Effect of Calcium Supplementation and TMEM16A Inhibition on Endoplasmic Reticulum Stress Induced by Dental Fluorosis in Mice

BACKGROUND

Dental fluorosis is a discoloration of the teeth caused by the excessive consumption of fluoride. It represents a distinct manifestation of chronic fluorosis in dental tissues, exerting adverse effects on the human body, particularly on teeth. The transmembrane protein 16a (TMEM16A) is expressed at the junction of the endoplasmic reticulum and the plasma membrane. Alterations in its channel activity can disrupt endoplasmic reticulum calcium homeostasis and intracellular calcium ion concentration, thereby inducing endoplasmic reticulum stress (ERS). This study aims to investigate the influence of calcium supplements and TMEM16A on ERS in dental fluorosis.

METHODS

C57BL/6 mice exhibiting dental fluorosis were subjected to an eight-week treatment with varying calcium concentrations: low (0.071%), medium (0.79%), and high (6.61%). Various assays, including Hematoxylin and Eosin (HE) staining, immunohistochemistry, real-time fluorescence quantitative polymerase chain reaction (qPCR), and Western blot, were employed to assess the impact of calcium supplements on fluoride content, ameloblast morphology, TMEM16A expression, and endoplasmic reticulum stress-related proteins (calreticulin (CRT), glucose-regulated protein 78 (GRP78), inositol requiring kinase 1α (IRE1α), PKR-like ER kinase (PERK), activating transcription factor 6 (ATF6)) in the incisors of mice affected by dental fluorosis. Furthermore, mice with dental fluorosis were treated with the TMEM16A inhibitor T16Ainh-A01 along with a medium-dose calcium to investigate the influence of TMEM16A on fluoride content, ameloblast morphology, and endoplasmic reticulum stress-related proteins in the context of mouse incisor fluorosis.

RESULTS

In comparison to the model mice, the fluoride content in incisors significantly decreased following calcium supplements (p < 0.01). Moreover, the expression of TMEM16A, CRT, GRP78, IRE1α, PERK, and ATF6 were also exhibited a substantial reduction (p < 0.01), with the most pronounced effect observed in the medium-dose calcium group. Additionally, the fluoride content (p < 0.05) and the expression of CRT, GRP78, IRE1α, PERK, and ATF6 (p < 0.01) were further diminished following concurrent treatment with the TMEM16A inhibitor T16Ainh-A01 and a medium dose of calcium.

CONCLUSIONS

The supplementation of calcium or the inhibition of TMEM16A expression appears to mitigate the detrimental effects of fluorosis by suppressing endoplasmic reticulum stress. These findings hold implications for identifying potential therapeutic targets in addressing dental fluorosis.

ERS Mediated by GRP-78/PERK/CHOP Signaling Is Involved in Fluoride-Induced Ameloblast Apoptosis

Fluoride can be widely ingested from the environment, and its excessive intake could result in adverse effects. Dental fluorosis is an early sign of fluoride toxicity which can cause esthetic and functional problems. Though apoptosis in ameloblasts is one of the potential mechanisms, the specific signal cascade is in-conclusive. High-throughput sequencing and molecular biological techniques were used in this study to explore the underlying pathogenesis of dental fluorosis, for its prevention and treatment. A fluorosis cell model was established. Viability and apoptosis rate of mouse ameloblast-derived cell line (LS8 cells) was measured using cell counting kit-8 (CCK-8) assay and flow cytometry analysis. Cells were harvested with or without 2-mM sodium fluoride (NaF) stimulation for high-throughput sequencing. Based on the sequencing data, subcellular structures, endoplasmic reticulum stress (ERS), and apoptosis related biomarkers were verified using transmission electron microscopy, quantitative real-time polymerase chain reaction, and Western blotting techniques. Expression of ERS markers, apoptosis related proteins, and enamel formation enzymes were detected using Western blotting after addition of 4-phenylbutyrate (4-PBA). NaF-inhibited LS8 cells displayed time- and dose- dependent viability. Additionally, apoptosis and morphological changes were observed. RNA-sequencing data showed that protein processing in endoplasmic reticulum was obviously affected. ERS and apoptosis were induced by excessive NaF. Downregulation of kallikrein-related peptidase 4 (KLK4) was also observed. Inhibition of ERS by 4-PBA rescued the apoptotic and functional protein changes in cells. Excessive fluoride induces apoptosis by activating ERS, which is mediated by GRP-78/PERK/CHOP signaling. Key proteinase is present in maturation-stage enamel; KLK4 was also affected by fluoride, but rescued by 4-PBA. This study presents a possibility for therapeutic strategies for dental fluorosis, while further exploration is required.

Role of Glycolysis/Gluconeogenesis and HIF-1 Signaling Pathways in Rats with Dental Fluorosis Integrated Proteomics and Metabolomics Analysis

Fluoride is widely distributed, and excessive intake will lead to dental fluorosis. In this study, six offspring rats administrated 100 mg/L sodium fluoride were defined as the dental fluorosis group, and eight offspring rats who received pure water were defined as the control group. Differentially expressed proteins and metabolites extracted from peripheral blood were identified using the liquid chromatography tandem mass spectrometry and gas chromatography mass spectrometry, with the judgment criteria of fold change >1.2 or <0.83 and p < 0.05. A coexpression enrichment analysis using OmicsBean was conducted on the identified proteins and metabolites, and a false discovery rate (FDR) < 0.05 was considered significant. Human Protein Atlas was used to determine the subcellular distribution of hub proteins. The Gene Cards was used to verify results. A total of 123 up-regulated and 46 down-regulated proteins, and 12 up-regulated and 2 down-regulated metabolites were identified. The significant coexpression pathways were the HIF-1 (FDR = 1.86 × 10−3) and glycolysis/gluconeogenesis (FDR = 1.14 × 10−10). The results of validation analysis showed the proteins related to fluorine were mainly enriched in the cytoplasm and extrinsic component of the cytoplasmic side of the plasma membrane. The HIF-1 pathway (FDR = 1.01 × 10−7) was also identified. Therefore, the HIF-1 and glycolysis/gluconeogenesis pathways were significantly correlated with dental fluorosis.

Progress of Signaling Pathways, Stress Pathways and Epigenetics in the Pathogenesis of Skeletal Fluorosis

Fluorine is widely dispersed in nature and has multiple physiological functions. Although it is usually regarded as an essential trace element for humans, this view is not held universally. Moreover, chronic fluorosis, mainly characterized by skeletal fluorosis, can be induced by long-term excessive fluoride consumption. High concentrations of fluoride in the environment and drinking water are major causes, and patients with skeletal fluorosis mainly present with symptoms of osteosclerosis, osteochondrosis, osteoporosis, and degenerative changes in joint cartilage. Etiologies for skeletal fluorosis have been established, but the specific pathogenesis is inconclusive. Currently, active osteogenesis and accelerated bone turnover are considered critical processes in the progression of skeletal fluorosis. In recent years, researchers have conducted extensive studies in fields of signaling pathways (Wnt/β-catenin, Notch, PI3K/Akt/mTOR, Hedgehog, parathyroid hormone, and insulin signaling pathways), stress pathways (oxidative stress and endoplasmic reticulum stress pathways), epigenetics (DNA methylation and non-coding RNAs), and their inter-regulation involved in the pathogenesis of skeletal fluorosis. In this review, we summarised and analyzed relevant findings to provide a basis for comprehensive understandings of the pathogenesis of skeletal fluorosis and hopefully propose more effective prevention and therapeutic strategies.

Elemental Status and Lipid Peroxidation in the Blood of Children with Endemic Fluorosis

The study aimed to assess the levels of trace elements, minerals, and toxic elements as well as lipid peroxidation biomarkers (lipid acyl hydroperoxides, 2-thiobarbituric acid reactive substances (TBARS)) in the blood of children with chronic fluorosis from endemic fluorosis areas (Sosnivka village, Lviv region, western Ukraine). The results were compared with healthy children from Staryi Sambir (Lviv region, western Ukraine), whose drinking water contained permissible levels (< 1 ppm) of fluoride. Thirty-one children from the Sosnivka village in the Lviv region, including 16 females and 15 males aged 7-10 years, with clinically diagnosed fluorosis, were recruited for the study. The children had been exposed to fluoride (> 1.5 ppm) through drinking water for more than 5 years. In the blood, eight macro- and microelements (calcium, zinc, potassium, iron, copper, selenium, manganese, chromium), five additional elements (sulfur, bromine, chlorine, nickel, strontium), and four toxic elements (lead, mercury, cadmium, mercury) were assessed with the X-ray fluorescence method. The results of our study demonstrated a 14-fold decrease in the copper level, a 2.5-fold decrease in the calcium and zinc levels, and a 2-fold decrease in the selenium level in the blood of children with chronic fluorosis compared with the healthy children from the non-fluorosis area. In turn, a 1.7- and 1.4-fold increase in the strontium and lead content, respectively, was noted. The sulfur, chlorine, potassium, calcium, copper, zinc, and selenium levels in the blood samples of children with chronic fluorosis were lower than the reference value. The children had higher blood TBARS levels, while the acyl hydroperoxide levels were non-significantly increased in comparison with healthy children living in the non-fluorosis area. Additionally, the bromine level was correlated positively with the selenium level and acyl hydroperoxides. However, more studies are needed to clarify the relationship between blood mineral status, oxidative stress biomarkers, and chronic fluorosis.

