Osteoblasts Subjected to Mechanical Strain Inhibit Osteoclastic Differentiation and Bone Resorption in a Co-Culture System

Bone remodeling is strictly mediated by the coupled activities of osteoblasts and osteoclasts, which are responsible for bone formation and resorption, respectively. Although many papers have been published on the mechanical responses of osteoblasts and osteoclasts, little is known about their communication during mechanical loading. In this study, a novel co-culture system was first established using Transwell culture inserts; MC3T3-E1 cells were embedded in the lower compartment of the inserts, and RAW264.7 cells were co-cultured in the upper compartment. The MC3T3-E1 cells were subjected to a mechanical strain of 2500 με at 0.5 Hz to investigate the effect of strain-loaded osteoblasts on co-cultured osteoclasts. The results showed that osteoblast-like cells were activated with an increase of alkaline phosphatase (ALP) activities. The strain-conditioned medium caused decreased activity of tartrate-resistant acid phosphatase and reduced the number of mature multinucleated osteoclasts, which subsequently resulted in the suppressed formation of resorption pits. The expression levels of cathepsin-K and matrix metalloproteinase-9 were also depressed by the strain-conditioned medium. In addition, we found that the expression ratio between osteoprotegerin (OPG) and receptor activator of NF-kB ligand in osteoblasts was significantly up-regulated due to the enhanced levels of OPG. In summary, we conclude that the strain-stimulated osteoblasts inhibited the differentiation and bone resorption of osteoclasts and that the mechanism was associated with the increased secretion of OPG in osteoblasts.