Effects on protein and mRNA expression levels of p53 induced by fluoride in human embryonic hepatocytes

Fluoride-induced apoptosis in non-skeletal tissues of experimental animals: A systematic review and meta-analysis

Different studies have suggested that fluoride can induce apoptosis in non-skeletal tissues, however, evidence from these experimental studies is still controversial. This meta-analysis aims to clarify the mechanism of fluoride-induced apoptosis in non-skeletal tissues of experimental animals. Primary studies which measured apoptosis were identified through exhaustive database searching in PubMed, Embase, Web of Science Core Collection, Scopus, and references of included studies. A random effects model with standardized mean difference (SMD) was used for meta-analyses. The heterogeneity of the studies was evaluated using Higgin's I2 statistics. The risk of bias and publication bias were assessed using the SYRCLE's risk of bias tool and Egger's test, respectively. There was an increase in total apoptotic cells, and the expression of Bax, Bax/Bcl-2 ratio, caspase-3, caspase-8, caspase-9, Cyt c, and p53, and a decrease in the expression of Bcl-2 in the fluoride-treated groups as compared to the control groups. However, there was no evidence of a difference in the expression of APAF-1 in the two groups. The subgroup analysis highlighted the role of the intervention period in modification of the apoptotic effect of fluoride and that the susceptibility and tolerance of different animal species and tissues vary. Meta-regression analysis indicated that the studies' effect size for total apoptotic cells was influenced by animal species and that of Bax by the sample source. The results of this meta-analysis revealed that fluoride causes apoptosis by up-regulating caspase-3, -8, and -9, Cyt c, p53, Bax, and down-regulating Bcl-2 with a concomitant up-regulation of the Bax/Bcl-2 ratio.

Inflammation- and cancer-related microRNAs in rat renal cortex after subchronic exposure to fluoride

The proximal tubule is a target of subchronic exposure to fluoride (F) in the kidney. Early markers are used to classify kidney damage, stage, and prognosis. MicroRNAs (miRNAs) are small sequences of non-coding single-stranded RNA that regulate gene expression and play an essential role in developing many pathologies, including renal diseases. This study aimed to evaluate the expression of Cytokine-Chemokine molecules (IL-1α/1β/4/6/10, INF-γ, MIP-1α, MCP-1, RANTES, and TGF β1/2/3) and inflammation-related miRNAs to evidence the possible renal mechanisms involved in subchronic exposure to F. Total protein and miRNAs were obtained from the renal cortex of male Wistar rats exposed to 0, 15 and 50 mg NaF/L through drinking water during 40 and 80 days. In addition, cytokines-chemokines were analyzed by multiplexing assay, and a panel of 77 sequences of inflammatory-related miRNAs was analyzed by qPCR. The results show that cytokines-chemokines expression was concentration- and time-dependent with F, where the 50 mg NaF/L were the main altered groups. The miRNAs expression resulted in statistically significant differences in thirty-four miRNAs in the 50 mg NaF/L groups at 40 and 80 days. Furthermore, a molecular interaction network analysis was performed. The relevant pathways modified by subchronic exposure to fluoride were related to extracellular matrix-receptor interaction, Mucin type O-glycan biosynthesis, Gap junction, and miRNAs involved with renal cell carcinoma. Thus, F-induced cytokines-chemokines suggest subchronic inflammation; detecting miRNAs related to cancer and proliferation indicates a transition from renal epithelium to pathologic tissue after fluoride exposure.

Role of oxidative stress-mediated cell death and signaling pathways in experimental fluorosis

Free radicals and other oxidants have enticed the interest of researchers in the fields of biology and medicine, owing to their role in several pathophysiological conditions, including fluorosis (Fluoride toxicity). Radical species affect cellular biomolecules such as nucleic acids, proteins, and lipids, resulting in oxidative stress. Reactive oxygen species-mediated oxidative stress is a common denominator in fluoride toxicity. Fluorosis is a global health concern caused by excessive fluoride consumption over time. Fluoride alters the cellular redox homeostasis, and its toxicity leads to the activation of cell death mechanisms like apoptosis, autophagy, and necroptosis. Even though a surfeit of signaling pathways is involved in fluorosis, their toxicity mechanisms are not fully understood. Thus, this review aims to understand the role of reactive species in fluoride toxicity with an outlook on the effects of fluoride in vitro and in vivo models. Also, we emphasized the signal transduction pathways and the mechanism of cell death implicated in fluoride-induced oxidative stress.