Protections against toxicity in the brains of rat with chronic fluorosis and primary neurons exposed to fluoride by resveratrol involves nicotinic acetylcholine receptors

Protection of Resveratrol (RSV) against the neurotoxicity induced by high level of fluoride was investigated. Sprague-Dawley (SD) rats and their offspring, as well as cultures of primary neurons were divided randomly into four groups: untreated (control); treated with 50 mg RSV/kg/ (once daily by gavage) or (20 M in the cultured medium); exposed to 50 ppm F^- in drinking water or 4 mmol/l in the cultured medium; and exposed to fluoride then RSV as above. The adult rats were treated for 7 months and the offspring sacrificed at 28 days of age; the cultured neurons for 48 h. For general characterization, dental fluorosis was assessed and the fluoride content of the urine measured (by fluoride-electrode) in the rates and the survival of cultured neurons monitored with the CCK-8 test. The spatial learning and memory of rats were assessed with the Morris water maze test. The levels of α7 and α4 nicotinic acetylcholine receptors (nAChRs) were quantified by Western blotting; and the activities of superoxide dismutase (SOD) and catalase (CAT), and the levels of malondialdehyde (MDA) and H₂O₂ assayed biochemically. The results showed that chronic fluorosis resulted in the impaired learning and memory in rats and their offspring, and more oxidative stress in both rat brains and cultured neurons, which may be associated the lower levels of α7 and α4 nAChR subunits. Interestingly, RSV attenuated all of these toxic effects by fluorosis, indicating that protection against the neurotoxicity of fluoride by RSV might be in mechanism involved enhancing the expressions of these nAChRs.

Fluoride exposure alters Ca2+ signaling and mitochondrial function in enamel cells

Fluoride ions are highly reactive, and their incorporation in forming dental enamel at low concentrations promotes mineralization. In contrast, excessive fluoride intake causes dental fluorosis, visually recognizable enamel defects that can increase the risk of caries. To investigate the molecular bases of dental fluorosis, we analyzed the effects of fluoride exposure in enamel cells to assess its impact on Ca2+ signaling. Primary enamel cells and an enamel cell line (LS8) exposed to fluoride showed decreased internal Ca2+ stores and store-operated Ca2+ entry (SOCE). RNA-sequencing analysis revealed changes in gene expression suggestive of endoplasmic reticulum (ER) stress in fluoride-treated LS8 cells. Fluoride exposure did not alter Ca2+ homeostasis or increase the expression of ER stress-associated genes in HEK-293 cells. In enamel cells, fluoride exposure affected the functioning of the ER-localized Ca2+ channel IP3R and the activity of the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) pump during Ca2+ refilling of the ER. Fluoride negatively affected mitochondrial respiration, elicited mitochondrial membrane depolarization, and disrupted mitochondrial morphology. Together, these data provide a potential mechanism underlying dental fluorosis.

Condition of mineralized tooth tissue in a population of 15-year-old adolescents living in a region of Ukraine with slightly exceeded fluorine concentration in the water

INTRODUCTION

Fluorine is a common element in nature; however, the difference between a beneficial dose and a toxic dose for the organisms is small. The main source of fluoride for humans is water in addition to food.

OBJECTIVE

The aim of this study was to estimate the degree of severity of pathological changes, namely, caries or fluorosis, in the mineralized tooth tissue of 15-year-old adolescents with respect to their hygienic and nutritional habits, and the content of fluorine in drinking water, soil and plant products.

MATERIAL AND METHODS

100 adolescents aged 15 from schools in Sosniłka, L'viv region, were examined. The condition of mineralized tooth tissue was evaluated by the caries severity, expressed by the mean number D3MFT, caries frequency and value of the SIC index. Fluorine in plant material and soil were determined according to the PN-G-04543:1982 standard, and water according to the PN-EN ISO 10304 - 1: 2009+ AC: 2012 standard.

RESULTS

Severity of caries disease expressed by the D3MFT number in the examined group of 15-year-olds was 3.39; in the group of girls - 3.08, and in the group of boys - 3.76. In the examined group, the average number of teeth with fluorosis was 7.59. Value of the SIC index among the examined population of students (n-31) was 6.26: 5.89 (n-18) for girls and 7.31 (n-13) for boys. Fluorine concentration in the water was 0.78 - 1.25(mg·dm-3). In the soil, it also did not vary across the sampled areas and amounted, on average, to 176 mg·dm-3. The biggest fluorine content noted in the dry mass of beetroots was 3.50 (mg F· kg), and the lowest - 3.34 (mg F· kg).

CONCLUSIONS

Close to optimal fluorine content lowers caries severity and frequency of fluorosis. Optimal fluorine content in drinking water and food does not require additional diet supplementation.

Effects of drinking water fluorosis on L-type calcium channel of hippocampal neurons in mice

The study aimed to investigate the effects of drinking water fluorosis on L-type calcium channels (LTCCs) in mouse hippocampal neurons. A total of 60 newly weaned ICR male mice were randomly divided into control, low fluoride and high fluoride groups. After 3 and 6 months of exposure to fluoride, the patch clamp technique was used to detect the peak and relative values (I/Imax), steady-state activation curve ratio (G/Gmax), decay time constant, and tail current time constant of LTCCs currents in hippocampal CA1 region of mouse brain slices. Fluoride greatly reduced the serum and urinary calcium concentrations in mice, and the chronic fluorosis has a greater impact than subchronic fluorosis. The peak value of LTCCs current in pyramidal neurons of hippocampal CA1 area was significant and increased with the prolonged exposure time. The relative values of current and steady-state coefficients were changed greatly. The decay and tail current time increased significantly. High fluorine concentration indicates great peak value and open time of LTCCs opening. LTCCs are sensitive to fluoride exposure. The activation voltage of calcium channels induced by fluoride exposure is decreased, the opening time of calcium channels is prolonged, and the calcium influx per unit time increased, thereby overloading calcium concentration in neurons and this may be an explanation for intracellular calcium overload caused by fluoride. The imbalance of calcium metabolism caused by fluorosis may be a pathogenesis of brain injury induced by fluoride. Furthermore, the risk of brain damage from low-fluorine exposure cannot be ignored.

Fluoride, Thyroid Hormone Derangements and its Correlation with Tooth Eruption Pattern Among the Pediatric Population from Endemic and Non-endemic Fluorosis Areas

AIM

To comparatively evaluate the status of fluoride in the body with thyroid activity in the pediatric population of endemic fluorosis areas. The present study also attempted to elucidate whether any correlation exists between fluoride and thyroid hormone derangement with delayed tooth eruption.

MATERIALS AND METHODS

A total of 400 pediatric subjects were included in the present study. All the patients were divided into two broad groups; groups A and B. Group A included 200 subjects who belonged to the endemic fluorosis area while Group B included remaining 200 subjects, who belonged to the fluorosis non-endemic area. Group B subjects were taken as control. Group A subjects were further divided into two study groups as follows: Group A1: 100 Pediatric subjects with dental fluorosis, and Group A 2: A total of one hundred pediatric subjects without dental fluorosis. Dean's index of fluorosis was calculated in all the patients. Blood samples were collected and were sent to a laboratory for assessment of thyroid hormone levels. All the results were subjected to statistical analysis by Statistical Package for the Social Sciences (SPSS) software.

RESULTS

Mean thyroid stimulating hormone (TSH), water fluoride levels, urine fluoride levels and serum fluoride levels of subjects in group 1 were found to be significantly higher than that of subjects of group 2. Delayed tooth eruption was absent in subjects of group B while it was present in 100 subjects of group A. Thyroid hormone level derangement was seen in 54 percent subjects of group B, while it was seen in 67.5% subjects of group A.

CONCLUSION

Positive correlation exists between fluorosis and thyroid functional activity. However; the tooth eruption pattern is independent up on the thyroid hormone derangement.

CLINICAL SIGNIFICANCE

Delayed tooth eruption and alteration in thyroid hormone levels can occur in subjects of the endemic fluoride areas. Therefore, adequate measures should be taken for controlling such adverse effects.

The Effect of Chronic Fluorosis on Calcium Ions and CaMKIIα, and c-fos Expression in the Rat Hippocampus

This study investigated neurotoxicity of chronic fluorosis in the rat hippocampus. Newly weaning, male, Sprague-Dawley (SD) rats were administered 15, 30, and 60 mg/L sodium fluoride (NaF) solution (fluorine ion concentration 8.25, 16.50, and 33.00 mg/L, respectively), and tap water, for 18 months. The neurotoxicological mechanism was examined with a focus on intracellular calcium overload. Results showed that as the fluoride concentration increased, calcium ion concentration [Ca²⁺], the expression of calcium/calmodulin-dependent protein kinase II α (CaMKIIα), and the expression of catus proto-oncogene protein c-fos (c-fos) all tend to increase. Compared to the control group, Ca²⁺, CaMKIIα, and c-fos significantly increased (P < 0.05) in the moderate-fluoride and the high-fluoride groups. These results indicate that Ca²⁺/CaMKIIα/c-fos channel signal may be the molecular mechanism of central nervous system damage caused by chronic fluoride intoxication. Moreover, elevated Ca²⁺ concentration in the hippocampus may be the initiating factor of neuronal apoptosis induced by fluoride.

