Dental fluorosis is characterized by subsurface hypomineralization and
increased porosity of enamel, associated with a delay in the removal of enamel
matrix proteins. To investigate the effects of fluoride on ameloblasts, A/J
mice were given 50 ppm sodium fluoride in drinking water for four weeks,
resulting serum fluoride levels of 4.5 µM, a four-fold increase over control
mice with no fluoride added to drinking water. MicroCT analyses showed delayed
and incomplete mineralization of fluorosed incisor enamel as compared to
control enamel. A microarray analysis of secretory and maturation stage
ameloblasts microdissected from control and fluorosed mouse incisors showed
that genes clustered with Mmp20 appeared to be less downregulated in
maturation stage ameloblasts of fluorosed incisors as compared to control
maturation ameloblasts. One of these Mmp20 co-regulated genes was the global
chromatin organizer, special AT-rich sequence-binding protein-1 (SATB1).
Immunohistochemical analysis showed increased SATB1 protein present in
fluorosed ameloblasts compared to controls. In vitro, exposure of human
ameloblast-lineage cells to micromolar levels of both NaF and AlF3 led to a
significantly increase in SATB1 protein content, but not levels of Satb1 mRNA,
suggesting a fluoride-induced mechanism protecting SABT1 from degradation.
Consistent with this possibility, we used immunohistochemistry and Western
blot to show that fluoride exposed ameloblasts had increased phosphorylated
PKCα both in vivo and in vitro. This kinase is known to phosphorylate SATB1,
and phosphorylation is known to protect SATB1 from degradation by caspase-6.
In addition, production of cellular diacylglycerol (DAG) was significantly
increased in fluorosed ameloblasts, suggesting that the increased
phosphorylation of SATB1 may be related to an effect of fluoride to enhance
Gαq activity of secretory ameloblasts