Introduction: Mineral trioxide aggregate (MTA) is a therapeutic, endodontic repair material that is reported to exhibit calcified tissue-conductive activity although the mechanisms remain unclear. We hypothesize that the dissolution of calcium from MTA into the surrounding environment may play an important role in the osteoblastic/cementoblastic differentiation of human periodontal ligament cells (HPLCs). Methods: Two populations of HPLCs were obtained from two patients, respectively, and were cultured in the presence or absence of MTA discs and/or CaCl2 in order to investigate calcium release, calcification activity, calcium-sensing receptor (CaSR) gene expression and bone morphogenetic protein-2 (BMP-2), and BMP-2 receptor protein and gene expression. Results: MTA released a substantial accumulation of calcium (4 mmol/L) within 14 days into culture media. After 4 weeks, the two populations of HPLCs independently exhibited calcification as well as BMP-2 distribution in the vicinity of MTA. HPLCs inherently expressed genes encoding for the CaSR and BMP-2 receptors. Exogenous CaCl2 media supplementation induced CaSR gene expression in HPLCs and calcification and BMP-2 synthesis throughout the entire HPLC cultures, whereas MgCl2 had no effect. Both MTA and CaCl2 stimulated BMP-2 gene expression above that of baseline levels. Conclusion: Here we show the first report showing that HPLCs cocultured directly with MTA up-regulated BMP2 expression and calcification. These results may be through CaSR interactions that were potentially activated by the release of calcium from MTA into the culture environment.
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