Role of endoplasmic reticulum stress in aberrant activation of fluoride-treated osteoblasts

Vitamin D may assist the UPR against sodium fluoride-induced damage by reducing RIPK1, ATG5, BECN1, oxidative stress and increasing caspase-3 in the osteoblast MC3T3-E1 cell line

BACKGROUND

Out of all measure systemic exposure to fluorides can cause defect of skeletal and dental fluorosis. Endoplasmic reticulum (ER) stress is caused by fluorine-induced oxidative stress and importance of vitamin D in its prevention is not known enough in bone cells. This study was carried out to investigate fluorine-induced oxidative stress, ER stress, and death pathways and the effect of vitamin D on them.

METHODS

MC3T3-E1 mouse osteoblast cell line was used as the material of the study. The NaF and vitamin D concentrations were determined by the MTT assay. NaF treatments and vitamin D supplementation (pre-add, co-add, and post-add) was administered in the cell line at 24th and 48th hours. The expression of the genes in oxidative stress, ER stress, and death pathways was determined using RT-qPCR and Western blotting techniques.

RESULTS

Vitamin D significantly reduced mRNA expression levels of SOD2, CYGB, ATF6, PERK, IRE1, ATG5 and BECN1 whereas caused an increase in levels GPX1, SOD1, NOS2 and Caspase-3 in MC3T3-E1 mouse osteoblast cell line of NaF-induced. In addition, GPX1, SOD1, ATF6, PERK, IRE1, BECN1, Caspase-3 and RIPK1 protein levels were examined by Western blot analysis, and it was determined that vitamin D decreased IRE1 and PERK protein levels, but increased GPX1, SOD1, ATF6 and Caspase-3 protein levels.

CONCLUSION

The findings of the study suggest that vitamin D has protective potential against NaF-induced cytotoxicity reasonably through the attenuation of oxidative stress, ER stress, ATG5, IRE1 and by increasesing caspase-3 in vitro conditions.

The Role of Trace Elements and Minerals in Osteoporosis: A Review of Epidemiological and Laboratory Findings

The objective of the present study was to review recent epidemiological and clinical data on the association between selected minerals and trace elements and osteoporosis, as well as to discuss the molecular mechanisms underlying these associations. We have performed a search in the PubMed-Medline and Google Scholar databases using the MeSH terms "osteoporosis", "osteogenesis", "osteoblast", "osteoclast", and "osteocyte" in association with the names of particular trace elements and minerals through 21 March 2023. The data demonstrate that physiological and nutritional levels of trace elements and minerals promote osteogenic differentiation through the up-regulation of BMP-2 and Wnt/β-catenin signaling, as well as other pathways. miRNA and epigenetic effects were also involved in the regulation of the osteogenic effects of trace minerals. The antiresorptive effect of trace elements and minerals was associated with the inhibition of osteoclastogenesis. At the same time, the effect of trace elements and minerals on bone health appeared to be dose-dependent with low doses promoting an osteogenic effect, whereas high doses exerted opposite effects which promoted bone resorption and impaired bone formation. Concomitant with the results of the laboratory studies, several clinical trials and epidemiological studies demonstrated that supplementation with Zn, Mg, F, and Sr may improve bone quality, thus inducing antiosteoporotic effects.

Unraveling molecular characteristic of fluoride neurotoxicity on U87 glial-like cells: insights from transcriptomic and proteomic approach

The potential of fluoride (F) as a neurotoxicant in humans is still controversial in the literature. However, recent studies have raised the debate by showing different mechanism of F-induced neurotoxicity, as oxidative stress, energy metabolism and inflammation in the central nervous system (CNS). In the present study, we investigated the mechanistic action of two F concentration (0.095 and 0.22 μg/ml) on gene and protein profile network using a human glial cell in vitro model over 10 days of exposure. A total of 823 genes and 2,084 genes were modulated after exposure to 0.095 and 0.22 μg/ml F, respectively. Among them, 168 were found to be modulated by both concentrations. The number of changes in protein expression induced by F were 20 and 10, respectively. Gene ontology annotations showed that the main terms were related to cellular metabolism, protein modification and cell death regulation pathways, such as the MAP kinase (MAPK) cascade, in a concentration independent manner. Proteomics confirmed the changes in energy metabolism and also provided evidence of F-induced changes in cytoskeleton components of glial cells. Our results not only reveal that F has the potential to modulate gene and protein profiles in human U87 glial-like cells overexposed to F, but also identify a possible role of this ion in cytoskeleton disorganization.

