Mineralized bone nodules formed in vitro from enzymatically released rat calvaria cell populations

Effects of high levels of fluoride on bone formation: an in vitro model system

In order to develop an in vitro model for the study of the effects of different agents on biomineralization, a three-dimensional cell culture system was investigated at different levels of fluoride. Rat fetal osteoblasts were seeded onto collagen discs and maintained in a culture medium for 40 days. Results showed that, at 40 days, the cultured matrices had a Ca:P ratio, mineral content and Fourier transform infrared (FTIR) spectrum that were close to those seen for normal rat bone. Viable cells, observed by light microscopy, were present in the matrix at 40 days. The formation of a mineralized matrix in this experimental set-up provided a model for exploring in vitro the effects of high levels of fluoride on bone. The fluoride content of the mineral formed in the cultures showed a dose-dependent increase in fluoride content with time. Also, an increase in the crystallinity of the apatite in the presence of fluoride, was observed by FTIR. The Ca:P ratio and percentage mineral by weight showed no apparent differences among the groups. The three-dimensional model used for this study has the potential to be a powerful tool in the study of time-dependent effects of drugs and other factors on osteoblast cell functions and subsequently on matrix mineralization.

Effect of prostaglandin E2 on mineralization of bone nodules formed by fetal rat calvarial cells

The effects of PGE2 on mineralized bone nodule formation were studied in fetal rat calvarial (RC) cells in vitro. Continuous exposure of RC cells to 3 x 10(-8) M PGE2 induced a twofold increase in mineralized bone nodule formation and a 1.5-fold increase in alkaline phosphatase (ALPase) activity without affecting RC cell growth. These stimulatory effects were evoked by concentrations of 3 x 10(-9)-3 x 10(-6) M PGE2 and the maximal effect was observed with 3 x 10(-8) M PGE2. The in vitro effects of PGE2 were evident when RC cells were exposed to it on days 8-14 and 8-21, which correspond to the post-confluent culture stage, but no effects were observed when the cells were exposed on days 1-7, the growth stage. The ALPase activity was also higher (1.2-1.4-fold) when 3 x 10(-8) M PGE2 was added during the post-confluent stage. In order to determine the effect of PGE2 during the mineralization phase of bone nodules in the presence of a large population of osteoprogenitor cells, RC cells were exposed to dexamethasone for 7 days before PGE2 was added during the post-confluent stage. A significantly higher percentage of nodules mineralized were observed with 3 x 10(-8)-3 x 10(-9) M PGE2 (1.6- and 1.4-fold, respectively), than in control cultures. Analysis of the mineral-related proteins by EDTA extraction of bone nodules followed by electrophoresis and Stains-All staining revealed an increased total amount of osteopontin extracted from the mineralized matrix after PGE2 treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Osteoblast function on synthetic biodegradable polymers

Rat osteoblasts were cultured on films of biodegradable poly(L-lactic acid) (PLLA), 75:25 poly(DL-lactic-co-glycolic acid) (PLGA), 50:50 PLGA, and poly(glycolic acid) (PGA) for up to 14 days. Osteoblasts attached equally well to all the polymer substrates after 8 h in culture. By day 4 in culture, osteoblasts had exceeded confluency numbers, and their proliferation leveled off by day 7. An increase in alkaline phosphatase (ALP) activity from 1.92 (+/- 0.47) x 10(-7) for day 7 to 5.75 (+/- 0.12) x 10(-7) mumol/cell per min for day 14 was reported for osteoblasts cultured on 75:25 PLGA, which was comparable to that observed for tissue culture polystyrene (TCPS) controls. The ALP activities expressed by osteoblasts cultured on PLLA, 50:50 PLGA, and PGA films did not significantly increase over time. Collagen synthesis for osteoblasts cultured on all polymer substrates was similar to that of TCPS and did not vary with time. The morphology of cultured osteoblasts was not affected by the continuous degradation of the polymer substrates. These results demonstrate that poly(alpha-hydroxy esters) can provide a suitable substrate for osteoblast culture and hold promise in bone regeneration by osteoblast transplantation.

Thyroid hormone suppresses the differentiation of osteoprogenitor cells to osteoblasts, but enhances functional activities of mature osteoblasts in cultured rat calvaria cells

The effects of thyroid hormone on osteoblastic differentiation and activity were studied in fetal rat calvaria (RC) cells cultured for up to 30 days in medium supplemented with thyroid hormone-depleted serum. In this condition, the cells proliferated and differentiated to form mineralized bone nodules (BN) and expressed osteoblastic markers such as alkaline phosphatase (ALP), osteocalcin (OCN), and osteopontin (OPN). The continuous presence of triiodothyronine (T3) at 10(-9)-10(-8) M in the medium inhibited the osteoblastic differentiation: 34% decrease in ALP activity on day 12 and 60% decrease in BN formation on day 15 at 10(-8) M. T3 at these doses had no effect on the DNA content of RC cells at confluence (day 6). Short-term (48-h) exposure of T3 at 10(-9) M or higher decreased ALP activity when RC cells were differentiating (days 7-11). However, when BN formation by the cells had already reached a plateau (day 28), the activity was increased by treatment with T3 at 10(-7)-10(-6) M. OCN production was increased dose dependently by this treatment with T3 (2.1-fold and 1.3-fold of control at 10(-8) M on days 11 and 28, respectively). Similar increases were observed in the levels of OCN mRNA. In addition, increases in phosphorylated OPN in the medium (day 11) and mineralized matrix (day 28) were observed (1.5-fold at 10(-8)-10(-6) M), while OPN synthesis and the level of its mRNA were depressed by T3 (60-70% of control at 10(-8) M). These results suggest that T3 regulates osteoblastic differentiation and activity depending on the state of cell differentiation: T3 suppresses the differentiation of osteoprogenitor cells to osteoblasts, but enhances the functional activity of mature osteoblasts.

Formation of afibrillar acellular cementum-like layers induced by alkaline phosphatase activity from periodontal ligament explants maintained in vitro

Fibroblasts of the periodontal ligament, by their alkaline phosphatase (ALP) activity, are considered to play a role in the formation of acellular cementum. As a means of exploring this hypothesis, periodontal ligament explants from rat incisors were cultured in direct contact with bovine dentin slices in the presence of 10 mmol/L beta-glycerophosphate. Periosteal and pericardial tissue explants were maintained under similar conditions. After two weeks, the slices were harvested and processed for electron microscopic examination. Controls included periodontal ligament explants to which the ALP-inhibitor levamisole was added. The results suggest that only ALP-positive cultures from periodontal ligament and periosteum form mineralized layers along the dentin. After demineralization, layers consisted of fine filamentous or granular material of moderate electron-density and resembled afibrillar acellular cementum. Our findings support the hypothesis that periodontal ligament fibroblasts, by means of their ALP activity, play a pivotal role in the formation of acellular cementum.

