Cytotoxicity of fluoride

Inhibitory effects of NaF on mitochondrial energy generation in human platelets in vitro

Background

fluoride is a beneficial ion that has been used in various fields, from industrial products to therapeutics. However, due to its narrow therapeutic index, fluoride sometimes acts as a toxic agent at relatively higher concentrations in the human body. Based on the interest in genetic stability, its cytotoxic effects have been investigated mainly in nucleated, adherent cells, such as fibroblasts. However, the sensitivity of blood cells, especially anucleate platelets, to fluoride is poorly understood. To fill this gap in the literature, we investigated the effects of relatively low levels of fluoride on platelet energy metabolism, function, and viability.

Methods

Platelet-rich plasma (PRP) was prepared from 15 non-smoking healthy male adults (age: 28-63) and treated with NaF (0.5 or 1.0 mM) in microtubes for up to 3 days. Platelet function was evaluated based on aggregation and adhesion activities. Platelet energy metabolism was evaluated based on intracellular ATP levels, extracellular lactate levels, and respiration activities. The mitochondrial membrane potential (Em) and localization of reactive oxygen species (ROS) were visualized using cytochemical methods. Platelet viability was evaluated by cell counting and tetrazolium reduction.

Result

NaF (1 mM) significantly reduced platelet viability and inhibited functions. Behind these phenomena, NaF substantially decreased mitochondrial Em and increased ROS production along with significant decreases in oxygen consumption and ATP levels. Simultaneously, NaF increased the lactate levels. Although not statistically significant, similar effects were observed at 0.5 mM NaF.

Conclusion

At relatively low levels, NaF has the potential to attenuate platelet function probably primarily through the inhibition of mitochondrial energy generation. Cytotoxicity may be directly related to ROS production. These findings suggest that when used topically, for example, for caries prevention in the oral cavity, NaF could interfere with wound healing and tissue regeneration by endogenous and exogenously added platelets in the form of PRP.

Fluoride resistance capacity in mammalian cells involves complex global gene expression changes

Fluorine is a bone-seeking element ubiquitously present in the environment and widely used in many oral hygiene products. In humans, excessive intake of fluoride may cause dental and skeletal fluorosis. However, endemic fluorosis does not appear to develop in a proportion of individuals exposed to the same levels of fluoride. The mechanisms by which mammalian cells resist fluoride are still unclear. In this study, we developed strains of mouse L-929 cells resistant to different levels of fluoride. High-throughput RNA-sequencing analyses of the fluoride-resistant L-929 cells indicated that massive changes in global gene expression occurred, compared with the wild-type L-929 cells. The main biological processes and functions changed were associated with the extracellular region and matrix, response to stress, receptor binding, and signal transduction. This indicated that high doses of fluoride not only exerted stress on L-929 cells but also induced functional pathways that helped them adapt to the presence of fluoride or to expel it. These data should prove useful in identifying cellular processes or transporters/channels that play central roles in adaptation to or expulsion of fluoride in humans.

Effect of Sodium Fluoride on the Proliferation and Gene Differential Expression in Human RPMI8226 Cells

Although fluoride is known to reduce the incidence of caries, chronic excessive fluoride exposure can impair human health, even resulting in fluorosis. Now the underlying mechanisms of fluoride-induced toxicity are not fully understood. So, we conducted this study with the purpose of investigating the effect of sodium fluoride (NaF) in human RPMI8226 cells. In this experiment, human RPMI8226 cells were cultured with varied doses of fluoride (10, 20, 40, 80, 160, 320 μM). After 48 h exposure, the change of cell viability was examined by CCK-8 assay, and also the messenger RNA (mRNA) expression of relevant genes was assessed by QRT-PCR. Compared to the control group, fluoride exposure increased the human RPMI8226 cells viability at relatively lower levels (10-160 μM); however, when the concentration reached to 320 μM, the cell proliferation was significantly inhibited (p < 0.05). In addition, the genes mRNA expression, including ANKRD1, CRSP6, KLF2, SBNO2, ZNF649, FANCM, PDGFA, RNF152, CDK10, and CETN2 changed in a concentration-dependent manner and increased with fluoride exposure concentration. The results suggest that overexposure to fluoride (160-320 μM) can induce cytotoxicity and regulate relevant genes expression. Our findings provide novel insights into the mechanisms of action of fluoride-induced toxicity.

