Effects of Fluoride on Oxidative Stress Markers of Lipid, Gene, and Protein in Rats

Impact of Fluoride Exposure on Rat Placenta: Foetal/Placental Morphometric Alterations and Decreased Placental Vascular Density

Inorganic fluoride is a geogenic and anthropogenic contaminant widely distributed in the environment and commonly identified in contaminated groundwater. There is limited information on the effect of fluoride exposure on pregnancy. The aim of this study was to evaluate possible placental alterations of fluoride exposure in a rat model simulating preconception and pregnancy exposure conditions in endemic areas. Fluoride exposure was administered orally to foetuses of dams exposed to 2.5 and 5 mg fluoride/kg/d. Foetal weight, height, foetal/placental weight ratio, placental zone thickness, levels of malondialdehyde (MDA) and vascular endothelial growth factor-A (VEGF-A) and vascular density in placental tissue were evaluated. The results showed a nonlinear relationship between these outcomes and the dose of fluoride exposure. In addition, a significant increase in the fluoride concentration in placental tissue was observed. The group that was exposed to 2.5 mg fluoride/kg/d had a greater increase in both MDA levels and VEGF-A levels than the higher dose group. A significant increase in the thickness of the placental zones and a decrease in the vascular density of the labyrinth zone area were also observed in the fluoride-exposed groups. In conclusion, the data obtained demonstrate that fluoride exposure results in morpho-structural alterations in the placenta and that non-monotonic changes in MDA, VEGF-A levels and placental foetal weight ratio were at environmentally relevant concentrations.

Protective effects of sodium butyrate on fluorosis in rats by regulating bone homeostasis and serum metabolism

Fluorosis due to high fluoride levels in drinking water profoundly affects the development of human skeletal and dental structures. Sodium butyrate (NaB) has been found to regulate overall bone mass and prevent pathological bone loss. However, the mechanism of NaB action on fluorosis remains unclear. In this study, a rat model of fluorosis induced by 100 mg/L sodium fluoride was used to investigate the impact of NaB on bone homeostasis and serum metabolomics. It was found that NaB significantly reduced the levels of bone resorption markers CTX-Ⅰ and TRACP-5B in fluorosis rats. Moreover, NaB increased calcium and magnesium levels in bone, while decreasing phosphorus levels. In addition, NaB improved various bone microstructure parameters, including bone mineral density (BMD), trabecular thickness (Tb. Th), trabecular bone separation (Tb. SP), and structural model index (SMI) in the femur. Notably, NaB intervention also enhanced the antioxidant capacity of plasma in fluorosis rats. Furthermore, a comprehensive analysis of serum metabolomics by LC-MS revealed a significant reversal trend of seven biomarkers after the intervention of NaB. Finally, pathway enrichment analysis based on differential metabolites indicated that NaB exerted protective effects on fluorosis by modulating arginine and proline metabolic pathways. These findings suggest that NaB has a beneficial effect on fluorosis and can regulate bone homeostasis by ameliorating metabolic disorders.

Exploration of the SIRT1-mediated BDNF-TrkB signaling pathway in the mechanism of brain damage and learning and memory effects of fluorosis

Introduction

Fluoride is considered an environmental pollutant that seriously affects organisms and ecosystems, and its harmfulness is a perpetual public health concern. The toxic effects of fluoride include organelle damage, oxidative stress, cell cycle destruction, inflammatory factor secretion, apoptosis induction, and synaptic nerve transmission destruction. To reveal the mechanism of fluorosis-induced brain damage, we analyzed the molecular mechanism and learning and memory function of the SIRT1-mediated BDNF-TrkB signaling pathway cascade reaction in fluorosis-induced brain damage through in vivo experiments.

Methods

This study constructed rat models of drinking water fluorosis using 50 mg/L, 100 mg/L, and 150 mg/L fluoride, and observed the occurrence of dental fluorosis in the rats. Subsequently, we measured the fluoride content in rat blood, urine, and bones, and measured the rat learning and memory abilities. Furthermore, oxidative stress products, inflammatory factor levels, and acetylcholinesterase (AchE) and choline acetyltransferase (ChAT) activity were detected. The pathological structural changes to the rat bones and brain tissue were observed. The SIRT1, BDNF, TrkB, and apoptotic protein levels were determined using western blotting.

Results

All rats in the fluoride exposure groups exhibited dental fluorosis; decreased learning and memory abilities; and higher urinary fluoride, bone fluoride, blood fluoride, oxidative stress product, and inflammatory factor levels compared to the control group. The fluoride-exposed rat brain tissue had abnormal AchE and ChAT activity, sparsely arranged hippocampal neurons, blurred cell boundaries, significantly fewer astrocytes, and swollen cells. Furthermore, the nucleoli were absent from the fluoride-exposed rat brain tissue, which also contained folded neuron membranes, deformed mitochondria, absent cristae, vacuole formation, and pyknotic and hyperchromatic chromatin. The fluoride exposure groups had lower SIRT1, BDNF, and TrkB protein levels and higher apoptotic protein levels than the control group, which were closely related to the fluoride dose. The findings demonstrated that excessive fluoride caused brain damage and affected learning and memory abilities.