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.

Effects of fluoride on the proliferation and activation of osteoblasts by regulating methylation of the DNA repair genes MGMT and MLH1

Introduction

Fluoride can induce the proliferation and activation of osteoblasts, resulting in skeletal fluorosis progression; however, the specific mechanism is unclear.

Methods

Cell proliferation was examined using the MTT assay. Flow cytometry was performed to detect the cell cycle distribution. Alkaline phosphatase (ALP) was calculated to evaluate bone formation and turnover. Gene methylation was examined using the MSP assay. mRNA and protein expression levels were assessed using qRT-PCR and Western blot assays.

Results

Low-concentration NaF treatment promoted the cell cycle progression of osteoblasts to S-phase, thus accelerating cell proliferation and activation in a concentration-dependent manner. In addition, the methylation of the MGMT and MLH1 genes was increased, and their mRNA expression was reduced. Furthermore, the DNA methyltransferase inhibitor 5-AZA-dC suppressed cell viability, cell number in S-phase, ALP activity and osteogenesis-related protein levels in osteoblasts treated with low doses of NaF. Meanwhile, 5-AZA-dC suppressed the increase in MGMT and MLH1 gene methylation in osteoblasts treated with low doses of NaF, leading to enhanced expression of MGMT and MLH1 mRNA.

Conclusion

NaF treatment led to methylation of the DNA repair genes MGMT and MLH1 in osteoblasts, resulting in cell proliferation and activation and causing the development of skeletal fluorosis.

Downregulation of miR-4755-5p promotes fluoride-induced osteoblast activation via tageting Cyclin D1

BACKGROUND

Endemic fluorosis remains a major public health issue in many countries. Fluoride can cause abnormalities in osteoblast proliferation and activation, leading to skeletal fluorosis. However, its detailed molecular mechanism remains unclear. Based on a previous study, the aim of this study is to explore the role of miRNA in osteoblast activation of skeletal fluorosis via targeting of Cyclin D1.

METHODS

A population study of coal-burning fluorosis and in vitro experiments were performed in this study. Urine fluoride (UF) concentrations of the participants were determined using a national standardized ion selective electrode approach. Based on our previous miRNA sequence results, bioinformatic analysis was used to predict miR-4755-5p targeting Cyclin D1. Quantitative real-time PCR (qRT-PCR) was used to verify the expression of miR-4755-5p. The expression of Cyclin D1 mRNA was detected by qRT-PCR. The expression of Cyclin D1 protein was detected by enzyme-linked immunosorbent assay (ELISA) and Western blotting, respectively. Cell viability was detected by CCK-8 method. The distribution of the cell cycle was analyzed by flow cytometry. The alkaline phosphatase (ALP) activity and bone Gla protein (BGP) content were detected by micronutrient enzymes standard method and ELISA. The target binding between miR-4755-5p and Cyclin D1 was verified using dual-luciferase reporter assay.

RESULTS

In the fluoride-exposed population, the results showed that with the increase in UF content, the expression of miR-4755-5p decreased gradually, while the mRNA transcription and protein expression of Cyclin D1 increased gradually. The relative miR-4755-5p expression showed a negative correlation with Cyclin D1 expression. Subsequently, in human osteoblasts treated with sodium fluoride (NaF), the results also showed that NaF caused low expression of miR-4755-5p and increased expression of Cyclin D1. Further, the results of miR-4755-5p mimic transfection confirmed that under the action of NaF, miR-4755-5p overexpression reduced Cyclin D1 protein expression within osteoblasts and further inhibited cell proliferation and activation. Simultaneously, luciferase reporter assays verified that Cyclin D1 was the miR-4755-5p direct target.

CONCLUSION

The results demonstrate that fluoride exposure induced the downregulation of miR-4755-5p and downregulated miR-4755-5p promoted fluoride-induced osteoblast activation by increasing Cyclin D1 protein expression. This study sheds new light on biomarkers and potential treatment for endemic fluorosis.