Changes of DNA repair gene methylation in blood of chronic fluorosis patients and rats

To investigate the relationship between DNA repair gene methylation and chronic coal-burning fluorosis. The methylation rates of O⁶-methylguanine-DNA- methyltransferase gene MGMT, a DNA repair gene and mismatch repair gene MutL homolog 1 (MLH1) were analysed by methylation of specific PCR (MSP), and the levels of mRNA in the blood of the chronic fluorosis rats and the patients in the region of endemic coal-burning fluorosis were determined by real-time PCR. The levels of mRNA and protein of MGMT and MLH1 in the liver tissue of the chronic fluorosis rats were determined by real-time PCR and Western blot respectively. The results showed an increased methylation of the MGMT and MLH1 genes in the blood of the patients in the fluorosis region that correlated positively with the severity of fluorosis. The mRNA levels of MGMT and MLH1 genes from the patients in fluorosis region were lower than those of a control group, and also showed a positive correlation with the severity of fluorosis. Both the protein and mRNA levels of MGMT and MLH1 genes from the blood of rats and liver tissue in a fluoride-treated group were lower than those of a control non-fluoride treated group. These results indicate that the degree of methylation of MGMT and MLH1 genes is altered in fluorosis disease, the resulting changed expression of these repair genes may play a role in the liver damage caused by fluoride.

A mini review of fluoride-induced apoptotic pathways

Fluorine or fluoride can have toxic effects on bone tissue and soft tissue at high concentrations. These negative effects include but not limited to cytotoxicity, immunotoxicity, blood toxicity, and oxidative damage. Apoptosis plays an important role in fluoride-induced toxicity of kidney, liver, spleen, thymus, bursa of Fabricius, cecal tonsil, and cultured cells. Here, apoptosis activated by high level of fluoride has been systematically reviewed, focusing on three pathways: mitochondrion-mediated, endoplasmic reticulum (ER) stress-mediated, and death receptor-mediated pathways. However, very limited reports are focused on the death receptor-mediated apoptosis pathways in the fluoride-induced apoptosis. Therefore, understanding and discovery of more pathways and molecular mechanisms of fluoride-induced apoptosis may contribute to designing measures for preventing fluoride toxicity.

Sodium fluoride causes hepatocellular S-phase arrest by activating ATM-p53-p21 and ATR-Chk1-Cdc25A pathways in mice

In this study, experimental pathology, flow cytometry (FCM), quantitative real-time polymerase chain reaction (qRT-PCR), and western blot (WB) were used to evaluate the effects of sodium fluoride (NaF) on hepatocellular cell cycle progression in mice. A total of 240 ICR mice were divided equally into four groups; the experimental groups received 12, 24, or 48 mg/kg NaF intragastrically for 42 days, while the control group received distilled water. Doses of NaF above 12 mg/kg increased the percentage of cells in S phase (S-phase arrest), reduced percentages of cells in G0/G1 or G2/M phase, and activated the ATM-p53-p21 and ATR-Chk1-Cdc25A pathways. Activation of these pathways was characterized by up-regulation of ATM, p53, p21, ATR, and Chk1 mRNA and protein expression, and down-regulation of Cdc25A, cyclin E, cyclin A, CDK2, CDK4, and proliferating cell nuclear antigen (PCNA) mRNA and protein expression. These results indicate that NaF caused S-phase arrest by activating the ATM-p53-p21 and ATR-Chk1-Cdc25A pathways.

Melatonin-mediated upregulation of Sirt3 attenuates sodium fluoride-induced hepatotoxicity by activating the MT1-PI3K/AKT-PGC-1α signaling pathway

Mitochondrial reactive oxygen species (ROS) production has been implicated in the pathogenesis of fluoride toxicity in liver. Melatonin, an indolamine synthesized in the pineal gland, was previously shown to protect against sodium fluoride (NaF)-induced hepatotoxicity. This study investigated the protective effects of melatonin pretreatment on NaF-induced hepatotoxicity and elucidates the potential mechanism of melatonin-mediated protection. Reducing mitochondrial ROS by melatonin substantially attenuated NaF-induced NADPH oxidase 4 (Nox4) upregulation and cytotoxicity in L-02 cells. Melatonin exerted its hepatoprotective effects by upregulating Sirtuin 3 (Sirt3) expression level and its activity. Melatonin increased the activity of manganese superoxide dismutase (SOD2) by promoting Sirt3-mediated deacetylation and promoted SOD2 expression through Sirt3-regulated DNA-binding activity of forkhead box O3 (FoxO3a), thus inhibiting the production of mitochondrial ROS induced by NaF. Notably, increased peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α) by melatonin activated the Sirt3 expression, which was regulated by an estrogen-related receptor (ERR) binding element (ERRE) mapped to Sirt3 promoter region. Analysis of the cell signaling pathway profiling systems and specific pathway inhibition indicated that melatonin enhances PGC-1α expression by activating the PI3K/AKT signaling pathway. Importantly, inhibition of melatonin receptor (MT)-1 blocked the melatonin-activated PI3K/AKT-PGC-1α-Sirt3 signaling. Mechanistic study revealed that the protective effects of melatonin were associated with down-regulation of JNK1/2 phosphorylation. Our findings provided a theoretical basis that melatonin mitigated NaF-induced hepatotoxicity, which, in part, was mediated through the activation of the Sirt3 pathway.

