The effects of fluoride on osteoblast progenitors in vitro

Sintered fluorapatite scaffolds as an autograft-like engineered bone graft

Hydroxyapatite (HA)-based materials are widely used as bone substitutes due to their inherent biocompatibility, osteoconductivity, and bio-absorption properties. However, HA scaffolds lack compressive strength when compared to autograft bone. It has been shown that the fluoridated form of HA, fluorapatite (FA), can be sintered to obtain this desired strength as well as slower degradation properties. Also, FA surfaces have been previously shown to promote stem cell differentiation toward an osteogenic lineage. Thus, it was hypothesized that FA, with and without stromal vascular fraction (SVF), would guide bone healing to an equal or better extent than the clinical gold standard. The regenerative potentials of these scaffolds were tested in 32 Lewis rats in a femoral condylar defect model with untreated (negative), isograft (positive), and commercial HA as controls. Animals were survived for 12 weeks post-implantation. A semi-quantitative micro-CT analysis was developed to quantify the percent new bone formation within the defects. Our model showed significantly higher (p < .05) new bone depositions in all apatite groups compared to the autograft group. Overall, the FA group had the most significant new bone deposition, while the differences between HA, FA, and FA + SVF were insignificant (p > .05). Histological observations supported the micro-CT findings and highlighted the presence of healthy bone tissues without interposing capsules or intense immune responses for FA groups. Most importantly, the regenerating bone tissue within the FA + SVF scaffolds resembled the architecture of the surrounding trabecular bone, showing intertrabecular spaces, while the FA group presented a denser cortical bone-like architecture. Also, a lower density of cells was observed near FA granules compared to HA surfaces, suggesting a reduced immune response. This first in vivo rat study supported the tested hypothesis, illustrating the utility of FA as a bone scaffold material.

The Impact of Dental Implant Surface Modifications on Osseointegration and Biofilm Formation

Implant surface design has evolved to meet oral rehabilitation challenges in both healthy and compromised bone. For example, to conquer the most common dental implant-related complications, peri-implantitis, and subsequent implant loss, implant surfaces have been modified to introduce desired properties to a dental implant and thus increase the implant success rate and expand their indications. Until now, a diversity of implant surface modifications, including different physical, chemical, and biological techniques, have been applied to a broad range of materials, such as titanium, zirconia, and polyether ether ketone, to achieve these goals. Ideal modifications enhance the interaction between the implant's surface and its surrounding bone which will facilitate osseointegration while minimizing the bacterial colonization to reduce the risk of biofilm formation. This review article aims to comprehensively discuss currently available implant surface modifications commonly used in implantology in terms of their impact on osseointegration and biofilm formation, which is critical for clinicians to choose the most suitable materials to improve the success and survival of implantation.

The effect of surface pre-reacted glass-ionomer filler eluate on dental pulp cells and mineral deposition on dentin: In vitro study

The effects of surface pre-reacted glass-ionomer (S-PRG) filler on pulpal cells and on the composition of dentinal deposits were investigated. Proliferation (CCK-8), cytotoxicity (LDH), and differentiation activity (ALP) tests, along with cell morphology observations, were conducted at 6 and 24 h after treatment of pulpal cells with different S-PRG filler eluate concentrations. Dentinal surfaces were immersed in deionized water or S-PRG filler eluate followed by immersion in deionized water or simulated body fluid and observed under scanning electron microscope and elemental analysis using energy dispersive x-ray spectrometer. At 24 h, there were significant differences in CCK-8 and ALP activity values between the groups in a concentration-dependent manner. LDH test data were not significantly different among the groups. Cell morphology was not altered at either exposure time. However, decreased cellular density was observed with the highest eluate concentration. Crystalline deposits and occluded dentinal tubules were observed in samples immersed in S-PRG filler with a later immersion in simulated body fluid, which also showed higher concentrations of certain ions compared to surfaces that were not initially treated with S-PRG filler. The lowest two eluate concentrations did not show significant toxicity. S-PRG enhanced the effect of simulated body fluid in the formation of mineral deposits.

Antibacterial coating of tooth surface with ion-releasing pre-reacted glass-ionomer (S-PRG) nanofillers

OBJECTIVES

Surface pre-reacted glass-ionomer (S-PRG) fillers release antibacterial borate and fluoride ions. We fabricated nanoscale S-PRG fillers (S-PRG nanofillers) for antibacterial coating of tooth surfaces and assessed the antibacterial effects of this coating in vitro. In addition, we creating a canine model of periodontitis to evaluate the effectiveness of S-PRG nanofiller application on tooth roots and improvement of periodontal parameters.

METHODS

Human dentin blocks were coated with S-PRG nanofiller (average particle size: 0.48 μm) and then characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometer (EDX), and ion-releasing test. Antibacterial effects of dentin blocks coated with S-PRG nanofiller were examined using bacterial strains, Streptococcus mutans and Actinomyces naeslundii. Next, we created an experimental model of periodontitis in furcation of premolars of beagle dogs. Then, S-PRG nanofiller coating was applied onto exposed tooth root surfaces. Periodontal parameters, gingival index (GI), bleeding on probing (BOP), probing pocket depth (PPD), and clinical attachment level (CAL), were measured from baseline until 4 weeks. In addition, bone healing was radiographically and histologically examined.