Greater inhibition of in vitro bone mineralization with metabolic than respiratory acidosis

At a similar decrement in pH, acidosis produced by lowering the concentration of medium bicarbonate (metabolic acidosis) induces greater net calcium efflux from cultured neonatal mouse calvariae than acidosis produced by increasing the partial pressure of carbon dioxide (respiratory acidosis). This differential effect is due, at least in part, to enhanced cell-mediated bone mineral resorption during metabolic acidosis. To determine the effect of acidosis on osteoblastic bone formation we utilized primary cultures of neonatal mouse calvarial cells which produce calcified nodules in culture. Cells were plated at 4.5 x 10(4) cells/35 mm dish and incubated until confluent (day 9). Nodule formation was then induced by addition of beta-glycerophosphate and ascorbic acid and the cultures were randomly divided and then cultured in control (Ctl, N = 18) medium or in medium simulating metabolic (Met, N = 17) or respiratory (Resp, N = 19) acidosis. Medium was changed and calcium (Ca) measured every 48 hours until day 23. The mean initial medium pH of all Resp cultures (7.186 +/- 0.002) was lower than Met (7.243 +/- 0.006, P < 0.01), which was lower than Ctl (7.502 +/- 0.002, p < 0.01), yet the number of discrete nodules formed in Met (22 +/- 4 nodules/cm2) was lower than Resp (43 +2- 7, P < 0.01), and both were lower than Ctl (88 +/- 6, P < 0.01 vs. both Met and Resp).(ABSTRACT TRUNCATED AT 250 WORDS)

In situ hybridization to show sequential expression of osteoblast gene markers during bone formation in vivo

We investigated the sequence of expression of osteoblast gene markers during bone formation in vivo by in situ hybridization. Cylindrical lesions were induced in the femora of sheep with titanium analytic bone implants that allow removal of serial core samples to study bone formation. At 2 weeks (2W), granulation tissue made up of spindle-shaped cells had partially replaced the blood clot. Islands of osseous tissue, first noted in the periphery of the ingrowing tissue at 3W, became the predominant tissue by 6W. The surfaces of newly forming bone at 3W were apposed by cuboidal cells, which in some areas were several layers thick. By 6W, most of the cells lining bone trabeculae had assumed a flattened morphology. The temporal and spatial distribution of osteoblast gene markers was examined by in situ hybridization with nonradioactive digoxigenin probes for alpha 1(I) procollagen, alkaline phosphatase (ALP), osteopontin (OP), and bone Gla protein (BGP). The spindle-shaped cells in the granulation tissue expressed mRNA for alpha 1(I) procollagen, ALP, and OP but not BGP, suggesting that they may be osteoblast precursor cells. alpha 1(I) procollagen mRNA was strongly expressed by all cells on the surface of bone, with a peak intensity at 3W and then reducing sharply by 6W. Initially, only pockets of cuboidal cells on bone surfaces expressed ALP mRNA, with a peak intensity at 5W. Similarly, only a proportion of cuboidal cells expressed OP mRNA early in bone formation, but the number of cells expressing OP mRNA increased with time. Clumps of cuboidal cells expressed BGP mRNA only when bone was present, and the degree of expression increased with the amount of bone formed. This model allows the study of temporal and spatial sequence of gene expression in cells participating in osteogenesis. The temporal sequence is similar to that shown in vitro in other models of mineralization. The geographic localization of cells expressing mRNA for alpha 1(I) procollagen, ALP, OP, and BGP implies subspecialization of osteoblasts in bone formation.

Mineralization of marrow-stromal osteoblasts MBA-15 on three-dimensional carriers

The present study describes a new three-dimensional (3-D) culture system that enables the maintenance and phenotypic expression of bone marrow stromal osteoblasts. This culture substratum is advantageous in that it provides suitable conditions for attachment, growth, and differentiation of cells forming 3-D layers. The MBA-15 cell line was grown in unlimited quantities on 3-D Fibro-Cel carriers. These cells mineralized when exposed to ascorbic acid and beta-glycerophosphate (beta GP). Under these mineralization conditions, mRNA expressions of procollagen alpha 2(I) and [3H]-proline-labeled protein were increased. The expression of mRNA for osteonectin and to a lesser extent, for osteopontin was increased, whereas alkaline phosphatase and biglycan remained unaffected under similar conditions. Exposure of mineralizing cultures to dexamethasone reduced mRNA of procollagen alpha 2 (I) and osteonectin to control level. Scanning electron microscopy revealed that cells were grown along the fabric's fibers and produced collagen fibrils. Under appropriate conditions, extensive mineralization had taken place. The mineralization process involves the formation of calcospherites, and correlates with an increase in calcium content. The Fibro-Cel carriers enable formation of 3-D architecture and mineralized tissue in vitro.

Characterizations of titanium implant surfaces. III

There are several reports in the literature concerning the similarities and the differences between the oxide on cpTi and Ti-6A1-4V alloy; however, their biological sequelae are not entirely known. In this work, a series of surface characterization techniques were used in conjunction with short term in vitro biological assays to assess the effects of materials selection (cpTi and Ti alloy) on osteoblast-like cell responses. Surface analysis indicated that with the exception of oxide thickness, there were no significant differences in surface characteristics between the two implant materials. These results were reflected in the biological studies, where the levels of cell attachment and adaptation of the attached cells to the titanium surfaces were similar. These results are in general agreement with previous in vivo studies and continue to indicate that cpTi and Ti alloy are suitable, biologically compatible materials for fabrication of dental implants.

TGF beta alters growth and differentiation related gene expression in proliferating osteoblasts in vitro, preventing development of the mature bone phenotype

This study examines the mechanism by which TGF-beta 1, an important mediator of cell growth and differentiation, blocks the differentiation of normal rat diploid fetal osteoblasts in vitro. We have established that the inability for pre-osteoblasts to differentiate is associated with changes in the expression of cell growth, matrix forming, and bone related genes. These include histone, jun B, c-fos, collagen, fibronectin, osteocalcin, alkaline phosphatase, and osteopontin. Morphologically, the TGF-beta 1-treated osteoblasts exhibit an elongated, spread shape as opposed to the characteristic cuboidal appearance during the early stages of growth. This is followed by a decrease in the number of bone nodules formed and the amount of calcium deposition. These effects on differentiation can occur without dramatic changes in cell growth if TGF-beta 1 is given for a short time early in the proliferative phase. However, continuous exposure to TGF-beta 1 leads to a bifunctional growth response from a negative effect during the proliferative phase to a positive growth effect during the later matrix maturation and mineralization phases of the osteoblast developmental sequence. Extracellular matrix genes, fibronectin, osteopontin and alpha 1(I) collagen, are altered in their expression pattern which may provide an aberrant matrix environment for mineralization and osteoblast maturation and potentiate the TGF-beta 1 response throughout the course of osteoblast differentiation. The initiation of a TGF-beta 1 effect on cell growth and differentiation is restricted to the proliferative phase of the culture before the cells express the mature osteoblastic phenotype. Second passage cells that are accelerated to differentiate by the addition of dexamethasone or by seeding cultures at a high density are refractory to TGF-beta 1. These in vitro results indicate that TGF-beta 1 exerts irreversible effects at a specific stage of osteoblast phenotype development resulting in a potent inhibition of osteoblast differentiation at concentrations from 0.1 ng/ml.

Aberrant gene expression in cultured mammalian bone cells demonstrates an osteoblast defect in osteopetrosis

Osteopetrosis is a skeletal condition in which a generalized radioopacity of bone is caused by reduced resorption of bone by osteoclasts. However, it has recently been shown that during skeletal development in several osteopetrotic rat mutations specific aberrations occur in gene expression reflecting the activity of the bone forming cells, osteoblasts, and the development of tissue organization. To evaluate their pathogenetic significance, progressive osteoblast differentiation was studied in vitro. Primary cultures of normal osteoblasts undergo a sequential expression of cell growth and tissue-related genes associated with development of skeletal tissue. We report that osteoblast cultures can be established from one of these mutants, toothless; that these cells in vitro exhibit similar aberrations in gene expression during cell proliferation and extracellular matrix formation and mineralization observed in vivo; and that an accelerated maturation sequence by mutant osteoblasts mimics the characteristic skeletal sclerosis of this disease. These data are the first direct evidence for an intrinsic osteoblast defect in osteopetrosis and establish an in vitro model for the study of heritable skeletal disorders.

