NADPH oxidase participates in the oxidative damage caused by fluoride in rat spermatozoa. Protective role of α-tocopherol

Effects of fluoride toxicity on the male reproductive system: A review

BACKGROUND

Fluoride toxicity and fluorosis is an emerging global problem. Fluoride has long been added to water for dental caries prevention; however, it has a variety of damaging consequences on human bodies. The aim of this paper is to analyse all the literature available on the effects of fluoride toxicity on male reproductive system.

METHODS

Research papers were collected using various methods of data collection like Pubmed, Scopus, and Google Scholar from 1980 to 2024, and then reviewed thoroughly.

RESULTS

Fluoride is known to cause various histopathological and biochemical alterations in the male reproductive system. It also affects fertility, semen quality, sperm number and quality,the process of spermatogenesis and spermiogenesis. Key changes caused by fluoride in male reproductive system include structural defects in the flagellum, acrosome, and nucleus of spermatids and epididymal spermatozoa. Degenerative changes in Leydig cells result in reduced testosterone production, causing regression of seminiferous tubules and structural damage to the epididymis, ultimately terminating spermatogenesis which leads to infertility. Decrease in levels of testosterone and activities of various antioxidant enzymes resulting in greater oxidative stress was also seen.

CONCLUSIONS

Fluoride has various detrimental effects on male reproductive system and overall reproductive health. This type of study is important for understanding the effects of fluoride toxicity so that health officials can guide public about safe fluoride exposure limits and the damages it can cause in higher concentrations. Studies using various natural and synthetic ameliorative substances mentioned in the text later can prove to be helpful for development of various therapeutic approaches to mitigate the effects of fluoride toxicity.

Effect of fluoride-induced testicular alteration in rats fed a high-fat diet

Previous research on the well-known environmental pollutant fluoride has demonstrated that fluoride exposure can lead to oxidative stress-related male infertility. Obesity is another public health issue that has a detrimental impact on male fertility. Previously, findings on fluoride toxicity in high-fat diet (HFD) conditions associated with oxidative stress have been evidenced. This study aimed to evaluate the impact of subchronic fluoride exposure (5 mg/kg) plus a HFD on testicular alteration in Wistar rats. Animals were divided into four groups (control, HFD, fluoride, and fluoride 5 mg/kg plus HFD). The HFD contained a 50% kcal increase in fat (saturated fat), after 90 days of co-exposure to fluoride plus HFD, the animals showed a significant decrease in the adiposity index. The co-exposed group showed oxidative damage assessed through decreased glutathione (GSH) concentration (p < 0.0001), increased concentrations of malondialdehyde (MDA) (p < 0.0001), and the oxidation of proteins (p < 0.0001) vs the control group. Finally, testicular histology exhibited a reduction in spermatogonia and spermatocytes. The results of the study indicate that under these conditions, subchronic co-exposure to fluoride under HFD conditions could protect against the accumulation of epididymal fat, however, oxidative alteration at the testicular level is maintained.

L-arginine and lisinopril supplementation protects against sodium fluoride-induced nephrotoxicity and hypertension by suppressing mineralocorticoid receptor and angiotensin-converting enzyme 3 activity

Sodium fluoride (NaF) is one of the neglected environmental toxicants that has continued to silently cause toxicity to both humans and animals. NaF is universally present in water, soil, and atmosphere. The persistent and alarming rate of increase in cardiovascular and renal diseases caused by chemicals such as NaF in mammalian tissues has led to the use of various drugs for the treatment of these diseases. The present study aimed at evaluating the renoprotective and antihypertensive effects of L-arginine against NaF-induced nephrotoxicity. Thirty male Wistar rats (150-180 g) were used in this study. The rats were randomly divided into five groups of six rats each as follows: Control, NaF (300 ppm), NaF + L-arginine (100 mg/kg), NaF + L-arginine (200 mg/kg), and NaF + lisinopril (10 mg/kg). Histopathological examination and immunohistochemistry of renal angiotensin-converting enzyme (ACE) and mineralocorticoid receptor (MCR) were performed. Markers of renal damage, oxidative stress, antioxidant defense system, and blood pressure parameters were determined. L-arginine and lisinopril significantly (P < 0.05) ameliorated the hypertensive effects of NaF. The systolic, diastolic, and mean arterial blood pressure of the treated groups were significantly (P < 0.05) reduced compared with the hypertensive group. This finding was concurrent with significantly increased serum bioavailability of nitric oxide in the hypertensive rats treated with L-arginine and lisinopril. Also, there was a significant reduction in the level of blood urea nitrogen and creatinine of hypertensive rats treated with L-arginine and lisinopril. There was a significant (P < 0.05) reduction in markers of oxidative stress such as malondialdehyde and protein carbonyl and concurrent increase in the levels of antioxidant enzymes in the kidney of hypertensive rats treated with L-arginine and lisinopril. The results of this study suggest that L-arginine and lisinopril normalized blood pressure, reduced oxidative stress, and the expression of renal ACE and mineralocorticoid receptor, and improved nitric oxide production. Thus, L-arginine holds promise as a potential therapy against hypertension and renal damage.

