In vitro evidence that local and systemic skeletal effectors can regulate 3[H]-thymidine incorporation in chick calvarial cell cultures and modulate the stimulatory actions(s) of embryonic chick bone extract

Effects of doxorubicin on human dental pulp cells in vitro

There is substantial information concerning the effects of continuous exposure to supratherapeutic or therapeutic concentrations of doxorubicin on human molar pulpal cells; the effects of continuous exposure to subtherapeutic concentrations of this agent are undetermined. To this end, we studied the proliferation of human fibroblasts and pulpal cells and their pattern of mineralized nodule deposition in vitro. Cell proliferation was assessed at 1, 3, 5, and 7 days from populations with either no exposure (control) or exposure to 10(-6)-10(-9) mol/L doxorubicin. Mineralized nodule deposition and calcium-45 incorporation were assessed at 7 and 21 days of culture. Data were compared by factorial ANOVA and a post-hoc Tukey test. 10(-6) and 10(-7) mol/L doxorubicin significantly reduced the total number of viable pulpal cells in cultures from days 1 to 3 (p < 0.05); doxorubicin 10(-6)-10(-9) mol/L significantly inhibited cell proliferation (p < 0.05) and DNA synthesis 5 days after plating (p < 0.001). After 21 days, doxorubicin 10(-6)-10(-8) mol/L significantly decreased calcium-45 incorporation into pulpal cultures (p < 0.001); all dilutions significantly reduced the number of mineralized nodules within the 21-day pulpal cultures (p < 0.05). In addition, all dilutions of doxorubicin significantly inhibited fibroblast cell proliferation and incorporation of [(3)H]thymidine. In contrast, the fibroblast cultures did not produce mineralized nodules, suggesting that the mineralized nodules within the pulpal cell cultures did not result from dystrophic calcification. Thus, exposure to subtheraputic doxorubicin concentrations has potential adverse effects on mineralized tissue formation within the pulp, which could affect the rates of reparative dentin deposition within the tooth pulps of patients receiving this chemotherapeutic agent.

Apoptosis may determine the release of skeletal alkaline phosphatase activity from human osteoblast-line cells

Although quantitative measurement of skeletal alkaline phosphatase (sALP) activity in serum can provide an index of the rate of bone formation, the metabolic process that determines the release of sALP - from the surface of osteoblasts, into circulation-is unknown. The current studies were intended to examine the hypothesis that the release of sALP from human osteoblasts is a consequence of apoptotic cell death. We measured the release of sALP activity from human osteosarcoma (SaOS-2) cells and normal human bone cells, under basal conditions and in response to agents that increased apoptosis (TNF-a, okadiac acid) and agents that inhibit apoptosis (IGF-I, calpain, and caspase inhibitors). Apoptosis was determined by the presence of nucleosomes (histone-associated DNA) in the cytoplasm of the cells by using a commercial kit. The results of these studies showed that TNF-a and okadiac acid caused dose- and time-dependent increases in apoptosis in the SaOS-2 cells (r = 0.78 for doses of TNF-a and r = 0.93 for doses of okadiac acid, P <0.005 for each), with associated decreases in cell layer protein (P <0.05 for each) and concomitant increases in the release of sALP activity (e.g., r = 0.89 for TNF-a and r = 0.75 for okadiac acid, P <0.001 for each). In contrast, caspase and calpain inhibitors reduced apoptosis, increased cell layer protein, and decreased the release of sALP activity (P <0.05 for each). Exposure to IGF-I also decreased apoptosis, in a time- and dose-dependent manner (e.g., r = 0.93, P <0.001 for IGF-I doses), with associated proportional effects to increase cell layer protein (P <0.001) and decrease the release of sALP activity (P <0.001). IGF-I also inhibited the actions of TNF-a and okadiac acid to increase apoptosis and sALP release. The associations between apoptosis and sALP release were not unique to osteosarcoma (i.e., SaOS-2) cells, but also seen with osteoblast-line cells derived from normal human bone. Together, these data demonstrate that the release of sALP activity from human osteoblast-line cells in vitro is associated with, and may be a consequence of, apoptotic cell death. These findings are consistent with the general hypothesis that the appearance of sALP activity in serum may reflect the turnover of osteoblast-line cells.

