-MSCs can better restores trabecular bone mass in osteoporotic mice. The double calcein labeling assay, trap staining of bones and ELISA analysis in peripheral serum showed enhanced bone formation and weakened bone resorption after transplantation. Antibody array and immunohistochemical analysis showed that several cytokines including OPG, Wnt2b, Noggin, VCAM-1 and RANKL might be involved in this process.

Conclusion: Human HF-MSCs transplantation can combat trabecular bone loss induced by menopause and aging in mice. And the above mechanism that hHF-MSCs transplantation inhibits bone resorption and promote bone formation is related to OPG, Wnt2b, VCAM-1, Noggin and RANKL.

Osteoimmuno-Oncology: Therapeutic Opportunities for Targeting Immune Cells in Bone Metastasis

Immunotherapies provide a potential treatment option for currently incurable bone metastases. Bone marrow is an important secondary lymphoid organ with a unique immune contexture. Even at non-disease state immune cells and bone cells interact with each other, bone cells supporting the development of immune cells and immune cells regulating bone turnover. In cancer, tumor cells interfere with this homeostatic process starting from formation of pre-metastatic niche and later supporting growth of bone metastases. In this review, we introduce a novel concept osteoimmuno-oncology (OIO), which refers to interactions between bone, immune and tumor cells in bone metastatic microenvironment. We also discuss therapeutic opportunities of targeting immune cells in bone metastases, and associated efficacy and safety concerns.

Vascularization and osteogenesis in ectopically implanted bone tissue-engineered constructs with endothelial and osteogenic differentiated adipose-derived stem cells

BACKGROUND

A major problem in the healing of bone defects is insufficient or absent blood supply within the defect. To overcome this challenging problem, a plethora of approaches within bone tissue engineering have been developed recently. Bearing in mind that the interplay of various diffusible factors released by endothelial cells (ECs) and osteoblasts (OBs) have a pivotal role in bone growth and regeneration and that adjacent ECs and OBs also communicate directly through gap junctions, we set the focus on the simultaneous application of these cell types together with platelet-rich plasma (PRP) as a growth factor reservoir within ectopic bone tissue engineering constructs.

AIM

To vascularize and examine osteogenesis in bone tissue engineering constructs enriched with PRP and adipose-derived stem cells (ASCs) induced into ECs and OBs.

METHODS

ASCs isolated from adipose tissue, induced in vitro into ECs, OBs or just expanded were used for implant construction as followed: BPEO, endothelial and osteogenic differentiated ASCs with PRP and bone mineral matrix; BPUI, uninduced ASCs with PRP and bone mineral matrix; BC (control), only bone mineral matrix. At 1, 2, 4 and 8 wk after subcutaneous implantation in mice, implants were extracted and endothelial-related and bone-related gene expression were analyzed, while histological analyses were performed after 2 and 8 wk.

RESULTS

The percentage of vascularization was significantly higher in BC compared to BPUI and BPEO constructs 2 and 8 wk after implantation. BC had the lowest endothelial-related gene expression, weaker osteocalcin immunoexpression and Spp1 expression compared to BPUI and BPEO. Endothelial-related gene expression and osteocalcin immunoexpression were higher in BPUI compared to BC and BPEO. BPEO had a higher percentage of vascularization compared to BPUI and the highest CD31 immunoexpression among examined constructs. Except Vwf, endothelial-related gene expression in BPEO had a later onset and was upregulated and well-balanced during in vivo incubation that induced late onset of Spp1 expression and pronounced osteocalcin immunoexpression at 2 and 8 wk. Tissue regression was noticed in BPEO constructs after 8 wk.

CONCLUSION

Ectopically implanted BPEO constructs had a favorable impact on vascularization and osteogenesis, but tissue regression imposed the need for discovering a more optimal EC/OB ratio prior to considerations for clinical applications.

Bone lesions in systemic mastocytosis: Bone histomorphometry and histopathological mechanisms

Osteoporosis is considered the most frequent skeletal manifestation of systemic mastocytosis (SM). We performed a retrospective analysis of sixty patients (37 males and 23 females) who underwent a bone biopsy in the assessment of SM or in the assessment of unexplained bone fragility. Thirty-three had simultaneously a bone marrow biopsy with a Jamshidi's needle; this sample was used for immunohistochemical analysis (tryptase, c-KIT. CD20, VCAM-1). Bone biopsy was realized in 42 cases in the assessment of SM to provide histologic proof of the disease and in 18 cases in the assessment of unexplained bone fragility and surprisingly revealed a SM. An increased bone turnover was observed in patients with SM with elevated eroded surfaces, osteoclast number and bone formation rate. In addition to nodules of mast cells (MC), a high number of MC was directly apposed on the trabeculae, affixed on the osteoblasts or the lining cells. The VCAM-1 adhesion protein recognizing α4β7 and α4β1 integrins may be a candidate to explain this particular adherence. One third of the bone marrow biopsies did not exhibit MC nodules or MC infiltration and led to a false negative diagnosis for SM. SM can be discovered in the assessment of fracture or osteoporosis. Transiliac bone biopsy allows for the diagnosis of the disease more accurately than bone marrow biopsy; it also provides a histomorphometric analysis of bone remodeling.

Coordination of early cellular reactions during activation of bone resorption in the rat mandible periosteum: An immunohistochemical study

The activation step of bone remodeling remains poorly characterized. Activation comprises determination of the site to be remodeled, osteoclast precursor recruitment, their migration to the site of remodeling, and differentiation. These actions involve different compartments and cell types. The aim of this study was to investigate events and cell types involved during activation. We used a bone remodeling model in rats where extractions of the upper jaw molars initiate remodeling of the antagonist lower jaw (mandible) cortex along the periosteum. In this model osteoclastic resorption peaks 4 days after extractions. We previously reported that mast cell activation in the periosteum fibrous compartment is an early event of activation, associated with recruitment of circulating monocyte osteoclast precursors. By using immunohistochemistry, we observed 9 hours after induction a spatially oriented expression of InterCellular Adhesion Molecule-1 in the vessels that was inhibited by antagonists of histamine receptors 1 and 2. It was followed at 12 hours by the recruitment of ED1+ monocytes. In parallel, at 9 hours, Vascular Cellular Adhesion Molecule-1+ fibroblast-like cells scattered in the fibrous compartment of the periosteum between the vessels and the osteogenic compartment increased; these cells may be implicated in osteoclast precursor migration. Receptor Activator of NF KappaB Ligand+ cells increased at 12 hours in the osteogenic compartment and reached a peak at 18 hours. At 24 hours the numbers of osteogenic cells and subjacent osteocytes expressing semaphorin 3a, a repulsive for osteoclast precursors, decreased before returning to baseline at 48 hours. These data show that during activation the two periosteum compartments and several cell types are coordinated to recruit and guide osteoclast precursors towards the bone surface.

