Fluoride induces hypomethylation of BMP2 and activates osteoblasts through the Wnt/β-catenin signaling pathway

Zebrafish larvae as a model for investigating dual effects of fluoride on bone development

Long-term excessive ingestion of fluoride is a severe public health threat globally. Skeletal fluorosis, a significant manifestation of prolonged fluoride exposure, is characterized by aberrant bone structure and alterations in bone function. However, there is currently a shortage of an efficient, fast, and easy-to-operate biological model for application in the field of fluorosis research. Zebrafish larvae, with human - like skeletal traits, high reproduction, rapid development, and transparency, are commonly used in bone disease studies. This study evaluates the potential of zebrafish larvae as a novel model for fluoride-induced bone impairment. Results showed dose-dependent differences in cranial and spinal bone mineralization in zebrafish larvae exposed to sodium fluoride (NaF). The detection results of bone formation-related indicators indicated a considerable increase in alkaline phosphatase (ALP) activity in zebrafish larvae at doses of 0.5 and 1 mg/L. Simultaneously, the expression of critical bone formation proteins (BMP2, and β-catenin) was elevated in the 1 and 4 mg/L groups, which is largely consistent with the results of cranial bone mineralization. Fluoride - exposed zebrafish also showed abnormal bone metabolism markers. The total phosphorus (TP) content in the zebrafish larvae of the 100 mg/L group was markedly reduced. The total calcium (TCa) content in the zebrafish of the NaF group zebrafish was slightly decreased, although the tartrate-resistant acid phosphatase (StrACP) activity increased. In conclusion, different fluoride doses cause osteoporosis and osteosclerosis in zebrafish larvae, linked to enhanced osteogenic and osteoclastic activities and abnormal key bone - forming protein expression.

Knockdown of SMYD3 by RNA Interference Regulates the Expression of Autophagy-Related Proteins and Inhibits Bone Formation in Fluoride-Exposed Osteoblasts

This study aimed to explore the role of histone methyltransferase SET and MYND domain containing 3 (SMYD3) in bone metabolism of osteoblasts exposed to fluoride. The levels of urine fluoride, BALP, and OC and the mRNA expression of SMYD3 were determined in patients with skeletal fluorosis and non-fluoride-exposed people on informed consent. The expression of SMYD3 protein, OC contents, and BALP activities were detected in human osteoblast-like MG63 cells and rat primary osteoblasts treated with sodium fluoride (NaF) for 48 h. The autophagosomes were observed by transmission electron microscopy. Then, we knocked down SMYD3 to confirm whether it was involved in the regulation of bone formation and related to autophagy and Wnt/β-catenin pathway. We observed that OC and BALP levels in patients with skeletal fluorosis significantly increased, while the mRNA expression of SMYD3 significantly decreased in the skeletal fluorosis groups. In vitro, the OC contents, BALP activities, and expression of SMYD3 significantly increased, and many autophagosomes were observed in NaF treated osteoblasts. The downregulation of SMYD3 significantly inhibited OC contents, BALP activities, and expression of autophagy-related proteins, but with no significant changes in the Wnt/β-catenin pathway. Our results demonstrated that fluoride exposure with coal-burning pollution caused orthopedic injuries and abnormalities in the levels of OC and BALP and hindered normal bone metabolism. Silencing the SMYD3 gene could significantly reduce OC and BALP levels via inhibiting the increase in autophagy induced by fluoride.

Apoptosis and Inflammation Involved with Fluoride-Induced Bone Injuries

BACKGROUND

Excessive fluoride exposure induces skeletal fluorosis, but the specific mechanism responsible is still unclear. Therefore, this study aimed to identify the pathogenesis of fluoride-induced bone injuries.

METHODS

We systematically searched fluoride-induced bone injury-related genes from five databases. Then, these genes were subjected to enrichment analyses. A TF (transcription factor)-mRNA-miRNA network and protein-protein interaction (PPI) network were constructed using Cytoscape, and the Human Protein Atlas (HPA) database was used to screen the expression of key proteins. The candidate pharmacological targets were predicted using the Drug Signature Database.

RESULTS

A total of 85 studies were included in this study, and 112 osteoblast-, 35 osteoclast-, and 41 chondrocyte-related differential expression genes (DEGs) were identified. Functional enrichment analyses showed that the Atf4, Bcl2, Col1a1, Fgf21, Fgfr1 and Il6 genes were significantly enriched in the PI3K-Akt signaling pathway of osteoblasts, Mmp9 and Mmp13 genes were enriched in the IL-17 signaling pathway of osteoclasts, and Bmp2 and Bmp7 genes were enriched in the TGF-beta signaling pathway of chondrocytes. With the use of the TF-mRNA-miRNA network, the Col1a1, Bcl2, Fgfr1, Mmp9, Mmp13, Bmp2, and Bmp7 genes were identified as the key regulatory factors. Selenium methyl cysteine, CGS-27023A, and calcium phosphate were predicted to be the potential drugs for skeletal fluorosis.