RESULTS

SEM and EDX revealed that S-PRG nanofillers uniformly covered the dentin surface after coating. Dentin blocks coated with S-PRG nanofiller showed ion-releasing property, bacterial growth inhibition, and sterilization effects. In the experimental periodontitis model, S-PRG nanofiller coating significantly reduced clinical inflammatory parameters, such as GI (P < 0.01) and BOP (P < 0.05), compared to uncoated samples. In addition, PPD and CAL significantly decreased by S-PRG nanofiller coating (2 weeks: P < 0.05; 3 and 4 weeks: P < 0.01), suggesting the improvement of periodontitis. Micro-CT and histology revealed that bone healing of furcation defects was enhanced by S-PRG nanofiller coating.

CONCLUSION

S-PRG nanofiller coating provides antibacterial effects to tooth surfaces and improves clinical parameters of periodontitis.

Modifications of Dental Implant Surfaces at the Micro- and Nano-Level for Enhanced Osseointegration

This review paper describes several recent modification methods for biocompatible titanium dental implant surfaces. The micro-roughened surfaces reviewed in the literature are sandblasted, large-grit, acid-etched, and anodically oxidized. These globally-used surfaces have been clinically investigated, showing survival rates higher than 95%. In the past, dental clinicians believed that eukaryotic cells for osteogenesis did not recognize the changes of the nanostructures of dental implant surfaces. However, research findings have recently shown that osteogenic cells respond to chemical and morphological changes at a nanoscale on the surfaces, including titanium dioxide nanotube arrangements, functional peptide coatings, fluoride treatments, calcium–phosphorus applications, and ultraviolet photofunctionalization. Some of the nano-level modifications have not yet been clinically evaluated. However, these modified dental implant surfaces at the nanoscale have shown excellent in vitro and in vivo results, and thus promising potential future clinical use.

The Inverted U-Curve Association of Fluoride and Osteoclast Formation in Mice

The effect of fluoride on osteoclasts is still controversial. In the past, researchers thought that the effects of fluoride on osteoclast and osteoblast formation occurred in a dose-dependent pattern. However, our previous in vitro study showed fluoride elicited a notably different effect on osteoclast formation. To further verify the relationship between fluoride and osteoclast formation in vivo, 60 male C57BL/6 mice were randomly divided into three groups: two treatment groups consuming water supplemented with 50 and 100 mg/L of fluoride, and a third control group with nonsupplemented water. Ion selective electrode method analysis was used to detect bone fluoride content, and the effects of fluoride on bone tissue were assessed with hematoxylin and eosin (HE) staining. Additionally, the expression of BGP and ALP were examined by Western blot analysis, and tartrate-resistant acid phosphatase (TRAP) was assessed with immunohistochemistry. Osteoclasts in bone tissue were identified with a combination method of TRAP staining and cell morphology assessment. Results showed increasing expression of BGP among treatment groups as fluoride exposure increased, and ALP expression in the 100 mg/L treatment group was significantly higher than that for both the 50 mg/L treatment and control groups. The number of osteoclasts in the 50 mg/L group was highest amongst the three groups, followed by the 100 mg/L treatment and then by the control group, with the latter showing significantly fewer osteoclasts than in either treatment group. These results suggest that fluoride enhances bone formation at increasing levels of fluoride exposure. However, the inverted U-curve association was found between fluoride exposure and osteoclast formation, with the higher dose of fluoride having slightly reduced osteoclast formation. The results from this study may provide key insights towards understanding the role of osteoclasts in the progression of skeletal fluorosis.

Histomorphometric and Histopathologic Evaluation of the Effects of Systemic Fluoride Intake on Orthodontic Tooth Movement

Objectives  The aim of this study was to determine the effects of systemic fluoride intake on orthodontic tooth movement with histomorphometric and histopathologic methods.

Materials and Methods  Forty-eight Wistar albino rats were randomly divided into four groups of 12 rats each. Group I received fluoridated water and underwent orthodontic tooth movement. Group II received fluoridated water and did not undergo orthodontic tooth movement. Group III received nonfluoridated water and underwent orthodontic tooth movement. Group IV received nonfluoridated water and did not undergo orthodontic tooth movement. At the beginning of the experiment (T1), impressions were taken from the maxilla of the rats in groups I and III under general anesthesia, and a NiTi closed coil spring appliance was ligated between the left maxillary central incisors and maxillary first molar. The orthodontic force applied was approximately 75 g, and the duration of the experimental period was 18 days. During the experimental period, appliances were controlled daily. At the end of the experimental period (T2), the rats were sacrificed with an overdose of a ketamine/xylasine combination, and their impressions were obtained. The upper first molars were subsequently dissected for histological examination. Incisor–molar distance, number of osteoblasts, number of osteoclasts and periodontal ligament (PDL) space widths on the compression and tension sides were measured.

Statistical Analysis  All measurements were statistically analyzed with SPSS for Windows version 18.0 (SPSS Inc., Chicago, IL, USA). Repeated measures ANOVA and posthoc Tukey tests were used to compare the groups.

Results  No statistically significant difference was found with respect to the amount of tooth movement between the fluoridated and nonfluoridated groups ( p > 0.05). Orthodontic force application increased the number of osteoblasts at the tension sides and reduced it at the compression sides ( p < 0.001). An increased number of osteoclasts was observed in the nonfluoridated group relative to the fluoridated group ( p < 0.01).

