Effects of Fluoride on Autophagy in Mouse Sertoli Cells

Analysis of Toxic Effects of Fluoride on Ovine Follicular Granulosa Cells Using RNA-Seq

Fluoride is abundant in the environment and is an essential trace element in living organisms. However, prolonged excessive fluoride intake can lead to fluorosis, which poses a threat to the reproductive health of animals and humans. Although previous research has mainly focused on animal models, the impact of fluoride on ovine follicular granulosa cells (GCs) has not been comprehensively elucidated. This study employed RNA-Seq technology to elucidate the toxic effects of fluoride on ovine follicular GCs and its mechanism of action. Culturing primary ovine follicular GCs in vitro and subjecting them to fluoride treatment revealed 3218 differentially expressed genes (DEGs), with 2278 upregulated and 940 downregulated. Significantly, this study unveiled fluoride's induction of endoplasmic reticulum (ER) stress in cells, triggering a cascade involving the PERK pathway factor ATF4, leading to cell death via DDIT3/CHOP activation and the subsequent upregulation of CHAC1, ATF3, ERO1α, and TRIB3. These findings provide crucial insights into the toxicity of fluoride in ovine, offering a foundation for mitigating fluoride-related losses in the farming industry.

Fluoride-Induced Mitochondrial Dysfunction and Approaches for Its Intervention

Fluoride is present everywhere in nature. The primary way that individuals are exposed to fluoride is by drinking water. It's interesting to note that while low fluoride levels are good for bone and tooth growth, prolonged fluoride exposure is bad for human health. Additionally, preclinical studies link oxidative stress, inflammation, and programmed cell death to fluoride toxicity. Moreover, mitochondria play a crucial role in the production of reactive oxygen species (ROS). On the other hand, little is known about fluoride's impact on mitophagy, biogenesis, and mitochondrial dynamics. These actions control the growth, composition, and organisation of mitochondria, and the purification of mitochondrial DNA helps to inhibit the production of reactive oxygen species and the release of cytochrome c, which enables cells to survive the effects of fluoride poisoning. In this review, we discuss the different pathways involved in mitochondrial toxicity and dysfunction induced by fluoride. For therapeutic approaches, we discussed different phytochemical and pharmacological agents which reduce the toxicity of fluoride via maintained by imbalanced cellular processes, mitochondrial dynamics, and scavenging the ROS.

The influence of NQO2 on the dysfunctional autophagy and oxidative stress induced in the hippocampus of rats and in SH-SY5Y cells by fluoride

INTRODUCTION

For investigating the mechanism of brain injury caused by chronic fluorosis, this study was designed to determine whether NRH:quinone oxidoreductase 2 (NQO2) can influence autophagic disruption and oxidative stress induced in the central nervous system exposed to a high level of fluoride.

METHODS

Sprague-Dawley rats drank tap water containing different concentrations of fluoride for 3 or 6 months. SH-SY5Y cells were either transfected with NQO2 RNA interference or treated with NQO2 inhibitor or activator and at the same time exposed to fluoride. The enrichment of gene signaling pathways related to autophagy was evaluated by Gene Set Enrichment Analysis; expressions of NQO2 and autophagy-related protein 5 (ATG5), LC3-II and p62, and mammalian target of rapamycin (mTOR) were quantified by Western-blotting or fluorescent staining; and the levels of malondialdehyde (MDA) and superoxide dismutase (SOD) assayed biochemically and reactive oxygen species (ROS) detected by flow cytometry.

RESULTS

In the hippocampal CA3 region of rats exposed to high fluoride, the morphological characteristics of neurons were altered; the numbers of autophagosomes in the cytoplasm and the levels of NQO2 increased; the level of p-mTOR was decreased, and the levels of ATG5, LC3-II and p62 were elevated; and genes related to autophagy enriched. In vitro, in addition to similar changes in NQO2, p-mTOR, ATG5, LC3 II, and p62, exposure of SH-SY5Y cells to fluoride enhanced MDA and ROS contents and reduced SOD activity. Inhibition of NQO2 with RNAi or an inhibitor attenuated the disturbance of the autophagic flux and enhanced oxidative stress in these cells exposed to high fluoride.

