Effects of transforming growth factor-β and mechanical strain on osteoblast cell counts: An in vitro model for distraction osteogenesis

Factors known to regulate bone production during distraction osteogenesis include mechanical strain on bone forming cells and up- regulation of transforming growth factor-β (TGF-β) during the distraction, or strain phase of distraction osteogenesis. In the present study, an in vitro model was used to evaluate the functional effect of exogenous TGF-β1 on mitogenesis in murine-derived MC3T3 osteoblasts during the period of active mechanical strain. The first hypothesis to be tested was that mitogenic suppression of MC3T3 osteoblasts by TGF-β1 is further enhanced when these cells are also subjected to mechanical strain. To test this hypothesis, MC3T3 osteoblasts were seeded on flexible and rigid membranes. These were subjected to cyclic, vacuum-induced strain, simulating physiologic stress loads. After 24 hours, all cells were transferred to media containing TGF-β1, and strain was continued for an additional 48 hours. The study was repeated by using two doses of TGF-β1. This study demonstrated that final cell counts were significantly decreased in the presence of TGF-β1 in both the nonstrained and strained groups (p < 0.0001). The final cell count in the strained group was significantly less than that in the nonstrained group (p < 0.0001) for both concentrations of TGF-β1 tested, confirming the initial hypothesis. The second hypothesis to be tested was that alteration in the mitogenic response of MC3T3 osteoblasts after strain is not directly due to autocrine factors produced by the strained osteoblasts. To test this hypothesis, a proliferation assay was performed on nonconfluent MC3T3 osteoblasts by using conditioned media collected from strained and nonstrained osteoblasts. This study demonstrated no significant differences in cell counts after addition of conditioned media collected from strained versus nonstrained cells, confirming the latter hypothesis. The present study demonstrates the functional significance of mechanical strain on osteoblast cell counts. Furthermore, this may help to explain the temporal relationship observed during the early distraction (strain) phase of distraction osteogenesis in rodent models in which peak up-regulation of TGF-β1 gene expression correlates with peak suppression of osteoblast function as measured by gene expression of extracellular matrix proteins.