Conclusions  No difference was observed with respect to the amount of tooth movement between the fluoridated and nonfluoridated groups. Fluoride significantly reduced the number of osteoclasts in the experimental groups.

Micromolar sodium fluoride mediates anti-osteoclastogenesis in Porphyromonas gingivalis-induced alveolar bone loss

Osteoclasts are bone-specific multinucleated cells generated by the differentiation of monocyte/macrophage lineage precursors. Regulation of osteoclast differentiation is considered an effective therapeutic approach to the treatment of bone-lytic diseases. Periodontitis is an inflammatory disease characterized by extensive bone resorption. In this study, we investigated the effects of sodium fluoride (NaF) on osteoclastogenesis induced by Porphyromonas gingivalis, an important colonizer of the oral cavity that has been implicated in periodontitis. NaF strongly inhibited the P. gingivalis-induced alveolar bone loss. That effect was accompanied by decreased levels of cathepsin K, interleukin (IL)-1β, matrix metalloproteinase 9 (MMP9), and tartrate-resistant acid phosphatase, which were up-regulated during P. gingivalis-induced osteoclastogenesis. Consistent with the in vivo anti-osteoclastogenic effect, NaF inhibited osteoclast formation caused by the differentiation factor RANKL (receptor activator of nuclear factor κB ligand) and macrophage colony-stimulating factor (M-CSF). The RANKL-stimulated induction of the transcription factor nuclear factor of activated T cells (NFAT) c1 was also abrogated by NaF. Taken together, our data demonstrate that NaF inhibits RANKL-induced osteoclastogenesis by reducing the induction of NFATc1, ultimately leading to the suppressed expression of cathepsin K and MMP9. The in vivo effect of NaF on the inhibition of P. gingivalis-induced osteoclastogenesis strengthens the potential usefulness of NaF for treating periodontal diseases.

Anti-inflammatory effects of bamboo salt and sodium fluoride in human gingival fibroblasts--An in vitro study

Dental caries preventive agents, such as sodium fluoride (NaF) and bamboo salt (BS), are known to cause cellular growth that is characterized by morphological and gene expression changes. This study was designed to investigate the dual effect of NaF and BS on interleukin (IL)-1β-induced gingival inflammation. Under in vitro experimental conditions, exposure to a subcytotoxic dose of IL-1β enhanced human gingival fibroblast inflammation, as characterized by increased levels of inflammation-associated proteins. A combination of NaF and BS significantly protected fibroblasts from IL-1β-induced cellular deterioration. Exposure to NaF and BS induced the cell growth and no changes in viability were found with the Lactate Dehydrogenase Assay (LDH) assay at the NaF and BS concentration analyzed. Molecular analysis demonstrated that NaF and BS increased resistance to inflammation by reduction of IL-1β, IL-8, and tumor necrosis factor (TNF)-α production. In addition, NaF and BS decreased the expression of IL-1β, IL-8, and TNF-α mRNA in IL-1β-induced human gingival fibroblast cells. The study identifies a new role for NaF and BS in the IL-1β-induced inflammation of gingival fibroblasts and provides a potential target for gingival protection.

Role of unfolded protein response in affecting osteoblast differentiation induced by fluoride

The objective of this study was to determine the expression of classic bone markers and unfolded protein response (UPR) signaling factors through MC3T3-E1 cells exposed to varying concentrations of fluoride. Excessive fluoride exposure caused the skeletal disease. During this process, osteoblasts played a critical role in the advanced skeletal fluorosis. Recent literature showed that endoplasmic reticulum (ER) stress and UPR were involved in numerous aspects of bone biology. Our results indicated that co-exposure of low-dose fluoride and mineral induction medium stimulated the expression of alkaline phosphatase, runt-related transcription factor 2 (Runx2), and osterix in MC3T3-E1 cells. Accordingly, the expression of double-stranded RNA-activated protein kinase (PKR)-like ER kinase, activating transcription factor 6, and X-box binding protein 1 also increased under the same fluoride exposure condition. The upregulation of three UPR factors was similar with osteogenic differentiation markers and transcription factors, which implied the relation between osteoblast differentiation and UPR pathways. Moreover, the role of UPR affecting osteoblast differentiation was investigated by decreasing the expression of binding immunoglobulin protein (BiP) mRNA through small interfering RNA (siRNA) technique. BiP knockdown led to suppress activation of UPR pathways. The deletion of BiP expression hardly stimulated the osteogenic cells differentiation but inhibited cell function under fluoride and mineralization induction exposure. In conclusion, fluoride had dual effect on osteogenic action. The UPR possibly involved in the mechanism of osteoblasts differentiation induced by fluoride.