Effects of sodium acetate on rat bone-nodule formation and mineralization in vitro

Sodium acetate reportedly promotes bone atrophy by inducing resorption and inhibiting osteoprogenitor-cell proliferation, but little is known about its effects on bone-matrix deposition and mineralization by a population containing osteoprogenitor cells. The objective here was to assess the effects of 1-20 mM sodium acetate on the proliferation and differentiation of these cells and their resultant bone-nodule formation and mineralization in an in vitro assay. Exposure to 10 mM sodium acetate had no effect on cellular proliferation but significantly increased the production and mineralization of bone nodules (p < 0.01), suggesting that it affected osteoprogenitor differentiation and subsequent metabolism. However, 10 mM acetate did not increase net bone mass. Dilutions of 1-5 and 20 mM inhibited cellular proliferation and resultant bone-nodule formation and mineralization, significantly reducing the percentage bone area as compared to controls (p < 0.001). These data suggest that 1-5 and 20 mM sodium acetate significantly inhibit bone deposition, whereas 10 mM has no effects, which could contribute to iatrogenic metabolic bone disease in patients receiving either renal dialysis or total parenteral nutrition.

The mitogenic effect of parathyroid hormone is associated with E2F-dependent activation of cyclin-dependent kinase 1 (cdc2) in osteoblast precursors

Injections of parathyroid hormone (PTH) have been reported to stimulate skeletal accretion through increased bone formation in several species, and osteoblast proliferation is a critical component of bone formation. However, the biological mechanisms of PTH-stimulated bone cell proliferation are largely unknown. In this study, we demonstrated that PTH stimulates proliferation of the osteoblast precursor cell line, TE-85, in association with increasing cdc2 protein levels and its kinase activity. cdc2 antisense oligonucleotides blocked PTH-induced DNA synthesis and cell cycle progression. Analysis of the time course of PTH-stimulated cdc2 message levels demonstrated that cdc2 mRNA levels were increased 1.5- to 4-fold between 3-18 h following release from cell synchronization. Transfections of TE-85 cells with a series of cdc2 promoter-luciferase deletion constructs revealed PTH stimulation of the cdc2 promoter. Promoter constructs containing a mutation in the E2F binding site were not stimulated by PTH. Gel mobility shift assays demonstrated increased free E2F levels in TE-85 nuclear extracts in response to PTH. Furthermore, the ratios of hyperphosphorylated to hypophosphorylated forms of Rb protein were increased by PTH treatment. These results demonstrate that PTH-stimulated cdc2 expression was associated with TE-85 cell proliferation and that the mechanism of stimulating cdc2 gene expression involved increasing the levels of free E2F.

Enhancement by sodium orthovanadate of the formation and mineralization of bone nodules by chick osteoblasts in vitro

Orthovanadate is a known inhibitor of phosphotyrosyl protein phosphatase and is reported to stimulate osteogenic cell proliferation and differentiation when administered during the logarithmic growth phase and to potentiate the mitogenic effects of several growth factors. There is little information concerning the effects of orthovanadate on bone matrix deposition and mineralization, although there is some evidence that it increases collagen synthesis by osteogenic cells. To test the effects of orthovanadate on bone nodule formation and mineralization, osteogenic cells were exposed to 5-50 microM orthovanadate or 10(-7) M insulin-like growth factor-1 for 3, 7, and 21 days after plating. Exposure to orthovanadate produced differential effects on cellular proliferation and alkaline phosphatase activity following completion of the logarithmic growth phase, and on resultant bone nodule formation and mineralization by these populations. The effects of orthovanadate on osteogenic cultures were concentration dependent: 5 microM concentrations produced by a relatively large quantity of poorly mineralized matrix, while 30-50 microM concentrations produced a smaller quantity of heavily mineralized matrix. Thus, orthovanadate could possibly be used as a growth factor for bone, if administered at the critical dosage at the proper stage of the life cycle of the osteoblast.