Effects of fluoride on hepatic antioxidant system and transcription of Cu/Zn SOD gene in young pigs

Thirty-two barrows (Duroc x Landrace x Yorkshire) were randomly divided into four groups, each of which included eight pigs. The groups received the same basal diet supplemented with 0, 100, 250 and 400mg/kg fluoride, respectively. The malondialdehyde (MDA) and glutathione (GSH) levels, antioxidant enzymes activities and zinc/copper superoxide dismutase (Cu/Zn SOD) mRNA content in the liver were determined to evaluate the fluoride hepatic intoxication. Results showed the increased lipid peroxides (LPO) level and the reduced GSH content, along with a concomitant decrease in the activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px). Moreover, the level of hepatic Cu/Zn SOD mRNA was also significantly reduced. We suggest the mechanism of fluoride injuring the liver as follows: fluoride causes a decrease in Cu/Zn SOD mRNA and the reduced activities of antioxidant enzymes, leads to the declined ability of scavenging free radicals with excessive production of LPO, which seriously damages the hepatic structure and function.

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.

Cytotoxicity and apoptosis induction of sodium fluoride in human promyelocytic leukemia (HL-60) cells

The role of sodium fluoride (NaF) in cytotoxicity and induction of apoptosis was investigated by treating human promyelocytic leukemia (HL-60) cells with varying concentrations of NaF, from 0 to 250 ppm for different periods (0-72 h). At lower concentrations (0-50 ppm), no significant cytotoxicity was observed in response to NaF treatment. However, at higher concentrations (100-250 ppm), NaF reduced cell viability, and decreased DNA and protein biosynthesis capability in cultured HL-60 cells. The growth inhibitory and antiproliferative effects of NaF appear to be attributable to its induction of apoptotic cell death, as NaF induced morphological changes, internucleosomal DNA fragmentation, and increased the proportion of hypodiploid cells. NaF treatment also gradually decreased the expression of the anti-apoptotic protein Bcl-2, and increased activation of caspase-3 and cleavage of poly (ADP-ribose) polymerase. These results provides important information towards understanding the mechanism by which NaF mediates cytotoxicity and apoptosis.

The distribution of stimulated saliva in children

OBJECTIVES

The need for an adequate quantity of saliva to maintain oral health has been widely reported. It is not only the quantity that is important, but also how the saliva is distributed once it enters the mouth. Several studies have looked at how saliva is distributed around adult mouths. The aim of this experiment was to describe the distribution of stimulated saliva around the mouth in children.

SAMPLE AND METHODS

In order to demonstrate the distribution of saliva around the mouth under stimulated conditions, 25 child subjects were asked to chew a piece of chewing gum containing 1.5 mg erythrosin for 3 min on one side of the mouth. The distribution of dye in the mouth was recorded on standardized charts.

RESULTS

In all cases, dye was present ipsilateral to the chewing side. In 13 cases (52%) the dye did not cross the midline and of the remaining 12 children the dye only reached the contralateral canines in four of them (16%).

CONCLUSIONS

Evidence is presented which suggests that during unilateral chewing in children, saliva is not well distributed around the mouth and has a tendency to stay on the side of the mouth from where it was secreted (P < 0.01).

Permeability changes of dentine treated with titanium tetrafluoride

OBJECTIVES

The aim of this research was to evaluate the effect of titanium tetrafluoride on dentine permeability in comparison with sodium fluoride and acidulated phosphate fluoride to determine their contribution to acid resistance of dentine.

