Senescent dental pulp cells express decreased Bmi-1 and undergo hypermineralization

Objectives: Bmi-1 is a polycomb group transcription repressor implicated in the maintenance of stem cell phenotype in various tissues. The current study was undertaken to examine the association between Bmi-1 expression and the mineralization or differentiation capacity of human dental pulp stem cells (DPSCs). Methods: DPSCs were maintained in serial subcultures until replicative senescence and were assayed for the onset of cellular senescence, in vitro mineralization capacity, and the expression of Bmi-1. Senescent cells were detected in situ by staining for the senescence-associated â-galactosidase (SA â-Gal) activity, a cytochemical marker of cellular senescence. Mineralization and cellular differentiation were detected by staining the cells with Alizarin Red or for alkaline phosphatase activity (ALP). Also, subculture-induced change in the expression level of Bmi-1 was determined by Western blotting. Results: Serially subcultured DPSCs demonstrated spontaneous replication arrest, SA â-Gal staining, and flattened morphology, which are the features of senescence. The mineralization capacity of the senescent culture of DPSCs was markedly increased compared to that of pre-senescent DPSCs, as evinced in increased formation of mineralized nodules positively stained for Alizarin Red. Also, senescent DPSCs expressed significantly reduced level of Bmi-1 and higher level of p16INK4A, a cell cycle inhibitor and downstream target of Bmi-1, compared with those of the replicating cells. CONCLUSION: These findings indicate that the loss of Bmi-1 expression in senescent DPSCs is associated with the enhanced mineralization capacity of cells. This study was supported by the grants (DE15316) from the NIDCR/NIH.