Cell cycle arrest and gene expression profiling of testis in mice exposed to fluoride

Exposure to fluoride results in low reproductive capacity; however, the mechanism underlying the impact of fluoride on male productive system still remains obscure. To assess the potential toxicity in testis of mice administrated with fluoride, global genome microarray and real-time PCR were performed to detect and identify the altered transcriptions. The results revealed that 763 differentially expressed genes were identified, including 330 up-regulated and 433 down-regulated genes, which were involved in spermatogenesis, apoptosis, DNA damage, DNA replication, and cell differentiation. Twelve differential expressed genes were selected to confirm the microarray results using real-time PCR, and the result kept the same tendency with that of microarray. Furthermore, compared with the control group, more apoptotic spermatogenic cells were observed in the fluoride group, and the spermatogonium were markedly increased in S phase and decreased in G2/M phase by fluoride. Our findings suggested global genome microarray provides an insight into the reproductive toxicity induced by fluoride, and several important biological clues for further investigations. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 1558-1565, 2017.

Dental Fluorosis and Catalase Immunoreactivity of the Brain Tissues in Rats Exposed to High Fluoride Pre- and Postnatally

This study evaluated dental fluorosis of the incisors and immunoreactivity in the brain tissues of rats given chronic fluoride doses pre- and postnatally. Female rats were given drinking water with 0, 30 or 100 ppm fluoride ad libitum throughout gestation and the nursing period. In addition, 63 male offspring were treated with the same water regimens as the mothers after weaning and were followed for 1, 3 or 5 months. The upper and lower incisors were collected, and all teeth were examined under a stereomicroscope and scored by two blinded examiners using a modified rodent enamel fluorosis index. Cortical, hippocampal and cerebellar brain samples were evaluated morphologically and immunohistochemically. All fluoride-treated pups were born with low body weight (p = 0.001). All animals from the fluoride groups had enamel fluorosis with defects of various degrees. The increase in the dental fluorosis scores in the fluoride treatment groups was significant (p < 0.01). The catalase immunoreactivity in the 30- and 100-ppm fluoride groups was significantly higher than that in the controls after 1, 3 and 5 months (p < 0.001). In conclusion, this study showed that rats with dental fluorosis had catalase immunoreactivity in the brain tissues, which may reflect the neurobehavioral toxicity of fluoride.

The cellular death pattern of primary haemocytes isolated from the black tiger shrimp (Penaeus monodon)

A key to successfully generate the penaeid shrimp cell line is to find out how primary cells died. The most suitable period to culture Penaeus monodon haemocytes was in the first 48 h of culture because cells had normal morphology, high percent of viable cells (65.29 ± 5.43%), low percent of early (11.75 ± 1.30%) and late apoptotic cells (15.47 ± 11.71%) determined by Annexin V and TUNEL including constant IAP (0.06 ± 0.01-0.07 ± 0.01) and caspase-3 expression (0.30 ± 0.06-0.39 ± 0.10) by real-time PCR throughout the experiment. Moreover, adding 50 and 250 μM of the cell permeable pan caspase inhibitor Z-VAD-FMK produced some melanised cells since the 48(th) hour, while percent of viable cells was decreased since the 24(th) hour with no difference in percent of early and late apoptotic cells compared to control at each time point. No difference of IAP and caspase-3 expression level in both Z-VAD-FMK groups was found compared to control and vehicle groups at each time point, excluding caspase-3 in 250 μM Z-VAD-FMK at the 24(th) hour was higher than control and vehicle. Supplementing sodium fluoride (NaF) induced cell membrane damage and cellular shrinkage of primary haemocytes within 2 h. Even percent of viable cells was reduced down to zero and percent of late apoptotic cells was increased by 2 h of incubation in 25 and 50 mM NaF, IAP and caspase-3 in all NaF groups was not different from control. These results indicate that a number of primary haemocytes derived in this study die through the apoptotic process.