CONCLUSION

Our findings indicate that NQO2 may be involved in regulating autophagy and oxidative stress and thereby exerts an impact on brain injury caused by chronic fluorosis.

Fluoride-Induced Sperm Damage and HuR-Mediated Excessive Apoptosis and Autophagy in Spermatocytes

It is critical to determine the mechanism underlying fluoride (F)-induced damage of the testes to develop appropriate strategies for monitoring and intervention. In the present study, exposure to 50 mg/L sodium fluoride (NaF) for 90 days damaged the normal structure of the testes and quality of the sperm, particularly the spermatocytes, and triggered overexpression of human antigen R (Elavl1/HuR) according to western blotting and immunofluorescence. Furthermore, 0.5 mM NaF exposure for 24 h exposure increased the proportion of apoptosis and expression of caspase-3 and caspase-9 in mouse spermatocytes (GC-2spd cell line), whereas inhibition of HuR reduced apoptosis and the expression of caspase-3 and caspase-9. Additionally, inhibition of HuR alleviated F-induced autophagy based on observation of the autophagy bodies, detection of autophagy activity, and analysis of the expression of the LC3II/LC3I and p62 proteins. These results reveal that excessive F can lead to overexpression of HuR, resulting in high levels of apoptosis and autophagy in spermatocytes. These findings improve the understanding of the mechanisms underlying F-induced male reproductive toxicity, and HuR may be explored as a treatment target for certain conditions. Excessive fluoride can induce overexpression of HuR in testis and result in excessive apoptosis and autophagy in spermatocytes as well as male reproductive damage, such as a decreased sperm count, decreased sperm motility, and increased deformity rate.

Effects of neuron autophagy induced by arsenic and fluoride on spatial learning and memory in offspring rats

There are numerous studies showing that exposure to arsenic (As) or fluoride (F) damages the nervous system, but there is no literature investigating the effects of combined As and F exposure to induce autophagy on neurotoxicity in the offspring. In this study, we developed a rat model of As and/or F exposure through drinking water from before pregnancy to 90 days postnatal. The offspring rats were randomly divided into nine groups. Sodium arsenite (NaAsO₂) (0, 35, 70 mg/L) and Sodium fluoride (NaF) (0, 50, 100 mg/L) were designed according to 3 × 3 factorial design. Our results suggested that the presence of F might antagonize the excretion of total As in urine, and As-F co-exposure led to severe pathological damage in brain tissue and reduced spatial learning and memory ability. At the same time, the experiments showed that As and F increased Beclin1 expression and LC3B ratio to activate autophagy; both P62 and Lamp2 expression were increased, suggesting that autophagy lysosomal degradation was blocked; SYN and JIP1 expression were significantly decreased, disrupting synaptic structure and function. Axonal autophagosome reverse transport regulation might be affected by combined As-F exposure, exacerbating neuronal synaptic damage and inducing neurotoxicity. Further analysis showed that there was an interaction between As and F exposure-induced changes in autolysosome-related proteins in the hippocampus, which showed antagonism, and the antagonism of the high As combined exposure groups were stronger than that of the low As combined exposure groups. In conclusion, our study showed that combined As and F exposure might induce reverse transport impairment of autophagy on axons, leading to autophagy defects, which in turn led to disruption of synaptic morphology and function, induced neurotoxicity, and there was an interaction between As and F, the type of its combined effect was antagonism.

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.

