Cyclic Mechanical Strain Increases Production of Regulators of Bone Healing in Cultured Murine Osteoblasts

Background: The adaptive response of bone to mechanical strain, for which angiogenesis is required, is underscored during fracture healing. Vascular endothelial growth factor (VEGF) and transforming growth factor β-1 (TGF-β1) are critical regulators of angiogenesis. The purpose of this study was to examine the effect of strain on the production of VEGF and TGF-β1. Study design: MC3T3-E1 mouse osteoblasts underwent cyclic strain (low, 0.1 Hz, or high, 0.2 Hz) for 24 or 48 hours. VEGF and TGF-β1 protein levels were determined by ELISA, and Northern blot analysis was performed for VEGF mRNA. Alkaline phosphatase (an osteoblast differentiation marker) activity was determined by functional enzymatic assay. All measurements were standardized for cell number by crystal violet colorimetric assay. Statistical significance was determined by t-test, ANOVA, and the Tukey-Kramer test. Results: Protein production of VEGF and TGF-β1 was dose-dependently elevated by strain (p < 0.05); alkaline phosphatase did not rise significantly. Northern blot analysis of strained osteoblast cells demonstrated increased VEGF mRNA. Cyclic strain was found to be progressively destructive in a dose-dependent manner, causing 51% and 70% decreases in cell number under low and high strain, respectively (p < 0.01). Conclusions: We demonstrated simultaneous, dose-dependent increases in VEGF and TGF-β1 protein production by osteoblastic cells in response to increasing strain. VEGF mRNA also increased in response to strain. This strain-induced increase in angiogenic cytokines suggests a potential mechanism by which injured bone may recruit a new blood supply. But we also found increasing strain to increase cellular toxicity, suggesting that cyclic mechanical strain may select for a subpopulation of osteoblasts.