Stimulation of signal transduction pathways in osteoblasts by mechanical strain potentiated by parathyroid hormone

Second-messenger systems have been implicated to transmit mechanical stimulation into cellular signals; however, there is no information on how mechanical stimulation is affected by such systemic factors as parathyroid hormone (PTH). Regulation of adenylyl cyclase and phosphatidylinositol pathways in rat dentoalveolar bone cells by mechanical strain and PTH was investigated. Two different cell populations were isolated after sequential enzyme digestions from dentoalveolar bone (group I and group II) to study potential differences in response. Mechanical strain was applied with 20 kPa of vacuum intermittently at 0.05 Hz for periods of 0.5, 1, 5, 10, and 30 minutes and 1, 3, and 7 days using the Flexercell system. Levels of cAMP, measured by RIA, and levels of inositol 1,4,5-triphosphate (IP3) and protein kinase C activity (PKC), measured by assay systems, increased with mechanical strain. When PTH was added to the cells, there was a significant increase in levels of all the intracellular signals, which appeared to potentiate the response to mechanical strain. IP3 levels (0.5 minute) peaked before those of PKC activity (5 minutes), which in turn peaked before those of cAMP (10 minutes). Group II cells showed higher levels of cAMP and IP3 than the group I cells. This suggests that the former may ultimately play the predominant roles in skeletal remodeling in response to strain. Immunolocalization of the cytoskeleton proteins vimentin and alpha-actinin, focal contact protein vinculin, and PKC showed a marked difference between strained and nonstrained cells. However, the addition of PTH did not cause any significant effect in cytoskeleton reorganization. Staining of PKC and vimentin, alpha-actinin, and vinculin suggests that PKC participates actively in the transduction of mechanical signals to the cell through focal adhesions and the cytoskeleton, although only PKC seemed to change with short time periods of strain. In conclusion, dentoalveolar osteoblasts responded to mechanical strain initially through increases in levels of IP3, PKC activity, and later cAMP, and this response was potentiated when PTH was applied together with mechanical strain.

Formation and mineralization of extracellular matrix secreted by an immortal human osteoblastic cell line: modulation by tumor necrosis factor-alpha

Tumor necrosis factor-alpha (TNF-alpha), a 17-kDa cytokine produced by stimulated macrophages/monocytes, modulates the functions of a variety of cells and has been shown to induce bone resorption in vitro. However, the effects that TNF-alpha may have on the process of bone formation are not completely understood. In order to study the effects of TNF-alpha on matrix development and mineralization, we utilized a human osteoblastic cell line, HOS TE85. Our results show that HOS TE85, which has been shown to be responsive to hormones active on normal osteoblasts, forms an extensive extracellular matrix (ECM) that mineralizes during extended culture. Treatment during the development of the matrix with TNF-alpha has little effect on cell number and DNA synthesis, showing thereby that TNF-alpha is not cytotoxic to the cells. However, TNF-alpha inhibits the formation of alkaline phosphatase (AP)-positive foci in a dose-dependent manner at concentrations of 0.1-10 ng/ml. TNF-alpha treatment caused a significant decrease in the incorporation of collagen into the developing matrix. In addition, TNF-alpha treatment resulted in a significant decrease in the synthesis of AP by HOS TE85 cells during the process of ECM formation and resulted in a pronounced lack of mineralization of the ECM. These results indicate that TNF-alpha may be acting as an uncoupler by decreasing the synthesis and incorporation of proteins required for bone formation, and inhibiting matrix formation and mineralization in vitro.

Effects of transforming growth factor beta on bone nodule formation and expression of bone morphogenetic protein 2, osteocalcin, osteopontin, alkaline phosphatase, and type I collagen mRNA in long-term cultures of fetal rat calvarial osteoblasts

Transforming growth factor beta (TGF-beta) is one of the most abundant of the known growth regulatory factors stored within the bone matrix. When bone is resorbed, TGF-beta is released in an active form and is a powerful bone growth stimulant. When injected into the subcutaneous tissue over the calvarial surface of rodents, it rapidly causes proliferation of the periosteal layer and accumulation of new woven bone. In this report, we describe the effects of TGF-beta 1 on first subcultures of fetal rat osteoblasts obtained from calvarial bones and cultured from confluence with ascorbic acid and beta-glycerophosphate. Under these conditions, nodules with characteristics of normal bone appear by day 8. Similar to experiments described by Antosz et al., TGF-beta added to confluent cultures inhibited the formation of bone nodules. Both the number and total area of the nodules were quantitated and shown to be completely inhibited by 2 ng/ml of TGF-beta 1. TGF-beta also impaired the expression of genes associated with bone formation, including type I collagen, alkaline phosphatase, osteopontin, and osteocalcin. TGF-beta also inhibited the expression of mRNA for the bone morphogenetic protein 2 (BMP-2). These results, showing suppression of markers representative of osteoblast differentiation, suggest that the effects of TGF-beta to stimulate bone formation in vivo are not likely a result of effects on differentiated mineralizing osteoblasts but, as suggested by previous studies, more likely are caused by effects on osteoblast precursors. These results also suggest that endogenous BMP-2 expression in fetal rat calvaria cells is important for bone cell differentiation.

Transcriptionally active nuclei isolated from intact bone reflect modified levels of gene expression in skeletal development and pathology

Transcriptional regulation of gene expression in vivo in bone, associated with normal development or skeletal disorders, to date, has not been studied. We report the successful isolation of nuclei that are transcriptionally active from normal and osteopetrotic rat bone. Transcription rates of cell growth and bone-related genes (including histone H4, c-fos, c-jun, TGF beta 1, beta 2 macroglobulin, collagen, fibronectin, osteocalcin, osteopontin, and tartrate resistant acid phosphatase) change as a function of calvarial development from birth to 6 weeks and are selectively modified in osteopetrotic animals. Additionally, nuclei isolated from intact bone yield promoter binding factors. Bone nuclei, which transcribe faithfully and contain the normal complement of nuclear protein factors, offer a powerful approach for investigating in vivo gene regulation in skeletal development and pathology.

Cell stage-dependent effects of ascorbic acid on cultured porcine bone cells

Pig bone cells were isolated from fetuses or young animals. In culture, they proliferated for 6 days, became confluent and began differentiating. The effects of ascorbic acid (AsA) on cell proliferation, alkaline phosphatase (ALP) activity, non-collagenous protein (NCP) and collagen synthesis, were studied by adding AsA to the medium at different times during culture. AsA affected fetal and post-natal cells similarly: ALP activity, NCP and collagen synthesis were markedly reduced in cells treated before confluence, but were strongly and dose-dependently stimulated in cells treated after confluence. AsA also stimulated cell proliferation. The cell stage-dependent action of AsA suggests that it may interfere with differentiation. The effects of AsA on ALP activity and DNA content were not coupled to its effect on collagen synthesis, raising the question of whether AsA action is matrix-mediated.

Bone cell expression on titanium surfaces is altered by sterilization treatments

Phenotypic responses of rat calvarial osteoblast-like cells (RCOB) were evaluated on commercially pure titanium (cpTi) surfaces when cultured at high density (5100 cells/mm2). These surfaces were prepared to three different clinically relevant surface preparations (1-micron, 600-grit, and 50-microns-grit sand-blast), followed by sterilization with either ultraviolet light, ethylene oxide, argon plasma-cleaning, or routine clinical autoclaving. Osteocalcin and alkaline phosphatase, but not collagen expression, were significantly affected by surface roughness when these surfaces were altered by argon plasma-cleaning. In general, plasma-cleaned cpTi surfaces demonstrated an inverse relationship between surface roughness and phenotypic markers for a bone-like response. On a per-cell basis, levels of the bone-specific protein, osteocalcin, and the enzymatic activity of alkaline phosphatase were highest on the smooth 1-micron polished surface and lowest on the roughest surfaces for the plasma-cleaned cpTi. Detectable bone cell expression can be altered by clinically relevant surfaces prepared by standard dental implant preparation techniques.