Principles of fluoride toxicity and the cellular response: a review

Fluoride is ubiquitously present throughout the world. It is released from minerals, magmatic gas, and industrial processing, and travels in the atmosphere and water. Exposure to low concentrations of fluoride increases overall oral health. Consequently, many countries add fluoride to their public water supply at 0.7-1.5 ppm. Exposure to high concentrations of fluoride, such as in a laboratory setting often exceeding 100 ppm, results in a wide array of toxicity phenotypes. This includes oxidative stress, organelle damage, and apoptosis in single cells, and skeletal and soft tissue damage in multicellular organisms. The mechanism of fluoride toxicity can be broadly attributed to four mechanisms: inhibition of proteins, organelle disruption, altered pH, and electrolyte imbalance. Recently, there has been renewed concern in the public sector as to whether fluoride is safe at the current exposure levels. In this review, we will focus on the impact of fluoride at the chemical, cellular, and multisystem level, as well as how organisms defend against fluoride. We also address public concerns about fluoride toxicity, including whether fluoride has a significant effect on neurodegeneration, diabetes, and the endocrine system.

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.

Excessive ER stress and the resulting autophagic flux dysfunction contribute to fluoride-induced neurotoxicity

Fluoride is capable of inducing neurotoxicity, but its mechanisms remain elusive. This study aimed to explore the roles of endoplasmic reticulum (ER) stress and autophagy in sodium fluoride (NaF)-induced neurotoxicity, focusing on the regulating role of ER stress in autophagy. The in vivo results demonstrated that NaF exposure impaired the learning and memory capabilities of rats, and resulted in histological and ultrastructural abnormalities in rat hippocampus. Moreover, NaF exposure induced excessive ER stress and associated apoptosis, as manifested by elevated IRE1α, GRP78, cleaved caspase-12 and cleaved-caspase-3, as well as defective autophagy, as shown by increased Beclin1, LC3-II and p62 expression in hippocampus. Consistently, the in vitro results further verified the findings of in vivo study that NaF induced excessive ER stress and defective autophagy in SH-SY5Y cells. Notably, inhibition of autophagy in NaF-treated SH-SY5Y cells with Wortmannin or Chloroquine decreased, while induction of autophagy by Rapamycin increased the cell viability. These results were correlated well with the immunofluorescence observations, thus confirming the pivotal role of autophagic flux dysfunction in NaF-induced cell death. Importantly, mitigation of ER stress by 4-phenylbutyrate in NaF-treated SH-SY5Y cells inhibited the expressions of autophagy markers, and decreased cell apoptosis. Taken together, these data suggest that neuronal death resulted from excessive ER stress and autophagic flux dysfunction contributes to fluoride-elicited neurotoxicity. Moreover, the autophagic flux dysfunction was mediated by excessive ER stress, which provided novel insight into a better understanding of fluoride-induced neurotoxicity.

Fluorosis induces endoplasmic reticulum stress and apoptosis in osteoblasts in vivo

The present study investigated the effects of fluoride on endoplasmic reticulum (ER) stress (ERS) and osteoblast apoptosis in vivo. Forty-eight Wistar rats were randomly divided into four groups (12/group) and exposed to 0, 50, 100, and 150 mg/L of fluoride in drinking water for 8 weeks, respectively. Peripheral blood samples and bilateral femurs were used to monitor the progression of fluorosis in the animals. Hematoxylin and eosin (H&E) staining of the bone tissues was used to determine the severity of osteofluorosis. The expression of ERS chaperones (glucose-regulated protein 78 (GRP78), X-box binding protein l (XBP1), cysteine aspartate specific protease-12 (caspase-12), and growth arrest and DNA damage-inducible gene 153 (Gadd153/CHOP) was analyzed by immunohistochemistry staining, and osteoblast apoptosis was determined by TUNEL staining and flow cytometry. Accumulation of fluoride in bone was associated with the severity of osteofluorosis. The expression of GRP78, XBP1, caspase-12, and CHOP was increased in a dose-dependent manner. Fluoride-induced apoptosis in osteoblasts was also dose-dependent. High concentrations of fluoride induced ERS and osteoblast apoptosis in vivo. The increased expression of GRP78 and XBP1 increased the adaptation of osteoblasts to ERS to a certain extent. Caspase-12 and CHOP activation was associated with ERS and osteoblast apoptosis.