The effects of smokeless tobacco extract on bone nodule formation and mineralization by chick osteoblasts in vitro

Short-term exposure to smokeless tobacco extracts (STE) reportedly inhibits osteoblast metabolism. The objective of this study was to determine the effects of serial dilutions of a water-soluble extract of smokeless tobacco on osteoblast proliferation and their potential to form and mineralize bone nodules. STE significantly stimulated cell proliferation when diluted 10(2)-10(4) times; 10(3) and 10(4) dilutions produced the greatest effect. 10(2)-10(4) STE dilutions significantly increased alkaline phosphatase activity at day 7 but 10(6) STE dilutions significantly decreased it. 10(3) and 10(4) dilutions significantly increased bone nodule formation, but inhibited their mineralization. In contrast, 10(5) and 10(6) dilutions significantly decreased bone nodule formation, but increased their mineralization. Stimulation of in vitro bone nodule formation by STE was similar to that produced by 10(-7) M insulin-like growth factor 1 (IGF-1) in vivo. Heat and acid treatment of STE significantly reduced its beneficial effect on cell proliferation, suggesting that a peptide within STE may be responsible for enhancement of osteogenic cell proliferation. Thus, STE may contain a peptide capable of significantly stimulating osteoblast proliferation, differentiation and metabolism, similar to the effects of IGF-1. This peptide could have potential therapeutic benefits.

Responsiveness of gene expression markers of osteoblastic and osteoclastic activity to calcitonin in the appendicular and axial skeleton of the rat in vivo

We have previously shown that calcitonin (CT), an inhibitor of bone resorption, increases vertebral, but not femoral bone density in the rat. To address the physiologic responses associated with these effects on bone mineral density (BMD), we assessed mRNA transcripts reflecting activities of osteoblasts (type I collagen, osteocalcin, osteopontin, and alkaline phosphatase), osteoclasts [tartrate-resistant acid phosphatase (TRAP)], and cell proliferation (histone H4) in the spine and femur of these rats. CT increased spine BMD while increasing type I collagen and decreasing TRAP and histone mRNAs. In the femur, where CT had no effect on BMD, it decreased type I collagen and histone H4 mRNA but did not affect TRAP. CT had no effect on the gene expression of osteocalcin, osteopontin, or alkaline phosphatase at either site. The results indicate that selective alterations in gene expression, as reflected by steady state mRNA levels, are consistent with the changes observed by BMD measurement, and can more clearly define the specific contribution from osteoblast and osteoclast activity. This study demonstrates a heterogeneity in response of the axial and appendicular skeleton to CT, reflected by alterations in gene expression that provide a basis for understanding the observed BMD responses to various pharmacologic interventions.

Effects of bone-seeking hormones on DNA synthesis, cyclic AMP level, and alkaline phosphatase activity in cultured cells from human posterior longitudinal ligament of the spine

In a study of the osteogenesis capability of the human posterior longitudinal ligament of the spine, ligament cells were isolated and cultured. The effect of bone-seeking hormones, such as parathyroid hormone (PTH), calcitonin (CT), prostaglandin E2 (PGE2), and 1,25-dihydroxycholecalciferol [1,25-(OH)2D3], on the ligament cells was investigated with respect to DNA synthesis, adenosine-3',5'-cyclic monophosphate (cAMP) levels, alkaline phosphatase (ALP) activity, and acid phosphatase (ACP) activity. Cell lines obtained from nonossified sites in patients with ossification of the posterior longitudinal ligament of the spine (OPLL) were found to have several different phenotypic characteristics for osteoblasts: high ALP activity, PTH- and PGE2-stimulated increases in cAMP, and responses to both CT and 1,25-(OH)2D3. It is clear that proliferation and differentiation in such ligament cells are controlled by various types of bone-seeking hormones, and it was suggested that many cells with osteoblast-like characteristics are present. These results are considered important with respect to the etiology of OPLL, and an experimental system using cultured ligament cells appears to be useful in research on OPLL.

