Fluoride Induces Endoplasmic Reticulum Stress and Inhibits Protein Synthesis and Secretion

BACKGROUND: Exposure to excessive amounts of fluoride (F(−)) causes dental fluorosis in susceptible individuals; however, the mechanism of F(−)-induced toxicity is unclear. Previously, we have shown that high-dose F(−) activates the unfolded protein response (UPR) in ameloblasts that are responsible for dental enamel formation. The UPR is a signaling pathway responsible for either alleviating endoplasmic reticulum (ER) stress or for inducing apoptosis of the stressed cells. OBJECTIVES: In this study we determined if low-dose F(−) causes ER stress and activates the UPR, and we also determined whether F(−) interferes with the secretion of proteins from the ER. METHODS: We stably transfected the ameloblast-derived LS8 cell line with secreted alkaline phosphatase (SEAP) and determined activity and localization of SEAP and F(−)-mediated induction of UPR proteins. Also, incisors from mice given drinking water containing various concentrations of F(−) were examined for eucaryotic initiation factor-2, subunit alpha (eIF2α) phosphorylation. RESULTS: We found that F(−) decreases the extracellular secretion of SEAP in a linear, dose-dependent manner. We also found a corresponding increase in the intracellular accumulation of SEAP after exposure to F(−). These changes are associated with the induction of UPR proteins such as the molecular chaperone BiP and phosphorylation of the UPR sensor PKR-like ER kinase, and its substrate, eIF2α. Importantly, F(−)-induced phosphorylation of eIF2αwas confirmed in vivo. CONCLUSIONS: These data suggest that F(−) initiates an ER stress response in ameloblasts that interferes with protein synthesis and secretion. Consequently, ameloblast function during enamel development may be impaired, and this may culminate in dental fluorosis.