Dental fluorosis, nutritional status, kidney damage, and thyroid function along with bone metabolic indicators in school-going children living in fluoride-affected hilly areas of Doda district, Jammu and Kashmir, India

A case-control study was undertaken among the school children aged 8-15 years to know the presence and severity of dental fluorosis, nutrition and kidney status, and thyroid function along with bone metabolic indicators in Doda district situated at high altitude where drinking water was contaminated and heat stress. This study included 824 participants with an age of 8-15 years. The results of the study reviled that dental fluorosis was significantly higher in affected than control area children. Urinary fluoride was significantly higher (p < 0.05) in affected children as compared to the control area school children. Nutritional status of affected children was lower than control area children. The chronic kidney damage (CKD) was higher in affected than control school children. Thyroid function was affected more in affected than control area schools. Serum creatinine, total alkaline phosphatase, parathyroid hormone, 1, 25(OH)₂ vitamin D, and osteocalcin were significantly higher in affected school children (p < 0.05) as compared to control school children, whereas there was no significant difference in triiodothyronine (T3), thyroxine (T4), and 25-OH vitamin D among the two groups. There was a significant decrease in thyroid-stimulating hormone (TSH) in the affected area school children compared to control. In conclusion, fluorotic area school children were more affected with dental fluorosis, kidney damage, along and some bone indicators as compared to control school children.

Molecular mechanism of brain impairment caused by drinking-acquired fluorosis and selenium intervention

This study investigated the molecular mechanism of brain impairment induced by drinking fluoridated water and selenium intervention. Results showed that the learning and memory of rats in NaF group significantly decreased. Moreover, the number of apoptotic cells, the expression levels of Cytc mRNA and protein, and the expression levels of Caspase-9 and Caspase-3 mRNA significantly increased; by contrast, Caspase-9 and Caspase-3 protein levels significantly decreased. Compared with the NaF group, the mRNA levels of Cytc and Caspase-9, as well as the protein levels of Cytc in NaF+Se group, significantly decreased. Conversely, the protein levels of Caspase-3 and Caspase-9, as well as the mRNA levels of Caspase-3, significantly increased. Thus, the mitochondrial CytC-Caspase-9-Caspase-3 apoptosis pathway in the hippocampus was one of the mechanisms leading to fluorosis-induced brain damage. Furthermore, the Cytc signaling molecules were possibly the key target molecules in fluorosis-induced apoptosis, and selenium could alleviate fluorosis-induced brain injury.

PI3K-Akt1 expression and its significance in liver tissues with chronic fluorosis

This study was to explore the effect and significance of PI3K signal pathway on mechanism of liver injury in chronic fluorosis. We used 48 Sprague-Dawley rats which were randomly divided into 4 groups according to the body weight, 12 in each group, half of male and female. The control group was fed with the solid feed (the fluorine content was 1.5 mg/kg). The fluorosis animals were fed with the corn containing fluorine content of 17 mg/kg from the endemic fluorosis areas. Blocking agent LY294002 was injected in the blocking group and phosphate buffer solution was injected in the blocking control in the caudal vein with 10 mg/kg once every other day in the one week before the end of the experiment. The animals were drunk by tap water freely. The fluoride contents of urinary and skeletal were determined by the F-ion selective electrode method. The mRNA and protein expressions of PI3K, Akt1 in the liver tissues were determined by real-time polymerase chain reaction, and streptavidin-perosidase and Western blot, respectively. Results showed that fluoride contents of the urine and bone were increased in the fluorosis compared to those in the control. The expression of PI3K and Akt1 mRNA and proteins was significantly increased in fluorosis hepatocytes, and lower than that of the fluorosis in the blocking. The apoptosis and the intracellular calcium concentration were increased. Therefore, we conclude that PI3K-Akt signaling pathway may be one of the signaling pathways in the pathogenesis of liver injury caused by fluorosis.

[Influence of chronic fluorosis on the expression of mitochondrial fission protein dynamin-related 1 in the cortical neurons of rats]

OBJECTIVE

To explore the changes of protein expression of mitochondrial fission gene dynamin-related 1(Drp 1) in the cortical neurons of rats with chronic fluorosis.

METHODS

A total of 120 one-month-old SD rats (each weighing approximately 100-120 g at the beginning of the experiment) were randomly divided into three groups, and fed with the different doses of fluoride containing in drinking water (untreated control containing 0 mg/L fluoride, and low-fluoride & high-fluoride supplemented with 10 and 50 mg/L fluoride,respectively). After 3 or 6 months exposure, 20 rats from each group were killed. Then the protein expression of mitochondrial fission gene, Drp1, was detected by immunohistochemistry and western-blotting method.

RESULTS

Dental fluorosis and urinary fluorosis were obviously found in the rats exposed to fluoride. At the experiment period of 3 months, the numbers of positive cells of Drp1 detected by immunohistochemistry changed. Compared with the control group (36.3 ± 5.8), the changes in low-fluoride group (34.7 ± 4.1) showed no significant difference (t = 1.5, P > 0.05),but the increase in high-fluoride group (45.0 ± 4.7) had statistical significance (t = 8.8, P < 0.05). The western-blotting method had consistent results. Compared with the control group (0.59 ± 0.03), a significant increase of the average topical density in low- fluoride (0.62 ± 0.03) and high-fluoride (0.71 ± 0.02) groups were found (t = 0.02,0.11, P < 0.05). At the experiment period of 6 months, the numbers of positive cells of Drp1 detected by immunohistochemistry significantly changed. Compared with the control group (33.2 ± 4.4), the number in low- fluoride and high-fluoride groups were separately (36.6 ± 3.8) and (39.4 ± 4.2),both increased significantly (t = 3.5,6.3, P < 0.05). Same results could be found in western-blotting method,compared with the control group (0.65 ± 0.06), the average topical density in low- fluoride (0.80 ± 0.09) and high-fluoride (0.76 ± 0.08) groups both increased significantly (t = 0.1,0.1, P < 0.05).

CONCLUSIONS

Taking excessive amount of fluoride might result in the changes of expression of Drp1, and the neurons damage from the chronic fluorosis might be associated with the hyperfunction of mitochondrial fusion.

Excessive fluoride induces endoplasmic reticulum stress and interferes enamel proteinases secretion

Protein retention in the enamel layer during tooth formation is well known to be associated with dental fluorosis but the underlying mechanism is unclear. The functions of the endoplasmic reticulum (ER) correlate directly with secreted protein metabolism. We used an ameloblast-derived cell line to determine whether excessive amounts of fluoride cause ER stress, and whether this interferes with the secretion of enamel matrix proteinases. ER stress activates a signaling network called the unfolded protein response (UPR). Here, we used real-time RT-PCR and immunofluorescence to study the effect of fluoride on the expression, translation, and secretion of UPR transcription factors in ameloblast-like cells. Measurement of both the gene and protein expression of UPR transcription factors indicated that high-dose fluoride increases the expression of UPR transcription factors in a dose-dependent manner. We also used ELISA to detect and quantify the enamel proteinases secreted by ameloblasts. We found a corresponding decrease in extracellular secretion of the enamel proteinases matrix metalloproteinase-20 and kallikrein-4, after exposure to fluoride. Furthermore, correlation analysis indicated that the expression of UPR transcription factors showed a strong inverse correlation with that of enamel proteinases. The results suggest that high-dose fluoride initiates an ER stress response in ameloblasts and induces the UPR, which interferes with the synthesis and secretion of enamel proteinases. Taken together, these results suggest that excessive ingestion of fluoride during tooth formation can decrease the secretion of proteinases, thus causing protein retention in the enamel layer, indicating that the ER stress response may be responsible for dental fluorosis.

Effect of fluoride and low versus high levels of dietary calcium on mRNA expression of osteoprotegerin and osteoprotegerin ligand in the bone of rats

The ratio of osteoprotegerin ligand (OPGL) to osteoprotegerin (OPG) determines the delicate balance between bone resorption and synthesis. The main objective of the present study is to investigate the possible role of OPGL and OPG in the bone metabolism of rats exposed to fluoride and the protective or aggravating effect of calcium (Ca). In a 6-month study, 270 weanling male Sprague-Dawley rats weighing between 70 and 90 g were divided randomly into six groups of 45 rats in each group. Three groups (groups I, III, and V)served as controls and drank deionized water and were fed purified rodent diets containing either 1,000 mg Ca/kg (low Ca), 5,000 mg Ca/kg (normal Ca), or 20,000 mg Ca/kg (high Ca). The three experimental groups (groups II, IV, and VI) were given the same diets but they drank water containing 100 mg F ion/L (from NaF). Every 2 months 15 rats were randomly selected from each group and sacrificed for the study. The ratio of OPGL mRNA to OPG mRNA was significantly increased by the sixth month in the distal femur joints of the F-exposed rats. Serum tartrate-resistant acid phosphatase activity and serum calcitonin activity in the F-exposed groups was increased, although changes were not apparent in the serum alkaline phosphatase or Gla-containing proteins, especially in the low calcium and high calcium diet F-exposed groups. The results indicated that OPG and OPGL may play important roles in skeletal fluorosis, and that fluoride may enhance osteoclast formation and induce osteoclastic bone destruction. A high Ca diet did not play a protective role, but rather may aggravate the damage of fluoride.

Soluble fluoride levels in drinking water-a major risk factor of dental fluorosis among children in Bongo community of Ghana

BACKGROUND

The purpose of the study was to investigate the relationship between fluoride ions in drinking water and the incidence of dental fluorosis in some endemic areas of Bongo District, Ghana.

