Strontium (Sr) elicits odontogenic differentiation of human dental pulp stem cells (hDPSCs): A therapeutic role for Sr in dentine repair?

Abstract

Strontium (Sr) forms a significant component of dental restorative materials and although it is widely used in toothpastes, the biological effects of Sr on the dentine-pulp complex have not been investigated. In this first study, we characterise the Sr elicited effects on human dental pulp stem cells (hDPSC) in vitro using exogenously Sr added to culture medium, and bioavailable Sr derived from a novel bioactive glass (BG). The related mechanisms were also investigated. Our results indicate that low dose Sr (between 0.1 and 2.5mM) induces proliferation and alkaline phosphatase (ALP) activity of hDPSCs, but has no effect on colony formation or cell migration. Sr at specific concentrations (1 and 2.5mM) stimulated collagen formation and mineralisation of the hDPSC generated matrix. In addition, qRT-PCR, Western blotting and immunocytochemistry revealed that Sr regulates gene expression and the protein secretion of the odontogenic markers: dentine sialophosphoprotein (DSPP) and dentine matrix protein 1 (DMP-1) and protein localisation (DSPP was localised to the Golgi, while no apparent changes occur in DMP-1 distribution which remains in both cytosol and the nucleus). Additionally, the calcium sensing receptor (CaSR) and downstream pathway MAPK/ERK signalling pathway in hDPSCs were activated by Sr. Bioavailable Sr from the BG revealed novel biological insights of regulating metabolic and ALP activities in hDPSCs. Taken together, these results suggest that Sr at specific doses significantly influences proliferation, odontogenic differentiation and mineralisation of hDPSCs in vitro via the CaSR using a pathway with similarities to osteoblast differentiation. These are the first such studies and indicate that Sr treatment of hDPSCs could be a promising therapeutic agent in dental applications. In conclusion, we propose that Sr from a substituted BG could be used more effectively in biomaterials designed for dental applications. Statement of SignificanceDespite the fact that strontium (Sr) is used widely in dental practise, its potential effects on odontoblasts have been ignored. Our study provides the first evidence that Sr (exogenous and that derived from a bioglass (BG)) can stimulate dentinogenesis in human dental pulp stem cells (hDPSCs) by promoting their proliferation, differentiation and mineralisation in vitro. Therefore, while previously unrecognised, Sr BG is likely to be beneficial in atraumatic dentistry practise and maintenance of a competent tooth in conditions such as caries. Repair of defected dentine is still one of the main challenges in dental research and annually untreated caries results in the loss of productivity equivalent to US$ 27 billion. Advances in tissue engineering technology, alongside the use of dental pulp stem cells provide an approach to achieve dentine regeneration. Understanding the actions of Sr will permit a more controlled application of Sr in the clinic. These data are thus likely to be of great interest to the material scientists, biological researchers, clinicians and manufacturers of dental products.