Surface properties and ion release from fluoride-containing bioactive glasses promote osteoblast differentiation and mineralization in vitro

Bioactive glasses (BG) are suitable for bone regeneration applications as they bond with bone and can be tailored to release therapeutic ions. Fluoride, which is widely recognized to prevent dental caries, is efficacious in promoting bone formation and preventing osteoporosis-related fractures when administered at appropriate doses. To take advantage of these properties, we created BG incorporating increasing levels of fluoride whilst holding their silicate structure constant, and tested their effects on human osteoblasts in vitro. Our results demonstrate that, whilst cell proliferation was highest on low-fluoride-containing BG, markers for differentiation and mineralization were highest on BG with the highest fluoride contents, a likely effect of a combination of surface effects and ion release. Furthermore, osteoblasts exposed to the dissolution products of fluoride-containing BG or early doses of sodium fluoride showed increased alkaline phosphatase activity, a marker for bone mineralization, suggesting that fluoride can direct osteoblast differentiation. Taken together, these results suggest that BG that can release therapeutic levels of fluoride may find use in a range of bone regeneration applications.

Simultaneous incorporation of carbonate and fluoride in synthetic apatites: Effect on crystallographic and physico-chemical properties

The mineral in bone is an impure hydroxyapatite, with carbonate as the chief minor substituent. Fluoride has been shown to stimulate osteoblastic activity and inhibit osteoclastic resorption in vitro. CO(3)- and F-substituted apatite (CFA) has been considered as potential bone graft material for orthopedic and dental applications. The objective of this study was to determine the effects of simultaneously incorporated CO(3) and F on the crystallographic physico-chemical properties of apatite. The results showed that increasing CO(3) and Na content in apatites with relatively constant F concentration caused a decrease in crystallite size and an increase in the extent of calcium release; increasing F content in apatites with relatively constant CO(3) concentration caused an increase in crystallite size and a decrease in the extent of Ca release. These findings suggest that CFAs as bone graft materials of desired solubility can be prepared by manipulating the relative concentrations of CO(3) and F incorporated in the apatite.

The influence of fluoride on the cellular morphology and synthetic activity of the rat dentine-pulp complex in vitro

Exposure to high fluoride concentrations in the immediate environment of the tissue is recognized to result in the post-translational modification of non-collagenous dentine extracellular matrix (ECM) components, potentially altering dentine mineralization. However, less is known about the effects of fluoride exposure on the morphology or metabolism of the cells associated with the dentine-pulp complex. This study examined the effects of fluoride exposure at defined concentrations on the cellular morphology and ECM synthetic activities of odontoblasts and pulpal fibroblasts by the culture of tooth sections from male Wistar rat incisors in Trowel-type cultures for up to 14 days, in the presence and absence of 6mM sodium fluoride. Histomorphometric analysis of the dentine-pulp complex of sodium fluoride-exposed tooth sections demonstrated no obvious gross morphological differences with respect to the odontoblasts and pulpal fibroblasts throughout the 14-day culture period, in comparison with unexposed tooth sections. No significant differences in odontoblast and pulpal fibroblast cell numbers were determined in the absence and presence of fluoride. Image analysis examination of odontoblast cytoplasmic:nuclear (C/N) ratios also showed no significant differences in fluoride-exposed and unexposed tooth sections, although reductions in the C/N ratios of pulpal fibroblasts were evident in fluoride-exposed sections at days 10 and 14. No significant differences in predentine width were observed in fluoride-exposed and unexposed tooth sections over the 14-day culture period. Autoradiography following [3H]proline incorporation into the dentine-pulp complex demonstrated inhibition of collagen synthesis, particularly by the odontoblasts in tooth sections exposed to 6mM sodium fluoride. These findings, in association with those from previous studies, imply that dentine ECM alterations may contribute to the altered mineralization of dentine during fluorosis, rather than secretory-related changes in odontoblast morphology.

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 role of fluoride in the prevention of osteoporosis

Osteoporosis defined as low bone mass and increased susceptibility to fracture is a reflection of the sum of peak bone mass and any bone that has been lost once peak mass has been attained. Several strategies have been applied to optimize peak bone mass and to prevent bone loss. Fluoride has greatest potential as a therapy for osteoporosis once bone has been lost. It has been demonstrated both experimentally and clinically to stimulate bone formation directly and to increase bone mass in patients who already have osteoporosis. Several bone formation/stimulation therapies are under development, and some of these have reached the stage of clinical trial. None of these therapies has been as extensively studied as fluoride, and none is sufficiently advanced in development to be clinically available in the next 3 to 5 years. Fluoride therapy for osteoporosis is already performed in many countries, and approval for use in osteoporosis in the United States is pending. The first clinical trials of NaF therapy for osteoporosis were reported by Rich and Ensinck in 1961. Since then, hundreds of reports on the successes and failures of fluoride therapy have appeared in the literature. At first glance, it seems disappointing and inexplicable that, after 40 years of research, fluoride is still considered an experimental drug in the United States. One plausible explanation is that much of the early research on this drug was suboptimal, including the author's contributions. Fluoride as a naturally occurring element is difficult to patent, and this has kept major pharmaceutical companies from investing heavily in fluoride therapy despite its obvious potential. As a result, pharmacologic and pharmacokinetics studies of fluoride are limited in scope, as are phase I and phase II human toxicology and dose-finding studies. Most early studies of large doses of plain NaF were unable to demonstrate a consistent effect on fracture rate despite a consistent and dramatic effect on bone density. Once this became obvious and as new technologies for measuring bone density became available, it became equally clear that future clinical trials would have to be performed using different formulations of fluoride and lower doses. This approach has not resulted in uniformly positive clinical trials, and one must look elsewhere for answers. The most compelling explanation is that the trials have included patients with different severity of disease, suggesting that there is point in the bone loss spectrum at which even a potent bone-stimulating agent such as fluoride is ineffective. This possibility should provoke a reappraisal of the earlier negative studies: was the failure a result of the drug or of patient selection? The answer to this question is crucial, because these failures have cast a long shadow over the safety of fluoride and are contributing more to the absence of this drug from the pharmacopoeia than any other factor.

