Biological Responses of Cementoblasts to Mechanical Loading in Vitro

External apical root resorption (EARR) occurs in 75-80% of patients undergoing orthodontic treatment. Mechanical loading has been shown to be largely contributive to the formation of EARR, however its mechanism remains unknown. Cementum, the shielding layer covering the dental root, bears a dynamic mechanical load during orthodontic tooth movement. Cementoblasts secrete mineral matrix and ultimately become cementocytes embedded in cementum. resembling the morphology and functions of osteocytes in bone matrix. Comparable to osteoblasts and osteocytes, which have been found to play an active role in modulating bone metabolism, we hypothesize that cementoblasts actively respond to mechanical loading and thus play a pivotal role in the control of cernentogenesis and cementum repair. To investigate whether, and how, cementoblasts respond to mechanical loading in vitro, cells of OCCM-30, an immortalized murine cementoblastic cell line, were subjected to a physiological level of 12 dyne/cm2 fluid shear stress (FSS) for 30 and 60 minutes. To study the early signaling events after the onset of flow, the cell culture medium was sampled at 5 minutes to test ATP and 15 minutes to test PGE2 releases. To investigate the functional protein changes, OCCM-30 cells were lysed to test mitogen-activated protein kinase (MAPK) activation after 30 minutes of FSS and 60 minutes of FSS followed by 6 hours of post incubation to examine cyclooxygenase (COX)-2, osteopontin (OPN), osteoprotegerin (OPG) and Receptor Activator of NFkB Ligand (RANKL) productions. FSS increased ATP at 5 minutes and PGE2 release at 15 and 60 minutes. FSS also increased COX-2 and OPN production. Different from osteoblasts, both ERK l/2 and p38 were phosphorylated within 30 min by FSS in cementoblasts. Blocking either ERK 1/2 or p38 signaling pathway inhibited FSS-induced increased release of PGE2 and OPN production. Furthermore, FSS reduced RANKL in cementoblasts, which was dependent on ERK1/2 MAPK signaling pathway. The data suggest that, like osteoblasts, cementoblasts are mechanosensitive. Both cell types activate similar signaling pathways upon mechanical loading. A physiological level of FSS might be a promising anti-resorptive factor. Furthermore, a potential role for cementoblasts in the adaptation of cementum to mechanical load as occurred in EARR during orthodontic tooth movement is suggested