Characterization of an osteoblast-like clonal cell line which responds to both parathyroid hormone and calcitonin

Integration of Epigallocatechin Gallate in Gelatin Sponges Attenuates Matrix Metalloproteinase-Dependent Degradation and Increases Bone Formation

Matrix metalloproteinase (MMP)-2 and MMP-9 are well-known gelatinases that disrupt the extracellular matrix, including gelatin. However, the advantages of modulating MMP expression in gelatin-based materials for applications in bone regenerative medicine have not been fully clarified. In this study, we examined the effects of epigallocatechin gallate (EGCG), a major polyphenol catechin isolated from green tea, on MMP expression in gelatin sponges and its association with bone formation. Four gelatin sponges with or without EGCG were prepared and implanted into bone defects for up to 4 weeks. Histological and immunohistological staining were performed. Micro-computed tomography was used to estimate the bone-forming capacity of each sponge. Our results showed that EGCG integration attenuated MMP-2 (70.6%) and -9 expression (69.1%) in the 1 week group, increased residual gelatin (118.7%), and augmented bone formation (101.8%) in the 4 weeks group in critical-sized bone defects of rat calvaria compared with vacuum-heated gelatin sponges without EGCG. Moreover, vacuum-heated gelatin sponges with EGCG showed superior bone formation compared with other sponges. The results indicated that integration of EGCG in gelatin-based materials modulated the production and activity of MMP-2 and -9 in vivo, thereby enhancing bone-forming capacity.

Brief exposure to full length parathyroid hormone-related protein (PTHrP) causes persistent generation of cyclic AMP through an endocytosis-dependent mechanism

Parathyroid hormone (PTH)-related protein (PTHrP) (gene name Pthlh) was discovered as the factor responsible for the humoral hypercalcemia of malignancy. It shares such sequence similarity with PTH in the amino-terminal region that the two are equally able to act through a single G protein-coupled receptor, PTH1R. A number of biological activities are ascribed to domains of PTHrP beyond the amino-terminal domain. PTH functions as a circulating hormone, but PTHrP is generated locally in many tissues including bone, where it acts as a paracrine factor on osteoblasts and osteocytes. The present study compares how PTH and PTHrP influence cyclic AMP (cAMP) formation through adenylyl cyclase, the first event in cell activation through PTH1R. Brief exposure to full length PTHrP(1-141) in several osteoblastic cell culture systems was followed by sustained adenylyl cyclase activity for more than an hour after ligand washout. This effect was dose-dependent and was not found with shorter PTHrP or PTH peptides even though they were fully able to activate adenylyl cyclase with acute treatment. The persistent activation response to PTHrP(1-141) was seen also with later events in the cAMP/PKA pathway, including persistent activation of CRE-luciferase and sustained regulation of several CREB-responsive mRNAs, up to 24 h after the initial exposure. Pharmacologic blockade of endocytosis prevented the persistent activation of cAMP and gene responses. We conclude that full length PTHrP, the likely local physiological effector in bone, differs in intracellular action to PTH by undergoing endosomal translocation to induce a prolonged adenylyl cyclase activation in its target cells.

Icariin, but Not Genistein, Exerts Osteogenic and Anti-apoptotic Effects in Osteoblastic Cells by Selective Activation of Non-genomic ERα Signaling

Genistein and icariin are flavonoid compounds that exhibit estrogen-like properties in inducing bone formation and reducing bone loss associated with estrogen deficiency in both preclinical and clinical studies. However, the mechanisms that are involved in mediating their estrogenic actions in bone cells are far from clear. The present study aimed to study the signaling pathways that mediate the estrogenic actions of genistein and icariin in osteoblastic cells. The effects of genistein and icariin on the activation of estrogen receptor (ER) and the downstream mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway in murine osteoblastic MC3T3-E1 cells and rat osteoblastic UMR-106 cells were studied. As expected, genistein displayed higher binding affinity toward ERβ than ERα and significantly induced estrogen response element (ERE)-dependent transcription in UMR-106 cells in a dose-dependent manner. In contrast, icariin failed to bind to ERα or ERβ and did not induce ERE-dependent transcription in UMR-106 cells at 10^-10 to 10^-7 M. The effects of genistein (10 nM) and icariin (0.1 μM) on cell proliferation and differentiation in osteoblastic UMR-106 cells were abolished in the presence of ER antagonist ICI 182,780 (1 μM), MAPK inhibitor U0126 (10 μM), and PI3K inhibitor LY294002 (10 μM). Genistein at 10 nM rapidly induced ERK1/2 phosphorylation at 5–10 min in UMR-106 cells and the phosphorylation of ERα at both Ser118 and Ser167 in both MC3T3-E1 and transfected UMR-106 cells whereas icariin at 0.1 μM rapidly activated both ERK1/2 and Akt phosphorylation in UMR-106 cells and subsequent ERα phosphorylation at both Ser118 and Ser167 in MC3T3-E1 and transfected UMR-106 cells. Confocal imaging studies confirmed that the phosphorylation of ERα at Ser 118 and Ser 167 by genistein and icariin in MC3T3-E1 cells was mediated via MAPK- and PI3K-dependent pathway, respectively. Furthermore, our studies showed that icariin exerted stronger anti-apoptotic effects than genistein and 17β-estradiol (E2) and inhibited the cleavage of downstream caspase-3 in MC3T3-E1 cells induced by a potent PI3K inhibitor, PI828 (at 2 μM). These results indicated that the mechanisms that mediate the estrogenic actions of icariin in osteoblastic cells are different from those of genistein.

Pueraria mirifica extract and puerarin enhance proliferation and expression of alkaline phosphatase and type I collagen in primary baboon osteoblasts

Phytoestrogen-rich Pueraria mirifica (PM) tuberous extract is a promising candidate for the development of anti-osteoporosis drugs for postmenopausal women, but its action has never been validated in humans or in non-human primates, which are more closely related to humans than rodents. In vitro study of non-human primate osteoblasts is thus fundamental to prepare for in vivo studies of phytoestrogen effects on primate bone. This study aimed to establish a culture system of baboon primary osteoblasts and to investigate the effects of PM extract and its phytoestrogens on these cells. Primary osteoblasts from adult baboon fibulae exhibited osteoblast characteristics in regard to proliferation, differentiation, mineralization, and estrogen receptor expression. They responded to 17β-estradiol by increased proliferation rate and mRNA levels of alkaline phosphatase (ALP), type I collagen, and osteocalcin. After being exposed for 48 h to 100 μg/ml PM extract, 1000 nM genistein, or 1000 nM puerarin, primary baboon osteoblasts markedly increased the rate of proliferation and mRNA levels of ALP and type I collagen without changes in Runx2, osterix, or osteocalcin expression. PM extract, genistein, and puerarin also decreased the RANKL/OPG ratio, suggesting that they could decrease osteoclast-mediated bone resorption. However, neither PM extract nor its phytoestrogens altered calcium deposition in osteoblast culture. In conclusion, we have established baboon primary osteoblast culture, which is a new tool for bone research and drug discovery. Furthermore, the present results provide substantial support for the potential of PM extract and its phytoestrogens to be developed as therapeutic agents against bone fragility.

Functional heterogeneity of osteocytes in FGF23 production: the possible involvement of DMP1 as a direct negative regulator

Fibroblast growth factor 23 (FGF23) and dentin matrix protein (DMP1) are hallmarks of osteocytes in bone. However, the mechanisms underlying the actions of DMP1 as a local factor regulating FGF23 and bone mineralization are not well understood. We first observed spatially distinct distributions of FGF23- and DMP1-positive osteocytic lacunae in rat femurs using immunohistochemistry. Three-dimensional immunofluorescence morphometry further demonstrated that the distribution and relative expression levels of these two proteins exhibited reciprocally reversed patterns especially in midshaft cortical bone. These in vivo findings suggest a direct role of DMP1 in FGF23 expression in osteocytes. We next observed that the inoculation of recombinant DMP1 in UMR-106 osteoblast/osteocyte-like cells and long-cultured MC3T3-E1 osteoblastic cells showed significant downregulation of FGF23 production. This effect was rescued by incubation with an focal adhesion kinase (FAK) inhibitor or MEK (mitogen-activated protein kinase (MAPK)/extracellular signal regulated kinase (ERK)) inhibitor but not inhibitors of phosphoinositide 3-kinase or Rho kinase. Consistently, the levels of phosphorylated FAK, ERK and p38 were significantly elevated, indicating that exogenous DMP1 is capable of activating FAK-mediated MAPK signaling. These findings suggest that DMP1 is a local, direct and negative regulator of FGF23 production in osteocytes involved in the FAK-mediated MAPK pathway, proposing a relevant pathway that coordinates the extracellular environment of osteocytic lacunae and bone metabolism.