Role of protein kinase C alpha, Arf, and cytoplasmic calcium transients in phospholipase D activation by sodium fluoride in osteoblast-like cells

The effect of fluoride on phospholipase D (PLD) activation was studied using in vitro culture of Saos-2, MG-63 osteosarcoma cells, and normal osteoblast-like cells derived from human bone explants. Millimolar concentrations of NaF induced a significant accumulation of phosphatidylethanol (PEt) in Saos-2 cells but not in MG-63 and normal osteoblast-like cells. PLD activation was evident at 15 mM and concentration-dependent up to 50 mM. This stimulation was inhibited by deferoxamine, a chelator of Al3+, suggesting that PLD activation involves fluoride-sensitive G proteins. A good correlation was found between the levels of intracellular free Ca2+ and the activation of PLD. The time courses of the two responses were nearly identical. The ability of NaF to induce both responses was largely dependent on the presence of extracellular calcium. The calcium ionophore A23187 reproduced the effect of NaF, and this effect was antagonized by EGTA, suggesting that PLD activation was, at least in part, a calcium-regulated event. Phorbol 12-myristate 13-acetate (PMA) also stimulated PLD activity in human bone cells. Protein kinase C alpha (PKC alpha) and epsilon were expressed in Saos-2 cells. Acute pretreatment of cells with PMA reduced concomitantly the amounts of PKC alpha, but not of PKC epsilon, and the subsequent activation of PLD elicited by PKC activators. The PLD response to NaF was not attenuated but rather enhanced by down-regulation of PKC alpha. Therefore, PKC-alpha-induced PLD activation is unlikely to mediate the effect of NaF. Moreover, PMA and NaF showed a supraadditive effect on PLD activation in Saos-2 cells. This stimulation, in contrast to NaF alone, was not reduced by EGTA. Hence, mobilization of calcium by NaF cannot account for the enhanced PLD activation in response to PMA stimulation. Membrane Arf and RhoA contents were assessed by Western immunoblot analyses. Membranes derived from NaF-stimulated Saos-2 cells contained more Arf and RhoA when compared with membranes derived from control or PMA-stimulated cells. Translocation of the small GTPases was calcium-independent. We conclude that PLD activation by NaF in Saos-2 cells includes a fluoride-sensitive G protein, increases in the levels of intracellular calcium, and Arf/RhoA redistribution to membranes. The results also indicate that the NaF-induced Arf/RhoA translocation exerts in concert with PMA-activated PKC alpha a synergistic effect on the activation of PLD in Saos-2 cells.

Dexamethasone enhances the osteogenic effects of fluoride in human TE85 osteosarcoma cells in vitro

The in vitro osteogenic effects of fluoride have not always been consistently observed in human bone cells. The present study sought to test if dexamethasone (Dex) could potentiate the action of fluoride to increase the detectability of the stimulatory effects of fluoride on [3H]thymidine incorporation, alkaline phosphatase (ALP) specific activity, collagen synthesis, and osteocalcin secretion in human TE85 osteosarcoma cells. Neither Dex at 10(-10)-10(-6) M or fluoride at a mitogenic dose (100 microM) had any consistent stimulatory effects on thymidine incorporation. When the cells were treated with both agents simultaneously, significant and highly reproducible stimulations were observed. The mitogenic effects of the two agents were confirmed with cell number counting. Analysis of variance (ANOVA) revealed a significant interaction (P < 0.001) between fluoride and Dex on cell proliferation. The enhancing effect of Dex on [3H]thymidine incorporation was not due to a shift of the optimal dose response of fluoride. Though fluoride alone or Dex alone also had no consistent effect on ALP specific activity, the co-treatment with fluoride and Dex for 24 hours produced significant (P < 0.001, ANOVA) stimulation in ALP specific activity. Fluoride alone had no consistent effect on collagen synthesis and on 1, 25(OH)2D3-dependent osteocalcin secretion, whereas Dex treatment consistently inhibited these two osteoblastic parameters in a dose-dependent manner. However, both the collagen synthesis and osteocalcin secretion rates were significantly higher (P < 0.001 ANOVA for each) when the cells were co-treated with Dex and fluoride (100 microM) than when they were treated with Dex alone. Thus, these data indicate that the response in collagen synthesis and osteocalcin secretion to fluoride stimulation was more readily observed in the presence of Dex than in its absence. ANOVA analysis revealed that the interaction between fluoride and Dex on collagen synthesis, but not the 1,25(OH)2D3-dependent osteocalcin secretion, was significant (P < 0.02). In summary, we have demonstrated for the first time that in TE85 cells (1) Dex potentiated the effects of fluoride on cell proliferation, ALP specific activity, and collagen synthesis; (2) while Dex at 10(-7)-10(-6) M alone inhibited the collagen synthesis and at 10(-9)-10(-6) M reduced osteocalcin secretion, Dex at 10(-8)-10(-6) M significantly stimulated the proliferation of TE85 cells; and (3) Dex interacted with fluoride to increase the percentage of experiments showing an osteogenic action of fluoride. In conclusion, the in vitro osteogenic actions of fluoride in human TE85 cells are more consistently observed in the presence than in the absence of Dex.

