Fluoride induces endoplasmic reticulum stress and inhibits protein synthesis and secretion

Environ Health Perspect. 2008 Sep;116(9):1142-6. doi: 10.1289/ehp.11375.

Abstract

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 (eIF2alpha) 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, eIF2alpha. Importantly, F(-)-induced phosphorylation of eIF2alphawas 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.

Keywords: ER stress; LS8 cells; SEAP; UPR; ameloblasts; eIF2α; enamel; endoplasmic reticulum; fluoride.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Alkaline Phosphatase / metabolism
  • Animals
  • Cell Line
  • Dose-Response Relationship, Drug
  • Endoplasmic Reticulum / drug effects*
  • Endoplasmic Reticulum / metabolism
  • Fluorides / pharmacology*
  • Immunohistochemistry
  • Mice
  • Protein Synthesis Inhibitors / pharmacology*

Substances

  • Protein Synthesis Inhibitors
  • Alkaline Phosphatase
  • Fluorides