Fluoride induces endoplasmic reticulum stress in ameloblasts responsible for dental enamel formation.
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.
Kubota
Kaori
K
Department of Cytokine Biology, Forsyth Institute, Harvard School of Dental Medicine, Boston, Massachusetts 02115, USA.
Lee
Daniel H
DH
Tsuchiya
Masahiro
M
Young
Conan S
CS
Everett
Eric T
ET
Martinez-Mier
Esperanza A
EA
Snead
Malcolm L
ML
Nguyen
Linh
L
Urano
Fumihiko
F
Bartlett
John D
JD
D000565:Ameloblasts / Q000378:metabolism*; D000818:Animals; D018797:Cell Cycle Proteins; D002460:Cell Line; D003743:Dental Enamel / Q000378:metabolism; D000091342:Endoplasmic Reticulum Chaperone BiP; D059865:Endoplasmic Reticulum Stress / Q000187:drug effects / Q000502:physiology; D005347:Fibroblasts / Q000378:metabolism / Q000648:ultrastructure; D005459:Fluorides / Q000378:metabolism / Q000494:pharmacology*; D051379:Mice; D011485:Protein Binding; D013552:Swine; D051742:Transcription Factor CHOP; D000071717:X-Box Binding Protein 1; D000097642:GADD45 Proteins
D018797:Cell Cycle Proteins; D000091342:Endoplasmic Reticulum Chaperone BiP; D005459:Fluorides; D051742:Transcription Factor CHOP; D000071717:X-Box Binding Protein 1; C497250:Ddit3 protein, mouse; C499072:Gadd45a protein, mouse; D000097642:GADD45 Proteins; C000717248:Hspa5 protein, mouse; C000605544:Xbp1 protein, mouse