Breast cancer metastasis: Putative therapeutic role of vascular cell adhesion molecule-1

BACKGROUND

Breast cancer is a notable cause of cancer-related death in women worldwide. Metastasis to distant organs is responsible for ~90% of this death. Breast cells convert to malignant cancer cells after acquiring the capacity of invasion/intravasation into surrounding tissues and, finally, extravasation/metastasis to distant organs (i.e., lymph nodes, lungs, bone, brain). Metastasis to distant organs depends on interactions between disseminated tumor cells (DTCs) and the endothelium of blood vessels present in the tumor microenvironment. Among several known endothelial adhesion molecules, vascular cell adhesion molecule-1 (VCAM-1) has been found to be involved in this process. It has been shown that VCAM-1 is aberrantly expressed in breast cancer cells and that it can bind to its natural ligand α4β1integrin, also denoted as very late antigen 4 (VLA-4). This binding appears to be responsible for the metastasis of breast cancer cells to lung, bone and brain. The α4β1 integrin - VCAM-1 interaction thus represents a potential therapeutic target for metastatic breast cancer cells. The development of inhibitors of this interaction may be instrumental for the clinical management of breast cancer patients.

CONCLUSIONS

This study focuses on recent progress on the role of VCAM-1, an important glycoprotein belonging to the immunoglobulin (Ig) superfamily of cell surface adhesion molecules in breast cancer angiogenesis, survival and metastasis. Targeting VCAM-1, expressed on the surface of breast cancer cells, and/or its specific ligand VLA-4/α4β1 integrin, expressed on cells at the site of metastasis, may be a useful strategy to reduce breast cancer cell invasion and metastasis. Various approaches to therapeutically target VCAM-1 and VLA-4 are also discussed.

Immune impairments in multiple myeloma bone marrow mesenchymal stromal cells

In multiple myeloma (MM), bone marrow mesenchymal stromal cells (BM-MSCs) play an important role in pathogenesis and disease progression by supporting myeloma cell growth and immune escape. Previous studies have suggested that direct and indirect interactions between malignant cells and BM-MSCs result in constitutive abnormal immunomodulatory capacities in MM BM-MSCs. The aim of this study was to investigate the mechanisms that underlie these MM BM-MSCs abnormalities. We demonstrated that MM BM-MSCs exhibit abnormal expression of CD40/40L, VCAM1, ICAM-1, LFA-3, HO-1, HLA-DR and HLA-ABC. Furthermore, an overproduction of IL-6 (1,806 ± 152.5 vs 719.6 ± 18.22 ng/mL; p = 0.035) and a reduced secretion of IL-10 (136 ± 15.02 vs 346.4 ± 35.32 ng/mL; p = 0.015) were quantified in culture medium when MM BM-MSCs were co-cultured with T lymphocytes compared to co-cultures with healthy donor (HD) BM-MSCs. An increased Th17/Treg ratio was observed when T cells were co-cultured with MM BM-MSCs compared to co-cultures with HD BM-MSCs (0.955 vs 0.055). Together, these observations demonstrated that altered immunomodulation capacities of MM BM-MSCs were linked to variations in their immunogenicity and secretion profile. These alterations lead not only to a reduced inhibition of T cell proliferation but also to a shift in the Th17/Treg balance. We identified factors that are potentially responsible for these alterations, such as IL-6, VCAM-1 and CD40, which could also be associated with MM pathogenesis and progression.

Activation of VCAM-1 and its associated molecule CD44 leads to increased malignant potential of breast cancer cells

VCAM-1 (CD106), a transmembrane glycoprotein, was first reported to play an important role in leukocyte adhesion, leukocyte transendothelial migration and cell activation by binding to integrin VLA-1 (α4β1). In the present study, we observed that VCAM-1 expression can be induced in many breast cancer epithelial cells by cytokine stimulation in vitro and its up-regulation directly correlated with advanced clinical breast cancer stage. We found that VCAM-1 over-expression in the NMuMG breast epithelial cells controls the epithelial and mesenchymal transition (EMT) program to increase cell motility rates and promote chemoresistance to doxorubicin and cisplatin in vitro. Conversely, in the established MDAMB231 metastatic breast cancer cell line, we confirmed that knockdown of endogenous VCAM-1 expression reduced cell proliferation and inhibited TGFβ1 or IL-6 mediated cell migration, and increased chemosensitivity. Furthermore, we demonstrated that knockdown of endogenous VCAM-1 expression in MDAMB231 cells reduced tumor formation in a SCID xenograft mouse model. Signaling studies showed that VCAM-1 physically associates with CD44 and enhances CD44 and ABCG2 expression. Our findings uncover the possible mechanism of VCAM-1 activation facilitating breast cancer progression, and suggest that targeting VCAM-1 is an attractive strategy for therapeutic intervention.

