Sodium fluoride-induced autophagy of ameloblast-like cells via the p-ULk1/ATG13/LC3B pathway in vitro

OBJECTIVE

To investigate the effects of sodium fluoride on the ameloblast and reveal the mechanism of dental fluorosis.

MATERIALS AND METHODS

Mouse ameloblast-like cell line (ALC) cells were treated with various concentrations of NaF, and subjected to Incucyte, fluorescence immunoassay, transmission electron microscopy, reverse transcription quantitative polymerase chain reaction (RT-qPCR), western blot for autophagy examination, alkaline phosphatase and alizarin red staining for mineralization after osteogenic induction.

RESULTS

NaF exerts a dose-dependent inhibitory effect on ALC cell growth. TEM and fluorescence immunoassay showed that 1.5 mM or higher concentrations of NaF could induce a fusion of lysosome and mitochondria, finally increasing the number of autophagosome. RT-qPCR and western blot showed that the upregulation of autophagy related gene 13 (ATG13), downregulation of phosphorylated Unc-51-like kinase 1 (p-ULK1) were found in NaF-induced autophagy of ALC cells. The knockdown of ATG13 could rescue it as well as the expression of p-ULK1 and LC3B. Besides, alizarin red staining showed that fluoride under these concentrations could promote the mineralization of ALC.

CONCLUSIONS

The data show that fluoride in higher concentration can induce autophagy via the p-ULk1/ATG13/LC3B pathway of ALCs than lower ones promote mineralization in vitro, which provides insight into the function of NaF in the autophagy and mineralization of ameloblast.

Excessive fluoride impairs autophagy flux in ameloblasts which is prevented by the autophagy activator rapamycin

Excessive fluoride intake can cause dental fluorosis during teeth development and growth. However, the mechanisms underlying fluoride-induced enamel damage are still not fully elucidated. Previously, we observed fluoride-induced autophagy in ameloblasts, but the effects of fluoride on autophagy flux in ameloblasts remain unclear. Hence, this study aimed to clarify the effects of fluoride and rapamycin, an autophagy activator, on autophagy flux in ameloblasts. This in vitro study used the murine ameloblast-derived cell line LS8. Cells were treated with different concentrations of sodium fluoride (NaF) to evaluate NaF-induced cytotoxicity. Using transmission electron microscopy, we observed an increase in the number of autophagosomes with increasing fluoride concentrations. Western blot analyses showed increases in microtubule-associated protein 1 light chain 3 (LC3) and SQSTM1 (p62) expression after NaF treatment and an increase in LC3II expression after bafilomycin A1 administration. Together with changes in RFP-GFP-LC3 lentivirus expression, this demonstrated that fluoride impaired autophagy flux. Furthermore, we evaluated whether rapamycin can alleviate fluoride-induced cytotoxicity by restoring autophagy flux. Compared to the NaF-treated group, LS8 cells cotreated with NaF and rapamycin grew considerably better and had significantly decreased p62 expression. Taken together, these data suggest that fluoride-induced impaired autophagosome degradation may damage ameloblasts. This provides experimental in vitro evidence and an explanation for the observed NaF-induced toxicity of ameloblasts. Rapamycin probably alleviates this impairment by decreasing the expression of p62, thereby preventing autophagy defects.

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.

The Effects of Sodium Fluoride (NaF) Treatment on the PI3K/Akt Signal Pathway in NRK-52E Cells

