Fluoride is a common element in nature and our daily life, and excessive intake of this element can cause fluorosis and irreversible brain damage. The toxic effects of fluoride on the central nervous system may be attributed to the release of inflammatory cytokines and ROS. GSK3B is a key protein that modulates NF-kB activity and inflammatory cytokine levels and plays an important role in the Wnt signaling pathway. In this study, we found that fluoride altered the inflammatory status and oxidative stress by inhibiting Wnt signaling pathway activity. This study thus provides a valid basis for the fluorine-induced neuroinflammation injury theory.
Keywords: DKK1; GSK3B; NaF; Wnt signaling pathway; neuroinflammation.
*Original abstract online at https://link.springer.com/article/10.1007/s10753-017-0556-y
Fluoride exposure increases IL-1B and TNF-a production and changes the cellular oxidation and antioxidation status [5, 19]. Neuroinflammation seems to be an important mediator of the effects of fluoride. Neuroinflammation, which is mediated by microglia, contributes to most pathological neurologic processes, including CNS infections, ischemic stroke, neurodegenerative disease, and anesthetic neurotoxicity. Increased pro-inflammatory mediator production is thought to cause neuronal cell death. As previously mentioned, inflammation is characterized by activation of the pro-inflammatory transcription factor NF-kB, a process mediated by GSK3B, a critical protein in the Wnt signaling pathway . Therefore, we conducted the present study to investigate the changes in the Wnt signaling pathway in BV2 cells exposed to NaF.
Microglia are the resident macrophages in the brain and play critical roles in the development and maintenance of the neural environment . To exclude the possibility that NaF directly affects microglial viability and proliferation, we examined the effects of different NaF concentrations on BV2 viability at different time points. As shown in Fig. 1, BV2 cell viability increased after treatment with lower concentrations of NaF for 6 h. This increase in cell viability was likely caused by a stress response to NaF. However, cell viability decreased significantly when the NaF concentration increased, indicating that high NaF concentrations are toxic to cells.
Microglia can be activated in response to perturbations in the brain microenvironment or changes in neuronal structure. Activated microglia cells break the balance between oxidation and antioxidation. MDA levels reflect the degree of lipid peroxidation and indirectly reflect the degree of cell damage. SOD, an antioxidant, can defend against ROS activity and reduce theMDA level. ROS can cause oxidative damage to cells and DNA structure . As shown in Fig. 2, the MDA levels increased in the NaF groups. SOD activity decreased significantly at higher NaF concentrations. The intracellular ROS levels increased as the NaF concentration increased. Activated microglia also produce various inflammatory mediators, including IL-6 and TNF-a, which are the two main pro-inflammatory cytokines produced by these cells. As shown in Fig. 3, higher NaF concentrations stimulated IL-6 and TNF-a production in BV2 cells. This result could indicate that NaF-induced oxidative stress and inflammation play an important role in fluorosis.
Previous studies have shown that the Wnt signaling pathway can affect cognitive function by regulating the proliferation, differentiation, and migration of neural stem cells…
Wnt/B-catenin signaling in the rat hippocampus . DKK1 has been described as an extracellular antagonist of the Wnt pathway, and studies have shown that DKK1 is required for the pathogenesis of many neurodegenerative diseases, including AD in patients and animal models , frontotemporal dementia in transgenic mice , ischemic insults , and mesial temporal lobe epilepsy with hippocampal sclerosis . However, the relationship between DKK1 level and fluorosis remains unknown. In this study, we established a model of NaF-induced neuroinflammation to measure DKK1 expression and found increased DKK1 levels in BV2 cells treated with various concentrations of NaF. This result suggested that DKK1, an inhibitor of the canonical Wnt signaling pathway, might be involved in NaF-induced neuroinflammation.
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This study was supported by BK20151159 from the Natural Science Foundation of Jiangsu Province, China, Project 81501185 of National Natural Science Foundation of China, Jiangsu Provincial Medical Youth Talent No.QNRC2016369, Xuzhou Medical Talents Project and Xuzhou technological and scientific project No. KC14SH050.