Different sympathetic pathways control the metabolism of distinct bone envelopes

Bone remodeling, the mechanism that modulates bone mass adaptation, is controlled by the sympathetic nervous system through the catecholaminergic pathway. However, resorption in the mandible periosteum envelope is associated with cholinergic Vasoactive Intestinal Peptide (VIP)-positive nerve fibers sensitive to sympathetic neurotoxics, suggesting that different sympathetic pathways may control distinct bone envelopes. In this study, we assessed the role of distinct sympathetic pathways on rat femur and mandible envelopes. To this goal, adult male Wistar rats were chemically sympathectomized or treated with agonists/antagonists of the catecholaminergic and cholinergic pathways; femora and mandibles were sampled. Histomorphometric analysis showed that sympathectomy decreased the number of preosteoclasts and RANKL-expressing osteoblasts in mandible periosteum but had no effect on femur trabecular bone. In contrast, pharmacological stimulation or repression of the catecholaminergic cell receptors impacted the femur trabecular bone and mandible endosteal retromolar zone. VIP treatment of sympathectomized rats rescued the disturbances of the mandible periosteum and alveolar wall whereas the cholinergic pathway had no effect on the catecholaminergic-dependent envelopes. We also found that VIP receptor-1 was weakly expressed in periosteal osteoblasts in the mandible and was increased by VIP treatment, whereas osteoblasts of the retromolar envelope that was innervated only by tyrosine hydroxylase-immunoreactive fibers, constitutively expressed beta-2 adrenergic receptors. These data highlight the complexity of the sympathetic control of bone metabolism. Both the embryological origin of the bone (endochondral for the femur, membranous for the mandibular periosteum and the socket wall) and environmental factors specific to the innervated envelope may influence the phenotype of the sympathetic innervation. We suggest that an origin-dependent imprint of bone cells through osteoblast-nerve interactions determines the type of autonomous system innervating a particular bone envelope.

JunB as a potential mediator of PTHrP actions: new gene targets Ephrin B1 and VCAM-1

Parathyroid hormone-related protein (PTHrP) is an integral mediator of physiologic and pathologic processes and has demonstrated actions in the periodontium. PTHrP functions via AP-1, and specifically through JunB. This study identified JunB-dependent downstream mediators of PTHrP using OCCM cementoblastic transfectants with JunB over- or reduced expression. Over-expressing cells showed an increase in proliferation, while the opposite was seen in siRNA transfected cells. Microarray analysis of over-expressing cells revealed more than 1000 regulated genes. Three genes were investigated in more detail. The PTH/PTHrP receptor (PTHR1) and ephrin B1 (EfnB1) were down-regulated, and vascular cell adhesion molecule-1 (VCAM-1) was up-regulated with JunB over-expression. JunB siRNA transfectants had increased PTHR1, but reduced ephrin B1 and unaltered VCAM-1 in vitro. To validate these targets, parental OCCM cells and primary osteoblasts were treated with PTHrP, resulting in reduced PTHR1 and ephrin B1, and increased VCAM-1. Cell transfectants were implanted subcutaneously in vivo, and microarray analysis and RT-PCR performed. Over-expression of JunB down-regulated PTHR1 and ephrin B1, and increased VCAM-1. JunB siRNA transfectant implants had increased PTHR1 and ephrin B1, but no altered VCAM-1. These data highlight new gene targets for PTHrP and indicate JunB is a critical mediator of PTHrP actions.

Accentuated osteoclastic response to parathyroid hormone undermines bone mass acquisition in osteonectin-null mice

Matricellular proteins play a unique role in the skeleton as regulators of bone remodeling, and the matricellular protein osteonectin (SPARC, BM-40) is the most abundant non-collagenous protein in bone. In the absence of osteonectin, mice develop progressive low turnover osteopenia, particularly affecting trabecular bone. Polymorphisms in a regulatory region of the osteonectin gene are associated with bone mass in a subset of idiopathic osteoporosis patients, and these polymorphisms likely regulate osteonectin expression. Thus it is important to determine how osteonectin gene dosage affects skeletal function. Moreover, intermittent administration of parathyroid hormone (PTH) (1-34) is the only anabolic therapy approved for the treatment of osteoporosis, and it is critical to understand how modulators of bone remodeling, such as osteonectin, affect skeletal response to anabolic agents. In this study, 10 week old female wild type, osteonectin-haploinsufficient, and osteonectin-null mice (C57Bl/6 genetic background) were given 80 microg/kg body weight/day PTH(1-34) for 4 weeks. Osteonectin gene dosage had a profound effect on bone microarchitecture. The connectivity density of trabecular bone in osteonectin-haploinsufficient mice was substantially decreased compared with that of wild type mice, suggesting compromised mechanical properties. Whereas mice of each genotype had a similar osteoblastic response to PTH treatment, the osteoclastic response was accentuated in osteonectin-haploinsufficient and osteonectin-null mice. Eroded surface and osteoclast number were significantly higher in PTH-treated osteonectin-null mice, as was endosteal area. In vitro studies confirmed that PTH induced the formation of more osteoclast-like cells in marrow from osteonectin-null mice compared with wild type. PTH treated osteonectin-null bone marrow cells expressed more RANKL mRNA compared with wild type. However, the ratio of RANKL:OPG mRNA was somewhat lower in PTH treated osteonectin-null cultures. Increased expression of RANKL in response to PTH could contribute to the accentuated osteoclastic response in osteonectin-/- mice, but other mechanisms are also likely to be involved. The molecular mechanisms by which PTH elicits bone anabolic vs. bone catabolic effects remain poorly understood. Our results imply that osteonectin levels may play a role in modulating the balance of bone formation and resorption in response to PTH.

Estradiol inhibits adhesion and promotes apoptosis in murine osteoclasts in vitro

Osteoporosis caused by estrogen deficiency is characterized by enhanced bone resorption mediated by osteoclasts. Adhesion to bone matrix and survival of differentiated osteoclasts is necessary to resorb bone. The aim of our study was to investigate the in vitro effects of estradiol on murine osteoclasts. RAW 264.7 cells treated with 30 ng/ml RANK-L were used as a model for osteoclastogenesis. Estradiol (10(-8)M) for 5 days induced an inhibition of osteoclast differentiation and beta3 expression. Estradiol inhibited significantly the adhesion of mature osteoclasts by 30%. Furthermore estradiol-induced apoptosis shown by with nuclear condensation and Bax/Bcl2 ratio. In addition, estradiol enhanced caspase-3, -8 and -9 activities. This effect completely disappeared using specific caspase-8 inhibitor. However, increased caspase-3 activity by estradiol was observed in the presence of caspase-9 inhibitor, indicating the preferential involvement of caspase-8 pathway. Fas and FasL mRNA expression was not regulated by estradiol. However, estradiol enhanced caspase-3 activity in Fas-induced apoptosis on mature osteoclasts, suggesting that this might interact with the Fas-signaling pathway. These data suggest that estradiol decreases bone resorption by several mechanisms including adhesion and apoptosis of osteoclasts.