Homologous mesenchymal stem cells promote the emergence and growth of pulmonary metastases of the rat osteosarcoma cell line UMR-106

Osteosarcoma (OS) is the most frequent primary bone sarcoma and tends to develop pulmonary metastasis. Studies have shown that mesenchymal stem cells (MSCs) are involved in OS growth and metastasis, but the mechanism remains unclear. The aim of the present study was to identify whether homologous MSCs could promote the growth and metastasis of OS in rats with a normal immune system. The OS cell line, UMR-106, which originally derives from a Sprague-Dawley (SD) rat-transplantable osteogenic sarcoma with an osteoblastic phenotype, has a strong carcinogenic capability and a high lung metastasis. Xenotransplanted models of UMR-106 with or without MSCs injected through the tibia (IT) or caudal vein (IV) were established. SD rats were randomly divided into six groups: Control, UMR-106 (IT), MSCs (IV), UMR-106 (IT) + MSCs (IV), UMR-106 (IV) and UMR-106 (IV) + MSCs (IV). Following injection, all rats were sacrificed at week 5, and the volume and quantity of metastatic sarcoma and the serum alkaline phosphatase levels were measured. There was no metastatic sarcoma in the liver, spleen and kidney in all groups. The rats in the MSCs (IV) + UMR-106 (IV) group showed a significantly higher volume and number of pulmonary metastatic tumors than those of the UMR-106 (IV) group. In pulmonary metastatic tissues, MSCs were found in the MSCs (IV) + UMR-106 (IV) group, but not in the UMR-106 (IT) + MSCs (IV) group. Notably, the expression of vascular endothelial growth factor (VEGF) was increased in the MSCs + UMR-106 cells co-culture system. The present study indicated that MSCs can significantly promote the pulmonary metastasis of the rat OS cell line, UMR-106, with a normal immune system, and VEGF was involved in MSC-promoted UMR-106 emergence and growth of pulmonary metastasis.

Calcitonin substitution in calcitonin deficiency reduces particle-induced osteolysis

Background

Periprosthetic osteolysis is a major cause of aseptic loosening in joint arthroplasty. This study investigates the impact of CT (calcitonin) deficiency and CT substitution under in-vivo circumstances on particle-induced osteolysis in Calca -/- mice.

Methods

We used the murine calvarial osteolysis model based on ultra-high molecular weight polyethylene (UHMWPE) particles in 10 C57BL/6J wild-type (WT) mice and twenty Calca -/- mice. The mice were divided into six groups: WT without UHMWPE particles (Group 1), WT with UHMWPE particles (Group 2), Calca -/- mice without UHMWPE particles (Group 3), Calca -/- mice with UHMWPE particles (Group 4), Calca -/- mice without UHMWPE particles and calcitonin substitution (Group 5), and Calca -/- mice with UHMWPE particle implantation and calcitonin substitution (Group 6). Analytes were extracted from serum and urine. Bone resorption was measured by bone histomorphometry. The number of osteoclasts was determined by counting the tartrate-resistant acid phosphatase (TRACP) + cells.

Results

Bone resorption was significantly increased in Calca -/- mice compared with their corresponding WT. The eroded surface in Calca -/- mice with particle implantation was reduced by 20.6% after CT substitution. Osteoclast numbers were significantly increased in Calca -/- mice after particle implantation. Serum OPG (osteoprotegerin) increased significantly after CT substitution.

Conclusions

As anticipated, Calca -/- mice show extensive osteolysis compared with wild-type mice, and CT substitution reduces particle-induced osteolysis.

Antiosteoporotic compounds from seeds of Cuscuta chinensis

ETHNOPHARMACOLOGICAL RELEVANCE

The seeds of Cuscuta chinensis (Tu-Si-Zi, TSZ) have long been used for the treatment of osteoporosis in China and some Asian countries. The compounds in TSZ responsible for the antiosteoporotic activity are still poorly understood.

AIM OF THE STUDY

The present study was designed to investigate the osteogenic compounds in TSZ, and to evaluate their antiosteoporotic effects in osteoblastic cells.

MATERIALS AND METHODS

Osteoblast-like UMR-106 cells were used for bioactivity-guided isolation of the active compounds. The activity of alkaline phosphatase (ALP) in UMR-106 cells was measured by p-nitrophenyl sodium phosphate assay. The proliferation of UMR-106 cells was assayed by Alamar-Blue method. Estrogenic activity of the extracts and isolated compounds was evaluated by activation of estrogen response element (ERE) luciferase reporter expression in HeLa cells co-transfected with human estrogen receptor subtypes (ERα or ERβ) expression vectors and 5×ERE luciferase reporter plasmid. Antiestrogenic activity of the extracts and isolated compounds were evaluated by activation of activator protein-1 (AP-1) luciferase reporter expression in HeLa cells co-transfected with human estrogen receptor subtypes (ERα or ERβ) expression vectors and 6×AP-1 luciferase reporter plasmid.

RESULTS

ALP-guided fractionation led to the isolation of five known flavonoids, quercetin, kaempferol, isorhamnetin, hyperoside and astragalin from the crude ethanolic extract of TSZ. Further study showed that kaempferol and hyperoside significantly increased the ALP activity in UMR-106 cells. Astragalin promoted the proliferation of UMR-106 cells whereas other compounds had no such effect. The isolated compounds showed estrogenic activity but quercetin, kaempferol and isorhamnetin showed more potent ERβ agonist activity. However, compared with their ER agonist activity, only quercetin and kaempferol showed potent ER antagonist activity by activating ERα/β-mediated AP-1 reporter expression.

CONCLUSIONS

Our findings validated the clinical use of TSZ in the treatment of osteoporosis, and demonstrated that kaempferol and hyperoside are the active compounds in TSZ for the osteogenic effect.

Parathyroid hormone gene family in a cartilaginous fish, the elephant shark (Callorhinchus milii)

The development of bone was a major step in the evolution of vertebrates. A bony skeleton provided structural support and a calcium reservoir essential for the movement from an aquatic to a terrestrial environment. Cartilaginous fishes are the oldest living group of jawed vertebrates. In this study we have identified three members of the parathyroid hormone (Pth) gene family in a cartilaginous fish, the elephant shark (Callorhinchus milii). The three genes include two Pth genes, designated as Pth1 and Pth2, and a Pthrp gene. Phylogenetic analysis suggested that elephant shark Pth2 is an ancient gene whose orthologue is lost in bony vertebrates. The Pth1 and Pth2 genes have the same structure as the Pth gene in bony vertebrates, whereas the structure of the Pthrp gene is more complex in tetrapods compared with elephant shark. The three elephant shark genes showed distinct patterns of expression, with Pth2 being expressed only in the brain and spleen. This contrasts with localization of the corresponding proteins, which showed considerable overlap in their distribution. There were conserved sites of localization for Pthrp between elephant shark and mammals, including tissues such as kidney, skin, skeletal and cardiac muscle, pancreas, and cartilage. The elephant shark Pth1(1-34) and Pthrp(1-34) peptides were able to stimulate cAMP accumulation in mammalian UMR106.01 cells. However, Pth2(1-34) peptide did not show such PTH-like biologic activity. The presence of Pth and Pthrp genes in the elephant shark indicates that these genes played fundamental roles before their recruitment to bone development in bony jawed vertebrates.

Calcitonin impairs the anabolic effect of PTH in young rats and stimulates expression of sclerostin by osteocytes

The therapeutic goal of increasing bone mass by co-treatment of parathyroid hormone (PTH) and an osteoclast inhibitor has been complicated by the undefined contribution of osteoclasts to the anabolic activity of PTH. To determine whether active osteoclasts are required at the time of PTH administration, we administered a low dose of the transient osteoclast inhibitor salmon calcitonin (sCT) to young rats receiving an anabolic PTH regimen. Co-administration of sCT significantly blunted the anabolic effect of PTH as measured by peripheral quantitative computer tomography (pQCT) and histomorphometry in the femur and tibia, respectively. To determine gene targets of sCT, we carried out quantitative real time PCR and microarray analysis of metaphyseal samples 1.5, 4 and 6.5h after administration of a single injection of PTH, sCT or PTH+sCT. Known targets of PTH action, IL-6, ephrinB2 and RANKL, were not modified by co-administration with sCT. Surprisingly, at all time points, we noted a significant upregulation of sclerostin mRNA by sCT treatment, as well as down-regulation of two other osteocyte gene products, MEPE and DMP1. Immunohistochemistry confirmed that sCT administration increased the percentage of osteocytes expressing sclerostin, suggesting a mechanism by which sCT reduced the anabolic effect of PTH. Neither mRNA for CT receptor (Calcr) nor labeled CT binding could be detected in sclerostin-enriched cells differentiated from primary calvarial osteoblasts. In contrast, osteocytes freshly isolated from calvariae expressed a high level of Calcr mRNA. Furthermore immunohistochemistry revealed co-localization of CT receptor (CTR) and sclerostin in some osteocytes in calvarial sections. Taken together these data indicate that co-treatment with sCT can blunt the anabolic effect of PTH and this may involve direct stimulation of sclerostin production by osteocytes. These data directly implicate calcitonin as a negative regulator of bone formation through a previously unsuspected mechanism.