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

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

Characterization of cultured cells derived from ossification of the posterior longitudinal ligament of the spine

Ossification of the posterior longitudinal ligament of the spine (OPLL) is a common cause of spinal canal stenosis and myelopathy in Orientals. OPLL is characterized by heterotopic new bone formation in ligamentous tissue. To investigate the pathogenesis of OPLL, human posterior longitudinal ligament cells were cultured and their in vitro morphological and biochemical characteristics were studied. Cell cultures from control subjects with normal spinal ligaments did not show any osteoblastic properties. In contrast, cell lines (OG1-OG5) obtained from an OPLL patient showed several different phenotypic characteristics for osteoblasts. OG1 cells showed typical osteoblast-like phenotypic characteristics (i.e., in vitro calcification, high alkaline phosphatase [ALP] activity, and elevation of cAMP levels by parathyroid hormone [PTH]). All cell lines (OG1-OG5) responded to PTH and PGE2 by markedly increasing cAMP levels, ALP activities varied among the cell lines. The OG1 and OG2 cells exhibited a high level of ALP activity. Compared with cell lines from the non-ossification group, the activities were higher in the OG3 and OG4 cells, but not significantly in the OG5 cells. Only in the OG3 cells, CT caused an increase in cAMP level and ALP activity, and its stimulatory effects demonstrated that CT had a direct, in vitro action on ligament cells of OPLL patients to stimulate osteoblastic differentiation. It is clear that some cells from ligaments with OPLL had several phenotypes characteristic of osteoblasts, but cells from ligaments without ossification did not show any osteoblastic properties. This observation is considered to be an important clue to understanding the pathophysiology of OPLL.

In vitro exposure to sodium fluoride does not modify activity or proliferation of human osteoblastic cells in primary cultures

The anabolic effects of sodium fluoride (NaF) on trabecular bone mass in osteoporosis is now well established. In vivo histologic studies performed in humans and other animals have shown that fluoride induces an increase in osteoblast number at the tissue level. To determine the mechanisms of action of fluoride on osteoblasts, we studied the effects of NaF on short- and long-term cultures of human osteoblastic cells derived from bone explants obtained from 21 donors. In short-term experiments, bone-derived cells were exposed to NaF for 4 days. At doses ranging from 10(-11) to 10(-5) M, NaF did not modify the alkaline phosphatase (AP) activity or osteocalcin secretion. In long-term experiments, half the bone samples from 15 donors were cultured for 4 months in the presence of 10(-5) M NaF and the other half were maintained in NaF-free medium. Observations by light and electron microscopy disclosed no morphologic modification in bone explants after 4 months of exposure to NaF, despite an increase in the bone fluoride content. After the first month of culture, slight but not significant increases were noted in 6 of 10 cases for AP activity, 4 of 10 for osteocalcin secretion, and 5 of 7 for [3H]thymidine incorporation. After 4 months of culture in the presence of NaF, no change in AP activity or cell proliferation was noted. In contrast, the osteocalcin secretion significantly decreased (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Fluoride-stimulated [3H]thymidine uptake in a human osteoblastic osteosarcoma cell line is dependent on transforming growth factor beta

Controversy exists regarding the effect of fluoride on human osteoblast proliferation. To learn more of the cellular action of fluoride, we chose the clonal osteoblast cell line HOS TE85 as a model system. In these phenotypically osteoblast-like cells, sodium fluoride stimulated [3H]thymidine incorporation in a dose-dependent manner over the concentration range 1 x 10(-5)-2 x 10(-4) M. The fluoride-induced stimulation of [3H]thymidine uptake was dependent on cell density, being optimal at subconfluent cell numbers. Stimulation of [3H]thymidine uptake was inhibited by anti-transforming growth factor beta but not by antibody to insulin-like growth factor I or beta 2-microglobulin. Transforming growth factor beta was shown to be a biphasic stimulator of [3H]thymidine uptake in HOS TE85, with maximal stimulation occurring at 0.5 nM transforming growth factor beta. In the presence of fluoride the cells were more sensitive to stimulation by this growth factor, with maximum effect occurring at 0.1 nM. Fluoride did not increase mRNA for transforming growth factor beta following either 8 or 24 h of exposure. We conclude that fluoride activates osteoblast proliferation by modulating the cellular sensitivity to transforming growth factor beta, a known stimulator of bone growth.