Therapeutic anti-integrin (alpha4 and alphaL) monoclonal antibodies: two-edged swords?

Anti-alpha4 and anti-alphaL integrin chain monoclonal antibodies have shown a clear-cut beneficial effect in different animal models of autoimmune and inflammatory disorders as well as in human diseases, including multiple sclerosis, inflammatory bowel disease, and psoriasis. It has been widely assumed that this therapeutic effect is mainly consequence of the blockade of leucocyte adhesion to endothelium, inhibiting thus their extravasation and the inflammatory phenomenon. However, it is evident that both alpha4beta1 (very late antigen-4) and alphaLbeta2 (leucocyte function-associated antigen-1) integrins have additional important roles in other immune phenomena, including the formation of the immune synapse and the differentiation of T helper 1 lymphocytes. Therefore, it is very feasible that the long-term administration of blocking agents directed against these integrins to patients with inflammatory/autoimmune conditions may have undesirable or unexpected effects.

Dominant negative N-cadherin inhibits osteoclast differentiation by interfering with beta-catenin regulation of RANKL, independent of cell-cell adhesion

We studied the effects of dominant negative N-cadherin (NCadDeltaC) expression in ST2 cells on their ability to support osteoclastogenesis. Expression of NCadDeltaC in ST2 cells did not decrease cell-to-cell adhesion but significantly reduced osteoclast formation when co-cultured with BMMs. NCadDeltaC inhibited beta-catenin/TCF signaling, resulting in decreased RANKL expression, which could contribute to the reduced osteoclast formation.

INTRODUCTION

Cadherin is a calcium-dependent cell adhesion molecule that plays major roles during embryonic development and morphogenesis. Classic cadherins interact with beta-catenin, which is also involved in the Wnt signaling pathway. We tested whether disruption of N-cadherin function in stromal cells by dominant negative N-cadherin affects their ability to support osteoclastogenesis by altering heterotypic interaction with osteoclast precursors.

MATERIALS AND METHODS

ST2 cells were transduced with retrovirus encoding extracellular domain-truncated, dominant negative N-cadherin (NCadDeltaC) and co-cultured with bone marrow macrophages (BMMs) to study the ability to support osteoclastogenesis. As a downstream target of NCadDeltaC, beta-catenin/T-cell factor (TCF) transcriptional activity was analyzed using TOPflash reporter construct. Real-time RT-PCR analysis and RANKL-luciferase reporter assays were performed to study the effects of NCadDeltaC on the osteoprotegerin (OPG)/RANKL system.

RESULTS

Immunoblotting analysis showed that primary bone marrow stromal cells, ST2 cells, and BMMs expressed N-cadherin. Retroviral expression of NCadDeltaC in ST2 cells did not significantly inhibit cell adhesion but markedly impaired the formation of TRACP(+) osteoclasts (>40%) when co-cultured with BMMs. However, the inhibition of osteoclastogenesis was not reproduced by neutralizing antibody against N-cadherin. Expression of NCadDeltaC, however, strongly suppressed beta-catenin/TCF transcriptional activity in ST2 cells, which was rescued by constitutively active beta-catenin adenovirus (Ad DeltaN46 beta-catenin) or constitutively active TCF mutant (pCS2-VP16DeltabetaXTCF-3). As a potential downstream target of Wnt signaling, we found that the expression of RANKL was reduced in ST2 cells expressing NCadDeltaC. Moreover, Wnt-3A, Ad DeltaN46 beta-catenin, and VP16DeltabetaXTCF-3 increased the expression of RANKL and enhanced the transcriptional activity of mouse RANKL promoter in ST2 cells.

CONCLUSIONS

Our data suggest that expression of dominant negative N-cadherin in ST2 cells suppressed osteoclastogenesis by interfering with beta-catenin regulation of RANKL independent of cell-cell adhesion.

The Wnt signaling inhibitor dickkopf-1 is required for reentry into the cell cycle of human adult stem cells from bone marrow

Adult human mesenchymal stem cells from bone marrow stroma (hMSCs) differentiate into numerous mesenchymal tissue lineages and are attractive candidates for cell and gene therapy. When early passage hMSCs are plated or replated at low density, the cultures display a lag phase of 3-5 days, a phase of rapid exponential growth, and then enter a stationary phase without the cultures reaching confluence. We found that as the cultures leave the lag phase, they secrete high levels of dickkopf-1 (Dkk-1), an inhibitor of the canonical Wnt signaling pathway. The addition of recombinant Dkk-1 toward the end of the lag period increased proliferation and decreased the cellular concentration of beta-catenin. The addition of antibodies to Dkk-1 in the early log phase decreased proliferation. Also, expression of Dkk-1 in hMSCs decreased during cell cycle arrest induced by serum starvation. The results indicated that high levels of Dkk-1 allow the cells to reenter the cell cycle by inhibiting the canonical Wnt/beta-catenin signaling pathway. Since antibodies to Dkk-1 also increased the lag phase of an osteosarcoma line that expressed the gene, Dkk-1 may have a similar role in some other cell systems.