The role of peptides and receptors of the calcitonin family in the regulation of bone metabolism

The 'calcitonin family' is a group of peptide hormones that share structural similarities with calcitonin, and includes calcitonin gene-related peptide (CGRP), amylin, adrenomedullin and adrenomedullin 2 (intermedin). These hormones are produced by different tissues, with calcitonin being produced in thyroid C cells, alphaCGRP predominantly in neural tissue, amylin in beta-islet cells of the pancreas and adrenomedullin in many tissues and cell types. Bone appears to be a common target for all the peptides of the calcitonin family, although the specific bone effects of the peptides vary. Administration of calcitonin produces rapid lowering of serum calcium levels, mainly through inhibition of bone resorption by osteoclasts. In vitro and in a number of animal experimental models, amylin and CGRP are also effective in inhibiting osteoclast activity and bone resorption. Amylin, adrenomedullin and CGRP can also affect cells of the osteoblast lineage, inducing osteoblast proliferation and promoting bone formation. Receptors for the peptides of the calcitonin family are formed by heterodimerization of the calcitonin receptor (CTR) or calcitonin receptor-like receptor (CLR) with receptor activity modifying proteins (RAMPs). Although the different combinations of these proteins create receptors with distinct ligand specificities, there is a degree of cross-reactivity and the receptors are able to bind other ligands from the family, usually with lower affinity. Analysis of the expression of the receptors for the calcitonin family in 16 samples of human osteoblasts showed high levels of CLR and RAMP1, low levels of RAMP2 and no expression of RAMP3 or CTR. Recent studies of the bone phenotype of knockout animals lacking the calcitonin, alphaCGRP or amylin gene indicated that in this experimental system the main physiological role of amylin in bone is the inhibition of bone resorption, that of CGRP is the activation of bone formation, while calcitonin, unexpectedly appears to be inhibiting bone formation without affecting bone resorption. Further investigations will be required to determine the mechanisms of action of calcitonin peptides in bone and their significance to human bone physiology.

Diversity of the osteoblastic phenotype

Studies of bone cells in culture have raised two salient questions: are the findings representative of the in vivo situation and can the conflicting data from different cell models be reconciled? Review of the literature indicates that all osteoblastic cells, defined by their origin or by their ability to produce mineralized matrix, have a few common properties: production of type I collagen; increased alkaline phosphatase activity; and parathyroid hormone-stimulated adenylate cyclase. Other features, such as osteocalcin and prostaglandin E production and the response to prostaglandin E, are selectively expressed by certain cell types. Pilot studies on mRNA levels of 'bone proteins' in developing calvaria suggest that such differences may reflect stages in osteoblastic differentiation. Immortalization of calvaria-derived cells using a SV40 large T antigen vector, which may freeze the cells in their particular state of differentiation (as proposed for leukaemia cells), yields phenotypes consistent with that hypothesis. Immortal cell lines may thus help to characterize osteoblastic differentiation. The diversity of osteoblast responses in culture to hormones and growth factors could be due to these phenotype differences but could also represent a subspecialization of differentiated cells. In addition, in the organism regulatory agents act in concert on a heterogeneous interactive cell population. Nonetheless cell cultures can be useful in screening for and predicting in vivo responses, as was shown by the 1,25-(OH)2D3 stimulation of osteocalcin, and for studying the molecular mechanisms of regulatory effects. Cell lines are also convenient for the production of specific proteins and cDNA libraries, and for the expression of specific genes.

Lignans from the stems of Sambucus williamsii and their effects on osteoblastic UMR106 cells

Three new lignans, sambucunol A (8) ((+)-erythro-1-(4-hydroxy-3-methoxyphenyl)-2-[4-(4-hydroxy-3-methoxycinnamoyloxypropanyl)-2-hydroxyphenoxy]-1, 3-propanediol), sambucunol B (9) ((+)-threo-1-(4-hydroxyl-3-methoxyphenyl)-2-[4-(4-hydroxy-3-methoxy-cinnamoyloxy propanyl)-2-hydroxyphenoxy]-1, 3-propanediol) and buddlenol G (10) (2-{4-[2, 3-dihydro-3-hydroxymethyl-7-hydroxy-5-(4-hydroxy-3-methoxycinnamoyloxypropanyl)-2-benzofuranyl]-2,6-dimethoxyphenoxy}-1-(4- hydroxy-3-methoxyphenyl) -1, 3-propanediol), along with seven known ones, including ( - )-syringaresinol (1), ( - )-pinoresinol (2), 1, 2-bis(4-hydroxy-3-methoxy phenyl)-1, 3-propanediol (3), ( - )-erythro-1-(4-hydroxy-3-methoxyphenyl)-2-[4-(3-hydroxy propanyl)-2-methoxyphenoxy]-1, 3-propanediol (4), ( - )-threo-1-(4-hydroxy-3-methoxyphenyl)-2-[4-(3-hydroxypropanyl)-2-methoxy phenoxy]-1, 3-propanediol (5), ( - )-lariciresinol (6) and ( - )-dihydrodehydrodiconiferyl alcohol (7), were isolated from the 60% ethanol extract of stems of Sambucus williamsii Hance by chromatographic methods. Their structures were established by spectral analysis. The effects of isolated compounds on the osteoblast-like UMR106 cell proliferation and ALP activities were determined. Compounds 2, 7 and 10 showed stimulating effects both on UMR106 cell proliferation and ALP activity. Compounds 1, 3, 6 and 8 stimulated UMR106 cell proliferation, while compounds 4 and 5 induced ALP activity in UMR106 cell.

Calcitonin and prednisolone display antagonistic actions on bone and have synergistic effects in experimental arthritis

We tested here the hypothesis that calcitonin and glucocorticoids, known to modulate bone metabolism, could have opposite actions on bone cells regulating expression of cytokine receptor activator of nuclear factor-kappaBeta ligand (RANKL) and osteoprotegerin (OPG). In the U2OS osteosarcoma cell line, calcitonin (10(-11) to 10(-9) mol/L) reduced RANKL and augmented OPG both at the mRNA and protein levels. Cell incubation with prednisolone (10(-8) to 10(-6) mol/L), the glucocorticoid chosen for this study, produced opposite results. These molecular studies prompted more functional analyses whereby osteoclast bone resorptive activity was determined. Calcitonin (10(-10) mol/L) abrogated the stimulating effect of 10 ng/ml RANKL or 10(-9) mol/L prednisolone; similar results were obtained with OPG. Assessment of calcitonin and prednisolone effects in an in vivo model of rheumatoid arthritis revealed partially surprising results. In fact, calcitonin not only preserved bone morphology (as assessed on day 18) in rats subjected to arthritis and treated with prednisolone (0.8 to 4 mg/kg daily from day 13) but also synergized with the steroid to elicit its antiarthritic effects. These results suggest that calcitonin could be used as a novel cotreatment to augment efficacy and reduce side effects associated with the prolonged use of steroids.

Commonly used mouse models of osteosarcoma

Osteosarcoma is the commonest primary tumour of bone and the second highest cause of cancer-related death in the paediatric age group. Little is known of the aetiology of human osteosarcoma and lesser still of the various interactions that occur between host and tumour cells that govern growth and progression of osteosarcoma in vivo. Although numerous osteosarcoma cell lines have been established and characterized in vitro, some as far back as in the 1960s, there is a scarcity of reliable and reproducible in vivo animal models that mimics all aspects of the human condition at the temporal, physiological and histopathological level, hence, making the accurate testing of therapeutic strategies difficult. Given that osteosarcoma is a disease that affects young people and better disease management strategies are essential, development of a robust human osteosarcoma model is long overdue.

Increase in bone mass and bone strength by Sambucus williamsii HANCE in ovariectomized rats

Herbal Sambucus williamsii HANCE (SWH) is a folk medicine with a long history of safe use for treatment of bone fractures and joint diseases in China. The present study was designed to investigate if SWH extract could be used for treatment of postmenopausal osteoporosis. SWH extracts (30 or 60 mg/100 g body weight/d) were orally administrated to four-months-old ovariectomized (OVX) rats for 3 months. SWH extracts did not alter weight gain and uterus weight in OVX rats. SWH extracts significantly increased serum Ca levels (p<0.05, vs. OVX control group) as well as decreased urinary Ca excretion (p<0.01, vs. OVX control group) in OVX rats. The upregulation of serum alkaline phosphatase, serum osteocalcin as well as urinary deoxypyridinoline levels by OVX was suppressed by treatment with SWH extracts in rats (p<0.05, vs. OVX control group). SWH extract increased the stiffness of femur at both dosage (p<0.05, vs. OVX control group) and increased tibial bone mineral density at 60 mg/100 g body weight/d (p<0.05, vs. OVX control group) in OVX rats. Our results indicate that orally administrated SWH extracts can decrease urinary calcium excretion and bone turnover rate in OVX rats, resulting in positive effects on biomechanical strength of bone and bone mineral density. This study is the first to report that SWH could be considered as a potential candidate for management of postmenopausal osteoporosis. Then in vitro experiments were performed to determine the potential molecular mechanism of the anti-osteoporotic effect of SWH. Results suggested that chloroform fraction and ethyl acetate fraction of SWH can inhibit osteoclastogenesis osteoclast by modulating the expression of osteoprotegrin (OPG) and receptor activator of NF-kappaB ligand (RANKL) mRNA in osteoblastic UMR 106 cells. Both of them increased OPG mRNA and decreased RANKL mRNA expression, resulting in a dose-dependent increase in OPG/RANKL mRNA ratio (p<0.01, vs. vehicle-treated). Taken together, SWH treatment can effectively suppress the OVX-induced increase in bone turnover and its effects might be mediated by a decrease in osteoclastogenesis.