Exposure of Fluoride with Streptozotocin-Induced Diabetes Aggravates Testicular Damage and Spermatozoa Parameters in Mice

Diabetes mellitus is the most common chronic disease worldwide that causes numerous complications, including male infertility. The prevalence of DM is 451 million people and estimated that would increase to 693 million in 2045. Fluorosis caused by drinking water contaminated with inorganic fluoride is a public health problem in many areas around the world. Previous studies have shown that fluoride exposure damages the male reproductive function. This study aimed to evaluate the fluoride sub-chronic exposure on the spermatozoa function in streptozotocin (STZ)-induced diabetic mice. After confirming diabetes by measuring blood glucose levels, the male mice received 45.2 ppm of fluoride added or deionized water. We evaluated several parameters in diabetic mice exposed to fluoride: standard quality analysis, the mitochondrial transmembrane potential (ψm), the caspase activity in spermatozoa, urinary fluoride excretion, and histological evaluation in the testes. After 60 days of fluoride-exposure, diabetic mice, significantly decreased sperm quality (motility, viability, and concentration). Spermatozoa from fluoride-exposure in diabetic mice presented a significant decrease in ψm and a significant increase in activity caspase 3/7. Urinary fluoride excretion was decreased in diabetic mice exposed to fluoride. Subchronic fluoride exposure of mice with STZ-induced diabetes aggravated testicular damage and the spermatozoa function.

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.

Caffeic acid, a phyto polyphenol mitigates fluoride induced hepatotoxicity in rats: A possible mechanism

Fluoride induced hepatotoxicity has been extensively studied in both humans and animals. However, the mechanism underlying in the hepatotoxicity of experimental fluorosis remains obscure. The severity of fluoride toxicity was reduced by oral administration of certain plant derived antioxidants. Caffeic acid (CA) a polyphenolic compound has diverse range of pharmacological activity in the biological system. Therefore, the present study was aimed to investigate the protective mechanism of CA, against fluoride induced hepatotoxicity in rats. The rats were treated with 300 ppm of NaF via drinking water ad libitum alone and in combination with CA at a dose of 50 mg/kg daily for 30 days by oral intubation. CA treatment significantly prevented fluoride induced hepatic damage as evident from the histopathological studies and declined levels of serum fluoride and liver marker enzymes. A significant decrease in the levels of enzymatic (SOD2, CAT, GPx, and GSTpi class) and nonenzymatic (GSH and Vitamin C) antioxidants along with increased ROS, lipid peroxidation, protein carbonyl content, and nitrate levels by fluoride were also prevented in these groups. In addition, CA inhibits fluoride induced apoptosis by altering the Bax and caspase-3p20 expressions. Further, fluoride induced upregulation of Nox4, p38α MAPK, Hsp60, and downregulation of Hsp27 are the indicators for the detection of oxidative damage, apoptosis, and mitochondrial stress was also modulated by CA. These findings reveal that CA supplementation has a protective effect against fluoride induced hepatotoxicity in rats.

Stress response pathways in ameloblasts: implications for amelogenesis and dental fluorosis

Human enamel development of the permanent teeth takes place during childhood and stresses encountered during this period can have lasting effects on the appearance and structural integrity of the enamel. One of the most common examples of this is the development of dental fluorosis after childhood exposure to excess fluoride, an elemental agent used to increase enamel hardness and prevent dental caries. Currently the molecular mechanism responsible for dental fluorosis remains unknown; however, recent work suggests dental fluorosis may be the result of activated stress response pathways in ameloblasts during the development of permanent teeth. Using fluorosis as an example, the role of stress response pathways during enamel maturation is discussed.

Fluoride induces oxidative stress and ATP depletion in the rat erythrocytes in vitro

The present study was designed to examine an ability of inorganic fluoride (F) to induce oxidative stress and energy depletion in the rat erythrocytes in vitro. Accumulation of ROS and alterations in glutathione (GSH) and ATP contents were estimated in the cells incubated with 0.1-10mM NaF for 1, 5 and 24h. Exposure of the rat erythrocytes to NaF was accompanied by progressive accumulation of peroxides, while superoxide (O(2)(-)) production was insignificant. Intracellular GSH content was reduced following 5-h incubation, but considerably elevated after 24h, although GSH/GSSG ratio decreased in both cases. ATP concentration in the NaF-treated cell exhibited a dose- and time-dependent decline, diminishing to extremely low levels within 24h. Thus, exposure of the rat erythrocytes to NaF leads to impairment of the cellular antioxidant system and severe energy depletion, the latter probably being the primary toxic effect.

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.