IL-13 regulates vascular cell adhesion molecule-1 expression in human osteoblasts

Activated T cells (Act T) produce multiple cytokines that affect osteoblast function as well as osteoclastogenesis. One of these cytokines, IL-13, is a multifunctional cytokine elaborated by Act T that regulates vascular cellular adhesion molecule (VCAM)-1 expression in endothelial cells. VCAM-1 has also been implicated in osteoclast formation by myeloma cells. We therefore studied whether IL-13 regulates VCAM-1 in human osteoblastic cells since these cells express RANKL, the major osteoclastogenic factor and osteoclast precursors are found adjacent to osteoblasts. Human T cells were activated in the absence or presence of Cyclosporin A (CsA), an inhibitor of the production of most activated T cell cytokines. Conditioned media were assayed for IL-13 by ELISA. Act T produced IL-13 and, unlike other T cell cytokines, this was elevated 3-fold by CsA. Exposure of human osteoblasts (hOB) to doses of recombinant human IL-13 (rhIL-13, 0-10 ng/ml) resulted in an increase of VCAM-1 mRNA (up to 5-fold) within 4 h with a maximum stimulation at 1 ng/ml. CsA had no effect on basal hOB VCAM-1 mRNA expression. Examination of VCAM-1 on the cell surface of hOB, by immunocytochemistry, revealed increasing levels of surface expression of the protein within 16 h after stimulation with doses of rhIL-13 (0.1-10 ng/ml) which were reflective of the mRNAs. IL-6 production was also stimulated in a dose dependent manner with a maximum of 2.5-fold with 1 ng/ml rhIL-13 within 16 h. Since both VCAM-1 and IL-6 showed similar responses to IL-13, IL-6 was examined for its ability to induce VCAM-1. Immunocytochemistry demonstrated no effect of IL-6 on VCAM-1 expression. These data demonstrate that during pathological processes associated with T cell activation, such as rheumatoid arthritis or possibly post-menopausal osteoporosis, T cells may play a pivotal role in osteoclast precursor adhesion to osteoblasts as a first step prior to RANKL signaling.

Osteoclastogenesis-related antigen, a novel molecule on mouse stromal cells, regulates osteoclastogenesis

Osteoclastogenesis is regulated by RANKL expressed on stromal cells. In this study, we sought to isolate a new surface molecule regulating osteoclastogenesis on stromal cells by generating monoclonal antibodies. A rat was immunized with the mouse stromal cell line, TSB13, which can support osteoclastogenesis, and a monoclonal antibody, A15-1, was obtained. A15-1 bound to a surface antigen on TSB13 cells, termed osteoclastogenesis-related antigen (OCRA), and immunoprecipitation with this antibody revealed that OCRA was a 220-kDa molecule. By means of flow cytometry, the A15-1 antigen (OCRA) was found to be expressed on various mesenchymal cell lines but not on hematopoietic cell lines, and the expression level of OCRA on the TSB13 cells was slightly increased by treatment with 1alpha,25(OH)2D3. When osteoclast progenitors and TSB13 cells were co-cultured in the presence of 1alpha,25(OH)2D3, the addition of A15-1 inhibited osteoclast differentiation in a dose-dependent manner; however, no significant inhibition of soluble RANKL-induced osteoclastogenesis was observed, suggesting that A15-1 inhibited only stromal cell-dependent osteoclastogenesis. The same inhibitory effect of A15-1 was also observed when primary bone marrow-derived stromal cells were used. The osteoclastogenesis-promoting effects of other osteotropic factors, such as parathyroid hormone (PTH) and interleukin (IL)-1beta, were also inhibited by A15-1. Time-course analysis of osteoclast differentiation in vitro indicated that the initial 2 days of treatment with A15-1 was sufficient for inhibition, suggesting that A15-1 inhibits the early stages of osteoclast differentiation. Finally, we investigated the in vivo effects of A15-1 on PTH-induced hypercalcemia in mice. Treatment with A15-1 significantly decreased the osteoclast surface in the PTH-administered mice. Taken together, our data indicate that OCRA, a novel A15-1-detected antigen, regulates stromal cell-dependent osteoclastogenesis.

Cell-to-Cell adhesion via intercellular adhesion molecule-1 and leukocyte function-associated antigen-1 pathway is involved in 1alpha,25(OH)2D3, PTH and IL-1alpha-induced osteoclast differentiation and bone resorption

Cell-to-cell interaction is required for the differentiation of osteoclast precursors as well as for osteoclast function. The present study was undertaken to determine whether 1,25 dihydroxyvitamin D3 (1,25D), PTH, IL-1alpha and PGE2 depend on cell-to-cell interactions through the intercellular adhesion molecule (ICAM)-1/leukocyte function-associated antigen (LFA)-1 pathway in osteoclast formation and bone resorption. We found that mouse osteoblasts expressed ICAM-1 and that the expression was increased by treatment with PTH, IL-1alpha or 1,25D, but not by PGE2. In resorption assays measuring either 45Ca release from bone organ cultures or pit formation in bone cell cultures, 1,25D-, PTH- and IL-1alpha-stimulated resorption was inhibited by anti-ICAM-1 monoclonal antibody (mAb) and/or anti-LFA-1 mAb, while basal and PGE2-stimulated bone resorbing activities were not affected by these mAbs. Furthermore, in a mouse bone marrow culture system, stimulation of osteoclast-like (OCL) cell formation by 1,25D (10 nM), PTH (10 ng/ml) or IL-1alpha (10 ng/ml) was inhibited by the addition of anti-ICAM-1 mAb and/or anti-LFA-1 mAb. In a coculture system of murine spleen cells and osteoblasts, the ICAM-1/LFA-1 interaction was also involved in 1,25D-, PTH- and IL-1alpha-stimulated TRAP-positive MNC formation. However, anti-ICAM-1 mAb and anti-LFA-1 mAb did not alter either 1,25D- or PTH-stimulated receptor activator of NF-kappaB ligand (RANKL) mRNA transcription in bone marrow cultures. Taken together, we here propose that ICAM-1-mediated cell-to-cell adhesion of osteoblasts and osteoclast precursors is involved in RANKL-dependent osteoclast maturation stimulated by 1,25D, PTH, and IL-1alpha.