DNA Methylation Profiles of Ovarian Granular Cells from Fluorosis Female Patients Suffering Reproductive Dysfunctions

Fluorosis often causes female reproductive dysfunction. A rapid turnover of DNA methylation is a pathological change in many human diseases, including female reproductive dysfunction. The role of DNA methylation in fluorosis was unknown and investigated in this experiment. Fifty fluorosis women patients were selected as High F group and forty-six healthy women were recruited as Control group were enrolled. In addition, ovarian granulosa cells were obtained from five women in High F group and five women in Control group. All ten women went through in vitro fertilization (IVF) process with DNA methylation sequencing. KGN cells (human granulosa cell line) were cultured with different concentrations of sodium fluoride (0-8 mM NaF) for 24 h for the in vitro study. The level of DNA methylation in blood samples was significantly higher in High F group than that in Control group. The level of serum estradiol (E₂) was significantly lower in women from High F group, while the levels of serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in High F group were significantly higher than that in Control group. The methylation sequences of KGN cells relating to autophagy were significantly changed by NaF treatment dose-dependently. Based on these results, it is concluded that DNA methylation and autophagy may play a significant role in the pathophysiology of reproductive dysfunction caused by fluorosis.

Effects of fluoride exposure on mitochondrial function: Energy metabolism, dynamics, biogenesis and mitophagy

Fluoride is ubiquitous in the environment. Furthermore, drinking water represents the main source of exposure to fluoride for humans. Interestingly, low fluoride concentrations have beneficial effects on bone and teeth development; however, chronic fluoride exposure has harmful effects on human health. Besides, preclinical studies associate fluoride toxicity with oxidative stress, inflammation, and apoptosis. On the other hand, it is well-known that mitochondria play a key role in reactive oxygen species production. By contrast, fluoride's effect on processes such as mitochondrial dynamics, biogenesis and mitophagy are little known. These processes modulate the size, content, and distribution of mitochondria and their depuration help to counter the reactive oxygen species production and cytochrome c release, thereby allowing cell survival. However, a maladaptive response could enhance fluoride-induced toxicity. The present review gives a brief account of fluoride-induced mitochondrial alterations on soft and hard tissues, including liver, reproductive organs, heart, brain, lung, kidney, bone, and tooth.

Effect of Fluoride on Cytotoxicity Involved in Mitochondrial Dysfunction: A Review of Mechanism

Fluoride is commonly found in the soil and water environment and may act as chronic poison. A large amount of fluoride deposition causes serious harm to the ecological environment and human health. Mitochondrial dysfunction is a shared feature of fluorosis, and numerous studies reported this phenomenon in different model systems. More and more evidence shows that the functions of mitochondria play an extremely influential role in the organs and tissues after fluorosis. Fluoride invades into cells and mainly damages mitochondria, resulting in decreased activity of mitochondrial related enzymes, weakening of protein expression, damage of respiratory chain, excessive fission, disturbance of fusion, disorder of calcium regulation, resulting in the decrease of intracellular ATP and the accumulation of Reactive oxygen species. At the same time, the decrease of mitochondrial membrane potential leads to the release of Cyt c, causing a series of caspase cascade reactions and resulting in apoptosis. This article mainly reviews the mechanism of cytotoxicity related to mitochondrial dysfunction after fluorosis. A series of mitochondrial dysfunction caused by fluorosis, such as mitochondrial dynamics, mitochondrial Reactive oxygen species, mitochondrial fission, mitochondrial respiratory chain, mitochondrial autophagy apoptosis, mitochondrial fusion disturbance, mitochondrial calcium regulation are emphasized, and the mechanism of the effect of fluoride on cytotoxicity related to mitochondrial dysfunction are further explored.