Fluoride and the skeleton

Fluoride has the potential to increase skeletal mass to a greater extent than any other pharmacologic agent, yet it has proven difficult to translate this into therapeutic benefit for patients with low bone mass in diseases such as osteoporosis. This apparent paradox can be explained in part by toxic actions of the ion on skeletal mineralization, impairment of the normal processes of bone resorption, and fluoride-induced decreases in strength per unit of bone (mass or volume). In part, the paradox can be explained by the late stage of osteoporosis in most patients enrolled in controlled clinical trials of fluoride, with alterations in skeletal microarchitecture beyond which restoration of mechanical integrity is not likely. Exposure of calcified tissues to environmental fluoride (water supply, dentifrices) also offers paradoxes. The anticaries effects are well documented as are the deleterious skeletal effects of endemic fluorosis when environmental exposure is too high. More controversial is the effect of seemingly nontoxic levels of exposure on the prevalence of osteoporotic fractures of the hip. This review attempts to provide a balanced overview of the conflicting literature concerning therapeutic and environmental effects of fluoride on the skeleton.

Aluminum inhibits both initiation and progression of mineralization of osteoid nodules formed in differentiating rat calvaria cell cultures

Osteoid nodules form in cultures of fetal rat calvaria (RC) cells grown in medium containing 10% fetal bovine serum (FBS) and 50 microns/ml of ascorbic acid. When 10 mM beta-glycerophosphate (beta-GP) is added, the nodules mineralize in two phases: an initiation phase that is dependent upon alkaline phosphatase activity for cleavage of beta-GP to inorganic phosphate (P(i)) and a progression phase that proceeds independently of the activity of alkaline phosphatase and does not require exogenous phosphate. We have used this system to investigate the effects of aluminum (Al3+)on mineralization. When AlCl3 was added to culture medium at concentrations of 0, 3, 10, 30, 100, and 300 muM, the total concentrations of aluminum were 0.98, 6.07, 16.82, 40.19, 88.45, and 284.52 muM, respectively. The corresponding free Al3+ concentrations, assessed after ultrafiltration, were found to be 1.11, 1.75, 3.40, 6.22, 5.38, and 12.11 muM. In cultures in which osteoid was formed and mineralization initiated in the presence of added Al+ (3-300 muM), a dose-dependent inhibition of mineralization occurred. Osteoid formed in the presence of added Al3+ mineralized normally when Al3+ was removed from cultures at the time of initiation of mineralization with beta-GP. In osteoid nodules grown in the absence of Al3+, addition of Al3+ (3-300 muM) at the start of the initiation phase of mineralization resulted in a dose-dependent inhibition of mineralization. Addition of Al3+ to cultures after mineralization had been initiated in the absence of Al3+ inhibited progression of mineralization at added Al3+ concentrations of 10 muM and above. Al3+ did not decrease the conversion of beta-GP to P(i) and caused a small but significant increase in alkaline phosphatase activity at added concentrations of 100 muM or greater. The data show that Al3+ inhibits both the initiation and progression phases of mineralization starting at added concentrations of 3-10 muM (approximately 1.7-3.4 muM free Al3+) and that mineralization of osteoid formed in the presence of Al3+ is unaffected if Al3+ is removed prior to the initiation of mineralization.

Pretreatment with low doses of norethindrone potentiates the osteogenic effects of fluoride on human osteosarcoma cells

We recently reported that picomolar doses of norethindrone (NET), a synthetic analog of 19-nortestosterone, significantly stimulated human TE85 osteosarcoma cell proliferation, differentiation, and activity in vitro. In the present study, we investigated the possibility that NET interacts with another osteogenic agent, i.e., fluoride, to stimulate human TE85 osteosarcoma cell proliferation, differentiation, and activities. Bone cell proliferation was measured by the stimulation in [3H]thymidine incorporation. Differentiation was monitored by the increase in alkaline phosphatase-specific activity. Osteoblastic activity was assessed by the stimulations in collagen synthesis and in osteocalcin secretion (in the presence of 1 nM 1,25-dihydroxyvitamin D3). When the human TE85 cells were incubated with mitogenic doses of NET and fluoride concurrently, the stimulatory effects of the two agents on these parameters exhibited no significant interaction. The enhancing effect of NET on the osteogenic effect of fluoride was not due to a shift of the fluoride dose response curve. Pretreatment with NET for 24 h followed by a treatment with a mitogenic dose (i.e., 100 microM) of fluoride for an additional 24 h significantly and synergistically potentiated the effects of fluoride on the [3H]thymidine incorporation, alkaline phosphatase-specific activity, collagen synthesis, and osteocalcin secretion, compared with those with the subsequent vehicle (0.05% ethanol) treatments. In contrast, pretreatment with fluoride for 24 h before the addition of NET for 24 h did not produce significant synergistic stimulations in the test parameters. Pretreatment of TE85 cells with the same doses of dihydrotestosterone or progesterone prior to treatment with fluoride under the same conditions did not induce synergistic potentiation of fluoride in [3H]thymidine incorporation, suggesting that the synergistic interaction with fluoride is probably not a common property of anabolic sex steroids. In summary, we found that: (1) the osteogenic effects of fluoride and NET were additive when cells were treated with both agents concurrently; (2) a 24-h pretreatment with picomolar doses of NET potentiated the osteogenic actions of fluoride in human TE85 osteosarcoma cells; and (3) pretreatment with NET produced a subsequent fluoride response that was synergistic. In conclusion, these findings led us to speculate that the osteogenic actions of NET and fluoride act through different mechanisms, and that NET at low doses has a permissive effect on the osteogenic effects of fluoride, and as such NET may be used in concert with fluoride to increase osteoblast proliferation, differentiation, and activity.

