Mechanoregulation of osteoblast-like MG-63 cell activities by cyclic stretching

Mechanical loading plays an important role in regulating bone formation and remodeling. Relevant mechanical stretching can increase the proliferation and differentiation of osteoblastic cells in vitro. However, little is known about the effects of supraphysiological high-level mechanical stretching on the growth and cell cycle progression of osteoblastic cells. Methods: Osteoblast-like MG-63 cells were seeded onto flexible-bottomed plates and subjected to cyclic mechanical stretching (15% elongation, 0.5 Hz) for 24 and 48 hours in a Flexercell FX-4000 strain unit. Cellular activities were measured by an assay based on the reduction of the tetrazolium salt, 3-[4,5-dimethyldiazol-2-yl]-2,5-diphenyl tetra-zolium bromide (MTT). The number of viable cells was also determined by the trypan blue dye exclusion technique. Cell cycle progression was checked by flow cytometry. mRNA expressions of apoptosis- and cell cycle-related genes (Bcl2, Bax, cdc2, cdc25C, and cyclin B1) were analyzed using an RT-PCR technique. Results: The number of viable cells significantly decreased in osteoblast-like MG-63 cells subjected to cyclic mechanical stretching for 24 or 48 hours. The MTT activity of stretched cells did not change at 24 hours, whereas a significant decrease was noted at 48 hours in comparison to the unstretched controls. The flow cytometry showed that mechanical stretching induced S-phase cell cycle arrest. Furthermore, exposure to mechanical stretching led to apoptotic cell death, as shown by the increase in the hypodiploid sub-G0/G1 cell population. Furthermore, a decreased cdc25C mRNA level was consistently noted in stretched cells. However, the mRNA expressions of Bcl2, Bax, cdc2, and cyclin B1 genes were not significantly altered compared to the unstretched control cells. Conclusion: High-level mechanical stretching induced S-phase cell cycle arrest and apoptotic cell death in osteoblastic cells. The results suggest that heavy tensional force is a negative regulator of osteoblastic activities and should, therefore, be minimized if bone formation is attempted during orthodontic/orthopedic treatment