Tumor Necrosis Factor Alpha Stimulates Proliferation of Dental Pulp Stem Cells via Akt/Glycogen Synthase Kinase-3β/Cyclin D1 Signaling Pathway

Introduction: It has been widely accepted that dental pulp stem cells (DPSCs), which are a class of self-renewal and differentiation potential of adult stem cells, play an important role in the repair procession of pulp's inflammation. We investigated whether tumor necrosis factor alpha (TNF-alpha) could induce the proliferation of DPSCs and clarified the potential mechanism of this proliferation. Methods: Cell Counting Kit-8 assay (Dojindo Laboratories, Mashiki-machi, Kumamoto, Japan) and 5-ethyny1-2'-deoxyuridine based proliferation assays were determined to investigate various concentrations or hours of TNF-alpha inducing a cell number change of DPSCs. Next, flow cytometry analysis was performed to investigate the main cell cycle phase process of DPSCs. Furthermore, the signaling pathway of TNF-alpha-induced proliferation of DPSCs was analyzed using Western blot analysis. Then, inhibitors were added to confirm the mechanism of this signaling pathway. Results: TNF-alpha induced the proliferation of DPSCs in a dose- and time-dependent manner. Cyclin D1, which controlled the cell cycle process from the G1 to the S phase, was up-regulated by TNF-alpha in a time-dependent manner, whereas its overexpression alone increased DPSC proliferation. Furthermore, TNF-alpha was capable of inducing Akt/GSK-3 beta signaling pathway activation. Blockage of phosphoinositide 3-kinase/Akt by their kinase or genetic inhibitors could significantly reduce TNF-alpha induced proliferation of DPSCs. Conclusions: This study confirmed that TNF-alpha induced the proliferation of DPSCs by regulating the Akt/GSK-3 beta/cyclin D1 signaling pathway and then provided a suitable number for the requirements of cell differentiation.