Bone cell responses to high-frequency vibration stress: does the nucleus oscillate within the cytoplasm?

Mechanosensing by cells directs changes in bone mass and structure in response to the challenges of mechanical loading. Low-amplitude, high-frequency loading stimulates bone growth by enhancing bone formation and inhibiting disuse osteoporosis. However, how bone cells sense vibration stress is unknown. Hence, we investigated bone cell responses to vibration stress at a wide frequency range (5-100 Hz). We used NO and prostaglandin E₂ (PGE₂) release, and COX-2 mRNA expression as parameters for bone cell response since these molecules regulate bone adaptation to mechanical loading. NO release positively correlated whereas PGE₂ release negatively correlated to the maximum acceleration rate of the vibration stress. COX-2 mRNA expression increased in a frequency-dependent manner, which relates to increased NO release at high frequencies, confirming our previous results. The negatively correlated release of NO and PGE₂ suggests that these signaling molecules play different roles in bone adaptation to high-frequency loading. The maximum acceleration rate is proportional to ω³ (frequency=ω/2π), which is commensurate with the Stokes-Einstein relation for modeling cell nucleus motion within the cytoplasm due to vibration stress. Correlations of NO and PGE₂ with the maximum acceleration rate then relate to nucleus oscillations, providing a physical basis for cellular mechanosensing of high-frequency loading.--Bacabac, R. G., Smit, T. H., Van Loon, J. J. W. A., Doulabi, B. Z., Helder, M., Klein-Nulend, J. Bone cell responses to high-frequency vibration stress: does the nucleus oscillate within the cytoplasm?