The effects of the element fluorine on the phosphoinositide-3-kinase-protein kinase B/Akt (PI3K/Akt) pathway has a significant role in regulation of intracellular molecular mechanisms. NRK-52E rat kidney epithelial cell line was selected as the material of the study. NaF was used as the fluorine source in the study. The NaF dose was determined with the MTT assay. The NaF concentrations were determined as the proliferation concentration of 10 μM and IC₂₅ (2250 μM) and IC₅₀ (4250 μM) for 24 h. In the study, the erb-b2 receptor tyrosine kinase 2 (ERBB2), phosphoinositide-3-kinase (PI3K), Protein kinase B (PKB,Akt), Mammalian target of rapamycin (mTOR), and the Tumor protein 53 (TP53) genes were considered as the target genes. NaF concentration was administered on the cells. Total mRNA was isolated. mRNAs were turned into cDNA. The expression levels of the target genes were determined by RT-qPCR method. According to the results obtained in the study, the low NaF concentration increased the expression levels of the ERBB2, PI3K, and Akt genes, while the higher concentrations did not significantly affect these levels. The expression of mTOR decreased at all given concentrations. The expression of the TP53 gene did not change at the low concentration, while it increased at the high concentrations. Based on the results, it may be stated that fluorine may inhibit the kinase enzymes in the PI3K/Akt pathway. In summary, in the pathogenesis of the cell damage caused by fluorine in the NRK-52E cell line, the PI3K/Akt/mTOR pathway is an important signal pathway.

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.

Altered miRNA expression profiling in enamel organ of fluoride affected rat embryos

Evidence has shown that miRNAs could play a role in dental fluorosis, but there is no study has investigated the global expression miRNA profiles of fluoride-exposed enamel organ. In this study, we analysed the differentially expressed (DE) miRNAs between fluoride-treated and control enamel organ for the first time and found several candidate miRNAs and signaling pathways worthy of further research. Thirty Wistar rats were randomly distributed into three groups and exposed to drinking water with different fluoride contents for 10 weeks and during the gestation. The three groups were a control group (distilled water), medium fluoride group (75 mg/L NaF), and high fluoride group (150 mg/L NaF). On the embryonic day 19.5, the mandible was dissected for histological analysis, and the enamel organ of the mandibular first molar tooth germ was collected for miRNA sequencing (miRNA-seq) and quantitative real-time PCR analysis (qRT-PCR). Typical dental fluorosis was observed in the incisors of the prepregnant rats. In addition to the disorganized structure of enamel organ cells, 39 DE miRNAs were identified in the fluoride groups compared with the control group, and good agreement between the miRNA-seq data and qRT-PCR data was found. The functional annotation of the target genes of 39 DE miRNAs showed significant enrichment in metabolic process, cell differentiation, calcium signaling pathway, and mitogen-activated protein kinase(MAPK) signaling pathway terms. This study provides a theoretical reference for an extensive understanding of the mechanism of fluorosis and potential valuable miRNAs as therapeutic targets in fluorosis.

Sodium fluoride induces splenocyte autophagy via the mammalian targets of rapamycin (mTOR) signaling pathway in growing mice

Fluoride is known to impair organism’s development and function via adverse effects, and autophagy plays a regulation role in human or animal health and disease. At present, there are no reports focused on fluoride-induced autophagy in the animal and human spleen. The objective of this study was to investigate sodium fluoride (NaF)-induced splenocyte autophagy and the potential mechanism via regulation of p-mTOR in growing mice by using the methods of transmission electron microscopy (TEM), immunohistochemistry (IHC), quantitative real-time polymerase chain reaction (qRT-PCR) and western blot. A total of 240 ICR mice were equally allocated into four groups with intragastric administration of distilled water in the control group and 12, 24, 48 mg/kg NaF solution in the experimental groups for 42 days. Results revealed that NaF increased autophagosomes or autolysosomes in spleen. Simultaneously, the autophagy marker LC3 brown punctate staining was increased with NaF dosage increase. On the other hand, NaF caused inhibition of mTOR activity, which was characterized by down-regulation of PI3K, Akt and mTOR mRNA and protein expression levels. And the suppression of mTOR activity in turn resulted in the significantly increased of ULK1 and Atg13 expression levels. Concurrently, NaF increased the levels of mRNA and protein expression of autophagy markers LC3, Beclin1, Atg16L1, Atg12, Atg5 and decreased the mRNA and protein expression levels of p62. The above-mentioned findings verify that NaF induces autophagy via mTOR signaling pathway. The inhibition of mTOR activity and alteration of autophagy-related genes and proteins are the potential molecular mechanism of NaF-induced splenocyte autophagy.