PTH stimulates the proliferation of TE-85 human osteosarcoma cells by a mechanism not involving either increased cAMP or increased secretion of IGF-I, IGF-II or TGF beta

Injections of parathyroid hormone (PTH) result in increased bone formation in several species. Work in our laboratory and others has shown a stimulation of bone cell proliferation and growth factor production by PTH. Our purpose was to study the effects of PTH on a human bone cell line using TE-85 human osteosarcoma cells as a model. After 24 h treatment, PTH caused an increase in cell proliferation as measured by cell counts and [3H]-thymidine incorporation. Proliferation was not inhibited by an anti-transforming growth factor beta (TGF beta) antibody which could abolish stimulation by exogenous TGF beta. PTH did not stimulate cAMP production, alkaline phosphatase activity or production of insulin-like growth factors I or II (IGF-I or IGF-II) in TE-85 cells. Although basal TE-85 proliferation was slowed by incubation with the calcium channel blocking agent verapamil, PTH still caused an increase in growth rate. We conclude that PTH directly stimulates TE-85 proliferation via a mechanism not involving increased adenylate cyclase activity or increased secretion of IGF-I, IGF-II or TGF beta and may stimulate bone formation in vivo by activating some other mitogenic signal to increase bone cell proliferation.

Stimulation of bone formation in osteoporosis patients treated with fluoride associated with increased DNA synthesis by osteoblastic cells in vitro

In this study we evaluated whether the fluoride-induced increased bone formation in osteoporosis is mediated by stimulation of bone cell proliferation and/or differentiation. We analyzed the kinetics of DNA synthesis and the phenotypic features of osteoblastic cells isolated from the trabecular bone surface in relationship to histomorphometric indices of bone formation evaluated on the same bone biopsy in 12 osteoporotic patients treated with fluoride. Osteoblastic cells isolated from patients with a higher than normal bone formation rate, increased mean wall thickness of trabecular bone packets, and high trabecular bone volume after fluoride therapy displayed a higher than normal rate of DNA synthesis in vitro. The peak of [3H]thymidine incorporation into DNA, the maximal DNA synthesis, and the area under the growth curve of osteoblastic cells isolated from these patients were higher than the values in normal bone cells obtained from age-matched controls. By contrast, in vitro parameters of osteoblastic cell proliferation were not different from normal in fluoride-treated osteoporosis patients in whom bone formation was not increased, although the duration of treatment and bone fluoride content were not different. Parameters of bone cell proliferation in vitro were increased in correlation with the mean wall thickness, and the latter correlated with the trabecular bone volume, indicating that the augmentation of bone formation and bone volume induced by fluoride was paralleled by an increased proliferation of osteoblastic cells. Basal osteocalcin production (corrected for cell protein) and alkaline phosphatase activity in vitro were comparable, and the response to 1,25-dihydroxyvitamin D3 (10 nmol/liter, 48 h) was not different in normal osteoblastic cells and in cells from fluoride-treated osteoporosis patients whether they had high or normal bone formation. The results show that the fluoride-induced increased bone formation in osteoporotic patients is associated with an increased in vitro proliferative capacity of osteoblastic cells lining the trabecular bone surface, whereas parameters of osteoblast differentiation are not affected. The data also suggest that induction of a higher than normal bone cell proliferation is prerequisite for the stimulation of bone formation by fluoride.

