Coexposure to fluoride and sulfur dioxide aggravates enamel mineralization disorders in mice by disrupting calcium homeostasis-mediated endoplasmic reticulum stress

Abstract

Prevalence of fluoride and sulfur dioxide (SO₂) cocontamination in the environment poses a serious threat to various human organs, especially the teeth. However, direct evidence linking coexposure to fluoride and SO₂ with enamel mineralization disorders is lacking. Here, we investigated the mechanisms through which fluoride and SO₂ exposure, either alone or in combination, affects enamel mineralization in mouse and LS8 cell models. Coexposure to fluoride and SO₂ resulted in more severe enamel mineralization disorders compared with those in the control or individual exposure groups. The coexposure caused significant pathological changes and retention of enamel matrix. Furthermore, the coexposure upregulated the expression of membrane calcium channels (Cav1.2), calmodulin-dependent protein kinase II (CaMKII), endoplasmic reticulum calcium ion(Ca²⁺)-release channel (IP₃R), and endoplasmic reticulum stress (ERS) marker protein (GRP78), and significantly downregulated the expression of endoplasmic reticulum (ER) Ca²⁺-uptake pump protein (SERCA2) and calreticulin (CRT). Investigations using Amlodipine (Am), Tunicamycin (Tm) and CDN1163 revealed that the coexposure exacerbated enamel mineralization disorders by disrupting calcium homeostasis and subsequently triggering ERS. Overall, this study highlights that coexposure to fluoride and SO₂ affects ER Ca²⁺ content through cytoplasmic calcium overload, triggers ERS, and increases the risk of enamel mineralization disorders. Activation of ERS, induced by disruption of calcium homeostasis, may play a key role in fluoride and SO₂-induced enamel mineralization disorders. The insights obtained from this study should be valuable for devising strategies to mitigate the effects of fluoride and SO₂ coexposure on enamel mineralization disorders.