The role of LFA-1 in osteoclast development induced by co-cultures of mouse bone marrow cells and MC3T3-G2/PA6 cells

Interactions between leukocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) influence the development of osteoclasts. However, little is known about how these adhesion molecules are involved in the process of osteoclast development. This study evaluated the role of LFA-1 and its ligands in osteoclast development and bone resorption. Co-cultures of bone marrow cells from LFA-1-deficient mice and MC3T3-G2/PA6 (PA6) cells were cultured in the presence of 1alpha,25(OH)2D3 and dexamethasone for 7 days. The number of TRAP-positive cells that were generated by bone marrow cells from LFA-1-deficient mice was smaller than that generated by bone marrow cells from wild-type mice. In addition, the bone-resorbing activity of osteoclast-like cells that were generated from LFA-1-deficient mice was lower than that generated by osteoclast-like cells from wild-type mice. Immunofluorescence flow cytometry showed that osteoclast stromal PA6 cells expressed the cell adhesion molecules, ICAM-1 and VCAM-1. When monoclonal antibodies to mice VCAM-1, CD11b or CD18 were added separately to the co-culture system, the number of TRAP-positive cells that were generated from LFA-1-deficient mice was 20-30% smaller than that generated from wild-type mice. The formation of TRAP-positive cells from both LFA-1 deficient and wild-type mice was especially inhibited by anti-CD18 antibody, in comparison to the addition of normal IgG serum. These results suggest that LFA-1 adhesion molecules play a role in osteoclast development by affecting adhesion between stromal cells and osteoclast progenitors before the occurrence of ODF-ODF receptor signaling. CD18 appears to be a key adhesion molecule in cell-to-cell contacts during the early stage of osteoclast development.

beta(2)-Microglobulin increases the expression of vascular cell adhesion molecule on human synovial fibroblasts

BACKGROUND

beta(2)-Microglobulin (beta(2)m) amyloidosis is a destructive articular disease that affects patients on dialysis. The disease presentation is similar to other forms of arthritis in which adhesion molecules are felt to be pathogenic. Therefore, we hypothesized that beta(2)m directly increases the expression of vascular cell adhesion molecule-1 (VCAM-1) by synovial fibroblasts. We also examined the effect of alteration of beta(2)m by advanced glycation end products on this cellular response.

METHODS

Human synovial fibroblasts were isolated and incubated with beta(2)m with and without alteration with advanced glycation end products. VCAM-1 expression was determined by immunohistochemistry, flow cytometry, and Western blot and Northern blot analyses.

RESULTS

beta(2)m increased the protein expression of VCAM-1 by synovial fibroblasts in a dose-dependent manner. beta(2)m altered with advanced glycation end products had no effect. However, all forms of beta(2)m increased VCAM-1 mRNA. beta(2)m also increased the adhesion of peripheral blood mononuclear cells to synovial fibroblasts.

CONCLUSION

beta2m directly increases the expression of VCAM-1 by synovial fibroblasts, indicating that synovial fibroblasts may play a key role in the pathogenesis of beta(2)m amyloidosis.

The role of the synovium and cartilage in the pathogenesis of beta(2)-microglobulin amyloidosis

The predilection for beta(2)-microglobulin (beta(2)M) amyloid deposition in articular structures is unique compared to other forms of amyloid; this article focuses on possible pathogenic mechanisms. The synovium and/or cartilage appear to be important in the pathogenesis of beta(2)M amyloidosis (A beta(2)M), as amyloid is not found in the shafts of long bones. The concentration of beta(2)M in the joint fluid parallels that in serum. Once in the joint space, evidence suggests that the beta(2)M binds to collagen in cartilage as the initial site of deposition. This binding may serve as the first step in subsequent amyloid formation, although this remains to be proven. beta(2)M has been shown to have many direct effects on synovial fibroblasts, including induction of the release of cytokines, metalloproteinases, cyclooxygenase-2, and vascular cell adhesion molecule-1 (VCAM-1). The release of these inflammatory mediators that lead to tissue degradation is also observed in other forms of arthritis. Thus beta(2)M itself may elicit the release of inflammatory mediators from synovial fibroblasts even in the absence of cellular infiltrates.

Increasing membrane-bound MCSF does not enhance OPGL-driven osteoclastogenesis from marrow cells

Macrophage colony-stimulating factor (MCSF) and osteoprotegerin ligand (OPGL), both produced by osteoblasts/stromal cells, are essential factors for osteoclastogenesis. Whether local MCSF levels regulate the amount of osteoclast formation is unclear. Two culture systems, ST-2 and Chinese hamster ovary-membrane-bound MCSF (CHO-mMCSF)-Tet-OFF cells, were used to study the role of mMCSF in osteoclast formation. Cells from bone marrow (BMM) or spleen were cultured with soluble OPGL on glutaraldehyde-fixed cell layers; osteoclasts formed after 7 days. Osteoclast number was proportional to the amount of soluble OPGL added. In contrast, varying mMCSF levels in the ST-2 or CHO-mMCSF-Tet-OFF cell layers, respectively by variable plating or by addition of doxycycline, did not affect BMM osteoclastogenesis: 20-450 U of mMCSF per well generated similar osteoclast numbers. In contrast, spleen cells were resistant to mMCSF: osteoclastogenesis required > or = 250 U per well and further increased as mMCSF rose higher. Our results demonstrate that osteoclast formation in the local bone environment is dominated by OPGL. Increasing mMCSF above basal levels does not further enhance osteoclast formation from BMMs, indicating that mMCSF does not play a dominant regulatory role in the bone marrow.

Expression of adhesion molecules LFA-I and ICAM-I on osteoclast precursors during osteoclast differentiation and involvement of estrogen deficiency

OBJECTIVES

Estrogen deficiency caused by the menopause or ovariectomy leads to stimulation of osteoclastogenesis. The adhesion molecules, leukocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1), are necessary for osteoclast formation. In this study, the expression of LFA-1 and ICAM-1 on osteoclast precursors during osteoclast differentiation, and the involvement of ovariectomy in the expression, were investigated.

METHODS

Spleen cells isolated from normal or ovariectomized (OVX) mice were co-cultured with TMS14, stromal cells derived from mouse bone marrow, in the absence or presence of 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) for 7 days. On days 3, 5 and 7 of culture, the expression of LFA-1 and ICAM-1 on osteoclast precursors was quantitated using indirect immunofluorescence and confocal laser cytometry, and, on day 7, the number of formed osteoclasts was measured by tartrate-resistant acid phosphatase (TRAP) stain.

RESULTS

The level of ICAM-1 expression on osteoclast precursors gradually increased with osteoclast differentiation, whereas that of LFA-1 did not change. A high level of ICAM-1 was observed on the integrin beta 3-positive mononuclear cells. On the osteoclast precursors isolated from OVX mice, both the level of ICAM-1 expression per cell and the number of cells showing a high expression of ICAM-1 significantly increased, with an increase in the number of osteoclast-like cells. However, the level of LFA-1 did not change.

