Fluoride induces endoplasmic reticulum stress in ameloblasts responsible for dental enamel formation
Program in Gene Function and Expression; Program in Molecular Medicine
Medical Subject Headings
Alternative Splicing; Ameloblasts; Animals; Apoptosis; Blotting, Northern; Blotting, Western; CCAAT-Enhancer-Binding Proteins; Carbonic Anhydrases; Caspase 3; Caspases; Cell Cycle Proteins; Cell Line; Cell Proliferation; Cell Survival; Coloring Agents; DNA Damage; DNA Fragmentation; DNA, Complementary; DNA-Binding Proteins; Dental Enamel; Dose-Response Relationship, Drug; Down-Regulation; Endoplasmic Reticulum; Endoribonucleases; Enzyme-Linked Immunosorbent Assay; Epithelium; Fibroblasts; Fluorides; Heat-Shock Proteins; Immunohistochemistry; In Situ Nick-End Labeling; Membrane Proteins; Mice; Mice, Transgenic; Molecular Chaperones; Nuclear Proteins; Protein Binding; Protein Biosynthesis; Protein Folding; Protein-Serine-Threonine Kinases; RNA, Small Interfering; Reverse Transcriptase Polymerase Chain Reaction; Sodium Fluoride; Swine; Tetrazolium Salts; Thiazoles; Time Factors; Transcription Factor CHOP; Transcription Factors
Genetics and Genomics
The mechanism of how fluoride causes fluorosis remains unknown. Exposure to fluoride can inhibit protein synthesis, and this may also occur by agents that cause endoplasmic reticulum (ER) stress. When translated proteins fail to fold properly or become misfolded, ER stress response genes are induced that together comprise the unfolded protein response. Because ameloblasts are responsible for dental enamel formation, we used an ameloblast-derived cell line (LS8) to characterize specific responses to fluoride treatment. LS8 cells were growth-inhibited by as little as 1.9-3.8 ppm fluoride, whereas higher doses induced ER stress and caspase-mediated DNA fragmentation. Growth arrest and DNA damage-inducible proteins (GADD153/CHOP, GADD45alpha), binding protein (BiP/glucose-responsive protein 78 (GRP78), the non-secreted form of carbonic anhydrase VI (CA-VI), and active X-box-binding protein-1 (Xbp-1) were all induced significantly after exposure to 38 ppm fluoride. Unexpectedly, DNA fragmentation increased when GADD153 expression was inhibited by short interfering RNA treatment but remained unaffected by transient GADD153 overexpression. Analysis of control and GADD153(-/-) embryonic fibroblasts demonstrated that caspase-3 mediated the increased DNA fragmentation observed in the GADD153 null cells. We also demonstrate that mouse incisor ameloblasts are sensitive to the toxic effects of high dose fluoride in drinking water. Activated Ire1 initiates an ER stress response pathway, and mouse ameloblasts were shown to express activated Ire1. Ire1 levels appeared induced by fluoride treatment, indicating that ER stress may play a role in dental fluorosis. Low dose fluoride, such as that present in fluoridated drinking water, did not induce ER stress.
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Citation: J Biol Chem. 2005 Jun 17;280(24):23194-202. Epub 2005 Apr 23. Link to article on publisher's site