Cell lines and primary cell cultures in the study of bone cell biology

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite and amorphous calcium phosphate crystals deposited in an organic matrix. Bone has two main functions. It forms a rigid skeleton and has a central role in calcium and phosphate homeostasis. The major cell types of bone are osteoblasts, osteoclasts and chondrocytes. In the laboratory, primary cultures or cell lines established from each of these different cell types provide valuable information about the processes of skeletal development, bone formation and bone resorption, leading ultimately, to the formulation of new forms of treatment for common bone diseases such as osteoporosis.

“Culture shock” from the bone cell's perspective: emulating physiological conditions for mechanobiological investigations

Bone physiology can be examined on multiple length scales. Results of cell-level studies, typically carried out in vitro, are often extrapolated to attempt to understand tissue and organ physiology. Results of organ- or organism-level studies are often analyzed to deduce the state(s) of the cells within the larger system(s). Although phenomena on all of these scales—cell, tissue, organ, system, organism—are interlinked and contribute to the overall health and function of bone tissue, it is difficult to relate research among these scales. For example, groups of cells in an exogenous, in vitro environment that is well defined by the researcher would not be expected to function similarly to those in a dynamic, endogenous environment, dictated by systemic as well as organismal physiology. This review of the literature on bone cell culture describes potential causes and components of cell “culture shock,” i.e., behavioral variations associated with the transition from in vivo to in vitro environment, focusing on investigations of mechanotransduction and experimental approaches to mimic aspects of bone tissue on a macroscopic scale. The state of the art is reviewed, and new paradigms are suggested to begin bridging the gap between two-dimensional cell cultures in petri dishes and the three-dimensional environment of living bone tissue.

Effects of 1alpha,25-dihydroxyvitamin D3 and growth hormone on apoptosis and proliferation in UMR 106 osteoblast-like cells

Mechanisms maintaining a correct balance between bone-forming osteoblasts and bone-resorbing osteoclasts are essential for bone formation. Apoptosis has been proposed to play a key role in controlling osteoblast homeostasis. 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and GH, which are important regulators of bone growth and bone metabolism, also play pivotal roles in regulation of mitogenesis, differentiation, and apoptosis. We have recently shown that 1,25(OH)2D3 prolongs GH signaling via the Janus kinase 2 (JAK2)/STAT5 (signal transducer and activator of transcription 5) pathway in UMR 106 osteoblast-like cells. In the present study, we have investigated the effects of GH and 1,25(OH)2D3 on proliferation and apoptosis in UMR 106 cells. We found that 1,25(OH)2D3 and GH, separate or in combination, inhibited apoptosis. GH also had profound effects on cell cycle distribution and proliferation. In addition, pretreatment of cells with 1,25(OH)2D3 was necessary to detect GH-induced MAPK activation. We hypothesize that these hormones separately regulate the processes of apoptosis and proliferation, which may be important for maintaining osteoblast cell number.

Impairment of the collagenase-3 endocytotic receptor system in cells from patients with osteoarthritis

OBJECTIVE

Collagenase-3, a matrix metalloproteinase (MMP-13) that can degrade collagen II and aggrecan, is produced by osteoarthritic (OA) chondrocytes and may contribute to matrix destruction in this disease. Our laboratory has previously identified a specific endocytotic receptor for collagenase-3 on osteoblastic and fibroblastic cells, which couples with the low-density lipoprotein receptor-related protein (LRP1) to mediate the internalization and degradation of this enzyme. We hypothesized that the activity of this receptor system is reduced in OA chondrocytes which may lead to increased local extracellular levels of collagenase-3 and increased destruction of the cartilage matrix at pericellular sites.

METHODS

Human chondrocytes and synoviocytes were obtained from OA knees at the time of joint replacement surgery and from non-arthritic control specimens following autopsy or surgery. Enzyme-linked immunosorbant assay (ELISA) was used to measure collagenase-3 secreted from primary cultures. Iodinated collagenase-3 was used to analyze the cell-surface binding, internalization and intracellular degradation of collagenase-3. Reverse-transcriptase polymerase chain reaction was used to confirm chondrocyte phenotype and the expression of collagenase-3 and LRP1 mRNAs.

RESULTS

OA chondrocytes and synoviocytes demonstrated significantly reduced (75-77%) binding of recombinant 125I collagenase-3. Internalization and degradation of the ligand was also significantly reduced (64-72%) in OA cells. Collagenase-3 removal was inhibited by the LRP1 receptor-associated protein (RAP).

CONCLUSION

These results suggest a mechanism whereby impaired receptor-mediated removal of collagenase-3 in OA chondrocytes may lead to enhanced local degradation of the cartilage matrix. This work also implicates an LRP family member in endocytotic receptor-mediated collagenase-3 processing and suggests a novel therapeutic target for OA.

IL-1beta induces and TGF-beta reduces vitamin D3-induced bone resorption in mouse calvarial bone cells

Bone cells produce multiple growth factors and cytokines that have effects on bone metabolism and can be incorporated into the bone matrix. The present study was designed to extend these observations by examining the interactions between transforming growth factor-beta (TGF-beta) or interleukin-1beta (IL-1beta) and bone cells in a rat long bone culture model. IL-1beta regulates several activities of the osteoblast cells derived from rat long bone explants in vitro. IL-1beta stimulated cellular proliferation and the synthesis of prostaglandin E2 and plasminogen activator activity in the cultured cells in a dose-dependent manner. TGF-B is present in the bone matrix and potentially can be released during bone resorption. TGF-beta reduced basal bone resorption and inhibited vitamin D3 [1,25(OH)2D3]-induced bone resorption in rat long bone cells. These studies support the role of IL-1beta in the pathological modulation of bone cell metabolism, with regard to implication in the pathogenesis of osteoporosis by IL-1beta, and that TGF-beta is positively inhibiting the bone resorption.

Gene expression profiles and transcription factors involved in parathyroid hormone signaling in osteoblasts revealed by microarray and bioinformatics

Parathyroid hormone (PTH) binds to its receptor PTH1R (parathyroid hormone 1 receptor) in osteoblastic cells to regulate bone remodeling and calcium homeostasis. While prolonged exposure to PTH causes increased bone resorption, intermittent injections of PTH have an anabolic effect on bone. The molecular mechanisms regulating these processes are still largely unknown. Here, we present our results on gene expression profile changes in the PTH-treated osteoblastic cell line, UMR 106-01, using DNA microarray analysis. A total of 125 known genes and 30 unknown expressed sequence tags (ESTs) were found to have at least 2-fold expression changes after PTH treatment at 4, 12, and 24 h. 14 genes were previously known to be PTH-regulated but many were unknown to be regulated by PTH prior to our experiments. Real-time reverse transcriptase-PCR confirmed that 90 and 50% of the genes are regulated more than 2-fold by PTH in UMR 106-01 and rat primary osteoblastic cells, respectively. Most genes belong to the following protein families: hormones, growth factors, and receptors; signal transduction pathway proteins; transcription factors; proteases; metabolic enzymes; structural and matrix proteins; transporters; etc. These results provide a comprehensive and deeper knowledge about PTH regulation of osteoblastic gene expression. Next, we designed a computational method to extract information about transcription factors likely involved in regulating these genes. These factors include those previously known to be involved in PTH signaling (AP-1 and the cAMP response element-binding protein), those that were identified by microarray data (C/EBP), and some novel transcription factors (AP-2, AP-4, SP1, FoxD3, etc.). Our results suggest that a reliable bioinformatics approach can be easily applied for other systems.

Osteoclast formation from circulating precursors in osteoporosis

OBJECTIVE

An imbalance between bone formation and bone resorption is thought to underlie the pathogenesis of reduced bone mass in osteoporosis. Bone resorption is carried out by osteoclasts. which are formed from marrow-derived cells that circulate in the monocyte fraction. The aim of this study was to determine the role of osteoclast formation in the pathogenesis of bone loss in osteoporosis.

METHODS

The proportion of circulating osteoclast precursors and their relative sensitivity to the osteoclastogenic effects of M-CSF. 1,25(OH)2D3 and RANKL were assessed in primary osteoporosis patients and normal controls.

RESULTS

Although there was no difference in the number of circulating osteoclast precursors in osteoporosis patients and normal controls. osteoclasts formed from osteoporosis patients exhibited substantially increased resorptive activity relative to normal controls. Although no increased sensitivity to the osteoclastogenic effects of 1,25(OH)D3 or M-CSF was noted, increased bone resorption was found in osteoporosis peripheral blood mononuclear cell (PBMC) cultures to which these factors were added.

CONCLUSION

Our findings suggest that osteoclast functional activity rather than formation is increased in primary involutional osteoporosis and that dexamethasone acts to increase osteoclast formation.

