Fluoride 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.


OBJECTIVE
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.
Sharma
Ramaswamy
R
Department of Cytokine Biology, Forsyth Institute, Boston, Massachusetts, USA.
Tsuchiya
Masahiro
M
Bartlett
John D
JD
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D004721:Endoplasmic Reticulum / Q000187:drug effects* / Q000378:metabolism; D005459:Fluorides / Q000494:pharmacology*; D007150:Immunohistochemistry; D051379:Mice; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology*
D011500:Protein Synthesis Inhibitors; D000469:Alkaline Phosphatase; D005459:Fluorides
J Dent Res. 1992 May;71(5):1238-43
1607440
Gene. 1988 Jun 15;66(1):1-10
3417148
FEBS Lett. 2007 May 15;581(10):2055-9
17475259
J Mol Biol. 1966 Oct 28;21(1):51-69
5969765
J Mol Biol. 1970 Feb 14;47(3):335-52
5418164
J Mol Biol. 1966 Aug;19(2):525-40
5969077
J Dent Res. 1998 Feb;77(2):377-86
9465170
Toxicol Sci. 2001 May;61(1):83-91
11294978
Bone. 1989;10(2):89-99
2765315
Arch Toxicol. 1996;70(7):420-9
8740536
J Dent Res. 2004 Jan;83(1):76-80
14691118
Neuromolecular Med. 2003;4(1-2):73-94
14528054
Bone. 2007 Dec;41(6):1036-44
17936699
MMWR Recomm Rep. 1995 Sep 29;44(RR-13):1-40
7565542
Arch Oral Biol. 1988;33(3):159-62
3178535
Arch Toxicol. 2000 Feb;73(12):611-7
10741471
J Dent Res. 1996 Dec;75(12):1936-41
9033447
Caries Res. 1984;18(1):25-32
6580951
Curr Opin Cell Biol. 1995 Aug;7(4):523-9
7495572
Arch Oral Biol. 1968 Aug;13(8):969-77
5251694
Arzneimittelforschung. 2005;55(8):455-60
16149713
Mol Cell Biol. 2001 Dec;21(23):7971-80
11689689
Am J Pathol. 1997 Feb;150(2):735-45
9033286
J Immunol Methods. 2006 Aug 31;315(1-2):202-7
16904124
Scand J Dent Res. 1976 Sep;84(5):276-85
1068504
Cell Biol Int Rep. 1979 Dec;3(9):701-5
389448
Reprod Toxicol. 2002 Jul-Aug;16(4):385-90
12220599
J Dent Res. 2002 Nov;81(11):794-8
12407097
Calcif Tissue Int. 1990 May;46(5):333-8
2110856
J Dent Res. 1990 Feb;69 Spec No:685-91; discussion 721
2179330
Anat Rec. 1993 Oct;237(2):243-58
8238976
J Cell Sci. 2006 May 1;119(Pt 9):1833-42
16608874
Caries Res. 2004 May-Jun;38(3):268-76
15153700
J Biol Chem. 1976 Jan 25;251(2):292-301
1245473
Int J Biochem Cell Biol. 1999 Jan;31(1):25-9
10216940
J Biol Chem. 2005 Jun 17;280(24):23194-202
15849362
J Dent Res. 1986 Oct;65(10):1272-7
3463596
Nucleic Acids Res. 2006 Jul 28;34(13):e93
16877567
Arch Oral Biol. 1996 Aug-Sep;41(8-9):739-47
9022911
Crit Rev Oral Biol Med. 2002;13(2):155-70
12097358
Community Dent Oral Epidemiol. 1999 Feb;27(1):41-7
10086925
Mol Cell. 2000 May;5(5):897-904
10882126
Acta Pharmacol Toxicol (Copenh). 1979 Aug;45(2):96-101
40394