Fluoride Induces Endoplasmic Reticulum Stress and Inhibits Protein Synthesis and Secretion.

PHYSIOLOGICAL effects of fluorides; FLUOROSIS; DENTAL discoloration; ENDOPLASMIC reticulum; HEAT shock proteins; SECRETION; PROTEIN synthesis

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 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 function during enamel development may be impaired, and this may culminate in dental fluorosis. [ABSTRACT FROM AUTHOR]