Adrenomedullin--a regulator of bone formation

Bone growth and maintenance are highly regulated processes. Throughout life, bone constantly undergoes remodelling, maintaining a balance between bone formation by osteoblasts and bone resorption by osteoclasts. This balance depends on the coordinated activities of many systemic hormones and locally acting factors in the bone microenvironment. Understanding the mechanisms of action of these factors provides a better appreciation of the cellular and molecular basis of bone remodelling. Adrenomedullin has recently been found to stimulate the proliferation of osteoblasts in vitro, and to increase indices of bone formation when administered either locally or systemically in vivo. Adrenomedullin receptors, as well as adrenomedullin itself, are expressed in primary osteoblasts and in osteoblast-like cell lines. In this paper we discuss the anabolic effect of adrenomedullin in bone, and present new evidence for a possible role of adrenomedullin in the regulation of cartilage cells. We show that adrenomedullin stimulates proliferation of primary chondrocytes in culture and that mRNA for adrenomedullin and for adrenomedullin receptors are expressed in these cells. Studies of structure-activity relationships have demonstrated that osteotropic effects of adrenomedullin can be retained in peptide fragments of the molecule which lack the parent molecule's vasodilatory properties. Thus, these small peptides, or their analogues, are attractive candidates as anabolic therapies for osteoporosis.

Increased bone mass is an unexpected phenotype associated with deletion of the calcitonin gene

Calcitonin (CT) is a known inhibitor of bone resorption. Calcitonin gene-related peptide-alpha (CGRPalpha), produced by alternative RNA processing of the CT/CGRP gene, has no clearly defined role in bone. To better understand the physiologic role of the CT/CGRP gene we created a mouse in which the coding sequences for both CT and CGRPalpha were deleted by homologous recombination. The CT/CGRP(-/-) knockout (KO) mice procreated normally, there were no identifiable developmental defects at birth, and they had normal baseline calcium-related chemistry values. However, KO animals were more responsive to exogenous human parathyroid hormone as evidenced by a greater increase of the serum calcium concentration and urine deoxypyridinoline crosslinks, an effect reversed by CT and mediated by a greater increase in bone resorption than in controls. Surprisingly, KO mice have significantly greater trabecular bone volume and a 1.5- to 2-fold increase in bone formation at 1 and 3 months of age. This effect appears to be mediated by increased bone formation. In addition, KO mice maintain bone mass following ovariectomy, whereas wild-type mice lose approximately one-third of their bone mass over 2 months. These findings argue for dual roles for CT/CGRP gene products: prevention of bone resorption in hypercalcemic states and a regulatory role in bone formation.

Increased bone mass is an unexpected phenotype associated with deletion of the calcitonin gene

Calcitonin (CT) is a known inhibitor of bone resorption. Calcitonin gene-related peptide-alpha (CGRPalpha), produced by alternative RNA processing of the CT/CGRP gene, has no clearly defined role in bone. To better understand the physiologic role of the CT/CGRP gene we created a mouse in which the coding sequences for both CT and CGRPalpha were deleted by homologous recombination. The CT/CGRP(-/-) knockout (KO) mice procreated normally, there were no identifiable developmental defects at birth, and they had normal baseline calcium-related chemistry values. However, KO animals were more responsive to exogenous human parathyroid hormone as evidenced by a greater increase of the serum calcium concentration and urine deoxypyridinoline crosslinks, an effect reversed by CT and mediated by a greater increase in bone resorption than in controls. Surprisingly, KO mice have significantly greater trabecular bone volume and a 1.5- to 2-fold increase in bone formation at 1 and 3 months of age. This effect appears to be mediated by increased bone formation. In addition, KO mice maintain bone mass following ovariectomy, whereas wild-type mice lose approximately one-third of their bone mass over 2 months. These findings argue for dual roles for CT/CGRP gene products: prevention of bone resorption in hypercalcemic states and a regulatory role in bone formation.

Characterization of an osteoblast-specific enhancer element in the CBFA1 gene

Cbfa1 is a critical regulator of cell differentiation expressed only in the osteochondrogenic lineage. To define the molecular basis of this cell-specific expression we analyzed the murine Cbfa1 promoter. Here we show that the first 976 bp of this promoter are specifically active in osteoblastic cells. Within this region DNase I footprinting delineated a 40-bp area (CE1) protected differently by nuclear extracts from osteoblastic cells and from non-osteoblastic cells. When multimerized, CE1 conferred an osteoblast-specific activity to a heterologous promoter in DNA transfection experiments; this enhancing ability was conserved between mouse, rat, and human CE1 present in the respective Cbfa1 promoters. CE1 site-specific mutagenesis determined that it binds NF1- and AP1-like activities. Further analyses revealed that the NF1 site acts as a repressor in non-osteoblastic cells due to the binding of NF1-A, a NF1 isoform not expressed in osteoblastic cells. In contrast, the AP1 site mediates an osteoblast-specific activation caused by the preferential binding of FosB to CE1 in osteoblastic cells. In summary, this study identified an osteoblast-specific enhancer in the Cbfa1 promoter whose activity is achieved by the combination of an inhibitory and an activatory mechanism.

1Alpha,25-dihydroxyvitamin D3 inhibits GH-induced expression of SOCS-3 and CIS and prolongs growth hormone signaling via the Janus kinase (JAK2)/signal transducers and activators of transcription (STAT5) system in osteoblast-like cells

Growth hormone (GH) and 1alpha,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) are regulators of bone growth and bone metabolism. In target cells, GH activates several signaling pathways, among them the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway. GH mainly activates JAK2 and STAT5a and b. The effects of 1,25-(OH)(2)D(3) are mediated via a nuclear receptor, the vitamin D receptor, which, when bound by 1,25-(OH)(2)D(3), activates the transcription of target genes. In earlier studies (Morel, G., Chavassieux, P., Barenton, B., Dubois, P. M., Meunier, P. J., and Boivin, G. (1993) Cell Tissue Res. 273, 279-286) synergistic interaction between 1,25-(OH)(2)D(3) and GH regarding expression of osteoblastic markers has been described. The UMR 106 cell line is a rat osteosarcoma cell line with osteoblast-like properties. We have recently shown (Morales, O., Lindgren, U., and Haldosen, L. A. (2000) J. Bone Miner. Res. 15, 2284-2290) that UMR 106 cells express a GH-responsive JAK2/STAT5 signaling system. These cells also express the vitamin D receptor and respond to 1,25-(OH)(2)D(3). In the present study we have investigated whether 1,25-(OH)(2)D(3) influences GH signaling via the JAK2/STAT5 pathway in UMR 106 cells. We found that 1,25-(OH)(2)D(3) prolonged GH signaling via the JAK2/STAT5 pathway. Pretreatment of cells with 1,25-(OH)(2)D(3) was also necessary in order to detect GH-induced STAT5 transcriptional response. Furthermore, the pretreatment of cells with 1,25-(OH)(2)D(3) rendered to the cells the capacity to respond to repetitive GH-stimulation. In UMR 106 cells, GH induced the expression of the JAK/STAT negative regulatory proteins SOCS-3 and CIS. Interestingly, pretreatment with 1,25-(OH)(2)D(3) inhibited GH-induced expression of these proteins. From these results we propose that 1,25-(OH)(2)D(3) has an inhibitory effect on negative regulatory pathways acting on JAK2 and/or STAT5 in UMR 106 cells and that this, in all or partly, explains the effects of 1,25-(OH)(2)D(3) on GH-signaling via the JAK/STAT pathway.

Parathyroid hormone (hPTH 1-38) stimulates the expression of UBP41, an ubiquitin-specific protease, in bone

Parathyroid hormone (PTH) stimulates bone formation in both animals and humans, and the expression of a number of genes has been implicated in the mediation of this effect. To discover new bone factors that initiate and support this phenomenon, we used differential display reverse transcription polymerase chain reaction (DDRT-PCR) and screened for genes, which are differentially expressed in osteoblast-enriched femoral metaphyseal primary spongiosa of young male rats after a single subcutaneous (s.c.) injection of hPTH (1-38) (8 microg/100 g). We found and cloned one full-length cDNA, which encodes a putative 348 amino acid protein. Sequence analysis of this protein demonstrates a 98, 93.7, and 82.5% identity with mouse, human, and chicken ubiquitin-specific protease UBP41, respectively. Northern blot analysis confirmed that a 3.8-4 kb UBP41 mRNA transcript was rapidly increased 1 h after acute hPTH (1-38) exposure in both metaphyseal (6- to 8-fold) and diaphyseal (3-fold) bone, but returned to control levels by 24 h after exposure. In contrast, continuous exposure to hPTH (1-38), resulted in a rapid and sustained elevation of UBP41 mRNA. PTH (1-31), which stimulates intracellular cAMP, and PTHrP (1-34) both induced UBP41 mRNA expression; whereas PTH analogs (3-34) and (7-34), that do not stimulate cAMP, had no effect on UBP41 expression. UBP41 mRNA expression was also rapidly induced 1 h after injection of PGE2, but returned to the control level by 6 to 24 h. In vitro, UBP41 mRNA is expressed in primary osteoblasts (metaphyseal and diaphyseal derived) and in the osteoblast-like cell lines UMR106, ROS17/2.8, and BALC. PTH (1-38) treatment induced UPB41 expression (3.6- to 13-fold) in both primary cultures of osteoblasts and in UMR106 cells. Further analysis in UMR 106 cells demonstrated that PGE2, forskolin and dibutyryl cAMP increased UBP41 mRNA expression 4-, 4.5-, and 2.4-fold, respectively. Tissue distribution analysis of UBP41 mRNA detected transcripts in brain, heart, skeletal muscle, kidney, liver, and testis. Together, these results demonstrate that UBP41, an ubiquitin-specific protease, is selectively upregulated in bone by the osteotropic agents PTH, PTHrP, and PGE2, possibly via the PKA/cAMP pathway. We speculate that the rapid induction of UBP41 in response to these physiological regulators contributes to the mechanism by which either the structure, activity, half-life or localization of essential proteins are modified to maintain bone homeostasis.