CONCLUSIONS

These results indicate that the expression level of ICAM-1, but not that of LFA-1, is involved in osteoclast differentiation. Estrogen deficiency results in an increase in ICAM-1 expression on osteoclast precursors, which may be one of the mechanisms underlying bone loss following the menopause or ovariectomy.

Intercellular adhesion molecule 1 discriminates functionally different populations of human osteoblasts: characteristic involvement of cell cycle regulators

The concept of differential regulation of certain adhesion molecules on different cell subsets and their relevance to cell functions has emerged in recent years. The initial event in bone remodeling is an increase in osteoclastic bone resorption and cell adhesion between osteoclastic precursors and bone marrow stromal cells or osteoblasts is known to commit the osteoclast development. Here, we show that human osteoblasts can be divided into two subsets based on the expression of the intercellular adhesion molecule (ICAM)-1; ICAM-1+ osteoblasts highly adhered to monocytes, including osteoclast precursors, produced osteoclast differentiation factor (ODF), and induced multinuclear osteoclast-like cell formation. Anti-ODF monoclonal antibody (mAb) did not inhibit the adhesion of monocytes to osteoblastic cells, whereas anti-leukocyte function-associated antigen (LFA)-1, a receptor for ICAM-1, mAb blocked the adhesion. We thereby propose that the higher affinity adhesion via LFA-1/ICAM-1 is prerequisite for efficient function of membrane-bound ODF during osteoclast maturation. The functional characteristics of ICAM-1+ osteoblasts were emphasized further by cell cycle regulation, as manifested by (i) up-regulation of p53 and p21, (ii) reduction of activity of cyclin-dependent kinase (cdk) 6, (iii) underphosphorylation of retinoblastoma protein, (iv) increased Fas but reduced bcl-2 expression, and (v) majority of cells remained at G0/G1 phase. Furthermore, ICAM-1+ osteoblasts were induced by interleukin-1beta (IL-1beta). Taken together, we propose that the differentiation of osteoblasts to ICAM-1+ subpopulation by inflammatory cytokines plays an important role in osteoporosis, which is observed in patients with chronic inflammation, because ICAM-1+ osteoblasts can bias bone turnover to bone resorption, committing osteoclast maturation through cell adhesion with its precursor, and the majority of ICAM-1+ osteoblasts arrested at G0/G1 phase. Such regulation of cell cycle arrest also is an important determinant of the life span of cells in bone in which continuous bone remodeling maintains its homeostasis.

Cell–cell contact between marrow stromal cells and myeloma cells via VCAM-1 and α4β1-integrin enhances production of osteoclast-stimulating activity

Myeloma is a unique hematologic malignancy that exclusively homes in the bone marrow and induces massive osteoclastic bone destruction presumably by producing cytokines that promote the differentiation of the hematopoietic progenitors to osteoclasts (osteoclastogenesis). It is recognized that neighboring bone marrow stromal cells influence the expression of the malignant phenotype in myeloma cells. This study examined the role of the interactions between myeloma cells and neighboring stromal cells in the production of osteoclastogenic factors to elucidate the mechanism underlying extensive osteoclastic bone destruction. A murine myeloma cell line 5TGM1, which causes severe osteolysis, expresses α4β1-integrin and tightly adheres to the mouse marrow stromal cell line ST2, which expresses the vascular cell adhesion molecule-1 (VCAM-1), a ligand for α4β1-integrin. Co-cultures of 5TGM1 with primary bone marrow cells generated tartrate-resistant acid phosphatase-positive multinucleated bone-resorbing osteoclasts. Co-cultures of 5TGM1 with ST2 showed increased production of bone-resorbing activity and neutralizing antibodies against VCAM-1 or α4β1-integrin inhibited this. The 5TGM1 cells contacting recombinant VCAM-1 produced increased osteoclastogenic and bone-resorbing activity. The activity was not blocked by the neutralizing antibody to known osteoclastogenic cytokines including interleukin (IL)-1, IL-6, tumor necrosis factor, or parathyroid hormone-related peptide. These data suggest that myeloma cells are responsible for producing osteoclastogenic activity and that establishment of direct contact with marrow stromal cells via α4β1-integrin/VCAM-1 increases the production of this activity by myeloma cells. They also suggest that the presence of stromal cells may provide a microenvironment that allows exclusive colonization of myeloma cells in the bone marrow.

Cell–cell contact between marrow stromal cells and myeloma cells via VCAM-1 and α4β1-integrin enhances production of osteoclast-stimulating activity

Myeloma is a unique hematologic malignancy that exclusively homes in the bone marrow and induces massive osteoclastic bone destruction presumably by producing cytokines that promote the differentiation of the hematopoietic progenitors to osteoclasts (osteoclastogenesis). It is recognized that neighboring bone marrow stromal cells influence the expression of the malignant phenotype in myeloma cells. This study examined the role of the interactions between myeloma cells and neighboring stromal cells in the production of osteoclastogenic factors to elucidate the mechanism underlying extensive osteoclastic bone destruction. A murine myeloma cell line 5TGM1, which causes severe osteolysis, expresses α4β1-integrin and tightly adheres to the mouse marrow stromal cell line ST2, which expresses the vascular cell adhesion molecule-1 (VCAM-1), a ligand for α4β1-integrin. Co-cultures of 5TGM1 with primary bone marrow cells generated tartrate-resistant acid phosphatase-positive multinucleated bone-resorbing osteoclasts. Co-cultures of 5TGM1 with ST2 showed increased production of bone-resorbing activity and neutralizing antibodies against VCAM-1 or α4β1-integrin inhibited this. The 5TGM1 cells contacting recombinant VCAM-1 produced increased osteoclastogenic and bone-resorbing activity. The activity was not blocked by the neutralizing antibody to known osteoclastogenic cytokines including interleukin (IL)-1, IL-6, tumor necrosis factor, or parathyroid hormone-related peptide. These data suggest that myeloma cells are responsible for producing osteoclastogenic activity and that establishment of direct contact with marrow stromal cells via α4β1-integrin/VCAM-1 increases the production of this activity by myeloma cells. They also suggest that the presence of stromal cells may provide a microenvironment that allows exclusive colonization of myeloma cells in the bone marrow.