METHOD

Two hundred children were randomly selected from various homes and taken through a questionnaire. Their teeth were examined for the detection of dental fluorosis using the Dean's specific index. Samples of their permanent sources of water were taken for the determination of soluble fluoride levels by SPADNS spectrophotometric method.

RESULTS

The study revealed that the incidence of dental fluorosis among the children in the main Bongo township was 63.0%, whereas villages outside the township recorded less than 10.0%. The respondents from the various communities had similar age group, educational background, sources of drinking water, oral hygiene habits and usage of oral health products, p-value > 0.05. However, there were statistically significant differences in the cases of dental fluorosis and fluoride ions among the communities, p-value < 0.05. The fluoride ion concentration in the Bongo township was above the WHO requirement of 1.50 ppm, whereas the nearby villages showed acceptable fluoride levels. Statistically, there was no significant relationship between the presence of dental fluorosis and the other characteristics, except the age group and fluoride ion concentration of the area.

CONCLUSION

These findings strongly support the association between the dental fluorosis and the high fluoride levels in the underground water of Bongo community. Therefore, policy makers need to consider an alternative source of drinking water for the area.

[Changed expression of mito-fusion 1 and mitochondrial fragmentation in the cortical neurons of rats with chronic fluorosis]

OBJECTIVE

To observe the mitochondrial fragmentation and the expression of mito-fusion 1 gene in the cortical neurons of rats with chronic fluorosis, and to reveal their roles in mitochondria damage to neurons due to chronic fluorosis.

METHODS

SD rats were divided randomly into three groups of 20 each (a half females and a half males housed individually in stainless-steel cages), and fed with the different doses of fluoride containing in drinking water (untreated control containing 0 mg/L fluoride, and low-fluoride and high supplemented with 10 and 50 mg/L fluoride, respectively). After 3 or 6 months exposure, the mitochondrial morphology of the neurons in rat brains were observed by transmission electron microscopy (TEM), then the expression of mitochondrial fusion gene, Mfn1, were detected by immunohistochemistry and western-blotting, respectively.

RESULTS

Dental fluorosis was obvious in the rats exposed to excessive fluoride in their drinking water, that is, (16 rats out of 20) numbers of I° detal fluorosis in the low-fluoride group, and (11 rats out of 20) numbers of I° and (9 rats out of 20) numbers of II° detal fluorosis in the high-fluoride group were observed after 3 months exposure. Moreover, (14 rats out of 20) numbers of I° and (6 rats out of 20) numbers of II° detal fluorosis in the low-fluoride group and (6 rats out of 20) numbers of Io, (13 rats out of 20) numbers of II°, and (1 rats out of 20) numbers of III° detal fluorosis in the high-fluoride group were observed after 6 months exposure. And both of untreated controls without detal fluorosis were also observed. The urinary level of fluoride in the low-fluoride group (3.30 ± 1.18) mg/L and in the high-fluoride group (5.10 ± 0.35) were observed after 3 months exposure (F = 3.18, P < 0.05). Moreover, the urinary level of fluoride in the low-fluoride group (4.16 ± 1.39) mg/L and in the high-fluoride group (5.70 ± 1.70) mg/L were also observed after 6 months exposure (F = 3.17, P < 0.05). The normal mitochondrial morphology of neurons in rats without fluorosis was observed after 3 and 6 months, while the abnormal mitochondrial morphology of neurons with fluorosis was shown, presenting mitochondrial fragmentation with swollen cristae and even the fragmented, shortened or stacked punctuate membranes (section observation of three bullous mitochondrial-mitochondrial fission process) by TEM. As compared with controls (53.0 ± 4.54 and 1.21 ± 0.18) at the experiment period of 3 months, Mif1 protein analysis with immunocytochemical (the numbers of positive cells: 51.09 ± 6.25) and western-blotting (1.22 ± 0.26) were no significant difference for low fluoride group (t = 1.7, 1.1, P > 0.05); Mif1 protein analysis with immunocytochemical (the numbers of positive cells: 59.71 ± 5.64) and western-blotting (1.66 ± 0.20) were significantly increasing for high fluoride group (t = 2.1, 2.1, P < 0.05). As compared with controls (36.43 ± 4.04 and 1.00 ± 0.13) at the experiment period of 6 months, Mif1 protein analysis with immunocytochemical (the numbers of positive cells 20.05 ± 4.55 and 17.10 ± 3.86) and western-blotting (0.64 ± 0.08 and 0.39 ± 0.06) were significantly decreasing for the two fluoride group (t = 2.1, 2.2; 2.2, 2.2 respectively, all P value were < 0.05).

CONCLUSIONS

Taking excessive amount of fluoride might result in the mitochondrial fragmentation for the changed expression of Mfn1, and the neurons damage from the chronic fluorosis might be associated with the dysfunction of mitochondrial fusion.

Renal proteome in mice with different susceptibilities to fluorosis

A/J and 129P3/J mouse strains have different susceptibilities to dental fluorosis due to their genetic backgrounds. They also differ with respect to several features of fluoride (F) metabolism and metabolic handling of water. This study was done to determine whether differences in F metabolism could be explained by diversities in the profile of protein expression in kidneys. Weanling, male A/J mice (susceptible to dental fluorosis, n = 18) and 129P3/J mice (resistant, n = 18) were housed in pairs and assigned to three groups given low-F food and drinking water containing 0, 10 or 50 ppm [F] for 7 weeks. Renal proteome profiles were examined using 2D-PAGE and LC-MS/MS. Quantitative intensity analysis detected between A/J and 129P3/J strains 122, 126 and 134 spots differentially expressed in the groups receiving 0, 10 and 50 ppmF, respectively. From these, 25, 30 and 32, respectively, were successfully identified. Most of the proteins were related to metabolic and cellular processes, followed by response to stimuli, development and regulation of cellular processes. In F-treated groups, PDZK-1, a protein involved in the regulation of renal tubular reabsorption capacity was down-modulated in the kidney of 129P3/J mice. A/J and 129P3/J mice exhibited 11 and 3 exclusive proteins, respectively, regardless of F exposure. In conclusion, proteomic analysis was able to identify proteins potentially involved in metabolic handling of F and water that are differentially expressed or even not expressed in the strains evaluated. This can contribute to understanding the molecular mechanisms underlying genetic susceptibility to dental fluorosis, by indicating key-proteins that should be better addressed in future studies.

Increased plasma neopterin and hs-CRP levels in patients with endemic fluorosis

Although fluoride induced inflammatory reactions have been shown in animals and in vitro humans, there are few studies about fluoride induced inflammatory reactions in human beings at clinical setting. We aimed to measure the plasma neopterin, a marker of activation of the monocyte/macrophage system, and high sensitivity C-reactive protein (hs-CRP) levels in patients with endemic fluorosis to investigate the possible role of inflammatory processes (monocyte/macrophage activity) in the underlying pathophysiology of fluoride toxicity at clinical level. Plasma neopterin and hs-CRP levels were determined in endemic fluorosis patients and control subjects. Plasma neopterin levels were significantly higher among patients with endemic fluorosis when compared with control group (2.40 ± 0.66 vs. 1.63 ± 0.27 ng/mL respectively; p < 0.001) and plasma hs-CRP levels were also significantly higher among patients with endemic fluorosis when compared with control group (2.41 ± 1.23 vs. 1.93 ± 0.64 mg/L respectively; p < 0.001). Plasma neopterin levels were positively correlated with urine fluoride levels (r = 0.67, p < 0.001) and serum hs-CRP levels were positively correlated with urine fluoride levels (r = 0.36, p < 0.001). We have found that plasma neopterin and hs-CRP levels are increased in patients with endemic fluorosis. We have concluded that inflammation play an important role in the pathophysiology of fluoride toxicity in patients with endemic fluorosis.

The influence of chronic fluorosis on mitochondrial dynamics morphology and distribution in cortical neurons of the rat brain

The present study was designed to evaluate the effects of chronic fluorosis on the dynamics (including fusion and fission proteins), fragmentation, and distribution of mitochondria in the cortical neurons of the rat brain in an attempt to elucidate molecular mechanisms underlying the brain damage associated with excess accumulation of fluoride. Sixty Sprague-Dawley rats were divided randomly into three groups of 20 each, that is, the untreated control group (drinking water naturally containing <0.5 mg fluoride/l, NaF), the low-fluoride group (whose drinking water was supplemented with 10 mg fluoride/l) and the high-fluoride group (50 mg fluoride/l). After 6 months of exposure, the expression of mitofusin-1 (Mfn1), fission-1 (Fis1), and dynamin-related protein-1 (Drp1) at both the protein and mRNA levels were detected by Western blotting, immunohistochemistry, and real-time PCR, respectively. Moreover, mitochondrial morphology and distribution in neurons were observed by transmission electron or fluorescence microscopy. In the cortices of the brains of rats with chronic fluorosis, the level of Mfn1 protein was clearly reduced, whereas the levels of Fis1 and Drp1 were elevated. The alternations of expression of the mRNAs encoding all three of these proteins were almost the same as the corresponding changes at the protein levels. The mitochondria were fragmented and the redistributed away from the axons of the cortical neurons. These findings indicate that chronic fluorosis induces abnormal mitochondrial dynamics, which might in turn result in a high level of oxidative stress.