Low-intensity pulsed ultrasound stimulates a bone-forming response in UMR-106 cells

Low-intensity (<100 mW/cm(2)) pulsed ultrasound (US) is an established therapy for fracture repair. In both animal and human trials, such US has been shown to facilitate fresh fracture repair and initiate healing in fractures with repair defects. However, the mechanism by which US achieves these outcomes is not clear. One possible mechanism is the direct stimulation of bone formation. To investigate this hypothesis, the current study investigated the mRNA response of isolated bone-forming cells (UMR-106 cells) to a single 20-min dose of low-intensity pulsed US. Using a novel US-cell coupling method, US was found to stimulate expression of the immediate-early response genes c-fos and COX-2 and elevate mRNA levels for the bone matrix proteins ALP and OC. These findings suggest that low-intensity pulsed US has a direct effect on bone formation. This may contribute to the beneficial effect of low-intensity pulsed US on fracture repair.

Discovery of a non-peptide small molecule that selectively mimics the biological actions of calcitonin

Calcitonin (CT), a 32-amino acid peptide hormone secreted mainly from the thyroid gland, plays an important role in maintaining bone homeostasis. To discover non-peptide small molecules with biological actions similar to those of CT, a cell-based screening of an in-house chemical library was performed and a pyridone derivative (SUN B8155) was identified. Like CT, it elevated cyclic AMP (cAMP) levels in T47D and UMR106-06 cells which endogenously express human and rat CT receptor, respectively. SUN B8155 also stimulated cAMP formation in cells expressing recombinant human CT receptor, but not in those expressing human parathyroid hormone/parathyroid hormone-related peptide receptor. Accumulation of cAMP in T47D cells was blocked by a selective antagonist of CT receptor, salmon CT(8-32), whereas SUN B8155 did not displace the specific binding of [(125)I]CT to the receptor. Our results suggested that the compound selectively interacts with the CT receptor by a mechanism similar to but probably different from that of CT itself. In rats, intraperitoneal administration of SUN B8155 significantly lowered serum calcium levels, like CT. Our results demonstrate, for the first time, that the biological activities of the newly identified small molecule can mimic that of CT, acting via the CT receptor.

ADAMTS-1: A cellular disintegrin and metalloprotease with thrombospondin motifs is a target for parathyroid hormone in bone

PTH stimulates bone formation in animals and humans, and the expressions of a number of genes have been implicated in the mediation of this effect. To discover new bone factors that initiate and support this phenomenon we used differential display RT-PCR and screened for genes that are selectively expressed in osteoblast-enriched femoral metaphyseal primary spongiosa of young male rats after a single s.c. injection of human PTH-(1-38) (8 microg/100 g). We show that one of the messenger RNAs that is up-regulated in bone is ADAMTS-1, a new member of the ADAM (A disintegrin and metalloprotease) gene family containing thrombospondin type I motifs. ADAMTS-1 consists of multiple domains common to ADAM family of proteins, including pro-, metalloprotease-like, and disintegrin-like domains. However, unlike other ADAMs, ADAMTS-1 does not possess a transmembrane or cytoplasmic domain and is a secreted protein. Northern blot analysis confirmed that ADAMTS-1 was up-regulated in both metaphyseal (14- to 35-fold) and diaphyseal (4.2-fold) bone 1 h after PTH-(1-38) injection and returned to control levels by 24 h. We also analyzed the regulation of ADAMTS-1 in response to various PTH/PTH-related peptide (PTHrP) analogs and found that PTH-(1-31) and PTHrP-(1-34), which activate the protein kinase A (PKA) pathway, induce ADAMTS-1 expression 1 h after injection, whereas PTH-(3-34) and PTH-(7-34), which do not activate the PKA pathway, did not regulate expression. To investigate the effect of other osteotropic agents, we analyzed ADAMTS-1 expression after a single dose of PGE2 (6 mg/kg) and found that it was up-regulated 1 h after injection and returned to control levels by 6 h. In vitro ADAMTS-1 is expressed in primary osteoblasts and osteoblastic cell lines, but was not detectable in osteoclasts generated from macrophage colony-stimulating factor/receptor activator of NF-kappaB ligand/transforming growth factor-beta1-treated bone marrow cells. Treatment of UMR 106 osteosarcoma cells with PTH, PGE2, forskolin, or (Bu)2cAMP increased ADAMTS-1 expression 7-, 4-, 5-, and 5-fold, respectively. Also, in vitro treatment with 1alpha,25-dihydroxyvitamin D3 increased ADAMTS-1 expression 3-fold. Tissue distribution analysis showed that ADAMTS-1 is expressed at high levels in many tissues, including the heart, lung, liver, skeletal muscle, and kidney. Taken together, these results demonstrate that ADAMTS-1 is specifically up-regulated in bone and osteoblasts by the osteotropic agents PTH, PTHrP, and PGE2 possibly via the cAMP/PKA pathway. We speculate that the rapid and transient increase in ADAMTS-1 expression may contribute to some of the effects of PTH on bone turnover.

Growth hormone-regulated intracellular signaling in UMR 106 osteosarcoma cells

Growth hormone (GH) and insulin-like growth factor 1 (IGF-1) are important growth factors for postnatal longitudinal bone growth. Although many effects of GH on bone growth are mediated by IGF-1, GH can directly influence bone cells. Limited knowledge exists regarding specific intracellular signaling pathways and genes activated by GH in bone cells. GH is known to activate several intracellular signaling pathways, among them the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway. GH mainly activates JAK2 and both isoforms of STAT5, A and B. STAT5 gene deletion experiments have shown the importance of these transcription factors for growth. To understand the molecular mechanism(s) behind this, different experimental models are needed. The UMR 106 cell line is a rat clonal osteosarcoma cell line with osteoblast-like phenotypic properties, one is the endogenous expression of GH receptor (GHR). The present study focused on whether these cells express a functional GH-responsive JAK2/STAT5 pathway. Analysis of cell extracts by immunoprecipitation and Western blot showed that physiological concentrations of GH activated JAK2. Western blot analysis of nuclear extracts from GH-stimulated UMR 106 cells showed that physiological concentrations of GH induced nuclear translocation of both STAT5 isoforms, but with STAT5A being predominant. Both isoforms displayed similar nuclear turnover after GH stimulation of cells. Gel electrophoretic mobility shift assay (GEMSA) of nuclear extract revealed that both STAT5A and STAT5B obtained DNA-binding capacity after GH stimulation. Thus, we have shown, for the first time, the expression and GH-induced activation of JAK2 and STAT5A/B in UMR 106 osteoblast-like cells. This study also shows that this cell line is a suitable experimental model to study unique GH effects in osteoblasts mediated by STAT5.

Development of an In Vitro Screening Assay for Compounds that Increase Bone Formation

Although parathyroid hormone (PTH) has the ability to stimulate bone growth in both rats and humans, its mechanism of action is not fully understood at the molecular level. An in vitro marker that reflects the in vivo anabolic actions of PTH would facilitate the discovery of small-molecule compounds that stimulate bone growth. We therefore compared the patterns of gene expression in three cell lines treated with PTH. The levels of c-fos, collagenase, interleukin-6 (IL-6), and collagen mRNA were determined by reverse transcription-polymerase chain reaction (RT-PCR) in three osteoblast-like cell lines. The most responsive marker was c-fos, which was induced 5-10-fold after 1 h of PTH treatment in the UMR106-01 cell line. Because it is a critical early response gene in bone growth, we investigated the possibility of using c-fos stimulation as a method to screen for compounds that can stimulate bone formation. A highly sensitive, medium-throughput RT-PCR assay for c-fos mRNA expression was established using the Taqmantrade mark Detection System (Perkin Elmer, Mississauga, Ontario). Cells were treated with a series of compounds to determine the specificity of c-fos stimulation. Of the compounds tested, only PTH, prostaglandin E(2), 8-bromo-cAMP, and forskolin induced c-fos mRNA levels, indicating that this assay was specific for compounds that are known to induce cAMP and stimulate bone growth. These results indicate that a simple in vitro assay for c-fos may be a reliable method for the screening of compounds that stimulate bone growth in vivo.