Differential effects of fluoride during osteoblasts mineralization in C57BL/6J and C3H/HeJ inbred strains of mice

The behavior of fluoride ions in biological systems has advantages and problems. On one hand, fluoride could be a mitogenic stimulus for osteoblasts. However, high concentrations of this element can cause apoptosis in rat and mouse osteoblasts. Toward an understanding of this effect, we examined the role of sodium fluoride (NaF) in two mouse calvaria osteoblasts during the mineralization process. The animals used were C3H/HeJ (C3) and C57BL/6J (B6) mice. The calvaria cells were cultured for 28 days in the presence of several doses of NaF (0, 5, 10, 25, 50, and 75 μM), and we performed the assays: mineralized nodule measurements, alkaline phosphatase (ALP) activity, determination of type I collagen, and matrix metalloproteinase-2 (MMP-2) activity. The results showed no effects on alkaline phosphatase activity but decreased mineralized nodule formation. In B6 cells, the NaF effect was already seen with 10 μM of NaF and a greater increase of cellular type I collagen, and MMP-2 activity was upregulated after 7 days of NaF exposure. C3 osteoblasts showed a reduction in the mineralization pattern only after 50 μM of NaF with a slight increase of type I collagen and downregulation of MMP-2 activity during the mineralization period. In conclusion, fluoride affects the production and degradation of the extracellular matrix during early onset and probably during the mineralization period. Additionally, the genetic factors may contribute to the variation in cell response to fluoride exposure, and the differences observed between the two strains could be explained by an alteration of the bone matrix metabolism (synthesis and degradation).

Genes and gene networks involved in sodium fluoride-elicited cell death accompanying endoplasmic reticulum stress in oral epithelial cells

Here, to understand the molecular mechanisms underlying cell death induced by sodium fluoride (NaF), we analyzed gene expression patterns in rat oral epithelial ROE2 cells exposed to NaF using global-scale microarrays and bioinformatics tools. A relatively high concentration of NaF (2 mM) induced cell death concomitant with decreases in mitochondrial membrane potential, chromatin condensation and caspase-3 activation. Using 980 probe sets, we identified 432 up-regulated and 548 down-regulated genes, that were differentially expressed by >2.5-fold in the cells treated with 2 mM of NaF and categorized them into 4 groups by K-means clustering. Ingenuity^® pathway analysis revealed several gene networks from gene clusters. The gene networks Up-I and Up-II included many up-regulated genes that were mainly associated with the biological function of induction or prevention of cell death, respectively, such as Atf3, Ddit3 and Fos (for Up-I) and Atf4 and Hspa5 (for Up-II). Interestingly, knockdown of Ddit3 and Hspa5 significantly increased and decreased the number of viable cells, respectively. Moreover, several endoplasmic reticulum (ER) stress-related genes including, Ddit3, Atf4 and Hapa5, were observed in these gene networks. These findings will provide further insight into the molecular mechanisms of NaF-induced cell death accompanying ER stress in oral epithelial cells.

Fluoride promotes osteoblastic differentiation through canonical Wnt/β-catenin signaling pathway

Although fluoride is known to stimulate bone formation, the underlying mechanisms are not fully understood. Recent studies have implicated the Wnt/β-catenin pathway as a major signaling cascade in bone biology. Our earlier studies highlighted a probable role of canonical Wnt pathway in bone formation of chronic fluoride-exposed rats, but the mechanism remains unclear. The current study determined the involvement of Wnt/β-catenin signaling in fluoride-induced osteoblastic differentiation. Using primary rat osteoblasts, we demonstrated that fluoride significantly promoted osteoblasts proliferation and alkaline phosphate (ALP) expression as well as the mRNA expression levels of bone differentiation markers, including type I collagen (COL1A1), ALP and osteonectin. We further found fluoride induced phosphorylations at serine 473 of Akt and serine 9 of glycogen synthase kinase-3β (GSK3β), which resulted in GSK-3β inhibition and subsequently the nuclear accumulation of the β-catenin, as shown by Western blot and immunofluorescence analysis. Moreover, fluoride also induced the expression of Wnt-targeted gene runt-related transcription factor 2 (Runx2). Importantly, the positive effect of fluoride on ALP activity and mRNA expressions of COL1A1, ALP, osteonection and Runx2 was abolished by DKK-1, a blocker of the Wnt/β-catenin receptor. Taken together, these findings suggest that fluoride promotes osteoblastic differentiation through Akt- and GSK-3β-dependent activation of Wnt/β-catenin signaling pathway in primary rat osteoblasts. Our findings provide novel insights into the mechanisms of action of fluoride in osteoblastogenesis.