Study on blood biochemical diagnostic indices for hepatic function biomarkers in endemic skeletal fluorosis

The aim of the study was to determine the relationship of fluoride in drinking water to liver function in individuals living in normal and seven endemic fluorosis areas of Punjab, India. The concentration of fluoride in drinking water of different areas varied from 5.9 to 24.5 mg/L. Study group consisted of 705 patients in the age group between 20 and 60 years (mean age of 39.35±11.27) affected with osteodental fluorosis were compared with 300 age- and sex-matched controls (with mean age of 35.28±8.25 years). Biochemical data was analyzed by one-way analysis of variance (ANOVA) with post hoc Tukey-Kramer and Bonferroni multiple comparison tests. The relationship between hepatic enzymes was calculated by Pearson's correlation and linear regression. The results revealed significantly (P<0.001) higher concentration of serum fluoride in patients when compared to control. The mean activities of cyclic adenosine monophosphate (AMP), alkaline phosphatase (ALKP), acid phosphatase (ACP), aspartate aminotransaminase (AST), and alanine aminotransaminase (ALT) were significantly (P<0.05-0.001) elevated in patients from all fluoride areas. ANOVA with post hoc Turkey-Kramer and Bonferroni multiple comparison test demonstrated a significant (P<0.0001) variance in the activities of cAMP, ALKP, ACP, AST, and ALT in fluorotic patients, with elevation in water fluoride levels. Maximum elevation of 196.14% (ACP), 99.31% (cyclic adenosine monophosphate; cAMP), 72.08% (ALT), 60.14% (AST), and least 21.35% (ALKP) was recorded in patients exposed to 24.5 mg/L fluoride in drinking water. There was positive correlation between water fluoride, serum fluoride and AST (r=0.77, 0.91), ALT (r=0.82, 0.90), ALKP (r=0.88, 0.97), and ACP (r=0.74, 0.85). Pearson's correlation demonstrated highly significant (P<0.05) positive relationship between water fluoride and cAMP (regression equation: Y=0.9807×+22:081, =0.84; r=0.92, P<0.05). The increased levels of transaminases in fluorotic patients suggest alteration in liver functions. The level of alkaline and acid phosphatase was increased during fluoride intoxication which is also an early marker of hepatic cell damage because of its specificity and catalytic activity. The elevated levels of enzymes are reflective of bone disorders, which are characterized by increased osteoblastic activity. There levels increased several times if cellular damage occurs in the liver. The results suggest that fluoride exposure intensifies the activities of hepatic function enzymes in osteofluorosis.

[Effect of inorganic fluorine on living organisms of different phylogenetic level]

The presented review summarizes literature data on pathways of the inorganic fluoride intake into the plant, animal, and human organisms, on its metabolism, distribution, and accumulation in the organism, forms of fluoride in biological tissues, toxic effects of fluoride on physiological and reproductive functions of living organisms of various phylogenetic groups, as well as clinical symptoms of deficient and excessive fluoride intake into the human organism.

[The influence of fluoride on expression of OPGL and M-CSF genes and their proteins in rats with experimental fluorosis and the therapeutic effect of Danlan Xianpeng Liaofu caspule]

OBJECTIVE

To investigate mRNA and protein expressions of OPGL and M-CSF mRNA in bones of rats with experimental fluorosis induced by intake of fluoride in the drinking water, and to study the antagonizing effects of Danlan Xianpeng Liaofu capsules on the gene expression.

METHODS

Totally 72 SD rats were randomly assorted into 6 groups including the control group, the fluoride group, the high-dosage (0.8 g/kg×d), mid-dosage (0.4 g/kg×d) and low dosage (0.2 g/kg×d) medication groups and the borax group (borax, 0.8 g/kg×d). The distribution of female and male rats in each group was divided up on a fifty-fifty basis. Except the control group, a NaF containing water (NaF 50 mg/L in concentration) was supplied as the drinking water for all the experimental rats in order to establish experimental fluorosis. The thickness and density of trabecula and the thickness of bone cortex were measured by light microscopy. The fluoride content in urine and bone were analyzed by using fluoride ion selective electrode method. Expressions of OPGL and M-CSF mRNA and protein were studied using RT-PCR and immuno-histochemistry, respectively.

RESULTS

(1) 10/12 of the experimental fluorosis rats developed dental fluorosis, and 2/12 of dental fluorosis rats occurred in the low-dosage medication group. Fluoride content in urine and bone of the fluorosis rats increased (P<0.05). (2) Compared with that of the control rats, the bone trabecular depth, cortical thickness and trabecular density in experimental fluorosis rats were remarkably reduced. (3) Compared with that of the control group, mRNA expression of both OPGL and M-CSF was increased in the fluoride group rats. The difference was statistically significant (P<0.05). (4) Compared with that of the fluoride group animals, the expression intensity of OPGL mRNA decreased in animals of the control group, the high, mid- and low- dosage medication groups and the borax group. Among them, except the low-dosage group, the difference between all the other groups and the fluoride group was statistically significant, respectively (P<0.05). There was also a decrease of M-CSF mRNA in all the 3 medication groups and the borax group animals in comparing with that of the fluoride group and the difference was also statistically significant (P<0.05), respectively. (5) Compared with that of the control group. There were an increase of OPGL and a decrease of M-CSF protein expression; and in addition, there were a decrease of OPGL and an increase of M-CSF protein expression in all 3 medication groups and the borax group in comparing with that of the fluoride group anima (P < 0.05).

CONCLUSIONS

Excessive fluoride induces an accelerated bone turnover and may promote the absorption activity of osteoclasts by increasing the expression of OPGL and M-CSF. Danlan Xianpeng Liaofu capsule may be capable of regulating bone remodeling through a down-regulation on OPGL and M-CSF expression.

Organic and inorganic content of fluorotic rat incisors measured by FTIR spectroscopy

Details on how fluoride interferes in enamel mineralization are still controversial. Therefore, this study aimed at analyzing the organic contents of fluorosis-affected teeth using Fourier Transformation Infrared spectroscopy. To this end, 10 male Wistar rats were divided into two groups: one received 45 ppm fluoride in distilled water for 60 days; the other received distilled water only. Then, the lower incisors were removed and prepared for analysis by two FTIR techniques namely, transmission and micro-ATR. For the first technique, the enamel was powdered, whereas in the second case one fluorotic incisor was cut longitudinally for micro-ATR. Using transmission and powdered samples, FTIR showed a higher C-H content in the fluorotic enamel compared with control enamel (p<0.05, n=4 in the flurotic, and n=5 in the control group). Results from the micro-ATR-FTIR spectroscopic analysis on one longitudinally cut incisor carried out at six points reveal a higher C-H bond content at the surface of the enamel, with values decreasing toward the dentine-enamel junction, and reaching the lowest values at the subsuperficial enamel. These results agree with the morphological data, which indicate that in the rat incisor the fluorotic lesion is superficial, rather than subsuperficial, as in the case of human enamel. The results also suggest that the increased C-H bond content may extend toward the more basal enamel (intraosseous), indicating that fluorotic enamel may intrinsically contain more protein. Finally, particularly when coupled to ATR, FTIR is a suitable tool to study the rat incisor enamel, which is a largely used model of normal and abnormal amelogenesis. Further studies along this line may definitely answer some questions regarding protein content in fluorotic enamel as well as their origin.

[Study on demineralization and remineralization of human fluorosed teeth in vitro]

This study sought to determine the impact of dental fluorosis severity on demineralization and remineralization of human fluorosed teeth in vitro. Surface enamel microhardness was measured on the enamel blocks before and after demineralization and after remineralization. The results showed that after demineralization, the sequence of % Surface microhardness demineralization (% SMHD) was TFI4 (18.92 +/- 1.31) < TFI3 (20.50 +/- 1.32) < TFI2 (25.08 +/- 1.69) < TFI1 (27.77 +/- 1.79) < TFI0 (30.70 +/- 1.35) (P < 0.05), and there was no statistically significant differences between TFI1 (27.77 +/- 1.79) and the normal group TFI0 (30.70 +/- 1.35). After remineralization, the sequence of % Surface microhardness remineralization (% SMHR) was TFI1 (55.17 +/- 1.23) > TFI0 (53.97 +/- 3.05) > TFI2 (49.17 +/- 1.81) > TFI3 (44.85 +/- 1.89) > TFI4 (36.51 +/- 2.95) (P < 0.05). Moderately fluorosed enamel showed a significatnt resistance to caries, but mildly fluorosed enamel could get better remineralization. These facts and figures deserve clinicians' attention.

[Effect of broken black tea on the formation of dental enamel and the contents of twelve kinds of chemical elements]

The aim of this study was to investigate the effect of broken black tea with slightly hyper-normal fluoride content which was near the level of people tea-drinking habits on enamel morphological structure and its content of chemical elements. Thirty six rats were divided randomly into 3 groups: one was control group and another two groups fed with broken black tea infusion with F- content of 8.2 mg/l and 16.4 mg/l, respectively. After 360 days, collected 144 teeth, observed their morphological structure by electron micrograph, Scanning electron micrographs (SEM) and also analyzed concentrations of several chemical elements in tooth by x-ray fluorescence spectrometry (XRF). Chronic dental fluorosis in rats was induced by treatment with broken black tea with slightly hyper-normal fluoride content. Hyper-calcification and hypo-calcification appeared in enamel of those teeth from both broken black tea treated groups in dose-dependent manner. Twelve kinds of chemical elements, such as Ca, Mg, P, Al, Cl were examined. The contents of Ca, P, Mg were reduced and that of Al and Cl were increased significantly. Compared with control group, the levels of Si, S, Fe were lower in those teeth from treated group (broken black tea with the F- content of 8.2 mg/l), while higher in those teeth from treated group (broken black tea with the F- content of 16.4 mg/l). Long-term drinking broken black tea with hyper-normal fluoride content could cause chronic dental fluorosis, and its injury in enamel was related with Hyper-calcification and hypo-calcification mainly. Those changes of several chemical elements level in enamel, such as Ca, P, Al, Cl, were suggested that these chemical elements have influences on the development and mineralization of enamel.