Discussion

Currently, there is no effective treatment method for the tissue damage caused by fluorosis. Therefore, the effective method for preventing and treating fluorosis damage is to control fluoride intake.

Effect of Fluoride on the Expression of 8-Hydroxy-2'-Deoxyguanosine in the Blood, Kidney, Liver, and Brain of Rats

Excessive exposure of fluoride not only leads to damage on bone, but also has an adverse effect on soft tissues. Oxidative DNA damage induced by fluoride is thought to be one of the toxic mechanisms of fluoride effect. However, the dose-response of fluoride on oxidative DNA damage is barely studied in organisms. This study investigated the concentration of fluoride in rat blood, kidney, liver, and brain as well as the dose-time effect of fluoride on the expression of 8-hydroxy-2'-deoxyguanosine (8-OHdG) in the above tissues. Rats were exposed to 0 mg/L, 25 mg/L, 50 mg/L, and 100 mg/L of fluorine ion and treated for one and three months. The results showed that the accumulation of fluoride in soft tissues was very different. At the first month, blood fluoride was increased, liver and brain fluoride showed a U-shaped change, and kidney fluoride was not significant. At the third month, blood fluoride was altered with an inverted U-shaped change, kidney and brain fluoride increased, but liver fluoride decreased. Both the exposure concentration and the time of exposure had a significant effect on the expression of 8-OHdG in the above tissues. However, the effect patterns of fluoride on these tissues were notably different at different times. At the first month of fluoride treatment, blood, kidney, and liver 8-OHdG decreased with the increasing fluoride concentration. At the third month, blood 8-OHdG showed a U-shaped change, but kidney 8-OHdG altered with an inverted U-shaped change. Liver 8-OHdG increased, while brain 8-OHdG decreased at the third month. Correlation analysis showed that only blood 8-OHdG was significantly inversely correlated with blood fluoride and dental fluorosis grade in both the first and third months. Liver 8-OHdG was negatively and significantly correlated with liver fluoride. There was a weak but nonsignificant correlation between kidney and brain 8-OHdG and fluoride in both tissues. Additionally, blood 8-OHdG was positively correlated with kidney and liver 8-OHdG at the first month and positively correlated with brain 8-OHdG at the third month. Taken together, our data suggests that concentration and time of fluoride exposure had a significant effect on 8-OHdG, but the effect patterns of fluoride on 8-OHdG were different in the tissues, which suggests that the impact of fluoride on 8-OHdG may be a tissue-specific, as well as a non-monotonic positive correlation.

Fluoride subacute testicular toxicity in Wistar rats: Benchmark dose analysis for the redox parameters, essential elements and DNA damage

Excessive fluoride (F^-) levels in the environment could induce different pathological changes, including comorbidities in reproductive functions. Hence, the aim of the present in vivo study was to explore F^- subacute toxicity mechanisms via Benchmark dose (BMD) methodology on rat's testicles. The experiment was conducted on thirty male Wistar rats for 28 days, divided into six groups (n = 5): 1) Control (tap water); 2) 10 mg/L F^-; 3) 25 mg/L F^-; 4) 50 mg/L F^-; 5) 100 mg/L F^-; 6) 150 mg/L F^-. Testicles were dissected out and processed for the determination of F^- tissue concentrations, redox status parameters, essential elements level, and DNA damage. PROASTweb 70.1 software was used for determination of external and internal dose-response relationship. The results confirmed a significant increase in superoxide anion (O₂^.-), total oxidative status (TOS), copper (Cu), zinc (Zn), iron (Fe), DNA damage levels, and decrease in superoxide dismutase activity (SOD1) and total thiol (SH) groups. The dose-dependent changes were confirmed for SOD1 activity and DNA damage. The most sensitive parameters were SOD1 activity and DNA damage with the lowest BMDLs 0.1 μg F^-/kg b. w. Since human and animal populations are daily and frequently unconsciously exposed to F^-, this dose-response study is valuable for further research regarding the F^- health risk assessment.

Fluoride Exposure and Salivary Glands: How Is Glandular Morphology Susceptible to Long-Term Exposure? A Preclinical Study

Despite a strong body of evidence attesting to the effectiveness of fluoride (F) in preventing and controlling caries, some studies have sought to investigate the influence of F exposure on the salivary glands, organs that are essential for the maintenance of cavity homeostasis through salivary production, finding that exposure to F can cause biochemical and proteomic changes. Thus, this study aimed to investigate the morphological effects of prolonged exposure to F on the salivary glands of mice, at concentrations that would correspond to optimally fluoridated water (suitable for human consumption) and to fluorosis-endemic regions. Twenty-four male mice (Mus musculus) were divided into three groups, according to F levels in the drinking water: 0 (control), 10, or 50 mg F/L, with an exposure period of 60 days. The glands were morphometrically analyzed for the total acinar area, parenchyma area, and stromal area, as well as for the immunohistochemical analysis of myoepithelial cells. The results showed that prolonged exposure to F at 10 mg F/L did not promote significant changes in the morphometry of the salivary glands of mice, which reinforces the safety of the chronic use of F in low doses.