Reversible suppression of in vitro biomineralization by activation of protein kinase A

Parathyroid hormone (PTH-(1-34)) potently suppresses apatite deposition in osteoblastic cultures. These inhibitory effects are mediated through signaling events following PTH receptor binding. Using both selective inhibitors and activators of protein kinase A (PKA), this study shows that a transient activation of PKA is sufficient to account for PTH's inhibition of apatite deposition. This inhibition is not a result of reduced cell proliferation, reduced alkaline phosphatase activity, increased collagenase production, or lowering medium pH. Rather, data suggest a functional relationship between matrix assembly and apatite deposition in vitro. Bone sialoprotein (BSP) and apatite co-localize in the extracellular matrix of mineralizing cultures, with matrix deposition of BSP temporally preceding that of apatite. Transient activation of PKA by either PTH-(1-34) or short term cAMP analog treatment blocks the deposition of BSP in the extracellular matrix without a significant reduction in the total amount of BSP synthesized and secreted. This effect is reversible after allowing the cultures to recover in the absence of PKA activators for several days. Thus, a transient activation of PKA may suppress mineral deposition in vitro as a consequence of altering the assembly of an extracellular matrix permissive for apatite formation.

Prevention of osteocyte and osteoblast apoptosis by bisphosphonates and calcitonin

Glucocorticoid-induced osteoporosis may be due, in part, to increased apoptosis of osteocytes and osteoblasts, and bisphosphonates (BPs) are effective in the management of this condition. We have tested the hypothesis that BPs suppress apoptosis in these cell types. Etidronate, alendronate, pamidronate, olpadronate, or amino-olpadronate (IG9402, a bisphosphonate that lacks antiresorptive activity) at 10(-9) to 10(-6) M prevented apoptosis of murine osteocytic MLO-Y4 cells, whether it was induced by etoposide, TNF-alpha, or the synthetic glucocorticoid dexamethasone. BPs also inhibited apoptosis of primary murine osteoblastic cells isolated from calvaria. Similar antiapoptotic effects on MLO-Y4 and osteoblastic cells were seen with nanomolar concentrations of the peptide hormone calcitonin. The antiapoptotic effect of BPs and calcitonin was associated with a rapid increase in the phosphorylated fraction of extracellular signal regulated kinases (ERKs) and was blocked by specific inhibitors of ERK activation. Consistent with these in vitro results, alendronate abolished the increased prevalence of apoptosis in vertebral cancellous bone osteocytes and osteoblasts that follows prednisolone administration to mice. These results suggest that the therapeutic efficacy of BPs or calcitonin in diseases such as glucocorticoid-induced osteoporosis may be due, in part, to their ability to prevent osteocyte and osteoblast apoptosis.

Regulation of adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in UMR-106 osteoblast-like cells: role of cAMP-phosphodiesterase and cAMP efflux

The present study aimed to define the role of adenosine 3',5'-cyclic monophosphate (cAMP)-phosphodiesterase (PDE) activity and the possible involvement of cAMP efflux on parathyroid hormone (PTH)-stimulated intracellular cAMP accumulation in cultured osteoblast-like UMR-106 cells. Treatment of the cells with 10 nM PTH (1-84) rapidly increased the level of intracellular cAMP. PTH stimulation also increased the cAMP efflux rate. The efflux of cAMP could only account for a minor part of the decrease in intracellular cAMP. Six peaks of cAMP-hydrolyzing PDE activity were separated by Q-Sepharose chromatography. The first peak to elute was stimulated by Ca2+/calmodulin and provided less than 2% of the total eluted cAMP-PDE activity. The second peak, providing less than 4% of the cAMP-PDE activity, was stimulated 3-fold by 4 microM cyclic GMP (cGMP) and was sensitive to the PDE2 isoenzyme-selective inhibitor erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA). The third peak, providing less than 10% of the cAMP-PDE activity, was insensitive to rolipram, EHNA, Ca2+/calmodulin, and cGMP. Peaks 4, 5 and 6 were sensitive to rolipram (IC50 < 0.1 microM) and provided approximately 85% of the total cAMP-hydrolyzing activity. It is concluded that cAMP-PDE activity in UMR-106 cells plays a major role in the control of intracellular cAMP accumulation, whereas only moderate amounts of cAMP are extruded from the cells through cAMP efflux. The main cAMP-hydrolyzing PDE isozyme is cAMP-specific/rolipram-sensitive. Ca2+/calmodulin-stimulated PDE, cGMP-stimulated PDE, and presently unidentified cAMP-specific/rolipram-insensitive PDE are also present in UMR-106 cells.

Gap junctional communication modulates gene expression in osteoblastic cells

Bone-forming cells are organized in a multicellular network interconnected by gap junctions. In these cells, gap junctions are formed by connexin43 (Cx43) and connexin45 (Cx45). Cx43 gap junctions form pores that are more permeable to negatively charged dyes such as Lucifer yellow and calcein than are Cx45 pores. We studied whether altering gap junctional communication by manipulating the relative expression of Cx43 and Cx45 affects the osteoblast phenotype. Transfection of Cx45 in cells that express primarily Cx43 (ROS 17/2.8 and MC3T3-E1) decreased both dye transfer and expression of osteocalcin (OC) and bone sialoprotein (BSP), genes pivotal to bone matrix formation and calcification. Conversely, transfection of Cx43 into cells that express predominantly Cx45 (UMR 106-01) increased both cell coupling and expression of OC and BSP. Transient cotransfection of promoter-luciferase constructs and connexin expression vectors demonstrated that OC and BSP gene transcription was down-regulated by Cx45 cotransfection in ROS 17/2. 8 and MC3T3-E1 cells, in association with a decrease in dye coupling. Conversely, cotransfection of Cx43 in UMR 106-01 cells up-regulated OC and BSP gene transcription. Activity of other less specific osteoblast promoters, such as osteopontin and osteonectin, was less sensitive to changes in gap junctional communication. Thus, altering gap junctional permeability by manipulating the expression of Cx43 and Cx45 in osteoblastic cells alters transcriptional activity of osteoblast-specific promoters, presumably via modulation of signals that can diffuse from cell to cell. A communicating intercellular network is required for the full elaboration of a differentiated osteoblastic phenotype.

Casein kinase 2 phosphorylation of recombinant rat osteopontin enhances adhesion of osteoclasts but not osteoblasts

Osteopontin (OP) is a highly phosphorylated bone matrix protein and contains the RGD cell-binding motif, which mediates cell adhesion through integrin receptors that include alpha(v)beta3. Casein kinase 2 (CK2) is a factor-independent serine/threonine kinase, which may be the predominant physiologically relevant kinase for OP phosphorylation. This study was designed to examine the effects of unphosphorylated recombinant rat OP, and CK2-phosphorylated OP (P-OP), on the adhesion and function of mouse osteoclasts (OC) and osteoblast-like cells (UMR 201-10B and UMR 106-06) in vitro. OP significantly increased OC adhesion compared to plastic alone, and cell attachment was further increased at least twofold on OP phosphorylated with CK2. Attachment was dependent on the integrity of the RGD domain and was completely abolished in the presence of 1 mM RGD peptide. Neither CK2 phosphorylation of mutant OP, in which the RGD was converted to RGE or RAD, nor protein kinase C (PKC) phosphorylation of wild-type OP enhanced OC attachment. An antibody to the beta3 integrin subunit, but not anti-mouse CD44 antibody, specifically blocked the proportion of attachment due to phosphorylation of OP. Actin ring formation in OC was increased by plating cells onto OP, with no further increase by phosphorylation. Both OP and CK2-phosphorylated OP enhanced attachment of the two osteoblastic cell lines, compared to plastic, but in contrast to OCs, there was no significant difference with phosphorylation. Osteoblast attachment was totally blocked by 1 mM RGD peptide, but was not influenced by the beta3 integrin antibody. Plating of UMR 201-10B cells onto OP further increased retinoic acid-induced alkaline phosphatase expression. The results suggest that specific phosphorylation of OP is important for interaction with OCs, compared with osteoblastic cells, and that alternative integrins may be important in the interaction between osteoblastic cells and OP compared with OCs.

The role of the alpha3(VI) chain in collagen VI assembly. Expression of an alpha3(VI) chain lacking N-terminal modules N10-N7 restores collagen VI assembly, secretion, and matrix deposition in an alpha3(VI)-deficient cell line

Collagen VI is a microfibrillar protein found in the extracellular matrix of virtually all connective tissues. Three genetically distinct subunits, the alpha1(VI), alpha2(VI), and alpha3(VI) chains, associate intracellularly to form triple-helical monomers, which then assemble into disulfide-bonded dimers and tetramers before secretion. Although sequence considerations suggest that collagen VI monomers composed of all three chains are the most stable isoform, the precise chain composition of collagen VI remains controversial and alternative assemblies containing only alpha1(VI) and alpha2(VI) chains have also been proposed. To address this question directly and study the role of the alpha3(VI) chain in assembly, we have characterized collagen VI biosynthesis and in vitro matrix formation by a human osteosarcoma cell line (SaOS-2) that is deficient in alpha3(VI) production. Northern analysis showed an abundance of alpha1(VI) and alpha2(VI) mRNAs, but no detectable alpha3(VI) mRNA was apparent in SaOS-2 cells. By day 30 of culture, however, small amounts of alpha3(VI) mRNA were detected, although the level of expression was still much less than alpha1(VI) and alpha2(VI). Collagen VI protein was not detected in SaOS-2 medium or cell layer samples until day 30 of culture, demonstrating that despite the abundant synthesis of alpha1(VI) and alpha2(VI), no stable collagen VI protein was produced without expression of alpha3(VI). The alpha1(VI) and alpha2(VI) chains produced in the absence of alpha3(VI) were non-helical and were largely retained intracellularly and degraded. The critical role of the alpha3(VI) chain in collagen VI assembly was directly demonstrated after stable transfection of SaOS-2 cells with an alpha3(VI) cDNA expression construct that lacked 4 of the 10 N-terminal type A subdomains. The transfected alpha3(VI) N6-C5 chains associated with endogenous alpha1(VI) and alpha2(VI) and formed collagen VI dimers and tetramers, which were secreted and deposited into an extensive network in the extracellular matrix. These data demonstrated that alpha3(VI) is essential for the formation of stable collagen VI molecules and subdomains N10-N7 are not required for molecular assembly.