Sodium fluoride regulates the osteo/odontogenic differentiation of stem cells from apical papilla by modulating autophagy

Fluoride (sodium fluoride) is thought to be essential in the development of tooth, and research shows that fluoride can modulate the differentiation of dental stem cells. However, the effects of fluoride on the committed differentiation of stem cells from apical papilla (SCAPs) and the underlying mechanisms remain unclear. Here, SCAPs were isolated from healthy extracted human third molars with immature roots and then were cultured with NaF conditioned media. Cell Counting Kit-8, EdU staining, and flow cytometry were performed to detected the proliferation activity. Alkaline phosphatase (ALP) activity, Alizarin Red staining, Western blot assay, and real-time reverse-transcription polymerase chain reaction were applied to assess the osteo/odontogenic differentiation NaF-treated SCAPs. Western blot assay and transmission electron microscope were used to evaluate the autophagy involved in the differentiation of SCAPs. ALP activity, ALP protein, and messenger RNA (mRNA) expression showed that 0.5 mM was the optimal concentration for the induction of SCAPs by NaF. 0.5 mM NaF-treated SCAPs induced more mineralized nodules as compared with untreated cells. Moreover, the osteo/odontogenic markers (RUNX2, OSX, DSP, and OCN) in mRNA levels were upregulated while the protein levels of these markers increased considerably in 0.5 mM NaF-treated SCAPs. Furthermore, the autophagy-related proteins (LC3, ATG5, and Beclin1) increased in NaF-treated SCAPs, and the osteo/odontogenic makers significantly decreased while silencing ATG5 to block autophagy. In all, sodium fluoride can regulate the osteo/odontogenic differentiation of SCAPs by modulating autophagy.

The pathogenesis of endemic fluorosis: Research progress in the last 5 years

Fluorine is one of the trace elements necessary for health. It has many physiological functions, and participates in normal metabolism. However, fluorine has paradoxical effects on the body. Many studies have shown that tissues and organs of humans and animals appear to suffer different degrees of damage after long-term direct or indirect exposure to more fluoride than required to meet the physiological demand. Although the aetiology of endemic fluorosis is clear, its specific pathogenesis is inconclusive. In the past 5 years, many researchers have conducted in-depth studies into the pathogenesis of endemic fluorosis. Research in the areas of fluoride-induced stress pathways, signalling pathways and apoptosis has provided further extensive knowledge at the molecular and genetic level. In this article, we summarize the main results.

Fluoride Alters Klk4 Expression in Maturation Ameloblasts through Androgen and Progesterone Receptor Signaling

Fluorosed maturation stage enamel is hypomineralized in part due to a delay in the removal of matrix proteins to inhibit final crystal growth. The delay in protein removal is likely related to reduced expression of kallikrein-related peptidase 4 (KLK4), resulting in a reduced matrix proteinase activity that found in fluorosed enamel. Klk4 transcription is known to be regulated in other cell types by androgen receptor (AR) and progesterone receptors (PR). In this study, we determined the possible role of fluoride in down-regulation of KLK4 expression through changes in AR and PR. Immunohistochemical localization showed that both AR and PR nuclear translocation was suppressed in fluoride exposed mice. However, when AR signaling was silenced in mouse ameloblast-lineage cells (ALCs), expression of both Pgr and Klk4 were increased. Similar to the effect from AR silencing, fluoride also upregulated Pgr in ALCs, but downregulated Klk4. This finding suggests that though suppression of AR transactivation by fluoride increases Prg expression, inhibition of PR transactivation by fluoride has a much greater effect, ultimately resulting in downregulation of Klk4 expression. These findings indicate that in ameloblasts, PR has a dominant role in regulating Klk4 expression. We found that when AR was retained in the cytoplasm in the presence of fluoride, that co-localized with heat shock protein 90 (HSP90), a well-known chaperone for steroid hormone receptors. HSP90 also known to regulate TGF-β signaling. Consistent with the effect of fluoride on AR and HSP90, we found evidence of reduced TGF-β signaling activity in fluorosed ameloblasts as reduced immunolocalization of TGFB1 and TGFBR-2 and a significant increase in Cyclin D1 mRNA expression, which also possibly contributes to the reduced AR signaling activity. In vitro, when serum was removed from the media, aluminum was required for fluoride to inhibit the dissociation of HSP90 from AR. In conclusion, fluoride related downregulation of Klk4 is associated with reduced nuclear translocation of AR and PR, and also reduced TGF-β signaling activity, all of which are regulated by HSP90. We suggest that a common mechanism by which fluoride affects AR, PR, and TGF-β signaling is through inhibiting ATP-dependent conformational cycling of HSP90.