[Effect of fluorosis on the expression of basic fibroblast growth factor in rat incisors]

OBJECTIVE

To investigate the effects of overdose fluoride on the expression of basic fibroblast growth factor (bFGF) in rat incisors.

METHODS

Twenty Wistar rats were randomly divided into 2 groups: group I (control group) distilled water was given; group II (experimental group) 100 mg/L NaF was given. The rats were killed at the end of 8 th week. Immunohistochemical staining was used to study the expression of bFGF in rat incisors.

RESULTS

Immunohistochemical results demonstrated the presence of bFGF in ameloblasts, odontoblasts of rat incisors. The expression of bFGF was reduced in group II (P < 0.01).

CONCLUSIONS

Overdose fluoride inhibits the expression of bFGF and affects the interaction between dental epithelium and dental mesenchyme, which leads to the enamel demineralization.

Dose-effect relationship between drinking water fluoride levels and damage to liver and kidney functions in children

Although a dose-effect relationship between water fluoride levels and damage to liver and kidney functions in animals has been reported, it was not demonstrated in humans. To evaluate the effects of drinking water fluoride levels on the liver and kidney functions in children with and without dental fluorosis, we identified 210 children who were divided into seven groups with 30 each based on different drinking water fluoride levels in the same residential area. We found that the fluoride levels in serum and urine of these children increased as the levels of drinking water fluoride increased. There were no significant differences in the levels of total protein (TP), albumin (ALB), aspartate transamine (AST), and alanine transamine (ALT) in serum among these groups. However, the activities of serum lactic dehydrogenase (LDH), urine N-acetyl-beta-glucosaminidase (NAG), and urine gamma-glutamyl transpeptidase (gamma-GT) in children with dental fluorosis and having water fluoride of 2.15-2.96 mg/L and in children having water fluoride of 3.15-5.69 mg/L regardless of dental fluorosis were significantly higher than children exposed to water fluoride of 0.61-0.87 mg/L in a dose-response manner. In contrast to children with dental fluorosis and having water fluoride of 2.15-2.96 and 3.10-5.69 mg/L, serum LDH activity of children without dental fluorosis but exposed to the same levels of water fluoride as those with dental fluorosis were also markedly lower, but the activities of NAG and gamma-GT in their urine were not. Therefore, our results suggest that drinking water fluoride levels over 2.0mg/L can cause damage to liver and kidney functions in children and that the dental fluorosis was independent of damage to the liver but not the kidney. Further studies on the mechanisms and significance underlying damage to the liver without dental fluorosis in the exposed children are warranted.

Fluoride down-regulates the expression of matrix metalloproteinase-20 in human fetal tooth ameloblast-lineage cells in vitro

Fluoride is associated with a decrease in the incidence of dental caries, but excessive fluoride intake during tooth enamel formation can result in enamel fluorosis. Fluorosed enamel has increased porosity, which has been related to a delay in the removal of amelogenin proteins as the enamel matures. This delay in protein removal suggests that fluoride may affect either the amount or the activity of enamel matrix proteinases. In this study, we investigated the role of fluoride in the synthesis and secretion of matrix metalloproteinase-20 (MMP-20), the proteinase primarily responsible for the initial hydrolysis of amelogenin during the secretory stage of enamel formation. Cultured human fetus tooth organ ameloblast-lineage cells were exposed to 10 microM fluoride and analyzed for synthesis of MMP-20. Immunoblotting showed that 10 microM NaF down-regulated the synthesis of MMP-20 by 21% compared with control cells, but did not alter the amount of amelogenin or kalikrein-4 (KLK-4) synthesized by the cells. Real-time polymerase chain reaction (PCR) showed that 10 microM NaF down-regulated MMP-20 mRNA expression to 28% of the levels found in the non-treated cells. These in vitro results suggest that fluoride can alter the expression of MMP-20 by ameloblasts, resulting in a disturbance of the balance between MMP-20 and its substrate that may contribute to the retention of amelogenins in the formation of fluorosed enamel.

Fluorosis: a new model and new insights

Fluoride is an effective agent for the prevention of dental caries. However, the mechanism of how excessive fluoride exposure causes fluorosis remains uncertain. Zebrafish (Danio rerio) exhibit periodic tooth replacement throughout their lives, thereby providing continuous access to teeth at developmental stages susceptible to fluoride exposure. Zebrafish teeth do not contain true enamel, but consist of a hard enameloid surface. Therefore, we asked whether zebrafish could be used as a model organism for the study of dental fluorosis. Scanning electron microscopy of fluoride-treated teeth demonstrated that the enameloid was pitted and rough, and FTIR analysis demonstrated that the teeth also contained a significantly higher organic content when compared with untreated controls. Furthermore, we demonstrate for the first time that decreased expression of an important signaling molecule (Alk8) in tooth development may contribute to the observed fluorotic phenotype, and that increased cell apoptosis may also play a role in the mechanism of fluorosis.

Fluoride metabolism and fluorosis

OBJECTIVES

This paper is primarily concerned with the only proven risk associated with water fluoridation: enamel fluorosis. Its purpose is to review current methods of measuring enamel fluorosis, its aetiology and metabolism. A further objective is to identify risk factors to reduce the prevalence of enamel fluorosis and employ methods to manage such risk factors.

DATA

The prevalence of enamel fluorosis is increasing in Ireland and internationally. A critical period has been identified at which teeth are most at risk of developing enamel fluorosis: 15-24 months of age for males and 21-30 months of age for females. The data included took these two factors into account.

SOURCE

A thorough narrative review of published literature was conducted to identify studies concerning the aetiology and metabolism of enamel fluorosis. Risk factors for fluorosis were identified from these studies.

STUDY SELECTION

As it is the pre-eruptive phase of enamel development which represents the greatest risk to developing enamel fluorosis, studies examining sources of fluoride ingestion for young children were selected. These included studies on ingestion of fluoride toothpaste by young children, fluoride supplementation and infant formula reconstituted with fluoridated water.

CONCLUSIONS

There is evidence that the age at which tooth brushing with fluoride toothpastes is commenced and the amount of fluoride placed on the brush are important risk factors in the incidence of dental fluorosis. It is recommended that brushing should not commence until the age of 2 and that a pea-sized amount (0.25 g) of toothpaste should be placed on the brush.

[Fluoride tolerance in cases of dental caries and fluorosis]

32 volunteers (18-20-year-olds) without concomitant diseases were divided into 3 groups: with high caries incidence (1), with dental fluorosis of different degrees (2) and with completely intact dentition (3). Dynamics of ionized F in blood serum and mixed saliva after single sodium fluoride burden (F-dosage=0.1 mg/kg body weight) was determined 1, 2, 3, 5, 7 and 24 hours after per os consumption by Orion-9609-BN F-selective electrode. F bound by organic material was assayed after breaking covalent bond C-F by sodium diphenyl. It was common for all participants that wide range of sensitivity to ingested F but F-tolerance had reversed trends in caries and fluorosis. Content of organic "fluoride" in blood serum changed insignificantly. In mixed saliva the dynamics of ionized F in dental caries and fluorosis conditions followed to some extent blood serum trends.

The effect of fluoride on the developing tooth

This review aims to outline the effects of fluoride on the biological processes involved in the formation of tooth tissues, particularly dental enamel. Attention has been focused on mechanisms which, if compromised, could give rise to dental fluorosis. The literature is extensive and often confusing but a much clearer picture is emerging based on recent more detailed knowledge of odontogenesis. Opacity, characteristic of fluorotic enamel, results from incomplete apatite crystal growth. How this occurs is suggested by other changes brought about by fluoride. Matrix proteins, associated with the mineral phase, normally degraded and removed to permit final crystal growth, are to some extent retained in fluorotic tissue. Fluoride and magnesium concentrations increase while carbonate is reduced. Crystal surface morphology at the nano-scale is altered and functional ameloblast morphology at the maturation stage also changes. Fluoride incorporation into enamel apatite produces more stable crystals. Local supersaturation levels with regard to the fluoridated mineral will also be elevated facilitating crystal growth. Such changes in crystal chemistry and morphology, involving stronger ionic and hydrogen bonds, also lead to greater binding of modulating matrix proteins and proteolytic enzymes. This results in reduced degradation and enhanced retention of protein components in mature tissue. This is most likely responsible for porous fluorotic tissue, since matrix protein removal is necessary for unimpaired crystal growth. To resolve the outstanding problems of the role of cell changes and the precise reasons for protein retention more detailed studies will be required of alterations to cell function, effect on specific protein species and the nano-chemistry of the apatite crystal surfaces.