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.

Intrauterine and Postnatal Exposure to High Levels of Fluoride Is Associated with Motor Impairments, Oxidative Stress, and Morphological Damage in the Cerebellum of Offspring Rats

Fluoride (F) is abundantly present on Earth and plays a beneficial role in human health. However, exposure to high doses of F can be a risk, mainly in endemic fluorosis regions. In light of this, we investigated the effects of F exposure during the intrauterine and postnatal periods of rats, in doses similar to those recommended in drinking water and the levels of F in regions with endemic fluorosis, on the offspring rats’ cerebellum. Pregnant rats were divided into three groups: control (received ultrapure water only), 10 mg F/L, and 50 mg F/L for a period of 42 days (21 days gestation and 21 days lactation). At the end of the lactation period, the male pups were evaluated by behavioral tests, morphological markers, and biochemistry assays. The results pointed out that 50 mg F/L exposure during the intrauterine and lactational period of rats is capable of promoting oxidative stress in the cerebellum with a decrease in Purkinje cell density and myelin basic protein compromise, which could be associated with functional motor impairments. In addition, although 10 mg F/L exposure promoted redox alterations, it did not affect other parameters evaluated, highlighting the safe use of F in low doses.

The role of Nrf2 pathway in alleviating fluorine-induced apoptosis by different selenium sources in the chicken duodenum and jejunum

In order to evaluate the alleviative effects and molecular mechanisms of sodium selenite (SS) and selenomethionine (SM) on excessive apoptosis induced by high fluorine (HF) in the duodenum and jejunum of broilers, 720 1 day old Lingnan Yellow broilers were randomly divided into 4 groups (each group assigned 180 chickens with 6 replicates) and offered either a control diet or test diets (800 mg/kg F, HF group; 800 mg/kg F + 0.15 mg selenium (Se)/kg as SS (SS group) or SM (SM group)) for 50 days. High F intake significantly increased (P < 0.05) apoptosis rates of duodenum and jejunum by inducing oxidative stress and leading to mitochondrial damage. Selenomethionine supplementation effectively alleviated mitochondrial damage and severe apoptosis of duodenum and jejunum caused by HF through decreasing oxidative stress parameters. Selenomethionine added group significantly increased (P < 0.05) nuclear factor erythroid 2-related factor 2 (Nrf2) mRNA and nuclear Nrf2 protein levels as well as Nrf2 downstream antioxidant enzymes expressions in the duodenum and jejunum when compared with the HF group. Selenomethionine was superior to SS in activating the Nrf2 pathway and reducing the apoptosis rate of duodenum. It was concluded that dietary SM supplementation could ameliorate F-induced excessive apoptosis by inducing the Nrf2 pathway. Our findings will bring a promising tactics for the utilization of SM as an efficient antioxidant additive for reducing the intestinal damage caused by fluorosis in poultry.

Borassus flabellifer Linn haustorium methanol extract mitigates fluoride-induced apoptosis by enhancing Nrf2/Haeme oxygenase 1 -dependent glutathione metabolism in intestinal epithelial cells

Fluoride is the most common cause of drinking water-associated toxicity and is known to induce various metabolic imbalances and dental/skeletal fluorosis. The present study analyzed the protective effect of Borassus flabellifer Linn. haustorium extract (BHE) against fluoride-induced intestinal redox metabolism and apoptosis. The total polyphenols and total flavonoids present in BHE were estimated to be 39.67 ± 5.14 mg gallic acid equivalent/g extract and 8.59 ± 0.74 mg quercetin equivalent. In cultured intestinal epithelial cells (IEC-6), sodium fluoride exposure-induced apoptosis mediated through antioxidant enzyme inhibition and subsequent oxidative damages. Further, there observed an increased expression of caspase-3, caspase-7, and apoptotic protease activating factor-1 (apaf-1) genes, increased cytochrome C release, and caspase 3/7 activity indicating the apoptosis- mediated cell death (p < 0.05). Upon pretreatment with BHE, the cytotoxic effect of fluoride was reduced by decreasing the expression of apoptotic genes and increased the cytochrome release as well as caspase 3/7 activity (p < 0.01). Providing the mechanistic basis, the expression of nuclear factor erythroid 2-related factor-2 (Nrf2)/haeme oxygenase-1 (HO1) gene was increased in the BHE pretreated cells; corroborating to these, there observed increased activity of glutathione biosynthetic enzymes (p < 0.05) and glutathione reductase. Hence, the protective effect of BHE may be mediated through Nrf2-mediated glutathione biosynthesis, the subsequent establishment of redox balance, and inhibition of apoptosis in intestinal epithelial cells.