Direct effects of metabolic products and sonicated extracts of Porphyromonas gingivalis 2561 on osteogenesis in vitro

It is well documented that oral microorganisms play a significant role in the initiation and progression of periodontal disease. By using various in vitro models, it has been shown that some bacteria considered periodontal pathogens or their products can stimulate bone resorption and some other parameters of osteoblast-like cell activity. However, the effects of these organisms and their products on osteogenesis itself are not known. This study was undertaken to determine the direct effects of metabolic products and sonicated extracts of Porphyromonas gingivalis on bone formation in the chick periosteal osteogenesis model. Cultures of P. gingivalis 2561 were grown under standard anaerobic culture conditions. The spent medium was collected, and following centrifugation, sonicated bacterial extracts were prepared from the bacterial pellet. These were added in various proportions to the chick periosteal osteogenesis cultures. Sonicated extracts were further fractionated into five molecular-size ranges and similarly tested. Parameters of osteogenesis, including alkaline phosphatase activity, calcium and Pi accumulation, and collagen synthesis, were measured on 6-day-old cultures. Compared with controls devoid of bacterial products, osteogenesis was inhibited significantly in cultures treated with either conditioned medium or extracts obtained from P. gingivalis. Various amounts of inhibitory activity were observed in the different ultrafiltration molecular-size fractions, with very profound inhibitory effects observed in the < 5-kDa range. Histological observations indicated the presence of cells, some bone, and/or new fibrous connective tissue at all concentrations, indicating that toxicity was not a factor. These results suggest that periodontal pathogens such as P. gingivalis might contribute to the bone loss in periodontal diseases not only by stimulating resorption but, possibly, by inhibiting bone formation directly.

Expression of bone morphogenetic protein messenger RNA in prolonged cultures of fetal rat calvarial cells

In addition to structural proteins of bone, such as type I collagen, bone cells synthesize a number of growth regulatory peptides that are also stored in the bone matrix, presumably as a consequence of local production by osteoblasts. Among the bone growth regulatory peptides found in the bone matrix are the recently described bone morphogenetic proteins (BMPs). These factors were purified from bone matrix by their capacity to stimulate ectopic bone formation, but it is not known whether they are produced by normal bone cells and influence normal bone formation. To determine whether they are expressed by normal osteoblasts during differentiation, we used the technique of prolonged primary culture of fetal rat calvarial osteoblasts. These cultures have been shown to be an informative model for studying expression of bone-related genes by cultured osteoblasts, since specific genes are expressed as the cells undergo proliferation and differentiation. We found that the bone morphogenetic proteins 1, 2, 4, and 6 are expressed by cultures of fetal rat calvarial osteoblasts before they form mineralized bone nodules and as they express alkaline phosphatase, osteocalcin, and osteopontin. This model can be used for study of regulation of expression of bone morphogenetic proteins by osteoblasts.

Isolation of cDNA clones from an osteosarcoma-ROS17/2.8 library by differential hybridization

We have used differential hybridization to isolate and characterize two novel cDNAs expressed in chondrocytes and some osteoblastic cells. A rat osteosarcoma ROS17/2.8 cDNA library was screened and cDNA clones hybridizing strongly to radiolabeled porcine calvaria cDNA but weakly to a control radiolabeled cDNA were isolated. Two clones were obtained--p.6.1 and p.10.15. A radiolabeled probe of p10.15 was shown to hybridize specifically to a 2.3 Kb message RNA from a chondrogenic clonal cell population from rat calvaria-RCJ 3.1C5.18, and the mRNA was downregulated by 1,25 (OH)2D3, which inhibits chondrogenesis in these cells. The other clone, p6.1, was found to hybridize to a 0.95 Kb message that is expressed in rat liver, kidney, lung, muscle, and brain, but not expressed in spleen and expressed only in low levels in thymus.

Cellular expression of bone-related proteins during in vitro osteogenesis in rat bone marrow stromal cell cultures

Rat bone marrow stromal cells comprise a heterogeneous mixture of cell lineages including osteoblastic cells. When grown in the presence of ascorbic acid, beta-glycerophosphate and 10(-8) M dexamethasone, osteoprogenitor cells within the population divide and differentiate to form bone nodules (Maniatopoulos et al., 1988, Cell Tissue Res., 254:317-330; Aubin et al., 1990, J. Bone Miner. Res., 5:S81) providing a useful model to investigate temporal and spatial changes in expression of osteoblastic markers. Immunocytochemistry was combined with Northern blotting, enzymatic assay, and radioimmunoassay to analyze the expression of bone-related proteins during the growth and differentiation sequence. By mRNA levels, protein production and/or enzymatic activity, expression of osteocalcin, bone sialoprotein, and alkaline phosphatase increased concomitantly with the development of bone nodules, while osteopontin mRNA levels decreased and those of SPARC/osteonectin did not change significantly. In older cultures with mineralizing nodules, mRNA levels for alkaline phosphatase and bone sialoprotein, but not osteocalcin, declined. Immunolabeling revealed that cells in early cultures stained poorly for SPARC/osteonectin and strongly for thrombospondin. Later, SPARC/osteonectin staining increased in most cells, while thrombospondin staining could be seen in both matrix and in cells, but with marked intercellular variability in intensity. At all time points studied, osteoblasts within bone nodules stained homogeneously for thrombospondin and alkaline phosphatase, and with marked heterogeneity of intensity amongst cells for SPARC/osteonectin and osteocalcin. Labelling with RCC455.4, a monoclonal antibody raised against rat calvaria cells which intensely labels osteoblasts and osteocytes (Turksen et al., 1992, J. Histochem. Cytochem., 40:1339-1352), co-localized with osteocalcin. Alkaline phosphatase activity and the amount of osteocalcin determined by both radioimmunoassay and immunolabelling decreased in very late cultures, a time corresponding to appearance of fully mineralized nodules. These studies indicate that the bone marrow stromal cell system is a useful model to study the temporal and spatial expression of bone-related proteins during osteogenesis and formation, mineralization, and maturation of bone nodules. Further, immunolabelling at the individual cell and single bone nodule level allowed discrimination of marked variability of expression of osteoblast markers during the differentiation sequence.

Temporal changes in matrix protein synthesis and mRNA expression during mineralized tissue formation by adult rat bone marrow cells in culture

To characterize the bone-like tissue produced by rat bone marrow cells (RBMC) from young adult femurs, the synthesis of bone proteins and the expression of their mRNA were studied in vitro. RBMC plated at a density of 5 x 10(3) cells/cm2 and grown in the presence of 10(-8) M dexamethasone (Dex) and 10 mM beta-glycerophosphate (beta-GP) produced mineralized bone nodules, which were first evident at day 3 and increased markedly to day 13. However, in the absence of dexamethasone, few mineralized nodules were observed. The formation of mineralized nodules was reflected by the uptake of 45Ca, which also increased markedly to day 13. Analysis of bone protein expression by Northern and slot-blot hybridizations revealed an increase in mRNA levels of collagen type I (Col I), osteonectin/SPARC (ON), alkaline phosphatase (ALP), osteopontin (OPN), bone sialoprotein (BSP), and osteocalcin (OC) during the formation of mineralized nodules. Whereas the Col I, ON, ALP, and OPN mRNAs were expressed before the formation of mineralized nodules was evident and were also expressed at various levels in the absence of Dex, the expression of BSP and OC mRNA was induced in the bone-forming cultures. The expression of BSP mRNA was correlated temporally with bone tissue formation, reaching maximal levels on day 16. In contrast, OC mRNA was expressed later and, following induction, increased over the 28 day culture period. Production of matrix proteins during the rapid formation of the bone tissue appeared to reflect the levels of the respective mRNAs. However, whereas some of the collagen and almost all of the SPARC were secreted into the culture medium, virtually all of the OPN and most of the BSP were extracted from the mineralized tissue matrix with EDTA. Some OPN and BSP were present in the medium, especially early in the culture, and a significant amount of BSP was also found associated with the collagenous tissue matrix. These studies point to the importance of Col I, ALP, OPN, and BSP, but not ON or OC, in the initial formation of bone tissue.