CONCLUSIONS

These results suggested that the PI3K-Akt signaling pathway being involved in the apoptosis of osteoblasts, with the IL-17 and the TGF-beta signaling pathways being involved in the inflammation of osteoclasts and chondrocytes in fluoride-induced bone injuries.

Crosstalk between Wnt and bone morphogenetic protein signaling during osteogenic differentiation

Mesenchymal stem cells (MSCs) originate from many sources, including the bone marrow and adipose tissue, and differentiate into various cell types, such as osteoblasts and adipocytes. Recent studies on MSCs have revealed that many transcription factors and signaling pathways control osteogenic development. Osteogenesis is the process by which new bones are formed; it also aids in bone remodeling. Wnt/β-catenin and bone morphogenetic protein (BMP) signaling pathways are involved in many cellular processes and considered to be essential for life. Wnt/β-catenin and BMPs are important for bone formation in mammalian development and various regulatory activities in the body. Recent studies have indicated that these two signaling pathways contribute to osteogenic differentiation. Active Wnt signaling pathway promotes osteogenesis by activating the downstream targets of the BMP signaling pathway. Here, we briefly review the molecular processes underlying the crosstalk between these two pathways and explain their participation in osteogenic differentiation, emphasizing the canonical pathways. This review also discusses the crosstalk mechanisms of Wnt/BMP signaling with Notch- and extracellular-regulated kinases in osteogenic differentiation and bone development.

Fluoride exposure confers NRF2 activation in hepatocyte through both canonical and non-canonical signaling pathways

Due to the high abundance in the Earth's crust and industrial application, fluoride is widely present in our living environment. However, excessive fluoride exposure causes toxicity in different organs. As the most important detoxification and excretion organ, liver is more easily involved in fluoride toxicity than other organs, and oxidative stress is considered as the key mechanism related with fluoride hepatotoxicity. In this study, we mainly investigated the role of nuclear factor erythroid-derived 2-like 2 (NRF2, a core transcription factor in oxidative stress) in fluoride exposure-induced hepatotoxicity as well as the related mechanism. Herein, liver cells (BNL CL.2) were treated with fluoride in different concentrations. The hepatotoxicity and NRF2 signaling pathway were analyzed respectively. Our results indicated that excessive fluoride (over 1 mM) resulted in obvious toxicity in hepatocyte and activated NRF2 and NRF2 target genes. The increased ROS generation after fluoride exposure suppressed KEAP1-induced NRF2 ubiquitylation and degradation. Meanwhile, fluoride exposure also led to blockage of autophagic flux and upregulation of p62, which contributed to activation of NRF2 via competitive binding with KEAP1. Both pharmaceutical activation and genetic activation of NRF2 accelerated fluoride exposure-induced hepatotoxicity. Thus, the upregulation of NRF2 in hepatocyte after fluoride exposure can be regarded as a cellular self-defense, and NRF2-KEAP1 system could be a novel molecular target against fluoride exposure-induced hepatotoxicity.

Biomimetic Porous Magnesium Alloy Scaffolds Promote the Repair of Osteoporotic Bone Defects in Rats through Activating the Wnt/β-Catenin Signaling Pathway

In this study, biomimetic porous magnesium alloy scaffolds were prepared to repair femoral bone defects in ovariectomized osteoporotic rats. The purpose of the study was to investigate the effect of biomimetic porous magnesium alloy scaffolds on repairing osteoporotic bone defects and possible mechanisms. The animal model of osteoporosis was established in female SD rats. Three months later, a bone defect of 3 mm in diameter and 3 mm in depth was created in the lateral condyle of the right femur. The rats were then randomly divided into two groups: an experimental group and a control group. Four weeks after surgery, gross specimens were observed and micro-CT scans were performed. The repair of osteoporotic femoral defects in rats was studied histologically using HE staining, Masson staining, and Goldner staining. The expression of Wnt5a, β-catenin, and BMP-2 was measured between groups by immunohistochemical staining. The bone defect was repaired better after the application of biomimetic porous magnesium alloy scaffolds. Immunohistochemical results showed significantly higher expression of Wnt5a, β-catenin, and BMP-2. To conclude, the biomimetic porous magnesium alloy scaffolds proposed in this paper might promote the repair of osteoporotic femoral bone defects in rats possibly through activating the Wnt/β-catenin signaling pathway.