Potential function of TGF-β isoforms in maturation-stage ameloblasts

Immunohistochemical expression of beta-catenin, BMP4 and TGF-beta in odontomas

Changes in the expression of nuclear β-catenin are responsible for tumorigenesis. Beta-catenin acts synergistically with the TGF-β/BMPs pathway. This interaction leads to greater dentin deposition and may explain the differences between distinct tooth morphologies and hamartomas. The aim of this study was to investigate the role of β-catenin, BMP4 and TGF-β in the development of odontomas. This cross-sectional, retrospective, immunohistochemical study evaluated 30 compound odontomas, 30 complex odontomas and 17 tooth germs. The results showed that BMP4 and TGF-β were more immunoexpressed in the ectomesenchyme of complex odontomas (median = 33.7, p < 0.001; median = 76.4, p = 0.002, respectively). Higher immunoexpression of BMP4 and TGF-β was also observed in the epithelium of tooth germs (median = 2.0, p < 0.001; median = 120.3, p < 0.001, respectively). TGF-β and BMP4 showed a positive and significant correlation (p < 0.001). Both TGF-β and BMP4 were positively correlated with nuclear β-catenin in ectomesenchyme (p = 0.047 and p = 0.023, respectively). Developing teeth exhibited higher concentrations of the proteins studied in odontogenic epithelium, especially during the bud and cap stages. Higher immunoexpression in odontomas occurred mainly in the ectomesenchyme. We therefore suggest that changes in the ectomesenchyme can lead to the development of odontomas.

The role of FGF9-mediated TGF-β1/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-β1 (TGF-β1) 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-β1. First, sodium fluoride (NaF) and SO2 derivatives were used to construct rat models and evaluate the enamel development of rats. Then, TGF-β1 (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 chromatin aggregation. The protein expression of FGF9 was higher and the protein expression of AMBN, TGF-β1 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-β1 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-β1. In addition, knockdown of FGF9 upregulated TGF-β1 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-β1/Smad signaling pathway.

Response of TGF-β isoforms in epithelial-mesenchymal transition of enamel epithelial cells

OBJECTIVE

During enamel formation, transforming growth factor-beta (TGF-β) isoforms exhibit different activities for gene expression, apoptosis, and endocytosis. This study aimed to investigate the differential response of TGF-β isoforms to epithelial-mesenchymal transition (EMT) in enamel epithelial cells.

DESIGN

Using a mouse enamel epithelial cell line (mHAT9d) cultured in the presence of each TGF-β isoform, (1) the morphological changes in EMT were explored, (2) EMT-related genes were analyzed by next-generation sequencing (NGS), (3) TGF-β pathway for EMT was identified by inhibition experiments, and (4) the expression of the TGF-β receptor gene in response to the binding affinity of the TGF-β isoform were analyzed.

RESULTS

EMT was observed in mHAT9d cultured in the presence of TGF-β1 and β3 but not TGF-β2. The expression of both epithelial and mesenchymal marker genes was observed in mHAT9d exhibiting EMT. NGS analysis suggested extracellular signal-regulated kinase (ERK) and Rho pathways as TGF-β signaling pathways associated with EMT. However, EMT in mHAT9d cultured in the presence of TGF-β1 or β3 occurred even in presence of an ERK1/2 inhibitor and was suppressed by Rho-kinase inhibitor. The expression of co-receptors for TGF-β signaling in mHAT9d cells reduced following stimulation with each TGF-β isoform. In contrast, endoglin levels increased following TGF-β1 or β3 stimulation, but no change was noted in response to TGF-β2.

CONCLUSIONS

We propose that in TGF-β-stimulated enamel epithelial cells, EMT mainly occurred via the Rho signaling pathway, and the differences in response across TGF-β isoforms were due to their endoglin-mediated binding affinity for the TGF-β receptor.

4-Hexylresorcinol Administration Increases Dental Hard Tissue Formation and Incisor Eruption Rate in Rats

Dental hard tissue formation and bone turnover are required for tooth eruption. 4-Hexylresorcinol (4HR) accelerates tooth movement by increasing bone turnover in orthodontic treatment. This study aimed to evaluate the following: (1) the effect of 4HR application on the expression of proteins associated with tooth formation, and (2) the effect of 4HR application on mandibular incisor eruption rate in a rat model. Primary cultured pulp cells received either 4HR (1 to 100 µM) or solvent only; western blotting was performed for transforming growth factor-beta 1 (TGF-β1), bone morphogenic protein-2/4 (BMP-2/4), runt-related transcription factor 2 (Runx2), osterix (OSX), dentin sialophosphoprotein (DSPP), and parathyroid hormone-related protein receptor (PTHrP-R). In in vivo study, rats (15 males and 15 females) received either solvent or 0.128 mg/kg or 12.8 mg/kg of 4HR via subcutaneous injection; mandibular incisor eruption rate was subsequently recorded. Immunohistochemical staining and western blotting for TGF-β1, BMP-2/4, Runx2, OSX, DSPP, and PTHrP-R were performed in the mandibular tissue samples. 4HR administration was found to increase TGF-β1, BMP-2/4, Runx2, OSX, DSPP, and PTHrP-R expression in both cell culture and tissue samples. Immunohistochemical staining of some markers showed site-specific expression, thereby indicating programmed differentiation of odontoblasts and ameloblasts. The eruption rate was significantly higher in the 12.8 mg/kg 4HR-administered group than in the untreated control (p = 0.001 and 0.010 for males and females, respectively). Collectively, 4HR administration increased the expression of markers related to dental hard tissue formation and accelerated the eruption rate of incisors in rats.

Tooth Formation: Are the Hardest Tissues of Human Body Hard to Regenerate?

With increasing life expectancy, demands for dental tissue and whole-tooth regeneration are becoming more significant. Despite great progress in medicine, including regenerative therapies, the complex structure of dental tissues introduces several challenges to the field of regenerative dentistry. Interdisciplinary efforts from cellular biologists, material scientists, and clinical odontologists are being made to establish strategies and find the solutions for dental tissue regeneration and/or whole-tooth regeneration. In recent years, many significant discoveries were done regarding signaling pathways and factors shaping calcified tissue genesis, including those of tooth. Novel biocompatible scaffolds and polymer-based drug release systems are under development and may soon result in clinically applicable biomaterials with the potential to modulate signaling cascades involved in dental tissue genesis and regeneration. Approaches for whole-tooth regeneration utilizing adult stem cells, induced pluripotent stem cells, or tooth germ cells transplantation are emerging as promising alternatives to overcome existing in vitro tissue generation hurdles. In this interdisciplinary review, most recent advances in cellular signaling guiding dental tissue genesis, novel functionalized scaffolds and drug release material, various odontogenic cell sources, and methods for tooth regeneration are discussed thus providing a multi-faceted, up-to-date, and illustrative overview on the tooth regeneration matter, alongside hints for future directions in the challenging field of regenerative dentistry.