Compressive force induces PGE₂ and its receptors production in osteoblasts

Objective: In orthodontic tooth movement, prostaglandin E₂ (PGE₂) released from osteoblasts can alter the normal process of bone remodeling. The aim of this study was to examine the effect of compressive force (CF) on PGE₂ production, the expression of PGE receptors Ep1–4, phosphorylation of protein kinase A (p-PKA), and calcium content of the extracellular matrix in osteoblastic Saos-2 cells. We also examined the effect of PGE₂ on p-PKA and calcium content. Methods: The cells were cultured for up to 24 h with or without continuous CF (98–294 Pa) and in the presence or absence of 10^-6 M indomethacin. The expression level of PGE receptors was estimated by determining mRNA levels using real-time PCR, and protein levels were examined by immunohistochemical staining. Results: PGE₂ production increased as CF strength. However, the simultaneous addition of indomethacin inhibited PGE₂ production. Applying CF of 98 or 294 Pa caused the cells to produce approximately 700 and 1400 pg/mL PGE₂, respectively. CF of 98 Pa increased Ep2 gene expression, and 98 and 294 Pa CF increased Ep4. Ep1–4 expression was not affected by the presence or absence of indomethacin. Immunohistochemical staining showed strong expression of Ep2 under 98 Pa, and Ep4 under 98 and 294 Pa. The p-PKA increased as the strength of CF or PGE₂ concentration. The simultaneous addition of indomethacin inhibited this increase in p-PKA. Culturing Saos-2 cells in conditioned medium from cells exposed to 98 Pa CF increased the calcium content of the extracellular matrix, whereas conditioned medium from cells exposed to 294 Pa CF decreased the calcium content. The calcium content was increased by the addition of 700 pg/mL PGE₂, but was decreased by 1400 pg/mL. Conclusion: Mechanical stress controls bone formation by stimulating PGE₂ production and Ep2 and/or Ep4 expression in osteoblasts.