Elevated extracellular calcium increases expression of bone morphogenetic protein-2 gene via a calcium channel and ERK pathway in human dental pulp cells

Dental pulp cells, which have been shown to share phenotypical features with osteoblasts, are capable of differentiating into odontoblast-like cells and generating a dentin-like mineral structure. Elevated extracellular Ca²⁺Ca_o^{2 +} has been implicated in osteogenesis by stimulating the proliferation and differentiation of osteoblasts; however, the role of Ca_o^{2 +} signaling in odontogenesis remains unclear. We found that elevated Ca_o^{2 +} increases bone morphogenetic protein (BMP)-2 gene expression in human dental pulp cells. The increase was modulated not only at a transcriptional level but also at a post-transcriptional level, because treatment with Ca²⁺ increased the stability of BMP-2 mRNA in the presence of actinomycin D, an inhibitor of transcription. A similar increase in BMP-2 mRNA level was observed in other human mesenchymal cells from oral tissue; periodontal ligament cells and gingival fibroblasts. However, the latter cells exhibited considerably lower expression of BMP-2 mRNA compared with dental pulp cells and periodontal ligament cells. The BMP-2 increase was markedly inhibited by pretreatment with an extracellular signal-regulated kinase (ERK) inhibitor, PD98059, and partially inhibited by the L-type Ca²⁺ channels inhibitor, nifedipine. However, pretreatment with nifedipine had no effect on ERK1/2 phosphorylation triggered by Ca²⁺, suggesting that the Ca²⁺ influx from Ca²⁺ channels may operate independently of ERK signaling. Dental pulp cells do not express the transcript of Ca²⁺-sensing receptors (CaSR) and only respond slightly to other cations such as Sr²⁺ and spermine, suggesting that dental pulp cells respond to Ca_o^{2 +} to increase BMP-2 mRNA expression in a manner different from CaSR and rather specific for Ca_o^{2 +} among cations.