2,3,7,8-Tetrachlorodibenzo-p-dioxin inhibits differentiation of normal diploid rat osteoblasts in vitro

The influence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent halogenated aromatic hydrocarbon, on the development of bone tissue-like organization in primary cultures of normal diploid calvarial-derived rat osteoblasts was examined. Initially, when placed in culture, these cells actively proliferate while expressing genes associated with biosynthesis of the bone extracellular matrix. Then, post-proliferatively, genes are expressed that render the osteoblast competent for extracellular matrix mineralization and maintenance of structural as well as functional properties of the mature bone-cell phenotype. Our results indicate that, in the presence of TCDD, proliferation of osteoblasts was not inhibited but post-confluent formation of multicellular nodules that develop bone tissue-like organization was dramatically suppressed. Consistent with TCDD-mediated abrogation of bone nodule formation, expression of alkaline phosphatase and osteocalcin was not upregulated post-proliferatively. These findings are discussed within the context of TCDD effects on estrogens and vitamin D-responsive developmental gene expression during osteoblast differentiation and, from a broader biological perspective, on steroid hormone control of differentiation.

Interfacial behavior of PEO/PBT copolymers (Polyactive) in a calvarial system: an in vitro study

Polyactive, a polyethylene oxide/polybutylene terephthalate (PEO/PBT) copolymer, has been reported to display bone-bonding behavior. Although a detailed description of the in vivo bone/Polyactive interface is available, the underlying bone-bonding mechanism is still largely unknown. In this in vitro study, a calvarial envelope method has been adopted to reproduce the in vivo bone-bonding phenomenon and subsequently to obtain information on the biological effect of varying PEO/PBT segment ratios. The following PEO/PBT ratios were examined: 70/30, 60/40, 55/45, 40/60, and 30/70. Light microscopy (LM) and scanning (SEM), transmission (TEM), and backscatter electron microscopy (BSE), as well as X-ray microanalysis (XRMA), were employed. Within the period of analysis (3 weeks), an intimate contact between mineralized deposition and the 70/30, 60/40, and, to a lesser extent, the 55/45 surface was observed. Calcified areas developed within the surface of these PEO/BPT proportions during the culture period. Needle-shaped crystals from the mineralized tissue compartment and from calcified areas within the materials surface were intermingled at the interface, providing a morphologic continuity. A cellular layer was interposed with the mineralization front and the noncalcified 40/60 and 30/70 substrates. Apparently, the percentage of PEO is important for calcification within the near surface of the polymer. This relation is such that the higher the PEO content in PEO/PBT ratios, the more rapid the calcification. The occurrence of material calcification is considered to be largely responsible for the subsequent interfacial interactions. The calvarial envelope culture method allows not only reproduction of the in vivo bone/Polyactive interface, but also a relatively rapid differentiation within the range of PEO/PBT ratios.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytokines and growth factors in the regulation of bone remodeling

Osteoporosis and periodontal disease both represent examples of abnormal bone remodeling. As knowledge of the cellular and molecular events in the normal bone remodeling process has accumulated in the last decade, better understanding of the pathophysiology of bone loss associated with periodontal disease and with aging has occurred. This short review does not attempt to include all aspects of this topic but covers specific areas in which there have been recent advances. (1) Observations made in the last few years have indicated that a hierarchy of both receptor and nonreceptor tyrosine kinases may be involved in normal osteoclastic bone resorption and that certain members of these tyrosine kinase families may mediate cytokine effects. Studies in the op/op variant of murine osteopetrosis have shown that normal production of monocyte-macrophage colony-stimulating factor 1 (M-CSF, also called CSF-1) and activation of its receptor (the receptor tyrosine kinase c-fms) are required for normal osteoclast formation. (2) Studies in mice made deficient in nonreceptor tyrosine kinase by gene knockout have shown that expression of this nonreceptor tyrosine kinase is required for normal osteoclast action and ruffled border formation, although not for osteoclast formation. (3) Recent studies have shown that in addition to prostaglandins of the E series, other arachidonic acid metabolites may be involved in normal and abnormal osteoclastic bone resorption. 5-Lipoxygenase metabolites, the leukotrienes, stimulate isolated osteoclasts to form resorption pits as well as cause osteoclastic bone resorption in organ cultures of neonatal mouse calvariae. These compounds, which are unstable in tissue culture media, are readily inhibitable by agents that inhibit 5-lipoxygenase enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Human osteogenic protein-1 induces both chondroblastic and osteoblastic differentiation of osteoprogenitor cells derived from newborn rat calvaria

Osteogenetic protein-1 (OP-1), a member of the TGF-beta superfamily, induces endochondrial bone formation at subcutaneous sites in vivo and stimulates osteoblastic phenotypic expression in vitro. Primary cultures of newborn rat calvarial cells contain a spectrum of osteogenic phenotypes ranging from undifferentiated mesenchymal osteoprogenitor cells to parathyroid hormone (PTH)-responsive osteoblasts. We examined whether treatment of this cell population with recombinant human osteogenic protein-1 could induce chondrogenesis in vitro. Markers of chondroblastic versus osteoblastic differentiation included alcian blue staining at pH 1, alkaline phosphatase-specific activity, osteocalcin radioimmunoassay, and expression of collagen mRNAs. 6 d of treatment (culture days 1-7) with 4-100 ng OP-1/ml caused dose-dependent increases in alcian blue staining intensity and alkaline phosphatase activity (4.7- and 3.4-fold, respectively, at 40 ng/ml), while osteocalcin production decreased twofold. Clusters of round, refractile, alcian blue-stained cells appeared by day 3, increased in number until day 7, and then became hypertrophic and gradually became less distinct. Histochemically, the day 7 clusters were associated with high alkaline phosphatase activity and became mineralized. mRNA transcripts for collagen types II and IX were increased by OP-1, peaking at day 4, while type X collagen mRNA was detectable only on day 7 in OP-1-treated cultures. Delay of OP-1 exposure until confluence (day 7) amplifies expression of the normal osteoblastic phenotype and accelerates its developmental maturation. In contrast, early OP-1 treatment commencing on day 1 strongly amplifies chondroblastic differentiation. In the same protocol, TGF-beta 1 alone at 0.01-40 ng/ml fails to induce any hypertrophic chondrocytes, and in combination with OP-1, TGF-beta 1 blocks OP-1-dependent chondroinduction. OP-1 is believed to act on a subpopulation of primitive osteoprogenitor cells to induce endochondrial ossification, but does not appear to reverse committed osteoblasts to the chondrocyte phenotype.

Differential effects of fluoride during initiation and progression of mineralization of osteoid nodules formed in vitro

Osteoid nodules form in cultures of fetal rat calvarial (RC) cells grown in medium containing 10% FBS and 50 micrograms/ml of ascorbic acid. When 10 mM beta-glycerophosphate (beta-GP) is added, osteoid nodules mineralize in two phases: an initiation phase, which is dependent upon alkaline phosphatase activity for conversion of beta-GP to P(i), and a progression phase that proceeds independently of alkaline phosphatase activity and does not require exogenous phosphate. We have now used this system to investigate the effects of fluoride (F-) on mineralization. In cultures in which osteoid was formed and mineralization initiated in the presence of F-, a dose-dependent inhibition of the initiation of mineralization occurred over a concentration range of 25-500 microM F- (p < 0.001 in all cases). The initiation of mineralization was not inhibited if F- was removed from the cultures at the time when mineralization was initiated with beta-GP. In osteoid nodules grown in the absence of F-, addition of F- resulted in a dose-dependent inhibition of the initiation of mineralization, with significant decreases in 45Ca uptake occurring at F- concentrations of 3 microM (p < 0.01) and higher. However, if F- was added to cultures after mineralization was initiated in the absence of F-, a stimulation of 45Ca uptake was observed at F- concentrations of 250 microM and above (p < 0.001). F- (1-1000 microM) did not affect the conversion of beta-GP to P(i) or alkaline phosphatase activity in the cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

Interleukin-11 inhibits bone formation in vitro

The effects of interleukin-11(IL-11) on the differentiation of osteoblast precursors was tested using a bone nodule forming assay in rat calvaria cell cultures. IL-11 caused a dose dependent inhibition of nodule formation, with 500 U/ml IL-11 resulting in complete inhibition of nodule formation. IL-11 also caused a reduction in alkaline phosphatase expression in these cultures. These effects are similar to, but more potent than, the actions of IL-6 on these cells. These results indicate that IL-11 is an osteotropic cytokine and suggest that IL-11 may be an important inhibitor of bone formation in health and disease.

