Insight Into the Stimulatory Effects of IGF1 on Odontoblast Differentiation

Objectives: Dentin formation in the developing teeth is regulated by various growth factors modulating odontoblast formation and differentiation. Our previous observations have shown that overexpression of insulin-like growth factor 1 (Igf1) under the control of the 3.6-kb Col1a1 promoter resulted in a distinct phenotype of molar teeth in 4- and 8-week-old mice. Those effects included significant increases in dentin thickness, volume, and mineralized area without changes in pulp chamber dimensions. However, the underlying mechanisms of these effects remain unknown. The purpose of the present study was to examine the underlying mechanisms by which overexpression of IGF1 in odontoblasts leads to the increased dentin thickness.
Methods: Primary cultures were established from coronal pulp tissue of unerupted molars of 5-7-day-old non-transgenic and 2.3-GFP and DSPP-Cerulean transgenic mice. Between days 0-7, the cells were cultured under proliferation-supporting conditions, and between days 7-14, the cells were cultured under conditions supporting their differentiation. The cultures were exposed to 20 ng/ml recombinant human IGF1 between days 3-7, 7-14, and 3-14 (referred to as early, late, and continuous exposures, respectively). Cell proliferation, odontoblast differentiation, and extent of mineralization were examined using various assays. The α level of ≤0.01 was considered statistically significant.
Results: IGF1 significantly increased pulp cell proliferation (~1.2 fold) as compared to control (vehicle). Early exposure did not affect mineralization, expression of odontoblast differentiation markers, 2.3-GFP transgene intensity, and the percentage of DSPP-Cerulean+ odontoblasts as compared to control. Both late and continuous exposures markedly increased mineralization (~1.3 fold), expression of odontoblast differentiation markers, 2.3-GFP intensity (~1.3-fold), and the percentage of DSPP-Cerulean+ odontoblasts (~1.2 fold) as compared to control. In all experiments, no statistically significant differences were observed between late and continuous exposures.
Conclusions: IGF1-mediated increases in dentin thickness are mediated via increases in odontoblast differentiation and formation of odontoblasts but not in the proliferation of odontoblast progenitors in dental pulp.