Skeletal alkaline phosphatase specific activity is an index of the osteoblastic phenotype in subpopulations of the human osteosarcoma cell line SaOS-2

During continuous culture with serial passage, the human osteosarcoma cell line SaOS-2 showed a time-dependent decrease in skeletal alkaline phosphatase (ALP) activity. Because this was indicative of heterogeneity, subpopulations of SaOS-2 cells were isolated from replicate low-density cultures. The subpopulations were less heterogeneous and more stable (with respect to ALP) than the parent population. ALP specific activity in the subpopulations ranged from 0.05 to 2.3 U/mg protein, and cytochemical analyses indicated multiple steady-state levels of ALP activity per cell. The amount of ALP activity in SaOS-2 subpopulations was proportional to collagen production ([3H]proline incorporation into collagenase-digestible protein; r = .84, P less than .005), and to parathyroid hormone (PTH)-linked synthesis of cyclic adenosine monophosphate (cAMP) (r = .88, P less than .01). From these data, we inferred that ALP activity in SaOS-2 cells can provide a useful index of the osteoblastic phenotype, and that ALP activity, collagen production, and PTH-linked adenylate cyclase were coordinately regulated in these osteoblast-like osteosarcoma cells (ie, selection of subpopulations for ALP activity coselected for collagen synthesis and PTH-linked synthesis of cAMP). Further comparative studies showed that micromolar fluoride concentrations stimulated cell proliferation ([3H]thymidine incorporation into DNA) in low-ALP SaOS-2 subpopulations, but not in high-ALP cells (P less than .001), and that this differential sensitivity to fluoride was associated with an inverse correlation between fluoride-sensitive acid phosphatase and ALP activities (r = -.91, P less than .001).

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)

Partial gastrectomy and mineral metabolism: effects on gastrin-calcitonin release

Bone mineral metabolism was studied in 20 male patients, between 8 and 18 years, after surgical treatment for peptic ulcer (ten Billroth 1 and ten Billroth 2 gastrectomies) and in 16 sex- and aged-matched healthy controls. The bone mineral content was statistically reduced only in the Billroth 2 group. Serum 25(OH)D was lower in all patients, but fractional calcium absorption was similar to the control value. This may be due to increases in 1,25(OH)2D and parathyroid activity (particularly in Billroth 2). Serum osteocalcin levels and hydroxyproline excretion were higher than in the controls. A positive linear correlation emerged not only between serum 1,25(OH)2D and PTH levels but also between each of these and serum osteocalcin and urine hydroxyproline. Both PTH and calcitriol were inversely correlated with the bone mineral mass in Billroth 2, confirming a trend observed in Billroth 1. Although calcitonin values were normal, basal gastrin levels were severely impaired in all patients. In response to a mixed meal, increases in gastrin and calcitonin were significantly lower than in the controls. The calcitonin response to intravenous calcium and pentagastrin infusion was not significantly different to the controls. The percentage increase in gastrin and calcitonin responses to oral calcium correlated positively with the reduction in bone mineral content only in the Billroth 2 group, suggesting a reduction in calcitonin release may contribute to gastric surgery osteopenia in these patients.

Sodium fluoride does not increase human bone cell proliferation or protein synthesis in vitro

The efficacy of sodium fluoride therapy for osteoporosis remains controversial. Evidence from clinical studies and from animals receiving fluoride has suggested that fluoride might act on bone to increase osteoblast numbers and matrix synthesis. In order to examine the hypothesis that fluoride has a direct mitogenic or anabolic action on the osteoblast, we tested the effect of fluoride on first passage human osteoblastic bone cells grown from collagenase-treated trabecular bone fragments. Under a variety of culture conditions, including both medium supplementation with serum and with a chemically defined medium, fluoride in doses ranging from 10(-6) to 10(-3) M did not alter cell proliferation as measured either by thymidine incorporation or by direct cell counting. Furthermore, exposure to fluoride under conditions of low serum supplementation did not alter either the total protein synthesis of the cells or their biosynthetic profile. These data suggest that fluoride does not act in vitro upon differentiated osteoblastic bone cells derived from adult human patients.

Sodium fluoride lacks mitogenic activity for fetal human bone cells in vitro

Sodium fluoride has been shown to be effective therapy for some patients with vertebral osteoporosis. Data from histomorphometric studies in patients and animals suggest that at least part of this effect may be a consequence of a proliferative effect of fluoride, either direct or indirect, on the osteoblast or on an osteoblastic precursor cell. Experiments with osteoblastic cells derived from embryonic chick calvaria have demonstrated a mitogenic effect of fluoride. The present study examined whether fluoride affects in a similar way fetal human bone cells derived from femur or calvaria. Under a variety of culture conditions, including medium supplemented with serum and in serum-free medium, fluoride did not alter the proliferative rate of human bone cells as measured by thymidine incorporation and direct cell counting.