Preliminary characterization of porcine bone marrow stromal cells: skeletogenic potential, colony-forming activity, and response to dexamethasone, transforming growth factor beta, and basic fibroblast growth factor

Neonatal pig bone marrow stromal cells (PBMSC) were tested in vivo and in vitro to establish their use as a large-animal model for the study of skeletogenesis. When implanted in diffusion chambers in athymic mice for 6-8 weeks, both freshly isolated pig bone marrow and passage 2 PBMSC formed partially mineralized cartilage, bone-like material, and fibrous tissue. The cartilage showed metachromatic, perilacunar staining with toluidine blue and safronin O, alcian blue staining for chondroitin and keratan sulfate, and intense immunostaining for type II collagen. Osteocalcin was immunolocalized to the mineralized regions, consistent with the formation of bone. Alkaline phosphatase was primarily observed in cell layers at boundaries between tissue types. Unstimulated monolayer cultures of PBMSC produced type I but not type II collagen, responded to dexamethasone (10(-8) M) with a 1.7-fold increase in alkaline phosphatase activity, and were stimulated to divide by basic fibroblast growth factor (1.5-fold; EC50 1 ng/ml). Transforming growth factor beta (TGF-beta) blocked both dexamethasone-induced alkaline phosphatase expression (EC50, 1 ng/ml of TGF-beta) and the mitogenic effects of bFGF (EC50 0.06 ng/ml of TGF-beta). When incubated for 10-14 days in medium containing dexamethasone, beta-glycerophosphate and ascorbate PBMSC formed mineralized nodules. Calcification occurred in the middle of the aggregates and was associated with intensely alkaline phosphatase positive cells and a dense type I collagen-rich matrix. PBMSC also displayed colony-forming unit-fibroblastic activity, with approximately 1 in 80 of the plated cells formed colonies > 128 cells over 14-21 days. PBMSC therefore mimic the known activities of stromal cells from other species, including the human, suggesting that they are a valid model for skeletal research.

Commitment of the teratocarcinoma-derived mesodermal clone C1 towards terminal osteogenic differentiation

The mesodermal clone C1 was derived from the multipotent embryonal carcinoma 1003 cell line transformed with the plasmid pK4 carrying SV40 oncogenes under the control of the adenovirus E1A promoter. We have shown that the C1 clone becomes committed to the osteogenic pathway when cultured in aggregates in the presence of mediators of the osteogenic differentiation. To further validate C1 as a model with which to study osteogenesis in vitro the kinetics of its differentiation was studied, focusing on the histology of the aggregates and on the expression of a set of genes corresponding to representative bone matrix proteins. The presence of ascorbic acid and beta- glycerophosphate specifically leads to mineralization in almost 100% of the aggregates. Transcription of the above genes, silent in exponentially growing cells, specifically occurred with the establishment of cell-cell contacts independently of the presence of ascorbic acid and inorganic phosphate. The latter, however, were absolutely required for matrix deposition and mineralization. In their presence, one observed an overall decline in type I collagen and alkaline phosphatase transcripts while osteocalcin and osteopontin transcripts preferentially accumulated in cells lining the mineralizing foci. Concomitantly, type I collagen and osteocalcin became extracellularly deposited. The osteogenic differentiation of C1 occurred while cells were still proliferating. The C1 clone thus behaves as a mesodermal stem cell, becoming committed to the osteogenic pathway upon: firstly, establishment of cellular contacts; and secondly, addition of ascorbate and beta-glycerophosphate. It therefore appears to be a promising in vitro system for deciphering the molecular basis of osteoblast ontogeny.(ABSTRACT TRUNCATED AT 250 WORDS)

Transgenic expression of COL1A1-chloramphenicol acetyltransferase fusion genes in bone: differential utilization of promoter elements in vivo and in cultured cells

To directly compare the patterns of collagen promoter expression in cells and tissues, the activity of COL1A1 fusion genes in calvariae of neonatal transgenic mice and in primary bone cell cultures derived by sequential digestion of transgenic calvariae was measured. ColCAT3.6 contains 3.6 kb (positions -3521 to +115) of the rat COL1A1 gene ligated to the chloramphenicol acetyltransferase (CAT) reporter gene. ColCAT2.3 and ColCAT1.7 are 5' deletion mutants which contain 2,296 and 1,672 bp, respectively, of COL1A1 DNA upstream from the transcription start site. ColCAT3.6 activity was 4- to 6-fold lower in primary bone cell cultures than in intact calvariae, while ColCAT2.3 activity was at least 100-fold lower in primary bone cells than in calvariae. These changes were accompanied by a threefold decrease in collagen synthesis and COL1A1 mRNA levels in primary bone cells compared with collagen synthesis and COL1A1 mRNA levels in freshly isolated calvariae. ColCAT3.6 and ColCAT2.3 activity was maintained in calvariae cultured in the presence or absence of serum for 4 to 7 days. Thus, when bone cells are removed from their normal microenvironment, there is parallel downregulation of collagen synthesis, collagen mRNA levels, and ColCAT3.6 activity, with a much greater decrease in ColCAT2.3. These data suggest that a 624-bp region of the COL1A1 promoter between positions -2296 and -1672 is active in intact and cultured bone but inactive in cultured cells derived from the bone. We suggest that the downregulation of COL1A1 activity in primary bone cells may be due to the loss of cell shape or to alterations in cell-cell and/or cell-matrix interactions that normally occur in intact bone.

Expression of bone protein mRNA at physiological fluoride concentrations in rat osteoblast culture

Fluoride causes an increase in the amount of unmineralized osteoid. To determine whether the increase in osteoid is due to greater protein expression in the presence of fluoride, we measured the relative amount of mRNA expressed by fetal rat calvaria cells maintained in culture for either 18 or 26 days in the presence of 0, 5, 20 or 300 microM fluoride. There were no differences in the level of expression of mRNA for collagenous or non-collagenous proteins in fluoride-treated cells as compared with control cells at 18 days in culture. Expression of mRNA for osteocalcin and alpha 1-type 1 collagen was decreased at 300 microM fluoride after 26 days culture. The amount of [3H]thymidine incorporation in cells exposed to the different amounts of fluoride was measured at various time points. Fluoride did not alter the time at which rapid cell proliferation ended. These studies indicate that at physiological serum levels, fluoride does not increase expression of mRNA by osteoblasts. The relative increase in osteoid in bone may be related to other mechanisms such as altered matrix mineralization.

Transgenic expression of COL1A1-chloramphenicol acetyltransferase fusion genes in bone: differential utilization of promoter elements in vivo and in cultured cells

To directly compare the patterns of collagen promoter expression in cells and tissues, the activity of COL1A1 fusion genes in calvariae of neonatal transgenic mice and in primary bone cell cultures derived by sequential digestion of transgenic calvariae was measured. ColCAT3.6 contains 3.6 kb (positions -3521 to +115) of the rat COL1A1 gene ligated to the chloramphenicol acetyltransferase (CAT) reporter gene. ColCAT2.3 and ColCAT1.7 are 5' deletion mutants which contain 2,296 and 1,672 bp, respectively, of COL1A1 DNA upstream from the transcription start site. ColCAT3.6 activity was 4- to 6-fold lower in primary bone cell cultures than in intact calvariae, while ColCAT2.3 activity was at least 100-fold lower in primary bone cells than in calvariae. These changes were accompanied by a threefold decrease in collagen synthesis and COL1A1 mRNA levels in primary bone cells compared with collagen synthesis and COL1A1 mRNA levels in freshly isolated calvariae. ColCAT3.6 and ColCAT2.3 activity was maintained in calvariae cultured in the presence or absence of serum for 4 to 7 days. Thus, when bone cells are removed from their normal microenvironment, there is parallel downregulation of collagen synthesis, collagen mRNA levels, and ColCAT3.6 activity, with a much greater decrease in ColCAT2.3. These data suggest that a 624-bp region of the COL1A1 promoter between positions -2296 and -1672 is active in intact and cultured bone but inactive in cultured cells derived from the bone. We suggest that the downregulation of COL1A1 activity in primary bone cells may be due to the loss of cell shape or to alterations in cell-cell and/or cell-matrix interactions that normally occur in intact bone.