Molecular Mechanisms of Cytotoxicity and Apoptosis Induced by Inorganic Fluoride

Fluoride (F) is ubiquitous natural substance and widespread industrial pollutant. Although low fluoride concentrations are beneficial for normal tooth and bone development, acute or chronic exposure to high fluoride doses results in adverse health effects. The molecular mechanisms underlying fluoride toxicity are different by nature. Fluoride is able to stimulate G-proteins with subsequent activation of downstream signal transduction pathways such as PKA-, PKC-, PI3-kinase-, Ca2+-, and MAPK-dependent systems. G-protein-independent routes include tyrosine phosphorylation and protein phosphatase inhibition. Along with other toxic effects, fluoride was shown to induce oxidative stress leading to excessive generation of ROS, lipid peroxidation, decrease in the GSH/GSSH ratio, and alterations in activities of antioxidant enzymes, as well as to inhibit glycolysis thus causing the depletion of cellular ATP and disturbances in cellular metabolism. Fluoride triggers the disruption of mitochondria outer membrane and release of cytochrome c into cytosol, what activates caspases-9 and -3 (intrinsic) apoptotic pathway. Extrinsic (death receptor) Fas/FasL-caspase-8 and -3 pathway was also described to be implicated in fluoride-induced apoptosis. Fluoride decreases the ratio of antiapoptotic/proapoptotic Bcl-2 family proteins and upregulates the expression of p53 protein. Finally, fluoride changes the expression profile of apoptosis-related genes and causes endoplasmic reticulum stress leading to inhibition of protein synthesis.

Effects of fluoride on expression of bone-specific genes in developing Xenopus laevis larvae

The effect of fluoride treatment on the expression of a panel of osteogenic and stress markers in Stage 55 premetamorphic Xenopus larvae was examined at the precise onset of replacement of the larval cartilaginous skeleton with bone. A dosing regimen of 10 mmol/L sodium fluoride over 8 days was followed, during which time larvae developed to Stage 58, when the process of progressive ossification takes place in the vertebral column and membranous bones of the skull, pelvic, and pectoral girdles and portions of the appendicular skeleton. Markers of bone formation, including COL1A1, the transcription factors Osterix, RUNX2-II, and matrix metalloproteinases MMP1 and MMP13, decreased relative to age-matched controls, though the osteoblast marker BGLAP was not significantly altered. Expression of the pro-osteoclastogenic factor RANKL decreased, whereas expression of the anti-osteoclastogenic factor osteoprotegerin increased. Altered expression of oxidative stress markers, with the exception of superoxide dismutase, was generally not observed. These data demonstrate the potent effects of fluoride on the expression of factors required for osteoblast and osteoclast differentiation, as well as on the expression of osteoblast products, including MMP1 and collagen. Importantly, these effects were observed in the absence of significant changes in the expression of oxidative stress markers. The results provide the first molecular insights into the mechanisms underlying skeletal fluorosis in a whole organism developmental model.

Effects of the Fas/Fas-L pathway on fluoride-induced apoptosis in SH-SY5Y cells

The mechanisms underlying fluoride-induced apoptosis in neurons still remain unknown. To investigate apoptosis, caspase-3 activity, and mRNA expression of Fas, Fas-L, and caspases (-3 and -8) induced by fluoride, human neuroblastoma (SH-SY5Y) cells were incubated with 0, 20, 40, and 80 mg/L sodium fluoride (NaF) for 24 h in vitro. The data show that cell viability in the 40 and 80 mg/L fluoride groups were significantly lower than that of the control group. The percentages of apoptosis in the 40 and 80 mg/L fluoride groups were markedly higher than those in the control group, and they increased with the increase in fluoride concentration. The activity of caspase-3 and mRNA expression levels for Fas, Fas-L, and caspases (-3 and -8) in the 40 and 80 mg/L fluoride groups were significantly higher than those in the control group. An agonistic anti-Fas monoclonal antibody (CH-11) significantly augmented apoptosis induction by fluoride, showing a synergistic effect, while a Fas-blocking antibody (ZB4) partly inhibited fluoride-induced apoptosis of SH-SY5Y cells. The results indicate that fluoride exposure could induce apoptosis in SH-SY5Y cells, and the Fas/Fas-L signaling pathway may play an important role in the process.