Prevention and treatment of osteoporosis

Because the lifetime risk of fragility fracture for a 50-year-old Caucasian woman is about 40 per cent, a whole-life strategy of osteoporosis prevention is necessary. In childhood, primary prevention of osteoporosis is based on exercise and adequate dietary calcium. In women undergoing menopause, hormone replacement therapy administered for at least ten years remains the preventive treatment of choice, and is associated with a substantial reduction in vertebral and non-vertebral fractures. Intranasal salmon calcitonin and bisphosphonates are effective alternatives, but their effects on fracture rate and their long-term safety require further evaluation. Regarding the prevention of the late bone loss leading to senile osteoporosis, there is now evidence that the reduction of the secondary hyperparathyroidism induced by calcium and vitamin D insufficiencies through the administration of calcium and vitamin D supplements significantly decreases the hip fracture incidence. There is no general consensus about the efficacy of treatment for established osteoporosis with fractures. Fluoride salts have proven their direct stimulating effects on bone formation; dosage must be moderate, and the duration of treatment should be limited to 2-3 years in order not to impair the quality of the new bone. Cyclical therapy with etidronate induces beneficial effects on bone mass in the spine, but its effect on the vertebral fracture rate is not yet established. The new bisphosphonates seem to be promising for the management of osteoporosis. Several other agents such as growth factors, silicon derivatives and strontium salts are in various stages of testing. The new definition of osteoporosis proposed by a WHO study group, no longer based on the fracture but on a low bone mass, is of major interest, because it should make possible to have a more effective therapeutic approach, before the occurrence of an irreversible degree of bone loss.

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)

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.

Fluoride increases net 45Ca uptake by SaOS-2 cells: The effect is phosphate dependent

Previous in vitro studies have shown that the effect of fluoride to increase avian osteoblast-like cell proliferation was dependent on the phosphate concentration. In vitro studies have further revealed that fluoride could also have direct effects on osteoblast-like cells to increase phosphate uptake and transiently increase cytosolic calcium. The current studies were intended to determine whether fluoride could increase net 45Ca uptake by human osteosarcoma (SaOS-2) cells and, if so, whether those effects would also be phosphate dependent. The results of these studies indicate that fluoride increased net 45Ca uptake by SaOS-2 cells, with biphasic dose and time dependencies. After 30 minutes of exposure, net 45Ca uptake was increased to a greater extent by 50 microM fluoride (217 +/- 16% of control, P < 0.001) than by 200 microM fluoride; and the stimulatory effect of 100 microM fluoride on net 45Ca uptake was greater after 20 minutes (187 +/- 22% of control, P < 0.001) than after 60 minutes (122 +/- 7% of control, P < 0.05). These effects of fluoride to increase net 45Ca uptake were dependent on the phosphate concentration in the medium. Fluoride had no effect on net 45Ca uptake in medium containing 0.4 mM phosphate, but increased net 45Ca uptake in medium containing 1.2 or 2.0 mM phosphate (P < 0.005). As the phosphate concentration was increased, the biphasic fluoride dose-response curve was shifted to a lower range of fluoride concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of controlled local release of sodium fluoride on bone formation: filling a defect in the proximal femoral cortex

To assess the effect of sodium fluoride (NaF) in the healing of a defect in cortical bone, an experimental model was created by the drilling of 5.0 mm holes in the proximal ends of both femora of 12 adult male New Zealand White rabbits. An interlocking intramedullary implant constructed of poly(d,l-lactic acid) containing NaF was placed in the right femur and an identical implant without NaF (sham), in the left. The implant in the right femur was designed to release NaF in a controlled manner over the duration of the experiment. Ten weeks after implantation, the specimens were removed and were tested in torsion. The mechanical properties were not significantly different between the groups. The femora exposed to NaF had an 18.6% increase in intact cortex near the defect (p = 0.023), however, the deposition of mineralized bone within the defect was not significantly greater. In fact, healing appeared to be impaired by the presence of NaF. There was complete closure of the defect in all but one of the femora with a sham implant, but the tissue had not yet calcified. In contrast, only one femoral defect exposed to NaF had closed. Examination of the material filling the defects of the femora exposed to NaF showed that it was predominantly uncalcified osteogenic mesenchymal tissue.

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.

