Flouride 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 (elf2α) 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 eIF2α was 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 ftinction during enamel development may be impaired, and this may culminate in dental fluorosis.