Clofibrate, a Peroxisome Proliferator-Activated Receptor-Alpha (PPARα) Agonist, and Its Molecular Mechanisms of Action against Sodium Fluoride-Induced Toxicity

Sodium fluoride (NaF) is one of the neglected environmental pollutants. It is ubiquitously found in the soil, water, and environment. Interestingly, fluoride has been extensively utilized for prevention of dental caries and tartar formation, and may be added to mouthwash, mouth rinse, and toothpastes. This study is aimed at mitigating fluoride-induced hypertension and nephrotoxicity with clofibrate, a peroxisome proliferator-activated receptor-alpha (PPARα) agonist. For this study, forty male Wistar rats were used and randomly grouped into ten rats per group, control, sodium fluoride (NaF; 300 ppm) only, NaF plus clofibrate (250 mg/kg) and NaF plus lisinopril (10 mg/kg), respectively, for 7 days. The administration of NaF was by drinking water ad libitum, while clofibrate and lisinopril were administered by oral gavage. Administration of NaF induced hypertension, and was accompanied with exaggerated oxidative stress; depletion of antioxidant defence system; reduced nitric oxide production; increased systolic, diastolic and mean arterial pressure; activation of angiotensin-converting enzyme activity and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB); and testicular apoptosis. Treatment of rats with clofibrate reduced oxidative stress, improved antioxidant status, lowered high blood pressure through the inhibition of angiotensin-converting enzyme activity, mineralocorticoid receptor over-activation, and abrogated testicular apoptosis. Taken together, clofibrate could offer exceptional therapeutic benefit in mitigating toxicity associated with sodium fluoride.

Autophagy: a multifaceted player in the fate of sperm

BACKGROUND

Autophagy is an intracellular catabolic process of degrading and recycling proteins and organelles to modulate various physiological and pathological events, including cell differentiation and development. Emerging data indicate that autophagy is closely associated with male reproduction, especially the biosynthetic and catabolic processes of sperm. Throughout the fate of sperm, a series of highly specialized cellular events occur, involving pre-testicular, testicular and post-testicular events. Nonetheless, the most fundamental question of whether autophagy plays a protective or harmful role in male reproduction, especially in sperm, remains unclear.

OBJECTIVE AND RATIONALE

We summarize the functional roles of autophagy in the pre-testicular (hypothalamic–pituitary–testis (HPG) axis), testicular (spermatocytogenesis, spermatidogenesis, spermiogenesis, spermiation) and post-testicular (sperm maturation and fertilization) processes according to the timeline of sperm fate. Additionally, critical mechanisms of the action and clinical impacts of autophagy on sperm are identified, laying the foundation for the treatment of male infertility.

SEARCH METHODS

In this narrative review, the PubMed database was used to search peer-reviewed publications for summarizing the functional roles of autophagy in the fate of sperm using the following terms: ‘autophagy’, ‘sperm’, ‘hypothalamic–pituitary–testis axis’, ‘spermatogenesis’, ‘spermatocytogenesis’, ‘spermatidogenesis’, ‘spermiogenesis’, ‘spermiation’, ‘sperm maturation’, ‘fertilization’, ‘capacitation’ and ‘acrosome’ in combination with autophagy-related proteins. We also performed a bibliographic search for the clinical impact of the autophagy process using the keywords of autophagy inhibitors such as ‘bafilomycin A1’, ‘chloroquine’, ‘hydroxychloroquine’, ‘3-Methyl Adenine (3-MA)’, ‘lucanthone’, ‘wortmannin’ and autophagy activators such as ‘rapamycin’, ‘perifosine’, ‘metformin’ in combination with ‘disease’, ‘treatment’, ‘therapy’, ‘male infertility’ and equivalent terms. In addition, reference lists of primary and review articles were reviewed for additional relevant publications. All relevant publications until August 2021 were critically evaluated and discussed on the basis of relevance, quality and timelines.

OUTCOMES

(i) In pre-testicular processes, autophagy-related genes are involved in the regulation of the HPG axis; and (ii) in testicular processes, mTORC1, the main gate to autophagy, is crucial for spermatogonia stem cell (SCCs) proliferation, differentiation, meiotic progression, inactivation of sex chromosomes and spermiogenesis. During spermatidogenesis, autophagy maintains haploid round spermatid chromatoid body homeostasis for differentiation. During spermiogenesis, autophagy participates in acrosome biogenesis, flagella assembly, head shaping and the removal of cytoplasm from elongating spermatid. After spermatogenesis, through PDLIM1, autophagy orchestrates apical ectoplasmic specialization and basal ectoplasmic specialization to handle cytoskeleton assembly, governing spermatid movement and release during spermiation. In post-testicular processes, there is no direct evidence that autophagy participates in the process of capacitation. However, autophagy modulates the acrosome reaction, paternal mitochondria elimination and clearance of membranous organelles during fertilization.

