Senescence and odontoblastic differentiation of dental pulp cells

Cellular senescence has been suggested to be involved in physiological changes of cytokine production. Previous studies showed that the concentration of tumor necrosis factor-alpha (TNF-alpha) is higher in the blood of aged people compared with that of young people. So far, the precise effects of TNF-alpha on theodontoblastic differentiation of pulp cells have been controversial. Therefore, we aimed to clarify how this cytokine affected pulp cells during aging. Human dental pulp cells (HDPCs) were cultured until reaching the plateau of their growth, and the cells were isolated at actively (young HDPCs; gamma HDPCs) or inactively (senescent HDPCs; sHDPCs) proliferating stages. sHDPCs expressed senescence-related molecules while gamma HDPCs did not. When these HDPCs were cultured in anodontoblast-inductive medium, both young and senescent cells showed mineralization, but mineralization in sHDPCs was lower compared with gamma HDPCs. However, the administration of TNF-alpha to this culture medium altered these responses: gamma HDPCs showed downregulated mineralization, while sHDPCs exhibited significantly increased mineralization. Furthermore, the expression of tumor necrosis factor receptor 1 (TNFR1), a receptor of TNF-alpha, was significantly upregulated in sHDPCs compared with gamma HDPCs. Downregulation of TNFR1 expression led to decreased mineralization of TNF-alpha-treated sHDPCs, whereas restored the reduction in TNF-alpha-treated gamma HDPCs. These results suggested that sHDPCs preserved the odontoblastic differentiation capacity and TNF-alpha promoted odontoblastic differentiation of HDPCs with the progress of their population doublings through increased expression of TNFR1. Thus, TNF-alpha might exert a different effect on the odontoblastic differentiation of HDPCs depending on their proliferating activity. In addition, the calcification of pulp chamber with age may be related with increased reactivity of pulp cells to TNF-alpha.