Osteoblasts are target cells for transformation in c-fos transgenic mice

We have generated transgenic mice expressing the proto-oncogene c-fos from an H-2Kb class I MHC promoter as a tool to identify and isolate cell populations which are sensitive to altered levels of Fos protein. All homozygous H2-c-fosLTR mice develop osteosarcomas with a short latency period. This phenotype is specific for c-fos as transgenic mice expressing the fos- and jun-related genes, fosB and c-jun, from the same regulatory elements do not develop any pathology despite high expression in bone tissues. The c-fos transgene is not expressed during embryogenesis but is expressed after birth in bone tissues before the onset of tumor formation, specifically in putative preosteoblasts, bone-forming osteoblasts, osteocytes, as well as in osteoblastic cells present within the tumors. Primary and clonal cell lines established from c-fos-induced tumors expressed high levels of exogenous c-fos as well as the bone cell marker genes, type I collagen, alkaline phosphatase, and osteopontin/2ar. In contrast, osteocalcin/BGP expression was either low or absent. All cell lines were tumorigenic in vivo, some of which gave rise to osteosarcomas, expressing exogenous c-fos mRNA, and Fos protein in osteoblastic cells. Detailed analysis of one osteogenic cell line, P1, and several P1-derived clonal cell lines indicated that bone-forming osteoblastic cells were transformed by Fos. The regulation of osteocalcin/BGP and alkaline phosphatase gene expression by 1,25-dihydroxyvitamin D3 was abrogated in P1-derived clonal cells, whereas glucocorticoid responsiveness was unaltered. These results suggest that high levels of Fos perturb the normal growth control of osteoblastic cells and exert specific effects on the expression of the osteoblast phenotype.

Kinetics of in vitro mineralization by an osteogenic clonal cell line (C1) derived from mouse teratocarcinoma

We have previously reported the isolation of an osteogenic clonal cell line (C1) derived from mouse teratocarcinoma and immortalized by the SV 40 oncogenes. In this report we describe the kinetics of osteogenic differentiation of aggregated C1 cells by following the matrix deposition and mineralization and the expression of alkaline phosphatase. We show that after addition of beta-glycerophosphate and ascorbic acid, more than 95% of C1 aggregates synthesize a bone matrix which is deposited as early as 2 days and increases progressively with time in culture. Matrix calcification is evidenced by von Kossa staining and tetracycline incorporation into the mineral whereas no calcification appears in control cultures. Calcium is detectable in mineralizing aggregates at 2 days and calcium content increases linearly with time in culture, being 125-fold higher in mineralizing nodules than in control aggregates at 30 days. Aggregated C1 cells are characterized by a high activity of the bone type isoenzyme of alkaline phosphatase, a marker of osteoblast phenotype. Upon addition of inducers, alkaline phosphatase activity decreases by five-fold after the onset of mineralization and remains stable thereafter. The down-regulation of alkaline phosphatase activity is confirmed at the cellular level by histochemical staining. The mRNA levels for alkaline phosphatase decline during osteogenesis, following a pattern similar to the decrease in protein activity. Analysis of DNA synthesis by (3H)-thymidine incorporation and quantification of labelled nuclei on autoradiographs shows that C1 cells proliferation is not down-regulated during the time course of differentiation and that proliferating C1 cells still express alkaline phosphatase activity during osteogenic differentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Differentiation and mineralization in osteogenic precursor cells derived from fetal rat mandibular bone

The process of mineralization in cells prepared either by neutral protease digestion (Pro I) or by collagenase digestion (fifth cycle, Col V) from fetal rat mandible was studied in vitro. Alkaline phosphatase (ALPase) activity of cells in Pro I was low on day 3, increased rapidly from day 8, and reached a maximum on day 16, whereas that in Col V was high on day 2, then declined and thereafter elevated to reach a maximum on day 13. Both cell populations synthesized type I collagen in cell matrix and medium. Type III collagen was observed in cell matrix of Pro I on day 14 and 21. There was alpha 2 band of type V collagen in cell matrix of Pro I on day 21. Calcium deposition could be detected from day 14 in Pro I and from day 19 in Col V. The von Kossa-positive nodules were found on day 17 in Pro I and day 21 in Col V, respectively. The extracellular matrix in Pro I electron-microscopically consisted of well-banded collagen fibrils with a large number of calcified spherules. An elevation of ALPase activity, collagen synthesis, and mineral deposition occurred sequentially with a time lapse in Col V, and almost simultaneously in Pro I. The number of mineralized nodules was correlated with the density of plated cells in Pro I, but not in Col V. Dexamethasone caused an increase in the number of mineralized nodules in Pro I, but not in Col V, suggesting that Pro I contained osteoprogenitor cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Interleukin-6 inhibits bone formation in vitro

Interleukin-6 (IL-6) is a pluripotent cytokine which is made by osteoblasts, but its role in bone metabolism is uncertain. The aim of this study was to test the effect of IL-6 on bone formation in vitro using a nodule-forming assay. Osteoblast-enriched calvaria cells were isolated from 2-day-old Sprague-Dawley rats and cultured in the presence of 10(-8) M dexamethasone. After 2 days, calvaria cells were treated with recombinant human IL-6 for 72 h, washed and maintained for a further 18 days before fixation. IL-6 caused a dose-dependent inhibition of bone nodule formation, with a maximum reduction of 53% with 5000 U/ml IL-6. IL-6 also inhibited alkaline phosphatase activity in a dose-dependent manner (e.g. control: 114 +/- 9.2; IL-6: 68 +/- 10.6 nmol p-nitrophenol (pNP)/mg/min). IL-6 did not affect cell numbers during early cell growth up to 6 days but caused a small but significant reduction in cell number at confluence (8 days). These results demonstrate that IL-6 inhibits bone nodule formation by rat calvaria cells in vitro and suggest that IL-6 may inhibit osteoblast differentiation.

Constitutive transcription of the osteocalcin gene in osteosarcoma cells is reflected by altered protein-DNA interactions at promoter regulatory elements

The bone-specific osteocalcin (OC) gene is transcribed only after completion of proliferation in normal diploid calvarial-derived osteoblasts during extracellular matrix mineralization. In contrast, the OC gene is expressed constitutively in both proliferating and nonproliferating ROS 17/2.8 osteosarcoma cells. To address molecular mechanisms associated with these tumor-related modifications in transcriptional control, we examined sequence-specific interactions of transactivation factors at key basal and hormone-responsive elements in the OC gene promoter. In ROS 17/2.8 cells compared to normal diploid osteoblasts, the absence of a stringent requirement for cessation of proliferation to support both induction of OC transcription and steroid hormone-mediated transcriptional modulation is reflected by modifications in transcription factor binding at (i) the two primary basal regulatory elements, the OC box (which contains a CCAAT motif as a central core) and the TATA/glucocorticoid-responsive element domain, and (ii) the vitamin D-responsive element. Particularly striking are two forms of the vitamin D receptor complex that are present in proliferating osteoblasts and osteosarcoma cells. Both forms of the complex are sensitive to vitamin D receptor antibody and retinoic X receptor antibody. After the down-regulation of proliferation, only the lower molecular weight complex is found in normal diploid osteoblasts. Both forms of the complex are present in nonproliferating ROS 17/2.8 cells with increased representation of the complex exhibiting reduced electrophoretic mobility that is phosphorylation-dependent.

