The role of FGF9-mediated TGF-f31/Smad signaling in enamel hypoplasia induced by exposure to fluoride and SO2 in rats

Many geographical areas of the world are polluted by both fluoride and sulfur dioxide (SO2). However, the effects of simultaneous exposure to fluoride and SO2 on teeth are unknown. Fibroblast growth factor-9 (FGF9) and transforming growth factor -f31 (TGF-f31) are key signaling molecules in enamel development. The purpose of the study was to explore the effects of co-exposure to fluoride and sulfur dioxide on enamel and to investigate the role and mechanism of FGF9 and TGF-f31. First, sodium fluoride (NaF) and SO2 derivatives were used to construct rat models and evaluate the enamel development of rats. Then, TGF-f31 (cytokine) treatment, SIS3 (inhibitor) treatment and FGF9 gene knockdown were used to explore the mechanism of enamel damage in vitro. The results showed that enamel column crystals in the exposed group were characterized by enamel hypoplasia, as indicated by alterations such as disarrangement of enamel column crystals, space widening and breakage. Ameloblasts also showed pathological changes such as ribosome loss, mitochondrial swelling, nuclear fragmentation and chro-matin aggregation. The protein expression of FGF9 was higher and the protein expression of AMBN, TGF-f31 and p-Smad2/3 protein was lower in the groups treated with fluoride and SO2 individually or in combination compared with the control group. Further studies showed that TGF-f31 significantly upregulated p-Smad2/3 and AMBN protein expression and reduced the inhibitory effects of fluoride and SO2; furthermore, SISI blocked the effect of TGF-f31. In addition, knockdown of FGF9 upregulated TGF-f31 protein expression, further activated Smad2/3 phosphorylation, eliminated the inhibitory effects of fluoride and SO2, and increased the protein expression of AMBN. In brief, the study confirms that co-exposure to fluoride and SO2 can result in enamel hypoplasia in rats and indicates that the underlying mechanism may be closely related to the effect of FGF9 on enamel matrix protein secretion through inhibition of the TGF-f31/Smad signaling pathway.