Mitogenic action(s) of fluoride on osteoblast line cells: determinants of the response in vitro

Clinically effective (osteogenic) concentrations of fluoride (5-30 microM) also have direct effects on skeletal tissues in vitro, to increase bone formation and osteoblast line cell proliferation. The effect on cell proliferation was specific for bone cells, modulated by systemic skeletal effectors, and dependent on (a) the [Pi] in the medium, (b) the presence of a bone cell mitogen, and (c) mitogen-responsive osteoprogenitor cells. Together, these data indicate that fluoride increases bone formation in vitro by increasing osteoprogenitor cell proliferation and that fluoride increases osteoprogenitor cell proliferation by enhancing the activity of bone cell mitogens.

Calcitonin (but not calcitonin gene-related peptide) increases mouse bone cell proliferation in a dose-dependent manner, and increases mouse bone formation, alone and in combination with fluoride

Previous in vitro studies have shown that salmon calcitonin had direct effects to increase parameters associated with embryonic chicken bone formation and to increase mouse and chicken osteoblast-line cell proliferation. The current studies demonstrate increased cell proliferation (i.e., [3H]-thymidine incorporation into DNA and tetrazolium salt reduction/deposition) in the osteoblastic murine cell line MC-3T3-E1 in response to salmon calcitonin (P less than 0.005) and to human calcitonin (P less than 0.005), but not to human calcitonin gene-related peptide. The current studies also show that salmon calcitonin increased several indices of murine bone formation. We found that 72 hours of exposure to salmon calcitonin [at 5 mU/ml-about 0.37 nM; mU/ml = milliunits of calcitonin activity/ml incubation medium (at 4,000 U/mg protein)] increased net 45Ca deposition (121% of control, P less than 0.05), net [3H]-proline incorporation 149% of control, P less than 0.001), and alkaline phosphatase activity (146% of control, P less than 0.01), in neonatal mouse half-calvaria. The calcitonin-dependent increase in alkaline phosphatase activity was not affected by co-incubation with 1 nM parathyroid hormone. Co-incubation with fluoride (which also increased net [3H]-proline incorporation and alkaline phosphatase activity in neonatal mouse half-calvaria, P less than 0.05, for each) enhanced the osteogenic response to low-dose calcitonin, (i.e., co-incubation with fluoride shifted the biphasic calcitonin dose-response curve to a range of lower calcitonin concentrations). The calcitonin-fluoride combinations had proportional effects on net [3H]-proline incorporation and alkaline phosphatase in the treated mouse calvaria (r = 0.78, P less than 0.005).

A proposed mechanism of the mitogenic action of fluoride on bone cells: inhibition of the activity of an osteoblastic acid phosphatase

Fluoride (F) is a potent inhibitor of osteoblastic acid phosphatase activity with an apparent Ki value (10 to 100 mumol/L) that corresponds to F concentrations that increase bone cell proliferation and bone formation in vivo and in vitro. This high sensitivity of acid phosphatase to F inhibition appeared to be specific for skeletal tissues. Mitogenic concentrations of F did not increase cellular cAMP levels but significantly stimulated net protein phosphorylation in intact calvarial cells and in isolated calvarial membranes. These concentrations of F also stimulated net membrane-mediated phosphorylation of angiotensin II (which contains tyrosyl but no seryl or threonyl residues), suggesting that some of the F-stimulated protein phosphorylations could occur on tyrosyl residues. F had no apparent effect on thiophosphorylation of membrane proteins, suggesting that the F-stimulated net protein phosphorylation in bone cells was probably not mediated via activation of protein kinases. Orthovanadate or molybdate at concentrations that inhibit bone acid phosphatase activity also stimulated bone cell proliferation, supporting the idea that inhibition of bone acid phosphatase would lead to stimulation of bone cell proliferation. Mitogenic concentrations of F potentiated the mitogenic activities of insulin, EGF, and IGF-1 (ie, growth factors the receptors of which are tyrosyl kinases) to a greater extent than they potentiated the action of basic FGF (a growth factor that does not appear to stimulate tyrosyl protein phosphorylation). Based on these findings, a model is proposed for the biochemical mechanism of the osteogenic action of F in which F stimulates bone cell proliferation by a direct inhibition of an osteoblastic acid phosphatase/phosphotyrosyl protein phosphatase activity, which in turn increases overall cellular tyrosyl phosphorylation, resulting in a subsequent stimulation of bone cell proliferation.