WIDER IMPLICATIONS

Deciphering the roles of autophagy in the entire fate of sperm will provide valuable insights into therapies for diseases, especially male infertility.

Fluoride regulates chondrocyte proliferation and autophagy via PI3K/AKT/mTOR signaling pathway

Fluorine is an essential trace element for human health. However, excessive fluoride intake causes skeletal fluorosis which affects cartilage development. Fluoride inhibited chondrocyte proliferation which is the initial and critical step of endochondral ossification, but the underlying mechanism has not been clearly illustrated. Mammalian target of rapamycin (mTOR) is an important protein kinase which modulates various cellular processes and is believed to be a central regulator of chondrocyte proliferation and autophagy. In this study, we explored the effect of fluoride on the proliferation and autophagy of chondrocytes and the regulatory role of mTOR signaling pathway. Our results suggested that NaF inhibited the protein expressions of proliferating cell nuclear antigen (PCNA) and pS6 in cultured fetal rat tibias. Furthermore, NaF significantly downregulated the expressions of mTOR signaling pathway-related genes, including PI3K, AKT, mTOR, 4EBP1 and S6K1 in mouse ATDC5 chondrogenic cell line. We also found that NaF increased autophagy in ATDC5 cells. The mRNA and protein levels of autophagy-related genes LC3, Beclin1 and p62 were significantly changed after NaF treatment. Further studies demonstrated that MHY1485, a small-molecular mTOR activator, totally reversed fluoride-induced promotion of autophagy. MHY1485 also recovered the downregulation of proliferative chondrocytes markers Sox9 and Type Ⅱ Collagen (Col2a1) induced by fluoride in ATDC5 cells. Taken together, our result demonstrate that fluoride suppressed proliferation and facilitated autophagy via PI3K/AKT/mTOR signaling pathway in chondrogenesis.

In Vitro Evaluation of the Apoptotic, Autophagic, and Necrotic Molecular Pathways of Fluoride

Prolonged exposure to high doses of fluoride causes chronic poisoning called fluorosis, which affects many tissues and causes serious health problems. This study was planned to investigate the apoptotic, autophagic, and necrotic molecular pathways of fluoride. Sodium fluoride (NaF) was administered to normal rat kidney epithelial (NRK-52E) cells. The NaF IC₅₀ value was determined using the MTT assay. The expression of the genes in the autophagic, apoptotic, and necrotic pathways was determined by real-time PCR. It was determined that there were significant changes in NaF-induced molecular pathways depending on the time. There were no increases in apoptotic and necrotic pathway markers except for Atg3, an autophagy gene, at the 3rd and the 12th hours. However, there was an induction in all cell death signaling pathways at 24 h. The molecular mechanisms demonstrated NaF-induced cellular death in the NRK-52E cell line. It was concluded that these molecular mechanisms were activated with NaF, and different mechanisms accelerated the cellular death at the 24th hour.

Co-exposure to fluoride and arsenic disrupts intestinal flora balance and induces testicular autophagy in offspring rats