METHODS

Thirty-two dentine discs were prepared from extracted mandibular molars. The hydraulic conductance of all discs was measured before and after the formation of smear layer. The discs were then randomly divided into five experimental and one-control groups. Coronal surfaces of dentine discs were treated with either fluoride solutions of NaF, APF, 1% TiF4, 0.5% TiF4 and 0.1% TiF4 or de-ionised water. Following the measurement of hydraulic conductance, treated surfaces were subjected to 25% citric acid application. Then, final permeability measurements were made. The data were statistically analysed using ANOVA and Tukey's HSD multiple comparisons.

RESULTS

While smear layer formation considerably reduced dentinal permeability of dentine discs, fluoride or de-ionised water application to smeared surfaces did not cause any significant change in hydraulic conductance (p > 0.05). After citric acid application, control and NaF groups showed higher permeability values than 1%, 0.5% and 0.1% TiF4 and APF groups (p < 0.01).

CONCLUSION

Clinical use of acidic solutions of titanium tetrafluoride in dentine cavities may be considered since smeared dentine surfaces are modified to a stabile and acid-resistant state.

Chromosome aberrations in cultured rat bone marrow cells treated with inorganic fluorides

The genotoxic effects of inorganic fluorides were investigated by treating cultured rat bone marrow cells with varying concentrations (0.1-100 microM) of potassium fluoride (KF) and sodium fluoride (NaF) for different durations (12, 24 and 36 h) and measuring the incidence of cells with aberrations and number of breaks per cell. Both forms of fluoride were found to be weak mutagens relative to the positive control N-methyl-N-nitro-N-nitrosoguanidine (MNNG). A specificity of fluoride ion in inducing chromosome aberrations (CA) was indicated by the observation that both NaF and KF behaved almost equivalently in this study and at significantly higher variations from the results with potassium chloride (KCl) and sodium chloride (NaCl).

Aluminofluoride- and epidermal growth factor-stimulated DNA synthesis in MOB 3-4-F2 cells

In attempt to study the mechanism of F(-)-induced, osteoblast-mediated bone formation, we tried to show the characteristics of Al-F complex-induced mitogenesis in osteoblastic cells. The MOB 3-4-F2 cell line, an osteoblast-like cell line derived from neonatal mouse calvaria, responded to F- (1-2 mM) combined with Al3+ and epidermal growth factor (EGF, 0.01-100 ng/ml) with increased DNA synthesis. Of the several types of Al-F complexes, AlF4- is thought to act as a mitogenic factor. On the other hand, NaF at high concentrations (greater than 2 mM) markedly decreased cell viability. The AlF(4-)-stimulated DNA synthesis at least with a delay of 48 hr, while EGF stimulated DNA synthesis within a few hours (4-6 hr). Both 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) and staurosporine, inhibitors of protein kinase C (PKC), further enhanced DNA synthesis in AlF(4-)-treated cells, whereas 12-O-tetradecanoyl-13-acetate (TPA), an activator of PKC, decreased the DNA synthesis. In EGF-treated cells, staurosporine and TPA, but not H-7, decreased DNA synthesis. In addition, indomethacin, an inhibitor of cyclooxygenase, partly inhibited the EGF-induced mitogenesis, which, however, was restored by addition of PGE2. AlF4-, as well as EGF, stimulated the release of arachidonic acid and its metabolites. Indomethacin failed to inhibit the AlF(4-)-induced mitogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium fluoride-induced chromosome aberrations in different stages of the cell cycle: a proposed mechanism