Expression of Src family kinases and their putative substrates in the human preosteoclastic cell line FLG 29.1

Several lines of evidence suggest that the c-Src tyrosine kinase has a specific role in bone-resorbing osteoclasts. To investigate this further, we examined the expression of c-Src, its kinase family members, and their putative substrates in the human leukemia cell line FLG 29.1. Western blot analysis with specific antibodies against Src family members showed expression of Src, Fyn, and Lyn, lower levels of Yes and Hck, and the absence of Lck tyrosine kinase. During a 3-day treatment with phorbol 12-myristate, 13-acetate (PMA), which induces differentiation of FLG 29.1 cells toward an osteoclast-like phenotype, the levels of Src and Fyn increased and the levels of Lyn decreased. In a similar leukemia cell line, HL-60, Src protein was not constitutively expressed and not induced by PMA treatment, which leads to monocytic differentiation. PMA treatment of FLG 29.1 cells induced a strong increase in the expression of p120 Cbl and Pyk2 kinase, which are putative Src substrates. Pyk2 phosphorylation increased upon adherence of FLG 29.1 cells to fibronectin and to ST2 stromal cells. The expression of other Src substrates and interacting proteins, such as p120 Cas, p130 Cas, vinculin, Fak kinase, and the p85 phosphatidylinositol 3-kinase subunit either did not change or slightly increased during PMA treatment. The elevated total protein tyrosine phosphorylation in PMA-treated FLG 29.1 cells was abolished by herbimycin A, a Src inhibitor. These data are consistent with the proposed role of Src in the osteoclastic function and support the use of FLG 29.1 cells as a model to study Src substrates in the cells of the osteoclastic lineage.

Bone marrow cell development and trabecular bone dynamics after ovariectomy in ddy mice

To clarify the relationship between the sequential changes of trabecular bone turnover and bone marrow cell development in ovariectomized (ovx) mice, bilateral tibiae of 8-week-old ddy mice were obtained. Histomorphometric analyses of the trabecular bone of the proximal tibia of ovx mice revealed increases in the bone formation rate and the osteoclast surface for the first 28 days postovariectomy. The trabecular bone volume showed a rapid decrease for the first 28 days and a steady state for the subsequent 14 days. In bone marrow cell culture experiments, the numbers of total and nonadherent bone marrow cells per tibia obtained from the ovx mice increased. The formation of osteogenic nodules and osteoclast-like multinucleated cells in the marrow cultures obtained from ovx limbs showed a significant increase on days 14 and 28 and returned to the sham-operated level by day 42. The numbers of colony forming units (fibroblastic) and colony forming units (granulocytes and macrophages) that developed from the marrow cells did not differ between the ovx and sham limbs at any time during the study period. Fluorescence-activated cell-sorter analysis revealed no population changes in the cell development of macrophages. These results demonstrate that there are two stages in the development of osteopenia after ovx. During the first 28 days after ovx, the ovariectomy enhances the developmental process from bone marrow stromal cells to osteoblasts and the terminal differentiation from osteoclast precursors to mature osteoclasts. The trabecular bone turnover also increases. In the subsequent 14 days, the changes in the osteogenic and osteoclastogenic potentials of the bone marrow cells are alleviated and the trabecular bone dynamics are in a steady state. The changes in bone marrow cell development are closely associated with those at the trabecular bone surface.

Involvement of lymphocyte function-associated antigen-1 and intercellular adhesion molecule-1 in osteoclastogenesis: a possible role in direct interaction between osteoclast precursors

In our search for molecules involved in the process of osteoclast differentiation, we examined the surface phenotypes of the preosteoclast-like cells and osteoclast-like multinucleated cells (MNCs) formed in bone marrow cultures, using monoclonal antibodies recognizing different antigen molecules expressed on hematopoietic cells. Among these cell surface antigens, lymphocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) were highly expressed on mononuclear cells in the cultures for forming preosteoclast-like mononuclear cells. The double detection of these two antigen molecules with osteoclast-specific antigen and with calcitonin receptor, using a fluorescence-activated cell sorter or autoradiography technique, revealed that LFA-1 and ICAM-1 were expressed on the preosteoclasts. The expression of ICAM-1 was detected on both preosteoclasts and osteoclast-like MNCs, whereas the expression of LFA-1 was restricted to preosteoclasts. We designed a peptide with the sequence of the binding site of ICAM-1 against the ligand LFA-1. In the whole bone marrow culture system for forming osteoclast-like MNCs, a significant inhibition of MNC formation was observed by the addition of this peptide. These results strongly suggest the involvement of an LFA-1/ICAM-1-interaction in osteoclastogenesis.

Increased B-lymphopoiesis by interleukin 7 induces bone loss in mice with intact ovarian function: similarity to estrogen deficiency

Estrogen deficiency caused by ovariectomy (OVX) results in a marked bone loss due to stimulated bone resorption by osteoclasts. During our investigations of the pathogenesis of bone loss in estrogen deficiency, we found that OVX selectively stimulates B-lymphopoiesis which results in marked accumulation of B220-positive pre-B cells in mouse bone marrow. To examine the possible correlation between stimulated B-lymphopoiesis and bone loss, 8-week-old female mice were treated with interleukin (IL) 7, which stimulates B-lymphopoiesis in bone marrow. We also examined bone mass in IL-7 receptor-knockout mice that exhibit marked suppression of B-lymphopoiesis in the bone marrow. The increased B-lymphopoiesis induced by IL-7 administration resulted in marked bone loss by stimulation of osteoclastic bone resorption in mice with intact ovarian function. The changes in both B-lymphopoiesis and bone mass in IL-7-treated female mice were similar to those in age-matched OVX mice. In contrast, the trabecular bone volume of the femur was greatly increased in both female and male IL-7 receptor-knockout mice when compared with the respective wild-type and heterozygous littermates. These results show that the perturbation of B-lymphopoiesis in the bone marrow is closely linked to the change in bone mass. We propose here that the increased B-lymphopoiesis due to estrogen deficiency is involved in the mechanism of stimulated bone resorption.