While numerous studies have shown that fluoride or arsenic exposure may damage the reproductive system, there are few reports of co-exposure to fluoride and arsenic. In addition, the literature on autophagy and intestinal flora composition in reproductive toxicity studies of co-exposure to fluoride and arsenic is insufficient. In this study, we developed a rat model of fluoride and arsenic exposure via drinking water from pre-pregnancy to 90 days postnatal. Sprague-Dawley rats were randomly divided into sterile water control group, fluoride group (100 mg/L NaF), arsenic group (50 mg/L NaAsO₂) and combined exposure group (100 mg/L NaF+50 mg/L NaAsO₂). Our results showed that fluoride and arsenic exposure caused a reduction in testicular weight and significant pathological damage to tissue. We found that the levels of follicle-stimulating hormone, luteinizing hormone, and testosterone were reduced to varying degrees. Meanwhile experiments showed that fluoride and arsenic exposure can modulate autophagic flux, causing increased levels of Beclin1 and LC3 expression and decreased p62 expression. Analogously, by performing 16S sequencing of rat feces, we found 24 enterobacterial genera that differed significantly among the groups. Furthermore, the flora associated with testicular injury were identified by correlation analysis of hormonal indices and autophagy alterations with intestinal flora composition at the genus level, respectively. In summary, our study shows that fluoride and arsenic co-exposure alters autophagic flux in the testis, causes testicular injury, and reveals an association between altered intestinal flora composition and testicular injury.

Arsenic induces dysfunctional autophagy via dual regulation of mTOR pathway and Beclin1-Vps34/PI3K complex in MLTC-1 cells

Arsenic poisoning and induced potential lesion is a global concern. However, the exact mechanisms underlying its toxicity especially in male reproductive system still remain unclear. Hence, this study aimed to explore the roles of mTOR and Beclin1-Vps34/PI3K complex during As-induced-toxicity using Rapamycin (mTOR inhibitor), Beclin1 siRNA and 3-methyladenine (3-MA, Vps34/PI3K inhibitor) in testicular stromal cells. For this, mouse testis Leydig Tumor Cell lines (MLTC-1) were challenged with As₂O₃ (0, 3, 6 and 9 μM) exposure for 24 hs. Lyso-Tracker Red and Monodansylcadaverine (MDC) staining results depicted a significant accumulation of autophagosomes in MLTC-1 cells exposed to arsenic. Meanwhile, arsenic treatment up-regulated autophagic markers including LC3, Atg7, Beclin1 and Vps34 expressions, mTOR downstream autophagy related genes and the Beclin1-Vps34/PI3K complex associated members. Furthermore, silencing of Beclin1, and inhibition of Vps34/PI3K and mTOR altered the arsenic-induced autophagosomes formation. However, p62, the substrate protein of autophagy, was also up-regulated by arsenic administration independent on Beclin1-Vps34/PI3K complex. Altogether, our results revealed that arsenic exposure induced autophagosomes formation via regulation of the Beclin1-Vps34/PI3K complex and mTOR pathway; the blockage of autophagosomes degradation maybe due to impaired function of lysosomes. Thus, this study provides a novel mechanistic approach with respect to As-induced male reproductive toxicity.

Effect of arsenic and/or fluoride gestational exposure on renal autophagy in offspring mice

Both arsenic (As) and fluorine (F) are toxic substances widely found in the environment, which threaten to various organs of both human and animals, especially the kidney. In this study, to investigate the individual and combined effects of arsenic (15 mg/L As₂O₃(III)) and fluoride (100 mg/L NaF), arsenic (15 mg/L As₂O₃(III)) and fluoride-arsenic (15 mg/L As₂O₃(III)+100 mg/L NaF) on the renal autophagy during early life, a mouse model of gestationally exposed to As and/or F was established. The results showed that the mRNA expression levels of LC3, LC3I, LC3II, Beclin-1, ULK1, Atg13 and Atg14 were significantly increased with a concomitant decrease in mTOR and Bcl-2 up on individual exposure to As and F rather than in combined (As + F) exposure. In addition, the protein expression levels of LC3-II/LC3-I, Beclin-1, and LAMP1 were significantly increased with a concomitant decrease in mTOR and Bcl-2 in the mice subjected to individual exposure than the combined exposure. Based on the results, it was observed that renal tissue of mice was highly sensitive to F than As. Moreover, the toxicity of the combined (As + F) exposure was significantly lower than that of the individual exposure, which could be attributed due to the antagonism between As and F.