In an attempt to clarify the controversy about sodium fluoride (NaF) clastogenicity, the induction of chromosome aberrations in Chinese hamster ovary cells (CHO) by NaF was investigated. Following a protocol used for screening chemicals for clastogenic activity, significant increases of aberrant cells were observed when cells were exposed to NaF for 4 h and harvested 8 h later. Cell-cycle kinetic studies demonstrated most cells were exposed in G2 of the cell cycle. Smaller increases in aberrant cells were observed when cells were harvested 20 h later (most cells were exposed in G1/S). The sensitivity of G2 cells to NaF was investigated further, along with the induction of aberrations at low doses. The results indicated that G2 cells are sensitive to NaF and the percent of aberrant cells increased with dose and length of exposure. With a 3-h exposure until harvest, no statistically significant increase in aberrant cells was observed at doses below 10 micrograms/ml NaF. These data are consistent with a threshold for NaF-induced clastogenicity around 10 micrograms/ml, as has been proposed previously (Scott and Roberts, 1987). It thus may be predicted that clastogenic effects would not occur in humans exposed to the levels of fluoride that are present in drinking water or dentifrices. An understanding of the mechanism of NaF-induced clastogenicity would help to clarify this point. It has previously been reported that NaF inhibits DNA synthesis/repair. The types of aberrations, mostly deletions and gaps, the induction of endoreduplicated cells, the cell-cycle delay and the sensitivity of G2 cells to NaF observed are similar to that reported in the literature for DNA synthesis/repair inhibitors like aphidicolin (APC). Similarities in the induction of aberrations by NaF and APC were confirmed in experiments with G2 cells. Based on these results and those previously reported for NaF and APC, it is proposed that NaF-induced aberrations may occur by an indirect mechanism involving the inhibition of DNA synthesis/repair.

Topical fluorides: a discussion of risks and benefits

Inadvertent ingestion of fluoride from topical procedures poses the risk of dental fluorosis in age-susceptible users. This risk can be minimized for mouthrinsing by restricting it to those above the age-susceptible limit. Operators administering topical fluoride treatments can take practical precautions that have been shown to reduce the amount of fluoride retained and, thus, reduce ingestion. Clinical and epidemiological studies have failed to establish that dentifrice ingestion by young children constitutes a serious fluorosis risk. Discussion of risk must consider the effect of perceived risks upon the public.

Effects of sodium fluoride on uptake of T-2 mycotoxin in cultured cells

We examined the effect of sodium fluoride on uptake of tritium-labeled T-2 toxin (molecules of toxin/cell) in Chinese hamster ovary (CHO) and African green monkey kidney (VERO) cells. Correlations were made to temperature (22 and 37 degrees C) and toxin concentration (0.001 and 0.01 microgram/ml) over time (0-180 min). As expected, toxin uptake increased in both cell types with increasing time and temperature. VERO cells exhibited significant (P less than 0.05) increases in the rate (i.e. slope) of toxin uptake under all parameters, while the rate of toxin uptake in both cell types was generally greater at 37 degrees C compared to 22 degrees C. The rate of equilibrium was affected by both temperature and sodium fluoride. At 37 degrees C toxin uptake plateaued by 30 min in the presence of sodium fluoride. At 22 degrees C the rate of toxin uptake was slower, with or without sodium fluoride present. Statistical analysis of individual time points along the curve demonstrated that sodium fluoride significantly increased cell-associated toxin at most time points. Analysis of the slopes of uptake curves from 0 to 20 min indicated significant (P less than 0.05) differences in the rates of T-2 uptake in both cell types and toxin doses in the presence of sodium fluoride. The increase in toxin uptake in the presence of sodium fluoride was not due to altered cell membrane permeability caused by sodium fluoride. This study demonstrates that sodium fluoride significantly increases cell-associated T-2 toxin and the rate of toxin uptake in two cultured cell lines.

Distribution and retention of salivary fluoride from a sodium fluoride tablet following various intra-oral dissolution methods

The distribution and retention of salivary fluoride were investigated according to the following intra-oral dissolution methods of a sodium fluoride tablet: active chewing/swishing, active sucking/swishing, and passive dissolution in the maxillary and mandibular labial vestibule. The results suggested that the oral cavity was somewhat compartmentalized, in that homogeneous distribution of a dissolved fluoride tablet did not occur throughout the mouth, but rather each site sampled was influenced by various rates of clearance and retention. Tablet chewing may have an advantage over tablet sucking in terms of salivary fluoride retention, and there was a tendency for fluoride to be preferentially retained in the maxillary labial vestibule following swishing. The passive tablet dissolution methods resulted in a less homogeneous distribution of fluoride. Although the fluoride retention values for passive dissolution were impressively superior to either active dissolution method, the alarmingly high salivary fluoride concentrations (4000 ppm F) recorded at the passive dissolution sites should discourage its use until the potential cytotoxicity of this approach is thoroughly investigated.