Cyclic stretch enhances gap junctional communication between osteoblastic cells

Mechanical loading is essential to maintain skeletal integrity. Because gap junctions in bone are affected by mechanical factors, we studied whether stretch, an anabolic stimulus for osteoblasts, modulates direct intercellular communication in these cells. Gap junctional communication during stretch was assessed using a newly developed method, the "parachute assay," which allows monitoring of dye diffusion without disruption of the plasma membrane. Application of cyclic stretch for 2 or 24 h to well-coupled ROS 17/2.8 cells resulted in a 56.5% and 30.4% increase in dye coupling, respectively, compared with resting conditions. Stretch increased dye diffusion less dramatically (12.4% compared with unstimulated cells) in the poorly coupled UMR 106-01 cells. The stretch-induced increase of cell coupling was abolished in the presence of the gap junctional inhibitor, heptanol. Steady-state mRNA levels of connexin43 (Cx43), the gap junction protein that mediates cell-to-cell diffusion of negatively charged dyes between osteoblasts, were not different between control and stretched ROS 17/2.8 or UMR 106-01 cultures after various periods of cyclic stretch. However, phosphorylated forms of Cx43 protein were more abundant in stretched ROS 17/2.8 than in controls. This was associated with increased punctate Cx43-specific immunostain at appositional membranes of stretched cells. Thus, cyclic stretch increases gap junctional communication between osteoblastic cells by modulating intracellular localization of Cx43.

Regulation of connexin43 expression and function by prostaglandin E2 (PGE2) and parathyroid hormone (PTH) in osteoblastic cells

Connexin43 (Cx43) forms gap junctions that mediate intercellular communication between osteoblasts. We have examined the effects of prostaglandin E2 (PGE2) and parathyroid hormone (PTH) on gap junctional communication in the rat osteogenic sarcoma cells UMR 106-01. Incubation with either PGE2 or PTH rapidly (within 30 min) increased transfer of negatively charged dyes between UMR 106-01 cells. This stimulatory effect lasted for at least 4 h. Both PGE2 and PTH increased steady-state levels of Cx43 mRNA, but only after 2-4 h of incubation. Transfection with a Cx43 gene construct linked to luciferase showed that this effect of PTH was the result of transcriptional upregulation of Cx43 promoter. Stimulation of dye coupling and Cx43 gene transcription were reproduced by forskolin and 8Br-cAMP. Exposure to PGE2 for 30 min increased Cx43 abundance at appositional membranes in UMR 106-01, whereas total Cx43 protein levels increased only after 4-6 h of incubation with either PGE2 or PTH. Inhibition of protein synthesis by cycloheximide did not affect this early stimulation of dye coupling, but it significantly inhibited the sustained effect of PTH and forskolin on cell coupling. In summary, both PTH and PGE2, presumably through cAMP production, enhance gap junctional communication in osteoblastic cell cultures via two mechanisms: initial rapid redistribution of Cx43 to the cell membrane, and later stimulation of Cx43 gene expression. Modulation of intercellular communication represents a novel mechanism by which osteotropic factors regulate the activity of bone forming cells.

Differential purinergic receptor signalling in osteoclasts and osteoblastic cells

There are differences between osteoclasts and osteoblastic cells in their cytosolic calcium responses to purinergic receptor activation. Application of 50 or 100 microM extracellular ATP inhibits the calcium response to a second application of ATP in osteoblastic rat osteosarcoma UMR 106 cells, but not in rabbit osteoclasts. This shows that there is adaptation to the extracellular, ATP in osteoclasts, but not in the UMR 106 cells. Extracellular washing of the UMR 106 cells restores the calcium response to ATP partially but not completely, indicating that there is a purinergic receptor activation-induced desensitisation of the receptor or its linked signalling pathways. In contrast to these results, if extracellular UTP is applied first, application of ATP produces no calcium response in osteoclasts, with or without washing, while in the UMR 106 cells there is some response to the ATP, which is greatly enhanced by washing. This indicates that UTP induces a complete desensitisation of the purinergic receptor/calcium signalling system in osteoclasts, but not in the osteoblastic cells, in which there is simply competition between UTP and ATP for the same receptors. Taken together, these results demonstrate that ATP and UTP could differentially regulate osteoblasts and osteoclasts.

Expression of the CT/CGRP gene and its regulation by dibutyryl cyclic adenosine monophosphate in human osteoblastic cells

There is general agreement that calcitonin (CT) inhibits bone resorption by its effects on osteoclast function. CT was also found to have direct effects on osteoblast-like cells. In this study, we investigated the expression of CT and calcitonin gene-related peptide (CGRP), the two peptides encoded by the CT/CGRP gene, in human osteosarcoma cell lines and in normal human trabecular osteoblastic cells (HOB), and we studied the modulation of CT/CGRP gene expression by dibutyryl cyclic adenosine monophosphate ((Bu)2, cAMP), a cAMP analog. We first detected by Northern blot hybridization the presence of CT and CGRP mRNAs in different osteosarcoma cell lines (OHS-4, MG-63, Saos-2, HOS-TE85) and HOB cells. In the steady state, OHS-4 cells express slightly more CT and CGRP mRNAs than other cell lines or normal human osteoblasts, in parallel with messengers of differentiated osteoblasts, such as osteocalcin (OC) and alkaline phosphatase (ALP). OHS-4 cells also express CT and CGRP proteins, as demonstrated by immunocytochemistry. Stimulation of OHS-4 cells with 1 mM (Bu)2 cAMP induced a significant increase in mRNA levels for CT (x 2.5) and CGRP (x 3), as determined by a semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) procedure. The involvement of a transcriptional mechanism in this effect was evidenced by nuclear run-off transcription assay. In addition, (Bu)2 cAMP increased OC (x 4) and ALP (x 3) mRNA levels in OHS-4 cells. These effects were observed at 24 h and were maximal at 48 h, indicating that (Bu)2, cAMP induced cell differentiation and increased the transcription of the CT/CGRP gene in OHS-4 osteoblast-like cells. The results indicate that human osteosarcoma cells and primary human osteoblastic cells express CT and CGRP mRNA and proteins, and that (Bu)2 cAMP, an activator of protein kinase A, induces up-regulation of osteoblastic phenotypic genes and enhances CT and CGRP gene transcription, indicating that induction of osteoblastic differentiation by (Bu)2 cAMP is associated with enhanced expression of CT and CGRP in human osteoblastic cells.

Rodent osteoblast-like cells support osteoclastic differentiation of human cord blood monocytes in the presence of M-CSF and 1,25 dihydroxyvitamin D3

Fracture repair requires the involvement of osteoclasts (OC), multinucleated cells which are responsible for bone resorption and form by fusion of circulating mononuclear haemopoietic precursors. The nature of these circulating precursor cells, in particular their relationship to blood monocytes, is uncertain. To define further the nature of the circulating human OC precursor, and to determine the role bone stromal cells and humoral factors play in the differentiation of OCs, we co-cultured human umbilical cord blood monocytes with UMR106.01 osteoblast-like cells in the presence and absence of 1,25 dihydroxyvitamin D3 [1,25 (OH)2D3], macrophage-colony stimulating factor (M-CSF) and dexamethasone on both bone slices and coverslips. Isolated cells were positive only for monocyte/macrophage markers (CD11a, CD11b, CD14 and HLA-DR) and negative for OC markers [tartrate resistant acid phosphatase (TRAP), vitronectin receptors (VNR) and calcitonin receptors (CT receptors)] and did not form resorption pits on bone slices after 24 hr in culture. However, after 14 days in co-culture with UMR106.01 cells, in the presence of 1,25 (OH)2D3 and M-CSF, numerous TRAP, CT receptor and VNR positive multinucleated cells capable of extensive lacunar bone resorption were formed in these co-cultures. The presence of 1,25 (OH)2D3, M-CSF and a bone-derived stromal cell population were absolute requirements for OC differentiation. It is concluded that mononuclear phagocytes are capable of differentiating into mature functional OCs when cultured under specific cellular and hormonal conditions. This is vitro model of human OC differentiation should prove useful in further analysing factors controlling OC generation in bone remodelling and repair.