Bone-like nodules formed in vitro by rat periodontal ligament cells

The periodontal ligament has been shown to possess the ability to regenerate both new cementum and alveolar bone as well as a self-regenerative capacity; however, the source of cementoblasts and osteoblasts is not still clear. We investigated the development of bone-like tissue in vitro by periodontal ligament cells, in order to determine whether the periodontal ligament contains osteoprogenitor cells. Periodontal ligament cells were obtained from periodontal ligament tissue attached to the maxillary incisors of 6-week-old WKA rats by means of the explant technique. Cells at passage #3 were cultured for long term in alpha-minimum essential medium containing 10% fetal bovine serum, antibiotics, and 50 micrograms/ml ascorbic acid, and were then examined using phase-contrast microscopy, histochemistry, transmission electron microscopy, X-ray microanalysis, and electron diffraction. Nodules were formed in the cultures, and when 10 mM Na-beta-glycerophosphate was added, these nodules became mineralized. The mineralized nodules were identified as bone-like elements in view of the presence of osteoblast-like and osteocyte-like cells, collagenous matrix, a mineral composed of hydroxyapatite, and intense alkaline phosphatase activity. The results show that the periodontal ligament contains osteoprogenitor cells, which differentiate into osteoblasts and produce bone-like tissue.

Pattern of gene expression following rat tibial marrow ablation

Following injury to bone marrow there is a phase of osteogenesis in which bone trabeculae replace the initial blood clot and fill the marrow cavity. The newly formed bone is subsequently fully resorbed by osteoclasts and normal bone marrow is restored. In this study we correlated the morphologic events with the pattern of gene expression that defines this sequence. Following marrow ablation, the trabecular bone volume in the affected section of the marrow cavity increased from control to 27% at day 6, declined to 18% at day 8, and eventually returned to control levels at day 14. Osteoblast number increased up to day 6 and declined substantially by day 8, but the number of osteoclasts peaked between days 8 and 10. Histologic analysis of alkaline phosphatase (AP) and tartrate-resistant acid phosphatase (TRAP) activity correlated with the observed cellular changes. Northern blot analysis of the levels of AP, osteocalcin (OC), and osteopontin (OP) mRNA shows a specific pattern of regulated gene expression, with AP mRNA maximal at day 6, OC mRNA very low until days 6-8, and OP mRNA expressed at very high levels throughout. In addition, procollagen alpha 1(I) and alpha 1(III) mRNAs show a regulated pattern of expression, with procollagen alpha 1(I) maximally expressed between days 4 and 10 and procollagen alpha 1(III) expressed at lower levels between days 4 and 6. The mRNA encoding insulin-like growth factor I (IGF-I) was found to be highly expressed between days 5 and 12; however, transforming growth factor beta 1 (TGF-beta 1) and TGF-beta 3 mRNA were only weakly expressed between days 4 and 10. These data demonstrate a temporal pattern of gene expression consistent with the observed morphologic profile, identify changes in growth factor mRNA that may be related to this repair process, and suggest that this is a suitable model for studying in vivo a synchronized sequence of bone formation and resorption at a well-defined anatomic site.

Postproliferative transcription of the rat osteocalcin gene is reflected by vitamin D-responsive developmental modifications in protein-DNA interactions at basal and enhancer promoter elements

In the osteocalcin (OC) gene promoter, both independent positive and negative regulatory elements, as well as others with contiguous [TATA/glucocorticoid-responsive elements (GRE)] or overlapping [TATA/GRE, vitamin D-responsive enhancer elements (VDRE)/AP-1, and OC box/AP-1] domains, are sites for modifications in protein-DNA interactions. In the present studies, we have examined nuclear protein extracts from fetal rat calvarial cells that undergo a developmental sequence of bone cell differentiation. Our results demonstrate modifications in protein-DNA interactions that relate to the developmental stages of the osteoblast and support developmental regulation of OC gene transcription. Basal expression of the OC gene is associated with sequence-specific protein-DNA interactions at the OC box, VDRE, and TATA/GRE box. Distinct differences are observed in proliferating osteoblasts, where the OC gene is not transcribed compared to postproliferative, differentiated osteoblasts that transcribe the OC gene. Furthermore, the protein-DNA complexes that reflect hormonal control are also developmentally regulated, mediating both the transcriptionally active and repressed states of the OC gene. For example, in proliferating osteoblasts, a vitamin D receptor-antibody-sensitive complex is formed that is different from the DNA binding complex induced by vitamin D postproliferatively when the OC gene is transcribed. Mutational analysis of the steroid hormone binding domain and the overlapping AP-1 site at the VDRE supports mutually exclusive occupancy by Fos-Jun heterodimers and vitamin D receptor. Such protein-DNA interactions at the VDRE are consistent with repression of competency for vitamin D-mediated transcriptional enhancement in proliferating osteoblasts expressing high levels of Fos and Jun.

Formation of mineralized nodules by bone derived cells in vitro: a model of bone formation?

The identification of the factors which regulate the proliferation and differentiation of cells of the osteoblast lineage remains one of the major challenges in the field of bone cell biology. Although considerable progress has been made in the isolation and culture of cells of the osteoblast lineage from both animal and, more recently, human bone, uncertainties have persisted as to the extent to which these cell populations retain the ability to differentiate into functional osteoblasts in vitro. The formation in vitro of mineralized nodules that exhibit the morphological, ultrastructural and biochemical characteristics of embryonic/woven bone formed in vivo, represents the first evidence that the differentiation of functional osteoblasts can occur in cultures of isolated animal bone-derived cell populations. It is clear, however, that the culture conditions employed at present only permit a small number of cells to differentiate to the extent of being capable of organising their extracellular matrix into a structure that resembles that of bone. Moreover, it has generally been found that the reproducible mineralization of this extracellular matrix requires supplementation of the culture medium with mM concentrations of beta-GP, which raises doubts as to the physiological relevance of this process. The formation of nodules has also been observed in cultures of human bone-derived cells. As found in cultures of animal bone-derived cells, reproducible mineralization of these nodules will occur in the presence of beta-GP. We have shown, however, that in the presence of the long acting ascorbate analogue Asc-2-P, the formation and mineralization of nodules can occur in the absence of beta-GP. The nodules formed in human bone-derived cell cultures have yet to be characterized as rigorously as those formed in cultures of animal bone-derived cells and thus it remains to be shown that they resemble bone formed in vivo.

Two cell lines from bone marrow that differ in terms of collagen synthesis, osteogenic characteristics, and matrix mineralization

Two cloned cell lines were isolated from cultures of mouse bone-marrow cells. One of the lines, D1, exhibited osteogenic properties and synthesized type-I collagen (alpha 1)2 alpha 2. The second cell line, D2, was not osteogenic and produced a collagen homotrimer (alpha 1)3. Whereas the extracellular matrix of the D1 cell cultures contained striated collagen fibrils, presumably composed of type-I collagen, the homotrimer-producing D2 cells did not demonstrate striated collagen fibrils. Instead, they had thin filaments without detectable striations. Sodium ascorbate stimulated collagen synthesis at the transcriptional level in both the D1 and the D2 cells. The bone-producing characteristics of D1 in vitro included high levels of alkaline phosphatase, increased cyclic adenosine monophosphate on treatment with parathyroid hormone, and expression of osteocalcin mRNA. The D1 cells, unlike the D2 cells, produced a mineralized matrix in vitro. Mineralization in the cultures of the D1 cells occurred in nodules of increased cell density, which also contained the cells with the highest concentrations of collagen mRNA, as shown by in situ hybridization. When the D1 cells were implanted in a diffusion chamber in vivo, a mixture of both osteogenic and adipogenic tissues was formed. This indicates that the D1 cell line is derived from an early marrow stromal precursor that is multipotential.