Inorganic phosphate added exogenously or released from beta-glycerophosphate initiates mineralization of osteoid nodules in vitro

Rat calvaria (RC) cells grown in medium containing ascorbic acid form nodules of osteoid and cells. When 10 mM beta-Glycerophosphate (beta-GP) is added, the osteoid mineralizes in two phases: an initiation phase that is dependent upon alkaline phosphatase activity and a progression phase that proceeds independently of the activity of alkaline phosphatase and does not require added beta-GP (Bellows et al., Bone Miner 1991;14:27-40). The present experiments were performed to determine whether beta-GP is converted to inorganic phosphate (Pi) during the initiation phase of the mineralization process and whether increased Pi can replace beta-GP in the initiation phase. Measurements of Pi concentrations in the culture medium showed that during the first 8 h of the initiation phase of mineralization, 10 mM beta-GP was rapidly degraded resulting in Pi concentrations of 9-10 mM. The production rate of Pi from beta-GP was linear (r = 0.996) and the alkaline phosphatase activity in the same cultures indicated a potential for conversion of beta-GP to Pi that was greater than the actual conversion rate. The addition of 2-5 mM Pi in the absence of beta-GP also initiated mineralization. Mineralization initiated by either beta-GP or Pi progressed in the absence of added beta-GP or Pi. 100 microM Levamisole inhibited the initiation of beta-GP-induced mineralization and the conversion of beta-GP to Pi, but did not affect Pi-induced initiation of mineralization. The addition of 1-5 mM Pi to cultures in which mineralization had been initiated by 10 mM beta-GP had no significant effect on the progression phase of mineralization. Neither beta-BP nor Pi initiated 45Ca uptake in cultures without nodules (RC population I) and the histological appearance of the mineralized tissue in either phosphate source appeared identical. The present experiments show that beta-GP is rapidly and virtually completely degraded to Pi during the initiation phase of mineralization and that the addition of increased concentrations of Pi can replace beta-GP in the initiation phase of mineralization in the absence of non-specific 45Ca uptake or apparent cellular toxicity.

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.

Modulation of plasminogen activators and plasminogen activator inhibitors by TGF-beta, IL-1 alpha and EGF in fetal rat calvaria cells at different times of culture

Fetal rat calvaria cells (RC cells) grown in long term culture in the presence of ascorbic acid and organic phosphate proliferate and differentiate to form mineralized nodules of bone. Since transforming growth factor beta (TGF-beta), interleukin 1-alpha (IL-1 alpha) and epidermal growth factor (EGF) affect both bone resorption and bone formation, we have studied the ability of these growth factors to affect plasminogen activators and plasminogen activator inhibitors release by RC cells at different times throughout this proliferation/differentiation sequence. Cultures in log phase growth (day 4), when first multilayering (day 7) and when bone nodules were forming (day 13) were exposed to either TGF-beta, IL-1 alpha, EGF or vehicle. Conditioned medium was collected after 6 and 24 h and plasminogen activators and plasminogen activator inhibitors were analysed by fibrin autography and reverse fibrin autography. TGF-beta-mediated changes in plasminogen activator were apparent at day 4. By day 7 two molecular weight species of plasminogen activator were noted; a 65 kDa species, prominent at 24 h exposure was blocked by anti-tPA antibody, and a 38 kDa plasminogen activator, prominent after 6 h of stimulation was not blocked by anti-tPA antibody. Plasminogen activator-plasminogen activator inhibitor complexes are also increased. IL-1 alpha caused similar increases in plasminogen activator and plasminogen activator inhibitor with maximal activity measured at day 13, coincident with the time when bone nodules were forming. EGF-mediated changes were less by comparison. TGF-beta significantly decreased bone nodule formation after both a 6 and 24 h serum-free exposure, whereas IL-1 alpha and EGF decreased nodule number only after the 24 h exposure. The data suggest that the three factors influence the expression of plasminogen activator and plasminogen activator inhibitor by RC cells and their effect is different at different times of culture.

Determination of the capacity for proliferation and differentiation of osteoprogenitor cells in the presence and absence of dexamethasone

Osteoprogenitor cells present in single-cell suspensions prepared from fetal rat calvaria (RC) form discrete mineralized three-dimensional bone nodules when cultured long-term in the presence of ascorbic acid and beta-glycerophosphate. These cells (CFU-O) constitute less than 1% of the total cell population under standard culture conditions and their number is increased in the presence of dexamethasone. Using the formation of the bone nodule as a marker for CFU-O, we have now analyzed the proliferation and differentiation capacity of these CFU-O by redistribution and continuous subculture experiments in the presence and absence of dexamethasone. Cell redistribution experiments showed no increase in nodule number after one population doubling with either treatment. After 5.4 population doublings of the entire RC population, nodule number increased up to 2.0-fold in control cultures and 4.5-fold in cultures containing 10 nM dexamethasone. Continuous subculture experiments in which cultures were split 1:3 every 3 day for up to seven subcultures showed that nodule number decreased in parallel with the split ratio in the absence of dexamethasone, while with dexamethasone nodule number was elevated above the number present in primary cultures for 1 or 2 subcultures after which nodule number decreased with the split ratio. Bone nodules were present for up to 18 population doublings. Measurements of nodule area by automated image analysis showed that dexamethasone increased nodule size and that nodule size decreased from primary to 1st to 2nd subculture with or without dexamethasone. The data suggest that dexamethasone selectively stimulates the proliferation of osteoprogenitor cells and that these progenitor cells have a limited capacity for generating daughter cells capable of expressing the bone phenotype.