Oxygen-independent killing by alveolar macrophages

We have found that normal alveolar macrophages can kill an intracellular parasite by a mechanism that does not involve toxic metabolites of oxygen. We studied the interaction between Toxoplasma gondii and rat alveolar macrophages in vitro. We were interested in Toxoplasma because it causes pneumonia in immunosuppressed patients but not in healthy individuals, and we chose the rat because it resembles immunocompetent human subjects in being resistant to T. gondii. Resident rat alveolar macrophages could kill large numbers of T. gondii. This occurred without a respiratory burst as judged by intracellular reduction of nitroblue tetrazolium and quantitative release of superoxide. Furthermore, scavengers of toxic oxygen metabolites had no effect on the toxoplasmacidal activity of the alveolar macrophages, nor did prior exhaustion of their respiratory burst with PMA. Whereas acid pH (e.g., 4.5-6.0) rapidly kills extracellular T. gondii, raising of the intralysosomal acid pH of rat alveolar macrophages by incubating them with weak bases did not inhibit their ability to kill T. gondii. Killing of Toxoplasma occurred within 1 h of initial exposure to the alveolar macrophages. However, there was no evidence that killing preceded ingestion; Toxoplasma attached to the surface of the cell appeared viable, and when phagocytosis was blocked with sodium fluoride the organisms survived. These results indicate that rat alveolar macrophages possess a powerful nonoxidative microbicidal mechanism, which is distinct from acidification of the phagolysosome but which probably involves phagosome formation. This mechanism may be clinically relevant, for we have recently observed that human alveolar macrophages also kill T. gondii by an oxygen-independent process.

The effects of chronic high fluoride levels on forming enamel in the rat

Sixty-gramme rats were given either 0, 75, 100 or 150 parts/10(6) fluoride in their drinking water. After five weeks, the fluoride, the phosphorus and the protein contents of the enamel were compared in control and experimental animals at three stages of enamel development. The mineral content was reduced in pigmented enamel from animals given 75 parts/10(6) or more fluoride in their drinking water. The fluoride content was elevated in all stages of fluorosed enamel development. At the lowest fluoride level (75 parts/10(6], a larger proline content was found in the proteins of the maturing, fluorosed enamel but there was no increase in the protein content. In animals given 100 parts/10(6) fluoride in their drinking water, the proline content of the protein was greater in maturing, fluorosed enamel, and the total protein content of the post-secretory enamel (maturing and pigmented) was greater than in the controls. These observations indicate that, with increasing levels of fluoride in drinking water, there was an initial delay in the loss of the amelogenin proteins followed by a decreased removal of total protein from the enamel. These results indicate that fluoride interfered with the normal post-secretory, pre-eruptive development of enamel.

Reduced fluoride sensitivity of liver cells from rats chronically exposed to fluoride

The fluoride sensitivity, determined as effect on protein synthesis (incorporation of 14C-leucine), of liver and kidney cells in suspension culture was exposed. The cells were freshly prepared by collagenase perfusion from rats given drinking water with or without addition of 100 p.p.m. (5.26 mM) fluoride for 9-28 weeks. The fluoride sensitivity of the liver cells from rats given fluoride sensitivity of the kidney cells from fluoride exposed and control rats appeared similar. Fluoride resistance (i.e. decreased sensitivity) may thus develop also in cells in vivo. When exposed to 3 mM NaF for 1 hour the intracellular concentration of fluoride in liver cells from fluoride exposed and controls animals were similar.