Enhancement of Osteoblast Proliferation via ERK Signaling When Cultured on Electrically Polarized Hydroxyapatite

Living cells are subjected to various biomechanical stimuli from their local microenvironments. The information transforms into intracellular biochemical signals, and the stimulus induces a change in cell behaviors. Electrical stimulation is known to affect bone formation; in particular, in vivo studies have shown that hydroxyapatite (HAp) electret stimulation could enhance bone formation. However, the molecular mechanisms underlying the electret-induced influences on osteogenic cells have not been clarified. In this study, we investigated the molecular mechanotransduction cascade that mediates the cell responses to the electric stimuli produced by the electret in order to regulate its safe application in a clinical setting. The HAp electret was fabricated to be applied through a DC field on HAp at high temperature to enhance proton conduction in the HAp lattice. Osteoblast-like cells were cultured on the electret, which induced cell adhesion and proliferation responses. In addition, the adaptor protein p130Cas (Cas), a known mechanosensor protein, sensed the electret-induced stimuli and activated a downstream signal cascade involving extracellular signal-regulated kinase (ERK). Entry to the S-phase of the cell cycle was subsequently promoted. Further, activation of Cas and ERK was maintained throughout the cell culture period. These results likely arise from the interactions between osteoblasts and the HAp electret, whose surface charges and electrostatic generator function are assumed to function in promoting cell adhesion and proliferation under static dynamic conditions, respectively. Thus, HAp electret-induced stimuli are beneficial for enhancing osteoblast proliferation, suggesting that an electret could be a maintenance-free bone substitute to promote bone regeneration.