Oral toxicity to high level sodium fluoride causes impairment of autophagy

Abstract

Background: Gingival recession and concomitant alveolar bone resorption are hallmarks of periodontal diseases and excessive fluoride intake has some deleterious effects on teeth, bone and soft tissues. Autophagy is a highly conserved intracellular digestion process that degrades damaged proteins and organelles but the biological roles of autophagy in pathological aspects of oral tissues remain largely unknown. We sought to elucidate the function of autophagy, especially its interplay with apoptosis and oxidative stress, in the oral toxicity induced by exposure to 5 mM sodium fluoride (NaF). Methods: HCEM2 cementoblast cells in culture were exposed to 5 mM NaF for 5 min, after which cell viability and cell apoptosis were assessed using the MTS assay and an Annexin V-FITC/PI apoptosis detection kit, respectively. Real-time quantitative RT-PCR and Western blotting were performed to characterize the mRNA and protein expression levels of markers for autophagy, apoptosis, and oxidative stress. C57BL/6 mice were exposed to 5 mM NaF in their drinking water from 12 to 58 weeks. Micro-computed tomography was used to measure changes in their alveolar bone while immunohistochemistry and immunofluorescence staining was used to evaluate protein expression levels of autophagy, apoptosis, and oxidative stress markers.Results: Cementoblasts exposed to 5 mM NaF had decreased levels of autophagy, as shown by reduced expression levels of ATG5, Beclin1 and LC3-II, elicited excessive apoptosis, which in turn induced oxidative stress and inflammation, as manifested by elevated levels of Bax, cleaved caspase-3, SOD1 and phospho NF-κB. Consistently, the in vivo results verified the findings of the in vitro study and treatment of mice with 5 mM NaF resulted in histological abnormalities in periodontal tissues, induced excessive oxidative stress and apoptosis, and reduced autophagy. These results correlated with the immunofluorescence observations, thus confirming the pivotal role of autophagic flux dysfunction in 5 mM NaF-induced cell death. Micro-computed tomography analysis demonstrated that 5 mM NaF caused a decrease in bone areas of mice compared with controls. Exposure to 5 mM NaF induced RANKL (receptor activator of nuclear factor κB ligand) and cathepsin K expression in periodontal tissues, while ATG5 and Beclin1 expression was abrogated by 5 mM NaF.Conclusion: Taken together, our findings suggest that 5 mM NaF elicits oral toxicity that contributes to excessive apoptosis, oxidative stress, and defective autophagy, which aggravates periodontal tissue damage.

Programmed cell death of periodontal ligament cells

“…Exposure of human cementoblast‐like cells (HCEM‐2) to 5 mM sodium fluoride reduced autophagy levels and induced apoptosis, oxidative stress, and inflammation, as evidenced by increased levels of Bax, cleaved caspase‐3, SOD1, and phosphorylated NF‐κB. Thus, exposure to 5 mM sodium fluoride can cause oral toxicity, leading to excessive cellular apoptosis, oxidative stress, and autophagy defects, which exacerbate periodontal tissue damage (Oka et al, 2020).…”

Biphasic Functions of Sodium Fluoride (NaF) in Soft and in Hard Periodontal Tissues

“…One study demonstrated that a high concentration of NaF induces the apoptosis of human cementoblasts through the inhibition of autophagy. Autophagy-related genes ATG5 and Beclin-1 were suppressed, resulting in alveolar bone resorption [101].…”