Effect of chronic fluorosis on lipid peroxidation and histology of kidney tissues in first- and second-generation rats

This experiment was designed to investigate the lipid peroxidation and histological effects of chronic fluorosis on first- and second-generation rat kidney tissues. Sixteen virgin female Wistar rats were mated with eight males (2: 1) for approx 12 h to obtain first-generation rats. Mating was confirmed by the presence of sperm in vaginal smears. Sperm in vaginal smears was observed in 10 of 16 rats (d 0). These rats were identified as pregnant and included in this experiment. Pregnant rats were divided into two experimental groups (control and fluoride-supplemented), each containing five rats. The pregnant rats in the fluoride-supplemented group were exposed to 30 mg/L sodium fluoride (NaF) in commercial drinking water containing 0.07 mg/L NaF throughout the gestation and the lactation periods. After the lactation period, young animals (first generation [F1]) were exposed to the same amount of NaF in drinking water for 4 mo. At the end of the 4-mo experimental period, nine randomly chosen male rats (F1) were sacrificed, and the kidneys were removed for the histological and lipid peroxidation examinations. The remaining eight female rats were mated with four males (2: 1) for approx 12 h to obtain second-generation rats. Six female were identified as pregnant, and treated similarly throughout the gestation and the lactation periods. After the lactation period, the young male rats (second-generation male rats [F2]) were also treated similarly for 4 mo. At the end of the 4-mo experimental period, nine randomly chosen male rats (F2) were sacrificed, and the kidneys were removed for the histological and lipid peroxidation examinations. The rats in the control groups underwent the same procedure without NaF supplementation. It was found that the plasma fluoride and kidney TBARS levels of fluoride-supplemented F1 and F2 rats were higher than controls. Hydropic epithelial cell degenerations and moderate tubular dilatation were observed in some proximal and distal tubules. There were markedly focal mononuclear cell infiltrations and hemorrhage at some areas of the interstitium, especially at the corticomedullar junction. Mononuclear cell infiltrations were also evident in some peritubular and perivascular areas. Most of the vascular structures were congestive. Many Bowman capsules were narrowed. The severe degenerative changes in most of the shrunken glomerules and vascular congestion were also observed.

Dental fluorosis: chemistry and biology

This review aims at discussing the pathogenesis of enamel fluorosis in relation to a putative linkage among ameloblastic activities, secreted enamel matrix proteins and multiple proteases, growing enamel crystals, and fluid composition, including calcium and fluoride ions. Fluoride is the most important caries-preventive agent in dentistry. In the last two decades, increasing fluoride exposure in various forms and vehicles is most likely the explanation for an increase in the prevalence of mild-to-moderate forms of dental fluorosis in many communities, not the least in those in which controlled water fluoridation has been established. The effects of fluoride on enamel formation causing dental fluorosis in man are cumulative, rather than requiring a specific threshold dose, depending on the total fluoride intake from all sources and the duration of fluoride exposure. Enamel mineralization is highly sensitive to free fluoride ions, which uniquely promote the hydrolysis of acidic precursors such as octacalcium phosphate and precipitation of fluoridated apatite crystals. Once fluoride is incorporated into enamel crystals, the ion likely affects the subsequent mineralization process by reducing the solubility of the mineral and thereby modulating the ionic composition in the fluid surrounding the mineral. In the light of evidence obtained in human and animal studies, it is now most likely that enamel hypomineralization in fluorotic teeth is due predominantly to the aberrant effects of excess fluoride on the rates at which matrix proteins break down and/or the rates at which the by-products from this degradation are withdrawn from the maturing enamel. Any interference with enamel matrix removal could yield retarding effects on the accompanying crystal growth through the maturation stages, resulting in different magnitudes of enamel porosity at the time of tooth eruption. Currently, there is no direct proof that fluoride at micromolar levels affects proliferation and differentiation of enamel organ cells. Fluoride does not seem to affect the production and secretion of enamel matrix proteins and proteases within the dose range causing dental fluorosis in man. Most likely, the fluoride uptake interferes, indirectly, with the protease activities by decreasing free Ca(2+) concentration in the mineralizing milieu. The Ca(2+)-mediated regulation of protease activities is consistent with the in situ observations that (a) enzymatic cleavages of the amelogenins take place only at slow rates through the secretory phase with the limited calcium transport and that, (b) under normal amelogenesis, the amelogenin degradation appears to be accelerated during the transitional and early maturation stages with the increased calcium transport. Since the predominant cariostatic effect of fluoride is not due to its uptake by the enamel during tooth development, it is possible to obtain extensive caries reduction without a concomitant risk of dental fluorosis. Further efforts and research are needed to settle the currently uncertain issues, e.g., the incidence, prevalence, and causes of dental or skeletal fluorosis in relation to all sources of fluoride and the appropriate dose levels and timing of fluoride exposure for prevention and control of dental fluorosis and caries.

[Fundamental and applied studies on transport and metabolism of electrolytes and glucose--aim to contact with molecular biology]

The authors' research focuses on polyuria, natriuresis, glucosuria, glycemia, and renal calcification in occupational lead poisoning and endemic fluorosis. Changes in electrolyte mobilization and in glucose metabolism and transport following the administration of lead compounds or fluoride were examined to elucidate these mechanisms. The results suggest fundamental approaches to the mechanism of aging and life style diseases. Our results show that: 1) Natriuresis and polyuria in lead poisoning and fluorosis are due to a decrease in renal Na/K-ATPase activity; 2) Renal calcification in fluorosis is due to stimulation of parathyroid function and activation of the renal phosphatidylinositol cascade; 3) Glycemia in fluorosis is due to elevation of renal and hepatic glucose-6-phosphatase activities; 4) Glusosuria in fluorosis is due to decreased renal Na/K-ATPase activity (but fluoride administered directly did not damage the renal Na/glucose cotransporter (SGLT); 5) Renal calcification in fluorosis is due to stimulation of parathyroid function; and 6) The decrease in renal Na/K-ATPase and SGLT activities with aging and hypertension is due to a decrease in phosphorylation activity by protein kinase C (PKC) etc. (decrease in PKC productivity with aging and hypertension).

Changes in the structure of dentine from cheek teeth of deer chronically exposed to high levels of environmental fluoride

The purpose of this study was to examine, using backscattered electron imaging, the changes occurring in the structure of the dentine including giant tubules in the teeth of deer chronically exposed to fluoride. The primary and secondary dentine was characterised by the presence of interglobular dentine and regular bands of hypo- and hypermineralised dentine. Giant tubules in the unworn teeth of animals exposed to low and high levels of fluoride were characterised by a large lumen and hypermineralised rim in which there were calcospherites. With the occlusal wear of the teeth tubules beneath the immediate surface of the exposed dentine in teeth from fluorotic animals became occluded with mineral, but in low fluoride animals some tubules at the surface were not occluded with mineral. It is suggested that mineral which occludes the tubules may come from both intrinsic and extrinsic sources. The presence of some open tubules at the immediate surface in the worn teeth of animals exposed to low fluoride suggests an extrinsic source for this mineral.

A laboratory assessment of enamel hypoplasia of teeth with varying severities of dental fluorosis

The effect of lifelong exposure to drinking-water containing fluoride on tooth enamel microhardness was investigated. Dental fluorosis of teeth from adult subjects, who lived continuously since birth in areas characterized by the hot climate of India, supplied by drinking-water containing between 0.5 and 8.7 parts/10 F, was estimated by the Dean score. Tooth enamel sections were examined from the enamel surface towards the dentino-enamel junction (DEJ) for microhardness. Separated enamel crowns were pulverized and analysed for fluoride. Regarding the distribution of the fluoride concentrations in the drinking-water, we selected a partition below and above 0.8 parts/10 F as a criterion for comparison between the results. Tooth enamel of humans from Israel living in a drinking-water area containing about 0.5 parts/10 F was also examined. Positive associations were evident between fluoride in drinking-water, fluoride concentrations in the bulk enamel and fluorosis selection. A significantly high inverse relationship was evident between the fluoride concentration of the drinking-water and the subsurface enamel microhardness. In similar fluoride-concentration drinking-water areas of India and Israel the microhardness of the subsurface enamel was less for the Indian teeth. The microhardness of the enamel near the DEJ for the three different water fluoride-concentration areas was not significantly different.

[Quantitative analysis of thirty-one elements in the stained dental fluorosis of enamel]

OBJECTIVE

To study the relationship between concentrations of thirty elements and fluorine element in teeth with stained dental fluorosis from high fluoride water area and teeth from normal fluoride area.

METHODS

The concentration of thirty-one elements in 43 teeth was measured by plasmatron spectrographic analysis and fluoride selective electrode.

RESULTS

1. Content of F, P, Ca, Mn, As, Se, Al, Ti, Mo, B elements in enamel had a direct relationship to severity of enamel staining. 2. Content of Mn, As, Se, Sr, Nb, Ca, P elements were high positively related to content of fluorine in enamel (P < 0.05).

CONCLUSION

Mn, As, Se, F content is closely related to the pigment formation in dental fluorosis of enamel.