Acetate inhibition of chick bone cell proliferation and bone growth in vitro

A hypothesis has been advanced that parenteral solutions as commonly formulated for use in clinical practice have a toxic effect on cell metabolism. A specific component of these solutions, sodium acetate, has been suggested to disrupt normal bone turnover and therefore to contribute to the osteopenia observed in patients receiving hemodialysis and parenteral nutrition (PN). We developed an in vitro model to test the hypothesis that sodium acetate at concentrations that are infused in PN solutions has a deleterious effect on bone metabolism. Osteoblasts and preosteoblasts from 16- to 17-day-old embryonic chick calvaria, and tibiae and femora from 10-day-old embryonic chicks were grown in BGJb medium (control) or in BGJb medium plus sodium acetate (5, 10, or 20 mM). Calvarial cell proliferation was quantified by direct cell counts as well as by incorporation of [3H]TdR into DNA as an index of cell proliferation. Calvarial cell alkaline phosphatase activity was quantified by the ability of extracts of the cultured cells to hydrolyze p-nitrophenyl phosphate to p-nitrophenol, and bone growth was determined by measuring final dry weight. Calvarial cell counts as well as DNA synthesis showed a dose-dependent decrease in the presence of sodium acetate (5-20 mM) compared with controls. [3H]TdR incorporation was decreased a mean 19% with 5 mM, 38% with 10 mM, and 63% with 20 mM acetate. Alkaline phosphatase activity per cell increased 48% with 5 mM, 140% with 10 mM, and 355% with 20 mM acetate. Cell viability as assessed by trypan blue exclusion was identical for test and control media (greater than 95%).(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that fluoride-stimulated 3[H]-thymidine incorporation in embryonic chick calvarial cell cultures is dependent on the presence of a bone cell mitogen, sensitive to changes in the phosphate concentration, and modulated by systemic skeletal effectors

In previous studies we have shown that clinically effective concentrations of fluoride (5 to 30 mumol/L) could also have direct effects in vitro on skeletal tissues to increase embryonic chick bone formation and bone cell proliferation (3[H]-thymidine incorporation into DNA). From these observations, we hypothesized that fluoride-stimulated bone formation might be mediated by a direct effect of fluoride to increase bone cell proliferation. The current studies were intended to investigate the mechanism of fluoride-stimulated 3[H]-thymidine incorporation, in chick calvarial cell cultures, by assessing mitogenic interactions between fluoride and inorganic phosphate, bone-derived growth factors, and systemic skeletal effectors. With respect to fluoride-phosphate interactions, the results of our studies indicate that the effect of fluoride was dependent on the phosphate concentration in the medium. Fluoride did not increase 3[H]-thymidine incorporation in BGJb medium containing 1 mmol/L (total) phosphate; but, in 1.6 mmol/L phosphate medium, fluoride caused a dose-dependent increase in 3[H]-thymidine incorporation, between 1 and 20 mumol/L (P less than .001). The action of fluoride was also dependent on the presence of a bone cell mitogen. Fluoride increased 3[H]-thymidine incorporation when added to calvarial cell cultures in the cell-conditioned medium, but had no effect in unconditioned (ie, fresh) medium. The action of fluoride could be restored by adding an exogenous growth factor (ie, concentrated cell-conditioned medium, bone-derived growth factors, or a systemic bone cell mitogen) to the unconditioned culture medium, P less than .05 for each effector.(ABSTRACT TRUNCATED AT 250 WORDS)