Exposure to sodium fluoride produces signs of apoptosis in rat leukocytes

Fluoride is naturally present in the earth's crust and can be found in rocks, coal, and clay; thus, it can be found in small quantities in water, air, plants, and animals. Therefore, humans are exposed to fluoride through food, drinking water, and in the air they breathe. Flouride is essential to maintain bone strength and to protect against dental decay, but if it is absorbed too frequently, it can cause tooth decay, osteoporosis, and damage to kidneys, bones, nerves, and muscles. Therefore, the present work was aimed at determining the effect of intake of sodium fluoride (NaF) as an apoptosis inducer in leukocytes of rats treated for eight weeks with 1 or 50 parts per million (ppm) NaF. Expression of p53, bcl-2, and caspade-3 were used as apoptotic and general metabolism indicators of leukocyte-like indicators of the (INT) oxidation system. Male rats were exposed to NaF (1 and 500 ppm) for eight weeks, and then sacrificed weekly to obtain blood samples. Expression of p53, bcl-2, and caspase-3 were determined in leukocytes by Western blot, and general metabolism of leukocytes was analyzed with a commercial kit. We found changes in the expression of the proteins described, especially when the animals received 50 ppm of NaF. These results indicate that NaF intoxication can be an apoptosis inducer in rat leukocytes treated with the compound for eight weeks.

The effects of fluoride on cell migration, cell proliferation, and cell metabolism in GH4C1 pituitary tumour cells

The consumption of drinking water rich in fluoride has toxic effects on the central nervous system. In cell biology research, fluoride is currently used as a phosphatase inhibitor. The aim of the present study was to evaluate the effect of fluoride on different physiological processes in GH4C1 pituitary tumour cells. We used a range of different fluoride concentrations, from levels below normal human serum concentrations (0.23 and 1.2 micromol/L) to those observed in chronically exposed persons (10.7 micromol/L) and above (107 and 1072 micromol/L). Treatment of 10.7 micromol/L fluoride resulted in a discrete induction of DNA synthesis, without a change in cell number. Cell migration, a behaviour stimulated by growth factors, was increased in cells treated with 2.4 micromol/L. At this fluoride concentration, changes in phosphorylation status of both cytoskeletal and cytosolic protein fractions, as well as in actin cytoskeletal arrangements were observed. The GH4C1 fluoride treated cells had significantly less cellular protein than control cells, suggesting an effect of fluoride on hormone secretion and protein synthesis in this endocrine cell. The bioreduction of MTT was significantly increased with a wide range of fluoride concentrations. With the highest fluoride concentration, 1072 micromol/L, all of the analysed parameters were significantly reduced, suggesting that this dose is highly toxic in GH4C1 cells. Our results show that biologically relevant concentrations of fluoride are capable of increasing cell migration in tumour cells, suggesting that exposure to fluoride could stimulate tumour invasion.

Effects of fluoride on the expression of NCAM, oxidative stress, and apoptosis in primary cultured hippocampal neurons

The mechanisms underlying the neurotoxicity of endemic fluorosis still remain unknown. To investigate the expression level of neural cell adhesion molecules (NCAM), oxidative stress, and apoptosis induced by fluoride, the primary rat hippocampal neurons were incubated with 20, 40, and 80 mg/l sodium fluoride for 24 h in vitro. The results showed that the cell survival rate in the 80 mg/l fluoride-treated group was significantly lower than that of the control group. Forty and 80 mg/l of fluoride induced significantly increased lactate dehydrogenase release, intracellular reactive oxygen species, and the percentage of apoptosis. Compared with control group, the malondialdehyde levels were significantly elevated while glutathione levels and glutathione peroxidase activities were decreased in all fluoride-treated groups, accompanied by the markedly reduced superoxide dismutase activity in 80 mg/l fluoride-treated group. With respect to NCAM mRNA expression levels, a significant dose-dependent decrease was observed in 40 and 80 mg/l fluoride-treated groups against the control group. In addition, as compared to the control group, the protein expression levels of NCAM-180 in 40 and 80 mg/l fluoride-treated groups, NCAM-140 in all fluoride-treated groups, and NCAM-120 in the 80 mg/l fluoride-treated group were significantly decreased. Our study herein suggested that fluoride could cause oxidative stress, apoptosis, and decreased mRNA and protein expression levels of NCAM in rat hippocampal neurons, contributing to the neurotoxicity induced by fluoride.