Effect of simulated orthodontic forces on fluoride-induced cytotoxicity in MC3T3-E1 osteoblast-like cells

The effects of simulated orthodontic forces such as centrifugal force or water pressure on sodium fluoride (NaF)-induced cytotoxicity against mouse osteoblast-like cells MC3T3-E1 were investigated. Loading with centrifugal force (44.5 g/cm2) or water pressure (5 g/cm2) slightly reduced the cell proliferation and additively enhanced the cytotoxic activity of millimolar concentrations of NaF. NaF induced the appearance of phosphatidylserine at outer cell membrane (detected by Annexin staining) but failed to induce caspase-3 activation even under the water pressure. On the other hand, NaF induced autophagic phenotype characterized by the formation of acidic organelles (detected by acridine orange staining). NaF did not increase, but rather dose-dependently reduced the alkaline phosphatase activity, with or without the loading of water pressure. The present study demonstrates that centrifugal force and water pressure partially enhanced the caspase-independent cytotoxicity of NaF against osteoblasts. These simulated orthodontic forces may be a new factor that affects the physiological activity of NaF.