Fluoride-Induced Autophagy via the Regulation of Phosphorylation of Mammalian Targets of Rapamycin in Mice Leydig Cells

Fluoride is known to impair testicular function and decrease testosterone levels, yet the underlying mechanisms remain inconclusive. The objective of this study is to investigate the roles of autophagy in fluoride-induced male reproductive toxicity using both in vivo and in vitro Leydig cell models. Using transmission electron microscopy and monodansylcadaverine staining, we observed increasing numbers of autophagosomes in testicular tissue, especially in Leydig cells of fluoride-exposed mice. Further study revealed that fluoride increased the levels of mRNA and protein expression of autophagy markers LC3, Beclin1, and Atg 5 in primary Leydig cells. Furthermore, fluoride inhibited the phosphorylation of mammalian targets of rapamycin and 4EBP1, which in turn resulted in a decrease in the levels of AKT and PI3K mRNA expression, as well as an elevation of the level of AMPK expression in both testes and primary Leydig cells. Additionally, fluoride exposure significantly changed the mRNA expression of the PDK1, TSC, and Atg13 regulator genes in primary Leydig cells but not in testicular cells. Taken together, our findings highlight the roles of autophagy in fluoride-induced testicular and Leydig cell damage and contribute to the elucidation of the underlying mechanisms of fluoride-induced male reproductive toxicity.

[Role of Beclin 1 gene in autophagy and apoptosis of SW620 cells]

OBJECTIVE

To study the role of Beclin1 gene in autophagy and apoptosis of SW620 cells.

METHODS

RNA interference was used to knockdown the expression of Beclin 1 in SW620 cells, and the cell viability was measured by MTT assays. The autophagic activity in serum-starved SW620 CRC cells was evaluated by assessing endogenous microtubule-associated protein 1 light chain 3 (LC3) protein levels using immunofluorescence assay. Flow cytometry was used to measure the apoptosis rate of SW620 cells treated with serum deprivation, staurosporine or etoposide, and the protein expression of Beclin1 was detected using Western blotting.

RESULTS

The viability of cells in pSUPER-Becl group was significantly reduced after serum deprivation (P<0.05). Serum deprivation for 24 h resulted in a stronger apoptotic resistance in cells in the control and pSUPER-non groups than in pSUPER-Becl group (P<0.05). Treatment with staurosporine the most significantly increased the cell apoptosis in pSUPER-Becl group (P<0.05), and similar effect was observed with etoposide treatment (P<0.05). As the time of serum deprivation extended, the expression of Beclin 1 in control group and pSUPER-non group increased progressively (P<0.05), which was consistent with the changes in LC3 expression; LC3 expression in pSUPER-Becl group decreased significantly with a prolonged serum starvation (P<0.05).

CONCLUSION

Beclin 1 is a crucial regulator of autophagy and apoptosis in colorectal cancer cells to maintain the balance between autophagy and apoptosis. Beclin 1 may serve as a protective mechanism to protect colorectal cancer cells from injury caused by low nutrition and chemotherapy byregulating cell autophagy.