Altered phosphorylation of rat dentine phosphoproteins by fluoride in vivo

Dentine phosphoproteins have been proposed to have an important role in mineralization. This study focused on the influence of fluoride on the biochemical composition of dentine phosphoproteins and attempts to relate changes to the altered mineralization witnessed during fluorosis. Wistar rats were rendered fluorotic by the administration of 20 ppm sodium fluoride in their drinking water ad libitum, a nonfluorotic group received double-distilled, deionized water only. After 17 weeks, the teeth showed signs of fluorosis. The incisors were removed, split longitudinally, and the pulps were removed. Teeth were powdered and demineralized in 10% EDTA with protease inhibitors, after which the organic matrix was extracted with 4 M guanidinium chloride. Phosphoproteins were selectively precipitated from the soluble extract by the addition of 1.0 M calcium chloride and further purified by anion exchange chromatography. SDS-PAGE revealed two protein bands with molecular weights of 130 kDa and 66 kDa in the nonfluorotic fraction and 116 kDa and 66 kDa in the fluorotic fraction. Western blotting analysis identified the 66 kDa band as alpha2-HS glycoprotein which co-precipitated with phosphoproteins. Electroelution of the protein bands was performed with subsequent biochemical analyses. Phosphate content was determined for each protein band and was detectable in the 116 kDa and 130 kDa bands from the fluorotic and nonfluorotic samples, respectively, with a decreased level noted in the 116 kDa band. The presence of phosphate and the amino acid analysis of these bands suggested their identity to be dentine phosphoproteins. No changes in the ratio of amino acids was detected in fluorotic samples. The fluoride-induced alterations to the biochemical structure of dentine phosphoproteins would appear to influence the phosphorylation of these macromolecules only, possibly affecting posttranslational events. Such alterations may play a role in disrupting the patterns of mineralization seen during fluorosis.

Biological mechanisms of dental fluorosis relevant to the use of fluoride supplements

Fluorosis occurs when fluoride interacts with mineralizing tissues, causing alterations in the mineralization process. In dental enamel, fluorosis causes subsurface hypomineralizations or porosity, which extend toward the dentinal-enamel junction as severity increases. This subsurface porosity is most likely caused by a delay in the hydrolysis and removal of enamel proteins, particularly amelogenins, as the enamel matures. This delay could be due to the direct effect of fluoride on the ameloblasts or to an interaction of fluoride with the proteins or proteinases in the mineralizing matrix. The specific mechanisms by which fluoride causes the changes leading to enamel fluorosis are not well defined; though the early-maturation stage of enamel formation appears to be particularly sensitive to fluoride exposure. The development of fluorosis is highly dependent on the dose, duration, and timing of fluoride exposure. The risk of enamel fluorosis is lowest when exposure takes place only during the secretory stage, but highest when exposure occurs in both secretory and maturation stages. The incidence of dental fluorosis is best correlated with the total cumulative fluoride exposure to the developing dentition. Fluoride supplements can contribute to the total fluoride exposure of children, and if the total fluoride exposure to the developing teeth is excessive, fluorosis will result.

[Effects of overdose fluoride on metabolism of extracellular matrix collagen and proteoglycan in condylar cartilage]

The study was conducted to evaluate the effects of chronic fluorosis on the metabolism of extracellular matrix in the development of rabbit condylar cartilage using the technique of pathology, biochemistry and radiographs. The results demonstrated that collagen content of condylar cartilage and the bone under condylar cartilage decreased (P < 0.01), and the proteoglycar contents increased significantly during the process of fluorosis (P < 0.01). The chondroprogenitor was obviously thin, and the cells of maturation zone increased in number. All the results indicated that overdose of fluoride retarded the growth of the condyle and ramus of the mandible.

The protein composition of normal and developmentally defective enamel

The development of human enamel involves a complex series of events including the secretion and degradation of a unique extracellular matrix. Ameloblasts progress through a succession of cellular phenotypes executing specialized secretory and regulatory functions. When performing optimally, ameloblasts produce a highly structured and mineralized tissue. Given the elaborate developmental events required for normal enamel formation, it is not surprising that a variety of enamel malformations arise from defects in matrix synthesis, secretion and extracellular processing. Normal matrix secretion and post-secretory processing by ameloblasts can be affected by a variety of hereditary and environmental conditions. These disturbances can result in an abnormal amount and/or composition of matrix proteins, and subsequently, an altered enamel structure and/or mineral content. For example, abnormal matrix removal during enamel maturation apparently contributes to hypomineralization associated with dental fluorosis. Incomplete matrix removal can also occur in several different forms of the hereditary condition amelogenesis imperfects. Specific types of this condition can have retention of substantial enamel protein (e.g. 5% by weight) that is, at least in part, composed of amelogenin and/or its breakdown products. Characterization of the enamel proteins in teeth affected by developmental disturbances can provide insight into the pathogenesis and normal formation of this highly specialized tissue.

Determinants and mechanisms of enamel fluorosis

Enamel fluorosis occurs when fluoride concentrations in or in the vicinity of the forming enamel are excessive during its pre-eruptive development. Fluoride concentrations in plasma, enamel and other tissues reflect the difference between intake and excretion, i.e. fluoride balance. In addition to the diet, modern sources of ingested fluoride include a variety of dental products, some of which have been identified as risk factors for fluorosis. Fluoride absorption is inversely related to dietary calcium which, at high concentrations, may cause net fluoride secretion into the gastrointestinal tract. The excretion of absorbed fluoride occurs almost exclusively via the kidneys, a process which is directly related to urinary pH. Thus, fluoride balance and tissue concentrations and the risk of fluorosis are increased by factors such as high protein diets, residence at high altitude, and certain metabolic and respiratory disorders that decrease pH. Factors that increase urinary pH and decrease the balance of fluoride include vegetarian diets, certain drugs and some other medical conditions. Although several other fluoride-induced effects might be involved in the aetiology of fluorosis, it now appears that inhibition of enzymatic degradation of amelogenins, which may delay their removal from the developing enamel and impair crystal growth, may be of critical importance. In addition to the effects of fluoride, disturbances in enamel formation that can be confused with fluorosis are caused by chronic acidosis and hypoxia independently of the level of fluoride exposure.

Enamel maturation

Enamel maturation is characterized by massive crystal growth in both width and thickness, resulting in the most highly mineralized of all mammalian skeletal tissues. The control of this process is mediated via a carefully orchestrated series of events that are temporally and spatially regulated, and it requires the co-ordinated degradation and removal of the endogenous enamel matrix. This is affected by both neutral metalloproteases and serine proteases, which are developmentally restricted and may be further modulated by changes in the chemistry of the enamel crystals themselves. Failure of these mechanisms, or the adventitious entry of mineral-binding proteins during the later stages of maturation, may result in the incomplete maturation of the enamel crystals and the eruption of dysplastic tissue.

The effect of fluoride on apatite structure and growth

Fluoride participates in many aspects of calcium phosphate formation in vivo and has enormous effects on the process and on the nature and properties of formed mineral. The most well-documented effect of fluoride is that this ion substitutes for a column hydroxyl in the apatite structure, giving rise to a reduction of crystal volume and a concomitant increase in structural stability. In the process of enamel mineralization during amelogenesis (a unique model for the cell-mediated formation of well-crystallized carbonatoapatite), free fluoride ions in the fluid phase are supposed to accelerate the hydrolysis of acidic precursor(s) and increase the driving force for the growth of apatitic mineral. Once fluoride is incorporated into the enamel mineral, the ion likely affects the subsequent mineralization process by reducing the solubility of the mineral and thereby modulating the ionic composition in the fluid surrounding the mineral, and enhancing the matrix protein-mineral interaction. But excess fluoride leads to anomalous enamel formation by retarding tissue maturation. It is worth noting that enameloid/enamel minerals found in vertebrate teeth have a wide range of CO3 and fluoride substitutions. In the evolutionary process from elasmobranch through enameloid to mammalian enamel, the biosystems appear to develop regulatory functions for limiting the fluoridation of the formed mineral, but this development is accompanied by an increase of carbonate substitution or defects in the mineral. In research on the cariostatic effect of fluoride, considerable emphasis is placed on the roles of free fluoride ions (i.e., preventing the dissolution and accelerating the kinetics of remineralization) in the oral fluid bathing tooth mineral. Fluoride also has been used for the treatment of osteoporosis, but much still remains to be learned about maximizing the benefit and minimizing the risk of fluoride when used as a public health measure.

Protein characterization of fluorosed human enamel

Despite extensive investigation, the development mechanism or mechanisms resulting in dental fluorosis are unknown. Several hypotheses suggest abnormal matrix synthesis, secretion, and delayed and/or defective matrix degradation with retention of enamel protein. The purpose of this study was to characterize the protein composition of fluorosed human enamel. Nine permanent moderately fluorosed (developed in a 3.2 ppm H2O area) and ten permanent normal control teeth (from individuals with < 0.2 ppm F in their drinking water) were evaluated. The enamel fluoride concentration, protein content, and amino acid composition were determined for each tooth. The enamel proteins were further characterized by gel electrophoresis and by Western blot analysis by means of polyclonal antibodies raised against recombinant amelogenin protein. Fluorotic enamel had significantly elevated (p = 0.0001) F levels compared with normal enamel (mean [F-] fluorosed = 431 ppm; mean [F-] control = 62 ppm). While there was a significantly greater protein content by weight in fluorosed enamel compared with normal enamel (mean fluorosed = 0.27%; mean control = 0.11%), the amino acid profiles were similar for fluorosed and normal enamel. Gel electrophoresis showed fluorosed enamel to have a greater diversity of primarily low-molecular-weight proteins compared with normal enamel. Western blot analysis did not indicate retention of amelogenin in either fluorosed or normal enamel. This investigation showed that the protein content of fluorosed enamel was greater than that of normal enamel; however, the amino acid compositions were similar for fluorosed and normal enamel. Furthermore, there does not appear to be retention of significant amounts of amelogenin in fully mature, moderately fluorosed human enamel. Although delayed removal of the enamel matrix proteins may play a role in the hypomineralization defects seen in fluorosed enamel, the majority of these proteins are absent in the mature tissue of these moderately fluorosed teeth.